ION IMPLANTATION AND CHEMICAL MODIFICATION OF CuO NANOPARTICLES AND THEIR ELECTROCHEMICAL APPLICATIONS

Abstract

Metal oxide nanoparticles (NPs) are finding wide range of applications in all the major domains of science and technology, e.g. energy, environment, catalysis, sensors and magnetism. Metal oxides for energy applications are further classified into different categories, viz. as fuel cells, solar cells, batteries, photoelectrochemical cells and as supercapacitors to meet the future energy demands. In view of these metal oxides, particularly ruthenium oxide exhibited very good supercapacitor behavior. Furthermore modification of these metal oxides revealed improved supercapacitor behavior. Owing to high cost of the ruthenium precursors alternate metal oxides are being explored. The variable valency of manganese led to development of manganese oxide as supercapacitors. Though copper also exhibit variable valency, but CuO and its nanoparticles are not widely studied as supercapacitor. This thesis work is therefore focused on the studies of modified CuO NPs to establish it as supercapacitor. The thesis comprises of FIVE chapters. The Chapter 1 consists of general introduction about different types of modified metal oxide nanoparticles and their applications. The focus has been on CuO NPs as it is known to exhibit excellent conductivity and has tremendous potential to be a supercapacitor and electrochemical sensor. The basic concepts involved in electrochemical properties for glucose sensing and supercapacitor behaviors are discussed. A brief literature survey on CuO NPs as electrocatalytic sensor and as supercapacitor is discussed. The importance of modifying the surface and lattice environment of metal oxide has been outlined in the context of their electrochemical properties. The focus has been on modification by irradiating metal oxides with low energy (keV) ion beams and by suitable capping and doping. From these discussions, the aim and scope of present thesis work is identified and the research objectives are outlined. ii The Chapter 2 deals with the effect of low ion implantation of 50 keV N5+ on copper oxide thin films. Pulsed laser deposition technique was used with different partial pressure of oxygen for preparing batches of copper oxide thin films on glass substrates kept at 350 oC. The N5+ ion beams of particle fluences of 2.5 1015, 1.0 1016 and 4 1016 were used. The as-deposited thin films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Their optical properties were studied by measuring the transmittance spectra in the UV-Vis-NIR region and the band gap of the materials were determined from Tauc’s plot using diffused reflectance spectroscopy (DRS) data. The as-deposited copper oxide thin film corresponding to 80 mTorr partial pressure of oxygen was transformed to single Cu2O phase of crystallite size 20.2 nm when implanted at 1.0 1016 particles/cm2. On the other hand the film deposited at 100 mTorr partial pressure of oxygen resulted in formation of mixed Cu2O and CuO phases. Interestingly, this film upon implantation with 2.5 1015 particles/cm2 resulted in conversion to a single Cu2O phase with enhanced crystallinity and larger crystallite size (23.5 nm). The phase transformation is attributable to thermal effect due to stopping of incident beam. Implantation with higher particle fluence led to transformation to CuO phase with decrease in crystallinity and increase in electrical conductivity. Chapter 3 deals with synthesis, characterization and electrochemical applications of CuO NPs. Different capping agents e.g., triphenylphosphine oxide (TPPO), Mercaptoacetic acid (MAA), Triton X-100, polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP) were used for preparing respective batches of capped CuO NPs by precipitation method. These as-synthesized batches of capped CuO NPs were characterized by XRD that revealed monoclinic phase with crystallite size in the range of 7.1 nm to 11.4 nm. The FE-SEM and TEM images iii revealed cluster of nanoparticles of sizes similar to crystallite sizes. The photoluminescence studies revealed more defects in MAA capped and PVP capped CuO NPs as compared to rest batches of capped CuO NPs. All these batches of capped CuO NPs were investigated by cyclic voltammetry (CV), galavanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) studies. The CV plot was quasi-rectangular in shape and the specific capacitance was high (i.e., 94 F/g and 103 F/g) for TPPO capped and MAA capped CuO NPs at the scan rate of 5 mV/s. The CV studies revealed involvement of both electric double layer capacitance (EDLC) and pseudocapacitance (PC) characters. The supercapacitor behavior of the capped CuO NPs was studied by recording potential vs. time plot at different current densities. The conductivity of CuO NPs and electrolyte ion interactions with electrode material were analyzed by EIS spectrum. However, the cyclic stability of the TPPO capped CuO NPs (highest specific capacitance) was poor. Because of this, the capped CuO NPs were calcined at 450 oC for 3 hours. All the electrochemical studies were repeated. The specific capacitance for calcined TPPO capped CuO NPs was 125 F/g at the current density of 0.2 A/g and its capacitance retention was significantly improved to 102%. Better specific capacitance for TPPO capped CuO NPs was attributed to lesser defects which are likely to be the charge tapping sites. The supercapacitor behavior was mostly due to pseudocapacitance character which is attributed to faradic process. Hence these capped CuO NPs were used as anode for electrochemical sensing of glucose. The limits of detection for calcined TPPO capped and MAA capped CuO NPs were 0.04 μM and 0.33 μM, respectively. Their linear detection range were 9 μM to 4.2 μM (for MAA capped) and 9 μM to 3.2 μM (for TPPO capped). The batches of calcined MAA capped and TPPO capped CuO NPs were for glucose determination in human blood and urine samples. iv Chapter 4 deals with synthesis, characterization and supercapacitor behavior of increasing concentration of selenium doped (Se-doped) in CuO NPs. The exact concentrations of doped selenium in different batches were determined by inductive coupled plasma optical emission spectroscopy (ICP-OES). The XRD measurements revealed formation of monoclinic phase of CuO with minor shift in the 2θ value due to lattice modification by Se species. The FE-SEM and TEM studies confirmed morphology and size distribution of the nanoaprticles. All the batches of Se-doped CuO NPs were investigated for supercapacitor behavior by recording CV, GCD and EIS spectra. The batch corresponding to 0.45 wt% Se-doped CuO NPs revealed maximum specific capacitance of 140 F/g at current density of 0.2 A/g. However the capacitance retention after 500 cycles was poor. For this reason, as-synthesized Se-doped CuO NPs were calcined at 450 oC and were further characterized and investigated as supercapacitor. The supercapacitor behavior was mostly due to pseudocapacitance character. Furthermore, the supercapacitor behavior of the calcined 0.45 wt% Se-doped CuO NPs was studied for its performance at elevated cell temperature, i.e., in the range of 25 oC to 75 oC. The CV plots reflected the role of pseudocapacitance behavior due to faradic processes. The stability studies at respective cell temperature were performed for 500 cycles at 3 A/g. The capacitance retention after 500 cycles was 98% for cell temperature upto 55 oC. Chapter 5 deals with synthesis, characterization and supercapacitor studies of different batches of cobalt doped (Co-doped) CuO NPs. The exact concentrations of doped cobalt were determined by ICP-OES. The as-synthesized batches of Co-doped CuO NPs were characterized by XRD, FE-SEM, TEM, BET surface area. The photoluminescence spectroscopy study revealed the vacancy based defects due to Co-doping. Similarly, supercapacitor behaviors of these Co-doped CuO NPs were studied from CV, GCD, EIS spectra. The best result was v obtained for 5.77 wt% Co-doped CuO NPs which corresponded to 160 F/g specific capacitance at current density of 0.2 A/g. However due to poor cyclic stability, this batch of Co-doped CuO NPs was calcined at 450 oC and was studied for supercapacitor behavior. As expected, the calcined batch of Co-doped CuO NPs revealed high specific capacitance (140 F/g) and high capacitance retention of 92-95%. Such high specific capacitance and excellent capacitance retention are so far better specifications than those available in literature. For this reason, the batches of calcined 5.77 wt% Co-doped CuO NPs was used for fabricating a symmetric solid state supercapacitor and successfully tested between a working potential window of 0 and 2.0 V. The energy density was determined as 5.2 Wh/kg at a current density of 0.2 A/g. This is an encouraging result for optimizing Co-doped CuO NPs for real time application as supercapacitor. This thesis work has been concluded by summarizing the salient features of modification of CuO NPs and their impact towards electrochemical applications e.g., as glucose sensing and as supercapacitor.