Facile Fabrication of Dumbbell-Like β-Bi₂O₃/Graphene Nanocomposites and Their Highly Efficient Photocatalytic Activity

Abstract

β-Bi₂O₃ decorated graphene nanosheets (β-Bi₂O₃/GN) were prepared by a facile solution mixing method. The crystal structure, surface morphology, and photo absorbance properties of the products were characterized by XRD, SEM, and UV-VIS diffuse reflection, respectively. Moreover, the effect of graphene content on photocatalytic activity was systematically investigated, and the results indicated that these composites possessed a high degradation rate of Rhodamine B (RhB), which was three times higher than that of bare β-Bi₂O₃ when graphene content was 1 wt %. This high photocatalytic activity was attributed predominantly to the presence of graphene, which served as an electron collector and transporter to efficiently lengthen the lifetime of the photogenerated charge carriers from β-Bi₂O₃.

1. Introduction

The semiconductor can be used as a photocatalyst due to its unique electronic component structure (a valence band full of electrons and an empty conduction band). Bi₂O₃ is a common important semiconductor material, which is widely used in various fields, such as electronic ceramics, sensors, and high-temperature superconductivity, to name a few [1,2]. As a photocatalyst, it has gained more and more attention [3]. Bi₂O₃ mainly has four crystal structures, including α, β, γ, and δ. Due to a lower band gap and a unique electronic structure, the β-Bi₂O₃ has higher photocatalytic activity than the other configurations of Bi₂O₃ [4,5]. However, the photocatalytic activity of pure Bi₂O₃ is still restricted. In order to further enhance its activity, researchers have made efforts, such as using Pt, Au, Ag, and other noble metals, to improve the conductivity of the electrons and reduce the recombination probability of charge carrier [6,7]. Chai et al. [8] used Bi₂O₃ as a precursor to prepare the BiOCl/Bi₂O₃ complex and finally form a heterojunction so as to significantly improve its activity in the photocatalytic degradation of pollutants.

In recent years, graphene-based semiconductor photocatalysts have gained great attention due to their highly efficient electronic conduction force, larger specific surface area, and good adsorption performance [9]. To date, various graphene–semiconductor composites with enhanced photocatalytic performance have been designed, such as graphene–TiO₂ nanocomposite [10], graphene/zirconium oxide [11] and RGO–Bi₂O₃ nanocomposite [12]. As far as we know, only a few research studies with graphene-based Bi₂O₃ used as a photocatalyst have been reported. For example, Som et al. [13] and Maruthamani et al. [14] introduced a co-precipitation route to prepare GO/α-Bi₂O₃ or rGO/Bi₂O₃ rods. Cao et al. [15] explored an organic electrolyte-assisted method to prepare the GR/β-Bi₂O₃ composites. Therefore, in this paper, we consider it important to undertake the green chemical synthesis of the composites; a simple method of solution mixing and thermal reduction was used to prepare β-Bi₂O₃ decorated graphene nanosheets (β-Bi₂O₃/GN) in one step. Rhodamine B (RhB) is used as the model organic dye to investigate the activity of as prepared samples, as it is an important factor in environmental pollution and its degradation mechanism has been studied quite well [16,17]. In addition, the effect of the content of graphene was also studied systematically, and finally, a possible photocatalytic mechanism of β-Bi₂O₃/GN composite was proposed.

2. Experimental

2.1. Synthesis of Bi₂O₃/GN

Graphite oxide was prepared by a modified Hummers method [18]; then, the aqueous solution of graphene oxide (GO) could be obtained by ultrasonic stripping from graphite oxide for 1 h. β-Bi₂O₃-decorated graphene nanosheets (β-Bi₂O₃/GN) were prepared by a facile solution mixing method and thermal reduction. In a typical process, 8 mmol of Bi(NO₃)₃·5H₂O was dissolved in 20 cm³ of nitric acid (1 mol/dm³); then, a different volume of GO (2 mg/dm³) was added dropwise into the solution, which was then continuously stirred for 30 min. Then, the above solution was added dropwise into 80 cm³ (0.6 mol/dm³) of saturated sodium carbonate solution. The reaction mixture was stirred for 5 h before filtration. After being washed by water and ethanol several times respectively, the whole product was dried at 60 °C for 10 h. The dried product was then transferred into the muffle furnace after being ground. Finally, the dried product was roasted at 360 °C for 10 min under the nitrogen atmosphere protection, then, it was natural cooled to room temperature to obtain the resultant product. Here, it was worth noting that the heating rate was 4 °C/min. During the reaction, GO was reduced to GN. Pure β-Bi₂O₃ was synthesized by the same experimental process, except that GO was not added.

2.2. Materials’ Characterization

The samples’ crystal structure was characterized by XRD (Bruker Advance D8, Cu Kα irradiation, Bruker, Germany). Scanning electron microscopy (SEM, JSM-7800F, Japan electronics, Japan) was used to observe the morphology of the prepared samples. The Fourier transform infrared spectroscopy (FTIR) spectra of samples were recorded on a 5DX FTIR (5DX, Nicolet. Co., Rhinelander, WI, USA) spectrometer using KBr powder-pressed pellets. The Brunauer–Emmett–Teller (BET) special surface area was determined through N₂ adsorption at 77 K using an adsorption instrument (ASAP-2020, Micromeritics, Norcross, GA, USA). The UV-VIS diffuse reflectance spectra (UV-vis DRS) of samples were measured using a UV-VIS spectrophotometer (TU1901, Beijing Purkinje, Beijing, China).

2.3. Test of Photocatalytic Activity

The photodegradation test was carried out by using a 300-W xenon lamp (the corresponding emission spectrum see Figure S1) (CEL-HXF300, AULTT, Beijing, Country), the self-made circulating water system maintained the temperature of the reaction system at 25 ± 5 °C. RhB solution (50 cm³, 10 mg/dm³) containing 50 mg of catalyst was put in a glass beaker and stirred in the dark overnight to ensure adsorption–desorption equilibrium. After light illumination at regular time intervals, the absorbance of the RhB solution was monitored by a UV-VIS spectrophotometer.

3. Results and Discussion

3.1. Crystal Structure Characterization

Figure 1 shows the XRD patterns of pure β-Bi₂O₃ and composites with different masses of GN. The peak positions of 27.9°, 31.7°, 32.6°, and 33.8° correspond to the crystal plane diffractions of (201), (002), (220), and (102) of the tetragonal β-Bi₂O₃ (JCPDS 27-0050), respectively. The diffraction peaks of the β-Bi₂O₃/GN sample and β-Bi₂O₃ are essentially the same, except for a few diffraction peaks of Bi₂O₂CO₃ (the inverted triangle as shown in Figure 1. This may be due to the coating effect of GN, which caused Bi₂O₂CO₃ to not be completely converted into β-Bi₂O₃ in the thermal decomposition process. However, Bi₂O₂CO₃ has a similar electronic structure to Bi₂O₃, which is also a well-known photocatalyst. When Bi₂O₂CO₃ was used as the photocatalytic material, it can play a synergistic effect. For example, Chai et al. reported that one β-Bi₂O₃/Bi₂O₂CO₃ nanosheet composite exhibits much higher photodegradation activity than single phase [5]. It was worth noting that there were no significant carbon-related diffraction peaks in XRD, which is because of the low GN content and low diffraction intensity [19].

3.2. FTIR Spectra

In order to better analyze the state of GN on the surface of β-Bi₂O₃, the FTIR of β-Bi₂O₃/GN (1%) and β-Bi₂O₃/GO are displayed in Figure 2. It can be seen that the composites both have a peak at 500–700 cm⁻¹, which belongs to the telescopic vibration of the Bi–O bond of BiO₆ octahedron, and the telescopic vibration peak at 840 cm⁻¹ belongs to Bi–O–Bi [20]. In addition, the O–H stretching vibration of adsorbed water corresponds to 3450 cm⁻¹, and the O–H stretching vibration peak in C–O–H corresponds to 1408 cm⁻¹ [21,22]. The peaks at 1440–1630 cm⁻¹ can be attributed to the antisymmetric stretching vibration and symmetry stretching vibration of the C=O bond in –COOH, respectively. It is worth noting that the C=O antisymmetric stretching vibration at 1450 cm⁻¹ has disappeared after the thermal reduction, and what’s more, the peak intensity becomes weaker after reduction, indicating the effectiveness of the thermal reduction. The results of FTIR show the existence of GO and Bi₂O₃ in the composites, and GO is effectively reduced to GN after thermal reduction.

3.3. Surface Morphology Characterization

Surface morphology of the samples was characterized by SEM (Figure 3). The pure β-Bi₂O₃ is as shown in Figure 3a. β-Bi₂O₃ has a dumbbell-like morphology (the insert is dumbbell). After the 1% GN was introduced, it can be clearly seen that the β-Bi₂O₃ was coated by GN (Figure 3b) or embedded in the GN sheets (Figure 3c). In Figure 3b, it can also be seen there is very little Bi₂O₂CO₃, which was consistent with the XRD results. The elemental mapping (Figure 3d) of the ternary as-prepared β-Bi₂O₃/GN obtained by EDS (the mapped region highlighted with red frame in Figure 3b) indicates that the weight ratios of the elements are close to the Bi₂O₃ molar mass ratios. Meanwhile, only the peaks of C, O, Bi, Au were detected, which means that the as-prepared composites are composed of graphene and Bi₂O_3.

3.4. Surface Areas and Pore Size Distributions

In general, the specific surface area of catalysts and its surface structure have significant influences on catalytic activity. Therefore, the specific surface analyzer is utilized to implement further research on the specific surface area and the pore diameter distribution of pure β-Bi₂O₃ and β-Bi₂O₃/GN (1%). Figure 4 shows the N₂ adsorption–desorption isotherms and the corresponding curves of the pore size distribution (inset) for samples β-Bi₂O₃ and β-Bi₂O₃/GN (1%). According to the Brunauer–Deming–Deming–Teller (BDDT) classification, pure β-Bi₂O₃ isotherms can be categorized as type III (Figure 4a), which is convex to the P/P₀ axis over its entire range, indicating that the pure β-Bi₂O₃ belong to a nonporous structure. Meanwhile, samples of β-Bi₂O₃/GN (1%) have isotherms of type IV, suggesting the presence of mesopores. Upon observing the pore diameter distribution diagram obtained from desorption isotherms, pore diameter mainly distributes in 5.28 nm (Figure 4b inset). The specific surface area of β-Bi₂O₃ and β-Bi₂O₃/GN (1%) are given by BET measurement as 3.37 m²/g, 4.53 m²/g, respectively, which conforms to the results. In other words, when graphene is introduced, mesopores begin to appear in samples and the specific surface area increases, which is because the prepared nanocomposite is composed of sheet-like graphene decorated with β-Bi₂O₃, similar to the previous report [23]. Consequently, the introduction of graphene can increase the specific surface area, and graphene may play a role in enhancing the photocatalytic activity.

3.5. UV-Vis Diffuse Reflectance and Photocatalytic Activity

The photocatalytic activity tests are shown in Figure 5. From Figure 5a, the absorption of visible light by the samples can be seen as the content of GN increases, and the sample color gradually changing from yellow to deeper brown can also be seen in Figure 5c. This is another proof to the increasing absorption of visible light. The band gap of calculated β-Bi₂O₃ and composites are shown in Figure 5b. Although the 5% GN is not narrowest band gap, the absorbed threshold value of sample’s visible light region is significantly greater than the other samples. Therefore, it is hypothesized that the introduction of graphene can better absorb the visible light and enhance the photocatalytic activity.

The tests of photocatalytic degradation of RhB (Figure 5d) show that all of the composites have higher degradation rates than bare β-Bi₂O₃, suggesting that there is a synergistic effect between the GN sheet and β-Bi₂O₃ nanoparticles. The sequential dye degradation rates are as follows: 1% GN > 0.5% GN > 2% GN > 3% GN > 5% GN > bare β-Bi₂O₃. It seen that only the low content (1%) of GN is introduced, the dye degradation rate can be significantly increased by three times in comparison with the bare β-Bi₂O₃. The kinetics of photodegradation reaction are investigated in Figure 5e, and the results show that 1% GN composite has the highest constant photodegradation reaction rate, which is 0.130 min⁻¹. With the increase of GN content, the dye degradation rate did not always increase; the activity of 5% GN was significantly less than that of 1% GN. The reason why the degradation ratio decreased as the content of graphene increased was that the introduction of a large mass of black graphene would result in a rapid decrease of the light absorption of the reaction solution [23,24]. Thus, in order to achieve an optimal photocatalytic performance, it is crucial to control the composition ratio in the nanocomposite of β-Bi₂O₃/GN.

3.6. Possible Mechanisms Speculation

According to the above experimental results, the possible photocatalytic mechanisms of β-Bi₂O₃/GN were estimated, as shown in Figure 6. Under simulated sunlight irradiation, the excited electrons of the β-Bi₂O₃ semiconductor divert a part of the electrons out due to the good conductivity of the graphene layers in the course of being conducted to the conduction band. In other words, in the system of β-Bi₂O₃/GN, the graphene, as the receiving body and conductor of the electrons, effectively separated the photogenerated electron-hole pairs to avoid the recombination of electrons and holes. It’s similarly compared with a previously studied system in which the GO embedded into TiO₂ nanofiber and served as a conduit of electron transfer [25]. Posa et al. [10] and Li et al. [23] prepared graphene–TiO₂ nanocomposite and CdS-cluster-decorated graphene nanosheets respectively; the similar mechanisms were proposed, too. Furthermore, the unique features of graphene also contribute to the improvement of photocatalytic activity, which allow photocatalytic reactions to take place not only on the surface of semiconductor catalysts, but also on the graphene sheet, greatly enlarging the reaction space.

4. Conclusions

With the simple method of solution mixing, β-Bi₂O₃/GN composite photocatalysts with different qualities were successfully prepared. In the test of degradation rate, it was found that the sample with 1% GN on its surface had the highest photocatalytic activity, with its dye degradation efficiency being three times higher than the pure β-Bi₂O₃. However, when the content of GN increased, the degradation rate decreased. This was because the introduction of more graphene may decrease the light absorption of the reaction solution. Overall, the introduction of appropriate quantity of graphene could significantly increase the photocatalytic degradation rate of the catalyst.