Vacuum-Deposited Porphyrin Protective Films on Graphite

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Vacuum-Deposited Porphyrin Protective Films on Graphite: Electro-Chemical-AFM Investigation during Anion Intercalation. Rossella Yivlialin, Gianlorenzo Bussetti, Marta Penconi, Alberto Bossi, Franco Ciccacci, Marco Finazzi, and Lamberto Duò ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12359 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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ACS Applied Materials & Interfaces

Vacuum-Deposited Porphyrin Protective Films on Graphite: ElectroChemical-AFM Investigation during Anion Intercalation. Rossella Yivlialin1, Gianlorenzo Bussetti*,1, Marta Penconi2, Alberto Bossi2, Franco Ciccacci1, Marco Finazzi1 and Lamberto Duò1 (1) Department of Physics, Politecnico di Milano, p.za Leonardo da Vinci 32, I-20133 Milano (Italy) (2) Institute of Molecular Science and Technologies of the CNR (ISTM), PST via G. Fantoli 16/15, 20138 Milano and SmartMatLab Centre@ISTM, Via C. Golgi 19, 20133 Milano (Italy). KEYWORDS. Anion Intercalation, Graphite, Protective Film, Porphyrins, Electrochemical AFM.

ABSTRACT: The development of graphene products is promoting a renewed interest towards the use of graphite, in addition to the historical one for its proven viability as battery electrode. However, when exposed to harsh conditions, the graphite surface ages in ways that still need to be fully characterized. In applications to batteries, to optimize the electrode performances in acid solutions, different surface functionalization have been studied. Among them, aromatic molecules have been recently proposed. In this communication, we report on the protective effect exerted by a physical-vapor-deposited porphyrin layer. Metal-free tetra-phenyl-porphyrins were deposited on a highly oriented pyrolytic graphite crystal to study the modifications occurring during anion intercalation in graphite. The graphite electrode was plunged in an electrolyte solution of 1 M sulfuric acid and subjected to cyclic voltammetry. The results indicate that blister formation, the characteristic swelling of graphite surface induced by anion intercalation, is significantly perturbed by the porphyrin overlayer; the process is inhibited in those areas where the protective porphyrin film is still present. We ascribe the inhibition of the anion intercalation to the protective porphyrin wetting-layer.

INTRODUCTION. Graphite crystals play a key role in modern technology.1 Among different applications, the use of graphite as an electrode in modern batteries2 and the delamination of the crystal for graphene production3,4 have received an increasing interest in the last years. In both these applications, graphite crystals are in contact with acid solutions (batteries) or plunged in an acid electrolyte (graphene production). In these environments, graphite undergoes an oxidation process that can damage the properties of the pristine crystal surface,5 influencing the performances of electrodes (batteries6) or the quality of the delaminated surface (graphene production 3). Strategies to control or prevent graphite oxidation are thus important and require an in-depth knowledge of the molecular processes acting at the graphite surface.7,8 Graphite delamination can be monitored in-situ and in real time, by checking the quality of the produced graphene sheets.5 When graphite is used as an electrode, it can be protected by suitable molecular coatings.1 Surface protection can be achieved by a layer of organic molecules,9,10 whose structure can be tailored to display specific chemical properties. This type of protection can involve a great variety of molecular frameworks, including porphyrins and their derivatives.1,11 An organic film is considered a good protective layer if (i) it is resistant to acid environment (ii) it is water-insoluble and impenetrable.12 Recently, we proved that vacuum-sublimated metal-free tetraphenyl-porphyrin (H2TPP) grows on highly oriented pyrolytic graphite (HOPG) following a layer-plus-islands mode.13,14 H2TPP, in addition to be insoluble in water-based solvents, can be grown through a physical vapor deposition process, which

allows to finely control the growth of engineered films and is broadly used in the optoelectronic industry of OLEDs. The organic 2D phase, i.e. the wetting-layer (single or multi-layer), covers the graphite substrate, while porphyrin crystals are randomly distributed on the wetting-layer. We showed that the crystal 3D phase is unstable while the 2D layer is stable when the sample is exposed to acid vapors (namely hydrochloric acid).13. The porphyrin 2D phase thus results potentially appealing as a protective layer for graphite. Bare HOPG undergoes a degradation process when subject to a proper positive electrochemical (EC) potential in acid solutions (e.g. H2SO4 and HClO45,15). Graphite degradation is flanked by a process known as blister growth: anion intercalation on the positively charged graphite produces gases (CO, CO2, O2), which swell the graphite surface and produce bubbles. 16 Without any surface protection blisters cover the whole sample surface. Therefore, the absence of blistering can be considered a marker to evaluate the real efficacy of a protective layer. In this paper, owing to its appealing and interesting properties, we study from a phenomenological point of view the ability of a H2TPP film to protect the graphite surface, when the latter is plunged inside a sulfuric acid electrolyte. We grow an ultra-thin (nominal thickness 0.5 Å) and a thin (12 Å) H2TPP film onto a HOPG substrate. The former shows only a 2D wetting layer, while the latter is characterized by the coexistence of 2D and 3D phases in air.14 To mimic possible working environments, we induced and forced anion intercalation exposing the samples to severe conditions into an EC cell filled

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with 1 M H2SO4 solution and studied the protective action provided by a H2TPP film.5,17 By means of atomic force microscopy (AFM) measurements, we give evidence that blister formation and growth are inhibited in those sample areas where a porphyrin layer is still present after the EC process. This opens the route towards new strategies for graphite protection by porphyrin compounds. EXPERIMENTAL SECTION. H2TPP films are grown by physical vapor deposition (PVD) at a pressure of 5 × 10-7 mbar. The Kenositec KE-500 chamber includes four effusive (Knudsen) sources and two thermal (Joule) sources. The sample holder is positioned 300 mm away from the Knudsen cells (Kcell) and is kept at room temperature. The K-cells are thermally controlled via a thermocouple inserted into the crucible. H2TPP was purchased from Sigma-Aldrich and used as received. Usually, 30 mg of H2TPP powder are placed inside the K-cell crucible and used for multiple depositions. Porphyrin is heated in high vacuum conditions (base pressure 0.8 - 1.8 × 10-6 mbar) at around 160 °C for one hour and then for 10 min at a temperature close to its sublimation point. No difference could be analytically detected comparing the material prior the deposition, the one left in the crucible and the deposited sample film. Porphyrin is sublimated on a graphite substrate kept at room temperature. α-grade HOPG (Optigraph) was exfoliated with an adhesive tape before molecule deposition. A quartz microbalance, placed in proximity of the sample, monitors the flux from the cell, allowing one to measure the average nominal film thickness. To define the nominal thickness, a test sample is grown by keeping a constant flux rate of the porphyrin cell. The deposited thick sample is measured ex-situ by a capacitive profilometer. The thickness of the thinner porphyrin films is evaluated by comparing the sample deposition time with the test sample. Two kind of samples are prepared: (i) a 0.5 Å-thick sample grown with a porphyrin flux of 0.06 - 0.11 Å/s (crucible temperature = 320 °C). (ii) a 12 Å-thick film grown either at 0.5 Å/s or at 1.0 Å/s (crucible temperature = at 350 °C or 367 °C, respectively). A commercial three-electrode EC cell, equipped with a potentiostat, was used to acquire characteristic cyclic-voltammetry (CV) curves of the sample. The EC potential of the sample, here considered as the working electrode (WE), is set according to a platinum quasi-reference electrode (PtQRef). The latter, commonly used in acid solutions, shows a stable potential shift of +0.743 V with respect to the standard hydrogen electrode (SHE).13A Pt counter electrode (CE) completes the cell. A 1 M solution of H2SO4 is prepared and de-aerate by bubbling a 5.0 grade Ar gas. A Keysight 5500 AFM is used both ex-situ and in-situ to characterize the surface morphology before and after the anion intercalation in graphite. The acquired images are collected in tapping mode.

Figure 1. AFM images (2 x 4 μm2) of the 0.5 Å-thick H2TPP sample. a) Topography acquired in air in tapping mode. The white dashed line represents the profile cross-section. b) Phase-contrast image. The blue areas indicate the porphyrin wetting-layer, while the yellow ones represent HOPG regions. The two panels refer to the same scanned area.

The AFM phase-contrast image allows revealing the presence of porphyrin molecules as demonstrated in a previous work (see ref. 13). The abrupt change in the measured phase signal gives evidence of the porphyrin wetting-layer on the surface (blue areas),18 while the graphite substrate corresponds to yellow regions. The porphyrin coverage is clearly uncompleted, leaving part of the HOPG exposed to the acid when the substrate is plunged into the electrolyte. In liquid, a phase-contrast analysis of the sample is more critical, because the quality of the image is in general poorer than in air; on the other hand, the topography is comparable to Figure 1a. Consequently, the persistence of the wetting-layer inside the EC cell cannot be assessed from these preliminary results. The 12 Å-thick H2TPP sample is characterized by wide 3D structures, which show sharp edges and a mean thickness of 15 Å (see Figure 2a).

RESULTS AND DISCUSSION. The 0.5 Å-thick H2TPP sample shows an AFM topography very similar to the pristine HOPG surface (not reported here), characterized by wide terraces and the absence of 3D crystals (see Figure 1a).

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ACS Applied Materials & Interfaces These peaks are related to electrochemical processes occurring on porphyrins and suggests that the protective layer has a good conductivity.19 Consequently, the HOPG electrode covered by porphyrins is still acting as an electrode. In the 0.5 Å-thick film such spectroscopic features are not detectable. Nonetheless, it is worth noting that the voltammograms of panels a and b display a reduction of the current densities (about 0.2 mA/cm2) and an anodic upward shift (50 mV) of the oxidation onset with respect to the clean surface (Figure 3c). This suggests the presence of porphyrins also on the working electrode of the 0.5 Å sample, which can be definitively proven by the morphological AFM analysis.

Figure 2. AFM image (2 x 4 μm2) of the 12 Å-thick H2TPP sample, acquired in tapping mode. The white dashed lines represent the profile cross-section. a) Sample measured in air. Wide porphyrin crystals characterize the surface morphology. b) Sample measured in liquid (1 M H2SO4). The surface morphology is significantly changed and the wide crystals do not characterize the sample surface anymore.

We ascribe these new features to porphyrin crystals (generally thicker than the very first deposited wetting-layer), grown on a H2TPP 2D phase, as already proven and discussed by the authors.13,14 When this sample is plunged in the liquid cell, the morphology significantly changes (Figure 2b). 3D crystals are completely dissolved: the surface appears comparable to the one displayed in Figure 1a (presence of the graphite steps, only) and “dusty”. Despite the use of a different acid and experimental protocols, this phenomenon mimics what was already observed on H2TPP films exposed to vapors of HCl.13 In that case, the acid is was able to modify the 3D porphyrin phase, while the wetting-layer was preserved. Nonetheless, in the current study we cannot ensure the stability of the 2D phase in the electrolytic bath, because of the difficulty of performing phasecontrast characterization in liquid, as previously discussed for the 0.5 Å sample. Following the surface morphological characterization in air, both 0.5 and the 12 Å-thick films are plunged inside the EC cell and submitted to a single CV oxidation cycle, up to 1.3 V (see Figure 3 a and b, respectively). The reported voltammograms are acquired at 25 mV/s. This scan rate represents a typical value at which the involved electrochemical process can be considered quasi-reversible.5 On the other hand, this rate is high enough to allow ion diffusion in the pristine graphite and observing the blistering. Both voltammograms in panels a and b show an irreversible broad anodic wave between 1.1 V and 1.2 V, matching the characteristic range of anion intercalation of graphite in H2SO4 solution (see Figure 3c).5 This indicates that intercalation takes place on the surface. The CV voltammogram reported in Figure 3a shows irreversible anodic peaks in the 0.4 – 0.7 V range, which are not detected in the pristine HOPG.

Figure 3. Cyclic voltammograms 1 M H2SO4 at 25 mV/s: a) 12 Åthick H2TPP sample; b) 0.5 Å-thick H2TPP sample; c) pristine (no porphyrin layer) HOPG.

The morphological changes induced on the electrode surface after the EC treatment are investigated ex-situ by AFM. Initially, we study a not-protected graphite as a benchmark to evaluate the role of porphyrins. When a clean HOPG undergoes a single CV cycle, the overall graphite surface is affected by significant changes in its morphology. The AFM image reveals blisters, twin blisters and aggregation of blisters distributed uniformly on the overall surface (see Figure 4).

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round-shaped structures observed on the thinner sample (Figure 5a), suggesting a role of the film thickness on the evolution of the 2D phase during anion intercalation. Other areas (roughly 50 % labeled as “region B” in Figure 6b) of the sample are damaged by blisters (typical 15 nm height and 300 nm width), which statistically cover 20 - 30 % of region B. These observations justify the decrease of the faradaic current as reported in Figure 3a in close analogy with the 0.5 Å-thick case.

Figure 4. AFM topography image (4 x 4 μm2) acquired in air of the clean HOPG, after a single CV in 1 M H2SO4.

The situation is significantly different for the AFM images (see Figure 5) obtained on the 0.5 Å-thick sample, after the acquisition of the CV reported in Figure 3b. We observe wide areas (labeled as “region A” in Figure 5a) completely unaffected by surface swelling and blister formation. This proves the presence of the porphyrin wetting-layer even in liquid. However, the round-shaped structures in Figure 5a suggest a possible re-arrangement of the as-deposited organic layer. This is not surprising if we consider the high EC potential applied to an ultra-thin organic film, which can degrade the H2TPP layer during the anodic process. Other areas of the sample (labeled as “region B” in Figure 5b) are highly affected by blisters (on average 15 nm height and 600 nm width), similarly to the case of the clean HOPG. The observed reduction of the CV current density in the protected HOPG (Figure 3b) with respect to the clean HOPG (Figure 3c) is reasonably due to the presence of porphyrins in the regions A, which covers roughly 50 % of the sample area. Considering that, blistering is a direct consequence of anion intercalation inside graphite, we conclude that regions A are preserved from this process as a consequence of the presence of a porphyrin layer inhibiting further degradation of the electrode surface.

Figure 5. AFM topography images (2 x 4 μm2) of the 0.5 Å-thick H2TPP sample after anion intercalation acquired on: a) a sample area (labeled as “region A”) where graphite is covered by porphyrins and no blisters are observed; b) a sample area, labeled as “region B”, where the surface is affected by blisters (see text for details).

When the 12 Å-thick H2TPP sample undergoes the CV reported in Figure 3a, we observe a comparable phenomenology as for the thinner sample (see Figure 6), allowing one to clearly identify regions of type A (no blisters or swelling) or B also for this sample. Here, we observe terraces clearly covered by porphyrins, which gives a direct evidence of the persistence of the porphyrin 2D phase in liquid. In addition, the morphology proves that porphyrins modify the local mechanism of anion intercalation and can thus act as a protective layer for the underneath graphite substrate. The porphyrin layer does not show the

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Figure 6. AFM topography images (2 x 4 μm2) of the 12 Å-thick H2TPP sample after anion intercalation acquired on: a) a sample area (labeled as “region A”) where graphite terraces are still covered by porphyrins and no blisters are observed; b) a sample area (labeled as “region B”), where the surface is affected by blisters.

In Figure 7, we report an image of the 12 Å sample after anion intercalation, acquired on a region where a coexistence of blisters and porphyrin film is observed. In fact, the phase-contrast analysis reveals areas covered by porphyrins (blue regions) placed on a wide blister, characterized by a different phase signal ascribed to graphite (yellow regions). If the porphyrin 2D phase is not uniform (region A-B borders, defects and/or subsequent CV cycles), anions can intercalate near the border of a solid porphyrin domain and produce lateral graphite swelling together with blister development, which run over the adjacent porphyrin-covered areas. This process causes the H2TPP film fragmentation and accelerates the 2D phase aging.

Figure 8. a) AFM topography image (4 x 4 μm2) of the 12 Å-thick H2TPP sample, after a CV in 1 M H2SO4 at 5 mV/s. The white dashed line represents the profile cross section on a blister. b) voltammogram of CV at 5 mV/s, where the current intensity is multiplied by a factor 7 (full line) for a better comparison with the voltammogram of the CV at 25 mV/s (dashed line).21 Figure 7. AFM topography image (2 x 4 μm2) of the 12 Å-thick H2TPP sample, after anion intercalation. The scanned area corre-

sponds to a region where the porphyrin film is damaged and blisters affect the graphite surface. In the inset, the phase-contrast analysis of the area on the top of a blister (blue regions are porphyrins, surrounded by yellow regions of non-protected graphite).

Finally, we note that the intercalation process is enhanced if the CV scan rate is reduced below that of the quasi-reversible process (e.g. 25 mV/s): in these conditions the protective action of the porphyrin layer is partially lost and blistering increases. As an example, we report in Figure 8a the 12 Å-thick H2TPP sample topography after a CV scan at 5 mV/s (panel b). In this case, regions A (in analogy to the previous nomenclature used) are not observed, hence suggesting that CV scan rates as slow as 5 mV/s define an important threshold to secure the protection efficiency of an organic layer.

CONCLUSION. In this paper we have proven that a H2TPP 2D wetting-layer grown on HOPG by physical vapor deposition meets the requirements a possible organic protective layer needs to possess. By analyzing HOPG protected with an over-layer of porphyrin under highly oxidative conditions (i.e. anion intercalation), it is found that the CV voltammograms undergo a shift of the anodic wave onset of 50 mV toward larger values, while a 0.2 mA/cm2 decrease of the current densities is observed. From the morphological standpoint, after a CV cycle, we deduce that the porphyrin overlayer can initially protect the graphite surface. We found wide sample areas still covered by porphyrins and not affected by blisters. Considering the renewed interest in graphite electrode protection and very recent results regarding the role of organic molecules in the EC properties of graphite,20 this work represents an attempt to explore the characteristic of an electrode covered by porphyrins. The inhibition of the anion intercalation, ascribed to the protective wetting-layer, opens interesting perspectives in the application of porphyrinic compounds on the working electrode of EC cells. For these reasons, the H2TPP/HOPG interface represents a prototypical system suitable for further investigation and optimization in the field of electrode protection. To this purpose, impedance characterization and electrode modeling are required to fully understand the performances of a real electrochemical electrode covered by molecules. An interesting

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question emerging from the study concerns the possibility to reduce the damaged regions to vanishingly small areas. In this way, it could be possible -at least in principle- to completely avoid anion intercalation in graphite.

AUTHOR INFORMATION Corresponding Author * Corresponding author email: [email protected].

ACKNOWLEDGMENT MP and AB gratefully acknowledge the use of instrumentation purchased through the Regione Lombardia—Fondazione Cariplo joint project ‘SmartMatLab Centre’ (decreti 12689/13, 7959/13; Azione 1 e 2).

REFERENCES (1) Alkire, R. C.; Bartlett, P. N.; Lipkowski, J. Advances in Electrochemical Science and Engineering. Electrochemistry of Carbon Electrodes. 2015, 16 (Wiley). (2) Jung, Y. S.; Lu P.; Cavanagh, A. S.; Ban C.; Kim, G.-H.; Lee S.-H.; George, S. M.; Harris, S. J.; Dillon, A. C. Unexpected Improved Performance of ALD Coated LiCoO2/Graphite LiIon Batteries. Adv. En. Mater. 2013, 3, 213-219. (3) Niu, L.; Coleman, J. N.; Zhang, H.; Shin, H.; Chhowalla, M.; Zhang, Z. Production of Two-Dimensional Nanomaterials via Liquid-Based Direct Exfoliation. Small 2016, 12, 272-293. (4) Xia, Z. Y.; Pezzini, S.; Treossi, E.; Giambastiani, G.; Corticelli, F.; Morandi, V.; Zanelli, A.; Bellani, V.; Palermo, V. The Exfoliation of Graphene in Liquids by Electrochemical, Chemical and Sonication-Assisted Techniques: a Nanoscale Study. Adv. Function. Mater. 2013, 23, 4684-4693. (5) Bussetti, G.; Yivlialin, R.; Alliata, D.; Li Bassi, A.; Castiglioni, C.; Tommasini, M.; Casari, C. S.; Passoni, M.; Biagioni, P.; Ciccacci, F.; Duò, L. Disclosing the Early Stages of Electrochemical Anion Intercalation in Graphite by a Combined Atomic Force Microscopy/Scanning Tunneling Microscopy Approach. J. Phys. Chem. C 2016, 120, 6088-6093. (6) The stability of graphite electrodes in working batteries (e.g. flow batteries) is tested in different kind of solutions by characterization in electrochemical potential range comparable with that one used in this paper. Cfr. Park, J. H.; Park, J. J.; Park, O. O.; Jin, C.-S.; Yang, J. H. Highly accurate apparatus for electrochemical characterization of the left electrodes used in redox flow batteries. J. Pow. Sour. 2016, 310, 137-144. (7) Park, J.-W.; Kim, E.-S.; Kim, J.-U.; Kim, Y.; Windes, W. E. Enhancing the Oxidation Resistance of Graphite by Applying a SiC Coat with Crack Healing at an Elevated Temperature. Appl. Surf. Sci. 2016, 378, 341-349. (8) Yang, X.; Huang, Q.; Su, Z.; Chang, X.; Chai, L.; Liu, C.; Xue, L.; Huang, D. Resistance to Oxidation and Ablation of SiC Coating on Graphite Prepared by Chemical Vapor Reaction. Corr. Sci. 2013, 75, 16-27. (9) Leroux, Y. R.; Hapiot, P. Nanostructured Monolayers on Carbon Substrates Prepared by Electrografting of Protected Aryldiazonium Salts. Chem. Mater. 2013, 25, 489-495.

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(10) Malmos, K.; Dong, M.; Pillai, S.; Kingshott, P.; Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. Using a Hydrazone-Protected Benzenediazonium Salt to Introduce a Near-Monolayer of Benzaldehde on Glassy Carbon Surfaces. J. Am. Chem. Soc. 2009, 131, 4928-4936. (11) Tu, W.; Lei, J.; Ju, H. Functionalization of Carbon Nanotubes With Water-Insoluble Porphyrins in Ionic Liquid: Direct Electrochemistry and Highly Sensitive Amperometric Biosensing for Trichloroacetic Acid. Chem. Eur. J. 2009, 15, 779-784. (12) Funke, W.; Haagen, H.; Empirical or Scientific Approach to Evaluate the Corrosion Protective Performance of Organic Coatings. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 50-53. (13) Bussetti, G.; Campione, M.; Riva, M.; Picone, A.; Raimondo, L.; Ferraro, L.; Hogan, C.; Palummo, M.; Brambilla, A.; Finazzi, M.; Duò, L.; Sassella, A.; Ciccacci, F. Stable Alignment of Tautomers at Room Temperature in Porphyrin 2D Layers. Adv. Funct. Mater. 2014, 24, 958-963. (14) Bussetti, G.; Campione, M.; Ferraro, L.; Raimondo, L.; Bonanni, B.; Goletti, C.; Palummo, M.; Hogan, C.; Duò, L.; Finazzi, M., Sassella, A. Probing Two-Dimensional vs Three-Dimensional Molecular Aggregation in Metal-Free Tetraphenylporphyrin Thin Films by Optical Anisotropy. J. Phys. Chem. C 2014, 118, 15649-15655. (15) Choo, H.S.; Kinumoto, T.; Joeng, S.K.; Iriyama, Y.; Abe, T.; Ogumi, Z. Mechanism of Electrochemical Oxidation in Highly Oriented Pyrolytic Graphite in Sulfuric Acid Solution. J. Electrochem. Soc. 2007, 154, B1017-B1023. (16) Goss, C. A.; Brumfield, J. C.; Irene, E. A.; Murray, R. W., Imaging the Incipient Electrochemical Oxidation of Highly Oriented Pyrolytic Graphite. Anal. Chem. 1993, 65, 1378. (17) Yivlialin, R.; Brambilla, L.; Bussetti, G.; Tommasini, M.; Li Bassi, A.; Casari, C. S.; Passoni, M.; Ciccacci, F.; Duò, L.; Castiglioni, C. Evolution of the Graphite Surface in Phosphoric Acid: an AFM and Raman Study. Beilstein J. Nanotechnol. (accepted) (18) The absence of 3D crystals with respect to the previous reported results (see ref. 13) is due to a decreased of the deposition rate (consequent of a different sample-effusion cell geometry) and a better film growth control. (19) H2TPP in CH2Cl2 solution displays two reversible oxidation peaks at 0.53 and 0.87 V vs Fc/Fc+ (i.e. 0.98 V and 1.32 V vs SHE), which are compatible, within 200 mV shift, with the peaks in Figure 3a. Paul-Roth, C.; Rault-Berthelot, J.; Simonneaux, G.; Poriel, C.; Abdalilah, M.; Letessier, J.; Electroactive films of poly(tetraphenylporphyrins) with reduced bandgap. J. Electroanal. Chem. 2006, 597, 19. (20) Patel, A. N.; Collignon, M. G.; O’Connell, M. A.; Hung, W. O. Y.; McKelvey, K.; Macpherson, J. V.; Unwin, P. R. A New View of Electrochemistry at Highly Oriented Pyrolytic Graphite. J. Am. Chem. Soc. 2012, 134, 20117-20130. (21) The decreasing of the current density measured on covered electrodes as a function of the CV scan rate is observed and related to an effect of the thin-layer diffusion. For instance, see Sims, M. J.; Rees, N. V.; Dickinson, E. J. F.; Compton, R. G. Effects of thin-layer diffusion in the electrochemical detection of nicotine on basal plane pyrolytic graphite (BPPG) electrodes modified with layers of multi-walled carbon nanotubes (MWCNT-BPPG). Sens. Actuators B 2010, 144, 153-158.

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AFM images (2 x 4 µm2) of the 0.5 Å-thick H2TPP sample. a) Topography acquired in air in tapping mode. The white dashed line represents the profile cross-section. b) Phase-contrast image. The blue areas indicate the porphyrin wetting-layer, while the yellow ones represent HOPG regions. The two panels refer to the same scanned area. 187x153mm (150 x 150 DPI)



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AFM image (2 x 4 µm2) of the 12 Å-thick H2TPP sample, acquired in tapping mode. The white dashed lines represent the profile cross-section. a) Sample measured in air. Wide porphyrin crystals characterize the surface morphology. b) Sample measured in liquid (1 M H2SO4). The surface morphology is significantly changed and the wide crystals do not characterize the sample surface anymore. 186x135mm (150 x 150 DPI)


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Cyclic voltammograms in 1 M H2SO4 at 25 mV/s: a) 12 Å-thick sample; b) 0.5 Å-thick H2TPP sample; c) pristine (no porphyrin layer) HOPG. 144x101mm (150 x 150 DPI)



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AFM topography image (4 x 4 µm2) acquired in air of the clean HOPG, after a single CV in 1 M H2SO4. 136x135mm (150 x 150 DPI)


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AFM topography images (2 x 4 µm2) of the 0.5 Å-thick H2TPP sample after anion intercalation acquired on: a) a sample area (labeled as “region A”) where graphite is covered by porphyrins and no blisters are observed; b) a sample area, labeled as “region B”, where the surface is affected by blisters (see text for details). 151x138mm (150 x 150 DPI)



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AFM topography images (2 x 4 µm2) of the 12 Å-thick H2TPP sample after anion intercalation acquired on: a) a sample area (labeled as “region A”) where graphite terraces are still covered by porphyrins and no blisters are observed; b) a sample area (labeled as “region B”), where the surface is affected by blisters. 151x138mm (150 x 150 DPI)


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AFM topography image (2 x 4 µm2) of the 12 Å-thick H2TPP sample, after anion intercalation. The scanned area is acquired in a region where the porphyrin film is damaged and blisters affect the graphite surface. In the inset, the phase-contrast analysis of the area on the top of a blister (blue regions are porphyrins, surrounded by yellow regions of not-protected graphite). 135x68mm (150 x 150 DPI)



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a) AFM topography image (4 x 4 µm2) of the 12 Å-thick H2TPP sample, after a CV in 1 M H2SO4 at 5 mV/s. The white dashed line represents the profile cross section on a blister. b) voltammogram of CV at 5 mV/s, where the current intensity is multiplied by a factor 7 (full line) for a better comparison with the voltammogram of the CV at 25 mV/s (dashed line).21 159x171mm (144 x 144 DPI)


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Vacuum-Deposited Porphyrin Protective Films on Graphite

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