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Article

Design and Preparation of ZnIn2S4/g-C3N4 Z-Scheme Heterojunction for Enhanced Photocatalytic CO2 Reduction

by
Jinghong Fang
,
Min Wang
,
Xiaotong Yang
,
Qiong Sun
* and
Liyan Yu
*
College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(1), 95; https://doi.org/10.3390/catal15010095
Submission received: 4 December 2024 / Revised: 30 December 2024 / Accepted: 16 January 2025 / Published: 20 January 2025
(This article belongs to the Special Issue Functional Nanomaterials in Catalysis and Sensing)
Browse Figures
Figure 1
<p>XRD patterns of g-C<sub>3</sub>N<sub>4</sub>, ZnIn<sub>2</sub>S<sub>4</sub> and 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub>.</p> "> Figure 2
<p>FESEM images of (<b>a</b>) ZnIn<sub>2</sub>S<sub>4</sub> and (<b>b</b>) 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub>, along with (<b>c</b>–<b>h</b>) the corresponding elemental mappings of Zn, In, S, C, and N in 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub>.</p> "> Figure 3
<p>XPS spectra of g-C<sub>3</sub>N<sub>4</sub>, ZnIn<sub>2</sub>S<sub>4</sub>, and 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub>: (<b>a</b>) survey spectra, (<b>b</b>) C 1s, (<b>c</b>) N 1s, (<b>d</b>) Zn 2p, (<b>e</b>) In 3d, and (<b>f</b>) S 2p spectra.</p> "> Figure 4
<p>(<b>a</b>) UV-vis absorption spectra, (<b>b</b>) Tauc plots, and (<b>c</b>) PL spectra of g-C<sub>3</sub>N<sub>4</sub>, ZnIn<sub>2</sub>S<sub>4</sub>, and 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub>.</p> "> Figure 5
<p>Mott–Schottky plots of (<b>a</b>) g-C<sub>3</sub>N<sub>4</sub>, (<b>b</b>) ZnIn<sub>2</sub>S<sub>4</sub>, and (<b>c</b>) 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub> measured at different frequencies.</p> "> Figure 6
<p>(<b>a</b>) Transient photocurrent response curves of g-C<sub>3</sub>N<sub>4</sub>, ZnIn<sub>2</sub>S<sub>4</sub>, and 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub> under light on/off cycles. (<b>b</b>) Nyquist plots of g-C<sub>3</sub>N<sub>4</sub>, ZnIn<sub>2</sub>S<sub>4</sub>, and 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub>.</p> "> Figure 7
<p>CO<sub>2</sub> reduction performance under various conditions: (1) photocatalytic CO<sub>2</sub> reduction using a 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub> composite under light irradiation; (2) reaction with light and CO<sub>2</sub> but without the catalyst; (3) reaction without light, but with the catalyst and CO<sub>2</sub>; (4) reaction with light and the catalyst, where CO<sub>2</sub> is replaced by Ar.</p> "> Figure 8
<p>(<b>a</b>) Photocatalytic performance of the catalyst with varying mass ratios of ZnIn<sub>2</sub>S<sub>4</sub> to g-C<sub>3</sub>N<sub>4</sub>; (<b>b</b>) time-dependent gas yield for the 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub>; (<b>c</b>) photocatalytic performance of the composite materials at different dosages; (<b>d</b>) results of the cyclic stability experiments.</p> "> Figure 9
<p>TEMPO spin-trapping EPR spectra of g-C<sub>3</sub>N<sub>4</sub> and 1:2 ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub>, showing photoinduced electron signals recorded in water.</p> "> Figure 10
<p>Proposed charge transfer mechanism for the efficient photocatalytic CO<sub>2</sub> reduction process facilitated by the ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub> heterojunction.</p> "> Scheme 1
<p>The preparation process of ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub>.</p> "> Scheme 2
<p>Detailed flow chart of the photocatalytic CO<sub>2</sub> reduction process.</p> ">
Versions Notes

Abstract

:
In this study, a novel Z-scheme heterojunction photocatalyst was developed by integrating g-C3N4 nanoplates into ZnIn2S4 microspheres. X-ray photoelectron spectroscopy analysis revealed a directional electron transfer from g-C3N4 to ZnIn2S4 upon heterojunction formation. Under irradiation, electrochemical tests and electron paramagnetic resonance spectroscopy demonstrated significantly enhanced charge generation and separation efficiencies in the ZnIn2S4/g-C3N4 composite, accompanied by reduced charge transfer resistance. In photocatalytic CO2 reduction, the ZnIn2S4/g-C3N4 composite achieved the highest CO yield, 1.92 and 5.83 times higher than those of pristine g-C3N4 and ZnIn2S4, respectively, with a notable CO selectivity of 91.3% compared to H2 (8.7%). The Z-scheme heterojunction mechanism, confirmed in this work, effectively preserved the strong redox capabilities of the photoinduced charge carriers, leading to superior photocatalytic performance and excellent long-term stability. This study offers valuable insights into the design and development of g-C3N4-based heterojunctions for efficient solar-driven CO2 reduction.

1. Introduction

With the rapid development of modern industry, the emission of CO2 and other greenhouse gasses has increased significantly, contributing to global warming and posing a severe threat to the survival of humans and other organisms [1,2]. Compared to conventional carbon dioxide capture strategies, such as physical adsorption and geological storage, which are often associated with high energy consumption, the conversion or reuse of CO2 offers a more sustainable and promising approach. This strategy not only addresses energy and environmental challenges, but also holds significant potential for practical and feasible applications [3,4,5].
Various technologies have been developed to convert CO2 into hydrocarbons or high-value-added chemicals, including thermal catalysis, biocatalysis, photoelectrocatalysis, electrocatalysis, and photocatalytic reduction [6,7]. Converting CO2 into CO and other useful low-carbon fuels in a renewable and environmentally friendly manner not only helps reduce atmospheric CO2 concentrations but also facilitates “closed-loop” carbon fixation. Among these methods, the photocatalytic reduction of CO2, which mimics the natural photosynthesis process, uses solar energy and photocatalysts (artificial photosynthesis) to catalytically convert CO2 and H2O into solar fuels and high-value chemicals, such as C1 products (CO, CH4, CH3OH, HCOOH) and C2 products (C2H4, C2H6, C3H6, C2H5OH) [8,9].
However, despite the demonstrated activity of various semiconductor photocatalysts for CO2 reduction, their practical applications are hindered by the slow separation and transport kinetics of photogenerated carriers and poor product selectivity. These limitations necessitate further advancements [10,11]. Among photocatalysts, graphite-phase carbon nitride (g-C3N4) has gained attention due to its visible light response (bandgap of about 2.7 eV), non-toxicity, good biocompatibility, excellent chemical and thermal stability, and ease of synthesis. These properties make g-C3N4 suitable for applications such as water splitting for hydrogen production, artificial photosynthesis, the degradation of organic pollutants, and CO2 reduction [12,13,14]. Like most photocatalysts, pure g-C3N4 synthesized via conventional calcination suffers from rapid electron–hole recombination and limited exposed active sites. These shortcomings can be addressed through nanostructure modification, heterojunction construction, or combination with cocatalysts. Among these strategies, constructing heterojunction structures has proven to be one of the most effective methods for enhancing photocatalytic performance due to its ability to facilitate efficient electron–hole pair separation and transfer [15,16]. For example, Luo et al. [17] prepared ultrathin CsPbBr3/g-C3N4 nanosheets using a simple electrostatic self-assembly process. Under AM 1.5 G light source irradiation, these nanosheets effectively reduced CO2 to CH4 and CO. In situ X-ray photoelectron spectroscopy (XPS) revealed a direct Z-scheme charge transfer mechanism at the CsPbBr3 and g-C3N4 interface, achieving efficient charge separation and high redox potential.
Zinc indium sulfide (ZnIn2S4, ZIS), a chemically stable and non-toxic material, possesses suitable energy band positions (2.06–2.85 eV) for the reduction of CO2 into hydrocarbon fuels. However, a high charge recombination rate and a short lifetime of photogenerated electrons caused by the bandgap limit its photocatalytic efficiency [18,19]. The formation of heterojunctions by combining ZnIn2S4 with other wide bandgap semiconductors has been widely recognized as an effective strategy to enhance photocatalytic performance [20,21,22].
Shao et al. [23] constructed a 3D/2D g-C3N4/ZnIn2S4 hollow spherical heterostructure by growing modified ZnIn2S4 nanosheets on the surface of g-C3N4 microspheres. This composite material combines the advantages of a hollow structure, including enhanced light absorption and increased active sites, with the heterostructure’s ability to improve photogenerated electron migration and separation. These features effectively address the significant issue of photogenerated electron recombination observed in the individual components. Similarly, Chen et al. [24] fabricated a spatial distribution heterojunction via the in situ growth of ZnIn2S4 nanosheets (ZIS) on g-C3N4 microtubes (T-CN), which demonstrated excellent performance in photocatalytic CO2 reduction. Their experimental findings revealed that the spatial distribution of ZIS nanosheets on T-CN enhances light absorption, accelerates interfacial charge transfer, and increases CO2 adsorption capacity, collectively contributing to the superior catalytic activity of the composite.
In this study, two-dimensional g-C3N4 nanoplates were integrated with ZnIn2S4 featuring a distinctive nanoflower structure using a simple mechanical stirring method. The photocatalytic activity and stability of the resulting ZnIn2S4/g-C3N4 composite were systematically evaluated in CO2 reduction reactions. The findings revealed that the ZnIn2S4/g-C3N4 heterojunction significantly outperformed pure g-C3N4 nanoplates and ZnIn2S4 nanoflowers in photocatalytic CO2 reduction, achieving an enhanced yield. Moreover, the composite exhibited high selectivity for CO over H2, underscoring its potential for efficient and targeted photocatalytic applications.

2. Results and Discussion

2.1. Characterization of the Samples

The crystal structures of the samples were analyzed using XRD patterns, as shown in Figure 1. Bare g-C3N4 exhibits two characteristic diffraction peaks at approximately 13.1° and 27.4°, corresponding to the (100) and (002) planes, respectively (JCPDS No.87-1526). These peaks represent the in-plane tri-s-triazine unit repetition and the interlayer stacking of aromatic ring structures. For ZnIn2S4 and the ZnIn2S4/g-C3N4 composite, five prominent diffraction peaks at 21.29°, 27.66°, 47.35°, 52.19°, and 55.57° are assigned to the (006), (102), (110), (116), and (202) planes of ZnIn2S4, respectively (JCPDS No.72-0305) [25]. In the composite, a weak diffraction peak corresponding to the g-C3N4 (100) plane is observed, while the (002) plane of g-C3N4 partially overlaps with the (102) plane of ZnIn2S4.
The microstructures of ZnIn2S4 and ZnIn2S4/g-C3N4 were examined through FESEM images. As shown in Figure 2a, ZnIn2S4 exhibits a distinct three-dimensional layered spherical nanoflower structure. In Figure 2b, the integration of ZnIn2S4 with g-C3N4 results in g-C3N4 nanosheets being inserted into the folds of the ZnIn2S4 structure, leading to a significantly rougher surface. The energy-dispersive spectroscopy (EDS) analysis (Figure 2c–h) confirms the presence of C, N, Zn, In, and S elements in the composite material. Moreover, the distribution maps reveal that the C and N elements from g-C3N4 are uniformly dispersed and surround the Zn, In, and S elements originating from ZnIn2S4.
XPS spectra were employed to investigate the structure and surface elemental composition of the different samples. In the full survey spectrum (Figure 3a), the elements C, N, Zn, In, and S are detected in the ZnIn2S4/g-C3N4 composites, which is consistent with the EDS results. In Figure 3b, the C1s peaks in both g-C3N4 and ZnIn2S4/g-C3N4 can be deconvoluted into two components: one at approximately 284.8 eV corresponding to graphite carbon (C-C), and another at 288.0 eV attributed to sp2-bonded carbon (N-C=N). Compared to pristine g-C3N4, the intensity of the N-C=N peak in ZnIn2S4/g-C3N4 is weaker, likely due to the shielding effect of ZnIn2S4 [24]. In Figure 3c, the N 1s spectra of g-C3N4 and ZnIn2S4/g-C3N4 exhibit three peaks at 398.4 eV, 399.9 eV, and 401.0 eV, which correspond to C-N=C, three-coordinate C-(N)3, and surface amino groups, respectively. A small peak at 404.1 eV is attributed to the π-excitation of the C-N heterocyclic ring. Figure 3d–f show the XPS spectra of the constituent elements in ZnIn2S4. Specifically, the Zn 2p peaks are observed at 1044.9 eV (Zn 2p1/2) and 1021.8 eV (Zn 2p3/2), the In 3d peaks appear at 451.9 eV (In 3d3/2) and 444.3 eV (In 3d5/2), and the S 2p peaks are located at 161.6 eV (S 2p1/2) and 160.5 eV (S 2p3/2) [26]. Furthermore, by comparing the binding energy shifts in key elements before and after the combination of ZnIn2S4 and g-C3N4, electron transfer can be inferred. The binding energies of C 1s and N 1s in ZnIn2S4/g-C3N4 both increase by approximately 0.1 eV compared to bare g-C3N4, suggesting a decrease in electron cloud density around the g-C3N4 in the composite. In the contrast, the binding energies of Zn 2p, In 3d, and S 2p decrease by 0.2–0.7 eV, indicating an increase in electron cloud density around the ZnIn2S4 component. These XPS results provide evidence for directional electron transfer from g-C3N4 to ZnIn2S4 upon excitation by external energy. A more detailed understanding of the electron transfer pathway will be further explored using in situ XPS, ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations in future studies [27,28].
The response ranges of different materials to incident irradiation were evaluated using UV-Vis absorption spectra. As shown in Figure 4a, the light absorption edges of pristine g-C3N4, ZnIn2S4, and the ZnIn2S4/g-C3N4 composite extend to approximately 430 nm, 530 nm, and 450 nm, respectively. The bandgap energy (Eg) was determined by plotting the relationship between (αhν)2 and photon energy (hν). Based on the absorption spectra shown in Figure 4b, the calculated bandgap energies of g-C3N4, ZnIn2S4, and ZnIn2S4/g-C3N4 are 2.86 eV, 2.34 eV, and 2.74 eV, respectively. These results demonstrate that the composite material can be excited by visible light. Photoluminescence (PL) emission, which is typically attributed to the recombination of free carriers, provides insight into the degree of electron–hole recombination. Figure 4c shows the PL spectra of the materials under 320 nm excitation. The primary emission peak of pristine g-C3N4 is observed at 453 nm, resulting from bandgap luminescence. ZnIn2S4 exhibited the weakest PL intensity among them, reflecting its inherently poor photoluminescence properties compared to g-C3N4. This can be attributed to the lower total amount of photogenerated carriers produced under identical irradiation conditions. These observations are consistent with findings reported in previous studies on ZnIn2S4, g-C3N4, and ZnIn2S4/g-C3N4 composites [24,29,30]. After forming the ZnIn2S4/g-C3N4 composite, the significant reduction in emission peak intensity indicates that the recombination of photogenerated carriers is effectively suppressed.
Electrochemical tests provide further insight into the energy band positions of the samples. Mott–Schottky (M-S) curves were employed to determine the semiconductor type and the conduction band (CB) potential. As shown in Figure 5, the positive slopes of the tangent lines drawn from the M-S curves of g-C3N4, ZnIn2S4, and ZnIn2S4/g-C3N4 confirm their n-type semiconductor characteristics. Moreover, for all three samples, the tangent lines at different test frequencies consistently intersect at the same point on the x-axis. This horizontal intercept corresponds to the flat band position (Efb) of the semiconductor, which is typically approximately equal to the CB position for n-type semiconductors. After correction for the reference electrode (Ag/AgCl, +0.199 eV), the CB positions of g-C3N4, ZnIn2S4, and ZnIn2S4/g-C3N4 were determined to be −0.52 eV, −0.69 eV, and −0.63 eV, respectively.
The transient photocurrent response under alternating light and dark conditions, as well as the impedance characteristics of the samples, were evaluated. As shown in Figure 6a, the ZnIn2S4/g-C3N4 composite exhibits the highest transient photocurrent response compared to the pristine samples, indicating improved charge separation efficiency under illumination. Additionally, EIS was used to compare the electron transfer resistance of the samples. Figure 6b shows that the Nyquist plot of ZnIn2S4/g-C3N4 has a significantly smaller semicircular arc diameter than those of the other two samples, indicating the lowest charge transfer resistance. This result highlights the superior photogenerated carrier transfer capability of the composite material.

2.2. Photocatalytic Reduction Activity of CO2

To validate the CO2 reduction process, several essential blank tests were conducted. As Shown in Figure 7, a comparison between Group 1 and the control groups (Groups 2 to 4) revealed no significant production of CO or H2 in the latter three groups. This indicates that no observable CO2 transformation occurred under those conditions. These results confirm that the production of CO and H2 in Group 1 is solely attributed to the photocatalytic reduction of CO2 facilitated by the 1:2 ZnIn2S4/g-C3N4 composite.
The photocatalytic properties of the g-C3N4, ZnIn2S4, and ZnIn2S4/g-C3N4 heterojunction were evaluated through the photocatalytic reduction of CO2, with the primary gas products (CO and H2) monitored over an eight-hour irradiation period. Figure 8a compares the photocatalytic performance of pristine g-C3N4, ZnIn2S4, and ZnIn2S4/g-C3N4 composites with various mass ratios. After 8 h of irradiation, all composite samples displayed significantly higher CO2 reduction product yields than pure g-C3N4 or ZnIn2S4, demonstrating the superior catalytic efficiency of the heterostructure. Compared to similar systems listed in Table 1, the ZnIn2S4/g-C3N4 heterojunction demonstrates exceptional high photocatalytic performance, further highlighting its superiority and effectiveness [24,31,32,33,34]. With increasing mass ratios of ZnIn2S4 to g-C3N4, the product yields initially increased, reaching a maximum at a 1:2 ratio, and then decreased. The best-performing composite achieved a high selectivity for CO (91.3%) over H2 (8.7%) and a maximum CO yield of 3743.14 μmol·g−1, which is 1.92 and 5.83 times higher than those of pristine g-C3N4 (1941.49 μmol·g−1) and ZnIn2S4 (641.68 μmol·g−1), respectively. The reduced efficiency at higher loading is likely due to material agglomeration and light shielding, which hinder photogenerated carrier separation and reaction progress.
As shown in Figure 8b, CO and H2 were continuously produced during the CO2 reduction process. Figure 8c explores the effect of photocatalyst loading on CO2 reduction. The yields of CO and H2 exhibit a volcano-shaped trend with increasing amounts of ZnIn2S4/g-C3N4 coated on the glass plate, reaching the maximum at a photocatalyst dosage of 10 mg.
For practical applications, the long-term photocatalytic stability of ZnIn2S4/g-C3N4 was assessed in cyclic experiments (Figure 8d). Over three cycles, the CO yield remained stable after each 8 h reaction period, demonstrating excellent durability of the ZnIn2S4/g-C3N4 composite. These results further confirm the stability and potential of the heterojunction for long-term photocatalytic CO2 reduction applications.

2.3. Photocatalytic Mechanism of ZnIn2S4/g-C3N4 Heterojunction

Electron paramagnetic resonance (EPR) spectroscopy was employed to identify the active radicals generated during the reaction. TEMPO, a spin-labeling agent for photoinduced electrons and holes, is reduced by electrons to form hydroxylamine (TEMPOH), resulting in a suppression of TEMPO’s EPR signals [35]. As shown in Figure 9, under dark conditions, the sample dispersed in water with soluble TEMPO exhibits a characteristic triple-splitting peak with an intensity ratio of 1:1:1. Upon 30 s of xenon light irradiation, the peak intensities in both the g-C3N4 and ZnIn2S4/g-C3N4 samples significantly decrease, indicating the generation of photoinduced electrons. Notably, the reduction in peak intensity is much greater for ZnIn2S4/g-C3N4 than for bare g-C3N4. This result provides compelling evidence of the enhanced production of photoinduced electrons in ZnIn2S4/g-C3N4, correlating with its improved photocatalytic performance in CO2 reduction.
Based on the experimental results, a possible mechanism for the photocatalytic reduction of CO2 by ZnIn2S4/g-C3N4 is proposed and illustrated in Figure 10. Under xenon light irradiation, electrons (e−) in the valence band (VB) of both ZnIn2S4 and g-C3N4 are excited to their respective conduction band (CB), leaving behind positive holes (h+) in the VB. Considering the CB and VB energy levels of these two semiconductors, two potential electron transfer pathways can occur between ZnIn2S4 and g-C3N4: the traditional Type-II heterojunction or the direct Z-scheme heterojunction. In the Type-II mechanism, excited electrons tend to transfer to the CB with the more negative potential, while holes move to the VB with the more positive potential. Although this pathway enhances the separation of charge carriers, it reduces the oxidation and reduction capabilities of the photocatalyst. Conversely, in the direct Z-scheme mechanism, electrons in the CB of one semiconductor recombine directly with holes in the VB of the other, preserving the strong reduction and oxidation abilities of the remaining charge carriers [36].
Evidence from XPS spectra (Figure 3) confirms electron transfer from g-C3N4 to ZnIn2S4, indicating the formation of a Z-scheme heterojunction in the ZnIn2S4/g-C3N4. Many studies [37,38,39,40] have shown that the Z-scheme configuration improves photoinduced charge generation and reduces charge transfer resistance, as corroborated by the electrochemical and EPR results in this work. Additionally, the composite exhibits a significantly suppressed recombination of photogenerated electron–hole pairs compared to its single components. In summary, the effective separation and transfer of electron–hole pairs in the Z-scheme heterojunction significantly enhance the photocatalytic CO2 reduction performance of ZnIn2S4/g-C3N4.

3. Experimental

3.1. Materials

All reagents were used as received without further purification. Urea (CH4N2O, 99%), zinc sulfate heptahydrate (ZnSO4·7H2O, 99.5%), ethanol (C2H5OH, 99.7%), and thioacetamide (C2H5NS, TAA, 99%) were purchased from Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China). Indium chloride (InCl3, 99.99%) was obtained from Shanghai Aladdin Co., Ltd. (Shanghai, China). Carbon dioxide (CO2, 99.999%) was supplied by Qingdao Deyi Gas Co., Ltd. (Qingdao, China). 2,2,6,6-tetramethylpiperidine nitrogen oxide (C9H18NO, TEMPO) was purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd. (Shanghai, China). Double-distilled deionized water was used throughout this study.

3.2. Preparation of g-C3N4

g-C3N4 was synthesized via a conventional calcination method [41]. Specifically, 10.0 g of urea was placed in a crucible and calcined at 550 °C for 2 h in an air atmosphere at a heating rate of 5 °C/min. After cooling to room temperature, a light-yellow powder (g-C3N4) was collected.

3.3. Preparation of Flower-like ZnIn2S4

ZnIn2S4 was prepared using an improved hydrothermal method based on previous reports [42]. Briefly, 1 mmol of ZnSO4·7H2O, 2 mmol of InCl3 and 2 mmol of TAA were dissolved sequentially in 60 mL of deionized water and stirred magnetically for 4.5 h. The solution was then transferred to a 100 mL Teflon-lined stainless-steel autoclave and heated at 120 °C for 2 h. After cooling to room temperature, the precipitate was washed with deionized water, centrifuged several times, and dried under vacuum at 60 °C for 12 h, yielding a yellow powder (ZnIn2S4).

3.4. Preparation of ZnIn2S4/g-C3N4 Composite

Firstly, y mg of g-C3N4 was dispersed in 300 mL of deionized water and sonicated for 30 min. Then, x mg of ZnIn2S4 was added, and the mixture was magnetically stirred at 80 °C for 2 h. The resulting yellow powder was washed with deionized water, centrifuged several times, and dried in an oven at 60 °C for 12 h. The final product was labeled as x: y ZnIn2S4/g-C3N4 and stored for subsequent use. The overall preparation process is illustrated in Scheme 1.

3.5. Characterizations

The crystal structure and phase composition of the samples were analyzed using an X-ray diffractometer (XRD, D/MAX-2500V, Rigaku, Tokyo, Japan) with Cu Kα radiation, scanning from 10° to 80° at a rate of 5 °min–1. The morphologies of ZnIn2S4 and ZnIn2S4/g-C3N4 were examined using a field emission scanning electron microscope (FESEM, Regulus8100, Rigaku). Elemental composition and valence states were determined via X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, Waltham, MA, USA) under high vacuum conditions (>6 × 10−9 mbar). UV-Vis absorption spectra were recorded on a UV-Vis-NIR spectrophotometer (CARY500UV-VIS-NI, Varian, Palo Alto, CA, USA) to assess the optical absorption properties of the samples. Photoluminescence (PL) spectra were measured using a spectrometer (FLS100, Edinburgh, Livingston, UK) with an excitation wavelength of 310 nm to evaluate the separation efficiency of photogenerated carriers. Free radicals generated during the reaction were detected via electron paramagnetic resonance (EPR) spectroscopy (EPR200 Plus, CIQTEK, Hefei, China) at X-band frequency (microwave frequency: 9.5 GHz; microwave power: 1 mW).

3.6. Electrochemical Measurements

Electrochemical properties were measured using a CHI 750E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) in a standard three-electrode configuration. The working electrode was prepared by dropping a homogeneous suspension—containing 4 mg of catalyst, 30 μL of Nafion, 50 μL of isopropanol, and 200 μL of deionized water—onto fluorine-doped SnO2 (FTO) glass (Yingkou OPV Tech New Energy Co., Ltd., Yingkou, China) with an active area of 1 cm2. An Ag/AgCl electrode and a carbon electrode were used as the reference and counter electrodes, respectively, with 1 M NaSO4 solution serving as the electrolyte. The semiconductor energy band structure was determined using Mott–Schottky plots, photogenerated carrier generation was evaluated using photocurrent response curves, and electron transfer resistance was analyzed via electrochemical impedance spectroscopy (EIS).

3.7. Photocatalytic Reduction of CO2

The catalyst was coated onto a circular glass sheet (China Luoyang Float Glass Group Co., Ltd., Luoyang, China, diameter: 5 cm, thickness: 4.5 mm) for photocatalytic experiments. A specific amount of g-C3N4, ZnIn2S4, or ZnIn2S4/g-C3N4 was mixed with 2 mL of C2H5OH and ultrasonicated for 5 min. The suspension was then evenly spread on quartz glass and dried at 60 °C until all C2H5OH evaporated.
For a typical photocatalytic reaction, 20 mL of deionized water was added to an 80 mL gas cylinder, which was connected to a sealed 250 mL glass reactor covered with quartz glass. The circular glass sheet with the catalyst was placed flat in the reactor, ensuring the catalyst side faced upward. Prior to illumination, high-purity CO2 was bubbled into the reactor for 30 min to expel air and create a CO2 atmosphere. A 300 W xenon lamp (CEL-HXF300, Beijing China Education Au-light Co., Ltd., Beijing, China) served as the light source. The gas products (CO and H2) were analyzed and quantified every 30 min using a gas chromatograph (GC, PANNA A60 with a thermal conductivity detector and a flame ionization detector, a 5A packed column, a box temperature of 50 °C, an inlet temperature of 380 °C, and N2 as the carrier gas, Panna Instruments Co., Ltd., Changzhou, China). The total product yield was calculated by summing the detected values over an 8h reaction period [43,44]. The flow chart for photocatalytic CO2 reduction is presented in Scheme 2.

4. Conclusions

In this study, a novel ZnIn2S4/g-C3N4 heterojunction photocatalyst was successfully synthesized and demonstrated excellent performance for the photocatalytic reduction of CO2. Compared to pristine g-C3N4 and ZnIn2S4, the composite exhibited significantly enhanced separation and transfer efficiency in photogenerated charge carriers, leading to a substantial improvement in photocatalytic CO2 reduction efficiency. Under 8 h of irradiation, the composite achieved a maximum total yield of CO at 3743.14 μmol·g−1, 1.92 and 5.83 times higher than that of pristine g-C3N4 (1941.49 μmol·g−1) and ZnIn2S4 (641.68 μmol·g−1), along with a remarkable CO selectivity of 91.3%. Furthermore, this study confirmed a Z-scheme charge transfer pathway in the ZnIn2S4/g-C3N4 heterojunction, which preserved the strong redox capabilities of the charge carriers, resulting in superior photocatalytic performance. These findings highlight the potential of ZnIn2S4/g-C3N4 as a highly active, stable, and durable Z-scheme heterojunction photocatalyst for solar-driven chemical energy conversion. This work provides valuable insights into the design of advanced photocatalysts for sustainable energy applications.

Author Contributions

J.F.: Data Management; Methodology; Survey; Writing—original drafts; M.W.: Methodology; Visualization; X.Y.: Visualization; Q.S.: Resources; Writing—review and editing; and L.Y.: Resources; Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by the National Natural Science Foundation of China (51402161 and 21776147) and the Natural Science Foundation of Qingdao (24-4-4-zrjj-178-jch).

Data Availability Statement

The data will be made available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00095 g001
Figure 1. XRD patterns of g-C3N4, ZnIn2S4 and 1:2 ZnIn2S4/g-C3N4.
Figure 1. XRD patterns of g-C3N4, ZnIn2S4 and 1:2 ZnIn2S4/g-C3N4.
Catalysts 15 00095 g001
Catalysts 15 00095 g002
Figure 2. FESEM images of (a) ZnIn2S4 and (b) 1:2 ZnIn2S4/g-C3N4, along with (c–h) the corresponding elemental mappings of Zn, In, S, C, and N in 1:2 ZnIn2S4/g-C3N4.
Figure 2. FESEM images of (a) ZnIn2S4 and (b) 1:2 ZnIn2S4/g-C3N4, along with (c–h) the corresponding elemental mappings of Zn, In, S, C, and N in 1:2 ZnIn2S4/g-C3N4.
Catalysts 15 00095 g002
Catalysts 15 00095 g003
Figure 3. XPS spectra of g-C3N4, ZnIn2S4, and 1:2 ZnIn2S4/g-C3N4: (a) survey spectra, (b) C 1s, (c) N 1s, (d) Zn 2p, (e) In 3d, and (f) S 2p spectra.
Figure 3. XPS spectra of g-C3N4, ZnIn2S4, and 1:2 ZnIn2S4/g-C3N4: (a) survey spectra, (b) C 1s, (c) N 1s, (d) Zn 2p, (e) In 3d, and (f) S 2p spectra.
Catalysts 15 00095 g003
Catalysts 15 00095 g004
Figure 4. (a) UV-vis absorption spectra, (b) Tauc plots, and (c) PL spectra of g-C3N4, ZnIn2S4, and 1:2 ZnIn2S4/g-C3N4.
Figure 4. (a) UV-vis absorption spectra, (b) Tauc plots, and (c) PL spectra of g-C3N4, ZnIn2S4, and 1:2 ZnIn2S4/g-C3N4.
Catalysts 15 00095 g004
Catalysts 15 00095 g005
Figure 5. Mott–Schottky plots of (a) g-C3N4, (b) ZnIn2S4, and (c) 1:2 ZnIn2S4/g-C3N4 measured at different frequencies.
Figure 5. Mott–Schottky plots of (a) g-C3N4, (b) ZnIn2S4, and (c) 1:2 ZnIn2S4/g-C3N4 measured at different frequencies.
Catalysts 15 00095 g005
Catalysts 15 00095 g006
Figure 6. (a) Transient photocurrent response curves of g-C3N4, ZnIn2S4, and 1:2 ZnIn2S4/g-C3N4 under light on/off cycles. (b) Nyquist plots of g-C3N4, ZnIn2S4, and 1:2 ZnIn2S4/g-C3N4.
Figure 6. (a) Transient photocurrent response curves of g-C3N4, ZnIn2S4, and 1:2 ZnIn2S4/g-C3N4 under light on/off cycles. (b) Nyquist plots of g-C3N4, ZnIn2S4, and 1:2 ZnIn2S4/g-C3N4.
Catalysts 15 00095 g006
Catalysts 15 00095 g007
Figure 7. CO2 reduction performance under various conditions: (1) photocatalytic CO2 reduction using a 1:2 ZnIn2S4/g-C3N4 composite under light irradiation; (2) reaction with light and CO2 but without the catalyst; (3) reaction without light, but with the catalyst and CO2; (4) reaction with light and the catalyst, where CO2 is replaced by Ar.
Figure 7. CO2 reduction performance under various conditions: (1) photocatalytic CO2 reduction using a 1:2 ZnIn2S4/g-C3N4 composite under light irradiation; (2) reaction with light and CO2 but without the catalyst; (3) reaction without light, but with the catalyst and CO2; (4) reaction with light and the catalyst, where CO2 is replaced by Ar.
Catalysts 15 00095 g007
Catalysts 15 00095 g008
Figure 8. (a) Photocatalytic performance of the catalyst with varying mass ratios of ZnIn2S4 to g-C3N4; (b) time-dependent gas yield for the 1:2 ZnIn2S4/g-C3N4; (c) photocatalytic performance of the composite materials at different dosages; (d) results of the cyclic stability experiments.
Figure 8. (a) Photocatalytic performance of the catalyst with varying mass ratios of ZnIn2S4 to g-C3N4; (b) time-dependent gas yield for the 1:2 ZnIn2S4/g-C3N4; (c) photocatalytic performance of the composite materials at different dosages; (d) results of the cyclic stability experiments.
Catalysts 15 00095 g008
Catalysts 15 00095 g009
Figure 9. TEMPO spin-trapping EPR spectra of g-C3N4 and 1:2 ZnIn2S4/g-C3N4, showing photoinduced electron signals recorded in water.
Figure 9. TEMPO spin-trapping EPR spectra of g-C3N4 and 1:2 ZnIn2S4/g-C3N4, showing photoinduced electron signals recorded in water.
Catalysts 15 00095 g009
Catalysts 15 00095 g010
Figure 10. Proposed charge transfer mechanism for the efficient photocatalytic CO2 reduction process facilitated by the ZnIn2S4/g-C3N4 heterojunction.
Figure 10. Proposed charge transfer mechanism for the efficient photocatalytic CO2 reduction process facilitated by the ZnIn2S4/g-C3N4 heterojunction.
Catalysts 15 00095 g010
Catalysts 15 00095 sch001
Scheme 1. The preparation process of ZnIn2S4/g-C3N4.
Scheme 1. The preparation process of ZnIn2S4/g-C3N4.
Catalysts 15 00095 sch001
Catalysts 15 00095 sch002
Scheme 2. Detailed flow chart of the photocatalytic CO2 reduction process.
Scheme 2. Detailed flow chart of the photocatalytic CO2 reduction process.
Catalysts 15 00095 sch002
Table 1. Comparison of photocatalytic performance with other Z-scheme systems.
Table 1. Comparison of photocatalytic performance with other Z-scheme systems.
CatalystReactant SolutionLight SourceProductActivityRef.
g-C3N4/Au/ZnIn2S4bpy+CoCl2+TEOA+solvent (CH3CN:H2O=3:2)300 W Xe lampCO242.3[31]
bulk g-C3N4/ZnIn2S4CH3CN+H2O+TEOA300 W Xe lampCO1453[24]
nanosheet g-C3N4/ZnIn2S4CH3CN+H2O+TEOA300 W Xe lampCO970[24]
microtubes g-C3N4/ZnIn2S4CH3CN+H2O+TEOA300 W Xe lampCO342[24]
Nb doped TiO2/g-C3N4H2O30 W white bulbsCO420[32]
g-C3N4/ZnIn2S4H2O300 W Xe lampCO467.8
Cu2V2O7/g-C3N4H2O20 W white bulbsCO166[33]
SnS2/S-CTFsTEOA300 W Xe lamp (Visible)CO123.6[34]
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Fang, J.; Wang, M.; Yang, X.; Sun, Q.; Yu, L. Design and Preparation of ZnIn2S4/g-C3N4 Z-Scheme Heterojunction for Enhanced Photocatalytic CO2 Reduction. Catalysts 2025, 15, 95. https://doi.org/10.3390/catal15010095

AMA Style

Fang J, Wang M, Yang X, Sun Q, Yu L. Design and Preparation of ZnIn2S4/g-C3N4 Z-Scheme Heterojunction for Enhanced Photocatalytic CO2 Reduction. Catalysts. 2025; 15(1):95. https://doi.org/10.3390/catal15010095

Chicago/Turabian Style

Fang, Jinghong, Min Wang, Xiaotong Yang, Qiong Sun, and Liyan Yu. 2025. "Design and Preparation of ZnIn2S4/g-C3N4 Z-Scheme Heterojunction for Enhanced Photocatalytic CO2 Reduction" Catalysts 15, no. 1: 95. https://doi.org/10.3390/catal15010095

APA Style

Fang, J., Wang, M., Yang, X., Sun, Q., & Yu, L. (2025). Design and Preparation of ZnIn2S4/g-C3N4 Z-Scheme Heterojunction for Enhanced Photocatalytic CO2 Reduction. Catalysts, 15(1), 95. https://doi.org/10.3390/catal15010095

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