ZnO nanomaterials have been extensively used as photocatalysts for the removal of pollutants in aqueous environments. This study explores the enhanced photocatalytic performance of porous ZnO coral-like nanoplates synthesized via a one-pot wet-chemical method and subsequent annealing treatment. Characterization through scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, photoluminescence (PL) spectroscopy, and Brunauer-Emmett-Teller (BET) measurements confirmed the nanoplates' porous structure, single-crystal structure, 100 nm thickness, and 80 nm pore size. These unique structural characteristics of the ZnO coral-like nanoplates enabled effective photodegradation of the organic dye rhodamine B (RhB) under visible light irradiation. Under simulated sunlight, the ZnO photocatalyst exhibited exceptional performance, achieving a 97.3% removal rate of RhB after 210 minutes of irradiation. The prepared ZnO photocatalyst also showed remarkable photostability and regeneration capability for RhB photodegradation with a decreased efficiency of less than 15% after eight testing cycles. The potential mechanism of the ZnO photocatalyst toward RhB degradation was also studied and is discussed in detail.
Acetone is a well-known volatile organic compound that is widely used in different industrial and domestic areas, but it can cause dangerous effects on human health. Thus, the fabrication of highly sensitive and selective sensors for recognition of acetone is incredibly important. Here, we prepared the SnO2/Pd-NiO (SPN) nanowires-based gas sensor for the detection of acetone, in which, the amount of Pd nanoparticles were varied to enhance the performance of the devices. We demonstrated that the acetone gas sensing performance of the SPN device was significantly enhanced, showing increases of 3.72 and 6.53 folds compared to pristine SnO2 and NiO sensors, respectively. The Pd-NiO 0.01% wt Pd SPN sensor (SPN-1) exhibited an excellent response (Ra/Rg = 14.88) toward 500 ppm acetone gas. The SPN-1 sensor also showed a fast gas response time of 11/150 seconds with 500 ppm Acetone at 450 °C, while the recovery time was 468/526 seconds. Additionally, the sensor showed good selectivity toward acetone over other reducing gases, such as NH3, CH4, and VOCs. With those results, the SPN-1 sensor shows superiority compared to sensors based on pure materials.
The real-time monitoring of food freshness in refrigerators is of significant importance in detecting potential food spoiling and preventing serious health issues. One method that is commonly reported and has received substantial attention is the discrimination of food freshness via the tracking of volatile molecules. Nevertheless, the ambient environment of low temperature (normally below 4 °C) and high humidity (90% R.H.), as well as poor selectivity in sensing gas species remain the challenge. In this research, an integrated smart gas-tracking device is designed and fabricated. By applying pump voltage on the yttria-stabilized zirconia (YSZ) membrane, the oxygen concentration in the testing chamber can be manually tailored. Due to the working principle of the sensor following the mixed potential behavior, distinct differences in sensitivity and selectivity are observed for the sensor that operated at different oxygen concentrations. Typically, the sensor gives satisfactory selectivity to H2S, NH3, and C2H5OH at the oxygen concentrations of 10%, 30%, and 40%, respectively. In addition, an acceptable response/recovery rate (within 24 s) is also confirmed. Finally, a refrigerator prototype that includes the smart gas sensor is built, and satisfactory performance in discriminating food freshness status of fresh or semi-fresh is verified for the proposed refrigerator prototype. In conclusion, these aforementioned promising results suggest that the proposed integrated smart gas sensor could be a potential candidate for alarming food spoilage.
Cold Temperature , Food , Humidity , Oxygen
As a source of clean energy, hydrogen (H2) is a promising alternative to fossil fuels in reducing the carbon footprint. However, due to the highly explosive nature of H2, developing a high-performance sensor for real-time detection of H2 gas at low concentration is essential. Here, we demonstrated the H2 gas sensing performance of Ag/Pd nanoparticle-functionalized ZnO nanoplates. Bimetallic Ag/Pd nanoparticles with an average size of 8 nm were prepared and decorated on the surface of ZnO nanoplates to enhance the H2 gas sensing performance. Compared with pristine ZnO, the sensor based on ZnO nanoplate doped with Ag/Pd (0.025 wt%) exhibited an outstanding response upon exposure to H2 gas (R a/R g = 78 for 500 ppm) with fast response time and speedy recovery. The sensor also showed excellent selectivity for the detection of H2 over the interfering gases (i.e., CO, NH3, H2S, and VOCs). The superior gas sensing of the sensor was dominated by the morphological structure of ZnO, and the synergistic effect of strong adsorption and the optimum catalytic characteristics of the bimetallic Ag/Pd enhances the hydrogen response of the sensors. Thus, bimetallic Ag/Pd-doped ZnO is a promising sensing material for the quantitative determination of H2 concentration towards industrial applications.
Thiol functionalization of two-dimensional (2D) metal sulfides has been demonstrated as an effective approach to enhance the sensing performances. However, most thiol functionalization is realized by multiple-step approaches in liquid medium and depends on the dispersity of 2D materials. Here, we utilize a three-dimensional (3D) In2S3 nano-porous structure that self-assembled from 2D components as the nanoreactor, in which the surface-absorbed thiol molecules from the chemical residues of the nanoreactor are used for the in-situ covalent functionalization. Such functionalization is realized by facile heat the nanoreactor at 100 °C, leading to the recombing sulfur vacancies with thiol-terminated groups. The NO2 sensing performances of such functionalized nanoreactor are investigated at room temperature, in which In2S3-100 exhibits a response magnitude of 21.5 towards 10 ppm NO2 with full reversibility, high selectivity, and excellent repeatability. Such high-performance gas sensors can be attributed to the additional electrons that transferring from the functional group into the host, thus significantly modifying the electronic band structure. This work provides a guideline for the facile in-situ functionalization of metal sulfides and an efficient strategy for the high performances gas sensors without external stimulus.
The adverse effects of NO2 on the environment and human health promote the development of high-performance gas sensors to address the need for monitoring. Two-dimensional (2D) metal chalcogenides have been considered an emerging group of NO2-sensitive materials, while incomplete recovery and low long-term stability are the two major hurdles for their practical implementation. The transformation into oxychalcogenides is an effective strategy to alleviate these drawbacks, but usually requires multiple-step synthesis and lacks controllability. Here, we prepare tailorable 2D p-type gallium oxyselenide with the thicknesses of 3-4 nm, through a single-step mechanochemical synthesis that combines the in-situ exfoliation and oxidation of bulk crystals. The optoelectronic NO2 sensing performances of such 2D gallium oxyselenide with different oxygen contents are investigated at room temperature, in which 2D GaSe0.58O0.42 exhibits the largest response magnitude of 82.2% towards 10 ppm NO2 at the irradiation of UV, with full reversibility, excellent selectivity, and long term stability for at least one month. Such overall performances are significantly improved over those of reported oxygen-incorporated metal chalcogenide-based NO2 sensors. This work provides a feasible approach to prepare 2D metal oxychalcogenides in a single-step manner and demonstrates their great potential for room-temperature fully reversible gas sensing.
Gas sensor-embedded smartphones would offer the opportunity of on-site tracking of gas molecules for various applications, for example, harmful air pollutant alarms or noninvasive assessment of health status. Nevertheless, high power consumption and difficulty in replacing malfunctioned sensors as well as limited space in the smartphone to host the sensor restrain the relevant advancements. In this article, we create a smartphone case-based sensing platform by integrating the functional units into a smartphone case, which performs a low detection limit of 117 ppb to acetone and high specificity. Particularly, dimming glass-regulated light fidelity (Li-Fi) communication is successfully developed, allowing the sensing platform to operate with relatively low power consumption (around 217 mW). Experimental proof on harmful gas sensing and potential clinic application is implemented with the sensing platform, demonstrating satisfactory sensing performance and acceptable health risk pre-warning accuracy (87%). Thus, the developed smartphone case-based sensing platform would be a good candidate for realizing harmful gas alarms and noninvasive assessment of health status.
Acetone , Smartphone
Gas sensors play an important role in many areas of human life, including the monitoring of production processes, occupational safety, food quality assessment, and air pollution monitoring. Therefore, the need for gas sensors to monitor hazardous gases, such as ammonia, at low operating temperatures has become increasingly important in many fields. Sensitivity, selectivity, low cost, and ease of production are crucial characteristics for creating a capillary network of sensors for the protection of the environment and human health. However, developing gas sensors that are not only efficient but also small and inexpensive and therefore integrable into everyday life is a difficult challenge. In this paper, we report on a resistive sensor for ammonia detection based on thin V2O5 nanosheets operating at room temperature. The small thickness and porosity of the V2O5 nanosheets give the sensors good performance for sensing ammonia at room temperature (RT), with a relative change of resistance of 9.4% to 5 ppm ammonia (NH3) and an estimated detection limit of 0.4 ppm. The sensor is selective with respect to the seven interferents tested; it is repeatable and stable over the long term (four months). Although V2O5 is generally an n-type semiconductor, in this case the nanosheets show a p-type semiconductor behavior, and thus a possible sensing mechanism is proposed. The device's performance, along with its size, low cost, and low power consumption, makes it a good candidate for monitoring freshness and spoilage along the food supply chain.
The effect of MoS2 nanosheet (NS) decoration on the gas-sensing properties of SnO2 nanofibers (NFs) was investigated. The decorated sensors were fabricated by facile on-chip electrospinning technique and subsequently dropping MoS2 NSs-dispersed solution. The MoS2 NS decoration resulted in enhanced the response and reduced the operating temperature of SnO2 NFs towards SO2 gas. The SnO2 NF sensor decorated with the optimum density of MoS2 NSs exhibited about 10-fold enhancement in gas response to 10 ppm SO2 at 150 °C as compared with the bare SnO2 NF sensor. Furthermore, the decorated sensors exhibited an extremely low detection limit and good selectivity for SO2 gas against other interfering gases, such as CO, NH3, and H2. The enhanced SO2 gas-sensing performance of MoS2 NSs-decorated SnO2 NFs was attributed to the chemical sensitization of MoS2 NSs and charge transfer through heterojunctions between the NSs and SnO2 nanograins. The classification of toxic gases such as CO, H2, and NH3 by the MoS2 NSs-decorated SnO2 NF sensors can achieve high accuracy with linear discriminant analysis (LDA). Our results suggest that the one-dimensional nanostructures of semiconductor metal oxides decorated with two-dimensional transition metal dichalcogenides are attractive candidates for the detection of hazardous gases.
The selective detection and classification of NH3 and H2S gases with H2S gas interference based on conventional SnO2 thin film sensors is still the main problem. In this work, three layers of SnO2/Pt/WO3 nanofilms with different WO3 thicknesses (50, 80, 140, and 260 nm) were fabricated using the sputtering technique. The WO3 top layer were used as a gas filter to further improve the selectivity of sensors. The effect of WO3 thickness on the (NH3, H2, and H2S) gas-sensing properties of the sensors was investigated. At the optimal WO3 thickness of 140 nm, the gas responses of SnO2/Pt/WO3 sensors toward NH3 and H2 gases were slightly lower than those of Pt/SnO2 sensor film, and the gas response of SnO2/Pt/WO3 sensor films to H2S gas was almost negligible. The calcification of NH3 and H2 gases was effectively conducted by machine learning algorithms. These evidences manifested that SnO2/Pt/WO3 sensor films are suitable for the actual NH3 detection of NH3 and H2S gases.
Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.
In the current work, we report the on-chip fabrication of a low-temperature H2S sensor based on p-type Co3O4 nanofibers (NFs) using the electrospinning method. The FESEM images show the typical spider-net like morphologies of synthesized Co3O4 NFs with an average diameter of 90 nm formed on the comb-like electrodes. The EDX data indicate the presence of Co and O elements in the NFs. The XRD analysis results confirm the formation of single-phase cubic spinel nanocrystalline structures (Fd3 m) for the synthesized Co3O4 NFs. The Raman results are in agreement with the XRD data through the presence of five typical vibration modes of the nanocrystalline Co3O4. The gas sensing properties of the fabricated Co3O4 NF sensors are tested to 1 ppm H2S within a temperature range of 150 °C to 450 °C. The results indicate a highest sensor response to 1 ppm H2S with the gas response of aproximately 2.1 times and the gas response/recovery times of 75 s/258 s at a low temperature of 250 °C. The fabricated sensor also demonstrates good selectivity and a low detection limit of 18 ppb. The overall results suggest a simple and effective fabrication process for the p-type Co3O4 NF sensor for practical applications in detecting H2S gas at low temperature.
Chemoresistive gas sensors play an important role in detecting toxic gases for air pollution monitoring. However, the demand for suitable nanostructures that could process high sensing performance remains high. In this study, hollow ZnO nanorices were synthesized by a simple hydrothermal method to detect NO2 and SO2 toxic gases efficiently. Material characterization by some advanced techniques, such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy, demonstrated that the hollow ZnO nanorices had a length and diameter size of less than 500 and 160 nm, respectively. In addition, they had a thin shell thickness of less than 30 nm, formed by an assembly of tiny nanoparticles. The sensor based on the hollow ZnO nanorices could detect low concentration of NO2 and SO2 gasses at sub-ppm level. At an optimum operating temperature of 200 °C, the sensor had response values of approximately 15.3 and 4.8 for 1 ppm NO2 and 1 ppm SO2, respectively. The sensor also exhibited good stability and selectivity, suggesting that the sensor can be applied to NO2 and SO2 toxic gas detection in ambient air.
In this work, four identical micro sensors on the same chip with noble metal decorated tin oxide nanowires as gas sensing material were located at different distances from an integrated heater to work at different temperatures. Their responses are combined in highly informative 4D points that can qualitatively (gas recognition) and quantitatively (concentration estimate) discriminate all the tested gases. Two identical chips were fabricated with tin oxide (SnO2) nanowires decorated with different metal nanoparticles: one decorated with Ag nanoparticles and one with Pt nanoparticles. Support Vector Machine was used as the "brain" of the sensing system. The results show that the systems using these multisensor chips were capable of achieving perfect classification (100%) and good estimation of the concentration of tested gases (errors in the range 8-28%). The Ag decorated sensors did not have a preferential gas, while Pt decorated sensors showed a lower error towards acetone, hydrogen and ammonia. Combination of the two sensor chips improved the overall estimation of gas concentrations, but the individual sensor chips were better for some specific target gases.
Fabrication of a high-performance room-temperature (RT) gas sensor is important for the future integration of sensors into smart, portable and Internet-of-Things (IoT)-based devices. Herein, we developed a NO2 gas sensor based on ultrathin MoS2 nanoflowers with high sensitivity at RT. The MoS2 flower-like nanostructures were synthesised via a simple hydrothermal method with different growth times of 24, 36, 48, and 60 h. The synthesised MoS2 nanoflowers were subsequently characterised by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, energy-dispersive X-ray spectroscopy and transmission electron microscopy. The petal-like nanosheets in pure MoS2 agglomerated to form a flower-like structure with Raman vibrational modes at 378 and 403 cm-1 and crystallisation in the hexagonal phase. The specific surface areas of the MoS2 grown at different times were measured by using the Brunauer-Emmett-Teller method. The largest specific surface area of 56.57 m2 g-1 was obtained for the MoS2 nanoflowers grown for 48 h. This sample also possessed the smallest activation energy of 0.08 eV. The gas-sensing characteristics of sensors based on the synthesised MoS2 nanostructures were investigated using oxidising and reducing gases, such as NO2, SO2, H2, CH4, CO and NH3, at different concentrations and at working temperatures ranging from RT to 150 °C. The sensor based on the MoS2 nanoflowers grown for 48 h showed a high gas response of 67.4% and high selectivity to 10 ppm NO2 at RT. This finding can be ascribed to the synergistic effects of largest specific surface area, smallest crystallite size and lowest activation energy of the MoS2-48 h sample among the samples. The sensors also exhibited a relative humidity-independent sensing characteristic at RT and a low detection limit of 84 ppb, thereby allowing their practical application to portable IoT-based devices.
SnO2 nanowires (NWs) are used in gas sensors, but their response to highly toxic gas H2S is low. Thus, their performance toward the effective detection of low-level H2S in air should be improved for environmental-pollution control and monitoring. Herein, Ag2O nanoparticle decorated SnO2 NWs were prepared by a simple on-chip growth and subsequent dip-coating method. The amount of decorated Ag2O nanoparticles on the surface of SnO2 NWs was modified by changing the concentration of AgNO3 solution and/or dipping times. Gas-sensing measurements were conducted at various working temperatures (200-400 °C) toward different H2S concentrations ranging within 0.1-1 ppm. The selectivity of Ag2O-decorated SnO2 NW sensors for ammonia and hydrogen gases was tested. Results confirmed that the Ag2O-decorated SnO2 NW sensors had excellent response, selectivity, and reproducibility. The gas-sensing mechanism was interpreted under the light of energy-band bending by sulfurization, which converted the p-n junction into n-n, thereby significantly enhancing the sensing performance.
Multisensor systems with low-power consumption are emerging for the Internet of Things. In this work, we demonstrate the use of self-heated networked Ag-decorated SnO2 NW sensors integrated into a portable module for selective detection of H2S gas at low power consumption, and the integrated system is simulated as a virtual multisensor under varying heating powers for identifying and quantifying different reducing gases. The H2S gas-sensing characterisations at the different self-heating powers of 2-10â¯mW showed that the gas response significantly increased with the increase in Ag density decoration and the heated power strongly affected the gas-sensing performance and sensor stability. Excellent response of 21.2 to 0.5â¯ppm H2S gas was obtained at a low heating power of 2â¯mW with an acceptable response/recovery time of 18/980â¯s. The increase of the heating power over 20â¯mW can destroy the devices. The integrated system could selectively detect H2S at the heating power below 4â¯mW and H2, C2H5OH and NH3 gases at the heating power upon 4â¯mW. The virtual multisensor could discriminate qualitatively (with an accuracy of 100%) and quantitatively H2S, H2, NH3, C2H5OH (Ethanol) and CH3COCH3 (Aceton) gases with average errors of 13.5%, 14.7%, 16.8%, 16.9%, and 14.8%, respectively. The proposed sensing platform is a promising candidate for selective detection of H2S gas and virtual multisensor with low power consumption for mobile or wireless network devices.
The highly toxic hydrogen sulphide (H2S) present in air can cause negative effects on human health. Thus, monitoring of this gas is vital in gas leak alarms and security. Efforts have been devoted to the fabrication and enhancement of the H2S-sensing performance of gas sensors. Herein, we used electron beam evaporation to decorate nickel oxide (NiO) nanoparticles on the surface of tin oxide (SnO2) nanowires to enhance their H2S gas-sensing performance. The synthesised NiO-SnO2 materials were characterised by field-emission scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy analysis. H2S gas-sensing characteristics were measured at various concentrations (1-10 ppm) at 200-350 °C. The results show that with effective decoration of NiO nanoparticles, the H2S gas-sensing characteristics of SnO2 nanowires are significantly enhanced by one or two orders compared with those of the bare material. The sensors showed an effective response to low-level concentrations of H2S in the range of 1-10 ppm, suitable for application in monitoring of H2S in biogas and in industrial controls. We also clarified the sensing mechanism of the sensor based on band structure and sulphurisation process.
ZnFe2O4 nanofiber gas sensors are cost-effectively fabricated by direct electrospinning on microelectrode chip with Pt interdigitated electrodes and subsequent calcination under different conditions to maximize their response to H2S gas. The synthesized nanofibers of approximately 30-100 nm in diameter show typical spider-net-like morphology of the electrospun nanofibers. The ZnFe2O4 nanofibers comprise many 10-25 nm nanograins, which results in multi-porous structures. Moreover, the nanofibers exhibit the single phase of cubic-spinel-structure ZnFe2O4. The density, crystallinity and grain size of ZnFe2O4 nanofiber that strongly affect gas-sensing properties can be optimized by controlling electrospun time, annealing temperature, annealing time and heating rate. Under optimal conditions, the ZnFe2O4 nanofiber sensors exhibit high sensitivity and selectivity to H2S at sub-ppm levels. Excellent gas-sensing performances are attributed to effects of multi-porous structure, nanograin size and crystallinity, which is explained by the sensing mechanisms of ZnFe2O4 nanofiber sensors to H2S gas.
