Photoelectrochemical Performances of Hematite (α‐Fe2O3) Films Doped with Various Metals

Abstract

Hematite (α‐Fe2O3 ) film electrodes doped with 14 different metal cations are fabricated using one‐step co‐electrodeposition techniques and their photoelectrochemical (PEC) performances are compared in a 1 M NaOH electrolyte under AM 1.5 irradiation. Identical doping levels of Pt(II and IV), Ce(III), Cd(II), Al(III), and Sn(IV) at 5 at.% improve the PEC performance of hematite, whereas Co(II), Ni(II), Cu(II), Ru(III), and Pd(II) reduce the performance. Doping with Ag(I), Cr(III), Ir(IV), and Au(III) have little effect on the performance. The doping effects of Pt(II), Cd(II), Ag(I), and Cr(III) are studied in greater detail using various surface analysis techniques (scanning electron microscopy, X‐ray photoelectron spectroscopy, X‐ray diffraction, and UV–Vis diffuse reflectance) and electrochemical analysis techniques (Mott–Schottky and impedance) by varying the doping levels (0–10%). Metal doping increases the primary particle sizes of hematite from ca. 50 nm to 70–100 nm due to interparticle agglomeration, and shifts the binding energies of the O 1s bands in the XPS spectra. The energetics of the bare and doped hematite (bandgap, conduction, and valence band levels) are estimated from the spectral responses and Mott–Schottky analyses. Pt, Cd, and Ag increase the bandgap by 0.06 eV but Cr reduces it by 0.06 eV. All doped hematite electrodes have valence band levels positive (~1.1 V) of water oxidation. At optimal doping levels, Pt(8%) improves the PEC performance of hematite approximately 3.2 times, whereas Cd(2%) and Ag(5%) increase it around twofold and 60%, respectively. The different optimal doping levels are discussed in terms of the depletion layer, electrical conductivity, and energy levels. The enhanced PEC performance of the doped hematite electrodes are attributed primarily to the increased electrical conductivity.