Phase-Controlled Synthesis of Ru Supported on Carbon Nitride and the Application in Photocatalytic H₂ Evolution

Abstract

This work aims to explore the influence of crystal phase engineering on the photocatalytic hydrogen evolution activity of Ru/C₃N₄ systems. By precisely tuning the combination of Ru precursors and reducing solvents, we successfully synthesized Ru co-catalysts with distinct crystal phases (hcp and fcc) and integrated them with C₃N₄. The photocatalytic hydrogen evolution experiments demonstrated that hcp-Ru/C₃N₄ achieved a significantly higher hydrogen evolution rate (24.23 μmol h⁻¹) compared to fcc-Ru/C₃N₄ (7.44 μmol h⁻¹), with activity reaching approximately 42% of Pt/C₃N₄ under the same conditions. Photocurrent and electrochemical impedance spectroscopy analyses revealed that hcp-Ru/C₃N₄ exhibited superior charge separation and transfer efficiency. Moreover, Gibbs free energy calculations indicated that the hydrogen adsorption energy of hcp-Ru (ΔG_H* = −0.14 eV) was closer to optimal compared to fcc-Ru (−0.32 eV), enhancing the hydrogen generation process. These findings highlight that crystal-phase engineering plays a critical role in tuning the electronic structure and catalytic properties of Ru-based systems, offering insights for the design of highly efficient noble metal catalysts for photocatalysis.

1. Introduction

The development of more efficient nanomaterial catalysts is crucial in fields such as environmental protection, food processing, petrochemicals and energy utilization. Catalyst performance can be enhanced by controlling the nanostructures through strategies involving size [1], shape [2], alloys [3], crystal phase [4], and defects [5]. Phase engineering of nanomaterials (PEN), as an effective strategy for the rational design and precise synthesis of nanomaterials with controllable phases, has gained significant attention in recent years [6,7,8,9]. Phase engineering modulates performance, functionality, and applications by altering the catalyst’s surface electronic and geometric structures through adjustments in reaction kinetics and surface energy [10,11,12]. For instance, bulk Nickel (Ni) typically shows the face-centered cubic (fcc) phase, but non-conventional hexagonal close-packed (hcp) Ni nanoparticles can be synthesized via chemical methods or one-pot procedures [13,14]. Experimental results indicate that hcp Ni exhibits superior catalytic performance compared to fcc Ni across various reactions. Additionally, studies have explored the impact of crystallinity on catalytic performance, revealing that the amorphous 1T phase of MoSe₂ rich in unsaturated coordination sites, facilitates proton coupling to form hydrogen [15]. This provides new insights into phase engineering for fine-tuning growth modes. Nevertheless, the practical application of phase engineering is still in its early stages, and there is an urgent need for the development of environmentally friendly and versatile synthetic methods for phase regulation. Furthermore, the influence mechanisms of template, reduction kinetics, and capping agents on atomic stacking require further investigation.

Photocatalytic hydrogen production is considered a green, clean, safe, and low-cost renewable energy technology. Due to their high electron capture efficiency, excellent chemical stability, and abundant surface active sites, noble metals play an essential role in the preparation of photocatalysts for hydrogen production [16,17,18,19]. Among them, platinum (Pt), with its optimal Fermi level and zero-approaching hydrogen adsorption Gibbs free energy, is one of the most promising co-catalysts for photocatalytic hydrogen production [20]. However, due to its high cost, large-scale use remains impractical. To reduce the dependence on expensive Pt catalysts for photocatalytic water splitting, researchers have explored various types of catalysts and optimized their nanostructures to develop high-activity, low-cost co-catalysts. Ruthenium (Ru) has attracted significant attention in the field of photocatalysis due to its lower cost and the ability to achieve activity comparable to Pt under certain optimized conditions [21,22,23]. Zhu et al. optimized the asymmetric electronic properties of Ru to lower the energy barrier for Ruⁿ⁺ in water splitting, and the resulting Ru/NC catalyst exhibited high alkaline hydrogen evolution reaction (HER) activity (21.9 mV @ 10 mA cm⁻², 29.03 mV dec⁻¹) [24]. Dong et al. discovered that the incorporation of copper (Cu) in the alloy altered the reaction pathway for Ru in photocatalysis, achieving a 96% selectivity for CO₂ to CH₄ [25]. Despite numerous successful cases of Ru in various catalytic reactions, studies on the synthesis and optimal crystal phase of Ru co-catalysts for hydrogen production are scarce, and the mechanisms underlying the impact of Ru crystal phase on photocatalytic performance remain largely unexplored. Therefore, it is essential to investigate the feasibility of phase control in Ru for enhancing hydrogen production.

In this study, we employ a rational and controllable chemical reduction method to prepare hcp and fcc Ru nanoparticles and successfully load them onto C₃N₄ ultrathin nanosheets (hcp-Ru/C₃N₄, fcc-Ru/C₃N₄) to demonstrate the feasibility of improving the photocatalytic hydrogen production performance of Ru-based co-catalysts using phase engineering. Experimental results show that the Ru-based co-catalysts exhibit distinct phase-dependent behavior. hcp-Ru/C₃N₄ displays more efficient photocatalytic hydrogen production activity (24.23 µmol h⁻¹) and stability, significantly outperforming fcc-Ru/C₃N₄ (7.44 µmol h⁻¹). This superior performance is attributed to its higher charge separation efficiency and lower Gibbs free energy, as supported by detailed physicochemical characterization and theoretical calculations. This study highlights the vast potential and versatility of Ru phase-controlled synthesis for improving photocatalytic hydrogen production.

2. Materials and Methods

2.1. Materials

Ruthenium acetylacetonate (Ru(acac)₃, C₁₅H₂₁O₆Ru, ≥97%), triethylene glycol (C₆H₁₄O_4, >99%), triethanolamine (TEOA, C₆H₁₅NO₃, >99%), polyvinylpyrrolidone ((C₆H₉NO)_n, K30, MW ≈ 40,000) were purchased from Shanghai Aladdin Biochemical Technology (Shanghai, China). Melamine (C₃H₆N₆, ≥99.0%), ruthenium (III) chloride hydrate (RuCl₃·xH₂O), ethanol absolute (C₂H₆O, ≥99.7%), ethylene glycol (C₂H₆O₂, ≥99.5%), potassium bromide (KBr, ≥99.0%) and sodium sulfate (Na₂SO₄, ≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Methods

2.2.1. Synthesis of C₃N₄ Nanosheets

An amount of 2 g of melamine was weighed and added to a covered ceramic crucible. The crucible was then placed in a muffle furnace and heated to 550 °C at a rate of 2 °C/min, where it was held for 4 h. After the reaction, the resulting yellow block was obtained as the bulk material. The bulk material was ground into a powder, and 800 mg of the powder was spread evenly in a partially open ceramic boat. The boat was then placed in the muffle furnace and heated to 550 °C at a rate of 10 °C/min, where it was held for 40 min. The resulting white powder was the desired C₃N₄ for the experiment.

2.2.2. Synthesis of C₃N₄-Ru Samples

In the synthesis of hcp-Ru/C₃N₄, 10 mL of an ethylene glycol (EG) solution containing 50 mg of C₃N₄ nanosheets and 0.5 mmol of polyvinylpyrrolidone (PVP) was sonicated in a 25 mL round-bottom flask for 30 min. Then, 0.025 mmol of RuCl₃·xH₂O was added to the flask and sonicated until well dispersed. The flask containing the reaction mixture was then transferred to an oil bath and heated to 200 °C for 3 h. After the reaction was complete, the sample was washed with ethanol and deionized water, and the product was separated by centrifugation, followed by freeze-drying to obtain hcp-Ru/C₃N₄.

In the synthesis of fcc-Ru/C₃N₄, 10 mL of a triethylene glycol (TEG) solution containing 50 mg of C₃N₄ nanosheets and 0.5 mmol of PVP was sonicated in a 25 mL round-bottom flask for 30 min. Then, 0.025 mmol of Ru(acac)₃ was added to the flask and sonicated until well dispersed. The flask containing the reaction mixture was then transferred to an oil bath and heated to 200 °C for 3 h. After the reaction was complete, the sample was washed with ethanol and deionized water, and the product was separated by centrifugation, followed by freeze-drying to obtain fcc-Ru/C₃N₄.

2.3. Photocatalytic Hydrogen Activity Measurement

The photocatalytic hydrogen evolution reaction was carried out in an online reaction detection system (CEL-SPH2N, China Education Au-Light, Beijing, China). The reaction solution consisted of 50 mL of TEOA/H₂O (10% vol TEOA). The photocatalyst used in the measurement was 10 mg. The reaction was irradiated with a 300 W xenon lamp (CEL-HX F300/CEL-HX UV300, China Education Au-Light, Beijing, China) without any filters. Prior to irradiation, the system was evacuated to a vacuum state, and the reaction temperature was maintained at 15 °C using a circulating cooling water system. The hydrogen production rate was analyzed by gas chromatography (GC 7920) equipped with a TCD detector and a 5Å molecular sieve column (all components purchased from China Education Au-light, Beijing, China), using argon as the carrier gas.

The external quantum efficiency (EQE) values were determined using the following equation:

$$EQE={\displaystyle \frac{the number of reacted electrons}{the number of incident photons}}\times 100\%$$

A more detailed calculation process can be found in the Supplementary Materials, including the information of the area of light spot, light intensities per unit area, and wavelength-dependent hydrogen evolution rates per unit area.

3. Results

The premise for studying the impact of crystal phase structure on photocatalytic reactions is the development of methods for controlling the crystal phase synthesis of Ru. We first prepared ultrathin C₃N₄ nanosheets as a support using a top-down thermal exfoliation method. Their coiled two-dimensional structure and large surface area are expected to provide an ideal environment for Ru loading. Different precursors and reducing agents can synergistically regulate the reduction kinetics rate and play an important role in selectively forming hcp and fcc phases of Ru [26,27]. The strong reducing nature of EG and the more easily dissociable chloride ligands in RuCl₃ molecules favor the formation of hcp Ru seeds. In contrast, TEG, with its weaker reducing ability, and the more stable acetylacetonate ligands in Ru(acac)₃ act to uniformly release Ru atoms, promoting the formation of the fcc structure. Therefore, in the synthesis of hcp-Ru/C₃N₄, RuCl₃·xH₂O was used as the Ru source, EG as the reducing agent and solvent, and PVP as the stabilizer, to chemically reduce and load hcp phase Ru nanoparticles onto the C₃N₄ nanosheets. The synthesis of fcc-Ru/C₃N₄ followed a similar method, with the only difference being the use of Ru(acac)₃ as the Ru source and TEG as the reducing agent and solvent (Figure 1a).

To understand the basic chemical structure of the prepared catalysts, we studied their crystal structure using X-ray diffraction (XRD). The XRD patterns of the prepared hcp-Ru/C₃N₄ and fcc-Ru/C₃N₄ exhibited features similar to those of C₃N₄ (Figure 1b). Two characteristic diffraction peaks of C₃N₄ were detected at 2θ angles of 12.8° and 27.8°, corresponding to the stacking units of the in-planar repeating tri-s-triazine unit and the conjugated aromatic segments, respectively. These peaks correspond to the (100) and (002) crystal planes [28]. No characteristic peaks of Ru were detected in the XRD patterns of hcp-Ru/C₃N₄ and fcc-Ru/C₃N₄ due to the low content and small particle size, which were undetectable [29]. This was later confirmed through transmission electron microscope (TEM, Tecnai G2 F30 S-TWIN, purchased from FEI COMPANY, Hillsboro, OR, USA), which calculated the average particle size of Ru. Fourier transform infrared (FT-IR, Nicolet iS5, purchased from Thermo Fisher Scientific, Waltham, MA, USA) spectra were used to identify the surface functional groups of hcp-Ru/C₃N₄, fcc-Ru/C₃N₄, and C₃N₄ (Figure 1c). The characteristic peaks of C₃N₄ were clearly observed in all the catalysts. The absorption peak at 811 cm⁻¹ corresponds to the characteristic vibration of the triazine units. The absorption peaks at 1410 and 1644 cm⁻¹ are attributed to the stretching vibrations of C-N heterocyclic bonds, and the absorption peak at 3158 cm⁻¹ corresponds to the stretching vibration of the O-H bond [30]. These results demonstrate that the introduction of Ru does not alter the original fundamental functional group structure of C₃N₄.

After determining the basic chemical structure of the prepared catalysts, we further investigated their microstructure and chemical composition through a series of electron microscopy characterizations. Scanning electron microscope (SEM, S-4800II, purchased from HITACHI, Tokyo, Japan) results show that the prepared C₃N₄ exhibits irregular porous coiled folds (Figure S1a,b). Atomic force microscope (AFM, SPM-9700HT, purchased from Shimadzu, Tokyo, Japan) observed that its thickness is approximately 2.8 nm (Figure 2d,e), providing further evidence of its typical ultrathin two-dimensional nanosheet structure. The larger lateral dimensions and Brunauer–Emmett–Teller (BET, ASAP2460, purchased from Micromeritics Instruments Corporation, Atlanta, GA, USA) surface area (Figure S7b–d) of C₃N₄ facilitate Ru loading, while the ultrathin thickness effectively shortens the charge migration path from the bulk to the surface, thereby reducing electron–hole recombination [31,32]. After loading with metallic Ru nanoparticles, the morphology of C₃N₄ did not show significant changes, and both hcp-Ru/C₃N₄ and fcc-Ru/C₃N₄ composite photocatalysts still maintained an irregular nanosheet morphology (Figure S1c–f). TEM results of hcp-Ru/C₃N₄ and fcc-Ru/C₃N₄ show that the Ru nanoparticles are uniformly dispersed on the flat surface of C₃N₄, with no apparent aggregation (Figure 2a–c), which can be attributed to the appropriate amount of reducing agent and PVP that control the uniform growth of metal Ru nanoparticles, preventing aggregation on C₃N₄ [33]. In addition, we have supplemented TEM-based size distribution analyses for both hcp-Ru (1.03 nm) and fcc-Ru (1.54 nm) samples (Figure S2a,b). While a minor size difference exists between the two phases (~0.5 nm), we propose that this subtle variation has negligible impacts on catalytic performance trends [34,35].

To reveal the fine-phase structure of the Ru loaded on the catalyst, we performed high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30 S-TWIN, purchased from FEI COMPANY, Hillsboro, OR, USA) characterization. The lattice spacings of 0.211 and 0.229 nm in Figure 2f correspond to the (002) and (100) planes of hcp Ru, while the lattice spacings of 0.189 and 0.220 nm in Figure 2g correspond to the (200) and (111) planes of fcc Ru. Additional HRTEM images are presented in Figure S3 to further clarify the phase purity of Ru nanoparticles in the catalysts. This indicates that the method used in this work successfully controls the crystal phase of Ru. Elemental mapping images further confirmed the uniform distribution of C, N, and Ru elements (Figure 2h,i), providing additional evidence that Ru nanoparticles of both crystal phases are uniformly loaded on C₃N₄. Furthermore, the actual Ru loading amount on hcp-Ru/C₃N₄ and fcc-Ru/C₃N₄ was determined to be 3.53 wt% and 3.64 wt% by ICP-MS (Elan DRC-e, purchased from PerkinElmer, Waltham, MA, USA) (Table S1).

X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, purchased from Thermo Fisher Scientific, Waltham, MA, USA) was used to characterize the element valence states and surface composition of the catalysts. Full XPS spectra of hcp-Ru/C₃N₄ and fcc-Ru/C₃N₄ clearly showed signals corresponding to C, N, and Ru, further confirming the successful synthesis of the catalysts (Figure 3a). The C 1s spectrum of hcp-Ru/C₃N₄ (Figure 3b) can be deconvoluted into two peaks at 284.8 eV and 287.9 eV, corresponding to graphitic carbon (C-C) and sp²-bonded aromatic structure (N-C=N), respectively. In the N 1s spectrum (Figure 3c), binding energy of 398.5 eV and 400.3 eV correspond to pyridinic N (N1) and pyrrolic N (N2) [36]. The Ru 3p spectrum (Figure 3d) reveals the Ru 3p_3/2 and Ru 3p_1/2 spin–orbit peaks, with fitting peaks at 461.2 eV and 483.5 eV assigned to metallic Ru (Ru⁰) [37]. In contrast, the C 1s and N 1s spectra of fcc-Ru/C₃N₄ show a more negative binding energy shift (~0.3 eV), and the signal peaks in the Ru 3p spectrum are shifted positively by 0.3 eV. This suggests that C₃N₄ interacts more strongly with hcp Ru, with more electrons transferred to hcp Ru [38]. Despite the differences in the crystal phase structure of Ru loaded on the surfaces of hcp-Ru/C₃N₄ and fcc-Ru/C₃N₄, their similar XPS spectra indicate that both catalysts possess similar chemical valence states.

After confirming the fine chemical structure of the prepared catalysts, we further discussed the rationality of constructing the Ru/C₃N₄ photocatalytic system using density functional theory (DFT). From a thermodynamic perspective, the direction of charge transfer is from the component with a higher Fermi level (i.e., lower work function) to the component with a lower Fermi level (i.e., higher work function) [39]. As shown in Figure 4a–c, we first calculated the theoretical work functions of C₃N₄, hcp-Ru, and fcc-Ru using DFT, which were found to be 4.69, 4.99, and 5.38 eV, respectively. The data indicate that after loading Ru onto C₃N₄, the photogenerated electrons produced upon light excitation can thermodynamically transfer to Ru, thereby enhancing charge separation efficiency and the electron concentration on the catalyst surface, providing theoretical support for Ru’s role as a co-catalyst.

Next, we assessed the light absorption ability of the catalysts using UV-Vis diffuse reflection spectra (DRS) (Figure 4d). It was found that C₃N₄ has a typical absorption edge of around 450 nm. After loading Ru, the optical absorption properties of the material were improved, and the visible light absorption range increased. This is mainly due to the synthesized catalyst being black in color. However, it should be noted that this enhancement in absorption is not significant, as the increased light absorption comes from Ru, which cannot be directly excited by light. The light absorption and charge generation capacity are solely determined by C₃N₄. Therefore, it can be concluded that both hcp-Ru/C₃N₄ and fcc-Ru/C₃N₄ exhibit similar charge generation behaviors.

Based on the theoretical analysis, we further determined the electric potential of C₃N₄ and speculated on its charge transfer pathway through Tauc curves and Mott–Schottky (M–S) plots. From the Tauc curve data, the bandgap of C₃N₄ was calculated to be 3.03 eV (Figure S4a). The flat band (FB) potential of C₃N₄ was determined to be −1.71 V vs. Ag/AgCl, pH = 7, using the M–S curves (Figure 4e) at 500, 1000, and 1500 Hz. Assuming that for n-type semiconductors, the gap between the flat band potential and conduction band (CB) potential can be neglected [40]. The conduction band potential of C₃N₄ was converted to −1.1 V vs. NHE, pH = 0 [41]. Thus, its valence band (VB) potential was determined to be 1.93 V vs. NHE, pH = 0, and the band structure was obtained (Figure S4b). Therefore, we can speculate on the charge transfer pathway in the Ru-C₃N₄ catalyst during photocatalytic hydrogen evolution (Figure 4f). Upon light irradiation, C₃N₄ generates electron–hole pairs. The photogenerated electrons migrate through the interface to the Ru surface. Due to the rich hydrogen evolution active sites at the edge and basal plane of Ru, the electrons gathered on its surface react with adsorbed protons to produce H₂. Meanwhile, the holes generated in C₃N₄ are consumed by the sacrificial agent, preventing the recombination of electron–hole pairs before charge separation and consumption.

In actual photocatalytic experiments, the hydrogen evolution activity of both Ru/C₃N₄ catalysts (with a theoretical loading of 5 wt%) was significantly improved compared to the pure C₃N₄. The average hydrogen evolution rates are shown in Figure 5a. Among them, hcp-Ru/C₃N₄ (24.23 μmol h⁻¹) exhibited much higher catalytic activity than fcc-Ru/C₃N₄ (7.44 μmol h⁻¹), indicating that the crystal structure of Ru significantly affects the photocatalytic hydrogen production performance of Ru/C₃N₄. To better understand the hydrogen production performance of hcp-Ru/C₃N₄, it is essential to evaluate its external quantum efficiency (EQE). As shown in Table S2, the EQE value of hcp-Ru/C₃N₄ is measured to be 5.28% at 420 nm. Clearly, compared to the catalysts listed in Table S3, hcp-Ru/C₃N₄ demonstrates superior performance in both hydrogen evolution rate and quantum efficiency, further highlighting its exceptional photocatalytic activity. We also tested the hydrogen evolution activity of Pt/C₃N₄ under the same conditions, which reached 57.46 μmol h⁻¹. This result indicates that the performance of the synthesized Ru co-catalyst is approximately 42% of that of Pt, showing a distinct advantage over most non-Pt co-catalysts. Additionally, we investigated the stability of hcp-Ru/C₃N₄ and fcc-Ru/C₃N₄ by conducting five photocatalytic reactions (25 h), as shown in Figure 5b,c. The results revealed that hcp-Ru/C₃N₄ demonstrated superior stability in hydrogen production compared to fcc-Ru/C₃N₄. Moreover, after the photocatalytic reaction, we performed characterizations on hcp-Ru/C₃N₄, which showed good chemical and structural stability (Figure S5). The slight decline in activity of hcp-Ru/C₃N₄ during the reaction could be attributed to the consumption of the sacrificial agent over prolonged reaction times.

To further explore the reasons for the significant difference in activity between the two Ru crystal phase catalysts, we continued to investigate their photoelectrochemical properties. We combined Linear sweep voltammetry (LSV) curves, photoelectrochemical measurements (photocurrent and electrochemical impedance spectroscopy (EIS)), steady-state fluorescence (PL), and transient-state fluorescence (FL) to further evaluate the charge dynamics of the two catalysts. As observed from the LSV curves (Figure S6b), the photocurrent density of hcp-Ru/C₃N₄ is higher than that of fcc-Ru/C₃N₄, and both are greater than that of C₃N₄. This indicates that Ru can significantly reduce the surface energy barrier of C₃N₄, enhances the consumption rate of photogenerated holes, and increases the density of free photogenerated electrons required for hydrogen evolution at lower potentials. The electrochemical hydrogen evolution activity is consistent with the photocatalytic performance, further demonstrating that hcp-Ru exhibits superior water reduction kinetics. As shown in Figure 5e, compared to C₃N₄, the introduction of Ru effectively enhanced the photocurrent response under light illumination, indicating that Ru loading promotes charge transfer in C₃N₄. hcp-Ru exhibited stronger photocurrent intensity than fcc-Ru, with the photocurrent response intensity order being: hcp-Ru/C₃N₄ > fcc-Ru/C₃N₄ > C₃N₄. The EIS results are consistent with the photocurrent results (Figure S6a), indicating that electrons in hcp-Ru/C₃N₄ exhibit more efficient transfer. Additionally, both materials showed a significant decrease in charge lifetime (Figure 5d) and a marked reduction in FL intensity (Figure S7a), further evidencing the enhanced charge separation efficiency [42,43]. Notably, although theoretical calculations of the Fermi level (Figure 4a–c) and charge difference distribution (Figure S6c) indicate that photogenerated electrons can effectively migrate from the conduction band of C₃N₄ to Ru nanoparticles, the electron density at the fcc-Ru/C₃N₄ interface is higher than that of hcp-Ru, which contradicts the actual photoelectric response and EIS results. We speculate that fcc-Ru can capture more electrons, but the water reduction kinetics on the surface of fcc-Ru is relatively poor, resulting in fewer electrons participating in surface hydrogen evolution, leading to a higher internal electron–hole recombination rate; thus, a reduced number of effective electrons transferred to the electrode. The comparison of Gibbs free energy results further supports our hypothesis.

The high photogenerated charge separation efficiency could be one of the reasons for the superior photocatalytic hydrogen evolution activity of hcp-Ru/C₃N₄. However, the surface molecular conversion barrier should also be considered an important factor [44]. It is evident that hcp-Ru possesses more active surface catalytic sites, corresponding to lower reaction energy barriers. Specifically, the rate-determining step for hydrogen evolution on the Ru surface is the formation of H* species. From the Gibbs free energy of this step, hcp-Ru (ΔG_H* = −0.14 eV) is significantly lower than fcc-Ru (ΔG_H* = −0.32 eV), and even approaches the efficient catalytic behavior of Pt (Figure 5f), making it easier to undergo reduction reactions at the CB edge to generate H₂. This indicates that, compared to fcc-Ru/C₃N₄, hcp-Ru/C₃N₄ not only exhibits higher charge separation efficiency but also demonstrates better surface molecular conversion efficiency. Based on these favorable physicochemical properties induced by phase engineering, hcp-Ru/C₃N₄ shows higher activity and better stability in photocatalytic hydrogen evolution reactions.

4. Conclusions

This study highlights the significant impact of crystal phase engineering on the photocatalytic performance of Ru/C₃N₄ systems for hydrogen evolution. By manipulating the choice of Ru precursors and reducing solvents, we achieved precise control over the synthesis of hcp- and fcc-phase Ru catalysts. Experimental results demonstrated that the hcp-Ru/C₃N₄ composite displayed markedly superior hydrogen evolution activity and stability compared to its fcc counterpart, benefiting from enhanced charge separation and transfer capabilities. Thermodynamic analysis further revealed that the surface catalytic properties of hcp-Ru, characterized by an optimized hydrogen adsorption energy, play a pivotal role in improving its photocatalytic efficiency. This work provides valuable insights into the rational design of photocatalysts via crystal phase engineering, emphasizing the synergistic effects of electronic structure and surface reactivity for effective hydrogen production.