Growth of Ruthenium-Based Functional Nanomaterials and their Catalytic Applications

Abstract

In materials science and engineering, the term ‘material growth’ carries significant value. In fact, the synthesis of materials, either in bulk or at the nano level, involves growth. Material growth is not possible without nucleation, a phenomenon that precedes growth. Therefore, nucleation and growth play a key role in the formation of successful bulk or nanostructured materials. Functional nanomaterials are among the smallest materials known to mankind, presenting intriguing as well as fascinating properties that have been extensively explored by the scientific community for decades. Their excellent performance in a variety of applications, such as next-generation computing chips, energy storage, catalysis, sensors, biomedicine, hydrogen production, etc., has made them their primary choice._x000D_ <br>With a dimension less than 100 nm, these classes of materials exhibit physiochemical properties in between atomic and bulk materials. Their property is highly dominated by the exposed surface area and surface chemical environment. Specifically, transitional metal-based nanomaterials are functioning well in the field of catalysis. Their abundant outer shell electrons, multivalent nature, and high electrical conductivity, which are intrinsically required attributes for catalysis, made them the ideal choice. _x000D_ <br>Functional nanomaterials in general are up to the test in a quest for clean practical energy resources like hydrogen due to the adverse effects on the environment of existing energy sources like fossil fuel. Several endeavors have been undertaken to produce hydrogen from various sources including hydrocarbons, biomass, metal hydride, and water. These methods involve steam reforming, gasification, hydrolysis, and electrochemical water splitting. Metal hydride hydrolysis and electrochemical water splitting have demonstrated the ability to serve as alternatives to energy-intensive and emission-prone processes such as steam reforming and gasification. This is due to their straightforward design and little emissions. Likewise, there has been significant focus on finding effective catalysts with favorable characteristics for promoting the production of hydrogen. Various catalysts, primarily composed of transitional metal compounds, have been employed to promote both metal hydride hydrolysis and hydrogen evolution processes. In particular, functional nanomaterials derived from platinum group metals and related derivatives, such as transitional metal dichalcogenides (TMDC), exhibit exceptional performance compared to their other counterparts._x000D_ <br>Ruthenium-based functional nanomaterials, which are the most abundant and highly stable among platinum-group nanomaterials have become a primary choice as a catalyst in hydrogen generation from metal hydride hydrolysis and electrolytic hydrogen evolution reactions in alkaline media. Even though the potential of functional nanomaterials in addressing fundamental challenges in hydrogen generation has been demonstrated, we are a long way from efficient and practical catalysts that can assist in the realization of a better hydrogen generation system. In this regard, the author addresses the challenges of preparing practical catalysts from the fundamental perspective of material growth. To do so, the author proposed several catalytic structures designed by top-down and bottom-up nanomaterial growth methods, which are presented in the thesis as follows:_x000D_ <br>Chapter 1 provides a general theoretical framework for the thesis, starting with the basic nucleation and growth theory that extends to non-classical growth mechanisms of materials. In the later part, thin film and nanoparticle formation, as well as basic definitions, classifications, and specific theoretical frameworks for functional platinum group nanomaterials, are given._x000D_ <br>Chapter 2 presents experimental details and brief details of the characterization methods used in this study._x000D_ <br>Chapter 3 describes the growth of graphene on polycrystalline Ru-thin via chemical vapor deposition (CVD) and the possible application of the obtained carbon layer for position-selective growth._x000D_ <br>Chapter 4 introduces the concept of using a thermal release tape-supported Ru on graphene sheet (Ru/G/TRT) as a catalyst for the dehydrogenation of sodium borohydride (NaBH4). The hybrid film was realized by a simple yet efficient method of laminating Ru/G by TRT and etching, taking advantage of the strong adhesion between Ru and graphene as well as the supporting TRT. The water displacement method was utilized to assess the catalytic efficiency of the proposed structure, which exhibited significant enhancement compared to the etched Ru/G structure while maintaining stability even after repeated use. Surface analysis using X-ray photoelectron spectroscopy ultimately showed the presence of metallic Ru on both Ru/G and Ru/G/TRT surfaces after repeated use, providing more evidence of the structure's stability._x000D_ <br>Chapter 5 presents the successful growth of RuSe2 nanoparticles (NPs) directly on commercial carbon paper utilizing a low-temperature, low-pressure and short reaction time CVD. A structural study using X-ray diffraction (XRD) and high-resolution transmission electron microscopy revealed the polycrystalline nature of the NPs. The catalyst demonstrated good catalytic ability in facilitating the hydrogen evolution reaction (HER) in an alkaline solution, characterized by minimal overpotential._x000D_ <br>Chapter 6 focuses on the growth of a RuSe2-based multi-metallic catalyst. The possibility of CVD to form catalysts with modified surface chemistry was demonstrated by growing cobalt nanoparticle-decorated ruthenium di-selenide nanorods on carbon paper. A reactive CVD process with low temperature, low pressure, and a short reaction time was employed to synthesize the Co-RuSe2/C catalyst. Structural analysis via high-resolution transmission electron microscopy (HRTEM) and elemental distribution using scanning transmission electron microscopy energy dispersive spectroscopy (STEM-EDS) confirmed the uniform distribution of cobalt nanoparticles (Co NPs) on densely grown RuSe2 nanorods (NRs). Surface studies using X-ray photoelectron spectroscopy (XPS) revealed the presence of electron-deficient Ru and Se species, indicating successful surface electronic structure modification of the Co-RuSe2/C structure. Furthermore, the HER performance of the Co-RuSe2/C catalysts was evaluated in 1M KOH, demonstrating excellent performance sustained over an extended period._x000D_ <br>The thesis ends with the concluding remarks and prospects of the author presented in Chapter 7.