Abstract

A series of different ratios of Ag2S/ZnO/ZnS nanocomposites with visible light response were prepared by a microwave-assisted hydrothermal two-step method, whose composition, crystalline structure, morphology and surface physicochemical properties were well-characterized via XRD, XPS, UV–vis/DRS, PL, SEM, HR-TEM and N2 adsorption–desorption measurements. Results showed that as-composites mainly consisted of ZnS crystal phase, whose grain size increased obviously compared with non Ag2S samples. At the same time, due to the introduction of narrow band gap Ag2S, the synthesized composite can effectively increase the visible optical absorption of ZnO/ZnS composites. Among them, 1% Ag2S/ZnO/ZnS showed a mixed structure of nano-line and nano-particle, of which BET value increased significantly, and the morphology was more excellent. Photocatalytic activities of a series of Ag2S/ZnO/ZnS composites under different light sources were studied using methyl orange as a model molecule, and 1% Ag2S/ZnO/ZnS was taken as the best one. Meanwhile, 1% Ag2S/ZnO/ZnS also showed a good degradation effect on other dyes with different structures, and its degradation efficiency did not change significantly after three cycles, showing certain stability. In addition, composites with Ag2S loading of 1% possessed the highest hydrogen production ability of photolysis water, indicating that the introduction of Ag2S had significantly enhanced the catalytic performance.

INTRODUCTION

With the rapid development of modern industry and the increasing consumption of daily life, solving environmental pollution and developing renewable energy have become one of the main subjects of human society. Since photocatalytic treatment of organic pollutants can directly use light at room temperature for the reaction, and the use of the catalyst is generally non-toxic, harmless and easy to recover, the resulting product is usually inorganic ions, CO2 and H2O, no secondary pollution (Gutierrez-Mata et al. 2017). Therefore, the photocatalysis technology is becoming a kind of green environmental protection technology with some practical applications (Li et al. 2014).

At present, many semiconductor materials have been used for photocatalysts in photocatalysis, such as TiO2, ZnO, ZnS and transition metal sulfides Ag2S etc (Akpan & Hameed 2009). As an important sulfur compound, Ag2S has been widely used in optical cells, photoconductors, solar selective coatings and infrared detectors. This is mainly due to the low band gap of Ag2S about 0.9 eV, which makes it possible to make full use of solar energy and lead to a higher optical performance (Konstantinou & Albanis 2004). ZnS is a type II–VI family sulfide semiconductor material with a certain photocatalytic activity. The band gap of ZnS is wide, about 3.67 eV, which belongs to a good indirect bandgap semiconductor. In recent years, a large number of studies have found that ZnS shows a better ability for photocatalytic degradation and photolysis of water to hydrogen, which has gradually become a hot spot in domestic and foreign research (Li et al. 2015). ZnO is a type II–VI oxide semiconductor material, which has other materials incomparable advantages of high photosensitivity, non-toxicity, low cost and environmental sustainability, and has also become one of the most important semiconductor photocatalytic materials (Sakurai et al. 2016).

Although the above photocatalytic monomers possess certain photocatalytic properties, the photocatalytic efficiency of ZnS and ZnO is limited to only ultraviolet absorption, and the narrow Ag2S band gap leads to the rapid recombination of electrons and holes (Chen et al. 2014). To this end, a variety of methods have been explored to improve the photocatalytic efficiency, such as ion doping, the deposition of precious metals and the construction of heterostructures (Chouhan et al. 2016). In these methods, the combination of ZnO with ZnS or other visible light response semiconductors to form heterogeneous structures has been proved to be a very effective method (Ma et al. 2017). By controlling the structural parameters of nanocomposites, the compound effect can make the material obtain excellent performance in all aspects. Among these, the use of a binary or multi-component composite photocatalyst is one of the important means of improving the photocatalytic activity. Therefore, this paper will use the combination of different nanomaterials to make up for the shortcomings of semiconductor monomers and improve their photocatalytic properties, of which the study has certain prospects (Kandula & Jeevanandam 2014). For many years, the traditional method of hydrothermal synthesis of multi-component materials has been the main means of synthesis. However, with the maturing of microwave technology in recent years, the microwave-assisted synthesis method has been favored by researchers for its rapidity and simplicity. Compared with other traditional synthesis methods, microwave-assisted synthesis has unique advantages, such as selective heating, fast heating, high reaction efficiency, low energy consumption, microwave induced polarization growth and so on, and it is widely used in the preparation of semiconductor nanomaterials (Sharma et al. 2011). For example, Huaqiang Wu et al. (Wu et al. 2008) used microwave irradiation to prepare carbon nanotubes wrapped with uniform and small diameter ZnS nanospheres, which had excellent photocatalytic properties. For the current research, the use of microwave-assisted technology can often optimize the product morphology and enhance the activity.

Accordingly, Ag2S/ZnO/ZnS nanocomposites were synthesized by a microwave-assisted two-step method. On the one hand, it is hoped that the absorption of light can be effectively changed by the combination of wide and narrow bandgap semiconductors, and the absorption sidebands are red-shifted so as to achieve higher sunlight utilization efficiency. On the other hand, we hope to improve the morphology and particle distribution of the synthesized products through the unique polarization of microwaves to improve the photocatalytic activity of Ag2S/ZnO/ZnS nanocomposites. In addition, through the combination of three semiconductor materials, we can change the photoelectron migration path in photocatalysis to a certain extent, reduce the recombination probability of photogenerated electron–hole pairs and improve the efficiency of the photocatalytic reaction. Moreover, in this paper we firstly determine the best performance of binary ZnO/ZnS ratio by changing the amount of ZnO material, then by changing the amount of Ag2S material to further optimize the performance of the catalyst. At the same time, in the research into photocatalytic performance, the degradation of methyl orange under the photocatalytic conditions of UV light or visible light and photolysis of water to produce hydrogen with Ag2S/ZnO/ZnS were studied, and some better results were obtained.

EXPERIMENTAL

Materials

Ethylene glycol was purchased from Kaitong Chemical Reagent Co., Ltd., Tianjin, China. Thiourea was obtained from Fuchen Chemical Reagent Co., Ltd., Tianjin, China. Anhydrous zinc acetate was purchased from Sinopharm Chemical Reagent Co., Ltd. Silver nitrate was obtained from Guangfu Technology Development Co., Ltd, Tianjin, China. Methyl orange (MO) was purchased from Beijing Chemical Inc., China. All reagents were analytical grade. Doubly distilled water was used in all experiments.

Synthesis of ZnS

In 28 mL ethylene glycol, the excess thiourea was added to the mixture, and then stirred until all the particles were dissolved into zinc acetate, and continue to be stirred for 30 min. Put in the microwave reactor and kept 75 min at 150 °C. After cooling to room temperature, the obtained light yellow solid product was washed three times with deionized water and ethanol, respectively, and finally dried at 60 °C for 12 h to obtain a final sample for comparative experiments.

Synthesis of ZnO/ZnS with different proportions

Different amounts of thiourea were added to 28 mL of ethylene glycol respectively. When the particles were completely dissolved, a certain amount of zinc acetate was added into the mixture during stirring, and stirring was continued for 30 min. This was put in the microwave reactor and then maintained at 150 °C for 75 min. After cooling to room temperature, the light yellow solid product obtained from each reactor was washed with deionized water and ethanol three times, respectively, and the resulting products were dried at 60 °C for 12 h to obtain the final sample. Among them, the ratio of the amounts of substances of zinc acetate and thiourea was taken as the amount of change, and the samples were labeled as ZnO/ZnS (2: 1), ZnO/ZnS (1: 1), ZnO/ZnS (1: 3) and ZnO/ZnS (1: 4), respectively.

Synthesis of Ag2S/ZnO/ZnS composites with different proportions

The above synthesized ZnO/ZnS (1: 2) nanocomposites were placed in 40 mL of deionized water and stirred for 30 min, the solution was suspended. Weighed AgNO3 can produce Ag2S in the amount of 0.5%, 1%, 1.5% 2%, and 5% to dissolve it in distilled water, it was added dropwise to the suspension, which was brown at this time and became deeper as the content increased. Several different suspensions were put into the microwave reactor, and then maintained at 150 °C for 75 min. After cooling to the room temperature, the brown solid obtained in each reactor was washed three times with deionized water and ethanol, respectively, and finally the resulting products were dried at 60 °C for 12 h to obtain a final sample, marked as X% Ag2S/ZnO/ZnS (X = 0.5, 1, 1.5, 2, 5).

Characterization

The phase and composition of the samples were detected by Bruker-AXS (D8) X-ray diffractometer (XRD) studies. The optical properties of the samples were analyzed by UV–vis diffuse reflectance spectroscopy (UV–vis/DRS) using a UV–vis spectrophotometer (TU-1901). The photoluminescence spectrum (PL) was recorded by a Hitachi F-7000 fluorescence spectrophotometer. The particle microstructure was investigated by high-resolution transmission electron microscopy (HRTEM) (JEM-2100F). The chemical composition analysis of the sample surfaces was made by X-ray photoelectron spectra (XPS) using an ESCALAB 250 Xi spectrometer. The morphologies and microstructures of as-prepared samples were investigated by scanning electron microscope (SEM) (Hitachi S-4300). The specific surface areas of the samples were measured by a Brunauer − Emmett − Teller specific surface area instrument (Beishide Instrumentation Technologies (Beijing) Ltd., Model 3H-2000PS2).

Photocatalytic experiment

The photocatalytic experimental device consists of a built-in light source surrounded by a cylindrical quartz outer tube and a quartz sleeve. The distance between the solution and the light source is about 10–15 mm. The visible light source is a 400 W Xe lamp (the main emission line is greater than 410 nm); the UV light source is a 125 W high pressure Hg lamp (the main emission line has a wavelength of about 313.2 nm). The concentration of reaction solution (such as methyl orange, MO) was 50 mg/L and the amount of photocatalyst was 0.3 g, and the volume of reaction solution was 220 mL under visible light irradiation. When UV light was used, the amount of catalyst was 0.15 g and the volume of reaction liquid was 90 mL. Photocatalytic reaction process: the catalyst was dispersed in methyl orange solution to form a suspension, ultrasound ca. 10 min, and then the suspensions were magnetically stirred for 30 min in the dark to ensure the adsorption − desorption equilibrium between MO and photocatalyst powders. The light source with stabilized light intensity was placed in the reaction solution. The photocatalytic experiment was carried out in the photocatalytic reaction device. The catalysts were removed by centrifugation after the completion of the reaction. The absorption spectrum of the centrifuged solution was recorded using a TU-1901 UV–vis spectrophotometer (China). The concentration change of MO was determined by monitoring the optical intensity of absorption spectra at λmax (460 nm) of methyl orange.

Photocatalytic hydrogen evolution test

The photocatalytic hydrogen production was carried out in a vacuum reactor connected to a closed loop system (labsolar-III AG system). 100 mg of photocatalyst was dispersed in 50 mL of distilled water and a certain amount of Na2S·H2O and Na2SO3 were added as a sacrificial agent. After degassing in vacuum, the light of the hydrogen production experiment of photolysis water was tested under stirring. A 300 W Xe lamp was the light source, fixed at a distance of 10 cm from the reaction solution; high purity nitrogen as a carrier gas at a flow rate of 0.5 mL/s; an output pressure of 0.4–0.5 MPa; and an operating voltage and operating current of about 20 mV and 50 mA. During the reaction, circulating cooling water kept the temperature of the reactor at around 5 °C. Gas was collected after a certain light exposure and the hydrogen evolution process was analyzed by gas chromatography. According to the peak area of the different reaction time, the amount of hydrogen production was calculated, and the catalytic activity of the catalyst was determined by the total amount of hydrogen produced in 8 hours.

REAULTS AND DISCUSSION

XRD analysis

The crystal structure and the composition of Ag2S/ZnO/ZnS composites with different proportions of ZnO/ZnS synthesized with different ratios of zinc acetate and thiourea, and microwave-assisted hydrothermal treatment, were studied by X-ray diffraction (XRD); the results are shown in Figure 1. Figure 1(a) is the XRD patterns comparison chart for the synthesized 1% Ag2S/ZnO/ZnS composites with ZnS and ZnO. First, the characteristic peak of Ag2S was not detected by 1% Ag2S/ZnO/ZnS composites prepared by the microwave solvothermal method and subsequent microwave hydrothermal method. The reason may be that the amount of Ag2S in the composite materials is extremely small and cannot be detected (Li et al. 2016). The ZnO is a cubic crystal phase and the ZnS is a six square crystal phase in 1% Ag2S/ZnO/ZnS composites (Li et al. 2015). ZnO and ZnS have similar diffraction peaks, which can easily lead to the overlapping of diffraction peaks, showing the emergence of a wide peak in the composites. As shown in Figure 1(b), ZnO/ZnS diffraction peaks of different proportions are relatively wide. After the secondary microwave reaction, Ag2S is added to the ZnO/ZnS materials, and the characteristic peaks of ZnS are sharper. The reason is that the second heating of the microwave reaction causes the crystallinity of the product to change. The free rotation of the ion clusters in the crystal leads to higher symmetry, thus changing the growth direction of the crystal (Liu et al. 2015). According to the Scherrer formula: Dc = 0.89λ/(Bcosθ) (λ is the X-ray wavelength, B is the full width at half maximum of the diffraction peak and θ is the diffraction angle). The grain sizes of the synthesized products are calculated as shown in Table S1. The grain size changes, due to the different proportions of the raw materials and the synthesis methods in the experiment, which lead to the change in the crystallinity of the composite material.

Figure 1

XRD patterns of 1%Ag2S/ZnO/ZnS compared with standard ZnO and ZnS (a); XRD patterns of different proportion composites (b).

Figure 1

XRD patterns of 1%Ag2S/ZnO/ZnS compared with standard ZnO and ZnS (a); XRD patterns of different proportion composites (b).

UV–vis/DRS analysis

In order to investigate the optical property of Ag2S/ZnO/ZnS composites, in this paper, UV–visible diffuse reflectance analysis of the monomer ZnS, different proportions of ZnO/ZnS composites and X% Ag2S/ZnO/ZnS were analyzed, and the results are shown in Figure S1. Among them, in Figure S1a, the absorption edge of the synthesized ZnS is located in the ultraviolet region with a wavelength less than 340 nm. However, in the preparation process, the band gap of the complexed ZnO and ZnS is a wide band gap, and the absorption edges of both of these are mostly in the ultraviolet region. And with the increase in ZnO content, the absorption edge redshifts (Dai et al. 2011). In the compounding process, the ratio of zinc acetate to thiourea increases, and the ratio of ZnO decreases and the absorption edge blueshifts. However, adding the narrow bandgap Ag2S compound into the ZnO/ZnS system, it can be seen from Figure S1b, the increase in Ag2S ratio in the UV region and the visible region is larger, indicating that the composite of Ag2S can help to enhance the photocatalytic performance of ZnO/ZnS composites in the ultraviolet and visible regions. The bandgap of the sample can be calculated by the formula: (αhv)n = K(hv – Eg). α is the absorption coefficient, K is the parameter related to the valence band and conduction band, n = 2 is the indirect band gap semiconductor, n = 1/2 is the direct band gap semiconductor, hv is the absorbed energy, and Eg is the bandgap energy (Nguyen et al. 2016). In this paper, we introduce the UV–visible diffuse reflectance data of ZnO/ZnS (1:2) and 1%Ag2S/ZnO/ZnS into the above formula, and calculate the relation diagram of the Kubelka–Munk function and energy obtained when n = 2, as shown in Figure S1c. The bandgap energy values of different composites are shown in Table S1. It can be seen that the introduction of Ag2S into ZnO/ZnS composites leads to a decrease in the bandgap energy of nanocomposites, indicating that Ag2S compounding can help to increase the photoresponse range of composites.

PL analysis

In order to study the separation and recombination of photogenerated electrons and holes on Ag2S/ZnO/ZnS composites, the PL spectra were tested (Ma et al. 2017). The PL spectra of ZnS, ZnO/ZnS (1:2) and 1% Ag2S/ZnO/ZnS composites excited at 425 nm are shown in Figure S2. Due to the rapid recombination of photogenerated electrons and holes, ZnS shows the strongest emission at a wavelength of 515 nm. After loading ZnO, the PL intensity of ZnO/ZnS composites decreases, indicating that the formation of ZnO/ZnS composites has a significant inhibitory effect on the recombination of photogenerated electrons and holes. After continuous loading of Ag2S, the PL intensity of Ag2S/ZnO/ZnS composites continues to decrease. The electron transfer between Ag2S and ZnO/ZnS suppresses the recombination of electrons and holes, thereby improving the photocatalytic activity and the stability of 1% Ag2S/ZnO/ZnS composites.

XPS determination

In order to study the elements on the surface of composites and their valence states, XPS analysis of 1% Ag2S/ZnO/ZnS composites was carried out, as shown in Figure S3. Figure S3a shows Zn, O, S and other elements clearly. However, compared with other elements, there are fewer Ag elements, which are hard to find in the full spectrum of XPS and can be analyzed by the XPS spectrum of S. As can be seen from Figure S3b, the highest binding peak energy of Zn in the sample is 1,023.23 eV and 1,045.97 eV, respectively, corresponding to Zn 2p3/2 and Zn 2p1/2, with a peak center-to-center spacing of 22.74 eV, which proves that the Zn element in the sample is in the Zn2+ form (Bora et al. 2016). Figure S3c shows the XPS spectrum of the element S, from which the peak corresponding to the highest binding energy is located at 162.5 eV; in contrast to the previous literature, the S element in the sample corresponds to −2 valence states (Wang et al. 2015). Through the analysis of the oxygen in the sample obtained in Figure S3d, the results show that the binding energies are 533.1 eV, 531.8 eV and 530.4 eV, respectively, corresponding to the hydroxyl oxygen adsorbed on the composite surface, the adsorption oxygen and the valence oxygen ions existing in the form of ZnO (Wang et al. 2016). The trace elements presenting in the sample can be confirmed by XPS analysis. The results show that Ag is successfully loaded into the ZnO/ZnS composite system through the microwave hydrothermal reaction, and the peaks at 368.43 eV and 374.39 eV at the binding energies correspond to Ag 3d5/2 and Ag 3d3/2, respectively, of which the center of the peak height is an approximation of 6.0 eV, indicating that Ag is +1 in the composites. The results of XPS analysis show that 1% Ag2S/ZnO/ZnS composite is consistent with the expected product in Figure S3e.

SEM and HR-TEM analysis

In this paper, the morphology of samples was analyzed by the scanning electron microscopy (SEM), and the results are shown in Figure S4. Figure S4(a, b) is the scanning electron microscope images of the binary ZnO/ZnS composites. It can be clearly seen that the samples show irregular clusters of particles, particles form large clusters, and few particles exist alone. By the microwave-assisted hydrothermal method, Ag2S is compounded to ZnO/ZnS composites, and the results are shown in Figure S4(c and d). Nanowires are grown in larger clusters of aggregates, showing the coexistence of nanoparticles and nanowires. And from Figure S4d, we can see that the rough nanowires grow on the nanoclusters, and the surface of the nanowires is irregularly granular, indicating that the growth of the nanowires is caused by the combination of the nanoparticles on the clusters and other dispersed nanoparticles, while the crystal structure is oriented to form smooth surface nanowires (Masatoshi et al. 1998). Ag2S nanocrystals prepared by the hydrothermal method have the function of regulating the growth of semiconductors and complexes. In the process of the compounding of Ag2S and ZnO/ZnS, the formation of Ag2S particles leads to the self-assembly of ZnO/ZnS nanoparticles. Meanwhile, Ag2S/ZnO/ZnS is prepared by the microwave assisted process under hydrothermal reaction, and the second heating is beneficial to the secondary growth of the crystalline form (Kim et al. 2015).

In addition, the morphology and crystal structure of the 1% Ag2S/ZnO/ZnS composite were further analyzed by HR-TEM, as shown in Figure S5. Figure S5a is a 1% Ag2S/ZnO/ZnS TEM image of the composite. Moreover, the composition of 1% Ag2S/ZnO/ZnS was studied by HR-TEM analysis, and the results are shown in Figure S5b. Using software (Gatan Digital Micro group) for calculation, the lattice fringe spacings in Figure S5b were 0.24 nm, 0.28 nm and 0.31 nm, respectively. This can be attributed to the Ag2S (112), ZnO (001) and ZnS (002) crystal faces.

N2 adsorption–desorption analysis

Figure S6 (a–d) show the nitrogen adsorption–desorption isotherms and pore size distribution curves of ZnS, ZnO/ZnS (1: 2), 1% Ag2S/ZnO/ZnS and 5% Ag2S/ZnO/ZnS composites, respectively. As shown in Figure S6, the monomer ZnS presents a type III isotherm with a hysteresis loop of type H4. The composites of ZnO/ZnS (1: 2), 1% Ag2S/ZnO/ZnS and 5% Ag2S/ZnO/ZnS all exhibit a type IV isotherm and H4 type hysteresis loop. According to the definition by IUPAC, the International Union of Pure and Applied Chemistry, the sample has an H4-type hysteresis loop, mostly in a solid of microstructured pores with narrow slits in the sample, and no adsorption properties in the higher relative pressure region. Due to the phenomenon of capillary condensation, the adsorption isotherms do not coincide with the desorption isotherms, resulting in the appearance of hysteresis loops (Dedryvère et al. 2006). The type III isotherm shows that the adsorption of nitrogen is smaller. However, the high specific pressure zone and the aperture zoning curve indicate that the sample has mesoporous existence. The IV isotherm and the H4 type hysteresis loop are characterized by their mesoporous structure. However, when ZnO is combined, the isotherm has a significant transition in low specific pressure area. The main adsorption is single molecule adsorption, corresponding to some mesoporous materials or pores caused by particle accumulation. Moreover, with the combination of Ag2S, the formation of Ag2S cocatalyst during the composite process can affect the composite morphology changes, resulting in the change in the pore size distribution and specific surface area (Zhang et al. 2015). When Ag2S loading is 1%, the specific surface area is larger than that of other semiconductors and most of ZnO/ZnS at different ratios; however, with the increase in Ag2S loading, more Ag2S particles are formed and the specific surface area is smaller. The 1% Ag2S/ZnO/ZnS composites have the highest specific surface area, indicating a better photocatalytic activity.

Photocatalytic results

In order to investigate the photocatalytic performance of ZnO/ZnS composites of different proportions and the samples containing different proportions of Ag2S cocatalyst, using methyl orange as a model molecule in adsorption experiments in Figure S7, the dye solution was irradiated with different light sources, and the photocatalytic activities of different ratios of ZnO/ZnS nanocomposites and different proportions of Ag2S/ZnO/ZnS composites under ultraviolet light and visible light were studied. The results are shown in Figure S8. From Figure S8a, it can be seen that the ZnO/ZnS (1:2) prepared by ethylene glycol as a solvent presents the maximum degradation efficiency in different proportions of samples under ultraviolet light. And with the increase in thiourea, added in the preparation of the solvent, the amount of ZnS in the corresponding sample (ZnO/ZnS (1: 1), ZnO/ZnS (1: 2) and ZnO/ZnS (1: 3)) increases, and the photocatalytic activities of the samples are changed, of which the efficiency of ZnO/ZnS (1: 2) irradiated by ultraviolet light is the best. That is, with the increase in the amount of thiourea, the ZnS contained in the sample increases, and the S2− ions are displaced with the O2− of the ZnO in the nanomaterial to form more ZnS. However, with the increase in thiourea, excess S2− ions are generated, and all of the ZnO is completely replaced. It is seen from the nitrogen adsorption–desorption that the specific surface area is changed and the ultraviolet light activity is reduced. At the same time, in order to investigate the influence of the cocatalyst Ag2S on the light source, different ratios of Ag2S and binary ZnO/ZnS were compounded. From Figure S8b, 5% Ag2S/ZnO/ZnS has the lowest activity. On the contrary, the activity of 1% Ag2S/ZnO/ZnS is highest. The reason is that Ag2S has lower bandgap energy (0.92 eV). A narrower bandgap can utilize more wavelengths of sunlight, and increase the photoresponse range of the composite, but with more Ag2S compound on the material, excessive Ag2S will become the aggregation center for photogenerated electron–hole pairs. As a result, the recombination rate of photogenerated electron–hole pairs is increased and the photocatalytic activity of the composite is decreased. The appropriate amount of Ag2S is conducive to promoting the transfer path of photogenerated electron–hole pairs and reducing the inactivation of the active group in the reaction. Therefore, an appropriate amount of Ag2S cocatalyst facilitates the photocatalytic activity of Ag2S/ZnO/ZnS composite under UV light. The results of UV experiments showed that ZnO/ZnS (1: 2) and 1% Ag2S/ZnO/ZnS had the best UV irradiation activity, which was higher than the direct solution of methyl orange under ultraviolet light and other proportions of composites. It can be clearly seen from Figure S8c that the maximum absorption wavelength of methyl orange gradually decreases with the increase in reaction time under the condition of UV irradiation and 1% Ag2S/ZnO/ZnS, which proves that the methyl orange molecule is obviously degraded.

In order to further investigate the photocatalytic activity of different proportions of products under different light sources, in this paper, different proportions of Ag2S/ZnO/ZnS composites were also degraded under visible light, as shown in Figure S8d. In Figure S8d, it can be found that the photocatalytic activity of 1% Ag2S/ZnO/ZnS composites under visible light is significantly higher than that of other Ag2S/ZnO/ZnS composites and ZnO/ZnS (1: 2); the results show that the cocatalyst Ag2S obviously improved the photocatalytic activity in visible light.

From the above experimental results, it is not difficult to find that the samples with an Ag2S doping amount of 0.5% and 1.5% are significantly less active than the sample with a doping amount of 1%. In addition, we made up the ICP test for the above samples. The specific results show that the Ag2S doping amount in the samples with 0.5%, 1% and 1.5% feeding ratio are 0.43%, 0.89% and 1.46%, respectively. This is close to the theoretical value.

To further study the photocatalytic properties of 1%Ag2S/ZnO/ZnS composites, the recovery of 1% Ag2S/ZnO/ZnS was also carried out by UV photocatalysis. After being washed with deionized water and ethanol, the cycle experiment of photocatalytic degradation of methyl orange dye was carried out three times, and the obtained data are shown in Figure S9. The photocatalytic degradation of methyl orange in the three photocatalytic results was 82.07%, 77.35% and 74.43%, respectively. The reduction of photocatalytic effect is likely to be because there is a trace amount of residual pollutants on the surface of the material used during the recovery process, resulting in the activity being reduced. Moreover, in the recovery process, by deionized water, ethanol washing and the drying process of postprocessing, there is still dye adsorbed on the surface of the catalyst, causing the pores of the catalyst be blocked, resulting in the catalyst surface's active sites being decreased, and thus the degradation effect is decreased.

In addition, in order to study the applicability of the samples, we investigated the effects of various reagents (RhB, CV, MO, SA, and CR) with 1% Ag2S/ZnO/ZnS degradation under UV light; the results are shown in Figure S10. It can be seen that the samples have obvious degradation of RhB, MO, CR, CV and SA, indicating that 1% Ag2S/ZnO/ZnS has photocatalytic activities for different structural dye molecules, which does show some applicability.

The data measured by high performance liquid chromatography were measured as shown in Figure S11, and the data of methyl orange stock solution, which was diluted 100 times and after adding a certain amount of catalyst were analyzed under ultraviolet light for 4 h, 6 and 8 h, respectively. The retention time of the methyl orange was ca. 0.8 minutes. After the catalyst was added for 8 h, the characteristic peak of methyl orange was not obvious after separation by high performance liquid chromatography, indicating that the methyl orange model molecule was degraded. It also proves that the composites synthesized in this paper have good photocatalytic properties.

The detection of reactive species in the photocatalytic process is the key to recognizing the photocatalytic reaction mechanism. In order to investigate the active groups produced by 1% Ag2S/ZnO/ZnS composite in the photocatalytic reaction system, in this paper, by adding different capture agents for photocatalytic results, the photocatalytic process condition is similar to the UV experimental condition, and the difference is the capture agent which is added. The capture reagents used in the capture experiments were EDTA-2Na (capture h+), isopropanol (IPA, capture ·OH) and p-benzoquinone (BQ, capture ·O2) (Nawaz et al. 2016); the results are shown in Figure S12a, indicating that the active groups, such as ·O2, h+ and ·OH, played a certain role in the degradation of methyl orange with 1% Ag2S/ZnO/ZnS composites under UV light irradiation. In order to facilitate the understanding of the effects of degradation of different capture agents on methyl orange under ultraviolet light, the kinetic diagram of photocatalytic reaction is presented (Jaafarzadeh et al. 2017). The first-order reaction equation is −lnCt/C0 = kt, where Ct is the concentration of the dye solution at time t (mg/L) under UV light irradiation, C0 is the initial concentration of the dye in mg/L, k is the rate constant (min−1). Figure S12b shows the photocatalytic kinetics under different capture agent degradation conditions. As can be seen from the figure, the reaction time and −lnCt/C0 basically showed a linear relationship, indicating that the photocatalytic results were in line with quasi-first order reaction kinetics. According to the slope can be seen the degradation rate of different capture agents under ultraviolet light.

The effect of different catalysts synthesized in the experiment was studied. We synthesized 1% Ag2S/ZnO/ZnS using the temperature programmed method under the same experimental conditions, comparing XRD, UV–visible diffuse reflection and activity experiments of different synthesis methods. It can be seen from Figure S13a that the characteristic peak of the photocatalyst synthesized by temperature-programming is not sharp enough compared with the characteristic peak synthesized by the microwave method, indicating that the characteristic peak of the catalyst synthesized by the microwave method is strong. It can be seen from the ultraviolet–visible diffuse reflection of Figure S13b that the absorption of the catalyst synthesized by the microwave method in the visible region is superior to that of the catalyst synthesized by the temperature programmed method. In addition, the UV photoactivity experiment and the photohydrolysis water hydrogen production experiment from Figure S13(c, d) further prove that the photocatalytic performance of the photocatalyst synthesized by microwave method is better than that of the temperature programmed method.

Photolysis water hydrogen properties

In order to test the hydrogenation ability of photolysis water in the synthesized sample, the different catalysts synthesized in this paper were placed in the reaction apparatus under the same conditions. Hydrolysis of hydrogen required for the experimental composite was 0.1 g, and 3 g of Na2S·H2O, and 2.2 g of Na2SO3 were also added, and the light source used was a 300 W xenon lamp. Before the start of the reaction, the photolysis water hydrogen production unit was evacuated to remove the dissolved oxygen from the reactor and the solution. The system was in the light of the reaction after 8 h; the results are shown in Figure S14. In the Ag2S/ZnO/ZnS system, the composite ZnO/ZnS increases the carrier lifetime and decreases the electron content, while the larger specific surface area promotes the efficiency of photolysis and hydrogen production. At the same time, with the proper amount of Ag2S compounding, the bandgap of the composite can be reduced, and the conduction band and valence band of Ag2S help to promote the efficient transfer of carriers. Ag2S with a loading of 1% has the highest photolysis hydrogen production capacity.

Possible photocatalytic mechanism of Ag2S/ZnO/ZnS composites

According to the results of the capture experiments, the possible mechanism of the photocatalytic reaction in 1% Ag2S/ZnO/ZnS photocatalytic system is shown in Figure 2. The conduction band and valence band of ZnS are −1.04 eV and +2.56 eV, respectively, calculated by the formulas ECB = X-EC-0.5Eg and EVB = ECB + Eg. Correspondingly, the conduction band and valence band of ZnO are −0.31 eV and +2.89 eV, and the conduction band and valence band of Ag2S are −0.04 eV and +0.92 eV, respectively (Liu et al. 2017). Differences in the bandgap of different semiconductors result in the transfer of photogenerated electrons and photogenerated holes, thereby extending electron–hole pair binding (Wang et al. 2008). After ZnO and ZnS are compounded by microwave assisted solvothermal method, photogenerated electrons can move to the lower semiconductor conduction band, while photogenerated holes can move to the higher valence band. After recombination, the transfer path of the carrier increases, reducing the electron–hole pairs' recombination. Secondly, the addition of Ag2S in Ag2S/ZnO/ZnS composites increases the transfer path again. The conduction band of the Ag2S is lower than the conduction band of ZnO and ZnS, and the Ag2S valence band is higher than that of ZnO and ZnS, and the photogenerated holes can also transfer. Therefore, the presence of excessive Ag2S in Ag2S/ZnO/ZnS composites will greatly increase photogenerated electron–hole recombination and less Ag2S helps carrier transfer. In the photoreaction system, the excited electrons combine with O2 adsorbed on the surface of the semiconductor to form superoxide radicals (·O2). Holes generated in the valence band combine with H2O to form hydroxyl radicals (·OH). It has been demonstrated that ·O2, ·OH and strongly oxidizing holes present in the system can mineralize methyl orange dye to produce CO2 and H2O.

Figure 2

Possible photocatalytic reaction mechanism of 1% Ag2S/ZnO/ZnS composites.

Figure 2

Possible photocatalytic reaction mechanism of 1% Ag2S/ZnO/ZnS composites.

Conclusion

In this paper, different proportions of Ag2S/ZnO/ZnS composites were prepared by a microwave-assisted two-step method. Due to the unique polarization of microwaves, the synthesized product enhanced absorption of light after microwave irradiation, and the morphology is further improved, so that the surface physicochemical properties of the sample are more excellent. At the same time, in microwave-assisted action, with the extension of microwave time, the grain size of the composites increases obviously; the bandgap energy decreases slightly, and its shape is arranged more regularly and is more orderly, indicating that the microwave field is conducive to the growth of crystals. The experimental results of photocatalytic degradation of methyl orange and photolysis of hydrogen show that ZnO/ZnS (1:2) and 1%Ag2S/ZnO/ZnS composites have better photocatalytic activity. The reasons leading to this situation are as follows: the difference between the conduction band and the valence band in the semiconductor leads to an increase in the transfer path of photogenerated electrons and holes, reducing the recombination rate; during the recombination process, the crystals interact with each other to promote the growth in specific directions, promote the formation of specific morphologies, increase the specific surface area and provide more active sites for the reaction molecules. The narrow bandgap of Ag2S, to a certain extent, increases the light absorption in the visible region of the composite material, thereby contributing to the improvement of photocatalytic activity and the increase in the hydrogen production capability of the photolysis water.

ACKNOWLEDGEMENTS

This study was supported by the National Natural Science Foundation of China (21376126), The Fundamental Research Funds in Heilongjiang Provincial Universities (135209105), Government of Heilongjiang Province Postdoctoral Grants, China (LBH-Z11108), Postdoctoral Researchers in Heilongjiang Province of China Research Initiation Grant Project (LBH-Q13172), Innovation Project of Qiqihar University Graduate Education (YJSCX2016-ZD06), College Students' Innovative Entrepreneurial Training Program Funded Projects of Qiqihar University (201810232056) and Qiqihar University in 2016 College Students Academic Innovation Team Funded Projects.

REFERENCES

REFERENCES
Chen
D. M.
,
Wang
Z. H.
,
Ren
T. Z.
,
Ding
H. Q.
,
Yao
W. Q.
,
Zong
R. L.
&
Zhu
Y. F.
2014
Influence of defects on the photocatalytic activity of ZnO
.
J. Phys. Chem. C.
118
(
28
),
15300
15307
.
Chouhan
N.
,
Ameta
R.
,
Meena
R. K.
,
Mandawat
N.
&
Ghildiyal
R.
2016
Visible light harvesting Pt/CdS/Co-doped ZnO nanorods molecular device for hydrogen generation
.
Int. J. Hydrogen. Energ.
41
(
4
),
2298
2306
.
Dedryvère
R.
,
Leroy
S.
,
Martinez
H.
,
Blanchard
F.
,
Lemordant
D.
&
Gonbeau
D.
2006
XPS valence characterization of lithium salts as a tool to study electrode/electrolyte interfaces of Li-Ion batteries
.
J. Phys. Chem. B.
110
(
26
),
12986
12992
.
Gutierrez-Mata
A. G.
,
Velazquez-Martínez
S.
,
Álvarez-Gallegos
A.
,
Ahmadi
M.
,
Hernández-Pérez
J. A.
,
Ghanbari
F.
&
Silva-Martínez
S.
2017
Recent overview of solar photocatalysis and solar photo-fenton processes for wastewater treatment
.
Int. J. Photoenergy.
2017
(
2017-7-9
),
1
27
.
Li
L.
,
Zhang
X. L.
,
Zhang
W. Z.
,
Wang
L. L.
&
Chen
X.
2014
Microwave-assisted synthesis of nanocomposite Ag/ZnO-TiO2 and photocatalytic degradation Rhodamine B with different modes
.
Colloid. Surface. A.
457
(
1
),
134
141
.
Li
C.
,
Chen
W.
,
Wu
D.
,
Quan
D. H.
,
Zhou
Z. M.
,
Hao
J. J.
,
Qin
J.
,
Li
Y. W.
,
He
Z. B.
&
Wang
K.
2015
Large Stokes shift and high efficiency luminescent solar concentrator incorporated with CuInS2/ZnS quantum dots
.
Sci. Rep-Uk.
5
,
17777
17785
.
Li
M. F.
,
Zhao
Q.
,
Yi
X.
,
Zhong
X. Y.
,
Song
G. H.
,
Chai
Z. F.
,
Liu
Z.
&
Yang
K.
2016
Au@MnS@ZnS core/shell/shell nanoparticles for magnetic resonance imaging and enhanced cancer radiation therapy
.
Acs. Appl. Mater. Inter.
8
(
15
),
9557
9564
.
Liu
Y. P.
,
Geng
P.
,
Wang
J. X.
,
Yang
Z. S.
&
Lu
H. D.
2017
In-situ ion-exchange synthesis Ag2S modified SnS2 nanosheets toward highly photocurrent response and photocatalytic activity
.
J. Colloid. Interf. Sci.
512
,
784
791
.
Ma
D. D.
,
Shi
J. W.
,
Zou
Y. J.
,
Fan
Z. Y.
,
Ji
X.
&
Niu
C.
2017
Highly efficient photocatalyst based on a CdS quantum dots/ZnO nanosheets 0D/2D heterojunction for hydrogen evolution from water splitting
.
Acs. Appl. Mater. Inter.
9
(
30
),
25377
25386
.
Nawaz
F.
,
Xie
Y. B.
,
Xiao
J. D.
,
Cao
H. B.
,
Li
Y. P.
&
Zhang
D.
2016
Insights into the mechanism of phenolic mixture degradation by catalytic ozonation with a mesoporous Fe3O4/MnO2 composite
.
Rsc. Adv.
6
(
35
),
29674
29684
.
Sharma
D.
,
Sharma
S.
,
Kaith
B. S.
,
Rajput
J.
&
Kaur
M.
2011
Synthesis of ZnO nanoparticles using surfactant free in-air and microwave method
.
Appl. Surf. Sci.
257
(
22
),
9661
9672
.
Wang
X. B.
,
Liu
J.
,
Leong
S.
,
Lin
X. C.
,
Wei
J.
&
Kong
B.
2016
Rapid construction of ZnO@ZIF-8 heterostructures with size-selective photocatalysis properties
.
Acs. Appl. Mater. Inter.
8
(
14
),
9080
9087
.
Wu
H. Q.
,
Wang
Q. Y.
,
Yao
Y. Z.
,
Qian
C.
,
Zhang
X. J.
&
Wei
X. W.
2008
Microwave-assisted synthesis and photocatalytic properties of carbon nanotube/zinc sulfide heterostructures
.
J. Phys. Chem. C.
112
(
43
),
16779
16783
.
Zhang
Q.
,
Wei
C.
,
Li
X.
,
Haffz
M.
&
Gan
L.
2015
Polar-surface-driven growth of ZnS microsprings with novel optoelectronic properties
.
Npg. Asia. Mater.
7
(
9
),
213
220
.

Supplementary data