Transition metal (Mn, Fe, Co,) doped TiO2 nanoparticles were synthesized by the sol-gel method. All the prepared samples were calcined at different temperatures like 200°C to 800°C and characterized by X-ray diffraction (XRD) and energy dispersive X-ray (EDX) analysis. The studies revealed that transition metal (TM) doped nanoparticles have smaller crystalline size and higher surface area than pure TiO2. Dopant ions in the TiO2 structure caused significant absorption shift into the visible region. The results of photodegradation of formaldehyde in aqueous medium under UV light showed that photocatalytic activity of TiO2 nanoparticles was significantly enhanced by the presence of some transition metal ions. Chemical oxygen demand (COD) of formaldehyde solutions done at regular intervals gave a good idea about mineralization of formaldehyde. 1. Introduction Titanium dioxide is one of the most efficient photocatalysts for degradation of azo dyes. Anatase has higher photocatalytic activity and has been studied more than the other two forms of TiO2 [1], but its wide band gap and high electron-hole recombination rate limit the use of TiO2 [2]. The photocatalytic activity of the TiO2 can be controlled by the following factors: (i) light absorption wavelength; (ii) rate of the electron or hole-induced redox reaction; (iii) recombination of the electron-hole. Much of the effort has been focused on the two latter factors. The competition between the influ-surface charge-transfer processes and recombination of electron-hole is strongly related to the size, surface area, crystallinity, and surface structure of the photocatalyst. In order to enhance the photocatalytic activity of TiO2, interfacial charge-transfer reaction should be increased and electron-hole recombination decreased by modifying the properties of TiO2 colloids [3, 4]. Several methods have been developed such as increasing its surface to volume ratio, optimization of particle size, coupling of TiO2 particles with other semiconductor particles, and doping of metals and nonmetals [5, 6]. The presence of metal ion dopants in the TiO2 crystalline significantly influences photoreactivity by changing charge carrier recombination rates and interfacial electron-transfer rates by shifting the band gap of the catalysts into the visible region [7]. A dopant ion may act as an electron trap or hole trap. This would prolong the life-time of the generated charge carriers, resulting in an enhancement in photocatalytic activity [8]. Many works have recently been made to prepare solar-driven photocatalysts by doping
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