Epitaxial growth of Bi₂S₃ nanowires on BiVO₄ nanostructures for enhancing photoelectrochemical performance

Abstract

In this paper, a novel Bi₂S₃/BiVO₄ heterojunction film was prepared by a facile drop-casting and hydrothermal method for the first time. The as-prepared films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultraviolet visible spectrometry (UV-Vis). Interestingly, the heterojunction film was formed by epitaxial growth of Bi₂S₃ nanowires on BiVO₄ nanostructures and exhibited a good visible light absorption performance. Photoelectrochemical (PEC) hydrogen generation was demonstrated using the prepared films as photoanodes. The heterojunction photoelectrode showed an excellent PEC activity and generated a photocurrent density of 7.81 mA cm⁻² at 0.9761 V vs. RHE (0.1 V vs. Ag/AgCl) in the electrolyte solution containing 0.35 M Na₂SO₃ and 0.25 M Na₂S. The present study provides new insight into the design of highly efficient heterojunction photoelectrodes for hydrogen generation.

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1. Introduction

The photoelectrochemical (PEC) cell is considered to be one of the most promising devices to convert solar energy into hydrogen by water splitting. Since it was first reported as a concept in 1972,¹ more and more attention has been paid to PEC water splitting using semiconductor materials as photoanodes.^2–5 Although TiO₂ is most often investigated as a photoanode for PEC water splitting, its photocatalytic activity is only in the UV region due to the large band gap of 3.2 eV.^6,7 Thus, recent research has focused on the development of new photoanode materials with a narrow band gap.^4,8–10 As a member of the narrow bandgap semiconductor class, BiVO₄ (band gap ∼2.4 eV) has been widely studied as a photoanode for PEC water splitting because of its high theoretical conversion efficiency of 9.1% and good photochemical stability.^11–15 However, the PEC activity of the BiVO₄-based photoanode is not ideal enough for practical application. To date, the BiVO₄-based photoanodes have exhibited quite low incident photon conversion efficiencies (IPCE) and poor photocurrent density due to rapid recombination of photogenerated electrons and holes.¹² Meanwhile, its band gap is still too wide to harvest more solar energy (λ > 520 nm).¹⁶ Recently, considerable efforts have been made to reduce the recombination of photogenerated electrons and holes and expand the spectral response region, such as elemental doping (e.g. Mo,^17–19 W¹⁴ and P¹¹), loading co-catalysts^20–23 and heterojunction structure formation.^24–27

In contrast to single-component photoelectrodes, composite heterojunction of two semiconductors with different energy levels can form an ideal system with rapid photogenerated charge separation and decreased recombination of electron–hole pairs by the synergetic effect.²⁸ In addition, it is a significant influence on PEC activity to the structure of semiconductor electrode. However, the simple bilayer structure of heterojunction system now is the most prevalent. Especially, the heterojunction nanostructure formed by epitaxial growth on the surface of one material has many distinct advantages, such as the reduced surface states of the nanocrystal, low lattice mismatch between the two materials and fast charge transport.^29–31

Narrow bandgap semiconductor coupled wide bandgap semiconductors have shown excellent PEC hydrogen generation performance using sulphide as a hole sacrificial agent.^7,32–37 Among the various narrow bandgap semiconductor materials, Bi₂S₃ is a well-known semiconductor material (∼1.3 eV) and considered as a promising material for the fabrication of PEC devices and photovoltaic cells because of its large absorption coefficient, narrow band-gap and reasonable IPCE.^35,38–41 The studies pioneered by Ma have shown that the Bi₂S₃/BiVO₄ heterojunction photocatalyst prepared by anion exchange could not only extend the spectral responsive range but also promote the separation of photogenerated carriers.⁴² Recently, Gao synthesized heterostructured BiVO₄/Bi₂S₃ hollow discoids that exhibited significantly enhanced photocatalytic activity for reduction of Cr^VI under visible-light illumination.²⁸ However, the current architectures used for Bi₂S₃/BiVO₄ heterojunction systems are only limited to a simple coating structure. Moreover, the Bi₂S₃/BiVO₄ heterojunction has not been effectively combined into a semiconductor electrode and its photocatalytic effect on PEC hydrogen generation has not been demonstrated and studied yet.

Herein, we report the synthesis of Bi₂S₃/BiVO₄ heterostructures film through an epitaxial growth of Bi₂S₃ nanowires on BiVO₄ nanostructures using low-cost techniques. The as-prepared films were characterized by SEM, TEM, XRD and UV-Vis. The PEC hydrogen generation was demonstrated using the prepared films as photoanodes. The PEC properties of the photoelectrodes were evaluated. A high photocurrent density of 7.81 mA cm⁻² is achieved at 0.9761 V vs. RHE under the irradiation of visible light.

2. Experiment section

2.1. Synthesis and characterization

The BiVO₄ films were prepared by a cost-effective drop-casting method. Bi(NO₃)₃·5H₂O in glacial acetic acid (0.2 M, 0.9 mL) and vanadyl acetylacetonate in acetylacetone (0.03 M, 6 mL) were mixed with a 1 : 1 mol ratio of Bi : V. 0.15 g polyethylene glycol 1000 was added to the above solution and dissolved completely to form a precursor solution. Next, 120 μL of precursor solution was drop-cast on FTO glass substrates (1 × 2 cm² Japan NSG 8 glass) that were cleaned by acetone, ethanol and distilled water, respectively. The sample was dried in 25 °C for 24 h and then annealed at 500 °C in air for 1 hour, obtaining a yellow film on the FTO glass substrates.

Bi₂S₃/BiVO₄ heterojunction films were synthesized by the hydrothermal method. In a typical synthesis, 0.1 g thiourea as the sulphur source was dissolved in 50 mL of DI water at room temperature. Then, the solution was transferred into an 80 mL of Teflon-lined stainless autoclave. The as-prepared BiVO₄ film was placed into the autoclave. The hydrothermal synthesis was carried out at 170 °C for 6 h. The as-prepared films were dried at 200 °C for 2 h.

The crystalline phase of the film was characterized by X-ray powder diffraction (XRD, Rigaku D/Max2500, Japan). Scanning electron microscope (SEM, Nova NanoSEM 230) was used to investigate the surface morphology of the thin films. Transmission electron microscopy (TEM, TECNAI G2 F20, FEI) was operated at an accelerating voltage o 200 kV to observe the microstructure and the crystallinity of samples. The UV-Vis absorption spectra were obtained using a spectrophotometer (DR-UVS, Shimadzu 2450 spectrophotometer).

2.2. Electrical and photoelectrochemical measurements

The PEC properties were investigated in a typical three-electrode electrochemical cell using an electrochemical analyzer (Zennium, Zahner, Germany). The synthesized films were employed as the working electrode and a platinum foil and an Ag/AgCl/satd. KCl electrode were employed as the counter and the reference electrodes, respectively. The illumination source was a 150 W xenon lamp (CHF-XM35, Beijing Trusttech Co. Ltd) with a 400 nm cutoff filter to remove UV irradiation. IPCE measurements were conducted using a xenon lamp (150 W, Oriel) with an AM 1.5 filter and a monochromator with a bandwidth of 5 nm.

3. Results and discussion

3.1. SEM, EDS, HR-TEM, XRD and UV-Vis optical absorption analysis

In order to better understand the possible formation process of Bi₂S₃ nanowires, the possible mechanism is shown in Fig. 1. During the hydrothermal reaction process, the S²⁻ can be released by the decomposition of thiourea first, and then the BiVO₄ particles with a good crystallographic structure will react with the S²⁻ to form the nucleation of Bi₂S₃ on the surface of BiVO₄ particles, due to the smaller solubility of Bi₂S₃. With the reaction time increasing, the Bi₂S₃ wires will be selectively formed slowly on the BiVO₄ particles because of its intrinsic nature.^43,44 Bi₂S₃ is a highly anisotropic semiconductor material with layer structure that parallel to the growth direction. So the Bi₂S₃ wires are formed preferentially.⁴⁵

Fig. 1 Depiction of the possible formation process of Bi₂S₃/BiVO₄ films.

The SEM image of a bare BiVO₄ film on a FTO glass substrate is shown in Fig. 2a, in which porous structures are observed, consisting of wormlike particles with sizes of 50–500 nm. The cross-section SEM image of bare BiVO₄ film shows that the thickness of the film is about 2 μm (Fig. 2a inset). In comparison of bare BiVO₄ film, the surface morphology of Bi₂S₃/BiVO₄ film is different. It is evident that a lot of nanowires with ∼1 μm length form on the surface of wormlike particles (Fig. 2b). The local composition of Bi₂S₃/BiVO₄ film was investigated by energy dispersive X-ray spectrometer (EDS), and the result in Fig. S1† shows the existence of Bi, V, O and S. Fig. 3a shows low magnification TEM image of the Bi₂S₃/BiVO₄ sample. It is evident that the nanowires are interlaced with wormlike particles and a number of small flakes and dots cover the entire surface of the wormlike particles. High-resolution TEM images of the wormlike particles and the nanowires show d-spacings of 0.237 nm, which can be assigned to the (202) plane of monoclinic BiVO₄ (Fig. 3b), and 0.326 and 0.565 nm, corresponding to the (102) and (002) planes of Bi₂S₃ (Fig. 3b and c), respectively. It indicates that the nanowire is a large near single crystal Bi₂S₃ and a number of Bi₂S₃ nanoparticles with ∼10 nm size cover the entire surface of the wormlike BiVO₄ particle.

Fig. 2 SEM images of (a) the BiVO₄ and (b) Bi₂S₃/BiVO₄ films.

Fig. 3 (a) TEM image of Bi₂S₃/BiVO₄ sample, and HRTEM image of (b) a worm like particle and (c) a nanowire.

The crystallographic structure and phase purity of the as-prepared films were first examined by XRD analysis (Fig. 4a). The high intensity peaks at 26.5, 33.7, 37.9, 51.6, 54.6, 61.7 and 65.7° of all samples are due to FTO substrate. The bare BiVO₄ films show a monoclinic scheelite structure of BiVO₄ (JCPDS no. 75-2480). After growth of Bi₂S₃ on BiVO₄, new peaks appear at 17.6, 22.4, 25.1, 27.4, 28.6 and 33.1°, which can be indexed to Bi₂S₃ (JCPDS no. 84-0279). It indicates that BiVO₄ can be turned into Bi₂S₃ after the hydrothermal process. The optical behaviour of the as-prepared films was investigated by the ultraviolet-visible absorbance spectra. As shown in Fig. 4b, a clear absorption edge around 510 nm in the absorption spectrum of the BiVO₄ film can be observed, corresponding to its band gap energy. By contrast, the absorption edge of Bi₂S₃/BiVO₄ film at the near-infrared region (>850 nm), which is red-shifted from the 510 nm, is due to the absorption caused by Bi₂S₃. The photograph of the as-prepared film (Fig. 4b inset) can also support the above result. Compared with the bright yellow BiVO₄ film, the colour of Bi₂S₃/BiVO₄ film obviously turns to black.

Fig. 4 (a) XRD and (b) ultraviolet-visible absorbance spectra of the samples.

3.2. Photoelectrochemical study

The PEC properties of the as-prepared films were checked by linear sweep voltammetry (LSV). Fig. 5a shows the LSV collected from bare BiVO₄ film and Bi₂S₃/BiVO₄ heterojunction film in an aqueous solution containing 0.35 M Na₂SO₃ and 0.25 M Na₂S (pH ≈ 11.5) under and without illumination. The photocurrents increase with increasing potential revealing a typical n-type semiconductor behavior, because both BiVO₄ and Bi₂S₃ are n-type semiconductors. Compared to the bare BiVO₄ (4.20 mA cm⁻², 0.9761 V vs. RHE), the Bi₂S₃/BiVO₄ heterojunction films exhibit an enhanced PEC performance and the photocurrent density at 0.9761 V vs. RHE (0.1 V vs. Ag/AgCl) is up to 7.81 mA cm⁻². To evaluate possible synergistic effects between Bi₂S₃ and BiVO₄, IPCE were measured at 0.4761 V vs. RHE (−0.4 V vs. Ag/AgCl). Fig. 5b shows the IPCE spectra for the bare BiVO₄ film and Bi₂S₃/BiVO₄ heterojunction film. As shown in Fig. 5b, the Bi₂S₃/BiVO₄ film shows higher IPCE over the entire testing wavelength region than that of bare BiVO₄. But a photoresponse over the entire testing wavelength region was also observed for the bare BiVO₄. It may be due to the reaction between BiVO₄ and S²⁻ ions leading to formation of Bi₂S₃ on the surface of BiVO₄, which can be supported from the XRD pattern of BiVO₄ film after LSV measurement (Fig. S2†). The colour of the bare BiVO₄ films changed to black from the bright yellow once the films were dipped into the sulfide solution, which also supported the above idea. To check the possibility that the higher PEC activity obtained on the Bi₂S₃/BiVO₄ heterojunction film was merely due to the heterojunction formed by the hydrothermal method, we repeated the PEC tests in a mixed solution of water and ethanol (V_water : V_ethanol = 1 : 2) containing 0.2 M NaCl, in which ethanol can act as hole sacrificial agent instead of S²⁻. Fig. 5c and d show LSV and IPCE spectra, respectively, for the pure BiVO₄ film and Bi₂S₃/BiVO₄ heterojunction film in this mixed solution. It can be seen in Fig. 5c that the Bi₂S₃/BiVO₄ heterojunction films still present higher photocurrent than the bare BiVO₄ films. At 1.0106 V vs. RHE (0.4 V vs. Ag/AgCl), the photocurrent densities of Bi₂S₃/BiVO₄ and BiVO₄ are 0.51 and 0.29 mA cm⁻², respectively, which implies that the photocurrent was improved by 76% when the Bi₂S₃ nanoparticles and nanowires were coated on top of a BiVO₄ film. Fig. 5d shows the IPCE spectra for the bare BiVO₄ film and Bi₂S₃/BiVO₄ heterojunction film at 1.0106 V vs. RHE in 0.2 M NaCl mixed water–ethanol solution. The bare BiVO₄ film can generate charge carriers only below 510 nm, which corresponds to the 2.4 eV band-gap energy of BiVO₄. For the Bi₂S₃/BiVO₄ film, the photoresponse extends to the infrared region and IPCE values are much higher than those shown by bare BiVO₄ film over the whole visible range. The result is direct evidence that the formation of heterojunction can extend the spectral responsive range and promote the separation of photogenerated carriers.

Fig. 5 The LSV scans and IPCE of the samples in (a) and (b) an aqueous solution containing 0.35 M Na₂SO₃ and 0.25 M Na₂S (pH ≈ 11.5) and (c) and (d) a mixed solution of water and ethanol (V_water : V_ethanol = 1 : 2) containing 0.2 M NaCl, respectively.

To verify the stability of the photoanodes, the I–t curves were recorded under continuous illumination at 0.6761 V vs. RHE (−0.2 V vs. Ag/AgCl) in the solution containing 0.35 M Na₂SO₃ and 0.25 M Na₂S (pH ≈ 11.5). As shown in Fig. S3,† both the Bi₂S₃/BiVO₄ and BiVO₄ photoanodes show a gradual decrease in the photocurrent by more than 60% after a 7200 s operation. This decrease is due to the lower stability of chacogenide materials during PEC water splitting. Further, the photoconversion efficiencies of the photoanodes were calculated according to the following equation:⁴⁶

where ΔG^°_rev is the Gibbs free energy per photon required to spilt water, J_p is the photocurrent density, E_bias is the bias voltage and J₀ is the intensity of the incident light. As can be seen from Fig. 6a, the maximum photoconversion efficiency of Bi₂S₃/BiVO₄ photoelectrodes is 3.9%, which is almost double that of BiVO₄ photoelectrodes (1.9%).

Fig. 6 (a) Calculated photoconversion efficiency as a function of applied voltage vs. Ag/AgCl (b) the amounts of evolved H₂ for Bi₂S₃/BiVO₄ and BiVO₄ photoelectrodes in the solution containing 0.35 M Na₂SO₃ and 0.25 M Na₂S at −0.2 V vs. Ag/AgCl.

It is essential for PEC water splitting to evaluate the amount of the hydrogen evolution in the electrolytes that contain sacrificial species. Hydrogen generation was measured at 0.6761 V vs. RHE in the solution containing 0.35 M Na₂SO₃ and 0.25 M Na₂S as electrolyte. The evolved H₂ was collected and analyzed using gas chromatography (GC). As shown in Fig. 6b, the faradaic efficiency for H₂ production was calculated to be about 82%. The hydrogen evolution rates for Bi₂S₃/BiVO₄ and BiVO₄ electrode are 69.9 and 36.6 μmol cm⁻² h⁻¹, respectively.

To understand the origin of PEC performance enhancement in the heterojunction film, Mott–Schottky plots were measured at a frequency of 1 kHz. The Fig. 7 shows the Mott–Schottky plots of Bi₂S₃/BiVO₄ and bare BiVO₄ photoelectrodes in the 0.2 M NaCl mixed water–ethanol solution. Both photoelectrodes show an n-type characteristic with the positive slop. As we all known, the charge carrier density (N_d) can be calculated from the slops of the Mott–Schottky plots and the flat band potential (V_fb) is equal to the value of x-intercept. The slops of the Mott–Schottky plots of Bi₂S₃/BiVO₄ photoelectrodes is greater than that of bare BiVO₄ photoelectrodes (Fig. 6), suggesting the higher charge carrier density for Bi₂S₃/BiVO₄ photoelectrodes. The Bi₂S₃/BiVO₄ photoelectrode shifts the flat band potential to the negative direction, compared to that of the bare BiVO₄ photoelectrode. It is because of the higher conduction band (CB) position for the Bi₂S₃. Usually, the more negative flat band potential for semiconductor electrode is indicative of larger accumulation of electrons in the photoelectrodes and exhibits the better ability to facilitate the charge separation.⁴⁷

Fig. 7 Mott–Schottky plots of photoelectrodes obtained using a mixed solution of water and ethanol (V_water : V_ethanol = 1 : 2) containing 0.2 M NaCl.

4. Mechanism discussion

Fig. 8 shows the energy diagram of the Bi₂S₃/BiVO₄ heterojunction film and electron transport process. Under visible-light irradiation, both BiVO₄ and Bi₂S₃ are excited and the photoelectrons and holes are generated. The photogenerated electrons in the CB of Bi₂S₃ are easily transferred to the CB of BiVO₄ due to its higher CB position,⁴⁰ while holes are migrated in the opposite direction in the VB and react with sacrificial ion in the interface to avoid the photo-induced corrosion of Bi₂S₃.^28,42 Such a transfer way can prolong the lifetime of charge carriers by separating the photogenerated charges and reducing the chance for their recombination, resulting in an enhanced PEC activity.

Fig. 8 The energy diagram and electron transport process of the Bi₂S₃/BiVO₄ film.

5. Conclusion

In summary, the Bi₂S₃/BiVO₄ heterojunction films were prepared by a facile drop-casting and hydrothermal method for the first time. The heterojunction film was formed by epitaxial growth of Bi₂S₃ nanowires on BiVO₄ nanostructures and a high photocurrent density of 7.81 mA cm⁻² is achieved at 0.9761 V vs. RHE (0.1 V vs. Ag/AgCl) under the irradiation of visible light. This may be because the introduction of Bi₂S₃ can extend the spectral responsive range and the formation of heterojunction can promote the separation of photogenerated carriers.

Conflict of interest

The author's declare that there are no conflicts of interest.

Acknowledgements

This study was supported by the National Nature Science Foundation of China (No. 51304253), China Scholarship Council (CSC File No. 201406370157) and the Fundamental Research Funds for the Central Universities of Central South University (2014zzts015).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13171e

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