Fabrication of Bi-Doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>T</mml:mi><mml:msub><mml:mi>iO</mml:mi><mml:mi>2</mml:mi></mml:msub></mml:mrow></mml:math> Spheres with  Ultrasonic Spray Pyrolysis and Investigation of  Their Visible-Light Photocatalytic Properties

Huang, Jianhui; Cheuk, Wahkit; Wu, Yifan; Lee, Frank S. C.; Ho, Wingkei

doi:https://doi.org/10.1155/2012/214783

Journal of Nanotechnology

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Research Article | Open Access

Volume 2012 | Article ID 214783 | https://doi.org/10.1155/2012/214783

Fabrication of Bi-Doped TiO2 Spheres with Ultrasonic Spray Pyrolysis and Investigation of Their Visible-Light Photocatalytic Properties

Jianhui Huang,^1,2Wahkit Cheuk,²Yifan Wu,³Frank S. C. Lee,³and Wingkei Ho^2,3

Academic Editor: Reda M. Mohamed

Received08 Mar 2012

Accepted20 Apr 2012

Published05 Jun 2012

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 [1–7]. 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 [15–17]. 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 [18–20]. 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 [23–25], 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 [28–31]. 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)₃6H₂O) 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 irradiation, = 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 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.

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.

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.

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 and Ti 2p_1/2 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 4f_5/2 and Bi 4f_7/2, 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⁺ [39–41]. 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.

(a)

(b)

(c)

(d)

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 ( nm) after the adsorption-desorption equilibrium was reached. Figure 5 shows the MO degradation ( and 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.

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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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.

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