New insights into Ag-doped BiVO₄ microspheres as visible light photocatalysts

Abstract

This study describes the synthesis of Ag–bismuth vanadate (Ag–BiVO₄) microspheres, a highly efficient visible light photocatalyst for the degradation of methylene blue, via a one-step hydrothermal method. Multiple characterization techniques showed that bulk, monoclinic, needle-like BiVO₄ and Ag nanoparticles (50 nm diameter) formed microspheres (3–7 μm diameter) with a uniform size distribution. Compared with pure BiVO₄, the Bi–Ag microspheres showed significantly enhanced absorption of visible light (480 to 700 nm) during measurement of UV-Vis diffuse reflectance. Electron paramagnetic resonance measurements indicated that Ag doping enhanced photocatalytic performance because it facilitated separation and transfer of photo-generated electrons and electron holes. This study provides a cost-effective approach for synthesizing Bi–Ag microspheres with enhanced photocatalyst performance for environmental and energy applications.

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1. Introduction

Photocatalysis technologies based on semiconductor materials that convert solar energy into chemical energy can be used to neutralize organic contamination.^1,2 More than 190 different semiconductors are suitable photocatalysts for photoelectron chemical applications.³ Bismuth vanadate (BiVO₄), a promising photocatalyst that uses visible light, has received considerable attention. BiVO₄ has three primary crystal structures: tetragonal zircon, tetragonal scheelite, and monoclinic scheelite. The monoclinic scheelite phase of BiVO₄ has a band gap of 2.4 eV and exhibits much higher photocatalytic activity in visible light in comparison with that of the other two phases.^4–6 Nevertheless, the monoclinic scheelite phase of BiVO₄ has very low quantum efficiency because photo-generated electron–hole pairs recombine relatively rapidly.^7–9

Recombination of electron–hole pairs can be inhibited by the deposition of noble metals onto the surface of BiVO₄.¹⁰ In comparison with nanoparticles of other noble metals, Ag nanoparticles have attracted considerable attention because of their unique physicochemical properties and their potential in optical applications and as co-catalysts of semiconductors.^11,12 The incident visible light on the surface of Ag nanoparticles can induce collective oscillations of conduction electrons with a resonant frequency in a phenomenon known as localized surface plasmon resonance (SPR), which is associated with strong absorption of visible light.^13–15 Ag nanoparticles can facilitate electron transfer from the conduction band of BiVO₄.¹⁶ Thus, Ag nanoparticles can significantly improve the quantum efficiency of semiconductor materials.¹³ However, a hydrothermal method of synthesizing small, highly monodisperse Ag–BiVO₄ microspheres has not been reported.

Good morphology also can improve catalyst's activity as well as absorption capacity of light.^17,18 Here, we describe a hydrothermal method for synthesizing Ag–BiVO₄ microspheres. Compared with other morphology, this 3D microspheres structure, with reflecting and scattering effects, can increase the response range and absorption of light.¹⁹ Our results demonstrate that bulk, monoclinic, needle-like BiVO₄ and Ag nanoparticles form microspheres (3–7 μm diameter) with a uniform size distribution, facilitating transport of photo-generated electrons in BiVO₄ and thereby reducing the recombination rate of photo-generated charge carriers in the coupled Ag–BiVO₄ composite system. Moreover, we demonstrate separation of the electron–hole pair. Finally, we show that photodegradation of methylene blue (MB) catalyzed by Ag–BiVO₄ microspheres was more efficient than that catalyzed by pure BiVO₄ under visible light irradiation.

2. Experimental

2.1. Experimental materials

The following analytically pure chemicals were used: bismuth nitrate (Bi(NO₃)₃·5H₂O, Chengdu Area of the Industrial Development Zone Xinde Mulan, Chengdu, China), silver nitrate (AgNO₃, 99.0%, Chongqing Chuandong Chemical Company, Chongqing, China), 25% ammonia solution (NH₃·H₂O, Chongqing Chuandong Chemical Company), sodium hydroxide powder (NaOH, Chongqing Chuandong Chemical Company), polyvinylpyrrolidone (PVP, Chengdu Area of the Industrial Development Zone Xinde Mulan), ammonium metavanadate (NH₄VO₃, Chongqing Chuandong Chemical Company), nitric acid (HNO₃, Chengdu Area of the Industrial Development Zone Xinde Mulan), MB dye (Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China), and ethylene glycol (Chongqing Chuandong Chemical Company, Chongqing, China).

2.2. Synthesis of Ag–BiVO₄ microspheres

In a typical experiment, 10 mmol of Bi(NO₃)₃·5H₂O and 2.0 mL of PVP (0.5%) were dissolved in 50 mL of 2 mol L⁻¹ HNO₃ solution, after which the pH was adjusted to 7 with a solution of NH₃·H₂O. Next, 10 mmol of NH₄VO₃ was dissolved in 50 mL of 2 mol L⁻¹ NaOH solution. These two solutions were combined and stirred vigorously for 30 min while 0.02 g of sodium dodecyl benzene sulfonate was added to the resulting solution. The pH of the mixed solution was adjusted to 7 with a solution of NH₃·H₂O. Separately, 323.9 mg AgNO₃ powder was dissolved in 10 mL of ethylene glycol, after which the resulting solution was added to the mixed BiVO₄ solution. The resulting solution of BiVO₄ and AgNO₃ was sonicated for 1 h, forming a homogeneous suspension, which was transferred to a 100 mL Teflon-sealed autoclave, which was maintained at 473.15 K for 18 h to achieve crystallization of the composite. The resulting precipitate was centrifuged, washed eight times with ethanol, and dried in a vacuum freeze drier at −60 °C for 40 h. The synthesized binary composite was designated as Bi–Ag microspheres. The process used to produce Bi–Ag microspheres is illustrated in Scheme 1.

Scheme 1 Procedure for the synthesis of Bi–Ag microspheres.

2.3. Characterization

Powder X-ray diffraction (XRD) spectra were acquired with a Rigaku D/Max-rB diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was performed with an Al Kα X-ray source (Thermo Fisher Scientific, K-Alpha, UA), and all binding energy values were corrected by calibration to the C 1s peak at 284.6 eV. Scanning electron microscopy (SEM) images were acquired with a JSM-7800F JEOL microscope. Energy dispersive X-ray (EDX) images were acquired with an EDX-100A-4. UV-Visible diffuse-reflectance spectroscopy (UV-Vis DRS) was performed with a Hitachi U-3010 UV-Vis spectrometer. Electronic paramagnetic resonance (EPR) analyses were performed using a JES FA200 X-band EPR spectrometer operating in the X-band at 0.907 GHz and 0.998 mW. Portions of the solid samples (50 mg) were introduced into a spectroscopic quartz probe cell, after which measurements were obtained at room temperature under visible light irradiation in air.

2.4. Evaluation of visible light photocatalytic activity

Photocatalytic degradation of MB was assessed in a water-cooled aqueous solution illuminated with visible light at room temperature. The as-prepared semiconductor material (50 mg) was mixed with an aqueous solution of MB (200 mL, 5 mg L⁻¹), followed by magnetic stirring in the dark for at least 30 min to establish the adsorption–desorption equilibrium of MB on the surface of pure BiVO₄ and Bi–Ag microspheres. The reaction system was then subjected to visible-light irradiation using a Xe lamp (500 W; λ ≥ 400 nm). A series of aqueous samples (8 mL) were collected and centrifuged at 10 000 rpm for 10 min to remove essentially all of the catalyst. The concentration of MB was determined by monitoring its characteristic absorption at 665 nm on a UV-Vis spectrophotometer.

3. Results and discussion

3.1. X-ray diffraction and XPS spectra

Fig. 1a shows the XRD patterns of pure BiVO₄ and Bi–Ag microspheres. All primary diffraction peak patterns had good correspondence with that of monoclinic scheelite BiVO₄ (JCPDS 14-0688), indicating that highly pure monoclinic scheelite BiVO₄ was obtained using a one-step method.²⁰ The Ag–BiVO₄ diffractogram showed some new peaks that were not present in that of pure BiVO₄. The peaks at 38.1°, 44.3°, and 64.4° were assigned to the (111), (200), and (220) planes of face-centered cubic (FCC) Ag (JCPDS card no. 65-2871),²¹ respectively.

Fig. 1 (a) XRD patterns of pure BiVO₄ and Bi–Ag microspheres; (b) survey XPS spectrum of the as-obtained Bi–Ag microspheres; (c) Ag 3d spectrum; (d) Bi 4f spectrum; (e) V 2p spectrum; (f) O 1s spectrum.

The survey XPS spectrum (Fig. 1b) shows that the composite contained Ag, Bi, O, and V. The high-resolution XPS spectrum of the Bi–Ag microspheres (Fig. 1c) shows that the peaks at binding energies of 374.1 and 368.1 eV can be ascribed to the Ag 3d_3/2 and Ag 3d_5/2 signals, respectively, suggesting the presence of metallic Ag.²² The high-resolution XPS spectra of Bi 4f, V 2p, and O 1s are shown in Fig. 2d–f. The strong peaks at 164.66 eV and 159.36 eV (Fig. 2d) corresponding to Bi 4f_5/2 and Bi 4f_7/2 are characteristic of Bi³⁺, indicating that the Bi in the composite is Bi³⁺.²³ The V 2p peaks at 524.26 eV and 516.74 eV are assignable to V 2p_1/2 and V 2p_3/2 signals, respectively (Fig. 2e), indicating that the V species in the composite is V⁵⁺.²⁴ The XRD and XPS results demonstrate conclusively that the tested sample consisted of Bi–Ag microspheres.

Fig. 2 SEM images of (a) Bi–Ag microspheres at low magnification and (b) high magnification.

3.2. SEM and energy dispersive X-ray

Fig. 2 shows SEM images of Bi–Ag microspheres. Fig. 2a demonstrates that microspheres with diameters ranging from 3–7 μm were formed. Fig. 2b shows the structure of a single microsphere. The Bi–Ag microsphere structure creates a good interface between BiVO₄ and the Ag nanoparticles; the Ag nanoparticles can facilitate electron transfer from the conduction band of BiVO₄, which reduces the recombination rate of the photo-generated electron–hole pairs. In addition, Fig. 2b shows mass-dispersive Ag nanoparticles in the composite, which produce SPR and enhance absorption of visible light. Therefore, we expect Bi–Ag microspheres to have relatively good photocatalytic activity.

To determine the spatial distribution of elements in the Bi–Ag microsphere structure, elemental mapping was performed using a single Bi–Ag microsphere. The composite was composed primarily of Ag, Bi, O, and V (Fig. 3). A comparison of the spectra indicated that the uniformly distributed microspheres consisted primarily of BiVO₄ and Ag nanoparticles. The SEM and EDX results suggest that the BiVO₄ and Ag nanoparticle composites were successfully synthesized using the one-step hydrothermal method. These results are consistent with, and support, those of the XRD and XPS analyses.

Fig. 3 Energy dispersive X-ray spectrum of the Bi–Ag microspheres and corresponding EDX elemental mapping results.

3.3. Light absorption and band structure

The light absorption characteristics of the samples were investigated (Fig. 4a). The UV-Vis DRS spectrum of the Bi–Ag microspheres was red-shifted in comparison with that of pure BiVO_4. Ag nanoparticles show intense SPR absorption in the visible region, which is highly sensitive to their diameter and length-to-diameter ratio, as well as the optical and electronic properties of their surroundings. In this study, incorporation of Ag slightly reduced the size of the band gaps and enhanced visible light absorption, suggesting that utilization of visible light was enhanced by combining Ag nanoparticles with BiVO₄.²⁵

Fig. 4 (a) UV-Vis DRS spectra. (b) Relationship between (Ahν)² and the photon energy (hν) of the as-synthesized pure BiVO₄ and Bi–Ag microspheres. (c) VB of BiVO₄ and Bi–Ag microspheres. (d) Schematic illustration of the band gap structure of BiVO₄ and Bi–Ag microspheres.

Moreover, the energy band structures of a semiconductor continue to be important in determining its photocatalytic activity. The relationship of absorbance and incident photon energy hν can be described by eqn (1)

Ahν = C(hν − E_g)^1/2 (1)

where A, E_g, h, and ν represent the absorption coefficient, the band gap energy, Planck constant, and incident light frequency, respectively, and C denotes a constant.^26,27 The band-gap energy E_g of the obtained photocatalysts can therefore be estimated from a plot depicting (Ahν)² versus hν. The estimated band-gap energies of pure BiVO₄ and Bi–Ag were measured to be 2.39 eV and 2.30 eV, respectively. Notably, the absorption edge for the Bi–Ag microspheres was red-shifted compared to those of pure BiVO₄, given that the absorption edges were measured to be at 518.83 nm for pure BiVO₄ and 539.13 nm for the Bi–Ag microspheres.

The valence band (VB) edges of pure BiVO₄ and the Bi–Ag microspheres were estimated to be 1.94 eV and 1.51 eV, respectively (Fig. 4c). According to Fig. 4b and c, the bandgap structures of pure BiVO₄ and the Bi–Ag microspheres were determined as shown in Fig. 4d. The conduction band (CB) of the Bi–Ag microsphere sample was shifted to a more negative position in comparison with that of pure BiVO₄. In general, as the CB value becomes more negative, the number of reductive photo-excited electrons generated increases, and good charge carrier transport results in little electron–hole recombination.^28,29 This finding suggests that Bi–Ag microspheres have photocatalytic performance better than that of pure BiVO₄.^28–30 Therefore, the minimum CB value of the Bi–Ag microspheres was increased because of the Ag nanoparticles.³¹ These results indicate that the Bi–Ag microspheres have a unique electronic structure and significantly enhanced photocatalytic redox ability and photocatalytic performance under visible light irradiation in comparison with pure BiVO₄.

3.4. Photocatalytic activity and stability

The photocatalytic activities of the samples were evaluated by analyzing the degradation of MB under visible light irradiation. As shown in Fig. 5b, the photodegradation efficiency of MB by the Bi–Ag microspheres was approximately 76% within 6 h. The superior photocatalytic performance of the Bi–Ag microspheres can be explained by their enhanced visible light absorption and high-efficiency separation of photogenerated charge carriers. P25 only absorbs UV light, so its photo-degradation efficiency is reduced in visible light.

Fig. 5 (a) Photocatalytic performance of the Bi–Ag composite for MB degradation as measured by UV-Vis DRS. (b) Degradation of MB over different catalysts under visible light irradiation. (c) Photo-catalytic reaction with linear fitting modes and the reaction rate constant k. (d) Cycles of photocatalytic degradation of MB over Bi–Ag microspheres.

The stability of the Bi–Ag microspheres was investigated by assessing cyclic degradation of MB under visible light irradiation (Fig. 5d). The proportion of MB that was degraded was essentially constant over five cycles. Therefore, the Bi–Ag microspheres have good photocatalytic stability.

3.5. Reaction mechanism

The photoactivity of a photocatalyst is determined primarily by its capacity to absorb sunlight and separate excited electron–hole pairs.³² To identify the radical species that evolve during the photocatalytic reaction, EPR analysis was employed to detect ˙OH and O₂˙⁻ radicals under visible light irradiation^33,34 (Fig. 6a and b). The ESR spectra of Bi–Ag microspheres irradiated with visible light revealed the four characteristic peaks of DMPO–˙OH adducts (1:2:2:1 quartet pattern), as well as the four characteristic peaks of DMPO–O₂˙⁻ adducts. Fig. 6a and b show that no EPR spectral peaks were observed when the reaction was performed in the dark, whereas signals with intensity corresponding to the characteristic peaks of DMPO–˙OH and DMPO–O₂˙⁻ adducts were observed when the reaction was performed under visible light irradiation, and the intensity gradually increased as the reaction time was prolonged.³⁵ Generation of O₂˙⁻ inhibited recombination of photo-induced charge carriers and enhanced degradation of MB. The hydroxyl radical OH is formed via the e⁻ → O₂˙⁻ → H₂O₂ → ˙OH route, whereas ˙OH radicals are formed by multistep O₂˙⁻ reduction.^36,37 These results suggest that O₂˙⁻ and ˙OH are active species formed in the presence of Bi–Ag microspheres and oxygen under visible light irradiation (Fig. 6a and b).

Fig. 6 EPR spectra of radical adducts trapped by DMPO in Bi–Ag microsphere dispersions: (a) DMPO–O₂˙⁻ formed in irradiated methanol dispersions; (b) DMPO–˙OH formed in irradiated aqueous dispersions; (c) EPR spectra of degassed BiVO₄ and Bi–Ag microsphere powder recorded under illumination for 15 min; and (d) EPR spectra of Bi–Ag microsphere powder recorded in the dark and under illumination for 15 min.

The EPR spectra confirm the separation of electron–hole pairs in the Bi–Ag microsphere powder and pure BiVO₄ powder^37–39 (Fig. 6c and d). The obvious peaks associated with trapped electrons in the Bi–Ag microspheres were noticeably higher than those of fully hydrated, pure BiVO₄ (Fig. 6c). For better visualization, the peaks corresponding to holes/oxidation products were normalized for the spectra presented in Fig. 6c, and lines were drawn to indicate the relative intensities of the photo-generated electrons.⁴⁰ The electron–hole pairs of the Bi–Ag microspheres had better separation than those of pure BiVO₄ under illumination with visible light (Fig. 6c). The Bi–Ag microspheres illuminated for 10 min exhibited a stronger signal intensity at the 278 mT peak (g = 2.002) in comparison with those in the dark (Fig. 6d), indicating that visible light illumination induced separation of electron–hole pairs. Because the EPR spectra were recorded under illumination, the intensities of the signals correspond to the steady-state concentrations of products, which depend on the formation and recombination of electrons, holes, and the products of their subsequent reactions.²⁰

In photocatalysts, reactive species such as O₂˙⁻, h⁺, e⁺, and ˙OH function as bridges during photodegradation of organic pollutants under visible light irradiation.⁴¹ To investigate the photocatalytic mechanism of the Bi–Ag microspheres and determine the reasons for their improved performance, the effects of scavengers on MB degradation was examined to clarify the contributions of various reactive species during photocatalysis. We used p-benzoquinone (PBQ, 0.1 mmol), isopropanol (IPA, 0.1 mmol L⁻¹), silver nitrate (AgNO₃, 0.1 mmol), and potassium sodium tartrate (C₄H₄O₆KNa·4H₂O, 0.1 mmol) as O₂˙⁻,˙OH, hole, and electron scavenger, respectively.^42,43 These scavengers were added to the MB solution together with the Bi–Ag microspheres before irradiation. The results of these experiments indicate essentially complete inhibition of MB degradation in the presence of PBQ (Fig. 7), suggesting that O₂˙⁻ is the major reactive species contributing to photocatalytic degradation of MB over Bi–Ag microspheres. The dramatic decrease in degradation of MB in the presence of C₄H₄O₆KNa·4H₂O demonstrates that h⁺ also plays an important role in photodegradation of MB over Bi–Ag microspheres, which was further confirmed by slight enhancement of MB degradation after the addition of AgNO₃ to capture photogenerated electrons. These results also demonstrate that MB can be degraded by the hole directly. The removal yields of Bi–Ag microspheres were decreased in the presence of IPA, indicating that ˙OH, produced from the reaction of O₂˙⁻ with H₂O, has an important effect on MB degradation. The results of the scavenging experiments were consistent with the results of the EPR analysis.

Fig. 7 Photocatalytic degradation of MB over Bi–Ag microspheres in the presence of scavengers.

A reaction mechanism was posited based on the results of the experiments described above (Fig. 8). The adsorbed MB would be photo-activated by visible light, so that electrons can transfer from the singlet excited MB (MB*) to the CB of BiVO₄, Ag nanoparticles, and the shallow trap levels in the band gap of BiVO₄. The electrons on the BiVO₄ surface are subsequently trapped by the Ag nanoparticles, separating the MB˙⁺ and electrons, thus preventing recombination.^30,33 The SPR of the Ag nanoparticles excited by solar light enhances the excitation of surface electrons and the transfer of interfacial electrons.³¹ Under visible light excitation, the BiVO₄ surface generates electron–hole pairs, followed by rapid transfer of photo-generated electrons to Ag nanoparticles. Next, the negatively charged Ag nanoparticles activate O₂ to produce O₂˙⁻, which reacts with H₂O to produce ˙OH. Finally, the holes, ˙OH, and O₂˙⁻ oxidize the dye molecules adsorbed on the active sites of the Bi–Ag photocatalyst.^44,45 The entire sequence is summarized in the following five reactions:

BiVO₄ + hν → BiVO₄ (h + e) (2)

BiVO₄ (e) + Ag → BiVO₄ + Ag (e) (3)

Ag (e) + O₂ → O₂˙⁻ + Ag (4)

O₂˙⁻ + H₂O → H₂O₂ + ˙OH (5)

BiVO₄ (h) + ˙OH + O₂˙⁻ + MB → degradation product (6)

Fig. 8 Photocatalytic reaction mechanism of Bi–Ag microspheres.

4. Conclusions

Bi–Ag microspheres were synthesized by a facile one-step hydrothermal method and shown to function as a photocatalyst. The Bi–Ag microspheres have a lower band gap comparing with pure BiVO₄ as a result of the VB and CB shifting in the same direction. The band structure variation was caused by Ag nanoparticles associated with BiVO₄ after the hydrothermal reaction. These features resulted in a unique electronic structure that prolonged the duration of photocatalysis and improved the photo-oxidation ability of the charge carriers, promoting visible light photocatalysis. Consequently, the photocatalytic activity and oxidation capacity of the Bi–Ag microspheres toward MB was significantly enhanced. Bi–Ag microspheres have superior, persistent photocatalytic performance and are expected to be utilized in a wide range of industrial applications to eliminate organic pollutants from wastewater. The unique electronic structure identified in this study can be applied to develop novel high-performance photocatalysts.

Conflicts of interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Author contributions

Xuan Xu, Mao Du, and Zihong Fan conceived of the study and designed the experiments; Mao Du, Tianhui Wu, and Shimin Xiong performed the experiments; Mao Du, Deqiang Zhao, and Tian Chen analyzed the data; Zihong Fan contributed reagents/materials/analysis tools; Mao Du wrote the paper.

Acknowledgements

We gratefully acknowledge support from the 111 Project (No. B13041), the Science and Technology Innovation Special Projects of Social Undertakings and Livelihood Support, Chongqing (cstc2015shmsztzx20003, cstc2016shmszx20009), the graduate scientific research and innovation foundation of Chongqing, China (grant NO. CYB16008), the Science and Technology Project of Chongqing Education Commission (KJ1500604) and the Chongqing Research Program of Basic Research and Frontier Technology (cstc2015jcyjA20013).

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