Electronic transport in semiconductor nanoparticles for photocatalytic and photovoltaic applications M. Kunst a, ⁎ , F. Goubard b , C. Colbeau-Justin c , F. Wünsch a a Hahn-Meitner Institut, SE5, Glienickerstrasse 100, D-14109 Berlin, Germany b Université de Cergy-Pontoise, LPPI, Neuville sur Oise- rue d'éragny,F-95031 Cergy Pontoise Cedex, France c Université Paris 13, Lab. d'Ingénierie des Matériaux et des Hautes Pressions- CNRS (UPR 1311), 99 Avenue J.B. Clément, F-93430 Villetaneuse, France Received 7 May 2006; received in revised form 18 September 2006; accepted 18 September 2006 Available online 24 October 2006 Abstract It is shown by contactless transient photoconductance measurements that electron transport in TiO 2 is highly dispersive and is strongly influenced by surface treatments. A fast recombination process is observed after band to band excitation. Oxalic acid absorbed at the surface slows the decay appreciably, as does alcohol. The electron transport in TiO 2 films and powder of the same composition is nearly identical pointing to an only minor importance of inter grain transport for the present measurements (up to 100 μs). Dye sensitization of TiO 2 leads not only to charge carrier generation by light in the visible range but also seems also to improve the electron transport properties. © 2006 Published by Elsevier B.V. Keywords: Nanomaterials; Charge carrier kinetics; Photocatalysis 1. Introduction One of the most promising applications of semiconductor nanomaterials is in photocatalysis and photovoltaics, where already important practical results have been obtained. The much larger surface-to-volume ratio compared to single crystals or films enables a stronger interaction between absorbed mole- cules and excess charge carriers leading to a better photo- catalysis and also a better excess charge carrier separation and so a higher photovoltaic efficiency. For the moment TiO 2 is the usual material for both appli- cations and for this reason the present work will be concentrated on this material. In photocatalysis and in photovoltaics excess charge carrier transport plays an important role and so transient photoconductance measurements yield relevant information on these processes. In this work contactless transient photocon- ductance measurements in the microwave frequency range by the Time Resolved Microwave Conductivity (TRMC) tech- nique will be presented. TRMC measurements have the addi- tional advantage that also measurements on powder samples can be performed. 2. Experimental TRMC measurements have been performed at about 30 GHz in a Ka band equipment as described previously [1]. TRMC signals have been excited by 10 ns (FWHM) pulses at 532 nm and at 355 nm of a Nd:YAG laser. The TRMC signal, ΔP(t)/P, i.e. the relative change of the microwave power reflected by the sample induced by a pulsed change of the conductance ΔS(t), is proportional to ΔS(t): D Pðt Þ=P ¼ A D S ðt Þ; where A is a proportionality constant and represents ΔS(t) the photoconductance induced by the exciting laser pulse at time t after the start of the laser pulse. In general ΔS(t) is determined by all mobile excess species but for TiO 2 it will be assumed that only electrons at the bottom of the conduction band with mobility μ n contribute to the photoconductance [2]: D Pðt Þ=P ¼ Ae l n D N ðt Þ Because a one-dimensional model is used the total number of excess electrons at time t, ΔN(t), refers to an integration of the excess electron concentration, Δn(t), only over the thickness d of the sample. So the TRMC signal represents excess electron Materials Science and Engineering C 27 (2007) 1061 – 1064 www.elsevier.com/locate/msec ⁎ Corresponding author. E-mail address: kunst@hmi.de (M. Kunst). 0928-4931/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.msec.2006.09.028