Experiment

The system of electrochemistry scanning tunneling microscopy (EC-STM) is shown in Figure 2-3.

Figure 2-3. The system picture of EC-STM. (1) is the EC-STM chamber, (2) the supported electrolyte, (3) the bipotentiostat, (4) the displayer of the CV curve, (5) the controller of STM, (6) the displayer of the STM image, and (7) is damping

The (1) represents the aluminum STM chamber included the input argon gas, the electrolyte, the pump out tubes. In order to maintain the chamber clean, the chamber is filled with argon gas during the experiments of the CV and the STM. The input electrolyte tubes have two separate tubes. One is for the H₂SO₄ solution (10 mM) and another is for the HCl (10 mM) or KBr solution (10 mM). The H₂SO₄ solution makes the concentration of the reference electrode maintained. Before the experiments, the H₂SO₄ solution is first injected into the cell through the reference electrode and then replaced by HCl or KBr solution. Because the chloride and bromide anions are stronger absorbed on the Cu(100) surface, the sulfate anions are not absorbed on the Cu(100) surface anymore. Therefore, once the HCl or KBr solution is on the cell, the c(2 2) surface structure is immediately formed instead of the c(6 2) or c(4 2) phase from the sulfate anions.¹³⁹ Before the HCl or KBr experiment, the tube for the HCl or KBr solution is clear by the high purity water in several times to make sure the chloride or bromide not exist in the tube channel. (2) shows the H₂SO_4, HCl, and KBr electrolytes are in the three individual separator funnel. (3) is the bipotentiostat and corresponding displayer is shown in (4) for the CV curve. (5) is the STM controller for setting and adjusting the parameters (i.e. tunneling current, tip bias.) and the STM image is displayed in (6) monitor. The CV and STM controllers are corresponded to the individual computer. The (7) describes the damping apparatuses. The white arrow indicates the rubber pieces in the middle position between the sample stage and corresponding based plate to protect the larger 100 Hz frequency vibration. The black arrow shows the STM chamber placed on the haven granite plate which is suspended with the four steel springs on the plate corners to protect the less 1 Hz frequency vibration.

The simplest circuit of the bipotentiostat is constituted of the several basic circuits as shown in Figure 2-5. The basic circuits included the adder (a), current follower (b), scaler (c), voltage follower (d), and control functions (e) are shown in Figure 2-4.¹⁴⁰ The effects of (a) to (e) are the summing of voltages from the supporting voltages, the current-to-voltage converter, the inverted input voltage, supporting the current at the same voltage, the loading current. The four electrodes of the counter, reference, work , and tip electrodes are corresponded to the E_c, E_r, and E_w as shown in the Figure 2-4(e) and 2-5 and hence the bipotentiostat major construct from the control function device and then expands the other devices. The current signal can be readied out from the control function at the E_w electrode.

Figure 2-4. The circuits of (a) the adder, (b) current follower, (c) scaler, (d) voltage follower, and (e) control functions.

At the CV operation, only the three electrodes are used besides the tip electrode.

The setting potential of the work electrode marked as E_w1. The adder and scalar devices make the potential equated with E_w1 at the reference electrode, but invert (-E_w1). Further, the adder device is accepted any wave function of the potential with varying the time. Therefore, the work potential has the respective potential E_w1 to apply on the sample. The reference electrode is connected the large resistance to avoid the current crossing and hence the voltage follower is placed to make the point A equated with 0. As the work electrode provides the sample the setting potential value, the current i from the solution is detected at the work electrode and then the obtained current signal is transferred to the voltage signal for the advanced treatment by the current follower. Therefore, the CV curve can be carried out by the bipotentiostat. As the potentiostat is combined with more than one work electrode, it is called the bipotentiostat. The case is for the STM experiment and the new work electrode is the tip electrode. In order to avoid the tip bias influencing the work potential and getting non-tunneling current, the circuit of the tip electrode is separated as shown in the right side of Figure 2-5. The OA5 and OA6 make only the bias potential (E_w2) applied on the tip without the work electrode. The OA4 and OA7 provide the only the E_w2 signal can be cross, and then the tunneling current is transferred to the voltage signal for the advanced treatment. More detail information can be found in the textbook of reference 141.

( ) d

( ) a ( ) b ( ) e

( ) c

Ec

Ew Er

Figure 2-5. The simplest circuit of the bipotentiostat.¹⁴¹

The illustration of the apparatus for fabrication the tip in the EC-STM is shown in Figure 2-5. The STM tip is produced from a tungsten wire of a 0.25 mm diameter. The tungsten wire is placed at the center of a platinum ring filled with a 2M potassium hydroxide electrolyte and then electrochemically etched until the end of the tungsten wire fall off. At the beginning, the square function wave with AC 12 voltage is applied and then switch to AC 4 voltage for fine the tip before the tip falling down.

Subsequently, the formed tungsten tips are coated with the hot-melt adhesive to reduce the Faradic current as shown in the insert image of Figure 2-5. This fabrication tip method with the small cone angle of the tip radius is better and easier to product in the EC-STM than that in the UHV-STM system.^142,143 However, the volume for the tip mounting is too small for UHV-STM. Therefore, the tip cannot be stably mounted into the tip holder of the UHV-STM.

Figure 2-5. The illustration of the apparatus for fabrication the tip in EC-STM.

Er Ew

Et A Ec

High purity water (Milli-Q purification system; conductivity < 18 M_{Ω cm} ; TOC <

5ppb) and reagent grade chemicals are used for all solutions. Before all experiments, all electrolyte solutions are desecrated with argon gas for several hours. All given potentials (E) in the part II of this thesis are relative to the reversible hydrogen electrode (RHE). The platinum is treated as the counter electrode in the sample cell.

The Cu(100) electrode is the single crystal cylinders and the surface orientation was better than 0.5^o off the (100) plane to certify a repeatable smooth surface. The oxide film on the copper surface which naturally is formed in oxidizing atmosphere have to be removed before installing the sample into the sample cell. The total volume of the cell is around 10 ml. In order to prepare a clean and smooth surface, the copper surface is immersed into the 50% orthophosphoric acid. Afterward, apply a 2V potential between the copper and the platinum was applied for 1 min to 2min. After polishing, the orthophosphoric acid on the copper surface is replaced with the sulfuric acid and then mounts into the chamber which is filled with argon gas to guarantee pure conditions. The Highly Ordered Pyrolytic Graphite (HOPG) surface is cleaned by peeling of the topmost layers with adhesive tape in air. At the beginning of the CV and the STM measurements, the hydrochrolic acid (10 mM) is injected into the copper electrode. As chloride anions specifically adsorb to form an order c(2×2) structure adlayer on the Cu(100) surface, this treatment can reduce surface defects which formed during the sample polishing process. This kind of process is called the electrochemical annealing.¹⁴⁴ For X mM carboxylated viologens on the Cu(100) and HOPG surfaces, the working electrolytes included 10 mM HCl and X mM dicarboxylated viologens from the group of Takamasa Sagara, Japan (X = 0.1 and 1).

The dilute concentration electrolyte of carboxylated viologens is prepared by re-injecting about 60 ml HCl into the cell to reduce the concentration of the 0.1 mM viologen electrolyte, and hence the minimum concentration is diluted six times compared to the original concentration. All experiments are carried out at room temperature.

Chapter 3 Results and Discussions of the self-assemble layers of