MOCVD Of Carbonaceous MnO Coating : Electrochemical And Charge Transport Studies

Abstract

Metalorganic Chemical Vapour Deposition (MOCVD) is a versatile technique for the deposition of thin films of oxide materials as it offers advantages, such as deposition over large surface area, conformal coverage, selective area deposition, and a high degree of compositional control. The MOCVD process uses metalorganic (MO) complexes, such as β-diketonate and alkoxide-based complexes, as precursors. These complexes are stable and moderately volatile. Because of the direct bond between metal and oxygen, MO complexes are natural precursors for oxide coatings. As the process involves chemical reactions taking place on the substrate surface, growth of thin films by MOCVD depends on various parameters such as the chemical nature and concentration of precursors, reaction pressure, reaction temperature, and the nature of the substrate. Such a large parameter space of the CVD process, when combined with the dynamics (thermodynamics and fluid dynamics) and kinetics, makes it rather complex. This complexity allows one to make thin films of metastable phases, including amorphous materials. One of the important findings of the work is that MOCVD process is capable of making composite coatings of carbonaceous metal oxide. Manganese is multivalent and forms various stable oxides, such as MnO, Mn2O3, Mn3O4 and MnO2. There are various potential applications of manganese oxides. MnO2 is a very well studied material for its electrochemical applications in dry cells, lithium-ion batteries, and in supercapacitors. Hence, it becomes pertinent to explore the properties of thin films of manganese oxides prepared by MOCVD for various electrochemical and other applications. The thesis work is divided into two parts. Part 1 describes the synthesis of manganese complexes, their characterization, and their application to the CVD of coatings, especially those of carbonaceous MnO. Part 2 is devoted to a detailed study of electrochemical aspects of the carbonaceous MnO coatings, followed by a report on their unusual transport properties. Chapter 1 begins with a brief introduction to thin film deposition processes. In particular, the CVD process is described with reference to various parameters such as carrier gas flow, pressure, temperature and most importantly, the CVD precursor. The chapter ends with a description of the scope of the work undertaken for the present thesis. Chapter 2 deals with “Synthesis and Characterization of MO complexes”. It begins with a description of the classification of CVD precursors with the description of MO complexes such as β-diketonates, which are generally subliming crystalline solids. Manganese β-diketonate complexes are discussed in detail. Due to the multivalent nature of Mn, there are two possible complexes namely Mn(acac)2(H2O)2 and Mn(acac)3. These complexes have been synthesised and characterized (confirmed) by various techniques, such as elemental analysis (CHN), X-ray diffraction (XRD), FTIR spectroscopy, and mass spectroscopy. Thermal analysis of the complexes shows that they are suitable as MOCVD precursors. We have used Mn(acac)2(H2O)2 as a precursor in the present work. Metalorganic complexes, where metal ion is directly bonded with both nitrogen and oxygen, can be potential candidates for the precursor for oxynitrides coatings. We have therefore studied solid crystalline anthranilate complexes of various metal ions, such as Mn2+, Co2+, Cu2+ and Zn2+ and confirmed their formation. Thermal analysis shows that anthranilate complexes are fairly volatile below 250oC and decompose below 500oC. These complexes were pyrolysed in open air and in sealed tube at different temperatures, and the resulting powder product examined by XRD, SEM, EDAX and FTIR. This preliminary study shows that anthranilate complexes yield different oxides of Mn, Co and Cu under different pyrolysis conditions, with very interesting morphological features. Pyrolysis of Zn(aa)2 in a sealed tube leads to the formation of a nanocomposite of carbon and zinc oxide (wuerzite), rich in carbon, with potential for applications in catalysis. On the other hand, the pyrolysis of Zn(aa)2 in air at the same temperature leads to leads to crystalline, nanostructured zinc oxide (wuerzite). However, no attempt has been made to use these anthranilates as CVD precursors. Chapter 3 deals with “MOCVD of Manganese Oxides and their Characterization”. It begins with a brief review of various manganese oxides and their properties. This is followed by description of the CVD reactor used for the present work, together with the conditions employed for the deposition of MnOx films. Depositions have been carried out on different substrates such as SS-316, ceramic alumina and Si (111), while varying various deposition parameters, viz., substrate, reactor pressure, carrier gas (argon) flow rate, and the duration of deposition. Significantly, depositions are divided into two categories: one, carried out in argon ambient, in the absence of a supply of oxygen (or any other oxidant) and the second one, under oxygen flow, using argon as carrier gas. The films deposited in the absence of oxygen flow are thick, black in colour, and electrically conducting, indicating the presence of carbon. The growth rate follows a typical thermal pattern, with activation energy of ~ 1.7 eV. Detailed characterization by XRD, TEM/ED, Raman, FTIR and XPS (X-ray photoelectron spectroscopy) shows that these films are composed of MnO in a carbon-rich amorphous matrix. High-resolution SEM (fig. 1) reveals a fractal pattern of cauliflower morphology, comprising very fine particles (4 – 10 nm), characteristic of very large specific surface area of the film, which is confirmed by volumetric BET measurement (~2000 m2/g). We conclude that growth in argon ambient leads to a homogenous nanocomposite film of hydrated MnO in carbon-rich matrix. Thus, our study reveals that MOCVD is a novel one-step chemical method to produce homogenous composite thin films, wherein all components of the nanocomposite film emerge from the same chemical precursor. Carbon incorporation is generally avoided by empirical process design, as it is viewed as an impurity. The potential advantages of carbon incorporation are thus not examined and the composite nature of carbonaceous films not recognized in the literature. Carbonaceous nanocomposite film can be significant as an electrode in supercapacitors, as discussed in part 2 of the thesis. Chapter 3 describes films deposited under oxygen flow, which are no longer black and are highly resistive, indicating the absence of carbon in the film, as confirmed by Raman spectroscopy. XRD, FTIR and Raman spectroscopy reveal that the films obtained under oxygen flow are more crystalline than the ones obtained in the absence of oxygen flow, and that the films are generally nanocrystalline composites of two manganese oxides, such as MnO and Mn3O4. Given the context of the carbonaceous MnO films described above, chapter 4 begins with a review of electrochemical capacitors (also called supercapacitors or ultracapacitors), which are emerging as important energy storage devices. Until now, in the Mn-O system, hydrated MnO2 has been well-studied as an electrode material due to its low cost and environmental compatibility, but the low electrical conductivity of MnO2, together with irreversible redox reactions, reduces its performance. In electrochemical capacitor applications, metal-oxide/carbon composites are finding importance. Chapter 4 deals with “MnO/C Nanocomposite Coatings as Electrodes for Electrochemical Capacitor”. In this chapter, we have examined the novel EM, i.e., the hydrated MnO/C nanocomposite coating prepared by the MOCVD process on a conducting substrate (current collector) such as SS-316 as an electrode. Electrochemical measurements have been carried out for both the 3-electrode assembly (for basic aqueous electrolyte) and 2-electrode assembly (for gel polymer electrolyte) using cyclic voltammetry (CV), AC impedance and charge-discharge techniques. The studies lead to a maximum specific capacitance of 230 – 270 F/g at 1 mA/cm2 discharge current density for the MnO/C nanocomposite coating grown at 680oC. The Bode plot shows a maximum phase angle of around 74 – 82o, indicating capacitive behaviour. The MnO/C nanocomposite film shows a very small time constant (0.5 – 3 msec), which is good for high frequency applications. The pulse power figure of merit is found to be 650 – 2000 W/g. Capacitance determined for a large number of charge-discharge cycles (~20000), and at large current densities (50 mA/cm2) show promising results. The energy density (5 - 32 Wh/kg) and power density (2 – 4 kW/kg) estimated from charge-discharge data at 1 mA/cm2 shows the potential of the nanocomposite MnO/C as electrode for superior capacitor devices. Gel polymer electrolytes (GPE) offer the advantage of large electrochemical potential window due to its structural and chemical stability. Studies have been carried out to show that the MnO/C nanocomposite film is compatible with gel polymer electrolytes based on poly(methyl methacrylate) (PMMA) and poly(acrylonitrile) (PAN) with salts of magnesium triflate and magnesium perchlorate, respectively) and plasticizers (ethylene carbonate (EC) + propylene carbonate (PC)), in a 2-electrode assembly. Chapter 5 deals with “Magnetoconductance in MnO/C Nanocomposite Coatings on Alumina”. Amorphous systems, such as MnO/C composites wherein carbon is amorphous and MnO is nearly so, are highly symmetric condensed phases, which do not possess long range translational or orientational order. Disorder in the system creates Anderson localized states just above the valence band, which lead to reduced electrical conductivity. Amorphous systems show either a small negative magnetoresistance (~ 5%) or a small positive magnetoconductance (~ 7%) at very low temperatures (~ 10 K). As such, the transport properties of the MnO/C nanocomposite film have been investigated, and are reported in chapter 5. Transport and magnetotransport measurements have been made on the MnO/C nanocomposite film grown on alumina. It is found that the MnO/C nanocomposite coating exhibits a giant negative MR (22.3%) at a temperature as high as 100 K, which is unusual because pure MnO is anti-ferromagnetic and does not ordinarily show any magnetoresistance (MR), while amorphous carbon is known to show a small MR at very low temperatures (~7 K), due to weak-localization. The present results mean that a mechanism other than weak-localization plays a role in this nanocomposite material. Further study of this material is called for, which can perhaps lead to giant magnetoresistance (GMR) at room temperature in a metal-oxide/carbon nanocomposite. A summary of the work and an outlook for further research are given in the concluding chapter 6.