In this paper, (TiO2/Fe2TiO5)@ polyoxometalates core-shell nanofibers were prepared by coaxial electrospinning technology to obtain an effective gas sensitive material utilizing the synergistic effect of the three components. The introduced polyoxometalates (POMs) can work as electrons acceptor and separate electrons and holes, finally improved gas sensing properties. Under optimal test condition, the (TiO2/Fe2TiO5)@PW12 sensor delivered a gas sensing response of 6.5 to 100 ppm acetone gas, which is obviously higher than TiO2/Fe2TiO5 and TiO2/Fe2TiO5/PW12 sensors. Selectivity, repeatability, and other sensing parameters were systematically investigated. The heterojunctions formed between the three components can extend electron lifetime, while allowing more electrons to migrate towards the surface to react with oxygen molecules in the air, thus improving the gas sensitivity. This work provides important theoretical and practical significance for the practical application of polyoxometalates in the field of gas sensing.

Gas sensors based on metal oxide semiconductors are increasingly recognized for their vital role in industrial and environmental applications, with their performance being heavily influenced by the morphology of the sensing materials. Herein, we employed a two-step hydrothermal method to synthesize ZnO nanomaterials with diverse morphologies, including solid spheres, porous hollow spheres, flocculent spheres, and snow flower structures. The ethanol sensing capabilities of these ZnO nanomaterials were systematically evaluated. Notably, the porous hollow sphere ZnO exhibited the highest gas response, achieving a detection limit of 500 ppb and a response of similar to 51.7 to 100 ppm ethanol at 250 degrees C, significantly outperforming the solid sphere ZnO. These findings demonstrate that controlled morphological variation via two-step hydrothermal process can effectively enhance gas sensing performance, offering a promising approach for the development of high-performance gas sensors.

In the field of gas sensor, the continuous pursuit is to select suitable noble metal modifications and apply them to excellent oxide semiconductor matrix materials to achieve high sensitivity and low detection limits for specific gases. In this study, silver (Ag) was deliberately chosen as a noble metal modification and applied to pure (3 ilmenite type CdSnO3 to achieve high-performance detection of ethanol gas. The pure phase of (3 ilmenite CdSnO3 was obtained through the addition of slightly excess cadmium salt during the solvothermal synthesis process, combined with suitable sintering temperature. The experimental results indicate that the sensors exhibited relatively lower resistances compared to those made of ZnO and SnO2 materials. Especially, the 3 at% Ag modified CdSnO3 nanoparticles has the highest response (86.2) to 100 ppm ethanol vapor at 225 degrees C, and the response value was about 6 times higher than that of pure CdSnO3 nanoparticles. The addition of 3 at% Ag modifying in CdSnO3 nanoparticles further lowered the detection limit for ethanol vapor from 1 ppm to approximately 100 ppb. In addition, the sensors based on 3 at%Ag modified CdSnO3 showed higher gas response than that based on 3 at% Pd modified material. All of those results demonstrate that modifying CdSnO3 materials with appropriate amounts of Ag species might be a meaningful strategy for developing high-performance ethanol gas sensors.

The selectivity for ethanol and acetone detection has always been the hot topic in gas sensor field, and developing high sensing performance gas sensor is also crucial for gas sensor practical application. Herein, mesoporous hierarchical ZnO materials with a series of Sn4+ doping have been prepared via a facile and reproducible hydrothermal method. Material characterizations indicate that all samples can be divided into two parts. One is the samples with low Sn4+ doping (L-Sn4+-D), which has inconspicuous phase (SnO2 and ZnO) separation. The other is the materials with high Sn4+ doping (H-Sn4+-D), which has obvious phase separation. Gas sensing results indicate that the optimum working temperature for ethanol and acetone detection is same at 250oC or 225oC, and gas sensor has relative high selectivity to acetone when sensing materials are L-Sn4+-D. While the optimal working temperatures further decrease to 200oC and 225oC for ethanol and acetone detection and gas sensors show better selectivity to ethanol, when the samples are H-Sn4+-D. Moreover, this work also presents the gas sensor based on 10 at%SnO2-ZnO has high acetone response and low detection limit (100 ppb) at 250oC. The selectivity variation for ethanol and acetone can be explained by Sn4+ incorporation.

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 degrees C to 250 degrees C, while the optimal working temperature for acetone detection will maintain at same temperature of 300 degrees 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 degrees 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.

Gas sensing performances of homogenous semiconductors materials are limited by their high carriers recombination rate. Polyoxometalates (POMs), as a kind of electron acceptors, comes into sight in gas sensing field. In this study, three series of SnO2 @POMs@WO3 layered core-middle-shell nanofibers with tandem heterojunctions are first prepared utilizing one-step coaxial electrospinning. The tandem heterojunctions are continuously distributed in a one-dimensional line along the nanofibers. For the first time the gas sensing performances of one-dimensional tandem heterojunctions are investigated. Furthermore, the effects of different contents and types of POMs on gas sensing performance are studied. The response of POMs-modified nanofibers can be significantly improved compared to SnO2 @WO3 nanofibers. The optimized response to 100 ppm ethanol can reach 8.8. The enhancement can be attributed to that the addition of POMs electron acceptor, the construction of POMs/semiconductor tandem heterojunctions and the one-dimensional nano-structure can together promote the separation of carriers, which could remarkably improve gas sensing performances. These results provide a new strategy for developing high-performance gas sensors by synthesizing one-dimensional tandem heterojunctions as well as introducing POMs.

Local structural engineering is an endogenous approach to modulate upconversion luminescence (UCL) from upstream to meet the needs of specific application scenarios. Herein, high pressure is utilized as a means to modulate the local structure, and the designed LiErF 4 :0.5%Tm 3+ @LiYF 4 (Er:Tm@Y) nanoparticles with fast energy transfer rates, abundant cross‐relaxation processes, and multiple near‐infrared wavelengths (808, 980, 1530 nm) excitation properties are tailored as local structure‐sensitive hosts. A unique excitation wavelength‐dependent UCL enhancement of Er:Tm@Y upconversion nanoparticles is observed by pressure‐induced local structure distortions. When the pressure of ≈6 GPa is applied, the UCL is enhanced by a factor of 2.6 at 980 nm excitation only. After pressure release, the luminescence diminishes and recovers. Density functional theory calculations show that the symmetry distortion of the LiErF 4 crystal reaches a maximum at pressurization to 6 GPa, while a new Er‐4f state emerges, greatly reducing the bandgap from 8.3 to 5.7 eV. Comparative experiments demonstrate that the local symmetry distortion caused by 0.5%Tm 3+ doping and the different energy transfer patterns of Er 3+ to Tm 3+ at different excitations are responsible for this wavelength‐dependent luminescence enhancement.

Oxide semiconductors with multi-shell hollow architecture have been considered as favorable candidates for sensing material applied in high-performance gas sensors. Herein, the solid, core-shell, and double-shell ZnO microspheres were successfully prepared via a facile template-free method. Compared with solid and core-shell sphere, the gas sensor based on double-shell sphere exhibited the highest response to 100 ppm ethanol at 275°C (~47.4), which was about 4.8 folders higher than that of the solid sphere (~9.8) under the same conditions. Furthermore, the detection limit of the double-shell ZnO microsphere could reach 1 ppm. The enhancement mechanism of the ethanol gas-sensing performance was discussed systematically.

The triethylamine (TEA) sensors with high response and selective detection have been widely reported. However, sluggish kinetics of recovery process, high operating temperature, unsatisfied long-term stability and high detection limit still restrict their further application. In this work, NiO nanofibers with different contents of Sn2+ are prepared via electrospinning method. Gas sensing investigation indicates that the sensor based on 6 at% Sn2+ doped NiO nanofibers with an elevated baseline resistance exhibits the highest gas response and excellent recovery characteristic to TEA at 200 degrees C. The sensor shows a low detection limit at ppb level and good long-term stability within three months. In addition, the sensor only has a small variation on baseline resistance, gas response and response/recovery speed under different relative humidity. The promising gas sensing performance can be mainly attributed to the increase of the oxygen vacancies defects caused by the difference of Sn2+ and Ni2+, resistance changes and the hollow structure of as-prepared nanofibers.

The development of mesoporous heterogeneous structure through a facile and repeatable method with high gas sensing performance is always highly desirable in the chemical sensor field. Herein, the mesoporous In2O3-ZnO heterogeneous structure has been successfully synthesized via a reproducible hydrothermal method. The characterizations of sensing materials demonstrate that the porous heterogeneous structure can be easily constructed. Gas sensing investigation indicates that the gas sensor based on 15 at. % In2O3-ZnO exhibits the obvious enhanced ethanol gas sensing performances compared with that based on pure ZnO material in our previous work, including lower detection limit (200 ppb), lower base line resistance (-1 Mohm), satisfying long term stability and improved moisture tolerance. The enhanced gas sensing properties can be mainly attributed to the In2O3-ZnO heterogeneous structure and unique microstructure, which will be deliberately discussed in the sensing mechanism part.

In this work, Sn4+ doped NiO nanofibers are modified by different contents of Rh2O3 through a facile electrospinning method to improve the gas sensing properties, especially the humidity tolerance, response value and detection limit. The morphological characterization indicates that the hollow nanostructures are still maintained after loading Rh2O3. Gas sensing investigation demonstrates that 6 at% Sn4+ doped NiO nanofibers modified with 6 at% Rh2O3 exhibit the highest response to triethylamine (28.3-20 ppm) with low detection limit (50 ppb) at a relatively low working temperature (165 degrees C), which are obviously superior to those (16.6-100 ppm, 500 ppb and 180 degrees C) of 6 at% Sn4+ doped NiO nanofibers without Rh2O3 modification. The response value to 20 ppm triethylamine is only changed about 35.7% as the relative humidity increases from 35% to 95%. The humidity tolerance of the optimal sensor is also improved after Rh2O3 modification including the baseline resistance and gas response changes. Furthermore, the baseline resistances of the optimal gas sensors don't become higher after Rh2O3 modification. These phenomena can be mainly attributed to the improved catalytic effect and increased surface defects caused by Rh2O3 modification.

Carbon monoxide (CO) is one of the most toxic gases to human life. Therefore, the effectively monitoring of it down to ppb level is of great significance. Herein, a series of In 2 O 3 nanofibers doped with Au, Pd or simultaneous Au and Pd have been prepared by electrospinning combined with 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-doped In 2 O 3 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 In 2 O 3 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 In 2 O 3 nanofibers. In addition, the sensor also shows well stability within 19 days. These demonstrate that co-doping suitable amount of Pd and Au to In 2 O 3 nanofibers might be a meaningful strategy for the development of high-performance carbon monoxide gas sensors.

The increasing demand for hazardous gas detections has triggered the enormous efforts for the development of mesoporous materials for gas sensors. Especially, a universal method to synthesize such structure for semiconductor oxides is highly desirable. In this work, ZnO microspheres assembled by mesoporous nanosheets are prepared via a one-step hydrothermal method successfully. It is found that the morphology of ZnO and the thickness of ZnO mesoporous nanosheets, which affect the gas sensing response to ethanol, can be regulated by adjusting the amount of PVP. In addition, this method is also suitable for the synthesis of NiO, CuO and Co3O4 mesoporous nanosheets and the repeatability is excellent. Gas sensing investigation indicates that the gas sensors based on mesoporous ZnO nanospheres synthesized with 3.3 g PVP have the highest response (similar to 58.4) to 100 ppm ethanol at 250 degrees C, which is about 9.4 times higher than that of the ZnO nanosheets obtained without PVP under the same test conditions (similar to 6.2). Furthermore, the detection limit could reach the ppb level with the response of 1.17-500 ppb ethanol. The possible mechanism of the formed structures and the enhanced ethanol sensing properties are discussed systematically.

Porous microstructures are playing more and more important role in developing new gas sensor with high gas-sensing performance due to their large specific surface area, high void fraction and many active sites. Therefore, a stable and facile synthesis method to synthesize porous microstructures is beneficial for a wide range of applications. Herein, we have used a facile hydrothermal method to achieve the controlled synthesis of various morphologies of ZnO such as solid sphere; mesoporous sphere; flocculent sphere and snow flower-like by simply regulating the temperature and time of the reaction, and apply these materials to gas sensors. As a result, gas sensing investigation demonstrates that the gas sensor based on the mesoporous sphere, flocculent sphere and snow flower zinc oxide microstructures can detect as low as 500 ppb ethanol at 250 o C with the response value of ~1.3. Especially, the gas sensor based on the mesoporous sphere zinc oxide has the highest gas response of ~51.7 to 100 ppm ethanol at 250 o C, which is about 7 times higher than that of solid sphere zinc oxide (~7.3) under the same condition. We believe that this work is meaningful for the synthesis of metal oxides with peculiar morphologies and provide a new strategy for the enhancement of gas sensing properties. The mechanisms of the structure formation and the enhanced gas-sensing properties are also discussed systematically.

Gas sensing performance has significant relationship with the morphology of the sensing materials. Therefore, it is always desirable to develop a new strategy for the synthesis of semiconductor oxides with unique microstructure and high gas sensing performance. Herein, the ladder hydrothermal temperatures are adopted to achieve the controlled synthesis of ZnO with various morphologies. Gas sensing investigation demonstrates that the gas sensor based on ZnO with the mesoporous sphere, flocculent sphere and snow flower microstructures can detect as low as 500 ppb ethanol at 250oC with the response value about 1.3. Especially, the gas sensor based on the mesoporous sphere ZnO has the highest gas response of ~51.7 to 100 ppm ethanol, which is about 7 times higher than that based on solid sphere ZnO (~7.3) under the same condition. The results indicate that the ladder hydrothermal temperatures are helpful for the synthesis of semiconductor oxides with unique structure and high gas sensing performance.

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.

Designing high-performance triethylamine gas sensors with the stable gas response and low resistance variation in air under a wide relative humidity range is expected for human health and environmental surveillance. Here, a novel porous NiO/NiFe2O4 fiber-in-tube nanostructure is prepared by the electrospinning process. The characterizations related to microstructure and surface morphology are carried out. Meanwhile, the gas sensing performance of the porous fiber-in-tube NiO/NiFe2O4 materials is evaluated and compared systematically. The results indicate that the introduction of NiO as the second component can not only reduce the baseline resistance of NiFe2O4 gas sensors dramatically but also optimize the gas sensing performance to a significant extent. Especially, the fabricated sensor based on the NiO/NiFe2O4 fiber-in-tube with a Ni/Fe molar ratio of 1.5 exhibits the best performance. The gas response while detecting 50 ppm triethylamine at 300 °C is about 3.6 times higher than that with Ni/Fe molar ratio of 0.5. Moreover, the response values become more stable, and the baseline resistance has a lower variation under a wide relative humidity range, demonstrating the excellent humidity resistance. These phenomena might be ascribed to the distinctive fiber-in-tube nanostructure as well as the heterojunction between NiFe2O4 and NiO.

Tungsten oxides with different crystal facet ratios were successfully prepared through one-spot solvothermal approach using alcohol solvents with different C chains accompanied by annealing treatment. The sensing performances for detecting triethylamine have been systematically investigated among the as-obtained WO3 samples. Benefiting from the unique microstructure with bimodal pores, the highest work function for the lowintensity ratio of the (002) WO3 facet, the smallest average crystallites sizes and abundant grain boundaries, the sensor based on WO3 microflowers assembled by tiny nanoparticles exhibits appealing sensing performance including high response, excellent selectivity, rapid response/recovery time (3 s/5 s) to 1 ppm triethylamine and extremely low detection limitation (200 ppb) at 325 degrees C.

Microwave gas sensor (MGS) becomes a highlight recently because it can work at the room temperature (20 celcius) and has potential wireless application. However, there are still some problems such as low sensitivity and linearity. In this short communication, a novel analysis method based on variation of energy loss was proposed as sensor response for the first time. The as-prepared device is successfully used for NH3 detection based on commercial graphite powder which can achieve a low detection limit (1 ppm, 20 celcius), excellent selectivity and humidity-resistance (20-95% RH). Moreover, the sensor shows good linearity for low concentration range of NH3 from 1 to 5 ppm (R-2 = 0.999) or high concentration range from 10 to 200 ppm (R-2 = 0.99) through analyzing energy loss calculated by both reflection and transmission, which is more linear compared with that only using reflection or transmission. Therefore, this study provides a simple and universal method for response signal analysis, which can amplify the weak sensing signal then realize high-performance MGS.