Increased Charge Storage Capacity of Titanium Nitride Electrodes
by Deposition of Boron-doped Nanocrystalline Diamond Films
Suzan Meijs
1
, Matthew McDonald
2
, Søren Sørensen
3
, Kristian Rechendorff
3
, Václav Petrák
4
,
Miloš Nesládek
2
, Nico Rijkhoff
1
and Cristian P Pennisi
5
1
Center for Sensory-Motor Interaction, Aalborg University, Fredrik Bajers vej 7D, Aalborg, Denmark
2
Institute for Materials Research, University of Hasselt, Agoralaan, Diepenbeek, Belgium
3
Danish Technological Institute, Kongvangs Alle 29, Århus, Denmark
4
Nano6 s.r.o., Kleinerova 1469, Kladno, Czech Republic
5
Laboratory for Stem Cell Research, Aalborg University, Fredrik Bajers vej 3B, Aalborg, Denmark
Keywords: Diamond, Titanium Nitride, Cyclic Voltammetry, Electrical Stimulation, Neural Prostheses.
Abstract: The aim of this study was to investigate the feasibility of depositing a thin layer of boron-doped nanocrys-
talline diamond (B-NCD) on titanium nitride (TiN) coated electrodes and the effect this has on charge injec-
tion properties. The charge storage capacity increased by applying the B-NCD film, due to the wide poten-
tial window typical for B-NCD. The impedance magnitude was higher and the pulsing capacitance lower for
B-NCD compared to TiN. Due to the wide potential window, however, a higher amount of charge can be in-
jected without reaching unsafe potentials with the B-NCD coating. The production parameters for TiN and
B-NCD are critical, as they influence the pore resistance and thereby the surface area available for pulsing.
1 INTRODUCTION
Neural stimulation with implantable electrodes is
used for many applications (Zhou, 2009). The sur-
face to area ratio of these electrodes can be increased
by applying a porous coating (Cunha, 2009; Specht,
2007). This gives the electrodes the following ad-
vantages: fast stabilization of the open circuit poten-
tial (OCP), making stimulation and sensing with the
same electrode possible (Specht, 2007), high charge
injection capacity, which allows for electrode minia-
turization (Cogan, 2008; Zhou, 2009).
Titanium nitride coatings can be made smooth or
porous by controlling deposition parameters, such as
nitrogen (N
2
) flow. At a low N
2
flow a smooth Ti-
rich metallic film can be obtained. Increasing the
flow results in a smooth (stoichiometric) TiN film
and further increasing the N
2
flow results in a porous
(over-stoichiometric) N-rich film (Cunha, 2009).
The pores extend deep into the coating, resulting in a
high surface to area ratio and a high charge storage
capacity (CSC) (Cunha, 2009; Norlin, 2005).
An increased amount of nitrogen in the coating,
however, also results in increased dissolution rates
and oxide thickness when high anodic potentials are
reached (Cunha, 2009). In addition, higher potentials
were observed when applying stimulation pulses
using implanted TiN electrodes (Meijs, 2015), which
may compromise safety during electrical stimula-
tion. It is thus advantageous to apply an additional
coating to improve the corrosion resistance and in
vivo electrochemical properties.
Boron-doped nanocrystalline diamond (B-NCD)
was selected, to combine its be mechanical stability,
biocompatibility (Tang, 1995; Garrett, 2015) and
corrosion resistance (Hupert, 2003) with the large
sufe to area ratio of TiN (Cunha, 2009). B-NCD
further has metal-like electrical properties and a
wide safe potential window (Garret, 2012; Meijs,
2013). In addition, B-NCD is fouling resistant (Hu-
pert, 2003), which may decrease the electrode poten-
tial during pulsing with implanted electrodes.
2 METHODS
2.1 Electrode Fabrication
The TiN coatings were deposited on the tip of 16
Ti6Al4V electrode pins (6 mm
2
) by reactive DC
106
Meijs, S., McDonald, M., Sørensen, S., Rechendorff, K., Petrak, V., Nesladek, M., Rijkhoff, N. and Pennisi, C..
Increased Charge Storage Capacity of Titanium Nitride Electrodes by Deposition of Boron-doped Nanocrystalline Diamond Films.
In Proceedings of the 3rd International Congress on Neurotechnology, Electronics and Informatics (NEUROTECHNIX 2015), pages 106-109
ISBN: 978-989-758-161-8
Copyright
c
2015 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
magnetron sputtering using an industrial CC800
coating unit (CemeCon AG, Germany). Sputtering
was done from four Ti targets (88 x 500 mm
2
) with
99.5% purity in an Ar/N
2
mixture atmosphere. The
N
2
flow was varied from 120 to 300 sccm, while the
Ar flow was constant. The deposition time was var-
ied in order to obtain different coating thicknesses.
Titanium-nitride electrodes were then coated
with B-NCD using an Astex AX6500 microwave
plasma enhanced chemical vapor deposition system.
The electrodes were first immersed in a 0.33 g/L
solution of diamond nanoparticles (3.8 ± 0.7 nm)
from Shinshu University to seed the surface for
diamond growth. Hydrogen gas with an addition of
1% CH4 was added to the chamber at a total flow
rate of 500 sccm. Tri-methyl boron (TMB) was
added to the gas as the dopant source, at boron to
carbon concentrations of 10,000ppm. The plasma
was maintained at a temperature of ~750 °C by us-
ing a pressure of 25 torr and a microwave power of
2500 W for the plasma.
2.2 Electrochemical Measurements
All electrochemical measurements were carried out
in a 3-electrode set-up, using the TiN/B-NCD pins
as working electrodes (0.06 cm
2
), a platinum foil
counter electrode (50 cm
2
) and a Ag|AgCl reference
electrode (1.6 cm
2
). Measurements were performed
in Ringer solution at room temperature.
The impedance spectrum was measured from 0.1
Hz-100 kHz, 5 points/decade using a sinusoidal
measurement current of 5.0 µA. Impedance spec-
troscopy was performed using Solartron, Model
1294 in conjunction with 1260 Impedance/gain-
phase Analyzer (Solartron Analytical, UK).
Cyclic voltammetry was performed by cycling the
electrode potential between the water window limits.
These limits were determined by increasing and
decreasing the electrode potential until an exponen-
tially increasing current was observed using a sweep
rate of 0.05 V/s. Measurements were made at 0.05,
0.1, 0.5 and 1.0 V/s; 10 cycles were recorded at each
sweep rate. The cathodic charge storage capacity
(CSC) of the electrodes was found by calculating the
surface area under the 0 A axis.
Voltage transient measurements were made us-
ing a cathodic-first bipolar symmetric current pulse
with an inter-phase, during which no current was
applied. Each phase had a phase width of 200 µs and
the duration of the inter-phase was 40 µs. For analy-
sis of the voltage transients, the OCP was set to 0 V
and the IR drop was subtracted. The IR-drop was
calculated for each phase by subtracting the potential
at 20 µs after pulse cessation from the last data point
of the respective phase. The pulsing capacitance
(C
pulse
) was calculated for each pulse using the fol-
lowing equation:
I
stim
= C
pulse
ௗ௏
ௗ௧
(1)
where I
stim
is the stimulation current and
ௗ௏
ௗ௧
is the
slope of the last 90% of the cathodic phase of the
voltage transient.
Cyclic voltammetry and voltage transient meas-
urements were performed with VersaSTAT 3 poten-
tio- galvanostat (Princeton applied research, USA).
3 RESULTS
Two of the pins were coated with a smooth (stoichi-
ometric) TiN coating, while all others were over-
stoichiometric and displayed an open pyramidal
columnar structure. The surface to area ratios of the
porous electrodes ranged from 89 to 295, calculated
by dividing the CSC of the porous coatings by the
CSC of the smooth coating. The coating thicknesses
varied with N
2
flow and deposition time (table 1).
Diamond films were deposited on the TiN coatings.
These films had uniform coverage on the TiN with
grain size of ~50nm (Fig. 1). The film thickness was
measured using silicon substrates coated under the
same conditions, which gave a film thickness on the
order of the grain size, around 50-70nm.
Table 1: Deposition parameters and coating thicknesses
for porous TiN coatings.
N
2
flow
(sccm)
Deposition
time (10
3
s)
Coating thick-
ness (µm)
Ref.
nr.
120 27.5 5.2 45
180 10 2.1 46
180 27.5 6.3 52
180 60 13.1 54
240 10 2.0 47
240 27.5 4.6 53
300 27.5 1.6 55
Figure 1: SEM of B-NCD coated TiN electrode (ref. 52).
Increased Charge Storage Capacity of Titanium Nitride Electrodes by Deposition of Boron-doped Nanocrystalline Diamond Films
107
Figure 2: The CV of the B-NCD coated TiN electrodes
had wider safe potential limits, which increased the CSC.
Figure 3: CSC increased linearly with coating thickness.
The blue markers correspond to TiN (r
2
=0.71) and red
markers to B-NCD (r
2
=0.74).
Figure 4: C
pulse
did not increase with coating thickness.
The blue markers correspond to TiN (r
2
=0.32) and red
markers to B-NCD (r
2
=0.01).
Figure 5: C
pulse
of B-NCD coated TiN electrodes was
decreased compared to uncoated TiN. This caused a steep-
er voltage rise and greater electrode potentials.
The water window was -0.6 V – 0.9 V (Ag|AgCl)
for TiN and -1.3 V – 1.2 V for B-NCD. Due to the
wide potential window typical for B-NCD, the CSC
of the B-NCD coated electrodes was 2-3 times larger
than the CSC of TiN electrodes (Fig. 2). Further-
more, the CSC of both B-NCD and TiN electrodes
increased with increasing coating thickness (Fig. 3).
The CSC of B-NCD coated electrodes was consist-
ently higher than the CSC of TiN electrodes
The impedance magnitude for the B-NCD and
the TiN electrodes decreased with increasing thick-
ness. All B-NCD coated electrodes had a higher
impedance than without the B-NCD coating.
Contrary to the CSC, C
pulse
did not increase with
coating thickness. The trend lines even have a slight-
ly negative slope for both TiN and B-NCD elec-
trodes. Also, C
pulse
of the B-NCD coated electrodes
was lower than C
pulse
of the corresponding TiN elec-
trodes (Fig. 4). Fig. 5 shows that a lower C
pulse
re-
sulted in in higher electrode potentials. The potential
limit for B-NCD, however, is higher than for TiN.
The highest CSC and lowest impedance magni-
tude for TiN with and without B-NCD coating were
obtained using the thickest coating (ref. nr. 54). The
main parameter that influenced the coating thickness
was the coating time. The highest C
pulse
, however,
was obtained with thinner coatings.
4 DISCUSSION
We have coated 16 TiN electrodes with various high
surface to area ratios with a thin layer of B-NCD.
Depositing a B-NCD thin film on a porous TiN
substrate improved the previously reported electro-
chemical performance of the B-NCD electrodes
(Garrett, 2012; Meijs, 2013) to a level that is compa-
rable to porous TiN electrodes.
The TiN electrodes had a high surface to area ra-
tio (89-295), which resulted in a high CSC and low
impedance magnitudes. A thicker coating resulted in
a higher CSC due to the increased surface to area
ratio. This suggests that pores extend into the entire
depth of the coating (Cunha, 2009; Norlin, 2005).
Depositing B-NCD on top of the TiN further in-
creased the CSC. There was a linear relation be-
tween the TiN CSC and the B-NCD CSC (fig. 6).
This indicates that the B-NCD coating did not block
the pores, but covered the inner surface of the TiN
columns without obstructing the pores.
The highest C
pulse
for TiN electrodes was found
for a 6.3 µm thick coating, while the lowest C
pulse
for
B-NCD coated electrodes was found for a 2.1 µm
thick coating. Both TiN coatings were produced un-
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
-1,4-1,2 -1 -0,8-0,6-0,4-0,2 0 0,2 0,4 0,6 0,8 1 1,2
Current (mA)
Potential (V)
TiN 46
TiN 54
B-NCD 46
B-NCD 54
0
50
100
150
200
250
300
0 5 10 15
CSC (mC/cm
2
)
Thickness (µm)
120 ml/min
180 ml/min
240 ml/min
300 ml/min
0
0,5
1
1,5
2
2,5
0 5 10 15
C
pulse
(mF/cm
2
)
Thickness (µm)
120 ml/min
180 ml/min
240 ml/min
300 ml/min
-0,07
-0,05
-0,03
-0,01
0,01
0,03
0,05
-0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7
Potential (V)
Time (ms)
TiN 46
TiN 54
B-NCD 46
B-NCD 54
NEUROTECHNIX 2015 - International Congress on Neurotechnology, Electronics and Informatics
108
Figure 6: There is a linear relationship between the CSC of
TiN and B-NCD coated TiN electrodes (r
2
=0.91).
der similar conditions and the increased thickness
was achieved by increasing the coating time. As the
B-NCD coating is added, however, the pores of the
electrodes become narrower. This increases the pore
resistance and decreases the pore depth that can be
used under pulsing conditions (Cogan, 2008). This
makes it less advantageous for electrical stimulation
purposes to increase the coating thickness beyond a
certain level. For TiN electrodes, the optimal thick-
ness is between 6.3 and 13.1 µm, while for B-NCD
coated TiN electrodes it is between 2.1 and 5.2 µm
with the settings used in this study. A thicker coating
may be more advantageous, if the pores are wider
without comprising mechanical stability.
The electrochemical properties of the B-NCD
coated TiN electrodes are far better than those of
conventional B-NCD electrodes (Garret, 2012;
Meijs, 2013). This is due to the large surface to area
ratio gained by the TiN on which B-NCD was
grown. B-NCD with a high surface to area ratio was
also made by growing diamond on carbon nano-
tubes, resulting in great improvements in impedance
and CSC (Piret, 2015). The CSC of the current elec-
trodes is, however, 3-10 times higher than the CSC
of B-NCD coated carbon nanotube electrodes.
Although C
pulse
is decreased for B-NCD coated
electrodes compared to TiN, it is important to view
this result in the light of wide safe potential window
of B-NCD (Garrett, 2012, Piret, 2015). The decrease
in C
pulse
after depositing B-NCD ranged from 67% to
less than 1%, while the cathodic potential limit was
more than doubled (-0.6 V vs Ag|AgCl for TiN and -
1.3 V vs Ag|AgCl for B-NCD). This means that the
amount of charge that can be injected without reach-
ing unsafe potentials is doubled by applying a B-
NCD coating on top of a porous TiN coating.
In order to achieve increased charge injection
(Q
inj
), the production parameters are of critical im-
portance, as the extra coating increases the pore
resistance, which may deteriorate Q
imj
. This study
suggests that specific deposition parameters are
optimal for stimulation electrodes, as increased
thickness and N
2
flow only result to a certain extent
in larger C
pulse
and Q
inj
. These settings, however, also
depend on the thickness of the diamond film.
5 CONCLUSIONS
The charge storage and charge injection capacity of
porous TiN electrodes can be improved by adding a
B-NCD coating. We further expect that the B-NCD
coating will improve the corrosion and fouling re-
sistance of porous TiN electrodes.
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50
100
150
200
250
300
25 50 75 100 125
B-NCD CSC
TiN CSC
Increased Charge Storage Capacity of Titanium Nitride Electrodes by Deposition of Boron-doped Nanocrystalline Diamond Films
109