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

: Pretreated (anodized) Pt and Au electrode surfaces derivatized with (1,1'- ferrocenediyl)dichlorosilane have been analyzed by X-ray photoelectron spectroscopy (XPES). The derivatized surfaces exhibit Fe 2p3/2 bands with binding energies consistent with ferrocene iron and with large companion satellite peaks. Using a layer model, relative Fe 2p3/2 and electrode substrate intensities (Au or Pt) can be correlated with ferrocene coverage measured by cyclic voltammetry. The results indicate that electrochemical charge transfer can occur through ferrocene/silane layers of thicknesses exceeding 100 Angstrom. (Author)

An x-ray photoelectron spectroscopic study of multilayers of an electroactive ferrocene derivative attached to platinum and gold electrodes

Using the differential capacitance technique, the flat-band potential of n-type TiO2, SnO2, SrTiO3, KTaO3, and KTa(0.77)Nb(0.23)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 0.059 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(0.77)Nb(0.23)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

Inter- and intramolecular quenching of the lowest singlet excited state of three porphyrins by ferrocene derivatives is reported. 5,15-Bis(4-tolyl)-2,3,7,8,12,13,17,18-octamethylporphyrin (1) and two derivatives of 1 where one of the tolyl methyl groups was replaced by a ferrocenylvinyl group, 2, or by a ferrocenylethyl group, 3, were prepared. Porphyrin 2 was isolated as a mixture of cis (73%) and trans (27%) isomers. Singlet excited state properties were studied by steady-state emission spectroscopy and by emission lifetime measurements. The relative quantum yields of fluorescence for 2 and 3 compared to 1 are 0.38 and 0.84, respectively. Fluorescence decay lifetimes of 1 and 3 are 15 and 14 ns, respectively. Fluorescence of 2 is revealed to be due to the emission of two species (cis and trans isomers) with lifetimes of 4 and 13 ns. The shorter fluorescence lifetimes and smaller fluorescence quantum yields for 2 and 3 compared to 1 are attributed to quenching of the singlet excited state of the porphyrin by the ferrocenyl centers. However, the fraction of quenching by electron transfer and energy transfer could not be quantitatively measured. The rate constant for quenching is no more than 10[sup 8] s[sup [minus]1], consistent with electron-transfer quenching. Intermolecular quenching ratemore » constants for the quenching of the porphyrin singlet excited state by ferrocene derivatives were also found to be consistent with an electron transfer quenching mechanism. 28 refs., 3 figs., 1 tab.« less

Inter‐ and Intramolecular Quenching of the Singlet Excited State of Porphyrins by Ferrocene.

Stabilization of n-type CdSe to photoanodic dissolution is reported. The stabilization is accomplished by the competitive oxidation of S(--) or S(n)(--) at the CdSe photoanode in an electrochemical cell. Such stabilized cells are shown to sustain the conversion of low energy (not less than 1.7 eV) visible light to electricity with good efficiency and no deterioration of the CdSe photoelectrode or of the electrolyte. The electrolyte undergoes no net chemical change because the oxidation occurring at the photoelectrode is reversed at the cathode. Conversion of monochromatic light at 633 nm to electricity is shown to be up to approximately 9% efficient with output potentials of approximately 0.4 V. Conversion of solar energy to electricity is estimated to be approximately 2% efficient.

Conversion of visible light to electrical energy - Stable cadmium selenide photoelectrodes in aqueous electrolytes

: This document reports the fabrication of a chemically derivatized microelectrode array that can function as a transistor when immersed in an electrolyte solution. The key finding is that a small signal (charge) needed to turn on the device can be amplified. The device to be described MIMICS the fundamental characteristics of solid state transistor, since the resistance between to contacts can be varied by a signal to be amplified. The chemical transistor is the set of three (drain, gate, and source) Au microelectroces covered by polypyrrole.

Mark S. Wrighton

Papers

An x-ray photoelectron spectroscopic study of multilayers of an electroactive ferrocene derivative attached to platinum and gold electrodes

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

Inter‐ and Intramolecular Quenching of the Singlet Excited State of Porphyrins by Ferrocene.

Conversion of visible light to electrical energy - Stable cadmium selenide photoelectrodes in aqueous electrolytes

Chemical derivatization of an array of three gold microelectrodes with polypyrrole: fabrication of a molecule-based transistor