FIGURE 6 Dependence of CG on a)tD and b) ω .
concentration, C0 – initial
concentration of dispersion, and τ – constant oftD ). On the basis of monitoring data,Cs and τ are determined
respectively to be 0.22 mg mL-1 and 25 h, indicating
92% of FLG sheets still remaining in the dispersion even after 250 h of
deposition. Such good stability is thought to result mainly from
electrostatic repulsion of VIB-co -VI-co -Py adsorbed on the
FLG surface. Zeta potential (ξ ), a parameter usually used for
evaluation of repulsion, is measured as 37.8 mV for the FLG dispersion,
while |ξ| > 30 mV is a widely
accepted cut-off range for colloidal stability.51 The
dependence of CG on centrifugation speed
(ω ) was also evaluated (Figure 6b). After 30 min
centrifugation, the dispersion shows a negligible decrease ofCG at ω ≤ 2,000 rpm. It
is because the dispersion has already experienced a centrifugation of
2,000 rpm/90 min in its production process. Over 2,000 rpm,CG decreases sharply from 0.228 to 0.073
and 0.049 mg mL-1 at 8,000 and 12,000 rpm,
respectively, exhibiting an empirical scaling ofCG ∝
ω -1. Despite undesirable decrease ofCG , a positive phenomenon is observed.
That is, the content of single-layer graphene improves to 33% and 76%
respectively in 8,000 rpm- and 12,000 rpm-centrifuged supernatants,
compared with 11% in 2,000 rpm-centrifuged one (SI: Section IV). This
offers an alternative method to produce the single-layer graphene.
FLG-P hybrid prepared from scalably produced dispersion,
hereafter named as FLG-P-80 hybrid, displays an unusual ability
of being invertibly dried and re-dispersed in pure water (by 5 min
sonication) for many rounds. The thus re-obtained dispersion has a
maximal CG of 3.87 mg
mL-1, still reaching 3.23 mg mL-1even after being deposited for 240 h. Such re-dispersing ability, which
is believed to be contributed by VIB-co -VI-co -Py adsorbed
on the FLG surface, is absent from reduced graphene oxide (rGO) and most
of surfactant-exfoliated FLG, highly facilitating the storage, delivery,
and applications of FLG. The content of VIB-co -VI-co -Py inFLG-P-80 hybrid is determined to be 6.8 wt% by TGA (SI:
Section V).
Taking the aqueous dispersion of FLG-P-80 hybrid as a typical
sample, AFM was carried out to examine the exfoliation state of FLG.
Figure 7a presents a typical AFM height image with a cross-section
analysis. The analytical results on 107 different flake-like structures
reveal that the thickness is mostly within 1.1−2.8 nm (Figure 7b-i). In
light of the AFM thickness of pristine graphene usually within 0.6−0.9
nm,52,53 the determined thickness of 1.1−2.8 nm
signifies that 90% of detected flakes are single- (9%), double-
(17%), or few-layer (3- to 5-layer, 64%) graphene sheets adsorbed with
VIB-co -VI-co -Py thin layers. Furthermore, the average
lateral size of flakes is determined to be 533 ± 102 nm (Figure 7b-ii),
in good agreement with the DLS value of 583 ± 115 nm. Within
instrumental resolution, no defects like holes were observed on the
basal planes, indicative of high quality of the FLG sheets.