Journal of The Electrochemical Society, 162 (3) H93-H101 (2015) H93 0013-4651/2015/162(3)/H93/9/$31.00 © The Electrochemical Society De-Colorization Activity Enhancement of Degussa P25 TiO 2 via the Formation of p-n Heterojunction by Carboxylic Modification Yu-Cheng Hsiao, Chih-Hao Su, Chi-Chang Hu, *, z and Muniyandi Rajkumar Department of Chemical Engineering, National TsingHua University, Hsin-Chu 30013, Taiwan The visible-light-driven photocatalytic activity of Degussa P25 TiO 2 is effectively promoted by modifiers containing carboxylic groups through impregnating cyclohexanol and a low-temperature heating treatment. The superficial TiO 2 modified with the organic modifiers creates certain low band-gap states to enhance its visible-light activity. The carboxylic-modified TiO 2 dispersed on the P25 surface shows the p-type semiconductor characteristics, confirmed by the photocurrent responses under the open-circuit state and blue-light irradiation. Accordingly, a p-n heterojunction between the superficial modified TiO 2 and underneath P25 enhances the separation of photo-excited electron/hole pairs, resulting in the higher photocatalytic activity. During the photo-electrocatalytic de-colorization test, the photo-excited electrons are further withdrawn by the positive potential bias toward the graphite cathode through the electric circuit, which favors the oxygen reduction reaction (ORR) of two-electron transfer to effectively generates H 2 O 2 for organic pollutants decomposition. Therefore, methylene blue (MB) can be efficiently de-colorized on the carboxylic-modified P25 under the visible-light irradiation meanwhile the de-colorization rate is enhanced under the photo-electrocatalytic mode. © 2014 The Electrochemical Society. [DOI: 10.1149/2.0031503jes] All rights reserved. Manuscript submitted September 17, 2014; revised manuscript received November 10, 2014. Published December 9, 2014. During 1970s, water splitting was found to occur upon illumi- nation of ultraviolet (UV) light on the TiO 2 electrode with a small potential bias signaled the start of extensive research efforts on the semiconductor-based photocatalysis. 1 In addition, photocatalytic degradation of organic hazardous pollutants on n-type semiconduc- tors, especially TiO 2 , is an important research field for environment protection and control. Owing to the interesting photo-induced physic- ochemical properties, excellent chemical and optical stability, non- toxicity, and low cost of TiO 2 , 1,2 extensive studies on the photocat- alytic decomposition and mineralization of organic wastes or air pol- lutions, such as dyes, NO x , etc., on TiO 2 have been conducted and several reviews have been published. 2–5 The photocatalytic degradation of pollutants on TiO 2 has been recognized as one of the advanced oxidation processes (AOPs) for wastewater treatments, which usually employs hydroxyl radicals (·OH) or other strong oxidants to effectively remove or decompose organic compounds. 1 Among various forms of TiO 2 , Degussa P25, containing 20–30% and 80-70% TiO 2 in the rutile and anatase phases, respectively, is the most acceptable photocatalyst with a relatively high photocatalytic activity. 6 However, the bandgap of P25 is ca. 3.0– 3.2 eV, 7 so the photo-excited electron/hole pairs on P25 for generating ·OH and other strong oxidants (e.g., ·O 2 − ) only can be generated by the UV light irradiation. Unfortunately, the solar energy associated with the UV light region accounts for about 4–5% in the solar light spectrum on the planet. 8 Therefore, many efforts have been paid for extending its optical absorption edge from UV to visible (Vis) light regions to utilize more solar energy. Various modification methods have been employed to enhance the photocatalytic activity and performance of TiO 2 under the solar-light irradiation. One is the usage of photo-sensitizers, such as dyes or quantum dots (CdS), 9,10 meanwhile some researches tried to modify TiO 2 with metals or non-metallic elements. 11–14 Among the non-metal doped TiO 2 , carbon doping has been claimed to take an advantage over the nitrogen doping because of the significant overlap between the O2p state and the carbon states near the valence band edge. 15 In addi- tion, the carbon-doped TiO 2 was reported to produce a larger valence band redshift than the nitrogen-doped TiO 2 . 15 For carbon-modified TiO 2 , the carbonaceous species formed during the heat-treatment were reported to show photo-electrochemical responses because of their visible-light photo-sensitization. 16 However, how to develop a reliable, wide-spread, and large-scale process for manufacturing C- modified TiO 2 for practical applications is still a challenge; especially the cost of manufacture has to be considered. Here the difference be- tween carbon doping and modification on TiO 2 can be distinguished by the bonding structure between C and TiO 2 . 4,17 ∗ Electrochemical Society Active Member. z E-mail: cchu@che.nthu.edu.tw In this work, we develop a new type of carboxylic-modified TiO 2 which can be prepared by a cost-effective impregnation process with low-temperature calcination. Because of the cost consideration, the starting materials are commercially available TiO 2 (P25, Degussa) and cyclohexanol. The textural properties of carboxylic-modified P25 (de- noted as C-P25) are systematically characterized by X-ray diffraction (XRD), UV-Vis absorption spectroscopic, photoluminescence spec- troscopic (PL), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopic, high-resolution transmission electron mi- croscopic (HR-TEM) and field emission scanning electron micro- scopic (FE-SEM) analyses. Since the photocatalytic degradation of organic compounds under the electrochemical assistance for elec- tron/hole pairs separation on semiconductors is an AOP with syn- ergistic benefits, 2,18,19 the enhanced photo-electrocatalytic activity of C-P25 under the visible-light irradiation and a positive potential bias for dye de-colorization is clearly demonstrated in this work. Experimental Preparation of carboxylic-modified P25.— Carboxylic-modified P25 samples were prepared from commercially available P25 (Degussa) and cyclohexanol (Acros) through the impregnation pro- cess with a low-temperature heating treatment. Briefly, 0.2 g P25 dried at 85 ◦ C under vacuum oven for 24 h was added into a 5-mL cyclo- hexanol solution and dispersed in an ultrasonic bath for 15 min. The mixture was heated at various temperatures such as 150, 200, 300 and 400 ◦ C in air for 3 h,which were denoted as C-P25–150, C-P25–200, C-P25–300, and C-P25–400, respectively. Electrode preparation.— A 10 × 10 × 3 mm graphite (Nippon Carbon EG-NPL, N.C.K., Japan) was used as the substrate and its pre-treatment procedure completely followed our previous work. 20 In preparing TiO 2 -coated electrodes, 0.5 mg C-P25 powders were well mixed with 5 wt% polyvinylidenefluoride (PVDF). This mixture was supplemented with 40 μLC 5 H 9 NO under an ultrasonic bath. The suspension was dropped onto the substrate and dried at 85 ◦ C. The exposed surface area of all photo-electrodes is 1 cm 2 (insulated with polytetrafluoroethylene (PTFE) films) for the photo-electrochemical characterization. Textural and photo-electrochemical characterization.— X-ray diffraction (XRD) patterns were obtained from an X-ray diffrac- tometer (Ultima IV, Rigaku) using a Cu target (CuK α = 1.5418 Å) at an angle speed of 1 ◦ (2θ) min −1 at 40 kV and 20 mA from 20 to 80 ◦ . A diffuse-reflectance scanning spectrophotometer (UV-2450, SHIMADZU) was employed to obtain the UV-Vis absorption spec- tra of the powders from 300 to 800 nm. The reflectance data was ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 140.114.47.2 Downloaded on 2015-05-21 to IP