Enhanced Photocatalytic Hydrogen Production of ZnIn2S4 by Using Surface-Engineered Ti3C2Tx MXene as a Cocatalyst

Developing efficient and stable photocatalysts is crucial for photocatalytic hydrogen production. Cocatalyst loading is one of the effective strategies for improving photocatalytic efficiency. Here, Ti3C2Tx (Tx = F, OH, O) nanosheets have been adopted as promising cocatalysts for photocatalytic hydrogen production due to their metallic conductivity and unique 2D characterization. In particular, surface functionalized Ti3C2(OH)x and Ti3C2Ox cocatalysts were synthesized through the alkalization treatment with NaOH and a mild oxidation treatment of Ti3C2Fx, respectively. ZnIn2S4/Ti3C2Tx composites, which were fabricated by the in-situ growth of ZnIn2S4 nanosheets on the Ti3C2Tx surface, exhibited the promoted photocatalytic performance, compared with the parent ZnIn2S4. The enhanced photocatalytic performance can be further optimized through the surface functionalization of Ti3C2Fx. As a result, the optimized ZnIn2S4/Ti3C2Ox composite with oxygen functionalized Ti3C2Ox cocatalyst demonstrated excellent photocatalytic hydrogen evolution activity. The characterizations and density functional theory calculation suggested that O-terminated Ti3C2Ox could effectively facilitate the transfer and separation of photogenerated electrons and holes due to the formation of a Schottky junction, with the largest difference in work function between ZnIn2S4 and Ti3C2Ox. This work paves the way for photocatalytic applications of MXene-based photocatalysts by tuning their surface termination groups.


Introduction
Hydrogen is regarded as an ideal energy with the advantages of a high energy capacity and zero pollutants. Among the various H 2 production strategies, solar-light-driven photocatalysis for H 2 production from water splitting is a promising route to alleviating the energy crisis [1][2][3]. Developing highly efficient photocatalysts is the key to realizing the industrialization of photocatalytic H 2 production. Regarding photocatalysts, ZnIn 2 S 4 has attracted more attention in recent years because of its low toxicity, visible-light response, and considerable photostability [4,5]. However, the rapid recombination and tardy migration of the photogenerated electrons and holes restricts the photocatalytic H 2 production efficiency of bare ZnIn 2 S 4 [6,7]. To address this issue, diverse approaches, including cocatalyst loading, vacancy engineering and heterojunction construction, have been systematically developed to improve the photocatalytic performance of ZnIn 2 S 4 materials [8][9][10]. Among them, cocatalyst loading has been verified to be a feasible and efficient method to promote the photocatalytic efficiency by accelerating the separation and transfer of photogenerated charge carriers while simultaneously acting as active sites to facilitate the photocatalytic H 2 production reaction kinetics. The employment of noble metals (such as Pt, Au, Pd and Rh) as cocatalysts, has been proven to be highly efficient in improving the photocatalytic performance, but their high price largely limits their widespread application [11,12]. Therefore, it Materials 2023, 16 is urgent to explore an inexpensive and efficient noble metal-free cocatalyst to replace Pt, Au, Pd and Rh to achieve large-scale photocatalytic H 2 production. MXene, as an emerging family of 2D transition metal carbides/nitrides, has gained intensive scientific interest in photocatalysis, ascribed to its excellent metal conductivity, large specific surface area with abundant active sites and hydrophilicity [13][14][15]. The 2D planar structure of Ti 3 C 2 T x MXene is beneficial to highly dispersing the host photocatalyst with a strong interfacial contact [16][17][18]. On the other hand, owing to its high conductivity and abundant exposed metal sites, Ti 3 C 2 T x could act as a cocatalyst to facilitate the separation and migration of photogenerated charge carriers and lower the reaction energy barriers for accelerating the reaction kinetics. Therefore, Ti 3 C 2 T x was widely used as a cocatalyst in photocatalytic H 2 production [18][19][20]. For instance, Zhao et al. [17] reported the construction of hierarchical 2D Bi 2 MoO 6 @Ti 3 C 2 T x by in-situ growing Bi 2 MoO 6 onto the surface of Ti 3 C 2 T x nanosheets. Ti 3 C 2 T x , as the cocatalyst, could not only suppress the agglomeration of Bi 2 MoO 6 nanosheets and increase the reaction active sites, but also endow the photocatalyst with the Schottky junction. As a result, the Bi 2 MoO 6 @Ti 3 C 2 T x exhibited enhanced photocatalytic activity. Zuo et al. [18] found that the ZnIn 2 S 4 -Ti 3 C 2 T x -ZnIn 2 S 4 sandwich-like hierarchical heterostructures exhibited a superior photocatalytic H 2 production performance due to the construction of the Schottky junction between ZnIn 2 S 4 nanosheets and Ti 3 C 2 T x . Ran et al. [19] reported that Ti 3 C 2 , as a potential cocatalyst, could efficiently improve the photocatalytic hydrogen production performance by forming the Schottky junction at the Ti 3 C 2 /CdS interface to facilitate the separation of the photogenerated electrons and holes. Meanwhile, they found that the Gibbs free energy for H adsorption (∆G H* ) of O-terminated Ti 3 C 2 is close to zero. With the near-zero ∆G H* , the favorable Fermi level position and electrical conductivity, O-terminated Ti 3 C 2 could serve as an alternative to noble metals in photocatalytic H 2 production. Liu et al. [20] utilized Ti 3 C 2 nanosheets acting as the substrate and cocatalyst to synthesize a CdLa 2 S 4 /Ti 3 C 2 photocatalyst, which could not only promote the dispersion of CdLa 2 S 4 , but also enhance the photogenerated charge carriers separation and transfer, leading to a significant enhancement in photocatalytic H 2 evolution. In most cases, Ti 3 C 2 with a large work function could act as electron sink to facilitate the separation and transfer of the photogenerated charge carriers in photocatalytic H 2 production. In contrast, Peng et al. [21] proposed a dualcarrier-separation mechanism for photocatalytic H 2 evolution within Cu/TiO 2 @Ti 3 C 2 T x , where -OH-terminated Ti 3 C 2 T x with a lower work function than TiO 2 served as the hole trap to accelerate the holes migration from TiO 2 to Ti 3 C 2 T x . Obviously, the surface termination groups of Ti 3 C 2 T x could arise tunable electronic properties (such as work function) to impact on the photocatalytic performance of the Ti 3 C 2 T x -based photocatalysts.
Tailoring the surface termination groups of Ti 3 C 2 T x could alter their work function, electronic and optoelectronic properties [22][23][24]. Recently, the theoretical calculations from Khazaei revealed that the work function of Ti 3 C 2 T x was strongly dependent on the surface termination groups, and the work function of Ti 3 C 2 T x could adjust in a wide range from 1.6 eV to 6.0 eV [24]. Jiang et al. [25] investigated the effect of the surface terminations of Ti 3 C 2 T x on the electrocatalytic H 2 evolution. They found that O-terminated Ti 3 C 2 T x nanosheets exhibited much higher H 2 evolution activity than other Ti 3 C 2 T x , and the -O termination groups on the basal plane of Ti 3 C 2 were the H 2 evolution reaction active sites. Especially, the -O termination groups could promote the adsorption of H and accelerate the H 2 evolution reaction. However, the insights into the effect of the surface termination groups in Ti 3 C 2 T x MXene-based photocatalysts on the photocatalytic H 2 production are not established experimentally. Herein, we designed a series of Ti 3 C 2 T x (T x = F, OH, O) with different surface termination groups, and then the 2D ZnIn 2 S 4 was in-situ grown on the surface of Ti 3 C 2 T x using a facile hydrothermal synthesis method to synthesize ZnIn 2 S 4 /Ti 3 C 2 T x composites. Specifically, the as-synthesized ZnIn 2 S 4 /Ti 3 C 2 O x with the O-terminated Ti 3 C 2 T x exhibited the superior photocatalytic H 2 production activity. When the content of Ti 3 C 2 O x was 1.0 wt%, the ZnIn 2 S 4 /Ti 3 C 2 O x presented the optimal photocatalytic H 2 production rate of 363 µmol g −1 h −1 . This work provides us with a paradigm for the rational design of Ti 3 C 2 T x MXene with tailored surface termination groups and the development of efficient MXene-based composites for photocatalytic applications.

Materials and Methods
2.1. Samples Preparation 2.1.1. Synthesis of Ti 3 C 2 F x Typically, 2 g LiF was added into 40 mL HCl aqueous solution (9 M) and stirred for 1 h until the LiF was completely dissolved. A total of 2 g of Ti 3 AlC 2 powder was then added to the above solution and stirred for 0.5 h. The suspension was stirred at 53 • C for 41 h. Upon cooling, the mixture was centrifuged and washed with deionized water until the pH was close to 7. The product was dried at 60 • C under vacuum for 48 h.

Synthesis of Surface Functionalized Ti 3 C 2 T x
In order to obtain the surface functionalized Ti 3 C 2 T x , the pristine Ti 3 C 2 F x were treated with a different functionality-modification strategy. To achieve Ti 3 C 2 (OH) x with −OH rich termination groups, according to the previous literature [25], 0.2 g of the pristine Ti 3 C 2 F x was dispersed in 100 mL of 1 M NaOH aqueous solution in order to replace the −F surface termination groups with −OH. After stirring for 2 h at room temperature, the product was centrifuged and washed with deionized water until the pH was close to 7. Then, the product was collected and dried at 60 • C under vacuum for 12 h. To obtain O-terminated Ti 3 C 2 O x , the Ti 3 C 2 F x was calcined under 300 • C in Ar gas flow for 2 h.

Synthesis of ZnIn 2 S 4 and ZnIn 2 S 4 /Ti 3 C 2 T x
Typically, 0.176 g InCl 3 ·4H 2 O, 0.041 g ZnCl 2 and 0.120 g thioacetamide (TAA) were added consecutively into 40 mL glycerol aqueous solution (20 vol%) and stirred for 0.5 h. The quantitative Ti 3 C 2 T x (T x = F, OH, O) (1.0 wt%) was added into the above solution. The mixed suspension was heated at 80 • C with stirring for 2 h. After centrifugation, the products were collected and dried at 60 • C for 12 h.
For comparison, the preparation of the pristine ZnIn 2 S 4 was similar to that of ZnIn 2 S 4 /Ti 3 C 2 T x without the introduction of Ti 3 C 2 T x .

Photocatalytic H 2 Production Experiments
The photocatalytic H 2 production tests were carried out in a Pyrex glass reaction (Beijing Perfectlight Labsolar-6A, Perfectlight Technology, Beijing, China) with a circulated cooling water system to maintain the temperature at 8 • C. A total of 100 mg of the photocatalyst was dispersed in 100 mL aqueous solution, containing 10 vol% triethanolamine (TEOA) as the sacrificial agent. Before irradiation under a Xe lamp (CEL-HXUV300, Perfectlight Technology, Beijing, China), the suspension was evacuated by the vacuum pump. The produced H 2 volume was analyzed using an on-line gas chromatograph (GC5190, TCD, A column, Ar carrier).

Characterization
The powder X-ray diffraction (XRD) patterns of the prepared samples were collected using a Rigaku SmartLab (9 kW, Tokyo, Japan) diffractometer with Cu Kα radiation (λ = 0.15418 nm) operating at 40 kV and 4 mA. The morphological analysis of the samples were recorded with scanning electron microscopy (SEM) using a Regulus 8230 scanning electron microscope (Hitachi, Ltd., Tokyo, Japan) at an acceleration voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) was carried out to investigate the surface chemical environment of the samples using an Escalab 250Xi X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Measuring with an ultraviolet photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was performed with a −5 V bias voltage. The data were calibrated with a C1s spectrum of 284.6 eV. The Fourier transform spectrophotometer (Vertex80 + Hyperion2000, Bruker, Billerica, MA, USA) was employed to acquire IR spectra with the standard KBr disk method. Transmission electron Materials 2023, 16, 2168 4 of 12 microscopy (TEM), high resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns of the samples were collected with a field-emission electron microscope (JEM-2100F, JEOL, Tokyo, Japan). UV-visible diffuse reflectance spectroscopy (UV-vis DRS) was recorded to study the optical absorption ability of photocatalysts with Hitachi U-4100 UV-visible spectrometer using a reference standard of BaSO 4 . The photoluminescence (PL) spectra and time-resolved fluorescence spectra were conducted on an Edinburgh FLS 1000 spectrometer (Edinburgh Instruments Ltd., Livingstone, UK) over an exaction wavelength of 375 nm. Electrochemistry impedance spectroscopy (EIS), Mott-Schottky analyses and transient photocurrent spectra were measured using a CHI660E analyzer (CH Instruments, Inc., Bee Cave, TX, USA) with a standard three-electrode system.

Schematic Illustration of the Synthesis
The schematic illustration in Scheme 1 shows the synthesis process for the ZnIn 2 S 4 /Ti 3 C 2 T x (T x = F, OH, O) samples, which consists of three steps: the preparation of the Ti 3 C 2 by the selective etching of Ti 3 AlC 2 , surface post-treatment (the alkalization treatment with NaOH and the mild oxidation treatment with Ar calcination) of Ti 3 C 2 F x to replace the -F termination groups with -OH or -O groups and the in-situ hydrothermal synthesis of ZnIn 2 S 4 on surface of Ti 3 C 2 T x .
(Thermo Fisher Scientific, Waltham, MA, USA). Measuring with an ultraviolet photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was performed with a −5V bias voltage. The data were calibrated with a C1s spectrum of 284.6 eV. The Fourier transform spectrophotometer (Vertex80 + Hyperion2000, Bruker, Billerica, MA, USA) was employed to acquire IR spectra with the standard KBr disk method. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns of the samples were collected with a field-emission electron microscope (JEM-2100F, JEOL, Tokyo, Japan). UV-visible diffuse reflectance spectroscopy (UV-vis DRS) was recorded to study the optical absorption ability of photocatalysts with Hitachi U-4100 UV-visible spectrometer using a reference standard of BaSO4. The photoluminescence (PL) spectra and time-resolved fluorescence spectra were conducted on an Edinburgh FLS 1000 spectrometer (Edinburgh Instruments Ltd., Livingstone, UK) over an exaction wavelength of 375 nm. Electrochemistry impedance spectroscopy (EIS), Mott-Schottky analyses and transient photocurrent spectra were measured using a CHI660E analyzer (CH Instruments, Inc., Bee Cave, TX, USA) with a standard three-electrode system.

Schematic Illustration of the Synthesis
The schematic illustration in Scheme 1 shows the synthesis process for the ZnIn2S4/Ti3C2Tx (Tx = F, OH, O) samples, which consists of three steps: the preparation of the Ti3C2 by the selective etching of Ti3AlC2, surface post-treatment (the alkalization treatment with NaOH and the mild oxidation treatment with Ar calcination) of Ti3C2Fx to replace the -F termination groups with -OH or -O groups and the in-situ hydrothermal synthesis of ZnIn2S4 on surface of Ti3C2Tx.

Samples Characterization
The X-ray diffraction (XRD) patterns of Ti3AlC2 and the as-prepared Ti3C2Tx (Tx = F, OH, O) samples in Figure S1 demonstrated a typical Ti3AlC2 and Ti3C2Tx MXene phase. No crystal structure variation was observed for the Ti3C2(OH)x and Ti3C2Ox, indicating that the surface functionalization treatments just modulated the termination groups without changing the crystalline structure of Ti3C2Fx. The XRD pattern of Ti3C2Ox showed no peaks of TiO2. Meanwhile, the morphology of the Ti3C2Tx nanosheets was maintained even after the alkalization and oxidation treatments ( Figure S2).
To confirm the surface termination groups of the as-prepared Ti3C2Tx (Tx = F, OH, O) samples, we performed X-ray photoelectron spectroscopy (XPS), as shown in Figure 1ac. Figure 1a showed the high-resolution XPS spectrum of F 1s, the binding energy at 685.8 Scheme 1. Illustration for the formation of ZnIn 2 S 4 /Ti 3 C 2 T x (T x = F, OH, O) photocatalysts.

Samples Characterization
The X-ray diffraction (XRD) patterns of Ti 3 AlC 2 and the as-prepared Ti 3 Figure S1 demonstrated a typical Ti 3 AlC 2 and Ti 3 C 2 T x MXene phase. No crystal structure variation was observed for the Ti 3 C 2 (OH) x and Ti 3 C 2 O x , indicating that the surface functionalization treatments just modulated the termination groups without changing the crystalline structure of Ti 3 C 2 F x . The XRD pattern of Ti 3 C 2 O x showed no peaks of TiO 2 . Meanwhile, the morphology of the Ti 3 C 2 T x nanosheets was maintained even after the alkalization and oxidation treatments ( Figure S2).
To confirm the surface termination groups of the as-prepared Ti 3 C 2 T x (T x = F, OH, O) samples, we performed X-ray photoelectron spectroscopy (XPS), as shown in Figure 1a-c. Figure 1a showed the high-resolution XPS spectrum of F 1s, the binding energy at 685.8 eV was assigned to the Ti-F bond [26]. After the alkalization treatment and mild oxidation treatment of Ti 3 C 2 F x , the Ti-F peak intensity in Ti 3 C 2 (OH) x and Ti 3 C 2 O x both significantly decreased, indicating that the surface functionalization treatments did not change its crystal structure, while the termination groups had modulated noticeably. The elemental composition result determined by XPS (Table S1) also confirmed the decrease of the -F termination groups. As seen from the Ti 2p XPS spectra in Figure 1b, more detailed structural variation could be obtained, four doublets were fitted for Ti 2p 3/2 and Ti 2p 1/2 , which indicated that the Ti species in Ti 3 C 2 T x exhibited four kinds of chemical environment. The Ti 2p 3/2 binding energies at approximately 455.1, 455.8, 456.9 and 459.1 eV could be assigned to C-Ti-C, C-Ti-OH, C-Ti-O and O-Ti-O bonds, respectively [23,27,28]. Obviously, compared to Ti 3 C 2 F x , the intensity of the C-Ti-O peak for Ti 3 C 2 O x increased and the intensity of the C-Ti-OH peak for Ti 3 C 2 (OH) x increased, which indicated that the -F terminations in the Ti 3 C 2 F x were replaced by -O and -OH after the oxidation treatment and alkalization treatment, respectively. The intensity of the O-Ti-O peak increased in Ti 3 C 2 O x and Ti 3 C 2 (OH) x , which was attributed to the surface oxidation with the transform C-Ti-C band to O-Ti-O. Furthermore, the O 1s XPS spectra (Figure 1c) exhibited Ti-O, Ti-OH and C-OH peaks at the binding energies of 530.1, 531.8 and 533.5 eV, respectively [29,30]. In particular, the peak at 531.8 eV demonstrated the highest proportion of -OH groups on the surface of Ti 3 C 2 (OH) x , while Ti 3 C 2 O x showed the highest concentration of Ti-O due to O-terminated surfaces. The XPS results showed the coexistence of Ti-F, Ti-OH and Ti-O bonds in all Ti 3 C 2 T x samples. It should be noted that the Ti 3 C 2 O x , Ti 3 C 2 (OH) x and Ti 3 C 2 O x represented a higher density of termination groups -F, -OH and -O on the surface, respectively. After the alkalization treatment, the Ti-F peak intensity significantly decreased while the Ti-OH peak intensity increased in Ti 3 C 2 (OH) x , implying the successful replacement of -F with -OH. Similarly, the -F groups in Ti 3 C 2 F x were successfully replaced by -O with the mild oxidation treatment to form the Ti 3 C 2 O x .
eV was assigned to the Ti-F bond [26]. After the alkalization treatment and mild oxidation treatment of Ti3C2Fx, the Ti-F peak intensity in Ti3C2(OH)x and Ti3C2Ox both significantly decreased, indicating that the surface functionalization treatments did not change its crystal structure, while the termination groups had modulated noticeably. The elemental composition result determined by XPS (Table S1) also confirmed the decrease of the -F termination groups. As seen from the Ti 2p XPS spectra in Figure 1b, more detailed structural variation could be obtained, four doublets were fitted for Ti 2p3/2 and Ti 2p1/2, which indicated that the Ti species in Ti3C2Tx exhibited four kinds of chemical environment. The Ti 2p3/2 binding energies at approximately 455.1, 455.8, 456.9 and 459.1 eV could be assigned to C-Ti-C, C-Ti-OH, C-Ti-O and O-Ti-O bonds, respectively [23,27,28]. Obviously, compared to Ti3C2Fx, the intensity of the C-Ti-O peak for Ti3C2Ox increased and the intensity of the C-Ti-OH peak for Ti3C2(OH)x increased, which indicated that the -F terminations in the Ti3C2Fx were replaced by -O and -OH after the oxidation treatment and alkalization treatment, respectively. The intensity of the O-Ti-O peak increased in Ti3C2Ox and Ti3C2(OH)x, which was attributed to the surface oxidation with the transform C-Ti-C band to O-Ti-O. Furthermore, the O 1s XPS spectra (Figure 1c) exhibited Ti-O, Ti-OH and C-OH peaks at the binding energies of 530.1, 531.8 and 533.5 eV, respectively [29,30]. In particular, the peak at 531.8 eV demonstrated the highest proportion of -OH groups on the surface of Ti3C2(OH)x, while Ti3C2Ox showed the highest concentration of Ti-O due to O-terminated surfaces. The XPS results showed the coexistence of Ti-F, Ti-OH and Ti-O bonds in all Ti3C2Tx samples. It should be noted that the Ti3C2Ox, Ti3C2(OH)x and Ti3C2Ox represented a higher density of termination groups -F, -OH and -O on the surface, respectively. After the alkalization treatment, the Ti-F peak intensity significantly decreased while the Ti-OH peak intensity increased in Ti3C2(OH)x, implying the successful replacement of -F with -OH. Similarly, the -F groups in Ti3C2Fx were successfully replaced by -O with the mild oxidation treatment to form the Ti3C2Ox.  The surface termination groups of the Ti 3 C 2 T x samples were further analyzed using Fourier transform infrared spectroscopy (FTIR), as displayed in Figure 2d. The FTIR spectrum of Ti 3 C 2 T x samples showed two peaks at approximately 3430 and 1625 cm −1 , which assigned to the −OH band on the surface of Ti 3 C 2 T x . In addition, a peak at 657 cm −1 can be observed, which is attributed to the Ti-O band [31]. It is notable that Ti 3 C 2 (OH) x showed the strongest −OH vibration intensity and that the Ti 3 C 2 O x exhibited a significantly increased Ti-O vibration, which was consistent with the XPS results. These results indicated Materials 2023, 16, 2168 6 of 12 that the surface functionalized Ti 3 C 2 (OH) x and Ti 3 C 2 O x were successfully synthesized with the alkalization treatment and mild oxidation treatment, respectively.
The surface termination groups of the Ti3C2Tx samples were further analyzed using Fourier transform infrared spectroscopy (FTIR), as displayed in Figure 2d. The FTIR spectrum of Ti3C2Tx samples showed two peaks at approximately 3430 and 1625 cm −1 , which assigned to the −OH band on the surface of Ti3C2Tx. In addition, a peak at 657 cm −1 can be observed, which is attributed to the Ti-O band [31]. It is notable that Ti3C2(OH)x showed the strongest −OH vibration intensity and that the Ti3C2Ox exhibited a significantly increased Ti-O vibration, which was consistent with the XPS results. These results indicated that the surface functionalized Ti3C2(OH)x and Ti3C2Ox were successfully synthesized with the alkalization treatment and mild oxidation treatment, respectively.  Figure S3). It was found that all ZnIn2S4/Ti3C2Tx samples presented similar diffraction peaks with ZnIn2S4. The missing Ti3C2Tx diffraction peaks could be ascribed to the low content and high dispersion of Ti3C2Tx in the composites. The morphology of the ZnIn2S4 and ZnIn2S4/Ti3C2Ox samples were investigated using scanning electron microscopy (SEM). The ZnIn2S4 presented a morphology of nanoflowers stacked with nanosheets ( Figure S4). From the SEM image of ZnIn2S4/Ti3C2Ox sample in Figure 3a, it can be seen that the ZnIn2S4 particles are uniformly dispersed and anchored onto the Ti3C2Ox surface. The more detailed microstructure of the ZnIn2S4/Ti3C2Ox composite were further demonstrated using the transmission electron microscopy (TEM) technique. TEM observation confirmed such hierarchical ZnIn2S4/Ti3C2Ox structure (Figure 2b). Furthermore, as shown in Figure 2c, the lattice distances of the ZnIn2S4/Ti3C2Ox photocatalyst were measured, and the lattice fringes spacing of 0.32 and 0.41 nm were corresponded to the (102) and (006) planes of ZnIn2S4, while the lattice fringes spacing of 0.26 nm was assigned to the (0110) crystal plane of Ti3C2Ox. Moreover, there was an obvious interface contact observed between the ZnIn2S4 and the Ti3C2Ox, which could contribute to the faster transfer of the photogenerated charge. In addition, the corresponding EDX elemental mapping (Figure 2d) displayed that the Zn, In, S, Ti and C elements were uniformly distributed in the ZnIn2S4/Ti3C2Ox sample. The above results powerfully confirmed that the ZnIn2S4/Ti3C2Ox photocatalyst was successful constructed. The ZnIn 2 S 4 /Ti 3 C 2 T x (T x = F, OH, O) composites were obtained through the in-situ growth of ZnIn 2 S 4 onto the surface of Ti 3 C 2 T x . To acquire the crystallinity phase of the ZnIn 2 S 4 and ZnIn 2 S 4 /Ti 3 C 2 T x composites, the XRD analysis was introduced ( Figure S3). It was found that all ZnIn 2 S 4 /Ti 3 C 2 T x samples presented similar diffraction peaks with ZnIn 2 S 4 . The missing Ti 3 C 2 T x diffraction peaks could be ascribed to the low content and high dispersion of Ti 3 C 2 T x in the composites. The morphology of the ZnIn 2 S 4 and ZnIn 2 S 4 /Ti 3 C 2 O x samples were investigated using scanning electron microscopy (SEM). The ZnIn 2 S 4 presented a morphology of nanoflowers stacked with nanosheets ( Figure S4). From the SEM image of ZnIn 2 S 4 /Ti 3 C 2 O x sample in Figure 3a, it can be seen that the ZnIn 2 S 4 particles are uniformly dispersed and anchored onto the Ti 3 C 2 O x surface. The more detailed microstructure of the ZnIn 2 S 4 /Ti 3 C 2 O x composite were further demonstrated using the transmission electron microscopy (TEM) technique. TEM observation confirmed such hierarchical ZnIn 2 S 4 /Ti 3 C 2 O x structure (Figure 2b). Furthermore, as shown in Figure 2c, the lattice distances of the ZnIn 2 S 4 /Ti 3 C 2 O x photocatalyst were measured, and the lattice fringes spacing of 0.32 and 0.41 nm were corresponded to the (102) and (006) planes of ZnIn 2 S 4 , while the lattice fringes spacing of 0.26 nm was assigned to the (0110) crystal plane of Ti 3 C 2 O x . Moreover, there was an obvious interface contact observed between the ZnIn 2 S 4 and the Ti 3 C 2 O x , which could contribute to the faster transfer of the photogenerated charge. In addition, the corresponding EDX elemental mapping (Figure 2d) displayed that the Zn, In, S, Ti and C elements were uniformly distributed in the ZnIn 2 S 4 /Ti 3 C 2 O x sample. The above results powerfully confirmed that the ZnIn 2 S 4 /Ti 3 C 2 O x photocatalyst was successful constructed. The optical properties of pristine ZnIn2S4 and ZnIn2S4/Ti3C2Fx (T = F, OH, O) composites were analyzed using the UV-vis diffuse reflectance spectra (UV-vis DRS). As shown in Figure 3a, the pristine ZnIn2S4 showed an absorption edge at 560 nm, while the absorption edge of the ZnIn2S4/Ti3C2Tx composites exhibited a slightly red shift with the intro- The optical properties of pristine ZnIn 2 S 4 and ZnIn 2 S 4 /Ti 3 C 2 F x (T = F, OH, O) composites were analyzed using the UV-vis diffuse reflectance spectra (UV-vis DRS). As shown in Figure 3a, the pristine ZnIn 2 S 4 showed an absorption edge at 560 nm, while the absorption edge of the ZnIn 2 S 4 /Ti 3 C 2 T x composites exhibited a slightly red shift with the introduction of Ti 3 C 2 T x . Moreover, compared to that of ZnIn 2 S 4 , the absorption intensities of the ZnIn 2 S 4 /Ti 3 C 2 T x composites increased in the whole visible light region, suggesting that the Ti 3 C 2 T x loading increased the visible light utilization efficiency of ZnIn 2 S 4 . In addition, the UV-vis DRS spectra of ZnIn 2 S 4 and ZnIn 2 S 4 /Ti 3 C 2 T x composites were converted into Tauc's band gap plots (Figure 3b), the band gaps of ZnIn 2 S 4 , ZnIn 2 S 4 /Ti 3 C 2 F x , ZnIn 2 S 4 /Ti 3 C 2 (OH) x and ZnIn 2 S 4 /Ti 3 C 2 O x were measured to be 2.64 eV, 2.60 eV, 2.59 eV and 2.63 eV, respectively.

Photocatalytic H 2 Evolution Activity
The photocatalytic H 2 evolution activity of the as-obtained pure ZnIn 2 S 4 and ZnIn 2 S 4 /Ti 3 C 2 T x composites were evaluated under visible light irradiation using triethanolamine (TEOA) as a sacrificial reagent. It was well known that Ti 3 C 2 T x were not semiconductors and they could not generate electrons and holes upon light irradiation [32]. Therefore, Ti 3 C 2 T x had no photocatalytic H 2 evolution activity. In Figure 4a, the pure ZnIn 2 S 4 exhibited the poor H 2 evolution rate of 253 µmol h −1 g −1 . Inspiringly, after loading the Ti 3 C 2 T x cocatalysts, the ZnIn 2 S 4 /Ti 3 C 2 T x composites all exhibited the improved photocatalytic H 2 evolution activity, and the order of photocatalytic activity was ZnIn 2 S 4 /Ti 3 C 2 O x > ZnIn 2 S 4 /Ti 3 C 2 F x > ZnIn 2 S 4 /Ti 3 C 2 (OH) x > ZnIn 2 S 4 . Furthermore, the photocatalytic H 2 evolution rate of the ZnIn 2 S 4 /Ti 3 C 2 O x composites strongly depended on the amount of Ti 3 C 2 O x . The ZnIn 2 S 4 /Ti 3 C 2 O x composite with 1.0 wt% Ti 3 C 2 O x achieved the optimal H 2 evolution rate of 363 µmol h −1 g −1 (Figure 4b). By further increasing the Ti 3 C 2 O x content, the H 2 evolution rate of the ZnIn 2 S 4 /Ti 3 C 2 O x composite decreased, which could be due to the excessive amount of Ti 3 C 2 O x covering the active sites and hindering the light absorption of ZnIn 2 S 4 [33]. The photocatalytic stability test of ZnIn 2 S 4 /Ti 3 C 2 O x for photocatalytic H 2 evolution was carried out for four consecutive cycles (Figure 4c). It can be seen that ZnIn 2 S 4 /Ti 3 C 2 O x maintained the photocatalytic H 2 evolution activity during the four consecutive cycles, indicating the excellent photostability of ZnIn 2 S 4 /Ti 3 C 2 O x .

The Mechanism of Enhanced Photocatalytic Activity
To shed light on the fundamental reasons for the enhanced photocatalytic performance of ZnIn2S4/Ti3C2Ox, fluorescence property and photoelectrochemical measurements were employed. It is well known that the transfer efficiency of photogenerated electrons and holes was an important influencing factor for the photocatalytic performance. The photoluminescence (PL) spectrum was employed to illustrate the transfer efficiency of the photogenerated electrons and holes. Figure 5a showed the PL spectra of the ZnIn2S4 and ZnIn2S4/Ti3C2Tx composites measured at 375 nm. The order of the PL signal intensity at 565 nm was ZnIn2S4 > ZnIn2S4/Ti3C2(OH)x > ZnIn2S4/Ti3C2Fx > ZnIn2S4/Ti3C2Ox. The loading of Ti3C2Tx lead to the decreased PL intensity of ZnIn2S4, and the ZnIn2S4/Ti3C2Ox composite

The Mechanism of Enhanced Photocatalytic Activity
To shed light on the fundamental reasons for the enhanced photocatalytic performance of ZnIn 2 S 4 /Ti 3 C 2 O x , fluorescence property and photoelectrochemical measurements were employed. It is well known that the transfer efficiency of photogenerated electrons and holes was an important influencing factor for the photocatalytic performance. The photoluminescence (PL) spectrum was employed to illustrate the transfer efficiency of the photogenerated electrons and holes. Figure 5a showed the PL spectra of the ZnIn 2 S 4 and ZnIn 2 S 4 /Ti 3 C 2 T x composites measured at 375 nm. The order of the PL signal intensity at 565 nm was ZnIn 2 S 4 > ZnIn 2 S 4 /Ti 3 C 2 (OH) x > ZnIn 2 S 4 /Ti 3 C 2 F x > ZnIn 2 S 4 /Ti 3 C 2 O x . The loading of Ti 3 C 2 T x lead to the decreased PL intensity of ZnIn 2 S 4 , and the ZnIn 2 S 4 /Ti 3 C 2 O x composite showed the lowest PL intensity, which indicated that the addition of the Ti 3 C 2 O x cocatalyst could effectively facilitate the transfer of the photogenerated electrons and hole on the ZnIn 2 S 4 photocatalyst. The time-resolved photoluminescence (TRPL) spectra (Figure 5b) further certified this result. The calculated average fluorescence lifetime (Ave. τ) of ZnIn 2 S 4 /Ti 3 C 2 O x (0.594 ns) was significantly longer than that of ZnIn 2 S 4 (0.167 ns), which demonstrated that the Ti 3 C 2 O x cocatalyst loading greatly reduced the recombination rate of the photogenerated electrons and holes on ZnIn 2 S 4 . In addition, electrochemical impedance spectroscopy (EIS) and transient photocurrent response analyses were carried out to further investigate the separation and transfer ability of the photogenerated charge carriers. The EIS Nyquist plots were shown in Figure 5c and the arc radius on the EIS Nyquist plot of ZnIn 2 S 4 /Ti 3 C 2 O x was the smallest among these four samples, which indicated its lowest resistance for the charge carriers on the ZnIn 2 S 4 /Ti 3 C 2 O x composite. This also confirmed that the Ti 3 C 2 O x cocatalyst enhanced the separation and transfer efficiency of the photogenerated electrons and holes of ZnIn 2 S. The transient photocurrent densities of the as-prepared samples were displayed in Figure 5d. Compared with that of the blank ZnIn 2 S 4 , the photocurrent densities of the ZnIn 2 S 4 /Ti 3 C 2 T x samples exhibited remarkable increases; in particular, ZnIn 2 S 4 /Ti 3 C 2 O x exhibited the highest photocurrent density, further confirming the excellent photogenerated carriers transfer and separation ability of ZnIn 2 S 4 /Ti 3 C 2 O x . All of these results proved that the ZnIn 2 S 4 /Ti 3 C 2 O x exhibited the fastest transfer and separation ability of photogenerated electrons and holes, further resulting in the excellent photocatalytic H 2 production performance. In terms of the band theory, electron transfer behavior is closely related to the work functions of ZnIn2S4 and Ti3C2Tx (Tx = F, OH, O). In order to determine the work functions (Φ) of the ZnIn2S4 andTi3C2Tx samples, the ultraviolet photoelectron spectroscopy (UPS) technique was employed, as shown in Figure 6. The incident photon energy (hν) was 21.22 eV. As for ZnIn2S4 (Figure 6a), the secondary electron cutoff energy (Ecutoff) was 9.32 eV and the Fermi energy (EFermi) was 25.92 eV. The work function of ZnIn2S4 was calculated to be 3.33 eV using the formula: Work function (WF) = hν + Ecutoff − EFermi. Similarly, the work functions for Ti3C2Fx, Ti3C2(OH)x and Ti3C2Ox were calculated to be 4.22 eV, 3.73 eV and In terms of the band theory, electron transfer behavior is closely related to the work functions of ZnIn 2 S 4 and Ti 3 C 2 T x (T x = F, OH, O). In order to determine the work functions (Φ) of the ZnIn 2 S 4 andTi 3 C 2 T x samples, the ultraviolet photoelectron spectroscopy (UPS) technique was employed, as shown in Figure 6. The incident photon energy (hν) was 21.22 eV. As for ZnIn 2 S 4 (Figure 6a), the secondary electron cutoff energy (E cutoff ) was 9.32 eV and the Fermi energy (E Fermi ) was 25.92 eV. The work function of ZnIn 2 S 4 was calculated to be 3.33 eV using the formula: Work function (WF) = hν + E cutoff − E Fermi . Similarly, the work functions for Ti 3 C 2 F x , Ti 3 C 2 (OH) x and Ti 3 C 2 O x were calculated to be 4.22 eV, 3.73 eV and 4.57 eV, respectively (Figure 6b-d). Obviously, the work functions of the Ti 3 C 2 T x samples were all higher than that of ZnIn 2 S 4 . Therefore, the photogenerated electrons could transfer from ZnIn 2 S 4 to Ti 3 C 2 T x . Meanwhile, the Schottky barrier could be formed at the ZnIn 2 S 4 /Ti 3 C 2 T x interface due to the difference in the work function and the band alignment between ZnIn 2 S 4 and Ti 3 C 2 T x , which could greatly accelerate the separation and transfer of the photogenerated electrons and holes [34]. The electrostatic potentials of Ti 3 C 2 F x , Ti 3 C 2 (OH) x and Ti 3 C 2 O x were obtained from a density functional theory (DFT), as shown in Figure S5. The order of work function values obtained from the DFT calculations was in accordance with that from the UPS characterization. Moreover, the difference in work function between ZnIn 2 S 4 and Ti 3 C 2 T x was associated with the photogenerated electrons' transfer ability [35,36]. The largest difference in the work function between ZnIn 2 S 4 and Ti 3 C 2 O x indicated that Ti 3 C 2 O x showed the strongest electron capture capability from ZnIn 2 S 4 in the ZnIn 2 S 4 /Ti 3 C 2 O x heterojunction, leading to the significantly high photocatalytic activity. Based on the aforementioned results, a probable photocatalytic mechanism fo ZnIn2S4/Ti3C2Ox was proposed (Figure 7). The conduction band potential of the paren ZnIn2S4 was estimated by the Mott-Schottky method ( Figure S6). Under visible light irra diation, the photogenerated electrons on the valence band (VB) of ZnIn2S4 were excited to the conduction band (CB). Because the work function of Ti3C2Ox was higher than that o ZnIn2S4, photogenerated electrons in the CB of ZnIn2S4 could quickly migrate to the sur face of Ti3C2Ox across the intimate interface, the Schottky junction formed between ZnIn2S and Ti3C2Ox could further prevent the recombination of photogenerated electrons and holes in the ZnIn2S4/Ti3C2Ox. Subsequently, the photogenerated electrons in ZnIn2S4/Ti3C2Ox were available to react with water to evaluate H2, while the holes on th VB of ZnIn2S4 are consumed by the sacrificial agent TEOA. Based on the aforementioned results, a probable photocatalytic mechanism for ZnIn 2 S 4 /Ti 3 C 2 O x was proposed (Figure 7). The conduction band potential of the parent ZnIn 2 S 4 was estimated by the Mott-Schottky method ( Figure S6). Under visible light irradiation, the photogenerated electrons on the valence band (VB) of ZnIn 2 S 4 were excited to the conduction band (CB). Because the work function of Ti 3 C 2 O x was higher than that of ZnIn 2 S 4 , photogenerated electrons in the CB of ZnIn 2 S 4 could quickly migrate to the surface of Ti 3 C 2 O x across the intimate interface, the Schottky junction formed between ZnIn 2 S 4 and Ti 3 C 2 O x could further prevent the recombination of photogenerated electrons and holes in the ZnIn 2 S 4 /Ti 3 C 2 O x . Subsequently, the photogenerated electrons in ZnIn 2 S 4 /Ti 3 C 2 O x were available to react with water to evaluate H 2 , while the holes on the VB of ZnIn 2 S 4 are consumed by the sacrificial agent TEOA. face of Ti3C2Ox across the intimate interface, the Schottky junction formed between and Ti3C2Ox could further prevent the recombination of photogenerated electro holes in the ZnIn2S4/Ti3C2Ox. Subsequently, the photogenerated electro ZnIn2S4/Ti3C2Ox were available to react with water to evaluate H2, while the holes VB of ZnIn2S4 are consumed by the sacrificial agent TEOA.

Conclusions
In summary, we have successfully designed and synthesized the surface fun ized Ti3C2(OH)x and Ti3C2Ox using the surface post-treatments of Ti3C2Fx; then Ti3C = F, OH, O) were employed as the substrate and cocatalysts for the in-situ gro ZnIn2S4 to obtain ZnIn2S4/Ti3C2Tx heterojunctions for photocatalytic H2 producti markably, the photocatalytic H2 production activity of ZnIn2S4/Ti3C2Tx was grea proved, compared to that of ZnIn2S4. Due to the differences in work function b

Conclusions
In summary, we have successfully designed and synthesized the surface functionalized Ti 3 C 2 (OH) x and Ti 3 C 2 O x using the surface post-treatments of Ti 3 C 2 F x ; then Ti 3 C 2 T x (T x = F, OH, O) were employed as the substrate and cocatalysts for the in-situ growth of ZnIn 2 S 4 to obtain ZnIn 2 S 4 /Ti 3 C 2 T x heterojunctions for photocatalytic H 2 production. Remarkably, the photocatalytic H 2 production activity of ZnIn 2 S 4 /Ti 3 C 2 T x was greatly improved, compared to that of ZnIn 2 S 4 . Due to the differences in work function between ZnIn 2 S 4 and Ti 3 C 2 T x , the formation of the Schottky junction could effectively accelerate the separation and migration of photogenerated electrons and holes, and thus boost the photocatalytic H 2 evolution activity. In particular, among Ti 3 C 2 T x (T x = F, OH, O), the work function of Ti 3 C 2 O x was the largest, and the Ti 3 C 2 O x showed the strongest electron capture ability from ZnIn 2 S 4 . Experimental characterization analyses also demonstrated the rapid separation and transfer of photogenerated electrons and holes of ZnIn 2 S 4 /Ti 3 C 2 O x . This work paves the way for photocatalytic applications of MXene-based photocatalysts by tuning their surface termination groups.

Conflicts of Interest:
The authors declare no conflict of interest.