MAPA DE RECONSTRUCCIÓN
REFLEXION CRÍTICA
This chapter provides details of experimental methods, chemicals and materials utilized.
3.1 SECCM imaging of graphene and HOPG
Double barrel pipettes used for imaging were fabricated from borosilicate glass capillaries
(ID = 1.0 mm, OD = 1.5 mm, Harvard Apparartus, UK) or quartz capillaries (ID = 0.9 mm,
OD = 1.2 mm, Intracel, UK) by pulling the capillaries on a laser puller (P-2000, Sutter
Instrument Co., USA) to a desirable diameter of the sharp tip (from 0.1 to 0.5 μm at the end).
The exact shape and dimensions were determined from the images of the tips obtained with a
scanning electron microscope (Supra 55-VP, Zeiss). To prevent leaking of aqueous solutions
on the outer walls of pipettes, and thus have a more confined/well-defined meniscus, the
pipettes were silanized by immersing their tips into dimethyltrichlorosilane for 2 min. A
pressure of argon of 5-8 bars was applied to the pipette to avoid the silane leaking inside.
Finally, the pipettes were dried in air and filled with the solution of interest.
Chloridized silver wires or H2-saturated palladium wires served as quasi-reference counter
electrodes (QRCEs) that were inserted in each barrel and connected to the voltage source, E1
(see Figure 2.1) supplying 0.2 - 0.5 V (exact value will be quoted for each imaging
experiment in the ‘Results’) . The pipettes and samples (described below) were mounted on
the in-house-built Warwick Electrochemical-Scanned Probe Microscopy setup so that z-piezo positioner controlled the pipette and the x,y-piezo moved the sample laterally. The pipette
78 was oscillated in the vertical direction with a frequency of 233 or 266 Hz, using the AC
signal from a lock-in amplifier (SR830, Stanford Research Systems).
For fixed potential imaging, the following parameters were used. The amplitude of the
oscillation (as defined in Section 2.3.2) was 20 nm for borosilicate tips and 12 nm for the
quartz ones. The data were recorded at a speed of 10 μs per data point that were averaged
over 512 points to yield one datum every 5.12 ms. For imaging in SECCM-CV/LSV mode,
the tip was held at each pixel of the image for as long as needed for a potential scan (CV or
LSV) to be complete and then was moved in x direction by 0.4 μm. The potential was swept at a rate of 0.2 V s-1 for imaging with Ru(NH3)63+ (200 data points in LSV) and 0.3 V s-1 for
imaging with FcTMA+ (230 data points per CV). Total time per entire image scan was around
2.5 h.
The following solutions were utilized for the imaging. For graphene samples, the solution
was 5 mM Ru(NH3)63+ in 25 mM KCl with 50 mM phosphate buffer (pH = 7.2). HOPG was
imaged with three redox mediators: i) 1 mM Ru(NH3)63+ in 100 mM KCl; ii) 1 mM FcTMA+;
and iii) 0.4 mM FcCOOH, both in 25 mM KCl with 50 mM phosphate buffer (pH = 7.2).
3.2 Preparation of graphene and HOPG samples
HOPG samples for imaging or CV were prepared by scotch tape exfoliation as routinely
done in the literature,1,2 in which top layers were taken by the scotch tape, leaving behind the
fresh pristine surface. HOPG of different grades – from high to low – was used in this study:
ZYB, SPI-3 and ungraded but of high quality sample referred to as AM HOPG was courtesy
of Prof. R. L. McCreery (University of Alberta, Canada).
To prepare ME graphene samples, AM HOPG was peeled off with the scotch tape as just
described and the layers (flakes) stuck to the tape were pressed against a SiO2/Si substrate
79 for imaging. Electrical contact between a copper wire and a graphene flake was made with
conducting silver paint. Apart from electrochemical imaging, the samples were characterized
with an optical microscope, AFM and (micro-)Raman spectroscopy.
3.3 Macroscopic CV on HOPG
All CV measurements, except for the grafting diazonium radicals, were carried out using a
droplet-cell arrangement and 760 C potentiostat (CH Instruments). Specifically, a droplet of
an electrolyte solution with a redox couple of interest (volume = 20 μL) was placed on either
a freshly cleaved surface (within seconds after cleavage) or one “aged” in air, or aged in a
glove box (nitrogen atmosphere) for a certain time (to be specified in the Results and
Discussion). The droplet was contacted with two electrodes: chloridized silver wire (a bare
wire or a wire with PTFE cladding that was coated with AgCl at the exposed disc-shape end)
that served as a reference electrode (RE) and platinum wire that served as a counter electrode
(CE). An HOPG block, firmly glued to a piece of gold-coated silicon wafer with silver paste
and contacted by a copper wire, was connected as a working electrode (WE). The
voltammetric scan rate varied between 0.05 and 10 V s-1. Redox mediators used for kinetic
and adsorption studies on HOPG were Ru(NH3)63+, Fe(CN)64-, IrCl62-, FcTMA+, FcCH2OH
and FcCOOH in various concentrations in sub-mM range in either 0.1 M or 1 M KCl (to be
specified in the ‘Results’). FcTMA+ in the form of FcTMA+PF6- was prepared by exchange
reaction of FcTMA+I- with AgPF6. All the solutions were prepared with Millipore Mili-Q
water (18.2 MΩ cm) and used on the day of preparation.
For the diazonium radical grafting experiments, a three electrode configuration was also
employed but the solution containing 1 mM 4-CBD (synthesized in-house according to ref3)
80 freshly cleaved HOPG surface. A H2-saturated Pd wire served as a RE and Pt wire as a CE.
The scan rate was 0.2 V s-1.
3.4 Micro-Raman analysis
Raman measurements were performed using a HeNe 633 nm micro-Raman spectrometer
(inVia micro-Raman, Renishaw, UK) equipped with an automated piezo-stage and a 100x
lens (Leica NA 0.85). For Raman mapping, the laser beam was raster-scanned across the area
of interest, acquiring spectra every 0.5 μm. To determine the number of graphene layers, the
signal at the 2D band region (around 2650 cm-1) was used.
3.5 Chemicals and materials
Chemicals and materials used in this thesis are listed.
Table 3.1. Chemical reagents
Name, purity grade Formula/Acronim Commercial source
Chlorotrimethylsilane, 98% (CH3)3SiCl ACROS Organics
(Ferrocenylmethyl)trimethylammonium
hexafuorophosphate
FcTMA+ prep. in-house (see text)
(Ferrocenylmethyl)trimethylammonium
iodide, 99%
FcTMA+I- Strem Chemicals
Ferrocenylcarboxylic acid, 98% FcCOOH Alfa-Aesar
Ferrocenylmethanol, 97% FcCH2OH Sigma-Aldrich
Potassium chloride, 99% KCl Sigma-Aldrich
Potassium hexachloroirridate (IV),
99.99%
81 Potassium hexacyanoferrate (II), 99.99% K4Fe(CN)6·3H2O Sigma-Aldrich
Ruthenium (III) hexamine chloride, 99% Ru(NH3)6Cl Aldrich
Silver hexafluorophosphste, 99% Ag[PF6] Strem Chemicals
Sulfuric acid, 99.999% H2SO4 Aldrich
4-carboxybenzenediazonium
tetrafluoroborate
4-CBD prep. in-house (see text)
Table 3.2. Materials
Materials Commercial source
Ag wire with PTFE cladding, 0.25mm,
99.99%
Goodfellow
Ag wire, 0.25 mm, 99.99% Goodfellow
Conductive silver paint RS Components
HOPG, AM GE Advanced Ceramics
HOPG, SPI-3 SPI Suppliers, West Chester, PA
HOPG, ZYA GE Advanced Ceramics
HOPG, ZYB NT-MDT (Moscow, Russia)