Due to its low bandgap, Earth abundance, and scalable manufacturability, Si has attracted considerable attention for use in integrated solar water splitting devices. However, one of the major limitations of Si is its inherent photoelectrochemical instability in aqueous conditions, particularly at neutral and high pH. Recently, stabilization of this material using thin corrosion protection layers, such as TiO2, has been reported for both anodic and cathodic operation. In this work, we investigate the effect of TiO2 coating layers with controllable properties (thickness, crystallinity, conductivity) on the photoelectrochemical performance of pn-Si devices for hydrogen evolution. These thin films were realized by using different deposition methods, including atomic layer deposition (plasma-enhanced and thermal), physical sputter deposition, and e-beam evaporation, and stability and activity were evaluated. The mechanism enabling carrier transport, while simultaneously providing corrosion protection, is evaluated based on composition and energy band alignment analysis, which were obtained by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES), as well as contact potential difference, transient microwave photoconductivity, and surface photovoltage measurements. Furthermore, we demonstrate improved Si stability for oxygen evolution by directly engineering the catalyst/Si interface. Deposition of catalyst directly on nano-textured pn junction Si surfaces, enables effective protection of Si from photocorrosion at high pH conditions, without need for an additional corrosion protection layer. Using Ir or CoOx as catalyst, photoelectrochemical water oxidation was observed to occur below the reversible thermodynamic potential. These results demonstrate that, although bare Si is inherently unstable under conditions required for artificial photosynthesis, corrosion protection provides a viable approach to operation of buried junction Si on both photoanodic and photocathodic reaction conditions.

Tandem devices are one of the most promising routes for harvesting a large part of the solar spectrum. On one hand the different components allow for a wider choice of materials particularly optimized for a specific part of the operation - whether that is on the anode or the cathode side. On the other hand it result in a number of interfacing challenges. In this presentation we shall discuss some elements of these choices and in particular how they may be optimized using the correct combination of protection layers and catalysts. Hydrogen is the simplest solar fuel to produce and while noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives will be needed for large-scale use. We have shown that bioinspired molecular clusters based on transition metal sulfides mimics nature's enzymes for hydrogen evolution when deposited on various supports [1, 2, 3]. When these catalysts are deposited on p-type Si they can harvest the red part of the solar spectrum [3, 4]. Such a system could constitute the cathode part of a tandem dream device where the red part of the spectrum is utilized for solar fuel evolution, while the blue part is reserved for the more difficult oxygen evolution. Recently we have found that this system can be improved considerably using a np-Si system which is protected by a relative thick layer of TiO2 [5]. This improvement in corrosion protection by deposition of TiO2will be discussed [6, 7] in greater detail. Finally we will also discuss the latest progress in combining this with a large band semiconductor and how this can be coupled with necessary protection layers and appropriate catalysts.
1. B. Hinnemann et al., JACS 127, (2005) 5308.
2. T. F. Jaramillo et al., Science 317, (2007) 100.
3. Y. Hou et al., Nature Materials 10, (2011) 434.
4. A. B. Laursen et al. ,Energy Environ. Sci., 5 (2012) 5577.
5. B. Seger, et al. “Sustainable Hydrogen Production from a Molybdenum Sulfide Catalyst on
protected n+p -Silicon Photocathode. ” Angew. Chem. Int. Ed., 51 (2012) 9128.
6. B. Seger, et al. ” Using TiO2 as a Conductive Protective Layer for Photo-cathodic H2
evolution”, JACS 135 (2013) 1057.
7. B. Seger, et al. “Silicon Protected with Atomic Layer Deposited TiO2 Part B- Conducting
Versus Tunneling Through the TiO2.”, Accepted Journal of Materials Chemistry A (2013).

Bismuth vanadate (BiVO4) is an n-type semiconductor with a bad gap of 2.4 eV that has demonstrated promise as a photoanode for photoelectrochemical (PEC) water splitting. The unmodified BiVO4 surface suffers from poor water oxidation kinetics and surface recombination, but the incorporation of co-catalysts has been shown to improve the efficiency of the device. The most common catalysts for high performance BiVO4 devices are non-precious metals and metal oxides including Co3O4, FeOOH, NiFe, and Co-Pi. A photocurrent onset as early as 0.3 V vs. RHE in pH 7 electrolyte was achieved with the Co-Pi catalyst. However, these devices do not perform as well in strongly acidic or strongly basic electrolytes, a technical challenge that would help match BiVO4 photoanodes with known photocathodes for a tandem PEC water splitting device.
This paper describes our efforts in developing acid-stable BiVO4 photoanodes that can be coupled with Si-based photocathode to achieve unassisted water splitting. In order to achieve that goal, an acid stable catalyst, IrO2, must be integrated on the BiVO4 surface instead of non-precious metal catalysts. Our experimental results as well as those in literature [1],[2], have shown that even though IrO2 catalyst is intrinsically more active for water oxidation catalysis than Co-Pi, the IrO2/BiVO4 devices do not exhibit as high activity as the Co-Pi/BiVO4 device. We hypothesize that there exists an interfacial problem between the BiVO4 surface and the IrO2, which is metallic in nature. Semiconductor surface treatments with different metals and metal oxides have shed light on the surface characteristics of BiVO4, allowing us to appropriately engineer the semiconductor-catalyst interface for improved PEC activity and stability of the IrO2/BiVO4 system.
[1] Seabold, J. A.; Choi, K. S. J. Am. Chem. Soc. 2012, 134, 2186-2192.
[2] Ye, H.; Park, H. S.; Bard, A. J. J. Phys. Chem. C 2011, 115, 12464-12470.

Oxide epitaxy with abrupt interfaces on Si and Ge (100) surfaces is a promising route towards incorporating high-κ dielectrics and functional oxide films into traditional semiconductor devices. Besides an atomically sharp interface with the substrate, the successful integration of high-κ crystalline oxides in field-effect transistors requires sufficiently large electron and hole injection barriers (band offsets) for both n and p channels. Recent studies, however, have shown that band discontinuities for crystalline oxide/Ge(100) and oxide/Si(100) epitaxial heterojunctions occur entirely at the valance band edges and that conduction band edges are virtually aligned for both pairs. While the absence of a large injection barrier for electrons is unfavorable in terms of gate oxide applications, the conduction band alignment and large valance band offsets can be useful in applications where efficient electron-hole separation and diode-like behavior is desired, as in photocatalysis and photovoltaics.
To test this, we investigate the photocatalytic properties of BaTiO₃ (BTO) epitaxial nano-films grown on Ge (100). The advantageous properties of Ge (optical absorption, large carrier diffusion lengths, high carrier mobility) and the chemical stability of BTO are coupled with a conveniently aligned band structure and a perfectly abrupt epitaxial interface between the two materials. The junction shows strong rectifying behavior and slight temperature dependence of its current-voltage (I-V) characteristics. Our initial results indicate that this nano-composite is capable of catalyzing the photoreduction of Ag⁺ on the BTO surface using visible light only. Potential designs for metal-insulator-semiconductor (MIS) solar cells using this and other similar heterostructures will also be discussed.

The photoelectrochemical splitting of water has the potential to efficiently store solar energy in the form of chemical bonds; however, for this to become economically feasible, certain material challenges must be met. One major task is finding catalysts for the oxygen evolution reaction (OER) that are highly active, stable, and inexpensive. Metal oxides are promising candidates. Alloying and doping different metal oxide films may increase the activity and enhance the stability of the catalysts.
In this presentation, we explore the impact on catalytic activity and stability of mixtures of MnOx and TiO2. Catalysts are synthesized by atomic layer deposition (ALD), allowing for fine control over film thickness and composition. We have previously shown that ALD-MnOx is an active catalyst for OER, but suffers from poor stability. TiO2 has poor OER catalytic activity, but is often associated with high stability. Here, we show that the catalytic activity for OER increases with increasing MnOx composition when films are below a critical thickness. The addition of up to 10% TiO2 (by cycle number) has little impact on the activity of MnOx. In thicker films, charge transport limitations begin to dominate, and the inclusion of TiO2 dramatically decreases the activity compared to unalloyed MnOx. In these films, charge transport becomes limiting on length scales of nanometers. We explore the effects of annealing, and find that while annealing has little effect on the thin films, annealing activates thicker films with high TiO2 content, pointing to an enhancement in conductivity of the film. Despite the addition of TiO2, electrochemical cycling still reduces the activity of the MnOx thin films.
We then utilize ALD&’s ability to deposit layers of material to investigate the effects of material segregation in TiO2/MnOx mixtures by applying overlayers of TiO2 on MnOx films and overlayers of MnOx on TiO2 films. Deposition of ~0.3 nm TiO2 on ~10 nm MnOx has little impact on the catalytic activity of bare MnOx, but addition of 1.5 nm TiO2 to MnOx begins to decrease this activity. Electrochemically cycling MnOx coated with 1.5 nm of TiO2 initially increases the activity, suggesting either instability of the TiO2 or mobility of the MnOx. X-ray photoelectron spectroscopy on these samples confirms a higher Mn content is present on the surface of the film after testing. Placing TiO2 under MnOx has been seen to reduce activity, likely due to charge-transport limitations. Work on applying these catalysts as protective coatings to photoanodes is also discussed.

Noble metal nanoparticles supported on nanocrystalline oxides can facilitate surface reactions and enhance catalytic selectivity and activity. Using ambient pressure x-ray spectroscopies, we study a nanoscale Pt-TiO2 system, where Pt nanoparticles are selectively photodeposited onto ordered arrarys of TiO2 nanoparticles. X-ray absorption and transmission electron microscopy indicate a predominant rutile phase of the TiO2 nanoparticles prior to Pt depostion, but the anatase phase can coexist on the surface after Pt photodeposition. Valence photoemission results demonstrate a band gap narrowing after Pt is loaded on the surface of TiO2. Upon in-situ heating, surface defects and oxygen vacancies develop, and the amount of metallic Pt grows on top of TiO2. In contrast, under hydrogen annealing the Pt4+ species increases, which is related to the strong metal support interactions that have been observed for the Pt/TiO2 system. The reduced band gap and the enhanced contact surface in the Pt-TiO2 nanostructures can potentially improve their performance in solar absorption and photocatalysis applications.

The use of ferroelectric templates, such as lithium niobate (LN), for the photoreduction of metallic nanostructures has received attention for potential applications in nanoelectronics and biosensing. In this work we show that by using a chemical patterning technique (proton exchange, PPELN) we can create ferro- and non-ferroelectric regions, thus altering the electric fields at the surface. Therefore, we can control the location of the photoreduced nanoparticle nucleation sites and create arrays of reproducible, spatially-defined nanostructures.
By varying the parameters in the chemical reactions (i.e., controlling the concentration of AgNO3, super band-gap illumination time, and exposure time to the proton source) we can alter the nucleation and the coverage of SERS-active metallic nanostructures that are formed during the reduction reaction. For a fixed illumination wavelength (254 nm) we found that the height of the nanostructures can be controlled by concentration (heights of 4.5 nm to 6.9 nm for concentrations of 10-2 M and 10-4 M, respectively). The width of the nanostructures can be controlled by the proton exchange depth (widths of 0.5 to 4.6 µm for a range of proton exchange depths of 0.6 to 3.1 µm for a given concentration), thereby increasing the coverage of nanostructures present at the surface.
The SERS-activity of the photoreduced Ag was probed through functionalization of Ag using target molecules. We measured the signal from target molecules on photoreduced metal nanostructures and found that signal was enhanced as the coverage increased. Reproducible, spatially-defined signal was visualized using fluorescence lifetime imaging, suggesting the templates are suitable for practical applications. From the results, we conclude that we can create highly sensitive (through a 4-fold increase in signal) and highly reproducible tuneable templates for biomedical sensing applications.

Optical control over transport through molecular junctions is of growing interest as a fundamental scientific question and for potential applications in computation and sensing, due to the versatility and nano-scale of molecular monolayers. We study the visible-range optoelectronic properties of photo-active molecular monolayer/ indium tin oxide (ITO) junctions.
ITO is a degenerate n-type semiconductor that is commonly used as a transparent conductive electrode in organic optoelectronic devices. Porphyrins have attracted much attention as dyes for photovoltaic applications due to their remarkable visible light harvesting properties and tunability of electronic behaviour. We have shown that porphyrin adsorption on ITO surface results in photo-induced charge injection across the ITO-porphyrin interface, whose direction depends on the porphyrin structure, i.e., peripheral substituents. Therefore, porphyrin derivatives can be used for modulating photo-induced interfacial charge transfer at ITO/ organic layer interfaces in a predefined, controllable way, relevant for artificial vision and for interfaces in organic optoelectronic/ photovoltaic devices.
Transport across porphyrin monolayers was studied in the dark and under illumination in the visible range, where only the porphyrin monolayer is photoexcited, in all-solid, metal-free junctions. Porphyrin/ ITO junctions showed photoconductance whose magnitude depends on the illuminating wavelengths and the molecular structure of the bridging monolayers. Moreover, the junctions displayed persistent photoconductance (PPC) that lasts many hours after illumination turn-off. PPC is attributed to charge trapping in ITO surface states and long-lived charge separated states at the ITO-porphyrin interface. Photoconductive molecular junctions can be miniaturized to the nano-scale and utilized as logic and memory elements.

Complex oxides cover a broad spectrum of intriguing functionalities. Heterostructures containing complex oxides provide a powerful route to manipulate the functionalities and offer tremendous opportunities for next-generation electronic device. For example, Au-BiVO4 heteroconjugated have been extensively studied as promising heterogeneous catalysts for a variety of oxidation reactions under visible light irradiation. However, the mechanism is still not thoroughly understood. In order to provide more fundamental understanding, a thin film approach is demonstrated in this study.
We synthesized the series of Au-BiVO4 heteroconjugation in which the size and density of the (111)-oriented Au NPs on the (001)-oriented BiVO4 thin film were successfully controlled by tuning the amount of the gold deposited and annealing process. The photocatalytic pollutant degradation results reveal that the photocatalytic activity of the Au-BiVO4 significantly increases as the mean size of Au NPs increases at 10le;dle;30nm and decreases as the mean size of Au NPs bigger than 30nm. The band alignment in the Au-BiVO4 heterostructurewas determined by high resolution x-ray photoemission and UV- visible absorption in which the fermi level of Au is 1.02eV below the conduction band minimum level and 1.6eV above the valance band maximum level of BiVO4 . The dynamics of relaxation processes for Au-BiVO4 with varying size of Au NPs have been investigated by femtosecond laser photolysis techniques. By analyzing the XPS and femtosecond laser spectroscopy, we found that the interfacial electrons transfer from BiOV4 to Au NPs, which reduces charge recombination, therefore, with BiVO4 surface modification by Au nanoparticales, the Au-BiVO4 heterostructure presents a giant enhanced photocatalytic activity. This study delivers a general approach to probe the photochemistry of complex oxide heteroconjugation

Titanium (IV) oxide, TiO₂, is well known for its excellent photocatalytic activity, however it absorbs very weakly in the visible part of the spectrum. Much effort has been expended to shift the absorption from the ultraviolet into the visible region. Recent work has shown that the band gap of mesoporous TiO₂ can be greatly narrowed by co-doping with transition metals and nitrogen, hereby designated as (M,N) TiO₂. The greatest shift of the absorption edge, which is attributable to the effective narrowing of the band gap, occurred with a 2.5% cobalt/nitrogen combination, followed by 2.5% manganese/nitrogen. The smallest shift occurred with 2.5% zinc/nitrogen. To test how this band gap narrowing affects the photoreactive properties of TiO₂, a systematic study was performed by measuring the photodegradation of methylene blue by co-doped TiO₂ in an isopropanol solution. The doping levels of the transition metals samples - Cr, Mn, Fe, Co, Ni, Cu, and Zn - were 2.5 percent and 5 percent. While most of the (M,N) TiO₂ samples&’ photocatalytic activity exceeded that of TiO₂ doped with just nitrogen, there was no clear correlation between UV-vis diffuse reflectivity and photocatalytic activity.

The possible use of proteins in solid-state electronic devices is intriguing because of their versatile structure and function but requires activity under dry environment. The photoactive reaction center PS I, a nano-sized (9x15 nm) protein-chlorophyll complex that harvests photons with a quantum efficiency of ~1 is functional in a dry environment. We report on the use of a robust cyanbacterial membrane protein photosystem I (PS I) with its outstanding photoelectronic properties to fabricate active Au-PS I, SiC-PS I and GaAs-PS I electronic junctions. The stable functional junctions were achieved by covalently binding genetically engineered cysteine mutants of PS I to a chemisorbed monolayer of small connecting molecules on the electrodes surface.
When illuminated, the PS I monolayer generated a photovoltage of ~0.1V, measured by Kelvin force probe microscopy (KPFM). In order to enhance the small light absorption by the PS I monolayers, stable micrometer thick plant PS I crystals have been grown. Such crystals made of serially arranged photosynthetic protein-chlorophyll complexes-the plant photosystem I (PS I), have generated unprecedented high photovoltage ~50 Volt, which give rise to internal electric fields as large as 100 kV cm -1 which is among the highest values ever reported in any inorganic or organic material system. A quantitative theory shows that the crystal polarization is due to trapping of photogenerated electrons in deep acceptor-type traps near the FB (Fe-S) cluster. By measuring the photovoltage as a function of temperature, we find that the electron thermal ionization in this trap is 0.56±0.03eV.

Artificial photosynthetic systems capable of exploiting solar energy for photocatalysis and electrical energy production have attracted considerable interest in recent years. A possible approach to such systems consists in the assembly of hybrid architectures combining a synthetically tailored antenna for effective light harvesting with a natural photoconverter optimized by billion years of evolution. We have designed and synthesized a hybrid complex combining the photosynthetic reaction center (RC) of the bacterium Rhodobacter Sphaeroides R26 with tailored π-conjugated fluorophores, which can act as antennas to extend the light harvesting capability of the natural RC in a wavelength range where the unmodified biological enzyme does not efficiently absorb [1]. The bio conjugation protocol developed enables to selectively functionalize the lysine residues of the RC that are best located for efficient energy transfer, and the molecular structures of the dyes preserve the enzyme full activity. The resulting hybrid systems are capable of efficient photoconversion in a wavelength range where the non-conjugated protein does not absorb, thus outperforming the natural system in light harvesting and conversion ability. Our study shows that it is possible to design effective organic/biological hybrid photosynthetic machines for energy conversion, and paves the way to a new generation of hybrid materials for artificial photosynthesis.
References
F. Milano, R.R. Tangorra, O. Hassan Omar, R. Ragni, A. Operamolla, A. Agostiano, G.M. Farinola, M. Trotta Angew. Chem. Int. Ed. 51, 11019 (2012)

Functionalization of the iron oxide surface with the light harvesting protein C-phycocyanin from blue-green algae can significantly enhance the water splitting photo current in photoelectrochemical cells. Such a bio-hybride photoelectrode assembly is thus an interesting component for artificial photosynthesis.
The functionality of electrodes depends on their transport properties, which is determined by their electronic structure, which in turn is determined by synthesis and processing. In the context of bio-hybride electrodes, not only immobilization but covalent attachment is of utmost importance.
We present electrode synthesis and processing strategies along with photocurrent and hydrogen evolution data, along with charge carrier dynamics and valence band spectroscopy data which allow determination of charge transfer between protein and inorganic semiconductor under various physiological and physico-chemical conditions.
Financial support by the VELUX Foundation (project no° 790), the Strategic Korean-Swiss Cooperative Program in Science and Technology, Korean NRF-2013K1A3A1A14055158; Swiss National Science Foundation (projects 137868; 132126; 121306). Portions of this research were carried out at Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of U.S. DOE, BES.
References
1. J. Ihssen, A. Braun, G. Faccio, K. Gajda-Schrantz, L. Thöny-Meyer, Light harvesting proteins for solar fuel generation in bioengineered photoelectrochemical cells; Current Protein and Peptide Science, accepted for publication.
2. D.K. Bora, A. Braun, E.C. Constable, “In rust we trust”. Hematite the prospective inorganic backbone for artificial photosynthesis, Energy Environ. Sci., 2013,6, 407-425.
3. D. K. Bora et al., Between Photocatalysis and Photosynthesis: Synchrotron spectroscopy methods on molecules and materials for solar hydrogen generation, J. Electron Spectr. Rel. Phenom., in press
http://www.sciencedirect.com/science/article/pii/S0368204812001600
4. K. Gajda-Schrantz et al., Formation of an electron hole doped film in the α-Fe2O3 photoanode upon electrochemical oxidation, Phys. Chem. Chem. Phys., 2013, 15, 1443-1451.
5. A. Braun et al., Direct observation of two electron holes in hematite during photo-electrochemical water splitting, J. Phys. Chem. C 2012, 116 (23) 16870-16875.
6. D.K. Bora et al., Functionalization of Nanostructured Hematite Thin-Film Electrodes with the Light-Harvesting Membrane Protein C-Phycocyanin Yields an Enhanced Photocurrent, Advanced Functional Materials 2012, 22 (3) 490-502.

Silicon is the ubiquitous element upon which the semiconductor industry is reliant. The surface chemistry of this material has been widely studied with respect to oxidation, but there is still a rich degree of new chemistry to be uncovered with respect to silicon-carbon bond formation. From a practical standpoint, the interfacing of functional molecules to silicon surfaces and devices is of enormous interest for applications that include molecular electronics, interfacing of biological targets with silicon transistors, MEMS tribology, and others. Hydrosilylation is a convenient means of producing Si-C bonds on surfaces since one starts with a stable alkene or alkyne, and the metastable Si-H-terminated surface. Light-promoted surface hydrosilylation is desirable since it occurs at room temperature, does not require extraneous reagents (or fewer), and can be patterned. The initial reports of light-promoted hydrosilylation of alkenes on silicon surfaces assumed a silane-like radical mechanism, but over the past decade, a surprising diversity of mechanisms has been shown to be operative, many of which result from the underlying electronics of the bulk or nanocrystalline silicon material. The chemistry can therefore be entirely different from that seen in silicon-containing molecular systems, and is thus truly materials chemistry. In this talk, we will describe the latest results from the field of light-promoted surface hydrosilylation reactions on silicon surfaces.

The photochemical production of H2 from water is a rapidly expanding research field that aims to store solar energy into a chemical fuel. This reaction requires the combination of a light harvesting unit and a catalytic component capable of carrying out the two-electron reduction of H+ into H2. Increasing attention is being paid to heterogeneous systems that combine molecular catalysts and semiconductors. These hybrid systems combine the high selectivity towards H+ reduction offered by the molecular catalysts, with an enhanced charge separation due to electron transfer between the different components.
In this conference, we aim to present the spectroscopic study of two different proton reduction systems based on molecular catalysts: a cobaloxime (CoP)1,2 and a nickel complex containing cyclic phosphine ligands (NiP)3. We have studied the photo-induced electron transfer mechanisms by monitoring the intermediates of the catalytic reaction using transient absorption spectroscopy in homogeneous media and when anchored onto the surface of a nanoparticle (TiO2 and ZrO2). We have demonstrated that the photocatalytic activity of NiP can be achieved either through an oxidative or a reductive quenching of the dye, depending upon the system employed.
For hybrid systems based on the functionalization of TiO2 with molecular catalysts, the H2 evolution mechanism requires a two-electron transfer from the semiconductor to a single molecular catalyst. We have used transient absorption spectroscopy to study the parameters that affect the kinetics of each electron transfer to necessary to reduce the molecular catalyst by monitoring the lifetime of charge carriers (e- and h+) of TiO2. While the first reduction of the molecular catalyst takes place quantitatively in the microsecond timescale, slower electron transfer kinetics are observed at low catalyst loadings and high excitation densities. We have assigned this observation to a 104 times slower second reduction of the molecular catalyst, which is due to a smaller thermodynamic driving force associated with the second electron transfer.
In this conference, we would like to discuss that the study of the electron transfer mechanisms, the control of the two-electron transfer kinetics and the design of molecular catalysts with a low overpotential for water reduction are critical issues for the effective implementation of hybrid systems requiring multiple reduction reactions. We believe that understanding and addressing this challenge is likely to be critical in the development of solar to fuel conversion systems.
References
1 Lakadamyali, F., Reynal, A., et al. Chem. Eur. J. 2012, 18 (48), 15464.
2 Reynal, A., et al. Energy Environ. Sci. 2013, DOI: 10.1039/c3ee40961a.
3 Gross, M. A., Reynal, A. et al. in press.

Solar hydrogen production through photoelectrochemical (PEC) water splitting is a possible clean and renewable solution to society&’s future energy needs. Due to the slow kinetics of the water splitting reactions, however, optimal integration of semiconductors (SCs) and electrocatalysts (ECs) is required for high-efficiency PEC devices. It is therefore important to understand how charge transport across the semiconductor-solution interface is affected by the addition of EC. For example, when a photoanode is coated with an electrolyte-permeable EC will holes oxidize water or EC first? How does the addition of an EC to to the surface of an SC affect the equilibrium and non-equilibrium interface energetics? We developed a dual-working electrode (DWE) PEC technique to directly control / monitor the charge transport process and energetics at the SC|EC interface [1]. By directly following the flow of photo-generated holes in a model TiO2|Ni(OH)2/NiOOH photoanode we found that holes preferentially oxidize the EC before oxidizing water. Based on results from further DWE experiments we propose that ion-permeable ECs such as Ni(OH)2/NiOOH form adaptive SC|EC Schottky junctions where the barrier height changes in-situ, leading to increased effective photovoltage. Single working electrode measurements on photoanodes coated with different ion-permeable ECs support this adaptive junction model. Our findings suggest that, in general, ion-permeable electrocatalysts are desirable for constructing SC|EC photoanodes compared to ion-impermeable electrocatalysts.
(1) Lin, F.; Boettcher, S. W. Adaptive semiconductor-electrocatalyst junctions in water
splitting photoanodes. Nat. Mater. 2013, Accepted.

Solar cells can provide clean and sustainable electricity, but high costs have limited their implementation. The use of sensitized nanostructured architectures may enable both low-cost processing and high efficiency by decoupling the functions of light harvesting and charge transport into different materials. We report on extremely thin absorber (ETA) solar cells that use ZnO nanowire arrays coated with a thin CdSe layer and filled with a liquid electrolyte. CdSe absorbs visible light, and photoexcited electrons are injected into the ZnO while photoexcited holes oxidize the redox species. Nanowire arrays provide direct pathways for electron transport as well as sufficient surface area for sensitization. The CdSe coatings should be crystalline and conformal with well-controlled thickness. With this ETA architecture, interfacial recombination is the dominant loss process, so controlling the interfacial chemistry, morphology, and microstructure of the materials during processing is critical.
Our approach utilizes a combination of solar cell measurements and ultrafast transient absorption spectroscopy to understand the effects of CdSe thickness, annealing conditions, and interfacial treatments on the dynamics and efficiency of charge carrier separation, and ultimately on the solar-to-electric energy conversion efficiency. These studies provide guidelines for architecture design and materials selection for ETA solar cells.
For a given set of deposition conditions, planar absorbers thinner than the carrier collection length suffer from poor light harvesting, while thicker absorbers suffer from poor charge collection. For electrodeposited planar CdSe films annealed at 400 °C, both Jsc and internal quantum efficiency indicated an optimal thickness of 50 - 70 nm, which is much smaller than the absorption depth. The nanowire geometry was designed to provide sufficient surface area so that efficient light absorption could be achieved with this coating thickness. Coated ZnO nanowire arrays showed 3-fold increase in Jsc to 8 mA/cm2 due to increased light harvesting. Ultrafast transient absorption spectroscopy showed that the characteristic time for electron transfer into ZnO (~2 ps) and hole transfer into the electrolyte (~100 ps) are much faster than the lifetime of photoexcited carriers (~500 ps). Therefore charge separation is very efficient. However, extremely thin coatings are subject to pinholes and shunt pathways that cause Voc and fill factor to be much smaller in ETA cells than planar cells. Deposition of an ultrathin (<5 nm) CdS interfacial layer reduced interfacial recombination and increased Voc from 0.31 V to 0.51 V and FF from 36% to 40%. With appropriate selection of coating thickness and interfacial treatment, nanostructured liquid-junction ETA cells achieved efficiencies of ~2%, which was more than twice those of the best planar cells with the same absorber.

Water splitting takes place at semiconductor electrodes that absorb visible light and have the appropriate band edges positions to straddle the water redox potential. The photolysis of water is performed by photogenerated holes that are injected from the valence band or from intermediate states in the bandgap (surface states). Meanwhile the electrons are evacuated towards a cathode by diffusive transport or by a drift field (if available). The recombination of carriers competes strongly with their injection for the useful anodic and cathodic electrochemical reactions. Here we discuss the application of impedance spectroscopy experimental and theoretical tools to identify the mechanism of operation of photoanodes for solar fuel production. A basic energetic model for a semiconductor layer that absorbs photons, with the generation of electrons and holes, extraction of electrons, and electrochemical reaction of holes, is developed into a full model for impedance spectroscopy analysis, and compared with a range of experimental results in hematite thin film electrodes as well as in surface functionalized films.

Our recent studies of single crystal oxide sensitization have concentrated on covalently bound quantum dot (QD) sensitizers. QDs have been investigated as sensitizers because of their potential for enhanced stability compared to conventional dyes, as well as high light absorption cross sections that can be tuned to cover a large fraction of the solar spectrum simply by varying the particle size. Despite such beneficial attributes, quantum dot sensitized solar cells (QDSSCs) have not achieved efficiencies or stabilities competitive with conventional dye sensitized solar cells. One reason for this is that the surface chemistry for the chemical attachment of the QDs to the TiO2 surface was not well understood or controlled. In several of our recent studies we used single crystals of both the anatase and rutile forms of TiO2 as simple model systems to evaluate the influence of different QD attachment procedures on the electronic coupling of CdSe QDs and CdSe/ZnS core/shell QDs to the TiO2 surface by measuring the photocurrent yields due to electron transfer from photoexcited QDs into TiO2. We utilized a surface chemistry strategy whereby short-chain, bifunctional passivating ligands such as 3-mercaptopropionic acid (MPA) stabilize the QDs in water while chemically binding the nanocrystals to the TiO2 surface via thiolate and carboxylic acid moieties, respectively. Atomic force microscopy (AFM) confirmed that our surface chemistry strategy reproducibly resulted in a single layer of QDs covalently bound to the atomically flat single crystal substrates with no three dimensional QD clusters.
Once we had established that we could reproducibly bind and characterize CdSe QDs to TiO2 crystal surfaces, we turned our attention to another possible benefit of QD sensitization where multiple carriers could be generated from a single photon of energy greater than twice the energy gap of the QD. The multiple exciton generation (MEG) process had been well established in colloidal QD systems but there were no demonstrations of actually collecting the current due to MEG (MEC - multiple electron collection) in a photovoltaic system. The energy band alignment needed for MEG collection could be obtained from switching to PbS QDs that have a low bulk band gap value of 0.37-0.41 eV at 300 K. PbS QDs are readily synthesized with band gap energies ranging from 0.5 to 2.0 eV, making it possible to measure sensitized photocurrents associated with MEG using photons sufficiently low in energy to preclude direct excitation of the TiO2 band gap (3.0 eV for rutile and 3.2 eV for anatase). Efforts to extend MEG to three electrons per photon will also be presented.

To achieve sustainable water resources, new treatment technologies are needed that can be applied to a broad range of undesirable constituents in water over a broad range of water chemistries. In this work, we have synthesized electrospun nanofibers of photocatalytic TiO2 for use as a building block in the construction of a multi-component nanofiber membrane that can be used for simultaneous filtration and chemical oxidation of impaired water supplies. Through systematic tuning of the parameters during electrospinning, TiO2 nanofibers of controlled diameter (ranging from 30-200 nm), crystallinity and grain size (ranging from 20-50 nm) were developed and their photocatalytic activity was tested toward phenol as a model pollutant. Results from reactivity studies reveal that under irradiation of wavelength greater than 305 nm, the reactivity of TiO2 nanofibers was the greatest at an average diameter of 30 nm and with mixed crystal phase composition. Notably, the most reactive TiO2 nanofibers synthesized thus far outperformed commercially available TiO2 Aeroxide® P25, considered by many the gold standard in photocatalysis due to its high photoactivity. Doping the TiO2 nanofibers with the noble metal silver showed additional physical and optical property changes as well as increased reactivity towards phenol. In addition to TiO2, BiVO4 nanofibers are also being synthesized. In lieu of TiO2&’s activity and efficiency with UV light, its performance in the visible realm, which is vastly more abundant in the solar spectrum, is fairly poor. BiVO4 is a viable non-titania photocatalyst that has shown great promise in photoactivity in the visible light region. Work on developing BiVO4 nanofibers will be the same, including characterization of its dimensional, morphological and optical properties and analysis of its photocatalytic activity for optimization towards remediation of organic pollutants as well as artificial photosynthesis.

Quantum confined nanomaterials, such as semiconductor nanocrystals (NCs), have emerged in the past decade as a new class of light harvesting and charge separation materials for solar energy conversion. An appropriate model for describing photoinduced charge transfer in these systems is, however, still lacking. Recently it was observed [1] that the rate of photoinduced electron transfer from CdSe NCs to molecular acceptors (methylene blue, MB) increased with decreasing NC size (and increasing driving force), exhibiting a lack of Marcus inverted regime behaviour over an apparent driving force range of ~ 0-1.3 V. Our atomistic semiempirical pseudopotential calculations show that an Auger assisted ET mechanism, in which the transfer of the electron is coupled to the excitation of the hole, can circumvent the unfavorable Frank-Condon overlap (that is a signature of inter- or intra- molecular electron transfer) in the Marcus inverted regime, reproducing the observed ET rates with remarkable accuracy. These results are also consistent with time-domain ab-initio calculations [1] on model systems.
We conclude that electron transfer from quantum dots differs from electron transfer originating from both molecules and bulk semiconductors. It proceeds via a novel Auger-assisted pathway which we believe is applicable to many excitonic nanomaterials, including quantum dots, nanorods, graphene, and carbon nanotubes. As a consequence, this new finding will have a major impact on our understanding of exciton dissociation in these systems and on the design of next generation solar energy harvesting devices
References:
[1] H. Zhu,Y. Yang, K. Hyeon-Deuk, M. Califano, N. Song, Y. Wang, W. Zhang, O. V. Prezhdo, T. Lian (unpublished).

Charge separation in quantum dots upon illumination can not only be used for a size-dependent fluorescence but by coupling these nanoparticles to electrodes also for the generation of a photocurrent. This allows their use in a light-switchable layer on the sensor surface. The quantum dots exchange electrons with the electrode but can also interact with reaction partners in solution providing access to the construction of photoactivated signal chains starting from an analyte molecule to be detected [1].
In this study the dependence of the photocurrent on several factors such as electrode polarization, pH and oxygen content of the solution has been investigated. Based on these results the combination of CdSe/ZnS quantum dot-electrodes with enzyme reactions has been evaluated.
It is found first that the QD-modified electrode can be used to follow enzymatic reactions in solution. For example, the system is applied to monitor the activity of glucose oxidase down to about 0.025 U/ml. In an alternative approach NADH as a cofactor of several dehydrogenases is detected [2].
In order to develop a photoelectrochemical biosensor, glucose oxidase (GOD) is immobilized on top of the CdSe/ZnS-electrode. Two different approaches have been followed based on covalent cross-linking and a layer-by-layer deposition of the enzyme by means of a polyelectrolyte [3]. It can be clearly shown that the photocurrent is a function of the concentration of glucose in solution. Photocurrent measurements show an increasing glucose sensitivity with a raising enzyme concentration fixed on the QD layer. In order to extend the applicability of the approach sarcosine oxidase has also been coupled to the QD electrode.
Furthermore it can be shown that direct protein interaction with illuminated QDs is feasible provided the surface properties of the nanocrystalls are properly adjusted. This has been exemplified with the small redox protein cytochrome c which is a suitable reaction partner of several enzymes and CdSe/ZnS QDs modified by thiol compounds [4].
[1] F. Lisdat, D. Schäfer, A. Kapp, Anal. Bioanal. Chem. 405 (2013) 3739.
[2] K. Schubert, W. Khalid, Z. Yue, W. Parak, F. Lisdat, Langmuir 26 (2) (2010) 1395
[3] J. Tanne, D. Schäfer, W. Khalid, W.J. Parak, F. Lisdat, Anal. Chem. 83 (2011) 7778.
[4] C. Stoll, C. Gehring, K. Schubert, M. Zanella, W.J. Parak, F. Lisdat, Biosens. Bioelectron. 24 (2008) 260.

Graphite nanodots (G-dots, <10 nm) possess many unique and novel properties.1-2The strong and tunable luminescence of G-dotsis particularly interesting both fundamentally and technologically.2 We report simple synthetic methods of G-dots with different sizes, which exhibit size-dependent photoluminescence (PL), excellent up-conversion PL, photo-induced electron transfer and electron reservoir properties. Forenergy, environmental, and biological applications,G-dots are attractive in terms of high aqueous solubility, robust chemical inertness, easy functionalization, high resistance to photobleaching, and low toxicity.We demonstrate the different applications of photo-activated G-dotsas follows:
(1)G-dots/semiconductor (such as TiO2,SiO2, Fe2O3, Cu2O, or Ag3PO4) composites exhibit enhanced photocatalytic activities, can harnessthe full spectrum of sun light, and are stable in visible light.
(2)G-dots/metal(Au, Cu, or Ag) composites show unprecedented high photocatalytic activity for selective oxidation of cyclohexane to cyclohexanone witha conversion efficiency of63.8% and selectivity of over 99.9%using H2O2 as oxidant in the absence of solvents.
(3) G-dotscan photo-catalyse a series of organic reactions (Esterification, Beckmann rearrangement and Aldol condensation) with high conversion efficiencies in water solution under visible light irradiation.
(4) G-dots-loadedTiO2 nanotube arrays can serve as a photo-anode for efficient hydrogen generation under visible light.
(5) G-dotscan photo-modulate enzyme(porcine pancreatic lipase, PPL)activities.Undervisible light irradiation, the activity of PPL/G-dotsis10%higher than that of free PPL, but is40%lowerthan that of free PPLwithout light source. Based on Michaelis-Menten kinetics,G-dotsare confirmed to play an important role of non-competitive inhibitor.
(6)G-dots can increase the permeability of HeLa cell membrane in the presence of different surfaces(G-dots terminated with-PEG for 8%, -OH/-COOH for 13%,and -NH2for19%, respectively). The permeability change was irreversiblewhileG-dotsshow low toxicity towardsHeLa cells.
Reference:
1. Li, H. T.; He, X. D.; Kang, Z. H.;* Huang, H.; Liu, Y.;* Liu, J. L.; Lian, S. Y.; Tsang, C. C. A.; Yang, X. B.; Lee, S. T.* Angew. Chem. Int. Ed. 2010, 49, 4430-4434.
2. Li, H. T.; Kang, Z. H.;* Liu, Y.; Lee, S. T. J. Mater. Chem.2012, 22, 24230-24253.

For TiO2-based photoanodes, the interfacial coupling between TiO2 and conductive materials (e.g., carbon), plays a vital role in determining the electron transfer efficiency and thus photoelectrical performance. In this paper, we describe a facile approach to effectively engineering the interfacial coupling between reduced graphene oxide (RGO) and TiO2 in well-designed one-dimensional (1D) RGO-wrapped TiO2 nanofibers, which act as ultrafast electron transfer bridges when implanted in photoanodes.The 3-5 nm RGO nanoshells were hybridized with TiO2 nanofibers as an electron donor component via d-π electron orbital overlap between C and Ti atoms, by adopting a thermal reduction at 450 °C. Remarkable photoelectric improvement in term of high photocurrent density by 2.2-fold and ultralow charge transfer resistance (Rct) by 0.2-fold is ascribed to the interfacial charge transfer. Completely reduced RGO in RGO/TiO2 nanofibers was not necessary at the expense of their hydrophilicity, as it led to unexpected isolation in the photoanodes. The thermal reduction temperature of RGO/TiO2 nanofibers was found to be critical, and a maximal photocurrent density could be achieved by 2.7-fold at 530 °C. The excessive RGO/TiO2 nanofibers more than 5 wt % had degrading effect on photoelectrical activity, largely due to the light-block effect and isolation in matrix. This strategy provides new insight for tuning intrinsically chemical and/or physical properties of well-designed semiconductor nanostructures with promising photoactivities in highly efficient photovoltaic devices.

Photocatalytic conversion of carbon dioxide (CO2) to hydrocarbons such as methanol makes possible simultaneous solar energy harvesting and CO2 reduction, two birds with one stone for the energy and environmental issues. This work describes a high photocatalytic conversion of CO2 to methanol using graphene oxides (GOs) as a promising photocatalyst. The modified Hummer's method has been applied to synthesize the GO based photocatalyst for the enhanced catalytic activity. The photocatalytic CO2 to methanol conversion rate on modified graphene oxide is 0.172 mu;mol/g-cat/h under visible light, which is six-fold higher than the pure TiO2. Meanwhile, we have developed a novel one-step and effective electrochemical (EC) method to directly exfoliate graphite into thin reduced graphene oxide (RGO) nanosheets at room temperature. The oxidation degree of the RGOs depends on the switching potentials of the EC synthesis. The high switching potential can significantly increase the C/O ratio of the RGOs. The ability to control the light-absorption of the RGOs by simply adjusting the switching potentials can be further achieved. Moreover, we also construct an RGO-ZnO heterojunction and investigate its photoelectrochemical (PEC) properties. The results show that highly photoactive RGO as a photosensitizer can make H2 evolution easier and improve the photoconversion ability of ZnO under visible-light irradiation. This approach presents us with a possibility for the environmentally friendly, ultrafast, low-cost, and large-scale production of RGOs and great potential in solar-energy conversion applications of graphene-based materials. Further, Cu and MoS2 nanoparticles were deposited on GO as co-catalysts to enhanced the photocatalysis reaction. Not only methanol, but also acetaldehyde was detected. Total solar to fuel yield of 6.8 mu;mole/g-cat/h has been achieved, which is 170 times enhancement relative to the commercial P-25 photocatalyst. In all the above-mentioned hybrids, the photo- catalytic performance is always much better than that of constituent component when used alone. Detailed preparation and characterization of the catalysts will be presented. The role and interplay of the constituent components will also be discussed in this paper.

Graphene-based hybrid materials have the potential to cater to a variety of nanotechnological applications ranging from optoelectronics, to biomedical as well as, industrial. Herein, we present an array of novel graphene-based hybrid materials targeted for energy conversion systems. Multichromophores as photoactive electron donors were integrated onto exfoliated graphene by diverse strategies based on either covalent or non-covalent approaches. While with the former methodology stable and robust anchoring of the photoactive component on graphene is achieved, however, generating insulating sp3 regions, with the latter strategy utilizing van der Waals pi-pi stacking and Coulombic interactions the advantage of keeping intact the skeleton of graphene, thus retaining the novel electronic properties, is achieved. The purpose of the current study is to comprehensively evaluate how the number and nature of the photoactive electron donors integrated onto graphene (eg. covalent vs non-covalent), affect the photoconversion efficiency of the so-formed donor-acceptor systems.
Initially, graphene sheets with minimal defects were produced by ultrasonication. Then, properly modified pyrene, porphyrin and oligo-thiophene adducts were anchored onto graphene under conventional heating or microwave irradiation. On the other hand, pyrene and porphyrins were immobilized onto graphene by supramolecular means forming multi-chromophore water-soluble graphene-based ensembles. Time-resolved photoluminescence experiments verify the promotion of electronic communication of the chromophores with the graphene. All the synthesized hybrids have been extensively characterized with thermogravimetric analysis, microRAMAN, electrochemical and optical techniques (absorption, steady state and time-resolved photoluminescence spectroscopy) among others to verify, the structural and optoelectronic properties.
The authors would like to thank GSRT/ESPA 2007-2013 through action “Postdoctoral support” project GRAPHCELL PE5(2126) for financial support.

NOx in the exhaust gas from stationary emission sources is removed by using selective catalytic reduction with NH3 (NH3-SCR) over V2O5 based catalysts at high temperatures above 573 K. In terms of saving energy on heating the catalyst bed, a novel catalyst which works at low temperature is now strongly desired. Photocatalyst is one of promising candidates for the NH3-SCR at low temperature. We have reported photo-assisted SCR of NO with NH3 in the presence of O2 over TiO2 photocatalysts under UV-light irradiation (photo-SCR). 90% of the conversion of NO and 99% of the selectivity to N2 were achieved at a gas hourly space velocity (GHSV) of 16,000 h-1 in the photo-SCR system. Unfortunately, TiO2 did not function as an effective photocatalyst for the photo-SCR under visible-light irradiation. Dye-sensitization of wide band gap semiconductors such as TiO2 is an effective way to develop the visible-light-driven photocatalyst. In this study, we fabricated various dye modified TiO2 photocatalysts and compared these photocatalytic activity for the photo-SCR under visible-light irradiation.
Fifteen kinds of dyes (e.g. Ru complex, porphyrin, methylene blue, rose bengal, etc.) were used for the modification of TiO2. The dye-modified TiO2 photocatalysts were prepared by an impregnation method. Photo-SCR was carried out in a conventional fixed bed flow system. The reaction gas composition was as follows: NO 1000 ppm, NH3 1000 ppm, O2 2%, He balance. A 300 W Xe lamp equipped with a L42 cut-off filter was used as a light source (lambda; > 400 nm).
All dye-modified TiO2 photocatalysts showed higher activity than non-modified TiO2 under visible-light irradiation. The N3 dye (RuL2(NCS)2 L=2,2'-bipyridyl-4,4'-dicarboxylic acid) modified TiO2 photocatalyst showed the highest activity for the photo-SCR (both the conversion of NO and the selectivity to N2 were 100% at high GHSV of 100,000 h-1). The turnover number of N3 dye was 1700 after 30 hours of the reaction at GHSV of 250,000 h-1, indicating that almost all N2 in the outlet gas were originated from N atoms of NO and NH3 molecules. The reaction hardly proceeded over N3 dye modified TiO2 without a substrate such as NO, NH3, or O2. The reasonable reaction mechanism was proposed by means of ESR, IR, and UV-Vis spectroscopies.

Zinc oxide (ZnO) is an n-type semiconductor with high electron mobility. Its nanorods structure has been studied and applied to hybrid solar cells using p-type polymers such as P3HT. In the solar cells, ZnO nanorods provide a continuous pathway for electron transportation as well as a large interface area that is useful for dissociation of excitons photogenerated in polymers. Although seemingly having an ideal interdigitated structure, devices with a ZnO-nanorods/P3HT junction show low efficiency. This is attributed to low exciton dissociation efficiency at the ZnO/P3HT interface and low carrier transporting property of the nanorods. In order to solve these problems, we tried to increase the crystallinity of ZnO nanorods by applying heat treatment. We prepared a ZnO seed layer by the sol-gel spin coating method and ZnO-nanorods were formed on the seed layer by chemical bath deposition. When the ZnO nanorods layer was deposited on the seed layer, which had been annealed at 300 °C in air, the nanorods were well-aligned and became more crystalline. To investigate the charge separation at the ZnO/P3HT interface, we measured the degree of fluorescence quenching of P3HT films at the interface. P3HT films deposited on ZnO nanorods formed on an annealed seed layer showed a larger degree of quenching than those deposited on ZnO nanorods formed on a non-annealed seed layer, indicating improvement in charge separation at the ZnO nanorods/P3HT interface by using ZnO nanorods grown on the annealed seed layer.
To confirm usefulness of annealing on improvement of solar cell properties, we fabricated devices with a structure of ITO/ZnO seed layer/ZnO nanorods layer/MoO₃/Ag and examined the external quantum efficiency (EQE) of photocurrent for devices with and without the annealing process. EQE values were 23%, 32% and 37.5% for a device without any annealing, for that with a ZnO seed layer annealed at 400 °C, and for that with a ZnO nanorods layer annealed at 300 °C, respectively. The EQE reached even 41% for the device with a ZnO seed layer annealed at 400 °C and the ZnO nanorods layer annealed at 300 °C. The annealing processes probably contribute to improve both charge separation at the ZnO/P3HT interface and charge transportation in the ZnO nanorods. In conclusion, we found that the simple annealing method is useful to improve the properties of solar cells with a ZnO nanorods/P3HT junction.

Titianium dioxide (TiO2) has long been pursued as a suitable material for various photocatalytic applications because of its strong oxidizing power, high chemical stability, and low cost. On the other hand, two-dimensional nanomaterials such as graphene (GR) and graphene oxide (GO) have received great attention due to their unique electrical, optical, and chemical properties. Recently, we demonstrated highly visible light-active, low bandgap graphene-wrapped TiO2 nanoparticles (GR-TiO2 NPs) that was synthesized by one-step reduction of GO and crystallization of TiO2 via hydrothermal reaction. Here, we report on a synthesis of a UV-activated colorimetric O2 sensor that is made of graphene oxide-wrapped TiO2 nanoparticles (GO-TiO2 NPs) as an adaptation of the GR-TiO2 NPs. The GO-TiO2 NPs were used not only as a photosensitizer in the O2 sensor that comprised methylene blue (MB) that was a redox dye and glycerol that was a sacrificial electron donor, but also as a scaffold for MB adsorption. When GO-TiO2 NPs were mixed with MB, MB molecules were adsorbed to GO by both ionic interaction and π-π stacking and we confirmed that that MB was bonded to the surface of GO-TiO2 NPs effectively through zeta potential, UV-Vis spectrometer, and cyclic voltammetry analysis. We found that the chemical interaction between GO and MB significantly reduced the amount of dye leakage in the O2 sensor, which has been regarded as a conventional problem of the sensor. The amount of MB leakage in the O2 sensor was decreased to 2.86 times lower compared with TiO2 sensor that was physically mixed with MB, without GO. After making a film with GO-TiO2 NPs that adsorbed MB (MB/GO-TiO2), we also examined the ability of the film as an O2 detector. Under UV light irradiation and N2 atmosphere, the O2 indicator was reduced to its colorless form, leuco MB, which completed in 6 min. When the sensor was exposed to O2 atmosphere without light, the color was returned to blue as the leuco MB was reoxidized to its original form and it took 18 min, which showed the MB/GO-TiO2 could act as a novel and stable O2 sensor.
Our Recent Publications Related to This Presentation:
J. S. Lee, K. H. You, and C. B. Park. Advanced Materials 24: 1084-1088 (2012).

From the advent of photoelectrochemical water splitting, many different electrodes have
been developed. Though the efficiency of some of the electrode such as hematite in pristine condition is quite low. To get rid of this discrepancy, various methods viz. doping, nano structuring and surface modification strategy have been adopted to increase the efficiency of hematite electrode [J. Am. Chem. Soc. 2006, 128, 15714-15721]. In our case we have developed a novel surface functionalization strategy whereby we can modify the hematite surface with naturally found c - phycocyanin molecule. This molecule helps in the panchromatic light harvesting of hematite film which resulted in 2 fold increase in photocurrent. [ Adv. Funct. Mater. 2012, 22, 490-502]. Now the question arises: What factor affects the extra 2 fold increase in photocurrent? To answer it, we have performed detailed photoelectrochemical investigation of the hybrid system. From the transient photoresponse study we found out that in the chemically functionalized system, the magnitude of cathodic spike get increases pointing to the removal of conduction band electrons by hydroxyl radical and it continued until all the hydroxyl radical used up. In the next step, we have performed long term chronoamperometric study to find out the number of electrons coming from parasitic carbon oxidation on the surface of electrode. From here, it is evident that the number of moles of electrons resulting from the dark oxidation of organic material reduced to lower number over the period of time. To validate the individual role of phycocyanin for enhancing the current density, we have performed transient study of phycocyanin adsorbed
FTO film. Hereby, we observed that phycocyanin do show anodic spike signifying the photoresponsive behavior of species under light on and off condition. From the long term sustainability study, phycocyanin showed good operational stability.

The two-dimensional conjugated structure, high surface area, and excellent electronic transport property of graphene make it an ideal promoter for catalytic reactions. In this study, graphene-based nanocomposites are synthesized via a hydrothermal process to anchor copper on GO and also to restore the graphene (RG) structure. The photodegradation efficiency of methyl orange (MO) solution by those synthesized nanoparticles under 400-Watt UV illumination is selected as an indicator to evaluate the photocatalytic activities of RG-Cu nanocomposites. The composition, crystalline structure, surface components, and morphology of RG-Cu nanocomposites are correlated to the process parameters and the MO degradation rates. XPS and XRD results confirm the crystalline structures of copper species and also graphene on each RG-Cu sample. On the surface of RG-Cu, the particle size of CuO or Cu2O increases gradually with hydrothermal period. More Cu2O particles form when ethanol or ethylene glycol presents during hydrothermal process. An optimum copper loading and chemical states of copper on RG enhance the MO degradation rate under illumication.

In this contribution, we report our research into coupling organic chromophores to single site ruthenium water oxidation catalysts. We have designed chromophore/catalyst assemblies that employ broadly absorbing organic chromophores for improving the light sensitization of existing water oxidation catalysts. These assemblies exploit the use of organic chromophores instead of the widely used Ru polypyridyl-based chromophores. The organic chromophores are synthesized via a donor-acceptor approach utilizing electron-donating thiophene units and electron-accepting units such as benzothiadiazole and isoindigo. Through appropriate choice of the donor and acceptor, the energy levels of the organic chromophores are tuned for facile electron injection into TiO2 and hole transfer to neighboring ruthenium water oxidation catalysts. Using ultrafast transient absorption spectroscopy, we will describe the charge transfer events and lifetimes of the transient species following photoexcitation of the chromophore/catalyst assemblies.

Due to its unique properties, TiO2 has found numerous applications in a wide range of fields such as optics, electronics and biotechnology. Recent advances have facilitated control over the nanometer scale structures of TiO2 surfaces. By forming nanostructured TiO2, novel properties can be obtained that are not associated with the bulk material, such as enhanced surface area and control over the band gap. Furthermore, TiO2 surfaces can be easily functionalised with a wide range of molecules and biomolecules, further optimising its use in science and technology. In the anatase phase, TiO2 surfaces are photocatalytically active, causing the degradation of organic molecules when irradiated with UV photons. Provided the photon energy is larger than the TiO2 band-gap energy, electron-hole pairs are formed which can react with surface water to form radicals that can break down organic matter. This property has been exploited in various technologies such as self-cleaning windows and water purification units.
In this work ‘self-cleaning&’ titanium nanostructures have been formed by interference lithography (IL) of self-assembled monolayers of alkylphosphonates on titania and by nanoimprint lithography (NIL). Exposure of the SAM by IL leads to the photocatalytic removal of the phosphonic adsorbate in regions of the surface coincident with maxima in the interferogram, allowing the underlying titanium to be removed subsequently by wet-etching. A Lloyd&’s mirror interferometer was used in conjunction with a frequency-doubled argon ion laser (244 nm). By varying the number of beams, the number of exposures, the angle between the sample and mirror in the interferometer and the amount of rotation of the sample between exposures, it is possible to fabricate a wide variety of Ti structures, including nanowires and nanodots, with varying full widths at half maximum (FWHM) and periodicities. Ti wires with dimensions ranging from 1um to 24 nm having been produced. NIL also yields structures over macroscopic areas. A titanium methacrylate monomer solution is spin-coated onto the substrate, imprinted and calcined, to yield Ti nanostructures. In both IL and NIL the Ti-free silica substrate regions are passivated with a protein-resistant oligo(ethylene glycol) functionalised silane, enabling protein to be adsorbed selectively to the titania regions. Once the protein has adsorbed onto the TiO2 pattern it can be easily removed by exposing the sample to light from a HeCd laser (325 nm), which causes photocatalytic breakdown of the biomolecule while leaving the passivating silane film on the silica regions intact. The regenerated titania surface may be used to adsorb a second protein and the process repeated multiple times, providing a very useful tool for biologists exploring fundamental problems associated with the molecular-scale organisation of proteins at interfaces.

Organic ligands are widely used to enhance the ability of CdSe quantum dots (QDs) to resist photodegradation processes such as photo-oxidation. Because long alkyl chains may adversely affect the performance of QD devices that require fast and efficient charge transfer, shorter aromatic ligands are of increasing interest. We show that small aromatic thiol ligands, particularly ones bearing an amino-electron donating group can be very effective in reducing photodegradation of CdSe QDs. Using photoluminescence and density functional theory calculations, we show that the enhanced stability results from hole transfer from the QD to the ligand and delocalization of the resulting positive charge on the aromatic ring and amino group instead of the sulfur atom that links the molecule to the CdSe (published in J. Phys. Chem. C, 117, pp 313-320). We further extended the photostability studies to phenyl dithiocarbamate (DTC) ligands on CdSe surfaces. First, we characterized the formation of these DTCs on the CdSe surface and with a series of para-substituted phenyl DTCs, we subsequently show that ligands bearing electron-donating substituents are the most effective in enhancing the photostability of CdSe QDs. A comparison of the QD photostability resulting from use of ligands bearing DTC versus thiol binding groups shows that the DTC group provides greater QD photostability.

Zinc oxide nanowires (ZnO NWs) have extensively been studied for their use in energy conversion devices due their unique physical properties, wide bandgap, and ease of synthesis with abundant resources. Device implementation of ZnO NWs has, ideally, controllable growth directly on conductive substrates such as copper. We synthesize an efficient hierarchical structure composed of an electro-initiated polymerization of acrylamide and acrylonitrile as carbon source on ZnO NWs grown directly on Cu foil current collector via solution based methods (hydrothermal and electropolymerization). We report an almost two-fold enhanced UV photodegradation of methylene blue by this carbon coated ZnO nanostructure compared to ZnO counterpart. We will discuss the enhancement mechanism of carbon doping in ZnO NWs. The realization of this carbon-ZnO hierarchy can be applied in the health sector as toxin decomposition and water purification processes, or in dye sensitized solar cells to increase their efficiency.

When metal nanoparticles are exposed to electromagnetic radiation, they produce heat due to the surface plasmon resonance. This conversion of electromagnetic energy to thermal energy is called the photothermal effect. The heat produced by metal nanoparticles could be used to control the properties of the medium they are embedded in such as a polymer matrix. Our research involves the study of the response of gold (Au) and silver (Ag) nanoparticles embedded in poly(ethylene oxide), PEO, under various conditions.
Au and Ag nanoparticles were synthesized in a refluxing oil solvent in the presence of oleylamine and oleic acid as surfactants. The synthesized nanoparticles were characterized by Dynamic Light Scattering (DLS), UV-Visible Spectroscopy and Transmission Electron Microscopy (TEM) experiments. The average size of both Au and Ag nanoparticles was found to be 8.9±2.7 and 8.4±1.7 nm, respectively. The Au/PEO and Ag/PEO nanocomposites were prepared by solution casting. The concentration of Au and Ag in PEO was varied between 0.01 and 2% by mass. Mechanical and thermal properties of the nanocomposites were studied by both static and dynamic tests with Dynamic Mechanical Analysis (DMA). The Young&’s modulus was found to increase with increasing nanoparticle concentration, however the modulus values reached a plateau at high nanoparticle concentrations. The glass transition temperature of the nanocomposites decreased with nanoparticle concentration. Dispersion of nanoparticles was found to be one of the most important factors affecting the thermomechanical properties of the nanocomposites.
The photothermal effect of Au nanoparticles was characterized using Raman spectroscopy (WITec alpha300 with 532 nm wavelength and 16 mW power). To correlate the temperature of the sample with radiation duration, Raman peak characteristics obtained at known temperatures were used as calibration data. Results showed that temperature changes obtained during photothermal heating could be calibrated with a priori knowledge of Raman peak characteristics at known temperatures. However, the calibration cannot be performed at temperatures above the melting temperature of PEO.

The semiconductor photocatalysts have attracted great interest over the past decade because of their unusual optical propertie and their application as alternatives for hydrogen generation, water disinfection and purification. The photocatalytic process originates from the generation of charge carriers under UV irradiation, producing electrons and holes in the conduction band and valence band, as a result of photoexcitation of semiconductors. Nevertheless, a high e^minus; -h⁺ recombination rate (~10 ns) certainly reduces the quantum efficiency and represents the major drawback for photocatalysis. Therefore, various strategies have been adopted to decreasing the recombination rate of electron-hole pairs by careful design of materials at nanoscale level, including: (i) chemical doping with elements that are able to alter the electronic structure and charge-transfer processes; (ii) coupling metal nanoparticles on semiconductor surface to improve electronic properties . Several authors have reported the synthesis of doped semiconductors materials using numerous techniques. One of the most interesting wet-chemical methods for the synthesis of lead-based nanometric materials was developed by Camargo and Kakihana , which they called the oxidant peroxo method (OPM). This wet-chemical method the hydrogen peroxide is replaced by an inorganic peroxo complex, such as peroxytitanato [Ti(OH)₃O₂]^-. The reaction with bismuth ions occurs in the presence of peroxo-complex with the formation of a non-crystalline yellow precipitate. This precipitate can be described as a mixture of amorphous Bi₂O₃ and TiO₂ at the low temperature, which can be obtained at any desired molar ratio of Bi:Ti, free of all the contaminants commonly found in materials synthesized by other chemical routes. In this sense, the objective of this study was to evaluate the properties and the photocatalytic activity of Bi_12-La_xTiO₂₀ and Bi_12-xPb_xTiO₂₀ (x= 0.025, 0.05, 0.10 and 0.15) prepared by oxidant peroxide method (OPM). The morphology and microstructure of the photocatalyst were characterized using X-ray diffraction (XRD), Raman spectroscopy, Diffuse Reflectance Spectroscopy (DRS-UVvis), Scanning Electronic Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). The La and Pb doped Bi₁₂TiO₂₀ nanoparticles exhibited a pale yellow color. The UV-vis diffuse reflection spectra of materials, which was calcined at 700 °C for 1h, showed a continuous and tailing absorption in the visible region. XRD patterns and Raman spectroscopy of all materilas showed the presence of sellenite phase. The photodegradation of rhodamine B dye showed a better efficiency for the bismuth titanate doped with lanthanum having approximately 90% descoloration solution with 210 min. However with the material doped with lead was a large decrease of photocatalytic activity of the materials presented only 50% of discoloration of the solution with 210 min. Work supported by FAPESP (2010/20129-0), CMDMF, CNPq and CAPES.

ZnSn(OH)₆ (ZHS), a perovskite-structured hydroxide, is a semiconductor which presents a band gap of 4,3 eV. Despite its high band gap value, some works have shown its potential as photocatalyst for water treatment.¹ However, few studies about the synthesis and application of ZHS have been performed so far. Therefore, the aim of this paper was the synthesis of Zn based hydroxides structures by the sol gel method and the comparison of their photocatalytic properties. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and N₂ physisorption to determine specific surface area (SSA). The synthesis of Zn based hydroxides were performed in the same conditions, only varying the presence of Sn²⁺. XRD analysis showed that in the synthesis performed without Sn2+, Zn(OH)₂ phase was formed, whereas in a synthesis environmental containing Sn²⁺, pure ZnSn(OH)₆ phase was formed. As could be seen by SEM analysis, there was remarkable difference in the particles morphology. Zn(OH)₂ showed anisotropic structure. On the other hand ZnSn(OH)₆ showed well-faceted cubes morphology, in agreement to described in the literature. ¹ Both materials presented similar specific surface area of 33.8 and 38.3 m²/g for Zn(OH)₂ and ZnSn(OH)₆ respectively. Rhodamine B (RhB) photodegradation was used as a probe reaction to access materials photoactivity and we used the method described by Ishibashi et al.² to measure the rate of OH radical formation during UV irradiation. ZnSn(OH)₆ was the most active specie in RhB photodegradation. The rate constant of RhB photodegradation in the reaction catalyzed by the ZnSn(OH)₆ samples was approximately 7 times higher than the rate observed for RhB self-photolysis and approximately 2 times higher than the rate observed for RhB photodegradation catalyzed by Zn(OH)₂. Interestingly, sample&’s photoactivity showed the same trend compared to the rate of OH radical formation. In this sense, we can propose that the mechanism of RhB degradation catalyzed by the as synthesized samples followed by an indirect process, i.e., oxidation of water or hydroxyl groups attached onto photocatalyst surface, OH radical formation and dye oxidation by those radicals, instead direct dye oxidation over semiconductor particle.
Acknowledgments: This work was supported by CNPq and FAPESP.
References
1 - FU, X.; et al. Hydroxide ZnSn(OH)₆: A promising new photocatalyst for benzene degradation. Applied Catalysis B: Environmental. v. 91, p. 67-72, 2009.
2 - ISHIBASHI, K.; et al. Detection of active oxidative species in TiO₂ photocatalysis using the fluorescence technique. Electrochemistry Comunications. v.2, p. 207-210, 2000.

Titanium dioxide is a wide band-gap semiconductor (3.2 eV) with strong absorption in the UV region. Used commercially in sunscreen, paints, and ceramincs, current research focuses on energy and environmental applications of TiO2. Recent studies suggest that {001} facet-dominated anatase TiO2 crystals exhibit superior capabilities in photocatalytic degradation of organic pollutants and water splitting. However, powders are less useful for practical applications because the TiO2 must later be separated from the mixture. Thus, we have focused on preparing {001} faceted anatase thin films.4 Anatase films exhibiting ~100% (001) reactive facets at the surface were grown hydrothermally on gold substrate from homogeneous solutions of 50 mM TiF4 and varying amounts of NH4F. The resulting polycrystalline anatase films are continuous, range from 0.2-1.0 mu;m thick, and evenly coat the substrate. As the amount of NH4F in solution increases, SEM shows that the facets progress from flat square facets to rounded ones with potentially greater surface area and reactivityGrazing angle XRD measurements show that the films exhibit a high degree of preferred orientation with the c-axis normal to the substrate surface. In order to understand the photocatalytic properties of the TiO2, thin films were placed in a solution of terephthalic acid and then photolyzed with 365 nm light. Photolysis of TiO2 in water produces hydroxyl radicals (OH) that can be measured by by fluorometric assay of the TA-OH adduct . Measurements of hydroxyl radical concentration were taken every 10mins for the first hour, every 20mins for the second hour, and every 30 minutes for the final hour. It was found {001} faceted anatase films produce a significant amount of OH comparable to the photocatalytic efficiencies of P25 (standard TiO2 reference) when normalized for surface area. Anatase films thus have promise to remove organic and biological contaminants to produce clean water in point of use applications.

Chalcogenide glasses are semiconducting materials containing sulfur, selenium, or tellurium. A unique aspect of these elements is the availability of lone pair electrons that can easily react with light, creating an electron-hole pair by which changes in the chemical and optical properties of the material occur. These changes can be applied to the formation of diffraction gratings. Recently the effect of relief formation in As2S3 due to light illumination has been reported. It opens opportunities for novel applications and the importance to study other members of the chalcogenide glass family, specifically the Ge containing films, which have the best thermal stability among these glasses. Studies of the surface relief gratings from direct holographic recording have been conducted in amorphous Ge-Se chalcogenide thin films and bulk materials. A He-Ne gas laser and laser diode, configured in opposite polarizations, were used to create relief formation on the surface of the chalcogenide material. The addition of a second radiation source with orthogonal polarization increases the efficiency of the recording process with respect to time by which relief gratings are formed. The efficiency of relief formation was observed by measuring the change in profile height by varying exposure time. Experiments were conducted using Si and glass substrates to determine the relationship between the reflective index and thermal conductivity of the substrate, and the resulting surface relief grating. Atomic force microscopy was used to measure photoinduced volume changes, and nano-indentation measurements were conducted to determine the hardness difference between the trough and crest of the profile. Structural data of the grating was collected using Raman Spectroscopy, and the composition of the grating was characterized by Energy Dispersive Spectroscopy. The studies indicate Ge20Se80 to be the most sensitive materials for grating formation, which is related to the floppy character of the material structure. The results are discussed based on the structural characteristics of the studied glasses and the chemical bonds which are broken during formation of the grating. Electron exchange is discussed as a mechanism for grating creation and volume change.

The selective oxidation of alkanes to more valuable products is of great importance in the chemical and pharmaceutical industries and, thus, many scientists and engineers are involved in the challenging task of preferential oxidation at specific C-H bonds. Attempts have been made to improve the terminal oxidation selectivity through rendering spatial constraints on n-alkanes. Thomas et al. have demonstrated the oxidation of terminal CH3 and penultimate CH2 of linear n-alkanes (C5-C8) by regioselective catalysts using O2 in a liquid-phase reaction under mild conditions (373-403 K). Molecular sieves CoAlPO-18 and MnAlPO-18, in which Co and Mn were introduced into the framework of aluminophosphate (AlPO), showed unprecedented high oxyfunctionalization selectivities (sim;65.5%) at the terminal CH3 group when conversion was kept low (sim;7%). In contrast, Modeuml;n et al. could not observe any specific regioselectivity in the catalytic oxidation of n-hexane and cyclohexane using MnAlPO-5 or MnAlPO-18.7 Their terminal oxidation selectivity of 7-8% was similar to that predicted from relative C-H bond energies in n-hexane.
In a completely different approach, we have discovered that photocatalytic gas phase oxidation of long chain alkanes by molecular O2 can produce OHC with mesoporous TiO2-SiO2 mixed oxide photocatalysts, in which Ti is ultimately dispersed in atomic scale under UV light irradiation. In particular, we obtained the surprisingly high terminal selectivity of 80.5-92.9% among formed aldehydes, carboxylic acids and ketones (37-62% of the products) at higher 29.1-47.2% conversion of dodecane because these oxygenated products were stabilized by diluting contiguous Ti sites present on the surface of SiO2 matrix. As we selected dodecane with 12 carbons or a 1.4 nm-long straight chain as a substrate, the synthesis of mesoporous TiO2-SiO2 mixed oxides was critical as pores were required to be big enough for dodecane to penetrate into the inner pores freely. We realized the synthesis of the mesoporous TiO2-SiO2 mixed oxides by adopting a relatively smaller tetramethyl ammonium hydroxide solution (TMAOH) molecule compared with the one used in our previous work1 as a structure directing agent. In addition to the extraordinarily high selectivity for the oxyfunctionalization of the terminal methyl groups, the results have a fundamental significance because the photocatalytic oxidation of organic compounds has been traditionally used for total oxidation or mineralization for environmental protection rather than selective partial oxidation.dagger;
1. J. Y. Kim, Y. H. Kim, S. Han, S. H. Choi and J. S. Lee, J. Catal., 2013, 302, 58-66.
dagger; Green Chem., DOI: 10.1039/c3gc41343h, First Published Online 12 SEP 2013.

The photocatalytic reduction of CO2 has long been an attractive route to generating useful compounds (e.g. formic acid, formaldehyde, methane, methanol, among others). The ability to achieve this using sunlight is of particular interest in order to develop a process that works with renewable energy sources. Inorganic compounds, such as TiO2, ZnO, WO3, SiC, CdS, ZnS, CaFe2, Ti-MCM-41, as well as organic compounds such as transition-metal complexes, have been studied under UV-light as potential materials for CO2 reduction. Metal doped, dye sensitized and coupled semiconductors have been preferred using visible light [1].
In particular, zinc sulfide can be considered as a good candidate for CO2 reduction due to comparatively high potentials of conduction band electrons and valence band holes (ECB= -1.75 V, EVB= + 1.85 V, NHE), with respect to other semiconductors. However, this material can only be activated with UV light. Therefore, a strategy for its use with visible light, is the addition of metal nanoparticles who act as sensitizers. In this work, a novel reductive photodeposition of metal nanoparticulas (M= Ag, Cu or Ni) on ZnS was employed to improve the electronic and photocatalytic properties of ZnS under visible light.
This method consists in two steps, in the first one, is carried out the metal precursor
(acetylacetonate) photolytic decomposition and second, it performs the photocatalytic reduction of metal ion assisted by the ZnS, both processes in presence of UV-light and ethanol. The obtained photocatalysts were characterized by XRD, UV-Vis diffuse reflectance, photoluminescence, XPS and HRTEM and tested in the photocatalytic reduction of CO2.
According to the results, pure, homogeneous and well distributed Ag, CuO and NiO nanoparticles deposited on ZnS (mean particle size of 2-8 nm) were obtained by the photodeposition method. All prepared materials were active for the photocatalytic reduction of CO2 to methane and methanol.
[1] Renew. Sustain. Energy Rev., 25 (2013) 560.

The negative electron affinity of diamond makes it uniquely suited as a facile emitter of photoelectrons. While electron emission from diamond in vacuum has long been studied,we show that llumination of diamond with ultraviolet light results in direct emission of electrons into water, lead to solvated electrons. These solvated electrons are capable of induced new, high-energy reactions such as the fixation of nitrogen and the reduction of CO₂. The formation of solvated electrons in water can be detected directly via transient absorption spectroscopy. Our results show that the ability to reduce N₂ to NH₃ is directly correlated with the negative electron affinity of diamond. A unique aspect of this approach is that instead of requiring molecular adsorption at the surface, we are able to directly inject electrons into a reactant medium where they can initiate reduction reactions not accessible to traditional electrochemical processes.

The development of semiconductor-based photochemical systems capable of mimicking the natural photosynthesis by driving useful chemical transformations has attracted significant interest motivated by the need to meet various environmental concerns and to secure the future supply of clean and sustainable energy. The talk will focus on two very different examples of artificial photosynthetic systems from our recent research. The first example are hybrid photoanodes for solar-driven water splitting. Mainly our attempts to improve the efficiency of kinetic charge separation and the coupling between the absorber and different co-catalysts will be discussed [1-4]. The second example will be rather unconventional, it will deal with surface-modified photocatalysts prepared by a photosynthetic route involving visible light-induced activation of benzene or toluene at the surface of TiO₂. Surprisingly, such photo-synthesized photocatalysts exhibited enhanced performance in degradation of phenolic water pollutants [5-6]. Mechanistic details of the photosynthetic procedure as well as of the enhanced photocatalytic performance will be elucidated.
References
[1] M. Bledowski, L. Wang, A. Ramakrishnan, A.; O.V. Khavryuchenko, V.D. Khavryuchenko, P.C. Ricci, J. Strunk, T. Cremer, C. Kolbeck, R. Beranek, Phys. Chem. Chem. Phys.2011, 13, 21511
[2] L. Wang, M. Bledowski, A. Ramakrishnan, D. König, A. Ludwig, R. Beranek, J. Electrochem. Soc.2012, 159 (7), H616.
[3] M. Bledowski, L. Wang, A. Ramakrishnan, A.; O.V. Khavryuchenko, A. Bétard, R. Beranek, ChemPhysChem2012, 13, 3018.
[4] M. Bledowski, L. Wang, A. Ramakrishnan, R. Beranek, J. Mater. Res.2013, 28, 411.
[5] A. Ramakrishnan, S. Neubert, B. Mei, J. Strunk, L. Wang, M. Bledowski, M. Muhler, R. Beranek, Chem. Commun.2012, 48, 8556.
[6] S. Neubert, A. Ramakrishnan, J. Strunk, H. Shi, B. Mei, L. Wang, M. Bledowski, D. A. Guschin, M. Kauer, Y. Wang, M. Muhler, R. Beranek, ChemPlusChem, in revision.

Photocatalysis is a well-established and growing area of research and innovation. It is also the basis of a number of commercial products, including: self-cleaning/easy clean glass, tiles, concrete, paints and fabrics. In this talk the essential feature of these commercial materials will be discussed briefly along with rapid methods of assessing their activities in the lab and field, using smart inks. In addition, photocatalysis can be used to create a range of novel, smart inks, such as those for the detection and measurement of: oxygen, time and UV. The first two have commercial promise in food packaging and the latter can be sued as a sun-burn warning indicator. The basic principles of operation of these different indicators will be discussed briefly.

Photoelectrochemical water splitting is an important technology for directly conversion from solar to chemical energy. A semiconductor for a photo-illuminating working electrode and a metal for a counter electrode are electrically connected and dipped into an electrolyte in order to split water via the reaction. The carrier transfer between a reactant in an electrolyte and an electrode is basically explained by the energy matching of the redox potential of the reactant and the carrier state of the electrode by the Gericher model. For the metal electrode, the carrier transfer is easy to occur due to the Fermi energy level can be controlled by an applied bias, and matched the chemical potential of the reactant. The energy of the conduction and valence band edge at the surface of the semiconductor is, however, pinned due to the surface adsorption molecule. This indicates that the carrier transfer between an electrolyte and a semiconductor electrode requires that the band edge energy satisfying the Gerisher relationship even changing the applying bias to the electrode. Especially for the wide band gap semiconductors like TiO₂, ZnO and GaN, the valence band edge energy is much more negative (positive potential) than that satisfying the relationship. Therefore, the photo-induced anodic current is explained to flow via surface intermediate levels. The experimental investigation is not easy because the intermediate level detection technique has not been proposed. Recently, the authors found that the photoluminescence of the semiconductor is one of the detection techniques of the levels. Thus, the properties of the intermediate levels related to the anodic photo-current of n-type GaN are discussed in this report.
The higher photocurrent density and the longer lifetime have been observed using the lower Si-doped (lower carrier concentration) n-type GaN photo-illuminated anodic electrode by the authors. Thus, the photoluminescence properties were investigated excited by 325 nm He-Cd laser at room temperature. The intermediate level related to the n-type GaN anodic photo-current is so called “yellow luminescence”. The luminescence was extremely long and over “ms” order for the longer lifetime case. The carrier lifetime did not show single exponential decay, thus, the life time was not defined. The intensity of yellow luminescence at 0.1 ms after the excited light turn-off was proportional to the photocurrent density and independent of the before or after the photoelectrochemical reaction. The ratio of the band edge emission and the yellow luminescence (not only the intensity of the yellow luminescence) was also proportional to the photocurrent density. These results show that not only the carrier lifetime at the intermediate state but also the electron-hole separation mechanism probably relates the carrier transfer process via intermediate levels for the photoelectrochemical reaction.

Solar energy is the ultimate solution that can provide carbon-neutral and sustainable energy to mitigate current and future global energy appetite. Solar energy needs to be stored to make it highly distributed from small to large scale practical applications. Mimicking the natural photosynthesis process, solar energy can be stored in the form of chemical energy through solar-water splitting. However, the energy-conversion machine of green plants is not efficient enough to satiate human energy appetite. Therefore, an artificial photosynthesis system which converts solar energy into chemical fuels in an efficient way is an urgent need. One of the main challenges in artificial photosynthesis is the production of stable H2 and O2 from water splitting in a way which is much efficient than that of green plants. Therefore, high efficiency and stable water splitting under has been of great challenge over the last four decades. Recently, metal nitrides (i.e., GaN, InGaN) have attracted considerable attention, as they possess the thermodynamic and kinetic potential requirements for water splitting. On the other hand, we have recently shown that one-dimensional (1D) GaN nanowires offer significantly improved photocatalytic activity over commonly used GaN nanoparticles for overall water splitting. Moreover, the surface Fermi level and therefore carrier transport properties of nanowires can be tuned by controlled dopant (Mg) incorporation, which can further enhance the photocatalytic activity. In this context, we have studied the effect of Mg ion dopants on the water splitting activity of GaN nanowire arrays in presence of sacrificial reagent.
GaN nanowires are grown on Si (111) substrate using radio frequency plasma assisted molecular beam epitaxy (MBE). The as-grown nanowires are doped divalent (Mg2+) ions to make it p-type. The doping density and therefore surface Fermi level is tuned by controlling the effusion cell temperature of Mg. The photocatalytic activity of Mg doped GaN nanowires are tested by individually performing H2 and O2 half reactions in the presence of respective sacrificial reagents. The H2 half reaction demonstrates significantly enhanced photocatalytic activity for Mg doped GaN nanowires, compared to non-intentionally doped nanowires. The enhanced activity of Mg doped nanowires is further correlated with X-ray photoelectron spectroscopy (XPS) valence spectrum and micro-Raman analysis, which indicate the presence of downward band bending at the nanowire surface. The doping density is further optimized to achieve high efficiency water splitting. Additionally, we will report on water splitting with below bandgap excitation of GaN. A detailed correlation between the enhanced photocatalytic activity and Fermi level tuning, due to controlled dopant incorporation is being investigated and will be reported.
[1] Kibria, M. G. et al. ACS Nano, 7, 7886 (2013).
[2] Wang, D. et al. Nano Lett. 11, 2353 (2011).

GaN is a chemically very stable wide bandgap semiconductor which holds great promise for future applications in photocatalysis and biocatalysis. This is due to the very favourable positioning of the GaN valence and conduction band edge energies with respect to the Fermi level of important catalytic metals such as Pt, Pd, or Rh and to many redox levels of liquid electrolyte systems. In addition, the alloying of GaN with Al or In increases the range of accessible energies even more, covering the entire energy range from the vacuum level to about 6 eV below the vacuum level. Finally, GaN and most of its alloys can be doped both p-type and n-type, providing a unique tunability of the Fermi level position.
In this presentation we will discuss fundamental prospects of GaN-based photocatalysis as well as recent efforts to make use of the flexibility of the GaN material system to control the catalytic activity of Pt nanoparticles on GaN substrates in a model reaction (ethene hydrogenation).

In the last decades, much research has been devoted to developing the so-called hydrogen economy in which hydrogen gas is used as a clean fuel for various energy needs. Current methods for hydrogen fuel generation rely predominantly on processing fossil fuels, so more sustainable hydrogen harvesting schemes are clearly needed. One way to attain this goal is to split water into its constituent hydrogen and oxygen components via solar-driven photocatalysis. Much research in this area has focused on the development/investigation of novel materials with optoelectronic and physical properties that yield efficient absorption of the solar-spectrum, favorable electronic energy level alignment with the redox potentials of water, and resistance to degradation. These desired properties are difficult to balance: for example, materials with bandgaps in the visible region of the spectrum tend to degrade fast under typical photoelectrochemical (PEC) conditions, while those that resist degradation tend to have large bandgaps, and are transparent across most of the solar spectrum. In this work, we investigate nanowires (NWs) grown from gallium nitride (GaN) as a potential solution to these conflicting requirements. Although bulk GaN has a bandgap (3.4 eV), the NWs exhibit strong absorption and fluorescence emission across the entire visible spectrum. Preliminary photoluminescence experiments and density functional theory calculations points at surface states as the main reason for this sub-bandgap absorption and luminescence.. Furthermore, the NWs surface terminations can be synthetically controlled during MOCVD growth, resulting in different crystallographic surface terminations with different electronic structures and thus optical properties. We are using high-resolution microscopy techniques (SEM,TEM, AFM, confocal spectroscopy) to determine the energetics and dynamics of the surface states associated with the various crystallographic terminations, and their relative overlap with the redox potentials for converting water into hydrogen and oxygen. In the future, NWs will be decorated with gold nanoparticles or other small molecules to tune the rates and efficiency for water-splitting.

Due to its mechanical stability and chemical inertness, silicon carbide (SiC) is a promising material for electrochemical applications involving harsh environments. Different mechanisms for the functionalization of its surfaces with self-assembled monolayers (SAMs) have been developed. Covalent attachment of organosilane and organophosphonate SAMs are particularly relevant for bioelectronic and biosensing applications, but also for chemical and electrical surface passivation and stabilization.
In this work, we carry out a (photo)electrochemical characterization of n-type 4H-SiC and 6H-SiC electrodes in aqueous electrolytes. Cyclic voltammetry and electrochemical impedance spectroscopy measurements are conducted to examine fundamental properties of the SiC electrodes such as the flatband potential, which is essential for determining the energetic positions of the conduction and valence band edges at the semiconductor/electrolyte interface. Further, SAMs are covalently grafted to SiC electrodes and the influence on the capacitive behavior of the SiC substrates is analyzed as well as changes in charge transfer processes across the semiconductor/electrolyte interface. Complementary investigation of surface and interface properties, such as near-surface band bending and changes of interfacial dipoles upon chemical binding, is performed by contact potential difference and surface photovoltage measurements. Finally, the photoelectrochemical characteristics of SiC electrodes under UV illumination in the presence of suitable redox couples are determined. At anodic bias potentials, photo-generated holes in the depletion region of the semiconductor are driven to the interface by the surface band bending and an anodic photocurrent is observed. Furthermore, hole transfer to the reduced component of the redox couple leads to a reversible charge transfer process between the SiC electrodes and the redox active species in solution.
The results of this study provide a fundamental understanding of charge transfer processes across the SiC/electrolyte interface, which is essential for a possible utilization of SiC in photoelectrochemical cells for applications in photocatalysis.

In this talk, the synergies between electro- and photocatalysis will be underlined with the dioxygen reduction and hydrocarbon oxidation as representative examples. The importance of conjugate processes occurring at the counterelectrode will be emphasized in both cases. Approaches based on the use of electrodes and photoelectrodes as well as those based on the use of colloidal suspensions will be compared and contrasted. Ideas on how we can learn from the intricate self-assembled architectures that Nature has evolved over millions of years, will be discussed with specific examples
The talk will then turn toward a discussion of work in the author&’s laboratory on the use of carbon and oxide semiconductor nanocomposites for driving catalytic processes of interest both in the dark and under irradiation of the oxide semiconductor component. The reactions of interest here include dioxygen reduction and the reduction of carbon dioxide to fuel such as methanol. The role of the nanocomposite components and their complementary functionality within the material architecture will again be discussed within the context of systems that Nature has evolved.
Finally the talk will turn toward the very recent work in the author&’s laboratory on the solar photoelectrosynthesis of methanol from CO2. This process was driven on hybrid CuO/Cu2O semiconductor nanorod arrays for the first time at potentials ~800 mV below the thermodynamic threshold value and at Faradaic efficiencies of ~95%. The CuO/Cu2O nanorod arrays were prepared on Cu substrates by a two-step approach consisting of the initial thermal growth of CuO nanorods followed by controlled electrodeposition of p-type Cu2O crystallites on their walls. No co-catalysts (such as pyridine, imidazole or metal cyclam complexes) were used contrasting with earlier studies on this topic using p-type semiconductor photocathodes. The roles of the core/shell nanorod electrode geometry and the copper oxide composition were established by varying the time of electrodeposition of the Cu2O phase on top of the CuO nanorod core.

Mixed metal oxide photocatalysts and composite photocatalysts were synthesized by a sol-gel based citrate route or low cost molten salt methods, being advantageous over solid state reactions providing highly crystalline materials at moderate calcination temperatures, ensuring particle sizes smaller than 100 nm.
In case of the layered perovskite Ba5Ta4O15, no or very small amounts (0.0125 wt.-%) of Rh are needed as a co-catalyst to achieve high amounts of evolving hydrogen from alcoholic solutions, but also from pure water using Rh/Cr2O3 core-shell co-catalyst. The citrate route for this material can be further tuned to realise a composite photocatalyst material, exhibiting enhanced photocatalytic activity due to improved charge carrier separation.
CsTaWO6 was for the first time prepared in a sol-gel synthesis approach to decrease its particle diameter and increase its specific surface area, reducing the recombination probability of photogenerated charge carriers leading to improved photocatalytic activities. Sol-gel-derived CsTaWO6 was calcined over a wide range of temperatures (600-850 °C) and times (4-20 hours), and the mild reaction conditions yield smaller primary particle sizes (<50 nm) and larger specific surface areas than the conventional solid state reaction (SSR), leading to improved hydrogen evolution.
Composites with a phase composition of either cubic α-CaTa2O6/hexagonal Ca2Ta2O7, or cubic CaTa2O6/hexagonal Ca2Ta2O7/orthorhombic β-CaTa2O6, or cubic α-CaTa2O6/β-orthorhombic CaTa2O6 showed drastically enhanced photocatalytic activity, which is attributed mainly to the significantly improved separation of photo-excited charges via the junctions and interfacial contacts. Pure water splitting was achieved by using NiO as co-catalytst, resulting in stoichiometric amounts of hydrogen and oxygen.
To ensure the formation of clean H2 from photocatalytic processes without the necessity of expensive gas purification, we developed a two-compartment cell working without any external electrical or chemical bias. Moreover, tailored composite polymer membranes act as compartment separator as well as support for electrodes and photocatalyst. Stable H2 rates were produced from pure water in a separated half-cell, low concentrated hydrochloric acid enhances the photocatalytic hydrogen generation proving the efficient proton transport via the composite membrane.

Heterogeneous photocatalysis is recognized as possibly one of the cheapest and most efficient methods for solar decontamination of water and air from toxic organic pollutants. However, real-life applications of photocatalytic water treatment are still rather scarce, particularly since the photocatalytic degradation rates are not high enough.
In this context it is important to realize that the oxygen reduction reaction is essential for achieving high rates in environmental photocatalysis [1-2]. Deposition of suitable co-catalysts able to catalyze two-electron reduction of dioxygen can render the transfer of electrons from TiO₂ to oxygen more efficient and thus significantly enhance charge separation and diminish recombination. Apart from well-known work on expensive Pt and Pd co-catalyts, very recently some promising results on enhanced rates of photocatalytic degradation of 2-propanol in gas phase on TiO₂ particles decorated with small Fe₂O₃ and CuO clusters have been reported [3-4].
In our own work, we impregnated rutile TiO₂ with FeO_x and CuO_x clusters and observed significantly enhanced rates of photocatalytic degradation of 4-chlorophenol in aqueous phase. The preparation, structural properties, and mechanistic investigations elucidating the reasons of the photocatalytic enhancement will be presented [5].
References
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[5] S. Neubert, P. Pulisova, C. Wiktor; P. Weide, B. Mei, D. A Guschin, R. A. Fischer, M. Muhler, R. Beranek, Catal. Today, submitted.

To date, most studies on the development and understanding of electrodes for use in a water-splitting photoelectrochemical cell have been performed using simple binary systems. However, there are a far greater number of ternary systems that have not been studied extensively although they are predicted to be potentially excellent photoelectrodes. Ternary systems can also offer more possibilities for band gap and band position tuning. In this presentation, we report our recent efforts on the electrochemical synthesis of photoelectrodes based on ternary oxides (both n-type oxides that can serve as photoanodes and p-type oxides that can serve as photocathodes). Various synthesis strategies to tune doping levels and morphologies will be presented. The systems we will discuss include a BiVO₄-based photoanode that achieves a current density of 2.73 mA/cm² for water oxidation in a stable manner under AM 1.5G, 100 mW/cm² illumination using a record low applied bias of 0.6 V vs. RHE. We will also discuss interfacing these photoelectrodes with appropriate oxygen evolution and hydrogen evolution catalysts. Critical issues for optimizing the photoelectrode/catalyst interface as well as the catalyst/electrolyte interface will be examined in detail.

Nanostructured metal oxides such as tin-doped indium oxide (ITO) or reduced oxides (e.g. substoichiometric Ti-Oxides) are of interest for diverse applications and fundamental research, based on their electronic conductivity and distinct electronic properties. If thin films of ITO, ATO (antimony-tin oxide), and other oxides are additionally endowed with nanoscaled porosity, they can be used for electrochemical applications, e.g. the immobilization of electroactive species such as redox-active enzymes, or as host materials for light-absorbing semiconductor building units, for (photo)electrochemical applications.
In particular, substoichiometric oxides (e.g. Ti-Oxides) are discussed as conducting matrices in battery applications, serving as alternative for carbon. Additionally, strongly reduced Ti-Oxides (so-called Magnéli-phases) absorb the full spectrum of visible light, exhibiting semiconductor behaviour for photocatalytic and photoelectrochemical hydrogen generation. Substoichiometric Ti0.91O2-nanosheets can be prepared in the form of electrospun titanate nanofibers for photocatalytic and photoelectrochemical applications. Those nanosheets are ideal starting materials for band gap engineering, improving the light absorption into the visible-light range by non-metal doping.
The presentation will review recent process in the sol-gel based synthesis, aiming at generating nanostructured thin films and hierarchical pore structures, taking advantage of suitable organic templates. In particular, electrochemical properties and applications of mesostructured transparent conducting oxide films (ITO, ATO) are presented. It is discussed if indeed nanostructuration results in advanced properties, focusing on photoelectrochemical devices and properties.

The fabrication of strong photocatalysts applied to the degradation of organic pollutants is necessary in environmental applications. In a single-stage method, acetate precursor and poly vinyl pyrolydine are used to produce ZnO nanostructures with various morphologies in annealing temperatures ranging from 300 oC to 900 oC. The physical properties of the prepared nanostructures were characterized by SEM, TEM, XRD, BET, DRS, CHN analysis and PL spectroscopy. The SEM images exhibit a variety of the as-prepared hexagonal zinc oxides including wires, rods, particles and porous network of welded particles of ZnO nanoparticles. The results of the photocatalytic degradation of methylene blue as an organic dye in aqueous suspension showed that the morphology of ZnO nanostructures influences on the photocatalytic efficiency of ZnO nanostructures, greatly. For the best result, the highest MB degradation occurs by ZnO nanowires within 16 minutes and in others samples, degradation of higher than 95 percent occurs within 20 minutes. The XRD and PL spectroscopy revealed neither VZn nor Oi are in all of samples but only VOminus;, VO2minus; and Zni exist in ZnO nanostructures.

Layered perovskites of the Aurivillius structure type were chemomechanically exfoliated into two dimensional perovskite nanosheets without the use of organic exfoliation agents. A unique aspect of these materials is that they contain a large fraction of Bi in the perovskite sheet (~20%). The freely-suspended nanosheets produce hydrogen from methanol solution at a rate of 41,000 micro-mol/h/g, without any added co-catalysts. The sheet layer thickness can be varied from 2 to 5 octahedral units, and comparisons among the 2-, 3-, 4- and 5-layer nanosheets show no trends, suggesting that quantum confinement effects are similar between the layers of thickness 0.8 to 2 nm. Inelastic neutron scattering shows that the oxide nanosheets are proton terminated with hydration of those protons, producing hydronium ions. The Zundel cation (H5O2+) is found as the dominant hydrated proton and we suggest a dynamic surface characterized by exchange of the terminal protons on the niobate sheet and hydrated protons formed in the presence of water.

Charge transport in mesoporous and high surface area semiconductor electrodes has been extensively investigated, particularly in the field of dye sensitized solar cells. For instance, the transient time of injected electrons was predicted to be significantly shorter in oriented TiO2 nanotubes than in sintered TiO2 nanoparticle films. However, this expectation does not appear to be supported by experimental evidences [1]. The picture could be even more complex in the case of photogeneration of carriers in the semiconductor nanostructures, as deep trap states could be acting on majority as well as on minority carriers. This point is particularly crucial for the development of stable and efficient photoelectrodes for water-splitting. In this contribution, we shall contrast the photoelectrochemical responses of oriented TiO2 nanotube and ZnO nanorod assemblies, focusing on the effect of potential and bias illumination on the dynamics of charge transport/trapping.
TiO2 nanotubes were obtained by anodization of Ti in solutions of NH4F in ethylene glycol, followed by annealing in air at 500 °C. ZnO nanorod assemblies were prepared by hydrothermal growth in the presence of hexamethylenetetramine. Analysis by XRD, TEM and electron diffraction studies reveals significant differences in the crystallinity of these materials. Annealed TiO2 nanotubes are essentially composed of anatase crystallites with dimensions in the range of 20 to 30 nm. On the other hand, ZnO nanorods are essentially single crystals growing along the c-axis. Impedance studies of the ZnO nanorods show clear evidence of n-type doping with donor densities of 10^18 cm-3.
In order to avoid corrosion of ZnO nanorods, photoelectrochemical studies were performed in the presence of Na2SO3 as a hole-scavenger. Under steady-state illumination, photocurrent-potential curves exhibit a conventional sigmoidal shape, indicating an increase in the efficiency of hole-capture by sulfite and collection of majority carriers at the back contact for both photoanodes. Under chopped illumination and lock-in detection, a maximum in the photocurrent-voltage curve can be observed at ca. 0.5 V more positive than the photocurrent onset potential for TiO2 nanotubes. The decay of the transient photocurrent magnitude at more positive potentials is strongly dependent on the frequency of light perturbation. Analysis of the transient photocurrent responses and impedance data show that these unusual dynamic responses are associated with a broad distribution of deep trap states extending as far as 0.5 eV below the conduction band edge. A qualitatively similar behavior is observed on as-grown ZnO nanorod assemblies. However, the effect of trap states can be effectively diminished by thermal treatment of the ZnO rods at 340 °C in air. The implication of these findings in areas such as water-splitting and dye-sensitized solar cells will be briefly discussed.
[1] L.M. Peter, J. Phys. Chem. Lett. 2 (2011) 1861