Synthesis and characteization of mesoporous nickel oxide and nickel cobaltite thin films

Abstract

In this thesis work, molten-salt assisted self-assembly (MASA) approach was adopted to synthesize mesoporous nickel oxide (m-NiO) and nickel cobaltite (m-NiCo2O4) thin films. The m-NiO and m-NiCo2O4 films were obtained by coating clear ethanol solutions of nickel salt and two surfactants (charged, CTAB and neutral, 10-lauryl ether), and nickel and cobalt salts with the same surfactants, respectively, followed by calcination at different temperatures (between 250 and 500 oC). The method has been established in a very broad range of salt concentrations in the lyotropic liquid crystalline (LLC) mesophase that can be calcined to produce mesoporous thin films. Both Ni(II) and Ni(II)/Co(II) systems form stable and oriented LLC mesophases in a broad range of salt concentrations (salt surfactant mole ratio of 2-8) upon evaporation of ethanol from the media. This can be achieved by either spin coating of the clear solutions (this ensure immediate evaporation of ethanol, leaving the LLC gel phase as thin film) or drop casting and evaporation of ethanol (the gelation process takes more time). At higher salt concentrations (10-30 salt/surfactant mole ratios), the mesophase is disordered and leach out salt crystals. However, those compositions can still be used for the synthesis of mesoporous metal oxides, if the samples are calcined immediately after the gelation step. The mesophase is 2D hexagonal at low salt concentrations and disordered or cubic at higher salt concentrations. The calcined films were characterized by recording x-ray diffraction (XRD), N2-adsorption desorption measurements, imaging (SEM, TEM, and POM) and spectroscopic (UV-Vis, XPS, EDX, and ATR-FTIR) techniques. The N2 adsorption-desorption isotherms are type IV and characteristic for mesoporous materials. The XRD data show that the crystalline m-NiO and m-NiCo2O4 form at around 300 and 250 oC, respectively, with a pore-wall thickness of around 3-4 nm. The pore-walls grow with increasing the calcination/annealing temperature up to 20 nm at around 500 oC. It accords well with the BET surface area that decreases with increasing calcinations temperature; it is 223 m2/g at 300 oC and drops to 20 m2/g at 500oC in mesoporous nickel oxide, and 223 m2/g at 250 oC and drops to 31 m2/g at 500 oC in mesoporous nickel cobaltite. The observed diffraction patterns can be indexed to rock salt cubic structure of NiO and cubic spinel structure of NiCo2O4. The diffraction lines gradually become sharper indicating crystallization and growth of the pore-walls that accord well with the reduction on the surface area. The m-NiO and m-NiCo2O4 films can be coated over FTO glass to use as an electrochromic electrode (oxidation dark-reduction clear) and electrode for water oxidation reactions (WOR) and WOR, respectively. In nickel oxide case, during cyclic voltammograms cycling, water oxidation process, and electrochromic switching, a few atomic layer of nanocrystalline NiO pore-wall is converted to NiOOH in oxidation and Ni(OH)2 upon reduction processes; initially formed nanocrystalline NiO (after calcination) pore-walls become NiO coated Ni(OH)2 (core-shell structure) upon electrochemical treatments. Both NiO and NiCo2O4 having high surface area and electrochemical stability show promising capacitive properties and can be used as electrocatalysts. From the Tafel slope analysis, it has been shown that nickel cobaltite can oxidize water at low overpotentials and therefore can be used as a promising water splitting catalyst.