Rapid detection of low concentration CO using Pt-loaded ZnO nanosheets

Current Advances of CO Sensing Based on Low Dimensional Materials

Carbon monoxide (CO) is a harmful gas with significant impacts on human health and the environment. Its timely detection, especially in the event of thermal runaway in automotive lithium batteries, is crucial to prevent casualties. This paper reviews the progress in the development of efficient, sensitive, and reliable CO sensors, focusing on electrochemical, optical, and resistive sensing materials. Low-dimensional materials have a large specific surface area, providing an abundant number of active sites, which has drawn extensive attention from researchers. According to the different sensor signals, we categorized these sensors into electrical and optical signal sensors. We hope that by systematically introducing the sensing mechanism and sensing performance of these two kinds of sensors, appropriate CO sensors can be developed in different application scenarios so as to realize early warning and monitoring to the maximum extent, reduce industrial losses, and ensure the life and health of personnel.

Metal-Organic Framework-Derived Au-Doped In2O3 Nanotubes for Monitoring CO at the ppb Level

Achieving selective detection of ppb-level CO is important for air quality testing at industrial sites to ensure personal safety. Noble metal doping enhances charge transfer, which in turn reduces the detection limit of metal oxide gas sensors. In this work, metal-organic framework-derived Au-doped In2O3 nanotubes with high electrical conductivity are synthesized by pyrolysis of the Au-doped metal-organic framework (In-MIL-68) as a template. Gas-sensing experiments reveal that the detection limit of 0.2% Au-doped In2O3 nanotubes (0.2% Au, mass fraction) is as low as 750 ppb. Meanwhile, the sensing material shows a response value of 18.2 to 50 ppm of CO at 240 °C, which is about 2.8 times higher than that of pure In2O3. Meanwhile, the response and recovery times are short (37 s/86 s). The gas-sensing mechanism of CO is uncovered by in situ DRIFTS through the reaction intermediates. In addition, first-principles calculations suggest that Au doping of In2O3 significantly enhances its adsorption energy for CO and improves the electron transfer properties. This study reveals a novel synthesis pathway for Au-doped In2O3 nanotubular structures and their potential application in low concentration CO detection.

Simple synthesis of flower-like ZnO@Pt composites for dual-mode colorimetric detecting Hg2+ with smartphone and UV-vis

BACKGROUND

Mercury is one of the most toxic heavy metal contaminants that can be harmful to human health through the food chain. Recently, the colorimetric detection of heavy metals based on nanozyme catalytic activity has received extensive interest due to the simplicity, signal visibility and suitability for in situ detection. However, the majority of these nanozymes that can be utilized for detecting mercury with high synthesis temperature and complicated synthesis methods, which limited their practical application.

RESULTS

In this work, flower-like ZnO@Pt composites were simply synthesized at room temperature, the flower-like structure and the high electron mobility of ZnO endow ZnO@Pt with stronger peroxidase-like activity. Consequently, dual-mode (UV-vis and smartphone) colorimetric sensors were designed to detect Hg2+. In UV-vis mode, the Hg2+ concentration linear range was 10-400 nM, and the limit of detection (LOD) was 0.54 nM. In smartphone mode, the Hg2+ concentration linear range was 50-1250 nM, and the LOD was 29.8 nM. A parallel analysis in 3 real water samples was confirmed by ICP-MS, the results showed good correlations (R2 > 0.98), indicating the practical reliability of these sensors.

SIGNIFICANCE

The novel flower-like ZnO@Pt composites with high stability, catalytic activity and Hg2+ response were simply synthesized at room temperature, simplifying the synthesis steps and reducing costs. The sensitivity of the developed colorimetric sensor in UV-vis mode was 3-145 times higher than that of the similar methods. The colorimetric sensor in smartphone mode broadened the detection range and improved the portability of Hg2+ detection. Thus, the dual-mode (UV-vis and smartphone) colorimetric sensors providing new detection modes for rapid monitoring of Hg2+ in environmental water.

Advances in 2D Materials Based Gas Sensors for Industrial Machine Olfactory Applications

The escalating development and improvement of gas sensing ability in industrial equipment, or "machine olfactory", propels the evolution of gas sensors toward enhanced sensitivity, selectivity, stability, power efficiency, cost-effectiveness, and longevity. Two-dimensional (2D) materials, distinguished by their atomic-thin profile, expansive specific surface area, remarkable mechanical strength, and surface tunability, hold significant potential for addressing the intricate challenges in gas sensing. However, a comprehensive review of 2D materials-based gas sensors for specific industrial applications is absent. This review delves into the recent advances in this field and highlights the potential applications in industrial machine olfaction. The main content encompasses industrial scenario characteristics, fundamental classification, enhancement methods, underlying mechanisms, and diverse gas sensing applications. Additionally, the challenges associated with transitioning 2D material gas sensors from laboratory development to industrialization and commercialization are addressed, and future-looking viewpoints on the evolution of next-generation intelligent gas sensory systems in the industrial sector are prospected.

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.

In situ assembly of one-dimensional Pt@ZnO nanofibers driven by a ZIF-8 framework for achieving a high-performance acetone sensor

To obtain a high-performance gas sensor, it is essential to ingeniously design sensing materials containing the features of high catalytic performance, abundant oxygen vacancies, and splendid grain dispersibility through a simple method. Inspired by the fact that ZIF-8 contains semiconductor metal atoms, well-arranged ZnO nanoparticle (NP)-in situ assembled one-dimensional nanofibers (NFs) are obtained by one-step electrospinning. By incorporating Pt NPs into the cavity of ZIF-8 NPs, well-dispersed Pt@ZnO NPs driven by Pt@ZIF-8 composites are obtained after annealing. The well-arranged Pt@ZnO NP-assembled NFs not only exhibit abundant oxygen vacancies but also avoid the self-aggregation of ZnO and Pt NPs. Meanwhile, the small Pt NPs could improve the catalytic effect in return. Therefore, the gas sensor fabricated based on the above materials exhibits an acetone sensitivity of 6.1 at 370 °C, compared with pristine ZnO NFs (1.6, 5 ppm). Moreover, the well-arranged Pt@ZnO NP-assembled NFs show exceptional sensitivity to acetone with a 70.2 ppb-level detection limit in theory. The synergistic advantages of the designed sensing material open up new possibilities for non-invasive disease diagnosis.

Recent Progress in Spinel Ferrite (MFe2O4) Chemiresistive Based Gas Sensors

Gas-sensing technology has gained significant attention in recent years due to the increasing concern for environmental safety and human health caused by reactive gases. In particular, spinel ferrite (MFe2O4), a metal oxide semiconductor with a spinel structure, has emerged as a promising material for gas-sensing applications. This review article aims to provide an overview of the latest developments in spinel-ferrite-based gas sensors. It begins by discussing the gas-sensing mechanism of spinel ferrite sensors, which involves the interaction between the target gas molecules and the surface of the sensor material. The unique properties of spinel ferrite, such as its high surface area, tunable bandgap, and excellent stability, contribute to its gas-sensing capabilities. The article then delves into recent advancements in gas sensors based on spinel ferrite, focusing on various aspects such as microstructures, element doping, and heterostructure materials. The microstructure of spinel ferrite can be tailored to enhance the gas-sensing performance by controlling factors such as the grain size, porosity, and surface area. Element doping, such as incorporating transition metal ions, can further enhance the gas-sensing properties by modifying the electronic structure and surface chemistry of the sensor material. Additionally, the integration of spinel ferrite with other semiconductors in heterostructure configurations has shown potential for improving the selectivity and overall sensing performance. Furthermore, the article suggests that the combination of spinel ferrite and semiconductors can enhance the selectivity, stability, and sensing performance of gas sensors at room or low temperatures. This is particularly important for practical applications where real-time and accurate gas detection is crucial. In conclusion, this review highlights the potential of spinel-ferrite-based gas sensors and provides insights into the latest advancements in this field. The combination of spinel ferrite with other materials and the optimization of sensor parameters offer opportunities for the development of highly efficient and reliable gas-sensing devices for early detection and warning systems.

A sigh-performance hydrogen gas sensor based on Ag/Pd nanoparticle-functionalized ZnO nanoplates

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.

Advances in Noble Metal-Decorated Metal Oxide Nanomaterials for Chemiresistive Gas Sensors: Overview

Highly sensitive gas sensors with remarkably low detection limits are attractive for diverse practical application fields including real-time environmental monitoring, exhaled breath diagnosis, and food freshness analysis. Among various chemiresistive sensing materials, noble metal-decorated semiconducting metal oxides (SMOs) have currently aroused extensive attention by virtue of the unique electronic and catalytic properties of noble metals. This review highlights the research progress on the designs and applications of different noble metal-decorated SMOs with diverse nanostructures (e.g., nanoparticles, nanowires, nanorods, nanosheets, nanoflowers, and microspheres) for high-performance gas sensors with higher response, faster response/recovery speed, lower operating temperature, and ultra-low detection limits. The key topics include Pt, Pd, Au, other noble metals (e.g., Ag, Ru, and Rh.), and bimetals-decorated SMOs containing ZnO, SnO2, WO3, other SMOs (e.g., In2O3, Fe2O3, and CuO), and heterostructured SMOs. In addition to conventional devices, the innovative applications like photo-assisted room temperature gas sensors and mechanically flexible smart wearable devices are also discussed. Moreover, the relevant mechanisms for the sensing performance improvement caused by noble metal decoration, including the electronic sensitization effect and the chemical sensitization effect, have also been summarized in detail. Finally, major challenges and future perspectives towards noble metal-decorated SMOs-based chemiresistive gas sensors are proposed.

Cross-Interference of VOCs in SnO2-Based NO Sensors

In this work, we studied the influence of cross-interference effects between VOCs and NO on the performance of SnO₂ and Pt-SnO₂-based gas sensors. Sensing films were fabricated by screen printing. The results show that the response of the SnO₂ sensors to NO under air is higher than that of Pt-SnO₂, but the response to VOCs is lower than that of Pt-SnO₂. The Pt-SnO₂ sensor was significantly more responsive to VOCs in the NO background than in air. In the traditional single-component gas test, the pure SnO₂ sensor showed good selectivity to VOCs and NO at 300 °C and 150 °C, respectively. Loading noble metal Pt improved the sensitivity to VOCs at high temperature, but also significantly increased the interference to NO sensitivity at low temperature. The explanation for this phenomenon is that the noble metal Pt can catalyze the reaction between NO and VOCs to generate more O^-, which further promotes the adsorption of VOCs. Therefore, selectivity cannot be simply determined by single-component gas testing alone. Mutual interference between mixed gases needs to be taken into account.

Selective detection of trace carbon monoxide at room temperature based on CuO nanosheets exposed to (111) crystal facets

In recent years, carbon monoxide (CO) intoxication incidents occur frequently, and the sensitive detection of CO is particularly significant. At present, most reported carbon monoxide (CO) sensors meet the disadvantage of high working temperature. It is always a challenge to realize the sensitive detection of carbon monoxide at room temperature. In this study, CuO nanosheets exposed more (111) active crystal facets and oxygen vacancy defects were synthesized by a simple and environmentally friendly one-step hydrothermal method. The sensor has good comprehensive gas sensing performance, compared with other sensors that can detect CO at room temperature. The response value to 100 ppm CO at room temperature is as high as 39.6. In addition, it also has excellent selectivity, low detection limit (100 ppb), good reproducibility, moisture resistance and long-term stability (60 days). This excellent gas sensing performance is attributed to the special structural characteristics of 2D materials and the synergistic effect of more active crystal facets exposed on the crystal surface and oxygen vacancy defects. Therefore, it is expected to become a promising sensitive material for rapid and accurate detection of trace CO gas under low energy consumption, reduce the risk of poisoning, and then effectively protect human life safety.

Effect of Morphological Modification over g-C3N4 on Photocatalytic Hydrogen Evolution Performance of g-C3N4-Pt Photocatalysts

In this study, the morphological properties of g-C3N4 in g-C3N4-Pt photocatalysts were modified by a simple hydrothermal treatment for photocatalytic hydrogen evolution. In addition, the morphological modification effect of g-C3N4 on the hydrogen evolution performance was investigated. The long-time hydrothermal treatment clearly changed the morphology of g-C3N4 by building extended melem units with more oxygen functional groups at the defect edges of the extended melem units, which was evidenced by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) measurements. The different morphological features of g-C3N4 resulted in lower photoluminescence (PL) emission intensity in PL spectra and a smaller semicircle radius in electrochemical impedance spectroscopy (EIS) data. This indicates the more efficient charge separation of the g-C3N4-Pt photocatalyst with a modified morphology. Consequently, morphologically modified g-C3N4-Pt showed a higher photocatalytic hydrogen evolution rate due to the better charge separation efficiency.

Conductometric ppb-Level CO Sensors Based on In2O3 Nanofibers Co-Modified with Au and Pd Species

Carbon monoxide (CO) is one of the most toxic gases to human life. Therefore, the effective monitoring of it down to ppb level is of great significance. Herein, a series of In₂O₃ nanofibers modified with Au or Pd species or simultaneous Au and Pd species have been prepared by electrospinning combined with a calcination process. The as-obtained samples are applied for the detection of CO. Gas-sensing investigations indicate that 2 at% Au and 2 at% Pd-co-modified In₂O₃ nanofibers exhibit the highest response (21.7) to 100 ppm CO at 180 °C, and the response value is ~8.5 times higher than that of pure In₂O₃ nanofibers. More importantly, the detection limit to CO is about 200 ppb with a response value of 1.23, and is obviously lower than that (6 ppm) of pure In₂O₃ nanofibers. In addition, the sensor also shows good stability within 19 days. These demonstrate that co-modifying In₂O₃ nanofibers with suitable amounts of Pd and Au species might be a meaningful strategy for the development of high-performance carbon monoxide gas sensors.

Crystal-facet and microstructure engineering in ZnO for photocatalytic NO oxidation

Photocatalysis is believed to be an important way of reducing NO pollutant in air and the facet engineering of semiconducting oxides could enhance the efficiency of the photocatalysis. ZnO nanoparticles with different exposed crystalline facets were successfully synthesized using a hydrothermal method and their photocatalytic degradation towards NO was investigated. The crystals from ZnCl₂ precursor were hexagonal mesoporous ones with exposed (0002) facet, while those from zinc acetate were in the form of flakes or wheat ears with enhanced exposure of (101(-)1) facet. Calcination in air imparted an enhanced the textural coefficient of the orientated facets as well as the oxygen defects. The nanocrystals with enhanced (0002) facet and lower flat-band energy did better in photoelectrochemical water-oxidation than those with exposed (101(-)1) facet that showed superior photocatalytic activity (approaching 76.7 ± 0.6% under 365 nm photons) for NO oxidation. According to theoretical calculations, (101(-)1) facet with O termination showed much higher affinity to NO molecules than other configurations, and the oxygen vacancy in ZnO played an minor role in the photocatalytic oxidation of NO. A high quantum efficiency approaching 97.5 ± 1.4% under 275 nm photons was obtained for the ZnO crystals from zinc acetate with mixed (0002) and (101(-)1) facets. This research explores the special characteristics of ZnO with different exposed facets and is important for the future design of highly efficient photocatalyst for hazardous material removal.

A dimethyl disulfide gas sensor based on nanosized Pt-loaded tetrakaidecahedralα-Fe2O3nanocrystals

Surface modification by employing precious metals is one of the most effective ways to improve the gas-sensing performance of metal oxide semiconductors. Pureα-Fe₂O₃nanoparticles and Pt-modifiedα-Fe₂O₃nanoparticles were prepared sequentially using a rather simple hydrothermal synthesis and impregnation method. Compared with the originalα-Fe₂O₃nanomaterials, the Pt-α-Fe₂O₃nanocomposite sensor shows a higher response value (R_a/R_g = 58.6) and a shorter response/recovery time (1 s/168 s) to 100 ppm dimethyl disulfide (DMDS) gas at 375 °C. In addition, it has better selectivity to DMDS gas with the value of more than 9 times higher than the other target gases at 375 °C. This study indicates that the Pt-α-Fe₂O₃nanoparticle sensor has good prospects and can be used as a low-cost and effective DMDS gas sensor.

Highly efficient ethanol vapour detection using g-C3N4/ZnO micro flower-like heterostructural composites

This work proposes precursor pyrolysis, ultrasonic exfoliation and hydrothermal methods as well as high-temperature calcination strategies to fabricate heterostructured g-C₃N₄/ZnO composites with excellent ethanol vapour sensing properties. The structure, composition and morphology of the as-prepared g-C₃N₄/ZnO composites were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). Then, the sensing properties of the g-C₃N₄/ZnO composites for ethanol (C₂H₅OH) were studied, and g-C₃N₄ doping with different mass ratios was used to control the gas-sensing properties of the composites. Compared with pure ZnO and g-C₃N₄, the performance of g-C₃N₄ with 1% doping content is the best, and the gas sensing activity of the 1% g-C₃N₄/ZnO composite is greatly improved at the optimal working temperature (280 °C). The response to 100 ppm ethanol reaches 81.4, which is 3.7 times that of the pure ZnO-based sensor under the same conditions. In addition, the sensor has good selectivity as well as fast response and recovery speeds (24 s and 63 s, respectively). Finally, a reasonable gas sensing enhancement mechanism is proposed, and it is believed that the constructed g-C₃N₄/ZnO micro flower-like heterostructure and the distinct positions of the valence and conduction bands of ZnO and g-C₃N₄ lead to the obtained sensor exhibiting a large specific surface area and increased conductivity, thereby improving the g-C₃N₄/ZnO-based sensor sensing performance.

Heterostructural g-C₃N₄/ZnO composites were synthesized through a facile hydrothermal strategy using as-prepared g-C₃N₄ nanosheets and precursor solutions of ZnO for effective ethanol detection.

Size-Controlled Au Nanoparticles Incorporating Mesoporous ZnO for Sensitive Ethanol Sensing

Zinc oxide (ZnO) as a commonly used semiconductor material has aroused extensive research attention in various fields, such as field-effect transistors, solar cells, luminescent devices, and sensors, because of its excellent light-electrical features and large exciton bonding energy. Herein, ultrasmall Au nanoparticles with tunable size decorated mesoporous ZnO nanospheres were synthesized via facile formaldehyde-assisted metal-ligand cross-linking strategy, where these active Au species could be transferred into Au nanoparticles in the frameworks by various reduction strategies. Typically, mesoporous ZnO-Au with a photoreduction technique showed superior ethanol sensing performance (ca. 159 for 50 ppm at 200 °C) because of its high surface area, dual-mesoporous structure, and interface effect (electron effect, surface catalytic/adsorption). Moreover, the mesoporous ZnO-Au composites by photoreduction show much better performance than those via H2 reduction and NaBH4 reduction, which is ascribed to the providential size of Au nanoparticles (ca. 6.6 nm) and abundant oxygen defects in the composites. In particular, the selectivity and sensitivity of mesoporous ZnO-Au far exceeds those of materials loaded with other noble metals (Pt, Pd, and Ag). The sensing mechanism of mesoporous ZnO-Au for ethanol is attributed to classical surface adsorption/catalytic reaction, where strong sensitization effect (electron and chemical) and the spillover effect of Au nanoparticles in the catalytic reaction cause superior ethanol sensing performances. In situ FTIR and GC-MS measurement revealed that the catalytic oxidation of ethanol follows the process of dehydrogenation and deep oxidation, that is, dehydrogenation to acetaldehyde, and then further oxidation to carbon dioxide and water.

Effect of Al and Mg Doping on Reducing Gases Detection of ZnO Nanoparticles

In this work, the main objective is to enhance the gas sensing capability through investigating the effect of Al and Mg doping on ZnO based sensors. ZnO, Mg1% doped ZnO, Al5% doped ZnO and (Al5%, Mg1%) co-doped ZnO nanoparticles (NPs) were synthesized by a modified sol-gel method. The structural characterization showed the hexagonal crystalline structure of the prepared samples. Morphological characterizations confirmed the nanometric sizes of the NPs (27–57 nm) and elemental composition investigation proved the existence of Al and Mg with low concentrations. The optical characterization showed the high absorbance of the synthesized samples in the UV range. The gas sensing performances of the synthesized samples, prepared in the form of thick films, were investigated. Sensing tests demonstrated the high influence of the Al and Mg on the sensing performances towards H2 and CO gas, respectively. The 5A1MZO-based sensor exhibits high sensitivity and low detection limits to H2 (<2 ppm) and CO (<1 ppm). It showed a response around 70 (at 250 °C) towards 2000 ppm H2 and 2 (at 250 °C) towards CO.

A high-response formaldehyde sensor based on fibrous Ag-ZnO/In2O3 with multi-level heterojunctions

Timely detection of formaldehyde is pivotal because formaldehyde is slowly released from the indoor decorative materials, jeopardizing our healthy. Herein, a high-response formaldehyde gas sensor based on Ag-ZnO/In₂O₃ nanofibers was successfully fabricated. Compared with all the control samples, the hybrid exhibits superior sensitivity (0.65 ppm^-1), excellent selectivity (≥ 12.5) and durable stability (the deviation value ≤ 3%). Particularly, an ultra-high response value of about 186 towards 100 ppm of formaldehyde at 260 °C was achieved, heading the list of outstanding candidates. Additionally, the limit of detection is as low as 9 ppb. The enhanced gas sensing properties can be mainly attributed to multi-level heterojunctions (n-n heterojunction and Ohmic junction) and the "spill-over" effect of Ag, ultimately increasing the adsorption of gas molecules on the surface of sensing material. This work verifies that proper design of multi-level heterojunctions significantly upgrade the sensing performance.

Recent Advances in ZnO-Based Carbon Monoxide Sensors: Role of Doping

Monitoring and detecting carbon monoxide (CO) are critical because this gas is toxic and harmful to the ecosystem. In this respect, designing high-performance gas sensors for CO detection is necessary. Zinc oxide-based materials are promising for use as CO sensors, owing to their good sensing response, electrical performance, cost-effectiveness, long-term stability, low power consumption, ease of manufacturing, chemical stability, and non-toxicity. Nevertheless, further progress in gas sensing requires improving the selectivity and sensitivity, and lowering the operating temperature. Recently, different strategies have been implemented to improve the sensitivity and selectivity of ZnO to CO, highlighting the doping of ZnO. Many studies concluded that doped ZnO demonstrates better sensing properties than those of undoped ZnO in detecting CO. Therefore, in this review, we analyze and discuss, in detail, the recent advances in doped ZnO for CO sensing applications. First, experimental studies on ZnO doped with transition metals, boron group elements, and alkaline earth metals as CO sensors are comprehensively reviewed. We then focused on analyzing theoretical and combined experimental-theoretical studies. Finally, we present the conclusions and some perspectives for future investigations in the context of advancements in CO sensing using doped ZnO, which include room-temperature gas sensing.

One-dimensional In2O3nanorods as sensing material for ppb-level n-butanol detection

Effectively and quantificationally detecting hazardous gas n-butanol is very significant in daily life, which can bring about a safe living condition for humans. In this study, the one-dimensional In₂O₃nanorods were successfully synthesized via hydrothermal route and post-heat treatment. Noticeably, one-dimensional nanorods structures were obtained and the products presented a superior growth orientation along with (222) plane. Additionally, systematical gas-sensing measurements of the sensor made from In₂O₃nanorods towards hazardous n-butanol gas were conducted. Results exhibited that the fabricated sensor showed excellent n-butanol sensing properties, with aspects to a superior response value of 342.20 with concentration 100 ppm at 240 °C, remarkable selectivity, fast response/recovery times (77.5/34.2 s) and good stability. More interestingly, the detection limit of sensor as low as 500 ppb and a good linearity relationship between response values and n-butanol concentrations was presented. Gas-sensitive properties of this sensor are better than previously reported in n-butanol detection. All results demonstrate that one-dimensional In₂O₃nanorod is a promising sensor material to practical applications in effectively detecting n-butanol gas.

0D/2D CdS/ZnO composite with n-n heterojunction for efficient detection of triethylamine

It is imperative to seek for novel materials with pronounced gas sensing performance towards triethylamine for the sake of human health. Herein, we successfully fabricate an outstanding triethylamine sensor based on CdS/ZnO composite with 0D/2D structure, which are prepared by in-situ growth of CdS quantum dots on ultra-thin ZnO nanosheets. The ratios between the two ingredients are adjusted and their effect is evaluated. The optimal sample exhibits the lowest operating temperature of 200 °C, the highest response value of ~20 and the fastest response time of 2 s. Besides, it also has the virtues of durable stability, excellent selectivity and superior anti-interference ability. The mechanism behind the aforementioned intriguing performance is investigated by X-ray photoelectron spectroscopy, Kelvin probe and density function theory (DFT) simulation. All the results verify that the enhanced gas sensing properties are derived from splendid 0D/2D structure, n-n heterojunction and large specific surface area. Additionally, this study opens an avenue for designing sensors with 0D/2D structure.

Hierarchically interconnected ZnO nanowires for low-temperature-operated reducing gas sensors: experimental and DFT studies

The larger surface area and increased point contacts of hierarchically arranged ZnO nanowires (upper panel) offered more potential barriers for the gas molecule exhibiting binding strength CO < C₂H₅OH < NH₃ (lower panel) and provided excellent gas-sensing performance at 100 °C for NH₃ gas.

Correlation between Microstructure and Chemical Composition of Zinc Oxide Gas Sensor Layers and Their Gas-Sensitive Properties in Chlorine Atmosphere

In this article, we present results concerning the impact of structural and chemical properties of zinc oxide in various morphological forms and its gas-sensitive properties, tested in an atmosphere containing a very aggressive gas such as chlorine. The aim of this research was to understand the mechanism of chlorine detection using a resistive gas sensor with an active layer made of zinc oxide with a different structure and morphology. Two types of ZnO sensor layers obtained by two different technological methods were used in sensor construction. Their morphology, crystal structure, specific surface area, porosity, surface chemistry and structural defects were characterized, and then compared with gas-sensitive properties in a chlorine-containing atmosphere. To achieve this goal, scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL) methods were used. The sensing properties of obtained active layers were tested by the temperature stimulated conductance method (TSC). We have noticed that their response in a chlorine atmosphere is not determined by the size of the specific surface or porosity. The obtained results showed that the structural defects of ZnO crystals play the most important role in chlorine detection. We demonstrated that Cl2 adsorption is a concurrent process to oxygen adsorption. Both of them occur on the same active species (oxygen vacancies). Their concentration is higher on the side planes of the zinc oxide crystal than the others. Additionally, ZnO sublimation process plays an important role in the chlorine detection mechanism.

Alkali metal-incorporated spinel oxide nanofibers enable high performance detection of formaldehyde at ppb level

Sensing material with high sensitivity, excellent selectivity and ultra-low detection limit is crucial for monitoring formaldehyde, which is a kind of hazardous gas to human health at very low concentration. Some one-dimensional semiconductor metal oxides show acceptable responses towards formaldehyde. However, the detection limit and selectivity of these sensors are still not satisfied, especially at ppb level. Herein, alkali metals (K, Na) doped CdGa₂O₄ nanofibers with excellent formaldehyde sensing performance are prepared by an electrospinning method. These nanofibers have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance spectroscopy (EPR), elemental mapping and other techniques. As a result, the sensor based on 7.5 at.% K doped CdGa₂O₄ gives remarkably improved formaldehyde sensing properties compared with that of pristine CdGa₂O₄. The greatly increased sensitivity and selectivity should be attributed to the increased chemisorbed oxygen and the enhanced basicity caused by the additional alkali metal, respectively. All in all, the 7.5 at.% K doped CdGa₂O₄ is a good candidate for the rapid detecting formaldehyde at ppb level.

Effect of the Morphology and Electrical Property of Metal-Deposited ZnO Nanostructures on CO Gas Sensitivity

The development of a highly sensitive gas sensor for toxic gases is an important issue in that it can reduce the damage caused by unexpected gas leaks. In this regard, in order to make the sensor accurate and highly responsive, we have investigated which morphology is effective to improve the sensitivity and how the deposited nanoparticle affects the sensitivity by controlling the morphology of semiconductor oxides—either nanorod or nanoplate—and depositing metal nanoparticles on the semiconductor surface. In this study, we compared the CO gas sensitivity for sensors with different morphology (rod and plate) of ZnO nanostructure with metal nanoparticles (gold and copper) photodeposited and investigated the correlation between the gas sensitivity and some factors such as the morphology of ZnO and the properties of the deposited metal. Among the samples, Au/ZnO nanorod showed the best response (~86%) to the exposure of 100 ppm CO gas at 200 °C. The result showed that the electrical properties due to the deposition of metal species also have a strong influence on the sensor properties such as sensor response, working temperature, the response and recovery time, etc., together with the morphology of ZnO.