Applied Surface Science 351 (2015) 950–961 Contents lists available at ScienceDirect Applied Surface Science journal h om epa ge: www.elsevier.com/locate/apsusc Evaluation of surface energy state distribution and bulk defect concentration in DSSC photoanodes based on Sn, Fe, and Cu doped TiO 2 Rajour Tanyi Ako a , Piyaisiri Ekanayake a,∗ , David James Young a,b,c , Jonathan Hobley a , Vijila Chellappan b , Ai Ling Tan a , Sergey Gorelik b , Gomathy Sandhya Subramanian b , Chee Ming Lim a a Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, BE1410, Negara, Brunei Darussalam b Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 3 Research Link, 117602, Singapore c Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, Locked Bag 4, Maroochydore DC, Queensland, 4558, Australia a r t i c l e i n f o Article history: Received 20 March 2015 Received in revised form 31 May 2015 Accepted 3 June 2015 Available online 11 June 2015 Keywords: Metal doped TiO2 Sn Fe Cu Charge recombination Energy state distribution a b s t r a c t Electron transfer dynamics in the oxide layers of the working electrodes in both dye-sensitized solar cells and photocatalysts greatly influences their performance. A proper understanding of the distribu- tion of surface and bulk energy states on/in these oxide layers can provide insights into the associated electron transfer processes. Metal ions like Iron (Fe), Copper (Cu) and Tin (Sn) doped onto TiO 2 have shown enhanced photoactivity in these processes. In this work, the structural, optical and transient properties of Fe, Cu and Sn doped TiO 2 nanocrystalline powders have been investigated and compared using EDX, Raman spectroscopy, X-ray Photoelectron spectroscopy (XPS), and Transient Absorption spec- troscopy (TAS). Surface free energy states distributions were probed using Electrochemical Impedance spectroscopy (EIS) on Dye Sensitized Solar Cells (DSSC) based on the doped TiO 2 photoanodes. Raman and XPS Ti2p 3/2 peak shifts and broadening showed that the concentration of defects were in the order: Cu doped TiO 2 > Fe doped TiO 2 > Sn doped TiO 2 > pure TiO 2 . Nanosecond laser flash photolysis of Fe and Cu doped TiO 2 indicated slower transient decay kinetics than that of Sn doped TiO 2 or pure TiO 2 . A broad absorption peak and fast transient decay at 430 nm for Fe doped TiO 2 was ascribed to an increase in surface hole concentration resulting in poor current density in the Fe doped TiO 2 photoanodes relative to pure TiO 2 , Sn or Cu doped anodes. The charge transfer capacitance and the calculated electron lifetimes correlated well with the trend in current density of the photoanodes (Sn > Cu > pure TiO 2 ). The poor per- formance of Fe doped cells is due to faster recombination of injected electrons with surface holes while those of Sn and Cu were more influenced by the concentration of their bulk defects. These results demon- strate that the choice of selected metal ions doping onto TiO 2 for a desired application should take into consideration the influence of bulk defect concentrations, the energy state distribution and the electron transfer properties in/on the oxide photoanodes. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Efficient photovoltaic and photocatalytic processes are gov- erned by the rate of charge transport in the oxides that are used as photoanodes. These semiconductor metal oxides act as sup- ports to directly or indirectly harness sunlight energy. Significant improvements have been achieved by tailoring the bulk/surface defect concentrations and band gap energies with controlled ∗ Corresponding author. Tel.: +673 2463001x1322; fax: +673 2461502. E-mail address: piyasiri.ekanayake@ubd.edu.bn (P. Ekanayake). amounts of different impurities. On the one hand, illuminated semiconductor oxides in photocatalytic processes can attain a state of high concentration of interfacial charges and enhance the charge exchange between the oxide surface and an electron donor/acceptor molecules in proximity, thus facilitating the desired chemical processes [1,2], such as water splitting [3] and photocat- alytic degradation of environmental pollutants [4,5]. On the other hand, semiconductor oxides with appropriate bulk defects will act as charge traps to free charges and prevent electron–hole recom- bination that enhances charge transport within the oxide [6,7] and are thus used as oxide layers in photovoltaics such as dye-sensitized solar cells (DSSC) for sunlight to current conversion [8]. Titanium http://dx.doi.org/10.1016/j.apsusc.2015.06.015 0169-4332/© 2015 Elsevier B.V. All rights reserved.