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Mechanistic Insights Gained by Monitoring Carbon Nanotube/prussian Blue Nanocomposite Formation with in Situ Electrochemically Based Techniques Edson Nossol, Arlene Bispo dos Santos Nossol, Muhammad E Abdelhamid, Lisandra L. Martin, Aldo Jose Gorgatti Zarbin, and Alan M Bond J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp501442h • Publication Date (Web): 24 May 2014 Downloaded from http://pubs.acs.org on June 1, 2014

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The Journal of Physical Chemistry

Mechanistic Insights Gained by Monitoring Carbon Nanotube/Prussian Blue Nanocomposite Formation With in Situ Electrochemically Based Techniques Edson Nossol,†* Arlene B. S. Nossol,‡ Muhammad E. Abdelhamid,‡ Lisandra L. Martin,‡ Aldo J. G. Zarbin†* and Alan M. Bond‡*

Federal University of Paraná, CP 19081, CEP 81531-990, Curitiba, Paraná, Brazil. School of Chemistry, Monash University, Clayton, 3800, Melbourne, Victoria, Australia.

* Corresponding author Edson Nossol e-mail: [email protected] phone: +55-41-33613176 fax: +55-41-33613186 1 ACS Paragon Plus Environment


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ABSTRACT Mass loadings and mechanistic insights into the stepwise formation of Prussian blue (PB), in a carbon nanotube nanocomposite electrode formed by an in situ electrochemical reaction between iron species filling the carbon nanotubes cavities and ferricyanide ions in solution, have been probed by electrochemical quartz crystal microbalance (EQCM), electrochemical surface plasmon resonance (SPR) and scanning electrochemical

microscopy

(SECM).

Changes

to

the

interfacial

nanotube

‘electrode/electrolyte’ (aqueous KCl) region during PB oxidation and reduction and the influence of the applied potential also have been assessed by EQCM mass and SPR refractive angle changes that reflect the local redox activity. The data obtained confirm that KCl present as the supporting electrolyte participates in PB formation and SECM studies reveal that redox activity take place at both metallic centers in PB. Large changes in the SPR angle with variation in applied potential and electrolyte cation in the carbon nanotube/PB film suggest that the nanocomposite material represents a promising material for the development of nanostructured optical devices.

Keywords: Electrochemical study, hexacyanoferrate, redox activity, in situ techniques

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1 Introduction Some of the most attractive and widely investigated nanomaterials in recent times have been based on carbon nanotubes (CNTs). These one dimensional carbon structures with an extremely high aspect ratio, remarkable strength and excellent mechanical, electrical, thermal, optical and electrochemical properties play a central role in the development of advanced materials needed for photoconversion and electrical energy storage devices,1 fuel cells,2 flexible field emitters,3 sensors,4 optical nanoantennas5 and radiosurgery.6 Electrochemical data suggest that there are advantages to be gained by using CNTs in the construction of electrodes.

In particular, the high current density and fast

heterogeneous electron transfer rates along with high mechanical stability and electrocatalytic properties are available for a range of important redox reactions at CNT based electrodes.7-9 Prussian blue (PB), Fe4[Fe(CN)6]3.xH2O (x=14-16), has an open framework structure formed by alternating face centered cubic lattices of Fe3+ and Fe2+ cations, distinguishable by distinct coordination of the CN- ligand; Fe3+ high spin sites with S= 5/2 are nitrogen coordinated and Fe2+ low spin sites with S= 0 are carbon coordinated.10 The relative quantities of Fe3+, Fe2+, [Fe(CN)6]3- and associated water content are nonstoichiometric values as a result of defects in the structure. Thin films of PB electrodeposited from aqueous electrolytes have the capacity to reversibly intercalate potassium and other hydrated cations (e.g. Rb+, Cs+, NH4+). Furthermore the cations may be interchanged using electrochemical methods.11-13 In this context, PB has emerged as a multifunctional material having applications in catalysis,14 environmental removal of radionuclides,15 battery components,16 sensors,17-19 electrochromic devices20 and photomagnetism.21 Recent studies have indicated that PB nanostructures can be prepared in a range of isoforms (e.g., nanocubes,22 nanospheres,23 nanotubes,24



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nanosheets25 and nanowires)26 and these variations can provide improvements in commercial applications. However, to achieve application advances, fine control of crystallinity and composition is required.27 In recent years, carbon nanotube-Prussian Blue nanocomposites have been shown to exhibit interesting material properties. However, the vast majority of this work has been focused on sensor development for the determination of acetylcholinesterase,28 diethylstilbestrol,29 dopamine, 30 glucose,31 hydroxylamine32 and hydrogen peroxide.33 Classic methods for electrochemical preparation of PB involve the use of a ferricyanide solution and a source of Fe3+ (generally FeCl3) in an acidic media.34-36 Recently, some of us have developed a new methodology for the preparation of Prussian blue/carbon nanotubes nanocomposites, based on an in situ electrochemical reaction between iron species present in the cavities of carbon nanotubes and ferricyanide ions in solution.37, 38 The CNT samples used to prepare the CNT/PB composite were synthesized by a method based on thermal decomposition of ferrocene. As a consequence, the CNTs have their internal cavity filled with metallic iron (Fe) and iron oxide species (primarily Fe2O3 and Fe3O4). These iron species can migrate to the CNT/ferricyanide interface, at neutral pH, and act as a Fe3+/Fe2+ source during the electrochemical cycling of the CNT electrode to form PB nanocubes. This innovative method provides selective deposition and control of the morphology and has been extended by other researchers to PB analogue materials such as carbon nanotube/zinc hexacyanoferrate to achieve excellent electrical charge/discharge characteristics.39 Thin and transparent films of Prussian Blue/carbon nanotube, multifunctional composite films have also been prepared and used in other applications to form a hydrogen peroxide sensor, which shows very low detection and quantification limits;37 an electrochromic electrode with high electrochemical cycling stability;40 and an electrode for environmental treatment of


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water samples which displays a high level of catalytic activity in the degradation of methyl orange dye.41 Applications described above require high nanocomposite film stability and a synergic effect between CNTs and PB. These characteristics are derived from the intimate contact achieved with the new preparation method.37,38 In our previous papers, the electrochemical, spectroscopic and microscopic characterization of the CNT/PB nanocomposite were reported.

However, a full understanding of the unique

characteristics and exceptional properties achieved with this new method of PB formation on carbon nanotubes requires a more detailed examination of the nature of the redox chemistry. Employment of non-traditional in situ techniques to investigate the formation and electrochemical properties of films on electrodes surfaces has been a key factor in the development of new materials.42,43 Thus, the physicochemical techniques of quartz crystal microbalance (QCM),44,45 in combination with electrochemistry (EQCM) surface plasmon resonance (SPR)46-48 and scanning electrochemical microscopy (SECM),49-51 are now introduced to elucidate optical, redox, electrical and structural properties of the electrochemically prepared PB-CNT materials. In this paper we report the results of an in situ investigation of Prussian blue electrochemical formation on carbon nanotubes films by EQCM and SPR methods, along with provision of new knowledge on the nature of PB redox processes obtained by scanning electrochemical microscopy (SECM). Information on the morphology was also provided by ex situ application of scanning electron microscopy (SEM) and atomic force microscopy (AFM).



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2 Materials and Methods

2.1 Materials Potassium chloride (KCl) and potassium ferricyanide K3[Fe(CN)6] (Aldrich), NaCl (BDH Chemicals) and chloroform (Merck) were used as received. Deionized water from a Milli-Q-MilliRho purification system (18.2MΩ cm-1) was used to prepare all aqueous electrolyte solutions. Indium tin oxide (ITO) (surface resistivity: 8-12 Ω/sq) (Aldrich) surfaces were cleaned by sequentially sonication in acetone and ethanol (Merck) for 10 min.

2.2 Carbon Nanotube Synthesis Carbon nanotubes were prepared by the Chemical Vapor Deposition (CVD) method52 which provides a large percentage of multi-walled carbon nanotubes (MWCNTs) filled with crystalline iron components, mainly α-Fe, α-Fe2O3 (hematite), and Fe3O4 (magnetite). The CNTs were treated with trifluoroacetic acid (TFA).53 Approximately 20.0 mg of the CNTs was dispersed in a mixture containing 50.0 mL of toluene and 5.0 mL of TFA. The dispersion was subsequently placed in an ultrasound (Unique, 154 W, 37 kHz) ice-bath for 2 h. The insoluble CNTs were then separated by centrifugation (3000 rpm for 5 min), washed three times with toluene, three times with acetone, and dried at 50 °C. Afterwards the solid was dispersed in chloroform (0.3 mg mL-1) using an ultrasound bath for 2 h.

2.3 Carbon nanotube/Prussian blue films synthesis The preparation of CNT/PB nanocomposite films was carried out according to the method described previously.37,38 The procedure consists of cycling the CNT film in a


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solution containing 1.0×10-3 mol L-1 K3Fe(CN)6 and 0.1 mol L-1 KCl, without the addition of Fe3+ ions, at a scan rate of 0.05 V s-1. The potential range and number of cycles were set according to the characteristics required for the particular film (see below). After the cycling process, the films were rinsed with distilled water and subjected to thermal treatment at 150 °C.

2.4 EQCM Experiments EQCM measurements were performed at 293 K with a Q-SENSE E4 system (QSENSE, Västra Frölunda, Sweden). The sensor crystals used were 5 MHz, AT-cut, polished quartz discs (chips) with an evaporated gold sensor surface.

Before

assembling into the chamber, sensors were rinsed with ethanol and dried under a stream of N2 gas, after which they were placed into a 1:1.3 solution of ammonia, hydrogen peroxide, and water at ∼75°C for 20 min. Subsequently, the chips were thoroughly rinsed with water and ethanol, dried, and immediately assembled into the EQCM. The modification of the sensor with a carbon nanotube film was carried out using the dip coating technique, in which the gold sensor was dipped 10 times into a CNT dispersion in chloroform (0.5 mg mL-1). The change of resonance frequency (∆f) upon mass deposition was recorded for the first, third, fifth, seventh and ninth harmonics and recorded simultaneously with potential cycling of the CNT modified sensor in an aqueous solution containing 1 mmol.L-1 K3[Fe(CN)6] and 0.1 mol.L-1 KCl, using a QSense Electrochemistry Module, QEM 401, having a Pt counter electrode, Ag/AgCl (3 mol.L-1 KCl), (WPI, Dri-REF) reference electrode and a CNTs modified sensor as the working electrode, all connected to a µAUTOLAB III (ECO-Chemie) potentiostat. This solution was then flushed out of the system and replaced by aqueous 0.1 mol.L-1 KCl.



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2.5 In Situ Electrochemical SPR Studies In situ SPR experiments were performed using an Autolab ESPRIT (ECO-Chemie) system in combination with the µAUTOLAB III (ECO-Chemie) potentiostat. SPR substrates (50 nm gold film coated glass) were placed onto a hemi-cylindrical lens with index matching oil (all supplied by ECO-Chemie). The experimental setup was based on the Kretschmann optical configuration and used a monochromatic p-polarized laser (λ = 670 nm) as the light source. A gold film was used as the working electrode with a Ag/AgCl (KCl 3 mol.L-1) reference electrode and a platinum wire counter electrode. Prior to CNT film formation, the gold working electrode was treated with a hydrogen peroxide based solution (v/v 1:1:3 mixture of NH3, H2O2 and H2O), heated for 20 min, followed by thorough rinsing with water and nitrogen drying. Finally, 10 µL of a 0.3 mg mL-1 dispersion of CNTs in chloroform was drop casted onto the gold electrode and dried at room temperature (22±2 oC).

2.6 Scanning Electrochemical Microscopy Measurements SECM experiments were carried out with a CH Instruments model CHI910B scanning electrochemical microscope. The four-electrode configuration mode consisted of a glassy carbon (GC) substrate electrode, a 25 µm diameter Pt working electrode with an insulating sheath-to-tip ratio RG=10, a Pt wire counter electrode, and a Ag/AgCl (3 mol.L-1 NaCl) reference electrode. In the approach curve method of analysis a solution containing 1 m mol L-1 K3[Fe(CN)6] in 0.1 mol L-1 KCl was used as a redox mediator to position the UME on the GC/CNTs/PB modified substrate electrode.54 This mediator solution was then flushed out of the cell and replaced with a 0.1 mol L-1 KCl electrolyte solution which was used as the supporting electrolyte in experiments used to monitor the surface redox chemistry of the PB film. The electrochemical preparation of 8 ACS Paragon Plus Environment

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CNT/PB modified electrodes was carried out according to the procedure described above by drop casting 50 µL of the 0.3 mg mL-1 dispersion of CNTs in chloroform onto the GC electrode and drying at room temperature. The potential of the CNT electrode was cycled from -0.3 to 1.4 V at a scan rate of 50 mV s-1 in a solution containing 1 m mol L-1 K3[Fe(CN)6] and 0.1 mol.L-1 KCl.

2.7 Scanning Electron Microscopy Scanning electron microscopy (SEM) analysis was performed by collecting images directly from the surface of the electrodes, using a Mira FEG-SEM (TESCAN) instrument equipped with an energy-dispersive X-ray (EDX) spectrometer.

SEM

images of the CNTs/PB films on indium tin oxide (ITO) electrosynthesized by potential cycling, were mounted on stubs with double-sided sticky carbon tape, (inner surface facing upward).

2.8 Atomic Force Microscopy (AFM) AFM images were obtained in the semi-contact mode using a NT-MDT NTEGRA AFM instrument (NT-MDT, Russia), in air and at room temperature. For image acquisition, a silicon tip (Veeco nanoprobe) was utilized with a radius of curvature

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