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.