α-NiS/g-C₃N₄ Nanocomposites for Photocatalytic Hydrogen Evolution and Degradation of Tetracycline Hydrochloride

Abstract

α-NiS/g-C₃N₄ nanocomposites were synthesized and used for photocatalytic hydrogen (H₂) evolution and tetracycline hydrochloride (TC) degradation. The fabricated nanocomposites were characterized by XRD, XPS, SEM, TEM, UV-vis DRS, TRPL, and PEC measurements. Photocatalytic studies show that the hydrogen generation rate of the 15%-α-NiS/g-C₃N₄ nanocomposite reaches 4025 μmol·g⁻¹·h⁻¹ and TC degradation rate 64.6% within 120 min, both of which are higher than that of g-C₃N₄. The enhanced performance of the nanocomposite is attributed to the formation of a heterojunction between α-NiS and g-C₃N₄ that enhances visible light absorption, promotes the separation and transfer of charges, and inhibits the recombination of carriers. The photocatalytic mechanism of the α-NiS/g-C₃N₄ heterojunction nanocomposite is discussed in terms of relevant energy levels and charge transfer processes.

1. Introduction

g-C₃N₄ is a promising material for photocatalytic applications in environmental purification, CO₂ reduction, hydrogen evolution, organic synthesis, and sterilization [1,2,3,4,5,6,7]. However, due to the rapid recombination of photogenerated electron and hole pairs, the photocatalytic efficiency of g-C₃N₄ is low, and modification is usually required to improve its performance [8]. Strategies for modification include morphology manipulation and metal or non-metal element doping [1,2,9,10,11,12,13,14]. However, the synthetic conditions for modification are often harsh, and the improvement of photocatalytic performance is limited [15].

One of the most effective strategies to improve photocatalytic hydrogen production is to load a cocatalyst on the surface of g-C₃N₄ to provide more active sites, rapidly separate photogenerated electrons and holes, and reduce the recombination probability [16]. The cocatalyst is low in content and does not affect the crystal shape and size of g-C₃N₄. Common cocatalysts include noble metals or non-noble metals. Supported noble metal cocatalysts, such as Pt, Au, Pd, and Ag, significantly improve the photocatalytic activity of g-C₃N₄, but their high price and scarcity greatly limit their application in practice [17].

As non-precious metal cocatalysts, metal sulfides, such as NiS [18,19], NiS₂ [20], MoS₂ [21,22,23], Ag₂S [24], and CoS_x [25], have been widely reported for their application in photocatalytic hydrogen production and pollutant degradation. For example, Yu et al. prepared amorphous MoS_x (a-MoS_x) nanoparticles modified g-C₃N₄ photocatalyst [23]. Compared with pure g-C₃N₄, the photocatalytic hydrogen production performance of 3 wt%-MoS_x/g-C₃N₄ is improved by 91 times. This is attributed to the amorphous MoS_x cocatalyst being conducive to the transfer of photogenerated electrons and effectively reducing the recombination of photogenerated carriers. Similarly, Jiang et al. designed Ag₂S/g-C₃N₄ photocatalyst and demonstrated a photocatalytic hydrogen production rate of 10 μmol·h⁻¹ for g-C₃N₄ loaded with 5 wt% Ag₂S, two orders of magnitude higher than that of pure g-C₃N₄ [24]. These studies show that metal sulfides are promising low-cost cocatalysts.

Among the metal sulfide cocatalysts, nickel sulfide (NiS) exhibits complicated structure, composition, and magnetic phase behavior [26,27,28,29]. For instance, NiS has two crystal structures: hexahedron NiS (α-NiS) and rhombohedron NiS (β-NiS) [30]. The α-NiS is beneficial to decompose H₂O into H⁺ and OH⁻ [31], and β-NiS shows better conductivity than other phases of NiS [32]. In previous research, we have studied the influence of cocatalysts with different crystalline phases, α-NiS-β-NiS and β-NiS, on hydrogen production using Cd_0.5Zn_0.5S as a photocatalyst [33]. The 17%-α-NiS-β-NiS/Cd_0.5Zn_0.5S nanocomposite showed a photocatalytic H₂ evolution rate of 3113.0 μmol·h⁻¹·g⁻¹, better than that of 17%-β-NiS/Cd_0.5Zn_0.5S. These results showed that moderate loading of NiS cocatalyst can greatly enhance the photocatalytic hydrogen production activity of Cd_0.5Zn_0.5S, and the enhancement depends on the different NiS crystalline phases.

For g-C₃N₄, most research has focused on the development of new cocatalysts to improve their photocatalytic activity, and the influence of α-NiS cocatalyst has not been systematically studied. In this work, α-NiS/g-C₃N₄ nanocomposite photocatalysts were prepared for hydrogen generation with triethanolamine as a sacrificial agent and photodegradation of tetracycline hydrochloride (TC). Their crystal structure, morphology, light-harvesting capacity, and surface chemical states were characterized. The 15%-α-NiS/g-C₃N₄ showed an optimal photocatalytic hydrogen rate of 4025 μmol·h⁻¹·g⁻¹, which was 35.7 times as high as that of pure g-C₃N₄, and the photocatalytic degradation rate of TC reached 64.6% under visible light irradiation, respectively. The recyclability of as-synthesized 15%-α-NiS/g-C₃N₄ nanocomposite was also evaluated. Based on ESR experiments, a possible photocatalytic mechanism is proposed.

2. Results and Discussion

2.1. Synthesis, Structure, and Morphology

The synthesis route of α-NiS/g-C₃N₄ photocatalyst is shown schematically in Figure 1a. Firstly, dicyandiamide and ammonium chloride were selected as the precursors to obtain g-C₃N₄ in thin layers. With g-C₃N₄ as the base material, α-NiS was grown on g-C₃N₄ with close contact. The crystal structure of the obtained sample was determined by XRD. As shown in Figure 1b, the diffraction peaks at 30.3°, 34.8°, 46.0°, and 53.7° correspond to the (100), (101), (102), and (110) crystal planes of NiS, respectively, which are consistent with the standard card (PDF No. 02-1280), indicating that the NiS have been successfully prepared. Meanwhile, the diffraction peak at ~27.3° corresponds to the (002) crystal plane, which is caused by the 2D g-C₃N₄ interlayer stacking reflection [34]. However, the diffraction peak (~13.1°) of the (100) crystal plane corresponding to 2D g-C₃N₄ is disappeared due to the 2D ultra-thin structure of 2D g-C₃N₄ [35,36]. The typical peaks of α-NiS and g-C₃N₄ are observed for the α-NiS/g-C₃N₄ composites with no impurity phases. In addition, there is no shift for the diffraction peak position of α-NiS or g-C₃N₄ in the α-NiS/g-C₃N₄ composites, suggesting the crystal structure remains during the process of synthesis. In addition, the spacing between layers of 2D g-C₃N₄ is about 0.335 nm, and the average crystallite size of NiS is calculated to be 17.5 nm according to Scherrer’s equation [37].

The pristine g-C₃N₄ and 15%-α-NiS/g-C₃N₄ were characterized by FT-IR. As observed in Figure 1c, the characteristic peaks at 810 and 878 cm⁻¹ are attributed to the triazine units and N-H, respectively. The absorption bands around 1110–1730 cm⁻¹ are related to the C-N heterocycle stretching vibration modes, and the broad peaks between 3000–3500 cm⁻¹ are N-H and O-H stretching vibrations [38]. In particular, the FT-IR spectrum of 15%-α-NiS/g-C₃N₄ nanocomposite was the same as that of 2D g-C₃N₄, indicating that g-C₃N₄ in 15%-α-NiS/g-C₃N₄ retains its chemical structure, consistent with XRD analysis.

The morphologies of g-C₃N₄, α-NiS, and 15%-α-NiS/g-C₃N₄ were characterized by SEM. As shown in Figure 2a,b, g-C₃N₄ showed a folded sheet structure, and NiS showed nanoparticles (NPs). In Figure 2c, both g-C₃N₄ and α-NiS are observed in the 15%-α-NiS/g-C₃N₄ nanocomposite, and α-NiS and g-C₃N₄ seem to be in close contact.

The morphologies of all photocatalysts were further determined by HRTEM. As shown in Figure 3a, many α-NiS NPs (dark color) are decorated on the surface of transparent g-C₃N₄ nanosheets. Though no lattice fringe of g-C₃N₄ was observed, its wavy structure is discernable. The d-spacing of 0.258 nm corresponds to the (101) lattice plane of α-NiS (PDF No. 02-1280) [39]. These results suggest that α-NiS NPs are in good contact with g-C₃N₄ forming heterojunctions. In addition, the element composition of 15%-α-NiS/g-C₃N₄ was analyzed with HAADF-STEM EDX mapping images, as shown in Figure 3b. The elements C, N, Ni, and S are shown and evenly distributed in 15%-α-NiS/g-C₃N₄, which further confirms the successful preparation of α-NiS/g-C₃N₄ nanocomposite.

XPS analysis of α-NiS/g-C₃N₄ was conducted with results shown in Figure 4. In Figure 4a, the C 1s spectra of α-NiS/g-C₃N₄ are deconvoluted into three peaks centered at 287.9, 286.5, and 284.8 eV, corresponding to sp² hybridized carbon (N=C-N), π–π* of adventitious carbon cycles and graphitic carbon (C–C), respectively [40]. As for N 1s shown in Figure 4b, its high-resolution XPS spectra can be fitted into three peaks centered at 398.5, 400.3, and 404.3 eV, which arose from sp² hybridized N (C–N–C, N₂C), tertiary N of three-fold coordination (N–C₃, N₃C) and the charge effect or the localization of the positive charge in the heterocyclic ring, respectively [41]. The connection of C–N–C and N–C₃ constitutes a tri-s-triazine conjugated skeleton of g-C₃N₄. Figure 4c shows the high-resolution of Ni 2p fitted with two spin-orbit doublets and two shakeup satellite peaks. The dominant peaks of 855.4 and 873.1 eV are related to Ni²⁺, and the lower peaks of 852.6 and 870.0 eV originated from Ni^δ+ with very small positive charges of NiS [42,43,44]. Two main shake-up satellite peaks at 860.7 and 879.3 eV were also observed. From Figure 4d, the S 2p spectra show two peaks at 162.3 and 161.0 eV, related to S 2p_3/2 and S 2p_1/2, respectively [45].

2.2. Optical and Photoelectrochemical Properties

The light absorption of a photocatalyst directly affects its photocatalytic performance. The UV-vis DRS spectra of the prepared photocatalysts are shown in Figure 5a. Compared with g-C₃N₄, the absorption edges of the α-NiS/g-C₃N₄ nanocomposite were redshifted with increased α-NiS content. For α-NiS/g-C₃N₄, light absorption is stronger than that of g-C₃N₄ in the 500–800 nm range, which is due to the introduction of NiS that contributes to visible light absorption. Based on the absorption spectra, E_g was calculated to be 2.76 and 2.52 eV for g-C₃N₄ and 15%-NiS/g-C₃N₄ using Equation (1), respectively [46,47].

$$\mathrm{B}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{g}\mathrm{a}\mathrm{p}=\frac{1240}{\mathrm{W}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}}$$

(1)

The Mott–Schottky (M−S) plot was measured at 1000 Hz in 0.10 M Na₂SO₄ solution to determine the conduction band (CB) of the g-C₃N₄. In Figure 5b, the positive slope of the Mott–Schottky plot indicates that g-C₃N₄ is an n-type semiconductor. The flat band potential (V_fb) of g-C₃N₄ was calculated to be −0.37 V vs. NHE at pH 7 based on the linear potential curves. The bottom of the CB is −0.1 V compared to V_fb for n-type semiconductors, and the E_CB of g-C₃N₄ is −0.47 V vs. NHE at pH 7 [48]. The valence band (VB) edge potential was determined to be 2.29 V vs. NHE at pH 7 based on E_CB = E_VB − E_g.

Electrochemical studies of g-C₃N₄, 5%-α-NiS/g-C₃N₄, 10%-α-NiS/g-C₃N_4, and 15%-α-NiS/g-C₃N₄ were conducted. Figure 6a shows the transient photocurrent–time (I–t) curves obtained. The results show that 15%-α-NiS/g-C₃N₄ has the highest photocurrent intensity, indicating that α-NiS loading leads to more efficient electron transfer [49]. Therefore, 15%-α-NiS/g-C₃N₄ has the optimal hydrogen evolution performance. Electrochemical impedance spectroscopy (EIS) provides more evidence for the charge transfer efficiency of catalysts. In Figure 6b, the 15%-α-NiS/g-C₃N₄ nanocomposite has the minimum resistance curve, indicating minimum resistance [50].

Figure 6c shows the TRPL profiles of α-NiS, g-C₃N₄, and 15%-α-NiS/g-C₃N₄, which are used to determine the average lifetime of photogenerated carriers τ using the following Equation (2) [34,51,52]:

$$\mathsf{\tau }=\frac{{\mathrm{A}}_1{\mathsf{\tau }}_1^2+{\mathrm{A}}_2{\mathsf{\tau }}_2^2}{{\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2}$$

(2)

where A₁ and A₂ are the constants obtained after fitting the decay curves with τ₁ and τ₂ representing fast and slow components. The average lifetime of the charge carriers of 15%-α-NiS/g-C₃N₄ (τ = 1.35 ns) is longer than that of pure g-C₃N₄ (τ = 0.76 ns). Therefore, α-NiS loaded on the surface of g-C₃N₄ lengthened the average carrier lifetime, which is desired for photocatalytic applications [53]. The result is consistent with that of electrochemical studies.

2.3. Evaluation of Photocatalytic Performance

2.3.1. Photocatalytic H₂ Generation and Stability of α-NiS/g-C₃N₄ Nanocomposites

Photocatalytic H₂ evolution over g-C₃N₄, α-NiS, and different proportions of α-NiS/g-C₃N₄ nanocomposites was evaluated under visible light irradiation. Figure 7a shows the photocatalytic H₂ evolution from pure g-C₃N₄. With the increase of α-NiS in the α-NiS/g-C₃N₄ nanocomposite, the performance of H₂ evolution is improved. When the mass ratio of α-NiS and g-C₃N₄ is 15 wt%, the H₂ production amount reaches 20,125 μmol·g⁻¹, which is about 35.7 times higher than that of pure g-C₃N₄. With further increase of α-NiS loading in α-NiS/g-C₃N₄, the photocatalytic performance is decreased because excessive α-NiS would suppress the H₂ production activity [20,54]. Hence, α-NiS with moderate loading can accelerate electron separation, which will provide more reaction sites and promote H₂ production. Figure 7b shows the hydrogen evolution rates of different photocatalysts. The photocatalytic hydrogen evolution rates of pure g-C₃N₄, 5%-α-NiS/g-C₃N₄, 10%-α-NiS/g-C₃N₄, 15%-α-NiS/g-C₃N₄, 20%-α-NiS/g-C₃N₄, and 25%-α-NiS/g-C₃N₄ are 113 μmol·g⁻¹·h⁻¹, 2959 μmol·g⁻¹·h⁻¹, 3165 μmol·g⁻¹·h⁻¹, 4025 μmol·g⁻¹·h⁻¹, 3598 μmol·g⁻¹·h⁻¹, 3047 μmol·g⁻¹·h⁻¹. Too high a loading of α-NiS likely decreased the light absorption of g-C₃N₄, which will reduce the photocatalytic H₂ evolution activity. In addition, the cycling stability of the 15%-α-NiS/g-C₃N₄ nanocomposite was measured every 5 h as one cycle, and the results are shown in Figure 7c. The photocatalytic H₂ production activity of 15%-α-NiS/g-C₃N₄ nanocomposite does not significantly decrease, indicating good photocatalytic stability. The photocatalyst was characterized using XRD after the stability test. As shown in Figure 7d, no evident change in the XRD pattern of the 15%-α-NiS/g-C₃N₄ nanocomposite was observed after the photocatalytic reaction, indicating no structural change.

2.3.2. Photocatalytic TC Degradation

Figure 8a shows the TC removal rate curve in terms of C/C₀ as a function of light irradiation time. After adsorption and photocatalytic reaction, the removal rate of TC for the optimized 15%-α-NiS/g-C₃N₄ was 67.6%. The degradation ratio of TC through the photocatalytic process for 15%-α-NiS/g-C₃N₄ was 64.6%. The result suggests that adsorption played a negligible role in TC removal. The improved photocatalytic activity contributed to the loading of α-NiS cocatalyst that effectively promoted the separation of charges. Figure 8b shows the absorption spectra of TC with time evolution over 15%-α-NiS/g-C₃N₄. The photocatalytic performance of 15%-α-NiS/g-C₃N₄ was monitored by measuring the absorbance peak at 375 nm, which decreased with increasing the irradiation time [53].

As shown in Figure 8c, the kinetics of photodegradation is described by the pseudo-first-order equation (−ln(C/C₀) = kt), suggesting the photocatalytic degradation activity of the photocatalyst [55]. In the inset of Figure 8c, the apparent kinetic constants (k) of TC degradation for pure g-C₃N₄ and 15%-α-NiS/g-C₃N₄ were 0.00199 min⁻¹ and 0.00968 min⁻¹, respectively. The k value of 15%-α-NiS/g-C₃N₄ was about 4.86 times as high as that of the pure g-C₃N₄, indicating that the 15%-α-NiS/g-C₃N₄ nanocomposite possessed enhanced photocatalytic performance. Figure 8d shows the reusability of the prepared 15%-α-NiS/g-C₃N₄ nanocomposite for photocatalytic TC degradation. After four operations, the photocatalytic activity was not decreased obviously, and the photocatalytic activity of 15%-α-NiS/g-C₃N₄ still remained at about 64.5% for TC, and the removal rate was decreased only by 2.2% compared with the initial value. The results show that the as-prepared 15%-α-NiS/g-C₃N₄ has good reusability and stability in the photocatalytic process.

The results of the radical scavenging study are shown in Figure 9a. The activities of hydroxyl radicals (·OH), photogenerated holes (h⁺), and superoxide anion radical (·O₂⁻) were inhibited by isopropyl alcohol (IPA), ethylene diamine tetra acetic acid (EDTA), and 1,4-benzoquinone (BQ), respectively [56]. The photocatalytic degradation activities are significantly reduced after the addition of IPA and BQ, which shows that ·OH and ·O₂⁻ play a dominant role in the photocatalytic TC degradation. The inhibition effect of EDTA on the degradation efficiency for TC was weaker than that of BQ and IPA, suggesting that the contribution of h⁺ was less than ·OH and ·O₂⁻. In addition, ESR was used to further investigate the active components involved [57]. In Figure 9b,c, no obvious signals of DMPO-·OH and DMPO-·O₂⁻ adducts can be probed under dark conditions, but signals for these two adducts appeared under light irradiation with intensity increasing with irradiation time. The ESR results show that both ·OH and ·O₂⁻ radicals were generated in the photocatalytic process with the 15%-α-NiS/g-C₃N₄, which is in agreement with the results of quenching experiments [58,59].

2.4. Proposed Photocatalytic Mechanism

Based on UV-Vis DRS and Mott–Schottky results, the conduction band (CB) of g-C₃N₄ is −0.47 V (vs. NHE), and the valence band (VB) of g-C₃N₄ is 2.29 V (vs. NHE). As illustrated in Figure 10a, light excitation of g-C₃N₄ results in photogenerated electrons (e⁻) in the CB and holes (h⁺) in the VB. The photogenerated e⁻ in the CB of g-C₃N₄ is expected to transfer to α-NiS. O₂ in the solution can react with electrons in the CB of g-C₃N₄ or be transferred to α-NiS to form an active species ·O₂⁻, which can react with TC causing its degradation. Meanwhile, the photogenerated holes in the VB of g-C₃N₄ react with H₂O to form ·OH, which can also react with TC to result in its degradation.

Figure 10b shows an illustration of the photocatalytic hydrogen evolution with EY for sensitization. The bandgap of 2.76 eV of g-C₃N₄ allows charge carrier generation. The e⁻ in the CB of g-C₃N₄ is transferred to α-NiS, and h⁺ can accumulate in the VB of g-C₃N₄, allowing efficient carrier separation. In addition, the EY molecules are excited to form EY¹*, which can lead to the formation of a triplet excited state (EY³*) via an intersystem crossing. In the TEOA solution, EY³* can be reduced, forming free radical EY⁻ with strong reducibility. The EY⁻ can transfer electrons to the CB of g-C₃N₄, and the regenerated EY takes part in the next electron transfer cycle. The photogenerated e⁻ in both g-C₃N₄ and EY can be transferred to α-NiS to reduce H⁺ to produce H₂. Finally, the h⁺ accumulated in the VB of g-C₃N₄ is consumed by TEOA.

3. Materials and Methods

3.1. Chemicals

Dicyandiamide (C₂H₄N₄), ammonium chloride (NH₄Cl), nickel acetate tetrahydrate (Ni(CH₃COO)₂·4H₂O), and thioacetamide (CH₃CSNH₂), sodium dodecyl sulfate (CH₃(CH₂)₁₁OSO₃Na), triethanolamine (C₆H₁₅NO₃) were supplied by Aladdin Reagent Company (Shanghai, China). Ethyl alcohol (C₂H₅OH) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and deionized water was used throughout this work.

3.2. Photocatalyst Preparation

3.2.1. Synthesis of g-C₃N₄

A total of 1.00 g dicyandiamide was ground with 10.0 g ammonium chloride, and the mixture was then transferred to the corundum crucible and covered and calcined in a Muffle furnace. The Muffle furnace was heated to 550 °C at 3 °C/min and maintained for 4 h, and g-C₃N₄ nanosheets were obtained when the furnace was cooled to room temperature.

3.2.2. Synthesis of α-NiS/g-C₃N₄ Photocatalysts

The as-prepared 0.100 g g-C₃N₄ was added into a beaker filled with 60 mL of ethanol for ultrasonic treatment for 40 min. Then, an appropriate amount of Ni(CH₃COO)₂·4H₂O, sodium dodecyl sulfate (SDS), and CH₄N₂S were added into the solution with continuous stirring for one hour. Then, the mixture solution was transferred to a 100 mL Teflon-lined autoclave and heated to 190 °C in an oven for 10 h. Finally, the product was cooled to room temperature and washed with deionized water and ethanol three times before being dried in a vacuum oven at 60 °C overnight. x%-α-NiS/g-C₃N₄ nanocomposites were prepared by varying the amount of NiS (x = 5, 10, 15, 20, 25), where x represented the mass ratio of α-NiS to g-C₃N₄.

3.3. Characterizations

The crystal structure of the obtained samples was characterized by an X-ray diffractometer (XRD-7000, Shimadzu Corporation, Kyoto, Japan). FT-IR spectra of the samples were obtained using Fourier transform infrared spectrometer (Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA). The morphology and microstructure were determined by emission electron microscopy (S-4800, Hitachi, Tokyo, Japan) and a high-resolution transmission electron microscope (Talos F200X, Thermo Fisher Scientific, Waltham, MA, USA). The surface composition and chemical state of the samples were measured by an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). Ultraviolet-visible diffuse reflection spectra (UV-vis DRS) were obtained using a UV-vis near-infrared spectrometer (Cary 5000, Agilent, Santa Clara, CA, USA). The time-resolved PL (TRPL) profiles were determined with a time-resolved PL spectrometer (FLS980, Edinburgh Instrument, Livingston, UK).

3.4. Evaluation of Photoelectrochemical Performance

Transient photocurrent and electrochemical impedance spectroscopy (EIS) measurements were conducted using a standard three-electrode system. First, 5.00 mg of the photocatalyst sample was added into 1.00 mL of mixture solution containing 800 µL isopropyl alcohol and 200 µL deionized water. Then, 40–60 μL of 5 vol% Nafion solution was added into the above-mentioned mixture solution. The mixture was dispersed evenly by ultrasonication and then spun onto FTO glass. The FTO glass coated with as-prepared samples was used as the working electrode, the standard Ag/AgCl electrode as the reference electrode, and the platinum electrode was used to construct a three-electrode system. The electrolyte was 0.100 M Na₂SO₄ solution, and the photoelectrochemical (PEC) performance of the sample was evaluated by an electrochemical workstation (CHI-660E, Shanghai Chenhua, Shanghai, China).

3.5. Evaluation of Photocatalytic Performance

The photocatalytic H₂ production and TC degradation were used to evaluate the photocatalytic performance of the prepared samples. In the experiment of H₂ production, 10 mg photocatalyst was ultrasonically dispersed into 70 mL aqueous solution including triethanolamine (TEOA, 15 vol%) with 25 mg of EY as a sacrificial agent, then the reactor was sealed, pumped, and purged with nitrogen for 30 min, and the oxygen in the reactor was emptied. A 300 W Xe lamp (λ > 400 nm) was placed at the top of the photoreactor, and the light source was turned on to start the photocatalytic reaction. Throughout the reaction, cooling water is passed through the reactor jacket to maintain a constant reaction temperature (15 °C). The reaction lasted for five hours, and samples were taken once every hour. The generated H₂ was measured by gas chromatography (5 Å molecular sieve-packed column). The detector was a thermal conductivity detector (TCD), and the carrier gas was high-purity nitrogen.

In addition, TC was selected as the target pollutant to evaluate the photodegradation performance of the α-NiS/g-C₃N₄ nanocomposites. Typically, a 30 mg photocatalyst was ultrasonically dispersed in 50 mL TC aqueous solution (20 mg/L) and stirred in the dark for 60 min to ensure that TC reached adsorption–desorption equilibrium. Then, the 300 W Xe lap source (λ > 400 nm) is activated to start the photodegradation reaction, and cooling water is injected to maintain a constant reaction temperature throughout the process. In the process of photodegradation, the photocatalyst was removed by 0.22 μm polyether sulfone membrane after taking samples at regular intervals. The absorbance of TC in the filtrate was measured at the maximum absorption wavelength λ_max = 375 nm by a UV-visible spectrophotometer.

4. Conclusions

α-NiS/g-C₃N₄ nanocomposites were constructed and evaluated for photocatalytic applications. By comparing x%-α-NiS/g-C₃N₄ (x = 5, 10, 15, 20, and 25) with various loading amounts of α-NiS, the photocatalytic H₂ evolution, and TC degradation first increased with the optimal α-NiS content for α-15%-α-NiS/g-C₃N₄ and then decreased with further increment of α-NiS content. The 15%-α-NiS/g-C₃N₄ showed the highest photocatalytic activities with an H₂ evolution rate of 4025 µmol·g⁻¹·h⁻¹ and 67.6% for TC removal rate. The heterojunctions formed between α-NiS and g-C₃N₄ increased absorption of visible light, facilitated charge carrier separation, and prolonged the lifetime of charge carriers, thus improving the photocatalytic efficiency. The ESR spectra and active species trapping experiments indicated that h⁺, ·OH, and ·O₂⁻ all contributed to TC photodegradation. A possible photocatalytic mechanism was proposed for the α-NiS/g-C₃N₄ nanocomposites based on their relative electronic energy levels and associated charge transfer processes.