Ag₂S sensitized mesoporous Bi₂WO₆ architectures with enhanced visible light photocatalytic activity and recycling properties

Abstract

To harvest solar energy more efficiently, novel Ag₂S/Bi₂WO₆ heterojunctions were synthesized by a hydrothermal route. This novel photocatalyst was synthesized by impregnating Ag₂S into a Bi₂WO₆ semiconductor by a hydrothermal route without any surfactants or templates. The as prepared structures were characterized by multiple techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmet–Teller (BET) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), UV-vis diffuse reflection spectroscopy (DRS) and photoluminescence (PL). The characterization results suggest mesoporous hierarchical spherical structures with a high surface area and improved photo response in the visible spectrum. Compared to bare Bi₂WO₆, Ag₂S/Bi₂WO₆ exhibited much higher photocatalytic activity towards the degradation of dye Rhodamine B (RhB). Although silver based catalysts are easily eroded by photogenerated holes, the Ag₂S/Bi₂WO₆ photocatalyst was found to be highly stable in the cyclic experiments. Based on the results of BET, Pl and DRS analysis, two possible reasons have been proposed for the enhanced visible light activity and stability of this novel photocatalyst: (1) broadening of the photoabsorption range and (2) efficient separation of photoinduced charge carriers which does not allow the photoexcited electrons to accumulate on the conduction band of Ag₂S and hence prevents the photocorrosion.

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1. Introduction

The growing environment concerns and energy demand are the main driving forces for the sustained fundamental and applied research in the area of semiconductor photocatalysis for organic pollutant degradation and hydrogen generation from water splitting using solar light.^1–3 Semiconductor photocatalysis involves the photoinduced generation of electron–hole pairs, therefore an ideal photocatalyst should have an extended spectral responsive range and low recombination rate of photogenerated charge carriers.⁴ Coupling of two photocatalysts to form a heterojunction may provide a chance to develop a new photocatalyst with enhanced separation of photoinduced charge carriers and an extended photoabsorption range.^5,6

Bi₂WO₆ is one of the simple aurivillius oxide with perovskite-like slabs of WO₄²⁻ and Bi₂O₂²⁺.^7,8 Owing to its non-toxicity and strong oxidizing power, Bi₂WO₆ has been used as one of the promising and excellent photocatalyst for the decomposition of organic pollutants under visible light.^9–12 To improve the photocatalytic activity of bare Bi₂WO₆, many different morphologies of Bi₂WO₆ with different hierarchical architecture and surface area have been reported. These include the development of nanoplates,¹³ porous thin films,¹⁴ 3D nest like mesoporous Bi₂WO₆ architectures¹⁵ and flower sphere like complex structures.^16,17 However there are still two important factors which limits the photocatalytic efficiency and thus hinders the practical application of bare Bi₂WO₆ (1) the absorption edge of pure Bi₂WO₆ is ca., 450 nm^18,19 which overlaps a small part of solar spectrum leading to the unsatisfactory photoresponse range and (2) the recombination rate of photoinduced charge carriers is large which leads to the low quantum efficiency because of the short lifetimes of the electron hole pairs.^20,21

Recent studies have shown that coupling of Bi₂WO₆ with other semiconductors improves the photocatalytic performance of Bi₂WO₆ to a substantial extent by promoting the effective separation of photoinduced charge carriers and broadening the visible light responsive range, For e.g. α-Fe₂O₃/Bi₂WO₆,²² Bi₂S₃/Bi₂WO₆,⁴ Bi₂O₃ decorated Bi₂WO₆ and TiO₂ modified flower like Bi₂WO₆,²³ Bi₂WO₆/BiOBr,²⁴ WO₃–Bi₂WO₆ ²⁵ and CuPc sensitized Bi₂WO₆.²⁶

As an important chalcogenide Ag₂S with a narrow band gap of 1.1 eV has been investigated extensively for the numerous applications such as photovoltaic device construction,^27,28 photocatalytic decomposition of organic pollutants^29,30 and photocatalytic H₂ production.³¹ Moreover Ag₂S has high absorption coefficient^31–36 and besides Ag₂S is free from toxic heavy elements like Pb and Cd and thus possess negligible toxicity compared to other narrow band gap materials.³⁷ Owing to the high chemical stability, narrow band gap, high absorption coefficient and excellent optical limiting properties of Ag₂S, we choose to couple it with Bi₂WO₆ to form a heterojunction with enhanced visible light photocatalytic activity and high stability.

Herein for the first time we report novel Ag₂S/Bi₂WO₆ heterojunction photocatalyst to overcome the drawbacks of low photocatalytic efficiency of Bi₂WO₆ brought by narrow photoresponsive range and high rate of recombination of photoinduced charge carriers. The photocatalytic activity of the as prepared photocatalysts was evaluated by decomposing the dye Rhodamine B. The effect of Ag/Bi weight ratio on the photocatalytic activity was also studied. The stability of the catalyst which is a major concern for catalysts containing silver was also investigated.

2. Experimental

2.1 Synthesis of Ag₂S/Bi₂WO₆ photocatalyst

All the reagents were of analytical purity, purchased from Sigma-Aldrich India and used without further purification. The hierarchical structures of Ag₂S/Bi₂WO₆ were synthesized by a two-step process. In the first step the mesoporous Bi₂WO₆ microspheres were fabricated by a simple hydrothermal process via following procedure: 1 mmol of Na₂WO₄·2H₂O was dissolved in 15 ml of ethylene glycol under constant stirring. Then 2 mmol of Bi(NO₃)₃·5H₂O was added into the solution and the mixture was stirred for 20 min. Thereafter absolute ethanol (25 ml) was added into the solution and the suspension was transferred into a 50 ml Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. The autoclave was subsequently cooled to room temperature and the products were collected by filtration. The as obtained products were washed with de-ionized water and absolute ethanol for several times and then dried at 80 °C for 12 h.

In the second step, impregnation of Bi₂WO₆ surface with Ag₂S was carried out by in situ growth of Ag₂S on Bi₂WO₆ substrate at room temperature. In a typical experimental procedure, 0.6 g of Bi₂WO₆ microspheres and different amounts of AgNO₃ were dispersed in 50 ml of absolute ethanol and ultrasonicated for 30 min. After ultrasonication, appropriate amount of Na₂S was added to the suspension with continuous stirring. The obtained suspension was vigorously stirred for 8 h. The precipitates were collected and washed with de-ionized water and absolute ethanol for several times and then dried at 80 °C for 12 h. The different weight ratios of Ag₂S to Bi₂WO₆ samples were prepared and noted as 3% Ag₂S/Bi₂WO₆, 5% Ag₂S/Bi₂WO₆, 7% Ag₂S/Bi₂WO₆ and 9% Ag₂S/Bi₂WO₆. For comparison pure Ag₂S was also synthesized in absence of Bi₂WO₆.

2.2 Characterization

To examine the phase purity and crystal structure of as prepared samples, XRD analysis was carried out using PANalytical, XPert PRO powder X-ray diffractometer with CuKα radiation (λ = 1.5418 Å) having a tube voltage of 40 kV and current of 30 mA at room temperature with the scan range 2θ = 15 to 80° and step size 0.02666°. X-ray photoelectron spectroscopy (XPS) was used to analyse the surface properties of samples using MK II photoelectron spectrometer having Al-K(alpha) (1486.6 eV) as the X-ray source. The particle morphology and microstructure of the samples was investigated by FESEM using Carl-Zeiss field-emission scanning electron microscope (FESEM) equipped with EDS attachment and transmission electron microscopy (TEM) using JEOL, JEM 2100F instrument. UV-vis DRS spectra of samples in the region of 300–800 nm was recorded by UV-vis NIR spectrometer (Perkin Elmer) equipped with an integrating sphere assembly. The surface area and pore size distribution of the as prepared samples was characterized by N₂ adsorption–desorption isotherm using Quantachrome Instruments Autosorb 1C. Adsorption of samples was done at 77 K and the samples were degased at 150 °C for 3 h before analysis. PL spectra was recorded on fluorospectrometer.

2.3 Photocatalytic activity

To evaluate the photocatalytic activity of the samples, RhB and phenol aqueous solution was degraded under visible light irradiation. A 500 W, tungsten halogen lamp was used as the light source. Immersion well photoreactor made of pyrex glass was used to perform the experiments. Refrigerated circulating liquid bath was used to keep the temperature of the reaction constant at 20 ± 0.3 °C. The experiments were performed as follows. 0.18 g of the as prepared catalyst was added into 180 ml RhB aqueous solution. Prior to the illumination, the suspension was magnetically stirred for at least 30 min in the dark to attain adsorption–desorption equilibrium between dye and catalyst. 5 ml suspensions were sampled at 5 min time intervals and centrifuged to remove the catalyst particles. The concentration of catalyst free dye solution was analysed at λ_max (554 nm) using Shimadzu UV-vis 1601, UV-vis spectrophotometer.

To determine the effect of scavengers on photocatalytic activity, appropriate quantity of scavenger species were introduced in the reaction system in a manner similar to photocatalytic experiment.

Photoluminescence (PL) technique with terephthalic acid (TA) as a probe molecule was used for the detection of ˙OH radicals. The experimental procedure was referred to previous studies.⁵

3. Results and discussions

3.1 XRD analysis

XRD analysis was used to determine the purity, phase structure and crystallinity of the as prepared samples. Fig. 1 presents the XRD pattern of pristine Bi₂WO₆ and Ag₂S loaded Bi₂WO₆. As can be seen from the Fig. 1a, the diffraction peaks can be categorised into a set of 2θ at 28.3°, 32.8°, 47.1° and 55.9° corresponding to the indices (131), (200), (202) and (331) which indicates a perfectly orthorhombic structure for pure Bi₂WO₆.³⁸ Compared with diffraction pattern of bare Bi₂WO₆, no significant diffraction peaks of any other phase or impurity was observed on Ag₂S/Bi₂WO₆ composites indicating that the characteristic peaks associated with Ag₂S are not obviously detected (Fig. 1b–e). This may be due to the limited amount of Ag₂S (3–9%) used, small size of Ag₂S nanoparticles and high dispersion of Ag₂S on the surface of Bi₂WO₆ spherical architectures. More importantly the absence of individual Ag₂S aggregates in SEM images of as prepared samples indicate high dispersion of Ag₂S in composites. Fig. 1f displays the characteristic XRD pattern of pure Ag₂S sample. All peaks of pure Ag₂S can be assigned to monoclinic Ag₂S crystal structure.³⁹ The average crystallite size of the samples was calculated by using Scherer formula given below⁵ and results are listed in Table 1.where D is taken as crystallite size, k is a constant, λ is X-ray wavelength, β is the full width at half maximum (FWHM) measured in radians on the 2θ scale, θ is the Bragg angle for diffraction peaks. From the results of crystallite size displayed in Table 1, it is clear that there is negligible change in the crystallite size of pure Bi₂WO₆ and Ag₂S loaded Bi₂WO₆ samples.

Fig. 1 XRD patterns of (a) the pure Bi₂WO₆, (b) the 3% Ag₂S/Bi₂WO₆ catalyst, (c) the 5% Ag₂S/Bi₂WO₆ catalyst, (d) the 7% Ag₂S/Bi₂WO₆ catalyst, (e) the 9% Ag₂S/Bi₂WO₆ catalyst, and (f) the pure Ag₂S.

Table 1 Pseudo-first-order rate constants (K_app) calculated from the plots of ln(C₀/C_t) versus irradiation time for decolorization of RhB over different catalysts, %decolorization of RhB and average crystallite size of pure Bi₂WO₆ and Ag₂S/Bi₂WO₆ catalysts with different Ag₂S contents

Sample	Kapp (min^−1)	% Decolorization	Average crystallite size (nm)
Bi2WO6	0.0160	56.42994	11.12
3% Ag2S/Bi2WO6	0.0185	61.11649	11.28
5% Ag2S/Bi2WO6	0.0243	71.10206	10.56
7% Ag2S/Bi2WO6	0.0373	84.96666	10.66
9% Ag2S/Bi2WO6	0.0300	78.2553	11.42

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3.2 XPX analysis

To further investigate the composition and oxidation states of as prepared samples, XPS analysis of 7% Ag₂S/Bi₂WO₆ heterostructure (Fig. 2) was carried out. Fig. 2a displays the overall XPX spectra for the 7% Ag₂S/Bi₂WO₆ in which peaks for Bi, W, O, Ag and S could be detected. The high resolution XPS spectra of Bi4f, W4f, O2s, S2p and Ag3d is shown in Fig. 2b–f. The Bi4f peaks (Fig. 2b) at 157.8 and 163.10 eV are attributed to the binding energies of Bi4f_7/2 and Bi4f_5/2 respectively which corresponds to previous results.⁴⁰ The W4f peaks at 34.00 and 36.1 eV are ascribed to binding energies of 4f_7/2 and 4f_5/2 (Fig. 2c). The splitting energy of 2.1 eV for 4f doublet of W indicates the +6 oxidation state of W in composite catalyst.⁴¹ The O1s peak (Fig. 2d) at 528.8 eV is in good agreement with previous results.⁴⁰ The high resolution Ag3d XPS spectrum is displayed in Fig. 2e. The two peaks corresponding to Ag⁺ at approximately 366.50 and 372.60 eV are attributed to the Ag3d_5/2 and Ag3d_3/2 respectively.⁴² Since in previous results the peaks at 368.3 and 374.2 eV are attributed to metallic Ag whereas the peaks at 367.8 and 373.8 eV are attributed to Ag⁺,^43,44 it implies that peaks obtained in our XPS results can be ascribed to Ag⁺ only suggesting that no metallic Ag⁰ is formed. Fig. 2f displays the S2p XPS spectrum. The peak at around 163 eV can be attributed to S2p_1/2 indicating the presence of S²⁻ which further suggests the formation of Ag₂S.⁴⁵ From these results it can be confirmed that Ag₂S/Bi₂WO₆ composites have been successfully synthesized.

Fig. 2 XPS spectra of 7% Ag₂S/Bi₂WO₆. (a) Full survey spectra, and the high resolution XPS spectra of the sample: (b) Bi4f, (c) W4f, (d) O1s, (e) Ag3d, and (f) S2p.

3.3 SEM, EDS and TEM analysis

To investigate the surface morphology of as prepared samples SEM characterization was used. Fig. 3 presents the FESEM images of pure Bi₂WO₆ and 7% Ag₂S/Bi₂WO₆ composite. As clear from the Fig. 3a, the FESEM image of bare Bi₂WO₆ consist of abundant hierarchically structured architectures of Bi₂WO₆ microspheres. The higher magnification (Fig. 3b) reveals that Bi₂WO₆ architectures are built from numerous nanoplates aligned to the spherical surface forming a microsphere. Compared with bare Bi₂WO₆ the Ag₂S loading affects the surface morphology of Bi₂WO₆ in composite (Fig. 3c) to some extent. As can be seen clearly from the higher magnification FESEM image of 7% Ag₂S/Bi₂WO₆ composite (Fig. 3d), it is somewhat different from spherical structure showing that Ag₂S has combined well with Bi₂WO₆ to form a heterostructure. EDX spectrum of Bi₂WO₆ and 7% Ag₂S/Bi₂WO₆ was collected to confirm the presence of Ag₂S in hybrids. The EDS spectrum in Fig. 3e shows clearly that bare Bi₂WO₆ is composed of Bi, W and O elements while as Ag₂S/Bi₂WO₆ (Fig. 3f) composite is composed of Bi, W, O, Ag and S elements indicating the formation of Ag₂S/Bi₂WO₆ composite. To further confirm the high dispersion of Ag₂S in composite, EDX elemental mapping of 7% Ag₂S/Bi₂WO₆ composite was performed as shown in Fig. 4. It can be seen from the Fig. 4, that Ag (Fig. 4b) and S (Fig. 4e) are highly dispersed in 7% Ag₂S/Bi₂WO₆ hybrid.

Fig. 3 FE-SEM images of samples. (a) Pure-Bi₂WO₆, (b) the high magnification SEM image of pure-Bi₂WO₆, (c) 7% Ag₂S/Bi₂WO₆, (d) the high magnification SEM image of 7% Ag₂S/Bi₂WO₆, (e) EDS spectrum of pure-Bi₂WO₆ sample, and (f) EDS spectrum of 7% Ag₂S/Bi₂WO₆ sample.

Fig. 4 EDS elemental mapping of 7% Ag₂S/Bi₂WO₆.

TEM analysis was further used to investigate the detailed structure as shown in Fig. 5. It can be seen that Bi₂WO₆ structures are composed of nanoplates with a thickness around 10–20 nm (Fig. 5a). Fig. 5b shows that Bi₂WO₆ microspheres are mesoporous in nature which helps Ag₂S particles to bind in this composite system. The TEM image for 7% Ag₂S/Bi₂WO₆ (Fig. 5e and f) shows subtle particles of Ag₂S grown on the surface of Bi₂WO₆ microsphere. The presence of lattice fringes (Fig. 5c) confirms the crystalline nature of Bi₂WO₆. Furthermore the spacing of lattice fringes was found to be 0.313 nm which corresponds to the 131 planes of orthorhombic Bi₂WO₆.³⁸ The SAED (Selected Area Diffraction Pattern) demonstrates the presence of clear diffraction spots (Fig. 5d) which further confirms the crystalline structure of Bi₂WO₆. All these above results are in good agreement with the results of XRD and XPS analyses and strongly demonstrate the formation of Ag₂S/Bi₂WO₆ heterojunction.

Fig. 5 TEM images of samples. (a and b) Pure Bi₂WO₆, (c and d) SAED pattern of pure Bi₂WO₆, and (e and f) 7% Ag₂S/Bi₂WO₆.

3.4 N₂ adsorption–desorption

Fig. 6 shows the nitrogen adsorption isotherm and pore size distribution of pure Bi₂WO₆ (Fig. 6a) and 7% Ag₂S/Bi₂WO₆ (Fig. 6b). As can be seen from the figure, the shape of both the isotherms seem to be nearly of type IV isotherm with hysteresis loop at higher relative pressure which is usually associated with capillary condensation in mesopores.^46,47 The type H3 hysteresis loop at higher relative pressure according to IUPAC classification reflect the presence of slit like pores indicating the presence of mesopores in the region of 2–10 nm which is further confirmed by the BJH pore size distribution diagram in the inset of Fig. 6. The BET surface area of bare Bi₂WO₆ and 7% Ag₂S/Bi₂WO₆ are 77.6 m² g⁻¹ and 66 m² g⁻¹ respectively indicating that the loading of Ag₂S leads to the decrease in surface area to some extent which is probably because Ag₂S particles are embedded in the pores of Bi₂WO₆.⁴⁸ The presence of mesoporous structure with large surface area provides more active sites and transport paths for the decomposition of contaminants in photocatalytic reaction.

Fig. 6 N₂ adsorption–desorption isotherm of the samples. (a) Pure Bi₂WO₆, and (b) 7% Ag₂S/Bi₂WO₆. The inserts are the pore size distribution of the respective catalysts.

3.5 Optical properties

Fig. 7 displays the UV-vis diffuse reflectance (DRS) spectra of bare Bi₂WO₆ and Ag₂S/Bi₂WO₆ heterostructures. As shown in Fig. 7a the bare Bi₂WO₆ has an absorption edge around 450 nm, where as weak and broadened peaks of Ag₂S (Fig. 7b) are similar to the reported UV-vis spectra of pure Ag₂S.^48,49 With increase in Ag₂S loading the absorption edge of heterostructure samples is shifted to longer wavelengths from 450–490 nm.

Fig. 7 UV-vis diffuse reflectance spectra of (a) the pure Bi₂WO₆, and Ag₂S/Bi₂WO₆ composites (b) the pure Ag₂S catalyst. The insets in the (a) and (b) are band gap energies of the pure Bi₂WO₆, and pure Ag₂S respectively.

The optical band gaps (insets of Fig. 7a and b) of Bi₂WO₆ and Ag₂S were calculated using the equation reported by Butler.⁵⁰

hν × α = (Ahν − E_g)^n/2 (2)

Since α is proportional to Kubelka–Munk function F(R), the equation becomes

hν × F(R) = (Ahν − E_g)^n/2 (3)

where, h is the planks constant, ν is the frequency, A is the proportionality constant and E_g is the band gap energy. The value of n is determined by the type of transition (n = 1 for direct and n = 4 for indirect transition). The n for Bi₂WO₆ has been reported by some researchers as 1 (direct transition),^51,52 and by some other researchers as 4 (indirect transition).^53,54 In our case we plotted (F(R) × hν)^1/2 versus hν for Bi₂WO₆ (inset of Fig. 7a) and (F(R) × hν)² versus hν for Ag₂S (inset of Fig. 7b). The band gaps for Bi₂WO₆ and Ag₂S were estimated as 2.74 eV and 1.1 eV respectively.

3.6 Photocatalytic properties

To assess the photocatalytic activity of as prepared samples, RhB was selected as a probe molecule. The characteristic absorption band of RhB at 554 nm was employed to determine the process of degradation. Fig. 8 displays the photocatalytic performances of all Ag₂S/Bi₂WO₆ heterojunctions with different Ag₂S contents over degradation of RhB (C_t/C₀) under visible light irradiation. As shown in Fig. 8b, the blank experiment (without photocatalyst) shows that the concentration of RhB remains unchanged even after 50 min of visible light irradiation indicating the significance of photocatalyst. The adsorption of RhB dye by 7% Ag₂S/Bi₂WO₆ was also checked for 50 min and it was found that the adsorption in dark is insignificant. As is shown the pure mesoporous Bi₂WO₆ exhibit photocatalytic efficiency of 56.5% in 50 min. The better performance of pure mesoporous Bi₂WO₆ photocatalyst was attributed to the high surface area and mesoporous structure of Bi₂WO₆. The high surface area and mesoporous structure not only provide more active sites and transport paths in photocatalytic reaction but also improves the contact of photocatalyst with organic pollutants. When Ag₂S and Bi₂WO₆ are combined to construct a heterostructure, the photocatalytic activity of the composites is dramatically enhanced even though the Ag₂S content is very low. The photocatalytic activity of all Ag₂S/Bi₂WO₆ heterostructures with Ag₂S weight percentage of 3%, 5%, 7% and 9% is higher than the bare Bi₂WO₆ and are listed in Table 1. The best photocatalytic performance was obtained for 7% Ag₂S/Bi₂WO₆ for which almost 85% of RhB was degraded in 50 min. The reason for enhanced visible light activity is the formation of heterojunction which results into the generation of natural energy bias at heterojunction with subsequent transfer of photoinduced charge carriers and thereby impeding the electron hole recombination, which is the main factor responsible for low visible light activity of Bi₂WO₆. With further increase in Ag₂S content to 9%, the photocatalytic activity decreases but is still higher than bare Bi₂WO₆. Therefore the optimal content of Ag₂S in Ag₂S/Bi₂WO₆ composite is 7%. The optimum content of AgBr in the heterojunction can be related to two factors. (1) For efficient electron–hole separation the space charge region potential must be certain.³⁸ When Ag₂S content was above 7% (optimal value) the space charge region might become narrow and lead to the recombination of photoinduced charge carriers. (2) The loading of Ag₂S leads to decrease in surface area as discussed in section 3.4. When Ag₂S content was above optimal value the surface area might decrease substantially leading to decrease in photocatalytic activity. Fig. 8a displays the absorption spectral changes of the photocatalytic degradation of RhB aqueous solution over 7% Ag₂S/Bi₂WO₆. As can be seen from the Fig. 8a, the absorbance of RhB at 554 nm decreases significantly with increase in reaction time accompanied by a slight shift of absorption band towards blue region which is attributed to the step wise de-ethylation process of RhB into intermediates as reported in previous works.^55,56 The apparent pseudo first order kinetics model given by the eqn (4) was applied in experiments to understand the reaction mechanism.⁵⁷

ln(C₀/C_t) = K_appt (4)

Fig. 8 (a) UV-vis spectral changes of the degradation of RhB by the 7% Ag₂S/Bi₂WO₆ catalyst, (b) the degradation efficiency (C_t/C₀) of RhB in presence of pure Bi₂WO₆ and Ag₂S/Bi₂WO₆ composites with different Ag₂S contents and (c) Rhodamine B decolourization curves of ln(C₀/C_t) versus irradiation time for different catalysts.

K_app is apparent pseudo-first order reaction constant and its value for bare Bi₂WO₆ and all heterojunctions was calculated via the first order linear fit data (Fig. 8c) and are displayed in Table 1. From Table 1, it is clear that photocatalytic activity of 7% Ag₂S/Bi₂WO₆ is 2.4 times higher than the bare Bi₂WO₆ suggesting that Ag₂S/Bi₂WO₆ is an excellent photocatalyst under visible light.

3.7 Proposed mechanism

The trapping experiments of active species formed during photocatalytic oxidation process were carried out to investigate the possible mechanism involved in RhB degradation over 7% Ag₂S/Bi₂WO₆ heterojunction. Benzoquinone (BQ)^58,59 isopropyl alcohol (IPA)^58,60 and ammonium oxalate (AO)⁶¹ were used as superoxide (˙O₂⁻), hydroxyl (˙OH) and hole (h⁺) scavengers respectively. Fig. 9a displays the effect of different scavengers and it can be seen from the figure that the rate of RhB degradation decreases substantially in presence of AO. However addition of IPA had no obvious effect on rate of RhB degradation whereas addition of BQ slightly decreases the photocatalytic performance of 7% Ag₂S/Bi₂WO₆ compared with no scavenger under same conditions. These results indicate that h⁺ are the main active species rather than ˙O₂⁻ and ˙OH in the RhB degradation over 7% Ag₂S/Bi₂WO₆ system under visible light irradiation.

Fig. 9 (a) Effect of different scavengers on the degradation of RhB over 7% Ag₂S/Bi₂WO₆ catalyst; BQ = benzoquinone, IA = isopropyl alcohol, AO = ammonium oxalate. (b) The degradation efficiency (C_t/C₀) of phenol in presence of pure Bi₂WO₆ and 7% Ag₂S/Bi₂WO₆ composite.

To further confirm that ˙OH did not play any role, we carried out the terephthalic acid photoluminescence studies. The PL spectral changes observed during irradiation of Ag₂S/Bi₂WO₆ in a basic terephthalic acid solution (315 excitation wavelength) are displayed in Fig. 10. As clear from the figure no Pl signals were observed at 425 nm throughout the photocatalytic oxidation process, indicating that no ˙OH is formed on the surface of catalyst.

Fig. 10 ˙OH trapping PL spectral changes over 7% Ag₂S/Bi₂WO₆ catalyst in a basic terephthalic acid solution (excitation at 315 nm) with irradiation time.

To ensure that the reactive species originate from the photocatalytic process of catalysts and not form the sensitization of dye, we selected a colourless molecule phenol as a probe. Fig. 9b displays the photocatalytic performances of pure Bi₂WO₆ and 7% Ag₂S/Bi₂WO₆ heterojunctions over degradation of phenol (C_t/C₀) under visible light irradiation. As can be seen from the Fig. 9b the concentration of phenol decreases with increasing reaction time in presence of both pure Bi₂WO₆ and 7% Ag₂S/Bi₂WO₆. The concentration (C_t/C₀) decreases sharply in presence of 7% Ag₂S/Bi₂WO₆ under visible light irradiation while as in case of pure Bi₂WO₆ the decrease in the concentration (C_t/C₀) of phenol is lower than 7% Ag₂S/Bi₂WO₆. Since phenol cannot absorb visible light, it follows directly that reactive spies originate from the photocatalytic process of samples under visible light ruling out the possibility of sensitization of dye. The results obtained are in accordance with the results obtained for the degradation of RhB.

The BET surface area of bare Bi₂WO₆ was calculated to be 77.6 m² g⁻¹. The high surface area and mesoporous structure of Bi₂WO₆ makes it an excellent visible light photocatalyst but the surface area of 7% Ag₂S/Bi₂WO₆ was calculated to be only 66 m² g⁻¹ which is obviously less than the bare Bi₂WO₆. The Ag₂S loading on the surface of Bi₂WO₆ enhances the visible light photocatalytic activity of Bi₂WO₆ substantially although the surface area is decreased. Therefore it can be assumed that large surface area may be responsible for the high photocatalytic activity of bare Bi₂WO₆ but it is not the influence factor for the enhancement of visible light activity of Ag₂S/Bi₂WO₆ composites up to 7% loading. Therefore to analyse the mechanism of improved photocatalytic activity of Ag₂S/Bi₂WO₆ composites the relative positions of the conduction band and valence band edges of Ag₂S and Bi₂WO₆ were investigated for the flow chart of photoinduced charge carriers in a heterojunction. The relative position of valence band (VB) of Ag₂S and Bi₂WO₆ was evaluated by the empirical formula in eqn (5).⁶²

E_VB = X − E^c + 0.5E_g (5)

where X is the electronegativity of semiconductor expressed as geometric mean of absolute electronegativity of constituent atoms, E^c is the energy of free electrons on hydrogen scale (4.5 eV) and E_g is the band gap energy. The E_VB for Ag₂S and Bi₂WO₆ were calculated to be 1.1 eV and 3.23 eV respectively. The relative position of conduction band (CB) of Ag₂S and Bi₂WO₆ were estimated from eqn (6).

E_CB = E_VB − E_g (6)

The E_CB for Ag₂S and Bi₂WO₆ were calculated to be 0 eV and 0.49 eV respectively.

Based on the results of BET analysis, DRS, trapping experiments and TA PL spectra we presented a schematic diagram (Fig. 11) to illustrate the photocatalytic reaction mechanism and charge transfer of Ag₂S/Bi₂WO₆ system. We propose that under visible light irradiation both Ag₂S and Bi₂WO₆ are excited and generate electron hole pairs. Since the CB energy level of Ag₂S and Bi₂WO₆ is 0 and 0.49 eV (vs. SHE), the single electron reduction of O₂ to ˙O₂⁻(−0.046 vs. SHE)^63,64 is not possible. When two semiconductors are in contact, the electrons from less positive conduction band of Ag₂S (0 eV) are injected with high efficiency into more positive conduction band of Bi₂WO₆ (0.49 eV). On the other hand holes from more positive valence band of Bi₂WO₆ (3.23 eV) are transferred to the less positive VB of Ag₂S (1.1 eV). The photoinduced charge carriers are thus effectively separated at the interface of Ag₂S/Bi₂WO₆. The holes (h⁺) on the valence band of Ag₂S could not react with OH⁻/H₂O to form ˙OH because VB of Ag₂S is more negative (1.1 eV) than that of ˙OH/H₂O (2.30 eV vs. SHE).³⁸ Hence holes from valence band of Ag₂S migrate to the surface of a photocatalyst where they take part in photocatalytic reaction to mineralize the organic pollutants and thereby enhancing the photocatalytic activity.

Fig. 11 Schematic diagram showing the band structure and separation of photoinduced electron hole pairs at interface of 7% Ag₂S/Bi₂WO₆ catalyst under visible light irradiation.

Photoluminescence spectra of hierarchically structured bare Bi₂WO₆ and 7% Ag₂S/Bi₂WO₆ was carried out to further demonstrate the enhancement of separation of charge carriers. The lower intensity of PL peak represents the lower probability of electron–hole recombination. Fig. 12 presents the PL spectra of bare Bi₂WO₆ and 7% Ag₂S/Bi₂WO₆ with an excitation wavelength of 380 nm. From the Fig. 12 it is clear that the intensity of PL spectra decreases significantly in 7% Ag₂S/Bi₂WO₆ compared to the bare Bi₂WO₆ indicating that the loading of Ag₂S on Bi₂WO₆ has greatly restrained the recombination of photoinduced charge carriers between Bi6s and O2p to the empty W4d orbitals.²²

Fig. 12 PL spectra of samples. (a) Pure-Bi₂WO₆ and (b) 7% Ag₂S/Bi₂WO₆ (E_x = 380 nm).

3.8 Stability of the catalyst and cycling runs

The stability of silver based compounds has always been a matter of concern,⁶⁵ particularly the metal chalcogenides get easily eroded by the photogenerated holes.⁶⁶ Therefore cyclic experiments with 7% Ag₂S/Bi₂WO₆ sample were carried out to determine the stability of a photocatalyst. Fig. 13 displays the results of RhB degradation by 7% Ag₂S/Bi₂WO₆ after 4 repeated cycles. It can be seen from the Fig. 13 that the degradation of RhB by 7% Ag₂S/Bi₂WO₆ is maintained at 81.3% even after 4 repeated cycles indicating the high stability of photocatalyst. Fig. 14 presents the Ag3d XPS spectrum and XRD analysis of used 7% Ag₂S/Bi₂WO₆ catalyst after 4 repeated cycles. It is clear from the Fig. 14a that the Ag3d XPS spectrum of used catalyst is similar to that of fresh catalyst (Fig. 2e) with no peak detected for metallic Ag in used catalyst after 4 repeated cycles. The XRD analysis of used catalyst (Fig. 14b) after 4 repeated cycles also confirms that the structure and phase of the used catalyst remains intact with no additional peak detected for Ag metal. From these results it is clear that the Ag₂S/Bi₂WO₆ is a stable photocatalyst under visible light irradiation and the main reason for the enhanced stability is the transfer of CB electrons from Ag₂S to Bi₂WO₆ which reduces the chances of erosion of catalyst.

Fig. 13 The repeated experiments of photocatalytic degradation of RhB over 7% Ag₂S/Bi₂WO₆ catalyst.

Fig. 14 (a) Ag3d XPS spectra of used 7% Ag₂S/Bi₂WO₆ catalyst after 4 recycling runs, and (b) XRD pattern of used 7% Ag₂S/Bi₂WO₆ catalyst after 4 recycling runs.

4. Conclusion

Mesoporous Bi₂WO₆ was modified with Ag₂S by hydrothermal method without any surfactant or template. The introduction of Ag₂S extended the photoabsorption range of Bi₂WO₆ and the composite showed red shift compared to the pure Bi₂WO₆. The Ag₂S/Bi₂WO₆ heterostructures showed enhanced photocatalytic activity in the degradation of RhB under visible light irradiation which is 2.4 times higher than the pure Bi₂WO₆. The Ag₂S/Bi₂WO₆ composite even after 4 recycling runs did not show any significant decrease in photocatalytic activity except for about 4%. The enhanced photocatalytic activity and stability was attributed to the broadening of photoabsorption range and efficient separation of the photoinduced electron hole pairs as a result of natural energy bias at heterojunction. This study inspires a thought of utilizing visible spectrum efficiently and simultaneously presents a way to promote separation of photo induced electron hole pairs.

Acknowledgements

The authors are highly thankful to Department of Physics AMU Aligarh, USIF AMU Aligarh and Department of Chemistry AMU Aligarh for providing the instrumentation facility. DST and UGC is duly acknowledged for financial support in the form of FIST, PURSE and DRS II research grants. The authors are also thankful to UGC, New Delhi, India for providing financial assistance. Niyaz A. Mir thanks Department of Science & Technology, India for the award of Young Scientist grant (SERB/F/1214/2014-2015).

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