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.






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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

Mapping the spatial arrangement of chemical functional groups and their interactions is of significant importance to problems ranging from lubrication and adhesion to recognition in biological systems. A force microscope has been used to measure the adhesive and friction forces between molecularly modified probe tips and organic monolayers terminating in a lithographically defined pattern of distinct functional groups. The adhesive interactions between simple CH(3)/CH(3), CH(3)/COOH, and COOH/COOH functional groups correlate directly with friction images of sample surfaces patterned with these groups. Thus, by monitoring the friction between a specifically functionalized tip and sample, one can produce friction images that display predictable contrast and correspond to the spatial distribution of functional groups on the sample surface. Applications of this chemically sensitive imaging technique are discussed.

https://science.sciencemag.org/content/sci/265/5181/2071.full.pdf

Functional Group Imaging by Chemical Force Microscopy

: Anodic growth of polyaniline films on a Au microelectrode array has been carried out to add to the characterization of polyaniline and to fabricate polyaniline-based microelectronic devices, diodes and transistors, that function when the polyaniline-functionalized microelectrode array is immersed in an electrolyte solution. The microelectrode array is a set of eight Au electrodes, 0.1 micrometer thick, 4.4 micrometers wide and 50 micrometers long, each individually addressable and separated from each other by 1.7 micrometers. Polyaniline can be deposited in controlled amounts by electrochemical oxidation of aniline. When the polyaniline is deposited in sufficient amounts, two or more of the eight Au microelectrodes can be connected in the electrical sense. Current can pass between two connected microelectrodes when there is an applied potential between them. The magnitude of the current at a given applied potential depends on the electrochemical potential of the polyaniline. In 0.5 M NaHsSO4/H20 the current (at a fixed applied potential) is maximum at an electrochemical potential of approx. +0.4 V vs. SCE and declines by a factor of greater than 1,000,000 upon reduction to +0.1 or oxidation to +0,7 V vs. SCE. Owing to the large change in resistance upon change in electrochemical potential, the functionalized microelectrodes is crucial to device function, owing to the limited conductivity of the polyaniline. The switching time of a transistor-like device is faster than 0.1 s.

/pdf/resistance-of-polyaniline-films-as-a-function-of-hy6h0chvwy.pdf

Resistance of Polyaniline Films as a Function of Electrochemical Potential and the Fabrication of Polyaniline-Based Microelectronic Devices.

Nature of the lowest excited state in tricarbonylchloro-1,10-phenanthrolinerhenium(I) and related complexes

Chemical force microscopy (CFM) has been used to measure adhesion and friction forces between probe tips and substrates covalently modified with self-assembled monolayers (SAMs) that terminate in distinct functional groups. Robe tips have been modified with SAMs using a procedure that involves coating commercial Si3N4 cantilevedtip assemblies with a thin layer of polycrystalline Au followed by immersion in a solution of a functionalized thiol. This methodology provides a reproducible means for endowing the probe with different chemical functional groups. The spring constants and radii of the chemically modified cantilevedtip assemblies have been characterized to allow for quantitative friction and adhesion measurements. Au-coated Si and Si substrates have been treated with functionalized thiols and silanes, respectively, to produce SAM coated substrates terminating with different functional groups. A force microscope has been used to characterize the adhesive interactions between probe tips and substrates that have been modified with SAMs which terminate with COOH, CH3, and N H 2 functional groups in EtOH and HzO solvents. Force vs distance curves recorded under EtOH show that the interaction between functional groups decreases as follows: COOWCOOH > CHdCH3 > COOWCH3. The measured adhesive forces were found to agree well with predictions of the Johnson, Kendall, and Roberts (JKR) theory of adhesive contact and thus show that the observed adhesion forces correlate with the surface free energy of the molecular groups in EtOH. Electrostatic contributions to adhesive forces have also been studied using a COO-/NH3+ tip/surface in aqueous solution. Force vs distance curves recorded as a function of ionic strength show that the observed adhesive interaction decreases with increasing ionic strength. These results have been interpreted in terms of contact and noncontact contributions to the experimentally measured adhesive force. The friction forces between tips and samples modified with COOH and CH3 groups have also been measured as a function of applied load. The magnitude of the friction force was found to decrease in the following manner with different tip/sample functionalities: COOWCOOH > CH3/CH3 > COOWCH3. Friction forces between different chemical functional groups thus correlate directly with the adhesion forces between these same groups. Specifically, high friction is observed between groups that adhere strongly, while low friction is observed between weakly interacting functional groups. The dependence of friction forces on the tip and sample functionality is shown to be the basis for chemical force microscopy in which lateral force images are interpreted in terms of the strength of both adhesive groups.

Chemical force microscopy: Exploiting chemically-modified tips to quantify adhesion, friction, and functional group distributions in molecular assemblies

: 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

Functional Group Imaging by Chemical Force Microscopy

Resistance of Polyaniline Films as a Function of Electrochemical Potential and the Fabrication of Polyaniline-Based Microelectronic Devices.

Nature of the lowest excited state in tricarbonylchloro-1,10-phenanthrolinerhenium(I) and related complexes

Chemical force microscopy: Exploiting chemically-modified tips to quantify adhesion, friction, and functional group distributions in molecular assemblies

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