Experimental and Computational Study of Dopamine as an

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Experimental and Computational Study of Dopamine as an Electrochemical Probe of the Surface Nanostructure of Graphitized N-Doped Carbon James A. Behan, Md. Khairul Hoque, Serban N. Stamatin, Tatiana S. Perova, Laia Vilella-Arribas, Max Garcia-Melchor, and Paula E. Colavita J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05484 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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The Journal of Physical Chemistry

Figure 1. (a) Survey scans of anC, anC:N1 and anC:N2 electrodes. Insets correspond to high resolution scans of the N 1s regions. (b) High resolution C 1s scans of anC and anC:N electrodes. (c) Deconvolution of the C 1s envelope of anC; the raw data and envelope are offset relative to the components for clarity. 165x115mm (300 x 300 DPI)

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Figure 2. High resolution N 1s scans of (a) anC:N1, and (b) anC:N2, showing their deconvolution into two contributions; raw data and envelope are offset relative to the components for clarity. 82x57mm (300 x 300 DPI)


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Figure 3. (a) Raman spectra of anC and anC-N carbons; spectra are background corrected and offset for clarity. (b) Deconvolution of anC, anC:N1 and anC:N2 spectra. 82x115mm (300 x 300 DPI)



Figure 4. (a) CVs of GC, anC and anC:N films in solutions of 1.0 mM DA/0.1 M H2SO4 at 5 mV s-1. (b) Peak current densities (top) and ∆E values (bottom) derived from voltammetric measurements at 5 mV s-1 on all annealed electrodes studied. 82x115mm (300 x 300 DPI)


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Figure 5. CVs of 1.0 mM DA in 0.1 M H2SO4 for (a) anC, (b) anC:N1, (c) anC:N2 and (d) GC electrodes; ν = 50 1000 mV s-1. Insets in (a)-(c) are plots of peak current density (jp) vs. ν, showing a linear relationship. The inset in (d) corresponds to a plot of jp vs. ν1/2. 165x115mm (300 x 300 DPI)



Figure 6. (a) Logarithmic plot of the anodic peak current, ip,a, vs. ν, for anC and anC:N electrodes in a solution of 1.0 mM DA in 0.1 M H2SO4. The dashed lines represent theoretical slopes of m = 1 and m = 0.5 and are present to guide the eye. (b) Coverage values, Γ, of DA on the electrode surfaces calculated using equation (2) in the text. 82x115mm (300 x 300 DPI)


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Figure 7. Ball and stick representation of the DFT-modelled structures: (a) DA, (b) graphene, (c) graphene with a single carbon vacancy, (d) N-graphene with a graphitic centre site, (e) N-graphene with a graphitic valley site, and (f) N-graphene with a pyridinic vacancy.



Figure 8. Calculated adsorption Gibbs energies (∆Gads), in kcal mol-1 for DA. Top views of the optimized structures for the different adsorption modes and their corresponding ∆Gads values on graphene (a-c) and Ndoped graphene (d-f) model systems. 166x173mm (300 x 300 DPI)


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Table of Contents Graphic 85x35mm (221 x 221 DPI)



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Experimental and Computational Study of Dopamine as an Electrochemical Probe of the Surface Nanostructure of Graphitized N-doped Carbon James A. Behan,a Md. Khairul Hoque,a Serban N. Stamatin,a,b Tatiana S. Perova,c Laia VilellaArribas,a Max García-Melchora* and Paula E. Colavitaa* a

School of Chemistry, CRANN and AMBER Research Centres, Trinity College Dublin, College

Green, Dublin 2, Ireland. b

University of Bucharest, Faculty of Physics, 3Nano-SAE Research Centre, 405 Atomistilor

Str., Bucharest-Magurele 077125, Romania c

Department of Electronic and Electrical Engineering, Trinity College Dublin, Dublin 2, Ireland

and ITMO University, 49 Kronverskiy pr., Saint Petersburg, 197101, Russia.


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Abstract

N-doped carbon nanomaterials have received increased attention from electrochemists due to their applications in the metal-free electrocatalysis of important redox processes. In this work, a series of graphitized undoped and nitrogen-doped carbon electrodes prepared by thermal annealing of sputtered amorphous carbon films were prepared and characterized using a combination of X-ray photoelectron spectroscopy and Raman spectroscopy. Adsorption of the surface-sensitive redox probe dopamine at each electrode surface was then studied using cyclic voltammetry and the results correlated to the physico-chemical characterisation. Results indicate that dopamine adsorption is influenced by both the nitrogen surface chemistry and the degree of graphitization of the carbon scaffold. N-doping, with predominantly graphitic-N sites, was found to increase adsorption of dopamine more than 6 fold on carbon surfaces when the introduction of N atoms did not result in substantial alterations to the sp2 network. However, when an identical type and level of N-doping is accompanied by a significant increase in disorder in the carbon scaffold, adsorption is limited to levels comparable to those of nitrogen-free carbon. Density functional theory studies of dopamine adsorption on graphene and N-doped graphene model surfaces showed that dopamine interacts via π-stacking at the graphene surface. The Gibbs free energy of adsorption on N-doped graphenes were estimated at 12-13 kcal mol-1, and found to be approximately twice that of undoped graphenes. Results suggest that chemical changes resulting from N-doping enhance adsorption; however, high coverage values depend on the availability of sites for π-stacking. Therefore, the structurally disruptive effects of N-incorporation can significantly depress the dopamine response by limiting the availability of basal sites, ultimately dominating the overall electrochemical response of the carbon electrode.


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1. Introduction Nitrogenated carbon materials have received much attention in recent years, in large part due to the discovery of their activity in the metal-free electrocatalysis of important reactions including the oxygen reduction reaction (ORR) 1-2 and oxygen evolution reaction (OER).3-4 The synthesis of nitrogenated carbons is facile, versatile and can be carried out using low cost reactants, with nitrogen having been successfully incorporated into a variety of different nanocarbons including amorphous carbon,5-7 graphene,8-11 and carbon nanotubes.4,

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literature has been reviewed recently.13-14 Most nitrogenation protocols result in different nitrogen sites incorporated into the material. Nitrogen may be substitutionally incorporated into the carbon scaffold as a so-called graphitic-N site, resulting in the N-doping of the carbon scaffold, or in other chemical forms such as pyridinic-N and pyrrolic-N, both of which are necessarily associated with the formation of vacancies and edge sites within the carbon matrix.15 The existence of different N-sites poses a challenge to researchers carrying out structureactivity studies for electrocatalytic applications. A good example is the extensive work done on the ORR at nitrogenated carbon surfaces, which has focused on elucidating active sites for oxygen adsorption or attempting to correlate electrochemical activity to the presence of one or more N-moieties (most commonly pyridinic-N and graphitic-N) in the carbon matrix.6,

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contrast, factors such as the degree of graphitization, the size and packing of graphitic clusters and the number of defect sites present at the surface have not been considered in great detail, which may be attributed to the lack of a convenient electrochemical means of probing carbon nanostructures.


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The electrochemistry of catechols such as dopamine (DA) is notable for its high degree of sensitivity to the surface chemistry of electrodes. The work by McCreery and co-workers17-18 on glassy carbon electrodes shows that catechol adsorption on electrode surfaces may have a dramatic effect on the charge transfer kinetics, due to the self-catalysis of one or more of the steps in the ‘scheme of squares’ mechanism often used to describe the 2-electron/2-proton process of catechol oxidation.19-20 Recently, catechol has been shown to adsorb on graphene nanoplatelets21 and the adsorption of DA on graphene, in particular, has been investigated by means of theoretical calculations.22-23 Due to its biological relevance as an important neurotransmitter, DA has also been intensely studied in the context of electroanalysis and simultaneous detection in the presence of co-analytes such as ascorbic acid.24-26 DA also has relevance to the nitrogenated amorphous carbon literature in this context,26-28 because these carbons may be engineered to have low background currents and wide potential windows. 29 Interestingly, despite intense focus on tailoring carbon electrode composition and preparation for electroanalytical determinations of DA, comparatively less work has been devoted to investigating the potential application of DA as a probe of surface nanostructuring for N-doped carbons. In this work we prepared carbon model systems based on topographically smooth graphitized amorphous carbon with and without incorporated nitrogen, and characterized them using a combination of cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. We report what we believe to be the first use of DA as a probe for the surface structure of a disordered nitrogenated carbon material. Results indicate that DA adsorbs on both nitrogenated and nitrogen-free graphitized carbon surfaces, with both the surface chemistry and carbon nanostructure influencing the DA coverage. These results were corroborated by


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computational studies of DA adsorption on model graphitic clusters via density functional theory (DFT). Our results are expected to have relevance to both the electrochemical detection of DA at nitrogenated carbon surfaces, and the structure-activity characterization of carbon based materials for surface-catalysed processes such as the ORR. 2. Experimental Methods Chemicals and Materials. Dopamine hydrochloride (98%, Aldrich), Sulfuric Acid (≥95%, Ultratrace) Hexane (analytical standard) and Methanol (semiconductor grade) were used without further purification. Substrate Preparation. Glassy carbon (GC) disks (HTW Sigradur radius 0.25 ± 0.05 cm) were prepared by polishing with progressively finer grades of alumina slurry (Buehler), sonicating and rinsing with copious Millipore water as reported previously. 4 Clean disks were either used immediately for electrochemical measurements or, in the case of amorphous carbon and nitrogenated amorphous carbon depositions, mounted in a custom-made Teflon holder and placed in the vacuum chamber for coating via magnetron sputtering prior to characterization, as previously described.5 In the case of substrates for Raman and XPS measurements, B-doped silicon wafers (MicroChemicals; resistivity 5–10 Ω-cm) were prepared via previously reported protocols.5 Preparation of Carbon Electrodes. Undoped and N-doped amorphous carbon thin film electrodes were prepared by magnetron sputtering followed by a thermal annealing treatment. Briefly, the films were deposited via DC magnetron sputtering in a chamber (Torr International Inc.) with a base pressure ≤ 2 × 10-6 mbar and a deposition pressure in the range (2–7) × 10-3 mbar using a graphite target (99.999%, Lesker) as reported previously.5 Annealed amorphous carbon films with no N-doping, which will be denoted an-C, were deposited using an Ar plasma;


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N-doped films, denoted as an-C:N, were prepared using two different fluxes of N 2 gas in the deposition chamber: anC:N1 was prepared using 2% N2 gas in a total flux of 50 sccm Ar/N2 during the deposition, whilst 10% N2/Ar was used for an-C:N2. After deposition, the resulting films were transported directly to a tube furnace and annealed under N 2 atmosphere for 1 h at 900 °C. Characterization. Electrochemical measurements were carried out using a Metrohm Autolab AUT50324 potentiostat using a 3-electrode setup. A static disk holder (Pine Instruments) enclosing the GC disk with the carbon thin film was used as the working electrode as reported previously.5 A Hydroflex hydrogen electrode (Gaskatel) and graphite rod were used as reference and counter electrodes, respectively. The electrochemical cell was a five-necked jacketed cell (Pine Instruments) which had its temperature held constant at 25 °C using a recirculator. Prior to experiments, the cell was cleaned using Piranha solution (3:1 H 2SO4:H2O2 CAUTION: Piranha solution is a strong oxidant which may react explosively with organic solvents and must always be used in a fumehood) followed by rinsing with copious amounts of Millipore water. The cell was then rinsed three times with the electrolyte solution used during the experiment immediately prior to the analysis. Cyclic voltammograms (CVs) were acquired in a potential window of 0.48-1.2 V vs RHE in deaerated solutions of 0.1 M H2SO4 with and without DA in concentrations ranging from 25 μM to 1 mM. All voltammograms were taken with iR compensation using commercial software (NOVA) with the uncompensated resistance of 18 ± 1 Ω determined prior to each experiment using Electrochemical Impedance Spectroscopy (EIS). X-ray photoelectron spectroscopy (XPS) characterization was performed at 1 × 10-10 mbar base pressure in an ultrahigh-vacuum system (Omicron). The X-ray source was a monochromatized Al Kα source (1486.6 eV). Spectra were recorded at 45° takeoff angle with an


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analyser resolution of 0.5 eV. Spectra were baseline corrected using a Shirley background and fitted with Voigt functions using commercial software (CasaXPS); atomic percent compositions were determined by calculating peak area ratios after correction by relative sensitivity factors (C 1s = 1.0, N 1s = 1.8, O 1s = 2.93). Raman spectra were measured in backscattering configuration using a Renishaw 1000 microRaman system equipped with an Ar+ laser for 488 nm excitation. The incident beam was focused by a Leica microscope with a 50× magnification objective and short-focus working distance; incident power was kept

Experimental and Computational Study of Dopamine as an

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