## 2 Materials and Methods  
### Sample Preparation  
The following process steps were required to fabricate our novel ZnO nanorod/nanotube gas sensors. In this research, a 30 nm ZnO seed layer was deposited on p-type Si substrates by ALD system at a deposition temperature of 200 \({}^{\circ}\)C. The two ALD precursors for ZnO seed  
Figure 1: Schematic 3D drawing illustrating the concept of the coaxial nested tube-in-tube nanostructure design after removal of the sacrificial Al2O3 film separating the outer nanotube from the center nanotube, thereby exposing two new ZnO gas sensing surfaces. layer were diethylzinc (Zn(C\({}_{2}\)H\({}_{5}\))\({}_{2}\)) (Sigma Aldrich, Rockville, MD, USA, CAS# 557-20-0) and deionized (DI) water, which were alternately introduced into the ALD reaction chamber by high-purity N\({}_{2}\) serving as a carrier gas. After ALD deposition, the ZnO samples were annealed at a temperature of 350 \({}^{\circ}\)C for 30 min in a thermal furnace. Subsequently, the hydrothermal solution crystal growth method was utilized for the initial ZnO nanorod growth on polycrystalline ALD ZnO seed layer, as shown in Figure 2a. The precursor solution for the ZnO nanorod hydrothermal growth was prepared with 0.03756 g zinc nitrate hexahydrate (Zn(NO\({}_{3}\))6H\({}_{2}\)O) (Alfa Aesar, Ward Hill, MA, USA, CAS# 10196-18-6) and 0.0176 g hexamethylenetetramine ((CH\({}_{2}\))\({}_{6}\)N\({}_{4}\)) (Sigma Aldrich, CAS# 100-97-0) dissolved in 60 mL DI water. This ratio ensured that each chemical precursor had an exactly equal molar concentration of 20.8 mM. Afterwards, vapor phase ALD technology was employed for conformal wrap-around coating of the anchoring ZnO nanorods, first with a 20 nm Al\({}_{2}\)O\({}_{3}\) sacrificial film followed by a 20 nm ZnO film using the ALD precursors trimethylamine (Al\({}_{2}\)(CH\({}_{3}\))\({}_{6}\)) (Sigma Aldrich, CAS# 75-24-1), diethylzinc (Zn(C\({}_{2}\)H\({}_{5}\))\({}_{2}\)) and DI water, as shown in Figure 2b,c. The sacrificial film serves as a \"gapfiller\" to separate the central hydrothermally grown anchoring ZnO nanorods from the surrounding coaxial nested nanotube film. Because of the surface saturating growth mechanism of ALD, all surfaces of the ZnO nanorods were completely covered with 20 nm Al\({}_{2}\)O\({}_{3}\) sacrificial films, followed by another 20 nm ZnO thin film coating. To expose the Al\({}_{2}\)O\({}_{3}\) sacrificial film from the top for wet chemical etch dissolution, a precision ion polishing system from Gatan Inc. was utilized to remove the surface ZnO/Al\({}_{2}\)O\({}_{3}\) ALD films which conformally coated the entire outer surface of the hydrothermal ZnO nanorods, as shown in Figure 2d. After removing the accumulated ALD films only from the top surface, the sacrificial Al\({}_{2}\)O\({}_{3}\) annular layer was selectively removed by immersion in an alkali sodium hydroxide solution, taking advantage of the different solubility range of Al\({}_{2}\)O\({}_{3}\) and ZnO, as shown in Figure 2e. Solid  
Figure 2: Schematic of process sequence fabricating coaxial nested ZnO nanorods starting with (**a**) single-crystal ZnO nanorods by hydrothermal solution growth, followed by (**b**) ZnO nanorods coated with ALD wrap-around Al\({}_{2}\)O\({}_{3}\) sacrificial film, (**c**) ZnO nanorods receiving an additional coaxial wrap-around ALD ZnO film outer coating, (**d**) surface ALD film accumulation removed by PIPS and (**e**) final structure of coaxial nested wrap-around ZnO film nanostructure following the selective removal of the sacrificial film by NaOH dissolution, thereby creating the empty annular gap to expose additional sensing surfaces of the ZnO film. alumina can exist in solutions with pH values between 4.2 and 9.8, which means that the solubility of Al\({}_{2}\)O\({}_{3}\) can be considered as below pH 4.2 and above pH 9.8 [10]. However, crystalized ZnO can only exist in a solution with a pH from 9.2 to 11.5 [10].