Journal of Electroanalytical Chemistry 458 (1998) 99 – 105 The scanning microscope for semiconductor characterization (SMSC): electrolyte electroreflectance and photovoltage imaging study of the electrochemical activation of RuS 2 photoelectrodes for oxygen evolution M. Turrio ´n a , N. Alonso-Vante a , H. Tributsch a , A. Mir b , P. Salvador b, * a Hahn -Meitner -Institute, Abt. Solare Energetik, 14109 Berlin, Germany b Departamento de Matema ´ticas e Informa ´tica, Uniersidad de las Islas Baleares, 07071 Palma, Spain Received 16 April 1998; received in revised form 28 July 1998 Abstract The photoelectrochemical behaviour of the oxygen evolving n-RuS 2  H 2 SO 4 interface was investigated with the help of the scanning microscope for semiconductor characterization. Electrolyte electroreflectance and photovoltage digital images were obtained as a function of the pretreatment of the RuS 2 surface (polishing and electrochemical etching). Polishing gives rise to an inhomogeneous, scarcely photoactive surface, while electrochemical etching destroys the damaged overlayer generated by polishing, leaving an inhomogeneous, highly photoactive surface in contact with the electrolyte. In general, more photoactive zones are characterized by a higher electrolyte electroreflectance signal (EER-s) generated at the interfacial hydroxide layer, and a smaller signal coming from the space charge layer. According to the observed influence of the donor concentration on the EER-s signal amplitude, the contrast observed in the EER-s image was attributed to lateral heterogeneities in the donor concentration. The difficulty of modulating the electric field in the hydroxide layer at relatively high frequencies was attributed to the low relaxation time constant of surface states involved in the EER-s signal. The fact that at low light modulation frequencies those zones with the highest EER-s amplitude also show the highest photovoltage indicates that the photovoltage intensity depends on the time constant. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Electrolyte electroreflectance; Photovoltage digital images; RuS 2 1. Introduction RuS 2 is the first discovered photostable, low-energy gap semiconductor (SC) (E g 1.3 eV), with energy bands derived from Ru-d states, able to photooxidize water under infrared illumination and supporting bias potential [1]. Under illumination, photogenerated va- lence-band holes are trapped at bandgap surface states (SS) associated with lattice Ru atoms, producing sur- face oxidation and inducing interfacial, coordination chemical reactions with water molecules, which finally leads to molecular oxygen evolution [2]. The presence of an interfacial hydroxide layer (IHL) when RuS 2 is in contact with 0.5 M H 2 SO 4 , explains the complex elec- trochemical [3] and photoelectrochemical [4,5] be- haviour of this semiconducting compound. The role of this interfacial phase and the nature of the complexes involved in the reaction of oxygen evolution has been a subject of recent investigations [5,6], although there is no entirely clear explanation up to now. Freshly mounted RuS 2 electrodes show high dark currents and few photoeffects. Dark currents are par- tially due to the oxidation of interfacial states associ- ated with the lattice Ru atoms coordinated to water molecules [5]. A dark current decrease and the simulta- * Corresponding author. Fax: +34 71 173003/173426.. 0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII:S0022-0728(98)00317-9