Photocatalytic performance of Ag₂S under irradiation with visible and near-infrared light and its mechanism of degradation

Abstract

Ag₂S has only rarely been investigated as a photocatalyst on its own, although it has been widely used as an important component of composite photocatalysts. We synthesized Ag₂S by a facile ion-exchange method at room temperature and used it directly as an effective photocatalyst. Our results confirmed the excellent performance of Ag₂S in completely photodegrading methyl orange within 30 min under irradiation with visible light and within 70 min under irradiation with near-infrared light. This good performance is ascribed to the narrow band gap (1.078 eV) of Ag₂S and the lower recombination efficiency of the photogenerated electron–hole pairs of Ag₂S in the photocatalytic process. The active species in the photo-oxidation process was identified as anioic ozone radicals. This simple preparation method and high photocatalytic performance increases the possible future applications of Ag₂S.

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1. Introduction

TiO₂ is often used as a stable and inexpensive photocatalyst active under ultraviolet (UV) radiation;^1–3 however, its application is restricted as it cannot be used with visible light. Photocatalysts that work with visible light, which makes up about 48% of the solar spectrum, or even infrared light (44% of solar spectrum)⁴ have therefore been the focus of much research. To achieve a high efficiency over a wide range of the solar spectrum, various photocatalysts, such as non-metals⁵ or metals⁶ doped with TiO₂, composite semiconductor photocatalysts⁷ and new photocatalysts with a small band gap,⁸ have been investigated. Photocatalysts that work under visible and near-infrared (NIR) irradiation include Cu₂(OH)PO₄,⁹ YF₃:Yb³⁺Tm³⁺/TiO₂,¹⁰ Bi₂WO₆/TiO₂,¹¹ CQDs/Cu₂O,¹² BiVO₄/CaF₂:Er³⁺, Tm³⁺, Yb^3+ 13 and NaYF₄:Yb, Tm/CdS/TiO₂.¹⁴ The preparation of most of these photocatalysts is too complex for mass production and the time taken to photodegrade organic compounds under NIR irradiation can be as long as 2, 4 or even 50 hours. It is therefore desirable to develop an efficient photocatalytic material with a simple method of preparation and higher activity in both the visible and NIR regions of the solar spectrum.

Ag₂S is an important chalcogenide compound and has been investigated for use in photoconductors, photovoltaic cells and superionic conductors as a result of its good chemical stability and excellent optical limiting properties.¹⁵ Ag₂S has often been combined with other semiconductor catalysts to form a composite photocatalyst, improving the performance of the bulk photocatalyst. Xie et al.¹⁶ reported that TiO₂ nanocomposites coupled with Ag₂S absorbed visible light. Subash et al.¹⁷ proved that the photocatalytic efficiency of ZnO was enhanced by loading with Ag₂S and Shen et al.¹⁸ confirmed that CdS nanostructures with highly dispersed Ag₂S had the highest photocatalytic activity for hydrogen evolution when the concentration of Ag₂S was 5% by weight. Other composite photocatalysts containing Ag₂S, such as Ag₂S/ZnIn₂S₄,¹⁹ ZnS–In₂S₃–Ag₂S,²⁰ Ag₂S/MCM-41 ²¹ and Ag₂S-coupled ZnO/ZnS,²² all exhibit good photocatalytic activity.

These successful examples of increased photocatalytic performance with the addition of Ag₂S suggested that Ag₂S may be an effective photocatalyst. However, there have only been a few reports of the performance of pure Ag₂S as a photocatalyst. Yang et al.²³ reported that only 7.8% of methyl orange (MO) was degraded by pure Ag₂S after 30 min of irradiation with visible light, whereas Pourahmad²¹ reported 41% degradation of methylene blue by pure Ag₂S within 60 min under UV light. Reddy et al.²⁴ reported the photodegradation of rhodamine B with Ag₂S with a 21.78% rate of conversion after 120 min of irradiation with sunlight.

These results suggest that the photocatalytic performance of pure Ag₂S is unsatisfactory. However, the low band gap energy (0.9–1.05 eV)²⁵ of Ag₂S with a conduction band (CB) at about 0 eV vs. NHE²⁶ means that it absorbs in the UV, visible and NIR regions, making it a possible semiconductor material for photocatalytic applications under visible and NIR irradiation. Thus we thought that it was worth systematically investigating the photocatalytic ability of pure Ag₂S for potential application to the degradation of persistent organic pollutants.

We prepared a pure Ag₂S photocatalyst by a facile ion-exchange method and used it to decompose MO under visible and NIR irradiation. The morphology and structure of the as-prepared Ag₂S were characterized and its photocatalytic performance was investigated in detail. The working mechanism of Ag₂S was determined by simulation, electron spin resonance (ESR) analysis and photodegradation experiments.

2. Experimental

2.1 Chemicals

All the reagents used were of analytical-reagent grade and were used without further purification. All the reagents were obtained from Chengdu Ke Long Chemical of China.

2.2 Preparation of Ag₂S photocatalyst

The Ag₂S photocatalyst was prepared by a facile ion-exchange method at room temperature. In a typical process, 30 mL of Na₂S·9H₂O (0.1 M) was dropped into 40 mL of AgNO₃ (0.2 M) solution using an automatic titrator at a speed of 3.2 mL min⁻¹. After 30 min of magnetic stirring, the black products were filtered and washed with deionized water at least five times to remove the unreacted components and the NaNO₃ produced. Finally, the black powders were vacuum-dried overnight at 60 °C in the dark. All these operations were carried out as far away from light as possible.

2.3 Characterization

X-ray diffraction (XRD) patterns for Ag₂S powder samples were obtained by glancing-angle X-ray diffraction (X'Pert proMPD, the Netherlands) with a Cu Kα 40 kV/40 mA X-ray source (λ = 0.15406 nm). Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectrometry (EDS) analysis were carried out using a JSM-7500F field-emission scanning electron microscope (JEOL). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a Tecnai G2 F20 S-TWIN microscope.

X-ray photoelectron spectrometry (XPS) measurements were performed on a VG ESCALAB MKII XPS system with an Al Kα source and a charge neutralizer. UV-visible diffusion reflection spectrometry (UV-visible DRS) was performed using a UV-visible spectrophotometer (TU-1901, Ge Beijing Spectrometer, China) and a fine-grained BaSO₄ powder was used as the standard. Photoluminescence (PL) spectra were recorded with a fluorescence spectrometer (F-7000, Hitachi, Japan). The absorption spectrum of the MO solution was obtained using a UV-visible spectrophotometer (TU-1901, Ge Beijing Spectrometer, China). The particle size distribution of the prepared Ag₂S samples was determined with a JL-1198 laser particle size analyser (Chengdu Jingxin Powder Analyser Instruments Co. Ltd, China). The surface areas of the photocatalysts were determined by the nitrogen adsorption method at −196 °C with a BET analyser (SSA-3500, China). The FTIR spectra of the samples were recorded with KBr pellets in the range 4000–400 cm⁻¹ on an FTIR spectrometer (Spectrum II L1600300, PerkinElmer) at room temperature. The active species were investigated with an ER200-SRC ESR analyser.

2.4 Measurement of photocatalytic activity

The photocatalytic activities of the samples were evaluated by the degradation of MO under visible and NIR light irradiation in an OCRS-IV photoreactor system, which included a 500 W Xe lamp and a 500 W Hg lamp, a cut-off filter with a specified light wavelength and a water filter. About 0.1 g of the Ag₂S photocatalyst was placed in a quartz reactor tube with 10 mL of aqueous MO solution (16 mg L⁻¹). The suspension was stirred for 120 min in the dark to reach adsorption–desorption equilibrium. Visible light (λ > 380 nm) or NIR light (λ > 850 nm) irradiation was switched on to start the photodegradation reaction. The photoreaction was terminated after the scheduled time and the suspension was sampled and centrifuged to remove the solid particles. The concentration of the MO solution was determined by UV-visible spectrophotometry. Fig. S3† shows the corresponding standard absorption curve of the MO solution.

2.5 Theoretical calculations

Theoretical calculations were carried out using the plane wave pseudopotential density functional theory (DFT) method. The ion–electron interaction was modelled by the ultrasoft pseudopotential in the Vanderbilt form. The energy cut-off for a plane wave basis set was 400 eV and a Monkhorst–Pack k-mesh of 6 × 6 × 6 was used. Geometry optimizations were performed before the single-point energy calculation and self-consistent convergence accuracy was set at 1 × 10⁻⁶ eV per atom. The convergence criterion for the maximum force between atoms was 0.01 eV. The maximum displacement was 5 × 10⁻⁴ Å and the stress was 60 GPa. The U_p parameter for S was 7 eV and the U_d parameter for Ag was 13.2 eV. As a result of the complete symmetry between the spin-up and spin-down states, we only report the spin-up results here.

In general, U_d + U_p increases the splitting between the occupied and empty energy levels because the correction depends on the occupancy of the orbitals. The energy band structure using GGA + U, where U_S,p = 7 eV and U_Ag,d = 13.2 eV, was used.

3. Results and discussion

3.1 Characterization of photocatalyst

The as-prepared Ag₂S samples were black or dark brown powders. The surface morphology, structure and band structure were characterized using XRD, XPS, SEM, EDS and UV-visible DRS.

3.1.1 XRD analysis. Fig. 1a shows the crystallization and phase identification of Ag₂S using XRD. The diffraction peaks were assigned to monoclinic Ag₂S (JCPDS no. 65-2356) with the lattice constants a = 4.230 Å, b = 6.910 Å and c = 7.870 Å. No obvious impurity peak was observed. Based on the XRD data, the crystallite size of Ag₂S was determined to be 32.6 nm using the Debye–Scherrer equation:
where D is the crystallite size for the catalyst, k is a dimensionless constant, λ is the wavelength of the X-rays (0.15406 nm), β is the full-width at half-maximum of the diffraction peak and θ is the diffraction angle.

Fig. 1 Structure and component analysis. (a) XRD pattern of Ag₂S; (b) XPS survey spectra of Ag₂S; (c) Ag_3d peaks of Ag₂S; and (d) S_2p peaks of Ag₂S.

3.1.2 XPS analysis. XPS was used to determine whether impurity elements such as N and Na were present and to determine the valence state of the Ag atoms. Fig. 1b confirms that no heteroatom was introduced into the crystal, despite the fact that the catalyst was obtained from the reaction of AgNO₃ and Na₂S in aqueous solution. This agrees with the results of the EDS analysis given in Fig. S1.†

The peaks at 368.21 and 374.26 eV in Fig. 1c can be assigned to the Ag_3d5/2 and Ag_3d3/2 of Ag⁺ ions in the Ag₂S photocatalyst.²⁷ The peaks at 161.3 and 162.5 eV of the S_2p photoelectron spectrum in Fig. 1d were in perfect agreement with the binding energy of S_2p3/2 and S_2p1/2 of S²⁻.²⁸ It was concluded that the as-prepared sample consisted of pure Ag₂S without any Ag and S atoms of other valencies.

3.1.3 Microscopic surface structure and morphology of Ag₂S. The crystalline structure of a photocatalyst affects its photocatalytic activity. We therefore investigated the typical microscopic surface structure and morphology of the as-prepared Ag₂S by SEM and TEM.

Fig. 2a is a typical SEM image of the Ag₂S samples showing that the Ag₂S particles were an aggregation of crystal grains with diameters of 30–80 nm, consistent with the XRD results. The TEM image in Fig. 2b confirms that the size of the Ag₂S aggregates was about 0.2–0.7 μm, consistent with the results obtained with the laser particle size analyser (Fig. S2†) and BET analysis; the average particle size was 0.7226 μm and the specific surface area was 6.062 m² g⁻¹. Fig. 2c shows the HRTEM image of an Ag₂S sample in which the lattice fringes can be clearly seen. The clear fringes with intervals of 0.34 and 0.38 nm can be indexed to the (012) and (002) lattice planes of Ag₂S, respectively.

Fig. 2 (a) SEM, (b) TEM and (c) HRTEM images of Ag₂S.

3.1.4 UV-visible DRS analysis. Fig. 3a shows the UV-visible absorption spectra of the Ag₂S sample. The as-prepared Ag₂S sample had a large amount of absorption in the UV and visible regions, suggesting that Ag₂S should have good photocatalytic activity under irradiation with UV and visible light. The absorption edge of the as-prepared Ag₂S samples was >800 nm, suggesting possible photocatalytic activity in the NIR region.

Fig. 3 (a) The UV-vis DRS spectra of Ag₂S, (b) plot of (ahν)²–hν.

Fig. 3b showed the results of UV-visible DRS, which is used to estimate the band gap of semiconductors. By plotting the graph of (ahv)² versus hv, where a is the absorption coefficient and hv is the photon energy,²⁹ the band gap of the as-prepared Ag₂S samples was estimated to be 1.078 eV, which was in agreement with previously reported values.³⁰

3.1.5 PL analysis. The PL signals of semiconductor materials result from the recombination of photoinduced charge carriers. In general, the lower the PL intensity, the lower the recombination rate of the photoinduced electron–hole pairs and the higher the photocatalytic activity of the semiconductor photocatalysts.^31,32 PL was therefore used to identify the photocatalytic activity of Ag₂S.

The PL emission spectra of pure Ag₂S and pure TiO₂ were examined in the wavelength range 300–700 nm (Fig. 4). The intensity of the PL spectrum of pure TiO₂ was much higher than that of the Ag₂S samples. It can be reasonably inferred that pure Ag₂S should have a higher photocatalytic performance than TiO₂.

Fig. 4 PL spectra of Ag₂S and TiO₂.

3.2 Photocatalytic activity

A typical persistent organic dye, MO, was selected as the target compound in the photodecomposition experiment to evaluate the photo-oxidation ability of Ag₂S. The adsorption and photodegradation of MO were determined with Ag₂S as a photocatalyst under irradiation with visible and NIR light.

3.2.1 Adsorption of MO on Ag₂S. The adsorption of MO on the surface of the Ag₂S nanoparticles in the dark was determined first to exclude the extra effect of this on the performance of the photocatalyst. Fig. 5 shows that there was a sharp decrease in the MO concentration from the start time to time zero. However, adsorption–desorption equilibrium was achieved after 1 h and no significant decrease in the MO concentration was observed after this time.

Fig. 5 Photodegradation of MO in aqueous solution (a) with fresh Ag₂S under irradiation with visible light and (b) with fresh Ag₂S under irradiation with NIR light.

3.2.2 Photodegradation of MO with Ag₂S. After the adsorption equilibrium of MO with Ag₂S had been achieved, we conducted a photocatalytic decomposition experiment of MO with Ag₂S under irradiation with visible and NIR light to evaluate the photocatalytic performance of Ag₂S. The degradation curve for MO (Fig. 5) shows that Ag₂S completely degraded MO within 30 min under irradiation with visible light and within 70 min under irradiation NIR light. The repeatability experiments for the degradation of MO over Ag₂S are shown in Fig. S5.†

The fast rate of degradation of MO with Ag₂S under irradiation with NIR light can be attributed to the huge surface area resulting from the aggregated structure of the Ag₂S crystal grains and the strong absorption in visible and NIR regions. However, the relatively slow decomposition rate of MO under irradiation with NIR light compared with visible light can be ascribed to the weak illumination of the NIR light from the same Xe lamp, which supplied a visible light illumination of 18–21 klx and an NIR light illumination of 0.09–0.12 klx.

These excellent properties confirmed the proposal that Ag₂S could work under NIR irradiation with a high degradation rate because of the narrow band gap (1.078 eV) and high quantum efficiency. This outstanding photocatalytic activity and wide working spectrum of Ag₂S is attractive for further development.

3.2.3 FTIR analysis. It was necessary to determine the degradation of the adsorbed MO on the surface of Ag₂S under light irradiation as about 40% of the MO had been adsorbed, despite the fact that the MO in aqueous solution had been completely degraded under light irradiation. Two Ag₂S samples were analysed with FTIR spectrometry after reaching MO adsorption equilibrium to determine the functional groups adhered to the surface of the Ag₂S and to confirm the photodegradation of the adsorbed MO. One sample was exposed to light for 30 min (sample 1) and the other sample was kept in the dark (sample 2).

The results in Fig. 6 confirm the complete decomposition of the MO adsorbed on the surface of the Ag₂S. The peaks at 1601 and 814 cm⁻¹ of sample 2 can be assigned to the framework vibrations of the benzene ring and the two substituted phenyl groups, respectively. These two characteristic peaks of MO were not observed in the FTIR spectrum of sample 1.

Fig. 6 FTIR spectra of Ag₂S with adsorbed MO: after (sample 1) and before (sample 2) light illumination.

Samples 1 and 2 were both soaked in deionized water. This phenomenon (Fig. S4†) and the results of FTIR analysis proved that MO can be completely photodegraded with Ag₂S under light irradiation in both the adsorbed and dissolved states.

3.3 Photocatalytic mechanism of Ag₂S photocatalyst

˙OH radicals and photogenerated holes are both possible active species in the photocatalytic degradation of organic pollutants. However, as the band gap of Ag₂S is only 1.078 eV and ˙OH has a large electrode potential of 2.80 eV, ˙OH may not play a part in the working mechanism for Ag₂S. It was therefore necessary to determine the active species and work out the photocatalytic mechanism for Ag₂S under light irradiation.

3.3.1 Determination of radical species. A spin-trapping technique is often used to detect free radicals by trapping a short-lived radical to generate a long-lived nitroxide radical, which can then be monitored using conventional ESR methodology.³³ The photocatalytic mechanism of Ag₂S with and without light irradiation was investigated by ESR (Fig. 7). No significant ESR signal was detected for Ag₂S in the dark. In contrast, a septet peak group with an intensity ratio of 1 : 2 : 2 : 2 : 2 : 2 : 1 was observed, consistent with the nitroxide DMPO–˙O₃⁻. The determined free radical should therefore be the ozone anion radical.³⁴ As the unique signal of ozone anion radicals was the only signal observed in solution with Ag₂S under light irradiation, we concluded that this free radical was photogenerated by Ag₂S in aqueous solution.

Fig. 7 ESR analysis of Ag₂S under irradiation with visible light and in the dark.

3.3.2 Radical-trapping experiments. Although the ozone anion radical was detected during the light irradiation of Ag₂S, it was also necessary to determine the role that this radical played in the photo-oxidation process. A radical-trapping experiment was conducted to identify the major active species in the degradation of MO on the surface of Ag₂S. Two scavengers, methanol and indigo carmine, were used to trap the photogenerated holes and ozone. Possible disturbances caused by the masking effect of the two scavengers in the detection of MO with UV-visible spectrophotometry was excluded based on the results shown in Fig. 8a. The degradation of MO after the addition of various scavengers is shown in Fig. 8b. The addition of methanol decreased the rate of photodegradation of MO significantly because the photogenerated holes were trapped. However, the addition of indigo carmine stopped the photo-oxidation process completely. This clear result confirmed that the combined function of the photogenerated holes and the ozone anion radicals under light irradiation dominated the oxidative degradation of MO. The ozone anion radicals may be the major active species in the photocatalytic degradation of Ag₂S.

Fig. 8 Photodegradation of MO with Ag₂S. (a) Spectrophotometric spectra of different mixtures composed of pure MO, MO + methanol and MO + indigo carmine. (b) Degradation curve of Ag₂S with MO + indigo carmine, MO + methanol and pure MO.

3.3.3 DFT simulation of Ag₂S. To further understand the electronic structure and mechanism of chemical bonding of the prepared Ag₂S, ab initio DFT calculations were carried out. Fig. 9 shows the crystal cell model used for the DFT simulation of Ag₂S. The space group was based on P21/c (no. 14), corresponding with the measured XRD results of the as-prepared samples. The provided bond length of the Ag₂S bond was 2.692 Å and the bond angle was 105.894°.

Fig. 9 Unit cell structure of Ag₂S. Yellow and blue spheres represent S and Ag atoms, respectively.

The calculated band structure (Fig. 10) confirmed that Ag₂S was a semiconductor with a direct band gap of 1.063 eV at the G point. This calculated value for the Ag₂S band gap was slightly lower than the determined value of 1.078 eV, but closer to the previously reported value of 1.05 eV.³⁰ This slight discrepancy may be ascribed to the underestimation of the DFT, which considered only the excited states in the calculation.

Fig. 10 Energy band structure and density of states of Ag₂S.

Fig. 11 plots the calculated total density of states (TDOS) and the partial density of states (PDOS) for Ag₂S. The valence atomic configurations were 3s²3p⁶ for S and 4d¹⁰ for Ag, respectively. The valence band of Ag₂S is clearly split into two regions. The lower region probably has S_3p features, whereas the upper region is dominated by the Ag_4d states. There were hybridized S_2p states near the Fermi level, with little practical contribution from the Ag_5p states that have a dominant weight at the bottom of the CB. The width of the valence band for Ag₂S was 10 eV. The S_3p states had an intensity peak at 0.68 eV. In the upper region, two Ag_4d maxima were found at −9.2 and −7.18 eV.

Fig. 11 TDOS and PDOS of Ag₂S.

3.3.4 Working mechanism of Ag₂S. The photocatalytic activity of a photocatalyst is determined by the band gap and the oxidation potential of the photogenerated holes. The CB and valence band (VB) potentials of Ag₂S at the point of zero charge were calculated by the following equation³⁵ to investigate the oxidation potential of the photogenerated holes:

E_VB = X − E_e + 0.5E_g

where X is the Pearson absolute electronegativity of the semiconductor, defined as the geometric mean of the absolute electronegativity of the constituent atoms,³⁶ E_e is the energy of free electrons on the hydrogen scale (about 4.69 eV), E_VB is the VB edge potential and E_g is the band gap of the semiconductor. The CB position can be determined by:

E_CB = E_VB − E_g

The value of X for Ag₂S is about 4.968 eV.³⁷ Based on these equations, the top of the VB and the bottom of the CB of Ag₂S were calculated to be 0.992 and −0.072 eV, respectively. This result agrees with previously published data.³⁸

Based on this analysis, the photo-oxidation mechanism of Ag₂S driven by light can be established and described as follows. (1) Under light irradiation, Ag₂S is transformed into the excited state, generating excited electrons and holes on the surface or in the interior simultaneously. (2) The electrons and holes react with the oxygen dissolved in aqueous solution and the water molecules absorbed on the surface of Ag₂S particles, producing ozone anion radicals. (3) The ozone anion radicals, extremely reactive species, oxidize the organic molecules. (4) The photogenerated holes can also oxidize the organic molecules directly.

Fig. 12 is a schematic diagram of the charge-transfer process in Ag₂S.

Ag₂S + hv → e⁻ + h⁺ (1)

O₂ + 2OH⁻ + 2h⁺ + e⁻ → O₃⁻ + H₂O (2)

O₃⁻ + dye → oxidation products (3)

h⁺ + dye → oxidation products (4)

Fig. 12 Schematic diagram of the charge-transfer process in Ag₂S.

4. Conclusion

An Ag₂S photocatalyst has been successfully synthesized by a facile ion-exchange method. The as-prepared Ag₂S was an aggregation of crystal grains with diameters of 30–80 nm and was an excellent photocatalyst for the decomposition of MO under light irradiation. The complete degradation of MO with Ag₂S was reached within 30 min under visible light illumination and within 70 min under NIR light illumination. MO was completely photodegraded by Ag₂S under light irradiation whether it was in the adsorbed or dissolved state. Such an attractive performance is ascribed to the narrow band gap of Ag₂S (1.078 eV) obtained both by theoretical calculations and by experiment. The PL spectrum of pure Ag₂S confirmed its high quantum efficiency. The combined function of the photogenerated holes and the ozone anion radicals of Ag₂S in aqueous solution under light irradiation dominated the reaction process of the oxidative degradation of MO. The simple preparation method, excellent photocatalytic performance and ability to work under NIR light irradiation broadens the possible applications of Ag₂S as a promising photocatalyst.

Acknowledgements

We appreciate the financial support from the National Natural Science Foundation of China Project (no. 21176157 and 21476146).

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Footnote

† Electronic supplementary information (ESI) available: EDS and the particle size distribution of Ag₂S; spectrum of methyl orange solution at different concentrations; extraction of adsorbed MO on Ag₂S with DI water; repeatability experiments of MO degradation with Ag₂S under light irradiation. See DOI: 10.1039/c4ra15774e

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