A New Class of Molecular-Based Photoelectrochemical Cell for Solar

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A New Class of Molecular-Based Photoelectrochemical Cell for Solar Hydrogen Production Consisting of Two Mesoporous TiO2 Electrodes Kohei Morita, Ken Sakai, and Hironobu Ozawa ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01992 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 21, 2019

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A New Class of Molecular-Based Photoelectrochemical Cell for Solar Hydrogen Production Consisting of Two Mesoporous TiO2 Electrodes Kohei Morita,† Ken Sakai,*†‡§ and Hironobu Ozawa*†‡ †

Department of Chemistry, Faculty of Science, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan § Center for Molecular Systems (CMS), Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan ‡

ABSTRACT: A new class of molecular-based photoelectrochemical cell for solar hydrogen production consisting of a TiO2based photoanode modified with a polypyridyl ruthenium photosensitizer (Ru-qpy) and a TiO2-based cathode modified with a platinum porphyrin H2 evolution catalyst has been investigated, showing that the electron accumulation at the conduction band of TiO2 at the photoanode is promoted via electron injection from the Ru-qpy together with the hole scavenging by a sacrificial donor. Our study here for the first time unveils that the upward shift given in the Fermi level of the TiO2 at the photoanode provides an electromotive force required to flow electrical current leading to solar hydrogen production from water even without applying external electrical bias.

Solar water splitting into H2 and O2 has attracted great attention in recent years towards a sustainable energy future. In order to achieve highly efficient solar water splitting, various efforts have been made to develop molecular-based photoanodes for O2 production1-4 and molecular-based photocathodes for H2 production.58 Moreover, efforts have also been made to construct molecularbased dye-sensitized photoelectrochemical cells (DSPECs) enabling the overall solar water splitting by the combined use of a photoanode and a photocathode.9-11 Generally, some n-type semiconductors, such as TiO21-4 and BiVO412,13, are used as anode materials, and various kinds of p-type semiconductors, such as NiO5-8, GaP14,15 and p-doped Si16,17, are employed as cathode materials. Until now, a large number of n-type and p-type semiconductors have been examined as electrode materials. However, only a limited number of reports have demonstrated the combined use of different electrode materials in constructing DSPECs, in which a n-type TiO2-based photoanode and a p-type NiO-based photocathode are often adopted.9,10 In these molecular-based DSPECs, the bottleneck reaction for the overall solar water splitting is H2 production at the NiO-based photocathode because NiO has some drawbacks as a cathode material.9,10 For example, the charge recombination due to electron transfer from the reduced dye or catalyst into the valence band holes is generally quite rapid.7,10,18 Moreover, H2 production competes with the reduction of a large amount of Ni3+ ions originally contained in NiO.7,10,19 These drawbacks are intrinsic limitations to develop a highly efficient NiO-based photocathode. Thus, there is a demand to develop or realize alternative cathode materials that are free of such drawbacks. In this context, several novel p-type semiconductors have

been recently reported as alternative cathode materials for either H2 production20 or CO2 reduction.21 e-

ee-

e-

e hv

2+

CB N

EDTA(ox)

N N

Ru

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TiO2

Ru-qpy

CB N

N N

Pt N

N

N

EDTA

e

H2O

N

N

FTO

e-

-

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FTO

TiO2

Aqueous acetate buffer (pH 5)

Scheme 1. Schematic representation of a molecular-based PEC for solar hydrogen production consisting of two mesoporous TiO2 electrodes. Visible light was irradiated only at the anode (irradiation of cathode did not affect the results).

On the other hand, we recently reported on the effectiveness of TiO2 (n-type) as a cathode material for H2 production.22 In the study, a TiO2-based cathode modified with a platinum porphyrin H2 evolution catalyst (PtP-py, Scheme 1) was developed, and the catalytic performance of the FTO/TiO2/PtP-py cathode was successfully demonstrated. It was found that PtP-py can catalyze H2 production from water with an extremely small onset overpotential (less than 50 meV), and H2 production proceeds with an almost quantitative Faradaic efficiency when the electrode potential is held at the conduction band edge (CBE) potential of TiO2.22 The merit of using TiO2 instead of other semiconductor materials lies in the well advanced technique enabling the preparation of extremely high surface mesoporous films, as a consequence of intensive studies on DSSCs (Dye-Sensitized Solar Cells). Moreover, we briefly reported that visible light-driven H2 production can be promoted when the FTO/TiO2/PtP-py cathode is connected to a TiO2-based photoanode modified with a polypyridyl ruthenium photosensitizer,23 i.e., [Ru(dmbpy)2(qpy)]2+ (dmbpy = 4,4'dimethyl-2,2'-bipyridine, qpy = 4,4':2',2'':4'',4'''-quarterpyridine) in the presence of a sacrificial electron donor, EDTA (ethylenediaminetetraacetic acid disodium salt).22 Although some reports previously demonstrated the use of TiO2 as a cathode material for H2 production11 or CO2 reduction24, our previous report became the

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ACS Applied Energy Materials first example of a molecular-based photoelectrochemical cell (PEC) consisting of two TiO2-based electrodes. Furthermore, we also reported on our preliminary results showing that our PEC, depicted in Scheme 1, actually evolves H2 even without applying external electrical bias (hereafter, we denote this condition as ‘bias-free condition’).22 In spite of these intriguing findings, our previous study still lacked evidences for further discussion on the operation mechanism of our PECs. In this study, we attempt to clarify the origin of the electromotive force (EMF) required to flow electrons between the two TiO2-based electrodes in our PEC. Here we report on the experimental details showing the electron accumulation behaviors at the photoanode in our PEC. The relevance of the upward shift in the Fermi level of TiO2 upon electron accumulation at the conduction band of TiO2 (CB@TiO2) will also be discussed. The relationship between the EMF and the H2 evolution characteristics under biasfree condition, together with the quick responses in all observable events upon the light-on and -off actions, will be described in detail. Here we employ a new ruthenium photosensitizer having two pyridyl anchors23 [Ru(dpbpy)2(qpy)]2+ (dpbpy = 4,4'-diphenyl2,2'-bipyridine) (Ru-qpy, Scheme 1), designed to increase the molar absorption coefficients at the MLCT band. This aims at achieving higher light harvesting efficiency even in a mesoporous TiO2 film having a relatively thin thickness (ca. 12 µm, Figures S2,S6b). Rectangle or square shape of mesoporous TiO2 thin films (1.0 cm2) using transparent conductive FTO glass substrates (i.e., FTO/TiO2 electrode) were fabricated by a screen-printing and a sintering technique.25 Modification of the pristine FTO/TiO2 electrode was carried out by an immersion method, and the amount of either Ru-qpy or PtP-py adsorbed over the TiO2 surfaces was estimated from the absorbance change in the immersion bath (determined as 0.12 and 0.10 μmol/cm2 for Ru-qpy and PtP-py, respectively).

Photocurrent density (mA/cm2)

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FTO/TiO2/Ru-qpy - FTO/TiO2/PtP-py FTO/TiO2/Ru-qpy - FTO/TiO2 FTO/TiO2 - FTO/TiO 2/PtP-py

2.5 2

Scannin

1.5 1 0.5 0 -0.2

0 0.2 0.4 0.6 0.8 Potential (V vs. cathode)

1

Figure 1. Photocurrent density observed while scanning a linear sweep voltammetry under intermittent irradiation of visible light (λ > 400 nm) at a scan rate of 10 mV/s. The measurements were carried out using a two-electrode configuration PEC by scanning the anode potential vs the cathode potential with the reference terminal short connected to the cathode. The electrolyte solution was an aqueous acetate buffer solution (0.1 M, pH 5.0) containing 30 mM EDTA and 30 mM NaClO4. The light was only irradiated at the photoanode using a solar simulator (AM 1.5G, 100 mW/cm2) equipped with a L-42 cutoff filter.

The photoelectrochemical measurements in the present study were carried out in a two-electrode configuration PEC by scanning the anode potential vs the cathode potential with the reference terminal short connected to the cathode terminal, leading to the description of “V vs. cathode” for the horizontal axis in Figure 1. The electrolyte solution contained acetate buffer (pH 5) and EDTA. Visible light (λ > 400 nm) was only irradiated at the photoanode with the cathode shielded from the light. For the combination of the pristine FTO/TiO2 photoanode and the FTO/TiO2/PtP-py cathode, photocurrent was confirmed negligible for the entire potential domain (Figure 1, blue). On the other hand, when the FTO/TiO2/Ru-qpy electrode is adopted as the photoanode (Figure 1, red and green lines), the anodic photocurrent increases as the potential is raised regardless of the presence or absence of PtP-py H2 evolution catalyst at the cathode. In both cases, the photocurrent responds quickly to the light-on and -off actions, revealing that the observed photocurrent is undoubtedly derived from the electron injection from the photosensitizer to the CB@TiO2 at the photoanode, where the electron injection can be due to either oxidative quenching of the photoexcited Ru-qpy (i.e., Ru-qpy*) or oxidation of the reductively quenched product of Ru-qpy (i.e., Ru-qpy-), as described elsewhere.26 Moreover, since the photocurrent density drastically decreases when the same experiments were carried out in the absence of EDTA (less than 0.1 mA/cm2, see Figure S7), effective reduction of either RuIII-qpy or Ru-qpy* by EDTA is essential for the enhancement in the anodic photocurrents. An important realization is that sufficient electron accumulation at the cathode occurs even in the absence of the platinum catalyst. Although the anodic photocurrent observed for the PEC using FTO/TiO2/Ru-qpy photoanode and FTO/TiO2 cathode (Figure 1, green) does not lead to H2 evolution at the cathode (confirmed by gas analysis), the color of the TiO2 film at the FTO/TiO2 cathode changes from white to deep blue (Figure S8a), clearly indicating that the electrons injected into the CB@TiO2 at the photoanode flow to the cathode through the external circuit, and are stored in the CB@TiO2 at the cathode.11,22,27-29 For the PEC using FTO/TiO2/Ru-qpy photoanode and FTO/TiO2/PtP-py ‘dark’ cathode (Figure 1, red), the anodic photocurrent increases as the applied potential is increased from -0.3 to 0.6 V vs. cathode, and reaches a plateau at the applied potential larger than 0.6 V vs. cathode with a photocurrent density reaching ca. 1.7 mA/cm2. In this case, during the measurement, H2 bubbles at the surface of the FTO/TiO2/PtP-py cathode without showing any color change in the TiO2 film (Figure S8b), revealing that the electrons stored in the CB@TiO2 at the cathode are effectively consumed in H2 production catalyzed by PtP-py. Importantly, it is obvious that H2 production proceeds even under bias-free condition, for the photocurrent density of ca. 0.4 mA/cm2 is attained even at 0 V vs. cathode (Figure 1, red). Under these conditions, the Faradaic efficiency for H2 evolution is confirmed to be almost quantitative (Figure S9). Thus, the electrons injected into the CB@TiO2 at the photoanode are actually transferred to the CB@TiO2 at the dark cathode even without applying external electrical bias. It must be noted here that irradiation of both electrodes with the same visible light source does not lead to any essential change in the outcome (Figure S10). When triethanolamine is used as a sacrificial electron donor, the photocurrent density of ca. 0.05 mA/cm2 is attained at 0 V vs. cathode with the Faradaic efficiency for H2 production nearly quantitative (Figure S11). On the other hand, hydroquinone did not attain any measurable photocurrent density at 0 V vs. cathode, during which the H2 production could not be quantified. These results strongly suggest that an irreversible sacrificial electron donor is critical to achieve the effective photocurrent density at 0

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FTO/TiO2/Ru-qpy - FTO/TiO2/PtP-py FTO/TiO2 - FTO/TiO2/PtP-py

50

Light ON 40 OFF

pose, either the FTO/TiO2 or FTO/TiO2/Ru-qpy electrode was located inside a quartz spectrophotometer cuvette, submerged into the same electrolyte solution, and sealed under Ar. For the pristine FTO/TiO2 electrode, the bandgap excitation of TiO2 using a UVvisible light source (λ > 300 nm) leads to growth of broad absorption in the visible to NIR region (500~1200 nm), resulting in the apparent color change of the TiO2 film from white to deep blue (Figure S14a), characteristic of the charge accumulation in the [email protected] The blue color can be bleached by scavenging the electrons accumulated at the CB@TiO2 by exposure to air (Figure S14b), as previously described.27,28,30-34 We also confirmed that neither growth of broad absorption nor color change into blue is observed when an EDTA-free electrolyte solution is employed (Figure S15). These observations clearly indicate that effective electron accumulation in the CB@TiO2 is only promoted by the hole scavenging by EDTA after the bandgap excitation. As expected, electron accumulation is much less promoted when the same experiment is carried out using visible light irradiation (λ > 400 nm; Figure S16). On the other hand, similar experiments using the FTO/TiO2/Ru-qpy electrode result in growth of the similar broad absorption even under visible light irradiation (λ > 400 nm; Figures 3, S17a). As noted above for the discussion of Figures 2 and S16, the bandgap excitation is only weakly enhanced with this visible light source. The bleaching characteristics of the band under air was also confirmed to be similar (Figure 3, inset, Figure S17b), as observed for the pristine FTO/TiO2 electrode. Thus, the electron accumulation at the FTO/TiO2/Ru-qpy electrode primarily occurs via electron injection from the Ru-qpy. Similarly, the presence of EDTA is confirmed to be crucial to promote the electron accumulation in the CB@TiO2 at the photoanode (Figure S18).

1 0.8 0.6 0.4

Absorbance at 900 nm

V vs. cathode in order to drive H2 production under the bias-free condition in our PEC. In order to clarify the reason for the observed anodic photocurrent at 0 V vs. cathode, the negative shift in the photoanode potential vs. the cathode potential (i.e., EMF) was monitored without using the electrochemical analyzer but with the photoanode simply connected to the cathode using a conducting wire (i.e., under bias-free condition), with the remaining conditions used for Figure 1 preserved. For the combination of the pristine FTO/TiO2 photoanode and FTO/TiO2/PtP-py cathode, only a small EMF (ca. 2~3 µV) is observed (Figure 2, blue). On the other hand, the combination of FTO/TiO2/Ru-qpy photoanode and FTO/TiO2/PtP-py cathode results in a relatively large EMF (ca. 28 µV) during the light-on stage (Figure 2, red). During the light-off stage, slightly negative EMF is observed. This suggests that, upon the light-off action, rapid discharge of electrons at the CB@TiO2 occurs only at the photoanode. In other words, back electron transfer of the electrons accumulated at the CB@TiO2 to either RuIII-qpy or EDTA+● upon the light-off action is rapid, leading to the positive shift in the photoanode potential vs. the cathode potential. When an EDTA-free electrolyte solution is employed, the EMF becomes substantially small (ca. 2~3 µV, Figure S12). In addition, rapid responses are seen in the EMF upon light-on and -off actions (see Figure 2), consistent with the rapid response behaviors observed for the anodic photocurrent in Figure 1. These results provide a solid evidence for the photoinduced EMF generated between the two electrodes. On the other hand, the photoinduced EMF decreases to about one fourth (ca. 7 µV) by changing the pH value of the electrolyte solution from 5 to 7, and it further decreases down to ca. 1~2 µV at pH 9 (Figure S13). These results clearly indicate that the electron injection from the photoexcited state of Ru-qpy into the CB@TiO2 is not effectively promoted under the neutral and basic conditions because the driving force for the electron injection decreases due to the negative shift in the CBE potential of TiO2 at the photoanode (Figure S4).

Electromotive force (µV)

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Absorbance

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0.2 0.15 0.1 0.05 0

10 20 30 40 Irradiation time (min)

0.2

30

0 500 600 700 800 900 1000 1100 1200 Wavelength (nm)

20 10 0 0

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40 60 Time (s)

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Figure 2. Photoresponse behaviors in the EMF (the photoanode potential vs. the cathode potential), observed by multimeter, upon light-on and -off actions. The measurements were carried out using a two-electrode configuration PEC, detached from the electrochemical analyzer, with the two electrodes simply connected using a conducting wire. Other experimental conditions are same to those described in Figure 1.

To further gain an insight into the origin of photoinduced EMF, the electron accumulation at the photoanode itself while detached from the cathode has been separately investigated. For this pur-

Figure 3. Spectral changes during the visible light irradiation (λ > 400 nm) to the FTO/TiO2/Ru-qpy electrode submerged in an aqueous acetate buffer solution (0.1 M, pH 5.0) containing 30 mM EDTA and 30 mM NaClO4 under Ar. Inset shows changes in absorbance at 900 nm during the irradiation.

The above electron accumulation behaviors must be viewed as related to the so-called “Burstein-Moss shift”, which states that the semiconductor nanoparticles may experience move up of the Fermi level above the CBE potential by filling a substantial amount of electrons at the [email protected] Moreover, such upward shifts in the Fermi level allow the systems to gain a driving force high enough to promote the reactions that must be driven at potentials more negative than their CBE potentials.29,34,38,39 In the case of the FTO/TiO2/Ru-qpy electrode, electron accumulation in the CB@TiO2 actually takes place. We therefore assume that the upward shift in the Fermi level of the TiO2 at the photoanode is

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ACS Applied Energy Materials responsible for the enhancement in the EMF required to operate our PEC. Finally, the relationship between the EMF and the amount of H2 actually evolved over the cathode has been examined in detail. The measurements were carried out in a two-electrode configuration PEC consisting of the FTO/TiO2/Ru-qpy photoanode and the FTO/TiO2/PtP-py cathode with the two electrodes simply connected through a conducting wire (i.e., under bias-free condition), and only the photoanode was irradiated by visible light (λ > 400 nm). Importantly, the results indicate that the light-on and -off responses in H2 evolution are also confirmed to be quite rapid, as is the case for the responses induced in the EMF (Figure 4). In this experiment, the photoinduced EMF (ca. 20 µV) is maintained over 20 min, and H2 production is only promoted during each light-on period. Under these conditions, the continued irradiation of visible light up to 60 min leads to evolve ca. 4.2 µmol of H2 (Figure S19), where the TON based on the amount of adsorbed PtP-py (0.10 μmol/cm2) can be calculated as 42 and the external quantum yield is estimated as ΦH2 = 1.2%.

Electromotive force (µV)

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Amount of H2 evolved (µmol)

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Figure 4. Time course of the EMF between the FTO/TiO2/Ruqpy photoanode and the FTO/TiO2/PtP-py cathode (red), and the amount of H2 evolved (blue) under intermittent irradiation. Experimental conditions are same to those described for Figure 2.

In summary, we have demonstrated, for the first time, that the upward shift in the Fermi level of TiO2, caused by electron accumulation in the CB@TiO2, plays an important role in giving a small but effective portion of EMF (ca. 20 μV) which is required to make a flow of electrons through a conducting wire connecting the two TiO2-based electrodes. Moreover, solar hydrogen production from water without applying external electrical bias has been demonstrated in our molecular-based PEC. The PECs consisting of two TiO2-based electrodes open a new avenue for achieving overall solar water splitting under bias-free condition. The extended studies involve our attempts to incorporate water oxidation catalysts over the photoanode in order to eliminate the use of sacrificial electron donor in our hope to conduct the overall solar water splitting. Such studies are now under way.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed experimental procedures and additional results for photoelectrochemical measurements (PDF)

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AUTHOR INFORMATION Corresponding Author *[email protected] (K.S.) *[email protected] (H.O.)

ORCID Hironobu Ozawa: 0000-0002-1393-2670

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by Grants-in-Aid for Scientific Research (B) (No. 15H03786 and No. 18H01996 to K.S.) and (C) (No. 16K05726 to H.O.), a Grants-in-Aid for Scientific Research on Innovative Areas 'Artificial Photosynthesis' (No. 2406, 24107004 to K.S.), and 'Innovations for Light-Energy Conversion' (No. 18H05171 to K.S. and H.O.) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. This was further supported by the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the World Premier International Research Center Initiative (WPI), MEXT, Japan.

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ACS Applied Energy Materials Copper(II) Meso-tetra(4-carboxyphenyl)porphyrin. Chem. Commun. 2016, 52, 3665-3668. (13) Liu, B.; Li, J.; Wu, H.-L.; Liu, W.-Q.; Jiang, X.; Li, Z.-J.; Chen, B.; Tung, C.-H.; Wu, L.-Z. Improved Photoelectrocatalytic Performance for Water Oxidation by Earth-Abundant Cobalt Molecular Porphyrin Complex-Integrated BiVO4 Photoanode. ACS Appl. Mater. Interfaces 2016, 8, 18577-18583. (14) Krawicz, A.; Yang, J.; Anzenberg, E.; Yano, J.; Sharp, I. D.; Moore, G. F. Photofunctional Construct That Interfaces Molecular Cobalt-Based Catalysts for H2 Production to a Visible-Light-Absorbing Semiconductor. J. Am. Chem. Soc. 2013, 135, 11861-118868. (15) Khusnutdinova, D.; Beiler, A. M.; Wadsworth, B. L.; Jacoba, S. I.; Moore, G. F. Metalloporphyrin-Modified Semiconductors for Solar Fuel Production. Chem. Sci. 2017, 8, 253-259. (16) Seo, J.; Pekareka, R. T.; Rose, M. J. Photoelectrochemical Operation of a Surface-Bound, Nickel-Phosphine H2 Evolution Catalyst on pSi(111): a Molecular Semiconductor|Catalyst Construct. Chem. Commun. 2015, 51, 13264-13267. (17) Lee, C.-Y.; Park, H. S.; Fontecilla-Camps, J. C.; Reisner, E. Photoelectrochemical H2 Evolution with a Hydrogenase Immobilized on a TiO2Protected Silicon Electrode. Angew. Chem. Int. Ed. 2016, 55, 5971-5974. (18) Morandeira, A.; Fortage, J.; Edvinsson, T.; Pleux, L. L.; Blart, E.; Boschloo, G.; Hagfeldt, A.; Hammarström, L.; Odobel, F. Improved Photon-to-Current Conversion Efficiency with a Nanoporous p-Type NiO Electrode by the Use of a Sensitizer-Acceptor Dyad. J. Phys. Chem. C 2008, 112, 1721-1728. (19) Boschloo, G.; Hagfeldt, A. Spectroelectrochemistry of Nanostructured NiO. J. Phys. Chem. B 2001, 105, 3039-3044. (20) Creissen, C. E.; Warnan, J.; Reisner, E. Solar H2 generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst. Chem. Sci. 2018, 9, 1439-1447. (21) Kumagai, H.; Sahara, G.; Maeda, K.; Higashi, M.; Abe, R.; Ishitani, O. Hybrid Photocathode Consisting of a CuGaO2 p-Type Semiconductor and a Ru(II)-Re(I) Supramolecular Photocatalyst: Non-Biased VisibleLight-Driven CO2 Reduction with Water Oxidation. Chem. Sci. 2017, 8, 4242-4249. (22) Morita, K.; Takijiri, K.; Sakai, K.; Ozawa, H. A Platinum Porphyrin Modified TiO2 Electrode for Photoelectrochemical Hydrogen Production from Neutral Water Driven by the Conduction Band Edge Potential of TiO2. Dalton Trans. 2017, 46, 15181-15185. (23) Takijiri, K.; Morita, K.; Nakazono, T.; Sakai, K.; Ozawa, H. Highly Stable Chemisorption of Dyes with Pyridyl Anchors over TiO2: Application in Dye-Sensitized Photoelectrochemical Water Reduction in Aqueous Media. Chem. Commun. 2017, 53, 3042-3045. (24) Rosser, T. E.; Windle, C. D.; Reisner, E. Electrocatalytic and SolarDriven CO2 Reduction to CO with a Molecular Manganese Catalyst Immobilized on Mesoporous TiO2. Angew. Chem. Int. Ed. 2016, 55, 73887392. (25) Ozawa, H.; Sugiura, T.; Kuroda, T.; Nozawa, K.; Arakawa, H. Highly Efficient Dye-Sensitized Solar Cells Based on a Ruthenium Sensitizer Bearing a Hexylthiophene Modified Terpyridine Ligand. J. Mater. Chem. A 2016, 4, 1762-1770. (26) Suneesh, C. V.; Balan, B.; Ozawa, H.; Nakamura, Y.; Katayama, T.; Muramatsu, M.; Nagasawa, Y.; Miyasaka, H.; Sakai, K. Mechanistic Studies of Photoinduced Intramolecular and Intermolecular Electron Transfer Processes in RuPt-Centred Photo-Hydrogen-Evolving Molecular Devices. Phys. Chem. Chem. Phys. 2014, 16, 1607-1616. (27) Kölle, U.; Moser, J.; Grätzel, M. Dynamics of Interfacial ChargeTransfer Reactions in Semiconductor Dispersions. Reduction of CobaltoCeniumdicarboxylate in Colloidal Titania. Inorg. Chem. 1985, 24, 22532258. (28) Kamat, P. V.; Bedja, I.; Hotchandani, S. Photoinduced Charge Transfer between Carbon and Semiconductor Clusters. One-Electron Reduction of C60 in Colloidal TiO2 Semiconductor Suspensions. J. Phys. Chem. 1994, 98, 9137-9142. (29) Mohamed, H. H.; Dillert, R.; Bahnemann, D. W. Reaction Dynamics of the Transfer of Stored Electrons on TiO2 Nanoparticles: A Stopped Flow Study. J. Photochem. Photobiol. A Chem. 2011, 217, 271-274. (30) Bahnemann, D.; Henglein, A.; Lilie, J.; Spanhel, L. Flash Photolysis Observation of the Absorption Spectra of Trapped Positive Holes and Electrons in Colloidal Titanium Dioxide. J. Phys. Chem. 1984, 88, 709711. (31) Rothenberger, G.; Moser, J.; Grätzel, M.; Serpone, N.; Sharma, D. K. Charge Carrier Trapping and Recombination Dynamics in Small Semiconductor Particles. J. Am. Chem. Soc. 1985, 107, 8054-8059.

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SYNOPSIS TOC

e-

ee-

e-

e

2+

CB N

EDTA(ox)

N N

Ru

N

N N

Ru-qpy

EDTA

CB

N N

Pt

N N

e

H2O

N

N

TiO2

e-

-

N

H2 PtP-py

Aqueous acetate buffer (pH 5)

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TiO2