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Article

Synthesis of g-C3N4@ZnIn2S4 Heterostructures with Extremely High Photocatalytic Hydrogen Production and Reusability

by
Yu-Cheng Chang
1,*,
Yung-Chang Chiao
1 and
Chi-Jung Chang
2
1
Department of Materials Science and Engineering, Feng Chia University, Taichung 40724, Taiwan
2
Department of Chemical Engineering, Feng Chia University, Taichung 40724, Taiwan
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(8), 1187; https://doi.org/10.3390/catal13081187
Submission received: 4 July 2023 / Revised: 2 August 2023 / Accepted: 3 August 2023 / Published: 4 August 2023
(This article belongs to the Special Issue Photocatalysts for Hydrogen Evolution Reaction Based on ZnIn2S4 Materials)
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Abstract

:
The g-C3N4@ZnIn2S4 heterostructures were successfully synthesized through a combination of thermal annealing and hydrothermal methods. To enhance the photocatalytic hydrogen production performance and explore the interface between charge carriers, heterostructures of g-C3N4@ZnIn2S4 were fabricated using varying weights of g-C3N4 nanostructures under visible light irradiation. Remarkably, the photocatalytic hydrogen production efficiency of g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures was significantly improved, showing approximately 228.6 and 2.58 times higher than that of g-C3N4 nanostructures and ZnIn2S4 nanostructures, respectively. This enhancement in photocatalytic performance is attributed to the effective utilization of visible light and the efficient separation of photogenerated electron-hole pairs facilitated by the heterojunction structures. Moreover, the reusability test validated the outstanding performance of g-C3N4@ZnIn2S4 heterostructures, as they maintained high photocatalytic hydrogen production even after undergoing eight cycles without any noticeable decrease in efficiency. This study offers a promising strategy for designing and synthesizing an environmentally friendly g-C3N4@ZnIn2S4 heterojunction with potential applications in photocatalytic hydrogen evolution.

1. Introduction

Photocatalytic water splitting, a promising solution for addressing the limitations of fossil fuels, utilizes abundant and renewable energy sources [1]. This process harnesses solar energy to produce hydrogen in a clean and environmentally friendly manner, offering economic and ecological advantages. However, the efficient utilization of the solar spectrum is crucial, and developing cost-effective, stable, and efficient photocatalysts that can harness visible and UV light is a crucial challenge [2]. Factors such as crystallinity, particle size, band gap, and redox stability influence the performance of photocatalysts in water splitting [3,4]. Achieving efficient, scalable, and sustainable hydrogen production requires photocatalysts that meet specific criteria, including suitable band gaps, precise band alignment, stability, low production cost, recyclability, abundance, corrosion resistance, and suitability for large-scale production [5,6].
Graphite-like carbon nitride (g-C3N4) exhibits exceptional electric, optical, structural, and physiochemical properties, rendering it a versatile nano-platform for diverse electronic, catalytic, and energy applications [7,8,9]. Since its discovery in 2009, g-C3N4-based photocatalysts, particularly for generating H2 and O2, have garnered significant attention, leading to a surge in publications and citations [10,11]. These nanostructures and graphene-based photocatalysts are increasingly considered prime contenders for numerous energy and environmental endeavors, including photocatalytic water splitting, pollutant degradation, and carbon dioxide mitigation [12,13,14,15]. The insufficient photocatalytic activity of bulk g-C3N4, attributed to limited solar-light absorption, low electrical conductivity, and fast charge carrier recombination, poses challenges for practical applications [11,16,17,18]. Extensive research has been conducted to enhance its photoactivity through various modification approaches, such as doping, nanostructure design, and integration with different materials [19,20,21,22,23]. Promising outcomes have been achieved by constructing nanostructures through heterojunction formation or controlling morphology and incorporating g-C3N4 with semiconductors, carbon-based materials, or metal nanoparticles [24,25,26,27,28]. Moreover, achieving superior performance in g-C3N4 modification relies on precise control of interfacial contacts at heterojunctions and the customization of nanostructures with surface functionality [19,29].
Ternary semiconductor chalcogenides, such as ZnIn2S4, are gaining attention for their potential as visible light active photocatalysts due to their excellent chemical stability and optical band gap [30,31]. ZnIn2S4 stands out with its layered structure, where Zn and In atoms are arranged in distinct environments, leading to improved photocatalytic performance. In3+ metal ions with a d10 configuration further enhance the photocatalytic activity, making ZnIn2S4 a promising candidate for water splitting [30,32,33]. The photocatalytic activity of ZnIn2S4 nanosheets needs improvement due to the short lifetime and high recombination rate of electron-hole pairs and the agglomeration of nanosheets [34,35]. To address these issues, coupling ZnIn2S4 nanosheets with hollow nanostructures can alleviate agglomeration and promote charge separation [36,37]. One promising approach is the construction of a composite hollow heterojunction photocatalyst based on ZnIn2S4 nanosheets and g-C3N4, which has a favorable energy band structure and easy loading capability [37,38]. Previous studies have reported the successful synthesis of ZnIn2S4/g-C3N4 nanocomposites with enhanced photocatalytic performance for various essential applications [39,40,41,42]. Despite their remarkable efficiency in photocatalytic hydrogen production, no research has investigated the extended durability of g-C3N4@ZnIn2S4 heterostructures under low-wattage light sources. By exploring this aspect, we can optimize photocatalyst utilization and achieve significant energy conservation from the light source.
In this study, we successfully synthesized g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures and thoroughly investigated their photocatalytic performance for water splitting. The results demonstrate that the flower-like C3N4@ZnIn2S4 heterostructures exhibit significantly enhanced photocatalytic activity compared to g-C3N4 nanostructures. Furthermore, we elucidated the mechanism underlying the superior photocatalytic performance of C3N4@ZnIn2S4 heterostructures.

2. Results and Discussion

Figure 1 shows the synthesis processes of g-C3N4@ZnIn2S4 heterostructures prepared by combining thermal annealing and hydrothermal methods. First, g-C3N4 nanostructures were synthesized by thermal annealing at a high temperature of 550 °C for 3 h. Second, the different weights of g-C3N4 nanostructures were dispersed in the ZnIn2S4 reaction precursors (1 mM ZnCl2, 2.5 mM InCl3, and 5 mM TAA) and heated at 160 °C for 12 h by a facile hydrothermal method. In order to investigate the morphological features of the synthesized g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures, field-emission scanning electron microscopy (FESEM) was utilized and depicted in Figure 2. Figure 2a illustrates the stacked arrangement of g-C3N4 nanostructures composed of irregularly layered nanosheets. The FESEM image in Figure 2b depicts the flower-like microsphere structure of the g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. These microspheres exhibit a hierarchical architecture composed of numerous ultrathin nanosheets. A sheet-like structure enhances the availability of active surface sites, making it highly favorable for facilitating photocatalytic reactions [43].
Figure 3 displays the XRD patterns, providing insights into the composition and structure of the synthesized g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. Notably, the diffraction peak observed in the XRD pattern of g-C3N4 nanostructures (Figure 3a) is positioned at 27.3°, corresponding to the crystal plane (002) of g-C3N4 (JCPDS No. 87–1526). In the g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures (Figure 3b), the XRD analysis reveals distinct characteristic peaks corresponding to various crystal planes of the hexagonal phase ZnIn2S4 (JCPDS No. 72–0773). These peaks are observed at 2θ angles of 21.6°, 27.7°, 30.5°, 39.8°, 47.2°, 52.4°, and 55.6°, corresponding to the (006), (102), (104), (108), (110), (116), and (202) crystal planes, respectively. However, in the g-C3N4@ZnIn2S4 heterostructures, the diffraction peak of g-C3N4 at 2θ = 27.3° is either of weak intensity or obscured by other diffraction peaks. These characteristic peaks and the absence of impurity peaks indicate the successful synthesis of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures.
The N2 adsorption–desorption isotherms were utilized to investigate the specific surface areas of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. Figure 4a,b show that both g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures exhibit an H3 model hysteresis loop (P/P0 = 1) in their N2 adsorption–desorption isotherms, indicating the presence of mesoporous features in the synthesized materials, respectively. In addition, the Brunauer–Emmett–Teller (BET) of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures are 73.13 m2g−1 and 91.65 m2g−1, respectively. g-C3N4 nanostructures decorated with ZnIn2S4 nanostructures to form g-C3N4@ZnIn2S4 heterostructures can increase their specific surface area. The Fourier Transform Infrared spectroscopy (FTIR) spectra of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures are depicted in Figure 4c. In these FTIR spectra, a distinct peak at 811 cm−1 originates from the characteristic breathing vibration of the triazine ring in g-C3N4 [37,44]. Additionally, absorption bands within the 1200–1640 cm−1 range signify the stretching vibration modes of the C, N-heterocyclic groups present in g-C3N4 [45,46]. Weaker peaks appearing in the 3000–3600 cm−1 range can be attributed to the N–H characteristic vibration of -NHx and the O–H characteristic vibration of residual hydroxyl groups or absorbed water molecules [46,47]. These observations validate the presence of g-C3N4 nanostructures within the g-C3N4@ZnIn2S4 heterostructures, as evidenced by the FTIR spectrum.
Figure 5a displays the FETEM image of g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures, exhibiting a consistent morphology with the nanosheet structure observed in the FESEM image formed by lamellar stacks. Furthermore, Figure 5b illustrates the selected area electron diffraction (SAED) patterns, revealing distinct polycrystalline diffraction rings that substantiate the polycrystalline nature of the g-C3N4@ZnIn2S4 heterostructures. These diffraction rings correspond to crystal planes (006), (102), (104), (108), (110), (116), and (202), indicating the typical hexagonal structure of ZnIn2S4 (JCPDS card No. 72-0773), which aligns with the XRD analysis results and confirms the polycrystalline properties of the ZnIn2S4 nanosheets. In Figure 5c, the high-resolution transmission electron microscopy (HRTEM) image reveals the presence of lattice fringes exhibiting spacings of 0.226 nm and 0.193 nm, corresponding to the (108) and (110) planes of hexagonal ZnIn2S4, respectively. The amorphous regions observed in the image are attributed to g-C3N4. Furthermore, Figure 5d displays the mapping images obtained from energy dispersive spectroscopy (EDS) analysis, which demonstrate a uniform and interconnected distribution of nitrogen (N), zinc (Zn), indium (In), and sulfur (S) elements. These comprehensive findings provide conclusive evidence for successfully synthesizing g-C3N4@ZnIn2S4 heterostructures, showcasing the well-defined interface between the two materials.
X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical states of the synthesized samples, including g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. The survey spectra (Figure 6a) revealed the presence of primary elements, such as C, N, Zn, In, S, and O. The appearance of the O 1s signal suggests the adsorption of oxygen atoms [48]. The XPS spectrum (Figure 6b) illustrates the C 1s signal for g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures. Three distinct peaks are observed, corresponding to C-C bonds, sp3-hybridized C, and sp2-hybridized C, respectively [42]. The XPS spectra of N 1s for g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures are shown in Figure 6c. Three distinct peaks are observed in the XPS spectra, corresponding to the N-H group, tertiary N in the aromatic ring, and sp2-hybridized N in the triazine ring, respectively [49,50]. The high-resolution XPS spectrum (Figure 6d) of Zn 2p displays two distinct peaks at 1021.4 eV and 1044.4 eV, corresponding to Zn 2p3/2 and Zn 2p1/2, respectively [51]. These peaks indicate the presence of Zn2+ in the ZnIn2S4 structure. The XPS spectrum of In 3d (Figure 6e) reveals the presence of two distinct peaks at 444.6 and 452.1 eV, corresponding to the In 3d3/2 and In 3d5/2 states, respectively, indicating the presence of In3+ in ZnIn2S4 [51]. The high-resolution XPS spectrum (Figure 6f) of S 2p reveals the presence of two prominent peaks at 161.3 and 162.6 eV, which can be attributed to the S 2p3/2 and S 2p1/2 states in ZnIn2S4, respectively [52].
Figure 7a illustrates the assessment of the recombination ability of electrons and holes based on the photoluminescence intensity of the g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. A higher intensity in the PL emission spectra indicates a diminished carrier separation ability. Remarkably, the PL spectrum of g-C3N4@ZnIn2S4 heterostructures exhibited lower emission intensity than that of g-C3N4 nanostructures, reducing the recombination frequency of electrons and holes within the heterostructures. A lower peak in the photoluminescence spectra suggests the potential for superior catalytic performance. Therefore, g-C3N4@ZnIn2S4 heterostructures may outperform g-C3N4 nanostructures regarding photocatalytic efficiency.
UV-visible diffuse reflectance spectroscopy (UV-vis DRS) was employed to investigate the optical properties of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. As depicted in Figure 7b, the g-C3N4@ZnIn2S4 heterostructures exhibit significantly enhanced light absorption capability compared to g-C3N4 nanostructures across the wavelength range of 300 nm to 800 nm. This broader light harvesting ability of g-C3N4@ZnIn2S4 heterostructures is advantageous for efficient solar utilization, facilitating photocatalytic hydrogen production.
The energy band gaps (Eg) were determined using the Kubelka–Munk method, which is expressed as follows [53]:
αhν = A(Eg − hν)n
In this equation, A, α, ν, Eg, and h represent constants, the absorption coefficient, the frequency of light, the band gap energy, and Planck’s constant, respectively. The parameter “n” denotes the characteristic of the semiconductor material, where it equals 1 for indirect bandgap semiconductors and 1/2 for direct bandgap semiconductors. Previous literature indicates that g-C3N4 possesses a direct band gap, thus setting the value of “n” as 1/2. The energy band gap value of g-C3N4 nanostructures was calculated at about 2.90 eV. The g-C3N4@ZnIn2S4 heterostructures exhibited a narrower band gap (2.48 eV), indicating the influence of incorporating ZnIn2S4 and g-C3N4.
Figure 8a depicts the results of electrochemical impedance spectroscopy (EIS) for g-C3N4 nanostructures, g-C3N4@ZnIn2S4 heterostructures with different weights of g-C3N4 nanostructures, and ZnIn2S4 nanostructures. A notable difference in the EIS Nyquist curves’ arc radii is that g-C3N4 nanostructures exhibit larger arc radii than g-C3N4@ZnIn2S4 heterostructures. The g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures reveal the lowest arc radii. In addition, the charge transfer resistance values of g-C3N4 nanostructures, g-C3N4@ZnIn2S4 heterostructures with different weights of g-C3N4 nanostructures, and ZnIn2S4 nanostructures, as shown in Table S1. The g-C3N4@ZnIn2S4 heterostructure with 0.01 g g-C3N4 nanostructure exhibits the lowest charge transfer resistance values. This result indicates that g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures possess the lowest charge transfer resistance to enhance efficiency in separating charge carriers, facilitating the fastest electron transfer process. The reduced arc radius in g-C3N4@ZnIn2S4 heterostructures signifies improved charge transfer kinetics and highlights the potential for efficient photocatalytic performance [54,55].
In order to investigate the charge-transfer properties, transient photocurrent responses of g-C3N4 nanostructures, g-C3N4@ZnIn2S4 heterostructures with different weights of g-C3N4 nanostructures, and ZnIn2S4 nanostructures were conducted, as depicted in Figure 8b. Notably, the g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures exhibited significantly higher photocurrent density than g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with other weights, indicating a substantial improvement in the efficiency of separating photogenerated electrons and holes. The obtained result aligns with the earlier findings from PL and EIS measurements, further supporting the conclusion. Incorporating ZnIn2S4 onto g-C3N4 can promote the efficient transfer of electron-hole pairs and enhance the potential for photocatalytic hydrogen production.
The photocatalytic performance of the synthesized g-C3N4 nanostructures, g-C3N4@ZnIn2S4 heterostructures with different weights of g-C3N4 nanostructures, and ZnIn2S4 nanostructures was evaluated by measuring the hydrogen evolution rate (HER) in a 50 mL deionized (DI) water solution with 50% triethanolamine (TEOA) serving as a scavenger under visible light irradiation, as shown in Figure 9a. The pH value of the solution was not adjusted. The average HER values of as-prepared photocatalysts were 10.4 (g-C3N4 nanostructures), 2056.2 (0.005 g g-C3N4 nanostructures), 2377.6 (0.01 g g-C3N4 nanostructures), 1355.1 (0.025 g g-C3N4 nanostructures), 448.7 (0.05 g g-C3N4 nanostructures), and 921.2 μmolh−1g−1L−1 ZnIn2S4 nanostructures, respectively. The average HER of g-C3N4@ZnIn2S4 heterostructures gradually increased as the weight of g-C3N4 nanostructures increased. However, a notable decline in the average HER was observed when the weight of g-C3N4 nanostructures exceeded 0.01 g. This outcome could be attributed to the higher weights of g-C3N4 nanostructures, which might cause an excessive generation of g-C3N4 nanostructures and subsequently decrease the efficiency of electron-hole pair transfer, thereby inhibiting the overall photocatalytic hydrogen production efficiency. This result is consistent with the above EIS and photocurrent response measurements. In addition, g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures revealed almost 228.6 and 2.58 times higher than g-C3N4 nanostructures and ZnIn2S4 nanostructures, respectively.
In order to investigate the impact of sacrificial agents on the hydrogen production performance of the hybrid system, various sacrificial agents, including methanol, ethanol, ethylene glycol (EG), and TEOA, were employed in the g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. Figure 9b demonstrates the average HER of g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures in the presence of methanol, ethanol, ethylene glycol (EG), and TEOA solutions, which were recorded as 16.3, 96.5, 10.9, and 2377.6 μmolh−1g−1L−1, respectively. These results indicate that TEOA is a more suitable choice as a sacrificial agent for the g-C3N4@ZnIn2S4 heterojunction photocatalyst in hydrogen evolution. The observed outcome can be attributed to the effective binding of TEOA on the catalyst surface, which helps prevent photocorrosion and degradation of the g-C3N4 base photocatalysts [56,57,58]. TEOA efficiently scavenges photogenerated holes, enhances the dispersion of photocatalysts, and acts as a binding ligand to improve the interaction between g-C3N4 and water molecules [58].
Using concentrated sacrificial reagents is advantageous in promoting efficient diffusion of reacting species towards the surface of photocatalysts [59]. However, it is essential to consider that achieving the highest hydrogen evolution rate is impossible with diluted or highly concentrated sacrificial reagents due to their respective limitations [48]. Balancing the concentration of sacrificial reagents is crucial to optimize the performance of the photocatalytic system. Figure 9c illustrates the influence of TEOA concentrations on the photocatalytic efficiency of g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. The average HER values for the g-C3N4@ZnIn2S4 heterostructures were recorded as follows: 0 (without TEOA), 1152.3 (20% TEOA), 2377.6 (50% TEOA), and 671.5 μmolh−1g−1L−1 (100% TEOA). It was observed that g-C3N4@ZnIn2S4 heterostructures with appropriate TEOA concentrations displayed the highest HER under visible-light irradiation.
The efficient recycling of catalysts is a crucial consideration in photocatalysis [60]. In order to reduce waste and ensure sustainable processes, it is desirable to design photocatalysts that maintain consistent photoactivity throughout each cycle [61]. If photocatalysts gradually lose activity over time, they contribute to waste generation during their life cycle. Furthermore, it is essential to develop easily separable and recyclable photocatalysts to prevent the loss of valuable materials in the waste stream [62]. Herein, eight cycles of photocatalytic hydrogen generation were conducted to assess the stability of g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. The average HER of the g-C3N4@ZnIn2S4 heterostructures remained consistently high throughout the eight cycles (Figure 10a). Furthermore, the XRD pattern of the sample after eight cycles (Figure 10b) showed no new peaks. The FESEM (Figure 10c) image and FESEM-EDS mapping image (Figure 10d) of the sample after eight cycles still reveal a similar morphology and uniform element distribution. These results indicate the exceptional stability of the g-C3N4@ZnIn2S4 heterostructures. Comparative analysis of the photocatalytic hydrogen production activity of g-C3N4@ZnIn2S4 heterostructures with other reported photocatalysts strongly supports their favorable application prospects, as shown in Table S2 [25,63,64,65]. The promotion of stable performance can be attributed to the enhanced photoinduced charge separation achieved through efficient electron transfer from g-C3N4 to ZnIn2S4. These findings confirm the excellent stability and reusability of g-C3N4@ZnIn2S4 heterostructures and highlight their potential for broader and diverse applications in various fields.
Figure 11 presents the photocatalytic hydrogen production mechanism of g-C3N4@ZnIn2S4 heterostructures, as deduced from the results above. In this study, we employ ion exchange resin to coat the materials (g-C3N4 and ZnIn2S4) onto indium tin oxide (ITO) glass, followed by measuring the flat band potential using cyclic voltammetry [66,67]. The conduction band (CB) positions of g-C3N4 and ZnIn2S4 are −1.40 eV and −0.58 eV, respectively, while their valence band (VB) positions are 1.50 eV and 1.78 eV [68,69]. Under visible light irradiation (λmax = 420 nm), g-C3N4 and ZnIn2S4 materials undergo excitation, generating photogenerated electrons in the VB transitioning to the CB, consequently creating holes in the VB. The construction of the heterojunction facilitates the transfer of electrons from the conduction band (CB) of g-C3N4 to the lower surface of ZnIn2S4, while the photogenerated holes are transferred from the VB of ZnIn2S4 to the VB of g-C3N4. This formation of a type II heterojunction at the interface between g-C3N4 and ZnIn2S4 greatly enhances the separation efficiency of the photogenerated electron-hole pairs in g-C3N4. Additionally, due to the more negative conduction potential of ZnIn2S4 compared to the reduction potential of H+/H2, the photogenerated electrons accumulated in the CB of ZnIn2S4 undergo a reduction reaction, effectively reducing H+ in an aqueous solution to produce H2. Simultaneously, the presence of TEOA serves to consume the photogenerated holes. This synergistic effect significantly reduces the possibility of carrier recombination, resulting in a notable increase in the photocatalytic activity of g-C3N4@ ZnIn2S4 and promoting efficient hydrogen generation. This intricate photocatalytic process facilitates efficient charge carrier separation and promotes hydrogen production. Additionally, the g-C3N4@ZnIn2S4 heterostructures substantially enhance light-harvesting capacity, further boosting the efficiency of photocatalytic hydrogen production.

3. Material and Methods

3.1. Chemicals

Urea (CH4N2O, 99.3+%), thioacetamide (TAA, C2H5NS, 98%), indium (III) chloride anhydrous (InCl3, 98%), zinc chloride anhydrous (ZnCl2, 98%), and ethylene glycol (EG, C2H6O2, 99%), were purchased from Alfa Aesar without further purification. Methanol (CH3OH, 99.8%), ethanol (C2H5OH, 99.8%), and triethanolamine (TEOA, C6H15NO3, 99+%) were purchased from Sigma-Aldrich without further purification. Deionized (DI) water (>18 MΩ·cm) was used throughout the experimental processes.

3.2. Synthesis of g-C3N4

To synthesize g-C3N4, a facile thermal annealing method was employed using urea as the nitrogen-rich compound precursor. About 10 g of urea was loaded into a crucible and heated at 550 °C for 3 h [70]. After the process, the resulting g-C3N4 in a yellow color was collected as a solid and ground into a fine powder using a mortar, preparing it for further modification.

3.3. Synthesis of g-C3N4@ZnIn2S4 Heterostructures

The g-C3N4@ZnIn2S4 heterostructures were synthesized via a hydrothermal approach, following a modified procedure reported in the literature [65]. A ZnIn2S4 reaction solution containing 1 mM ZnCl2, 2.5 mM InCl3, and 5 mM TAA was dissolved in a 20 mL mix solvent (15 mL DI water and 5 mL ethanol). Then, different amounts (0.005, 0.01, 0.025, and 0.05 g) of g-C3N4 were uniformly dispersed in the reaction solution using an ultrasonic treatment for 15 min. The reaction solution was transferred to a Teflon-lined autoclave and heated at 160 °C for 12 h. These products were washed several times with DI water and collected by centrifugation. Finally, the products were dried at 75 °C for 2 h.

3.4. Characterization

The surface morphologies and microstructures of the samples were observed using field-emission scanning electron microscopy (FESEM, Hitachi S-4800, Kyoto, Japan) and transmission electron microscopy (FETEM, JEOL 2100 F, Kyoto, Japan). The X-ray diffraction (XRD) analysis was performed using a Bruker D2 phaser X-ray diffractometer (Massachusetts, USA) with Cu Ka radiation (40 kV, 20 mA). The ASAP 2020 Instrument (Micromeritics, Norcross, GA, USA) was utilized to conduct surface area measurements based on the Brunauer–Emmett–Teller (BET) method. The Fourier Transform Infrared (FTIR) spectra were measured using an FTIR spectrophotometer (DIGILAB FTX 3500, PerkinElmer, Waltham, MA, USA) with KBr disks at room temperature. In order to evaluate the light absorption characteristics of the photocatalysts, the diffuse reflectance spectra (DRS) were measured using a spectrometer (JASCO V-770, Tokyo, Japan). Photoluminescence (PL) spectra were measured using a LabRAM HR Evolution spectrophotometer with a 325 nm He-Cd laser. The as-prepared samples’ surface chemical composition and valence were detected using X-ray photoelectron spectroscopy (XPS, ULVAC-PHI Versa Probe 4, Chigasaki, Japan) with an Al Kα X-ray source. For electrochemical impedance spectroscopy (EIS) measurements, a three-electrode system was employed in conjunction with the Zennium electrochemical workstation (Zahhner, Kronach, Germany).

3.5. Photocatalytic Hydrogen Production Measurement

For the photocatalytic hydrogen production using the prepared photocatalysts, a multi-channel reaction system was employed, consisting of a 5 W blue LED light (λmax = 420 nm) as a visible light source powered by PCX50 B Discover with Perfect Light technology, while magnetic stirring was applied. In a typical experiment, 50 mg of the prepared photocatalysts were placed into a solution containing various sacrificial reagents (e.g., methanol, ethanol, EG, and TEOA) and 50 mL of DI water. Before the experiment, degassing pretreatment was performed for 30 min to remove air from the system. Subsequently, the hydrogen amounts were measured using gas chromatography (GC, Shimadzu GC-2014) equipped with a thermal conductivity detector (TCD).

4. Conclusions

A combination of thermal annealing and hydrothermal methods successfully synthesizes g-C3N4@ZnIn2S4 heterostructures. Incorporating ZnIn2S4 nanosheets introduces additional sites for H+ reduction and increases the number of active sites within the material. The synergistic effect between ZnIn2S4 nanosheets and g-C3N4 nanostructures plays a crucial role in modifying the electronic structure, narrowing the bandgap, facilitating efficient charge transfer, and suppressing the recombination of photogenerated electron-hole pairs. This combined effect significantly enhances the hydrogen evolution activity of the system. The g-C3N4@ZnIn2S4 heterostructures exhibit an optimal hydrogen evolution rate, reaching a significantly higher value of 2377.6 μmolh−1g−1L−1. This HER is approximately 228.6 times greater than that observed for g-C3N4 nanostructures. This enhanced performance can be attributed to the efficient utilization of visible light and the effective separation of photogenerated electron-hole pairs facilitated by the heterojunction structures. Furthermore, the reusability test confirms the exceptional performance of g-C3N4@ZnIn2S4 heterostructures, as they maintain high photocatalytic hydrogen production even after eight cycles without any noticeable decrease in efficiency. This study presents a promising approach for designing and synthesizing environmentally friendly g-C3N4@ZnIn2S4 heterojunctions for applications in photocatalytic hydrogen evolution.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13081187/s1, Table S1: The charge transfer resistance values of g-C3N4 nanostructures, g-C3N4@ZnIn2S4 heterostructures with different weights of g-C3N4 nanostructures, and ZnIn2S4 nanostructures. Table S2: List of photocatalytic hydrogen evolution for the g-C3N4@ZnIn2S4 heterostructures and other similar photocatalysts reported in the literature.

Author Contributions

Formal analysis, investigation, and data curation, Y.-C.C. (Yung-Chang Chiao); funding acquisition, methodology, project administration, resources, software, supervision, validation, writing—original draft, and writing—review and editing, Y.-C.C. (Yu-Cheng Chang); funding acquisition, resources, C.-J.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Technology, Taiwan (MOST 109-2221-E-035-041-MY3) and the National Science and Technology Council, Taiwan (NSTC 112-2221-E-035-017-MY3).

Data Availability Statement

Where no new data were created or where data are unavailable due to privacy or ethical restrictions.

Acknowledgments

The authors appreciate the Precision Instrument Support Center of Feng Chia University for providing the fabrication and measurement facilities.

Conflicts of Interest

The authors declare no competing financial interests.

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Figure 1. Schematic illustration for the synthesis of g-C3N4@ZnIn2S4 heterostructures.
Figure 1. Schematic illustration for the synthesis of g-C3N4@ZnIn2S4 heterostructures.
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Figure 2. FESEM images of (a) g-C3N4 nanostructures and (b) g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
Figure 2. FESEM images of (a) g-C3N4 nanostructures and (b) g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
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Figure 3. XRD pattern of (a) g-C3N4 nanostructures and (b) g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
Figure 3. XRD pattern of (a) g-C3N4 nanostructures and (b) g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
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Figure 4. N2 adsorption–desorption spectra of (a) g-C3N4 nanostructures and (b) g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. (c) FTIR spectra of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
Figure 4. N2 adsorption–desorption spectra of (a) g-C3N4 nanostructures and (b) g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures. (c) FTIR spectra of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
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Figure 5. (a) FETEM image, (b) SAED pattern, (c) HRTEM image, and (d) EDS-mapping images of g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures (N, Zn, In, S).
Figure 5. (a) FETEM image, (b) SAED pattern, (c) HRTEM image, and (d) EDS-mapping images of g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures (N, Zn, In, S).
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Figure 6. XPS (a) survey, (b) C 1s, (c) N 1s, (d) Zn 2p, (e) In 3d, and (f) S 2p spectra of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
Figure 6. XPS (a) survey, (b) C 1s, (c) N 1s, (d) Zn 2p, (e) In 3d, and (f) S 2p spectra of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
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Figure 7. (a) PL spectra, (b) UV-vis DRS spectra, and (c) Kubelka–Munk function vs. the energy of incident light plots of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
Figure 7. (a) PL spectra, (b) UV-vis DRS spectra, and (c) Kubelka–Munk function vs. the energy of incident light plots of g-C3N4 nanostructures and g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures.
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Figure 8. (a) EIS spectra and (b) photocurrent response of g-C3N4 nanostructures, g-C3N4@ZnIn2S4 heterostructures with different weights of g-C3N4 nanostructures, and ZnIn2S4 nanostructures.
Figure 8. (a) EIS spectra and (b) photocurrent response of g-C3N4 nanostructures, g-C3N4@ZnIn2S4 heterostructures with different weights of g-C3N4 nanostructures, and ZnIn2S4 nanostructures.
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Figure 9. (a) The average HER of g-C3N4 nanostructures, g-C3N4@ZnIn2S4 heterostructures with different weights of g-C3N4 nanostructures and ZnIn2S4 nanostructures. The average HER of g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures under the different (b) sacrificial agents and (c) TEOA concentrations.
Figure 9. (a) The average HER of g-C3N4 nanostructures, g-C3N4@ZnIn2S4 heterostructures with different weights of g-C3N4 nanostructures and ZnIn2S4 nanostructures. The average HER of g-C3N4@ZnIn2S4 heterostructures with 0.01 g g-C3N4 nanostructures under the different (b) sacrificial agents and (c) TEOA concentrations.
Catalysts 13 01187 g009
Catalysts 13 01187 g010
Figure 10. (a) Reusability test of g-C3N4@ZnIn2S4 heterostructures for eight cycles. (b) XRD spectrum, (c) FESEM image, and (d) EDS-mapping image of g-C3N4@ZnIn2S4 heterostructures after the eighth cycle.
Figure 10. (a) Reusability test of g-C3N4@ZnIn2S4 heterostructures for eight cycles. (b) XRD spectrum, (c) FESEM image, and (d) EDS-mapping image of g-C3N4@ZnIn2S4 heterostructures after the eighth cycle.
Catalysts 13 01187 g010
Catalysts 13 01187 g011
Figure 11. Schematic diagram of g-C3N4@ZnIn2S4 heterostructures for enhanced photocatalytic hydrogen production.
Figure 11. Schematic diagram of g-C3N4@ZnIn2S4 heterostructures for enhanced photocatalytic hydrogen production.
Catalysts 13 01187 g011
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Chang, Y.-C.; Chiao, Y.-C.; Chang, C.-J. Synthesis of g-C3N4@ZnIn2S4 Heterostructures with Extremely High Photocatalytic Hydrogen Production and Reusability. Catalysts 2023, 13, 1187. https://doi.org/10.3390/catal13081187

AMA Style

Chang Y-C, Chiao Y-C, Chang C-J. Synthesis of g-C3N4@ZnIn2S4 Heterostructures with Extremely High Photocatalytic Hydrogen Production and Reusability. Catalysts. 2023; 13(8):1187. https://doi.org/10.3390/catal13081187

Chicago/Turabian Style

Chang, Yu-Cheng, Yung-Chang Chiao, and Chi-Jung Chang. 2023. "Synthesis of g-C3N4@ZnIn2S4 Heterostructures with Extremely High Photocatalytic Hydrogen Production and Reusability" Catalysts 13, no. 8: 1187. https://doi.org/10.3390/catal13081187

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