Photocatalytic Degradation of Organic Dye by Sol-Gel-Derived Gallium-Doped Anatase Titanium Oxide Nanoparticles for Environmental Remediation

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2012, Article ID 201492, 14 pages doi:10.1155/2012/201492 Research Article Photocatalytic Degradation of Organic Dye by Sol-Gel-Derived Gallium-Doped Anatase Titanium Oxide Nanoparticles for Environmental Remediation Arghya Narayan Banerjee,1 Sang Woo Joo,1 and Bong-Ki Min2 1 School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea 2 Center for Research Facilities, Yeungnam University, Gyongsan 712-749, Republic of Korea Correspondence should be addressed to Arghya Narayan Banerjee, banerjee and Sang Woo Joo, Received 20 January 2012; Revised 12 March 2012; Accepted 13 March 2012 Academic Editor: Vo-Van Truong Copyright © 2012 Arghya Narayan Banerjee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Photocatalytic degradation of toxic organic chemicals is considered to be the most efficient green method for surface water treatment. We have reported the sol-gel synthesis of Gadoped anatase TiO2 nanoparticles and the photocatalytic oxidation of organic dye into nontoxic inorganic products under UV irradiation. Photodegradation experiments show very good photocatalytic activity of Ga-doped TiO2 nanoparticles with almost 90% degradation efficiency within 3 hrs of UV irradiation, which is faster than the undoped samples. Doping levels created within the bandgap of TiO2 act as trapping centers to suppress the photogenerated electron-hole recombination for proper and timely utilization of charge carriers for the generation of strong oxidizing radicals to degrade the organic dye. Photocatalytic degradation is found to follow the pseudo-first-order kinetics with the apparent 1storder rate constant around 1.3 × 10−2 min−1 . The cost-effective, sol-gel-derived TiO2 : Ga nanoparticles can be used efficiently for light-assisted oxidation of toxic organic molecules in the surface water for environmental remediation. 1. Introduction Titanium dioxide (TiO2 ) is one of the most important wide bandgap metal oxides which is having a vast range of applications from paint to sunscreen to food coloring to photocatalyst, hydrogen production, storage medium, sensors, solar cells, organic waste management, and various biological and health-related applications [1–13]. Because of its wide range of properties, TiO2 bulk films as well as nanostructured materials become the subject of intense research within the global scientific community. In general, TiO2 has two stable crystalline structures: anatase and rutile [14]. Rutile is preferred to anatase for optical applications because of its higher refractive index, whereas anatase is preferred for all the applications related to photocatalytic activity, gas sensing, and solar cells, due to its higher mobility and catalytic properties [15, 16]. Amongst various TiO2 nanostructures, titania nanoparticles have specific advantages in the enhancement of light absorption due to the large fraction of surface atoms. Interband electron transition is the primary mechanism of light absorption in pure semiconductors. These transitions are direct as the momentum gain by the electron from light wave is small in comparison with πh/a (“a” is the lattice constant). This absorption is small in direct-forbidden gap semiconductors, as in the case for TiO2 , for which the direct electron transitions between the band centers are prohibited by the crystal symmetry. However, momentum is not conserved if the absorption takes place at the boundary of the crystal, for example, at the surface or at the interface between two crystals, which leads to the indirect electron transitions that can result in the essential enhancement of light absorption. This means that considerable enhancement of the absorption can be observed in small nanocrystals where the surface-to-volume ratio is very high and the fraction of the surface atoms is sufficiently large. The particle size at which the interface enhancement of the absorption becomes significant is around 20 nm or less. An additional advantage obtained in nanoparticles in the few nanometer 2 size regimes is that the large surface-to-volume ratio makes possible the timely utilization of photogenerated carriers in interfacial processes [1, 17]. Additionally, the doping of TiO2 nanoparticles is performed for improved photocatalytic activities by reducing the band gap of TiO2 to utilize the wider fraction of solar radiation, especially the visible and near infrared (NIR) parts [4, 18–20]. Many efforts have been expended to narrow the TiO2 band gap by substitutional doping. According to the crystal structure of TiO2 , it appears that replacement of Ti4+ with any cation is relatively easier than to substitute O2− with any other anion due to the difference in the charge states and ionic radii. Cationic doping of TiO2 with transition and rare earth metals has been extensively studied [18–24]. While several authors have reported that transition metal ion doping decreases the photothreshold energy of TiO2 , there is also an increase in thermal instability and a decrease in carrier lifetimes [25], which limits overall conversion efficiencies. Therefore, it is clear that there is always scope for improvement in the photoactivity of TiO2 nanostructures either by applying various new dopants or (and) adopting different doping parameters through different deposition processes and conditions. Amongst various photocatalytic applications of TiO2 , photocatalytic degradation of toxic organic chemicals (especially organic dyes generated as industrial wastes and released in the surface water without proper treatment) is considered to be the most efficient green method for organic waste management in terms of photosensitized TiO2 -assisted oxidation of organic pollutants for surface water treatment, recovery of precious metals via TiO2 -assisted reduction, organic synthesis, photokilling activity, and self-cleaning activity among others [26–35]. As far as the syntheses of undoped and doped TiO2 nanostructures are concerned, both solution-based chemical techniques as well as vacuum-based physical techniques [1, 7, 9] have been adopted. In the current study, we have reported the sol-gel syntheses and characterizations of Ga-doped anatase TiO2 nanoparticles (TiO2 : Ga) and investigated the photocatalytic oxidation of organic dyes for environmental remediation. Apparently, sol-gel deposition process is preferred (at least in the research scale) over vacuumbased as well as hydrothermal syntheses, mainly because of its simplicity and cost-effectiveness in terms of materials, design, process, and implementation. As far as the reason behind the adoption of Ga as the doping material is concerned, previously few authors reported the improved photocatalytic activities of TiO2 : Ga (and TiO2 : Ga/I codoped) nanom (...truncated)