Looking Outside the Square: The Growth, Structure, and Resilient Two‐Dimensional Surface Electron Gas of Square SnO₂ Nanotubes

Abstract

Nanotechnology has delivered an amazing range of new materials such as nanowires, tubes, ribbons, belts, cages, flowers, and sheets. However, these are usually circular, cylindrical, or hexagonal in nature, while nanostructures with square geometries are comparatively rare. Here, a highly scalable method is reported for producing vertically aligned Sb‐doped SnO2 nanotubes with perfectly‐square geometries on Au nanoparticle covered m‐plane sapphire using mist chemical vapor deposition. Their inclination can be varied using r‐ and a‐plane sapphire, while unaligned square nanotubes of the same high structural quality can be grown on silicon and quartz. X‐ray diffraction measurements and transmission electron microscopy show that they adopt the rutile structure growing in the [001] direction with (110) sidewalls, while synchrotron X‐ray photoelectron spectroscopy reveals the presence of an unusually strong and thermally resilient 2D surface electron gas. This is created by donor‐like states produced by the hydroxylation of the surface and is sustained at temperatures above 400 °C by the formation of in‐plane oxygen vacancies. This persistent high surface electron density is expected to prove useful in gas sensing and catalytic applications of these remarkable structures. To illustrate their device potential, square SnO2 nanotube Schottky diodes and field effect transistors with excellent performance characteristics are fabricated.

Ultrathin Boundary-Less SnO2 Films with Surface-Activated Two-Dimensional Nanograins Enable Fast and Sensitive Hydrogen Gas Sensing

“…As has been reported, the root hydroxyl groups (OH R ) on the surface of the SnO 2 can serve as the electron donor by either direct ionization or the evolving binding to a neighboring Sn atom to produce oxygen vacancies. 22,23 A 2D electron accumulation layer will therefore be formed near the surface region of the SnO 2 ultrathin layers. Consequently, a decrease in the OH surface content (confirmed by XPS) will lead to a reduction in the electron density and result in the decrease of the Fermi level (E F ) in SnO 2 .…”

“…18−21 The OH groups will dominate the surface electron states and hinder the gas−solid interaction, leading to unsatisfied gas sensing performance. 22,23 Besides that, the majority of the nanocrystalline 2D oxides consist of numerous nanograins with in-plane grain sizes down to a few nanometers. 24,25 The 2D electrical transportation behaviors are dominated by grain boundaries, which lead to an inefficient signal transduction from the surface chemical process to the bulk conductivity.…”

“…Moreover, it has been revealed that the oxygen species can be more easily adsorbed on the metastable (101) facets of rutile SnO 2 rather than the most stable (110) and other low-index facets. , In principle, the highly reactive dangling bonds on the (101)-dominated surfaces together with the 2D electrical transportation behavior can provide better gas–solid reactivity and high-performance sensor response toward the target gases, making them one of the most promising candidates for developing high-performance semiconductor gas sensors. However, the defective (101) surfaces of the ultrathin SnO 2 can be steadily passivated by the undesirable surface adsorbents (mostly the OH groups) under ambient conditions. − The OH groups will dominate the surface electron states and hinder the gas–solid interaction, leading to unsatisfied gas sensing performance. , Besides that, the majority of the nanocrystalline 2D oxides consist of numerous nanograins with in-plane grain sizes down to a few nanometers. , The 2D electrical transportation behaviors are dominated by grain boundaries, which lead to an inefficient signal transduction from the surface chemical process to the bulk conductivity. Various new studies have been reported in the literature to enhance the gas reception and transduction performance of the SnO 2 -based ultrathin films, e.g., by doping, noble metal decoration, and UV illumination .…”