P. Salvador

Analysis of the transient photocurrent-time behaviour of a sintered n-SrTiO3 electrode in water photoelectrolysis

Abstract The transient photocurrent-time behaviour shown by a sintered n-SrTiO 3 photanode during water photoelectrolysis has been studied. A model based on the existence of surface states in the bandgap region, generated by illumination, is supported by the experimental results. At sufficiently low band bending these surface states can be filled with conduction band electrons giving rise to a back-reaction opposed to the anodic photocurrent, which results in a photocurrent transient. Furthermore photogenerated holes can be trapped by intrinsic surface states, producing a decrease of the negative surface charge, and consequently a positive shift of the flat-band potential, which also decreases the photocurrent efficiency. The nature of the surface states involved in these photocurrent transients and their implication in the quantum efficiency of the system are discussed. The conclusions reached are applicable to n-TiO 2 and probably to other semiconducting oxide photoanodes.

Photoelectrochemical properties of n‐type NiTiO3

The photoelectrochemical behavior of a sintered Nb doped n‐NiTiO3 electrode in relation to the photoassisted oxidation of water has been studied. Three bands mediate the different photoprocesses observed: A conduction band (Ti4+:3d) several tenths of one electron volt above the H+/H2 redox level; the O2−:2u2009p6 band about 3 eV below the conduction band; and the Ni2+:3d8 ‘‘band’’ above the oxygen band that seems just to overlap the O2/H2O level. Electronic transitions induced by UV illumination are mainly O2−→Ti4+ transitions; holes photogenerated in the oxygen band move through this band, by the effect of the internal electric field, to the electrode surface where they react with water molecules with an efficiency similar to that observed in n‐TiO2 or SrTiO3. Visible light produces Ni2+→Ti4+ electronic transitions. Hole mobilities in the Ni2+:3d8 ‘‘band’’ appear to be adequate for the photogenerated holes to reach the s.c. surface, where they become trapped at Ni3+ surface centers at an energy level too hig...

Photoresponse of n‐type semiconductor NiTiO3

The 60‐Hz photocurrent of 103‐Ωu2009cm, Nb‐doped n‐TiO3 has been measured by lock‐in amplification as a function of wavelength and applied potential. NiTiO3 is oxidized anodically in the dark producing Ni2+ ions in solution; however, this corrosion is but a fraction of the total charge passed either with visible or UV light, which indicates that photogenerated holes are able to oxidize water. The spectral response is shifted towards the visible region, as compared with TiO2, and the flatband potential is 0.4 V more negative than that of the H+/H2 redox couple, which would make possible the use of NiTiO3 for water photoelectrolysis. However, the quantum efficiency for visible light is much lower than that for UV light, primarily because of a low oscillator strength for the visible light Ni2+‐Ti4+ charge‐transfer excitation in this structure.

Photoelectrochemical properties of n-type NiTiO/sub 3/

The photoelectrochemical behavior of a sintered Nb doped n‐NiTiO3 electrode in relation to the photoassisted oxidation of water has been studied. Three bands mediate the different photoprocesses observed: A conduction band (Ti4+:3d) several tenths of one electron volt above the H+/H2 redox level; the O2−:2u2009p6 band about 3 eV below the conduction band; and the Ni2+:3d8 ‘‘band’’ above the oxygen band that seems just to overlap the O2/H2O level. Electronic transitions induced by UV illumination are mainly O2−→Ti4+ transitions; holes photogenerated in the oxygen band move through this band, by the effect of the internal electric field, to the electrode surface where they react with water molecules with an efficiency similar to that observed in n‐TiO2 or SrTiO3. Visible light produces Ni2+→Ti4+ electronic transitions. Hole mobilities in the Ni2+:3d8 ‘‘band’’ appear to be adequate for the photogenerated holes to reach the s.c. surface, where they become trapped at Ni3+ surface centers at an energy level too hig...

Analysis of the photocurrent transient behaviour associated with flatband potential shifts during water splitting at n-TiO2 electrodes

Abstract The effect of local pH changes and surface hole accumulation on the photocurrent transients observed during water photoelectrolysis at TiO2 electrodes has been studied. The diffusion of pH-determining electrolyte species is found to affect not only the time-dependent photocurrent decay but also the photocurrent vs. potential dependence. Whereas the instantaneous initial photocurrent follows the theoretically expected dependence on the band-bending, the stationary photocurrent deviates from the predicted behaviour, depending on the electrolyte pH. With the help of the rotating disk electrode technique, the ratio between the stationary and the initial photocurrent was studied as a function of the rotation rate, at both pH 10.5 and 12. Variations of band bending, associated with Vfb shifts due to local pH changes, are invoked to explain the observed behaviour. At sufficiently high and low pH values, where photocurrents are not limited by the diffusion of OH− and H+ electrolyte species, the transients are due exclusively to Vfb shifts associated with accumulation of photogenerated holes at the semiconductor surface. These results are consistent with those obtained recently in our laboratory from electrolyte electroreflectance experiments, which allowed for the first time direct measurement of the Vfb shifts occurring during photoelectrochemical water splitting.

The scanning microscope for semiconductor characterization (SMSC) : study of the influence of surface morphology on the photoelectrochemical behavior of an n-MoSe2 single crystal electrode by photocurrent and electrolyte electroreflectance imaging

Abstract Photocurrent and electrolyte electroreflectance (EER) measurements have been carried out, on a microscopic scale, in order to characterize the photoelectrochemical behavior of an n-MoSe 2 single crystal as a function of its surface morphology. Digital images of the surface distribution of these photoeffects have been obtained with the help of the scanning microscope for semiconductor characterization (SMSC). From the mathematical expressions describing both photoeffects, it is shown that the corresponding images yield complementary information about the semiconductor (sc)electrolyte interface, which can be related to the semiconductor topology. A comparative study is made on three different types of surface: (1) “as grown”, produced just after growing the single crystal; (2) “aged”, which results from the as-grown surface after working the electrode in the photoelectrochemical cell for some time; and (3) “freshly-peeled”, which is obtained by peeling off the upper sandwich layers of the aged sample. In the case of the aged surface, it is observed that electrochemical corrosion is detrimental for the photoelectrochemical behavior of the semiconductor, except on the sharp defects of its van der Waals surface. An explanation of this aging effect is proposed on the basis of the high anisotropy and structural parameters of this material.From EER images of the sc ¦electrolyte interface, the spatial distribution of the drop of the applied voltage at the Helmholtz layer is shown for the first time.

New semiconducting oxide photoanodes for water splitting in a photoelectrochemical cell Electrochemistry of n-type NiTiO3

Abstract The electrochemical behaviour of NiTiO 3 has been studied. The interest in this mixed-metal oxide lays in its possible use for solar-energy conversion. Anodic oxidation of NiTi0 3 was very irreversible. The electrode became aged, probably due to formation of a porous film of an irreducible higher oxide. The stirring-independent Cottrell behaviour of the anodic oxidation of NiTiO 3 should be due to current limitation by diffusion of a reaction product away from the NiTi0 3 surface through this irreducible higher oxide, which should be of a porous nature. The diffusion-limiting species could not be OH − , as the Cottrell slopes only increased by about five times for a pH increase from 3 to 14. Five and two processes were involved in the anodic oxidation of NiTiO 3 at pH 14 and 3 respectively, as determined by chronopotentiometry. The same number of processes appeared upon inversion of the current.

Photoelectrochemical study of the TiO2Cr system. Observation of strong(001) rutile photoetching in the presence of Cr(VI)

Abstract The photoassisted electroreduction of Cr(VI) to Cr(III) at (001) TiO2 rutile with UV light has been studied by using conventional photoelectrochemical techniques. Specific adsorption of Cr(VI) on the TiO2 surface is only possible at high chromium concentrations (≈ 0.2 M K2Cr2O7) and very acidic electrolyte (pH ≈ 1). With diluted solutions a weaker interaction seems to dominate Cr(VI) adsorption. This explains the 1 2 order kinetics found in the literature for the photocatalytic reduction of Cr(VI) at suspended TiO2 powders. Cr(VI) electroreduction was found to be higher at acidic than at neutral or basic electrolytes, and attributed to the interaction of dissolved Cr(VI) anions with the pH dependent electric charge of the TiO2 surface. Strong anisotropic TiO2 photocorrosion has been observed for the first time in Cr(VI) containing electrolytes, at pH ≈ 1.0. TiO2 photoetching is believed to be a catalytic process associated with the photo-oxidation of oxygen coordinated Cri3+ interstitial ions, directly generated on the semiconductor surface by reduction of dissolved dichromate ions. Key points in rutile photocorrosion are the weakness of surface Tiue5f8O bonds, owing to hole trapping at O:2p valence band orbitals, and the further surface bonding reorganization, involving the generation and simultaneous breaking of Criue5f8O and Tiue5f8 O bonds (TiO2 dissolution) respectively.

The scanning microscope for semiconductor characterization: photocurrent, photovoltage and electrolyte electroreflectance imaging at the n-MoSe2/I− interface

Abstract Digital images of photocurrent, photovoltage and electrolyte electroreflectance at the n-MoSe 2 /electrolyte interface, with micrometric lateral resolution, are presented under different experimental conditions. Inhomogeneities in the surface distribution of the parameters controlling both the semiconductor photoresponse and the kinetics of transfer of photogenerated charge carriers to the electrolyte can be inferred from these images. A clear influence of the semiconductor topography on these photoeffects is evidenced. So, the photoresponse is observed to decrease in regions with structural surface defects, due to the existence of associated bulk recombination centers. The state of the van der Waals surface in contact with the electrolyte is shown to be a main factor determining the photoelectrochemical behavior of the n-MoSe 2 electrode.

On the photoelectrochemical etching of polycrystalline CdS thin film electrodes in SO2-3 electrolytes

Donnees sur la photocorrosion de couches minces, polycristallines, de sulfure de cadmium, dans des solutions de SO 3 2− , dans differentes conditions experimentales (intensite dillumination, potentiel de polarisation et pH de lelectrolyte). Etudes par voltammetrie cyclique