Cobalt Metal–Organic Framework Ultrathin Cocatalyst Overlayer for Improved Photoelectrochemical Activity of Ti-Doped Hematite

The poor conductivity and sluggish kinetics of hematite (α-Fe2O3) limit its photoelectrochemical (PEC) performance. Herein, a cobalt metal–organic framework (Co-MOF) ultrathin overlayer is in situ-grown onto a Ti-doped hematite nanorod array via the chemical bath deposition. The optimal Co-MOF/Ti:Fe2O3 achieves a photocurrent density of 2.24 mA/cm2 at 1.23 V [vs reversible hydrogen electrode (RHE)], which is 2.4-folds that of pristine Fe2O3. When compared with the onset potential of Ti:Fe2O3, Co-MOF/Ti:Fe2O3 exhibits a cathodic shift of 310 mV. Co-MOF/Ti:Fe2O3 also shows a photocurrent density retention of 98.1% after the 6 h stability test. The improved PEC activity is mostly ascribed to the increased charge separation and surface charge injection. Ti doping increases the electron density in Fe2O3 and lowers the Fermi level (vs RHE), facilitating the charge transport in the bulk. The enhanced photovoltage and favorable distribution of surface states after Co-MOF modification and the catalytic/conductive properties of Co-MOF induce the enhanced charge separation/injection and the low onset potential.