Accelerating photocatalytic hydrogen production by anchoring Pt single atoms on few-layer g-C₃N₄ nanosheets with Pt–N coordination

Abstract

Graphitic carbon nitride (g-C₃N₄) has gained considerable attention as a promising photocatalyst for hydrogen production through water splitting. However, its catalytic efficiency remains severely limited due to the rapid recombination of charge carriers and poor charge-transfer properties. Here, g-C₃N₄ is subjected to modification through the introduction of well-isolated Pt single atoms using a low-temperature incipient wetness impregnation method. The Pt single atoms exhibit a maximum weight ratio of 1.26%, resulting in a giant enhancement of the photocatalytic H₂ evolution rate (336.8 μmol h⁻¹), approximately two orders of magnitude higher than that of pristine g-C₃N₄ (1.8 μmol h⁻¹) during a 22-h-long test with an apparent quantum yield (AQY) of 13.5% at 405 nm. The improved performance and excellent stability in photocatalytic H₂ evolution can be attributed to the formation of Pt–N bonds between Pt single atoms and g-C₃N₄, which creates a new energy level of the N 2p–Pt 5d hybrid orbital for remarkably inhibiting the recombination of photogenerated electron–hole pairs and reducing interfacial charge-transfer resistance.

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1. Introduction

Photocatalytic water splitting is anticipated to be an eco-friendly and enduring method for generating hydrogen (H₂) devoid of detrimental byproducts or emissions.^1–3 Extensive research has been conducted to identify the most suitable semiconductor-based photocatalysts that possess visible-light responsivity, stability, and reusability. In particular, carbon nitride allotropes have received extensive attention in photocatalysis due to their extended conjugation for fast separation and transfer of the charges.^4–6 Some studies indicate that graphitic carbon nitride (g-C₃N₄) has emerged as a highly promising candidate for photocatalytic water splitting due to its distinctive electronic structure, exceptional photoelectric characteristics, remarkable stability, and cost-effectiveness.^7–11 However, the efficiency of g-C₃N₄ in H₂ production is hindered by the limited carrier mobility and the rapid recombination of electron–hole pairs generated during the photocatalytic process.^12–14 To enhance the efficiency of photocatalytic H₂ production, a significant approach is the incorporation of cocatalysts, which can facilitate charge transfer and provide reactive sites.

Platinum (Pt) has been widely acknowledged as one of the most efficient cocatalysts for photocatalytic H₂ production due to its low overpotential and favorable Gibbs free energy in H₂ adsorption.^15–17 Traditional large-scale particulate Pt cocatalysts exhibit limited low atomic utilization and a scarcity of active sites, significantly constraining the utilization efficiency of Pt atoms. Consequently, it becomes imperative to incorporate Pt species in the form of clusters or even isolated single atoms, as this approach offers a viable strategy to optimize atomic efficiency.¹⁸ With the reduction in the volume of the particulate cocatalyst, there is a progressive enhancement in both the specific surface area and atomic utilization. When the volume is decreased to atomic clusters comprising merely a few dozen atoms, a pronounced interaction between the support and the Pt atoms emerges, exerting a substantial effect on the photocatalytic process.^19–21 Ultimately, upon reducing the cocatalysts to a state of single atom dispersion, the ideal atom utilization attains 100% and the selectivity achieves its utmost level.^22–25 The presence of homogenous pores in layer-structured g-C₃N₄ offers stable coordination sites for anchor atoms, while the N atoms within the structure facilitate bonding between pyridine N and active atoms. Thus, g-C₃N₄ can serve as an appropriate support for the deposition of Pt single atoms, enabling efficient photocatalytic H₂ production.²⁶ In recent years, numerous investigations have documented diverse approaches for synthesizing single-atom Pt catalysts on g-C₃N₄, including ion exchange,²⁷ reflux reaction,²⁸ photoreduction,^29–33 and high-temperature pyrolysis.^34–38 Nevertheless, these methods face constraints when it comes to attaining high loading levels of single-atom Pt catalysts. In this respect, preparing Pt single atom anchored g-C₃N₄ with high loading and secure trapping is necessary to develop highly efficient photocatalysts.

In this study, the synthesis of Pt single atom anchored g-C₃N₄ nanosheets with varying Pt contents (Pt_x/g-C₃N₄) has been successfully achieved through a low-temperature incipient wetness impregnation method followed by high-temperature pyrolysis. The incipient wetness impregnation method is frequently employed to enhance the loading capacity of single atoms,^39–41 and lowering the impregnation temperature can additionally suppress the occurrence of agglomeration during the impregnation process. By employing the sequence of low-temperature incipient wetness impregnation, lyophilization, and high-temperature pyrolysis, the dispersion of high-density single atoms can be effectively improved. Characterization analyses have revealed that the isolated Pt single atoms are uniformly dispersed on the surface of g-C₃N₄, and the formation of Pt–N bonds notably enhances the charge separation and transfer capabilities. The photocatalytic rate of H₂ evolution for Pt_x/g-C₃N₄ demonstrates a notable increase compared to both pristine g-C₃N₄ and g-C₃N₄ modified with Pt nanoparticles.

2. Experimental

2.1. Synthesis of bulk g-C₃N₄ powder

Bulk g-C₃N₄ powder was synthesized through the thermal polymerization of urea. Specifically, 10 g of urea powder was placed in a closed alumina crucible within a muffle furnace (KSL-1100X-S, MTI, China) under a N₂ atmosphere. The furnace was gradually heated to 550 °C at a rate of 1 °C min⁻¹ and maintained at this temperature for a duration of 2 h. Subsequently, the resulting yellow g-C₃N₄ powder was collected and allowed to cool to room temperature before further use.

2.2. Synthesis of few-layer g-C₃N₄ nanosheets

Few-layer g-C₃N₄ nanosheets were obtained through the process of ultrasonic-assisted liquid exfoliation. 100 mg of bulk g-C₃N₄ powder were dispersed in 100 mL of isopropanol and subjected to exfoliation using an ultrasonic crusher with a power of 1 kW for 12 h at room temperature. The resulting dispersion was then centrifuged using a high-speed centrifuge (HC-3018, Zonkia, China) at a speed of 8000 rpm for 10 min. The upper liquid was dried in a vacuum at 50 °C for 6 h to obtain pale yellow few-layer g-C₃N₄ nanosheets.

2.3. Synthesis of Pt_x/g-C₃N₄

The g-C₃N₄ nanosheets were loaded with Pt single atoms through low-temperature incipient wetness impregnation. A stable suspension was prepared by dispersing 100 mg of the as-prepared few-layer g-C₃N₄ nanosheets in 100 mL of deionized water and subjecting to ultrasonic treatment at a power of 200 W for 30 min. Subsequently, 0.5, 1, and 1.5 mL of H₂PtCl₆ solutions with a Pt concentration of 1 g L⁻¹ were added drop by drop to the suspension. The resulting mixture was then transferred to a refrigerator and allowed to stand for 12 h at 0 °C to facilitate abundant adsorption, followed by rapid cooling to −60 °C. After undergoing freeze-drying in a freeze dryer (SCIENTZ-10N, Ningbo Scientz, China), the as-dried powders were transferred to a muffle furnace under a N₂ atmosphere and subjected to a heating process at 450 °C for 2 h to facilitate the generation of Pt single atoms on the surface of g-C₃N₄. Finally, the resulting products were dissolved in deionized water, filtered with a 0.22 μm PTFE filter membrane, and dried using a freeze dryer to remove any impurities. The as-prepared samples were designated as Pt_0.5/g-C₃N₄, Pt₁/g-C₃N₄, and Pt_1.5/g-C₃N₄, respectively. To establish a control, 10 mL of H₂PtCl₆ solution with a Pt concentration of 1 g L⁻¹ was introduced into 100 mL of g-C₃N₄ solution and stirred for 30 min under xenon lamp irradiation to form Pt nanoparticles on few-layer g-C₃N₄.

2.4. Characterization

The Pt content of the samples was quantified using inductively coupled plasma-optical emission spectroscopy (ICP–OES, Agilent 5510, USA). The surface morphology was characterized with a scanning electron microscope (SEM, Regulus8230, Hitachi) and a transmission electron microscope (TEM, JEM-F200, JEOL) at an acceleration voltage of 200 kV. The identification of crystalline phases, chemical functional groups, and surface chemical states was performed using X-ray diffraction (XRD, Rigaku Miniflex 600, Japan), Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS20, USA), and X-ray photoelectron spectrometry (XPS, Thermo Scientific K-Alpha, USA), respectively. The surface area analyzer (Quantachrome Nova 4000e, USA) was utilized to obtain the Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution. Diffuse reflectance spectroscopy (DRS, Hitachi UH5700, Japan) was employed to acquire the UV-vis absorption spectroscopy data. High angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) images were captured using an aberration corrected JEOL JEM-ARM200F operating at 200 keV. X-ray absorption spectroscopy (XAS) measurements of the Pt element were conducted at the XAS beam line of the Shanghai Synchrotron Radiation Facility (SSRF) utilizing an Si(111) double-crystal monochromator, where the experimental data were acquired in the fluorescence mode using an ionization chamber. The steady and time-resolved photoluminescence (PL) spectra were measured using a photoluminescence spectrometer (Edinburgh FLS1000, UK).

2.5. Photoelectrochemical measurements

Electrochemical measurements were performed using an electrochemical workstation (CHI 660E, China) with a standard three-electrode system. The Ag/AgCl electrode and Pt foil were used as the reference electrode and counter electrode in 0.5 M Na₂SO₄ aqueous solution, respectively. Fluorine–tin oxide (FTO) glass coated with the photocatalyst was used as the working electrode. To prepare the working electrode, 5 mg of the photocatalyst was dispersed in 1 mL of ethanol and 20 μL of Nafion anisole solution (5%) under ultrasonic treatment for 30 min to form a homogeneous slurry. Then, 50 μL of the obtained slurry was dropped on the conductive surface (10 mm × 10 mm) of FTO (10 mm × 30 mm × 1.1 mm) using a pipette and dried at 60 °C for 2 h. The transient photocurrent curves were measured in the i–t mode by periodically switching a 300 W Xe-lamp (PLS-SXE300, Perfectlight, China) at a 20 s interval under a continuous bias of 0.2 V. The electrochemical impedance spectroscopy (EIS) was conducted at an open circuit voltage with frequency ranging from 0.01 to 100 kHz and an amplitude of 5 mV.

2.6. Photocatalytic H₂ evolution

Photocatalytic water splitting reactions were implemented in a photocatalytic trace-gas generation and analysis system (Labsolar-6A, Perfectlight, China), involving a sealed top-irradiation quartz vessel connected to a closed gas circulation system. In a typical experiment, 50 mg of photocatalysts were added to 90 mL of deionized water and 10 mL of triethanolamine (TEOA) was added as a sacrificial agent. Prior to the photocatalytic test, the air in the reactor is evacuated using a vacuum pump. The evolved H₂ gas was analyzed every 30 min using gas chromatography (GC-2060, FULI, China) with N₂ serving as carrier gas. During the photocatalytic test, a simulated sunlight source was provided by the 300 W Xe-lamp. The reaction temperature was maintained at 2 °C using a cooling water circulator. The apparent quantum yield (AQY) of Pt_1.5/g-C₃N₄ was determined using the same equipment under irradiation passing through different bandpass filters (365, 405, 420 and 500 nm, respectively) for 4 h. The AQY value was calculated using eqn (1),where R represents the H₂ evolution rate, N_A is the Avogadro constant, h is the Planck constant, c is the speed of light, I is the light intensity, A is the irradiation area, and λ is the illumination wavelength.

3. Results and discussion

g-C₃N₄ samples loaded with varying Pt contents have been successfully synthesized using the low-temperature incipient wetness impregnation method, and the ICP–OES analysis reveals that the actual Pt loading percentages of Pt_0.5/g-C₃N₄, Pt₁/g-C₃N₄, Pt_1.5/g-C₃N₄ and Pt_NP/g-C₃N₄ are determined to be 0.38%, 0.90%, 1.26%, and 3.95%, respectively. The observed loss in Pt content can potentially be attributed to incomplete adsorption of Pt species on the surface of g-C₃N₄ during low-temperature treatment. The XRD patterns of all the samples exhibit two characteristic diffraction peaks at 12.9° and 27.7° (Fig. 1a), which correspond to the (100) and (002) planes of hexagonal g-C₃N₄. No discernible peaks or peak-shifts associated with Pt species can be observed for the Pt_x/g-C₃N₄ samples, suggesting that the presence of Pt does not significantly alter the crystal structure of g-C₃N₄. The FTIR spectrum of few-layer g-C₃N₄ demonstrates distinct peaks at approximately 3100–3500, 1200–1600, and 810 cm⁻¹, which correspond to the vibrational modes of N–H/O–H bonds, aromatic C–N heterocyclic units, and triazine units, respectively (Fig. 1b).³⁴ The FTIR spectra of the Pt_x/g-C₃N₄ samples exhibit similar peaks to that of pristine g-C₃N₄, suggesting that the introduction of Pt species does not alter the chemical composition of g-C₃N₄.

Fig. 1 (a) XRD patterns. (b) FTIR spectra.

The BET analyses are depicted in Fig. 2. The N₂ adsorption–desorption isotherms of the samples exhibit a type-IV behavior (Fig. 2a), indicating the presence of mesopores. The BET surface areas of g-C₃N₄ and Pt_0.5/g-C₃N₄ are 71.097 and 132.904 m² g⁻¹, respectively. Fig. 2b displays a quite wide distribution of pore size, confirming the existence of both micropores and mesopores. Interestingly, the pore volumes of pristine g-C₃N₄, Pt_0.5/g-C₃N₄, Pt₁/g-C₃N₄, and Pt_1.5/g-C₃N₄ are 0.322, 0.539, 0.268, and 0.302 cm³ g⁻¹, while the mean pore diameters of these samples are 2.571, 1.897, 2.197, and 1.963 nm, respectively, implying that the adsorption behavior is primarily influenced by micropores and the presence of Pt atoms can enhance the formation of micropores to provide more active sites. The significant increase in the surface area and pore volume of Pt_0.5/g-C₃N₄ can be attributed to the fact that the uniform dispersion of isolated Pt atoms can enlarge the interlayer distance of g-C₃N₄ structures.^27,42,43 As the Pt content exceeds 0.5%, the specific surface area and pore volume exhibit a declining trend, but Pt_x/g-C₃N₄ still demonstrates the ability to offer an increased number of micropores as active sites for the photocatalytic reaction.

Fig. 2 (a) N₂ adsorption–desorption isotherm. (b) Pore size distributions.

Fig. 3 presents the XPS analysis conducted on the samples to examine their chemical composition and oxidation states. The survey spectra indicate the presence of C and N in the g-C₃N₄ sample and Pt in the Pt_x/g-C₃N₄ and Pt_NP/g-C₃N₄ samples (Fig. 3a). The high-resolution C 1s spectra are well fitted to three distinct peaks at 287.98, 286.02, and 284.6 eV, corresponding to the N–C=N, C–(N)₃, and C=C bonds, respectively (Fig. 3b).⁴⁴ In the N 1s spectrum of g-C₃N₄, there are three characteristic peaks including 398.36 eV for C–N=C, 400.04 eV for N–(C)₃, and 401.00 eV for C–N–H, respectively (Fig. 3c).⁴⁴ Compared to g-C₃N₄, the three peaks of N 1s shift towards higher energy with increasing Pt loading.⁴⁰ For Pt_1.5/g-C₃N₄, the shift values assigned to C–N=C, N–(C)₃, and C–N–H groups are 0.09, 0.14, and 0.13 eV, respectively. The binding energy of N 1s in Pt_NP/g-C₃N₄ exhibits a similar shift.⁴⁵ In Fig. 3d, the Pt 4f spectrum of Pt_NP/g-C₃N₄ is divided into two sets of double peaks, with the high-energy and low-energy peaks corresponding to the high-oxidation state of Pt clusters and metallic Pt nanoparticles, respectively.^28,46,47 The Pt 4f_7/2 and 4f_5/2 orbitals related to Pt nanoparticles are located at 72.05 and 75.25 eV, while those of Pt_x/g-C₃N₄ are located at 72.45 and 75.65 eV, respectively.^46,48 The Pt 4f orbitals of Pt_x/g-C₃N₄ roughly corresponding to Pt²⁺ species positively shift by 0.4 eV compared to Pt nanoparticles, which is ascribed to the weakened shielding effect due to the lack of 5d electrons of the outer orbital.^47,49 Therefore, the alteration in the electron density of the N atom is ascribed to the modification of the Pt atom, thereby confirming the robust electronic interaction between Pt and N atoms (Pt–N bond) at higher density of Pt single atoms.

Fig. 3 XPS spectra of the samples. (a) Survey. (b) C 1s. (c) N 1s. (d) Pt 4f.

The distribution and configuration of Pt atoms at high loading density in Pt_1.5/g-C₃N₄ are accurately determined through the characterization of an HAADF–STEM image, as illustrated in Fig. 4. Fig. 4a and b are the SEM images of Pt_1.5/g-C₃N₄ and Pt_NP/g-C₃N₄, indicating that the nanosheets remain ultra-thin after the introduction of Pt species. The corresponding high-resolution TEM images are shown in Fig. 4c and d, respectively. The Pt nanoparticles can only be observed in Pt_NP/g-C₃N₄. Fig. 4e provides a low-resolution STEM depiction of the crumpled few-layer structure of Pt_1.5/g-C₃N₄. The elemental mapping profiles presented in Fig. 4f–h demonstrate the homogeneous distribution of C, N, and Pt elements, confirming the presence of trace Pt atoms dispersed throughout g-C₃N₄. Furthermore, the HAADF–STEM image in Fig. 4i reveals densely dispersed bright spots corresponding to Pt atoms, which exist in an isolated single-atom state on the surface of g-C₃N₄. To quantify the dispersion of Pt single atoms, the distance between adjacent Pt single atoms is determined to be approximately 0.5 nm (Fig. 4j), suggesting a high level of dispersion and individualization of Pt atoms.

Fig. 4 Morphology of Pt_1.5/g-C₃N₄ and PtNP/g-C₃N₄. (a) and (b) SEM images of Pt_1.5/g-C₃N₄ and PtNP/g-C₃N₄. (c) and (d) TEM images of Pt_1.5/g-C₃N₄ and PtNP/g-C₃N₄. (e) Low-resolution STEM image. (f)–(h) Elemental mapping of C, N, and Pt. (i) HAADF-STEM image. (j) Intensity profile of two Pt single atoms along the middle of the red rectangle in (i).

The detailed atomic-level structure and coordination information on Pt species is provided by synchrotron-based X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) at the Pt L₃-edge. Pt₁/g-C₃N₄ and Pt_1.5/g-C₃N₄ exhibit identical XANES spectra with the absorption edge intensity falling between that of Pt foil and PtO₂ (Fig. 5a), confirming that the Pt single atoms in Pt_x/g-C₃N₄ are highly reduced to an oxidation state between Pt⁰ and Pt⁴⁺.⁵⁰ The correlation between the peak intensity of L₃-edge absorption and the unoccupied density of states of Pt 5d orbitals is evident.⁵¹ Thus, the Pt atoms in Pt_x/g-C₃N₄ possess a greater abundance of unoccupied 5d-electron states, which aligns with the observed electron transfer indicated by the Pt 4f XPS spectra. To further analyze the local coordination environment of the Pt single atoms, Fourier-transform EXAFS analysis is performed (Fig. 5b). The peaks observed in the spectra correspond to distinct coordination shells surrounding the Pt atoms. The prominent peaks of Pt foil and PtO₂ are clearly observed at 2.55 and 1.60 Å, respectively. However, these peaks are absent for Pt₁/g-C₃N₄ and Pt_1.5/g-C₃N₄, confirming the presence of Pt single atoms, which is consistent with the HAADF–STEM observation. Previous studies have shown that Pt single atoms can be stabilized through covalent bonds with adjacent N and C atoms.^27,47,50,52 Therefore, the dominant peak at approximately 1.9 Å can be assigned to the Pt–N/C bonds. In this case, the experimental EXAFS spectra of Pt₁/g-C₃N₄ and Pt_1.5/g-C₃N₄ exhibit a satisfactory fit while employing the Pt–N/C pathway. Although the exact coordination of Pt atoms with either N or C presents difficulties, it is more probable to observe Pt–N coordination by examining the significant energy shift of N 1s spectra of Pt_x/g-C₃N₄. Based on the EXAFS fitting outcomes (Fig. 5c and d), both the Pt₁/g-C₃N₄ and Pt_1.5/g-C₃N₄ demonstrate comparable coordination numbers (Pt₁–N: 6.3 ± 0.4; Pt_1.5–N: 6.4 ± 0.4) and Pt–N bond lengths (∼2.41 Å). It should also be noted that when the Pt atom is situated within the central position of a sixfold cavity in g-C₃N₄, it is expected to exhibit a Pt–N bond length of approximately 2.38 Å.⁴⁰ The EXAFS wavelet transforms provide a method to obtain both the radial distance resolution (R space) and k space resolution simultaneously. Fig. 6a and b show that the maximum intensities of Pt₁/g-C₃N₄ and Pt_1.5/g-C₃N₄ centered at about 6.55 and 6.30 Å⁻¹ (k space) are resolved at 1.90 Å (R space), respectively, which are both assigned to the Pt–N coordination structure.^45,53 Note that the intensities corresponding to Pt–Pt (10.05 Å⁻¹, Fig. 6c) and Pt–O (6.05 Å⁻¹, Fig. 6d) coordination for the Pt foil and PtO₂ are not observed for Pt₁/g-C₃N₄ and Pt_1.5/g-C₃N₄. The coordination analyses provide confirmation of the highly reduced state of the Pt single atoms and their strong interaction with N atoms in an isolated state.

Fig. 5 XAFS of Pt₁/g-C₃N₄ and Pt_1.5/g-C₃N₄. (a) L₃-edge XANES spectra. (b) k²-weighted FT-EXAFS spectra. (c) Fitted FT-EXAFS spectra of Pt₁/g-C₃N₄. (d) Fitted FT-EXAFS spectra of Pt_1.5/g-C₃N₄.

Fig. 6 WT-EXAFS of (a) Pt foil, (b) PtO₂, (c) Pt₁/g-C₃N₄, and (d) Pt_1.5/g-C₃N₄.

The band structures of the samples are depicted in Fig. 7. The DRS results indicate an increasing absorption intensity in the UV region and a slight red-shift in the absorption edge at approximately 430 nm with increasing Pt content (Fig. 7a), which can be ascribed to the presence of a higher concentration of Pt–N bonds.³⁴ By extrapolating the intercept of the x-axis in the linear region of Tauc plots, the band gaps (E_g) of g-C₃N₄ and Pt_x/g-C₃N₄ are calculated to be 2.67, 2.63, 2.60, and 2.59 eV, respectively (Fig. 7b).⁵⁴ The corresponding valence band potentials (E_VB) are calculated using valence band XPS (V_B-XPS), yielding values of 1.89, 1.75, 1.20, and 1.17 eV, respectively (Fig. 7c). The E_VB and the standard hydrogen electrode (E_VB,_NHE) exhibit a correlation expressed as E_VB,NHE = φ + E_VB,XPS − 4.44, where φ represents the work function of the XPS analyzer (4.2 eV).⁵⁵ Thus, the E_VB,NHE values of the respective samples are determined to be 1.65, 1.51, 1.02, and 0.93 eV. The band structures of g-C₃N₄ and Pt_x/g-C₃N₄ are shown in Fig. 7d, where the conduction band potential (E_CB) is calculated using the empirical formula: E_CB = E_VB − E_g. It is noteworthy that the E_CB of Pt_x/g-C₃N₄ tends to be more negative than that of g-C₃N₄, suggesting that Pt_x/g-C₃N₄ has a stronger reduction in protons compared to g-C₃N₄ in thermodynamics.

Fig. 7 (a) UV-vis DRS. (b) Tauc plots. (c) VB-XPS. (d) Schematic band structures.

The PL spectra reveal that all of the samples show a similar intrinsic fluorescence emission peak within the range of 400–550 nm (Fig. 8a).³⁰ The PL intensity of Pt_x/g-C₃N₄ is significantly diminished as a result of the incorporation of Pt single atoms, indicating a pronounced suppression of recombination of photogenerated charge carriers.^22,46,56 The time-resolved PL-decay spectrum fitted by third-order exponential terms provides direct evidence of the duration of the photoexcited charge carriers (Fig. 8b), showing that the presence of Pt single atoms leads to a shorter lifetime (6.00, 5.80, and 5.90 ns for Pt_0.5/g-C₃N₄, Pt₁/g-C₃N₄, and Pt_1.5/g-C₃N₄, respectively) compared to g-C₃N₄ (8.09 ns). The rapid migration of charge carriers between Pt single atoms and g-C₃N₄ enhances exciton dissociation, thereby effectively inhibiting the recombination of electron–hole pairs.⁴⁹ To assess the charge-separation ability, the photocurrent and EIS properties are examined. In Fig. 8c, the photoelectrode of Pt_x/g-C₃N₄ shows a higher photocurrent density in comparison to g-C₃N₄, indicating superior charge-transfer properties and more efficient photogenerated charge separation in Pt_x/g-C₃N₄. Furthermore, the EIS Nyquist plot of Pt_x/g-C₃N₄ displays a smaller semicircle compared to g-C₃N₄ (Fig. 8d), suggesting enhanced charge-transfer ability at the photocatalyst interface and improved electronic utilization. These results indicate that the incorporation of well-dispersed Pt single atoms can effectively augment optical absorption, diminish the band gap and valence band, impede photogenerated carrier recombination, and facilitate interfacial charge transfer. Moreover, the improving effect becomes stronger with the increasing content of Pt single atoms. These outcomes hold significant implications for enhancing the efficiency of the photocatalytic H₂ evolution reaction.

Fig. 8 (a) PL spectra. (b) Time-resolved PL spectra. (c) Transient photocurrent responses. (d) EIS Nyquist plots.

The advantage of loading Pt single atoms in photocatalytic H₂ evolution is evaluated by varying the Pt content and aggregation. As shown in Fig. 9a, the initial H₂ production rate of pristine g-C₃N₄ is relatively low (1.8 μmol h⁻¹), which experiences an increase to 26.0 μmol h⁻¹ upon loading Pt nanoparticles. Once g-C₃N₄ is modified with Pt single atoms at a loading content of merely 0.5%, a substantial enhancement of the photocatalytic H₂ production rate is achieved (208.7 μmol h⁻¹). The H₂ production rate of Pt_0.5/g-C₃N₄ is ∼116 times that of g-C₃N₄ and ∼8 times that of Pt_NP/g-C₃N₄, despite the amount of Pt source being only 1/20th of Pt_NP/g-C₃N₄. With an increase in Pt concentration, Pt₁/g-C₃N₄ and Pt_1.5/g-C₃N₄ exhibit elevated H₂ production rates of 238.9 and 336.8 μmol h⁻¹, respectively (Fig. 9b). These results indicate a positive correlation between the H₂ production rate and the loading content of Pt single atoms within a reasonable range. The AQY of Pt_1.5/g-C₃N₄ under irradiation at different wavenumbers is shown in Fig. 9c. Against irradiation wavenumbers of 365, 405, 420, and 500 nm, the AQYs are 10.30%, 13.46%, 9.35%, and 6.32%, respectively. The high AQY value and the good match between AQY and the absorption spectrum indicate that most of the captured photons are involved in the photocatalytic reaction.^45,57 Compared with other Pt single-atom modified g-C₃N₄ photocatalysts prepared by different methods, Pt_1.5/g-C₃N₄ reported in this work exhibits a relatively high loading content of Pt single atoms and excellent photocatalytic H₂ evolution performances, as shown in Table 1.^{28,40,49,58,59}

Fig. 9 (a) Time courses for photocatalytic H₂ evolution of Pt_x/g-C₃N₄ under Xe-lamp irradiation. (b) Corresponding photocatalytic H₂ evolution rate. (c) AQY of Pt_1.5/g-C₃N₄. (d) Stability tests for 22 h.

Table 1 A comparison of the photocatalytic H₂ production efficiency of Pt_1.5/g-C₃N₄ reported in this work with those of other Pt single-atom modified g-C₃N₄ photocatalysts

Samples	Pt loading	Methods	Light sources	H2 production rates (μmol h^−1)	AQYs	Ref.
Pt–CN	0.74%	Immersion	30 W LED (λ > 520)	34.2	0.84% λ = 520 nm	28
Pt/CN	0.16%	Immersion	300 W Xe lamp (λ > 420)	318	—	40
Pt/CN	0.11%	Photo-chemical reduction	300 W Xe lamp (AM1.5)	40.1	—	49
Pt-CN	0.38%	Immersion	300 W Xe lamp (AM1.5)	2.9	—	58
PtHD-CNNT	0.32%	Pyrolysis	300 W Xe lamp (λ > 420)	106.1	13.1% λ = 420 nm	59
Pt1.5/g-C3N4	1.26%	IWI	300 W Xe lamp	336.8	13.5% λ = 405 nm	This work

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To study the stability of photocatalytic H₂ production, an extended duration of the H₂ evolution test is conducted. Fig. 9d shows that the H₂ production rate of Pt_0.5/g-C₃N₄ experiences a noticeable decline after 10 h, which may be attributed to the rupture of the Pt–N bond resulting from inadequate Pt filling between 3-s-triazines. Interestingly, the H₂ production rates of Pt₁/g-C₃N₄ and Pt_1.5/g-C₃N₄ remain relatively constant for 22 h, indicating that the sufficient dispersion of Pt single atoms facilitates an optimal and robust interaction with g-C₃N₄. To further investigate the reusability, the photocatalytic test of Pt_1.5/g-C₃N₄ is repeated four times, and the used catalyst after the reaction is characterized again, as depicted in Fig. 10. Fig. 10a shows the H₂ production rate of Pt_1.5/g-C₃N₄ during four cycles. After four cycles, the H₂ production capability is 5.8% lower than the initial value, exhibiting excellent photocatalytic stability. The HAADF–STEM image of Pt_1.5/g-C₃N₄ after photocatalysis is shown in Fig. 10b, indicating that the Pt single atoms maintain a high dispersion in an isolated state without apparent aggregation. Comparing the XRD patterns (Fig. 10c) and Pt 4f XPS spectra (Fig. 10d) before and after 4-cycle-photocatalysis, both the structure of g-C₃N₄ and the valence state of Pt single atoms are not significantly changed. These results demonstrate that Pt_x/g-C₃N₄ exhibits excellent photocatalytic stability after a long photocatalytic reaction even when loaded with a high density of Pt single atoms.

Fig. 10 Reusability of Pt_1.5/g-C₃N₄. (a) The photocatalytic H₂ production rate during four cycles. (b) HAADF-STEM image after reaction. (c) and (d) Comparison before and after photocatalytic reaction of (c) XRD and (d) XPS Pt 4f.

The enhanced mechanism of photocatalytic H₂ production of Pt_x/g-C₃N₄ is illustrated in Fig. 11. Low-temperature treatment reduces the average pore size of the small pores in g-C₃N₄, which may promote the adsorption of H⁺. Furthermore, the strong interaction between Pt 5d and N 2p for the formation of the Pt–N bond plays a key role in the kinetics of the H₂ evolution reaction by effectively accelerating charge transfer and stabilizing active sites for photocatalysis.^34,40,42,60 Pt single atoms incorporated in the center of the sixfold cavity and forming robust Pt–N₆ coordination introduce new energy levels within the band structure of g-C₃N₄, leading to improved separation of photo-induced electron–hole pairs. Under light irradiation, the photogenerated electrons migrate to Pt single atoms for participating in the H₂ evolution reaction. The stable and efficient production of H₂ is attributed to the optimal dispersion of Pt atoms and the improved charge carrier dynamics facilitated by the interaction between Pt and g-C₃N₄.

Fig. 11 Mechanism of photocatalytic H₂ production of Pt_x/g-C₃N₄.

4. Conclusions

In summary, Pt_x/g-C₃N₄ photocatalysts with different Pt contents are synthesized using a low-temperature incipient wetness impregnation method. Pt atoms loaded on g-C₃N₄ in the form of the isolated single atoms even at a high density of 1.5 wt% are demonstrated by HAADF–STEM and EXAFS results. The well-dispersed Pt single atoms provide active sites for H⁺ adsorption. The Pt single atoms incorporated in the sixfold cavity between the three adjacent triazines form strong interaction between Pt and N₆, which generates new doping energy levels and boosts higher transportation and separation efficiency of photo-generated charge carriers for improving photocatalytic H₂ evolution. The photocatalytic H₂ production rates of Pt_x/g-C₃N₄ are roughly two orders of magnitude better than that of the pristine g-C₃N₄, and the stability increases with increasing Pt loading content. This research reflects a significant advancement in photocatalyst design and opens new avenues for the development of efficient hydrogen production systems.

Author contributions

Qi Zhang: resources, investigation; Miao Yue: investigation, writing – original draft; Peng Chen: investigation, formal analysis; Qingmiao Ren: investigation; Weihu Kong: investigation; Chenxia Jia: visualization, formal analysis; Qianyu Lu: resources, writing – reviewing and editing; Jizhou Wu: formal analysis, writing – original draft; Yuqing Li: methodology; Wenliang Liu: writing – reviewing and editing; Peng Li: visualization; Yongming Fu: conceptualization, investigation, writing – original draft; supervision; and Jie Ma: writing – reviewing and editing, supervision, project administration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (2017YFA0304203), the National Natural Science Foundation of China (62020106014, 62175140, 61901249, 92165106, 12104276), PCSIRT (IRT—17R70), 111 project (D18001), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi (OIT), the Applied Basic Research Project of Shanxi Province, China (201901D211191, 201901D211188), and the collaborative grant by the Russian Foundation for Basic Research and NSF of China (62011530047, 20-53-53025 in the RFBR classification).

Notes and references

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