Formation of Silicon / Carbon Core-Shell Nanowires Using Carbon Nitride Nanorods Template and Gold Catalyst

In this experiment, silicon/carbon (Si/C) core-shell nanowires (NWs) were synthesized using gold nanoparticles (Au NPs) coated carbon nitride nanorods (CN NRs) as a template. To begin with, the Au NPs coated CN NRs were prepared by using plasmaenhanced chemical vapor deposition assisted with hot-wire evaporation technique. Fourier transform infrared spectrum confirms the C–N bonding of the CN NRs, while X-ray diffraction pattern indicates the crystalline structure of the Au NPs and amorphous structure of the CNNRs.TheAuNPs coated CNNRs were thermally annealed at temperature of 800C in nitrogen ambient for one hour to induce the growth of Si/C core-shell NWs. The growth mechanism for the Si/C core-shell NWs is related to the nitrogen evolution and solid-liquid-solid growth process which is a result of the thermal annealing. The formation of Si/C core-shell NWs is confirmed by electron spectroscopic imaging analysis.


Introduction
Researches about silicon nanowires (Si NWs) have recently attracted much interest because of their novel physical properties that are suitable for many applications such as in solar cells, lithium-ion batteries, thermoelectric devices, and biological sensors [1][2][3][4].However, Si NWs-based devices still encounter several drawbacks such as surface oxidation [5][6][7] and rapid volume expansion upon lithium alloying [8,9], which could significantly reduce their efficiency.In order to avoid this, surface passivation of the NWs by more stable materials are necessary.One of these is carbon (C) which serves as a promising candidate for protective shells due to its high chemical stability and superior physical properties [10].By comparing the performance of Si NWs with and without C coating in lithium-ion batteries, Chen et al. [11] observed a higher capacity and better cycle stability when Si NWs with C coating are used as anode materials.On the other hand, Das et al. [12] demonstrated an improvement in the field emission of Si NWs when the NWs are coated with an amorphous C layer.Several efforts have been carried out to prepare silicon/carbon (Si/C) core-shell NWs [11][12][13].
Nevertheless, a second carbon coating is usually required after the preparation of Si NWs [11,12], which can cause contamination of the Si NWs during the sample transfer process.
In this work, we report a synthesis of Si/C core-shell NWs by using carbon nitride nanorods (CN NRs) template coated with gold nanoparticles (Au NPs).The Au NPs and CN NRs coating were prepared separately on an Si(111) substrate using a hot-wire-assisted plasma-enhanced chemical vapor deposition system.The as-grown Au NPs coated CN NRs sample was then annealed at a temperature of 800 ∘ C to form the Si/C core-shell NWs.800 ∘ C is selected as a preferred annealing temperature ( a ) because the nucleation of NWs at the Au/Si interface begins at this temperature [14].The effects of  a on the structural and morphological changes in the sample were then analyzed.vapor deposition method assisted with hot-wire evaporation technique.Prior to deposition process, the substrates were chemically cleaned using Radio Corporation of America (RCA) cleaning procedure [15].The experiment began with a deposition of CN NRs.The radio frequency power, substrate temperature, process pressure, and deposition time were kept at 60 W, 100 ∘ C, 1.9 mbar, and 90 minutes, respectively.At the same time, the flow rate ratio of methane, nitrogen, and hydrogen gases was maintained at 1 : 4 : 5 throughout the experiment.Hydrogen treatment was carried out on the substrates for 10 minutes prior to the deposition process in order to remove the contamination on the substrates' surface.After that, an Au wire measuring 4 mm in length and 1 mm in diameter was heated by a tungsten filament in vacuum ambient (10 −3 mbar) to form Au NPs on the surface of the deposited CN NRs.The distance from the Au wire to the substrate was fixed at 3 cm.The evaporation process was kept at 10 seconds and was controlled by a shutter assembled below the tungsten filament.The as-prepared Au NPs coated CN NRs were then thermally annealed in a quartz tube (Carbolyte CFM 12/1 furnace system) at different  a of 600 and 800 ∘ C for 1 hour.The annealing process was carried out in nitrogen atmosphere.
The structural properties of the CN NRs were characterized by a Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer System 2000).A Siemens D 5000 X-Ray Diffractometer (XRD) with Cu-K radiation (  = 1.54056Å) was used to determine the crystallinity of the Au NPs and the CN NRs.Field emission scanning electron microscope (FESEM) images and energy dispersive X-ray (EDX) spectra for as-prepared and annealed samples of Au NPs coated CN NRs were obtained by using a Zeiss Auriga FIB-SEM to determine the morphological properties and composition of the nanostructures.As a complementary technique, energy-filtered transmission electron microscopy (EFTEM, Zeiss Libra 120) was employed for electron spectroscopic imaging (ESI) on the samples to obtain specific and improved elemental information of the nanostructures.evolution from the CN NRs during the annealing process [22].In addition, an absorption peak located at ∼800 cm −1 corresponding to the Si-C stretching mode [23] appears in the FTIR spectrum of the Au NPs coated CN NRs annealed at 800 ∘ C.This suggests that the NWs formed at  a of 800 ∘ C consist of Si and C atoms in the NWs matrix.Meanwhile, the increase in the C-O peak intensity with  a is due to the oxidation of the surface C atoms during the annealing process [24,25].

Results and Discussion
EDX analysis was carried out in order to study the composition of the grown NWs at  a of 800 ∘ C.This was done by obtaining EDX spectra from the catalyst droplet, NW stem, and substrate, which are labeled as Spots 1, 2,   Based on the previous results, we propose a mechanism for phase transformation from Au NPs coated CN NRs templates into Si/C core-shell NWs through the annealing process (Figures 6(a)-6(c)).During the annealing step, the Au NPs and CN NRs could undergo two processes, namely, Au agglomeration [27] and nitrogen evolution [22], respectively.It has been reported that [28,29] nitrogen atoms can be evolved from the covalent sp 3 C-N bond upon an  a of 500 ∘ C onward, leaving small amount of stable graphite-like sp 2 C=N [30] and mostly graphite-like sp 2 C=C structures [31,32] at temperature lower than 700 ∘ C. The amorphous CN films are morphologically more compact after annealing process [33].This explains the slanting and bundling of the CN NRs at  a of 600 ∘ C as a result of a tendency towards more stable graphitic sp 2 C=C film structure formation (Figure 6(b)).The agglomeration of Au NPs usually occurs when annealed  a is above 500 ∘ C [34,35], even for those embedding in a dielectric matrix (SiO 2 or TiO 2 ).It was found that two NWs can join together if their catalysts were combined [36].This phenomenon could also result in the formation of the CN NRs bundles.Au is one of the most effective catalysts for the growth of Si NWs [37].However, in most cases, Aucatalyzed Si NWs grown from the Si source that originated from Si substrate can only be achieved at a high temperature of ∼1000 ∘ C [38].Recently, Márquez et al. [39] demonstrated the growth of Si NWs using Au as catalyst and Si substrate as source via porous anodic alumina mask at a much lower temperature of 800 ∘ C. In this process, the porous anodic alumina mask allows confinement of energy within the nanosized pores, thus lowering the growth temperature.Similarly, in our study, the CN NRs template acts in reducing the growth temperature of Si NWs.Au NPs distributed between the interspace of the CN NRs form eutectic with Si substrate, followed by diffusion of Si and eventually the precipitation and growth of Si NWs.Additionally, we found that graphite was able to lower the formation temperature of SiO x NWs on Au-coated Si substrate [40].Hence, the transformation of CN NRs into graphite-like structures also assists the growth of Si NWs.In other words, the Au-induced Si NWs were preferably grown within the bundles of graphite-like NRs as shown in Figure 6(c).The grown Si NWs were then joined with the graphite-like NRs bundles via the combining of Au NPs, eventually forming Si/C core-shell NWs.

Conclusions
Si/C core-shell NWs were successfully synthesized from Au NPs coated CN NRs template on Si substrate by using a simple thermal annealing process.The Au NPs acted as a catalyst for the growth of Si NWs from the Si substrate at a temperature of 800 ∘ C. Annealing process results in N evolution and transforms the CN NRs into bundled structures.The grown Si NWs combined with the graphitelike sp 2 C=C bundles form the Si/C core-shell NWs.EDX and ESI analyses confirm the Si/C core-shell structure of the worm-shaped NWs.

Figure 1 :
Figure 1: Au NPs coated CN NRs template.Schematic diagram of (a) CN NRs and (c) Au NPs coated CN NRs template.(b) FTIR spectrum of the CN NRs.(d) XRD pattern of the as-grown Au NPs coated CN NRs template.

Figure 2 :− 1 )Figure 3 :
Figure 2: Morphological analysis of the as-grown and annealed samples.(a) Surface and (b) cross-sectional FESEM images of the as-grown and annealed Au NPs coated CN NRs samples at different  a of 600 and 800 ∘ C.

Figure 1 (
Figure 1(a) shows a schematic diagram of CN NRs grown on an Si(111) substrate.FTIR absorbance spectrum of the CN NRs (Figure 1(b)) revealed three major peaks located at 1080-1160 cm −1 , 1210-1350 cm −1 , and 1500-1800 cm −1 .These findings correspond to C-O (ketones group) stretching mode[16], sp 3 C-N[17], and the overlapping of sp 2 C=N and C=C stretching modes[18][19][20], respectively.This FTIR analysis confirmed the presence of C-N bond for the CN NRs.A schematic diagram of Au NPs deposited on the CN NRs template is shown in Figure1(c).Figure1(d)shows the XRD pattern of the Au NPs coated CN NRs.The diffraction peaks are well indexed to the face-centered cubic of Au crystal (JCP2:00-001-1174).The absence of CN crystalline peaks clearly indicates that the CN NRs are amorphous in structure.Using Scherrer equation, the average Au crystallites size calculated from the Au(111) crystalline peak[21] was about 10 ± 1 nm.Figure2shows the FESEM images of as-prepared and annealed Au NPs coated CN NRs template.The as-prepared sample reveals high density of vertically aligned CN NRs with a diameter and length of ∼60 and ∼240 nm, respectively (Figure2(a) as-grown).Whereas Au NPs with size varying from 40-120 nm can be seen from the plane view FESEM image (Figure2(b) as-grown).Backscattered electron image (inset in Figure2(a) as-grown) further shows that the Au NPs are freely distributed on the (i) and (ii) tip, (iii) surface, and (iv) interspacing between the CN NRs.Here we observe that the CN NRs tend to slant, forming an inclination angle of ∼70-85 ∘ from the substrate, while some of them started to merge with each other, forming bundled structures after annealing at  a = 600 ∘ C for 1 hour (Figure2(a) 600 ∘ C).In addition, the Au NPs on the tip of the CN NRs were found to agglomerate with each other to form Au droplets in larger sizes.This can be seen from plane view FESEM image (Figure2(b) 600 ∘ C) where the Au NPs on the cluster of two or three CN NRs tips combined with each other.However, the rod-like structure is absent at  a of 800 ∘ C. Worm-like structure of Si/C NWs with Au NPs capping on tip of the NWs is grown throughout the substrate (Figures 2(a) and 2(b) 800 ∘ C).The length of the NWs varies from 100 to 500 nm with an average diameter of 70 ± 10 nm.XRD and FTIR measurements were carried out on the annealed Au NPs coated CN NRs samples to study the structural and bonding configuration change of the samples upon annealing process.Figure 3(a) shows the XRD patterns of the as-grown and annealed Au NPs coated CN NRs at  a of 600 and 800 ∘ C. The increase in the intensity and decrease in width of the Au diffraction peaks with  a indicate an increase in crystallite size of the Au NPs according to Scherrer relation [21].This is consistent with observation of the Au NPs agglomeration in FESEM analysis.No other diffraction peak is observed in the XRD pattern except that Au indicates that the NWs formed at  a 800 ∘ C are amorphous in structure.FTIR absorbance spectra of the as-grown and annealed Au NPs coated CN NRs are shown in Figure 3(b).A reduction in the intensity of C-N and C=N absorption peaks with  a is observed.This can be attributed to the nitrogen

Figure 5 :
Figure 5: ESI mapping of the Si/C core-shell NWs.(a) TEM image of Si/C core-shell NWs.ESI patterns of the Si/C core-shell NWs in (a) for the elements of (b) Si, (c) C, and (d) N.

Figure 4 (
b) shows the weight percentage (wt%) of each element detected from the mentioned regions.Generally, high concentration of Si signal was obtained for all spectra due to the high penetration depth (<1 m) of X-ray source[26].The catalyst droplet (Spot 1) contains high signal of Au and Si as well as traces of C and O elements.This confirms the initial growth of the NWs from the Au NPs.The NW stem (Spot 2) consists of higher Si and C concentration and similar amount of O element as the catalyst droplet.Meanwhile, only Si and O elements were observed in the substrate side (Spot 3).This indicates that C and Si are the main elements within the grown NWs.Similar concentration of O element (wt% ∼14-18%) was observed in all scanning regions, indicating a postoxidation of the sample while being exposed to air.The structure and composition of the Si/C core-shell NWs were further confirmed using an ESI analysis.TEM micrograph of NWs is shown in Figure5(a).Figures 5(b), 5(c), and 5(d) represent the ESI patterns of Si, C, and N elements, respectively, in the NWs.The ESI patterns confirm that the NWs are structurally composed by an Si core and cladded by a C layer.Diameter of the C shell is about 10 nm.Only a small quantity of N element is detected from ESI pattern which again confirms the N evolution from CN NRs via the annealing process.

Figure 6 :
Figure 6: Growth mechanism of the Si/C core-shell NWs.Schematic diagram of (a) as-prepared, (b) annealed at 600 ∘ C and (c) 800 ∘ C Au NPs coated CN NRs template.Arrows illustrated in (b) present agglomeration process of Au NPs by joining of smaller sized Au NPs into larger sized Au NPs.