Fabrication of Bi₂Sn₂O₇-ZnO heterostructures with enhanced photocatalytic activity

Abstract

ZnO microspheres synthesized by a hydrolysis method were sensitized with Bi₂Sn₂O₇ (BSO) nanoparticles prepared using a hydrothermal method at different concentrations. Various characterization methods were employed to study the microstructural, morphological, optical and photocatalytic properties of the BSO-ZnO heterostructures. The effect of the BSO concentration on the photocatalytic activity of the as-prepared samples was also investigated. The 12.5BSO-ZnO sample exhibits the highest photocatalytic efficiency. The enhanced photocatalytic efficiency is ascribed to an effective separation of the photogenerated electrons and holes due to the presence of the BSO-ZnO heterojunction.

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1 Introduction

With the rapid development of industry and the economy of the human society over the past few decades, organic pollution has become one of the most serious environmental problems.^1–3 In order to solve this problem, semiconductor photocatalysis is regarded as one of the most ideal methods to eliminate toxic chemicals in the environment. Also, semiconductor materials have aroused a great interest among scientists all over the world. To date, various semiconductor materials have been synthesized and studied as photocatalysts, such as metal oxides,⁴ sulfides,⁵ phosphides,⁶ and their mixtures or solid solutions.^7–10 However, their application under the irradiation of sunlight has been seriously impeded due to their large bandgap along with the rapid recombination of the photogenerated charge carriers.^11,12 Heterostructures and p–n junctions formed by coupling wide band semiconductors with other semiconductors with different absorption bands, such as BiOI, CdS and Bi₂O₃, have proved to be promising candidates to overcome these drawbacks.^13–15 In this case, the energy levels of the composite semiconductors must match each other to form a staggered heterojunction that meets the requirements for practical applications.

In the field of photocatalysis, ZnO with a direct bandgap of 3.2 eV is renowned as one of the most admirable semiconductor photocatalysts due to its high photosensitivity, non-toxic nature and low cost, compared to titanium dioxide (TiO₂).^16–18 However, owing to its large bandgap along with a fast recombination rate of the photogenerated electron–hole pairs, its photocatalytic activity is hindered. Therefore, various semiconductors with narrow bandgaps have been coupled with ZnO nanostructures to extend its light absorption from the UV region to the visible region. Hence, many systems, including ZnO/Fe₂O₃,¹⁸ ZnO/ZnSe,¹⁹ CuInSe₂/ZnO,²⁰ and CuInS₂/ZnO,²¹ have been reported and proved to be efficient photocatalysts.^22,23 Recently, bismuth-containing photocatalysts, such as Bi₂WO₆, BiVO₆ and Bi₂O₂CO₃,^24–26 have attracted considerable attention. Similarly, Bi₂Sn₂O₇ (BSO) is not only a novel semiconductor with a pyrochlore structure and a bandgap of 2.86 eV, but has also been used as a catalyst and a gas sensor.^27–29 Theoretically, the formation of a coupled semiconductor structure can efficiently improve the photocatalytic efficiency.

Herein, we report the synthesis and characterization of BSO-ZnO heterostructures with high photocatalytic activity. Nanocrystalline BSO was synthesized by a facile hydrothermal method, whereas ZnO microspheres were prepared through the hydrolysis of zinc salts in polyol. The two semiconductors were then mixed together and annealed at 500 °C for 1 h so as to form BSO-ZnO composites with a heterogeneous structure. The visible-light-mediated photocatalytic behavior of the BSO-ZnO composites for the photodegradation of RhB has been studied, the relationship between the composition and the photocatalytic activity of the BSO-ZnO composites has been systematically investigated, and the formation process and the heterogeneous structure of the BSO-ZnO composites are discussed.

2 Experimental section

All the reactants used in this study were of analytical-grade and used without any further purification treatment.

2.1 Synthesis of ZnO microspheres

ZnO microspheres were synthesized through the hydrolysis of a zinc salt as described in our previous work.³⁰ Typically, 0.01 mol Zn(CH₃COO)₂·2H₂O was added to 400 mL diethylene glycol with constant stirring at 130 °C until complete dissolution was achieved, followed by heating at 180 °C for 15 min. The obtained white solution was then centrifuged and rinsed with absolute ethanol for several times to obtain highly pure ZnO microspheres.

2.2 Synthesis of Bi₂Sn₂O₇ and preparation of BSO-ZnO heterostructures

Stoichiometric amounts of K₂SnO₃·3H₂O and Bi(NO₃)₃·5H₂O were dissolved in 10 mL dilute HNO₃ (1 : 10 v/v).³¹ Mixed hydroxides were then precipitated from this solution by adding NH₃·H₂O till the pH value was 12. Subsequently, the mixed precipitates were transferred into a Teflon-lined stainless-steel autoclave, which was sealed and heated at 180 °C for 24 h. Finally, the product was filtered, washed thoroughly with water and ethanol, and dried at 100 °C for 1 h.

BSO modified ZnO microspheres were prepared as follows. The as-prepared BSO powders were first dispersed in ethanol and then mixed with the ZnO microspheres at the required molar ratio. After being thoroughly mixed through strong stirring, solid powders were thus obtained by washing and drying in air at 75 °C, which were then directly transferred into an oven and calcined at 500 °C for 1 h. The BSO-ZnO composite photocatalysts were thus obtained. Similarly, pure BSO without ZnO microspheres was also prepared for comparison purposes.

2.3 Characterization

X-ray diffraction (XRD, D/max-2200, Rigaku, Japan) with Cu Kα radiation was used to characterize the phase composition of the ZnO microspheres, the BSO-precursors and the BSO-ZnO composites. Scanning electron microscopy (SEM, Quanta F250, FEI, USA) coupled with an energy-dispersive X-ray (EDX) spectrometer and transmission electron microscopy (TEM JEM-2100, JEOL Inc., Japan) were employed to observe morphological properties of the as-prepared materials. UV-Vis absorption spectra of all the samples were measured by using a JASCO Model V-570 UV/VIS/NIR spectrometer equipped with a diffuse reflectance accessory. The specific surface area and average pore diameter of the nanoparticles were measured using N₂ adsorption–desorption isotherms with an Automated Surface area and Porosity Analyser. Photoluminescence spectra were recorded with a Jobin-Yvon Fluorolog-3 spectrofluorimeter at room temperature, with an excitation wavelength of 251 nm.

2.4 Photocatalytic activity measurement

Rhodamine B (RhB) was selected as the model pollutant to evaluate the photocatalytic performance of the as-prepared photocatalysts. Here, the photocatalytic activity of the as-prepared samples was evaluated by observing the decomposition of a RhB aqueous solution. Typically, 35 mg photocatalyst was added to a 50 mL of RhB aqueous solution with an initial concentration of 10⁻⁵ M in a glass reactor with a 115 cm² cross section and 5 cm height. The reactor was then kept in the dark with continuous stirring for 0.5 h to reach the adsorption–desorption equilibrium, prior to light irradiation from a 500 W Xe arc lamp with a 420 nm cutoff filter. 5 mL of the suspension solution was taken out at certain time intervals and centrifuged to remove the catalyst. Thus, the efficiency of the degradation processes was determined by monitoring the decrease of the absorbance intensity at λ = 554 nm.

3 Results and discussion

3.1 Structural and morphological properties

Fig. 1 shows the XRD patterns of pure ZnO, pure BSO and BSO-ZnO composites. All diffraction peaks shown in spectra (a) and (b) can be indexed to the hexagonal phase of ZnO (JCPDS card 36-1451) and the cubic phase of BSO (JCPDS card 87-0284).^27,32 The sharp and intense diffraction peaks of BSO and ZnO confirm their highly crystalline nature. In addition, no impurity peaks are detected, indicating the high purity of the two components. However, for the BSO-ZnO samples, two sets of XRD peaks corresponding to ZnO and BSO can be clearly observed in Fig. 1(c)–(h), where the peaks indicated by asterisks (*) are indexed as the cubic structure of BSO. It can also be seen that most of the peaks of the BSO-ZnO samples match those of ZnO, except the three peaks assigned to the (311), (222) and (400) planes of cubic BSO. The diffraction peak intensity of BSO increases gradually as the amount of BSO increases from 3% to 15%. The relatively higher intensity of the (222) peak in the composite samples than that in pure BSO indicates its anisotropic growth behavior and implies the preferred orientation of the crystallites.³³ However, the peak intensity of ZnO decreases significantly as the BSO content increases, suggesting that the presence of BSO inhibits the crystal growth of ZnO. In addition, the broadening of the peaks of the composite samples indicates the formation of nanostructures in the material.³⁴ Especially, the significant broadening of the (002) peak of ZnO may result from the merging of the (400) plane of cubic BSO and the (002) peak of ZnO. Similarly, the same mergence is observed for the (102) peak of ZnO and the (440) peak of BSO, and the (110) peak of ZnO and the (622) peak of BSO.

Fig. 1 XRD patterns of pure ZnO, pure BSO, and the BSO-ZnO composites: (a) ZnO, (b) BSO, (c) 3BSO-ZnO, (d) 5BSO-ZnO, (e) 7.5BSO-ZnO, (f) 10BSO-ZnO, (g) 12.5BSO-ZnO, and (h) 15BSO-ZnO.

Fig. 2 shows SEM images of pure ZnO, pure BSO and the BSO-ZnO composites. It can be observed from Fig. 2(a) that the as-prepared ZnO consists of dispersed spherical particles with a smooth surface and diameter of about 200 nm. In contrast, BSO, as seen in Fig. 2(b), shows also relatively dispersive but irregular particles. However, the morphology of the BSO-ZnO composites as shown in Fig. 2(c)–(h) highly depends on the content of BSO. It can be also clearly observed that some spherical nanoparticles, which have a diameter of about 20 nm, are deposited on the ZnO microspheres, and the amount of these adhesive nanoparticles increases with the increasing BSO content. Fig. 3 shows the EDX spectrum of the ZnO microspheres with adhered nanoparticles in the 12.5BSO-ZnO sample, which only shows O, Zn, Bi and Sn elements, suggesting that Bi and Sn elements are present in the adhered nanoparticles. Considering the contents of the elements as presented in Table S1, ESI,† it can be inferred that the adhered nanoparticles must be BSO.

Fig. 2 SEM images of (a) ZnO, (b) BSO, (c) 3BSO-ZnO, (d) 5BSO-ZnO, (e) 7.5BSO-ZnO, (f) 10BSO-ZnO, (g) 12.5BSO-ZnO and (h) 15BSO-ZnO samples.

Fig. 3 EDX spectrum of the 12.5BSO-ZnO sample.

In order to further confirm the BSO-ZnO heterostructures, TEM and HRTEM images were obtained to investigate the detailed structure of the BSO-ZnO composites. Fig. 4(a) shows a typical TEM image of a microsphere of the 12.5BSO-ZnO nanocomposite sample. Obviously, the result is consistent with the SEM observations from Fig. 2, in which some nanoparticles were clearly anchored to surface of the ZnO microspheres. These nanoparticles are so strongly attached to the microspheres that it is difficult to separate them by ultrosonication. In addition, TEM images of pure ZnO and BSO are also presented in Fig. S1, ESI.† Fig. 4(b) is the HRTEM image of the blue square region in Fig. 4(a), revealing the highly crystalline nature of the BSO-ZnO heterostructure. Here, the well lattice fringes, with interplanar spacings of 0.28 nm and 0.32 nm, correspond to the (100) plane of hexagonal ZnO³⁵ and the (444) plane of BSO,²⁷ respectively. These TEM results are in good agreement with the XRD patterns. Therefore, it can be concluded that an heterojunction between the two materials has been formed.

Fig. 4 (a) TEM and (b) HRTEM images of the 12.5BSO-ZnO sample.

3.2 UV-Vis light absorption

The UV-Vis absorption spectra of the BSO-ZnO nanocomposites with different BSO amounts are shown in Fig. 5(a). For comparison, the UV-Vis absorption spectra of pure ZnO and BSO are also included. They exhibit a clear absorption edge at 380 nm and 480 nm, which belong to the ultraviolet light region and the visible light region, respectively. In contrast, all the BSO-ZnO nanocomposites have a pronounced absorption in the visible light region and the absorbance intensity increases gradually with the increasing BSO content from 3 to 15%.

Fig. 5 (a) UV-Vis diffuse reflectance spectra and (b) estimated bandgap energies of ZnO, BSO and all BSO modified ZnO samples.

The bandgaps of all the samples were estimated according to the following equation:³⁴

F(R)hv = A(hv − E_g)ⁿ (1)

where F(R), v, E_g and A are the absorption coefficient, light frequency, bandgap energy and the proportionality constant, respectively. The value of n describes the type of transition, with n = 1/2 for direct transition and n = 2 for indirect transition. All the BSO-ZnO nanocomposites have an n value of 2, which means that they have an indirect transition.³⁴ The bandgap energy of all these samples, which were estimated from the plot of (F(R)hv)^1/2 versus hv, are shown in Fig. 5(b) and Table 1. It should be mentioned here that the value of pure ZnO is slightly smaller than that reported in the open literature, which is probably related to quantum confinement effects in the ZnO microspheres.³⁶ Due to the involvement of SnO₂ in the structure of Bi₂O₃, the bandgap of pure BSO sample is 2.62 eV, which is lower than the value of 0.22 as reported in ref. 37. However, the bandgap energy of all the BSO-ZnO nanocomposite samples are distributed in the range of 2.8–3.01 eV. Actually, it can be also seen that the shift of their absorption edges to longer wavelengths is in agreement with the calculated bandgap energies.

Table 1 Bandgaps of all the samples

Samples	Bandgap energy/eV
ZnO	3.05
BSO	2.62
3BSO-ZnO	3.00
5BSO-ZnO	2.98
7.5BSO-ZnO	2.92
10BSO-ZnO	2.89
12.5BSO-ZnO	2.85
15BSO-ZnO	2.80

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3.3 Pore size distribution and specific surface area

Fig. 6 shows the N₂ absorption–desorption isotherms and pore size distribution of the 12.5BSO-ZnO sample, which displays a type-IV isotherm with a hysteresis loop according to the classification by the IUPAC, indicating the presence of a mesoporous structure.³⁸ The formation of this mesoporous structure can be ascribed to the aggregation of BSO nanoparticles adhered to the surface of the ZnO microspheres. The specific surface area of the sample was calculated from Fig. 6(b), with a value of 43.3 m² g⁻¹.

Fig. 6 (a) N₂ adsorption–desorption isotherms and (b) pore size distribution of the 12.5BSO-ZnO sample.

3.4 Photocatalytic activity

Fig. 7(a) shows the photocatalytic activity of all the samples, including ZnO, BSO, P25 and the BSO-ZnO nanocomposites. It can be seen that ZnO, BSO and P25 show relatively low photocatalytic activity under simulated sunlight irradiation. The low photocatalytic activity of ZnO is due to the large bandgap (3.05 eV) and the rapid recombination of the photogenerated charge carriers.³⁹ In addition, as reported by Xu et al., BSO can absorb visible light, but its high recombination rate still leads to a low photocatalytic efficiency.²⁷ However, our results as shown in Fig. 7 indicate that the BSO-ZnO nanocomposites have a much higher photocatalytic activity. In particular, the 12.5BSO-ZnO nanocomposite sample shows the best photocatalytic activity of all measured samples, which also shows a better photocatalytic efficiency than CTAB-Bi₂Sn₂O₇, nano-Bi₂Sn₂O₇ and Bi₂O₃/Bi₂Sn₂O₇ nanocomposites.^27,32,37 The photocatalytic activity of the BSO-ZnO nanocomposites increases with the increasing BSO content, indicating the beneficial effect of the heterostructure on promoting photocatalytic efficiency. In addition, the first-order kinetics of RhB degradation of different samples is shown in Fig. S2, ESI.† According to the first-order kinetics, the rate constants of 12.5BSO-ZnO, ZnO and BSO were calculated to be 0.011 min⁻¹, 0.0037 min⁻¹ and 0.0044 min⁻¹, respectively. It is clear that the rate constant of 12.5BSO-ZnO is about 3 and 2.5 times that of pure ZnO and BSO, respectively. Therefore, it can be concluded that more electron–hole pairs are separated which can then react with RhB molecules. However, it should be mentioned that the 15BSO-ZnO nanocomposite sample displays a decrease in activity.

Fig. 7 (a) RhB concentration versus visible-light irradiation time, (b) representative degradation profile of RhB in the presence of the 12.5BSO-ZnO sample.

In order to propose a reasonable photocatalytic mechanism, the main active species responsible for the degradation of RhB were investigated. Sodium hydrogen carbonate (NaHCO₃, ˙OH radical and h⁺ scavenger) and disodium ethylenediaminetetraacetate (EDTA, a hole scavenger) are unstable under certain conditions and so they may not be used commonly for this purpose. However, the conditions in our experiments are moderate and therefore, these two important scavengers have been used to probe the reactive species in the system.^40–42 Fig. 8 shows the as-obtained results, where the photocatalytic activity is slightly depressed when EDTA (5 mM) and NaHCO₃ (5 mM) are added. This means that both ˙OH and h⁺ act as main reactive species. As seen in Fig. 8, although the scavenging effects of EDTA and NaHCO₃ are not strong, which is mainly due to the low concentration of the scavengers, the photoactivity is suppressed more notably by EDTA than NaHCO₃, indicating that h⁺ plays a more important role in promoting RhB degradation. Moreover, Fig. S3 in the ESI† shows the first-order kinetics of RhB degradation with or without EDTA and NaHCO₃. The rate constant of 12.5BSO-ZnO in RhB aqueous solution is 0.011 min⁻¹, which is 1.27 times and 1.67 times that in NaHCO₃ or EDTA containing solutions.

Fig. 8 Photodegradation of RhB in the presence of different radical scavengers.

Furthermore, the pH value of the RhB solution during degradation was also recorded, and the values are shown in Table S2 (ESI†). It can be seen that, after stirring in the dark for 1 h to establish the adsorption–desorption equilibrium, the initial pH value of the RhB solution changed due to the varying content of hydroxyl ions adsorbed on the surface of the sample. However, only a small change of the pH value can be observed in the whole photodegradation process. It is therefore further proved that h⁺ provides the largest contribution for the degradation of RhB.

Photoluminescence (PL) spectra of the samples were obtained to observe the separation rate of the photo-generated electron–hole pairs directly. It is well known that PL signals are derived from the recombination of electrons and holes. As shown in Fig. 9, the intensity of the PL signal for the 12.5BSO-ZnO sample is the weakest, lower than those of the pure ZnO and pure BSO samples, indicating that the recombination of electron–hole pairs is effectively inhibited. Obviously, these results are consistent with the conclusion that the 12.5BSO-ZnO sample has the highest photocatalytic activity for RhB degradation.

Fig. 9 PL spectra of ZnO, BSO and the 12.5BSO-ZnO sample.

Similar to the formation process of ZnO/BiOI,³⁵ the BSO nanoparticles are attached to the surface of the ZnO microspheres as shown in Fig. 10. Therefore, it is possible to suggest that there exists an optimized content of BSO. An excess of BSO would decrease the photocatalytic activity of the sample due to a longer pathway for the migration of the photogenerated charge carriers.⁴⁰

Fig. 10 Schematic illustration of the BSO-ZnO heterojunction system.

Based on above results, a mechanism for the formation of the heterostructure is proposed. Fig. 11(a) shows the energy bands of BSO and ZnO. It is well known that ZnO is an n-type semiconductor, and BSO is a p-type semiconductor whose Fermi energy level is close to the conduction band and valence band of ZnO.^43,44 Thus, the enhanced photocatalytic activity of the BSO-ZnO nanocomposites is mainly ascribed to the presence of the heterojunction, which inhibits the recombination of the photogenerated electron–hole pairs. Fig. 11(b) shows the formation process of the heterojunction. That is, a p–n junction is formed as BSO comes in contact with ZnO, so that the electrons diffuse from ZnO to BSO and the holes diffuse from BSO to ZnO, resulting in a positive section in the ZnO side and a negative section in the BSO side. When the Fermi levels of BSO and ZnO reach equilibrium, the charge diffusion stops. Furthermore, the energy band of ZnO shifts downward along its Fermi level, whereas the energy band of BSO shifts upward as shown in Fig. 11(b). Hence, under irradiation of visible light, the photogenerated electrons transfer from the conduction band of BSO to ZnO, while the photogenerated holes transfer from the valence band of ZnO to BSO.

Fig. 11 Diagram of (a) the bandgaps of ZnO and BSO before contact, and (b) the proposed electron–hole separation process of the BSO-ZnO heterojunction structure.

The photodegradation of RhB in aqueous solution has been extensively studied and two possible mechanisms have been proposed.⁴⁴ It can be seen from Fig. 7(b) that the main degradation process of RhB is the photodegradation under visible light irradiation.³¹ Therefore, the electrons in ZnO can react with oxygen, which is located on the surface of ZnO and acts as an electron acceptor, to form a superoxide radical anion (O₂˙⁻).¹⁹ Subsequently, this superoxide ion can not only react with RhB molecules, but also further form hydroxyl radicals with a strong oxidation ability for degrading organic molecules. Meanwhile, the holes in BSO can potentially react with water molecules adhered to the surface of BSO nanoparticles to generate hydroxyl radicals (OH˙) with high reactivity. The whole electron transfer process in the degradation of the RhB aqueous solution is depicted in Fig. 11(b).

4 Conclusions

BSO-ZnO nanocomposites with a heterostructure have been synthesized using both a solvothermal method and a hydrolysis method. The structural, morphological, and optical properties of the BSO-ZnO nanocomposites can be controlled by tuning the content of BSO. The BSO-ZnO nanocomposites exhibit high photocatalytic activity for the degradation of RhB in aqueous solution. The enhanced photocatalytic activity is mainly attributed to the formation of a heterojunction, which inhibits the recombination of the photogenerated electron–hole pairs. It is believed that this result is of great importance in the development of new heterojunction photocatalysts for pollutant mitigation.

Acknowledgements

This work was supported by the Research Fund for the Doctoral Program of Higher Education of China under grant 20120201130004, partially by the National Natural Science Foundation of China Major Research Plan on Nanomanufacturing under Grant no. 91323303, the National Natural Science Foundation of China under Grant no. 61078058, and the 111 Project of China (B14040). The SEM and TEM work was conducted at International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, China. The authors also thank Ms Feng and Mr Ma for their help in using SEM and TEM.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra16616g

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