SEM images of MSH, MSH@C and M@C-ON are shown in Fig. 1. The results show that the morphology of MSH changed from plate-shape with slightly curl to granular or elongated structure after adding glucose alone which maybe result from the existence of glucose or the extension of reaction time as shown in Fig. 1(a) and (b). However, the morphology of M@C-ON in Fig. (c) is flaky with clear curved. Therefore, we can confirm that the addition of ON can inhibit the curling of the sheet despite the prolonged reaction time or the presence of glucose.
From the XRD image (Fig. 2) of the M@C-ON, the diffraction peaks at 12°, 19.6°, 24.4°, 35°, and 60.5° correspond to (200), (-111), (400), (-202) and (-513) lattices of MSH (PDF#25–0645), respectively, which indicates that the crystalline structure of inner MSH in M@C-ON doesn’t change. However, it can be easily seen that the intensity of some diffraction peaks of M@C-ON changes compared with that of MSH. The diffracted intensity of (-202) plane becomes weak to a great extent while the diffraction peaks of (-242) and (-803) planes almost disappear. Besides, it can be seen from the spectrum that there is a wide peak near 20°-30° which is due to the amorphous carbon introduced by ON and glucose. It is worth noting that the intensity of this amorphous carbon peak in M@C-ON is far strong than that in MSH@C [25], which indicates more amorphous carbon could be produced after the adding of ON.
Figure 3 shows the FTIR spectra of MSH and M@C-ON nanoparticles. After the MSH was hydrothermally treated with glucose and ON, some new bands appear in the spectrum of M@C-ON compared with that of pure MSH. A new broad peak at 3300 cm− 1 is assigned to v(N-H) stretching mode [26] which means ON has been grafted onto the surface of MSH during the hydrothermal process. Due to the long alkyl chain of ON or the hydrolysis of glucose, two peaks appear at 2927 cm− 1 and 2856 cm− 1, responding to the asymmetric and symmetric CH2 stretching modes, respectively [26]. Besides, the wider peak at 1641 cm− 1 of M@C-ON compared with that of MSH indicates that there are some other peaks under it. The C = O peak from the hydrothermal carbonization of glucose at 1650 cm− 1 [27]and the C = C peak at 1590 cm− 1 is considered being responsible for this wide peak. In the enlarged spectrum, the peaks at 1407 cm− 1 and 1460 cm− 1 are attributed to the carbonyl groups (COOH) [27] and C = C stretching [28] respectively which confirms the aromatization of glucose under hydrothermal condition. The peak at 1375 cm− 1 is due to C-N vibration[29] which may be from the ON or the connection between the hydrocarbon from glucose and ON. Besides, the broad peak at about 1300 cm− 1 [30] is due to the C-OH stretching. The peaks related to MSH still exist, such as the peak of Mg-O at 556 cm− 1 [31], the Si-O-Si peak at 458 cm− 1 [32] and the inner surface hydroxyl groups at 3700 cm− 1 [33]. The results show that ON is grafted to the carbon on the surface of M@C-ON and the surface of M@C-ON contains some COO- groups.
The chemical valence states of the elements in the MSH@C nanoparticles were detected by XPS measurements, and the results are displayed in Fig. 4. From the survey scan spectrum (Fig. 5(a)), the MSH@C is mainly composed of four elements: Mg, Si, O, C and N whose contents are 8.50 at%, 10.63 at%, 41.29 at%, 36.82 at%, and 1.13 at% respectively. The first three elements are the main elements in magnesium silicate hydroxide, while the element of C is from glucose or ON and the N is only derived from the ON. The C 1 s core level spectrum obtained, with the peak fitting of its envelope, is shown in Fig. 4(b), which is deconvoluted into 4 peaks: CHx, C-C/C = C (284.3 eV)[34], aliphatic C (284.9 eV), C-N/C = O (286 eV)[35]and COO- (287.8 eV)[36]. The peaks at CHx, C-C/C = C, C = O, and COO- are produced by the hydrothermal reaction of glucose. As for the aliphatic C, it derives from the long alkyl chain of ON. The C-N peak is from the connection between the N element and the C element in carbon skeleton chain of ON or between the N element and C coated in MSH. The O1s spectrum (Fig. 4(c)), it can be deconvoluted into 5 peaks: C-O-R (533.29 eV), C = O (532.56 eV), Si-O (531.94 eV), O-H (531.39 eV), Mg-O (530.98 eV) in which the band of C-O-R may be associated with C-O-C or C-O-Si. The broad peak at 399.9 eV in the N1s spectrum (Fig. 4(d)) is not sure yet. According to the analysis above, it may be attributed to N-C and N-H. The results of XPS and FTIR are in good agreement and further prove the presence of long alkyl chains and COO- functional groups on the surface of M@C-ON.