This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references)

Heterogeneous photocatalyst materials for water splitting

Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

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Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

The field of self-assembled monolayers (SAMs) has witnessed tremendous growth in synthetic sophistication and depth of characterization over the past 15 years.1 However, it is interesting to comment on the modest beginning and on important milestones. The field really began much earlier than is now recognized. In 1946 Zisman published the preparation of a monomolecular layer by adsorption (self-assembly) of a surfactant onto a clean metal surface.2 At that time, the potential of self-assembly was not recognized, and this publication initiated only a limited level of interest. Early work initiated in Kuhn’s laboratory at Gottingen, applying many years of experience in using chlorosilane derivative to hydrophobize glass, was followed by the more recent discovery, when Nuzzo and Allara showed that SAMs of alkanethiolates on gold can be prepared by adsorption of di-n-alkyl disulfides from dilute solutions.3 Getting away from the moisture-sensitive alkyl trichlorosilanes, as well as working with crystalline gold surfaces, were two important reasons for the success of these SAMs. Many self-assembly systems have since been investigated, but monolayers of alkanethiolates on gold are probably the most studied SAMs to date. The formation of monolayers by self-assembly of surfactant molecules at surfaces is one example of the general phenomena of self-assembly. In nature, self-assembly results in supermolecular hierarchical organizations of interlocking components that provides very complex systems.4 SAMs offer unique opportunities to increase fundamental understanding of self-organization, structure-property relationships, and interfacial phenomena. The ability to tailor both head and tail groups of the constituent molecules makes SAMs excellent systems for a more fundamental understanding of phenomena affected by competing intermolecular, molecular-substrates and molecule-solvent interactions like ordering and growth, wetting, adhesion, lubrication, and corrosion. That SAMs are well-defined and accessible makes them good model systems for studies of physical chemistry and statistical physics in two dimensions, and the crossover to three dimensions. SAMs provide the needed design flexibility, both at the individual molecular and at the material levels, and offer a vehicle for investigation of specific interactions at interfaces, and of the effect of increasing molecular complexity on the structure and stability of two-dimensional assemblies. These studies may eventually produce the design capabilities needed for assemblies of three-dimensional structures.5 However, this will require studies of more complex systems and the combination of what has been learned from SAMs with macromolecular science. The exponential growth in SAM research is a demonstration of the changes chemistry as a disciAbraham Ulman was born in Haifa, Israel, in 1946. He studied chemistry in the Bar-Ilan University in Ramat-Gan, Israel, and received his B.Sc. in 1969. He received his M.Sc. in phosphorus chemistry from Bar-Ilan University in 1971. After a brief period in industry, he moved to the Weizmann Institute in Rehovot, Israel, and received his Ph.D. in 1978 for work on heterosubstituted porphyrins. He then spent two years at Northwestern University in Evanston, IL, where his main interest was onedimensional organic conductors. In 1985 he joined the Corporate Research Laboratories of Eastman Kodak Company, in Rochester, NY, where his research interests were molecular design of materials for nonlinear optics and self-assembled monolayers. In 1994 he moved to Polytechnic University where he is the Alstadt-Lord-Mark Professor of Chemistry. His interests encompass self-assembled monolayers, surface engineering, polymers at interface, and surfaces phenomena. 1533 Chem. Rev. 1996, 96, 1533−1554

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Formation and Structure of Self-Assembled Monolayers.

A fundamental aim in the field of catalysis is the development of new modes of small molecule activation. One approach toward the catalytic activation of organic molecules that has received much attention recently is visible light photoredox catalysis. In a general sense, this approach relies on the ability of metal complexes and organic dyes to engage in single-electron-transfer (SET) processes with organic substrates upon photoexcitation with visible light.

Many of the most commonly employed visible light photocatalysts are polypyridyl complexes of ruthenium and iridium, and are typified by the complex tris(2,2′-bipyridine) ruthenium(II), or Ru(bpy)32+ (Figure 1). These complexes absorb light in the visible region of the electromagnetic spectrum to give stable, long-lived photoexcited states.1,2 The lifetime of the excited species is sufficiently long (1100 ns for Ru(bpy)32+) that it may engage in bimolecular electron-transfer reactions in competition with deactivation pathways.3 Although these species are poor single-electron oxidants and reductants in the ground state, excitation of an electron affords excited states that are very potent single-electron-transfer reagents. Importantly, the conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes.






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Figure 1


Ruthenium polypyridyl complexes: versatile visible light photocatalysts.

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Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

Self-assembly of a ferrocenyl thiol and a quinone thiol onto Au microelectrodes forms the basis for a new microsensor concept: a two-terminal, voltammetric microsensor with reference and sensor functions on the same electrode. The detection is based on measurement of the potential difference of current peaks for oxidation and reduction of the reference (ferrocene) and indicator (quinone) in aqueous electrolyte in a two-terminal, linear sweep voltammogram in which a counterelectrode of relatively large surface area is used. The quinone has a half-wave potential, E½, that is pH-sensitive and can be used as a pH indicator; the ferrocene center has an E1/2 that is a pH-insensitive reference. The key advantages are that such sensors require no separate reference electrode and function as long as current peaks can be located for reference and indicator molecules.

https://science.sciencemag.org/content/sci/252/5006/688.full.pdf

Molecular Self-Assembly of Two-Terminal, Voltammetric Microsensors with Internal References

Using the differential capacitance technique, the flat-band potential of n-type TiO2, SnO2, SrTiO3, KTaO3, and KTa(077)Nb(023)O3 electrodes has been determined as a function of pH in aqueous electrolytes Plots of flat-band potential vs pH are linear in all cases and have a slope of approximately 0059 V/pH unit The flat-band potential correlates nicely with the onset for photoanodic currents corresponding to O2 evolution at the n-type semiconductor and H2 at the dark Pt cathode The ordering of flat-band potentials at a given pH is SrTiO3 of the order of KTaO3 of the order of KTa(077)Nb(023)O3 greater than TiO2 greater than SnO2 (SnO2 most positive vs a saturated calomel electrode)

Correlation of photocurrent-voltage curves with flat-band potential for stable photoelectrodes for the photoelectrolysis of water

The nature of the lowest excited state in fac-tricarbonylhalobis(4-phenylpyridine)rhenium(I) and fac-tricarbonylhalobis(4,4'-bipyridine)rhenium(I): emissive organometallic complexes in fluid solution

We show that the standard potential for anodic dissolution, , of n‐type semiconductors (eg, for is + 008V vs SCE) plays a key role in ultimate efficiency of thermodynamically stable n‐type semiconductor‐based photoelectrochemical cells The selection of redox active substances that can be used for competitive capture of photogenerated holes is limited to those systems where the product does not have the potential to oxidize the semiconductor Where and represent the position of the valence and conduction band positions at the interface, respectively, we must conclude that is at a more negative potential (vs a reference) than , if the semiconductor undergoes photoanodic dissolution Quenching of the photoanodic dissolution by competitive hole capture by some electrolyte component, say A, is possible if lies at a potential more negative than  But additionally, if the A+, the oxidation product, is to be incapable of oxidizing the semiconductor, must be more negative than Consequently, for the semiconductor to be thermodynamically stable in the A/A+ electrolyte the maximum photovoltage output, , can be no greater than for more negative than Naturally, kinetic inertness of the semiconductor to an oxidant with more positive than may allow its presence and/or use in a more efficient cell N‐type and semiconductors in aqueous electrolytes are treated in detail Some preliminary comments are made concerning p‐type materials

https://iopscience.iop.org/article/10.1149/1.2133140/pdf

Thermodynamic Potential for the Anodic Dissolution of n‐Type Semiconductors A Crucial Factor Controlling Durability and Efficiency in Photoelectrochemical Cells and an Important Criterion in the Selection of New Electrode/Electrolyte Systems

Studies of CdX-based photoelectrochemical cells in X/sup 2 -//X/sup 2 -//sub n/ electrolytes are reported for X = S, Se, and Te. For eight of the nine electrode/electrolyte combinations we have demonstrated that the n-type semiconducting single-crystal CdX photoelectrodes are stable to anodic dissolution. Only for CdTe in S/sup 2 -//S/sup 2 -//sub n/ do we find that oxidation of the added chalcogenide does not quench the decomposition of CdX typically found in aqueous electrolytes. For all eight remaining elecrolyte/electrode combinations the added chalcogenide is oxidized at the photoelectrode at a rate which precludes anodic dissolution of the CdX. For the stable combinations each electrolyte is capable of being oxidized at the photoelectrode and subsequently reduced at the dark counter electrode to complete a cycle where no net chemical change obtains. For all nine electrolyte/electrode combinations and for the CdX in alkaline H/sub 2/O, the redox level associated with the oxidation of X/sup 2 -/ or with O/sub 2/ evolution is between the valence band and conduction band positions at the semiconductor--electrolyte interface. Thus, energetic requirementsfor X/sup 2 -/ oxidation or O/sub 2/ evolution from H/sub 2/O are met in all cases, but apparently kinetic factors control whether oxidation ofmore » X/sup 2 -/ or of H/sub 2/O will be fast compared to anodic dissolution, which is also energetically feasible. For the stable electrode/electrolyte combinations, conversion of optical to electrical energy can be accomplished with efficiencies of > 10 percent for monochromatic visible light. For CdTe or CdSe in the Te/sup 2 -//Te/sub 2//sup 2 -/ electrolyte input power densities of >500 mW/cm/sup 2/ can be converted with a few percent efficiency with no deterioration of properties. Output voltages at maximum power conversion efficiency are of the order of 0.4 V.« less

Mark S. Wrighton

Papers

Molecular Self-Assembly of Two-Terminal, Voltammetric Microsensors with Internal References

Correlation of photocurrent-voltage curves with flat-band potential for stable photoelectrodes for the photoelectrolysis of water

The nature of the lowest excited state in fac-tricarbonylhalobis(4-phenylpyridine)rhenium(I) and fac-tricarbonylhalobis(4,4'-bipyridine)rhenium(I): emissive organometallic complexes in fluid solution

Thermodynamic Potential for the Anodic Dissolution of n‐Type Semiconductors A Crucial Factor Controlling Durability and Efficiency in Photoelectrochemical Cells and an Important Criterion in the Selection of New Electrode/Electrolyte Systems

Study of n-type semiconducting cadmium chalcogenide-based photoelectrochemical cells employing polychalcogenide electrolytes