Figure 1. Schematic illustration of ACP/RuSAC+C.
Scanning electron microscopy (SEM) analysis revealed that the morphology of ACP was similar to that of the bare CP (Figure S1, Supporting Information). Acid treatment modified the surface of the CP, thereby transforming its hydrophobic nature into a hydrophilic nature. This modification enabled the efficient removal of gas bubbles generated during electrode operation.[22,23] The enhancement in the electrolyte wettability of ACP compared to that of CP was confirmed by contact angle measurements, which decreased from 57.2 to 29° (Figure 2 a). Moreover, the activation step generated a significant number of oxygenated functional groups,[24-26] and defects[12] on the CP surface that are essential for anchoring metal atoms. Table S2 substantiates the presence of a greater amount of oxygenated functional groups on ACP. Additionally, the successful formation of defects was confirmed through Raman spectroscopy analysis (Figure S2, Supporting Information). It is clear that the ID/IG ratio of ACP is higher than that of the bare CP, indicating that a higher concentration of defects was formed on ACP. Figure S3, Supporting Information, depicts the corresponding atomic structures of bare CP and ACP. After acid activation process, ACP was immersed in a well-dissolved RuCl3-H2O solution for 15 h. During the immersion process, the Ru ions were anchored to the oxygen-containing groups and defects were formed on the surface of ACP in the preceding steps. Oxygenated functional groups possess lone pairs of electrons that can coordinate with metal ions through coordination bonds, allowing for the immobilization of metal ions. Therefore, the increased number of functional groups and defects allows a larger amount of Ru metal ions to anchor to the surface of ACP than that in bare CP. To support these hypotheses, a control sample denoted as CP/Ru was synthesized with the same amount of Ru doping on unactivated bare CP. Moreover, Ab initio molecular dynamics (AIMD) simulations were conducted to elucidate the effect of CP activation on the anchoring of Ru metal ions. As illustrated in Figure 2c,d, RuCl3 in the solution was rapidly adsorbed onto the defect sites of ACP within 0.1 ps, whereas this process was not observed for the pristine graphene support. Additionally, RuCl3 and carbon atoms at the defect sites maintained chemical bonding for longer than 10 ps (Figure 2d). While it requires a significantly extended time frame to observe the rare event of RuCl3 adsorption on bare CP, the instantaneous adsorption of RuCl3 on ACP suggests a more favorable and abundant formation of Ru nanoclusters and Ru single atoms on ACP. Finally, high-temperature pyrolysis was conducted at 800 ℃ for 1 h under an N2 atmosphere in the presence of DCD. DCD serves as a source of nitrogen, stabilizing Ru single atoms through coordination. This overall synthetic approach resulted in the formation of two distinct samples: a self-standing electrode containing a combination of Ru single atoms and Ru nanoclusters (labelled ACP/RuSAC+C), and a control sample consisting of only Ru nanoclusters (labelled ACP/RuC). Detailed experimental information is provided in the supporting information. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis confirmed a greater amount of Ru doping on ACP than that on CP (Table S3, Supporting Information), and contact angle measurements (Figure 2b) revealed the more hydrophilic nature of ACP/RuSAC+C (CA = 32°) compared to that of CP/Ru (CA = 50°). Thus, these distinct properties are anticipated to exert a positive impact on HER performance.