Scanning photoelectrochemical microscopy (SPECM) integrates an illumination system into a conventional SECM setup. This technique enables the local investigation of photoelectrochemical processes. Localized illumination can be achieved in different ways.
There are different options for the integration of light. Here, we present three configurations which can be directly realized with HEKA ElProScan without the fabrication of special tips.
1. Top-illumination through an optical fiber
This setup is very simple and requires very little rearrangement of a conventional SECM setup. An optical fiber is mounted on the z-axis and brought in close proximity to the photoactive surface. No microelectrode is involved in this measurement. The optical fiber is scanned over the surface to drive the photoreaction locally. A typical size of the fiber is 200 µm. The resolution of the measurement is slightly larger than the fiber owing to the size of the cone of light. The substrate is connected as the working electrode and the photocurrent is measured at the sample. The measured current reflects the local photoactivity under the light beam. A high background current can occur if a large area of the substrate is immersed in solution. This can make it harder to see small photoactivity. Reducing the size of the electrochemical cell and therefore the size of the immersed substrate can greatly improve the quality of the measurement. This method allows large areas to be scanned to give a good overview of inhomogeneities.
Fig.1: Schematic representation of the experiment configuration (left) and the resulting photocurrent map with local inhomogeneities of photoactivity in a 2D color plot (middle) and a 3D color plot (right). Currents are given in A.
2. Transparent samples with illumination from the bottom
In this setup the photoactive substrate is illuminated from the bottom through an inverse microscope. HEKA offers a small spot illumination where the spot of illumination is restricted to an area of 2.3 – 5 µm depending on the size of the optical fiber. The measured photocurrent is in the nA range, due to the small active surface area under illumination.
In this configuration, the photocurrent measurement at the sample and the detection of reaction products (e.g. oxygen in a water splitting reaction) at a microelectrode can be conducted simultaneously. The microelectrode is aligned with the small illuminated spot and the stage with the photoactive sample is moved in XY direction. The photocurrent can therefore be correlated with the detected products.
A bipotentiostat is used to connect one working electrode to the sample to measure photocurrents and the second working electrode to the microelectrode. A shared counter and reference electrode are used. HEKA uses a special potentiostat, the PG 618 USB which is designed for SECM experiments. It has a high current channel for large samples and a low current channel for the microelectrode detection.
The detection at the microelectrode can be combined with Shear Force Sensing to perform constant-distance scan. This results to an additional topography map.
3. Scanning microdroplet contact method combined with local illumination
In the scanning microdroplet contact method a micropipette is filled with diluted electrolyte solution and the droplet which is forming at the pipette opening is brought into contact with the sample. This way, a miniaturized electrochemical cell is formed, where only the wetted area inside the droplet is active surface area. The sample is commonly connected as the working electrode and a Ag/AgCl wire inside the micropipette serves as a combined pseudo reference counter electrode. For this SPECM experiment, an optical fiber is also inserted into the micropipette to illuminate the sample within the droplet. The area of the wetted surface depends on the pipette opening and the surface properties of the sample and is usually in the range of a few micrometer. Currents in the nA range are expected.
The micropipette is scanned in hopping mode. The resulting images are one for the topography, because the position of the micropipette is tracked and one for the photocurrent at the sample.
The advantage of this technique is that large capacitive background currents at extended surfaces can be avoided due to the small wetted area.