Journal of Nanomaterials

Volume 2013 (2013), Article ID 125409, 6 pages

http://dx.doi.org/10.1155/2013/125409

Research Article

Hydrothermal Synthesis of Bi₂S₃ Nanorods from a Single-Source Precursor and Their Promotional Effect on the Photocatalysis of TiO₂

Juan Lu, Zuoshan Wang, Ying Zhang, and Xiufeng Zhou

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Soochow 215021, China

Received 30 January 2013; Accepted 22 April 2013

Academic Editor: Anukorn Phuruangrat

Copyright © 2013 Juan Lu 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.

As a direct bandgap semiconductor, Bi₂S₃ has the potential ability to improve the photocatalytic activity of nano-TiO₂ due to its low energy gap ( eV). In this study, large-scale uniform Bi₂S₃ nanorods were synthesized by a hydrothermal treatment, using Bi[S₂P(OC₄H₉)₂]₃ as the single-source precursor. Characterization results show that as-prepared samples belong to an orthorhombic phase of Bi₂S₃, and the products mainly crystallize in the form of nanorods which measure ca. 200 nm in length and ca. 50 nm in diameter. The photo-catalytic experiments for the degradation of methyl orange under visible irradiation revealed that a small amount of as-prepared Bi₂S₃ in our study would significantly improve the photo-catalytic activity of nano-TiO₂, whether Bi₂S₃ is introduced by a physical way or a chemical way. However, excess Bi₂S₃ will lead to a decrease in the catalytic efficiency of TiO₂ when Bi₂S₃ was introduced by a chemical way; it never happened when Bi₂S₃ was introduced by a physical way. Even so, among all as-prepared samples, the TiO₂-based photo-catalyst with 3 wt.% Bi₂S₃ introduced by a chemical way exhibits the best catalytic performance under visible irradiation.

1. Introduction

As a direct bandgap semiconductor, Bi₂S₃ has the potential ability to improve the photocatalytic activity of nano-TiO₂ due to its low energy gap ( eV) [1–9]. As we all know, material’s properties strongly depend on its structure which in turn has a close relationship with its preparation method. Therefore, in the case of the same TiO₂, the photocatalytic activity of Bi₂S₃/TiO₂ heterojunction will mainly rely on two aspects, including the different preparation methods for Bi₂S₃ and the different ways to introduce Bi₂S3 into TiO₂.

Currently, there are many classic preparation methods for Bi₂S₃ nanoparticles or superstructures, such as hydrothermal method [10–12], solvothermal method [13], electro-deposition technique [14], chemical deposition [15, 16], spray pyrolysis deposition [17], microwave refluxing [18–21], and single-source precursor approach [22–28]. Among them, we select the single-source precursor approach to prepared Bi₂S₃ in our study because this method is effective in synthesizing a large number of products with uniform size.

Besides, in most of the previous reports, Bi₂S₃ was introduced into TiO₂ by a chemical way. There are few articles that simultaneously related to the introduction of Bi₂S₃ into TiO₂ by a physical way and a chemical way and discussed their different effects on photocatalytic efficiency. To the best of our knowledge, only Bessekhouad et al. [8] reported the relational research. Nevertheless, their study was a little rough for the proportion of Bi₂S₃ in Bi₂S₃/TiO₂ heterojunction increased from 10 wt.% to 50 wt.%, with an increase of 20 wt.% each time. Therefore, it is still interesting to study in detail the influence of different preparation methods for Bi₂S₃/TiO₂ heterojunctions on their photocatalytic activity.

In this study, Bi[(S₂P(OC₄H₉)₂]₃ was selected as the single precursor to prepare Bi₂S₃ nanorods by a hydrothermal approach. As-prepared samples would significantly improve the photocatalytic activity of nano-TiO₂ for degradation of methyl orange (MO) under visible irradiation, whether Bi₂S₃is introduced by a physical way or a chemical way. Meanwhile, the influences of different proportions of Bi₂S₃ on the catalytic efficiency of TiO₂ were discussed in detail.

2. Experimental

All reactants and solvents are in analytical grade and are used without further purification.

2.1. Preparation of Bi[S₂P(OC₄H₉)₂]₃

At the beginning, 0.4 mol sec-butyl alcohol (C₄H₁₀O) and 0.1 mol phosphorus pentasulfide (P₂S₅) were mixed together and stirred at the room temperature for 3 h. 0.5 mol of NaOH was added into the system by every 20 min during this period. Then, the obtained product and bismuth nitrate (Bi(NO₃)₃·5H₂O) were dissolved in deionized water and DMF, respectively. Bismuth (III) dialkyldithiophosphate complex (Bi[S₂P(OC₄H₉)₂]₃) was finally precipitated by mixing the two solutions with stirring.

2.2. Preparation of Bi₂S₃ Nanorods

1.5 g of clean and dry Bi[S₂P(OC₄H₉)₂]₃ was dissolved in 16 mL of deionized water and was transferred into a teflon-liner autoclave of 20 mL capacity, maintained constantly at 180°C for 12 h. After the reaction, the mixture was cooled naturally to room temperature, and the precipitate was filtered, washed with water and absolute ethanol for several times, and dried in air for characterization.

2.3. Preparation of Bi₂S₃/TiO₂ Heterojunctions

Bi₂S₃/TiO₂ heterojunction was prepared by two different methods. One is designed as a physical way, in which the two constituents (as-prepared Bi₂S₃ and commercial TiO₂-P25) were directly mixed together with different concentrations (1–20 wt. % for Bi₂S₃) and ground for about 5 min, and the other is designed as a chemical way, carried out by adding commercial TiO₂-P25 into the reaction system during the preparation of Bi₂S₃.

2.4. Photocatalytic Experiments

The promotion effect of as-prepared samples on the photocatalytic efficiency of TiO₂ was evaluated by measuring the degradation of MO under visible irradiation (500 W Xe lamp), using as-prepared Bi₂S₃/TiO₂ heterojunction as the catalyst. A cut filter (ZJB 420) was inserted between the Xenon lamp and reactor to eliminate ultraviolet light, when the experiments were performed under visible light irradiation. In a typical experiment, 0.2 g of Bi₂S₃/TiO₂ heterojunction was dispersed in 400 mL of methyl orange (MO, 20 mg·L⁻¹) solution, with a 300-W high pressure Hg lamp providing irradiation with a wavelength centered at 365 nm and an air flow at the rate of 10 mL·min⁻¹. The actual effect of photocatalytic activity by chemical reaction was studied by maintaining the solution in dark for 1 h before irradiation to reach the balance between adsorption and desorption. At given irradiation time intervals, the samples (5 mL) were taken out every 5 min and analyzed by UV-Vis spectrophotometer. The measure of the maximum absorbance was taken at 465 nm. The percentage of degradation is calculated via the formula (), where is the absorbance of original MO solution after being maintained in dark for 1 h before irradiation, and is the absorbance of MO solution measured every 5 min during the process of photodegradation.

2.5. Characterization

The X-ray diffraction patterns (XRD) were recorded on a Bruker D8 Advanced X-ray diffractometer using Cu Kα radiation ( nm) with the range of the diffraction angle of 2θ = 15~75°. Energy dispersion X-ray spectra (EDS) were performed with a GENESIS 2000 X-ray energy spectrometer (EDAX). Transmission electron microscopic (TEM) and scanning electron microscopic (SEM) images were carried out on a JEM-2100 microscope (JEOL) and JSM-6380LV scanning electron microscope, respectively. Ruili 1100 spectrophotometer was used to record the UV-Visible absorption spectra of the as-prepared samples.

3. Results and Discussion

3.1. XRD and EDS Analyses of Bi₂S₃

Figure 1(a) shows the XRD patterns of the samples obtained at different temperatures. The main diffraction peaks are labeled, and all the reflections can be indexed to an orthorhombic phase of Bi₂S₃ (JCPDS Files, No.17-320). No impurities such as Bi₂O₃, Bi and S are detected. Obviously, the shapes of the diffraction peaks indicate that the product should be well crystallized at 180°C. Therefore, the purity and composition of the sample obtained at 180°C are reflected by EDS analysis. The detected peaks in the EDS spectrum, shown in Figure 1(b), are assigned to Bi, S, C, and O, implying that there are no obvious impurities except trace amount of CO₂ and O₂ absorbed on the surface of the sample. Quantification of the EDS peaks gives the atomic ratio of Bi : S as 56.48 : 34.76 which is nearly consistent with the given formula of Bi₂S₃.

(a)

(b)

Figure 1: (a) XRD patterns of as-prepared samples at different temperatures; (b) EDS spectrum of the sample obtained at 180°C.

3.2. SEM and TEM Images of Bi₂S₃

The morphology of as-prepared sample obtained at 180°C is revealed by SEM and TEM images, and the results are shown in Figure 2. The product mainly consists of many short rods with an average length of ca. 400 nm and a diameter of ca. 50 nm. The typical HRTEM image (Figure 2(c)) shows that the crystal lattice fringes, with an average neighboring distance of 0.42 nm, correspond to the crystal plane of orthorhombic-structured Bi₂S₃, indicating as-prepared Bi₂S₃ nanorods grow along the direction.

(a)

(b)

(c)

Figure 2: (a) and (b) SEM and TEM images of as-prepared sample obtained at 180°C; (c) the HRTEM image taken from an individual Bi₂S₃ nanorod and its FFTs (inset).

3.3. TEM Images of Pure TiO₂ and Bi₂S₃ (3 wt.%)/TiO₂ Heterojunctions

Figure 3 shows the TEM images of the pure commercial TiO₂ (Figure 3(a)), Bi₂S₃ (3 wt.%)/TiO₂ heterojunctions prepared, respectively, by a physical way (Figure 3(b)) and a chemical way (Figure 3(c)). It is obviously observed that the pure TiO₂ was assembled particles with smooth borderlines, and the heterojunction contained TiO₂ particles and as-prepared Bi₂S₃ nanorods, whether Bi₂S₃ introduced by a physical way or a chemical way. The only difference is that the chemically introduced Bi₂S₃ nanorods seemed to be growing on the surface of the TiO₂ particles.

(a)

(b)

(c)

Figure 3: TEM images of pure TiO₂ (a) and Bi₂S₃/TiO₂ heterojunctions prepared by different methods: (b) a physical way; (c) a chemical way.

3.4. Photocatalytic Activity

The photocatalytic activity of the Bi₂S₃/TiO₂ heterojunction under visible irradiation is showed in Figure 4(a) (prepared by a physical way) and Figure 4(b) (prepared by a chemical way). It can be observed from Figure 4(a) that when Bi₂S₃ was introduced by a physical way, all samples exhibited a higher efficiency than that of the pure TiO₂, and the catalytic efficiency of the heterojunction increased with increasing the proportion of Bi₂S₃ from 1 wt.% to 20 wt.%. This indicates that as-prepared Bi₂S₃ nanorods introduced by a physical way have an indubitable promotional effect on the photocatalytic activity of TiO₂. When the Bi₂S₃ was introduced by a chemical way, the Bi₂S₃/TiO₂ heterojunction also showed a better efficiency than that of the pure TiO₂ as the proportion of Bi₂S₃ was in the range of 1 wt.% to 10 wt.%. However, further increasing Bi₂S₃ till 15 wt.% and 20 wt.% caused the catalytic efficiency to be decreased instead. Above all, the chemical introduced Bi₂S₃ with a proportion of 3 wt.% most significantly improved the catalytic efficiency of TiO₂ under visible irradiation.

(a)

(b)

Figure 4: Degradation efficiency of MO for Bi₂S₃/TiO₂ heterojunctions under visible irradiation: (a) prepared by a physical way; (b) prepared by a chemical way.

3.5. Mechanism

Bi₂S₃ was able to efficiently improve the photocatalytic performance of TiO₂ mainly because it has a narrower bandgap than that of TiO₂ (1.3 eV and 3.2 eV, resp.). When the two semiconductors get in contact, the photo-generated electrons in conduction band (CB) of TiO₂ will transfer to valence band (VB) of Bi₂S₃ first, but not to recombine with the photo-generated holes in VB of TiO₂, as the energy level of VB of Bi₂S₃ is located between those of the VB and CB of TiO₂, as shown in Figure 5. In addition, a small number of electrons would further transfer to the higher CB of Bi₂S₃ after being excited by UV irradiation. Consequently, more and more positive holes would be left and take part in the reactions of oxidizing OH⁻ and H₂O into hydroxyl radical () which is finally responsible for the degradation of pollutants.

Figure 5: Energetic diagrams of Bi₂S₃/TiO₂ heterojunction.

However, excess Bi₂S₃ will lead to a decrease in the catalytic efficiency of TiO₂, when Bi₂S₃ was introduced by a chemical way, it never happened when Bi₂S₃ was introduced by a physical way. The most likely explanation can be elaborated from the structure of the material as follows. Originally, adding more Bi₂S₃ would bring an increase of the probability of interparticle collisions which is beneficial for the improvement of the degradation efficiency. After that, the two constituents—Bi₂S₃ and TiO₂—were separated (Figure 3(b)) in the heterojunction prepared by a physical way. That is why the catalytic efficiency of the physically prepared Bi₂S₃/TiO₂ heterojunction increased with increasing the proportion of Bi₂S₃. But if the photocatalyst was prepared chemically, some active points on the surface of TiO₂ might be covered by excess Bi₂S₃ and lose their catalytic activity, since Bi₂S₃ nanorods were growing on the surface of TiO₂ particles (Figure 3(c)), and Bi₂S₃ itself has few effects on the degradation of MO (Figure 4(a)). That is why the chemically prepared heterojunction with too much Bi₂S₃ will show a decreasing efficiency. In fact, it can be observed from Figures 4(a) and 4(b) that in the case of a small amount of Bi₂S₃ added, the chemically prepared photo-catalyst exhibits a higher efficiency than that prepared by a physical way, and vice versa.

So far, some researchers have studied the photocatalytic activity of Bi₂S₃/TiO₂ heterojunction, but there are few reports related to its photocatalytic activity using MO solution as the target pollutant. Compared with the reported value [8], although the final efficiency is very close, less Bi₂S₃ (3 wt.%) is needed than that (10 wt.%) in their work. It can be concluded that the as-prepared Bi₂S₃ in our study has an enhanced effect on the promotion of the photocatalytic activity of TiO₂ mainly due to its different preparation method.

4. Conclusions

Large-scale uniform Bi₂S₃ nanorods were synthesized by a hydrothermal treatment, using Bi[S₂P(C₄H₉O)₂]₃ as a single-source precursor. Results of photocatalytic experiments showed that a small amount of as-prepared Bi₂S₃ would significantly improve the photocatalytic activity of nano-TiO₂ under visible irradiation, whether Bi₂S₃is introduced by a physical way or a chemical way. After that, excess Bi₂S₃ will make the catalytic efficiency of TiO₂ decreased when Bi₂S₃ was introduced by a chemical way, but it never happened when Bi₂S₃ was introduced by a physical way, mainly due to the different structures of Bi₂S₃/TiO₂ heterojunction resulted from different preparation methods. Even so, among all as-prepared samples, the TiO₂-based photo-catalyst prepared chemically with 3 wt.% Bi₂S₃ exhibits the best catalyst efficiency under visible irradiation.

Acknowledgments

The authors are grateful for the financial support provided by the Universities Natural Science Research Project of Jiangsu Province (11KJD430004) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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