X-ray diffraction (XRD) patterns were collected to evidence the successful synthesis of Cu NPs and then the uniform coating of C shells. As shown in
Fig. 2a, the obtained Cu NPs present intensive diffraction peaks at 2θ = 43.2°, 50.4°, and 74.1°, assigned to the (111), (200), and (220) planes of well crystallized zero-valent copper (#99-0034), respectively. In comparison, with C shells coated, the XRD peaks related to Cu NPs are significantly weakened for Cu@C-8, and even hardly observed for Cu@C-25 and Cu@C-80, with the increasing thickness of the coated C shells. The disappearance of diffraction characteristics implies that C shells are well coated onto Cu NPs, which might block X-ray detection. In addition, one would note a broad peak at 2θ = 20-28° for all the Cu@C core-shell NPs, which suggests the amorphous phase of the coated C shells, as previously revealed by TEM images. It has been well recognized that the optical absorption ability of the light-absorbing materials is determinative to the photothermal efficiency and then the solar-driven interfacial steam generation (SISG) performance [
2,
6,
26]. As shown in
Fig. 2b, both Cu NPs and Cu@C core-shell NPs present excellent light absorption ability in visible light and even infrared light (> 750 nm), well covering the wide-range solar spectrum. Specifically, Cu NPs exhibit excellent optical absorption in visible light, and especially an obvious absorption peak is easily observed at ca. 590 nm, which should be attributed to the LSPR effect of Cu NPs [
20,
27]. Meanwhile, one would also note a broad optical absorption peak located at ca. 1150 nm. This wide-range optical absorption in near-infrared region is believed to be induced by the high density of hybridized LSPR between the neighboring Cu NPs [
11,
28]. With C shells coated, both the peaks at ca. 590 nm and ca. 1150 nm are remarkably weakened, owing to the C shells inhibiting the LSPR effects of Cu NPs. Especially, depending on the increasing C shell thickness, the optical absorption in the near-infrared region (ca. 1150 nm) is gradually reduced, given the thickened C shells separating the neighboring Cu NPs and thus eliminating the hybridized LSPR effect that induces the electromagnetic field coupling [
6,
28,
29]. In addition, as compared to Cu NPs, the enhanced and extended optical absorption in visible light region (< 900 nm) is observable for all the Cu@C core-shell NPs, attributed to the excellent optical absorption of amorphous C shells derived from the hydrothermal carbonization of glucose. These results indicate that the coating of C shells onto Cu NPs would be beneficial to photothermal conversion and then SISG. The LSPR effect of Cu@C core-shell NPs depending on the C shells thickness was further validated by the spatial distribution of electromagnetic fields simulated with COMSOL Multiphysics, based on the grid independence study on optical simulations (Fig. S1). As shown in
Fig. 2c, under solar light irradiation, Cu NPs display the intensive localized electromagnetic field at surface, induced by the LSPR effect. However, the electromagnetic field intensity is gradually decreased at the surface of Cu@C core-shell NPs, with the increasing thickness of the coated C shells. This comparative result again reveals that the coated C shells would weaken the hybridized LSPR effect of Cu NPs, corresponding well with the C shell thickness dependent optical absorption profiles of Cu@C core-shell NPs in the near-infrared region. Interestingly, the electromagnetic field intensity at the Cu/C interface is gradually strengthened, depending on the increasing C shell thickness. This phenomenon could be explained by the photon resonance accompanying with the LSPR effect of Cu NPs. With C shells coated, the incident photons would experience multiple scattering or refraction at the interface of Cu core and C shell, giving rise to the enhanced localized electromagnetic field [
1]. Moreover, the thickening C shells could further confine the incident photons at the interfacial region by intensifying their scattering or refraction. Thus, the most intensive localized electromagnetic field is achieved at the Cu/C interfacial region of Cu@C-80 NPs.