Cobalt–Salen Complexes as Catalyst Precursors for Electrocatalytic

Article pubs.acs.org/JPCC

Cobalt−Salen Complexes as Catalyst Precursors for Electrocatalytic Water Oxidation at Low Overpotential Haiyan Chen, Zijun Sun, Xiang Liu, Ali Han, and Pingwu Du* CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, and the Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), University of Science and Technology of China, Hefei, 230026, China

J. Phys. Chem. C 2015.119:8998-9004. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/16/18. For personal use only.

S Supporting Information *

ABSTRACT: Water oxidation is an important half-reaction to achieve overall water splitting. In this present study, we show that a series of molecular cobalt−salen complexes can serve as catalyst precursors to form nanostructured and amorphous cobalt-based thin films during electrodeposition, which can catalyze the water oxidation reaction at low overpotentials. Cyclic voltammetry and bulk electrolysis using the cobaltbased film electrodes demonstrated obvious catalytic currents in 0.1 M KBi solution at pH 9.2. The onset catalytic potentials of the catalyst films are at ∼0.84 V (vs Ag/AgCl) with a film made by electrodeposition of cobalt−salen complex 2 on FTO and at ∼0.85 V for complex 4. Oxygen gas bubbles were clearly seen on the FTO electrode when the applied potential was above the onset potential. The Tafel plots using a catalyst film made of complex 4 showed that appreciable catalytic current was observed starting at η = 0.26 V for the film (a current density of 0.01 mA/cm2 required η = 290 mV), accompanied by a Faradaic efficiency >93% at 1.2 V. The catalyst film was further characterized by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS).

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INTRODUCTION Water splitting to produce hydrogen is considered to be an ideal pathway to provide clean energy in the future. In nature, the oxidation of water (2H2O → O2 + 4H+ + 4e−) is achieved in photosystem II (PSII), in which the active site contains a CaMn4Ox cluster.1−4 It is believed that water oxidation reaction is more difficult than the process of water reduction because the former reaction involves a 4e−/4H+ process.5,6 To mimic the structure and function of the CaMn4Ox cluster in photosystem II, extensive research efforts have been devoted to the development of PSII model complexes for the oxidation of water.6−10 For instance, various manganese complexes have been studied.6,11−14 In addition, a series of synthetic complexes and metal oxides of ruthenium and iridium have been developed as water oxidation catalysts (WOCs) since the late 1970s.15−20 However, due to the low abundance and high cost of these noble metals, limitation may arise for the usage of the catalysts in constructing artificial photosynthetic systems.21 As a consequence, developing cheap and abundant materials as WOCs with low overpotential and high efficiency is highly desirable.22 In the literature, various homogeneous and heterogeneous systems based on other first-row transition metals have attracted much attention for the oxygen evolution reaction (OER), such as cobalt (Co),23−25 nickel (Ni),26−29 copper (Cu),30−32 and iron (Fe).33−35 More recently, it has been found that active WOCs can be prepared by electrodeposition or photodeposition from organic metal complexes. The Spiccia group reported nickel oxide © 2015 American Chemical Society

(NiOx) water oxidation catalysts can be electrodeposited from [Ni(en)3]Cl2 (en = 1,2-diaminoethane) in a 0.10 M borate buffer (NaBi) solution (pH = 9.2).36,37 A stable current of 1.8 mA/cm2 was achieved at 1.1 V (vs Ag/AgCl), in contrast to 1.2 mA/cm2 for films derived from [Ni(OH2)6](NO3)2 in a 0.60 M NaBi buffer. In 2012, Fukuzumi, Nam, and co-workers reported on the use of organic cobalt complexes as precursors for photocatalytic water oxidation (λ > 420 nm) at pH 6.0−10.38 In a system containing [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine), Na2S2O8, and water-soluble cobalt complexes with various organic ligands as precatalysts, the optimal turnover numbers of photocatalytic water oxidation to evolve O2 reached 420 at pH 9.0. The formation of CoOx nanoparticles, as the real catalyst was confirmed by 1H NMR measurements, dynamic light scattering (DLS), and transmission electron microscopy (TEM) experiments. Our group recently reported that nanostructured CoOx films could be facilely prepared by electrodeposition from a series of cobaloximes,39,40 which showed yellow or green colors with high activity as WOCs. In this present study, we report that cobalt−salen complexes can serve as precursors to deposit nanostructured amorphous catalyst films for catalytic water oxidation with high activity. In the literature, molecular cobalt−salen complexes have been well studied for sensing,41 asymmetric catalysis,42,43 polymerization Received: November 19, 2014 Revised: February 27, 2015 Published: April 7, 2015 8998

DOI: 10.1021/jp511584z J. Phys. Chem. C 2015, 119, 8998−9004

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The Journal of Physical Chemistry C catalysis,44,45 and so on. Although a recent study reported on the use of a cobalt−salen complex for electrocatalytic hydrogen production,46 very few studies have been reported on the use of these complexes for water oxidation.

(60 nmol/cm2). In a three-electrode electrochemical system, the FTO plates coated with cobalt−salen complexes were used as working electrodes for electrocatalysis, with an Ag/AgCl electrode (3 M KCl, 0.21 V vs NHE) as the reference electrode and a platinum wire as the counter electrode. The cyclic voltammograms (CVs) were measured with a scan rate of 50 mV/s, ranging from 0 to 1.5 V with an iR compensation and no stirring. Generally, the electrolyte was a 0.1 M potassium borate (KBi) buffer solution at pH 9.2. Bulk electrolysis was performed in a 40 mL of 0.1 M KBi solution at 1.2 V. The Tafel plots were done during bulk electrolysis in a 40 mL 0.1 M KBi solution with different applied anodic potentials. The solution resistance was detected by iR compensation function. The applied potential varied from 0.60 to 1.13 V with an interval of 25 mV. The Faradaic efficiency was measured by a fluorescence-based O2 sensor (Ocean Optics). Bulk electrolysis was performed in a 40 mL 0.1 M KBi solution in a threeelectrode electrochemical cell at pH 9.2. Prior to the measurement, the electrolyte was degassed by bubbling with high purity N2 for 20 min. After initiating bulk electrolysis at 1.2 V, the percentage of O2 detected in the headspace readily rose. The Faradaic efficiency was calculated by assuming that all of the charges were caused by 4e− oxidation of water to produce O2. Characterization. UV−vis spectra of cobalt−salen complexes were measured using 3802 UV−vis spectrophotometer. Electrocatalysis using cobalt-salen complexes resulted in the formation of heterogeneous catalyst films. The electrodeposited films were washed three times with both Millipore water and THF, then dried in air. Scanning electron microscopy (SEM) images and energy-dispersive X-ray analysis (EDX) spectra of the films were obtained with a SIRION200 Schottky field emission scanning electron microscope (SFE-SEM) equipped with a Rontec EDX system. Before loading into the instrument, the samples were covered with Au to make the samples conductive. Then SEM images and EDX spectra were obtained at an acceleration voltage of 15 kV. The valence states of the elements on catalyst films were probed with the ESCALAB 250 X-ray photoelectron spectroscopy (XPS) instrument.

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EXPERIMENTAL SECTION Materials. All chemical reagents, including ethylene diamine (99.0%), salicylic aldehyde (98.0%), o-phenylenediamine (99.0%), 1,2-hexamethylene diamine (98.0%), 3,5-ditertbutylsalicylaldehyde (98.0%), cobalt acetate tetrahydrate (Co(OAc)2·4H2O; 99.5%), boric acid (H3BO3; 99.99%), potassium hydroxide (KOH; 85.0%), anhydrous tetrahydrofuran (THF; 99.0%), and ethanol (EtOH; 99.7%) were purchased from Aldrich or Acros. All reagents were used without further purification. The aqueous solutions were freshly prepared using Millipore water (resistivity: ∼18 MΩ·cm). The fluorine-doped tin oxide (FTO, surface resistivity of 8−12 Ω/sq) glass plates were purchased from Zhuhai Kaivo Electronic Components Co., Ltd. Prior to the electrochemical experiments, the FTO plates were cleaned by ultrasonication in Millipore water and ethanol, respectively, at least three times (5 min). Synthesis of the Salen Ligands. The salen ligands were synthesized according to a method reported in the literature.47−50 For instance, the ligand 6,6′-((1E,1′E)(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(2,4-ditert-butylphenol) was synthesized by adding 20 mmol of 3,5-ditert-butylsalicylaldehyde and 10 mmol of ethylene diamine into 25 mL ethanol at room temperature (RT), then stirring for 1 h. The orange precipitate was filtered and recrystallized in ethanol. The difference for synthesis of other ligands was only in the starting materials. The obtained products were fully characterized and consistent with the previously published results. Synthesis of Cobalt−Salen Complexes. Cobalt complexes were synthesized in a 50 mL round-bottom flask under N2 by adding 300 mg corresponding ligand and Co(OAc)2· 4H2O (molar ratio 1:1.2) to 30 mL of ethanol solution.47−50 The mixture was refluxed at 353 K for 2 h. During refluxing, the color of the reaction solution changed compared with the color of the starting ligand. After cooling to RT, the precipitate was filtered with a Buchner funnel and washed three times each with deionized water and ethanol. The obtained precipitate was dried in a vacuum oven for 5 h at RT. Catalyst Films Preparation. The catalyst films were synthesized by coating the cobalt−salen complexes onto FTO surfaces using the amount of 60 nmol/cm2 (note: the area of FTO surface was controlled as 1 cm2). Then, the FTO plates were used as the working electrode for bulk electrolysis in 0.1 M KBi solution (pH 9.2) at 1.2 V versus Ag/AgCl. After 11 h of bulk electrolysis, the FTO electrode was washed three times with both Millipore water and THF. Thereafter, the catalyst films electrodeposited on FTO were dried in air for a few hours for subsequent electrochemical studies. Co3O4 and Co2+ (Co(OAc)2·4H2O) were dissolved in ethanol with nafion to make a 1.5 mM solution, which was ultrasonicated at RT for 10 min and then used to drop on the FTO surface. Electrochemical Methods. Electrochemical experiments were performed in a three-electrode electrochemical cell with a CHI 602D instrument potentialstat (purchased from Shanghai Chenhua Instrument Co., Ltd.) at RT. Before the tests, the cobalt−salen complexes were dissolved in THF to make a stock solution with the concentration at 1.5 mM. FTO glass plates were coated with 40 μL of the stock solution and dried in air

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RESULTS AND DISCUSSION The cobalt−salen complexes 1−6 were facilely synthesized, as shown in Scheme 1, according to the reported methods.47−51 The details for OER catalyst film preparation can be found in Scheme 1. Molecular Structures of Cobalt−Salen Complexes 1−6

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Figure 1. (a) CV scans of complex 4 coated on conductive FTO plate from the first cycle to 150th cycle. (b) CV scans of catalysts films (Films 1−6, Co3O4, and Co2+). (c) Charges passed through the catalyst films as the working electrode during bulk electrolysis. (d) Bulk electrolysis of Film 4 under different applied potentials (from 1.0 to 1.3 V). All the experiments were performed in a 40 mL KBi solution at pH 9.2 and bulk electrolysis was run at a stirring rate of 200 r/min.

the Experimental Section. The as-prepared catalyst films were named Films 1−6, corresponding with the precursor complexes 1−6. Figure 1a shows the cyclic voltammetry (CV) scans of complex 4 on FTO plate from the first cycle to 50th cycle to 150th cycle under an applied anodic potential of 1.5 V (vs Ag/AgCl, note: all the potentials reported in this paper are vs Ag/AgCl). The plots have appreciable changes during CV scans, indicating the decomposition of the cobaltsalen complexes. Similar CV scans can be observed for complex 2, as shown in Figure S1. Figure 1b shows CV scans of these films electrodeposited on FTO plates from the cobalt−salen complexes as the working electrodes in 0.1 M KBi electrolyte at pH 9.2. Commercial Co3O4 and Co2+ were used for comparison. Obviously, a catalytic wave is observed during a CV scan from 0 to 1.5 V for each catalyst film. Gas bubbles were clearly observed on the FTO electrode surface. The bubbles were further confirmed to be oxygen by a fluorescence-based oxygen sensor, indicating that the water oxidation catalysis occurred during the anodic scan. The onset catalytic potentials are shown at ∼0.98 V for Film 1 and ∼1.05 V for Film 3. Moreover, the onset potentials are negatively shifted to ∼0.92 V for Film 5 and ∼0.90 V for Film 6. Impressively, the onset catalytic potentials are even much lower, as evidenced by ∼0.84 V for Film 2 and ∼0.85 V for Film 4. The onset potentials of Films 2 and 4 are much lower than the catalyst films electrodeposited from cobaloximes (0.92 V vs Ag/AgCl for Co(dmgBF2)2(OH2)2, dmgBF2 = difluoroboryl-dimethylglyoxime).40 For a fixed catalytic current of 100 μA/cm2, the observed potentials are at 1.11, 0.89, 1.03, 0.91, 0.98, and 0.95 V for Films 1−6, respectively. The same amount of Co3O4 and Co2+ were further used as control experiments for comparison. The results showed that the catalytic activities of Films 2 and 4 are much higher than Co3O4 and Co2+ under the same conditions.

Bulk electrolysis of Films 1−6 was performed at 1.2 V in 0.1 M KBi electrolyte. The passed charges during water oxidation catalysis are shown in Figure 1c, and the results are consistent with the activity observed from the CV scans in Figure 1b. After 11 h of bulk electrolysis, the FTO electrode was washed three times each with Millipore water and THF to remove any soluble metal ions and cobalt complexes. Clearly, a thin film (light yellow) was generated on FTO plates for all these six cobalt-salen complexes, confirming the decomposition of the present metal complexes. The deposited catalyst films show similar catalytic performance before and after washing by Millipore water and THF. Taking complex 4 as an example, as shown in Figure 1d, the as-deposited catalyst film (Film 4) showed a catalytic current density of ∼2.4 mA/cm2 under 1.2 V for bulk electrolysis. Furthermore, when the applied potentials were 1.3, 1.1, and 1.0 V, the stable current densities reached ∼3.3, ∼1.4 and ∼0.8 mA/cm2, respectively. The morphologies of the catalyst films were characterized by SEM, as shown in Figure 2. The catalyst films were grown by electrodeposition under an anodic potential at 1.2 V in KBi buffer solution (pH = 9.2). Figure 2a shows the SEM image of the film using complex 4 as the precursor. Noticeable nanosized materials were observed on the FTO substrate. The film became thicker as the operating time increased. A similar result was obtained when using complex 2 as the catalyst precursor (Figure 2b). For comparison, the SEM image of bare FTO was shown in Figure S2. After bulk electrolysis of 11 h, the SEM images of Films 1, 3, 5, and 6 were also measured (Figure S3). All these images showed the formation of heterogeneous nanomaterials on FTO. Therefore, these observations clearly demonstrated that nanostructured materials were formed during the catalytic water oxidation reaction using cobalt− salen complexes. As for the reason why different cobalt−salen complexes had variable catalytic activities, it is not very clear so far and we try to discuss as follows. The structures of these 9000

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Figure 2. (Top) SEM images of catalyst Films 4 (a) and 2 (b) after 11 h of bulk electrolysis at 1.2 V in 0.1 M KBi buffer solution (pH 9.2). (Bottom) EDX spectra of catalyst Films 4 (c) and 2 (d).

cobalt−salen complexes may have a significant effect on the deposition of catalyst films. Since the precursors of Films 1, 3, and 5 contain four t-butyl groups, the planarity of these complexes is worse than the precursors of Films 2, 4 and 6. The results showed that Films 2 and 4 have better electrocatalytic activity for water oxidation than other films, probably attributing to the better planarity of these precursors for close contact between the complex molecules and the FTO surface before electrodeposition. In addition, due to their different coordination environments, these cobalt complexes may have different propensities to release cobalt ions to form active catalysts for water oxidation. The EDX data of the catalyst Films 4 and 2 in Figure 2c,d showed that the film deposited in KBi solution mainly contained Co, O, Sn, and Au. Co element probably resulted from the decomposition of the cobalt complexes. In addition, Sn was from the FTO substrate, and Au was artificially sprayed on the film to increase the conductivity. Powder XRD data showed no typical diffraction peak of cobalt species could be observed, as seen in Figure S4. The catalyst Film 4 has nearly identical XRD pattern as bare FTO, indicating that the catalyst film is probably amorphous. Subsequently, the catalyst Films 4 and 2 were analyzed by Xray photoelectron spectroscopy (XPS), as shown in Figures 3 and S5. The survey data of catalyst Film 4 reveal that both catalyst films mainly contained Co, O, Sn, and C elements (Figure 3a). The Co 2p peaks are probably attributable to the catalyst films. In addition, the Sn 3d peak should be from the FTO substrate. Figure 3b shows the high-resolution XPS spectra of the Co 2p region. In both Co 2p spectra, obvious shakeup satellite peaks are observed, which is consistent with previously reported results of CoOx.39,40 The presence of Co(II) or Co(III) oxide species is also confirmed by the binding energies of Co 2p3/2 and Co 2p1/2 located at 779.95 and 794.89 eV, respectively (Figure 3b).51−53 The O 1s peak is at 530.65 eV, which can be attributed to the oxide on the surface of the films.52 The ratios of Co to O is around 1:6.76. The C 1s spectrum is shown in Figure S6, which has three main peaks maximized at 285.0, 286.3, and 288.2 eV. The main peak at 285.0 eV is the reference peak for XPS correction. Other two peaks probably came from the ligands in cobalt salen

Figure 3. (a) XPS survey data of the catalyst Film 4. (b) Co 2p highresolution region.

complexes. The catalyst Film 2 showed very similar XPS data for both the survey and the high-resolution Co 2p spectrum (Figure S5), indicating the as-deposited films have very similar components. The catalytic current densities for water oxidation are highly dependent on pH values in aqueous solution, which is consistent with thermodynamic requirements for water electrolysis. Higher catalytic current intensities were observed when the pH was increased for Film 2, as shown in Figure 4a. When the pH values increased, the current density was enhanced while the onset catalytic potential obviously decreased. Figure 4b shows the fitting line obtained from Figure 4a with anodic peak potential versus pH values.36 The slope is ∼−87 mV per pH unit, which may indicate a 2e−/3H+ coupled transfer process.26 The pH dependence of the catalyst Film 2 has similar properties, but it is difficult to observe the anodic peak potentials under this scan rate (Figure S7). These results demonstrate that the catalytic activities of the electrodeposited films are highly dependent on the pH values, indicating that a proton-coupled electron transfer process is probably involved in the water oxidation reaction.26,54 The Tafel plots for catalyst films were measured in a 40 mL 0.1 M KBi solution at pH 9.2, as shown in Figures 5a and S8. The Tafel plots were obtained by measuring current density (j) of the films during bulk electrolysis with an applied potential (Eapplied) ranging from 0.60 to 1.13 V with an interval of 25 mV. The current density was measured as a function of the overpotential (η). Appreciable catalytic current was observed starting at η = 0.26 V for both Films 2 and 4. For Film 4, a current density of 0.01 mA/cm2 required η = 290 mV and a current density of 0.1 mA/cm2 required η = 350 mV. In contrast, a current density of 0.1 mA/cm2 required η = 385 mV for catalyst film electrodeposited from Co(dmgBF2)2(OH2)2.40 The Tafel plots were fitted with nearly linear relationships over the range from η = 0.26 to 0.38 V, and the slope was ∼68 and 9001

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The Faradaic efficiency of the as-deposited catalyst Film 4 was measured by a three-electrode system, where oxygen was rapidly produced on the surface of the working electrode. Oxygen gas was detected by a fluorescence-based oxygen sensor, as shown in Figure 5b. After initiating electrolysis at 1.2 V, oxygen bubbles were rapidly generated, accompanied by a rise in oxygen concentration in the headspace. The theoretical yield of oxygen was calculated by assuming that all charges were used for 4e− oxidation of water following the Faraday law (n = Q/4F). In the equation, n is mole number of oxygen, Q is the charges from bulk electrolysis, and F is the Faraday constant. The amount of oxygen produced during bulk electrolysis was a good match with the theoretical calculation of the oxygen during water oxidation reaction, corresponding to a Faradaic efficiency of >93% in 110 min (Figure 5b). The Faradaic efficiency of the catalyst Film 2 electrodeposited from complex 2 was about ∼91%, as shown in Figure S8.

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CONCLUSIONS In conclusion, cobalt−salen complexes were studied as catalyst precursors for electrocatalytic water oxidation reaction. The results clearly show that highly active cobalt-based catalyst films were deposited on conductive FTO substrates during bulk electrolysis. The cobalt-based films exhibit obvious catalytic currents and their catalytic activities are highly dependent on the coordination salen ligands. The optimal performance was found by using catalyst Film 4, which demonstrated an overpotential of only ∼350 mV for water oxidation. The Tafel plot showed a low slope of ∼62 mV/decade, indicating that the films are highly efficient for water oxidation. The Faradaic efficiency of the catalyst Film 4 is about 93%. Overall, the simple electrodeposition of cobalt−salen complexes is appealing method to prepare active catalysts for water oxidation.

Figure 4. (a) CVs of Film 2 in a 40 mL 0.1 M KBi solution, at various pH values from 6.69 to 9.93. (b) The linear plot obtained for pH vs anodic peak potential of the Film 2 with a slope of ∼−87 mV per pH unit.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details including electrochemical methods, electrochemical data, and surface characterization (XPS data and SEM images). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: 86-551-63606207. Notes

The authors declare no competing financial interest.

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Figure 5. (a) Tafel plots of Film 4 with a slope of ∼62 mV per decade varied from 0.73 to 0.88 V (vs Ag/AgCl). The applied potential varied from 0.60 to 1.13 V. The overpotential η was calculated by the equation η = Vapplied − EpH − iR, where Vapplied is the applied potential vs NHE (Vapplied = Eapplied + 0.21), EpH = 1.23−0.059 pH vs NHE, i is the stable current under the corresponding potential and R is the solution resistance measured by iR compensation function. (b) Theoretical and experimental oxygen by bulk electrolysis at 1.2 V using the catalyst film at pH 9.2 in 0.1 M KBi solution.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21271166, 21473170), the Fundamental Research Funds for the Central Universities, the Program for New Century Excellent Talents in University (NCET), and the Young Thousand Talents Program.

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REFERENCES

(1) Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.; Iwata, S. Architecture of the Photosynthetic Oxygen-evolving Center. Science 2003, 303, 1831−1838. (2) Barber, J. Crystal Structure of the Oxygen-Evolving Complex of Photosystem II. Inorg. Chem. 2008, 47, 1700−1710. (3) Barber, J. Photosynthetic Energy Conversion: Natural and Artificial. Chem. Soc. Rev. 2009, 38, 185−196.

∼62 mV/decade for Films 2 and 4, respectively. The relatively low slope of the Tafel plots probably indicate that the asdeposited cobalt-based catalyst films from complexes 2 and 4 are quite active for electrocatalytic water oxidation. 9002

DOI: 10.1021/jp511584z J. Phys. Chem. C 2015, 119, 8998−9004

Article

The Journal of Physical Chemistry C (4) Umena, Y.; Kawakami, K.; Shen, J. R.; Kamiya, N. Crystal Structure of Oxygen-Evolving Photosystem II at a Resolution of 1.9 A. Nature 2011, 473, 55−60. (5) Bricker, T. M.; Roose, J. L.; Fagerlund, R. D.; Frankel, L. K.; Eaton-Rye, J. J. The Extrinsic Proteins of Photosystem II. Biochim. Biophys. Acta, Bioenerg. 2012, 1817, 121−142. (6) McEvoy, J. P.; Brudvig, G. W. Water-Splitting Chemistry of Photosystem II. Chem. Rev. 2006, 106, 4455−4483. (7) Hammarström, L.; Hammes-Schiffer, S. Artificial Photosynthesis and Solar Fuels. Acc. Chem. Res. 2009, 42, 1859−1860. (8) Sun, L.; Hammarström, L.; Åkermark, B.; Styring, S. Towards Artificial Photosynthesis: Ruthenium-Manganese Chemistry for Energy Production. Chem. Soc. Rev. 2001, 30, 36−49. (9) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis. ChemCatChem 2010, 2, 724− 761. (10) Tran, P. D.; Artero, V.; Fontecave, M. Water Electrolysis and Photoelectrolysis on Electrodes Engineered using Biological and Bioinspired Molecular Systems. Energy Environ. Sci. 2010, 3, 727−747. (11) Tamaura, Y.; Kojima, M.; Sano, T.; Ueda, Y.; Hasegawa, N.; Tsuji, M. Thermodynamic Evaluation of Water Splitting by a CationExcessive (Ni, Mn) Ferrite. Int. J. Hydrogen Energy 1998, 23, 1185− 1191. (12) Tagore, R.; Crabtree, R. H.; Brudvig, G. W. Oxygen Evolution Catalysis by a Dimanganese Complex and its Relation to Photosynthetic Water Oxidation. Inorg. Chem. 2008, 47, 1815−1823. (13) Dismukes, G. C.; Brimblecombe, R.; Felton, G. A. N.; Pryadun, R. S.; Sheats, J. E.; Spiccia, L.; Swiegers, G. F. Development of Bioinspired Mn4O4-Cubane Water Oxidation Catalysts: Lessons from Photosynthesis. Acc. Chem. Res. 2009, 42, 1935−1943. (14) Najafpour, M. M.; Heidari, S.; Amini, E.; Khatamian, M.; Carpentier, R.; Allakhverdiev, S. I. Nano-Sized Layered Mn Oxides as Promising and Biomimetic Water Oxidizing Catalysts for Water Splitting in Artificial Photosynthetic Systems. J. Photochem. Photobiol. B 2014, 133, 124−139. (15) Gersten, S. W.; Samuels, G. J.; Meyer, T. J. Catalytic-Oxidation of Water by an Oxo-Bridged Ruthenium Dimer. J. Am. Chem. Soc. 1982, 104, 4029−4030. (16) Harriman, A.; Pickering, I. J.; Thomas, J. M.; Christensen, P. A. Metal Oxides as Heterogeneous Catalysts for Oxygen Evolution under Photochemical Conditions. J. Chem. Soc., Faraday Trans. I 1988, 84, 2795−2806. (17) Concepcion, J. J.; Jurss, J. W.; Brennaman, M. K.; Hoertz, P. G.; Patrocinio, A. O. v. T.; Iha, N. Y. M.; Templeton, J. L.; Meyer, T. J. Making Oxygen with Ruthenium Complexes. Acc. Chem. Res. 2009, 42, 1954−1965. (18) Hull, J. F.; Balcells, D.; Blakemore, J. D.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. Highly Active and Robust Cp* Iridium Complexes for Catalytic Water Oxidation. J. Am. Chem. Soc. 2009, 131, 8730−8731. (19) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S. Cyclometalated Iridium(III) Aquo Complexes: Efficient and Tunable Catalysts for the Homogeneous Oxidation of Water. J. Am. Chem. Soc. 2008, 130, 210−217. (20) Grätzel, M. Artificial Photosynthesis: Water Cleavage into Hydrogen and Oxygen by Visible Light. Acc. Chem. Res. 1981, 14, 376−384. (21) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729−15735. (22) Gray, H. B. Powering the Planet with Solar Fuel. Nature 2009, 1, 7. (23) Jiao, F.; Frei, H. Nanostructured Cobalt and Manganese Oxide Clusters as Efficient Water Oxidation Catalysts. Energy Environ. Sci. 2010, 3, 1018−1027. (24) Artero, V.; Chavarot-Kerlidou, M.; Fontecave, M. Splitting Water with Cobalt. Angew. Chem., Int. Ed. 2011, 50, 7238−7266.

(25) Du, P.; Eisenberg, R. Catalysts Made of Earth-Abundant Elements (Co, Ni, Fe) for Water Splitting: Recent Progress and Future Challenges. Energy Environ. Sci. 2012, 5, 6012−6021. (26) Dincă, M.; Surendranath, Y.; Nocera, D. G. Nickel-Borate Oxygen-Evolving Catalyst that Functions under Benign Conditions. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 10337−10341. (27) Bediako, D. K.; Lassalle-Kaiser, B.; Surendranath, Y.; Yano, J.; Yachandra, V. K.; Nocera, D. G. Structure-Activity Correlations in a Nickel-Borate Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2012, 134, 6801−6809. (28) Chen, G.; Chen, L.; Ng, S. M.; Lau, T. C. Efficient Chemical and Visible-Light-Driven Water Oxidation using Nickel Complexes and Salts as Precatalysts. ChemSusChem 2014, 7, 127−134. (29) Yu, X.; Hua, T.; Liu, X.; Yan, Z.; Xu, P.; Du, P. Nickel-Based Thin Film on Multi-Walled Carbon Nanotubes as an Efficient Bifunctional Electrocatalyst for Water Splitting. ACS Appl. Mater. Interfaces 2014, 6, 15395−15402. (30) Chen, Z. F.; Meyer, T. J. Copper(II) Catalysis of Water Oxidation. Angew. Chem., Int. Ed. 2013, 52, 700−703. (31) Zhang, M. T.; Chen, Z. F.; Kang, P.; Meyer, T. J. Electrocatalytic Water Oxidation with a Copper(II) Polypeptide Complex. J. Am. Chem. Soc. 2013, 135, 2048−2051. (32) Liu, X.; Jia, H. X.; Sun, Z. J.; Chen, H. Y.; Xu, P.; Du, P. W. Nanostructured Copper Oxide Electrodeposited from Copper(II) Complexes as an Active Catalyst for Electrocatalytic Oxygen Evolution Reaction. Electrochem. Commun. 2014, 46, 1−4. (33) Ellis, W. C.; McDaniel, N. D.; Bernhard, S.; Collins, T. J. Fast Water Oxidation Using Iron. J. Am. Chem. Soc. 2010, 132, 10990− 10991. (34) Fillol, J. L.; Codola, Z.; Garcia-Bosch, I.; Gomez, L.; Pla, J. J.; Costas, M. Efficient Water Oxidation Catalysts Based on Readily Available Iron Coordination Complexes. Nat. Chem. 2011, 3, 807− 813. (35) Hong, D.; Mandal, S.; Yamada, Y.; Lee, Y. M.; Nam, W.; Llobet, A.; Fukuzumi, S. Water Oxidation Catalysis with Nonheme Iron Complexes under Acidic and Basic Conditions: Homogeneous or Heterogeneous? Inorg. Chem. 2013, 52, 9522−9531. (36) Singh, A.; Chang, S. L. Y.; Hocking, R. K.; Bach, U.; Spiccia, L. Highly Active Nickel Oxide Water Oxidation Catalysts Deposited from Molecular Complexes. Energy Environ. Sci. 2013, 6, 579−586. (37) Singh, A.; Chang, S. L. Y.; Hocking, R. K.; Bach, U.; Spiccia, L. Anodic Deposition of NiOx Water Oxidation Catalysts from Macrocyclic Nickel(II) Complexes. Catal. Sci. Technol. 2013, 3, 1725−1732. (38) Hong, D.; Jung, J.; Park, J.; Yamada, Y.; Suenobu, T.; Lee, Y. M.; Nam, W.; Fukuzumi, S. Water-Soluble Mononuclear Cobalt Complexes with Organic Ligands Acting as Precatalysts for Efficient Photocatalytic Water Oxidation. Energy Environ. Sci. 2012, 5, 7606− 7616. (39) Han, A.; Wu, H.; Sun, Z.; Jia, H.; Du, P. Facile Deposition of Nanostructured Cobalt Oxide Catalysts from Molecular Cobaloximes for Efficient Water Oxidation. Phys. Chem. Chem. Phys. 2013, 15, 12534−12538. (40) Han, A.; Wu, H.; Sun, Z.; Jia, H.; Yan, Z.; Ma, H.; Liu, X.; Du, P. Green Cobalt Oxide (CoOx) Film with Nanoribbon Structures Electrodeposited from the BF2-Annulated Cobaloxime Precursor for Efficient Water Oxidation. ACS Appl. Mater. Interfaces 2014, 6, 10929− 10934. (41) Lee, J. H.; Jeong, A. R.; Shin, I. S.; Kim, H. J.; Hong, J. I. Fluorescence Turn-On Sensor for Cyanide Based on a Cobalt(II)Coumarinylsalen Complex. Org. Lett. 2010, 12, 764−767. (42) Li, L. S.; Wu, Y. K.; Hu, Y. J.; Xia, L. J.; Wu, Y. L. Asymmetric Hetero-Diels-Alder Reaction of l-Alkyl-3-silyloxy-1,3-dienes with Ethyl Glyoxylate Catalyzed by a Chiral (Salen)cobalt(II) Complex. Tetrahedron: Asymmetry 1998, 9, 2271−2277. (43) Cozzi, P. G. Metal-Salen Schiff Base Complexes in Catalysis: Practical Aspects. Chem. Soc. Rev. 2004, 33, 410−421. 9003

DOI: 10.1021/jp511584z J. Phys. Chem. C 2015, 119, 8998−9004

Article

The Journal of Physical Chemistry C (44) Peretti, K. L.; Ajiro, H.; Cohen, C. T.; Lobkovsky, E. B.; Coates, G. W. A Highly Active, Isospecific Cobalt Catalyst for Propylene Oxide Polymerization. J. Am. Chem. Soc. 2005, 127, 11566−11567. (45) Ajiro, H.; Peretti, K. L.; Lobkovsky, E. B.; Coates, G. W. On the Mechanism of Isospecific Epoxide Polymerization by Salen Cobalt(III) Complexes: Evidence for Solid-State Catalysis. Dalton Trans. 2009, 8828−8830. (46) Kochem, A.; Thomas, F.; Jarjayes, O.; Gellon, G.; Philouze, C.; Weyhermü l ler, T.; Neese, F.; Gastel, M. v. Structural and Spectroscopic Investigation of an Anilinosalen Cobalt Complex with Relevance to Hydrogen Production. Inorg. Chem. 2013, 52, 14428− 14438. (47) Fraile, J. M.; Garcia, N.; Herrerias, C. I.; Mayoral, J. A. Integration of Heterogeneous Catalysts into Complex Synthetic Routes: Sequential vs One-Pot Reactions in a (Knoevenagel + Mukaiyama-Michael + Hydrogenation + Transesterification) Sequence. Catal. Sci. Technol. 2013, 3, 436−443. (48) Rhodes, B.; Rowling, S.; Tidswell, P.; Woodward, S.; Brown, S. M. Aerobic Epoxidation via Alkyl-2-oxocyclopentanecarboxylate CoOxidation with Cobalt or Manganese Jacobsen-type Catalysts. J. Mol. Catal. A: Chem. 1997, 116, 375−384. (49) Felicio, R. C.; Cavalheiro, E. T. G.; Dockal, E. R. Preparation, Characterization and Thermogravimetric Studies of (N,N′-cis-1,2Cyclohexylene bis(salicylideneaminato)]cobalt(II) and [N,N′(±)-trans-1,2-Cyclohexylene bis(salicylideneaminato)]Cobalt(II). Polyhedron 2001, 20, 261−268. (50) Dzygiel, P.; Reeve, T. B.; Piarulli, U.; Krupicka, M.; Tvaroska, I.; Gennari, C. Resolution of Racemic N-Benzyl α-Amino Acids by Liquid−Liquid Extraction: A Practical Method Using a Lipophilic Chiral Cobalt(III) Salen Complex and Mechanistic Studies. Eur. J. Org. Chem. 2008, 1253−1264. (51) Bomben, K. D.; Moulder, J. F.; Sobol, P. E.; Stickel, W. F. Handbook of X-Ray Photoelectron Spectra: A Reference Book of Standard Spectra for Identification; Perkin Elmer: Eden Prairie, MN, 1992. (52) Mcintyre, N. S.; Cook, M. G. X-Ray Photoelectron Studies on Some Oxides and Hydroxides of Cobalt, Nickel, and Copper. Anal. Chem. 1975, 47, 2208−2213. (53) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (54) Dogutan, D. K.; McGuire, R., Jr.; Nocera, D. G. Electocatalytic Water Oxidation by Cobalt(III) Hangman β-Octafluoro Corroles. J. Am. Chem. Soc. 2011, 133, 9178−9180.

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DOI: 10.1021/jp511584z J. Phys. Chem. C 2015, 119, 8998−9004