Synthesis and Characterisation of Mesoporous Transition Metal Oxides Based on Soft-Templating Method

Mesoporous materials have attracted extensive interest in the past few decades because of their diverse applications in different fields, including chemical, environmental, energy, optics, electronics, medical and biotechnological applications. They display unique properties presumably because of their pores, which are sufficiently large to host large and/or multiple molecules, but not large enough for bulk properties to influence surface interaction. The mesoporous transition metal oxides group is among the mesoporous materials with several properties, including d-shell electrons confined to nanosized walls, redox active internal surfaces and connected pore networks. Among various transition metal oxides, nickel oxide (NiO), a wide bandgap (3.6–4.0 eV) p-type semiconductor, has gained significant attention owing to its exciting intrinsic properties, such as electrochromic, antiferromagnetic and high capacitive properties. In addition, NiO can be utilised in a wide range of applications, such as electrochromic display devices, smart windows, active optical fibres, gas sensors, solar thermal absorbers, catalysis, fuel cell electrodes, supercapacitors and energy storage devices. The soft-templating method is used to synthesise mesoporous materials because it offers many benefits, including achieving cost-effectiveness, creating various porous networks with a wide range of pore sizes and providing access to well-defined morphologies and customisation for various applications. However, this method is extremely sensitive to hydrothermal conditions, such as concentration, temperature and pH. In addition, the synthesis process for some transition metal oxides has extra difficulties, including high tendency to build a stable structure with high lattice and possibility of structure collapsing on template removal. In the present study, an asymmetric poly (styrene-block-acrylic acid-block-ethylene glycol) (PS-b-PAA-b-PEG) triblock copolymer is used as a soft template to synthesise the mesoporous NiO. In aqueous solutions, the block copolymer forms a micelle containing a PS core, a PAA shell and a PEG corona. The PS block serves as the core of the micelles because of its hydrophobicity, while the anionic PAA block interacts with the cationic Ni2+ ions in the solution to form the shell and the PEG block forms the corona of the micelles. The goal of the corona is to stabilise the micelles by avoiding secondary aggregation through steric repulsion between the PEG chains. The synthesised mesoporous NiO was characterised with different techniques, including x-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and the Brunauer–Emmett– Teller method. The NiO shows a large average pore size of 35 nm with a large specific surface area (97.0 m2 g-1) and pore volume (0.411 cm3 g-1). It is expected that the proposed soft-templating strategy could be generalised to other metal oxides/sulphides in the future for potential applications in gas sensors, environmental applications, catalysis, energy storage and conversion, optoelectronics and biomedical applications.