Template-free formation of oriented oxide nanowalls via topotactic-like pseudomorphic transformation: [110]-MgO(111) nanowall arrays

Abstract

Oxide nanowall arrays having aligned growth orientation and aligned wall crystal planes are a challenge to form on substrates without the use of templates. Here, oriented magnesium oxide (MgO) nanowall (NW) arrays with a [110] growth direction and (111) wall surfaces are prepared using a template-free spontaneous route: electrodeposition of magnesium hydroxide (Mg(OH)₂) and its topotactic-like pseudomorphic transformation (TPT). Oriented Mg(OH)₂ NWs were prepared on a conductive substrate using simple electrochemical deposition from a Mg(NO₃)₂ aqueous solution, and were characterized using scanning electron microscopy, X-ray diffraction, Fourier-transform infrared (FTIR) spectroscopy and Raman spectroscopy. The obtained Mg(OH)₂ is a vertical nanowall array structure (∼660 nm height and 20–40 nm thickness) intersecting each other, with a [110] growth orientation and a (0001) wall surface. The size of NWs can be controlled by electrodeposition time and deposition current density. Heat treatment of the [110]-Mg(OH)₂(0001) NW arrays at 500 °C and 800 °C in air led to the formation of [110]-MgO(111) NW arrays via the TPT process. The chemical state analysis of NW surfaces using X-ray photoelectron microscopy and FTIR spectroscopy revealed that MgO NWs heated at 500 °C exhibit a superior CO₂ adsorption property compared to the Mg(OH)₂ NWs and MgO NWs heated at 800 °C.