Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review

Metal oxide resistive-type nano-scale gas sensors have been investigated for their low cost, high sensitivity, and environmentally friendly fabrication. In these sensors, electrical resistance measurements are used to detect the presence of gas. In n-type metal oxides, resistance is increased by coverage of adsorbed oxygen and lowered by removal of adsorbed oxygen through reactions with reducing gasses. The sensitivity and selectivity of these sensors have been improved by incorporation of heterostructures. Heterostructures may improve sensor performance through facilitating catalytic activity, increasing adsorption, and creating a charge carrier depletion layer that produces a larger modulation in resistance. Synergistic effects in these gas sensors describe the improved sensor signal due to these combined effects which act to amplify the reception and transduction of the sensor signal. Receptive mechanisms may be improved by increasing adsorption and reactivity. Transduction mechanisms may be improved by restriction of the major charge conduction channels which helps to maximize resistance modulation. In this review, the synergistic effect achieved by combining these two mechanisms are examined. Fundamental properties of the metal oxide surface are used to provide insight for the large body of experimental evidence available for metal oxide resistive-type gas sensors. This review aims to connect experimental evidence to conceptual mechanistic descriptions by examining adsorption processes, charge transfer, reaction mechanisms, morphology, and ambient gas interactions.

Synergistic effects in gas sensing semiconducting oxide nano-heterostructures: A review

In this study, SnO2 nanosheets (NSs) was firstly prepared by the hydro-solvothermal treatment, and then decorated with Pd, Au and PdAu bimetallic nanoparticles (NPs) by an in situ reduction with ascorbic acid (AA). Their morphology, chemistry, and crystal structure were characterized at the nanoscale. It was found that SnO2 NSs were flower-like with thickness of 7–12 nm, and PdAu NPs with the size of 3–10 nm were dispersed uniformly on the surface of SnO2 NSs. Their gas sensing properties were carefully studied. The results demonstrated that the PdAu/SnO2 sensor can not only effectively detect acetone at 250 °C with response of 6.6 to 2 ppm acetone, but also detect formaldehyde at 110 °C with response of 4.1–2 ppm formaldehyde, and the corresponding detection limit is as low as 45 ppb and 30 ppb, respectively. Moreover, the PdAu/SnO2 sensor exhibited excellent reusability, and reliability to the low concentration of acetone and good anti-interference to humidity and other biomarkers in human breath. Compared with that decorated with their parent metal (Pd or Au), the enhanced response of SnO2 NSs decorated with PdAu bimetallic NPs may be ascribed to the chemical sensitization of Au, the electronic sensitization of Pd and the synergistic effect of PdAu bimetallic NPs. The PdAu/SnO2 sensor has a great potential application in detecting formaldehyde and diabetes diagnosis.

Bimetal PdAu decorated SnO2 nanosheets based gas sensor with temperature-dependent dual selectivity for detecting formaldehyde and acetone

The alarming rise of indoor pollution and the need to combat the associated negative effects have promoted increasing attention in modernizing the chemical sensing technologies by newly designed materials with rich and tunable functionalities at atomic or molecular levels. With the appealing physical, chemical, optical, and electronic properties for various potential applications, the state-of-art gas-sensing nanomaterials and their future perspectives are well-documented and summarized in this paper. Specifically, the key performance attributes are addressed in detail such as the sensitivity, selectivity, reversibility, operating temperature, response time, and detection limit. As such, this review provides both critical insights in exploring and understanding various gas sensing nanomaterials and points out limitations and opportunities for further developments, such as morphology control, doping and surface alteration, atomic-scale characterization, and applications in different fields. Finally, the challenges and outlooks are discussed on the basis of the current developments.

Functional gas sensing nanomaterials: A panoramic view

Tin oxide microspheres were successfully obtained through a facile hydrothermal method without any polymer templates or surfactant. The as-synthesized SnO 2 microspheres are composed of large amount of small spheres with average diameters of about 250 nm, and every small sphere consists of numerous primary nanocrystallites with average sizes of about 8 nm. The resultant product was used as sensing material for gas sensor to detect the formaldehyde (HCHO) gas. The gas response, response and recovery time, selectivity and stability were carefully studied. It was found that the response value of the sensor to 100 ppm HCHO was 38.3 at the operating temperature of 200 °C. The gas sensor based on SnO 2 microspheres has excellent gas response, good response-recovery properties, linear dependence, repeatability and selectivity, making it to be a promising candidate for practical detectors for HCHO.

Formaldehyde detection: SnO2 microspheres for formaldehyde gas sensor with high sensitivity, fast response/recovery and good selectivity

Preparation of Pd nanoparticle-decorated hollow SnO2 nanofibers and their enhanced formaldehyde sensing properties

Nanofibers of p-type NiO/n-type ZnO heterojunction with different NiO content were fabricated by a facile electrospinning technique. The as-synthesized nanofibers were characterized by differential thermal analyzer (DTA) and thermal gravimetric analyzer (TGA), X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The p–n junction formed between cubic NiO particle and hexangular ZnO particle in the NiO/ZnO nanofibers was confirmed and highly improved the trimethylamine (TMA) sensing properties. The influence of NiO content in nanofibers on TMA sensing properties was studied. The results showed that the NiO/ZnO nanofibers exhibited the significantly enhanced response, good selectivity, fast response and recovery rate (less than 30 s and 35 s, respectively), and excellent linearity (in a relatively wide range of 0.5–200 ppm). Especially, the NiO/ZnO nanofibers with 4 mol% of NiO has the best performance, whose response to 100 ppm TMA could reach 892 and detection limit could extend down to 0.5 ppm at 260 °C. The mechanism of the enhanced TMA sensing performance caused by p-type NiO/n-type ZnO heterojunctions in the NiO/ZnO nanofibers was also discussed.

Electrospun nanofibers of p-type NiO/n-type ZnO heterojunction with different NiO content and its influence on trimethylamine sensing properties

Highly stabilized and rapid sensing acetone sensor based on Au nanoparticle-decorated flower-like ZnO microstructures

Synergistically improved formaldehyde gas sensing properties of SnO2 microspheres by indium and palladium co-doping

The chemical and optical properties of 1D single-crystalline cadmium sulfide (CdS) nanowires (NWs) synthesized by a solvothermal method were discussed systematically. The CdS NW was characterized using different analytical techniques. In our work, CdS was employed as the active nanomaterial to detect ethanol gas for the first time and showed good gas sensing performance. Especially, the fast response (0.4 s) and recovery speed (0.2 s) to 100 ppm ethanol were much faster than the reported values. The visible-light detector based on CdS NWs demonstrated ultrafast decay speed (3.77 ms), which was the fastest in the reported photodetectors (PDs) based on randomly oriented CdS NW networks. This research indicates that the CdS NW is an excellent nanomaterial for high performance gas sensors and PDs.

Ying Lin

Papers

Preparation of Pd nanoparticle-decorated hollow SnO2 nanofibers and their enhanced formaldehyde sensing properties

Electrospun nanofibers of p-type NiO/n-type ZnO heterojunction with different NiO content and its influence on trimethylamine sensing properties

Highly stabilized and rapid sensing acetone sensor based on Au nanoparticle-decorated flower-like ZnO microstructures

Synergistically improved formaldehyde gas sensing properties of SnO2 microspheres by indium and palladium co-doping

Excellent gas sensing and optical properties of single-crystalline cadmium sulfide nanowires