Oxygen-Free High-Resolution Electrochemical SPM

Application Note

Wenhai HanAgilent Technologies

IntroductionFor two decades, scanning probemicroscopy (SPM) has providedscientists a unique tool to studyin situ electrochemical processeswith atomic/molecular resolution.During this time, remarkableprogress has been made at theinterface of electrochemistry (EC)and SPM. New discoveries contin-ue to be reported as the researchboundary expands wider anddeeper. While the field steadilyadvances, the requirements forelectrochemical scanning probemicroscopes also become greater.In addition to clean liquid imagingand atomic resolution, an oxygen-free environment is now consid-ered critical for many EC SPMstudies.

In this note, the oxygen-elimina-tion capability of the standardenvironmental chamber of theAgilent 5500 AFM is thoroughlycharacterized. As an example ofoxygen-free applications, reduc-tive desorption potentials of alka-nethiol self-assembled monolayers(SAMs) on Au (111) surface weremeasured. In situ high-resolutioncapability was demonstrated withthe well-known transition betweenAu (1 x 1) atomic lattice and Cu(√3 x √3) under potential deposi-tion (UPD) structure on Au (111)surface at different electrochemi-cal potentials.

ExperimentFor oxygen measurements, a 5500AFM base with an atomic STMscanner was set up on an environ-mental chamber the same way asfor imaging. The chamber waspurged with high-purity nitrogengas through one of the eight built-in ports. The oxygen concentra-tion inside the chamber wasmeasured with a TPI 708 oxygenanalyzer, which can detect theoxygen level down to 0.1%. Lowerlevels of oxygen were monitoreddirectly from cyclic voltammetry.

An electrochemical cell using 0.1 M H2SO4 electrolyte was con-trolled by an internal potentialstatfor the 5500 AFM. The workingelectrode was hydrogen flame-annealed Au (111) surface, thequasi-reference electrode was Agwire, and the counter electrodewas Pt wire. All potentials weremeasured against the quasi-refer-ence electrode. The ramp range ofcyclic voltammetry was from thebeginning of hydrogen evolutionto the Au oxidation peak at a rateof 20 mV/sec. The liquid cell wasabout 6 mm in diameter, provid-ing ~28 mm2 working electrodearea. The cell and electrodes werethoroughly cleaned in detergentand acid and rinsed with18 MΩ•cm-1 water before the electrolyte was applied.

For the experiment of reductivedesorption of alkanethiol SAMs,hexanethiol (C6), octanethiol (C8),decanethiol (C10), and octade-canethiol (C18) were purchasedfrom Sigma-Aldrich. Cyclic voltam-metry was measured in 0.5 M KOHsolution, also using Ag for thequasi-reference electrode and Ptfor the counter electrode. Theramp was approximately from -0.1 V to -1.2 V with a rate of 10mV/sec.

For Cu UPD, the electrolyte was0.1 mM H2SO4 containing 5 mMCu2+. Sample potentials weremeasured against a Cu quasi-reference electrode. Images wereacquired with a small AFMscanner in contact mode.

Results and discussionI. Oxygen characterization of theenvironmental chamber

Figure 1 shows a set of cyclicvoltammograms for 0.1 M H2SO4

on Au(111) at different oxygenconcentrations. Green plots repre-sent ramp-down potential and blue plots represent ramp-up. In

ambient, where the oxygen is20.9%, the electrochemical currentmeasured from the counter elec-trode started to increase as thepotential was decreased to 0.1 V,as shown in Figure 1(a). Its ampli-tude reached a maximum at about-0.02 V (versus Ag quasi-referenceelectrode). This potential corre-sponds to the reduction of the oxygen in the electrolyte tohydrogen peroxide.

After the oxygen-reduction peak,the strong background currentcontinues. Sensitive electrochemi-cal processes in this range wouldbe very difficult to detect becauseof the presence of oxygen.

With the decrease of the oxygenconcentration inside the chamber,the amplitude of the backgroundcurrent below 0.1 V was decreasednoticeably, as shown in Figures1(b) and (c). When the oxygen wasnot detectable, the voltammogramwas stabilized (Figure 1(d)). Thestabilized voltammogram remained

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Figure 1. Cyclic voltammograms for 0.1 M H2SO4 on Au (111) at different oxygen concentrations.

O2 + 2H+ + 2e- H2O2

-

unchanged for hours with continu-ous purge of nitrogen.

It took less than 5 minutes of con-tinuous nitrogen purge at a pres-sure of 10 to 20 psi to reduce theoxygen concentration of the cham-ber from ambient to 0.1%. In orderto reach the level where the oxy-gen was not detectable, as shownin the stabilized voltammogram,approximately an additional 15-minute purge was needed.

Figure 2 shows six continuouscyclic voltammograms in 21 min-utes of nitrogen purge, startingfrom ~5% oxygen concentration.Slight changes on the backgroundcurrent below 0.1 V were stillobservable in the first 12 minutes,as shown in Figures 2(a) to 2(c).From Figure 2(d), 15 minutes afterpurge, the voltammograms becamestale, indicating that there was no detectable oxygen inside thechamber.

The standard environmental cham-ber for the 5500 AFM is notdesigned for vacuum. If the nitro-gen flow is completely stopped,oxygen outside can slowly flowinto the chamber, driven by theentropy force. Figure 3 shows con-tinuous changes in the cyclicvoltammogram after the nitrogenflow was stopped. Oxygen becamedetectable after 20 to 30 minutes,as seen from the slightly observ-able background current below 0.1 V in Figures 3(e) and (f). Witha slightly positive pressure, thechamber can be kept oxygen-freefor many hours.

II. Reductive desorption of alkanethiol SAMs

As an example, an oxygen-freeenvironment was used to study thereductive desorption of alkanethiolSAMs on Au(111) surface.Depending upon the number ofcarbon atoms and the type of endgroup on the alkanethiol molecule,the desorption potentials of alka-nethiol SAMs vary from around

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Figure 2. Continuous cyclic voltammograms in 21 minutes of purge from ~5% oxygen concentration.

-0.7 V to -1.0 V (versus Ag quasi-reference electrode).

Each of the C6, C8, C10, and C18SAMs was prepared by leaving aAu(111) film in ethanol solutioncontaining 2 mM of the correspon-ding molecules overnight. Figure 4plots the reductive desorptionvoltammograms of all four SAMs.The measured desorption poten-tials of C6, C8, C10, and C18 were-0.69 V, -0.74 V, -0.85 V, and -0.98 V,respectively. These potentials areall below the oxygen-reductionpotential. Without an oxygen-freeenvironmental chamber, the peakswill not be visible because of thestrong background current in thepresence of oxygen, as shown inFigure 1.

III. High-resolution in situ EC SPM

Figure 5 demonstrates a typicalexperimental result of routinelyobserved Cu UPD deposition onAu (111). Figure 5(a) shows a con-tinuous change of the topographyimage while the potential wasramped from 327 mV (top) to 157mV (bottom). At 327 mV, the Au(1 x 1) atomic lattice was clearlyresolved. After passing the UPDpeak at ~0.24 V, SO4

2- ions startedto form (√3 x √3) structure. Nearthe bottom of the frame, when thepotential reached 157 mV, a stableSO4

2- lattice was fully developedand clearly resolved. Figure 5(b)shows the reverse process from(√3 x √3) SO4

2- structure to Au (1x 1) atomic lattice while thepotential was ramped from 157mV (bottom) to 300 mV (top).Both atomic and molecular struc-tures were clearly resolved. Thisin situ transition between the twostates was very reproducible andeasy to control.

Figure 3. Continuous change in cyclic voltammograms after the nitrogen flow was completely stopped.

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Figure 4. Voltammograms showing reductive desorption of C6, C8, C10, and C18 SAMs on Au (111).

Figure 5. Transition between Au (1 x 1) atomic lattice (top) and SO42- (√3 x √3) structure (bottom)

on Au (111) surface during ramp of the sample potential.

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ConclusionThe standard environmental chamber of the Agilent 5500 AFM has beenfully characterized on its oxygen-removal ability. Reductive desorptionpotentials of alkanethiol SAMs were measured as an example to test theoxygen-free environmental chamber. Reproducibly observed atomic andmolecular structures in the Cu UPD experiment clearly demonstrated thecapability of the 5500 AFM for high-resolution in situ electrochemicalSPM research.