In2O3 Nanofibers and Nanoribbons: Preparation by Electrospinning and Their Formaldehyde Gas-Sensing Properties

Electrospun Metal Oxide Nanofibers and Their Conductometric Gas Sensor Application. Part 2: Gas Sensors and Their Advantages and Limitations

Electrospun metal oxide nanofibers, due to their unique structural and electrical properties, are now being considered as materials with great potential for gas sensor applications. This critical review attempts to assess the feasibility of these perspectives. This article discusses approaches to the manufacture of nanofiber-based gas sensors, as well as the results of analysis of the performances of these sensors. A detailed analysis of the disadvantages that can limit the use of electrospinning technology in the development of gas sensors is also presented in this article. It also proposes some approaches to solving problems that limit the use of nanofiber-based gas sensors. Finally, the summary provides an insight into the future prospects of electrospinning technology for the development of gas sensors aimed for the gas sensor market.

Graphene-Doped Tin Oxide Nanofibers and Nanoribbons as Gas Sensors to Detect Biomarkers of Different Diseases through the Breath

This work presents the development of tin oxide nanofibers (NFs) and nanoribbons (NRs) sensors with graphene as a dopant for the detection of volatile organic compounds (VOCs) corresponding to different chronic diseases (asthma, chronic obstructive pulmonary disease, cystic fibrosis or diabetes). This research aims to determine the ability of these sensors to differentiate between gas samples corresponding to healthy people and patients with a disease. The nanostructures were grown by electrospinning and deposited on silicon substrates with micro-heaters integrated. The morphology of NFs and NRs was characterized by Scanning Electron Microscopy (SEM). A gas line was assembled and programmed to measure a wide range of gases (ethanol, acetone, NO and CO) at different concentrations simulating human breath conditions. Measurements were made in the presence and absence of humidity to evaluate its effect. The sensors were able to differentiate between the concentrations corresponding to a healthy person and a patient with one of the selected diseases. These were sensitive to biomarkers such as acetone and ethanol at low operating temperatures (with responses above 35%). Furthermore, CO and NO response was at high temperatures (above 5%). The sensors had a rapid response, with times of 50 s and recovery periods of about 10 min.

Integrated sensing layer of bacterial cellulose and polyethyleneimine to achieve high sensitivity of ST-cut quartz surface acoustic wave formaldehyde gas sensor

Surface acoustic wave (SAW)-based formaldehyde gas sensor using bi-layer nanofilms of bacterial cellulose (BC) and polyethyleneimine (PEI) was developed on an ST-cut quartz substrate using sol-gel and spin coating processes. BC nanofilms significantly improve the sensitivity of PEI films to formaldehyde gas, and reduces response and recovery times. The BC films have superfine filamentary and fibrous network structures, which provide a large number of attachment sites for the PEI particles. Measurement results obtained using in situ diffuse reflectance Fourier transform infrared spectroscopy showed that the primary amino groups of PEI strongly adsorb formaldehyde molecules through nucleophilic reactions, thus resulting in a negative frequency shift of the SAW sensor due to the mass loading effect. In addition, experimental results showed that the frequency shifts of the SAW devices are determined by thickness of PEI film, concentration of formaldehyde and relative humidity. The PEI/BC sensor coated with three layers of PEI as the sensing layer showed the optimal sensing performance, which had a frequency shift of 35.6 kHz for 10 ppm formaldehyde gas, measured at room temperature and 30 % RH. The sensor also showed good selectivity and stability, with a low limit of detection down to 100 ppb.

Electrospun one-dimensional nanostructures: a new horizon for gas sensing materials

Electrospun one-dimensional (1D) nanostructures are rapidly emerging as key enabling components in gas sensing due to their unique electrical, optical, magnetic, thermal, mechanical and chemical properties. 1D nanostructures have found applications in numerous areas, including healthcare, energy storage, biotechnology, environmental monitoring, and defence/security. Their enhanced specific surface area, superior mechanical properties, nanoporosity and improved surface characteristics (in particular, uniformity and stability) have made them important active materials for gas sensing applications. Such highly sensitive and selective elements can be embedded in sensor nodes for internet-of-things applications or in mobile systems for continuous monitoring of air pollutants and greenhouse gases as well as for monitoring the well-being and health in everyday life. Herein, we review recent developments of gas sensors based on electrospun 1D nanostructures in different sensing platforms, including optical, conductometric and acoustic resonators. After explaining the principle of electrospinning, we classify sensors based on the type of materials used as an active sensing layer, including polymers, metal oxide semiconductors, graphene, and their composites or their functionalized forms. The material properties of these electrospun fibers and their sensing performance toward different analytes are explained in detail and correlated to the benefits and limitations for every approach.

Ammonia gas sensors based on In2O3/PANI hetero-nanofibers operating at room temperature

Indium nitrate/polyvinyl pyrrolidone (In(NO3)3/PVP) composite nanofibers were synthesized via electrospinning, and then hollow structure indium oxide (In2O3) nanofibers were obtained through calcination with PVP as template material. In situ polymerization was used to prepare indium oxide/polyaniline (In2O3/PANI) composite nanofibers with different mass ratios of In2O3 to aniline. The structure and morphology of In(NO3)3/PVP, In2O3/PANI composite nanofibers and pure PANI were investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and current-voltage (I-V) measurements. The gas sensing properties of these materials towards NH3 vapor (100 to 1000 ppm) were measured at room temperature. The results revealed that the gas sensing abilities of In2O3/PANI composite nanofibers were better than pure PANI. In addition, the mass ratio of In2O3 to aniline and the p-n heterostructure between In2O3 and PANI influences the sensing performance of the In2O3/PANI composite nanofibers. In this paper, In2O3/PANI composite nanofibers with a mass ratio of 1:2 exhibited the highest response values, excellent selectivity, good repeatability and reversibility.

Mesoporous In2O3 nanofibers assembled by ultrafine nanoparticles as a high capacity anode for Li-ion batteries

As a lithium storage material, In₂O₃ has been hindered by its rapid capacity degradation due to the large volume change during the repeated lithiation and delithiation process, although an initial discharge capacity of more than 1600 mA h g⁻¹.

A rapid ammonia sensor based on lysine nanogel-sensitized PANI/PAN nanofibers

Macrogel was biodegraded into nanogel using trypsin PBS solution, the NH₃ sensing properties of the PANI/PAN/4-Lys-4 nanogel thin film were measured after the nanogels were electrospun into PANI/PAN nanofibers.

A Single Eu-Doped In₂O₃ Nanobelt Device for Selective H₂S Detection

Eu-doped In₂O₃ nanobelts (Eu-In₂O₃ NBs) and pure In₂O₃ nanobelts (In₂O₃ NBs) are synthesized by the carbon thermal reduction method. Single nanobelt sensors are fabricated via an ion beam deposition system with a mesh-grid mask. The gas-sensing response properties of the Eu-In₂O₃ NB device and its undoped counterpart are investigated with several kinds of gases (including H₂S, CO, NO₂, HCHO, and C₂H₅OH) at different concentrations and different temperatures. It is found that the response of the Eu-In₂O₃ NB device to 100 ppm of H₂S is the best among these gases and the sensitivity reaches 5.74, which is five times that of pure In₂O₃ NB at 260 °C. We also found that the former has an excellent sensitive response and great selectivity to H₂S compared to the latter. Besides, there is a linear relationship between the response and H₂S concentration when its concentration changes from 5 to 100 ppm and from 100 to 1000 ppm. The response/recovery time is quite short and remains stable with an increase of H₂S concentration. These results mean that the doping of Eu can improve the gas-sensing performance of In₂O₃ NB effectually.

Width-controlled M-type hexagonal strontium ferrite (SrFe12O19) nanoribbons with high saturation magnetization and superior coercivity synthesized by electrospinning

Width-controlled M-type hexagonal SrFe12O19 nanoribbons were synthesized for the first time via polyvinylpyrrolidone (PVP) sol assisted electrospinning followed by heat treatment in air, and their chemical composition, microstructure and magnetic performance were investigated. Results demonstrated that as-obtained SrFe12O19 nanoribbons were well-crystallized with high purity. Each nanoribbon was self-assembled by abundant single-domain SrFe12O19 nanoparticles and was consecutive on structure and uniform on width. PVP in the spinning solution played a significant influence on the microstructure features of SrFe12O19 nanoribbons. With PVP concentration increasing, the ribbon-width was increased but the particle-size was reduced, which distributed on a same ribbon were more intensive, and then the ribbon-surface became flat. The room temperature magnetic performance investigation revealed that considerable large saturation magnetization (Ms) and coercivity (Hc) were obtained for all SrFe12O19 nanoribbons, and they increased with the ribbon-width broadening. The highest Ms of 67.9 emu · g(-1) and Hc of 7.31 kOe were concurrently acquired for SrFe12O19 nanoribbons with the maximum ribbon-width. Finally, the Stoner-Wohlfarth curling model was suggested to dominate the magnetization reverse of SrFe12O19 nanoribbons. It is deeply expected that this work is capable of opening up a new insights into the architectural design of 1D magnetic materials and their further utilization.

Synthesis of CaTiO3 Nanofibers with Controllable Drug-Release Kinetics

Calcium titanate (CaTiO3) nanofibers with controlled microstructure were fabricated by a combination of sol-gel and electrospinning approaches. The fiber morphology has been found to rely significantly on the precursor composition. Altering the volume ratio of ethanol to acetic acid from 3.5 to 1.25 enables the morphology of the CaTiO3 nanofibers to be transformed from fibers with a circular cross section to curved ribbon-like structures. Ibuprofen (IBU) was used as a model drug to investigate the drug-loading capacity and drug-release profile of the nanofibers. It was found that the BET surface area and the pore volume decrease markedly with the utilization of F127 surfactant. The nanofibers synthesized without F127 surfactant present the highest drug-loading capacity and the most sustained release kinetics. This study suggests that calcium titanate nanofibers can offer a promising platform for localized drug delivery.

Ag-Modified In₂O₃/ZnO Nanobundles with High Formaldehyde Gas-Sensing Performance

Ag-modified In₂O₃/ZnO bundles with micro/nano porous structures have been designed and synthesized with by hydrothermal method continuing with dehydration process. Each bundle consists of nanoparticles, where nanogaps of 10–30 nm are present between the nanoparticles, leading to a porous structure. This porous structure brings high surface area and fast gas diffusion, enhancing the gas sensitivity. Consequently, the HCHO gas-sensing performance of the Ag-modified In₂O₃/ZnO bundles have been tested, with the formaldehyde-detection limit of 100 ppb (parts per billion) and the response and recover times as short as 6 s and 3 s, respectively, at 300 °C and the detection limit of 100 ppb, response time of 12 s and recover times of 6 s at 100 °C. The HCHO sensing detect limitation matches the health standard limitation on the concentration of formaldehyde for indoor air. Moreover, the strategy to synthesize the nanobundles is just two-step heating and easy to scale up. Therefore, the Ag-modified In₂O₃/ZnO bundles are ready for industrialization and practical applications.

Temperature effect on electrospinning of nanobelts: the case of hafnium oxide

Electrospinning is a convenient and versatile method for fabricating different kinds of one-dimensional nanostructures such as nanofibres, nanotubes and nanobelts. Environmental parameters have a great influence on the electrospinning nanostructure. Here we report a new method to fabricate hafnium oxide (HfO(2)) nanobelts. HfO(2) nanobelts were prepared by electrospinning a sol-gel solution with the implementation of heating and subsequent calcination treatment. We investigate the temperature dependence of the products by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive x-ray (EDX) spectroscopy. The heating temperature of spinning ambient is found to be crucial to the formation of HfO(2) nanobelts. By tuning the temperature, the morphological transformation of HfO(2) from nanowires to nanobelts was achieved. It was found that the rapid evaporation of solvent played an important role in the formation process of HfO(2) nanobelts. It is shown that nanobelts can only be obtained with the temperature higher than 50 °C and they are in the high quality monoclinic phase. A possible growth mechanism of the nanobelts based on phase separation is proposed. The enhanced photoluminescence (PL) of HfO(2):Eu(3+) nanobelts is also illustrated.

Preparation of LaFeO3 nanofibers by electrospinning for gas sensors with fast response and recovery

LaFeO(3) nanofibers are successfully prepared by the electrospinning method. XRD patterns show that the materials belong to a cubic system. After calcination at 600 °C for 3 h, SEM photographs show that the diameters of the nanofibers are about 80-90 nm and their surfaces are smooth. The response-recovery properties of an LaFeO(3) nanofiber sensor to ethanol are better than those of an LaFeO(3) nanobelt and nanoparticle sensor. LaFeO(3) nanofibers have relatively low resistance, and they improve the weakness of LaFeO(3) nanoparticles upon application. An LaFeO(3) nanofiber sensor also has good reversibility and selectivity to ethanol and is a very good p-type semiconductor material.