Abstract
Nano TiO2 was synthesized by the hydrothermal method. The sample was doped with transition metal ions (V, Cr and Fe) and non-metal (N). Doped TiO2 samples were characterized by x-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and UV-Vis diffuse reflectance spectroscopy (UV-Vis). Photocatalytic activity in the mineralization of xylene (vapor phase), methylene blue and active dyer PR (liquid phase) was tested. In comparison with non-doped TiO2, V-, Cr-, Fe-doped TiO2 and N-doped TiO2 samples exhibited much higher photocatalytic activity using visible light instead of UV.
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1. Introduction
In recent decades, TiO 2 has been the most widely used and investigated photocatalyst because of its non-toxicity, inexpensiveness, chemical stability and favorable opto-electronic properties. However, it can only work under ultraviolet (UV) light (wavelength<388 nm) due to its wide band gap of 3.2 eV. Modification with the goal of improving the optical absorption and photocatalytic performances, e.g. extending the spectral response into the visible region and enhancing photocatalytic activity, seems to be the most important research interest [1, 2].
TiO 2 modification approaches for achieving visible-light-driven photocatalysis by incorporating transition metal ions (e.g. V, Cr or Fe) to lower the band gap has attracted much attention [3–5]. Other approaches, such as doping nitrogen atoms into substitutional sites in the crystal structure of TiO 2, have also been made [1, 6–9].
In this paper, we report on TiO 2 modification by doping transition metal ions (V, Cr and Fe) and non-metal (N), as well as their photacatalytic activity in the mineralization of xylene (in vapor phase), methylene blue and active dyer (in liquid phase) using visible light instead of UV.
2. Experiment
2.1. Synthesis of samples
Metal-doped TiO 2 was prepared by the hydrothermal method. An amount of fresh TiO 2·nH 2 O and isopropyl alcohol was dissolved in a hydrosulfuric acid solution to form a clear mixture with Ti-concentration of 0.25 M at room temperature. The precursor solution was transferred into a Teflon-lined stainless steel autoclave, heated to a temperature of 200 °C for 10 h. The obtained products were filtered and washed in distilled water (figure 1).
Figure 1 Scheme of the Me-doped TiO 2 synthesis procedure by the hydrothermal method.
For non-metal-doped TiO 2, the aim of our study is to use urea as a nitrogen source to synthesize N-doped TiO 2 powder by heating the mixture of urea and TiO 2. In the N-doped TiO 2 process, urea and TiO 2 were mixed with various molars for a few minutes in agate mortar. The resulting mixture was calcined at 450–500 °C for 3 h.
2.2. Characterization methods
The products were characterized by x-ray diffraction using Cu-Kα radiation (λ=0.14045 nm) at 30 KV and 20 mA, 0.02° step size and 0.1 s step time from 20° to 60° on a HUT-PCM-Bruker D8 Advance instrument. The size of titanium crystallites was calculated by means of Scherer's equation. The rutile and anatase ratio was determined by the following equation:
where x a is the ratio of anatase in the mixture, and I a and I r are the integral intensities of the (101) reflection of the anatase and the (110) reflection of the rutile [4]. The morphologies of the TiO 2 particles were observed using field emission scanning electron microscopy (FE-SEM) on a HITACHI S-4800 instrument operating at 10 kV and magnification of 80 000–100 000. The UV-Vis diffuse reflectance spectrum was measured in the wavelength range 250 to 650 nm using a Shimadzu UV-Vis spectrophotometer.
2.3. Catalytic test
In the photodegradation of methylene blue (MB) and active dyer (PR), 50 mg of TiO 2 was added to 30 mL MB (or PR) solution with a concentration of 50 mg l −1. The suspensions were magnetically stirred for 15 min in the dark to establish the equilibrium of MB and PR adsorption/desorption. The reaction aqueous solution was irradiated under visible light for 4 h with constant stirring speed.
Photocatalytic activity evaluation was carried out by the gas-phase photocatalytic oxidation of p-xylene. The reaction condition for each study was as follows: the gas-phase photocatalytic oxidation of p-xylene was carried out in the microflow region with the following reaction parameters: the concentrations of the reaction mixture of p-xylene (C p-xylene 0), steam (C H 2 O 0) and oxygen (C O 2 0) were 15.94, 11.5 and 285.7 mg l −1, respectively. The gas-flow velocity was 6 l h −1. The temperature was 400 °C. The catalyst was coated on the surface of the Pyrex tube with area of 68 cm 2 and the loading amount of TiO 2 was 30 mg. The analysis was carried out on the GC Agilent 6890 Plus, detector FID, capillary column HP-1 methyl siloxane (30 m; 0.32 mm; 0.25 μm).
3. Results and discussion
3.1. Characterization of samples
Figure 2 shows the XRD patterns of the TiO 2, V-, Cr-, Fe- and N-doped TiO 2 samples. The distinctive peaks at 2θ=25.3°, 37.8°, 47.7°, 54.0° and 62.4°, corresponding to the anatase crystal planes, are observed in all the samples. This indicated that after TiO 2 modification by doping transition metal ions such as V, Cr and Fe as well as doping non-metal (N), the crystalline structure of the anatase phase still remained. Moreover, the intensities of the peaks were almost the same, indicating that their crystallites after doping did not change.
Figure 2 XRD patterns of V-, Cr-, Fe- and N-doped TiO 2.
FE-SEM experiments were carried out to investigate the microstructure of the samples. Figure 3 presents the FE-SEM images of V-, Cr-, Fe- and N-doped TiO 2 samples. As seen in figure 3, no significant differences were observed when comparing the photographs of V-, Fe-, Cr- and N-doped TiO 2, consisting of nearly spherical nanoparticles of 20–30 nm. The particle form and size of these samples were similar to those of the non-doped TiO 2 sample. In the case of Cr-doped TiO 2, the morphology was different from that observed on V-, Fe- and N-doped TiO 2. The Cr-doped TiO 2 sample is composed of needle particles with size ranging from 20–50 nm.
Figure 3 FE-SEM image of (A) Fe-doped TiO 2, (B) V-doped TiO 2, (C) Cr-doped TiO 2 and (D) N-doped TiO 2.
UV-Vis absorption spectra of V-, Cr-, Fe- and N-doped TiO 2 samples are shown in figure 4. In the case of TiO 2, the fundamental absorption edge of TiO 2 appeared in the UV region at about 385 nm. In comparison to non-doped TiO 2, the absorption edge of V-, Cr-, Fe- and N-doped TiO 2 was broader and shifted to a higher wavelength. The fundamental absorption edge of V-, Cr-, Fe- and N-doped TiO 2 appeared in the visible light region at about 500–600 nm. This indicated that UV-Vis absorption is closely related to doping metal ions and non-metal atoms.
Figure 4 UV-Vis diffuse reflectance of samples V-, Cr-, Fe- and N-doped TiO 2.
3.2. Photocatalysis
Photocatalytic decomposition of VOCs has received particular interest. We have chosen p-xylene as the model molecule to test the photocatalytic activity. Here we define the yield conversion as grams of reactant completely converted to CO 2 per gram of photocatalyst.
Figure 5 presents the yield conversion of p-xylene over 0.7 %wt V-, Cr- and Fe-doped TiO 2 catalysts using UV light (λ of 365 nm) and visible light (λ of 470 nm). As observed in figure 5, photocatalytic activity using UV light was lower than that using visible light for all doped TiO 2 samples. In contrast, photocatalytic activity was higher than that using visible light (not shown here). From these results, it could be concluded that doped TiO 2 samples are more suitable for use with visible light than UV light.
Figure 5 Yield conversion of p-xylene over different catalysts.
The dependence of the yield conversion on the amount of doping transition metal (Fe) is shown in figure 6. It shows that the yield conversion increases with increasing Fe content from 0–1 %wt. Further increase of Fe content led to a decrease of the yield. It seems that the content of 1%wt Fe doping is optimal.
Figure 6 Yield conversion of p-xylene over Fe-doped TiO 2 samples.
Figure 7 shows the yield conversion of p-xylene over non-doped, Fe-, V-, Cr- and N-doped TiO 2 using visible light with wavelength of 470 nm. As observed in figure 7, the doped TiO 2 sample exhibited much higher yield conversion as compared to that of the non-doped TiO 2. Among doped TiO 2, the N-doped TiO 2 sample showed the highest yield conversion. This indicated that non-metal doped TiO 2 was a very effective photocatalyst, especially in visible light. Furthermore, we also investigate the photocatalytic activity of non-metal doped TiO 2 (N-doped TiO 2) in the mineralization of methylene blue (MB) and active dyer (PR).
Figure 7 Yield conversion of p-xylene over non-doped, Fe-, V-, Cr- and N-doped TiO 2.
Figure 8 plots the degradation of RP and MB of TiO 2 and N-doped TiO 2 samples. The UV-Vis spectra of the initial solution and product under visible light for 4 h reaction over TiO 2 samples were recorded to determine how deeply RP and MB was converted. As can be seen in figure 8, for the N-doped TiO 2 sample, the peak characteristics of the benzene ring and of the color bearing group at 500–700 nm almost disappeared in the spectrum of the product. Furthermore, peaks in the region of 200–300 nm characterized by the double link in the acid molecule were absent. This result indicated that both PR and MB were completely mineralized after 4 h reaction. In the case of the non-doped TiO 2 sample, both peaks at 500–700 nm and at 200–300 nm were still present, indicating incomplete mineralization of PR and MB. From the obtained results, the high potential of N-doped TiO 2 to be applied as an effective photocatalyst was revealed.
Figure 8 UV-Vis spectra of initial and reaction products of PR and MB over N-doped TiO 2.
4. Conclusion
Nano TiO 2 was successfully synthesized by hydrothermal treatment. The sample was modified by doping transition metal ions (V, Cr, Fe) and non-metal (N). Non-doped TiO 2 and doped TiO 2 samples were characterized by XRD, FE-SEM and UV-Vis. From the characterization results, it was revealed that after doping, the structure (anatase phase) still remained and the extension of the spectral response into the visible region was noted. Photocatalytic activity in the mineralization of xylene (vapor phase) and methylene blue and active dyer PR (liquid phase) was investigated. The obtained results showed that in comparison to non-doped TiO 2, the doped TiO 2 samples exhibited much higher photocatalytic activity using visible light instead of UV.
Acknowledgments
This work is supported by the Vietnam Academy of Science and Technology and National Basic Research Project on Nanostructured Carbon Materials. We thank the Vietnam–Korea and Vietnam–France Bilateral Science Collaboration Programs for their support. The research work was done with the help of the National Key Laboratory for Electronic Materials and Devices, Institute of Materials Science.