Electrochimica Acta 54 (2009) 3799–3805 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Photoelectrochemical oxidation of salicylic acid and salicylaldehyde on titanium dioxide nanotube arrays Min Tian, Brian Adams, Jiali Wen, R. Matthew Asmussen, Aicheng Chen ∗,1 Department of Chemistry, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada article info Article history: Received 17 December 2008 Received in revised form 27 January 2009 Accepted 28 January 2009 Available online 5 February 2009 Keywords: Photoelectrocatalysis Salicylic acid TiO2 nanotubes UV–vis spectroscopy Quantum computation abstract We report on the kinetics of photoelectrochemical oxidation of salicylic acid (SA) and salicylaldehyde (SH) on titanium dioxide nanotube arrays. The TiO 2 nanotubes were prepared by the electrochemical oxidation of titanium substrates in a nonaqueous electrolyte (DMSO/HF). Scanning electron microscopy (SEM) was employed to examine the morphology of the formed nanotubes. Linear voltammetry was used to study the electrochemical and photoelectrochemical behavior of the synthesized TiO 2 nanotube arrays. The photoelectrochemical oxidation of SA and SH on the TiO 2 nanotubes was monitored by in situ UV–vis spectroscopy, showing that the kinetics of the photoelectrochemical oxidation of SA and SH follows pseudo first-order and that the rate constant of SH oxidation is 1.5 times larger than that of SA degradation. Quantum chemical calculations based on the DFT method were performed on SA and SH to address the large difference in kinetics. The relatively higher E LUMO - E HOMO makes SA more stable and thus more difficult to be oxidized photoelectrochemically. The impact of temperature and initial concentrations on the kinetics of SA and SH photoelectrochemical degradation was also investigated in the present work. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction It is known that salicylic acid (SA) has an ototoxic effect and can induce transient hearing loss in zinc-deficient individuals. It can also cause fetal abnormalities and central nervous system depres- sion if swallowed, inhaled or absorbed through skin. Salicylic acid has been identified as a water pollutant which originates from the manufacturing activities of paper milling, cosmetic industries and as a landfill leachate. Thus, there is great interest in studying the kinetics of the degradation of SA in order to develop advance oxidation techniques to effectively treat the SA pollutant for the purification of wastewater and groundwater. A variety of techniques have been proposed for wastewater treatment including electrochemical oxidation [1,2], activated elec- trosorption [3], chemical oxidation, and biological digestion [4]. Heterogeneous photocatalysis is a promising alternative technique for eliminating organic pollutants from wastewater [5–8]. Among various oxide semiconductor photocatalysts, titania (TiO 2 ) is one of the most promising photocatalysts [9–12] because of its biological and chemical inertness, cost effectiveness, and the strong oxidizing power of its photogenerated holes [13,14]. TiO 2 has three natural phases: brookite, anatase and rutile; anatase is commonly consid- ∗ Corresponding author. Tel.: +1 807 3438318; fax: +1 807 3467775. E-mail address: aicheng.chen@lakeheadu.ca (A. Chen). 1 ISE member. ered to be the most active phase as a photocatalyst [15,16]. When TiO 2 is irradiated with near UV light ( < 400 nm), the ensemble of the electrons are excited from the valence band to the conduction band, generating electron–hole pairs which initiate the subsequent surface catalytic reactions. The generated electron–hole pairs have the tendency to recombine; this lowers the photocatalytical effi- ciency. TiO 2 suspension is widely used due to its large surface area/volume ratio which improves the photocatalytical efficiency. However, there is a serious drawback associated with this approach: it is tedious and costly to recycle TiO 2 suspensions from the treated wastewater, making it undesirable for large scale treatment. Using supported semiconductor films is an alternative solution to over- come the recycling problem. However, when photocatalyst films are used, the surface area exposed to the liquid phase is reduced by 2–3 orders of magnitude compared to powders. The diffusion boundary conditions change from binary collisions to that of a particle against a wall; and mass transfer can become a limiting step. It has been shown that using TiO 2 nanostructures with a high surface area can effectively improve the photocatalytic efficiency [17]. In addition, with TiO 2 nanostructures, the recombination between the photo- generated charge carriers can be effectively suppressed by applying an external potential bias [18,19], further increasing the efficiency of wastewater treatments. In the present study, for the first time, we report on the kinetics of the photoelectrochemical oxidation of SA and its related com- pound, salicylaldehyde (SH), on Ti-supported TiO 2 nanotube arrays. SA and SH were chosen as model organic pollutants since they can 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.01.077