‘‘In situ” XPS study of band structures at Cu 2 O/TiO 2 heterojunctions interface Lei Huang a,b , Feng Peng b , Fumio S. Ohuchi a, * a Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA b School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China article info Article history: Received 4 March 2009 Accepted for publication 21 July 2009 Available online 6 August 2009 Keywords: TiO 2 Cu 2 O Interface Band alignment X-ray photoelectron spectroscopy abstract In an attempt to investigate influence of the defects on electronic structure of Cu 2 O/TiO 2 heterojunctions, thin Cu 2 O layers were successively deposited on TiO 2 that has different levels of defect concentrations, and the resultant band bending and offset characteristics were studied by in situ X-ray photoelectron spectroscopy (XPS). The TiO 2 substrates with defects were prepared by Ar + sputtering, followed by annealing at different temperatures in oxygen atmosphere. Presence of the defects in TiO 2 surface dra- matically influences on the band bending and band offset at the interface: more defects are on TiO 2 sur- face, less band bending are at the interface, inducing smaller conduction band offsets. On the reduced TiO 2 surface, Cu 2 O was disproportionately decomposed to form CuO and Cu. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Cu 2 O thin films deposited on a TiO 2 substrate typically form a p– n heterojunction. This system has been widely investigated for pho- tocatalysis and solar cell applications [1–3]. The bandgap energies of Cu 2 O and TiO 2 are 2.0 and 3.2 eV [4–7], respectively. Both the conduction band minimum and the valence band maximum of Cu 2 O lie above those of TiO 2 ; therefore the electrons excited to the conduction band of Cu 2 O would transfer to TiO 2 , whereas the holes generated in the valence band in TiO 2 prefer opposite transfer to Cu 2 O. Charge carriers separated in different semiconductors effectively reduce the chance of electron-hole pair recombination also prolong their lifetime, thus increasing the quantum efficien- cies. In addition, working range of the wavelength is extended to a visible region due to absorption of visible light by Cu 2 O, further enhancing the efficiency of the solar energy transition. Synergistic cooperation of these effects enables the Cu 2 O/TiO 2 system exhibit great potential for solar cell and photocatalysis applications [8–10]. Transport properties of the heterojunctions are, however, lar- gely influenced by band discontinuities and interface states [11,12]. Band discontinuities at the interface are usually deter- mined by the barrier heights for charge carriers to be overcome at the interface; therefore their magnitudes dramatically influence the performance of the heterojunctions. Energy of the photogener- ated electrons will be also lost during the transfer process if the band discontinuities are large. In addition, the interface states act as the recombination centers or carriers traps. Defects in the TiO 2 surface therefore influence the resulting band states and interface structure. It is imperative to examine how the defects in TiO 2 influ- ence on the heterointerface characteristics. In this paper, Cu 2 O/TiO 2 heterojunctions were prepared by suc- cessive deposition of Cu 2 O thin layers on a single crystalline TiO 2 substrate in ultrahigh vacuum (UHV). X-ray photoelectron spec- troscopy (XPS) was used to investigate the electronic property and band offset at the interface. On the basis of the valence band and core level binding energy shifts [13–15], the band bending and energy levels at the Cu 2 O/TiO 2 interface were determined as a function of defects concentrations of the TiO 2 substrate. 2. Experimental Experiments were carried out in an UHV system (P 6 3.0  10 9 Torr) equipped with a monochoromatized Al Ka excited X-ray photoelectron spectroscopy (SSL 300 from Surface Science Inc.), an Ar + ion gun for surface cleaning, a Cu evaporation source, an oxygen source. and TiO 2 substrate holder with a heater. Exper- imental set-up is schematically shown in Fig. 1. The TiO 2 substrate holder consists of a transparent rutile TiO 2 (0 0 1) single crystal with 1 cm in length of a crescent shape and a molybdenum bottom heater. These two components are held to- gether by Ta strips (Fig. 1a). Indium beads were inserted between the TiO 2 sample and Mo heater. Heating the Mo heater to around 180 °C, the In was melt to be spread uniformly in-between the sub- strate and heater to enhance the heat transfer. For calibration, Au and Cu foils were spot-welded on a sample stage. The whole assem- bly was then fastened on an X Y Z sample manipulator and installed 0039-6028/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2009.07.030 * Corresponding author. Address: Department of Materials Science and Engi- neering, University of Washington, P.O. Box 352120, FB-10, Seattle, WA 98195- 2120, USA. Tel.: +1 206 543 8272; fax: +1 206 543 3100. E-mail address: ohuchi@u.washington.edu (F.S. Ohuchi). Surface Science 603 (2009) 2825–2834 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc