Zinc ferrite based gas sensors: A review

Flammable, explosive and toxic gases, such as hydrogen, hydrogen sulfide and volatile organic compounds vapor, are major threats to the ecological environment safety and human health. Among the available technologies, gas sensing is a vital component, and has been widely studied in literature for early detection and warning. As a metal oxide semiconductor, zinc ferrite (ZnFe₂O₄) represents a kind of promising gas sensing material with a spinel structure, which also shows a fine gas sensing performance to reducing gases. Due to its great potentials and widespread applications, this article is intended to provide a review on the latest development in zinc ferrite based gas sensors. We first discuss the general gas sensing mechanism of ZnFe₂O₄ sensor. This is followed by a review of the recent progress about zinc ferrite based gas sensors from several aspects: different micro-morphology, element doping and heterostructure materials. In the end, we propose that combining ZnFe₂O₄ which provides unique microstructure (such as the multi-layer porous shells hollow structure), with the semiconductors such as graphene, which provide excellent physical properties. It is expected that the mentioned composites contribute to improving selectivity, long-term stability, and other sensing performance of sensors at room or low temperature.     

Gas sensing properties of asymmetrically reduced Na1/2Bi1/2TiO3–based ceramics

We report the gas sensing properties of a type of new materials, Na_1/2Bi_1/2TiO₃ (NBT)-based ceramics. After the NBT-based ceramics were asymmetrically reduced and coated with Au electrodes, the materials exhibit relatively large electrical responses when exposed to oxygen and some oxidizable gases at a relatively low temperature (≤300 °C). An electric voltage ～60 mV is measured in the mixture of O₂ and N₂ (1% O₂). In oxidizable gases, a negative response can be obtained. The measured voltages are ?45 mV and ?98 mV in the mixtures of H₂/air (1000 ppm H₂) and C₂H₅OH/air (1000 ppm C₂H₅OH), respectively. The electrical responses are proportional to the logarithm of the concentrations of the analyzed gases. Also, the electrical responses to oxygen and oxidizable gases have opposite signs, and the model of mixed-potential is proposed to explain the gas sensing phenomenon. This study provides a new material and a simple design for gas sensors. The proposed gas sensor comprises a reduced NBT-based ceramic wafer with the same electrodes on the opposite surfaces. Additional components in traditional gas sensors, such as sensing or reference electrode, are unnecessary.     

NO2 gas sensing responses of In2O3 nanoparticles decorated on GO nanosheets

Nanomaterials offer a wide range of applications in environmental nanotechnology. Hazardous pollutants in the environment are needed to be detected and controlled effectively to avoid human health risks. In this paper, we described the fine-controlled growth of In₂O₃ nanoparticles embedded on GO nanosheets by a facile precipitation method. The In₂O₃@GO nanocomposites exhibited outstanding gas sensing performance as compared with pure In₂O₃ nanoparticles towards NO₂. At 225 °C, the sensor displayed high selectivity, best response (78) to 40 ppm NO₂, quick response, and recovery times of 106s/42s. The improved sensing performances of the nanocomposite were attributed to large surface area, high gas adsorption-desorption capability, and the formation of p-n heterojunctions between In₂O₃ nanoparticles and GO nanosheets. The excellent gas detecting activities validate In₂O₃@GO nanocomposites as a promising candidate in the NO₂ gas sensor industry.     

Fabrication and CO gas-sensing properties of Pt-functionalized Ga2O3 nanowires

Ga₂O₃ one-dimensional (1D) nanostructures were synthesized using an evaporation technique. The morphology, crystal structure and enhanced sensing properties of the Ga₂O₃ nanostructures functionalized with Pt to CO gas at 100 °C were examined. The diameter and lengths of the 1D nanostructures ranged from a few tens to a few hundreds of nanometers and up to a few hundreds of micrometers, respectively. Pt nanoparticles with diameters of a few tens of nanometers were distributed over the Ga₂O₃ nanowires. Multiple networked gas sensors fabricated from these Pt-functionalized Ga₂O₃ nanowires exhibited enhanced electrical responses to CO gas. The responses of the nanowires were improved 27.8, 26.1, 22.0 and 16.9 fold at CO concentrations of 10, 25, 50, and 100 ppm, respectively, Compared to the bare Ga₂O₃ nanowires. The mechanism responsible for the enhanced gas sensing properties of the Pt-functionalized Ga₂O₃ nanowires is discussed.     

Urchin-like PdO–Fe2O3 heterojunctions for high-performance hydrogen sulfide gas sensors

To realize a synergistic effect between composition engineering and morphology modulation, PdO–Fe₂O₃ heterojunction architectures with various morphologies were fabricated by facile hydrothermal and solution impregnation methods. Afterwards, gas sensing devices were constructed from the as-prepared PdO–Fe₂O₃ composites, and their parameters were systematically investigated. The obtained results revealed that all PdO–Fe₂O₃ sensors exhibited excellent selectivity properties and high responses to target gases. Importantly, an urchin-like PdO–Fe₂O₃ sensing device produced the largest response of 75 to 50 ppm hydrogen sulfide at a PdO concentration of 4 wt%, which was approximately four times greater than that of a spherical PdO–Fe₂O₃ sensor. The urchin-like sensor was also characterized by the highest response recovery efficiency, water resistance, and long-term stability. Furthermore, the sensing mechanism of the fabricated PdO–Fe₂O₃ composite-based gas sensors was discussed comprehensively.     

ZnO-enhanced In2O3-based sensors for n-butanol gas

A series of high-response and fast-response/recovery n-butanol gas sensors was fabricated by adding ZnO to In₂O₃ in varying molar ratios to form ZnO-In₂O₃ nanocomposites via a facile co-precipitation hydrothermal method. Morphological characterizations revealed that the shape of pure In₂O₃ was changed from irregular cubes into irregular nanoparticles, 30–50?nm in size, with the addition of ZnO. Compared with the pure In₂O₃ gas sensor, the ZnO-In₂O₃ gas sensor exhibits superior n-butanol sensing performance. With the introduction of ZnO, the response of the sensor to n-butanol was improved from 17 to 99.5 at 180?°C for a [Zn]:[In] molar ratio of 1:1. In addition, the ZnO-In₂O₃ gas sensors show a reduced optimal working temperature, excellent selectivity to n-butanol, and good repeatability. The response of the ZnO-enhanced In₂O₃-based sensors showed a strong linear relationship with the n-butanol gas concentration, allowing for the quantitative detection of n-butanol gas.     

Hierarchical assembly of SnO2 nanorod on spindle-like α-Fe2O3 for enhanced acetone gas-sensing performance

Preparing a heterojunction structure in different metal oxides is an efficacious method to improve the gas-sensing properties. In this article, a novelty SnO₂ nanorod/spindle-like Fe₂O₃ heterostructure was successfully fabricated through a simple two-step hydrothermal route. The morphological characterization revealed that the spindle-shaped Fe₂O₃ with length and diameter of 400 and 100 nm were firstly fabricated by a hydrothermal process, and then a large number of SnO₂ nanorods (lengths of 30 nm and diameterd of 8 nm) covered the spindle-shaped Fe₂O₃ uniformly. In order to facilitate better practical applications, the gas sensing performance of sensors based on SnO₂/Fe₂O₃ nanostructures and pure Fe₂O₃ nanospindles on volatile organic compounds were systematically studied. Gas sensing tests indicated that such hierarchical SnO₂/Fe₂O₃ heterostructures revealed improved acetone sensing performance compared to pure spindle-like Fe₂O₃, and the enhanced gas-sensitivity performance possibly be attributed to the synergistic effect and heterojunction of the interface between spindle-like Fe₂O₃ and SnO₂ nanorod. Additionally, this research on as-obtained SnO₂/Fe₂O₃ hierarchical assembly may provide a new insight and a rational strategy to upgrade the sensing performance of certain semiconductor metal oxide materials by rationally designing various novel layered nanostructures in the future.     

Realization of low-temperature and selective NO2 sensing of SnO2 nanowires via synergistic effects of Pt decoration and Bi2O3 branching

Metal oxide semiconductors with branched structures, such as branched nanowires (b-NWs), have promising properties for being used in gas sensors. In this work, we synthesized Pt-decorated Bi₂O₃-branched SnO₂ nanowires (NWs). NO₂ sensing studies revealed the superior capacity of a Pt-decorated Bi₂O₃-branched SnO₂ NWs gas sensor relative to pristine and branched SnO₂ gas sensors, and it worked at near room temperature (50 °C). The increased sensing capacity was related to the synergistic effects of Pt decoration and Bi₂O₃ branching, particularly the morphology of the gas sensor with branched structures, the promising effects of Pt as a noble metal with good catalytic activity, and the generation of homo- and heterojunctions in the Pt-decorated Bi₂O₃-branched SnO₂ NWs gas sensor. The results obtained in this work are useful for design and development of NO₂ gas sensors using a simple strategy, which can be easily extended to various metal oxides.     

Morphology-controllable synthesis of hierarchical hollow GaFeO3 microcubes with selective triethylamine gas-sensing properties

Hierarchical multi-metal oxide-based gas sensors with high surface area and abundant active sites have attracted intensely research interests for their highly sensitive and fast gas detection performance. Developing synthetic strategies for obtaining novel hierarchical metal oxides with high sensing performance remains eminently challenging. Herein, hierarchial hollow GaFeO₃ microcubes were successfully prepared via a Ga³⁺-modified Fe-based Prussian Blue (PB) mediated template conversion strategy. The microsized morphologies and hollow interior structures of GaFeO₃ microcubes can be feasibly modulated by controlling the thermolysis temperatures. The ultrasmall nanoparticle-assembly of GaFeO₃ architecture obtained at 500 °C exhibited an optimum response value (R_a/R_g) of 7.4, and rapid response/recovery times (9 s/49 s) toward 200 ppm triethylamine (TEA) at a working temperature of 200 °C, as well as remarkable selectivity and excellent long-term stability (for at least 31 days), which are intrinsically beneficial from the unique interior loose structure with good permeability for diffusion of target gases. This work provides a promising approach for synthesizing various hierarchical multimetal oxides with intriguing nanoparticle-assembled hollow structure and broad prospects for practical gas sensing applications.     

Enhanced H2S sensing performance of TiO2-decorated α-Fe2O3 nanorod sensors

Pristine and TiO₂ nanoparticle-decorated Fe₂O₃ nanorods were synthesized via thermal oxidation of Fe thin foils, followed by the solvothermal treatment with titanium tetra isopropoxide (TTIP) and NaOH for TiO₂ nanoparticle-decoration. Subsequently, gas sensors were fabricated by connecting the nanorods with metal conductors. The structure and morphology of the pristine and TiO₂ nanoparticle-decorated Fe₂O₃ nanorods were examined via X-ray diffraction and scanning electron microscopy, respectively. The gas sensing properties of the pristine and TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensors with regard to H₂S gas were examined. The TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensor showed a stronger response to H₂S than the pristine Fe₂O₃ nanorod sensor. The responses of the pristine and TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensors were 2.6 and 7.4, respectively, when tested with 200 ppm of H₂S at 300 °C. The TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensor also showed a faster response and recovery than the sensor made from pristine Fe₂O₃ nanorods. Both sensors showed selectivity for H₂S over NO₂, SO₂, NH₃, and CO. The enhanced sensing performance of the TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensor compared to that of the pristine Fe₂O₃ nanorod sensor might be due to enhanced modulation of the conduction channel width, the decorated nanorods’ increased surface-to-volume ratios and the creation of preferential adsorption sites via TiO₂ nanoparticle decoration. The dominant sensing mechanism in the TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensor is discussed in detail.     

Synthesis of tuneable porous hematites (α-Fe2O3) for gas sensing and lithium storage in lithium ion batteries

Tuneable porous α-Fe₂O₃ materials were prepared by using a selective etching method. The structure and morphology of the as-prepared porous hematites have been systematically characterised by X-ray diffraction, field emission scanning electron microscope, and transmission electron microscope. We found that the pore size and pore volume can be controlled by adjusting the etching time during the synthesis process. The porous hematites have been applied for gas sensing and lithium storage in lithium ion cells. The porous α-Fe₂O₃ materials demonstrated a reversible lithium storage capacity of 1269 mAh/g. When used as a sensing material in gas sensors, porous α-Fe₂O₃ exhibited a superior sensitivity towards toxic and flammable gases.     

In2O3:Ga and In2O3:P-based one-electrode gas sensors: Comparative study

Gas sensing characteristics of one-electrode sensors based on the In₂O₃ ceramics doped by gallium and phosphorus have been discussed. In₂O₃-based ceramic was prepared by sol–gel technology. Ozone, CO, CH₄ and H₂ were used as tested gases. The doping concentration effect on the sensor parameters such as magnitude of response, operating temperature, response and recovery times, sensitivity to the air humidity, and selectivity have been analyzed. It was shown that In₂O₃ doping by Ga and P could be used for the sensor performance optimization. It was assumed that the appearance of the second phase (InPO₄ and Ga₂O₃) and the change of structural parameters, taking place during doping process, were the main factors controlling the change of operating characteristics in In₂O₃:P and In₂O₃:Ga-based sensors.     

A review of recent developments in tin dioxide nanostructured materials for gas sensors

Gas sensors based on SnO₂ nanostructures have been extensively investigated in recent years. Many recent investigations have focused on synthesizing 0D, 1D, 2D and 3D SnO₂ nanostructures with high sensing capacity. This work presents a review of the recent developments in pure, doped and metal oxide functionalized SnO₂ nanostructured gas sensors, emphasizing the main SnO₂ preparation methods and the working principle of SnO₂ gas sensors. Most studies have shown that doping, coupled with a high surface area, can significantly improve SnO₂ sensing properties. Sensing response, response/recovery times, and operating temperature can be modulated by the synergistic effect between these two factors. In general, fine nanoparticles, mesoporous materials, hollow and 3D nanostructures combined with additives such as Pt, Pd, Cu, Ni, Ag and Al have shown the best improvements in gas sensing.     

Ti2CTx MXene: A novel p-type sensing material for visible light-enhanced room temperature methane detection

The development of semiconductor-based room-temperature methane (CH₄) gas sensors is appealing but challenging. Herein, we report a CH₄ gas sensor operating at room temperature based on Ti₂CT_x MXene, a novel p-type sensing material, achieving high-performance CH₄ detection with visible light assistance. The Ti₂CT_x MXene based device showed more than seven-fold improvement for CH₄ detection under visible-light irradiation, and the response/recovery times were also sharply decreased. The excellent CH₄ sensing performance at room temperature could be attributed to the visible-light photocatalytic CH₄ oxidation activity of the Ti₂CT_x sensing material. CH₄ oxidation was revealed by photocatalytic measurement, O₂-TPD and in-situ IR spectroscopies. The present work demonstrates the novel application of Ti₂CT_x MXene as a promising p-type sensing material for methane detection at room temperature. Moreover, the concept of “photocatalysis-enhanced gas sensing” can be employed in room-temperature gas sensors based on other novel semiconductors.     

Construction of pine-branch-like α-Fe2O3/TiO2 hierarchical heterostructure for gas sensing

For real-time environmental monitoring and gas detection under harsh conditions, gas sensors with high reliability, sensitivity, and selectivity are in increasing demand. Therefore, pine-branch-like α-Fe₂O₃/TiO₂ with a core–shell hierarchical heterojunction structure was designed and fabricated for ethanol sensing. The heterojunction increased the O adsorption, resulting in the formation of more chemisorbed O species for reaction with the target gas. Moreover, the pine-branch-like morphology formed three-dimensional hollow holes as channels for molecule diffusion and adsorption. Compared with gas-proof structures, the hollow regions can realize gas diffusion in the inner and outer surfaces of materials simultaneously. Further, in contrast to aggregated structures, the fibers were separated from each other, and charge conduction between the fibers needed to pass through the surface sensing layer at the fiber boundary; thus, the overall resistance change of the sensors was significantly affected by the response of the heterojunction to the target gas. Consequently, the α-Fe₂O₃/TiO₂ sensor exhibited a high response value of 40.4 toward 500 ppm ethanol gas and had excellent selectivity to ethanol. The proposed strategy may facilitate the design and construction of effective heterojunctions and nanostructures for gas sensing.     

Ethanol sensing properties of networked In2O3 nanorods decorated with Cr2O3-nanoparticles

In₂O₃ nanorods decorated with Cr₂O₃ nanoparticles were synthesized by thermal evaporation of In₂S₃ powder in an oxidizing atmosphere followed by solvothermal deposition of Cr₂O₃ and their ethanol gas sensing properties were examined. The pristine and Cr₂O₃-decorated In₂O₃ nanorods exhibited responses of ~524% and ~1053%, respectively, to 500-ppm ethanol at 200 °C. The Cr₂O₃-decorated In₂O₃ nanorod sensor showed stronger electrical response to ethanol gas at 200 °C than the pristine In₂O₃ nanorod counterpart. The former also showed faster response and recovery than the latter. The pristine and Cr₂O₃-decorated In₂O₃ nanorod sensors showed the strongest response to ethanol gas at 250 and 200 °C, respectively. The Cr₂O₃-decorated In₂O₃ nanorod sensor showed selectivity for ethanol gas over other reducing gases. The underlying mechanism for the enhanced response, sensing speed and selectivity of the Cr₂O₃-decorated In₂O₃ nanorod sensor for ethanol gas is discussed.     

Cadmium sulfide activated zinc oxide coatings deposited by liquid plasma spray for room temperature nitrogen dioxide detection under visible light illumination

Zinc oxide (ZnO) is well-known to be used as a gas sensing material. However, due to its high operation temperature, the chemical and thermal stability of ZnO based gas sensors are relatively low. In recent years, some researchers adopt light illumination as activated source to replace heating and obtain high gas sensing performance at low temperature. The study that follows is an attempt to use cadmium sulfide (CdS) as sensitizer to activate ZnO at room temperature with assistance of visible-light illumination. CdS–ZnO coatings were deposited by liquid plasma spray with aqueous solution containing zinc acetate and cadmium sulfide as precursors. The crystal structure of as-sprayed CdS–ZnO coatings was characterized by X-ray diffractometer (XRD) and field-emission scanning electron microscopy (FE-SEM). The sensing performance of sensors based on CdS–ZnO coatings were tested with 1 ppm nitrogen dioxide (NO₂) at room temperature illuminated under various visible lights. The results demonstrated that the LPS process was a straightforward method for deposition of high performance CdS–ZnO sensitive layers and the obtained sensors showed high responses to NO₂ at room temperature.     

Annealing temperature controlled crystallization mechanism and properties of gallium oxide film in forming gas atmosphere

Gallium oxide (Ga₂O₃) films had been fabricated on Al₂O₃(0001) substrate by employing pulsed laser deposition (PLD) and annealed at different temperatures under forming gas (FG) atmosphere (95% N₂ + 5% H₂). The influence of annealing temperature on the structural, optical, chemical composition, and surface morphological properties of the Ga₂O₃ thin films was investigated comprehensively. The annealing processes with hydrogen gas play a crucial role in the characteristics of Ga₂O₃ thin films. A crystallization mechanism of Ga₂O₃ films controlled by annealing temperature has been proposed firstly and analyzed systematically, which contains three kinds of competitive mechanism, namely the thermal enhanced crystallization, the enhanced H₂ dissociative adsorption on Ga₂O₃ surfaces, and the high-temperature decomposition of Ga₂O₃. Both Ga⁺ and Ga³⁺ oxidation valence states were presented in all samples, which indicated lattice oxygen deficiency in Ga₂O₃ films. The variation of the non-lattice oxygen proportion of Ga₂O₃ films related to the crystallization mechanism firstly increased and then decreased with the increase of annealing temperature. The detailed crystallization mechanism of PLD-Ga₂O₃ films annealed in FG offers a guideline and references for the further fabrication of high-quality Ga₂O₃ films and their applications in high-performance devices.     

Photoelectrochemical (PEC) etching of Ga2O3

In recent years, interest in the use of gallium oxide (Ga₂O₃) in semiconductor devices has increased due to its wide bandgap that permits device operation at high temperatures and high voltages. As the size of these devices decrease, it becomes more important to be able to produce features on the micro and nanoscale. Traditional etching (both wet and dry) have several limitations which either are unable to produce nano-features at the required scale or degrade device quality. Consequently, photoelectrochemical etching of Ga₂O₃ is of interest to researchers for its potential to produce features on the order of magnitude required while also causing minimal device degradation. Photoelectrochemical etching introduces a number of parameters that can be adjusted to control the etching process. In this work, we demonstrated photoelectrochemical etching of Ga₂O₃ by showing the effect of changing electrolyte concentration, anodic voltage, and etching time on the etching process. This etching method could be useful for a variety of applications which require complex patterning of Ga₂O₃ with high degrees of control compared to simple wet or dry etching processes.     

Electrochemical detection of volatile organic compounds using a Na3Zr2Si2PO12/Bi2Cu0.1V0.9O5.35 heterojunction device

A fast sodium ion conductor, NASICON (Na₃Zr₂Si₂PO₁₂), has been widely used for gas sensor applications. In this study, we demonstrate that a device combining NASICON with an oxygen-ion conductor of BiCuVOx (Bi₂Cu_0.1V_0.9O_5.35) can electrochemically detect volatile organic compounds (VOCs), such as ethanol, formaldehyde, and toluene. The sensing electrode made of BiCuVOx was attached onto a sintered NASICON disk at high temperature to produce an interfacial layer that had a different morphology and composition from those of NASICON and BiCuVOx, as observed by scanning electron microscopy-energy dispersive X-ray spectroscopy analysis. The device in which NASICON was fitted with the BiCuVOx-based electrode was found to efficiently detect VOCs in ppm concentrations. The sensor signal (electromotive force) exceeded 100 mV in response to 10 ppm HCOH at 400 °C, demonstrating the high sensitivity of the device. It also exhibited a relatively quick response, reproducible and stable sensor signals, and high selectivity to VOCs. The sensor responses followed behavior typical for mixed-potential-type gas sensors based on oxygen-ion conductors. It was thus suggested that the electrochemical oxidation of VOCs with oxide ions took place at the interfacial oxygen ion-conductive layer that was formed by the reaction of NASICON with BiCuVOx.