Over the last few years, knowledge of electrochemical methods such as electrodeposition, or electroplating, is reflected in the range of technologies and practical uses they are applied to, such as nanobiosystems, microelectronics, chemistry and solar energy conversion [1,2]. The chemical or physical surface characteristics can be altered to suit different applications using electrodeposition which involves passing an electrical current through an electrolyte solution.
The direct application of electrical currents into an electrolyte solution works by depositing particles onto the surface of the conductive substrate part of the material [3]. Founded on the theory of electrolysis, this technique is typically carried out in order to improve heat tolerance, electrical conductivity and corrosion resistance, as well as to increase the attractiveness of products.
The surface morphology of the substrate determines the quality of the deposition [4]. Therefore, a method that can collect information on and regulate the electrodeposition process in nanoscale would be highly sought after. There are a number of technologies that were implemented for the surface characterization, two candidates are scanning tunneling microscopy (STM) and scanning electron microscopy (SEM).
Although these methods enable data collection of structures that are nanometers in size, some issues with them are that they must be ex situ, it is common for them to need a high vacuum, and others have a slow image acquisition time meaning that they are not suitable for regulating samples that do not have consistent processes [2,5].
As a solution to these problems, electrochemistry put together with Atomic Force Microscopy to make EC-AFM was developed. This hybrid method enables researchers to carry out in situ imaging and the visualization of variations in surface morphology of the sample under investigation when in a particular electrochemical setting in nanoscale [6].
In the investigation discussed in this article, it is shown that copper particles can be deposited and dissolution can occur on a gold surface. The morphological changes of the copper particles in situ using Park NX10 AFM are shown clearly, and a current-voltage (CV) curve using the potentiostat was also produced during the investigation.
AFM Head and Tip
A liquid probehand shielded with glass was used instead of a conventional probehand, therefore allowing measurement in liquid. A NANOSENSORS PointProbe® Plus-Contact (PPP-CONTSCPt) cantilever (nominal spring constant k = 0.2 N/m and resonant frequency f = 25 kHz) coated with platinum and mounted on Teflon chip carrier was used in the experiment.
Platinum coated cantilever was chosen to maintain beam intensity when the tip is immersed in the solution. Moreover, the tip was mounted on Teflon chip carrier to protect the EC Cell from other unwanted electric signals that could affect the conditions of the electrolyte solution.
EC Cell Setup
A small EC Cell manufactured by Park Systems was utilized in the experiment. The cell is made of polychlorotrifluoroethylene (PCTFE), ensuring chemical stability. A sample is mounted with a top cover and sealed with a thin silicon O-ring, securely preventing leaking on the backside of the sample.
The working electrode (WE) that was used is gold (Au) in the 111 orientation evaporated on a mica surface while reference electrode (RE) and counter electrode (CE) are silver chloride (AgCl) and platinum-iradium (Pt-Ir).
The 3 electrodes were connected on a Solartron Modulab XM potentiostat. The aqueous solution contained 0.1 mM CuSO4 in 50 ml of 0.01 mM H2SO4. Sulfuric acid was added in the solution to stabilized and prevent copper to precipitate.
Figure 1. (a) Overall structure of the NX EC Cell (b) Actual setup of EC cell
EC-AFM Experiment Conditions
The deposition and dissolution of the copper particles in gold thin films with 111 orientation was monitored using a Park NX10 system. A reference image of the gold surface was attained in ambient air and in liquid condition without introducing chemical reaction to serve as a point of comparison of the sample surface prior to and after the experiment.
The images were acquired using non-contact mode, to get high quality images. After obtaining the reference image in-liquid, the 3 electrode was connected into the potentiostat. The CE and RE were bent before immersing in the liquid solution to make a good contact and prevent saturation of the signal.
AgCl electrode was utilized as RE since a chloride solution was not used. Furthermore, Pt-Ir electrode was selected as CE since this material is chemically stable, therefore it will not contaminate the solution.
In EC-AFM, the WE is the sample surface where the electrodeposition process takes place, the CE is where the electric current is expected to flow. EC cell is like an electronic circuit and the main function of CE is to close the current loop in the circuit. The RE is used as a fixed reference point and serves as a feedback to maintain a stable voltage in the solution.
An electric field is supplied in the working electrode to transmit electrons to the ions in the solution so that oxidation or reduction will occur. The type of chemical reaction depends on the amount of voltage supplied (either positive or negative voltage) in the EC cell. In this study, a cyclic voltammetry was applied in the EC cell to know the oxidation and reduction peaks of the solution.
After determining the threshold voltage where the oxidation and reduction occurs, linear sweep voltammetry was used to deposit and dissolve copper particles on the gold surface. Two scans were carried out with -0.2 V to -0.4 V to cover the complete surface of the gold with copper particles. On the other hand, 4 scans were performed with -0.2 V to 0 V to entirely dissolve the copper in the solution.
Results and Discussion
Figure 2 depicts the CV curve produced during cyclic voltammetry. When creating the graph, four cycles of oxidation and reduction process (known as the redox process) from beginning to completion were chosen.
The graph produced suggests that these reactions are reversible based on how much potential is put to the sample. It was found that the deposition of copper is initiated at the application of -0.2 V potential to the cell and the greatest decreased state occurs when there is an applied potential of -0.4 V.
Figure 2. Cyclic voltammograms. The negative peaks demonstrate the reduction reaction state where copper is deposited in the sample. Meanwhile, the positive peaks demonstrate the oxidation reaction state where copper is dissolved in the sample.
One conclusion that can be drawn from the findings is that a larger number of negative potentials applied on the cell will raise the amount of copper deposited on the gold surface. However, the dissolution of copper starts when the voltage is zero, and the highest oxidation state occurs when 0.1 V is applied.
The amount of the copper dissolved in the gold surface is higher as more positive potentials are applied on the cell. In addition, the CV curve shows that in -0.1 V, the solution is in a neutral state and there are no chemical reactions occuring.
Figure 3. (a) AFM image acquired in air, (b) AFM image acquired in liquid. Figure 3 (a) and (b) revealed that the 5 um by 5 um scanned region in ambient air and 1 um by 1 um scanned region in liquid condition of gold surface is made of individual grains which are believed to be crystalized. The high quality images show that no foreign particles are present on the surface prior introducing electrochemical reaction in the solution.
The redox process was confirmed using AFM and voltages were applied using linear sweep voltammetry. Figure 4(b) and 4(d) shows the CV curves acquired by applying voltages from -0.2 V to -0.4 V. The current density decreases as more negative voltages were applied. This phenomenon demonstrates that reduction reaction is occurring. Figure 4(a) and 4(c) confirms this process wherein the images clearly show that copper particles were successfully deposited on the gold surface.
XEI software developed by Park Systems was used to quantify copper particles, which marked each detected particles with different colors and numbers. The detection method used in XEI is Upper Threshold Grain Detection. In this method a threshold value is set and particles whose heights are smaller than the threshold value are not detected.
Figure 4. (a) and (c) AFM images acquired on 1st and 2nd deposition test, (b) and (d) CV curves acquired using linear sweep voltammetry with voltages from -0.2 V to -0.4 V on 1st and 2nd deposition test.
In this experiment 5.5 nm was used as a threshold value to quantify the copper particles. As the gold surface is composed of individual grains with various heights, most of the copper particles deposited on lower surface with height value smaller than the threshold value were not detected.
Figure 4a shows the image acquired during the first deposition test. The number of particles detected on this test was approximately 7 with mean area value being 2 nm. Additionally, there are particles deposited on lower regions with smaller height value that were not detected by XEI.
Furthermore, 199 particles were detected on the 2nd test with a mean area value of 36 nm as can be seen in Figure 4b. The data shows that the number of particles deposited on the 2nd test is 28 times more compared to the number in the 1st test with larger area which covered almost the entire region of gold surface.
Figure 5. Histogram plots of particles area distribution on 1st (left) and 2nd (right) deposition test.
The CV curves acquired by employing reverse voltage from -0.2 V to 0 V during dissolution process show that current density increases as more positive voltages were applied. This phenomenon demonstrates that an oxidation reaction is occurring. The AFM images attained in this process confirm that such phenomenon happened since the number of copper particles deposited on the gold surface decreases when dissolution tests were performed.
Figure 6 (a) displays the image acquired in the 1st dissolution test. The number of particles detected in this test was 180 with a mean area value of 37 nm2. The number of particles detected on this test was slightly fewer with a smaller mean area value compared to particles detected on the 2nd deposition test.
Almost similar results were observed in 2nd dissolution test, the number of particles detected was 181 but with a smaller mean area value of 24 nm2. However, the results on the 3rd dissolution test are far from the first two tests. The detected number of particles was only 19 with a mean area value of 7 nm2.
Lastly, the 4th and final dissolution test shows that almost the remaining copper particles on the gold surface including those deposited on the lower region were entirely dissolved in the solution.
Figure 6. (a), (c), (f), and (h) AFM images acquired on 1st to 4th dissolution test, (b), (d), (g) and (i) CV curves acquired using linear sweep voltammetry with voltages from -0.2 V to 0 V on 1st to 4th dissolution test.
Figure 7. Histogram plots of particles area distribution on 1st (upper-left), 2nd (upper-right), 3rd (bottom left), and 4th (bottom-right) dissolution test.
Conclusion
Here the use of electrochemical atomic force microscopy in situ monitoring of morphological changes of samples that are undergoing electrochemical process is demonstrated. The deposition and dissolution of the copper particles on the gold surface were successfully performed by applying the suitable voltages suggested by the CV curve acquired during cyclic voltammetry.
The data on the deposition process demonstrate that the magnitude of copper deposited on the surface increases tremendously on the 2nd deposition test. However, the dissolution data shows that greatest number of copper nanoparticles was dissolved on the 3rd dissolution test. Overall, the technique described in this study will successfully give researchers nanoscale information that is noteworthy in monitoring an electrochemical process.
Acknowledgments
Original authors: John Paul Pineda, Mario Leal, Gerald Pascual, Byong Kim, and Keibock Lee, Park Systems.
References and Further Reading
- Dryhurst G., et al. Application of Electrochemistry in the Studies of the Oxidation Chemistry of Central Nervous System Indoles. Chem. Rev. 1990.
- Schlesinger M., et al. Modern Electroplating. Fifth edition, pg. 27.
- Saidin N., et al. ELECTRODEPOSITION: PRINCIPLES, APPLICATIONS AND METHODS.
- Popoola A., et al. Effect of some process variables on zinc coated low carbon steel substrates. Scientific Research and Essays, Vol. 6 (20), pp. 4264-4272, 19 September, 2011.
- Smith T., et al. Electrochemical SPM Fundamentals and Applications.
- Reggente M., et al. Electrochemical atomic force microscopy: In situ monitoring of electrochemical processes.
This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.
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