**3.2 Polymer-assisted synthesis of nanofibers**

Metal oxide nanofibers can be synthesized by chemical bottom-up routes like the ones described in the previous sub-section, as well as by top-down routes either mechanical or spinning methods [80–82]. Among the latter, electrospinning is the most widely used method for the production of metal oxide nanofibers, mainly due to its high simplicity and ease of use, low-cost setup, ability to mass-production of continuous fibers, and flexibility in controlling the diameter, morphology, structure, and alignment of the fibers [82–84]. The typical setup for electrospinning is depicted in **Figure 4a**) [85]. This is rather simple and operates at ambient conditions, although the mechanism of electrospinning is complex [86, 87]. An

#### **Figure 4.**

*(a) Schematic illustration of the synthesis of SnO2 nanofibers by (step 1) electrospinning of the precursor solution followed by (step 2) high-temperature sintering of the as-spun fibers; adapted with permission from [85], Copyright 2018 Authors, licensee MDPI. (b) SEM micrograph of the hollow SnO2 nanofibers resulting from (a); reprinted with permission from [97], Copyright 2013 eXpress Polymer Letters.*

electrospinning system consists of three major components: a high voltage supply, a spinneret (e.g., glass capillary tube, metallic needle, pipette tip) and a grounded collector. The high voltage source injects the charge of a given polarity into a polymer solution, which is fed (e.g., syringe pump) at a constant rate to the spinneret. An electric field is thus established between the spinneret and the collector and when it reaches a critical value, the liquid reaching the spinneret tip forms a cone (Taylor cone) that emits a liquid jet through its apex. This charged liquid jet is stable only at the tip of the cone and undergoes an unstable and rapid stretching and whipping process downstream of the spinneret, which leads to the formation of a long and thin thread. As the liquid jet is continuously elongated and the solvent is evaporated, its diameter can be reduced from hundreds of micrometers down to the sub-micrometer scale. The electrospun polymer fibers are deposited randomly on the plate due to the attraction of the collector placed in front of the spinneret.

The diameter, morphology, and structure of the electrospun fibers are key factors that need to be controlled for practical applications [88, 89]. They depend on a multitude of parameters related to the solution, setup and electrospinning process [90–94]. Target fibers can be obtained by properly choosing the polymer and the solvent and their concentration in the solution, thereby adjusting the electrospinning-relevant solution properties such as viscosity, surface tension, and electrical conductivity. The setup orientation (e.g., vertical, horizontal), spinneret type (e.g., needleless, single-needle, coaxial-needle) and nozzle diameter, solution feed rate, applied voltage, and spinneret-to-collector distance tremendously influence the fiber features. These features are also affected by the evaporation conditions such as ambient temperature and humidity.

Nanofibers of the MOS used for chemical sensing can be produced by electrospinning of a solution containing a polymer (e.g., polyvinylpyrrolidone PVP, polyvinylalcohol PVA, polyvinylacetate PVAc, polyethylene oxide PEO) and an inorganic precursor (e.g., acetates, nitrates, chlorides) in a solvent (e.g., deionized water DIW, ethanol EtOH, isopropyl alcohol IPA, dimethyl formamide DMF) followed by a sintering step, also known as annealing or calcination, at high temperature [84, 95, 96]. After obtaining the electrospun polymer/metal composite fibers, the polymer is eliminated by the sintering process. The evaporation of residual solvent and water from the fibers occurs in the first place, at low temperature. As the temperature rises, the polymer decomposes gradually and fiber shrinkage takes place. Simultaneously the inorganic precursor undergoes a complex transformation including outwards radial diffusion, nucleation, condensation, crystallization and oxidation processes to finally form metal oxide nanograins aligned along the preceding as-spun fibers [84]. A simplified schematic of the process is shown in **Figure 4a**) [85], while **Figure 4b**) displays the SnO2 nanofibers obtained by sintering the as-spun fibers [97]. The nanofibers have diameters of less than 100 nm that are rather uniform along the entire length of the nanofibers, which are in the order of hundreds of micrometers. They are hollow nanofibers and have an ultra-thin porous granular wall consisting of SnO2 nanograins.

Sintering parameters such as the heating rate, sintering temperature and time, cooling rate, and atmosphere largely influence the diameter, structural morphology, crystallinity and grain size of the MOS nanofibers [93], which in turn determine their sensing performance [98–100]. Hollow nanofibers have about twice the active surface area of solid nanofibers of the same diameter, which increases the sensor response, as gases can interact with both the outer and inner surface of the nanofibers [101]. While MOS nanowires are single-crystalline with no noticeable grains, electrospun MOS nanofibers are composed of polycrystalline grains. Higher sintering temperatures or longer sintering times result in better crystallinity, which in turn leads to higher sensor response. Also, the grain size increases with the increasing

### *One-Dimensional Metal Oxide Nanostructures for Chemical Sensors DOI: http://dx.doi.org/10.5772/intechopen.101749*

sintering temperature and time, but the sensor response decreases with the increasing grain size due to the lower surface area. Hence, it is necessary to optimize the size and crystallinity of the grains in the MOS nanofibers simultaneously to attain superior sensing behavior [102–107]. The diameter and wall thickness of the hollow MOS nanofibers increase with the increasing concentration of polymer and inorganic precursors in the solution, respectively [108]. Reducing both the diameter and wall thickness also improves the sensing properties of the electrospun MOS nanofibers [98]. A decrease in the diameter of the nanofiber contributes to decreasing both the grain size and the time of gas diffusion into the nanofiber, resulting in enhanced sensor response and reduced response time. Nevertheless, there is a limit in the process of enhancing sensor performance by decreasing the grain size, since an excessive decrease of grain size (<10 nm) leads to a loss of structural stability and, as a consequence, to changes in both surface and catalytic properties of the material. Moreover, the porosity of the wall determines the accessibility of the gases to the inner surface of the hollow fibers, and several strategies have been adopted to increase the porosity of the electrospun MOS nanofibers. One strategy is to remove the polymer from the as-spun polymer/metal fibers by exposing the fibers to RF oxygen plasma before sintering [109, 110], as can be seen in **Figure 5**. This technique allows tuning the porous structure of the hollow MOS nanofibers by controlling the etching power and time. Finally, in a recent work 1D hierarchical structures of ZnO have been prepared by growing ZnO nanowires over electrospun polyacrylonitrile (PAN) fibers loaded with Au nanoparticles, as seeds for catalytic VLS growth [111]. **Figure 6** shows the resulting ZnO cactus-like structures, whose surface area is far larger than that of electrospun ZnO nanofibers of the same diameter.

#### **Figure 5.**

*SEM micrographs of electrospun PVA/Sn fibers (a) without post-treatment and (b) after exposure to RF oxygen plasma, (c) highly porous hollow SnO2 nanofibers obtained by sintering of the PVA/Sn fibers in (b). Reprinted with permission from [110], Copyright 2009 Elsevier.*

#### **Figure 6.**

*(a) SEM micrograph of a ZnO nano-cactus like structure obtained by VLS growth of ZnO nanowires over electrospun fibers of polyacrylonitrile (PAN) loaded with Au nanoparticles. (b) Higher magnification SEM micrograph of the dashed area in a), showing Au nanoparticles at the tip of ZnO nanowires (dotted circles) and EDX spectrum of the ZnO structure (inset). Reprinted with permission from [111], Copyright 2021 Elsevier.*

#### **3.3 Functionalization of 1D metal oxide nanostructures**

Functionalization or modification with second-phase constituents (modifiers or additives) can strongly change the physical and chemical properties of the 1D MOS nanostructures. Tailoring the properties (e.g., shape, size, load, dispersion) of the additives is a very challenging task because not all methods allow for the control of their properties and homogeneous dispersion over the 1D MOS nanostructures. The functionalization or modification of 1D MOS nanostructures with second-phase materials may be of the decorative-type, only at surface level, when incorporating low amounts of the additives at the surface. It can also involve single mixtures, when mixing 1D nanostructures and the additives randomly, or doping, when additive atoms incorporate in the intrinsic material structure [25, 112]. Generally, the methods to functionalize or modify 1D MOS nanostructures fall into two categories: one-step processes in which the additive materials are incorporated simultaneously during the 1D nanostructure formation or multiple-step processes in which the additives are incorporated over the pre-synthetized 1D MOS nanostructures [113]. In both cases, the incorporation may involve a precursor for the targeted additive or pre-formed particles, as shown in **Figure 7**. Routes 1 and 2 (representing one-step processes) may be enabled by sol-gel, hydrothermal synthesis, CVD, and electrospinning. Routes 3 and 4 (representing multiple-step processes) rely on techniques, such as dip- or spin-coating, to introduce pre-formed additives or a broad type of techniques that can incorporate the additive from a precursor or as ions. For instance, high energy ion implantation, immersion in solutions containing the additive precursor followed by a photocatalytic reduction or heat treatment, sputtering, hydrothermal synthesis, CVD including atomic layer deposition (ALD), amongst others.

1D heteronanostructures have gained great attention due to their hybrid properties, which may induce synergies between the host material (MOS) and the guest material (modifier or additive), resulting in improved and/or new attributes for the chemical detection [114–116]. In the first place, p-type MOS of transition metals (e.g., CuO, NiO, Fe2O3) and noble metals (e.g., PdO, Ag2O) have been applied to develop ultrasensitive chemical sensors by tuning the electrical properties of the n-type MOS by forming n-p heterojunctions. Also, n-n heterojunctions may lead to enhanced sensing performance of the nanostructures composed of two n-type MOS with different work functions (e.g., SnO2-ZnO, WO3-SnO2, TiO2-SnO2, SnO2-In2O3), as compared to single-phase MOS nanostructures. In addition, the combination of MOS and graphene may significantly improve the performance of MOS gas sensors

**Figure 7.**

*Routes to the functionalization or modification of 1D MOS nanostructures with second-phase materials (modifiers or additives); adapted with permission from [113], Copyright 2006 American Chemical Society.*

#### *One-Dimensional Metal Oxide Nanostructures for Chemical Sensors DOI: http://dx.doi.org/10.5772/intechopen.101749*

at room temperature [48, 117]. Graphene is a family of two-dimensional (2D) nanomaterials (e.g., pristine graphene PG, graphene oxide GO, reduced graphene oxide rGO, graphene quantum dots GQD, graphene nanoplatelets GnP) that are obtained from natural graphite or synthesized chemically from organic compounds, and have different morphology, physical, chemical, and electronic properties. Among them, rGO is the best choice for gas sensing applications because it has oxygen functional groups, defects and vacancies on its surface, which favor the adsorption of gases. Moreover, rGO behaves as a p-type semiconductor and is stable at high temperature [118, 119]. When added to 1D MOS nanostructures, rGO nanosheets can increase the overall sensing surface and adsorption sites, and form n-p or p-p type heterojunctions with the MOS grains, thereby modulating the resistance of the MOS gas sensors. Also, noble metals (e.g., Au, Ag, Pt, Pd) and transition metals (e.g., Ni, Cu, Co) act as effective modifiers or additives, particularly due to their catalytic properties.

The functionalization or modification of 1D MOS nanostructures based on nucleation and growth processes usually rely on two-step processes, in which the second-phase material is incorporated in a subsequent step after the synthesis of the 1D nanostructure; routes 3 and 4 in **Figure 7**. Hence, the methods for the functionalization step are varied. They may include, for instance, sputtering as in recent reports that showed the incorporation of DC pulsed sputtered Au nanoparticles over the surface of hydrothermally synthesized ZnO nanowires, as displayed in **Figure 8a**) [120]. In this method, the size and density of the Au nanoparticles decrease as the sputtering pressure increases (e.g., from 5 to 20 mTorr) due to the dependency of the mean free path and rate of gas phase collisions on the process pressure. The routes to functionalize 1D nanostructures in a second-step process also involve a broad variety of CVD methods. Among them, for instance, aerosolassisted (AA) CVD has demonstrated to be useful to incorporate both metals and MOS nanoparticles over 1D nanostructures. This method allowed for the incorporation of dispersed nanoparticles based on n-type or p-type MOS from metals such as Pd [121], Ni, Co, or Ir [122]. It also allowed for the formation of core-shell 1D nanostructures based on WO3 nanowires covered by a Ce2O3 thin film [123]. Similarly, flame-assisted CVD has shown to functionalize 1D nanostructures including SnO2 nanowires with Au and Pd nanoparticles [124]. This method has also been used to modify the surface of SnO2 nanowires with an amorphous carbon layer [125]. Other methods for functionalization in a second-step may also combine the merits of several techniques, including sol-gel, dip-coating, and flame spray pyrolysis, as is the case of the sol-flame method. This method incorporates the second phase

#### **Figure 8.**

*(a) TEM micrograph of a ZnO nanowire functionalized with sputtered Au nanoparticles; reprinted with permission from [120], Copyright 2021 Elsevier. (b) Functionalization of CuO nanowires by sol-flame: HRTEM (left) and TEM (right) micrographs of a CuO nanowire functionalized with Co3O4 nanoparticles; reprinted with permission from [126], Copyright 2013 American Chemical Society.*

#### **Figure 9.**

*(a) SEM micrograph of WO3 nanowires functionalized with noble metal nanoparticles (Au, Pt) in a singlestep process by AACVD. HRTEM micrographs of WO3 nanowires functionalized with nanoparticles of (b) Au and (c) Pt, with insets showing the size distribution and lattice fringes of the nanoparticles; reprinted from [128].*

material by dip-coating the nanowires with a sol-gel precursor solution and annealing them over a flame for a few seconds. During the flame treatment, the metal salt precursor decomposes chemically to the final metal or MOS and nucleates locally over the nanowire. An example of this process used to functionalize CuO nanowires with Co3O4 is displayed in **Figure 8b**) [126].

The functionalization or modification of 1D MOS nanostructures based on nucleation and growth processes can also be achieved by one-step processes. However, their use is less common, despite the advantage of reducing processing steps. Examples of 1D nanostructures functionalized by a one-step process include those achieved by the AACVD of two metal precursors from one-pot simultaneously [127]. In this method, the precursor leading the formation of nanowires is in a higher concentration than the precursor for the modifier. **Figure 9** displays examples of the WO3 nanowires functionalized with Au and Pt nanoparticles obtained by this method [128]. The functionalization of nanowires with MOS from metals such as Fe [129] and Cu [31] was also achieved by this method.

There is much less published work on the functionalization or modification of electrospun MOS nanofibers for application in chemical sensors than on nanowires. Most works chose to incorporate the additives or their precursors into the solution containing the polymer and the inorganic precursor of the MOS (i.e., routes 1 and 2 in **Figure 7**). Thus, for example, composite nanofibers are prepared by dissolving inorganic precursors of the involved MOS in suitable solvents and mixing the solutions with the polymer solution, usually by magnetic stirring. Then, the inorganic precursors are distributed uniformly in the polymer by electrospinning and the metals are oxidized upon sintering of the electrospun polymer/metal fibers. Intimate contact between the MOS nanocrystals is achieved in the composite nanofibers [130–133]. Another method uses the coaxial-electrospinning configuration [84, 134], for which polymer solutions are prepared with each of the inorganic precursors separately. The solution with the precursor of the main metal oxide leaves the spinneret through a central circular nozzle, while the solution with the precursor of the second metal oxide exits the spinneret through an annular nozzle that surrounds and is concentric to the circular nozzle, as depicted in **Figure 10a**) [135]. After sintering of the electrospun fibers, composite nanofibers with a coreshell structure are obtained, in which the two metal oxides occupy distinct zones with a well-defined interface [136–138], as can be seen in **Figure 10b**).

The same strategy has been adopted for the functionalization of electrospun MOS nanofibers with additives such as graphene (rGO) and metals. In this case, the rGO flakes and metal nanoparticles are dispersed or their precursors are dissolved in a liquid (e.g., DIW, EtOH, IPA), usually by ultrasonic agitation. The colloidal dispersions or solutions so obtained are added to the solution with the polymer and the *One-Dimensional Metal Oxide Nanostructures for Chemical Sensors DOI: http://dx.doi.org/10.5772/intechopen.101749*

**Figure 10.**

*(a) Layout of a typical spinneret used for coaxial electrospinning; adapted with permission from [135], Copyright 2019 Authors, licensee MDPI. (b) TEM micrograph of an In2O3-SnO2 core-shell nanofiber obtained by sintering of coaxially electrospun PVP/In-PVP/Sn fibers; reprinted with permission from [138], Copyright 2016 American Chemical Society.*

inorganic precursor of the host MOS and, then, magnetically stirred until a homogeneous electrospinnable solution is achieved. As an example, **Figure 11** shows TEM images of an electrospun nanofiber of SnO2 loaded with rGO [139]. Double-shell hollow nanofibers are usually obtained, with the rGO nanosheets on top of the MOS nanograins [139–143]. Generally, it has been found that the rGO-loaded MOS nanofibers are more sensitive to specific gases and that the optimal operating temperature (i.e., the temperature at which the sensor response reaches a maximum) is lower than that of the pure MOS nanofibers. This improved sensing behavior is attributed to the formation of local n-p or p-p (MOS-rGO) heterojunctions.

Some authors have attempted to further improve the gas sensing capabilities of electrospun single-phase MOS nanofibers [144], MOS composite nanofibers [145, 146], and rGO-loaded MOS nanofibers [147] by functionalization of the nanofibers with metal nanoparticles. For this purpose, they chose also the routes 1 and 2 in **Figure 7**, leading to the dispersion of the metal nanoparticles, formed *in-situ* or pre-formed, in the solution with the polymer, inorganic precursors, and eventually rGO. Sensors based on hybrid nanofibers composed of ZnO, rGO and nanoparticles of Au or Pd prepared by electrospinning showed enhanced sensitivity towards reducing gases and volatile organic compounds, for the pure and rGO-loaded ZnO nanofibers [147].

#### **Figure 11.**

*(a) Low-magnification and (b) high-magnification TEM micrographs of an rGO-loaded SnO2 nanofiber obtained by sintering of an electrospun PVA/Sn/rGO fiber; reprinted with permission from [139], Copyright 2015 American Chemical Society.*

It was proven therefore the synergistic combination of the catalytic effects of the noble metal nanoparticles and the hybrid sensing mechanism, which combines the effects of radial resistance modulation, intergrain (ZnO/ZnO) modulation and local n-p (ZnO-rGO) heterojunctions. Moreover, it has been observed that there is an optimal value of the load of rGO (<1 wt%) and metal nanoparticles (1–4 wt%) in the nanofibers, above which the gas sensing performance of the hybrid nanofibers does not show further improvement or starts degrading. This result is attributed to the agglomeration of rGO nanosheets and metal nanoparticles in the polymer, resulting in the formation of agglomerates on the surface of the nanofibers and hence in an increased density of p-p (rGO-rGO) heterojunctions and metal-metal contacts, in detriment of n-p or p-p (MOS-rGO) heterojunctions and metal-MOS contacts. The electrospinning of polymer solutions containing nanomaterials is very challenging as to achieve an even distribution of the nanomaterials in the polymer fibers, since the large specific energy of the solution promotes the agglomeration of the nanomaterials [148, 149]. To overcome this problem, the nanomaterials can be loaded onto the surface of the fibers either after electrospinning or after sintering the as-spun fibers, i.e., routes 3 and 4 in **Figure 7**. This approach however adds complexity and costs as it introduces an additional process step (e.g., sputtering, ALD) for which dedicated equipment is required [150, 151].
