One-Step Synthesis and Characterization of Silica Nano-/Submicron Spheres by Catalyst-Assisted Pyrolysis of a Preceramic Polymer

Silica nanospheres have attracted tremendous interest due to their importance in extensive applications. However, the direct largescale fabrication of silica nanospheres with controlled morphology and high purity remains a significant challenge. In this work, silica nano-/submicron spheres were successfully synthesized by a simple method through pyrolysis of an amorphous polysilazane preceramic powder with catalyst FeCl 2 . The synthesized spheres possess well-designed shape with diameter of 600–800 nm and high purity. The surfaces of the spheres are smooth and clean without any flaws. Besides, the spheres are identified as amorphous silica, and their growth mechanism was also proposed.


Introduction
In the past few years, silica nanostructure materials have attracted tremendous interest due to their importance in basic scientific research and unique applications in nanoscale devices [1,2].In particular, extensive efforts have been focused on silica nanospheres because they can be widely used in electronic substrates, thermal and electrical insulators, humidity sensors, optoelectronic devices, photographic emulsions, the protection of environmentally sensitive materials, and so on [3][4][5][6][7][8].So, new and versatile routes to synthesize silica nanospheres, which have well-defined shape and high purity, are highly desirable [9].
Current strategies for preparing silica nanospheres rely heavily on hydrolysis and hydrolytic condensation of tetraethyl orthosilicate [9], by which, however, the prepared silica nanospheres are of relatively low purity, and the experiment procedures need several steps [10,11].To date, the large-scale fabrication of silica nanospheres with controlled morphology and high purity remains a significant challenge.In the newly reported methods, catalyst-assisted pyrolysis of polymeric precursors for nanostructures is simple and easy controlling, and the resultant products are of high purity.Theoretically, it can be applied in fabricating various forms of Si-based nanostructures by adjusting the composition of polymeric precursors, atmosphere, catalyst, and so on [12,13].For example, Peng et al. [12] reported the preparation of clustered one-dimensional and approximately stoichiometrical SiO 2 amorphous nanowires with high productivity via catalytic pyrolysis of a polymer precursor onto alumina wafers.And Li et al. [14] prepared SiO 2 straight nanowires, spring-shaped nanowires, twinborn nanospring, fishbone-shaped nanowires, and braided-like helical nanowires.In this paper, we report the preparation of silica nano-/submicron spheres of high purity and sphericity using catalyst-assisted pyrolysis of a preceramic polysilazane, which has never been reported before in the literature.

Experimental
Polysilazanes are chain-like polymers with Si backbone and organic side-chain groups.They can be called perhydropolysilazanes when the side groups are all hydrogen.A perhydropolysilazane was used as the starting precursor, which is a liquid with average molecular weight Mn = 900-1200 at room temperature.The precursor was first solidified by heat treatment at 260 ∘ C for 0.5 h in N 2 .After that, the resultant amorphous SiCN chunk was crushed into powder with an agate mortar and mixed with 20 wt% FeCl 2 powder as catalyst.Then the powder mixture was placed in a high-purity alumina crucible in a conventional furnace under flowing high-purity nitrogen (99.9 vol.%) of 120 sccm (standard cubic centimeter per minute).The powder mixture was heated to 1250 ∘ C with a speed of 10 ∘ C/min and pyrolyzed there for 2 h.Finally, the furnace was cooled naturally to room temperature.Experiments were also performed without FeCl 2 as catalyst in order to investigate the growth mechanism.

Results
The morphologies of the as-prepared products were first examined by SEM. Figure 1 shows two typical SEM images of the as-produced silica nano-/submicron spheres under different magnifications.From Figure 1(a), it can be seen that the products are of uniform morphology with high density, indicating that the present method can produce spheric materials in large yield.A closer examination of the spheres at higher magnification as shown in Figure 1(b) reveals that they own diameter less than 1 m, high sphericity, and smooth clean surface.
The structure of the spheres was further analyzed by TEM.TEM images confirm the conclusions about the morphology of the products drawn from SEM.From Figure 2(a), it can be seen that the products possess uniform high sphericity with diameter of 600-800 nm.Higher magnification TEM image as shown in Figure 2(b) reveals that the surface of the spheres is smooth without any cracks, pores, or holes, indicating that the present method can produce solid spheric materials of high quality.
Figure 2(c) illustrates a typical EDX spectrum of the sphere as shown in Figure 2(a), revealing that the chemical compositions of the as-produced materials are of mainly Si and O and a tiny Al absorbed from the crucible.The detected Cu element in the spectrum comes from the grid which supports the samples during TEM observation.In addition, the quantitative analysis on the spheres shows that the atomic ratio of Si : O is about 1 : 1.9, revealing that the spheres are almost stoichiometric silica.
Moreover, typical HRTEM image as shown in Figure 2(d), and the highly diffusive ring pattern in the corresponding selected area electron diffraction as shown in the inset of this figure display that the silica nano-/submicron spheres are amorphous.
In order to confirm the chemical composition of the as-produced nano-/submicron spheres, FT-IR spectra were recorded.The collected complex set of infrared vibration bands as illustrated in Figure 3 can be assigned separately as follows: the band at 797.60 cm −1 is associated with the Si-O-Si symmetric stretch, while the sharp one located at 475.27 cm −1 corresponds to the Si-O-Si or O-Si-O bending mode and the peak at 1093.92 cm −1 to the Si-O-Si asymmetric stretching vibrations.The strong peaks at 3445.19 and 1631.71cm −1 agree with O-H stretching and bending of absorbed H 2 O, respectively.From IR analysis, it can be concluded that the as-produced nano-/submicron spheres are of silica.

Discussion
In our experiments, without FeCl 2 , no nanostructures were observed, indicating that FeCl 2 directed the growth of the silica nano-/submicron spheres, and the most likely source of oxygen that contributed to the formation of the spheres might come from the atmosphere, the low content of O 2 in the carrier gas of N 2 , which could supply a constant oxygen source during the growth of nanomaterials.In our previous work [15], Fe-rich droplets were observed on the tips of nanowires from pyrolysis of polymeric precursors with 20 wt% FeCl 2 as catalyst.The only difference between the current study and our previous one is that the oxygen partial pressure in the current one is higher than that in the previous, which can attribute to the oxygen content in the N 2 .On the basis of the evidence mentioned above, it was proposed that Fe-rich droplets were also formed and played a significant role in the nucleation and growth of the present nanospheres.So, it can be inferred that the synthesis process goes as follows.The SiCN derived from the polymer precursor reacted with oxygen survival in N 2 to produce silica and CO 2 via Reaction (1).CO 2 reacted with C from pyrolytic product to produce CO through Reaction (2), which reduced Fe 2+ ions into Fe atoms during reaction.After atomic Fe was produced, it formed tiny eutectic liquid droplets of Fe, Si, and O as nuclei at above 1200 ∘ C [16].The nanospheres gradually developed from the nuclei when elemental Si and O saturated in the alloy via Reaction (3): So, the last question is why the silica grew into nanospheres instead of other morphologies such as nanowires.In the present experiment, the speed of flowing N 2 reached 120 sccm, much higher than that conducted in our previous experiment for producing nanowires in [15], which will lead to an increase in gas pressure in the synthesis system and solubility of the corresponding gases in the droplets at high temperature and thus a decrease in their supersaturation there.And the growth rate of the nanostructures is determined by the supersaturation in the catalyst droplet [17].Therefore, the nanostructures grew slowly from the droplets, and the high surface tension of the alloyed particles at high temperature promotes them to grow into nanospheres instead of nanowires.

Conclusions
Silica nano-/submicron spheres were synthesized via catalystassisted pyrolysis of a perhydropolysilazane precursor.
The spheres possess well-designed shape with diameter of 600-800 nm, high purity, and smooth clean surface without any flaws.The spheres were identified as amorphous silica.The growth of the nanospheres might be controlled  by the high pressure in the synthesis system and high surface tension of the alloyed particles at high temperature.The reported technique would provide a facile method to fabricate high-quality nanospheres.

Figure 1 :Figure 2 :
Figure 1: Typical SEM images of the as-produced nanospheres: (a) in low magnification and (b) in high magnification.

Figure 3 :
Figure 3: Typical FT-IR spectrum of the as-produced spheric materials.