pH Effect on the Optical Properties of Peroxo-Titanium Complex
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Effect of pH on the Optical Properties of Peroxo-titanium Complex Ruxiong Cai*, Kiminori Itoh¶ and Chang Qing Sun# Singapore Institute of Manufacturing Technology, 71 Nanyang Drive Singapore 638075. ¶Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokoham 240-8501, Japan. #School of Electrical& Electronic Engineering, Nanyang Technological University, Singapore 639798.
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ABSTRACT Interesting optical properties of peroxo-titanium hydrogen peroxide complex against pH changes were studied for the first time using an analytical UV-VIS spectrophotometer. A freshly prepared peroxo titanium complex with pH value of 2.21 exhibited an orange color. This color changed to light orange, cloudy yellow, and then translucent pale-yellow as its pH value increased to 3.9, 5.8 and 6.7 or above. UV-VIS spectra show that the fresh complex had an absorption band rising at wavelength around 400 nm to higher energy, but had no maximum peak. A new absorption peak appears at 245nm for the cloudy yellow samples. This is similar to that of colloidal TiO2, which suggests that TiO2 particles might form upon increasing pH value to or above 5.80. The formation of TiO2 particles was accelerated in the pH value range from 6 to 9, but not in the acidic environment. In H2SO4 acidified environment, the color turned red-orange instead, and a strong absorption at 397nm was observed only at pH =0.99. Based on the experimental observations, a model for the color-forming species and possible applications by using the techniques generated from the present study were proposed. INTRODUCTION Since 1870 when SchÖnn reported for the first time that an addition of hydrogen peroxide (H2O2) to an acidic solution of Ti(IV) could produce a liquid with an intense orange color[1], considerable efforts have been made to investigate this compound [2-10]. These include using this phenomenon to develop an analytical method for measurement of Ti4+ or H2O2 concentration[2,3]; studying the chemistry of this complex [4-6] and employing it as precursor for thin film deposition [7,8]. The first quantitative analytical method was based on an absorption peak at 400 nm, which can be detected using UV-VIS spectrophotometry. As this method is simple, rapid, and accurate, it is widely used in medical, food and pigment industries. However, the pH dependent UV-VIS spectrophotometry has never been studied systematically because precipitation always occurred during the change from low pH to neutral and to alkaline. As a precursor for thin film development, PTC has been mainly investigated in acidic environment [7, 11-12]. This acidic window is quite narrow and not user-friendly. An ideal precursor should possess characteristics such as stable (no precipitate) with a wider range of pH values and transparent ~ translucent. For special requirement like on-site use, a water based precursor with neutral pH is preferred. Although Ichinose et al developed a neutral precursor by using H+-type ion exchange resin,
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