Characterization of as of as-prepared photocatalysts. The morphology and microstructure of the as-synthesized ZIS, MoSe2 and Vs-ZIS/MoSe2 (the optimized sample) were analyzed by the SEM, TEM and HRTEM characterizations. As observed in Fig. 2a, the basic morphology of ZIS is flower-like hierarchical microsphere composed by plenty of intersecting nanoflakes, which benefits to the exposure of active surface. The TEM image in Fig. 2b further reveals the hierarchical microsphere of ZIS assembled by nanoflakes. Furtherly, as shown in the HRTEM image in Fig. 2c, the clear lattice stripes with interplanar spacing (d) of 0.32 nm can be well indexed to the (102) lattice plane of hexagonal ZnIn2S4 (JCPDS:65-2023)14. Fig. S1-S2 are the elements mapping and EDS spectrum of ZIS, it can be clearly seen the evenly distributed Zn, In and S elements, and the atomic ratio of Zn/In/S can be calculated to be about 1.00/1.85/4.13 (as listed in Table S1), very close to the stoichiometric ratio of ZnIn2S4. Fig. S3 presents the SEM, TEM and element mapping images of the S vacancies-rich ZIS (Vs-ZIS). It is found that the Vs-ZIS appears the identical morphology and structure with ZIS, suggesting that the N2H4∙H2O-assisted hydrothermal treatment cannot destroy the flower-like microsphere structure of ZIS. The atomic ratio of Zn/In/S in Vs-ZIS sample is approximately 1.00/1.92/3.35 (as displayed in Table S2), the distinctly deficient of S atom compared to that in ZIS confirms the existence of abundant S vacancies in ZIS. Fig. 2d is the TEM picture of MoSe2, which manifests the nanosheet feature. The HRTEM image (Fig. 2e and f) present the d-spacing of 0.65 and 0.24 nm, assigning to the (002) and (103) lattice planes of 2H-MoSe2 (JCPDS: 29-0914), respectively27. Fig. 2g is the SEM image of Vs-ZIS/MoSe2, which exhibits almost the same morphology with ZIS, moreover, the ZIS and MoSe2 in the Vs-ZIS/MoSe2 structure are undistinguishable, indicating that the MoSe2 was grown on the surface of ZIS intimately to form a 2D/2D contact, and the introduction of MoSe2 can hardly affect the hierarchical microsphere morphology of ZIS. The TEM image displaying in Fig. 2h and i further reveal the hierarchical flower-like microsphere structure of Vs-ZIS/MoSe2, which could lead to the enhanced light absorption by the multilevel reﬂection and scattering of the incident light28. Furthermore, the HRTEM picture displaying in Fig. 2j shows the different lattice stripes with d value of 0.32 and 0.24 nm, respectively, which can be indexed to the (102) crystal face of hexagonal ZnIn2S4 (JCPDS:65-2023) and the (103) lattice planes of 2H-MoSe2 (JCPDS: 29-0914), respectively. The HRTEM results indicate that MoSe2 are directly grown and attach on the ZIS nanosheets substrate. Fig. 2k-p are the EDS spectra and element mapping of Vs-ZIS/MoSe2, as displayed, the distribution of Zn, In, S elements are dense and uniform, meanwhile, the Mo and Se elements are relatively sparse but still evenly distributed. From the EDS spectrum, the mass ratio of MoSe2 to ZIS can be calculated to be about 4.8% (as presented in Table S4), which is very close to the ratio of the added raw materials. What’s more, the atomic ratio of Zn/In/S in Vs-ZIS/MoSe2 was determined to be 1.00/1.83/3.25, indicating that there is still a mass of S vacancies inside Vs-ZIS/MoSe2.
The ZIS, Vs-ZIS, MoSe2 and Vs-ZIS/MoSe2 were further characterized by X-ray diffraction (XRD) to determine the phase composition. As displayed in Fig. 2q, the XRD pattern of MoSe2 matches well with 2H-MoSe2 (JCPDS:29-0914)23. Meanwhile, ZIS displays the distinct peaks at 21.6°, 27.7°, 30.4°, 39.8°, 47.2°, 52.4°, 55.6° and 76.4°, which can be severally indexed to the (006), (102), (104), (108), (110), (116), (022) and (213) crystal planes of hexagonal ZnIn2S4 (JCPDS:65-2023)28. It is worth noting that the Vs-ZIS sample shows almost the same XRD pattern with ZIS, indicating that the introduction of S vacancies can hardly affect the size and crystal structure of ZIS. Moreover, in the XRD patterns of Vs-ZIS/MoSe2, in addition to the peaks of hexagonal ZIS, a new peak at about 13.7° can be well assigned to the (002) crystal face of MoSe2, reconfirming the successful synthesis of Vs-ZIS/MoSe2 composite.
To further characterize the chemical structures of the as-synthesized photocatalyst, the Raman spectra were carried out (shown in Fig. 2r). As observed in the Raman spectra of MoSe2, the peaks located at 235.4, 277.4 and 330.8 cm-1 stem from the A1g, E2g and B2g modes of 2H-MoSe2, respectively, while the peak at 142.1 cm-1 is associated to the E1g mode of the in-plane bending of Se atoms in 2H-MoSe229. For the Raman spectra of Vs-ZIS, the peaks located at 244.8 and 348.9 cm-1 can be severally assigned to the F2g and A1g modes of ZnIn2S4. Furtherly, as for the Vs-ZIS/MoSe2 (the red line), in addition to the E1g mode of 2H-MoSe2, and the F2g and A1g modes of ZnIn2S4, a new emerging peak situated at about 404.9 cm-1 can be indexed to the Mo-S bonding state30, suggesting that the Vs-ZIS and MoSe2 were combined intimately by Mo-S bond. Additionally, it can be observed that all the peaks in Vs-ZIS/MoSe2 exhibited evidently blue-shift compared to that in Vs-ZIS, further revealing the intense chemical coupling effect between the Vs-ZIS and MoSe231.
To further testify the existence of S vacancies, the electron paramagnetic resonance (EPR) was carried out (Fig. 2s). For the original ZIS sample, the EPR intensity can hardly be observed, in comparison, the Vs-ZIS sample shows the sharply increased EPR signal at a g-factor of 2.009, confirming the abundant S-vacancies in Vs-ZIS32,33. In addition, it is interesting to observe that the EPR intensity of Vs-ZIS/MoSe2 exhibits slightly decreased compared to that of Vs-ZIS, which should be contributed to the bonding effect among Mo and unsaturated S in Vs-ZIS, decreasing the number of unpaired electrons, but the S vacancies in ZIS haven’t been sewed up by compositing MoSe234.
The X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface composition and chemical states of ZIS, Vs-ZIS and Vs-ZIS/MoSe2, and the results are showing in Fig. 3. As can be found from the survey spectrum (Fig. 3a), the Zn, In and S peaks are coexisting in ZIS and Vs-ZIS, in comparison, Mo and Se peaks can also be observed in the Vs-ZIS/MoSe2, which is agree with the EDS test results. As observed in Fig. 3b, the S 2p3/2 and 2p1/2 of the original ZIS located at 161.72 and 162.97 eV, respectively, in accordance with the reported literature33. In comparison, the S 2p3/2 and 2p1/2 of Vs-ZIS presented evident negative-shift of about 0.14 eV and 0.19 eV, respectively, verifying the generation of S vacancies in ZIS. The S-vacancies can serve as strong electron-withdrawing group for facilitating the ZIS electrons transfer to S-vacancies, thus decreasing the equilibrium electron cloud density of S atoms inside ZIS, and further leading to the decreased binding energy35,36. Furtherly, it can be noted that the S 2p3/2 and 2p1/2 of Vs-ZIS/MoSe2 exhibited a positive-shift of about 0.13 and 0.17 eV compared to that of Vs-ZIS, which should be caused by the strong interfacial interaction between MoSe2 and Vs-ZIS31. Besides, as shown in Fig. 3c and d, the Zn 2p and In 3d in Vs-ZIS also exhibited a slightly negative-shift compared to that in ZIS, which could be explained that the generation of S vacancies leading to the decreased coordination number of Zn and In28.After combining with MoSe2, the Zn 2p and In 3d peaks re-shift to the high binding energy region, revealing that the bonding effect between Mo atoms in MoSe2 and unsaturated coordination S in Vs-ZIS contributing to the slightly increased electron cloud density around Zn and In. Interestingly, it can also be observed that the binding energy variation of Zn 2p in ZIS, Vs-ZIS and Vs-ZIS/MoSe2 are more notable than that of In 3d, revealing that the Mo were mainly bonded with the S around Zn sites34. What’s more, according to the XPS peak area, the actual atomic ratio of Zn/In/S in ZIS, Vs-ZIS and Vs-ZIS/MoSe2 are 1.00/2.15/3.87, 1.00/2.20/3.29 and 1.00/2.14/3.36, respectively. The lower S atom ratio in Vs-ZIS and Vs-ZIS/MoSe2 further confirm the presence of abundant S vacancies. As shown in Fig. 3e, the peaks at 228.05 and 230.5 eV can be attributed to Mo 3d5/2 and 3d3/2 of Mo4+ in MoSe2, meanwhile, the peak at 227.1 eV verified the formation of Mo-S bond38. Fig. 3f is the Mo 3p spectrum, as observed, four distinct XPS peaks can be distinguished, where the peaks at 400.55 and 390.3 eV can be corresponded to the Se Auger peaks, and the peaks at 395 and 416 eV can be assigned to the Mo 3p3/2 and 3p1/2 of Mo4+. The Se 3d spectrum presented in Fig. 3g shows two peaks at 54.4 and 55.35 eV, which can be indexed to Se 3d5/2 and 3d3/2 of Se2- in MoSe2, respectively39. The XPS results further confirm the successful synthesis of Vs-ZIS and Vs-ZIS/MoSe2 with abundant S-vacancies, and the MoSe2 is attached on the surface of Vs-ZIS through Mo-S bond.
Photocatalytic H2 evolution activity measurements. The photocatalytic H2 evolution were evaluated under the visible light (λ>420 nm) irradiation, the corresponding test results are showing in Fig. 4. As shown in Fig. 4a and b, all the tested samples exhibit H2 production activity except for MoSe2. The pristine ZIS exhibits the poor H2 production activity of about only 3.36 mmol∙g-1·h-1, in comparison, the Vs-ZIS presents a slightly improved H2 evolution rate of 4.77 mmol∙g-1·h-1. The improved photocatalytic performance of Vs-ZIS should be ascribed to the accelerated photocarriers separation induced by S vacancies as the electrons trap. Furtherly, the introduction of MoSe2 gave rise to the distinctly improved H2 evolution activity, and the H2 evolution rate of Vs-ZIS/MoSe2 increased with the mass ratio of MoSe2 to ZIS increasing. Until the mass ratio of MoSe2 to ZIS reaches to 5.0%, the H2 evolution rate reaches to the highest of 63.21 mmol∙g-1·h-1, which is about 18.8 and 13.3 times higher than that of pristine ZIS and Vs-ZIS, respectively, and superior to the recently reported ZnIn2S4-based photocatalytic system (as listed in Table S6). It can also be observed that the Vs-ZIS-5.0MoSe2 (synthesized by mixing Vs-ZIS and MoSe2 by ultrasound) performs obvious inferior H2 evolution property compared to that of Vs-ZIS/5.0MoSe2, indicating that the in-situ growth of MoSe2 on Vs-ZIS connecting by Mo-S bond plays critical influence on the photocatalytic performance of the ZIS-MoSe2 composite, which should be attributed to that the Mo-S bond could facilitate the charge transfer between Vs-ZIS and MoSe2. Fig. S6 shows the wavelength dependent hydrogen evolution efficiency of Vs-ZIS/MoSe2, and the corresponding test results and the light power of different monochromatic light are displaying in Table S5. The apparent quantum yield (AQY) of photocatalytic H2 evolution can be calculated (the detailed calculation process is shown in the Supporting Information) and displayed in Fig. 4c. As observed, the AQY values are about 93.08% (380 nm), 76.48% (420 nm), 29.7% (500 nm) and 0.15% (600 nm), match well with the UV-vis absorption spectra, indicating the outstanding optical absorption and utilization capacity of Vs-ZIS/MoSe2 photocatalyst. In addition to the excellent photocatalytic H2 evolution efficiency, the recycling stability is also a pivotal factor for the practical application of photocatalyst. As discerned in Fig. 4d, the H2 evolution amount of the optimized Vs-ZIS/MoSe2 photocatalyst remains about 90.5% after 20 hours of 5 cycles of photocatalytic tests, signifying the excellent photocatalytic stability of Vs-ZIS/MoSe2 photocatalyst, which maybe contributed to the favorable anti-photocorrosion ability of Vs-ZIS/MoSe2, and the strong combination between ZIS and MoSe2.
Photophysical and Electrochemical Properties. Fig. 5a is the UV-vis absorption spectra of ZIS, Vs-ZIS, MoSe2 and Vs-ZIS/MoSe2. It is apparent that the MoSe2 shows the intense light absorption in the whole UV-vis light range, which should be caused by its dark black color. Meanwhile, it can be observed that light absorption intensity of Vs-ZIS is higher than that of ZIS, indicating that the introduction of S vacancies can influence the band structure of ZIS. Furtherly, after combining with MoSe2, the light absorption of Vs-ZIS/MoSe2 increased again compared to Vs-ZIS. The improved light absorption is in favor of the generation of photocarriers, and beneficial for the enhancement of photocatalytic performance21. Fig. 5b is the PL spectroscopy. As displayed, under the 375 nm excitation wavelength, the pristine ZIS displays a prominent emission peak, indicating the intense recombination of photogenerated carriers inside ZIS. In comparison to ZIS, the emission peak intensity of Vs-ZIS decreases lightly, which should be contributed to that S vacancies can act as electrons trap for facilitating the photocarriers separation. It is worth noting that the PL signal of Vs-ZIS/MoSe2 sample is further quenched compared to that of Vs-ZIS, revealing the positive effect of MoSe2 for suppressing the recombination of photocarriers. Fig. 5c is the photocurrent response. As observed, all the tested samples exhibit the light-response characteristic under the FX-300 Xe lamp. Obviously, the photocurrent density is in the order of Vs-ZIS/MoSe2>Vs-ZIS>ZIS. The highest photocurrent density of Vs-ZIS/MoSe2 reveals the most accelerated photocarriers separation and migration efficiency. Fig. 5d is the electrochemical impedance spectroscopy (EIS). As compared, MoSe2 express the smallest semicircle, meanwhile, the semicircle of ZIS is the largest. Obviously, the semicircle of Vs-ZIS is slightly lower than that of ZIS, and the semicircle of Vs-ZIS/MoSe2 is significantly decreased than that of pristine ZIS and Vs-ZIS, manifesting that the introduction of S vacancies and the combination with MoSe2 can decrease the interfacial charge transfer resistance, which is in favor of photogenerated carriers transfer and separation, and finally facilitate the photocatalytic property.
In order to investigate the effects of the MoSe2 to ZIS mass ratio on the photocatalytic performance of Vs-ZIS/MoSe2 composites. The light absorption, photocarriers separation and photocurrent density of Vs-ZIS/MoSe2 photocatalysts with different mass ratio of MoSe2 to ZIS were also characterized by UV-vis absorption, steady-state PL spectroscopy and photocurrent response. As observed in Fig. S6, with increasing the mass ratio of MoSe2 to ZIS, the light absorption intensity enhance gradually. It is worth mentioning that the Vs-ZIS/7.0MoSe2 sample displays the strongest light absorption ability, but its photocatalytic H2 production performance is not the best (as known from Fig. 4a), suggesting that the light absorption is not the only decisive factor for the photocatalytic activity. Fig. S7 is the PL spectra, it can be observed that the PL peak of Vs-ZIS/5.0MoSe2 is the lowermost, revealing the most effective photocarriers separation when the mass ratio of MoSe2 to ZIS is 5%, which directly explains why the Vs-ZIS/5.0MoSe2 sample has the best photocatalytic performance. Fig. S8 shows the photocurrent response. As displayed, the Vs-ZIS/5.0MoSe2 shows the highest photocurrent density, which is the result of high-efficiency separation and transfer of photogenerated electron and hole, further revealing the optimum photocatalytic performance of Vs-ZIS/5.0MoSe2. As known from the above results, the prominent photocatalytic performance requires the coordination among the efficient light absorption, photocarrier separation and transfer ability.
Mechanism analysis. Furtherly, the bandgap value (Eg) of the tested sample can be obtained from the Kubelka-Munk function vs. the energy of incident light plots40. As displayed in Fig. 6a, the Eg of ZIS, Vs-ZIS and Vs-ZIS/MoSe2 can be estimated to be 2.35, 2.28 and 2.19 eV, respectively. The narrower Eg is beneficial for the incident light absorption and photocarriers generation, thereby contributing to the photocatalytic property41. The Mott-Schottky (M-S) plot can be obtained by the following formula of
in which CSC represents space charge capacitance, ɛ represents the dielectric constant, ɛ0 represents the permittivity of vacuum, e represents the single electron charge, ND represents the charge carrier density, Efb represents the flat band potential, kB represents the Boltzmann constant, and T represents the temperature, E represents the electrode potential21. As displayed in Fig. 6b-d, the Efb of ZIS, Vs-ZIS and MoSe2 can be determined to be -0.96, -0.9 and -0.1 V (vs. NHE), respectively, by extending the linear part of M-S plots. Besides, all the tested samples exhibit the positive slope of M-S plots, indicating the n-type semiconductor traits. As known, the conduction band potential (ECB) of n-type semiconductor is approximately 0.2 eV negative than the Efb37, thus the ECB of ZIS, Vs-ZIS and MoSe2 can be discerned to -1.16, -1.1 and -0.3 V (vs. NHE), respectively. According to the equation of EVB = ECB + Eg (EVB is the potential of valence band (VB)), the EVB of the ZIS and Vs-ZIS can be estimated to 1.19 and 1.18 V vs. NHE, respectively. According to the reported literature, the Eg of MoSe2 is about 1.89 eV, therefore, the EVB of MoSe2 can be determined to be 1.59 eV42.
The work function (ɸ) is an important nature for reflecting the escaping ability of free electron from fermi level (Ef) to vacuum level43. To investigate the mechanism for the excellent photocatalytic performance of Vs-ZIS/MoSe2, the ultraviolet photoelectron spectroscopy (UPS) with He Ⅰ as the excitation source was conducted. As displayed in Fig. 6e, the secondary cutoff binding energy (Ecutoff) of Vs-ZIS and MoSe2 can be respectively determined as 17.65 and 16.87 eV, by extrapolating the linear part to the base line of the UPS spectra. Based on the formula of ɸ=hv-Ecutoff, the ɸ of Vs-ZIS and MoSe2 can be calculated as 3.57 and 4.35 eV, respectively. Hence, the Ef of Vs-ZIS and MoSe2 can be determined as -0.93 and -0.15 V (vs. NHE), respectively. Based on the above calculation and analysis results, the detailed band structure of Vs-ZIS, MoSe2 and Vs-ZIS/MoSe2 were depicted in Fig. 6f. As observed, the Ef of MoSe2 is below that of Vs-ZIS, hence, when Vs-ZIS and MoSe2 contact and form an intimate interface, the free electrons in Vs-ZIS with high Ef would spontaneously diffuse to MoSe2 with low Ef, until a new equilibrium state Ef fabricated. The electron drifting from Vs-ZIS to MoSe2 result in the charge redistribution on the interface between Vs-ZIS and MoSe2, in which the interface near Vs-ZIS side is positively charged, while negatively charged near the MoSe2 side, as result, an internal electric field from Vs-ZIS to MoSe2 was built44.
Accordingly, the photocatalytic reaction mechanism of Vs-ZIS/MoSe2 can be elaborated in Fig. 7a. Under the irradiation of visible light, a mass of photoinduced electrons (e-) with enough energy would transfer from the VB of Vs-ZIS and MoSe2 to the CB of Vs-ZIS and MoSe2, respectively, while the holes (h+) be left on the VB of Vs-ZIS and MoSe2, respectively. It should be mentioned that the abundant S vacancies inside ZIS could introduce new donor level in the band gap of ZIS, which can act as efficient electrons trap to suppress the photogenerated electron-hole pairs recombination45. Furtherly, under the driving effect of the internal electric field, the electrons on the CB of MoSe2 would migrate to the VB of Vs-ZIS to recombine with the holes. The Mo-S bond acting as atomic-level interfacial “bridge” can promote the photoexcited carriers migration between Vs-ZIS and MoSe2, thus significantly accelerating the Z-scheme charge transfer. To validate the Z-scheme charge transfer mechanism, the SPV and EPR measurements were carried out. Fig. 7b is the SPV spectra of Vs-ZIS, MoSe2 and Vs-ZIS/MoSe2 samples. It is noted that the pristine MoSe2 presents no SPV signals in the whole wavelength, suggesting the poor photocarriers separation efficiency inside the MoSe2, that’s why MoSe2 performed very poor hydrogen evolution. In comparison, a significant positive photovoltage response can be observed in the SPV spectra of Vs-ZIS, suggesting that the holes migrate to the surface of Vs-ZIS, which is the typical trait of n-type semiconductor46. Meanwhile, the SPV response of Vs-ZIS/MoSe2 is significantly lower than that of Vs-ZIS, which means that fewer photogenerated holes migrate to the surface of Vs-ZIS/MoSe2. This phenomenon should be contributed to that the photogenerated electrons on the CB of MoSe2 transfer to the VB of Vs-ZIS and recombine with the photogenerated holes, that’s the Z-scheme mechanism47. EPR spin-trapping experiment with DMPO as spin-trapping reagent was further proceeded to support the Z-scheme charge transfer mechanism in Vs-ZIS/MoSe2. As displayed in Fig. 7c, almost no DMPO-∙O2- signals can be observed under dark conditions. However, under visible light irradiation, the characteristic peaks of DMPO-∙O2- (1:1:1:1) can be monitored for the Vs-ZIS/MoSe2 methanol dispersion liquid, and the peak intensity increase with the time extending, suggesting that the ∙O2- was generated in the reaction system48. In theory, the electrons on MoSe2 cannot reduce O2 to product ∙O2- due to the lower CB potential of MoSe2 (-0.3 V vs. NHE) than the redox potential of O2/∙O2- (-0.33 V vs. NHE)49. Therefore, the ∙O2- should be the reaction product between the photoinduced electrons on the CB of Vs-ZIS and O2 (the CB potential of Vs-ZIS is about -1.10 eV, lager than the redox potential of O2/∙O2-), indicating that a mass of photogenerated electrons were accumulated on the CB of Vs-ZIS under irradiation of visible light, which should be contributed by the recombination between the electron on the CB of MoSe2 and the hole on the VB of Vs-ZIS, thus verifying the direct Z-scheme charge migration mechanism. Above SPV and ESR spin-trapping technique provides the direct proof for the direct Z-scheme charge transfer mechanism inside the Vs-ZIS/MoSe2 photocatalyst.
In summary, we have successfully demonstrated an interfacial Mo-S bond and internal electric field modulated Z-scheme Vs-ZnIn2S4/MoSe2 photocatalyst through a defect-induced heterostructure constructing strategy for boosting the photocatalytic H2 evolution performance. The internal electric field provide the necessary driving force steering the photogenerated electrons on the conduction band of MoSe2 transfer to the valence band of Vs-ZnIn2S4 following the Z-scheme mechanism, while the interfacial Mo-S bond creates direct charge transfer channels between Vs-ZnIn2S4 and MoSe2, further accelerates the Z-scheme charge transfer process. What’s more, the abundant S-vacancies also contribute to the enhanced light absorption and accelerated photocarriers separation. The above factors together lead to the outstanding photocatalytic performance of the Vs-ZnIn2S4/MoSe2. Specifically, the optimized photocatalyst exhibits a high AQY of 76.48% at 420 nm, and an ultrahigh H2 evolution rate of 63.21 mmol·g-1∙h-1 under visible light (λ > 420 nm), which is about 18.8 times higher than that of pristine ZnIn2S4. Besides, the Vs-ZnIn2S4/MoSe2 also shows favorable recycling stability by remaining above 90% rate retention after 20 h of 5 continuous photocatalytic tests. This work not only provides an efficient direct Z-scheme ZnIn2S4-based heterostructure photocatalyst, but also affords a beneficial prototype for designing other Z-scheme photocatalyst for efficient green energy conversion.