Visible Light Driven Hydrogen Evolution with a Noble Metal Free CuGa2_{2}In3_{3}S8_{8} Nanoparticle System in Water

CuGa$_{2}$In$_{3}$S$_{8}$ (CGIS) nanoparticles were synthesised by a hot-injection method and rendered water dispersible by modification with the hydrophilic ligand 3-mercaptopropionic acid (MPA). The CGIS nanoparticles were characterised by X-ray diffraction, transmission electron microscopy, X-ray photoelectron, diffuse reflectance and infrared spectroscopy as well as inductively coupled plasma optical emission spectroscopy. Photocatalytic H$_{2}$ production using the MPA modified CGIS nanoparticles and a nickel salt under visible light irradiation was achieved from acidic solution (pH 2.6) with ascorbic acid as a sacrificial electron donor. Previously, CGIS required the presence of a precious metal co-catalyst and sulfide ions as a sacrificial reagent in alkaline solution to display photocatalytic activity for H$_{2}$ generation. In the reported system, visible light irradiation of the MPA modified CGIS nanoparticles with a Ni salt displayed even superior sacrificial H$_{2}$ evolution activity than when employing the precious metals Pt, Rh and Ru. An external quantum efficiency of more than 12% was achieved at λ = 540 nm, which is almost twice that previously reported for CGIS nanoparticles in the presence of a noble metal co-catalyst and sulfide ions as an electron donor.T.A.K. thanks the Science and Technology Development Fund (STDF) of the Arab Republic of Egypt and the British Council at Cairo for financially supporting his visit to the University of Cambridge, UK. G.A.M.H. was supported by a Cambridge Trust / Australia Poynton PhD scholarship.This is the author accepted manuscript. The final version is available from the Royal Society of Chemistry at http://dx.doi.org/10.1039/C6CY01103A

Long-term investigation of the photocatalytic hydrogen production on platinized TiO2: an isotopic study

Long-term investigations of the photocatalytic hydrogen production on platinized TiO2 photocatalysts have been carried out employing different solutions of (deuterated) water and (deuterated) methanol. The results indicate that methanol acts as a sacrificial reagent, i.e., as an “electron donor” and that the amount of evolved molecular hydrogen is equivalent to the amount of H2 expected from the complete reforming of methanol or even less depending on the used photocatalyst. No evidence for photocatalytic water splitting is observed even in the presence of very low methanol concentrations, i.e., no molecular oxygen has been detected. Based upon the isotopic labelling studies it was confirmed that H2 is mainly produced by the reduction of protons originating from water.BMBF/HyCats/01RC1012

Mesoporous TiO2 nanostructures: A route to minimize Pt loading on titania photocatalysts for hydrogen production

Mesostructured TiO2 nanocrystals have been prepared using Pluronic F127 as the structure-directing agent. Platinum nanoparticles at different contents (0.1-1.0 wt%) have been photochemically deposited onto the mesoporous TiO2. TEM investigation of 0.2 wt% Pt/TiO2 calcined at 450 °C reveals that the TiO2 particles are quite uniform in size and shape with the particle sizes of TiO2 and Pt being 10 and 3 nm, respectively. The photocatalytic activities of the Pt loaded TiO2 have been assessed and compared with those of nonporous commercial Pt/TiO2-P25 by determining the rates and the photonic efficiencies of molecular hydrogen production from aqueous methanol solutions. The results show that the amount of hydrogen evolved on Pt/TiO2-450 at low Pt loading (0.2 wt%) is three times higher than that evolved on Pt/TiO2-P25 and twelve times higher than that evolved on Pt/TiO 2-350. Despite the BET surface area of the TiO2-450 photocatalyst being 3.5 times higher than that of TiO2-P25, a 60% smaller amount of the Pt co-catalyst is required to obtain the optimum photocatalytic hydrogen production activity. The reduced Pt loading on the mesoporous TiO2 will be important both from a commercial and an ecological point of view. © 2011 the Owner Societies

Brookite versus anatase TiO2 photocatalysts: Phase transformations and photocatalytic activities

Titanium dioxide nanoparticles consisting of pure anatase, anatase-rich, brookite-rich, and pure brookite modifications were synthesized and characterized by X-ray diffraction, field emission-scanning electron microscopy and nitrogen adsorption. The phase transformations among the three modifications of TiO2 (anatase, brookite, and rutile) and the photocatalytic activities of these nanoparticles were investigated by heat treatment over the temperature range from 400 to 800 °C and by the photooxidation of methanol, respectively. Direct transformation of anatase and brookite to rutile was observed, while in the case of the anatase–brookite mixture, anatase transforms firstly to brookite and then to rutile. The photocatalytic activity measurements indicate that brookite nanoparticles exhibit higher photocatalytic activities than anatase, and a comparable activity to that of the anatase-rich nanoparticles. The phase transformations and photocatalytic results are discussed regarding their dependence on crystallite size, surface area, and phase composition

Structure-activity relationships of hierarchical three-dimensional electrodes with photosystem II for semi-artifcial photosynthesis

Semi-artificial photosynthesis integrates photosynthetic enzymes with artificial electronics, which is an emerging approach to reroute the natural photoelectrogenetic pathways for sustainable fuel and chemical synthesis. However, the reduced catalytic activity of enzymes in bioelectrodes limits the overall performance and further applications in fuel production. Here, we show new insights into factors that govern the photoelectrogenesis in a model system consisting of photosystem II and three-dimensional indium tin oxide and graphene electrodes. Fluorescence microscopy and in situ surface-sensitive infrared spectroscopy are employed to probe the enzyme distribution and penetration within electrode scaffolds of different structures, which is further correlated with protein film-photoelectrochemistry to establish relationships between the electrode structure and enzyme activity. We find that the hierarchical 1 structure of electrodes mainly affects the protein integration, but not the enzyme activity. Photoactivity is more limited by light intensity and electronic communication at the biointerface. This study provides guidelines for maximizing the performance of semi-artificial photosynthesis and also presents a set of methodologies to probe the photoactive biofilms in three-dimensional electrodes.CSC-Cambridge PhD Scholarship, EPSRC PhD studentship, Newton-Mosharafa Research Fellowship, ERC Consolidator Grant 'MatEnSAP

Mechanisms of Photocatalytic Molecular Hydrogen and Molecular Oxygen Evolution over La-Doped NaTaO3 Particles: Effect of Different Cocatalysts and Their Specific Activity

A better understanding of the mechanisms of H2 and O2 evolution over cocatalyst-loaded photocatalysts is an essential step in constructing efficient artificial systems for the overall water splitting. In this paper, La-doped NaTaO3 particles loaded with different cocatalysts (i.e., noble metals and metal oxides) have been synthesized and used as model photocatalysts to study the mechanisms of photocatalytic H2 and/or O2 evolution from pure water, aqueous methanol solution, and aqueous silver nitrate solution. It was found that the photocatalytic activity and selectivity toward H2 and/or O2 evolution strongly depend on the nature of the cocatalyst and the investigated system. For pure water and aqueous silver nitrate systems, the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) was found to be the main reason for the observed selective behavior for H2 and O2 evolution. The creation of active sites and subsequent decrease in activation energy is thought to play a secondary role. In the presence of methanol, when the dark injection of an electron into the conduction band of the photocatalyst is possible, the catalytic roles of the investigated cocatalysts toward the formation of H2 gas were found to be decisive, in addition to the charge separation and interfacial electron transfer processes. No overall water splitting into H2 and O2 can be achieved utilizing La-doped NaTaO3 loaded with only one cocatalyst; however, it was found that the loading of La-doped NaTaO3 with two different cocatalysts, i.e. RuO2 and CoO, enables the simultaneous formation of H2 and O2 from pure water. The modification of photocatalyst with two different cocatalysts seems to be essential for enhancing the efficiency of overall photocatalytic splitting. The interfacial electron transfer on the cocatalyst-loaded La-doped NaTaO3 was determined by measuring the cathodic and anodic photocurrents in the presence of Fe2+/Fe3+ electron shuttle. Methanol and bromate were used as electron donors and electron acceptors during the cathodic and anodic photocurrent measurements, respectively. By correlation of the photocurrent with the activity of the investigated cocatalysts, it was concluded that the creation of active sites and subsequent decrease in activation energy for H2 evolution is the main requirement for efficient H2 evolution from the aqueous methanol system, whereas the interaction with the photogenerated holes and the formation of intermediates allowing a multielectron transfer process seems to be an essential step for the water oxidation and O2 evolution. This information appears to be crucial for a rational design of a highly active photocatalyst for overall water splitting under UV-vis illumination. © 2018 American Chemical Society.11sciescopu

Electronic structure and photocatalytic activity of wurtzite Cu–Ga–S nanocrystals and their Zn substitution

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Modification of Hematite Photoanode with Cobalt Based Oxygen Evolution Catalyst via Bifunctional Linker Approach for Efficient Water Splitting

The modification of hematite photoanode with cheap, scalable, and efficient oxygen evolution catalyst is an essential step for its practical application for solar fuel production. In this paper, a simple and water-based method has been developed for the modification of hematite surface with cobalt based oxygen evolution catalyst using 3-aminopropionic acid (APA) as a bifunctional linker. APA exists in an aqueous solution at pH 6.1 in zwitterionic form and thus it has a positive charge on the protonated amino group and a negative charge on the carboxylate group. The carboxylate groups of APA molecules can thus interact with the protonated surface hydroxyl groups of hematite whereas the amino groups tether the cobalt ions. The hematite photoanodes were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy and inductively coupled plasma optical emission spectroscopy. The photoelectrochemical measurements indicated that Co/APA–hematite photoanode exhibit 3.8-, 3.2-, and 2-fold higher photocurrent at 1.23 V vs RHE than bare, Co–, and CoPi–hematite photoanodes, respectively. Moreover, the onset potential of photoelectrochemical water oxidation on Co/APA–hematite photoanode is cathodically shifted by 290 mV in comparison to that obtained on bare hematite photoanode. This finding has been explained by measuring the transient photocurrent and intensity modulated photocurrent spectroscopy responses. On the basis of these measurements, it was found that the rate constant of electron transfer at Co/APA–hematite photoanode/liquid interface is higher than that measured for bare and CoPi–hematite photoelectrodes. This explains the higher photoelectrochemical activity of Co/APA–hematite photoanode and reflects on the potential application of this simple approach for the modification of different metal oxide photoelectrodes with cobalt-based oxygen evolution catalyst