**2. Sensitive materials**

The heart of a chemical sensing device is the sensing element or sensor, which consists of two components: a receptor and a transducer. The receptor is the sensitive or active material that has an affinity for and interacts with a specific analyte, stimulating the transducer and causing a change in some physical or chemical property of the material that is ultimately converted into an electrical signal. The analyte may be chemical species (e.g., molecules, ions) or biological species (e.g., microorganisms, biomolecules) in the gas or liquid phase. The receptor function may rely on various principles: physical, where no chemical reaction takes place; chemical, in which a chemical reaction with the participation of the analyte gives rise to the analytical signal; and biochemical, when a biochemical process involving the analyte and a biological recognition element is the source of the analytical signal. The latter is called a biosensor, which is regarded as a subgroup within the group of chemical sensors. According to the operating principle of the transducer, chemical sensors may be classified as optical, electrochemical, electrical, gravimetric, magnetic or thermometric [27]. In electrically-transduced sensors, the signal arises from the changes in the properties of the sensitive material such as conductivity, work function, or permittivity. These changes are converted into variations in electrical parameters of the sensor such as capacitance, inductance or resistance and finally into changes in the device current or voltage. Electrically-based sensors are one of the most investigated chemical sensors due to their simplicity, portability, compatibility with standard electronics, non-line-of-sight detection, the capability of continuous monitoring and potential for wireless transmission.

#### **2.1 Metal oxide semiconductors**

MOS has been used as primarily sensitive materials in conductometric chemical sensors due to their outstanding physical and chemical properties. MOS can be produced by a large number of cost-effective synthetic methods, have shown to be active to detect chemical analytes, and their energy band alignment has proved to be suitable to immobilize biomolecules (e.g., enzymes, antibodies, DNA) [2, 18]. MOS are classified according to their conductivity as n-type and p-type, in which the charge carriers are electrons and holes, respectively. N-type MOS (e.g., SnO2, ZnO, TiO2, In2O3, WO3, V2O5) are the most representative materials for sensing gases and bioanalytes. Some of them are already commercially used, as it is the case in gas sensors, due to their higher sensitivity compared to p-type MOS (e.g., NiO, CuO, Co3O4, Cr2O3, Mn3O4) or other MOS that present both n- and

**Figure 1.**

*Schematic illustration of the sensing mechanisms in an n-type metal oxide semiconductor: (a) Interaction with ambient oxygen and with a reducing gas and variation of the sensor resistance; adapted with permission from [32], Copyright 2012 Authors, licensee IntechOpen. (b) Electron depletion layer at the grain surface and intergrain barrier potential; adapted with permission from [33], Copyright 2012 Authors. (c) Hydrogen detection mechanisms in a metal oxide semiconductor functionalized with metal nanoparticles; adapted with permission from [51], Copyright 2018 Elsevier.*

p-type behavior (e.g., Fe2O3, HgO2) [28, 29]. Also, n-type MOS are thermally stable and have the possibility to work at lower partial pressure, in contrast to p-type MOS which are less stable due to their tendency to exchange lattice oxygen easily with air. Another practical reason for the use of n-type MOS in conductometric sensors is related to the preferred direction for resistance change during detection of reducing gases (the vast majority of gaseous analytes), which simplifies the peripheral electronics for measurements and improves the reproducibility of the output signal. Nonetheless, p-type MOS should not be underestimated as sensitive materials, as significant improvement in sensor performance can be achieved by incorporating p-type MOS with commonly used n-type MOS [30]. As an example, p-type MOS have been used as good catalysts to promote selective oxidation of various volatile organic compounds. Moreover, the distinctive oxygen adsorption of p-type MOS may be used to design high-performance gas sensors that show low humidity dependence and rapid recovery speed [31].

**Figure 1a** and **b** illustrate the sensing mechanisms in an n-type MOS [32, 33]. The molecules of the ambient oxygen are adsorbed and then ionized by capturing electrons from the conduction band. Thus, an electron depletion layer of a given width, known as Debye length, forms at the surface of the MOS grains and a potential barrier develops at the boundaries of adjacent grains, caused by the negative surface charge due to the adsorbed oxygen ions. As a result, the density of free electrons is reduced and the electron conduction is hindered, increasing the sensor resistance. In the presence of a reducing gas (electron donor), the gas molecules react with the adsorbed oxygen ions, which are released to the ambient, while the trapped electrons return to the conduction band. Then, the density of free electrons increases and the intergrain barrier potential is reduced, resulting in a decrease in the sensor resistance. Many optimization strategies aim at modulating or triggering variations in the electron depletion region or barrier potential in a controlled manner to improve the receptor function in chemical sensors based on MOS.

#### **2.2 Functionalized metal oxide semiconductors**

Since the efficiency of the chemical receptor material is surface-dependent, previous research on MOS sensors proved that MOS with sizes within the Debye

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

length, typically of the order of 2–100 nm, are attractive in chemical sensing as they provide higher surface-area-to-volume-ratio, as compared to bulk materials. Also, it was found that nanostructures (e.g., 1D MOS) can provide specific crystal facets and electronic properties to the surface that enhance the performance of the receptor [34]. Further, the functionalization or modification of MOS nanostructures by loading, doping, surface-decoration or hybridization with second-phase constituents (e.g., noble metals, other MOS, carbon-based materials) showed other ways to enhance and extend the capabilities of the receptor [35, 36]. Similarly, in biochemical sensors, the functionalization of the surface by immobilizing biomolecules that act as biological recognition elements of specific analytes (e.g., glucose, urea, cancer cells, viruses) is mandatory to sensitize the MOS surface [2, 18, 37].

The rationale for these improvements or further sensitization is generally connected with an increase in the density of active sites (e.g., defects, oxygen vacancies) or charge carriers (doping effect). Also, it is related to the catalytic activation (chemical sensitization) and the formation of interfaces (electronic sensitization), either metal-semiconductor or semiconductor-semiconductor interfaces. The latter is known as heterojunctions as they involve two dissimilar semiconductors, whilst the combination of multiple heterojunctions together in a system is called a heterostructure. The materials chosen for these interfaces dictate the principles of sensing; for example, in gas sensing, adsorption, reaction, and electronic behavior. On one side, the intimate electrical contact at the interface between the two components equilibrates the Fermi level across the interface to the same energy, usually resulting in charge transfer and further extending the region of charge depletion/accumulation. On the other side, the mix of the two components leads to synergistic behavior. This means that each component serves a different purpose that is complementary to the other, so that the synergistic effect of the two-component system is greater than the effect of each element. The synergistic effect is possible due to three common features: geometric effects, electronic effects, and chemical effects. These are the basis for the improved sensing properties such as enhanced sensitivity and recovery speed or reduced operating temperature of the MOS heteronanostructures [38–42]. These approaches are also used to improve further the selectivity, particularly towards gas analytes. The usual combination includes wide bandgap MOS of n-type as the host material and second-phase constituents commonly chosen from noble metals (e.g., Pt, Pd, Ag, Au) [43–45] or other transition metal oxides (e.g., p-type: NiO, CuO, Cr2O3, Fe2O3, Co3O4, Mn3O4; n-type: SnO2, ZnO, In2O3, WO3, TiO2) [46, 47]. Recent combinations also make use of carbon-based materials (e.g., carbon nanotubes, graphene) [48, 49] and two-dimensional (2D) inorganic materials (e.g., transition metal dichalcogenides TMDC, transition metal carbides and nitrides MXenes, phosphorene) [50].

Generally, the modifiers or additives (i.e., metals, MOS, rGO) enhance the sensing properties of the host MOS by surface- (chemical sensitization) and/or interface- (electronic sensitization) dependent effects. As an example, **Figure 1c**) illustrates the synergistic mechanisms, both chemical and electronic, enabling the detection of hydrogen by a MOS functionalized with metal nanoparticles [51]. The chemical sensitization includes spill-over (i.e., the enrichment of the MOS surface with reactive species through catalysis), whereas the electronic sensitization involves Fermi level control (i.e., changes in the chemical state of the additive with active species). To date, there is no clear evidence of whether any of these mechanisms are superior. Nevertheless, the literature indicates the need to combine both mechanisms to induce a better performance of MOS. The possibility of tuning the sensor performance via chemical and electronic sensitization depends strongly on the characteristics (size, shape, distribution, composition, oxidation states) of the MOS and additives forming the heteronanostructure and the formation of an

intimate electric contact at the interface of the components. The optimal combination of components significantly improves the selectivity and sensitivity of MOS sensors to specific gases, especially for sensors operated at room temperature.

In summary, MOS nanomaterials modified with second-phase materials and controlled interfaces are essential for sensing chemical species more efficiently. Among several nanoscale materials, 1D structures have been shown to be most relevant to chemical sensing. They are projected as potential structures for molecularlevel sensing, particularly when integrated as single elements. Below are discussed the most common synthetic methods to achieve 1D MOS nanostructures and their integration with appropriate transducers for their application as chemical sensors.

#### **3. Synthesis of 1D metal oxide nanostructures**

Among a host of 1D nanostructures, nanowires and nanofibers bring the greatest potential for use in the next generation of chemical sensors based on MOS [52–60]. Both nanomaterials have a high specific surface area and a large surface-area-to-volume ratio. However, their aspect ratio, crystallinity and surface properties may differ markedly because of the different synthesis techniques and conditions used, which in turn result in different chemical sensing performances. The techniques for the synthesis of 1D MOS structures can be classified in two general approaches: top-down and bottom-up technologies. Top-down technologies are subtractive technologies that rely on microfabrication methods, mainly lithography and etching processes, to reduce the lateral dimensions of a MOS film to nanometer size. Usually, the 1D nanostructures produced by this approach are amorphous or polycrystalline structures. Top-down technologies are well developed in the semiconductor industry, but need expensive equipment limiting their broad application in the academic sector. Bottom-up technologies, in contrast, are additive technologies that consist of the assembly of molecular building blocks that lead to the formation of nanostructures. Bottom-up technologies are generally enabled by vapor- or solution-based techniques and are considered cost-effective solutions for large-scale production of 1D nanostructures [24]. The nanomaterials produced by this approach may have monocrystalline or polycrystalline characteristics depending upon the specific processing steps of the vapor- or solution-based techniques employed.

Different routes are commonly used to synthesize 1D MOS nanostructures [61–63]. Whist chemical vapor deposition (CVD) is one of the most representative vapor-phase methods for forming 1D nanostructures; hydrothermal synthesis and electrospinning-assisted synthesis are the most representative amongst the liquid-phase methods. However, CVD and hydrothermal synthesis share common mechanisms for 1D structure formation based on nucleation and growth processes, whereas 1D structures formed by electrospinning rely on the generation of guiding polymer-based fibers using an electrostatic field to shape the 1D structure. The following sub-sections discuss separately the synthesis of 1D MOS nanostructures based on nucleation and growth processes and those mediated by electrospinning, herein called nanowires and nanofibers, respectively. A sub-section dedicated to the functionalization of these 1D nanostructures with second-phase materials is also included in this section.

#### **3.1 Synthesis of nanowires based on nucleation and growth processes**

The group of synthetic methods employed to form chemical-sensitive 1D structures usually involves CVD and hydrothermal processes. Generally, CVD refers to

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

methods based on chemical reactions of gaseous reactants in an activated environment. In CVD synthesis, chemical precursors in the vapor phase are delivered to a reactor, where external energy (heat, light, or plasma) is added to the system to initiate the deposition reactions. These reactions can be homogeneous or heterogeneous; the first occurs in the gas phase whilst the second occurs between gas-phase species and a solid substrate (usually involving an initial gas phase reaction with the formation of reactive intermediate species). Homogeneous reactions form typically non-adherent powders and by-products whereas heterogeneous reactions lead to nucleation and growth processes on solid substrate surfaces that result in the formation of a solid material [64]. Tuning the reaction conditions by adjusting pressures, precursors, and type of activation, amongst other CVD-type related parameters, can promote the formation of either planar films or 1D structures such as nanowires. In contrast, hydrothermal synthesis refers to chemical reaction methods run in a sealed and heated aqueous solution at appropriate temperatures (100–1000 °C) and pressures (1–100 MPa); the name solvothermal is commonly used when organic solvents are involved in the solution. In hydrothermal/solvothermal synthesis, the properties of the synthesized structures are controlled by adjusting parameters such as the solution pH, the chemical species concentration, and the oxidationreduction potential, apart from the temperature and pressure. The nucleation of the structures can also be adjusted by adding inorganic additives. Thus, hydrothermal/ solvothermal synthesis can generate large good quality crystals while keeping control over the material composition [65]. Both CVD and hydrothermal/solvothermal methods have been suitable to deliver MOS nanoparticles and nanostructures including nanowires of ZnO [66–68], SnO2 [68, 69], or WO3 [70, 71], to cite a few examples. The formation of nanowires by these methods generally responds to two mechanisms: one assisted by adding catalyst seeds and the other without catalysts.
