Symposium NM03 : Rational Designed Hierarchical Nanostructures for Photocatalytic Systems

Water splitting and CO₂ fixation on semiconductor photocatalysts are importance reactions from the viewpoint of solar-to-fuel energy conversion to realize “artificial photosynthesis”. To achieve these reactions, it is important to improve both bulk and surface properties of a photocatalyst so as to suppress electron–hole recombination and promote surface redox catalysis. In this presentation, recent progress on the development of new photocatalysts that are active for such artificial photosynthetic reactions will be given. In particular, surface modification techniques developed by our group to construct active sites and light-absorbing centers using nanoparticles and metal complexes will be presented.
For example, we developed a new powdered photocatalyst consisting of Co(OH)₂ and TiO₂. It is well known that TiO₂ is an active photocatalyst, but only works under UV irradiation. By contrast, the Co(OH)₂/TiO₂ hybrid photocatalyst is capable of absorbing visible light with wavelengths of up to 850 nm and oxidizing water into oxygen gas, even though it consists of only earth-abundant elements only. To our knowledge, this system provides the first demonstration of a photocatalytic material capable of water oxidation upon excitation by visible light up to such a long wavelength.

The self-assembly of metal chalcogenide tetrahedral clusters can lead to a family of porous semiconducting materials with uniform pore sizes and high surface area. The single-sized tetrahedral clusters act as building blocks to form well-ordered three-dimensional superlattices in the presence of either organic or inorganic species (including optically active metal complexes) as structure directing agents. The structural analysis based on single crystals reveals detailed information that could help for the understanding of the unique optical and photocatalytic properties of the materials. The diversity of superlattices is achieved by modifying the cluster size, the cluster composition, and the inter-cluster linkage mode. The electronic band structures of the materials can be tuned over a wide range by controlling the chemical compositions and sizes of the clusters. Physical properties such as porosity and gas adsorption properties of the materials have been studied and will be presented. These porous semiconducting materials exhibit high surface area and uniform porosity. They can be used as photocatalytic materials for converting water and carbon dioxide into useful chemicals, as well as electrocatalytic materials for oxygen reduction reaction.

Photocatalytic water splitting by semiconductors has drawn significant attention as a potential means of hydrogen production using solar energy. Metal oxide photocatalysts have been widely studied because of high stability in water under irradiation. Metal oxides are easy to form oxygen defects by annealing under reductive condition, which have been studied in defect chemistry. However, there are very few that deal with quantitative discussion about the relationship between oxygen defect density and photocatalytic activity.
SrTiO₃ is a typical non-stoichiometric compound that has oxygen vacancies depending on the oxygen partial pressure. With decreasing the oxygen partial pressure, the density of electrons in SrTiO_3–δ is increased. With this fact in mind, we synthesized several SrTiO_3–δ samples with different electron densities, and examined photocatalytic activities.
Non-stoichiometric SrTiO_3–δ samples were prepared by a polymerized complex method followed by annealing in different oxygen partial pressure. The prepared samples were studied by powder X-ray diffraction, UV-visible diffuse reflectance spectroscopy, scanning electron microscopy and so on. Photocatalytic reactions were conducted using a top-irradiation type cell that was connected to a closed gas circulation system. 100 mg of SrTiO_3-δ was dispersed in methanol aqueous solution or silver nitrate aqueous solution. After outgassing, the solution was irradiated under a 300 W Xe lamp (λ >300 nm).
In X-ray diffraction patterns, all of the prepared samples showed single-phase diffraction patterns attributed to perovskite SrTiO₃, and no significant difference of structure with calcining oxygen partial pressure. Also, no significant variance in surface morphology, surface area and chemical compositions were confirmed by SEM images, BET measurement and ICP-MS measurement, respectively. However, in diffuse reflectance spectroscopy, the samples annealed in lower oxygen partial pressure than 10^-16 atm were gray powder, exhibiting a visible light absorption band longer than 400 nm. This absorption is more pronounced with decreasing oxygen partial pressure. The visible light absorption band can be assigned to reduced titanium due to formation oxygen defects. In addition, samples prepared in lower oxygen partial pressure prolonged the lifetime of photogenerated electron, as revealed by transient IR absorption spectroscopy.
Then we evaluated photocatalytic activities of the prepared SrTiO_3-δ samples. Hydrogen evolution and oxygen evolution half reactions were enhanced with decreasing oxygen partial pressure. Samples prepared in lower oxygen partial pressure have a high oxygen defect density, which increase the electron density of the samples. The high electron density shifts the Fermi level to more negative directions. The results of photocatalytic reactions suggest that the Fermi level shift and longer lifetime of photogenerated electron contributed the high photocatalytic activities.

The design of highly efficient, non-biological, molecular-level energy conversion “machines” that generate fuels directly from sunlight, water, and carbon dioxide is both a formidable challenge and an opportunity that, if realized, could have a revolutionary impact on our energy system. Basic research has already provided enormous advances in our understanding of the subtle and complex photochemistry behind the natural photosynthetic system, and in the use of inorganic photo-catalytic methods to split water or reduce carbon dioxide-key steps in photosynthesis. Yet we still lack sufficient knowledge to design solar fuel generation systems with the required efficiency, scalability, and sustainability to be economically viable.
In the DOE Energy Innovation Hub, the Joint Center for Artificial Photosynthesis, we are developing an artificial photosynthetic system that will only utilize sunlight and water as the inputs and will produce hydrogen and oxygen as the outputs. We are taking a modular, parallel development approach in which the three distinct primary components-the photoanode, the photocathode, and the product-separating but ion-conducting membrane-are fabricated and optimized separately before assembly into a complete water-splitting system. The design principles incorporate two separate, photosensitive semiconductor/liquid junctions that will collectively generate the 1.7-1.9 V at open circuit necessary to support both the oxidation of H₂O (or OH^-) and the reduction of H⁺ (or H₂O). The photoanode and photocathode will consist of rod-like semiconductor components, with attached heterogeneous multi-electron transfer catalysts, which are needed to drive the oxidation or reduction reactions at low overpotentials. This talk will discuss a feasible and functional prototype and blueprint for an artificial photosynthetic system, composed of only inexpensive, earth-abundant materials, that is simultaneously efficient, durable, manufacturably scalable, and readily upgradeable, including both the operational and technical scope of the JCAP Hub, as well as technical results towards this goal that has recently been developed at Caltech.

As a potentially low-cost, high-efficiency solar energy storage solution, solar water splitting faces great challenges. One of the issues is the poor catalytic activity and low stability. Applications of co-catalysts have been shown effective to correct the deficiency by promoting desired chemical reactions so as to minimize charge recombination at the surface and to reduce parasitic corrosion reactions. The detailed behaviors of the light absorber/catalyst interface, however, remain poorly understood. Here we present our recent research in this area. We show that the application of the co-catalysts may greatly influence the charge separation behaviors of the photoelectrode. Detailed thermodynamic and kinetic measurements support our understanding. Furthermore, we show that the photoelectrode substrate also exerts great influences on the co-catalyst behaviors. A strong interaction between the photoelectrode and the catalyst can be beneficial. The combined system may also serve as a new platform to understand heterogeneous catalysis such as water oxidation at a level previously inaccessible. The knowledge generated by our work will likely contribute significantly to the development of solar water splitting technology for a future powered by renewable energies.

We discuss our recent progress in the design and architectural control of complex nanostructured photocatalytic systems and their applications. We first review the synthesis, crystallinity control, and photocatalysis of titania porous nanostructures by discussing several methods for changing the structures from amorphous to crystalline and subsequently ways for enhancing the crystallinity. We also discuss the photocatalytic applications of the nanoshells and the methods for improving their catalytic activities. We will also report a new color switching system based on the reversible redox reaction that could be initiated by the photocatalytic response of semiconductor oxide nanocrystals. The excellent performance of the new color switching system promises their potential applications as attractive rewritable media to meet our society’s increasing needs for sustainability and environmental conservation. We further discuss the use of such photocatalytic systems for heavy metal removal from the wastewater.

To find practical techniques for manufacturing clean and sustainable energy is a globally demanding challenge due to the shortage of fossil fuels as well as the global climate changes. Solar water splitting directly to hydrogen and oxygen has become one of the most desirable methods for harvesting and conversion of solar energy into chemical energy.
Among various water splitting materials, metal oxide semiconductors are particularly appealing candidates for practical applications. Recently, in order to replace the noble metal to enhance hydrogen evolution performance, a variety of transition metal oxides and hydroxides are widely used as effective co-catalyst. In this work, we successfully synthesized multi-shelled heterostructural metal oxide hollow spheres by the sequential templating method. The advantages of our designed multi-shelled hollow spheres are as follows: 1) the porous structures enlarges the specific surface area, 2) a void cavity enables the solvent to access the reactive sites and enhances the matter transfer, 3) hollow structures enhances the light harvesting capability and 4) thin shells shorten the diffusion paths for photoexicited electrons and holes. These unique features of our designed morphology make it a promising candidate in solar water splitting. When tested as hydrogen evolution reaction materials for water splitting, the multi-shelled heterostructural metal oxide hollow spheres exhibited a higher oxygen evolution reaction rate and good stability, which are competitive among the TiO₂ based photocatalysts. This work opens up a new method to enhance hydrogen evolution performance by structural design.

It is still a great challenge to develop highly efficient photocatalysts for solar fuels to deal with the energy crisis and climate change. Layered double hydroxides (LDHs), are a very promising family of photocatalysts, due to their easily controllable metal cation composition and thicknesses that allow band gap tuning.¹ By carefully synthesizing LDH nanosheets of a few nanometers thick, oxygen-vacancies can be easily generated on the surface/edge of LDH nanosheets, which promote the adsorption of CO₂ on the surface of LDH nanosheets, thereby enhancing the rates of photocatalytic CO₂ reduction to CO in the presence of water vapor.² Furthermore, heterostructured catalysts consisting of nickel nanoparticles partially decorated with nickel oxide layers (NiOx/Ni) supported on alumina prepared based on the topological transformation of LDHs can be used to selectively synthesize hydrocarbons with chain lengths of up to seven carbon atoms at room temperature via carbon monoxide (CO) hydrogenation under mild reaction conditions in the presence of sunlight, mainly attributed to the controlled reaction path of intermediate species induced by the interfacial synergistic effect.³ A series of CoFe-based catalysts were successfully fabricated via direct H₂ reduction of a CoFeAl-LDH nanosheet precursor. By varying the reduction temperature, three unique catalysts were obtained, each of which showed distinct activity and product selectivity for CO₂ hydrogenation under simulated solar excitation. LDH precursor reduction at temperatures above 600 ^oC resulted in CoFe-alloy nanoparticle formation, thereby affording a remarkable CO₂ hydrogenation selectivity towards high value (C₂₊) hydrocarbons through photothermal effects.⁴ The above results provide a full understanding of surface and interfacial at the nanoscale for achieving improved photocatalytic performance.

References
1. Y. Zhao, T. R. Zhang, et al., Adv. Energy Mater. 2016, 6 (6), 1501974.
2. Y. Zhao, T. R. Zhang, et al., Adv. Mater. 2015, 27, 7824.
3. Y. Zhao, T. R. Zhang, et al, Angew. Chem. Int. Ed. 2016, 55, 4215.
4. Y. Zhao, T. R. Zhang, et al, Adv. Mater. 2017, 55, 1703828.

TiO2 has been widely studied as a photocatalyst for various solar environmental and energy application. Its photocatalytic performance is largely related to its optical and structural properties. For example, its UV-only optical absorption limits its main photocatalytic applications in the UV region and its low overall photocatalytic efficiency. Therefore, much effort has been devoted to improve its optical absorption properties. In this work, we present various strategies in the direction of manipulating its optical properties towards enhanced photocatalytic performances. Representative work from our group will be given to discuss the advantage and disadvantages of those strategies. Discussion of future effort will be proposed as well.

Reduced or black TiO_2-x materials were achieved by creating oxygen vacancies and/or defects at the surface using different methods. Fascinatingly, they exhibited an extended absorption in VIS and IR instead of only UV light with a band gap decreases from 3.2 (anatase) to ~1 eV. However, despite the dramatic enhancement of optical absorption of black TiO_2-x material, it fails to show expected visible light-assisted water splitting efficiency [1,2]. Therefore, reduced or black TiO₂ materials with optimized properties would be highly desired for efficient visible light photocatalysis.
In this presentation, we report H-doped reduced reduced of TiO_2-x nanoparticles prepared by a controlled reduction via the simultaneous presence of two active reducing species, [Mg] and [H] in confined microenvironment at TiO₂ surface for excellent photocatalytic H₂ production from methanol-water system [3.4]. It exhibits outstanding activity (16 mmolg^-1h^-1) and excellent stability after Pt deposition for photochemical H₂ generation from methanol-water in simulated sunlight. The excellent photoactivity of H:TiO_2-x is attributed to the oxygen vacancies and H doping at TiO₂ surface generated by [Mg] and [H]. The photocatalyst is working in the wavelength < 700 nm and exhibits reasonable visible-light activity with a quantum yield of 8.07, 3.92 and 2.02 % at 400, 420 and 454 nm, respectively along with the exceptionally high turnover number (122460) with respect to Pt. This outstanding activity can be correlated with the extended absorption in visible light, perfect band position, presence of an appropriate amount of Ti³⁺ and oxygen vacancy, and slower charge recombination.

References
X. Chen, L. Liu, P. Y. Yu, and S. S. Mao, Science, 2011, 331, 746-750
X. Chen, L. Liu, F. Huang, Chem. Soc. Rev., 2015. 44. 1861-19885.
A. Sinhamahapatra, J. P. Jeon, and J.-S. Yu, Energy Environ. Sci., 2015, 8, 3539-3444.
A. Sinhamahapatra, J. P. Jeon, J. Kang, B. Han and J.-S. Yu, Sci. Repts., 2016, 6, 27218.

Recently a great deal of attention has been focused on “black TiO₂”, a modified TiO₂ material that can absorb visible light much more efficiently than pristine TiO₂. Black TiO₂ consists of nanoparticles (NPs) with a crystalline TiO₂ core covered by a highly reduced outer shell, whose chemical composition and atomic structure are not known in detail. To obtain insight into the stability and properties of these NPs, we have carried out first principles calculations on model structures consisting of reduced overlayers on the majority (101) surface of anatase, the TiO₂ phase typically found in nanomaterials. The overlayers are formed by aggregation of extended defects known as crystallographic shear planes (CSPs), which are relatively frequent in TiO₂ and other reducible oxides.
Our results show that formation of a reduced overlayer (“shell”) on the anatase surface is thermodynamically favorable under a wide range of experimental conditions. This shell has Ti₂O₃ stoichiometry and its structure is not the well-known corundum-like Ti₂O₃ phase (the mineral ”tistarite”), but a novel phase that has not yet been reported. DFT calculations with various exchange-correlation functionals predict that this new phase − denoted csp-Ti₂O₃, to distinguish it from standard corundum Ti₂O₃ − is very close in stability to corundum Ti₂O₃ and has a band gap of 1.2-1.8 eV, which is consistent with the absorption of black TiO₂. These findings suggest a possible important role of csp-Ti₂O₃ in the properties black TiO₂ nanomaterials. In a broader context, analogous structures could be relevant for describing the reduced surface region of nanomaterials of other metal oxides as well.

Converting solar energy into storable chemical energy with photocatalysts represents a promising way of utilizing solar energy. Photocatalysis can realize the splitting of water to release hydrogen and reduction of CO₂ to produce solar fuels or chemicals such as methane/methanol/CO. Construction of solar-driven photocatalysts is one central task in this area. The photocatalysis efficiency is synergistically determined by three basic steps, namely light absorption, charge separation and surface catalysis. Understanding and controlling each step as much as possible is highly necessary in order to rationally design and constructing efficient solar-driven photocatalysts. To address the challenges each step facing, we focus on band engineering & exploring new photocatalysts to increase visible light absorption, microstructure controlling to promote charge separation, and selective exposure of different facets to mediate surface catalysis. In this talk, the key progress in each part will be introduced. Specifically, regarding increasing the visible light absorption, the significance of controlling spatial distribution of electronic structure modifiers in realizing band-to-band redshift of the light absorption edge will be illustrated. Regarding the promotion of charge separation, the advantages of several core-shell engineered photocatalysts with favorable features in endowing spatial separation of photogenerated charge carriers will be highlighted. Regarding the surface catalysis, the exposed facets dependent unique properties of metal oxide photocatalysts will be introduced.

Semiconductor photocatalysis is a promising approach to combat both environmental pollution and the global energy shortage. Advanced TiO2-based photocatalysts with novel photoelectronic properties are benchmark materials that have been pursued for their high solar energy conversion efficiency. Among the different morphological TiO2 nanostructures, TiO2 nanobelts (NBs) attract more attention due to their unique physical properties and ideal 1D ribbon-like morphology that is favorable for constructing heterostructures by assembling second-phase nanoparticles on the surface of the NBs. A large number of studies have proven that well-designed TiO2 NB heterostructures can not only broaden the photocatalytically active light band of TiO2 but also enhance the light absorption performance and the photo-induced carrier separation ability. The TiO2 NB heterostructure has become a versatile and powerful tool for building high-performance TiO2-based photocatalysts, which has stimulated intense research activities focused on the growth, properties, and applications of the 1D TiO2 NB and its heterostructures.
In this talk, we will summary all the above aspects, including the underlying principles and key functional features of TiO2 NBs and TiO2 NB heterostructures in a comprehensive way and also discuss the prospects of this type of novel hybrid photocatalyst. Several new NIR and full solar spectrum light photocatalysts will be introduced. In addition, the devices for full solar light photocatalysis base on TiO2 nanobelt heterostructures will be reviewed.

References
1. X Zhang, Y Wang, B Liu, Y Sang, H Liu, Heterostructures construction on TiO2 nanobelts: a powerful tool for building high-performance photocatalysts, Applied Catalysis B-environmental, 2017, 202, 620-641
2. J Tian, Y Sang , G Yu , H Jiang , X Mu , H Liu，A Bi2WO6-Based Hybrid Photocatalyst with Broad Spectrum Photocatalytic Properties under UV, Visible, and Near-Infrared Irradiation， Advance Materials, 2013, 25, 5075–5080
3. Y Sang, Z Zhao, M Zhao, P Hao, Y Leng, H Liu, From UV to Near-Infrared, WS2 Nanosheet: A Novel Photocatalyst for Full Solar Light Spectrum Photodegradation. Advance Materials, 2015, 27, 363–369.
4. J Tian, Z Zhao, A Kumar, R. I. Boughton and H Liu, Recent progress in design, synthesis, and applications of one-dimensional TiO2nanostructured surface heterostructures: a review, Chem. Soc. Rev., 2014,43, 6920-6937
5. Z Zhao, J Tian, Y Sang, A Cabot, H Liu. Structure, Synthesis, and Applications of TiO2 Nanobelts, Advanced Materials, 2015, 27 (16), 2557-2582
6. H Li, Y Sang, S Chang, X Huang, Y Zhang, R Yang, H Jiang, H Liu, Z Wang. Enhanced Ferroelectric-Nanocrystal-Based Hybrid Photocatalysis by Ultrasonic-Wave-Generated Piezophototronic Effect, Nano Letters, 2015, 15 (4), 2372–2379
7. X Wang, F Wang, Y Sang, H Liu. Full-Spectrum Solar-Light-Activated Photocatalysts for Light–Chemical Energy Conversion, Advanced Energy Materials, 2017, DOI: doi.org/10.1002/aenm.201700473

Solar energy conversion using the photocatalytic water splitting is considered to be a powerful strategy to solve the problems of energy crisis and environmental pollution.^1-4 However, the establishment of design guidelines to obtain the optimized structures for maximizing the photocatalytic performance remains a great challenge for artificial photosynthesis systems. Colloidal semiconductor nanocrystals provide us with a simple route for exploring the physical and chemical properties of nanomaterials for application in optoelectronics, energy conversion, and catalysis.⁵ In particular, the core–shell nanorod structures with a type-II band alignment exhibit spatial charge separation which has been widely investigated for improving the performance in photocatalytic activities. Furthermore, the efficient extraction of holes in type-II band alignment is favorable for the stable photocatalysts. However, in contrast to the intense researches about band offset tuning for photocatalysts, the relationship between nanostructures and photo-induced carrier dynamics has been still poorly explored.

In this work, we synthesized CdS core–mesoporous ZnSe shell “durian-shaped” nanocrystals (d-CdS/ZnSe NCs) with a type-II band alignment. The photo-generated carrier dynamics were investigated by fs- or ns-transient absorption spectroscopy and time resolved microwave conductivity (TRMC) technique. The d-CdS/ZnSe NCs exhibited cocatalyst-free high photocatalytic activity for H₂ evolution (14.8% of apparent quantum yield at 420 nm) and excellent stability (maintaining 80% activity after 72 h) under visible light illumination. The unique hierarchical mesoporous shell enables the high photo-carriers mobility, efficient spatial charge separation, and long-lived charge separation state in the d-CdS/ZnSe NCs. We demonstrated that the hierarchical structure is favorable for high photocatalytic activity and sustainability of nanomaterials for H₂ evolution.

References
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(5) Chica, B.; Wu, C.-H.; Liu, Y.; Adams, M. W. W.; Lian, T.; Dyer, R. B., Energy Environ. Sci. 2017, 10, 2245-2255.

A biomimetic blueprint emulating the growth pattern of a natural plant –peapod– was put forward for the first time for plasmonic photocatalyst design.^1-3 It was exemplified via a multi-core-shell (mCS-) nanocomposite consisting of a tubular protonated metaniobate (H_xK_1-xNbO₃) semiconductor sheath with spherical core-shell Au@Nb nanoparticles as the beans residing in the cavity. The biomimicry in such design endows Au@Nb@H_xK_1-xNbO₃ nanopeapods (mCS-NPPs) with exceptional light harvesting ability well befitting the solar spectrum. In particular, H_xK_1-xNbO₃ is responsible for the ultraviolet (UV) light absorption while the transverse-mode and longitudinal-analogous surface plasmon resonances (SPRs) stem from the strong near-field plasmon coupling of the Au@Nb nanopeas accounting for visible (VIS) to near-infrared (NIR) light harvesting. More importantly, the 3D Schottky junction arising from the peapodded configuration allows these SPR-triggered charge carries to efficiently inject into H_xK_1-xNbO₃ to participate in interesting chemical transformations. As a proof-of-concept, organic dye degradation for environment remediation and water splitting for hydrogen generation validate the significance.

[1] Adireddy, S.; Carbo, C. E.; Rostamzadeh, T.; Vargas, J. M.; Spinu L.; Wiley, J. B. Angew. Chem. Int. Ed. 2014, 53, 4614.
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[3] Chen, Y. C.; Popescu, R.; Gerthsen, D.; Hsu, Y. K.; Lin, Y. G.; Feldmann, C. 2017, Nature Commun., in revision.

It is a promising way to resolve the worldwide energy crisis and environmental pollution by converting solar energy into storable chemical energy through solar water splitting or CO₂ reduction. However, the conversion efficiency is still relatively low since complicated processes involved in photocatalysis, including charge generation, transportation and surface reaction. Given the fact that all these three processes could become the rate limiting step during solar-to-chemical energy conversion, different strategies have been taken to manipulate photogenerated excitons to enhance the photocatalytic efficiencies. Firstly, self-doping has been adopted to narrow the bandgap of TiO₂ to generate more charge carriers upon visible light illumination, while avoiding the introduction of excessive bulk defects that serve as charge recombination centers. Secondly, nanotube and 3-D junction structures have been realized for Fe₂O₃ photoanodes, which significantly improves the charge transportation of this semiconductor with a very short hole diffusion length. Finally, the particle size and distribution of Co₃O₄ surface co-catalysts have been carefully controlled to construct an effective p-n junction that facilitates the charge separation at the semiconductor/co-catalyst interface, obtaining a synergetic enhancement of surface reaction kinetics and bulk charge separation. With all the effort to better manipulate photogenerated excitons in semiconductors, the era of highly effective photocatalytic conversion process for practical applications will be realized.

Noble metal nanoparticle (NP) decorated ZnO nanowires (NWs) possess unique properties for photocatalytic transformations. The noble metal-ZnO nanostructures can be considered as a type of Schottky photochemical diode. When noble metal NPs are loaded onto ZnO NWs to form metal/ZnO nanocomposites the photocatalytic efficiency and photoelectric energy conversion can be significantly enhanced. The photocatalytic properties of noble metal-ZnO nanocomposites depend on the quality of their interfacial structures. Epitaxial growth of noble metal NPs onto ZnO NWs not only provides high-quality interfaces but may also stabilize the metal NPs during a liquid phase photocatalytic reaction. We studied the nucleation and growth processes of noble metal NPs onto ZnO NWs and investigated the stability of the metal NPs under aqueous environment and light illumination. We have explored a range of metal/ZnO NW systems including endotaxially grown Au NPs, PtZn and PdZn alloy NPs, Ag NPs and Ir single-atom chains. The photon-induced sintering behavior of these ZnO NW supported metal NPs in aqueous solutions will be discussed.

Today's human-race-scale desire for sustainable and renewable sources of energy lets us consider it rational and natural to pursue the sunlight, which have served as the invariant resource since living organisms started on the earth. A number of basic studies have been conducted for recovery, storage, and transport of solar energy towards development for mass-scale industry. Photocatalytic splitting of water into H₂ and O₂ is one of the promising technologies for solar energy harvesting. Both of the gases are valuable materials in chemical industry, and moreover, utilization of H₂ is strategically involved in CO₂ fixation to realize artificial photosynthesis on the industrial basis.
Design of future mass production plants for solar water splitting will involve solar catalytic panels, delivery system of reactant water, transportation system of product H₂ and O₂, separation and purification system of these gases. There may be two main categories for water photo-splitting devices. One is the photoelectrochemical design, in which photocathode and photoanode separately evolve H₂ and O₂ by the aid of gas separation/ion transmitting membrane. This scheme is rational for the product delivery to the following processes, and a high solar-to-H₂ energetic conversion efficiency is anticipated by optimal choice of the two electrodes. The other is application of a single powder photocatalyst, which evolve 2H₂+O₂ mixture. In the following process, first 2H₂+O₂ mixture must be separated into H₂ gas and O₂ gas by an efficient separation membrane, which might be additional sophistication. Nevertheless, the single power photocatalysis system is anticipated to be low cost, and therefore suitable for mass-scale plants.
This time we will discuss the latter system. So far the most developed and successful powder photocatalyst is SrTiO₃. The original study of stoichiometric 2H₂+O₂ evolution by UV light was simply achieved by NiO catalyzed SrTiO₃ [1]. Later, doped with various additives, SrTiO₃ exhibited p-type or n-type characteristics as a semiconductor, extending the range of light absorption wavelength. Recently, Al-doped SrTiO₃ was introduced as an UV-active stoichiometric water splitting photocatalyst that realizes a quantum efficiency of 69% at 320 nm by the aid of CrO_x+Rh cocatalyst [2]. Due to the small fraction of UV light within the sunlight on the earth, the solar-to-H₂ energetic conversion efficiency is approximately 0.6 ~ 0.7 %. Nonetheless this simplicity of water photo-splitting mechanism is a preferable feature in designing facile and low-cost solar H₂ panels.
In this talk, we will further discuss some novel visible-light active photocatalysts. Also, the following processes for safe transportation of explosive 2H₂+O₂ mixture and operation of H₂/O₂ separation membrane will be generalized to enumerate tasks to be tackled for this development.

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[2] Ham et al., Chem. Commn, 2016, 52, 5011.

Efficient conversion of solar energy into chemical energy (i.e. solar fuel production) requires a semiconducting photocatalyst that converts photons into excitons and provides charge separation, and an electrocatalyst that carries out proton-coupled electron transfer reactions. One of the main challenges is to better understand the relationship between the structure and the reactivity of electrocatalytic nanostructures. In this talk, we will discuss three examples of how nanostructuring an electrocatalyst can affect the charge transfer kinetics for each of the three main solar fuel reactions—the hydrogen evolution reaction, the oxygen evolution reaction, and the CO₂-reduction reaction.

First, we will show that 2D FeS₂ disc nanostructures are an efficient and stable hydrogen evolution electrocatalysts. By changing the Fe:S ratio in the precursor solution, we were able to preferentially synthesize either 1D wire or 2D disc nanostructures. The 2D FeS₂ disc structure has the highest electrocatalytic activity for the hydrogen evolution reaction, comparable to platinum in neutral pH conditions.

Second, we will show that a novel microwave-assisted synthesis route for creating nickel-iron oxyhydroxides produces a nanoamorphous structure that has increased activity for the oxygen evolution reaction compared to crystalline counterparts. We observed that the overpotential for oxygen evolution on Ni_0.8:Fe_0.2 oxyhydroxide decreased from 366 mV on the crystal-derived structure to 286 mV on the nanoamorphous structure at 10 mA cm^-2. We measured the kinetic rate constant of the active sites directly with the surface interrogation mode of scanning electrochemical microscopy (SI-SECM). We show that the microwave-assisted nanoamorphous structure has only one type of catalytic site with an OER kinetic rate constant of 1.9 s⁻¹ per site. We compared this to the crystalline sample and verified that it contained two types of catalytic sites–“fast” sites with an OER rate constant of 1.3 s⁻¹ per site and “slow” sites with an OER rate constant of 0.05 s⁻¹ per site. The percentage of “fast” sites in the crystalline sample matched to the total iron atom content, while 100% of the sites were “fast” in the microwave-assisted, nanoamorphous (Ni_0.8:Fe_0.2) oxide.

Third, we will show that electrochemically reducing In₂O₃ nanocatalysts will remove the metastable surface oxide layer, and create an In⁰–In₂O₃ composite. This In⁰–In₂O₃ composite material changes the selectivity and is able to electrochemically reduce CO₂ to CO with near 100% selectivity at relatively low overpotentials (c.a. −1.0 V vs. Ag/AgCl). We attribute the change in selectivity to the direct exposure of In⁰ to CO₂ in solution that typically does not exist due to the native oxide layer that forms on In metal. In addition, we observed that the first electron-transfer step to form the surface adsorbed intermediates is highly reversible on the In⁰–In₂O₃ composite; however it is irreversible on an In foil electrode.

Solar-driven photocatalytic water splitting has been regarded as a promising strategy for harnessing solar energy to supply clean and renewable hydrogen energy. It is well known that the photoinduced charge separation is a crucial driving force for all the photocatalytic reactions, while little is known about the interatomic electron transitions on photocatalysts that involved in water splitting process, probably due to the greater difficulty to experimentally access excitation state in comparison to ground ones. Thereby, the visualizing study concerning the interatomic electron transfer on photocatalysts for water splitting under photo-excitation should be not only more relevant for understanding photochemical mechanisms but also important for developing highly efficient photocatalysts. Our research mainly focus on the exploration of a novel synchronous illumination X-ray photoelectron spectroscopy for direct observation of the photocharge separation and transfer process of photocatalysts in water splitting process, and study the influence of sacrificial reagent and co-catalyst on the photocharge separation of photocatalysts. By providing interatomic electron excitation and transfer for the photocatalysts, these studies provide the possible way to the investigation of the elementary steps involved in water splitting operated under photo-excitation, which may promote the understanding and developing of practical photocatalytic techniques^[1-8].

References
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2. H. Hu, Z. Jiao, J.Ye, G. Lu, Y. Bi, Nano Energy, 2014, 8, 103.
3. Y. Bi, S. Ouyang, N. Umezawa, J. Cao, J. Ye, J. Am. Chem. Soc. 2011, 133, 6490.
4. X. Liu, G. Dong, S. Li, G. Lu, Y. Bi, J. Am. Chem. Soc. 2016, 138, 2917.
5. Y. Li, S. Ouyang, H. Xu, X. Wang, Y. Bi, Y. Zhang, J. Ye, J. Am. Chem. Soc., 2016, 138, 13289.
6. L. Wang, H. Hu, N. Nguyen, Y. Zhang, P. Schmuki, Y. Bi, Nano Energy, 2017, 35, 171.
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8. L. Wang, N. Nguyen, X. Huang, P. Schmuki, Y. Bi, Adv. Funct. Mater, 2017, In Press.

A core-shell structured CdS@C₃N₄ photocatalyst with a 4 nm thick shell was prepared using self-assembly, and its structure, composition and morphology were characterized using X-ray powder diffraction, scanning electron microscopy (SEM), transmission electron microscope (TEM) and high resolution TEM. The SEM image clearly shows that pristine CdS has well-crystallized nanowire morphology with lengths of 2–4 μm. The internal structure can be clearly observed using TEM due to the ability of electrons to penetrate between the shell and the core. It can be seen from the results that after formation of the CdS@C₃N₄ core-shell photocatalyst, the nanostructure of the CdS nanowires is well retained and they have a diameter of 30–50 nm. HRTEM observation shows that the crystalline core of CdS displays distinct lattice planes of (002) and (100) with d-spacings of 0.33 and 0.36 nm respectively. The thickness of the film-like C₃N₄ layer is ~4 nm with a lattice spacing of 0.33 nm corresponding to the (002) facet, indicating that no change occurred in the crystal structure of C₃N₄ after coating on the CdS. During the self-assembly process, C₃N₄ nanosheets adsorb on the surface of CdS NWs to minimize the total interfacial energy, and then the adsorbed C₃N₄ assembles into a smooth and compacted shell. Electron energy loss spectroscopy was also used to determine the elemental composition and distribution of the core-shell material, and the obtained results revealed the uniform distribution of C, N, Cd and S over the whole nanowire, supporting the successful fabrication of the core-shell photocatalyst.
The hybrid visible-light catalyst exhibited a high photocatalytic performance for oxidative coupling of amines under atmospheric conditions, and robust product selectivity up to > 99% for photodriven oxidation of various amines to corresponding imines was achieved. Superoxide radicals detected using electron spin resonance and a quenching experiment showed that superoxide radicals played a crucial part in amine oxygenation, and an investigation of the steric and electronic effects on conversion efficiency revealed that the former is more important in the studied system.

Copper is an attractive alternative for plasmonic photocatalysis due to its high abundancy as well as good spectral match of its Localized Surface Plasmon Resonance (LSPR) with solar radiation. In ambient conditions, Cu nanoparticles quickly oxidize to Cu/Cu₂O core/shell structures. In this hierarchical structure, the native oxide, Cu₂O, being a semiconductor of 2.2 eV band gap, has multiple roles. It allows for a longer lifetime for plasmon-excited hot electrons in its conduction band, resulting in more efficient electron transfer. While it is plasmons in the Cu core which strongly couple with solar photons and decay to hot electrons needed for catalytic activity, excited electron/hole generation may also occur in Cu₂O under the effect of LSPR-enhanced nearfield. Other potential mechanisms in this core/shell structure are: i) Resonance Energy Transfer (RET) between Cu and Cu₂O through plasmon-exciton dipolar coupling; and ii) Chemical Interface Damping (CID) at the Cu/Cu₂O interface. To elucidate these open questions, here, we perform single nanoparticle scattering spectroscopy and electromagnetic simulations (FDTD). The Cu/Cu₂O nanoparticles are synthesized by a modified polyol/microemulsion technique in the size range of 50-200 nm. The particles are immobilized on patterned glass slides, so scattering of a specific particle can be acquired in between sequential thermal oxidation steps (220 ^oC, 30 min) as the oxide thickness is increased systematically. The LSPR peaks can be conveniently resolved by single particle scattering spectroscopy in the absence of heterogenous broadening. Otherwise, the peaks are impossible to resolve from optical extinction spectra of the colloids. In larger nanoparticles, we observe the quadrupolar and hexapolar LSPR modes in addition to dipolar. By comparison of the peak widths for experiment and simulation, we infer the dipolar mode is by far more damped than the higher order modes. We explain the selective damping of the dipolar plasmon by its energy match with the Cu/Cu₂O interface states, resulting in CID. While our simulations show increase of Mie scattering with thickening of the Cu₂O shell, the single particle measurements reveal a systematic attenuation (i.e., ~4 times) during 90 min of thermal oxidation. Based on our simulations, we rule out reduction of the Cu core diameter as a cause for this attenuation of scattering. Therefore, an energy loss mechanism is inferred with increasing Cu₂O thickness. This loss mechanism is not the surface-enhanced optical absorption in Cu₂O, which is already included in the FDTD solver through imaginary component of the Cu₂O dielectric function. We infer the attenuation of scattering is due to RET. Investigation is under way to validate RET in Cu/Cu₂O nanostructures. The presence of RET should benefit plasmonic photocatalysis by providing an additional channel of energy transfer to catalytic electrons.

The oxygen evolution reaction (OER) is a major cause of energy losses in photocatalytic systems for solar fuels, but also in a number of other emergent technologies such as rechargeable metal-air batteries, water splitting devices, and unitized regenerative fuel cells. The low energy efficiencies of OER are the result of its sluggish reaction kinetics and large overpotentials. Significant improvements to the OER activity of transition metal oxides (TMOs) have been made by tailoring the morphology and crystal structure of the catalysts, incorporating dopants, as well as using conductive supports. However, clear structure-activity correlations remain elusive because of the complex composition and structure of TMO catalysts. Further understanding of these relationships for these promising catalysts for oxygen evolution may lead to additional developments that will enable TMOs to replace precious metal-based OER catalysts (e.g. IrO_x and RuO_x). Insights from two recent examples of our studies will be discussed. We have utilized a range of spectroscopic techniques for characterization of Ce-modified copper oxide (CuO_x) and Ni-modified cobalt (oxy)hydroxides to reveal the OER active species and structures of these catalysts. In the case of Ce-modified CuO_x, Ce incorporation (6.9 at%) into CuO_x led to 3.3 times greater OER activity compared to pure CuO_x and this was coincident with significant structural changes due to an increasing amount of disorder. By combining X-ray photoelectron and Raman spectroscopy techniques, a strong correlation between OER performance with tetravalent Ce (Ce⁴⁺) ions was observed up to a concentration corresponding to CeO₂ phase segregation. We propose a strong promoting effect of Ce⁴⁺ for OER in this system. In the case of Ni-modified CoO_xH_y, operando Raman spectroscopy was used to reveal a drastic transformation of a spinel Co₃O₄ like structure into a more active (oxy)hydroxide structure under applied potential. Such a transformation was only observed in the presence of uniformly distributed Ni ions. These two examples, i.e. the promoting effect of Ce⁴⁺ and the formation of active OER structures in Ni-modified CoO_xH_y, reveal the importance of chemical state and local structure considerations for the rational design of improved oxide-based OER catalysts.

In recent years, the increase of carbon dioxide (CO₂) in the atmosphere has become a global environmental issue because of the serious problems, such as the “greenhouse effect”. The idea of mimicking the overall natural photosynthetic cycle of chemical conversion of CO₂ into useful fuels has been consistently gaining attention for more than thirty years. Such artificial photosynthesis allows direct conversion of CO₂ and water on photocatalysts into valuable hydrocarbon using sunlight at room temperature and ambient pressure to serve to reduce atmospheric CO₂ concentrations while providing on a renewable carbon fixation and energy storage.

In this presentation, we will report the utilization of solar energy to highly efficient conversion of CO₂ into renewable hydrocarbon fuel over structured nanomaterials. The geometric shape and exposure of specific crystal planes of the nanostructures as well as combination of graphene as a good electron collector and transporter are a requisite for the high level of photocatalytic reduction of CO₂.

References
(1) Ping Li, Xingyu Chen, Hechao He, Xin Zhou, Yong Zhou, and Zhigang Zou, Adv. Mater. 2018, in press.
(2) Haijin Li, Yuying Gao, Yong Zhou, Fengtao Fan, Qiutong Han, Qinfeng Xu,Xiaoyong Wang, Min Xiao, Can Li, and Zhigang Zou, Nano Lett. 2016, 16, 5547.
(3) P. Li, Y. Zhou, Z. Zhao, Q. Xu, X. Wang, M. Xiao, and Z. Zou, J. Am. Chem. Soc. 2015, 137, 9547.
(4) W. Tu, Y. Zhou, and Z. Zou, Adv. Mater. 2014, 26, 4607
(5) H. Li, Y. Zhou, W. Tu, J. Ye, and Z. Zou, Adv. Funct. Mater. 2015, 25, 998.
(6) W. G. Tu, Y. Zhou, Z. G. Zou, Adv. Funct. Mater. 2013, 23, 4996
(7) W. Tu, Y. Zhou, Q. Liu, S. Bao, X. Wang, M. Xiao, and Z. Zou, Adv. Funct. Mater. 2013, 23, 1743
(7) Q. Liu, Y. Zhou, J. H. Kou, X. Y. Chen, Z. P. Tian, J. Gao, S. C. Yan, Z. G. Zou, J. Am. Chem. Soc. 2010, 132, 14385.

The ultimate goal of reduction of CO₂ is to convert CO₂ into more valued carbon compounds by green energy sources like solar light. To utilize CO₂ by solar light, photoelectrochemical system coupled with photocatalyst is typically used. Cuprous oxide (Cu₂O) is a promising material for a photocathode because of its small band gap (~ 2.2 eV) that enables absorbing visible light and its earth abundancy. However, achieving high efficiency in nanostructured Cu₂O was difficult because of phase mixing with other copper oxide; CuO. it is critical in CO₂ reduction in aqueous electrolyte because CuO makes CO₂ reduction inferior. Until now, nanostructure with mono phase Cu₂O has not been precisely controlled. We fabricated 100 % mono-phase Cu₂O with nanofiber structure by finding kinetic processing window in Cu₂O-unstable condition which precisely controls the chemical potential of oxygen and reaction kinetics during annealing process based on thermodynamic calculations.
Cu₂O nanofiber was fabricated on FTO glass substrate by electrospinning process using the precursor solution of mixture of 10 wt % of copper acetate (Cu(CH₃COO)₂), 10 wt% of PVA (Poly(vinyl alcohol)), and Distilled water. The heat treatment for the fabrication of Cu2O phase in the Cu-C-O ternary system which is a constituent of as-spun nanofibers required 2-step process. Firstly nanofibers were annealed at 500 °C for 3 hour at atmospheric atmosphere, and then annealed again with controlled oxygen partial pressure (pO₂) from 250 μTorr to 1 μTorr to tune the phase of nanofiber to 100 % mono phase Cu₂O. TiO₂ overlayer with 5 nm thickness was deposited by ALD process on Cu₂O nanofiber to prohibit the photocorrosion of Cu₂O electrode and then annealed again at 450 ^oC for 1 hour to annihilate defects from the as-deposited amorphous TiO2 layer.
In the first annealing step to combust all the carbons, annealing at the pO₂ of atmospheric pressure successfully combusted all the carbons forming nanofiber structure having the CuO phase. Then the reduction of CuO to Cu₂O is performed and pO₂ range during annealing process is derived by calculating the Gibbs free energy of oxidation in Cu-O system. We discovered the kinetic processing window of Cu₂O fabrication which is pO₂ of 10 μTorr with the annealing duration of 3 hours that successfully reduced all the CuO into Cu₂O. Though Cu₂O is thermodynamically unstable in this condition, it is kinetically stable around the reaction time of 3 hours. Treating the CuO in a Cu₂O-stable condition could not achieve 100 % mono phase Cu₂O because the driving force of reduction is not sufficiently large to turn all the CuO into Cu₂O in a realizable time scale. As a result, mono-phase Cu₂O nanofiber electrode showed 0.92 mA/cm² of photocurrent density at 0.4 V vs. RHE in aqueous solution with faradic efficiency for CO₂ reduction of 92 %, and the major product was methanol. pO₂ is controlled to Cu₂O-stable region and Cu-stable region for the reduction.

The spontaneous polarization is the intrinsic property of a ferroelectric material. The induced electric field can work as charge transfer booster and mass transport booster. Take the typical ferroelectric material of BaTiO₃ as an example, a well crystalline BaTiO₃ nanocube results in opposite induced piezoelectric charges on the +z and –z surfaces of the nanocube. In addition, by applying an alternative mechanical stress via ultrasonic waves to the BaTiO₃ nanocube, spontaneous polarization potential of the nanocube should have an alternate increase and decrease, which causes the charging and discharging in the surfaces of BaTiO₃ nanocubes. Recently, this piezotronics effect driven by ultrasonic excitation has been used in BaTiO₃-Ag₂O hybrid nanocube to realize the continuous photo-induced carrier separation and getting high sono-photocatalysis performance [1]. Piezoelectrochemical catalysis has been proposed and realized by the stress-induced piezoelectric potential of nanomaterials [2]. Combining the piezoelectric potential of BaTiO₃ nanocubes and the surface charge driven electrochemical polymerization mechanism, we suggest to use the piezoelectric potential of BaTiO₃ to build microscale pseudo-electrochemical cells in solution, and realize a micro scale electrochemical polymerization with PANI as a model polymer. The electrochemical potential was derived from the micro pseudo-electrochemical cells of end-to-end aligned BaTiO₃ nanocubes with a spontaneous polarization [3]. The piezotronics property based on ferroelectric nano-materials open a new window for the application of the micro-electric field.

References:
[1] Haidong Li, Yuanhua Sang, Sujie Chang, Xin Huang, Yan Zhang, Rusen Yang, Huaidong Jiang, Hong Liu, Zhong Lin Wang, Enhanced ferroelectric-nanocrystal-based hybrid photocatalysis by ultrasonicwave-generated piezophototronic effect, Nano Lett. 15 (2015) 2372–2379.
[2] K.-S. Hong, H. Xu, H. Konishi, X. Li, Piezoelectrochemical effect: a new mechanism
for azo dye decolorization in aqueous solution through vibrating piezoelectric microfibers, J. Phys. Chem. C 116 (2012) 13045–13051.
[3] Lili Zhao, Yan Zhang, Fulei Wang, Shicong Hu, Xiaoning Wang, Baojin Ma, Hong Liu, Zhong Lin Wang, Yuanhua Sang, BaTiO₃ nanocrystal-mediated micro pseudo-electrochemical cells with ultrasound-driven piezotronic enhancement for polymerization, Nano Energy 39 (2017) 461–469.

Molecular self-assembly is a powerful method to synthesize nanostructured materials. The unique property of molecular assemblies has been shown to depend not only on the size, shape, and composition of the molecular building blocks but also to a large extent on ordered spatial arrangement within an assembly. The synthesis of hierarchical structures leveraging the structural advantages of individual molecules still remains a significant challenge. Here, we developed interfacially driven microemulsion (μ-emulsion) method and micelle-confined method to initiate self-assembly and formation of hierarchically structured porphyrin nanocrystals. An optically active macrocyclic building block meso-tetraphenyl porphine dichloride (TPP) with different metal core are used to initiate non-covalent self-assembly confined within µ-emulsion droplets. In-situ studies of dynamic light scattering, UV-vis spectroscopy, and TEM, as well as optical imaging suggest an evaporation-induced nucleation and growth self-assembly mechanism. The resulted nanocrystals exhibit uniform shapes and sizes form ten to hundred nanometers. Due to the spatial ordering of (M)TPP, the hierarchical nanocrystals exhibit collective optical properties resulted from molecular (M)TPP and photocatalytic reduction of platinum nanoparticles and networks.

Semiconducting 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 metal oxides materials 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%; 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.

High efficiency artificial photosynthesis, that can convert solar energy directly into chemical fuels, is one of the key sustainable technologies to enable a carbon-free, storable and renewable source of energy. To date, however, success in finding abundant visible-light active photocatalyst has been very limited. In this context, we have investigated the photocatalytic and photoelectrochemical properties of InGaN nanowires. Compared to conventional metal-oxide and other semiconductor photocatalysts/photoelectrodes, the energy bandgap of InGaN can be tuned across nearly the entire solar spectrum. Moreover, InGaN is the only known semiconductor whose conduction and valence band edges can straddle water redox potentials under deep visible light irradiation. We have demonstrated that the quantum efficiency of solar-to-hydrogen conversion on InGaN nanowires can be enhanced by two orders of magnitude through precise tuning of the near-surface Fermi level. We have further demonstrated an integrated InGaN/Si nanowire photoelectrode system that can lead to significantly enhanced efficiency and stability in strong acidic solution. Moreover, we have demonstrated the reduction of CO₂ into methanol (CH₃OH) and syngas on InGaN nanowires utilizing sunlight.

In this work, InGaN nanowires are grown on Si substrate by molecular beam epitaxy. The water splitting reaction takes place on the nonpolar surface (m-planes) of InGaN nanowire photocatalysts. With increasing Mg-dopant incorporation, the efficiency for solar-to-hydrogen conversion is enhanced by nearly two orders of magnitude. The internal quantum efficiency reaches ~75% with optimum Mg doping concentration. The significantly enhanced efficiency is directly related to the optimized surface electronic properties that lead to both efficient water oxidation and proton reduction. A solar-to-hydrogen conversion efficiency of 3.4% was demonstrated without any energy input other than sunlight. We have further demonstrated multi-band InGaN/GaN nanowire photoelectrodes monolithically integrated on a Si solar cell wafer. The tandem PEC device consists of a planar n⁺-p Si solar cell wafer and p-InGaN nanowire segments. The p-InGaN nanowire arrays are designed to absorb the ultraviolet and visible solar spectrum. The remaining photons with wavelengths up to 1.1 µm are absorbed by the underlying planar Si p-n junction. Such a monolithically integrated photocathode promises solar-to-hydrogen conversion efficiency >20%. With the use of such a photoelectrode, we have also demonstrated that syngas, a key feedstock to produce methanol and liquid fuels in industry, can be produced from a CO₂ and H₂O with a benchmark turnover number of 1330 and a desirable CO/H₂ ratio of 1:2. Work is currently in progress to achieve high efficiency syngas and methanol generation in an aqueous photoelectrochemical cell.

Photosystem I (PS I), the photosynthetic membrane protein, undergoes light activated (λ=680 nm) charge separation and unidirectional electron transfer with near-unity quantum efficiency. The robust photoelectrochemical (PEC) activities of PSI make it an ideal biomaterial for bio-hybrid photovoltaic and/or, optoelectronic devices.^1,2 But, the first step towards rational design of such devices requires systematic electrochemical characterizations of PSI assembly in tailored biotic-abiotic interfaces.³ In the past, such interfaces have been created using plasmonic metal nanostructures to tune optoelectronic properties of molecular fluorophores. Herein, we investigate plasmon-enhanced photocurrents from PSI assembled with plasmonic Ag and Au nanopatterned structures. Based on our recent works, we present the first-ever experimental verification of plasmon-induced photocurrent enhancements from PSI attached to Fischer patterns of Ag nano-pyramids (Ag-NP) whose resonance peaks are tuned to match the PSI absorption peaks at ~450 and ~680 nm. Detailed atomic force microscopy (AFM) characterizations reveal both the background Fischer patterns and the PSI immobilization on them. The plasmon enhanced photocurrents indicate enhancement factors of ~6.5 and ~5.8 as compared to PSI assembly on planar Ag substrates for nominal excitation wavelengths of 660 and 470 nm respectively.⁴ The comparable enhancement factors from both 470 nm and 660 nm excitations, in spite of a significantly weaker plasmon absorption peak at ~450 nm for the Ag-NP structures, can be explained by previously reported observations of excessive plasmon-induced fluorescence emission losses from PSI in red region of the excitation wavelengths. Based on these results, our on-going efforts are directed towards the systematic investigation of the effect of varying plasmon peak positions tuned with designer nanopatterns on the plasmon-enhanced photocurrents from PSI assembly on these structures. We aim to carry out systematic studies on the role of tailored Ag and Au nanopatterned substrates designed with E-beam lithography for specific peak plasmonic resonance peaks and the distance of separation between PSI to the plasmonic surfaces on plasmon-induced photocurrents from PSI complexes. Aforementioned work will shine light on the fundamental biophysics behind the alterations in excitation energy transfer mechanism among chlorophylls in PSI under plasmon-induced localized electric field.<!--![endif]---->
References:
(1) D.Mukherjee, M. May, B. Khomami; J. Coll. Interf. Sci., 2011, 358, 477..
(2) D. Mukherjee, M. May, M. Vaughn, B. D. Bruce, B. Khomami; Langmuir, 2010, 26, 16048.
(3) T. Bennett, H. Nirooman, R. Pamu, I. Ivanov, D. Mukherjee, B. Khomami; PCCP, 2016, 18, 8512.
(4) R. Pamu, B. Lawrie, R. Kalyanaraman, D. Mukherjee, B. Khomami;Nature Comm., 2016, Submitted.

Energy conversion such as CO₂ reduction to fuel and water splitting needs catalysts with high activity. Nanostructured materials are promising for future application in this area. Although there are a large number of related publications, the catalytic activity and stability for energy conversion is still far from application. So far, Cu and CuO_x materials have been widely applied as catalysts for electrochemical, photochemical and photoelectrochemical CO₂ conversion. Additionally, Cu as a good conductive material works well for electrode substrate. In order to take advantage of Cu and CuxO materials and overcome their problems, we have prepared nanostructured Cu and CuxO based materials for photochemical, electrochemical and photoelectrochemical CO₂ reduction to organic fuel.^[1-4] Besides, Cu nanowires have been used as the substrate to fabricate a highly efficient three-dimensional (3D) bulk catalysts of core-shell structure, in which thin NiFe layered double hydroxide (LDH) nanosheets are grown on the substrate cores supported on Cu foams, toward overall water splitting.^[5-6] The preliminary conclusion can be reached that Cu and CuxO materials are prospective in future practical application in energy conversion and the 3D core-shell electrocatalysts significantly advances the research towards large-scale practical water electrolysis.

References:
[1] Y. Li, W. Zhang, X. Shen, P. Peng, L. Xiong and Y. Yu, Chin. J. Catal. 2015, 36, 2229-2236.
[2] L. Yu, G. Li, X. Zhang, X. Ba, G. Shi, Y. Li, P.K. Wong, J. C. Yu, and Y. Yu, ACS Catal. 2016, 6, 6444-6454.
[3] X. Ba, L. Yan, S. Huang, X. Xia and Y. Yu, J. Phys. Chem. C, 2014, 118, 24467-24478.
[4] G. Shi, L. Yu, X. Ba, X. Zhang, J. Zhou and Y. Yu, Dalton Trans. 2017, 46: 10569-10577.
[5] Luo Yu, Haiqing Zhou, Jingying Sun, Fan Qin, Fang Yu, Jiming Bao, Ying Yu, Shuo Chen and Zhifeng Ren, Energy Environ. Sci. 2017, 10: 1820-1827.
[6] Luo Yu, Haiqing Zhou, Jingying Sun, Fan Qin, Dan Luo, Lixin Xie, Fang Yu, Jiming Bao, Yong Li, Ying Yu, Shuo Chen and Zhifeng Ren, Nano Energy. 2017, 41: 327-336.

The rapid recombination of electrons-holes and low visible-light utilization of TiO₂ restrict the photocatalytic activity for hydrogen evolution.Various nano-carbon materials, serving as the charge transfer passway, are potential materials to overcome these obstacles. Very recently, 2D transition metal carbides (MXenes), are promising precursors for the synthesis of transition oxides/carbon hybrid materials. Firstly, we demonstrated a 2D-layered carbon/TiO₂ (C/TiO₂) architecture via CO₂ oxidation of 2D-Ti₃C₂, in which the 2D carbon layers provide electron transport channels and improve the hole-electron separation efficiency. The 2D layered C/TiO₂ delivers enhanced photocatalytic activity compared with pure TiO₂ catalysts. Furthermore, we designed a novel laminated defect-controlled sulfur-doped TiO₂ on carbon substrate (LDC-S-TiO₂/C) hybrid catalyst to enhance the HER activity, shorten the diffusion path of electrons, and broaden the absorption wavelength. The novel synthesis method involves a sulfur impregnation process of Ti₃C₂ MXenes and the subsequent oxidation, which can simultaneously achieve the doping of TiO₂ and the defect-engineering of carbon. The band-gap of LDC-S-TiO₂/C is reduced to 1.62 eV, which means the synthesized materials can be excited from UV to visible lights. The synergistic effects of porous carbon and S-TiO₂ result in a high photocatalytic H₂ evolution rate of 333 μmol/g/h under visible light irradiation with a high apparent quantum yield (AQY) of 7.36% at 400 nm and it is also active even at 600 nm,, which is 48 times that of C/TiO₂, and ≈1200 times that of TiO₂. These stirring results shed a new insight in the supports and their defects for the photocatalytic activity, which can provide an alternative design strategy for achieving high active hybrid catalysts for clean energy conversion and utilization.

TiO2 nanoparticles are well known for their photocatalytic activities that can be used for air purification and antibiotic purposes. These nanoparticles need to be well-crystalized, rather than amorphous and preferably in the crystal phase Anatase to produce the best photocatalytic efficiency. Oftentimes, these TiO2 particles are applied in a thin layer to a substrate but, without causing damage to substrate, they lack adequate adhesion. Conventional approaches to applying this coating involve using a TiO2 mixture followed by high temperature sintering. This method does not work very well if the substrate is a plastic or metal. In other approaches like sol-gel, CVD, or PVD the TiO2 tends to be amorphous or mix-phased, being both anatase and rutile phased, unless a high temperature is used. In this work, we will introduce a new, low-temperature coating method where an anodized metal substrate and a plastic air filter, which is pretreated with atomic layer deposition (ALD), are coated with an anatase-phased TiO2 and silica sol-gel mixture. This is then followed by a low-temperature curing and another ALD process. Investigated in this work are the photocatalytic air purification and antibiotic efficacy at various conditions where the TiO2 nanoparticles in the coating are still highly crystalline and photocatalytic.

Nanomaterials are a promising class of ideal high performance candidates for photocatalytic application owing to their unique optical, structural and electronic properties. In this respect, silicon carbide (SiC) has proven to be technologically important material. The occurrence of different polytypes of SiC along with their individual characteristic electrical, chemical and thermal properties made it suitable for photocatalytic material for hydrogen generation and environmental remediation. Here we demonstrate the significant effect of surface modification in enhancement of photocatalytic activity of SiC. The confinement effects of the different polytypes of crystalline SiC and amorphous SiC are also being addressed. Graphitization of SiC by high temperature thermal decomposition method has been employed to grow epitaxial graphene (EG) on silicon carbide (EG/SiC hybrid system) to design the surface and interface structure in controlled manner. The systems have been characterized by Raman and UV-vis spectroscopies along with the XRD, SEM and HRTEM analysis. Significant enhancement of the photocatalytic activity (~1000%) and bandgap narrowing (~30%) of EG/SiC systems were observed, relative to the bare SiC, depending on the quality and quantity of the EG and heterojunction interface structures. The effect of different types of SiC (crystalline and amorphous) at their different confinement levels (thin films, nanoparticles) have been studied further to explore the potential application in photocatalysis for renewable energy and environmental remediation (e.g., waste water treatment).

Abilities to control the size and shape of nanocrystals in order to tune functional properties are an important grand challenge. Here we report a surfactant self-assembly induced micelle encapsulation method to fabricate porphyrin nanocrystals using the optically active precursor zinc porphyrin (ZnTPP). Through confined non-covalent interactions of ZnTPP within surfactant micelles, nanocrystals with a series of morphologies including nanodisk, tetragonal rod, and hexagonal rod, as well as amorphous spherical particle are synthesized with controlled size and dimension. A phase diagram that describes morphology control is achieved via kinetically controlled nucleation and growth. 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 photo-degradation of methyl orange (MO) pollutants and hydrogen production. This simple ability to exert rational control over dimension and morphology provides new opportunities for practical applications in photocatalysis, sensing, and nanoelectronics.

According to previous researches, two-step hydrothermal method was used to deposit ZnSnO₃ on the different kinds of substrate and control their alignment with different conditions such as substrate, temperature, surfactant, and others. ^[1,2] In this research, a novel way was proposed to fabricate ZnSnO₃/polymer nanocomposites by simple hydrothermal and polymer epitaxy method. This research emphasized on improving the alignment of ZnSnO₃ nanorods^[3,4] by polymer epitaxy^[5] such as PVDF and its synergistic piezo-related performance of the ZnSnO₃/PVDF nanocomposites.^[6] PVDF was used to control the alignment of the fabricated ZnSnO₃ nanorods and enhance its piezo-related performance including piezopotential, piezotronic, piezophototronic, and piezophotocatalytic analyses.

XRD and SEM were used to characterize the ZnSnO₃/PVDF nanocomposites. The results from the XRD confirmed the presence of ZnSnO_3. SEM analysis showed the morphologies and alignments of the ZnSnO₃ nanorods and PVDF. These nanocomposites exhibited average piezopotentials. Piezotronic analysis was also conducted on ZnSnO₃/PVDF nanocomposites, exhibiting high current density when the ZnSnO₃ are well-aligned. When under UV light illumination, the output current density obtained were several times higher for ZnSnO₃/PVDF. These confirmed the alignment control and synergistic piezophototronic property of the material.

In a piezophotocatalytic experiment, the decomposition of methylene blue (MB) was also investigated. The ZnSnO₃/PVDF nanocomposites exhibited better degradation property than pure ZnSnO_3. All the promising enhancement was attributed to the well-aligned ZnSnO_3, which reduced the recombination of photogenerated electron−hole pairs and enhanced the mobility of these pairs resulting from the energy band distortion caused by applied stresses. Finally, we can use this nanocomposites or this epitaxially fabricating method to other materials on various electronic applications, such as multifunctional electronic-skin.^[7]

Keywords: ZnSnO₃/PVDF nanocomposites, epitaxy, ZnSnO₃ nanorods, piezophotocatalysis, electronic-skin

REFERENCES
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[6] H.M. Lin, K.S. Chang, RSC Adv., 7 (2017) 30513-30520
[7] H. He, Y. Fu, W. Zang, Q. Wang, L. Xing, Y. Zhang, X. Xue, Nano Energy, 31 (2017) 37-48

This paper reported an approach to the synthesis of prism-anchored octahedronal CeO₂ nanostructure using hydro-thermal method. The morphology, structure, photoelectrochemical property of prism-anchored octahedronal CeO₂ nanocomposite were investigated. The photoelectrochemical experiment revealed that growth of the prism arm on octahedron allowed to activate inert CeO2 octahedron for an increase in electrons transfer rates from bulk materials to solution under visible light. Furthermore, prism-anchored octahedronal CeO₂ was utilized to achieve degradation of methyl orange and methylene blue effectively. The reusability, stability, and other results suggests that the prism-anchored octahedronal CeO2 nanostructure could be exploited as potential candidates for visible light photocatalysis, photovoltaic, and photoelectrochemical device.

Solar water splitting using suitable photocatalysts is a potentially feasible technology for converting solar energy into chemical energy. g-C₃N₄-carbon dots (g-C₃N₄-C) composite, as an efficient photocatalyst for overall water splitting, has been explored. However, it remains unclear on the function of carbon dots. Here, we developed a novel synthesis route to prepare g-C₃N₄-C composite with uniform size and surface groups and state via one-step bottom-up approach. The band gap of g- C₃N₄-C can be tuned from 2.84 to 2.08 eV by changing the content of CDs due to CDs extends the conjugation length of g-C₃N₄ 2D sheets. The photo-deposition and PL lifetime decay illustrate that the photo-generated electron transfer to the CDs site, and holes are left at g-C₃N₄ site. The electron transfer rate and efficiency of g- C₃N₄-C are much higher than those of g-C₃N₄ physically loaded with Pt or CDs nanoparticle. We also confirm that CDs work not only as a cocatalyst for H₂ production, but also as a catalyst for H₂O₂ decomposition, which effectively prevent from H₂O₂ poison to g-C₃N₄. Our findings give a detailed understanding on the working mechanism of g-C₃N₄-C_x composites, which will beneficial to rational design highly efficient metal-free photocatalyst.

Flexible strain sensors have many applications such as structural health monitoring, mechanical testing, and pulse power suppliers. Piezotronic strain sensors, which consist of a metal–semiconductor– metal interface, are well-suited for these applications due to their high sensitivity and fast response times. Zinc oxide (ZnO) nanowires (NWs) are a popular material for use in piezotronic strain sensors.[3] However, Zinc oxide has relatively high work function, so we can enhance its field electron emission with titanium nitride (TiN) coating, which has good electrical conductivity and relatively low work function.[1] Therefore, TiN thin film makes it potential in ideal field emitters. In our research, we want to develop the new material which has piezo-related properties and low work function simultaneously.

In this work, piezophotocatalytic and piezoelectric performance of Titanium Zinc Nitride Nanorod thin films deposited by RF magnetron sputtering were described. TiN and ZnN have centrosymmetric structure. However, thin film capacitors fabricated by sputtering Zn doped TiN nanorods from Zinc and Titanium targets in N2 ambient has non-centrosymmetric structure, because electric polarization and relative permittivity measurements yield distinct ferroelectric properties.

Based on various measurements including piezopotential, piezotronic, piezophototronic, and piezophotocatalytic analyses obtained by characterization tools, (i.e. X-ray diffraction, X-ray photoelectron spectroscopy, Raman scattering, Scanning electron microscope, Transmission electron microscopy, Secondary-ion mass spectrometry, UV-Vis, and I-V methods) we found that the base pressure of vacuum chamber, the chamber pressure and temperature, the sputtering power, and gas flow significantly influenced this material’s crystallinity, morphology (i.e. surface roughness), structure properties (i.e. crystallite size), electrical properties (i.e. refractive index), optical, and mechanical properties. In addition, we use combinatorial methodology to fabricate the material [4], which has significant piezoelectric properties in the specific concentration of Zinc, for use as a piezoelectric sensor.

Keywords: Titanium Zinc Nitride, Zinc doped, nanocolumn, morphology control, composition spread, combinatorial magnetron sputtering, piezotronic / piezophototronic effects, photocatalysis / piezophotocatalysis.

REFERENCES
[1] Yasuhito Gotoh, Sho Fujiwara, and Hiroshi Tsuji,
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 031401 (2016)
[2] C. Tholander, C. B. A. Andersson, R. Armiento, F. Tasnádi, and B. Alling,
Journal of Applied Physics, 120(2016) 22
[3] Kory Jenkins, Vu Nguyen, Ren Zhu and Rusen Yang, Sensors, 15 (2015) 22914-22940
[4] Daniela Rende, Kerstin Schwarz, Ute Rabe, Wilhelm F. Maier, Walter Arnold,
Zeitschrift für Physikalische Chemie, 222(2008) 587-600

Using solar energy to produce clean hydrogen from water is a dream reaction that people are perusing. To efficiently utilize the whole range of wavelength of solar light, the photocatalysts are required to have a robust absorption of visible light, which means the bandgap of these semiconductors should be narrow enough. Unfortunately, most inorganic semiconductors, including most studied photocatalysts, like TiO₂, SrTiO₃, ZnO, CdS, suffer from limited optical absorption. Compared with inorganic semiconductors, organic materials show superior properties, like tunable band gap via structure design on molecular level and high absorption coefficient. Conjugated polymers (CPs) have been demonstrated to be excellent solar energy conversion materials, however, it is considered that pure CPs are not able to photocatalyze water splitting because of the fast recombination of photogenerated charge carriers. In our investigation, we found PTh synthesized at high temperature will introduce structure defects, such as, α–β’ coupling and end groups, which are beneficial to the charge separation. Meanwhile, PTh synthesized at low temperature has long polymer chain and ordered arrangement of chain, which enhanced the light adsorption. Considering both light adsorption and charge separation, PTh compound nanostructure was rationally designed, PTh synthesized at low temperature as core to obtain good light adsorption and PTh prepared at high temperature as sheath to form more structure defects to promote charge separation. This PTh compound nanostructure exhibit higher photocatalytic hydrogen production performance. Our finding may open a door for conjugated polymer for photocatalyst.

An active catalyst for the oxidation of CO at all temperatures is essential to ensuring safer air quality. In this study, we demonstrate the use of light to enhance CO oxidation over Mg(OH)₂ and MgO supported gold catalysts, resulting in high activity and more substantial lifetimes at all temperatures ranging from -50 to 250 °C. Catalysts are synthesized by impregnation with plasmonic gold nanoparticles with a known size and plasmon resonance frequency. Using experiments which couple light induced desorption of CO₂ and regeneration of catalytic activity, we propose a mechanism for light enhancement of gold catalysis. Light reactivates gold catalysts by removing surface carbonates which otherwise form in the active sites and poison the catalyst. This more efficient application allows for dramatically increased catalytic activity and catalyst lifetime, demonstrating a new path for highly active CO oxidation.

Heterojunction and direct Z-scheme are typical two types of composite photocatalyst. However, it remains open question on deliberately constructing these composites. Photo deposition is a common method to use the photo excited electron for reduction noble metal. The deposition site will be the photo generated electron active site, which can be potentially used for regulating electron flow direction. In this report, we proposed a method to construct heterojunction or direct Z-scheme by regulating the electron flow direction. We synthesized two CdS/g-C₃N₄ composites through photo deposition and chemical deposition synthesis route. In the former case, CdS was deposited at electron transfer site. It leads to the photo excited electron from g-C₃N₄ tends to transfer to CdS in the composite. Thus, type II heterojunction was constructed due to the band alignment. For comparison, the CdS was also deposited onto the g-C₃N₄ nanosheets through chemical deposition. There is no preference for deposition and the charge transfer in this case. The experiment results the electron of CdS tends to recombine with the hole from g-C₃N₄. Direct Z-scheme takes dominant place for the CdS/g-C₃N₄ prepared via chemical deposition route. Based on these results, we can deduce that photo deposition method can be used for regulating the electron transfer route. We expect this report shed light on the rational design of heterojunction or direct Z-Scheme type composites.

Photocatalytic water splitting is a burgeoning topic in energy conversion. Selectively oxygen permeable membranes with high flux and selectivity are important for this application when oxygen and hydrogen gases are generated. Reduced membrane thickness and precise pore size/chemistry control are the keys for achieving high flux and selectivity. Membranes in natural biological systems can be as thin as 4 nm and the pores are precisely constructed via molecular assembly, leading to unbeatable performance when compared to synthetic industrial membranes that are typically 100-1000 times thicker. In this work, we report a biomimetic ultra-thin membrane for oxygen separation fabricated by combined molecular self-assembly and atomic layer deposition (ALD). ALD is a layer-by-layer deposition method that builds up a thin film with atomic precision in structure and compositions. Here we introduce the membrane fabrication by using a “plasma-defined” ALD process where the location of ALD modification is confined by plasma irradiation. Using this approach, sub-10nm ultra-thin membranes with precisely defined pore size and pore surface chemistry have been successfully formed. Excellent performance in oxygen separation was achieved.

The attraction of hydrogen as a clean fuel has continued to stimulate interest and development in generation, storage and usage. One of the most attractive approaches for hydrogen generation is z-scheme solar water splitting using semiconductor electrodes and photocatalysts where solar light serves as the source of energy and water serves as the source of hydrogen. Although many oxide photocatalysts have been used in past for water splitting, they only respond to ultraviolet radiation. The number of photocatalysts that are active for water splitting under visible light irradiation is very limited. Therefore, it is important to develop visible-light-driven photocatalyst materials for solar water splitting via a suitable bandgap engineering process. The bandgap of a visible-light-driven photocatalyst should be narrower than 3.00 eV (λ > 415 nm). In this context, we are using TiN nanowires which is converted very controllably to TiN_1-xO_x (TNO) by bringing a trace amount of oxygen during or after the growth of TiN nanowires into the deposition chamber. TNO is semiconducting whose bandgap is a function of oxygen content. By controlling the oxygen content in TNO, we are able to tune its bandgap to a value where absorption of visible light is strong to generate hydrogen and oxygen from splitting of water. When N atoms in TiN are partially substituted by O atoms in TNO, the top of the valence band (highest occupied molecular orbital, HOMO) is shifts higher compared to the corresponding metal oxide (TiO₂) without affecting the level of the bottom of the conduction band (lowest unoccupied molecular orbital, LUMO). The potential of the HOMO for the oxinitride is located at higher potential energy than that for the corresponding oxide due to the contribution of N 2p orbitals, making the bandgap energy sufficiently small to respond to visible solar light (< 3eV).

Among the platinum group metals, it is Pd that provides the highest catalytic activity for the complete oxidation of hydrocarbons in automotive exhausts (particularly for gasoline engine applications) and the combustion of methane in gas-powered turbines.For diesel emissions, palladium is commonly used in combination with platinum to catalyze oxidation reactions. Together these two metals exhibit a synergistic relationship and are able to oxidize CO, hydrocarbons and NO produced by diesel engines while demonstrating remarkable hydrothermal stability not seen for Pt-only catalysts. With the push towards higher efficiency engines, there is a need to achieve conversion of hydrocarbons at lower temperatures. Hence, this study focuses on the hydrocarbon oxidation performance of palladium catalysts which are known to be very active for this reaction. The nature of the active sites is still a matter of debate and it has been suggested that the active sites could be attributed to bulk PdO, metallic Pd, a surface Pd oxide on metallic Pd,a PdO-rich surface,or co-existence of Pd and PdO. In this study we set out to reexamine the roles and relative reactivity of metallic Pd and PdO since this will help in the design and operation of low temperature catalysts for emission control. The contradictory assignments for active sites of Pd catalysts are due, in part, to the facile and reversible conversion between Pd metal and PdO and because observations are often made under very different experimental conditions, especially in the case of methane oxidation. Above the decomposition temperature of PdO (which depends on the prevailing oxygen pressure), the only observed phase is Pd metal. However, at the high temperatures encountered the kinetics are so rapid that both phases are found to be active for methane oxidation. This is because Pd metal exposed to oxygen at high temperatures forms a strongly bound surface oxide that passivates the metal and prevents the formation of bulk oxide. It was shown that an unreactive surface oxide was formed on Pd when exposed to ~700 °C at high oxygen pressures. The co-existence of Pd and PdO represents a metastable state due to the presence of passivated Pd metal which resists oxidation. Our previous work on catalysts that were quenched in liquid nitrogen from high temperatures shows that Pd and PdO can co-exist, but as separate phases. If PdO formed as a shell on the surface of Pd metal, we would not expect to see reactivity spanning that of metallic Pd and PdO. In this work, we follow industry-standard light off protocols to compare the performance of pre-reduced Pd and fully oxidized PdO. We also used in-situ X-ray absorption near edge spectroscopy (XANES), and temperature programmed reduction and oxidation (TPR/TPO) to correlate the reactivity with catalyst structure. The comparison of reactivity of metallic Pd and PdO provides insight into the factors responsible for low temperature hydrocarbon oxidation activity.

Quantum dots are promising building blocks for important applications including photovoltaic, light emission, transistors and bioimaging due to their unique size- and shape-dependent optical and electronic properties. The ability to tune optical and electronic properties of quantum dots by engineering their size, shape, and composition has proved to be a versatile way to interrogate structure–property relationships in quantum dots. Here we present a new method to engineer quantum dot assemblies and to probe their structure-property relationships through stress-induced phase transformation and their exchange coupling during high-pressure compression. We show that under hydrostatic pressure, the unit cell dimension of a 3-dimensional (D) ordered quantum dot superlattice can be manipulated to shrink and swell reversibly, allowing fine-tuning of interparticle separation to probe optical coupling in the supertlattice. Further, beyond a threshold pressure, quantum dots are forced to connect with neighboring dots to form new classes of chemically and mechanically stable 1-3D nanostructures including nanorods, nanowires, nanosheets, and nanoporous networks which cannot be achieved by traditional top-down or bottom-up methods. Moreover, through in situ high-pressure synchrotron-based x-ray scatterings and optical absorption measurements, we discovered Hall-Petch-like size-dependent elastic stiffness and size-dependent pressure coefficient of energy gap in quantum dots. Stress-induced phase transformation and exchange coupling provides new insights for fundamental understanding of chemical and physical properties of quantum dots.

Conventionally, semiconductors are employed for photocatalysis. An incident photon creates an electron-hole pair which then proceeds to engage in the desired chemical reaction. Inherent to this approach is the fact that only photons above the bandgap energy can be utilized. This has the drawback that photocatalytic highly active and stable materials such as titania can convert UV-photons into carriers. Thus, such photocatalysts cannot utilize the solar spectrum effectively.
It has been shown that the photocatalytic active wavelength spectrum of a semiconductor can be expanded by bringing it in contact with gold nanoparticles. These particles can be designed such that photons with energy below the semiconductor bandgap are absorbed resonantly. This process can create hot electrons which can be injected into the semiconductor’s conduction band and then take part in the catalytic reaction. While this approach is very promising, it has two drawbacks: First, the metal particles must be close to the semiconductor surface in order to inject electrons before these either lose their energy by electron-electron scattering or recombine with holes. Only those carriers which are successfully injected into the semiconductor and that reach its surface can take part in the catalytic reaction. This calls for short carrier diffusion paths, i.e. an essentially thin absorber layers that absorb a large fraction of the incoming light. Second, the nanoparticles absorb only at their resonance frequencies and thus the optical bandwidth of the system remains limited.
We will present a novel approach to mitigate both drawbacks. We replace the gold nanoparticles on the semiconductor surface with a nanoporous gold (NPG) network with extremely high specific surface area that is conformally coated with a thin layer of semiconductor such as titania. NPG represents a broad-band absorber (with tunable absorption under applied electric fields) and a surfactant-free surface. NPG manufacturing creates curved surfaces with a high number of low-coordinated atoms that reside far from their crystallographic equilibrium. Apart from that, residual elements are also currently discussed as possible origins for the found high catalytic activity. Consequently, we discuss to perspectively invert the classical approach with few NPs over a semiconductor electrode, such that a NPG carrier with homogeneous semiconductor coverage is envisaged as potentially more efficient water splitting electrode. Based on theoretical calculations (of hot electron mean free path and carrier insertion into a titania layer) we discuss structural parameters for water splitting electrodes to be introduced into synthesis approaches (involving thin film dealloying and atomic layer deposition) as well as possible methods for a direct prove of hot-electron (or hot hole) injection into the semiconductor (and the active electrolyte species) and the efficient utilization in electro-chemical conversion.

Bismuth oxychloride (BiOCl) is one of the promising photocatalysts, featured with the tetragonal layered structures of [Bi ₂O₂] slabs interleaved with double Cl slabs, endowed with an efficient separation of photoinduced charges. Hierarchical BiOCl microflowers have been synthesized by one-step solvothermal method, which are constructed from many thin nanosheets by exposing the highly active {001} facets. Considering the wide band gap of BiOCl microflowers (3.4 eV), amorphous Fe(III) or Rh(III) clusters were grafted on the surfaces through a simple impregnation method to extend the photocatalytic activity to the visible light region. The obtained products were characterized by X-ray diffraction, scanning electron and transmission microscopy, X-ray photoelectron spectroscopy, UV–vis diffuse reflectance spectroscopy, nitrogen adsorption–desorption, and electrochemical measurements. It is found that the Fe(III) or Rh(III) clusters are just deposited on the surfaces rather than doped in the lattices of BiOCl. The morphologies and crystal structures of BiOCl microflowers thus remain unchanged after modification of Fe(III) or Rh(III) clusters. The photocatalytic activity for the decomposition gaseous acetaldehyde can be significantly improved by the modificationof Fe(III) clusters under visible light irradiation. The surface Fe(III) or Rh(III) clusters play an important role in the interfacial charge transfer for the visible light absorption. Furthermore, together with the strong oxidative holes in the valence band of BiOCl, the Fe(III) or Rh(III) clusters can serves as the redox reactioncenters for the multi-electron reduction of oxygen molecules, resulting in the full decomposition of acetaldehyde into CO₂.

In this work, Ag/ZnFe₂O₄/rGO nanocomposites were synthesized and characterized by SEM, TEM, XRD, XPS, BET and TGA. Photocatalytic degradation of methylene blue, methyl orange and rhodamine B dyes under UV light source was studied using the nanocomposites. Nearly perfect degradation of the dyes at varying concentrations was achieved under both UV- and visible light irradiation. The nanocomposites were found to be stable and reusable even after five cycles of adsorption and photocatalytic degradation. The mechanism and kinetics of the photodegradation were also investigated.

We present a new method for probing ultrafast time-dependent currents in two-dimensional bilayer heterostructures by recording the associated emitted electromagnetic fields. This detection scheme offers direct sensitivity to the flow of charges at the atomic-scale and enables a real-time non-contact probe for investigating ultrafast charge transfer processes at molecular interfaces. Upon photo-excitation with above-gap photons, here we observe a burst of electromagnetic radiation from a bilayer transition metal dichalcogenide (TMDC) heterostructure (e.g., MoS₂ on WS₂) having a Type-II band alignment. The emitted electric field transients encode information about the charge transfer within this heterostructure. We detect the emitted electromagnetic fields via phase-sensitive free-space electro-optic sampling and show that these fields have spectral content at the terahertz (THz) frequency range. The polarity of the emitted field reflects the direction of the charge transfer and the polarity is reversed as the order of the monolayers within the heterostructure is inverted. Importantly, through analyzing the emitted field transients, we find that the charge transfer proceeds at an ultrafast rate (~100 fs) indicating a remarkable efficiency for the charge separation across these atomic-scale bilayers. Therefore, Type-II TMDC heterostructures could enable broadband THz emitters owing to the efficient and ultrafast charge transfer observed. Moreover, the detection of the emitted electromagnetic radiation presented here can be applied to a broad range of materials employed for solar light-harvesting and photo-catalysis to probe charge transfer processes at the atomic-scale in real-time with high time resolution.

Early attempts to make hematite electrodes for water oxidation focused on planar geometries, whose related surface and bulk properties were studied extensively. The conclusions of these studies resulted in a set of design criteria with the goal to maximize the electrode’s water oxidation performance. Nanostructuring is one of the most important design criterion, as it enhances light absorption and reduces the distance the charge carriers have to traverse before being collected and moved to a counter electrode. Furthermore, the interaction of light with ordered arrays of nanostructures results in resonance effects in both the material and voids, which enhance the charge carrier generation, increasing water oxidation efficiency. It was also shown that the surface properties of nanostructured hematite electrodes is strongly dependent on the patterning method, material growth and the annealing approaches.
In this work, we fabricate arrays of free-standing hematite nanorods and study their surface states with transmission electron microscopy. The fabrication process comprises electroplating of iron in anodized alumina membranes followed by etching of the latter to reveal an array of bare, free-standing iron nanorods. This enables an efficient thermal treatment, resulting in hematite nanorods. Annealing was performed in a box furnace in air at different temperatures and for different durations with slow heating and cooling rates. Raman spectroscopy was used to study the effects of the annealing conditions, revealing well defined hematite patterns for samples annealed at 350^oC and higher temperatures and 2 or more hours. This is a considerably lower annealing temperature than what was reported before. Using Electron Energy loss spectroscopy on individual rods, a trend of Fe³⁺ to Fe²⁺ valence states from the center to the very edges was found by evaluating the L₃/L₂ intensity line ratios and chemical shifts. This reduction of iron has been related in rapid annealing works to the reduction of surface trap states, improving performance. The combination of ordered nanostructuring, efficient fabrication process and surface states make the hematite nanorods electrodes excellent candidates for photocatalytic water splitting.

Chiral nanoarchitectonics to mimic beautiful biological structure available in nature by controlled hierarchical self-assembly of chiral molecules as building block have stimulated great interest. Here, we report a facile approach to prepare a new class of twisted chiral nanostructures based on organic nanofiber using beta-lactoglobulin protein templates. This protein possesses a unique properly of self assemble to form twisted nanofibrilar aggregates upon controlled heating at low temperature. The method involves one pot room temperature co-polymerization of 1,5-dihydroxynapthalene and 1,3,5-trimethyl 1,3,5-triazine in aqueous system containing dispersed protein filaments, which results in π electron rich polymer coated twisted nanofiber. Electron microscopic observation performed by SEM and HRTEM clearly demonstrates morphological transcription of twisted protein fibrils (ca. 20 - 30 nm in diameter) to organic twisted nanofibers. CD and UV-visible spectroscopies indicated that heat-treated protein filaments are constructed on the basis of S-chirally ordered molecular structures in aqueous system, which was transformed in organic nanofibers. The original chiral morphology was perfectly maintained after the co-polymerization. It was believe that the enhancement of chiral ordering of protein fiber by incorporation of polymer. We demonstrated a handedness control over the chiral morphology of molecular self-assembly and stabilization of chiral structure through rapid transcription. Further the obtained twisted organic nanofilaments were subjected to calcination at high temperature. A colloidal dispersion of twisted ultrashort carbon nanofibers was achieved in aqueous solution. The study demonstrated a custom design of chiral carbon nanostructures and making them promising platform for chiral optoelectronics.

Carbon dots have garnered attention for their interesting fluorescent properties and their biocompatibility, which make them ideal for applications in biological imaging and drug delivery. Specifically, carbon dots with emission in the yellow or further red-shifted wavelengths provide a promising platform for biological imaging with minimal interference from blood and tissue autofluorescence. Here we present the synthesis and characterization of multicolored inherently functionalized carbon dots from a variety of carbon sources and at multiple synthesis temperatures. The carbon dots were characterized using transmission electron microscopy (TEM), UV-Vis spectroscopy, fluorescence spectroscopy, fluorescence imaging, Fourier Transform Infrared Spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). We found that functionalized carbon dots could be obtained through a single-step synthesis method, and that small changes in synthesis temperature led to detectable red-shifting or blue-shifting of nanoparticle fluorescence. Additionally, the direction of fluorescence change based on temperature was found to vary between carbon sources. These results suggest the potential for synthesis temperature to be utilized as a simple tool for modulating carbon dot fluorescence.

Phosphonated compounds, in particular, bis-analogs are widely applied in the clinical settings for the treatment of severe bone turnovers and recently as imaging probes when conjugated with organic fluorophores. Herein, we introduce a bone seeking luminescent probe that shows high binding affinity towards bone minerals based on monophosphonated carbon dots (CDs). The spheroidal CDs tethered with PEG monophosphates are synthesized in one pot hydrothermal method and are physico-chemically characterized where the retention of phosphonates is confirmed by ¹³P NMR and XPS. Interestingly, the high abundance of multiple monodentate phosphonates exhibited strong binding to hydroxyapatite, the main bone mineral constituent. The remarkable opto-physical properties of monophosphonated-CDs were confirmed in an ex vivo model of bovine cortical bone where the imaging feasibility of microcracks as the calcium rich regions were demonstrated. Importantly, the in vivo studies specified the docking of monophosphonated-CDs on tibia. The biodigestible nature and cyto-compatibility of the probe presented here can obviate the demand for a secondary fluorophore while offering nanoscale strategy for bone targeting and can eventually be employed for potential bone therapy in the future.

Red blood cell like-mesoporous carbon hollow microspheres (RBC-PCHMs) have been synthesized by modified stöber method with following pyrolysis and etching procedures. The as-synthesized carbon spheres show unique red blood cell like morphology with interior void and mesoporous shell. The composite mixing phenolic resin with 10 wt. % RBC-PCHMs exhibits an efficient absorption bandwidth (reflection loss less than -10dB，> 90% absorption) more than 3 GHz with a sample thickness of < 2 mm in the X band at the temperature from 300 to 523 K. The fundamental mechanism based on polarization loss and conductive loss is discussed, the polarization loss decreases with rising temperature and compensates the increase of conductive loss, which significantly contributes to impedance match at elevated temperature. Our results demonstrated that the mesoporous carbon hollow spheres with red blood cell-like morphology have great potential to be lightweight microwave absorbers at elevated temperature.

Due to interesting electrical and optical characteristics of semiconductor nanowires, they have attracted enormous attention in various fields including color filters, photodetectors, non-linear optical converters, and microcavity lasers. Among recently investigated semiconductor nanowires, Si nanowires are the most important piece of the puzzle for a novel optical device with high performance since those can be produced with existing semiconductor fabrication procedures.
Recently, several studies on the multi-color generation and multispectral imaging with vertical Si nanowire arrays (Si NWAs) have been reported. The Si NWAs reveal wavelength dependence of spatial field distribution of the guide mode. The coupling of incident light and the guide mode of Si NWA provides wavelength-selective colors. However, those studies are only focused on the subtractive Si NWA color filters. Here, we present tunable color systems which show both subtractive and addictive colors by using Si NWA embedded in PDMS together with an adequate selection of substrate materials.
In order to investigate the interaction between the PDMS embedded Si NWA and the substrate, we transferred the PDMS embedded Si NWAs onto both Au thin film (100 nm, yellow colored) and ZnO film (100 nm, blue colored), respectively, deposited on the Si wafers. We measured reflectance spectra of the Si NWAs with different diameters (120 nm to 150 nm with 10 nm steps) integrated with two different substrates (i.e., Au and ZnO). For the Au film with a thickness of 100 nm, the Si NWA shows concave reflectance spectra equal to subtractive color from 600 to 900 nm wavelength due to the strong reflection properties of Au film. On the contrary, when Si NWA is mounted on the ZnO-coated Si wafer, the reflectance of the Si NWA is a convex function from 600 to 900 nm wavelength which yields addictive colors. Additional experiments with various colored substrates will be discussed in the presentation. We believe that this tunable color system can be used for various applications including flexible color filters, decorative devices, and photodetectors.

This study reports an effective strategy to enhance photocatalytic hydrogen evolution in MoS₂ by using ion radiation technique. The ultrathin MoS₂ nanosheets were obtained by solvothermal treating and ultrasonic dispersion. The scanning electron microscope (SEM)，transmission electron microscope (TEM) and Raman spectroscopy were used to characterize the morphology and sulfur vacancies of MoS₂ before and after radiation, respectively A photocurrent measurement experiment was performed in a standard three electrode system. The results show that the ion irradiated sample exhibited higher photocatalytic activity by exploiting hydrogen evolution reaction (HER) and the sample irradiated at fluence of 5x10¹² ions/cm² exhibited the best photocatalytic activity which is ~ 4 fold H₂ evolution enhancement than the original sample. All radiated sample are stable after severe durability measurements. TEM results show that high-density defects are induced in the ion irradiated MoS₂ and they act as active sites to enhance HER. Lattice expansion observed in high resolution TEM image of ion irradiated MoS₂ results in the increase of light absorption. The possible mechanisms for enhancing photocatalytic hydrogen in MoS₂ nanosheets are discussed. The current work presents an advanced defect-engineering approach to maximize the catalytic activity and light absorption of MoS₂ nanosheets by ion radiation, and may provide a new guidance for the rational design of highly efficient MoS₂ photocatalysts.

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.

Electrospinning is a well-known, simple and fast method to prepare polymer fibers with diameters of 100-500 nm and lengths up to several micrometers.[1] Since for many semiconductor materials the charge carrier diffusion length is a critical parameter restricting photocatalytic or photoelectrochemical performance, we use the electrospinning approach to prepare nanostructured metal oxide nanofibers.[2] Directly after electrospinning, such nanofibers still contain spinning polymer, after calcination crystalline metal oxide nanofibers with diameter of 100-200 nm can be prepared.[3] Using the electrospinning technique, it is also possible to prepare fibrous photoelectrodes directly onto conducting substrates in a one step process.[4,5]
Nanofibers of the (111)-layered perovskite materials Ba₅Ta₄O₁₅ are built up from small single crystals, and are able to generate hydrogen without any co-catalyst in photocatalytic reformation of methanol. After photodeposition of Rh-Cr₂O₃ co-catalysts, the nanofibers show better activity in overall water splitting compared to sol–gel-derived powders.[3]
Hollow a-Fe₂O₃ nanofibers and core–shell-like a-Fe₂O₃/indium-tin oxide (ITO) nanofiber composites were utilized as a photoanode for solar water splitting, the latter showing a doubled photocurrent compared to the hollow fiber photoanodes. This can be most likely be attributed to fast interfacial charge carrier exchange between the highly conductive ITO nanoparticles and a-Fe₂O₃, thus inhibiting the recombination of the electron–hole pairs in the semiconductor by spatial separation.[4]
CuO photocathodes were directly prepared via electrospinning onto FTO, and calcination studies were performed to systematically characterize their crystallographic and structural evolution.[5] The higher the annealing temperature, the more developed are the crystalline domains of the nanofibers, which results in better conductivity and less defect sites serving as trap states for the photo-excited charge carriers. Hence, the CuO nanofiber photocathodes annealed at 800 °C showed the highest photoresponse and stability. No decrease in the photocurrent density after prolonged operation in aqueous electrolyte was observed.

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The molecular modification of semiconductors has applications in energy conversion and storage, including solar fuels production. We have developed a synthetic methodology using surface-grafted polymers with discrete chemical recognition sites for assembling human-engineered catalysts in three-dimensional environments, providing additional control over the redox properties and stability of the composite material. This presentation will highlight the versatility of polymeric coatings to interface cobalt-containing catalysts with semiconductors for solar fuel production. Spectroscopic techniques, including ellipsometry, grazing angle attenuated total reflection Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, provide detailed information on the structure and composition of the assemblies at the nano and meso scales. Photoelectrochemical measurements confirm the hybrid photocathode uses solar energy to power reductive fuel-forming transformations in aqueous solutions without the use of organic acids, sacrificial chemical reductants, or electrochemical forward biasing.

Two-dimensional (2D) layered materials with weak interlayer coupling and sufficiently large electronic band gap as well as high carrier mobility are highly desired for future high-speed electronic devices due to their super high feasibility in experimental accessibility and functionality. Here we present a systematic study of 2D double-metal-layered scandium chloride carbides (Sc₂CCl₂), which displays extremely high carrier mobility and rather weak interlayer coupling, which endows Sc₂CCl₂ intrinsic layer-decoupled electronic, vibrational and optical properties in few-layer structures. Sc₂CCl₂ also possesses rather high and highly anisotropic carrier mobility of up to 72 × 10³ and 15 × 10³ cm² V^-1 s^-1 for electrons and holes, respectively, far higher than those of conventional 2D materials, such as molybdenum disulfide (60–200 cm² V^-1 s^-1) and few-layer black phosphorus (~10³ cm² V^-1 s^-1). The observed electronic and optical features suggest few-layer Sc₂CCl₂ are also promising donor materials for light harvesting in solar cell and photocatalysts for water splitting. These results enable probing of 2D-like systems without the requirements for the fine preparation of monolayers, and greatly enrich the family 2D transition-metal-based materials and hint for a new platform for the next-generation high-speed electronics.