A Newly Developed Inverted Rotating Disk Electrode (IRDE) Setup for the Investigation of Photoelectrochemical Reactions
Authors: Mairhofer Katharina , Larisegger Silvia , Fafilek Guenter 
Published: July 2022

A new setup for an inverted rotating disk electrode (IRDE) was developed for the investigation of photoassisted or photoelectrochemical processes involving mass transfer reactions, as in electrodeposition or etching processes. In this setup, rotation of the disk electrode is achieved by magnetic coupling between a driver magnet (i.e. a magnetic stirrer) and the follower magnet included in the sample holder. For photoelectrochemical experiments, the cell allows a homogeneous illumination and removal of gaseous reaction products, while still providing leak tightness. In this way, controlled flow conditions are provided with the possibility to simultaneously illuminate the sample surface from a distance as small as 20 mm. At the same time gases formed during the reaction can easily ascend through the electrolyte.
The light source is placed on top of the cell and illumination is done through the electrolyte. A low liquid level (≥ 15 mm) allows a small distance between the sample and the light source and therefore a high light intensity on the sample surface. Illumination is done using a high-power LED. Due to the high power, thermal effects need to be taken into consideration and especially an effective cooling of the LED is essential.
As a wide band gap semiconductor, silicon carbide (SiC) has gained large interest due to its high chemical and physical stability. Since photoassisted anodic etching of n-type 4H silicon carbide in potassium hydroxide solutions [1] requires both UV illumination and the removal of gaseous reaction products, this reaction is investigated in the newly developed IRDE setup. Possible applications for such a process are defect decoration [2], trench etching, and dicing.
Photoassisted anodic etching of n-type SiC occurs in three steps: (1) hole generation in the material through illumination with light with an energy higher than the bandgap of the semiconductor (2) oxidation of silicon and carbon through the holes driven to the semiconductor-electrolyte interface by an external potential and (3) dissolution of the oxidation products in the electrolyte. In accordance with the three steps of the reaction, different processes can be limiting: For low light intensities (either through a large distance between the sample and the LED or through low driving currents through the LED) the number of photon-induced holes is limiting. At low electrolyte concentrations, the transport of hydroxyl ions and the dissolution of the oxidation products determine the etch rate. For high light intensities and high electrolyte concentrations, the external potential and thereby the oxidation reaction are limiting for the overall current flow. These different reaction regimes can be distinguished in cyclic voltammetry measurements in the newly developed IRDE cell.
[1] D. H. van Dorp und J. J. Kelly, „Photoelectrochemistry of 4H–SiC in KOH solutions,“ Journal of Electroanalytical Chemistry, Bd. 599, Nr. 2, pp. 260-266, 2007.
[2] J. L. Weyher, S. Lazar, J. Borysiuk und J. Pernot, „Defect-selective etching of SiC,“ physica status solidi, Bd. 202, Nr. 4, pp. 578-583, 2005.

Figure 1