English

• INTRODUCTION

  The fast exhaustion of fossil fuels and deterioration of environment urgently call for the excavation of new renewable energy resources. Solar-driven photocatalytic H₂ evolution has long been deemed as an appealing approach to assure sustainable energy supply and achieve a carbon-neutral society.^[1-8] However, the typical strategy of photocatalytic overall water splitting generally suffers from a low H₂ production rate due to the sluggish water oxidation kinetics, while the photocatalytic H₂ evolution half reaction is essentially restricted by the use of sacrificial agent, which wastes the oxidizing power of the excited holes and raises the reaction cost.^[9] In this context, the emerging approach of coupling photocatalytic H₂ evolution with organic transformations, in which the oxidation of water or sacrifice agent is replaced by the selective oxidation of organics, provides an advanced alternative approach to tackle the challenges.^[10-15] To scale up the scheme, the design of high-efficient photocatalysts with excellent optoelectronic properties to allow both efficient capture of solar light and separation of photoexcited charges carriers is a focused task.

  Over the past decade, tremendous efforts have been devoted to the design and synthesis of ultrathin two-dimensional (2D) catalysts dictated by their distinctive physical-chemical properties different from bulk counterparts. Specifically, the 2D photocatalysts have been revealed with desirable features such as high specific surface area, short charge diffusion length, and fast charge mobility.^[16-18] Among diverse promising candidates, ZnIn₂S₄ (ZIS) is recognized as an outstanding material due to its intrinsic 2D structure, suitable bandgap, favorable chemical stability, and excellent visible light absorption properties.^[19-22] Nevertheless, the photoactivity of single component ZIS is still retarded by the undesirable surface charge recombination and lack of surface redox reaction sites.^[23]

  To ameliorate the limitation, cocatalyst loading onto semiconductor surface is one of the most effective means, which can not only provide a large amount of reactive sites but also capture the electrons generating from the semiconductor.^[24-28] Traditionally, noble metals (e.g., Pt, Pd, Au, Ru) are widely selected, but they suffer from resource scarcity and high cost.^{[20, 29-31]} Plenty of researches have been dedicated to exploring noble-metal-free materials.^[32-37] Transition metal nitrides (TMNs) are a kind of metallic interstitial compounds fabricated by inserting nitrogen atoms into metal lattice. The introduction of nitrogen atoms expands the crystal lattice and rearranges the density of states (DOS) near the Fermi level of the metal. Therefore, the TMNs possess electronic structures similar to that of noble metals, which are highly desirable for acting as advanced cocatalyst.^[38-41] However, to date, the application of TMNs for the photocatalytic coupled reaction of organic transformation and H₂ evolution is still sparse.

  Herein, we present the design and fabrication of a new ZIS/Ni₃FeN composite photocatalyst for efficient selective oxidation of aromatic alcohols to aldehydes along with the production of H₂. Bimetallic nitrides Ni₃FeN are integrated with ultrathin 2D ZIS nanosheets via a facile electrostatic self-assembly method, which forms well contacted heterointerface. A H₂ production rate of 2427.9 µmol h^-1 g^-1 and a benzaldehyde selectivity of 99% are obtained over the optimal ZIS/Ni₃FeN sample, which is about 7.8-folds as high as that of bare ZIS. On the basis of collective spectroscopic and photoelectrochemical measurements, the higher photocatalytic performance of the hybrid ZIS/Ni₃FeN composite than bare ZIS is majorly owing to that the Ni₃FeN efficiently captures electrons from ZIS and accelerates the surface H₂ reduction rates. The study demonstrates the great potential of transition metal nitrides as highly efficient cocatalysts for selective oxidation of aromatic alcohols into corresponding aldehydes coupled with H₂ evolution.

  RESULTS AND DISCUSSION

  The synthesis of the hybrid ZIS/M-Ni₃FeN composite is realized by an electrostatic self-assembly method, as schematically illustrated in Figure 1a. Based on Zeta potential analysis, the surface charge modification has shown important influence on the interfacial interaction of ZIS/M-Ni₃FeN. As revealed in Figure S1, both of the pristine Ni₃FeN and ZIS show negative surface charge, which is difficult to form a good interface contact between them due to the electrostatic repulsion. As such, to optimize the interfacial interaction, Ni₃FeN has been modified with surfactant APTES (marked as M-Ni₃FeN) to endow it with positive charge (Figure S2). After surface modification, the positive M-Ni₃FeN can spontaneously and strongly assemble onto the ZIS nanosheets by simply dropwise adding the M-Ni₃FeN dispersion into the aqueous solution of ZIS.^[42] This simple solution-based assembly method avoids high-energy input procedures and complex process control, which is easy to be scaled up for large-scale preparation. Moreover, the method also enables the facile modulation of constituent ratio to optimize the photocatalytic activity of the hybrid composites.

  Figure 1

  

  Figure 1.  (a) Schematic illustration for the synthesis of ZIS/M-Ni₃FeN hybrids. (b) SEM, (c) TEM and (d-e) HRTEM of ZIS/M-Ni₃FeN composite.

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  The morphological structures of the synthesized samples were firstly analyzed by scanning electron microscopy (SEM) analysis. As displayed in Figure S3a, the bare ZIS shows a uniform flake-like morphology. Transmission electron microscopy (TEM) image of the nanosheets shows transparent feature, denoting an ultrathin thickness of the ZIS (Figure S4). Additionally, atomic force microscope (AFM) further reveals the thickness of the nanosheet is around 2.7 nm (Figure S3b), which proves a single-unit-cell structure of ZIS. The ultrathin structure enables an immediate transfer of photogenerated charge carriers to the surface after photoexcitation, which would greatly inhibit the bulk charge recombination. For Ni₃FeN, the SEM analysis shows uniform particle morphology with diameters of approximately 125 nm (Figure S5). After APTES modification, no perceptible influence on the morphology of M-Ni₃FeN is detected (Figure S6). Figure 1b shows the SEM image of ZIS/M-Ni₃FeN, which discloses that Ni₃FeN nanoparticles are attached onto the surface of ZIS. More structural information of the composite is obtained via TEM and high-resolution TEM (HRTEM) analysis. As depicted in Figure 1c, obviously, the Ni₃FeN nanoparticles are intimately loaded on the surfaces of ZIS nanosheets, which is consistent with the SEM image. The HRTEM image (Figure 1d-e) of the ZIS/M-Ni₃FeN composite reveals two lattice fringes. The lattice stripe of 0.32 nm can be indexed to the (102) plane of ZIS, while another lattice spacing of 0.22 nm matches with the (111) crystal facet of Ni₃FeN.^{[38, 43]} The intersecting of ZIS and Ni₃FeN confirms the formation of a closely contacted interface, which would benefit the transfer of photoexcited electrons from ZIS to Ni₃FeN.

  To reveal the chemical element compositions and valence states of the composite, X-ray photoelectron spectroscopy (XPS) measurement is carried out. As manifested in Figure 2a, for Zn 2p spectrum, two characteristic peaks with binding energies of 1021.9 and 1044.9 eV are detected, which belong to the Zn 2p_3/2 and Zn 2p_1/2 of Zn²⁺, respectively.^[44] The In 3d spectrum of ZIS/M-Ni₃FeN can be divided into In 3d_3/2 (452.5 eV) and In 3d_5/2 (444.9 eV) regions (Figure 2b), corresponding to the In³⁺.^[45] The two peaks of S 2p located at 162.9 eV (S 2p_1/2) and 161.6 eV (S 2p_3/2) are matched with S^2- (Figure 2c).^[46] Meanwhile, in the Ni 2p region (Figure 2d), the peaks at 852.7 and 869.9 eV are related to metallic nickel (Ni⁰), while the binding energies of 855.9 and 873.4 eV are fitted into 2p_3/2 and 2p_1/2 of Ni²⁺, respectively.^[47] The satellite peaks are located at 861.7 and 879.9 eV. In the XPS pattern of Fe 2p (Figure 2e), the binding energies positioned at about 707.1 and 719.5 eV can be attributed to Fe⁰. The peaks of 711.5 and 724.5 eV can be assigned to Fe³⁺.^[48] Another two satellite peaks are observed at 716.1 and 732.3 eV. The states of Ni²⁺ and Fe³⁺ around the surface of composite are caused by inevitable surface oxidation of Ni₃FeN exposed to air.^{[38, 49]} As for the N 1s spectrum, it can be divided into four peaks at 396.2, 397.4, 398.7, and 400.4 eV, corresponding to the metal-N, pyridine-N, pyrrole N, and graphitic-N, respectively (Figure 2f).^{[50, 51]} Based on the above analysis, the successful synthesis of ZIS/M-Ni₃FeN composite is further confirmed.

  Figure 2

  

  Figure 2.  XPS spectra of (a) Zn 2p, (b) In 3d, (c) S 2p, (d) Ni 2p, (e) Fe 2p and (f) N 1S of ZIS/M-Ni₃FeN composite.

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  To verify the effectiveness of catalysts, the photocatalytic activity of ZIS/M-Ni₃FeN hybrids has been estimated by visible-light-driven selective alcohol oxidation integrated with H₂ evolution under anaerobic condition. Figure 3a-b show the H₂ evolution and benzaldehyde (BAD) formation rates from anaerobic oxidation of BA over ZIS/M-Ni₃FeN with different weight percentages of M-Ni₃FeN. The bare ZIS exhibits a quite low photoactivity (H₂ production rate of 309 µmol g^-1 h^-1 and BAD generation rate of 320 µmol g^-1 h^-1), which should be ascribed to the surface charge recombination and insufficient surface redox reaction sites of the bare ZIS. In addition, no H₂ is produced for pristine Ni₃FeN under visible light irradiation because of its metallic characteristic. After the integration of M-Ni₃FeN with ZIS, the hybrid composites reveal markedly enhanced photocatalytic performance. Thus, Ni₃FeN mainly acts as an efficient cocatalyst to boost the photoredox reaction in the ZIS/M-Ni₃FeN composite. The best photocatalytic performance is found on ZIS/1.5% M-Ni₃FeN, which shows a H₂ evolution rate of 2427.9 μmol g^-1h^-1 and a BAD generation rate of 2460 μmol g^-1h^-1, approximately 7.8 times higher than that of bare ZIS. The yield ratios of BAD and H₂ are close to 1:1, indicating a high selectivity of BAD generation.

  Figure 3

  

  Figure 3.  (a, b) Photocatalytic anaerobic oxidation of benzyl alcohol to benzaldehyde and H₂ over different ZIS/M-Ni₃FeN composites. (c) Long-term photoactivity test over ZIS/1.5% M-Ni₃FeN. (d) Photocatalytic performance of ZIS/1.5% M-Ni₃FeN for anaerobic oxidation of different aromatic alcohols.

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  Moreover, the apparent quantum efficiency (AQE) of ZIS/1.5% M-Ni₃FeN composite at 420 nm is calculated to be 8.6% (Figure S7), which is comparable to that of the recently reported state-of-the-art photocatalyst systems for the coupled selective alcohol oxidation and H₂ evolution (Table 2), further indicating high-performance of the ZIS/M-Ni₃FeN composite. Notably, the weight ratio of M-Ni₃FeN shows important effect on the photocatalytic performance. By varying the addition ratio of M-Ni₃FeN, the photocatalytic activities of the composites show a volcano-type trend, which can be ascribed to that at a low mass percentage, the lack of M-Ni₃FeN species leads to insufficient surface active sites. On the contrary, excessive addition of M-Ni₃FeN will cause an agglomeration of the nanoparticles, and induce a shading effect that blocks the visible light absorption of ZIS, thus depressing the photoactivities of the composites.^[52]

  Moreover, the stability of ZIS/M-Ni₃FeN composite has been examined through a long-term experiment. As can be seen from Figure 3c, taking the optimal ZIS/1.5% M-Ni₃FeN as an example, the sample retains stable photocatalytic H₂ evolution over a 10 h visible light irradiation, and the selectivity of BAD reaches up to 99% after the reaction test. The XPS and SEM analyses reveal that the chemical states and morphology of Ni₃FeN in the used composite are almost unchanged in comparison with those in the fresh sample (Figure S8 and S9). Furthermore, the cyclic test of the ZIS/1.5% M-Ni₃FeN composite has also been performed. As displayed in Figure S10, no apparent performance degradation is detected during the six recycle tests. These results validate high stability of the metallic Ni₃FeN and ZIS/M-Ni₃FeN composite for photocatalytic H₂ evolution coupled with organic transformation.

  To study the general applicability of ZIS/M-Ni₃FeN composite, the photocatalytic anaerobic oxidation of a series of aromatic alcohols that contain different substituent groups is tested under the same conditions. As presented in Figure 3d, it is obvious that for diverse aromatic alcohols, including 4-methylbenzyl alcohol, 4-fluorobenzyl alcohol, 4-chlorobenzyl alcohol, 4-bromobenzyl alcohol, 2-methylbenzyl alcohol and 3-methylbenzyl alcohol, the ZIS/M-Ni₃FeN can realize simultaneous H₂ evolution and aldehyde production. The result manifests that the ZIS/M-Ni₃FeN composite is universal towards the photocatalytic anaerobic oxidation of different aromatic alcohols.

  In order to illustrate the importance of interfacial interaction induced by surface modification on the photoactivity of the ZIS/M-Ni₃FeN composite, a series of ZIS/P-Ni₃FeN comparison samples with the same weight ratios of Ni₃FeN have also been synthesized. The process is similar to the preparation of ZIS/M-Ni₃FeN composite except that the Ni₃FeN is used without APTES modification. As shown in Figure S11, typical structures of NiFe₃N particle and ZIS nanosheets are observed in the SEM image, and the NiFe₃N particles are wrapped by the ZIS nanosheets. However, it is notable that abundant Ni₃FeN particles are aggregated into large agglomerate in the hybrid composite, which are separated from the ZIS nanosheets, implying an insufficient interfacial contact between the two components.

  Thereafter, the photocatalytic activity of ZIS/P-Ni₃FeN hybrids in different proportions has been tested under the same experimental conditions. As illustrated in Figure S12, the ZIS/P-Ni₃FeN hybrids show increased photocatalytic activities as compared with bare ZIS, indicating the simple mixture of Ni₃FeN can also effectively enhance the photoactivity of ZIS. Nevertheless, the photocatalytic performance for all of the ZIS/P-Ni₃FeN composites is much lower than that of the ZIS/M-Ni₃FeN hybrid, which are synthesized with different interfacial contact. The optimum photocatalytic H₂ evolution rate and BAD production rate obtained over the ZIS/1.5% P-Ni₃FeN hybrid are 704 µmol g^-1 h^-1 and 712.5 mmol g^-1 h^-1, respectively. The photoactivity is only a quarter of the ZnIn₂S₄/1.5% M-Ni₃FeN sample with the same content of Ni₃FeN (Figure 4a). The results highlight the importance of enhancing interfacial contact for fabricating advanced hybrid composites. The significantly decreased photoactivity might result from the weak interaction between the Ni₃FeN and ZIS nanosheets caused by their electrostatic repulsion with the same negative surface charge, which results in insufficient interfacial charge transfer and lower surface reaction rates. This has been verified in the following collective physicochemical and photoelectrochemical characterizations.

  Figure 4

  

  Figure 4.  (a) Photocatalytic activity for selective BA oxidation integrated with H₂ evolution over ZIS, ZIS/1.5% P-Ni₃FeN and ZIS/1.5% M-Ni₃FeN hybrids. (b) XRD patterns, (c) DRS spectra and (d) N₂ adsorption-desorption isotherms.

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  Figure 4b shows the X-ray diffraction (XRD) patterns of the bare ZIS, Ni₃FeN, ZIS/P-Ni₃FeN and ZIS/M-Ni₃FeN hybrid composites, which are characterized to study the crystal structures and phase composition of the samples. The three samples of ZIS, ZIS/P-Ni₃FeN and ZIS/M-Ni₃FeN display similar diffraction peaks at around 21.6°, 27.7°, 30.8°, 39.4°, 47.3°, 52.4° and 55.6° that are well indexed to a bare hexagonal ZnIn₂S₄ structure (standard card, JCPDS No. 65-2023).^[53] There is no phase change of ZnIn₂S₄ or generation of new component during the synthesis process after loading P-Ni₃FeN and M-Ni₃FeN due to that the assembly synthesis was operated under low temperature. In addition, the ZIS/M-Ni₃FeN with different addition ratios of Ni₃FeN exhibits similar XRD patterns, which are the same as that of ZIS (Figure S13). No characteristic diffraction peaks corresponding to Ni₃FeN are detected in XRD patterns (Figure S14) for all the ZIS/M-Ni₃FeN composite, which is inferred to be caused by the low content of Ni₃FeN.^[38] The above analyses reveal that the integration with Ni₃FeN and the APTES modification of Ni₃FeN both have negligible effect on the chemical properties of ZIS. The well-maintained crystal structures of ZIS and Ni₃FeN in the composites enable the study of different interfacial effects on the photocatalytic performance and simplifies the identification of respective roles of each component in the composite.

  The optical absorption properties of the samples have been measured by UV-vis diffuse reflectance spectroscopy (DRS), as displayed in Figure 4c. It is apparent that bare ZIS shows strong optical absorption in the visible light region and possesses an absorption edge at about 550 nm. Due to the metallic character of pristine Ni₃FeN, a strong absorption at 200-800 nm appears.^[54] After integrating with M-Ni₃FeN, the visible light absorption of ZIS/M-Ni₃FeN composite shows an enhancement of visible light absorption in the range of 550-800 nm. Similar light absorption enhancement is observed in ZIS/P-Ni₃FeN, which is caused by the inherent background absorption of black-colored Ni₃FeN.^[55] With the increase of M-Ni₃FeN contents in the samples, the absorbance of visible light also gradually rises (Figure S15), which coincides with the color change of the composites. Moreover, the band gaps (E_g) of the samples have been calculated according to the Tauc plot.^{[56, 57]} Figure S16 indicates that the band gaps of bare ZIS, ZIS/P-Ni₃FeN and ZIS/M-Ni₃FeN hybrid are about 2.2, 2.3 and 2.33 eV, respectively. The combination of Ni₃FeN with ZIS increases the band gaps of ZIS/Ni₃FeN, which can be ascribed to the quantum confinement effect induced by the inhibited agglomeration of nanosheets in the composites.^[58]

  To further study the microstructure, Brunauer-Emmett-Teller (BET) surface area, pore size distribution, and pore volume of the samples have been analyzed by N₂ adsorption-desorption mea-surement. All the three samples show characteristic type IV isotherms with a H3 hysteresis loop (Figure 4d), illustrating the presence of a mesoporous structure.^[59] This has also been proven by the pore diameter distribution curves (Figure S17). Moreover, the specific surface areas of bare ZIS, ZIS/P-Ni₃FeN and ZIS/M-Ni₃FeN are 11.59, 27.39 and 35.34 m² g^-1, respectively (Table S1). The increased BET surface area of ZIS/Ni₃FeN also indicates the prohibited agglomeration of ZIS nanosheets after the introduction of Ni₃FeN. In addition, the BET surface area of ZIS/M-Ni₃FeN is higher than that of ZIS/P-Ni₃FeN, indicating that the better dispersity of M-Ni₃FeN expands the contact area between M-Ni₃FeN and ZIS, thus further effectively impeding the agglomeration of ultrathin ZIS nanosheets and enlarging the surface area of the composites. Generally, a large surface area is in favor of enhancing the interfacial contact between the photocatalyst and reactant, exposing more surface active sites and improving the migration rate of photoexcited charge carrier, which could be contributed to enhancing the photoactivity.^[60]

  To explore the effect of Ni₃FeN in promoting the interfacial charge separation and transfer, a series of photoelectrochemical tests have been carried out. The transient photocurrent responses are measured by several on-off intermittent cycles under visible light irradiation, as shown in Figure 5a. The photocurrent responses of photocatalysts are proportional to the separation efficiency of interfacial charge.^{[61, 62]} It can be seen that all samples display a relatively stable light response and the photocurrent density is in an order of ZIS/M-Ni₃FeN > ZIS/P-Ni₃FeN > ZIS under an identical experimental condition. The ZIS/M-Ni₃FeN exhibits the highest photocurrent intensity, indicating that the integration with M-Ni₃FeN most effectively promotes the separation and transfer of photogenerated electron-hole pairs of ZIS.^[63-66] The optimized charge separation has been further confirmed by the electrochemical impedance spectroscopy (EIS) measurement.Ni₃FeN shows a small semicircle diameter of the Nyquist plot (Figure S18a). Moreover, as shown in Figure 5b, compared with ZIS and ZIS/P-Ni₃FeN, the ZIS/M-Ni₃FeN exhibits the smallest semi-circle diameter of the Nyquist plot, confirming the lowest resistance and highest charge transfer efficiency between the ZIS/M-Ni₃FeN electrode and electrolyte solution, which is beneficial for inhibiting the charge combination of the photocatalyst and hence improves the photocatalytic property.^{[67, 68]} The enhanced electrical conductivity of the composites can be mainly ascribed to the introduction of Ni₃FeN with intrinsic metallic conductivity.

  Figure 5

  

  Figure 5.  (a) Transient photocurrent response. (b) EIS Nyquist plots. (c) Steady-state PL spectra and (d) LSV curves. (e) A mechanism for photocatalytic H₂ evolution coupling with aldehydes production over ZIS/M-Ni₃FeN hybrids.

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  The steady-state photoluminescence (PL) spectroscopy is also carried out to verify the increased charge-carrier separation and transfer efficiency. The PL intensity reflects the recombination of photoexcited electron-hole pairs.^[69] Figure 5c shows that bare ZIS presents a broad PL emission peak at around 550 nm. In comparison, the emission peaks for ZIS/M-Ni₃FeN and ZIS/P-Ni₃FeN composites are notably quenched, denoting that the combination of Ni₃FeN with ZIS effectively suppresses the recombination of charge carriers. The lower PL signal of ZIS/M-Ni₃FeN than ZIS/P-Ni₃FeN can be attributed to the more efficient interfacial contact in the composite, which leads to faster electron transfer from photoexcited ZIS to Ni₃FeN.^[70-72] To further study the electron transfer behavior in the composite, the transient PL spectrum has been tested. As presented in Figure S19, the fluorescence lifetime of ZIS/M-Ni₃FeN (0.17 ns) and ZIS/P-Ni₃FeN (0.52 ns) is shorter than that of ZIS (0.74 ns), implying the formation of an electron transfer pathway from ZIS to Ni₃FeN in a non-radiative quenching manner.^{[8, 73]} The Ni₃FeN as a cocatalyst could effectively receive photogenerated charge carriers to inhibit the surface charge re-combination. Notably, compared with ZIS/P-Ni₃FeN, the apparent lifetime diminution of ZIS/M-Ni₃FeN could be attributed to the better interfacial contact of ZIS/M-Ni₃FeN, which more efficiently promotes the charge carrier migration from ZIS to Ni₃FeN. The result is consistent with the PL quenching test.

  Moreover, to confirm that the Ni₃FeN can provide active site to promote surface H₂ evolution reaction, the proton reduction activities of ZIS, ZIS/P-Ni₃FeN and ZIS/M-Ni₃FeN are investigated by the linear sweep voltammetry (LSV) approach. As shown in Figure 5d, the cathodic current observed in the range of 0 to -0.85 V (vs. NHE) can be attributed to H₂ evolution. The bare ZIS exhibits a low reduction current due to the lack of enough active sites. On the contrary, Ni₃FeN shows a strong reduction current under the same condition (Figure S18b). The obviously higher current density of ZIS/Ni₃FeN than that of bare ZIS at the same potential range demonstrates that the loading of Ni₃FeN can efficiently capture the electrons and facilitate the reduction of protons to H₂. Especially, ZIS/M-Ni₃FeN shows a much higher reduction current, illustrating the combination with M-Ni₃FeN optimizes the proton reduction ability of the composites.^{[74, 75]}

  To better elucidate the photocatalytic mechanism, gas chromatography-mass (GC-MS) and EPR measurement have been tested. As shown in Figure S20, three peaks are detected for the reaction solution after light irradiation of 1 h, which belongs to the solvent of acetonitrile, product of benzaldehyde and reactant of benzyl alcohol, respectively. The absence of other products and intermediates denotes the high selectivity of the reaction. More-over, the EPR measurement has been performed using 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical trapping reagent in acetonitrile solvent containing BA under Ar atmosphere. It is obvious that no free radical signals are detected in dark (Figure S21). After the visible light illumination, six characteristic signals are displayed in the EPR spectra of bare ZIS, which prove the generation of a carbon-centered radical.^{[76, 77]} However, it is unable to carry out the EPR test over the ZIS/M-Ni₃FeN composite due to its strong magnetism, as presented in Figure S22, which makes it incapable to detect the radical intermediate. In addition, the photo-activity test of ZIS/M-Ni₃FeN in the presence of DMPO as a radical scavenger has been carried out. As displayed in Figure S23, the photocatalytic performance decreased significantly. Based on these results, it can be inferred that the carbon-centered radical is the key intermediate in the photocatalytic process.

  To further study the photocatalytic charge transfer in ZIS/Ni₃FeN, the band structures and work functions of ZIS and Ni₃FeN are analyzed. In our previous work, the band structure of ZIS nanosheets has been investigated. The conduction band (CB) and valence band (VB) of ZIS are determined to be around -0.98 V and 1.22 V vs. reversible hydrogen electrode (RHE), respectively. Accordingly, the CB and VB values of ZIS vs. vacuum level are calculated to be -3.46 and -5.66 eV based on the equation of E_vac = -4.44 - E_RHE.^{[23, 78, 79]} Moreover, the work function (Φ) and Fermi level (E_f) of Ni₃FeN are determined by density functional theory (DFT) calculation. As displayed in Figure S24, Φ of Ni₃FeN is theoretically calculated to be about 4.97 eV (vs. vacuum level). As such, the E_f of Ni₃FeN is determined to be -4.97 eV (vs. vacuum level) according to the equation of E_f (vs. vacuum level) = E_vac - Φ, where E_vac is assumed as 0 eV.^[80] Based on literature reports, the Φ of ZIS is 4.75 eV (vs. vacuum level) and the corresponding E_f is calculated to be -4.75 eV (vs. vacuum level).^{[60, 81]} Therefore, a Schottky junction is formed between the metallic Ni₃FeN cocatalyst and the ZIS semiconductor.

  In conclusion, resulting from the above-mentioned analyses, a possible photocatalytic reaction mechanism towards the anaerobic oxidation of aromatic alcohols over ZIS/M-Ni₃FeN hybrids is presented. Before contact, the Ni₃FeN has a larger work function than ZIS (Figure S25). As displayed in Figure 5e, when they are in close contact, an energy difference is formed. The electrons in ZIS could spontaneously transfer to Ni₃FeN until E_f reaches equilibrium. The charge migration leads to an upward band bending of ZIS along the side of the Schottky contact as well as the formation of an internal electric field between ZIS and Ni₃FeN. Under visible light irradiation, the photogenerated electrons could transfer from ZIS to Ni₃FeN and suppress the backflow of electrons. Due to that the electron accumulation causes a negative shift of E_f for Ni₃FeN, the E_f of ZIS/Ni₃FeN composite shifts closer to the CB of ZIS. The higher negative potential of E_f corresponds to stronger reduction ability in the composite.^[82-84] Then, the isolated photogenerated electrons and holes would promote H₂ evolution and the oxidation of aromatic alcohol, respectively. Consequently, the introduction of Ni₃FeN effectively accelerates the separation and migration of the photogenerated electrons, and improves the photocatalytic activity.

  CONCLUSION

  In summary, we report a bimetallic transition metal nitride Ni₃FeN as a novel noble-metal-free cocatalyst for photocatalytic selective anaerobic oxidation of aromatic alcohols to form corresponding aldehydes pairing with H₂ in one redox cycle, which realizes simultaneous utilization of photogenerated electrons and holes. The synthesis of hybrid ZIS/M-Ni₃FeN composite is realized by a simple yet efficient electrostatic self-assembly method, which is easy to be scaled up. The optimal ZIS/M-Ni₃FeN sample achieves a H₂ evolution rate of 2427.9 µmol h^-1 g^-1 and a BAD production rate of 2460 μmol g^-1 h^-1, about 7.8-fold as high as that of bare ZIS. Photoelectrochemical characterizations show that Ni₃FeN not only accelerates interfacial charge separation and transfer, but also promotes surface H₂ evolution reaction. Moreover, a comparison study with ZIS/P-Ni₃FeN reveals that the tight interfacial contact and strong coupling effect in the composites are favorable for separating charges and utilizing active sites. The present study offers insight into the development of economical and efficient transition metal nitride cocatalysts to promote photocatalytic activity in solar energy conversion.

  EXPERIMENTAL

  Materials and Reagents. All chemicals in the work were analytic grade and used without further purification. Nickel acetate tetrahydrate (NiC₄H₆O₄·4H₂O) was purchased from Aladdin Biochemi-cal Technology Co., Ltd. (Shanghai, China). Trisodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), indium chloride tetrahydrate (InCl₃·4H₂O), potassium ferricyanide K₃[Fe(CN)₆], 3-aminopropyltriethoxysilane (APTES), thioacetamide (C₂H₅NS), acetonitrile (C₂H₃N), benzyl alcohol (C₆H₅CH₂OH), 2-methylbenzyl alcohol (C₈H₁₀O), 3-methylbenzyl alcohol (C₈H₁₀O), 4-methylbenzyl alcohol (C₈H₁₀O), 4-fluorophenyl alcohol (C₇H₇OF), 4-chlorobenzyl alcohol (C₇H₇ClO), 4-bromobenzyl alcohol (C₇H₇OBr), benzaldehyde (C₇H₆O), 2-methylbenzaldehyde (C₈H₈O), 3-methylbenzaldehyde (C₈H₈O), 4-methylbenzaldehyde (C₈H₈O), 4-fluorobenzaldehyde (C₇H₅OF), 4-chlorobenzaldehyde (C₇H₅OCl), and 4-bromobenzaldehyde (C₇H₅OBr) were all obtained from Shanghai Sinopharm Chemical Reagent Co., Ltd. The deionized (DI) water used throughout the experiments was from local sources.

  Synthesis of Ultrathin 2D ZnIn₂S₄ Nanosheets. Ultrathin 2D ZnIn₂S₄ nanosheets were prepared through a facile refluxing method.^[43] In a typical experiment, Zn(CH₃COO)₂·2H₂O (0.5 mmol) and InCl₃·4H₂O (1 mmol) were added into 150 mL DI water and stirred for 30 min at room temperature. Then, 3 mmol of thioacetamide (TAA) was added into the above solution. After being stirred for another 30 min, the mixture was maintained at 95 ℃ for 5 h under stirring. The product was collected by centrifugation and washed with DI water for three times. Finally, the sample was obtained by drying for further use.

  Synthesis of Ni₃FeN Nanoparticles. 0.3 g NiC₄H₆O₄·4H₂O and 0.441 g Na₃C₆H₅O₇·2H₂O were dissolved in 40 mL DI water and stirred for 15 min to get a green solution. At the same time, a yellow solution was obtained by dissolving 0.264 g K₃[Fe(CN)₆] in 60 mL DI water under stirring for 15 min. Then the yellow solution was transferred into the green solution rapidly and stirred for 10 min. The liquid was left overnight and filtered to get the precipitates. Subsequently, such precipitates were washed with DI water and ethanol for three times. The powder was dried at room temperature overnight. After that, the dried precursor was transferred into a muffle furnace and heated to 350 ℃ at a heating rate of 1 ℃ min^-1 and maintained for 1 h. The obtained brown-red powder of NiFeO_x was annealed in a NH₃ flow at 400 ℃ for 20 min, thus obtaining Ni₃FeN. This pristine sample was labeled as P-Ni₃FeN.

  Surface Modification of Ni₃FeN. In brief, 25 mg of Ni₃FeN was dispersed in 50 mL ethanol and sonicated for 1 h at room temperature. Then, 0.15 mL of 3-aminopropyltriethoxysilane (APTES) was added into the solution. The mixture was heated at 60 ℃ for 4 h. After that, the sample was collected by centrifugation, washed twice with ethanol, and re-dispersed in DI water to obtain the APTES-modified Ni₃FeN (marked as M-Ni₃FeN) suspension with the concentration of 1 mg mL^-1.

  Synthesis of ZIS/M-Ni₃FeN and ZIS/P-Ni₃FeN Hybrids. The ZIS/M-Ni₃FeN hybrids were prepared by slowly dropping the M-Ni₃FeN suspension into the ZIS suspension under ultrasonication for 1 h and shaking for 12 h. Then, the sample was centrifuged, washed with DI water for 3 times, and dried. By simply changing the adding amount of M-Ni₃FeN, a series of ZIS/M-Ni₃FeN composites with 1%, 3%, 5%, 7% and 10% weight ratios of M-Ni₃FeN were obtained. The ZIS/P-Ni₃FeN hybrids were fabricated via the same procedure except that P-Ni₃FeN without APTES modification was used.

  Characterizations. The zeta potential measurements of the samples were taken on a Zetasizer Nano ZS (Malvern Panalytical, UK) at room temperature of 298 K. The morphologies of the samples were measured on a Hitachi 8100 scanning electron microscopy (SEM). Transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM) images were recorded on a Jem-2100F Jeol. The crystal structures and crystallinity of the catalysts were measured through a Bruker D8 advance X-ray diffractometer (XRD) operated at 40 kV and 40 mA with Cu Kα radiation in the 2θ range from 10° to 80°. The UV-vis diffuse reflectance spectra (DRS) were recorded on an Agilent CARY-100 spectrophotometer (100% BaSO₄ was used as reference). X-ray photoelectron spectroscopy (XPS) spectra were investigated on Thermo Kalpha (Thermo Fisher Scientific) using a monochromatic Al Kα as the X-ray source. All binding energies were calibrated by the C 1s peak at 284.8 eV of surface adventitious carbon. The thickness of nanosheets was measured by atomic force microscope (AFM) (NanoWizard4, Bruker). Photoluminescence (PL) spectra of solid samples were obtained by RF-5301PC (Shimadzu, Japan) and transient PL was recorded by using a FLS920 Fluorescence Spectrometer equipped with a nanosecond hydrogen flash-lamp (nF920). The Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution were characterized using an adsorption analyzer (BELSORP-Mini-II, Bel, Japan). The electron paramagnetic resonance (EPR) measurements were performed by a Magnettech ESR5000 spectrometer. The products were detected by gas chromatography-mass (GC-MS) (Aligent 5977A GC/MSD, USA).

  Photocatalytic Activity Tests. Typically, 10 mg of photocatalyst and 0.1 mmol of aromatic alcohol were added into a quartz reactor containing 3 mL CH₃CN. Prior to light irradiation, the mixture was sonicated for 2 min and then purged with argon (Ar) gas for 10 min to remove air. After that, the reactor was irradiated by visible light (λ > 420 nm) with a 300 W Xe lamp (CEL-PF300-T8, Beijing China Education Au-light Co., Ltd). To keep the photocatalyst in suspension status, a continuous magnetic stirrer was applied at the bottom of the reactor. After reaction, the generated H₂ was measured by a gas chromatograph (GC 9790plus, Fu Li, China, TCD detector, Ar as carrier gas) and the liquid products were analyzed by another gas chromatography (GC-2030, Shimadzu, Japan, FID detector, nitrogen (N₂) as carrier gas) after centrifuging the suspension at 10000 rpm for 3 min.

  The conversion efficiency of alcohols and selectivity of aldehydes production were calculated as follows:

  $\text { Conversion (%) }=100 \times\left[\left(C_0-C_{\text {alcohol }}\right) / C_0\right] \%$

  $ \text { Selectivity }(\%)=100 \times\left[C_{\text {aldehyde }} /\left(C_0-C_{\text {alcohol }}\right)\right] \%$

  where C₀ is the initial concentration of alcohol, C_alcohol and C_aldehyde are the concentrations of alcohol and aldehyde measured after illumination for a certain time, respectively.

  Photoelectrochemical Measurements. All the photoelectro-chemical measurements were measured in a three-electrode cell using a CHI 760E instrument. A Pt plate and Ag/AgCl electrode were employed as the counter electrode (CE) and reference electrode (RE), respectively. The working electrode was prepared as follows: 5 mg of catalyst powder was dispersed in 0.5 mL of N, N-dimethylformamide (DMF) under ultrasonication. Then, the mixed solution was spread onto the conductive surface of FTO glass and then dried in air. The exposed area of the working electrode was 1 cm². The electrochemical impedance spectroscopy (EIS) measurement was carried out in 0.1 M KCl aqueous solution containing 0.01 M K₃[Fe(CN)₆]. The transient photocurrent was tested in 0.2 M Na₂SO₄ solution using a 300 W Xenon lamp (CEL-PF300-T8, Beijing China Education Au-light Co., Ltd) equipped with a 420 nm cutoff filter (λ > 420 nm). Linear sweep voltammetry (LSV) curves were also examined in 0.2 M Na₂SO₄ solution.

  Computational Methods. All calculations were performed by using the density functional theory (DFT) within the Vienna ab initio Simulation Package (VASP). The projector augmented wave (PAW) method, Perdew-Burke-Ernzerhof (PBE) function and generalized gradient approximation (GCA) potential were used for the exchange-correlation effect. A plane-wave cutoff energy of 600 eV, energy difference criteria of 1 × 10^-5 eV and Monkhorst-Pack k-mesh of 13 × 13 × 13 for unit cell were used. A vacuum region of 25 was used to separate the interactions between the neighboring cells of slab models.

  
  ACKNOWLEDGEMENTS: This work is financially supported by the National Natural Science Foundation of China (21905049 and 22178057), Natural Science Foundation of Fujian Province (2020J01201 and 2021J01197), and the Award Program for Minjiang Scholar Professorship. S. Liu thanks the support from the Fundamental Research Funds for the Central Universities (Grant No. DUT21RC(3)114). The authors declare no competing interests.
  COMPETING INTERESTS
  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0147
  For submission: https://www.editorialmanager.com/cjschem
  ADDITIONAL INFORMATION
  
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  Figure 1  (a) Schematic illustration for the synthesis of ZIS/M-Ni₃FeN hybrids. (b) SEM, (c) TEM and (d-e) HRTEM of ZIS/M-Ni₃FeN composite.

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  Figure 2  XPS spectra of (a) Zn 2p, (b) In 3d, (c) S 2p, (d) Ni 2p, (e) Fe 2p and (f) N 1S of ZIS/M-Ni₃FeN composite.

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  Figure 3  (a, b) Photocatalytic anaerobic oxidation of benzyl alcohol to benzaldehyde and H₂ over different ZIS/M-Ni₃FeN composites. (c) Long-term photoactivity test over ZIS/1.5% M-Ni₃FeN. (d) Photocatalytic performance of ZIS/1.5% M-Ni₃FeN for anaerobic oxidation of different aromatic alcohols.

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  Figure 4  (a) Photocatalytic activity for selective BA oxidation integrated with H₂ evolution over ZIS, ZIS/1.5% P-Ni₃FeN and ZIS/1.5% M-Ni₃FeN hybrids. (b) XRD patterns, (c) DRS spectra and (d) N₂ adsorption-desorption isotherms.

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  Figure 5  (a) Transient photocurrent response. (b) EIS Nyquist plots. (c) Steady-state PL spectra and (d) LSV curves. (e) A mechanism for photocatalytic H₂ evolution coupling with aldehydes production over ZIS/M-Ni₃FeN hybrids.

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