A series of boron-doped TiO2 photocatalysts (2% B-TiO2) with different water/alkoxide molar ratio were synthesized by conventional sol-gel method. The prepared samples were characterized by BET measurement, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIRS), and diffuse-reflectance UV-vis. The phase anatase was present, but unexpectedly a small amount of rutile phase was formed with low and excess water in the synthesis. Additionally it has been observed that the increase in the molar ratio of water significantly increases the values of band gap energy and the specific surface area. Results showed that degradation of Orange II azo dye increases with surface area, particle size, boron, and water content in photocatalysis. The boron species were introduced in the tricoordinated form. 1. Introduction Current emission of pollutants from textile industry to different bodies of water is a serious problem, not only since this industry uses large volumes of water, but also because most of these dyes are resistant to the conventional wastewater treatment processes. Different emerging technologies have been tested to try to solve this problem; among them, the advanced oxidation processes such as heterogeneous photocatalysis employing TiO2 have demonstrated to be the most viable alternative to eliminate several organic contaminants [1, 2]. On the other hand titanium dioxide (TiO2) has attracted growing scientific interest due to its good chemical and photochemical stability, nontoxicity, availability, low price, or ease of synthesis. However the structural, electronic, and photocatalytic properties of TiO2 depend on the method of synthesis, while the size, morphology, and microstructure of crystals can be controlled and modified during hydrolysis and condensation steps [2–4]; that is, the water content changes the rate of hydrolysis. In order to improve its photocatalytic activity and shift its absorption band towards the visible region, several attempts have been also made to narrow the band gap energy by doping with various metals ions [5, 6], where it is suggested that the incorporation of metal to TiO2 structure modifies both the charge carriers recombination rate and the interfacial electron-transfer rate, as well as the generation of recombination sites or the band gap energy, depending on where these ions are located in the TiO2 structure. On the other hand the doping of TiO2 with non metals atoms such as carbon, nitrogen or boron atoms [7–12] have shown a higher photoactivity performance under visible light irradiation. The
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