Elsevier

Materials Letters

Volume 94, 1 March 2013, Pages 147-149
Materials Letters

Synthesis of visible-light-active TiO2-based photo-catalysts by a modified sol–gel method

https://doi.org/10.1016/j.matlet.2012.11.137Get rights and content

Abstract

Pure TiO2 and C, N co-doped TiO2 (TiO2: C, N) nanoparticles were successfully synthesized by a modified sol–gel method, using urea as the carbon and nitrogen source. The entire preparation differs from the traditional sol–gel route employed to prepare nano-TiO2 for it does not require the use of any acid or alkali. The characterization results showed that as-prepared samples were nanoparticles with a diameter of 10–30 nm. Most of the carbon incorporated in the TiO2 matrix exists in the form of elemental carbon, and nitrogen was substituted for some of the lattice oxygen atoms. The PL spectra revealed that carbon and nitrogen dopants could enhance the separation of photo-excited electrons and holes, and improve the photo-catalytic efficiency of TiO2. The conjecture was demonstrated by degrading methyl blue under visible light, using the samples as the photo-catalyst.

Highlights

► TiO2: C, N nanoparticles were synthesized by a modified sol–gel method. ► The entire preparation does not require the use of any acid or alkali. ► As-prepared TiO2: C, N nanoparticles have a small and uniform size. ► As-prepared TiO2: C, N nanoparticles have an excellent visible-light photo-catalytic activity.

Introduction

As a photo-catalyst for degradation of organic compounds, TiO2 has attracted a great deal of attention due to its high photo-stability, nontoxicity, low cost and ready availability [1]. However, because bulk anatase TiO2 has a direct transition optical band gap of 3.2 eV, TiO2 has an excellent photo-catalytic efficiency solely under UV irradiations. Therefore, understanding how to expand the range of visible-light response of TiO2 and make full use of solar energy is essential. Previous researches already established that C, N co-doping improved the visible-light photo-catalytic activity of TiO2 [2], [3], [4].

Among the many preparation methods, such as sol–gel route [5], [6], homogenous precipitation [7], hydrothermal method [8], relative new molten salts method [9] or plasma spray [10], that have been used to prepare TiO2 nanoparticles – no matter whether doped or not – the sol–gel route was the most widely used due to its high homogeneity, low processing temperature, stability and versatility of processing [11], [12]. However, the preparation necessitates that liquid water be directly used to hydrolyze Tetra-n-butyl Titanate (TBOT), which causes the hydrolysis process of Ti4+ to be too quick to control and the products to easily agglomerate. Consequently, various acids or alkalis [13], [14], [15] were often used to adjust the pH value and decrease the hydrolysis speed.

We developed a new method for fabricating pure TiO2 and C, N co-doped TiO2 (TiO2: C, N) nanoparticles, in which gaseous water molecules are transported by air into the reaction system to hydrolyze TBOT at a predetermined low speed. The hydrolysis is thus controllable and the entire preparation requires no use of any acid or alkali.

Section snippets

Experimental

In the procedure, we used TBOT and alcohol with analytical reagent grade, and methyl blue (MB) with chemical reagent grade.

In our previous research [16], we successfully synthesized TiO2: C, N nanorods by combining spinning evaporation with the modified sol–gel method. However, we proceeded here to synthesize TiO2: C, N nanoparticles by solely using the modified sol–gel method, and a scheme of the preparation is listed in Fig. 1.

For this experiment, a flask containing a mixture of 8 mL TBOT, 40 

Results and discussion

Fig. 2(a) shows the XRD patterns of the samples. All diffraction peaks of the pure TiO2 can be well indexed to anatase (PDF21-1272). The rutile phase could not be detected until the calcination temperature reached 500 °C. Neither carbon nor nitrogen-derived peaks could be detected in any of the samples, mainly due to the low dosage of the dopant, and possibly because C and N are well dispersed in TiO2 particles [17], [18]. The average sizes of the samples (P-300 °C, D-300 °C, D-400 °C, and D-500 

Conclusions

To sum up, pure TiO2 and TiO2: C, N nanoparticles were successfully synthesized by using the modified sol–gel method, and without adding any acid or surfactants. The carbon and nitrogen dopants provided by the same source urea enhanced the separation of photo-excited electrons and holes and improved the visible-light photo-catalytic efficiency of TiO2 in degrading MB. In our experiment, as-prepared nanoparticles whose small and uniform size is mainly due to gaseous water molecules – and not to

Acknowledgment

The authors are grateful for the financial support provided by Explosion Science and Technology Key Laboratory Foundation of China (KFJJ09-7) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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