Self-assembled films of nickel hexacyanoferrate: Electrochemical properties
and application in potassium ion sensing
Nitin Bagkar a, C.A. Betty a, P.A. Hassan a, K. Kahali b, J.R. Bellare b, J.V. Yakhmi c,*
a
Chemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India
Department of Chemical Engineering, Indian Institute of Technology, Mumbai-400078, India
Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai-400 085, India
b
c
Abstract
Pressure-area isotherms of hexametaphosphate (HMP) stabilized nickel hexacyanoferrate (NiHCF) nanocrystals were studied under the
surfactants dioctadecyl dimethylammonium bromide (DODA), octadecyl amine (ODA) and tetrahexadecyl ammonium bromide. A transition from
liquid-expanded to liquid-condensed phase is observed for NiHCF under DODA and ODA. The higher stability is observed for the monolayer of
NiHCF under DODA. Ultrathin films of NiHCF were prepared by multiple adsorption of sodium-HMP stabilized nickel hexacyanoferrate
nanocrystals under a monolayer of DODA. Its electrochemical properties were analysed by cyclic voltammetry and AC impedance spectroscopy.
The successive transfer of HMP-stabilized NiHCF – DODA layers was demonstrated by increase in the current measured by cyclic voltammetry as
well as the decrease in the capacitance measured with increasing number of layers. The voltammetric response and capacitance response of the
film with change in the potential and varying potassium ion concentration (from 0.0001 M to 1.5 M) was also demonstrated.
Keywords: Nickel hexacyanoferrate; Impedance measurements; Potassium ion sensing
1. Introduction
The design and study of nano-phase molecular magnets
have been the center of extensive research in recent years.
Among them, polynuclear cyanometallates, especially the
Prussian blue and its analogues have also found applications
in catalysis [1], energy storage [2,3], solid state batteries [4],
electrochromism [5 – 8] and molecular sieving [9]. Films
required for these applications have generally been prepared
by electrochemical deposition [10,11], dip-coating [12], and
casting from colloidal solutions [13], all of which yield films
with rather heterogeneous structure, with no precise control of
thickness. Langmuir – Blodgett (LB) technique has been
extensively used for preparing ultrathin organized films with
precise control over molecular arrangement and thickness in
the nanometer range [14,15]. It utilizes either hydrophobic
forces to drive the self-assembly of monolayers/multilayers of
amphiphiles [16] or layer-by-layer assembly of cationic –
anionic pairs using electrostatic interactions [17]. Prussian
blue (Fe4[Fe(CN)6]3I15H2O) and related analogues exhibit an
open zeolite type structure consisting of a cubic network of iron
centers bound by cyanide bridge [18]. Thin films of Prussian
blue analogues can sense alkali metal ions when the latter
intercalate into the interstitial sites thereby squeezing an
appropriate number of water molecules out [19]. Electrode
surfaces modified with metal hexacyanoferrates have often
been employed as ion-sensitive electrodes [20 – 28]. Among
them, nickel hexacyanoferrate (NiHCF) shows good response
to alkali metal cations, the electrochemical behaviour depending selectively on the type and concentration of cations [29]. Its
cubic structure with a channel diameter of about 3.2 Å is
permeable to small hydrated ions such as Cs+ and K+, whereas
larger hydrated ions such as Na + get blocked [9,30].
Quantitative detection of K+ ion is considered important, since
it is the most essential intracellular cation for maintaining
osmotic pressure and electrodynamic cellular properties in
living organisms. Electrodes modified with thin films of
260
electroactive nickel hexacyanoferrate have been shown to act
as effective potentiometric sensors for the determination of
potassium ions [31 –35]. Voltammetric determination of formal
potentials of nickel hexacyanoferrates have been employed in
the sensing of potassium ions, quantitatively, since the formal
potential changes linearly with log [K+].
Electrochemical impedance spectroscopy has been successfully used in the study of charge transport processes that take
place at modified electrode/electrolyte interface [36]. Using
multi-frequency small signal ac impedance analysis, it is
possible to separate individual electrochemical phenomena
such as charge and mass transfer processes, since the time
constants for these processes are different. Impedance spectra
of electrochemically deposited as well as LB films of Prussian
blue have already been reported [37 – 39]. However hardly any
attempts have been made to study impedance of LB films of
Prussian blue analogues such as NiHCF. Cyclic voltammetry
and impedance spectroscopy can act as complementary
techniques to study composite films consisting of insulating
organic molecules dimethyl dioctadecyl ammonium bromide
(DODA) and NiHCF crystallites.
Recently, we reported the synthesis and characterization of
sodium hexametaphosphate stabilized nanocrystals of NiHCF
and fabrication of its multilayers using LB technique [40]. We
have now extended this study to investigate the effect of
number of alkyl chains on the amphiphile, which induce the
self-assembly of NiHCF nanocrystals. In this paper, we have
discussed the stability of NiHCF nanocrystals in the presence
of different surfactants. LB films of NiHCF nanocrystals were
characterized by cyclic voltammetry, impedance measurements
and transmission electron microscopy. Finally, the voltammetric and impedance response of the films were investigated
for application in K+ ion sensing.
2. Experimental details
2.1. Chemicals
Dimethyl dioctadecyl ammonium bromide, octadecyl amine
(ODA) and tetrahexadecyl ammonium bromide (THDA) were
A.R. grade and purchased from Fluka. Nickel chloride
(NiCl2I6H2O), Potassium ferricyanide (K3Fe(CN)6) and sodium hexametaphosphate (HMP) were also A.R. grade and
purchased from S.D. Fine Chemicals, India. Chloroform was
used as a spreading solvent. All solutions were prepared using
deionised water from a Millipore-MilliQ system (resistivity
¨ 18 MV cm).
2.2. Synthesis
Nanocrystals of sodium hexametaphosphate stabilized
NiHCF was prepared following a procedure reported earlier
[40]. In a typical bulk synthesis nickel chloride and
K3[Fe(CN)6] are mixed in 3:2 ratio, resulting in immediate
formation of yellowish brown precipitate of NiHCF. However,
for the synthesis of NiHCF nanoparticles, the precipitation was
avoided by using sodium HMP as a stabilizer. 15 ml of 10 mM
nickel chloride aqueous solution was added gradually with
constant stirring to a 10 ml solution of 10 mM of K3[Fe(CN)6]
and 5 mM of sodium HMP. A clear brown colored solution was
obtained, which indicates the formation of nanocrystals of
NiHCF. The role of HMP is to restrict the size of nanoparticles
and impart stability to the nanocrystals in aqueous phase. The
nanocrystals in the aqueous phase were then used for further
characterization and LB film deposition. The HMP stabilized
NiHCF nanocrystals, in aqueous phase, were taken in the subphase in Langmuir trough. After spreading the surfactants and
allowing sufficient time for evaporation of the solvent and
equilibration of the monolayer, it was compressed on the
NiHCF sub-phase at a linear speed of 5 mm/min. Indium tin
oxide (ITO) coated glass electrode was used as a substrate for
depositing the films. The films were obtained by the vertical
lifting method at a target pressure of 30 mN/m, with a dipping
speed of 1 mm/min. After each cycle the substrate was allowed
to dry for 10 min in air.
2.3. Characterization
LB films of nanocrystals were deposited using KSV5000
Langmuir double-barrier teflon trough where surface pressure
was measured with a platinum Wilhelmy plate microbalance.
TEM measurements were performed on a TECNAI G12
instrument operated at an accelerating voltage of 120 kV.
Samples were prepared by depositing 5 and 13 layers of
DODA – NiHCF using LB technique on carbon-coated copper
TEM grids. The films on the TEM grids were allowed to stand
for 5 min during each deposition cycle and allowed to dry prior
to measurement.
Electrochemical measurements were performed using Autolab
PGSTAT20 potentiostat/galvanostat equipped with a Frequency
Response Aanalyser (Eco-Chemie Ltd, The Netherlands).
Multifrequency impedance data were measured using 10 mV
peak to peak ac signal in the frequency range 0.1 Hz to 100 kHz
for the same potential range (0.2 V to +0.8 V). The cyclic
voltammetry and impedance measurements were conducted in a
single compartment cell using a standard three-electrode arrangement employing Ag/AgCl as reference electrode and
Platinum rod as a counter electrode. Cyclic voltammograms
were recorded in 0.1 M KCl at sweep rates between 10 and 150
mV/s and in the range of 0.2 V to + 0.8 V vs. Ag/AgCl.
3. Result and discussion
To study the interaction of HMP stabilized NiHCF
nanocrystals and DODA at the air – water interface, the
isotherms of DODA were recorded on pure water, HMP and
HMP-stabilized NiHCF. Surface pressure (p) vs. mean
molecular area isotherm of DODA monolayer floating on pure
water, on aqueous 2 104 M sodium HMP as well as on
HMP-stabilized NiHCF nanocrystals are illustrated in Fig. 1.
The isotherm of DODA monolayer on pure water showed
collapse pressure of 46 mN/m with the limiting area of 55 Å2
consistent with those reported in literature [41]. In the case of
the isotherm of DODA on sodium HMP, the extrapolated area
261
Surface pressure (π)
40
30
20
10
0
0.2
0.4
0.6
0.8
1.0
1.2
Mean molecular area (Å2)
Fig. 1. Surface pressure (p) vs. mean molecular area isotherm of DODA
monolayer floating on pure water, on aqueous 2 10 4 M sodium HMP as
well as on HMP-stabilized NiHCF nanocrystals.
per molecule of the condensed phase is 86 Å2. The increase
in the mean molecular area at the onset of isotherm of
HMP – DODA as compared to water –DODA might be due to
electrostatic interactions between negatively charged HMP
with positively charged DODA surfactant and steric hindrance
of bulky HMP ions in association with DODA molecules
[42 –44]. However it showed the similar collapse pressure as
that observed on pure water. The isotherm of DODA on
NiHCF nanocrystals showed slight increase in the molecular
area for the condensed phase as compared to HMP subphase
because of incorporation of NiHCF nanocrystals bound to
HMP at air –water interface. The shape of the isotherm is
reproducible with the same collapse pressure during each time
of the successive compression and expansion of the barrier
indicating the reproducibility and stability of the condensed
phase of DODA isotherm on HMP stabilized NiHCF subphase.
In order to elucidate the role of surfactant molecules in
imparting stability to NiHCF at air – water interface, the
isotherms were recorded for NiHCF with different surfactant
molecules. The three surfactants ODA, DODA and THDA
containing single, double and four hydrocarbon chains,
respectively, were chosen for this purpose. Surface pressure
(p) vs. mean molecular area isotherm for DODA, THDA and
ODA monolayers on HMP stabilized NiHCF nanocrystals are
presented in Fig. 2. The isotherm of DODA on NiHCF is
compared with those of ODA and THDA on NiHCF. The effect
of chain length on the stability was reflected in the difference in
the shape of the isotherms in this figure. As the number of alkyl
chains on the surfactant increases, the isotherm shifts to higher
mean molecular area, as is expected from the geometry of the
molecules. The expanded and condensed regions of the
isotherm of ODA on NiHCF have a mean molecular area of
43 and 19 Å2, respectively. The condensed phase of ODA on
NiHCF had a collapse pressure of 62 mN/m, less than that
observed for DODA on NiHCF. The isotherm of THDA on
NiHCF did not show any solid region because of the instability
of monolayer arising from conformational/orientational
changes of the hydrocarbon chains of THDA, upon compres-
sion. We also observed the presence of liquid expanded/liquid
condensed phases in the case of DODA and ODA surfactants,
which was not so prominent in the case of THDA on NiHCF
subphase. This points to the influence that the nature of a
surfactant has on the phase transitions. The stability of
condensed and expanded regions changes as the number of
hydrocarbon chains in the surfactant changes. The maximum
stability of condensed phase was observed for DODA on
NiHCF. Based on packing considerations stable lamellar layers
are preferred for surfactants having cylindrical geometry. This
is more favourable for double chain surfactants as opposed to
single and tetra alkyl ones. This could be the reason for the
increased stability of DODA – NiHCF nanocrystals. Thus, we
selected DODA for the deposition of NiHCF nanocrystals on
suitable substrates for further characterization and applications.
3.1. Cyclic voltammetry
3.1.1. DODA – NiHCF multilayers
The DODA monolayer on NiHCF subphase was transferred
onto ITO coated glass electrode at a surface pressure of 30 mN/
m and a dipping speed of 1 mm/min. The transfer ratio was
found to be close to 1. The successive number of layers, viz. 1,
5, 13 and 19, were deposited on an ITO coated glass electrode
and cyclic voltammetry (CV) was carried out to monitor the
adsorption of NiHCF onto this electrode. Fig. 3 (a) shows the
CV plots of 1, 5, 13, 19 layers of DODA transferred on ITO
electrode in 0.1 M KCl, which are in agreement with the
known electrochemical activity of NiHCF [45]. The data
consists essentially of one set of redox peaks corresponding to
the oxidation and reduction of hexacyanoferrate moiety, as
given by the following equation.
II
2þ III
þ
K2 Ni2þ
2 Fe ðCNÞ6 Y KNi2 Fe ðCNÞ6 þ e þ K ðat 0:56 VÞ
ð1Þ
The peak currents are proportional to square root of the scan
rate, which indicates surface confined redox behaviour of
70
NiHCF-THDA
NiHCF-DODA
NiHCF-ODA
60
Surface pressure (π)
NiHCF-DODA
Water-DODA
HMP-DODA
50
50
40
30
20
10
0
0
50
100
150
200
250
2
Mean molecular area (Å )
Fig. 2. Surface pressure (p) vs. mean molecular area isotherm of DODA,
THDA and ODA monolayers on HMP stabilized NiHCF nanocrystals.
262
8x10-6
6x10-6
19 L
4x10-6
I (A)
ratio of HMP concentration and NiHCF was kept constant and
concentration of both HMP and NiHCF were varied in the
same proportion. It was found that there is negligible change in
the nature of the isotherms for NiHCF – DODA monolayers
indicating that the nature of transition is similar under different
subphase concentrations. The DODA layers transferred on ITO
electrode were analyzed by cyclic voltammetry. Fig. 5 shows
CV of 5 layers of DODA – NiHCF films immersed in different
subphase concentrations at the scan rate of 50 mV/s. The
voltammetric response increases with NiHCF concentration
due to the increase in number of NiHCF crystallites under
DODA monolayer. The oxidation as well as reduction current
increases with HMP concentration. No saturation is observed
for the given range of HMP concentration. This is likely
because in the concentration range investigated, the surface
concentration of NiHCF may not be sufficient enough to
completely neutralize the surface charge of DODA monolayer.
(a)
13 L
2x10-6
5L
1L
0
-6
-2x10
-4x10-6
-6x10-6
0.0
0.2
0.4
0.6
0.8
E (V)
7x10-6
(b)
6x10-6
IPA (A)
5x10-6
3.1.3. Potassium ion sensing
The response of metal hexacyanoferrate modified electrodes
in the presence of different alkali metal cations has been
investigated in detail by different groups [25,46,47]. Sensors
for Cs+, K+, NH4+ and Rb+ using copper, iron and nickel
4x10-6
3x10-6
2x10-6
1x10-6
0
0
2
4
6
8
10
12
14
16
18
20
No of layers
Fig. 3. (a) CV of NiHCF nanocrystals of 1, 5, 13, 19 layers transferred on ITO
electrode in 0.1 M KCl, (b) The variation of anodic peak current (I PA) with
number of layers of NiHCF nanocrystals.
NiHCF nanocrystals in DODA multilayers. The variation of
anodic peak current with number of layers is shown in Fig.
3(b). The voltammetric response gradually increased with
increasing number of deposition of DODA layers on NiHCF
subphase, which indicates the successful transfer of NiHCF
nanocrystals along with the DODA layers on ITO electrode.
However, from the cyclic voltammograms, it is difficult to
comment on the nature of the layers of DODA and
nanocrystals on the substrate during each deposition cycle.
Hence, transmission electron microscopy (TEM) has been
carried out on the films containing 5 and 13 (Fig. 4) layers of
NiHCF –DODA transferred on carbon-coated 300 mesh copper
grids. All samples show well-defined nanocubes having
dimensions between 80 and 100 nm. It was found that the
average number density of NiHCF nanocrystals increases with
the number of layers transferred on the electrode. Thus, the
film with 13 layers is more densely packed than the film with 5
monolayers. This confirms the regular transfer of NiHCF
nanocrystals on ITO substrate.
3.1.2. Effect of subphase concentration
An investigation was made into the kinetics at different
subphase concentrations. For this, the pressure area isotherm of
DODA was recorded on different NiHCF concentrations. The
Fig. 4. (a) TEM images of 5 layers of DODA – NiHCF nanoparticles transferred
on TEM grid. Scale bar is 200 nm. (b) TEM images of 13 layers of DODA –
NiHCF nanoparticles transferred on TEM grid. Scale bar is 100 nm.
263
film. The formal potentials, EV as obtained from the average
values of the cathodic and anodic peak potetials, found to vary
linearly against log a K+ [27], which can be expressed as
2*10-4 M HMP
4x10-6
-4
1*10 M HMP
2x10-6
EV ¼ E 0 þ
I (A)
5*10-5 M HMP
RT
lnaKþ
F
For different concentrations of K+ in the solution, EVvalues are
different [27]. This explains shift in formal potentials observed
in the case of DODA –NiHCF. It shows Nernstian behaviour
with slope of 48 mV in the range of 103 to 1.5 M,
respectively.
0
-2x10-6
-4x10-6
3.2. Impedance characterization
0.2
0.4
0.6
0.8
E (V)
Fig. 5. CV of 5 layers of NiHCF films immersed in different concentrations of
NiHCF with HMP in the subphase.
hexacyanoferrates were developed [48 – 50]. However selectivity of these sensors is limited because of the similar redox
activity of hexacyanoferrates in the presence of alkali cations
[51]. There are reports on the use of K+-selective potentiometric sensors using copper and nickel hexacyanoferrates. Nickel
hexacyanoferrate modified electrodes, prepared by electrochemical deposition have been used efficiently in the determination of K+ ions [31]. In the present study, we investigate the
effect of K+ ions on DODA – NiHCF nanocrystals deposited as
LB films. The electrochemical response of DODA – NiHCF
multilayers to different K+-ion concentrations has been studied
using 0.001, 0.01, 0.1 and 1.5 M KCl solutions. Fig. 6 shows
CV plots of 19 layers of DODA –NiHCF in different KCl
concentrations. In this case also the peak potential shifted
towards more positive values with the increase in the K+
concentration. The important role of K+ ion is to neutralize
negative charge of HMP stabilized NiHCF and to make the
film maintain charge balance. The hydrated ionic radii, also
known as Stokes radii, is approximately 0.125 nm for K+ ion
which is significantly smaller than the radius of the zeolitic
pores of the nickel hexacyanoferrate of 0.32 nm and thus the
cations should be able to pass the pores easily [10].
The reduction of hexacyanoferrate centers in the film
requires an uptake of cations from the solution while the
oxidation forces the release of the cations from the film. During
oxidation and reduction reactions electrons are exchanged
between the electrode and NiHCF in the bulk of the electrode,
whereas K+ ions are exchanged between the NiHCF and
adjacent electrolyte [48]. The redox reaction of NiHCF in
solution containing K+ ions is given in Eq. (1). On applying the
Nernst equation one can obtain,
i
II III
a
K½Ni
Fe
ð
CN
Þ
6 ÞOx aKþ
RT
II
ln E ¼ E0 þ
II
F
a K2 Ni Fe ðCNÞ6 Re
where E is the potential applied to electrode, E 0 is standard
reduction potential and a(NiHCF)Ox, and a(NiHCF)Re refer to
the activities of oxidized and reduced form of NiHCF in the
3.2.1. Effect of impedance on LB layers of NiHCF nanocrystals
The real and imaginary impedance data as a function of
frequency obtained for DODA –NiHCF multilayers (1 and 19)
deposited on ITO in 0.1 M KCl are given in Fig. 7(a). Using
nonlinear least square algorithm, the measured impedance data
was fitted and an equivalent circuit, which explains the
electrical properties of the DODA –NiHCF films was obtained.
The equivalent circuit (Fig. 7(b)) extracted from this data is
similar to the ones reported for LB films of DODA –PB [52].
In this figure the R1 represents resistance due to the electrode
contacts and electrolyte. Cdl represents the capacitance due to
the double layer at the electrode/electrolyte interface, Rct is the
charge transfer resistance between ITO electrode and DODA –
NIHCF. Qf is a frequency dispersive constant phase element.
Qf in this form can include contributions from static disorder
such as porosity, random mixture of conductor and insulator. It
can also include contributions from dynamic disorder such as
diffusion either in electrolyte or in the solid [53]. The
impedance of Qf is given by 1 / ( Y 0 jx)n , where j = ( 1)1/2,
x = 2pf, f being the frequency and n 1, represents the porous
and/or inhomogeneous nature of the film. Y 0 and n values were
extracted after fitting the impedance data by keeping R1 and
Cdl constant. Rct was found to be almost constant with
increasing number of layers. The values of Y 0n increases with
0.001M
-6
8x10
0.01M
0.1M
4x10-6
I (A)
0.0
1.5M
0
-6
-4x10
-8x10-6
0.0
0.2
0.4
0.6
0.8
E (V)
Fig. 6. CV of 19 layers of DODA – NiHCF in different KCl concentrations.
264
9000
(a)
1 Layer
19 Layer
Fit to data
7500
Z'' (ohms)
6000
4500
3000
1500
0
0
200
400
600
800
1000
Z' (ohms)
(b)
Cdl
R1
Rct
measurement frequency can be represented as a combination
of equivalent resistance R p and capacitance C p in parallel with
series resistance representing the electrolyte and contact
resistance. We have observed larger change in C p for these
DODA – NiHCF LB films compared to the R p due to the
presence of surfactant molecules in the LB film. Therefore, we
have plotted the variation of C p with number of layers.
Considering the series resistance values for electrode and
electrolyte obtained after fitting the data we had to choose a
measurement frequency, which is less than 500 Hz. Therefore,
we have chosen 77 Hz as the frequency to observe maximum
sensitivity. Fig. 9(a) gives the C p for different number of
NiHCF –DODA monolayers as a function of applied voltages
(ranging from 0.2 to 1.0 V) and shows a decrease with
increasing number of layers. A decrease in capacitance is in
order when we consider the increasing thickness of the
insulating film. While cyclic voltammetric response depends
on the amount of NiHCF transferred with each monolayer
formation, capacitance behaviour shows the dependence on the
thickness of DODA layer. Thus the cyclic voltammetry and
capacitance – voltage measurements act as complementary
techniques.
Q
8x10-5
Fig. 7. (a) The real and imaginary impedance data as a function of frequency
obtained for 1 and 19 layers of DODA – NiHCF nanocrystals deposited on ITO
in 0.1 M KCl. (b) The equivalent circuit used to simulate the impedance spectra
of the DODA – NiHCF LB films.
1 Layer
5 Layer
13 Layer
19 Layer
6x10-5
CP (F)
the number of layers deposited as given in Fig. 8. Such increase
in Y 0n implies that the charge or mass transfer between LB film
and electrolyte decreases with increasing number of layers. For
different number of layers the value of n was observed to vary
from 0.76 for one monolayer to 0.88 for 19 layers. This
suggests the film is slightly porous for single layer and
becomes more compact with the increasing number of layers
as observed in TEM micrographs (Fig. 4).
The impedance of electrochemical system consisting of
NiHCF –DODA films on ITO electrode, electrolyte solution
(KCl) and reference and counter electrode at a single
(a)
4x10-5
-5
2x10
0
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
E (V)
8x10-5
(b)
-0.2V
1.0V
6.4x10-5
6x10-5
CP (F)
6.0x10-5
-5
4x10
Y0n
5.6x10-5
5.2x10-5
-5
2x10
4.8x10-5
0
0
-5
4.4x10
4
8
12
16
20
No of layers
0
2
4
6
8
10
12
14
16
18
No of layers
Fig. 8. Y n0 vs. number of layers for DODA – NIHCF LB films.
20
Fig. 9. The variation of equivalent capacitance Cp for (a) different number of
DODA – NiHCF layers as a function of applied voltages ranging from 0.2 to
1.0 V and (b) different number of layers at 0.2 and 1.0 V.
265
3.2.2. Potassium ion sensing
The impedance response of 5 and 19 multilayers of
DODA –NiHCF to different K+-ion concentrations have been
studied using 0.001, 0.01, 0.1 and 2 M KCl solutions. As a
representative plot, the dependence of KCl concentration on
the NiHCF – DODA LB film of 5 monolayers is shown in Fig.
10(a). In this case, C p increases with KCl concentration
indicating the effective thickness reduction of the film with
more K+ incorporation. The typical response of the film with
KCl shows two slightly differing slopes, which are more
prominent at higher concentrations of KCl. However, at lower
concentrations the change in the slope is less, indicating that C p
remains almost constant throughout the potential range. This
could be due to the lesser number of K+ ions incorporated and
removed from the film. At higher concentrations the rate of
removal of K+ ions is faster therefore faster decrease in the C p
in the reduction region, whereas the rate of incorporation of
water molecules might be at slower rate as potential becomes
7.5x10-5
(a)
0.001M KCl
0.01M KCl
0.05 MKCl
0.1M KCl
1M KCl
CP (F)
6.0x10-5
4.5x10-5
3.0x10-5
1.5x10-5
0.0
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
E (V)
7x10-5
6x10-5
(b)
5 Layers
19 Layers
5x10-5
CP (F)
It is known that at negative potential K+ ions gets
incorporated in the crystal structure replacing water molecules,
which are weakly coordinated in the interstitial positions in the
crystal [54]. When oxidation of NiHCF takes place (at more
positive potentials) K+ gets removed from the NiHCF
effectively decreasing the capacitance. Fig. 9(a) shows that
for NiHCF – DODA film with lesser thickness (5 monolayers),
when the potential is changed from negative to positive (FeII
getting oxidized to FeIII), the capacitance starts decreasing with
two slopes. The fast decreasing slope at the beginning of the
potential scan is due to the effect of K+ ion getting removed
from the NiHCF –DODA film. After oxidation of FeII to FeIII
the removal of K+ from the film should be more and it should
be replaced with more number of water molecules [54].
However, the reduced slope observed after oxidation of the
film might be due to the residual K+ ions, which still remain in
the film. For LB monolayer, the lower the thickness of the film,
the higher the percentage of defects and voids. Therefore, when
the thickness of the film is less the diffusion of the ions in and
out of the film also is easier, which explains the larger
sensitivity observed with lesser number of layers. For NiHCF –
DODA films with more number of layers, the capacitance
decreases slowly. This indicates the restricted diffusion of K+
ions into the film at negative potentials and into the solution at
positive potentials. Therefore it is preferable to choose
DODA –NiHCF LB films with thickness less than 10 monolayers to have maximum sensitivity.
Fig. 9(b) shows the variation of equivalent capacitance C p
for different number of layers at typical potential values of
0.2 and 1.0 V. At 1 V, which is in oxidation region, the
variation of C p with number of layers is linear indicating a
linear increase in the film thickness with increasing number
of layers as explained above. However at 0.2 V, which is
in the reduction region, the change in the capacitance is
more for DODA – NiHCF films with lesser number of layers.
This suggests that the reduction potential can be chosen as
more sensitive region than oxidation potential for ion sensing
studies.
4x10-5
3x10-5
2x10-5
1x10-5
0
1E-3
0.01
0.1
1
log [K+]
Fig. 10. (a) Dependence of equivalent capacitance Cp for different KCl
concentrations for the case of 5 layers DODA – NiHCF LB film deposited on
ITO. (b) Variation of Cp with concentration of KCl at specific potentials for 5
and 19 number of layers.
more positive [55]. We have also studied the effect of KCl
concentration for 19 layers. It was observed that the change in
capacitance is more in the case of 5 layers. The lesser change
observed in the case of film with 19 layers is due to the
restricted diffusion of the ions in thicker films. Fig. 10(b)
shows the variation of C p with concentration of KCl at specific
potentials for 5 and 19 layers. This figure indicates that C p of
the film with 5 layers varies linearly with KCl concentration,
whereas with 19 layers it is nonlinear. This study shows that
LB films with less than 10 layers can be chosen to get
maximum sensitivity.
4. Conclusions
The interaction of HMP stabilized NiHCF was studied with
different surfactants at the air – water interface. The monolayer
of NiHCF under DODA was found to be more stable and
reproducible than ODA and THDA. NiHCF – DODA LB layers
were transferred on ITO coated glass electrode. The quantitative transfer of NiHCF on ITO depends on the concentration of
NiHCF in the subphase. A linear dependence of the voltammetric and capacitive response of NiHCF/DODA LB films on
266
the number of deposited layers is demonstrated. The multilayer
film of NiHCF – DODA was shown to respond to K+ ion
concentration by employing cyclic voltammetry and impedance measurements.
Aknowledgement
Authors thank Mrs. Shilpa Sawant, Dr. Sipra Choudhury
and Dr. Rajib Ganguly of the Chemistry Division for carrying
out fruitful discussions.
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