ZnO Gas Sensor Research Articles

Constructing 2D Porous ZnO Gas Sensors Based on Polyvinylpyrrolidone-Assisted Zn-MOF Nanosheets for NO2 Detection.

Two-dimensional (2D) ZnO nanomaterials are promising for gas sensing, because of their large surface area, abundant active sites, and rapid charge transfer. However, it is challenging to prepare 2D ZnO nanosheet gas sensors with high sensing performance, due to the tight interlayer stack and low adsorptive property of ZnO for NO2 molecules. Herein, we synthesized Zn-MOF nanosheets employing polyvinylpyrrolidone (PVP) as the structure-directing agent, further through pyrolysis of the Zn-MOF to obtain 2D ZnO nanosheet gas sensors. As anticipated, the 2D ZnO gas sensors exhibited high sensitivity and selectivity for NO2, and the optimal sample could achieve a response value of 162 at the working temperature of 160 °C, which is 10 times higher than that of pristine ZnO. Meanwhile, experimental and DFT results showed that PVP plays critical roles in the lateral lattice growth of 2D Zn-MOF nanosheets, while the existence of PVP makes the ZnO gas sensors with rich porous property and more oxygen vacancy after the pyrolysis process, which promoted the adsorption, activity, and surface reaction for NO2 molecules. It provides a new approach for the application of 2D ZnO nanosheets in the NO2 detection field.

Investigation on oxygen vacancy regulation mechanism of ZnO gas sensors under temperature modulation mode to distinguish alcohol homologue gases

Homologous gases are difficult to be detected due to their similar chemical properties, especially for metal oxide semiconductor gas sensors whose selectivity has always been a significant challenge. Dynamic temperature modulation method with periodically changing operating temperature was used to detect alcohol homologous gases. However, there are still problems with the deviceization of heating control circuits. In this paper, dynamic temperature modulated method which needs a simple heating circuit is used to distinguish alcohol homologous gases in combination with oxygen vacancy modulation of ZnO gas sensors. Ethyl alcohol, n-propyl alcohol, isopropyl alcohol and n-butyl alcohol were measured by ZnO gas sensors with different oxygen vacancies content ratios under rectangular wave with a voltage range of 2–6 V. It was found that the greatest difference among the response curves when the material calcined at 600°C which has the largest oxygen vacancies content ratio. Finally, the recognition of the four homologue gases measured by ZnO-600 sensor was achieved by dynamic time warping and the recognition accuracy is 99.33 %.

Enhancement of n-butanol sensing performance of porous ZnO flakes by decorating Ag nanoparticles

This study designs to investigate a high-performance n-butanol gas sensor with a porous morphology. Porous ZnO flakes modified with Ag particles are prepared in two steps using hydrothermal and precipitation methods. Morphological analysis of the samples shows that the ZnO flakes were covered with mesopores with high porosity, and their edge lengths ranged from 3 μm to 6 μm, with a minimum thickness of 11.3 nm. Through rigorous gas sensing testing, we find that the optimal mass percentage Ag-ZnO sensor provided significantly better sensing behavior, with a 100.8 response for 100 ppm n-butanol, compared to 14.5 for the pure ZnO sensor, a 5.95-fold improvement. In addition, the sensor's optimal operating temperature is reduced from 290 °C to 190 °C, increasing its adaptability and versatility in practical applications. In subsequent tests, it is found that this Ag-ZnO sensor not only shows excellent selectivity to n-butanol but also favorable stability, with no significant degradation during 30 days of continuous testing. The experimental analysis shows that the formation of porous flake structure and the modification of Ag particles are effective approaches to improve the ZnO gas sensor.

Highly responsive double-shell ZnO hollow microspheres based gas sensor for acetic acid detection in vinegar

Acetic acid sensing materials for environmental and related food quality control requiring high sensitivity and selectivity, and ppb-level detection limit, remain challenging. Double-shell ZnO hollow microspheres were prepared by a surfactant assisted template method, and the resultant gas sensor showed excellent acetic acid sensing performance. The sensor response reached 116 to 100 ppm acetic acid at 270°C with a short response time of 3.2 s. The relationship between acetic acid concentration and the response was established, and the detection limit reached 100 ppb. DFT theory computational results demonstrated the highest adsorption energy of the ZnO gas sensor to acetic acid among all tested gases, proving its high selectivity to acetic acid. Besides, the unique double-shell hollow structure with abundant oxygen vacancy and large surface area might be the other factors that gave rise to its excellent acetic acid gas sensing performance. Based on the gas sensor, a method for fast quantitative detection of acetic acid content in vinegar was developed. The linear relationship between sensor response and acetic acid gas concentration of vaporized white vinegar was established, and the acetic acid content in the vinegar can be quickly determined. The result proved our acetic acid sensor to be a promising candidate for practical applications.

Sawtooth wave temperature modulation measurement method for recognizing five kinds of VOCs based on ZnO gas sensor

The dynamic measurement of temperature modulation can improve selectivity of semiconductor gas sensor. However, most of dynamic measurements only have one or two characteristic peaks, which makes great difficulties to subsequent gas identification. To solve this problem, this paper increased the characteristic peaks of dynamic response of ZnO sensor by adjusting the period of sawtooth wave, and expanded the difference of dynamic responses of different gases, so as to realize the efficient detection and identification of different VOCs. The experiments find that when the period of sawtooth wave is 90 s, the dynamic responses of the sensor to the five gases of ethanol, toluene, 2-butanone, acetone and ethyl ether are better, with many characteristic peaks. Combined with support vector machine, the recognition rate reaches 100 %, and it is difficult to misjudge the type and concentration of gas, which improves the selectivity.

Controlled evolution of surface microstructure and phase boundary of ZnO nanoparticles for the multiple sensitization effects on triethylamine detection.

In ZnO gas sensors, donor defects (such as zinc interstitials and oxygen vacancies) are considered active sites for the chemical adsorption and ionization of oxygen on the surface of ZnO, which can significantly enhance the sensor's response. However, the influence of the surface microstructure and phase boundaries of ZnO nanoparticles on the chemical adsorption and ionization of surface oxygen has rarely been explored. In this study, we developed a mixed-phase ZnO nanoparticle gas sensor with a rich phase boundary showing 198-50 ppm improvement in response to triethylamine at 340 °C. This is attributed to the generation of defects originating from lattice mismatch at the ZnO - zincite phase boundaries, which providing more active sites for adsorption of oxygen and triethylamine molecules. This work demonstrates a feasible method of combining surface microstructure regulation with pyrolysis strategies to develop ZnO sensors with significantly enhanced gas response performance.

Mn Doped ZnO Film for Ethanol Vapor Detection

It has become a major challenge to develop highly sensitive and selective ZnO gas sensors to detect the leakage of toxic and explosive gases due to their high operating temperature. This article reports the design, characterization, and application of a Mn-doped ZnO thin film for ethanol gas vapor detection at a relatively lower temperature. To achieve this, undoped (0%) and Mn doped ZnO thin films were prepared using a spin coating technique and their structural, morphological and optical characterizations were carried out using XRD, SEM, EDX and UV-vis spectrophotometer. The XRD analysis showed a polycrystalline ZnO structure with the preferred orientations along (100), (002), (101), (102), (110), (103), and (200) planes. Upon Mn doping, the average crystallite size and band gap of ZnO were found to be decreased, which indicated that Mn ions substitute into the lattice of ZnO. Interestingly, SEM images revealed the grainy like porous ZnO surfaces. The EDX results confirmed the presence of Mn dopant in the ZnO lattice. In the doped films, the changes in particle size and grain boundaries of nanocrystalline ZnO resulted changes in sensitivity and operating temperature of ZnO gas sensor. The sensitivity measurements towards ethanol vapor showed significant decrease of the operating temperature from 220°C to 160°C as Mn concentration was increased from 0 to 5%. As-prepared Mn-doped ZnO sensors will be useful for sensing ethanol vapor at relatively low temperature.

Fabrication of high-performance RT-NH3 gas sensor based on Cu and La co-doped ZnO films through a facile drop-casting method

In this work, undoped ZnO, 2 % Cu-doped ZnO, 2 % La-doped ZnO and 2 % (Cu:La) co-doped ZnO thin films are deposited on glass substrates using the facile drop-casting method, and their physical characteristics were investigated for the gas sensing application. The X-ray diffraction (XRD) patterns confirmed the hexagonal wurtzite structure and increase in crystallinity for the doped samples. Field Emission Scanning Electron Microscopy (FESEM) images revealed a significant change in morphology from the granular particles to densely packed nanoplates-like morphology for (Cu:La) co-doped ZnO. The doped samples exhibited a red shift in absorption edge and shrinkage of the optical bandgap. Multiple emission peaks in photoluminescence (PL) spectra indicated the presence of defect level in the material. It was discovered that Cu-doped and La-doped ZnO thin films had a considerable NH3 sensing response at ambient temperature. The gas response was remarkably enhanced to 341 % for (Cu:La) co-doped ZnO film and a quicker response/recovery time of 80/10 s for 250 ppm NH3. The (Cu:La) co-doped ZnO can serve as a feasible alternative for pure ZnO gas sensors.

Capacitively coupled contactless conductivity detection (C4D) of ZnO nanostructures gas sensor by adding Au:Pd metal with response to ethanol and acetone vapor

In this work, we present a new gas sensor detection method using the capacitively coupled contactless conductivity detection (C4D) technique. The ZnO nanostructures were prepared by the current heating technique and then added Au: Pd nanoparticles on their surface by a sputtering technique. The gas detector with a C4D system had been fabricated. A data acquisition system was designed to record the signal obtained from the detection electrode. The sensing properties of the sensor with different gas vapor concentrations and operating temperatures have been investigated. Sensitivity was defined as the ratio of signal data in air and vapor gas ambient. The results found that the signal data depend on gas concentration and temperature. However, the results showed lower sensitivity than other techniques, such as the resistive ZnO gas sensor. Finally, the gas sensing mechanism in terms of surface charge density was proposed and used to explain the sensitivity enhancement of C4D ZnO gas sensors.

High response ZnO gas sensor derived from Tb@Zn-MOFs to acetic acid under UV excitation

As an n-type metal oxide semiconductor with a wide band gap (3.37 eV), ZnO has been widely used as gas sensors, but pure ZnO gas sensors suffer from low sensitivity. In this work, Zn-metal organic frameworks (Zn-MOFs) were synthesized by introducing H3BTC and H2ABDC as organic ligands, and then the Zn-MOFs were combined with Tb and calcined at 530 ℃ to obtain the derived oxides named as Tb@ZnO-1 and Tb@ZnO-2, respectively, in order to improve the gas sensitivity. Meanwhile, the microstructures and gas sensing properties of the two MOFs and their derivatives were characterized. Interestingly, the Zn-ABDC MOFs were observed as plush spherical microspheres assembled from nanowires, more possibly favorable for gas adsorption. In fact, prominent oxygen defects and chemisorbed oxygen content were found in the derivative Tb@ZnO-2. As a result, it exhibited good gas-sensitive properties and selectivity towards acetic acid, with a high response value of 47.30 at 240 ℃ for 100 ppm acetic acid gas and a very short response time (1 s), and the response value increased to 77.80 under UV excitation. The gas-sensitive mechanism and UV excitation mechanism are discussed.

Nano Hotplate Fabrication for Metal Oxide-Based Gas Sensors by Combining Electron Beam and Focused Ion Beam Lithography.

Metal oxide semiconductor (MOS) gas sensors are widely used for gas detection. Typically, the hotplate element is the key component in MOS gas sensors which provide a proper and tunable operation temperature. However, the low power efficiency of the standard hotplates greatly limits the portable application of MOS gas sensors. The miniaturization of the hotplate geometry is one of the most effective methods used to reduce its power consumption. In this work, a new method is presented, combining electron beam lithography (EBL) and focused ion beam (FIB) technologies to obtain low power consumption. EBL is used to define the low-resolution section of the electrode, and FIB technology is utilized to pattern the high-resolution part. Different Au++ ion fluences in FIBs are tested in different milling strategies. The resulting devices are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and secondary ion mass spectrometry (SIMS). Furthermore, the electrical resistance of the hotplate is measured at different voltages, and the operational temperature is calculated based on the Pt temperature coefficient of resistance value. In addition, the thermal heater and electrical stability is studied at different temperatures for 110 h. Finally, the implementation of the fabricated hotplate in ZnO gas sensors is investigated using ethanol at 250 °C.

TeO2 doped ZnO nanostructure for the enhanced NO2 gas sensing on MEMS sensor device

Severe health issues occur when human beings are exposed to NO2 gas, which is formed from combustion processes and reactions facilitated by sunlight. In order for safety measurements, NO2 gas sensors are highly needed at low concentration gas detections. ZnO gas sensor with the doping of TeO2 is developed using co-sputtering technique. ZnO/TeO2 heterojunction nanostructure is analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) for structural study. Gas sensing properties were tested with Micro electro mechanical system (MEMS) devices, on which ZnO/TeO2 nanostructure is developed. This study focused on doping of 4 different variations of TeO2 with ZnO such as 0%, 2%, 4% and 8% atomic weight percentage (at.w%) of TeO2. Upon testing, 4% ZnO/TeO2 transpired to be best possible parameter for NO2 gas sensing. With maximum NO2 gas concentration of 1 ppm, 4% ZnO/TeO2 sensor exhibited 80% sensing response and with 200 ppb of minimum gas concentration, sensor had 42% response at optimal temperature of 100 oC. TeO2 doped ZnO sensor exhibits superior performance over pure ZnO sensor and has good selectivity over NO2 gas.

A ZnO Gas Sensor with an Abnormal Response to Hydrogen

ZnO is a commonly used material for hydrogen gas sensors. In this study, a ZnO nanofiber film with a diameter of approximately 60 nm was synthesized by the electrospinning method. Compared to previously reported ZnO hydrogen gas sensors, an abnormal phenomenon was observed here, where the resistance of the ZnO nanofiber film increased upon exposure to hydrogen gas in the temperature range from 210 °C to 330 °C. The physical mechanism of this phenomenon was explored through microstructure analysis and DFT simulation calculations that showed a total charge transfer of 0.65 e for the hydrogen molecule. This study can push forward the understanding of ZnO hydrogen sensing.

Insecticide Monitoring in Cattle Dip with an E-Nose System and Room Temperature Screen-Printed ZnO Gas Sensors

Taktic, an Amitraz-based insecticide, is commonly used in sub-Saharan Africa to treat cattle for ticks. Due to misuse in rural dipping pools, some ticks are showing resistance to Taktic. This work presents a low-cost e-nose with commercial sensors to monitor Taktic levels in dipping pool water. The device shows distinctly different measurements for the odours of air, distilled water, farm water, and four levels of Taktic insecticide in farm water. A naive Bayes algorithm with a Gaussian distribution is trained on the data and a validation set achieves a 96.5% accuracy. This work also compares two sol-gel ZnO nanoparticle solutions with an off-the-shelf ZnO nanoparticle ink for use as active material in chemiresistive gas sensors to be employed in an e-nose array. The ZnO solutions are screen-printed onto gold electrodes, auto-sintered with a built in heater, and used with UV illumination to operate as low-power, room temperature gas sensors. All of the screen-printed ZnO sensors show distinct changes in resistance when exposed to Taktic vapours under room temperature and humidity conditions. The custom room temperature ZnO gas sensors fabricated via facile and low-cost processes are suitable for future integration in a point-of-need microsystem for the detection of Taktic in water.

Role of potassium loading in ZnO-based gas sensors under NO2 exposure – Operando diffuse reflectance infrared Fourier transform spectroscopic study

Unloaded, 0.08 and 0.9 at% potassium (K)-loaded ZnO gas sensors were investigated using operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy combined with simultaneous direct current (DC) sensor resistance measurements. At 250 °C in humidified (10%) synthetic air, during NO2 exposure, K-loaded ZnO exhibits a resistance decrease, which is contrary to the usual response of unloaded ZnO. Using DRIFTS, it was observed that for K-loaded ZnO, under NO2 exposure, free nitrates (such as KNO3) are formed and surface oxygen is removed, which explains the observed decrease in resistance. Moreover, the presence of K-related carbonates was confirmed through X-ray photoelectron spectroscopy. In addition, decomposition of monodentate carbonates was observed during the formation of free nitrates. To the best of our knowledge, this is the first time the peculiar sensor response of K-loaded ZnO to NO2 is reported and explained.

Separated detection of ethanol and acetone based on SnO2-ZnO gas sensor with improved humidity tolerance

Developing separated detection of ethanol and acetone for semiconductor oxides based gas sensors with relative stable gas response under different humidity is of great significance to practical application. Herein, a unique mesoporous SnO2-ZnO hierarchical structure has been prepared by a facile one-step hydrothermal method. Several characterizations for SnO2-ZnO samples have been carried out and the gas sensing properties of all gas sensors have been systematically investigated and analyzed. The results indicate that the optimal working temperature for ethanol detection will decrease from 275°C to 250°C, while the optimal working temperature for acetone detection will maintain at same temperature of 300°C with Sn2+ doping ratio increasing. For ethanol detection, 10 at% SnO2 modified ZnO gas sensor has the highest gas response, and the humidity tolerance has been obviously improved compared with those of lower Sn element doping ratios (1 at% and 5 at%) at 250°C. Moreover, the detection limit for ethanol and acetone can reach as low as 200 ppb with high SnO2 contents (10 at% and 15 at%). The improved gas sensing performance could be mainly attributed to the unique morphology of sensing material and the synergistic effect of SnO2 and ZnO.

Surfactant modified hexagonal ZnO gas sensor for acetic acid

Surfactant modified hexagonal ZnO gas sensor for acetic acid

Investigation of fiber laser-induced porous graphene electrodes in controlled atmospheres for ZnO nanorod-based NO2 gas sensors

This study aims to develop fiber laser-induced graphene (LIG) on polyimide films fabricated under a chamber with air, vacuum, or nitrogen atmospheres. Moreover, ZnO nanorods with a large specific surface area were synthesized by a facile and low-cost hydrothermal method. The ZnO nanorod sensing solution was drop-cast on the LIG surface with porous structures as a sensing layer for the NO2 gas sensor. The characteristics of the LIGs and ZnO nanorods including the surface morphology, elemental content, crystal structure, elemental composition, and chemical bond were investigated. The ZnO gas sensor with LIG electrodes fabricated in air has outstanding sensing performance in a high response (251.71 %), fast response and recovery times (9.5 s and 8.3 s, respectively), and response stability at a NO2 concentration of 1000 ppb. In addition, the ZnO gas sensor with LIG electrodes fabricated in vacuum has an excellent sensitivity. Based on the ZnO nanorod/LIG-based NO2 gas sensor with a high response and sensitivity, the proposed gas sensors can be combined with the Internet of Things and widely used in real-time detecting harmful gases.

Comparison of Characteristics of a ZnO Gas Sensor Using a Low-Dimensional Carbon Allotrope.

Owing to the increasing construction of new buildings, the increase in the emission of formaldehyde and volatile organic compounds, which are emitted as indoor air pollutants, is causing adverse effects on the human body, including life-threatening diseases such as cancer. A gas sensor was fabricated and used to measure and monitor this phenomenon. An alumina substrate with Au, Pt, and Zn layers formed on the electrode was used for the gas sensor fabrication, which was then classified into two types, A and B, representing the graphene spin coating before and after the heat treatment, respectively. Ultrasonication was performed in a 0.01 M aqueous solution, and the variation in the sensing accuracy of the target gas with the operating temperature and conditions was investigated. As a result, compared to the ZnO sensor showing excellent sensing characteristics at 350 °C, it exhibited excellent sensing characteristics even at a low temperature of 150 °C, 200 °C, and 250 °C.

The Effect of Rare Earths on the Response of Photo UV-Activate ZnO Gas Sensors.

In this work, ZnO nanoparticle resistive sensors decorated with rare earths (REs; including Er, Tb, Eu and Dy) were used at room temperature to detect atmospheric pollutant gases (NO2, CO and CH4). Sensitive films were prepared by drop casting from aqueous solutions of ZnO nanoparticles (NPs) and trivalent RE ions. The sensors were continuously illuminated by ultraviolet light during the detection processes. The effect of photoactivation of the sensitive films was studied, as well as the influence of humidity on the response of the sensors to polluting gases. Comparative studies on the detection properties of the sensors showed how the presence of REs increased the response to the gases detected. Low concentrations of pollutant gases (50 ppb NO2, 1 ppm CO and 3 ppm CH4) were detected at room temperature. The detection mechanisms were then discussed in terms of the possible oxidation-reduction (redox) reaction in both dry and humid air atmospheres.