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Article

Construction of A NiS/g-C3N4 Co-Catalyst-Based S-Scheme Heterojunction and Its Performance in Photocatalytic CO2 Reduction

by
Qianyu Zhao
and
Hengbo Yin
*
Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(6), 599; https://doi.org/10.3390/catal15060599
Submission received: 18 May 2025 / Revised: 5 June 2025 / Accepted: 14 June 2025 / Published: 17 June 2025
(This article belongs to the Special Issue Catalytic Carbon Emission Reduction and Conversion in the Environment)
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Versions Notes

Abstract

:
NiS nanoparticles were chemically deposited on the surface of g-C3N4, in situ, followed by high-temperature calcination to prepare x-NiS/g-C3N4 co-catalyst-based S-scheme heterojunction photocatalysts. Due to the intrinsic charge accumulation preference on specific crystal planes of g-C3N4, NiS nanoparticles selectively deposited on its surface and formed a strong interfacial contact, thereby constructing an S-scheme heterojunction with co-catalytic functionality. This structure effectively suppressesd the recombination of electron–hole pairs in the valence band, significantly enhancing the separation efficiency of photogenerated charge carriers, and thereby improving performance in photocatalytic CO2 reduction. Compared with pure g-C3N4, the x-NiS/g-C3N4 photocatalysts exhibit superior CO2 reduction activity. Among them, the sample with 1.0% NiS loading showed the best performance, achieving CO and CH4 production rates of 27.34 μmol/g and 13.87 μmol/g, respectively, within 4 h.

1. Introduction

In the 21st century, humanity faces two major challenges: energy crises and the greenhouse effect [1,2]. These challenges have prompted scientists to seek green and sustainable new energy solutions. It is well known that carbon dioxide (CO2) is one of the primary gases responsible for the greenhouse effect. Inspired by the photosynthesis of plants in nature, scientists have developed a technology called photocatalytic CO2 reduction, which can convert CO2 into other usable hydrocarbon energy materials under light irradiation [3,4,5]. To date, numerous photocatalytic systems characterized by their green sustainability, high efficiency, low cost, and pollution-free nature have been extensively studied [6,7,8].
Among these, graphitic carbon nitride (g-C3N4), a metal-free, organic polymeric semiconductor, has emerged as a new and highly promising photocatalyst due to its unique properties [9,10,11]. Unlike many conventional inorganic semiconductors, g-C3N4 derives its exceptional thermal and chemical stability from the strong covalent bonds within its tri-s-triazine (heptazine)-based framework. Remarkably, it can be easily synthesized through direct thermal polycondensation of low-cost, readily available nitrogen-rich precursors (such as urea, melamine, or thiourea), making it both economical and scalable. Furthermore, g-C3N4 possesses an appropriate bandgap width (Eg ≈ 2.7 eV), with its conduction band (CB) and valence band (VB) positions providing sufficient overpotential for the effective photocatalytic reduction of CO2 under light irradiation. However, bulk-structured g-C3N4 exhibits limitations such as a small specific surface area, limited active sites, and excessively long carrier migration distances. These drawbacks lead to significant recombination and wasteful dissipation of photogenerated carriers during transport, thereby restricting its application in photocatalytic CO2 reduction [12,13,14,15].
Therefore, various modification strategies have been attempted by scientists to enhance the photocatalytic CO2 reduction performance of g-C3N4, including metal and non-metal doping to adjust the band structure, noble metal modification to enhance the localized surface plasmon resonance effect, and the construction of heterojunctions using other semiconductor materials [16,17,18]. Among these, ultrathin g-C3N4 nanosheets have attracted widespread attention due to their large specific surface area as well as their excellent optical and electrical properties. In addition, methods such as thermal oxidation, chemical exfoliation, and ultrasonic exfoliation have been employed to transform bulk g-C3N4 into ultrathin two-dimensional nanosheets. According to reports in the literature, two-dimensional g-C3N4 exhibits improved catalytic performance [19,20,21]. However, the π-conjugated planar structure of g-C3N4 results in low and random in-plane charge migration efficiency, making it difficult for photogenerated carriers to migrate to the material surface and react with CO2.
Among various modification strategies, constructing heterojunctions by incorporating other materials to reduce the recombination rate of photogenerated electron-hole pairs is widely regarded as an effective approach to enhance the photocatalytic CO2 reduction performance of g-C3N4-based photocatalysts [22,23,24]. This not only promotes the separation and transfer of photogenerated charge carriers, but also accelerates the surface reaction kinetics. Notably, compared with noble metals and most inorganic materials, transition metal sulfides have attracted significant attention due to their excellent physical and chemical properties [25]. Among them, nickel sulfide (NiS) possesses advantages such as low cost and outstanding electrochemical performance, making it suitable for application in photocatalytic systems. According to reports in the literature, various preparation methods-such as hydrothermal synthesis, ion exchange, and calcination-have been employed to establish good interfacial contact between the two materials [26]. However, in these photocatalytic systems, NiS is randomly dispersed on the material surface, and excessive loading may lead to aggregation, which prevents it from exhibiting optimal photocatalytic CO2 reduction performance [27].
In this study, the preferential charge accumulation characteristics of g-C3N4 were utilized to precisely deposit NiS nanoparticles at active sites on the g-C3N4 surface where photogenerated electrons are ready to transfer, using an in situ photochemical deposition method. The x-g-C3N4/NiS composite photocatalyst was then obtained after high-temperature calcination. The structure, morphology, optical properties, electrochemical properties, and photocatalytic CO2 reduction performance of the x-g-C3N4/NiS composite photocatalyst were systematically characterized, and a possible mechanism for the photocatalytic reduction of CO2 over the composite was proposed. Experimental results indicate that, compared with pure g-C3N4, the incorporation of NiS nanoparticles enables the formation of a co-catalyst-based S-scheme heterojunction between g-C3N4 and NiS, which effectively reduces the recombination rate of photogenerated electron–hole pairs and enhances the photocatalytic performance for CO2 reduction.

2. Results and Discussion

The surface morphology of the catalyst was observed using scanning electron microscopy (SEM), and the results are shown in Figure 1 [28]. As seen from Figure 1A, g-C3N4 consists of irregular thin nanosheets. From Figure 1B, we selected the most representative 1.0%-NiS/g-C3N4 composite catalyst for microscopic morphology analysis. It can be observed that the morphology of g-C3N4 does not undergo significant changes after the incorporation of NiS; however, the surface of the sheets became noticeably rougher, indicating successful loading of NiS nanoparticles [10]. This conclusion is further supported by Figure 1C–G, which show uniform distributions of Ni and S elements on the surface of g-C3N4. Notably, the Ni/S atomic ratio is approximately 1:1.1, which is close to the stoichiometric ratio of NiS particles. The C/N atomic ratio is slightly higher than that of pure g-C3N4, which is attributed to the use of carbon conductive adhesive during sample preparation.
To observe the microstructure and morphology of pure g-C3N4 flakes and the NiS/g-C3N4 composite photocatalyst, transmission electron microscopy (TEM) was employed to visually demonstrate their characteristics [29]. As shown in Figure 2, Figure 2A,B reveal that the g-C3N4 sheets exist in a wrinkled two-dimensional layered structure, consisting of single or fewer layers [11]. This unique structure results in a large surface area, making it suitable as a carrier for combining with other materials. Additionally, the energy-dispersive spectroscopy (EDS) spectrum (inset in Figure 2A) shows the presence of carbon and nitrogen elements, while the diffraction peaks of copper are due to the use of a microgrid film during testing, indicating pure g-C3N4 flakes. Figure 2C,D present TEM images of the 1.0% NiS/g-C3N4 composite photocatalyst, from which it can be observed that NiS nanoparticles are highly dispersed on the surface of the g-C3N4 flakes [30]. Due to the small particle size of NiS nanoparticles and their unique properties (such as surface effects), this specific structure facilitates the formation of heterojunctions between NiS and g-C3N4. These results indicate that the NiS/g-C3N4 composite photocatalyst was successfully prepared [31,32].
As shown in Figure 2E,F, HR-TEM observations reveal a close connection between NiS and g-C3N4, indicating the successful preparation of the sample [33,34]. The high-resolution TEM image (Figure 2F) shows a lattice fringe of NiS with a d-spacing of 0.201 nm, corresponding to the (102) plane of NiS.
The phase structure changes between the sheet-like g-C3N4 and the x-NiS/g-C3N4 composite catalysts were investigated using X-ray diffraction (XRD) [35,36,37]. As shown in Figure 3A, all characteristic peaks of g-C3N4 are present in the x-NiS/g-C3N4 composites, with no significant changes in peak profiles, indicating that the crystal structure remains largely unchanged after high-temperature calcination. The diffraction peaks at 2θ = 13.2° and 27.4° correspond to the (100) and (002) planes of g-C3N4, respectively, where the (100) peak is attributed to the in-plane repetition of tri-s-triazine units. Additionally, the intensity of the diffraction peak at 2θ = 27.4° in the x-NiS/g-C3N4 composites is enhanced compared to pure g-C3N4, suggesting increased interlayer stacking, mainly due to the aggregation of nanosheets during high-temperature calcination [19,38,39]. Moreover, no diffraction peaks corresponding to NiS are observed in the XRD patterns of the x-NiS/g-C3N4 composites, which can be attributed to the amorphous nature and low loading amount of the NiS particles. Notably, the narrow full width at half maximum (FWHM) of the diffraction peaks indicates a high degree of crystallinity in the synthesized samples, which facilitates the migration of photogenerated electrons within the lattice and thereby enhances the photocatalytic CO2 reduction performance.
To further investigate the chemical bonds and functional groups of the catalysts, Fourier-transform infrared (FTIR) spectroscopy was carried out [20,40]. As shown in Figure 3B, the FTIR spectra of CN and x-NiS/g-C3N4 are presented. Two peaks observed at 810 cm−1 and 890 cm−1 are attributed to the vibrations of the triazine ring and N–H stretching vibration, respectively. A broad absorption peak in the range of 1700–1200 cm−1 corresponds to the stretching vibrations of C=N and C–N bonds. These results confirm the formation of the graphitic carbon nitride structure in CN. Additionally, the peak in the range of 3000–3500 cm−1 is assigned to the stretching vibrations of adsorbed H2O molecules and N–H bonds [41,42,43]. Compared with pure g-C3N4, the main characteristic peaks of the x-NiS/g-C3N4 composite photocatalyst show no significant changes, indicating that x-NiS/g-C3N4 possesses the same chemical bonds and functional groups as pure g-C3N4 [23,44]. No obvious new peaks can be observed when comparing the spectra of pure g-C3N4 and the x-NiS/g-C3N4 composite photocatalyst, which may be due to the low loading amount of NiS nanoparticles.
To further analyze the composite catalyst and its surface chemical properties, Figure 4 presents the XPS spectra of the 1.0%-NiS/g-C3N4 catalyst [24]. Figure 4A shows the survey spectrum of the 1.0%-NiS/g-C3N4 composite photocatalyst, clearly indicating that it consists of C, N, Ni, and S elements. Figure 4B displays the high-resolution XPS spectrum of the C1s peak, where two peaks are observed at 288.1 eV and 284.8 eV. The main peak at 288.1 eV corresponds to sp2-hybridized carbon atoms in nitrogen-containing aromatic groups (N=C–N), while the peak at 284.8 eV is attributed to adventitious carbon or C–C groups. Figure 4C shows the high-resolution XPS spectrum of the N1s peak, with four peaks observed at 398.5 eV, 399.4 eV, 401.1 eV, and 404.6 eV, corresponding to the C=N–C groups, N–(C)3 groups, N–H groups, and π excitation, respectively [45,46]. Figure 4D shows the high-resolution XPS spectrum of the Ni 2p region, revealing two characteristic peaks at 856.2 eV and 862.1 eV, which can be assigned to the Ni2p3/2 and Ni2p₁/2 spin–orbit components, respectively [47,48,49]. The binding energy values, along with the spin–orbit splitting of approximately 6.0 eV, are consistent with the presence of Ni2+ species in the sample. Figure 4E shows the high-resolution XPS spectrum of the S2p peak, with two peaks observed at 163.9 eV and 168.9 eV, corresponding to the S2p3/2 electrons, which arise from the binding energy of S2− [20,50,51]. All these results confirm that the NiS/g-C3N4 composite photocatalyst is composed of both NiS and g-C3N4.
To evaluate the light absorption properties of the x-NiS/g-C3N4 composite photocatalyst, solid-state UV–Vis diffuse reflectance spectroscopy was performed [52,53]. As shown in Figure 5, the solid-state UV–Vis diffuse reflectance spectra of pure g-C3N4 and the x-NiS/g-C3N4 composite photocatalysts are presented. In Figure 5A, it can be observed that g-C3N4 nanosheets have an absorption edge at approximately 447 nm, indicating limited visible light absorption, which results in insufficient photogenerated electrons for CO2 reduction and consequently, reduced lower photocatalytic performance. After modification with NiS nanoparticles, the absorption edge of the x-NiS/g-C3N4 composite photocatalyst shifts to longer wavelengths (redshift), and the absorbance increases with the increasing NiS content. Because NiS is a black material that cannot be directly used for photocatalytic CO2 reduction on its own, the amount of loaded NiS is not necessarily beneficial in excess [54]. The bandgap of the catalysts was calculated using the Kubelka–Munk equation. For example, the absorption edge of the 1.0% NiS/g-C3N4 composite photocatalyst is around 464 nm, corresponding to a calculated bandgap of 1.9 eV, which is notably narrower than that of pure g-C3N4 sheets (2.1 eV). This change in bandgap can be attributed to the close integration and formation of heterojunctions between NiS and the g-C3N4 nanosheets. Generally, a narrower bandgap implies a broader visible light response range, and a wider light response range means more photogenerated carriers can be produced, potentially improving the photocatalytic CO2 reduction performance [55]. As shown in Figure 5B, the steady-state photoluminescence (PL) spectra of the prepared samples reveal that pure g-C3N4 exhibits the highest fluorescence intensity, indicating that its electron transport mainly relies on radiative recombination for energy dissipation. With the incorporation of NiS, the PL intensity gradually decreases, and the 1%-NiS/g-C3N4 composite shows the lowest fluorescence intensity. This suggests that the formation of a heterojunction effectively creates non-radiative charge transfer pathways. Consequently, the separation efficiency of photogenerated electron–hole pairs is significantly improved. Overall, the results indicate that the x-NiS/g-C3N4 composite photocatalyst has a slightly narrowed bandgap and an extended visible light absorption range. Moreover, the formation of the heterojunction successfully facilitates non-radiative transport channels, which likely contributes to enhanced photocatalytic CO2 reduction performance.
To further investigate the separation and transfer of photogenerated charge carriers in pure g-C3N4 and x-NiS/g-C3N4 composites, photoelectrochemical analyses were carried out [56]. As shown in Figure 6A, the transient photocurrent-time (I-t) curves of pure g-C3N4 and the x-NiS/g-C3N4 composite photocatalysts are presented. The photocurrent responses of all samples exhibit relatively stable periodic behavior. The photocurrent intensity increases with the loading amount of NiS nanoparticles, reaching a maximum at the 1.0%-NiS/g-C3N4 sample, after which it decreases for the 1.5%- and 2.0%-NiS/g-C3N4 composites, consistent with its optimal performance in photocatalytic CO2 reduction (Figure 7) [57]. These results indicate that the incorporation of NiS nanoparticles enhances the separation efficiency of photogenerated electron-hole pairs, thereby improving the photocatalytic CO2 reduction performance.
Electrochemical impedance spectroscopy (EIS) was employed to study the surface charge transfer resistance of different samples. Generally, the charge transfer resistance is related to the diameter of the semicircular region in the Nyquist plot-the smaller the radius, the lower the resistance, indicating higher efficiency in separation and transfer of photogenerated charge carriers [58]. As shown in Figure 6B, the Nyquist plots of pure g-C3N4 and the x-NiS/g-C3N4 composite photocatalysts reveal that pure g-C3N4 exhibits a large charge transfer resistance, which suppresses the separation and transfer of photogenerated carriers, leading to its poor photocatalytic CO2 reduction performance. After modification with NiS nanoparticles, the resistance of the composite photocatalysts significantly decreases, indicating greatly improved separation and transfer efficiencies of photogenerated charge carriers. This is attributed to the formation of an S-scheme heterojunction between g-C3N4 and NiS. Moreover, the 1.0%-NiS/g-C3N4 composite photocatalyst shows the smallest resistance, which corresponds to its best photocatalytic CO2 reduction performance. These findings demonstrate that the incorporation of NiS nanoparticles significantly enhances the separation efficiency and transfer rate of photogenerated carriers, resulting in markedly improved photocatalytic CO2 reduction performance [59].
Figure 6C presents the LSV polarization curves of pure g-C3N4 and the x-NiS/g-C3N4 composite photocatalysts at a scan rate of 10 mV/s [60]. It can be clearly observed that the x-NiS/g-C3N4 composites exhibit more active electrochemical behavior compared to pure g-C3N4. Among them, the 1.0%-NiS/g-C3N4 composite photocatalyst achieved the best performance, consistent with its superior photocatalytic CO2 reduction activity. The results indicate that the introduction of NiS nanoparticles effectively reduces overpotential and accelerates the reaction kinetics, which suggests that the heterojunction formed between g-C3N4 and NiS promotes the migration of photogenerated carriers. In conclusion, the co-catalyst-based S-scheme heterojunction structure formed by modifying g-C3N4 with NiS nanoparticles effectively improves the separation and transfer of photogenerated charge carriers, thereby enhancing the photocatalytic CO2 reduction performance.
As shown in Figure 7, the CO and CH4 production kinetics after CO2 reduction by pure g-C3N4 and x-NiS/g-C3N4 composites are presented. From Figure 7A, it can be observed that the CO production kinetics of pure g-C3N4 remains very low even after four h of reaction. However, after modification with NiS nanoparticles, the photocatalytic CO2 reduction rate for CO generation is significantly enhanced. This is attributed to the formation of a co-catalyst-type heterojunction between pure g-C3N4 and NiS, which improves the separation and transfer efficiency of photogenerated carriers and reduces the recombination rate of electron-hole pairs. Among them, the 1.0%-NiS/g-C3N4 composite photocatalyst exhibited the best performance, achieving a CO yield of 27.34 µmol/g after 4 h.
As shown in Figure 7B, the overall CH4 production kinetics from CO2 reduction by all catalysts are much lower than those for CO, and pure g-C3N4 shows almost no CH4 generation. The 1.0%-NiS/g-C3N4 composite photocatalyst achieved the highest CH4 yield of 13.87 µmol/g after 4 h. It is also worth noting that increasing the loading amount of NiS does not necessarily lead to higher photocatalytic performance. This may be due to the stacking of NiS nanoparticles, which increases their size and reduces or eliminates the original volume and quantum size effects. Additionally, the heterojunction formed between NiS and g-C3N4 does not exhibit optimal performance in reducing the recombination rate of photogenerated electron-hole pairs. In summary, the results indicate that the formation of a co-catalyst-type heterojunction through NiS nanoparticle modification effectively reduces the recombination of photogenerated electron–hole pairs, thereby enhancing the photocatalytic CO2 reduction performance.
To further investigate the stability of the composite catalyst, the best-performing 1.0%-NiS/g-C3N4 composite photocatalyst was selected for cyclic experiments. During the recycling process, any loss was compensated by additional 1.0%-NiS/g-C3N4 composite photocatalysts that underwent the same number of photocatalytic performance tests. As shown in Figure 8, after four cycles of photocatalytic reduction tests, the catalytic performance of the 1.0%-NiS/g-C3N4 composite photocatalyst demonstrated excellent stability, with no significant changes in the amounts of CO and CH4 produced from CO2 reduction. This stability is attributed to the strong structural stability of both the small-sized NiS nanoparticles and pure g-C3N4. The results indicate that the x-NiS/g-C3N4 composite photocatalyst not only improves photocatalytic CO2 reduction performance but also exhibits a certain degree of stability. Finally, control experiments under various conditions were performed. As shown in Figure 8B, activity tests were additionally carried out in the absence of the catalyst, under an Ar atmosphere, and in dark conditions, respectively. No detectable activity was observed in any of these control experiments, confirming that the CO2 products were generated via photocatalytic conversion by the catalyst. In addition, XRD tests were conducted on the samples after cycling (Figure 8C). It was found that there was no significant change in XRD before and after cycling, further proving the stability of the catalyst.
Pure g-C3N4 possesses a unique two-dimensional (2D) structure with very few exposed chemical bonds on its surface, making it difficult to recombine with other materials. However, by employing in situ chemical deposition combined with high-temperature calcination treatment, NiS nanoparticles can be tightly loaded onto the surface of g-C3N4 nanosheets. Under UV–Vis light irradiation, Ni2+ is first reduced to Niad (adsorbed nickel species), and upon the addition of sodium sulfide, NiS nanoparticles are immediately formed at the Niad sites. In this way, NiS is selectively deposited at electron-rich regions, forming a co-catalyst-type S-scheme heterojunction. According to the literature, preferential charge accumulation exists in g-C3N4; therefore, during the photochemical deposition process, NiS nanoparticles are precisely deposited at active sites where photogenerated electrons are ready to transfer to the g-C3N4 surface. This leads to an improvement in the separation and transfer efficiency of photogenerated carriers. Based on bandgap calculations and the electrochemical Mott–Schottky test and analysis results (Figure 9A–C), a possible electron transfer mechanism for the photocatalytic reduction of CO2 under UV-Vis light by the x-NiS/g-C3N4 composite photocatalyst is proposed, as shown in Figure 9D. On one hand, photogenerated electrons from the conduction band of g-C3N4 migrate to NiS and neutralize the holes present on NiS; on the other hand, electrons in the conduction band of NiS migrate to the catalyst surface and reduce CO2 under light irradiation. Additionally, TEOA serves as an efficient hole scavenger, which selectively captures photogenerated holes on the valence band of the semiconductor, thereby inhibiting charge carrier recombination and prolonging the lifetime of the electrons available for CO2 reduction. In summary, the x-NiS/g-C3N4 composite photocatalyst effectively reduces the recombination rate of photogenerated electron-hole pairs and enhances the photocatalytic performance for CO2 reduction.

3. Experimental Section

3.1. Preparation of g-C3N4

Ten grams of urea was weighed and placed into a crucible, which was then introduced into a muffle furnace and calcined at 550 °C for 4 h with a heating rate of 5 °C/min. After cooling down, the obtained sample (bulk CN) was transferred into a 250 mL beaker and mixed with 200 mL deionized water. Five drops of concentrated nitric acid (6 M) were added, followed by heating in a water bath at 80 °C for 10 h. After the water bath treatment, the solid product was collected by centrifugation, repeatedly washed with deionized water until neutral pH, and finally dried in an oven at 60 °C for 12 h to obtain the g-C3N4 (CN) sample (sheet-like structure).

3.2. Preparation of NiS/g-C3N4

A series of 0.46 g CN samples were weighed and placed into 100 mL beakers, each containing 50 mL of deionized water, and stirred evenly. Different volumes (x) of 0.05 M nickel nitrate solution were added, followed by magnetic stirring for 30 min. The mixture was then subjected to ultrasonication for another 30 min to ensure homogeneity. The suspension was irradiated with a 300 W xenon lamp for 1 h. Subsequently, sodium sulfide solution with a concentration of 0.05 M and a volume equal to 1.2 times x was added. After stirring for 30 min, the product was collected via centrifugation, washed three times with deionized water and ethanol, respectively, and dried at 60 °C for 12 h.
The as-prepared sample was loaded into a ceramic boat and calcined at 500 °C for 2 h under nitrogen atmosphere in a tube furnace with a heating rate of 5 °C/min. After cooling down to room temperature, the final product, denoted as x-g-C3N4/NiS (where x represents the mass percentage of NiS), was obtained.

4. Materials and Methods

4.1. Materials

The experimental chemicals included urea (AR grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), ethanol (AR grade, Sinopharm Chemical Reagent Co., Ltd.), sodium sulfide (AR grade, Shanghai Chemical Reagent Co., Ltd.), nickel nitrate hexahydrate (AR grade), and triethanolamine (AR grade, Sinopharm Chemical Reagent Co., Ltd.), all of analytical reagent (AR) purity. Specifically, urea, ethanol, and triethanolamine were procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), while sodium sulfide was supplied by Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). The manufacturer of nickel nitrate hexahydrate was not explicitly stated in the provided information.

4.2. Material Characterization

Material characterization and testing were performed using the following instruments: transmission electron microscope (Tecnai 12, Philips, Eindhoven, The Netherlands), high-resolution transmission electron microscope (Tecnai G2 F30 S-Twin, Philips, Eindhoven, The Netherlands), X-ray diffractometer (D/max-RA, Rigaku, Tokyo, Japan), Fourier transform infrared spectrometer (Nicolet Nexus 470, Thermo Fisher Scientific, Madison, WI, USA), X-ray photoelectron spectrometer (PHI 5300, Perkin-Elmer PHI, Waltham, MA, USA), UV–Vis diffuse reflectance spectrometer (Shimadzu UV-2450, Shimadzu, Kyoto, Japan), fluorescence spectrometer (F3500, Hitachi, Tokyo, Japan), X-ray energy dispersive spectrometer (Oxford Instruments X-Max 50, Oxford Instruments, Abingdon, UK), scanning electron microscope (Tecnai G2 F20, FEI, Eindhoven, The Netherlands; S-4800, Hitachi, Tokyo, Japan), and a magnetic stirrer (85-2C, Yuhua Instrument Co., Ltd., Hangzhou, China). The equipment encompassed structural, morphological, and optical analyses critical for comprehensive material characterization.

4.3. Photocatalytic Activity Measurement

Photocatalytic CO2 reduction experiments were carried out in a 300 mL closed photoreactor under a pressure of 0.4 MPa and at a temperature of 25 °C. A Xe lamp (PE300BFA, PerkinElmer, Waltham, MA, USA) with dominant visible light output was used as the irradiation source. The lamp was positioned vertically above the reactor at a distance of approximately 5 cm, ensuring direct and uniform illumination of the catalyst suspension. In each test, 20 mg of the sample, 45 mL of deionized water, 50 mL of 0.2 M NaOH solution, and 5 mL of triethanolamine (TEOA) were added into the reactor. Subsequently, CO2 gas was introduced for 20 min to remove any impurities, and the pressure in the reactor was stabilized at 0.4 MPa. Gas samples were collected every h after the reaction started and analyzed using a gas chromatograph (GC-7920, Nanjing Kejie Co., Ltd., Nanjing, China) equipped with an FID-II detector to determine the composition and concentration of gaseous products.

4.4. Photocatalytic Stability Test

In this experiment, two groups (1) the cyclic reduction group and (2) the sample supplementation group were subjected to photocatalytic CO2 reduction under identical conditions simultaneously. After each reduction cycle, the catalysts were recovered. The sample supplementation group was used to replenish the cyclic reduction group to ensure that the amount of catalyst remained consistent for each cycle. The photocatalytic reduction process was repeated four times.

4.5. Electrochemical Measurements

Transient photocurrent and electrochemical impedance spectroscopy (EIS) measurements were conducted using a standard three-electrode system. First, 0.05 g of the synthesized sample was mixed with 0.01 g of polyvinylpyrrolidone (PVP, k = 5800), followed by the addition of 30 μL of oleic acid. The mixture was then diluted with 5 mL of ethanol. This suspension was spin-coated onto an FTO conductive glass substrate (1 cm × 1 cm). (Zhuhai Kaivo Optoelectronic Technology Co., Ltd., Zhuhai, China). The electrochemical tests were performed in a 0.5 M Na2SO4 solution. A platinum sheet and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively, while the synthesized photocatalyst served as the working electrode. A 300 W xenon lamp (λ ≥ 420 nm) (300 W Xenon Lamp, Zhongjiao Jin Yuan, Beijing, China) was employed as the light source. It is noteworthy that the platinum sheet should be positioned opposite the working electrode. Data were collected at intervals of 30 s. For the transient photocurrent measurements, a constant bias potential of + 1.0 V vs. Ag/AgCl was applied. EIS measurements were carried out under open-circuit potential (OCP) conditions. For the linear sweep voltammetry (LSV) measurements, the scan rate was set to 10 mV/s over a potential range from 0 to −1 V.

5. Conclusions

The x-NiS/g-C3N4 co-catalyst-type S-scheme heterojunction photocatalyst was successfully synthesized by first depositing NiS nanoparticles onto the surface of g-C3N4 via in situ chemical deposition, followed by high-temperature calcination. Compared to pure g-C3N4, the x-NiS/g-C3N4 composite photocatalyst exhibited enhanced photocatalytic CO2 reduction performance, with the 1.0%-NiS/g-C3N4 composite showing the best activity, achieving CO and CH4 production rates of 27.34 µmol/g and 13.87 µmol/g, respectively, after 4 h. Based on the above results, it can be concluded that due to the preferential charge accumulation characteristics of g-C3N4, the selectively deposited NiS nanoparticles on its surface form a tight interface, leading to the formation of an internal electric field and the construction of a co-catalyst-type S-scheme heterojunction. This structure effectively reduces the recombination rate of photogenerated electron-hole pairs, thereby enhancing the photocatalytic performance for CO2 reduction.

Author Contributions

Conceptualization, H.Y.; methodology, H.Y.; investigation, Q.Z.; data curation, H.Y.; writing—original draft preparation, H.Y.; writing—review and editing, H.Y.; supervision, H.Y.; project administration, H.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Our data can only be obtained by contacting us by email.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM patterns of g-C3N4 (A); 1%-NiS/g-C3N4 (B); EDS patterns of 1%-NiS/g-C3N4 (C–G).
Figure 1. SEM patterns of g-C3N4 (A); 1%-NiS/g-C3N4 (B); EDS patterns of 1%-NiS/g-C3N4 (C–G).
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Figure 2. TEM patterns of g-C3N4 (A,B) and 2.0%-NiS/g-C3N4 (C,D); TEM (E) patterns and HR-TEM (F) patterns of 2.0%-NiS/g-C3N4.
Figure 2. TEM patterns of g-C3N4 (A,B) and 2.0%-NiS/g-C3N4 (C,D); TEM (E) patterns and HR-TEM (F) patterns of 2.0%-NiS/g-C3N4.
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Figure 3. XRD patterns (A) and FT-IR of all the g-C3N4 and NiS/g-C3N4 (B).
Figure 3. XRD patterns (A) and FT-IR of all the g-C3N4 and NiS/g-C3N4 (B).
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Figure 4. XPS (A) survey spectrum; (B) C 1s; (C) N 1s; (D); Ni 2p; (E) S 2p spectra of 1.0%- NiS/g-C3N4.
Figure 4. XPS (A) survey spectrum; (B) C 1s; (C) N 1s; (D); Ni 2p; (E) S 2p spectra of 1.0%- NiS/g-C3N4.
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Figure 5. (A) UV–Vis absorption spectra derived from diffuse reflectance measurements using the Kubelka–Munk function. and (B) PL emission spectra of CN and x-NiS/g-C3N4.
Figure 5. (A) UV–Vis absorption spectra derived from diffuse reflectance measurements using the Kubelka–Munk function. and (B) PL emission spectra of CN and x-NiS/g-C3N4.
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Figure 6. Transient photocurrent response (A); EIS spectra (B) and LSV (C) curves of g-C3N4 and x- NiS/g-C3N4.
Figure 6. Transient photocurrent response (A); EIS spectra (B) and LSV (C) curves of g-C3N4 and x- NiS/g-C3N4.
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Figure 7. (A) CO production yield of CN and x- NiS/g-C3N4 under ultraviolet light irradiation; (B) CH4 production yield of CN and x- NiS/g-C3N4 under ultraviolet light irradiation.
Figure 7. (A) CO production yield of CN and x- NiS/g-C3N4 under ultraviolet light irradiation; (B) CH4 production yield of CN and x- NiS/g-C3N4 under ultraviolet light irradiation.
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Figure 8. (A) Cyclic experiments of g-C3N4 and 1%-NiS/g-C3N4 under ultraviolet light irradiation; (B) conditional control experiment of 1%-NiS/g-C3N4; (C) comparison of XRD curves before and after the 1%-NiS/g-C3N4 cycle test.
Figure 8. (A) Cyclic experiments of g-C3N4 and 1%-NiS/g-C3N4 under ultraviolet light irradiation; (B) conditional control experiment of 1%-NiS/g-C3N4; (C) comparison of XRD curves before and after the 1%-NiS/g-C3N4 cycle test.
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Figure 9. (A) Band structure diagram of g-C3N4 and NiS; (B,C) Mott–Schottky plots of g-C3N4 and NiS; (D) photocatalytic mechanism.
Figure 9. (A) Band structure diagram of g-C3N4 and NiS; (B,C) Mott–Schottky plots of g-C3N4 and NiS; (D) photocatalytic mechanism.
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Zhao, Q.; Yin, H. Construction of A NiS/g-C3N4 Co-Catalyst-Based S-Scheme Heterojunction and Its Performance in Photocatalytic CO2 Reduction. Catalysts 2025, 15, 599. https://doi.org/10.3390/catal15060599

AMA Style

Zhao Q, Yin H. Construction of A NiS/g-C3N4 Co-Catalyst-Based S-Scheme Heterojunction and Its Performance in Photocatalytic CO2 Reduction. Catalysts. 2025; 15(6):599. https://doi.org/10.3390/catal15060599

Chicago/Turabian Style

Zhao, Qianyu, and Hengbo Yin. 2025. "Construction of A NiS/g-C3N4 Co-Catalyst-Based S-Scheme Heterojunction and Its Performance in Photocatalytic CO2 Reduction" Catalysts 15, no. 6: 599. https://doi.org/10.3390/catal15060599

APA Style

Zhao, Q., & Yin, H. (2025). Construction of A NiS/g-C3N4 Co-Catalyst-Based S-Scheme Heterojunction and Its Performance in Photocatalytic CO2 Reduction. Catalysts, 15(6), 599. https://doi.org/10.3390/catal15060599

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