A Mo-cation/O-anion doping strategy for creating vacancy defects and cation multivalency to enhance the hydrogen evolution of ZnS under visible light

Abstract

Herein, we designed a stable and photocorrosion-resistant Mo/O co-doped ZnS (labeled as ZnMoOS) catalyst with abundant sulfur vacancy (Vs) defects and bivalent Mo⁴⁺/Mo⁶⁺ states for an effective photocatalytic hydrogen evolution reaction (photo-HER) under visible light. The Mo/O co-doping reduces the band gap of ZnS and extends its visible light absorption range. The hydrazine-driven process adjusts ZnMoOS with appropriate bivalent n(Mo⁴⁺)/n(Mo⁶⁺) states and creates abundant Vs defects. The Vs defects are active sites to capture water molecules and weaken H–O–H bonds for producing protons and H₂ generation. The bivalent n(Mo⁴⁺)/n(Mo⁶⁺) states act as hosts for photogenerated electrons, facilitating the rapid hopping of photogenerated electrons between Mo⁴⁺ ↔ Mo⁶⁺ to transfer for the photo-HER, thereby improving the photo-HER efficiency. DFT calculations reveal that Mo/O co-doping of ZnMoOS with abundant Vs defects and heterovalent Mo⁴⁺/Mo⁶⁺ states significantly facilitates hydrogen desorption and enhances the surface H* generation rate. ZnMoOS-3 with appropriate Mo/O co-doping and regulation with an optimum hydrazine content exhibits an excellent photo-HER rate of 41.6 mmol g⁻¹ h⁻¹ and an AQE of 18.6% at 400 nm, along with good durability and stability. This work provides a strategy for vacancy defects and heterovalent states for designing sulfide catalysts with high photo-HER activity and inhibition of photocorrosion.

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1. Introduction

Hydrogen has recently been considered an alternative to fossil energy sources due to its green nature and better energy capacity.^1–3 Coal and natural gas reforming are currently used in hydrogen gas generation. However, due to the challenges associated with environmental remediation and the lack of adequate production inspections in such processes, it has become a great source of concern because this production technique invariably involves using fossil fuels and environmental degradation.^4,5

Photocatalytic hydrogen production has attracted significant attention due to its greater economic efficiency and environmental benefits compared to conventional hydrogen production strategies.^6,7 The photocatalytic water decomposition process comprises three primary steps. Firstly, upon light irradiation, electron–hole pairs are generated on the surface of the photocatalyst. Secondly, the charges undergo separation and migrate towards the photocatalyst surface. Finally, the electrons participate in redox reactions to generate hydrogen and reduce electron holes.^8,9 In 1972, Honda and Fujishima were the first to report hydrogen production by water decomposition on TiO₂ electrodes, and their research laid the theoretical groundwork for subsequent photocatalytic hydrogen production.¹⁰ Subsequently, numerous semiconductor materials, such as TiO₂, NiO, ZnO, SrTiO₃, La:NaTiO₃, CdS and Ta₂O₅, were applied in the study of photocatalytic decomposition of water to produce hydrogen.^11–14 However, these are wide-bandgap semiconductors that can only be excited by ultraviolet radiation, which accounts for about 5% of the solar energy. Consequently, the effectiveness of solar-to-hydrogen conversion (STH) is relatively poor.¹⁵ In contrast, narrow-bandgap photocatalysts can effectively utilize the photon energy of the visible light band for hydrogen production. Narrow band gap photocatalytic materials are highly promising for photocatalytic hydrogen production since the photon energy of the visible light band accounts for about 46% of the solar energy that reaches the Earth's surface. ZnS has been widely explored in the photo-HER, thanks to its rapid production rate of photo-induced e⁻ and h⁺ pairs and a more negative U_CB-edge potential.¹⁶ To improve the photocatalytic performance of ZnS, forming a Zn(O, S) solid solution by combining ZnS and ZnO has attracted considerable attention because of its abundant chemical resources, easy availability, environmental safety, and reusability.^17,18 Researchers such as Abdullah and Gultom have successfully enhanced the photocatalytic performance of the modified Zn(O, S) by doping it with Ho, Ni, Y, La, In, and Sn. The improved photocatalytic hydrogen evolution performance and the ability to reduce pollutants of Zn(O, S) indicate that Zn(O, S) materials have application prospects in the field of photocatalysis.^19–23 To further enhance the photocatalytic performance of Zn(O, S), researchers have adopted methods such as defect engineering.^24–26

Defect engineering represents a promising approach for improving the light-trapping ability of photocatalytic materials like ZnS.^27–30 Vacancy defects have been demonstrated to broaden the absorption edge towards visible wavelengths while narrowing the overall band gap. During this process, visible light initially excites electrons from the VB to Vo energy level, which acts as the intermediate transition state. The electrons then shift to the conduction band, speeding up charge separation and transport and eventually enhancing the photocatalytic activity.^31–33 For instance, Wang et al.³⁴ demonstrated that an increase in vacancy defects results in a red shift of the absorption edge and an enhanced visible-light response. Moreover, there are numerous reports on improving the photocatalytic performance of ZnS through doping with transition metals and rare earth ions. Shah et al.³⁵ successfully prepared Sn-doped ZnS nanomaterials via the hydrothermal method, decreasing the bandgap from 3.50 eV to 3.10 eV. Lee et al.³⁶ successfully synthesized In/Cu co-doped ZnS nanoparticles in deionized water and ethanol solvent by a sonochemical method using citric acid as a surfactant in an aqueous medium, and the bandgap decreased from 3.74 eV to 3.59 eV. Bao et al.³⁷ prepared a series of Co-doped ZnS catalysts, and Co-doping reduced the bandgap of ZnS from 3.31 eV to 3.20 eV. Furthermore, doping ZnS with a second metal can further improve the efficiency of photocatalysts, as it helps to slow down the recombination of photogenerated electron–hole pairs.³⁸ Noble metals like Pt, Ir, Ru, Au, etc. have been reported to be the most efficient co-catalysts when doped onto semiconductors. However, their high cost and limited availability obstruct commercializing current photocatalysts for hydrogen production.³⁹ Hence, the research on developing active photocatalysts that are both cost-effective and relatively abundant has become a focal point in scientific research.

Molybdenum is relatively abundant, of low cost, and non-toxic.⁴⁰ Mo-doping can accelerate the electron transfer from the VB to CB, diminish the recombination of e⁻ and h⁺, gain active sites on the photocatalyst surface, and significantly enhance the activity of the photocatalyst, thereby increasing the hydrogen generation rate.^41–43 Meanwhile, Mo-doping can also improve the stability of the photocatalyst, enabling it to maintain high activity even after repeated recycling.^44,45 For example, Zuo et al.⁴⁵ designed a Mo-doped ZnIn₂S₄ (MoZIS)-coated Ni-based Hofmann-type coordination polymer (Ni–NiHCP), and the H₂ yield of Ni–NiHCP/MoZIS reached 26.7 mmol g⁻¹ h⁻¹ under visible light irradiation, which was 10 times higher than that of pure ZIS. Tamboli et al.⁴⁶ fabricated a multi-component Mo-doped BiVO₄@graphite photoanode through a simple spin-coating and annealing process. By leveraging the synergistic effect between Mo and graphene, they improved the carrier density and inhibited photo-induced e⁻ and h⁺ recombination rates to enhance the photo-HER efficiency. Li et al.⁴⁷ prepared Mo/S co-doped g-C₃N₄, which extended the photoresponse range and achieved an H₂ yield of 294 μmol g⁻¹ h⁻¹ without a noble metal co-catalyst. Nevertheless, the abovementioned reports have limitations, such as a relatively low H₂ yield, complex catalyst synthesis methods, or stringent photo-HER conditions.

Based on the above limitations, ZnMoOS was synthesized using hydrolysis at atmospheric pressure. ZnS was doped with Mo/O, and hydrazine was used as a reducing agent to reduce Mo⁶⁺ to Mo⁴⁺, which broke the charge balance of the system to produce Vs. The synergistic effect of transition metal doping and sulfur defect engineering enhanced the photocatalytic performance of ZnMoOS.

2. Experimental

2.1 Preparation of catalysts

To a beaker containing 800 mL of distilled water, 20 mmol of Na₂MoO₄·2H₂O (A.R.), 10 mmol of ZnCl₂ (A.R.) and 10 mmol of thioacetamide (TAA, A.R.) were added sequentially, adding each component at an interval of 20 min. After increasing the temperature of the mixture to 98 °C, 0, 0.2, 0.4, and 0.6 mL of N₂H₄ (A.R. 85%) were individually added to the mixed solution to react at 98 °C for 3 h. The formed precipitates were washed with deionized H₂O and anhydrous C₂H₅OH five and three times, respectively, and dried at a rotary evaporator at 70 °C for 1 h. Then, the obtained solids were labeled as ZnMoOS-1, ZnMoOS-2, ZnMoOS-3, and ZnMoOS-4, respectively. For comparison, samples without Na₂MoO₄·2H₂O and with 0.4 mL of N₂H₄ (A.R. 85%) but without Na₂MoO₄·2H₂O were also prepared using the same synthetic process. These samples were labeled as ZnS and ZnOS, respectively.

2.2 Characterization of catalysts

The XPS test was performed on a PHI5700 ESCA system with Al Kα X-rays. UPS measurements were conducted with He I (21.22 eV) monochromatic light. The XRD test was carried out on a Rigaku Ultima IV phase-emission X-ray diffractometer (λ = 1.5405 Å). The EPR signal of DMPO was collected using a Bruker A300 spectrometer. A ZSX Primus II X-ray fluorescence spectrometer was applied for elemental analysis. Ultraviolet-visible spectroscopy was carried out using a Lambda 750S spectrometer to measure the visible absorption spectrum of ultraviolet light. The FE-SEM test was performed on a Hitachi SU-8010 scanning electron microscope, and a TEM test was accomplished on a TECNAI G2 F20 transmission electron microscope. Optical property analysis was conducted on a Lambda 750S spectrophotometer with BaSO₄ (A.R.) as the reference. The specific surface area (S_BET) analysis was performed on an ASAP 2020 analyzer, and the S_BET was calculated using the BET formula. The pore-size distribution was determined using the BJH formula. Photoluminescence (PL) was carried out on a fluorescence spectrometer (Varian Cary Eclipse 500) with an excitation wavelength of 460 nm. The sulfur vacancies were investigated by Raman spectroscopy (HORIBA LabRAM HR 800), and electrochemical analyses were conducted on a BioLogic SP-300 utilizing the method reported in the literature.¹ The detailed measurement procedure is provided in the ESI.†

2.3 Evaluation of photocatalytic hydrogen activity

The photo-HER activity was assessed on a Labsolar-6A system (Perfect Light Co.) with a 300 W Xe lamp filtered at 420 nm. During the experiment, 50 mg of the catalyst was dispersed in a 50 mL aqueous solution containing 10 mmol of Na₂S and Na₂SO₃ as sacrificial agents. A dilute nitric acid solution was added dropwise and mixed evenly, and the pH was adjusted to 6.5. The reactor was degassed by argon purging and had a H₂O circulation cooling system to maintain the temperature of the reactor at 25 °C. The H₂ formation was analyzed using an online Panna A91 gas analyzer with 99.999% helium as the carrier gas under visible light irradiation for 6 h. The detailed AQE calculations are provided in the ESI.†

2.4 Density functional theory calculation method

The DFT calculation procedure is provided in detail in the ESI.†

3. Results and discussion

3.1 Structure analysis

Fig. 1a presents XRD patterns of ZnMoOS, ZnOS, and ZnS catalysts. We observe that diffraction peaks at 28.909°, 48.110°, 57.105°, and 77.830° correspond to crystal planes (111), (220), (311), and (331), respectively, of monoclinic ZnS phases indexed in PDF#80-0020. Notably, the diffraction patterns of ZnOS and ZnMoOS catalysts do not match the crystal planes of monoclinic zinc oxide, indicating their single-phase structure. The crystal sizes of ZnMoOS, ZnOS, and ZnS are calculated from the peak broadening in XRD patterns, and the results are included in Table S1.† It was revealed that Mo/O co-doping decreased the crystallinity of ZnS. Still, with increased hydrazine content, the grain size of ZnMoOS increases, and the contact area increases due to the reduction of Mo⁶⁺ to Mo⁴⁺ by hydrazine.

Fig. 1 (a) XRD spectra of ZnMoOS, ZnOS, and ZnS. The XPS spectra of (b) Zn 2p, (c) S 2p, (d) O 1s, and (e) Mo 3d. (f) EPR spectra of ZnMoOS, ZnOS, and ZnS.

XPS has been employed to analyze the valence states and composition of elements on the ZnMoOS-3 surface. Fig. S1† displays the survey XPS spectrum of ZnMoOS-3, with peaks corresponding to Zn, Mo, S, and O elements. Fig. 1b shows that the peaks at 1022.3 and 1045.4 eV are related to Zn 2p_3/2 and Zn 2p_1/2, respectively.^48,49Fig. 1c presents the S 2p spectra of ZnMoOS-3 and ZnOS. For ZnMoOS-3, the binding energy peaks are located at 161.8 eV and 163.8 eV, which are attributed to S 2p_3/2 and S 2p_1/2, respectively. Compared with ZnOS, these two peaks of ZnMoOS-3 shift towards the direction of lower binding energy, a phenomenon likely to be caused by the presence of Vs. The Vs facilitate electron transfer, thus reducing the binding energy.^50–54Fig. 1d shows the photoelectron spectra of O 1s, indicating that oxygen is located in multiple chemical environments, with distinct binding energies observed for lattice oxygen (O_Lattice) and hydroxyl oxygen (O–H) at 529.9 and 531.4 eV, respectively.^55,56 By comparing the XPS spectra of Zn, O, and S in ZnMoOS-3 and ZnOS, it can be found that their binding energies all exhibit a negative shift. This indicates that Mo doping has altered the chemical environments of Zn, O, and S.⁵⁷Fig. 1e shows that Mo exists in heterovalent states, with binding energies at 234.7 and 231.58 eV related to Mo⁴⁺ 3d_3/2 and Mo⁴⁺ 3d_5/2, respectively, while binding energies at 235.6 and 232.4 eV correspond to Mo⁶⁺ 3d_3/2 and Mo⁶⁺ 3d_5/2, respectively. Mo in ZnMoOS exhibits two valence states: Mo⁶⁺ and Mo⁴⁺. The formation of Mo⁴⁺ leads to the creation of Vs and an increased charge transfer rate.^58,59 These results indicate an increase in Vs. Combined with the EPR data in Fig. 1f, the peak at g = 2.004 further confirms the formation of Vs in ZnMoOS.^60,61 The results further demonstrate that the formation of Vs can be enhanced by doping Mo and adjusting the valence states of Mo. The addition of hydrazine reduces Mo⁶⁺ to Mo⁴⁺ and leads to the reconstruction of valence charges. Therefore, Mo generates heterovalent states, forming a large amount of Vs to maintain the overall charge balance of the crystal. Notably, there is a significant synergistic effect between heterovalent states and Vs, which enhances the charge separation of photo-induced carriers, delays recombination, and prolongs their lifetime. Meanwhile, more active sites are formed on the catalyst surface, facilitating the adsorption, activation, and conversion of reactant molecules. This significantly boosts the photocatalytic performance, laying a solid foundation for its application in the photo-HER.

3.2 Morphology analysis

Fig. 2a and b depict the morphology of the ZnMoOS-3 catalyst at 50k and 30k magnifications, respectively, revealing a uniform accumulation of nanoparticles into powder clusters. Fig. 2c and d present SEM images of ZnOS, which exhibit a similar morphology to ZnMoOS-3, although the nanoparticle diameter of ZnOS is smaller. The TEM image of ZnMoOS-3 in Fig. 2e further confirms that ZnMoOS-3 comprises nanoparticles with an average size of 50 nm. Fig. 2f presents the HR-TEM image of ZnMoOS-3, with a d-spacing value of 3.09 Å related to the (111) crystal plane of ZnS (PDF# 80-0020). Fig. 2g shows a selected region for the SEM-EDS elemental mapping, and Fig. 2g–k display SEM-EDS elemental mapping images of ZnMoOS-3, indicating the uniform distribution of O and Mo elements in ZnMoOS-3. Meanwhile, Tables S1–S3† present Zn, Mo, O, and S elemental contents in the catalyst, as measured by XPS, XRF, and SEM-EDS, respectively. All the above-mentioned results were consistent.

Fig. 2 SEM images of (a and b) ZnMoOS-3 and (c and d) ZnOS. (e) TEM and (f) HR-TEM images of ZnMoOS-3. (g–k) SEM-EDS elemental mapping of ZnMoOS-3. (l) N₂ adsorption–desorption isotherms and (m) pore size distribution curves of ZnMoOS-3 and ZnOS.

BET and pore size distribution test results are presented in Fig. 2l. The BET curves display a hysteresis regression IV isotherm at a relative pressure (p/p^o) between 0.8 and 1.0.⁶² The BJH pore size distribution curves are shown in Fig. 2m. The S_BET of ZnMoOS-3 was determined to be 64.7 m² g⁻¹, the total pore volume was 0.032 cm³ g⁻¹, and the average pore diameter was 0.26 nm. In contrast, the S_BET, total pore volume, and average pore diameter of ZnOS were 35.9 m² g⁻¹, 0.014 cm³ g⁻¹, and 0.13 nm, respectively. These results confirm that ZnMoOS-3 has higher S_BET than ZnOS, indicating more active sites and potentially superior photo-HER activity.

3.3 Energy band structure analysis

UV-vis spectra of ZnS, ZnOS, and ZnMoOS are shown in Fig. 3a. It is observed that ZnMoOS has a substantial absorption range in the visible region, while ZnS and ZnOS only absorb ultraviolet light. However, UV light accounts for only 5% of sunlight, and ZnS and ZnOS cannot utilize sunlight efficiently. In this study, hydrazine was used as a dopant. The doping of Mo/O into ZnS significantly improved the optical properties of ZnS without changing its main phase. ZnMoOS-3 shows the best light absorption ability. The E_g was calculated using the formula: (αhν)² = k(hν − E_g), where k is the absorbance constant, ν is the optical frequency, and h is Planck's constant,.^63,64 As illustrated in Fig. S2,† the E_g1 values of ZnMoOS-1, ZnMoOS-2, ZnMoOS-3, ZnMoOS-4, ZnOS, and ZnS are 3.45, 3.38, 3.36, 3.37, 3.50, and 3.55 eV, respectively. Notably, upon the incorporation of Mo into the ZnOS lattice, new energy levels emerge at λ₂ for ZnMoOS-1, ZnMoOS-2, ZnMoOS-3, and ZnMoOS-4, with corresponding E_g2 values of 3.00, 2.92, 2.83, and 2.88 eV, respectively. These values are significantly lower than the bandgap widths of ZnOS and ZnS. This reduction in bandgap can be attributed to the lattice distortion of ZnOS induced by the co-doping of Mo and O. Meanwhile, hydrazine regulation gives rise to heterovalent states. As the ratio of n(Mo⁴⁺)/n(Mo⁴⁺ + Mo⁶⁺) increases, more Mo⁴⁺ ions replace Mo⁶⁺, creating a charge imbalance that leads to the formation of Vs. These defects act as electron-capture centers, enhancing the electron transfer and generating new energy levels within the forbidden band. Consequently, the bandgap decreases, and the wavelength range of light absorption expands. This enables ZnMoOS materials to absorb a broader spectrum of photons, thereby significantly enhancing the utilization efficiency of solar energy.

Fig. 3 (a) UV-vis absorption spectra, (b) MS curves of ZnMoOS-3, (c) UPS spectra of ZnMoOS-3 and ZnOS, (d) VB-XPS spectra of ZnMoOS-3 and ZnOS, and (e) energy band arrangement of ZnMoOS-3. (f) Schematic band structures of ZnOS and ZnMoOS-3.

Fig. 3b displays positive slopes of MS curves. A significant finding confirms that ZnMoOS-3 belongs to n-type semiconductors. This is a crucial characteristic of its application as a photocatalyst. The flat-band potential (U_fb) of ZnMoOS-3 is measured at −1.12 V vs. Ag/AgCl. Fig. S3† provides U_fb values of ZnMoOS-1, ZnMoOS-2, ZnMoOS-4, ZnOS and ZnS, which are −0.99, −1.04, −1.08, −1.50, and −1.33 V, respectively. Utilizing relevant formulae, U_CB = U_fb − 0.2 V vs. Ag/AgCl and V_NHE = V_Ag/AgCl + 0.2, U_VB = U_CB + E_g, the U_CB of ZnMoOS-3 is calculated to be −1.12 V, and its U_VB is determined to be 1.71 V. UPS measured the work function (Φ) of ZnMoOS-3 and ZnOS (Fig. 3c and d). The secondary electron cut-off edge (E_C) and Fermi-level edge (E_F) are 17.22 eV and 0.51 eV for ZnMoOS-3 and 16.72 eV and 0.95 eV for ZnOS-1. As the formula Φ = hν – (E_C − E_F) states, the Φ values of ZnMoOS-3 and ZnOS are 4.51 and 5.45 eV. VB-XPS spectra of ZnMoOS-3 further verified the value of the valence band top (V_BM), with a difference of 1 eV between the Fermi level and V_BM, estimating the V_BM level of ZnMoOS-3 to be 6.21 eV. With the definitions of vacuum at −4.5 eV and NHE at 0.0 eV, the E_VB value of ZnMoOS-3 at vacuum was 1.71 eV vs. NHE and the E_CB value was −1.12 eV, consistent with NHE results. ZnMoOS-3 has a CB position more negative than H⁺/H₂ relative to the H⁺/H₂ potential at 0.0 eV, enabling water decomposition to H₂.

For a comparative purpose, the energy band diagrams of ZnOS and ZnMoOS-3 are shown in Fig. 3f. For ZnOS, the E_g of is 3.50 eV, E_CB = −1.5 V, and E_VB = 2.0 V, and for ZnMoOS-3, the E_g is 2.83 eV, E_CB = −1.12 V, and E_VB = 1.71 V. The Mo/O doping resulted in a bandgap value reduction from 3.50 to 3.0 eV, and adding an optimum amount of hydrazine decreased the bandgap value from 3.50 eV to 2.83 eV. Herein, we demonstrate the suitability of ZnMoOS-3 for the photo-HER in visible light. ZnMoOS-3 was obtained via Mo/O co-doping and hydrazine modulation of the energy-band structure of ZnS, thereby significantly enhancing the activity of the photo-HER and convincing us of its potential in visible light photocatalysis.

3.4 Electrochemical analysis

The data in Fig. 4a demonstrate that the photoluminescence (PL) spectra of all samples are similar in shape. It can also be observed that ZnS exhibits a strong emission peak, indicating severe recombination of photo-induced carriers in ZnS.⁶⁵ In contrast, ZnMoOS-3 has the lowest PL intensity, suggesting that the incorporation of Mo suppresses the photo-induced electron–hole recombination and improves the charge separation efficiency. As shown in Fig. 4b, time-resolved photoluminescence (TRPL) spectroscopy was employed to evaluate the carrier lifetimes of ZnMoOS, ZnOS, and ZnS. The average lifetime was retrieved by curve fitting with the formula:⁵⁶τ_ave = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂), and A₁, τ₁, A₂, and τ₂ are given in Table S4.† The average carrier lifetime of ZnMoOS-3 is 4.250 ns, while those of ZnOS and ZnS are 1.314 and 1.216 ns, respectively. This indicates that Mo doping enhances the carrier separation efficiency, suppresses charge recombination, and thus prolongs the lifetime of photo-induced carriers to improve the photo-HER performance. Meanwhile, the effective separation of photo-induced electrons/holes pairs and interfacial charge transfer ability in catalysts were investigated through electrochemical measurements. Fig. 4c presents the EIS curves of ZnS, ZnOS, and ZnMoOS. The diameter of the semicircle in the Nyquist diagram determines the photo-induced carrier transfer resistivity. Among them, ZnMoOS-3 exhibits the smallest diameter, indicating that the resistivity is decreased and the charge transfer rate is accelerated after Mo doping.^66,67 To further certify the enhanced carrier transfer rate of ZnMoOS-3, TPC density analysis was employed to investigate photo-induced electrons/holes pair separation. Higher photocurrent can promote the separation and intensity of photoresponse, thereby enhancing the photo-HER performance.⁶⁸Fig. 4d shows the TPC densities of ZnS, ZnOS, and ZnMoOS with different Mo-doped contents. Compared to ZnOS, Mo-doped ZnOS exhibits a better photocurrent response intensity in the extended visible light response range after Mo-doping and formation of charge imbalance via hydrazine regulation of Mo heterovalence, which increases active sites and promotes photo-induced electron separation. Fig. 4e shows ZnMoOS-1, -2, -3, -4, ZnOS, and ZnS at a scan rate of 6 mV s⁻¹, with ZnMoOS-3 exhibiting the strongest current density. Fig. 4f depicts the CV curves of ZnMoOS-3 at scan rates of 1–6 mV s⁻¹, showing an increase in the current density with increasing scan rate. The current density values of ZnMoOS-1, ZnMoOS-2, ZnMoOS-4, ZnOS, and ZnS are presented in Fig. S4.† It can be observed that the order of the current density is as follows: ZnMoOS-3 > ZnMoOS-2 > ZnMoOS-4 > ZnMoOS-1 > ZnOS > ZnS. This result strongly indicates that the ZnMoOS-3 electrode possesses a favorable electron transfer rate. Fig. 4g depicts the specific capacitance curves of ZnMoOS, ZnOS, and ZnS. With the increase in the scanning speed, the specific capacitance decreases, resulting in large currents, and polarization at high scanning speeds results in low specific capacitance. At a considerable scanning speed, the ions in the catalyst move too slowly to respond, and the ions will have insufficient time to use the catalyst, leading to surface adsorption and low specific capacitance. However, the specific capacitance of ZnMoOS-3 was found to be larger than that of other catalysts, which was ascribed to possession of heterovalent states and Vs in ZnMoOS-3, increasing the reactive active sites to maintain the reoxidation state as much as possible, and at high scan speeds, ions had more time to utilize the material, making the specific capacitance of ZnMoOS-3 higher than that of other catalysts. As shown in Fig. 4h, the C_dl measurements for ZnS, ZnOS, and ZnMoOS-1, -2, -3, -4 are 1.25, 2.43, 4.22, 5.91, 11.24, and 7.2 μF cm⁻², respectively. The maximum C_dl was observed for ZnMoOS-3 and the minimum for ZnS. Due to the larger specific surface area and more heterovalent states of ZnMoOS-3, a charge imbalance is created, leading to the formation of sulfur vacancies, which increase the active sites. Fig. 4i presents the current density between −1.05 V and −0.4 V per electrode, as recorded by linear voltammetry (LSV). ZnMoOS-3 exhibited the highest cathodic current density for H₂ generation by water decomposition, indicating a fast photo-generated carrier transfer rate at the ZnMoOS-3 interface.^69,70 Furthermore, Fig. 4j displays a plot of Tafel slopes converted by LSV, showing that the Tafel slopes of ZnMoOS-1, ZnMoOS-2, ZnMoOS-3, ZnMoOS-4, ZnOS, and ZnS are −641.88, −552.59, −432.47, −454.40, −736.06, and −934.50 mV dec⁻¹, respectively, of which the overpotential of ZnMoOS-3 is the smallest, suggesting that it possesses better electron transfer performance in redox reactions.

Fig. 4 (a) PL, (b) TRPL, (c) EIS, (d) TPC, and (e) CV curves of ZnMoOS, ZnOS, and ZnS at a scan rate of 6 mV s⁻¹ (f) CV curves of ZnMoOS-3 at scan rates of 1, 2, 3, 4, 5, and 6 mV s⁻¹. (g) Specific capacitance, (h) ECSA, (i) LSV, and (j) Tafel curves of ZnMoOS, ZnOS, and ZnS.

Our findings indicate that ZnMoOS-3, particularly with the best hydrazine content, exhibits the fastest electron transfer rate and the least electrochemical impedance to reduce the photoelectron–hole recombination rate. These characteristics underscore the excellent photo-HER activity and stability of ZnMoOS-3, highlighting its potential in the field of photo-HER.

3.5 Photocatalytic activity and stability

Fig. 5a shows the photo-HER rate in the presence of 50 mg of catalyst as 1.53, 1.75, 2.08, 1.3, 1.09 and 0.89 mmol h⁻¹ for ZnMoOS-1, -2, -3, -4, ZnOS, and ZnS, respectively, and no H₂ was detected in the absence of light and a sacrificial agent. Fig. 5b shows the photo-HER activity of ZnMoOS, ZnOS, and ZnS under visible light. It can be observed that after 6 h of reaction, ZnMoOS-3 evolves the highest hydrogen amount of 12.48 mmol. This is attributed to hydrazine regulation, which produces heterovalent Mo⁴⁺/Mo⁶⁺ states, results in charge re-distribution in the system, promotes electron transfer rate and Vs formation, and thus improves the charge carrier separation and transfer efficiency. ZnMoOS-3 shows the best photo-HER activity, consistent with its electrochemical characteristics of the lowest resistance and highest photocurrent response. Fig. 5c shows the photo-HER results of ZnMoOS-3 after six runs, and it is observed that the photo-HER rate decreases from 12.82 mmol to 12.14 mmol without a noticeable significant change, further proving that ZnMoOS-3 has good stability during the photo-HER. Table S5† presents the research on the photo-HER of ZnS-based catalysts in recent years. In comparison with previous studies, it can be observed that ZnMoOS-3 has a superior photo-HER performance. Fig. S5† illustrates the variation of the photo-HER rate with the amount of catalyst for ZnMoOS-1, ZnMoOS-2, ZnMoOS-3, ZnMoOS-4, ZnOS, and ZnS. It can be observed from this figure that a nonlinear relationship exists between the amount of catalyst and the reaction rate. Fig. S6† demonstrates the influence of solution pH on the photo-HER rate. Specifically, under strongly acidic conditions, the photo-HER rate decreases. Under weakly acidic conditions with a pH value of 6.5, the photo-HER rate reaches its maximum value. Under alkaline conditions, the photo-HER rate also shows a downward trend. This can be attributed to the fact that under highly acidic conditions, the reaction between H⁺, Na₂S, and Na₂SO₃ leads to a reduction in the H⁺ concentration, which subsequently causes a decrease in the hydrogen ion adsorption efficiency.⁷¹ Under strongly alkaline conditions, the relatively low H⁺ concentration results in a low hydrogen ion adsorption efficiency. However, under weakly acidic conditions, the increase in the H⁺ content can promote adsorption and combination rates of H⁺ on the catalyst surface. Fig. S7† presents the relationship between UV-vis absorption and AQE in terms of radiation wavelengths of ZnMoOS-3. It can be observed that the AQE at 400 nm is 18.6%, and the AQE can be detected at 560 nm.

Fig. 5 (a and b) Hydrogen production from ZnMoOS, ZnOS and ZnS. (c) Photo-HER activity of the recycled ZnMoOS-3 catalyst. (d) XRD pattern and (e) Zn 2p, (f) Mo 3d, (g) O 1s, and (h) S 2p XPS spectra of the ZnMoOS-3 catalyst after six cyclic runs. (i) 100 CV cyclic tests at 50 mV s⁻¹ in a redox solution and (j) electrochemical long-term stability of ZnMoOS-3.

The stability of ZnMoOS-3, confirmed through multiple tests, provides a reliable foundation for its potential applications in photocatalysis and materials science, reassuring the audience of its performance consistency over time. To further validate the stability of ZnMoOS-3, we examined its XRD spectrum before and after six runs (Fig. 5d). The peak intensity and position showed no significant change, confirming the stability of ZnMoOS-3. Fig. 5e–h present the Zn 2p, Mo 3d, O 1s, and S 2p XPS spectra of ZnMoOS-3 after six repeated uses, revealing little change in XPS peaks with the progress of the photoreaction. Fig. 5i displays the electrochemical CV curves of ZnMoOS-3 after 100 cycles of a redox reaction, exhibiting stable redox peaks, indicating little change in the potential and current density of the electrochemical reduction/oxidation. Fig. 5j shows the long-term electrochemical stability of ZnMoOS-3 operated at 1.5 V and 30 mA cm⁻², further confirming its good stability.

3.6 Density functional theory calculations

To further validate that the Mo doping and heterovalent Mo⁴⁺/Mo⁶⁺ states enhance the photo-HER activity, ZnOS and ZnMoOS-3 models were selected as representative examples for detailed DFT investigations (Fig. 6 and S8†). Conductivity, a critical parameter for evaluating the electrochemical activity of electrocatalysts, was assessed by calculating the band structures of the ZnOS and ZnMoOS-3 models. As shown in Fig. S8c and d,† the computed bandgaps were 3.54 and 2.85 eV, respectively, closely matching the experimental values of 3.5 eV for ZnOS and 2.83 eV for ZnMoOS-3.

Fig. 6 The ELF plots for (a) ZnOS and (b) ZnMoOS-3. 0 and 0.8 represent the electron deficiency and electron abundance, respectively. (c) Calculated ΔG values of the HER on ZnOS and ZnMoOS-3. The CDD maps of (d) ZnOS and (e) ZnMoOS-3. The isovalue is 0.0008 eÅ⁻³.

To further elucidate the electronic structure of ZnOS and ZnMoOS-3, electron localization functions (ELFs) were computed, and the results are shown in Fig. 6a and b. ELF values close to 0.5 indicate ionic bonding, while values near 0.8 or 0 suggest covalent or metallic bonding. In both the ZnOS and ZnMoOS-3 models, atomic bonding is predominantly ionic, with Mo atoms in the ZnMoOS-3 model also exhibiting ionic characteristics. The results reveal that Mo⁴⁺/Mo⁶⁺ doping in ZnOS significantly enhances the electron distribution in the vacancy region (Fig. 6b). This moderate electron localization within the vacancy region facilitates effective interaction with H atoms, suggesting a strong orbital overlap between the catalyst and the H 1s orbital.^72–74 To better understand why ZnMoOS-3 exhibits the best catalytic performance, we calculated the band structure and ELF for ZnMoOS-1 (Fig. S9†). The results indicate that Mo⁴⁺/Mo⁶⁺ doping in ZnOS significantly enhances the electron distribution in the vacancy regions. However, the degree of electron concentration remains lower than that observed in ZnMoOS-3. Consequently, while photo-HER activity of ZnMoOS-1 is improved compared to pristine ZnOS, it remains inferior to that of ZnMoOS-3.

The adsorption behavior of H⁺ on Zn and non-Zn sites was evaluated for the ZnOS model, revealing that H⁺ adsorption is more energetically favorable on Zn sites, thereby promoting proton adsorption and hydrogen evolution. Similarly, in the ZnMoOS-3 model, H⁺ adsorption was more favourable on the Mo⁴⁺ site than on other sites. Reaction pathways for the photo-HER were further analysed using free energy diagrams for H* adsorption on ZnOS and ZnMoOS-3 (Fig. 6c). The free energy for H* adsorption on ZnMoOS-3 was calculated to be −0.063 eV, closer to 0 compared to −0.106 eV for ZnOS. According to the Sabatier principle,^75–81 these results suggest that ZnMoOS-3 achieves a more balanced hydrogen binding energy, which facilitates the desorption of H₂ and thereby enhances the overall photo-HER kinetics. These findings are consistent with experimental photo-HER measurements, where ZnOS exhibits lower hydrogen evolution efficiency than ZnMoOS-3.

Charge analysis was conducted to investigate further the stable adsorption of H* on ZnOS and ZnMoOS-3. The charge density difference (CDD) maps (Fig. 6d and e) and Bader charge analysis provide an intuitive representation of electron transfer and catalytic activity. Following Mo doping in ZnOS, enhanced electron transfer from ZnMoOS-3 to H* was observed. This enhancement is attributed to the reduced bandgap and increased electron distribution, which improve light absorption and enhance the photo-HER activity. These results prove that the Mo doping and heterovalent Mo⁴⁺/Mo⁶⁺ states significantly enhance the photo-HER activity in ZnMoOS.

3.7 Photo-HER kinetic mechanism

Fig. 7a illustrates the photo-HER mechanism of ZnMoOS, with Vs defects playing a crucial role. These defects provide reactive active centers that activate H₂O molecules, reducing their activation energy for the photo-HER. Additionally, Vs defects capture electrons and adsorb polar H₂O molecules under coulombic interaction to promote photo-generated charge separation and form active oxygen at Vs sites through the photo-HER, which weakens the O–H bonding of adsorbed H₂O molecules, producing H₂. Furthermore, the charge variation with the formation of Mo⁴⁺ accelerates the charge transfer and promotes the photo-HER.^82,83 The kinetic mechanism of ZnMoOS photo-HER include the following steps: (I) Vs defects trap H₂O;^84,85 (II) the Mo⁶⁺/Mo⁴⁺ valence transition occurs via an electron jump between Mo⁴⁺ and Mo⁶⁺, thus extending the e⁻ lifetime and promoting electron transfer for the photo-HER;^86,87 (III) H₂O donates electrons, breaking H–O to give active oxygen while releasing H₂. (IV) Vs is recovered by the h⁺/O_active/S²⁻/SO₃²⁻ reaction.^88,89

H₂O + Vs²⁺ → 2H⁺ + O_active (2)

2H⁺ + 2e^- → H₂↑ (3)

H₂O + Vs²⁺ + 2e^- → H₂↑ + O_active (4)

2S²⁻ + 2h⁺ → S₂²⁻ (5)

S₂²⁻ + SO₃²⁻ → S₂O₃²⁻ + S²⁻ (6)

S₂O₃²⁻ + 3O_active + 4h⁺ → 2SO₃²⁻ + 3 Vs²⁺ (7)

Fig. 7 (a) Schematic illustration of the reaction kinetic mechanism of the photo-HER over ZnMoOS-3 with Vs. (b) Raman spectra of ZnS and ZnMoOS-3. Local enlarged Raman spectra of (c) Zn–S bond and (d) O–H bond. (e) Correlation between the photo-HER and n(Mo⁴⁺)/n(Mo⁴⁺ + Mo⁶⁺) ratio in ZnS, ZnOS, and ZnMoOS.

The adsorption sites of water were further explored through Raman spectroscopy. As depicted in Fig. 7b, the Raman spectral peaks within the 200–400 cm⁻¹ range correspond to the A₁/E₁ transverse optical and A₁/E₁ longitudinal optical phonons of the Zn–S bonds in the catalyst.⁹⁰ Notably, for ZnMoOS-3, two new peaks emerge at 575 cm⁻¹ and 930 cm⁻¹. The peak at 575 cm⁻¹ can be attributed to the A₁ longitudinal optical phonons of Zn–O bonds,⁹¹ while the 930 cm⁻¹ peak corresponds to the symmetric stretching vibration of Mo–O bonds. Compared with the symmetric stretching vibration peak of standard Mo–O bonds (940 cm⁻¹), this peak exhibits a shift towards lower frequencies,⁹² which is likely attributable to the presence of sulfur vacancies in ZnMoOS-3. Fig. 7c presents an enlarged view of the Raman spectra in the 200–400 cm⁻¹ region for ZnMoOS-3 and ZnS. It is evident that the peak intensity of ZnMoOS-3 decreases, and there is a noticeable shift towards lower frequencies. This observation provides further evidence for sulfur vacancies in ZnMoOS-3.^93–95 Furthermore, as illustrated in Fig. 7d, distinct peaks are observed when water molecules adsorb on ZnMoOS-3 and ZnS. For ZnMoOS-3, the asymmetric stretching vibration of O–H at 3700 cm⁻¹ shifts to lower frequencies with increased intensity. Additionally, the symmetric stretching vibration peak of O–H not only shifts to lower frequencies but also splits. These changes indicate that the adsorption environment of water molecules on ZnMoOS-3 has been altered,⁹⁶ which can be attributed to the adsorption effect of sulfur vacancies present in ZnMoOS-3 on water molecules.

Fig. 7e shows the relationship between the photo-HER and change ratio in n(Mo⁴⁺)/n(Mo⁴⁺ + Mo⁶⁺) for various valence states. Fig. 7b indicates that with hydrazine introduction, the n(Mo⁴⁺)/n(Mo⁴⁺ + Mo⁶⁺) ratio increases, and photo-HER activities of ZnMoOS-1, ZnMoOS-2, and ZnMoOS-3 catalysts also increase. However, when n(Mo⁴⁺)/n(Mo⁴⁺ + Mo⁶⁺) reaches 26.88%, the photo-HER activity of ZnMoOS-4 is lower than that of ZnMoOS-3. According to the results of the PL and TRPL tests, it can be observed that the PL peak intensity of ZnMoOS-4 is higher than that of ZnMoOS-3, and the average carrier lifetime of ZnMoOS-4 is lower than that of ZnMoOS-3. This indicates that the separation efficiency of photo-generated carriers in ZnMoOS-4 is lower than that in ZnMoOS-3. Meanwhile, by analyzing the results of electrochemical tests, ZnMoOS-3 exhibits a better electron transfer efficiency. Therefore, when ZnMoOS-4 has a higher content of heterovalent states, it shows a lower photo-HER.

4. Conclusions

A novel Mo/O co-doped ZnS oxysulfide catalyst with abundant Vs defects and heterovalent Mo⁴⁺/Mo⁶⁺ states was synthesized via a facile method. ZnMoOS-3 with appropriate Mo/O co-doping and driven by the optimum hydrazine content exhibits an excellent photo-HER rate of 41.6 mmol g⁻¹ h⁻¹ and an AQE of 18.6% at 400 nm, along with good stability and durability. The Mo/O co-doping reduces the E_g of ZnS and extends its visible-light absorption range. The hydrazine-driven process adjusts ZnMoOS with appropriate bivalent n(Mo⁴⁺)/n(Mo⁶⁺) states and creates abundant Vs defects. The Vs defects are active sites to capture H₂O molecules and weaken H–O–H bonds for producing protons and H₂ generation. The bivalent n(Mo⁴⁺)/n(Mo⁶⁺) states act as hosts for photogenerated electrons, facilitating the rapid hopping of photogenerated electrons between Mo⁴⁺ ↔ Mo⁶⁺ to transfer for the photo-HER, thereby improving the photo-HER efficiency. DFT calculations indicate that Mo/O co-doping of ZnMoOS with abundant Vs defects and heterovalent Mo⁴⁺/Mo⁶⁺ states significantly facilitates hydrogen desorption and enhances the surface *H generation rate. This work provides a strategy for leveraging vacancy defects and heterovalent states to design stable and photocorrosion-resistant sulfide catalysts for an efficient photo-HER.

Data availability

The authors declare that all relevant data are included in the manuscript and ESI.†

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China under grant no. 31000269, the Natural Science Foundation of Fujian Province under grant no. 2021J01100, and the Forestry Department Foundation of Fujian Province under grant no. (2023TG17 and 2025FKJ18).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta02176f

‡ These authors contributed equally to this work.

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