*3.1. Structure and Morphology of Material*

Figure 1 shows the X-ray diffractometer (XRD) patterns of the BiFeO<sup>3</sup> film. The diffraction peaks of the pure BiFeO<sup>3</sup> sample are consistent with the standard chart of BiFeO<sup>3</sup> with rhombohedral *R*3c structure (JCPDS PDF # 86-1518), as shown in Figure 1. There is no impurity peak, which proves that the sample is a pure-phase perovskite structure BiFeO3. The scanning electron microscope image of the BiFeO<sup>3</sup> film is shown in the inset of Figure 1. The image reveals that the as-synthesized BiFeO<sup>3</sup> film had a porous structure, indicating its excellent ability to adsorb water molecules, which is essential for humidity sensing.

**Figure 1.** XRD pattern of BiFeO3 film. Inset: SEM image of BiFeO3 film. **Figure 1.** XRD pattern of BiFeO<sup>3</sup> film. Inset: SEM image of BiFeO<sup>3</sup> film. **Figure 1.** XRD pattern of BiFeO3 film. Inset: SEM image of BiFeO3 film.

The ferroelectric hysteresis loop of the BiFeO3 film is shown in Figure 2. A schematic diagram of the ferroelectric test circuit is shown in the inset of Figure 2. This test circuit was composed of two silver electrode points coated on the surface of the material to connect the wires. In Figure 2, the unsaturated ferroelectric hysteresis loop was obtained due to the serious leakage current [19]. The composition of the sample indicates that the BiFeO3 film had a serious electrical leakage problem due to the multiple valence states of Fe [19]. The ferroelectric hysteresis loop of the BiFeO<sup>3</sup> film is shown in Figure 2. A schematic diagram of the ferroelectric test circuit is shown in the inset of Figure 2. This test circuit was composed of two silver electrode points coated on the surface of the material to connect the wires. In Figure 2, the unsaturated ferroelectric hysteresis loop was obtained due to the serious leakage current [19]. The composition of the sample indicates that the BiFeO<sup>3</sup> film had a serious electrical leakage problem due to the multiple valence states of Fe [19]. The ferroelectric hysteresis loop of the BiFeO3 film is shown in Figure 2. A schematic diagram of the ferroelectric test circuit is shown in the inset of Figure 2. This test circuit was composed of two silver electrode points coated on the surface of the material to connect the wires. In Figure 2, the unsaturated ferroelectric hysteresis loop was obtained due to the serious leakage current [19]. The composition of the sample indicates that the BiFeO3 film had a serious electrical leakage problem due to the multiple valence states of Fe [19].

**Figure 2.** Ferroelectric hysteresis loop. Inset: schematic diagram of the test circuit. **Figure 2.** Ferroelectric hysteresis loop. Inset: schematic diagram of the test circuit. **Figure 2.** Ferroelectric hysteresis loop. Inset: schematic diagram of the test circuit.

### *3.2. Humidity-Sensing Properties 3.2. Humidity-Sensing Properties 3.2. Humidity-Sensing Properties*

The dependence of the capacitance of the BiFeO3 film on the RH was measured at the frequencies of 10, 40, 100, 300, 600 and 1200 Hz, as shown in Figure 3. The inset is a partial enlarged view of capacitance change with RH (RH 30−50%) at different frequencies. At low frequency (i.e., 10 Hz, 40 Hz, 100 Hz), the capacitance increased significantly with The dependence of the capacitance of the BiFeO3 film on the RH was measured at the frequencies of 10, 40, 100, 300, 600 and 1200 Hz, as shown in Figure 3. The inset is a partial enlarged view of capacitance change with RH (RH 30−50%) at different frequencies. At low frequency (i.e., 10 Hz, 40 Hz, 100 Hz), the capacitance increased significantly with The dependence of the capacitance of the BiFeO<sup>3</sup> film on the RH was measured at the frequencies of 10, 40, 100, 300, 600 and 1200 Hz, as shown in Figure 3. The inset is a partial enlarged view of capacitance change with RH (RH 30−50%) at different frequencies. At low frequency (i.e., 10 Hz, 40 Hz, 100 Hz), the capacitance increased significantly with increasing

RH. In particular, the capacitance of the BiFeO<sup>3</sup> film increased from 25 to 1410 pF as the RH increased from 30% to 90% at 10 Hz. This was due to the increase of physisorbed water molecules on the BiFeO<sup>3</sup> film surface with the increase of RH, which made more water molecules polarized. At high frequency (i.e., 300 Hz, 600 Hz, 1200 Hz), the capacitance remained almost constant with increasing RH, implying that frequency is a crucial factor in the humidity response. At high frequency, the dipoles of the water molecules slow their reorientation. The dipole rotation of water molecules no longer resonates with the external field at high frequencies, which means that the polarizability of the water molecules lags behind the frequency of the change of the external electric field. Therefore, the capacitance of the BiFeO<sup>3</sup> film had a high humidity response at frequencies range of 10–100 Hz, while RH is independent of the capacitance at frequencies in the range of 100 Hz to 1.2 kHz. The effect of RH on capacitance can be expressed by Equation (1) [20] increasing RH. In particular, the capacitance of the BiFeO3 film increased from 25 to 1410 pF as the RH increased from 30% to 90% at 10 Hz. This was due to the increase of physisorbed water molecules on the BiFeO3 film surface with the increase of RH, which made more water molecules polarized. At high frequency (i.e., 300 Hz, 600 Hz, 1200 Hz), the capacitance remained almost constant with increasing RH, implying that frequency is a crucial factor in the humidity response. At high frequency, the dipoles of the water molecules slow their reorientation. The dipole rotation of water molecules no longer resonates with the external field at high frequencies, which means that the polarizability of the water molecules lags behind the frequency of the change of the external electric field. Therefore, the capacitance of the BiFeO3 film had a high humidity response at frequencies range of 10–100 Hz, while RH is independent of the capacitance at frequencies in the range of 100 Hz to 1.2 kHz. The effect of RH on capacitance can be expressed by Equation (1) [20]

$$\mathcal{C} = (\varepsilon\_{\gamma} - i \times \frac{\gamma}{\omega \times \varepsilon\_{0}}) \times \mathbb{C}\_{0} \tag{1}$$

where *ε<sup>γ</sup>* and *γ* are the permittivity and the electrical conductivity of the BiFeO<sup>3</sup> film, respectively. *C*<sup>0</sup> and *ε*<sup>0</sup> denote the capacitance of an ideal capacitor and the vacuum permittivity, respectively. *C* and *ω* are the capacitance and the frequency, respectively. Equation (1) indicates that the capacitance of the BiFeO<sup>3</sup> film is inversely related to *ω* and is proportional to the material's *γ*. Both *γ* and *C* increase as RH increases [20]. where *εγ* and *γ* are the permittivity and the electrical conductivity of the BiFeO3 film, respectively. *C*0 and *ε*0 denote the capacitance of an ideal capacitor and the vacuum permittivity, respectively. *C* and *ω* are the capacitance and the frequency, respectively. Equation (1) indicates that the capacitance of the BiFeO3 film is inversely related to *ω* and is proportional to the material's *γ*. Both *γ* and *C* increase as RH increases [20].

**Figure 3.** RH dependence on the capacitance of BiFeO3 film. Inset: enlarged capacitance vs. %RH plot (30–50% RH range). **Figure 3.** RH dependence on the capacitance of BiFeO<sup>3</sup> film. Inset: enlarged capacitance vs. %RHplot (30–50% RH range).

In order to determine the optimal working frequency, the dependence of impedance on RH was measured using BiFeO3 film at 30–90% RH and frequencies of 10, 40, 100, 300, 600 and 1200 Hz, as shown in Figure 4. Since it is difficult to lead the adsorbed water molecules to modify the associated polarization at high frequencies, there was a weak response to humidity at these frequencies. Therefore, it is important to determine the optimal frequency for RH measurements [21]. Figure 4 shows that the impedance of the BiFeO3 film decreased from 1.7 × 105 to 1570 kΩ when RH increased from 30% to 90%. The impedance decreased significantly at 10 Hz, indicating that the optimum working frequency is 10 Hz. Over the entire frequency range, the impedance decreased with the increase of RH. At the same frequency, the impedance change was not obvious at low RH, In order to determine the optimal working frequency, the dependence of impedance on RH was measured using BiFeO<sup>3</sup> film at 30–90% RH and frequencies of 10, 40, 100, 300, 600 and 1200 Hz, as shown in Figure 4. Since it is difficult to lead the adsorbed water molecules to modify the associated polarization at high frequencies, there was a weak response to humidity at these frequencies. Therefore, it is important to determine the optimal frequency for RH measurements [21]. Figure 4 shows that the impedance of the BiFeO<sup>3</sup> film decreased from 1.7 <sup>×</sup> <sup>10</sup><sup>5</sup> to 1570 kΩ when RH increased from 30% to 90%. The impedance decreased significantly at 10 Hz, indicating that the optimum working frequency is 10 Hz. Over the entire frequency range, the impedance decreased with the increase of RH. At the same frequency, the impedance change was not obvious at low RH, while the impedance drop

was more significant at high RH. This is because the main conduction mechanism for humidity sensing is caused by proton hopping between the sensitive layer of the film and water molecules. At low RH, a small amount of water molecules are chemisorbed on the cations (Bi3+ and Fe3+) on the film surface [22]. Due to the lack of a complete adsorption layer, the low polarizability of water molecules eventually leads to high impedance. At high RH, multiple layers of physical adsorption are formed on the basis of the chemical adsorption layer, resulting in the movement of more protons in the water layer [22]. This results in a significant increase in the conductivity of the humidity sensor and a decrease in impedance. while the impedance drop was more significant at high RH. This is because the main conduction mechanism for humidity sensing is caused by proton hopping between the sensitive layer of the film and water molecules. At low RH, a small amount of water molecules are chemisorbed on the cations (Bi3+ and Fe3+) on the film surface [22]. Due to the lack of a complete adsorption layer, the low polarizability of water molecules eventually leads to high impedance. At high RH, multiple layers of physical adsorption are formed on the basis of the chemical adsorption layer, resulting in the movement of more protons in the water layer [22]. This results in a significant increase in the conductivity of the humidity sensor and a decrease in impedance.

**Figure 4.** RH dependence on the impedance of BiFeO3 film. **Figure 4.** RH dependence on the impedance of BiFeO<sup>3</sup> film.

Humidity hysteresis of the BiFeO3 film usually occurred during the desorption of samples. The humidity hysteresis is a critical characteristic for the application of humidity sensing, and is defined as the maximum difference between adsorption and desorption of the humidity sensor. The humidity hysteresis (*γH*) is expressed in Equation (2) as [21]: Humidity hysteresis of the BiFeO<sup>3</sup> film usually occurred during the desorption of samples. The humidity hysteresis is a critical characteristic for the application of humidity sensing, and is defined as the maximum difference between adsorption and desorption of the humidity sensor. The humidity hysteresis (*γH*) is expressed in Equation (2) as [21]:

$$
\gamma H = \pm \frac{\Delta R H\_{MAX}}{2F\_{FS}} \tag{2}
$$

where *RHMAX* is the maximum difference in the output of adsorption and desorption processes. *FFS* is the impedance change over the entire humidity range. The humidity hysteresis characteristics of the BiFeO3 humidity sensor at 10 Hz are shown in Figure 5. It can be seen from the figure that the BiFeO3 showed a narrow hysteresis loop. The BiFeO3 film had a small hysteresis during the entire humidity test with a maximum hysteresis of approximately 16%, mainly caused by residual moisture in the BiFeO3 film layer. With the decrease of RH, the number of water molecules between the layers of the BiFeO3 film gradually decreased, resulting in the gradual disappearance of the hysteresis phenomewhere *RHMAX* is the maximum difference in the output of adsorption and desorption processes. *FFS* is the impedance change over the entire humidity range. The humidity hysteresis characteristics of the BiFeO<sup>3</sup> humidity sensor at 10 Hz are shown in Figure 5. It can be seen from the figure that the BiFeO<sup>3</sup> showed a narrow hysteresis loop. The BiFeO<sup>3</sup> film had a small hysteresis during the entire humidity test with a maximum hysteresis of approximately 16%, mainly caused by residual moisture in the BiFeO<sup>3</sup> film layer. With the decrease of RH, the number of water molecules between the layers of the BiFeO<sup>3</sup> film gradually decreased, resulting in the gradual disappearance of the hysteresis phenomenon [23,24].

*FS*

non [23,24].

**Figure 5.** Humidity hysteresis characteristics of BiFeO3 film measured at 10 Hz. **Figure 5.** Humidity hysteresis characteristics of BiFeO<sup>3</sup> film measured at 10 Hz.**Figure 5.** Humidity hysteresis characteristics of BiFeO3 film measured at 10 Hz.

Based on the conversion circuit of a humidity sensor, RH changes in the environment can be converted into an electrical signal that is easy to control and identify. The ideal humidity sensor needs to meet the following characteristics: fast response speed, strong recovery ability and small humidity hysteresis error. The response and recovery times are the times required for the BiFeO3 film to reach 90% of the total impedance change during adsorption and desorption, respectively. Figure 6 shows that the humidity response and recovery times of the BiFeO3 film in the maximum humidity range (30–90% RH) were 60 s and 70 s at 10 Hz, respectively. The recovery time of the BiFeO3 film was higher than the response time due to the higher bonding energy between the adsorbed water molecules and the surface of the sensor material [25]. This result indicates that the BiFeO3 film could rapidly adsorb and desorb water molecules, indicating its potential value for practical applications. Based on the conversion circuit of a humidity sensor, RH changes in the environment can be converted into an electrical signal that is easy to control and identify. The ideal humidity sensor needs to meet the following characteristics: fast response speed, strong recovery ability and small humidity hysteresis error. The response and recovery times are the times required for the BiFeO<sup>3</sup> film to reach 90% of the total impedance change during adsorption and desorption, respectively. Figure 6 shows that the humidity response and recovery times of the BiFeO<sup>3</sup> film in the maximum humidity range (30–90% RH) were 60 s and 70 s at 10 Hz, respectively. The recovery time of the BiFeO<sup>3</sup> film was higher than the response time due to the higher bonding energy between the adsorbed water molecules and the surface of the sensor material [25]. This result indicates that the BiFeO<sup>3</sup> film could rapidly adsorb and desorb water molecules, indicating its potential value for practical applications. Based on the conversion circuit of a humidity sensor, RH changes in the environment can be converted into an electrical signal that is easy to control and identify. The ideal humidity sensor needs to meet the following characteristics: fast response speed, strong recovery ability and small humidity hysteresis error. The response and recovery times are the times required for the BiFeO3 film to reach 90% of the total impedance change during adsorption and desorption, respectively. Figure 6 shows that the humidity response and recovery times of the BiFeO3 film in the maximum humidity range (30–90% RH) were 60 s and 70 s at 10 Hz, respectively. The recovery time of the BiFeO3 film was higher than the response time due to the higher bonding energy between the adsorbed water molecules and the surface of the sensor material [25]. This result indicates that the BiFeO3 film could rapidly adsorb and desorb water molecules, indicating its potential value for practical applications.

**Figure 6.** Humidity response and recovery curve of BiFeO3 film measured at 10 Hz. **Figure 6. Figure 6.** Humidity response and recovery curve of BiFeO Humidity response and recovery curve of BiFeO33 film measured at 10 Hz. film measured at 10 Hz.
