Methods for cleaning substrates and glass chambers for electrochemical measurements

By Marcin Orzech

1. Introduction
Clean labware is essential in electrochemistry, as contamination will lead the most carefully executed experiment to fail and provide an invalid result. There are several methods of cleaning glassware and other items, which vary depending on laboratory preferences. Here are described some of the most common methods of cleaning glassware and substrates/electrodes for electrochemical measurements. For high precision measurements, the cleanliness of glass is verified by uniform wetting of the surface by distilled water. If the beads of water are formed or it doesn’t sheet cleanly off, the glass is not clean enough.

Health and safety warning – Many of the mentioned here chemicals are highly corrosive and dangerous. Remember to check any relevant safety documentation and act accordingly. Always wear appropriate PPE, particularly for most of the solutions listed here butyl gloves are recommended instead of latex or nitrile gloves. Always wear eye protection and a lab coat and work in the fume hood if possible.

2. Electrochemical cells and glassware
As a basic rule, always disassemble and wash cell immediately after each experiment. If a thorough cleaning is not possible immediately, put disassembled parts to soak in water. Electrochemical cells offered by redox.me can be cleaned according to following steps.

• Clean bulk residue by wiping them with a paper towel soaked in soapy water.
• Place all elements in a warm aqueous solution of neutral detergent (e.g. Alconox^®, Dural®,M&H^®, Lux^®, Tide^® and Fab^®) and let sit for several minutes.
• Rinse thoroughly with distilled water.
• Rinse with organic solvent – acetone, ethanol etc. in ultrasonic bath 3-4 times.
• Finally rinse in ultrapure water in ultrasonic bath 3-4 times.
• Before use with air/moisture sensitive materials dry in vacuum at 60°C for 2h.

In most cases this procedure would be sufficient. However, if contamination remains more aggressive cleaning methods may be used after above procedure.

Note: Many parts of the measuring cell are made of high-performance PEEK. This material is extremely resistant to many chemicals. However, it is not compatible with strong mineral acids, such as concentrated nitric and sulphuric acids, including piranha or aqua regia. However, PEEK can safely withstand the 20-30% nitric acid or 1M HCl solution.

Removal of organic residue from glassware:

• Base bath – it is mild cleaning method comparing to some others described here. Soak glassware in 2M KOH in isopropanol (methanol or ethanol are also suitable) solution for 10-15 min. Rinse with DI water 3-4 times. Heating to 60°C can make the process more effective. Note: Do not use this method for glassware contaminated with metal-containing compounds, glass fritted funnels and volumetric glassware.
• Permanganate-based method – This is very effective technique of removing organic species. Make 3 g/L KMnO₄ solution with H₂O-H₂SO₄ (1:1 vol./vol. - 2.25 mol/L) and soak glassware overnight. The solution should never be prepared at T<0°C because Mn₂O₇ would form, which is an olive-green liquid and can explodes at 10°C. After this process organic compounds are oxidized to form brown mixture with MnO₂. Finally, the glassware is soaked in an H₂O-H₂O₂-H₂SO₄ (1:1:1 vol./vol.) solution, followed by rinsing with cold and boiling ultra-pure water, successively. This step lead to reduction of MnO₂ and remaining MnO₄^-: bubbles must be seen during the process.
• Piranha etch – the piranha mixture with a variable concentration of H₂SO₄ and aqueous H₂O₂ (3:1, 4:1 or even 7:1 vol./vol.) is another strong oxidant and a strong reductant at the same time. When preparing the piranha solution, always add the peroxide to the acid very slowly. The H₂O₂ is added immediately before the etching process because it immediately produces an exothermic reaction with gas release. Immerse the glassware in the piranha solution for at least 30 min. Rinse with copious amount of DI water. Caution: Piranha solution reacts violently with organic compounds, which can lead to explosion. It should be handled with extreme care.
  Note: another used version is the base piranha: a 3:1 (vol./vol.) mixture of NH₄OH with H₂O₂. The reaction in the acid piranha is self-starting whereas the base piranha must be heated to 60°C before the reaction takes off.

Removal of metallic residue from glassware:

• Acid bath – For most metallic species soaking the glass in a 6 M HCl solution would be sufficient. Once the impurities are dissolved, rinse with tap water, and then repeat the general cleaning steps above. This method will also remove some organic residues.
• Aqua Regia - This is an extremely powerful oxidizing solution capable of dissolving even gold. It is prepared by mixing concentrated HCl and concentrated HNO₃ in 3:1 (vol./vol.) ratio. for at least 30 min. Similarly, to piranha solution, it is recommended to prepare fresh batch for each cleaning, however 1 part H₂O be added if the aqua regia will be stored to minimize the generation of Cl₂. Immerse pre-cleaned glassware in the solution for few minutes, then rinse it thoroughly with DI water [1]. Extreme caution must be used when working with aqua regia because it generates Cl₂ and NO_x gases in addition to causing severe tissue damage.

Use of chromic acid is popular method of cleaning glassware, however it is not recommended due to its highly carcinogenetic and teratogenic nature.

3. ITO/FTO substrates
redox.me offer specialised racks (see Figure 1) for easy handling and cleaning wide variety of substrates. It resists most of the acids, bases and organic solvents and can be used for cleaning substrates, etching, developing photoresist, solvent treatment, SAM deposition and baking (below 250°C).

Figure 1. redox.me PTFE adjustable substrate racks for beakers with volume of 25, 50, 100, 150 and 250 mL

For most applications the FTO and ITO substrate have to be extremely well cleaned and activated. The most common general method of cleaning involves sonicating in several solvents, as follows [2]:

• Ultrasonicate in soapy water for 10min
• Ultrasonicate in DI water for 10min
• Ultrasonicate in methanol (or ethanol) for 10min
• Ultrasonicate in ultrapure water for 10min
• Dry with nitrogen.

Another popular method developed by Werner Kern is RCA treatment. It is a mild cleaning process which helps to removes organic impurities from the surface. The substrate is heated at 80°C for 15-30 min in 1: 1: 5 vol. ratio of NH₃, 30% H₂O₂ and H₂O. This is followed by rinsing with ultrapure water 3-4 times and drying with a stream of nitrogen gas.

Also the described above base bath in KOH at 60°C is a suitable cleaning method.

4. Disk electrodes
Over time and after some experiments disk electrodes like those shown in Figure 2 (Pt, Au, Ag or Glassy Carbon) will cover with surface contamination. To restore the electrode to initial state, polishing procedure is recommended as the most common practice [2-3]. Do not over-polish the electrodes, as it would shorten their lifetime. In most cases the electrode remains clean after the experiment or could be cleaned with suitable organic solvent to remove greasy organic materials and with dilute acid or base (0.1M HCl, HNO₃, NaOH) to remove inorganics.

Figure 2. redox.me working disk electrodes with PEEK body

The polishing is performed with successively smaller alumina/water slurries (1.0, 0.3, and 0.05 µm) on individual felt polishing pads. For lightly contaminated electrode or just for maintenance just the last step with 0.05 µm will be sufficient. To maintain the even flat surface of electrode grip the electrode near its tip and hold it in a vertical position while making figure-8 motions on the polishing pad. Always use moistened pads with appropriate polishing paste. Turn the electrode by 90° in your hand every now and then. Between each polishing step and after finishing the electrode has to be sonicated in DI water for 5 min to remove all alumina and removed material residue. After this process the electrode should have smooth, mirror finish surface.

In most cases the general cleaning procedure, above, will give satisfactory results. Information on additional activation and pre-treatment methods can be found in ‘Handbook of Electrochemistry' [2].

5. References 
[1] Napporn, T. W. et al. Electrochemical Measurement Methods and Characterization on the Cell Level. in Fuel Cells and Hydrogen vol. 53 175–214 (Elsevier, 2018).
[2] Swain, G. M. Solid Electrode Materials: Pretreatment and Activation. in Handbook of Electrochemistry 111–153 (Elsevier, 2007). doi:10.1016/B978-044451958-0.50006-9.
[3] Gross, M. & Jordan, J. Voltammetry at glassy carbon electrodes. Pure Appl. Chem. 56, 1095–1129 (1984).

Please, note that the content of this article is constantly being updated to cover the topic in the possibly most reliable way. At the same time, we are doing our best to keep it in line with the state-of-the-art research.