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In this work, we report on the synthesis of four morphologies of ZnO, namely, nanoparticles, nanorods, nanosheets, and nanoflowers, from a single precursor Zn(CH3COO)2·2H2O under different reaction conditions. The synthesised nanostructured materials were characterised using powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy, UV-Vis, XPS analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and nitrogen sorption at 77 K. The XRD, FTIR, and Raman analyses did not reveal any significant differences among the nanostructures, but differences in the electronic properties were noted among the different morphologies. The TEM and SEM analyses confirmed the four different morphologies of the ZnO nanostructures. The textural characteristics revealed that the specific surface areas were different, being 1.3, 6.7, 12.7, and 26.8 m2/g for the nanoflowers, nanoparticles, nanorods, and nanosheets, respectively. The ZnO nanostructures were then mixed with carbon nanoparticles (CNPs) and cellulose acetate (CA) to make nanocomposites that were then used as sensing materials in solid-state sensors to detect methanol, ethanol, and isopropanol vapour at room temperature. The sensors' responses were recorded in relative resistance. When detecting methanol, 6 out of 12 sensors were responsive, and the most sensitive sensor was the composite with a mass ratio of 1:1:1 of ZnO nanorods:CNPs:CA with a sensitivity of 0.7740 Ω ppm-1. Regarding the detection of ethanol vapour, 9 of the 12 sensors were responsive, and the 3:1:1 mass ratio with ZnO nanoparticles was the most sensitive at 4.3204 Ω ppm-1. Meanwhile, with isopropanol, 5 out of the 12 sensors were active and, with a sensitivity of 3.4539 Ω ppm-1, the ZnO nanoparticles in a 3:1:1 mass ratio were the most sensitive. Overall, the response of the sensors depended on the morphology of the nanostructured ZnO materials, the mass ratio of the sensing materials in the composites, and the type of analyte. The sensing mechanism was governed by the surface reaction on the sensing materials rather than pores hindering the analyte molecules from reaching the active site, since the pore size is larger than the kinetic diameter of the analyte molecules. Generally, the sensors responded well to the ethanol analyte, rather than methanol and isopropanol. This is due to ethanol molecules displaying a more enhanced electron-donating ability.

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Identifying disease biomarkers and detecting hazardous, explosive, flammable, and polluting gases and chemicals with extremely sensitive and selective sensor devices remains a challenging and time-consuming research challenge. Due to their exceptional characteristics, semiconducting metal oxides (SMOxs) have received a lot of attention in terms of the development of various types of sensors in recent years. The key performance indicators of SMOx-based sensors are their sensitivity, selectivity, recovery time, and steady response over time. SMOx-based sensors are discussed in this review based on their different properties. Surface properties of the functional material, such as its (nano)structure, morphology, and crystallinity, greatly influence sensor performance. A few examples of the complicated and poorly understood processes involved in SMOx sensing systems are adsorption and chemisorption, charge transfers, and oxygen migration. The future prospects of SMOx-based gas sensors, chemical sensors, and biological sensors are also discussed.

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Glutaraldehyde disinfectant has been widely applied in aquaculture, farming, and medical treatment. Excessive concentrations of glutaraldehyde in the environment can lead to serious health hazards. Therefore, it is extremely important to develop high-performance glutaraldehyde sensors with low cost, high sensitivity, rapid response, fabulous selectivity, and low limit of detection. Herein, mesoporous lanthanum (La) doped SnO2 spheres with high specific surface area (52-59 m2 g-1), uniform mesopores (with a pore size concentrated at 5.7 nm), and highly crystalline frameworks are designed to fabricate highly sensitive gas sensors toward gaseous glutaraldehyde. The mesoporous lanthanum-doped SnO2 spheres exhibit excellent glutaraldehyde-sensing performance, including high response (13.5@10 ppm), rapid response time (28 s), and extremely low detection limit of 0.16 ppm. The excellent sensing performance is ascribed to the high specific surface area, high contents of chemisorbed oxygen species, and lanthanum doping. DFT calculations suggest that lanthanum doping in the SnO2 lattice can effectively improve the adsorption energy toward glutaraldehyde compared to pure SnO2 materials. Moreover, the fabricated gas sensors can effectively detect commercial glutaraldehyde disinfectants, indicating a potential application in aquaculture, farming, and medical treatment.

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The utilization of printing techniques for the development of high-performance humidity sensors holds immense significance for various applications in the fields of the Internet of Things, agriculture, human healthcare, and storage environments. However, the long response time and low sensitivity of current printed humidity sensors limit their practical applications. Herein, a series of high-sensing-performance flexible resistive-type humidity sensors is fabricated by the screen-printing method, and hexagonal tungsten oxide (h-WO3 ) is employed as the humidity-sensing material due to its low cost, strong chemical adsorption ability, and excellent humidity-sensing ability. The as-prepared printed sensors exhibit high sensitivity, good repeatability, outstanding flexibility, low hysteresis, and fast response (1.5 s) in a wide relative humidity (RH) range (11-95% RH). Furthermore, the sensitivity of humidity sensors can be easily adjusted by altering the manufacturing parameters of the sensing layer and interdigital electrode to meet the diverse requirements of specific applications. The printed flexible humidity sensors possess immense potential in various applications, including wearable devices, non-contact measurements, and packaging opening state monitoring.

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Liquefied petroleum gas (LPG), which is mainly composed of hydrocarbons, such as propane and butane, is a flammable gas that is considered a clean source of energy. Currently, the overwhelming use of LPG as fuel in vehicles, domestic settings, and industry has led to several incidents and deaths globally due to leakage. As a result, the appropriate detection of LPG is vital; thus, gas-sensing devices that can monitor this gas rapidly and accurately at room temperature, are required. This work reviews the current advances in LPG gas sensors, which operate at room temperature. The influences of the synthesis methods and parameters, doping, and use of catalysts on the sensing performance are discussed. The formation of heterostructures made from semiconducting metal oxides, polymers, and graphene-based materials, which enhance the sensor selectivity and sensitivity, is also discussed. The future trends and challenges confronted in the advancement of LPG room temperature operational gas sensors, and critical ideas concerning the future evolution of LPG gas sensors, are deliberated. Additionally, the advancements in the next-generation gas sensors, such as the wireless detection of LPG leakage, self-powered sensors driven by triboelectric/piezoelectric mechanisms, and artificial intelligent systems are also reviewed. This review further focuses on the use of smartphones to circumvent the use of costly instruments and paves the way for cost-efficient and portable monitoring of LPG. Finally, the approach of utilizing the Internet of Things (IoT) to detect/monitor the leakage of LPG has also been discussed, which will provide better alerts to the users and thus minimize the effects of leakages.

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Benzene is a potentially carcinogenic volatile organic compound (VOC) and its vapor must be strictly monitored in air. Metal-oxide semiconductors (MOS) functionalized by catalytic noble metals are promising materials for sensing VOC, but basic understanding of the relationships of materials composition and sensors behavior should be improved. In this work, the sensitivity to benzene was comparatively studied for nanocrystalline n-type MOS (ZnO, In2O3, SnO2, TiO2, and WO3) in pristine form and modified by catalytic PtOx nanoparticles. Active sites of materials were analyzed by X-ray photoelectron spectroscopy (XPS) and temperature-programmed techniques using probe molecules. The sensing mechanism was studied by in situ diffuse-reflectance infrared (DRIFT) spectroscopy. Distinct trends were observed in the sensitivity to benzene for pristine MOS and nanocomposites MOS/PtOx. The higher sensitivity of pristine SnO2, TiO2, and WO3 was observed. This was attributed to higher total concentrations of oxidation sites and acid sites favoring target molecules' adsorption and redox conversion at the surface of MOS. The sensitivity of PtOx-modified sensors increased with the surface acidity of MOS and were superior for WO3/PtOx. It was deduced that this was due to stabilization of reduced Pt sites which catalyze deep oxidation of benzene molecules to carbonyl species.

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Semiconductor metal oxide (SMO) gas sensors have attracted considerable attention for detecting environmental pollution, as well as the accidental leakage of flammable, explosive, and toxic gases. SMOs are known to exhibit high sensitivity, fast response time, and excellent selectivity towards various types of gases. Many new strategies have been implemented to improve these characteristics. Among the materials produced by these methods, nanomaterials (NMs) synthesized by electrospinning have unprecedented advantages, including catalyst introduction, morphological control, thermodynamic stability, unique physicochemical properties, composition adjustment, and rapid adsorption-desorption rates of the NMs, and are appealing for the designing highly sensitive and selective gas sensors. This review highlights the latest findings on the design and fabrication of electrospun gas sensors for detecting various gases including hydrogen (H2), methane (CH4), nitrogen monoxide (NO), hydrogen sulfide (H2S), ammonia (NH3), ethanol (C2H5OH), acetone (CH3COCH3), formaldehyde (HCHO) and toluene (C6H5CH3). Studies have indicated that NMs with different shapes (e.g., nanotubes, nanowires, nanoflowers, nanosheets, nanorods, nanofilms, and nanofibers) and compositions (single-phase SMOs, modified SMOs, nanocomposites of SMOs, and SMOs combined with carbon nanomaterials) display high response values, long-term stability, low humidity dependence, fast response/recovery times, and low detection limits for gases. Finally, conclusions and future perspectives for gas sensors based on the electrospinning technique are discussed.

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The increase in environmental pollution has led to an increased investigation in the development of novel ternary photocatalytic systems for remediation. These photocatalytic systems exhibit superior photocatalytic action for the removal of pollutants because of their visible light active bandgaps. A highly effective visible light active ternary heterojunction was fabricated using a hydrothermal method assisted by ultrasonication. Herein, we report the in situ hydrothermal synthesis of Mn-doped Bi2WO6-GO/ MoS2 photocatalyst, efficiently exhibiting greater photocatalytic activity for the wastewater treatment under solar light. The binary metal sulphide (MoS2) used as a co-catalyst, acted as an electron collector and graphene oxide (GO) as a support material for interfacial electron transfer to and from bismuth tungstate and MoS2. The as-prepared samples were characterized using SEM-EDX, FT-IR, XRD, XPS, BET, PL, and UV-Vis techniques. The bandgap of the novel photocatalyst was found in the visible region (2.2 eV) which helped in suppressing photoinduced electron-hole pairs recombination. The ternary Mn-doped Bi2WO6-GO/MoS2 showed 99% methylene blue removal after 60 minutes of sunlight irradiation at the optimum conditions of pH 8, catalyst dose 50 mg/100ml, and initial MB concentration of 10ppm under sunlight irradiation. The doped ternary heterostructure has proved to be an effective sunlight-active photocatalyst that can be reused without substantial loss in photocatalytic efficiency.

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Mnx - ZnO(1-x) nanopowders were successfully synthesised through a simple sol-gel method. The samples were annealed at 300 °C to enhance their crystallinity. The lattice structure, morphology and optical properties of the prepared powdered samples were extensively studied using different characterization techniques, confirming the formation of Mnx - ZnO(1-x). The inclusion of Mn did not cause any change to the wurtzite structure of ZnO; however slight peak shifting and increase in lattice parameters were indicated. The normal absorption spectra pointed to a cut-off edge extending beyond the UV region and a Burstein- Moss type band gap broadening induced by the Mn doping. ZnO showed excellent photodegradation activity against methylene blue (MB) upon UV irradiation. Intensifying the dopant concentration resulted in further diminution of photoactivity against MB. This reduction of photocatalytic activity of ZnO upon doping can be drawn to be due to the presence of Mn in the ZnO lattice, which acted as recombination sites for the photogenerated charge carriers. The results demonstrated that doping ZnO with Mn can be used to suppress the oxidative stress induced by reduced oxygen species (ROS) through generation of recombination centres. The suppression of toxic ROS generation implies possible application in fabrics and ointments for UV shielding applications.