Low-Dimensional Nanostructures Based on Cobalt Oxide (Co3O4) in Chemical-Gas Sensing

Highly sensitive, stable, low production costs, together with easy maintenance and portability, sensors are ever most demanded nowadays for monitoring and quantification of hazardous chemicals/gases in the environment. The utilization of one dimensional (1D) metal oxide nano structured chemical/gas sensors for environmental monitoring is vastly investigated because of their superior surface to volume ratio, stability, and low production costs, to provide information on the presence of chemical species. Several outstanding attempts have been pursued investigating 1D nano structures of Co3O4 over the past decades as an active material for chemical analytes detection owing to its superior catalytic effect together with its excellent stability. This article reviews the state-of-the-art of growth and characterization of Co3O4 1D nano structures and their functional characterization as chemical/gas sensors. Moreover, fundamental concepts and characteristic features, that enhance the key performances of chemical/gas sensors, are discussed. Finally, challenges and prospective for growth and fabrication of 1D Co3O4 chemical/gas sensors are discussed

Conductometric Gas Sensors

The main focus of this article is to describe the basic but important perspectives of gas sensors, especially, conductometry metal oxides (MOXs). Fundamental functions, drawbacks, and factors influencing the sensor’s performances are explained. Furthermore, the enrollment of nanostructured materials and novel technology to enhance the sensing performance is also discussed. MOXs gas sensing mechanism is described to provide guidelines and knowledge to improve the sensing performances. Moreover other gas sensing devices are briefly discussed to have a clear overall picture of gas sensors

P-Type Metal Oxide Semiconductor Thin Films: Synthesis and Chemical Sensor Applications

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

Recent Advancements in TiO₂ Nanostructures: Sustainable Synthesis and Gas Sensing

The search for sustainable technology-driven advancements in material synthesis is a new norm, which ensures a low impact on the environment, production cost, and workers’ health. In this context, non-toxic, non-hazardous, and low-cost materials and their synthesis methods are integrated to compete with existing physical and chemical methods. From this perspective, titanium oxide (TiO2) is one of the fascinating materials because of its non-toxicity, biocompatibility, and potential of growing by sustainable methods. Accordingly, TiO2 is extensively used in gas-sensing devices. Yet, many TiO2 nanostructures are still synthesized with a lack of mindfulness of environmental impact and sustainable methods, which results in a serious burden on practical commercialization. This review provides a general outline of the advantages and disadvantages of conventional and sustainable methods of TiO2 preparation. Additionally, a detailed discussion on sustainable growth methods for green synthesis is included. Furthermore, gas-sensing applications and approaches to improve the key functionality of sensors, including response time, recovery time, repeatability, and stability, are discussed in detail in the latter parts of the review. At the end, a concluding discussion is included to provide guidelines for the selection of sustainable synthesis methods and techniques to improve the gas-sensing properties of TiO2

Enhancement of Photo-Electrical Properties of CdS Thin Films: Effect of N₂ Purging and N₂ Annealing

The impact of N2 purging in the CdS deposition bath and subsequent N2 annealing is examined and contrasted with conventional CdS films, which were deposited without purging and annealed in ambient air. All films were fabricated using the chemical bath deposition method at a temperature of 80 °C on fluorine-doped tin oxide glass slides (FTO). N2 purged films were deposited by introducing nitrogen gas into the deposition bath throughout the CdS deposition process. Subsequently, both N2 purged and un-purged films underwent annealing at temperatures ranging from 100 to 500 °C for one hour, either in a nitrogen or ambient air environment. Photoelectrochemical (PEC) cell studies reveal that films subjected to both N2 purging and N2 annealing exhibit a notable enhancement of 37.5% and 27% in ISC (short-circuit current) and VOC (open-circuit voltage) values, accompanied by a 5% improvement in optical transmittance compared to conventional CdS thin films. The films annealed at 300 °C demonstrate the highest ISC, VOC, and VFB values, 55 μA, 0.475 V, and −675 mV, respectively. The improved optoelectrical properties in both N2-purged and N2-annealed films are attributed to their well-packed structure, enhanced interconnectivity, and a higher sulfur to cadmium ratio of 0.76 in the films