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Methane (CH4) gas sensors play an important role in industrial safety and detection of indoor gas quality. In general, metal oxide semiconductor sensing materials with nano-structure have high responses to the target gas. However, the sensor resistance is usually very high. Considering the practical application, it is vital to reduce base resistance and improve sensitivity for gas sensors. Herein, Pd-doped SnO2 nanoparticles were prepared as the basis material by a simple sol-gel method. In order to adjust the resistance, the pentavalent metal element (Sb) was introduced via a simple doping route. As CH4 sensing layers, the prepared SnO2-sensors doped with Pd and Sb exhibited the most obvious resistance reduction effect. Meantime, excellent sensing performances including high response, fast response/recovery time, excellent reproducibility and great stability were also obtained. In-depth research has shown that the ability to reduce resistance depends on the effective internal doping of cation with high valence. The enhanced sensing capability can be attributed to the 'synergistic effects' including catalytic effects of novel metals, increased oxygen vacancies and decreased band gap energy. This work can provide a new opportunity to design metal oxide sensing materials with low resistance and high sensitivity. [ABSTRACT FROM AUTHOR]

The α-MoO 3 -based materials have been intensively investigated as chemiresistive-type gas sensors due to their excellent sensing performances for gas sensing applications. However, there still remains formidable challenges for developing a simple and scalable approach to synthesize α-MoO 3 with controllable structures. Herein, we demonstrated a glucose-assisted combustion synthesis strategy for the synthesis of α-MoO 3 materials with controllable structures. They were obtained by heat treatment of the gels containing ammonium heptamolybdate tetrahydrate and glucose. Owing to the coordination interactions between glucose and heptamolybdate, α-MoO 3 -2 sample obtained by adding 1.8 g of glucose featured the highest oxygen vacancy concentration with an O/Mo atomic ratio of 2.61. The gas sensing measurements indicated that α-MoO 3 -2 had excellent sensing performances for detecting ethanol. Such ethanol sensor displayed the lowest detection concentration of 5 ppm ethanol, high response of 3.86 toward 20 ppm ethanol, and wide detection range of 5–100 ppm. This study sheds light on preparation of metal oxides with controllable structures on a scalable way for various applications. • A glucose-assisted combustion synthesis method have been developed to preparation of MoO3 samples. • The MoO3 sample thus obtained possesses high oxygen vacancy concentration and surface chemical-adsorbed oxygen species. • The MoO3 sample exhibits excellent sensing performances for ethanol sensing. [ABSTRACT FROM AUTHOR]

Abstract: Nitrogen-doped carbon (N-carbon) materials have been prepared by low temperature (300°C) heat treatment of hexamethylenetetramine in air. The combined characterizations of X-ray diffraction pattern, scanning electron microscopy, and X-ray photoelectron spectroscopy indicate the successful preparation of N-carbon materials. Most importantly, the N-carbon exhibits catalytic activity toward electrochemical reduction of H2O2, leading to a novel metal-free non-enzymatic H2O2 sensor. The linear detection range is estimated to be from 0.1mM to 40mM (r =0.996), and the detection limit is estimated to be 90μM at a signal-to-noise ratio of 3. [Copyright &y& Elsevier]

Optimally increasing the catalytic activity of Co3O4‐based materials is crucial for promoting their practical applications for acetone detection; however, this still remains challenging. Herein, a strategy is proposed in this work to increase the catalytic activity of Co3O4 toward the oxidation of acetone through the synergistic effect of boron doping and chemical reduction, which can substantially improve the sensing performance for acetone detection. The characterization results confirm that the present strategy facilitates the formation of more oxygen vacancies and increases the content of Co2+ ions serving as active sites. As expected, the optimal sensor yields a higher response. To extend the practical application to the auxiliary diagnosis of diabetes through exhaled breath, the optimal sensors distinguished between acetone concentrations in the breath of healthy individuals and in the simulated breath of patients with diabetes. Detailed gas‐sensing measurements reveal that the enhanced acetone sensing performance is attributed to the increased catalytic activity of materials for oxidation of acetone by the Mars–van Krevelen, Langmuir–Hinshelwood, and Eley–Rideal mechanisms. This study provides new insights into the fabrication of high‐performance metal oxide‐based gas sensors by improving the catalytic activity of sensing materials. [ABSTRACT FROM AUTHOR]

Reduced graphene oxide (rGO)-based materials are deemed as promising candidates for fabrication of room-temperature gas sensors, while a tougher challenge is to develop an effective strategy for improving gas responses. Here, a noble metal decoration strategy is developed to enhance NO2 responses of α-Fe2O3/SnO2–rGO hybrids. Noble metal decorated α-Fe2O3/SnO2–rGO hybrids were prepared by depositing noble metals onto α-Fe2O3/SnO2–rGO hybrids through reduction of noble metal ions by NaBH4. Most importantly, Pt modified α-Fe2O3/SnO2–rGO hybrids (designated as Pt-FSR) exhibit the highest response value of 21.3 toward 5 ppm NO2, which is much higher than that of α-Fe2O3/SnO2–rGO (8.8). Combined characterization studies indicate that the enhanced NO2 response of Pt-FSR is attributed to the narrow band gap of Pt-FSR, small particle size and excellent catalytic activity of Pt. The NO2 sensing mechanism is also investigated by FT-IR spectroscopy. The findings proposed in this work not only broaden the study of room-temperature gas sensors, but also provide suggestions for discovering the gas sensing mechanism. [ABSTRACT FROM AUTHOR]

Toluene is widely used as a solvent and reactant in a wide range of industrial fields; however, gaseous toluene is toxic to humans, causing severe acute respiratory effects. In this case, Co3O4-based chemiresistive gas sensors are promising candidates for monitoring toluene. However, it is challenging to prepare Co3O4-based sensing materials with good toluene sensing performances on a large-scale. In this study, oxygen vacancy-rich Co3O4 materials were successfully synthesized via the reduction of commercial Co3O4 using NaBH4 as the reducing agent (designated as Co3O4-R), and the oxygen vacancy concentrations in Co3O4-R materials were effectively modulated by regulating the NaBH4 concentration. With an increase in oxygen vacancies, the optimal Co3O4-R-5 samples displayed better sensing performances toward 100 ppm toluene at 190 °C than that of the pristine commercial Co3O4, including an enhanced response value of 3.18 (vs. 1.22 for commercial Co3O4) and fast response time of 48 s and recovery time of 38 s. The combination of band structure analysis and chemisorption analysis indicated that the enhanced toluene sensing performances of Co3O4-R is attributed to the synergistic effect of reducing the band gap and activating the surface oxygen species. This work demonstrates the first utilization of commercial Co3O4 for toluene sensing applications, providing a universal and high-throughput preparation method to prepare gas sensing materials using commercial metal oxides. [ABSTRACT FROM AUTHOR]

A novel NO 2 gas sensor has been constructed using Ag nanoparticles-SnO 2 nanoparticles-reduced graphene oxide (AgNPs-SnO 2 -rGO) hybrids as sensing materials. AgNPs-SnO 2 -rGO hybrids were prepared by a two-step wet-chemical method. Firstly, SnO 2 -rGO hybrids were synthesized by hydrothermal treatment of aqueous dispersion of GO in the presence of SnCl 4 . Then, AgNPs-SnO 2 -rGO hybrids were obtained by in situ reduction of AgNO 3 on the surface of SnO 2 -rGO hybrids. The combined characterizations of UV–vis spectroscopy, X-ray diffraction (XRD), energy-dispersive X-ray spectrometer (EDX), elemental mapping, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and Raman spectra were used to investigate the structure of AgNPs-SnO 2 -rGO hybrids. Most importantly, the sensor based on AgNPs-SnO 2 -rGO hybrids exhibits good sensing performance for NO 2 sensing operating at room temperature. For example, the response time and recovery time of the sensor based on AgNPs-SnO 2 -rGO hybrids for 5 ppm NO 2 are 49 s and 339 s, which are much shorter than that of SnO 2 -rGO hybrids (415 s and 740 s), indicating that the sensing performances for NO 2 sensing at room temperature have been tremendously enhanced by introduction of AgNPs into SnO 2 -rGO hybrids. [ABSTRACT FROM AUTHOR]

The α-Fe 2 O 3 /SnO 2 -RGO-15 possessed a competitive sensing performance with a high response of 7.4 towards 1 ppm NO 2 at room temperature. [Display omitted] As promising sensing materials, reduced graphene oxide (RGO)-based nanomaterials have drawn considerable attention in the fields of gas monitoring owing to their low operating temperature. However, constructing RGO-based room-temperature gas sensors possessing ppb-level limit of detection with high sensitivity remains challenging. In this work, a series of highly sensitive NO 2 sensors were fabricated using α-Fe 2 O 3 and SnO 2 co-decorated RGO hybrids (designated as α-Fe 2 O 3 /SnO 2 -RGO) as sensing materials. They were rationally synthesized by a one-pot hydrothermal method. Compared to SnO 2 modified RGO hybrids (SnO 2 -RGO with bandgap of 3.88 eV), the bandgap energy of α-Fe 2 O 3 /SnO 2 -RGO hybrids (3.53 eV) was reduced by adding α-Fe 2 O 3; the narrower bandgap facilitated the sensing materials to release more electrons and form more oxygen ions at room temperature. Besides, the high carrier migration of RGO, which served as continuous phase, identical structure with ultrasmall particle size of α-Fe 2 O 3 and SnO 2 (about 3–6 nm), and abundant chemisorbed oxygen species on the surface (20.8%) of the sensing materials, as well as their suitable bandgap (3.53 eV) in the sensing materials, significantly improved NO 2 response at room temperature. Among the sensors fabricated, α-Fe 2 O 3 /SnO 2 -RGO-15-based NO 2 sensor had the highest response of 7.4 with a short response time of 59 s towards 1 ppm NO 2 ; it could even reach a response of 2.6 towards 100 ppb NO 2. Notably, α-Fe 2 O 3 /SnO 2 -RGO-15 sample has excellent capability to recognize NO 2 , where the response value (7.4) towards 1 ppm NO 2 is about 7 times higher than that of 100 ppm ammonia and common volatile organic compounds (formaldehyde, toluene, ethanol and acetone). Such NO 2 sensor has superior repeatability with negligible response deviation towards 1 ppm NO 2 for four reversible cycles. This makes it to have a great potential application in the field of NO 2 detection. [ABSTRACT FROM AUTHOR]

[Display omitted] The metal oxides-based chemiresitive gas sensors have attracted enormous interest because of their exellent sensing perforamcnes, which have emerged as very promising candidates for gas monitoring. However, from the view of organophosphorus compounds detection, a unique combination of low detection limit and fast respons/recovery rate remains challenging. Herein, the synersgitic effects of oxygen vacancy engineering and surface gold decoration enabling excellent sensing performances for detection of dimethyl methyl phosphonate (DMMP, a typical organophosphorus) is reported. To demonstrate the proof of concept, Au nanoparticles (NPs) decorated oxygen vacancy-enriched SnO 2 hybrids (designated as Au-O-SnO 2) were designed as sensing materials, where the O-SnO 2 samples were fabricated by introduction of oxygen vacancies onto commercial SnO 2 through organometallic chemistry-assisted approach using (CH 3) 2 SnCl 2 as precursor, followed by deposition of Au NPs by an in-situ reduction routine. After optimizing Au NPs content in hybrids (1 wt%, 3 wt%, 5 wt% and 7 wt%), O-SnO 2 decorated with 5 wt% Au NPs (designated as Au-O-SnO 2 -5) exhibits excellent DMMP sensing performances, such as, an enhanced recoverable response of 1.67 to 680 ppb DMMP, low detection limit of 4.8 ppb, short response time of 26 s and recovery time of 32 s, as well as good selectivity, which are much better than that of commercial SnO 2 (C-SnO 2) and O-SnO 2 , and Au NPs decorated C-SnO 2. Based on the detailed investigation, the enhanced DMMP sensing performances of Au-O-SnO 2 hybrids can be mainly ascribed to the synergistic effect of increasing surface active sites induced by oxygen vacancies, the chemical and electronic sensitization of Au NPs. As a result, Au-O-SnO 2 -5 hybrids display relatively low activation energy of 24.11 kJ/mol for DMMP oxidization, which is lower than that of O-SnO 2 (35.54 kJ/mol). Our results provide a feasible method for boosting sensing performances for DMMP detection, paving new way for fabrication of metal oxides-based gas sensors for rapid detection of trace organic compounds with complexed structures. [ABSTRACT FROM AUTHOR]

Development of room-temperature gas sensors with excellent sensing performances for detection of dimethyl methylphosphonate (DMMP, a simulant of organophosphate) has aroused considerable attention. However, the weak adsorption ability of sensing materials toward DMMP molecules causes poor sensing performances. In this work, a multisite strategy for enhancing sensitivity was proposed by introduction of both N-doped carbon nanoparticles (N-CNPs) and copper ions onto the surface of rGO (designated as GNC). Impressively, optimal GNC-3 exhibits the response value of 10.2% toward 100 ppm DMMP, which is 5.3 times higher than that of pristine rGO. This mechanism for multisite-assisted enhanced DMMP sensing performances was investigated by combined characterizations of Fourier-transform infrared spectra, UV-Vis spectra, photoluminescence spectra and Mott-Schottky curves. Besides the construction of multisite, improvement of hydrogen bonding interactions between DMMP molecules and N-CNPs in hybrids assisted by copper ions, and decrease of conduction band potential by formation of ternary hybrids also contribute to enhance the sensitivity of DMMP sensors. These findings not only provided new insights into designing high-performance room-temperature gas sensors, but also promoted the fundamental research on sensing mechanism for room-temperature gas sensors. • A multisite strategy for enhancing sensitivity was proposed by constructing three kinds of DMMP adsorption active sites. • GNC hybrids demonstrate 5.3-fold improvement in response to 100 ppm DMMP compared to pristine rGO hybrids. • This mechanism for multisite-assisted enhanced DMMP sensing performances was confirmed by the spectroscopy. [ABSTRACT FROM AUTHOR]

The hydrogen bonding interactions are potentially employed to fabricate high-performance room-temperature gas sensors for detection of organophosphorus. Nevertheless, the low response toward target gases of conventional sensing materials hinders their further applications. Herein, an isolated Cu-N 5 sites engineering strategy is reported to improve sensing performances for polypyrrole-reduced graphene oxide hybrids (PPy-rGO)-based room-temperature dimethyl methyl phosphate (DMMP) sensors. The isolated Cu-N 5 sites were simply constructed by adsorption of Cu2+ ions with PPy-rGO hybrids owing to strong metal-support interactions. X-ray absorption fine structure analysis indicates that Cux+ ions with mean chemical valence of + 1.07 nearly atomically distributed on support with coordination structure of Cu-N 5. Benefiting from the structure regulation by Cux+ ions, as-constructed DMMP sensor demonstrated 4.5-fold improvement in response to 100 ppm DMMP and 2.5-fold improvement in limit of detection, compared to PPy-rGO hybrids. The density functional theory, spectroscopic characterizations and quartz crystal microbalance tests prove that the construction of Cu-N 5 sites not only heightens hydrogen bonding interactions between N H bonds and DMMP molecules induced by newly formed coordination interactions between Cux+ ions and N H bonds, but also serves as new active sites for adsorption of DMMP molecules. This work offers a new avenue for developing high-performance room-temperature gas sensors by heightening hydrogen bonding interactions. • Isolated Cu-N 5 sites-assisted heightening hydrogen bonds strategy is proposed to construct room-temperature DMMP sensors. • Cux+-PPy-rGO hybrids demonstrate 4.5-fold improvement in response to 100 ppm DMMP compared to PPy-rGO hybrids. • The limit of detection of the Cux+-PPy-rGO based sensor is 2 ppm. [ABSTRACT FROM AUTHOR]

Reduced graphene oxide (RGO)-based NO 2 sensors have attracted considerable attention due to their excellent advantages of low power consumption and manufacturability to facilitate massive deployment. However, it is still a great challenge to fabricate RGO-based room-temperatureNO 2 sensors with excellent sensing performances. Herein, we have demonstrated a combined surface modification and heteroatom doping approach to enhance the sensing performances of RGO-based room-temperature NO 2 sensors, where SnO 2 nanoparticles modified nitrogen-doped RGO (SnO 2 /N-RGO) hybrids had been used as sensing materials. The SnO 2 /N-RGO hybrids were prepared by hydrothermal synthesis method using SnCl 4 , GO and urea as precursors. The combined characterizations of X-ray diffraction (XRD), energy-dispersive X-ray spectrometer (EDS), elemental mapping, X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM), Raman spectra as well as N 2 sorption isotherm were used to characterize the materials thus obtained, indicating the successful preparation of SnO 2 /N-RGO hybrids. During the hydrothermal progress, SnCl 4 conversed into SnO 2 nanoparticles (NPs), and GO was reduced into RGO, while urea was decomposed into nitrogen-containing molecule and doped into RGO. It is found that SnO 2 NPs with the size of 3–5 nm are uniformly dispersed on N-RGO nanosheets. Most importantly, SnO 2 /N-RGO hybrids-based sensor exhibits superior sensing performances toward NO 2 operated at room temperature, which are better than those of pure RGO and SnO 2 /RGO hybrids. For example, SnO 2 /N-RGO hybrids show response of 1.38 to 5 ppm NO 2 with the response time and recovery time of 45 s and 168 s. The excellent sensing performances are attributed to incorporation of N atoms into RGO and the modification of RGO with SnO 2 NPs. This novel sensor based on SnO 2 /N-RGO hybrids promises to provide an essential sensing platform for the detection of NO 2 with excellent sensing performances at room temperature. [ABSTRACT FROM AUTHOR]

The development of metal oxides-based gas sensors for DMMP detection simultaneously possessing low operating temperature, low limit of detection and high sensitivity remains formidable challenges. In this work, a facile and efficient colloidal assembly approach is proposed to prepare Au modified SnO 2 hybrids (Au-SnO 2) with perfect metal-semiconductor interface and successfully used in DMMP detection application. After optimization, the best DMMP sensors based on Au-SnO 2 hybrids display outstanding sensing performances, including high response (1.88 to 680 ppb DMMP), low operating temperature (200 °C), extremely low limit of detection (34 ppb), as well as short response/recovery time (25 s/72 s). The sensing mechanism analysis reveals that the excellent sensing performances are mainly attributed to the synergy of chemical oxidation of DMMP catalyzed by Au-SnO 2 hybrids and physical adsorption of DMMP by hydrogen-bonding interactions. The present work not only provides an insight into the DMMP sensing mechanism, but also offers a promising general approach for development of high-performance gas sensors. [Display omitted] • A facile and efficient colloidal assembly approach waa proposed to synthesize Au modified SnO 2 hybrids. • The Au-SnO 2 -based sensor exhibits high response of 1.88 towards 680 ppb DMMP. • The limit of detection of the Au-SnO 2 -based sensor was 34 ppb. • An integrated chemical oxidation and hydrogen-bonding DMMP sensing mechanism model was first proposed. [ABSTRACT FROM AUTHOR]

Abstract: In2O3 micro/nanotubes (MNTs) with different diameters are successfully synthesized by an effective and template-free coaxial electrospinning method. The diameters of the MNTs can be simply tailored by controlling the ratio of reactants. The HCHO sensing tests of as-prepared MNTs reveal that the samples possess high response value, fast response and recovery rate, and favorable selectivity. Moreover, compared with the large diameter (∼1μm or ∼500nm) In2O3 MNTs, the small diameter (∼100nm) In2O3 MNTs exhibit highly enhanced sensing properties. The gas sensing mechanism of In2O3 MNTs has been discussed in detail. The promising HCHO-sensing properties enable In2O3 MNTs to be a competitive candidate for detecting poisonous HCHO in practice. The experiment also indicates that the coaxial electrospinning is an environmental friendly and easy method for constructing tubular structure of metal oxides with different diameters for various applications. [Copyright &y& Elsevier]

• Polypyrrole decorated-reduced graphene oxide (PPy-rGO) hybrids were successfully synthesized by a self-redox strategy. • The PPy-rGO-3-based sensor exhibits a higher response and fast response and recovery behavior to DMMP than pristine rGO. • The enhanced DMMP sensing properties were attributed to formation of hydrogen bonds between DMMP and PPy. The development of fast-response, high sensitivity and selectivity gas sensors for monitoring of organophosphorus is essential, where two dimensional graphene-based materials possessing exceptionsal carrier mobility are considered as promising candidates for room-temperature organophosphorus detection. However, it is challenging to fabrication of graphene-based gas sensors for detection of organophosphorus with excellent sensing performances. Herein, we develope a self-redox strategy to synthesize polypyrrole decorated-reduced graphene oxide hybrids (PPy-rGO) by redox reactions between pyrrole and GO during hydrothermal treatment process. This self-redox strategy results in the formation of perfact interfical strucutre between PPy and rGO through the π-π interactions without the impurity from the conventional oxidation/reducing agents. Most importantly, such PPy-rGO hybrids can be used as novel sensing materials for detection of dimethyl methylphosphonate (DMMP) at room temperature. Specially, the response of PPy-rGO-based sensor towards 100 ppm DMMP can reach 12.9 %, which is 3-fold higher than that pristine rGO-based sensor. Meanwhile, PPy-rGO-based sensor exhibits short response time/recovery time (43 s/75 s), low detection limit (5 ppm), excellent repeatability, and high selectivity. By combination of FT-IR and N 2 sorption isotherms, the enhanced DMMP sensing performances of PPy-rGO hybrids are concluded as following two aspects. Firstly, the formation of hydrogen bonds between PPy-rGO hybrids and DMMP molecules regulates the adsorption/desorption of DMMP. Secondly, increasing BET surface area by introduction of PPy into rGO matrix is beneficial to DMMP diffusion among the sensing materials. Our work would offer a new strategy for rational development of graphene-based materials for detection of organophosphorus at room temperature. [ABSTRACT FROM AUTHOR]