Controlled synthesis of Mg(OH)2 thin films by chemical solution deposition and their thermal transformation to MgO thin films

Abstract The chemical solution deposition of Mg(OH) 2 thin films on glass substrates and their transformation to MgO by annealing in air is presented. The chemical solution deposition consists of a chemical reaction employing an aqueous solution composed of magnesium sulfate, triethanolamine, ammonium hydroxide, and ammonium chloride. The as-deposited films were annealed at different temperatures ranging from 325 to 500 °C to identify the Mg(OH) 2 -to-MgO transition temperature, which resulted to be around 375 °C. Annealing the as-deposited Mg(OH) 2 films at 500 °C results in homogeneous MgO thin films. The properties of the Mg(OH) 2 and MgO thin films were analyzed by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, UV–Vis spectroscopy, and by circular transmission line model. Results by X-ray diffraction show that the as-deposited thin films have a brucite structure (Mg(OH) 2 ), that transforms into the periclase phase (MgO) after annealing at 500 °C. For the as-deposited Mg(OH) 2 thin film, a nanowall surface morphology is found; this morphology is maintained after the annealing to obtain MgO, which occurred with the evident formation of pores on the nanowall surface. The assessed chemical composition from X-ray photoelectron spectroscopy yields Mg 0.36 O 0.64 (O/Mg ratio of 1.8) for the as-deposited Mg(OH) 2 film, where the expected stoichiometric composition is Mg 0.33 O 0.67 (O/Mg ratio of 2.0); the same assessment yields Mg 0.60 O 0.40 (O/Mg ratio of 0.7) for the annealed thin film, which indicates the obtainment of a MgO material with oxygen vacancies, given the deviation from the stoichiometric composition of Mg 0.50 O 0.50 (O/Mg ratio of 1.0). These results confirm the deposition of Mg(OH) 2 films and the obtainment of MgO after the heat-treatment. The energy band gap of the films is found to be 4.64 and 5.10 eV for the as-deposited and the film annealed at 500 °C, respectively. The resistivity of both Mg(OH) 2 and MgO thin films lies around 10 8  Ω·cm.