Detection of Intermediate Species in Oxygen Evolution on Hematite

Article pubs.acs.org/JPCC

Detection of Intermediate Species in Oxygen Evolution on Hematite Electrodes Using Spectroelectrochemical Measurements Toshihiro Takashima,† Koki Ishikawa,‡ and Hiroshi Irie*,† †

Clean Energy Research Center, and ‡Special Doctoral Program for Green Energy Conversion Science and Technology, Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan S Supporting Information *

ABSTRACT: The development of active oxygen evolution catalysts based on understanding of the underlying reaction mechanisms is the key to achieving efficient solar-to-chemical energy conversion. In this study, we synthesized hematite nanoparticle film electrodes and applied them to spectro-electrochemical measurements to detect the intermediate species in oxygen evolution reaction. In situ UV−vis absorption spectra showed that the formation of the species exhibiting absorption at 580 nm was the rate-determining step of oxygen evolution reaction on hematite over a wide range of pH from 4 to 13. In addition, the pH dependences of the onset potentials for oxygen evolution and formation of the intermediate species revealed that there were two reaction mechanisms, which switched at approximately pH 10. On the basis of careful inspection of the observed spectra and the possible active species with reference to previous reports on anomalously oxidized iron compounds, the detected intermediate species was assigned to iron in the oxidation state of 4+.

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INTRODUCTION Solar-to-chemical energy conversion has long been attracting considerable attention as a means of storing renewable energy in the form of chemical bonds.1−5 In this energy conversion process, oxygen (O2) evolution reaction (OER) plays an essential role as the complementary half-reaction to fuel production reactions such as hydrogen evolution and carbon dioxide fixation and is generally considered to be the efficiencylimiting step because OER requires the coupled transfer of four protons and four electrons from two water molecules to release one O2 molecule. Therefore, significant effort has been devoted to obtaining a fundamental understanding of the mechanism of OER6−14 as well as to the development of highly active catalysts.15−21 The use of first-row transition metal oxides for the development of OER catalysts is of particular interest because these materials are advantageous in terms of scalability, nontoxicity, and cost-effectiveness compared with precious metal oxides. In particular, iron (Fe) oxide is an attractive material because Fe is the second most abundant metal in the Earth’s crust and functions as an active center in many biological enzymes owing to its multiple oxidation states.22,23 For a long time, hematite (α-Fe2O3) has been focused on as a promising photoanode material owing to the suitability of its band position for photoelectrochemical OER, and the reaction mechanism of photoelectrochemical OER on α-Fe2O3 has recently been investigated using spectroscopic methods such as transient absorption measurements, operando infrared spectroscopy, impedance spectroscopy, and in situ soft X-ray © XXXX American Chemical Society

absorption spectroscopy, and there has been intensive discussion about the role of Fe in the processes of surface redox chemistry and charge separation.24−37 In contrast, few studies have been conducted with the aim of understanding the mechanisms of electrochemical OER on Fe oxides including αFe2O338−40 although Durrant et al. reported that a spectroscopic change of α-Fe2O3 was observed also in the dark conditions by applying potential.25 Notably, some Fecontaining perovskites and metal oxyhydroxides composed of a mixture of nickel (Ni) and Fe have recently been reported to show excellent OER activity under alkaline conditions40−44 comparable to that of precious metal oxides, although the origin of the high activity remains unclear. Thus, these findings indicate that Fe oxides are potentially active OER catalysts, and we expect that understanding the reaction mechanism will be helpful for achieving high OER activity using Fe oxides under neutral pH conditions, which are desirable for solar-to-chemical energy conversion. In this study, we investigated the intermediate species in OER on an α-Fe2O3 electrode using spectroelectrochemical techniques. From the observed in situ absorption spectra, a new absorption band was found to appear simultaneously with the generation of anodic current for O2 evolution. Detailed investigation of the pH dependences of the onset potentials for the change in absorption and O2 evolution revealed that the Received: August 7, 2016 Revised: October 12, 2016

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DOI: 10.1021/acs.jpcc.6b07978 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) XRD pattern and optical image (inset) of α-Fe2O3 thin film on FTO substrate. (b) Top-view (b-1) and cross-sectional (b-2) SEM images of α-Fe2O3 thin-film electrode. (c) Current density (solid line) and dissolved O2 concentration (red squares) for α-Fe2O3 film electrode at pH 7.

adjusted using 0.1 M sulfuric acid (H2SO4, Kanto Kagaku, >99.0%) and 1 M sodium hydroxide (NaOH, Kanto Kagaku, >99.0%). No reagent for pH buffering was added to the electrolyte solution to avoid any effect of the adsorption of multivalent anions. The prepared electrolyte was bubbled with argon (Ar) gas prior to the measurement. To minimize changes in the pH near the electrode surface, the polarization curve was measured by performing a potential sweep from negative to positive. The concentration of O2 dissolved in the electrolyte solution was monitored simultaneously with the potential sweep measurement using a needle-type oxygen microsensor (Microx TX3-trace, PreSens).

species corresponding to the absorption band is the intermediate species of OER, and comparison of the spectra reported herein with those reported for Fe species with high oxidation states provided a plausible assignment of this species.

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EXPERIMENTAL SECTION Electrode Preparation. An Fe(OH)3 colloidal solution was synthesized by adding 20 mL of 1 M ferric chloride (FeCl3· 6H2O, Kanto Kagaku, >99.0%) solution dropwise to 230 mL of boiling water while vigorously stirring. After boiling for 10 min, the solution was allowed to cool to room temperature and was then dialyzed for 2 days to remove chloride ions. The thusprepared Fe(OH)3 solution was diluted and then sprayed onto a clean conducting glass substrate (FTO-coated glass, resistance 20 Ω/square) fixed on a 250 °C hot plate using an automatic spray gun (Lumina, ST-6). The red film that formed on the substrate was thoroughly rinsed with pure water and then calcinated at 500 °C in air for 2 h. Characterization. The crystal structure of the prepared film electrode was determined by X-ray diffraction (XRD, PW-1700, PANalytical). XRD patterns were recorded from 30° to 70° in 2θ at a step size of 0.02° and a scan rate of 0.25°/min. The optical absorption spectra were obtained in diffuse transmission mode using a UV−visible (UV−vis) spectrometer (V-650, JASCO) equipped with an integrating sphere. For the in situ acquisition of spectra, an α-Fe2O3 film electrode mounted in an electrochemical cell was placed in front of the integrating sphere to collect the diffused transmission light. A scanning electron microscope (SEM; JSM-6500F, JEOL) was used to observe the morphology of the prepared α-Fe2O3 film. For high-resolution transmission electron microscopy (HR-TEM; Tecnai Osiris, FEI) inspection, α-Fe2O3 powder was scratched off from the surface of the electrodes, dispersed in ethanol, and deposited on copper (Cu) microgrids. Electrochemical Measurements. Polarization curves were obtained with a commercial potentiostat and potential programmer (HZ-5000, Hokuto Denko) using a platinum (Pt) wire as the counter electrode and a silver/silver chloride electrode (Ag/AgCl/KCl(sat.)) as the reference electrode. The electrolyte solution (0.1 M sodium sulfate (Na2SO4)) was prepared by dissolving Na2SO4 (Kanto Kagaku, >99.0%) in highly pure Milli-Q water (18 MΩ·cm), and the pH was

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RESULTS AND DISCUSSION A transparent α-Fe2O3 nanoparticle film electrode prepared by spray deposition on fluorine-doped tin oxide (F-doped SnO2, FTO) was used for spectroelectrochemical measurements. The thin-film electrodes exhibited an X-ray diffraction (XRD) pattern corresponding to those of α-Fe2O3 and SnO2 as shown in Figure 1a. Scanning electron microscopy (SEM) inspection showed that the entire electrode surface of the FTO substrate was covered with nanoparticles with diameters ranging from 20 to 40 nm (Figure 1b-1). The network pattern shown in this image is considered to be formed by the aggregation of α-Fe2O3 nanoparticles during the evaporation of the sprayed droplets, and the thickness of the α-Fe2O3 layer was confirmed to be in the range from 200 to 500 nm (Figure 1b2). Thus, an α-Fe2O3 nanoparticle electrode was obtained (inset of Figure 1a), and such a transparent nanoparticle electrode is useful for in situ observation of the intermediate species because of its large surface area and applicability to diffuse transmission-mode absorption measurements.14,21 Figure 1c shows a polarization curve of an α-Fe2O3 electrode measured at pH 7. The simultaneous increase in current density and O2 concentration indicates that OER initiated from approximately 1.4 V, which corresponds to an overpotential of 0.59 V. This overpotential is a typical value for α-Fe2O3 electrodes measured under neutral pH conditions.38 To investigate the stability of the electrocatalyst, the α-Fe2O3 nanoparticles before and after electrocatalysis were subjected to inspection using transmission electron microscopy (TEM). For several Fe-containing catalysts, amorphization of their B


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The Journal of Physical Chemistry C surface structure has been observed after OER.41,45 In contrast, TEM images of the α-Fe2O3 nanoparticles showed that the surfaces of the α-Fe2O3 nanoparticles after electrocatalysis at 1.7 V for 1 h were highly crystalline, and little change in the crystal structure was observed between before and after electrolysis. (Figure 2). Therefore, the α-Fe2O3 nanoparticles

change was fully reversible in both positive- and negativedirection sweeps and remained stable at a constant electrode potential, indicating that this new absorption was caused by a change in the redox state of the surface Fe ions and not by a change in the physical state of α-Fe2O3 such as dissolution, phase transition, or amorphization, as confirmed by the TEM observation. The appearance of an absorption peak at 580 nm under applied potential was also observed in a paper of Durrant’s group25 while the absorption band observed in this study is much broader and unsymmetrical. We tentatively consider that the difference in the shape of the absorption band is due to a difference in preparation methods of α-Fe2O3 electrodes. As described above, we adopted a spray deposition method to prepare an α-Fe2O3 electrode, which was composed of assembly of nanoparticles (Figures 1b and 2). Thus, the prepared α-Fe2O3 film is considered to have rough surface, large surface area, and low crystallinity compared with that prepared by the atmospheric pressure chemical vapor deposition (APCVD) method,25 and those differences might cause the change in the shape of the absorption band. Figure 3c shows the potential dependence of the integrated absorbance of the absorption band formed at 580 nm (ΔAbs580) together with the polarization curve.46 From these results, ΔAbs580 was found to be negligible in the potential range of 0.8−1.0 V, while it increased rapidly at more positive potentials. Namely, ΔAbs580 and the O2 evolution current increased with similar potential dependences, and the onset potential of ΔAbs580 was slightly more negative than that of the O2 evolution. These results suggest that the transient species showing absorption at 580 nm is a precursor of OER. Notably, when the in situ measurement was carried out in the presence of phosphate ions, the peak position was shifted to a slightly longer wavelength (Figure 3d). This shift can be explained by the coordination of phosphate ions to the transient Fe species because the coordination of a

Figure 2. TEM images of nanocrystalline α-Fe2O3 (a) before and (b) after electrolysis at 1.7 V for 1 h.

were stable during the catalytic cycle, which ensured that spectroscopic data obtained by using in situ measurement techniques can be regarded as originating from the species involved in surface redox reactions, not from physical changes in the catalyst. Figure 3a shows a comparison of the diffuse transmission UV−vis absorption spectra of the α-Fe2O3 electrode measured at 0.7 V (open-circuit potential) and 1.7 V where O2 evolution proceeded on the electrode. From this comparison, a very small increase in absorption from 400 to 800 nm was found at 1.7 V. To show this change in absorption clearly, difference spectra were obtained by subtracting the reference spectrum measured at 0.7 V from spectra measured over an increasing range of the electrode potential (Figure 3b). Upon stepping the electrode potential from 0.8 to 1.7 V in 0.1 V increments, the formation of an absorption peak at 580 nm was observed. This spectral

Figure 3. (a) Diffuse transmission UV−vis absorption spectra of α-Fe2O3 film electrode held at 1.7 V (red line) and open-circuit potential (0.7 V, black line). The inset shows a magnification to clarify the increase in absorption upon changing the electrode potential from 0.7 to 1.7 V. (b) Changes in the UV−vis spectrum of α-Fe2O3 film electrode with increasing potential (1.1, 1.2, 1.3, 1.4, 1.5, 1.6, and 1.7 V). The spectrum measured at 0.7 V was used as a reference. The arrow indicates the direction of the spectral change with increasing potential from 1.1 to 1.7 V. The spectrum at 1.7 V is colored red. (c) Potential dependence of difference in the integrated absorbance of an absorption band at 580 nm (red squares) at pH 7. The solid line indicates a polarization curve. (d) Comparison of difference absorption spectra measured at 1.7 V in the presence (blue line) and absence (black line) of phosphate ions. The blue arrow indicates the direction of the peak shift upon the addition of phosphate ions. C


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The Journal of Physical Chemistry C phosphate ion to a metal ion can induce red shifts of the d−d transition absorption and ligand to metal transition absorption.47,48 Thus, the absorption can be expected to originate from the transition associated with surface-associated Fe ions. It is also notable that when an organic electrolyte, i.e., a chloroform solution containing tetra-n-butylammonium perchlorate (TBA(ClO4)), was used instead of an aqueous electrolyte, no reversible change in absorption appeared (Figure S2). Taken together, it is considered that the observed spectral change was a consequence of the formation of oxidized Fe species on the surface of α-Fe2O3 which participates in OER.31 We also conducted similar spectroelectrochemical measurements at pH values other than 7. Figure 4 representatively

Figure 5. pH dependences of onset potential for oxidation current (Uon,j, red squares) and optical absorption at 580 nm (Uon,A580, blue circles). The electrode potentials at which the current density and the increase in absorbance reached 50 μA cm−2 and 0.001 were adopted as Uon,j and Uon,A580, respectively. The solid line represents the standard potential for OER.

mechanism during the potential sweep because protons released as a reaction product acidified the interface. When the measurement was conducted under stirring conditions, current density continuously increased (Figure S6). Notably, the pH dependence of Uon,A580 closely reproduced that of Uon,j including both Nernstian and non-Nernstian behaviors. At pH 10, where the reaction mechanism switched during the potential sweep, not only current density but also the change in absorbance showed a two-step increase. In addition, it is also notable that Uon,A580 was located at the potential region slightly negative of Uon,j irrespective of the pH. Thus, it is considered that the species showing absorption at 580 nm is the intermediate species of electrochemical O2 evolution over the entire range of the examined pH and that the potential for the formation of this species determines the onset potential for O2 evolution on α-Fe2O3. When we added hydrogen peroxide (H2O2) to the electrolyte, two interesting changes were observed in the results of the spectroelectrochemical measurement. First, anodic current was generated from a more negative potential in the presence of H2O2 (Figure 6). This is reasonable since the standard potential of H2O2 oxidation (H2O2 → O2 + 2H+ + 2e−; +0.681 V) is located at more negative potential than that of water oxidation (2H2O → O2 + 4H+ + 4e−; +1.23 V); therefore, H2O2 is more easily oxidized than H2O. Second, the change in absorption at 580 nm was strongly suppressed, and

Figure 4. Potential dependences of current density (solid line) and difference in the integrated absorbance of an absorption band at 580 nm (red squares) measured at (a) pH 4, (b) pH 10, and (c) pH 13.

shows plots of ΔAbs580 against the electrode potential, together with the polarization curves measured at pH 4, 10, and 13.46 In all cases, the anodic current was attributable to the OER owing to the simultaneous detection of O2 (Figure S3), and the measured onset potentials were almost the same as the reported values (Figure S4). The optical absorption spectra observed during OER at each pH indicated the generation of essentially the same absorption band as that appearing at pH 7 (Figure S5). Figure 5 summarizes the pH dependences of the onset potentials for anodic current (Uon,j, red squares) and absorption (Uon,A580, blue circles) from the results presented in Figure 4. From these plots, we found that Uon,j remained almost constant at approximately 1.5 V between pH 4 and 9 but displayed a negative shift following the Nernst equation at pH ≥ 10. These trends clearly indicate that there are different reaction mechanisms for O2 evolution under neutral and alkaline pH conditions, which will be discussed later. The existence of the similar two pH-dependent reaction pathways was previously proposed on the basis of the evaluation of kinetic parameters, and the observation of switching of the reaction mechanism at pH of around 10 was consistent with our result.38 In this study, the switching of the reaction mechanism at pH 10 was also confirmed from the polarization curve, in which current density increased in a stepwise manner at approximately 1.1 and 1.5 V. This two-step increase in current density is considered to show the switching of the reaction mechanism from the alkaline mechanism to the neutral

Figure 6. Polarization curves of α-Fe2O3 film electrode measured at pH 13 in the presence (red line) and absence (black line) of 50 mM H2O2. Red plots indicate the potential dependence of the change in absorbance at 580 nm in a 50 mM H2O2 solution. D


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CaCu3Fe4O12 had superior activity to state-of-the-art catalysts such as RuO2 and Ba0.5Sr0.5Co0.8Fe0.2O3‑δ. In addition, we demonstrated that the availability of Fe4+ significantly affected OER activity of strontium ferrite (Sr3Fe2O7) derivatives, which contain Fe4+ but suffer from charge disproportionation (2Fe4+ → Fe3+ + Fe5+) at room temperature.56 When we suppressed the consumption of Fe4+ owing to this reaction by increasing the temperature or introducing a dopant element, its OER activity considerably increased. Thus, Fe4+ species contribute an efficient OER. Notably, direct evidence for formation of Fe4+ in NiFe hydroxide during electrochemical OER has recently been reported by Chen et al.57 Because Ni hydroxide is known to show enhancement of OER activity by introduction of Fe, they conducted in situ Mössbauer measurements of NiFe hydroxide to gain insight into the role of Fe and observed signals indicating the formation of Fe4+ during O2 evolution. On the basis of this result, they speculated that Fe4+ species generated at the edge, corner, or defect sites of Ni hydroxide were responsible for the enhanced OER activity. Fe4+ formation on NiFe hydroxide under oxidative conditions has also been reported in other previous papers.58,59 Notably, when we conducted preliminary experiments to observe in situ UV−vis absorption spectra of NiFe hydroxide, incorporation of Fe was found to induce an increase in absorption similar to that observed for α-Fe2O3 electrodes (Figure S7), which further supports our assignment of the detected intermediate species to Fe4+. Finally, we comment on the pH dependence of the catalytic activity of α-Fe2O3 measured in this study. First, as the onset potential for O2 evolution follows that of Fe4+ formation over a wide pH range, the pH dependence of the OER activity should reflect that of the availability of Fe4+. In other words, Fe4+ is presumed to be more easily accessible at pH values higher than 10. This expectation agrees with the results of previous pulse radiolysis studies, in which Fe4+ was found to be stabilized under alkaline conditions from pH 10 to 14.50,60 In addition, in these reports, Fe4+ was considered to be reduced to Fe3+ by oxidizing water through the formation of H2O2. Second, at a neutral pH, O2 evolution and Fe4+ formation were initiated at almost constant potential. This pH-independence is likely to show that the rate-determining step of the OER, namely Fe4+ formation, proceeds via the sequential proton and electron transfer at neutral pH, not via the concerted proton-coupled electron transfer (PCET) as observed at alkaline pH. The conversion of the electron transfer from the sequential process to the concerted process has been reported to be useful to reduce overpotential of electrochemical reactions13,61 because the concerted PCET process enables the avoidance of the formation of high-energy protonated or deprotonated intermediates that arise in the sequential process. Therefore, the induction of the concerted PCET on α-Fe2O3 would enable us to improve its OER activity at a neutral pH. Notably, Zhao et al. recently reported that the concerted PCET process prevails in photoelectrochemical OER at a neutral pH.62 Thus, this discrepancy suggests that a way of participation of protons in the rate-determining step is different between electrochemical and photoelectrochemical conditions.

no detectable change was observed even at a potential where OER proceeded in the absence of H2O2 (Figure 6). The absorption spectrum of the electrode itself was the same before and after the addition of H2O2. Therefore, we expect that the transient absorption did not appear because H2O2 oxidation preferentially occurred on the α-Fe2O3 electrode or because the intermediate species that appeared reacted with H2O2. In general, OER is often considered to proceed via the formation of a surface-bound peroxide species, and then the peroxide species is assumed to be further oxidized.40,41,49 However, the absence of the transient absorption both with and without the applied potential indicates that the intermediate species is not generated during the formation and oxidation of the peroxide species. Thus, the intermediate species is considered to be generated in the initial steps of OER prior to peroxide formation. To obtain an insight into the assignment of the observed intermediate species, we compared the measured difference absorption spectra with spectra reported for Fe species whose valence state is higher than Fe3+ because Fe3+ is the initial state of α-Fe2O3 and an Fe species with a higher oxidation state is considered to be responsible for OER. Among the oxidation states ranging from Fe4+ to Fe6+, ferrate compounds containing Fe5+ and Fe6+ have been examined in pulse radiolysis studies, and HFe5+O42− and Fe6+O42− were reported to show absorption peaks at 400 and 510 nm, respectively.50,51 As the spectral features of these compounds are inconsistent with that of the intermediate species reported herein, other possible species are assigned. In contrast, the transient absorption spectrum of an α-Fe2O3 photoanode was reported to show a broad absorption band with a peak at 580 nm, and its shape was very similar to the spectrum observed in this study.24−30 In addition, the species showing this broad absorption band has been considered to be responsible for photoelectrochemical OER because it has a long lifetime up to 2 s on an anodically poised α-Fe2O3 photoanode.24−28 Owing to the predominant Fe4+ character of photogenerated holes on α-Fe2O3, this transient species was assigned to Fe4+.52 In addition, Hamann et al. recently reported a spectroscopic evidence for the assignment of this transient species to Fe4+.31 They conducted operando infrared spectroscopy of α-Fe2O3 photoanode using isotope-labeling techniques and demonstrated the appearance of an infrared absorption signal during photoelectrochemical OER which is attributable to Fe4+O species. It was also reported that Fe4+ formed by holes trapped at Fe3+ sites of Fedoped SrTiO3 showed optical absorption bands with maxima at approximately 2.1 eV.53 Although this is not sufficient evidence for the conclusive identification of the intermediate species detected herein, we tentatively assign the intermediate species to Fe4+ generated at the surface. Following this assignment, the efficient formation of Fe4+ is considered to afford efficient O2 evolution. This assumption coincides with the prediction by Suntivich et al. that OER activity of transition metal oxides is correlated with the occupancy of the 3d electrons with eg symmetry and that having an eg filling close to unity leads to high activity.40 Generally, the Fe4+ ions of metal oxides have been reported to have the high-spin d4 configuration of t2g3eg1;54,55 thus, Fe4+ formed on α-Fe2O3 is likely to satisfy the preconditions to drive OER efficiently. In fact, there have been a few reports showing the effectiveness of Fe4+ for OER. Yagi et al. developed Fe4+based quadruple perovskite, CaCu3Fe4O12, and examined its OER activity under alkaline conditions.41 They found that

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CONCLUSIONS In summary, in situ spectroelectrochemical measurements of αFe2O3 electrodes revealed that a species showing absorption at 580 nm always formed prior to O2 evolution over a wide range of pH values from 4 to 13. The similar pH dependences of E


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onset potentials for O2 evolution and the formation of the transient species indicated that the observed species served as the precursor of OER. By comparison of the intermediate species spectrum with those reported for Fe compounds containing high-valency Fe such as a photoexcited α-Fe2O3 photoelectrode, Fe-doped metal oxides, and ferrate ions, we found that the intermediate species was attributable to Fe4+, and the assignment to Fe4+ can account for the high OER activity of CaCu3Fe4O12, Sr3Fe2O7 derivatives, and NiFe hydroxide. These findings suggest that the accessibility to Fe4+ is a key to designing Fe-based active OER catalysts. In particular, the pH independence of the Fe4+ formation potential at a neutral pH suggests that the induction of concerted PCET during the catalytic cycle can enhance OER activity of α-Fe2O3. Because the presence of a proton acceptor with an appropriate pKa regulates the concerted PCET,13,61,63 studies on the effect of PCET induction on OER activity of Fe oxides are ongoing in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07978. Spectroelectrochemical results measured in organic electrolyte, O2 evolution activity and in situ absorption spectra measured at different pH values, an effect of agitation on a polarization curve, and preliminary spectroelectrochemical results of NiFe hydroxide (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-55-220-8092. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Program to Disseminate Tenure Tracking System by MEXT and by JKA and its promotion funds from KEIRIN RACE(28-146).

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REFERENCES

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