Metal Chelation Assisted In Situ Migration and Functionalization of Catalysts on Peapod‐Like Hollow SnO2 toward a Superior Chemical Sensor

Rational design of nanostructures and efficient catalyst functionalization methods are critical to the realization of highly sensitive gas sensors. In order to solve these issues, two types of strategies are reported, i.e., (i) synthesis of peapod‐like hollow SnO₂ nanostructures (hollow 0D‐1D SnO₂) by using fluid dynamics of liquid Sn metal and (ii) metal–protein chelate driven uniform catalyst functionalization. The hollow 0D‐1D SnO₂ nanostructures have advantages in enhanced gas accessibility and higher surface areas. In addition to structural benefits, protein encapsulated catalytic nanoparticles result in the uniform catalyst functionalization on both hollow SnO₂ spheres and SnO₂ nanotubes due to their dynamic migration properties. The migration of catalysts with liquid Sn metal is induced by selective location of catalysts around Sn. On the basis of these structural and uniform functionalization of catalyst benefits, biomarker chemical sensors are developed, which deliver highly selective detection capability toward acetone and toluene, respectively. Pt or Pd loaded multidimensional SnO₂ nanostructures exhibit outstanding acetone (R _air/R _gas = 93.55 @ 350 °C, 5 ppm) and toluene (R _air/R _gas = 9.25 @ 350 °C, 5 ppm) sensing properties, respectively. These results demonstrate that unique nanostructuring and novel catalyst loading method enable sensors to selectively detect biomarkers for exhaled breath sensors.     

Oxide nanoparticle arrays for sensors of CO and NO2 gases

Metal oxide sensors with active films from Fe₂O₃ and CoFe₂O₄ hexagonally assembled nanoparticle (NP) arrays were studied. NPs were synthesized by high-temperature solution phase reaction. Sensing NP layers were deposited by Langmuir-Blodgett (LB) technique. LB layers were characterized by XRD, SEM and magnetic measurements. NPs are monodomain and superparamagnetic at RT. Sensor active films are formed from 1 or 7 LB monolayers deposited on the alumina substrates equipped with heating meander and interdigitated contacts. LB layers were heat-treated or UV irradiated to remove the insulating surfactant. Sensing properties were studied in a test chamber containing reducing (CO) or oxidizing (NO₂) gas in concentrations between 5 and 100 ppm in the mixture with dry air. Response current signal (I_gas/I_air) vs. temperature and response vs. gas concentration calibration curves were measured. Best response values were obtained with CoFe₂O₄ devices between 300 and 400 °C, being 3 and 10 for 100 ppm of CO and 5 ppm of NO₂, respectively. The response and recovery times of sensors are between 3 and 30 min. The Fe₂O₃ and CoFe₂O₄ sensors with response of 8 or 10 to 5 ppm of NO₂ might be of practical applications in the detection of explosives.     

Silver Nanowire Embedded Colorless Polyimide Heater for Wearable Chemical Sensors: Improved Reversible Reaction Kinetics of Optically Reduced Graphene Oxide

Optically reduced graphene oxide (ORGO) sheets are successfully integrated on silver nanowire (Ag NW)‐embedded transparent and flexible substrate. As a heating element, Ag NWs are embedded in a colorless polyimide (CPI) film by covering Ag NW networks using polyamic acid and subsequent imidization. Graphene oxide dispersed aqueous solution is drop‐coated on the Ag NW‐embedded CPI (Ag NW‐CPI) film and directly irradiated by intense pulsed light to obtain ORGO sheets. The heat generation property of Ag NW‐CPI film is investigated by applying DC voltage, which demonstrates unprecedentedly reliable and stable characteristics even in dynamic bending condition. To demonstrate the potential application in wearable chemical sensors, NO₂ sensing characteristic of ORGO is investigated with respect to the different heating temperature (22.7–71.7 °C) of Ag NW‐CPI film. The result reveals that the ORGO sheets exhibit high sensitivity of 2.69% with reversible response/recovery sensing properties and minimal deviation of baseline resistance of around 1% toward NO₂ molecules when the temperature of Ag NW‐CPI film is 71.7 °C. This work first demonstrates the improved reversible NO₂ sensing properties of ORGO sheets on flexible and transparent Ag NW‐CPI film assisted by Ag NW heating networks.     

WO3 Nanofiber‐Based Biomarker Detectors Enabled by Protein‐Encapsulated Catalyst Self‐Assembled on Polystyrene Colloid Templates

A novel catalyst functionalization method, based on protein‐encapsulated metallic nanoparticles (NPs) and their self‐assembly on polystyrene (PS) colloid templates, is used to form catalyst‐loaded porous WO₃ nanofibers (NFs). The metallic NPs, composed of Au, Pd, or Pt, are encapsulated within a protein cage, i.e., apoferritin, to form unagglomerated monodispersed particles with diameters of less than 5 nm. The catalytic NPs maintain their nanoscale size, even following high‐temperature heat‐treatment during synthesis, which is attributed to the discrete self‐assembly of NPs on PS colloid templates. In addition, the PS templates generate open pores on the electrospun WO₃ NFs, facilitating gas molecule transport into the sensing layers and promoting active surface reactions. As a result, the Au and Pd NP‐loaded porous WO₃ NFs show superior sensitivity toward hydrogen sulfide, as evidenced by responses (R_air/R_gas) of 11.1 and 43.5 at 350 °C, respectively. These responses represent 1.8‐ and 7.1‐fold improvements compared to that of dense WO₃ NFs (R_air/R_gas = 6.1). Moreover, Pt NP‐loaded porous WO₃ NFs exhibit high acetone sensitivity with response of 28.9. These results demonstrate a novel catalyst loading method, in which small NPs are well‐dispersed within the pores of WO₃ NFs, that is applicable to high sensitivity breath sensors.     

Novel method for fabrication of NiO sensor for NO2 monitoring

Nickel oxide (NiO) sensor films were prepared on glass substrate by a sol–gel spin coating technique. These films were characterized for their structural and morphological properties by means of X-ray diffraction, field emission scanning microscopy and atomic force microscopy. The NiO films are oriented along (200) plane with the cubic crystal structure. These films were utilized in nitrogen dioxide gas (NO₂) sensor. The dependence of the NO₂ response on operating temperature, NO₂ concentration was investigated. The NiO film showed selectivity for NO₂ over Cl₂ compared to H₂S $ \left( {{\text{S}}_{{{\text{NO}}_{ 2} }} /{\text{S}}_{{{\text{Cl}}_{ 2} }} = 3 7. 5,{\text{ S}}_{{{\text{NO}}_{ 2} }} /{\text{S}}_{{{\text{H}}_{ 2} {\text{S}}}} = 3. 4} \right) $ . The maximum NO₂ response of 23.3 % with 85 % stability at gas concentration of 200 ppm at 200 °C was achieved. The response time of 20 s and recovery time of 498 s was also recorded with same operating parameters.     

Highly improved toluene gas-sensing performance of mesoporous Ag-anchored cobalt oxides nanowires

Mesoporous cobalt oxides nanowires (Co₃O₄ NWs) were synthesized by the nanocasting method, and then Ag nanoparticles with the different content were anchored on the surface of Co₃O₄ NWs. The experimental results indicate that Ag nanoparticles hardly affect the morphology and microstructure of Co₃O₄ NWs and actually exist on the surface of Co₃O₄ NWs. It is worth mentioning that Ag-loading greatly improves the toluene gas-sensing performance of Ag-anchored Co₃O₄ NWs. The operating temperature of Ag-anchored Co₃O₄ NWs sensors decreases from 210 °C for Co₃O₄ NWs sensor to 190 °C, while the response to 100 ppm toluene gas increases 3-folds. Ag_0.166-Co₃O₄ NWs sensor exhibits the best gas-sensing performance due to the optimal Ag-loading content. Ag nanoparticles not only provide more effective oxygen adsorption sites to reduce resistance in air, but also form metal–semiconductor heterojunctions at the Ag/Co₃O₄ interface to increase resistance in toluene gas. In this way, Ag-loading can further improve the gas-sensing performance of mesoporous Ag-anchored Co₃O₄ NWs sensors to toluene gas.     

Detection of H2S gas at lower operating temperature using sprayed nanostructured In2O3 thin films

Nanostructured indium oxide (In₂O₃) thin films were prepared by spray pyrolysis (SP) technique. X-ray diffraction (XRD) was used to investigate the structural properties and field emission scanning electron microscopy (FESEM) was used to confirm surface morphology of In₂O₃ films. Measurement of electrical conductivity and gas sensing performance were conducted using static gas sensing system. Gas sensing performance was studied at different operating temperature in the range of 25–150 °C for the gas concentration of 500 ppm. The maximum sensitivity (S = 79%) to H ₂ S was found at lower temperature of 50 °C. The quick response (4 s) and fast recovery (8 s) are the main features of this film.     

Nanowatt-Level Photoactivated Gas Sensor Based on Fully-Integrated Visible MicroLED and Plasmonic Nanomaterials

Photoactivated gas sensors that are fully integrated with micro light-emitting diodes (µLED) have shown great potential to substitute conventional micro/nano-electromechanical (M/NEMS) gas sensors owing to their low power consumption, high mechanical stability, and mass-producibility. Previous photoactivated gas sensors mostly have utilized ultra-violet (UV) light (250–400 nm) for activating high-bandgap metal oxides, although energy conversion efficiencies of gallium nitride (GaN) LEDs are maximized in the blue range (430–470 nm). This study presents a more advanced monolithic photoactivated gas sensor based on a nanowatt-level, ultra-low-power blue (λ_peak = 435 nm) µLED platform (µLP). To promote the blue light absorbance of the sensing material, plasmonic silver (Ag) nanoparticles (NPs) are uniformly coated on porous indium oxide (In₂O₃) thin films. By the plasmonic effect, Ag NPs absorb the blue light and spontaneously transfer excited hot electrons to the surface of In₂O₃. Consequently, high external quantum efficiency (EQE, ≈17.3%) and sensor response (ΔR/R₀ (%) = 1319%) to 1 ppm NO₂ gas can be achieved with a small power consumption of 63 nW. Therefore, it is highly expected to realize various practical applications of mobile gas sensors such as personal environmental monitoring devices, smart factories, farms, and home appliances.     

Study of YSZ-based electrochemical sensors with oxide electrodes for high temperature applications

Potentiometric sensors based on yttria stabilized zirconia (YSZ) with WO₃ as sensing electrode were fabricated using either Pt or Au electrodes. The sensors were studied in the temperature range 550–700°C in the presence of different concentrations (300-1000 ppm) of NO₂ and CO in air. The response to NO₂ was very stable with fast response time (20-40 s). The best sensitivity (18.8 mV/decade) using Pt electrodes was observed at 600°C. At the same temperature a cross-sensitivity (-15 mV/decade) to CO gas was also noticed. The response to CO was decreased (-4 mV/decade) using Au electrode. The role played by WO₃ on the sensing electrode was discussed.     

Comparison of gas sensing properties based on hollow and porous α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanotubes

Hollow and porous α-Fe₂O₃ nanotubes were successfully synthesized by single nozzle electrospinning method followed by annealing treatment. The crystal structures and morphologies of the as-prepared materials were characterized by X-ray diffraction and scanning electron microscopy, respectively. The as-prepared materials were applied to construct gas sensor devices which gas sensing properties were further investigated. The obtained results revealed that porous α-Fe₂O₃ nanotube gas sensors exhibit a markedly enhanced gas sensing performance compared with hollow α-Fe₂O₃ nanotube gas sensors, which was about three times higher to 100 ppm acetone at 240 °C. Interestingly, hollow and porous α-Fe₂O₃ nanotube gas sensors both showed fast response–recovery time and good selectivity, but the porous ones possessed the shorter recovery time. The improved properties could be attributed to the unique morphology of porous nanotubes. Thus, further improvement of performance in metal-oxide-semiconductors materials could be realized by preparation the unique porous structures of nanotubes. Moreover, it is expected that porous metal-oxide-semiconductors nanotubes could be further design as promising candidates for gas sensing materials.     

Performance evaluation of ZnO–CuO hetero junction solid state room temperature ethanol sensor

A semiconductor ethanol sensor was developed using ZnO–CuO and its performance was evaluated at room temperature. Hetero-junction sensor was made of ZnO–CuO nanoparticles for sensing alcohol at room temperature. Nanoparticles were prepared by hydrothermal method and optimized with different weight ratios. Sensor characteristics were linear for the concentration range of 150–250 ppm. Composite materials of ZnO–CuO were characterized using X-ray diffraction (XRD), temperature-programmed reduction (TPR) and high-resolution transmission electron microscopy (HR-TEM). ZnO–CuO (1:1) material showed maximum sensor response (S = R_air/R_alcohol) of 3.32 ± 0.1 toward 200 ppm of alcohol vapor at room temperature. The response and recovery times were measured to be 62 and 83 s, respectively. The linearity R² of the sensor response was 0.9026. The sensing materials ZnO–CuO (1:1) provide a simple, rapid and highly sensitive alcohol gas sensor operating at room temperature.     

Facile synthesis of perfect ZnSn(OH)<Subscript>6</Subscript> octahedral microcrystallines with controlled size and high sensing performance towards ethanol

Uniform and monodisperse ZnSn(OH)₆ perfect octahedrons have been synthesized by a facile coprecipitation reaction process. The particle size of the as-prepared ZnSn(OH)₆ octahedral structure can be readily controlled by adjusting the reaction temperature (T), and the side length of ZnSn(OH)₆ octahedrons was tailored from 3 μm (40°C) to 4 μm (60°C) and 5 μm (80°C). The ethanol sensing properties of ZnSn(OH)₆ octahedrons were carefully investigated. The gas sensing experimental data show that the sensor based on ZnSn(OH)₆ (40°C) has good selectivity, fast response/recovery time and the highest response (R_a/R_g = 23.8) to 200 ppm ethanol at relatively low optimum operating temperature (200°C) compared to sensors based on ZnSn(OH)₆ (60°C) and ZnSn(OH)₆ (80°C), which might result from different specific surface areas. The study demonstrated that perfect octahedral ZnSn(OH)₆ with controlled crystalline size and desirable sensing performance can be synthesized by a simple fabrication procedure, and the octahedral ZnSn(OH)₆ could be a highly promising material for high-performance sensors.     

Effect of processing and palladium doping on the properties of tin oxide based thick film gas sensors

In the present work, solid-state reaction and sol–gel route derived pure tin oxide (SnO₂) powders have been used to develop the palladium (Pd)-doped SnO₂ thick film sensors for detection of liquefied petroleum gas (LPG). Efforts have been made to study the gas sensing characteristics i.e., sensor response, response/recovery time and repeatability of the thick film sensors. The response of the sensors has been investigated at different operating temperatures from 200 to 350 °C in order to optimise the operating temperature which yields the maximum response upon exposure to fixed concentration of LPG. The optimum temperature is kept constant to facilitate the gas sensing characteristics as a function of the various concentration (0.25–5 vol%) of LPG. The structural and microstructural properties of Pd-doped SnO₂ powder and developed sensors have been studied by performing X-ray diffraction and field emission electron microscopy measurements. The improvement in the response along with better response and recovery time have been correlated to the reduction in crystallite size of SnO₂ powder and morphology of printed sensor in thick film form. It is found that the thick film sensor developed by using sol–gel route derived SnO₂ powder with an optimum doping of 1 wt% Pd is extremely sensitive (86 %) to LPG at 350 °C.     

Highly sensitive free-base-porphyrin-based thin-film optical waveguide sensor for detection of low concentration NO<Subscript>2</Subscript> gas at ambient temperature

Meso-5,10,15,20-tetrakis-(4-tertbutyl phenyl) porphyrin was synthesized using Adler–Longo method and was served as sensing material. Electronic absorption spectra of the porphyrin chloroform solution and its thin film were studied comparatively. An optical waveguide sensor based on free-base porphyrin was fabricated by spin coating method. Absorption variation of porphyrin film was studied before and after exposure to NO₂, H₂S, SO₂, and volatile organic gases. XRD patterns of porphyrin film before and after exposure to analytes (NO₂, SO₂, and H₂S) were provided, and light source of the OWG testing system was selected. This facile-prepared sensor exhibited high sensitivity and selectivity to NO₂ with fast response time of 3 s and slow recovery time of 10 min or so and was capable of measuring NO₂ down to 10 ppb at ambient temperature. Scanning electron microscopy was employed to characterize film morphology before and after contact with NO₂. Film thickness was measured before (71.3 nm) and after (76.8 nm) exposure to NO₂, and film thickness variation value (5.20 nm) was calculated. The sensing behavior of the studied sensing device was tested through mixture of H₂S, SO₂, and VOC gases with NO₂ and without NO₂ for determination of the selectivity of the device. Film stability was probed by UV–Vis spectra, and response values of sensing element to NO₂ gas were detected after several days of film preparation, and its RSD value was 1.66%.     

Effect of variation of sintering temperature on the gas sensing characteristics of SnO2:Cu (Cu=9 wt.%) system

The effect of variation of sintering temperature (600–800 °C/4 h) on the gas sensing characteristics of a SnO₂:Cu (Cu=9 wt.%) system (a high-performance temperature-selective composition) in the form of pellets is investigated systematically for the CO, H₂ and LPG gases at a concentration level of 1000 ppm. The XRD, SEM and half-bridge techniques were employed to establish the structural, morphological and gas sensing characteristics of the materials, respectively. A very high value of sensitivity factor (SF) equal to 1400 is obtained for CO gas at an optimal operating temperature of 160 °C for the pellets sintered at 750 °C. The selectivity values of CO gas against H₂ and LPG (S_CO/S_H2∼14 and S_CO/S_LPG∼280) at an optimum temperature of 160 °C are also improved considerably. This material (SnO₂:Cu, Cu=9 wt.% sintered at 750 °C with an optimal temperature of 160 °C) may prove to have tremendous potential for CO gas sensing applications.     

A General and Straightforward Route to Noble Metal‐Decorated Mesoporous Transition‐Metal Oxides with Enhanced Gas Sensing Performance

Controllable and efficient synthesis of noble metal/transition‐metal oxide (TMO) composites with tailored nanostructures and precise components is essential for their application. Herein, a general mercaptosilane‐assisted one‐pot coassembly approach is developed to synthesize ordered mesoporous TMOs with agglomerated‐free noble metal nanoparticles, including Au/WO₃, Au/TiO₂, Au/NbO_x, and Pt/WO₃. 3‐mercaptopropyl trimethoxysilane is applied as a bridge agent to cohydrolyze with metal oxide precursors by alkoxysilane moieties and interact with the noble metal source (e.g., HAuCl₄ and H₂PtCl₄) by mercapto (? SH) groups, resulting in coassembly with poly(ethylene oxide)‐b‐polystyrene. The noble metal decorated TMO materials exhibit highly ordered mesoporous structure, large pore size (≈14–20 nm), high specific surface area (61–138 m² g^?1), and highly dispersed noble metal (e.g., Au and Pt) nanoparticles. In the system of Au/WO₃, in situ generated SiO₂ incorporation not only enhances their thermal stability but also induces the formation of ε‐phase WO₃ promoting gas sensing performance. Owning to its specific compositions and structure, the gas sensor based on Au/WO₃ materials possess enhanced ethanol sensing performance with a good response (R_air/R_gas = 36–50 ppm of ethanol), high selectivity, and excellent low‐concentration detection capability (down to 50 ppb) at low working temperature (200 °C).     

Sensor properties of Pt doped SnO2 thin films for detecting CO

Polycristalline Pt-doped SnO₂ thin films have been integrated to silicon substrate by ultrasonic spray deposition. This deposition technique differs from the usual SnO₂ deposition methods by using a liquid source. It allows one to obtain a very fine and homogeneous dispersion of Pt aggregates which act as a catalyst for the low temperature CO detection (25–100°C) by conductance change. The influence of synthesis temperature (460–560°C), concentration of Pt additive (0.1–5 at.%) on gas sensitivity has been studied. The realisation of gas sensor includes a gas sensitive highly porous layer (SnO₂/Pt, thickness: ∼1 μm). The results of electrical measurements under 300 ppm of CO for thin films in a dynamic and quasistatic regime are discussed. The narrow peak of gas sensitivity in the range of low temperatures (25–100°C) is obtained for about 2 at.% Pt in the SnO₂ film.     

Enhanced acetone detection using Au doped ZnO thin film sensor

Zinc oxide (ZnO) thin films are prepared using sol–gel method for acetone vapor sensing. Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) was taken as starting material and a stable and homogeneous solution was prepared in ethanol by deliquescing the zinc acetate and distinct amount of monoethanolamine as a stabilizing agent. The prepared solution was then coated on silicon substrates by spin coating method and then annealed at 650 °C for preparing ZnO thin films. The thickness of the film was maintained at 410 nm. The structural, morphological and optical studies were done for the synthesized ZnO thin films. The operating temperature and sensor response is considered to be an important parameter for the gas sensing behavior of any material. Therefore, the present study examined the effect of sensing behavior of 3% v/v gold (Au) doped ZnO thin films as a sensor. The response characteristics of 410 nm ZnO thin film for temperature ranging from 180 to 360 °C were determined for the acetone vapors. The reported study provides a significant development towards acetone sensors, where a very high sensitivity with rapid response and recovery times are reported with lowered optimal operating temperature as compared to bare ZnO nano-chains like structured thin films. In comparison to the bare ZnO thin films giving a response of 63 at an operating temperature of 320 °C, a much better response of 132.3 was observed for the Au doped ZnO thin films at an optimised operating temperature of 280 °C for a concentration of 500 ppm of acetone vapors.     

Sensitivity and selectivity of (Au,Pt, Pd)-loaded and (In,Fe)-doped SnO2 sensors for H2 and CO detection

(Au, Pt, Pd)-loaded and (In, Fe)-doped SnO₂ are synthesized by a sol–gel method. The composition, morphology and electrochemical property of the materials were characterized by XRD, SEM and electrochemical workstation, respectively. The results show that Au, Pd loading and In, Fe doping prefer to enhance the selectivity to CO against H₂, while Pt loading can enhance the selectivity to H₂ against CO. Furthermore, 1 mol% Pt-loaded SnO₂ sensor has preferable selectivity to H₂ against CO when Pt loading amount is changed. The response time of the Pt-loaded SnO₂ sensor to 5,000 ppm H₂ is 5 s at 400 °C, which is much shorter than that of pure SnO₂ sensor. Meanwhile the effect of operating temperature and Pt loading on n value (the slope of logarithm of response versus logarithm of gas concentration) is studied. The Pt-loaded SnO₂ sensor can detect H₂ down to 1 ppm. These results show that the Pt-loaded SnO₂ sensor is a good candidate for practical H₂ sensors.     

Preparation of ultrathin TiO2 single-crystal nanowires for high performance dye sensitized solar cells

Ultrathin TiO₂ anatase nanowires (NWs) were successfully prepared via a rapid and facile hydrothermal route. Consequently, the TiO₂ NWs and TiO₂ nanoparticles (NPs) composites electrodes were prepared with different weight ratios (25, 50 and 100 %) for a dye sensitized solar cell, and the photoelectrical performance has been systematically studied. It is observed that although the amount of absorption dye decreases, the composite solar cells exhibit a higher power conversion efficiency compared to either pure TiO₂ NP or NW solar cells by rationally tuning the weight ratios. The behavior was attributed to a combination of the rapid carrier transport in NW framework and the high dye loading on P25 surface.