Nanotextured Spikes of α-FeO/NiFeO Composite for Efficient Photoelectrochemical Oxidation of Water.

We demonstrate for the first time the application of p-NiFeO/n-FeO composite thin films as anode materials for light-assisted electrolysis of water. The p-NiFeO/n-FeO composite thin films were deposited on planar fluorinated tin oxide (FTO)-coated glass as well as on 3D array of nanospike (NSP) substrates. The effect of substrate (planar FTO and 3D-NSP) and percentage change of each component (i.e., NiFeO and FeO) of composite was studied on photoelectrochemical (PEC) water oxidation reaction. This work also includes the performance comparison of p-NiFeO/n-FeO composite (planar and NSP) devices with pure hematite for PEC water oxidation. Overall, the nanostructured p-NiFeO/n-FeO device with equal molar 1:1 ratio of NiFeO and FeO was found to be highly efficient for PEC water oxidation as compared with pure hematite, 1:2 and 1:3 molar ratios of composite. The photocurrent density of 1:1 composite thin film on planar substrate was equal to 1.07 mA/cm at 1.23 V, which was 1.7 times higher current density as compared with pure hematite device (0.63 mA/cm at 1.23 V). The performance of p-NiFeO/n-FeO composites in PEC water oxidation was further enhanced by their deposition over 3D-NSP substrate. The highest photocurrent density of 2.1 mA/cm at 1.23 V was obtained for the 1:1 molar ratio p-NiFeO/n-FeO composite on NSP (NF1-NSP), which was 3.3 times more photocurrent density than pure hematite. The measured applied bias photon-to-current efficiency (ABPE) value of NF1-NSP (0.206%) was found to be 1.87 times higher than that of NF1-P (0.11%) and 4.7 times higher than that of pure hematite deposited on FTO-coated glass (0.044%). The higher PEC water oxidation activity of p-NiFeO/n-FeO composite thin film as compared with pure hematite is attributed to the Z-path scheme and better separation of electrons and holes. The increased surface area and greater light absorption capabilities of 3D-NSP devices result in further improvement in catalytic activities.