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Journal of Nanotechnology
Volume 2012 (2012), Article ID 214783, 7 pages
http://dx.doi.org/10.1155/2012/214783
Research Article

Fabrication of Bi-Doped 𝐓 𝐢 𝐎 𝟐 Spheres with Ultrasonic Spray Pyrolysis and Investigation of Their Visible-Light Photocatalytic Properties

1Department of Environmental and Life Sciences, Putian University, Putian 351100, China
2Nano and Advanced Materials Institute Limited, The Hong Kong University of Science and Technology, Hong Kong
3Department of Civil and Structural Engineering, Research Center for Environmental Technology and Management, The Hong Kong Polytechnic University, Hong Kong

Received 8 March 2012; Accepted 20 April 2012

Academic Editor: Reda M. Mohamed

Copyright © 2012 Jianhui Huang 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

Bismuth-doped TiO2 submicrospheres were synthesized by ultrasonic spray pyrolysis. The prepared bismuth-doped titania was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), and X-ray photoelectron spectroscopy (XPS). Aqueous photocatalytic activity was evaluated by the decomposition of methyl orange under visible-light irradiation. The results indicate that doping of bismuth remarkably affects the phase composition, crystal structure, and the photocatalytic activity. The sample with 2% Bi exhibits the optimum photocatalytic activity.

1. Introduction

Photocatalytic oxidation of pollutants has increasing interests in recent years because of its advantages such as high redox capability, nonselectivity, and efficient solar utilization. Different kinds of photocatalytic materials have been studied such as metal oxides, nitrides, and sulfides [17]. Among them, TiO2 is the most commonly used photocatalyst owing to its high redox power, photostability, chemical inertness and low cost. However, the relatively low quantum efficiency of TiO2 photocatalysts limits its real application. To overcome this problem, a lot of efforts have been paid to improve the photocatalytic efficiency of TiO2 from the viewpoint of practical use.

It has been reported that the crystal size, specific surface area, morphology, and texture have great effects on the photocatalytic properties of semiconductors. Nano/microspheres structure has attracted great interests due to their thermodynamically favorable state in terms of surface energy. Recent researches have demonstrated their potential application in many fields such as photonic crystals [8], biomedicine [9, 10], sensing [11, 12], and solar cells [13, 14]. In particular, some studies found that the nano/microspheres structure of semiconductor have promising properties in the region of photocatalysis [1517]. Various approaches have been used for the preparation of spherical semiconductor materials. The most common approach is based on the use of various removable templates. The removal of template materials is complex and usually requires high temperature processes or wet chemical etching, which is expensive and not easy for mass application. Therefore, it is promising to develop a simple and inexpensive way without using template for the preparation of semiconductor nano/microspheres.

Many reports have shown that the photocatalytic properties of TiO2 can be modified strongly by doping with different elements. The doping element at low concentrations can act as separation centers for the light-induced electron-hole pairs, prohibiting the undesirable fast recombination of the electron-hole pairs. Besides, the doping of impurity atoms has also been proven for the extension of the optical absorption of photocatalysts [1820]. Thus, visible light in the solar energy could be utilized. Among various transition metals, bismuth is a good choice as the dopants in photocatalysis research. Many bismuth-based semiconductors such as Bi2O3 [21], BiOCl [22], BiVO4 [2325], Bi2WO6 [26], and Bi2MoO6 [27] have been found to be efficient as visible-light-driven photocatalysts. Research studies also reported that bismuth-doped materials have enhanced photocatalytic activity [2831]. For example, Li and his coworkers prepared Bi-doped TiO2 exhibiting high efficiency in the decomposition of benzene under the irradiation of visible-light [32]. Liu et al. reported that Bi2O3-TiO2 composite also showed high visible light photocatalytic activity in the degradation of methyl orange [33].

In this study, we described the synthesis of Bi-doped TiO2 spheres with a doping level in the range of 0.5–5% via a facile method of ultrasonic spray pyrolysis. Their photocatalytic performance in the decomposition of methyl orange (MO) dye under visible light irradiation was also investigated.

2. Experimental Section

2.1. Catalysts Preparation

All the chemicals were of commercially available analytical grade and used without further purification. Bi-doped TiO2 photocatalysts were prepared by an ultrasonic spray pyrolysis method. In a typical process, 2.2 mL TiCl4 was added to 200 mL HCl solution (0.5 M) under magnetic stirring. After stirring for 1 hour, a transparent colorless solution was formed. The calculated amounts of Bismuth nitrate (Bi(NO3)3 6H2O) were added to the solution under magnetic stirring. After the Bismuth nitrate dissolved completely, the resulting solution was nebulized at 1.7 MHz ± 10%. The produced aerosol was carried through a corundum tube surrounded by a thermostated furnace with pumping air flow of 10 L/min. The pyrolysis proceeded quickly as aerosol passed through the high-temperature tube maintained at the temperatures of 500°C. The products were collected with distilled water and then separated by centrifugation. Collected samples were washed with water and ethanol and then dried at 80°C. The as-prepared product was then further calcined at 500°C and held for 2 h with a ramp rate of 3°C/min. The samples prepared with the doping level of 0, 0.5, 1, 2 and 5% were designated as TBi0, TBi0.5, TBi1, TBi2, and TBi5, respectively.

2.2. Characterization

X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance X-ray diffractometer (Cu 𝐾 𝛼 1 irradiation, 𝜆 = 1.5406 Å). SEM images of the samples were performed on a JEOL JSM-6300 microscope operated at an accelerating voltage of 15 kV. Transmission electron microscopic (TEM) images were obtained on JEOL 2010F TEM microscope operated at an accelerating voltage of 200 kV. A Varian Cary 500 Scan UV/vis system equipped with a Labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of the catalysts. XPS experiments were performed on a Physical Electronics PHI 5600 multitechnique system, using monochromatized Al 𝐾 𝛼 radiation (1486.6 eV) at 350 W.

2.3. Evaluation of Photocatalytic Activity

The photocatalytic activities of the samples were evaluated by the decomposition of methyl orange (MO) in aqueous solution. Catalyst (0.04 g) was suspended in a 100 mL Pyrex glass vessel containing contaminant aqueous solution. The initial concentration of MO is 10 ppm. The visible-light source was a 300 W halogen lamp (Philips Electronics) positioned beside a cylindrical reaction vessel with a flat side. The system was water-cooled to maintain the temperature. A 400 nm cutoff filter was placed in front of the vessel to ensure irradiation by visible light. The suspension was stirred in darkness for 2 h to achieve adsorption equilibrium, and the reactor was irradiated to induce photocatalyzed decomposition reactions. At given irradiation time intervals, 3 mL of the reaction suspension was collected and centrifuged to remove the catalyst. The degraded solution was analyzed using a Varian Cary 50 Scan UV/vis spectrophotometer.

3. Results and Discussion

3.1. Crystal Structure and Morphology

Figure 1 shows the X-ray diffraction (XRD) patterns of the samples doped with different amounts of Bi. It indicates that the amount of Bi plays an important role in controlling the crystal structure of the products. For the undoped sample, both anatase and rutile are found while the doped samples only exist in anatase phase. The peak intensity of anatase becomes weaker with increasing the content of Bi dopant, while the width of the (101) peak becomes broader. The calculated average crystal sizes of the TBi0, TBi0.5, TBi1, TBi2, and TBi5 using the Scherrer equation are 12.7, 10.4, 9.8, 9.3, and 8.6 nm, respectively, indicating the nanocrystal nature of the prepared samples. The result also indicates that the doping of Bi could suppress the agglomeration of TiO2 particle during the thermal treatment and enhance the phase transformation temperature.

214783.fig.001
Figure 1: XRD patterns of Bi-doped TiO2 spheres with 0, 0.5, 1, 2, and 5% of Bi.

Figures 2(a) and 2(b) depict the SEM and TEM images of doped TiO2 with 2% Bi, respectively. The images show that the as-prepared samples consist entirely of spheres with a range of 100 nm to 1.5 μm. The ultrasonic nebulizer during the preparation is the key to form the spherical morphology. It is known that the droplets produced from nebulizer could serve as microreactors and yield the particle from each droplet when sprayed into a tubular reactor under pyrolysis conditions. In addition, the size of droplet determines the product dimension [34]. In Figure 2(c), the typical high magnified TEM image of Bi-doped TiO2 spheres is shown, which reveals that the prepared spheres are composed of small nanoparticles and have porous structure. Figure 2(d) shows the HRTEM image at the edge of the Bi-doped TiO2 spheres. Well-resolved lattice fringes are clearly observed, with an interplanar distance of 0.35 nm corresponding to the (101) d-spacing of the anatase phase. The selected area electron diffraction (SAED) pattern (Figure 2(e)) of the spheres demonstrates well crystalline nature of the doped TiO2 spheres, which is in well agreement with the XRD results. The composition of the doped TiO2 spheres was determined by energy dispersive X-ray spectroscopy (EDX). The results of EDX analysis (Figure 2(f)) imply that the as-prepared samples contain Ti, O, and a small amount of Bi element. The ratio of Bi to Ti in the produced nanocrystalline titania microspheres is 0.019, which is very close to that in the precursor solution. Therefore, the Bi concentration in the precursor solution could approximately represent the corresponding Bi concentration in the nanocrystalline titania spheres.

214783.fig.002
Figure 2: Representative structural characterizations and general morphologies of Bi-doped TiO2 with 2% of Bi (a) SEM image, (b) and (c) TEM images, (d) HRTEM image, (e) SAED pattern, and (f) Energy-dispersive X-ray analysis spectra.
3.2. Optical Properties

Figure 3 shows the UV spectra of the doped samples with different amounts of Bi. Compared with the undoped sample, the Bi-doped TiO2 samples show remarkable absorption in the visible-light region. The absorption in visible-light region is increased with the Bi-doping content. This wide visible-light response of Bi-doped TiO2 spheres could attribute to the formation of surface-defect centers, which are associated with existence of oxygen vacancies created by the doping process [35, 36]. Besides, the basic adsorption edge of the Bi-doped TiO2 is shifted to a shorter wavelength. These blue shifts of adsorption edge are possibly due to the quantum confinement effect, which is attributed to the smaller crystal size of Bi-doped samples.

214783.fig.003
Figure 3: Diffuse reflectance UV-vis spectra of Bi-doped spherical TiO2 and pure TiO2.
3.3. Surface Electronic States and Composition

The assessment of the surface chemical composition and electronic state of the product was studied by XPS analysis. Figure 4(a) is the XPS survey spectrum of TBi2, which contains the peaks of Ti, O, Bi, and C elements. The C element can be ascribed to the adventitious hydrocarbon from the XPS instrument itself. The high-resolution XPS spectra of the Ti 2p, Bi 4f, and O 1s region on the surface of samples are shown in Figure 4(b)4(d). In Figure 4(b), the two peaks at 459.0 and 464.8 eV are assigned to the T i 2 p 3 / 2 and Ti 2p1/2 states in TiO2, respectively. The binding energy of Ti 2p shows a positive shift of approximately 0.4 eV compared to those of pure anatase TiO2, [37] implying the successful incorporation of the bismuth atom into the TiO2 lattice. In Figure 4(c), four peaks could be found. The peaks centered at 164.9, 163.1, and 159.6, 157.9 eV could be assigned to Bi 4f5/2 and Bi 4f7/2, respectively, indicating two different states of Bi in the sample. The peaks of 163.1 and 157.9 eV could be attributed to the Bi3+, which is in good agreement with other studies [38]. The peaks at high binding energy of 164.9 and 159.6 eV could be assigned to the Bi doped into the TiO2 lattice. The doped Bi with a strong interaction between TiO2 is oxidized to Bi4+ [3941]. The presence of Bi4+/Bi3+ species in the catalyst favors the trap of electrons and benefits the separation of the electron-hole pairs in the photocatalytic process. The O1s XPS spectra in the Figure 4(d) are asymmetric, which can be fitted by two peaks. The strong peak at the low binding energy of 530.0 eV corresponds to crystal lattice oxygen (Ti–O). The shoulder peak at 531.6 eV is associated with hydroxyl groups (H–O) [42, 43]. The quantitative XPS analysis shows that the surface atomic number ratio of Bi to Ti is close to 1.9 : 100, which is also near the ratio in the precursor. Both XPS and EDX results indicate that Bi is uniformly dispersed into the TiO2.

fig4
Figure 4: (a) XPS survey spectrum of TBi2, (b) high-resolution XPS spectrum of Ti 2p region, (c) high-resolution XPS spectrum of Bi 4f region, and (d) high-resolution XPS spectrum of O1s region of TBi2.
3.4. Photocatalytic Activity

The photocatalytic performances of the doped photocatalysts were evaluated by comparing the degradation efficiency of methyl orange (MO) with otherwise identical conditions under visible-light irradiation ( 𝜆 > 4 0 0  nm) after the adsorption-desorption equilibrium was reached. Figure 5 shows the MO degradation 𝐶 𝑡 / 𝐶 0 ( 𝐶 0 and 𝐶 𝑡 are the equilibrium concentration of MO before and after visible-light irradiation, resp.) versus visible-light irradiation time in the presence of various photocatalysts. Control test (without catalyst) under visible-light irradiation showed that the photolysis of MO was negligible. As expected, the doping of Bi greatly affects the photocatalytic activity of TiO2 spheres. Only limited (~6%) MO was degraded over undoped TiO2 spheres. For the doped samples, they show much higher decomposition rate. For the optimal catalyst (TBi2), over 90% MO was degraded after the irradiation of 12 h. The visible-light activity of doped TiO2 may be attributed to two reasons. The first one is due to the formation of oxygen vacancy after introducing of bismuth leading to the visible-light response of TiO2. Another reason is the introduction of Bi existed in two states of Bi4+ and Bi3+ as shown in the result of XPS. The Bi4+/Bi3+ species trap the electrons and thus can be benefited from the separation of the electron-hole pairs.

214783.fig.005
Figure 5: Photocatalytic activity of the Bi-doped TiO2 spheres samples.

The test also shows that the activities of doped samples were greatly influenced by the level of doped Bi. For the sample of TBi0.5, the decomposition of MO after 12 h reaction is as high as 40%. When the concentration of Bi in the doped TiO2 is increased to 2%, the degraded MO is increased to 90%. Further increasing the level of Bi in the TiO2 spheres, the efficiency drops quickly. When the dopant concentration increases to 5%, only 50% MO was degraded after 12 h photocatalytic reaction. It has been reported that the doping of transition metal serving as electron/hole separation centers can eliminate the rapid recombination of excited electron/hole pair during photoreaction, thereby increasing the efficiency of the TiO2 photocatalyst. However, this effect is sensitive to dopant levels. The excessive dopants can act as recombination centers, promoting the recombination of electron/hole pairs. The activity of Bi-doped TiO2 indicates that 2% is the optimized level, which may slow down the recombination process.

4. Conclusions

The Bi-doped TiO2 spheres with variable Bi dopant concentrations were synthesized by using ultrasonic spray pyrolysis process. The doping of Bi effectively suppressed the formation of rutile phase and the crystal growth of TiO2 particles during preparation. Bi-doped TiO2 samples showed an extension of light absorption into the visible-light region. It exhibited improved photocatalytic activities in the degradation of methyl orange under visible-light irradiation. Compared with the traditional preparation technique of spherical materials, the ultrasonic spray pyrolysis technique presented in this paper is continuous and easily to be operated. We believe that this facile method is easy to scale up for industrial production. Similar doping or codoping with other nonmetal and metal ions with similar method would lead to the development of a new kind of materials.

Acknowledgments

The author would like to thank the support of the National Natural Science Foundation of China (21103095) and the Natural Science Foundation of Fujian Province (Grant no. 2010J05030).

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