Functionalized ZnO/ZnO₂ n–N straddling heterostructure achieved by oxygen plasma bombardment for highly selective methane sensing

Abstract

Metal oxide semiconductors have been extensively used as reducing gas sensors with major limitations regarding selectivity and operating temperature which is relatively high for most of the cases making the device unusable in some critical situations. Higher operating temperature is also associated with the higher power consumption, which goes against the miniaturization of the device. In order to resolve these problems, here we introduced a ZnO/ZnO2 straddling 'n-N' isotype heterostructure as a highly selective and sensitive methane sensor at moderately low operating temperature. ZnO-Zn(OH)2 precursor films were treated in oxygen plasma in a pulsed DC magnetron sputtering system. Morphological analyses by field emission scanning electron microscopy showed flake like growth of the grains with high surface roughness, whereas X-ray diffraction (XRD) showed polycrystalline nature of the films. Polycrystalline ZnO2 peaks were observed in the XRD pattern in addition to the existing ZnO, which indicates modification of the precursor to oxygen rich heterostructure of ZnO/ZnO2. This was further supported by the shifting of the O1s peak in the X-ray photoelectron spectroscopic analysis. Plasma treated ZnO/ZnO2 heterostructured films were found to show high selectivity towards methane (with respect to H2S and CO) and sensitivity (∼96%) at a comparatively low operating temperature.

ZnO/γ-Fe2O3 Charge Transfer Interface toward Highly Selective H2S Sensing at a Low Operating Temperature of 30 °C

“…In this case, the band gap of n-type γ-Fe 2 O 3 ( E g = 2.1 eV) is smaller than that of n -ZnO ( E g = 3.2 eV). Again, the electron affinity of γ-Fe 2 O 3 (χ = 4.7 eV) is larger than that of ZnO (χ = 4.1 eV), which indicates that these two semiconductors can create an N–n straddling type heterojunction at the interface. , The energy band diagrams of ZnO/γ-Fe 2 O 3 N–n straddling heterojunction before and after exposure to different gases have been presented in the Scheme . The individual energy-band diagrams of ZnO and γ-Fe 2 O 3 are shown in Scheme a.…”

“…12 There are also several reports on gas sensing by composite materials like ZnO−CuO, ZnO−In 2 O 3 , SnO 2 −CuO, and ZnO−ZnO 2 , to name a few. 14,15 Wang et al 16 have reported CuO nanoparticle decorated porous ZnO based H 2 S sensor, but the major drawback was long sensing and recovery time. Park et al 17 have fabricated H 2 S sensors based on In 2 O 3 -core/ ZnO-shell nanorods with an operating temperature of 300 °C and the device was tested at a very low concentration of 10− 100 ppm.…”

High performance acetone sensor based on γ-Fe2O3/Al–ZnO nanocomposites

“…In the composites, Zn 2+ ions can be substituted with Fe 3+ ions, decreasing the donor concentration and then the electrical conductivity [46] as for γ-Fe 2 O 3 (33%)/Al-ZnO NC. However, because ZnO and γ-Fe 2 O 3 are n-type semiconductor with different energy bands, when the loading of γ-Fe 2 O 3 is enough high, it can be assumed that a n-n straddling heterostructure is formed [47,48]. In this situation, due to the different work functions of the two phases, free electrons can easily migrate from γ-Fe 2 O 3 to the Al-ZnO because of the potential difference at the heterojunction interface [49], leading to the enhanced electrical conductivity as for γ-Fe 2 O 3 (56%, 83%)/Al-ZnO NCs.…”

Surface modification and direct plasma amination of L605 CoCr alloys: on the optimization of the oxide layer for application in cardiovascular implants

“…12,13 In regards to ion implantation processes, such as plasma immersion ion implantation (PIII), these can produce changes both on the surface and in the internal structure, producing an amorphous oxide layer, due to the acceleration of ions into the metallic substrate. 11,[14][15][16] Finally, chemical treatments, for example electropolishing, allow for the removal of any surface contaminant and the passivation of the surface whilst obtaining a nano-smooth surface with a mirror like nish. 7,12,17 This study focuses on the surface preparation step of L605 alloys for their direct plasma amination.…”