**1. Introduction**

Humid environments are essential in many fields, such as weather forecasting, agricultural production and personnel health [1,2]. In addition, trace amounts of water molecules can have a significant impact on industry and manufacturing [3,4]. Therefore, it is necessary to explore highly efficient and accurate humidity sensors. Humidity is a physical quantity that indicates the molecular content of water in the air, and is mainly measured by relative humidity. Over the past decade, many techniques for measuring humidity have been reported, including wet and dry bulb hygrometers, piezoelectric quartz films, resistive sensors, and sensors based on current, impedance and surface acoustic waves [2,5]. Among them, impedance-based humidity-sensing technology is the most convenient and commonly used [2]. Impedance humidity sensors work on the principle that changes in humidity can be reflected by changes in the impedance of a hygroscopic medium [6]. Impedance-type humidity sensors have been extensively reported in recent years due to their low cost, fast response speed and small size [6,7]. Impedance measurements indicate that suitable humidity-sensitive materials mainly include polymers, carbon materials and ceramic materials [8,9]. However, polymer films are not suitable for application at high temperatures. Ceramic films with good stability at high temperatures are considered to be the preferred materials for impedance-based humidity sensors due to their unique structure of grain boundaries, grains and pores [10].

For the convenience of microelectronics integration, film materials are often prepared for humidity sensors. Some ferroelectric perovskite (ABO3, where A is a rare earth, alkali or alkaline earth metal and B is a transition metal) humidity-sensing film materials, including BaTiO3, K0.5Bi0.5TiO3, K0.5Na0.5NbO<sup>3</sup> and LaFeO3, can behave with remarkable humiditysensing properties [11–13]. BiFeO<sup>3</sup> is a well-known lead-free ferroelectric material that has been regarded as a promising spintronic and information-storage receptor material in recent years due to its large remanent polarization and high Curie point [14–16]. BiFeO<sup>3</sup> is a distorted perovskite ferroelectric material with a non-stoichiometric ratio, which makes it

**Citation:** Zhang, Y.; Li, B.; Jia, Y. High Humidity Response of Sol–Gel-Synthesized BiFeO<sup>3</sup> Ferroelectric Film. *Materials* **2022**, *15*, 2932. https://doi.org/10.3390/ ma15082932

Academic Editor: Georgios C. Psarras

Received: 18 March 2022 Accepted: 13 April 2022 Published: 17 April 2022

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behave with *p*-type semiconductor behavior and makes it a promising material for highperformance humidity-sensing applications [17,18]. So far, there are few reports exploring the humidity-sensing behavior of BiFeO<sup>3</sup> films. In humidity sensors, morphology and cation distribution can be controlled by the synthesis method, which affects the surface reaction. The sol–gel technique is a simple, low-cost and promising method for the preparation of BiFeO<sup>3</sup> films [19].

In this work, the capacitance of BiFeO<sup>3</sup> film synthesized via the sol–gel method was found to increase from 25 to 1410 pF when RH increased from 30% to 90%. In particular, the impedance varied by more than two orders of magnitude when RH varied between 30% and 90% at 10 Hz, which extends the potential application of ferroelectric BiFeO<sup>3</sup> films to humidity-sensitive devices.
