NaNbO3 heterojunction for boosted humidity sensing ability

Journal Pre-proof TiO2 /NaNbO3 heterojunction for boosted humidity sensing ability Renjun Si, Xiujuan Xie, Tianyu Li, Jun Zheng, Chao Cheng, Chunchang Wang

PII:

S0925-4005(20)30150-7

DOI:

https://doi.org/10.1016/j.snb.2020.127803

Reference:

SNB 127803

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

11 October 2019

Revised Date:

13 January 2020

Accepted Date:

31 January 2020

Please cite this article as: Si R, Xie X, Li T, Zheng J, Cheng C, Wang C, TiO2 /NaNbO3 heterojunction for boosted humidity sensing ability, Sensors and Actuators: B. Chemical (2020), doi: https://doi.org/10.1016/j.snb.2020.127803

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TiO2/NaNbO3 heterojunction for boosted humidity sensing ability Renjun Si1, Xiujuan Xie1, Tianyu Li1, Jun Zheng2, Chao Cheng2*, Chunchang Wang1,3*

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Laboratory of Dielectric Functional Materials, School of Physics & Materials

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Science, Anhui University, Hefei 230601, China.

Institute of Physical Science and Information Technology, Anhui University, Hefei

230601, China

State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics,

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*corresponding author

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Tsinghua University, Beijing 100084, China.

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Research Highlights

Heterojunctions greatly enhance the humidity sensitivity of composites.

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E-mail: [email protected] (Cheng), [email protected] (Wang)

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Such composite sensors have high performance parameters and reliability.



Dc bias enhances the complex impedance plot giving rises to notable tunability of the sensitivity.

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Abstract TiO2, NaNbO3, and TiO2/NaNbO3 nanomaterials were fabricated by the hydrothermal method. The humidity sensing properties of impedance sensors based on the single and composite materials were systematically studied. Our results show that the TiO2/NaNbO3-based sensor shows superior sensing performances, including ultrahigh sensitivity of 125512, rapid response/recovery time (11/15 s), extremely low

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hysteresis (less than 2%) as well as excellent stability. These sensing parameters are much better than those of TiO2- and NaNbO3-based sensors. At the same time, we found that the sensitivity of the TiO2/NaNbO3-based sensor can be controlled by a

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small dc bias. Complex impedance analysis reveals that the TiO2/NaNbO3 heterojunction plays an important role in boosting the humidity sensing properties.

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Keywords: Hydrothermal method; Heterojunction; Impedance sensors; Humidity

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sensing

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1. Introduction As one of the vital parameters of environment, humidity is playing increasingly important role in our daily lives and human activities. Therefore, it motivates extensive research activities in developing high-performance humidity sensors for its on-line detection [1-13]. Hitherto, a large number of sensing parameters and measurement techniques, such as, capacitance, impedance, resistance, optical fiber, field

effect

transistor,

surface-acoustic

wave,

quartz

crystal

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fluorescence,

microbalance, and so on, have been used to characterize the humidity sensing

properties [14-20]. A variety of materials such as polymers [12-23], metal oxides

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[24-26], and composites [24,27,28] have been used as humidity sensing materials. A

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high-performance humidity sensor should exhibit excellent sensitivity, rapid response/recovery time, small hysteresis loop as well as good repeatability. Besides,

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with the development of micro/nanofabrication methods, more and more types of

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electronic sensors, including humidity sensor, are required to be biocompatible, wearable or flexible [29-31]. However, it is difficult to simultaneously achieve all

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satisfactory merits in a single materials. Therefore, tremendous advances have been made in constructing nanosized heterojunctions and fabricating nanocomposites

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[17,32,33,34].

Being an important n-type semiconductor, TiO2 has been extensively studied due

to its simple structure, low cost, high sensitivity, and excellent stability [35-45]. The TiO2 and its composites have previously been reported to exhibit exceptional sensing characteristics under different humidity conditions. For instance, TiO2 nanobelts,

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nanorods, and nanosheets have large surface areas and are candidates for the design and manufacture of

superior humidity sensors.

In consideration

of the

antiferroelectric-ferroelectric phase transition, NaNbO3 with perovskite structure is frequently used as ferroelectric compound for the production of piezoelectric and energy storage devices [46-47]. It has been reported that humidity has significant influence on the electrical properties of NaNbO3 ceramics [48-52], giving evidence of

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good humidity sensing behavior of the material.

In our previous work, we found that Na and Nb co-doped TiO2 showed giant humidity sensitivity. The best humidity sensing properties were achieved in the

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sample with a small amount of second NaNbO3 phase [37]. This fact indicates that the

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presence of NaNbO3 in TiO2 is favourable for boosting the humidity sensing response. The humidity sensing materials, therein, were prepared by solid state reaction method

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showing an average particle size of ~ 1 m. If a wet chemical route was used to

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nanosize the particle size, much better humidity sensing ability would be anticipated. This anticipation actuates us to construct the TiO2/NaNbO3 heterojunction and clarify

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its humidity sensing properties.

In this work the TiO2/NaNbO3 heterojunction was fabricated by hydrothermal

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method. The humidity sensing characteristics of the humidity sensor based on the heterojunction were measured at room temperature in the range of 12-94% relative humidity. Our results manifest that TiO2/NaNbO3 heterojunction greatly enhance the humidity sensing performance as compared to the components. The origin of this enhancement was discussed.

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2. Experimental 2.1. Material preparation TiO2 and NaNbO3 nanomaterials were firstly synthesized by hydrothermal method. Nb2O5 and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). TiCl4 (99.9%) was purchased from Rhawn. They are analytical grade and can be used without further purification.

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TiCl4 (0.02 mol) was added into beaker 1 containing 15 mL deionized water to

form a pale white solution (TiCl4 is highly volatile and hydrolyzed while producing irritating gases. It is recommended to carry out in a vacuum fume hood). NaOH (0.05

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mol) was added into beaker 2 containing 15 mL deionized water to form a clear

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solution. Nb2O5 (0.4 mol%) was added into a beaker 3 containing 15 mL anhydrous ethanol solution to form a turbid liquid. The solution in beaker 3 was added dropwise

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into beaker 1 under magnetic stirring for 20 min until evenly mixed to form a pale

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white solution. Then, the clear solution in beaker 2 was added dropwise to the pale white solution under magnetic stirring for 20 minutes until a homogeneous mixture

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was obtained. The resulting mixture was further magnetically stirred at 50℃ for 30 minutes and sonicated for 20 minutes and then transferred to a 50 mL Teflon-lined

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stainless steel autoclave, reacted at 180℃ for 8 h. After naturally cooling down to room temperature, the the obtained precipitate was washed several times with anhydrous ethanol and deionized water. Eventually, the precipitate was dried at 80℃ for 20 h, and was calcined in a muffle furnace at 500℃ for 6 h to obtain nanocomposite TiO2/NaNbO3. Nano TiO2 was obtained by adding TiCl4 into

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deionized water and nano NaNbO3 was obtained by adding the solution in beaker 2 into beaker 3, followed by the the same subsequent steps as described above. 2.2. Characterization of materials The purity of the samples was analyzed by X-ray diffraction (XRD) performed on a Rigaku SmartLab diffractometer (Rigaku Smartlab Beijing Co, Beijing, China) with Cu Kα radiation. The morphology and structure of these materials were analyzed

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by scanning electron microscope (SEM, Model S-4800, Hitachi Co.,Tokyo, Japan) and transmission electron microscope (TEM, JEOL, JEM-2100). The current-voltage (I-V) characteristics were collected using a Keithley 2400 SourceMeter. The valence

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states of the ions in the samples were determined by X-ray photon spectroscopy (XPS)

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of Thermo-Fisher ESCALAB 250Xi.

2.3. Fabrication and performance measurement of humidity sensors

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The TiO2/NaNbO3-based humidity sensor was fabricated on Au interdigitated

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electrodes. Figure 1(a) presents the schematic diagram of sensing material based sensor as well as the photo images of the sensor and the Al2O3 substrate with Au

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interdigitated electrodes. First, each of the obtained powders of TiO2, NaNbO3, and TiO2/NaNbO3 with anhydrous ethanol solution was ball-milled in an agate jar at a

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ratio of 1:50 for 1 h, and the resulting mixture was sonicated at room temperature (300 K) for 30 minutes to form homogeneous humidity-sensing pastes. Then, 10 mL of the paste was uniformly sprayed onto a pre-cleaned Al2O3 substrate covered with Au interdigital electrodes using a 0.2 mm caliber spray pen (Sao Tome V130) to prepare the humidity sensor. Finally, the sensors were dried at 100℃ for 30 minutes

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and naturally cooled down to room temperature.

Figure 1. (a) Schematic of the the TiO2/NaNbO3-based humidity sensor. (b) Photo

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images of the Al2O3 substrate with Au interdigitated electrodes and the

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TiO2/NaNbO3-based humidity sensor.

Humidity sensing measurement setup was shown in figure 2. The different

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relative humidity (RH) environments were achieved by saturated salt solutions of

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LiCl , MgCl2 , Mg (NO3)2 , NaCl , KCl, and KNO3 in closed vessels. The humidity sensing measurements were performed in an air-conditioned room by keeping the

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temperature of 27oC. At this temperature, these closed containers can provide stable RH levels of 12, 33, 54, 75, 86, and 94% with an accuracy of ±0.7% RH. The

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impedance of the sensor was measured with Wayne Kerr 6500B precise impedance analyzer (Wayne Kerr Electronic Instrument Co., Shenzhen, China).

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Figure 2. Schematic of the humidity sensing experimental setup: (a) PC, (b) Wayne

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Kerr 6500B meter, and (c) different humidity environments. 3. Results and discussion 3.1. Structure and morphology characterizations

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The prepared powders were examined by XRD characterizations and the results were shown in Fig. 3. The XRD peaks of TiO2 and NaNbO3 agree well with the

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standard JCPDS card nos. 21-1276 and 19-1221, respectively. The XRD pattern of

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(JCPDS 25-0854) phases.

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TiO2/NaNbO3 reveals a small amount of Nb2O5 (JCPDS 19-0862) and Na3NbO4

Figure 3. XRD patterns of TiO2, NaNbO3, and TiO2/NaNbO3 nanopowders. SEM images of TiO2, NaNbO3, and TiO2/NaNbO3 as well as high resolution TEM image of TiO2/NaNbO3 are shown in Fig. 4. It can be seen from the SEM images that 8

TiO2 and NaNbO3 show, respectively, a needle-shaped and cube-shaped structure. The structure of TiO2/NaNbO3 appears to be nano heterojunctions formed by stacking the cubic NaNbO3 on the needle TiO2 surface. To further confirm this feature, high-resolution TEM image of the TiO2/NaNbO3 powder was performed and the result was displayed in Fig. 4(d). The fringe spacing of TiO2 and NaNbO3 can be observed in the TEM image. The fringe spacing of 0.25 nm and 0.275 nm for the

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selected particle corresponding to the (1 0 1) plane of rutile TiO2 and the (1 1 0) plane

of NaNbO3, respectively. The NaNbO3 nanoparticle is on top of the needle TiO2 phase. TEM observation and element mapping reveal that the needle-shaped particle is TiO2

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phase and the cube-shaped particle is NaNbO3 phase [see Fig. S1 of the Supporting

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Information].

Figure 4. (a) SEM images of TiO2, (b)NaNbO3, (c) TiO2/NaNbO3 powders and (d) high-resolution TEM image of TiO2/NaNbO3 powder.

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To further evidence the heterojunction nature, I-V measurements on the TiO2-, NaNbO3-, and TiO2/NaNbO3-based sensors under different RH environments were conducted. The I-V curves of TiO2/NaNbO3-based sensor were shown in figure 5(a) and the full view of the curve at 12% RH was given in figure 5(b). From which two main feature can be extracted: (1) The curves at all tested RH levels exhibit linear behavior in the low-voltage range followed by notable increase in current for both

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positive and negative polarizations leading to pronounced nonlinear behavior in the

high-voltage range. (2) As shown in the inset of figure 5(b), the slope for the I-V curves reveals that the voltage range for the linear behavior drastically decrease with

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increasing the RH level. For example, the nonlinear behavior for the I-V curve at 12%

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RH occurs when the applied voltage is larger than 17 V. While for the curve at 94% RH, the voltage decreases to 0.06 V. On the contrary, only linear I-V behavior was

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observed in the TiO2-and NaNbO3-based sensors as shown in Fig. S2 of the

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Supporting Information. It is well-known that a Schottky barrier is associated with energy band bending owing to the mismatch of Fermi levels of the materials on both

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sides of a heterojunction. The I-V curve of a the heterojunction can feature both rectifying and non-rectifying behaviors. In the cases of PN junction and

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metal-semiconductor structure, the current exhibits polarity and I-V curve features rectifying behavior, i.e., the current is on state at positive bias and off state at negative bias. While in the case of a grain-boundary-like heterojunction such as the TiO2/NaNbO3 junction of the present work, the statistical distribution of TiO2/NaNbO3 and NaNbO3/TiO2 heterojunctions makes the current without polarity.

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Therefore, instead of the rectifying behavior, the I-V curve of this kind heterojunction is characterized by the nonlinear behaviour of

I  V



(  is the non-linear coefficient)

with almost symmetrical forward and backward branches. The above features firmly evidence the heterojunction nature resulting from the sensing materials [53]. At high RH levels ≥86% and high voltages above 0.09 V, the slope is greater than 2, implying that the energy barrier between the sensing materials can be overcome and the charge

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injection occurs. It is apparent that the transport properties of the TiO2/NaNbO3 heterojunction are affected by the energy barrier. Previous workers have described a humidity mechanism for heterojunction sensors [54,55], in which the increase in

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conductivity is due to proton migration, which is formed by electrolytic

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decomposition of adsorbed water molecules. I-V curves of TiO2 and NaNbO3 are

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shown in Fig. S2 of the Supporting Information.

Figure 5. Type I-V characteristics of the TiO2/NaNbO3-based sensor recorded at

different humidity levels (a) and (b) and the curve slope as a function of voltage in the positive polarization side (inset). 3.2. Humidity sensitive characteristics

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Figure 6(a) compares the impedance as a function of RH level recorded at 100 Hz of the TiO2-, NaNbO3-, and TiO2/NaNbO3-based sensors. The impedance of the TiO2-, NaNbO3-based sensors almost exponentially decreases with the increasing of RH level [note the logarithmic scale]. Whereas the impedance of the TiO2/NaNbO3-based sensor sharply decreases with the increasing of RH level. The impedance of the TiO2/NaNbO3-based sensor changes from 252 MΩ @ 12% RH to

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2010.96 Ω @ 94% RH, varying by more than five orders of magnitude. Which is

several orders of magnitude larger than those of the TiO2-based sensor (from 114 MΩ @12% RH to 3.19 MΩ @ 94% RH) and the NaNbO3-based sensor (from 91.71 MΩ

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@12% RH to 39.3 kΩ @ 94% RH). This finding indicates that the TiO2/NaNbO3

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heterojunction greatly boosts the humidity sensing ability. We, therefore, performed detailed characterizations on the TiO2/NaNbO3-based sensor. Figure 6(b) presents the

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impedance as a function of RH level of the TiO2/NaNbO3-based sensor recorded with

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different frequencies. The variations of impedance become weak with increasing measurement frequency. Therefore, 100 Hz is selected as the optimum test frequency

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and well be used in the following part.

Figure 6. (a) The impedance as a function of RH level measured at 100 Hz for all 12

sensors. (b) The impedance as a function of RH level of the TiO2/NaNbO3-based sensor recorded with different frequencies. Based on the data in Fig. 6(a), the humidity sensitivity of the sensor can be calculated according to the relation [56,57] S  R d /R

(1)

h

where Rd and Rh are the resistance values measured at 12% RH and at a specific RH

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level, respectively. The impedance and S values for all sensors are given in Table S1

of the Supporting information. The TiO2/NaNbO3-based sensor was found to show an

humidity sensors. 7

displays

a

comparison

of

the

performances

between

the

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Figure

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unprecedented large sensitivity of S=125512. This value rivals all known impedance

TiO2/NaNbO3-based sensor and the impedance-type humidity sensors reported in the

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literature [17, 38, 39, 41, 43, 44, 45, 46, 47]. The comparison highlights the largest

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sensitivity [note again the logarithmic scale] and comparable performances of recover

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and response times.

Figure 7. A comparison between the present sensor and other impedance-type humidity sensors from published literature. 13

In order to fully characterized the performances of the TiO2/NaNbO3-based sensor. The hysteresis behavior, recovery/response curves of the sensor were measured at the optimum frequency, and the results are given in figure 8. The hysteresis curve was obtained by successively switching the sensors between the vessels with the RH levels of 12, 33, 54, 75, 86, and 94%, and then shifting back. After exposing the sensor in each of the RH level for 5 min, the impedance values are

were calculated by the following formula [38]:

where

Z des

and

Z ads

%

(2)

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 log  Z des   log  Z ads      100   log Z des  

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recorded. Based on the measured impedance values, the humidity hysteresis values

represent the impedance of the desorption and adsorption

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processes, respectively. A spectacular hysteresis value of 0.71% is achieved for the

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TiO2/NaNbO3-based sensor. Hysteresis is caused by chemical adsorption and physical adsorption. The physical adsorption is relatively easy to be desorbed, whereas the

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chemical adsorption is weakly affected by environmental humidity. Compared to the component materials, the physical adsorption in composite materials is much greater

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than the chemical adsorption, especially at medium and high humidity level, which

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leads to the sensor showing good hysteresis effect in the whole humidity range [24,42,43]. In addition, the hysteresis is also related to the diffusion rate of water molecules and the amount of the sensing materials [58,59]. In the desorption process, the faster the water diffusion rate, the smaller of the hysteresis, and the thicker the sensing material, the larger of the hysteresis. Due to the excellent hydrophilicity of TiO2 [60,61], the diffusion rate of water molecules is fast. Meanwhile, the small 14

amount of the sensing materials we used in the process of sensor preparation. These factors may be the reasons for the negligible hysteresis exhibited by the TiO2/NaNbO3 sensor. The response and recovery characteristics were measured by placing the sensor back and forth between RH 12% and other humidity levels with a dwell time of 5 min in each RH level and then measuring the impedance. The humidity response/recovery

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time is defined as the time required for the sensor to reach 90% of the total impedance

change during adsorption and desorption processes [62,63]. Table S2 in the Supporting information displays the recovery/response time between each humidity

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and 12% RH. The results show that the TiO2/NaNbO3-based sensor exhibits excellent

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recovery/response time.

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Figure 8. (a) Hysteresis behavior and (b) recovery/response curves of the TiO2/NaNbO3-based sensor.

The repeatability and stability of the TiO2/NaNbO3-based sensor were further

tested. The repeatability of the sensor was evaluated by switching the RH level for 5 cycles between 12% and 94% RH with a dwell time of 5 min in each RH level. The impedance values of the sensor at different humidity levels were recorded every ten 15

days to assess the stability. The results of the repeatability and stability are shown in Fig. 9(a) and (b), respectively. In the five repeated tests, the final value of each time reaches more than 98% of value of the first time. Figure 9(b) shows that the sensor has excellent stability, with a fluctuation of about 10% at 75% RH and no more than 5% at the rest humidity

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Figure 9. Repeatability (a) and stability (b) curves of the TiO2/NaNbO3-based sensor. 3.3. Tuning the sensitivity of TiO2/NaNbO3-based sensor by a dc field

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In order to better apply the humidity sensitivity characteristics of TiO2/NaNbO3-based sensor. The possibility of tuning the humidity sensitivity was

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explored by applying an additional dc electric field at 12 and 94% RH. To this end,

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the sensor is successively exposed to the environments 12%-94%-12% RH with the duration time of 5 minutes in each environment, Then the impedance was recorded under different dc biases ranging from 0 to 7 V. The results were plotted in figure 10. From which. One notes that the impedance in the lower RH level of 12% RH is almost independent of the dc bias. While the impedance in the higher RH level of 94% RH can be notable enhanced by the dc bias. This fact endows the sensor with 16

good tunability of sensitivity, which can be estimated by the following expression: n 

ZV

 100

%

(3)

Z0

where

Z0

and

ZV

represent the impedance at 0 bias and V bias, respectively, in a

94% RH environment. As the dc bias voltage increases from 1 to 7 V, the values of n are calculated to be 13.55, 25.42, 32.2, and 35.59%, respectively. This means that the

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humidity sensitivity of the sensor can be adjusted by applying a small dc bias. This feature confers the sensor attractiveness for practical applications. Additionally, the

dc-bias-dependent impedance implies that the Maxwell-Wagner relaxation dominates

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the impedance at high RH level [64].

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Figure 10. The normalized impedance as a function of time for the

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TiO2/NaNbO3-based sensor measured in the environments of 12%-94%-12% RH under different dc biases.

3.4. Humidity sensing mechanism Collectively, our results demonstrate that the TiO2/NaNbO3-based humidity sensor has excellent humidity sensing performances. In order to gain deep information about the mechanism of the humidity sensitivity, XPS spectra were measured and the 17

results of TiO2/NaNbO3 were shown in figure 11. The XPS spectra of the TiO2 and NaNbO3 were given in figures S3 and S4 of the Supporting Information, respectively. It can be seen from the figure 11 that the composite material consists of Ti, O, Na and Nb elements. The Ti 2p spectrum shown in figure 11(a) can be fitted by four Gaussian peaks, indicating that Ti3+ and Ti4+ exist simultaneously [65-67]. The existence of Ti3+ is suitable for humidity sensing [68,69]. The O 1s spectrum [figure 11(b)] can be

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fitted by three Gaussian peaks with the binding energy from low to high

corresponding to the oxygen atom on Ti-O bond, oxygen vacancy, and chemisorption oxygen[65,70,71]. Meanwhile, due to the existence of Ti3+, oxygen vacancies adjacent

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to Ti3+ will be generated [72]. This can be proved by the slight shift of the O 1s peak

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from 529.7 eV to 529.9 eV. Ti3+ and oxygen vacancies play a very important role in increasing humidity sensitivity, as both of these defects can improve the

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chemisorption/desorption of H2O molecules [73]. For Na 1s [figure 11(c)], the peak at

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the binding energy of 1071.4 eV demonstrates that the oxidation state of Na is +1 [74]. As shown in figure 11(d), peaks corresponding to Nb 3d5/2 and 3d3/2 were observed at

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the binding energy of 206.6 eV and 209.4 eV respectively, withou no other 3d5/2 at low binding energy. This fact confirms that the oxidation state of Nb is +5. The

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binding energy difference between the two peaks is 2.8 eV, which is consistent with the previously reported data [65,70].

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Figure 11. XPS spectra of Ti 2p (a), O 1s (b), Na 1s (c) and Nb 3d (d) of TiO2/NaNbO3.

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Based on the above results, we can now discuss the humidity sensing mechanism. To better describe the mechanism, we first describe the sensing

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mechanisms of monolithic components (i.e., pure TiO2 and NaNbO3). Humidity

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sensing of TiO2 is main caused by oxygen vacancies [37], according to the defect reaction:

 

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H 2O  VO

 O O  2 OH



(4)

O

The alkali-ion-containing compounds display appreciable hydrophilic character

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as

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usually due to their surface basicity [75]. For NaNbO3, this process can be described

H O  Na  O  (Na



2

When NaNbO3 (band gap

Eg

 OH

-

)  (OH)



 e



(5)

=3.6 eV [76]) loads on TiO2 ( E g =3.2 eV [77]), a

potential barrier will develop at the TiO2/NaNbO3 heterojunction due to the band bending as illustrated in Fig. 12(a) (Φb stands for the barrier height). The TiO2/NaNbO3-based sensor, therefore, exhibits a very high impedance value. When 19

the sensor is exposed in moisture environment with low RH levels, water molecules are chemically adsorbed at the hydrophilic active sites as described by Eq. (4). The resulting electrons will transfer from NaNbO3 to TiO2 [Fig. 12(a)], leading to energy level rising in the TiO2 side the and falling in the NaNbO3 side, thereby reducing the potential barrier height [Fig. 12(b)]. Hence, the impedance decreases with the increasing of RH level. As the RH level increases, the water molecules are further

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adsorbed by electrostatic interaction due to the OH--groups, and a physical adsorption

water layer, i.e., a continuous aqueous layer is formed [78]. The layer facilitates the

transfer of H2O or H3O+ as proposed by Grotthuss [79] and Casalbore-Miceli [80] et



 H 3O



 H 2O

(6)

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H 2O  H 3O

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The rapid transfer of ions in the aqueous layer greatly reduces the impedance, giving

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rise to high sensitivity of the sensor. In addition, the second phases of Nb2O5 and

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Na3NbO4 may be another factor in achieving a higher level of humidity sensitivity of

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the TiO2/NaNbO3-based sensor.

Figure 12. Schematic diagrams showing the energy band variations of the

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TiO2/NaNbO3 heterojunction after adsorbing water molecules at (a) low and (b) high humidity levels. In order to further confirm the sensing mechanism, the complex impedance diagrams of TiO2/NaNbO3-based sensor were measured in different RH environments. The results are shown in Fig. 13(a). The sensor exhibits similar impedance diagrams at lower RH levels of 12 and 33%. Figure 13(b) shows, representatively, the full view

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of the complex impedance plot at 12% RH recorded with and without a dc bias of 5 V.

The plot exhibits as an arc indicative of very large impedance. The high-frequency range of the arc is independent of the bias, whereas the low-frequency range of the arc

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can be depressed by the bias, demonstrating that the low- and high-frequency ranges

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result from the bulk and heterojunction, respectively. Because the heterojunction is associated with Schottky barrier that can be depressed by an applied dc bias [64].

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Additionally, the slight depression of the complex impedance plots confirms the

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impedance related to the Schottky barrier is very large, and the its change induced by the dc bias can be negligible. In this case, the water molecules are chemically

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adsorbed by the sensor and the continuous aqueous layer is yet to take shape, as shown in figure 12(a). The impedance spectrum is mainly contributed by the bulk part

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and TiO2/NaNbO3 heterojunction.

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Figure 13. (a) Complex impedance diagrams of TiO2/NaNbO3-based sensor recorded

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at different RH levels. (b) Complex impedance plots recorded at 12% RH without and with a dc bias of 5 V. (c) Complex impedance plots at high humidity levels of 75, 86,

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and 94% RH.

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When the sensor is placed in a moderate RH level of 54%, the physical adsorption process of water molecules exceeds the chemical adsorption process. The

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chemisorbed layer is covered by the physical adsorption layer forming a continuous aqueous layer, as shown in figure 12(b). Based on the ion transport mechanism

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reported by Casalbore-Miceli et al., [79,80] the H2O molecules can be dissociated into

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H3O+ and OH-. The H3O+ ions become the main charge carriers in the physical adsorption layer and can freely transfer throughout this layer, resulting in a sharp drop in impedance, as is evidenced by the impedance plots. Meanwhile, a linear low-frequency “tail” indicative of Warburg impedance appears. When the sensor is placed in higher humidity environments, the physical adsorption layer is becoming saturated. The impedance,

Z 

(Z ' )

2

 (Z " )

22

2

as illustrated by the magnified view

of the complex impedance plots of 75, 86, and 94% RH presented in Fig 13(c), is dominated by the Warburg impedance and slightly decreases with the RH level increasing from 75 to 94%. It is worth noting that: (1) the applied dc bias will lead to electron injection. The injected electrons neutralize the charged oxygen vacancies in the OH--groups. This is harmful for water absorption. Thus, contrary to Schottky barrier, the dc bias at this stage enhance the complex impedance plot giving rise to

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notable tunability of the sensitivity. (2) The formation of the continuous aqueous layer leads to inhomogeneous distribution of conductivity, which, in turn, gives rise to

Maxwell-Wagner relaxation. Obviously, without the continuous aqueous layer at

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lower RH levels, the sensor is not very sensitive. Likewise, when a saturated aqueous

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layer is formed, the variations of the impedance will also trend to be saturated, i.e., the sensor is also not very sensitive at higher RH levels.

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4. Conclusions

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In this work, we present a comparative study on the TiO2-, NaNbO3-, and TiO2/NaNbO3-based humidity sensors. The TiO2/NaNbO3-based sensor shows

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excellent sensing properties of ultrahigh sensitivity, rapid response/recovery time, negligible hysteresis, good stability and tunability, which are superior to those of the

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pure TiO2- and NaNbO3-based sensors. The boosted humidity performances were argued to be caused by the TiO2/NaNbO3 heterojunction. Our results highlight that the TiO2/NaNbO3 nanocomposite is a promising humidity sensing material and would enable widespread applications in humidity sensor.

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Conficts of interest There are no conflicts to declare.

Declaration of Interest Statement: No conflict of interest exists in the submission of this manuscript, and this manuscript has been approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described is original research that has not been published previously, and not under consideration for

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publication elsewhere, in whole or in part.

Acknowledgements

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The authors acknowledge financial support from the National Natural Science

Foundation of China (Grant Nos. 51872001 and 51572001) and the Open Research

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Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics

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(Grant No. KF201803). References

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Author biographies Renjun Si: is a graduate student in Anhui University, China. His research focuses on the development of humidity-sensitive sensors and dielectric functional materials.

Xiujuan Xie: is a graduate student at Anhui University in China. She specializes in

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the development of chemical synthesis, stability, adsorption.

Tianyu Li: is a PhD student major in Material Science and Engineering at Anhui University. He has received his B.S. degree in Rare Earth engineering and the M. E.

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degree in Materials Science and Engineering from Inner Mongolia University of

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Science and Technology, Baotou, China, in 2015 and 2018, respectively. His research

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and varistor ceramics.

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interest is focused on dielectric energy storage, colossal permittivity, humidity sensing

Jun Zheng: is a professor of the Institute of Physical Science and Information

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Technology, Anhui University, China. He received his Ph.D. degree from Xiamen University. His current research interests include crystal structure, synthesis, stability,

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adsorption, nickel coordination polymer.

Chao Cheng: Chao Cheng is a professor of Institute of Physical Science and Information Technology, Anhui University, China. He received his Ph.D. degree from Tsinghua University. His current research interests include programming, homepage

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production and material microscopic fields.

Chunchang Wang: is a professor of School of Physics & Materials Science, Anhui University, China. He received his Ph.D. degree in material science and engineering from Tsinghua University in 2004. His current research interests include energy

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storage, dielectric and humidity sensing materials.

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