Role of silica in the self-assembly of salt-surfactant mesophases and synthesis of mesoporous metal oxides

Abstract

In recent years, mesoporous metal oxides have attracted great attraction due to their unique optical, electrochemical, and catalytic properties. Mesoporous nickel oxide (m-NiO) is a p-type semiconductor, versatile in its application due to its high surface area, and has been investigated towards electrochromic devices, electrodes, supercapacitors, and catalysts. The electrochemical properties of NiO depend on its morphology, surface area, and particle size. In this thesis, mesoporous nickel oxide has been synthesized by combining soft templating (molten salt-assisted self-assembly method) and hard templating methods to attain a high surface area. Homogeneous aqueous solutions of nickel(II) nitrate hexahydrate (Ni(H2O)62), TMOS (as silica source), and two surfactants, CTAB (charged surfactant) and C12E10 (nonionic surfactant) are stable only if a concentrated nitric acid is added before the TMOS addition. In the absence of nitric acid, TMOS hydrolyzes and condenses quickly, resulting in silica precipitation. The silica precipitation also occurs by using other salts, such as nickel(II) chloride hexahydrate, nickel(II) sulfate hexahydrate, cobalt(II) nitrate hexahydrate, and manganese(II) nitrate tetrahydrate. The silica precipitate is characterized by ATR-FTIR, small-angle, and wide-angle XRD and N2 adsorption-desorption measurements. The diffraction lines at 1.7 and 23o, 2θ, indicate the formation of mesostructured amorphous silica, in which the surfactant species fill the pores. . The silica precipitate is calcined at 450 oC for two hours to remove the surfactant completely, and characterized by ATR-FTIR, small-angle and wide-angle XRD measurements, N2- adsorption-desorption analysis and SEM-EDX techniques. The maximum surface area (1395 m2/g) is obtained from the cobalt(II) nitrate hexahydrate salt, and the EDX analysis confirms that there is no element other than silicon and oxygen in its elemental detection limit. The homogeneous, stable aqueous solutions of the nickel(II) nitrate hexahydrate (Ni(H2O)62), HNO3, TMOS (as silica source), and two surfactants, CTAB (charged surfactant) and C12E10 (nonionic surfactant) solution is drop-casted on a glass slide to form a mesophase and analyzed by small-angle XRD, ATR-FTIR and POM techniques. The diffraction lines at 1.5 and 1.6o, 2θ, show the formation of ordered lyotropic liquid crystalline mesophases. The mesophases are then calcined at different temperatures (from 250 to 500 °C), to obtain m-NiO/SiO2 powders and characterized by ATR-FTIR, XRD measurements, N2- adsorption-desorption analysis, and SEM-EDX techniques. The XRD patterns show broad lines at small- and wide-angles, indicating the formation of m-NiO/SiO2 at 300 °C, where the pore-walls are made up 2.6 nm crystalline NiO coated amorphous silica . The NiO particle size (on the pore wall) grows with increasing annealing temperature, and at 500 °C, the particle size reaches 7.9 nm. This is also supported by the BET surface area that decreases at higher temperatures. At 300 °C, the BET surface area is 305 m2/g, which drops to 174 m2/g at 500 °C. However, the pore size of m-NiO/SiO2 does not responds to annealing temperature. It means that the pore walls grow in 2D space rather than 3D due to the presence of silica as a hard template. Therefore, combining the hard- and soft-templating methods can efficiently synthesize the crystalline materials with a high surface area. The m-NiO/SiO2 films can be coated over the FTO glass and calcined at different temperatures to fabricate the electrodes for oxygen evolution reaction (OER). During CV measurement, the NiO pore-walls get oxidized to NiOOH and reduced to Ni(OH)2 in the back cycle. Moreover, overpotential that is determined for the OER improves with the usage of the electrode, independent of the electrode thickness.