**2. Materials and Methods**

The sol–gel method was used to successfully prepare a BiFeO<sup>3</sup> film. Powders of bismuth nitrate (Bi(NO3)3·5H2O) and ferric nitrate (Fe(NO3)3·9H2O) were dissolved in C3H8O<sup>2</sup> solution with a molar ratio of 1:1 and agitated at room temperature for 30 min. Afterwards, enough CH3COOH was added to the solution for dehydration. During continuous stirring, 5 mL of aminoethanol was added to BiFeO<sup>3</sup> solution in order to control the viscosity. Finally, a 0.3 mol/L red-brown mixed solution with a volume of 30 mL was obtained. The mixture was stirred on a magnetic stirrer for 2 h and left at room temperature for 12 h. The obtained reddish-brown BiFeO<sup>3</sup> solution was spin-coated on a Pt/Si(111) substrate and dried for 3 min at 180 ◦C. Then, films were calcined for 20 min at 490 ◦C. Finally, conductive silver glue was used to stick electrodes on the surface of the BiFeO<sup>3</sup> film for the electrical measurement.

The simple structure was determined via XRD (D/Max2550VB+/PC, Japan). The microstructure was characterized via SEM (Nova NanoSEM 450, Lincoln, NE, USA). A ferroelectric analyzer was used to explore the ferroelectric hysteresis loop (Precision Multiferroic, Radiant Technology, Albuquerque, NM, USA). The capacitance and impedance were measured using a precision impedance analyzer (Novocontrol GmbH, Montabaur, Germany). The current–voltage relationship was measured using a current–voltage meter (Agilent B2902A, Santa Clara, CA, USA). A humidifier in an enclosed space was employed to generate an environment with 30% to 90% relative humidity. The RH was measured using a hygrometer.
