Atomic layer deposition of ZnO on ultralow-density nanoporous silica aerogel monoliths

We report on atomic layer deposition of an $\sim 2$ -nm-thick ZnO layer on the inner surface of ultralow-density ($\sim 0.5\mathrm{\%}$ of the full density) nanoporous silica aerogel monoliths with an extremely large effective aspect ratio of $\sim {10}^5$ (defined as the ratio of the monolith thickness to the average pore size). The resultant monoliths are formed by amorphous-${\mathrm{SiO}}_2$ core/wurtzite-ZnO shell nanoparticles which are randomly oriented and interconnected into an open-cell network with an apparent density of $\sim 3\mathrm{\%}$ and a surface area of $\sim 100{\mathrm{m}}^2{\mathrm{g}}^{-1}$ . Secondary ion mass spectrometry and high-resolution transmission electron microscopy imaging reveal excellent uniformity and crystallinity of ZnO coating. Oxygen $K$ -edge and Zn $L_3$ -edge soft x-ray absorption near-edge structure spectroscopy shows broadened O $p$ - as well as Zn $s$ - and $d$ -projected densities of states in the conduction band.