Symposium ES10—Rational Design Hierarchical Nanostructures for Photocatalytic System

Silicon materials has many advantages, including earth-crust, wide spectra absorption, high solar to chemicals conversion efficiency, suitable band edge. However, the amorphous silicon-based photoelectrode is very easily to be corroded by electrolyte. In the presentation, cobalt oxide by magnetron sputtering at ambient temperature was introduced as a protective layer in water oxidation, and atomic Pt as HER cocatalysts prepared by magnetron sputtering method on thin film silicon was used for water reduction. The photoanode delivered a current density of 7.3 mA cm^-2 at 1.23V vs. RHE, and the onset potential was negative shift to 0.62 V. The photocathode current density at the 0 V vs RHE was increased to 12.03 mA cm^-2 in 1M KOH electrolyte with the onset potential of 0.82 V. The Pt/Si faradic efficiency was ~ 100 % and the fill factor was 38 % that was the highest value among Si-based PEC system. Pt and CoO_x strengthened the inner electric field and passivated the surface of photoelectrodes. The dual-amorphous silicon photoelectrodes integrated CoO_x and Pt realized the bias-free water splitting at 0.81 V with a photocurrent of 1.5 mA cm^-2, and 0.92 % of the solar to hydrogen efficiency. The novel amorphous silicon photoanode provides a promising strategy for photo-electrochemical energy conversion and storage.


Acknowledgement: This research was financially supported by the National Natural Science Foundation of China (61564003, 61774050), and the Guangxi Natural Science Foundation (2015GXNSFGA139002), and the Guangxi Bagui Scholar program.

References (12 pt)
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The promotion of the separation and suppression of the recombination of photo-generated charge carrier have attracted extensive attention in the field of photocatalysis, and constructing heterogeneous junction of photocatalyst by passive types as the core is one of the most effective ways. However, the construction of heterogeneous junction is an uncontrollable and spontaneous process, which may have many disadvantages to improve the photocatalytic performance. In addition, taking advantage of the external fields to enhance charge carrier separation such as electric field enhanced charge carrier separation will bring more energy consumption. Therefore, we proposed a non-power strategy to act on the process of charge carrier transport through Lorentz force and apply in the TiO₂ nanobelts to improve the photocatalytic performance. In this work, the results of photoluminescence spectra, photocurrent density and theoretical calculation proved the reaction mechanism that the Lorentz force can effectively promote the separation and inhibit the recombination in the process of the charge carrier transport. It is believed that the new strategy based on the magnetic effect will start a new thinking of the charge separation, and our work will be extremely illuminating to the future researches in the field of photocatalysis.

Traditional heterogeneous catalysis that requires high temperatures to overcome reaction barriers often activates undesired side reactions and shortens catalyst lifetimes. The use of illuminated metal nanoparticles in plasmonic photocatalysis has led to accelerated catalytic activities, reduced activation energies, and the ability to select for desired products at reduced temperatures for important chemical reactions. A key challenge in studying such reactions is the separation of the thermal vs. nonthermal contributions. Here, we implement rhodium nanoparticles as a compelling ultraviolet plasmonic photocatalyst for carbon dioxide hydrogenation. Using direct and indirect illumination in conjunction with precise temperature measurements, effects on the overall reactivity of the catalyst from photo-generated hot carriers can be isolated and extracted to reveal the true nonthermal contribution. It is demonstrated that light and heat work together to accelerate important chemical reactions.

Primary amines have been shown to increase the photoluminescence quantum yield (PLQY) of CdSe nanocrystals (NCs), yet little is known about how the binding of amines effects the energy transfer properties of these materials. It is well known that surface effects play the largest role in energy transfer efficiencies in nanocrystalline materials, better understanding, and subsequently controlling surface states is the key to unlocking the full potential of NCs in energy transfer processes, such as upconversion. In this study, CdSe NCs were placed in varying concentrations of 1-propylamine, with and without 2-anthracenecarboxylic acid transmitter ligands attached. This poor transmitter ligand allows for better observation of PL enhancement due to the primary amine. It is shown that adding a primary amine increases the PLQY of the NC, as well as increasing the photon upconversion quantum yield. This affect is maximized at low (11 mM) concentrations of propylamine, while the PLQY can be enhanced further by increasing the amine concentration. By using transient absorption, this work studies the underlying energy transfer processes both enhanced and suppressed by the addition of amine. Through this work the importance of surface trap states are quantified along with the band edge emission of the NC, allowing for a better understanding of important surface chemistry in these complicated systems.

TiO₂nanomaterials have over the last 30 years attracted tremendous scientific and technological interest. Particularly various 1D and highly defined TiO₂morphologies were explored for the replacement of nanoparticle networks and were found in many cases far superior to nanoparticles or their assemblies. Nanotubes or wires can be grown by hydrothermal or template methods, or even more elegantly, by self-organizing anodic oxidation. The latter is not limited to TiO₂but to a full range of other functional oxide structures that on various metals and alloys can be formed. These advanced and doped morphologies can be grown on conductive substrates as ordered layers and therefore can be directly used as functional electrodes (e.g. photo-anodes). The presentation will focus on these highly ordered nanotube arrays of TiO₂(and similar) and discuss most recent progress in synthesis, modification and applications towards photocatalytic and photoelectrochemical applications, such as noble-metal-free H₂generation or the site-selective placement of active centers onto/into these tube layers.

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Design and engineering of the size, shape, and chemistry of photoactive building blocks enable the fabrication of functional nanoparticles for applications in light harvesting, photocatalytic synthesis, water splitting, phototherapy, and photodegradation. Here, we report the synthesis of such nanoparticles through a surfactant-assisted interfacial self-assembly process using optically active porphyrin as a functional building block. The self-assembly process relies on specific interactions such as π–π stacking and ligand coordination between individual porphyrin building blocks. Depending on the kinetic conditions, resulting structures exhibit well-defined one- to three-dimensional morphologies such as nanowires, nanooctahedra, and hierarchically ordered internal architectures. At the molecular level, porphyrins with well-defined size and chemistry possess unique optical and photocatalytic properties for potential synthesis of metallic structures. On the nanoscale, controlled assembly of macrocyclic monomers leads to formation of ordered nanostructures with precisely defined size, shape, and spatial monomer arrangement so as to facilitate intermolecular mass and energy transfer or delocalization for photocatalysis. Due to the hierarchical ordering of the porphyrins, the nanoparticles exhibit collective optical properties resulted from coupling of molecular porphyrins and photocatalytic activities such as photodegradation of methyl orange (MO) pollutants and hydrogen production. The capability of exerting rational control over dimension and morphology provides new opportunities for applications in sensing, nanoelectronics, and photocatalysis.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Gold-palladium (Au-Pd) bimetallic nanoparticles were prepared with a series of alloy and core-shell nanostructures to synergistically couple a plasmonic (Au) and catalytic (Pd) metal to tailor the optical and catalytic properties. Catalysts utilizing plasmonic metals that exhibit a localized surface plasmon resonance (SPR) can be harnessed for light-driven enhancement for small molecule oxidation via augmented carrier generation/separation and photothermal conversion. The colloidal AuPd bimetallic nanoparticles were used as catalysts to study the ethanol (EtOH) photo-oxidation cycle, with an emphasis towards driving carbon-carbon (C-C) bond cleavage at low temperatures. The AuPd nanoparticles were coupled to semiconductor photocatalysts and irradiated with targeted wavelength ranges to probe their effects on plasmonically-assisted photocatalytic oxidation of EtOH. The oxidation of EtOH to CO₂ under solar simulated-light irradiation was confirmed by monitoring the yield of gaseous products during photo-oxidation experiments using a gas chromatography-mass spectrometry-multiple headspace extraction (GC-MS-MHE) analysis method. The coupling of Au to Pd in an alloy or core-shell nanostructure at higher Au content maintains SPR-induced charge separation, mitigates the carbon monoxide poisoning effects on Pd, and allows for improved hydrocarbon oxidation. Under visible light (>420 nm) irradiation, carrier generation/separation and photothermal conversion was achieved, resulting in the photogenerated “hot” holes driving the photo-oxidation of EtOH, providing a method to selectively cleave C-C bonds. Bimetallics provide a pathway for driving desired photocatalytic and photoelectrochemical reactions with superior catalytic activity and selectivity.

Amorphous semiconductors are useful as they can mimic the properties of their crystalline counterparts while having the ability of being deposited uniformly over a large area at a rapid rate in a thin film geometry. For a great many years, the non-dispersive transport of holes in glassy-amorphous selenium has been of interest. This interest originates from the importance of elemental Selenium as a high-resolution, large-area, wide bandgap, and room-temperature semiconductor with many successful commercial applications such as 2D and 3D medical x-ray imaging. Amorphous selenium is the only amorphous material that undergoes impact ionization where only holes avalanche at high electric fields and is emerging as a viable large-area imaging detector with avalanche multiplication gain for low-light and low-dose radiation detection applications. This leads to a small Excess Noise Factor which is a very important performance comparison matrix for avalanche photodetectors. Thus, there is a need to model high field avalanche in amorphous selenium. At high fields, the transport in amorphous selenium changes from low values of activated trap-limited drift mobility to higher values of band transport mobility, via extended states. When the transport shifts from activated mobility with high degrees of localization to extended state band transport, then the wavefunction of the amorphous material resembles that of its crystalline counterpart. Thus, we expect the general details of the extended-state hole-phonon interaction in the amorphous phase to be described by the band-transport lattice theory of its crystalline counterpart’s namely monoclinic and trigonal selenium. To that effect and due to the intrinsic meta-stability of the monoclinic phase and high trap density in prepared specimens, we study hole transport in crystalline trigonal selenium semiconductors using a bulk Monte Carlo technique to solve the semi-classical Boltzmann transport equation (MC-BTE).
In this work we first perform molecular dynamic simulations (MD) of vapor-deposited amorphous selenium thin-films using a well-established empirical three-body interatomic potential. On comparing the simulated reduced radial distribution function (RDF) obtained from the MD simulations of amorphous Selenium with monoclinic Selenium and trigonal Selenium, we saw strong correlations up to a distance of 10 A. Thus, we showed that the molecular arrangement for the amorphous phase has considerable local order which may be a plausible explanation for the similar general feature of the electronic structure with its crystalline counterparts. Next we utilized density functional theory (DFT) simulations to calculate the density of states and acoustic/optical deformation potentials for the crystalline phases. The accuracy of our DFT model was validated by the very similar results that was previously reported for the ab initio self-consistent energy band structure and previously measured phonon dispersion data for trigonal Selenium. The results from our DFT calculations were then used to calculate the phonon scattering rates which were utilized in the bulk MC algorithm to stochastically interrupt hole free flights. Using our MC-BTE model we calculated phonon-limited low-field drift mobilities which are comparable to published experimental values for trigonal Selenium in both perpendicular and parallel directions to the c-axis for a wide range of temperatures (100-500 k).

To summarize, we showed that holes in selenium can undergo both elastic and inelastic collisions and yet get `hot', thus gaining more energy from the electric field than they lose to lattice vibrations. This study makes a strong case for the need of a microscopic dynamical model to study hole transport in selenium semiconductor devices.

Colloidal assemblies of complementing nanoscale or molecular components represent a promising class of multifunctional materials. Their composite architecture can allow several reaction steps to progress at the same site with minimal mass and energy transfer distances, which is an important factor for optimizing energy conversion processes in artificial systems. Here, we would like to report on a general strategy for integrating arbitrary inorganic nanocrystals into multi-component colloids. The synthetic innovation lies in stimulating the viscoelastic behavior of inorganic nanocrystals, which causes dissimilar colloids to bond at hybrid interfaces in a controllable manner. This process is initiated by the introduction of ion-solubilizing molecules to colloidal solutions that, under thermal activation, promote nanoparticle coalescence into clusters of two, three, or more domains. Once a desired size of a nanoparticle cluster is formed, the process is thermally switched off. We demonstrate that the ability to assemble pre-fabricated inorganic colloids permits a programmable design of multifunctional nanostructures, where a particular selection of materials is optimized to perform catalytic, light-emitting, or other energy conversion functions.

Nanostructured porous electrodes offer significant advantages for chemical and energy transformations due to their extremely high internal surface area and flexible accommodation of active components, and they have found a wide range of applications including photoelectrocatalysis. Their performance is determined by the rate of transport of charge carriers and/or reactants through the porous network as well as their photophysical and chemical properties, however many details remain to be understood. In this talk I will present an analysis of how charge flow and reactant transport are interconnected in a specific type of system, a dye-sensitized solar cell (DSSC) with a I₃^-/I^- redox couple, using detailed reaction-diffusion simulations. The DSSC is chosen as a model system for the more general case because of the abundant literature available on it for model construction and validation. The stochastic, multiscale computational technique used in this work allows detailed modeling of processes over a very large dynamic range of distances and time, thereby predicting macroscopic observables such as current at the same time as revealing extensive detail about the processes occurring within the porous nanostructured material. The calculations show that at low dye photoexcitation frequencies, the electron distribution in the system and the currents reflect a balance between production of I^- at the cathode and dye redox cycling that consumes I^- at the pore-electrolyte interface in the anode. Electron generation and current flow at high photoexcitation frequencies are controlled by the distribution of I^- in the electrolyte, which becomes highly nonuniform both in the pores and the electrolyte bulk. Trends in photoelectrode characteristics with pore density and geometry will be described, and some comments on general implications of the DSSC results for other types of systems will be made.

Mesoporous photocatalysts have gained great attention in the last decade by demonstrating that increased surface area and porosity can strongly improve their performance. In fact, all reports on mesoporous semiconductors support this scenario. But is it possible to quantify and compare the reported advantages of the mesopores and the increased surface area between different works? In this contribution, we present a model that can evaluate the improvements in photocatalytic activity achieved by the introduction of mesoporosity independent on synthetic or test conditions. We exemplify this methodology focusing on photocatalytic hydrogen/oxygen evolution with sacrificial reagents, but also include examples of CO₂ reduction and electrocatalysis. By correlating the relative increase in surface area to the relative increase in activity – in comparison to non-porous counterparts – we show that the origin of mesoporosity can have a pronounced influence on the activity enhancement, and that different semiconductor materials behave quite differently. Our model can serve as a starting point for the community to extract and compare key information on mesoporous photocatalysts, to put results into context of existing data, and to compare the performances of various catalytic systems much better.

A. S. Cherevan, L. Deilmann, T. Weller, D. Eder, R. Marschall, Appl. Energy Mater. 2018, DOI:10.1021/acsaem.8b01123

Graphitic carbon nitrides (gCNs) offer immense potential as inexpensive photocatalysts for solar fuel generation (e.g., H₂ from water) owing to their facile synthesis from a range of precursors, ability to absorb visible light, and high surface area.¹ Recent experimental and theoretical work has highlighted the importance of native N-H₂ defects in facilitating interfacial charge transfer to supported Pt cocatalyst.² Typically, gCNs are functionalized with Pt via photodeposition at high loadings (e.g., ~1-5 wt%) but recent work has shown that single-atom Pt cocatalyst can also be used for H₂ evolution at much lower loadings.³ Nanoscale characterization should reveal more precise structure-function relationships leading to rational design strategies for Pt/gCN photocatalysts that maximize energy conversion efficiency and minimize Pt consumption.

A systematic study correlating the hydrogen evolution rates (HERs), bulk characterization of the gCN supports, and Pt dispersion determined from annular dark field scanning transmission electron microscopy (ADF-STEM) was performed to understand photocatalytic activity in Pt/gCNs. Three gCNs demonstrating a range in structural condensation were selected including a commercially-produced “Nicanite” and two urea-derived gCNs. The C/N/H stoichiometry and bandgap onset energies vary widely with the most structurally condensed gCN (i.e., Nicanite) exhibiting increased visible-light absorption while the lowest N/H ratio is achieved for the urea-derived gCN demonstrating moderate structural condensation (i.e., U₂₄₀-gCN). The HER of U₂₄₀-gCN under visible light is >2x higher than that of Nicanite or U₃₀-gCN; all were loaded with 1.6 wt% Pt via photodeposition and tested under identical conditions.

ADF-STEM imaging of the used Pt/gCN photocatalysts reveals that the Pt dispersion varies widely on each support with U₂₄₀-gCN exhibiting the highest Pt dispersion. Moreover, the large Pt nanoparticles and low percentage of Pt photodeposited from solution (i.e., 33% on all supports) widely observed points to an inefficient use of Pt. We find that a photocatalyst containing single-atom Pt, at a loading of only 0.15 wt%, supported on Nicanite can achieve a ~20% greater HER than that of the conventional preparation while reducing Pt consumption by >10x. While promising, deactivation of the photocatalyst occurred which was attributed to single-atom coarsening.

Due to the vast differences in Pt dispersion and visible light absorption among the gCN supports, we argue that the HER (per mass of photocatalyst) is not a suitable metric for photocatalytic activity, although commonly used in this manner. Rather, by normalizing the HER by the total Pt surface area and integrated photon flux above the bandgap onset energy, different gCNs’ photocatalytic activity can be reliably compared. In this view, the order of photocatalytic activity was as follows: U₂₄₀-gCN > U₃₀-gCN > Nicanite. This analysis suggests that “optimal” structural condensation in gCNs should maximize photocatalytic activity whereas both under-/over-polymerization results in poor charge separation/migration. In addition to a discussion of our findings, we will emphasize methods for determining photocatalytic activity and improving the stability of single-atom Pt supported on gCNs.

[1] T.S. Miller et al. Phys. Chem. Chem. Phys. 2017, 19, 15613. [2] V.W. Lau et al. Adv. Energy Mater. 2017, 7, 1. [3] X. Li et al. Adv. Mater. 2016, 28, 2427. [4] We gratefully acknowledge the support from the DOE (DE-SC0004954), ASU’s John M. Cowley Center for High Resolution Electron Microscopy and ASU’s Eyring Materials Center.

Photocatalytic water splitting is considered one of the promising ways to provide clean fuels. Extensive efforts have been made in the past to develop various inorganic and organic materials systems as photocatalysts for water splitting by using visible light. Among these photocatalysts, it was recently demonstrated that incorporation of carbon dots (CDs) into graphitic carbon nitride (g-C₃N₄) results in new metal-free composites (gC₃N₄-C) with excellent stability and impressive performance for photocatalytic water splitting. However, fundamental questions still remain to be addressed such as how added CDs influence the photocatalytic reaction through the bandgap tunability, charge transfer route, and efficiency, as well as the specific function of CDs in the photocatalytic process. Understanding the chemical and physical behaviors of added CDs for the control of gC₃N₄-C architecture is a critical need for its emergence from the fundamental design of more efficient photocatalysts to practical applications. In this article, we report new materials with well-controlled architecture allowing for fine-tuning band gaps for broader visible light absorption and controlled understanding of the photocatalytic process. The well-defined model materials allow us to address the fundamental question regarding chemically bonding of CDs and how the chemical bonded CDs promote charge separation and transfer for highly efficient generation of H₂.

Reference

Qu, Dan; Liu, Juan; Miao, Xiang; Han, Mumei; Zhang, Haochen; Cui, Ze; Sun, Shaorui; Kang, Zhenhui(*); Fan, Hongyou(*); Sun, Zaicheng(*), Applied Catalysis B: Environmental, 2018, 227: 418~424.

With improving efficiencies of metal halide perovskite-based photovoltaic and emissive devices, research interest is increasingly focused on understanding and improving device stability. In particular, the effects of mobile ions moving within and escaping from the perovskite material are of interest as they are posited to be a major source of device degradation. While significant research has been conducted on the interactions of these ionic species with the perovskite material as well as the metal contacts of the device, research on the interactions with organic carrier transport or blocking layers is lacking. Using methylammonium lead triiodide (MAPI) as a prototypical metal halide perovskite, we show that interactions between adjacent organic films with iodine-containing species ejected from the degrading perovskite are significant and material dependent. In the presence of iodine vapor, common small-molecule and polymer hole transport materials (HTMs) are shown to be permeable and susceptible to reversible doping based on their highest occupied molecular orbital (HOMO) energy. Similarly, when in contact with an illuminated MAPI film, the conductivity of the HTM after illumination increases depending on the HOMO level of the material, suggesting iodide doping of the HTM. The strength of the HTM interaction with iodide is found to be linked to the HOMO energy proximity to the iodide/triiodide redox couple energy, with materials that have HOMO levels closer to the redox energy being the most strongly interacting. Furthermore, as suggested by x-ray photoelectron spectroscopy, iodide with differing oxidation states to those of the iodide in MAPI exists in the HTM, emphasizing the significance of the HOMO energy proximity to the redox couple. These interactions suggest that the HTM could facilitate perovskite material degradation through loss of iodide, transmit corrosive iodide/triiodide from the perovskite to the device's metal contacts, and contribute to hysteresis via ionic capacitance from reversible HTM doping. Ultimately, our data bring to light the importance of the HOMO energy and interactivity with respect to the iodide/triiodide redox energy when designing or selecting organic HTMs for perovskite devices.

As a promising alternative to organo-metal halide perovskite, metal halide perovskite CsPbX₃ (X = Cl, Br, or I) has gathered great research interest because of its better stability against light and heat. Among this kind of perovskite materials, the black-phase CsPbI₃ possesses the optimal bandgap (~1.7 eV) for harvesting solar energy but suffers from the spontaneous phase transition to the photo-inactive yellow-phase in the ambient air at room temperature. In this work, we achieved improved phase stability by confining nanocrystals of black-phase CsPbI₃ in a solid-state matrix. Using microscopic imaging tools, we discovered the correlation between the improved stability and the microscopic morphology of the thin films. Remarkably, the electrical conductivity and photoresponse can be improved simultaneously using this method, indicating its potential for photovoltaic applications.

The unique bandgap tunability in mixed halide perovskites is desirable for multiple applications such as solar cells, light emitting diodes (LEDs), and sensors. By simply mixing halides ions, the bandgap can be tuned through the entire visible range, such as the blue emission of CsPb(Br_xCl_1-x)₃ can be achieved by controlling the ratio between Chlorine and Bromine ions. However, suffered from phase segregation, the desired bandgap is not sustainable under operation conditions such as continuous illumination when used in the down conversion LEDs. In this work, we found that the stability of mixed halide CsPb(Br_xCl_1-x)₃ phase is related to the morphology. The morphologically modified mixed halide perovskites exhibited stable photoluminescence peaks with desired wavelengths and narrow bandwidths under extended illumination. Our results demonstrate a facile approach of creating stable halide perovskites with all-inorganic composition and desirable optoelectronic functionalities such as blue LEDs, narrow-band sensors.

Since the discovery of superconductivity in certain cuprates, these materials have been widely used for research in probing the phenomena of high temperature superconductivity. In particular, YBCO has been extensively studied. To further investigate this material, we have developed and tested a novel approach to growing procedures of thin films. During reactive sputtering we will introduce a reactive ion beam source (Reactive Ion Beam Assisted Deposition). We then looked at the transport properties as well as surface morphologies via SEM and AFM and compare film composition and phases with EDX and EBSD. We then make a comparison between non ion beam assisted film growth and ion beam assisted samples.
We have developed unique single crystal thin films with rare earth series of cuprates containing Holmium and Lanthanum doped YBCO as well as PrBCO and others. We have combined these rare earth series of cuprates onto substrates that have not yet been investigated for rare earth cuprate growth such as NdGaO₃ as well as (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 We have shown transport properties of these materials that further back our claim of thin film single crystal materials.

The defect-pyrochlore stuctured semiconductor KTaWO₆ has been prepared via hydrothermal synthesis, resulting in single-crystalline nanoparticles with adjustable crystallite size between 15 and 24 nm.[1] With subsequent ion-exchange of K with Sn(II) the band gap of this complex semiconductor can be reduced by 1.3 eV. We show that the ion-exchange is greatly facilitated by the incorporation of water molecules into the crystal lattice.
Moreover, we have systematically investigated the effect of Sn(II) exchange conditions on the band structure and subsequent photocatalytic properties. Different tin precursors show varying influence on the resulting band gap. While the optimum conditions diminish the band gap by up to 1.4 eV, the increase in visible light absorption does not correlate with an increase of photocatalytic activity. The incorporation of Sn(II) may result in smaller band gaps and an increased absorbance of visible light, but this is most likely due to coordination of water or methanol molecules to the incorporated Sn(II) atoms, leading to reduced optical band gaps. This is supported by quantum-chemical calculations of optical spectra.

[1] M. Weiss, R. Marschall, Nanoscale 10 (2018) 9691-969
[2] M. Weiss, T. Bredow, R. Marschall, Chem. Eur. J. 2018, DOI: 10.1002/chem.201803276

YBCO is a ceramic superconductor discovered in the late 80s with a transition temperature much higher than conventional metal superconductors. It has been difficult to incorporate into electronic applications because of its complexity and difficulty to process. YBCO electronics fabricated utilizing ion beam disorder to directly write electrical patterns show great promise. The key to this method is that modest levels of ion irradiation causes YBCO to change from a metal superconductor to an insulator. Controlling the dose with a finely focused helium ion beam allows for patterning of nanowires and Josephson tunnel devices. The degree of irradiation can be controlled to inflict various levels of damage. As the damage is increased, changes in the normal-state resistivity and temperature dependence can be seen.

We present the superconductor insulator transition as a function of ion irradiation using a helium ion microscope (HIM) to irradiate very thin films of YBCO. To study the junction barrier properties, we use the HIM in the broad beam mode to irradiate a well-defined region. By holding the energy constant, and only varying the dose, we can study the impact at different levels of damage. As the YBCO shifts from superconductor to insulator, the resistivity also changes. To study this effect, we compare and contrast structural materials characterization with electrical transport measurements. Specifically, we measure the resistivity of the irradiated region as a function of temperature and analyze the superconducting transition, normal state resistivity, and temperature independent residual resistivity.

TiO₂ nanomaterials have potential electrical and photochemical applications because of their unique physical and chemical properties. One-dimensional TiO₂ nanofibers have been studied in particulate matter (PM) removal applications due to their large specific surface area and high photocatalytic reactivity. To improve the photocatalytic activity, ZnO/TiO₂ nanofibers having high adsorption is recently suggested. To fabricate the ZnO/TiO₂ nanofibers, electrospinning method which can efficiently produce a nanofiber at low cost was used. The precursor including zinc nitrate hexahydrate, polyvinyl acetate, and titanium isopropoxide was prepared and applied to various electrical fields. The in-situ ZnO/TiO₂ nanofibers were analyzed by Thermogravimetric Analysis (TGA) to get an information of combustion behavior. The rutile phase and diameter of 200 to 300 nm in annealed ZnO/TiO₂ nanofibers at 600 were observed by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), respectively. To analyze the photocatalytic activity in the atmosphere, NO gas was used as a reaction model material. The photochemical reaction rate of 7% was measured by the reaction time, and the photocatalytic activity in ZnO/TiO₂ nanofiber were higher than 3% that in TiO₂ nanofiber. Photocatalytic properties of ZnO/TiO₂ nanofiber will be able to apply to various fields such as moisture separation, environmental purification, super surface, sensor, and disinfection.

Diamond, via its extreme properties, presents itself as a material capable of pushing the electronics industry beyond current limitations. It’s ultra-high thermal conductivity, wear resistant, electrically insulating and chemically inertness are just some of diamond’s outstanding properties. Although diamond is an insulator by nature, its wide band gap should allow for incorporation of a variety of donors and acceptors. With an appropriate concentration of either, the material will take on n-type or p-type semiconductor characteristics that can lead to the fabrication of diamond p-n junctions. These p-n junctions have potential application as high temperature and high power operating devices.

In this research, we will detail the characteristics of boron doped p-type and phosphorus doped n-type polycrystalline diamond grown on 6H silicon carbide using a hot-filament chemical vapor deposition system (HF-CVD) in preparation of p-n junction fabrication. Diamond nanoparticles are used to prepare the surface of silicon carbide substrates to enhance nucleation during heteroepitaxial growth. Boron doping is performed by two different methods: 1. In situ during diamond growth via vaporization of solid boron and 2. a pre-deposition followed by a drive in using a boron oxide diffusion source. Phosphorus doping is performed in the same manner as the second method stated using a phosphorus oxide diffusion source. Raman spectroscopy, Hall measurements, and secondary ion mass spectroscopy (SIMS) analysis is conducted to verify diamond quality, active incorporation, and dopant concentrations.

This work is supported by the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319. This work was performed in part at the Harvard University Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. 1541959.

Thermal rectifier, which transports heat preferentially in one direction, plays a significant role in thermal circuits. An efficient thermal rectifier should exhibit great different thermal conductivity with the forward and reversed temperature gradient. Thus, the rectifying coefficient defined by the ratio of k_f and k_r. However, the majority of thermal rectifiers show low rectifying coefficient or have a rigorous temperature requirements. In this work, we show an efficient thermal rectifier in a temperature-sensitive hydrogel synthesized with melamine and 6,7-dimenthoxy-2,4[1H,3H]-quinazolinedione in molar ratio 1:1 at 303K. DSC and rheology results indicate that a thermoreversible first order phase transition occurs when the temperature rises above its upper-critical-solution-temperature(UCST), which can be owned to H-bonding breakage suggested by the Raman-spectra. In this study, the suspended micro-device based on the differential bridge method is applied to measure the thermal conductivity of single hydrogel nanofiber picked out from the dispersion. The thermal conductivity of hydrogel nanofiber reveals an abrupt change with the phase changes. This study provides an efficient realization of a thermal rectifier for heat management that could be implemented in moderate temperature with high rectifying coefficient.

Sunlight-driven water splitting has been studied actively for production of renewable solar hydrogen as a storable and transportable energy carrier [1,2]. Both the efficiency and the scalability of water-splitting systems are important factors because of the low areal density of solar energy. Particulate photocatalyst systems do not involve any secure external electric circuit and thus can be spread over a wide area by inexpensive processes.

The authors’ group has been developing panel-type reactors that can accommodate particulate photocatalysts thinly fixed on substrates [3]. Al-doped SrTiO₃ is a photocatalyst that can split water at an apparent quantum yield of up to 69% at 365 nm [4]. Photocatalyst sheets based on Al-doped SrTiO₃ contained in a panel-type reactor split water into hydrogen and oxygen and release gas bubbles at a rate corresponding to a solar-to-hydrogen energy conversion efficiency of 10% under intense UV illumination even when the water depth is merely 1 mm. Moreover, it is possible to maintain the intrinsic activity of the photocatalyst sheets when the size of the panel is extended to the scale of 1 m². It is likely feasible to build panel-type photocatalytic reactors suitable for scale-up using light-weight materials inexpensively. Accordingly, development of photocatalyst systems that can split water under visible light irradiation efficiently is a key issue of the research field.

We have developed particulate photocatalyst sheets consisting of hydrogen evolution photocatalysts (HEPs) and oxygen evolution photocatalysts (OEPs) embedded into conductive layers by particle transfer [5-9]. The photocatalyst sheet shows significantly higher water splitting activity than the corresponding powder suspension systems, because conductive materials transfer photogenerated electrons between HEP and OEP particles effectively. In addition, evolution of hydrogen and oxygen in close proximity prevents generation of the pH gradient during the water splitting reaction. Therefore, the photocatalyst sheet is scalable without sacrificing the high activity. A photocatalyst sheet consisting of La- and Rh-codoped SrTiO₃ as the HEP and Mo-doped BiVO₄ as the OEP embedded into a carbon conductor exhibits a solar-to-hydrogen energy conversion efficiency of 1.0% at ambient pressure [8]. Photocatalysts sheets can also be prepared by ambient-pressure processes, namely by screen-printing photocatalyst ink containing transparent conductive oxide colloids [9].

It has been known that Ta₃N₅ has a band structure suitable for water splitting under visible irradiation of up to 600 nm since 2002 [10,11], but overall water splitting had not been achieved because of high densities of trap states. We have activated Ta₃N₅ in overall water splitting under visible light by developing an innovative nitridation process [12]. Ta₃N₅ single crystal nanorods can be directly evolved on particulate KTaO₃ by short-time nitridation accompanied by gradual evaporation of K contents. Ta₃N₅ single crystal nanorods have low defect densities and can utilize photoexcited carriers in the water splitting reaction when being loaded with appropriate cocatalysts.

In this talk, development of photocatalyst systems involving hierarchical structures will be presented.

[1] Hisatomi et al., Chem. Soc. Rev. 2014, 43, 7520.
[2] Hisatomi et al., Faraday Discuss. 2017, 198, 11.
[3] Goto et al., Joule 2018, 2, 509.
[4] Chiang et al., 2018, 8, 2782.
[5] Wang et al., J. Catal. 2015, 328, 308
[6] Wang et al., Nat. Mater. 2016, 15, 611
[7] Wang et al., Faraday Discuss. 2017, 197, 491
[8] Wang et al., J. Am. Chem. Soc. 2017, 139, 1675.
[9] Wang et al., Joule. 2018, DOI: 10.1016/j.joule.2018.08.003.
[10] Hitoki et al., Chem. Lett. 2002, 31, 736.
[11] Chun et al., J. Phys. Chem. B 2003, 107, 1798.
[12] Wang et al., Nat. Catal. 2018, 1, 756.

Semiconductor materials hold the key for efficient photocatalytic and photoelectrochemical water splitting. In this talk, we will give a brief overview of our recent progresses in designing semiconductor nanomaterials for photoelectrochemical energy conversion including photocatalytic solar fuel generation. In more details, we have been focusing the following a few aspects; 1) band-gap engineering of layered semiconductor compounds including layered titanate, tantalate and niobate-based metal oxide compounds for visible light phtocatalysis, and 2) two-dimensional nanosheets/nanoplates of TiO₂, Fe₂O₃, WO₃, BiVO₄ as building blocks for new photoelectrode design，and 3) the combination of a high performance photoelectrode BiVO₄ with perovskite solar cells can lead to unassisted solar driven water splitting process with solar-to-hydrogen conversion efficiency of >6.5%.^1-6 The resultant material systems exhibited efficient visible light photocatalytic performance and improved power conversion efficiency in solar energy, which underpin important solar-energy conversion applications including solar fuel generation and simultaneous environmental application.

Photocatalytic water splitting using solar energy for hydrogen production offers a promising alternative form of storable and clean energy for the future. Light driven proton reduction requires a three-component system with a photosensitizer (light harvesting unit) that transfers electrons to a catalyst that reduces protons and a sacrificial donor. To make this process cost-effective, we need an inexpensive, durable and efficient catalyst that could reduce protons from water to produce hydrogen and we need to couple this catalyst to a photosensitizer that could supply the energy for water splitting.

Quantum dots (QDs) have high extinction coefficients across a broad range in the solar spectrum and can simultaneously absorb multiple photons, or continuously absorb multiple photons even after the electrons and holes are accumulated. These characteristics make QDs the perfect candidates for photosensitizers in solar fuel generation when they are properly coupled to efficient catalyst for hydrogen generation.

Nature provides an attractive inspiration for the reductive cycle of water splitting in the form of hydrogenases using earth abundant and inexpensive transition metals in the active site. Transition metals are useful because of their redox activity, their capability to bind and exchange ligands and their high charge density. As a result, many homogeneous catalytic systems incorporate transition metals.

The fundamental step for photocatalysis is to generate a charge separated state. In order to couple a catalyst for proton reduction to a photosensitizer, it is essential to understand the mechanism of charge transfer or photoinduced electron transfer from the photosensitizer to the catalyst. Hence our work is focused on the study of light driven production of hydrogen from QD systems by modification with reduction catalysts. We synthesized and characterized a variety of nickel and copper complexes of tetradentate ligands with amine and pyridine functionalities (N2/Py2) and studied their interactions with Cadmium Telluride QDs stabilized by 3-mercaptopropionic acid (MPA-CdTe QDs). The studies performed include absorbance and emission spectroscopic behavior as well as lifetime measurements that will give us a direct insight into the photo-induced charge transfer process.

It is well known that plasmonic gold nanostructures feature extraordinary capability of absorbing visible light and concentrating the excitation energy in subwavelength volumes. Recently, they have also been proposed as promising candidates for the production of chemical fuels from sunlight. Upon excitation of these nanostructures, the energy is transferred to single electrons that for a brief period of time become highly energetic. Recent scientific advances demonstrate that these highly energetic, “hot” electrons can be extracted and used to drive chemical reactions, such as the conversion of protons to molecular hydrogen. However, to greatly improve the production rate, expensive and rare cocatalysts such as platinum are required.
In order to make effective use of as little catalyst material as possible, it is therefore important to localize the cocatalyst at the places where it is best coupled to the photogenerated hot electrons. To this end, we use the hot electrons themselves to deposit the cocatalyst and to construct photocatalytically active nanostructures. Briefly, a photocathode consisting of ITO-gold nanoislands-TiO₂ was illuminated with red light in presence of PtCl₆, which resulted in the local deposition of platinum nanoparticles on the gold nanoislands. We furthermore compare the photocatalytic performance of these photocathodes, where the platinum nanoparticles were selectively deposited, with those that were fabricated by a random-deposition technique, i.e. electrodeposition. Indeed, the samples where the cocatalyst was locally-deposited, with hot-electron chemistry, showed higher catalytic activity in photocurrent measurements in presence of a phosphate buffer. Therefore, it seems that indeed the location of the cocatalytic material on the plasmonic photocathodes, and maybe generally on photoactive materials, plays a crucial role.
Overall, these results demonstrate that plasmonic hot electron chemistry can be used for fabricating photocatalytic nanostructures with sub-wavelength control over localization. This careful design of photoelectrodes with nanoscale precision, could open up a new way for higher photocatalytic efficiencies as well as lower fabrication costs.

Constructing Z-scheme type composites catalyst is an effective way to achieve highly efficient photocatalytic activity. In this presentation, we reported two examples for demonstration. CdTiO₃nanoparticles were treated with NaBH₄to obtain Cd/CdTiO3 nanoparticles. It finely tunes the sulfurization via hydrothermal reaction with thiourea. CdS/Cd/S doped TiO2 junction can be formed. Solid Z-scheme photocatalyst forms due to the presence of metal Cd. S doped TiO2 leads to much broad visible light absorption till to near 500 nm, which is close to light absorption range of CdS. The H2 production dependence of wavelength illustrates that H2 production activity is significantly enhanced in the visible light range.[1] Photo deposition technique is an effective method to regulate the charge flow direction. We construct CdS/g-C₃N₄by photo-deposition technique and chemical deposition. Charge tracking indicated the heterojunction composites form for the composites prepared by photo-deposition. Z-scheme type composites form by using chemical deposition technique.[2]
References
1. Z. Zhao, and Z. Sun et al., Sci. China Mater., 2018, 10.1007/s40843-017-9170-6
2. W. Jiang, and Z. Sun, et al., ACS Catal., 2018, 8, 2209.

Photocatalytic water splitting and CO₂ reduction are attractive reactions to convert photon energy to chemical energy, and hence they are called as artificial photosynthesis. The photocatalyst systems for water splitting and CO₂ reduction can be divided into powdered and photoelectrode types. In both types, photogenerated electrons have to migrate from photocatalyst particles to other particles or conductive substrates. In the present study, we successfully developed new photocatalyst systems for water splitting and CO₂ reduction upon combining photocatalysts and a conductive reduced graphene oxide (RGO).
In a powder-based photoelectrode system in which the photocatalyst particles are loaded on a conductive substrate, photogenerated carriers in a particle need to migrate to the conductive substrate through other particles. We found that the photoelectrochemical performance is improved upon incorporating the RGO in the photoelectrode due to the boosted electron transfer from the particles to the substrate by the RGO. The incorporation of the RGO was effective for both n- and p-type semiconductor materials.
A powdered Z-scheme system consists of two different photocatalysts and an electron mediator. Although ionic redox couples are usually used as the electron mediator, we found that the RGO works as a solid-state electron mediator. Additionally, we successfully utilized photocorrosive metal sulfides as an H₂-evolving photocatalyst when the RGO was used. The developed Z-scheme system showed the activity for CO₂ reduction as well as water splitting under visible light irradiation.

Janus nanoparticles (NPs) containing two chemically distinct materials in one system are of great significance in nanocatalysis in terms of harnessing catalytic synergies that are not exist in either component. We herein present a novel synthetic method of Janus-type MnO_x-AgI NPs consisting of n-type MnO_x octahedrons and p-type AgI NPs for efficient photochemical water oxidation. The synthesis of Janus-type MnO_x-AgI NPs is based on the oxidative nucleation and growth of Ag domains on MnO first and the subsequent iodization of Ag. A mild, non-disruptive iodization strategy is developed to yield Janus MnO_x-AgI NPs in which converting Ag to AgI domains with iodomethane (CH₃I) is achieved through the partial iodization. Simultaneously, Mn²⁺ species in the primary MnO octahedrons are oxidized during the growth of Ag NPs, leading to the formation of n-type MnO_x domains. Therefore, as-resultant Janus-type MnO_x-AgI NPs combining two semiconductors into an integrated nanostructure can be used as an efficient self-sensitized photocatalyst for visible light-driven water oxidation. Janus MnO_x-AgI NPs show a superior photocatalytic activity (358 mmol O₂ per mg_catalyst in 4 min) even in the absence of [Ru(bpy)₃]²⁺ as a photosensitizer. This intriguing synthesis may open up a new opportunity to develop silver halide asymmetric nanostructures that will potentially be efficient photocatalysts for solar-driven water splitting.

In spite of the potential of using photocatalysts to sustainably produce fuels and chemicals, overall reaction rates remain prohibitively low, limited by recombination of photogenerated electrons and holes. Extensive work can be found in the literature on suppressing this recombination by addition of co-catalysts. However, much of the reported data on photocatalytic activity are rendered ineffectual in the rational design of new, improved catalyst systems by the common practice of reporting activity on a catalyst mass basis. In this work, using a model system of TiO₂ and g-C₃N₄ with Pt, we show that normalizing photocatalytic rates by mass is non-physical and can even artificially enhance the apparent performance of a two-photoabsorber system relative to that of the individual components. A mathematical method is presented for approximating a system’s quantum yield from measured reaction rates under simulated solar or otherwise polychromatic light. A sensitivity analysis was performed on this method, demonstrating how the approximated quantum yield can be used to evaluate the degree of relative enhancement introduced by mass-normalizing photocatalytic reaction rates for any set of photoabsorbing co-catalysts.

Plasmonic metal nanocrystals can interact strongly with light, efficiently converting light into heat and generating hot charge carriers. Both plasmonic photothermal conversion and hot carrier generation can accelerate chemical reactions. The use of plasmons to drive chemical reactions has recently become a new and active research field. Plasmonic hot charge carriers can not only enhance the reaction yield and selectivity, but also introduce new reaction pathways.

The lifetime of plasmonic hot charge carriers is on the femtosecond scale. Without immediate usage, they will rapidly relax, converting their energy into heat. We have employed two approaches to enable the use of plasmonic hot charge carriers. The first is the integration of Au or Ag nanoparticles with Pd or Pt nanoparticles. Au and Ag nanoparticles possess strong localized plasmons, while Pd and Pt nanoparticles are excellent catalysts for many chemical reactions. Combination of the two types of metal nanoparticles can lead to efficient light absorption and generation of plasmonic hot charge carriers, which can subsequently inject into molecules that are adsorbed on the Pd or Pt component. We have applied this approach to enhance Suzuki coupling reactions by localized plasmons.

The second is the integration of plasmonic metals with semiconductors. A barrier is formed at the interface. The generated hot electrons and holes can quickly inject into the conduction or valence band and therefore get separated. The injected charge carriers can then drive chemical reactions. We have synthesized Au/TiO₂, Au/CeO₂ and Au/BiOCl hybrid structures as plasmonic photocatalysts to drive the photo-generation of reactive oxygen species, the selective oxidation of alcohols, and N₂ photofixation. Specifically, oxygen vacancies have been introduced in the metal oxides to “work in-tandem” together with plasmonic hot charge carriers. For example, during the use of Au/BiOCl hybrid structures for the photocatalytic selective oxidation of benzyl alcohol, oxygen vacancies on BiOCl facilitate the trapping and transfer of plasmonic hot electrons to adsorbed O₂, producing superoxide anion radicals, while plasmonic hot holes remaining on the Au surface mildly oxidize benzyl alcohol to corresponding carbon-centered radicals. The ring addition between these two radical species leads to the production of benzaldehyde along with an unexpected oxygen atom transfer from O₂ to the product. In contrast, the oxygen atom in the product is usually from the benzyl alcohol reactant.

In another example, Au nanoparticles are anchored on ultrathin TiO₂ nanosheets with oxygen vacancies. The oxygen vacancies on the nanosheets chemisorb and activate N₂ molecules, which are subsequently reduced to ammonia by hot electrons generated from plasmon excitation of the Au nanoparticles. The hybrid photocatalyst can accomplish photodriven N₂ fixation in the “working-in-tandem” pathway at room temperature and atmospheric pressure. The apparent quantum efficiency of 0.82% at 550 nm for the conversion of incident photons to ammonia is higher than those reported so far. The oxygen vacancies play three roles. They act as the active sites for N₂ molecules; they cause defect states within the bandgap for trapping plasmonic hot electrons and therefore lengthening their lifetime; and they function as a bridge between plasmonic hot electrons and the activated N₂ molecules. This work offers a new approach for the rational design of efficient catalysts towards sustainable N₂ fixation through a less energy-demanding photochemical process compared to the industrial Haber-Bosch process.

Photoelectrochemical (PEC) devices combine light capture, charge separation, and electrocatalysis to drive solar to chemical energy conversion. Silicon based photocathodes integrated with co-catalysts have been studied extensively for solar fuels production, especially for the hydrogen evolution reaction of PEC water splitting.¹ In contrast, Si photocathodes that drive the more challenging CO₂ reduction (CO₂R) reaction are fewer in number and, with only a few exceptions, produce 2 electron products such as CO or formate.^2,3
Here we show that Si photocathodes integrated with nanostructured bimetallic catalysts perform light-driven conversion of CO₂ to C₂ and C₃ products (up to 18 electron transfers). In contrast to conventional photocathode designs which employ p-type absorbers, we used a back illumination geometry with an n-type Si absorber to permit the use of absorbing metallic catalysts which would otherwise block the light. Back and front interfaces were configured by ion implantation and by surface passivation to achieve carrier selectivity. Surface texturing of the Si was used optimize light absorption on the illuminated side and increase the surface area available for catalysis on the electrolyte side. Based on our prior work with bimetallic cascade catalysis,⁴ we employed a hierarchical Au-Cu or Ag-Cu nanostructures to drive CO₂R to C-C coupled products.
The photovoltage, 550- 600 mV under simulated 1-sun illumination, confirms the carrier selectivity and passivation of the front and back interfaces. Compared to planar controls, textured photocathodes generate higher current densities, exceeding 30 mA/cm².Under simulated diurnal illumination conditions in CO₂-saturated 0.1 M CsHCO₃ electrolyte, over 60% faradaic efficiency to C₂₊ hydrocarbon and oxygenate products (mainly ethylene, ethanol, propanol) is maintained for several days.
Over longer testing periods, contamination from the counter electrode is observed, which causes an increase in hydrogen production. This effect is mitigated by a regeneration procedure which restores the original catalyst selectivity. A tandem, self-powered CO₂ reduction device was also demonstrated by coupling a Si photocathode with two series-connected semitransparent CH₃NH₃PbI₃ perovskite solar cells, achieving an efficiency for the conversion of sunlight to hydrocarbons and oxygenates of 1.5% (3.5% for all products).

1 K. Sun, S. Shen, Y. Liang, P. E. Burrows, S. S. Mao and D. Wang, Chem. Rev., 2014, 114, 8662–719.
2 J. T. Song, H. Ryoo, M. Cho, J. Kim, J.-G. Kim, S.-Y. Chung and J. Oh, Adv. Energy Mater., 2017, 7, 1601103.
3 S. K. Choi, U. Kang, S. Lee, D. J. Ham, S. M. Ji and H. Park, Adv. Energy Mater., 2014, 4, 1–7.
4 Y. Lum and J. W. Ager, Energy Environ. Sci., 2018, 11, 2935–2944.

We are integrating theory and experiment to design, synthesize, and characterize semiconductor heterostructures with programmable light harvesting and charge transfer for photocatalysis. Heterostructures consist of M_xV₂O₅ nanowires (NWs), where M is an intercalated metal cation, interfaced with quantum dots (QDs). Intercalation of metal cations gives rise to midgap electronic states in the NWs, which are well-positioned to accept photogenerated holes from QDs, facilitating charge separation and redox photocatalysis. We previously demonstrated that photoexcitation of cadmium chalcogenide QDs, within heterostructures, is followed by the transfer of holes to midgap states of NWs on sub-picosecond time scales.¹ This presentation will focus on recent research to evaluate the heterostructures in photoelectrochemical hydrogen evolution and photocatalytic water splitting. Photoelectrochemical measurements revealed that CdSe/β-Pb_0.33V₂O₅ heterostructures exhibited (3.2 ± 0.4)-fold greater photocurrents than corresponding CdSe/V₂O₅ heterostructures (wherein the V₂O₅ NWs lack midgap states) following selective excitation of CdSe QDs and (5.7 ± 0.3)-fold greater photocurrents under white-light illumination. Photocurrents arose from the reduction of aqueous H⁺ to H₂ with high Faradaic efficiencies of (92 ± 5)%. These results are consistent with a mechanism in which excited-state interfacial hole transfer facilitates charge separation and photocatalysis, and suggest that the CdSe/β-Pb_0.33V₂O₅ heterostructures are indeed promising architectures for light harvesting and photoelectrochemical water splitting. This presentation will highlight these results, as well as recent experiments on second-generation heterostructures with optimized interfacial energetics.

(1). Milleville, C. C.; Pelcher, K. E.; Sfeir, M. Y.; Banerjee, S.; Watson, D. F.: Directional charge transfer mediated by midgap states: A transient absorption spectroscopy study of CdSe quantum dot/β-Pb_{0. 33}V₂O₅ heterostructures. The Journal of Physical Chemistry C 2016, 120, 5221-5232.

Plasmonic materials have attracted much attention owing to their unique surface plasmon effect (SPR) by photoluminescence. As a typical anisotropic plasmonic material, gold nanorods(GNRs) have advantages of good biocompatibility, adjustable light response between 550 - 1550 nm and high chemical stability. In recent years, studies show gold nanorods can enhance the photocatalytic activity of semiconductor materials, but most of the inorganic semiconductors are used for photocatalysis which absorb spectrum lies in ultraviolet region. In order to improve the utilization of visible light, efficient photocatalysts with visible light responsive need to be designed and prepared. Porphyrin assembly is an organic semiconductor material with excellent photoelectric properties. Compared with traditional inorganic semiconductor materials such as TiO₂, the absorption spectrum of the porphyrin is located in the visible light region, which can make use of solar energy more effectively, but Pt must be added as co-catalyst. Therefore, the paper chooses gold nanorods and porphyrin assemblies to make a composite, in order to obtain more efficient solar energy utilization materials. Here, we synthesized a gold nanorods-porphyrin core-shell structure via a surfactant-assisted self-assembly induced acid-base neutralization micelle encapsulation method. The fabrication of composite was achieved by adjusting the length diameter ratio of GNRs, the type and concentration of surfactants, and pH. By regulating the reaction parameters, the composite structure with regular morphology, uniform size and good monodispersibility was prepared by using CTAB/NaOL coated GNRs with a length diameter ratio greater than 4. Combined with TEM and the dynamic tracking experiment indicated the composite structure was the core-shell structure of porphyrin assemblies covering GNRs. The XRD result showed the GNRs-ZnTPyP core-shell structure had good lattice structure. The photocurrent test showed that the photocurrent density of GNRs-ZnTPyPcore-shell structure and ZnTPyP self-assemblies was about 2.0 μA/cm² and 0.8 μA/cm² respectively, and the core-shell structure was 2.5 times as high as that of self-assemblies. Using ascorbic acid (AA) as electron donor, the visible light photocatalytic hydrogen production without Pt of different catalyst systems were carried out under pH ≈ 5.0. Under the same condition, the GNRs-ZnTPyP core-shell structure had a H₂ production rate of 37 mmol/g/h, which was 15 times of the physical mixing.

Single-atom catalysts where active metal atom is dispersed on the surface of the support have received great attention due to their high specific activity and unique selectivity. It is well known that single-atom is anchored on the defect site at the surface. However, there has been little effort to correlate the number and type of defects and the resultant catalytic activity.
Hematite is one of the most prominent photoelectrochemical catalysts for water oxidation reaction, in virtue of its good optical property and cost-effectiveness. However, its widespread adaptation to commercial usage has been limited due to its modest charge transport property and poor surface reaction kinetics. To overcome these, in this work, the concept of single atom catalyst is applied and the contribution of defect to the catalytic activity is investigated. Hematite is defect chemically modified by atmosphere change and doping, varying type (Fe-vacancy or O-vacancy) and number of defects. Subsequently, iridium, which is known to be the best water oxidation catalyst yet very expensive, is deposited at the trace amount, as a form of the single-atom anchored at the surface defect sites. Through this, both the carrier conduction in the bulk and the reaction rate at the hematite-electrolyte interface are optimized. As a result, photocurrent is enhanced up to 3 mA/cm³ at 1.23 V_RHE, and 5 mA/cm³ at 1.4 V_RHE by donor doping, while undoped or acceptor doped system show 3-fold or much less photocurrent. Materials properties related with these are analyzed and the defect chemical origin underlying these is elucidated.

Many industrial oxidation processes based on peroxide, have a difficulty of over oxidation in homogeneous solution principally due to faster oxidation rates of already oxidized species and high statistical probability of secondary oxidation near the completion of reaction. Ideal for stepwise oxidation is to limit the residence time of target molecule in a reaction zone to allow for single (or quantized) reaction events. This can be achieved in a membrane geometry where catalyst along a pore length and set flow velocity can precisely control residence time for oxidation. It is also known that Au nanoparticles can catalytically activate peroxide under light irradiation due to the formation of concentrated surface plasmon electric fields and thus hypothesized to be present in nanoporous planes. A plasmonic membrane was synthesized by evaporation of 25 thick Au films onto pore entrances anodized aluminum oxide membranes (AAO) with pore diameters of 20-200nm. This allowed solutions of peroxide, benzene, methyl blue to flow through membrane and interact with Au surface plasmon upon exit of the membrane. Under light illumination of 10-100 mW/cm2 quantum efficiencies (photon/peroxide radical) of 200-800% were seen, indicating a mechanism of field induced activation of peroxide compared to a hot electron injection mechanism. Dye and benzene to phenol oxidation are demonstrated.

Generating solar fuels such as H₂ gas from water will require photocatalysts that can absorb visible light and facilitate swift charge separation/migration for surface electrochemical reactions. Photocatalysts containing “mixed metal oxide” (MMO) interfaces between TiO₂ and CeO₂, both of which are UV-absorbers on their own, have garnered attention due to their ability to drive H₂/O₂ evolution from water under visible light.^1,2 Visible light absorption is hypothesized to originate from partially-occupied Ce-4f states, which act as donor states within TiO₂’s bandgap, that arises from an enrichment of Ce³⁺ at the MMO interface.² In this view, TiO₂-supported CeO_2-x photocatalysts may offer a unique hierarchical geometry that confines visible-light harvesting (and charge generation) near the surface. Direct probing of the optical properties about such MMO interfaces would provide insights into the unique supported CeO_2-x morphologies that produce strong, and potentially tunable, visible-light absorption.

TiO₂ supports were synthesized through a two-step hydrothermal process to yield batches of “small” (S) and “large” (L) anatase nanoparticles with average particle sizes of 15 and 66 nm, respectively, determined by powder X-ray diffraction (XRD). Subsequently, both were loaded with 6 wt% Ce via wet impregnation followed by appropriate calcination heat treatments to remove precursors. UV-visible diffuse reflectance spectroscopy shows no change in the bulk optical properties on the modified TiO₂(L) nanoparticles whereas a red-shifted bandgap from 3.2 to 2.7 eV is observed on the Ce-loaded TiO₂(S) powder which is accompanied by a white-to-yellow color change in the as-loaded powder. To compare the hydrogen evolution rates (HERs), both powders were photodeposited with 0.5 wt% Pt under visible light (λ>400 nm) for one hour followed by UV+visible illumination to measure evolved H₂ gas. The HER of 6 wt% Ce/TiO₂(S) is >2x higher than that of 6 wt% Ce/TiO₂(L) owing to its increased visible-light absorption.

Aberration-corrected annular dark field scanning transmission electron microscopy (ADF-STEM) was utilized to image the two powders at atomic resolution, revealing unique CeO_2-x morphologies dominating each support. On TiO₂(L), larger CeO₂ nanoparticles (e.g., ~10 nm) are ubiquitous. In contrast, TiO₂(S) is decorated by Ce single atoms and smaller CeO₂ nanoparticles. Thus, TiO₂(S) appears to contain more MMO interfaces which is consistent with its significant visible-light absorption.

Monochromated electron energy-loss spectroscopy (EELS), in the STEM, was then applied to 6 wt% Ce/TiO₂(S) to obtain valence EELS spectra from regions containing MMO interfaces. The valence EELS spectra from all regions shows the characteristic bandgap onset for TiO₂ at ~3.5 eV. However, when the probe is placed near areas containing TiO₂/CeO_2-x interfaces, a broad peak centered at ~1.5 eV (with almost half of the intensity as the bulk TiO₂ bandgap signal) emerges. The relative intensity of this peak decreases in thicker regions and disappears in areas where no CeO_2-x species are present. This preliminary evidence suggests the presence of “interfacial” bandgap state(s) that facilitate absorption of visible light as low as 1.5 eV, which is not apparent from bulk optical spectroscopy. Future work will extend the valance EELS analysis to more regions of the sample and attempt to correlate the nanoscale optical properties with local Ce³⁺ concentration and MMO morphology (e.g., single atom vs. nanoparticles).

[1] S. Luo et al. J. Phys. Chem. C 2015, 119, 2669. [2] S. Kundu et al. J. Phys. Chem. C 2012, 116, 14062. [3] We gratefully acknowledge the support from the DOE (DE-SC0004954) and NSF (CHE-1508667), ASU’s John M. Cowley Center for High Resolution Electron Microscopy and ASU’s Eyring Materials Center.

As the world population continues to grow and access to modern industry becomes increasingly common, humanity finds itself in the midst of an environmental and energy crisis. Natural resources such as clean water are becoming increasingly scarce. A recent development in the scientific community is the study of semiconducting photocatalytic nanomaterials as a potential solution. Photocatalysts are among the most promising solutions since these materials can directly convert abundant solar energy into usable energy resources via the photoelectric effect, and produce hydroxyl group radicals which can proceed to decompose pollutants or deactivate certain microbes in a given medium.

However, it is often difficult to find an individual photocatalyst that satisfies all the requirements for an exceptional photocatalyst, such as high redox ability, narrow energy bandgap, and efficient charge-separation. The artificial heterogeneous z-scheme photocatalytic systems made by amalgama of two or more photocatalysts is a replica of the natural photosynthesis process that enables the materials to overcome the shortcomings of a single component photocatalyst. In a Z-scheme, materials with high oxidation potential, as determined by their conduction and valence band position, are paired with those that have a higher reduction potential, Ag₃PO₄, and Bi₂MoO₆ in this instance, thus creating a channel for photo-induced electron-hole pairs to recombine sacrificially. This arrangement creates the Z-shaped path of the charge carriers and ensures the availability of electrons at the conduction band with the highest reduction potential and holes at the valence band with the highest oxidation potential. This mechanism enables composites to maintain better charge-separation and redox ability simultaneously.

Surface morphology, interface, and composition between component photocatalysts play a significant role in efficient light absorption and charge separation. Numerous strategies including morphology control have been developed recently to enhance the photocatalytic performance by maximizing the amount of light energy capture. Considering Bi₂MoO₆ and Ag₃PO₄ can absorb a broad visible light spectrum from the sun due to their relatively low bandgap energy, which makes them suitable for morphology manipulation. In this study, the correlation between the photocatalytic performance of Bi₂MoO₆/Ag₃PO₄ and their varied morphology and chemical characteristics is investigated. The Bi₂MoO₆/Ag₃PO₄ z-scheme is of particular interest for its exceptional photocatalytic performance, as reported by previous investigators. Therefore the purpose of this study is to design, synthesize, and test the best configuration of Bi₂MoO₆/Ag₃PO₄ and further improve upon an already impressive performance.

Bi₂MoO₆/Ag₃PO₄ z-scheme composite was synthesized through a facile in-situ chemical method. The crystallinity, morphology, microstructure, and light absorption wavelength range are characterized using, x-ray diffraction (XRD), field emission electron microscopy (FESEM), transfer electron microscopy (TEM), energy dispersive spectrometer (EDS) and UV–visible diffuse reflectance spectroscopy (DRS). Finally, photocatalytic performance is evaluated and reported by degrading Methylene Blue (MB) under a solar simulator with UV and visible wavelengths.

The solar industry has recently transitioned to manufacturing passivated emitter rear contact (PERC) cells, which are also more efficient than Al back-surface field (Al-BSF) cells because they provide rear-side surface passivation. The goal of this experiment was to enhance the short-circuit current of PERC solar cells by inserting a low-refractive index dielectric layer between the rear passivation layer and screen-printed aluminum paste, which parasitically absorbs near-infrared light, stealing current from the cell. We used a low-cost deposition method, based on an open-air atomizer, to deposit these coatings from solution. We coated the front side of textured silicon wafers with silicon nitride (SiNx), which is used for commercial cells, and then deposited aluminum oxide (Al2O3) onto the back of some these wafers to test against reference cells with standard atomic layer deposited (ALD) Al2O3. We then deposited silicon dioxide (SiO2) on top of the sprayed Al2O3 to test if the lower refractive index of SiO2 would enhance the reflectance of our spray deposited cells. The experiment showed that samples with 10 nm ALD Al2O3 were optically equivalent to samples with 84 nm spray deposited Al2O3, and by depositing SiO2 on top of Al2O3 we were able to get a higher reflectance at longer wavelengths than with Al2O3 alone.

The rare earth oxide Er₂O₃ is distinguished for its exceptional and diverse properties, which can benefit in the various fields of semiconductor technology. In particular, erbium oxide is characterized by a high dielectric constant (14) and a wide bandgap (5.6 eV). It is distinguished as a high thermal resistive and chemical stable compound. Erbium oxide grown on silicon substrates reacts poorly with Si and retains its properties unchanged up to 900°C, which makes it one of the most stable compound compared to other rare earth metal oxides. Additionally, erbium oxide is noted for its high conduction band offset relative to silicon. All of these mentioned above properties are favorable for an application as a high-k gate dielectric in complementary-metal-oxide-semiconductor (CMOS) technology. Due to its stability erbium oxide can serve as a buffer layer for III-group nitrides such as GaN, InGaN, InN epitaxial growth on silicon. Furthermore, because of a large difference in refractive index n between Si(n=3.5) and Er₂O₃(n=1.8) the stacks of Si/Er₂O₃ can serve as distributed Bragg reflectors for the optoelectronic devices.
The unique properties and a wide range of applications leads to necessity to investigate the patterning of erbium oxide. The cheapest way to perform patterning is through the wet chemical etching. Till now, the comprehensive research have not yet been proceeded on patterning of erbium oxide. Therefore, we report on the wet etching mechanism of erbium oxide in sulphuric acid solution. After detailed analysis of wet etching kinetics, the parameters were optimized to achieve well defined structures. The etching experiments were performed on 300 nm thick Er₂O₃ thin films prepared on Si substrate by molecular beam epitaxy (MBE). The surface morphology studies revealed that etching mechanism was different for Er₂O₃(111) on Si(111) compared to Er₂O₃(110) on Si(100) orientation samples. Finally, GaN epitaxial growth results by metal organic chemical vapour deposition (MOCVD) on Er₂O₃/Si will be shown. Those growth results will be mainly exploited to explain the wet etching mechanism in atomic scale.

In this study, we introduce a simple and effective method to induce directed self-assembly (DSA) of symmetric block copolymers (BCPs) on large area using soft grating patterns. By physically rubbing poly(tetrafluoro ethylene) (PTFE) at various temperatures near its melting point, the horizontally aligned PTFE grating patterns with ~ 20nm in amplitude and ~ 200nm in pitch distance are produced on flat Si substrates due to its low friction coefficient and high wear rate. Then thin films of symmetric polystyrene-block-poly(methyl methacrylate) copolymers (PS-b-PMMA) form on the patterned substrates as spin-coated at 2000 rpm. To induce BCP self-assembly on the patterned surface, the thin films are solvent-annealed in vapor of organic solvents like acetone, tetrahydrofuran, toluene. Even though initial morphology of the as-spun BCP thin films is irregular, the parallel orientation of lamellar nanostructures of PS-b-PMMA is generated after solvent-annealing process. Interestingly, it is observed that those lamellar nanostructures are aligned along the pitch of underlying PTFE grating patterns and their ordering behavior are surprisingly improved by the grating patterns as compared to the BCP morphology on flat substrates. As the BCP patterns are used as templates for metal (Au, Ag, Pt) deposition process, extremely aligned metal nano-wires can be produced on the Si substrates. The ordering behavior of BCP thin films on the patterned surface is characterized by using atomic force microscopy (AFM) and scanning electron microscopy (SEM).

Due to their unique opto-electronic properties, luminescent quantum dots (QDs) have been enhancing renewable energy efficient devices, i.e. solar cells. Although quantum dot-based organic solar cells exhibit lower efficiencies, their advantages of being light weight, cost effective, variable materials processing, and easily tunable optical energy gaps, make them an appealing alternative to inorganic solar cells. In order to enhance the opto-electrical properties, we doped rare earth and novel metals into the semiconductors nanocrystals. Cadmium telluride (CdTe) QDs is a direct band gap materials with band gap energy of 1.52 eV and emits electromagnetic radiation in the visible region by tuning the quantum confinement of charge carriers. One pot microwave irradiation technique is an efficient, highly favored, quicker and more cost-effective synthesis route of core type CdTe and doped CdTe QDs in an aqueous phase. Exposure of the synthesized QDs to ultraviolet radiation confirmed that time and temperature play an immense and important role to effectively control the particle size of the quantum dots. Fluorescence intensity shifts, absorption peaks, and particle size were characterized by photoluminescence (PL) and ultraviolet-visible spectroscopy (UV-VIS). Furthermore, the electronic behavior of the doped QDs were examined by measuring leakage current, capacitance and frequency dispersion data. Transmission electron microscopy (TEM) confirmed that QD sizes are in the range of few nanometers. The photovoltaic performance of synthesized P3HT/PCBM/CdTe QDs hetero-junction solar cells were observed when alternating the size of the QDs and their respective band gap. Hence, effects of doping is an efficient method of tuning the opto-electrical properties of CdTe QDs.

ZnO semiconductors are known as excellent materials for photocatalytic applications because of their high photosensitivity, nontoxic nature, and large bandgap. A variety of morphologies containing polar and non-polar surfaces can be achieved by controlling the crystal growth condition. Stability and efficiency are two important factors in determining the feasibility of a photocatalyst for oxygen evolution reactions in water splitting applications. Polar surfaces generally are known to have higher photoelectrochemical activity increasing water splitting efficiency. However, the question is whether these surfaces are also stable. ZnO pyramidal nanostructures are deposited through chemical vapor deposition. Herein, we perform a set of Density Functional Theory (DFT) calculations to compare the stability and electronic structure of the surfaces formed during different stages of the growth process. Although these pyramidal structures are generally not stable due to their relatively high surface energy, our calculations prove that under specific growth conditions, these morphologies are stable. Moreover, the differences in the local DFT computed electronic structure between surfaces are consistent with the nanopyramids enhanced photocatalytic activity.

Semiconductors, such as TiO₂, BiVO₄ and C₃N₄, are the most widely used photocatalysts for environmental remediation due to their outstanding photocatalytic ability and low cost. However, semiconductor-based photocatalysts have two major inherent drawbacks: limited light absorption and fast electron/hole recombination. The most widely used method to solve the issues is to create semiconductor heterojunctions, with which not only the light absorption can be expanded but also the electron/hole separation can be efficiently promoted. It should be noted that the surface area and porosity of the semiconductors are generally very small, which limits the amount and accessibility of the photoactive sites to the reactants, and the subsequent improvement in photocatalytic efficiency. In this sense, it would be rational to create hierarchical nanostructures between semiconductors with porous materials, such as metal-organic frameworks (MOFs). Given their unique properties (e.g., tunable surface chemistry and huge porosity), the incorporated MOFs have great potentials to improve the photocatalytic efficiency by enhancing the charge transfer and providing numerous reactive sites.

Herein, MIL-100(Fe)/TiO₂ composite photocatalysts with two distinct structures (i.e., nanoarrays and powders) were designed (MIL-100(Fe): a typical MOF; MIL: Materials Institute Lavoisier). Systematic measurements were carried out, including X-ray photoelectron spectroscopy (XPS), grazing-incidence wide-angle X-ray scattering (GIWAXS), and transient absorption spectroscopy (TAS) measurements, to characterize the as-prepared products and analyze the photocatalytic performances. With the incorporation of MIL-100(Fe) on the TiO₂ surface, the composite photocatalysts exhibited enhanced photocatalytic efficiency towards both tetracycline degradation and Cr(VI) reduction. The enhanced photocatalytic performance was attributable to promoted charge separation, which arose from the unique structure at the interface between MIL-100(Fe) and TiO₂. More specifically, during the in-situ growth of MIL-100(Fe), defect energy levels were created in the electronic structure of the composite photocatalyst, as demonstrated with the density functional theory (DFT) simulation. The defect energy levels served as sinks to capture excited charge carriers and thus retarded the recombination process, which eventually led to increased charge carrier density and enhanced photocatalytic efficiency. This work not only presents the rationally designed MOF/semiconductor composite structures for environmental remediation, but also provides mechanistic insights into the photocatalytic pathways.

Chemical modification of semiconductor surfaces with molecular electrocatalysts provides a strategy for developing homogeneous-heterogeneous materials capable of converting sunlight to fuels and other value-added products, but their development is hampered by an incomplete understanding of the factors limiting their performance. [1-5] Although kinetic models have been separately developed to describe photoelectrochemical or homogeneous electrocatalytic reactions, related modeling for molecular-modified hybrid photoelectrodes has not been as extensively elaborated. This presentation addresses the interplay between light absorption, charge transfer, and catalytic activity during photoelectrosynthetic transformations at a molecular-modified semiconductor. The analysis provides opportunities to better understand the principles governing these hierarchal constructs and develop improved photocatalytic assemblies.

1. B. L. Wadsworth, D. Khusnutdinova, G. F. Moore, J. Matter. Chem. A., 6, 21654–21665 (2018); DOI:10.1039/C8TA05805A.
2. D. Khusnutdinova, A. M. Beiler, B. L. Wadsworth, S. I. Jacob, G. F. Moore, Chem. Sci., 8, 253–259 (2017); DOI: 10.1039/c6sc02664h.
3. A. M. Beiler, D. Khusnutdinova, B. L. Wadsworth, G. F. Moore, Inorg. Chem., 56, 12178–12185 (2017); DOI:10.1021/acs.inorgchem.7b01509.
4. A. M. Beiler, D. Khusnutdinova, S. I. Jacob, G. F. Moore, ACS Appl. Mater. Interfaces, 8, 10038–10043 (2016); DOI: 10.1021/acsami.6b01557.
5. B. L. Wadsworth, A. M. Beiler, D. Khusnutdinova, S. I. Jacob, G. F. Moore, ACS Catal., 6, 8048–8057 (2016); DOI: 10.1021/acscatal.6b02194.

Porphyrins are a class of optically active biomacromolecular compounds that play critical roles in many biological processes including photosynthesis, and also serve as colorful pigments covering a wide range of the visible spectrum. As an effort to utilize these versatile porphyrins in advanced materials development, organized porphyrin nanostructures with photoactive properties have been obtained through a surfactant-assisted non-covalent self-assembly method through the cooperative interactions of the porphyrin building block zinc meso-tetra(4-pyridyl)porphine. Electron microscopy characterizations in combination with X-ray diffraction confirm that self-assembly of optically active porphyrin building blocks leads to formation of ordered nanostructures including nanorods, nanowires, nanocylinders, nanooctahedron. Investigations of variable parameters influencing the growth process show that the final product morphology is determined by reaction conditions including pH, reaction time, precursor concentration, and surfactant types. Optical characterizations using UV-vis spectroscopy and fluorescence imaging and spectroscopy show enhanced collective optical properties over the individual porphyrins, favorable for exciton formation and transport. With active and responsive optical properties, these porphyrin nanostructures are promising components for a wide range of practical applications including sensing, and photocatalysis, and phototherapy.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Electrowetting as a droplet actuation principle is used in many applications including digital microfluidics and reflective display devices.¹The manipulation of fluids on surfaces via electrowetting offers several advantages such as lower power consumption, high mobility of fluids and flexibility in the design of the substrates. Electrowetting on dielectric is very attractive due to larger changes in contact angle between the liquid droplet and the substrate. Nanofluids (fluids containing nanoparticles) are of great interest due to reported enhanced wetting characteristics on surfaces². Nanoparticles possess tunable optical, electrical and dielectric properties as well as varying chemical stability. They also cover a wide range of technological applications from infrared detectors, solar cells, displays to photorefractive materials. We have evaluated the electrowetting properties of nanofluids comprised of visible II-VI (CdSe), III-V (InP); near infrared IV-VI (PbS) and mid-wave infrared (Ag₂Se) active semiconductor nanomaterials as well as ferroelectric nanomaterials (BaTiO₃). We will present our results on the influence of surface chemistry (different hydrophilic ligands on nanoparticles) and nanomaterial type on the electrowetting phenomena of aqueous droplets of nanofluids. We observed pronounced differences in the wetting behavior of nanofluids for all the nanomaterials studied.
The study underscores the material- dependence of the wetting properties of nanofluids and the potential exploration of the optical properties of these materials in electrowetting -based platforms as well as optofluidics.
References
1) Nature Photonics 2009, 3,292–296
2) Langmuir 2015, 31, 5827−5835

Hybrid organic-inorganic metal halide perovskites have generated tremendous interest as low-cost semiconductors for optoelectronic applications such as photovoltaics. The rapid success of hybrid organic-inorganic perovskites has thus far been centered around the lead-based perovskite derivatives; however, in recent years a sub-field has developed that seeks to substitute lead with a less toxic and more environmentally friendly alternative. Here, the clear front runner is tin, which has successfully been used in photovoltaics to achieve over 9% power conversion efficiency. In conjunction with the active layer properties the performance of optoelectronic devices is also dependent on the energetic alignment between adjacent layers. To select or design appropriate transporting materials for use in tin hybrid perovskite based optoelectronics the energetics of the active layer must be understood. Herein lies an issue where current literature regarding the valence and conduction band energies cannot reach consensus, even for a material as studied as methylammonium lead iodide. For tin-based perovskites such studies are far fewer, and yet reports for the valence band maximum of formamidinium tin iodide, the most popular composition, already span a range from 4.7 to 5.9 eV. These discrepancies can potentially be attributed to the differences in surface stoichiometry, variations in air exposure of films prior to measurements, ratio of tin(II) to tin(IV) present, the additives and their quantity used, and even the methods used to determine band onsets. In this talk I will present our low energy ultraviolet and inverse photoelectron spectroscopy systems, which allow for us to minimize sample degradation, and apply them to the study of some the aforementioned factors that may contribute to the variation in reported energetics for formamidinium tin iodide. We find that air exposure has little effect on the measured energetics whereas the addition of tin(II) fluoride, which is used nearly universally in tin perovskite photovoltaics, can have great influence. Further, a series of fullerene derivatives with varying electron affinities were investigated to probe how the energetic alignment between the perovskite and electron transport layer influences photovoltaic performance.

Recently, the coupled systems of the degree of freedom among spin, lattice, charge and orbital have been suggested as a breakthrough to improve conventional magnetocaloric effect (CMCE) for magnetic refrigeration application [1]. We have investigated the MCE of rare earth tetraboride, RB₄ (R = Dy, Ho). These compounds show complex magnetic phase transition with highly degenerated system due to the interplay between magnetic and quadrupolar ordering. After the orbital degeneracy is lifted, the quadrupolar moments are geometrically frustrated through quadrupolar fluctuation. The network of quadrupolar moments forms the Shastry-Sutherland lattice (SSL) and the direction and angle of magnetic moments are limited from simple antiferromagnetic state by the strong spin-orbit coupling. Giant inverse MCE (IMCE) is observed at quadrupolar ordering temperature, which is strongly-coupled with the magnetic ground state. The exotic IMCE exhibits maximum values of 19.6 J/kgK, 19.0 J/kgK and 22.7 J/kgK at the critical applied magnetic fields of 5 T, 4 T, and 2.5 T for DyB₄, Dy_0.5Ho_0.5B₄, and HoB₄, respectively. In particular, the exotic IMCE, which is enhanced under the critical field, decreases above the critical fields, due to the disappearance of quadrupolar ordering. These results offer new insight into the mechanism of exotic giant MCEs and magnetic cooling applications.
[1] S. Pakhira et. al., Sci. Rep. 7, 7367 (2017))

Photocatalysis is a powerful way for tackling the increasingly severe environmental and energy concerns. Owing to their intriguing localized surface plasmon resonances, noble metal nanocrystals and nanostructures have shown a great potential for enhancing the photocatalytic efficiency and thereby have attracted rapidly growing interests recently. Au and Ag nanocrystals typically exhibit strong plasmon resonances, while Pt and Pd nanoparticles typically function as versatile catalysts for various chemical reactions. In order to employ plasmon to enhance photocatalysis, much effort has been devoted to the synthesis of metal heterostructures by integrating plasmonic Au and/or Ag nanocrystals with catalytically active Pt and/or Pd nanoparticles. Such heterostructures usually exhibit enhanced catalytic performances in comparison with their monometallic counterparts due to a concerted action between the different components. However, many reported metal heterostructures are often in core@shell configurations, which can suppress the plasmonic and/or catalytic performances of the inner components. Moreover, plasmonic nanocrystals have been well known to possess hot spots, where light energy is strongly squeezed. A few studies have shown that catalytic nanoparticles deposited at the hot spots can optimize the plasmonic enhancement effect and at the same time minimize the use of expensive Pt and Pd. In this regard, we have recently performed systematic studies on the site-selective deposition of catalytic metals on pre-grown plasmonic metal nanocrystals and on their plasmon-enhanced photocatalysis behaviors.

We have recently developed a synthetic method for the site-selective overgrowth of catalytic metals on Au nanorods (NRs) and Au nanobipyramids (NBPs) (Angew. Chem. Int. Ed. 2013, 52, 10344 and Adv. Funct. Mater. 2017, 27, 1700016). Silica is first selectively deposited at the two ends of Au NRs or Au NBPs owing to the higher curvature, or deposited on the side surface when the ends are blocked by thiol-terminated methoxy poly(ethylene glycol). The pre-deposited silica component on the Au nanocrystals guides the subsequent selective overgrowth of a second metal on the exposed Au surface. Au NBP/end Pd and Au NBP/side Pd heterostructures have been prepared for exploring the effect of the local electric field enhancement on the photocatalytic activity. Suzuki coupling reaction has been chosen as a model reaction to evaluate the photocatalytic performances of the different Au/Pd heterostructures. The results indicate that the photocatalytic activity is highly dependent on the site of the Pd nanoparticles, and that the plasmonic hot spots play an important role in hot-electron-driven plasmonic photocatalysis.

We have recently synthesized highly asymmetric Au nanocups, which possess a strong magnetic plasmon mode (Adv. Mater. 2016, 28, 6322). The electromagnetic field at the edge of the cup and in the opening region is strongly enhanced. We have therefore deposited Pt or Pd nanoparticles selectively at the opening region or inside the cavity of the Au nanocups and studied their photocatalytic performances. Taken together, our results offer a new insight into the rational design of highly anisotropic metal heterostructures out of two or more functional metals for various plasmon-enhanced applications, including photocatalysis, optics and biomedicine.

In recent years, ceramic and noble metals have been utilized as photocatalysts [1]. In the case of catalyst, the size, shape and electrical state of the catalyst are important for obtaining sufficient catalytic activity. Especially, the unique phenomena caused by ceramic materials supported by noble metal have been researched [2]. In this prospect, the composite of ceramic and metal in the shape of multi-layer structure with varying length could provide new possibilities in catalyst area.
Meanwhile, a view point of material, amorphous cobalt phosphate is an promising candidate by effectively promoting water degradation by sunlight [3]. Furthermore, it is well known that Au is an excellent electrical conductor and absorbs light of different wavelengths depending on their shape, size and interdistance of each other [4].
In this study, we propose a relatively facile and economical synthesis process fabricating 1D heterostructures, that is, core-shell/barcode nanowires (BNWs) of which each segment’s length, diameter and the thickness of shell can be easily tuned.
First, we synthesized CoAu BNWs by pulsed electrodeposition method using anodic aluminum oxide (AAO) template. After then, AAO was removed with chromium phosphate and phosphoric acid. At the same time, BNWs with a core shell - noble metal structure were formed by selectively reacting only with Co layers. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), Inductively coupled plasma atomic emission spectroscopy (ICP-AES), Ultraviolet-visible spectroscopy (UV-vis) and vibrating-sample magnetometer (VSM) were employed to observe shape and microstructure of core-shell/BNWs. The length of each part of nanowires were successfully adjusted by the applied current densities, duration time, and phosphorus containing solution. As the face centered cubic (FCC) Au dominates the foundation of subsequent layers, the transition of Co layers was observed from FCC to stable hexagonal closed pack (HCP) structures. Moreover, the Co phosphate shell was characterized as amorphous structures and if the reaction time is enough, Co layers are completely reacted by intruding phosphorus changing into a perfect ceramic-noble metal BNW. X-ray based measurement system and ICP-AES let us understand the diffusion of phosphorus. In addition, UV-vis provides optical property of synthesized nanowires. Finally, we verified the catalytic properties depending on the dimension of each part.

Reference
[1] R. Jiang et al., Adv. Mater, 26, 5274 (2014)
[2] K. Fujiwara et al., Environ. Sci. Nano, 4, 2076 (2017)
[3] X. L. Hu et al., ACS Nano, 6, 10497 (2012)
[4] S. Linic et al., Nat. Mater, 10, 911 (2011)

Direct solar-to-fuel generation using a photocathode-based photoelectrochemical cell requires a light absorber which can provide the photovoltage necessary to overcome the thermodynamic potential (1.23V for H₂/O₂, 1.33V for CO/O₂) as well as the catalyst overpotentials for both cathode and anode reactions. To realize high solar-to-fuel efficiency, it is necessary to maintain a catalytic current density close to the light limiting photocurrent density for a solar-driven light absorber, which can be fulfilled when catalyst ensembles are highly transparent. Recently, we reported a record solar-to-hydrogen efficiency using a tandem III-V semiconductor photoelectrode based solar photoelectrochemical cell with an optically transparent catalyst comprise of a dense array of Rh metal nanoparticles¹. For CO₂ reduction, a different approach is required, given the opaque nature and limited activity of most CO₂R catalysts. Here, we report and demonstrate two light management strategies to create highly active and effectively transparent catalyst structures for photocathodic CO₂ reduction: i) an effectively transparent catalyst consisting of arrays of micron-scale triangular cross-sectional silver grid fingers, which is capable of redirecting the incoming light to the open areas of the PEC cell without shadow loss, and ii) arrays of mesophotonic dielectric cone structures that serve as tapered waveguide light coupler to efficiently guide incident light through apertures in an opaque catalyst into the light absorber. To validate these designs, numerical calculations using full wave electromagnetic simulations were used to investigate the optical response. We find that a mesoscale silver grid array with triangular cross-section lines and metal coverage of > 50% exhibits negligible additional reflection loss. For the nanostructure dielectric cone light couplers, <10% reflection (>90% transmission) is achieved experimentally for metal (Cu) coverage as high as 70%. Both catalyst designs will be described in detail, including simulations and fabrication methods, as a guide to efficient catalyst design for photoelectrochemical solar fuels generation.

Reference
1. Cheng, W.-H. et al. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Lett.3, 1795–1800 (2018).

An important building block for detector devices are signal amplifiers, such as microchannel plates (MCPs). Due to the electron cascade produced within the high voltage-biased MCP pores, incident electrons, photons, or ions can be amplified by 10⁴, making MCPs highly efficient in photon-counting devices. Moreover, since the MCP comprises millions of parallel pores, these devices are ideal for imaging applications astronomy, high-energy physics, medicine, etc. Recently, MCPs have been fabricated by functionalizing capillary array glass using atomic layer deposition (ALD) coatings to impart the necessary electrical resistance and secondary electron emissive (SEE) properties.^{[1, 2]} Considering the complex geometry of the MCPs, ALD is ideal for depositing these functional coatings with high precision and conformity. Thus far, a limited number of ALD materials (i.e. Al₂O₃ and MgO) have been evaluated as SEE layers in MCPs. While these materials show great performance in real-life MCP applications, chemical reactions between these SEE materials and the underlying resistive coatings are a possible limitation to their ultimate performance. In addition, the reaction of MgO SEE layers with ambient H₂O and CO₂ changes the surface chemistry and causes a drop in the SEE value. These issues motivate the study of alternative ALD materials to function as SEE layers in MCPs.

We present here the development and evaluation of ALD metal fluoride and mixed metal oxy-fluoride coatings as potential SEE layers in MCPs. We also investigated the influence of H₂O and CO₂ exposure on the SEE surfaces to understand the effect of these ambient compounds on the properties of the coatings. The ALD mechanism was studied using in-situ quartz-crystal microbalance (QCM), quadrupole mass-spectrometry (QMS), and Fourier transform infrared spectroscopy (FTIR) measurements. The materials were analyzed to determine their structure and composition using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and X-ray reflectivity (XRR). The ALD SEE layers were deposited on commercial MCP substrates to evaluate the electronic properties and to measure the gain and stability during operation.

[1] A. U. Mane and J. W. Elam, Chem. Vap. Deposition, 2013 (19), 186-193
[2] M. J. Minot, B. W. Adams, M. Aviles, J. L. Bond, C. A. Craven, T. Cremer, M. R. Foley, A. Lyashenko, M. A. Popecki, M. E. Stochaj, W. A. Worstell, A. U. Mane, J. W. Elam, O. H. W. Siegmund, C. Ertley, H. Frisch and A. Elagin, Proceedings Volume 9968, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XVIII, 2016, DOI: 10.1117/12.2237331

Carbon nanotubes (CNTs) have many attractive properties, and show promise in a wide variety of applications. Demonstrated is a light absorption material, EMI shielding and or sound absorption material utilizing CNT materials in the form of a vertically aligned CNT forest.
The CNT forest is grown via chemical vapor deposition and requires catalyst, carbon source, inert gas and hydrogen. Hard substrates such as quartz, silicon wafer, stainless steel or aluminum are generally used as these can withstand the high temperatures required for CNT growth. Such hard substrates make it difficult to use the CNT forest in applications, particularly those requiring complex geometric configurations or flexibility.

The fragility of the CNT forests makes it a challenge to transfer them from their inflexible growth substrate onto a target substrate. In this work we report a novel method of transferring CNT forests onto an arbitrary substrate, including common polymer films such as PET. The developed technique is also able to alter the alignment angle of the CNT forest relative to the substrate: 90 degree (high angle), 45 degree (mid angle) and 25 degree (low angle). Typically, an adhesive is used to transfer the CNT forest, but in some cases the adhesive is replaced with a non-adhesive material.

The dye-sensitized photoelectrochemical cell (DSPEC) represents a novel approach to combining molecular chromophores and catalysts with semiconductors to effect light driven production of solar fuels. A multidisciplinary approach has been used to develop and study molecular and polymer assemblies for light driven water oxidation at a DSPEC photoanode and proton reduction at a photocathode. The work aims to understand mechanisms and dynamics for the photoprocesses occurring at the molecular/semiconductor interfaces. The talk will present an overview of work done during the past several years, involving the design, construction and study of molecular and polymer-based assemblies of light absorbing chromophores and catalysts, primarily aimed at water oxidation at the DSPEC anode.

Zinc oxide (ZnO) is an earth abundant, non-toxic, and low-cost material that has been used widely for photocatalytic water splitting, gas sensing, and dye degradation. In this study, several ZnO structures were synthesized, characterized, and tested for the photocatalytic reduction of bicarbonate to formic acid, an intermediate to methanol, a high-octane-number fuel. The different ZnO morphologies studied included micron- and nano-particulate ZnO, rods, wires, belts, and flowers. ZnO was also synthesized from the direct calcination of zinc acetate, which provided a cheap and large-scale synthesis method to produce ZnO. The photocatalytic efficiency of the synthesized ZnO was compared to commercial micron- and nano-particulate ZnO, and was proven to be just as efficient. ZnO flowers, possessing the largest surface area of 12.9 m²/g, were found to be the most efficient reaching an apparent quantum efficiency (AQE) of 10.04±0.09%, with a superior performance over commercial TiO₂ (P25), a benchmark photocatalyst. Green chemistry solvent, glycerol, proved to be a far superior hole scavenge in comparison to 2-propanol, which is derived from petroleum sources. This is the first study to compare different shapes and sizes of ZnO for bicarbonate reduction in an aqueous system with excellent photocatalytic performance.

Plasmon-enhanced photocatalytic reduction reaction on TiO₂ under solar light using alternative plasmonic titanium nitride(TiN) nanoparticles is studied. The excellent chemical stability at elevated temperatures and higher integrated absorption efficiency under a solar light in comparison to conventional plasmonic nanomaterials (silver and gold) make TiN a promising candidate for this application. We observed an efficient production of formic acid through simultaneous photoreduction of bicarbonate and oxidation of glycerol in the presence of TiO₂/TiN composite nanocatalysts in an isothermal condition. The photocatalytic productivity of formate with TiO₂/TiN composite is significantly (6 times) higher than TiO₂ alone. The enhancement is predicted to be due to the plasmon-induced excited charge transfer from TiN to the conduction band of TiO₂. <div>Interestingly, under solar light TiN alone can photo catalyze the reaction more efficiently than TiO₂ nanocatalyst (Degussa, P-25). The enhanced performance of TiN is attributed to the efficient hot electron transfer from TiN nanoparticle core to native thin (1-2 nm) amorphous titanium oxide shell, where the reaction takes place. The characterization of TiN nanoparticles after the reaction confirmed that TiN nanoparticles could remain stable under reaction conditions for extended periods of solar light exposure (8 hours). Comprehensive theoretical analyses including fully numerical solutions of Maxwell equations to understand the optical properties of the nanostructures are done to support the experimental observation.</div>

(Photo)electrocatalytic reduction of carbon dioxide offers an efficient strategy to reduce the presence of greenhouse gases in the atmosphere while concurrently producing valuable carbon-based products.^1,2 However, existing (photo)electrocatalysts for this process are insufficiently active or selective for attractive energy dense products, particularly in the face of the competing hydrogen evolution reaction (HER).^3,4 Here, we present design strategies for the synthesis of novel (photo)electrocatalysts for CO₂ reduction. We provide different examples of catalytic systems that can reveal design principles that enable development of active and selective catalysts and provide further insights into the reaction mechanism.
References:
Artz, J. et. al. Sustainable conversion of carbon dioxide: an integrated review of catalysis and life cycle assessment. Chem. Rev. 118, 434-504 (2018).
She, Z. W. et. al. Combining theory and experiment in electrocatalysis: insights into materials design. Science 355, eaad4998 (2017).
Qiao, J., Liu, Y., Hong, F., & Zhang, J. A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels. Chem. Soc. Rev. 43, 631-675 (2014).
Hori, Y. in Modern Aspects of Electrochemistry Vol. 42, CH. 3 (eds Vayenas, C. G., White, R. E., & Gamboa-Aldeco, M. E.) 89-189 (Springer, New York, 2008)

In this presentation recent efforts to create Si based photoanodes for photoelectrochemical applications will be discussed. Particular attention will be paid to WO₃ films, using an ITO interface. WO₃ on ITO/Si was found to rapidly loose performance in acidic media, which we assume to be due to the formation of adsorbed hydroperoxo species. When nanometer thin Pt was used to interface WO₃ with Si, stability and enhanced performance could be induced by deposition of TiO₂, prior to deposition of IrO₂.
The effect of the height and spacing of microwires of Si on the photoelectrochemical performance of deposited BiVO₄ will also be addressed. Microwire substrates with BiVO₄ underperformed compared to BiVO₄ on flat surfaces. We predominantly attribute this to detrimental diffusion limitations of the redox species within the internal volume of the microwire arrays, in agreement with existing literature and observations regarding the electrodeposition of BiOI, used as precursor for BiVO₄. Our results assist in developing high-efficiency PEC devices.

To accomplish a practical CO₂ reduction system aimed at artificial photosynthesis, a hybrid system composed of a metal complex catalyst and a semiconductor photosensitizer has become a feasible approach, and the conversion efficiencies of such systems have been improving. However, there is still little research focusing on material cost and operation in water. Hematite (α-Fe₂O₃) is one of the most abundant and low-cost semiconductor materials, and can absorb a substantial amount of solar light (Bandgap: 2.1 eV). Although a usual α-Fe₂O₃ exhibit n-type conductivity, doping with ions such as Mg²⁺ and Zn²⁺, and N^3- into α-Fe₂O₃ induce p-type conduction. We have reported that N,Zn-codoped α-Fe₂O₃ (N,Zn-Fe₂O₃) exhibited high cathodic photocurrent for O₂ reduction.[1] However, there are two issues with p-type Fe₂O₃ in that the charge separation property is inefficient and it easily corroded due to the self-reduction reaction under the reducing conditions in the CO₂ saturated aqueous electrolyte. To overcome the issues, a TiO₂ layer was introduced onto the surface of p-type N,Zn–Fe₂O₃ to passivate surface defects. In addition, to ensure efficient electron transfer, a thin Cr₂O₃ layer was also inserted between N,Zn–Fe₂O₃ and a bottom side conductive oxide layer to generate a favorable band alignment for hole transfer. To optimize a metal complex – semiconductor hybrid photoelectrode, a Ru complex polymer with a low CO₂ reduction potential and an electron network with polypyrrole chains was determined to be the best combination with TiO₂/N,Zn-Fe₂O₃/Cr₂O₃.[2] We have successfully accomplished stable CO₂ reduction reaction with p-type Fe₂O₃ in water, and demonstrated solar CO₂ reduction reaction coupled with H₂O oxidation in the absence of an external electrical bias by constructing a tandem cell reactor with an n-type SrTiO₃ photoanode.

Reference
[1] Morikawa, T.; Arai, T.; Motohiro, T., Photoactivity of p-Type α-Fe₂O₃ Induced by Anionic/Cationic Codoping of N and Zn. Appl. Phys. Express 2013, 6, 041201.
[2] Sekizawa, K.; Sato, S.; Arai, T.; Morikawa, T., Solar-Driven Photocatalytic CO₂ Reduction in Water Utilizing a Ruthenium Complex Catalyst on p-Type Fe₂O₃ with a Multiheterojunction. ACS Catal. 2018, 8, 1405-1416.

Heterojunction and direct Z-scheme nanostructures are two typical representatives of an efficient photocatalyst, which is composed of two semiconductors. However, it is a great challenge to construct each of them on purpose. Photo-deposition technique can be a potentially powerful tool to regulate the electron flow direction for constructing these nanostructures. So, can we refer to the controllability of photo-deposition on the direction of charge flow to achieve a direct Z-scheme nanojunction construction? Based on the above discussion, we synthesized Fe₂O₃/g-C₃N₄and TiO₂/CdS composites by photocatalytic oxidation deposition and photocatalytic reduction deposition, respectively. In order to verify both systems are the Z-scheme type junction, charge-tracking experiments were performed on both of them. The charge tracking experiments of Fe₂O₃/g-C₃N₄composites obtained by oxidative deposition of Fe₂O₃on g-C₃N₄substrates show that the oxidation sites of the composites are located in Fe₂O₃，which is the same as the expected results. At the same time, TiO₂/CdS composite material is obtained by using TiO₂nanorods reduction deposition of CdS, the charge tracking results showed that the reduction site of the composite was located in CdS, which proved that direct Z-scheme was reached through photo-deposition.

Innovations in photoelectrode architecture can continuously increase surface area, enhance light absorption and improve charge transport, and thereby increase overall power conversion efficiency. In the past, nanostructures of varying complexities, from one-dimensional nanotubes, nanowires and nanorods, to two-dimensional films and nanonets, and three-dimensional (3-D) porous structures, were reported with superior performance. In this presentation, branched 3-D nanostructured materials, especially ZnO, were discussed. To elevate the spatial occupancy of one-dimensional ZnO nanostructures and explore new structures as efficient electrodes for industry-level photoelectrochemical (PEC) water splitting into usable H₂ fuel, we have developed procedures to fabricate nanoforest and “caterpillar-like” ZnO nanostructured network (CZN) for PEC applications. Moreover, by fine-tuning the synthesis procedure and manipulating their growth process, the dependence of their PEC properties on geometry factors of the unique branched nanostructures consisting of branched ZnO nanowires onto ZnO nanofibers with tunable surface-to-volume ratio and roughness factor has been investigated. They offer mechanically and electrically robust interconnected networks with open micrometer-scale structures and short hole diffusion length. The preferential light-material interaction and charge separation to maximize the photo-to-hydrogen conversion efficiency were further studied. When used as photoanode, our branched nanostructures not only favor sunlight harvesting with multireflection ability, but also suppress the recombination of photogenerated charge. To sum, these novel branched nanostructures represent a new generation of photoelectrodes for high-efficiency solar energy harvesting and conversion to clean chemical fuels and hold bright potential for a wide range of practical applications in renewable energy.

The preparation of semiconductor nanomaterials and its composite formation with ternary chalcogenides (TC) for photocatalytic applications has been reported. Heterogeneous photocatalysis in the past decade has deemed to be an effective route for producing greener energy and environmental remediation. Ternary chalcogenides (TC) with remarkable visible light absorption, are identified as an ideal candidate to form heterostructure with classical semiconductors such as TiO₂. In the present investigation, a heterostructure nanocomposite of AgBiS₂-TiO₂ was synthesized using a solvothermal technique. Computational analysis was utilized to study the electronic and optical properties of the pristine parent samples. The XRD results show the formation of the cubic phase of AgBiS₂ and TiO₂ is in tetragonal phase. The XPS and the TEM results illustrate the heterostructure formation. The UV-DRS pattern for all the composites shows enhanced visible light absorption due to the coupling of TC. The band gaps of the composites were decreased with increased doping levels. These materials were further studied for their photocatalytic efficiency, by photocatalytic degradation of Doxycycline, photocatalytic hydrogen generation and photocatalytic antimicrobial disinfection. The composite samples illustrated more than 95% degradation results within 180 minutes and showed about 3 log reductions of bacterial strains (E. coli and S. aureus) within 30 minutes of irradiation. The hydrogen production results were interesting as the AgBiS₂ based composites illustrated a 1000-fold enhanced output. The enhanced photocatalytic activity is attributed to the decreased rate of recombination of the photogenerated excitons, as validated in the PL measurements.

In recent years, photocatalytic technology has attracted a great attention, due to its advantages such as thorough pollutant purification, nontoxicity, strong oxidation and reduction, and long-term stability, etc. Among various photocatalysis, ZnO has been recognized as a kind of excellent materials for photocatalysis, because of its high photosensitivity, nontoxic nature, and large band gap. However, improving the photocatalytic efficiency of ZnO to meet practical application requirements is still a challenge, due to the bottleneck of poor quantum yield. Generally, oxide semiconductor coupling is one of an effective process, which enhances the light utilization efficiency and electron-hole pair separation efficiency via forming a heterojunction between two different oxide semiconductors.
In this paper, we introduce some novel and facile methods to prepare the ZnO based heteroarchitecture. The experimental results exhibit the great improvement on the photocatalytic properties. It is expected that the 3D heteroarchitectured composite will have a potential applications in the areas of environmental improvement, green energy, water splitting and hydrogen generation, etc.
1) Au/ZnO/NiO–In this work, we present a novel physical route to fabricate a kind of Au/ZnO/NiO composite. That is, a Zn layer upon Ni foam substrate is prepared by using a pulse electro-deposition, then the ZnO nanoneedle/NiO heterostructure is obtain via thermal oxidation, and at last, the composite is modified with the dispersively deposited Au nanoparticles (Au NPs) by ion sputtering. The surface plasmon resonance effect of the Au NPs significantly enhances the light absorption. Meanwhile, the Au NPs form a Schottky barrier with ZnO nanoneedles and further inhibit the recombination of photo-generated electron-holes. In addition, due to the non-solvent conditions, the introduction of impurities is avoided, and it shows strong photocatalytic stability. The experimental results reveal that, the optimized Au/ZnO/NiO composite exhibits up to two times photocatalytic performance on RB degradation and higher stability than that of regular ZnO/NiO composite.
2) ZnO/CNF/NiO–In this work, we introduce a novel facile two-step chemical vapor deposition route as a straightforward protocol for preparing a kind of 3D reticulated ZnO/CNF/NiO heteroarchitectured composite. In the experiment, carbon nanofibers (CNFs) grew directly on porous Ni foam, and ZnO nanorods were seamlessly and uniformly grew from CNFs. Due to the good contact of CNFs to ZnO and NiO, the CNFs exhibited a role as a bridge to transport electrons and holes between ZnO and NiO during photocatalytic process, which resulted in an improvement on separation efficiency for electrons and holes. In addition, the CNFs also provided additional sites for ZnO adhesion, which therefore enhanced the light energy absorption efficiency. The experimental results revealed that this composite improved the photocatalytic performance 2.5 times higher than that of regular ZnO/NiO composite.
3) TiO₂ NRAs/graphene/ZnO NPs–In this work, a ternary hierarchical TiO₂ /graphene/ZnO nanocomposite is prepared by using graphene sheets as bridge between TiO₂ nanorod arrays (NRAs) and ZnO nanoparticles (NPs) via a facile combination of spin-coating and chemical vapor deposition techniques. The experimental study reveals that the graphene sheets provide a barrier-free access to transport photo-excited electrons from rutile TiO₂ NRAs and ZnO NPs. In addition, there generates an interface scattering effect of visible light as the graphene sheets provide appreciable nucleation sites for ZnO NPs. This synergistic effect in the ternary nanocomposite gives rise to a largely enhanced photocurrent density and visible light-driven photocatalytic activity, which is 2.6 times higher than that of regular TiO₂ NRAs/ZnO NPs heterostructure.

Recent years has witnessed the rocketed development of halide perovskite in photovoltaic technology. Further enabling the perovskite photovoltaic in versatile solar to fuel applications, in particular, the solar-CO₂ reduction to valuable chemicals, is of highly interest. In this work, we developed a lead halide perovskite photocathode protected by indium-based electrocatalytic alloys, which function as both protective layer in aqueous solution and highly selective CO₂ reduction electrocatalyst for formic acid formation. Moreover, the catalytic alloys were optimized via tuning the compositions and the catalytic performance of the alloys were carefully investigated. We then fabricated the photocathode using the alloys having the lowest melting point as well as high catalytic performance. Remarkably, under 1 sun irradiation, the faradic efficiency for formic acid formation reached nearly 100% at around -0.4 V vs. RHE.

Aluminum has attracted a great deal of attention as a plasmonic material in recent years due to its high abundance and low toxicity. Compared to frequently used gold and silver, it exhibits relatively high free carrier density, and the localized surface plasmon resonances (LSPRs) of aluminum nanoparticles can be tuned from the ultraviolet to the visible wavelength regime by controlling the nanoparticle size and shape. Aluminum nanostructures have been successfully used as sensitizers in photocatalysis for absorption enhancement and higher photocatalytic efficiency and selectivity. Since this process relies on hot carrier injection into the semiconductor photocatalyst, investigations on carrier relaxation dynamics in aluminum nanoparticle systems are critical in order to develop optimal architectures for efficient carrier injection.
In an early study, we synthesized aluminum nanoparticles with diameters of approximately 100 nm in the solution phase and characterized used high-resolution transmission electron microscopy to show that these particles contain a 3.7 nm thick self-limiting surface oxide layer. Using transient absorption spectroscopy (TAS), we demonstrated that these large nanocrystals show a decreased transmission in the visible and near infrared regions while displaying a bleach in the wavelength that corresponds to the aluminum interband transition. These large particles show fast energy transfer to the solvent, on a timescale of approximately 250 ps, which is mediated by the thin oxide shell according to an extended two interface model for thermal transport.
Here, we prepare aluminum nanoparticles with tunable sizes in the range of 70 nm to 130 nm in diameter with dipolar LSPRs ranging from 350 nm to 580 nm in wavelength. TAS is employed to study the size-dependent thermal relaxation dynamics. We show that the frequency of phonon oscillation is inversely proportional to the size of the particles which is consistent with acoustic modeling results. Furthermore, we find that the energy relaxation timescale decreases with decreasing nanoparticle diameter. For all particle sizes, the oxide shell mediates a faster energy exchange process than that predicted for metal-only particles. These energy dissipation studies are crucial for designing nanostructures with optimized sizes and surface profiles that will be favorable for utilizing local temperature variations and charge transfer to enhance photocatalytic rates in heterogeneous systems.

Exploring the synthesis of photosensitizers that not only have strong absorption in the near-infrared region but also achieve rapid temperature rise in a short period of time are required for excellent photothermal therapy. Porphyrins as a class of biological macrocyclic conjugated molecules with good biocompatibility are more and more used in biological diagnosis and treatment. However, how to make ordinary porphyrin molecules achieve strong absorption in 800 nm near infrared region and enhance penetration ability in PTT is still unresolved. Here we report a proton acid doping porphyrin self-assembly method to construct strong NIR absorbance near 1000 nm photothermal conversion agents using 5,10,15,20-tetrakis (4-aminophenyl) porphyrin (TAPP) as optical active precursor. A series of morphologies including nanosheets (NSs) and nanorods (NRs) as well as amorphous nanoparticles (NPs) are synthesized with controlled size and dimension. The morphology and absorption spectra can be regulated by adjusting the types of surfactant micelles and the addition amount of protonic acid though adjusting the weak interaction forces between the assembled molecules. Owing to the inner core protonated of the TAPP in NPs effectively improved NIR-absorption and complete quenching of fluorescence. Ultimately obtaining a morphologically dependent NPs capable of efficiently utilizing NIR light energy to thermal energy conversion is obtained. The NPs connected with cRGD have been used in the treatment of tumor-bearing mouse models to achieve excellent therapeutic effects. This method of adjusting the spectral changes by adjusting the weak interaction between the assembly elements has a good extension value.

Photocatalytic hydrogen production from water splitting is considered as one of the promising ways to provide clean fuels that can convert solar energy into chemical energy. In the past years, researchers have made great efforts to develop various inorganic and organic materials systems as photocatalysts for water splitting. Photocatalytic water splitting includes three steps: (i) photocatalyst absorbs light; (ii) photogenerated charge separation; (iii) surface chemical reaction. Only by enhancing the efficiency of each step can the efficiency of photocatalytic water splitting be improved. Directional separation of photogenerated charge is more studied in inorganic semiconductor photocatalysis, but little research has been done on organic photocatalysis. Here, we reported an emulsifier-assisted co-solvent method to fabricate porphyrin nanomaterials using Pd meso-tetra (4-carboxylphenyl) porphyrin as building blocks. The specific arrangement of aggregates were spontaneously constructed driven by confined noncovalent interactions of hydrogen bonds, π-π stacking, hydrophilic-hydrophobic. The four carboxyl groups and center metal Pd within the PdTCPP molecular endowed strong hydrogen bonding and axial coordination between porphyrin and emulsifier. These assemblies had a wide absorption spectrum at 400-700 nm, which was good for absorbing visible light. These assemblies were used for photocatalytic hydrogen production under visible light and the result exhibited the outstanding hydrogen evolution rate of 9.38 mmol/g/h in the absence of Pt particles as cocatalyst in our work. The rate of photohydrogen production increases to 138.94 mmol/g/h after photocatalytic deposition of Pt nanoparticles. What's interesting is that the Pt nanoparticles are mainly on the edge of the assemblies. This indicated that the edge of the assemblies was the electron transport channel. The hole transport channel remained to be studied.