Charge Transfer Process at Illuminated Semiconductor/Electrolyte Junctions Modified by Electrodeposition of Microscopic Metal Grain

Abstract

Layers made of microscopic Pt grains, optically transparent, with various morphologies are deposited on n-GaAs by electrocrystallization. It is shown that the modified GaAs photoanode is long-term stabilized under illumination in aqueous electrolytes provided that the film presents some required structural properties, e.g., size and density of Pt nuclei. In fact, correlations between the film morphology [studied by transmission electron microscopy (TEM)] and the stabilization of the modified electrode [studied by I-V, C-V measurements and the rotating ring-disk electrode (RRDE) technique] are observed. The mechanism of stabilization by Pt grains is also investigated by.considering the surface state distributions, determined by photocapacitance experiments, at both bare and modified interfaces.The use of n-type narrow bandgap semiconductor/electrolyte (SC/EL) junctions as practical photoelectrochemical (PEC) solar cells is still hampered by the degradation of the photoanode, i.e., the photocorrosion and the chemical attack of the semiconductor. Many years ago photoanodes (Si, GaAs, GAP...) were reported to be effectively protected by vacuum deposition of thin metal layers (1-4), but with some disadvantages: (i) a too large film absorption and (ii) the fact that the modified interface behaves like a semiconductor/metal Schottky barrier dipped in a liquid. Recently the concept of discontinuous metal films makes metal deposition more attractive because such films can be optically transparent (5) and the intrinsic properties of the SC/EL contact (e.g., photovoltage) can be preserved (6). At present discontinuous metal films proved to be suitable for catalyzing the hydrogen photoevolution at p-InP and p-Si (7, 8) and for stabilizing n-Si and n-GaAs in aqueous media (9, 10). Reference (9) puts forward a theory to interpret the spectacular photovoltage of n-Si electrodes modified with a discontinuous metallic film. As the Si metal-free surface is passivated in aqueous media the Pt grains act as the only pathway through the oxide so the invoked catalytic effect is not clear. Considering the above question the present paper intends to bring a better understanding of the role played by the Pt grains. For that purpose we must (i) consider a semiconductor which does not passivate and (ii) be able to vary easily the morphology (density, size of the grains, distribution) of the deposited film. The second point is essential because according to Ref. (9), we expect correlations between the photoelectrochemical behavior of the modified interface (I-V, stabilization...) and the film morphology.The present study deals with the n-GaAs/aqueous electrolyte junction modified by electrodeposition of Pt because GaAs does not passivate in water solution (unless the pH is close to 7); furthermore a previous work in our laboratory suggests that the use of electrocrystallization of Pt on n-GaAs should fulfill the second above requirement because the nucleation and growth modes of the film proved to be easily changed by just modifying some par...

Article

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Electrochemical deposition of metals onto silicon

“…In photocatalysis, metal islands on p-type semiconductors [30][31][32][33][34][35][36][37][38][39][40][41] have been shown to catalyse charge transfer processes such as the reduction of water. In this case the metal islands can be viewed as electronic surface states.…”

“…For deposition of metals onto semiconductors, the critical nucleus size has been reported to be close to unity, implying a barrierless nucleation process. However, the determination of |η| in equations (39) and (41) for semiconductor surfaces is not straightforward. The implications of this will be discussed in section 6.3.4.…”

“…Therefore, by determining AN ∞ as a function of the applied potential, the number of atoms required to form a critical nucleus can be obtained using equation (39) or (41). Experimentally, the mechanism of nucleation and growth can be obtained from analysis of current transients at different deposition potentials.…”

The enhancement of TiO2 photocatalytic activity by hydrogen thermal treatment

“…On the other hand, simple recombination of the e-h pairs is the major hindrance leading to a low quantum yield (<5%). Some recent research confirmed that a number of metal islands (Heller and Miller, 1980;Allongue et al, 1989; or metal oxides (Gopidas et al, 1990;Bedja et al, 1994;Anderson and Bard, 1995) were introduced into the TiO 2 lattice for accelerating the e-h separation. Alternatively, the thermal hydrogen (H 2 ) treatment of TiO 2 was also found to be capable to prolong the holesÕ lifetime by reducing the number of bulk recombination centers (Harris and Schumacher, 1980;Heller et al, 1987).…”