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Surface immobilisation of the Krebs type polyoxometalates with silver nanoparticles
Journal of Electroanalytical Chemistry 832 (2019) 493–499
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Surface immobilisation of the Krebs type polyoxometalates with silver nanoparticles Kevin Wearen, Shahzad Imar, Bushra Ali, Timothy McCormac
T
⁎
Electrochemistry Research Group, Department of Applied Science, Dundalk Institute of Technology, Dublin Road Dundalk, County Louth, Ireland
ARTICLE INFO
ABSTRACT
Keywords: Krebs polyoxometalates Layer by layer Ag nanoparticles
Modified electrodes composed of the anionic Kreb type polyoxometalates, K10[Bi2W20Co2O70(H2O)6]nH2O, K10[Sb2W20Co2O70(H2O)6]nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O, and cationic Poly(ethyleneimine) (PEI) capped silver nanoparticles have been constructed through the layer-by-layer (LBL) technique onto carbon electrode surfaces. The cyclic voltammograms recorded during film construction exhibited redox activity associated with the Ag nanoparticles and each POM's tungsten-oxo four electron redox process. The modified electrodes were then investigated for their stability towards redox cycling at pH 2.5, thin layer behaviour and pH dependent redox chemistry. Electrochemical impedance spectroscopy was employed for the modified electrode system showing the relatively low charge transfer resistance (RCT) of the POM based films.
1. Introduction Polyoxometalates (POMs) are a significant class of inorganic metal oxide cluster compounds that are of nanometer size and that are composed of a wide variety of types with varying anionic charge and shape [1–3]. They have been combined with other moieties to yield POM based materials with enhanced properties, such as, carbon nanotubes [4], encapsulation of Sm POMs by hydrophobic shells [5], Keggin POMs dispersed in Hexagonal microporous Silica [6], POM clusters capped by polyhedral [NiO8] [7], Dawson type POMs hybridised with a bis-pyridal-bis-amide ligand [8], Oxygen bonded cerium POM [9] and a Keggin POM polymerised with photosensitive N-vinylcarbazole [10]. Nanoparticles are of interest in many avenues of research, surfactant encapsulated Eu POMs have been investigated for their ability to increase the quantum yield in a polymerised layer [11]. CdS nanoparticles have been combined with Preyssler POM assemblies using the layer by layer technique [12]. Nanoporous graphene sheets have been incorporated with POMs as nano crosslinkers for possible use in hydrogen storage, gas adsorption and super capacitor electrode materials [13]. The catalytic [PW22O40] was combined with ammonium based cations giving monodisperse nanoparticles stabilised as an emulsion for efficient epoxidation reactions involving hydrogen peroxide [14]. A one pot synthesis of a tricomponent is utilised in the creation of encapsulated Au nanoparticles combined with graphene nanosheets (GNS) to enhance the catalytic efficiency of an enzyme free biosensor [15]. Keggin POMs protecteded Pt nanoparticles were investigated for
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their electrocatylitic properties towards the electroreduction of oxygen [16]. Graphene oxide(GO) nanosheets and Keggin POM clusters were assembled via layer by layer deposition and a reduced form of GO for use as microelectrodes in photodetector devices [17]. In this contribution poly-(ethylenimine) capped Ag nanoparticles were combined with the Kreb type POMs, namely, K10[Bi2W20Co2O70(H2O)6]nH2O, K10[Sb2W20Co2O70(H2O)6]nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O through the formation of structured films via employment of the layer by layer technique. The electrochemical and surface properties of the films were then investigated for their ability to be redox switched at varying pH values. The rationale for conducting this research was to see if organised layers could be constructed through the layer by layer technique with POMs and nanoparticles. This would lead to conductive films, as evidenced by the reversible redox behaviour and the RCT measurements observed here. The next steps in this work, which will be a further submission, will be to investigate the electrocatalytic abilities of such Kreb type POM nanoparticle films for the reduction of well-known water pollutants. 2. Experimental 2.1. Materials Potassium ferricyanide, potassium ferrocyanide, silver nitrate (99.99%), poly-(ethylenimine) (PEI, MW ~25,000), and all other chemicals were purchased from Sigma-Aldrich. The Kreb type POMs,
Corresponding author. E-mail address: [email protected] (T. McCormac).
https://doi.org/10.1016/j.jelechem.2018.11.035 Received 6 October 2018; Received in revised form 19 November 2018; Accepted 19 November 2018 Available online 20 November 2018 1572-6657/ © 2018 Published by Elsevier B.V.
K. Wearen et al.
Journal of Electroanalytical Chemistry 832 (2019) 493–499
Fig. 1. Cyclic voltammograms of the Krebs polyoxometalates showing the WI peaks 1 mM in electrolyte pH 2.5 using a glassy carbon working electrode (0.0707 cm2), Ag/AgCl (3 M KCl) reference electrode, Pt wire counter electrode and a scan rate of 10 mVs−1. A. [BiCo]; B. [SbCo]; C. [SbCu].
electrode (0.0707 cm2), platinum wire counter electrode and silver/ silver chloride (3 M KCl) reference electrode were used for all electrochemical experiments unless otherwise stated. The working electrode was polished using aluminium oxide powder of 1.0, 0.3 and 0.05 μm in succession, being rinsed thoroughly with deionised water following each polishing step. The electrochemical experiments were performed using a CHI660 potentiostat. Electrochemical impedance spectroscopy studies were performed in a 10 mM potassium ferricyanide, 10 mM potassium ferrocyanide with 0.1 M potassium chloride as the electrolyte solution at an applied potential +223 mV, frequency range 0.1 to 1 × 106 Hz and voltage amplitude 5 mV. The Nyquist plot has been recorded after each monolayer of POM deposit after 20 min deposition. After polishing the carbon electrode was dipped into the base layer (2% PEI) solution for 20 min to construct the cationic base layer. On removal from the PEI solution the electrode was rinsed thoroughly with deionised water and dried under a low pressure nitrogen gas stream. The modified electrode was then placed in a 1 mM POM solution for 20 min, rinsed and dried. The electrode was then immersed in the cationic solution of silver nanoparticles for 20 min to form a positively charged layer, then rinsed and dried. These POM and Silver depositions were repeated in a cyclic fashion to construct the desired number of layers. Up to 6 bilayers were constructed with the base layer being PEI in each case. The construction of the layers was verified by cyclic voltammetry, performed following the deposition of each layer.
namely, K10[Bi2W20Co2O70(H2O)6]nH2O, K10[Sb2W20Co2O70(H2O)6] nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O, employed in this work were pre-synthesised and characterised according to published literature [18,19]. PEI stabilised silver nanoparticles were produced according to the literature [20]. A mixture of 100 mL of 10 mM AgNO3 was heated and 3 mL of 2% w/w PEI Poly(ethyleneimine) (PEI: MW 250,000) was added and the solution was stirred constantly for approximately 10 min until the solution developed a bright amber colour during which the formation of the nanoparticles occurred. The synthesised AgNP's were characterised by UV-Vis, TEM and cyclic voltammetry. All other chemicals were of reagent grade and purchased from Aldrich. Alumina powders of sizes 0.05, 0.3 and 1.0 μm were received from CHI Instruments. Highly purified water with a resistivity of 18.2 MΩ cm (ELGA PURELAB Option-Q) was used for the preparation of the electrolyte and buffer solutions. The following buffer solutions have been employed for the electrochemical investigations: 0.1 M Na2SO4 (pH 2–3), 0.1 M Na2SO4 + 20 mM CH3COOH (pH 3.5 to 5) and 0.1 M Na2SO 4 + 20 mM NaH2PO4 (pH 5.5 to 7). The pH of the solutions was adjusted either 0.1 M NaOH or 0.1 M H2SO4. 2.2. Apparatus and procedure Electrochemical experiments were carried out using a single compartment three electrode electrochemical cell. A glassy carbon working 494
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Fig. 2. Cyclic voltammograms monitoring the growth of PEI/BiCo/[AgNP/BiCo]6 LBL multilayer film construction, A: W1; B: Ag peak. Using glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode in electrolyte pH 2.5, scan rate 10 mVs−1.
Fig. 3. Cyclic voltammograms monitoring the growth of PEI/SbCo/[AgNP/SbCo]6 multilayer film construction; A: W1 peak; B: Ag Peak both using glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode in electrolyte pH 2.5, scan rate 10 mVs−1.
3. Results and discussion
redox currents associated with both the W-O and Ag based redox processes with increasing layer number. For the PEI/SbCo/[AgNP/SbCo]6 multilayer system there appears to be uniform growth with increased bilayers as shown in Fig. 3. The increase in current from the first silver containing layer to the 6th is 1.0 to 3.0 μA. The silver growth for the [AgNP/SbCo]7 multilayer system in Fig. 3A and B, shows a steady increase as more bilayers were deposited on the electrode. A negative shift of the silver redox activity is also observed in the [AgNP/SbCo]7 multilayer system as the number of bilayers is increased. In the [AgNP/ SbCu] multilayer system growth is also evident in the silver and W-O redox regions. Table 1 shows that when comparing the W-O reduction peaks with the POM as the outer layer, they were −520 mV to −498 mV and −503 mV for the [BiCo]10−, [SbCo]10− and [SbCu]10− layers respectively. Overall it can be concluded that PEI capped Ag nanoparticles act as good positively charged candidates for the layer by layer (LBL) construction of Kreb's type polyoxometalates. The increase in surface coverage as a function of layer number for each of the films was calculated according to Eq. (1)
In solution, each of the Kreb type POMs displayed redox behaviour for the four electron redox couple associated with their tungsten-oxide frameworks as shown in Fig. 1, with measured E1/2 values of −467 mV, −461 mV and −452 mV (vs Ag/AgCl) respectively for [BiCo]10−, [SbCo]10− and [SbCu]8− at pH 2.5 [21]. These are reversible processes which are known to be dependent on the pH and the electrolyte solution and are in agreement with the literature [21]. The construction of the three POMs [BiCo]10−, [SbCo]10−, and [SbCu]8− immobilised as multi-layered films is investigated here, the POMs serve as the anionic layers and all use PEI+ as the base layer and alternated with PEI protected silver nanoparticles cationic layers, [PEI-Ag+] as discussed in the experimental section. Figs. 2 to 4 show the resulting cyclic voltammograms obtained for each multilayer during the deposition process. The layer order is accurately summarised by the following representation, PEI/POM/[AgNP/POM]X, where X is the number of repetitions of the bracketed bilayer. For the [BiCo]10− based silver nanoparticle doped multilayers, it is apparent that upon the successive deposition of each layer there is continual growth in the redox electrochemistry associated with both the POM's W-O framework (Fig. 2A) and the Ag nanoparticles (Fig. 2B). As can be seen in each of these figures there is a gradual increase in the
=
Q nFA
(1)
where Q is charge, n is the number of electrons, F is Faraday's constant (96,485 Cmol−1) and A is the area of the electrode surface in cm2 495
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Fig. 4. Cyclic voltammograms monitoring the growth of PEI/SbCu/[AgNP/SbCu]6 multilayer film, A: W1 peak; B: Ag peak both using glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode in electrolyte pH 2.5, scan rate 10 mVs−1. Table 1 Redox values following layer construction showing values for outermost silver and outermost POM layers after construction on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode in electrolyte pH 2.5, scan rate 10 mVs−1. Multilayer
PEI/POM/[Ag/POM]6 ″ ″ PEI/[POM/Ag]7 ″ ″
Outer layer
BiCo SbCo SbCu Ag(BiCo) Ag(SbCo) Ag(SbCu)
I/I′ W1 peak redox (mV)
II/II′ Ag redox peak (mV)
Epc
Epa
E½
ΔpE
Epc
Epa
E½
ΔpE
−543 −522 −523 −541 −521 −521
−496 −474 −481 −495 −482 −485
−519 −498 −502 −518 −502 −503
47 48 43 46 39 36
−7 −39 −6 −7 −56 −22
192 188 215 192 242 181
93 75 105 93 149 80
199 227 222 199 186 203
(0.0707 cm2). For each POM based layer there is an increase in surface coverage with each additional layer of POM when the POM is the outer layer. The total concentration of adsorbant, extrapolated using the W1 oxidation peak at pH 2.5, scan rate 10 mVs−1, for [BiCo]10−, [SbCo]10−, [SbCu]8− layers, respectively is 2.38 × 10−10 mol cm−2, 2.59 × 10−10 mol cm−2 and 2.98 × 10−10 mol cm−2. The average
increase per bilayer for the deposition of [BiCo]10−, [SbCo]10−, respectively is 3.10 × 10−11 mol cm−2, 3.48 [SbCu]8− −11 −2 −11 × 10 mol cm and 3.93 × 10 mol cm−2. It is well known that the W-O redox processes of the Kreb's type POMs are pH dependent [21]. Figs. 5–7 show the effect of the pH of the contacting electrolyte upon the redox activity of the Kreb based POM
Fig. 5. A: W1 peak over a range of pH values for PEI/BiCo/[AgNp/BiCo]6 multilayer film B: E1/2 dependency on pH for W1 reduction of BiCo in same film for range of pH 1.0–6.5; on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode, scan rate 10 mVs−1. Electrolytes used 0.5 M Na2SO4 and 0.5 M CH3COONa, over the pH range 1.00 to 3.50 and 4.00 to 6.50 with the pH adjusted by addition of 0.5 M H2SO4 or 0.5 M CH3COOH. 496
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Fig. 6. A: W1 peak over a range of pH values for PEI/SbCo/[AgNp/SbCo]6 multilayer film; B: E1/2 dependency on pH for W1 reduction of SbCo in same film for range of pH 1.0–6.5; on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode, scan rate 10 mVs−1. Electrolytes used 0.5 M Na2SO4 and 0.5 M CH3COONa, over the pH range 1.00 to 3.50 and 4.00 to 6.50 with the pH adjusted by addition of 0.5 M H2SO4 or 0.5 M CH3COOH.
multilayers whole pH range. The dependence of the E1/2 of the W-O redox process for each POM layer and the number of resulting protons involved in this redox step can be calculated by employing Eq. (2):
dE = dpH
59.1
m mVpH n
1
[Sb2 W20 Cu2 O70 ]8 + xH+ + 4e
where m is the number of protons and n is the number of electrons, 4 in this case, involved in the redox process. For each of the POM based layers presented the POM's W-O redox process moved in a negative direction as the pH was increased as expected. At pH 2.5 immobilised on a glassy carbon electrode, the three examined POMs within the multilayer systems have E1/2 values of 519, 498 and 502 mV for [BiCo]10−, [SbCo]10− and [SbCu]8− respectively for the W1 peak. The role of any added protons into the POM framework as part of the redox process is shown as follows:
Hx[Bi2 W20 Co2 O70 ](10 + 4
[Sb2 W20 Co2 O70 ]10 + xH+ + 4e
Hx[Sb2 W20 Co2 O70 ](10 + 4
x)
x)
x)
(5)
Each POM based multilayer system showed a pH dependent W1 peak with a change in E1/2 per unit pH of 80.4 mV, 86.0 mV and 76.4 mV for [BiCo]10−, [SbCo]10− and [SbCu]8− layers respectively for pH values from 1.0–3.5. In each case this change dropped as the electrolyte solution was adjusted for pH 4.0–6.5 where the change was 64.8 mV, 65.2 mV and 63.2 mV for [BiCo]10−, [SbCo]10− and [SbCu]8− respectively. These slope values equate to the addition of 4 to 5 protons per redox step. The stability of each multilayer system towards redox cycling was investigated at pH 2.5 with redox cycling through the W-O redox process for 300 cycles. For the PEI/BiCo/[AgNP/BiCo]6 multilayer film, the stability declined dramatically at the 20th cycle. For the PEI/SbCo/ [AgNP/SbCo]6 multilayer system, the current stabilised near the 10th cycle and remained quite stable for the duration of the cycling. The PEI/ SbCu/[AgNP/SbCu]6 multilayer system demonstrated stability beyond that of the [BiCo]10− and [SbCo]10− systems. It showed little change in the WI peak over the duration of 300 cycles.
(2)
[Bi2 W20 Co2 O70 ]10 + xH+ + 4e
Hx[Sb2 W20 Cu2 O70 ](8 + 4
(3) (4)
Fig. 7. A: W1 peak over a range of pH values for PEI/SbCu/[AgNp/SbCu]6 multilayer film; B: E1/2 dependency on pH for W1 reduction of SbCo in same film for range of pH 1.0–6.5; on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode, scan rate 10 mVs−1. Electrolytes used: pH 1.0–3.5, 0.5 M Na2SO4 adjusted with 0.5 M H2SO4 pH 4.0–6.5, 0.5 M CH3COONa adjusted with CH3COOH. 497
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The effect of scan rate upon the redox behaviour of each of the PEI/ POM/[AgNP/POM]6 LBL films was investigated. All three multilayer systems display thin layer behaviour within a range of scan rates. For [BiCo]10− the thin layer behaviour ranges from 20 to 150 mVs−1, the oxidation peak shifts positive above 150 mVs−1 indicating a change in the behaviour of the film. The [SbCo]10− behaves in a thin layer fashion between 5 and 150 mVs−1, while [SbCu]8− appears to have a thin layer behaviour within a less broad range of 5–100 mVs−1 than the other two systems. 3.1. Electrochemical impedance spectroscopy (EIS) characterisation Preliminary investigations utilising EIS were conducted to examine the electrical properties of the Kreb based LBL films during construction. EIS is a technique which has been previously employed [22] to study the self-assembled layers of an iron silicotungstate/pol(ethylenimime) modified electrode. The technique is primarily employed to study the interfacial properties of such films as a function of both layer number and thickness [22]. The impedance studies were performed in a 10 mM potassium ferricyanide, 10 mM potassium ferrocyanide with 0.1 M potassium chloride as the electrolyte solution during the layer construction. It is based on the AC signal and Randle's modified equivalent circuit that have been developed previously [21,22].This circuit works with charge transfer resistance (Rct), the resistance of electrolyte (Rs) and Warburg impedance (W), arises by the diffusion of electroactive species. It is negligible at higher frequency range and a semicircle will appears in the plot with higher Rct values while at lower frequency, Zw diffusion controlled region is dominant. (Cdl) is the double layer capacitance of the solution in series, in this case of the immobilised electrode, Cdl is replaced by constant phase electrode (CPE). The CPE is modelled as a non-ideal capacitor, given by Eq. (6), [22].
CPE =
1/(C i w) n
(6)
where C is the capacitance, which describes the charge separation at double layer interface, ω is the frequency in rad s−1 and n exponent is due to the heterogeneity of the surface, n = 1 for ideal capacitor. Fig. 8(a) shows the cyclic voltammogram obtained for the negatively charged redox probe Fe(CN)63− as a function of layer thickness for the PEI/BiCo/[AgNP/BiCo]2 film with an outer POM anionic layer. It has been previously reported that if the repulsion between the outer layer of similar charge to that of the redox probe will inhibit the free diffusion of the probe through the multilayer film to undergo electron transfer at the underlying electrode surface. This is apparent in Fig. 8(a) as with increasing layer number there is a general decrease in peak current and increase in the peak to peak separation for the monoelectronic Fe(CN)3−/Fe(CN)4− redox process moving from a reversible to a quasi-reversible redox system, thus indicating the increased difficulty of the probe to diffuse through the LBL film. This is reflected in the measured decrease in the measured rate constant, k°, for the Fe(CN)3−/ Fe(CN)4− redox process from 7.1 × 10−1 cms−1 to 8.6 × 10−2 cms−1 for the 1st and 6th outer POM layer, with the rate constant being measured according to the literature method [21]. EIS was then employed to investigate the interfacial properties of the PEI/BiCo/[AgNP/ BiCo]2 film as seen in Fig. 8(b). The impedance spectrum shown is characterised by two regions, a kinetically (semi-circle) and diffusion controlled region. The Randles equivalent model [22] has been previously employed to interpret such LBL POM based layers impedance spectra. What is apparent is that with increasing layer number there is a general increase in the charge transfer resistance value, RCT. Overall, due to the presence of the cationic AgNPs the RCT values appear to relatively low compared to other POM based LBL systems [22]. For example for the PEI/BiCo/[AgNP/BiCo]2, PEI/SbCu/[AgNP/SbCu]6 and PEI/SbCo/[AgNP/SbCo]6 LBL films the RCT values go from initial values with a PEI base layer of 10.8, 6.2 and 6.2 Ωcm2, respectively, to 79.4, 55.0 and 42.7 Ωcm2 with a 6th outer POM layer. For the PEI/
Fig. 8. A Cyclic voltammograms for the Ferricyanide/Ferrocyanide redox couple on PEI/BiCo/[AgNP/BiCo]2 film for the 2nd, 4th and 6th outer POM layers on the glassy carbon electrode (0.0707 cm2) in 0.1 M KCl. B. AC impedance voltammograms for the bare carbon electrode up to the outer POM 6th layer, for a PEI/BiCo/[AgNP/BiCo]2 film as the film is constructed, base layer: PEI, 2nd/4th/6th layers: [BiCo]10−, 3rd/5th/layers: AgNP. Film formed on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt counter electrode, electrolyte 10.0 mM potassium ferricyanide, 10.0 mM potassium ferrocyanide and 0.1 M KCl, applied potential +223 mV, frequency range 0.1 to 1 × 106 Hz, voltage amplitude 5 mV.
SbCu/[AgNP/SbCu]6 and PEI/SbCo/[AgNP/SbCo]6 LBL films this corresponds to a decrease in the heterogeneous rate constant, k°, for the Fe (CN)3−/Fe(CN)4− redox process from 9.9 × 10−1 cms−1 and 7.1 × 10−1 cms−1 for the first PEI base layer to 1.3 × 10−1 cms−1 and 2.7 × 10−1 cms−1 for the outer 6th POM layer. The RCT values and the heterogeneous rate constant, k°, values compare favourably to other POM based LBL films whereby nanoparticles are not immobilised within the film [21,22]. 4. Conclusion This article has successfully demonstrated the ability to surface immobilsie a range of Kreb's type polyoxometalates, K10[Bi2W20Co2O70(H2O)6]nH2O, K10[Sb2W20Co2O70(H2O)6]nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O through the employment of the Layer by Layer technique whereby PEI capped Ag nanoparticles are employed as the cationic layer within the nanaossemblies. The pH depedent redox properties of the immobilised K10[Bi2W20Co2O70(H2O)6]nH2O, 498
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K10[Sb2W20Co2O70(H2O)6]nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O POMs seen in solution are maintained in the LBL films. The films exhibited high stability towards redox cycling and thin layer behaviour across a range of scan rate ranges. Electrochemical impedance spectroscopy in conjunction with cyclic voltammetry showed that as the film thickness increased the ability of an anionic probe moiety, the potassium ferro/ferricyanide, to diffuse through the film reduced as evidenced by a decrease in the rate constant for the Fe(CN)3−/Fe (CN)4− redox process along with an increase in the measured charge transfer resistance, RCT, for the films. However on comparison to other POM based LBL films where nonmetallic cationic moieties were employed within the films the decreases in k° and the increases in the RCT values were of lower magnitude thus implying the conductive nature of the LBL films.
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Surface immobilisation of the Krebs type polyoxometalates with silver nanoparticles Kevin Wearen, Shahzad Imar, Bushra Ali, Timothy McCormac
T
⁎
Electrochemistry Research Group, Department of Applied Science, Dundalk Institute of Technology, Dublin Road Dundalk, County Louth, Ireland
ARTICLE INFO
ABSTRACT
Keywords: Krebs polyoxometalates Layer by layer Ag nanoparticles
Modified electrodes composed of the anionic Kreb type polyoxometalates, K10[Bi2W20Co2O70(H2O)6]nH2O, K10[Sb2W20Co2O70(H2O)6]nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O, and cationic Poly(ethyleneimine) (PEI) capped silver nanoparticles have been constructed through the layer-by-layer (LBL) technique onto carbon electrode surfaces. The cyclic voltammograms recorded during film construction exhibited redox activity associated with the Ag nanoparticles and each POM's tungsten-oxo four electron redox process. The modified electrodes were then investigated for their stability towards redox cycling at pH 2.5, thin layer behaviour and pH dependent redox chemistry. Electrochemical impedance spectroscopy was employed for the modified electrode system showing the relatively low charge transfer resistance (RCT) of the POM based films.
1. Introduction Polyoxometalates (POMs) are a significant class of inorganic metal oxide cluster compounds that are of nanometer size and that are composed of a wide variety of types with varying anionic charge and shape [1–3]. They have been combined with other moieties to yield POM based materials with enhanced properties, such as, carbon nanotubes [4], encapsulation of Sm POMs by hydrophobic shells [5], Keggin POMs dispersed in Hexagonal microporous Silica [6], POM clusters capped by polyhedral [NiO8] [7], Dawson type POMs hybridised with a bis-pyridal-bis-amide ligand [8], Oxygen bonded cerium POM [9] and a Keggin POM polymerised with photosensitive N-vinylcarbazole [10]. Nanoparticles are of interest in many avenues of research, surfactant encapsulated Eu POMs have been investigated for their ability to increase the quantum yield in a polymerised layer [11]. CdS nanoparticles have been combined with Preyssler POM assemblies using the layer by layer technique [12]. Nanoporous graphene sheets have been incorporated with POMs as nano crosslinkers for possible use in hydrogen storage, gas adsorption and super capacitor electrode materials [13]. The catalytic [PW22O40] was combined with ammonium based cations giving monodisperse nanoparticles stabilised as an emulsion for efficient epoxidation reactions involving hydrogen peroxide [14]. A one pot synthesis of a tricomponent is utilised in the creation of encapsulated Au nanoparticles combined with graphene nanosheets (GNS) to enhance the catalytic efficiency of an enzyme free biosensor [15]. Keggin POMs protecteded Pt nanoparticles were investigated for
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their electrocatylitic properties towards the electroreduction of oxygen [16]. Graphene oxide(GO) nanosheets and Keggin POM clusters were assembled via layer by layer deposition and a reduced form of GO for use as microelectrodes in photodetector devices [17]. In this contribution poly-(ethylenimine) capped Ag nanoparticles were combined with the Kreb type POMs, namely, K10[Bi2W20Co2O70(H2O)6]nH2O, K10[Sb2W20Co2O70(H2O)6]nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O through the formation of structured films via employment of the layer by layer technique. The electrochemical and surface properties of the films were then investigated for their ability to be redox switched at varying pH values. The rationale for conducting this research was to see if organised layers could be constructed through the layer by layer technique with POMs and nanoparticles. This would lead to conductive films, as evidenced by the reversible redox behaviour and the RCT measurements observed here. The next steps in this work, which will be a further submission, will be to investigate the electrocatalytic abilities of such Kreb type POM nanoparticle films for the reduction of well-known water pollutants. 2. Experimental 2.1. Materials Potassium ferricyanide, potassium ferrocyanide, silver nitrate (99.99%), poly-(ethylenimine) (PEI, MW ~25,000), and all other chemicals were purchased from Sigma-Aldrich. The Kreb type POMs,
Corresponding author. E-mail address: [email protected] (T. McCormac).
https://doi.org/10.1016/j.jelechem.2018.11.035 Received 6 October 2018; Received in revised form 19 November 2018; Accepted 19 November 2018 Available online 20 November 2018 1572-6657/ © 2018 Published by Elsevier B.V.
K. Wearen et al.
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Fig. 1. Cyclic voltammograms of the Krebs polyoxometalates showing the WI peaks 1 mM in electrolyte pH 2.5 using a glassy carbon working electrode (0.0707 cm2), Ag/AgCl (3 M KCl) reference electrode, Pt wire counter electrode and a scan rate of 10 mVs−1. A. [BiCo]; B. [SbCo]; C. [SbCu].
electrode (0.0707 cm2), platinum wire counter electrode and silver/ silver chloride (3 M KCl) reference electrode were used for all electrochemical experiments unless otherwise stated. The working electrode was polished using aluminium oxide powder of 1.0, 0.3 and 0.05 μm in succession, being rinsed thoroughly with deionised water following each polishing step. The electrochemical experiments were performed using a CHI660 potentiostat. Electrochemical impedance spectroscopy studies were performed in a 10 mM potassium ferricyanide, 10 mM potassium ferrocyanide with 0.1 M potassium chloride as the electrolyte solution at an applied potential +223 mV, frequency range 0.1 to 1 × 106 Hz and voltage amplitude 5 mV. The Nyquist plot has been recorded after each monolayer of POM deposit after 20 min deposition. After polishing the carbon electrode was dipped into the base layer (2% PEI) solution for 20 min to construct the cationic base layer. On removal from the PEI solution the electrode was rinsed thoroughly with deionised water and dried under a low pressure nitrogen gas stream. The modified electrode was then placed in a 1 mM POM solution for 20 min, rinsed and dried. The electrode was then immersed in the cationic solution of silver nanoparticles for 20 min to form a positively charged layer, then rinsed and dried. These POM and Silver depositions were repeated in a cyclic fashion to construct the desired number of layers. Up to 6 bilayers were constructed with the base layer being PEI in each case. The construction of the layers was verified by cyclic voltammetry, performed following the deposition of each layer.
namely, K10[Bi2W20Co2O70(H2O)6]nH2O, K10[Sb2W20Co2O70(H2O)6] nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O, employed in this work were pre-synthesised and characterised according to published literature [18,19]. PEI stabilised silver nanoparticles were produced according to the literature [20]. A mixture of 100 mL of 10 mM AgNO3 was heated and 3 mL of 2% w/w PEI Poly(ethyleneimine) (PEI: MW 250,000) was added and the solution was stirred constantly for approximately 10 min until the solution developed a bright amber colour during which the formation of the nanoparticles occurred. The synthesised AgNP's were characterised by UV-Vis, TEM and cyclic voltammetry. All other chemicals were of reagent grade and purchased from Aldrich. Alumina powders of sizes 0.05, 0.3 and 1.0 μm were received from CHI Instruments. Highly purified water with a resistivity of 18.2 MΩ cm (ELGA PURELAB Option-Q) was used for the preparation of the electrolyte and buffer solutions. The following buffer solutions have been employed for the electrochemical investigations: 0.1 M Na2SO4 (pH 2–3), 0.1 M Na2SO4 + 20 mM CH3COOH (pH 3.5 to 5) and 0.1 M Na2SO 4 + 20 mM NaH2PO4 (pH 5.5 to 7). The pH of the solutions was adjusted either 0.1 M NaOH or 0.1 M H2SO4. 2.2. Apparatus and procedure Electrochemical experiments were carried out using a single compartment three electrode electrochemical cell. A glassy carbon working 494
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Fig. 2. Cyclic voltammograms monitoring the growth of PEI/BiCo/[AgNP/BiCo]6 LBL multilayer film construction, A: W1; B: Ag peak. Using glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode in electrolyte pH 2.5, scan rate 10 mVs−1.
Fig. 3. Cyclic voltammograms monitoring the growth of PEI/SbCo/[AgNP/SbCo]6 multilayer film construction; A: W1 peak; B: Ag Peak both using glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode in electrolyte pH 2.5, scan rate 10 mVs−1.
3. Results and discussion
redox currents associated with both the W-O and Ag based redox processes with increasing layer number. For the PEI/SbCo/[AgNP/SbCo]6 multilayer system there appears to be uniform growth with increased bilayers as shown in Fig. 3. The increase in current from the first silver containing layer to the 6th is 1.0 to 3.0 μA. The silver growth for the [AgNP/SbCo]7 multilayer system in Fig. 3A and B, shows a steady increase as more bilayers were deposited on the electrode. A negative shift of the silver redox activity is also observed in the [AgNP/SbCo]7 multilayer system as the number of bilayers is increased. In the [AgNP/ SbCu] multilayer system growth is also evident in the silver and W-O redox regions. Table 1 shows that when comparing the W-O reduction peaks with the POM as the outer layer, they were −520 mV to −498 mV and −503 mV for the [BiCo]10−, [SbCo]10− and [SbCu]10− layers respectively. Overall it can be concluded that PEI capped Ag nanoparticles act as good positively charged candidates for the layer by layer (LBL) construction of Kreb's type polyoxometalates. The increase in surface coverage as a function of layer number for each of the films was calculated according to Eq. (1)
In solution, each of the Kreb type POMs displayed redox behaviour for the four electron redox couple associated with their tungsten-oxide frameworks as shown in Fig. 1, with measured E1/2 values of −467 mV, −461 mV and −452 mV (vs Ag/AgCl) respectively for [BiCo]10−, [SbCo]10− and [SbCu]8− at pH 2.5 [21]. These are reversible processes which are known to be dependent on the pH and the electrolyte solution and are in agreement with the literature [21]. The construction of the three POMs [BiCo]10−, [SbCo]10−, and [SbCu]8− immobilised as multi-layered films is investigated here, the POMs serve as the anionic layers and all use PEI+ as the base layer and alternated with PEI protected silver nanoparticles cationic layers, [PEI-Ag+] as discussed in the experimental section. Figs. 2 to 4 show the resulting cyclic voltammograms obtained for each multilayer during the deposition process. The layer order is accurately summarised by the following representation, PEI/POM/[AgNP/POM]X, where X is the number of repetitions of the bracketed bilayer. For the [BiCo]10− based silver nanoparticle doped multilayers, it is apparent that upon the successive deposition of each layer there is continual growth in the redox electrochemistry associated with both the POM's W-O framework (Fig. 2A) and the Ag nanoparticles (Fig. 2B). As can be seen in each of these figures there is a gradual increase in the
=
Q nFA
(1)
where Q is charge, n is the number of electrons, F is Faraday's constant (96,485 Cmol−1) and A is the area of the electrode surface in cm2 495
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Fig. 4. Cyclic voltammograms monitoring the growth of PEI/SbCu/[AgNP/SbCu]6 multilayer film, A: W1 peak; B: Ag peak both using glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode in electrolyte pH 2.5, scan rate 10 mVs−1. Table 1 Redox values following layer construction showing values for outermost silver and outermost POM layers after construction on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode in electrolyte pH 2.5, scan rate 10 mVs−1. Multilayer
PEI/POM/[Ag/POM]6 ″ ″ PEI/[POM/Ag]7 ″ ″
Outer layer
BiCo SbCo SbCu Ag(BiCo) Ag(SbCo) Ag(SbCu)
I/I′ W1 peak redox (mV)
II/II′ Ag redox peak (mV)
Epc
Epa
E½
ΔpE
Epc
Epa
E½
ΔpE
−543 −522 −523 −541 −521 −521
−496 −474 −481 −495 −482 −485
−519 −498 −502 −518 −502 −503
47 48 43 46 39 36
−7 −39 −6 −7 −56 −22
192 188 215 192 242 181
93 75 105 93 149 80
199 227 222 199 186 203
(0.0707 cm2). For each POM based layer there is an increase in surface coverage with each additional layer of POM when the POM is the outer layer. The total concentration of adsorbant, extrapolated using the W1 oxidation peak at pH 2.5, scan rate 10 mVs−1, for [BiCo]10−, [SbCo]10−, [SbCu]8− layers, respectively is 2.38 × 10−10 mol cm−2, 2.59 × 10−10 mol cm−2 and 2.98 × 10−10 mol cm−2. The average
increase per bilayer for the deposition of [BiCo]10−, [SbCo]10−, respectively is 3.10 × 10−11 mol cm−2, 3.48 [SbCu]8− −11 −2 −11 × 10 mol cm and 3.93 × 10 mol cm−2. It is well known that the W-O redox processes of the Kreb's type POMs are pH dependent [21]. Figs. 5–7 show the effect of the pH of the contacting electrolyte upon the redox activity of the Kreb based POM
Fig. 5. A: W1 peak over a range of pH values for PEI/BiCo/[AgNp/BiCo]6 multilayer film B: E1/2 dependency on pH for W1 reduction of BiCo in same film for range of pH 1.0–6.5; on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode, scan rate 10 mVs−1. Electrolytes used 0.5 M Na2SO4 and 0.5 M CH3COONa, over the pH range 1.00 to 3.50 and 4.00 to 6.50 with the pH adjusted by addition of 0.5 M H2SO4 or 0.5 M CH3COOH. 496
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Fig. 6. A: W1 peak over a range of pH values for PEI/SbCo/[AgNp/SbCo]6 multilayer film; B: E1/2 dependency on pH for W1 reduction of SbCo in same film for range of pH 1.0–6.5; on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode, scan rate 10 mVs−1. Electrolytes used 0.5 M Na2SO4 and 0.5 M CH3COONa, over the pH range 1.00 to 3.50 and 4.00 to 6.50 with the pH adjusted by addition of 0.5 M H2SO4 or 0.5 M CH3COOH.
multilayers whole pH range. The dependence of the E1/2 of the W-O redox process for each POM layer and the number of resulting protons involved in this redox step can be calculated by employing Eq. (2):
dE = dpH
59.1
m mVpH n
1
[Sb2 W20 Cu2 O70 ]8 + xH+ + 4e
where m is the number of protons and n is the number of electrons, 4 in this case, involved in the redox process. For each of the POM based layers presented the POM's W-O redox process moved in a negative direction as the pH was increased as expected. At pH 2.5 immobilised on a glassy carbon electrode, the three examined POMs within the multilayer systems have E1/2 values of 519, 498 and 502 mV for [BiCo]10−, [SbCo]10− and [SbCu]8− respectively for the W1 peak. The role of any added protons into the POM framework as part of the redox process is shown as follows:
Hx[Bi2 W20 Co2 O70 ](10 + 4
[Sb2 W20 Co2 O70 ]10 + xH+ + 4e
Hx[Sb2 W20 Co2 O70 ](10 + 4
x)
x)
x)
(5)
Each POM based multilayer system showed a pH dependent W1 peak with a change in E1/2 per unit pH of 80.4 mV, 86.0 mV and 76.4 mV for [BiCo]10−, [SbCo]10− and [SbCu]8− layers respectively for pH values from 1.0–3.5. In each case this change dropped as the electrolyte solution was adjusted for pH 4.0–6.5 where the change was 64.8 mV, 65.2 mV and 63.2 mV for [BiCo]10−, [SbCo]10− and [SbCu]8− respectively. These slope values equate to the addition of 4 to 5 protons per redox step. The stability of each multilayer system towards redox cycling was investigated at pH 2.5 with redox cycling through the W-O redox process for 300 cycles. For the PEI/BiCo/[AgNP/BiCo]6 multilayer film, the stability declined dramatically at the 20th cycle. For the PEI/SbCo/ [AgNP/SbCo]6 multilayer system, the current stabilised near the 10th cycle and remained quite stable for the duration of the cycling. The PEI/ SbCu/[AgNP/SbCu]6 multilayer system demonstrated stability beyond that of the [BiCo]10− and [SbCo]10− systems. It showed little change in the WI peak over the duration of 300 cycles.
(2)
[Bi2 W20 Co2 O70 ]10 + xH+ + 4e
Hx[Sb2 W20 Cu2 O70 ](8 + 4
(3) (4)
Fig. 7. A: W1 peak over a range of pH values for PEI/SbCu/[AgNp/SbCu]6 multilayer film; B: E1/2 dependency on pH for W1 reduction of SbCo in same film for range of pH 1.0–6.5; on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt wire counter electrode, scan rate 10 mVs−1. Electrolytes used: pH 1.0–3.5, 0.5 M Na2SO4 adjusted with 0.5 M H2SO4 pH 4.0–6.5, 0.5 M CH3COONa adjusted with CH3COOH. 497
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The effect of scan rate upon the redox behaviour of each of the PEI/ POM/[AgNP/POM]6 LBL films was investigated. All three multilayer systems display thin layer behaviour within a range of scan rates. For [BiCo]10− the thin layer behaviour ranges from 20 to 150 mVs−1, the oxidation peak shifts positive above 150 mVs−1 indicating a change in the behaviour of the film. The [SbCo]10− behaves in a thin layer fashion between 5 and 150 mVs−1, while [SbCu]8− appears to have a thin layer behaviour within a less broad range of 5–100 mVs−1 than the other two systems. 3.1. Electrochemical impedance spectroscopy (EIS) characterisation Preliminary investigations utilising EIS were conducted to examine the electrical properties of the Kreb based LBL films during construction. EIS is a technique which has been previously employed [22] to study the self-assembled layers of an iron silicotungstate/pol(ethylenimime) modified electrode. The technique is primarily employed to study the interfacial properties of such films as a function of both layer number and thickness [22]. The impedance studies were performed in a 10 mM potassium ferricyanide, 10 mM potassium ferrocyanide with 0.1 M potassium chloride as the electrolyte solution during the layer construction. It is based on the AC signal and Randle's modified equivalent circuit that have been developed previously [21,22].This circuit works with charge transfer resistance (Rct), the resistance of electrolyte (Rs) and Warburg impedance (W), arises by the diffusion of electroactive species. It is negligible at higher frequency range and a semicircle will appears in the plot with higher Rct values while at lower frequency, Zw diffusion controlled region is dominant. (Cdl) is the double layer capacitance of the solution in series, in this case of the immobilised electrode, Cdl is replaced by constant phase electrode (CPE). The CPE is modelled as a non-ideal capacitor, given by Eq. (6), [22].
CPE =
1/(C i w) n
(6)
where C is the capacitance, which describes the charge separation at double layer interface, ω is the frequency in rad s−1 and n exponent is due to the heterogeneity of the surface, n = 1 for ideal capacitor. Fig. 8(a) shows the cyclic voltammogram obtained for the negatively charged redox probe Fe(CN)63− as a function of layer thickness for the PEI/BiCo/[AgNP/BiCo]2 film with an outer POM anionic layer. It has been previously reported that if the repulsion between the outer layer of similar charge to that of the redox probe will inhibit the free diffusion of the probe through the multilayer film to undergo electron transfer at the underlying electrode surface. This is apparent in Fig. 8(a) as with increasing layer number there is a general decrease in peak current and increase in the peak to peak separation for the monoelectronic Fe(CN)3−/Fe(CN)4− redox process moving from a reversible to a quasi-reversible redox system, thus indicating the increased difficulty of the probe to diffuse through the LBL film. This is reflected in the measured decrease in the measured rate constant, k°, for the Fe(CN)3−/ Fe(CN)4− redox process from 7.1 × 10−1 cms−1 to 8.6 × 10−2 cms−1 for the 1st and 6th outer POM layer, with the rate constant being measured according to the literature method [21]. EIS was then employed to investigate the interfacial properties of the PEI/BiCo/[AgNP/ BiCo]2 film as seen in Fig. 8(b). The impedance spectrum shown is characterised by two regions, a kinetically (semi-circle) and diffusion controlled region. The Randles equivalent model [22] has been previously employed to interpret such LBL POM based layers impedance spectra. What is apparent is that with increasing layer number there is a general increase in the charge transfer resistance value, RCT. Overall, due to the presence of the cationic AgNPs the RCT values appear to relatively low compared to other POM based LBL systems [22]. For example for the PEI/BiCo/[AgNP/BiCo]2, PEI/SbCu/[AgNP/SbCu]6 and PEI/SbCo/[AgNP/SbCo]6 LBL films the RCT values go from initial values with a PEI base layer of 10.8, 6.2 and 6.2 Ωcm2, respectively, to 79.4, 55.0 and 42.7 Ωcm2 with a 6th outer POM layer. For the PEI/
Fig. 8. A Cyclic voltammograms for the Ferricyanide/Ferrocyanide redox couple on PEI/BiCo/[AgNP/BiCo]2 film for the 2nd, 4th and 6th outer POM layers on the glassy carbon electrode (0.0707 cm2) in 0.1 M KCl. B. AC impedance voltammograms for the bare carbon electrode up to the outer POM 6th layer, for a PEI/BiCo/[AgNP/BiCo]2 film as the film is constructed, base layer: PEI, 2nd/4th/6th layers: [BiCo]10−, 3rd/5th/layers: AgNP. Film formed on a glassy carbon working electrode (0.0707 cm2), Ag/AgCl reference electrode (3 M KCl), Pt counter electrode, electrolyte 10.0 mM potassium ferricyanide, 10.0 mM potassium ferrocyanide and 0.1 M KCl, applied potential +223 mV, frequency range 0.1 to 1 × 106 Hz, voltage amplitude 5 mV.
SbCu/[AgNP/SbCu]6 and PEI/SbCo/[AgNP/SbCo]6 LBL films this corresponds to a decrease in the heterogeneous rate constant, k°, for the Fe (CN)3−/Fe(CN)4− redox process from 9.9 × 10−1 cms−1 and 7.1 × 10−1 cms−1 for the first PEI base layer to 1.3 × 10−1 cms−1 and 2.7 × 10−1 cms−1 for the outer 6th POM layer. The RCT values and the heterogeneous rate constant, k°, values compare favourably to other POM based LBL films whereby nanoparticles are not immobilised within the film [21,22]. 4. Conclusion This article has successfully demonstrated the ability to surface immobilsie a range of Kreb's type polyoxometalates, K10[Bi2W20Co2O70(H2O)6]nH2O, K10[Sb2W20Co2O70(H2O)6]nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O through the employment of the Layer by Layer technique whereby PEI capped Ag nanoparticles are employed as the cationic layer within the nanaossemblies. The pH depedent redox properties of the immobilised K10[Bi2W20Co2O70(H2O)6]nH2O, 498
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K10[Sb2W20Co2O70(H2O)6]nH2O and K8[Sb2W20Cu2O70(H2O)6]nH2O POMs seen in solution are maintained in the LBL films. The films exhibited high stability towards redox cycling and thin layer behaviour across a range of scan rate ranges. Electrochemical impedance spectroscopy in conjunction with cyclic voltammetry showed that as the film thickness increased the ability of an anionic probe moiety, the potassium ferro/ferricyanide, to diffuse through the film reduced as evidenced by a decrease in the rate constant for the Fe(CN)3−/Fe (CN)4− redox process along with an increase in the measured charge transfer resistance, RCT, for the films. However on comparison to other POM based LBL films where nonmetallic cationic moieties were employed within the films the decreases in k° and the increases in the RCT values were of lower magnitude thus implying the conductive nature of the LBL films.
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