Physicochemical, Electrical and Electrochemical Studies on Titanium Carbide-Based Nanostructures
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Materials for studies related to nanoscience and nanotechnology have gained tremendous attention owing to their unique physical, chemical and electronic properties. Among various anisotropic nanostructures, one dimensional (1D) materials have received immense interest in numerous fields ranging from catalysis to electronics. Imparting multi-functionality to nanostructures is one of the major areas of research in materials science. In this direction, use of nanosized materials in energy systems such as fuel cells has been the subject of focus to achieve improved performance. Tuning the morphology of nanostructures, alloying of catalysts, dispersing catalytic particles onto various supports (carbon nanotubes, carbon nanofibers, graphene, etc.) are some of the ways to address issues related to electrochemical energy systems. It is worth mentioning that highly stable and corrosion resistant electrodes are mandatory as electrochemical cells operate under aggressive environments. Additionally, carbon, which is often used as a support for catalysts, is prone to corrosion and is subsequently implicated in reduced performance due to poor adherence of catalyst particles and loss in electrochemically active area. Hence, there is a quest for the development of stable and durable electrocatalysts / supports for various studies including fuel cells. The present thesis is structured in exploring the multi-functional aspects of titanium carbide (TiC), an early transition metal carbide. TiC, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrical and electrochemical studies. The current study explores the versatility of TiC in bulk as well as nanostructured forms, in electrical and electrochemical studies towards sensing, electrocatalytic reactions and active supports. 1D TiC nanowires (TiC-NW) are prepared by simple solvothermal method without use of any template and are characterized using various physico-chemical techniques. The TiC-NW comprise of 1D nanostructures with several µm length and 40 ± 15 nm diameter (figure 1). Electrical properties of individual TiC-NW are probed by fabricating devices using focused ion beam deposition (FIB) technique. The results depict the metallic nature of TiC-NW (figure 2). Figure 1. (a) SEM, (b) TEM and (c) HRTEM images of TiC-NW prepared by solvothermal method. Figure 2. (a) SEM image and (b) I-V characteristics of TiC-NW - based device as a function of temperature. The contact pads are made of Pt. Subsequently, oxidized TiC nanowires are prepared by thermal annealing of TiC-NW, leading to carbon - doped TiO2 nanowires (C-TiO2-NW) (figure 3). Photodetectors are fabricated with isolated C-TiO2-NW and the device is found to respond to visible light (figure 3) radiation with very good responsivity (20.5 A/W) and external quantum efficiency (2.7 X 104). The characteristics are quite comparable with several reported visible light photodetectors based on chalcogenide semiconductors. Figure 3. (a) HRTEM, (b) EDAX, (c) Scanning TEM-DF images of C-TiO2-NW along with (d) Ti (e) O and (f) C mapping. (g) Current – voltage curves of single C-TiO2-NW recorded in dark (black) and in presence of visible light radiation (red) of intensity 57.7 mW/cm2 at 25oC. Inset of (g) shows the SEM image of the device (top) and schematic illustration of fabricated photodetector (bottom). The next chapter deals with the electrochemical performance of TiC demonstrated for studies involving oxygen reduction and borohydride oxidation reactions. Electrochemical oxygen reduction reaction (ORR) reveal that TiC-NW possess high activity for ORR and involves four electron process while it is a two electron reduction for bulk TiC particles (figure 4). The data has been substantiated by density functional theory (DFT) calculations that reveal different modes of adsorption of oxygen on bulk and nanowire morphologies. Stable performance is observed for several hundreds of cycles that confirm the robustness of TiC. The study also demonstrates excellent selectivity of TiC for ORR in presence of methanol and thus cross-over issue can be effectively addressed in direct methanol fuel cells. In the chapter on borohydride oxidation, bare TiC electrode is explored as a catalyst for the oxidation of borohydride. One of the major issues in direct borohydride fuel cells (DBFC) is the hydrolysis of borohydride that happens on almost all electrode materials leading to low efficiency. The present study reveals that TiC is a very good catalyst for borohydride oxidation with little or no hydrolysis of borohydride [figure 5 (a)] under the experimental conditions studied. Further, shape dependant activity of TiC has been studied and fuel cell performance is followed [figure 5 (b)]. Polarization data suggests that the performance of TiC is quite stable under fuel cell experimental conditions. Figure 4. (a) Linear sweep voltammograms for ORR recorded using (i) bulk TiC particles and (ii) TiC-NW in O2-saturated 0.5 M KOH at 1000 rpm. Scan rate used is 0.005 Vs-1. (b) Variation of number of electrons with DC bias. Black dots correspond to TiC bulk particles while red ones represent nanowires. Figure 5. (a) Cyclic voltammograms of borohydride oxidation on TiC coated GC electrode in 1 M NaOH containing 0.1 M NaBH4. Scan rate used is 0.05 Vs-1. (b) Fuel cell polarization data at 70oC for DBFC assembled with (i) bulk TiC particles and (ii) TiC-NW as anode catalysts and 40 wt% Pt/C as cathode. Anolyte is 2.1 M NaBH4 in 2.5 M NaOH, and catholyte is 2.2 M H2O2 in 1.5 M H2SO4. Anode loading is 1.5 mg cm-2 and cathode loading is 2 mg cm-2. The corrosion resistance nature of TiC lends itself amenable to be used as an active support for catalytic particles (Pt and Pd) for small molecules oxidation reactions. In the present study, electro-oxidation of methanol, ethanol and formic acid have been studied. As shown in figure 6 (a), the performance of Pd loaded TiC (Pd-TiC) is found to be higher than that of Pd loaded carbon (Pd-C) suggesting the active role of TiC. The catalytic activities of TiC-based supports are further improved by tuning their morphologies. Figure 6 (c) reveals that the activities are higher in case of Pd-TiC-NW than that of Pd-TiC. Figure 6. (a) Cyclic voltammograms of Pd-TiC and Pd-C for ethanol oxidation, (b) T EM image of Pd-TiC-NW and (c) voltammograms of Pd-TiC-NW in N2-saturated 1 M ethanol in 1 M KOH medium, scan rate used is 0.05 Vs-1. The next aspect explored, is based on the preparation of C-TiO2 and its use as a substrate for surface enhanced Raman spectroscopy (SERS). Carbon doped titanium dioxide is prepared by thermal annealing of TiC. It is observed that the amount of dopant (carbon content) is dependent on the experimental conditions used. SERS studies using 4¬mercaptobenzoic acid (4-MBA) as the analyte, indicates that C-TiO2 [figure 7 (a)] enhances Raman signals based on chemical interactions between the analyte and the substrate. Raman signal intensities can be tuned with the amount of carbon content in C¬TiO2. Enhancement factors are calculated to be (7.7 ± 1.2) x 103 (for 4-MBA) and (1.7 ± 1.2) x 103 (for 4-nitrothiophenol). The SERS substrates are found to be surface renewable using visible light, a simple strategy to re-use the substrate [figure 7 (b)]. The regeneration of SERS substrates is based on self cleaning action of TiO2 that produces highly reactive oxygen containing radicals known to degrade the molecules adsorbed on TiO2. Thus, the versatility of TiC has been demonstrated with various studies. In addition to using TiC-based materials, nanoparticles of Rh, Ir and Rh-Ir alloy structures have also been used for borohydride oxidation reaction. This is explained in the last section. In Appendix-I, preliminary studies on the preparation of TiC-polyaniline (PANI) composites using liquid-liquid interfacial polymerization is explained. Raman spectroscopy results suggest that the presence of TiC-NW makes PANI to assume preferential orientation in the polaronic (conducting) form. Appendix-II discusses the role of TiC-NW as a fluorescence quencher for CdS semiconductor nanoparticles.
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