## 3 Results and Discussion
### Electrochemical and Photoelectrochemical Experiment Characteristics
#### 3.2.2 Photoelectrochemical Experiment  
Figure 8 presents the characterization of the synthesized \(CuO\) NRs used as the WE in the PEC measurements. The \(CuO\) characterization results comprise (a) the X-ray diffraction (XRD) pattern, confirming the crystal-line of synthesized \(CuO\) NRs; (b) the UV-vis spectrum of \(CuO\) NRs with an inset depicting the Tauc plot for bandgap determination based on light absorption properties; and (c,d) scanning electron microscope (SEM) images revealing the morphology of the synthesized \(CuO\) NRs. The XRD pattern confirms the synthesized material is \(CuO\). The pattern (Figure 8a) matches the characteristic lattice peaks of \(CuO\) referenced in the Crystallography Open Database (COD ID: 1011148). The experimental peaks are observed at positions close to 32.5\({}^{\circ}\) (110), 35.64\({}^{\circ}\) (002), 38.60\({}^{\circ}\) (111), 48.69\({}^{\circ}\) (112), 53.67\({}^{\circ}\) (020), 58.47\({}^{\circ}\) (202), 61.57\({}^{\circ}\) (113), 68.21\({}^{\circ}\) (220), and 75.29\({}^{\circ}\) (004). The UV-vis spectrum (Figure 8b) reveals a strong light absorption region for \(CuO\) NRs between 400 nm and 550 nm, indicative of its absorbance property. The Tauc plot analysis of the absorption spectrum yields a bandgap energy of 1.31 eV for the synthesized \(CuO\) NRs. In particular, the bandgap energy of \(CuO\) is known to vary depending on morphology (petal-like: 1.73 eV; bulk: 1.2 eV; ellipsoid-like: 1.371 eV; plate-like: 1.447 eV; boat-like: 1.429 eV; flower: 1.425 eV) [65]. This value falls within the reported range for various \(CuO\) morphologies documented in the literature [65], suggesting a good agreement with previous findings. The SEM analysis (Figure 8c,d) reveals that the synthesized \(CuO\) material exhibits a morphology dominated by rod-like structures with some interwoven particles. The absorption spectrum and bandgap analysis (Tauc plot) of the \(CuO\) photoactive material identified a 405 nm monochrome LED as the optimal excitation source for PEC measurements. Figure 9a depicts the photocurrent response (red line) of the system containing 5 mM \(K_{3}Fe(CN)_{6}/K_{4}Fe(CN)_{6}\) measured over 750 s. The black line represents the baseline current measured during the \"OFF state\" (no light excitation). The difference  
Figure 7: (a) Cyclic voltammogram of 10 mM \(K_{3}Fe(CN)_{6}/K_{4}Fe(CN)_{6}\) in 0.1 M \(KCl\) solution. The inset shows the ipa dependence on the concentration of \(K_{3}Fe(CN)_{6}/K_{4}Fe(CN)_{6}\). (b) Chronoamperogram of 1 mM \(K_{3}Fe(CN)_{6}/K_{4}Fe(CN)_{6}\) in 0.1 M \(KCl\) solution. The inset depicts the dependence of the differential current density between the pre\(-\)pulse state and the steady\(-\)state current following the pulse on the concentration of \(K_{3}Fe(CN)_{6}/K_{4}Fe(CN)_{6}\). between the \"ON state\" (light excitation) and \"OFF state\" currents signifies the photocurrent generated by light-induced redox reactions. As illustrated in Figure 9b, increasing the concentration of redox species in the electrolyte solution leads to a corresponding increase in the observed negative photocurrent density. This phenomenon is primarily attributed to the concentration dependence of the \(Fe(CN)_{6}^{3-}\) reduction reaction. Under light excitation, the \(CuO\) photoelectrode absorbs photons, promoting electrons to the conduction band. These photo-generated electrons then participate in the reduction reaction at the WE. The \(Fe(CN)_{6}^{3-}\) ions in the solution act as electron acceptors, undergoing reduction to \(Fe(CN)_{6}^{4-}\) according to the forward reaction in Equation (1). As the concentration of \(Fe(CN)_{6}^{3-}\) increases, the number of electron acceptors available increases proportionally. This leads to greater participation of photo-generated electrons  
Figure 8: (**a**) XRD pattern, (**b**) UV\(-\)VIS spectrum with inset of Tauc plot, and (**c.d**) SEM images of the synthesized \(CuO\) NRs.