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Ag-decorated SnO 2 hollow spheres with different mass ratios were synthesized through a two-step synthesis route. The morphology of as-prepared materials was characterized via FESEM and TEM microscopes, and the results proved the rough and porous spherical structures. The EDX and XPS patterns verified the elementary composition, especially for Ag/SnO 2 , the standard peaks of Ag were obviously observed. The N 2 isotherm of SnO 2 hollow spheres belongs to a type-IV isotherm with a large type H3 hysteresis loop. The pore size distribution covered a range of 35–80 nm and the sample showed a high BET surface area of 31.91 m 2 /g. During the systematic examinations of gas sensing performance, 5 wt% Ag/SnO 2 showed superior properties, including the ultrahigh response of over one hundred to methoxy propanol within a wide concentration range and the excellent selectivity for detecting methoxy propanol among other gases. Moreover, the synthesis mechanism and gas sensing mechanism were discussed, which indicated that both the morphology of SnO 2 hollow spheres and the catalytic effect of Ag nanoparticles played important roles on the gas sensing properties, and could be used for the further design of gas sensor utilizing other oxide semiconductors. [ABSTRACT FROM AUTHOR]

Hierarchical Cu n O nanoflowers were synthesized through the laser ablation of a CuO target in NaOH solutions for room-temperature (27 ℃) H 2 S detection. Notably, the pH value of NaOH solutions influenced both the micro-morphologies and compositions of the Cu n O products, as evidenced by XRD, XPS, SEM and TEM. In high pH solutions, the specific surface area of the Cu n O products increased, their thickness decreased, and the Cu 2 O content diminished, resulting in enhanced sensitivity, selectivity and stability of the Cu n O products' response to H 2 S. Notably, the pH14# sample synthesized using an NaOH solution with a pH value of 14 featured pure CuO nanoflowers comprising slightly curled nanosheets with a thickness of approximately 10 nm. This sensor demonstrated excellent H 2 S sensing performance at room temperature, exhibiting a response value of 1.17 for 10 ppb H 2 S, along with high selectivity and good long-term stability. However, after exposure to H 2 S, the resistance of the sensor did not recover to its baseline in air at room temperature. Thermogravimetry results revealed that a temperature of 300 °C was effective for recovery of the sensor. Consequently, the operation temperature of the pH14# sensor was controlled using a micro-hotplate. In the pulse heating mode, the sensor's response to 100 ppb H 2 S was 1.5, with a response time of 135 s and a recovery time of 137 s. • Cu n O nanoflowers were synthesized by laser ablation of CuO target in NaOH solutions. • The pH value influenced H 2 S sensing performance of the Cu n O products. • CuO nanoflower sensor showed response of 1.17 for 10 ppb H 2 S at RT. • CuO nanoflower sensor achieved fast recovery ability under pulse heating. [ABSTRACT FROM AUTHOR]

Ethanol and acetone sensors have a wide variety of applications across different industries. However, it is necessary to improve the performance of such sensors and to understand the underlying mechanisms. In this study, Pd/PdO-WO 3 nanoblocks are synthesized via hydrothermal growth and calcination. The Pd and PdO contents of the materials are tuned by varying the Pd doping concentration and annealing temperature. The gas sensing performance of the nanoparticles is investigated, which shows that Pd doping increases the sensitivity and reduces the optimum operating temperature. At 200 °C, Pd/PdO-WO 3 nanoblocks with different PdO ratios exhibit good sensitivity to acetone and ethanol. However, because PdO is an active catalyst for ethanol oxidation, the oxygen sensitivity increases as the PdO ratio increases. The prepared sensors exhibit good stability and excellent selectivity against a variety of interferents, and the detection limit of the target gas is 100 ppb. The chemical and electronic sensitization of Pd/PdO lowers the activation barrier and improves the gas sensing response. This study demonstrates that the selectivity of a gas sensor can be regulated by controlling the electronic states of the active species. • Pd doped WO 3 nanoblocks were prepared by hydrothermal method. • The doping amount and annealing temperature affect the final chemical state of Pd. • Showed good selectivity to ethanol and acetone, respectively, with detection limits below 100 ppb. [ABSTRACT FROM AUTHOR]

To improve the poor selectivity of a semiconductor gas-sensor system, an array of sensors can be utilized. However, this increases the system's size and power consumption. To overcome these limitations, we propose a high-integrated and highly selective electronic nose (E-nose) comprising two independent gas-sensing elements on a low-power microhotplate (MHP). Pd-SnO 2 nanoflowers and Pd-WO 3 microparticles were prepared and printed on a bridge-structured MHP 20 µm apart and over an area of 110 µm × 45 µm. This was achieved using electrohydrodynamic inkjet printing aided by in-situ infrared laser curing to form an array of sensors. A power of 17 mW was required to increase the temperature of the MHP to 300 °C, which is the optimal operating temperature of the two gas sensors. Thus, a high response and low cross-sensitivity were achieved for hydrogen, ammonia, hydrogen–ammonia, ethanol, acetone, ethanol–acetone, toluene, and formaldehyde. A wavelet transform was used to reduce the noise and dimensionality of the signals from the gas-sensor array. Qualitative identification of the eight gases with an accuracy of 99.86% was achieved using the k -nearest neighbor (k NN) model. A p neighbors back propagation neural network (p N-BPNN) model was established to remove interfering samples to quantitatively estimate the gas concentration. The quantitative identification accuracy of p N-BPNN was higher than that of the standard back propagation neural network (BPNN) model with the average absolute percentage error of hydrogen detection in the range of 15–500 ppm, decreasing from 5.44% to 2.08%. • E-nose comprising two independent gas-sensing elements on a low-power microhotplate was achieved. • Prepared Pd-SnO 2 nanoflowers and Pd-WO 3 microparticles achieved low cross-sensitivity to eight different gases. • Proposed p neighbors-backpropagation neural network model has high estimation accuracy. • The qualitative identification and quantitative estimation of the E-nose with high accuracy and reliability. [ABSTRACT FROM AUTHOR]

Ti 3 C 2 T x is a new two-dimensional material with attractive characteristics, but the disadvantage of easy oxidation hinders its practical application. In this paper, single/few-layer Ti 3 C 2 T x (ST) and multilayer Ti 3 C 2 T x (MT) are fabricated from severely oxidized Ti 3 C 2 T x by TMAOH intercalation. Simultaneously most TiO 2 nanoparticles are removed. This is a useful path to fabricate Ti 3 C 2 T x with oxygen-rich functional groups by reusing the oxidized Ti 3 C 2 T x. Ag-loaded Ti 3 C 2 T x (AgT) is then successfully synthesized by the self-reduction of AgNO 3 on MT. The morphology, surface functional groups, oxidation degree, and oxidation derived defects are characterized by XRD, SEM, TEM, XPS, and FTIR. The gas sensing test results illustrate that MT and AgT are more sensitive to NH 3 than ST. These sensors perform best in 40% RH N 2 and worst in dry N 2 , significantly related to humidity. In comparison, AgT exhibits the best humidity resistance due to its minor response degradation from 40% RH N 2 to dry N 2. The DFT calculation results reveal that with pre-adsorbed H 2 O, the adsorption capacity of Ti 3 C 2 O 2 and Ag-Ti 3 C 2 O 2 for NH 3 is considerably enhanced. In the absence of H 2 O, the adsorption of NH 3 by Ag-Ti 3 C 2 O 2 is much more effective than that of Ti 3 C 2 O 2 , indicating that Ag-Ti 3 C 2 O 2 's response decays less with humidity changes. The enhanced scattering of carriers by the Ti atom at the adsorption site on the Ti 3 C 2 O 2 surface also contributes to the gas sensing performance. These simulation results are consistent with the experimental results. In addition, the formation of heterojunction between Ag and Ti 3 C 2 T x is beneficial to improving its gas-sensing response to NH 3. • Ti 3 C 2 T x with oxygen-rich functional groups was prepared from oxidized Ti 3 C 2 T x by TMAOH treatment. • Ag decorated multilayer Ti 3 C 2 T x was fabricated by self-reduction of AgNO 3. • The synthesized materials showed sensitive NH 3 responses highly related to humidity. • Improved humidity resistance was obtained by Ag decoration on Ti 3 C 2 T x. • DFT simulation proved the promotion of NH 3 adsorption with H 2 O and Ag decoration. [ABSTRACT FROM AUTHOR]

Nanosheet-assembled microspheres of tungsten oxide doped with antimony were synthesized via hydrothermal method. Sb doping changed the valency of W, promoted a partial crystal phase transition from orthorhombic to hexagonal, and modified the surface morphology. Further, WO 3 is an n-type semiconductor and is sensitive to gases above 100 °C. The Sb-WO 3 sensors in this work were p-type semiconductors from 25 °C to 65 °C, and exhibited excellent sensitivity, fast response and good recovery properties to NH 3. At 35 °C, the detection limit of 1 at% Sb-WO 3 for ammonia was 200 ppb, and the sensor shows a good stability and selectivity toward several possible interferent. The substantially improved sensing performance of the Sb-WO 3 sensors at near room temperature could be mainly attributed to the change of the W valency promoted via Sb doping. [Display omitted] • Microspheres composed of Sb doped WO 3 nanosheets were prepared by hydrothermal method. • The Sb-WO 3 sensor in this work was a p-type semiconductor at near RT, and exhibits excellent sensitivity to NH 3. • The improved sensing performance of the Sb-WO 3 at near RT could be attributed to the change of the W valency by Sb doping. [ABSTRACT FROM AUTHOR]

• 3D Hierarchical structured PdO/PdO 2 -SnO 2 :Sb materials were uniformly deposited on microhotplate (MHP) gas sensor array incorporated with Al electrodes via advanced electrohydrodynamic inkjet printing method. • The sensitivity of the SnO 2 :Sb material is regulated by doping different concentrations of PdO/PdO 2 , and identification of benzene, ethanol, acetone, formaldehyde, and ethanol-acetone mixed gas (volume ratio 1:1) is achieved by two-step principal component analysis (PCA). • Simulation calculations on the I–V characteristic of the MHP gas sensors indicate the gas sensing properties are determined not only by gas sensing materials but also by the electrodes. A possible mechanism based on the Poole-Frenkel effect for electrode sensing behavior is proposed. High-performance, low-power microhotplate (MHP) gas sensors loaded with 3D hierarchical structured PdO/PdO 2 -SnO 2 :Sb materials on a pair of Al gas sensing electrodes were fabricated through advanced high-accuracy electrohydrodynamic printing method. The PdO/PdO 2 -SnO 2 :Sb materials with various PdO/PdO 2 concentrations were initially synthesized via a mild and simple hydrothermal reaction. Their micromorphology and chemical composition were characterized by SEM, XRD, TEM, and XPS, which proved the successful synthesis. The gas sensing performance of the sensors to benzene, ethanol, acetone, formaldehyde, and ethanol-acetone mixed gas (volume ratio 1:1) was tested. The sensing property of the materials is regulated with different content of PdO/PdO 2. The identification of the VOCs is realized by two-step principal component analysis (PCA). Simulation calculations on the I–V characteristic of the MHP gas sensors reveal that carriers have to get through Al 2 O 3 barriers by Poole-Frenkel emission, which significantly affects the carrier conduction as well as the gas sensing properties. The gas sensing mechanism is analyzed from the aspects of gas sensing material and electrodes. [ABSTRACT FROM AUTHOR]