Carrier density and interfacial kinetics of mesoporous TiO 2 in aqueous electrolyte determined by impedance spectroscopy Sixto Gimenez ⇑ , Halina K. Dunn, Pau Rodenas, Francisco Fabregat-Santiago, Sara G. Miralles, Eva M. Barea, Roberto Trevisan, Antonio Guerrero, Juan Bisquert ⇑ Photovoltaics and Optoelectronic Devices Group, Departament de Física, Universitat Jaume I, 12071 Castelló, Spain article info Article history: Received 12 December 2011 Received in revised form 21 December 2011 Accepted 23 December 2011 Available online 11 January 2012 Keywords: Water splitting Titanium oxide Porous semiconductors Transmission line Impedance spectroscopy abstract Water splitting at a semiconductor/solution interface with the only input of sunlight to generate hydro- gen is one of the most attractive strategies to produce and store chemical energy. In the present study we have investigated carrier dynamics and interfacial kinetics of mesoporous TiO 2 in an aqueous solution. The applicability of the transmission line model for mesoporous semiconductors has been validated to identify chemical capacitance, transport resistance and charge transfer resistance in this system by test- ing samples of different thicknesses in the dark and under illumination. We found that both transport resistance and chemical capacitance scale well with sample thickness, while charge transfer resistance scales with thickness when the FTO substrate is not exposed to the solution. Otherwise, there is a com- petition between charge transfer through TiO 2 and through the FTO substrate. Under illumination, the electron density is dominated by photogenerated carriers at biases below the open circuit potential, whereas at higher bias, the applied potential determines the electron density. Evidence of charge transfer via surface states has been experimentally observed and corroborated with a physical model, which explicitly includes charge transfer through a monoenergetic trap for electron and holes. This study may lay the basis for understanding more complex processes at anodic potentials on the TiO 2 /solution interface where water splitting reactions take place. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction With the increasing global demand for energy, the need to adopt alternative energy sources is concomitantly growing. Among all the different renewable energy sources presently available, so- lar energy is the only potential candidate to satisfy the global en- ergy needs [1]. Collecting and storing solar energy in chemical bonds, as nature accomplishes through photosynthesis, is an attractive means to solve the energy challenge [2]. One promising strategy to store energy from sunlight is the photo-assisted split- ting of water to generate hydrogen, a clean and portable energy carrier, which can be used to supply electricity upon demand [2,3]. Since the seminal report of Fushijima and Honda in 1972 [4], demonstrating the photocatalytic effect of TiO 2 for electrochemical water splitting, intensive efforts have been dedicated to enhance the efficiency of this material by different strategies: (i) extending the absorption range to the visible (C [5],N [6] and S [7] doping) (ii) tuning the energy of the conduction band (CB) to withstand the hydrogen evolution reaction (HER) by alloying with Ba or Sr). Indeed, SrTiO 3 has been the only material showing water splitting without external assistance, although with efficiencies lower than 1% [8]. Moreover, the molecular mechanisms for the involved reac- tions have been thoroughly studied. Particular attention has been given to the four-hole oxygen evolution reaction (OER) on TiO 2 surfaces, since this reaction has been identified as the rate limiting step of the process. Salvador suggested a mechanism for water oxidation based on the formation of OH Å radicals on the surface, followed by the formation of H 2 O 2 , which is finally oxidized to O 2 [9–12]. This type of intermediates could build up at the surface to allow a sufficient number of holes to accumulate in the same place, in line with the suggestion made by Cowan et al. [13] On the other hand, Nakato and co workers [14,15] reported that the OER is not initiated by the electron transfer-type oxidation, but by a nucleophilic attack of an H 2 O molecule (Lewis base) to a surface-trapped hole (Lewis acid), accompanied by bond breaking. Consequently, understanding charge carriers dynamics for both electrons and holes is essential for the evaluation of candidate materials and for the identification of relevant mechanisms in the process. Previous investigations of electrochemical impedance spectros- copy (EIS) employed to elucidate molecular mechanisms at the TiO 2 /solution interface highlighted the key role of surface states on the electrode kinetics [16–19]. The present study is aimed at 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.12.019 ⇑ Corresponding authors. Tel.: +34 964387554; fax: +34 964729218. E-mail addresses: sjulia@fca.uji.es (S. Gimenez), bisquert@fca.hji.es (J. Bisquert). Journal of Electroanalytical Chemistry 668 (2012) 119–125 Contents lists available at SciVerse ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem