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Growth and investigation of MoO3/WO3- based nanostructures and their applications

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Growth and investigation of MoO3/WO3-based nanostructures and their applications The structure–property relationship lies at the heart of materials science, providing the foundation for understanding how atomic arrangement, bonding characteristics, and microstructural features dictate a material's macroscopic behavior. Properties such as electrical conductivity, ionic mobility, optical response, catalytic activity, and mechanical stability all emerge from the underlying structural motifs, whether crystalline, amorphous, layered, or defect-rich. This principle becomes particularly critical in functional oxides, where even subtle variations in lattice symmetry, grain size, surface orientation, or defect concentration can dramatically alter performance. Transition-metal oxides (TMOs) such as MoO₃ and WO₃ exemplify this sensitivity: their partially filled d-orbitals, flexible oxidation states, and strong metal oxygen bonding allow them to host a broad spectrum of structural distortions, oxygen vacancies, and polymorphic transformations. These features govern key functionalities, including redox activity, proton conduction, adsorption selectivity, and electron transport, making TMOs highly responsive to changes induced during synthesis. Their performance depends critically on precise control over crystallographic orientation, oxygen stoichiometry, phase purity, and nanoscale morphology. Yet, achieving such control in a scalable thin-film form remains a significant challenge, as many traditional synthesis routes lack the ability to tune microstructure and defect concentration independently or to stabilize metastable phases needed for enhanced functionality. Physical vapor deposition offers a uniquely flexible, far- from-equilibrium pathway to engineer these oxides, but a comprehensive understanding of how plasma energetics, growth kinetics, and post- treatment conditions shape the resulting microstructure is still underdeveloped. This gap is particularly evident when attempting to relate the same synthesis principles to multiple device applications, highlighting the need for a unified framework that connects thin-film growth mechanisms with structure–property relationships across sensing and energy-storage technologies. Thus, the synthesis dependent evolution of crystal structure, morphology, and defect states directly determines the functional performance of MoO₃-based and MoO₃/WO₃ composite thin films. By controlling thin-film growth mechanisms, namely nucleation, diffusion, adatom mobility, oxygen incorporation, and annealing, one can tailor the structure-property relationship to achieve superior sensing and energy- storage characteristics. This central paradigm guides the experimental and analytical work presented throughout this thesis. Chapter 1 establishes the general theoretical foundation of the thesis by introducing the fundamental principles governing transition-metal oxide (TMO) growth, polymorphism, and defect chemistry. It begins with classical and non-classical nucleation and growth theories, then advances toward the physics of thin-film deposition, particularly magnetron sputtering, which serves as the primary synthesis route in this work. The chapter further provides essential definitions, classifications, and structural frameworks relevant to MoO₃, WO₃, and their mixed-oxide counterparts, linking crystal structure, thermodynamics, and electronic properties to their functional roles in humidity sensing and electrochemical energy storage. Chapter 2 presents the experimental methodologies and characterization techniques employed throughout the thesis. Details of RF and DC magnetron sputtering, co-sputtering configurations, and post- deposition annealing are provided alongside descriptions of the structural, morphological, and electrochemical characterization tools, including XRD, SEM, TEM, XPS, CV, and EIS. Chapter 3 describes the single-step growth of vertically aligned MoO₃ nanorods via RF magnetron sputtering and examines their application as highly sensitive humidity-sensing layers. The chapter discusses the influence of substrate temperature, plasma conditions, deposition dynamics, and oxygen partial pressure on nanorod morphology and crystallographic orientation. The humidity sensing performance is analyzed through dynamic response, recovery behavior, and conduction mechanisms governed by water adsorption and protonic transport on anisotropic MoO₃ surfaces. Chapter 4 introduces the co-sputtering strategy used to synthesize MoO₃/WO₃ composite thin films for supercapacitor applications. The chapter highlights the synergistic interactions between Mo and W oxides, demonstrating how co-sputtering and subsequent annealing at controlled temperatures tune defect chemistry, electronic conductivity, and redox- active sites. Structural evolution is examined using XRD and TEM, while XPS provides insights into mixed-valence states. Electrochemical performance, including capacitive behavior, charge-storage kinetics, and impedance characteristics, is evaluated to demonstrate the superiority of the annealed MoO₃/WO₃ composite—particularly the 700 °C annealed sample. Chapter 5 concludes the thesis by summarizing the key scientific contributions, emphasizing the advantages of controlled sputtering growth for tailoring hierarchical nanostructures, and outlining future research opportunities in advanced mixed-oxide systems. Potential directions include exploring additional metal-oxide combinations, tuning plasma chemistry for defect-engineered films, and extending sputtering-derived TMOs to emerging applications in sensors, catalysis, and next-generation energy storage devices. Together, these studies contribute a comprehensive mechanistic understanding of how sputtering parameters, oxygen partial pressure, nucleation mode, and annealing govern the structural and functional evolution of Mo- and W-based oxides. The thesis demonstrates that sputtering is not merely a deposition tool but a powerful platform for tailoring nanostructured TMOs across multiple applications. The dual demonstration of high-performance humidity sensing and enhanced supercapacitive energy storage positions sputtered MoO₃/WO₃ systems as versatile, scalable, and industrially relevant materials for next-generation multifunctional devices.

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목차

Chapter 1 . Introduction 1
1.1. Background and Theoretical Foundation 1
1.2. Transition-Metal Oxides as a Functional Platform 2
1.3. Group VI Transition Metals and Their Basic Properties 5
1.4. Crystal Structure and Polymorphs of MoO₃ and WO₃ 8
1.4.1. MoO₃ Polymorphs and Their Electrochemical Characteristics 8
1.4.2. WO₃ Polymorphs and Their Electrochemical Properties 10
1.5. Electronic Structure and Defect Physics 12
1.6. Thermodynamic principles and phase stability 14
1.7. Surface Chemistry and Electrochemical Functionality 16
1.7.1. Water Adsorption and Humidity Sensing Mechanism 16
1.7.2. Pseudocapacitance and Redox Thermodynamics 17
1.8. Thin-Film Engineering Through Magnetron Sputtering 18
1.9. Sputtering Growth Fundamentals and Nanorod Formation 19
1.10. Dissertation Overview 22
Chapter 2 . Experimental Details 24
2.1. Physical Vapor deposition 24
2.2. Chemical vapor deposition 28
2.3. Characterizations 29
2.3.1. Scanning electron microscopy (SEM) 29
2.3.2. Transmission Electron Microscopy (TEM) 31
2.3.3. X-ray diffraction (XRD) 32
2.3.4. X-ray Photoelectron Spectroscopy (XPS) 33
Chapter 3 Single-Step Magnetron Sputtering Growth of MoO₃ Nanorods for Humidity Sensing 35
3.1. Introduction 35
3.2. Experimental 36
3.2.1. MoO₃ nanorods growth 36
3.3. Results and discussions 37
3.4. Conclusion 45
Chapter 4 Co-Sputtered MoO₃/WO₃ Composite Thin Films for High-Performance Supercapacitor Electrodes 46
4.1. Introduction 46
4.2. Experimental 48
4.3. Result and discussion. 49
Chapter 5 Summary and prospect 64
5.1. Bibliography 69


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