Effects of peroxo precursors and annealing temperature on properties and photocatalytic activity of nanoscale titania
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Ratibor G. Chumakov Kurchatov Institute National Research Centre, Moscow 123182, Russia (Received 5 November 2017; accepted 12 February 2018)
Titania nanoparticles (anatase or anatase 1 rutile) with enhanced photocatalytic activity were successfully produced by treating titanyl sulfate with various peroxo compounds (hydrogen peroxide, ammonium persulfate, and urea hydrogen peroxide) with further annealing. Transformation of titanyl sulfate to titanium dioxide was investigated by X-ray diffraction, electron microscopy, X-ray microanalysis, IR, Raman, X-ray photoelectron, and UV/vis spectroscopy. The peroxo compound and annealing temperature play an important role in phase composition and properties of the samples. Correlations between phase composition, oxygen content, band gaps, and constant rates for methyl orange (MO) discoloration were found. The [TiOx(O2)2 x(H2O)m] phase, which forms on the first stage of the reaction, contains nanoparticles with small crystallites (1–2 nm) and promotes formation of titanium dioxide with the anatase structure. Thermal decomposition of the peroxo-containing phase results in formation of titanium dioxide. Oxygen excess prevents transformation of anatase to rutile, decreases band gap, and increases activity of titanium dioxide (anatase or anatase 1 rutile) in the model reaction of MO destruction.
I. INTRODUCTION
Nanoscale titania is now one of the most applicable heterogeneous photocatalysts.1 Among 12 polymorphous modifications of titanium dioxide, three natural modifications of titanium dioxide with anatase, rutile, and brookite structures are best known; the highest photocatalytic activity was found for anatase.2 Titanyl sulfate is often used for preparing nanoscale anatase.3–10 Thermal behavior of anhydrous titanyl sulfate TiOSO4 and titanyl sulfate monohydrate TiOSO4H2O is studied in detail.11,12 On heating, anhydrous titanyl sulfate undergoes a reversible phase transition at ;330 °C and decomposes into anatase at ;525 °C. Titanyl sulfate monohydrate loses its crystal water at ;310 °C; liberation of sulfur trioxide starts at ;40 °C lower temperature than for anhydrous titanyl sulfate. Thermal decomposition of titanyl sulfate dihydrate TiOSO42H2O is also well studied.13–15 Upon heating, titanyl sulfate dihydrate exhibits two mass losses, the loss of two water molecules and evolution of sulfur trioxide between 560 and 650 °C; at 650 °C, a crystalline phase starts to appear; at 700 °C, anatase is the major phase and the formation of rutile becomes apparent; upon further heating, anatase is converted to a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.52
rutile, at 900 °C little or no anatase can be detected. Temperatures of anatase–rutile transformations obtained by numerous authors are summarized in the review.16 In mesoporous TiO2/SiO2 (5 wt% TiO2), which was synthesized by the sol–gel method, anatase remains stable up to 1000 °C.17 Hydrogen peroxide has been often used for titania preparation by wet syntheses including variou
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