Construction of 0D/2D Schottky heterojunctions of ZnO and Ti3C2 nanosheets with the enriched transfer of interfacial charges for photocatalytic hydrogen evolution

Citation: Irfan, M.; Ahmad, I.; Shukrullah, S.; Hussain, H.; Atif, M.; Legutko, S.; Petru, J.; Hatala, M.; Naz, M.Y.; Rahman, S. Construction of 0D/2D Schottky Heterojunctions of ZnO and Ti3C2Nanosheets with the Enriched Transfer of Interfacial Charges for Photocatalytic Hydrogen Evolution. Materials 2022,15, 4557. https://doi.org/10.3390/ma15134557 Academic Editor: It-Meng (Jim) Low Received: 3 May 2022 Accepted: 25 June 2022 Published: 28 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Construction of 0D/2D Schottky Heterojunctions of ZnO and Ti3C2Nanosheets with the Enriched Transfer of Interfacial Charges for Photocatalytic Hydrogen Evolution Muhammad Irfan 1, Irshad Ahmad 2, Shazia Shukrullah 2,*, Humaira Hussain 3, Muhammad Atif 4, Stanislaw Legutko 5, Jana Petru 6, Michal Hatala 7, Muhammad Yasin Naz 2,* and Saifur Rahman 1 1Electrical Engineering Department, College of Engineering, Najran University Saudi Arabia, Najran 61441, Saudi Arabia; [email protected] (M.I.); [email protected] (S.R.) 2Department of Physics, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan; [email protected] 3Department of Chemistry, University of Okara, Punjab 56300, Pakistan; [email protected] 4Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan; resear[email protected] 5Faculty of Mechanical Engineering, Poznan University of Technology, 3 Piotrowo Street, 60-965 Poznan, Poland; stanislaw[email protected] 6Department of Machining, Assembly and Engineering Metrology, Mechanical Engineering Faculty, VŠB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava, Czech Republic; [email protected] 7Faculty of Production Technologies with a Seat in Prešov, Technical University of Kosice, 1 Bayerova Street, 080 01 Prešov, Slovakia; [email protected] *Correspondence: [email protected] (S.S.); [email protected] (M.Y.N.) Abstract: The development of cost-effective co-catalysts of high photocatalytic activity and recyclability is still a challenge in the energy transformation domain. In this study, 0D/2D Schottky heterojunctions, consisting of 0D ZnO and 2D Ti 3 C 2 , were successfully synthesized by the electrostatic self-assembling of ZnO nanoparticles on Ti 3 C 2 nanosheets. In constructing these heterojunctions, Ti 3 C 2 nanosheets acted as a co-catalyst for enhancing the transfer of excitons and their separation to support the photocatalytic response of ZnO. The as-prepared ZnO/Ti 3 C 2 composites demonstrate an abbreviated charge transit channel, a huge interfacial contact area and the interfacial electrons’ transport potential. The extended optical response and large reactive area of the ZnO/Ti 3 C 2 composite promoted the formation of excitons and reactive sites on the photocatalyst’s surface. The ZnO/Ti 3 C 2 Schottky heterojunction showed significantly high photocatalytic activity for hydrogen production from a water–ethanol solution under the light illumination in the visible region. The hydrogen evolution overoptimized the ZnO/Ti 3 C 2 composition with 30 wt.% of Ti 3 C 2 , which was eight times higher than the pristine ZnO. These findings can be helpful in developing 0D/2D heterojunction systems for photocatalytic applications by utilizing Ti3C2as a low-cost co-catalyst. Keywords: ZnO; Ti3C2; 0D/2D heterojunction; hydrogen evolution; photocatalytic activity 1. Introduction The ever-worsening energy problem caused by the rapid depletion of non-renewable fossil fuels has prompted researchers to develop photocatalysts that convert unlimited sunlight straight into H 2 fuel via photocatalytic water splitting [ 1 , 2 ]. Usually, the process of photocatalytic H 2 evolution involves three processes, namely, (i) the formation of excitons after absorbing photons of energy exceeding the band gap energy of the catalyst, (ii) the separation and transportation of photoinduced e − /h + pairs towards the surface of the catalyst; and (iii) the contribution of charge carriers in the redox reaction at surface-active sites to produce H 2 [ 2 ]. The H 2 fuel, being pollution free, sustainable and renewable, has Materials 2022,15, 4557. https://doi.org/10.3390/ma15134557 https://www.mdpi.com/journal/materials Materials 2022,15, 4557 2 of 14 earned a widespread focus as a substitute for conventional fossil fuels due to its maximum energy gradient among all the available chemical fuels. The efficiency of the hydrogen evolution reaction via water splitting greatly depends on the optical response range, recombination rate of excitons and redox capacity of the considered photocatalyst [ 3 ]. The recent decade has experienced massive development in nanomaterials used to explore an efficient and industrial-scale outlet for photocatalytic H 2 evolution. Up to now, numerous strategies and pathways have been adopted to search for economical, stable and wide light-responsive candidates for H 2 evolution. Several materials such as titanium oxide (TiO 2 ) [ 4 ], cerium oxide (CeO 2 ) [ 5 ], zinc sulfide (ZnS) [ 6 ], graphitic carbon nitride [ 7 ], zinc oxide (ZnO) [ 8 ], cadmium sulfide (CdS) [ 9 ], etc., have been extensively explored for their performance and viability in this emerging field. Among semiconductors, ZnO reveals diversified morphologies and an appropriate optical band gap (~3.37 eV) to effectively harvest the sunlight [ 10 ]. Although ZnO has shown highly photocatalytic efficiency for different environment treatment and energy applications, the pristine ZnO possesses some unavoidable constraints such as a visible light absorption inability, the swift recombination of excitons, photo-corrosion after light exposure and an insufficient number of active sites, which consequently reduces its utilization for industrial-scale H2evolution [8–11]. To address the outlined issues, diverse approaches, including elemental doping [ 12 ], loading of the co-catalyst [ 13 ], integrating with other semiconductors [ 14 ], etc., have been dynamically explored. Despite noteworthy advancements during the last decades in the betterment of photocatalytic H 2 evolution efficiency via the nano-structuring of ZnO [ 15 ], the accurate development of a ZnO-modified composite outlet is still a high challenge. Moreover, the present photocatalytic H 2 generating ZnO-based candidates do not satisfy the targeted expectations owing to the technical barrier in simultaneously enhancing the photocatalytic performance and stability and reducing the high price affiliated with costly noble metal co-catalysts [ 16 ]. A fruitful strategy for addressing the issue is to integrate ZnO with a conductive noble metal co-catalyst such as Pt, Au, etc., to design strong and integrated hybrid photocatalytic frameworks with an inhibited recombination of excitons, rapid transmission of charge carriers and the availability of numerous catalytic sites to induce swift redox reactions to trigger the H 2 evolution process [ 17 ]. Despite obtaining much higher photocatalytic H 2 evolution performance due to the utilization of these noble metal co-catalysts, the overpriced cost and extensive scarcity greatly restrict their large-scale application. Therefore, the search for an inexpensive and noble metal-free co-catalyst is essential in order to promote the activity of a photocatalyst for hydrogen evolution. Among ultrathin MXenes, titanium carbide (Ti 3 C 2 ) has emerged as a hot photocatalytic material owing to its strong conductivity, broad light-harvesting ability, enriched surface hydrophilic groups and strong reactive capacity stemming from the disclosed terminal metal sites [ 18 ]. The aforesaid unique features of Ti 3 C 2 make it highly appropriate for designing high-performance Ti 3 C 2 -modified hybrid photocatalysts. Considering the promising characteristics of 0D and 2D nanomaterials, the synthesis of the 0D ZnO/2D Ti 3 C 2 composite system for obtaining large-scale photocatalytic activity is appealing and significantly predicted. Li et al. [ 19 ] performed a photocatalytic reduction of CO 2 over Ti 3 C 2 /ZnO composites. The revealed reduction efficiency of the composite photocatalyst was higher than the pristine ZnO due to the swift transfer of electrons towards the co-catalyst Ti 3 C 2 [ 19 ]. Similarly, the ZnO/Ti 3 C 2 composite has also been documented as improving the photocatalytic degradation of methylene blue [ 20 ]. However, the construction of 0D/2D ZnO/Ti 3 C 2 heterojunction systems for the photocatalytic conversion of water into hydrogen fuel has not been documented well in the published literature. Herein, we use 2D Ti 3 C 2 and 0D ZnO materials to design 0D/2D ZnO/Ti 3 C 2 composites by the electrostatic assembly route to overcome the limitations of ZnO. The electrostatic interaction between 0D ZnO and 2D Ti 3 C 2 can result in a strong contact. By the integration of 0D ZnO and 2D Ti 3 C 2 , it is not only the electron-hole recombination rate that can be reduced but also the light absorption and charge transport capacities can be greatly improved compared with those in pure ZnO, resulting in superior photocatalytic H 2 evolution results. Materials 2022,15, 4557 3 of 14 2. Experimental Section 2.1. Reagents Titanium aluminum carbide (Ti 3 AlC 2 , 99.0%) and hydrofluoric acid (HF, 99.9%) were obtained from Sigma-Aldrich, Saint Louis, MO, USA. Dimethyl sulfoxide (DMSO) was bought from Merck. ZnO and DI-water were purchased from Sigma-Aldrich. All chemical agents were used as they were received without performing additional purification procedures. 2.2. Preparation of Ti3C2 The HF etching process was used to prepare Ti 3 C 2 MXene as follows: 1 g of bulk Ti 3 AlC 2 powder was steadily added into 20 mL of concentrated hydrofluoric acid (HF, 40%) and placed in oil bath under consistent and vigorous stirring at 60 ◦ C for 48 h to etch the Al layer. The obtained residue was refined with filter paper, centrifuged to eliminate any supernatant and preserved bulk product after centrifugation was cleansed with DI-water successively until the neutral pH was reached. The as-obtained powder was heated at 60 ◦ C in a vacuum furnace for 12 h to obtain few-layered Ti 3 C 2 , which was subsequently redispersed in 20 mL of DMSO and placed under sitting overnight with N 2 protection. Afterwards, the suspension was centrifuged, cleaned several times with ethanol and DI-water wiped out any remaining DMSO. Subsequently, 0.5 g of collected dry powder was once again re-dispersed in 50 mL of DI-water followed by ultrasonication under N 2 atmosphere preservation. Following ultrasonication for 60 min, the obtained suspension was centrifuged (3500 rpm, 60 min) to get rid of unexfoliated species. Finally, the black powder of ultrathin Ti3C2was obtained. This powder was calcined for 4 h at 700 ◦C. 2.3. Preparation of ZnO/Ti3C2Composites ZnO/Ti 3 C 2 composites (ZnO/TiC) were synthesized using an electrostatic self-assembly route. Firstly, 3 g of ZnO powder was added to 20 mL of DI-water under constant stirring for one hour, followed by sonication for 20 min to produce a uniform mixture, which was designated as solution A. Next, calculated amounts of Ti 3 C 2 were dissolved in 20 mL of DI-water with subsequent stirring for 30 min and ultrasonication for 40 min; the solution was labeled as solution B. Afterwards, both solutions were statically dissolved in each other, and the as-obtained suspension was stirred at 2500 rpm for 30 min. The prepared residue was centrifuged to wipe out dispersing species and cleansed with DI-water to obtain the powders, which were dried in an oven at 80 ◦ C for 12 h. Four different ZnO/TiC composites with varying mass content of Ti 3 C 2 (0.1%, 0.2%, 0.3% and 0.4%) were synthesized by following the same preparation method. The as-prepared composites were labeled as ZnO 0.9 /TiC 0.1 , ZnO 0.8 /TiC 0.2 , ZnO 0.3 /TiC 0.3 and ZnO 0.96 /TiC 0.4 , respectively, for characterization and photocatalytic activity. 2.4. Characterization The crystalline structures and phases of the as-prepared composites were recorded over XRD Bruker D8 diffractometer using CuK α radiation of wavelength 0.15046 nm with a scan rate of 2 ◦ per min in a 2 θ range of 5–80 ◦ and V: 40 kV, I: 100 mA. The morphological analysis of as-prepared composites was conducted through scanning electron microscopy (Hitachi S4800, Hitachi, Tokyo, Japan). The optical absorption spectra were recorded using UV-Visible diffuse reflectance spectroscopy (UV-Vis DRS, Perkin Elmer Lambda 950, Waltham, MA, USA) in the range of 200–800 nm with reference calibration in accordance with BaSO 4 . The room temperature photoluminescence spectra were produced with a fluorescence spectrophotometer (Hitachi, F-7000, Hitachi, Tokyo, Japan) in the range of 340–460 nm over an exciton wavelength of 320 nm. BET surface area was measured using BJH modal, NOVA 2200e. The electrochemical impedance spectroscopy was performed with a CH1760E analyzer (frequency: 10 Hz to 1 MHz, light source: 300 Xe lamp with wavelength > 400, the intensity of light: 40 mW · cm −2 , CH Instruments, Inc., Austin, TX, USA) to determine the charge separation capacity of charge carriers in as-prepared samples. The Mott–Schottky analysis was performed to find out the flat band potentials. The tests Materials 2022,15, 4557 4 of 14 were conducted using a conventional three-electrode system where photocatalyst was used to make a working electrode, a platinum foil was used as a counter electrode and Ag/AgCl, immersed in saturated KCl, worked as a reference electrode. Linear sweep voltammetry (LSV) tests were also measured in the same configuration using 1 M KOH as electrolyte. The EIS and Mott–Schottky analyses were carried out in 0.1 M Na 2 SO 2 solution as electrolyte. The FTO glass, immersed in the photocatalyst, was taken as a working electrode, as reported in our previous study [21]. 2.5. Photocatalytic Activity The hydrogen evolution experiments were carried out in a Pyrex reaction with a vessel volume of 100 mL and equipped with a water-cooling system to maintain the temperature of the reaction at 15 ◦ C. Typically, 10 mg of the as-prepared photocatalyst was mixed in 50 mL of DI-water (0.2 gL −1 ) under continuous stirring at 7000 rpm to achieve the homogenous mixture and afterward sonicated for 20 min. Then, the solution was added with 20 vol% of ethanol as holes scavenger. Before illumination to trigger each photocatalytic reaction, the reaction system was fully vacuumed with subsequent bubbling of N 2 gas for half an hour to completely expel the oxygen gas from the solution. Afterwards, the visible lightdriven photocatalytic reaction was commenced under the illumination of 300 W Xe lamp fitted with 400 nm UV cut-off filter and placed 12 cm away from the reactor system. The hydrogen production was quantified using a multi-gas analyzer. The stability experiments were conducted using as-prepared ZnO 0.7 /TiC 0.3 composite and ethanol was added before and after third cycle. 3. Results and Discussion 3.1. XRD Structural Analysis Figure 1a displays the XRD patterns of Ti 3 AlC 2 and Ti 3 C 2 in the 2 θ range of 5–60 ◦ . After the HF etching process, the obvious shift of the (002) and (004) diffraction planes from 2 θ = 9.54 ◦ and 18.8 ◦ to 2 θ = 8.94 ◦ and 17.9 ◦ , respectively, and the disappearance of the (104) diffraction planes of Ti 3 AlC 2 at 2 θ = 38.85 ◦ authenticated the successful transformation of Ti 3 AlC 2 to Ti 3 C 2 [ 22 ]. Figure 1b shows that the main diffraction planes (100), (002) and (101) of pristine ZnO are observed at 2 θ values of 31.69, 34.44 and 36.34 ◦ , respectively, along with other observed peaks at higher 2 θ ranges. These planes show the hexagonal wurtzite phase of ZnO, as confirmed from JCPDS 36-1451 [ 23 – 25 ]. The XRD profiles of the ZnO/Ti 3 C 2 composites possess similar XRD patterns to ZnO, with one additional (002) diffraction peak of Ti 3 C 2 . The absence of the remaining diffraction peaks of Ti 3 C 2 in the XRD patterns of the ZnO/Ti 3 C 2 composites may be because of a too low intensity of the diffraction peaks of Ti 3 C 2 compared to pristine ZnO. Moreover, the diffraction peak intensity of the ZnO/Ti 3 C 2 composites steadily reduced with the increasing content of Ti 3 C 2 in contrast to the pristine ZnO sample, identifying that the increasing content of Ti 3 C 2 effectively suppressed the growth of ZnO. These observations strongly confirmed the successful formation of the ZnO/Ti3C2composites with varying contents of Ti3C2. 3.2. SEM Analysis The SEM micrograph of pristine ZnO nanoparticles is shown in Figure 2a, while the SEM micrograph of Ti 3 C 2 with a conventional 2D-layered structure is shown in Figure 2b. The ultrasonic treatment and calcination destroyed the typical accordion-like morphology of Ti 3 C 2 , which was then modified into different stacked layers with a bed sheets-like morphology, and identified the intuitive fabrication of Ti 3 C 2 , consistent with the XRD results. Moreover, 0D ZnO nanoparticles were observed to be distributed over 2D Ti 3 C 2 , as identified by the SEM micrograph of the ZnO 0.7 /TiC 0.3 composite, as shown in Figure 2c. The SEM analysis confirmed the integration of the ZnO nanoparticles into Ti 3 C 2 , which provides the rapid separation and transfer of charge carriers. The integration of ZnO and Ti 3 C 2 results in a high aggregation of ZnO nanoparticles, which, consequently, will provide more active sites. Materials 2022,15, 4557 5 of 14 Materials 2022, 15, x FOR PEER REVIEW 5 of 14 Figure 1. XRD spectra of (a) Ti3AlC3 (red) and Ti3C2 (green); (b) (1) ZnO, (2) ZnO0.9/TiC0.1, (3) ZnO0.8/TiC0.2, (4) ZnO0.7/TiC0.3 and (5) ZnO0.6/TiC0.4 composites. 3.2. SEM Analysis The SEM micrograph of pristine ZnO nanoparticles is shown in Figure 2a, while the SEM micrograph of Ti3C2 with a conventional 2D-layered structure is shown in Figure 2b. The ultrasonic treatment and calcination destroyed the typical accordion-like morphology of Ti3C2, which was then modified into different stacked layers with a bed sheets-like morphology, and identified the intuitive fabrication of Ti3C2, consistent with the XRD results. Moreover, 0D ZnO nanoparticles were observed to be distributed over 2D Ti3C2, as identified by the SEM micrograph of the ZnO0.7/TiC0.3 composite, as shown in Figure 2c. The SEM analysis confirmed the integration of the ZnO nanoparticles into Ti3C2, which provides the rapid separation and transfer of charge carriers. The integration of ZnO and Ti3C2 results in a high aggregation of ZnO nanoparticles, which, consequently, will provide more active sites. Figure 1. XRD spectra of ( a ) Ti 3 AlC 3 (red) and Ti 3 C 2 (green); ( b ) (1) ZnO, (2) ZnO 0.9 /TiC 0.1 , (3) ZnO0.8/TiC0.2, (4) ZnO0.7/TiC0.3 and (5) ZnO0.6/TiC0.4 composites. Materials 2022, 15, x FOR PEER REVIEW 5 of 14 Figure 1. XRD spectra of (a) Ti3AlC3 (red) and Ti3C2 (green); (b) (1) ZnO, (2) ZnO0.9/TiC0.1, (3) ZnO0.8/TiC0.2, (4) ZnO0.7/TiC0.3 and (5) ZnO0.6/TiC0.4 composites. 3.2. SEM Analysis The SEM micrograph of pristine ZnO nanoparticles is shown in Figure 2a, while the SEM micrograph of Ti3C2 with a conventional 2D-layered structure is shown in Figure 2b. The ultrasonic treatment and calcination destroyed the typical accordion-like morphology of Ti3C2, which was then modified into different stacked layers with a bed sheets-like morphology, and identified the intuitive fabrication of Ti3C2, consistent with the XRD results. Moreover, 0D ZnO nanoparticles were observed to be distributed over 2D Ti3C2, as identified by the SEM micrograph of the ZnO0.7/TiC0.3 composite, as shown in Figure 2c. The SEM analysis confirmed the integration of the ZnO nanoparticles into Ti3C2, which provides the rapid separation and transfer of charge carriers. The integration of ZnO and Ti3C2 results in a high aggregation of ZnO nanoparticles, which, consequently, will provide more active sites. Figure 2. SEM micrographs of ( a ) ZnO nanoparticles, ( b ) Ti 3 C 2 with conventional 2D-layered structure and (c) ZnO/Ti3C2composite. 3.3. Optical Absorption The optical absorption spectra of the as-prepared ZnO, ZnO 0.9 /TiC 0.1 , ZnO 0.8 /TiC 0.2 , ZnO 0.7 /TiC 0.3 and ZnO 0.6 /TiC 0.4 composites are displayed in Figure 3a. It is evident that pristine ZnO showed only UV absorption with a cut-off wavelength of 375 nm, consistent with the reported literature [ 25 – 29 ]. With the integration of Ti 3 C 2 , the optical response of ZnO was significantly improved towards the visible region. In contrast to the optical response of pristine ZnO, the ZnO 0.9 /TiC 0.1 composite revealed high absorption intensity Materials 2022,15, 4557 6 of 14 in the UV light spectrum as well as a red shift in the absorption response, identifying that the integration of ZnO and Ti 3 C 2 significantly improved the photon harvesting capacity, which could be assigned to the black color and metallic nature of Ti 3 C 2 [ 29 ]. The absorption spectra of the ZnO0.8/TiC0.2 and ZnO0.7/TiC0.3 composites also demonstrated the steady enhancement in the optical absorption with an obvious red shift towards the visible zone. Moreover, the absorption edge of the ZnO 0.6 /TiC 0.4 composite showed a blue shift compared to the ZnO 0.7 /TiC 0.3 composite because of the synergism between ZnO and Ti3C2[30]. Materials 2022, 15, x FOR PEER REVIEW 6 of 14 Figure 2. SEM micrographs of (a) ZnO nanoparticles, (b) Ti3C2 with conventional 2D-layered structure and (c) ZnO/Ti3C2 composite. 3.3. Optical Absorption The optical absorption spectra of the as-prepared ZnO, ZnO0.9/TiC0.1, ZnO0.8/TiC0.2, ZnO0.7/TiC0.3 and ZnO0.6/TiC0.4 composites are displayed in Figure 3a. It is evident that pristine ZnO showed only UV absorption with a cut-off wavelength of 375 nm, consistent with the reported literature [25–29]. With the integration of Ti3C2, the optical response of ZnO was significantly improved towards the visible region. In contrast to the optical response of pristine ZnO, the ZnO0.9/TiC0.1 composite revealed high absorption intensity in the UV light spectrum as well as a red shift in the absorption response, identifying that the integration of ZnO and Ti3C2 significantly improved the photon harvesting capacity, which could be assigned to the black color and metallic nature of Ti3C2 [29]. The absorption spectra of the ZnO0.8/TiC0.2 and ZnO0.7/TiC0.3 composites also demonstrated the steady enhancement in the optical absorption with an obvious red shift towards the visible zone. Moreover, the absorption edge of the ZnO0.6/TiC0.4 composite showed a blue shift compared to the ZnO0.7/TiC0.3 composite because of the synergism between ZnO and Ti3C2 [30]. Figure 3. (a) UV–Vis spectra of ZnO, Ti3C2 and ZnO/Ti3C2 composites; (b) Tauc plot of (α*hV)2 vs. energy (hV) of ZnO and ZnO0.7/TiC0.3 composite. The Tauc graphs: (α hV)1/n vs. hV (h: Planck constant, V: frequency, α =()  is the absorption coefficient and n: 1 for the direct band gap (ZnO) and 2 for the indirect band gap (Ti3C2), respectively) drawn from the UV–Vis absorption data were used for further examination of the as-prepared composites [31]. The linear fit analysis (Figure 3b) showed that all prepared composites possessed the direct band gaps and respective band gap values obtained from the extrapolation of the corresponding tangents of the energy and were found to be 3.31, 3.1, 3.02, 2.92 and 2.98 eV for the bare ZnO, ZnO0.9/TiC0.1, ZnO0.8/TiC0.2, ZnO0.7/TiC0.3 and ZnO0.6/TiC0.4 composites, respectively. 3.4. Spatial Charge Separation and Transfer Ability The transmission and capturing of excitons were further studied by PL fluorescence emission spectra, as reported in Figure 4a. The photocatalyst samples showed a typical ZnO UV emission peak centered around 390 nm due to the fast decay of excitons except in Ti3C2 [32]. The absence of the emission peak in the PL spectra of Ti3C2 identified its metallic nature. However, the integration of ZnO with Ti3C2 greatly lowered the Figure 3. ( a ) UV–Vis spectra of ZnO, Ti 3 C 2 and ZnO/Ti 3 C 2 composites; ( b ) Tauc plot of ( α *hV) 2 vs. energy (hV) of ZnO and ZnO0.7/TiC0.3 composite. The Tauc graphs: ( α hV) 1/n vs. hV (h: Planck constant, V: frequency, α=(1−R)2 2R is the absorption coefficient and n: 1 for the direct band gap (ZnO) and 2 for the indirect band gap (Ti 3 C 2 ), respectively) drawn from the UV–Vis absorption data were used for further examination of the as-prepared composites [ 31 ]. The linear fit analysis (Figure 3b) showed that all prepared composites possessed the direct band gaps and respective band gap values obtained from the extrapolation of the corresponding tangents of the energy and were found to be 3.31, 3.1, 3.02, 2.92 and 2.98 eV for the bare ZnO, ZnO 0.9 /TiC 0.1 , ZnO0.8/TiC0.2, ZnO0.7/TiC0.3 and ZnO0.6/TiC0.4 composites, respectively. 3.4. Spatial Charge Separation and Transfer Ability The transmission and capturing of excitons were further studied by PL fluorescence emission spectra, as reported in Figure 4a. The photocatalyst samples showed a typical ZnO UV emission peak centered around 390 nm due to the fast decay of excitons except in Ti 3 C 2 [ 32 ]. The absence of the emission peak in the PL spectra of Ti 3 C 2 identified its metallic nature. However, the integration of ZnO with Ti 3 C 2 greatly lowered the recombination rate of excitons, identifying the effective role of Ti 3 C 2 to trap the charge carriers. Both the ZnO 0.9 /TiC 0.1 composite and the ZnO 0.8 /TiC 0.2 composite exhibited identical PL profiles with red-shifted peaks, identifying the enhanced absorption towards the visible region of the spectrum, in good agreement with the absorption spectra results. Moreover, the ZnO 0.7 /TiC 0.3 composite demonstrated the lowest emission intensity, which means the lowest recombination of excitons in contrast to other catalysts. Therefore, the PL spectra identified the importance of the construction of heterojunction for suppressing the recombination of excitons and extending the optical response. Materials 2022,15, 4557 7 of 14 Materials 2022, 15, x FOR PEER REVIEW 7 of 14 recombination rate of excitons, identifying the effective role of Ti3C2 to trap the charge carriers. Both the ZnO0.9/TiC0.1 composite and the ZnO0.8/TiC0.2 composite exhibited identical PL profiles with red-shifted peaks, identifying the enhanced absorption towards the visible region of the spectrum, in good agreement with the absorption spectra results. Moreover, the ZnO0.7/TiC0.3 composite demonstrated the lowest emission intensity, which means the lowest recombination of excitons in contrast to other catalysts. Therefore, the PL spectra identified the importance of the construction of heterojunction for suppressing the recombination of excitons and extending the optical response. Figure 4. (a) PL spectra of ZnO, Ti3C2 and ZnO/Ti3C2 composites; (b) EIS Nyquist plots of ZnO, ZnO0.9/TiC0.1, ZnO0.7/TiC0.3 and ZnO0.6/TiC0.4; (c) CV curves of ZnO, ZnO0.7/TiC0.3 and ZnO0.6/TiC0.4. To deeply analyze the separation and migration of charge carriers, different characterization such as EIS and CV were also conducted. EIS Nyquist analysis was carried out to further study the separation ad mobility of charge carriers over the as-prepared composites and the results are displayed in Figure 4b. It is evident that the diameters of the Nyquist plots corresponding to the ZnO/Ti3C2 composites were smaller than bare ZnO, revealing that the integration of Ti3C2 with ZnO effectively improved the interfacial charge transfer speed due to the high conductivity of Ti3C2 [33]. Among all the prepared samples, the ZnO0.7/TiC0.3 composite witnessed the lowest arc radius to identify the highest interfacial separation and mobility of charges, consistent with the PL and UV-Vis results [34]. Figure 4. ( a ) PL spectra of ZnO, Ti 3 C 2 and ZnO/Ti 3 C 2 composites; ( b ) EIS Nyquist plots of ZnO, ZnO 0.9 /TiC 0.1 , ZnO 0.7 /TiC 0.3 and ZnO 0.6 /TiC 0.4 ; ( c ) CV curves of ZnO, ZnO 0.7 /TiC 0.3 and ZnO0.6/TiC0.4. To deeply analyze the separation and migration of charge carriers, different characterization such as EIS and CV were also conducted. EIS Nyquist analysis was carried out to further study the separation ad mobility of charge carriers over the as-prepared composites and the results are displayed in Figure 4b. It is evident that the diameters of the Nyquist plots corresponding to the ZnO/Ti 3 C 2 composites were smaller than bare ZnO, revealing that the integration of Ti3C2with ZnO effectively improved the interfacial charge transfer speed due to the high conductivity of Ti 3 C 2 [ 33 ]. Among all the prepared samples, the ZnO 0.7 /TiC 0.3 composite witnessed the lowest arc radius to identify the highest interfacial separation and mobility of charges, consistent with the PL and UV-Vis results [ 34 ]. The CV curves of ZnO, ZnO 0.7 /TiC 0.3 and ZnO 0.6 /TiC 0.4 composites were also used to further evaluate the charge migration efficiency, and the results are displayed in Figure 4c. Although all three electrodes exhibited identical redox peaks at the same scan rate and range, the value of the current density was greater for the ZnO/TiC composites in contrast to bare ZnO. The current density for the ZnO 0.7 /TiC 0.3 composite was obviously higher than ZnO 0.6 /TiC 0.4 , which again confirmed that the most favorable composite with the optimized content of Ti 3 C 2 was ZnO 0.7 /TiC 0.3 , which can accelerate the transmission of the charge carriers and thereby improve the photocatalytic efficiency towards H2evolution [35]. Materials 2022,15, 4557 8 of 14 3.5. Photocatalytic H2Evolution Activity and Stability Test The photocatalytic capacity of the as-prepared catalysts was evaluated for H 2 evolution under visible light illumination using ethanol as the sacrificial agent. Figure 5a clearly shows that except for bare Ti 3 C 2 , all other catalysts showed H 2 evolution from the water– ethanol mixture. For comparison, we also investigated the photocatalytic H 2 evolution activity of P25 and it showed a photocatalytic result of 0.01 µ mol/h/g, which is quite small when compared to other photocatalysts. The bare ZnO catalyst showed a negligible H 2 evolution rate of 0.03 µ mol/h/g, which identified that bare ZnO is inactivated under visible light illumination. In contrast, the construction of the ZnO/TiC composites effectively enhanced the photocatalytic H 2 evolution even when the introduction of Ti 3 C 2 was not high. The most optimum composite, ZnO 0.7 /TiC 0.3 with 30 wt% of Ti 3 C 2 , exhibited more than an eight-fold larger photocatalytic performance than bare ZnO (3146 vs. 386 µ mol/h/g). However, further enhancing the content of Ti 3 C 2 is unfavorable to the photocatalytic H 2 evolution activity of the ZnO/TiC composite, which reduces to 2388 µ mol/h/g in the case of the ZnO 0.6 /TiC 0.4 composite. The introduction of the suitable content of a metallic nature, Ti 3 C 2 , can promote the performance of charge transfer and separation; however, the number of available catalytic sites and the absorption capacity of ZnO reduces due to the excessive loading of Ti3C2, which consequently decelerates the photocatalytic activity [36,37]. The photocatalytic H 2 evolution activity of all the as-prepared catalysts was also evaluated in the absence of a sacrificial agent and the results are displayed in Figure 5b. The H 2 evolution performance of the catalysts first enhanced and then reduced with the enhancing quantity of T 3 C 2 , which was in line with the H 2 evolution results from the water–ethanol mixture. Moreover, the H 2 evolution results of all the prepared samples were much lower than using ethanol as the holes scavenger, which identified the increased productivity of the H2evolution results from the significant role of the sacrificial reagent. The stability of the as-prepared optimum ZnO 0/7 /TiC 0/3 catalyst was evaluated for cyclic H 2 evolution. The H 2 evolution remained almost unchanged, showing no observable loss in the yield even after 18 h (Figure 5d), which authenticated the fact that the ZnO 0/7 /TiC 0/3 catalyst was appropriate for photocatalytic H 2 evolution, and this opens up a new pathway to design and improve the performance of sunlight-driven photocatalysts. Figure 5e shows XRD patterns of the ZnO 0/7 /TiC 0/3 catalyst composite before and after the photocatalytic H 2 evolution reaction. It is evident that the XRD patterns did not show any obvious difference, which confirms the strong stability of the ZnO0/7/TiC0/3 catalyst. Materials 2022,15, 4557 9 of 14 Materials 2022, 15, x FOR PEER REVIEW 9 of 14 Figure 5. The photocatalytic H2 evolution activity of the ZnO and ZnO/Ti3C2 composites using (a,b) water–ethanol mixture; (c) pure water; (d) the photocatalytic H2 evolution stability tests of ZnO0.7/TiC0.3 composite; and (e) XRD patterns of ZnO0.7/TiC0.3 composite before and after reaction. Figure 5. The photocatalytic H 2 evolution activity of the ZnO and ZnO/Ti 3 C 2 composites using ( a , b ) water–ethanol mixture; ( c ) pure water; ( d ) the photocatalytic H 2 evolution stability tests of ZnO 0.7 /TiC 0.3 composite; and ( e ) XRD patterns of ZnO 0.7 /TiC 0.3 composite before and after reaction.