Electrochemical Atomic Force Microscopy and First-Principles

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Electrochemical Atomic Force Microscopy and First Principles Calculations of Ferriprotoporphyrin Adsorption and Polymerization Jason A. Bennett, Daniel P Miller, Scott Simpson, Marcela Rodriguez, and Eva Zurek Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02059 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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Electrochemical Atomic Force Microscopy and First Principles Calculations of Ferriprotoporphyrin Adsorption and Polymerization

Jason A. Bennett*a, Daniel P. Millerb, Scott M. Simpsonc, Marcela Rodrigueza, Eva Zurekb

a b

School of Science, Penn State Behrend, 4205 College Drive, Erie, PA, 16563, USA

Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260, USA c

Department of Chemistry, St. Bonaventure University, St. Bonaventure, NY 14778, USA

____________________ * Corresponding author Tel.: 1-814-898-6123; Fax: 1-898-6213 Email address: [email protected] (J. A. Bennett)

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ABSTRACT

The adsorption and subsequent electrooxidative polymerization of ferriprotoporphyrin IX chloride (hemin; FePPCl) was investigated on highly ordered pyrolytic graphite (HOPG), glassy carbon (GC), and polycrystalline Pt electrodes using electrochemical atomic force microscopy, first principles calculations, and cyclic voltammetry. Hemin was shown to readily adsorb to all three surfaces, however it was more continuous over the carbon surfaces compared to the Pt surface. This disparity in adsorption appears to be a major contributing factor to differences observed between the electrodes following hemin electropolymerization. Despite differences in roughness and morphology, hemin polymerized as a continuous layer over each electrode surface. Periodic density functional theory calculations were used to model FePP (without Cl) on both the Pt(111) and graphite surfaces using the vdW-DF-optPBE functional to account for the dispersion interactions. Our calculations suggest that the FePP molecule chemisorbs to the Pt surface while at the same time exhibiting intramolecular hydrogen bonding between the carboxylic acid groups, which are extended away from the surface. In contrast to FePP-Pt chemisorption, FePP was found to physisorb to graphite. The preferred spin state upon adsorption was found to be S = 2 on Pt(111), whereas on graphite the high and intermediate spin states were nearly isoenergetic. Additionally, gas phase calculations suggest that much of the surface roughness observed microscopically for the polymerized porphyrin layer may originate from the nonparallel stacking of porphyrin molecules, which interact with each other by forming four intermolecular hydrogen bonds and through dispersion interactions between the stacked porphyrin rings. Regardless of polymer thickness, the underlying electrode appears to be able to participate in at least some redox processes. This was observed for the hemin-polymerized Pt electrode using the 2H+/H2 redox couple and was suspected to be due to some Pt surface atoms not being specifically coordinated to the hemin molecules and therefore available to react with H+ that was small enough to diffuse through the polymer layer.

Keywords: ferriprotoporphyrin, hemin, polymerization, density functional theory, surface adsorption, electrocatalyst 2 ACS Paragon Plus Environment

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INTRODUCTION Metalloporphyrins have been studied for a variety of applications including catalysis, chemical sensors, medicine, biotechnology, and water remediation to name a few.1-12 Additionally, there is specific interest in understanding the interaction between porphyrins and surfaces for a variety of new technologies.12-17 Interest in this class of compounds is generated by their stability, structural flexibility, and the ability to adjust the porphyrin properties for a particular application by altering either the central metal atom coordinated to the ring or the peripheral sustitutents.4,12,18 The metal atom is particularly important in electrocatalytic processes due to its redox potential.

The

electropolymerization

of

vinyl-substituted

metal

porphyrins

(e.g.,

metalloprotoporphyrins; MPP) has significantly aided in the application of these molecules. The procedure, originally described by Macor and Spiro, forms a MPP film via a radicalcation-initiated vinyl polymerization mechanism.19-20 While this was originally performed using protoporphyrin dimethyl esters in nonaqueous solution, Snyder and White adapted it to an aqueous sodium borate solution by using protoporphyrins with carboxylic acid groups instead of esters.21 They reported that polymerizing FePPCl from an aqueous Na2B4O7 solution by cycling the potential between 0 V and 1.0 V vs. SCE produced a continuous polymer layer over an HOPG electrode surface; however, it did possess a large number of deep holes or cavities.

The electrooxidative polymerization of MPPs is highly sensitive to the applied potential. For example, applying a moderately positive potential to FePPCl (e.g., 0.4 V vs. Ag/AgCl in Na2B4O7) produces the desired polymerization mechanism, and results in an electrically conducting film. However, applying too positive of a potential (e.g., 1 V vs. Ag/AgCl) results in oxidative degradation of the porphyrin through a 6-electron process that is initiated by the nucleophilic attack of H2O on FePPCl at the methine bridge position.20-21 This results in an insulating polymer film, which is not attractive for electrocatalytic applications. Our group has previously used the former procedure to polymerize FePPCl (from a 0.1 M Na2B2O7 solution) onto a Pt microelectrode followed by coordinating CN- ligands to the iron center in order to investigate its potential as an 3 ACS Paragon Plus Environment

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electrocatalytic material capable of selectively detecting H2S over CO and NO for physiological applications.3 In short, this material exhibited promise for successful detection of H2S at physiological levels, but lacked the response stability needed for practical applications. Scanning electron micrographs showed dark regions or holes in the polymerized porphyrin layer, similar to that observed by others,21-22 despite the less positive polymerization potential utilized. It was theorized that these suspected holes allowed the gases to still react at the exposed underlying electrode in addition to reacting at the porphyrin. This resulted in decreased selectivity and sensitivity (due to fouling of the Pt) over several trials.

The work presented herein focuses on studying the fundamental interactions between the FePPCl molecule and various electrode materials, and how these interactions influence the electrooxidative polymerization process. Electrochemical atomic force microscopy (ECAFM), in conjunction with first principles calculations, was used to study these interactions. The polymerization was monitored using ECAFM to study how the molecule polymerizes on each of the surfaces, with the overall goal of this study to determine whether the polymerization of hemin can be controlled and optimized in order to achieve a more stable response towards H2S oxidation. The adsorption and electrooxidative polymerization of hemin has been previously investigated using microscopy, however in those studies, the authors only investigated it on highly ordered pyrolytic graphite (HOPG), and scanned the potential to very positive potentials resulting in an insulating thin film.21-23 To our knowledge, this is the first microscopic investigation of the polymerization of a conducting FePPCl film, using an upper potential limit of 0.4 V, studied on different electrode surfaces. Density functional theory was used to better understand the interactions between the porphyrin and the electrode material, as well as between neighboring porphyrin molecules in an effort to more thoroughly explain the ECAFM observations.

EXPERIMENTAL Solutions


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All solutions were reagent grade and used as received. Sodium tetraborate decahydrate (borax; Alfa Aesar) was dissolved in ultrapure water and stirred for 10 min until fully dissolved. Ferriprotoporphyrin IX chloride (hemin; FePPCl; Frontier Scientific, Inc., Logan, UT) was then added to the borax solution and the resulting solution was sonicated for 10 min and stirred for 20 min; this yielded a solution containing 0.4 mM FePPCl in 0.1 M borax.

Electrochemical atomic force microscopy Electrochemical atomic force microscopy (ECAFM) was performed using Bruker’s closed electrochemical cell (EC Dimension Icon Cell) in conjunction with the Bruker Icon Atomic Force Microscope and a CH Instruments 842C Series Bipotentiostat (CH Instruments, Austin, TX). The working electrode was highly ordered pyrolytic graphite (HOPG), glassy carbon (GC), or polycrystalline Pt. A Pt and Ag wire served as the auxiliary and reference electrode, respectively. The details of the ECAFM set-up can be found in the Supporting Information and illustrated in Figure S1.

Once the cell and AFM were set-up, the EC cell was filled with the 0.1 M borax solution, and the electrode was subjected to potential cycling between 0.6 V and either 0 V (Pt) or -0.6 V (GC and HOPG) for 20 cycles at a scan rate of 0.1 V/s until a stable voltammetric i-E curve was obtained. The AFM probe was then engaged, and the electrode surface was imaged in the borax solution. All images in this study were acquired using peak force tapping mode and a silicon tip on a nitride cantilever (Bruker, SCANASYST-FLUID+). Once a representative surface image was obtained, the probe was disengaged, and the borax solution was replaced with fresh FePPCl solution. The AFM probe was then reengaged, and the surface was imaged in the FePPCl solution. After the initial image in the FePPCl solution, the probe was disengaged and mechanically raised out of the solution. Hemin was then polymerized using either a cyclic voltammetric or potential step approach. Once the potential was finished being applied to the working electrode, the AFM probe was lowered back into the solution and reengaged to image any changes on the electrode surface. An attempt was made at imaging the surface during the electropolymerization experiments; however, it was found that this resulted in fouling of the AFM probe tip. 5 ACS Paragon Plus Environment

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Therefore, the probe was mechanically raised during polymerization in only the z-direction and then reengaged to image the surface in close proximity (± 1 µm) of its previous location. All micrographs presented herein are 1 x 1 µm images with z-axis scales adjusted to be ±40 nm in order to make comparing the individual experiments easier. The root mean square roughness (RRMS) was used to quantitate the surface roughness in the micrographs.

Computational Methods It is worth noting that there is discrepancy in the literature on how hemin is ligated in aqueous solutions (e.g., -OH vs. -Cl under basic pH).16,21,24-29 This, as well as an effort to relate this work to our previous work on iron porphyrin (FeP),30 is the reason for our choice to model the FePP molecule without any ligands and protonated -COOH substituents. Spin polarized periodic DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) version 5.4.1.31 Each system was optimized with the vdWDF-optPBE functional,32 which self-consistently accounts for dispersion forces.33-34 The projector augmented wave (PAW) method35 was used to treat the core states along with a plane-wave energy cutoff of 500 eV. The C/N/O 2s/2p, H 1s, Fe 4s/3d, and Pt 6s/5d electrons were treated explicitly. Due to the computational expense involved, it was not feasible to include spin orbit coupling. In addition, it has been shown that spin orbit effects are not necessary to accurately model the adsorption of benzene to the Pt(111) surface.36-37 A Γ-centered Monkhort-Pack scheme was used to generate k-point grids. For computations involving iron, DFT+U was employed, with the Hubbard U term used to describe the dstates,38 where a Ueff = U – J = 3.0 eV was employed. This Ueff was previously shown to accurately reproduce the electronic and magnetic structure of iron porphyrin (FeP) in both the gas phase and on a Pt(111) surface, resulting in the correct S = 1 or S = 2 spin state of FeP, respectively.30,39-40

Geometry optimizations of FePP on models for the Pt(111) and graphite surfaces were performed using a 3x3x1 k-point mesh. A three-layer Pt(111) slab containing 168 Pt atoms with a ~30 Å vacuum space was used to simulate the Pt(111) surface. The two bottom layers of the surface were kept fixed, whereas the top layer and the molecule were


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allowed to relax during the optimization. The binding site study employed a graphite model that consisted of a two-layer system with 324 C atoms, wherein the bottom layer was kept fixed, but the top layer and the molecule were allowed to relax. For the spin state study, a single layer with 162 C atoms that was allowed to fully relax was used instead. A vacuum space of at least ~30 Å was employed in all of the calculations. Because the layers in graphite, whose experimental interlayer distance measures ~3.4 Å, interact with each other via weak dispersion forces, including a second layer is unlikely to affect the spin state of the adsorbate. For example, recent DFT calculations that used a finite C28H14 aromatic model for the surface successfully studied the interaction between FePPIX (iron protoporphyrin IX) and graphite, including the change in the spin state.17 The vdW-DFoptPBE optimized lattice constant for Pt and the experimental lattice constant for graphite were used for building the models for the Pt(111) and graphite surfaces. The surface coverage was approximately 0.258 and 0.236 molecules/nm2 for the models of the Pt(111) and graphite surfaces, respectively. The adsorption energy (∆Eads) of the molecule adhering to the surface was determined by:

∆ = − −

(1)

where EFePP – Surf is the energy of the ferriprotoporphyrin adsorbed to the respective surface, EFePP is the energy of the isolated FePP molecule, and ESurf is the energy of either the Pt(111) or graphite models.

Unrestricted molecular calculations were carried out using the Amsterdam Density Functional (ADF) software package41-42 with the revPBE generalized gradient density functional, in conjunction with the Grimme343 dispersion correction (revPBE-D3). The basis functions on all of the atoms consisted of a triple-ζ Slater-type basis set with polarization functions (TZP) from the ADF basis-set library. The core shells up to 1s for C/N/O and 3p for Fe were kept frozen. The binding energy (∆EBE) between two gas phase FePP molecules was determined by:


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∆ = ∙ − 2

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(2)

where E2·FePP is the energy of the optimized FePP dimer, and EFePP is the energy of an isolated FePP molecule. Since the FeP and FePP molecules prefer the S = 1 spin state in the gas phase, the FePP dimer geometries considered were constrained to the S = 1 state for each FePP in the overall calculation.

RESULTS AND DISCUSSION The overall goals for this study were to evaluate (i) FePPCl adsorption on different electrode materials, (ii) whether the electrode material impacts the FePPCl polymerization process and (iii) whether the polymerization process can be controlled in order to obtain a more electrochemically useful layer. Three working electrode materials were utilized in this study; highly ordered pyrolytic graphite (HOPG), glassy carbon (GC), and polycrystalline Pt. HOPG and GC were chosen as representative sp2 hybridized carbon surfaces that are either ordered or disordered, respectively. HOPG was chosen because it possesses two distinct electrochemical regions in its basal and edge planes. It is well known that the electrochemical behavior of this material at its basal plane is heavily dependent on the presence of defects (e.g., pits, step edges, etc.).44 A freshly cleaved basal plane with minimal surface defects results in a surface that exhibits lower background capacitance and generally slower electron transfer kinetics. In contrast, GC is a more disordered sp2 hybridized carbon material that possesses a ribbon-like structure.44-45 This disorder results in a more electrochemically homogeneous surface that, if properly polished, exhibits higher background capacitance and generally more facile electron transfer kinetics than the HOPG basal plane.44 These different carbon materials in addition to polycrystalline Pt were used to determine whether the hemin adsorption/polymerization processes are affected by the chemical and/or physical characteristics of the supporting electrode material.

Hemin Adsorption The adsorption of hemin was first monitored by imaging the respective electrode surface in the 0.1 M borax background solution, and then again after filling the cell with the 0.4 mM hemin/0.1 M borax solution. Figure 1 shows this comparison. In the borax solution, 8 ACS Paragon Plus Environment

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both the HOPG and Pt electrodes (Figures 1A and 1C, respectively) exhibit relatively ordered surfaces with some visible defects. The HOPG was the smoothest material with an average roughness of 0.35 nm and easily identifiable basal planes and step edges. We were particularly interested whether these two different attributes, which are known to behave very different electrochemically, would behave differently with regards to the hemin coating process. In contrast, the GC electrode surface (Figure 1B) is very homogeneous in nature (both physically and electrochemically), exhibiting an average roughness of 0.87 nm. As expected, both carbon electrodes were smoother than the polycrystalline Pt electrode, which exhibited a roughness of 1.9 nm.

The ECAFM images clearly show that upon introducing the hemin solution (no applied potential), the porphyrin readily adsorbs to all three of the electrode surfaces as each electrode surface becomes distinctly rougher. The presence of surface-adsorbed hemin was confirmed using X-ray photoelectron spectroscopy (XPS). The micrographs of HOPG and Pt surface no longer display the ordered surface with distinct defects as seen in the borax solution. This is similar to what others have previously observed on HOPG.21-23 Both carbon surfaces (Figures 1D and 1E) exhibit consistent adsorption of individual islands over the entire surface. The adsorption is independent of surface defects in the case of the HOPG, and the surface roughness is determined to be 0.63 nm, about double of that found in borax. The GC surface (Figure 1E) also exhibits relatively consistent adsorption over the entire surface with the roughness increasing to 1.2 nm.

The Pt electrode (Figure 1F), however, shows a very interesting adsorption phenomenon compared with the two carbon samples. While it appears that there is hemin over the entire surface, there seems to be more preferential adsorption sites, and this creates a much rougher surface than for either carbon electrode; exhibiting a roughness of 3.2 nm. Comparing the individual cross-section for each of the images further supports this. Figures 1G (HOPG), 1H (GC), and 1I (Pt) compare the cross-section for each electrode surface from Figures 1A – 1F, respectively. All three surfaces exhibit hemin adsorbed as aggregates that are ca. 5 nm wide at the tip, in agreement with what Snyder and White observed on HOPG via STM,21 and ca. 30 nm wide at the base. The peak height of the hemin 9 ACS Paragon Plus Environment

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layer is about 1 nm on the carbon electrodes and 3-4 nm on Pt. It is clear that while the roughness increases for all surfaces, the carbon surfaces appear to have a higher density of topographical peaks that are shorter than that observed on the Pt surface. In contrast, the Pt surface shows very defined peaks that are more separated than in the carbon materials. This suggests that while hemin readily adsorbs on all 3 surfaces, the carbon surfaces exhibit more consistent adsorption over the entire surface.

In an effort to better understand the interaction between the hemin and the underlying electrode surface, a series of computations were carried out to model the interaction between an FePP molecule and both the Pt(111) and graphite (sp2 carbon) surface models. The iron porphyrin (FeP) structure (no substituents) was previously optimized on Pt(111) using nonlocal density functional theory calculations that treat the strong on-site 3d electron-electron interactions on Fe via a Hubbard Ueff = 3.0 eV.30 The previous work on FeP concluded that both the vdW-DF-optPBE and vdW-DF-optB88 functionals yielded the same lowest energy binding site for the FeP on the Pt(111) surface, and comparable binding energies. It also showed that the stability of the binding sites depended on the number of Fe–Pt and C–Pt bonds that were formed, and that the elongation of the Fe–N bonds that occurred upon adsorption resulted in an increase in the spin state of the Fe atom from S = 1 to S = 2. This work was used as a starting point for optimizing the FePP molecule and its adsorption to both surfaces, especially because both the gas phase FeP molecule from ref. 30 and FePP molecule prefer the S = 1 electronic state. Recently Stochastic-CASSCF calculations on FeP in the gas phase showed that the S = 1 state is stabilized over the S = 2 via charge-transfer excitations.46

Due to the large computational cost involved in optimizing FePP adsorbed to Pt(111), we assumed that its most stable binding site would be the same as that found for FeP on Pt(111), where the iron atom lies on top of a bridge (B) site (along the center of the bond connecting two neighboring Pt atoms on the top layer and aligned with a Pt atom in the second layer) and rotated 45o (B45) with respect to the [100] lattice plane of the surface.30 The vdW-DF-optPBE optimized geometry of the FePP-Pt(111) system is displayed in Figure 2. The top view shows FePP adsorbed to the B45 site, while the side 10 ACS Paragon Plus Environment

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view images show the interactions between FePP and the Pt(111) surface as a function of the Fe spin state. This set of calculations yields important insight with regards to understanding the initial porphyrin adsorption in the microscopic images. First, the FePP exhibits intramolecular hydrogen bonding between the carboxylic acid groups, which are extended away from the Pt surface with O–H•••O hydrogen bond distances of ~1.65 Å. Second, in agreement with the FeP-Pt(111) system, the spin state of the iron changes from S = 1 to S = 2 upon adsorption.30 The FePP-Pt(111) system with a spin of S = 2 is calculated to be 4.20 kcal/mol more stable than the S = 1 spin state. This is comparable to the 10.20 kcal/mol difference in favor of the S = 2 spin state over the S = 1 state previously found for the FeP-Pt(111) system.30 The spin state change is a direct result of the Fe–N bonds elongating due to Fe–Pt bond formation in the S = 2 FeP and FePP geometries.

The FePP molecule appears to be strongly chemisorbed to the Pt surface in the computations. Classifying this interaction as chemisorption is justified by the significant distortion the molecule undergoes from its gas phase geometry, C–Pt and Fe–Pt bond formation and C–C bond length changes within the porphyrin ring. The adsorption energy (∆Eads) is calculated to be -139.36 kcal/mol for the S = 2 and -135.16 kcal/mol for the S = 1 state, respectively. This is similar to that calculated for the FeP-Pt(111) system of -105.13 kcal/mol and -94.93 kcal/mol for the S = 2 and S = 1 spin states, respectively.30 The interaction of FePP with the surface is stronger than that of FeP because of the increased number of C–Pt bonds (7 vs. 6 C–Pt distances shorter than 2.34 Å, respectively) and the larger dispersion forces inherently present between the larger molecule and the surface.

In comparison, the FePP molecule was found to adsorb weakly to the graphite surface model. The FePP was optimized on different binding sites of a two-layer graphite slab to determine the most stable binding configuration. The most energetically preferred binding site found was where the iron atom lies above a carbon atom (a top site, T) and rotated 0o (T0) with respect to the z-axis of the surface. The different possible sites considered, as well as both the top and side view of the optimized geometry of FePP to the graphite surface for the intermediate (S = 1) and high (S = 2) spin states are shown in


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Figure 3. For the sites considered herein, the corrugation energy was calculated to be at least 1.46 kcal/mol, highlighting a weak site preference for FePP to the carbon surface. The adsorbate geometry had minor distortions when compared to the gas phase FePP geometry, but the S = 2 spin state (Figure 3D) with a [(dx2

– y2)1(dxy)1(dπ)2(dz2)2]

configuration was slightly preferred (in the coordinate system used, the dx2

– y2

orbital

points towards the nitrogen atoms). The S = 1 spin state (Figure 3C) with a configuration of [(dxy)2(dπ)2(dz2)2] is only 0.38 kcal/mol less stable. Thus, considering the error inherent in our calculations, the two spin states are virtually isoenergetic. The Fe-N bond distances for the S=1 state was 2.01-2.02 Å, whereas for the S=2 state they were slightly longer, 2.062.08 Å. In a related study that considered the interaction of an iron porphyrin (FeP) molecule deposited at a divacancy site in graphene, the high spin state was clearly preferred because of the elongation in the Fe-N bond upon chemisorption.47 The reason why these results differ from ours is whereas we considered a pristine graphite surface, Ref 47 considered a divacancy site.

In our work, the iron center remains relatively planar with the porphyrin ring, in stark contrast to the Pt(111) system, even for the S = 2 spin state. The Fe–C distance (3.44 Å) is much longer than the Fe–Pt distance (2.72, 2.78 Å). The adsorption energy was determined to be -71.63/-72.01 kcal/mol for the S = 1/S = 2 FePP-graphite systems, respectively. This provides further justification that the molecule is physisorbed on the graphite surface model, as these adsorption energies are roughly half that of the FePPPt(111) adsorption energies mentioned earlier. It is important to note that the weaker FePP-graphite physisorption compared to the FePP-Pt(111) chemisorption may explain the differences in roughness observed between the Pt and carbon surfaces in Figure 1 for the adsorbed hemin ECAFM images.

To investigate the charge transfer that occurs upon adsorption, we calculated the Bader charges on the surface and adsorbate, along with the difference in the charge density that occurs upon adsorption. This information is presented in the SI Figures S2 and S3. As expected for a physisorbed species, there is little charge transfer between FePP and the


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graphite surface model (Figure S2) for either spin state, except at the Fe atom. On Pt(111) (Figure S3), however, the FePP assumes a charge of +0.73 e and +0.78 e for the S=1 and S=2 states, respectively. The significant charge reorganization between the surface and adsorbate is indicative of chemisorption, as seen through the change in charge at the Fe atom as the isolated FePP, S=1 FePP-Pt(111), and S=2 FePP-Pt(111) have charges of +1.06 e, +1.17 e, and +1.32 e, respectively.

It is worth noting that an alternative S = 2 spin state with a configuration of [(dx2 – y2)1(dxy)2(dπ)2(dz2)1]

was uncovered for the FePP-graphite system, which is 9.63 kcal/mol

less stable than the more preferred S = 2 spin state. The major difference between these S = 2 spin states is their electronic configuration and the Fe–C distance being 3.48 Å for the less preferred S = 2 spin state, a difference of only 0.04 Å. The energetically preferred state favors a configuration with a singly occupied dxy orbital and a doubly occupied dz2 orbital, while the unfavored S = 2 state favors the opposite occupation of these two d-orbitals. Worth noting is that the S = 1 spin state of the FePP-graphite system is also influenced by the substrate as one of the doubly populated d-orbitals is the dz2 orbital, whereas in the gas phase it is singly occupied. Thus, despite the fact that the molecule interacts with the substrate via dispersion interactions, the presence of the graphite surface is strong enough to produce ligand-field effects and perturb the orbital occupation.

The calculated relative energies of the FePP molecule in the gas phase with different spin states, as well as the modeled surface-adsorbate systems as a function of spin state are summarized in Table 1. Projected densities of states plots for the FePP-Pt(111) and FePPgraphite systems are shown in SI Figures S4 and S5, respectively.


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Table 1: The spin state preference for FePP in the gas phase and adsorbed to the Pt(111) and graphite model surfaces. The energy of different spin states is given relative to the most stable configuration found along with the adsorption energy (∆Eads) of the FePP molecule to the corresponding substrate.

Gas

∆E (kcal/mol)

S=0

36.93

S=1

0

S=2

10.36

Pt(111)

∆E (kcal/mol)

∆Eads (kcal/mol)

S=1

4.20

-135.16

S=2

0

-139.36

Graphite

∆E (kcal/mol)

∆Eads (kcal/mol)

S=1

0.38

-71.63

S=2

0 (9.63)*

-72.01 (-62.38)*

*denotes the value obtained for the unfavorable S = 2 state on graphite in parenthesis.

While in-depth spectroscopy of the hemin-surface interface is beyond the scope of this work, UV-vis spectroscopy was used to confirm a change in the electronic structure of the porphyrin upon surface adsorption. This is illustrated by the spectra shown in Figure S6 of the Supporting Information. The spectrum of a 5 µM hemin/borax solution resulted in a spectrum that possessed two characteristic Soret bands at 364 nm and 385 nm in addition to Q bands at 489 and 608 nm. Upon adsorption of hemin onto an optically transparent indium tin oxide electrode (ITO), only the 364 nm band exhibits a hypsochromic shift to 346 nm, while the other Soret band remains at 385 nm and the Q bands coalesce into a single peak around 588 nm. After polymerizing the hemin, the peak positions in the spectrum remain relatively unchanged from the adsorbed spectrum. This shift indicates a change in the electronic structure upon surface adsorption and would be consistent with the first principles calculations. It is important to note that the spectral changes observed here contradict that observed by Snyder and White for their adsorbed


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FePPCl film on SnO2 but do agree with their findings after electrooxidative polymerization.21

Electrooxidative Polymerization of Hemin We previously reported polymerizing hemin using cyclic voltammetry (25 cycles) followed by coordinating cyanide ligands to the iron center in order to utilize it as an electrocatalyst for selective H2S detection.3 To better understand the polymerization process, the modification of each electrode material was monitored during this cyclic voltammetric electrooxidative polymerization process. After the image of the adsorbed hemin was obtained (Figure 1D, 1E, or 1F, respectively), the AFM probe was raised out of the solution in only the z-direction (to keep the porphyrin from adsorbing to or polymerizing on the AFM tip) and the potential was cycled from -0.3 V to 0.4 V at a rate of 25 mV/s. The cycling was paused after the second cycle in order to image the surface before continuing the cycling for 23 additional cycles. Representative i-E curves for the polymerization process (cycles 1, 5, 10, 20, 25) on each electrode are shown in Figure 4. Quantitative comparisons between the current generated at the 3 materials is not possible because the nature of the cell and experimental set-up (see the Supporting Information for details) did not allow for a defined surface area. Additionally, the edges of the electrodes would contribute to the current observed and all electrodes possessed a different thickness. However, qualitatively, all electrodes exhibit a relatively flat background current, and an increase in anodic current at ca. 0.15 – 0.20 V with increasing scan number. This increase is consistent with the radical-cation vinyl coupling mechanism followed by attachment or precipitation of the product resulting in an increasingly thick conductive polymer layer.19,21 Interestingly, the i-E curve for HOPG (4A) showed the most well resolved oxidation signal that increased in both current and onset potential with increasing cycle number. This may be due to the more ordered surface of HOPG and subsequently more ordered hemin adsorption.

Figure 5 compares the three electrode surfaces in the hemin solution following 2 cyclic voltammetric polymerization cycles (A, C, and E) and then following the additional 23 cycles (B, D, and F). The two-cycle image for the carbon electrodes (5A and 5C) do not 15 ACS Paragon Plus Environment

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appear much different than their respective adsorbed image in Figure 1D and 1E; however, both do exhibit slightly rougher surfaces of 0.87 nm for the HOPG and 1.9 nm for the GC. Additionally, their individual cross-sections (not shown) both exhibit porphyrin peak heights of ca. 2-3 nm, similar to that reported by Duong et al. after cycling the potential once up to 1.1 V.22 The Pt electrode shows a much rougher surface and apparently more heterogeneous polymerization across the surface. Its cross-section (not shown) exhibited fewer aggregates that were wider at the base. This suggests that new nuclei are not forming with the applied potential, but rather aggregates are growing together. Interestingly, all 3 electrodes exhibit approximately 2-2.5x higher roughness than their initial roughness in borax. This would suggest that the initial adsorption of hemin is the primary factor in the morphology of the polymerized porphyrin surface as the rate of polymerization, at least initially, appears to be similar on all 3 surfaces.

After 25 cycles, all 3 electrodes exhibit extremely rough surfaces and it is clear that the porphyrin is heterogeneously polymerized over the surface of each electrode. Interestingly, HOPG (Figure 5B), which possessed the smoothest initial surface, exhibits the highest roughness of the three (15 nm), followed by Pt (10 nm) and GC (6.6 nm). The fact that HOPG exhibits the largest relative increase in roughness (by almost a factor of 23) likely suggests a faster overall growth rate on the time scale of the polymerization. This could be due to the high order of carbon atoms (compared to GC) and the most uniform adsorption (compared to Pt) over the surface.

It is clear that the porphyrin on either carbon electrode exhibits more individualized islands that have grown up as opposed to growing out and coalescing. This is further illustrated in Figure 6, which compares the representative cross-sections of the micrographs obtained after the 25-cycle polymerization shown in Figure 5B, 5D, and 5F, respectively. The cross-section comparison shows that the GC electrode (middle line; light grey) has a relatively consistent hill/valley profile over the entire scan, with most peaks not reaching heights greater than 10 nm. In contrast, the HOPG electrode (top line; black) possesses very tall, individualized peaks that reach heights approximately 40-50 nm high. The Pt surface (bottom line; dark grey) exhibits even larger features that appear as much 16 ACS Paragon Plus Environment

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wider agglomerates of several hundred nanometers. While Duong et al. concluded that much of the electrode surface was left uncovered by the porphyrin layer,22 we suspect this is not the case here when considering the rather continuous adsorption over each surface. This may be due to applying a lower potential and producing a conducting film here as opposed to the very positive potential and passivated film produced in their study. Additionally, the traces in Figure 6 show much more gradual changes in height as opposed to the well-defined peaks in Figure 1. It is important to note however, that the porphyrin surface is considerably rough and so it is possible that there are areas of the underlying electrode that remain uncovered by the porphyrin that cannot be reached by the AFM probe. Therefore, while the absolute coverage of the underlying electrode cannot be confirmed, it is clear from Figures 5 and 6 that a majority of the electrode surface is covered.

Cyclic voltammetry does not adequately control the polymerization process; however, it is unknown whether this impacts the electrochemical properties of the porphyrin layer. In an effort to better control the polymerization, a potentiostatic approach was investigated. The thought was that slower growth would allow for better control and minimize the extreme roughness exhibited through cyclic voltammetry, similar to the work performed by Penner and coworkers for controlling metal deposition.48-49 They applied a particular potential for a short duration to nucleate the particles, and then lowered the magnitude of the potential for longer times to grow the metal particles. This resulted in slower growth and particles of a narrower-size distribution because the depletion layer around each particle was reduced in size and therefore had less competition between nearby particles for diffusing metal ions. While this is not a direct comparison to the hemin polymerization here, it would be reasonable to assume a similar phenomenon could occur. By holding the potential at a lower magnitude, the polymerization would be controlled by the electron transfer kinetics and not by the diffusion of the porphyrin molecules to the electrode surface. This should result in a more uniform polymerization over the entire surface.


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Hemin polymerization was initially attempted using the lowest possible applied potential. Based on the increase in oxidation current shown in Figure 4, the onset for the polymerization process was determined to be ~ 0.2 V. However, holding the potential at values less than 0.4 V did not produce a polymerized porphyrin layer. It is clear that there is an energy barrier that must be overcome in order to get the adsorbed FePPCl to begin polymerizing and grow in thickness. In order to initiate the polymerization, but slow the growth rate, a potential of 0.4 V was applied for 2 s to begin the polymerization process and then stepped to 0 V for 20 s to replenish the diffusion layer before stepping to a potential of 0.3 V for 50 s to grow the polymer layer.

All three electrodes exhibited similarly more consistent polymer growth compared to their cyclic voltammetric counterpart; therefore, only HOPG is shown in Figure 7. As expected, the multi-potential step approach (Figure 7A) produces a significantly smoother polymer layer compared to the 25-cycle surface. This translates into more consistent polymerization over the entire surface. The HOPG electrode exhibits a roughness of only 1.3 nm (compared to a value of 0.80 nm after initial FePP adsorption for that individual sample confirming film growth), while the GC electrode, the most homogeneous surface of the three, exhibited a roughness of only 1.0 nm. The Pt electrode exhibited a roughness of 4.7 nm. These results also suggest that the heterogeneity of the surface plays an important role in the polymerization process. The most electrochemically homogeneous surface exhibited the smallest relative increase in roughness compared to that in borax (1.1x increase), while HOPG, the most electrochemically heterogeneous material exhibited a 3.7x increase in roughness. This is explained by an individual porphyrin molecule approaching the electrode surface and being equally likely to polymerize to several sites on the homogeneous surface as opposed to having preferential polymerization sites on the heterogeneous surface due to faster electron transfer kinetics.

Overall, the multi-potential step approach yielded a roughness slightly higher than that observed after 2 cyclic voltammetric cycles. It is important to note that the cyclic voltammetric polymerization time is estimated to be 300 s (assuming polymerization occurs between 0.1 and 0.4 V at a sweep rate of 25 mV/s for 25 cycles) compared to the 52 18 ACS Paragon Plus Environment

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s of polymerization used for Figure 7A. While a quantitative growth rate comparison is not possible, it is clear that the decrease in roughness is due to more controlled polymerization over the entire surface due to replenishing of the diffusion layer. This is further supported in Figure 7B, which shows the HOPG surface after an additional 50 s at 0.3 V. It is clear that while the surface roughness increases slightly to 1.9 nm, the overall look of the surface is quite similar to that in 7A and very different then that obtained using cyclic voltammetry. This suggests that the multi-potential step approach can help control the overall roughness of the polymer layer. This may be important in future applications.

In an effort to obtain a better understanding of the potential interactions between neighboring porphyrin molecules as they diffuse towards the electrode surface, a series of gas phase molecular calculations were conducted on dimers. It was determined that the two neighboring molecules could interact by forming anywhere from 2 to 4 H-bonds, and in the case of the 4 H-bonds, the molecules could be arranged in two different configurations; stacked on top of each other or lying apart. These four possible arrangements along with their calculated relative energies are shown in Figure 8. From the relative energies, it is clear that the FePP molecules prefer to form 4 H-bonds while at the same time stacking on top of each other so as to maximize dispersion interactions. The energy of the preferred geometry is set to 0 kcal/mol in Figure 8. The binding energy (∆EBE) of the 4 H-bond stacked FePP dimer is calculated to be -37.09 kcal/mol. The intermolecular hydrogen bonding is important in stabilizing the dimers as having only 2 or 3 H-bonds between the two FePP molecules weakens the binding energy with the relative energy to the most stable 4 H-bond stacked structure being 21.72 kcal/mol (∆EBE = -15.37 kcal/mol) or 11.65 kcal/mol (∆EBE = -25.44 kcal/mol), respectively.

The FePP molecules in the dimer are not perfectly parallel to each other in the stacked 4 H-bond arrangement in Figure 8, likely because this geometry attempts to maximize both the hydrogen bonding and dispersion interactions. This configuration is 23.55 kcal/mol more stable than if there was no stacking interaction (∆EBE = -13.54 kcal/mol), indicating the strength of the dispersion interactions in the dimer. Interestingly,


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the dimer with 4 H-bonds in a non-stacking motif was found to be less stable than a system with just 2 H-bonds with the FePP dimer stacked. The fact that the molecules are not parallel to each other in the stacked dimer may possibly contribute to the increased roughness with increased growth of the FePP layer. It appears that while maximizing the Hbonding between FePP molecules is important, the stacking arrangement is the more crucial driving factor that contributes to the overall appearance of the polymerized porphyrin. This computational analysis of FePP dimerization was only carried out using molecular calculations because periodic calculations would have been prohibitively expensive, and because including the surface is unlikely to change the conclusions. The most stable geometries for a single FePP chemisorbed to Pt(111) and physisorbed to graphite have the carboxylic acid groups well above the molecular plane. Thus, the monomer assumes an ideal configuration to dimerize with a second FePP molecule via hydrogen bonding and dispersion interactions in the same fashion as found within the gas phase as shown in Figure 8.

Electrochemical Probing of the Polymerized Hemin Electrode In addition to understanding the hemin polymerization, we were interested in the electrochemical response of the porphyrin-coated electrodes. The previous paper that focused on using cyanide-coordinated FePP for selective H2S detection theorized that a possible reason for insufficient selectivity over CO and NO interferences, as well as the lack of response stability towards H2S, was possibly due to the electropolymerized layer not sufficiently covering all areas of the underlying Pt electrode.3 Scanning electron micrographs (SEM) of both freshly-polymerized and cyanide-coordinated FePP surfaces showed dark regions that were thought to be holes in the polymer layer leaving the underlying Pt exposed to the solution analytes. These regions were consistent with what others had previously observed on carbon electrodes and what has been computationally predicted for aromatic molecules on various metal surfaces.21-22,50-53 However, the continuity observed in the conductive polymerized porphyrin layer in the AFM images in Figures 1 and 5 suggest that those dark regions observed in the previous SEM images were not large holes leaving the underlying electrode exposed, but rather deep cavities that result in a roughness beyond the resolution of the microscope. This would lead one to think 20 ACS Paragon Plus Environment

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that the underlying electrode would be minimally exposed to participate in surfacesensitive redox reactions as originally suspected. Therefore, the practical question remains as to why the observed response to H2S decreased despite the electrode surface possessing an extensive coating.

To investigate the ability of the underlying electrode to participate in redox reactions despite a complete porphyrin coating, the polymerized Pt electrode was probed using 5 mM HCl in 0.5 M KCl and cycling the potential between 0 V and -0.7 V at 50 mV/s. This is a surface-sensitive reaction that exhibits a very reversible redox couple at ca. -0.38 V vs. Ag/AgCl due to the 2H+/H2 redox reaction on clean Pt, but which becomes increasingly irreversible as the surface is coated.54-56 For example, this redox couple was previously used to ascertain the extent of electrochemically reduced graphene oxide (ERGO) coverage, an sp2 hybridized carbon, deposited on clean Pt electrodes.54,57 In those studies, an absence of this redox couple signified complete ERGO coverage of the underlying Pt electrode.

As with the ERGO studies, the hemin-coated Pt electrode was not expected to exhibit the 2H+/H2 redox couple, or at least exhibit a significantly lower peak current and increased peak separation. However, when FePPCl was polymerized using either 25 voltammetric cycles or the potentiostatic procedure described above, the electrode in each case still unexpectedly exhibited a reversible 2H+/H2 redox signal (not shown). To assure that this was not due to a relatively thin polymer layer, the FePPCl was then polymerized onto two Pt electrodes; one using 100 voltammetric cycles (this exhibited an increasing oxidation signal at ~0.2 V for all 100 cycles) and one using a potential step protocol where the potential was stepped to 0.4 V for 10 s to initiate polymerization, stepped to 0 V for 20 s, and then held at 0.3 V for 1200 s. These conditions were chosen so that electrodes coated by both methods had similarly long polymerization times. Both electrodes exhibited a visibly reddish surface confirming a very thick FePP layer.

Figure 9 compares the i-E curves in HCl/KCl for these electrodes to that obtained at a clean Pt electrode. Surprisingly, both porphyrin-coated electrodes still exhibit a well21 ACS Paragon Plus Environment

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defined reversible redox signal at ca. -0.375 V that is very similar to that exhibited by the clean Pt electrode. There is a slight increase in the ∆Ep between the 3 electrodes with the clean Pt exhibiting a Nernstian ∆Ep of 59 mV compared to 68 mV and 79 mV for the potentiostatic and cyclic voltammetric coatings, respectively. Interestingly, there is very little change in the peak current. We suspect this is due mainly to the slower diffusion through the porphyrin layer. The small increase in ∆Ep and decrease in peak current, as well as the increase in double layer current are all consistent with an increasingly thick FePP layer. Therefore, considering the extreme polymerization thickness here, one would expect this porphyrin layer to have a more profound impact on the reversibility of the reaction.

The observed 2H+/H2 redox couple is likely due to the ability of the small H+ ion to diffuse through the porphyrin layer and because not all of the underlying Pt surface atoms form bonds with atoms in the hemin molecule. For example, Denisevich et al. observed the Pt oxide formation/stripping waves in 1 M HClO4 at a poly-[Ru(vbpy)3]2+-coated Pt electrode.58 They attributed this signal to a relatively weak interaction between the polymer and Pt and that the film adherence was mainly due to their insolubility and possibly molding to the morphological surface defects. However, the relatively strong chemisorption of the FePP to the Pt surface calculated in this work suggests that this is not the case here. Rather, since our calculations show that every Pt atom does not necessarily form a bond with an atom in the porphyrin molecule, it is possible to conceive that they would be free to react with a redox species that reached it. Regardless of whether it is H+, as seen here, or HS- and H2S in our previous work, it seems likely that these species are small enough to diffuse through the porphyrin layer, independent of its thickness or polymerization method, and interact with the Pt atoms that are not directly coordinated to atoms in the porphyrin. This would certainly explain the decrease in sensitivity to sulfide observed in our previous study,3 as elemental sulfur, produced upon sulfide oxidation, is known to readily foul Pt electrodes.59-60

SUMMARY AND CONCLUSIONS


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A fundamental study was carried out to understand the adsorption and electrooxidative polymerization of hemin onto various electrode surfaces. The porphyrin was shown to readily adsorb to carbon and Pt electrode surfaces, however it adsorbs more uniformly over the carbon surfaces, and independent of surface defects. This difference in adsorption between carbon and Pt surfaces appears to set the stage for their different features upon polymerization. Hemin appears to polymerize on all surfaces as more individualized islands that grow upwards more than coalescing. However, due to the nonuniform adsorption on the Pt, the polymer layer on Pt exhibited very large features that appear as wide agglomerates of several hundred nanometers.

First principles computations showed that FePP chemisorbs to the Pt(111) surface while being physisorbed to the graphite surface. The difference in the way this molecule interacts with the two surfaces likely explains the difference in the roughness observed for the carbon (both HOPG and GC) and Pt surfaces in the ECAFM images. The FePP molecule exhibits intramolecular H-bonding between the carboxylic acid groups, which extend away from both surfaces. When an FePP molecule interacts with another FePP molecule, they prefer to maximize the number of H-bonds, with the stacking arrangement of the porphyrin rings also being important. The preferred non-parallel stacking between two FePP molecules on top of each other, coupled with the H-bonding possibly contributes to the increased roughness observed upon polymerization of hemin, especially with the Pt surface due to initial FePP chemisorption being more structurally distorting compared to the more orderly physisorption with FePP on graphite.

Finally, while polymerizing hemin using a potential step approach did allow for greater control of the polymerization process, and result in a smoother surface, this did not impact the electrochemical response towards the 2H+/H2 redox couple. Regardless of the thickness of the porphyrin layer, this redox couple was always observed. This is likely due to the small size of H+, giving it the ability to diffuse through the porphyrin layer and reacting with Pt atoms not specifically coordinated to an individual atom in the porphyrin layer. This likely suggests that Pt is not going to be a sufficient electrode support material


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for the purpose of an H2S amperometric sensor, since both HS- and H2S may possibly diffuse through the layer and react/foul the underlying Pt surface.


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Overall set-up for the AFM electrochemical cell, charge density difference isosurface plots and Bader charges for the FePP-graphite and FePP-Pt(111) systems, projected densities of states (PDOS) plots for the FePP-Pt(111) and FePP-graphite systems, UV-vis spectral comparison of hemin in solution to that adsorbed and polymerized onto an ITO optically transparent electrode, and optimized coordinates for the relevant FePP systems.

Acknowledgements The authors would like to thank Penn State Behrend for giving J. B. sabbatical time to complete this study as well as an award from the National Science Foundation (CMI; grant CHE-1305660) for the financial support for this work. J. B. would also like to acknowledge Dr. Timothy Tighe and Mr. Jeff Shallenberger at the Penn State University Materials Characterization Laboratory for their help carrying out the ECAFM and XPS experiments, respectively. D.P.M. thanks the University at Buffalo, SUNY for the Silbert Fellowship. Support from the Center of Computational Research at the University at Buffalo is also acknowledged.


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REFERENCES (1) Phougat, N.; Vasudevan, P.; Jha, N.; Bandhopadhyay, D. Metal porphyrins as electrocatalysts for commercially important reactions. Transition Met. Chem. 2003, 28, 838847. (2) Bedioui, F.; Trevin, S.; Albin, V.; Guadalupe, M.; Villegas, G.; Devynck, J. Design and Characterization of Chemically Modified Electrodes With Iron(III) Porphyrinic-Based Polymers: Study of Their Reactivity Toward Nitrites and Nitric Oxide in Aqueous Solution. Anal. Chim. Acta 1997, 341, 177-185. (3) Bennett, J. A.; Wheeler, C. D.; Sterling, K. L.; Chiodo, A. M. Exploring DicyanoFerriprotoporphyrin as a Novel Electrocatalytic Material for Selective H2S Gasotransmitter Detection. Electrochim. Acta 2013, 88, 86-93. (4) Nyokong, T. Electrodes modified with monomeric M-N4 catalysts for the detection of environmentally important molecules. In N4-Macrocyclic Metal Complexes, Zagal, J. H.; Bedioui, F.; Dodelet, J.-P., Eds.; Springer Science+Business Media, Inc.: New York, 2006, pp 315-361. (5) Peteu, S. F.; Bose, T.; Bayachou, M. Polymerized hemin as an electrocatalytic platform for peroxynitrite's oxidation and detection. Anal. Chim. Acta 2013, 780, 81-88. (6) Su, X.; Bromberg, L.; Tan, K.-J.; Jamison, T. F.; Padhye, L. P.; Hatton, T. A. Electrochemically Mediated Reduction of Nitrosamines by Hemin-Functionalized Redox Electrodes. Environ. Sci. Technol. Lett. 2017, 4, 161-167. (7) Pander, J. E.; Fogg, A.; Bocarsly, A. B. Utilization of Electropolymerized Films of Cobalt Porphyrin for the Reduction of Carbon Dioxide in Aqueous Media. ChemCatChem 2016, 8, 3536-3545. (8) Fuhrhop, J.-H. Porphyrin Assemblies and Their Scaffolds. Langmuir 2013, 30, 1-12. (9) Tom, R., T.; Pradeep, T. Interaction of Azide Ion with Hemin and Cytochrome c Immobilized on Au and Ag Nanoparticles. Langmuir 2005, 21, 11896-11902. (10) Shen, J.; Birdja, Y. Y.; Koper, M. T. W. Electrocatalytic Nitrate Reduction by a Cobalt Protoporphyrin Immobilized on a Pyrolytic Graphite Electrode. Langmuir 2015, 31, 84958501. (11) Duca, M.; Khamseh, S.; Lai, S. C. S.; Koper, M. T. M. The Influence of Solution-Phase HNO2 Decomposition on the Electrocatalytic Nitrite Reduction at a Hemin-Pyrolitic Graphite Electrode. Langmuir 2010, 26, 12418-12424. 26 ACS Paragon Plus Environment

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(12) Zhang, Q.; Zheng, X.; Kuang, G.; Wang, W.; Zhu, L.; Pang, R.; Shi, X.; Shang, X.; Huang, X.; Liu, P. N.; Lin, N. Single-Molecule Investigations of Conformation Adaptation of Porphyrins on Surfaces. J. Phys. Chem. Lett. 2017, 8, 1241-1247. (13) Stark, M.; Ditze, S.; Lepper, M.; Zhang, L.; Schlott, H.; Buchner, F.; Röckert; Chen, M.; Lytken, O.; Steinrück, H.-P.; Marbach, H. Massive Conformational Changes During Thermally Induced Self-Metalation of 2H-tetrakis-(3,5-di-tert-butyl)-phenylporphyrin on Cu(111). Chem. Commun. 2014, 50, 10225-10228. (14) Buchner, F.; Flechtner, K.; Bai, Y.; Zillner, E.; Kellner, I.; Steinrück, H.-P.; Marbach, H.; Gottfried, J. M. Coordination of Iron Atoms by Tetraphenylporphyrin Monolayers and Multilayers on Ag(111) and Formation of Iron-Tetraphenylporphyrin. J. Phys. Chem. C 2008, 112, 15458-15465. (15) Gottfried, J. M.; Flechtner, K.; Kretschmann, A.; Lukasczyk, T.; Steinrück, H.-P. Direct Synthesis of a Metalloporphyrin Complex on a Surface. J. Am. Chem. Soc. 2006, 128, 56445645. (16) Amreen, K.; Senthil Kumar, A.; Mani, V.; Huang, S.-T. Axial Coordination Site-Turned Surface Confinement, Electron Transfer, and Bio-Electrocatalytic Applications of Hemin Complex on Graphitic Carbon Nanometerial-Modified Electrodes. ACS Omega 2018, 3, 5435-5444. (17) Zhang, L.; Kepp, K. P.; Ulstrup, J.; Zhang, J. Redox Potentials and Electronic States of Iron Porphyrin IX Adsorbed on Single Crystal Gold Electrode Surfaces. Langmuir 2018, 34, 3610-3618. (18) Bennett, J. A.; Sterling, K. L.; Pander, J. E. Direct Metal Substitution of Electropolymerized Ferriprotoporphyrin: A Simple Electrode-Modification Process for Developing Electrocatalytic Materials. ECS Electrochemistry Letters 2013, 2, H37-H39. (19) Macor, K. A.; Spiro, T. G. Porphyrin electrode films prepared by electrooxidation of metalloprotoporphyrins. J. Am. Chem. Soc. 1983, 105, 5601-5607. (20) Macor, K. A.; Spiro, T. G. Oxidative electrochemistry of electropolymerized metalloprotoporphyrin films. J. Electroanal. Chem. Interfacial Electrochem. 1984, 163, 22336. (21) Snyder, S. R.; White, H. S. Electrochemistry and structure of thin films of (protoporphyrinato(IX))iron (III) chloride. J. Phys. Chem. 1995, 99, 5626-5632.


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D.; te Velde, G.; Vernooijis, P.; Versluis, L.; Visscher, L.; Visser, O.; Wang, F.; Wesolowski, T. A.; van Wezenbeek, E. M.; Wiesenekker, G.; Wolff, S. K.; Woo, T. K.; Yakovlev, A. L. ADF2013, SCM, Theoretical Chemistry, Vrije Universiteit: Amsterdam, The Netherlands, 2013. (43) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements HPu. J. Chem. Phys. 2010, 132, 154104 (1-19). (44) McCreery, R. L. Electrochemical Properties of Carbon Surfaces. In Interfacial Electrochemistry: Theory, Experiment, and Applications, Wieckowski, A., Ed.; Marcel Dekker, Inc.: New York, 1999, p 992. (45) Jenkins, G. M.; Kawamura, K. Structure of Glassy Carbon. Nature 1971, 231, 175-176. (46) Li Manni, G.; Alavi, A. Understanding the Mechanism Stabilizing Intermediate Spin States in Fe(II)-Porphyrin. J. Phys. Chem. A 2018, 122, 4935-4947. (47) Bhandary, S.; Ghosh, S.; Herper, H.; Wende, H.; Eriksson, O.; Sanyal, B. Graphene as a Reversible Spin Manipulator of Molecular Magnets. Phys. Rev. Lett. 2011, 107, 257202. (48) Liu, H.; Favier, F.; Ng, K.; Zach, M. P.; Penner, R. M. Size-selective electrodeposition of meso-scale metal particles: a general method. Electrochim. Acta 2001, 47, 671-677. (49) Liu, H.; Penner, R. M. Size-Selective Electrodeposition of Mesoscale Metal Particles in the Uncoupled Limit. J. Phys. Chem. B 2000, 104, 9131-9139. (50) Murphy, C. J.; Miller, D. P.; Simpson, S.; Baggett, A.; Pronschinske, A.; Liriano, M. L.; Therrien, A. J.; Enders, A.; Liu, S.-Y.; Zurek, E.; Sykes, E. C. H. Charge-Transfer-Induced Magic Cluster Formation of Azaborine Heterocycles on Noble Metal Surfaces. J. Phys. Chem. C 2016, 120, 6020-6030. (51) Peköz, R.; Johnston, K.; Donadio, D. Adsorption of Dichlorobenzene on Au and Pt Stepped Surfaces Using van der Waals Density Functional Theory. J. Phys. Chem. C 2012, 116, 20409-20416. (52) Simpson, S.; Zurek, E. Substituted Benzene Derivatives on the Cu(111) Surface. J. Phys. Chem. C 2012, 116, 12636-12643. (53) Camarillo-Cisneros, J.; Liu, W.; Tkatchenko, A. Steps or Terraces? Dynamics of Aromatic Hydrocarbons Adsorbed at Vicinal Metal Surfaces. Phys. Rev. Lett. 2015, 115, 086101 (1-5).


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Figure Captions

Figure 1.

Atomic force micrographs (A-F) and cross-section analysis (G-I) of HOPG (A, D, G), GC (B, E, H), and Pt (C,F, I) in borax (A-C) and hemin (D-F) solutions, respectively. Figures G, H, and I show the individual cross-sections for the respective electrode surface in borax (bottom; grey) and hemin (top; black).

Figure 2.

Top view and side view of optimized geometries of FePP adsorbed to the Pt(111) surface B45 site for the intermediate (S = 1) and high (S = 2) spin states obtained with the vdW-DF-optPBE functional. The Fe-Pt distances, two shortest O–Pt distances, and an average of the four Fe–N distances are provided. The C/N/O/H/Fe/Pt atoms are represented by black/blue/red/white/orange/grey spheres, respectively.

Figure 3.

Possible binding sites as well as top and side views of optimized geometries of FePP adsorbed to graphite. The binding sites considered for FePP adsorption to the graphite surface with the Fe atom was placed over a top (T) binding site or a hollow (H) binding site in two configurations (0- or 45-configuration) that differ by a rotation of the adsorbate by 45o about the z-axis. The top/bottom layer of carbon atoms is represented by green/purple balls, respectively (A). Top-view (B) and side-view (C and D) for the intermediate (S = 1, C) and high (S = 2, D) spin states of the most stable adsorption geometry (T0 site) of FePP on the graphite surface obtained with the vdW-DF-optPBE functional. The C/N/O/H/Fe atoms for FePP are represented by black/blue/red/white/orange spheres respectively. The C graphite atoms are represented by grey spheres. The shortest Fe–C and average of the four Fe–N distances are provided for the S = 1 and both S = 2 (P = preferred; U = unfavored) spin states.

Figure 4.

Voltammetric i-E curves (cycles 1, 5, 10, 20, and 25) for 0.4 mM FePPCl in 0.1 M borax at HOPG (A), GC (B), and Pt (C) electrodes. Scan rate = 25 mV/s.


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Figure 5.

Electrochemical atomic force micrographs of HOPG (A and B), GC (C and D) and Pt (E and F) following either 2 (A, C, and E) or 25 (B, D, and F) cyclic voltammetric hemin polymerization cycles.

Figure 6.

Cross-sectional analysis for ECAFM images of HOPG, GC, and Pt surfaces following 25 cyclic voltammetric hemin polymerization cycles shown in Figures 5B, 5D, and 5F, respectively.

Figure 7.

Electrochemical atomic force micrographs of hemin polymerized on HOPG using a multi-potential step approach of 0.4 V for 2 s, 0 V for 20 s, and then 0.3 V for 50 s (A) and then after an additional 50 s at 0.3 V (B).

Figure 8.

Computational models showing possible gas phase configurations between neighboring FePP molecules and the relative energy (kcal/mol) that is associated with each configuration compared to the most stable stacked 4 Hbond model (noted with 0 kcal/mol).

Figure 9.

Voltammetric i-E curves of 5 mM HCl in 0.5 M KCl at clean Pt (solid black) and Pt electrodes coated with FePP polymerized using either 100 voltammetric cycles (red dashed) or a multi-potential step protocol of 0.4 V for 10 s, 0 V for 20 s, and 0.3 V for 1200 s (blue dashed). Scan rate = 50 mV/s.


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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Figure 6.


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Figure 7.


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Figure 8.


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Figure 9.


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TOC Graphic


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Electrochemical micrographs showing the adsorption and polymerization of ferriprotoporphyrin (FePP) on Pt. The lower left figure shows the interactions between FePP and Pt and the ability of Pt to participate in the 2H+/H2 redox reaction. 44x22mm (600 x 600 DPI)

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Electrochemical Atomic Force Microscopy and First-Principles

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