Method Article

Reductive Electropolymerization of a Vinyl-containing Poly-pyridyl Complex on Glassy Carbon and Fluorine-doped Tin Oxide Electrodes

DOI:

10.3791/52035

⸱

January 30th, 2015

 ,  , 
1Department of Chemistry, Virginia Military Institute

In This Article

Summary

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A procedure for performing reductive electropolymerization of vinyl-containing compounds onto glassy carbon and fluorine doped tin-oxide coated electrodes is presented. Recommendations on electrochemical cell configurations and troubleshooting procedures are included. Although not explicitly described here, oxidative electropolymerization of pyrrole-containing compounds follows similar procedures to vinyl-based reductive electropolymerization.

Abstract

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Controllable electrode surface modification is important in a number of fields, especially those with solar fuels applications. Electropolymerization is one surface modification technique that electrodeposits a polymeric film at the surface of an electrode by utilizing an applied potential to initiate the polymerization of substrates in the Helmholtz layer. This useful technique was first established by a Murray-Meyer collaboration at the University of North Carolina at Chapel Hill in the early 1980s and utilized to study numerous physical phenomena of films containing inorganic complexes as the monomeric substrate. Here, we highlight a procedure for coating electrodes with an inorganic complex by performing reductive electropolymerization of the vinyl-containing poly-pyridyl complex onto glassy carbon and fluorine doped tin oxide coated electrodes. Recommendations on electrochemical cell configurations and troubleshooting procedures are included. Although not explicitly described here, oxidative electropolymerization of pyrrole-containing compounds follows similar procedures to vinyl-based reductive electropolymerization but are far less sensitive to oxygen and water.

Introduction

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Electropolymerization is a polymerization technique that utilizes an applied potential to initiate the polymerization of monomeric precursors directly at the surface of an electrode and has been exploited to produce thin electroactive and/or photochemically active polypyridyl films on electrode and semiconductor surfaces.1-4 Electrocatalysis,5-10 electron transfer,11,12 photochemistry,13-16 electrochromism,17 and coordination chemistry18 have been investigated in electropolymerized films. This technique was first developed at the University of North Carolina in a Meyer-Murray collaboration for the electropolymerization of vinyl3,5,7,8,11-15,19,20 and pyrrole6,9,21-24 derivatized metal complexes on a variety of conducting substrates. Figure 1 presents a number of common pyridyl based ligands that, when coordinated to metal complexes, have produced electropolymers. In reductive electropolymerization, electropolymerization of vinyl containing compounds occurs upon the reduction of pyridyl ligands conjugated to vinyl groups, while with pyrrole-functionalized ligands, electropolymerization is initiated by oxidation of the pyrrole moieties, resulting in oxidative electropolymerization (Figure 2). Electropolymerization technology was developed with the goal of providing a generalized methodology for directly attaching virtually any transition metal complex to any electrode. The versatility of the method opens the door to numerous investigations of electropolymer modified electrodes.

In contrast to other attachment strategies, which involve direct bonding to the electrode, electropolymerization offers the advantage of not requiring electrode surface pre-modification. Therefore it can be applied to any number of conducting substrates, regardless of surface composition or morphology.4,10,25,26 This versatility is a result of changing physical properties as the polymer length grows; the monomers are soluble in the electrolytic solution but as polymerization occurs and cross-linking rigidifies the film, precipitation and physical adsorption the electrode surface occurs (Figure 3).27

Compared to oxide surface-bound carboxylate, which are unstable on oxide surfaces in water, or phosphonate-derivatized complexes, which are unstable at elevated pH's, commonly used in solar fuels research, these interfacial electrode-polymer film structures offer the added benefit of stability in a variety of media including organic solvents and water over a large pH range (0-14).28-30 Electropolymerization can also deposit films with large ranges of apparent surface coverages, from sub-monolayer to dozens or hundreds of equivalents monolayer, whereas carboxylate or phosphonate-derivatized complexes-interface structures are limited to monolayer surface coverages.

Although any number of vinyl or pyrrole containing pyridyl and polypyridyl compounds are capable of polymerization, [RuII(PhTpy)(5,5’-dvbpy)(MeCN)](PF6)2, (1; PhTpy is 4'-phenyl-2,2':6',2''-terpyridine; 5,5’-dvbpy is 5,5'-divinyl-2,2'-bipyridine; Figure 4) will be utilized as a model complex to demonstrate reductive electropolymerization on glassy carbon and fluorine-doped tin oxide, FTO, electrodes in this report. 1 is an example of a modern electropolymer precursor that has potential electrocatalytic applications and, due to its metal-to-ligand charge transfer, MLCT, absorption spectrum lying in the visible region of the light spectrum, can be investigated with UV-Vis spectroscopy.18,30 Please note that some results presented here for 1 have already been published in a slightly modified form.18

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Protocol

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1. Synthesize 1

Synthesize 1 (PhTpy is 4'-phenyl-2,2':6',2''-terpyridine; 5,5’-dvbpy is 5,5'-divinyl-2,2'-bipyridine; Figure 4) according to the procedure outlined previously.18

2. Prepare 1.3 mM Monomer Solution of 1 in an Electrolyte Solution

  1. Prepare a 0.1 M stock electrolyte solution of tetra-n-butylammonium hexafluorophosphate, TBAPF6, in acetonitrile, MeCN.
    1. Place MeCN over activated 3 Å molecular sieves, or K2CO3, for 24 hr to remove adventitious H2O.
    2. Place TBAPF6 (0.969 g, 2.50 mmol) in a 25.00 ml flame dried volumetric flask.
    3. Filter the molecular sieve or K2CO3 particulates from the dried MeCN and bring the 25.00 ml volumetric flask containing TBAPF6 to volume.
  2. Place 1 (0.0049 g, 5.2 x 10-6 mol) in and dry 4 dram vial or a 10 ml round bottom flask and add 4.00 ml of the stock solution of 0.1 M TBAPF6 in MeCN.
  3. Transfer 3.5-4.0 ml of the red-orange colored electrolytic solution of 1 into the central compartment of a 3-compartment cell, with each compartment separated by a medium porosity glass frit.
  4. Quickly fill the outer compartments of the 3-compartment cell to an equal height as the central compartment stock solution, with some of the remaining dry 0.1 M TBAPF6 in MeCN to prevent leakage to the outer compartments. Note: Time is an important factor because solutions in the different compartments will slowly mix and significantly change the concentration of the main compartment if the solvent heights are not the same.

3. Electropolymerize 1 on a 3 mm Diameter Glassy Carbon Electrode or a 1.0 cm2 FTO Electrode

  1. Prepare septa for nitrogen/argon degassing tubes and for electrodes.
    1. Cut a slit into each of the 3 rubber septa and guide a thin polytetrafluoroethylene, PTFE, tube through the slit.
    2. Slide the Ag/AgNO3 reference electrode through one of the septa, place the reference electrode/PTFE tube/septum in one of the outer compartments, and seal the compartment with the septum.
    3. Guide the platinum wire/gauze counter electrode through a different septum place the platinum wire/PTFE tube/septum in one of the outer compartments, and seal the compartment with the septum. If the slit is not sufficiently large or wire sufficiently stiff to prevent bending the wire, use a wide bore needle to guide the platinum wire counter electrode through the septum.
    4. Guide a freshly polished 3 mm glassy carbon electrode through the remaining septum and place it such that the electrode is suspended in the solution or, for an FTO slide, guide a wire connected to an alligator clip through the septum, then clamp the FTO slide with the alligator clip and make sure that the conductive side of the slide is perpendicular to the counter electrode when submerged.
      1. Prior to inserting the glassy carbon electrode: polish the glassy carbon by placing alumina (0.5 μm) on a wetted polishing pad, then, move the electrode in a figure-8 motion for 30 sec while holding the electrode perpendicular to the pad — to polish all sides of the electrode evenly — and rinse any remaining alumina off with a H2O water squirt bottle followed by an MeCN squirt bottle rinse.
      2. Prior to clamping the FTO slide: wrap several layers of non-conductive Kapton tape around the center part of a 30 x 10 mm FTO slide such that a 10 x 10 mm portion of the slide is exposed.
      3. Collect a UV-Vis spectrum of the FTO slide by placing/holding the FTO slide in a position in the beam path of the spectrometer that has been predetermined to ensure consistency.
  2. De-aerate the solutions in the 3-compartment electrochemical cell.
    1. Connect one end of Tygon tubing to the nitrogen/argon supply and connect the other end to a gas washer containing MeCN.
    2. Cut another piece of Tygon tubing, connect one end to the outflowing MeCN washed nitrogen/argon, and connect the other end to a 4 way splitter.
    3. Connect the PTFE tubes to the 3 remaining connections of the 4 way splitter.
    4. Submerge the PTFE tubes into the solutions in each of the compartments and turn on the flow of nitrogen/argon such that a rapid bubbling of the solution commences.
    5. Continue de-aerating the solution for 5-10 min, then pull the PTFE tubes just above the surface of the solution, leaving the flow of nitrogen/argon on in order to keep a positive pressure of inert gas on the system and to prevent solution convection caused by bubbling.
  3. Perform electrochemical experiments.
    1. Connect electrodes from the potentiostat to the appropriate electrodes in the 3‑compartment cell.
    2. Perform a cyclic voltammetry, CV, experiment with the following parameters: switching potentials = 0 V and -1.81 V; scan/sweep rate = 100 mV/sec; number of cycles = 5.
    3. When the CV experiment is complete, remove the working (glassy carbon or FTO) electrode from the polymerization solution and gently rinse the surface of the electrode with MeCN from a pipette or a squirt bottle to remove any remaining monomer solution.

4. Surface Coverage Determination

  1. Place the rinsed working electrode in a freshly prepared solution of 0.1 M TBAPF6/MeCN in an electrochemical cell containing a counter electrode and a reference electrode (preferably the same reference electrode used in the electropolymerization).
  2. Perform a cyclic voltammetry, CV, experiment with the following parameters: switching potentials = 0 and +1.5 V; scan/sweep rate = 100 mV/sec; number of cycles = 15.
  3. Integrate the charge under the anodic and cathodic peaks for the adsorbed electropolymer Ru(III/II) couple, average the charge under the anodic and cathodic peaks, and using Equation 1 determine the surface coverage.
  4. For the FTO slide: place/hold the FTO slide in the predetermined position in front of the UV-Vis sample holder such that the beam-path passes through the colored film. The FTO slide can be wet or dry but make comparisons under the same conditions as the blank spectra was collected under.
  5. Subtract the spectrum obtained for the FTO spectrum that was collected for that particular slide prior to electropolymerization from the spectrum of the film-on-FTO in order to produce an absorption spectrum for the film itself.

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Results

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Electropolymer growth is most easily recognized when observing the progress of the prescribed CV experiment (Protocol Text STEP 3.3.2). Figure 5 exemplifies electropolymer growth on a 0.071 cm2 (3 mm diameter) glassy carbon electrode with 1. The first cycle of the experiment produces a voltammogram roughly resembling that which is expected for a ruthenium solution of similar concentration (Figure 5, black trace) but upon successive cycles, through the 1st...

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Discussion

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Electropolymerization offers a large range of controllable variables that are not common to other techniques. In addition to standard reaction variables like reagent (monomer) concentration, temperature, solvent, etc., electropolymerization can be additionally controlled by electrochemical experiment parameters common to electrochemical methods. CV scan rates, switching potentials, and number of cycles affect the deposition of electropolymers. For example, as the number of cycles through the ligand reduction wav...

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Disclosures

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No conflicts of interest declared.

Acknowledgements

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We acknowledge the Virginia Military Institute (VMI) Department of Chemistry for support of electrochemical experiments and instrumentation (L.S.C. and J.T.H.). The VMI Office of the Dean of Faculty supported production fees associated with JoVE publications. We acknowledge the UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001011, for support of compound synthesis and materials characterization (D.P.H).

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Tetrabutylammonium hexafluorophosphate for electrochemical analysis, ≥99.0%Sigma-Aldrich86879-25G
Acetonitrile (Optima LC/MS), Fisher ChemicalFisher ScientificA955-4
3 mm dia. Glassy Carbon Working ElectrodeCH InstrumentsCH104
Non-Aqueous Ag/Ag+ Reference Electrode w/ porous Teflon TipCH InstrumentsCHI112
Platinum gauzeAlfa AesarAA10282FF 
Electrode Polishing KitCH InstrumentsCHI120
Cole-Parmer KAPTON TAPE 1/2 IN x 36 YDFisher ScientificNC0099200
Fisherbrand Polypropylene Tubing 4-Way ConnectorsFisher Scientific15-315-32B
500 ml Bottle, Gas Washing, Tall Form, Coarse FritChemglassCG-1114-15
3-compartment H-Cell for electrochemistryCustom made H-cell with 3 compartments

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Tags

Reductive ElectropolymerizationVinyl containing Poly pyridyl ComplexGlassy Carbon ElectrodesFluorine doped Tin OxideElectrochemical Cell ConfigurationCyclic VoltammetryUV Vis SpectroscopySurface Coverage AnalysisElectrode Surface ModificationSolar Fuels Applications

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