Electrodeposition of WO 3 Nanoparticles for Sensing Applications

The motivation of using metal oxides is mainly due to its charge storage capabilities, and electrocatalytic, electrochromic and photoelectrochemical properties. But comparing with bulk, nanostructured materials present several advantages related with the spatial con ﬁ nement, large fraction of surface atoms, high surface energy, strong surface adsorp‐ tion and increased surface to volume ratio, which greatly improves the performances of these materials. The deposition of this materials can be accomplished by a variety of physical and chemical techniques but nowadays, electrodeposited metal oxides are gen‐ erally used in both laboratories and industries due to the flexibility to control structure and morphology of the oxide electrodes combined with a reduced cost. Tungsten oxide (WO 3 ) is a well-studied semiconductor and is used for several applications as chromo‐ genic material, sensor and catalyst. The major important features is its low cost and avail‐ ability, improved stability, easy morphologic and structural control of the nanostructures, reversible change of conductivity, high sensitivity, selectivity and biocompatibility. For the electrodeposition of WO 3 , more than one method can be adopted: electrodeposition from a precursor solution, anodic oxidation, and electrodeposition of already produced nanoparticles; however, in this case the mechanism of the electrodeposition is not fully understood. In this chapter, a review of the latest published work of electrodeposited nanostructured metal oxides is provided to the reader, with a more detailed explanation of WO 3 material applied in sensing devices.


Introduction
Over the past two decades, the revolution in materials science has driven great advances in all areas of science and engineering. Nanoscience and nanotechnology are leading this revolution fueled by the industrial progress, the scientific ability to fabricate, model, and manipulate objects with a small number of atoms, and the continuous discovery of new phenomena at the nanoscale [1,2]. Nanomaterials present unique properties, which are not found in the respective bulk materials [3]. Surface and quantum effects arise in nanostructures due to the large surface-to-volume ratio and to the dimensions that are comparable to the electron wavelength, respectively [4,5].
In the metal oxides field, the discovery of superconductivity [6] and large magnetoresistance [7] has raised researchers' attention, especially to those with transition metals. Moreover, in traditional electronics, oxides are widely used as semiconductors, dielectrics, and conductive electrodes [8]. In the last years, nanostructured metal oxides for sensing applications have achieved significant advances, mainly due to their better thermal and environmental stability compared with organic materials. These devices, based on nanomaterials, can operate with low power consumption and can be easily integrated with nanoelectronics. Furthermore, the construction of sensors in "low-cost" substrates, such as plastic, paper, or textile, is also in demand for application in portable consumer devices [9][10][11][12][13]. Electrodeposition, in this case, is of great interest due to its flexibility to control the structure and morphology of the oxide electrodes combined with the reduced cost [14,15].

Electrodeposition
The term electrodeposition is often used unclearly, referring either to electroplating or to electrophoretic deposition (EPD) [16]. The electroplating process is based on a solution of ionic species, usually in water, while EPD occurs in a suspension of particles. In electroplating, there is a charge transfer during the deposition to produce the metal or oxide layer in the electrode, while in EPD the deposition occurs without any reaction involved (Fig. 1). In fact, the principal driving force for EPD is the charge and the electrophoretic mobility of the particles in the solvent under the influence of an applied electric field, with the drawback that the solvent should be organic in order to avoid water electrolysis [16,17]. Another variation of an electrochemical deposition is the electroless (autocatalytic) deposition in which a reducing agent, dissolved in the electrolyte, is the electron source for the redox reaction, and no external power supply is needed [18]. Nevertheless, the electroless deposition will not be discussed in this chapter.
The first reports on the electrodeposition technique date back to the 19th century; however, the understanding of the process and the electrochemistry involved was only developed in the 20th century and it is believed that further research is still needed to optimize the process [16].
In electroplating, the relation between the current and the overpotential of electrodeposition is given by the Tafel equation (Equation 1), which describes the exponential dependence between the two parameters. Worth mentioning is that with the increase of the overpotential, the ionic current that the electrolyte can supply is limited either by material transport or electrical conductivity [15,19].
where i is the current, F is Faraday's constant, k a constant, C the concentration of metal ions in solution (which can be initially dissolved in the electrolyte or originated from the dissolution of the metallic anode), α the coefficient of symmetry (∼ 0.5), η the overpotential, R the ideal gas constant, and T the absolute temperature (K).
The first attempt to correlate the amount of particles deposited by EPD with the different parameters influencing electrophoresis was first described by Hamaker for electrophoretic cells with a planar geometry. Over the years, Hamaker's law has been adapted and more recently Equation 2 was derived, relating the weight (W) of the charged particles deposited per unit area of electrode in the initial period, with different parameters, and disregarding the charge of the free ions [20].
Here, C is the concentration of the particles;ε 0 and ε r the permittivity of vacuum and solvent, respectively; ξ the zeta potential of the particles;μ the viscosity of the solvent; E the applied potential; L the distance between the electrodes; and t the deposition time. Equation 2 demonstrates that the deposition weight of the charged particles under ideal EPD depends on all the previous parameters. However, if the solvent, the particles, and the apparatus for EPD are not changed, the weight of the deposited particles (W) is a function of C, E, and t. Therefore, the mass of the deposited particles, namely the thickness of the films, can be easily controlled by the concentration of the suspension, applied potential, and deposition time [17].
Electrodeposition of conventional metals for coatings has a very long history, with more than 200 years for some metals and alloys. Today, electrodeposition is much more than just a technique for coatings fabrication. In addition to applications such as decorative, wear, and corrosion-protective coatings, electrodeposition is also used for the manufacture of molds, functional coatings for magnetic and electronic applications, and microelectromechanical system components production [5]. In the future, though traditional applications will continue, new ones will rapidly develop, especially in the fields of nanoelectronics, biotechnology, and energy engineering. The electrodeposition of non-metallic materials will become more important and the combination of electrodeposition with other processes will lead to nanostructured materials with new and improved properties [21,22]. Electrodeposition is extremely versatile and different applications will keep being explored [23].

Metal oxide electrodeposition
Metal oxides are an important class of materials, which benefit from the large electronegativity of oxygen to induce strong bonding with nearby atoms [22]. At the same time, when compared with bulk materials, nanostructured metal oxides benefit from the spatial confinement, the large fraction of surface atoms, high surface energy, strong surface adsorption, and increased surface-to-volume ratio that greatly improves the performance of these materials [24].
The deposition of nanostructured metal oxides has been already reported by both physical and chemical methods [8,5]. The advantages of electrodeposition include its speed, low cost, high purity, industrial applicability, use of different types of substrates, and production of films with different morphologies and compositions, as multilayers and alloys [21,22].
In the electroplating of metal oxides, the reaction involved is usually defined by two consecutive steps (Equation 3). First, the hydroxide will precipitate in the surface of the electrode due to the reaction of the metal ion (M n+ ) in an alkaline solution, and secondly, the oxide is formed through a condensation/dehydration process. This last step can occur either during electrodeposition or by a subsequent annealing procedure [15].
Another alternative is the formation of metal oxides by anodic oxidation [15]. In this case, the source of the metal ions is the metallic anode and the metal oxide film will be deposited on top of the metal electrode. The general equation can be described as (Equation 4): In EPD, the metal oxide nanoparticles are generally synthesized by different solution based techniques, e.g., sol-gel, precipitation, and hydrothermal synthesis, prior to deposition. The main challenge of this technique is the preparation of a stable dispersion that originates a film with good properties, uniformity, and appropriate thickness. The use of dispersants, binders, or other additives that influences the agglomeration and charge of the particles contributes to the tuning of the properties of the deposited film and need to be considered in defining the EPD parameters [16].

Applications
Nowadays, electrodeposited nanostructured metal oxides are generally used for different applications in laboratories and industry [15]. The latest published reports on the field, listed in Table 1, evidentiate the diversity of areas where these materials can be applied, as presented below.
The deposition of metal/metal oxide nanoparticles composites allowed advances on the protective coatings field. Sajjadnejad et al. [25] improved the corrosion resistance of zinc by co-depositing TiO 2 nanoparticles, while Zeng et al. [26] incorporated CeO 2 nanoparticles to improve the corrosion behavior of nickel coatings. Charlot et al. [27] opened the discussion of the kinetics and mechanism of the anodic EPD of SiO 2 nanoparticles to improve the control of the thickness and properties of these coatings (Fig. 2).

Figure 2.
Scanning electron microscopy (SEM) images of a film cross-section obtained from a suspension with a mass fraction of 3% of nanoparticles under an applied electric field of (a) 6 V cm −1 and of (b) 60 V cm −1 . Reprinted from [27], with permission from Elsevier.
Metal oxide nanostructures are already known to show good catalytic properties. Tu et al. [28] produced Cu 2 O-Cu nanoparticles in carbon paper via electroplating. This procedure is an easy, one-step technique that can be an attractive candidate as a visible-light-driven photocatalyst. At the same time, Yoon et al. [29] studied the influence of 2D and 3D structures on electrodeposited Cu 2 O films by controlling the electrolyte pH and by using polystyrene (PS) beads as template, respectively. This techniques allowed the production of electrodes with increased surface area. Battaglia et al. [30] also improved the catalytic performance of different Ni electrodes by electrodepositing IrO 2 nanostructures through different electrochemical methods. The composites obtained by galvanostatic deposition of the oxide catalyst presented the best activity for water splitting applications.
Solid oxide fuel cells (SOFC) have shown to be a good alternative for electric power generation systems. SOFC show high energy conversion efficiency, clean power generation, reliability, modularity, fuel adaptability, noise-free, excellent long-term stability, and versatility for direct conversion of chemical energy to electrical energy. In this field, Das and Basu [31] applied the EPD technique to deposit yttria-stabilized zirconia (YSZ) nanoparticles on a NiO-YSZ substrate, which after sintering was suitable for application in SOFC (Fig. 3). EPD was also the technique used to deposit TiO 2 nanoparticles for dye-sensitized solar cells (DSSC) [32] and Li-ion micro-batteries applications [33]. For DSSC, the thickness of the TiO 2 films was controlled by changing the deposition time and the I 2 dosage that electrically charge the nanoparticles, while for batteries, the EPD was performed with different TiO 2 structures and different 3D aluminum collectors configurations (Fig. 4). The effect of the substrate was also tested in the EPD of ZnO nanoparticles for conductive fabrics applications [34]. Liu et al. [35] studied the EDP of metal oxides using celestine blue as charging and dispersing agent. The nanostructured MnO 2 films were applied for energy storage in electrochemical supercapacitors with high capacitance and excellent capacitance retention at high charge-discharge rates. The use of metal oxides offers functionalities that vary from electrically conducting to insulating and from highly catalytic to inert, which are useful for sensing applications. Different types of metal oxide sensors have been investigated for several decades, and it has been proved that the reduction of crystallite size provided a significant increase in the sensing performances. Even if less established, these type of sensors are very promising and new developments are being accomplished every day [36].
Recently, Cu 2 O nanostructures were electroplated to produce a facile and economic photoelectrochemical sensor [37], while Ir 2 O 3 was deposited in stretchable and multiplexed pH sensors [38]. This sensor combined electrochemical, microfabrication, and printing techniques and was successfully applied in beating explanted cardiac tissue, with accurate spatiotemporal monitoring of changes in pH (Fig. 5).
Monitoring analgesic drugs with the use of biosensors allows a rapid, reliable, and sensitive method without the requirement of a sample pre-treatment. For that, alloys deposition allows the combination of different materials properties without compromising thickness or surface area available. The biosensors developed by Narang et al. [39] were produced by EPD of an Fe 2 O 3 magnetic nanoparticle coated with ZrO suspension containing chitosan, prior to enzyme (horseradish peroxidase) immobilization. Also the combination of Fe 2 O 3 with carbon nanotubes and chitosan was earlier used by Batra et al. [40] to immobilize hemoglobin and were applied as an amperometric biosensor.

Nanostructured WO 3
Tungsten oxide (WO 3 ) is a well-studied semiconductor used for several applications such as chromogenic material, sensor, and catalyst [41]. The major advantages is its low cost and availability, improved stability, reversible change of conductivity and optical properties, high sensitivity, selectivity, and biocompatibility [42].
Transition-metal oxides, especially those with d 0 and d 10 electronic configurations, as WO 3 , TiO 2 , or ZnO show interesting properties and stability that are important for sensing applications [43]. The energy band gap of WO 3 corresponds to the difference between the energy levels of the valence band formed by the filled O 2p orbitals and the conduction band formed by empty W 5d orbitals, ranging from 2.6 to 3.25 eV [44]. In nanostructured WO 3 , the bandgap generally increases with the reduction of the grain size, which is attributed to the quantum confinement effect [45]. Tungsten oxide is also well known for its properties in a non-stoichiometric form, since its lattice can support a significant concentration of oxygen vacancies [44].

WO 3 electrodeposition
Many liquid and vapor phase synthesis methods have been used to synthesize WO 3 [45]. Nevertheless, for the electrodeposition of nanostructured WO 3 films, more than one method can be adopted: electroplating from a precursor solution [46,47], anodic oxidation from a metal layer [47][48][49], and electrodeposition from a WO 3 nanoparticles dispersion [50,51]. A list of the latest reports is presented in Table 2.
Depending on the electrochemical potential and solution pH, the WO 3 phase may also be involved in other reactions, as the formation of sub-stoichiometric oxide and tungsten bronze (Equations 7 and 8) or even re-dissolution of the oxide phase (Equation 9). The reduced phases formed by these reactions have higher conductivity and hydrophilicity than WO 3 and should be considered during characterization of the deposited films [53].
For the anodic oxidation procedure, the general equation can be expressed as Equation 10 [54] and the full mechanism is explained by the occurrence of different reactions simultaneously, as the synthesis of surface oxide films (e.g., W 2 O 5 , WO 2 ) and tungstate ions (WO 4 2-) [55,56]. This oxidation is usually followed by the slow dissolution of the oxide phase, as in Equations 11 or 12 depending on the solution pH [55,57].
In the case of the deposition from WO 3 nanoparticles dispersions (EPD), the mechanism is not yet fully understood. The majority of the authors agree that the deposition occurs through an electrophoretic mechanism driven by the surface charge of the particles [51,58], but in fact, the potential (or current) applied during deposition can also promote tungsten reduction from W 6+ to W 5+ that is counterbalanced by the cation intercalation into the oxide structure, as described in Equation 8, thus forming tungsten bronze (H x WO 3 ) [50]. In the work of Liu et al. [50], XRD and optical characterization showed that HWO 3 was obtained as the main phase of the deposited films, which supports the hypothesis of the mechanism via electrochemical reduction. Furthermore, since the reduced WO 3 is significantly more conductive than the oxidized form, it allowed continuous film growth. In the future, further analysis of the deposited films should be conducted to confirm the electrochemical deposition mechanism.

Gas sensors
Precise and affordable monitoring of chemical gases is a critical issue for human health, industrial processes, and environmental protection. For that, nanostructured WO 3 has been intensively studied due to its excellent sensing capabilities and reproducibility. These characteristics are mainly ascribed to the increased surface area and complete depletion of carriers within the nanostructure when exposed to the target gas [45]. The gas sensing mechanism is described by the increase or decrease of the conductance of the oxide layer when exposed to reducing (H 2 , H 2 S, CO) or oxidizing (NO 2 , O 3 , CO 2 ) gases, respectively. When NO 2 is targeted on the WO 3 surface, it not only reacts with the electrons from the conduction band (Equation 14) but also with the chemisorbed oxygen (Equation 15), thus promoting a depletion on the surface of the electrode and, consequently, the increase on resistance [70,71].
An example of a hydrogen gas sensor was built by Yang et al. [67] through anodic oxidation of a tungsten layer previously deposited by radio frequency magnetron sputtering on a sapphire substrate (Fig. 6). The nanoporous WO 3 film sensor, after annealing at 600°C, exhibited good sensitivity to H 2 gas in air.

Biosensors
The application of WO 3 to other sensing platforms, as in biosensors, is mainly due to the electrical and optical properties mentioned above [72]. In fact, it was already demonstrated that nanoparticles of metal oxides applied to suitable electrode surfaces allow protein immobilization and biocatalytic processes to be driven electrochemically [73]. However, to the best of the authors' knowledge, only Feng et al. [74] employed electrodeposited nanostructured WO 3 films to enhance the hemoglobin protein loadings, accelerate interfacial electron transfer, and improve thermal stability of the adsorbed protein. The influence of the electrodeposition time to the response time and peak current of the electrode is demonstrated in Fig. 7.

pH sensors
The pH value can be used as an indicator for disease diagnostics, medical treatment optimization, and monitoring of biochemical and biological processes [75]. Nevertheless, the integration of pH sensing systems into the next generation of wearable devices requires a different architecture than currently used in typical glass-type electrodes and a minimal electrode size [76]. In addition, technological and industrial efforts are under way to incorporate different sensors into our daily life by assembling these sensors on common substrates such as plastic, textile, and paper [9]. In the work reported earlier [12], flexible pH sensors were based on electrodeposited WO 3 sensing layer in a gold/polyimide substrate (Fig. 8). The pH sensing mechanism for this material, even if not fully understood, is believed to be dependent of the redox reaction involving the production of the tungsten bronze with a higher conductivity than the tungsten oxide (Equation 8).

Neural electrodes
Microtechnology allowed the arrangement of multiple microelectrodes on the same substrate over small distances (Fig. 9a). Nevertheless, in order to provide sufficient recording sensitivity to small electrodes for measuring neuron electrical activity, they are often coated with different nanostructured or conducting materials to increase the effective surface area and electrochemical interface capacitance [77][78][79]. The interest in utilizing transition metal oxide films is due to its pseudocapacitive character related to chemisorption processes and redox reactions that take place at the surface [80]. Since nanostructured WO 3 has already proved to enhance capacitive performances due to its large surface area and low charge transport resistance [52], it was used for neural recordings applications [69]. The optimization of the electrodeposition parameters led to a slight increase on the charge storage capacity (∼10%) and a decrease of the impedance values, of approximately 40% (Fig. 9b and 9c).  These preliminary results show the versatility of electrodeposition in different materials and configurations as well as in different sensing mechanisms.

Conclusions
Tungsten oxide (WO 3 ) is one of the most studied metal oxide and the sensing performance of this material is of great interest due to the capability of reversible change of both its optical and electrical properties. The evolution in the fields of nanoscience and nanotechnology allowed these materials to replace many organic and metallic materials in a huge range of applications besides creating new areas of development. The increased surface area and the quantum confinement effects in size ranges below 100 nm make nanostructured WO 3 a good platform for gas and pH sensors, along with neural electrodes and biosensors.
In the last decade, the use of electrodeposition for nanostructured metal oxide films has been growing due to the versatility of this method in different applications and materials. Just in the last year, applications varied from catalysts and sensors to capacitors. The use of different types of templates and the deposition of composites will contribute to the continued development of this technique.