In situ synthesis of Bi₂S₃/Bi₂SiO₅ heterojunction photocatalysts with enhanced visible light photocatalytic activity

Abstract

In order to improve the photocatalytic activity of Bi₂SiO₅ under visible light irradiation, Bi₂S₃/Bi₂SiO₅ heterojunctions were synthesized through a facile in situ ion exchange method. Thioacetamide (TAA) is regarded as the appropriate sulfur source. The as-prepared samples of Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅ were systematically characterized by XRD, SEM-EDS, TEM, XPS, PL, UV-vis DRS and BET techniques. The photocatalytic activities of the samples were evaluated by degrading rhodamine B (RhB) under visible light and UV-vis light irradiation, respectively. Bi₂S₃/Bi₂SiO₅ heterojunctions show enhanced photocatalytic activities under visible light irradiation. Further investigation reveals that ion exchange reaction time plays an important role in the photocatalytic efficiency. The mechanism of the enhanced photocatalytic activity was proposed.

---

1. Introduction

Semiconductor photocatalytic technology has received much attention as one of the most promising technologies in solving the worldwide energy shortage and environmental degradation. Photocatalysis occurs under mild reaction conditions and does not produce secondary pollution.^1,2 In general, improving the photocatalytic activity is mainly focused on two aspects: improving the carrier separation efficiency and extending the spectral response range. Heterojunction is an efficient strategy to overcome the drawbacks of fast charge recombination and the limited visible-light absorption of semiconductor photocatalysts.^3–7

Bismuth silicate (Bi₂SiO₅) is one of the newly found compounds within Aurivillius family, which was first reported in 1996. Bi₂SiO₅ is composed of (SiO₃)²⁻ pyroxene file layers and (Bi₂O₂)²⁺ layers, which are alternatively stacked. Bi₂SiO₅ was reported as a material with relatively good dielectric properties and piezoelectric and nonlinear optical effects.^8–10 In addition, Bi₂SiO₅ has been applied in photocatalytic degradation of organic compounds during the past several years. For example, Zhu and co-workers synthesized a phase junction Bi₂SiO₅ flower-like microsphere, which exhibited superior photocatalytic performance in the degradation of phenol.⁹ Chen and his co-workers prepared uniform sponge-like Bi₂SiO₅, which is an efficient photocatalyst for Cr(VI) removal.¹¹ However, the white Bi₂SiO₅ can only be responsive to the ultraviolet light due to its wide energy band gap (3.76 eV),¹² and the ultraviolet light accounts for as less as about 4% of the solar radiation energy. Therefore, low utilization of solar radiation energy severely restricts the application of Bi₂SiO₅ into photocatalysis.

Bismuth sulfide (Bi₂S₃) is a narrow band gap (∼1.3 eV) semiconductor which has been widely used as electrochemical hydrogen storage, thermoelectric, and photoresponsive materials, etc.^13–15 Moreover, Bi₂S₃ can absorb visible light with wavelength up to 800 nm and exhibit good photosensitization properties. Therefore, Bi₂S₃ attracts more and more attention in photocatalysis.¹⁶ Up to now, a large number of Bi₂S₃ based heterojunction photocatalysts have been synthesized for the degradation of organic pollutants and wastewater treatment, such as Bi₂S₃/BiOCl, Bi₂S₃/Bi₂O₂CO₃, Bi₂S₃/Bi₂WO₆, Bi₂S₃/BiOI, Bi₂S₃/ZnS and so on.^17–21 Thus, Bi₂S₃ is expected to be a potential candidate which can combine with Bi₂SiO₅ forming an effective heterojunction with enhanced charge separation and visible light responsive photocatalytic activity. However, the combination of Bi₂SiO₅ and Bi₂S₃ to construct novel Bi₂S₃/Bi₂SiO₅ heterostructures has not been reported yet.

In this study, Bi₂S₃/Bi₂SiO₅ heterostructures were synthesized through a facile in situ ion exchange method. A series of control experiments were carried out to investigate the influence of the reaction time on the content and size of Bi₂S₃ and ultimately the photocatalytic activity of Bi₂S₃/Bi₂SiO₅ heterojunctions. The photocatalytic properties were evaluated via the photocatalytic degradation of RhB dye under visible light (λ > 420 nm) and UV-vis light. The enhanced photocatalytic activity of Bi₂S₃/Bi₂SiO₅ was discussed in detail based on the calculated energy band positions.

2. Experimental section

2.1. Materials

Bismuth nitrate (Bi(NO₃)₃·5H₂O), sodium silicate (Na₂SiO₃·9H₂O), ethylene glycol methyl ether, sodium hydroxide, thiourea, cysteine and thioacetamide were analytic reagents and were used without further purification.

2.2. Synthesis of Bi₂SiO₅ nanosheets

In a typical procedure, 6 mmol (2.91 g) Bi(NO₃)₃·5H₂O was added into 40 mL ethylene glycol methyl ether with constant stirring until bismuth nitrate was dissolved, 3 mmol (0.852 g) Na₂SiO₃·9H₂O was dissolved into 40 mL deionized water under stirring. Then the Na₂SiO₃·9H₂O solution was added to the solution of Bi(NO₃)₃·5H₂O to form a white suspension. Finally, the pH value of the solution was adjusted to 12 using 2 mol L⁻¹ NaOH. After stirring for 30 min, the mixture was transferred into a Teflon-lined autoclave with a capacity of 100 mL and maintained at 200 °C for 10 h. After cooling down to room temperature naturally, the product was obtained by filtering and washed with deionized water and ethanol for three times and then dried at 60 °C for 12 h.

2.3. In situ synthesis of Bi₂S₃/Bi₂SiO₅ heterostructures using different sulfur source

The prepared Bi₂SiO₅ sample (0.5 mmol, 0.263 g) was dispersed in into 80 mL three different solutions containing thiourea (2 mmol, 0.152 g), cysteine (2 mmol, 0.244 g) or thioacetamide (2 mmol, 0.15 g) with constant stirring for 30 min at room temperature, respectively. And then, the same prepared Bi₂SiO₅ samples were dispersed in 80 mL solutions containing thiourea and cysteine with constant stirring for 3 h at elevated temperature (60 °C) with other experimental conditions unchanged. The obtained products were filtered, washed with water and ethanol for four times, and then dried at 60 °C for 6 h.

The Bi₂S₃/Bi₂SiO₅ sample obtained using TAA as sulfur source under stirring for 15 min, 30 min and 60 min, were marked as Bi₂S₃/Bi₂SiO₅-15 min, Bi₂S₃/Bi₂SiO₅-30 min and Bi₂S₃/Bi₂SiO₅-60 min, respectively.

2.4. Characterizations

The crystal structure of as-obtained samples was characterized by X-ray powder diffraction recorded on a Bruker AXS D8 diffractometer using Cu Kα radiation. The microstructure and morphology of the samples were characterized by transmission electron microscopy (JEOL JEM-2100F) and scanning electron microscopy (Hitachi S-4800) equipped with an Energy Dispersive Spectrometer (EDS). X-ray Photoelectron Spectroscopy (XPS) spectra was measured in a Thermo Fisher Scientific Escalab 250 spectrometer with monochromatized Al Kα excitation, and C_1s (284.6 eV) was used to calibrate the peak positions of the elements. The Brunauer–Emmett–Teller (BET) surface area was measured by a Micromeritics ASAP 2020 apparatus. The Diffuse Reflectance Spectroscopy (DRS) of the samples were recorded in the range from 200 to 800 nm using Shimadzu UV 2550 spectrophotometer equipped with an integrating sphere, using 100% BaSO₄ as reflectance sample. The PL spectra was carried out on a Hitachi F-4500 fluorescence spectrophotometer at room temperature and obtained with excitation wavelength at 300 nm.

2.5. Evaluation of photocatalytic performance

The RhB was used to evaluate the photocatalytic activity of the samples at room temperature. The light source is a 500 W Xe arc lamp (PLS-SXE500) equipped with a UV cut off filter (λ > 420 nm). 0.1 g photocatalyst was dispersed into 100 mL of 20 mg L⁻¹ RhB solution, and the suspensions were first sonicated for 5 min and then kept in the dark for 60 min to reach the adsorption/desorption equilibrium under constant stirring. At the given time interval, 3 mL of the suspension was taken from the beaker and centrifugated to separate the photocatalyst and solution. The residual concentration of RhB was measured by a UV-vis spectrophotometer (Xinmao UV-7502PC).

3. Results and discussion

3.1. The effect of sulfur source on the formation of Bi₂S₃/Bi₂SiO₅ heterojunctions

Fig. 1 shows the effect of sulfur source and reaction conditions on the color of the as-prepared samples, and according to which, it can be directly determined whether Bi₂S₃ is formed.¹⁵ Obviously, under the same conditions (room temperature and reacted for 30 min), the color of the sample that treated with TAA (d) is gray, while that treated with thiourea (b) and cysteine (c) is almost the same as that of pure Bi₂SiO₅ (a). In addition, negligible Bi₂S₃ formation is observed using thiourea and cysteine as sulfur source with increased reaction time and temperature, the visible light absorption of which is very low (Fig. S1 in the ESI†). The reason for above phenomenon is due to the different release rate of S²⁻, and TAA displays the fastest S²⁻ release speed among three sulfur source. Therefore, TAA is used as sulfur source for the preparation of Bi₂S₃/Bi₂SiO₅ heterojunctions. The K_sp of Bi₂S₃ is much lower than Bi₂SiO₅. Consequently, Bi₂S₃ formed on the surface of Bi₂SiO₅ once S²⁻ was released by TAA. It is noteworthy to point out that in situ formation enable close interaction between Bi₂S₃ and Bi₂SiO₅, which is advantageous for the separation of photo-generated charge carriers.

Fig. 1 The color of Bi₂SiO₅ that reacted with different sulfur source and under different reaction conditions, (a) pure Bi₂SiO₅, (b) Bi₂SiO₅ reacted with thiourea for 30 min at room temperature, (c) Bi₂SiO₅ reacted with cysteine for 30 min at room temperature, (d) Bi₂SiO₅ reacted with TAA for 30 min at room temperature, (e) Bi₂SiO₅ reacted with thiourea for 3 h at 60 °C, (f) Bi₂SiO₅ reacted with cysteine for 3 h at 60 °C.

3.2. Characterization

3.2.1. Optical properties of Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅ heterojunctions. The color of pure Bi₂SiO₅, Bi₂S₃/Bi₂SiO₅-15 min, Bi₂S₃/Bi₂SiO₅-30 min, and Bi₂S₃/Bi₂SiO₅-60 min gradually changes from white to gray, which means that the content of Bi₂S₃ increased with increasing reaction time. The UV-vis diffuse reflectance spectra (DRS) were shown in Fig. 2a. It can be seen that the absorption edge of pure Bi₂SiO₅ locates at about 330 nm; while with increasing Bi₂S₃ content the visible light absorption is enhanced greatly. Obviously, the light absorption of the Bi₂S₃/Bi₂SiO₅ heterojunction is broadened from UV to the visible light range. This result is attributed to the small band gap of Bi₂S₃, which also confirms the existence of Bi₂S₃. Fig. 2b shows the plot of (ahν)² vs. hν, from which the band gap of the samples can be estimated, and the E_g of Bi₂SiO₅, Bi₂S₃ in Bi₂S₃/Bi₂SiO₅-15 min, Bi₂S₃ in Bi₂S₃/Bi₂SiO₅-30 min, Bi₂S₃ in Bi₂S₃/Bi₂SiO₅-60 min are 3.76 eV, 2.96 eV, 2.56 eV and 2.21 eV, respectively (details are in the ESI Table S1†). Furthermore, the size of Bi₂S₃ in Bi₂S₃/Bi₂SiO₅ heterojunction photocatalysts with different reaction time were determined to be 3.41 nm, 3.92 nm and 4.61 nm, respectively (part 4 in the supporting information).

Fig. 2 (a) UV-vis diffuse reflectance spectra, (b) the plot of (ahν)² vs. hν of the samples.

3.2.2. XRD analysis of Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅ heterojunctions. The phase structure and crystallinity of the Bi₂S₃/Bi₂SiO₅ heterojunctions under different reaction times were determined by XRD patterns as shown in Fig. 3. All the diffraction peaks can be indexed to the pure orthorhombic Bi₂SiO₅ phase (JCPDS no. 36-0287, a = 1.5217 nm, b = 0.5477 nm, c = 0.5325 nm, space group Cmc2₁). The sharp and intense diffraction peaks show that the products are well crystallized and contain no other impurities. No significant diffraction peaks of Bi₂S₃ can be detected even for Bi₂S₃/Bi₂SiO₅ heterojunctions with stirring for 60 min. This may be due to the fact that the content of Bi₂S₃ was very low. The analysis of EDS spectra (part 3 in the ESI†) suggests the actual molar content of Bi₂S₃ in the Bi₂S₃/Bi₂SiO₅ heterojunctions synthesized with different reaction times are 0.110%, 0.247% and 0.427%, respectively. In addition, the growth of Bi₂S₃ on Bi₂SiO₅ lowers the diffraction intensity of Bi₂SiO₅.

Fig. 3 XRD patterns of the as-prepared Bi₂S₃/Bi₂SiO₅ heterojunctions and pure Bi₂SiO₅.

In order to further check the formation of Bi₂S₃, increased reaction time and temperature were applied to increase the content of Bi₂S₃, i.e. Bi₂SiO₅ reacting with TAA at 80 °C for 3 h. As expected, the diffraction peaks of Bi₂S₃ can be detected (Fig. S2 in the ESI†). This result further suggests the presence of Bi₂S₃ in Bi₂S₃/Bi₂SiO₅ heterojunctions under ambient anion exchange process.

3.2.3. XPS analysis of the Bi₂S₃/Bi₂SiO₅ heterojunctions. XPS spectra were further carried out to confirm the chemical state of the elements in the samples. From Fig. 4a, it can be seen that Bi, O and Si elements can be detected obviously. The weak peak of S 2p is covered by strong Bi 4f spectra, therefore S could not be detected by XPS easily. Bi 4f spectra (Fig. 4b) displays two strong peaks at 164.35 eV and 159.05 eV, which are corresponded to the 4f_5/2 and 4f_7/2 of Bi³⁺ in Bi₂S₃/Bi₂SiO₅ heterojunctions, respectively. Fig. 4c shows the XPS spectra of Si 2p, which displays a binding energy of 101.95 eV, and is ascribed to Si(IV). Fig. 4d presents the spectra for O 1s which can be fitted to two peaks. The peak at 530 eV is characteristic of crystal O in Bi₂SiO₅ and the other at 531.8 eV is due to the adsorbed H₂O on the surface.²²

Fig. 4 XPS spectra of Bi₂S₃/Bi₂SiO₅-30 min: (a) the wide-scan XPS spectra of Bi₂S₃/Bi₂SiO₅-30 min sample, (b) Bi 4f, (c) Si 2p, (d) O 1s.

3.2.4. SEM and TEM analysis of Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅ heterojunctions. The surface morphologies of pure Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅-30 min were investigated by SEM (Fig. 5). As can be seen from Fig. 5a and b, pure Bi₂SiO₅ is constructed by interlaced nanosheets with a thickness of about 20 nm. Fig. 5c and d shows the morphologies of Bi₂S₃/Bi₂SiO₅-30 min. No obvious change is observed between pure Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅-30 min. Bi₂S₃ can not be distinguished from the SEM images. The presence of sulfur in Bi₂S₃/Bi₂SiO₅-30 min is detected by EDS mapping (Fig. 6), which indicates the homogeneous distribution of bismuth, silicon, oxygen and sulfur.

Fig. 5 SEM images of as-prepared pure Bi₂SiO₅ (a and b) and Bi₂S₃/Bi₂SiO₅-30 min heterojunctions (c and d).

Fig. 6 EDS distribution maps for bismuth, silicon, oxygen and sulfur of Bi₂S₃/Bi₂SiO₅-30 min.

More detailed information of the microstructures of the pure Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅-30 min were further investigated by TEM and HRTEM. Fig. 7a and c confirm the SEM results that both pure Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅-30 min are composed of nanosheets. In Fig. 7b the layered lattice fringes with a spacing of 0.301 nm matches well with the spacing of the (311) crystal plane of Bi₂SiO_5. In Fig. 7d, the lattice fringes with a spacing of 0.199 nm which matches well with the spacing of the (002) crystal plane of Bi₂S₃, further confirming the presence of Bi₂S₃. In addition, the size of the Bi₂S₃ particles is approximately 5 nm. Meanwhile, the fringe spacing of 0.243 nm is indexed to the (021) crystal plane of Bi₂SiO₅. From Fig. 7d, the intimate contact between Bi₂S₃ and Bi₂SiO₅ can be observed thanks to the in situ ion exchange method.

Fig. 7 TEM and HETEM images of as-prepared pure Bi₂SiO₅ (a and b) and Bi₂S₃/Bi₂SiO₅-30 min (c and d).

3.3. Photocatalytic activity

RhB was used as the pollutant to evaluate the photocatalytic activity of the samples under visible light (λ > 420 nm) and UV-visible light irradiation. Fig. 8a exhibits the results of photocatalytic decomposition of RhB over different photocatalysts under visible light irradiation. As well known, RhB is quite stable, and its self-degradation without photocatalyst can be ignored (Fig. 8a). Only 35% RhB is decomposed under irradiation for 3 h over pure Bi₂SiO₅, due to the self-sensitization effect. However, the photocatalytic activity is greatly improved after in situ formation of Bi₂S₃ on Bi₂SiO₅. Among the different Bi₂S₃/Bi₂SiO₅ heterojunctions, Bi₂S₃/Bi₂SiO₅-30 min displays the best photocatalytic activity with 87% RhB is decomposed. The photocatalytic activity of Bi₂S₃/Bi₂SiO₅ heterojunctions decreases with further increasing Bi₂S₃ content. Fig. 8b shows the absorption profile of RhB in the presence of Bi₂S₃/Bi₂SiO₅-30 min under different irradiation time. The intensity of the maximum absorption decreases gradually, suggesting the decolorization of RhB. In addition, a blue shift of the maximum absorption is observed, which is consistent with the previously reported results and means the conjugated structure was destructed (de-ethylation of RhB).²³

Fig. 8 (a) Photocatalytic activities of the prepared samples under visible light irradiation, (b) the temporal evolution of the absorption spectra of RhB solution with Bi₂S₃/Bi₂SiO₅-30 min as a function of irradiation time under visible light irradiation.

The enhanced photocatalytic activity of these heterojunctions was further investigated under UV-vis light irradiation. The results are shown in Fig. 9a. As expected, Bi₂S₃/Bi₂SiO₅-30 min heterojunction displays the best photocatalytic activities, and nearly 100% of RhB (20 mg L⁻¹) are decomposed within 90 min. In addition, stability is significant for catalysts to be used in practical applications. In order to evaluate the stability of the Bi₂S₃/Bi₂SiO₅ heterojunctions, cycling test of Bi₂S₃/Bi₂SiO₅-30 min have been done (Fig. 9b). The photocatalytic activity reduced a little after 4 cycles, indicating that the heterojunctions are relative stable.

Fig. 9 (a) Photocatalytic activities of different catalysts under full light (UV-vis light) irradiation, (b) cycling tests of photocatalytic activity of Bi₂S₃/Bi₂SiO₅-30 min heterojunction for RhB degradation.

3.4. PL spectra

The recombination of free photogenerated carriers can cause photoluminescence (PL) emission. Therefore, PL spectroscopy is an effective technique to investigate the separation efficiency of the photogenerated charge carriers in a semiconductor. The PL spectra of Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅ heterojunctions are presented in Fig. 10. From Fig. 10, pure Bi₂SiO₅ displays the highest intensity of PL emission, indicating that the photogenerated carriers recombine quickly. The PL intensity of Bi₂S₃/Bi₂SiO₅ heterojunctions are lower than pure Bi₂SiO₅, and with the increase of reaction time, the PL intensity gradually decreases. This phenomenon demonstrates that the Bi₂S₃/Bi₂SiO₅ heterojunctions can deeply suppress the recombination of photogeneration carriers and lead to lower PL intensity. Therefore, the photocatalytic activity enhanced.

Fig. 10 PL spectra of Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅ heterojunctions (λ_exc = 300 nm).

3.5. Photocatalytic mechanism

The band positions of the samples can be calculated by the empirical formula E_VB = X − E^e + 0.5E_g,²⁴ and E_CB = E_VB − E_g. Where E_VB is valence band (VB) edge potential, E_CB is conduction band (CB) edge potential, X is the electronegativity of the semiconductor, E_g is the band gap energy of the semiconductor that is the geometric mean of the absolute electronegativity of the constituent atoms and E^e is the energy of free electrons on the hydrogen scale (4.5 eV). The positions of E_CB and E_VB of Bi₂SiO₅ and Bi₂S₃/Bi₂SiO₅ heterojunctions were calculated in ESI (Table S1†).

Fig. 11 show the energy level of Bi₂S₃/Bi₂SiO₅ heterojunctions and the possible charge transfer process of the heterojunctions under visible light irradiation. The photocatalytic mechanism was suggested as follows. Under visible light irradiation, Bi₂S₃ absorb photons giving rise to electrons and holes. The photogenerated electrons transfer to the conduction band of Bi₂SiO₅; while the photogenerated holes left at the valence band of Bi₂S₃. Therefore, the photo-induced electrons and holes are separated spatially. The photogenerated electrons react with adsorbed O₂ to produce O₂⁻, meanwhile, photogenerated holes left at the valence band of Bi₂S₃. Then photogenerated holes and O₂⁻ initiate the oxidative reaction of RhB.

Fig. 11 Diagram for energy level of Bi₂S₃/Bi₂SiO₅ heterojunctions and the possible charge transfer process of the heterojunctions under visible light irradiation.

Under UV-vis light irradiation, both Bi₂SiO₅ and Bi₂S₃ can absorb photons giving rise to electrons and holes. The photogenerated electrons from the conduction band of Bi₂S₃ transfer to the conduction band of Bi₂SiO₅; meanwhile, the photogenerated holes from the valence band of Bi₂SiO₅ transfer to the valence band of Bi₂S₃. Therefore, the photogenerated holes and electrons can be separated effectively, the photocatalytic activity enhanced a lot.

As mentioned above, Bi₂S₃/Bi₂SiO₅-30 min displays the best photocatalytic activity. The BET surface areas of all the samples are almost the same (Table S2 in the ESI†), therefore, surface area is not the main factor for the different photocatalytic activity. The reason may be attributed to the quantum size effect of Bi₂S₃. When the ion exchange reaction time is 15 min, the size of Bi₂S₃ is small. Therefore, the band gap is larger (E_g = 2.96 eV) and the conduction band (CB) position is more negative, which means the driving force for electrons transfer from the CB of Bi₂S₃ to the CB of Bi₂SiO₅ is stronger and the photogenerated holes at the valence band have higher oxidation capacity, but larger band gap cause less visible light absorption. With increasing the ion exchange reaction time up to 60 min, the size of Bi₂S₃ increase significantly (up to 4.61 nm). According to quantum size effect, the band gap of Bi₂S₃ decreased (E_g = 2.21 eV) which means more visible light can be absorbed. However, the CB position of Bi₂S₃ shifts to positive and the VB position of Bi₂S₃ shifts to negative, which mean both the driving force for electrons transfer from the CB of Bi₂S₃ to the CB of Bi₂SiO₅ and the oxidation capacity of photogenerated holes at the VB of Bi₂S₃ are decreased. The overmuch content of Bi₂S₃ can promote the recombination of photoinduced electrons and holes to reduce the photocatalytic activity.²⁵ Consequently, it is the combined effect of visible light absorption and the CB and VB position that is responsible for the best photocatalytic activity of Bi₂S₃/Bi₂SiO₅-30 min.

4. Conclusions

In this study, a novel Bi₂S₃/Bi₂SiO₅ heterojunctions was synthesized by a simple in situ ion exchange method. The Bi₂S₃/Bi₂SiO₅ heterojunctions exhibited enhanced visible light absorption and photocatalytic activity compared with pure Bi₂SiO₅. With the ion exchange time increasing, the content and size of Bi₂S₃ in Bi₂S₃/Bi₂SiO₅ heterojunctions increased but less than 10 nm, and the photocatalytic activity increased first and then decrease. Bi₂S₃/Bi₂SiO₅-30 min heterojunction displays the best photocatalytic activity that can degrade 87% of RhB within 3 h under visible light irradiation. The quantum size confinement of Bi₂S₃ nanoparticles cause the tunable energy band so that the suitable heterojunction can be formed. The enhanced photocatalytic activity can attribute to the extended spectral response range, the effective separation of photoinduced electrons and holes and the photosensitization of Bi₂S₃.

Acknowledgements

This work is financially supported by the Shandong Province Natural Science Foundation (ZR2014JL008), National Basic Research Program of China (the 973 Program: no. 2013CB632401), and the National Natural Science Foundation of China (no. 21333006, 11374190 and 51021062).

Notes and references

1. H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri and J. Ye, Adv. Mater., 2012, 24, 229–251.
2. S. B. Wang, H. M. Ang and M. O. Tade, Environ. Int., 2007, 33, 694–705.
3. W. J. Wang, H. F. Cheng, B. B. Huang, X. J. Lin, X. Y. Qin, X. Y. Zhang and Y. Dai, J. Colloid Interface Sci., 2013, 402, 34–39.
4. G. Liu, L. Z. Wang, H. G. Yang, H. M. Cheng and G. Q. Lu, J. Mater. Chem., 2010, 20, 831–843.
5. X. W. Wang, G. Liu, Z. G. Chen, F. Li, L. Z. Zhou, G. Q. Lu and H. M. Cheng, Chem. Commun., 2009, 3452.
6. Z. B. Yu, Y. P. Xie, G. Liu, G. Q. Lu, X. L. Ma and H. M. Cheng, J. Mater. Chem. A, 2013, 1, 2773.
7. I. Kornarakis, I. N. Lykakis, N. Vordos and G. S. Armatas, Nanoscale, 2014, 6, 8694.
8. I. Koiwa, T. Kanehara, J. Mita, T. Iwabuchi, T. Osaka, S. Ono and M. Maeda, Jpn. J. Appl. Phys., 1996, 35, 4946.
9. D. Liu, J. Wang, M. Zhang, Y. F. Liu and Y. F. Zhu, Nanoscale, 2014, 6, 15222–15227.
10. S. Geoges, F. Goutenoire and P. Lacorre, J. Solid State Chem., 2006, 179, 4020.
11. G. Cheng, J. Xiong, H. Yan, Z. Lu and R. Chen, Mater. Lett., 2012, 77, 25–28.
12. R. G. Chen, J. H. Bi, L. Wu, W. J. Wang, Z. H. Li and X. Z. Fu, Inorg. Chem., 2009, 48, 9072–9076.
13. H. Bao, C. M. Li, X. Cui, Y. Gan, Q. Song and J. Guo, Small, 2008, 4, 1125.
14. B. Zhang, X. C. Ye, W. Y. Hou, Y. Zhao and Y. Xie, J. Phys. Chem. B, 2006, 110, 8978–8985.
15. S. C. Liufu, L. D. Chen, Q. Yao and C. F. Wang, Appl. Phys. Lett., 2007, 90(11), 112106.
16. D. Robert, Catal. Today, 2007, 122, 20–26.
17. H. F. Cheng, B. B. Huang, X. Y. Qin, X. Y. Zhang and Y. Dai, Chem. Commun., 2012, 48, 97.
18. N. Liang, J. T. Zai, M. Xu, Q. Zhu, X. Wei and X. F. Qian, J. Mater. Chem. A, 2014, 2, 4208.
19. Z. J. Zhang, W. Z. Wang, L. Wang and S. M. Sun, ACS Appl. Mater. Interfaces, 2012, 4(2), 593–597.
20. J. Cao, B. Y. Xu, H. L. Lin, B. D. Luo and S. F. Chen, Dalton Trans., 2012, 11482.
21. Z. D. Wu, L. L. Chen, C. S. Xing, D. L. Jiang, J. M. Xie and M. Chen, Dalton Trans., 2013,(36), 12980.
22. F. Dong, X. Feng, Y. Zhang, C. Gao and Z. Wu, RSC Adv., 2015, 5, 11714.
23. J. Fu, Y. Tian, B. Chang, F. Xi and X. Dong, J. Mater. Chem., 2012, 22, 21159–21166.
24. Y. Zhu, D. Fan and W. Shen, Langmuir, 2008, 24, 11131.
25. Y. Guo, G. Zhang, H. Gan and Y. Zhang, Dalton Trans., 2012, 12697–12703.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09070a

---