# Metal-Oxide-Semiconductor Nanostructure/NiO Microparticle Heterojunctions Formed on Metallic Foils for Sensitive Chemiresistive Ion Sensors

## TL;DR

This paper introduces a new type of ion sensor using metal-oxide-semiconductor heterojunctions that can detect ions in solutions with high sensitivity.

## Contribution

The study introduces p–p and p–n heterojunctions on metallic foils to enhance the sensitivity of MOS-based ion sensors.

## Key findings

- ZnO sensors with high-number density NiO microparticles showed a detection sensitivity of 639 for chloride ions at 1 ppm.
- The use of p–p and p–n heterojunctions lowered activation energies for ion detection.
- NiO microparticles improved the sensitivity of detecting chloride, sodium, and calcium ions in solutions.

## Abstract

Chemiresistive metal-oxide-semiconductor
(MOS) sensors for routine
health and environmental monitoring are required to detect ion species
in solutions simply and highly sensitively. In this study, p–p and p–n heterojunctions, formed by using facile processing, were
introduced to enhance the detection sensitivity of MOS-based ion sensors.
NiO microparticle-bearing CuO

x

-, ZnO-, and SnO

x

-based ion sensors were thermally and hydrothermally fabricated on
metallic foils. On the MOS nanostructures, NiO microparticles affected
the sensitivity of chloride, sodium, and calcium ion detection in
solutions. The ZnO sensor with high-number density NiO microparticles
demonstrated an excellent detection sensitivity of 639 at 30 °C
in a 1 ppm chloride ion solution. These results suggest that bulk
and surface reactions between the MOS and ionic solutions were enhanced
by p–p and p–n heterojunctions, lowering activation energies.
MOS/NiO p–p and p–n heterojunctions fabricated using the proposed
process on metallic foils could contribute to the practical application
of chemiresistive MOS sensors for detecting ionic solutions.