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

/pdf/formation-and-structure-of-self-assembled-monolayers-5eorfomxfu.pdf

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.






Open in a separate window


Figure 1


Ruthenium polypyridyl complexes: versatile visible light photocatalysts.

/pdf/visible-light-photoredox-catalysis-with-transition-metal-5d1oxrsczo.pdf

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

: Described herein are the proof of concept results demonstrating a new approach to electrochemical sensors based upon chemically sensitive microelectrochemical devices. A typical device consists of at least two individually addressable electrodes. The working electrode is a microelectrode and is derivatized with at least one molecule that has a chemically sensitive formal potential and serves as the indicator, and one molecule that has a chemically insensitive formal potential and serves as a reference. The indicator and reference molecules are confined to the electrodes either by monolayer self assembly techniques or by dissolving in a thin film of solid electrolyte. Detection in these systems is accomplished by measuring either the potential difference associated with current peaks for oxidation (or reduction) of microelectrode confined redox reagents.

Chemically Sensitive Microelectrochemical Devices: New Approaches to Sensors

An 80-MeV/c negative muon beam from the Alternating Gradient Synchrotron at Brookhaven National Laboratory was used to investigate the stopping of muons inside Pd, Ti, and Y targets saturated with deuterium. Neutron emission from the targets was measured with an array of3He detectors, and in some runs, the temperature of the target was monitored as a function of time, with and without a flux of muons on the target. The neutron rates were also measured for Pd cathodes in an active electrochemical cell similar in design to those used in so-called “cold fusion” experiments, and the electrolyte solution was analyzed for excess tritium. No evidence was found for muon-catalyzed fusion at rates consistent with those claimed in “cold fusion” experiments. Neutron production from catalyzed fusion due to the presence of deuterium in palladium deuteride, PdD0.7, exposed to muons was determined to be 0.0±0.03 (stat.) ±0.25 (syst.) neutrons per stopped muon.

/pdf/measurements-of-neutron-emission-induced-by-muons-stopped-in-12igi3x7v2.pdf

Measurements of neutron emission induced by muons stopped in metal deuteride targets

Variation in the electrochemical potential of the electrode results in a change in the stretching frequency of CO bound to elemental Pt dispersed in an electrode-confined redox polymer, (PQ/sup 2+/+/)/sub n/, derived from an N,N'-dialkyl-4,4'-bipyridinium monomer. The stretching frequency of the CO, about 2000 cm/sup -1/, can be monitored by transmission infrared spectroscopy using a Fourier transform infrared (FTIR) spectrometer and moves to lower values with movement of the potential to more negative values. Spectroelectrochemical studies were conducted by using an IR-transparent n-Si/((PQ/sup 2+/+/Pt(CO)/sub x/)/sub n/) electrode assembly in an electrochemical cell with the derivatized surface exposed to the electrolyte solution. Experiments have been carried out with both natural-abundance and 99% /sup 13/C-enriched CO, and the potential dependence is the same, about 20 cm/sup -1/ V/sup -1/, over the potential range studied, approximately 0.0 to -1.4 V vs. Ag/sup +//Ag in CH/sub 3/CN/2 M LiClO/sub 4/. The stretching frequency of natural-abundance CO in the same assembly has been studied in H/sub 2/O/3 M LiCl and shows a slightly larger potential dependence, about 35 cm/sup -1/ V/sup -1/, in the range -0.2 to -1.0 V vs. AgCl/Ag. The data for the CO bound to the Pt dispersed in the polymer ismore » in accord with similar data obtained for CO bound to smooth Pt electrode surfaces.« less

Potential dependence of the stretching frequency of carbon monoxide bound to the surface of platinum dispersed in an electrode-confined redox polymer

Characterization of intrinsic amorphous hydrogenated silicon as a thin-film photocathode material. Efficient photoreduction processes in aqueous solution

: Interfacial energetics for n-type MoSe2 (E(g) = 1.4 eV, direct) and photoelectrochemical conversion of light to electrical energy in the presence of X(n)(-)/X(-) (X = Cl, Br, I) have been characterized in CH3CN electrolyte solution. Data for MoSe2 in H2O/I3(-)/I(-) are included for comparison, along with a comparison of MoSe2-based cells with MoS2- (E(g) = 1.7 eV, direct) based cells. Cyclic voltammetry for a set of reversible (at Pt electrodes) redox couples whose formal potential, E(o), spans a range -0.8 to +1.5 V vs. SCE has been employed to establish the interface energetics of MoSe2. For the redox couples having E(o) more negative than approximately -0.1 V. vs. SCE, we find reversible electrochemistry in the dark at n-type MoSe2. When E (o) is somewhat positive of -0.1 V vs. SCE we find that oxidation of the reduced form of the redox couple can be effected in an uphill sense be irradiation of the n-type MoSe2 with = or E(g) light; the anodic current peak is at a more negative potential than at Pt for such situations. The extent to which the photoanodic current peak is more negative than at Pt is a measure of the output photovoltage for a given couple. For E(o) more positive than approximately +0.7 V vs. SCE it would appear that this output photovoltage is constant at approximately 0.4 V. For a redox couple such as biferrocene (E(o)(BF(+)/BF = +0.3 V vs. SCE) we find a photoanodic current onset at approximately -0.2 V vs. SCE; a redox couple with E = 1.5 V vs. SCE shows an output photovoltage of 0.43 V under the same conditions.

N-type molybdenum diselenide-based photoelectrochemical cells: evidence for fermi level pinning and comparison of the efficiency for conversion of light to electricity with various solvent/halogen/halide combinations

Mark S. Wrighton

Papers

Chemically Sensitive Microelectrochemical Devices: New Approaches to Sensors

Measurements of neutron emission induced by muons stopped in metal deuteride targets

Potential dependence of the stretching frequency of carbon monoxide bound to the surface of platinum dispersed in an electrode-confined redox polymer

Characterization of intrinsic amorphous hydrogenated silicon as a thin-film photocathode material. Efficient photoreduction processes in aqueous solution

N-type molybdenum diselenide-based photoelectrochemical cells: evidence for fermi level pinning and comparison of the efficiency for conversion of light to electricity with various solvent/halogen/halide combinations