Preparation of TiO₂ photocatalyst loaded with V₂O₅for O₂ evolution

GAO You-liang(高友良)^1,2, CHEN Qi-yuan(陈启元)¹, TONG Hai-xia(童海霞)¹, HU Hui-ping(胡慧萍)¹,

QIAN Dong(钱 东)¹, YANG Ya-hui(杨亚辉)¹, ZHOU Jian-liang(周建良)¹

 (1. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, China)

Abstract: TiO₂ photocatalysts loaded with V₂O₅ were prepared via a modified hydrolysis process, and characterized by X-ray diffraction, transmission electron microscopy, Raman spectra and diffuse reflectance UV-Vis spectra measurements. The photocatalytic activity of V₂O₅/TiO₂ was investigated by employing splitting of water for O₂ evolution. The results indicate that V₂O₅ loading can pronouncedly improve the photocatalytic activity of TiO₂ with Fe³⁺ as an electron acceptor under UV or visible light irradiation. The optimum mass fraction of the loaded V₂O₅ is 8%, and the largest speed of O₂ evolution for 8%V₂O₅ (mass fraction) loaded TiO₂ catalyst is 118.2 ?mol/(L·h) under UV irradiation, and 83.7 ?mol/(L·h) under visible light irradiation.

Key words: TiO₂; V₂O₅; photocatalytic activity; O₂ evolution

1 Introduction

Nowadays, more and more attention is paid to the research on photocatalytic decomposition of water because of the high efficiency, cleanness and other advantages of hydrogen energy. FUJISHIMA and HONDA[1] used n-type semiconductor TiO₂ to split water and produce H₂ in the 20th century. As titania with large band gap is cheap, nontoxic, stable and reclaimable[2], it is a very promising kind of semiconductor material that can be photo-excited to generate electron-hole pairs on its surface and create a strong oxidation ability when it absorbs light with energy greater than its band gap, and the electrons in the valence band are excited to the conduction band. TiO₂ is an effective photocatalyst under UV irradiation for decomposing volatile organic compounds[3], smart-windows[4] and water electrolysis for hydrogen production[5]. However, pure titania has low photo-activity and slow photolysis speed for photocatalysis of water and is very difficult to be practically used. So it is necessary to improve its photo-activity and enhance the photolysis speed for water splitting. Generally, the photocatalytic activity of semiconductor can be improved through doping or loading semiconductor, such as Ru³⁺/TiO₂[6], TiO₂-Al₂O₃, V₂O₅/TiO₂-Al₂O₃[7], TiO₂-La₂O₃[8], TiO₂- WO₃[9], V₂O₅-WO₃-TiO₂[10]. And most of these doped semiconductors were used to photo-decompose organic pollutants, and some of them were used to split water with low photocatalytic efficiency[6]. When TiO₂ is combined with other oxides such as V₂O₅, the spectral range of absorption is increased due to the small band gap of V₂O₅ (2.2 eV, λ=564 nm) compared with that of TiO₂ (3.0 eV, λ=413 nm). This allows light with λ≤  564 nm to be absorbed by the heterogeneous structure, and the photocatalytic activity can be extended to a larger spectral range. In addition, V₂O₅ can act as an electron scavenger and pull the photo-generated conduction band electrons of TiO₂ to its own conduction band, which keeps the electron-hole pairs apart longer (charge separation), and improves the overall photocatalytic performance. Vanadium supported on titania is commonly used as a catalyst for a number of industrially important reactions, including the selective oxidation reactions of o-xylene[11], ammoxidation of hydrocarbons[11-12], as well as selective reduction of NO_x with NH₃ in the presence of O₂[13]. JIANG et al  [14] reported that the spreading of vanadium over TiO₂ support leads to a modification of the chemical-physical peculiarities of the former and to an enhancement of its catalytic properties. However, there are few reports about V₂O₅/TiO₂ used as a catalyst for splitting water. As you know, the reaction of photocatalytic splitting water is carried out via two half reactions, of which, one is the photo-reduction with H₂ evolution, the other is the photo-oxidation with O₂ evolution. And the latter is more difficult to achieve. In this work, by using Fe³⁺ as electron acceptors, V₂O₅-TiO₂ catalysts prepared by a modified hydrolysis process were employed for the photocatalytic oxidation of water with O₂ evolution.

2 Experimental

2.1 Synthesis of rutile TiO₂ and V₂O₅-TiO₂ catalysts

TiO₂ colloid solution was prepared by the hydrolysis of tetrabutyl titanate [Ti(OC₄H₉)₄]. All the chemical reagents were analytically pure and used without further purification. Ti(OC₄H₉)₄ was dissolved in ethanol, and then the mixture was added to distilled water drop by drop under magnetic stirring. The mixture was stirred for 2 h and then deposited for 6-8 h before being dried at 373 K to obtain a white precursor. The white precursor was ground in a carnelian mortar and loaded in a navicular quartz vessel to be calcined at    1 223 K for 5 h. The white precursor was ground again to obtain rutile TiO_2. Rutile TiO₂ was dipped in ammonium matavanadate (NH₄VO₃) solution, then dried, and calcined at 723 K for 5 h. A series of catalysts were obtained with concentrations of 5%, 8%, 10%, 20% and 40% of loaded V₂O₅, respectively.

2.2 Characterization of V₂O₅-TiO₂ catalysts

X-ray diffraction (XRD) analysis was used to check the coexistence of different crystal phases of the catalyst by a HATCHI D/max2250 powder X-ray diffractometer. The diffraction profiles were recorded with Cu K_a radiation (0.154 056 nm) over a 2θ range from 10? to 90?. A plumbaginous counter with monochromator was used. The X-ray tube was operated at 40 kV and 300 mA.

Diffuse reflectance UV-Vis spectra (UV-DRS) measurements were carried out on a Beijing Purkinje TU-1901 UV/Vis spectrophotometer equipped by a diffuse reflectance accessory with an IS19-1 integrating sphere, and BaSO₄ powder was used as the reference.

Raman spectra were recorded on a RT1-30 spectrophotometer. The line at 4 888 nm from an argon ion laser was used as the excitation source. The laser power applied was 326 mW, and the raster width was 201 μm. The scan was arranged in the wavenumber range of  100-1 100 cm^-1.

The results were examined by a HATCHI SP-2305 gas chromatography equipped with a thermal conductivity detector. Argon was used as the carrier gas and the fixed phase was molecule sieve (0.5 nm).

2.3 Photo-activity tests

The photocatalytic reaction was performed using a closed gas-circulating system with an inner irradiation reactor. A light source (250 W high-pressure Hg lamp or 250 W Xe lamp), was covered with a glass jacket made of quartz (cut-off wavelength less than 200 nm). The reactor temperature was kept constant at 293 K using cooling water. A mixture of catalyst (2 g), distilled water (640 mL) and a required amount of Fe₂(SO₄)₃ in the reactor was completely degassed, and then argon gas was introduced into the system. The catalyst powder was suspended by using a magnetic stirrer. The product gas oppressed the liquid in the reactor into the graduated cylinder through the outlet. So the volume of product gas can be read through the graduated cylinder indirectly. The evolution of O₂ was detected by gas chromatography.

3.1 Characterization of catalyst

Fig.1 shows XRD patterns for catalysts V₂O₅-TiO₂. From the patterns some distinct peaks for TiO₂ can be observed. From the left to right 2θ is 27.56?, 36.21?, 41.38?, 54.44?, 56.74? and 69.09? corresponding to the diffraction peak of rutile TiO₂ crystal faces (110), (101), (111), (211), (220), and (311), respectively, and which are consistent with literature reports[15]. XRD analysis does not present any characteristic diffraction peak of V₂O₅ phase or other vanadium oxide until the loading concentration of V₂O₅ is more than 8%(mass fraction). The XRD patterns for the catalysts with 10%, 20% and 40% of V₂O₅-loaded TiO₂ show the characteristic diffraction peak of V₂O₅ (2θ=20.26?). Therefore, V₂O₅ will be well dispersed on the TiO₂ phase when its concentration is small; and on the contrary, when the concentration of loading V₂O₅ is enhanced, V₂O₅ is gathered and crystallized.

Fig.1 XRD patterns for V₂O₅-TiO₂ catalysts: (a) 5%V₂O₅-TiO₂; (b) 8%V₂O₅-TiO₂; (c) 10%V₂O₅-TiO₂; (d) 20%V₂O₅-TiO₂;   (e) 40%V₂O₅-TiO₂

Raman spectra of V₂O₅-TiO₂ catalysts synthesized under different dehydrated conditions, are presented in Fig.2. The rutile TiO₂ possesses Raman bands at 440 and 612 cm^-1[16], and V₂O₅ possesses Raman bands at   143, 280, 486, 695 and 995 cm^-1[17]. Raman band at

Fig.2 Raman spectra of V₂O₅-TiO₂ catalysts: (a) 5%V₂O₅-TiO₂; (b) 8%V₂O₅-TiO₂; (c) 10%V₂O₅-TiO₂; (d) 20%V₂O₅-TiO₂;   (e) 40%V₂O₅-TiO₂

995 cm^-1 is assigned to the stretching mode of V—O in the spectra of 10% V₂O₅-TiO₂ catalysts from Fig.2(c), which indicates that V₂O₅ has dispersed on the surface of TiO₂ in monolayer. When the concentration of V₂O₅ is beyond its dispersion ability, the microcrystallite of V₂O₅ is formed. With the quantity increase of loading V₂O₅, the intensity of the characteristic peak of V—O becomes more obvious. WACHS and WECKHUYEN[18] reported that when V₂O₅ dispersed on the surface of TiO₂ in monolayer, the atomicity of vanadium would be   7-8 nm^-2. In the spectra of 8%V₂O₅-TiO₂ catalysts, there is no peak for V₂O₅ and the atomicity of vanadium on the surface of TiO₂ is about 6.8 nm^-2, which is close to the atomicity of the monolayer. However, when the coverage is beyond 8%, the peak for V—O appears, which is in a good agreement with XRD patterns.

Fig.3 shows an obvious absorption band in the wavelength range of 200-380 nm. It is the characteristic absorption of the charge transfer of O 2p→Ti 3d and the charge is from oxygen atom coordinated with titanium to the empty orbit of the center titanium atom[19]. As shown in Fig.3, the absorption band in the wavelength range of 200-600 nm results from electron transition of O 2p→V 3d, and the absorption band in the wavelength range of 300-350 nm results from electron transition of octahedron coordinate vanadium. But the absorption band in the wavelength range of 200-300 nm is from utterly isolated octahedron vanadium. So the absorption band in the wavelength range of 200-350 nm should be attributed to the electron transition interaction between  O 2p→Ti 3d and O 2p→V 3d[20]. Meanwhile, the absorption of visible light for V₂O₅-TiO₂ catalysts is enhanced obviously. So V₂O₅ loading can improve the absorption of visible light for TiO₂ photocatalyst markedly.

Fig.3 Diffuse reflectance UV-Vis spectra (UV-DRS) of catalysts: (a) 5%V₂O₅-TiO₂; (b) 8%V₂O₅-TiO₂; (c) 10%V₂O₅- TiO₂; (d) 20%V₂O₅-TiO₂; (e) 40%V₂O₅-TiO₂

There are several existing forms for vanadium- oxygen species on the surface of TiO₂ on account of the loading quantity of V₂O₅, such as isolated-VO_x, poly-VO_x and crystalloid-V₂O₅. So it is reasonable to consider that isolated-VO_x is dominant when loading quantity is very small, which is likely to be concerned with TiO₂ by V—O—Ti bond (shown in Fig.4(a)). With the increase of loading quantity of V₂O₅, vacancies near the isolated-VO_x are gradually occupied and the proportion of poly-VO_x that is formed by V—O—V chemistry bond connection increases gradually, leading to increasing the polymerization degree of VO_x. The proportion of V—O—Ti bond declines with the proportion of V—O—V bond mounting up[21] (shown in Figs.4(b) and (c)).

Fig.4 Bond graphs of V₂O₅ and TiO₂ on surface of TiO₂

Fig.5 shows the transmittance spectra of the suspending liquid of TiO₂ powder with different V₂O₅ loadings. It is clear from the transmittance curves that with the increase of the concentration of loading V₂O₅, the optical transmittance gradually decreases. The average transmittance in the visible region is larger than that in UV-light region for all samples, and the transmission in the UV-light region is less than 5%. This means that all catalysts can absorb almost all the UV-light in the photocatalytic reactions according to UV-DRS in Fig.3 and the transmitted spectra of the suspending liquid of V₂O₅-TiO₂ catalysts in Fig.5. In the wavelength region of 400-580 nm, for all the samples the refraction and the transmittance increase gradually from Fig.3 and Fig.5. This indicates that the catalysts can absorb visible light. But obviously when the red shift of the wavelength occurs the absorption will decrease. That is to say, the catalysts have a good absorption to UV-light and can also absorb visible light. But the transmittance does not change obviously with the increase of loading V₂O₅.

Fig.5 UV-Vis transmittance spectra of V₂O₅-TiO₂ catalysts:  (a) 5%V₂O₅-TiO₂; (b) 8%V₂O₅-TiO₂; (c) 10%V₂O₅-TiO₂; (d) 20%V₂O₅-TiO₂; (e) 40%V₂O₅-TiO₂

Under the illumination, the transition for electrons absorbing photons must keep conservation of momentum besides conservation of energy, which satisfies the selection rule. It is assumed that the primary wave vector of the electron is K, which will transit to the state with wave vector of K′. As for electrons in the energy band, hK is similar to the property of momentum, so during the process of transition K and K must satisfy the conditions as follows[22]:

hK′-hK=M                                 (1)

where  M denotes the photons’ momentum.

Generally, as photons’ momentum resulting from semiconductor absorbing is less than the electrons’ momentum, the photons’ momentum can be ignored. So Eq.(1) can be written as follows:

K′ = K                                                                                                                    (2)

This shows that the wave vector of electrons keeps equal after absorbing photon and the electrons’ energy increases, which is the electrons’ selection rule. In order to satisfy the selection rule and keep the wave vector constant during the transition, electrons in valence band in excited state A can only transit to state B in the conduction band. Because of states A and B on the same droop line in one dimension energy curve this kind of transition is called direct transition. The intrinsic absorption of semiconductor forms a continuous absorption band, and has a long wave absorption edge ν₀=E_g/h. So the band gap of semiconductor (E_g) can be extracted from the measurement of absorption. The common semiconductors such as GaAs and InSb in Ⅲ-Ⅴ and Ⅱ-Ⅵ subgroups are called direct band semiconductors because their minimum conduct band and maximum valence band are corresponding to the same wave vector. Theoretical calculation of electrons of this kind of semiconductors from their intrinsic absorption shows that if the transition for any K is permitted in the direct transition the relationship between the absorption coefficient and photon energy can be denoted as follows:

where  α is the absorption coefficient, and the relationship between α and transmissivity T is α=lnD- lnT (D is the thickness of solution); hv is the energy of photon with 1 240/λ; and A is a constant.

But both theory and experiment prove that the conduct bands and valence bands for many semiconductors such as Ge and Si are not corresponding to the same wave vector, which are called indirect semiconductors. For indirect semiconductors the relationship between the absorption coefficient and photon energy is shown as follows:

The curves of (αhv)^1/2 vs hv for V₂O₅-TiO₂ catalysts are shown in Fig.6. The linearity parts of the curves are extended to the point of α=0, and the intercept on abscissa is the band gap (E_g). V₂O₅ is an indirect gap semiconductor[23]. According to Eq.(4) and Figs.6(a)- (e), the optical energy band gap of the catalysts of 5%V₂O₅-TiO₂, 8%V₂O₅-TiO₂, 10%V₂O₅-TiO₂, 20% V₂O₅-TiO₂ and 40%V₂O₅-TiO₂ are 2.12, 2.14, 2.21, 2.25 and 2.26 eV, respectively. Rutile TiO₂ is a direct gap semiconductor, and according to Eq.(3) and Fig.6(f), the optical energy band gap of rutile TiO₂ is about 3.04 eV.

3.2 Photocatalytic activity of catalysts

The photocatalytic activities of V₂O₅-TiO₂ and pure rutile TiO₂ were studied through the splitting of water for O₂ evolution under UV-light irradiation. The results are presented in Fig.7, which shows that the rates of O₂ evolution of TiO₂ loaded with V₂O₅ are larger than that of pure rutile TiO₂ after irradiation for 12 h. With the increase of the quantity of loaded V₂O₅, the rate of O₂ evolution also increases, which reaches the maximum rate of 118.2 ?mol/(L·h) when the concentration of loaded V₂O₅ is 8%. The rate of O₂ evolution declines when the concentration of loaded V₂O₅ is more than 10%. Therefore, TiO₂ loaded with V₂O₅ in a suitable amount can improve the photocatalytic activity of TiO₂.

Fig.6 Absorption coefficient as function of incident photon energy of V₂O₅-TiO₂ catalysts: (a) 5%V₂O₅-TiO₂; (b) 8%V₂O₅-TiO₂;   (c) 10%V₂O₅-TiO₂; (d) 20%V₂O₅-TiO₂; (e) 40%V₂O₅-TiO₂; (f) Rutle TiO₂

The photocatalytic activities of V₂O₅-TiO₂ were investigated through the splitting of water for O₂ evolution under visible light irradiation. The results are shown in Fig.8. The lamp source is 250 W Xe, which is basically consistent with the characteristic spectrum of sun shine. Compared with the photocatalytic activity of V₂O₅-TiO₂ catalysts under UV-light irradiation, the order of O₂ evolution speed does not change. But the rates of O₂ evolution change, and the maximum rate is 83.7 ?mol/(L·h) for 8%V₂O₅-TiO₂. The band gap of pure TiO₂ is 3.0 eV, and the largest response wavelength is 413 nm according to formula E_g=1 240/λ. V₂O₅ loading makes TiO₂ response to visible light because the band gap of V₂O₅ is about 2.2 eV.

Generally, the photocatalytic activity of photocatalyst is determined by its ability of light absorbing, the efficiency of separation between photoelectrons and holes, and the increase of the transfer rate of charge carriers[24].

When the surface of the semiconductor is irradiated

Fig.7 Dependence of photocatalytic O₂ evolution on concentration of loaded V₂O₅ under UV-light irradiation

Fig8.Dependence of photocatalytic O₂ evolution on concentration of loaded V₂O₅ under visible light irradiation.  

by light whose energy is higher than the band gap of this semiconductor, electrons will obtain much energy to jump onto the conduction band and become free electrons named photoelectrons. Thus, holes are left in the valence band[25]. Because the conduction band of V₂O₅ is about 0.17 eV lower than that of rutile TiO₂, electrons can be gathered in the conduction band of V₂O₅ and holes are left in the valence band of TiO₂ after the photo-irradiation of the catalyst. The produced photoelectrons and holes can be separated efficiently. During the process of photocatalytic reactions, photo-electrons in conduction band of V₂O₅ can be captured by electron acceptor Fe³⁺, and anions OH^– in water can release their electrons in the valence band of TiO₂ with O₂ evolution.

4 Conclusions

(1) Rutile TiO₂ powder photocatalysts are synthesized by hydrolysis process, then rutile TiO₂ is dipped in the ammonium matavanadate solution, dried, and calcined, at last V₂O₅-TiO₂ powders are obtained.

(2) Raman spectra of V₂O₅-TiO₂ catalysts and XRD results show that 8%V₂O₅-TiO₂ is near to monolayer. The diffuse reflectance UV-Vis spectra show that V₂O₅ loading can improve the absorption of visible light for TiO₂ photocatalyst markedly. The optical energy band gaps of the catalysts of 5%V₂O₅-TiO₂, 8%V₂O₅-TiO₂, 10%V₂O₅- TiO₂, 20%V₂O₅-TiO₂ and 40 %V₂O₅-TiO₂ are 2.12, 2.14, 2.21, 2.25 and 2.26 eV, respectively.

(3) V₂O₅ loading can improve the photocatalytic activity of TiO_2. The maximum rate of water splitting for V₂O₅-TiO₂ photocatalysts is 118.2 ?mol/(L·h) when the concentration of loaded V₂O₅ is 8% using Fe³⁺ as an electron acceptor under UV irradiation and 83.7 ?mol/(L·h) under visible light irradiation.

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(Edited by CHEN Wei-ping)

Foundation item: Project(08JJ3022) supported by the Natural Science Foundation of Hunan Province, China

Received date: 2009-03-26; Accepted date: 2009-06-06

Corresponding author: GAO You-liang, Professor; Tel: +86-798-8499697; E-mail: gaocsu@163.com