Date Published: January 28, 2019
Publisher: John Wiley and Sons Inc.
Author(s): Hailong Xiong, Tunan Gao, Kaiqian Li, Yali Liu, Yali Ma, Jingwei Liu, Zhen‐An Qiao, Shuyan Song, Sheng Dai.
Mesoporous metal oxides (MMOs) have attracted comprehensive attention in many fields, including energy storage, catalysis, and separation. Current synthesis of MMOs mainly involve use of surfactants as templates to generate mesopores and organic reagents as solvents to hinder hydrolysis and condensation of inorganic precursors, which is adverse to adjusting the interactions between surfactants and inorganic precursors. The resulting products have uncontrollable pore structure, crystallinity, and relatively lower surface areas. Here, a facile and general polymer‐oriented self‐assembly strategy to synthesize a series of MMOs (e.g., TiO2, ZrO2, NbO5, Al2O3, Ta2O5, HfO2, and SnO2) by using cationic polymers as porogens and metal alkoxides as metal oxide precursors in a robust aqueous synthesis system are reported. Nitrogen adsorption analysis and transmission electron microscopy confirm that the obtained MMOs have ultrahigh specific surface areas and large pore volumes (i.e., 733 m2 g−1 and 0.485 cm3 g−1 for mesoporous TiO2). Moreover, the structural parameters (surface area, pore size, and pore volume) and crystallinity can be readily controlled by tuning the interactions between cationic polymers and precursors. The as‐synthesized crystalline mesoporous TiO2 exhibits promising performance in photocatalytic water splitting of hydrogen production and a high hydrogen production rate of 3.68 mol h−1 g−1.
Mesoporous metal oxides (MMOs) have been of great interest over the past decades for comprehensive applications including energy storage, selective oxidation, adsorption, and gas sensor.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 Most of these applications are strongly relied on their superior physical, chemical properties, and the combination of their advanced structural properties such as high specific surface areas, large pore volumes, and narrow pore size distributions.12 Various synthetic methods have been developed to synthesize MMOs. One promising route is nanocasting method,13, 14, 15, 16, 17, 18, 19, 20, 21 which depends on the usage of mesoporous silica, mesoporous carbon, or polymer as hard templates to provide the desired mesoporous structure and subsequent removal of templates by etching or calcination to finally obtain MMOs. However, the application of this method suffers from some limitations, for instance, the mesoporous characteristics of such oxides are completely relied on the framework of hard templates. Moreover, there are only a few templates available for use, and the procedure is tedious, costly as well as not suitable for mass production, which greatly hinders their practical applications.
A series of MMOs with ultrahigh surface areas have been synthesized by the polymer‐oriented self‐assembly strategy in a robust aqueous synthesis system. Mesoporous TiO2 labeled as MTx, x referred to the amount of HOAc, was took as a representative in the following discussion. In this approach, tetrabutyl titanate (TBOT) first hydrolyzed in H2O/PEI/HOAc solution under stirring at room temperature for 2 h to form mesoporous titanium/PEI composites (Figure S1 and Table S1, Supporting Information). Then, the resulting titanium/PEI composites were then hydrothermally treated at 100 °C for cross‐linking and condensation of titanium oligomers. Finally, the obtained composites were washed with distilled water and absolute ethanol several times in sequence to remove PEI, giving rise to mesoporous TiO2. When the amount of HOAc ranged from 0 to 1.5 mL, the ultrahigh surface area mesoporous TiO2 could be obtained. When the amount of HOAc was larger than 2.5 mL, we could get mesoporous TiO2 with highly crystallized framework. It was noteworthy that the yield of products was as high as ≈95% and grams of samples could be easily synthesized in one bath.
In summary, a facile and general polymer‐oriented self‐assembly strategy, using water as a solvent and nonsurfactant cationic polymers as porogens, for the fast and scalable synthesis of a series of MMOs (e.g., TiO2, ZrO2, NbO5, Al2O3, Ta2O5, HfO2, and SnO2) with ultrahigh specific surface areas and large pore volumes was developed. The presented strategy overcomes the defects associated with the traditional sol–gel‐based self‐assembly route in organic solvents synthesis system, which would strongly promote the industrial application of MMOs. These obtained MMOs possessing superior porous properties will be tailored for promising applications in energy conversion, catalyst, gas separation, and sensing. This approach may open up new opportunities for fabricating other high surface area mesoporous materials, such as mixed‐metal oxides, metal phosphates, metal sulfides, and oxide nanostructures.
Materials: TBOT, zirconiumn butoxide (Zr(OBu)4), hafnium ethoxide, niobium ethoxide, tantalum ethoxide, aluminum ethoxide, tin(II)ethoxide, PEI (average Mw 600), and PDADMAc were purchased from Sigma‐Aldrich Co. HOAc concentrated HCl and ammonium hydroxide (NH3·H2O) were of analytical grade and obtained from Shanghai Chemical Corp.
The authors declare no conflict of interest.