Synthesis and characterization of nitrogen-doped titanium dioxide nanomaterials derived from nanotube sodium titanate pr
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Nitrogen-doped titanium dioxide (denoted as N-doped TiO2) nanomaterials were prepared through the ion exchange of sodium titanate nanotube (the precursor; denoted as STN) with aqueous NH4Cl and follow-up sintering at different temperatures in air. The morphology, structure, surface component, and optical properties of as-obtained N-doped TiO2 nanomaterials have been analyzed by transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and ultraviolet–visible light diffuse reflectance spectrometry. The formation mechanism and the origin of the visible light absorption for N-doped TiO2 nanomaterials have been discussed. Moreover, the thermogravimetric analysis and differential thermal analysis of N-doped TiO2 nanomaterial calcined at 100 °C are conducted as an example to examine the thermal stability of as-synthesized N-doped TiO2. It has been found that, as the calcination temperature rises, the initial nanotubular morphology of STN is transformed to the final nanoscale granular one, accompanied by a phase transformation from orthorhombic crystalline system to anatase TiO2. The N content in N-doped TiO2 is 7.04%, 6.22%, 3.20%, 1.14%, 0.61%, and 0.40% (atomic percentage), depending on calcination temperature rising from 100 to 600 °C. Moreover, N-doped TiO2 samples experience three stages of weight losses, and that calcinated at 300 °C and above have strong visible light absorption, due to the formation of Ti–O–N bonds thereat.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.249
TiO2xNx powders containing a total N concentration of about 1 at.% by annealing TiO2 powder in the mixed gas of NH3 and Ar (volume ratio 67%:33%) at 550–600 °C. Another frequently used synthesis route of N-doped TiO2 is wet chemical method, which involves the hydrolysis of Ti- and N-containing precursors and follow-up sintering of the hydrolyzed product at elevated temperatures in air. For example, Ihara et al.16 fabricated visible-active TiO2 photocatalyst with a N concentration of 0.20 at.% by calcinating the hydrolyzed product of Ti(SO4)2 at 400 °C in air. Qiu and Burda17 and Zhu et al.18 synthesized N-doped TiO2 and Cr–N codoped TiO2 nanocrystals with a N concentration of 8% by the hydrolysis of titanium (IV) tetraisopropoxide in the presence of ethylenediamine as the N source and chromium acetylacetonate as the Cr source. Thanks to the research at-depth on nanotube titanic acid (NTA) and titanate, a novel approach has recently been developed for preparing N-doped TiO2 nanomaterials with titanate nanotubes as the precursors.19–24 For example, Feng et al.19 prepared N-doped anatase TiO2 by thermal treatment of NTA in a NH3 flow at 400, 500, and 600 °C, respectively. Jiang et al.20 synthesized N-doped anatase TiO2 nanotubes through a solvothermal treatment of protonated titanate nanotubes in the mixed solution of NH4Cl/ethanol/water and found that only ammonia chloride as nitrogen source could promote the formation of anatase phase, while chloride ion might
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