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.