Interstitially O-doped Cd_xZn_1−xS solid solution derived from chalcogenide molecular clusters for photocatalytic hydrogen evolution

Abstract

Interstitial oxygen dopants (O_i) in sulfide nanocrystals are more conducive to charge carrier separation for improving the photocatalytic hydrogen evolution (PHE) performance than substituted oxygen dopants (O_S). However, oxygen dopants exist dominantly in the form of O_S, rather than O_i, in oxygen-doped sulfides prepared via traditional methods. Herein, an O_i-doped Cd_xZn_1−xS solid solution was facilely synthesized through pyrolyzing a mixture of oxygen-containing zinc sulfide clusters and CdCl₂ salts. At a controlled pyrolysis temperature, the oxygen components inside the cluster were successfully converted into O_i dopants associated with sulfur vacancies. The as-synthesized O_i-doped Cd_xZn_1−xS solid solution exhibits good PHE performance due to its unique energy band structure caused by the two coexistent defects. The synthetic method of using the intrinsic components in molecular clusters to modulate the types of specific dopants in photocatalysts may provide a new synthetic strategy for creating doped photocatalysts with excellent PHE activities.

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Introduction

Solar water splitting to generate hydrogen over photocatalysts is considered as a promising approach to address the energy crisis and environmental pollution issues.^1–6 Various semiconducting materials, such as metal oxides,^7–9 metal sulfides,^10–16 and carbon nitrides,^17,18 have been investigated as potential candidates for solar water splitting. Among them, metal sulfide solid solutions have attracted the attention of the academic community for a long time because their band gaps can be adjusted in a wide range.^19,20 For example, Cd_xZn_1−xS solid solution bears a tunable band gap in the range from 3.6 eV (ZnS) to 2.4 eV (CdS) and exhibits better photocatalytic activity than single ZnS or CdS.²¹ Unfortunately, the rapid electron–hole recombination, weak visible light absorption, and poor photocatalytic stability in Cd_xZn_1−xS solid solution deteriorated its PHE activity. A simple and common method for solving the above problems is to modify Cd_xZn_1−xS by doping with a heterogeneous element, which may narrow the band gap for more light absorption and insert impurity energy levels for better charge separation.^22–25 For example, Chen's group doped Zn_xCd_1−xS solid solution with interstitial P element, and the S vacancies associated with the introduction of interstitial P prolonged the lifetime of charge carriers and enhanced the carrier separation efficiency.²⁴ However, the P atoms with a larger radius relative to S atoms may cause larger lattice distortion of Zn_xCd_1−xS solid solution during the doping process and result in the formation of new electron–hole pair recombination centers, which in turn degrade the PHE performance to a certain extent. We believe that introducing interstitial oxygen doping elements with a small atomic radius into Cd_xZn_1−xS solid solution is expected to reduce the carrier recombination probability caused by lattice distortion and simultaneously realize rapid photogenerated carrier separation and high PHE performance.

The common synthetic method for doping oxygen in metal sulfide nanocrystals is to add oxygen-containing raw materials during the preparation.^26–28 For instance, Xie's group prepared oxygen-doped ZnIn₂S₄ photocatalysts by adding oxygen-containing polyvinyl pyrrolidone during the synthesis, wherein the type of oxygen dopant was substituted oxygen (O_s) rather than interstitial oxygen (O_i).²⁸ Unfortunately, such substituted oxygen dopants are inferior to interstitial oxygen dopants in promoting carrier separation since interstitial oxygen dopants are often associated with S vacancies. These two different types of defects can trap electrons and holes, respectively, which is better for improving the charge separation efficiency.^24,29 Therefore, to improve the photocatalytic performance of metal sulfides, it is particularly necessary to seek synthetic methods for introducing interstitial oxygen dopants into metal sulfide nanocrystals.

Herein, we intentionally used well-defined oxygen-containing zinc sulfide molecular clusters (denoted as ZSP, Scheme 1a) as the precursor to prepare oxygen-doped metal sulfide semiconductors. The title sample of Cd_xZn_1−xS solid solution (denoted as 500-Cd) with abundant S vacancies (V_S) and interstitial oxygen (O_i) was synthesized by pyrolyzing a mixture of cluster-based crystals and CdCl₂ salts at 500 °C under N₂ (Schemes 1c and d). Comparative synthesis experiments showed that the temperature control and introduction of CdCl₂ salts were key to the conversion of oxygen elements in metal sulfide molecular clusters into O_i (Schemes S1 and 2†). The prepared O_i-doped Cd_xZn_1−xS photocatalyst (i.e., 500-Cd) exhibited an excellent PHE performance (1240 μmol h⁻¹ g⁻¹), which is 29.5 and 3.3 times higher than that of ZSP (41 μmol h⁻¹ g⁻¹) and Cd_xZn_1−xS solid solution without O_i dopants (denoted as 400-Cd, 380 μmol h⁻¹ g⁻¹) in the presence of a sacrificial agent. This is because O_i and V_s introduce new intermediate energy levels, which position near the valence band and between the conduction band and the Fermi level, respectively. Such a band structure may effectively and separately trap holes and electrons to improve the separation efficiency of photogenerated carriers, thereby improving the PHE performance.

Scheme 1 Illustration of the fabrication process of O_i-doped Cd_xZn_1−xS solid solution. (a) Polyhedral model of the [Zn–S–O] molecular cluster. (b) Simplified structure of the ZSP-packed crystal. (c) Simplified model of the mixture of ZSP-packed crystals and CdCl₂ salt. (d) O_i-doped Cd_xZn_1−xS solid solution with the coexistence of V_s defects.

Results and discussion

Morphology and structure characterization

The ZSP cluster-based microcrystals were prepared according to a previously reported procedure,³⁰ and their phase purity was also confirmed through powder XRD measurements (Fig. S1†). The ZSP cluster is a structurally well-defined oxygen-containing zinc sulfide molecular cluster with a formula [(Zn₂₅S₁₉)(SO₄)₆(Py)₁₈], which is constructed by a [Zn–S] inner core terminally capped by six sulfate groups and eighteen pyridine ligands. In each ZSP cluster, zinc ions have four types of coordination modes: ZnS₄, ZnS₃N, ZnS₃O, and ZnS₂NO (Scheme 1a). For convenience, the molecular cluster may be simplified as an easily understood structure motif, i.e., the [Zn–S] inner core is wrapped around by sulfate groups and pyridine ligands in the outer layer (Scheme 1b). Such a unique structure was anticipated to be a useful template for constructing an O-doped chalcogenide semiconductor.

To synthesize the Cd_xZn_1−xS solid solution, a mixture of ZSP cluster-based microcrystals and CdCl₂ powder was pyrolyzed at different temperatures (400 °C, 500 °C, and 600 °C) in N₂ for 2 hours (Scheme 1c). The obtained samples are denoted as 400-Cd, 500-Cd, and 600-Cd, respectively (Scheme 1 and S1†). Their structure, composition, and morphology were revealed by TEM and HRTEM images. It was suggested that 400-Cd exists in the form of nanoparticles (NPs) (Scheme S1a† and Fig. 1a), and is identified as Cd_xZn_1−xS solid solution due to the clear lattice fringes of 0.33 nm corresponding to the (002) lattice plane of Cd_xZn_1−xS (Fig. 1d).^23,31 When the heating temperature was elevated to 500 °C, 500-Cd exhibited a large sheet morphology with a few grain boundaries (Fig. 1b), which is also identified as Cd_xZn_1−xS solid solution (Fig. 1e). The elements Cd, Zn, and S were found to be evenly distributed in the solid solution of Cd_xZn_1−xS (Fig. S3†). Also, a small amount of oxygen element was present in the Cd_xZn_1−xS solid solution. The morphological difference between 400-Cd and 500-Cd may be ascribed to CdCl₂.^9,10 In the mixture of ZSP-based microcrystals and CdCl₂ salts, CdCl₂ wraps around ZSP microcrystals. When the pyrolysis temperature is set at 400 °C, partial CdCl₂ salts may diffuse into ZSP microcrystals, and granular Cd_xZn_1−xS NPs were gradually formed. Meanwhile, the unreacted CdCl₂ salts adsorbed on the surface of NPs may block their further aggregation because CdCl₂ is not molten under 400 °C. However, when the pyrolysis temperature is increased above 500 °C, the molten CdCl₂ salts combine with the Cd_xZn_1−xS NPs, thereby leading to the sheet morphology of 500-Cd and 600-Cd (Fig. 1c). Furthermore, the ZnO phase (PDF#36-1451) is observed in 600-Cd according to the clear lattice fringes of 0.28 nm corresponding to the (100) lattice planes in the HRTEM image (Fig. 1f). These results are consistent with the XRD results (Fig. S2†). Obviously, the oxygen element in ZnO stemmed from the decomposition of SO₄²⁻ anions in ZSP microcrystals. Also, due to the formation of molten CdCl₂ salts in 500-Cd and 600-Cd, the oxygen components inside the ZSP molecular clusters were retained during the pyrolysis of the intermediate mixture to form Cd_xZn_1−xS solid solution.

Fig. 1 TEM images of (a) 400-Cd, (b) 500-Cd, and (c) 600-Cd. HRTEM images of (d) 400-Cd, (e) 500-Cd, and (f) 600-Cd.

For comparison, pyrolyzed products by directly annealing pure ZSP microcrystals at different temperatures (400 °C, 500 °C, and 600 °C) in N₂ were also obtained (Scheme S2†), which are denoted as 400-D, 500-D, and 600-D, respectively. The morphologies of 400-D, 500-D, and 600-D are characterized using TEM and HRTEM images (Fig. S4†). It was clearly observed that 400-D and 500-D were prepared in the form of NPs, and the size of NPs in 400-D is smaller than that of NPs in 500-D (Fig. S4a and b†). The NPs in 600-D present signs of merging (Fig. S4c†). This is caused by the absence of CdCl₂ salts. The NPs in 400-D and 500-D are characterized as the ZnS phase because the clear lattice fringes of 0.31 nm corresponding to the (006) lattice planes of ZnS (PDF#89-2739) are observed in the HETEM images (Fig. S4d and e†). These results were consistent with the XRD results (Fig. S5†). In addition, the sample of 600-D contains a partial ZnO phase because the lattice fringes of 0.28 nm corresponding to the (100) lattice planes of ZnO (PDF#36-1451) are observed in the HRTEM images (Fig. S4f†). This phenomenon is similar to that observed in 600-Cd. Considering that the oxygen species are not found in the samples of 400-D, 500-D, 400-Cd and 500-Cd, we can't help but wonder where the oxygen components are at low temperatures (400 °C and 500 °C).

To determine the existing form of the oxygen species in the pyrolyzed samples, X-ray photoelectron spectroscopy (XPS) was conducted on all of the above samples. The XPS scan survey spectra of 400-Cd, 500-Cd, and 600-Cd confirm the presence of Zn, O, Cd, C, and S elements in the as-synthesized samples (Fig. S6†). Fig. S7† shows the high-resolution XPS spectra of C 1s, Cd 3d, Zn 2p, and N 1s. It is clearly observed that the peaks of Cd 3d in 400-Cd, 500-Cd, and 600-Cd shift slightly to lower binding energy (Fig. S7b†). However, the Zn 2p XPS peaks shift first to a lower binding energy, then to a higher binding energy with increasing pyrolysis temperature (Fig. S7c†). This may be attributed to the formation of ZnO at high pyrolysis temperature. Meanwhile, there are no N 1s XPS peaks observed in all samples (Fig. S7d†), suggesting that pyridine molecules are not retained in Cd_xZn_1−xS solid solution. In addition, the high-resolution XPS spectra of S 2p and O 1s are also characterized to determine the existing form of O species (Fig. 2a and b). There are two kinds of S 2p XPS peaks, corresponding to SO₄²⁻ and Zn–S bonds, and the weak XPS signals of SO₄²⁻ observed in all samples suggest that SO₄²⁻species are easily pyrolyzed in the presence of CdCl₂. As shown in Fig. 2b, 400-Cd exhibits three O 1s XPS peaks at 532.83, 531.99, and 531.29 eV, which are assigned to water, hydroxyl groups, and lattice oxygen, respectively.^26,28 This might be because SO₄²⁻ anions are not pyrolyzed completely under 400 °C. However, a new XPS peak at 530.29 eV is observed in 500-Cd. We conjecture that this peak is attributed to O_i, i.e. the O anions are doped into the interstitial lattice of Cd_xZn_1−xS solid solution. To test our guess, electron spin resonance (ESR) measurements were employed to further confirm the existence of interstitial oxygen dopants in all samples. As shown in Fig. 2c, pristine ZSP microcrystals have no ESR signal. However, 400-Cd shows a clear ESR signal at 327 mT (g-value of 2.001), and 500-Cd exhibits two ESR signals at 312 mT (g-value of 2.096) and 335 mT (g-value of 1.953), respectively. By contrast, a large number of other ESR signals are observed in 600-Cd. According to the literature,^32,33 the EPR signal at g = 2.096 was ascribed to S vacancies. Similarly, we think that the EPR signal at g = 1.953 is attributed to interstitial O (O_i) dopants in 500-Cd.^34,35 For further comparison, the existing type of oxygen species in 400-D, 500-D, and 600-D was also analyzed by XPS and ESR measurements (Fig. 2d–f and S8†). Clearly, the high-resolution Zn 2p XPS peaks show the same shift trend as that observed in 400-Cd, 500-Cd, and 600-Cd. According to the N 1s XPS peaks (Fig. S8d†), pyridine molecules are thought to be decomposed, and nitrogen atoms are doped into ZnS (Scheme S1†). SO₄²⁻ anions in pristine ZSP microcrystals are thought to be gradually pyrolyzed into S and O species with the increasing temperature (Fig. 2d). Also, three O 1s XPS peaks in 400-D are assigned to water, hydroxy, and lattice oxygen, respectively (Fig. 2e). In combination with the ESR results (Fig. 2f and Fig. S9†), the new O 1s XPS signal appearing in 500-D and 600-D is assigned to the substituted oxygen (O_s). This is because the EPR signal at g = 2.001 in 500-D was significantly different from that in 500-Cd, and, if the interstitial oxygen dopant cannot be formed in 500-D, the oxygen element located in ZnS can only replace the S element to form O_S dopants. Therefore, it was concluded that oxygen species exist in the form of O_i in 500-Cd and 600-Cd, along with the formation of V_s, however, in the form of O_s in 500-D and 600-D. Therefore, in the presence of CdCl₂ salts, the oxygen components in ZSP are easily converted into interstitial oxygen dopants rather than substituted oxygen dopants.

Fig. 2 High-resolution XPS signals of S 2p (a) and O 1s (b) in 400-Cd, 500-Cd, and 600-Cd. (c) ESR signals of ZSP microcrystals, 400-Cd, 500-Cd, and 600-Cd. High-resolution XPS signals of S 2p (d) and O 1s (e) in 400-D, 500-D, and 600-D. (f) ESR signals of ZSP microcrystals, 400-D, 500-D, and 600-D.

Photocatalytic hydrogen evolution performance

PHE experiments were performed under visible-light irradiation using triethanolamine (TEOA) as a hole scavenger. As shown in Fig. S10,† the pristine ZSP microcrystals exhibit a very low PHE activity with a rate of 42 μmol g⁻¹ h⁻¹, indicating that ZSP is less active for hydrogen evolution. As expected, 500-Cd delivers a high PHE rate of 1.24 mmol g⁻¹ h⁻¹, which is 29.5 times higher than that of pristine ZSP (Fig. 3a, S10, and S14†). This is ascribed to the formation of the O_i-doped Cd_xZn_1−xS solid solution. In addition, 500-Cd still exhibits a stable photoactivity after 10 consecutive cycles for a total of 50 h (Fig. S11†). The XRD patterns and TEM images of the cycled samples further confirm the PHE stability (Fig. S12 and S13†). To explain why 500-Cd has higher photocatalytic activity than other samples, photocurrent response experiments were explored. As shown in Fig. 3b, 500-Cd displays the highest photocurrent density, suggesting the fastest photo-generated carrier separation among all samples. Also, according to the results of photoluminescence (PL) and electrochemical impedance spectroscopy (EIS), 500-Cd exhibits the fastest carrier separation efficiency (Fig. 3c and d). For comparison, the PHE performance of 400-D, 500-D, and 600-D samples was also investigated (Fig. S14†). 500-D delivers a PHE rate of 0.71 mmol g⁻¹ h⁻¹, which is lower than that of 500-Cd. Also, the results of the photocurrent response, EIS, and PL reveal that the carrier separation efficiency in 500-D is higher than that in both 400-D and 600-D (Fig. S15–S17†), but lower than that in 500-Cd. This could be explained by the different types of oxygen dopants.

Fig. 3 (a) The PHE rate, (b) photocurrent response, (c) EIS plots, and (d) PL spectra of 400-Cd, 500-Cd, and 600-Cd.

Band structure and photocatalytic mechanism

To understand the PHE mechanism, the energy band structures of photocatalysts are analyzed. As discussed above, the major component in 400-Cd is Cd_xZn_1−xS solid solution without any oxygen dopants, 500-Cd consists of Cd_xZn_1−xS with O_i dopants and V_s defects, and 600-Cd is the mixture of ZnO and Cd_xZn_1−xS with O_i and V_s defects. The band gap (E_g) of 400-Cd (2.58 eV) was estimated using the Kubelka–Munk function:¹¹ (ahν)² = A(hν − E_g) (Fig. S18 and 19†). The valence band (VB) is determined based on the Mulliken electronegativity theory: E_VB = χ − E⁰ + 0.5E_g, where E_VB is the VB edge potential. Moreover, E⁰ is the energy of free electrons vs. NHE (4.5 eV). Finally, χ is the electronegativity of the semiconductor and is calculated using the following equation:³⁶χ = [χ(A)^aχ(B)^bχ(C)^c]^1/(a+b+c), where a, b, and c are the number of atoms in the compounds. The value of χ is calculated to be 5.25 eV. Therefore, E_CB and E_VB of 400-Cd are calculated to be −0.54 and 2.04 eV (vs. NHE), respectively. Next, the XPS VB spectra of 400-Cd are analyzed to determine the Fermi level (Fig. S21†).³⁷ Based on the value of the XPS VB plot (2.24 eV), the Fermi level of 400-Cd is determined to be −0.23 eV (vs. NHE), which is close to −0.25 eV using the M–S profile (Fig. S20†). The band energy structure diagram of 400-Cd is shown in Fig. 4a. Similarly, the band gap of 500-Cd is determined to be 2.43 eV. Also, the E_CB and E_VB of 500-Cd are calculated to be −0.47 and 1.96 eV (vs. NHE), respectively. Using the M–S plot in Fig. S20,† the Fermi level is determined to be −0.13 eV (vs. NHE).^11,38 It is worth noting that the difference between E_VB and E_f is 2.06 eV, while the value of the XPS VB plot of 500-Cd is 1.65 eV (Fig. S22†). This suggests that there is an intermediate level at 1.52 eV between E_f and E_VB, which may be ascribed to the energy level of O_i. Meanwhile, on the basis of previous references, the shallow trap state of V_s locates between the conduction band and the Fermi level at −0.25 eV.^23,29,39 Thus, the band energy structure diagram of 500-Cd is shown in Fig. 4b. This unique energy structure can not only improve the light absorption (Fig. S18†), but also accelerate the photoinduced carrier separation (Fig. 3b–d). This is because the coexistence of O_i and V_s may narrow the band gap to a certain extent. In addition, O_i and V_s are able to separately trap holes and electrons, resulting in fast carrier separation and preventing the recombination of electron–hole pairs. The band structure of 600-Cd is shown in Fig. 4c. The photoexcited electrons in Cd_xZn_1−xS solid solution could be transferred to ZnO because ZnO has a more positive conduction band position and more negative Fermi level than Cd_xZn_1−xS solid solution, but ZnO locates inside the whole sample (Fig. 1c and f); because H₂O molecules cannot reach the surface of ZnO for the reduction reaction, there is no PHE rate in 600-Cd (Fig. 3a).

Fig. 4 The diagrams of the energy band structure of 400-Cd (a), 500-Cd (b), and 600-Cd (c).

Conclusions

In summary, the O_i-doped Cd_xZn_1−xS solid solution (500-Cd) with rich S vacancies was fabricated by pyrolyzing a mixture of oxygen-containing chalcogenide microcrystals and CdCl₂ salt. The oxygen components in the ZSP cluster were proved to be successfully converted into interstitial oxygen dopants in Cd_xZn_1−xS solid solution by controlling the temperature at 500 °C and adding CdCl₂ salts. Notably, 500-Cd gives a PHE rate of 1.24 mmol h⁻¹ g⁻¹, which is 3.3 times higher than that (0.38 mmol h⁻¹ g⁻¹) of 400-Cd without O_i dopants. Also, the PHE rate of 500-Cd is 1.75 times higher than that (0.71 mmol h⁻¹ g⁻¹) of 500-D without O_i dopants. The enhanced photocatalytic activity is attributed to the rapid separation of photogenerated electron–hole pairs because the coexistence of O_i dopants and V_s defects allows for trapping holes and electrons separately and simultaneously. Meanwhile, the interstitially O-doped Cd_xZn_1−xS solid solution has greater stability than the undoped one during the PHE process. The adopted synthetic method for preparing the Cd_xZn_1−xS solid solution with O_i dopants may be a useful synthetic strategy for the creation of specific dopants into photocatalysts for boosting the PHE performance.

Experimental

Materials and chemicals

Zinc acetate dihydrate (Zn, 99%, powder), sulfur (S, 99.9%, powder), cadmium chloride (CdCl₂, 99%, powder), pyridine (Py, >99%, liquid), 3,5-dimethylpyridine (3,5-DMPy, >99%, liquid), N,N-dimethylformamide (DMF >99%, liquid), isopropyl alcohol (IPA, >99%, liquid), ethanol (>99%, liquid), and deionized water (H₂O, 18.5 MΩ cm) were used as received without further purification.

Synthesis of ZSP³⁰

Zinc acetate dihydrate (60 mg), sulfur (30 mg), pyridine (1.5 mL), and DMF (0.5 mL) were added into a 5 mL glass tube. The vessel was sealed, shaken at room temperature, and then heated at 120 °C for 24 h. The tube was subsequently cooled to room temperature, and then colorless regular rhombic prism crystals were obtained with the yield of 35.0 mg (76% based on the Zn source).

Synthesis of 400-D, 500-D, and 600-D

A group of samples were synthesized by pyrolyzing the as-synthesized ZSP-based microcrystals in a tube furnace for 2 h under N₂ at 400 °C, 500 °C, and 600 °C, respectively. The tube furnace was subsequently cooled to room temperature, and the obtained powders were labeled as 400-D, 500-D, and 600-D, respectively.

Synthesis of 400-Cd, 500-Cd, and 600-Cd

100 mg of ZSP-based microcrystals and 100 mg of CdCl₂ powder were uniformly mixed together. Then, the mixture was heated in a tube furnace for 2 h under a N₂ atmosphere at 400 °C, 500 °C, and 600 °C, respectively. After cooling to room temperature, the products were collected, washed several times with H₂O and ethanol, and dried at 60 °C under vacuum. The obtained powders were labeled as 400-Cd, 500-Cd, and 600-Cd, respectively, based on different pyrolysis temperatures.

Author contributions

Weijie Yang: conceptualization, methodology, investigation, resources, data curation, formal analysis, visualization, and writing – original draft preparation. Xiang Wang: HRTEM characterization of materials, and resources. Zhiqiang Wang: PL characterization of materials and resources. Peipei Sun: UV-vis DRS characterization of materials and resources. Jiaqi Tang: investigation and resources. Juan Li: UV-vis DRS characterization of materials and resources. Dong-Sheng Li: resources, supervision and funding acquisition. Tao Wu: resources, writing – reviewing & editing, supervision, project administration and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (No. 21875150 and 22071165) and the 111 Project (D20015).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qi00497f

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