Delafossite CuGaO2-Based Chemiresistive Sensor for Sensitive and Selective Detection of Dimethyl Disulfide.

Dimethyl disulfide (DMDS) is a common odor pollutant with an extremely low olfactory threshold. Highly sensitive and selective detection of DMDS in ambient humid air background, by metal oxide semiconductor (MOS) sensors, is highly desirable to address the increased public concern for health risk. However, it has still been a critical challenge up to now. Herein, p-type delafossite CuGaO2 has been proposed as a promising DMDS sensing material owing to its striking hydrophobicity (revealed by water contact angle measurement) and excellent partial catalytic oxidation properties (indicated by mass spectroscopy). The present CuGaO2 sensor shows a selective DMDS response, with satisfied humidity resistance performance and long-term stability at a relatively low operation temperature of 140 °C. An ultrahigh response of 100 to 10 ppm DMDS and a low limit of detection of 3.3 ppb could be achieved via a pulsed temperature modulation strategy. A smart sensing system based on a CuGaO2 sensor has been developed, which could precisely monitor DMDS vapor in ambient humid air, even with the presence of multiple interfering gases, demonstrating the practical application capability of MOS sensors for environmental odor monitoring.

Prompt Electronic Discrimination of Gas Molecules by Self-Heating Temperature Modulation.

Though considerable progress has been achieved on gas molecule recognition by electronic nose (e-nose) comprised of nonselective (metal oxide) semiconductor chemiresistors, extracting adequate molecular features within short time (<1 s) remains a big obstacle, which hinders the emerging e-nose applications in lethal or explosive gas warning. Herein, by virtue of the ultrafast (∼20 µs) thermal relaxation time of self-heated WO3-based chemiresistors fabricated via oblique angle deposition, instead of external heating, self-heating temperature modulation has been proposed to generate sufficient electrical response features. Accurate discrimination of 12 gases (including 3 xylene isomers with the same function group and molecular weight) has been readily achieved within 0.5-1 s, which is one order faster than the state-of-the-art e-noses. A smart wireless e-nose, capable of instantaneously discriminating target gas in ambient air background, has been developed, paving the way for the practical applications of e-nose in the area of homeland security and public health.

Decoupling the Conflicting Roles of Photoactivation and Boosting Chemiresistor Response by Pulsed UV Light Modulation.

UV photoactivation has been widely employed to trigger the response of semiconductor chemiresistors at room temperature (RT). Generally, continuous UV (CU) irradiation is applied, and an apparent maximal response could be obtained via optimizing UV intensity. However, owing to the conflicting roles of (UV) photoactivation in the gas response process, we do not think the potential of photoactivation has been fully explored. Herein, a pulsed UV light modulation (PULM) photoactivation protocol has been proposed. Pulsed UV-on facilitates the generation of surface reactive oxygen species and refreshes the surface of chemiresistors, while pulsed UV-off avoids the side effects of UV-induced desorption of the target gas and the decline of base resistance. PULM enables decoupling those conflicting roles of CU photoactivation, resulting in a drastic boost of response to trace (20 ppb) NO2 from 1.9 (CU) to 131.1 (PULM UV-off), and a decline of limit of detection from 2.6 ppb (CU) to 0.8 ppb (PULM) for a ZnO chemiresistor. This work highlights that PULM allows full exploitation of the potential of nanomaterials for sensitively detecting trace (ppb-level) toxic gas molecules and opens a new opportunity for designing highly sensitive, low-power consumed RT chemiresistors for ambient air quality monitoring.

Bifunctional role of PDMS membrane in designing humidity-tolerant H2S chemiresistors with high selectivity.

A thermally evaporated hydrophobic PDMS membrane could significantly mitigate humidity interference/poisoning (without a decline in response at 50% RH for nearly 3 months) and enhance the selectivity of a CuCrO2 chemiresistor toward erosive H2S, offering an avenue for the practical applications of (H2S) chemiresistors in an ambient humid air atmosphere.

Voltage driven chemiresistor with ultralow power consumption based on self-heating bridged WO3 nanowires.

Metal oxide semiconductor (MOS)-based chemiresistors have been widely used for detecting harmful gases in many industrial and indoor/outdoor applications, which possess the advantages of small size, low cost, integratability, and ease of use. However, power consumption has become a critical parameter for practical applications. Several methods have been explored to reduce power consumption including reducing the operation temperature, use of micro-electro-mechanical systems (MEMS), and self-heating working mode. Among them, the self-heating working mode has attracted significant attention. Herein, a facile approach of modulating bridged NW chemiresistor by Joule heating effect is proposed to combine both the superiority of single crystal nanowire (NW) carrier channels and power consumption optimization of the self-heating mode. The WO3-bridged NW chemiresistors and WO3 film NW chemiresistors are both constructed to investigate gas responses and power consumption. Substantially magnified electrical responses (Rg/Ra) of WO3 NW chemiresistor toward NO2 is demonstrated by constructing a bridged structure. Under the optimal external heating condition, the responses of chemiresistors toward 5 ppm NO2 can be boosted from 369.7 (film NW) to 1089.7 (bridged NW). The responses to 5 ppm NO2 under the self-heating mode also can be boosted from 13.6 (film NW) to 24.6 (bridged NW) with a drastically declined power consumption. Self-heating bridged NWs allows for localizing the Joule heat within the nanojunction, and thus substantially lowers the power consumption to 0.13 µW (300 °C). This provides an additional opportunity for reducing power consumption of oxide chemiresistors for air quality monitoring in future.

Boosting Electrical Response toward Trace Volatile Organic Compounds Molecules via Pulsed Temperature Modulation of Pt Anchored WO3 Chemiresistor.

Insufficient limit of detection (LoD) toward volatile organic compounds (VOCs) hinders the promising applications of metal oxide chemiresistors in emerging air quality monitoring and/or breath analysis. There is an inherent limitation of widely adopted strategies of creating sensitive chemiresistors then operating at the optimized temperature via a continuous heating (CH) mode. Herein, a strategy combining Pt single atoms anchoring (chemical sensitization) with pulsed temperature modulation (PTM, physical sensitization) is proposed. Apart from generating abundant surface asymmetric oxygen vacancy (Pt-VO -W) active sites at pulsed high temperature (HT) stage, inward diffusion of trace target VOCs across the sensing layer at pulsed low temperature stage (driven by PTM induced concentration gradient), can greatly enhance the charge interaction probability between the generated surface active species and the surrounding VOCs, and thus offers a novel avenue on addressing the bottleneck issue of low LoD by PTM. Triggered by HT of 300 °C, the responses of Pt anchored WO3 chemiresistor to 1 ppm trimethylamine (TMA) and xylene can be drastically boosted from 1.9 (CH) to 6541.5 (PTM) and 1.5 (CH) to 1001.1 (PTM), respectively. And ultra-low theoretic LoD of 0.78 ppt (TMA) and 0.18 ppt (xylene) are successfully achieved, respectively.

Pt-Anchored CuCrO2 for Low-Temperature-Operating High-Performance H2S Chemiresistors.

Recent advances in heterogeneous catalysts indicate that single atoms (SAs), anchored/stabilized on metal oxide nanostructures, exhibit not only high catalyst atom efficiency but also intriguing reactivity and selectivity. Herein, isolated Pt SA-anchored CuCrO2 (CCO) has been designed by a glycine-nitrate solution combustion synthesis (SCS) route. The density of isolated Pt SAs achieves the highest value of ∼100 µm-2 for the 1.39 wt % Pt-anchored CCO sample, which results in the drastically boosted H2S response characteristics, including a high response of 1250 (35 times higher than that of pure CCO) at 10 ppm H2S and a low operating temperature of 100 °C. Except for CH4S, the responses of a 1.39 wt % Pt-anchored CCO chemiresistor to diverse vapors with concentrations of 50-100 ppm are less than 2, exhibiting excellent selectivity. Various ex situ characterizations indicate that the spillover catalytic effect of Pt SA sites, other than the conventional sulfuration-desulfuration mechanism, plays a dominant role in the outstanding H2S response characteristics.

Unveiling low-temperature thermal oxidation growth of W18O49 nanowires with metastable ß-W films.

With features of innate tunnels and oxygen vacancies derived from a unique geometrical structure and sub-stoichiometric compositions, W18O49 nanowires have been explored as multifunctional materials with diverse applications. Though thermal oxidation offers a facile method to synthesize patterned W18O49 nanowires, the relatively high growth temperature (≳500 °C) hinders their emerging applications in flexible or wearable electronics. In this work, aiming to lower the growth temperature of W18O49 nanowires by thermal oxidation, the temperature and oxygen partial pressure dependent growth has been systematically investigated for both W films and powders. W18O49 nanowires could be steadily obtained with appropriate temperature and oxygen pressure ranges for both cases, while the growth temperature of a W film (metastable ß phase dominant) could be much lower than that of W powder (α phase). The structural analysis indicates that metastable ß-W is susceptible to oxidation in comparison with α-W and thus generates oxidation-induced chemical compression for nanowire growth. The growth temperature of W18O49 nanowires could be reduced to ca. 400 °C, which paves the way for the in situ patterning of W18O49 nanowires on indium-tin-oxide (ITO) glass substrates and flexible substrates.

High-Performance Planar-Type Photodetector Based on Hot-Pressed CsPbBr3 Wafer.

Considering the disadvantages of the common methods for CsPbBr3 single crystal growth including the high cost of the melt method and the low shape controllability of the solution method, a facile hot-pressed (HP) approach has been introduced to prepare CsPbBr3 wafers. The effects of HP temperature on the phase purity of HP-CsPbBr3 wafers and the performance of the corresponding photodetectors have been investigated. The HP temperature for preparing phase-pure, shape-regular, and dense CsPbBr3 wafers has been optimized to be 150 °C, and the HP-CsPbBr3 wafer based planar-type photodetectors exhibit an ultrasensitive weak light photoresponse. Under the illumination of a 530 nm LED with a light power density of 1.1 µW cm-2, the responsivity, external quantum efficiency, and detectivity of the devices reach 19.79 A W-1, 4634%, and 2.14 × 1013 Jones, respectively, and a fast response speed with a rise time of 40.5 µs and a fall time of 10.0 µs has been achieved.

In Situ Assembly of Ordered Hierarchical CuO Microhemisphere Nanowire Arrays for High-Performance Bifunctional Sensing Applications.

Seeking a facile approach to directly assemble bridged metal oxide nanowires on substrates with predefined electrodes without the need for complex postsynthesis alignment and/or device procedures will bridge the gap between fundamental research and practical applications for diverse biochemical sensing, electronic, optoelectronic, and energy storage devices. Herein, regularly bridged CuO microhemisphere nanowire arrays (RB-MNAs) are rationally designed on indium tin oxide electrodes via thermal oxidation of ordered Cu microhemisphere arrays obtained by solid-state dewetting of patterned Ag/Cu/Ag films. Both the position and spacing of CuO microhemisphere nanowires can be well controlled by as-used shadow mask and the thickness of Cu film, which allows homogeneous manipulation of the bridging of adjacent nanowires grown from neighboring CuO hemispheres, and thus benefits highly sensitive trimethylamine (TMA) sensors and broad band (UV-visible to infrared) photodetectors. The electrical response of 3.62 toward 100 ppm TMA is comparable to that of state-of-the-art CuO-based sensors. Together with the feasibility of in situ assembly of RB-MNAs device arrays via common lithographic technologies, this work promises commercial device applications of CuO nanowires.

Discriminating BTX Molecules by the Nonselective Metal Oxide Sensor-Based Smart Sensing System.

Discriminating structurally similar volatile organic compounds (VOCs) molecules, such as benzene, toluene, and three xylene isomers (BTX), remains a significant challenge, especially, for metal oxide semiconductor (MOS) sensors, in which selectivity is a long-standing challenge. Recent progress indicates that temperature modulation of a single MOS sensor offers a powerful route in extracting the features of adsorbed gas analytes than conventional isothermal operation. Herein, a rectangular heating waveform is applied on NiO-, WO3-, and SnO2-based sensors to gradually activate the specific gas/oxide interfacial redox reaction and generate rich (electrical) features of adsorbed BTX molecules. Upon several signal preprocessing steps, the intrinsic feature of BTX molecules can be extracted by the linear discrimination analysis (LDA) or convolutional neural network (CNN) analysis. The combination of three distinct MOS sensors noticeably benefits the recognition accuracy (with a reduced number of training iterations). Finally, a prototype of a smart BTX recognition system (including sensing electronics, sensors, Wi-Fi module, UI, PC, etc.) based on temperature modulation has been explored, which enables a prompt, accurate, and stable identification of xylene isomers in the ambient air background and raises the hope of innovating the future advanced machine olfactory system.

High Nitrate Accumulation in the Vadose Zone after Land-Use Change from Croplands to Orchards.

Additional evidence indicates that the nitrate stored in the deep soil profile has an important role in regulating the global nitrogen (N) cycle. This study assessed the effects of land-use changes from croplands to intensive orchards (LUCO) on N surplus, nitrate accumulation in deep soil, and groundwater quality in the kiwifruit belt of the northern slope region of the Qinling Mountains, China. LUCO resulted in comparatively high N surplus in orchards (282 vs 1206 kg ha-1 yr-1, respectively). The average nitrate accumulation within the 0-10 m profiles of orchards was 7113 kg N ha-1, which was equal to approximately the total N surplus of 6 years of the orchards. The total nitrate stock within 0-10 m soil profiles of the kiwifruit belt was 266.5 Gg N, which was 3.5 times higher than the total annual N input. The nitrate concentrations of 97% of groundwater samples exceeded the WHO standard. The LUCO resulted in large nitrate storage in the vadose zone and caused serious contamination of groundwater. Our study highlights that nitrate accumulation in the vadose zone of an intensive land-use system is one of the main fates of surplus N and also a hotspot of nitrate accumulation.

Heterostructural (Sr0.6Bi0.305)2Bi2O7/ZnO for novel high-performance H2S sensor operating at low temperature.

Exploring novel sensing materials enabling selective discrimination of trace ambient H2S at lower temperature is of utmost importance for diverse practical applications. Herein, heterostructural (Sr0.6Bi0.305)2Bi2O7/ZnO (SBO/ZnO) nanomaterials were proposed. Synergetic effect of promoting analyte adsorption (via multiplying oxygen vacancy defects) and reversible sulfuration-desulfuration reaction induced unique band alignment among SBO/ZnO/ZnS, contributes to the sensitive and selective response toward H2S molecules. Novel SBO/ZnO (10%) sensor possesses excellent sensing H2S performances, including a high response (107.6 for 10 ppm), low limit of detection of 20 ppb, good selectivity and long-term stability. Together with the merits of low operation temperature of 75 °C and weak humidity dependence (endowed by the hydrophobic SBO), present heterostructural SBO/ZnO sensor paves the way for the practical monitoring of trace H2S pollutants in diverse workplaces including petroleum and natural gas industries.

Generic Approach to Boost the Sensitivity of Metal Oxide Sensors by Decoupling the Surface Charge Exchange and Resistance Reading Process.

As one of the bottleneck parameters for practical applications of metal oxide semiconductor-based gas sensors, sensitivity enhancement has attracted significant attention in the past few decades. In this work, alternative to conventional strategies for designing sensitive surfaces via morphology/defect/heterojunction control (then operating at an optimized isothermal temperature with a maximal response), a facile enhancement approach by decoupling surface charge exchange and resistance reading process (possessing different temperature-dependent behaviors) through pulsed temperature modulation (PTM) is reported. Substantially magnifying electrical responses of a generic metal oxide (e.g., WO3) micro-electromechanical systems sensor toward diverse analyte molecules are demonstrated. Under the optimal PTM condition, the response toward 10 ppm NO2 can be boosted from (isothermal) 99.7 to 842.7, and the response toward 100 ppm acetone is increased from (isothermal) 2.7 to 425, which are comparable to or even better than most of the state-of-the-art WO3-based sensors. In comparison to conventional (isothermal) operation, PTM allows to sequentially manipulate the physisorption/chemisorption of analyte molecules, generation of surface reactive oxygen species, and sensor resistance reading and thus provides additional opportunities in boosting the electrical response of oxide sensors for advanced health and/or environment monitoring in future.

Insight into the Humidity Dependent Pseudo-n-Type Response of p-CuScO2 toward Ammonia.

Metal oxide semiconductor (MOS) gas sensors operated at room temperature (RT) hold great promise for environmental monitoring in the emerging smart society. An in-depth understanding of the gas/MOS interfacial charge exchange under the ambient moist atmosphere is essential for subsequent molecule recognition. Herein, delafossite p-CuScO2 with a unique oxygen intercalation property was utilized. Eight orders of resistance tuning could be rationally achieved via controlling oxygen intercalation realized by air annealing (without altering the size and morphological properties), which allows extraction of both the intrinsic (gas/MOS interaction, p-type response) and extrinsic (gas/H2O/MOS interaction, pseudo-n-type response) sensing mechanisms upon exposure to ammonia molecules. This work could provide an insight into the underlying sensing mechanism of RT sensors working in an ambient moist atmosphere, and the unique n- to p-type switching contains rich molecule related features that could be potentially explored for molecule recognition.

Oxygen Vacancy Defects Boosted High Performance p-Type Delafossite CuCrO2 Gas Sensors.

p-type ternary oxides can be extensively explored as alternative sensing channels to binary oxides with diverse structural and compositional versatilities. Seeking a novel approach to magnify their sensitivities toward gas molecules, e.g., volatile organic compounds (VOCs), will definitely expand their applications in the frontier area of healthcare and air-quality monitoring. In this work, delafossite CuCrO2 (CCO) nanoparticles with different grain sizes have been utilized as p-type ternary oxide sensors. It was found that singly ionized oxygen vacancies (Vo•) defects, compared with the grain size of CCO nanoparticles, play an important role in enhancing the charge exchange at the VOCs molecules/CCO interface. In addition to suppressing the hole concentration of the sensor channel, the unpaired electron trapped in Vo• provides an active site for chemisorptions of environmental oxygen and VOCs molecules. The synergetic effect is responsible for the observed increase of sensitivity. Furthermore, the sensitive (Vo• defect-rich) CCO sensor exhibits good reproducibility and stability under a moderate operation temperature (<325 °C). Our work highlights that Vo• defects, created via either in situ synthesis or postannealing treatment, could be explored to rationally boost the performance of p-type ternary oxide sensors.

Precision excimer laser annealed Ga-doped ZnO electron transport layers for perovskite solar cells.

Organic-inorganic hybrid perovskite solar cells (PSCs) continue to attract considerable attention due to their excellent photovoltaic performance and low cost. In order to realize the fabrication of PSCs on temperature-sensitive substrates, low-temperature processing of all the components in the device is required, however, the majority of the high-performance PSCs rely on the electron transport layers (ETLs) processed at high temperatures. Herein, we apply excimer laser annealing (ELA) to treat ETLs (Ga-doped ZnO, GZO) at room temperature. A synergetic improvement in optical transparency and electrical conductivity is achieved after ELA treatment, which in turn improves light absorption, enhances electron injection, and depresses charge recombination. Devices fabricated with ELA treated GZO ETL acheived a power conversion efficiency (PCE) of 13.68%, higher than that of the PSCs utilizing GZO with conventional high-temperature annealing (12.96%). Thus, ELA is a promising technique for annealing ETLs at room temperature to produce efficient PSCs on both rigid and flexible substrates.

Large eddy simulation and experimental study on vented gasoline-air mixture explosions in a semi-confined obstructed pipe.

In this work, LES simulation coupled with a TFC sub-grid combustion model has been performed in a semi-confined pipe (L/D=10, V=10L) in the presence of four hollow-square obstacles (BR=49.8%) with circular hollow cross-section, in order to study the premixed gasoline-air mixture explosions. The comparisons between simulated results and experimental results have been conducted. It was found that the simulated results were in good agreement with experimental data in terms of flame structures, flame locations and overpressure time histories. Moreover, the interaction between flame propagation process and obstacles, overpressure dynamics were analyzed. In addition, the effects of initial gasoline vapor concentration (lean (ϕ=1.3%), stoichiometric (ϕ=1.7%) and rich (ϕ=2.1%)), and the number of obstacles (from 1 to 4) were also investigated by experiments. Some of the experimental results have been compared with the literature data. It is found that the explosion parameters of gasoline-air mixtures (e.g. the maximum overpressure peaks, average overpressure growth rates, etc.) are different from some other fuels such as hydrogen, methane and LPG, etc.

Fabrication of Hierarchical Anatase TiO2 Nanostructure for Dye-Sensitized Solar Cells.

Hierarchical anatase TiO2 nanostructure was synthesized through a simple one-step hydrothermal method, and used as photoanodes in dye-sensitized solar cells (DSSCs). Different reaction time of 3, 6, 9, and 12 h were adopted to obtain photoanodes with different thickness and morphology. The surface area of the hierarchical structure increased significantly with the increase of reaction time, which can substantially promote the dye adsorption. The maximum energy conversion efficiency of 4.11 ± 0.02% was obtained with the reaction time of 9 h while the thickness of photoanode was only about 2 µm, this could be attributed to the hierarchical nanostructure with synergistic effects of high dye loading and small recombination resistance.

Credible evidence for the passivation effect of remnant PbI2 in CH3NHCH3PbICH3 films in improving the performance of perovskite solar cells.

The role of remnant PbI2 in CH3NH3PbI3 films is still controversial, some investigations have revealed that the remnant PbI2 plays a passivation role, reduces the charge recombination in perovskite solar cells (PSCs), and improves the performance of PSCs, but the opposing views state that remnant PbI2 has no passivation effect and it would deteriorate the stability of the devices. In our investigation, the CH3NH3PbI3 films have been prepared by a two-step spin-coating method and the content of the remnant PbI2 in CH3NH3PbI3 films has been tuned by varying the preparation temperature. It has been found that increasing the heating temperature could increase the coverage of spin-coated PbI2 films, which has led to high coverage CH3NH3PbI3 films and more remnant PbI2 in CH3NH3PbI3 films, and as a result, the performance of PSCs was enhanced obviously and the maximum power conversion efficiency of 14.32 ± 0.28% was achieved by the PSCs prepared at 130/120 °C (PbI2 films were heated at 130 °C and CH3NH3PbI3 films were heated at 120 °C). Furthermore, the dark current, electrochemical impedance spectroscopy and time-resolved fluorescence emission decay measurements revealed that the charge recombination in PSCs has been gradually suppressed and the fluorescence emission lifetime has gradually increased with the content of remnant PbI2 increasing. Thus, the passivation effects of the unreacted and decomposed PbI2 in improving the performance of PSCs have been confirmed unquestionably.