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

: The complexes Ph3ERe(CO)3( ,10-phenanthroline) (E = Sn, Ge) exhibit optical emission (approx. 750 nm) from the lowest excited state in CH2Cl2/CCl4 solutions at 298deg. K and also give E-Re bond cleavage (Phi approx. 0.25) upon photoexcitation under the same conditions. Both the relative emission quantum yield and the reaction quantum yield for ClRe(CO)3 (l,10-phenanthroline) formation are independent of excitation wavelength consistent with reaction and emission originating from the lowest excited state. The emission is now quenched by CCl4 in benzene solution, ruling out excited state, bimolecular, electron transfer to CCl4 as the reaction mechanism. Rather, the mechanism is likely slow (approx. 1/100000 sec) E-Re dissociation. This notion is in accord with a lowest excited state.

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Competitive Radiative Decay and Metal-Metal Bond Cleavage from the Lowest Excited State of Triphenyltin- and Triphenylgermanium-tricarbonyl (1,10-phenanthroline)rhenium.

Use of a redox-active ligand to reversibly alter metal carbonyl electrophilicity

RELATIVE IMPORTANCE OF DISSOCIATIVE LOSS OF CARBON MONOXIDE AND FORMATION OF BENZYL RADICALS FROM PHOTOEXCITATION OF (η5-C5R5)FE(CO)2(η1-CH2C6H5) AND EVIDENCE FOR REACTION OF CARBON MONOXIDE WITH 17-ELECTRON RADICALS

Organic thiols, RSH = cysteamine, 2-mercaptoethanol, penicillamine, or cysteine, effectively suppress photoanodic decomposition of n-CdS and n-CdSe in aqueous solution and undergo efficient controlled potential photoelectrochemical oxidation to the corresponding disulfides, RSSR Photovoltage greater than 700 mV is observed for certain thiols upon excitation of the semiconductor anode with light of energy greater than the band gap As electrodes for preparative electrosynthesis, both illuminated n-CdS and n-CdSe offer significant electrical energy savings compared to conventional electrochemical oxidation in the dark at a Pt electrode The oxidation of RSH to RSSR at n-CdS and n-CdSe has been studied under various conditions

Interaction of thiols with n-type cadmium sulfide and n-type cadmium selenide in aqueous solutions: adsorption of thiolate anion and efficient photoelectrochemical oxidation to disulfides

: This report provides a method for direct measurement of physical diffusion of molecular species in solution Movement of a small number of species in solution can be tracked as a function of space and time by using an array of individually addressable microelectrodes The arrays typically consist of 8 parallel microelectrodes approx 50 microns x 27 microns x approx 01 microns separated from each other by approx 14 microns Experimental studies involve electrochemical generation of as few as 6 x 10 to the minus 20 power moles of redox active species at one microelectrode (generator) and electrochemical detection of a significant percentage of these species at other microelectrodes (collection) 08 -26 microns away Measurements involve determining the time dependence of the collector current associated with detection of the diffusing species produced in short and long pulses at the generator Microelectrochemical experiments and numerical simulations are presented to show that the time dependence of the collector current allows determination of the diffusion coefficient of the diffusing species For the geometries of generator and collector used, an important relationship for short generator pulses is t(mt) = 022 (d sq)/D where t(mt) is the time associated with the peak in the collector current, d the distance from the center of the generator to the nearest edge of the collector, and D the diffusion coefficient Thus, by knowing d and measuring t(mt), D can be determined for a species generated at one microelectrode and detected at another Microelectrochemical arrays; Electrolyte aqueous media (edc)

Mark S. Wrighton

Papers

Competitive Radiative Decay and Metal-Metal Bond Cleavage from the Lowest Excited State of Triphenyltin- and Triphenylgermanium-tricarbonyl (1,10-phenanthroline)rhenium.

Use of a redox-active ligand to reversibly alter metal carbonyl electrophilicity

RELATIVE IMPORTANCE OF DISSOCIATIVE LOSS OF CARBON MONOXIDE AND FORMATION OF BENZYL RADICALS FROM PHOTOEXCITATION OF (η5-C5R5)FE(CO)2(η1-CH2C6H5) AND EVIDENCE FOR REACTION OF CARBON MONOXIDE WITH 17-ELECTRON RADICALS

Interaction of thiols with n-type cadmium sulfide and n-type cadmium selenide in aqueous solutions: adsorption of thiolate anion and efficient photoelectrochemical oxidation to disulfides

Direct Measurements of the Physical Diffusion of Redox Active Species: Microelectrochemical Experiments and their Simulation