Transition metal substituted Krebs-type Polyoxometalate doped

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Transition metal substituted Krebstype Polyoxometalate doped PEDOT films. Rashda Naseer, Bushra Ali, Fathima Laffir, Lekshmi Kailas, Calum Dickinson, Gordon James Armstrong, and Timothy McCormac Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03785 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Transition metal substituted Krebs-type Polyoxometalate doped PEDOT films. R. Naseer‡, B. Ali‡, F. Laffir†, L. Kailas†, C. Dickinson†, G. Armstrong† and T. McCormac‡* ‡Electrochemistry

Research Group, Department of Applied Science, Dundalk Institute of

Technology, Dublin Road Dundalk, County Louth, Ireland. †

Bernal Institute, University of Limerick, Limerick, Ireland.

KEYWORDS-POM: polyoxometalates, PEDOT: 3,4-ethylene dioxythiophene

ABSTRACT. The Krebs type transition metal substituted type polyoxometalates [Sb2W20M2O70(H2O)6]n-, M = Fe(III), Co(II) or Cu(II) were surface immobilized within the conducting polymer 3,4-ethylene dioxythiophene (PEDOT) on glassy carbon electrode surfaces. The immobilized films of different thickness were characterised by electrochemical and surface based techniques. The inherent redox activity for the Kreb POMs, [Sb2W20M2O70(H2O)6]n-, M = Fe(III), Co(II) or Cu(II), that were observed in the solution phase were maintained in the polymeric PEDOT matrix. The resulting films were found to be extremely stable towards redox switching between the various POM based redox states. The films exhibited pH dependent redox activity and thin layer behavior up to 100mVs-1. The films were found to be highly conductive through the employment of Electrochemical impedance spectroscopy (EIS). Surface characterization of the films was carried out by using X-ray photoelectron spectroscopy, atomic force microscopy and scanning electron microscopy.graph.

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INTRODUCTION. Polyoxometalates are a distinctive class of inorganic metal-oxygen cluster compounds, having a wide variety of structures and compositions [1, 2]. The magnetic, electrochemical and photochemical properties of polyoxometalates have attracted much attention because of their wide range of applications from medicines to molecular electronics [1]. Several strategies such as adsorption [3-4], electrodeposition [5-6], entrapment into polymeric matrices [7-8], layer – by – layer

assemblies

[9-10] and

Langmuir – Blodgett films [11-12] have been employed for the surface immobilisation of polyoxometalates. Among them entrapment into conducting polymer is most common because of facile and easy method. It is based on the electrostatic attraction between POM’s (anion) and oxidized polymer (cation). The sandwich-type polyoxometalates are an important family because of their unique catalytic and electrochemical properties [13]. In 1973, Weakley et al. synthesized the first sandwich-type polyoxometalate [Co4(H2O)2(B-α-PW9O34)2]10-[14]. The main group ion Sb(III) present novel stereochemical structural features by the lone pair of electrons located at the top of the trigonal pyramid [13]. In 2002, Kortz et. al.; synthesis and discuss the electrochemical property of the iron-containing species [FeIII4(H2O)10(α-SbW9O33)2]6- of sandwich type polyoxometalates[15]. The Kreb-type polyoxometalates are one of the sandwich type polyoxometalates,

with

general

formula

[X2W20M2O70(H2O)6].

The

Krebs

type

polyoxometalate, K6NaH[Sb2W20Fe2O70(H2O)6].29H2O was previously incorporated into polypyrrole by Kevin Foster, et. al., (2008) for the electrocatalytic reduction of hydrogen peroxide in water [16].

Herein, we present the successful immobilisation of the transition metal substituted Krebs type polyoxometalates, [Bi2W20M2O70(H2O)6]n-, M = Fe(III), Co(II) and Cu(II) in poly(3,4ethylenedioxythiophene), onto carbon electrode surfaces through cyclic voltammetry and


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chronocoulometry. The resulting polymeric films exhibited the stable electrochemical redox activity associated with the Fe(III), Cu(II) and W–O framework of the polyoxometalates in a wide pH range, while the cobalt centers in the cobalt substituted kreb-type polyoxometalate did not show any redox activity, similar electrochemical behavior of investigated polyoxometalates was observed in solution. Moreover, X-ray photoelectron spectroscopy was used to investigate the interfacial elemental composition of formed hybrid films. Their surface morphologies were studied by atomic force microscopy and scanning electron microscopy.

2. EXPERIMENTAL 2.1 MATERIALS. The Kreb-type polyoxometalates [Bi2W20M2O70(H2O)6]n-, where M = Fe(III), Co(II) and Cu(II) have been synthesized and characterized according to the literature [17]. 3,4ethylenedioxythiophene (EDOT) was received from ACROS Organics. Alumina powders of sizes 0.05, 0.3 and 1.0μm were received from IJ Cambria. Highly purified water a using Milli-Q water purification system (ELGA PURELAB Option-Q) with a resistivity 18.2 MΩ cm was used throughout for the preparation of all aqueous electrolytes and buffer solutions. All other chemicals were of reagent grade, purchased from Sigma Aldrich, and used as received. Buffer solutions were prepared from the following reagents: 0.1M Na2SO4 (pH 23), 0.1M Na2SO4 + 20 mM CH3COOH (pH 3.5-5), 0.1M Na2SO4 + 20mM NaH2PO4 (pH 5.57). The pH of the solutions was adjusted with either 0.1M NaOH or 0.1M H2SO4 depending on the pH required.


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2.2 APPARATUS AND PROCEDURE All electrochemical experiments were performed with a CHI-660c electrochemical work station (CH Instruments, Texas, USA, IUPAC) with a conventional three electrode electrochemical cell. The glassy carbon electrode (GCE) (d = 3mm, A = 0.0707cm-2) was used as the working electrode, a platinum wire as the auxiliary electrode (length = 5cm, d = 0.5mm) and Ag/AgCl (3M KCl, E = 0.223V ±0.13mV at 25ºC vs. SHE) as the reference electrode. The GCE was polished with 1.0, 0.3 and 0.05µm Al2O3 powders respectively and sonicated in water for about 1 minute after each polishing step. Finally, the electrode was washed with ethanol and then dried with a high purity nitrogen stream before use. The polymerisation procedure reported previously by our group [7] was employed with minor modifications to yield films of varying thicknesses for the electrochemical surface polymerisation of PEDOT on the GCE using the Kreb POMs 2mM solution (0.1M Na2SO4 electrolyte) as the polymer’s dopant anion at a constant potential of +0.88V. The deposition process was controlled by chronocoulometry of different charges (5, 10, 15 and 20mC). Once the polymer film had been grown electrochemically, the electrode was removed from the EDOT monomer/POM solution and washed with the same buffer solution (pH-2) that the film was going to be studied in. Cyclic voltammetry was performed to observe the redox behaviour of the PEDOT doped Kreb-type POM films modified electrode in pH 2 buffer solutions being freshly made and degassed prior to use for 15 minutes. Electrochemical Impedance spectra were carried out with bare glassy carbon electrode and every successive deposition on the electrode surfaces. Measurements were performed by placing the bare electrode and modified electrode with PEDOT doped POM. The recording of the spectra was performed at different (oxidised/reduced) applied potential (versus Ag/AgCl) from 0.01 to 1x105 Hz frequency with voltage amplitude of 5 mV. The electrolyte was freshly prepared before use.


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X-ray Photo-electron Spectroscopy (XPS) analysis was performed in a Kratos AXIS 165 spectrometer using monochromatic Al Kα radiation of energy 1486.6 eV. Survey spectra and high resolution spectra were acquired at fixed pass energies of 160 eV and 20 eV respectively. In the near-surface region the atomic concentrations of the chemical elements were evaluated after subtraction of a Shirley type background by considering the corresponding Scofield atomic sensitivity factors. Surface charge was efficiently neutralised by flooding the sample surface with low energy electrons. Core level binding energies were determined using C 1s peak at 284.8 eV as the charge reference. Atomic force microscopy experiments were conducted on as-prepared samples using an Agilent 5500 AFM microscope in AAC (“tapping”) mode. Micromasch NSC14 cantilevers (typical resonant frequency 160 kHz, 5 N/m spring constant) were used. The microscope was controlled using Agilent PicoView v1.12 software. The image size, height resolution and scan speeds were optimized to suit the features observed for each sample examined. The conditions used for each sample are noted alongside the images; typically, scans of 0.5 x 0.5 microns were obtained at 512 pixels/line resolution at a scan speed of 0.7 Hz, i.e. 0.7 lines/second. Images were processed using PicoImage Advanced 5.1.1 software. Raw data were levelled using a least squares algorithm. Noise was removed by applying a smoothing spatial filter and line noise arising from artefacts was removed. The resulting topography, amplitude and phase images are presented are pseudo-colour images. Height parameters according to ISO standard 25178 were determined for each topography image, post processing. SEM images were obtained using Hitachi SU-70 field emission scanning electron microscope operating at an accelerating voltage of 3 kV. The low voltage allowed for imaging the multilayer assemblies without the requirement of gold coating. 3. RESULTS AND DISCUSSION


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3.1 Solution Redox behaviour of [Bi2W20M2O70(H2O)6]nThe cyclic voltammogram for each of the [Bi2W20M2O70(H2O)6]n-, M = Fe(III), Co(II) or Cu(II) polyoxometalates at pH 2 are shown in figure. 1(A)–(C). Each polyoxometalate exhibited multiple pH dependent four electron well defined tungsten-oxide redox couples, I and II, for [BiCo]10− with E1/2 values of −431 mV and -548 mV (vs Ag/AgCl), for [BiCu]8− with E1/2 values of −431 mV and -547 mV (vs Ag/AgCl), and for [BiFe]8− with E1/2

values of

−394

mV and -532 mV (vs

Ag/AgCl), respectively. The [BiFe]8−

polyoxometalate exhibited and additional mono-electronic +78mV (vs

redox process with Ep,c of

Ag/AgCl) associated with the Fe(III) centres within the POM. The

polyoxometalates that contain copper (figure 1-C) displayed an additional redox activity corresponding to the Cu2+/Cu0 redox process with E1/2 value 108 mV, which indicated an adsorption and associated stripping processes from the electrode surface occurring during its

reduction and

oxidation processes,

respectively [16, 18]. The [BiCo]10−

polyoxometalates (figure 1-A) did not display redox activity associated with the cobalt ions present in the structure as would be expected. However the solution redox behaviour and electrochemistry of substituted Kreb-type polyoxometalates are in agreement with literature [16, 18].


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A

B

C


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Figure 1. Cyclic voltammogram of a 1mM solution of (A) K10[Bi2W20Co2O70(H2O)6].nH2O, (B) [K8[Bi2W20Fe2O70(H2O)6].nH2O and (C) K8[Bi2W20Cu2O70(H2O)6].nH2O in pH 2 buffer (0.5MNa2SO4) at a bare glassy carbon electrode (A = 0.0707 cm2). Scan rate = 10 mV s−1. 3.2 Electrochemical behaviour of Kreb-POM doped PEDOT films Kreb-POM /PEDOT hybrid films of various surface coverages were fabricated on glassy carbon electrode surfaces by employing the electrochemical technique of chronocoulometry. The redox behaviour of the resulting films were then investigated in pH 2 buffer by cyclic voltammetry. The formation of [BiCo]10−, [BiFe]8− and [BiCu]8− substituted Kreb-POM doped PEDOT films with deposition charges of 5mC and associated redox electrochemistry of the resulting hybrid film in pH 2 buffer at a scan rate of 10 mVs-1 are shown in figure 2 (A-C). All cyclic voltammograms show two typical POM based four electron tungsten-oxo redox couples, I and II, as described in figure 1. Figure 2B shows the redox couple with an E1/2 of +111mV (vs Ag/AgCl)) associated with the POM’s FeIII/II redox centre, whilst only the anodic peak of the Cu0 to Cu2+ process is seen with an Epa of +52 mV (vs Ag/AgCl) is seen


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in figure 2C with the reduction of the Cu2+ being unclear [18, 19]. The E1/2 values of metal redox W-O redox couples are in close agreement with solution behaviour for each of the three POM moieties and the ΔEp of modified electrode is typical for a surface confined species. Continuous cycling of the Kreb-type POM doped PEDOT modified electrodes through the POM based redox processses, showed only a small decrease in associated peak currents and charges thereby indicating the realtive stability of the PEDOT films towards redox switching. The quantities of the immobilized POM in each of the PEDOT films was calculated at a slow scan rate by employing equation (1):

Г= Q/nFA

(1)

Where Г (mol cm-2) is the surface coverage for the surface confined active species, Q (Coulomb) is the charge passed associated with a particular redox process, n is number of transferred electrons, A is the area of electrode in cm2 and F (96,485 Cmol-1) is Faraday’s constant. The measured surface coverages were found to be 3nmolcm-2 for [BiCo]10−, 1nmolcm-2 for [BiFe]8−, and 2nmolcm-2 for [BiCu]8− and substituted Kreb-type POM doped PEDOT films.


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A

B


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C

Figure 2. The cyclic voltammograms of (A) K10[Bi2W20Co2O70(H2O)6].nH2O–PEDOT, (B) [K8[Bi2W20Fe2O70(H2O)6].nH2O–PEDOT and (C) K8[Bi2W20Cu2O70(H2O)6].nH2O–PEDOT hybrid films (5mC) in pH 2 buffer (0.5MNa2SO4 + 0.5MH2SO4) at a glassy carbon electrode (A = 0.0707 cm2). Scan rate=10mVs−1. Scan rate studies were also performed on the films as shown in figure 3 (A-D). In all films investigated peak current densities associated with the POM’s first W-O redox process, I, were found to be linear versus scan rate up to 100 mVs-1 as shown in figure 3(D) which this being characteristic of a surface confined species with fast redox switching ability [16, 18]. It is however observed that with increasing scan rate the peak to peak separations (ΔEp) increased with an associated loss in peak symmetry with increasing scan rates.


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A 150 100 50

I/ µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

70 mV/s

-50 -100

10 mV/s

-150 -200 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

E/V vs. Ag/AgCl

B


Langmuir

C 120 100 80 60 40 20

I/ µA

0 -20

70 mV/s

-40 -60 -80

10 mV/s

-100 -120 -140 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

E/V vs. Ag/AgCl

D Co-Ipa: Y=1.65x+5.8

Cu-Ipa: Y=1.18x+2.1 2

2

R =0.999

R =0.997

250

Fe-Ipa: Y=2.34x+10.9

200

2

R = 0.995

150 100

Co-IPa Co-Ipc Cu-Ipc Cu-Ipa Fe-Ipc Fe-Ipa

50

I/ µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 -50 -100 Co-Ipc: Y=-1.64X-12.3

-150

2

R =0.994

-200

Fe-Ipc: Y=-2.25x-14.9 2

R = 0.995

-250 0

20

Cu-Ipc: Y=-1.29x-4.4 2

R =0.998 40

60

80

Scan rate mV/s


100

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Figure

3.

Cyclic

voltammograms

of

(A)

[Bi2W20Co2O70(H2O)6]10-/PEDOT

(B)

[Bi2W20Fe2O70(H2O)6]8-/PEDOT (C) [Bi2W20Cu2O70(H2O)6]8-/PEDOT hybrid film (5mC) in pH 2 aqueous buffer at scan rate between 10 to 100 mVs-1 (from inner to outer core). (D). Scan rate plot between 10 to 100 mVs-1 (from inner to outer core) of substituted Kreb-type POM/PEDOT hybrid film (5mC) in pH 2.0 buffer. It shows the linear relationship between the peak current and the scan rate for the POM based I redox couple.

It is well known that the reduction of the tungsten-oxo framework of the Kreb-type polyoxometalate is accompanied by protonation countering the polyoxometalates’ increased basicity [18]. Figures 4 (A-C) represents the effect of pH on the voltammetric responses of the POM doped PEDOT films. The tungsten-oxo redox activity for the investigated was found to shift towards more negative potentials with increasing pH. The number of protons involved in the redox process, I, can be calculated from the slope of the pH dependencies of the formal potentials by employing equation (2).

d[E] m (mV pH-1)  59.1 d [ pH ] n

(2)

Where m is the number of protons and n the number of electrons involved in the redox process. The slope values of the pH plots is shown in figure 4(D) with slope values of 65.5 mV pH-1 , 59.5 mV pH-1 and 60.2 mV pH-1 being obtained for the [Bi2W20Co2O70(H2O)6]10- , [Bi2W20Fe2O70(H2O)6]8- and [Bi2W20Cu2O70(H2O)6]8- doped films respectively. These values indicate that for the redox process, I, for each of these films there is the addition of 4 protons with this being in close agreement with the literature [16, 18]. In addition, it can be seen that the well-defined redox activity associated with POM moieties is maintained across the investigated pH domain.


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A

B


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C

D 10-

700

[BiCo] : Y=65.5x+370 2 R =0.998

650

8-

[BiCu] : Y=60.2x+364 2 R =0.995

600 8-

E1/2 (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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[BiFe] : Y=59.5x+347 2 R =0.999

550

500 10-

[BiCo] 8[BiCu] 8[BiFe]

450

400 1

2

3

4

5

pH number

Figure.

4.

Cyclic

voltammogram

of

(A)

[Bi2W20Co2O70(H2O)6]10-–PEDOT

(B)

[Bi2W20Fe2O70(H2O)6]8-–PEDOT (C) [Bi2W20Cu2O70(H2O)6]8-–PEDOT film with 5mC


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deposition charge in pH 1-3 aqueous buffer at scan rate 10 mVs-1. (D) The plot of pH versus redox wave potential for the WO-I redox processes. 3.3 Electrochemical Impedance spectroscopy (EIS). Electrochemical impedance spectroscopy was performed to investigate the electrical properties of Kreb-type POM doped PEDOT films by the Randle equivalent circuit. Ho et.al., [20] and Yongfang et.al., [21] used the Randle equivalent circuit to study the tungsten trioxide thin films behaviour and kinetics of PEDOT films, respectively. Diana M. et.al., present

the

electrochemical

impedance

study

of

self-assembled

iron–

silicotungstate/poly(ethylenimine) modified electrodes by following the Randle equivalent circuit with modification [22].

Figure 5. Randle equivalent circuit used to measure impedance data performed at GCE electrode modified with the POM-based multilayer films. In figure 5, Rs represents the uncompensated solution resistance between the electrolyte and electrode, Cdl is the double layer capacitance that is replaced here as constant phase element (CPE) and CPE is modelled as a non-ideal capacitor, Rct is the charge transfer resistance and Zw is the resistance by the diffusion of the electroactive species, also known as the Warburg impedance. Zw is negligible at the higher frequency range with higher Rct values being attained at such frequencies indicating the kinetically controlled region whilst at


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lower frequencies Zw is dominant thereby indicating a diffusion controlled process [21, 22]. The frequency responses for [BiCo]10-, [BiFe]8- and [BiCu]8-, doped PEDOT films was recorded over a range of potentials from +0.0V to -0.6V (vs. Ag/AgCl) in pH 2 buffer. It was found from figure 6(A-C) that the films show lower Rct values in the domain (+0.2V to +0.0V) as compared to when the films are further reduced to -0.6V, with these results being in agreement with literature [21, 23-24]. Moreover, the impedance data results as the solution resistance (Ri) by electrolyte of solution, charge transfer resistance (Rct) and RL (Zw) complex impedance or Warburg impedance, for both reduced and oxidised domain of POM +PEDOT doped film are presented in table 1. The Warburg impedance was modelled as an open circuit finite Warburg element which includes a diffusion resistance, Rdif. It is the resistance by the diffusion of the electroactive species ([BiCo]10−, [BiFe]8−, and [BiCu]8−) from modified probe to the analyte or vice versa. The exchange current density io is also calculated (table 1) by employing equation (3): [21, 23-25]. 𝑅𝑇

𝑅𝑐𝑡 = 𝑧𝐹𝑖𝑜

(3)

Where Rct represents the charge transfer resistance, io an exchange current density, T as the temperature taken as 300 K and F (96,485 Cmol-1) is Faraday’s constant. Exchange current densities reflect intrinsic rates of electron transfer between an analyte and the modified electrode. It can be observed from that the values of the charge density decrease with increasing negative electrode potential thereby indicating at more negative potentials the PEDOT films are more insulating in character [21].


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A

-600 -0.2V +0.0V +0.2V

-400 Z''(Ohm)

-200

Z''(Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.2V +0.0V +0.2V

75

78

81

84

87

Z' (Ohm)

0 70

75

80

85

90

95

100

Z' (Ohm)


105

110

115

120

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B

-34

-10000

-32 -30

Z''(Ohm)

-28

-8000

-26 -24 -22 -20

+0.0V -0.2V +0.2V

-18 -16 -14

-6000

Z''(Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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105

110

115

120

125

130

135

140

Z'(Ohm)

-4000

-2000

+0.0V -0.2V +0.2V

0 0

500

1000 Z'(Ohm)


1500

2000

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C

-1000

-800

-600 Z'' (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-400

+0.0V +0.2V -0.2V

-200

0 50

100

150

200

250

300

[Bi2W20Co2O70(H2O)6]10-

-PEDOT

Z'(Ohm)

Figure

6.

Nyquist

plots

of

(A)

(B)

[Bi2W20Fe2O70(H2O)6]8--PEDOT, (C) [Bi2W20Cu2O70(H2O)6]8- PEDOT, polyoxometalate doped PEDOT film (5mC) in pH 2 (0.5M Na2SO4) buffer on glassy carbon electrode. The frequency range is between 0.01 to 105 Hz. The amplitude of the applied sine wave potential in each case was 10mV and the applied potential was +0.0V, +0.2V, and -0.2V in the pH 2.0 (0.1M Na2SO4) buffer.


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Table 1. Results of the A.C. impedance measurement of [BiCo]10-, [BiFe]8-, [BiCu]8- , polyoxometalate doped PEDOT film (5mC) in pH 2 (0.5M Na2SO4) buffer. POM+PEDOT E

Ri

Rct

Zw

CPE

io (mA)

Hybrid film

(V)

(µΩ)

(Ω)

(Ω)

(µF.cm-2.sα=1)

+0.2

79

8

679

34.9

0.8

+0.0

79

9

617

34.9

0.7

-0.2

79

11

342

31.1

0.5

-0.6

89

38

-----

35.4

0.1

+0.2

78

4

821

44.1

16

+0.0

78

7.5

710

42.1

0.8

-0.2

80

13

301

43.5

0.5

-0.6

86

33

---

47.2

0.2

+0.2

81

95

247

34.5

0.07

+0.0

87

103

171

34.4

0.06

-0.2

87

110

90

35.1

0.05

-0.6

93

221

----

45.3

0.02

(5mC) [BiCo]10/PEDOT

[BiFe]8/PEDOT

[BiCu]8/PEDOT

Higher RCT values with lower diffusion control region Zw for the [BiCu]8-/PEDOT hybrid film are observed in table (1) as compared to the [BiFe]8-/PEDOT and [BiCo]10-/PEDOT hybrid films. Similarly it was noticed that the CPE increases with a subsequent increase in the RCT values as the potential is more negative again indicating the insulting nature of PEDOT at such negative potentials and the more compact and less porous nature of the POM doped PEDOT hybrid film in oxidised region [26]. 3.4. SURFACE ANALYSIS


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The surface morphologies of the as-prepared POM-PEDOT hybrid films were examined by both SEM and AFM, presented in figures 7 and 8, respectively. By SEM, the surfaces of all POM-PEDOT hybrid films exhibited a sponge-like structure, and appeared to be porous as may be seen from the representative high-magnification images presented in figure 7 (A-C). Similar surface morphology was observed previously in [P2W17V4O62]8−–PEDOT hybrid film, as reported by Górala et al [27]. At higher magnification under AFM, globular structures were observed in the topography images obtained from all samples examined.

These globular structures had an overall

hemispherical shape, which may be most readily seen in the 3D topography images presented in figure 8(A-D). A slight phase contrast shift was seen within the globular regions for all samples analysed, suggesting that a uniform film was present in these areas for the samples examined. Whereas the surfaces of the iron- and copper substituted Krebs-type POM doped PEDOT films (figure 8B and 8C, respectively), were nevertheless found to be rather flat and homogenous, the cobalt substituted film (figure 8A) presented a more pronounced globular morphology that was comparable both with analogous Dawson heteropolytungstate films based on poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(2,2-bithiophene) (PBT) [27], and polyoxometalates (POM), as reported by Anwar et al [7].

These variations were

reflected in the surface texture parameters determined from the AFM data for each film: whereas the surface roughness of the iron- and copper-substituted POM doped PEDOT films were comparable, as may be seen from the Sq (Root mean square height) and Sa (arithmetical mean height) values presented in table 2 below, the surface roughness of the cobaltsubstituted film was appreciably higher.

Unlike the other samples analysed, the lithium chlorate-PEDOT film did not appear to be deposited uniformly (figure 8D). The outermost layer was rather flat, with a variation in


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height of 5-10 nm across the image. Across the surface, depressions of 20-25 nm depth were observed, with some globular structure similar to those observed for the Co-doped and Fedoped PEDOT films (figures 8A and 8B) apparent around the edges of these depressions. Overall, the surface roughness of the lithium chlorate-PEDOT film was lower than that of all three metal-substituted films (cf. Table 2). There was, however, significant difference in phase angle for the outer film and depressions observed in the lithium chlorate-PEDOT film indicating that the upper most surface and depressions consisted of polymer lay down during the film formation process.

(A)

(B)


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Page 26 of 39

(C)

Figure

7.

SEM

micrographs

of

(A)

Bi2W20Co2O70(H2O)6]-10/PEDOT,

(B)

Bi2W20Fe2O70(H2O)6]-8 /PEDOT film, and (C) Bi2W20Cu2O70(H2O)6]-8 /PEDOT films at 20,000x magnification.

(B)

(A)


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(D)

(C)

Figure 8. AFM Topography images 2D and 3D images of (A) Bi2W20Co2O70(H2O)6]10/PEDOT (B) Bi2W20Fe2O70(H2O)6]8-/PEDOT film and (C) Bi2W20Cu2O70(H2O)6]8-/PEDOT film and (D) ClO4- - PEDOT film.

Table 2: Sq (Root mean square height) and Sa (arithmetical mean height) surface texture parameters for metal-substituted polyoxometalate doped PEDOT films and lithium chloratePEDOT film determined according to ISO 25178. Sample

Sq / nm

Sa / nm

Bi2W20Co2O70(H2O)6]-10 –PEDOT film

7.64

6.23

Bi2W20Fe2O70(H2O)6]-8 –PEDOT film

4.96

3.84

Bi2W20Cu2O70(H2O)6]-8 –PEDOT film

4.44

3.42

ClO4- - PEDOT film

3.86

3.00

The surface composition of the POM-PEDOT hybrid films were determined using X-ray photoelectron spectroscopy (XPS). The survey spectra from Fe, Cu and Co substituted KrebPOM-PEDOT, presented in figure 9, detected the presence of Bi 4f (~159 eV), W 4f (~35 eV), S 2p (~164 eV), C 1s (~285 eV) and O 1s (~531 eV). In the case of Cu and Co


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substituted POM, the survey spectra identified Cu 2p (~932 eV) and Co 2p (~781 eV) respectively but Fe-substituted POM did not show the presence of Fe, perhaps the concentration was close to or below the detection limit of XPS (~ 0.1 atomic %). Moreover, relative sensitivity factor of Fe 2p is the lowest compared to Cu 2p and Co 2p. The relative compositions of the elements are quantified in table 3, below. The presence of Bi and W are characteristic of POM and in the case of Cu and Cosubstituted POMs the presence of the respective metals further supports the incorporation of the POM layer in the polymer matrix. Bi 4f peak appears as a doublet due to spin orbit coupling with Bi 4f7/2 at a binding energy of 159.3 eV characteristic of Bi3+ [28]. The W 4f peaks also appear as a doublet with the principle W 4f7/2 peak appearing at 35.6 eV characteristic of tungstate [27-28]. High resolution spectra of S 2p in the hybrid films containing Fe and Cu consists of a dominant doublet with S 2p3/2 at 163.8 eV for thiophenic sulphur and minor doublet peaks at high binding energy corresponding to oxidised sulphur. The C 1s spectra as shown in figure 9 can be fitted with five peaks at 284.8 eV (C-C, C=C), 286.2 eV (C-O, C-S), 287.6 eV (C=O), 289 eV (O-C=O) and 290.7 eV (shake up satellites of C=C) [28]. The relatively high intensity of the component at 286.2 eV in Fe and Cu substituted hybrid films is reflective of the presence of C-S and C-O bonding in PEDOT. The sulphur content in Cobalt-substituted POM was a trace amount and carbon C 1s showed relatively low amount of C-S/C-O bonding thus indicating that a relatively low amount of PEDOT may have been incorporated into the matrix. From the above analysis report, sulphur may be taken as a marker for the presence of PEDOT and tungsten for POM layer. Fe and Cu hybrid films give W/S ratio of 0.8 and 0.7 respectively, whereas Co hybrid films give a ratio of 6.9.


Page 28 of 39

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Figure 9: High resolution XPS C1s spectra of the Krebs-type POM-PEDOT hybrid films. Top: Fe-PM/PEDOT; middle: Cu-POM/PEDOT; bottom: Co-POM/PEDOT.

Table 3: Relative elemental composition of POM-PEDOT films analysed by XPS.


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Atomic Percentage POM-PEDOT film

O

C

Bi

W

S

Fe

Cu

Co

Bi2W20Co2O70(H2O)6]10-–PEDOT film

49.1

46.2

0.5

3.3

0.5

----

----

0.4

Bi2W20Fe2O70(H2O)6]8-–PEDOT film

33.6

56.7

0.5

3.7

5.4

---

----

----

Bi2W20Cu2O70(H2O)6]8-–PEDOT film

29.7

60.8

0.4

2.9

3.5

---

0.4

----

**, Fe was not detected and may have been close to or below the detection limit of the XPS (~ 0.1 atomic %) in proposed [BiFe]8-POM-PEDOT film.**

4. CONCLUSION This article has successfully demonstrated the electrochemical polymerization of Krebmetal substituted type polyoxometalate [Sb2W20M2O70(H2O)6]n-, M = Co(II), Fe(III), Cu(II) with conducting polymer 3,4-ethylene dioxythiophene (PEDOT) on to glassy carbon electrode by Chronocoulometry. Impedance and cyclic voltammetry measurements applied to study the electrochemical properties of Kreb-POM doped PEDOT films of different thicknesses. Two W-O redox processes associated with incorporated POM were observed for the POM doped PEDOT film associated with the redox switching of the W-O centres within the POM . The dependence of POM doped PEDOT film on electrolyte pH and tungsten-oxo switching behaviour of the POM at different pH was also observed. The electrical parameters, charge transfer resistance (Rct), Solution resistance (Ri), Warburg Impedance (ZW) and the current density (io) for these films (in oxidized, partially reduced and reduced states) were calculated and analysed. It was observed that the films showed lower charge transfer resistance (Rct) in the domain (+0.2V to +0.0V) as compared to when the film is further reduced to -0.6V. Surface topography and morphology was also done by using different surface analysis techniques.


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AUTHOR INFORMATION Corresponding Author *Dr.Timothy McCormac. *Dundalk Institute of Technology, Dublin Road, Dundalk, IRELAND. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †These authors F. Laffir, L. Kailas, C. Dickinson, G. Armstrong equally contributed to the completion of this article in surface analysis section. Funding Sources Dundalk Institute of Technology fund used to support the research of the manuscript. ACKNOWLEDGMENT Author Acknowledge the Dundalk Institute of Technology to funding this project, MSSI and Bernal Institute, Limerick for the surface analysis support. ABBREVIATIONS PEDOT; 3,4-ethylenedioxythiophene, POM; polyoxometalates, PPY; polypyrrole, AFM; Atomic Force Microscopy, SEM; Scanning Electron Microscopy, XPS; X-ray Photoelectron Microscopy. REFERENCES

(1)

Ueda. T. Electrochemistry of Polyoxometalates: From Fundamental Aspects to

Applications. ChemElectroChem. 2018, 5, 823–838.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(2)

Page 32 of 39

Ammam, M. Polyoxometalates: formation, structures, principal properties, main

deposition methods and application in sensing. J. Mater. Chem. A. 2013, 1, 6291–6312. (3)

Song, G.; Hong-Yan, L.,; Wang, X.; Guo-Cheng, L.; Xing, R.; Xiu-Li, W. .

Unprecedented application of the oxazolo–containing ligand to construct a new Andersontype polyoxometalate-based copper(II) complex: Electrocatalytic and adsorption properties. Inorganic Chemistry Communications. 2018, 88, 42–46. (4)

Farhadi, S.; Amini, M. M.; Dusek, M.; Kucerakova, M.; Mahmoudi, F. A new

nanohybrid

material

constructed

from

Keggin-type

polyoxometalate

and

Cd(II)

semicarbazone Schiff base complex with excellent adsorption properties for the removal of cationic dye pollutants. Journal of Molecular Structure. 2017, 1130, 592-602. (5)

Guo, S-X.; Fengwang, L.; Chen, L.; MacFarlane, D. R.; Zhang, J. Polyoxometalate-

Promoted Electrocatalytic CO2 Reduction at Nanostructured Silver in Dimethyl formamide. ACS Appl. Mater. Interfaces. 2018, 10, 12690−12697. (6)

Liua, S.; Xiaoshu, Q. Construction of nanocomposite film of Dawson-type

polyoxometalate and TiO2 nanowires for electrochromic applications. Applied Surface Science. 2017, 412, 412, 189–195. (7)

Anwar, N.; Vagin, M.; Laffir, F.; Armstrong, G.; Dickinson, C.; McCormac, T.

Transition Metal Ion-substituted Polyoxometalates Entrapped in Polypyrrole as an Electrochemical Sensor for Hydrogen Peroxide. Analyst. 2012, 137, 624-630. (8)

Yaqub, M.; Walsh,J.; Laffir, F.; Olstoorn, P.; Kailas, L.; Forster,R.; Keyes, T. E.;

Vagin, M.; McCormac, T. Polypyrrole entrapped 18-molybdodisulphate anion for the detection of hydrogen peroxide. Electrochimica Acta. 2018, 287, 78-86.


Page 33 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(9)

Zhai, S. Y.; Gong, S. Y.; Jiang, J. U.; Dong, S. J.; Li, J. H. Assembly of multilayer

films containing iron(III)-substituted Dawson-type heteropolyanions and its electrocatalytic properties: cyclic voltammetry, electrochemical impedance spectroscopy and UV-Vis spectrometry. Analytica Chimica Acta, 2003, 486(1), 85-92. (10) Ali, B.; Imar, S.; Laffir, F. Kailas, L.; Maccato, C.; McCormac, T. Electrochemical, surface and electrocatalytic properties of layer-by-layer multilayer assemblies composed of silver nanoparticles and a Ni(II)-crown type polyoxometalate. Journal of Electroanalytical Chemistry. 2018, 824, 75-82. (11) Amiri, M.; Salavati-Niasari, M.; Akbari, A. A magnetic CoFe2O4/SiO2 nanocomposite fabricated by the sol-gel method for electrocatalytic oxidation and determination of Lcysteine. Microchimica Acta. 2017, 184, 825-833. (12) Rossini, C. V. T.; Molina, C.; Caseli, L. Langmuir and Langmuir-Blodgett films of diureasil hybrid compounds containing phosphotungstic acid. Colloids and Surfaces A. 2017, 524, 35–42. (13) Sun, M.; Wang, T.; Fengyan, L.; Suna, Z.; Lin, X.. A novel sandwichtungstoantimonate cluster with FeII ions: synthesis, magnetic property and electrochemical sensing of dopamine. New J. Chem., 2018, 42, 7480-7484. (14) Weakley, T. J. R. Heteropolyanions containing two different heteroatoms. Part III. Cobalto(II)undecatungstophosphate and related anion. J. Chem. Soc., Dalton Trans. 1973, 0(3), 341–346. (15) Kortz, U.; Savelieff, M. G.; Bassil, B. S.; Keita, B.; Nadjo, L. Synthesis and Characterization of Iron(III)-Substituted, Dimeric Polyoxotungstates, [Fe4(H2O)10(β-


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 39

XW9O33)2]n- (n = 6, X = AsIII, SbIII; n = 4, X = SeIV, TeIV). Inorg. Chem. 2002, 41(4), 783789. (16) Foster, K.; Bi, L. H.; McCormac, T. Immobilisation of the polyoxometallate cluster, K6NaH[Sb2W20Fe2O70(H2O)6]·29H2O, in a polypyrrole film. Electrochim. Acta. 2008, 54(2), 868-875. (17) Loose, I.; Droste, E.; Bosing, M.; Pohlmann, H.; Dickman. M.; Rosu. C.; Pope, M.; Krebs, B. Heteropolymetalate Clusters of the Subvalent Main Group Elements BiIIIand SbIII. Inorg. Chem. 1999, 38(11), 2688-2694. (18) Murphy, A. F.; McCormac, T. Surface immobilisation of transition metal substituted Krebs type polyoxometalates, [X2W20M2O70(H2O)6]n− (X = Bi or Sb, M = Co2+ or Cu2+), by the layer by layer technique. Electrochimica Acta. 2011, 56, 10751– 10761. (19) McCormac, T.; Fabre, B.; Bidan, G. Part II. Role of pH and the transition metal for the electrocatalytic reduction of nitrite with transition metal substituted Dawson type Heteropolyanions. Journal of Electroanalytical Chemistry. 1997, 427, 155-159. (20) Han, Z.; Gao, Y.; Wang, J.; Hu, C. Preparation and Electrochemistry Properties of a New PolyoxometalateBased Supramolecular Assembly. Z. Anorg. Allg. Chem. 2009, 635, 2665-2670. (21) Li, Y.; Qian. R. Studies on the electrode kinetics of polypyrrole in aqueous solution by a.c. impedance measurement. Synthetic Metals. 1994, 64, 241-245. (22) Fernandes, D. M.; Ghicab, M. E.; Cavaleiroa, A. M. V.; Brett, A. M. C. Electrochemical

impedance

study

of

self-assembled

layer-by-layer

iron-

silicotungstate/poly(ethylenimine) modified electrodes. Electrochimica Acta. 2011, 56, 7940-7945.


Page 35 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(23) Hallik, A.; Alumaa, A.; Tamm, J.; Sammelselg, V.; Vaartnou, M.; Janes, A.; Lust. E. Analysis of electrochemical impedance of polypyrrole|sulfate and polypyrrole|perchlorate films. Synthetic Metals. 2006, 156, 488-494. (24) Doyle, R.; Breslin, B. C.; Power, O.; Rooney, D. A. Electrochemical Characterisation of Polypyrrole Doped with p-Sulfonatocalix[4]arene. Electroanalysis. 2012, 24(2), 293-302. (25) Falla, M.; Diagne, A. A.; Guène, M.; Della Volpe, C.; Bonora, L. P.; Deflorian, F.; Rossi.

S.

ELECTROCHEMICAL

IMPEDANCE

SPECTROSCOPY

PROPERTIES OF

AND

ELECTROCHEMICAL

POLYPYRROLE-COATED

PLATINUM

ELECTRODES. Bull. Chem. Soc. Ethiop. 2006, 20(2), 279-293. (26) Anwar, N.; Vagin, M.; Naseer, R. Imar, Laffir, F.;

S.; Ibrahim, M.; Mal, S. S.; Kortz, U.;

McCormac. T. Redox Switching of Polyoxometalate−Methylene Blue-Based

Layer-by-Layer Films. Langmui, 2012, 28, 5480-5488. (27) Goral, M.; Jouini, M.; Perruchot, C.; Miecznikowski, K.; Rutkowska, I. A.; Kulesza, P. J. Integration of vanadium-mixed addenda Dawson heteropolytungstate within poly(3,4ethylenedioxythiophene) and poly(2,2′-bithiophene) films by electrodeposition from the nonionic micellar aqueous medium. Electrochimica Acta. 2011, 56, 3605-3615. (28) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation Physical Electronics Division, 1992, pp16-19.


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TOC on next page

Transition metal substituted Krebs-type Polyoxometalate doped PEDOT films. R. Naseer‡, B. Ali‡, F. Laffir†, L. Kailas†, C. Dickinson†, G. Armstrong† and T. McCormac‡* ‡Electrochemistry

Research Group, Department of Applied Science, Dundalk Institute of

Technology, Dublin Road Dundalk, County Louth, Ireland. †

Bernal Institute, University of Limerick, Limerick, Ireland.

KEYWORDS-POM: polyoxometalates, PEDOT: 3,4-ethylene dioxythiophene

ABSTRACT.

The

Krebs

transition

metal

substituted

type

polyoxometalate

[Sb2W20M2O70(H2O)6]n-, M = Fe(III), Co(II) or Cu(II) were surface immobilized within the conducting polymer 3,4-ethylene dioxythiophene (PEDOT) on glassy carbon electrode surfaces.

The

immobilized

thickness

films

were

electrochemical

of

different

characterised and

surface

polymeric

by based

techniques. The inherent redox activity for the [Sb2W20M2O70(H2O)6]n-, M = Fe(III), Co(II) or Cu(II) POMs observed in the solution phase were maintained in the


PEDOT

matrix.

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Page 38 of 39

The resulting films were found to be extremely stable towards redox switching between the various POM based redox states. The films exhibited pH dependent redox activity and thin layer behaviour up to 100mVs-1. The films were found to be highly conductive through the employment of AC impedance spectroscopy. Surface characterization of the films was carried out by using X-ray photoelectron spectroscopy, atomic force microscopy and scanning

electron

microscopy


graph.

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39

Transition metal substituted Krebs-type Polyoxometalate doped

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