Boron Carbide Nanowires from Castor Oil for Optronic Applications: A Low-Temperature Greener Approach

The development of one-dimensional nanostructures has revolutionized electronic and photonic industries because of their unique properties. The present paper reports the low-temperature green synthesis of boron carbide nanowires, of diameter 14 nm and length 750 nm, by the condensation method using castor oil as the carbon precursor. The nanowires synthesized exhibit beaded chain morphology, and bandgap energy of 2.08 eV revealed through the Tauc plot analysis. The structure of boron carbide nanowires is revealed by Fourier transform infrared spectroscopy and X-ray diffraction analyses. The photoluminescence study reveals the nanowire's blue light emission capability under ultraviolet excitation, which is substantiated by the CIE plot suggesting its potential in photonic applications. As an castor oil facilitates the dispersibility with boric acid, boron source, and forms the NWs through the condensation reaction. The FESEM analysis of the BC NWs exhibit beaded chain morphology of diameter 14 nm and length 750 nm. The UV-Vis spectroscopic analysis indicates a direct bandgap of energy 2.08 eV. The XRD analysis shows a high-intensity peak at 27.5° due to the orthorhombic BC, and the peaks at 30°, 34°, 39°, 42°, 54°, 60°, 66° suggests the presence of rhombohedral BC. The FTIR spectrum shows the B-C stretching vibration and C-B-C bending vibrations conrming the nanowire to be boron carbide. The PL spectrum recorded for the excitations 270 and 300 nm shows a peak at 410 nm. The CIE plot shows the coordinates corresponding to the emission (0.148,0.039) and (0.144,0.047) in the blue region. The attractive thermal, physical, and electrical properties of BC NWs make it suitable in power applications and as cathode materials. Thus, the study reveals the possibility of green synthesis of 1D BC NWs using castor oil as the natural carbon source for potential photonic and electronic applications.


Introduction
Nanoscale materials have received considerable attention employing their unique properties and a wide range of applications relative to their bulk counterparts [1,2]. Nanomaterials can be classi ed into zero-, one-, and two-dimensional structures depending on the shape and size. Among the one dimensional (1D) nanostructures, nanowires play a signi cant role in device fabrication [3][4][5]. The properties of the nanomaterials are strongly in uenced by their size, shape, and morphology. The diameter (< 10 nm) of the nanowires (NWs) introduces quantum-con ned size effects, modifying the structural, electronic, optical, thermal, and magnetic properties. This enables its use in micro and nanoelectromechanical systems, sensors, and photodetectors [1,[6][7][8][9]. The wide bandgap exhibited by NWs makes it suitable for applications in light-emitting diodes, photovoltaics, and nanoscale lasers [1,10]. NWs having hightemperature stability and high-frequency performance are essential in the development of electronic and optoelectronic systems.
Of different materials -metals, polymers, semiconductors, insulators, and ceramics -NWs of ceramic materials have gained attention recently due to their characteristic properties and widespread applications in the area of magnetic nanocomposites, quantum electronic materials, gas separations, and structural reinforcements. Ceramic materials are high strength materials and are chemically and thermally stable. Among ceramic materials, boron carbide (BC) nds widespread applicationslightweight armors, blast nozzles, ceramic bearing, brake linings, cutting tools, rocket propellant in the aerospace industry, and neutron absorber and shielding material in the nuclear industry [11][12][13][14] -due to its unique properties -greater hardness, high thermal and chemical stability, high neutron absorption cross-section, corrosion resistivity, and high melting point [15][16][17]. The lower fracture toughness of bulk BC is overcome by the 1D BC nanostructures having high elastic modulus [18]. The enhanced physical and electrical properties of BC NWs are responsible for the high temperature and power applications. The eld emission properties of BC NWs make them a potential candidate as cathode materials [19,20].
The performance of 1D BC nanostructures is greatly in uenced by the synthesis condition [21]. Several methods, such as template-based synthesis, microwave radiation, chemical vapor deposition, and synthesis using polymer precursors, are available to produce different morphological structures of BC [22][23][24][25][26][27][28]. Zhang et al. [29] synthesized BC nanowires using a plasma-enhanced chemical vapor deposition method, and high pure BC NWs are prepared by Ma et al. using the thermal evaporation method [30].
Most of these methods need pre-process conditions such as catalyst preparation, substrate, and template preparation. Despite these methods, a more convenient low-temperature method for producing BC NWs has high demand and interest.
The condensation method is one of the low-temperature synthesis methods used for BC's production using inexpensive organic precursors such as glycerin, citric acid, and polyvinyl alcohol (PVA) [31][32][33].
The usage of polymeric precursors enables control over the nal product's properties and structures by varying the precursor composition at low-temperatures [11,34,35]. Based on the above advantages of polymeric precursors, the bulk BC structure can be tailored to NWs by choosing the correct precursor ratio and suitable synthesis temperature. In this research work, a low-temperature condensation method is selected to synthesize BC NWs using readily available natural carbon source, castor oil.

Experimental
Of different synthesis methods for the formation of NWs, the solution-based approach is more suitable.
The structure can be controlled by varying the precursor ratio and synthesis temperature. The use of organic precursors, for the formation of BC, facilitates the dispersibility and uniformity of boron and carbon sources. Among boron sources, boric acid enhances the esteri cation process due to the liberation of hydroxyl groups [31,36]. For the synthesis of BC, boric acid (Sigma-Aldrich) and castor oil (Indian pharmaceuticals) are used as the boron and carbon sources in the ratio of 1:4, respectively, by condensation reaction method. Initially, boric acid is added to the heated castor oil. The mixture is re uxed with the help of a heating mantle at a temperature of 75-80 °C until all the moisture content is vaporized from the mixture. The mixture's golden yellow gel is transferred to white solid after the reaction and is subjected to pre-treatment at 300 °C for three hours to get the dry powder. The crushed powder is then heat-treated at 900 °C for two hours in a tubular furnace with a continuous nitrogen gas ow. The obtained powder is annealed at 600 °C in the air (1 h) to remove unreacted carbon to get the BC sample.
The BC sample synthesized is subjected to different morphological, structural, and compositional analyses using eld emission scanning electron microscope (FESEM -Nova Nano), X-ray diffractometer (XRD -Bruker d8 Advance), and Fourier transform infrared spectrometer (FTIR -Thermo Fisher iS50) in attenuated total re ectance mode in the region 4000 − 400 cm − 1 . The product's optical properties are also studied by UV-visible absorption (UV-Vis-Jasco V550) and photoluminescence (PL-Horiba Fluoromax) spectroscopic methods.

Results And Discussion
The formation of 1D BC NWs can be understood from the FESEM images shown in Fig. 1. The morphological modi cations of the sample before and after the heat treatment at 900 °C are analyzed.
From Fig. 1, the transformation of morphology to 1D NWs, upon annealing to 900 °C, is evident. The images given at different magni cation (nm to µm scale) indicate the formation of NWs over a large area. Figure 1 also reveals the formation of 1D NWs, of diameter 14 nm and length up to 750 nm, at 900 °C from the nanolayers appearing at 300 °C. A closer examination of the NWs (from Fig. 1(d)) reveals a beaded chain morphology responsible for the surface roughness.
The FTIR spectrum, Fig. 3 (c) the power spectrum for λ ex -270 nm and (d) power spectrum for λ ex -300 nm.

Conclusion
The present paper reports the low-temperature green synthesis of BC NWs by condensation method using castor oil as the carbon precursor. The 1D BC nanostructures have emerged as an important class of technologically signi cant material. Of various synthesis methods, employing sophisticated equipment and toxic chemicals, eco-friendly approaches are highly relevant and became the focus of research. As an organic precursor, castor oil facilitates the dispersibility with boric acid, boron source, and forms the NWs through the condensation reaction. The FESEM analysis of the BC NWs exhibit beaded chain morphology of diameter 14 nm and length 750 nm. The UV-Vis spectroscopic analysis indicates a direct bandgap of energy 2.08 eV. The XRD analysis shows a high-intensity peak at 27.5° due to the orthorhombic BC, and the peaks at 30°, 34°, 39°, 42°, 54°, 60°, 66° suggests the presence of rhombohedral BC. The FTIR spectrum shows the B-C stretching vibration and C-B-C bending vibrations con rming the nanowire to be boron carbide. The PL spectrum recorded for the excitations 270 and 300 nm shows a peak at 410 nm.
The CIE plot shows the coordinates corresponding to the emission (0.148,0.039) and (0.144,0.047) in the blue region. The attractive thermal, physical, and electrical properties of BC NWs make it suitable in power applications and as cathode materials. Thus, the study reveals the possibility of green synthesis of 1D BC NWs using castor oil as the natural carbon source for potential photonic and electronic applications.