Metal salts assisted thermoplastic polymer NIPAM in-situ carbonization on porous silica microspheres surface

The preparation of different carbon-shell morphology microspheres and controllable surface area via using thermoplastic polymers as carbon source was still a challenge. In addition, the unique structure of porous Silica@Carbon (Sil@C) microspheres not only can provide larger surface area and active site, but also have the characteristics of high mechanical strength and easy modification. Herein, we report a simple method to in-situ fixation a series of different morphology graphitized carbon shell (rosa roxburghii, bulk, and carbon sphere shapes) on porous silica microspheres surface. Metal salt assisted polymer carbonization will not only prepare graphitized carbon with different morphology on silica surface, but also facilitate the in-situ carbonization of thermoplastic polymer on the silica microspheres surface instead of internal pores; the more heat of metal salt released during carbonization, the larger the specific surface area of Sil@C.


Introduction
The excellent properties of Carbon materials (CMs) were height correlation to itself morphology, resulted in a variety of CMs morphology, such as activated carbon [1], graphite [2], graphene [3], fullerene [4], carbon nanotubes [5], carbon microspheres [6][7][8][9][10], carbon quantum dots [11] and so on. In addition, Silica@Carbon materials (Sil@C) microspheres were widely applied to various fields, such as wastewater treatment [12][13][14], gas storage and separation [15][16][17], catalysis [18][19][20], supercapacitor, and batteries manufacturing [21][22][23][24][25], nevertheless, how to prepare CMs with different morphologies on silica microspheres surface was still a difficult problem. So far, the strategies for preparing Sil@C with different morphologies can be summarized as follows: one was high-temperature carbonization Xiaojing Liang xjliang@licp.cas.cn 1 bulk, and carbon microsphere shapes) were prepared by insitu carbonization of thermoplastic polymer NIPAM under metal salts assisted condition. The shell morphology has no correlation with the NIPAM morphology itself, which greatly simplifies the preparation process; It had the potential of industrial production. It was worth noting that the thermoplastic polymer NIPAM was assisted by the additional heat release of metal salts in the high-temperature carbonization process, so that NIPAM will not collapse and block the pores of porous silica microspheres, but will only carbonize in-situ on the surface. In addition, the surface area of Sil@C has a positive correlation with the heat released by metal salt, in the process of high temperature carbonization, the more heat released by metal salt, the larger surface area of prepared material.

Reagents and experimental Reagents and instruments
The detail of reagents and instruments saw in supporting information.

Synthesis of porous Sil@C microspheres
As shown in Fig. 1a, the preparation of porous Sil@C composites microspheres with different morphology can be divided into two simple steps: (1) coating polymer NIPAM on bare porous silica microspheres surface via surface insitu polymerization, which were referring to our group previous works (denoted as NIPAM@Sil) [36][37][38]; (2) different metal salts assisted polymers in-situ carbonization on silica microspheres at high temperature (named as Sil@C). More experimental details saw in supporting information experimental section.

Direct carbonized NIPAM@Sil without metal salt assistance
After NIPAM in-situ polymerization on bare silica surface, scanning electron microscopy (SEM) images have showed that an irregular polymer layer formed on NIPAM@Sil surface and a slight adhesion phenomenon between the microspheres ( Fig. S1a-d). When the NIPAM@Sil was carbonized at 500 °C without the assistance of metal salt, the polymer coating on surface bare silica basically disappeared, but there were still some residues on silica microspheres surface, and the color changes from white to black (Fig. S2), indicated that NIPAM not completely pyrolyzed but formed carbon at 500 °C. In addition, the surface area and pore volume of Sil@C decreased sharply compared with bare silica and NIPAM@Sil. As a result, the carbonation process was deduced as follow: (1) the polymer NIPAM on silica microspheres was thermoplastic polymer; (2) the graphitized carbon formed by thermoplastic polymer occupied the pores of porous silica microspheres.

Effects of NiCl 2 assisted carbonization and carbonization temperature
Thermogravimetric analysis (TGA) decomposition results have shown that compared to bare silica with no significant mass loss, NIPAM@Sil had 11.97% mass loss at about 400 °C. The Sil@C was black (Fig. S2b) and there were residues on silica microspheres surface (Fig. S1e, f), indicated that the NIPAM at 400 °C was not pyrolysis completely. As shown in Fig. S3a and b, when NiCl 2 was introduced to carbonization process, adhesion phenomenon of NIPAM@ Sil has disappeared and collapsed accompanied with about 30 nm carbon microspheres formed on bare silica surface (noted as Sil@C(NiCl 2 )). Moreover, the number of carbon microsphere decreased sharply with the increase of carbonization temperature (Fig. S3a-f). The transmission electron microscopy (TEM) images also showed a sharp decline in carbon microspheres with carbonation temperatures rising (Fig. S4). Notably, with the increase of carbonization temperature, the color of Sil@C(NiCl 2 ) has changed from black to gray, which shown that the more complete the pyrolysis of NIPAM with the increase of temperature, and 500 °C was the more appropriate carbonization temperature.
Furthermore, compared to bare silica, broad and overlapping D (1351 cm -1 ) and G (1587 cm -1 ) bands were registered in the first order Raman spectra for Sil@C and Sil@C(NiCl 2 ) (Fig. 2b). These two overlapping bands were identified as poorly organized carbons: (i) 1351 cm -1 assigned to disordered crystal graphitized carbon or sp 3 -rich phase, and (ii) 1587 cm -1 ascribed to amorphous sp 2 -bonded forms of carbon [39,40]. Compared with the fourier transform infrared spectroscopy (FT-IR) of bare silica and NIPAM@Sil, FT-IR spectrum of Sil@C(NiCl 2 ) has shown the stronger absorptions of 3300-3400 cm -1 and absorptions of 1630 cm -1 belonged to the carboxyl (Fig. S6a) [37]. X-ray photoelectron spectroscopy (XPS) of Sil@C and Sil@C(NiCl 2 ) revealed stronger existence of C 1s peak (~ 287.8 eV) and weaker SiO 2 peak (~ 103.5 eV) compared with bare silica and NIPAM@Sil (Fig. S6b). The high-resolution C 1s XPS of Sil@C and Sil@C(NiCl 2 ) can be deconvoluted three individual peaks assignable to the graphitic C (284.5 and 285.2 eV) and O = C-O (289.6 eV) (Fig. S6c, d) [41][42][43]. In particular, as Fig. 2c and d shown that N 2 adsorption and desorption curves of all microspheres conform to the type IV adsorption isotherm; IUPAC classification with a hysteresis loop, which indicates that pore width greater than 4 nm [44]. The surface area and pore size of Sil@C(NiCl 2 ) only had a small reduction compared with NIPAM@Sil, but had larger surface area and smaller pore size than Sil@C. The surface area and pore size of Sil@C(NiCl 2 ) very close to non-carbonized NIPAM@Sil, this shown that with the assistance of NiCl 2 in the carbonization process, NIPAM almost only carbonized on silica surface of and not collapsed and shrinked into the silica microspheres inside pores.

Effects of different metal salts on assisted carbonization process
In order to verify that the in-situ carbonization of NIPAM on silica surface by NiCl 2 was not an accident. Six other metal salts, including Na 2 CO 3 , NaCl, Na 2 SO 4 , NiCO 3 , NiSO 4 and CoCl 2 , were selected to assisted carbonize NIPAM@ Sil under 500 °C. Only Na 2 CO 3 , NiCO 3 and NaCl could forming a specific morphology of graphite carbon, including rosa roxburghii and bulk, on porous silica microspheres surface, respectively (Fig. 3a, b, e). However, other metal collapse at NiCl 2 around and then be carbonized. Bare silica and NIPAM@Sil without obvious thermal effect in the heating process (Fig. S9a, b), that was no additional heat supply except the heat provided by the instrument. The differential scanning calorimetry (DSC) results of seven metal salts have shown that all of metal salts had obvious exothermic peaks higher than 150 °C, which directly explained the auxiliary pair of metal salts for NIPAM@Sil carbonization, in addition to the heat provided by the instrument, there was a certain amount of heat released by the metal salt itself (Fig. S9). For the integration of these exothermic peaks, the emission of each metal salt was different, ranging from 2.7 to 677.9 J g -1 (Table S7). Combined the surface area results of Sil@C with different metal salts to assist carbonization that the more heat the metal salt released; the larger the surface area of Sil@C. Na 2 CO 3 only released 2.7 J g -1 heat, which leads to the weak ability of Na 2 CO 3 to maintain the in-situ carbonization of NIPAM on silica microspheres surface, and the surface area of Sil@C(Na 2 CO 3 ) (76.47 m 2 g -1 ) was only a little larger than that of Sil@C (53.48 m 2 g -1 ). What was more noteworthy was that the flake graphitized carbon grown directly from silica microspheres pores can be directly captured by SEM (Fig. S7a), which shown that NIPAM would collapse and entered the internal silica microspheres pores during heating. In addition, NiSO 4 released 677.9 J g -1 heat, which caused the more complete pyrolysis of NIPAM, and resulting in the surface area of Sil@C(NiSO 4 ) (392.60 m 2 g -1 ) larger than NIPAM@Sil (289.37 m 2 g -1 ) and close to bare silica (370.72 m 2 g -1 ), and the color closer to white; it could showed that high heat was not conducive to the carbonization stability.
salts cannot remain graphite carbon on porous silica microspheres surface, which were same like bare porous silica microspheres surface (Fig. 3c, d and f). Though the colors of Sil@C(Na 2 CO 3 ), Sil@C(NiCO 3 ) and Sil@C(NaCl) were black with graphitic carbon (Fig. S7), the other metal salts assisted carbonized process was yellow or closer to white (Fig. S8). Thus, it can be speculated that some metal salts may lead to NIPAM instability during carbonization. In addition, compared with NiCl 2 assisted NIPAM to formed carbon microspheres on silica microspheres during carbonization, different metal salts were used for assisted carbonization to formed different morphologies, which may be related to the corresponding catalytic properties of metal salts.

Different metal salt assisted on NIPAM@Sil carbonization mechanism
NiCl 2 assisted carbonization not only generated carbon microspheres on silica surface, but also avoided the collapse of polymer into silica internal pores. Moreover, different metal salts including Na 2 CO 3 , NaCl, Na 2 SO 4 , NiCO 3 , NiSO 4 and CoCl 2 assisted carbonized NIPAM@Sil process was compared with the surface area, pore diameter. As shown in Fig. 4a, b and Table S5, different metal salts assisted carbonization Sil@C with different surface areas and pore sizes which indicated that metal salts had played an important role in maintaining the in-situ carbonization of thermoplastic polymer NIPAM on porous silica microspheres surface, and this maintenance ability must be related to metal salts properties. Without the assistance of metal salt NIPAM@Sil during carbonization, NIPAM will collapse and enter the internal pores of silica, while with the assistance of NiCl 2 , it was basically carbonized in-situ only on silica surface, which mean that in the whole carbonization heating process, thermoplastic NIPAM will preferentially

Conclusion
In summary, Sil@C microspheres with series carbon-shell morphologies (rosa roxburghii, bulk, and carbon sphere shapes) was successful prepared by in-situ carbonized the thermoplastic polymer NIPAM on porous silica microspheres surface. Metal salts assisted the carbonization of thermoplastic polymer NIPAM, which can not only prepared carbon-shell with different morphologies, but also maintain the in-situ carbonization of NIPAM on silica microspheres surface. In addition, more heat released by metal salts, the stronger ability to maintain in-situ carbonized NIPAM on silica surface. This simple strategy provided a theoretical and experimental basis for the preparation of carbon composites with different morphologies and surface areas by using thermoplastic polymers as carbon source.

Supplementary Information
The online version contains supplementary material available at https://doi.org/10.1007/s10934-022-01363-6. Fig. 4 (a, b) Nitrogen adsorption and desorption isotherms at 77 K and pore size distributions for different Sil@C with different metal salt assisted carbonization process. (c) The surface areas of different Sil@C with different metal salt assisted carbonization process. (d) The DSC analysis of different metal salts total heat released