Electrochemical Characterization of Surface Area

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Necessary Materials

1. np-Au/planar Au chips (22 mm X 22 mm) → working electrode

2. 0.05 M or 0.5 M H2SO4

3. Platinum wire → counter/auxillary electrode

  • Product Number: 45093
  • 0.25 mm (0.010 in) dia, 99.9% (metals basis) length: 50 cm

4. Reference Electrode

  • Product Number: ET073-1
  • Refillable miniature Ag/AgCl reference electrode

5. Gamry Reference 600 potentiostat

6. DI water

Procedure

Background

Nanostructured materials with high surface area maximize the current density and signal magnitude enabling sensitive detection. The traditional surface area measurement techniques, such as Brunauer-Emmett-Teller (BET), cannot be used for characterizing the surface area of sub- micron thin films as those techniques require a specific surface area of ~5 m2/g. Alternatively, measurement of charge required to strip the gold oxide layer via cyclic voltammetry (CV) provides an accurate measurement of the electrochemically-active surface area. This reaction is not mass transport-limited and therefore the entire nanostructured surface participates in the reaction, enabling a reliable estimate of surface area

Sample Preparation

1. Cut np-Au/planar Au chips (22 mm X 22 mm) into four squares. Use one square per experiment.


2. Prepare piranha solution (1 : 3; hydrogen peroxide: sulphuric acid) and leave it to cool for 10 minutes.

2a. Clean the electrodes by immersing for 20 seconds each.

2b. Rinse with DI water and store in DI water until surface area measurements.


3. Plasma treat electrodes for 1 minute prior to surface area measurements.


4. Place the electrode inside the custom-built Teflon electrochemical cell and assemble the cell.

Surface Area Measurements

1. Fill chamber with 150 μl of 0.05 M or 0.5 M H2SO4.

  • 0.5 M H2SO4 gives less noisy readings


2. Assemble Ag/AgCl reference electrode, platinum wire counter electrode and connect the cell to the Gamry Reference 600 potentiostat.


3. Check the open circuit potential (OCP) to make sure there is electrical continuity between the working and reference electrodes.

  • A high OCP > 1 V indicates a short. This may be due to poor working electrode contact or bubbles in the electrolyte or damaged reference electrode.
  • Start trouble shooting first by refilling electrolyte making sure to remove bubbles. If that doesn’t solve the problem, disassemble the cell and check working electrode contact. If this fails, finally test with a different reference electrode.


4. Measure OCP for 30 s to make sure the system is stable. 10 - 20 mV change in OCP in this 30 s is acceptable.


5. Run Cyclic Voltammetry (CV) with the following parameters:

5a. Scan rate: 50 mV/s

5b. Potential range: -0.25 mV to 1.75 mV


6. Copy CV file in the Gamry Data folder to the desired location under the desired file name.


7. Wash the el. cell 3 times with DI water to clean H2SO4 residue before storing the tested sample.


8. Wash el. cell thoroughly with DI water before testing another sample.

Surface Area Calculations

1. Find the electrical charge under the gold oxide reduction peak between the potentials 720 mV and 970 mV. Usually given in [mC].

1a.

1b.


2. Convert obtained electrical charge into the effective surface area by using 450 μC/cm2 as the specific charge required for gold oxide reduction.

  • Reference: Tan, Y.H., et al., Surface area and pore size characteristics of nanoporous gold subjected to thermal, mechanical, or surface modification studied using gas adsorption isotherms, cyclic voltammetry, thermogravimetric analysis, and scanning electron microscopy. Journal of materials chemistry, 2012. 22(14): p. 6733-6745.


3. The ratio of the effective surface areas of different np-Au samples to the effective surface area of control planar Au sample was defined as enhancement factor, Eh.

  • Usually, the effective surface area of np-Au is ~7 cm2, displaying a significant Eh of ~9.