Improving the Performance of ZnS Photocatalyst in Degrading Organic Pollutants by Constructing Composites with Ag2O

ZnS is a promising photocatalyst in water purification, whereas its low photon efficiency and poor visible-light response restrict its application. Constructing composites may help solve these problems. In this work, Ag2O was introduced to ZnS for the first time based on their energy band characteristics to form a novel ZnS/Ag2O composite photocatalyst. In the model reaction of degrading methylene blue, the as-designed catalyst exhibited high catalytic activity among a series of ZnS-based composite photocatalysts under similar conditions. The catalytic rate constant was up to 0.138 min−1, which is 27.4- and 15.6-times higher than those of ZnS and Ag2O. This composite degraded 92.4% methylene blue in 50 min, while the ratios were 31.9% and 68.8% for ZnS and Ag2O. Catalytic mechanism study based on photoluminescence and radical-scavenging experiments revealed that the enhanced photocatalytic activity was attributed to the composite structure of ZnS/Ag2O. The structure not only facilitated the separation and transmission of photogenerated carriers but also extended the light response range of the catalyst. The as-designed ZnS/Ag2O composite is promising in degrading organic pollutants in water.

Silver oxide (Ag 2 O), a p-type semiconductor with a narrow band gap, has been widely used as an antibacterial material, colorant, preservative, and electrode material [24][25][26][27]. It has also been a promising photocatalyst since Wang et al. [28,29] discovered its photocatalytic activity. Subsequently, a series of Ag 2 O-involved composites have been reported, such as TiO 2 /Ag 2 O [30], ZnO/Ag 2 O [31], Bi 2 WO 6 /Ag 2 O [32], and g-C 3 N 4 /Ag 2 O [33]. Benefitting from the efficient carrier separation of composites and extended light response range due to the narrow band gap of Ag 2 O, these composites display higher photocatalytic activities than respective components. Therefore, combining n-type semiconductor ZnS with p-type semiconductor Ag 2 O to form a composite may elevate the activity of pristine ZnS.
Herein, Ag 2 O was introduced to ZnS for the first time based on respective energy band characteristics. A novel ZnS/Ag 2 O photocatalyst was constructed and synthesized by a simple chemical precipitation method. The as-designed photocatalyst was composed of n-type semiconductor ZnS and p-type semiconductor Ag 2 O, exhibiting higher catalytic activities than pristine ZnS or Ag 2 O toward degrading the model pollutant methylene blue (MB). Furthermore, the composite exhibited a high-rate constant of 0.138 min −1 , indicating a high catalytic activity among a series of ZnS-based photocatalysts under similar conditions. Catalysis mechanism study of the enhanced activity revealed that the ZnS/Ag 2 O composite not only facilitated the separation and transmission of photoelectrons and holes but also extended the light response range of the photocatalyst. The as-designed ZnS/Ag 2 O composite is a promising photocatalyst for degrading hazardous organic pollutants in the water purification application.

Synthesis of ZnS Broccoli-Like Microspheres
ZnS broccoli-like microspheres were synthesized through the hydrothermal process. In a typical process, a certain amount of Zn(NO 3 ) 2 (0.1 M), CH 4 N 2 S (0.3 M), and 0.05 g PVP were mixed together, and stirred for 30 min. The suspension was transferred into a Teflon-lined autoclave and kept at 120 • C for 12 h. The final products were collected by centrifugation, washed with ethanol, and deionized water for several times. Then, the products were dried in an oven at 60 • C for 6 h. The products obtained under other temperatures were synthesized under corresponding temperatures while keeping other conditions consistent.

Synthesis of ZnS/Ag 2 O Composite
In a typical process, 0.2 g as-prepared ZnS was dispersed in 50 mL deionized water, and 0.117 g AgNO 3 was added to the suspension. After vigorous stirring for 30 min, a certain amount of 2 M NaOH was added dropwise to the suspension until it reached a pH of 14. Then, the composite was collected by centrifuging at 5000 rpm. The composite was then washed with deionized water and dried at 60 • C. In addition, the pristine Ag 2 O was synthesized through a similar approach at room temperature, which was used as a contrast sample.

Characterizations of Samples
The crystallographic phases of samples were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray powder diffractometer (Bruker Corp., Billerica, MA, USA) with Cu-Kα radiation (λ = 1.5418 Å). Morphological observations were performed by a Hitachi S-4800 field emission scanning electron microscope (SEM, Tokyo, Japan). The diffuse reflectance spectra (DRS) were recorded by a Shimadzu UV-2600 spectrophotometer equipped with an integrating sphere, using BaSO 4 as the reflectance standard. Photoluminescence (PL) spectra were obtained by an Edinburgh FLS920 fluorescence spectrom-eter (Edinburgh Instruments Ltd., Livingston, England) under an excitation wavelength of 325 nm.

Photocatalytic Activity Tests
The photocatalytic activities of the as-prepared samples were evaluated in terms of the model reaction of degrading MB. The 300 W xenon lamp (PLS-SXE300UV, Perfectlight Co., Ltd., Beijing, China) was positioned 10 cm away from the cuvette and was used as light source to trigger the photocatalytic reaction. In a typical test, 0.05 g of a certain sample was dispersed into 100 mL MB solution (10 mg/L) with magnetic stirring. The suspension was stirred constantly in the dark for 1 h to achieve the absorption equilibrium before irradiation. The residual concentration of MB was determined by UV-2600 UV-Vis spectroscopy (Shimadzu, Kyoto, Japan). Figure 1 shows the XRD patterns of the as-prepared ZnS, Ag 2 O, and ZnS/Ag 2 O composite. From the XRD pattern in black, it is evident that all the diffraction peaks can be well-indexed to the hexagonal wurtzite structure of ZnS (JCPDS 80-0007), with characteristic diffractions of the (100), (002), (101), (110), (103), and (112) crystal planes. The pattern in red can be assigned to the Ag 2 O in cubic phase (JCPDS 43-0997). The typical peaks in good crystallinity of Ag 2 O can be clearly identified as (111), (200), (220), and (311) facets. For the ZnS/Ag 2 O composite, the XRD pattern is composed of two sets of characteristic peaks, which are the hexagonal ZnS and cubic Ag 2 O phase. In addition, there is no characteristic peak relevant to another phase, which demonstrates the high purity of the product. Therefore, the as-prepared typical sample is indeed a composite composed of ZnS and Ag 2 O. standard. Photoluminescence (PL) spectra were obtained by an Edinburgh FLS9 rescence spectrometer (Edinburgh Instruments Ltd., Livingston, England) under tation wavelength of 325 nm.

Photocatalytic Activity Tests
The photocatalytic activities of the as-prepared samples were evaluated in t the model reaction of degrading MB. The 300 W xenon lamp (PLS-SXE300UV, Perf Co., Ltd., Beijing, China) was positioned 10 cm away from the cuvette and was light source to trigger the photocatalytic reaction. In a typical test, 0.05 g of a certa ple was dispersed into 100 mL MB solution (10 mg/L) with magnetic stirring. The sion was stirred constantly in the dark for 1 h to achieve the absorption equilibrium irradiation. The residual concentration of MB was determined by UV-2600 UV-V troscopy (Shimadzu, Kyoto, Japan). For the ZnS/Ag2O composite, the XRD pattern is composed of two sets of chara peaks, which are the hexagonal ZnS and cubic Ag2O phase. In addition, there is n acteristic peak relevant to another phase, which demonstrates the high purity of th uct. Therefore, the as-prepared typical sample is indeed a composite composed and Ag2O. The details of morphology and microstructure of as-prepared ZnS, Ag ZnS/Ag2O composite were studied by SEM. Figure 2a  the original broccoli-like microspheres after composition with Ag2O. The Ag2O nanoparticles were uniformly covered on the surface of ZnS microspheres, constituting the ZnS/Ag2O composite. In addition, EDX spectrum of ZnS/Ag2O composite was acquired simultaneously with SEM, as displayed in Figure 2f. The characteristic X-ray peaks of Zn, Ag, S, and O can be found without other element emerges, revealing the elemental composition of the composite. To explore the law of morphology evolution, condition control experiments on morphologies were carried out. Among these influencing factors, temperature showed more obvious regularity. Figure 3a displays the ZnS microspheres obtained at a temperature of 90 °C while the other experiment conditions were kept identical to the typical sample (120 °C). The as-obtained product was mainly ZnS microspheres with plenty of nanoparticles. The diameter of the microspheres was around 3 μm. As the temperature increased to 150 °C (Figure 3b), the morphology of the product did not change significantly, remaining spherical. However, the proportion of ZnS microspheres decreased while that of the nanoparticles increased. Besides, the diameter of ZnS microspheres became uneven. In Figure 3c, at a temperature of 200 °C, there were no separate ZnS microspheres in the product. Agglomerates emerged that were composed of ZnS microspheres, and the shape of the ZnS microspheres became blurred and uneven in size. To investigate whether the introduction of Ag2O improved the light absorption properties of ZnS, UV-Vis DRS and derived Tauc plots were acquired to obtain the band gap To explore the law of morphology evolution, condition control experiments on morphologies were carried out. Among these influencing factors, temperature showed more obvious regularity. Figure 3a displays the ZnS microspheres obtained at a temperature of 90 • C while the other experiment conditions were kept identical to the typical sample (120 • C). The as-obtained product was mainly ZnS microspheres with plenty of nanoparticles. The diameter of the microspheres was around 3 µm. As the temperature increased to 150 • C (Figure 3b), the morphology of the product did not change significantly, remaining spherical. However, the proportion of ZnS microspheres decreased while that of the nanoparticles increased. Besides, the diameter of ZnS microspheres became uneven. In Figure 3c, at a temperature of 200 • C, there were no separate ZnS microspheres in the product. Agglomerates emerged that were composed of ZnS microspheres, and the shape of the ZnS microspheres became blurred and uneven in size. the original broccoli-like microspheres after composition with Ag2O. The Ag2O nanoparticles were uniformly covered on the surface of ZnS microspheres, constituting the ZnS/Ag2O composite. In addition, EDX spectrum of ZnS/Ag2O composite was acquired simultaneously with SEM, as displayed in Figure 2f. The characteristic X-ray peaks of Zn, Ag, S, and O can be found without other element emerges, revealing the elemental composition of the composite. To explore the law of morphology evolution, condition control experiments on morphologies were carried out. Among these influencing factors, temperature showed more obvious regularity. Figure 3a displays the ZnS microspheres obtained at a temperature of 90 °C while the other experiment conditions were kept identical to the typical sample (120 °C). The as-obtained product was mainly ZnS microspheres with plenty of nanoparticles. The diameter of the microspheres was around 3 μm. As the temperature increased to 150 °C (Figure 3b), the morphology of the product did not change significantly, remaining spherical. However, the proportion of ZnS microspheres decreased while that of the nanoparticles increased. Besides, the diameter of ZnS microspheres became uneven. In Figure 3c, at a temperature of 200 °C, there were no separate ZnS microspheres in the product. Agglomerates emerged that were composed of ZnS microspheres, and the shape of the ZnS microspheres became blurred and uneven in size.  To investigate whether the introduction of Ag 2 O improved the light absorption properties of ZnS, UV-Vis DRS and derived Tauc plots were acquired to obtain the band gap energy for the ZnS, Ag 2 O, and ZnS/Ag 2 O composite. As-obtained plots are shown in Figure 4a. The absorption edge of pristine ZnS was around 340 nm in the UV region, while the pristine Ag 2 O exhibited strong light absorption performance both in the UV and visible light regions, which is ascribed to its much lower energy for band gap transition. Combining the light absorption properties of ZnS and Ag 2 O in UV and visible light regions of 300-800 nm, the ZnS/Ag 2 O composite achieved a great elevation in light absorption in visible light regions, forming an obvious contrast to pristine ZnS. Figure 4b shows the Tauc plot of the three samples. The band gap energy of semiconductors can be obtained from the x-axis intercept of the extended line. From the graph, the band gap energy values for the ZnS, Ag 2 O, and ZnS/Ag 2 O composite were determined to be 3.52 eV, 1.80 eV and 2.85 eV, respectively, which suggests that the band gap energy of ZnS was greatly reduced after introducing Ag 2 O to ZnS to form the composite. In particular, the band gap energy changed from 3.52 eV to 2.85 eV. The results demonstrate huge improvement by the introduction of Ag 2 O, which led to a higher photon efficiency and extended the light response range of the composite.  For photocatalysts, PL spectrum is crucial because it is related to the recombination of photogenerated electrons and holes. High fluorescence intensity means that there will be less carriers involved in the photocatalytic reaction, resulting in a low quantum efficiency of catalysts. Under the exciting wavelength of 325 nm, PL spectra of pristine ZnS and ZnS/Ag2O composite were obtained, as shown in Figure 5. The ZnS showed two strong emission peaks at 423 nm and 472 nm. For semiconductors, two emission peaks were observed, corresponding to interstitial and trapped surface state emission, respectively. The emission peak at 423 nm is evidence of sulfur vacancies due to the recombination of electrons from shallow trap state to the sulfur vacancies, which is consistent with the results reported previously [34]. The emission peak centered at 472 nm can be attributed to the zinc vacancies in ZnS lattice. After the addition of Ag2O, the fluorescence intensities of ZnS/Ag2O composite decreased greatly, indicating that photogenerated electrons and holes can rapidly migrate between ZnS and Ag2O, thus suppressing the recombination of carriers. For photocatalysts, PL spectrum is crucial because it is related to the recombination of photogenerated electrons and holes. High fluorescence intensity means that there will be less carriers involved in the photocatalytic reaction, resulting in a low quantum efficiency of catalysts. Under the exciting wavelength of 325 nm, PL spectra of pristine ZnS and ZnS/Ag 2 O composite were obtained, as shown in Figure 5. The ZnS showed two strong emission peaks at 423 nm and 472 nm. For semiconductors, two emission peaks were observed, corresponding to interstitial and trapped surface state emission, respectively. The emission peak at 423 nm is evidence of sulfur vacancies due to the recombination of electrons from shallow trap state to the sulfur vacancies, which is consistent with the results reported previously [34]. The emission peak centered at 472 nm can be attributed to the zinc vacancies in ZnS lattice. After the addition of Ag 2 O, the fluorescence intensities of ZnS/Ag 2 O composite decreased greatly, indicating that photogenerated electrons and holes can rapidly migrate between ZnS and Ag 2 O, thus suppressing the recombination of carriers. The photocatalytic activity of obtained ZnS/Ag2O composite was eva model reaction of degrading MB under UV-Vis irradiation. For compariso and Ag2O were also conducted under the same condition. Before irradiatio of MB was stirred evenly with certain catalyst in the dark for 60 min to a sorption-desorption equilibrium. The original UV-Vis spectra data of the samples recorded at different time during the tests are plotted in Figure 6. Th the overlapped UV-Vis absorption spectra with a major peak at 664 nm, wh the characteristic absorption peak of MB. In Figure 6a, the peak of MB dec and was catalyzed by ZnS, and the MB was degraded 31.9% in 50 min. Howe in Figure 6b, the Ag2O catalyst showed a faster degradation rate than Zn 664 nm decreased by 68.8% within the same time, whereas the ZnS/Ag2O hibited the highest activity. The concentration of MB decreased rapidly, an was degraded in 50 min. The results demonstrate that the ZnS/Ag2O compo ter performance than both of its components. The superior performance of lyst can be attributed to the improved visual light response of the composite photon efficiency. The photocatalytic activity of obtained ZnS/Ag 2 O composite was evaluated by the model reaction of degrading MB under UV-Vis irradiation. For comparison, tests of ZnS and Ag 2 O were also conducted under the same condition. Before irradiation, the solution of MB was stirred evenly with certain catalyst in the dark for 60 min to achieve the absorptiondesorption equilibrium. The original UV-Vis spectra data of the three typical samples recorded at different time during the tests are plotted in Figure 6. The data exhibit the overlapped UV-Vis absorption spectra with a major peak at 664 nm, which indexed to the characteristic absorption peak of MB. In Figure 6a, the peak of MB decreased slowly and was catalyzed by ZnS, and the MB was degraded 31.9% in 50 min. However, as shown in Figure 6b, the Ag 2 O catalyst showed a faster degradation rate than ZnS. The peak at 664 nm decreased by 68.8% within the same time, whereas the ZnS/Ag 2 O composite exhibited the highest activity. The concentration of MB decreased rapidly, and 92.4% of MB was degraded in 50 min. The results demonstrate that the ZnS/Ag 2 O composite had a better performance than both of its components. The superior performance of the photocatalyst can be attributed to the improved visual light response of the composite and its higher photon efficiency. The photocatalytic activity of obtained ZnS/Ag2O composite was evaluated by the model reaction of degrading MB under UV-Vis irradiation. For comparison, tests of ZnS and Ag2O were also conducted under the same condition. Before irradiation, the solution of MB was stirred evenly with certain catalyst in the dark for 60 min to achieve the absorption-desorption equilibrium. The original UV-Vis spectra data of the three typical samples recorded at different time during the tests are plotted in Figure 6. The data exhibit the overlapped UV-Vis absorption spectra with a major peak at 664 nm, which indexed to the characteristic absorption peak of MB. In Figure 6a, the peak of MB decreased slowly and was catalyzed by ZnS, and the MB was degraded 31.9% in 50 min. However, as shown in Figure 6b, the Ag2O catalyst showed a faster degradation rate than ZnS. The peak at 664 nm decreased by 68.8% within the same time, whereas the ZnS/Ag2O composite exhibited the highest activity. The concentration of MB decreased rapidly, and 92.4% of MB was degraded in 50 min. The results demonstrate that the ZnS/Ag2O composite had a better performance than both of its components. The superior performance of the photocatalyst can be attributed to the improved visual light response of the composite and its higher photon efficiency.     Figure 7 shows the degradation profiles of MB at 664 nm as a function of time. To quantitatively compare the catalytic efficiencies of the above samples, kinetic calculation was carried out. In terms of the Langmuir-Hinshelwood model, degradation can be regarded as a first-order reaction at low MB concentration. Hence, the fitting calculation of degrading kinetics can be simplified to be linear. Besides, this reaction satisfies the apparent first-order reaction rate equation: ln(C/C 0 ) = −k·t, where k is the apparent rate constant of first-order reaction, and ln(C/C 0 ) is a function of irradiation time t. Based on this simplified model, relevant fitting results were obtained, as summarized in Figure 7.  To further evaluate the composite photocatalyst we designed, some relevant reports were retrieved to assess similar catalytic systems for horizontal comparison. All of the selected catalysts used composite photocatalysts with ZnS as the main body, and the catalytic degradation of MB was used as the model reaction to investigate the ability to purify organic pollutants in water. Table 1 lists the catalytic activities of some representative ZnSbased photocatalysts with crucial catalytic properties. The rate constant of the catalyst reported in this work is also listed at the bottom of the table to facilitate horizontal comparison. It can be clearly seen that the as-prepared ZnS/Ag2O composite has a high-rate constant among a various of photocatalysts, indicating that it has higher photocatalytic activity than most existing photocatalysts. Therefore, it can be concluded that as-designed ZnS/Ag2O composite is a promising candidate in the field of wastewater purification.  To further evaluate the composite photocatalyst we designed, some relevant reports were retrieved to assess similar catalytic systems for horizontal comparison. All of the selected catalysts used composite photocatalysts with ZnS as the main body, and the catalytic degradation of MB was used as the model reaction to investigate the ability to purify organic pollutants in water. Table 1 lists the catalytic activities of some representative ZnS-based photocatalysts with crucial catalytic properties. The rate constant of the catalyst reported in this work is also listed at the bottom of the table to facilitate horizontal comparison. It can be clearly seen that the as-prepared ZnS/Ag 2 O composite has a high-rate constant among a various of photocatalysts, indicating that it has higher photocatalytic activity than most existing photocatalysts. Therefore, it can be concluded that as-designed ZnS/Ag 2 O composite is a promising candidate in the field of wastewater purification.

Results and Discussion
In addition, the recyclability of ZnS/Ag 2 O composite in MB degradation reaction under UV-Vis irradiation was studied, as shown in Figure 8. After three successive cycles, the sample retained nearly consistent photocatalytic efficiency without apparent deactivation, indicating that the ZnS/Ag 2 O composite is stable during the degrading process.
To reveal the photocatalytic mechanism, the main active species in the MB degradation reaction were investigated. Radical scavengers, such as benzoquinone (BQ), ammonium oxalate (AO), tertiary butanol (TBA), and AgNO 3 , have been used to scavenge superoxide radical anions (•O 2− ), holes (h + ), electrons (e − ), or hydroxyl free radicals (•OH) respective [35,36]. From Figure 9, the addition of TBA had no obvious influence on the degradation of MB, implying that •OH are not the primary active species. When BQ and AgNO 3 were added, the reaction rates were dramatically decelerated, indicating that •O 2− has a crucial role in the photocatalytic process (O 2 + e − → •O 2 -). Besides, the addition of AO also remarkably suppressed the reaction, suggesting that h + is involved in the degradation process.  To reveal the photocatalytic mechanism, the main active species in the MB degradation reaction were investigated. Radical scavengers, such as benzoquinone (BQ), ammonium oxalate (AO), tertiary butanol (TBA), and AgNO3, have been used to scavenge superoxide radical anions (•O 2− ), holes (h + ), electrons (e − ), or hydroxyl free radicals (•OH) respective [35,36]. From Figure 9, the addition of TBA had no obvious influence on the degradation of MB, implying that •OH are not the primary active species. When BQ and AgNO3 were added, the reaction rates were dramatically decelerated, indicating that •O 2− has a crucial role in the photocatalytic process (O2 + e − → •O2 -). Besides, the addition of AO also remarkably suppressed the reaction, suggesting that h + is involved in the degradation process.  To reveal the photocatalytic mechanism, the main active species in the MB degradation reaction were investigated. Radical scavengers, such as benzoquinone (BQ), ammonium oxalate (AO), tertiary butanol (TBA), and AgNO3, have been used to scavenge superoxide radical anions (•O 2− ), holes (h + ), electrons (e − ), or hydroxyl free radicals (•OH) respective [35,36]. From Figure 9, the addition of TBA had no obvious influence on the degradation of MB, implying that •OH are not the primary active species. When BQ and AgNO3 were added, the reaction rates were dramatically decelerated, indicating that •O 2− has a crucial role in the photocatalytic process (O2 + e − → •O2 -). Besides, the addition of AO also remarkably suppressed the reaction, suggesting that h + is involved in the degradation process. Based on the above results, a possible mechanism for the enhanced photocatalytic activity of ZnS/Ag2O composite was proposed ( Figure 10). The activity of a semiconductor photocatalyst mainly depends on the oxidation-reduction potentials of the valence band (VB) and conduction band (CB). Fermi level (Ef, dashed line in Figure 10) is the chemical potential of thermodynamic equilibrium. Before n-type semiconductor ZnS and p-type semiconductor Ag2O formed the composite, their Fermi levels had different potentials. When the ZnS/Ag2O composite is formed and irradiated by UV-Vis light, the photogen- Based on the above results, a possible mechanism for the enhanced photocatalytic activity of ZnS/Ag 2 O composite was proposed ( Figure 10). The activity of a semiconductor photocatalyst mainly depends on the oxidation-reduction potentials of the valence band (VB) and conduction band (CB). Fermi level (E f , dashed line in Figure 10) is the chemical potential of thermodynamic equilibrium. Before n-type semiconductor ZnS and p-type semiconductor Ag 2 O formed the composite, their Fermi levels had different potentials. When the ZnS/Ag 2 O composite is formed and irradiated by UV-Vis light, the photogenerated electrons are transferred from ZnS to Ag 2 O due to the initially higher CB potential of ZnS until the quasi-Fermi level (quasi-static equilibrium) is generated [37][38][39][40]. This causes the energy bands of ZnS to shift downward and those of Ag 2 O to shift upward, eventually making the Fermi levels of the two components equal. Because the Fermi level of ZnS is close to CB but that of Ag 2 O is close to VB [39], the final CB position of Ag 2 O is higher than that of ZnS [41]. Therefore, the photogenerated electrons transfer from Ag 2 O to ZnS, driven by the potential difference. Then, the photogenerated electrons reduce the surface chemisorbed O 2 to form oxidizing species •O 2 − , which can degrade MB into small molecules [41,42]. Conversely, photogenerated holes on the VB of ZnS can be transferred to Ag 2 O, which can be consumed to oxidize MB directly. In addition, pristine ZnS has a poor ability for visible-light response due to its high band gap energy. Ag 2 O, having a much lower band gap energy, can effectively extend the spectral response range to visual light, elevate the photon efficiency, and improve the activity by forming a composite with ZnS. Therefore, the photocatalytic performance can be greatly enhanced because photogenerated electrons and holes are more efficiently generated and transferred by the ZnS/Ag 2 O composite. of ZnS until the quasi-Fermi level (quasi-static equilibrium) is generated [37][38][39][40]. This causes the energy bands of ZnS to shift downward and those of Ag2O to shift upward, eventually making the Fermi levels of the two components equal. Because the Fermi level of ZnS is close to CB but that of Ag2O is close to VB [39], the final CB position of Ag2O is higher than that of ZnS [41]. Therefore, the photogenerated electrons transfer from Ag2O to ZnS, driven by the potential difference. Then, the photogenerated electrons reduce the surface chemisorbed O2 to form oxidizing species •O2 − , which can degrade MB into small molecules [41,42]. Conversely, photogenerated holes on the VB of ZnS can be transferred to Ag2O, which can be consumed to oxidize MB directly. In addition, pristine ZnS has a poor ability for visible-light response due to its high band gap energy. Ag2O, having a much lower band gap energy, can effectively extend the spectral response range to visual light, elevate the photon efficiency, and improve the activity by forming a composite with ZnS. Therefore, the photocatalytic performance can be greatly enhanced because photogenerated electrons and holes are more efficiently generated and transferred by the ZnS/Ag2O composite.

Conclusions
In summary, a novel photocatalyst composed of n-type semiconductor ZnS and ptype semiconductor Ag2O was designed and constructed by a simple chemical precipitation method. The as-obtained ZnS/Ag2O composite exhibited a high photocatalytic activity and favorable stability among a series of similar ZnS-based composite photocatalysts toward degrading MB. The catalytic rate constant reached up to 0.138 min −1 , which is much higher than pristine ZnS or Ag2O. Results of catalytic mechanism experiment revealed that the enhanced catalytic activity can be attributed to the efficient separation and transmission of photogenerated carriers and extended light response range brought about by the narrow band gap of Ag2O. The as-designed ZnS/Ag2O composite is a promising photocatalyst for removing hazardous organics from wastewater due to its high performance.

Conclusions
In summary, a novel photocatalyst composed of n-type semiconductor ZnS and p-type semiconductor Ag 2 O was designed and constructed by a simple chemical precipitation method. The as-obtained ZnS/Ag 2 O composite exhibited a high photocatalytic activity and favorable stability among a series of similar ZnS-based composite photocatalysts toward degrading MB. The catalytic rate constant reached up to 0.138 min −1 , which is much higher than pristine ZnS or Ag 2 O. Results of catalytic mechanism experiment revealed that the enhanced catalytic activity can be attributed to the efficient separation and transmission of photogenerated carriers and extended light response range brought about by the narrow band gap of Ag 2 O. The as-designed ZnS/Ag 2 O composite is a promising photocatalyst for removing hazardous organics from wastewater due to its high performance. Funding: This research was funded by the National Natural Science Foundation of China, grant number 51402157, and the project of the outstanding scientific research innovation team plan for universities of Shandong province.