Abstract
The study of selective metal oxide-based binary/ternary systems has received increasing interest in recent years due to the possibility of producing efficient new ceramic materials for relative humidity (RH) detection, given the superior properties of the mixed compounds in comparison with pristine ones. The aim of this work was focused on preparation and characterization of non-doped and Nb2O5-doped TiO2 : WO3 pair (in the pellet form) and evaluation of corresponding humidity-dependent electrical properties. The microstructure of the samples was analyzed from scanning electron microscopy, X-ray diffraction patterns, Raman spectra, BET surface area analysis, and porosimetry. The electrical characterization was obtained from impedance spectroscopy (100 Hz to 40 MHz) in the 10–100% RH range. The results showed that adequate doping levels of Nb2O5 introduce important advantages due to the atomic substitution of Ti by Nb atoms in highly doped structures with different levels of porosity and grain sizes. These aspects introduced a key role in the excursion (one order of magnitude) in the bulk resistance and grain boundary resistance, which characterizes these composite ceramics as a promising platform for RH identification.
1. Introduction
The development of new ceramic metal oxide materials provides a promising platform for diverse applications such as optoelectronics, microelectronics, dye-sensitized solar cells, and tunneling devices [1–3].
In particular, the production of moisture sensors of metal oxide materials requires improved selectivity and stability for water sorption. These processes are intrinsically dependent on microstructure and prevailing transport mechanisms of resulting materials. The doping process induced by metal oxides introduces atomic defects which affect the overall conduction mechanisms of blends, characterizing the relative concentration of a dopant as a tuning parameter in the optimization of electrical response in terms of RH variation [4–7].
The typical process of structural modification is provided by mechanical mixing of the powders, molding, and sintering of the pelletized samples [8, 9].
The introduction of a doping agent in a mixed metal oxide ceramic has been considered an interesting strategy for improvement in the dependence of impedance value with relative humidity (RH) variation [10–13]. The incorporation of dopants affects the structure and morphology of the ceramic, providing additional path-structural water layer interaction.
In this work, the authors explored the incorporation of niobium pentoxide (Nb2O5)—an n-type transition metal oxide (Eg of 3.2–4 eV) applied as a dopant for the TiO2 : WO3 pair. The microstructure was evaluated from SEM images, X-ray diffraction patterns, Raman spectra, BET analysis, and porosimetry. These techniques were explored in order to evaluate the influence of each component on the overall electrical response of the blends under controlled variation of RH.
2. Experimental
2.1. Materials and Methods
TiO2, WO3, and Nb2O5 (Fluka) were used as received. Grain size was determined using an Autosizer II C (Malvern Instruments). The X-ray diffraction patterns were obtained by means of a Philips X’Pert, PW 3040/00 using Cu-Kα radiation (Kα = 1.5418 Å) (20° < 2θ < 75°) with 0.04° of step and 0.5 s per point. Raman spectra of the samples were obtained in a HORIBA Jobin-Yvon Raman spectrometer in which the excitation wavelength was adjusted to 532 nm with a power of 25 mW. SEM images were obtained in a Philips scanning electron microscope (model XL 30 TMP), operated at 30 kV. The pore area was determined using a Micromeritics PoreSizer 9320 mercury porosimeter (as a standard procedure, chamber containing the samples was degasified, and then, mercury intrusion pressured analysis was performed in the pressure range from 0.5 up to 30000 psi). Brunauer–Emmett–Teller (BET) surface area experiments were provided by a Micrometrics ASAP using nitrogen gas. The impedance of the samples was measured in the range between 100 Hz and 10 MHz, with an AC voltage of 0.5 V—no bias, in a Hewlett-Packard (model HP4294A) impedance/gain-phase analyzer.
The experimental setup for impedance measurements consists in a 6.5-liter chamber in which the temperature is controlled with a 1°C precision in response to the RH variation with steps of 10% in an overall range from 10 to 100%. All the experiments were performed at 20°C. The RH values were obtained by mixing water-saturated air and dry synthetic air, in which the respective amount of each part was regulated by mass flow controllers. The impedance spectra for each RH value were reached at a continuous flow rate of 5 l/h, after at least 90 minutes of stabilization. The measuring electrical contacts were made of gold on opposite sides of the top surface of the samples.
2.2. Preparation of the Samples
The pristine TiO2 : WO3 powder was prepared by mechanical mixing of TiO2 and WO3 in the ratio of 48.92 : 51.08 wt.%, respectively. The dopant (Nb2O5) was incorporated by direct mixing with specific amounts (2, 4, and 6 wt.%), and named as TiW-Nb2, TiW-Nb4, and TiW-Nb6 samples. All the samples were prepared in the form of pellets for the microstructural and electrical characterization. The mixtures (pristine and doped powders) were initially pelletized in the samples of 8 mm × 6 mm × 1 mm under 8 MPa of uniaxial pressure and then isostatically pressed at 200 MPa. Afterwards, the thermal treatment (under air) was conducted at 700°C for 2 h with heating and cooling rates of 20°C/min in accordance with a previous used procedure [8, 9].
3. Results and Discussion
3.1. Morphology of the Mixed Pellets
SEM images of TiW-Nb2, TiW-Nb4, and TiW-Nb6 are shown in Figure 1. As can be seen, the resulting material is characterized by a porous surface with a distribution of different aggregation sizes of particles disposed in overlaid layers, providing free sites for molecule percolation along the structure. Statistical analysis of the images indicates a distribution of smaller grains with size around 225.4 ± 88.2 nm. A slight increase in the size of the smaller aggregates is observed for TiW-Nb4 sample, reaching 284.5 ± 131.6 nm. The aggregation level is favored by the progressive incorporation of a doping agent (Figure 1(c)) (TiW-Nb6 reveals aggregates with a diameter of about 386.7 ± 252.3 nm). By comparing the images, it is possible to identify a slight dependence of the aggregation level on the dopant concentration.
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These data can be confirmed by the differential mercury intrusion curves (shown in Figure 2). As shown, a broad peak in the range of 0.07–0.5 μm with maximum in 0.275 μm is observed for all the samples. A weak contribution is observed in the range of 3–30 μm, shown in details in the inset, characterizing the distribution of aggregates with different sizes, as observed in the previous SEM images. Using the total volume of intruded mercury, the corresponding values for the total porosity were calculated: percentages of 34.4, 37.1, and 37.0 were determined for the samples TiW-Nb2, TiW-Nb4, and TiW-Nb6, respectively, indicating that doping load introduces minimal differences on the pore structure of the resulting material, which remain distinct due to the previously defined ratio of the semiconductors used in the preparation, preserving the structural humidity properties of the template.
The BET surface area of samples is summarized in Table 1. The results (Table 1) confirm that progressive incorporation of the dopant until 4 wt.% introduces negligible influence on the surface area of samples. The higher level of the dopant in the sample TiW-Nb6 affects the response (32% in terms of undoped ones) affecting the water adsorption if compared with samples prepared under low doping level condition.
3.2. Microstructural Characterization
The X-ray diffraction patterns of a pristine sensor (mixed TiO2 : WO3) sintered at 700°C for 120 minutes in air are compared with those of a nonsintered sample in the inset of Figure 3(a). It is noteworthy that the anatase phase of the nonsintered sample is converted into rutile phase as a consequence of annealing. Both TiO2 polymorphs are characterized by tetragonal configuration consisting in octahedra that share four edges with anatase and two with rutile. The rutile is identified by the ICDD card no. 21-1276 according to the following crystal system: tetragonal space group: with the unit cell parameters a = b = 4.5933 Å and c = 2.9592 Å. The spectrum of tungsten trioxide, shown in the inset (B) of Figure 3(a), reveals the decrease in the intensity of the initial monoclinic phase due to the sintering process—by introduction of impurities and defects. It is also noticeable the appearance of the anorthic phase identified by ICDD card no. 083-0951 and of the monoclinic identified by ICDD card no. 071-0131. These diffraction patterns confirm the existence of a polycrystalline material that results from a mixture between the oxides. In Figures 3(b)–3(d), the XRD patterns of the samples TiW-Nb2, TiW-Nb4, and TiW-Nb6 are shown. The possible incorporation of ions into the crystalline structure of [14–16] can be assumed, once the corresponding ionic radius of and can be identified as a source for the absence of niobium peaks in the corresponding curves. The insets of Figures 3(b)–3(d) confirm the absence of the rutile phase and the incorporation of atoms into the structure. XRD peaks of anatase were identified at 2θ = 25.3°, 36.9°, 37.8°, 38.5°, 48.1°, 55.1°, 62.7°, and 68.6°. The latter phase also has tetragonal organization with the space group I41 and with the unit cell parameters a = b = 3.7842 Å and c = 9.5146 Å, all identified by the ICDD card no. 21-1272. Therefore, in order to state more clearly the substitution of atoms by atoms, the unit cell parameters of samples under study were calculated from the anatase peaks located at 2θ = 25.28° and 2θ = 48.05°. The calculated cell parameters are a = b = 3.7818 Å and c = 9.6261 Å for the sample TiW-Nb2, corresponding to an increase along c-direction and a decrease along a-b directions, when compared with the standard values. Such distortions reinforce the niobium incorporation: they were also observed for the sensors doped with 4 wt.% (a = b = 3.7822 Å and c = 9.5971 Å) and 6 wt.% (a = b = 3.7838 Å and c = 9.5410 Å). Moreover, the anatase phase showed a slight shift towards smaller 2θ angles as doping content increased, as verified in the changes in the peak positions 2θ = 48.08° (2 wt.%) and 2θ = 48.05° (6 wt.%). The average crystallite size of WO3 and TiO2 of doped and nondoped sensors was calculated by the Scherrer equation (, where denotes the crystallite size, corresponds to the full width at half maximum, is the Bragg peak, in radians, is the wavelength of incident radiation, and is the Scherrer constant). Table 1 summarizes the values obtained for the higher intensity reflections (101) and (002) of the TiO2 and WO3, respectively. Comparing both WO3 and TiO2 crystallite sizes of doping sensors with the value of nondoped sensor, it is observable that niobium incorporation promoted a slight decrease of crystallite dimensions for both oxides. These results indicate that niobium has been incorporated into the crystal lattice of titanium and provides considerable changes in the electrical response to humidity of the pellets.
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The Raman spectrum of nondoped and TiW-Nb2 samples is compared in Figure 4(a). The incorporation of niobium originates a less intense peak in the spectrum, corresponding to the position at 635 cm−1, and two other even less intense peaks at 513 and 390 cm−1, attributed to the anatase phase, confirming that niobium hinders the phase change of titanium dioxide. The peak at 440 cm−1 in the spectrum of doped pellet is assigned to tungsten trioxide ( = O), also visible in the Raman spectra of TiW-Nb4 and TiW-Nb6 (Figure 4(b)). The asymmetric shoulder formed is more evident for the higher doped sensor and might be associated with the proximity of the monoclinic (mode G) vibrational mode at 605 cm−1 [17, 18].
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3.3. Electrical Impedance Characterization of Samples
Electrical impedance spectroscopy has been progressively reported in the literature as a promising tool applied in the identification of phase transitions in materials [19–22] and transport mechanisms in structures of solid state (ceramics) and soft matter. The frequency-dependent excitation provides information about transport and polarization (transport, ionic diffusion, and charge separation). The graphical representation (Nyquist diagrams) offers a direct visualization of the different mechanisms (relaxation process—from a characteristic semicircle—and diffusion—from a straight line at low-frequency limit) [23].
In terms of surface conductivity in ceramics due to the progressive water adsorption [24], three different mechanisms are present [25]. At low RH range, the monomolecular adsorption process takes place as a response to surface modification due to water molecule incorporation, reaching a first complete layer, the chemisorbed one, in the range 20 to 40% RH. Above 40% RH, the proton conductivity of water dominates and diffusion effects tend to be more effective, improving the surface conductivity. The continuous formation of water layers, physisorbed ones, favors the ionic transport participation in the overall conduction process. The porous structure tends to be filled by water molecules, allowing proton transport between adjacent water molecules. With the increasing water adsorption rate, the surface conductivity tends to assume a constant value. Nyquist diagrams in Figures 5(a) and 5(b) reveal that a minimal variation is observed for all the samples if the considered RH concentration is above 70%, confirming the previous analysis; below 70% RH, the relaxation dominates over diffusion and two overlapping semicircles are present. At increasing RH (in the range of 40%–60%), diffusion contributions are added to the surface contribution due to the tunneling process along water layers. Above 70% RH, diffusion tends to be the dominant mechanism—as a result of mutual contribution of surface water layer and bulk water-filled pores. This pronounced behavior is similarly observed for the sample TiW-Nb4 (Figure 5(b)). Increasing dopant concentration affects the dependence of the electrical spectrum with RH variation. As expected, the minimal variation in the Nyquist curves’ characteristic diameter reveals that saturation in the transport is reached.
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These results are confirmed in Figure 6 (impedance measured at f = 1.3 kHz and 4 kHz, at 20°C for different doping levels). As shown, the incorporation of niobium in the composite reduces the range of variation of resulting devices as a function of RH due to the doping level established by the additive.
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3.3.1. Electrical Circuit Modelling for Sample’s Response
Equivalent circuits have been considered as an interesting source of parameters [25–27] that have been associated with different mechanisms in overall electrical response. The circuit represented in Figure 7 has been explored by the authors in recent works [8, 9], and the components are described below:
is used to represent the geometrical capacitance, while the bulky granular response is assigned to . The grain boundary contribution is assigned to the parallel circuit , while the component characterizes the surface contribution from electron tunnelling along the water layers disposed above the semiconductor surface. The charge diffusion is represented by a constant phase element, CPE [25, 28]. For both referred diffusion mechanisms, the interfacial character of their impedance makes it partly capacitive as well as resistive: has to do with the contribution of the pores, due to diffusion phenomena taking place inside the water-filled pores, and is related to the electrode-water layer interface diffusion phenomena that take place at that interface.
In Tables 2–4, the best-fit parameters for the proposed electrical equivalent circuit are summarized. and represent the two parameters of the impedance , while and represent the two parameters of the impedance .
Due to the strong variation observed in two of these parameters, R1 and R2 were chosen as relevant parameters for RH dependence of impedance data.
Figure 8 shows the dependence of R1 as a function of RH for different doping levels. A general tendency of reduction of the corresponding bulk resistance is observed with increasing RH concentration, as a consequence of progressive diffusion of water molecules into the bulk of the devices. In addition to this, it is possible to identify a niobium-dependent variation of R1 with RH. The minimal variation is observed for the sample TiW-Nb6, while the maximum one is reached for the sample TiW-Nb4.
The water diffusion in the corresponding structure introduces a competition for transport mechanisms with intrinsic electrical properties. The higher doping level samples are minimally affected by water incorporation, since efficient channels for current transport are established under doping; the water impregnation in the bulk of the samples represents a minimal perturbation in the corresponding transport process. In the opposite direction, for the sample TiW-Nb4, this process is extremely dependent on water incorporation.
The dependence of grain boundary resistance (R2)—shown in Figure 9—confirms the observed behavior. Maximum variation in R2 is observed for the sample TiW-Nb4, while negligible variation is observed for the sample TiW-Nb6.
The sensitivity of all three doped samples taken at 1.3 and 4 kHz is plotted in Figure 10. The sensitivity was calculated using the ratio between the conductivity of the sensor exposed to a certain moisture concentration and the conductivity of the sensor under a dry air atmosphere. As can be seen, the sample that exhibits the best sensitivity is the one doped with 4% of niobium, in accordance with the discussed morphology of the resulting material and with the impedance changes with moisture previously discussed.
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These results confirm that structural modification provided by niobium returns the best sensibility to RH at 4% of niobium. At this condition, the competition established between the doping level, induced by additive, and electrical response of structure to water layer incorporation is maximized, characterizing an optimal condition for TiW-Nb-based RH samples.
4. Conclusion
The doping level established by in composites preserves the anatase phase and provides modifications at the atomic level of the resulting structure (niobium modifies the crystal lattice of titanium) as detected by XRD data.
This induced structural modification requires specific and low concentration of the dopant (sample TiW-Nb4) in order to optimize the RH sensitivity of the resulting composite. Above this critical value, the high conductivity of the obtained devices affects the sensibility degree of the structure, due to the progressive aggregation of grains and higher surface conductivity at low RH condition.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.