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Abstract: Three different compositions of ZnO–In2O3 composite nanofibers were prepared by electrospinning, and their gas sensing characteristics were compared to those of pure ZnO and In2O3 nanofibers. All ZnO–In2O3 composite nanofibers showed high response to trimethylamine (TMA), relatively low cross-response to C2H5OH, and negligible cross-responses to CO and H2; pure ZnO or In2O3 nanofibers did not show selective detection of TMA. The maximum responses to 5ppm TMA of the ZnO–In2O3 composite nanofibers with the compositions of [Zn]:[In]=67:33, 50:50, and 33:67 by at% were 133.9 at 300°C, 82.9 at 350°C, and 119.4 at 375°C, respectively. Considering all the sensing characteristics such as gas response, selectivity, and sensing/recovering kinetics together, the operation of the ZnO–In2O3 nanofiber sensor with the composition of [Zn]:[In]=33:67 by at% at 375°C was determined to be in optimal condition to detect TMA. The significant enhancement of gas response and selectivity by the formation of composite nanofibers is discussed in relation to the variation of particle size, the formation of hetero-interfaces between ZnO and In2O3, and the combination of two sensing materials with different catalytic properties, gas adsorption behaviors, and acid–base properties. [Copyright &y& Elsevier]

Abstract: One-dimensional (1D) amorphous InGaZnO4 (a-IGZO) submicron-tubes were synthesized in a method involving an electrospun polymeric fiber template and the direct RF-sputter-coating of a-IGZO films combined with subsequent calcination at 450°C. The a-IGZO hollow fibers with a diameter of 300nm and a shell thickness of 20–30nm showed an amorphous structure, as confirmed by XRD and HR-TEM analyses. Gas sensors using semiconducting a-IGZO tube networks exhibited n-type gas sensing characteristics and a 3.7-fold higher gas response (R gas /R air =109.5 at 2ppm NO2) compared to (R gas /R air =29.4) planar a-IGZO thin films at an operating temperature of 300°C. The enhanced gas response of a-IGZO tubes is attributed to the greater space charge modulation depth associated with the thin shell structures and the porous networks which are readily accessible by gas. [Copyright &y& Elsevier]

Electrospinning–etching strategy was employed to synthesize Pt-loaded SnO 2 hollow microbelts (Pt_SnO 2 HBLs) based chemiresistive-type sensor demonstrating high sensitivity and selectivity toward acetone gas. This strategy overcomes a major hurdle toward achieving ubiquitous gas diffusion in electrospun structures, and provides highly porous thin-walled HBLs with large surface area and small crystallites. [Display omitted] • In situ templating of SnO 2 with SiO 2 was employed to form Pt-loaded SiO 2 @SnO 2 core–shell microbelts. • Etching away of SiO 2 in the Pt-loaded SiO 2 @SnO 2 microbelts yielded Pt-loaded SnO 2 hollow microbelts. • The Pt-loaded SnO 2 hollow microbelts exhibited good response (R a /R g = 93.5) and selectivity toward 2 ppm acetone at 350 °C. Challenges for ubiquitous diffusion of gas analytes into semiconducting metal oxides (SMOs) based sensing layers have necessitated the introduction of unprecedented synthesis techniques for thin, porous and high-performance gas sensing materials. In this work, electrospinning and calcination were employed to prepare unprecedented SiO 2 –SnO 2 core–shell microbelts (hereinafter referred to as SiO 2 @SnO 2 BLs) followed by etching away of SiO 2 in NaOH solution (pH 12) to achieve hollow SnO 2 BLs (hereinafter referred to as SnO 2 HBLs) with mean crystal size of 14.01 nm, large BET surface area (143.5 m2‧g–1), high porosity (mean pore size of 5.7 nm), and shell thickness of 58.3 ± 11.4 nm. Sensitization of SnO 2 HBLs with apoferritin-templated platinum nanoparticles (Pt NPs) enhanced their detection capability toward acetone. Response (defined as R a /R g , where R a and R g are sensor resistances in air and target gas, respectively) of Pt(0.12 %)_SnO 2 HBLs toward 2 ppm acetone was up to 7.8 times higher compared to that of pristine SnO 2 HBLs, and exhibited faster response (9.2 s). The Pt(0.12 %)_SnO 2 HBLs based sensor indicated a promising long-term stability, and outstanding repeatability of response (R a /R g = 93.7 ± 0.89) toward 25 cycles of 2 ppm acetone exposure in a humid environment of 90 % relative humidity. This performance could be attributed to the unique morphology of HBLs, sensitization effects of catalytic Pt NPs, and enlargement of the electron-depleted layer resulting from a Schottky barrier between Pt NPs and SnO 2 grains. [ABSTRACT FROM AUTHOR]

Functionalization of catalytic nanoparticles (NPs) on semiconductor metal oxide (SMO) sensing layer is an indispensable process to obtain improved sensitivity and selectivity for high performance chemical sensors. It is a critical challenge to achieve homogeneous distribution of nanoscale catalysts on SMO in consideration that gas sensing characteristics of SMO-based sensing layer are significantly influenced by the size and distribution of catalysts. Here, we propose a highly effective functionalization method to achieve well-distributed catalytic NPs onto one dimensional (1D) SMO nanofibers (NFs) using protein cage templates: apoferrtin. By simply replacing precursor in the apoferritin assisted method, not only precious catalyst such as Pt but also non-precious catalysts such as La and Cu were successfully synthesized in nanoscale ( i.e. , 3–5 nm). Furthermore, the apoferritin-encapsulated catalysts exhibited high dispersion property due to repulsive force between protein shells. For this reason, catalytic NPs were homogeneously decorated on ZnO NFs after electrospinning followed by calcination. Catalytic Pt NPs and Cu NPs functionalized ZnO NFs exhibited approximately 6.38-fold (R air /R gas = 13.07) and 2.95-fold (R air /R gas = 6.04) improved acetone response compared with the response (R air /R gas = 2.05) of pristine ZnO NFs. In the case of La NPs functionalized ZnO NFs, 9.31-fold improved nitrogen monoxide response (R air /R gas = 10.06) was achieved compared with the response of pristine ZnO NFs. The four catalyst-ZnO composite NFs successfully distinguished simulated breath components such as acetone, toluene, nitrogen monoxide, carbon monoxide, and ammonia with well-classified patterns by principal component analysis (PCA). This work demonstrated a robustness of synthetic and functionalization method using bio-inspired protein templates combined with electrospinning technique and a promising potential of using non-precious catalysts to establish diverse sensing material libraries that can be applicable to breath pattern recognition for diagnosis of diseases. [ABSTRACT FROM AUTHOR]

In this work, a novel catalytic synthesis and functionalization method using apoferritin is used to fabricate RuO 2 nanoparticles (NPs) loaded WO 3 nanofibers (NFs) for potential diagnosis of diabetes. Catalytic ruthenium (Ru) NPs with very small average diameters of 1.8 ± 0.9 nm were synthesized using apoferritin which is a hollow protein cage, and were easily functionalized on WO 3 NFs by introducing electrospinning solution with W precursor and polyvinylpyrrolidone (PVP). As-spun Ru NPs-loaded W precursor/PVP composite NFs were calcined at 600 °C for 1 h in air atmosphere to achieve RuO 2 -functionalized WO 3 NFs. The small size and uniform distribution of catalytic RuO 2 NPs were well maintained due to hollow nature of apoferritin cages after calcination. The chemo-resistive sensors using RuO 2 -functionalized WO 3 NFs showed significantly enhanced acetone (CH 3 COCH 3 ) sensing response (R air /R gas = 78.61–5 ppm), which was 7.4 times higher than the response (R air /R gas = 10.61–5 ppm) of pristine WO 3 NFs at highly humid atmosphere (95% RH). In addition, the RuO 2 -functionalized WO 3 NFs showed outstanding selectivity toward acetone gas in comparison with other gases such as hydrogen sulfide (H 2 S), toluene (C 6 H 5 CH 3 ), ethanol (C 2 H 5 OH), pentane (C 5 H 12 ), ammonia (NH 3 ), hydrogen (H 2 ), and water vapor (H 2 O) at 5 ppm. These results represent potential feasibility for the detection of acetone in exhaled breath for diagnosis of diabetes. [ABSTRACT FROM AUTHOR]

In this work, catalytic Rh 2 O 3 -functionalized WO 3 nanofibers (NFs) were synthesized via an electrospinning route and used as a highly selective acetone-sensing layer for potential diagnosis of diabetes. Catalytic rhodium nanoparticles (Rh NPs) with average diameters of 5.0 ± 0.52 nm, which were synthesized by the polyol process, were dispersed in water with W precursor and poly(vinylpyrrolidone) (PVP) for electrospinning. As-spun Rh NP-loaded W precursor/PVP composite NFs were calcined at 600 °C for 1 h in air atmosphere to achieve Rh 2 O 3 -decorated WO 3 NFs. Microstructure evolution and chemical composition of Rh 2 O 3 -decorated WO 3 NFs as a function of Rh-loading amounts, i.e., 0.01 wt%, 0.05 wt%, 0.10 wt%, and 0.15 wt%, were examined using energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The mean size (30 nm) of the WO 3 crystallites in Rh 2 O 3 -decorated WO 3 NFs was much smaller than that (60 nm) of the WO 3 crystallites in pristine WO 3 NFs. The Rh 2 O 3 -decorated WO 3 NFs showed outstanding acetone (CH 3 COCH 3 ) sensing response ( R air / R gas = 41.2 to 5 ppm), which was 4.6 times higher than the response ( R air / R gas = 9.0 to 5 ppm) of pristine WO 3 NFs at highly humid atmosphere (95% RH). In addition, superior acetone cross-sensitivity of the Rh 2 O 3 -decorated WO 3 NFs was observed in other interfering gases such as pentane ( n -C 5 H 12 ), ammonia (NH 3 ), toluene (C 6 H 5 CH 3 ), carbon monoxide (CO), and ethanol (C 2 H 5 OH) at 5 ppm. These results are highly promising for the accurate and selective detection of acetone in exhaled breath for potential diagnosis of diabetes. [ABSTRACT FROM AUTHOR]

In this work, we report a remarkably improved toluene response and superior cross-sensitivity against H2S molecules by combining Pd catalysts and highly porous WO3 nanofibers (NFs). We functionalized Pd catalysts inside and/or outside of WO3 NFs synthesized by electrospinning, which is a facile and versatile process for producing webs of metal oxide NFs. Pd-embedded WO3 NFs were obtained by the electrospinning of a Pd and W composite precursor/poly(methyl methacrylate) (PMMA) solution followed by calcination at 700°C. Pd nanoparticles (NPs) (6–10nm) synthesized by a polyol method were decorated on the WO3 NFs (Pd-NPs/WO3 NFs) by the attachment of the Pd NPs on as-prepared WO3 NFs. The gas sensing characteristics of pure WO3, Pd-embedded WO3, Pd-NPs/WO3, and Pd-NPs/Pd-embedded WO3 NFs were tested using H2S and toluene gases in a highly humid atmosphere (RH 90%), which is similar to human exhaled breath. The results showed that the Pd-NPs/Pd-embedded WO3 NFs whose inner and outer layers were decorated by Pd catalysts, exhibited a high toluene response (R air/R gas =5.5 at 1ppm) and remarkable selectivity against H2S (R air/R gas =1.36 at 1ppm) at 350°C, whereas pristine WO3 NFs showed superior H2S response (R air/R gas =11.1 at 1ppm) along with a negligible response toward toluene (R air/R gas =1.27 at 1ppm). The highly porous WO3 NFs decorated with Pd catalysts, exhibited potential feasibility, i.e., a low limit of detection (LOD) of 20ppb (R air/R gas =1.32) at 350°C, for application in VOCs sensors, particularly for diagnoses of lung cancer. [ABSTRACT FROM AUTHOR]

We report on facile synthesis of SnO2 nanofibers (NFs) functionalized by Pt nanoparticles (NPs) with various loading contents (0.05–0.4wt%) and their gas response toward H2, NO2, NH3, CO, and SO2 gases as a function of the operation temperature (100–450°C) and gas concentration (0.125–2.5ppm). Mesoporous SnO2 NFs with large surface-to-volume ratio, which facilitates sensitive gas response, were synthesized by electrospinning followed by subsequent calcination at 500°C for 2h. Colloidal Pt NPs synthesized by simple polyol method were immobilized on SnO2 NFs as a catalytic sensitizer to H2 oxidation. The gas sensing properties and response kinetics were significantly affected by the Pt NPs and the operation temperature. It was found that 0.1wt% Pt-loaded SnO2 sensor showed the highest H2 response (I gas/I air =16.6 at 2.5ppm) at 300°C. A remarkably high NO2 response (I air/I gas =57.0 at 2.5ppm) was achieved in the case of pristine SnO2 sensor at 150°C. This work demonstrates that sensitive and selective detection of H2 and NO2 gases with negligible cross-response to interfering gases such as NH3, CO, and SO2 can be obtained by decoration with Pt NPs on metal oxide NFs in conjunction with manipulation of the operation temperature. [ABSTRACT FROM AUTHOR]

Abstract: SnO2 hollow nanofibers (HNFs) with diameters of 300–500nm were fabricated via electrospinning of templating nanofibers and RF-sputtering of SnO2 thin overlayers (15–20nm thick and comparable grain size) followed by heat-treatment at 450°C. Gas sensors using these HNFs exhibited n-type gas sensing characteristics and an enhanced gas response to 2ppm NO2 (R gas /R air =81.4) as compared to that (R gas /R air =19.9) of planar SnO2 thin films. The enhanced response to NO2 is attributed to the greater accessible active area and the greater space charge modulation depth associated with the hollow thin walled structures. [Copyright &y& Elsevier]

Abstract: This work presents a new route to enhance the sensitivity of nanocrystalline TiO2 fibers based gas sensors. Pure and Pd-doped TiO2 nanofiber mats were synthesized by electrospinning and subsequent calcination. Highly porous fibrillar morphology was observed in the resultant nanocrystalline TiO2 fibers. In addition to anatase TiO2 crystallites, tiny (∼2nm) PdO crystallites were observed in the Pd-doped TiO2 fibers. The sensors using Pd-doped TiO2 fibers showed promising gas sensing characteristics, such as low operation temperature (180°C) and sufficient gas response (R/R o =38 to 2.1ppm NO2). The techniques outlined here offer new means for preparing novel metal-oxide nanofibers with catalytic dopants. [ABSTRACT FROM AUTHOR]

• Porosity control of nanofibers: The high porosity of the metal oxide nanofibers is the key to achieving the dual sensitization owing to their high gas permeability and numerous active sites. Both saponin and water-ethanol co-solvent play critical roles in modifying morphology of nanofibers into elliptical nanofibers during electrospinning. Additionally, saponin forms numerous pores in the nanofibers when being burnt out during calcination. These porous, elliptical nanofibers possess a larger surface area and porosity.Saponin and water-ethanol co-solvent enabled porous, elliptical nanofibers to emerge. • Bifunctional Ga doping: Ga dopants enhanced sensing performance via conductivity improvement and grain growth inhibition. • Functionalization of Pt catalysts : Spill-over effects and chemical sensitization of remarkably improved C 2 H 2 sensitivity. Herein, we demonstrate a sensitization strategy via solid-solution doping and catalytic metal nanoparticles (NPs) onto porous metal oxide nanofibers (NFs) for superior acetylene (C 2 H 2) sensing capabilities. The introduction of dopants and catalysts not only increases the inherent reactivity of metal oxides, but also induces the spillover of C 2 H 2 onto metal oxides, significantly improving the sensing properties. We elucidate that the high porosity of the metal oxide NFs, prepared by electrospinning and utilizing saponin as a sacrificial template, is the key to achieving high gas permeability and numerous active sites. Consequently, Ga-doped ZnO porous NFs functionalized with Pt catalysts exhibited the highest C 2 H 2 sensing responses among the current state-of-the-art C 2 H 2 sensors (R a /R g = 26.2 at 400 °C) with a fast response time (10.2 s), at a particularly low concentration of 5 ppm. Moreover, these NFs showed excellent selectivity against the interfering gas (H 2 , C 7 H 8 , CO, CH 4 , C 8 H 10 , CH 3 COCH 3 , HCHO and C 2 H 5 OH) and outstanding stability up to 20 sensing cycles to 0.4 ppm of C 2 H 2. These results provide an efficient strategy to tailor composites of metal oxide nanostructures, dopants, and catalysts for highly sensitive gas sensors. [ABSTRACT FROM AUTHOR]