Fabrication of α-hematite/ferrite composite thin films on planar and three-dimensional nanostructured substrates for photoelectrochemical water splitting

Abstract

Inspired from the natural photosynthesis in which solar radiation is being effectively utilized in photoconversion of simple compounds (H2O and CO2) into carbohydrates and oxygen, scientists are dragged toward artificial photosynthesis for obtaining important chemicals from cheap and sustainable sources. The efficient conversion of solar energy into clean form chemical energy is the concept that is considered to be effective solution of world’s growing issues i.e. increasing demand of the fossil fuels and global warming. Hydrogen is considered as the fuel of the future because it has more energy capacity than fossil fuel, yielding zero carbon emission and has competency to replace the fossil fuels being used in different sectors. Photoelectrochemical (PEC) splitting of water is considered as one of the most promising technology by which solar energy could be efficiently utilized in hydrogen generation from its cheap and abundantly available source i.e. water. In this work, we focussed on the use of α-hematite/ferrites composite thin films as electrode material for photoelectrochemical water oxidation. Based on the band gap and their band alignment knowledge, three different series of composite thin films i.e. (1) CuFe2O4/α-Fe2O3, (2) ZnFe2O4/α-Fe2O3 and (3) NiFe2O4/ α-Fe2O3 were deposited on planar and 3-dimentional (3-D) nanostructured substrates. The fabricated devices were then structurally and morphologically characterized by various techniques and evaluated for photoelectrochemical water oxidation applications. It has confirmed that the ratio between the components of the composite thin films is crucial, so the highest activity results were obtained by the thin films devices having equal molar ratio (1:1) between α-hematite and ferrites in all the three classes. Among CuFe2O4/α-Fe2O3 composite thin films series, the CF-1, having 1:1 molar ratio between CuFe2O4 and α-Fe2O3 showed the highest activity. This composite when deposited on planar FTO coated substrate showed the highest photocurrent density of 1.24 mA/cm2 at the applied voltage of 1.23 VRHE and retained the photoconversion efficiency of 0.14%. The same material when deposited on 3-D nanostructured substrate, an increase in the photocurrent density upto 2.2 mA/cm2 at the same applied voltage was recorded. Among ZnFe2O4/α-Fe2O3 composite thin films, we found that nanostructured device (ZF1-NSP) having molar ratio of 1:1 between their components retained the highest photocurrent density of 2.19 mA/cm2 and showed the photoconversion efficiency of 0.22%. This photocurrent density is 3.4 and 2.73 times higher than photocurrent density values of pure hematite on planar FTO and the highest reported value of ZnFe2O4/α-Fe2O3 composite, respectively. Among NiFe2O4/α-Fe2O3 composite thin films, the highest photocurrent density of 2.1 mA/cm2 at 1.23 VRHE was obtained for the composite device having 1:1 molar ratio of NiFe2O4/α-Fe2O3 iii deposited on 3-D nanostructured substrate (NF1-NSP), which was 3.3 times more photocurrent density than pure hematite. It has been verified by electrochemical impedance spectroscopy (EIS) that α-hematite/ferrite composite thin films have greater conductivities of charge carriers than α-hematite and the highest values of charge conductivities were obtained for the composites consisting of equal molar ratio between α-hematite and ferrite. Based on photoluminescence studies, the photogenerated charge recombination has also been decreased by increasing ferrite component in α-hematite/ferrite composite. So, the better activities of α-hematite/ferrite composite in PEC water oxidation is provided by greater electrical conductivity and reduced charged recombination as compared to pure α-hematite. The performance of 3-D nanostructured devices in photoelectrochemical water oxidation were much greater than planar devices with the similar composite thin films. The three dimensional architecture of the electrode offers large surface area for the redox reaction and larger capability to harvest visible light for enhancing the performance of 3-D nanostructured electrode as compared to planar electrode.