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

Authors: Juan Lu; Zuoshan Wang; Ying Zhang; Xiufeng Zhou
Journal: Journal of Nanomaterials
 (2013)

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## 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 (Eg=1.3 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.


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## Body

## 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 (Eg=1.3 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 Bi2S3into 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 molsec-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 (1-Ai/A0), where A0 is the absorbance of original MO solution after being maintained in dark for 1 h before irradiation, and Ai 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 (λ=0.1542 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.


## 2.1. Preparation of Bi[S2P(OC4H9)2]3

At the beginning, 0.4 molsec-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 (1-Ai/A0), where A0 is the absorbance of original MO solution after being maintained in dark for 1 h before irradiation, and Ai 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 (λ=0.1542 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

Figure1(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) XRD patterns of as-prepared samples at different temperatures; (b) EDS spectrum of the sample obtained at 180°C.
(a)
(b)


### 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 Figure2. 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 (220) crystal plane of orthorhombic-structured Bi2S3, indicating as-prepared Bi2S3 nanorods grow along the [001] direction.(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).
(a)
(b)
(c)


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

Figure3 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.TEM images of pure TiO2 (a) and Bi2S3/TiO2 heterojunctions prepared by different methods: (b) a physical way; (c) a chemical way.
(a)
(b)
(c)


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


### 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 (OH∙) 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.


## 3.1. XRD and EDS Analyses of Bi2S3

Figure1(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) XRD patterns of as-prepared samples at different temperatures; (b) EDS spectrum of the sample obtained at 180°C.
(a)
(b)


## 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 Figure2. 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 (220) crystal plane of orthorhombic-structured Bi2S3, indicating as-prepared Bi2S3 nanorods grow along the [001] direction.(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).
(a)
(b)
(c)


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

Figure3 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.TEM images of pure TiO2 (a) and Bi2S3/TiO2 heterojunctions prepared by different methods: (b) a physical way; (c) a chemical way.
(a)
(b)
(c)


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


## 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 (OH∙) 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.



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