Synthesis and Photocatalytic Study of NbC-C Nanocomposites

Abstract

Transition metal carbides (TMCs) are considered as excellent compounds for cutting tool and tribological applications with extremely high hardness and thermal stability. Among all TMCs, niobium carbide (NbC) exhibits better toughness than other carbides such as TiC, ZrC and WC and is extensively used in steel industries for better mechanical properties. The present work describes the versatility of NbC as a photocatalyst for the degradation of methylene blue as an organic pollutant. In this work, the synthesis of nano NbC using different carbon precursors and its photocatalytic behavior is reported. The entire work is presented in 8 chapters which are as follows: Chapter 1 presents the current status of water pollution and the treatment of polluted water using different techniques. Among all the available processes, the role of photocatalysis to treat organic pollutants has been described in this chapter. Moreover, some of the real-time limitations of semiconductors as photocatalyst has also been discussed. Apart from this, basic introduction of TMCs along with their various applications is presented. A detailed discussion of NbC is done considering its nature of bonding and solubility dependent phase transition (NbCx, α- Nb2C, β- Nb2C and γ- Nb2C). Further, the need of nano scaled NbC for some specific applications are also discussed. Chapter 2 gives details of the available literature pertaining to the synthesis and characterization of nano-scale NbC. This chapter includes the different synthesis methods where synthesis conditions such as precursors, temperature, pressure and environment are different to get defined morphologies of nano NbC. It also describes the effect of synthesis conditions on the composition of NbCx and its mechanical, electrochemical and capacitive characteristics. Thereafter, utilization of organic waste/discarded materials as carbon source in order to obtain NbC has also been discussed. One of the most common daily life utilities i.e. smoked cigarette filters and a hazardous agricultural weed i.e. parthenium hysterophorous are discussed on the basis of their adverse environmental effects and their utilization to develop an engineering material. Based on literature, the work plan for this thesis is also proposed. Chapter 3 presents the path for designing different experiments which include the synthesis and characterization of nano NbC using different carbon precursors. The adopted methodology along with a brief introduction of characterizations techniques (XRD, TEM, Raman, XPS, UV-visible spectroscopy and BET) are also presented in this chapter. Moreover, the detailed methodology of photocatalytic experiments is described at the end of this chapter. Considering the available literature, the methodology has been designed into two sections in which organic waste materials (smoked cigarette filters and parthenium hysterophorous) and the associated laboratory grade chemicals (cellulose acetate and activated charcoal) were used as carbon precursors as has been presented in subsequent chapters. Chapter 4 describes a brief introduction of cigarette usage, compositional and morphological features of smoked cigarette filters (CFs). The optimization of synthesis parameters using smoked cigarette filters as carbon source (temperature, holding time and CF content) is done to obtain nano NbC. In terms of lattice distortion and carbon content (x) in NbCx, the effect of temperature and holding time on the formation of NbC has been discussed in detail. TEM results revealed the formation of NbC-C nanocomposite in which NbC nanoparticles are embedded in graphitic carbon network. BET analysis has been done to understand the nature of pore distribution and specific surface area of the synthesized nanocomposite sample. Further, Raman and XP spectroscopy were carried out to understand the nature of carbon network and elemental composition on the surface of optimized sample. Since, absorption of visible radiation has been observed from UV-visible spectroscopy, detailed photocatalytic experiments were carried out to optimize the concentration of dye (methylene blue; MB) and catalyst (NbC-C nanocomposite) under the illumination of household CFL lamp. The degradation of MB molecule associated to decreased absorption of dye solution has been confirmed with the help of mass spectrometry (MS) and scavenger tests. Further, a detailed mechanism associated to photocatalytic activity of optimized NbC-C nanocomposite has been proposed at the end of the chapter. Chapter 5 describes the allelopathic and other adverse effects of parthenium hysterophorous (PH) on crop production and livestock health. The idea of the transformation of agricultural weed to get an engineering product for the treatment of pollutants is presented in this chapter. The optimization of synthesis parameters has been discussed to obtain single phase NbC-C nanocomposite sample. Different characterization techniques suggest that the synthesis of NbC involves the simultaneous reductioncarburization process with the incorporation of NbCxOy or NbOz centers inside the NbC nanoparticles. Moreover, the effect of nature of carbon network and composition of NbCx on the photocatalytic performance has been discussed in this chapter. Further, with the help of MS, total organic carbon (TOC) and scavenger tests, a detailed degradation mechanism of MB dye under household CFL illumination with the help of optimized NbC-C nanocomposite sample has also been discussed at the end of the chapter.Chapter 6 describes the optimization of synthesis parameters (temperature and holding time) to obtain NbC from cellulose acetate which is a major component of cigarette filter. This study was undertaken to understand the effect of processing parameters for the synthesis of NbC using a chemical grade cellulose acetate. This chapter explains the effect of carbon precursor and its morphology for achieving reduction and carburization at relatively lower temperature. Due to the solid-state carburization reaction, composition of NbCx and nature of carbon were modified as compared to those obtained using cigarette filters. Hence, the modification in the surface chemistry has also been observed affecting the photocatalytic performance of synthesized nano NbC-C nanocomposite. Further, the involved degradation parameters with degradation kinetics and responsible mechanism. Chapter 7 describes the synthesis of NbC-C nanocomposite using activated charcoal as carbon source. This work was undertaken to probe the pathway of reduction-carburization of Nb2O5 yielding NbC-C nanocomposite. The influence of amorphous carbon source as compared to polymeric carbon sources as studied in earlier chapters is described. This chapter also provides the carbon source dependent structural features such as particle size, composition of NbCx (distribution of oxygen centers inside the NbC inducing NbCxOy or NbOz) and specific surface area of the synthesized NbC-C nanocomposite sample. Moreover, Raman spectroscopy suggested the amorphous nature of in-situ produced carbon network engulfing the NbC nanoparticles in it. With the help of thermodynamic calculations (Gibb’s free energy; ΔG), a detailed discussion related to synthesis mechanism of NbC nanoparticles has been done. Higher specific surface area and smaller particle size of NbC-C nanocomposite sample enhanced the adsorption of MB dye on the surface of the nanocomposite. Further, the photodegradation efficiency of optimized NbC-C nanocomposite sample to degrade MB dye under similar conditions is discussed and the degradation mechanism is also proposed. Chapter 8 summarizes the entire work to understand the effect of synthesis parameters and carbon source for the synthesis of NbC-C nanocomposite at relatively low temperature. The influence of different carbon sources, composition and nature of carbon network on the photocatalytic activity of NbC-C nanocomposite is compared. Further, as an effect of carbon source it has been observed that direct conversion of waste product resulted efficient NbC-C nanocomposite as photocatalyst as compared to laboratory grade chemicals. At the end of this chapter, possible future work based on the present work is also given.