Electrospun Ceramic Nanofibers and Hybrid-Nanofiber Composites

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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Electrospun Ceramic Nanofibers and Hybrid-Nanofiber Composites for Gas Sensing Luiza A. Mercante,† Rafaela S. Andre,† Luiz H. C. Mattoso, and Daniel S. Correa*

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Nanotechnology National Laboratory for Agriculture (LNNA), Embrapa Instrumentaçaõ , 13560-970 São Carlos, São Paulo, Brazil ABSTRACT: Over the past few years, there has been a huge demand for developing sensors capable of monitoring and quantifying volatiles related to food quality analysis, medical diagnosis, and environmental monitoring. Sensors designed for such applications are required to present simultaneously high selectivity, low power consumption, fast response/recovery rate, low humidity dependence, and a low detection limit, which pose great challenges to be overcome in the development of suitable transducer nanomaterials for gas sensing. The nanostructure dimensionality certainly plays a key role for efficient gas detection. Thanks to the advantages derived from the nanoscaled size and the large surface-to-volume ratio, electrospun ceramic nanofibers (ECNs) have demonstrated great potential for the design of gas-sensing devices during the most recent years. Recently, studies have shown that the sensitivity, selectivity, and other important sensing parameters of ceramic nanofibers can be improved by designing heterojunctions at the nanoscale. As a consequence, outstanding composite and hybrid materials with unprecedented features have been recently reported for the detection of hazardous gases. In view of the importance and the potential impact of these results, here we review the latest findings and progress reported in the literature on the factors that influence the gassensing characteristics, as well as their application as gas sensors for monitoring various volatiles. This review will help researchers and engineers to understand the recent evolution and the challenges related to electrospun ceramic-based gas sensors and also further stimulate interest in the development of new gas-sensing devices. KEYWORDS: electrospinning, synthesis, nanomaterials, ceramic nanofibers, hybrid nanofibers, gas sensors, volatile monitoring field-effect transistors (FET).18 They can also be integrated to other configurations such as optical, surface work function (SWF) change transistors, surface acoustic wave (SAW), and quartz crystal microbalance (QCM) sensors.19−22 Among them, chemiresistor is the most widely used configuration for gas sensors. In this case, the gaseous analytes are detected by measuring the electrical resistance variation of sensing layers induced by the chemical and/or physical adsorption of a small amount of gas molecules.23,24 The advantageous properties of these types of sensors include simple fabrication and direct measurements.25 The performance of semiconductive metal oxides as sensing layers can be significantly influenced by modification of their size, shape, and operation temperature, for example.3,26 As a consequence, ceramic structures with varied types of morphologies have been reported to fabricate gas-sensing devices.1,27−29 Since the most common principle of gas sensing involves the adsorption/desorption of gas molecules onto the material surface, the sensitivity can be significantly enhanced by increasing the number of active sites available for the interaction between the analyte and sensing material.26 In this way, nanomaterials appear as promising candidates to improve gas-sensing properties due to their large surface−volume ratio

1. INTRODUCTION Gas sensors play a vital role in practical applications of our daily life. In the past few decades, gas sensing has become increasingly important for different areas, including food spoilage detection,1,2 human health,3,4 and the monitoring of environmental pollution/air quality.5,6 Nowadays, around 92% of the world’s population lives in regions where air pollutant levels are higher than the World Health Organization (WHO) specified limits.7 In addition, it is now widely recognized that exposure to some of these pollutants contributes to a broad array of health effects, ranging from minor physiological impacts to death from respiratory and cardiovascular diseases.8,9 The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD 2015) identified air pollution as a leading cause of global disease burden, especially in low-income and middle-income countries.10 Therefore, the detection of hazardous gases has become an important issue for environmental/human protection, which has boosted interest in the design of new sensing materials for the detection of a large variety of hazardous gases. Various sensing materials have been investigated in the past few years aiming to develop high-performance gas sensors.11,12 Among them, metal oxide semiconductors have evolved as powerful interface materials, being exhaustively employed for the detection of more than 150 hazardous gases.13−17 Currently, the commonly applied configuration of metal oxide-based gas sensor devices includes chemiresistors and © XXXX American Chemical Society

Received: June 20, 2019 Accepted: June 26, 2019 Published: June 26, 2019 A

DOI: 10.1021/acsanm.9b01176 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

for the detection of a large variety of gases. Furthermore, we exploit the use of ECNs to engineer innovative composite/ hybrid materials and their influence on the sensing device efficiency. The synergistic physicochemical properties and the structure−property correlation will also be highlighted in order to better understand the gas-sensing mechanisms involved for each case. Finally, we will provide an outlook of the challenges and opportunities for the use of electrospun ceramic-based hybrid nanofibers in the next generation of gas sensors.

and massive reactive sites, leading to greater adsorption of gas species and thus increasing sensing capability.30 In recent years, one-dimensional (1D) ceramic nanostructures have attracted much attention for gas detection owing to their large surface area and high crystalline structure, which potentially lead to miniaturized sensors with outstanding performance.31−33 Among the different strategies for producing 1D building blocks, electrospinning is a straightforward, versatile, and low-cost route to produce different kinds of nanocrystalline metal oxides with a highly porous fibrous morphology.34−37 Such porous nanostructures formed by nanograins with final diameters in small dimensions (D ≤ Debye length)38 greatly enhance the effective gas adsorption/ diffusion, increasing the sensor sensitivity. Therefore, the remarkable specific surface area (one or two orders of the magnitude larger than flat films) and high porosity (∼70− 90%) due to the presence of small and large pores make electrospun nanofibers highly attractive for designing ultrasensitive sensors.21,22,39 Furthermore, ceramic nanofibers derived from a variety of inorganic precursors/polymeric matrices can be fabricated in various assemblies (e.g., nanocomposite, core−shell or hollow) using the electrospinning technique.21,40 Tuning of the ECNs properties through the introduction of new functionalities is a fundamental strategy for producing highly sensitive, selective, and cost-effective gas sensors to operate at room temperature. Different strategies including doping/surface decoration with noble metals and combination of two or more distinct nanomaterials/inorganic phases have been exploited to achieve multi-heterojunction structures.41−46 Final properties (e.g., sensitivity, working temperature, selectivity, sensor response, stability, and low detection limit) of sensor devices using composite/hybrid ECNs can be synergistically improved by the distinct materials employed in these approaches. Although some interesting reviews about ceramic nanofibers are available in the literature,14,15,21−23,42,47−50 little attention has been given to composite/hybrid ECNs-based gas sensors. In this review, we provide an overview on the use of electrospinning technique to obtain different ECNs-based architectures for gas detection (see Figure 1). We aim to show how the crystalline porous nanofiber morphology can help enhance the sensor properties and offer exciting opportunities

2. OVERVIEW ON THE SYNTHESIS OF ECNs Several methods can be employed to produce nanofibers such as melt-blown,51 self-assembly,52 solution blow spinning,53 force spinning,54 and electrospinning 35 among others. However, electrospinning is the most commonly used technique for the fabrication of homogeneous ECNs.21,36,49,55 This is a straight consequence of specific characteristics of the electrospinning method such as the capability of mass production, simple and versatile configuration, and yet the ability to produce nanofibers with controlled diameters and orientations from a desired composition, as previously highlighted. Moreover, with the continuous research on the technique, various electrospinning setups have been developed allowing the customization of nanofiber assemblies to meet the requirement of specific applications. For instance, a review article by Teo and Ramakrishna56 discusses the advantages and disadvantages of numerous electrospinning setups. The formation of nanofibers by electrospinning method is based on a balance between the electrospinning parameters and the solution characteristics. Aiming at the production of homogeneous nanofibers, the solution characteristics such as viscosity, dielectric properties, and surface tension need to be modulated to work in consonance with the electrostatic forces generated by the applied high-voltage tension, the selected feed rate, and collector distance. When the electrostatic force overcomes the surface tension, the solution stretches first as a cone, known as Taylor’s cone, followed by further stretching, forming a jet. Such stretching is characteristic of spinnable solutions that present suitable rheological properties.37 Generally, inorganic precursor (metal salts or metal alkoxides) solutions present inappropriate viscosity and rapid hydrolysis rates that impair formation of stable jets during the spinning process. Consequently, two approaches have been proposed to overcome this drawback: (i) the use of metallic precursors, such as metal salts, dispersed in a polymeric solution for reaching proper viscosity, while the polymer should work as a structural matrix; or (ii) the use of metallic alkoxides that are submitted to polymerization forming a sol− gel, with a catalyst to control the hydrolysis rate and then combination with a polymeric solution for attaining suitable viscosity.55 Analogous nanofibers can be obtained using both strategies, as reported by Song et al., who fabricated 3Al2O3· B2O3·2SiO2 nanofibers using different silica precursors.57 Following the electrospinning process, the formed jet needs to reach the collector as a dried fiber in order to keep the morphology, which will be dependent on the traveled distance and on the solvent volatility. For that reason, the choice of the solvent needs to take into account not only the compatibility with the polymers and precursors but also its volatility.58 Besides, the polymer concentration will likewise influence the final viscosity and surface tension.59 Therefore, the preparation of a co-solution consisting of metallic precursor and polymer

Figure 1. Scheme of distinct electrospun ceramic nanofibers (ECNs), including pure ECNs, composite ECNs, doped ECNs, and hybrid ECNs employed for the detection of various volatiles. B


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Figure 2. Microscopy images of (A) solid, (B) porous, (C) hierarchical, (D) hollow, and (E) coaxial ECNs. Panels A, B, and E: Reprinted with permission from ref 85. Copyright 2017 Elsevier. Panel C: Reprinted with permission from ref 86. Copyright 2014 The Royal Society of Chemistry. Panel D: Reprinted with permission from ref 84. Copyright 2017 Elsevier.

inorganic materials (e.g., mixed metal oxides and metal oxides combined with metallic nanoparticles), while hybrid ECNs as metal oxide nanofibers combined with organic materials (e.g., conjugated polymers and carbon nanomaterials as graphene oxide or carbon nanotube). Mixed metal oxides’ nanofibers can be obtained by adding different metallic precursors into the polymeric solution before the electrospinning step, according to a conventional direct dispersion approach, while doped ECNs can be synthesized by suspending metallic nanoparticles into the metallic cation solution before the electrospinning process. In the last case, once the calcination is performed, the metal oxide crystalline structure will be formed by incorporating the metallic nanoparticles into the nanofiber core. Several composite ECNs have been reported, and among them, ZnOSnO2, In2O3-WO3, Fe2O3-SnO2, SnO2-TiO2, and NiO-SnO2 are remarkable examples.66−71 Hybrid ECNs can be prepared following two main strategies: (i) by incorporating the desirable material during the electrospinning process or (ii) by post-electrospinning functionalization of the crystalline ECNs.35 The postprocessing method is a powerful alternative to incorporate materials that are not resistant to high temperature and thus not viable for the heat treatment step. Examples of hybrid ECNs have been obtained by the combination of ECNs with conjugated polymers,72,73 such as polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT), and graphene derivative nanomaterials.74 In addition to the different ECNs’ composition, the nanofiber microstructure is a specific characteristic that can be modulated to enhance the sensor performance.75 Besides the conventional nanofibers with solid microstructure (Figure 2A), it is possible to obtain porous (Figure 2B) or hierarchical (Figure 2C) crystalline structures by adjusting the annealing temperature as demonstrated by Wang and co-workers.76 The authors showed that the variation of heat treatment parameters is a powerful strategy for the controllable synthesis of porous, hierarchical, and single-crystalline V2O5 nanofibers. In the past years, great attention has been paid to the design and control of the ZrO2 structure, in order to obtain porous ZrO2 materials with high surface area.77 In this context, Mao et al. reported the fabrication of ZrO2/ZnO composites with highly porous structure and highly improved properties compared to their pristine counterparts. Their superior performance might be due to the synergistic effect from the high surface area and charge separation at the interface of heterojunction and efficient charge transfer, which is greatly important for sensing applications.78 Hollow ceramic nanofibers (Figure 2D), remarkable by their exclusive open structure, have also been obtained by employing distinct approaches. The first one is based on a

matrix mixed in optimized ratio and combined to solvents appears as a good strategy to obtain polymeric nanofiber mats containing metallic cations throughout them. Following this approach, a subsequent step of calcination and annealing treatment is necessary to remove the organic matrix and sinter the metallic oxide as a crystalline ceramic nanofiber.39,60 The preservation of the nanofibrous mat structure after the calcination has been associated with the parameters employed during the heat treatment.49,55 Low heating rates are applied to remove the polymeric matrix without destroying the nanofibers morphology, neither causing breaks of the ECNs mat structure. While lower temperatures are ideal to form ECNs with smooth surfaces, higher temperatures tend to result in porous ECNs due to the organic compounds’ degradation.61 Further increase in the temperature can eliminate these pores through grain coarsening, evolving to a solid core.62 In view of the ceramic phase crystallization, when the metallic ions are supplied through ionic precursors, the metal oxide phase will be formed by co-precipitation process. Thus, the crystal formation occurs trough steps of nucleation, growth, and ripening, leading to the nanofiber’s arrangement. For metallic ions supplied by alkoxides, crystalline phase is formed by the sol−gel method, which is characterized by hydrolysis and condensation reactions. In other words, the alkoxide groups are replaced by hydroxyl groups followed by a condensation reaction where the metal−oxygen bond takes place and water molecules are formed as byproduct.55,62−64 It is worth noticing that all of the crystallization process will occur concurrently with the elimination of the organic phase, giving rise to the crystalline ceramic nanofiber as a co-dependent process. Simonsen and collaborators63 studied in detail the calcination and sintering temperature influence on the grain growth and nanofiber microstructure. The authors observed a reduction of the diameter of (La0.6Sr0.4)0.99CoO3−δ nanofibers as a function of the temperature, which was attributed to crystal water loss and decomposition of the metallic salts followed by polymer gradual decomposition from the surface to the core of the fiber. In situ transmission electron microscopy (TEM) images also showed the crystal formation, growth and coarsening as a function of the temperature. The crystal grain size increased with the temperature by the growth and coarsening process, so the porous structure decreased as a coarsening side effect, which could be confirmed by the strong contrast indicating the mass−thickness at the grain area and porous area. In another work, Senthamizhan et al.65 showed that the growth of nanograins severely altered the morphology of ZnO nanofibers and grain boundaries areas at different annealing temperatures. Considering multiphasic ECNs, it is worth designating composite ECNs as nanofibers composed by two or more C


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Figure 3. (A) Representative microstructures of the ZnO nanofibers after calcination in air at (a) 400, (b) 500, (c) 600, (d) 650, (e) 700, (f) 750, (g) 800, and (h) 850 °C. The inset panels are the corresponding low-magnification images that reveal the overall features of the nanofibers. (B) Summary of the (a) responses of the sensors fabricated using ZnO nanofibers as a function of calcination temperature (sensors exposed to 1 ppm CO), (b) grain size, and (c) degree of structural perfection of ZnO nanofibers in relation to calcination temperature. Reprinted with permission from ref 100. Copyright 2013 Elsevier.

with inorganic or metallic nanoparticles or yet when ECNs are combined with conjugated polymers, the resulting composite will present a hierarchical structure.89−92

sacrificial template that is removed from the nanofiber core after the electrospinning process. The second approach makes use of a coaxial needle system where two immiscible solutions yield hollow nanofibers by coaxial electrospinning followed by core removal.79,80 Furthermore, the nonremoval of the core is another alternative that results in multiphasic coaxial nanofibers with core−shell-like structure (Figure 2E).81 The successful fabrication of core−shell heterostructures by using a coaxial needle system can be directly related to the ejection rate and viscosity of each solution. The main strategy is to use the outer solution to drag the inner solution, forming a core− shell-like configuration. Additional adjustment can be made at the outer and inner tip alignment of the coaxial needle in order to facilitate the formation of a coaxial nanofiber.82 The heat treatment parameters will be adjusted according to the coaxial nanofiber composition. Once different polymers can be used for the preparation of the core and shell solution, the heat rate will be important to guarantee the concomitant inner and outer polymer elimination as well as the concomitant crystallization of the different inorganic phases in order to not lose the core−shell structure.79,83,84 Additionally, hierarchically structured ceramic nanofibers can be defined as 1D structures with secondary structure attached on its surface. This type of structure has been produced by both in situ route and post-treatment of the electrospun ceramic nanofibers route.87−89 An early study conducted by Hwang et al.88 reported the fabrication of hierarchically structured TiO2 nanofibers with increased surface area, pore volume four times higher than for nanoparticles, and high performance at room temperature for solid-state dye-sensitized solar cells. The nanofibers are formed by single-phase TiO2 anatase nanorods aligned as a nanofiber and called by the authors as “nanorod-in-nanofiber” morphology. However, multiphase nanofibers can be produced with hierarchic structure when more than one metallic element is employed. Besides, hierarchical structures are the majority of the composite and hybrid nanofibers structures obtained when constituent mixture is performed after the nanofiber production. For example, when nanofibers are decorated

3. FACTORS INFLUENCING THE GAS-SENSING CHARACTERISTICS OF ECNs Metal oxides are among the most investigated class of materials for gas sensor once they present high chemical stability, low cost, and great sensing performance.93−96 The sensing properties of metal oxides are highly dependent on their structure, phase, shape, size, and size distribution. For instance, the selectivity of a metal oxide-based sensor can be tailored by controlling the shape, as it will determine the crystallographic facets exposed on the surface of a nanocrystal and therefore the number of atoms located at the edges or corners.96 In addition, the design of a porous structure can also enhance the response/recovery rate and the selectivity.29 The nanofibrous structure in a web-like configuration is a perfect configuration for gas sensing once large surface areas are required for gas percolation and adsorption/desorption during the sensing process.21,97 Furthermore, the sensor sensitivity will be also dependent on characteristics such as the grain size, the presence or absence of crystal lattice defects such as oxygen vacancies, the type of the charge carriers (ptype or n-type), and the nature of the target gas (oxidizing or reducing).15,98,99 Since the gas/sensor interaction occurs through surface adsorption processes, the metal oxide grain size can interfere on the sensitivity of the sensor once the grain boundary acts as electrons scattering centers.96 In other words, grains larger than the Debye length (2−100 nm) will not have the bulk affected by the adsorption/desorption reactions. Meanwhile, ECNs with nanosized diameters and formed by nanograins will act as a 1D conductive channel effectively improving the transmission of electrons and allowing effective sensing performance, once the whole grain (surface and bulk) will be affected by the target gas/sensor surface interaction.62,75 Back to 2013, Katoch and co-workers100 conducted a detailed study on the ZnO nanofiber grain size and crystallinity and D


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combination of similar materials only results in satisfactory resistance modulation when the constituents possess different work functions.94,107 Specific systems and the role of the shared interfaces on hetero- and homojunctions will be further discussed.

their influence on gas sensing. First, the grain size and crystallinity were controlled through variation of the calcination temperature and then tested toward 1 ppm of CO. Thus, the sensor response was correlated with the grain size and with the degree of structural order observed for the nanofibers. The authors identified two domains of influence, one where the crystallinity is dominant and another where the grain size is the dominant influence on the response behavior (Figure 3). The crystallinity-dominant domain was observed for nanofibers treated with temperatures up to 700 °C, and despite the grain size increase, the sensor response showed improvement with the temperature increase. Therefore, the author demonstrated that the enhanced crystallinity overcomes the grain growth effects. For nanofibers obtained at temperatures above 700 °C, grain growth was considered the dominant characteristic for sensor performance once it was impaired, although the crystallinity was still increasing. The results indicated the grain size and crystallinity progress simultaneously and need to be optimized in order to achieve superior sensor performance. Oxygen vacancies, such as crystal defects, have also important influence on electronic structures and physical characteristics of metal oxide semiconductors and play a crucial role in gas-sensing performance.101 Usually, the oxygen vacancy content can be easily tuned by varying the calcination atmosphere.102 For gas sensing, existing oxygen vacancies not only act as an electronic charge carrier capable of enhancing the electronic conductivity but also adsorb more oxygen molecules to form active sites.103 The investigation of oxygen vacancy engineering is a good strategy to achieve excellent sensing performance of ECNs, as demonstrated by Kim and co-workers.104 The authors reported an engineering process to control the amount of oxygen vacancies in ZnO nanofibers aiming to improve their H2-sensing capacity. They showed that the increase on the number of oxygen vacancies led to an enhancement of the sensor response by increasing the adsorption probability of gas molecules. Sensors based on semiconductor metal oxides are usually resistive or yet conductometric. Considering n-type semiconductor metal oxides, the majority of charge carriers are electrons and generally bring oxygen adsorbed species from the synthesis processing. This results in a surface layer with a smaller concentration of electrons than the bulk, a depletion layer, and therefore, the charge transfer will be characterized by bulk conductance effects. In the case of p-type semiconductors’ metal oxides, the charge carriers are dominated by holes and the charge transfer will be characterized by superficial conductance effects.14,105 The sensing properties of ECNs depend not only on their structure but also on their composition. Additional gain in sensitivity, for example, can be obtained through the incorporation of a second constituent as a composite or hybrid material. The combination of two dissimilar materials forming a heterostructure with a shared interface and intimate contact allows the charge transfer between constituents and a band bending up to equalized Fermi levels by electron flow from the n-type ECNs to the ptype ECNs.106 Heterostructures formed by p−n composites are the most studied type of composites for gas sensors based on metal oxide semiconductor.107 The exposition of the heterostructure to the target gas (reducing or oxidizing gas) will result in resistance modulation greater than the modulation observed for materials with homointerfaces or yet for pristine ECNs.108 It is worth mentioning that

4. GAS-SENSING MECHANISM OF ECNs As previously mentioned, semiconductor metal oxides are the most common materials employed in the development of gas sensors based on variation in electrical properties.15 For this type of sensor, the sensing material is usually deposited on a substrate with integrated electrodes and the change in their electrical conductance/resistance caused by the interaction with the target gas molecules depends on the working temperature, the nature of the target gas (oxidizing or reducing), and the type of the majority charge carriers.93,109 The mechanisms responsible for ECNs’ gas sensing are not completely understood and have been a subject of ongoing discussions. This complexity arises from the various parameters that may affect the function of the ECNs gas sensors, which include the adsorption ability, physical and chemical properties, catalytic activity, and thermodynamic stability, as well as the adsorption/desorption properties of the surface.42,73 As a consequence, several aspects have been used to understand the ECNs’ gas-sensing mechanisms, including the adsorption of oxygen on the surface of semiconductor oxides, band gap, Schottky barrier contact, the catalysis-based sensing mechanism, and the heterojunction-based sensing mechanism.28,85,93 The most popular and widely accepted gas-sensing mechanism of ECNs is based on the change of resistance caused by adsorption and desorption of target gas molecules on the surface of the sensing material.110−112 Generally, upon exposure to reactive gases, the volatile species are adsorbed on the surface of the sensing material, changing the resistance of the ECN due to the surface interactions and charge transfer processes. When the sensor is again exposed to air (or to an inert environment), the gas molecules desorb from the sensor surface and the sensor recovers its original electrical resistance.113 The ability of the metal oxide surface to adsorb and react with the gas molecules depends on the physisorption and chemisorption processes. The physisorption involves the adsorption of the gas molecules on the surface of the metal oxide that dominates at low temperatures. On the other hand, chemisorption occurs when adsorbed molecules bond to the surface atoms by charge transfer.110 It is important to emphasize the different conducting materials (n- or p-type) will exhibited different sensing behaviors to the same detecting gas. For instance, upon exposure to oxidizing gases, the gas species act as electron acceptors, leading to a resistance increase for n-type semiconductors. On the other hand, for the reducing gases, the gas species act as electron donors, leading to a resistance decrease for p-type semiconductors.29 5. ECNs FOR GAS SENSING The rational design of ECNs-based materials for gas sensing is still hindered by the lack of detailed knowledge on the relation between material properties/composition and their performance. In order to assist researchers to further engineer the materials’ composition to overcome the issues related to operating conditions and sensing response parameters, a survey on ECN materials’ compositions that have been explored as active gas-sensing elements is displayed in Figure 4. Among all E


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Figure 4. (A) Survey on recent metal oxides used for the formation of ECNs as gas-sensing units (for composites each individual metal oxide was counted). (B) Number of publications related to the different ECNs’ configurations and gases/vapors analyzed. Data obtained from Web of Science for the period 2014−2019 using the keywords “electrospun ceramic nanofibers” and “gas sensor” (accessed June 2019).

Table 1. Sensing Performance of Recent (Past 3 years) ECNs-Based Gas Sensor toward Acetone configuration pure ECNs composite

doped

hybrid doped/hybrid doped/composite

ECN-based sensing material

response

operating temp (°C)

PrFeO3 SmFeO3 SnO2/ZnO NiO-WO3 MoO3-WO3 V2O5/CuO PtO2-doped SnO2 Cu-doped α-Fe2O3 CS-Pt@SnO2 Ag-decorated SnO2 Rh-doped SnO2 La2O3-doped Zn2SnO4 Eu-doped α-Fe2O3 Ca2+/Au co-doped SnO2 GO−WO3 rGO-α-Fe2O3 Pt-loaded GO-SnO2 Pt-ZnO-In2O3 SnO2/Au-doped In2O3 PdO@ZnO-SnO2

73.8 at 100 ppm 9.98 at 100 ppm 12.5 at 100 ppm 22.5 at 100 ppm 26.5 at 100 ppm 3.8 at 100 ppm 194 at 5 ppm 99.4 at 100 ppm 142 at 5 ppm 70 @ 100 ppm 133 @ 100 ppm 68 @ 100 ppm 84 @ 100 ppm 62 @ 100 ppm 35.9 @ 100 ppm 8.9 @ 100 ppm 245 @ 5 ppm 57.1 @ 100 ppm 17 @ 100 ppm 10.1 @ 5 ppm

180 140 350 375 375 440 400 164 350 160 200 200 240 180 375 375 350 300 300 400

of the ECNs reported in the literature for gas-sensing applications, the most widely used metal oxides are SnO2, ZnO, In2O3, and TiO2, as illustrated in Figure 4A. All of these materials are low cost, capable of detecting a wide range of gases, and have a wide band gap, which is beneficial for reducing background signals due to thermally activated conduction.15,27 Other key points include the high chemical and thermal stability,114 the large exciton binding energy,115 and the high intrinsic dopability.18 As previously mentioned, to enhance the sensitivity and selectivity to certain gases or to reduce the operating temperature, various ECNs’ architectures have been explored to fabricate sensing devices. The most common configurations of ECNs reported in the literature are pure, composite (more than one metal oxide is combined), doped, and hybrid (when combined with organic materials) (Figure 4B).

detection limit (ppm)

response time (s)

recovery time (s)

4 17 12 6

4 16 27 11

30 12 5 12 6 2 7 11 8 4 3 12.7 1 2 20

107

0.019

0.1 0.005 5 0.1

0.5 0.01

18 44 10 64 9 36 5 10 9 44 58 64

ref 116 117 118 119 120 121 122 123 124

111 125 126 127 128 129 41 130 112 83 131

The different types of ECNs-based gas sensors have been found to enable sensing of different gases/vapors, as illustrated in Figure 4B. Most of these works are related to the development of sensing devices with predominant responses toward acetone and ethanol. For each of these gases an overview of the sensing performance of recent ECNs-based gas sensor is given in Tables 1 and 2, respectively. The response of gas sensors is defined as the resistance ratio Rair/Rgas or Rgas/ Rair, depending on the metal oxide (p-type or n-type) and target species (oxidizing or reducing). Literature values based on different definitions were converted to facilitate comparison. 5.1. Pure ECNs. ECNs based on pure semiconducting metal oxides produced by electrospinning were the focus of the earliest reports on ECNs.147,148 However, it is still possible to find recent works reporting gas sensors based on pristine ECNs, as shown in Table 3. F


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ACS Applied Nano Materials Table 2. Sensing Performance of Recent (Past 3 years) ECNs-Based Gas Sensor toward Ethanol configuration pure ECNs

composite

doped

hybrid doped/composite

ECN-based sensing material

response

operating temp (°C)a

detection limit (ppm)

response time (s)

recovery time (s)

TiO2 α-Fe2O3 ZnO WO3@SnO2 LaMnO3/SnO2 ZnO@In2O3 α-Fe2O3/SnO2 Eu2O3−In2O3 NiO−In2O3 NiO@ZnO Ho-doped In2O3 Pd-doped α-Fe2O3 Ba-doped LaFeO3 SnO2-PPy Ag-doped ZnO-SnO2 Pd@Co3O4-ZnO

5.6 at 100 ppm 3 at 1000 ppm 78 at 100 ppm 5.1 at 10 ppm 20 at 100 ppm 31.9 at 100 ppm 22.5 at 100 ppm 68 at 100 ppm 78 at 100 ppm 15.8 at 100 ppm 60 at 100 ppm 65.4 at 50 ppm 31.3 at 100 ppm 103 at 100 ppm 129 at 100 ppm 59 at 200 ppm

450 340 RT 280 260 225 300 260 300 325 240 250 210 RT 200 240

0.7

5

9 18.5 6 3.7

0.2

3

>600 5 12 282 34 52 20 21

0.2 0.1

13 5 8 42

498 28 30 40

1 1

5 6

5 12

ref 132 133 134 135 136 137 138 139 140 141 142 143 144 145 84 146

a

RT = room temperature.

Table 3. Sensing Performance of Pure ECNs-Based Gas Sensor ECNs material ZnO

CuO WO3 α-Fe2O3 SmFeO3 PrFeO3 NiGa2O4

target gas H2 CO, NO2 ethanol ethanol, methanol, propanol acetone ethanol ethanol acetone acetone trimethylamine benzene

selective against CO, benzene, toluene, ethanol acetone, acetaldehyde, methanol

toluene, methanol, benzene NO2, CO, acetone, CO2, CH4, NH3, H2, O2 ethanol, formaldehyde, NH3, methanol acetic acid, ethanol, ammonia, DMF, methanol, benzene ethanol, acetic acid, ammonia, formaldehyde, acetone acetone, acetaldehyde, formaldehyde, nitrobenzene, CO, NH3, H2, CH4, NOx

structure

working temp (°C)a

ref

solid hollow solid solid

350 375 RT RT

104 149 134, 150 151

porous porous solid solid hollow solid porous

270 340 250 140 180 RT 260

152 133 153 117 116 154 155

a

RT = room temperature.

was exhibited by NiGa2O4 nanofibers at 260 °C. Besides, gas response sensitivity decreased for NiGa2O4 > CuGa2O4 > CoGa2O4. These results highlight the importance of the material composition considering the intended target gas. Despite the successful fabrication of pure metal oxide nanofibers, the temperature dependent sensitivity raised additional concerns about adhesion, electrical contacts, reliability, reproducibility, and costs of operation. Very few works report the use of pure ECNs as gas sensor at room temperature. As a spotlight Shankar and co-workers134 reported a comparison between ZnO nanoparticles’ and ZnO nanofibers’ performances for gas sensing without using high temperature. In order to overcome these issues, research on the design of a rich variety of metal oxides, including binary and ternary compositions, core-shell and hollow structures as well as doped, surface decorated, and hybrid structures, have been performed. 5.2. Composite ECNs. For multiphase ECNs the composition can be planned in such way to form a shared interface with a charge-depletion layer and resistance modulation. For instance, Kim et al.156 reported the fabrication by electrospinning of SnO2-NiO composite nanowebs and their NO2-sensing ability according to the Sn:Ni ratio. The

For instance, Leonardi and co-workers153 investigated the influence of the ECN microstructure on the gas-sensing performance by comparing α-Fe2O3 nanofibers with α-Fe2O3 nanoparticles. The obtained α-Fe2O3 were tested for ethanol sensing at temperatures varying from 200 up to 400 °C. The results showed gas-sensing properties dependent on the materials’ structure (for both nanofiber and nanoparticle), and a strong dependence on the operation temperature. αFe2O3 nanofibers showed faster response when compared with α-Fe2O3 nanoparticles, which indicates a better and more effective gas diffusion through the porous web-like structure combined to the large specific surface area and the different adsorbed oxygen species according to the temperature of operation. Wei and collaborators152 proposed an acetone gas sensor based on pristine WO3 nanofibers that presented excellent response and selectivity toward acetone at 270 °C. Metal spinel is another promising group of materials to be applied as gas sensor with nanofibrous microstructure. Chen and co-workers155 reported the fabrication of mesoporous MGa2O4 (M = Ni, Cu, Co) nanofibers via electrospinning process and demonstrated the sensing process is decisively influenced by the transition metal ion. All compositions were sensitive to benzene gas though the best sensing performance G


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Figure 5. (A) (i) Dynamic normalized response of xSnO2−(1 − x)NiO (x = 0.1, 0.3, 0.5, 0.7, and 0.9) composite nanoweb sensors to 1, 5, and 10 ppm NO2 gas at 300 °C and (ii) schematic illustration of the change in the energy barrier of the SnO2−NiO heterojunctions upon the introduction of oxidizing and reducing gases, respectively. Reprinted with permission from ref 156. Copyright 2018 Elsevier. (B) Schematic diagram of (i) the coaxial electrospinning setup used in this experiment and the formation process of core−shell nanofibers and (ii) ZnO@In2O3 nanofibers exposed to air and ethanol. Reprinted with permission from ref 137. Copyright 2018 Elsevier.

showed a great potential for NO2 gas monitoring with sensitive response 9 and 5.2 times higher than the pristine ZnO and SnO2 nanofibers, respectively, besides selectivity toward NO2 against ammonia, carbon monoxide, methanol, ethanol, and formaldehyde. Similarly, the fabrication of SnO2 hollow nanofibers followed by ZnO shell growth on the SnO2 nanofiber surface showed modulated response to ethanol.67 Although the heterostructures were based on the same composition, they were microstructurally organized in different ways with local and long-range heterojunction. Thus, the modulation of the response behavior observed for the heterostructures relies on the magnitude order of the intimate interaction between the shared interface, as well as on the thickness of each structure wall. Multichannel hollow nanofibers can be even more interesting with several inner channels capable of adsorbing and entrapping the gaseous molecules.160 In this way, ECNs based on multichannel SnO2 porous nanofibers loaded with PtO2 nanoparticles have been reported as an alternative to enhance the electron-depletion layer, which leads to increased relative response for acetone gas detection.122 Dense coaxial nanofibers, also designated as core−shell structures, are also a promising alternative to promote an interface junction between dissimilar materials. Core−shell nanofibers are 1D nanostructures with more than one composition, but each phase has a distinct space delimitation (Figure 5B(i)). Besides, the shared interface in core−shell nanofibers is continuous with a high length-to-width ratio. Advantages such as better control of the shared interface and the possibility of selecting just one material exposed to the target gas, but still with a shared interface, and the synergistic effect contributing to the sensing ability are the major

authors demonstrated that the sensing response could be modulated by the change on the nanofiber stoichiometry. In this sense, ECNs’ composition can be optimized in order to obtain materials more selective and sensitive to a specific target gas by a modulation of the interface junction. The versatility and modulation ability of the ECNs can be better understood when comparing Kim et al.’s work with the research reported by Li and co-workers.157 The second one also obtained NiO (p-type) nanofibers but now combined with ZnO (n-type) with several Ni:Zn ratios. The NiO-ZnO composite was optimized and showed a greater performance to detect trimethylamine, a reducing gas, instead of an oxidizing gas such as NO2 (Figure 5A). In addition to the synergistic effect between the interface junctions present in composite nanofibers, the intrinsic high surface area can be increased by porous structured nanofibers, which can improve the gas-sensing performance.119,140 In this sense, Zhang and collaborators119 prepared NiO-WO3 electrospun composite nanofibers with optimized Ni:W molar ratio and porous structure for acetone detection. The composite nanofibers’ performance was compared with that of pristine WO3 nanofibers and showed a response 2.1 times higher than solid and pure WO3 nanofibers. Moreover, the porosity, when extrapolated to the macroscale, can be interpreted as hollow and multichannel nanofibers where the target gas can reach not only the outer active surface but also the inner active surface, as reported by Choi and collaborators.158 Besides the surface area increase, hollow nanofibers can present unique properties such as transport charge associated with confinement effects and 1D transport phenomena.158 ZnO-SnO2 hollow composite nanofibers with different Zn percentage were prepared and tested for NO2 gas sensing.159 The 1D hollow nanostructure H


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Figure 6. (A) Schematic representation of the NO2-sensing mechanism for (a) undoped WO3 nanofibers and (b) Ag-doped WO3 nanofibers. Reprinted with permission from ref 163. Copyright 2018 Elsevier. (B) (i) HRTEM images of ZnO/Au heterojunction nanofibers and (ii) sensors response of ZnO nanofibers and ZnO/Au heterojunction nanofibers. Reprinted with permission from ref 164. Copyright 2018 John Wiley and Sons.

performance with higher response to ethanol than any of those structure configurations. Such behavior was associated with the intimate contact provided at the shared interface with radial modulation of the shell depleted layers and the electron flow triggered by the SnO2 and WO3 different work function. 5.3. Doped ECNs. Another strategy that contributes to the formation of ECNs with outstanding gas-sensing performance is the incorporation of dopants into the nanofibers’ structure. Noble metal and substitutive metallic cations have been investigated as ECN dopants in order to improve the sensor performance.84,90,128 Feng and collaborators162 synthesized a sensor by electrospinning process and investigated the sensing ability toward methanol. Since no other phase was observed in the XRD analysis, the authors managed to prove the successful doping of NiO nanofiber without phase segregation. The best sensing response to methanol detection was provided by the In-doped NiO nanofibers with 3% In3+ (molar ratio). The doped ECN performance was compared with that of pure NiO nanofibers, which suggests a great potential of doped ECN for gas sensing. The sensing mechanism proposed regards the methanol gas molecules adsorption and modulation of the depletion layer increasing the resistance. The In3+ addition results in a lower concentration of holes, which in turn leads to higher variation of resistance when in the presence of reducing gas such as methanol. In other words, modulating the density of the charge carriers by doping the ECNs provides materials with high performance for gas sensor. Considering noble metal dopants, the microstructure will be composed by ECNs combined with metallic nanoparticles,

attracting features for core−shell nanofibers in gas-sensing applications. Core−shell ZnO@In2O3 nanofibers fabricated by coaxial electrospinning process with well-defined structure have been reported.137 The authors were able to demonstrate the potential of the heterostructure for ethanol sensing, and the modulated response was ascribed to the conversion of the In2O3 shell in a complete depletion layer (Figure 5B(ii)). Gao and co-workers161 fabricated a formaldehyde sensor based on a core−shell ECN with Co3O4(p-type) as core and ZnO (ntype) as shell. The authors showed the superior performance of the core−shell ECNs when compared to the pristine ECNs. Moreover, not just the core−shell configuration compared to pristine ECNs was examined but also the core−shell composition variation where Co3O4-ZnO showed a formaldehyde-sensing response two times higher than that of ZnOCo3O4. This result indicates the influence of the electrons and holes flow direction on the p−n heterojunction and therefore determines the behavior type of the sensing response. Besides, Co3O4-ZnO presented a p-type behavior even having an n-type shell, showing no dependence on the type of shell but relying on the synergistic effect between the p−n heterojunction. In order to compare the performance of pristine with composite and core−shell architectures, Li and collaborators135 obtained SnO2, WO3, SnO2WO3, and SnO2@WO3 nanofibers. Through the fabricated ECNs, the authors reported an enhancement of the sensing response for the composite composition with 25% SnO2 and 75% WO3 when compared with pristine SnO2 and WO3 nanofibers. However, the SnO2@ WO3 core−shell-like heterostructure presented even better I


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Figure 7. (A) Schematic illustration of the electrospinning process for Pt-functionalized SnO2 nanosheet-assembled nanofibers. Reprinted with permission from ref 130. Copyright 2018 Royal Society of Chemistry. (B) Selective characteristics of (a) rGO (0.01 wt %)−SnO2 toward H2S at 200 °C and (b) rGO (5 wt %)−SnO2 toward acetone at 350 °C with respect to interfering gases including ethanol, toluene, carbon monoxide, ammonia, and pentane. Reprinted with permission from ref 167. Copyright 2014 American Chemical Society. (C) (a) FEG-SEM image of the electrospun nanofiber and (b) low- and (c) high-resolution FEG-SEM images of the tube-in-tube SnO2. (d) Low- and (e) high-resolution TEM images of the tube-in-tube SnO2. (f) Selected area electron diffraction pattern of the tube-in-tube SnO2. (g) FEG-SEM and (h) TEM and HRTEM images of the tube-in-tube PPy/SnO2. Reprinted with permission from ref 170. Copyright 2017 Royal Society of Chemistry.

exhibited enhanced NO 2 gas-sensing properties when compared with ZnO nanofibers due to the electron transfer process between Au and ZnO at the interfaces and the presence of surface defects in the nanograins (Figure 6B(ii)). 5.4. Hybrid ECNs. Among the various sensing materials that have been explored to fabricate hybrid ECNs-based gas sensors, 2D carbon nanomaterials, including graphene oxide (GO) and reduced graphene oxide (rGO), have attracted intense attention owing to their distinct gas-sensing capability.30,129 The superiority of graphene for the development of gas-sensing devices relies on two basic factors associated with its 2D dimensions: (i) the ultrahigh surface area per atom and (ii) high electron transport along the graphene base-plane.25 Moreover, graphene exhibits excellent anchoring ability for gas molecules and can act as a substrate for other nanomaterials, enabling the fabrication of novel graphene-based nanohybrids with superior gas-sensing properties.165 The (i) electron transfer, (ii) bonding chemistry, and (iii) charged defects between components of a graphene/ ECNs heterostructure are reported to be critical parameters that can significantly affect the sensing performance of hybrid materials.166

once it is not energetically favorable for these dopants to be inserted in the crystal lattice. Jaroenapibal et al.163 reported the fabrication of Ag-doped WO3 nanofibers with Ag addition ranging from 1 up to 10% in molar ratio. The WO3 nanofibers microstructure were formed by WO3 nanoparticles combined with small Ag particles in the metallic state with a thin layer of Ag2O onto the surface, as indicated by XPS analysis. As expected, the Ag-doped WO3 nanofibers showed an improved performance for NO2 sensing when compared with pristine WO3. The proposed mechanism suggested the electron injection from Ag to WO3, expanding the depletion layer compared to pristine WO3 (Figure 6A). Since NO2 is an oxidizing gas, when in contact with Ag-doped WO3 nanofibers, it will improve the depletion layer modulation. Besides, the additional Ag2O also contributes as a chemical sensitizer when in the presence of NO2. The enhanced response of doped ECN with noble metals is attributed to both electronic and chemical modulation of the material. Similar behavior was reported by Ponnuvelu and co-workers164 for ZnO/Au heterojunction nanofibers. The nanofibers exhibited a typical nanograined structure with 1D morphology consisting of two distinct lattice fringes corresponding to Au [111] and ZnO [101], as illustrated in Figure 6B(i). The ZnO/Au nanofibers J


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ACS Applied Nano Materials Recently, Kim et al.130 reported the fabrication of ultrathin 2D SnO2 nanosheets upon electrospinning of Sn2+ ions coated GO solution and subsequent heat treatment. After subsequent pyrolysis, highly porous oxide nanofibers were obtained and functionalized with Pt nanoparticles (Figure 7A). The Ptfunctionalized SnO2 nanosheet-assembled nanofibers exhibited high acetone response at 350 °C (Rair/Rgas = 79.4 at 1 ppm), excellent selectivity, and fast response speed (12.7 s). The exceptional acetone-sensing performance of the hybrid material, when compared with their counterparts, was attributed to three distinct benefits: (i) a high surface area and broad pore size distribution arising from the coexistence of 1D and 2D structures; (ii) numerous reaction sites derived from the ultrasmall thickness of SnO2 NSs and small crystallite size of SnO2(

Electrospun Ceramic Nanofibers and Hybrid-Nanofiber Composites

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