Microstructure - dependent photoelectrocatalytic activity of heterogeneous ZnO – ZnS nanosheets

: ZnS crystallite - loaded ZnO sheet composites were successfully synthesized through vulcanization of hydrothermally derived porous ZnO sheet templates. The sulfur precursor ( Na 2 S: 0.05 – 0.25 M ) concentration a ﬀ ects the ZnS loading content and surface morphology of the ZnO – ZnS composites. A higher sulfur precursor concen tration increased the ZnS loading content and decreased the porosity of the ZnO – ZnS composites. The ZnO – ZnS sheet composites with the atomic composition ratio of ZnO larger than that of the decorated ZnS exhibited an enhanced photoactivity. By contrast, the overloading of ZnS crystallites on the ZnO template decreased photo activity. The ZnO – ZnS sheet composite with a S/O atomic ratio of 0.61 exhibits the highest photoactivity among var ious samples. The enhanced charge separation e ﬃ ciency because of the formation of ZnO/ZnS heterojunctions and porous structure allowed the synthesis of the ZnO – ZnS composite via hydrothermal vulcanization with 0.05 M Na 2 S, and shows the higher photoelectrochemical ( PEC ) degradation ability towards Rhodamine B solution among various samples. The scavenger tests and the proposed PEC - degradation mechanism demonstrate that superoxide radicals are the main active species for the degradation of the RhB solution. The experimental results herein show that the porous ZnO – ZnS sheet composite with a suitable phase ratio is promising for photoelectrocata lyst applications.


Introduction
ZnO is widely used for degrading organic pollutants due to its variety of preparation methods, low cost, nontoxicity, and strong redox ability [1,2]. Several routes have been established for the synthesis of low-dimensional ZnO crystals with various morphologies, such as hydrothermal method, chemical bath deposition, vapor deposition, and sputtering deposition [3][4][5][6]. It has been shown that the advantages of a high specific surface area, abundant surface active sites, and strong adsorption capacity make low-dimensional ZnO nanomaterials suitable for catalytic reaction applications. Recently, the development of porous ZnO nanostructures shows that the tiny pores in ZnO substantially improve their specific surface area size, light absorption ability, and surface active site number in comparison with the ZnO with solid crystal characteristics [3,7]. Therefore, porous ZnO nanomaterials are a good candidate for use in photoactive devices with high performance. Furthermore, ZnS because of its remarkable chemical stability against oxidation and hydrolysis also receives much attention for application in photoactive devices [8]. Great efforts have been steadily devoted to synthesize ZnS nanostructures, aiming to enhance their photoactive performance via controlling their specific surface area and photoinduced charge separation efficiency. Several approaches have been adopted for synthesizing ZnS nanostructures with different morphologies for high-performance applications in photoactive devices [9,10].
Photocatalytic activation of semiconductors is mainly dominated by the number of excitons under irradiation to generate free radicals to degrade target pollutants [11][12][13][14][15][16][17]. Therefore, the control of photoinduced charge separation efficiency is a key for ZnO or ZnS to perform high photocatalytic performance. Several approaches including the incorporation of impurity, crystal defect, and formation of a heterostructure have been widely adopted for ZnO or ZnS for improved photoactive performance [18][19][20]. Among these approaches, the formation of ZnO-or ZnSbased heterostructures with a suitable band alignment between the constituent compounds is of potential importance to enhance the photoactive performance of ZnO or ZnS. This is due to various choices of coupling semiconductors to be integrated into the ZnO or ZnS, and the formation of a heterostructure has a high degree of freedom to manipulate the photoactive performance of the semiconductors. More importantly, due to the suitable Type II band alignment between the ZnO and ZnS, the construction of the ZnO/ZnS heterostructure has been shown to be a promising combination to improve the photoactive performance of the constituent counterparts [21].
The morphology, microstructure, composition ratio, and band alignment are factors that affect the final photoactive performance of the ZnO-ZnS heterostructures [22,23]. The control of ZnO-ZnS heterostructures with abundant active surfaces and interfaces for the photocatalytic process and design of the heterojunction to promote the separation and transfer of photogenerated electrons and holes with suitable bandgap positions are still a challenge to synthesize ZnO-ZnS composites with high photoactive performance. In this study, porous ZnO-ZnS sheet-like nanostructures are synthesized using a hydrothermal vulcanization method with porous ZnO sheet templates. The porous ZnO sheet templates can improve photocatalytic activity due to their high specific surface area. Moreover, the formation of ZnO-ZnS porous nanosheets by varying the parameters of hydrothermal vulcanization utilizes the control of microstructures of the ZnO-ZnS sheet composites to obtain high photoactive performance at a low-temperature and low-cost process. The systematic investigation on the vulcanization process-dependent microstructure evolution of ZnO-ZnS sheet composites in this study provides a promising approach to synthesize and design ZnO-ZnS sheet composites for photoelectrocatalytic device applications with high efficiency.

Material analysis and characterization tests
The crystallographic structures of the products were characterized by X-ray diffraction (XRD) using a Bruker D2 PHASER. The morphologies of the products were investigated using field emission scanning electron microscopy (FE-SEM; JEOL JSM-7900F). High-resolution lattice images, composition, and selected area electron diffraction of the products were recorded using high-resolution transmission electron microscopy (HR-TEM; JEOL JEM-2100F) equipped with an energy-dispersive X-ray spectroscope (EDS). X-ray photoelectron spectroscopy (XPS; PHI 5000 VersaProbe) was used to analyze the elemental binding energies of the products. The analysis of absorbance spectra of the products was conducted using a UV-Vis spectrophotometer (Jasco V750). The photoelectrochemical (PEC) and impedance spectroscopy properties of the products were characterized in a three-electrode electrochemical quartz cell using an electrochemical analyzer (SP-150, BioLogic). The assynthesized product was used as the working electrode as well as a Pt wire and an Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. The electrolyte is a 0.5 M Na 2 SO 4 solution and the irradiation source for measurements is excited with a 100 W Xe arc lamp. The light intensity on the sample is approximately 3.12 mW/cm 2 . The photoelectrocatalytic activities of the products were evaluated by PEC-degrading rhodamine B (RhB) solution with different irradiation durations of 15, 30, 45, and 60 min. Moreover, the dark balance absorption tests of various samples for 30 min are also conducted. A total of 10 mL of RhB solution (10 −5 ) M was added to 10 mL of 0.5 M Na 2 SO 4 solution to form a homogeneous aqueous solution for PEC-degradation system use, and the applied potential was varied from 0 to 1 V during PEC-degradation tests. The concentration of the degraded RhB solution was evaluated by recording the absorbance spectra intensity variation ratio of the RhB solution before and after the test. The C t /C 0 is evaluated from I t /I 0 , in which C 0 is the initial concentration of the RhB solution, C t is the concentration of that at a different interval during the degradation process, I 0 is the absorbance spectrum intensity of the initial RhB solution, and I t is the absorbance spectrum intensity of the RhB solution at a different interval during the degradation process.   Figure 1(b) and (c) indicate that the morphology of the ZnO nanosheet template did not show a marked change after the vulcanization process with the lower sulfur concentration below 0.1 M. The sheet template still maintained a porous structure with abundant tiny pores on it. Notably, when the sulfur precursor concentration was substantially increased to 0.25 M, the morphology changed to be more solid, and no distinct porous structure was observed for the ZnO sheet template after vulcanization. The addition of Na 2 S into the hydrothermal reaction solution results in the ion exchange between S 2− from the dissolved Na 2 S and O 2− from the ZnO template, and this ion exchange process promotes the formation of ZnS crystallites on the surfaces of the ZnO sheet template [24][25][26]. The inward diffusion of S 2− to interact with Zn 2+ in the ZnO template forms the ZnS phase during the hydrothermal vulcanization process. Notably, even the vulcanization process was conducted for the ZnO sheet template with an increased sulfur precursor concentration ranging from 0.01 to 0.25 M, the sheet-like feature of the ZnO template was still maintained in this study, revealing a gradual dissolve of the ZnO phase and the formation of ZnS phase reached an equilibrium state during the current hydrothermal vulcanization processes. With the increase of the sulfur ion source, more ZnO might convert into ZnS with an increased reaction degree of ion exchange, and the generated ZnS crystallites will cover the residual ZnO surface and reorganize the surface topography of the porous ZnO sheet template. Therefore, the porous ZnO sheet template treated with the highest sulfur concentration herein causes more ZnS crystallites to form, leading to the disappearance of the porous surface of the ZnO sheet template. Figure 2 shows the XRD patterns of the ZnO sheet template treated with and without various vulcanization processes. In Figure 2 The XRD results reveal that the vulcanization processes herein are effective to induce the partial ZnO template crystal to transfer into the ZnS phase. Furthermore, the intensity of the characteristic crystallographic plane of cubic ZnS (111) was substantially increased with the increased sulfur precursor concentration, revealing an increased content of ZnS crystals in the sample [21,26].

Results and discussion
From the comparison and investigation of the main characteristic crystallographic planes (100) and (101) of the ZnO sheet and (111) of ZnS crystals, the ZS-1, ZS-2, and ZS-3 show the clear coexistence of ZnO and ZnS phases in the products and are in a crystalline composite structure. Notably, the substantially weak Bragg reflections of ZnO ( 100) and (101) together with a markedly intense Bragg reflection of ZnS (111) in Figure 2(d) demonstrates that the ZnO template has been massively vulcanized for the ZS-3. Figure 3(a) shows the low-magnification TEM image of the ZS-1 nanosheet. Visible pores in a nanometer scale existed in the sheet structure, and the diameter of the sheet is approximately 1 µm. The HRTEM images taken from the outer and inner pore regions of the sheet are displayed in Figure 3 The SAED pattern presents the coexistence of ZnO and ZnS in the ZS-1 nanosheet and the result is consistent with the aforementioned XRD pattern. The EDS spectra in Figure 3(g) clarify the elemental composition of the ZS-1 nanosheet. In addition to Cu and C signals from the TEM grid, no impurity elements are detected herein. The atomic percentages of Zn, O, and S for the ZS-1 nanosheet is 48, 32.2, and 19.8, respectively. Furthermore, the spatial elemental distribution of Zn, S, and O in the ZS-1 sheet structure is shown in Figure 3(h). The elemental distribution of Zn, O, and S atoms over the selected nanosheet reveals that the ZS-1 has a homogeneous composition of ZnO and ZnS that coexisted in the nanosheet.
Similarly, the ZS-2 nanosheet also presents a porous sheet structure in Figure 4   nanosheet. This might be associated with an increased crystalline quality of ZnS crystallites at the given vulcanization process [26]. The SAED pattern in Figure 5 The narrow scan XPS spectra of Zn2p regions for the ZS-1, ZS-2, and ZS-3 are presented in Figure 6(a)-(c), respectively. The Zn2p 3/2 and Zn2p 1/2 peaks are centered at approximately 1021.1 and 1044.1 eV, respectively. The spin-orbit splitting of approximately 23 eV between the Zn2p 3/2 and Zn2p 1/2 core-level components reveals the Zn 2+ state nature herein [21,28]. Notably, the core-level Zn2p binding energies of Zn-O are close to those of Zn-S [21]. Due to the similar Zn 2+ binding energy of ZnO and ZnS phases, there are no substantial differences in the obtained Zn2p spectra among various samples. Figure  6(d)-(f) presents XPS S2p spectra of various samples. Notably, asymmetric S2p spectra can be further deconvoluted into two subpeaks that are centered at approximately 161.1 and 162.1 eV and are attributed to S2p 3/2 and S2p 1/2 of S 2− binding states, respectively [21,25]. Moreover, the peak intensity ratio of S2p 1/2 to S2p 3/2 was approximately 1:2, revealing that the ZnS phase has been successfully synthesized by O 2− and S 2− ion exchange during the given vulcanization processes for the ZnO sheet template. Moreover, the higher S2p spectrum intensity of ZS-3 reveals higher content of the ZnS phase in the ZS-3 composite. The O1s XPS spectra of ZS-1, ZS-2, and ZS-3 are shown in Figure 6(g)-(i), respectively. The decreased O1s spectrum intensity for the composite structure with an increased sulfur precursor concentration demonstrates an increased sulfidation degree of the ZnO template to form more Zn-S bonds [29]. The O1s spectra are further split into two subpeaks. The subpeak centered at approximately 530.7 eV (blue line) is assigned to the crystal lattice oxygen ions in ZnO, and the peak centered at approximately 531.5 eV (green line) is associated with the oxygen vacancy and/or absorbed OH − in the ZnO lattice [30]. The XPS results herein reveal that a composite structure of ZnO-ZnS was formed for the ZS-1, ZS-2, and ZS-3 samples.
The optical absorption spectra of various samples are shown in Figure 7(a). It can be clearly seen that the ZnO template has a strong absorption edge at approximately 380 nm. The optical absorption edge gradually shifted to a shorter wavelength for the ZnO template treated by vulcanization processes with an increased sulfur precursor concentration. Notably, dual optical absorption edges are visibly observed for the ZS-1, ZS-2, and ZS-3. The dual absorption edges are associated with the contribution of two constituent phases of ZnO and ZnS, and this has also been demonstrated in the ZnO-ZnS heterostructure synthesized via a solid-state low-temperature synthesis method [31]. Moreover, from Figure 7(a), all the prepared products have optical absorption edges located in the UV range, revealing they are UV-light sensitizer materials. The band gap energy (E g ) of the products can be further calculated according to the equation (αhv) 2 = A(hv − E g ), where A is a constant, E g is the band gap energy, h is the Planck constant, and α is the absorption coefficient. Figure 7    in the ZnO-ZnS composite affects the photo-induced charge separation efficiency in the composite structure. The fewer ZnS crystallites in the porous ZnO-ZnS sheetlike composite structure improves the separation of electron-hole pairs between ZnO and ZnS under irradiation, and thus, have a better photoresponse. By contrast, the photo-induced electron-hole pairs are unable to be separated effectively for massive ZnS crystallites in the solid ZnO-ZnS sheet-like composite structure. It has been shown that the thick ZnS shell layer in the ZnO-ZnS core-shell nanofibers suppresses the tunneling efficiency of the holes from the ZnO core to the ZnS shell [29], and this work supports the observed deteriorated photoresponse performance of the ZS-3 herein. Notably, the photocorrosion phenomenon was observed from the ZS-1 and ZS-2 composites. The photocorrosion causes the slightly decreased photocurrent with the irradiation on/off cyclic number, and this phenomenon might be associated with the accumulation of excessive light-induced holes on the ZnS surface at the given test condition [33]. Figure 8(b) shows the Nyquist plots of various products. Comparatively, the ZS-1 displays the smallest semicircle radius among various products. The smaller radius of the Nyquist plot's semicircle implies lower interfacial charge transfer resistance in semiconductor heterostructures [34][35][36]. The corresponding equivalent circuits for the pristine ZnO template and ZnO-ZnS composite are shown in Figure 8(c). Notably, R s represents the series resistance of the electrochemical device. C s and R CTs represent the capacitance phase element and charge transfer resistance of the depletion layer of the semiconductor, respectively. C d and R CT are the capacitance phase element and the charge transfer resistance at the double layer of the semiconductorelectrolyte interface, respectively [37]. According to the fitting results from the Nyquist plots using the proposed circuit models, the R CT is approximately 6,510, 2,594, 3,566, and 12,665 Ω for the pristine ZnO, ZS-1, ZS-2, and ZS-3 respectively. The smaller R CT of ZS-1 is ascribed to a faster charge transfer and higher separation rate of photogenerated electron-hole pairs in the composite structure [34], which supports the aforementioned photoresponse performance of the samples. Similarly, in ZnS or ZnIn 2 S 4-modified ZnO nanorods, the decoration of suitable sulfides substantially decreases the R CT of the ZnO-sulfides system, and therefore, obtains an enhanced photoresponse in comparison with the pristine ZnO nanorods [38] Figure 9(a) shows the Mott-Schottky (M-S) plots of the pristine ZnO template. The positive slope in the M-S plots indicates the n-type nature of ZnO [27]. Moreover, the flat band potential at the electrolyte/electrode interface can be estimated from the M-S curves according to the M-S equation [34]. By extrapolating the X-axis intercepts of the linear region in the M-S curve, the flat band potential of ZnO has been determined to be approximately −0.5 eV (vs NHE). The evaluated value is close to the hydrothermally derived ZnO nanorods that have a flat band potential of −0.55 V (vs NHE) [39]. Compared with the flat band potential of the ZnO template, the value of the representative ZnO-ZnS heterogeneous structure (ZS-1) was more negative in Figure 9(b). The flat band potential of ZS-1 is estimated to be −0.63 eV. A more negative flat band reflects a smaller barrier to the charge transfer and can induce more electrons accumulation in the conduction band of the sample, thus suppressing the recombination of photogenerated electron-hole pairs [40]. A negative shift of the flat band level of the ZnO-ZnS composite in comparison with that of the ZnO template means a lifted Fermi level after vulcanization of the ZnO sheet template, and the more negative flat band potential of the ZnO sheet template after vulcanized modification means an enlarged degree of band bending in the material system [38]. Similarly, the M-S result of the g-C 3 N 4modified TiO 2 heterostructure also exhibits the more negative flat band potential in the heterostructure with respect to that of the TiO 2 template, and an enhanced charge separation efficiency is demonstrated in the g-C 3 N 4 -modified TiO 2 heterostructure [41]. Furthermore, it has been shown that the conduction band energy (E CB ) for the n-type semiconductor is more negative, i.e., about 0.1 eV than its flat band potential. [42]; therefore, the E CB of the pristine ZnO template herein can be evaluated as −0.6 eV vs NHE. Notably, for the construction of the ZnO/ ZnS heterogeneous band structure for the ZS-1, the referenced M-S plot of a single ZnS phase is displayed in Figure 9(c), and the E CB of the ZnS is found to be roughly up to −0.89 eV vs NHE. Combined with the band gap energy obtained from the previous UV-vis results, the valence band energies (E VB ) for the ZnO and ZnS are calculated to be 2.6 eV and 2.51 eV vs NHE, respectively, according to the equation E VB = E CB + E g . Figure 10(a) presents the impressed potential-dependent degradation efficiency of the RhB solution by ZS-1 under 30 min irradiation. The PEC-degradation level of the RhB solution increased with increasing the impressed potential from 0 V to 0.5 V, and PEC-degradation efficiency of RhB solution reached a peak value of 58% at 0.5 V. Once the impressed potential exceeded 0.5 V, the degradation rate decreased (54%; 0.75 V). The result of Figure 10(a) demonstrates that a higher impressed potential (larger than 0.5 V) might result in reallocation of the space charge layers, further hindering the degradation efficiency [43]. Moreover, the ZS-1 was difficult to efficiently separate the photoinduced charges if the impressed voltage was lower than 0.5 V. Therefore, the optimal impressed potential for the ZnO-ZnS composite to the PEC-degrade RhB solution was chosen to be 0.5 V in this study. The photoelectrocatalytic performance of the ZnO template and various ZnO/ZnS composites toward the RhB solution is shown in Figure 10(b)-(e). The absorbance spectra intensity of the aqueous RhB solution decreased with irradiation duration for all the photoelectrocatalysts. In the PEC-degradation system, the absorbance peak of the RhB solution decreased during degradation without blue shift indicating the destruction of C]N, C]C, and C]O of the dye molecules due to the enhanced catalytic activity, and the final degradation products might consist of low-molecular-weight organic, CO 2 , H 2 O, etc. [43]. Figure 10(f) shows the RhB degradation efficiencies of ZnO, ZS-1, ZS-2, and ZS-3 are 57, 82, 72, and 43% under 60 min irradiation, respectively. Comparatively, the ZS-1 shows the highest degradation level toward the RhB solution among various control samples at the same given test convictions. Figure 10(g) presents the kinetic behaviors of the RhB solution degraded by ZnO and various ZnO-ZnS composites; the degradation rates of the RhB solution follow the pseudo-first-order reaction according to the simplified Langmuir-Hinshelwood model, −ln(C t /C 0 ) = kt, where C 0 is the initial concentration of the RhB solution, C t is the concentration of that at different intervals during the degradation process, and k is the reaction rate constant (min −1 ) [44]. The evaluated PEC-degradation rate constant of ZS-1 is 0.02493 min −1 , which is 1.8, 1.2, and 2.8 times that of ZnO, ZS-2, and ZS-3, respectively. The PEC-degradation performance of ZS-1 and ZS-2 towards RhB solution is superior to that of the ZnO template, revealing the construction of ZnO-ZnS composite structure with suitable vulcanization conditions is beneficial to improve the PEC-degradation activity of the ZnO template. Furthermore, the recycling tests are conducted for the ZS-1 five times (Figure 10(h)). Notably, after five cycles, only a slight decrease of the photoelectrocatalytic activity is found, and 73% of RhB is degraded. The ZS-1 still maintains good photocatalyst activity (>70%) for RhB dye degradation herein, inferring that the prepared ZS-1 porous composite sheet catalyst is effective and reusable under sunlight irradiation.
Based on the experimental results, the possible photogenerated electron-hole pair transfer steps inside the ZnO/ZnS photoanode, and the degradation mechanism toward RhB dyes is shown in Figure 11(a). In the ZnO/ ZnS composite system, the CB position of ZnO was located at −0.6 eV and that of ZnS was located approximately −0.89 eV from the M-S results. Figure 11(a) indicates that the constructed ZnO/ZnS system exhibited a staggered type-II band alignment configuration, which can lead to a spatial separation of the photoinduced electrons and holes. Notably, the oxidation ( O O , 2 2 / ⋅ − ) and reduction (˙OH/H 2 O) potentials were −0.33 and +2.27 eV (vs NHE), respectively [2]. Excitation of electrons will take place in ZnO and ZnS simultaneously under irradiation. As the CB of ZnO is at a lower energy level than ZnS, the photogenerated electron will be transferred from CB of ZnS to the CB of ZnO, and further transported to the Pt counter electrode under an applied potential. By contrast, holes will migrate from the valence band (VB) of ZnO to the VB of ZnS, leading to the improved separation efficiency of photogenerated electron-hole pairs under the PEC system. The electrons at the Pt electrode and holes at the VB of ZnS would subsequently react with oxygen and water, respectively, to produce active superoxide and hydroxyl radicals for degradation processes. For further understanding of the photoelectrocatalytic mechanism, it is important to investigate the active species formed during the photoelectrocatalytic process toward the RhB solution. Benzoquinone (BQ), edentate disodium (EDTA), and tert-butyl alcohol (t-BuOH) were employed to scavenge the superoxide radicals ( O 2 ⋅ − ), the hole (h + ), and hydroxyl radicals (˙OH), respectively, herein. As shown in Figure 11(b), the photoelectrocatalytic activity of ZS-1 has been significantly decreased when BQ is added, indicating the superoxide radicals ( O 2 ⋅ − ) are the main reactive species in the photodegradation process. Comparatively, when introducing EDTA and t-BuOH, the degradation has been decreased for a lower degree, suggesting that h + and˙OH also participate in the photocatalytic reaction for the ZS-1 toward RhB solution, but the effect is smaller than that of the O 2 ⋅ − species. Based on the above results, a schematic diagram of the possible degradation mechanisms of the photoinduced charges in the ZS-1 heterostructure is also constructed in Figure 11(a  The aforementioned potential enhanced photoinduced charge separation is similar to the PEC-degradation of In 2 O 3 -ZnO composites with a type II band alignment toward the MB solution [46]. The electron drift and hole capture in Type II band alignment will suppress the recombination of photoinduced electron-hole pairs. Meanwhile, the applied voltage will rapidly prompt the photogenerated electrons to move to the external circuit rapidly, which tremendously reduces the recombination of electronhole pairs. The improved photoelectrocatalytic activity of ZS-1 and ZS-2 composites with suitable ZnS/ZnO phase ratios than that of the pristine ZnO template herein was mainly ascribed to the formation of heterojunctions, which improved the absorption efficiency of light and inhibited the recombination of the photogenerated electron-hole pairs and the apparent synergetic effect between photo-and electrocatalysis, which facilitated charge generation, separation, and transfer. It has been shown that the comparative component ratio in the heterostructures is highly associated with their photocatalytic performances [46]. Moreover, the porous structure of the composites because of the high specific surface area is also beneficial to the photocatalytic activity [47]. Based on the aforementioned points, the porous structure and the optimal ZnS/ZnO phase ratio in the ZS-1 sheet impacted the superior photoelectrocatalytic performance toward degrading RhB dyes with respect to other ZnO-ZnS composites in this study. Similar porous structure and composition ratio effects dominating the optimal photocatalytic activity have also been demonstrated in ZnS-SnS 2 porous nanosheets [47].

Conclusion
The sheet-like ZnO-ZnS composites were successfully synthesized by the hydrothermal vulcanization method of porous ZnO sheet templates. The microstructure and composition analyses results reveal that the sulfur precursor concentration affected the ZnO/ZnS phase ratio and surface morphology of the as-synthesized ZnO-ZnS sheet composites. Compared with pristine ZnO, the ZnO-ZnS composites with the S/O atomic composition ratios of 0.61 and 0.85 substantially reduced the recombination rate of photogenerated electron-hole pairs by enhanced charge separation. By contrast, the ZnO-ZnS composite with the higher S/O atomic composition ratio of 3.63 decreased the photoactivity. The suitable ZnO/ZnS phase ratio and the porous surface feature made the ZS-1 porous sheet composite exhibit superior photoactivity among various control samples. In this study, the ZS-1 photoanode had a high photocurrent density of 1.22 mA/cm 2 at 0.5 V (vs Ag/AgCl), and highly efficient PEC-degradation ability toward RhB solution to over 80% at 0.5 V. The integration of the hydrothermally derived porous ZnO sheet template with a vulcanization process to control surface porous features and the ZnO/ZnS phase ratio by varying sulfur precursor concentrations is a promising approach to design the ZnS-ZnO sheet composite with various degrees of photoactive performances for photoelectrocatalyst applications. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.