Vincent Laporte

Highly active oxide photocathode for photoelectrochemical water reduction

A clean and efficient way to overcome the limited supply of fossil fuels and the greenhouse effect is the production of hydrogen fuel from sunlight and water through the semiconductor/water junction of a photoelectrochemical cell, where energy collection and water electrolysis are combined into a single semiconductor electrode. We present a highly active photocathode for solar H(2) production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electrodeposited Pt nanoparticles. The roles of the different surface protection components were investigated, and in the best case electrodes showed photocurrents of up to -7.6 mA cm(-2) at a potential of 0 V versus the reversible hydrogen electrode at mild pH. The electrodes remained active after 1 h of testing, cuprous oxide was found to be stable during the water reduction reaction and the Faradaic efficiency was estimated to be close to 100%.

Intermediate temperature embrittlement of copper alloys

Abstract Copper and its alloys generally display a severe reduction in ductility between roughly 300 and 600°C, a phenomenon variously called ‘intermediate temperature embrittlement’ or ‘ductility trough behaviour’. This review of the phenomenon begins by placing it in the wider context of the high-temperature fracture of metals, showing how its occurrence can be rationalised in simple terms on the basis of what is known of intergranular creep fracture and dynamic recrystallisation. Data in the literature are reviewed to identify main causes and mechanisms for embrittlement, first for pure copper, and then for monophase and multiphase copper alloys. Coverage then turns to the ‘grain boundary embrittlement’ phenomenon, caused by the intergranular segregation of even minute quantities of alloying additions or impurities, which appears to worsen dramatically the intermediate temperature embrittlement of copper alloys. Finally, metal-induced embrittlement, including in particular liquid metal embrittlement, is presented as a second mechanism leading to an exacerbation of the intermediate temperature embrittlement of copper and its alloys.

Diffusion-controlled liquid bismuth induced intergranular embrittlement of copper

The consequences of the contact between liquid bismuth and a copper bicrystal are investigated at 500°C. Atoms of bismuth are shown to penetrate and embritlle the copper grain boundary. Grain boundary concentration profiles of bismuth are obtained on fracture surfaces by both Auger electron spectroscopy and He4+ Rutherford backscattering spectroscopy. The maximum bismuth intergranular concentration is calculated from experimental data to be about 1.7 monolayers (near the liquid bismuth / solid copper interface). The overall profiles are significantly different from typical erfc profiles and an interpretation is proposed, based on the coupling effect between grain boundary diffusion and non-linear segregation. These results allow us to conclude on the absence of grain boundary wetting for the Cu / Bi system at 500°C.

Reducing recombination in ZnO photoanodes for dye sensitised solar cells through simple chemical synthesis

A method has been developed to synthesise zinc oxide nanowire arrays that demonstrate superior device performance compared to conventional ZnO nanowires within dye sensitised solar cells (DSSCs). This simple yet effective methodology has produced nanowires with triple device performance when compared to conventional ZnO nanowire DSSCs. We have fully characterised the morphology and physical properties of these wires and have demonstrated that the device improvements are due to a reduction in the recombination centres associated with the ZnO crystal.

Evidence for a Diffusion-Based Mechanism of Liquid Metal Intergranular Penetration: Case Study of a Ni-Bi Model System

A model Ni-Bi system has been used to investigate intergranular penetration (IGP) phenomenon. All experiments have been done on Ni 26°<110> bicrystal at 700°C using bismuth vapour condensation as a source of liquid bismuth. Such a procedure results at room temperature in either partial or total Liquid Metal Induced Embrittlement (LMIE) of a unique grain boundary, depending on the duration of liquid Bi / solid Ni contact at 700°C. Auger Electron Spectrometry (AES) and Rutherford Backscattering Spectrometry (RBS) have been used to measure the Bi concentration profile between the source of liquid bismuth and the penetration front. Two zones have been clearly identified : the first one of almost constant Bi concentration called nanometrethick film which is interpreted in terms of Fowler-Guggenheim multi-layer segregation under local equilibrium conditions and the second one with a progressive decrease of Bi concentration over a distance of the order of 20-200µm. Such a long transition zone, together with parabolic diffusion kinetics indicates diffusion-based mechanism of intergranular penetration as opposed to the direct grain boundary wetting.