Semiconductor devices harness the power of light to produce other types of energy: electrical or chemical. One of the applications of this technology is to produce hydrogen molecules from water, in a process known as the photoelectrochemical (PEC) splitting of water, as a source of ‘renewable energy.
Photoelectrolytic cells include semiconductor electrodes, in which the hole and the electron are spatially separated when irradiated with light with an energy above a certain threshold. Charge flow initiates PEC reactions. Titanium dioxide (TiO2) nanotubes are widely used semiconductors for this purpose. However, the charge flux distribution at the surface of TiO2 tubes is unclear. Now, Marina Makarova at the University of Kanazawa and her colleagues in Japan and Europe have used an innovative technique called scanning electrochemical microscopy (SECCM) to identify this distribution.
To first measure the success of PEC water separation, the release of oxygen (the reaction product accompanying hydrogen) was measured. Its production was further characterized by lead(II) ions added to the cell, such that lead(IV) oxide (PbO2) particles were found deposited on the walls and tops of the TiO2 nanotubes, suggesting similar electrochemical reactivity at these two sites. The research team then used SECCM to clarify the photoreactivity at the two sites at different electrode potentials.
SECCM involves the use of a narrow pointed pipette probe to measure electrical changes at specific locations. This probe is filled with a conductive fluid called an electrolyte which sits between two charged electrodes. One electrode is immersed in the electrolyte, while the second is connected to the sample surface. Measurements were taken perpendicular and parallel to the length of the tubes to determine the differences in reactivity on the walls and top, respectively. Now the free electrons generally move along the nanotubes towards the positive electrode inside the cell. When these electrons delocalize and jump to the conductive strip, they leave behind “holes” which are just pockets of positive charge that can move around. The SECCM results showed the presence of similar electrical activity along the walls and top of the nanotubes. Since the activity at the top could be preferentially attributed to the movement of electrons, this suggested that the holes instead traveled shorter distances and moved perpendicular to the length of the tube towards the walls. This flow in a direction orthogonal to the length of the tubes explains why PbO2 deposits were also found on the walls of the tubes.
“This information could be used to establish further correlations between photocurrent and microstructure of 1D nanostructures, and for site-specific decoration with co-catalysts or visible-light sensitive sensitizers,” the team explain. Understanding the spatial patterns of electrochemical activity is important for designing efficient and cost-effective photovoltaic cells. Additionally, a combination of oxide deposition and SECCM might be the most sensitive tool for identifying active sites.
Figure 1. Local analysis by scanning electrochemical cell microscopy (SECCM).
The photoelectrochemical responses of the top and sides of the TiO2 nanotube array were measured using the nanopipette. ©Takahashi, Kanazawa University
Figure 2. Three-dimensional topographic image of the TiO2 nanotube array and local photocurrent response.
There were no significant differences in photocurrent values between the top and the side of the tube. ©Takahashi, Kanazawa University
Photoelectrochemical (PEC) separation of water
Hydrogen is increasingly becoming a cleaner and more sustainable source of energy. One method of obtaining it is the PEC splitting of water to produce hydrogen and oxygen molecules. This reaction can also be carried out industrially using a photovoltaic system coupled to an electrolyser. The PEC cell uses light to produce high-energy electrons and uses these electrons to initiate the chemical reaction that breaks up water molecules. Since semiconductors are the main source of electrons in photoelectrolytic cells, understanding their PEC properties is essential for designing optimal systems.
Scanning electrochemical cell microscopy (SECCM)