Photoassisted reduction of metal ions and organic dye by titanium dioxide nanoparticles in aqueous solution under anoxic conditions Ruey-An Doong a, ⁎, Tien-Chin Hsieh a , Chin-Pao Huang b a Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu, 30013, Taiwan b Department of Civil and Environmental Engineering, University of Delaware, Newark, 19716, Delaware, USA abstract article info Article history: Received 21 September 2009 Received in revised form 17 March 2010 Accepted 19 March 2010 Available online 6 May 2010 Keywords: Metal ions Methylene blue (MB) Photoreduction Titanium dioxide nanoparticle (TiO 2 ) Photocatalytic activity The photoassisted reduction of metal ions and organic dye by metal-deposited Degussa P25 TiO 2 nanoparticles was investigated. Copper and silver ions were selected as the target metal ions to modify the surface properties of TiO 2 and to enhance the photocatalytic activity of TiO 2 towards methylene blue (MB) degradation. X-ray powder diffraction (XRPD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were used to characterize the crystallinity, chemical species and morphology of metal-deposited TiO 2 , respectively. Results showed that the particle size of metal-deposited TiO 2 was larger than that of Degussa P25 TiO 2 . Based on XRPD patterns and XPS spectra, it was observed that the addition of formate promoted the photoreduction of metal ion by lowering its oxidation number, and subsequently enhancing the photodegradation efficiency and rate of MB. The pseudo-first-order rate constant (k obs ) for MB photodegradation by Degussa P25 TiO 2 was 3.94×10 -2 min -1 and increased by 1.4–1.7 times in k obs with metal-deposited TiO 2 for MB photodegradation compared to simple Degussa P25 TiO 2 . The increase in mass loading of metal ions significantly enhanced the photodegradation efficiency of MB; the k obs for MB degradation increased from 3.94 × 10 -2 min -1 in the absence of metal ion to 4.64–7.28×10 -2 min -1 for Ag/TiO 2 and to 5.14–7.61×10 -2 min -1 for Cu/TiO 2 . In addition, the electrons generated from TiO 2 can effectively reduce metal ions and MB simultaneously under anoxic conditions. However, metal ions and organic dye would compete for electrons from the illuminated TiO 2 . © 2010 Elsevier B.V. All rights reserved. 1. Introduction Heterogeneous photocatalysis is an emerging technology that has been widely used for the decomposition of a variety of pollutants in gaseous and aqueous phases (Hoffmann et al., 1995; Doong et al., 2001; Sirtori et al., 2006; Fujishima et al., 2008). Several semiconduc- tors such as TiO 2 , SnO 2 , Fe 2 O 3 , CdS, and ZnO, have been widely employed as photocatalysts in water and wastewater treatment to remove priority organic pollutants as well as inorganic ions (Abrams and Wilcoxon, 2005; Sirtori et al., 2006; Doong et al., 2007). Of various photocatalysts used, TiO 2 is the most commonly investigated due to its high stability, nontoxicity, high photocatalytic activity, and excellent dielectric properties (Barakat et al., 2004; Nagaveni et al., 2004; Zhang and Chen, 2009). Several studies have demonstrated that TiO 2 can form electron-hole pairs under the illumination of near-UV light, which encompasses energies higher than the corresponding band gap (Chen and Mao, 2007; Černigoj et al., 2009). However, the high degree of recombination of electrons and holes is a rate-limiting factor controlling the photocatalytic efficiency. Addition of transition metal ions as dopants to prolong the recombination time or shift the adsorption wavelength to visible region is one of the promising methods to improve the degradation efficiency and rate of organic pollutants in aqueous solutions (Stylidi et al., 2004; Chatterjee and Dasgupta, 2005; Tayade et al., 2006; Zhou et al., 2007). Several transition or noble metal ions at different mass loadings have been used to improve the photodegradation efficiency of organic compounds by TiO 2 (Kapoor et al., 2005; Chang and Doong, 2006). However, the enhanced efficiency of metal-doped TiO 2 toward organic photodegradation is highly dependent on the metal ions used. A reduced photoactivity of TiO 2 nanoparticles was reported when Fe and V was doped (Di Paola et al., 2002; Nagaveni et al., 2004). In contrast, several studies have demonstrated that doping of metal ions including V, Zr, Fe, Ag, Co, and Cu in TiO 2 particles led to an enhanced photoactivity (Zhu et al., 2004; Colmenares et al., 2006; Kubacka et al., 2007; Chang et al., 2009). This discrepancy may mainly result from the different experimental parameters used for photodegradation. In general, the enhancement of photodegradation efficiency by the addition of metal ions may be attributed to the rapid transfer of the photogenerated electrons from the semiconductors to the dopants, resulting in the effective separation of the electrons and holes. Another strategy for modification of TiO 2 surface with noble metals is the photoreduction (Murakata et al., 2002; Iliev et al., 2006). Addition of noble metals such as Pt, Au, Pd, and Ag has been proven to Science of the Total Environment 408 (2010) 3334–3341 ⁎ Corresponding author. Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu, 30013, Taiwan. Tel.: +886 3 5726785; fax: +886 3 5718649. E-mail address: radoong@mx.nthu.edu.tw (R.-A. Doong). 0048-9697/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2010.03.032 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv