Microwave-Epoxide-Assisted Hydrothermal Synthesis of the CuO/ZnO Heterojunction: a Highly Versatile Route to Develop H2S Gas Sensors

Biogenic synthesis of CuO/ZnO nanocomposite from Bauhinia variegate flower extract for highly sensitive electrochemical detection of vitamin B2

In this study, we report the preparation of bio-inspired binary CuO/ZnO nanocomposite (bb-CuO/ZnO nanocomposite) via the biological route using Bauhinia variegata flower extract following hydrothermal treatment. The prepared bb-CuO/ZnO nanocomposite was electrophoretically deposited (EPD) on indium tin oxide (ITO) substrate to develop bb-CuO/ZnO/ITO biosensing electrode which is employed for the determination of vitamin B2 (Riboflavin) through electrochemical techniques. Physicochemical assets of the prepared bb-CuO/ZnO nanocomposite have been extensively evaluated and make use of different characterization techniques including powder XRD, FT-IR, AFM, SEM, TEM, EDX, XPS, Raman, and TGA. Electrochemical characteristics of the bb-CuO/ZnO/ITO biosensing electrode have been studied towards vitamin B2 determination. Furthermore, different biosensing parameters such as response time, reusability, stability, interference, and real sample analysis were also estimated. From the linear plot of scan rate, charge transfer rate constant (Ks), surface concentration of electrode (γ), and diffusion coefficient (D) have been calculated, and these are found to be 6.56 × 10-1 s-1, 1.21 × 10-7 mol cm-2, and 6.99 × 10-3 cm2 s-1, respectively. This biosensor exhibits the linear range of vitamin B2 detection from 1 to 40 μM, including sensitivity and limit of detection (LOD) of 1.37 × 10-3 mA/μM cm2 and 0.254 μM, respectively. For higher concentration range detection linearity is 50-100 μM, with sensitivity and the LOD of 1.26 × 10-3 mA/μM cm2 and 0.145 μM, respectively. The results indicate that the bio-inspired nanomaterials are promising sustainable biosensing platforms for various food and health-based biosensing devices.

Recent Advances on Metal Oxide Based Sensors for Environmental Gas Pollutants Detection

Optimizing materials and associated structures for detecting various environmental gas pollutant concentrations has been a major challenge in environmental sensing technology. Semiconducting metal oxides (SMOs) fabricated at the nanoscale are a class of sensor technology in which metallic species are functionalized with various dopants to modify their chemiresistivity and crystalline scaffolding properties. Studies focused on recent advances of gas sensors utilizing metal oxide nanostructures with a special emphasis on the structure-surface property relationships of some typical n-type and p-type SMOs for efficient gas detection are presented. Strategies to enhance the gas sensor performances are also discussed. These oxide material sensors have several advantages such as ease of handling, portability, and doped-based SMO sensing detection ability of environmental gas pollutants at low temperatures. SMO sensors have displayed excellent sensitivity, selectivity, and robustness. In addition, the hybrid SMO sensors showed exceptional selectivity to some CWAs when irradiated with visible light while also displaying high reversibility and humidity independence. Results showed that TiO2 surfaces can sense 50 ppm SO2 in the presence of UV light and under operating temperatures of 298-473 K. Hybrid SMO displayed excellent gas sensing response. For example, a CuO-ZnO nanoparticle network of a 4:1 vol.% CuO/ZnO ratio exhibited responses three times greater than pure CuO sensors and six times greater than pure ZnO sensors toward H2S. This review provides a critical discussion of modified gas pollutant sensing capabilities of metal oxide nanoparticles under ambient conditions, focusing on reported results during the past two decades on gas pollutants sensing.

Synthesis and characterization of ZrO2-ZnO heterojunction composite for isopropanol detection

We prepared ZrO2-ZnO heterojunction composites by a simple hydrothermal method as materials sensitive to isopropanol gas. The 5% ZrO2-ZnO sample presented a uniform rod-like structure. The optimum operating temperature, sensitivity and response/recovery times were measured to investigate the response of ZrO2-ZnO composites to isopropanol. The sensor based on 5% ZrO2-ZnO composites at an optimum temperature of 260 °C had a response to 100 ppm isopropanol of up to 172.46, which was about 3.6 times higher than that of pure ZnO. The sensor also exhibited fast response and recovery times of 5 s and 11 s, respectively. The gas-sensitive properties can be attributed to the rod-like structure, heterojunction structure and catalytic activity of ZrO2. These results would contribute in expanding the application of ZrO2 in gas sensors.

Bismuth ferrite based acetone gas sensor: evaluation of graphene oxide loading

We report a BiFeO3/graphene oxide (BFO/GO) perovskite, synthesized using a CTAB-functionalized glycine combustion route, as a potential material for acetone gas sensing applications. The physicochemical properties of the developed perovskite were analysed using XRD, FE-SEM, TEM, HRTEM, EDAX and XPS. The gas sensing performance was analysed for various test gases, including ethanol, acetone, propanol, ammonia, nitric acid, hydrogen sulphide and trimethylamine at a concentration of 500 ppm. Among the test gases, the developed BFO showed the best selectivity towards acetone, with a response of 61% at an operating temperature of 250 °C. All the GO-loaded BFO samples showed an improved gas sensing performance compared with pristine BFO in terms of sensitivity, the response/recovery times, the transient response curves and the stability. The 1 wt% GO-loaded BiFeO3 sensor showed the highest sensitivity of 89% towards acetone (500 ppm) at an operating temperature of 250 °C. These results show that the developed perovskites have significant potential for use in acetone gas sensing applications.

Cu/CuO-Doped ZnO Nanocomposites via Solution Combustion Synthesis for Catalytic 4-Nitrophenol Reduction

The synthesis of optoelectrically enhanced nanomaterials should be continuously improved by employing time- and energy-saving techniques. The porous zinc oxide (ZnO) and copper-doped ZnO nanocomposites (NCs) were synthesized by the time- and energy-efficient solution combustion synthesis (SCS) approach. In this SCS approach, once the precursor-surfactant complex ignition point is reached, the reaction starts and ends within a short time without the need for any external energy. The TGA-DTA analysis confirmed that 500 °C was the point at which stable metal oxide was obtained. The doping and heterojunction strategy improved the optoelectric properties of the NCs more than the individual constituents, which then enhanced the materials' charge transfer and optical absorption capabilities. The porosity, nanoscale crystallite size (15-50 nm), and formation of Cu/CuO-ZnO NCs materials were confirmed from the XRD, SEM, and TEM/HRTEM analyses. The obtained d-spacing values of 0.275 and 0.234 nm confirm the formation of ZnO and CuO crystals, respectively. The decrease in photoluminescence intensity for the doped NCs corroborates a reduction in electron-hole recombination. On the Mott-Schottky analysis, the positive slope for ZnO confirms the n-type character, while the negative and positive slopes of the NCs confirm the p- and n-type characters, respectively. A diffusion-controlled type of charge transfer process on the electrode surface was confirmed from the cyclic voltammetric analysis. Thus, the overall analysis shows the applicability of the less expensive and more efficient SCS for several applications, such as catalysis and sensors. To confirm this, an organic catalytic reduction reaction of 4-nitrophenol to 4-aminophenol was tested. Within three and a half minutes, the catalytic reduction result showed the great potential of NCs over ZnO NPs. Thus, the energy- and time-saving SCS approach has a great future outlook as an industrial pollutant catalytic reduction application.

Synthesis and H2S-Sensing Properties of MOF-Derived Cu-Doped ZnO Nanocages

Metal–organic framework (MOF)-derived pure ZnO and Cu-doped ZnO nanocages were fabricated by calcining a zeolitic imidazole framework (ZIF-8) and Cu-doped ZIF-8. The morphology and crystal structure of the samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). It was found that Cu doping did not change the crystal structures and morphologies of MOF-derived ZnO nanocages. The H2S-sensing properties of the sensors based on ZnO and Cu-doped ZnO nanocages were investigated. The results indicated that the H2S-sensing properties of MOF-derived ZnO nanocages were effectively improved by Cu doping, and the optimal doping content was 3 at%. Moreover, 3 at% Cu-doped ZnO nanocages showed the highest response of 4733 for 5 ppm H2S at 200 °C, and the detection limit could be as low as 20 ppb. The gas-sensing mechanism was also discussed.

A review of the microwave-assisted synthesis of carbon nanomaterials, metal oxides/hydroxides and their composites for energy storage applications

Currently, nanomaterials are considered to be the backbone of modern civilization. Especially in the energy sector, nanomaterials (mainly, carbon- and metal oxide/hydroxide-based nanomaterials) have contributed significantly. Among the various green approaches for the synthesis of these nanomaterials, the microwave-assisted approach has attracted significant research interest worldwide. In this context, it is noteworthy to mention that because of their enhanced surface area, high conducting nature, and excellent electrical and electrochemical properties, carbon nanomaterials are being extensively utilized as efficient electrode materials for both supercapacitors and secondary batteries. In this review article, we briefly demonstrate the characteristics of microwave-synthesized nanomaterials for next-generation energy storage devices. Starting with the basics of microwave heating, herein, we illustrate the past and present status of microwave chemistry for energy-related applications, and finally present a brief outlook and concluding remarks. We hope that this review article will positively convey new insights for the microwave synthesis of nanomaterials for energy storage applications.

CuxO Nanostructure-Based Gas Sensors for H2S Detection: An Overview

H2S gas is a toxic and hazardous byproduct of the oil and gas industries. It paralyzes the olfactory nerves, with concentrations above 100 ppm, resulting in loss of smell; prolonged inhalation may even cause death. One of the most important semiconducting metal oxides for the detection of H2S is CuxO (x = 1, 2), which is converted to CuxS upon exposure to H2S, leading to a remarkable modulation in the resistance and appearance of an electrical sensing signal. In this review, various morphologies of CuxO in the pristine form, composites of CuxO with other materials, and decoration/doping of noble metals on CuxO nanostructures for the reliable detection of H2S gas are thoroughly discussed. With an emphasis to the detection mechanism of CuxO-based gas sensors, this review presents findings that are of considerable value as a reference.

RGO/WO3 hierarchical architectures for improved H2S sensing and highly efficient solar-driving photo-degradation of RhB dye

Surface area and surface active sites are two important key parameters in enhancing the gas sensing as well as photocatalytic properties of the parent material. With this motivation, herein, we report a facile synthesis of Reduced Graphene Oxide/Tungsten Oxide RGO/WO3 hierarchical nanostructures via simple hydrothermal route, and their validation in accomplishment of improved H2S sensing and highly efficient solar driven photo-degradation of RhB Dye. The self-made RGO using modified Hummer's method, is utilized to develop the RGO/WO3 nanocomposites with 0.15, 0.3 and 0.5 wt% of RGO in WO3 matrix. As-developed nanocomposites were analyzed using various physicochemical techniques such as XRD, FE-SEM, TEM/HRTEM, and EDAX. The creation of hierarchic marigold frameworks culminated in a well affiliated mesoporous system, offering efficient gas delivery networks, leading to a significant increase in sensing response to H2S. The optimized sensor (RGO/WO3 with 0.3 wt% loading) exhibited selective response towards H2S, which is ~ 13 times higher (Ra/Rg = 22.9) than pristine WO3 (Ra/Rg = 1.78) sensor. Looking at bi-directional application, graphene platform boosted the photocatalytic activity (94% degradation of Rhodamine B dye in 210 min) under natural sunlight. The RGO's role in increasing the active surface and surface area is clarified by the H2S gas response analysis and solar-driven photo-degradation of RhB dye solution. The outcome of this study provides the new insights to RGO/WO3 based nanocomposites' research spreadsheet, in view of multidisciplinary applications.

Design, Development and Validation of a Portable Gas Sensor Module: A Facile Approach for Monitoring Greenhouse Gases

We report the unique design and prototype of a portable gas sensor module for monitoring greenhouse gases. The commercially available gas sensors (MQ-02, MQ-135, and TGS2602) were adopted in designing the module using Arduino Uno. Different locations in the city of Solapur, India (17.6599° N, 75.9064° E), were scanned for the usability of the developed prototype of the mobile gas sensor module. The choice of gas sensors in combination with Arduino Uno led to an excellent prototype for measuring the concentration of greenhouse gases, and therefore the wrong alarm for toxic gases. The prototype model and corresponding greenhouse gas concentrations (ppm) are described using an interplay of sensor design, software program, and greenhouse gases sites.