Hollow BiVO₄/Bi₂S₃ cruciate heterostructures with enhanced visible-light photoactivity

Abstract

Novel BiVO₄/Bi₂S₃ cruciformities with hollow heterostructures were synthesized by an anion exchange method. The cruciate BiVO₄ acted as a template for the formation of special BiVO₄/Bi₂S₃ heterostructures. The proportion of Bi₂S₃ in the heterostructures could be simply controlled by adjusting the amount of Na₂S during the reaction with BiVO₄ cruciate flakes. The as-prepared BiVO₄/Bi₂S₃ heterostructures exhibit superior photo-responsive range and enhanced visible-light photoreduction activity for Cr⁶⁺ ions. A possible enhanced photoreduction mechanism is proposed.

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Introduction

Monoclinic bismuth vanadate (m-BiVO₄) with a band gap of 2.4 eV has been widely used for visible-light photocatalytic evolution of O₂ and organic matter degradation.^1–4 As a consequence of serious recombination of photogenerated electrons and holes, its specific applications still suffer from poor quantum yield.^5,6 Therefore, a series of BiVO₄-based heterostructures are designed to improve the separation of photoinduced carriers owing to the well-matched energy levels between BiVO₄ and semiconductors.^7–10

Ion exchange is a novel approach for the synthesis of photocatalysts with heterostructures through an ion exchange reaction at the interface of two semiconductors.^11–15 Based on the Kirkendall effect, the movement of atoms is from one semiconductor with a lower diffusion coefficient into the other one with a higher diffusion coefficient.^16–18 Hollow structures can be eventually generated because of the different diffusion rates of the two components.^19–24

Orthorhombic bismuth sulfide (Bi₂S₃) is a direct semiconductor with a band gap of 1.3 eV, and has received increasing attention owing to its potential applications in many fields such as electrochemical hydrogen storage,²⁵ hydrogen sensors,²⁶ X-ray computed tomography imaging,²⁷ biomolecule detection,²⁸ and lithium ion batteries.²⁹ Due to its narrow band gap and large absorption coefficient, Bi₂S₃ can be applied as an efficient sensitizer to extend light absorption of semiconductors into the visible-light region.³⁰

Herein, unique hollow BiVO₄/Bi₂S₃ cruciate heterostructures are constructed via a simple anion exchange method assisted by hydrothermal treatment of BiVO₄ cruciate flakes in Na₂S aqueous solution. The cruciate BiVO₄ plays an important template role in the formation of novel BiVO₄/Bi₂S₃ hollow heterostructures. The hollow structure of the special cruciate unities also enlarges the area for electron transfer. The proportion of BiVO₄ to Bi₂S₃ can be easily adjusted by controlling the amount of Na₂S during the anion exchange process. Purposeful spatial isolation of photogenerated electrons and holes from the heterojunction interface area significantly increases the survivability of the electrons crossing therein. The optimized visible-light absorption range of the heterostructures by loading an appropriate amount of Bi₂S₃ is the prerequisite to obtain high photoreduction efficiency for Cr⁶⁺ ions.

Experimental section

Synthesis of the NH₄V₄O₁₀ nanoribbon

All the reagents were analytical pure and used as received without further purification. Typically, 0.315 g of BiCl₃ and 0.117 g of NH₄VO₃ were dissolved in 50 mL of deionized water (DI, the corresponding suspension is abbreviated as A). Then 1.0 mL of 1.0 mol L⁻¹ ethanolamine aqueous solution was dropped into A under magnetic stirring. After static settlement for 30 min, solution B was obtained and then transferred into a 100 mL Teflon-lined stainless-steel autoclave at 160 °C for 24 h. After the autoclave was cooled to room temperature, a dark green sponge was collected, and washed with ethanol and DI water more than 3 times.

Synthesis of BiVO₄ cruciate flakes

BiVO₄ is prepared with the assistance of hydrothermal treatment using NH₄V₄O₁₀ as the source of vanadium. Typically, 0.096 g of the above as-prepared NH₄V₄O₁₀ nanoribbon and 0.315 g of BiCl₃ were dissolved in 50 mL of DI water under magnetic stirring for 24 h, and the dark green solution C was obtained. Then, solution C was transferred into a 100 mL Teflon-lined stainless-steel autoclave at 200 °C for 1 h. Finally, yellow powder was separated by centrifugation, washed with ethanol and DI water more than 3 times, and dried in a lyophilizer for 2 days.

Synthesis of BiVO₄/Bi₂S₃ cruciate heterostructures

0.015 g of BiVO₄ cruciate flakes were dissolved in 50 mL of DI water. After a certain amount of Na₂S·9H₂O aqueous solution was added dropwise to the suspension above, the resulting mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave at 150 °C for 5 h. By controlling the volume of the 0.024 M Na₂S aqueous solution during the anion exchange process and the calculated weight ratio of BiVO₄ : B₂S₃, R is selected as 5.6, 2.8, 0.8 and 0.4, and the corresponding samples were designated as S-1, S-2, S-3 and S-4, respectively.

Materials characterization

The crystallographic phases of the prepared samples were analyzed by powder X-ray diffraction (XRD, D8 ADVANCE, Bruker) at room temperature. The XRD curves were described with a sweep rate of 10° min⁻¹ using Cu-K_α radiation at 40 kV and 40 mA. The morphologies and structures were characterized by field emission scanning electron microscopy (FE-SEM) on an XL30 ESEM FEG scanning electron microscope conducted at 15.0 kV. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken on a JEM 200CX TEM apparatus. Energy dispersive X-ray spectroscopy (EDX) and high-angle annular dark field (HAADF) images were also obtained with a spectroscope attached to the HRTEM. The diffuse reflectance graphs were obtained over the range of 300–800 nm with a UV-vis spectrophotometer (UV-2550, Shimadzu) and transformed to the absorption spectrum on the basis of the Kubelka–Munk relationship. The chemical states were analysed by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific) and were calibrated using the reference binding energy of C 1s at 284.6 eV.

Photoelectrochemical measurement

The photoelectrochemical response was measured using an electrochemical workstation (CHI-630D, Shanghai Chenhua) in a three-electrode cell under AM 1.5G illumination (standard 100 mW cm⁻²) cast by an Oriel 92251A-1000 sunlight simulator and was calibrated using the standard reference of a Newport silicon solar cell. The as-prepared BiVO₄ or BiVO₄/Bi₂S₃ cruciate heterostructures were coated onto a FTO glass with an area of 1 × 1 cm² and employed as the working electrode. The BiVO₄ and BiVO₄/Bi₂S₃ electrodes were then annealed at 450 °C and 350 °C for 30 min, respectively. A Pt foil served as the counter electrode, Ag/AgCl in saturated KCl as the reference electrode, and 0.02 M Na₂SO₄ aqueous solution as the working electrolyte. The active area of the BiVO₄ sample was fixed to 0.28 cm² by using a black mask. The photocurrent of the sample was measured from the back side (electrolyte–substrate interface). A cyclic voltammetry method was adopted with a scan rate of 10 mV s⁻¹.

Measurement of photocatalytic activity

The photocatalytic activity of the as-prepared BiVO₄ and BiVO₄/Bi₂S₃ cruciate heterostructures was evaluated by photocatalytic reduction of Cr⁶⁺ irradiated with a visible-light source (λ > 420 nm). Before the photocatalytic test, 300 mL of 10 mg L⁻¹ K₂Cr₂O₇ aqueous solution (pH 5.5) and 100 mg BiVO₄ or BiVO₄/Bi₂S₃ cruciate heterostructures were mixed and magnetically stirred for 60 min in the dark to achieve Cr⁶⁺ adsorption equilibrium. 4.0 mL of the suspension was taken out of the reactor every 30 min and separated by centrifugation. The Cr⁶⁺ concentration was measured by a colorimetric method with diphenylcarbazide.³¹

Results and discussion

BiVO₄ cruciate flakes are prepared by hydrothermal treatment of NH₄V₄O₁₀ (Fig. S1, see the ESI†). The crystallographic structure of BiVO₄ can be confirmed by XRD analysis (Fig. 1). All the diffraction peaks can be indexed to the monoclinic phase of BiVO₄ with lattice constants of a = 5.195, b = 11.70 and c = 5.092 (JCPDS card no. 14-0688). A series of BiVO₄/Bi₂S₃ cruciate heterostructures are prepared by a facile anion exchange method using BiVO₄ cruciate flakes as the precursor and Na₂S as the sulfidation reagent (Fig. S1†). For samples S-1–S-4, the diffraction peaks can be well assigned to the monoclinic phase of BiVO₄, as well as the orthorhombic phase of Bi₂S₃ (JCPDS card no. 17-0320, a = 11.14, b = 11.30 and c = 3.981), confirming the formation of BiVO₄/Bi₂S₃ heterostructures. With the increasing amount of Na₂S, the diffraction intensity of the (040) plane of BiVO₄ deceases, while the intensity of that of Bi₂S₃ becomes stronger, indicating the increasing content of Bi₂S₃ in the BiVO₄/Bi₂S₃ heterostructures.

Fig. 1 XRD patterns of BiVO₄ and the series of BiVO₄/Bi₂S₃ heterostructures.

Fig. S2† shows the high-resolution XPS spectra of BiVO₄ and S-3. The peaks with binding energies of 163.8 eV and 158.6 eV are attributed to Bi 4f7/2 and Bi 4f5/2 in BiVO₄ (Fig. S2a₂†). The protruding part in Fig. S2a₁† (161.3 eV) located between Bi 4f5/2 and Bi 4f7/2 could be assigned to S 2p.^32,33 According to Fig. S2b,† the peaks with binding energies of 529.6 eV and 531.7 eV are attributed to O 1s in BiVO₄ and BiVO₄/Bi₂S₃, respectively. The V 2p peaks with binding energies of 523.5 eV and 516.3 eV for V 2p are in good agreement with those in a previous report (Fig. S2c†).³⁴ The appearance of Bi, S, O and V elements in XPS confirms the formation of Bi₂S₃ on the BiVO₄ flakes. Relative intensities of V 2p in the BiVO₄ flakes are obviously stronger than those in the BiVO₄/Bi₂S₃ heterostructures, indicating a decrease in the amount of BiVO₄ in BiVO₄/Bi₂S₃ due to the anion exchange reaction.³⁵

As shown in Fig. 2, the structure and morphology of the as-obtained BiVO₄ are characterized by FE-SEM and TEM. The FE-SEM images show that the BiVO₄ cruciformities used as precursors have a flake-like structure with a diameter of 600–900 nm and a thickness of 80 nm (Fig. 2a and b). The high-resolution FE-SEM images further suggest the smooth top and bottom surfaces of these cruciate flakes (Fig. 2c). From the selected area electron diffraction (SAED) results, the patterns can be assigned to the (200) and (002) planes of the monoclinic BiVO₄ phase (Fig. 2f). Thus, the surface of the BiVO₄ flakes can be confirmed as the [010] plane.

Fig. 2 (a)–(c) FE-SEM, (d and e) TEM and (f) SAED images of BiVO₄ cruciate flakes.

Sample S-3 of the as-prepared BiVO₄/Bi₂S₃ heterostructures is characterized in detail to study the formation process of the special hollow structures. The size and shape of the cruciate BiVO₄ flakes are well duplicated and the precursors are converted into hollow heterostructures (Fig. 3). Some broken ones clearly confirm the hollow cruciate feature of the BiVO₄/Bi₂S₃ heterostructures (Fig. 3b and c), which can enable the full use of visible light and the illumination from the entries and cavities through multiple reflections. The surface of the heterostructures becomes much rougher compared with that of the BiVO₄ flakes (Fig. 3d). With the increasing amount of Na₂S, the surfaces of obtained BiVO₄/Bi₂S₃ heterostructures become much rougher (Fig. S3†). The detailed structure of the as-prepared BiVO₄/Bi₂S₃ heterostructures is analyzed by TEM observation (Fig. 4). It is anticipated that the initial anion exchange reaction with the assistance of hydrothermal treatment generates a thin Bi₂S₃ layer around the surface of the BiVO₄ flakes. A further anion exchange reaction between BiVO₄ and S²⁻ ions leads to the evolution of solid cruciformities into hollow heterostructures. The slower inward diffusion rate of S²⁻ anions than the outward diffusion rate of Bi³⁺ cations leads to the evacuation of Bi³⁺ in the inner region.^16–18 As a result, the surface components of BiVO₄/Bi₂S₃ are mainly Bi₂S₃ while the residual BiVO₄ is inside.²³

Fig. 3 (a)–(d) FE-SEM images of S-3. (b) and (c) indicate BiVO₄/Bi₂S₃ with hollow structures. The circle in (d) shows the entry of the tube.

Fig. 4 (a) TEM image, (b) HRTEM image and the inset SAED pattern, (c) HAADF images of a single BiVO₄/Bi₂S₃ cruciate flake (S-3), and the corresponding elemental mappings of (d) Bi, (e) O, (f) S, and (g) V elements.

The TEM images shown in Fig. 4 further indicate the hollow structure of the BiVO₄/Bi₂S₃ heterostructures. Lattice fringes with a d-spacing of 0.278 nm and 0.282 nm are assigned to the (400) and (040) planes of the orthorhombic Bi₂S₃ phase, respectively, which confirms the formation of Bi₂S₃ on the surface of BiVO₄ (Fig. 4b). The SAED result shown in the inset of Fig. 4b indicates the top surface of the BiVO₄/Bi₂S₃ flake as the [001] plane. The HAADF images clearly show the curling of flakes into tubular structures (Fig. 4c). Elemental mapping analysis of the BiVO₄/Bi₂S₃ heterostructure confirms the homogeneous distribution of Bi, V, O and S elements. According to the results shown in Fig. S3,† the S/V atomic ratio keeps increasing as the amount of Na₂S increases, which is consistent with the XRD result shown in Fig. 1. Furthermore, the series of BiVO₄/Bi₂S₃ heterostructures exhibit a similar cross-like shape except for the surface roughness.

By UV-visible diffuse-reflectance spectroscopy, BiVO₄ cruciate flakes and a series of BiVO₄/Bi₂S₃ heterostructures are investigated (Fig. 5). The BiVO₄ cruciate flakes exhibit a weak visible-light absorption range of ∼525 nm because of their intrinsic bandgap. When R is 5.6, S-1 exhibits nearly the same absorption band as that of BiVO₄. On further increasing the amount of Bi₂S₃, the obtained BiVO₄/Bi₂S₃ heterostructures exhibit an obvious red shift into the visible region. A broad visible-light absorption band at ∼800 nm is detected in S-4, when the weight ratio of V to S is decreased to 0.4. This indicates a possible electronic coupling effect between BiVO₄ and Bi₂S₃. The color change from yellow to black also indicates the broadening visible light absorption edge of the series of BiVO₄/Bi₂S₃ heterostructures (Fig. S1†). Electron–hole pairs can therefore be greatly generated by BiVO₄/Bi₂S₃ heterostructures under visible-light irradiation compared to BiVO₄ alone.

Fig. 5 UV-vis diffuse reflectance spectra of BiVO₄ and BiVO₄/Bi₂S₃ heterostructures.

Fig. 6 shows the photocatalytic reduction of Cr⁶⁺ with BiVO₄ and hollow BiVO₄/Bi₂S₃ cruciate heterostructures under visible-light irradiation. With the increasing weight ratio of S to V, the photocatalytic activity of BiVO₄/Bi₂S₃ heterostructures is significantly improved. S-3 with an R of 0.8 has the highest photocatalytic activity, and Cr⁶⁺ can be completely degraded within 120 min. Further loading B₂S₃ contrarily decreases the photoreduction activity of S-4, probably due to excess Bi₂S₃ blocking the charge transport.³⁶ The CB and VB for BiVO₄ are estimated to be 0.35 and 2.71 eV and 0.12 and 1.42 eV for Bi₂S₃.³⁷ The combination of BiVO₄ and Bi₂S₃ will form an ideal heterostructure with rapid photoinduced charge separation and decreased chance of recombination of electron–hole pairs by the synergetic effect.^{32,33,36–38} The band alignment of the elegant heterostructure facilitates the photogenerated electrons and holes in BiVO₄ and Bi₂S₃, respectively, and results in a spatial charge separation for long survival (Fig. S4†).

Fig. 6 Photocatalytic reduction of Cr⁶⁺ with BiVO₄ and the series of BiVO₄/Bi₂S₃ heterostructures under visible-light irradiation.

A kinetic model is also discussed to better understand the enhanced photoreduction activity of the series of BiVO₄/Bi₂S₃ heterostructures. When the initial concentration of the target is relatively lower, the Langmuir–Hinshelwood (L–H) model can be expressed as:³⁹

The apparent first-order constant is determined to be 0.00141 (BiVO₄), 0.00185 (S-1), 0.0036 (S-2), 0.02804 (S-3) and 0.02647 (S-4). It is clear that the reaction rate of S-3 has increased by ∼20 times compared with BiVO₄ alone, indicating the exact role of the BiVO₄/Bi₂S₃ heterostructures. Furthermore, the heterostructures of S-3 with a broader visible-light absorption range have higher photocatalytic performance than S-1 and S-2, implying that the enhanced photoreduction activity for Cr⁶⁺ is also subject to the broadened visible-light absorption range. Meanwhile, the balance between the amount of B₂S₃ and the increased visible-light absorption range should be considered, as the excess Bi₂S₃ could block the transportation of the photo-induced charge.³⁶

According to the results of the transient photocurrent response, S-3 shows highly promoted current generation, which is approximately 6.0 times higher than that of BiVO₄ (Fig. 7). The prominently enhanced photocurrent intensity of the BiVO₄/Bi₂S₃ heterostructures further proves the efficient separation of the photogenerated electrons and holes and the lower recombination probability under visible-light irradiation.⁴⁰

Fig. 7 Transient photocurrent response of BiVO₄ and S-3 under visible-light illumination.

Conclusions

In summary, unique BiVO₄/Bi₂S₃ cruciate hollow heterostructures are successfully constructed via an anion exchange reaction with BiVO₄ used as the precursors. The hollow BiVO₄/Bi₂S₃ cruciate heterostructures exhibit excellent photocurrent response and photocatalytic activity for reduction of Cr⁶⁺ under visible-light illumination. The work here provides a new design and further exploration of novel heterostructures.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by 973 Programs (No. 2014CB239302 and 2013CB632404), the NSF of China (No. 21773114, 21603183 and 21473091), the NSF of Jiangsu Province (No. BK2012015 and BK20130425), the Jiangsu Postdoctoral Science Foundation (1601062B) and the National Laboratory of Solid State Microstructures (M31044).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cy01899e

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