Carboxylic-rich graphene oxide (CGO) and MnIII tetrakis(N-methyl-4-pyridinio)porphyrin (MnTMPyP) form self-as-sembled leaf-like structures. These were obtained throughelectrostatic and π–π stacking interactions, as indicated bythe redshift of the metalloporphyrin Soret band throughoutthe pH range of 6–12. Voltammetry and XPS measurementsconfirmed that CGO does not significantly affect the chemi-cal environment of the metal ion in MnTMPyP. However, theeffect of CGO is apparent when determining by rotating ring
Introduction
Graphene is composed of a single layer of carbon atomsarranged in a two-dimensional honeycomb network of six-membered rings. This material has generated increasedinterest both in fundamental science and its potential appli-cations due to its unique structure and properties, such asexcellent conductivity, high electron mobility, superiorchemical stability, large surface-to-volume ratio,[1–6] andhigh transparency.[7] The surface properties of graphene canalso be adjusted by chemical modifications and this offersopportunities for the preparation and possible applicationsof functionalized materials.[8] Such applications have beendemonstrated in a variety of fields, such as energy stor-age,[9,10] catalysis,[11] and biosensors.[12] Recent comprehen-sive reviews concerning the properties and applications ofgraphene are available.[9,12–15]
Porphyrins and metalloporphyrins are vital molecules inmany biological processes and synthesized porphyrin mo-lecules have been used as models to understand porphyrin-based natural systems.[16] They are also exploited, for exam-ple, as catalysts, NMR image enhancement agents, and anti-cancer drugs.[17] Metalloporphyrin-modified carbon materi-als have been extensively studied in the context of chemi-cally modified electrodes and particularly as O2 reductioncatalysts for fuel cell cathodes.[18] Porphyrin derivatives have
[a] Department of Chemical Engineering, Ben-Gurion Universityof the Negev,P. O. Box 653, Beer-Sheva 84105, IsraelE-mail: [email protected]://in.bgu.ac.il/enSupporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejic.201400054.
disk electrode (RRDE) voltammetry the ability of CGO-MnIVTMPyP films to catalyze water oxidation and evolveoxygen. The amount of O2 evolved at pH 10 and 1.6 V versusAg/AgCl is 50% higher than that of MnTMPyP films. Whenused for the photoelectrochemical oxidation of water, CGO-MnTMPyP films on iron/hematite foams exhibited substan-tial activity, as evidenced by the current density (0.54 mA/cm2 at 1.23 V vs. NHE) and incident photons to current effi-ciency (IPCE; 9.0% at 1.43 V vs. NHE) measured at pH 10.
also been used for modifying carbon nanotubes[19] and havebeen covalently attached to graphene,[20] resulting in materi-als with novel optical and electronic properties.
The remarkable chemical reactivity and catalytic proper-ties of synthetic manganese porphyrins have been related tothe central metal ion which can exhibit various oxidationstates.[21] Manganese porphyrins have been studied asmodel compounds for oxygen transfer reactions of cyto-chrome P-450.[22] High oxidation states of oxomanganese(IV or V) porphyrins have been suggested to be involvedin the catalytic oxidation reactions of a variety of organiccompounds.[23] In addition, MnV(O) species are of specialinterest because they have been postulated as important in-termediates in the conversion of water into dioxygen duringwater oxidation in photosynthesis.[24,25] Although it hasbeen recognized that some monomeric manganese por-phyrins,[26,27] including water-soluble ones,[27–29] are inactivetowards water oxidation, high-valent MnV=O intermediatesin covalently bound porphyrin dimers have been suggestedto be involved in the generation of O2.[26] Another approachhas been to concentrate a monomeric, water-soluble MnIII
porphyrin [manganese(III) tetrakis(4-sulfonatophenyl)por-phyrin, MnIIITPPS], which is inactive towards water oxi-dation,[27,28] into a polymeric matrix to yield a light-assistedcatalyst for this reaction.[27] This has been explained as astatistical proximity effect, which allows cooperative cataly-sis by adjacent Mn–oxo porphyrins.[27,30] We have exploiteda similar effect for the concerted four-electron reduction ofoxygen to water by concentrating cobalt porphyrins in themicropores of aerogel carbon electrodes.[31]
This paper describes the spectroscopic and electrochemi-cal properties of self-assembled structures obtained by the
interaction of negatively charged carboxylic-rich grapheneoxide (CGO) and a positively charged MnIII porphyrin[manganese(III) tetrakis(N-methyl-4-pyridinio)porphyrin,MnTMPyP]. Bearing in mind the special characteristics ofgraphene derivatives and the catalytic capabilities of man-ganese porphyrins, we examined the application of CGO-MnTMPyP in the photoelectrochemical splitting of water.Systems consisting of porphyrins and carbon materials(such as fullerenes[32] or graphene[33]) have previously beenreported in the context of artificial photosynthesis. Photo-electrochemical cells mimic the processes of photosystems Iand II[34] and work on the simple principles of semiconduc-tor electrochemistry.[35] This topic has been reviewed in re-cent reports (e.g., see refs.[36–42]). The real problem is thehalf-reaction of water oxidation near the potential of themultielectron transfer process in oxygen evolution. The abil-ity of iron oxide, and particularly α-Fe2O3 (hematite), toabsorb solar irradiation, coupled with its abundance andnontoxicity, make it an attractive photoanode mate-rial.[36,39,42,43] We previously showed[44] that it is possible touse the open 3D structure of iron foams, which permit lightpenetration, and the simple procedures to coat them withhematite and a catalytic layer, to exploit them as photo-anodes. In this report we demonstrate the improved photo-electrochemical water-splitting performance of these foamsafter the adsorption of CGO-MnTMPyP in the hematitelayer.
Results and DiscussionClear solutions of CGO were obtained by diluting by a
factor of 100 an aqueous dispersion of 5 mg/mL CGO inwater or methanol. The UV/Vis spectra of the diluted aque-ous CGO solutions showed a broad peak with λmax ataround 223 nm and a shoulder at around 290 nm. Theaqueous and methanolic solutions obtained after the ad-dition of MnTMPyP (0.01 mg/mL; 1.1�10–5 m) remainedclear for a period of around 30 min. Samples of the meth-anolic solutions obtained during this period were used toprepare films of CGO-MnTMPyP, the morphologies ofwhich were examined by SEM and TEM and comparedwith those of CGO. Figure 1A,B show the SEM images ob-tained for CGO and CGO-MnTMPyP, respectively. Theformer has a layered structure with a smooth surface,whereas the latter shows a morphology characterized bywrinkles. TEM analysis revealed a leaf-like structure for theCGO-MnTMPyP aggregates that does not exist for CGO(Figure 1, D, C, respectively). A similar structure has alsobeen observed for J-aggregates of amphiphilic por-phyrins.[45] The complex, branched structure of the graph-ene-porphyrin species extends to distances in the micro-meter range (inset of Figure 1, D). Floculation of large-sized aggregates of CGO-MnTMPyP occurs over longerperiods of time (see Figure S1 in the Supporting Infor-mation), and such effects, which have been reported forgraphene oxide sheets in electrolyte solutions, have been at-tributed to a change in surface charge.[46]
Figure 1. SEM and TEM images obtained for CGO (A and C,respectively) and CGO-MnTMPyP (B and D, respectively).
Graphene oxide (GO) is characterized by two differentcarboxylic acidic groups: Those that are in close proximityto a phenol or hydroxy group and those that are not.[47]
The pKa values of these groups and the phenol functionali-ties have been determined to be 4.4, 6.6, and 9.0, respec-tively.[47] CGO-MnTMPyP was studied spectroscopicallyand electrochemically in the pH range 6–13, in which mostof the carboxy groups are deprotonated. The UV/Vis spec-tra of aqueous solutions of CGO-MnTMPyP at differentpHs were compared with those of MnTMPyP. It can beseen from the spectra in Figure 2 (A,B) obtained at pH 6and 10 that the Soret band of the metalloporphyrin is red-shifted in the presence of CGO. This is indicative of theformation of complexes through electrostatic and π–πstacking interactions. This has also been suggested for sup-ramolecular structures obtained between chemically re-duced graphene, which is negatively charged due to its resid-ual carboxy groups, and TMPyP.[48] Bathochromic (red)shifts of the Soret band of cationic porphyrins deposited onclay layers have also been reported previously and associ-ated with flattening of the porphyrin molecule, that is, thefour cationic moieties become parallel to the porphyrinring,[49] leading to enhanced π conjugation. The dependenceof the Soret band position on pH for both MnTMpyP andCGO-MnTMPyP is depicted in Figure 2 (D). The redshiftexhibited by the CGO-MnTMPyP species persists through-out the whole pH range of 6–12. The maximum shift,around 19 nm at pH 12, is comparable to the value of22 nm reported for CGO-TMPyP.[50] However, no signifi-cant shift was observed for CGO-MnTMPyP at pH 13 (Fig-ure 2, C,D). Because at pH 13, MnIIITMPyP converts tot-ally from an aqua-hydroxo into a dihydroxo species (pKa1
and pKa2 of 10.5 and 11.4, respectively),[51] this may be dueto a decrease in the electrostatic attraction between CGOand MnIIITMPyP(OH–)2 and the release of the latter intosolution.
A UV/Vis spectroscopic analysis of CGO-MnTMPyPfilms was carried out by applying methanolic solutions ofthis species on to indium tin oxide (ITO) and exposing themto aqueous electrolyte solutions. The position of the Soret
Figure 2. UV/Vis absorbance spectra for a solution of 1.1�10–5 mMnTMPyP (a) and a solution of CGO (0.05 mg/mL)-MnTMPyP(1.1�10–5 m) (b) at pH 6 (A), 10 (B), and 13 (C). The variation ofthe Soret band position as a function of pH is shown in (D).
band of the CGO-MnTMPyP films does not change signifi-cantly in the pH range 6–10 (λmax ≈ 467 nm, see Figure S2in the Supporting Information). This is similar to the smallspectral changes observed for the dissolved species in thispH range (curve b, Figure 2, D; λmax ≈ 464 and 463 nm forpH 6 and 10, respectively). Moreover, this band completelydisappears at pH 13 for the CGO-MnTMPyP film. Thisseems to indicate that an excess of OH– ions causes therelease of MnIIITMPyP(OH–)2 from the film and corrobo-rates the detachment of the metalloporphyrin from graph-ene at high pH, as was deduced from the spectroscopicanalysis of dissolved CGO-MnTMPyP.
The electrochemical behavior of CGO-MnTMPyP solu-tions was investigated by differential pulse voltammetry(DPV) and compared with that of MnTMPyP solutions.The voltammograms of the dissolved species at pH 8 (Fig-ure 3, A) show two distinct peaks at –0.14 and +0.72 V,both for the MnTMPyP (curve a) and CGO-MnTMPyPspecies (curve b). The same behavior is observed at pH 13(Figure 3, B); the oxidation potentials of the two processesattributed to MnII/MnIII and MnIII/MnIV[52] (–0.2 and+0.16 V, respectively) are almost the same for the two spe-cies. Thus, it can be inferred that the electrostatic interac-tion between the metalloporphyrin and CGO does not sig-nificantly affect the redox behavior of the metal center ofthe porphyrin in dissolved CGO-MnTMPyP.
Figure 3. Differential pulse voltammograms obtained with glassycarbon electrodes (0.07 cm2) at a scan rate of 4 mV/s and 100 mVpulse for solutions of MnTMPyP (1.1�10–5 m) (a) and CGO(0.05 mg/mL)-MnTMPyP (1.1�10–5m) (b) at pH 8 (A) and 13 (B).Voltammograms of films of MnTMPyP (a) and CGO-MnTMPyP(b) on graphite (A = 0.28 cm2) at pH 13 (C).
Voltammograms of CGO-MnTMPyP films obtained byapplying methanolic solutions of this species on to glassycarbon (GC) electrodes and exposing them to aqueous elec-trolyte show similar features in the pH range 8–12 as thoseobtained for MnTMPyP solutions and films. However, atpH 13 the voltammogram of the MnTMPyP film (curve a)shows different characteristics to those of dissolvedMnTMPyP (Figure 3, B, curve a). The broad wave ataround –0.2 V can be attributed to MnII/MnIII as for dis-solved MnTMPyP. However, the two other processes lead-ing to higher oxidation states and which appear at higher
potentials, + 0.48 and + 0.75 V, seem to indicate that theproducts in this case are oxo monomers or dimers of MnIV
or MnV.[29,52,53] It seems that the potentials of these pro-cesses, not observed for dissolved MnTMPyP, are shifted tolower potentials due to the surface stabilization of high-valence states. A featureless voltammogram is obtained forCGO-MnTMPyP (Figure 3, C, curve b), which indicatesthe release of MnTMPyP from the films, as was also ob-served by UV/Vis spectroscopy.
Films of CGO-MnTMPyP, as prepared for the spectro-scopic and electrochemical experiments, were characterizedby X-ray photoelectron spectroscopy (XPS). The N 1s spec-trum of such a film on graphite is shown in Figure 4 (B)and can be compared with that of a MnTMPyP film inFigure 4 (A). Three peaks are observed in the two spectraafter deconvolution. In metalloporphyrins, the four pyrrolecore nitrogen atoms typically generate a single N 1s signalwith a binding energy of around 400 eV.[54] The MnTMPyPand CGO-MnTMPyP films both show two peaks attributedto the core nitrogen atoms (399.4 and 400.5 eV for themetalloporphyrin and 398.3 and 400.1 eV for the CGO-metalloporphyrin). This could be the result of different lig-ation of the metal ion,[55] such as by H2O molecules or OH–
ions. The additional N 1s peak at 402.4 and 402.7 eV forMnTMPyP and CGO-MnTMPyP, respectively, is attrib-uted to alkylated peripheral nitrogen atoms and have alsobeen observed (BE = 401.9 eV) for non-metallatedTMPyP.[56]
Figure 4. N 1s XPS spectra for MnTMPyP (A) and CGO-MnTMPyP (B) films on graphite and the peaks (a,b,c) obtainedafter deconvolution of the broad spectrum signals.
The Mn 2p XPS spectra for the MnTMPyP and CGO-MnTMPyP films are shown in Figure 5 (a and b, respec-tively). A broad 2p3/2 peak centered at 642.0 �0.2 eV is ob-served for these species, in agreement with the values re-ported for MnIII complexes of salen[57,58] and porphyrins.[59]
The peak observed at 653.2 �0.1 eV for both films is attrib-uted to 2p1/2.[59] The similarity of the Mn 2p signal posi-tions for MnTMPy and CGO-MnTMPyP, unlike the largeshift (1.8 eV) observed for the interaction of CoII tetraphen-ylporphyrin with an AgIII surface,[60] seems to indicate thatthe chemical environment of the metal center in the metall-oporphyrin is not significantly affected by the electrostaticinteraction with CGO. This is in accordance with the elec-trochemical results, which indicate no significant potentialshifts for the MnIII/MnII and MnIII/MnIV redox couples forCGO-MnTMPyP as compared with those of MnTMPyP.
Figure 5. Mn 2p XPS spectra for MnTMPyP (a) and CGO-MnTMPyP (b) films on graphite. The full black lines represent theaveraged signals.
Films of CGO-MnTMPyP and MnTMPyP were testedfor their possible ability to catalyze water electrooxidationat pH 10. The potentials of rotating ring disk electrodes(RRDE; ED), on which the films were applied, werescanned to water oxidation potentials and the potential ringelectrode (ER) was set at a potential of –0.5 V at which oxy-gen evolving from the disk could be reduced. It can be seenfrom Figure 6 that both MnTMPyP and CGO-MnTMPyPfilms increase the water oxidation currents obtained at thedisk electrode at potentials exceeding 1.3 V (curves b and c,respectively) compared with the current observed for a bareelectrode (curve a). Although the disk currents for bothfilms are almost identical, a significant increase in oxygenreduction current at the ring is observed for graphene-por-phyrin (450 μA at ED = 1.6 V, curve f) compared with thebare or porphyrin-coated disk electrode (300 μA at ED =1.6 V, curves d and e). It seems, therefore, that a significantincrease (50%) in the amount of oxygen obtained by theoxidation of water at a bare electrode under these condi-tions (pH 10 and E � 1.3 V) is obtained only by combiningCGO with MnTMPyP. These results are similar to litera-ture reports concerning water oxidation catalysis by dimericmanganese porphyrins yielding O2 at potentials above 1.2 Vversus Ag/AgCl.[26] This has been explained by a concertedinteraction between two short-lived, high-valent MnV=Ointermediates in each of the porphyrins.[52,60] The effect ofthe CGO on the metalloporphyrin catalytic behavior is at-tributed to the nature of the self-assembled CGO-MnTMPyP aggregates, as observed by TEM, which proba-bly increases the probability of the encounter of neigh-boring MnTMPyP molecules. This would in turn favor asynchronous bimolecular mechanism, as also suggested for
dimeric Mn tetraarylporphyrins linked by a 1,2-phenylenebridge[26] and for sulfonated monomeric Mn porphyrin af-ter being hosted in a polymer.[27]
Figure 6. RRDE voltammograms (rotation rate: 500 rpm; scanrate: 5 mV/s) obtained at pH 10 for water oxidation at bare (a),MnTMPyP-coated (b), and CGO-MnTMPyP-coated (c) disks, andoxygen reduction at the ring held at –0.5 V (d, e, and f are the ringresponses corresponding to the disk voltammograms a, b, and c,respectively).
In a previous study[44] it was established that heating aniron foam (cell diameter of 800 μm) in air at 500 °C coatsthe foam with a 300 nm oxide layer consisting of a 30 % α-Fe2O3 (hematite) outer layer and a 70% Fe3O4 (magnetite)inner layer. Aiming to enhance their photoanode responsetowards water splitting, in this work MnTMPyP or CGO-MnTMPyP were incorporated into the oxide layer by ad-sorption for 1 min from their respective aqueous solutions(prolonged adsorption caused diminished photoresponse,probably due to semiconductor masking) The water oxi-dation electrochemical response of the electrodes was inves-tigated in buffer solution at pH 10. The linear sweep vol-tammograms obtained for the nonilluminated foam in theabsence and presence of CGO-MnTMPyP (Figure 7,curves a and b) reveal that the adsorbed species increasesthe dark oxidation current at 1.23 V versus NHE (ca.0.43 V vs. Ag/AgCl) by a factor of around 2.8 (0.11 and0.31 mA/cm2, respectively). Moreover, the illuminated heat-treated foam with adsorbed CGO-MnTMPyP shows an in-creased photoresponse over the whole potential range of 0–1.0 V in comparison with illuminated bare and MnTMPy-containing foams (curves e, c, and d, respectively). With thelack of any direct evidence, it is suggested that the higheractivity of the CGO-MnTMPyP films stems from the pro-motion of MnIIITMPyP dimer formation by CGO. Theseare oxidized to MnIV-oxo-bridged dimers by holes obtainedby the illumination of the semiconductor. This is followed
by O2 release and recovery of the MnIIITMPyP (Scheme 1),as also proposed for the electrooxidation of heterodimers ofMnIII porphyrins at high anodic potentials and at pH 12.[52]
Figure 7. Linear sweep voltammograms obtained at a scan rate of5 mV/s in buffer solution at pH 10 for an iron foam/hematite elec-trode. Curves (a) and (b) are voltammograms obtained before illu-mination of the bare and CGO-MnTMPyP-coated electrode,respectively. Curves (c), (d), and (e) are voltammograms obtainedduring illumination of the bare, MnTMPyP-coated, and CGO-MnTMPyP-coated electrodes, respectively.
Scheme 1. Schematic description of the reactions involved in theelectro- and photoelectrocatalytic oxidation of water by CGO-MnTMPyP.
The chronoamperometric dark and photoresponses forthe bare, MnTMPy-, and CGO-MnTMPyP-containingfoams at +1.23 V versus NHE are depicted in Figure 8. Thephotocurrent obtained with CGO-MnTMPyP at this po-tential is substantially higher than that obtained in the ab-sence of any catalyst or in the presence of MnTMPyP (0.57,0.15, and 0.21 mA/cm2, respectively). The same trend wasobtained with the incident photons to current efficiency(IPCE) measured at 370 nm and 1.43 V versus NHE forilluminated bare, MnTMPyP-, and CGO-MnTMPyP-coated foams: 6.8, 6.2, and 9.0%, respectively (Figure 9).These values are within the range of those obtained at360 nm and 1.23 V versus NHE for undoped and Zr-dopedhematite nanorod arrays on fluorine-doped tin oxide (FTO)glass (3.8 and 13.6%, respectively).[61] The IPCE values for
these and similar systems reach a maximum at around 350–370 nm due to an increase in charge separation in hematiteat short wavelengths.[42] The CGO-MnTMPyP-coated pho-toanode shows improved performance in comparison withthose observed with other modifications of the foam oxidelayer, as investigated in our previous study (0.2 mA/cm2 at1.23 V vs. NHE for silica as dopant in hematite and CoII
ions as water oxidation catalyst).[44]
Figure 8. Dark (D) and photocurrents (L) obtained by chronoam-perometry in buffer solution at pH 10 at 1.23 V vs. NHE for a bare(a), MnTMPyP-coated (b), and CGO-MnTMPyP-coated (c) ironfoam/hematite electrode.
Figure 9. IPCE for a bare (a), MnTMPyP-coated (b), and CGO-MnTMPyP-coated (c) iron foam/hematite photoanode at 0.6 V vs.Ag/AgCl (1.42 V vs. NHE).
Conclusions
Self-assembled structures were obtained when mixingaqueous or methanolic solutions of the negatively chargedCGO and the positively charged MnTMPyP. Based uponUV/Vis spectroscopy, XPS, and electrochemical findings, ithas been concluded that electrostatic and π–π stacking in-teractions form between CGO and MnTMPyP, which haveno significant effect on the chemical environment of the Mnion in the metalloporphyrin. The CGO-MnTMPyP struc-tures show increased capability over MnTMPyP in the ca-talysis of water oxidation and evolve oxygen at high pH andE � 1.3 V versus Ag/AgCl. Substantial photoelectrochem-ical catalytic activity towards water splitting was also exhib-ited by adsorbed CGO-MnTMPyP on iron foam photoan-odes. These effects are attributed to CGO sheets serving asscaffolds for the accommodation of associated MnTMPyP
molecules which promote the evolution of oxygen. The in-vestigation of other assemblies consisting of reduced graph-ene derivatives and Mn-based porphyrins and other metal-loporphyrins is now in progress.
Experimental SectionMaterials: The chloride salt of manganese(III) meso-tetrakis(N-methyl-4-pyridyl)porphyrin (abbreviated: MnTMPyP) and a carb-oxy-rich graphene oxide (CGO, 1–5 μm diameter, 0.8–1.2 nm thick)water dispersion (5 mg/mL) were obtained from Midcentury andACS Material, respectively. Iron foams were obtained from Alan-tum Corp., Germany.
Preparation of Solutions and Films: Aqueous solutions were pre-pared with deionized water (18.2 MΩcm, Millipore). Electrolytesolutions for the spectroscopic and electrochemical experimentsconsisted of 0.05 m Na2SO4 and KOH to attain pH 6–8, whereasthe pH in the range 10–13 was adjusted by using 0.1 m Na2CO3/0.05 m NaHCO3 buffer and KOH. Concentrated solutions ofMnTMPyP or CGO in water (1 and 5 mg/mL, respectively) wereused to prepare diluted solutions in water or methanol. Dispersionscontaining both MnTMPyP and CGO were prepared by mixingsolutions of the two species so that their final concentrations were0.01 mg/mL (1.1�10–5 m) and 0.05 mg/mL, respectively, in water ormethanol. Films of these dispersions were obtained by applyingfresh solutions on the surface and were left to dry at room tempera-ture.
The adsorption of MnTMPyP and CGO-MnTMPyP on ironfoams was performed from aqueous solutions of these species andwas followed by measuring the optical density at λmax = 461–463 nm. The surface coverages of MnTMPyP and CGO-MnTMPyP for the photoelectrochemical experiments were deter-mined to be 3.7�10–12 and 2.1�1–10–10 mol/cm2, respectively,1 min after exposure of the foams to the respective solutions. Ad-sorption for a period of 5 min significantly increased the surfacecoverage of MnTMPyP in comparison with that of CGO-MnTMPyP (1.6�10–10 and 2.7�1–10–10 mol/cm2, respectively).
Methods: UV/Vis spectra were determined by using an Ocean Op-tics USB4000 spectrophotometer. The spectra of films were ob-tained by applying a dispersion (40 μL) of a methanolic mixture ofMnTMPyP and GO on transparent indium tin oxide (ITO) coatedpolyethylene tetraphthalate (PET; 4 cm2) and exposing them toaqueous electrolyte solutions at different pH.
Transmission electron microscopy (TEM, Tecnai 12 G2 TWIN)and scanning electron microscopy (SEM, JSM-7400F) were per-formed with coatings obtained by applying methanolic solutionsof MnTMPyP or MnTMPyP-CGO on to copper grids and glass,respectively.
X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with anAl X-ray source and monochromator was used to determine theoxygen CGO contents originating from the OH and COOH func-tionalities (ca. 41.5 and 7.0%, respectively) according to their C 1speaks at 286.8 and 288.5 eV, respectively. XPS spectra were ob-tained for MnTMPyP and MnTMPyP-GCO films on alumina-pol-ished graphite rods (60 μL of solution or dispersion on 0.28 cm2).
Differential pulse voltammetry (DPV), linear sweep voltammetry(LSV), and chronoamperometry (CA) experiments were performedwith a PCI4/300 potentiostat (Gamry). The working electrode inthe DPV experiments was glassy carbon (GC; A = 0.07 cm2) for theexamination of aqueous solutions of MnTMPyP and dispersions of
MnTMPyP-GCO, and was replaced by graphite rods (A =0.28 cm2) for the examination of films. All the electrochemical ex-periments used a platinum wire counter electrode and potentials(unless noted) are reported versus Ag/AgCl/KClsatd.
Rotating ring disk electrode (RRDE) experiments were performedwith a bipotentiostat (AFCBP1, Pine Instrument Co.) coupled witha Pine Instrument rotator and a RRDE electrode consisting of aGC disk (A = 0.19 cm2) and Pt ring. The disk was coated with anaqueous solution of MnTMPyP (90 μL, 0.01 mg/mL, 1.1�10–5 m)or MnTMPyP-GCO dispersion (0.01 and 0.05 mg/mL, respec-tively). The disk potential was scanned to water oxidation poten-tials (0 to +1.6 V) at a rate of 50 mV/s and the ring potential wasset to detect oxygen by its reduction at –0.5 V.
Photoelectrochemistry was conducted using a three-compartmentglass cell kept at 25 °C (Quantum Northwest TC 125 thermostat).The working electrode in this case was an iron foam with cell dia-meter of 800 μm, 2.0� 0.5 mm thick, which was first heated for 1 hin air (Carbolite STF 16/180 tubular furnace). The surface area ofthe electrode exposed to the electrolyte solution, 2.0�0.5 cm2, wascalculated from the surface density, as determined by the supplier(500 g/m2) This electrode faced a quartz window, through whichit was illuminated [Newport Oriel Product, 200 W Hg(Xe) lamp,100 mW/cm2]. The incident photons to current efficiencies (IPCE)were measured by using a xenon lamp (450 W Osram® xenon arclamp) coupled to a monochromator (Jobin-Yvon single-grating).Dark and photocurrents were measured with CH Instruments650C electrochemical workstations in a three-electrode experimen-tal configuration with a platinum gauze counter electrode andpseudo Ag/AgCl reference electrode. Light intensity was measuredat each wavelength tested by using a calibrated silicon diode detec-tor (Newport Corp. model 818-UV) to obtain the power densityspectrum. IPCE(%) versus wavelength measurements were ob-tained in 10 nm increments. The photocurrents at pH 10 and anapplied bias of 0.6 V versus Ag/AgCl (1.42 V vs. NHE) were deter-mined by subtracting the average dark current from the averagelight current.
Supporting Information (see footnote on the first page of this arti-cle): Floculation occurring in aged CGO-MnTMPyP solutions,UV/Vis spectra of CGO-MnTMPyP films.
Acknowledgments
The authors would like to thank the office of the Chief Scientist ofthe Israel Ministry of Energy and Water Resources for fundingthis study. Professor Devens Gust, Department of Chemistry andBiochemistry, Arizona State University, USA, is thanked for al-lowing one of the authors (A. K.) to use the IPCE equipment inhis laboratory.
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A. Kaplan, E. Korin, Carboxylic-rich graphene oxide and MnIII
A. Bettelheim* .................................. 1–9 tetrakis(N-methyl-4-pyridinio)porphyrinform self-assembled structures through π–π stacking interactions. When adsorbed onStructures Self-Assembled from Anioniciron/hematite photoanodes, these struc-Graphene and Cationic Manganese Por-tures show substantial activity towards thephyrin: Characterization and Applicationphotoelectrochemical oxidation of water.in Artificial Photosynthesis
