[Image above] Schematic showing the benefits of the new low-temperature synthesis process of mesoporous metal oxides. Credit: Kim et al., Nanowerk
By Laurel Sheppard
In the upcoming April 2024 issue of Ceramic & Glass Manufacturing, the growing marketplace for nanomaterials and nanotechnology takes center stage. The possibilities attainable with nanoscience have started to materialize in many industrial sectors, and continued research and development in this field will further accelerate the adoption of these materials in other areas.
Mesoporous metal oxides (MMOs) play an important role in the emerging nano industry. These materials have well-defined structures consisting of interconnected pores ranging in size between 2 and 50 nm. Because of their large surface area and pore volume, MMOs find application as supports for nanoparticles in electrocatalysis, sensing, adsorption, and energy storage devices.
The first synthesis of an ordered mesoporous material was patented in 1969, but it wasn’t until 1992 that a similar material was commercialized by Mobil Oil Corporation. Several years later, the University of California, Santa Barbara developed 3D mesoporous silica, which is still in use today.
The synthesis of MMOs can be achieved through various methods, including hydrothermal, electrochemical, and microwave-assisted synthesis. Solvothermal synthesis, in which a solvent is subjected to high pressure and temperature in an autoclave, is another option. Other research groups have used chemical surfactants or molten salt to produce MMOs with improved properties.
Of all these methods, block copolymer (BCP) template-assisted sol-gel synthesis is considered the most effective in synthesizing MMOs due to the highly tunable chemical composition of BCPs and metal oxide precursors. However, the high temperatures used in this method to condense the precursors and remove the BCP template can have some undesirable effects.
For one, rapid crystallization can occur in certain MMO compositions at high temperatures, including vanadium pentoxide (V2O5) and molybdenum trioxide (MoO3), and this process destroys the mesoscale structure. Additionally, the high temperatures are incompatible with most flexible substrates, preventing the integration of MMOs onto flexible electronics.
To overcome these difficulties, researchers from various universities in the Republic of Korea recently developed a low-temperature process (150–200°C) to remove the BCP template. Their method, described in an Advanced Materials paper, combines thermal activation and oxygen plasma.
The method
The researchers used a BCP called PS-b-PEO as the template and vanadyl isopropoxide as the metal oxideprecursor; these materials were dissolved in a mixture of toluene and 1-butanol.
After casting the solution onto a fluorine-doped tin oxide glass, evaporation-induced self-assembly occurred, resulting in a mesostructured composite consisting of core polystyrene blocks surrounded by poly(ethylene oxide) shells containing hydrolyzed precursor.
Heat and oxygen plasma were then simultaneously applied to the composite.
Observations
Mesoporous V2O5 was synthesized at temperatures higher than 100°C and pressures below 100 mTorr. Both temperature and plasma pressure played a significant role in metal oxide formation and template removal.
Regarding temperature, complete removal of the template occurred at 200°C throughout the entire film, which was attributed to oxygen radicals diffusing into the V2O5 framework. Heat promoted this interaction between the plasma species and the composite, which consisted of fully interconnected mesopores.
Temperatures above 350°C did not produce mesoporous V2O5. According to the researchers, this situation was due to template removal and rapid crystal growth occurring simultaneously at high temperatures, and “thus the template does not sufficiently support its mesostructured composite,” they write.
Regarding plasma pressure, the researchers reported no effect on the template at 300 mTorr but partial removal at 50 mTorr; the template was completely removed at 10 mTorr. This finding indicates that both metal oxide formation and template removal can occur “even in the absence of plasma,” they write.
Further analysis showed that the low-temperature process suppressed preferential crystal growth in the MMO, resulting in the formation of small V2O5 nanocrystals within an amorphous matrix. The reason for this suppression is the numerous nucleation sites created by ion bombardment, which suppressed rapid crystal growth along the (010) direction. Also, the high kinetic energy transferred to the V2O5 framework during ion bombardment was enough to remove the BCP template.
The method in application
Using different types of inorganic precursors (metal alkoxide and chloride), the researchers synthesized a wide range of MMOs with this method, including those based on titanium, niobium, tungsten, and molybdenum. MoO3 is normally difficult to synthesize using conventional thermal methods at high temperatures because of its fast crystallization.
The researchers also successfully fabricated a flexible microsupercapacitor by directly synthesizing a mesoporous V2O5 electrode onto an indium tin oxide-coated colorless polyimide film. The interdigitated electrode pattern was created through laser ablation and encapsulated with the mesoporous V2O5.
The energy storage performance of this device was well maintained under severe bending conditions. No cracks or film detachment was observed after 3,000 cycles and a bending radius of 1.5 cm.
Compared to V2O5 synthesized by high-temperature calcination (350°C), the mesoporous material had superior electrochemical performance. It exhibited a capacitance retention of 91.4% after 10,000 charge/discharge cycles compared to the capacitance retention of 76.9% exhibited by the high-temperature calcined sample.
The researchers attribute these properties “… to the large surface area and interconnected mesopores, which provide numerous active sites and effective ion transport pathways.” In addition, the mesopores “…can easily accommodate volume expansion and strain relaxation during repeated charge/discharge cycles.”
In conclusion, “By utilizing the synergistic effect of thermal activation and plasma reactions, various MMOs were successfully synthesized at a low-temperature range … [and exhibited] reliable energy storage performance with excellent flexibility,” the researchers write.
The paper, published in Advanced Materials, is “Low-temperature, universal synthetic route for mesoporous metal oxides by exploiting synergistic effect of thermal activation and plasma” (DOI: 10.1002/adma.202311809).