Investigation of the Catalytic Activity
of Plasma-Treated
Fe, Ni, and Co Foil for Water
Splitting
Nick Lavrov, Olivia Watson
Introduction
• Increasing evidence for climate change,
increasing cost of fossil fuels, and
diminishing natural resources motivate
today’s alternative energy research.
• An ideal alternative energy would be
cost-effective and renewable.
• Our project focuses on storing solar
energy in chemical bonds, using the
body of research in surface science and
electrochemical catalysis as our
platform.
2
Introduction
• A common mechanism used to store
energy from sunlight is called the water
splitting reaction
H2O  O2 + H2 (E0 =1.23 V)
• Reactive groups called oxides and
(oxy)hydroxides can improve the
efficiency of oxygen production
• Plasma (ionized gas) can be used to
grow thin layers of these materials on
metal electrode surfaces
3
Sample Processing Procedure
●
●
●
●
Measure, cut and clean foils by sonication in
acetone, ethanol, and water, then load and
ground sample.
Turn on water cooling, leak in Ar gas, and
ignite plasma to sputter.
Main treatment:
○ Leak in Ar or O2, then H2O
○ Heat sample with resistive heating.
○ Turn up ion energy and treat for 5-15
minutes.
Break vacuum with N2 gas between runs.
4
Electrochemistry
•Electrolyte solution was a solution
of 1M NaHCO3 (NaHCO3 solid
dissolved in DI water).
•Sample mounted as the working
electrode, platinum coil counter
electrode,
Ag/AgCl (4 M KCl) reference
electrode
•Active area of samples marked off
using chemically-resistant tape
from 3M
5
Results
• For each foil, we implemented some of the
following characterization techniques:
Linear Sweep Voltammetry
Raman Spectroscopy
X-ray Photoelectron Spectroscopy (XPS)
Scanning Electron Microscopy (SEM)
6
Linear Sweep Voltammetry
• Current at working electrode
is measured while potential
between the working
electrode and the reference
electrode is swept linearly in
time.
7
Tafel Plot
• Voltammetry data is
converted to fit the Tafel
equation.
• From the graph we can
extract the Tafel slope
and exchange current
density parameters.
8
Raman Spectroscopy
•
•
“Fingerprinting” method to identify
the unique inelastic scattering of
monochromatic light by chemical
bonds
Instrument contains optical
microscope, useful for initial
assessment of changes in surface
appearance
9
X-ray Photoelectron Spectroscopy (XPS)
•
•
•
Analysis technique that
focuses X-ray beam on a
surface to eject electrons
Energy of ejected electron
gives information about how
tightly it was bound to its
atom
Every atom has a specific
corresponding pattern of
electrons at various energy
levels
10
Scanning Electron Microscopy
•
•
Special type of microscope that
focuses an electron beam on a
surface to give a topographical
image of a sample
SEM has many advantages over
light microscopes including:
greater depth of field, allow more
of a surface image to be in focus
at once; very high resolution; very
high magnification
11
Fe Foil Color Changes
•
•
•
•
At RT, unchanged
At 200°C and 250°C, gold
At 300°C and 450°C, blue
Tempering colors due to thin film interference of
iron oxide.
12
Iron Linear Voltammetry
Current Density (A/mm2)
8.E-05
7.E-05
0kV, 450C
6.E-05
5.E-05
4.E-05
BLANK
2kV, 250C
3.E-05
2kV, 250C
0.1kV, 150C
1kV, 20C
2.E-05
1.E-05
0.E+00
1.7
1.75
1.8
1.85
Volts vs RHE
1.9
1.95
2
13
14
15
SEM: Fe surfaces
Untreated Fe
Hematite
Magnetite
50 000x ; 5x5 μm
•
Very strong morphological differences between untreated Fe and
hematite; hematite grain size ~160 nm.
•
No obvious morphological difference in magnetite.
16
Nickel Linear Voltammetry
Current Density (A/mm2)
1.E-04
9.E-05
2kV
8.E-05
7.E-05
blank
6.E-05
5.E-05
4.E-05
1kV
3.E-05
0.1kV, 450C
2kV, 200C
2.E-05
1.E-05
0.E+00
1.6
1.7
1.8
Volts vs RHE
1.9
2
17
SEM: Ni surfaces
Untreated Ni
25 000x
•
10x10 μm
Treated Ni
20 000x
13 x 13 μm
No obvious morphological differences
18
Ni: X-ray Photoelectron Spectroscopy
oxide
XPS data confirms an increasing presence of oxidized Ni with samples treated
with O2/H2O plasma. An increased XPS oxide peak corresponds to a small
19
cathodic shift in water oxidation onset potential.
Current Density (A/mm2) x 0.001
Cobalt Linear Scan (300C)
0.14
1.2 kV
0.12
1.0 kV
0.1
1.6 kV
0.08
0.5 kV
0.06
1.4 kV
0.04
0.02
0
1.4
1.6
1.8
Volts vs RHE
2
20
Cobalt Tafel Plot (300C)
0.7
0.5
0.4
1.4
0.3
0.2
Overpotential (V)
0.6
1.6
1.0
0.1
1.2
0.5
0
-6.5
-6
-5.5
-5
-4.5
Log Current Density
-4
-3.5
21
Log Exchange Current Density (I0)
Exchange current density (I0) vs Ion Energy
0
0.5
1
1.5
2
-6
-6.5
-7
-7.5
-8
-8.5
-9
-9.5
-10
Ion Energy (kV)
22
Tafel slope (A) vs Ion energy
Tafel Slope (V/decade)
0.25
0.2
0.15
0.1
0.05
0
0
0.5
1
Ion Energy (kV)
1.5
2
23
Co: Raman Spectroscopy
expected peaks at 191, 470, 510, 608, 675 cm-1
24
Co: Raman Spectroscopy
Co blank
Co sample 4
Co sample 5
Images taken at 10x magnification during Raman spectroscopy
25
SEM: Co surfaces
Co sample 4
25,000 x
10 x 10 µm
Co sample 5
25,000 x
10 x 10 µm
There are many scattered grains across the surface of sample 4,
compared to sample 5, which has none.
26
SEM: Co surfaces
Co sample 4
50,000 x
5 x 5 µm
Co sample 5
50,000 x
5 x 5 µm
There are not any obvious morphological differences between the
two Co samples at a higher magnification.
27
Conclusions
• Plasma treatment seems to be localized to surface and
thus too thin to be characterized with Raman spectroscopy
• Plasma treatment is complex, with many parameters to
optimize
• Future experiments would involve exploring other regimes
of plasma treatment including increasing sputtering time,
hydrogen plasma, chamber pressure.
28
Appendix
Gen2 Tectra® Plasma Source
●
●
●
●
Microwave plasma source with magnets at 0.0875T to enhance plasma by ECR.
Hybrid mode.
One positive grid to accelerate ions to grounded sample by controlling ion energy.
High vacuum chamber kept at 10^(-6) Torr.
FARADAY CUP
● Aperture of 3mm^2
● Biased to 54V
● Connected in series to picoammeter to
measure current at sample location in
relation to ion energy and pressure.
● Materials: stainless steel, sapphire ceramic, tantalum wire.
3
0
Ion Beam Characterization
● Variables:
○ Gas: argon, oxygen, water vapor
○ Gas mixture
○ Partial and total pressure ( 0 - 1.0 mTorr )
○ Ion Energy
Ar (mTorr)
O2 (mTorr)
H2O (mTorr)
1:1
0.46
0.45
0.42
2:3
0.46
0.3
0.75
3
1
Pure Plasma Results
Sputtering Conditions:
- 1 kV
- 10 minutes
- 0.46 mTorr
- Room Temperature
- Used water contact
angle to confirm surface
cleaning.
3
2
Mixed Plasma Ion Flux
3
3