Highly sensitive and selective acetylene sensors based on p-n heterojunction of NiO nanoparticles on flower-like ZnO structures

Acetylene (C₂H₂) gas concentration is a key parameter in transformer monitoring. In current work, the selective acetylene sensors which based on flower-like ZnO structures with NiO nanoparticles were successfully fabricated. The NiO–ZnO composites were synthesized by two-step hydrothermal method. And various of characterization analyses had been applied to the exploration of crystal structure and the p-n heterojunction. According to the systematic gas sensitivity tests, the response of NiO–ZnO (5%) to 50 ppm C₂H₂ was 15.23 at 200 °C whereas the response of pure ZnO was 4.1 in the same condition. In addition, the response value of NiO–ZnO (5%) to 50 ppm C₂H₂ was 3.6 times to 50 ppm H₂. Such a good gas-sensing property of NiO–ZnO composites is due to p-n heterojunction and high catalytic activity of NiO.     

Low-temperature and highly sensitivity H2S gas sensor based on ZnO/CuO composite derived from bimetal metal-organic frameworks

The bimetallic metal-organic frameworks (MOF) Zn/Cu-BTC were prepared by a facile solvothermal method in one step and used as a self-sacrificed template to obtain the ZnO/CuO composites. The composites with different Cu/Zn molar ratios were characterized by XRD, FESEM, and XPS. The ZnO/CuO composite exhibited an octahedral structure, and a p-n heterojunction may be formed between p-type CuO and n-type ZnO. To prove its functional characteristics, the ZnO/CuO composite was used as a sensing material to test its gas sensitivity. The effect of Cu/Zn molar ratios was examined, and the results showed that the optimized ZnO/CuO (1: 0.33) composite based gas sensor exhibited reasonable selectivity to 10 ppm H₂S, operated at 40 °C. The sensitivities were improved by 17.1 times and 327.8 times compared with the pristine CuO and ZnO based gas sensors, respectively. Moreover, the detection limit to H₂S of such sensors could be reduced as low as 300 ppb. The sensing mechanism has been thoroughly studied and such ZnO/CuO composite is an ideal candidate for highly sensitive detection for H₂S with low power consumption in the real application.     

Low-temperature H2S removal from gas streams with SBA-15 supported ZnO nanoparticles

Mesoporous silica SBA-15 with zinc oxide (ZnO) nanoparticles was prepared via incipient wetness impregnation and ultrasonic method, followed by in situ activation at 523 K. The mesoporous materials obtained were characterized by ICP, XRD, FTIR, nitrogen adsorption, TEM and XPS. The prepared materials showed a superior ability to remove H₂S down to parts per billion (ppb) from gas stream at lower temperature (298 K), and the highest H₂S breakthrough capacity, 436 mg S/g adsorbent, was observed for SBA-15 with 3.04 wt% zinc loading. The enhancement of H₂S removal capacity was attributed to the integration of the high surface area of the mesoporous material and the promising desulphurization properties of ZnO nanoparticles. It was believed that ZnO-modified SBA-15 is a promising adsorbent for H₂S cleaning at ambient conditions, which will extend the application of the mesoporous materials to the environmental protection area.     

Hydrogen sulfide gas detection by Au-decorated ZnO nanotube: a computational study and comparison to experimental observations

Based on the experimental reports, Au-decoration on the ZnO nanostructures dramatically increases the electronic sensitivity to H₂S gas. In the current study, we computationally scrutinized the mechanism of Au-decoration on a ZnO nanotube (ZON) and the influence on its sensing behavior toward H₂S gas. The intrinsic ZON weakly interacted with the H₂S gas with an adsorption energy of ?11.2 kcal/mol. The interaction showed no effect on the HOMO–LUMO gap and conductivity of ZON. The predicted response of intrinsic ZON toward H₂S gas is 6.3, which increases to 78.1 by the Au-decoration at 298 K. The corresponding experimental values are about 5.0 and 80.0, indicating excellent agreement with our findings. We showed that the Au atom catalyzes the reaction 3O₂?+?2H₂S?→?2SO₂?+?2H₂O. Our calculated energy barrier (at 298 K) is about 12.3 kcal/mol for this reaction. The gap and electrical conductance Au-ZON largely changed by this reaction are attributed to the electron donation and back-donation processes. The obtained recovery time is about 1.35 ms for desorption of generated gases from the surface of the Au-ZON sensor.     

WO3/La0.8Pb0.2FeO3 perovskite heterostructure for highly active and selective hydrogen sulfide detection

In this study, tungsten oxide (WO₃) nanofibers were prepared using electrospinning technology and combined with La_0.8Pb_0.2FeO₃ (LPFO) perovskite materials to form a heterostructure film, and then used to evaluate the potential as a gas sensing material. The results show that the pure WO₃ nanofiber gas sensor has an excellent sensing effect on nitrogen dioxide (NO₂) and hydrogen sulfide (H₂S), and the WO₃/LPFO heterostructure film gas sensor still has a high response to H₂S, but the response to NO₂ is suppressed. This WO₃/LPFO heterostructure film gas sensor greatly improves the gas selectivity, making the selectivity more specific. The WO₃/LPFO heterostructure film gas sensor exhibits excellent gas selectivity for H₂S gas, the optimal operating temperature is 175 °C, and the response is about 89.5% under 1.25 ppm H₂S gas.     

One step from nanofiber to functional hybrid structure: Pd doped ZnO/SnO2 heterojunction nanofibers with hexagonal ZnO columns for enhanced low-temperature hydrogen gas sensing

In this paper, a novel hybrid structure of Pd doped ZnO/SnO₂ heterojunction nanofibers with hexagonal ZnO columns was one step synthesized from electrospun precursor nanofibers. Due to the synergistic effect of hexagonal ZnO, SnO₂ and Pd, the structure exhibited excellent hydrogen (H₂) gas sensing properties. At low-temperature of 120 °C, the response (R_a/R_g) to 100 ppm H₂ gas exceeded 160, the response/recovery time was only 20 s and 6 s respectively and the limit of detection was only 0.5 ppm. Meanwhile, it also had good selectivity for H₂ gas and excellent linearity. In addition, the materials were characterized by XRD, FESEM, HRTEM, XPS, and the synthesis mechanism and gas sensing mechanism were proposed.     

Preparation of hollow SnO2/ZnO cubes for the high-performance detection of VOCs

Detecting volatile organic compounds is essential to improving the environment and human health. This study prepared a novel composite of hollow SnO₂/ZnO cubes using a self-template hydrothermal method followed by a calcination process. The morphology and structure of the composites were characterized using a series of analysis techniques, and the formation mechanism of a hollow cube-like structure was explored. Compared to the hollow SnO₂ cube sensor, the hollow SnO₂/ZnO cube sensor exhibited a strong response (148–100 ppm formaldehyde), fast response/recovery time (15 s/25 s), good linearity (R² = 0.995), good repeatability, and excellent stability. The superior gas sensing property of the hollow SnO₂/ZnO cubes was attributed to the combined advantages of hollow structures and heterojunctions.     

Nanoarchitectonics of three-dimensional ZnO–BiVO4 for trace nitrogen dioxide gas detection

In the present study, several novel three-dimensional (3D) ZnO–BiVO₄ composites were prepared using the hydrothermal method, and their ability to detect nitrogen dioxide (NO₂) gas was assessed. The fabricated sensing materials were examined using X-ray diffraction, transmission electron microscopy, energy dispersive spectrometry, scanning electron microscopy, ultraviolet–visible spectrophotometry, X-ray photoelectron spectroscopy, and surface-area and porosity analysis. The sensing material assessments verified that 3D ZnO–BiVO₄ was successfully prepared; the sensing data revealed that 3D ZnO–BiVO₄ exhibited excellent sensing response (24.6) to 1-ppm NO₂ at room temperature. The composites exhibited a fast response (28 s) and short recovery time (75 s) when detecting 1-ppm NO₂; they also exhibited long-term stability (decline of approximately 7% after 30 days) and selectivity to 10-ppm NO₂ gas. A reasonable NO₂ gas sensing mechanism for 3D ZnO–BiVO₄ heterojunction was also proposed in this paper.     

A new synergistic effect in one step sputtered ZnO/Zn2SnO4 heterojunction films for H2 sensing related to crystal structure and film compactness

In this paper, ZnO/Zn₂SnO₄ heterojunction films were one step fabricated by magnetron sputtering and the dependence of crystal structures, film compactness and H₂ sensing properties on annealing process were investigated and discussed. The results showed that three typical surface morphologies can be controlled by adjusting annealing temperatures and periods. The films annealed at the temperature of 550 °C for 6 h showed the best H₂ sensing properties. It exhibited a response (R_a/R_g) of 28.3–100 ppm H₂ at the temperature of 230 °C and the detection limit is 30.2 ppb. Meanwhile, it also showed a good selectivity and long-term stability to H₂. The H₂ sensing mechanism is attributed to the synergistic effect between ZnO (0001) signal crystal facets and ZnO/Zn₂SnO₄ heterojunction structures which enhanced the gas reactivity and resistance modulation range. On the contrary, insufficient annealing restricts the film crystallinity and the growth of hexagonal ZnO while undue annealing destroys the compactness of the films, leading to poor H₂ sensing properties.     

CSA doped polyaniline/CdS organic–inorganic nanohybrid: Physical and gas sensing properties

Camphor sulfonic acid (CSA) doped polyaniline/CdS nanohybrid materials were prepared by chemical oxidative polymerization method and characterized by field emission scanning electron microscopy (FESEM) and Fourier transform infrared (FTIR) spectroscopy. It is proved that there is a strong synergetic interaction between the CSA and polyaniline–CdS nanohybrid. Gas sensing measurements showed that the gas sensor based on the CSA doped PANi–CdS nanohybrid had high sensor response (75%), good selectivity (for H₂S) and stability (97.34%), as well as comparatively short recovery time to H₂S, operating at room temperature. The enhanced gas sensing performance of the nanohybrid is due to the high surface area of the CSA doped PANi–CdS hybrids and the p–n heterojunction formed between p-type polyaniline and n-type CdS nanoparticles.     

Highly sensitive C2H2 gas sensor based on Ag modified ZnO nanorods

The sliver (Ag) modified zinc oxide (ZnO) nanorods were successfully obtained with a simplified and environmentally friendly solvothermal method. Materials characterization indicated that the metallic Ag was located on the outside of ZnO nanorods after annealing. In comparison with ZnO nanorods, Ag modified ZnO (Ag–ZnO) nanorods exhibited a considerably enhanced response to C₂H₂. The response of the 3 at% Ag–ZnO based sensor operating at 175 °C is 539 (R_a/R_g), which is the highest value among all the sensors in detecting 100 ppm C₂H₂. The Ag–ZnO based sensors exhibited fast response speed, lower operation temperature and higher selectivity.     

Unusual electrochemical response of ZnO nanowires-decorated multiwalled carbon nanotubes

A novel type of ZnO nanowires-modified multiwalled carbon nanotubes (MWCNTs) nanocomposite (ZnO-NWs/MWCNTs) has been prepared by a hydrothermal process. The ZnO-NWs/MWCNTs nanocomposite has a uniform surface distribution and large coverage of ZnO nanowires onto MWCNTs with 3D configuration, which was characterized by scanning electron microscopy. Cyclic voltammetry and electrochemical impedance spectroscopy methods were applied to investigate the electrochemical properties of ZnO-NWs/MWCNTs nanocomposite. Surprisingly, unlike the conventional n-type semiconducting ZnO nanowires grown on Ta substrate, the ZnO-NWs/MWCNTs nanocomposite exhibits excellent electron transfer capability and gives a pair of well-defined symmetric redox peaks towards ferricyanide probe. What's more, the ZnO-NWs/MWCNTs nanocomposite shows remarkable electrocatalytic activity (current response increased 4 folds at 0.3 V) towards H₂O₂ by comparing with bare MWCNTs. The ZnO-NWs/MWCNTs nanocomposite could find applications in novel biosensors and other electronic devices.     

C2H2 gas sensor based on Ni-doped ZnO electrospun nanofibers

Pure and Ni-doped ZnO nanofibers were synthesized using the electrospinning method. The morphology, crystal structure and optical properties of the nanofibers were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy, respectively. It is found that Ni doping does not change the morphology and crystal structures of the nanofibers, and the ultraviolet emissions of ZnO nanofibers present red shift with increasing Ni doping concentration. C₂H₂ sensing properties of the sensors based on the nanofibers were investigated. The results show that the C₂H₂ sensing properties of ZnO nanofibers are effectively improved by Ni doping, and 5 at% Ni-doped ZnO nanofibers exhibit a maximum sensitivity to C₂H₂ gas.     

Effect of low H2 concentrations on the current-voltage characteristic of ZnO gas sensor

Abstract

Abstract

A self-heated ZnO gas sensor was prepared through thermal oxidation of Zn metal for 30 and 60?min in oxygen atmosphere. The XRD results confirmed the conversion of Zn metal to ZnO. The current-voltage measurements showed changes in forward current in the presence of hydrogen gas, with the concentration ranging from 40 to 160?ppm. A better resolution of current response due to H₂ concentrations was obtained for the samples with 30?min oxidation, although both samples showed enhanced response with the applied voltage. The barrier height of both samples was found to decrease with H₂ concentrations, thus aiding in the increase in current response.     

Removal of minor concentration of H2S on MDEA-modified SBA-15 for gas purification

A promising adsorbent for H₂S removal of minor concentration for gas purification was prepared by synthesizing and modifying the mesoporous molecular silica of SBA-15 with methyl-diethyl-amine (MDEA). Removal performance of minor concentration of H₂S on the adsorbent was experimentally studied in a dynamic setup. The adsorbents showed good performance in removing H₂S from gas steams. The loaded amines did not change the ordered structure of SBA-15, but enhanced its removing H₂S. The adsorbents were characterized by X-ray powder diffraction (XRD) and N₂ adsorption/desorption. Effect of the R (the loading ratio) of MDEA, effect of initial H₂S concentration and effect of moisture on removal performance of H₂S were studied respectively. With increase of the R of MDEA, the H₂S removal performance of the adsorbent was improved obviously. When R of MDEA was 0.6, the removal performance was optimum. Initial H₂S concentration had a large effect on the removal performance. Both breakthrough capacity and saturation capacity increased as the initial H₂S concentration increased. In the presence of moisture, the experimental results showed the improvement in the removal performance of H₂S. In addition, not only was the adsorbent regenerable by purging with the purified gas, but also the removal performance was stable in removal adsorption cycles.     

Effects of Pretreatments of Cu/ZnO-Based Catalysts on Their Activities for the Water–Gas Shift Reaction

The effects of the pretreatments of Cu/ZnO-based catalysts prepared by a coprecipitation method on their activities for the water–gas shift reaction at 523K were investigated. The activity of a Cu/ZnO/ZrO₂/Al₂O₃ catalyst for the water–gas shift reaction was less affected by calcination at temperatures ranging from 673-973K and by H₂ treatment at 573 or 723K than that of a Cu/ZnO/Al₂O₃ catalyst. The catalyst activity could be correlated mainly to the Cu surface area of the catalyst.     

Preparation,characterization of 1D ZnO nanorods and their gas sensing properties

The 1D ZnO nanorods (NR's) were grown with Zinc (Zn) ion precursor concentration variation on seed layer glass substrate by the low temperature hydrothermal method and utilized for nitrogen dioxide (NO₂) gas sensing application. Zn ion precursor concentration varied as 0.02, 0.03, 0.04, 0.05 and 0.06 M and thin films were characterized for structural, morphological, optical, electrical, surface defect study and gas sensing properties. All the film showed dominant orientation along the (002) direction, the intensity of the peak vary with the length of the nanorods. SEM cross images confirmed that nanorods had vertical alignment perpendicular to the plane of the substrate surface. The PL intensity of oxygen vacancy related defects for prepared samples was found to be linearly proportional to gas sensing phenomena. This result in good agreement with the theoretical postulation that, oxygen vacancies plays the important role for adsorption sites to NO₂ molecule. The gas sensing performance was studied as a function of operating temperature, Zn ion precursor concentration variation, and gas concentration. The maximum gas response is 113.32–100 ppm NO₂ gas at 150 °C for 0.05 M sample out of all prepared samples. Additionally, ZnO thin film sensor has potential to detect NO₂ as low as 5 ppm.     

Gas sensing properties of conducting polymer/Au-loaded ZnO nanoparticle composite materials at room temperature

In this work, a new poly (3-hexylthiophene):1.00 mol% Au-loaded zinc oxide nanoparticles (P3HT:Au/ZnO NPs) hybrid sensor is developed and systematically studied for ammonia sensing applications. The 1.00 mol% Au/ZnO NPs were synthesized by a one-step flame spray pyrolysis (FSP) process and mixed with P3HT at different mixing ratios (1:1, 2:1, 3:1, 4:1, and 1:2) before drop casting on an Al₂O₃ substrate with interdigitated gold electrodes to form thick film sensors. Particle characterizations by X-ray diffraction (XRD), nitrogen adsorption analysis, and high-resolution transmission electron microscopy (HR-TEM) showed highly crystalline ZnO nanoparticles (5 to 15 nm) loaded with ultrafine Au nanoparticles (1 to 2 nm). Film characterizations by XRD, field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy, and atomic force microscopy (AFM) revealed the presence of P3HT/ZnO mixed phases and porous nanoparticle structures in the composite thick film. The gas sensing properties of P3HT:1.00 mol% Au/ZnO NPs composite sensors were studied for reducing and oxidizing gases (NH₃, C₂H₅OH, CO, H₂S, NO₂, and H₂O) at room temperature. It was found that the composite film with 4:1 of P3HT:1.00 mol% Au/ZnO NPs exhibited the best NH₃ sensing performances with high response (approximately 32 to 1,000 ppm of NH₃), fast response time (4.2 s), and high selectivity at room temperature. Plausible mechanisms explaining the enhanced NH₃ response by composite films were discussed.     

锰系可再生高温脱硫剂的制备及其性能测试

煤气的高温脱硫净化是 IGCC 和 DRI 生产的瓶颈,直接影响整个过程的热效率。在50℃、pH值约为9的条件下采用硝酸锰、硝酸铝混合溶液与氨水进行共沉淀,制备了锰含量不同的脱硫剂,在固定床反应器中考察了脱硫剂的硫化及再生性能,并利用XRD、SEM、BET等手段表征了脱硫剂在硫化/再生过程中的物相和结构变化。共沉淀法制备的脱硫剂Mn/Al分散性好,在850℃高温下进行脱硫反应可以定量快速进行。脱硫硫容与脱硫剂锰含量呈正比,Mn-S/Mn-O交换原子比在0.90~0.95之间,改变空速和进口H₂S含量并不改变脱硫硫容。采用O₂浓度为3%的稀释空气在850℃下再生,再生后的硫容稳定,说明所制备的脱硫剂可用于高温可再生脱硫。     

Synthesis and microstructure of (LaSr)MnO3 and (LaSr)FeO3 ceramics by a reaction-sintering process

(La_xSr_1−x)MnO₃ (LSMO) and (La_xSr_1−x)FeO₃ (LSFO) (x = 0.2–0.4) ceramics prepared by a simple and effective reaction-sintering process were investigated. Without any calcination involved, La₂O₃ and SrCO₃ were mixed with MnO₂ (LSMO) or Fe₂O₃ (LSFO) then pressed and sintered directly. LSMO and LSFO ceramics were obtained after 2 and 4 h sintering at 1350–1400 and 1200–1280 °C, respectively. Grain size decreased as La content increased in LSMO and LSFO ceramics.