World energy consumption is bound to increase with the increasing population. Fossil fuels widely used today, are limited in their supply. Moreover, high CO2 emission from their widespread usage is believed to have caused the global warming. Clearly, in the long run an economy based entirely on the fossil fuels is not a sustainable economy. It is therefore very important than ever before, to innovate new and sustainable ways to harvest the solar energy. With this view, work presented here investigates the electro- catalytic and photo-electrochemical properties of Co3O4 thin films prepared by the sol-gel method, for the purpose of water splitting and solar cells. The thin films synthesized were characterized by using x-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS).
The electrocatalysis studies were done for the oxygen evolution reaction (OER). OER reaction is the half reaction occurring at anode in water splitting cell. It has large overpotential, which brings the overall efficiency of the electrochemical water splitting down. Therefore, better OER electrocatalysts are required.
In the OER experiments, we used the electrochemical milling (ECM) on Co3O4 thin films, to generate the nanoporous Co3O4. Electrochemical measurements were done in 1M NaOH solution with the thin films as working electrode in a typical three-electrode assembly. OER performance for these ECM treated sample increased compared to samples without ECM treatment. Scanning electron microscopy (SEM) images of samples were taken at different stages of experiments to detect any change in surface morphology. The planar surface of the thin films was found to have changed to hexagonal plates after ECM treatment, and they were only formed after exposing ECM treated samples to 1M NaOH. XPS investigation revealed the formation of CoO from Co3O4 because of ECM.
These hexagonal plates were identified to be cobalt oxyhydroxide, using Raman spectroscopy and X-ray diffraction (XRD). CoO(OH) is known to have higher conductivity. Hence increased OER performance of CoO(OH) thin films is maybe due to the overall increase in effective active sites.
Wide band gap semiconductors such as TiO2 or ZnO are limited to UV region of solar spectrum. Therefore, small band gap semiconductors such as Co3O4 with absorption in visible solar spectrum can potentially harvest higher amount of the solar energy. Reported studies of photo-electrochemical (PEC) properties of Co3O4 are very scarce. Hence, this work is partly focused on developing working procedure of stable photocurrent measurement for Co3O4 thin films. High absorption in the visible region of the solar spectrum was the initial motivating factor for pursuing the PEC study of Co3O4. Photocurrent measurements were done using a Xe lamp as the light source. In all electrochemical measurements, a typical three-electrode assembly was used with Co3O4 used as working electrode. Many different electrolytes - aqueous as well as non-aqueous - were used. The photocurrent obtained was found to be very low, ~ 10-20 μA at the maximum. Transient photocurrent was observed, maybe due to the surface trapped minority carriers. Large background dark current was present in almost all systems. It could be due to porous nature of the thin films, exposing FTO to the electrolyte.