Fabrication of size-tunable graphitized carbon spheres with hierarchical surface morphology on p -Si (100) by chemical vapour deposition

Graphitized carbon spheres (GCSs) with varied diameters (500 nm to 4.5μm) and hierarchical surface morphologies were successfully produced on iron-particles coated silicon (100) substrate at 750°C by chemical vapour deposition (CVD). By varying the mass flow rate of the precursor gasses and the method of catalyst coating on silicon (100), GCSs with varied diameters and differing morphologies were obtained. When the mass flow rate of the precursor gasses was altered, the mean diameter of GCSs increases until it reaches an optimum value (~3.1  m) suggesting a size-tunability of GCSs. Changing the catalyst coating method on silicon (100) from dip coating to spin coating produces larger-sized GCSs on silicon (100). Field Emission Scanning Electron Microscopy (FE-SEM) images show that GCSs possess a regular and uniform shape with the formation of a hierarchical morphology. The analysis of the variation of the surface roughness laterally across the substrate showed that the increased surface roughness resulting in from catalyst spin coating increases the mass transfer rates leading to the formation of larger-sized GCSs on Si (100). Raman spectroscopy and X-ray diffraction spectra obtained from the catalyst spin-coated and dip-coated samples confirmed the presence of graphitized hexagonal carbon networks in CSs. The surface functionality of GCSs was examined using FTIR spectroscopy. Synthesized GCSs were then used to fabricate an anode material in sodium ion rechargeable batteries and the performance of GCSs as an anode material in rechargeable battery system was investigated and the results obtained are also discussed here.


INTRODUCTION
The inimitable intricacy in the configuration of binding resulting in the ability of carbon to form sp, sp 2, and sp 3 hybridizations with wide-ranging materials results in an immensely diverse range of structures and morphologies with extensive technical applications. The nano-science and nanotechnology of carbonaceous structures received an exciting further development with the discovery of fullerene 1 and its elongated cousin the carbon nanotube 2 .
In recent years, various forms of spheroidal carbon structures have been discovered with extraordinary properties and multidisciplinary applications 3 . Carbon nano-capsules 4,5 carbon onions 6 carbon spheres [7][8][9] and carbon beads 10,11 are just a few but to name them.
Two different schemes have been proposed for the classification of spherical carbonaceous structures: the first such scheme was proposed by Inagaki 12,13 who classified the spherical carbon structures based on the arrangement of the number of carbon layers. The second approach, made by Serp 14 , classifies the spheroidal carbon structures based on their particle size, into fullerene (< 2 nm), carbon onions with closed graphitic layers (2-20 nm), carbon spheres (50 nm-1000 nm), and carbon beads (> 1000 nm). Among these different forms of spherical carbons, carbon spheres (CSs) are unique materials because they can be fabricated with significant characteristics such as adjustable porosity, uniform geometry, surface functionality, flexible particle size distribution, and outstanding chemical and thermal stability 15,16 Moreover, CSs have similar properties to fullerenes and graphite. Due to remarkable characteristics, CSs have been used in different fields including, composites material 17 , sodium-ion batteries 18 , adsorbents 19 , and water purification 20 . The surface morphology and graphitic structure of CSs are playing a vital role in enhancing the performance of these applications. Therefore, attempts focusing onto produce CSs with tunable surface morphologies (hence size) and more graphitized structures are receiving attention of physicists as well as chemists.
Several approaches have been made in the past to produce CSs which includes chemical vapour deposition (CVD) 14 , mixed-valent oxide catalytic carbonization (MVOCC) 21 highpressure carbonization 9,11 and pyrolysis of carbon sources in the presence of transition metals on kaolin support 22,23 .However, some of these processes must be operated at high pressure (usually up to 10 MPa). Under high-pressure conditions, the production rate could be enhanced but the purity of the carbon spheres produced decreases. In terms of economy and versatility, chemical vapour deposition (CVD) is the most popular technique to produce carbon spheres (CSs), which can be divided into three types as (i) catalytic CVD 24 , (ii) noncatalytic CVD 12 , and (iii) template CVD methods 25 . Although each method has its own advantages and disadvantages, methods capable of producing CSs at relatively low cost and low temperature make their selectivity and suitability in economically viable applications.
With this regard, the catalytic CVD method is important as it can produce carbon spheres at temperatures as low as 650 0 C with transition metals as catalysts deposited on a template 24 (in this experiment, the template is p-Si/SiO2). Jin  reported on the high yield one-step synthesis of CSs produced by dissociating individual hydrocarbons at their autogenic pressure and low temperatures 9 . These techniques are however limited by several factors. For example, in a few cases, the methods are not able to control the size distribution of the CSs and to remove the catalyst encapsulated, which are difficult to separate from the spheres. In the present work, we report the production, characterization and a possible application of size-tunable CSs via CVD by varying the process of catalyst coating on the substrate (which is reported for the first time), the source of catalyst precursor, and the mass flow rate of the precursor gasses at low temperature (750 0 C).
Even though the Lithium-ion batteries (LIBs) are at the peak of their performances as the most popular type of rechargeable batteries, Lithium deposits are depleting rapidly due to the increasing demand for the LIBs. This has made the cost of rechargeable batteries made out with Lithium to be a victim of inflation. 27 Due to this reason, researchers are paying their attention in the quest of finding a suitable substitution for LIBs. Sodium shares the same electrochemical properties as Lithium. On the other hand, sodium being more abundant has a great advantage in cost-cutting while in the production process. Hence developing a commercially applicable Sodium-ion battery (SIBs) has taken the interest of modern researchers in the last few decades. The developed Sodium-ion batteries have shared the same rocking chair mechanism as Lithium-ion batteries. 28 Therefore it is predicted that achieving high capacities with SIBs is also within the reach in near future. The CSs produced in this work therefore were used to fabricate an anode material in sodium-ion rechargeable batteries and the results are also discussed here.

MATERIALS AND METHODS
p-type silicon (100) wafer having a diameter 450 mm (18 inches and the surface orientate was confirmed by the XRD) was purchased from international vendors. Subsequently, the wafer was cut into pieces each having an area of 2x2 cm 2 . The silicon (100) was prepared for deposition of catalyst as follows. First the silicon substrates were degreased in acetone, and cleaned ultrasonically with deionized water at 40 °C for 10 mins, followed by ultrasonically cleaning with methanol for 5 min, washed again with deionized water, and   Fig.1 shows the image of GCSs where the catalyst (FeCl3) was dip-coated while the right panel in Fig.1 displays the image of GCSs where the catalyst (FeCl3) was spin coated. It is clearly evidenced from the images that GCSs grown on Si with catalyst dip-coating have smaller sized spheres with variation in their sizes leading to the formation of a hierarchical structure which appears to be emanating from the bottom to upward. The smaller spheres are seemed connected with each other on the surface. Typically the smaller GCSs made by CVD, are found to be connected (accreted) in a 2D strand that can extend well over tens of spheres 30 .

Process of catalyst coating on the formation of GCSs
However, there is a minor amount of deformation and destruction of spheres and the tendency to form larger spheres at the bottom of the hierarchical structure. This nature is more evident in the image shown in the right panel in Fig. 1 where the catalyst was spincoated on Si. In any case, the formation of larger spheres normally happens as discrete carbon structures that can interact with other spheres through van der Waals bonding.
Further, if the temperature is high enough, sometimes, the discrete carbon structures can form amalgamated structures via sphere coalescence 30 . However, in our case, the probability of having former is higher than the latter on the observed morphological differences seen in the images as a relatively low temperature was used in our case.

Mass flow rate of the precursor gases on the mean diameter of GCSs
As was pointed out earlier 30 , the mechanism of formation of GCSs involves the nucleation of pentagonal or heptagonal carbon flakes followed by the formation of spiral shell carbon particles proposed by Kroto and Mckey 30 . The growth of the GCSs, i.e., the addition of added carbon layers to the already formed spiral shell carbon particle, is due to the deposition of carbon flakes from the gas phase. This growth is influenced by the carbon source and the reaction conditions such as temperature, pressure, mass flow rate, and time in the reactor (feeding time). In CVD synthesis of GCSs, the carbon source will decompose into fragments and radicals and these fragments will provide the building blocks for the formation of flakes. Fig.4 shows the plan view of FE-SEM images recorded from GCS grown on Si by varying the mass flow rate of precursor gasses. Other parameters T (750 0 C), P (1 atm) and carbon source (acetylene), feeding time (30 minutes)) were kept unchanged.
The mass flow rate of the precursor gasses was changed (acetylene/nitrogen ratio) from 250/100, 500/200, 650/250, and 850/350 sccm, and the resulting GCSs were labeled as GCS250, GCS500, GCS650, and GCS850. Histogram showing the variation in the size distribution across each SEM image is also shown with the same image. The mean diameter of each sample was obtained by averaging the diameters of GCSs spread over each image.
The plot showing the variation of the mean diameter against each sample is shown in Fig.5. can continue to do so assembling eventually into a hierarchical structure. The second layer always is observed to have smaller particles, hence smaller mean diameter, than the first layer in a hierarchical structure. Further, the growth of densely packed GCSs at higher mass flow rates seen in Fig. 4 (d) provides corroborative evidence to this explanation.

Process of catalyst coating on the degree of graphitization of GCSs
Raman spectroscopy is one of the powerful techniques for characterizing carbon materials.
The peak in approximately 1592-1600 cm −1 (G band) corresponds to an E2g mode of graphite and is related to the vibration of sp 2 -bonded carbon atoms in a 2-dimensional hexagonal lattice, such as in a graphite layer. 34 The peak in approximately 1308-1323 cm −1 (D band) is associated with the vibrations of carbon atoms having dangling bonds in-plane terminations of a disordered graphitic network. The intensity ratio of the D and G-bands ID/IG is commonly used to characterize the graphitized degree (the degree of graphitization) of carbonaceous materials which is proportional to the number of defects in graphitic carbon within the analyzed spectrum. 35 Intense peaks are seen at 1316 cm -1 and 1592 cm -1 Raman shifts and their shapes agree well with previous observations 35 and are assigned as to originating from typical D band and G band peaks for carbon material. However, when the mass flow rate of the carbon source is increased, the intensity of both peaks diminishes, but only with slight changes to the ID/IG ratio. The ID/IG ratio manifest the degree of graphitization of GCSs produced in two different catalyst coating methods.
As can be observed in the tabulated data in Table 1, the ID/IG ratio of GCSs produced by both methods lies around 1.00 which reflects the presence of somewhat lower degree of graphitization and non-graphitizable carbon (amorphous carbon structure with a high content of lattice edges or defects). However, a slight increase in the graphitization can be observed with samples grown at a flow rate of 500/200 on Si where the catalyst was spin-coated.      mA. The resulting graph shows a steady increment of the voltage until it reaches a voltage of 2.35 V. By observing the curve pattern, it can be predicted that the cell achieves its full charge capacity in this voltage. If the charging time is increased further, the curve shows a decline which indicates that the charging time has exceeded the limit and the cell is unable to retain its capacity any further due to overcharging. The discharging curve of the cell shown in Fig. 9 (b) was obtained for a discharging current of 0.5 mA. The curve shows a rapid voltage drop starting with around 2.35 V and ending at around 0.25 V. After that, the discharging rate decreases rapidly and retains a steady value until it is fully discharged. The discharge capacity of the cell was calculated to be 107.8 mAh g -1 . The high discharging capacity obtained for this cell can be predicted to be arising due to efficient intercalation process involved between sodium ions and the GCSs. The FE-SEM images obtained show that the diameter of the prepared GCSs is in the range of 500 nm ~4.5µm. The grinding process can further minimize the particle size of the prepared composite which was used as the active material for the anode. Such porous nanoparticles can increase the contact area between the electrode material and the electrolyte. This process will be resulting in a favourable immersion of the electrolyte leading to high-quality electrochemical performances in the final count 40 . Hence it is evident that the cell capacity can be further increased by reducing the particle size and increasing the porosity of the prepared CSs in future work about its cyclability performances. The prepared cell was tested with electrochemical impedance measurements to investigate the sodium-ion migration dynamics. Figure 10 (b) shows the Nyquist plot prepared using the result of the impedance measurements. It consists of a small semi-circle at the high-frequency region, a large semi-circle, and a linear part in the high-frequency region that fits with the inserted circuit. The semi-circle in the highfrequency region indicates the charge transfer resistance and the straight line in the lowfrequency region represents the sodium-ion diffusion within the electrode respectively. 18,41 The linear behaviour of the low-frequency region further illustrates the typical Warburg behaviour associated with the sodium ion conductivity in the electrolyte. 42 The equitant circuit which entails the values of charge transfer resistances and impedance of constant phase elements is also included in the inserted circuit.

CONCLUSION
In summary, the growth of GCSs was achieved using state-of-the-art CVD technique over an iron-catalyst coated silicon substrate with C2H2 as a carbon precursor and N2 as a carrier gas at 750ºC under atmospheric pressure. The method of catalyst coating on Si has an effect on the growth and the formation of GCSs with size-tunable distribution. The carbon spheres produced by CVD have a regular shape with reasonably high yield, and diameters ranging from 500 nm to 4.5μm. Raman, FTIR, and XRD analysis confirmed the presence of graphite hexagonal carbon networks of the products. The results further reveal that the size distribution of the GCSs is dependent on the mass flow rate of the carbon source as well.
Further studies are underway to produce graphitized nano carbon spheres (nGCS) which could potentially have interesting physical and chemical properties and wide range of applications due to size confinement in the nanoscale.