Tunable synthesis of enhanced photodegradation activity of brookite/anatase mixed-phase titanium dioxide
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Using titanium sulfate, Ti(SO4)2, as precursor and sodium hydroxide, NaOH, as adjusting reagent, pure brookite, pure anatase, and mixed-phase titanium dioxide (TiO2) with tunable brookite/anatase ratios were synthesized via a hydrothermal process. The samples were characterized by x-ray diffractionspectrometry, ultraviolet-visible diffuse reflectance spectrometry, transmission electron microscopy, and Brunauer-Emmett-Teller measurement. Photocatalytic degradation of Rhodamine B in aqueous solution served as a probe reaction to evaluate the photocatalytic activity of the asprepared nanocomposites under visible irradiation (k . 400 nm). The mixed-phase TiO2 exhibits higher photodegradation activity than single phase TiO2. The sample with 63.1% brookite and 36.9% anatase shows the highest degradation activity. Possible mechanism attributing to the enhanced activity was proposed based on the strucutre and surface property of the samples.
I. INTRODUCTION
In recent years, environmental pollution has become an issue calling for urgent attention. Semiconductor photocatalytic process, known as a novel, highly efficient and “green” pollution-controlling process,1,2 has shown great potential to align with the “zero” waste scheme in the water/wastewater industry due to its low cost, environmental friendly, and sustainable treatment. This advanced redox technology has been widely demonstrated to remove persistent organic compounds and microorganisms in water.3 Among various semiconductors, titanium dioxide (TiO2) has been regarded as one of the most promising photocatalysts due to its excellent photoactivity, chemical stability, and nontoxicity.4–6 TiO2 is an important functional inorganic material and it exists mainly in three crystal phases in nature: rutile (tetragonal space group P42/mnm), anatase (tetragonal, space group I41/amd) and brookite (orthorombic, space group Pbca).7 In most photocatalytic applications, anatase is usually regarded as the most active crystal phase among the three occurring forms of TiO2.8–10 Most of the synthetic processes favor the forming of anatase, rutile, or a mixture of anatase/rutile. These two phases are extensively studied as photocatalysts mainly due to their easy synthesizability. In contrast, the brookite phase is rarely studied, ascribing the lack of interest to the difficulties in obtaining its pure form; furthermore, it is reported that brookite has higher rate of photocatalytic activities than that of rutile and anatase.11 Recently, the synthesis of pure brookite a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.226 400
J. Mater. Res., Vol. 28, No. 3, Feb 14, 2013
http://journals.cambridge.org
Downloaded: 14 Mar 2015
TiO2 with different morphologies such as nanorods, nanotubes, and nanoflowers has been reported.10,12,13 It has been known that anatase powders with a small fraction of rutile or brookite phase, such as commercial TiO2 Degussa P25, show enhanced photocatalytic activity compared with pure anatase powders due to the “mixed-ph
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