Article pubs.acs.org/cm
Robust, Highly Thermally Stable, Core−Shell Nanostructured Metal Oxide Aerogels as High-Temperature Thermal Superinsulators, Adsorbents, and Catalysts Guoqing Zu,*,† Jun Shen,*,† Wenqin Wang,† Liping Zou,† Ya Lian,† Zhihua Zhang,† Bin Liu,‡ and Fan Zhang‡ †
Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, P. R. China ‡ Aerospace Research Institute of Special Material and Processing Technology, Beijing 100074, P. R. China S Supporting Information *
ABSTRACT: Robust, highly thermally stable, MOx/(MOx− SiO2)/SiO2 core−shell nanostructured metal oxide aerogels with a MOx core and (MOx−SiO2)/SiO2 shell are produced via novel alkoxide chemical liquid deposition techniques. The core−shell nanostructure not only significantly reinforces the nanoparticles but also effectively inhibits the crystal growth and phase transition of metal oxide upon heat treatment, which enhances the heat resistance from approximate 400− 800 °C up to 1000−1300 °C. The resultant core−shell nanostructured Al2O3, ZrO2, and TiO2 aerogels can support at least 5800 times their weight and exhibit high surface areas of 139, 186, and 154 m2/g after fired at 1300, 1000, and 1000 °C, respectively, which are the highest surface areas for metal oxide aerogels ever reported. We demonstrate that the core−shell ZrO2 and TiO2 aerogels show enhanced adsorption and photocatalytic performances, respectively, for dye after fired at 1000 °C. The core−shell Al2O3 aerogel/mullite fiber/TiO2 composite possesses ultralow thermal conductivities of 0.058, 0.080, and 0.11 W/mK at 800, 1000, and 1200 °C, respectively, which are the lowest values for inorganic aerogels ever reported. The resulting materials are promising candidates as hightemperature (400−1300 °C) thermal superinsulators, adsorbents, and catalysts.
1. INTRODUCTION Aerogels are highly porous materials synthesized by the sol−gel method and subsequent removal of the liquid solvent from within the pores of the gel without collapsing the networks by using supercritical fluid drying, freeze-drying, or special ambient drying.1,2 Owing to their special nanoporous structure, high porosity, high specific surface area (SSA), and low density, metal oxide aerogels exhibit unique thermal, electronic, chemical, optical, and mechanical properties and have drawn great interest in a wide range of applications such as thermal insulations, catalysts, catalyst supports, adsorption, sensors, etc.3−9 However, there are many challenges in preparation of metal oxide aerogels and enhancement of their thermal stability and mechanical strength. The first challenge is that metal oxide aerogels are usually prepared from either metal alkoxides or metal inorganic salts, and it is difficult to obtain monolithic metal oxide aerogels from metal alkoxides because of the complex chemical pathways leading to gelation, high reactivity of the metal alkoxides to water, and the susceptibility to cracking during drying.10−17 Many crack free metal oxide aerogel monoliths (Al2O3, ZrO2, SnO2, etc.) have been made from metal inorganic salts via the epoxide method.7,12,13 According to reports, the © 2014 American Chemical Society
alkoxide derived aerogels, such as alumina aerogel, show better thermal stability at high temperatures than those derived from metal inorganic salts.9,11,12 However, metal alkoxides such as Al(OR)3, Zr(OR)4, or Ti(OR)4 are much more reactive toward water than alkoxysilanes due to the lower electronegativity, higher Lewis acidity, as well as the possibility of increasing the coordination number.1 In order to obtain transparent and rigid metal oxide wet gels with uniform microstructure from metal oxide alkoxides, a chelating agent such as acetylacetone, ethyl acetoacetate, or acetic acid is usually used to control the hydrolysis and condensation rates of metal oxide alkoxides.18−22 Although it lowers the reaction rate and produces the transparent wet gels, it introduces organic ligands bonded to metallic elements such as Al, Zr, Ti, etc., which lowers the cross-linking density and skeleton strength. In order to solve this problem, we recently have developed an acetone−aniline in situ water formation (AISWF) sol−gel technique, which can effectively slow down the hydrolysis and condensation rate of the highly Received: August 6, 2014 Revised: September 18, 2014 Published: September 22, 2014 5761
dx.doi.org/10.1021/cm502886t | Chem. Mater. 2014, 26, 5761−5772
Chemistry of Materials
Article
Table 1. AISWF Sol-Gel Parameters of Typical Metal Oxide Wet Gels Obtained in the Present Study step one
step two
step three
sample
precursor (g)
EtOH (mL)
H2O (mL)
EtOH (mL)
HNO3 (mL)
Al2O3 ZrO2 TiO2
ASB 4.64 ZBO 5.25 TBO 4.83
3.5 15 3
0.1
0.3 1.0 0.3
0.03 0.1 0.03
reactive metal alkoxide by controlling water formation through slow dehydration reaction of acetone and aniline. We have successfully prepared monolithic Al2O3 aerogel via this method in previous work;23 here we apply this method to prepare other metal oxide aerogels such as ZrO2 and TiO2 aerogels. Another challenge is that metal oxide aerogels are usually fragile, readily to sinter, and lose most of their SSAs under heat treatment at high temperatures, which badly limits their practical thermal insulation, adsorption, and catalyst applications, especially at high temperatures. Many research works focus on various metal oxides composites or metal oxide/silica composites (such as Al2O3/SiO2, Al2O3/ZrO2, ZrO2/SiO2, and TiO2/SiO2) in order to enhance the strength and thermal stability of metal oxide aerogels.24−34 Most of these composite methods are performed by simply mixing various precursors and allowing them to react with each other during sol−gel process. The enhancement of strength and thermal stability is limited via this route. The SSA of pure Al2O3 aerogel is reduced to as low as 10− 40m2/g at 1300 °C.35 After the addition of silica during the sol− gel process, the SSA is increased to only 80 m2/g.25 The SSAs of ZrO2 and TiO2 aerogels decrease to
