Abstract
This review focuses on the synthesis of p-type metal-oxide (p-type MOX) semiconductor thin films, such as CuO, NiO, Co3O4, and Cr2O3, used for chemical-sensing applications. P-type MOX thin films exhibit several advantages over n-type MOX, including a higher catalytic effect, low humidity dependence, and improved recovery speed. However, the sensing performance of CuO, NiO, Co3O4, and Cr2O3 thin films is strongly related to the intrinsic physicochemical properties of the material and the thickness of these MOX thin films. The latter is heavily dependent on synthesis techniques. Many techniques used for growing p-MOX thin films are reviewed herein. Physical vapor-deposition techniques (PVD), such as magnetron sputtering, thermal evaporation, thermal oxidation, and molecular-beam epitaxial (MBE) growth were investigated, along with chemical vapor deposition (CVD). Liquid-phase routes, including sol–gel-assisted dip-and-spin coating, spray pyrolysis, and electrodeposition, are also discussed. A review of each technique, as well as factors that affect the physicochemical properties of p-type MOX thin films, such as morphology, crystallinity, defects, and grain size, is presented. The sensing mechanism describing the surface reaction of gases with MOX is also discussed. The sensing characteristics of CuO, NiO, Co3O4, and Cr2O3 thin films, including their response, sensor kinetics, stability, selectivity, and repeatability are reviewed. Different chemical compounds, including reducing gases (such as volatile organic compounds (VOCs), H2, and NH3) and oxidizing gases, such as CO2, NO2, and O3, were analyzed. Bulk doping, surface decoration, and heterostructures are some of the strategies for improving the sensing capabilities of the suggested pristine p-type MOX thin films. Future trends to overcome the challenges of p-type MOX thin-film chemical sensors are also presented.
Highlights
Publisher’s Note: MDPI stays neutralIn recent years, emergency scenarios have led to increased demand for high-performance chemical gas sensors due to industrialization and population growth [1]
This review investigates several methods for growing p-type MOX, including magnetron sputtering, thermal oxidation, thermal evaporation, molecular-beam-epitaxy (MBE), chemical vapor deposition (CVD), sol–gel-assisted by dip-and-spin coating, spray pyrolysis, and electrodeposition
Chemisorbed oxygen species cause electron trapping from the MOX valence band, which leads to a hole-accumulating layer (HAL) near the semiconductor surface [14]
Summary
Emergency scenarios have led to increased demand for high-performance chemical gas sensors due to industrialization and population growth [1]. P-type MOX, such as CuO, NiO, Cr3 O4 , and Co3 O4 exhibit important advantages, such as low humidity dependency, high chemical stability, and excellent catalytic properties, which make them ideal catalysts for improving the performance of conductometric gas sensors. The sensors have demonstrated detection capability at room temperature (RT), opening up the possibility of portable applications for p-type MOX gas sensors [8,10] Due to their advantages, p-type MOX sensors using thick film materials are available on the market [11]. As a result of these properties, thin films are ideal for a variety of applications, including optoelectronic devices, catalyst nanomedicine, and, in particular, chemical sensors. Many reducing gases are considered, such as volatile organic compounds (VOCs) (such as acetone and ethanol), hydrogen, and NH3 , as well as oxidizing gases, such as NO2 , CO2 , and O3
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