Figure 1 Structural characterization of EDTA-CDs and UPy-CDs.
(a) TEM image and inset of High-resolution TEM of EDTA-CDs. Inset:HRTEM
image of EDTA-CDs.(b) The corresponding specially filtrated image of
High-resolution TEM in EDTA-CDs.(c) size distribution of EDTA-CDs.(d)
XRD pattern of EDTA-CDs. (e) TEM image and inset of High-resolution TEM
of UPy-CDs. Inset:HRTEM image of UPy-CDs. (f) The corresponding
specially filtrated image of High-resolution TEM in UPy-CDs.(g) size
distribution of EDTA-CDs.(h) XRD pattern of UPy-CDs.
prepared EDTA-CDs and UPy-CDs were first characterized by transmission
electron microscopy (TEM) (Fig. 1a and Fig. 1e). The EDTA-CDs showed
monodisperse spherical particles with an average size of 3.53 nm (Fig.
1c). Obvious aggregation could be observed for the UPy-CDs, which is
also reflected by the significantly increase of particle size (6.40 nm,
Fig. 1g). This phenomenon could be ascribed to the enhanced
interparticle interaction induced by UPy association, which could be
further evidenced by the substantial increase of hydrodynamic diameter
from EDTA-CDs (5.92 nm) to UPy-CDs (138.2 nm) (Fig. S3). Meanwhile, the
Zeta potential of EDTA-CDs and UPy-CDs were determined to be 0.377 and
0.262 mV, respectively (Fig. S4), demonstrating the positive charge
character of the particle surface. High-resolution TEM images showed
that the lattice spacing of
EDTA-CDs was 0.218 nm (Fig. 1b), consistent with the (100) plane of
graphitic carbon [30]. This value exhibits a
little increase to 0.334 nm for UPy-CDs (Fig. 1f). According to X-ray
powder diffraction (XRD) patterns, both
EDTA-CDs and UPy-CDs have a broad
peak centered at 22°, corresponding to the graphite (002). Notably, the
XRD peak show an obvious broadening from EDTA-CDs to UPy-CDs. Both the
increase of lattice spacing and the broadening of XRD peak should be
ascribed to the assembly of UPy-CDs (Fig. 1d and 1h).
Raman spectroscopy was then used to evaluate the
graphitization degree. For both
EDTA-CDs and UPy-CDs, the Raman spectra show two peaks at 1350
cm-1 (D band) and 1600 cm-1 (G
band), corresponding to the disordered sp 3carbon domains and graphitized sp 2 carbon
domains, respectively [31]. The intensity ratio of
D band to G band (I D/I G)
of EDTA-CDs and UPy-CDs was determined to be 1.06 and 1.18, respectively
(Fig. 2a and Fig. 2e). This result suggests that the introduce of UPy
exerts little effect on the graphitization degree of carbon core. In
addition, the A band can also be found in the Raman spectrum, which
could be attributed to the five or seven membered carbon ring defects in
the CDs [32]. The compositions of EDTA-CDs and
UPy-CDs were analyzed by X-ray photoelectron spectroscopy (XPS). The
full spectrum of EDTA-CDs and UPy-CDs showed that three peaks were found
at 285, 400 and 532 eV, which could be indexed to C 1s, N 1s and O 1s
respectively (Fig. 2b) [28]. For EDTA-CDs, the
element atomic percent of C, O and N was 66.61%, 16.33% and 17.05%,
respectively. The C 1s (Fig. 2d) band of EDTA-CDs can be fitted into
C=C/C-C (284.75 eV), C-O/C-N (286.10 eV) and C=O (288.65 eV)[33], while the N 1s (Fig. 2c) band could be
deconvoluted to three characteristic peaks corresponding to pyridine N
(398.95 eV), pyridine N (400.10 eV) and graphite N (402.15 eV)[34]. C-O/O-H
(532.65 eV) and C=O (531.55 eV)
were found in the XPS spectra of O 1s (Fig. S5), indicating that the
surface of EDTA-CDs was rich in hydroxyl and carboxyl groups. These
phenomena indicate that the carbon core of EDTA-CDs was mainly composed
of N-doped polyaromatic structures derived from the condensation ofo -PD precursor. Different from o -PD, another precursor,
EDTA, contributes more to the surface state of CDs. The abundant
oxygen-containing functional groups derived by EDTA residues provides
reactive sites for surface decoration of UPy [35].
The XPS results of UPy-CDs show similar compositions to EDTA-CDs, with
an atomic percent of C, O and N of 70.44%, 14.19% and 15.36%,
respectively (Fig. 2f and Fig. 2g). Nevertheless, the amounts of C-O/O-H
species (Fig. 2h and S6) decreases from EDTA-CDs to UPy-CDs, which
should be attributed to the isocyanate reaction between C-OH and N=C=O
moieties.