Ppm NH3 Research Articles

A comparative study of various transition metal overlayer catalysts for low-temperature NH3 oxidation under dry and wet conditions

In this study, several nanometer-thick transition metal overlayers (M) were formed on an Fe–Cr–Al metal (SUS) foil by pulsed cathodic arc-plasma deposition. The thin-film materials prepared, exhibiting a full-coverage M overlayer (M/SUS), were applied as catalysts for NH3 oxidation in the presence of excess O2 (300 ppm NH3, 8% O2, and He balance). Under dry conditions (0% H2O in the gas feed), the observed NH3 oxidation activity decreased in the order of Ir > Pt > Co > Pd > Cu > Rh when the product selectivity was not taken into consideration. The activities of the materials composed of Co and Cu decreased significantly in the presence of 10% H2O, while those of the catalysts containing Pt and Ir remained nearly unchanged. Based on the kinetic analyses and X-ray photoelectron spectrometry experiments conducted, it was determined that the contrasting effect of H2O on the activity of M/SUS was related to the ease of oxidation of the non-precious metals studied. The resulting hydrophilic oxide surface was more susceptible to the competitive adsorption of H2O and thus inhibited the NH3 reaction than a metallic surface. The negative effect of H2O was evident by the decrease in the NH3 concentration in the gas feed. Notably, the metallic surface of the Pt overlayer was stable even under oxidizing reaction conditions; therefore, the oxidation of NH3 was less susceptible to the effects of high H2O concentrations.

Highly sensitive and chemically stable NH3 sensors based on an organic acid-sensitized cross-linked hydrogel for exhaled breath analysis

Due to interference by the high moisture content and complicated compositions of human exhaled breath, the trace-level detection of ammonia (NH3) with desirable selectivity and stability is a large challenge for exhaled breath analysis. Carboxyl-sensitized hydrogels can be activated by moisture to exhibit a significant response and excellent selectivity to NH3. However, the high activity of carboxyl groups in hydrogels is a double-edged sword, resulting in poor chemical stability during NH3 detection. Herein, organic acids were embedded into a cross-linked poly(ethylene glycol) diacrylate (PEGDA) hydrogel via thiol−ene photochemistry to form stable hydrogels for NH3 detection in a humid atmosphere. As a result, under high humidity conditions (80% RH), the optimal sensors exhibited superior selectivity to NH3 among various interfering gas species, a remarkably high NH3 response (Za/Zg=6.20) towards 20 ppm NH3, and an extremely low actual detection limit (50 ppb) at room temperature. Moreover, the sensors exhibited excellent chemical stability due to the moderate equilibrium water content of the hydrogel composites and acid dissociation constant of the acid groups. The moisture-activated NH3 sensing mechanism was thoroughly investigated by complex impedance spectroscopy (CIS), quartz crystal microbalance (QCM) measurements, Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). To explore the application prospects of cross-linked hydrogel sensors for detecting NH3 in exhaled breath, a simulated exhaled breath test was also performed.

Sensing by Surface Work Function Modulation: High Performance Gas Sensing using van der Waals Stacked Bipolar Junction Transistor

We propose and demonstrate a new principle for sensing. Impact of light, radiation, charge particle and gas molecular etc. can induce the change of the surface work function of a solid. This will result in a profound effect to a device’s output, such as current and voltage. Such an effect is in particular outstanding in a nanodevice using van der Waals stacked two-dimensional (2D) atomic crystals. In this paper, a study of a gas sensing device relying on such a principle is presented. Van der Waals bipolar junction transistors (V2D-BJT) based on vertically stacked MoS2/WSe2/MoS2 and WSe2/MoS2/WSe2 heterostructures are separately fabricated to detect NH3 and NO2 gas. Kelvin probe force microscope measurement shows that the work function change at the overlapped region of MoS2/WSe2/MoS2 heterostructure upon exposure to 50 ppm NH3 gas is 80 meV. In addition, the MoS2/WSe2/MoS2 V2D-BJT manifest superior sensing performances in detecting NH3, while the WSe2/MoS2/WSe2 V2D-BJT exhibited superior NO2 sensing properties in terms of high sensitivity (18), low power dissipation (12 nW), fast response (26 s) and recovery (14 s) time. We demonstrate that the NH3 and NO2 gas can be selectively detected by the V2D-BJT through changing the van der Waals heterostructure stacking sequences.

Stable Cobalt Porphyirn Ometed Type Small Molecule Sensor for the Sensitive and Selective Detection of Ammonia Gas at Room Temperature

AbstractThe ammonia (NH3) gas device sensors at room temperature rapid‐response (RTRR) selective discovery is a challenging and massive task that is restricted due to reduced room‐temperature (RT) detecting materials and their unsatisfactory gas detecting performance features. In this work, an organic small molecule 5, 10, 15, 20‐tetrakis {4‐[N,N‐di(4‐methoxyphenyl) amino‐phenyl]‐porphyrin (PO) and its cobalt complex cobalt‐porphyrin (PCo) based twisted type structure sensor is established for the detection of NH3 gas at RT. Interestingly, the introduction of Co atom into porphyin ring center increases overall performance, including high stability maximum response of 85% to 20 ppm NH3, and high selectivity towards ammonia compared to PO molecule. In terms of selectivity, the PCo sensor has a great affinity for NH3 due to the unpaired electron in dx2‐y2 with the porphyrin‐system, which boosts NH3 selectivity even in a mixture of gases. The detection range is 1 ppm, which is an 11% response with outstanding fast response (180 s) and recovery (10 s) containing excellent reproducibility to 20 ppm NH3. The superior gas‐detecting characteristics of PCo are ascribed to the interface between NH3 and cobalt Co‐group of PCo. The results from sequential studies deliver a novel approach and procedure for exploring RT ammonia gas device sensors.

Hierarchical flower-like TiO2 microspheres for high-selective NH3 detection: A density functional theory study

TiO2 is proved to be a promising material in the detection of various hazardous gases at the high temperature. However, gas adsorption-diffusion capacity at the room temperature is limited by their insufficient specific surface area and surface active sites. Herein, we report a strategy to synthesis of flower-like TiO2 microspheres assembled by nanosheets and characterize the selective adsorption of NH3 by using density functional theory (DFT). The as fabricated flower-like TiO2 microspheres show an outstanding gas sensitivities (∼122 to 50 ppm NH3) and high selectivity for NH3 detection at room temperature. The higher adsorption energy and smaller adsorption distance of NH3 on TiO2 (001) indicate the much stronger interaction NH3-TiO2 system. The present studies provide useful gas sensing materials with fine morphologies for the development of high-performance room temperature NH3 sensors.

Bismuth-based lead-free perovskite film for highly sensitive detection of ammonia gas

Lead halide perovskite materials have received increased attention for sensing applications, due to their environmental sensitivity. However, the lead toxicity represents a potential obstacle to their practical sensing application. Here, we report a lead-free (phenethylammonium)3Bi2Br9-based sensor for ammonia (NH3) gas detection. The sensor exhibits high gas response (R0/Rg = 1.76, at 30 ppm NH3), short response/recovery time (39/130 s), low detection limit (0.2 ppm), and good reversibility. The NH3 sensing mechanism is established using a combination of X‐ray diffraction (XRD), absorption spectra, Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and differential thermal analysis (DTA) measurements. The results demonstrate that the electron-donating NH3 molecules may first absorb onto the surface of the (phenethylammonium)3Bi2Br9 film for electron-injection, and then infiltrate it to dissociate the Bi2Br93− bi-octahedra. It will form a new NH4Br substance, different from an NH3-induced phase transformation in perovskite CH3NH3PbI3. This work reveals that it is feasible to design high-performance gas sensors based on environmentally-friendly Bi-based perovskites.

Demonstrating the impact of Band Gap Modilation on Semiconductor Metal Oxide Gas-sensing Performance

IntroductionGas sensors as the front-end device is the core component of indoor air quality monitoring system and plays an important role in detecting toxic gases. Due to various characteristics of simple structure, all solid state, well stability, and low cost, the semiconducting metal oxides (SMOs) based gas sensors have showed potential [1,2]. Recently, some researches have found that the alteration of electronic band structure will impact the gas sensing characteristics via controlling the charge carrier concentration and enhancing the surface adsorbed oxygen species [2-5].Herein, we reported the synthesis of honeycomb-like 3D inverse opal (IO) CdO-CdGa2O4 microspheres (MS) by using the combination of the one-step USP method with self-assembled S-PS spheres template. Such a novel structure provides both inside and outside reaction sites with high gas accessibility. Since the CdGa2O4 is an n-type semiconductor metal oxide with suitable band gap, and have been found to be a promising material as a transparent electronic conductor [6], we use it as base material of sensitive element. Importantly, the incorporation of CdO with narrow band gap (2.15-2.7 eV) and high electrical conductivity [7] is implied to deeply investigate the relationship between sensing materials’ electronic band structure and gas sensing characteristics. Accordingly, we successfully prepare 3D IO CdO-CdGa2O4 MS based chemiresistor sensor with alterable selectivity and high sensitivity via tuning the ratio of Cd/Ga.Results and ConclusionsIn order to demonstrate the sensing capability of the 3D IO CdGa2O4, CdO-CdGa2O4 MS based gas sensor, we measure their gas sensing response to 100 ppm benzene, toluene, acetone, methanol, formaldehyde, and ethanol, as well as 1 ppm NH3, CH3, H2S, CO, SO2, and NO2 at the range of 175-275 °C, respectively. As shown in Figure 1(A)-(C), the CdGa2O4 based gas sensor exhibits the highest response to 100 ppm formaldehyde at 225 °C, however, after adding of CdO element, the CdO-CdGa2O4 based gas sensor shows lower sensitivity to formaldehyde than primary CdGa2O4 at 225 °C, and possesses higher sensitivity to low concentration of NO2 at 175 °C.Fig.1 (A-C) Gas responses of the 3D IO CdGa2O4, CdO-CdGa2O4 MS based gas sensor to various gases at 175-275 °C, respectively.The UV-vis absorption spectra of the CdGa2O4, CdO-CdGa2O4, and Cd-abundant CdO-CdGa2O4 samples show continuous red shifts of absorption edges with increasing CdO element content (Figure 2(A)), suggesting an decrease in band gap. In addition, the band gaps of all above samples can be calculated from the transformation of UV-vis diffuse reflectance spectra by utilizing the Tauc plot equation (Figure 2(B)). The calculated results further show that the band gaps tend to be narrower values compared with incorporation of CdO (from 4.16 to 2.03 eV). Thus, the baseline resistances in air of Cd-abundant CdO-CdGa2O4 sensor is the lowest (Figure 2(C)). Accordingly, this study indicates that the gas-sensing mechanism of semiconductor metal oxide chemiresistor should consider electron energy level structure. Meantime, we will further study this relationship in subsequent experiments based on above analysis.Fig. 2 (A) UV/visible diffuse reflection spectra and (B) corresponding band gaps of CdGa2O4, CdO-CdGa2O4, and Cd-abundant CdO-CdGa2O4 samples; (C) Resistance in air of gas sensors based on CdGa2O4, CdO-CdGa2O4, and Cd-abundant CdO-CdGa2O4 at 200 °C.

Machine Learning for the Quantification and Identification of Natural Gas from Mixed Potential Electrochemical Sensors

Methane emissions from natural gas pipeline infrastructure contribute to over 2 billion dollars in losses in the US and 1.6 x 108 metric tons of CO2-equivalent per year.[1-2] Widely deployable sensors are needed to quickly identify and repair leaks so that their environmental and economic impact can be minimized. Mixed potential electrochemical sensors (MPES) are a low cost, highly robust system which can be deployed in the field. Mixture identification and quantification using sensor signals produced by MPES devices is challenging due to cross interference effects and complicated by the presence of interferent methane sources including wetlands and agricultural resources.We have developed a machine learning system with artificial neural networks using MPES devices as inputs and either concentration or mixture label as outputs. We have previously demonstrated using this approach to automatically decode sensor signals associated with automotive emissions gas mixtures [3-4]. Artificial neural networks created using the Google Tensorflow library for the Python programming language were trained on MPES sensor signals to identify and quantify mixtures as natural gas, CH4 only, and a CH4+NH3 mix as a simulant for bovine emissions. The results we have collected showed >99% test accuracy in identification of these gas mixtures as shown in the confusion matrix in Figure 1 in the concentration range of 1000-4000 PPM CH4 and 75-255 PPM NH3. Optimization of the ANN architecture to minimize processing time and facilitate deployment of machine learning onto portable hardware will also be presented. We will also compare the performance of ANNs to alternate ML techniques.This work was supported by the US Department of Energy under award DE-FOA-0031864.[1] A.J. Marchese, T. L. Vaugh, D. J. Zimmerle, D. M. Martinez, L. L. Williams, A. L. Robinson, A. L. Mitchell, R. Subramanian, D. S. Tkacik, J. R. Roscioli, S. C. Herndon. Science. 7204 (2018).[2] Kort, E. A.; Frankenberg, C. Costigan, K.R.; Lindenmaier, R.; Dubey, M. K; Wunch, D. Geophys. Res. Lett, 2014, 41, 19, 6898[3] L. Tsui, A. Benavidez, P. Palanisamy, L. Evans, F. Garzon, Electrochim. Acta 283 (2018) 141–148.[4] L. Tsui, A. Benavidez, P. Palanisamy, L. Evans, F. Garzon, Sens. Act. B: Chem. 249 (2017), 673-684Figure 1. Confusion matrix for (a) natural gas simulant, (b) CH4, (c) NH3, and (d) CH4+NH3. Mixtures (b) and (d) serve as surrogates for wetlands and agricultural bovine emissions respectively. Figure 1

The Room-Temperature Planar Type Gas Sensor Based on DPA-Ph-Dbpzdcn/TPA-Dcpp for NH3 Monitoring Application

IntroductionAmmonia (NH3) gas sensor is the indispensable element in safety, air quality monitoring and respiratory analysis fields. The chemoresistor sensors are mainly focused in the investigation of NH3 sensors attributing to the high stability, simple sensing structure, low cost and portable device. However, solving the high working temperature issue and achieving reproducible and stable gas sensing properties by designing novel sensitive material is highly desirable for widespread development of ammonia sensor [1]. Here, the planar room-temperature NH3 sensors are fabricated through spin-coating DPA-Ph-DBPzDCN/TPA-DCPP organic semiconductor materials onto the Al2O3 substrate with Au interdigital electrodes. The two sensors exhibit favorable sensing response as 72.73%/76.4% to 100 ppm NH3 under 98% RH, respectively, also demonstrates the low detection limits, good reproducibility, selectivity and long-term stability. The DFT calculation results further demonstrate that the excellent NH3 sensing characteristics are attributed to the enhanced absorption between NH3 and the N groups of organic molecules, which formed the H2O pre-adsorption under the high RH. This work is expected it can offer some guidance and inspiration for the rational design of sensing materials and further development of room temperature gas sensor. Results and Conclusions The synthesis process of DPA-Ph-DBPzDCN/TPA-DCPP were detailed reported previously [2, 3]. The sensor devices were successful fabricated via spin-coated sensitive materials onto the Al2O3 substrate with Au interdigital electrodes and illustrated in Figure 1. It was illustrated from scanning electron microscopy (SEM) image that the organic sensing films were formed uniformly and tightly. Then, the response transients of sensors in different NH3 concentration under 98% and 25% RH were monitored (Figure 2). The response value improved alone with the increasing of NH3 concentration and performed the highest response value as 72.73%/76.4% to 100 ppm NH3 under the 98% RH, respectively. Furthermore, there had a decreasing trend of the lowest detectable NH3 concentration along with the increasing RH, especially, the TPA-DCPP sensor exhibited the lowest detection limit as 500 ppb under 98% RH.Accordingly, the density functional theory (DFT) calculations were performed to gain further insight into the interaction between gas molecules and the sensitive materials, also the enhanced ammonia sensing properties under the high RH. The three most likely ammonia adsorbing sites (A: trihpenylamine-ortho C on the benzene ring; B: pyrazine-N; C: cyano-N) were selected due to the negative charge was mainly located. The configuration and the charge density difference after NH3 adsorption (Figure 3) demonstrated that the charge transfer occurred through the NH3 given electrons to organic compounds, thereby the conductivity of organic semiconductors increased until to an equilibrium state. Simultaneously, the NH3, H2O, NH3-H2O adsorption energies of the corresponding adsorbing sites were given in Table 1. It was shown that the NH3 adsorbing to organic compounds after H2O pre-adsorption were more capable compared with the NH3 adsorption only. Meanwhile, the NH3 adsorption energy before and after H2O pre-adsorption of the pyrazine-N on TPA-DCPP (-0.1809 eV, -0.4011 eV) was obviously higher than the DPA-Ph-DBPZdCN molecule (-0.14362 eV, -0.15679 eV), resulting in the response value of TPA-DCPP sensor was apparently improved compared to the DPA-Ph-DBPZdCN sensor under the low/high RH, that were attributed to the increasing of the negative charge on pyrazine-N owing to the directly link between and the pyrazine the cyano group, the change of molecular structure leading to a significantly stronger coulombic effect between pyrazine-N with NH3. Taken together, the excellent NH3 sensing characteristics were attributed to the enhanced absorption between NH3 and H2O pre-adsorption organic molecules under the high RH [4]. The DPA-Ph-DBPZdCN/TPA-DCPP could be the promising NH3 organic sensitive materials for the room temperature sensor, meanwhile, the discussion of sensing mechanism bring the further understanding of the organic gas sensors, providing the new concept of organic materials which has potential application value in gas monitoring of production and daily life at room temperature.

Two Dimensional Mxene (Ti3C2) Decorated By SnO2 Nanocrystals for Enhanced Chemical Gas Sensors at Room Temperature

IntroductionA low-cost chemical sensor for ammonia (NH3) detection is highly demanded nowadays for several important applications. Two-dimensional (2D) nanomaterials such as graphene/reduced graphene oxide (rGO), transitional metal dicharcogenides, black phosphor and transition metal carbonitride (MXene) have found a promising application in the fields of gas sensing electronics at room temperature due to their unique microstructure, electrical properties and excellent gas adsorptions ability. Decoration of the secondary phase such as noble metals and metal oxide nanocrystals to form heterojunctions with two-dimensional materials can effectively improve the inadequacy of two-dimensional materials based gas sensors. It takes advantages of the difference in the Fermi level of each phase to make the electrons transfer from one material to the other and thus enriching the electron concentration on the surface of one material that is dominant in the electrical transport. In this way, more ionized oxygen is chemisorbed, leading to the improvement of the gas sensing properties [1].In this work, 2D MXene heterojunctions incorporated with SnO2 nanoparticles have been synthesized by using hydrothermal method. The as-fabricated MXene/SnO2 heterojunction based chemiresistive-type sensor showed excellent sensitivity to different concentrations of ammonia from 0.5-100 ppm at room temperature. The response has about 20 times higher than those reported in literature to 100 ppm NH3 and an excellent selectivity.Preparation and characterizations The Ti3AlC2 was purchased directly from Beijing Forsman Corp. The concentrated hydrochloric acid (HCl) was diluted to 9 M. 2 g of lithium fluoride (LiF) was added into 20 ml diluted hydrochloric acid and stirred with magnetic mixer. 2 g Ti3AlC2 was slowly and evenly dispersed in the solution over a period of several minutes and subsequently stirred at 35 ℃ for 48 h to obtain the MXene. 70 mg of MXene powder was dispersed in 30 ml of deionized water after grinding and sonicated for 30 min. 73.5 mg of stannic chloride pentahydrate (SnCl4·5H2O) was added into the suspension and stirred with magnetic stirrer at room temperature. The suspension was then loaded in a 50 ml Teflon-lined autoclave. The autoclave was sealed and heated in an KSL-1200X hydrothermal system at 180 ℃ for 12 h to obtain the MXene-SnO2 hybirds. The samples were then characterized by XRD, SEM, XPS, Raman and TEM-ED. The MXene/SnO2 based chemirestive-type sensor was measured by a static volumetric method. The resistance of the sensor is directly measured by a digital multimeter (Agilent 34410A). The responses of the sensors were defined as the relative change in the resistances of the sensors in the air and those in the tested gas. Results and Conclusions Fig.1a shows the XRD of the prepared samples. Appearance and down-shift of the peak at ~10 oC indicates the successful achievements of MXene and thus the MXene-SnO2 composite heterojunctions [2-3].Fig.1 (b-c) show the micrographs of the surface morphology of the MXene and heterojunctions. The etched MXene material has obvious layered structure with sizes ranging from 2 μm to 4 μm. The thickness of MXene is about 75 nm indicating the sample contains about 100 single layers of MXene. The granular SnO2 nanoparticles are intimately attached to the surface of MXene. Fig.2 (b) shows the Raman spectra of MXene and Mxene/SnO2 heterojunction. The appearance of both D-band and G-band in the MXene/SnO2 heterojunction sample indicated that MXene remained well during the hydrothermal reaction. Fig.2(c) shows the Ti core level (2p) spectra of the MXene and MXene/SnO2. Ti - X corresponds to titanium carbide or titanium oxynitride. Fig.2d indicates that the peaks at 530.1 and 532 eV correspond to oxygen vacancies and O=C-OH, respectively, suggesting that these and surface-adsorbed oxygen tend to form more active sites, thereby enhancing sensing capability.The sensors using MXene/SnO2 heterojuction shows excellent response and selectivity to low-level NH3 from 0.5-9 ppm at RT (Fig.3). Since the work function of MXene (3.4 eV) was slightly lower than that of SnO2 (4.7 eV), Schottky-type junctions were formed across MXene/SnO2 interfaces (Fig.4). Electrons would transfer from SnO2 to MXene, and the depletion layer was deepened on the surface of MXene. MXene could then chemically adsorb more oxygen, and the response of the MXene/SnO2 heterojunction was thus significantly enhanced.

YSZ-Based Mixed Potential Type NH3 Sensor Attached with Ag Doped FeVO4 Sensing Electrode

Introduction In an automobile selective catalytic reduction (SCR) system, the monitoring of NH3 concentration is of vital importance. NH3 detection in such harsh environment of high temperature and humidity is an almost impossible task for most testing methods. Nevertheless, YSZ-based mixed potential type gas sensor is an incomparable choice. Moreover, the development and utilization of sensing electrode materials is an effective method to develop high performance YSZ-based mixed potential type ammonia gas sensor [1]. FeVO4, a n-type semiconductor with direct band gap, is considered as a promising sensing electrode material due to its special three-dimensional layered structure [2]. Ag was reported to have excellent catalytic activity. Through Ag collaborating with the sensing electrode, the response of the NH3 sensor could be enhanced accompanied by the improved sensitivity and detection limit [3]. In this study, we prepared FeVO4 through the sol-gel method. Then, Ag doped FeVO4 in different molar ratio (1.5, 2.5, 3.5 and 4.5 mol.%) was synthesized through the sodium borohydride reduce method and used as sensing electrode material of the YSZ-based mixed potential type NH3 gas sensor. The effects of different molar ratios Ag doped on the NH3 sensing performance were studied. The sensor attached with 3.5 mol.% Ag doped FeVO4 displays high response to target NH3 gas, short response and recovery time, satisfying humidity resistance and good repeatability, indicating its potential for in-situ ammonia monitoring for automotive applications [4]. Method Fabrication and measurement of the gas sensor The sensor was fabricated on a YSZ plate (8 mol. % Y2O3 doped, 2 mm × 2mmsquare, 0.2 mm thickness), provided by Anpeisheng Corp. China. A point-shaped electrode and a narrow stripe-shaped electrode were formed on the two sides using a commercial Pt paste (Sino-platinum Metals Co. Ltd.), and sintered at 1000°C. The sensing material was mixed with a minimum quantity of deionized water and the resultant paste was applied on the point-shaped Pt to form stripe-shaped SE and then sintered at 800 °C for 2 h. The testing method of the sensor was as showed in our previous work [5]. Results and Conclusions A series of sensing tests showed the sensor attached with 3.5 mol.% Ag doped FeVO4-SE had the highest response to NH3, as Fig.1a shows. The reason that the sensor attached with 3.5 mol.% Ag doped FeVO4-SE have the highest sensing performance is due to best electrochemical catalytic activity to NH3 of the sensing electrode materials. The best electrochemical catalytic activity of the material will be proved by the polarization curves and complex impedance test in the future. The NH3 sensing performance of the sensor attached with 3.5 mol.% Ag doped FeVO4 was studied at different operating temperatures and the optimal operating temperature was determined at 550 °C, as Fig.1b shows. The sensor could even detect as low as 200 ppb NH3 with the response of -3.2 mV, as shown in Fig.1 c. Also, the sensor had a good reproducibility as shown in Fig.1 d. Moreover, the sensor displayed good selectivity to NH3 when compared to other interfering gases such as NO2, SO2, C2H2 and CO, as shown in Fig.1 e. For accurate and rapid detection of NH3, the response and recovery time is a vital indicator. In the present research, the typical 90% response time to 50 ppm NH3 of the sensor was less than 3 s as shown in Fig.1 f. In the actual working condition, the exhaust gas is always of high humidity. Therefore, the environmental humidity’s influence on the NH3 sensor was studied and the result in F ig.1g and h showed that the sensor had an acceptable humidity stability. Based on the above results, the sensor we fabricated is prospective in automobile selective catalytic reduction (SCR) exhaust gas treatment system.

Variation of Sputtered WO3 Film Thickness in Ag (NPs)/WO3/Au (NPs) System for Optimizing Sensing Behaviors to NH3

Introduction Ammonia (NH3) is one of the major air pollutants. It can cause serious damage to the eyes, skin, and respiratory system when its concentration exceeds 300 ppm [1]. Therefore, it is necessary to develop an efficient NH3 gas sensor to monitor its concentrations in the environment and varies industrial sectors. So far, several nanostructured materials have been investigated to detect NH3 gas, such as ZnO, SnO2, and WO3 [2]. However, these materials usually suffer from a lack of sensitivity, especially at low operating temperatures. Several strategies have been applied in the literature to improve the NH3 sensing performance, including surface treatment, doping with other materials, and forming a heterojunction. Furthermore, it is generally accepted that the decoration of the thin films will noble metals such as Au, Pt, or Pd will improve the sensor response owing to the increased depletion layer thickness of gas sensing materials and additional electrical resistance changes due to the built-in barrier high formed at noble metal nanoparticles and sensing material junctions. In our previous studies, we developed a method to decorate oxide thin-film surfaces with well-dispersed and uniform noble metal nanoparticles and tested their gas sensing performance toward different gases such as [3-4]. Additionally, the flow of charge carriers between the metal oxide and the attached nanoparticles was found to depend on the work function difference between them. For instance, silver (work function = 4.33 eV), or gold (work function = 5.1 eV) decorated metal oxide has a different impact on the gas sensing properties. The flow of electrons between metal oxides such as WO3 and the attached silver, or gold nanoparticles has two opposite directions: one from metal oxide to the Au and the second one from Ag to the metal oxide owing to the work function difference between Ag/ metal oxide, and Au/metal oxide. This work intends to fabricate double side noble metals activated sputtered WO3 thin film and investigate the effect of the WO3 thickness on the NH3 gas sensing properties. Method Ag (NPs)/WO3/Au (NPs) films were fabricated by DC sputtering followed by post annealing treatment in nitrogen atmosphere. The deposition of the Ag (NPs)/WO3/Au (NPs) films was carried out into three steps. First, Ag thin film was deposited for a period of 30 s by applying 20W. Afterward, WO3 films having a different thickness ranging from 5nm to 150 nm were deposited via DC reactive sputtering at 100 W in oxygen. In the third step, Au was sputtered using DC sputtering at 25 W for 30 s. Subsequently, the samples were heated at 600 ⁰C in a nitrogen atmosphere for 3 h to convert the Ag, and Au layer to nanoparticles. Different analytical techniques were used to study the physical and the chemical properties of the fabricated films. Results and Conclusions Figure 1(a) shows the FESEM image of the Ag (NPs)/WO3/Au (NPs) film. As can be observed, two different size distributions of nanoparticles can be seen which could be ascribed to the formation of Ag and Au nanoparticles. The formation of WO3 film, Ag and Au nanoparticles was also confirmed using FESEM-EDX mapping (not shown). Gas sensing results showed Ag (NPs)/WO3/Au (NPs) sensor with a thickness of 30 nm demonstrated the highest gas response to NH3. Fig. 1(b) shows the dynamic response-recovery curves of the Ag (NPs)/WO3/Au (NPs) sensor for different concentrations of NH3. It can be noticed that all the sensing responses increased with the increase of NH3 concentration. The repeatability of the fabricated sensor was implemented up to five cycles in 100 ppm. As can be observed, the developed sensor displayed high repeatability under the tested conditions. Figure 1(c) shows the five-cyclic response of the fabricated sensor towards 100 ppm NH3 at 250 ⁰C. As can be noticed, the sensor capable of complete recovery upon removal of NH3. The deviation in the sensing response was less than 2% thus ensuring excellent repeatability. The long-term stability of a sensor is of great importance for almost all practical applications. To investigate the long-term stability of the sensor, the gas sensitivity was recorded at 100 ppm NH3 for over 20 days and the obtained sensing results plotted in Figure 1 (d). The response of the fabricated sensor decreased by about 5% in comparison to its original characteristic, confirming its excellent stability.

Sensitivity and selectivity enhancement of the YSZ-based mixed-potential ammonia sensors with flame-spray-made double-sensing electrodes

YSZ-based mixed-potential sensors with double-sensing electrodes were fabricated for ammonia (NH3) detection. ZnO and LaCoO3 nanoparticles prepared by flame spray pyrolysis (FSP) method were used as the sensing materials. The sensing responses of ZnO and LaCoO3 sensing electrodes can be overlapped based on the double-sensing-electrode structure, thereby significantly enhancing the sensitivity compared with the traditional mixed-potential sensors using Pt reference electrode. The sensitivities of this sensor reached up to -102.85, -75.312, -70.165, -48.616 mV/decade to 10–50 ppm NH3 at 450, 475, 500, 525 ℃, respectively. Moreover, the NO2 interference effects on NH3 detection of ZnO and LaCoO3 electrodes were opposite and can be counteracted based on the double-sensing-electrode structure, eliminating most of the cross-sensitivity to NO2. The double-sensing-electrode sensor exhibited good performance under CO, CH4, NO, NO2 interference, oxygen fluctuation, and humidity variations, and showed good long-term stability. The results of sensing performance exhibited the sensors with double-sensing electrodes have a great potential for application on SCR system of diesel engines. In further studies, each sensing electrode can be optimized respectively and the sensitivity and selectivity of NH3 sensors could be enhanced to a higher level.

High temperature polymer electrolyte membrane fuel cell degradation provoked by ammonia as ambient air contaminant

High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are used from stationary to mobile applications and have the advantage of increased tolerances against fuel impurities like H2S and CO. However, air impurities can limit their performance and durability. Here, the impact of NH3-contaminated air is studied during 500 h of operation. 10 ppm NH3 in air provokes a voltage decay of at least −279.3 μV h−1 compared to −18.1 μV h−1 during operation without NH3 demonstrating strong sensitivity of the HT-PEM technology to this air pollutant. Cyclic voltammetry shows a selectively poisoned catalyst, whereby the loss of electrochemical surface area seems to be of no importance. Impedance spectroscopy reveals affected electrode charge transfer processes and strongly affected proton conductivity. μ-computed tomography illustrates significant membrane thinning being significantly larger compared to the blank reference cell. Ion chromatography further indicates that ammonium is incorporated into the cell, so that ammonia is believed to trap the protons stemming from phosphoric acid and hydrogen oxidation reaction. In conclusion, HT-PEMFC degradation caused by ammonia naming formation and incorporation of ammonium species and formation of nitrogen species interacting with the catalyst are identified.

Extraordinary H2S gas sensing performance of ZnO/rGO external and internal heterojunctions

External and internal heterojunctions of reduced graphene oxide (rGO) and ZnO nanofibers (NFs) are developed for ultrasensitive H2S sensing. The gas response of both heterojunctions is significantly higher than that of bare ZnO NFs. The internal heterojunction of 0.1 wt% rGO/ZnO NFs exhibited the highest gas response, reaching 1353–1 ppm H2S at 350 °C and increasing up to 25-fold, compared to bare ZnO NFs. Whereas response of the former to 1000 ppm H2, 1000 ppm NH3, and 1 ppm SO2 gases is negligible (1.5–9). Moreover, the present sensor exhibits an ultralow detection limit of 4.9 ppt and excellent stability towards H2S. A novel sulfuration-desulfuration sensing mechanism is proposed to explain the high response and selectivity to H2S.

Ultrathin Nb2CTx nanosheets-supported polyaniline nanocomposite: Enabling ultrasensitive NH3 detection

A novel NH3 sensor enabled by two dimensional (2D) niobium carbide MXene (Nb2CTx) nanosheets-supported polyaniline (PANI) nanocomposite (PANI/Nb2CTx) with enriched three dimensional (3D) spatial structure was successfully developed to achieve ultrasensitive NH3 detection under humidity atmosphere. The 2D ultrathin Nb2CTx nanosheets with abundant hydrophilic groups exhibited excellent regulation effect on the polymerization of PANI nanofibers and highly enhanced the morphology of PANI/Nb2CTx nanocomposite. The surface elemental analysis of Nb2CTx nanosheets was characterized through XPS, and the morphology images, crystal structure and chemical composition of the as-fabricated samples were investigated by FE-SEM, TEM, XRD and FTIR. According to the test results, the as-developed PANI/Nb2CTx sensor exhibits superior selectivity, high sensitivity, low detection limit (20 ppb) and good long-term reliability toward 0.1−10 ppm NH3 with 87.1 % RH. Owing to numerous hydrogen bonds between PANI and Nb2CTx, humidity affects on the sensing properties of PANI/Nb2CTx sensor was effectively reduced. Finally, the gas-sensing mechanism has been systematically studied to illustrate the improved sensing properties combining p-n junction with hydrogen bonds. This research provides a novel PANI/Nb2CTx NH3 sensor with excellent gas sensing properties, which can open up a new way to explore the application potential of Nb2CTx-based nanocomposite in both atmospheric and breath NH3 detection.

An enhanced flexible room temperature ammonia gas sensor based on GP-PANI/PVDF multi-hierarchical nanocomposite film

Resistive-type polyaniline (PANI) based sensing film was fabricated by a simple in-situ polymerization method with graphene (GP) as additives on flexible porous polyvinylidene fluoride (PVDF) substrate. The fabricated flexible GP-PANI/PVDF membrane demonstrates a multi-hierarchical porous microstructure, enhancing its sensing performance towards ammonia (NH3). The optimized GP-PANI/PVDF sensor shows 60% response towards 1 ppm NH3 with a response time of 46 s, and 10% response towards 0.1 ppm NH3 at 24℃, respectively. Moreover, the GP-PANI/PVDF sensor presents excellent flexibility, exhibiting only small response values’ difference upon several bending angles and after 1500 bending/extending cycles. Besides, the GP-PANI/PVDF sensor exhibits a strong linear relationship between the NH3 response values and working temperatures with a range of 24℃∼50℃. The results demonstrate that the flexible GP-PANI/PVDF film sensor holds great promise for the application of portable, lightweight, and room temperature sub-ppm level NH3 detection devices.

Humidity-activated ammonia sensor with excellent selectivity for exhaled breath analysis

Abstract High ammonia (NH3) concentration in the exhaled breath can be used as a crucial biomarker for end-stage of renal disease. Detecting exhaled ammonia via gas sensors remains a huge challenge, because of the complex components and high humidity environment of exhaled gases. Herein, a creative design for NH3 sensing is realized using a humidity-activated mechanism at room temperature. The investigation reveals that the carboxylate formation reaction specific recognition of NH3 can be activated under high humid environment. An environmentally friendly and non-toxic biomass hydrogel NH3 sensor was developed, with poly- l -aspartic acid (PAA) and l -glutamic acid (GA) as the sensing material. The response to 50 ppm NH3 can reach 9.2 under 80 % RH at room temperature. The humidity-activated sensing mechanism is proposed as a complex process involving the acid-base adsorption, the formation of ionic conduction and the reaction between carboxylic acid groups and NH3. It is believed that the PAA/GA sensor presents a new pathway for highly selective detection of NH3 under high humidity condition, which is a promising method for analyzing the exhaled ammonia.

Comparative study on the gas-sensing performance of ZnO/SnO2 external and ZnO–SnO2 internal heterojunctions for ppb H2S and NO2 gases detection

• ZnO-SnO 2 composite NFs and ZnO/SnO 2 hetero-NFs were synthesized by electrospinning. • Gas sensing characteristics of fabricated NFs were studied to H 2 S and NO 2 gases. • Internal-junction and external-junction can result in a high response of NF sensors. • The high response of NF sensors was governed by internal-junction. The design of hetero-nanojunctions can greatly amplify the gas-sensing performance of conventional metal oxide gas sensors. In this study, the composite nanofibers (NFs) of ZnO–SnO 2 internal heterojunctions (HJs) and the mixed NFs of ZnO/SnO 2 external HJs were realized by using the electrospinning technique. Then, the gas-sensing characteristics of the as-synthesized NFs toward H 2 S and NO 2 gases were systematically investigated and compared. The effects of the internal and external nanojunctions of the hetero-NF-based gas sensors were also explored. Results showed that the ZnO–SnO 2 composite NFs demonstrate a higher response than those of the ZnO/SnO 2 mixed NFs, as well as bare ZnO and SnO 2 NFs. Also, the cross responses of the hetero-NF sensors toward 200 ppm CO, 250 ppm H 2 , and 250 ppm NH 3 were examined. The gas-sensing mechanism of the internal and external HJs of composite ZnO/SnO 2 and mixed ZnO/SnO 2 NFs was analyzed. The remarkable enhanced gas-sensing performance of hetero-NFs was mainly attributed to the formation of internal HJs.

Mixed potential type NH3 sensor based on YSZ solid electrolyte and metal oxides (NiO, SnO2, WO3) modified FeVO4 sensing electrodes

YSZ-based mixed potential type NH3 sensor has a good application foreground in on-board SCR (Selective Catalytic Reduction) system due to its good thermal and chemical stability. In this study, we greatly improved the NH3 sensing performance of this type of sensor to meet the detection requirement through the development of high-performance sensing electrode materials. FeVO4 was synthesized through a traditional sol-gel method and used as basis material. In order to improve the electrochemical catalytic activity, different molar ratio of metal oxides (NiO, SnO2, WO3) modified FeVO4 were prepared. The cyclic voltammetry (CV), the linear sweep voltammetry (LSV) and dc polarization (I–V) measurements were carried out to confirm the electrochemical catalytic activity of the sensing electrode materials. Eventually, the sensor attached with 20 mol.% NiO modified FeVO4-SE exhibited the highest response (−83 mV) to 100 ppm NH3 and had a low detection limit of 5 ppm. What is more, the sensor also displayed short response and recovery time, satisfying oxygen resistance and good stability over 20 days at 650 °C, indicating its potential for in-situ ammonia monitoring in industrial and automotive applications.