Protonic Conduction Induced Selective Room Temperature Hydrogen Response in ZnO/NiO Heterojunction Surfaces

TL;DR

This study demonstrates that surface ionic conduction through chemisorbed moisture enables highly selective room temperature hydrogen sensing in ZnO/NiO heterojunctions, with enhanced response due to heterostructure effects.

Contribution

It reveals that surface ionic conduction and heterojunction barriers in ZnO/NiO structures lead to high, selective hydrogen response at room temperature, a novel insight.

Findings

Response increases from 10% to 190% for 1200 ppm H2

Maximum response at 50-50% NiO/ZnO composition

Surface ionic conduction is primary for high sensitivity

Abstract

In this paper we show that the ionic conduction through surface chemisorbed ambient moisture leads to the remarkably high room temperature selective response towards hydrogen gas. The surface adsorbed moisture acts as surface states and shows ionic conduction, as a result of smaller size of ZnO nanoparticles of 20 +/- 5 nm. This response is enhanced remarkably i.e. from 10% to 190% for 1200 ppm H2 gas when p-type NiO quasi-nanowires (width ~50 nm) are mixed with these n-type ZnO nanoparticles to form a homogenous NiO/ZnO nano-bulk p-n heterostructure. The maximum response is obtained for about 50-50 % composition of NiO/ZnO although it is of still n-type character. The dominant carrier type reversal from n to p type takes place at rather high NiO content of about 60-80% in ZnO, depicting dominating contribution of ZnO into the response. The parallel surface ionic current through chemisorbed moisture (surface states) has been identified as a primary factor for high sensitivity at room temperature. Thus, the presence of heterojunction barrier at the NiO-ZnO interface assisted with the surface ionic conductivity due to adsorbed moisture results in large, selective response to hydrogen at room temperature.