Synthesis and band structures of photocatalysts. ZrS3 NBs were synthesized via a chemical vapor transport of S powder to Zr powder using iodine as a transport agent. ZrS3 with S22- vacancies (ZrSS2-x) was obtained by the re-annealing of the as-grown ZrS3 NBs at 700 ℃ for 15 min under vacuum. ZrSS2-x with S2- vacancies (ZrS1-yS2-x) was prepared through a low-temperature solvothermal treatment by using Li-dissolved ethanediamine. The x-ray diffraction (XRD) pattern indicates the formation of ZrS3 in the monoclinic phase (ICCD PDF no. 30-1498), and the vacuum annealing and further Li treatment did not induce any phase transition in ZrSS2-x and ZrS1-yS2-x NBs (Figure S1). The obtained ZrS3 was formed as NBs with the width ranging from 400 nm to 3 μm and length in tens of micrometers (Figure 1a-c). The individual ZrS3 NB is confirmed as the single crystal along  direction by the transmission electron microscopy (TEM) and corresponding selected area electron diffraction (SAED) characterization (Figure 1d). It is demonstrated that the ZrS3 layer is parallel to the axial direction of NB, which is in favor of charge carrier transport.34 As shown in the diffuse reflectance UV–vis spectra (Figure 1e), both ZrS3 and ZrSS2-x NBs absorb light with the wavelength up to ~650 nm, corresponding to a bandgap of 2.02 eV (Figure S2). ZrS1-yS2-x NBs present a slight red-shift of absorption spectrum, revealing a smaller bandgap of 1.98 eV. The Mott−Schottky plots for all three samples exhibit positive slopes, indicating the n-type behavior of ZrS3 (Figure 1f). The flat band potentials (Efb) of ZrS3, ZrSS2-x, and ZrS1-yS2-x are estimated to be -0.10, -0.11, and -0.18 V versus reversible hydrogen electrode (VRHE), respectively (Figure 1g and S3). Efb is commonly used to estimate the CBM for a series of n-type semiconductors at the surface in an aqueous environment, which agreed with their theoretically determined values.1,38,46-48 Previsous stduies on the energy positions of semicondutors have shown that the CBM of zirconium-based sulfides is very close to their Efb,31,47 and therefore the CBM of ZrS3, ZrSS2-x, and ZrS1-yS2-x can be directly determined by their Efb. Based on the Mott−Schottky (Figure 1f, g and S3) and UV–vis spectra results, the CBM and valance band maximum (VBM) for ZrS3, ZrSS2-x, and ZrS1-yS2-x were revealed to be -0.10, -0.11, -0.18 VRHE (CBM) and 1.92, 1.91, and 1.80 VRHE (VBM), respectively (Figure 1h and S4). The CBMs of ZrS3, ZrSS2-x, and ZrS1-yS2-x are higher than the potential for two-electron reduction of O2, and their VBMs lie far below the oxidation potential of benzylamine,12,15,48,49 indicating that these photocatalysts are applicable to the photocatalytic O2 reduction and benzylamine oxidation.
Characterizations of vacancy structure. Four characteristic Raman modes of ZrS3 located at ~ 147, 274, 315, and 524 cm-1 were observed in Figure 2a, which are assigned to the rigid chain vibration (I: Agrigid), internal out-of-plane vibrations (II: Aginternal and III: Aginternal), and S–S diatomic motion (IV: Ags–s), respectively.29 The Raman spectra show an obvious red-shift of Ags–s mode by ~5 cm-1 from ZrS3 to ZrSS2-x and ZrS1-yS2-x, originating from the introduction of S22− vacancies.34 We also observed a 3 cm-1 red-shift of Agrigid mode from ZrS3 and ZrSS2-x to ZrS1-yS2-x. Since the Agrigid is correlated to the vibration of quasi-one-dimensional chains in the direction of c axis (Figure S5a), the shift of Agrigid mode in ZrS1-yS2-x results from the introduction of S2− vacancies, which alters the length of Zr–S bonds within each chain. The similar shift of Agrigid mode was also identified from ZrS3 and ZrSS2-x to the only Li-treated ZrS3 (ZrS1-yS) (Figure S5b). The XPS characterization was conducted on these samples to further confirm the vacancy type. After the vacuum annealing, the ZrSS2-x NBs exhibit a slightly higher binding energy of the Zr 3d core level than ZrS3 NBs, consistent with the results from ZrS3 to ZrS2 (Figure S6a).50 Furthermore, ZrSS2-x shows a significant attenuation of S22− 2p peaks with the nearly unchanged S2− 2p peaks compared to ZrS3 (Figure 2b and S6b), indicating the mere increase of S22- vacancies in ZrSS2-x. After further Li treatment, both Zr 3d and S 2p core levels of ZrS1-yS2-x NBs apparently shifted to the higher binding energy by ~0.3 eV with reference to ZrSS2-x (Figure S6a and b) due to the increased electron densities around the Zr sites induced by the S2− vacancies.51 In particular, the intensity of S2− 2p peaks in the ZrS1-yS2-x was clearly lower than that of ZrSS2-x (Figure 2b and S6b), revealing the increase of S2− vacancies. The similar variation of Zr 3d and S 2p spectra observed from ZrS3 to ZrS1-yS further suggest the separate introduction of S2- vacancies by the Li treatment (Figure S6a and b). In addition, the electron paramagnetic resonances (EPR) investigation was also carried out to detect the vacancy structure. Both ZrS1-yS2-x and ZrSS2-x NBs show a similar characteristic peak located at g = 2.004, suggesting the formation of sulfur vacancies.52 The higher signal intensity of ZrS1-yS2-x than that of ZrSS2-x indicates more sulfur vacancies existing in ZrS1-yS2-x NBs. In order to have a direct view of the atomic arrangement for ZrSS2-x and ZrS1-yS2-x NBs, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were obtained, where the atomic sites can be determined by comparing the HAADF-STEM image with the crystal structure of ZrS3 lattice along the  direction (Figure S5c and d). The ZrSS2-x demonstrates the missing atoms only emerging on the S22- sites, as indicated by the red dashed circles in Figure 2d, while the atomic vacancies exist on both S2- (indicated by yellow dashed circles) and S22- sites for ZrS1-yS2-x (Figure 2e).
Photocatalytic performance. The photocatalytic capability of the defective ZrS3 NBs for reducing O2 to create the reactive oxygen species (ROS) was first evaluated by the EPR trapping experiment using 5,5-dimethyl-1-pyrroline N-oxide (DMPO). As illustrated in Figure 3a, four characteristic peaks of DMPO−O2•− were observed for all NBs, confirming the generation of O2•−.52-54 The introduction of S22- vacancies was found to enhance the reduction of O2 to O2•−, and the additional introduction of S2- vacancies led to a further increased photocatalytic activity. Thus, ZrS1-yS2-x NBs possess a high H2O2 evolution rate of 90 μmol h-1 with the presence of benzyl alcohol as the hole scavenger (entry 5 in Table S1), which is higher than most previous reports (Table S2). The wavelength-dependent apparent quantum yield (AQY) for the H2O2 generation on ZrS1-yS2-x agrees well with its absorption spectrum, revealing that the photocatalytic activity originates from the bandgap excitation of ZrS1-yS2-x (Figure 3b). In particular, ZrS1-yS2-x produces an AQY of 11.4 and 10.8 % for the incident light of 400 and 500 nm respectively, and demonstrates a good activity even with the excitation extended to the near-infrared region of ~700 nm. Furthermore, the photocatalyst of ZrS1-yS2-x is able to maintain its activities after being recycled for the same reaction, as presented in Figure 3c. Based on the high activity of ZrS1-yS2-x for H2O2 generation, we further utilized benzylamine to substitute the hole scavenger. The H2O2 evolution rate of ZrS1-yS2-x was decreased to 78 μmol h-1 with the same molar amount of benzylamine as benzyl alcohol, due to the slower oxidation kinetics of benzylamine than that of benzyl alcohol. Simultaneously, the benzylamine was oxidized and converted to BN at a rate of 32 μmol h-1 with a high selectivity of > 99% (entry 2 in Table S1 and Figure 3d). Similar photocatalytic behaviors were also identified on both ZrSS2-x and ZrS3 NBs, which produced the H2O2 at a rate of 58 and 30 μmol h-1 with the hole scavenger (entry 3 and 1 in Table S1), respectively. As a comparison, ZrSS2-x and ZrS3 show a decreased H2O2 evolution rate of 48 and 18 μmol h-1 with the use of benzylamine, and the corresponding BN generation rates are 21 and 7 μmol h-1, respectively (Figure 3d). As a result, the comparison of photocatalytic performance among ZrS1-yS2-x, ZrSS2-x, and ZrS3 reveals the key role of S22- and S2- vacancies on the O2 reduction and benzylamine oxidation.
To provide a deep insight into the effect of defective structures in ZrS3 NBs on its photocatalytic performance, the transient open-circuit potential measurements were performed on ZrS3, ZrSS2-x, and ZrS1-yS2-x NBs to reveal the lifetime of photo-induced charge carriers (Figure S7 and Equation S3). After introducing S22- vacancies, the carrier lifetime of ZrSS2-x was significantly increased to 0.69 s as compared to 0.3 s of ZrS3, while the ZrS1-yS2-x exhibits a further enhanced lifetime of 0.82 s, as shown in Figure 4a. The increased photocurrent for the defective ZrS3 also suggests the role of S22- and S2- vacancies on improving the carrier lifetime and dynamics (Figure S8). In order to explore the underlying mechanism for the lifetime enhancement, the charge carrier dynamics of these samples were extracted through the Mott–Schottky method. According to the Mott–Schottky equation (Equation S1), the electron concentrations of ZrS3, ZrSS2-x, and ZrS1-yS2-x NBs were calculated to be 4.00 × 1018, 5.35 × 1018 and 4.58 × 1019 cm-3, based on the estimated width of the depletion region (wd) under the illumination of 55, 46, and 17 nm, respectively (Equation S2). The similar band bending between ZrS3 and ZrSS2-x suggests that the significantly enhanced carrier lifetime in ZrSS2-x is attributed to the role of S22- vacancies in reducing electron-hole recombination rather than band bending, in agreement with the previous theoretical calculation.35 The significantly reduced wd in ZrS1-yS2-x indicates a large electric field strength on the surface of ZrS1-yS2-x, which can accelerate the extraction of photogenerated holes towards the surface and limit the internal band-to-band recombination. Moreover, the small wd in ZrS1-yS2-x results in a large conduction region for the free electrons compared to ZrS3 and ZrSS2-x, which is beneficial for the electron transport. On the other hand, the reaction kinetics of benzylamine oxidation on the photocatalysts were also investigated by the intensity-modulated photocurrent spectroscopy (IMPS). The typical IMPS plots are shown in Figure S9, and the details for the calculation of rate constant of charge transfer (kt) and surface recombination (krec) are discussed in the supporting information. The higher kt/(kt+ krec) of ZrSS2-x and ZrS1-yS2-x than that of ZrS3 indicates a more efficient benzylamine oxidation for the defective ZrS3 (Figure 4b).37 The ZrSS2-x exhibits a decreased krec compared to ZrS3 (Figure 4c and d), revealing that the more efficient benzylamine oxidation on ZrSS2-x stems from the enhanced carrier lifetime induced by the S22- vacancies. The krec of ZrS1-yS2-x was further decreased due to its efficient hole extraction induced by the large surface band bending. Furthermore, the increase of kt for ZrS1-yS2-x compared to ZrSS2-x suggests that the S2- vacancies can act as an additional photocatalytic layer for the benzylamine oxidation (Figure 4d).