3.1 Structural characteristics of HOPG and CVD graphene
To confirm the presence or absence of defects in the HOPG and CVD graphene samples, their surfaces were examined by in-lens FE-SEM detection and Raman spectroscopy. Figure 3 shows the SE and in-lens SE images of the HOPG and CVD graphene specimens, whereas Figure 4 shows their Raman spectra.
The SE and in-lens SE images of HOPG did not reveal any differences, and the corresponding Raman spectrum showed an intense peak at 1580 cm−1. Additionally, the Raman spectrum did not exhibit D bands around 1350 cm−1, which are caused by defects in graphene. Therefore, the HOPG sample was confirmed to have a defect-free sp2 network. In contrast, the in-lens SE image of the CVD graphene showed numerous black-lined patterns, although the SE image did not show any contrasting details. The hexagonal shape observed in the in-lens SE image represented bilayer graphene [49]. The higher the work function in graphene, the darker the contrast detected in the in-lens SE image [52]; therefore, the black-lined pattern represented sites with higher work functions. Additionally, the work function is known to increase with increasing exposure of the HOPG to argon plasma, indicating an increase in the number of dangling bonds in the sp2-bonded network [53]. Therefore, defects derived from the sp2-bonded network, such as dangling bonds, presumably existed along the black-lined patterns. Additionally, the Raman spectrum of the sample with the black-lined pattern showed a D band at 1350 cm−1. Therefore, the black-lined pattern in CVD graphene contained defects derived from the sp2-bonded network. Therefore, these results indicate that defects could be identified by in-lens SE detection as contrast.
3.2 Morphologies of silica adhered onto HOPG and CVD-synthesized single-layer graphene
SEM observations were performed to confirm the correlation between the silica adhesion sites and the defect distributions in HOPG and CVD graphene. Figure 5 shows the SE and in-lens SE images of the HOPG and CVD graphene specimens after the silica adhesion tests.
The SE images showed silica that was several tens of nanometers in size adhered onto HOPG and CVD graphene. Moreover, the number of silica particles adhered onto CVD graphene was greater than that on HOPG. Neither dark contrast nor roughness was observed in the in-lens SE and SE images of the silica adhesion sites in HOPG, respectively. In contrast, silica selectively adhered onto CVD graphene along the black-lined pattern seen in the in-lens SE image. Therefore, silicic acid tended to preferentially adhere to defects derived from the sp2-bonded network. In other words, this result suggests that silica adhered selectively to graphene at defects derived from the sp2-bonded network, such as dangling bonds, rather than to the sp2 bonds themselves.
3.3 Adsorption of silica onto different graphene models
Based on the aforementioned results of the silica adhesion tests, first-principles calculations of the adsorption energy between a silica molecule and the sp2-bonded network were performed to confirm whether the silica adhesion sites were determined inherently by the sp2-bonded network or the defects derived from the sp2-bonded network. Dangling bonds were artificially created as defects derived from the sp2-bonded network by extracting one carbon atom from the sp2-bonded network in the calculation model. Then, the adsorption energies of the silica molecule for the defect sites and sp2-bonded network were calculated. Figure 6 shows results obtained before and after the calculations of the sp2-bonded networks in graphene systems with no defects, with dangling bonds, and with hydrogen-terminated dangling bonds.
The atomic arrangement observed in the calculated results is described henceforth. In the defect-free graphene model, the calculated distance between the O atom in the silicic acid ion and the C atom of graphene was 2.66 Å, which is considerably greater than that in the initial arrangement (1.7 Å). However, in the graphene sheet with the dangling bond defects derived from the sp2-bonded network, the O− of the silicic acid ion approached graphene, resulting in an interatomic distance of 1.59 Å between the O− of the silicic acid ion and the C atom with the dangling bond. Additionally, the interatomic distance between a H-atom-terminated C atom with a dangling bond and the O− of the silicic acid ion was 2.08 Å.
To confirm the adsorption mode of the silicic acid ion onto the graphene sheet, Bader charge [67–70] calculations were performed using the results of the respective models; the results are depicted in Figure 7. In the defect-free graphene sheet (Figure 7(a)), the charge distribution between the silicic acid ions and graphene was not shared. However, in the graphene sheet with dangling bonds (Figure 7(b)), the charge distribution was shared between the silicic acid ion and the dangling-bond-containing C atom, indicating the occurrence of chemical adsorption. Additionally, the charge distribution between silicic acid ions and graphene was not shared in the graphene with hydrogen-terminated dangling bonds (Figure 7(c)). Therefore, physical adsorption occurred onto the defect-free and hydrogen-terminated graphene specimens. Furthermore, the graphene with dangling bonds and hydrogen-terminated possibly exhibited covalent bonding and hydrogen bonding, respectively.
The adsorption energies and modes for the investigated graphene sheet models are listed in Table 2. The absolute value of adsorption energy of graphene with dangling bonds (−1.04 eV) was higher than that of the defect-free graphene (−0.25 eV). Therefore, a strong adsorptive force was exerted because of the high adsorption energy and the occurrence of chemical adsorption. However, the hydrogen terminations of the dangling bonds lowered the adsorption energy to −0.69 eV, and the adsorption mode changed to physical adsorption. These results indicate that although the graphene sheets with the sp2-bonded network did not inherently attach to the silica, defects derived from the sp2-bonded network, such as dangling bonds, functioned as chemisorption sites for the silica. Furthermore, by terminating the defects derived from the sp2-bonded network with hydrogen, the adsorption mode of silicic acid presumably changed to physical adsorption, and the adhesion of silica could be restricted.
The aforementioned experimental and theoretical calculation results suggest that silica readily adhered to the defects derived from the sp2-bonded network, such as dangling bonds.
Table 2 Adsorption energies and types of reaction between graphene and a silicic acid ion
Defect type
|
Interatomic distance (Å)
|
Adsorption energy (eV)
|
Adsorption type
|
Defect-free
|
2.66
|
−0.25
|
Physical
|
Dangling bonds
|
1.59
|
−1.04
|
Chemical
|
Hydrogen
terminations
|
2.08
|
−0.69
|
Physical
|