Synthesis of vanadium-modified rutile TiO₂ nanoparticle by reactive grinding method and its photocatalytic activity under solar light at room temperature

Among various semiconductors, TiO₂ is the most promising photocatalyst because it is biologically and chemically inert; its band gap and position of valence band (VB) and conduction band (CB) are appropriate for the photocatalytic oxidation and reduction of a wide range of environmental pollution as well as for water splitting to generate hydrogen and oxygen [1–3]. However, because the band gap of anatase is 3.2 eV, TiO₂ anatase only shows photocatalytic activities under the UV light of wavelength λ < 387 nm; the band gap of rutile is 3.0 eV and TiO₂ rutile exhibits activity in a very small visible range near to the UV one with wavelength λ ⩽ 413 nm. So TiO₂ can utilize only a small UV fraction of the solar light, about 2–5%. Many publications have focused on doping TiO₂ by metals or non-metals, so that its photocatalytic activities exhibit in the visible range of light [4–9]. The authors show that activities of materials depend on their surface properties such as particle size, specific surface area, surface structure and active reaction sites. Those properties are dependent on the methods of preparation. The authors [4, 8, 10] indicated that Cr-doped TiO₂ prepared by an impregnation method exhibits photocatalytic activities less than those of the Cr ions implanted TiO₂. A majority of publications concentrates on modifying of TiO₂ anatase. Since they demonstrated that the density of TiO₂ octahedral in anatase is smaller than that in rutile (creating more hollow spaces inside the anatase for the process of transportation and diffusion of photogenerated holes and photogenerated electrons) and the anatase can be obtained at low temperature (<500 °C) with small particle size, the TiO₂ production for application purposes requires controlling over reaction conditions to obtain anatase. When the preparation temperature is increased, not only is there a rise in the particle size but the anatase also converts into rutile. This means we always obtain TiO₂ rutile with large size under normal calcination at high temperature and hence the photocatalytic activities of rutile are less than those of anatase.

In fact, the most common and stable form of TiO₂ is rutile. If the rutile form of TiO₂ can be utilized as a photocatalyst, the cost can be reduced, and also too much concern about controlling the conditions during production is not necessary.

For these above reasons, by the special method, we prepared TiO₂ rutile with sufficiently small size and doping TiO₂ with metal or metalloid ions can be expected to result in the total effects that decrease the band gap of material. According to the published works [7, 11], doping V₂O₅ into TiO₂ anatase can reduce the band gap, shifts the absorption wavelength of the material to the visible range, thus, their photocatalytic activities significantly increase. Some authors have made an effort to obtain rutile and modified rutile by in situ hydrothermal method and they show effective catalytic properties [12].

By the top-down preparation route, Kaliagine et al [13] received many complex nano-oxides as catalysts using reactive grinding method. In the grinding process, solid reactions occurred by high energy and nanometer particles were created. So, in this work, we used this grinding reactive method for vanadium-doped rutile TiO₂ and prepared a product that has high photocatalytic activities.

According to [7, 9], the increase of vanadium doping promoted the particle growth, and enhanced 'red-shift' in the UV–Vis absorption spectra. It was found that the samples containing 2.5–5% of V₂O₅ have the highest degradation of MB. Accordingly, in the present work we prepared 5% V₂O₅/TiO₂ rutile and studied its characterizations and photocatalytic activity.

2.1. Preparation of TiO₂ rutile

2.2. Preparation V₂O₅/TiO₂ rutile

3.1. Crystal structure, morphology, particle size and the BET specific surface area

Figure 1 shows XRD patterns of TiO₂ sample before grinding (T0), TiO₂ samples after grinding for 2, 4, 6 h (T2, T4 and T6) and V₂O₅/TiO₂ sample ground for 4 h (VT). There are only specific peaks of TiO₂ rutile and no peak of V₂O₅. The XRD patterns showed that samples synthesized are single phase—rutile phase. Peaks of sample T0 are higher and narrower than those of TiO₂ particles T2, T4, T6 and VT. Thus, the particle size of TiO₂ sample before grinding (T0) is larger than that of TiO₂ ground samples (T2, T4, T6). This result is clear and suitable with a decrease in particle size when increasing grinding time. The average crystal sizes and the BET specific surface area values of these samples are given in table 1.

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Table 1. Crystal size and BET values of samples.

Sample	Symbol	Grinding time (h)	Average crystal size (nm)	BET (m2 g−1)
TiO2	T0	0	130	7.18
	T2	2	40	9.84
	T4	4	14	15.12
	T6	6	13	15.20
V2O5/TiO2	VT	4	22	20.80

From table 1 it is seen that the ground particle size decreased significantly as grinding time increased. However, when grinding time increased from 4 to 6 h, the TiO₂ particle sizes and BET specific surface area nearly did not change. So the grinding time chosen to synthesis V₂O₅/TiO₂ sample was 4 h. In the case of V₂O₅/TiO₂, the BET specific surface area increased noticeably (20.8 m² g⁻¹) when compared with TiO₂ which was also ground for 4 h (15.12 m² g⁻¹).

There is no peak of vanadium in XRD pattern of V₂O₅/TiO₂ sample. This phenomenon was like that in [9]. There was also no peak of vanadium oxide in the XRD patterns but x-ray absorption spectroscopy (XAS) analysis indicating V⁴⁺ instead of V⁵⁺ implied that vanadium either substituted Ti⁴⁺ site or embedded in the vacancy of TiO₂ structure. Therefore, either vanadium was incorporated in the crystalline of TiO₂ or vanadium oxide was very small and highly dispersed.

3.2. UV–Vis absorption spectra and band gap energy calculation

To study the light absorption ability of materials, we determined UV–Vis absorption spectra of TiO₂ and V₂O₅/TiO₂ samples. The results are given in figure 2.

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Figure 2 shows that the sample T0 absorbs light having wavelength below 420 nm, while the samples T4 and VT absorb light at longer wavelengths and induce red-shift. The samples T4 and VT can absorb mostly at the region below 480 and 570 nm, respectively, with much higher absorption coefficients. These samples show even the absorption at the longer wavelength regions. The reason perhaps is the difference in particle size of the same sample. In the case of VT, the interaction between V₂O₅ and TiO₂ is the explanation for its red-shift and for the increase in its absorption coefficient.

In figure 2 we can see that the UV–Vis absorption spectrum of V₂O₅/TiO₂ (figure 2(c)) is really spectrum of a composite of TiO₂ and V₂O₅ but not that of separated TiO₂ (figure 2(b)) or V₂O₅ (we can find the absorption spectrum of V₂O₅ in [9]). This evidence demonstrates the interaction between V₂O₅ and TiO₂ after grinding and V₂O₅ may incorporate into the lattice of TiO₂ rutile.

On the other hand, as we know, the bulk TiO₂ crystal is an indirect semiconductor, its absorption index α(hν) should closely correlate with the behavior of the form (hν − E_g)^3/2. Normally nanomaterials exhibit an absorption spectrum such as that of the direct band gap semiconductor, and its absorption coefficient α(hν) follows the (hν − E_g)^1/2 law [14]. Based on the absorption data (from 400 to 800 nm) presented in figure 2 we plot the curves of (αhν)² in dependence on energy hν in figure 3.

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It is shown that the T0 and T4 curves contain AB and CD linear lines, respectively. The prolongations of these lines cut the x-axis at 3.04 and 2.99 eV, respectively. These energy values approximate to the band gap energy value of TiO₂ rutile (3.0 eV). Similarly, the VT curve includes the EF and GH linear lines that the x-axis intersects at 2.43 and 2.79 eV, respectively. The former value 2.43 eV is close to the band gap energy value of V₂O₅. The latter is considered the band gap energy value of TiO₂ doped by vanadium. This value is between the band gap energy values of TiO₂ rutile and V₂O₅. Doping vanadium could result in the 3d orbital of vanadium changing the band gap of titania. Thus, the optical absorption edge of TiO₂ shifts to visible light and the band gap of vanadium-doped TiO₂ is narrower than that of TiO₂ [15].

As above, V₂O₅/TiO₂ material can absorb the light not only in the ultraviolet region but also in the visible region. In order to determine photocatalytic activity of the V₂O₅/TiO₂ material, we carried out degradation of methylene blue on VT under solar light.

A weight 0.1 g of catalyst powder was suspended in 20 ml of 5.10⁻⁵ M methylene blue (MB) aqueous solution and the mixture was stirred under solar light at room temperature. The concentration changes of MB were determined by the UV–Vis spectrometer Jasco 630.

Figure 4 shows the absorption spectra of MB with concentration 5.10⁻⁵ M (a), of the degraded MB by T0 (b), T4 (c) and VT after 15 min (d) and by VT after 30 min (e). Whereas TiO₂ material with large particle size mostly shows inactivity in reaction with methylene blue under solar light, the TiO₂ milled in 4 h has smaller size and exhibits some more photocatalytic activity. The degradation of methylene blue on VT was very fast and most of the methylene blue was degraded after 30 min (curve (e) in figure 4). So, the obtained V₂O₅/TiO₂ material has high photocatalytic activity in degradation of methylene blue due to the good combination of V₂O₅ and TiO₂ in the nano crystalline lattice. These results open the way for a new application in environmental treatment.

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The photocatalytic activity of present vanadium-doped TiO₂ rutile could be compared to that of some TiO₂ anatase and TiO₂ rutile which had been published previously by the author groups of Anpo et al [8] and Liu et al [12], respectively.

The grinding reactive method exhibits successful technique to synthesize vanadium-doped titania rutile TiO₂. By this route, we can obtain nano composite particles of metal-doped TiO₂ rutile without controlling the calcination procedure or using expensive starting chemicals.

The vanadium-doped titania rutile shows a red-shift in the UV–Vis spectra due to the small size of nano rutile particles (with BG more than that of anatase TiO₂) and promotion of vanadium.

The results of methylene blue photocatalytic degradation indicate that the nano vanadium-doped TiO₂ rutile obtained has remarkable higher activity than that of pure nano TiO₂ rutile under solar light at room temperature.

This work is partially financially supported by Vietnam Academy of Science and Technology. The authors are thankful to Professor Acad. Nguyen Van Hieu for his help and encouragement.