Improved photocatalytic activity of titanium dioxide coatings : a study on the effect of silver nanoparticle deposition on the photoactivity of TiO2 nanotubes and the implications of pH on reaction efficiency

Abstract

The photocatalytic properties of TiO2 nanotube arrays (TNT) synthesised by researchers within the Department of Metallurgy and Materials were ameliorated by doping with silver nanoparticles to expand their photoactivity to the visible. The photocatalytic properties of the treated and untreated samples were tested by measuring the degradation of methylene blue (MB) and methyl orange (MO). In addition, the effects of pH conditions on the degradation of the cationic and anionic dyes were also tested. The nanotubes were silver doped using a facile electroless deposition method consisting of sodium citrate and tannic acid. To investigate the effects of Ag-deposition on the photocatalytic properties of the nanotube arrays, the morphological effects of doping were investigated using Scanning Electron microscopy and Electron Dispersive Spectroscopy. Ultra Violet Visible Spectroscopy and Total Organic Carbon measurement were used to quantify improvements in degradation efficiency. Ag-doped samples were found to be 25-30 % more efficient than undoped samples for degradation of MB under UV light and as much as twice as efficient for degrading MO under UV. The band gap energy of the materials was found to decrease from 3.2 eV to 2.8 eV after Ag-doping and visible light irradiation tests showed that the doped TNT were at least twice as effective at degrading methylene blue when irradiated with visible light only The effects of pH were found to be mixed. MB was found to degrade most readily under isoelectronic conditions (pH 6) whereas MO was found to degrade most efficiently in acidic conditions (pH 3). These results were due to charging effects imparted onto the polar anatase TNT surfaces and the target molecules (MB or MO). This phenomenon may cause electrostatic repulsion or attraction between the NT surfaces and the target molecules under different pH conditions. These results elucidate that, it is important that such effects are considered to maximise the efficiency of photocatalytic treatment of target molecules.