Fabrication and enhanced visible-light photocatalytic activities of BiVO₄/Bi₂WO₆ composites

Abstract

BiVO₄/Bi₂WO₆ composite photocatalysts were synthesized by coupling a homogeneous precipitation method with hydrothermal techniques. The as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), UV-Vis diffusion absorption spectroscopy (DRS) and photoluminescence (PL), respectively. The photocatalytic activities were evaluated by the degradation of methyl blue (MB) under visible-light irradiation. The results reveal that BiVO₄/Bi₂WO₆ composites exhibit higher photocatalytic activities than either pure Bi₂WO₆ or BiVO₄. The 0.3BiVO₄:0.7Bi₂WO₆ sample shows the best photocatalytic performance. The enhancement of photocatalytic activity is attributed to the improvement of light absorption, the excellent adsorbability, the narrow band gap and increasing separation rate of photo-generated charge carriers. The possible photocatalytic mechanism is discussed on the basis of the band structures of BiVO₄ and Bi₂WO₆.

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1. Introduction

In the past decades, semiconductor photocatalysts have attracted much attention due to their promising application in the area of solar energy conversion and degradation of environmental pollutants.^1,2 To date, TiO₂ has been proven to be the most popular photocatalyst for its high photocatalytic activity, good chemical stability, non-toxicity and low cost.³ Unfortunately, the excellent photocatalytic performance of TiO₂ has been confined to the ultraviolet light (λ < 400 nm) because of the large band gap (3.2 eV), which greatly restrict its practical application.⁴ Therefore, it is quite urgent to exploit new photocatalysts with visible-light-driven photocatalytic ability. In the study of semiconductor materials responding to the visible-light, many attempts have been carried out in the last decade. One method to obtain visible-light-driven photocatalysts is to broaden the photo-response of TiO₂ into visible light region, such as metal ion doping,⁵ nonmetal doping,⁶ noble metal deposition,⁷ and semiconductors coupling.⁸ In another approach, several researchers have developed novel photocatalysts with intrinsic visible-light-driven photocatalytic ability, for examples CaIn₂O₄,⁹ InVO₄,¹⁰ BiVO₄,¹¹ Bi₂WO₆¹² and bismuth oxyhalides.^13,14

Among various novel photocatalysts, the typical Aurivillius oxide Bi₂WO₆ has been considered as an excellent visible light photocatalyst used for degradation of organic pollutants and water splitting.^15,16 However, the photocatalytic activity of the pure Bi₂WO₆ is relatively low owing to its narrow absorption range of visible light (shorter than 450 nm) and rapid recombination of photo-induced electron–hole pairs.^17,18 To enhance its photocatalytic performance, doping Bi₂WO₆ with TiO₂,¹⁹ Fe,²⁰ BiIO₄,²¹ CdSe,²² SiO₂²³ have been taken to inhibit the recombination of photogenerated holes and electrons successfully.

As is known, BiVO₄ with a monoclinic scheelite structure shows photocatalytic activity for O₂ evolution and organic pollutants photodegradation under visible light irradiation.²⁴ What's more, BiVO₄ has suitable band edges (E_CB = 0.385 eV, E_VB = 2.685 eV), matching well with Bi₂WO₆ (E_CB = 0.515 eV, E_VB = 3.205 eV), which makes BiVO₄ to be a suitable material for constructing heterojunction with Bi₂WO₆. It's hoped that the BiVO₄/Bi₂WO₆ composite will exhibit good photocatalytic activity under visible light irradiation. Until now, few works have been concerned with BiVO₄ modification on visible-light photocatalyst Bi₂WO₆.

In this study, a series of BiVO₄/Bi₂WO₆ composites were synthesized by the combination of the homogeneous precipitation and hydrothermal methods and characterized by XRD, FTIR, SEM, EDS, DRS, and PL techniques. The visible light photocatalytic activities of BiVO₄/Bi₂WO₆ for degradation of MB were greatly improved as compared to either pure BiVO₄ or Bi₂WO₆. The mechanism of enhanced photocatalytic activity based on the band structures of BiVO₄ and Bi₂WO₆ was also discussed.

2. Experimental

2.1. Preparation of photocatalysts

The pure BiVO₄ was prepared by EDTA assisted coprecipitation method. In a typical synthesis, 10 mmol Bi(NO₃)₃·5H₂O was dissolved in 2 M nitric acid to obtain solution A. Meanwhile, 10 mmol of NH₄VO₃ was dissolved in 2 M NaOH solution, and then 2 g of EDTA was added into the solution to form solution B. The resulting solution B was added drop-wise into solution A under ultrasound and the pH of the final suspension was adjusted to 7 by using NaOH solution. After reacting for some time, the as-formed precipitates were filtration, washed, dried and finally calcined at 500 °C for 3 h.

The BiVO₄/Bi₂WO₆ composites were prepared by the combination of the homogeneous precipitation and hydrothermal process. In a typical synthesis, 5 mmol Bi(NO₃)₃·5H₂O and 2.5 mmol Na₂WO₄·2H₂O were first dissolved in 2 M HNO₃ solution and deionized water, respectively, and these two solutions were mixed together to get a stable mixture. Then, certain amounts of BiVO₄ powders were added to the prepared mixture solution to form suspensions with different BiVO₄ : Bi₂WO₆ mole ratios. The pH of the suspension was then adjusted to 7 by NaOH solution. After reacting for 1 h under ultrasound, the suspension was transferred into a Teflon-lined stainless steel autoclave and the hydrothermal reaction was carried out at 180 °C for 10 h. Then, the precipitates were filtered, washed and dried. Besides, the pure Bi₂WO₆ were also prepared for comparison by the same method.

2.2. Characterization of photocatalysts

The micro-structure and crystallinity of the photocatalysts were investigated by a D-MAX 2500/PC (Rigaku, Japan) X-ray diffractometer with the 2θ ranging from 10° to 70°. The morphology of the samples was examined by JSM-6700F Cold Field Emission Scanning Electron Microscope (JEOL, Japan). The energy-dispersive spectroscopy (EDS) was also performed during the SEM measurement. The Fourier transform infrared spectra (FT-IR) of the samples were obtained through a Nicolet 510P FT-IR spectrometer (Nicolet, America), using KBr as diluents. UV-Vis diffuse reflection spectroscopy (DRS) was determined with a UV-Vis-NIR spectrophotometer (Cary 500, America) in the range of 350–800 nm using BaSO₄ as the reference. The photoluminescence (PL) spectra, was recorded on a F-7000 fluorescence spectrophotometer (Hitachi, Japan) at room temperature with an excitation wavelength of 370 nm.

2.3. Photocatalytic activity tests

The photocatalytic activity of the samples was evaluated by the degradation of MB under visible light irradiation. A 300 W iodine-wolfram lamp was used as the light source, and a cut-filter was placed between the lamp and the reaction solution to remove the light with wavelengths less than 400 nm. In each experiment, 0.1 g photocatalyst was added into 100 mL MB solution (10 mg L⁻¹). Prior to illumination, the suspension was magnetically stirred in darkness for 30 min to ensure an adsorption/desorption equilibrium for MB solution and photocatalyst. At an irradiation interval of every 30 min, 5 mL suspensions were collected and centrifuged to remove the photocatalyst particles. Then the absorbance of MB was monitored with a UV-Vis spectrophotometer (UV 5100 B) at 664 nm during the photodegradation process.

3. Results and discussion

3.1. XRD analysis

Fig. 1 presents the XRD patterns of the as-prepared photocatalysts with different BiVO₄ contents. Fig. 1a shows that the peaks of pure Bi₂WO₆ were readily indexed to the orthorhombic phase of Bi₂WO₆ (JCPDS no. 39-0256). As revealed in Fig. 1e, the diffraction peaks of pure BiVO₄ agreed well with those of the monoclinic BiVO₄ (JCPDS card no. 14-0688). As for BiVO₄/Bi₂WO₆ composites (Fig. 1b–d), it can be seen that all of them exhibited a coexistence of both monoclinic BiVO₄ and orthorhombic Bi₂WO₆ phases, indicating that Bi₂WO₆ and BiVO₄ doped together successfully. The calcined BiVO₄ sample was less crystallized, resulting in more defects in the crystal, which were beneficial to the separation of the photo-generated electron–hole pairs in a sense.²⁵

Fig. 1 XRD patterns of different samples: (a) Bi₂WO₆, (b) 0.1BiVO₄:0.9Bi₂WO₆, (c) 0.3BiVO₄:0.7Bi₂WO₆, (d) 0.5BiVO₄:0.5Bi₂WO₆, (e) BiVO₄.

3.2. FT-IR analysis

FT-IR spectra of BiVO₄, Bi₂WO₆, and 0.3BiVO₄:0.7Bi₂WO₆ are shown in Fig. 2. The broad absorptions at 3445 cm⁻¹ and 1629 cm⁻¹ were related to H–O–H band of the adsorbed water molecules. Fig. 2a shows an intense and broad band that included the characteristic bands of BiVO₄ oxide. The symmetric and asymmetric stretching vibrations of V–O at 730 cm⁻¹ and 829 cm⁻¹, and the bending vibration band of Bi–O at 560 cm⁻¹.²⁶ The characteristic bands of Bi₂WO₆ (Fig. 2b) assigned to the symmetric and asymmetric stretching vibration of W–O (820 cm⁻¹, 734 cm⁻¹) and the stretching vibration of Bi–O (579 cm⁻¹) were found.

Fig. 2 FT-IR spectra of (a) BiVO₄, (b) Bi₂WO₆, and (c) 0.3BiVO₄:0.7Bi₂WO₆.

For the 0.3BiVO₄:0.7Bi₂WO₆ composite (Fig. 2c), the symmetric and asymmetric stretching vibrations of V–O shifted to 708 cm⁻¹ and 816 cm⁻¹ while the peak existing at 731 cm⁻¹ could be attributed to stretching vibrations of W–O. Besides, the bending vibration band of Bi–O at 576 cm⁻¹ was also observed. As compared to the peaks of functional groups of BiVO₄ and Bi₂WO₆, the 0.3BiVO₄:0.7Bi₂WO₆ had a similar spectrum, which indicated the BiVO₄ was successfully doped into Bi₂WO₆ in the hydrothermal reaction.

3.3. SEM and EDS observations

The SEM and EDS photographs of the samples are presented in Fig. 3. As shown in Fig. 3a, BiVO₄ was consisted of irregular blocky structures with the size ranging from 0.2 μm to 1.8 μm. the surfaces and edges of the mono-dispersed lump are not smooth, which was advantageous to form the heterojunctions between Bi₂WO₆ and BiVO₄. Bi₂WO₆ nanoplates with rod-shaped structures could be clearly observed in Fig. 3c. It could be speculated that these nanoplates with uneven edges may be formed by the bar agglomeration during hydrothermal process.

Fig. 3 SEM images and corresponding EDS patterns of BiVO₄ (a and b), Bi₂WO₆ (c and d), 0.3BiVO₄:0.7Bi₂WO₆ (e–g).

Low-magnification image of BiVO₄/Bi₂WO₆ compounds in Fig. 3e exhibited some flower-like aggregations. High magnification SEM image in Fig. 3f further revealed that these aggregations were formed by Bi₂WO₆ growth on the surface of BiVO₄. More importantly, Bi₂WO₆ scattered itself evenly over the surfaces of BiVO₄ blocks and developed close interactions with BiVO₄ which would further facilitate the formation of heterojunctions.

From Fig. 3b and d, one can see that the as-synthesized BiVO₄ and Bi₂WO₆ were consisted of Bi, V, O and Bi, W, O elements, respectively. EDS spectrum (Fig. 3g) was also performed to determine the Bi, W, V and O as major elements for BiVO₄/Bi₂WO₆ compounds.

3.4. UV-Vis DRS

Fig. 4 displays the diffuse reflectance spectra of BiVO₄/Bi₂WO₆ composite photocatalysts with different BiVO₄ contents. It can be noted that with the increasing of BiVO₄ content, the absorption intensity of composites increased with varying degrees as compared to pure Bi₂WO₆ in the region of 350–550 nm and the absorption edge had a significantly red shift. Additionally, the steep shape of the spectrum indicated that the visible light absorption was not due to the transition from the impurity level but was due to the band gap transition.²⁷

Fig. 4 UV-Vis diffuse reflectance spectra of the as-synthesized samples.

The absorption onsets of the samples were measured by linear extrapolation from the inflection point of the curve to the baseline, reaching a quantitative estimate of the band gap energies.²⁸ The band gap energy values (E_g, eV) for different samples were calculated using the equation:²⁹

E_g (eV) = 1240/λ (nm) (1)

where λ represents the wavelength of the absorption onset. The E_g of Bi₂WO₆, 0.1BiVO₄:0.9Bi₂WO₆, 0.3BiVO₄:0.7Bi₂WO₆, 0.5BiVO₄:0.5Bi₂WO₆ and BiVO₄ were 2.83, 2.79, 2.21, 2.29 and 2.34 eV, respectively. From the result, we can see that the 0.3BiVO₄:0.7Bi₂WO₆ sample showed the minimum band gap energy value, which implied it had the highest photocatalytic activity. In other words, the modification of BiVO₄ tended to narrow the band gap of Bi₂WO₆ photocatalyst, which would result in improvement of visible light responses and higher photocatalytic activities.

3.5. Photocatalytic degradation of MB under visible light irradiation

Photocatalytic performance of as-fabricated catalysts was evaluated by decomposition of MB under visible light irradiation. Fig. 5A shows the efficiencies of the photocatalytic degradation, where C_t is the concentration of MB during the reaction and C₀ is the initial concentration of MB solution (before adsorption equilibrium on BiVO₄/Bi₂WO₆ composites), respectively. As depicted in Fig. 5A, the removal rates of MB concentration in the presence of pure Bi₂WO₆ or pure BiVO₄ were only about 35.9% and 42.0% after 120 min under visible light irradiation. As for BiVO₄/Bi₂WO₆ composites, obvious enhancement of degradation efficiency was found. Obviously, the highest photocatalytic activity was achieved when the mole ratio of BiVO₄ and Bi₂WO₆ was 0.3 : 0.7. The adsorption quantity of Bi₂WO₆, 0.1BiVO₄:0.9Bi₂WO₆, 0.3BiVO₄:0.7Bi₂WO₆, 0.5BiVO₄:0.5Bi₂WO₆ and BiVO₄ after magnetically stirred in darkness for 30 min reaches 25.2%, 29.3%, 55.2%, 42.2% and 23.0%, respectively. The adsorbability of 0.3BiVO₄:0.7Bi₂WO₆ catalyst was also the best among all the samples, which proved that better adsorbability was also favorable for excellent photocatalytic performance.

Fig. 5 Photocatalytic degradation efficiency of MB by different photocatalysts under visible light (A) and the dynamic of MB degradation reaction (B).

It is well known that when the pollutant is within the millimolar concentration range, photocatalytic oxidation of organic pollutants follows first-order kinetics,³⁰ as shown by eqn (2):

ln (C₀/C_t) = kt + a (2)

where C₀ and C_t are the dye concentrations in solution at times 0 and t, respectively, and k is the apparent first-order rate constant. In Fig. 5B, it can be seen that the k was maximum when the BiVO₄ content was 0.3 (mole ratio), showing that the reaction rate of 0.3BiVO₄:0.7Bi₂WO₆ sample was the highest. Furthermore, the k for MB photodegradation over 0.3BiVO₄:0.7Bi₂WO₆ catalyst was about 4.0 and 2.3 times higher than that of pure Bi₂WO₆ and pure BiVO₄, respectively.

3.6. Photocatalytic mechanism

To further investigate the effect of the BiVO₄ modification, the PL spectra of BiVO₄/Bi₂WO₆ with 370 nm excitation wavelength was carried out. PL spectra was commonly employed to study the immigration, transfer, and charge carrier trapping processes and to investigate the fate of e⁻–h⁺ pairs in semiconductor particles.³¹ The lower PL intensity usually indicates the higher separation efficiency of photoinduced charge, thus the higher photocatalytic activity.³²

Fig. 6 PL spectra of pure BiVO₄, 0.3BiVO₄:0.7Bi₂WO₆ and pure Bi₂WO₆ photocatalysts recorded at room temperature with the excitation wavelength of 370 nm.

From Fig. 6, it was found that the tested three catalysts showed the main peaks at similar position but with different intensities: a strong sharp peak at 487 nm and a broad weak peak at 587 nm. The BiVO₄/Bi₂WO₆ composite showed diminished intensity in comparison to pure Bi₂WO₆ before 550 nm, indicating that the introduction of BiVO₄ would improve the separation efficiency of the carriers generated in Bi₂WO₆. Interestingly, the PL peak of pure BiVO₄ was lower than that of the BiVO₄/Bi₂WO₆ composite though BiVO₄ exhibited much lower photocatalytic activity under visible light, which was similar to the results of the reported document.³³ Further researches are still being carried out to reveal relevant mechanisms.

As we all know, the process of photocatalysis was propelled by the absorption of photon of the band gap to excite electrons from valence band to conduction band, and subsequently, separated electrons and holes moved to composite surface and react. Thus, the narrower the band gap is, the higher activity the photocatalyst has. After decorated by BiVO₄, the forbidden band of Bi₂WO₆ whose width was 2.83 eV dropped to 2.21 eV, the light absorption was enhanced and the recombination of electron–hole pairs was reduced. Based on the existing results, we can conclude that the enhancement of photocatalytic activity of BiVO₄/Bi₂WO₆ composite photocatalysts was due to the improvement of light absorption, the better adsorbability, the decrease of band gap energy and the lower recombination efficiency of e⁻–h⁺ pairs.

On the basis of reported researches^34,35 and our experiment results, we have proposed a possible photodegradation mechanism of the BiVO₄/Bi₂WO₆ composites:

BiVO₄/Bi₂WO₆ + MB → BiVO₄/Bi₂WO₆ − MB (3)

BiVO₄ + hν → BiVO₄ (h⁺ + e⁻) Bi₂WO₆ + hν → Bi₂WO₆ (h⁺ + e⁻) (4)

BiVO₄ (e⁻) + Bi₂WO₆ → Bi₂WO₆ (e⁻) + BiVO₄

Bi₂WO₆ (h⁺) + BiVO₄ → BiVO₄ (h⁺) + Bi₂WO₆ (5)

Bi₂WO₆ (e⁻) + O₂ → Bi₂WO₆ + O₂˙⁻

BiVO₄ (h⁺) + OH⁻ → BiVO₄ + ˙OH (6)

O₂˙⁻/˙OH + MB → degraded product (7)

The possible photo-catalytic mechanism can be schematically described in Fig. 7. The photogenerated electrons in Bi₂WO₆ were captured by O₂ to yield O₂˙⁻ and the holes in BiVO₄ are trapped by OH⁻ to produce ˙OH. O₂˙⁻ and ˙OH play an important role in the photodegradation process, and would further oxidize the MB molecules to form CO₂ and H₂O completely.

Fig. 7 Mechanism of photodegradation over BiVO₄/Bi₂WO₆ composite photocatalyst under visible light irradiation.

4. Conclusions

Visible-light-driven BiVO₄/Bi₂WO₆ photocatalysts with different mole ratios have been successfully prepared by the combination of the homogeneous precipitation and hydrothermal process. XRD patterns revealed that the as-prepared composites were composed of monoclinic BiVO₄ and orthorhombic Bi₂WO₆. The addition of BiVO₄ significantly improved the photocatalytic activity of Bi₂WO₆ under visible light irradiation. The optimized catalyst with a mole ratio of 0.3 : 0.7 (BiVO₄ : Bi₂WO₆) had the highest activity. It can be concluded that the enhancement of photocatalytic performance of BiVO₄/Bi₂WO₆ composite photocatalysts was because of the improvement of light absorption, the excellent adsorbability, the narrow band gap and high separation efficiency of photo-induced charges.

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