Hollow bowl shaped materials are further designed based on hollow spherical materials, which not only inherit the advantages of large specific surface area and small specific gravity of hollow spheres, but also have unique properties brought about by weakly symmetric geometric structures. For example, stacking behavior can occur between particles to increase the compacted density of the powder. As an electrode material, it can increase the volume specific capacity; Similar in shape to red blood cells, it has better biocompatibility compared to spherical materials; Its concave surface can effectively concentrate light and electron beams. Therefore, bowl shaped materials have broad application prospects and market value in specific fields such as sensing, electrodes, catalysis, and drug transportation. Due to the complex structure of hollow bowl shaped powder particles (hollow, weak symmetry), the large-scale preparation of high-quality powder is still very difficult, especially the lack of in-depth understanding of its growth behavior and mechanism, which limits the widespread application of this material. The template method for bowl shaped powder material synthesis Because the bowl shaped powder material has structural specificity (hollow, weak symmetry), when synthesizing bowl shaped structural materials, it is usually necessary to use a structure directing agent template. When the precursor nucleation is induced, a specific nucleation location occurs and its growth direction is limited.

Therefore, the template method for preparing bowl shaped materials is one of the main research topics for researchers. In the template method, templates can be designed based on the size and morphology of the synthesized material, and the size, morphology, structure, and layout of the synthesized material can also be controlled based on the spatial confinement effect of the template and the regulatory effect of the template agent. Under different experimental conditions, different component bowl shaped materials were synthesized using the role of templates. The template required for preparing bowl shaped materials can be bowl shaped or spherical, usually in three modes: mold method, cutting method, and concave method. The mold method refers to using the concave (or outer) part of the bowl as the mold to directly obtain the bowl shaped powder; The cutting method refers to the growth of a layer of target product on the surface of a spherical template, forming a core-shell structure, cutting off half and removing the remaining template to obtain a bowl shaped material; The concave method also requires the formation of a core-shell mechanism. During the process of removing the template, by controlling the escape speed of the template, capillary forces pointing inward can be generated on the shell, causing the shell to gradually concave inward, ultimately forming a bowl shape.

The template method can be divided into two types based on its own characteristics and different domain limiting capabilities: hard template and soft template. Hard template, including inorganic and polymer materials. Since the discovery that hard templates have the advantages of direct and easy control of material size, shape, and structure, the hard template method has now become a widely used method for preparing bowl shaped materials. Soft templates are generally small molecule organics and some surfactants, including various ordered polymers formed by amphiphilic molecules, such as liquid crystals, micelles, micro lotion, supramolecular micelles, polymer aggregationesicles, bubbles, etc. Among them, micro lotion is an important form of soft template method, and micro lotion is a liquid-liquid homogeneous thermodynamic stable system composed of surfactant, cosurfactant (amine or alcohol), oil and water, which is homogeneous in macro and heterogeneous in micro, transparent, non-conductive and isotropic. Vapor deposition method, divided into chemical vapor deposition and physical vapor deposition, is a technology applied to various thin film materials.

For the preparation of bowl shaped materials, due to its unique structure, it is usually necessary to combine it with the template method. A layer of spherical templates, usually hard templates, such as silicon balls, silica balls, and polystyrene balls, are first deposited on the surface of the solid substrate. The target substance is then deposited on the surface of the template and processed to remove the template to obtain the bowl shaped material. The liquid phase deposition method uses the liquid phase deposition method to prepare bowl shaped materials, similar to the vapor phase deposition method. It usually needs to be combined with the template method to make the precipitation reaction occur on the surface of the template, and obtain the bowl shaped material through subsequent processing. Hard templates can be silicon spheres, silica spheres, and polystyrene spheres. The dynamic swelling method is a concept proposed by Okubo et al., who used this method to prepare monohollow and monodispersed polyvinylbenzene micron-sized colloidal particles and can control the size of the hollow pores. The dynamic swelling method is usually divided into three steps, namely swelling, copolymerization, and extraction. Firstly, seed polymerization is used to synthesize latex particles. Then, toluene and divinylbenzene are used to swell the latex particles and initiate the polymerization of divinylbenzene. After polymerization, toluene is removed to form hollow microspheres as shells.

The application of bowl shaped powder materials for energy storage. Bowl shaped structural materials have many advantages, such as large specific surface area, small specific gravity, relatively high compaction density, and structural stability. They have shown enormous application potential in supercapacitors, lithium-ion batteries, lithium-sulfur battery systems, and lithium-oxygen battery systems. The catalytic material of fuel cells. Fuel cells typically generate current through electrochemical catalytic oxidation of fuels such as hydrogen, methanol, or ethanol at the battery cathode, and their performance mainly depends on the electrocatalytic effect of the oxygen reduction reaction at the electrode.

Traditional carbon loaded precious metal catalysts are very expensive, which hinders the widespread application and commercialization of fuel cells. For this reason, many efforts are focused on reducing or even avoiding the use of precious metals without compromising catalytic performance and energy efficiency. The nitrogen doped single-layer porous bowl shaped carbon spheres prepared by the soft template method exhibit better catalytic performance than carbon supported platinum in oxygen reduction reactions.

The low efficacy and significant toxic side effects of drug delivery materials in the chemotherapy of major diseases such as cancer are fatal defects. Drug delivery systems (DDS) are one of the most promising solutions to address this deficiency, with carrier materials being the key technology of DDS.

At present, various nanomaterials, including micelles, liposomes, and inorganic nanomaterials, have been attempted to construct joint anti-tumor nanocarriers and have achieved good expected results. The porous hollow bowl shaped carbon spheres prepared by the hard template method, as carriers of anticancer drugs, exhibit excellent performance in drug adsorption, release, and targeted release.

Source: Zhang Zili Template synthesis and regulation of bowl shaped powder materials [D]. Beijing University of Science and Technology, 2019