Preparation of mesoporous tin oxide for electrochemical applications

Following the discovery of the MCM family of mesoporous silicates using the supramolecular templating approach,1 mesoporous materials have attracted considerable attention because of their remarkably large surface areas and narrow pore size distributions, which make them ideal candidates for catalysts, molecular sieves, and as electrodes in solid-state ionic devices. A number of related synthetic strategies have been developed and a variety of materials, in terms of both composition and structure, have been prepared.2–4 Tin oxide is a wide-energy-gap semiconductor and has been widely used as a catalyst for oxidation of organic compounds, and for applications such as solid-state gas sensors, rechargeable Li-batteries, and optical electronic devices. The success in many of these applications relies critically on the preparation of crystalline SnO2 with uniform nanosize pore structure. Consequently, the synthesis of thermally stable mesoporous SnO2 is of great importance. To date, several preparative approaches utilizing a supramolecular templating mechanism have been reported for the preparation of mesoporous tin oxide.5–7 Upon removal of the surfactant, however, the mesoporous structures were destroyed; in other words, the preparation of mesoporous SnO2 without the support of a surfactant has not yet been achieved. For example, upon hydrolysis of SnCl4 in the presence of sodium dioctylsulfosuccinate (AOT, an anionic surfactant), Rao and Ulagappan5 obtained a mesoporous SnO2–AOT material with an average pore size of 3.2 nm. Attempts to remove the surfactant either by calcination at ca. 400 °C or by solvent extraction, however, resulted in collapse of the mesoporous structure. Similarly, upon hydrolysis of SnCl4 in the presence of sodium dodecyl sulfonate (another anionic surfactant), Qi et al.6 obtained mesoporous tin oxide with an average pore size of 4.1 nm. Again, the mesostructure collapsed when the surfactant was removed at 400 °C. Starting with tin isopropoxide and tetradecylamine (a neutral surfactant), Pinnavaia and coworkers7 obtained mesoporous tin oxide with an average pore size of 5.6 nm. The mesoporous structure was stable up to 350 °C, but was destroyed upon calcination at 400 °C and the surface area was greatly reduced. For electrochemical applications such as gas sensors, however, thermal stability of a mesoporous structure at high temperatures and without the support of a surfactant is critical for high catalytic reactivity, fast charge and mass transport, and long-term microstructural stability and durability. Thus, the objective of this study was to develop synthesis procedures for the preparation of mesoporous SnO2, stable at high temperatures ( > 400 °C), without the support of a surfactant. We have explored both neutral (S0I0) and electrostatic (S+I2) templating approaches to prepare mesoporous SnO2. In the neutral templating approach,† tetradecylamine (a neutral primary amine) was used as the surfactant (S0) or structure director and tin isopropoxide as the inorganic precursor (I0). Fig. 1 shows X-ray diffraction (XRD) patterns of the as-synthesized product and the mesostructured SnO2 after calcination at 500 °C for 2 h. While the as-synthesized product showed a sharp diffraction peak at low angle (characteristic of a mesostructured material), the calcined sample displayed only a very broad diffraction peak. However, TEM analysis indicated that the calcined solid was indeed mesoporous; the short-range hexagonal order can be seen from the TEM images shown in Fig. 2. The average d-spacings of the as-synthesized and the ascalcined mesostructures are 4.8 and 3.4 nm, respectively, as calculated from the positions of the XRD peaks, which are also consistent with the pore sizes as determined from the TEM micrographs. Thermogravimetric analysis of the as-synthesized tin oxide mesoporous powder under N2 flow indicated that the weight loss occurred between 200 and 500 °C, probably resulting from the decomposition of the surfactant and subsequent removal of the carbon residue. There was no detectable weight loss above 500 °C, indicating that the amine surfactant was completely removed from the mesoporous SnO2 structure upon calcination at 500 °C for 2 h. Thus, we have obtained mesoporous SnO2 (without the support of a surfactant) which is stable up to 500 °C. In the electrostatic templating approach, cetyltrimethylammonium bromide (CTAB) was used as the structure director (S+) and [Sn(OH)6] used as the inorganic precursor (I2).‡ This approach is similar to that employed by Suib and coworkers.8§ Fig. 3 shows the XRD patterns of the assynthesized product and the mesostructured SnO2 after calcination at 500 °C for 2 h. A sharp diffraction peak at small angle corresponding to the (100) diffraction was observed in both cases. Small peaks due to the (110) and (200) reflections are also observable before and after the removal of the surfactant. Fig. 1 XRD patterns of tin oxide synthesized via a neutral templating approach: (a) as-synthesized and (b) after calcination at 500 °C for 2 h.