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Microwave Effects on Co-Pi Co-catalysts Deposited on #FeO for Application to Photocatalytic Oxygen Evolution 2

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Masato M. Maitani, Takuya Yamada, Hisanori Mashiko, Kohei Yoshimatsu, Takayoshi Oshima, Akira Ohtomo, and Yuji Wada ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16319 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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ACS Applied Materials & Interfaces

Microwave Effects on Co-Pi Co-catalysts Deposited on α-Fe2O3 for Application to Photocatalytic Oxygen Evolution

Masato M. Maitania†*, Takuya Yamadaa, Hisanori Mashikoa, Kohei Yoshimatsua, Takayoshi Oshimaa§, Akira Ohtomoa*, Yuji Wadaa* a

Department of Chemical Science and Engineering, School of Materials and Chemical

Technology, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8550, Japan.

Keywords: photocatalysis, co-catalysts, microwave, interface, water splitting, pulsed laser deposition, α-Fe2O3, Co-Pi

ACS Paragon Plus Environment

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ABSTRACT

We analyzes the effects of microwave applied in the process of photo-electrochemical deposition of cobalt-based co-catalysts, Co–Pi, onto well-orientated flat α-Fe2O3 thin films, which were fabricated by pulsed laser deposition.

As compared with conventional heating, microwave

significantly affects the morphology, chemical composition, and photocatalytic activity of Co-Pi / α-Fe2O3 composite. A significant enhancement in photocurrent related to photocatalytic water oxidation is achieved by the Co–Pi catalyst prepared under microwave irradiation. This, along with its interfacial electron-transfer properties, is studied by means of electrochemical impedance spectroscopy.


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Artificial photosynthesis has been recently considered as a great importance for one of renewable energy sources. Among those, water splitting is an ideal clean system for creating hydrogen and oxygen from water by using solar energy (eq. 1) 1-3. In a typical water splitting system, semiconductor electrodes immersed in an aqueous solution are used to absorb solar photons to produce electrons and holes, which possess sufficient energy to enable the reduction of protons (eq. 2) and oxidation of water molecules (eq. 3) in the surrounding solution for hydrogen and oxygen evolution, respectively4. Although the required bias of electrochemical water splitting for hydrogen and oxygen evolution, in theory, is 1.23 V, the reaction typically requires larger bias, typically 1.8-2.0V, due to large over potentials. Therefore, the reported photocatalytic water splitting typically requires relatively large band gap due to the large over-potential of surface reactions especially in the process of oxygen evolution associated with the four-electron reaction (eq. 3). Even though the band alignment satisfy the condition of water splitting, e. g. each CB and VB locate high and low enough to reduce protons and oxidize water, respectively, complete water splitting under visible irradiation especially with narrow bandgap semiconductor electrodes is a quite difficult task due to the large over potential of reactions, although the most photon flux is available in visible region in the solar spectrum. 2Hଶ O ሱሮ 2Hଶ + Oଶ

(eq. 1)

2H ା + 2eି → Hଶ

(eq. 2)

୦ν

2Hଶ O → 4H ା + Oଶ + 4eି

(eq. 3)

To overcome the difficulty, co-catalysts lowering the over-potential have been applied to relatively low-bandgap semiconductors such as WO3, α-Fe2O3, TaON, TiON, and C3N4 which are applicable under visible irradiation. 1, 5-13 A co-catalyst, cobalt oxide with phosphate moiety


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(Co–Pi), was first reported by Nocera et al. and considered as one of the most promising cocatalysts not only for its high reactivity but also for its cost effectiveness and the natural abundance of CoOx.5, 14- 18 Additionally, a series of cobalt based co-catalysts, Co(OH)2, CoSe2, etc., have been also intensively investigated as promising candidates for photocatalytic oxygen evolution14. Co–Pi co-catalysts are typically deposited on the semiconductor surface by either photo-electrochemical10, 14,15 or electrochemical16, 17 deposition from the buffered aqueous Co ion solutions. Despite the promising characteristics, the nature of the Co–Pi is not fully understood, since the amount and size of Co–Pi is quite small with typical diameter of a few nanometers. In addition, assembled semiconductor substrates typically possess nanostructure to maximize the surface area for achieving high conversion efficiency.11, 13, 14 Furthermore, Co–Pi with a bulk content of CoOx is in the amorphous phase16, 19, and therefore, the characterization method of Co–Pi is quite limited. Consequently, the deposition process including the insertion of phosphate anions in bulk CoOx is still unclear.16, 19--23 Therefore, a novel strategy to improve Co– Pi co-catalyst deposition is still desired; e.g., precise and independent control of the composition and morphology of each element in addition to dispersion and adhesion on the semiconductor surface. Here, we apply microwave irradiation during the photo-electrochemical deposition of Co–Pi catalysts on α-Fe2O3, and in particular, explain the effects of microwave irradiation during photoelectrochemical deposition by utilizing the well-defined epitaxial grown α-Fe2O3 films (Figure S1 and Figure S2) exposing the specific crystal orientation of (0001) facet prepared by pulsedlaser deposition (PLD) 24, 25. The effects of microwave irradiation were examined with morphologies and chemical compositions of Co–Pi co-catalysts in addition to the photocatalytic properties. Electrochemical impedance spectroscopy (EIS) was also performed under visible


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illumination in order to elucidate the carrier transport properties. The significant enhancement of photocurrent directly related to the photocatalytic reactions with Co–Pi prepared under MW irradiation is attributed to the carrier transport properties at Co–Pi/α-Fe2O3 interfaces. The Co–Pi nanoparticles were photo-electrochemically deposited by photo-electrochemical deposition on α-Fe2O3 substrates in an aqueous precursors solution (pH = 7) containing Co(NO3)2 (0.5 mM) and potassium phosphate (0.1 M) in a quartz cell under illumination of filtered monochromatic UV light (λ = 300 nm) for 10 min. During Co-Pi deposition, the whole sample vessel was heated either by microwave (MW) irradiation (2.45 GHz, 5 W) or in an oil bath in a conventional heating (CH) system. Since the solution temperature increases under MW irradiation due to MW absorption by the aqueous precursor solution, the system temperature was controlled at 97 oC during the Co–Pi deposition, both under MW and CH. After washing the sample with deionized water three times, atomic force microscopy (AFM) revealed the morphology of Co–Pi catalysts on α-Fe2O3 in Figure 1 and Figure S3. Clear morphological differences were observed with Co–Pi / α-Fe2O3 prepared either under MW or CH. AFM images clearly indicate the finer and more uniformly distributed Co–Pi nanoparticles on MW sample. The histograms in inset exhibit distributions of the height of Co–Pi nanoparticles in Figure 1. These uniform Co–Pi nanoparticles may be induced by the nucleation–dissolution equilibria of Co–Pi in the precursor solution during photo-electrochemical deposition, as discussed later. The same Co–Pi deposition was carried out either by MW or CH onto α-Fe2O3 films on the sapphire substrate and conductive Ta:SnO2/sapphire substrate, for fundamental studies and practical photo-electrochemical measurements, respectively(Figure S1 and Figure S3).


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Figure 1 AFM images of (a) CoPi on α-Fe2O3/c-sapphire substrate by photo-electrochemical deposition under MW, (b) CoPi on the substrate by photo-electrochemical deposition under CH, and (c) bare α-Fe2O3/c-sapphire substrate with scale bars indicating 200nm. The distributions of Co-Pi nanoparticle diameter prepared under (d) MW and (e) CH, where the blue and red indicate the data taken from two independent samples indicating a good sample-to-sample reproducibility.

As the deposition condition may influence the composition of the resulting Co–Pi catalysts 5, 15, 16, 23

, Co–Pi / α-Fe2O3 films were characterized by x-ray photoelectron spectroscopy (XPS) as

results are given in Figure 2 and Table 1. The peaks of Co 2p and P 2p in Figure 2 indicate the difference in the composition of P and Co as the atomic fraction with different preparation conditions under MW or CH (Table 1) oneach α-Fe2O3 substrates. The atomic fraction of P/Co is significantly smaller with MW sample than that with CH sample. As previous studies show, the fraction of P/Co in Co–Pi majorly affects the photo-electrochemical properties of water


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oxidation. Therefore, MW is a novel method to control the morphology and chemical composition of Co–Pi probably by owing to the characteristic non- thermally equivalent condition 26, while the detailed study of the effects of the fraction is out of scope of this study. On the other hand, the atomic fraction of Co/Fe is similar for both MW and CH samples. Although XPS cannot be clearly quantify the atomic fractions due to the morphological difference, similar atomic fraction of Co/Fe on each sample qualitatively indicates that the amount of Co–Pi catalyst on α- Fe2O3 would be quite similar with MW and CH smaples. The observed differences in the atomic fractions of each P/Co, and Co/Fe exhibit a similar trend of difference between MW and CH samples with both α-Fe2O3 substrates on sapphire and on Ta:SnO2/sapphire. Although α-Fe2O3 on sapphire and on Ta:SnO2/sapphire exhibited a large difference in the fraction of P/Co (Table 1), this is probably due to significant difference in surface morphology between bare α- Fe2O3, either on sapphire and on Ta:SnO2/sapphire (Figure S1). Therefore, we do not focus on the variation dependence on the substrates of α- Fe2O3, either on sapphire or on Ta:SnO2/sapphire.


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Figure 2 XPS results of Co-Pi deposited on α-Fe2O3/c-sapphire substrates (a and b) and on αFe2O3/Ta:SnO2/c-sapphire substrates (c and d) under MW irradiation (a and c) and conventional oil bath heating (b and d).


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Table 1 Compositional analysis of Co, P, and Fe of each sample in XPS spectra. Co-Pi deposition method

P/Co XPS P/Co atomic Co/Fe XPS Co/Fe atomic fractiona fractionb fractionc fractionb

α-Fe2O3/ Sapphire

MW

0.0774

0.503

0.136

0.161

CH

0.141

0.914

0.136

0.161

α-Fe2O3/ Ta:SnO2/ Sapphire

MW

0.0934

0.607

0.290

0.344

CH

0.124

0.805

0.293

0.347

Sample substrate

a

Values were calculated with Co 2p3/2 and P 2p. b Values were calculated with atomic sensitivity factors (ASF) of each element (S3)27. Intensity of Co 2p3/2 was multiplied with a factor of 1.5 due to the theoretical intensity of 2p as a summation of singlet and triplet of P 2p component. c Values were calculated with a summation of Fe 2p1/2 and Fe 2p3/2 including the satellite peak. We ignore the instrument-specific ASF for the purpose of only the relative compositional analysis.

Photocatalytic activity of water oxidation by α-Fe2O3 was evaluated as the anodic photocurrent by using a conventional experimental setup5-13. Co–Pi / α-Fe2O3 / Ta:SnO2 samples prepared under MW and CH, and bareα-Fe2O3 / Ta:SnO2 as a control, were immersed in 0.1 M NaOH aqueous solutions (pH = 13.0) in a three-component photo-electrochemical cell utilizing a quartz window with reference and counter electrodes of Ag/AgCl in (3 M NaCl, E0 = 0.203 V at 25°C) and a platinum wire, respectively under Xe lamp illumination (>420 nm) The current-potential plot in Figure 3 indicates that onsets potential of anodic current of each samples locate around 0.7–1.0 V vs. RHE (pH = 13.0) under illumination, while the onsets potential of dark current locate around 1.5 V vs. RHE (pH = 13.0). This difference onset potential is attributed to the photocatalytic water oxidation. Since the working α- Fe2O3 electrodes are connected to a potentiostat, the reaction rate is directly related to the current. We therefore correlate the


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photocurrent to the activity of photocatalytic water oxidation, although the Faradic efficiency of the reaction has to be considered in detailed analysis of oxygen evolution. Both Co–Pi / α-Fe2O3 samples prepared under MW and CH exhibited a significant improvement of anodic current onset potential under illumination as compared with bare α-Fe2O3. As generally discussed, this is the typical effect of Co–Pi co-catalysts reducing the over potential of water oxidation. Moreover, comparing between MW and CH samples, the anodic photocurrent is significantly higher with MW sample than CH sample within a whole range of measurement. Furthermore, CH sample results in a lower photocurrent at higher potential range as compared with bare sample, although the anodic onset potential is more negative with Co–Pi/α-Fe2O3 than bare α-Fe2O3. Therefore, we applied electrochemical impedance spectroscopy (EIS) in order to further understand the observed differences.


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Figure 3 Photocatalytic reactions under visible light illumination with bias sweep bare αFe2O3/Ta:SnO2/c-sapphire substrate (green), photo-electrochemically deposited CoPi on the substrate under MW (red), and photo-electrochemically deposited Co-Pi on substrate under CH (blue) against reference electrode of Ag/AgCl in the aqueous solution of NaOH (0.1M). RHE was calculated form the theoretical difference of 0.97 V from Ag/AgCl (3 M NaCl, E0 = 0.203 V at 25°C) at pH13.

EIS was applied to study the interfacial electron transfer in each MW and CH sample as compared with a reference bare sample. The photocurrent response at each applied modulated AC potential coupled with a certain DC potential was examined as the amplitude and phase shift. The obtained frequency responses of the photocurrent were analyzed with curve fitting on the Nyquist plots based on the transmission line model (Figure S4), in which the surface resistance and surface capacitance components were examined as a function of the DC potential (Figure 4 and Figure S5)28.


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As compared with bare sample, the surface resistance is significantly lower with both Co–Pi/αFe2O3 samples at more positive potential range than the anodic onset potential, ~0.7V vs. RHE (pH = 13.0). In addition, both Co–Pi/α-Fe2O3 samples exhibited a drastic drop of surface resistance at more positive potential range than the anodic onset potential. In this region, relatively constant resistance was observed with Co–Pi/α-Fe2O3 samples, while bare α-Fe2O3 revealed a continuous drop of surface resistance. This difference probably arises from the properties of the hole accumulation process at α-Fe2O3 surface. In case of Co–Pi/α-Fe2O3, the surface resistivity of the sample was determined by the hole transport properties from α-Fe2O3 to Co–Pi and the reaction rate on Co–Pi. On the other hand, in case of bare α-Fe2O3, the resistance was determined by the surface reaction rate; therefore, higher applied potential decreased the surface resistance more, since the reaction is more likely to overcame the over potential of surface reactions under high potential. Furthermore, bare sample reveals lower resistivity than CH sample in the high potential range < ~1.2 V (vs. RHE (pH = 13.0)). Comparison between MW and CH sample ed clearly exhibits that MW sample consistently showed a lower resistance than CH sample in the potential range of 0.5–1.0 V (vs. RHE (pH = 13.0)). In addition, MW sample exhibited much smaller capacitance of ~10-5 F/cm-2 with a factor of more than an order of magnitude smaller than CH sample possessing the capacitance of ~10-4 F/cm-2. Since bare sample exhibited the least value of capacitance, the observed capacitance could be associated with the defect-related trap states of the semiconductor generated at the interface of Co–Pi/α-Fe2O3. The same component of surface capacitance of bare sample exhibited a value of capacitance ( ~10-5 F/cm-2) similar to that with MW sample, and which is an order of magnitude smaller than CH sample as well. These results indicate that Co–Pi/α-Fe2O3 prepared under MW does not create major additional interfacial defects resulting in trap states as


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compared with bare α- Fe2O3. On the other hand, CH builds up an order of magnitude more trap states at the Co–Pi/α-Fe2O3 interface in addition to the intrinsic trap states on bare α-Fe2O3 surface. This large trap related resistance and capacitance with CH sample attributes the hysteresis of photocurrent in the photocurrent-potential sweep (Figure 3 and 4).

Figure 4 EIS results of each film, bare α-Fe2O3/Ta:SnO2/c-sapphire substrate (green), Co-Pi deposited under MW(red), and CH (blue) on the same α-Fe2O3 substrates. EIS were performed under visible light irradiation with varied bias against reference electrode of Ag/AgCl in the aqueous solution of NaOH (0.1M). RHE was calculated form the theoretical difference of 0.97 V from Ag/AgCl (3 M NaCl, E0 = 0.203 V at 25°C) at pH13 (Figure S4). Dotted lines are replicated data from Figure 3 as the photocurrent indicating the photocatalytic water oxidation reaction.


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These results can be clearly attributed to the difference in the bottleneck of photocatalytic water oxidation in case of bare α-Fe2O3 and Co–Pi/ α-Fe2O3. In case of bare sample , the charge separation is not efficient due to fast recombination of the photo-generated carries in the bulk αFe2O3 films. Therefore, efficient charge separation needs a strong electric field provided by the applied potential12. On the other hand, Co–Pi significantly suppresses the recombination rate in general due to the hole accumulation on Co-Pi12, and thus leads to a lowering of the applied potential for water oxidation. Our results of α-Fe2O3 with and without Co–Pi showed the same effects as a result of lowering applied potential for water oxidation As the comparison between CH and MW prepared Co-Pi /α-Fe2O3 samples, CH sample exhibited a higher density of interfacial traps than MW sample and even bare sample , as indicated by EIS capacitance component. This is also reflected as the surface resistance of CH sample, and therefore the photocurrent of bare sample overcame the CH sample at a higher current region due to trap states providing more recombination and higher surface resistance at higher carrier density and applied voltage. Since Co–Pi/α-Fe2O3 interface does not create the additional capacitance by MW application, Co–Pi retains the similar level of trap density to bare sample ; thus, MW sample exhibited the highest photocurrent among the samples including the high potential region. Since the interfacial trap state could play a key role in interrupting the charge transport through the Co– Pi/α-Fe2O3 interface as well, the surface resistance was higher with CH sample than MW sample. Therefore, MW irradiation improves the interfacial resistivity and interfacial trap states at Co– Pi/α-Fe2O3 interface as compared with CH process. We hypothetically attribute the improvement of the interface created under MW as compared with CH to the characteristic interfacial heating at Co–Pi/α-Fe2O3 29,30. According to the evaluation of MW absorptivity of each material, α-


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Fe2O3, Co–Pi catalysts, and Co–Pi / α-Fe2O3, the sample of Co–Pi / α- Fe2O3 substrate shows the most effective heating under MW irradiation among samples (Figure S6). . Therefore, the Co– Pi/α-Fe2O3 interface could be selectively heated under our Co–Pi deposition condition under MW irradiation.29,30 These results indicate that the charge transport can be engineered by MW interface heating by achieving low resistance and capacitance related to the defects at the Co– Pi/α-Fe2O3 interface. In addition to the interface, MW affects to the composition of Co-Pi (Table 1). We, at this point, do not think the relatively low fraction of P/Co observed with MW sample is the origin of the higher photocatalytic activity, since previous results suggested opposite effect of P/Co fraction, i. e. the high fraction of P/Co is preferable for photocatalytic activity.10 To conclude this, further systematic study is required with controlling the compositions of P/Co, which could be possible with non-thermal equilibrium under MW26, as a future work. We consequently introduced MW irradiation to the photo-electrochemical deposition of Co–Pi co-catalysts on α-Fe2O3 semiconducting thin films. As compared with the controlled CH heating condition, Co–Pi/α-Fe2O3 prepared under MW irradiation revealed higher photocatalytic activity evaluated by the photocurrent associated with the water oxidation under photo irradiation. EIS also revealed that MW irradiation provides more favorable properties of photocatalysts in terms of the interfacial resistivity and capacitance related to the defects at the Co–Pi/α-Fe2O3 interface. In addition, we propose the origin of the improvement of Co–Pi/α-Fe2O3 interface attributed to selective interfacial heating under MW irradiation.


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ASSOCIATED CONTENT Experimental section indicating details of preparation and characterization of α-Fe2O3 thin films (S1), photo-electrochemical deposition of Co-Pi nanoparticles (S2), characterization of Co-Pi deposited films (S3), photo-electrochemical characterization (S4), and Co-Pi/Fe2O3 heating properties by microwave(S5) are included in a supporting file, SuppVer3(PDF)

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (MM), [email protected] (AO), [email protected] (YW), Phone/FAX: +81-3-5734-2879(MM and YW), +81-3-5734-2145 (AO). Present Addresses †Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8904, Japan. § Department of Electrical and Electronic Engineering, Saga University, Saga 840-8502, Japan.

ACKNOWLEDGMENT This research was supported in part by Grant-in-Aid for Scientific Research (A) 25249113, Grant-in-Aid for Young Scientists (B) 25790013, and Quantum Basic Research Coordinated Development Program by MEXT, Japan, ASPIRE League Research Grant 2014-5, Tokyo Tech, Research Grants of TEPCO Memorial Foundation, JFE 21st Century Foundation, and NEDO.


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H.M. acknowledges financial support by JSPS Research Fellowships for Young Scientists and the Program for the MEXT Leading Graduate Schools Academy for Cocreative Education of Environment and Energy Science (ACEEES).


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REFERENCES (1) Kärkäs, M. D.; Verho, O.; Johnston, E.V.; Åkermark, B., Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev. 2014, 114, 11863−12001. (2) Esswein, A. J.; Nocera, D. G. Hydrogen Production by Molecular Photocatalysis, Chem. Rev. 2007, 107, 4022-4047. (3) Ravelli, D.; Dondi, D.; Fagnoni, M.; Albini, A., Photocatalysis. A Multi-faceted Concept for Green Chemistry. Chem. Soc. Rev., 2009, 38, 1999–2011. (4) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37 - 38. (5) Kanan, M. W.; Surendranath, Y.; Nocera, D. G. Cobalt–phosphate Oxygen-evolving Compound. Chem. Soc. Rev., 2009, 38, 109–11 (6) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev., 2014, 43, 7520-7535. (7) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76 – 80. (8) Hara, M.; Hitoki, G.; Takata, T.; Kondo, J. N.; Kobayashi, H.; Domen, K. TaON and Ta3N5 as New Visible Light Driven Photocatalysts. Catal. Today, 2003, 78, 555-560.


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Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Higashi, M.; Domen, K.; Abe, R. Highly Stable Water Splitting on Oxynitride TaON Photoanode System under Visible Light Irradiation. J. Am. Chem. Soc. 2012, 134, 6968−6971. (10)

Zhong,D.K.; Cornuz,M.; Sivula,K.; Gratzel, M.; Gamelin D. R. Photo-assisted

electrodeposition of cobalt–phosphate (Co–Pi) catalyst on hematite photoanodes for solar water oxidation Energy Environ. Sci., 2011, 4, 1759–1764. (11)

Barrosoa, M.; Mesaa, C. A.; Pendleburya, S. R. Cowana, A. J.; Hisatomi, T.;

Sivula, K.; Grätzel, M.; Kluga, D. R.; Durrant, J. R. Dynamics of Photogenerated Holes in Surface Modified α-Fe2O3 Photoanodes for Solar Water Splitting. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15640–15645. (12)

Barrosoa, M.; Cowana, A. J.; Pendleburya, S. R.; Grätzel, M.; Kluga, D. R.;

Durrant, J. R. The Role of Cobalt Phosphate in Enhancing the Photocatalytic Activity of α- Fe2O3 toward Water Oxidation. J. Am. Chem. Soc. 2011, 133, 14868–14871 (13)

Warren, S. C.; Voïtchovsky, K.; Dotan, H.; Leroy, C. M.; Cornuz, M.;

Stellacci, F.; Hébert, C.; Rothschild, A.; Grätzel, M. Identifying Champion Nanostructures for Solar Water-splitting. Nat Mater. 2013, 12, 842-849. (14)

Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M. Sun, X. Recent Progress in

Cobalt-Based Heterogeneous Catalysts for Electrochemical Water Splitting. Adv. Mater. 2016, 28, 215–230. (15)

Steinmiller, E. M. P.; Choi, K. S. Photochemical Deposition of Cobalt-based

Oxygen Evolving Catalyst on a Semiconductor Photoanode for Solar Oxygen


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Production. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 20633–20636. (16)

Kanan, M. W.; Nocera, D. G. In Situ Formation of an Oxygen-Evolving

Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 10721075. (17)

Surendranath, Y.; Dinca, M.; Nocera, D. G. Electrolyte-dependent

Electrosynthesis and Activity of Cobalt-based Water Oxidation Catalysts. J. Am. Chem. Soc. 2009, 131, 2615–2620. (18)

Surendranath, Y.; Lutterman, D. A.; Liu, Y.; Nocera, D. G. Nucleation,

Growth, and Repair of a Cobalt-based Oxygen Evolving Catalyst. J. Am. Chem. Soc. 2012, 134, 6326−6336. (19)

Surendranath, Y.; Kanan, M. W.; Nocera, D. G. Mechanistic Studies of the

Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. J. Am. Chem. Soc. 2010, 132, 16501–16509. (20)

Kanan,M. W.; Yano, J.; Surendranath, Y.; Dinca, M.; Yachandra,V. K.;

Nocera, D. G.Structure and Valency of a Cobalt-Phosphate Water Oxidation Catalyst Determined by in Situ X-ray Spectroscopy.J. Am. Chem. Soc. 2010, 132, 13692– 13701. (21)

Gonzalez-Flores, D.; Sanchez, I.; Zaharieva, I.; Klingan, K.; Heidkamp, J.;

Chernev, P.; Menezes, P. W.; Driess, M.; Dau, H.; Montero, M. L. Heterogeneous Water Oxidation: Surface Activity versus Amorphization Activation in Cobalt Phosphate Catalysts. Angew. Chem. Int. Ed. 2015, 54, 2472 –2476


20

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22)

Zhang, G.; Zang, S.; Lin, L.; Lan, Z. A.; Li, G.; Wang,X. Ultrafine Cobalt

Catalysts on Covalent Carbon Nitride Frameworks for Oxygenic Photosynthesis. ACS Appl. Mater. Interfaces, 2016, 8, 2287–2296. (23)

Risch, M.; Khare, V.; Zaharieva, I.; Gerencser, L.; Chernev, P.; Dau, H.

Cobalt-oxo Core of a Water-oxidizing Catalyst Film. J. Am. Chem. Soc. 2009, 131, 6936–6937 (24)

Mashiko, H.; Oshima, T.; Ohtomo A. Band-gap Narrowing in α-(CrxFe1-x)2O3

Solid-solution Films. Appl. Phys. Lett. 2011, 99, 241904. (25)

Mashiko,H.; Yoshimatsu, K.; Oshima, T.; Ohtomo, A. Fabrication and

Characterization of Semiconductor Photoelectrodes with Orientation-controlled α‑Fe2O3 Thin Films. J. Phys. Chem. C 2016, 120, 2747−2752.

(26)

Tsukahara, Y.; Higashi, A.; Yamauchi, T.; Nakamura, T.; Yasuda, M.; Baba,

A.; Wada, Y. In Situ Observation of Nonequilibrium Local Heating as an Origin of Special Effect of Microwave on Chemistry. J. Phys. Chem. C 2010, 114, 8965-8970. (27)

Briggs D.; Seah, M.P.; Practical Surface Analysis. Volume 1. Auger and X-ray

Photoelectron Spectroscopy, John Wiley and Sons, Chichester, UK, (1990). (28)

Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann T. W.

Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with “Co−Pi”-coated Hematite Electrodes. J. Am. Chem. Soc. 2012, 134, 16693−16700 (29)

Maitani, M. M.; Inoue, T.; Tsukushi, Y.; Hansen, N. D. J.; Mochizuki, D.;


21


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Page 22 of 23

Suzuki, E.; Wada, Y. Collaborational Effect of Heterolytic Layered Configuration for Enhancement of Microwave Heating. Chem.Commun. 2013 49, 10841–10843. (30)

Maitani, M. M.; Tsukushi, Y.; Hansen, N. D. J.; Sato, Y.; Mochizuki, D.;

Suzuki, E.; Wada, Y. Low-temperature Annealing of Mesoscopic TiO2 Films by Interfacial Microwave Heating Applied to Efficiency Improvement of Dye-sensitized Solar Cells. Sol. Energy Mater. Sol. Cells 2016, 147, 198–202


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