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

Reversible addition of carbon monoxide to tetracarbonylbis(.eta.5-pentamethylcyclopentadienyl)dimolybdenum(I)

: This article reports the fabrication and characterization of a microelectrochemical transistor derived from coating and connecting two closely spaced (< 1.7 micrometers) microelectrodes ( 80 micrometers thick) with poly (4-vinylpyridine ), (4-VPy)n. The transistor turns on only when two chemical criteria are met: the pH of the medium in contact with the (4-VPy)n must be sufficiently low to protonate the polymer to give (4-VPyH+)n and an anionic redox couple must be present to be electrostatically bound into the polymer, e.g. ferrie cyanide. By using conventional redox species as the mechanism for transporting charge from one microelectrode (source) to the other microelectrode (drain) the device shows a narrow region of gate voltage, VG, where the source-drain current, ID, is non-zero: ID (peak) occurs at VG E (ferrie cyanide) for a small difference (<50 mV) in potential between source and drain, VD. The temperature dependence of ID shows an Arrhenius activation energy of 50 kJ/mole, consistent with previous results for the (4-VPyH+ . 1/3 Fe(CN)63-)n polymer. The transistor can be turned alternately on and off in a reproducible manner by alternate exposure of the device to pH = 3 Fe(CN)63-/4- and pH = 3 cobalt cyanide, respectively, at VG= +0.2 V vs. SCE, consistent with dynamic exchange in and out of the (4-VPyH+)n by electroactive (Fe(CN)63-/4-) and non-electroactive (Co(CN)63-) anions. At pH's above the pka of the (4-VPy)n exposure of the device to Fe(CN) 63-/4- does not turn on the device.

Microelectrochemical transistors based on electrostatic binding of electroactive metal complexes in protonated poly(4-vinylpyridine): devices that respond to two chemical stimuli

Potential-Dependent Nucleophilicity of Polyaniline

: Treatment of a single crystal Si wafer (n- or p-type, 100 face) having a thin surface SiO(x) layer with Cl3Si-(eta5-C5H4)Mn(CO)3, I, yields the Si-confined complex (Si)-O-Si-eta5-C5H4)Mn(CO)3 Coverage of about a monolayer is typically achieved Near-UV irradiation of the Si-confined tricarbonyl in L = PPh2(n-octyl) gives the surface-confined substitution product (Si)-O-Si-eta5- C5H4)Mn(CO)2L The identities of the Si-confined complexes were determined by comparison of their IR CO stretching frequencies with those of (CH3)3Si-(eta5- C5H4)Mn(CO)2L, and of the corresponding high surface area SiO2-confined species, (SiO2)-O-Si (CH3)2-eta5-C5H4)Mn(CO)2L (L = CO, PPh(n-octyl)) Treatment of a Au- coated Si wafer with HS(CH2)11-eta5-C5H4)Mn(CO)3, II, yields about a monolayer of the Au-confined complex (Au)S(CH2)11-eta5-C5H4)Mn(CO)3 Near-UV irradiation of the Au-confined tricarbonyl in the presence of L = PPh2(n-octyl) yields the surface-confined substitution product (Au)-S(CH2)11-(eta5-C5H4)MN(CO)2L The photochemistries of surface-confined I and II differ from the photochemistries of the corresponding solution species Patterning of the surface of a Si wafer modified with I or of Au modified with II is readily accomplished, since photosubstitution occurs only on irradiated portions of the surface

Patterned Functionalization of Gold and Single Crystal Silicon via Photochemical Reaction of Surface-Confined Derivatives of (Eta5-C5H5)MN(CO)3

Potential-dependent conductivity of conducting polymers yields opportunities for molecule-based devices: a microelectrochemical push-pull amplifier based on two different conducting polymer transistors

Mark S. Wrighton

Papers

Reversible addition of carbon monoxide to tetracarbonylbis(.eta.5-pentamethylcyclopentadienyl)dimolybdenum(I)

Microelectrochemical transistors based on electrostatic binding of electroactive metal complexes in protonated poly(4-vinylpyridine): devices that respond to two chemical stimuli

Potential-Dependent Nucleophilicity of Polyaniline

Patterned Functionalization of Gold and Single Crystal Silicon via Photochemical Reaction of Surface-Confined Derivatives of (Eta5-C5H5)MN(CO)3

Potential-dependent conductivity of conducting polymers yields opportunities for molecule-based devices: a microelectrochemical push-pull amplifier based on two different conducting polymer transistors