Novel Bi₁₂ZnO₂₀–Bi₂WO₆ heterostructures: facile synthesis and excellent visible-light-driven photocatalytic activities

Abstract

Novel sillenite-type Bi₁₂ZnO₂₀–Bi₂WO₆ heterostructure photocatalysts were successfully prepared via a controllable partial precipitate conversion strategy, employing previously-prepared Bi₂WO₆ nanosheets as precursors of Bi³⁺. The phases and morphologies of the products were characterized by powder X-ray diffraction, energy dispersive spectrometry, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and scanning electron microscopy. Compared to the sample (Bi₁₂ZnO₂₀ + Bi₂WO₆) obtained by physical mixing technology and the single-component photocatalyst (Bi₁₂ZnO₂₀ and Bi₂WO₆), such sillenite-type Bi₁₂ZnO₂₀–Bi₂WO₆ heterostructures exhibited higher visible-light-driven photocatalytic activity, which could be ascribed to matching band positions and intimate interfacial contact between different components that could benefit the efficient separation of photo-generated carriers.

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1. Introduction

In the past decades, semiconductor photocatalysis has been attracting much attention due to its potential applications in the decomposition of organic waste-water, H₂ production by water splitting and CO₂ reduction.^1–5 As is well known, a prerequisite for the development of photocatalysis applications is to gain access to efficient photocatalysts. While TiO₂ can be recognized as the “star photocatalyst” owing to its high oxidizing efficiency, high chemical stability, nontoxicity and low-cost,^6–8 it only responds to UV light, that accounts for about 4% of the solar radiation energy, which inevitably leads to a huge waste of energy.⁹ Therefore, a great deal of effort has been devoted to the development of highly efficient visible-light-driven photocatalysts.

Bi₁₂(Bi_xM_1−x)O₂₀ (M = Zn, Si, V, Fe, Ge, Ti) belongs to a family of sillenite compounds with a body-centered cubic crystal structure (space group I23).^10–12 Several years' research have shown that sillenite-type semiconductors usually possess excellent mobility of photo-induced carriers, strong oxidizing ability of photo-generated holes, and a relative narrow band-gap (<2.8 eV).¹³ Therefore, these kind photocatalysts should exhibit excellent visible light activity based on the theoretical analysis.^14–17 Actually, many research groups have been working on the structure optimization and performance control of this kind photocatalytic material and have made encouraging progress in this field. For example, Hou and coworkers reported the successful preparation of visible-light-driven Bi₁₂TiO₂₀ photocatalysts with different morphology through a facile solution-phase hydrothermal process.¹⁸ Wan et al. synthesized uniform Bi₁₂GeO₂₀ microspheres and microtetrahedrons enclosed by four {111} facets.¹⁹ Photocatalytic experiments showed that the as-obtained samples could degrade some organic pollutants such as RhB and gaseous formaldehyde under visible light irradiation. Unfortunately, the reported photocatalytic activity of these materials is usually relatively low. The main limiting factor can be ascribable to the unfavorable recombination rate of photo-generated charges.^20–26

Recently, semiconductor coupling technology has been demonstrated as an effective strategy for efficient separation of photo-generated electron–hole pairs, leading to the enhancement of photocatalytic activity. For example, some composite photocatalysts including Bi₂O₃–Bi₂WO₆,²⁷ Bi₂₅FeO₄₀–graphene,²⁸ CuO–In₂O₃ (ref. 29) and Bi₁₂TiO₂₀–TiO₂ (ref. 30) all exhibit much better photocatalytic activity than the corresponding single semiconductors. In addition, among various synthesis approaches of these heterostructures, it has been demonstrated that the partial precipitate conversion method exhibits great advantages for the construction of heterostructure photocatalyst with intimate interfacial contacts between different components.^31–33 This feature is particularly important for the migration, transfer and separation of photo-generated charges and the remarkable improvement of photocatalytic activity.

Therefore, we now report the successful construction of novel Bi₁₂ZnO₂₀–Bi₂WO₆ (denoted as BZO–BWO) heterostructure photocatalysts via a controllable partial precipitate conversion strategy, employing previously-prepared BWO nanosheets as a precursor of Bi³⁺. Significantly, our photocatalytic experiments reveal that these heterostructures exhibit excellent visible-light-driven photocatalytic activity. To the best of our knowledge, report on the preparation of such sillenite-type heterostructures is still quite rare, specially for BZO. More importantly, this work not only enriches the preparation technology and improves the photocatalytic activity of BZO, but also presents a general and effective method to improve the visible light activity of other sillenite-type photocatalytic materials.

2. Experimental section

Preparation of BWO nanosheets

The synthesis method of BWO is based on ref. 34. In a typical preparation procedure, Bi(NO₃)₃·5H₂O (0.001 mol) was dissolved in 15 mL of deionized water. Then, 15 mL of deionized water containing 0.0005 mol of Na₂WO₄·2H₂O was slowly dropped into the above solution. Finally, 1 g of poly(vinylpyrrolidone) were introduced. The as-obtained solution was stirred for 10 min and transferred into stainless-steel autoclave with a Teflon liner of 40 mL capability. After treating the mixture at 180 °C for 24 h, it was cooled to room temperature naturally. The product was collected, washed with deionized water and absolute ethanol, and dried in a vacuum at 60 °C for 6 h with a yield of 90%. Fig. S1† show the XRD pattern, EDS spectrum and SEM images of the product, which demonstrates the successful preparation of BWO nanosheets (see ESI†).

Preparation of BZO microcrystals

In a typical preparation procedure, Bi(NO₃)₂·5H₂O (0.0012 mol) was dissolved in 15 mL of deionized water. Then, 15 mL of deionized water containing 0.0001 mol of Zn(CH₃COO)₂·2H₂O was slowly dropped into the above solution. Finally, 0.2 g of NaOH was introduced. The as-obtained solution was stirred for 10 min and transferred into stainless-steel autoclave with a Teflon liner of 40 mL capability. After treating the mixture at 180 °C for 24 h, it was cooled to room temperature naturally. The product was collected, washed with deionized water and absolute ethanol, and dried in a vacuum at 60 °C for 6 h with a yield of 90%. Fig. S2 and S3† show the XRD pattern, SEM images and XPS spectra of the product, which demonstrates the successful preparation of BZO microcrystals (see ESI†).

Preparation of BMO–BWO heterostructures

0.5 g of BWO nanosheets, a certain amount of Zn(CH₃COO)₂·2H₂O and 0.3 g of NaOH were introduced into Teflon liner. Then, 30 mL of deionized water was slowly dropped into the above container. The as-obtained solution was stirred for 10 min and transferred into stainless-steel autoclave. After treating the mixture at 180 °C for 24 h, it was cooled to room temperature naturally. The product was collected, washed with deionized water and absolute ethanol and dried in a vacuum at 60 °C for 6 h. The BZO–BWO heterostructures with different BZO–BWO weight ratios (9, 16 and 22%, this data is calculated from the feed ratio) were denoted as S1, S2 and S3, respectively.

Characterization

The products were characterized by XRD with a Shimadzu XRD-6000 powder X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å), recorded with 2θ ranging from 15° to 70°. SEM images of the products were obtained on field emission scanning electron microanalyser (Hitachi S-4800), employing the accelerating voltage of 5 kV. Energy dispersive X-ray spectroscopy, being attached to the scanning electron microscope, was used to analyze the composition of the sample. HRTEM were carried out on a JEM-2100 high resolution transmission microscope, employing an accelerating voltage of 200 kV. XPS spectrum was recorded on a PHI5000 VersaProbe X-ray photoelectron spectrometer. The UV-vis diffusion reflectance spectrum of the sample was analyzed with a UV-vis spectrophotometer (UV-3600, Shimadzu, Japan). Photoluminescence spectrum was recorded employing an Edinburgh FLSP 920 fluorescence spectrophotometer at room temperature. Photocurrent was also measured using a CHI 660B electrochemical workstation in a standard three-electrode system using the prepared samples as the working electrodes with an active area of ca. 0.5 cm², a platinum wire as counter electrode, and a standard calomel electrode (SCE) as reference electrode. A 300 W xenon lamp with a 400 nm cut-off filter was used as the source of visible-light irradiation. Potentials were given with reference to SCE. The photoresponses of the photocatalysts at light on and off were measured at 0.0 V.

Photocatalytic evaluation

A 240 W Xe lamp (BL-GHX-Xe-300, Shanghai BILON Co., Ltd.) was used as the light source and equipped with an ultraviolet cutoff filter to provide visible light (λ ≥ 400 nm). The distance between the liquid surface of the suspension and the light source was set about 15 cm. The photocatalytic experiments were performed with the sample powder (40 mg) suspended in RhB (10 mg L⁻¹, 40 mL) with constant stirring. Before light was turned on, the solution was continuously stirred for 60 min in the dark to ensure the establishment of an adsorption–desorption equilibrium. During irradiation, ∼3 mL of the suspension was continually taken from the reactor at given time intervals. The photocatalyst powders and the pollutions solution were separated by a centrifugal machine. The RhB concentration was analyzed through a UV-vis spectrophotometer (UV-3600, Shimadzu, Japan).

3. Results and discussion

The successful construction of BZO–BWO heterostructures (S1) was confirmed by the XRD pattern (Fig. 1a). In addition to the characteristic diffraction peaks assigned to orthorhombic BWO (JCPDS file Card no. 79-2381), the remaining peaks can be well-indexed to cubic BZO (JCPDS file Card no. 78-1325). These experimental results demonstrate that the as-synthesized products are composite materials. The strong and sharp diffraction peaks suggest good crystallinity of the product and no other impurity peak is detected. The microstructure and morphology of the product were investigated by SEM. As shown in Fig. 1b, one can find that the as-prepared product is composed of cube-like microstructures and nanosheets. High magnification SEM image shown in Fig. 1c reveals that some nanosheets exist in the surface of cube-like microstructures, indicating the formation of BZO–BWO heterojunctions.

Fig. 1 XRD pattern (a), SEM images (b and c), EDS spectra (d and e) and TEM image (f) of BZO–BWO heterostructures (S1) prepared under hydrothermal synthesis method.

Because it is difficult to obtain the EDS spectrum of “pure” cube, whole heterostructure (cube and nanosheet) is selected as information acquisition area. As shown in Fig. 1d, one can find that weak Zn peak appears accompanied by peaks of other elements such as Bi, W, O. Furthermore, the EDS analysis of nanosheets attached to the surface of the cube reveals that these sheet-like structures can be assigned to BWO (Fig. 1e). Therefore, it is reasonable to believe that the remaining cube-like structure can be attributable to BZO. The further evidence of the formation of BZO–BWO heterostructures comes from HRTEM. The HRTEM image from the heterostructures displays two types of clear lattice fringes as shown in Fig. 1f. One set of the fringes spacing is ca. 0.321 nm, corresponding to the (310) plane of orthorhombic BWO. Another set of the fringes spacing measures ca. 0.361 and 0.362 nm, which corresponds to the (220) and (2–20) lattice spacing of cubic BZO, respectively. The above experimental results fully prove the formation of BZO–BWO heterojunction.

More detailed structural information regarding the chemical and bonding environment of the BZO–BWO heterostructures (S1) was ascertained using X-ray photoelectron spectroscopy. Fig. 2a shows the survey spectrum of the sample in the range of 2–1200 eV. This overview spectrum demonstrates that Bi, W and O exist in the final products. The Bi 4f peaks centered at 158.5 (Bi 4f_7/2) and 163.9 eV (Bi 4f_5/2) imply the existence of Bi in the Bi³⁺ oxidation state (Fig. 2b). Two peaks at 34.7 (W 4f_7/2) and 36.7 eV (W 4f_5/2) indicate that W is in its [WO₄]²⁻ (W⁶⁺) state (Fig. 2c). A strong oxygen 1s peak centered at 529.9 eV corresponds to the oxygen in its O²⁻ oxidation state (Fig. 2d). Two weak peaks taken for the Zn region at 1021.7 eV (Zn 2p_3/2) and 1044.2 eV (Zn 2p_1/2) are assigned to the Zn 2p binding energy (Fig. 2e), which infers the existence of Zn in the Zn²⁺ oxidation state. Compared to pure BZO and BWO, the XPS peak position of Bi 4f, Zn 2p and W4f in the BZO–BWO heterostructures shift to high binding energies (Table 1),³⁴ which may be ascribed to the weak interaction between BZO and BWO resulting from their intimate interfacial contacts.³⁵ The weak peaks of Zn 2p can be ascribed to the low content of BZO (BZO–BWO = 9%, wt%, S1) in the final heterostructures photocatalyst.

Fig. 2 XPS spectra of BZO–BWO heterostructures (S1) prepared under hydrothermal method.

Table 1 The XPS peak positions of Bi 4f, Zn 2p and W 4f in different photocatalysts (the XPS peak positions of pure BWO were obtained from ref. 34)

Sample	Bi 4f (eV)	Zn 2p (eV)	W 4f (eV)
S1	158.5, 163.9	1021.8, 1044.2	34.7, 36.8
Pure BZO	158.2, 163.6	1020.7, 1043.9	—
Pure BWO	158.2, 163.5	—	34.5, 36.5

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If the weight percentage of BZO is increased from 9 to 30%, such XPS peak intensity of Zn 2p significantly increases but with no phase change (see ESI, Fig. S4 and S5†). For the BZO–BWO heterostructure, it is actually composed of BWO and BZO. Therefore, it should show the diffraction peaks of each component in the XRD pattern of heterostructure photocatalyst. In addition, this XRD pattern of heterostructures should also be consistent with the sample (BZO + BWO) prepared by the physical mixing method. As given in Fig. S6 (see ESI†), one can find that the sample prepared by physical mixture has almost the same XRD pattern as the BZO–BWO heterostructures. Therefore, these results further confirm the successful construction of BZO–BWO heterojunction photocatalyst with close interconnection.

The UV-vis diffuse reflection data is used to calculate the absorption coefficient from the well-known Kubelka–Munk (KM) function, defined as:

F(R_∞) = α/S = (1R_∞)²/R_∞ (1)

here α, S and F(R_∞) is the optical absorption coefficient, scattering coefficient and KM function, respectively. As a crystalline semiconductor, the optical absorption near the band edge follows the formula:

αE = C(E − E_g)ⁿ (2)

here α, E, E_g and C is the absorption coefficient, incident photon energy, the optical absorption edge energy and a constant, respectively.³⁵ The exponent n is dependent on the type of optical transition as a result of photon absorption. The exponent n is assigned a value of 1/2 and 2 for direct allowed and indirect allowed, respectively. For the diffused reflectance spectra, the KM function can be used instead of α for estimation of the optical absorption edge energy. In order to determine E_g, the diffuse reflectance spectra of the pure BWO, BZO and BZO–BWO heterostructures (S1) were measured (Fig. 3). The intercept of the line on abscissa [F(R_∞)E = 0] give the values of optical absorption edge energy to be 2.97, 2.72 and 2.88 eV for the above three samples, respectively.

Fig. 3 UV-vis diffuse reflectance spectrum and the plots of F(R) vs. photo energy for the estimation of the optical absorption edge energy of the products: (a) pure BWO, (b) pure BZO and (c) BZO–BWO heterostructures (S1).

To demonstrate the potential application of the BZO–BWO heterostructures in the decomposition of organic pollutants, the photocatalytic activities of different photocatalysts were investigated by choosing the photocatalytic degradation of RhB with high concentrations as a model reaction (Fig. 4a). In our experiments, one can find that the concentration of RhB hardly changes after 90 min reaction in the absence of any photocatalyst. When bare BZO or BWO is employed as photocatalyst, the degradation rate of RhB is 10 and 23%, respectively. These show that the photocatalytic activity of single-component photocatalyst (BZO or BWO) is relatively low due to the high recombination rate of photo-generated carriers. When BZO and BWO are combined to construct BZO–BWO heterostructures, all three composite photocatalysts (S1, S2 and S3) exhibit excellent photocatalytic activity and S1 possesses the highest photocatalytic degradation efficiency (91%). The above experimental results indicate that the introduction of BWO into BZO can significantly inhibit the recombination of photo-induced electron–hole pairs and lead to the remarkable improvement of the photocatalytic activities. Recently, Shenawi-Khalil reported the preparation of BiOBr/bismuth oxyhydrate photocatalyst via a facile and simple hydrothermal method.³⁵ Research indicated that excess bismuth oxyhydrate in the composite photocatalyst could act as a recombination center and impeded the improvement of photocatalytic activity, which might explain the difference of photocatalytic degradation efficiency in S1, S2 and S3. Furthermore, if the sample prepared by the physical mixture of BZO and BWO is used as photocatalyst, only 29% of RhB can be decomposed. Usually, composite photocatalyst obtained by physical mixing technology does not have intimate interfacial contacts between different components. So, it is difficult to realize the efficient separation of photo-generated charges as well as the remarkable improvement of photocatalytic efficiency, which proves the superiority of partial conversion strategy. To analyze in detail the photocatalysis kinetics of the RhB degradation, we apply the pseudo-first order model as expressed by eqn (3), which is usually used for photocatalytic degradation process if the initial concentration of pollutant is low.^36–38

ln(C₀/C) = kt (3)

Fig. 4 (a) Photodegradation efficiencies of RhB as a function of irradiation time and (b) kinetics of RhB decolorization in solutions.

The C₀ and C are the concentrations of organic dye in solution at time 0 and t, respectively, and k is the pseudo first-order rate constant. The rate constants calculated from the data plotted in Fig. 4a are summarized in Table 2. The experimental data obviously shows that the apparent rate constant k is 0.0010, 0.0029, 0.0037, 0.0250, 0.0169 and 0.0096 min⁻¹ for pure BZO, BWO, physical mixture sample, S1, S2 and S3, respectively. In other words, BZO–BWO heterojunction photocatalysts (S1) exhibits the highest photocatalytic activity, which is almost 25.0, 8.6, 6.8, 1.5 and 2.6 times higher than those of pure BZO, BWO, physical mixture of BZO and BWO, S2 and S3, respectively.

Table 2 Pseudo-first-order rate constant for RhB photocatalytic degradation under different photocatalysts

Series	Photocatalysts	First-order kinetic equation	k (min^−1)	R
1	BZO	ln(C0/C) = 0.0010t	0.0010	0.992
2	BWO	ln(C0/C) = 0.0029t	0.0029	0.996
3	Mixture of BZO and BWO	ln(C0/C) = 0.0037t	0.0037	0.997
4	S1	ln(C0/C) = 0.0250t	0.0250	0.991
5	S2	ln(C0/C) = 0.0169t	0.0169	0.997
6	S3	ln(C0/C) = 0.0096t	0.0096	0.998

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Photoluminescence (PL) spectrum arises from the migration, transfer, and separation efficiency of the photo-generated charge carriers in a semiconductor photocatalyst. In general, there is a close correlation between the photocatalytic activity and PL intensity. The lower the PL intensity, the lower the recombination rate of photo-induced carriers, and the higher the photocatalytic activity of photocatalysts.^39–41 Fig. 5 shows the PL spectra of pure BZO, pure BWO, physical mixture of BZO and BWO, and BZO–BWO heterostructures (S1) at room temperature. It can be seen that BZO–BWO composite photocatalyst (S1) displays the lowest emission peak, suggesting that such heterojunctions can effectively inhibit the recombination of photogenerated charge carriers and possess the highest photocatalytic activity, which is in good agreement with the result from photodegradation experiment.

Fig. 5 Room temperature PL spectra of different photocatalysts.

To better understand the effect of the heterojunction on the high photocatalytic activity of the BZO–BWO heterostructure, we investigated its photoinduced charge transfer properties. Fig. 6 shows the photocurrent responses of BWO, BZO and BZO–BWO heterostructures (S1), under visible light irradiation. Experimental results reveal that BZO–BWO heterojunctions exhibit higher current density than the single-component semiconductors, which demonstrate that a more effective separation of photogenerated electron–hole pairs and faster interfacial charge transfer occur in the heterostructures.³⁵

Fig. 6 Photocurrent responses of different photocatalysts under visible-light irradiation.

In order to study the photocatalytic degradation mechanism over the BZO–BWO composite photocatalyst (S1), some sacrificial agents such as isopropanol (IPA), sodium oxalate (Na₂C₂O₄) and benzoquinone (BQ) were introduced (2.0 mM) into the RhB solution and served as scavengers of ˙OH, h⁺ and ˙O₂⁻, respectively.⁴² The corresponding experimental results are shown in Fig. 7. Na₂C₂O₄ has a strong influence on the degradation efficiency of RhB. However, the degradation rate of RhB slightly decreases after the addition of IPA and BQ. The above experimental results indicate that h⁺ played more important roles than ˙OH and ˙O₂⁻ in the photodegradation of RhB.

Fig. 7 Photocatalytic degradation of RhB over the BZO–BWO heterostructures (S1) with or without quenchers.

To fully understand the mechanism of the enhanced photocatalytic efficiency of the BZO–BWO heterostructures, the band edge positions of the valence and conduction band of the two semiconductors need to be determined. The conduction band bottoms are estimated in this study according to the below empirical formula:⁴³

E_CB = X − 0.5E_g − E₀ (4)

where E_g is the band gap energy of the semiconductor photocatalyst, E₀ is the energy of free electrons on the hydrogen scale (about 4.5 eV), and X is the electronegativity of the semiconductor, which can be expressed as the geometric mean of the absolute electronegativity of the constituent atoms (the absolute electronegativity of Bi, W, O and Zn is 4.69, 4.40, 7.54 and 4.45 eV, respectively).^44–46 Therefore, the X values for BZO and BWO are calculated to be 6.24 and 6.39 eV, respectively, and the band gap energies of BZO and BWO are 2.74 and 2.98 eV, respectively. By following this equation, the conduction band bottom of BZO and BWO is calculated to be 0.37 and 0.40 eV, respectively. Correspondingly, the valence band tops of BZO and BWO are 3.11 and 3.38 eV, respectively. Thus both the conduction band bottom and the valence band top of BZO are higher than those of BWO. These data indicate that the coupling between BZO and BWO is favorable for the separation and transportation of charge carriers. As is illustrated in Fig. 8, both BZO and BWO can be easily excited under visible-light irradiation and lead to the formation of corresponding photo-induced electrons and holes. Then, photo-generated electrons on the surface of BZO can directly inject into the conduction band of BWO due to the intimate contact between the two semiconductors. Meanwhile, holes on the valence band of BWO can be easily transferred to that of BZO under the band energy potential difference. In such a way, the redistribution of electrons on one side of the junction (BWO) and holes on the opposite side (BZO) greatly reduces the electron–hole pairs recombination.

Fig. 8 Diagram for energy band levels of BZO–BWO heterostructures and the possible charge separation process.

4. Conclusions

In summary, visible-light-driven BZO–BWO heterostructure photocatalyst has been fabricated via a controllable partial conversion strategy. Photocatalytic experiments demonstrate that such heterojunctioned material exhibit higher photocatalytic activity than the single-component photocatalyst. The significant improvement of photocatalytic activity is closely associated with the efficient separation of photo-generated carriers resulting from the matching band positions and intimate interfacial contacts. This work reports a general and effective method to improve the visible light activity of sillenite-type photocatalytic materials and may find important applications in decomposition of organic wastewater.

Acknowledgements

We gratefully acknowledge financial support from the Natural Foundation of Anhui Province (no. 1308085QB34 and 1408085QB31), the National Natural Science Foundation of China (no. 21201006 and 21301005) and Special Research Found for the Doctoral Program of Higher Education (no. 269 20123415120002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01327e

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