Cathodic shift in flatband potential of hematite based photoelectrodes: Evaluating and correlating the redshift to solar driven photocatalysis

As a promising metal oxide semi-conductor, hematite (α-Fe 2 O 3 ) has interesting optical properties to be regarded as photoactive material. In this study, we evaluated the cathodic shift in onset potential ( E onset ) and the negative shift in flat band potential ( E fb ). It was observed that the E onset value of −0.457 V for α-Fe 2 O 3 based electrode shifted to −0.638 V for the ternary nanocomposite (PNFC) [Pani/Ni-α-Fe 2 O 3 /CNT] based electrode. The flat band potential for α-Fe 2 O 3 in 1 M NaOH is at −0.49 V which shifts to −0.68 V for the ternary nanocomposite (PNFC). This photoelectrochemical enhancement appears due to electron-hole pair separation. Therefore, it apparently increases the photocurrent trace for PNFC at −0.60 V to 7.04 mA cm -2 , compared to the photocurrent of 4.52 mA cm -2 observed for pristine α-Fe 2 O 3 . According to the Mott-Schottky plot, the donor concentration ( N d ) for the α-Fe 2 O 3 is 4.83 × 10 9 cm -3 , whereas for the ternary nanocomposite (PNFC) is 7.68 × 10 9 cm -3 , respectively. As evident, the N d value increase for the ternary nanocomposite PNFC points to the proximity of the Fermi ( E f ) level below the CB edge when Ni doped α-Fe 2 O 3 comes in contact with polyaniline (Pani). Indeed, the ternary nanocomposite PNFC with lower E g value (1.32 eV) and higher photocurrent (7.04 mA cm -2 ) show higher photo-activity with percent degradation of 98.42 % for photocatalytic degradation of Sunset Yellow under visible light irradiation. The aforementioned correlations of the energetic levels of band edges with the shifting of E fb to more negative potentials provide insight into the high photo-activity observed. • Hematite (α-Fe 2 O 3 ) based ternary nanocomposite PNFC was prepared using Ni doped α-Fe 2 O 3 and polyaniline. • Optical band gap ( E g ) and electrochemical analysis evaluated the relationship between redshift and E fb negative shift. • Optical band gap ( E g ) values decreased from 1.76 eV for α-Fe 2 O 3 to 1.32 eV for PNFC due to Ni-α-Fe 2 O 3 photosensitization NPs. • Cathodic shift in onset potential ( E onset ) and negative shift in flat band potential ( E fb ) was observed for PNFC. • E onset value of −0.457 V for α-Fe 2 O 3 based electrode shifted to −0.638 V for ternary nanocomposite (PNFC) based electrode.