International Journal of Photoenergy

Volume 2012 (2012), Article ID 208987, 7 pages

doi:10.1155/2012/208987

Research Article

Synthesis and Photocatalytic Activity of TiOX Powders with Different Oxygen Defects

Leini Wang,¹ Fei Lu,¹ and Fanming Meng1,2,3

¹Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230039, China

²Department of Physics and Astronomy, University of Arkansas at Little Rock, Little Rock, AR 72204, USA

³Key Laboratory of Materials Modification by Laser, Ion and Electron Beams Dalian University of Technology, Ministry of Education, Dalian 116024, China

Received 10 February 2012; Revised 28 March 2012; Accepted 30 March 2012

Academic Editor: Weifeng Yao

Copyright © 2012 Leini Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The novel carbon- or chromium-doped TiOX photocatalysts with different oxygen defects were synthesized by mechanochemical technique and heating process. The samples were characterized by X-ray diffraction, UV-vis spectrophotometer, and fluorescence spectrometer. Carbon and chromium species were incorporated into TiOX crystal matrix. The mass fraction of Ti₇O₁₃ in TiOX photocatalysts could be tunable through carbon or chromium doping. The mass fraction of Ti₇O₁₃ could be an indication of the degree of oxygen defects (the concentration of Ti³⁺) in the TiOX. The degree of oxygen defects increased for carbon doping, while the degree of oxygen defects decreased for chromium doping. The photocatalytic activity measurement results showed that photodegradation rate of methyl orange reached the maximum value with mass fraction of Ti₇O₁₃ of about 66.93%, but the photodegradation rate decreased when mass fraction of Ti₇O₁₃ is raised further. In addition, the origin of absorption in the visible spectral range for carbon-doped TiOX as well as the effect of band gap on photocatalytic activity has also been discussed in this paper.

1. Introduction

Currently, extensive research has been carried out on oxide semiconductor photocatalysis in the conditions of aggravation of environmental pollution and resources shortage [1–5]. In this sense, semiconductor photocatalysis can change solar energy to electrical and chemical energy, drive redox reactions, degrade organic substance, and improve environment. As the leading candidate semiconductor photocatalyst, titania has attracted most attention due to its unique physicochemical properties, including good chemical stability, inexpensiveness, relatively good reactivity, and notoxicity [6, 7]. However, the large band gap (3.2ev) of titania allows it to absorb only the ultraviolet light, which seriously restricts its utilization efficiency for the solar photons. Furthermore, the low-quantum yield that results from high frequency of recombination of photoinduced current carriers limits its practical applications [8, 9]. Hence, significant efforts, including metal or nonmetallic ions doping [10, 11], semiconductor coupling [12], and deposition of noble metals [13], have been devoted to extend the spectral response region of titania and enhance its photocatalytic activity.

The photocatalytic activity of doped TiO₂ is a complex function of the dopant concentration [14], crystal structure [11], surface area [15], and lattice defects. Especially oxygen vacancies play an important role in photocatalytic efficiency of TiO₂ [16–18]. However, the photocatalyst of the TiOX with oxygen defects was little reported in previous literatures.

In this paper, TiOX powder including Ti₇O₁₃ and TiO₂ was used as precursor. Carbon or chromium doping was used to control the degree of oxygen defects of samples. The photocatalytic activities dependence on the degree of oxygen defects was evaluated in terms of the photodegradation of methyl orange (MO) under UV light irradiation. Furthermore, a facile green synthetic route of C-doped TiOX was developed, which could provide an effective technique for industrial production due to its low cost.

2. Experimental

2.1. Preparation of Samples

The TiOX powder and chromic oxide (Cr₂O₃) were purchased from Tianjin Guangfu Fine Chemical Research Institute, China. Glucose (C₆H₁₂O₆) was purchased from Tianjin Guangfu technology development Co. Ltd., China. All the reagents were of analytical grade purity and used without any further purification. Distilled water was used in all experiments. A planetary ball mill was used for sample synthesis.

Carbon- and chromium-codoped TiOX sample was prepared by following process. Appropriate amount of C₆H₁₂O₆ and Cr₂O3   were added to TiOX powder. The mass ratio of Cr₂O₃/C₆H₁₂O₆/TiOX was kept constant at 2 : 9 : 200. Total 40 g of above mixture was put into the bottle, and then 25 mL distilled water was added. After being milled for 180 min at a speed of 400 rpm, the wet powder was dried in air over at 90°C for 10 h. The ground powder was subsequently heated at 200°C for 300 min. The prepared catalyst was named C/Cr-TOV. Carbon-doped and chromium-doped and pure TiOX samples were also prepared through the same method, without adding the corresponding dopant, named as C-TOV, Cr-TOV, and TOV, respectively. Namely, for the synthesis of Carbon-doped TiOX, the mass ratio of C₆H₁₂O₆/TiOX was kept constant at 9 : 200. Total 40 g of above mixture was put into the bottle followed by the addition of 25 mL of distilled water.

2.2. Characterizations

The crystal phases of the samples were analyzed by X-ray diffraction (XRD) with CuKα. The crystalline sizes of anatase and Ti₇O₁₃ were calculated by Scherrer formula. The average crystalline size was obtained using following equation [15]:

where is average crystalline size, and are crystalline sizes of anatase and Ti₇O₁₃, respectively. and are peak intensity of anatase (1 0 1) and Ti₇O₁₃ ( 2 5), respectively.

The mass fraction of anatase (), Ti₇O₁₃ (), and brookite () were calculated using following equations [11, 19]:

where , , and are peak intensity of anatase (1 0 1), Ti₇O₁₃ ( 2 5), and brookite (2 1 1), respectively. , , and are constant (taken as 1, 0.1963, and 0.3354, resp.).

The UV-vis absorption spectroscopy of the sample was measured using Ultraviolet-Visible-Near Infrared Spectrophotometer (U-4100), while BaSO₄ was used as a reference. Photoluminescence (PL) spectra were obtained using a fluorescence spectrophotometer (F-4500) at room temperature.

2.3. Photocatalytic Activity

Photocatalytic activity of samples was characterized by decolorization of methyl orange (MO). The photocatalyst (0.3 g) was dispersed in 25 mL MO aqueous solution with a concentration of 5 ppm in a dish with diameter of 8 cm. The mixture was kept in the dark for 30 min to obtain the absorption-desorption equilibrium before UV-light irradiation. A 100 W mercury lamp was used as a light source. The distance between the lamp and the reaction solution was 9 cm. The absorbance of MO solution at 464 nm was measured with UV-vis spectrophotometer at intervals of 15 min and the total irradiation time was 45 min.

3. Results and Discussion

Figure 1 (a, b, c, d) shows the XRD patterns of the samples. It is found that all the samples consist of mixed phases of anatase, Ti₇O₁₃, and brookite. For doped TiOX (b, c, d), the characteristic peaks of carbon and Cr₂O₃ are not observed. It may be attributed to the small amount of dopant or carbon, and Cr₂O₃ is dispersed uniformly into the TiOX matrix [20, 21]. The average crystalline sizes of the samples and the mass fraction of three phases are calculated by equations (1)–(4), as listed in Table 1. As can be seen from Table 1, the average crystalline sizes of doped TiOX are lower than that of undoped TiOX. This result implies that the existence of impurity prevents the agglomeration of particles [15]. The samples show different mass fraction of three phases for different doping. Especially the mass fraction of Ti₇O₁₃ is changed through carbon and Cr₂O₃ doping, which suggested that carbon or chromium doping can change the degree of oxygen defects for samples. This is because that the mass fraction of Ti₇O₁₃ can be an indication of the degree of oxygen defects (the concentration of Ti³⁺) in the TiOX.

Table 1: Physicochemical properties of the TiOX samples.

Samples
Crystalline Size (nm)
Anatase (wt %)
Ti₇O13   (wt %)
Brookite (wt %)
Band gap (eV)

TOV
42.3
20.01
66.93
13.06
3.13

Cr-TOV
40.5
19.79
66.87
13.34
3.09

C-TOV
38.4
22.02
67.73
10.25
3.02

C/Cr-TOV
38.5
18.41
68.19
13.40
2.99

Figure 1: XRD patterns of samples with different oxygen defects: (a) undoped TiOX, (b) C-doped TiOX, (c) Cr-doped TiOX, and (d) C/Cr-doped TiOX. (e)–(h) are magnified XRD spectra of (a)–(d), respectively.

From Figure 1 (e, f, g, h), it can be seen that, compared with TOV, anatase peaks (1 0 1) and Ti₇O₁₃ peaks ( 2 5) of other samples shift to lower 2θ value, which imply that the d-spacing of TiOX increases. Furthermore, it is suggested that some of C/Cr atoms or ions have entered into the interstitial sites of TiOX host and that results in the expansion of TiOX lattice [20, 22].

Figure 2 shows the UV-vis absorption spectra of all the samples. In contrast to the pure TiOX, carbon- or chromium-modified TiOX show, broader absorption shoulders in the visible light region. The relation between absorption coefficient () and band gap () can be written as , where is the frequency and is Planck’s constant [14]. The band gap energies can be estimated by the plot of  versus photon energy (hv), as shown in Figure 3 and listed in Table 1. It can be seen that the band gap for carbon- or Cr₂O₃-doped TiOX is less than that of undoped TiOX. Two small absorption peaks located at 450 and 600 nm can be observed clearly for Cr-TOV. Based on related documents [11, 23, 24], the peak located around 450 nm can be attributed to the charge transfers band Cr³⁺→Ti⁴⁺  or ⁴A2g→⁴T1g of Cr³⁺   in an octahedral environment, another due to ⁴A2g→⁴T2g d-d transitions of Cr³⁺. However, in the case of carbon-doped TiO₂, there is controversial reports in the literature for the origin of absorption in the visible spectral range. Some researchers proposed that this red shift was ascribed to the presence of localized states of the dopants in the band gap [14, 25]. While others have suggested that the formation of oxygen vacancies and the appearance of color centers were responsible for obvious absorption in the visible light range for nonmetal doped titania [16, 26]. It is well known that carbon atoms or ions that diffused into the interstitial sites of TiOX host will “plunder” oxygen in TiOX attributed to the chemical bond strength of C–O (1076 kJ/mol) stronger than that of Ti–O (662 kJ/mol) [21]. Oxygen vacancies state between the conduction and valence bands will be easier to form in the carbon-doped TiOX   [16]. We assume that carbon doping is consistent with the increase of oxygen vacancies that result in red shift for C-TOV and C/Cr-TOV. Similar results have been observed in nonmetal doped TiO₂ [20, 27, 28].

Figure 2: UV-vis absorption spectra of the samples with different oxygen defects.

(a)

(b)

(c)

(d)

Figure 3: The plot of versus photon energy of the samples with different oxygen defects.

PL spectrum analysis is an effective tool to discern defect-related transitions in the samples. To further explore the degree of oxygen defects in the samples, PL measurements were done for all of the samples. Figure 4(a) shows the PL spectra of TOV and Cr-TOV measured at room temperature at an excitation wavelength of 320 nm. It is found that the emission peaks are mainly centered on 400, 470, and 530 nm. The emission peaks around 400 nm are related to the band-edge free excitons. Peaks around 470 and 530 nm originate from the bound excitons [29, 30]. The luminescence bands ranging from 460 to 580 nm for TiO₂ sample are ascribed to the transition from different exciton energy levels arising from oxygen vacancies to the valence band [29–31]. Obviously, the emission intensity increases with the degree of oxygen defects. Compared with the Cr-TOV sample, the enhanced PL intensity for TOV reflects the increases of the degree of oxygen defects in the TOV sample. The result is in good accord with the XRD analysis about the mass fraction of Ti₇O₁₃ phase between TOV and Cr-TOV (Table 1). Also, it can be concluded from Figure 4(b) that the degree of oxygen defects of C/Cr-TOV is more than that of C-TOV. Combined with the XRD analysis, it is believed that the degree of oxygen defects of the samples is determined in the following order: C/Cr-TOV > C-TOV > TOV > Cr-TOV.

(a)

(b)

Figure 4: Photoluminescence spectra of samples: (a) TOV and Cr-TOV, (b) C/Cr-TOV and C-TOV at room temperature ().

Figure 5 shows the reaction constant for MO photodegradation of samples under UV light irradiation. The reaction constant is evaluated by equation: [32], where and are the initial concentration and the reaction concentration of MO, t is the time of light irradiation. It is clear that the samples with different oxygen defects exhibit the superior photocatalytic activity compared with blank sample (the absence of photocatalyst) for decolorization rate of MO. In addition, the is 0.135, 0.112, 0.104, and 0.081 for TOV, Cr-TOV, C-TOV, and C/Cr-TOV, respectively. Obviously, the loss of oxygen plays a significant role in improving the photodegradation rate for various catalysts.

Figure 5: The comparison of reaction constant of different samples.

The dependence of the mass fraction of Ti₇O₁₃ phase in the samples is showed in Figure 6. The mass fraction of Ti₇O₁₃ can be an indication of the degree of oxygen defects (the concentration of Ti³⁺) in the TiOX. As can be seen from Figure 6, the photocatalytic activity increases and then decreases with the increase of the mass fraction of Ti₇O₁₃. The photocatalytic reaction rate of MO reaches the maximum value with mass fraction of Ti₇O₁₃ of about 66.93%. The relationship between the decolorization of MO and the degree of oxygen defects (the concentration of Ti³⁺) can be explained as follows. For TOV sample, appropriate degree of oxygen defects (or appropriate concentration of Ti³⁺) would act as electron-trapping centers, which inhibit the recombination of photoinduced electron-hole pairs. And thus more photoinduced holes are removed to the surface of photocatalyst to produce more hydroxyl radicals and participate in the redox reaction that results in the enhancement of photocatalytic activity [17, 20, 26]. For Cr-TOV sample, lower concentration of oxygen defects (or lower concentration of Ti³⁺) will lead to higher photocatalytic activity. However, in the case of C-TOV and C/Cr-TOV samples, the highest degree of oxygen defects (or the highest concentration of Ti³⁺) results from carbon doping that suppresses the photocatalytic activity. This is because excess amount of oxygen vacancies would become recombination centers of the photoinduced charge carriers, leading to the depressed quantum yield [20].

Figure 6: The photocatalytic activity as a function of the mass fraction of Ti₇O₁₃.

In addition, there are also many factors [3] such as phase structure, band gap, and surface state that affect the photocatalytic activity. Combining absorption spectra analysis (band gap) with photocatalysis mechanism, photocatalytic activity of TiOX is also dependent on energy gap. From above band gap of Table 1, it can be reasonably deduced that proper band gap (around 3.13 eV) is beneficial to creation of electronic-hole pairs that make excellent photocatalytic activity of TiOX. When energy gap is smaller than 3.13 eV, it is beneficial to creation of electronic-hole pairs, but at the same time, recombination chance of electronic-hole increases, as a result, photocatalytic activity is lower.

4. Conclusions

In summary, some of C/Cr atoms or ions are successfully incorporated into the TiOX host by mechanochemical and heating process. The mass fraction of Ti₇O₁₃ phase of samples can be changeable by C/Cr doping. The mass fraction of Ti₇O₁₃ increased for carbon- doping, while the mass fraction of Ti₇O₁₃ decreased for chromium doping. The photocatalytic activity of samples is closely related to the degree of oxygen defects (the concentration of Ti³⁺) and band gap. However, the photodegradation rate will be suppressed with excess oxygen defects. Furthermore, carbon doped TiOX shows a broader absorption shoulder in the visible light region. It is attributed to the increase of oxygen vacancies and the advent of color centers.

Acknowledgments

This work was supported by the Natural Science Foundation of Anhui province of China (no. 1208085ME81), the Scientific Research Foundation of Education Ministry of Anhui province of China (nos. KJ2011A010 and KJ2012A029), the Teaching Research Foundation of Education Ministry of Anhui province of China (no. 20100185), the Doctor Scientific Research Starting Foundation of Anhui University of China, the Foundation of “211 Project” of Anhui University of China (no. KJTD004B), and the Foundation of Construction of Quality Project of Anhui University of China (nos. XJ200907, xj201140, and 39020012).

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