1 Introduction

Humidity detection and measurement are essential for agriculture, industry, health care, meteorology, air conditioning, and environmental monitoring [1,2,3,4,5]. The criteria such as rapid response and recovery time, good repeatability, excellent sensitivity, linearity, and a small hysteresis loop are general requirements for a good humidity sensor. There are several materials used as a sensitive layer for humidity sensors devices. The humidity sensors are classified into two categories, relative humidity sensors and absolute humidity sensors. Most of the sensors are relative humidity sensors, which are further classified into ceramic and polymer humidity sensors [6]. The polymer and ceramic types of humidity sensing materials operate via a proton conduction mechanism [7, 8]. Metal oxide ceramics-based humidity sensors possess high sensitivity high chemical and mechanical stability compared to polymer materials counterparts [9,10,11]. TiO2, SiO2, SnO2, ZnO, Al2O3, WO3 are the most popular ceramic-type metal oxide materials used for humidity sensing devices [12, 13]. Among these, TiO2 is the most studied material, which has many applications in diverse fields such as anti-fogging surface coatings, solar cells [14], gas sensors [15], energy harvesting/storage [16], photocatalysis [17], etc. It also exhibits good electrochemical stability, hydrophilicity, and surface roughness. It is an n-type transition metal oxide semiconductor material with three phases: anatase, brookite, and rutile. Anatase and rutile phases are more stable than brookite. Anatase and rutile TiO2 have tetragonal structures, whereas brookite has an orthorhombic structure. The rutile phase is more stable than anatase at high temperatures and pressure. Nanostructures of TiO2 can be synthesized by various methods such as physical vapor deposition [18], wet chemical method [19], sol–gel method [20], electrodeposition method [21], chemical vapor deposition [22], solution combustion method [23], and hydrothermal method [24], etc. The morphology of the sensing material also plays a vital role in humidity sensing performance. The performance of different nanostructures such as nanowires [25], nanobelts [26], nanotetrapods [27], and nanorods [28] was studied for humidity sensing. The hydrothermal method is the most popular for the large-scale synthesis of materials among these methods. Different morphologies of TiO2, like nanobelts [29], nanowires [30], nanoflowers [31], and nanorods [32], have been successfully synthesized by the hydrothermal method. Chow et al. [33] had prepared TiO2 thin films by reactive sputtering. Nevertheless, the response time of the humidity sensor obtained by this method is relatively long. Zhang et al. [34] also prepared highly ordered TiO2 nanotubes by anodic oxidation. However, the humidity sensor's response and recovery time were quite large (100 s and 190 s, respectively). Wu et al. [35] fabricated a TiO2 and TEOS-Nafion composite by hydrothermal method. They reported that the response time at 97% of relative humidity was much longer than the humidity range between 12 and 76%. Most of the reports show that TiO2 has an excellent response to high humidity level (above 50% RH), and fail to gain response to low humidity level (below 30% RH) [36, 37]. In order to enhance the response to low humidity level we have used interdigital electrode. Zhao et al. [38] found that the interdigital electrode gap size influences the humidity sensor's response to surface electrical conduction mechanisms. After the synthesis process, the annealing treatment improves lattice structure and crystallinity and reduces material defects. In the present study, we have fabricated IDE by a simple copper itching process. The synthesized TiO2 microspheres were deposited on these IDEs and thereby we have studied the humidity response. The sensitivity, as well as response to humidity, mainly depends on distance between two electrodes. In fabricated IDEs the distance between electrodes enable fast charge transfer rate which results in better sensitivity, fast response and recovery time.

Most research on TiO2 for humidity sensors focuses on Ti3+ defect sites. The presence of Ti3+ defects and the oxygen vacancies on the surface of TiO2 directly affects the performance of the humidity sensor. A large number of oxygen vacancies for TiO2 material can enhance the ionization of adsorbed water molecules and speed up the conduction process [39]. Huali et al. [40] reported that TiO2 sample annealed in an atmosphere of lower oxygen content such as Ar, N2 contains a high density of oxygen vacancies which was also confirmed by XPS and ESR results. With this motivation, the present study attempts to synthesize and characterize TiO2 microspheres and explore their possible use as a humidity sensor. In the present work, the TiO2 microspheres are synthesized by hydrothermal method and subsequently annealed in Ar at 200 °C, 400 °C, and 600 °C. The influence of annealing temperature in the Ar atmosphere on surface morphology structural and optical properties are investigated. Finally, humidity sensors are fabricated using as-synthesized and annealed TiO2 microspheres on interdigital electrodes instead of conventional FTO substrate to study the humidity sensing performance. We found that TiO2 microspheres and IDEs based humidity sensors had significantly enhanced response and recovery time compared to conventional ones.

2 Experimental

2.1 Synthesis of TiO2 microspheres

TiO2 microspheres were synthesized via the hydrothermal method. In a typical procedure, 10 ml of titanium butoxide [Ti(C4H9O)4, Sigma Aldrich] was drop-wise added into the mixture of 20 ml hydrochloric acid [HCl, Sigma Aldrich] and 30 ml distilled water in a ratio of 1:2:3. The above solution was stirred at room temperature for an hour using a magnetic stirrer. The solution was then transferred into a 100 ml stainless steel autoclave. The autoclave was sealed tightly and placed in an oven at 150 °C for 24 h. After completing the reaction, the autoclave was allowed to cool naturally, and the white precipitate was separated by centrifugation. Next, the residue was washed several times with distilled water and ethanol. Finally, the washed residue was dried under an IR lamp. The as-synthesized TiO2 powder was then annealed in a tube furnace at 200 °C, 400 °C, and 600 °C in the Ar atmosphere for 2 h. The schematic of the synthesis of TiO2 powder is depicted in Fig. 1.

Fig. 1
figure 1

Schematic of the synthesis process of TiO2 powder by hydrothermal method

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2.2 Material characterization

As-synthesized and annealed TiO2 powder were characterized using various characterization techniques. The crystallinity and phase formation of TiO2 was confirmed by an x-ray diffractometer (Bruker D8 Advance, Germany) using CuKλ (λ = 1.54 Å) radiation source in the range 2θ = 20°–80°. Raman spectroscopy instrument (Reinshaw microscope) in the range 100–800 cm−1 was used to record the Raman spectra using a source wavelength of 532 nm. The UV–Visible optical absorption spectra were measured using JASCO V-670 (Japan) UV–Visible spectrophotometer. Scanning electron micrographs were recorded using JEOL JSM-6360-LA scanning electron microscopy (SEM). The FTIR spectra of TiO2 were recorded with JASCO, FTIR-6100 FTIR spectrophotometer in the range of 500–4000 cm−1. The BET (Braunauer–Emmet–Teller) studies were carried out using Quantachrome Instrument to analyze the compounds' pore size and surface area.

2.3 Sensor design, fabrication, and characterization

The interdigital electrode (IDE) finger pattern was initially printed on a toner transfer paper (Glossy Paper) by the HP LaserJet Pro M202dw printer. This IDE pattern was then transferred to a clean Cu clad plate by heat treatment. The copper-clad plate was then dipped in 1 M of ferric chloride (FeCl3) solution for the Cu etching process. After completing the etching process, the Cu IDE finger pattern was obtained. The schematic of the IDE pattern is shown in Fig. 2.

Fig. 2
figure 2

Schematic of the interdigital electrode (IDE) pattern prepared and used in the present study

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The sensing area of the substrate is 1.08 cm2, with 20 interdigitated fingers. The gap between two consecutive fingers is ~ 500 μm. The TiO2 powder was mixed with ethanol and ground in a mortar to form a paste. The paste was then transferred and coated on IDE substrate by a thin brush. The film was dried naturally in the air overnight before testing. The thickness of all TiO2 films calculated by the gravimetric method was found to be 9.5 μm. Figure 3 shows the schematic of the humidity sensing performance measurement setup used in the present study.

Fig. 3
figure 3

Schematic of the humidity sensing performance measurement setup used in the present study

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The humidity sensing performance of the TiO2 sensor was tested at different humidity levels. The saturated salts are used to maintain different relative humidity levels, LiCl (11%), CH3CO2K (22%), MgCl2 (33%), K2CO3 (43%), Mg(NO3)2 (52%), KI (68%), NaCl (75%), (NH4)2SO4 (80%), KCl (84%), KNO3 (93%) and K2SO4 (97%). These salts' relative humidity (% RH) was tested on a portable Dewpoint hygrometer HI 9565 (HANNA instruments) with a HI706023 sensing probe. All the electrical measurements were carried out using a source meter (Keithley 2450).

3 Results and discussion

3.1 Morphological studies

The surface morphology of as-synthesized and annealed rutile-TiO2 nanoparticles was studied using scanning electron microscopy (SEM) analysis. Figure 4 shows SEM images of as-synthesized and annealed rutile-TiO2 nanoparticles.

Fig. 4
figure 4

SEM images of as-synthesized and annealed rutile-TiO2 prepared by the hydrothermal method

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The morphology of rutile-TiO2 is microspheres. The SEM images show that the average microsphere size increases with annealing temperature; however, the surface morphology remains the same. The average diameter of the TiO2 microspheres is ~ 4.46 μm for as-synthesized rutile-TiO2 and ~ 5.23 μm, ~ 6.71 μm and ~ 7.38 μm for rutile-TiO2 annealed at 200 °C, 400 °C, and 600 °C, respectively.

3.2 X-ray diffraction (XRD) analysis

Figure 5 shows the XRD pattern of as-synthesized and annealed TiO2 nanoparticles. The presence of multiple peaks in the diffraction pattern indicates that all films are polycrystalline. As seen, major diffraction peaks are observed at 2θ ~ 27.22°, 35.83°, 38.94°, 41.03°, 43.85°, 54.07°, 56.38°, 62.50°, 63.77°, 68.71° and 69.49° which are associated with (110), (101), (200), (111), (210), (211), (220), (002), (310), (301) and (112) planes respectively. These peaks are compared with the standard JCPDS data card #01-078-1510 and imply the tetragonal rutile-TiO2 phase formation. No diffraction peaks of anatase or brookite phase have been detected in the XRD pattern, ensuring the synthesis of high-purity rutile-TiO2 nanoparticles using the hydrothermal method. The most intense peak corresponds to the (110) plane, and its intensity increases with an increase in annealing temperature, signifying an increase in crystallinity.

Fig. 5
figure 5

XRD pattern of as-synthesized and annealed TiO2 microspheres powder prepared by hydrothermal method

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The following equation determines the lattice parameters a and c [41],

$$\frac{1}{{d_{hkl}^{2} }} = \frac{{h^{2} + k^{2} }}{{a^{2} }} + \frac{{l^{2} }}{{c^{2} }}.$$
(1)

The calculated values of lattice constants are a = b = 4.60 Å and c = 2.96 Å, which agree with the standard reported values for tetragonal rutile-TiO2. For the as-deposited rutile-TiO2 film, the absence of diffraction peaks at 2θ ~ 68.71° and 69.49° corresponding to the (301) and (112) planes may be due to insufficient heat energy to complete the crystallization process.

The crystallite size (D) of rutile-TiO2 was calculated from (110) peak using Debye–Scherrer’s equation,

$$D = \frac{k }{{ \beta \cos \theta }},$$
(2)

where k is the particle shape factor and taken as 0.9 for spherical shape nanoparticles, λ is the x-ray source wavelength, β is the full width at half maximum (FWHM), and θ represents Bragg’s diffraction angle. As seen, the values of FWHM for rutile-TiO2 films decreases with an increase in annealing temperature, suggesting an increase in crystallite size with annealing temperature. The values of FWHM and crystallite size of rutile-TiO2 samples are listed in Table 1. The crystallite size for as-synthesized rutile-TiO2 is found ~ 12 nm and it increases with annealing temperature.

Table 1 Crystallite size, dislocation density, and microstrain of rutile TiO2 at the various annealing temperatures
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A dislocation is a crystallographic defect or irregularity within a crystal structure that strongly influences the material properties. The dislocation density (δ) is the length of dislocation lines per unit volume of the crystal and was calculated using the relation [42],

$$\delta = \frac{n}{{D^{2} }},$$
(3)

where n is the factor that is equal to unity for minimum dislocation density and D is the crystallite size.

The calculated values of dislocation density are listed in Table 1. It is observed that the dislocation density decreases with an increase in annealing temperature, implying a decrease in lattice imperfections in rutile-TiO2 nanoparticles.

Lattice micro-strain (ε) in a crystal occurs due to the unit cell's displacements from their normal position. It arises in the rutile-TiO2 due to lattice misfit, such as imperfections like dislocation, stacking fault probability, and lattice distortion. The micro-strain was calculated using the equation,

$$\varepsilon = \frac{\beta }{4 \tan \theta },$$
(4)

where β is the full width at half maximum and θ is the Bragg angle. The micro-strain for as-deposited rutile-TiO2 is found ~ 12.2, and it decreases with an increase in annealing temperature, implying a decrease in lattice imperfections in rutile-TiO2. The calculated values of micro-strain are listed in Table 1.

3.3 Raman spectroscopy studies

The Raman spectra of as-synthesized and annealed TiO2 are shown in Fig. 6. As seen, the Raman spectra consist of four Raman bands at ~ 604 cm−1, ~ 441 cm−1, ~ 225 cm−1, and ~ 136 cm−1, which are characteristic vibrations of rutile-TiO2. The distinct stretching peaks at ~ 441 cm−1 and ~ 604 cm−1 correspond to the symmetries of Eg and A1g modes of rutile-TiO2, respectively.

Fig. 6
figure 6

Raman spectra of as-synthesized and annealed rutile-TiO2 microspheres powder prepared by hydrothermal method

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The band positions of TiO2 are in good agreement with previous reports for the rutile phase [43]. The intensity of Eg and A1g modes increases, whereas FWHM decreases with an increase in annealing temperature. The FWHM values of the mentioned modes are listed in Table 2.

Table 2 FWHM values of rutile TiO2 at the various annealing temperatures
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Another tiny peak at ~ 225 cm−1 arises from the multiple phonon scattering processes [44]. Furthermore, no Raman active modes due to other organic impurity phases are observed, indicating the formation of pure rutile-TiO2 phase by the hydrothermal method. These results are analogous to the XRD analysis.

3.4 Fourier transform infrared (FTIR) spectroscopy analysis

To investigate the presence of various chemical groups in as-synthesized and annealed TiO2, Fourier transform infrared (FTIR) spectroscopy was used. Figure 7 shows the FTIR spectra of hydrothermally synthesized TiO2 in 500–4000 cm−1. The strong absorption broad-band below 1000 cm−1 corresponds to the characteristic Ti–O and Ti–O–Ti stretching and bending vibrational modes for rutile-TiO2. The vibrational peak ~ 3412 cm−1 corresponds to the O–H stretching vibrations, whereas the peak ~ 1632 cm−1 corresponds to the hydroxyl group of the water molecule. With an increase in annealing temperature, the absorption intensity in all bands gradually reduced.

Fig. 7
figure 7

FTIR spectrum of hydrothermally synthesized rutile-TiO2 microspheres powder and annealed at different temperatures

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3.5 UV–Visible spectroscopy analysis

The optical properties of rutile-TiO2 were investigated using UV–Visible spectroscopy. Figure 8a shows the optical absorption spectra of as-synthesized and annealed rutile-TiO2 nanoparticles in the range of 200–800 nm. It exhibits a sharp absorption edge in 397 nm to 410 nm. A red-shift in absorption edge has been observed with increase in annealing temperature, which may be due to increase in crystallite size with increase in annealing temperature. The XRD analysis support this conjecture.

Fig. 8
figure 8

a Optical absorption spectra of as-synthesized and annealed rutile-TiO2 microspheres and b Typical Tauc’s plot used to estimate the direct bandgap using Kubelka–Munk transformation

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To calculate the band gap of rutile-TiO2, the diffuse reflectance (R) value has been converted to equivalent absorption and extinction coefficient value [F(R)] using Kubelka–Munk transformation [45],

$$F\left( R \right) = \frac{{\left( {1 - R} \right)^{2} }}{2 R},$$
(5)

Figure 8b shows a typical Tauc plot used to estimate the direct band gap using Kubelka–Munk transformation for rutile-TiO2 synthesized at different annealing temperatures by hydrothermal method. No significant change in the band gap has been observed in rutile-TiO2 with increased annealing temperature. However, it decreases slightly from 3.08 eV for the sample annealed at 600 °C to 3.03 eV for the as-synthesized rutile-TiO2 sample. These values are shown in Fig. 8b.

3.6 BET analysis

The Brunauer–Emmett–Teller (BET) method is commonly applied to calculate the specific surface area based on nitrogen adsorption isotherm measurements [46,47,48]. Figure 9a–d represents the BET and Barrett–Joyner–Halenda (BJH) analysis of as-synthesized and annealed TiO2 at various temperatures. Figure 9a shows the nitrogen adsorption–desorption isotherms for TiO2. It exhibits a type IV isotherm corresponding to a mesoporous structure with a relative pressure between 0 and 1.

Fig. 9
figure 9

a Nitrogen adsorption–desorption isotherms for TiO2 at different annealing temperatures b and c BJH pore size distribution curve of TiO2 d BET adsorption isotherm

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In addition, the presence of the hysteresis loop provides information about the pore size. The values of BET results for surface area, average pore radius, and pore volume of TiO2 at the various annealing temperature are summarized in Table 3.

Table 3 BET and BJH analysis parameters for rutile TiO2 at the various annealing temperatures
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As seen from Table 3, the surface area of TiO2 decreased with an increase in annealing temperature. It can be attributed to the increase in crystallite size and crystallinity of TiO2 with an increase in annealing temperature. The XRD analysis further supports this (Fig. 5). As the temperature is elevated to 600 °C, the total pore volume decreases. We think that decrease in pore volume may be due to the collapse of pore structure and aggregation/merging of multiple grains into bulk due to crystal growth. Therefore, the porosity decreases with an increase in annealing temperature. As a result, there is a reduction in the defect density, which improves the electrical conductivity. These results agree with the previously reported work [49, 50]. Therefore, the obtained BET and BHJ results are desirable for the enhanced sensing performance of TiO2 based humidity sensors.

3.7 Humidity sensor performance

The humidity sensing behavior of the TiO2 microsphere was investigated by fabricating the two probe IDE devices and then introducing them into different relative humidity (RH) conditions, which were attained using saturated salt solutions at room temperature. Figure 10 shows the current–voltage (I–V) characteristics of the TiO2 microsphere (annealed at 600 °C) at different RH levels. As seen, the current increases with an increase in RH. These results confirm the semiconducting behavior of the as-synthesized and annealed rutile-TiO2 microsphere. Due to double hydrogen bonding, the charge carrier cannot move freely in the chemisorbed layer at lower humidity. On the other hand, increasing humidity improves the bonding of water molecules with a hydroxyl group. As a result, it forms a physisorbed layer where carriers can quickly move [51] and increases the current. The humidity sensing properties of as-synthesized and annealed rutile-TiO2 thin films were measured by monitoring the change in resistance with the corresponding change in relative humidity at room temperature (25 °C). The sensitivity was calculated using [52, 53],

$$S = \frac{{R_{{{\text{RH}}}} - R_{{\text{D}}} }}{{R_{{\text{D}}} }} \times {1}00 \, \% ,$$
(6)

where RRH is the saturated resistance of rutile-TiO2 microsphere thin film at a given relative humidity and RD is the stabilized resistance of dry air.

Fig. 10
figure 10

I-V characteristics of hydrothermally grown rutile-TiO2 microsphere (annealed at 600 °C) at different RH levels

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Figure 11 shows the variation of resistance of rutile-TiO2 microsphere thin film as a function of change in relative humidity.

Fig. 11
figure 11

Variation of resistance of rutile-TiO2 microsphere thin film as a function of change in relative humidity. The error bars shown in the figure are derived from the differences obtained in resistance calculated for all RH levels under the same set of prevailing experimental environments

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The error bars shown in the figure are derived from the differences obtained in resistance calculated for different relative humidity under the same set of prevailing experimental conditions. As seen, the resistance decreases with increase in relative humidity, which can be attributed to the mobility of charged carriers. When the rutile-TiO2 nanoparticles film is exposed to low humidity, the water molecules are adsorbed on the TiO2 surface and form a chemisorbed layer [54]. The tunneling effect of the electrons overcome the barrier between the chemisorbed layer and grain boundaries, which results in enhanced conduction in the rutile-TiO2. Figure 12 shows the schematic illustration of the sensing mechanism in the rutile-TiO2 films.

Fig. 12
figure 12

Schematic illustration of TiO2 humidity sensing mechanism (Grothuss conduction mechanism)

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Figure 13 shows the sensitivity of the humidity sensor fabricated using as-synthesized and annealed TiO2 microsphere thin films at various relative humidity levels. The error bars shown in the figure are derived from the differences obtained in sensitivity calculated for different relative humidity under the same set of prevailing experimental conditions. The sensitivity monotonically increases with increase in relative humidity except for relative humidity of 11%. At low relative humidity (11%), few water molecules are adsorbed on TiO2 microsphere film surfaces. As a result, a continuous water layer does not form. The proton hopping between the adjacent water molecules is difficult because of the immense activation energy [55, 56]. As the humidity level increases (22%), the water molecules adsorbed on the TiO2 microsphere are enough to form a continuous water layer initially. As a result, proton conduction occurs, and the sensor shows relatively good sensitivity. When the relative humidity increased gradually to 97%, more water molecules were adsorbed on the TiO2 microsphere film surface, and the water layer thickness increased. Increasing adsorbed water thickness on the sensing surface allows the water molecules to be ionized gradually under the electric field, and charge transport in the adsorbed water layer becomes convenient. Therefore, the sensitivity of the humidity sensor increases with increase in relative humidity.

Fig. 13
figure 13

Variation of the sensitivity of humidity sensor fabricated using as-synthesized and annealed TiO2 microsphere thin films at various relative humidity levels. The error bars shown in the figure are derived from the differences obtained in sensitivity calculated for all RH levels under the same set of prevailing experimental environments

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Hysteresis is the non-coincidence between a particular plot's loading and unloading behavior. For example, a perfect humidity sensor's loading and unloading curves usually follow the same path. Figure 14 shows the hysteresis curves for humidity sensors fabricated using as-synthesized and annealed rutile-TiO2 microspheres. When the relative humidity level increases from 11 to 97%, the water molecules are adsorbed on the TiO2 surface, while they are desorbed when relative humidity decreases from 97 to 11%. As seen in Fig. 14a–d, the hysteresis loop becomes narrower with an increase in annealing temperature, signifying a decrease in the hysteresis loss. Humidity sensor fabricated using rutile-TiO2 annealed at 600 °C show least hysteresis width hence better performance than the others.

Evaluating the performance of humidity sensors, response time, and recovery time are the most significant features. The response time of the humidity sensor is defined as the time required to reach 90% of the maximum resistance value observed under a higher humidity level. Similarly, the recovery time is defined as the time needed for the sensor when the resistance value drops to 10% of its maximum value [54, 57]. Thus, a good humidity sensor must have a rapid response and recovery time. The response and recovery were studied at a higher relative humidity (97%) and lower humidity level (22%) at room temperature at a bias voltage of 1 V. Figure 15 shows the variation of humidity sensors resistance as a function of time for humidity sensors fabricated using as-synthesized and annealed rutile-TiO2 microspheres. The humidity sensors response time is found to be 14.4 s, 12.1 s, 15.8 s, and 11.1 s for as-synthesized and annealed TiO2 at 200 °C, 400 °C, and 600 °C respectively, while the recovery time is found to be 1.5 s, 2.9 s, 1.9 s, and 0.8 s for as-synthesized and annealed TiO2 at 200 °C, 400 °C, and 600 °C respectively. The rapid response and recovery time (11.1 s and 0.8 s) observed the humidity sensor fabricated using rutile-TiO2 microspheres annealed at 600 °C. The fast recovery time may be due to the fast desorption process of H2O molecules from the TiO2 surface. Besides, it is observed that the rutile-TiO2 microspheres humidity sensor produces a repeatable change in resistance on exposure to RH and shows a constant value of baseline resistance, indicating stable device behavior. Table 4 shows the response and recovery time of the hierarchical nanostructure of TiO2 based humidity sensors reported earlier.

Fig. 14
figure 14

Hysteresis curves for humidity sensors fabricated using as-synthesized and annealed rutile-TiO2 microspheres

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Table 4 Comparison of response and recovery time of the hierarchical nanostructure of TiO2-based humidity sensors
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As seen, the hydrothermally grown TiO2 microsphere humidity sensor shows a fast response and recovery time compared with other oxide-based humidity sensors at higher humidity.

Stability and repeatability are important parameters for the humidity sensors. The present work tested the TiO2 microsphere humidity sensor for stability and repeatability under fixed relative humidity levels (22% and 97%). The results are shown in Fig. 16. It is observed that the current in the TiO2 microsphere humidity sensor increases with increase in annealing temperature. Thus, enhancement in current and humidity sensing performance is due to an improvement in the crystallinity in TiO2 with increase in annealing temperature. Furthermore, it is observed that the TiO2 microsphere humidity sensor produces a repeatable change in current on exposure to relative humidity and shows a constant value of baseline current, indicating stable device behavior.

Fig. 15
figure 15

Variation of sensors resistance as a function of time for humidity sensors fabricated using as-synthesized and annealed rutile-TiO2 microspheres

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Fig. 16
figure 16

Stability and repeatability tests under fixed relative humidity levels (22% and 97%) of TiO2 microsphere humidity sensors

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4 Conclusions

This paper reports the synthesis of rutile-TiO2 microsphere thin films using the hydrothermal method. The effect of annealing temperature on structural, optical, morphological, and humidity sensing properties have been investigated. No diffraction peaks due to anatase or brookite phase have been detected in the X-ray diffraction pattern, ensuring the formation of high-purity rutile-TiO2 using the hydrothermal method. The most intense peak corresponds to the (110) plane, and its intensity increases with an increase in annealing temperature, signifying an enhancement in the film's crystallinity. Furthermore, the highly intense and sharp Raman peaks indicate the highly crystalline nature of as-synthesized and annealed TiO2 materials. Surface morphology investigated by scanning electron microscopy analysis shows the formation of uniform and homogeneous microspheres-like morphology throughout the substrate surface without flaws and cracks. UV–Visible spectroscopy analysis showed no significant change in the band gap with an increase in annealing temperature. Finally, humidity sensors using as-synthesized and annealed TiO2 were fabricated on the interdigital electrode pattern to evaluate their sensing performance. The sensors showed good sensing properties in a wide relative humidity range from 11 to 97%. The humidity sensor fabricated with rutile-TiO2 microspheres annealed at 600 °C showed the highest sensitivity (~ 72%), having a fast response time (~ 11.1 s) and recovery time (~ 0.8 s). The results demonstrate that the rutile-TiO2 microsphere-based humidity sensors fabricated on an interdigital electrode pattern can be better for humidity sensors than fabricated on the traditional FTO substrate.