Applied Surface Science 339 (2015) 22–27 Contents lists available at ScienceDirect Applied Surface Science journal h om epa ge: www.elsevier.com/locate/apsusc Charge transfer behavior of graphene-titania photoanode in CO 2 photoelectrocatalysis process Md. Rakibul Hasan a,∗ , Sharifah Bee Abd Hamid a , Wan Jeffrey Basirun b a Nanotechnology & Catalysis Research Centre (NANOCAT), 3rd Floor, Block A, Institute of Postgraduate Studies (IPS), University of Malaya, 50603 Kuala Lumpur, Malaysia b Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia a r t i c l e i n f o Article history: Received 30 October 2014 Received in revised form 8 February 2015 Accepted 23 February 2015 Available online 1 March 2015 Keywords: Charge transfer Recombination rate CO2 conversion Photoelectrocatalysis a b s t r a c t In the present study, a graphene-titania composite photoelectrode was synthesized, characterized and examined for the photoelectrocatalytic (PEC) response. The charge transfer process on the semicon- ductor/electrolyte interface was investigated via electrochemical impedance spectroscopy (EIS) and voltammetry. In addition, the influence of pH toward the photoanode performance was also investi- gated and it was noticed that a high pH condition was favorable higher photocurrent response from the EIS measurements. The main reason could be attributed to the decrease of recombination process at the photoanode with fast quenching of the photogenerated holes with OH - ions at high pH. The experiment was also run for CO 2 photoreduction and increased photocurrent was observed. © 2015 Elsevier B.V. All rights reserved. 1. Introduction With the increase of population growth, sky-rocketing depend- ence on fossil fuels are placing unprecedented constraints on energy supplies for domestic, commerce and industrial sectors. The dependency on fossil fuels enhances carbon dioxide (CO 2 ) emission to the atmosphere which eventually plays a major role in long term climate change and global warming [1,2]. Since the CO 2 molecule is highly stable, it requires very high energy input for the successful recycling to synthetic fuels. The photoelectrocatalytic reduction of CO 2 is a very promising catalytic process where direct solar energy is utilized to activate the photocatalyst for the desired reactions [3,4]. Much of the important research in photoelectrochemistry is aimed to develop these photocatalysts by utilizing semiconductors with a suitable energy band-gap and favorable electron transfer kinetics [5]. For this purpose, TiO 2 nanomaterials have shown some promise although some significant improvements are mandatory for a successful photocatalytic activity. Recently researchers have reported about graphene-TiO 2 nanocomposites for the application of various optical and electronic devices. This composite mate- rial has also shown tremendous photocatalytic activity and charge transfer properties [6,7]. ∗ Corresponding author. Tel.: +60 149012391. E-mail addresses: rakibacctdu@gmail.com (Md.R. Hasan), sharifahbee@um.edu.my (S.B.A. Hamid), wjeffreyb@yahoo.com (W.J. Basirun). The most important mechanism for the photo-assisted reduc- tion of CO 2 is the photogeneration of electrons and holes on the electrode surface. Upon illumination with photon energies higher than the band gap energy of the semiconductor photocatalyst, the photogenerated electrons are excited from the valence band to conduction band, while the holes migrate to the surface [8]. The photogeneration of electrons and holes and their subsequent redox reactions can be summarized as follows: TiO 2 (e CB - - h VB + ) → e CB - + h VB + (1) The holes oxidize the H 2 O molecule: 2H 2 O + 4 h + → 4H + + O 2 (2) A small biased potential is necessary to drive the oxidation pro- cess at the photoanode. This helps to reduce the recombination of electron (e CB - ) and hole (h VB + ) and results in an improved photo- catalytic performance. At the same time, the photoexcited electrons are drawn to the counter cathode where the actual CO 2 electrore- duction occurs. Several reports have been published on CO 2 photo-conversion process using TiO 2 photocatalysts. The doping of TiO 2 with various metal, non-metal and other semiconductors has been per- formed to extend the absorption range in the visible region [9]. Recently, graphene showed tremendous performance in photocat- alytic activity due to its high optical transmittance, large specific surface area and unique electronic properties [10]. It was demon- strated earlier that the electrochemical redox reactions could be http://dx.doi.org/10.1016/j.apsusc.2015.02.162 0169-4332/© 2015 Elsevier B.V. All rights reserved.