Synthesis and characterization of transition metal ion doping on the photocatalytic activity of TiO<inf>2</inf> nanoparticles

Abstract

Transition metal ion (V, Fe, or Cu) doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles were synthesized from titanium tetraisopropoxide (TTIP) in absolute ethanol by the modified sol-gel method. The precursor of the transition metal to be doped was added to an alcoholic solution containing TTIP. This solution was loaded into a pouch type cellophane membrane and placed in a clear solution containing 1:1 (v/v) ratio of absolute ethanol and distilled water with 0.5-1.0% concentrated ammonia solution for 1 h. After the completion of the dialysis process (1 h), the suspension was centrifuged (7500 rpm, 10 min), washed with milli-Q water and then dried in an oven at 60degC for 24 h. The powder was finally calcined in a furnace at a temperature of 400degC for 3 h. The use of cellophane membrane offered the advantage of a well-controlled diffusion rate. UV-Vis absorption studies confirmed that the spectral responses of metal ion doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were shifted to the visible light region. Phase composition, crystallinity, crystal size and morphology of V, Fe, and Cu doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanopaticles were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The crystalline size of V, Fe, Cu doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanopaticles was found to be in the range of 10-20 nm. The Brunauer, Emmett and Teller (BET) adsorption-desorption of nitrogen gas for specific surface area determination at the temperature of liquid nitrogen was performed on V-, Fe-, and Cu-doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles. The photocatalytic activities of the various doped samples were evaluated by studying the mineralization of sucrose, phenol, oxalic acid, formic acid, methanol, and malonic acid under UVA, solar spectrum, and visible light irradiation. Under UVA and solar spectrum illumination, bare TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> outperformed all doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> photocatalysts in mineralizing the various organic compounds. Under visible light illumination, it was found that Fe-doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles could significantly mineralize only oxalic acid. This indicates that the enhanced performance of the Fe-doped TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles might not be due to the red shift of the semiconductor. It is likely due to the formation of the easily photolyzed ferrioxalate complex. Various factors such as particle size, crystallinity, and amount of transition metal ion dopant in TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were evaluated for the enhancement of the photocatalytic activity.