Synthesis and tuning of nanomaterials through photothermal effects and its application

Abstract

Nanomaterials have been actively studied in energy storage, catalysts, and gas sensors due to superior surface electrical and chemical reaction properties resulting from considerable surface-to-volume ratio to overcome limit of existing materials’ properties. Along with massive researches on nanomaterials towards commercialization, a furnace annealing system has been widely used for synthesis and properties modulation of nanomaterials. The furnace annealing method normally provides heating condition by inductor coils surrounding the system, which is not a productive method to solely treat nanomaterials itself. In the furnace system, the temperature of materials is reached to the specified value through thermal equilibrium on thermal radiation and heat conduction, leading to waste of thermodynamic energy. As a result, furnace annealing causes redundant thermal budget. Accordingly, study on an effective and productive annealing approach is required. Photothermal effects can be achieved by heat generated from nonradioactive recombination of photo-excited electrons. In surface-reaction-dominant applications, two main materials are broadly utilized: host materials to lead main reactions and guest materials such as catalysts to accelerate and improve the reaction rates by reducing activation energies. In this thesis, accordingly, a host-materials-heating (HMH) approach using heat from interaction with light was firstly introduced to achieve photo-pyrolysis of guest materials and properties modulation of host materials. Along the way, new ambient-air processes were proposed on synthesis and tuning of nanomaterials for the potential replacement of convention vacuum furnace system. Besides, the feasibility of promising applications in gas sensing and electrocatalysts was cautiously examined with the obtained samples. First, a method based on phothermal effects for doping and reduction of graphene oxide (GO) as host materials, which features high photothermal efficiency, was studied in terms of nanomaterials property modulation. To do so, flash light irradiation was implemented to a coated mixture of graphene oxide and doping source (DS) on a glass substrate, allowing for increase in temperature higher than 1600 °C within 10 ms. It was confirmed that the momentary high temperature annealing could be responsible for the subsequent reduction and doping in ambient air. In addition, detail mechanism of optical doping on temperature was elucidated. Next, it was further verified that the identical process using flash light irradiation facilitated uniform formation of single atom catalysts at the doping sites of reduced GO even in ambient air by adding metal ion precursors to GO with DS. Lastly, apart from carbon-based materials with high photothermal efficiency, nano-structuring of oxides led to significant enhancement in photothermal efficiency. In turn, oxides with wide band gaps (> 3.0 eV) such as SnO$_2$ could be exploited as host materials for direct synthesis of catalytic alloy nanoparticles on the surface on the host oxides. Through the empirical knowledge and possibility from the preceded studies, it was concluded that the combination of the HMH concept and photothermal effects can be applied as a novel approach for high-throughput and ambient-air synthesis and tuning of nanomaterials. After the thesis, the scope of the research will be extending to researches on a light-to-heat conversion thin film layer for heating platform for improvement in performance of electronic devices and 2D materials synthesis.