Metallic plasmons significantly boosted visible-light photocatalytic hydrogen evolution from water splitting

Abstract

Under visible-light irradiation, graphitic carbon nitride (g-C₃N₄) is considered a favorable photocatalyst for hydrogen (H₂) production from water splitting. However, the poor H₂ production and fast recombination rate of charge carriers prevent its practical applications. Therefore, the integration of g-C₃H₄ with suitable plasmonic materials to develop a nanocomposite photocatalyst is worthwhile for enhancing H₂ evolution. Herein, a highly efficient g-C₃N₄/Ni@N-doped C (termed as CN/Ni@C) plasmonic photocatalyst is developed by the combination of g-C₃N₄ and nickel supported on nitrogen-doped carbon (Ni@C) for H₂ production from water splitting. The results show that photocatalytic performance is enhanced by Ni metallic plasmons over the CN/Ni@C nanocomposite. The optimized CN/Ni@C-1 (1 wt% Ni@C loading) plasmonic heterojunction achieves an efficient H₂ evolution rate of 56.67 μmol h⁻¹, which is 4-fold higher than that of bare g-C₃N₄ (13.55 μmol h⁻¹) with an apparent quantum yield (AQY) of 5.20% under visible-light irradiation (λ ≥ 420 nm). This improved performance is associated with the efficient charge separation, charge transfer, and surface plasmon resonance (SPR) effect of metallic Ni nanoparticles. Additionally, the optimal CN/Ni@C-1 plasmonic heterojunction exhibits excellent photocatalytic stability toward H₂ generation. We believe that this study will open the door to constructing and developing other plasmonic material decorated g-C₃N₄ photocatalysts for potential applications in sustainable and renewable energy.

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Introduction

The world's energy crisis is growing at a shocking rate owing to severe environmental problems; therefore, the development of economical and environment-friendly energy sources is essential nowadays.^1–3 Hydrogen energy is an attractive source of renewable energy due to its clean, sustainable, and environment-friendly features.^4–6 In the past few decades, non-renewable sources have been used to produce H₂; however, this depletes fossil fuel supplies and causes serious environmental issues.⁷ In 1972, Fujishima et al.⁸ discovered photo-electrocatalytic H₂ generation from water splitting over a TiO₂ electrode. Recently, researchers have developed various semiconductor-based photocatalysts for H₂ evolution, including conjugated polymers,⁹ nitrides,¹⁰ sulfides,¹¹ and oxides;¹² because of their low efficiency, poor stability, low charge separation, and high cost, such materials are restricted in practical applications. Thus, great efforts are being made to find promising materials with extraordinary photocatalytic efficiency and stability.

At present, g-C₃N₄ conjugated polymer has been considered a potential visible-light photocatalyst for H₂ generation from water splitting. Polymeric g-C₃N₄ is attractive as compared to conventional semiconductors, owing to its remarkable photocatalytic stability, easy synthesis, narrow bandgap, and capability of visible-light absorption.^13–15 Moreover, g-C₃N₄ is considered as a rich polymer source and readily synthesized by direct calcination of cheap, nitrogen-rich, and reliable compounds, in particular urea,¹⁶ thiourea,¹⁷ dicyanamide,^18,19 and melamine.²⁰ Despite its advantages, bare g-C₃N₄ appears to be restricted in terms of low photocatalytic efficiency and applicability because of poor light absorption and fast recombination of photoexcited charge carriers. To overcome these shortcomings, researchers have endeavoured to develop new approaches, including protonation, polymerization,²¹ increasing surface area,²² doping,²³ controlling morphology,²⁴ and coupling with other semiconducting materials²⁵ to enhance charge transfer and separation, thus improving the photocatalytic activity of g-C₃N₄.

The effective fabrication of heterojunctions with plasmonic metals is regarded as an effective strategy to overcome the above-mentioned limitations since it could instantaneously broaden the light absorption of materials and inhibit charge carrier recombination.^26,27 Plasmonic photocatalysts with a distinctive surface plasmon resonance (SPR) effect have a unique light absorption ability in the visible range. Additionally, plasmonic metals could be employed as electron-donating agents to improve photocatalytic performance.²⁸ Reported studies proved that under visible-light excitation, metallic plasmons generate hot electrons that could transfer to the attached semiconductor through the Schottky barrier.^29–32 Yi et al.³³ investigated the influence of SPR on enhancing photocatalytic performance over the constructed Ag/g-C₃N₄ plasmonic heterostructure. Zhao et al.³⁴ designed a plasmonic Au/g-C₃N₄ nanocomposite and explored the role of the SPR effect in enhancing H₂ evolution. However, these photocatalysts suffer from the shortcoming of the use of noble metals and relatively low photocatalytic activity. Ni is commonly used for different applications, including batteries, coins, and catalysts.^35–38 Recently, some studies have been reported to construct Ni-based heterostructured photocatalysts, which exhibited an efficient H₂ evolution,^39–42 but according to our understanding, metallic Ni plasmons are still less explored for the hydrogen evolution reaction under visible-light. Thus, the incorporation of plasmonic Ni metal into a semiconductor photocatalyst to form a heterostructure is essential to enhance optical absorption, and electron transfer and separation, which in turn increases the performance of the photocatalyst.

Herein, we design and fabricate g-C₃N₄/Ni@N-doped C (termed as CN/Ni@C) nanocomposites as proficient visible-light photocatalysts for H₂ generation from water splitting. The as-synthesized CN/Ni@C nanocomposites show a remarkable photocatalytic H₂ evolution rate with improved photostability due to strong interfacial interaction between g-C₃N₄ and plasmonic Ni@C NPs. The successful incorporation of plasmonic Ni@C onto g-C₃N₄ nanosheets favours the formation of an internal built-in electric field, which in turn boosts charge transport and reduces electron–hole pair recombination, which is beneficial for boosting H₂ generation. A series of CN/Ni@C-x samples (x is the weight content of Ni@C) were synthesized, displaying a higher H₂ production rate than bare g-C₃N₄. This work could encourage the development of other heterostructured plasmonic photocatalysts for high performance.

Experimental section

Materials

Urea (CH₄N₂O, ≥99%), triethanolamine (C₆H₁₅NO₃, TEOA, ≥78%), lactic acid (C₃H₆O₃, ≥85%), ascorbic acid (C₆H₈O₆, ≥95%), methanol (CH₃OH, ≥99.5%), ethanol (C₂H₅OH, ≥99.7%), chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, 37.5% Pt), dicyanamide (C₂H₄N₄, DCDA, ≥98%), ammonium sulfate (N₂H₈SO₄, ≥99%), nickel powder (Ni, ≥99.5%), sodium sulfate (Na₂SO₄, ≥99%), and Nafion solution (C₇HF₁₃O₅S·C₂F₄, 5%) were obtained from Sinopharm Chemical Reagent Co. Ltd. All materials were used without additional treatment.

Preparation of g-C₃N₄ nanosheets

In a conventional synthetic route, g-C₃N₄ was fabricated by calcining urea at 550 °C for 4 h. The light-yellow powder was cooled down to ambient temperature, and then labelled as g-C₃N₄.

Preparation of Ni@C nanoparticles

First, DCDA powder (20 g) and ammonium sulfate (0.08 g) were added in deionized water (20 mL), followed by stirring in an oil bath at 85 °C until the solvent was removed. After that, the resultant white powder (16 g) and nickel powder (0.14 g) were ground in an agate mortar and then annealed at 900 °C for 4 h in an N₂ flow. The resultant material was named Ni@C.

Preparation of g-C₃N₄/Ni@C (CN/Ni@C-x) nanocomposites

The g-C₃N₄/Ni@C plasmonic photocatalysts were prepared by grinding specified amounts of g-C₃N₄ and Ni@C in a mortar for 40 min. Then, the mixture was heated at 500 °C for 2 h in an N₂ flow. The resultant samples were marked as CN/Ni@C-x, (x = 0.5, 1, 2, and 3 wt% of Ni@C).

Results and discussion

Bare g-C₃N₄ was synthesized by calcination of urea at 550 °C for 4 h, while metallic Ni supported on nitrogen-doped carbon (Ni@C) was prepared via a sulfur-diffusion method; for construction of CN/Ni@C plasmonic photocatalysts, Ni@C was well-mixed with g-C₃N₄ in an agate mortar and annealed at 500 °C for 2 h under a N₂ flow (see the Experimental section, Fig. S1†). The structure and crystallinity of Ni@C, g-C₃N₄, and CN/Ni@C-1 samples were investigated by X-ray diffraction (XRD) measurements. As displayed in Fig. 1a, one broad peak at 27.7° is attributed to the (002) crystal plane induced by interlayers of conjugated aromatic structures, while the weak peak which appeared at 13.3° can be assigned as the (100) plane, which is associated with the compositional pattern of g-C₃N₄.^43,44 From the XRD pattern of Ni@C, it is clearly seen that metallic Ni⁰ was formed. The XRD pattern of the CN/Ni@C-1 sample displays similar peaks to that of g-C₃N₄, which suggests that the incorporation of Ni@C did not alter the structure of g-C₃N₄. The FTIR spectra of Ni@C, g-C₃N₄, and CN/Ni@C-x samples are illustrated in Fig. 1b. It is clearly seen that CN/Ni@C-x nano-heterostructures have similar bands to those of g-C₃N₄ with no shift. The bands located at 3000 to 3500 cm⁻¹ are attributed to the –NH and –OH stretching vibrations, respectively, owing to unconverted amino groups and water physisorption.⁴⁵ The extensive bands starting from 1220 to 1645 cm⁻¹ are attributed to the conventional extending forms of CN heterocycles.⁴⁶ Furthermore, the band which appeared at around 813 cm⁻¹ is associated with the stretching vibration of s-triazine modes.⁴⁷ Consequently, the incorporation of Ni@C has no obvious influence on the molecular arrangement of g-C₃N₄ nanosheets. Fig. 1c shows UV-vis diffuse reflectance spectra (DRS) of g-C₃N₄ and CN/Ni@C-x photocatalysts. In comparison to bare g-C₃N₄, the absorption band edges of the CN/Ni@C-x nanocomposites show small blue shifts. Moreover, CN/Ni@C heterostructured photocatalysts have increased light-absorption in the visible range as compared to g-C₃N₄, as a result of metallic Ni@C plasmons. This result confirms the successful incorporation of Ni@C onto the g-C₃N₄ surface. The UV-DRS spectra of Ni@C NPs is depicted in Fig. S2.† Furthermore, the electron paramagnetic resonance (EPR) test was conducted to evaluate the role of Ni@C in charge transport and the inherent electronic configuration of the samples. As depicted in Fig. 1d, the samples present an isotropic signal (g = 2.0034), which is ascribed to the delocalized conducting electrons.⁴⁸ The EPR intensity of the CN/Ni@C-1 nanocomposite is enhanced as compared to that of g-C₃N₄, thus providing the maximum concentration of conducting electrons and electronic conductivity of CN/Ni@C-1. It is concluded that incorporating plasmonic Ni@C NPs onto g-C₃N₄ could help the formation of unpaired electrons and radicals, thereby optimizing the electronic band structure for effective charge separation and transfer. Consequently, more photoinduced conducting electrons are helpful for enhanced photocatalytic hydrogen evolution.

Fig. 1 (a) XRD patterns of Ni@C, g-C₃N₄, and CN/Ni@C-1 photocatalysts. (b) FTIR spectra of Ni@C, g-C₃N₄, and CN/Ni@C-x samples (x represents the weight content of Ni@C in the samples). (c) UV-vis DRS of g-C₃N₄ and CN/Ni@C-x samples. (d) EPR plot of g-C₃N₄ and CN/Ni@C-1 photocatalysts.

The morphology and structure of the g-C₃N₄, Ni@C, and CN/Ni@C-1 samples were examined by transmission electron microscopy (TEM). As presented in Fig. 2a–c, Ni@C shows a nanotube like structure, and g-C₃N₄ shows a traditional layered sheet-like structure. While the CN/Ni@C-1 nanocomposite photocatalyst exhibits similar morphology to g-C₃N₄, this result obviously reveals that the incorporation of plasmonic Ni@C did not change the morphology of g-C₃N₄. The scanning electronic microscopy (SEM) images of Ni@C, g-C₃N₄, and CN/Ni@C-1 are displayed in Fig. S3,† which is in agreement with the TEM results. In addition, high-angle annular dark-field scanning transmission microscopy (HAADF-STEM) and its corresponding EDS elemental mapping images (Fig. 2d–g) show that C, N, and Ni elements are present in CN/Ni@C-1, confirming the formation of CN/Ni@C-1 heterostructures.

Fig. 2 TEM images of (a) Ni@C NPs, (b) g-C₃N₄ nanosheets, and (c) the CN/Ni@C-1 nanocomposite. (d–g) HAADF-STEM image and its corresponding EDS elemental mapping images of the CN/Ni@C-1 photocatalyst.

The X-ray photoelectron spectroscopy (XPS) measurements were conducted to investigate the chemical state and composition of the as-synthesized samples. XPS survey spectra prove the presence of C, N, and Ni elements in the optimal CN/Ni@C-1 heterojunction (Fig. 3a). The C 1s spectrum can be deconvoluted into three peaks that occur at 284.4, 285.3, and 288.3 eV (Fig. 3b). The peak at 284.4 eV is ascribed to sp²-hybridized carbon contamination, such as C=C, whereas the peaks at 285.3 and 288.3 eV are readily assigned to covalently bonded carbon located within N–C=N and C–NH₂ in the carbon nitride interlayers.⁴⁹ The N 1s spectra can be split into four peaks that appeared at 398.8, 400.2, 401.3, and 404.5 eV (Fig. 3c). The peaks positioned at 398.8, 400.2, and 401.3 eV are attributed to sp²-hybridized confined nitrogen in the core of C-atoms from the conjugated ring (C=N–C) and tertiary nitrogen in the C3–N groups, respectively.⁵⁰ The weak peak at 404.5 eV is attributed to the existence of π–π localization of the C–N heterocycles.⁵¹Fig. 3d illustrates the Ni 2p spectra of the CN/Ni@C-1 sample displaying two main peaks at 854.7 (Ni 2p_3/2) and 872.4 eV (Ni 2p_1/2). This result indicates the existence of Ni²⁺ in the CN/Ni@C-1 sample, which is ascribed to the surface oxidation of metallic Ni⁰ upon exposure to air. Consequently, the XPS results confirm that plasmonic Ni@C was successfully introduced into g-C₃N₄.

Fig. 3 (a) XPS survey spectra of Ni@C, g-C₃N₄, and CN/Ni@C-1 photocatalysts. (b) High-resolution XPS spectra of C 1s, (c) N 1s, and (d) Ni 2p of the CN/Ni@C-1 photocatalyst.

Photoluminescence (PL) measurements were carried out to evaluate the efficiency of charge separation and recombination. The PL spectra of g-C₃N₄ and CN/Ni@C-1 exhibit wide emission peaks around 450 nm (Fig. 4a). Specifically, the decreased PL intensity of the CN/Ni@C-1 sample as compared to that of g-C₃N₄, suggests that efficient inhibition of photoinduced e⁻/h⁺ recombination is found for the CN/Ni@C-1 heterojunction, which is beneficial in promoting H₂ production. The time-resolved photoluminescence (TRPL) spectra were conducted to evaluate the lifetime of photoinduced charge carriers. As illustrated in Fig. 4b, the average lifetimes for g-C₃N₄ and CN/Ni@C-1 are 2.27 ± 0.39 ns and 1.69 ± 0.27 ns, respectively. This reduction in the charge carrier lifetime implies the efficient transport and separation of photoinduced e⁻/h⁺ pairs.⁵² Furthermore, transient photocurrent tests were performed in a conventional three-electrode arrangement with several on/off visible-light irradiation cycles to investigate charge transfer over g-C₃N₄ and CN/Ni@C-1 samples (Fig. 4c). The transient photocurrent response of the CN/Ni@C-1 sample is higher than that of g-C₃N₄, demonstrating that in the presence of plasmonic Ni@C NPs, the separation and transfer of photoinduced charge carriers is highly accelerated,⁵³ which agrees well with the PL and TRPL results. Fig. 4d represents the interfacial charge transfer resistance measured by electrochemical impedance spectroscopy (EIS) Nyquist analysis. Clearly, the arc diameter of the CN/Ni@C photocatalyst is relatively narrower than that of g-C₃N₄, demonstrating that the introduction of plasmonic Ni@C NPs can reduce charge transfer resistance and enhance charge separation.⁵⁴ As such, the photoelectrochemical results confirm that plasmonic Ni@C anchored on g-C₃N₄ could prevent the recombination of photoinduced e⁻/h⁺ pairs and in turn improve H₂ generation.

Fig. 4 (a) PL spectra of g-C₃N₄ and CN/Ni@C-1 samples. (b) TRPL spectra of g-C₃N₄ and CN/Ni@C-1 photocatalysts. (c) Transient photocurrent response curves of g-C₃N₄ and CN/Ni@C-1 samples under on/off visible-light irradiation cycles. (d) EIS Nyquist curves of pristine g-C₃N₄ and CN/Ni@NC-1 samples under visible-light irradiation.

We carried out comparative photocatalytic H₂ evolution experiments over the obtained photocatalysts under visible-light irradiation (Fig. 5). TEOA is found to be a more efficient sacrificial agent as compared to other sacrificial reagents after evaluating the H₂ evolution rate over the optimal CN/Ni@C-1 sample (see Fig. S4†). Pristine g-C₃N₄ without the Pt cocatalyst achieves a minimum H₂ generation rate of 0.53 μmol h⁻¹ owing to its quick charge recombination (Fig. 5a). In the presence of 1 wt% Pt cocatalyst, the H₂ evolution rate is increased to 13.55 μmol h⁻¹ over g-C₃N₄. To confirm the role of plasmonic Ni@C, a comparison test was conducted on CN/Ni@C-1 without 1 wt% Pt, which shows 2.07 μmol h⁻¹ H₂ evolution rate. Moreover, the H₂ evolution rate increased significantly to 56.67 μmol h⁻¹ over the CN/Ni@C-1 plasmonic photocatalyst with 1 wt% Pt, which is 4 times higher than that of g-C₃N₄. As such, it is concluded that plasmonic Ni@C loaded on g-C₃N₄ to construct heterojunctions significantly improves visible-light absorption, charge transport and separation, thus leading to an enhanced H₂ evolution rate. We next studied the influence of the weight contents of Ni@C in CN/Ni@C-x samples on photocatalytic activity. The hydrogen evolution rate of 0.45 μmol h⁻¹ is achieved over the pure Ni@C sample (Fig. 5b). Furthermore, the H₂ evolution rate increased with an increase of Ni@C contents in the CN/Ni@C photocatalysts, and then declined when the content of Ni@C further increased. The optimized H₂ evolution rate of 56.67 μmol h⁻¹ is achieved over the CN/Ni@C-1 sample, which is 4 times to that of bare g-C₃N₄ (13.55 μmol h⁻¹), and higher than that of g-C₃N₄ photocatalysts fabricated earlier (see Table S1†). As the weight content of Ni@C increased up to 3 wt%, the H₂ evolution rate reduced to 27.76 μmol h⁻¹. A possible reason for this might be that excess Ni@C could contribute to the shielding effect, which blocks active centers and inhibits visible-light from the surface of g-C₃N₄.⁵⁵ It is concluded that a proper amount of plasmonic Ni@C is vital to boost the performance of CN/Ni@C plasmonic photocatalysts. Additionally, Brunauer–Emmett–Teller (BET) surface area of CN/Ni@C-1 (91 m² g⁻¹) is higher than that of g-C₃N₄ (74 m² g⁻¹) (Fig. S5a†), and the pore size distribution curve reveals that CN/Ni@C-1 possesses more mesopores than g-C₃N₄ (Fig. S5b†); this result suggests that the incorporation of plasmonic Ni@C onto g-C₃N₄ leads to an increase in surface areas. Furthermore, the AQY of CN/Ni@C-1 was measured via an extensive range of monochromatic light irradiation wavelength (λ = 420, 450, 500, 550 and 600 nm ± 5 nm). As depicted in Fig. 5c, the AQY decreased with increasing wavelength, which well agrees with the UV-vis absorption spectrum of CN/Ni@C-1, and it is noted that an AQY of 5.20% is obtained at λ ≥ 420 nm. The stability and recycling are the most crucial factors to estimate the possibility of photocatalysts for practical photocatalytic applications. The recycling tests were carried out to evaluate the stability of the CN/Ni@C-1 sample. As presented in Fig. 5d, after 5 consecutive cycles (each cycle of 4 hours) under the same reaction conditions, no distinguishable decline is noticed in photocatalytic H₂ evolution. Besides, no noticeable alteration is detected in the XRD patterns of the CN/Ni@C-1 sample before and after the photocatalytic reaction (Fig. S6†). This result proves that the CN/Ni@C-1 plasmonic photocatalyst has outstanding photocatalytic stability and reusability without structural deformation.

Fig. 5 (a) Control experiments of H₂ evolution over various synthesized photocatalysts (a₁, g-C₃N₄ in the absence of Pt, a₂, g-C₃N₄ in the presence of Pt, a₃, CN/Ni@C-1 in the absence of Pt, and a₄, CN/Ni@C-1 in the presence of Pt). (b) Photocatalytic H₂ evolution over the Ni@C, g-C₃N₄, and CN/Ni@C-x samples. (c) The wavelength-dependent AQY of H₂ evolution over the CN/Ni@C-1 sample. (d) The stability test of the CN/Ni@C-1 photocatalyst.

Taken together, the improved H₂ evolution from water splitting over CN/Ni@C-x photocatalysts is indeed steered by the construction of the heterostructures from g-C₃N₄ and plasmonic Ni@C NPs which leads to highly efficient charge carrier generation, separation and migration. A possible mechanism is proposed for g-C₃N₄/Ni@C plasmonic photocatalysts to understand the phenomena of photocatalytic H₂ generation from water splitting. The valence band (VB) and conduction band (CB) of g-C₃N₄ are located at −6.1 eV and −3.4 eV vs. the absolute vacuum scale, respectively.⁵⁶ The measured optical bandgap of g-C₃N₄ is 2.75 eV (Fig. S7†). No band gap is detected for Ni@C, indicating its metallic characteristics. While the Fermi level of metallic Ni is positioned at −4.59 eV vs. vacuum level.⁵⁷ As such, plasmonic Ni is promoted to excited states via 3d electron excitation due to its strong surface plasmon absorption under visible-light irradiation (Fig. 6). The excited Ni nanoparticles produced a lot of surface plasmons and these surface plasmons decay quickly and generate hot electrons, which jump to the above Fermi level of Ni, and plasmonic hot electrons are transported to the CB of g-C₃N₄ because of the surface plasmon resonance (SPR) effect, then further transferred to Pt active sites for H₂ evolution (reductive product). Simultaneously, to retain charge equilibrium, photoinduced holes in g-C₃N₄ are quenched by TEOA to generate TEOA⁺ (oxidative product). Therefore, continuously photoinduced hot electrons are transported from plasmonic Ni NPs, and effective separation and transport of photoexcited e⁻/h⁺ leads to a significantly improved H₂ evolution rate.

Fig. 6 Schematic presentation of the energy level, charge separation and transfer over g-C₃N₄/Ni@C plasmonic photocatalysts under visible-light irradiation for H₂ evolution from water splitting.

Conclusions

In summary, CN/Ni@C-x plasmonic photocatalysts have been successfully developed and fabricated. The optimal CN/Ni@C-1 plasmonic heterojunction displays an optimized photocatalytic H₂ evolution rate of 56.67 μmol h⁻¹ with an AQY of 5.20% under visible-light irradiation (λ ≥ 420 nm). The PL, TRPL, transient photocurrent response, and EIS analysis of the CN/Ni@C-1 sample show higher charge separation efficiency as compared to bare g-C₃N₄, and a possible mechanism of plasmon-enhanced photocatalytic performance is proposed. Moreover, the as-synthesized CN/Ni@C-1 nano-heterostructure exhibits good photocatalytic stability under visible-light irradiation for 20 h. The boosted photocatalytic activity is associated with efficient charge separation, charge transport, and the SPR effect of Ni NPs. Overall, our study could provide a creative strategy for developing other metallic plasmon-modified graphitic carbon nitride photocatalytic systems to fulfil the renewable energy demands.

Conflicts of interest

The authors confirm no conflict of interest.

Acknowledgements

The authors appreciatively acknowledge the special financial support from the National Natural Science Foundation of China (22271266, 21771171, and 82102905), the Scientific Research Grant of the Hefei National Synchrotron Radiation Laboratory (UN2017LHJJ), and the Fundamental Research Funds for the Central Universities (YD2340002001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Characterization studies, photoelectrochemical tests, and photocatalytic H₂ evolution measurements. Schematic illustration of the as-synthesized samples. UV-DRS specturm of Ni@C sample. SEM images of Ni@C, g-C₃N₄, and CN/Ni@C-1 photcatalysts. Photocatalytic H₂ evolution rate over CN/Ni@C-1 nanocomposite under various sacrificial agents. Comparison table of the photocatalytic H₂ evolution rate of our study with previous reports. Nitrogen adsorption–desorption isotherms and pore size distribution of g-C₃N₄ and CN/Ni@C-1 samples. XRD spectra of CN/Ni@C-1 before and after the recycling experiment. Plot of energy band gap of g-C₃N₄. See DOI: https://doi.org/10.1039/d2se01523d

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