Study on the preparation of porous TiO2 heterostructures using organonanogel-base template and their catalytic characteristics

Abstract

TiO2-based electrode has attracted great attention for eletrocatalytic applications due to superior thermal and chemical stability for many years. For example, as a catalyst support, electronic and geometric interaction between a metal catalyst and TiO2 support (SMSI) boost up catalytic activity and stability. In addition, the chemical and physical property of TiO2 can be easily tuned by various processing techniques including doping, decoration of nanoparticles, and morphology control. However, the catalytic efficiency of TiO2 is still very low for industrial application. To realize the electrocatalytic use of TiO2, efficient catalytic system need to be designed. Porous structure of TiO2 play an important role in catalytic performance since overall rate of catalysis highly depends on the mass transport and catalytic surface area. A myriad of efforts have been made to design and prepare TiO2-based catalyst with well-defined porous structure with high surface area, including nanocasting, sol-gel method, atomic layer deposition (ALD), electrospinning, and 3D printing. However, current existing methods does not provide an efficient system to prepare porous metal oxide with high surface area and to finely tune its property for target reactions. In this thesis work, we report the synthetic methods of porous TiO2 electrode using amine-based organic nanogel and the catalytic performance of prepared electrodes is discussed. Nitrogen-bonded polymeric network such as polyamide and polyurea exhibits high thermal and chemical stability compared to other polymeric materials. The utilization of N-containing organic networks as a porous template enables to synthesize porous TiO2 catalyst with well-defined porous structure and large surface area. Furthermore, the multi-dentate structure of N-containing monomers evolves into various geometric bonding structures, resulting in unique property. In the light of remarkable characteristics, efficient synthetic systems of porous TiO2 electrode using nanogel phases of N-containing polymeric network as a template are developed here. High stability and easy processability of amine-based organonanogel provide the opportunity for the subtle control of synthetic parameters for TiO2 growth to modify electronic and optical property of TiO2 for enhanced catalytic activity. In first part, porous SiO2/Cu2O/TiO2 heterostructure was prepared from nanoporous hybrid film (NHF) as a template by impregnating metal oxide precursors with multiple sequence. Previously, we reported the synthesis of tetrakis(4-aminophenyl)methane (TAPM)-based hybrid porous network using sol-gel method. TAPM-hexamethylene diisocyanate (HDI)-organosilica precursor terary system produces core/shell-like urea network coated with organosilica on pore surface. The hybrid structure of NHF serve as a good template for porous metal oxide growth due to high thermal and chemical stability. It not only gives the opportunity of multiple chemical processing to prepare complex structures but also leads to pure anatase phase. In addition, core-shell like hybrid network directly gives porous silica coating on the catalyst surface without post-treatment for silica deposition, resulting in high catalytic stability and unique electronic structure on catalyst surface. Here, NHFs with different organosilica were prepared for TiO2 templates. The effect of pore structure of the SiO2 overlayer on the catalytic activity was investigated. In second part, dual doped porous TiO2 electrode was prepared with modification of Pechini method by replacing ethylenglycol with ethylenediamine dihydrochloride. The modification produces 3D nanogel arrays at a precursor film stage instead of macroscopic polymeric network, which prevents mechanical deformation by film shrinkage during heat treatment and assist to prepare highly crystalline porous TiO2 film with uniform dopant distribution. With the method, Al and Nb dual doped TiO2 was synthesized and the catalytic activity and selectivity for nitrogen reduction reaction was investigated. This nanogel templating for porous metal oxide growth provides synthetic advantages to finely tune electrocatalytic property. In ionic nanogels templating, metal ions are confined in ionic network of nanogels, which prevents metal aggregations and leads to uniform metal distribution. These enables controlled heteroatom doping with fine control of compositional variation. Defect clusters by finely controlled heeteroatom doping leads to selectivity control with high activity. In UN nanogel templating, high thermal and chemical stability of nanoporous template synthesized with UN nanogel and polymer blending enable multiple loading process of metal precursor. The multiple loading porocess on one pot helps to directly create heterointerface, which can design the metal oxide heterostructure for enhanced charge separation efficiency. The synthetic advantages and the resulting electrocatalytic property with nanogel templating paves way to overcome problems associated with current polymer-based templating methods.