Fast and complete degradation of Congo red under visible light with Er 3 + and Nd 3 + ions doped TiO

We report fast and complete destruction of the dye Congo red (CR) under visible light in the presence of Nd3+ and Er3+ doped TiO2 nanocomposites (NCs). TiO2 NCs with general composition TiO2[R2O3]x, {x=0.1, 0.2; R≡ Nd, Er} and particle size 12-16 nm were synthesized through co-precipitation/hydrolysis (CPH). A set of similar composites was also prepared through solid state reaction (SSR) route, which produced 3147 nm particles. After characterization, visible light photocatalytic activity of all the samples was recorded for the degradation of CR. NCs with molar concentration x=0.2 of both doping types produced close to 100% degradation in 180 min. The apparent rate constant (kobs) was found to be 2.91×10–2 min–1 and 2.36×10–2 min–1, for these Nd3+ and Er3+ doped NCs, respectively. The other NCs with x=0.1, also showed significant degradation of CR, but the samples prepared through SSR performed worse. The excellent degradation obtained with the NCs may be attributed to their small particle size. Moreover, the doping of Nd3+ and Er3+ ions further supported the photocatalysis through formation of intermediate energy levels within the band gap of TiO2. These new levels not only acted as electron traps for efficient suppression of the undesired e–/h+ recombination, but also facilitated to a certain extent the absorption of visible irradiation. Introduction Discharge of waste products from various industries leading to environmental pollution is a worldwide problem of great concern. Waste products, whether solid or non-solid, influence the natural processes of the ecosystem. The most undesirable fate of almost all waste products, especially the water-soluble ones, is that they end up polluting the water systems. Currently the coloration of water in rivers and streams is due to the large amounts of colored organic effluents (mainly the synthetic dyes) discharged from industries, such as the textile, rubber, cosmetic, paper industries, etc., where coloration of products is of great importance. As a result, such industries are major contributors to environmental pollution.1-5 More than 60% of the world production of dyes is utilized in textile industries, and during the processing about 10-20% is discharged as effluents.1,2 These dye effluents distract the food web in water because their intense color reduces the light transmission, and thus inhibits the growth of most aquatic plants, as well as raises the biological oxygen demand (BOD) and the chemical oxygen demand (COD) in the water. Furthermore, they are potentially carcinogenic and mutagenic, and may also cause allergenic dermatitis and skin irritation even in small amounts.1,2 Because of these reasons, the discharge of dye effluents requires special handling and proper and effective treatments, not just for the possible reuse of disinfected water, but also to promote a cleaner and healthier environment. Photocatalytic destruction of organic pollutants based on semiconductor photochemistry (advanced oxidative processes such as heterogeneous photocatalysis) has been an attractive method since the 1970s and in recent years, this method has been investigated by various groups for degrading synthetic dyes. In heterogeneous photocatalysis, photo-induced molecular reactions or transformations occur at the surface of a catalyst. The process involves initial absorption of photons by the catalyst substrate or the pollutant molecule in order to produce highly reactive chemical species. Depending on where the first excitation takes place, photocatalysis can be divided into two fundamental processes: the catalyzed photoreaction and sensitized photoreaction, respectively. The first process takes place when the initial excitation occurs in the catalyst particle followed by electron or energy transfer from the catalyst to the pollutant molecule and the second, when an adsorbed molecule gets photo-excited initially leading to an interaction with the catalyst.2,5-7 Titanium oxide (TiO2), a wide band gap (3.2 eV) semiconductor with several attractive optical and electronic properties, has been recognized as the most effective photocatalyst for the purpose of degradation. When TiO2 is irradiated with ultraviolet (UV) radiations of suitable wavelengths (<390 nm), the electrons in the valence band (VB) of the semiconductor get excited and promoted to the conduction band (CB). Consequently, pairs of negativeelectron and positive-holes (e–/h+) are created. The generation of e–/h+ pairs is a crucial feature that initiates primary reactions involving the hydroxide ions, water and oxygen to form highly active species (e.g. the hydroxyl radicals OH* and superoxide anion radicals O2). These species interact directly with the pollutants leading to their oxidative degradation.5-7 In this mechanism, obviously the band gap of the photocatalyst is the controlling factor. Another and perhaps more important mechanism so far as the visible light induced photocatalysis is concerned, is that of photosensitization.8-10 In this process, the pollutant molecule (e.g. dye molecule) must initially get adsorbed onto the surface of TiO2 and start absorbing visible radiations. This eventually results in the electrons of adsorbed molecule being photochemically excited and finally transferred to the catalyst particle. This transfer produces superoxide anion radicals from reduction of the molecular oxygen and, at the same time, a cation radical is also produced from the pollutant molecule. Both of these radicals lead to the degradation of other pollutant molecules. In this case, the degradation of the organic pollutant is dependent on the effective Nanotechnology Development 2012; volume 2:e2 Correspondence: Himanshu Narayan, Department of Physics and Electronics, National University of Lesotho, Roma 180, Lesotho. E-mail: h.narayan@nul.ls