Room-temperature gas sensors based on ZnO nanorod/Au hybrids: Visible-light-modulated dual selectivity to NO2 and NH3

MOF-derived xPd-NPs@ZnO porous nanocomposites for ultrasensitive ppb-level gas detection with photoexcitation: Design, diverse-scenario characterization, and mechanism

Metal-organic frameworks (MOFs) have been regarded as a potential candidate with great application prospects in the field of gas sensing. Although plenty of previous efforts have been made to improve the sensitivity of MOF-based nanocomposites, it is still a great challenge to realize ultrafast and high selectivity to typical flammable gases in a wide range. Herein, porous xPd-NPs@ZnO were prepared by optimized heat treatment, which maintained the controllable morphology and high specific surface area of 471.08 m2g-1. The coupling effects of photoexcitation and thermal excitation on the gas-sensing properties of nanocomposites were systematically studied. An ultrafast high response of 88.37 % towards 200 ppm H2 was realized within 1.2 s by 5.0Pd-NPs@ZnO under UV photoexcitation. All xPd-NPs@ZnO exhibited favorable linearity over an extremely wide range (0.2-4000 ppm H2) of experimental tests, indicating the great potential in quantitative detection. The photoexcited carriers enabled the nanocomposites a considerable response at lower operating temperatures, which made diverse applications of the sensors. The mechanisms of high sensing performances and the photoexcitation enhancement were systematically explained by DFT calculations. This work provides a solid experimental foundation and theoretical basis for the design of controllable porous materials and novel photoexcited gas detection.

Effects of Visible Light on Gas Sensors: From Inorganic Resistors to Molecular Material-Based Heterojunctions

In the last two decades, many research works have been focused on enhancing the properties of gas sensors by utilising external triggers like temperature and light. Most interestingly, the light-activated gas sensors show promising results, particularly using visible light as an external trigger that lowers the power consumption as well as improves the stability, sensitivity and safety of the sensors. It effectively eliminates the possible damage to sensing material caused by high operating temperature or high energy light. This review summarises the effect of visible light illumination on both chemoresistors and heterostructure gas sensors based on inorganic and organic materials and provides a clear understanding of the involved phenomena. Finally, the fascinating concept of ambipolar gas sensors is presented, which utilised visible light as an external trigger for inversion in the nature of majority charge carriers in devices. This review should offer insight into the current technologies and offer a new perspective towards future development utilising visible light in light-assisted gas sensors.

The enhanced sensing properties of MOS-based resistive gas sensors by Au functionalization: a review

Gas sensors are essential for detecting toxic gases that can harm social life or industrial production. Traditional metal oxide semiconductor (MOS)-based sensors suffer from shortcomings such as high operating temperature and slow response time, which limits their detection capabilities. Thus, there is a need to improve their performance. One useful technique is noble metal functionalization, which can effectively enhance the response/recovery time, sensitivity and selectivity, sensing response, and optimum operating temperature of MOS gas sensors. Among the noble metals, Au NPs are considered a promising material for forming composite sensing materials to achieve better sensing performance. This paper aims to review and discuss the recent research on Au-decorated MOS-based sensors, including Au/n-type MOS-based sensors, Au/p-type MOS-based sensors, Au/MOS/carbon composite materials, and Au/MOS/perovskite composite materials. The sensing mechanism of Au-functionalized MOS-based materials will also be examined.

Functionalized Hydrogel-Based Wearable Gas and Humidity Sensors

Breathing is an inherent human activity; however, the composition of the air we inhale and gas exhale remains unknown to us. To address this, wearable vapor sensors can help people monitor air composition in real time to avoid underlying risks, and for the early detection and treatment of diseases for home healthcare. Hydrogels with three-dimensional polymer networks and large amounts of water molecules are naturally flexible and stretchable. Functionalized hydrogels are intrinsically conductive, self-healing, self-adhesive, biocompatible, and room-temperature sensitive. Compared with traditional rigid vapor sensors, hydrogel-based gas and humidity sensors can directly fit human skin or clothing, and are more suitable for real-time monitoring of personal health and safety. In this review, current studies on hydrogel-based vapor sensors are investigated. The required properties and optimization methods of wearable hydrogel-based sensors are introduced. Subsequently, existing reports on the response mechanisms of hydrogel-based gas and humidity sensors are summarized. Related works on hydrogel-based vapor sensors for their application in personal health and safety monitoring are presented. Moreover, the potential of hydrogels in the field of vapor sensing is elucidated. Finally, the current research status, challenges, and future trends of hydrogel gas/humidity sensing are discussed.

Controllable Synthesis of Sheet-Flower ZnO for Low Temperature NO2 Sensor

ZnO is a wide band gap semiconductor metal oxide that not only has excellent electrical properties but also shows excellent gas-sensitive properties and is a promising material for the development of NO₂ sensors. However, the current ZnO-based gas sensors usually operate at high temperatures, which greatly increases the energy consumption of the sensors and is not conducive to practical applications. Therefore, there is a need to improve the gas sensitivity and practicality of ZnO-based gas sensors. In this study, three-dimensional sheet-flower ZnO was successfully synthesized at 60 °C by a simple water bath method and modulated by different malic acid concentrations. The phase formation, surface morphology, and elemental composition of the prepared samples were studied by various characterization techniques. The gas sensor based on sheet-flower ZnO has a high response value to NO₂ without any modification. The optimal operating temperature is 125 °C, and the response value to 1 ppm NO₂ is 125. At the same time, the sensor also has a lower detection limit (100 ppb), good selectivity, and good stability, showing excellent sensing performance. In the future, water bath-based methods are expected to prepare other metal oxide materials with unique structures.

Design and Simulation of Au/SiO2 Nanospheres Based on SPR Refractive Index Sensor

In this paper, three different structures of surface plasmon resonance (SPR) sensors based on the Kretschmann configuration: Au/SiO₂ thin film structure, Au/SiO₂ nanospheres and Au/SiO₂ nanorods are designed by adding three different forms of SiO₂ materials behind the gold film of conventional Au-based SPR sensors. The effects of SiO₂ shapes on the SPR sensor are investigated through modeling and simulation with the refractive index of the media to be measured ranging from 1.330 to 1.365. The results show that the sensitivity of Au/SiO₂ nanospheres could be as high as 2875.4 nm/RIU, which is 25.96% higher than that of the sensor with a gold array. More interestingly, the increase in sensor sensitivity is attributed to the change in SiO₂ material morphology. Therefore, this paper mainly explores the influence of the shape of the sensor-sensitizing material on the performance of the sensor.

Three-dimensional hierarchical flower-like bimetallic–organic materials in situ grown on carbon cloth and doped with sulfur as an air cathode in a microbial fuel cell

Herein, the output power density produced by Zn/Co-S-3DHFLM as the cathode catalyst of an MFC was higher than that of Co-3DHFLM.

Progress and Perspectives of Single-Atom Catalysts for Gas Sensing

Single-atom catalysts (SACs) attract extensive attention in the field of heterogeneous catalysis in recent years due to the maximum atom utilization and unique physical and chemical properties. The gas sensing is actually a heterogeneous catalysis process but the SACs are new to this area. Although SACs show huge potential in gas sensing, the SACs gas sensing area currently is still at the infancy stage. This work critically reviews the recent advances and current status of single-atom gas sensing materials. General synthesis routes, characterization methods, and sensing performance indexes are introduced. At the end, the challenges and future prospects on SACs gas sensing are presented from the authors' perspectives. This work is anticipated to provide insights and guideline for the chemical sensing community.

High-Performance Room-Temperature NO2 Gas Sensor Based on Au-Loaded SnO2 Nanowires under UV Light Activation

Optical excitation is widely acknowledged as one of the most effective means of balancing sensor responses and response/recovery properties at room temperature (RT, 25 °C). Moreover, noble metals have been proven to be suitable as photosensitizers for optical excitation. Localized surface plasmon resonance (LSPR) determines the liberalization of quasi-free electrons in noble metals under light irradiation, and numerous injected electrons in semiconductors will greatly promote the generation of chemisorbed oxygen, thus elevating the sensor response. In this study, pure SnO₂ and Au/SnO₂ nanowires (NWs) were successfully synthesized through the electrospinning method and validated using XRD, EDS, HRTEM, and XPS. Although a Schottky barrier led to a much higher initial resistance of the Au/SnO₂ composite compared with pure SnO₂ at RT in the dark, the photoinduced resistance of the Au/SnO₂ composite became lower than that of pure SnO₂ under UV irradiation with the same intensity, which confirmed the effect of LSPR. Furthermore, when used as sensing materials, a detailed comparison between the sensing properties of pure SnO₂ and Au/SnO₂ composite toward NO₂ in the dark and under UV irradiation highlighted the crucial role of the LSPR effects. In particular, the response of Au/SnO₂ NWs toward 5 ppm NO₂ could reach 65 at RT under UV irradiation, and the response/recovery time was only 82/42 s, which far exceeded those under Au modification-only or optical excitation-only. Finally, the gas-sensing mechanism corresponding to the change in sensor performance in each case was systematically proposed.

Recent Progress on Flexible Room-Temperature Gas Sensors Based on Metal Oxide Semiconductor

With the rapid development of the Internet of Things, there is a great demand for portable gas sensors. Metal oxide semiconductors (MOS) are one of the most traditional and well-studied gas sensing materials and have been widely used to prepare various commercial gas sensors. However, it is limited by high operating temperature. The current research works are directed towards fabricating high-performance flexible room-temperature (FRT) gas sensors, which are effective in simplifying the structure of MOS-based sensors, reducing power consumption, and expanding the application of portable devices. This article presents the recent research progress of MOS-based FRT gas sensors in terms of sensing mechanism, performance, flexibility characteristics, and applications. This review comprehensively summarizes and discusses five types of MOS-based FRT gas sensors, including pristine MOS, noble metal nanoparticles modified MOS, organic polymers modified MOS, carbon-based materials (carbon nanotubes and graphene derivatives) modified MOS, and two-dimensional transition metal dichalcogenides materials modified MOS. The effect of light-illuminated to improve gas sensing performance is further discussed. Furthermore, the applications and future perspectives of FRT gas sensors are also discussed.

Temperature-Modulated Selective Detection of Part-per-Trillion NO2 Using Platinum Nanocluster Sensitized 3D Metal Oxide Nanotube Arrays

Semiconductor chemiresistive gas sensors play critical roles in a smart and sustainable city where a safe and healthy environment is the foundation. However, the poor limits of detection and selectivity are the two bottleneck issues limiting their broad applications. Herein, a unique sensor design with a 3D tin oxide (SnO₂ ) nanotube array as the sensing layer and platinum (Pt) nanocluster decoration as the catalytic layer, is demonstrated. The Pt/SnO₂ sensor significantly enhances the sensitivity and selectivity of NO₂ detection by strengthening the adsorption energy and lowering the activation energy toward NO₂ . It not only leads to ultrahigh sensitivity to NO₂ with a record limit of detection of 107 parts per trillion, but also enables selective NO₂ sensing while suppressing the responses to interfering gases. Furthermore, a wireless sensor system integrated with sensors, a microcontroller, and a Bluetooth unit is developed for the practical indoor and on-road NO₂ detection applications. The rational design of the sensors and their successful demonstration pave the way for future real-time gas monitoring in smart home and smart city applications.

A Room Temperature ZnO-NPs/MEMS Ammonia Gas Sensor

This study uses ultrasonic grinding to grind ZnO powder to 10−20-nanometer nanoparticles (NPs), and these are integrated with a MEMS structure to form a ZnO-NPs/MEMS gas sensor. Measuring 1 ppm NH3 gas and operating at room temperature, the sensor response for the ZnO-NPs/MEMS gas sensor is around 39.7%, but the origin-ZnO powder/MEMS gas sensor is fairly unresponsive. For seven consecutive cycles, the ZnO-NPs/MEMS gas sensor has an average sensor response of about 40% and an inaccuracy of <±2%. In the selectivity of the gas, the ZnO-NPs/MEMS gas sensor has a higher response to NH3 than to CO, CO2, H2, or SO2 gases because ZnO nanoparticles have a greater surface area and more surface defects, so they adsorb more oxygen molecules and water molecules. These react with NH3 gas to increase the sensor response.

Cu2O induced Au nanochains for highly sensitive dual-mode detection of hydrogen sulfide

Colorimetric and chemoresistive gas sensing methods have aroused great interest in H₂S monitoring due to their unique merits of naked-eye readout, and highly sensitive and rapid detection. However, combining these two methods for gas detection, especially utilizing one material as their common sensing material is a grand challenge because they are inconsistent in sensing mechanism. Taking advantage of the strong chemical affinity of Cu₂O for H₂S and the excellent performance of localized surface plasmon resonance (LSPR) of Au nanoparticles (NPs) in the visible regions and its ability as a noble metal to enhance gas sensing property, the Cu₂O-Au nanochains (NCs) were prepared for dual-mode detection of H₂S gas. The Cu₂O-Au chemoresistive gas sensor shows a 5-fold higher response than Cu₂O sensor at room temperature with a low detection limit of 10 ppb. Such good performance is attributed to the spillover effect and catalytic activity of Au NPs, and the enhanced H₂S adsorption after Au loading as revealed by density functional theory calculation. Test strips containing Cu₂O-Au produced for gaseous H₂S detection show superior color gradient changes (blue, yellow, and brown). Finally, the practicability of the method was validated by real-time monitoring H₂S released from cell culture.

DFT insights into the selective NH3sensing mechanism of two dimensional ZnTe monolayer

Exploring novel NH₃sensing materials is crucial in chemical industries, fertilizing plants and medical fields. Herein, for the first time, the NH₃sensing behaviors and sensing mechanisms of two dimensional (2D) ZnTe monolayer are systematically investigated by density functional theory calculations. It is shown that 2D ZnTe monolayer exhibits excellent selective NH₃sensing properties. (220) crystal facet of ZnTe possesses a higher NH₃adsorption energy (-1.59 eV) and a larger charge transfer (0.195e) than (111) and (311) crystal facets. The positive charges could enhance NH₃sensing while the negative charges could reduce NH₃sensing. The NH₃adsorption strengths are significantly improved in O₂atmosphere while it is negligibly affected by N₂atmosphere and H₂O atmosphere. Moreover, the presence of Zn vacancy and Fe, Co, Ni doping could improve the NH₃sensing of ZnTe. Additionally, the experimental results confirms that ZnTe possesses a low detection limit of 0.1 ppm NH₃. These theoretical predictions and experimental results present a wide range of possibilities for the further development of ZnTe monolayer in NH₃sensing fields.

Visible Light-Induced Room-Temperature Formaldehyde Gas Sensor Based on Porous Three-Dimensional ZnO Nanorod Clusters with Rich Oxygen Vacancies

Oxygen vacancy (V_O) is a kind of primary point defect that extensively exists in semiconductor metal oxides (SMOs). Owing to some of its inherent qualities, an artificial manipulation of V_O content in one material has evolved into a hot research field, which is deemed to be capable of modulating band structures and surface characteristics of SMOs. Specific to the gas-sensing area, V_O engineering of sensing materials has become an effective means in enhancing sensor response and inducing light-enhanced sensing. In this work, a high-efficiency microwave hydrothermal treatment was utilized to prepare a V_O-rich ZnO sample without additional reagents. The X-ray photoelectron spectroscopy test revealed a significant increase in V_O proportion, which was from 9.21% in commercial ZnO to 36.27% in synthesized V_O-rich ZnO possessing three-dimensional and air-permeable microstructures. The subsequent UV-vis-NIR absorption and photoluminescence spectroscopy indicated an extension absorption in the visible region and band gap reduction of V_O-rich ZnO. It turned out that the V_O-rich ZnO-based sensor exhibited a considerable response of 63% toward 1 ppm HCHO at room temperature (RT, 25 °C) under visible light irradiation. Particularly, the response/recovery time was only 32/20 s for 1 ppm HCHO and further shortened to 10/5 s for 10 ppm HCHO, which was an excellent performance and comparable to most sensors working at high temperatures. The results in this work strongly suggested the availability of V_O engineering and also provided a meaningful candidate for researchers to develop high-performance RT sensors detecting volatile organic compounds.

Carbonized lotus leaf/ZnO/Au for enhanced synergistic mechanical and photocatalytic bactericidal activity under visible light irradiation

Nowadays, bacterial resistance has continued to be a troublesome issue caused by the abuse of antibiotics, and it is the paramount difficulty in resolving the bacterial proliferation and infection. In this study, fresh lotus leaf was treated with Zn²⁺ followed by sintered and modification with gold nanoparticles through the photoreduction process sequentially, and thus a composite of micro/nanostructured carbonized lotus leaf/ZnO/Au (C-LL/ZnO/Au) was obtained to explore its bactericidal properties. C-LL/ZnO/Au retained the papillary structure of fresh lotus leaf and showed great mechanical bactericidal performance and photocatalytic sterilization. The antibacterial rate of mechanical sterilization for C-LL/ZnO/Au amount to 79.5% in 30 min, 4.7 times of fresh lotus leaf's figure under the same conditions. Furthermore, in C-LL/ZnO/Au, the introduction of gold nanoparticles heightened light absorbance through localized surface plasmon resonance (LSPR) effect and separation efficiency of photogenerated electron-hole pairs, which showed improved photocatalytic sterilization than that of carbonized lotus leaf/ZnO (C-LL/ZnO). Carbonized lotus leaf/ZnO/Au exhibited prominent photocatalytic and mechanical synergistic antibacterial performance against E. coli: all the bacteria were inactivated within 30 min under visible light. The approach presented here could be applied to a variety of biomass materials, which holds a promising application potential in biomedical, public health and other fields.

A Fully Integrated Flexible Tunable Chemical Sensor Based on Gold-Modified Indium Selenide Nanosheets

In this work, a novel light-modulated bifunctional gas sensor based on Au nanoparticles-modified 2D InSe nanosheets was demonstrated. The prepared sensor displayed a reversible and extremely high response for recognition of nitrogen dioxide (NO₂) under visible-light illumination. The sensitivity (1192%) was about 10 times higher than that under dark condition, and the limit of detection (LOD) was 0.17 ppb. In contrast, when sensing ammonia (NH₃), higher sensitivity and selectivity were obtained in darkness rather than in light, with sensitivity and LOD of 11% and 0.2 ppm. Furthermore, the sensor possesses decent stability, repeatability, and anti-interference ability. The tunable sensing behavior with light modulation has been clearly studied with the help of density functional theory. A new principle called "carrier storage box" of Au nanoparticles was proposed to explain the change in surface state of InSe under light modulation. Finally, the prepared sensor has been successfully applied to construct a fully integrated wearable device to measure NH₃ and NO₂ in ambient environment. In all, this work provides a highly competitive gas detection method and paves the way for designing 2D materials-based optoelectronic devices with tunable and multifunctional features.

Superior acetone sensor based on hetero-interface of SnSe2/SnO2 quasi core shell nanoparticles for previewing diabetes

To improve gas sensing performance of SnO₂ sensor, a heterostructure constructed by SnO₂ and SnSe₂ is designed and synthesized via hydrothermal method and post thermal oxidation treatment. The obtained SnSe₂/SnO₂ composite nanoparticles demonstrate a special core-shell structure with SnO₂ nanograins distributed in the shell and mixed SnSe₂ and SnO₂ nanograins in the core. Owning to the promoted charge transfer effect invited by SnSe₂, the sensor based on SnSe₂/SnO₂ composite nanoparticles exhibit expressively enhanced acetone sensing performance compared to the pristine SnO₂ sensor. At the working temperature of 300 °C, the SnSe₂/SnO₂ composite sensor with optimized composition exhibits superior sensing property towards acetone, including high response (10.77-100 ppm), low theoretical limit of detection (0.354 ppm), high selectivity and good reproducibility. Moreover, the sensor shows a satisfactory sensing performance in trace acetone gas detection under high humidity condition (relative humidity: 70-90%), making it a promising candidate to constructing exhaled breath sensors for acetone detection.

Fast and recoverable NO2 detection achieved by assembling ZnO on Ti3C2Tx MXene nanosheets under UV illumination at room temperature

Recently, Ti₃C₂T_x MXenes have begun to receive attention in the field of gas sensors owing to their characteristics of high conductivity and abundant surface functional groups. However, Ti₃C₂T_x-based gas sensors still suffer from the drawbacks of low sensitivity and sluggish response/recovery speed towards target gases, limiting their development in further applications. In this work, Ti₃C₂T_x-ZnO nanosheet hybrids were fabricated through a simple sonication method. The Ti₃C₂T_x-ZnO nanosheet hybrids exhibited a short recovery time (10 s) under UV (ultraviolet) illumination, a short response time (22 s), a high sensitivity (367.63% to 20 ppm NO₂) and selectivity. Furthermore, the Ti₃C₂T_x-ZnO sensor has prominent anti-humidity properties, as well as superior reproducibility in multiple tests. The abundant active sites in the Ti₃C₂T_x-ZnO nanosheet hybrids, including surface groups (-F, -OH, -O) of Ti₃C₂T_x and oxygen vacancies of ZnO, the formation of Schottky barriers between Ti₃C₂T_x and ZnO nanosheets and the rich photogenerated charge carriers of ZnO under UV illumination, together result in excellent gas-sensing performance. Density functional theory calculations have been further employed to explore the sensing performance of Ti₃C₂T_x and ZnO nanosheets, showing strong interactions existing between the NO₂ and ZnO nanosheets. The main adsorption sites for NO₂ were present on the ZnO nanosheets, while the Ti₃C₂T_x played the role of the conductive path to accelerate the transformation of charge carriers. Our work can provide an effective way for improving the gas-sensing performances of Ti₃C₂T_x-based gas sensors.

Visible Light Driven Ultrasensitive and Selective NO2 Detection in Tin Oxide Nanoparticles with Sulfur Doping Assisted by l-Cysteine

In the pandemic era, the development of high-performance indoor air quality monitoring sensors has become more critical than ever. NO₂ is one of the most toxic gases in daily life, which induces severe respiratory diseases. Thus, the real-time monitoring of low concentrations of NO₂ is highly required. Herein, a visible light-driven ultrasensitive and selective chemoresistive NO₂ sensor is presented based on sulfur-doped SnO₂ nanoparticles. Sulfur-doped SnO₂ nanoparticles are synthesized by incorporating l-cysteine as a sulfur doping agent, which also increases the surface area. The cationic and anionic doping of sulfur induces the formation of intermediate states in the band gap, highly contributing to the substantial enhancement of gas sensing performance under visible light illumination. Extraordinary gas sensing performances such as the gas response of 418 to 5 ppm of NO₂ and a detection limit of 0.9 ppt are achieved under blue light illumination. Even under red light illumination, sulfur-doped SnO₂ nanoparticles exhibit stable gas sensing. The endurance to humidity and long-term stability of the sensor are outstanding, which amplify the capability as an indoor air quality monitoring sensor. Overall, this study suggests an innovative strategy for developing the next generation of electronic noses.

Flexible All-Inorganic Room-Temperature Chemiresistors Based on Fibrous Ceramic Substrate and Visible-Light-Powered Semiconductor Sensing Layer

As the most extensively used gas-sensing devices, inorganic semiconductor chemiresistors are facing great challenges in realizing mechanical flexibility and room-temperature gas detection for developing next-generation wearable sensing devices. Herein, for the first time, flexible all-inorganic yttria-stabilized zirconia (YSZ)/In2 O3 /graphitic carbon nitride (g-C3 N4 ) (ZIC) gas sensor is designed by employing YSZ nanofibers as substrate, and ultrathin In2 O3 /g-C3 N4 heterostructures as active sensing layer. The YSZ substrate possesses small nanofiber diameter (310 nm), ultrafine grain size (23.9 nm), and abundant dangling bonds, endowing it with striking mechanical flexibility and strong adhesion with In2 O3 /g-C3 N4 sensing layer. Meanwhile, the ultrathin thickness (≈7 nm) of In2 O3 /g-C3 N4 ensures that the inorganic sensing layer has tiny linear strain along with the deformation of flexible YSZ substrate, thereby enabling unusual bending capacity. To address the operating temperature issue, the gas sensor is operated by using a visible-light-powered strategy. Under visible-light illumination, the flexible ZIC sensor exhibits a perfectly reversible response/recovery dynamic process and ultralow detection limit of 50 ppb to toxic nitrogen dioxide at room temperature. This work not only provides an insight into the mechanical flexibility of inorganic materials, but also offers a valuable reference for developing other flexible inorganic-semiconductor-based room-temperature gas sensors.

UV Light Activated Multi-Cycle Photoelectric Properties of TiO₂ and CdS/TiO ₂ Films in Formaldehyde

In this work, UV light activated multi-cycle photoelectric properties of TiO₂ and CdS/TiO₂ films in formaldehyde were researched. TiO₂ film was prepared by screen printing, CdS/TiO₂ compounded film was synthesized by SILAR method. XRD and FE-SEM was used to characterize the TiO₂ and CdS/TiO₂ samples. Multi-cycle photoelectric properties of TiO₂ and CdS/TiO₂ with uv light on and off were evaluated by testing the photocurrent. On one hand, under the same bias voltage, CdS/TiO ₂showed a higher photocurrent than that by TiO₂. The reason for this result should be ascribed to the compounded structure in CdS/TiO₂, with which the separation and transfer of photogenerated electron-hole pairs could be improved. On the other hand, with the testing cycle number increased, the photocurrent amplitudes of TiO₂ and CdS/TiO₂ increased. These results suggested that the time to reach a stable photocurrent value for TiO₂ and CdS/TiO₂ is much longer than one cycle time (300 S). To illustrate the increased photocurrent amplitude value cycle by cycle, the photocurrent of CdS/TiO₂ to a much longer time (more than 4000 seconds) was also tested. To explain these results, corresponding possible illustrations were presented.

rGO decorated ZnO/CdO heterojunction as a photoanode for photoelectrochemical water splitting

A ternary photoanode of ZnO/CdO heterojunction decorated with reduced graphene oxide (rGO) was firstly fabricated by electrochemical deposition and thermal decomposition that is simple and effective compared with other method reported in literature. The structure and morphology of the photoanode were systematically characterized by various spectrum technologies. The photoanode expands the visible light absorption range to 428 nm, the photocurrent density reaches 1.15 mA·cm^-2 at 1.23 V (vs. RHE) that is 3 times and 1.85 times of pure ZnO (0.38 mA·cm²) and ZnO/CdO (0.62 mA·cm²) photoanodes. The highest IPCE value reaches 42.63% at 380 nm. The enhancement is attributed to the architecture of semiconductor heterojunctions and the decoration of rGO nanosheets, the former promotes charge separation, while the latter accelerates electron transfer thus both synergistically enhance PEC water splitting efficiency. Here fabricated photoanode has never been reported before, only Cd and other metal elements doped ZnO photoanodes were reported in the literature.

Highly Efficient SO2 Sensing by Light-Assisted Ag/PANI/SnO2 at Room Temperature and the Sensing Mechanism

Sulfur dioxide (SO₂) is one of the most hazardous and common environmental pollutants. However, the development of room-temperature SO₂ sensors is seriously lagging behind that of other toxic gas sensors due to their poor recovery properties. In this study, a light-assisted SO₂ gas sensor based on polyaniline (PANI) and Ag nanoparticle-comodified tin dioxide nanostructures (Ag/PANI/SnO₂) was developed and exhibited remarkable SO₂ sensitivity and excellent recovery properties. The response of the Ag/PANI/SnO₂ sensor (20.1) to 50 ppm SO₂ under 365 nm ultraviolet (UV) light illumination at 20 °C was almost 10 times higher than that of the pure SnO₂ sensor. Significantly, the UV-assisted Ag/PANI/SnO₂ sensor had a rapid response time (110 s) and recovery time (100 s) to 50 ppm SO₂, but in the absence of light, the sensors exhibited poor recovery performance or were even severely and irreversibly deactivated by SO₂. The UV-assisted Ag/PANI/SnO₂ sensor also exhibited excellent selectivity, superior reproducibility, and satisfactory long-term stability at room temperature. The increased charge carrier density, improved charge-transfer capability, and the higher active surface of the Ag/PANI/SnO₂ sensor were revealed by electrochemical measurements and endowed with high SO₂ sensitivity. Moreover, the light-induced formation of hot electrons in a high-energy state in Ag/PANI/SnO₂ significantly facilitated the recovery of SO₂ by the gas sensor.

Localized Energy Band Bending in ZnO Nanorods Decorated with Au Nanoparticles

Surface decoration by means of metal nanostructures is an effective way to locally modify the electronic properties of materials. The decoration of ZnO nanorods by means of Au nanoparticles was experimentally investigated and modelled in terms of energy band bending. ZnO nanorods were synthesized by chemical bath deposition. Decoration with Au nanoparticles was achieved by immersion in a colloidal solution obtained through the modified Turkevich method. The surface of ZnO nanorods was quantitatively investigated by Scanning Electron Microscopy, Transmission Electron Microscopy and Rutherford Backscattering Spectrometry. The Photoluminescence and Cathodoluminescence of bare and decorated ZnO nanorods were investigated, as well as the band bending through Mott–Schottky electrochemical analyses. Decoration with Au nanoparticles induced a 10 times reduction in free electrons below the surface of ZnO, together with a decrease in UV luminescence and an increase in visible-UV intensity ratio. The effect of decoration was modelled with a nano-Schottky junction at ZnO surface below the Au nanoparticle with a Multiphysics approach. An extensive electric field with a specific halo effect formed beneath the metal–semiconductor interface. ZnO nanorod decoration with Au nanoparticles was shown to be a versatile method to tailor the electronic properties at the semiconductor surface.

Thin-layered MoS2 nanoflakes vertically grown on SnO2 nanotubes as highly effective room-temperature NO2 gas sensor

The unique properties of heterostructure materials make them become a promising candidate for high-performance room-temperature (RT) NO₂ sensing. Herein, a p-n heterojunction consisting of two-dimensional (2D) MoS₂ nanoflakes vertically grown on one-dimensional (1D) SnO₂ nanotubes (NTs) was fabricated via electrospinning and subsequent hydrothermal route. The sulfur edge active sites are fully exposed in the MoS₂@SnO₂ heterostructure due to the vertically oriented thin-layered morphology features. Moreover, the interface of p-n heterojunction provides an electronic transfer channel from SnO₂ to MoS₂, which enables MoS₂ act as the generous electron donor involved in NO₂ gas senor detection. As a result, the optimized MoS₂@SnO₂-2 heterostructure presents an impressive sensitivity and selectivity for NO₂ gas detection at RT. The response value is 34.67 (R_a/R_g) to 100 ppm, which is 26.5 times to that of pure SnO_2. It also exhibits a fast response and recovery time (2.2 s, 10.54 s), as well as a low detection limit (10 ppb) and as long as 20 weeks of stability. This simple fabrication of high-performance sensing materials may facilitate the large-scale production of RT NO₂ gas sensors.

Green light-driven acetone gas sensor based on electrospinned CdS nanospheres/Co3O4 nanofibers hybrid for the detection of exhaled diabetes biomarker

Morphological and structural characteristics of semiconductors have a significant impact on their gas sensing characteristics. Reasonable design and synthesis of heterojunctions with special structures can effectively improve sensor performance. Herein, a cobalt oxide (Co₃O₄) nanofibers/cadmium sulfide (CdS) nanospheres hybrid was synthesized by an electrospinning method combined with a hydrothermal method to detect acetone gas. By adjusting loading amount of CdS, the sensing performance of CdS/Co₃O₄ sensor for acetone at room temperature (25 °C) was greatly ameliorated. In particular, the response of CdS/Co₃O₄ to 50 ppm acetone gas increased by 25% under 520 nm green light, meanwhile, the response/recovery time was shortened to 5 s/4 s. This is attributed to the heterojunction formed between CdS and Co₃O₄ as well as the influence of light excitation on the carrier concentration of the surfaces. Meanwhile, the unique high-porosity fiber structure and the catalytic action of cobalt ions also play an essential role in improving the performance. Furthermore, practical diabetic breath was experimentally simulated and proved the potential of the sensor in the future application of disease-assisted diagnosis.

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.

Gold-Carbon Nanocomposites for Environmental Contaminant Sensing

The environmental crisis, due to the rapid growth of the world population and globalisation, is a serious concern of this century. Nanoscience and nanotechnology play an important role in addressing a wide range of environmental issues with innovative and successful solutions. Identification and control of emerging chemical contaminants have received substantial interest in recent years. As a result, there is a need for reliable and rapid analytical tools capable of performing sample analysis with high sensitivity, broad selectivity, desired stability, and minimal sample handling for the detection, degradation, and removal of hazardous contaminants. In this review, various gold–carbon nanocomposites-based sensors/biosensors that have been developed thus far are explored. The electrochemical platforms, synthesis, diverse applications, and effective monitoring of environmental pollutants are investigated comparatively.

Gas sensing materials roadmap

Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.

Edge-enriched WS2 nanosheets on carbon nanofibers boosts NO2 detection at room temperature

Two-dimensional (2D) transition metal dichalcogenides (TMDs) hold great promise for room temperature (RT) NO₂ sensors. However, the exposure of the edges of TMDs with high adsorption capability and electronic activity remains a great obstacle to achieve high sensor sensitivity. Herein, we demonstrate a high-performance RT NO₂ gas sensor based on WS₂ nanosheets/carbon nanofibers (CNFs) composite with abundant intentionally exposed WS₂ edges. Few-layer WS₂ nanosheets are anchored on CNFs through a hydrothermal process. The approach permits to achieve a coating presenting an optimized active surface area and accessibility of the sensing layers. The exposure of WS₂ edges remarkably improves the sensing properties. Consequently, the WS₂@CNFs composite exhibits excellent selectivity to NO₂ at RT with improved response and much lower detection limit in comparison to the WS₂ and CNFs counterparts. Density functional theory (DFT) calculations verify a surprisingly strong NO₂ adsorption on WS₂ edge sites (adsorption energy 3.40 eV) with a partial charge transfer of 0.394e, while a week adsorption on the basal surface of WS₂ (adsorption energy 0.25 eV) with a partial charge transfer of 0.171e. The strategy proposed herein will be instructive to the design of efficient material structures for low-power NO₂ sensors with optimized performances.

Gold nanorods doping induced performance improvement of room temperature OTFT NO2sensors

Solution-processed organic thin-film transistors (OTFTs) are regarded as the promising candidates for low-cost gas sensors due to their advantages of high throughput, large-area and sensitive to various gas analytes. Microstructure control of organic active layers in OTFTs is an effective route to improve the sensing performance. In this work, we report a simple method to modify the morphology of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) thin films via doping gold nanorods (Au NRs) for enhancing the performance of the corresponding OTFT sensors for nitrogen dioxide (NO₂) detection. With the optimized doping ratio of Au nanorods, the TIPS-pentacene OTFT snesors not only exhibit a 3-fold increase in mobility, but also obtain a high sensitivity of 70% to 18 ppm NO₂with a detection limit of 270 ppb. The microstructures and morphologies of the modified TIPS-pentacene thin film characterized by atomic force microscopy and field scanning electron microscope. The experimental results indicate that the proper addition of Au NRs could effectively regulate the grain size of TIPS-pentacene, and therein control the density of grain boundaries during the crystallization, which is essential for the high-performance gas sensors.

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.

Light Activation of Nanocrystalline Metal Oxides for Gas Sensing: Principles, Achievements, Challenges

The review deals with issues related to the principle of operation of resistive semiconductor gas sensors and the use of light activation instead of thermal heating when detecting gases. Information on the photoelectric and optical properties of nanocrystalline oxides SnO2, ZnO, In2O3, and WO3, which are the most widely used sensitive materials for semiconductor gas sensors, is presented. The activation of the gas sensitivity of semiconductor materials by both UV and visible light is considered. When activated by UV light, the typical approaches for creating materials are (i) the use of individual metal oxides, (ii) chemical modification with nanoparticles of noble metals and their oxides, (iii) and the creation of nanocomposite materials based on metal oxides. In the case of visible light activation, the approaches used to enhance the photo- and gas sensitivity of wide-gap metal oxides are (i) doping; (ii) spectral sensitization using dyes, narrow-gap semiconductor particles, and quantum dots; and (iii) addition of plasmon nanoparticles. Next, approaches to the description of the mechanism of the sensor response of semiconductor sensors under the action of light are considered.

Boosting the sensing properties of resistive-based gas sensors by irradiation techniques: a review

The ongoing need to detect and monitor hazardous, volatile, and flammable gases has led to the use of gas sensors in several fields to improve safety and health issues. Conductometric type gas sensors, which have considerable advantages over other gas sensors, have thrived in numerous gas sensing fields. The ever-present key challenges and requirements of these sensors are to achieve excellent performance, including high sensitivity, good selectivity, low working temperature, and durability. Therefore, tremendous research effort has focused on improving these properties, and various state-of-the-art techniques have been reported. This review article discusses the recent advances and utilization of various irradiation techniques, including electron-beam, microwave, ion-beam, and gamma-ray irradiation, along with their investigation of the effects on the physicochemical properties of pre-synthesized nanomaterials, sensing performances, and related gas sensing mechanisms. A review of the progress on the effects of different irradiation techniques for boosting the sensing properties can contribute to the evolution of highly reliable sensors to assess the environment and health. For researchers, who work on gas sensors, this paper provides information on the current trends on the advances in the novel state-of-art of irradiated materials and their promising application in the sensitive detection of various toxic and VOCs.

Design of size-controlled Au nanoparticles loaded on the surface of ZnO for ethanol detection

Schematic diagram of the reaction mechanism of the sensor in air and ethanol.

Ammonia Gas Sensor Response of a Vertical Zinc Oxide Nanorod-Gold Junction Diode at Room Temperature

Conventional metal oxide semiconductor (MOS) gas sensors have been investigated for decades to protect our life and property. However, the traditional devices can hardly fulfill the requirements of our fast developing mobile society, because the high operating temperatures greatly limit their applications in battery-loaded portable systems that can only drive devices with low power consumption. As ammonia is gaining importance in the production and storage of hydrogen, there is an increasing demand for energy-efficient ammonia detectors. Hence, in this work, a Schottky diode resulting from the contact between zinc oxide nanorods and gold is designed to detect gaseous ammonia at room temperature with a power consumption of 625 μW. The Schottky diode gas sensors benefit from the change of barrier height in different gases as well as the catalytic effect of gold nanoparticles. This diode structure, fabricated without expensive interdigitated electrodes and displaying excellent performance at room temperature, provides a novel method to equip mobile devices with MOS gas sensors.

rGO modified nanoplate-assembled ZnO/CdO junction for detection of NO2

The triadic composite of ZnO/CdO heterojunction decorated with reduced graphene oxide (rGO) was prepared using a one-step hydrothermal method. The characterizations of morphology, structure and composition to the composite were undertaken by XRD, Raman, SEM, TEM, XPS, UV-vis spectra. The sensing experimental data indicate that the highest response of the ZnO/CdO/rGO (1.0 wt%) composite to ppm-level NO₂ is 8 times and 2 times higher than pure ZnO and ZnO/CdO junction, respectively. The composite not only exhibits fast response time and recovery time, high response, but also reveals outstanding stability and repeatability at an operating temperature of 125 °C. The sensing mechanism also has been discussed in detail in the work. The enhancement in gas sensing properties is credited to the development of ZnO/CdO heterojunction and the decoration of rGO with high conductivity. The logarithm of sensitivity in the range of 0.4-2.4 ppm NO₂ shows good linear dependence, indicating that the composite based sensor can be used to quantificationally detect low concentration of NO₂.

Characterization and Sensing of Inert Gases with a High-Resolution SPR Sensor

It is generally difficult to characterize inert gases through chemical reactions due to their inert chemical properties. The phase interference-sensing system based on high-resolution surface plasmon resonance (SPR) has an excellent refractive index detection limit. Based on this, this paper presents a simple and workable method for the characterization and detection of inert gases. The phase of light for the present SPR sensor is more sensitive to the change in the external dielectric environment than an amplitude SPR sensor. The limit of detection (LOD) is usually in the order of 10⁻⁶ to 10⁻⁷ RIU, which is superior to LSPR (Localized Surface Plasmon Resonance) sensors and traditional SPR sensors. The sensor parameters are simulated and optimized. Our simulation shows that a 36 nm-thick gold film is more suitable for the SPR sensing of inert gases. By periodically switching between the two inert gases, helium and argon, the resolution of the system is tested. The SPR sensing system can achieve distinguishable difference signals, enabling a clear distinction and characterization of helium and argon. The doping of argon in helium has a detection limit of 1098 ppm.

Temperature-controlled resistive sensing of gaseous H2S or NO2 by using flower-like palladium-doped SnO2 nanomaterials

Palladium-doped SnO₂ nanomaterials, with palladium in fractions from 0 to 10 mol% were hydrothermally synthesized and characterized by XRD, FESEM, TEM, and XPS. Their gas sensing properties were studied in two temperature ranges of 75-95 °C and 160-210 °C. The sensor using 5 mol% Pd-doped SnO₂ exhibits temperature-dependent sensing property. NO₂ can be detected at 80 °C, while H₂S is preferably detected at 180 °C. The response to 10 ppm H₂S is 50 times higher than that of the undoped sample. Its detection limit is 500 ppb. For NO₂, the sensor exhibited strong response and a lower detection limit of 20 ppb. In view of the selective detection of H₂S and NO₂ by regulating the temperature, palladium-doped SnO₂ has great prospects in the detection of H₂S and NO₂. Graphical abstract Schematic of the gas sensing mechanism of the S-5% Pd doped SnO_2.

Low-temperature operating ZnO-based NO2 sensors: a review

A comprehensive review on designs and mechanisms of ZnO-based NO₂ gas sensors operated at low temperature.