Epitaxial Single and Double Nanolayers of ${\rm SnO}_{2}$ and ${\rm TiO}_{2}$ for Resistive Gas Sensors

Rutile TiO
₂
(1 0 1) and cassiterite SnO
₂
(1 0 1) epitaxial single and double nanolayers, the latter stacked in either sequence, are atomic layer deposited on
r
-cut α-Al
₂
O
₃
(0 1 ̅ 2) substrates. Thickness of the layers is varied. Epitaxial quality of the films is characterized by X-ray diffraction (XRD), reflection high-energy electron diffraction, and transmission electron microscopy. In gas response measurements, as-grown films and the films coated with electron-beam evaporated Pt nanoclusters are exposed, at 350
^°
C, to H
₂
, CO, and CH
₄
diluted in air. In response to test gas concentrations of 30 parts per million (ppm), the films with a thickness of order of 10 nm exhibit, depending on the makeup and gas, as high as two- to five-fold decrease in the resistance. It is shown that the platinum surface catalyst is effective in accelerating the response and recovery processes. The transition times of the order of a few tens of seconds are observed. The results demonstrate the feasibility of gas sensing with single-crystal-like nanolayer films. Comparison of sensor characteristics of such quasi-2D nanostructures and the literature data relevant to individual nanowires, nanorods, and nanobelts, i.e., typical representatives of the quasi-1D structures, shows that, as to H
₂
, CO, and CH
₄
, both structures are worthy competitors.