Functionalized Cobalt Triarylcorrole Covalently Bonded with

Jul 25, 2017 - The electrocatalytic activity of GO-CorCo toward the oxygen reduction reaction (ORR) was then examined in air-saturated 0.1 M KOH and 0...
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Functionalized Cobalt Triarylcorrole Covalently Bonded with Graphene Oxide: A Selective Catalyst for the Two- or Four-Electron Reduction of Oxygen Jijun Tang,†,‡ Zhongping Ou,*,†,§ Rui Guo,† Yuanyuan Fang,† Dong Huang,‡ Jing Zhang,‡ Jiaoxia Zhang,‡ Song Guo,*,∥ Frederick M. McFarland,∥ and Karl M. Kadish*,§ †

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013 China National Demonstration Center for Experimental Materials Science and Engineering Education, Jiangsu University of Science and Technology, Zhenjiang, 212003 China ∥ Department of Chemistry and Biochemistry, University of Southern Mississipi, Hattiesburg, Mississippi 39406, United States § Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States ‡

S Supporting Information *

ABSTRACT: A cobalt triphenylcorrole (CorCo) was covalently bonded to graphene oxide (GO), and the resulting product, represented as GO-CorCo, was characterized by UV−vis, FT-IR, and micro-Raman spectroscopy as well as by HRTEM, TGA, XRD, XPS, and AFM. The electrocatalytic activity of GO-CorCo toward the oxygen reduction reaction (ORR) was then examined in air-saturated 0.1 M KOH and 0.5 M H2SO4 solutions by cyclic voltammetry and linear sweep voltammetry using a rotating disk electrode and/or a rotating ring-disk electrode. An overall 4-electron reduction of O2 is obtained in alkaline media while under acidic conditions a 2-electron process is seen. The ORR results thus indicate that covalently bonded GO-CoCor can be used as a selective catalyst for either the 2- or 4-electron reduction of oxygen, the prevailing reaction depending upon the acidity of the solution.

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INTRODUCTION The oxygen reduction reaction (ORR) plays a crucial role in electrochemical energy conversion technologies.1−9 Electrocatalysts are required, both to minimize overpotential and to lower the activation energy barrier of the sluggish ORR. Platinum and platinum alloys have long been accepted as the best catalyst materials for ORR due to their good catalytic efficiency and high stability in natural environments, but these materials still have problems10−12 related to their high price and scarcity, which has limited their use in large scale commercialization of fuel cells and other energy conversion devices.13,14 For this reason, much research effort has been dedicated over the past two decades to exploring the use of new and better ORR electrocatalysts which are based on either low cost nonnoble metals and their oxides or metal-free compounds to replace platinum.15−27 Graphene oxide (GO) is a solution-dispersible form of graphene that has a unique structure and outstanding optical, electrical, and mechanical properties.28−32 In particular, the presence of epoxy and hydroxyl functional groups on graphene oxide can act as structural nodes in metal organic frameworks or they can bind to organometallic and free-base macrocycles, such as porphyrins, resulting in GO functionalized electrocatalysts which may possess improved electron transfer efficiency for ORR.33−41 © 2017 American Chemical Society

Cobalt corroles are one of the most studied porphyrin analogues because of their unique electrochemical properties42 and their potential application as catalysts for a variety of reactions,43−52 especially for ORR under different solution conditions.53−63 The functionalization of metallocorroles with selected electron-donating or electron-withdrawing substituents can significantly alter the catalytic activity of the compounds for ORR.53−63 In recent years, carbon-supported transition metal macrocyclic complexes, such as nickel-/cobalt-porphyrins,41 iron phthalocyanines25 and porphyrins,25,33 cobalt porphyrins,37 manganese corroles,50 and cobalt corroles,48,50 have attracted a great interest as alternative cathode catalysts for ORR.3,6,8 However, to the best of our knowledge, metallocorroles functionalized by GO have never been prepared and examined as electrocatalysts for ORR. This is addressed in the present manuscript where cobalt 5,15-di(4-methylphenyl)-10-(4-aminophenyl)corrole triphenylphosphine, represented as CorCo 2, was functionalized by graphene oxide, GO 1, and examined as to its catalytic activity for ORR in both basic and acidic media. The covalently bonded cobalt corrole, represented as GO-CorCo 3, was characterized Received: April 11, 2017 Published: July 25, 2017 8954

DOI: 10.1021/acs.inorgchem.7b00936 Inorg. Chem. 2017, 56, 8954−8963

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Inorganic Chemistry

Figure 1. Structures of GO 1, CorCo 2, and GO-CorCo 3.

Figure 2. (a) TGA and (b) Raman spectra of GO 1 and GO-CorCo 3.

GO, which agrees with published data in the literature,36,66,67 while CorCo exhibits a split Soret band at 394 and 425 nm and a weak Q-band at 566 nm. The GO functionalized CorCo exhibits a broad band at 320 nm under the same solution conditions, but bands assigned to the corrole are too weak to be observed in the functionalized material because the molar absorptivity of corroles is known to be much lower than that of GO. However, a small (2 nm) difference in the position of the 318−320 nm band is seen between GO 1 and GO-CorCo 3 (Figure S1b) and, when combined with data from IR, XRD, TGA, Raman spectra, and XPS measurements, provides additional evidence to confirm that GO-CorCo is the material characterized. Figure 2a shows TGA data for GO 1 and covalently bonded GO-CorCo 3 between room temperature (∼22 °C) and 800 °C. In the case of GO, two steps of weight loss are observed, the first from 25 to 100 °C and the second from 150 to 230 °C, corresponding to 14 and 22% losses, respectively. The first step in the TGA spectrum of the GO sample has been assigned to evaporation of adsorbed water while the second step is due to a loss of the labile oxygen groups on GO.68 A different TGA curve is obtained for GO-CorCo, where a 15% weight loss occurs from 230 to 480 °C, but no weight loss is seen for the oxygen groups, consistent with the functionalizing of CorCo and GO after a reaction between the NH2 groups of CorCo and the COCl groups of GO. Raman measurements were carried out on GO 1 and GOCorCo 3, and the spectra are shown in Figure 2b. In a typical Raman spectrum, the G band is assigned to the first-order

by a variety of spectral techniques and also by TEM, TGA, XRD, XPS, and AFM. The electrocatalytic activity of GOCorCo toward the oxygen reduction reaction (ORR) was also examined in air-saturated 0.1 M KOH and 0.5 M H2SO4 solutions by cyclic voltammetry and linear sweep voltammetry using a rotating disk electrode (RDE). Chemical structures for GO 1, CorCo 2, and GO-CorCo 3 are schematically illustrated in Figure 1.

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RESULTS AND DISCUSSION Characterization of GO-CorCo 3. The functionalized GOCorCo 3 and its precursors, GO 1 and CorCo 2, were characterized by Fourier transform infrared spectrometry (FTIR) and UV−vis spectra in DMF (Figure S1). The FTIR spectrum of GO (Figure S1a) has signals attributable to a stretching of the carbonyl bonds (CO) at 1730 cm−1 and O−H deformations at 3398 and 1370 cm−1. A C−O−C stretching vibration is located at 1063 cm−1. A band at 1628 cm−1 is also seen and was earlier assigned to the stretching vibration of a CC bond or a skeletal vibration of the aromatic ring of GO.36,64,65 The peak of GO which is located at 1730 cm−1 is not seen in the spectrum of GO-CorCo. The 1628 cm−1 band of GO shifts to 1630 cm−1 in GO-CorCo, and may correspond to the characteristic CO stretching frequency of the linked NH−CO amide group from the covalently bonded GO-CorCo.36,66 A C−N stretching band of the amide also appears at 1260 cm−1 in the spectrum of GO-CorCo. Figure S1b shows the UV−vis spectra of GO 1, CorCo 2, and GO-CorCo 3 in DMF. A band at 318 nm is observed for 8955

DOI: 10.1021/acs.inorgchem.7b00936 Inorg. Chem. 2017, 56, 8954−8963

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Figure 3. XPS spectra of (a) GO 1 and (b) GO-CorCo 3.

Figure 4. AFM images and height profiles of (a) GO 1 and (b) GO-CorCo 3 on silica surface and TEM of (c) GO 1 and (d) GO-CorCo 3.

scattering of the E2g mode observed for the sp2 carbon domains, and the pronounced D band is connected with disordered structural defects.36,69,70 The Raman spectra of GO 1 and GOCorCo 3 are both characterized by two major peaks at 1346 and 1592 cm−1, which correspond to the D and G bands, respectively. The relative intensity of the D and G bands (ID/IG) can be used to estimate the degree of structural disorder. As seen in Figure 2b, the ID/IG ratio of GO-CorCo 3

(0.99) is much larger than that of GO 1 (0.40). The increased ratio of ID/IG ratio for GO-CorCo 3 indicates the a conversion of the sp2 carbons to sp3 carbons on the graphene oxide surface,36,69,70 thus suggesting that the cobalt corrole was successfully bonded with GO to generate GO-CorCo. Figure S2 illustrates the XRD patterns of GO 1 and GO-CorCo 3. The XRD pattern of GO is characterized by a reflection peak at 2θ = 10.4° corresponding to the plane of 8956

DOI: 10.1021/acs.inorgchem.7b00936 Inorg. Chem. 2017, 56, 8954−8963

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Figure 5. (a) Cyclic voltammograms of GO-CorCo 3 coated on EPPG electrode in N2 (- - -) and air-saturated () 0.1 M KOH at a scan rate of 50 mV/s and (b) linear sweep voltammograms in the air-saturated KOH solution of 3 with different rotating rates at a scan rate of 10 mV/s.

sheet with dark spots (Figure 4d), which is probably due to extensive stacking of CorCo on the GO surface. It should be noted that the AFM and TEM results, by themselves, are not sufficient to confirm formation of a covalent bond between GO and the corrole, and other techniques are needed. Nonetheless, in the AFM measurements, significant morphological differences are observed between GO and GO-CorCO, and this strengthens the conclusion that CorCo is covalently bonded with GO. The catalytic activity of covalently bonded GO-CorCo 3 toward the reduction of O 2 was examined by cyclic voltammetry and rotating ring-disk electrode voltammetry in the presence of base. Cyclic voltammograms of 3 adsorbed on an EPPG disk electrode under N2 (- - -) and in an air-saturated () KOH solution are illustrated in Figure 5a. A single broad surface reduction is observed at 0.05 V for GO-CorCo 3 in the N2-saturated solutions, while two irreversible reductions with high peak currents are observed at Epc = 0.70 and 0.10 V in the presence of air which contains O2. The more positive cathodic reduction peak at 0.70 V which is not observed in the N2-saturated solution is assigned to the catalytic reduction of dissolved O2. Linear sweep voltammetric measurements were performed in the air-saturated solution (Figure 5b) using a rotating disk electrode (RDE) for calculating the number of electrons transferred during the catalytic electroreduction of dioxygen. The disk rotation rate varied from 100 to 1600 rpm as shown in Figure 5b. The number of electrons transferred was calculated from the magnitude of the steady-state limiting current values, which were taken at a fixed potential on the catalytic wave plateau of the current−voltage curves. The slope of the diagnostic Koutecky−Levich plot76 obtained by linear regression was then used to estimate the average number of electrons involved in the catalytic reduction of O2. The Koutecky−Levich plots are interpreted on the basis of eq 1, where jlim is the measured limiting current density (mA· cm−2), jk is the kinetic current, and jlev is the Levich current which is used to measure the rate of the current-limiting chemical reaction as defined by eq 2. The value of n in eq 2 corresponds to the number of electrons transferred in the overall electrode reaction, F is the Faraday constant (94685 C mol−1), A is the electrode area (cm2), D is the dioxygen diffusion coefficient (cm2 s−1), c is the bulk concentration of O2 (M) in 0.1 M KOH, v is the kinematic viscosity of the solution, and ω is the angular rotation rate (rad s−1) of the electrode.

graphene oxide with a calculated interlayer spacing of 0.84 nm according to the Prague formula.71−73 However, graphene has a diffraction peak at 2θ = 24.6° which may possess an interlayer spacing of 0.35 nm.72 The increased d-spacing is attributed to the presence of oxygen functionalities in the gallery spacing of GO. However, after formation of GO-CorCo, the peak at 10.4° disappears completely while a broad peak is seen at 26.5° due to the GO being partially reduced and leading to the diffraction peak shifting to the peak of graphene.71 This result further indicates that the corrole has been successfully attached to the surface of GO. The XPS spectra of GO 1 and GO-CorCo 3 are shown in Figure 3. GO exhibits distinct C 1s and O 1s peaks with no other elements being detected from 0 to 800 eV (Figure 3a). In contrast, the XPS spectrum of covalently bonded GO-CorCo (Figure 3b) exhibits Co 2p (780.5 eV) and N 1s (401.8 eV) peaks in addition to signals for the O and C elements. A similar XPS spectrum with a low intensity Co 2p peak was previously reported for a covalently bonded cobalt porphyrin and GO.36 The C 1s signal of GO encompasses three types of carbons as shown in the lower spectrum of Figure 3a. There are a nonoxygenated C−C signal at 283.3 eV, a C−O signal at 285.3 eV, and an O−CO signal at 286.8 eV, all three of which are consistent with assignments reported in the literature.36 Only two types of carbons are detected in the C 1s XPS spectrum of GO-CorCo (lower spectrum in Figure 3b). There are a C−C signal at 284.8 eV and an N−CO signal at 290.0 eV. As expected, the C−O peaks of GO are no longer present because CorCo is attached to the surface of GO through the amidation reaction and the reduction of oxygen.36,74 The atomic force microscopy (AFM) images are also consistent with CorCo being covalently bonded to the GO. The average thickness of GO-CorCo 3 is significantly larger than that of GO 1. The insets shows cross-section profiles taken along with the dotted line marked in the AFM pographic images (Figures 4a and 4b). The average thickness of GO was measured as 1.4 nm (Figure 4a), which is similar to the value previously reported in the literature,34,75 while the measured thickness of covalently bonded GO-CorCo is 2.3 nm (Figure 4b). In order to understand the morphology of GO and GO-CorCo, transmission electron microscopy (TEM) measurements were carried out. The samples were prepared by placing a few drops of dispersion onto a Cu grid. The TEM image of GO is a few micrometers in length with a fold as seen in Figure 4c. The GO-CorCo retains the same layered structure of graphene oxide, but it possesses a wrinkled graphene oxide

1/jlim = 1/jlev + 1/jk 8957

(1) DOI: 10.1021/acs.inorgchem.7b00936 Inorg. Chem. 2017, 56, 8954−8963

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Inorganic Chemistry jlev = 0.62nFAD2/3cν−1/6ω1/2

reduction of dioxygen at reduction potentials more negative than 0.20 V vs RHE in 0.1 M KOH. In order to demonstrate how the catalytic activity for ORR changes upon covalently bonding the corrole and GO, the individual components GO 1 and CorCo 2 were investigated as catalysts for O2 reduction. Also investigated under the same solution conditions was a physical mixture of GO and CorCo, which is labeled as 4. The cyclic and linear sweep voltammograms of GO 1, CorCo 2, and 4 in air- and nitrogen-saturated 0.1 M KOH are given in Figures 7 and S3, while Figure 8 compares cyclic voltammograms for GO 1, CorCo 2, GO-CorCo 3, and a mixture of GO and CorCo 4 in airsaturated KOH. As seen in this latter figure, a catalytic reduction peak is located at Ep = 0.61 V for 1 and 0.59 V for 2 in the cyclic voltammograms, but two reduction peaks are observed for 3 at 0.70 and 0.10 V. Both peaks of 3 are shifted positively by 50−80 mV as compared to the peaks of 4 (a mixture of 1 and 2) under the same solution conditions. The voltammograms in Figure 8 indicate that ORR can be catalyzed not only by GO-CorCo but also by each component itself, but the data in Figure 6 shows that only GO-CorCo 3 is able to function as a 4-electron catalyst for ORR. This contrasts with 1, 2, and 4 where the Koutecky−Levich plots demonstrate that the number of electrons transferred for ORR at 0.0 V is 2.5 for 1, 2.7 for 2, and 2.8 for the mixture 4 (Figure S4). It should be noted that a significant difference is seen in the cyclic voltammograms of covalently bonded GO-CorCo 3 and a mixture of GO 1 and CorCo 2 (see Figures 5 and S3), with the number of electrons transferred being n = 2.8 for the mixture and 4.0 for 3 as shown in Figure S4. This result indicates that covalently bonding the functionalized corrole with GO will significantly enhance the measured catalytic activity for ORR. This is consistent with what has previously been reported in the literature where an enhanced catalytic performance for ORR was observed for functionalized metallomacrocycles supported on GO, carbon nanotubes, and other carbon materials.25,27,34,36,41,48,49 The catalytic activity of the covalently bonded GO-CorCo was also examined in 0.5 M H2SO4. Figure 9a illustrates the cyclic voltammograms of GO-CorCo 3 in N2- (···) and airsaturated () acid solutions at a scan rate of 50 mV/s. Again, the high peak current in air indicates that a catalytic reduction of the O2 occurs under the experimental conditions. The linear sweep voltammograms of 3 coated on an RDE with different rotation rates at a scan rate of 10 mV/s show that the catalytic currents increase at 0.60 V and reach diffusion limited values at 0.35 V (Figure 9b). The Koutecky−Levich plots at different electrode potentials were then constructed and are shown in Figure 9c. As seen in this figure, the value of n is 2.0 in each case, consistent with a 2e− rather than 4e− reduction occurring during the ORR in acid media. Thus, H2O2 is the only O2 reduction product and no H2O is formed in airsaturated 0.5 M H2SO4. The relevant reaction is given by eq 6.

(2)

The Koutecky−Levich plots are shown in Figure 6a for GO-CorCo 3 and indicate that the number of electrons

Figure 6. (a) Koutecky−Levich plots at different electrode potentials of GO-CorCo 3 and (b) plots of electrons transferred vs potential of ORR using catalysts 1−4 coated EPPG electrode in air-saturated 0.1 M KOH.

transferred (n) in the electroreduction process of O2 is 2.0−2.9 over the potential range of 0.5 to 0.3, but this value increases to 3.9−4.0 over the potential range of 0.2 to 0.0 V. It is well-known that ORR may involve multiple electrochemical reactions8 and can proceed through a two-step, twoelectron (2e−) pathway with the initial formation of HO2− as the intermediate species (eq 3) followed by further 2e− reduction of HO2− to give OH− (eq 4). Alternatively, a fourelectron (4e−) process can also occur to reduce dioxygen into OH− in a single overall process (given by eq 5).6−8 2e− process:

O2 + H 2O + 2e− → HO2− + OH−

H 2O + HO2− + 2e− → 3OH− −

4e process:

(3) (4)

−

O2 + 2H 2O + 4e → 4OH

−

(5)

As indicated above, the measured number of electrons transferred (n) varied between 2.0 and 2.9 for GO-CorCo 3 over the potential range of 0.5 to 0.3 V (Figure 6b), proving that the catalytic reduction of O2 gives a mixture of HO2− and OH− under these experimental conditions. This contrasts with the n = 3.9−4.0 seen for the same catalyst 3 at potentials between 0.20 and 0.0 V (Figure 6b), suggesting that an overall four-electron process occurs over this potential range. However, a 2e− + 2e− process rather than a single 4e− process is also possible in an air-saturated solution using GO-CorCo as the catalyst. Whatever the case, the results of this study indicate that a functionalized cobalt corrole, when coated on the EPPG electrode, is a highly efficient electrocatalyst for the 4e−

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O2 + 2e− + 2H+ → H 2O2

(6)

CONCLUSION In summary, we have prepared and characterized a functionalized cobalt corrole which was covalently bonded with graphene oxide via a reaction of the corrole and chloridefunctionalized GO. The catalytic activity of the GO-functionalized corrole for the oxygen reduction reaction was examined by cyclic voltammetry using a rotating disk electrode in both 8958

DOI: 10.1021/acs.inorgchem.7b00936 Inorg. Chem. 2017, 56, 8954−8963

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Inorganic Chemistry

Figure 7. Cyclic voltammograms (CV) and linear sweep voltammograms (LSV) of (a) GO 1 and (b) CorCo 2 coated EPPG electrode in 0.1 M KOH. CV at a scan rate of 50 mV/s and LSV in air-saturated KOH solution with rotating rate at 100−1600 rpm at a scan rate of 10 mV/s. purchased from Sinopharm Chemical Reagent Co. or Sigma-Aldrich and used as received. Preparation of GO-CorCo 3. GO-CorCo was prepared from the cobalt corrole and graphene oxide via the following procedure. Graphene oxide (GO) was prepared by a modified Hummers method.77 1 g of graphite, 1 g of NaNO3, 4.6 g of KMnO4, and 50 mL of H2SO4 were added very slowly into a Teflon reactor and stirred at 0 °C for 2 h followed by another 3 h at 100 °C. About 500 mL of deionized water was slowly added into the reaction mixture under vigorous stirring, which was followed with a drop by drop addition of 30% H2O2 into the mixture until the color became bright yellow. The reaction mixture was washed with 5 wt % hydrochloric acid, and, after centrifugation (4000 rpm), the solid was washed with deionized water and freeze-dried, resulting in pure GO. 30 mg of GO was then transferred into the solution of SOCl2 (20 mL) and DMF (0.5 mL) and refluxed at 70 °C for 24 h under an argon atmosphere. After removal of excess SOCl2 and solvent by distillation, pure acyl chloride functionalized graphene oxide (GOCl) was obtained. Cobalt 5,15-di(4-methylphenyl)-10-(4-aminophenyl)corrole triphenylphosphine (CorCo) was synthesized following a procedure described in the literature.78,79 5-(4-Methylphenyl)dipyrromethane (475 mg) and 4-nitrobenzaldehyde (150 mg) were dissolved in CH3OH (100 mL), and HCl (0.9%, 100 mL) was added to the solution, which was stirred at room temperature for 1 h. The products were then extracted with CHCl3. The organic layer was collected and washed twice with H2O, then dried with Na2SO4, filtered, and diluted to 200 mL with CHCl3. After addition of p-chloranil (490 mg), the mixture was refluxed for 1 h and then chromatographed on a silica column using CH2Cl2/hexane as eluent. The free-base nitrocorrole was obtained by evaporating the solvent to dryness. The nitrocorrole was added to a flask containing SnCl2·2H2O (450 mg, 2 mmol) in concentrated HCl (50 mL), after which the mixture was stirred at 75 °C for 2 h and then neutralized with an ammonia solution (25%) to pH 7. The product was extracted with ethyl acetate, and the organic layer was collected and washed three times with water,

Figure 8. Cyclic voltammograms of GO 1, CorCo 2, GO-CorCo 3, and a mixture of GO and CorCo 4 in air-saturated 0.1 M KOH. Scan rate = 50 mV/s.

alkaline and acid solutions. The Koutecky−Levich equation was used to determine the number of electrons transferred, and the results indicated that the GO-CorCo can be used as a selective catalyst for the 2e− ORR in acid media or the 4e− ORR in alkaline media.

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EXPERIMENTAL SECTION

Chemicals. N,N′-Dimethylformamide (DMF) was purchased from Sigma-Aldrich and used as received for electrochemical measurements. Other solvents for synthesis and characterization are analytical grade, 8959

DOI: 10.1021/acs.inorgchem.7b00936 Inorg. Chem. 2017, 56, 8954−8963

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In the presence of triethylamine (∼0.4 mL), GOCl (20 mg) was allowed to react with CorCo (20 mg) in DMF (8 mL) at 130 °C for 72 h under argon. The solution was then cooled to room temperature and poured into ether (200 mL). The precipitate was isolated by filtration on a nylon membrane (0.22 mm). Excess CorCo and other impurities were removed by washing several times with tetrahydrofuran and CHCl3, resulting in the GO-CorCo final product. Instrumentation. UV−vis absorption spectra were recorded with a ZF-I spectrophotometer (Gucun Dianguang Instruments, Baoshan, Shanghai). Fourier transformed infrared (FT-IR) measurements were carried out with a Nexus-470 spectrometer (Nicolet Co., USA). Surface morphology was examined by field emission scanning electron microscopy (TEM, JSM-6700F, Japan). The thermal stability was assessed by thermogravimetric analysis (TGA Q500, PE Co., USA) from room temperature to 800 °C under a nitrogen atmosphere at heating rate of 20 °C/min. Raman spectra were recorded on a surface enhanced Raman spectrophotometer (Invia, Renishaw Co., U.K.). Xray diffraction (XRD) measurements were carried out on an XRD6000X (Shimadzu Co., Japan) diffractometer at 40 kV with Cu Kα radiation (λ = 1.541 Å) with 2θ ranging from 5 to 60°. X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB 250Xi X-ray photoelectron spectrometer (Thermo Fisher Scientific, Inc., USA). The excitation source was monochromatic Al Kα radiation. The height profiles of the catalysts were characterized by atomic force microscopy (AFM, NT-MDT NTEGRA Prima). For AFM measurements, graphene oxide (GO) was dispersed by sonication in DI water and then dip-cast onto a Piranha treated SiOx wafer. GO-CorCo was similarly dispersed by sonication in DMF and then dip-cast onto a Piranha treated SiOx wafer. AFM topography measurements were done in a semicontact mode using silicon tips with a force constant of 1.2−6.4 N/m purchased from MikroMasch. All electrochemical measurements were carried out at 298 K using an EG&G Princeton Applied Research (PAR) 173 potentiostat/ galvanostat or a CHI-730C Electrochemistry Workstation. A threeelectrode system was used and consisted of a RDE working electrode for cyclic voltammetry and rotating disk voltammetry. A platinum wire served as the auxiliary electrode and a homemade saturated calomel electrode (SCE) as the reference electrode, which was separated from the bulk of the solution by means of a salt bridge. The potentials measured vs SCE were then converted to potentials vs the reversible hydrogen electrode (RHE). The relevant conversion is given by the equation E(RHE) = E(SCE) + 0.998 V in alkaline media and E(RHE) = E(SCE) + 0.304 V in acid media.80,81 The RDE was purchased from Pine Instrument Co. and consisted of an edge-plane pyrolytic graphite (EPPG) disk (A = 0.196 cm2). A Pine Instrument MSR speed controller was used for the RDE experiments. The catalysts (∼2 mg) were dissolved into a 20 mL mixture of H2O/DMF (v/v = 1/1) and treated with ultrasound for 24 h prior to use. The catalyst was then adsorbed onto the electrode by transferring aliquots of the solution directly onto the surface of the electrode, followed by evaporation of the solvent.

Figure 9. (a) Cyclic voltammograms of GO-CorCo 3 coated on an EPPG electrode in air- and nitrogen-saturated 0.5 M H2SO4 at a scan rate of 50 mV/s, (b) linear sweep voltammograms of GO-CorCo 3 in air-saturated KOH with different rotation rates at a scan rate of 10 mV/s, and (c) Koutecky−Levich plots at different electrode potentials.

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ASSOCIATED CONTENT

S Supporting Information *

dried with Na2SO4, and then chromatographed on a basic alumina column to afford the free-base amino corrole. The amino-substituted free-base corrole was dissolved in 30 mL of CH3OH containing excess Co(OAc)2·4H2O and triphenylphosphine. The mixture was heated to reflux for 1 h and the progress of the reaction monitored by thin-layer chromatography until the amino containing corrole was consumed. The sample was evaporated to dryness and chromatographed on a basic alumina column using ethyl acetate/hexane as eluent. The red fraction was collected and the solvent evaporated to dryness. The pure (NH2Ph)(CH3Ph)2CorCo(PPh3) product was obtained with a yield of 5.4%. UV−vis (DMF): λmax = 394, 425, 566 nm. 1H NMR (400 MHz, CDCl3): δ = 8.61 (d, J = 12.4 Hz, 2H), 8.36 (m, 2H), 8.16 (s, 2H), 8.04 (s, 4H), 7.85 (s, 2H), 7.70 (m, 2H), 7.44 (s, 6H), 7.05 (d, J = 6.6 Hz, 3H), 6.71 (m, 6H), 4.75 (m, 6H), 3.57 (m, 2H), 2.59 (s, 6H). MS (MALDI-TOF): m/z calcd for C57H43CoPN5 932.30; found [M − PPh3]+ 625.10.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00936. FT-IR and UV−vis spectra, XRD patterns, cyclic and linear sweep voltammograms, and Koutecky−Levich plots (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. 8960

DOI: 10.1021/acs.inorgchem.7b00936 Inorg. Chem. 2017, 56, 8954−8963

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Inorganic Chemistry ORCID

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Frederick M. McFarland: 0000-0002-8191-962X Karl M. Kadish: 0000-0003-4586-6732 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the Natural Science Foundation of China (21501070), Jiangsu University Foundation (05JDG051, 15JDG131), startup funds from University of Southern Mississippi (S.G.), NSF-NRT: INTERFACE (F.M.M., NSF Award No. 1449999), and the Robert A. Welch Foundation (K.M.K., Grant E-680) are gratefully acknowledged.

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DOI: 10.1021/acs.inorgchem.7b00936 Inorg. Chem. 2017, 56, 8954−8963