*3.2. EIS and Photoelectrochemical Characterization of Bi2S3/BiVO4/FTO Platform*

Electrochemical impedance spectroscopy (EIS) is a powerful tool for monitoring the charge transfer resistance, which can be used to demonstrate the successful fabrication of a PEC-sensing platform [25]. EIS measurements were taken to evaluate the charge transfer characteristics of the semiconductor materials. EIS measurements were also performed to evaluate the electrocatalytic effect of the semiconductor materials and the increase in the electroactive area of the working electrodes.

Figure 2A shows the Nyquist plots of the FTO, BiVO4/FTO, and Bi2S3/BiVO4/FTO electrodes, recorded at the open circuit potential and modeled using an adapted Randles equivalent circuit (inset in Figure 2A) consisting of a cell resistance (Rs) in series with a parallel combination of a constant phase element (CPE), considered a non-ideal capacitance, and a charge transfer resistance (Rct) with a Warburg impedance (Zw). Following the fitting of the parameters presented in Figure 2A. The BiVO4/FTO electrode presented a charge transfer resistance value of 515 Ω, while the Bi2S3/BiVO4/FTO electrode exhibited an Rct value of only 93 Ω. These results suggest that the sensitization of the platform by Bi2S3 enhanced the electron transfer in the electrode–solution interface, contributing to the improved electrocatalytic properties of the sensing platform. Furthermore, the capacitance values of the BiVO4/FTO and Bi2S3/BiVO4/FTO electrodes were determined to be 0.0629 mF and 3.08 mF, respectively. These results indicate an increase in the electroactive area of the surface due to increase in the capacitances.

The effects of the LED light on the electron transfer processes and the lifetime of the electron were evaluated for the different sensing platforms to investigate the nature of their PEC responses. Figure 2B presents the Nyquist plots of the Bi2S3/BiVO4/FTO electrode obtained in 0.1 mol L−<sup>1</sup> Na2SO4 solution in the absence and in the presence of visible LED light. As can be seen, there was a decrease in the semicircle diameter of spectra in the presence of LED light, indicating that the Rct value decreased. These results suggest that light irradiation enhances the electrocatalytic potential of the sensing platform since it enables the formation of electron–hole pairs in the Bi2S3/BiVO4 composite material. Bode phase plots were obtained for the BiVO4/FTO and Bi2S3/BiVO4/FTO electrodes in order to estimate the lifetime of the electron through the following equation [26]:

$$\pi\_{\mathfrak{c}} = \frac{1}{(2\pi f\_{\text{max}})} \tag{1}$$

where *τ<sup>e</sup>* is the lifetime of the electron and *fmax* is the maximum frequency in the Bode phase diagram.

**Figure 2.** (**A**) Nyquist plots for the FTO (black), BiVO4/FTO (red), and Bi2S3/BiVO4/FTO (blue) electrodes, recorded in 0.1 mol L−<sup>1</sup> KCl solution containing 5 mmol L−<sup>1</sup> Fe[(CN6)]3-/4-. (**B**) Nyquist plots for the Bi2S3/BiVO4/FTO photosensor in aqueous 0.1 mol L−<sup>1</sup> Na2SO4 solution, recorded at an open-circuit potential in the dark (black) and under the visible LED light irradiation (red). (**C**) Bode phase plots for BiVO4/FTO (black) and Bi2S3/BiVO4/FTO (red) electrodes in 0.1 mol L−<sup>1</sup> KCl solution containing 0.03 mol L−<sup>1</sup> AA donor molecules. (**D**) Photocurrent response of FTO (black line), BiVO4/FTO (red line), and Bi2S3/BiVO4/FTO (blue line) photoelectrodes. Amperometric measurements were performed in a 0.1 mol L−<sup>1</sup> phosphate buffer (pH 7.4) containing 0.03 mol L−<sup>1</sup> AA, Eappl = 0 V vs. Ag/AgCl/KClsat.

Figure 2C shows that the *fmax* in the Bode phase plots of the BiVO4 platform decreased from 179.6 Hz to 70.38 Hz after sensitization by Bi2S3, reflecting the increase in the lifetime of the electron from 0.886 ms to 2.26 ms. The enhancement of the lifetime of the electron suggests that the formation of the Bi2S3/BiVO4 heterojunction enabled a slower recombination of the electron–hole pairs, which also explains the more favorable electron transfer between composite material and FTO electrode, as shown in the study of Figure 2A.

Additionally, the photocurrent intensity obtained for each individual component of the developed PEC platform was evaluated in the presence of an AA donor molecule (Figure 2D). As can be seen, there was an increase in the photocurrent response for the BiVO4/FTO platform after sensitization by Bi2S3 film, confirming an improvement in the photo–current conversion efficiency due to a greater absorption of visible light, an increase in the electroactive area, and an improvement in the electrochemical properties of the platform. Although BiVO4 is a semiconductor with good photocatalytic activity, its photoelectrochemical efficiency may not reach a desirable level due to the occurrence of charge recombination [27], which is minimized after the conjunction of energy bands in the Bi2S3/BiVO4 composite. Furthermore, Bi2S3 is a material that presents a high surface activity, further improving the catalytic activity of the PEC platform [28].
