Improvement of humidity sensing performance of BiFeO3 nanoparticles-based sensor by the addition of carbon fibers

Journal Pre-proof Improvement of humidity sensing performance of BiFeO3 nanoparticles-based sensor by the addition of carbon fibers Rachida Douani, Nouara Lamrani, M’hand Oughanem, Malika Saidi, Yannick Guhel, Ahc`ene Chaouchi, Bertrand Boudart

PII:

S0924-4247(19)32196-X

DOI:

https://doi.org/10.1016/j.sna.2020.111981

Reference:

SNA 111981

To appear in:

Sensors and Actuators: A. Physical

Received Date:

2 December 2019

Revised Date:

14 March 2020

Accepted Date:

21 March 2020

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Improvement of humidity sensing performance of BiFeO3 nanoparticles-based sensor by the addition of carbon fibers Douani Rachida1*, Lamrani Nouara1, Oughanem M’hand1, Saidi Malika1, Guhel Yannick2, Chaouchi Ahcène1, Boudart Bertrand2 1 2

Laboratoire deChimie Appliquée et Génie Chimique /U. de Tizi Ouzou, Tizi Ouzou, Algérie Normandie Univ, UNICAEN, ENSICAEN, CNRS, GREYC, 14000 Caen, France

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Corresponding author’s e-mail address [email protected]

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Graphical abstract

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Water vapor adsorption on the sensor’s surface

Highlight 

The nanocomposites BFO/CF based sensor has a high sensitivity to humidity of up to 12640 % in the relative humidity range (16-92 %).

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The maximum sensitivity reached by the BFO-based sensitive layer does not exceed 4652 % in the same RH range.

Abstract This paper presents humidity sensing elements based on bismuth ferrite nanoparticles BiFeO3 (BFO) synthesized by a sol-gel method and bismuth ferrite / carbon fibers nanocomposites (BFO/CFs) prepared by a hydrothermal process. The morphological and structural characteristics of the nanomaterials were realized by SEM, BET, XRD, and Raman

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spectroscopy. The electrical properties of the sensing elements were studied by impedance

spectroscopy. The humidity sensing properties of the two elements (BFO and BFO/CFs) were studied in the frequency range 300Hz-1 MHz, at room temperature and over the relative

humidity (RH) range between 16 and 92%. The results showed that the impedance of the

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developed sensors decreases considerably when the sensitive layers absorb water vapor. We

also noticed that they have a small hysteresis and good stability. Compared to the BFO-based

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humidity sensor, the BFO/CFs based humidity sensor shows a high sensitivity of 12640%. Finally, in order to understand the mechanisms of adsorption of water molecules on the surface of the sensitive layers, we plotted the variation of the imaginary part of the impedance

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versus the real part. The results showed that the relaxation mechanism is predominant. The results presented in this paper demonstrate the potential of BFO/CFs composite in the design

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of high-performance humidity sensors.

Keywords: humidity sensor, nanoparticles, BFO/CFs nanocomposite, impedance, bismuth ferrite, carbon fibers.

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1. Introduction

Water is one of the essential components of all living organisms on Earth, in the

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biosphere, as well as most materials used by people. Even in trace amounts, it can have a considerable influence on the physical, chemical, mechanical and technological properties of natural or artificial materials [1]. The measurement and control of humidity is important in many fields, including industry, medicine and food production, environmental protection and physico-chemical processes in pharmaceuticals [2]. Desirable characteristics of humidity sensors are high sensitivity, low hysteresis, high chemical and physical stability, fast response and recovery times.

Humidity sensing elements based on oxidized materials are widely studied. Materials based on CeO2 [3], TiO2 [4, 5], MgTiO3 [6], ZnO [7, 8], Si-Bi-O [9], Li − CuFe2O4 [10] and Sn-NiFe2O4 [11] are used. Sensor elements based on polymers and inorganic/organic composite materials are also developed [12-16]. Furthermore, humidity sensors based on nano-sized grains and nanoporous structures have aroused great interest due to the high surface exposure for the adsorption of water molecules [17-19]. Recently, carbon-based materials have attracted considerable interest in gas/vapor detection and other applications due to their high surface volume ratio, high surface activity, high stability and mechanical rigidity [20, 23].A series of oxide compounds with a simple or doped perovskite-likes tructure

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have also been investigated as materials for resistive humidity sensors [24]. Perovskite BiFeO3 is well known for its multiferroic properties at room temperature and has been investigated as a new material for non-volatile memories, spintronics and

magnetoelectric sensor devices [25, 26]. It is also used as a promising material for gas sensing applications due to its p-type semiconductor behavior [25, 27, 28].However, this multiferroic

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material has rarely been used for humidity sensors [29].

Motivated by the possibility of improving the parameters of humidity sensors based on

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perovskite BFO structures by using suitable additives, this paper presents a humidity sensor element based on the BiFeO3 surface layer, prepared by a sol-gel process. The influence of

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the addition of carbon fibers on the humidity sensing characteristics of the resulting layer (BiFeO3/carbon fibers) was also investigated. 2. Experimental

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In this research, we used chemical reagents of high analytical purity. The chemical reagents used were ferric nitrate (III) nanohydrate (Fe(NO3)3, 9H2O) (99%), bismuth nitrate pentahydrate (Bi(NO3)3, 5H2O) (99%), nitric acid HNO3(69%), citric acid monohydrate

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(C6H8O7, H2O) (99.5%), carbon fibers, polyvinyl alcohol (PVA) and deionized water. Sol-gel and hydrothermal methods were chosen to synthesize BiFeO3 nanoparticles and BiFeO3/FC

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nanocomposites, respectively.

2-1. Synthesis of BiFeO3 nanoparticles The sol-gel method has been used to prepare the BiFeO3 nanoparticles. An aqueous

solution of bismuth nitrate with a concentration of 0.105 M is prepared by dissolving 2.5468 g of (Bi(NO3)3, 5H2O) in 50 ml of deionized water, then 20ml of nitric acid is added by dripping until a transparent solution is obtained. Thereafter, 2.02g of (Fe(NO3)3, 9H2O) were added to the solution. Finally, 5.0708g of citric acid (chelating agent) were added to the mixture. The mixture was kept under magnetic stirring at 80°C for about 3 hours to obtain a

gel, which was dried in an oven at 100°C for 16 hours. The recovered powder was then ground with a mortar and calcined at 550°C for 2 hours. Finally, the product was washed several times with deionized water and then dried at 70°C for 8 hours. The different steps of synthesis are illustrated in Figure 1. 2-2. Synthesis of nanocomposite BiFeO3 /Carbon Fibers (BFO/CFs) As shown in Figure 2, a suitable mass of finely ground carbon fibers was weighed and dispersed in 5 ml of deionized water in a beaker. In another beaker, 0.25 g of BFO nanoparticles were also dispersed in 5 ml of deionized water. Both beakers were placed under ultrasound for 15 minutes. The suspensions were then mixed and transferred to hermetically

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sealed vials immersed in silicone oil heated to 100°C for 6 hours. The resulting BiFeO3/CFs nanocomposite was retrieved by filtration and was rinsed with deionized water and finally dried at 60°C. 2-3. Preparation of sensors

The sensors were developed using copper metal spiral electrodes with a surface area of

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180 mm2 and sensing films. Ahomogeneous solution was prepared by dispersing 0.2 g of the powders as synthesized (BFO or BFO/CFs) in polyvinyl alcohol (about 3 drops). The mixture

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was then manually applied on the pre-cleaned electrodes and dried at 160°C for 1 hour to remove 95% of the PVA matrix. The polyvinyl alcohol polymer was used as the matrix to

2-4. Sensing measurements

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ensure the adhesion of the powders to the surface of the structures.

In order to produce a controlled wet environment, a series of standard saturated salt

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solutions were used. These salts and the relative humidity generated are listed in Table 1. These relative humidity levels were measured using a standard thermo-hygrometer. Electrical measurements were made using the HP 2484A LCR, at various relative humidity levels at

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room temperature (22°C) and atmospheric pressure. The applied voltage was 1 V and the frequency was adjusted from 300 Hz to 1 MHz. The set up used for the measurements is

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shown in Figure 3.

2-5. Material characterization The surface morphology of prepared BFO nanoparticles and BFO/CFs

nanocomposites was analyzed by scanning electron microscopy (SEM) (PHILIPS ESEM XL30 with tungsten filament). The crystallographic structure and the effect of the addition of carbon fibers on the crystallinity of BFO nanoparticles were studied by X-ray diffraction (XRD) (XPERTPRO type diffractometer with Cu Kα radiation at 45 kV).The perovskite structure of the nanoparticles and the presence of carbon fibers in the nanocomposite were

determined by Raman spectroscopy (Renishaw’sIn Via spectrometer using a visible laser with a wavelength of 632.8 nm).The BET specific surface area of the materials was determined by measuring the nitrogen adsorption-desorption contents. The measurements were carried out using a Quanta chrome instrument driven by Nova Win software. Impedance measurements of the samples were carried out by the HP 2484A LCR measuring instrument. 3. Results and discussion 3-1. Structural and morphological characterization Figure 4 shows the X-ray diffraction patterns of BFO nanoparticles and synthesized BFO/CFs nanocomposites. The Bragg angles 2θ observed for both BFO and BFO/CFs are in good agreement with the data of the standard diffraction model (JCPDS n°96-210-2910) of

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the hexagonal phase R3c whose strongest peak is observed at 2θ = 32°. It represents the preferential axis of crystal growth oriented along the (104) plane. All diffraction peak

positions for the BFO/CFs nanocomposite samples were almost identical to those of pure BFO powder, indicating that addition of carbon fibers into BFO nanoparticles does not

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change their crystal structure.

Diffractograms show the appearance of a secondary phase corresponding to the

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selenite (Bi25FeO39) at the 2θ positions of about 28°, 30.5°, and 33° (JCPDS n°96-901-1269) for both systems but no obvious carbon peak was observed in the XRD patterns of the

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BFO/CFs nanocomposites. These results are in good agreement with those described in the literature [20, 30].The crystallite size of the BFO particles is calculated using the semiempirical Debye-Scherrer formula (equation 1): (1).

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0.9𝜆

𝐷 = 𝛽𝑐𝑜𝑠𝜃

Where D is the crystallite size,  is the X-ray wavelength,  is the full width at half maximum

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and θ is the half diffraction angle. The calculated crystallite size was approximately 2.3 nm for the BFO and 4.7 nm for the BFO/FCs.

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The room-temperature Raman spectra of BFO nanoparticles and BFO/CFs nanocomposites are shown in Figure 5. The observed spectral peaks can be attributed to the BFO. The peaks observed at 141, 170 and 220 cm-1 can be attributed to Bi-O band vibration modes A1-1, A1-2 and A1-3, respectively, and the Raman peaks recorded at 274, 350.7, 469.9 and 529.6 cm-1 can be attributed to the Fe-O band vibration modes E3, E5, E7 and E8, respectively [31]. Compared to the values given in the literature, there is a small difference [32, 33], which can be related to the difference between the powders in terms of crystallinity or size.

Analyses confirmed the presence of carbon fibers in BFO/CFs nanocomposites by the appearance of two Raman bands located at approximately1352.5 and 1592.3 cm-1. These correspond to the D and G bands of carbon [34]. The specific surface area of the as-prepared BFO nanoparticlesand BFO/CFs nanocomposites was determined from the BET linear plot. A high BET specific surface area of 253m2/g was obtained for BFO/CFs nanocomposites, while only 2.3 m2/g was recorded for BFO nanoparticles. This will probably increase the number of adsorption sites for water vapor molecules on the sensor surface. Figure 6 shows the SEM observations of pure BFO nanoparticles and composite

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BFO/CFs. The micrographs show that the BFO nanoparticles are nanoscale. In addition, Figure 6(b) clearly shows an inhomogeneous dispersion of BFO nanoparticles on the surface of the carbon fibers. 3-2. Electrical characterization

Figure 7 shows the impedance as a function of the relative humidity of the BFO and

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BFO/CFs sensors. The measurements were performed with a frequency of 1 kHz. We can see that the impedances of the BFO/CFs samples are lower than those of the BFO samples for the

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studied relative humidity range. The results show that sensitive materials have high impedance which varies very slightly for low humidity values.

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On the other hand, we note that the impedance values of the sensor based on BFO/CFs nanocomposites decrease significantly from the humidity level of 57%, while those of the BFO layer decrease gradually.

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For the BFO/CFs sensing layer, between 57 and 70% relative humidity, the impedance decreases abruptly from 19 to 3 MΩ to reach 166 kΩ at 92% relative humidity (see Figure 7). But for the BFO sensing layer, the impedance value is about 23 MΩ at 57% RH and decreases

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to 20 MΩ at 70% RH to reach 534 kΩ at 92% RH. Possible reasons for this improvement in detection are the creation of more active sites such as defects and the heterojunction that can

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be created at the interface of nanoparticles and carbon fibers [35]. This shows the positive effect of the incorporation of carbon fibers into the samples, which contributes considerably to the adsorption process of water moleculesandto the improvement ofthe sensitivity of our devices. The decrease in the impedance value of the sensitive layers with increasing RH level means an increase in the conductivity of the layers, which is mainly due to the adsorption of water molecules on their surface. It can be seen that for the same frequency value: at low humidity (few water molecules adsorbed on the surface) the impedance variation is

insignificant, but it is more pronounced at high RH values (high number of adsorbed water molecules). It is believed that the main mechanism of conductivity is the exchange of protons between the sensing films and the adsorbed water molecules (Figure 8). At low RH, the water molecules on the surface of the sensitive materials were mainly chemisorbed on Bi3+ and Fe3+ cations [29] and the layer of water molecules was not complete, resulting in low polarization and high impedance. In this case, we can conclude that the conductance of the sensor was provided by the intrinsic impedance of the sensing materials. On the other hand, a considerable decrease in impedance is observed at high humidity level, which can be explained by the physical adsorption process of water molecules and the type of conduction of

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proton [36-39]. The conduction process is the same as that of pure water and is called Grotthuss chain reaction(figure 9).With the increase in humidity, i.e.the number of layers of water molecules formed during physical adsorption, the number of protons increases

considerably and they begin to move freely in the water layer. This leads to a considerable

increase in conduction and a decrease in the impedance of the humidity sensing elements [4,

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39, 40].

The relationship between the impedance of BFO and BFO/CFs based sensors and

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frequency was investigated at different RH levels. The results are shown in Figure 10. It was observed that at low RH, the impedance of the humidity sensors sharply decreases with

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increasing frequency. The impedance difference between two adjacent curves becomes progressively smaller as the RH increases. However, at high frequency, the impedance hardly drops with RH, indicating that the impedance of the sensors becomes independent of RH.

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When the frequency is low, the direction of the electrical field changes slowly and the space-charge polarization of the adsorbed water appears [41]. At high RH level, more water molecules are adsorbed and the polarization is therefore stronger. As a result, the device

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impedance decreases. At high frequencies, the impedance changes a little, this phenomenon is based on the fact that the direction of the electric field changes rapidly. In fact, the

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polarization of water molecules cannot catch up with the direction of the electric field [42-43, 17].

The hysteresis effect is one of the most important characteristics to evaluate the

stability and reliability of humidity sensors. It is defined as the maximum difference between the adsorption and desorption curves [44]. Figure 11 shows the humidity hysteresis of the sensors, recorded by scanning the RH from low to high values then scanning backwards. It can be seen that during cyclic humidity measurement, the BFO and BFO/CFs layers reveal a

narrow hysteresis–loop.The maximum humidity hysteresis (𝐸𝑚𝑎𝑥 ) is calculated using the expression (equation 2) [45]: 𝐸𝑚𝑎𝑥 =

∆𝑚 2𝑌𝐹𝑆

(2)

Where, Δm is the maximum hysteresis and YFS is the full scale output.The BFO-based humidity sensor showed a maximum hysteresis of about 3% corresponding to 70% RH, while it is about 3.5% for the BFO/CFs-based sensor corresponding to 57% RH, indicating good sensor’s reliability. Response and recovery time are other crucial parameters for evaluating sensor performances. The response time is the time taken by the device to reach 90% of the total

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impedancein the case of adsorption, or the recovery time in case of desorption.

Figure 12 shows the humidity response and recovery times of BFO and BFO/CFs

sensors, measured by switching the humidity level of the test environment between 16 % and 92%.

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Response and recovery times for BFO nanoparticles were determined to be 50 s and 150 s, respectively, so those of BFO/CFs were approximately130 s and 75 s, respectively.

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Compared to humidity sensors based on nanomaterials, the balance between the response and recovery time of the two sensors is greatly improved [16, 46]. Table 2 shows the comparison

reported humidity sensors.

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of the humidity sensing properties of BFO and BFO/CFs humidity sensors and impedance-

The sensitivity of the humidity sensor was calculated using the impedance with the

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following equation (equation 3) [52]: 𝑆=

𝑍0 − 𝑍∆𝑍% 𝑍∆𝑍%

(3)

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Where S is the sensitivity, 𝑍0 and 𝑍∆𝑍% are the impedance of the sensor at 16% RH and after the RH change, respectively.

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The graphical representation of the increase in sensitivity as a function of relative humidity is shown in Figure 13. It can be seen that the sensitivity of BFO/CFs layers was about 2% at 34% RH, while it was 2.3% for BFO nanoparticles-based layers. The results show that the sensitivity value increases with the increase of RH. It can be noted that at 92% RH, the sensitivity values for BFO/CFs nanocomposites and BFO nanoparticles are 12640% and 4652%, respectively (table 3). This indicates that carbon fibers can significantly improve the sensitivity of BFO-based humidity sensors.

Long-term stability graphs are shown in Figure 14. It can be seen that the impedance values of the sensors under different RH did not show obvious change for 180 days. This result demonstrates that these sensors have good stability. Complex impedance spectroscopy has been widely used to characterize the electrical response of humidity sensing layers. Impedance spectra explain the different physical phenomena of electrical conduction and polarization that occur in these materials in the presence of water molecules. To understand the sensing mechanism of our humidity sensors, complex impedance spectra (CIS) were measured at different RH levels and at different operating frequencies

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(from 300 Hz to 1 MHz). 𝑍′ and 𝑍′′ are the real and imaginary parts of the CIS, respectively,and some of the real and imaginary parts were enlarged in the same figure to make the CIS comparison more convenient.

Figures 15 and 16 show the complex impedance spectra of BFO and BFO/CFs films at different RH, respectively. At low RH (˂57%), it can be seen that both spectra exhibit an

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arc with a very large radius of curvature, so that it looks like a line.Thus, when the RH

increases to 70% the radius of curvature of the arc reduces and most of the semicircle is

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observed in the case of BFO-based sensors. On the other hand, in the case of BFO/CFs nanocomposite film, at high RH (˃70%), the beginning of a second semicircle is observed

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which is clearly visible at 92% RH.

The semicircle is typical of the relaxation mechanism expressed by the parallel resistance–capacitance circuits [53]. The relaxation mechanism results mainly from the

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intrinsic impedance of the sensing film due to the contributions of the grains (BFO) and grain boundaries (BFO - carbone fibers). The decrease in the curvature of the semi circle with increasing RH reflects the decrease in the intrinsic impedance, which is mainly due to the

[53-55].

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interaction between the sensing nanomaterials and water molecules (charge transfer process)

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At low humidity, only a few discontinuous chemisorbed water molecules are formed

on the sensing film which results in the difficult transmission of protons (H+) hopping and high impedance of the sensor [47], the chemisorbed monolayer water is localized by a double hydrogen bonding, and there is no hydrogen bond formation between the water molecules. As the RH increases, the layers of physisorbed water gradually exhibit liquid-like behavior. In bulk water, the hopping of protons between adjacent water molecules occurs, with charge transport by a Grotthuss chain reaction for conductivity [2, 39, 52].

The appearance of two semicircles with very different relaxation times at high RH indicates that the dominant mechanisms of the BFO/CFs as a prepared sensor are mainly derived from the interaction of water molecules with grains (BFO) and grains boundaries BFO- carbone fibers [18, 55]. The impedance module |Z| of the C parallel circuit can be given by equation 4: 1

|𝑍| = √

1 𝑅2

+

(4)

(𝜔𝐶)2

the resistance R was high at low RH because that little water molecules were adsorbed on the 1

sensing film, so 𝑅2 ≪ (𝜔𝑐)2 , hence the impedance Z of the sensor was mainly dominated by

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the term of “ωC”. So the curve of impedance versus frequency at low RH shows more capacitive behavior [56].

4. Conclusion

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BFO nanoparticles and BFO/CF nancomposites have been successfully synthesized by sol-gel and hydrothermal processes, respectively.The formation of BFO nanoparticles was

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confirmed by XRD characterization and the presence of carbon fibers in the nanocomposite was demonstrated by RAMAN analysis. The BET specific surface area of BFO nanoparticles

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and BFO/CFs nanocomposites is 2.3 m²/g and 253m²/g, respectively. Investigation of the humidity sensing properties reveals that the BFO/CF nanocomposite-based sensor has a high sensitivity to humidity of up to 12640% over the entire relative humidity range. While the

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maximum sensitivity achievedby the BFO-based sensor layer does not exceed 4652% in the same RH range.The developed sensing films have a small hysteresis (BFO: 3 % and BFO/CFs: 3.5 %) and very good stability. In addition, compared to pure BFO, the response

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time was increased from 50 s to 130 s for the nanocomposite, while its recovery time was improved from 150 s to 75 s. We also concluded that the relaxation mechanism in our samples

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is the dominant mechanism, which is expressed by the appearance of a semicircle in the case of the BFO and two semicircles with distinct relaxation times in the case of theBFO/CFs. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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[40] W. D. Lin, H. M. Chang, R. J. Wu. Applied novel sensing material graphene/polypyrrole for humidity sensor. Sensors and Actuators B, 181 (2013), 326-331. [41] Q. Qi, T. Zhang, Q. Yu, R. Wang, Y. Zeng, L. Liu and H. Yang. Properties of humidity sensing ZnO nanorods-base sensor fabricated by screen-printing. Sensors and Actuators B, 133 (2008) 638–643. [42] W. Xuan, M. He, N. Meng, X. He, W. Wang, J. Chen, T. Shi, T. Hasan, Z. Xu, Y. Xu and J. K. Luo. Fast response and high sensitivity ZnO/glass surface acoustic wave humidity sensors using graphene oxide sensing layer. Scientific Reports, 4 (2014), 1-9. [43] P. G. Su and C. C.Shiu. Electrical and sensing properties of a flexible humidity sensor

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Author Biographies De : Boudart Bertrand Date : 18/11/2019 15:21 (GMT+01:00) À : Nouara Lamrani ép Amaouz Cc : Yannick Guhel

Rachida DOUANI received the Magister diploma in 2004 and Phd in chemical Materials in 2013 from the University of Mouloud Mammeri Tizi Ouzou, Algeria. She is currently Doctor in Mouloud Mammeri University of Tizi-Ouzou (Algeria). Her research disciplines are based on nanomaterial, sol-gel synthesis, sensors, dielectric materials, and AC impedance.

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Nouara LAMRANI received here Magister in physical chimistry from Houari Boumediene University of Science and Technology, Algiers, Algeria, in 2000 and Ph.D. degrees in material chemistry from Mouloud Mammeri University, Tizi Ouzou, Algeria, in 2011. She is currently a research professor at the Mouloud Mammeri University in Tizi-Ouzou (Algeria).

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Her research activities include sol-gel synthesis, hydrothermal synthesis, nanomaterials, nanocomposites, piezoelectric material, dielectric material, AC impedance and sensor

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characterization.

Ahcéne Chaouchi received the Magister diploma in 2000 and Phd in Materials science

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electronics in 2007 from university Caen (France) and the University of Mouloud Mammeri Tizi Ouzou, Algeria. He is currently Professor in Mouloud Mammeri University of TiziOuzou (Algeria). His research disciplines are based on nanomaterial, sol-gel synthesis,

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piezoelectric material, dielectric material, AC impedande and sensor characterization. M’hand OUGHANEM received the Licence on fundamental chemistry diploma in 2015 and

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master 2 in physic-chemical of materials in 2017 from the University of Mouloud Mammeri Tizi Ouzou, Algeria. H’is currently Doctorant study in Mouloud Mammeri University of Tizi-

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Ouzou (Algeria). His research disciplines are based on nanomaterial, sol-gel synthesis, sensors, dielectric materials, and AC impedance. SAIDI Malika received the Magister diploma in 2012 and Phd in chemical de l’envirennement in 2017 from the University of Mouloud Mammeri Tizi Ouzou, Algeria. She is currently Doctor in Mouloud Mammeri University of Tizi-Ouzou (Algeria). Her research disciplines are based on nanomaterial, sol-gel synthesis, sensors, dielectric materials, and AC impedance.

Bertrand Boudart received his M.S. and Ph.D. degrees in electronic engineering and solidstate physics from Louis Pasteur University, Strasbourg, France, in 1988 and 1992, respectively. In 1992, he joined the Institut d’Electronique et de Microélectronique du Nord (IEMN), Lille, France, where he worked on power HEMTs for millimeter-wave amplification. His investigations included GaAs, InP, and GaN-based devices. He has been a professor at the University of Caen, France, since 2001 and conducts research at the Groupe de Recherche en Informatique, Image, Automatique & Intrumentation de Caen (GREYC). He is primarily involved in GaN and Raman spectroscopy activities. Yannick Guhel received a Ph.D. in electronics from the IEMN, Lille, France, in 2002. He

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then joined the University of Caen and conducts research at the GREYC. His research activities include the technological process of devices, deposition of thin films by sputtering, impact of electrical stress and gamma and neutron irradiation on the electrical behavior of

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GaN-based devices, and Raman spectroscopy.

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BFO xerogel

BFO Gel

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Solution of selts of Bi and Fe

Calcined BFO powder

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Figure 1: Schematic illustration of preparation of BFO nanoparticles.

(3) (2) (1)

BFO+ 10% FC

Hydrothermal treatememt (100°C/6 hours)

BFO/FC powders

(1) : stirring lacquer heating ; (2) : silicone oil ; (3) : hermetically sealed.

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Figure 2: Schematic illustration of preparation of BFO/CFs nanocomposites

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(1) Measurment chamber (2) Sensitive sensor (3) Saturated salt (4) Thermo-hygrometer (5) LCR meter

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Figure 3: Schematic diagram of the electrical measurements of the sensors

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208

125

018

122

006

211

113

003

116

204

202

214

012

104 110

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Figure 5: Room-temperature Raman spectra of the BFO/CFs nanocomposites.

(a)

(b)

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CF

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Figure 6: SEM micrographs: (a) BFO, (b) BFO/CFs.

30000

Impedance (kΩ)

BFO/CFs BFO

20000

10000

0 16

26

36

46

56

66

76

86

96

Relative Humidity (%)

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Figure 7: Impedance versus relative humidity of BFO and BFO/CFs at 1 kHz.

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Figure 8: Illustration of water vapor adsorption on the sensor’s surface

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Figure 9: Grotthuss chain reaction

100000

(a) 1 kHz 10 kHz 100 kHz

1000

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Log (Z)(kΩ)

10000

100 16

36 56 76 Relative Humidity (%)

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(b)

1 kHz

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10 kHz

100 kHz

500

50 16

36

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Log (Z)(kΩ)

50000

56

76

96

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Relative Humidity (%)

Figure 10: Impedance-relative humidity of BFO(a) and BiFeO3/CFs (b) at different

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frequencies (1 kHz, 10 kHz and 100 kHz)

30000

(a)

Impedance (kΩ)

25000 sorption 20000

desorption

15000 10000 5000 0 16

26

36

46

56

66

76

86

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Relative Humidity (%)

25000

(b)

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Sorption

15000

Desorption

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Imedance (kΩ)

20000

0 16

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Relative Humidity (%)

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Figure 11: Hysteresis of (a) BFO and (b) BFO/CFs at 1 kHz.

86

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16% RH

(a) 16% RH

response recovery

92% RH 92%RH

0

50

100

150

200

250

300

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24 22 20 18 16 14 12 10 8 6 4 2 0

16% RH

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(b)

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16%RH

92%RH

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Impedance MΩ)

Time (sec)

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Impedance (MΩ)

26 24 22 20 18 16 14 12 10 8 6 4 2 0

Response Recovery

92%RH

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Time (sec)

Figure 12: Response and recovery time of: (a) BFO and (b) at 1 kHz at RH (16-92%)

1.4E+04 1.2E+04 BFO

Sensitivity (%)

1.0E+04

BFO/CFs 8.0E+03 6.0E+03 4.0E+03 2.0E+03 0.0E+00 54

74

94

114

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Relative humidity (%)

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Figure 13: Sensitivity-relative humidity of BFO and BFO/CFs at 1 kHz.

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(a)

25000

16%

Impedance (kΩ)

34% 20000

45% 57%

15000

70% 10000

80% 92%

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30

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Time(day)

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(b)

16% 34%

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45% 57% 70%

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Impedance (kΩ)

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0

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80% 92% 150

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Time (day)

Figure 14: Long term Stability of sensors: BFO (a) and BFO/CFs (b).

(a) 16% 34% 45% 57%

0

1000

2000

3000

4000

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Z'(kΩ)

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50000 40000 30000

(b) 70% 80% 92%

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-Z"(kΩ)

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-Z"(kΩ)

100000 90000 80000 70000 60000 50000 40000 30000 20000 10000 0

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10000

15000

20000

25000

30000

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Z'(kΩ)

Figure 15: Complex impedance plots of BFO under different humidity levels. (a) at lower humidity range (16-57%), (b) at higher humidity levels (70-92%).

70000

(a)

60000

-Z''(kΩ)

50000 16%

40000

34%

30000

45% 20000

57%

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1800 1600 1400 1200 1000 800 600 400 200 0

(b)

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70% 80%

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Z'(kΩ)

3000

92%

4000

5000

Z'(kΩ)

Figure 16: Complex impedance plots of BFO/CFs under different humidity levels. (a) at lower

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humidity range (16-57%), (b) at higher humidity levels (70-92%).

Table.1: Saline solutions and corresponding humidity levels

Salt

CH3COOK

MgCl2

MgNO3

NaCl

KCl

KNO3

16

34

45

57

70

80

92

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RH (%)

KOH

Table 2: Comparison of the humidity sensing properties of BFO and BFO/CFs humidity sensors and impedance-reported humidity sensors. Hysteresis (%)

0-91.5 5-75 17-93 11-95 12-95 10-90 11-95 11-98

Response time/recovery time(s) 3/70 1.78/0.45 4-5/19-24 32/131 30/50 384/0.1 16/133 10/3

attapulgite Cs2BiAgBr6 Si-Bi-O Porous TiO2 Mn3.15Co 0.3Ni 0.8 O4 PVA-ZnO/SnO2 CoO(OH) CeO 2 Nanoparticles ZnO/La(OH) BFO BFO/CFs

15-95 16-92 16-92

400/300 50/150 130/75

2 3 3.5

3.4 3.1-4.5 4 5.5 2

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Reference

[47] [48] [9] [5] [49] [46] [39] [50]

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RH range (%)

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Sensing material

[51] This work This work

Table.3: Sensitivity for different ranges of relative humidity for BFO and BFO/CFs sensing films. 16-34 0.7 0.29

34-45 2.3 2

45-57 11 11

57-70 29 565

70-80 103 2695

80-92 4652 12640

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Humidity rang (%RH ) BFO sensitivity (%) BFO /CFs sensitivity (%)