A Self-Assembled Organic/Metal Junction for Water Photo-Oxidation.

We report the in situ self-assembly of TTF, TTF•+, and BF4- or PF6- into p-type semiconductors on the surface of Pt microparticles dispersed in water/acetonitrile mixtures. The visible light photoactivation of these self-assemblies leads to water oxidation forming O2 and H+, with an efficiency of 100% with respect to the initial concentration of TTF•+. TTF•+ is then completely reduced to TTF upon photoreduction with water. The Pt microparticles act as floating microelectrodes whose Fermi level is imposed by the different redox species in solution; here predominantly TTF, TTF•+, and HTTF+, which furthermore showed no signs of decomposition in solution.

Photosensitized Hydrogen Evolution on a Floating Electrocatalyst Coupled to Electrochemical Recycling.

Photoexcited protonated tetrathiafulvalene (HTTF+) was found to act as a photosensitizer, injecting electrons into Pt microparticles (floating electrocatalysts) to produce H2 and TTF•+ in acidic acetonitrile. In addition, TTF•+ was electrochemically reduced back to TTF on a carbon electrode, to be further protonated to continuously produce H2 photochemically. The onset potential for the electrochemical recycling of TTF•+ on carbon was set at a potential 500 mV more positive than the potential required for the direct reduction of protons. HTTF+ showed no signs of decomposition after 51 h of continuous recycling and photoinduced production of H2, proving stability and reversibility.

Boosting water oxidation layer-by-layer.

Electrocatalysis of water oxidation was achieved using fluorinated tin oxide (FTO) electrodes modified with layer-by-layer deposited films consisting of bilayers of negatively charged citrate-stabilized IrO2 NPs and positively charged poly(diallyldimethylammonium chloride) (PDDA) polymer. The IrO2 NP surface coverage can be fine-tuned by controlling the number of bilayers. The IrO2 NP films were amorphous, with the NPs therein being well-dispersed and retaining their as-synthesized shape and sizes. UV/vis spectroscopic and spectro-electrochemical studies confirmed that the total surface coverage and electrochemically addressable surface coverage of IrO2 NPs increased linearly with the number of bilayers up to 10 bilayers. The voltammetry of the modified electrode was that of hydrous iridium oxide films (HIROFs) with an observed super-Nernstian pH response of the Ir(III)/Ir(IV) and Ir(IV)-Ir(IV)/Ir(IV)-Ir(V) redox transitions and Nernstian shift of the oxygen evolution onset potential. The overpotential of the oxygen evolution reaction (OER) was essentially pH independent, varying only from 0.22 V to 0.28 V (at a current density of 0.1 mA cm(-2)), moving from acidic to alkaline conditions. Bulk electrolysis experiments revealed that the IrO2/PDDA films were stable and adherent under acidic and neutral conditions but degraded in alkaline solutions. Oxygen was evolved with Faradaic efficiencies approaching 100% under acidic (pH 1) and neutral (pH 7) conditions, and 88% in alkaline solutions (pH 13). This layer-by-layer approach forms the basis of future large-scale OER electrode development using ink-jet printing technology.

Catalysis of water oxidation in acetonitrile by iridium oxide nanoparticles.

Water oxidation catalysed by iridium oxide nanoparticles (IrO2 NPs) in water-acetonitrile mixtures using [RuIII(bpy)3]3+ as oxidant was studied as a function of the water content, the acidity of the reaction media and the catalyst concentration. It was observed that under acidic conditions (HClO4) and at high water contents (80% (v/v)) the reaction is slow, but its rate increases as the water content decreases, reaching a maximum at approximately equimolar proportions (≈25% H2O (v/v)). The results can be rationalized based on the structure of water in water-acetonitrile mixtures. At high water fractions, water is present in highly hydrogen-bonded arrangements and is less reactive. As the water content decreases, water clustering gives rise to the formation of water-rich micro-domains, and the number of bonded water molecules decreases monotonically. The results presented herein indicate that non-bonded water present in the water micro-domains is considerably more reactive towards oxygen production. Finally, long term electrolysis of water-acetonitrile mixtures containing [RuII(bpy)3]2+ and IrO2 NPs in solution show that the amount of oxygen produced is constant with time demonstrating that the redox mediator is stable under these experimental conditions.