Modification of Hematite Photoanode with Cobalt Based Oxygen

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Modification of Hematite Photoanode with Cobalt Based Oxygen Evolution Catalyst via Bifunctional Linker Approach for Efficient Water Splitting Amira Y Ahmed, Mahmoud G. Ahmed, and Tarek A. Kandiel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08010 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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The Journal of Physical Chemistry

Modification of Hematite Photoanode with Cobalt Based Oxygen Evolution Catalyst via Bifunctional Linker Approach for Efficient Water Splitting Amira Y. Ahmed,a Mahmoud G. Ahmed, a and Tarek A. Kandiela,* a

Department of Chemistry, Faculty of Science, Sohag University, Sohag 82524, Egypt

Email: [email protected]

ACS Paragon Plus Environment

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Abstract:

The modification of hematite photoanode with cheap, scalable, and efficient oxygen evolution catalyst is an essential step for its practical application for solar fuel production. In this paper, a simple and water-based method has been developed for the modification of hematite surface with cobalt based oxygen evolution catalyst using 3-aminopropionic acid (APA) bifunctional linker. APA exists in an aqueous solution at pH 6.1 in zwitteric ion form and thus it has a positive charge on the amino group and a negative charge on the carboxylate group. The carboxylate groups of APA molecules can thus interact with the hydroxyl groups of hematite while the amino groups tether the cobalt ions. The hematite photoanodes were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy and inductively coupled plasma optical emission spectroscopy. The photoelectrochemical measurements indicated that Co/APA-hematite photoanode exhibited 3.8, 3.2, and 2-fold higher photocurrent at 1.23 V vs. RHE than bare-, Co-, and CoPi-hematite photoanodes, respectively. Moreover, the onset potential of the photoelectrochemical water oxidation on Co/APA-hematite photoanode is cathodically shifted by 290 mV in comparison to that obtained on bare hematite photoanode. This finding has been explained by measuring the transient photocurrent and intensity modulated photocurrent spectroscopy responses. Based on these measurements, it was found that the rate constant of electron transfer at Co/APA-hematite photoanode/liquid interface is higher than that measured for bare- and CoPi-hematite photoelectrodes. This explains the higher photoelectrochemical activity of Co/APA-hematite photoanode and reflects the potential application of this simple approach for the modification of different metal oxide photoelectrodes with cobalt based oxygen evolution catalyst.


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1- Introduction The daylight and intermittent nature of the solar energy hinders its use for night time and transportation. Therefore, the search for economic and efficient technologies for storing the solar energy in the form of chemical fuel becomes urgent.1 Photoelectrochemical (PEC) cell is a promising route for utilizing and storing the solar energy in the form of carbon-free fuel such as H2, via water splitting.2-4 The simple PEC cell consists of a single photoelectrode and a metal counter electrode. The photoelectrode is the main component of the PEC cell and it converts incident photons to electron–hole pairs. The photogenerated holes oxidize the water in case of ntype semiconductors (photoanodes) and the photogenerated electrons are swept toward the conducting back contact, and are transported to the metal counter-electrode via an external wire where they will reduce water to form hydrogen gas.2 Many metal oxides and oxynitrides have been investigated;5-9 but they still suffer from some drawbacks, e.g., incongruity of conduction band energetic position with that for water reduction, large bandgap or low stability. Unfortunately, to date, there is no ideal material gathering the characteristics of stability, appropriate bandgap, and proper band alignment relative to the redox potential of water. Among many metal oxides, hematite (α-Fe2O3) exhibits great features which make it a good candidate as photoanode for photoelectrochemical water splitting. It possess an optical bandgap around 2.2 eV, thus it can in principle convert 16.8% of the solar energy into hydrogen which corresponds to maximum photocurrent 12.6 mA cm˗2 under standard illumination (AM 1.5G, 100 mW cm‒ 2 10

).

It has an excellent chemical stability against photocorrosion in alkaline aqueous solutions

and it is also abundant and non-toxic.11 However, hematite has some downsides, e. g., short-hole diffusion length (2-4 nm), low mobility of minority carriers, short life time of photogenerated charge carriers (~ 10 ps), and lower conduction band energetic level than the proton redox


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potential.10, 12 Moreover, the kinetic of water oxidation on hematite surface is slow and there is an energy loss due to the high overpotential. For instance, the encountered potential onset for water oxidation on hematite photoanode is 0.9 V versus reversible hydrogen electrode (RHE), however its flatband potential is located at 0.4 V vs. RHE.13-14 The high overpotential observed on hematite surface is usually attributed to the sluggish kinetics of oxygen evolution reaction (OER). This slow kinetics of water oxidation resulted in large build-up of photogenerated holes at the photoelectrode surface leading to high surface and bulk recombination rates.15 The modification of hematite surface with oxygen evolution catalyst for enhancing the kinetics of water oxidation is thus an important step for enhancing the efficiency of PEC cell. Two approaches have recently been introduced to decrease the catalytic overpotential on hematite photoanode and accelerate the OER, either through passivation of surface states or via coating the surface with oxygen evolution catalyst. It was reported that deposition of metal oxide overlayer, e.g., Al2O3,13 Ga2O3 or In2O3,16 or TiO217 on hematite can effectively passivate the surface trapping centres and prevent the accumulation of the photogenerated holes thus reducing surface recombination and enhancing the photoactivity. Modification of hematite with oxygen evolution catalyst, e.g., IrO215 or Ru based catalyst,18 has also been proved to enhance the catalytic activity of hematite towards water oxidation. Recently, cobalt based catalysts have attracted great attention due to the ease of their synthesis, their low cost and their globe abundant which make them more superior for large scale application. The recent study indicates that cobalt treated hematite exhibits a cathodic acceleration of OER as reported by Grätzel group.19 This cathodic shift is further improved by electrochemical modification of hematite with cobalt phosphate “CoPi” deposited under illumination.20 Recently, the coating of hematite with cobalt based sub-monolayer using atomic layer deposition (ALD) leaded to a dramatic potential onset


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shift, i.e., 100-200 mV.21 Modification of hematite surface with perfluorinated Cophthalocyanine (CoFPc) molecular electrocatalyst was also found to enhance the photocurrent and significantly shift the onset of photoelectrochemical water oxidation.22 It is apparent that the reactivity of the cobalt based catalyst strongly depends on the preparation method and thus the discovery of a simple method for the modification of photoelectrodes with cobalt based catalyst is desirable. In this work, a novel and solution-based method has been developed for the modification of hematite surface with cobalt based oxygen evolution catalyst using 3-aminopropionic acid (APA) bifunctional linker. The hematite surface was first functionalized with APA followed by soaking in cobalt solution. Interestingly, the hematite photoanodes modified with cobalt based catalyst via this simple method showed 3.8 and 2-fold higher photocurrent density than bare- and CoPi-hematite photoanodes at 1.23 V vs. RHE, respectively. Moreover, the onset potential for water oxidation is cathodically shifted by 290 mV which is comparable to that observed when CoPi was used and better that that observed for cobalt based sub-monolayer deposited using the sophisticated atomic layer deposition technique (ALD).21 The role of the cobalt based catalyst has been explored and discussed based on the transient photocurrent spectroscopy and intensity modulated photocurrent spectroscopy (IMPS) measurements.

2. Experimental Preparation of hematite films. Hematite films on fluorine doped tin oxide (FTO) glass were prepared by a template-less film processing technique as previously reported.23 FTO glass pieces (1×2 cm2) were cleaned by acetone, ethanol and water in an ultrasonic bath, and then they were immersed in aqueous solution of ferric chloride (0.15 mol L‒1) in


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tightly-closed glass vessel and heated at 95 °C for 6 h. The grown uniform yellow films of β-FeOOH on the FTO glass pieces were rinsed with water to remove the un-deposited inorganic salts, and heat treated at 550 °C for 1 h and at 800 °C for 20 min to obtain crystalline hematite films. Functionalization of hematite films with APA. The hematite films were functionalized with APA by immersing them in aqueous solution of APA (10 mmol L‒1) at pH 6.1 for different time periods (5−180 min). Afterwards, the films were rinsed with water to remove the unbounded APA molecules and re-immersed in cobalt nitrate aqueous solution for different time periods (5−90 min), then rinsed with water, and dried naturally at room temperature. Characterization. XRD patterns of hematite films were collected by using a Bruker D8 diffractometer operating with a CuKα1,2 energy source at 40 kV and 40 mA. SEM measurements were done by using a Nova Nano 630 (FEI Company) using a TLD detector at an accelerating voltage of 3 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Nova instrument (Kratos Analytical, with the CasaXPS software) using a high power monochromatic Al Kα radiation (1486.6 eV, 400 µm spot size, 36 W). The Attenuated total reflectance Fourier transform infrared (ATRFTIR) spectroscopy was carried out on a Bruker ALPHA FT-IR spectrometer. Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements were performed by digesting the cobalt modified hematite films in a mixture of concentrated HCl (6 mL) and concentrated HNO3 (2.0 mL). After dilution, the ICP-OES data were recorded using a Thermo Scientific spectrometer (iCAP 7000 series).


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Hematite photoanode fabrication. Hematite photoanode was fabricated by connecting a copper wire to the uncoated area of the FTO conductive surface using silver paste. After drying of the paste, the connection between the wire and the FTO glass substrate except 0.5 cm2 of the coated area was sealed with non-conductive epoxy resin and connected to plastic tube to be easily handled. Photoelectrochemical measurements. All photoelectrochemical measurements were carried out using an Autolab PGSTAT302N potentiostat in a three electrode system with hematite photoanode as a working electrode, a platinum wire as counter electrode, and Hg/HgO (1.0 mol L−1 NaOH) as a reference electrode. The electrolyte was an aqueous solution of NaOH (1.0 mol L−1). All the three electrodes were put in a three-electrode Teflon photoelectrochemical cell equipped with a quartz window on one side for illumination. The photocurrent measurements were performed under simulated solar irradiation. Sunlight was simulated by an Osram XBO 70 W xenon lamp in Müller LXH 100 lamp housing coupled with an air mass 1.5 global filter (Sciencetech Inc.). Light intensity of the simulated light was adjusted to 1 sun (100 mW cm−2) using a Thorlab digital handheld energy meter console (PM100D) connected with a calibrated highsensitivity thermal sensor (S401C, Thorlabs). Spectroscopic measurements. IMPS responses were measured using the same photoelectrochemical workstation except that the light source was a triple LED array (470 nm) driven by the output current of the Autolab LED Driver. The DC output of the LED Driver is controlled by the DAC164 of the Autolab and the AC output of the LED Driver is controlled by the FRA32M module. The AC amplitude was set to 10 % of the DC output. All the measurements were carried out with the NOVA software. The IMPS


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responses were examined at a frequency ranging from 0.1 Hz to 10 KHz at different applied potential, i.e., 0.7 to 1.3 V vs. RHE at 0.1 V intervals. The light intensity of the LED array at the electrode surface was 52.0 mW cm−2.

3. Results and Discussion Hematite films were prepared according to the previous report17 and characterized by scanning electron microscope (Figure S1) and X-ray diffraction (Figure S2). The SEM and XRD analysis indicated that the hematite films consist of condensed particles with a rod-like morphology and average diameters of ca. 90 nm and it has a hematite structure as evidenced from its (110) and (300) diffractions located at 35.7 and 64.05° (2θ).17,

23

Scheme 1 depicted the strategy used to modify the hematite surface with active cobalt species for water oxidation.

Scheme1. Schematic illustration for the modification of hematite photoanode with cobalt species employing APA bifunctional linker.


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APA has a carboxyl and amino functional groups and thus can act as a bifunctional linker. Under the appropriate condition, the carboxylate groups of APA molecules can interact with the surface hydroxyl groups of hematite over the ester linkage formation while the free amino groups interact with cobalt ions. To optimize the functionalization conditions, hematite films were immersed in an aqueous solution of APA (10 mmol L−1) for different time periods (5−180 min) at different pH values (pH 4–8). After washing the films with bi-distilled water to remove the unattached APA molecules, the hematite films were again immersed in cobalt nitrate aqueous solutions (1–10 mmol L−1) for different periods of times (15–90 min). The photoelectrochemical responses of the cobalt modified hematite photoanodes, modified under different experimental conditions, were measured and presented in Figures S3, S4, S5, and S6. It was found that hematite photoanodes modified by immersion in APA aqueous solution for 120 min at pH 6.1 followed by soaking in cobalt nitrate aqueous solution (5 mmol L−1) for 60 min exhibit the highest activity. This behavior can be explained by the fact that at pH 6.1, APA exists in zwitteric ion form24 and thus it has a positive charge on the amino group and a negative charge on the carboxylate group. Under the same condition, hematite surface is positively charged as its point of zero charge (pHPZC) lies in the range of pH 7−9.5.11,

25

Accordingly, a good chemical interaction via ester formation or at least electrostatic interaction between hematite and APA is highly expected. It was not possible to detect directly the characteristic infrared absorption bands of APA attached to the hematite film using the ATR-FTIR mode. The grown β-FeOOH was thus collected by scratching and heat treated under the same condition as for hematite films and then it was functionalized with APA. The attachment of APA to the surface of hematite was proven by FTIR analysis as shown in Figure 1. The APA functionalized hematite (APA-hematite) has two


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characteristic peaks at 2855 and 2920 cm−1 which can be readily assigned to the asymmetric and symmetric C–H stretching originates from the attached APA ligand whereas these two peaks are not exist in the case of bare hematite evidencing the successful linking of APA to the surface of hematite.26 The peaks at 1630 and 1530 cm−1 which can be assigned to the asymmetric stretching mode of COO− and the symmetric deformation mode of NH3+, respectively, further confirm the functionalization of hematite with APA.27-29 Characteristic peaks appear at 3400 cm−1 are attributed to the O–H stretching originates from the physisorbed water molecules.30 When the APA functionalized hematite is soaked in cobalt aqueous solution, APA interacts with cobalt ions to form Co-APA complex. By measuring the FTIR spectrum for Co/APA modified hematite, it was found that it exhibits corresponding peaks to that of APA functionalized hematite. In addition, a new peak appeared at 1165 cm−1 which might be assigned to the NH2 twist and indicate the interact of cobalt ions with APA.29 However, it is worth mentioning that the nature and stoichiometry of the Co-APA complex is still unclear and scheme 1 is for illustration purpose. According to this approach a monolayer of cobalt can be tethered to the hematite surface using APA as an organic bifunctional linker. This CoAPA layer might act as a catalyst for water oxidation or more likely it decomposes under the photoelectrochemical water oxidation reaction in basic condition leading to the formation of highly dispersed (CoOx)-based species on the surface of hematite photoanode. In fact the dispersed cluster of (CoOx)-based catalyst is expected to exhibit high activity towards water oxidation.31-32 Figure 2 shows the photoelectrochemical responses of bare hematite and hematite photoanode modified with cobalt by sequential soaking in APA and cobalt aqueous solutions (Co/APA-hematite) at the optimized


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conditions. For comparison, the photoelectrochemical activities of hematite photoanodes modified with cobalt in the absence of the APA linker (Co-hematite), with just APA (APA-hematite) and with CoPi catalyst (CoPi-hematite) were also presented.

Figure 1. FTIR for bare hematite, APA-hematite, and Co/APA-hematite.

Figure 2. Photoelectrochemical responses obtained on bare-, CoPi-, Co-, APA-, and Co/APAhematite photoanodes; conditions: scan rate (50 mV/s), electrolyte (NaOH, 1.0 mol L−1), light intensity (1 sun, AM 1.5G).


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Interestingly, it was found that the Co/APA-hematite photoanode exhibits 3.8, 3.2, and 2-fold higher photocurrent at 1.23 V vs. RHE than bare-, Co-, and CoPi-hematite photoanodes, respectively. The observed enhancement in the photocurrent density measured at 1.23 V vs. RHE for bare hematite photoanode after modification with Co/APA catalyst is higher than that reported when CoPi was employed.20 The achieved photocurrent on Co/APA-hematite photoanode, however, strongly depends on the photoactivity of bare hematite photoanode. By defining the potential needed to sustain 0.01 mA cm−2 as the photocurrent onset potential, it was found that the photocurrent onset potential of photoelectrochemical water oxidation on Co/APA-hematite is cathodically shifted by 290 mV in comparison with that obtained on bare hematite. APA-hematite photoanode showed a little enhanced activity which might be attributed to the oxidation of APA. To study the nature of the cobalt species, the Co/APA-hematite film has been analyzed by using the X-ray photoelectron spectroscopy (XPS), but unfortunately, the amount of cobalt tethered to the surface of hematite seems to be small and lies below the detection limit of the XPS spectrometer. To prove the existence of cobalt, a set of Co/APA-hematite films were digested in aqua regia and the concentration of cobalt was determined by inductively coupled plasma optical emission spectroscopy (ICP−OES). The concentration of cobalt was found to be 0.5 ng per cm2. To explore the role of cobalt species, the transient photocurrent (TPC) responses for bare- and Co/APA-hematite photoanodes were measured at low bias potential (0.9 V vs. RHE, Figure 3). Both TPC responses showed a characteristic decay from a spike to a steady state during the illumination period, followed by an overshoot and a decay back to zero during the dark period. In case of n-type semiconductor, this spike is generally


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attributed to the trapping and accumulation of the photogenerated holes in surface states. Afterwards, a steady-state photocurrent is reached when the rate of arrival of holes to the surface is exactly balanced by interfacial charge-carrier transfer and recombination.33 By integration the area under the spike the amount of trapped and accumulated photogenerated holes in the surface states can be calculated. It was found that the amounts of the photogenerated holes accumulated at the surface are 11.8 and 16.6 µC cm−2 for bare- and Co/APA-hematite photoanodes, respectively. The higher amount of trapped and/or accumulated photogenerated holes on the later than that on the former implies that cobalt species acts as an electrocatalyst rather than a passivation layer. This might explain the significant cathodic shift in the water oxidation onset potential and the enhanced kinetic of charge transfer.

Figure 3. Transient photocurrent (TPC) responses for bare- and Co/APA-hematite photoanodes at 0.9 V vs. RHE; conditions: electrolyte (NaOH, 1.0 mol L−1), light intensity (1 sun, AM 1.5G). To investigate this assumption in details, the rate constants of charge transfer and recombination at photoanode/liquid interface (PLI) were determined using the IMPS technique. In this technique, the photoelectrode is irradiated using a modulated light source leading to


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modulated flow of photogenerated holes toward the surface known as Gärtner flux. These holes might recombine with their counterparts at the PLI or transfer to the electrolyte. The relaxation of photogenerated holes at the surface by charge transfer and recombination gives rise to a semicircle in the complex plane IMPS plot with a radial frequency (ωmax) at the maximum.33-36 Figure 4 shows the typical IMPS responses for bare-, CoPi- and Co/APA-hematite photoanodes measured at 1.1 V vs. RHE. The IMPS responses were normalized to the high frequency intercept which represents the hole flux toward the surface (Gärtner flux, g). By analysis of the IMPS response, the rate constants of charge transfer (ktr) and charge recombination (krec) can then be calculated from the intersection of semicircle with the real axis at low frequency and ωmax as illustrated in Figure 4. It can see from the size of the semicircle in the positive/positive plane that the rate of charge transfer is greatly enhanced after modification with cobalt species. It is obvious also that the Co/APA is even more efficient in charge transfer than CoPi.

Figure 4. Normalized IMPS responses measured for bare-, CoPi-, and Co/APA-hematite photoanodes at 1.1 V vs. RHE; conditions: electrolyte (NaOH, 1.0 mol L−1), light intensity (monochromatic light 52 mW cm−2).


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The dependence of the krec and ktr on the applied potential for bare-, CoPi-, and Co/APAhematite photoanodes were calculated and presented in Figures 5a and b, respectively. It can be seen from Figure 5a that the recombination rate constant (krec) for bare hematite is much higher than that calculated for CoPi- and Co/APA-hematite photoanodes. It was also found that the krec of CoPi-hematite is less than that observed for Co/APA-hematite; however, the latter has higher photoelectrochemical activity (see Figure 2). CoPi was deposited under illumination and thus it can be selectively deposited at the active sites where the photogenerated holes are trapped at the surface. CoPi can thus act as an oxygen evolution catalyst and it can also form a Schottky-type heterojunction and hence reduce the surface recombination more efficiently than Co/APA.20, 37 By comparing the Ktr values at different applied potential (Figure 5b), it is obvious that the Co/APA-hematite exhibits higher Ktr than bare- and CoPi-hematite. These results indicate that the kinetics of water oxidation on Co/APA-hematite surface is much faster than that on CoPihematite. This explains the higher photoelectrochemical activity of Co/APA-hematite photoanodes.

Figure 5. (a) Recombination (krec) and (b) transfer rate constants for bare-, CoPi-, and Co/APAhematite photoanodes at different applied potential vs. RHE.


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4. Conclusions In conclusion, a simple and novel method for the modification of hematite photoanode with cobalt based electrocatalyst has been developed. APA was employed as a bifunctional linker. The APA molecules interact with the hydroxyl groups of hematite over the ester linkage formation while the amino groups tether the cobalt ions. Via this simple bifunctional linker approach, the surface of hematite photoanode was modified with cobalt monolayer. Interestingly, at the optimum condition, it was found that Co/APA-hematite photoanode exhibited 3.8, 3.2, and 2-fold higher photocurrent at 1.23 V vs. RHE than bare-, Co-, and CoPi-hematite photoanodes, respectively. Moreover, the onset potential of photoelectrochemical water oxidation on Co/APAhematite is cathodically shifted by 290 mV in comparison to that obtained on bare hematite. This finding was explained by measuring the transient photocurrent and intensity modulated photocurrent spectroscopy responses. Based on these measurements, it was concluded that Co/APA electrocatalyst facilitate the electron transfer at the hematite/solution interface more efficiently than CoPi.

Associated Content Supporting Information SEM image for hematite film, XRD patterns for bare- and Co/APA-hematite films, I−V curves for photoelectrochemical water oxidation on cobalt modified hematite photoanodes modified under different experimental conditions. These materials are available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author *Tel: +20 93 457 0000 ext. 2342; e-mail: [email protected]


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Notes The authors declare no competing financial interest.

Acknowledgements The authors thank the Science and Technology Development Fund (STDF, Egypt) for financial support through the Egyptian-German Research Fund (GERF III, Project ID 5064).

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22. Joya, K. S.; Morlanes, N.; Maloney, E.; Rodionov, V.; Takanabe, K. Immobilization of a Molecular Cobalt Electrocatalyst by Hydrophobic Interaction with a Hematite Photoanode for Highly Stable Oxygen Evolution. Chem. Commun. 2015, 51, 13481-13484. 23. Ahmed, M. G.; Kandiel, T. A.; Ahmed, A. Y.; Kretschmer, I.; Rashwan, F.; Bahnemann, D. Enhanced Photoelectrochemical Water Oxidation on Nanostructured Hematite Photoanodes via p-CaFe2O4/n-Fe2O3 Heterojunction Formation. J. Phys. Chem. C 2015, 119, 5864−5871. 24. Kannappan, V.; Vinayagam, S. C. Ultrasonic Studies on Molecular Interaction of α-Amino Acids in Aqueous Solutions at Different pH. Indian J. Pure Appl. Phys. 2013, 51, 471-478. 25. Parks, G. A.; Bruyn, P. L. d. The Zero Point of Charge of Oxides. J. Phys. Chem. 1962, 66, 967-973. 26. Lee, J.; Petruska, M. A.; Sun, S. Surface Modification and Assembly of Transparent Indium Tin Oxide Nanocrystals for Enhanced Conductivity. J. Phys. Chem. C 2014, 118, 12017-12021. 27. Jinnah, M. M. A.; Umadevi, M.; Ramakrishnan, V. Vibrational Spectral Studies of (βalanine) β-alaninium nitrate. J. Raman Spectroscopy 2004, 35, 956-960. 28. Leifer, A.; Lippincott, E. R. The Infrared Spectra of Some Amino Acids. J. Am. Chem. Soc. 1957, 79, 5098-5101. 29. Celap, M. B.; Niketic, S. R.; Janjic, T. J.; Nikolic, V. N., Synthesis and Characterization of the Geometrical Isomers of Tris(.beta.-alaninato)cobalt(III) Complexes. Inorg. Chem. 1967, 6, 2063-2065. 30. Sharma, G.; Jeevanandam, P. Synthesis of Self-assembled Prismatic Iron Oxide Nanoparticles by a Novel Thermal Decomposition Route. RSC Adv. 2013, 3, 189-200. 31. Esswein, A. J.; McMurdo, M. J.; Ross, P. N.; Bell, A. T.; Tilley, T. D. Size-Dependent Activity of Co3O4 Nanoparticle Anodes for Alkaline Water Electrolysis. J. Phys. Chem. C 2009, 113, 15068-15072. 32. Deng, X.; Tüysüz, H. Cobalt-Oxide-Based Materials as Water Oxidation Catalyst: Recent Progress and Challenges. ACS Catal. 2014, 4, 3701-3714. 33. Peter, L. M.; Wijayantha, K. G. U.; Tahir, A. A. Kinetics of Light-Driven Oxygen Evolution at α-Fe2O3 Electrodes. Faraday Discuss. 2012, 155, 309-322. 34. Xiao, S.; Chen, H.; Yang, Z.; Long, X.; Wang, Z.; Zhu, Z.; Qu, Y.; Yang, S. Origin of the Different Photoelectrochemical Performance of Mesoporous BiVO4 Photoanodes Between the BiVO4 and the FTO Side Illumination. J. Phys. Chem. C 2015, 119, 23350-23357. 35. Thorne, J. E.; Jang, J.-W.; Liu, E. Y.; Wang, D. Understanding the Origin of Photoelectrode Performance Enhancement by Probing Surface Kinetics. Chem. Sci. 2016, 7, 3347-3354.


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36. Peter, L. M. Energetics and Kinetics of Light-Driven Oxygen Evolution at Semiconductor Electrodes: The example of Hematite. J. Solid State Electrochem. 2013, 17, 315-326. 37. Barroso, M.; Cowan, A. J.; Pendlebury, S. R.; Grätzel, M.; Klug, 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.

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Modification of Hematite Photoanode with Cobalt Based Oxygen

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