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Photo-induced electron transfer for water splitting and redox biocatalytic synthesis = 광전자 전달계 기반 인공광합성을 통한 에너지 변환 소재 연구
서명 / 저자 Photo-induced electron transfer for water splitting and redox biocatalytic synthesis = 광전자 전달계 기반 인공광합성을 통한 에너지 변환 소재 연구 / Dong Heon Nam.
발행사항 [대전 : 한국과학기술원, 2015].
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Green plants successfully convert solar energy into chemical energy via natural photosynthesis through a series of photo-induced electron transfer reactions in nanoscale architectures that contain light-harvesting complexes, protein-metal clusters, and many redox biocatalysts. This process has unparalleled, unique features, such as near-unity quantum yield and environmental compatibility, thus, the utilization of solar energy through photo-induced electron transfer remains a target model for the development of artificial photosynthetic systems that utilize solar light as a sustainable and environmentally benign energy source. The key requirement in the design of artificial photosynthesis is an efficient and forward electron transfer between each photosynthetic component. In this thesis, nanobiocatalytic assemblies inspired from the photosynthetic units were studied for the development of artificial photosynthesis by means of efficient light harvesting and photo-induced electron transfer originated from the structure of nanobiocatalytic assemblies. Chapter 1 reviews the key principles and issues in the design of biocatalytic artificial photosynthesis and introduces recent advances in the development of nanobiocatalytic assemblies and light-harvesting materials for efficient photo-induced electron transfer toward biomimetic photosynthesis. Natural photosynthesis, a solar-to-chemical energy conversion process, occurs through a series of photo-induced electron transfer reactions in nanoscale architectures that contain light-harvesting complexes, protein-metal clusters, and many redox biocatalysts. Artificial photosynthesis in nanobiocatalytic assemblies aims to reconstruct man-made photosensitizers, electron mediators, electron donors, and redox enzymes for solar synthesis of valuable chemicals through visible light-driven cofactor regeneration. The key requirement in the design of biocatalyzed artificial photosynthetic process is an efficient and forward electron transfer between each photosynthetic component. Recent research outcomes that serve as a proof of the concept are summarized and current issues are discussed to provide a future perspective. Chapter 2 describes the first application of proflavine as a molecular photosensitizer for visible-light-driven regeneration of NADH in the presence of electron mediator (M) and sacrificial electron donor. An efficient regeneration of NADH with a high turnover rate was achieved. Both the wavelength and intensity of incident light were found to significantly affect the efficiency of photochemical NADH regeneration. Photochemical regeneration of the nicotinamide cofactor, coupled with enzymatic reaction catalyzed by glutamate dehydrogenase, was successfully conducted as a model dye-sensitized artificial photosynthetic system. In Chapter 3, we have first demonstrated photoenzymatic synthesis of chemicals using upconversion nanoparticles through NIR light-driven cofactor regeneration. From the zeta potential and spectrofluorometric analyses, we verified successful binding of RB molecules to NaYF4:Yb,Er nanoparticles through electrostatic interaction under close proximity enough for FRET. NIR-induced electron transfer was observed through linear sweep voltammetric analysis, which indicated photoexcited electrons of RB/Si-NaYF4:Yb,Er were transferred to NAD+ through M, an electron mediator. By comparing RB/Si-NaYF4:Yb,Er with RB/Si-NaYF4:Yb,Tm and Si-NaYF4:Yb,Er, we investigated the effect of FRET efficiency on redox enzymatic reaction coupled with NIR light-driven NADH regeneration. RB/Si-NaYF4:Yb,Er nanoparticles, which exhibited higher FRET efficiency due to more spectral overlap than RB/Si-NaYF4:Yb, resulted in better performance for photoenzymatic conversion of α-ketoglutarate to L-glutamate. Chapter 4 deals with autonomous photovoltaic cell for spontaneous water splitting and biocatalytic CO2 fixation achieved with no external electrical bias. To construct the autonomous photovoltaic cell, we considered the ways to overcome thermodynamic barrier for water oxidation reaction and ensure efficient electron transfer without external bias. Spontaneous water splitting in the photovoltaic cells has hindered because of a lack of suitable photoanode materials. In this regard, hematite, the most popular and promising candidate of photoanode material for solar water splitting, could be the key to solve this problem. To ensure efficient electron transfer without external bias, we adopted the principles of natural photosynthesis and mimicked the photo-induced electron transport chain called the Z-scheme by using man-made materials. We observed that autonomous photovoltaic cell composed serial coupling of light excitation steps in hematite and eosin Y was successfully performed biocatalytic CO2 fixation under visible light. Through the photoelectrochemical analyses, we also demonstrated photo-induced electron transfer pathway in autonomous photovoltaic cell. Lastly in Chapter 5, We constructed self biased photoelectrochemical cell for spontaneous water splitting achieved with no external electrical bias by mimicking the Z-scheme. We observed that serial coupling of light excitation steps in hematite and BiFeO3 is the key to overcome thermodynamic barrier for water oxidation and ensure efficient electron transfer. We also found that buffer solution pH in the anode compartment is important to overcome thermodynamic barrier for water oxidation, because of higher water oxidation activity of Co-Pi deposited hematite in basic condition. Through the photochemically transforming CO2 into formate coupled with cofactor regeneration, self biased photoelectrochemical cell was successfully performed biocatalytic CO2 fixation under visible light. This work shows potential of self biased photoelectrochemical cell for solar to chemical energy conversion.

녹색식물은 연속적인 광에 의한 전자 전달과정을 이용한 광합성 기작을 통해 화학에너지 형태인 탄수화물로 태양에너지를 저장하여 사용하는데, 이러한 자연계의 광합성은 광을 소모하는 안테나와 촉매 역할을 하는 금속 입자들이 단백질에 고정화된 정교한 나노구조인 광계에 의해서 성공적으로 이루어진다. 따라서 지속가능하고 친환경적인 태양에너지를 광에 의한 전자 전달을 통한 인공광합성 방법으로 모사하는 연구는 많은 관심을 받고 있다. 인공광합성 설계에 있어서 가장 중요한 조건은 전자가 사용되는 위치로 전하재결합에 의한 전자 손실을 최소화 할 수 있는 구성 성분들 간의 효율적인 전자 전달이다. 본 학위 논문에서는 광계를 모사한 나노바이오소재를 이용하여 이들의 구조에서 기인하는 효율적인 전자 전달 및 광을 소모하는 시스템을 이용하여 효율적인 인공광합성으로의 응용에 관한 연구를 진행하였다. 제 1장에서는 광합성 기작을 모사한 광촉매와 산화환원효소의 결합을 통한 생체 촉매 인공 광합성의 개념 및 응용 가능성에 관한 문헌을 조사하였다. 특히, 광합성을 위한 구성성분인 전자 주개, 광감응제, 그리고 전자 전달체 간의 가장 중요한 원칙인 광에 의한 효율적인 전자 전달 및 에너지 상관관계에 대하여 기술하였으며 최근에 발표된 연구 결과를 바탕으로 실용 가능화하기 위하여 과학적 그리고 경제적으로 개발이 필요한 분야에 대하여 기술하였다. 제 2장에서는 전자전달 매개체와 희생 전자주개의 존재하에서 가시광 유래 보조인자 재생을 위한 분자 광감응제로서 프로플라빈의 첫 응용에 대해 기술하였다. 프로플라빈에 의한 효율적인 보조인자 재생은 글루타메이드 탈수소화효소를 통한 효소촉매반응과 연결되었으며, 이를 통해 우리는 염료감응형 인공광합성 모델 시스템을 성공적으로 제시하였다. 제 3장에서는 업컨버전 나노입자를 이용하여 적외선을 에너지 소스로 사용하는 인공광합성 시스템에 대해 기술하였다. 지금까지 개발된 대부분의 광감응제는 자외선이나 가시광을 흡수하는데 적합하여 태양광 에너지의 46%를 차지하는 적외선을 사용할 수 없었다. 이러한 측면에서 볼때, 적외선을 흡수하여 자외선 또는 가시광을 방출하는 업컨버전 나노입자는 태양광 에너지를 보다 효울적으로 활용하는데 도움을 줄 수 있으며, 이에 적외선을 에너지 소스로 사용하는 인공광합성 시스템을 개발하였다. 또한 업컨버전 나노입자 표면에 로즈벤갈을 고정화하여 이들 사이의 프렛 현상을 이용함으로써 보다 효과적인 광화학적 효소촉매반응을 수행하였다. 제 4 장에서는 물을 전자 공여체로 사용하는 자발적 광전극 셀 시스템을 개발에 대해 기술하였다. 물을 전자 공여체로 사용하는 통상적인 광전극 셀 시스템에서는 반응을 수행하기 위해 외부전압을 인가하여야만 하는데, 이는 4개의 전자와 정공이 동시에 참여하는 물분해 반응이 가진 열역학적 배리어에 기인한다. 따라서 물분해 반응을 효과적으로 수행할 수 있는 코발트 포스페이트-해마타이트 전극을 사용하여 물분해 반응에 대한 열역학적 베리어를 극복하였으며, 자연계의 광합성을 모방한 해마타이트 및 이오신 와이로 이루어진 광전자 전달계를 구성함으로써 광전극 셀 시스템에서의 안정적인 전자 전달을 가능케 하였다. 또한 산화환원 효소와의 결합을 통해 이산화탄소를 포르메이트로 변환함으로써 자발적 광전극 셀 시스템이 지속가능한 광효소적 합성반응에 이용될 수 있음을 보여주었다. 마지막으로 제 5장에서는 광전극 셀 시스템을 통해 외부 전압의 인가없이 자발적인 물분해 반응을 수행할 수 있는 방법에 대하여 기술하였다. 자연계의 광합성을 모방한 광전자 전달 시스템의 효과는 앞선 4 장에서 확인하였으므로, 5 장에서는 효과적인 물분해 반응이 일어날 수 있는 조건을 찾는데 주력하였다. 그 결과, 광전극 셀 시스템에는 외부 전압의 인가보다 수용액 상의 pH가 물분해 반응에 더 큰 영향을 끼치는 것을 확인하였으며, 이를 바탕으로 태양 에너지만으로도 물분해 반응으로부터 전자를 얻을 수 있는 자발적 광전극 시스템을 구현하였다.

서지기타정보

서지기타정보
청구기호 {DMS 15005
형태사항 ix, 100 p : 삽화 ; 30 cm
언어 영어
일반주기 저자명의 한글표기 : 남동헌
지도교수의 영문표기 : Chan Beum Park
지도교수의 한글표기 : 박찬범
수록잡지명 : "Visible Light-Driven NADH Regeneration Sensitized by Proflavine for Biocatalysis". ChemBioChem, v. 13. no. 9, pp. 1278-1282(2012)
Including Appendix
학위논문 학위논문(박사) - 한국과학기술원 : 신소재공학과,
서지주기 References : p.
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Schematic illustration of(a) natural photosynthesis and (b) biocatalytic artificial photosynthesis. In both processes, photosystems transfer photo-induced electrons from electron donors to nicotinamide cofactors under visible light, which are coupled to the redox biocatalytic cycle for solar-to-chemical conversion.

Natural photosynthesis occurs through the electron transport chain, which includes many electron mediators such as plastoquinone (Pq), Cytochrome complex, and plastocyanine (Pc). The reaction center in photosystem II triggers a photochemical water oxidation reaction at the oxygen-evolving center (OEC). Photosystem I transfers excited electrons to NADP reductase through the primary acceptor, ferred

The structure of the thylakoid membrane in chloroplasts, where efficient photo-excited electron transfer occurs through precise arrangement and coordination ofPS I and II components.

(a) Nanobiocatalytic artificial photosynthesis is designed by replacing natural photosynthetic components with man-made OECs, photosensitizers, and electron mediators for visible light-driven cofactor regeneration. (b) Porphyrins encapsulated within a porous lignocellulosic support to serve as light-harvesting pigment (Lee et al. 2011).

(a) Graphene-based photosensitizer for photoenzymatic synthesis of l-phenylethanols. Adapted with permission from Ref. (Choudhury et al. 2012); Copyright (2012) Wiley-VCH. (b) Quantum-dot-sensitized TiO2 nanotube arrays for redox enzymatic synthesis coupled with NADH photo-regeneration (Ryu et al. 2011).

(a) Peptide self-assembly with porphyrins and further self-metallization with platinum nanoparticles to form photosynthetic nanotubes that can mimic natural PS I (Kim et al. 2012). (b) P450- catalyzed O-dealkylation reaction coupled with sustainable NADPH regeneration under visible light (Lee et al. 2013).

Nanobiocatalytic assemblies for mimicking PS II. (a) Molecular arrangement ofBIP-modified IrO2 2+ nanoparticles and Ru(bpy)3 in dye-sensitized TiO2 with photoanode. Adapted with permission from Ref. (Zhao et al. 2012); Copyright (2012) National Academy of Sciences, USA. (b) M13 virus-templated assembly of porphyrins for the fabrication of light-harvesting nanoantenna. Adapted with permission from

Structure and mechanism of Pt, Co, and TiOz-decorated Au nanorod for solar water splitting, Adapted with permission from Ref. (Mubeen etal. 2013); Copyright (2013)Nature Publishing Group.

Proflavine-sensitized NADH regeneration and photoenzymatic synthesis ofL-glutamate. The light- excited electrons generated by proflavine are transferred to a rhodium (III)-based mediator (M [Cp*Rh(bpy)H+이] Cp* = CsMes, bpy = 2,2'-bipyridine), and then used to reduce NAD+ to NADH for use in enzymatic synthesis.

Temporal changes of NADH regeneration yield with different concentrations of proflavine. The irradiation of visible light triggered rapid NADH regeneration. The rate ofphotocatalytic NADH reduction was proportional to the concentration of proflavine. The maximum yield and initial turnover frequency for NADH regeneration with 10 uM proflavine amounted to 63 4%and 127.8h-1 respectively.

Emission spectra of three different LEDs and absorption spectrum of proflavine. Proflavine can only absorb photon energy at above approximately 2.36 eV, while LEDs possess energy bands as follows: 2.34~2.95 eV forthe blue LED,2.07~2.70eVfor the green LED, and 1.84~2.16eV for the red LED.

(a) Redox enzymatic synthesis ofL-glutamate coupled with NADH regeneration photosensitized by proflavine under visible light. When the proflavine and M were present together in the reaction medium, conversions of 12.5% and 65.3% were obtained after an hour of reaction under proflavine concentrations of2 pM and 10 nM, respectively. In contrast, control experiments conducted in the absence of either

The effect ofthe intensity and wavelength ofincident light upon the photochemical regeneration of NADH.[a] [a] For all of the experiments, 10 nM proflavine, 1 mM NAD+, and 250 nM M were used in a phosphate buffer (100 mM, pH 7.0) containing 15% w/v TEOA under the irradiation ofeither a Xelamp or an LED (blue, green, or red LED). The light intensity of each LED was fixed at 1.0 mW/cm2. [b] Turnover

Molecular structures ofproflavine and flavin derivatives (FAD, FMN, lumichrome, and riboflavin), which are based on a similar tricyclic ring.

(a) Photo-induced reduction offlavin derivatives through sacrificial electron donors transfers photo- excited electrons to an acceptor. The star indicates the light-activated oxidation state of flavin derivatives. (b) The performance of proflavine in NADH photoregeneration in comparison with four different derivatives of flavin molecules. Onlyproflavine enabled the photochemical regeneration ofNAD

Cyclic voltammograms of (a) proflavine, (b) FAD, (c) riboflavin, (d) lumichrome, and (e) FMI with M in the absence (solid line) and presence (dotted line) ofNAD+. Only proflavine plus M displays catalytic effect on NAD+ reduction.

Molecular structure of three different electrochemical states of M during NADH regeneration photosensitized by proflavine. The Mox is reduced to Mredby accepting two photo-excited electrons from proflavine, and then Mred is chemically converted to the M by taking up one proton from an aqueous solution. NADH is produced from NAD by accepting hydride from M and returning M to the Mox.

Schematic diagram of NIR light-driven biocatalytic artificial photosynthesis using FRET- conjugated upconversion nanoparticles. The photoexcited electrons generated by RB/Si-NaYF,:Yb,Er are transferred to NAD through rhodium (III)-based mediator (M =[Cp*Rh(byy)H+O];Cp*=CsMes, bpy =2,2'- bipyridine), and then used to synthesize L-glutamate by L-glutamate dehydrogenase (GDH).

The absorption band of RB overlaps well with the green emission band of NaYF.:Yb,Er nanoparticles. Hence, RB/silica-coated NaYFu:Yb,Er nanoparticles (RB/Si-NaYFA:Yb,Er) exhibited red emission band that was the same as the emission band ofRB under 980 nm excitation.

Morphology analysis of NaYF,:Yb,Er nanoparticles by (a) SEM and (b) TEM. NaYFu:Yb,Er nanoparticles exhibited polyhedral shape, good uniformity, and monodispersity with an average diameter of approximately 55 nm. (c) HRTEM images ofa NaYF,:Yb,Ernanoparticle show crystal lattice ofthe (100) plane with interplanar spacing of0.52 nm. (d) SAED patterns ofNaYF.:Yb,Er nanoparticles indicate that the crys

XRD patterns of NaYF,:Yb,Er nanoparticles confirmed the crystal structure of NaYF.:Yb,Er nanoparticles. All of the diffraction peaks matched well with JCPDS PDF peak (#28-1192), suggesting hexagonal phase with cell parameters ofa = 5.96 A and C = 3.51 A. No impurity peak was observed indicating high purity ofthe final material.

(a) TEM images of silica-coated NaYF.:Yb,Er nanoparticles (Si-NaYF,:Yb,Er) dispersed well in aqueous solutions. (b) Zeta potentials ofbare, NH2-treated, and rose bengal (RB)-bound Si-NaYF,:Yb,Er show successful binding ofRB molecules on NaYF.:Yb,Ernanoparticles.

(a) Absorption spectra of Si-NaYF,:Yb,Er with and without RB binding. In contrast to RB/Si- NaYFq:Yb,Er, Si-NaYF,:Yb,Er showed no absorption peak at 550 nm and exhibited green emission band. (b) Digital camera images ofupconversion nanoparticles upon the irradiation ofa 980 nm laser.

(a) The silica shell thickness of RB/Si-NaYF,,Yo,Er is the same with that of Si-NaYF,:Yb,Er which indicated that electrostatic interaction ofNaYF,:Yb,Ernanoparticles and RB ensures the close proximity between them for FRET. TEM images of (b) NaYF.:Yb,Tm, and (c) Si-NaYF::Yb,Tm NaYF.:Yb,Tm nanoparticles were uniformly distributed with a mean size ofapproximately105 nm, and well-dispersed in water a

Linear sweep voltammograms ofRB/Si-NaYF2:Yb,Erin the absence and presence ofM and NAD RB/Si-NaYF:Yb,Er with M exhibited a strong increment in its reduction peak current with the addition NAD indicating their catalytic effect on the reduction reaction ofNAD+

Photochemical NADH regeneration with upconversion nanoparticles. RB/Si-NaYF,,Yo,EI exhibited the highest performance, which indicates the FRET between RB and upconversion nanoparticles is critical for photon-conversion efficiency and NIR light-driven biocatalytic artificial photosynthesis.

Energy level diagram showing the upconversion mechanism ofNaYF.:Yb,Tm nanoparticles. The 3+ higher electronic levels ofTm (e.g. 'D2and G4 level) were populated by energy transfer from excited Yb3+ ions upon the irradiation ofNIR light, then 'D2and 1G4 level can decay nonradiatively to the3F4,and 3H.levels By these multiphonon nonradiative relaxations, Tm3+ emits in the blue (450 and 480 nm) and re

(a) Fluorescence spectrum of NaYF.:Yb,Tm nanoparticles and absorption spectrum of RB. The emission band of NaYF.:Yb,Tm nanoparticles slightly overlaps with the absorption band of RB. Note that emission peaks marked with arrows (360, 450, 480, and 660 nm) correspond to multiphonon nonradiative relaxation from 'D2 and 'G4 levels to 3F4 and 3H. levels. (b) Digital camera image of Si-NaYF.:Yb,Tm nanop

Photoenzymatic synthesis ofL-glutamate with upconversion nanoparticles. The conversion yield ofL-glutamate with RB/Si-NaYF,:Yb,Er was 20.25% after 5 hr reaction, while those with Si-NaYF,:Yb,Erand RB/Si-NaYF,Yb,Imm were 0.45% and 1.89%, respectively, which demonstrates that redox enzymatic conversion of a-ketoglutarate to L-glutamate was highly facilitated with RB/Si-NaYF,,Yb,Er through NIR light-

Adequate band gap energy ofhematite (2.14 eV) for visible light absorption, which calculated form Kubelka-Munk function versus the light energy.

(a) SEM image and (b) XRD pattern of hematite, exhibiting worm-like structure and high crystallinity.

Schematic illustration of(a) natural photosynthesis and (b) biocatalytic artificial photosynthesis by autonomous photovoltaic cell. The natural photosynthesis have unparalleled advantages ofnear-unity quantum yield, efficient photo-induced electron transfer, and environmental compatibility, originated from photo-induced electron transport chain called the Z-scheme. Autonomous photovoltaic cell con

Visible light absorption of eosin Y with maximum absorption peak intensity at 516 nm. EY is a promising photosensitizer, having high quantum yields in photocatalytic hydrogen evolution and dye-sensitized solar cells.

Molecular structures ofM with three different electrochemical states during NADH regeneration. The Moxis reduced to Mredby accepting two photo-excited electrons from eosin Y, and then Mredis chemically converted to the M by taking up one proton from an aqueous solution. NADH is produced from NAD by accepting hydride from M and returning M to the Mox·

Linear sweep voltammogram of bare hematite under chopped light illumination by using three- electrode configuration setup, employing Ag/AgCl electrode and a platinum wire as reference and counter electrodes, respectively.

Faradaic efficiency of water oxidation reaction at Co-Pi deposited hematite calculated from simultaneously measured oxygen production and photocurrent. At least 80% of faradaic efficiency observed during the whole measurements, which indicates that most of electrons occurred from water oxidation reaction are transferred to eosin Y through the external wire with no external electrical bias.

Photocurrent of hematite during oxygen production measured by three-electrode configuration setup, employing Ag/AgCl electrode and a platinum wire as reference and counter electrodes, respectively.

Photocurrent of autonomous photovoltaic cell consisted of serial coupling oflightexcitation ste in hematite and eosin Y. Illumination exclusively on the hematite results in a decrease of the photocurrer which restored by switching the light on again. This result also proved the contribution ofthe Co-Pi deposit hematite on electron transfer in autonomous photovoltaic cell.

Linear sweep voltammogram of (a) eosin Y (EY) and M solution and (b) mixture of two components in the absence and presence ofNAD+,implying co-reduction ofeosin Y with M and their catalytic effect on the reduction reaction ofNAD+.

The spectral changes ofEY fluorescence in the presence ofM in various concentrations. Emiss maximum of EY fluorescence are red-shift and their intensity decreased when the concentration ofM v increased.

(a) Time profiles of visible light-driven NADH regeneration performed by autonomous photovoltaic cell with maximum yield and turnover frequency amounted to 63.5%and 57.4h-1, respectively. (b) Photochemically transforming CO2 into formate coupled with cofactor regeneration conducted by TsFDH. Amount of0.55 mM formate were obtained after five hours offraction in the presence ofreaction components su

The influence of light intensity by varying the light intensity from 0 to 140 mW/cm2. The concentration offormate converted from CO2 was proportional to the intensity ofincident light, indicating vital role ofincident lighton autonomous photovoltaic cell.

Schematic illustration ofbiocatalytic artificial photosynthesis by self biased photoelectrochemica cell. The natural photosynthesis have unparalleled advantages of near-unity quantum yield, efficient photo induced electron transfer, and environmental compatibility, originated from photo-induced electron transpor chain called the Z-scheme. Selfbiased photoelectrochemical cell consisted ofserial cou

XRD patterns of BiFeO3. All ofthe diffraction peaks matched well with JCPDS PDF peak (#20- 0169), indicating high crystallinity ofBiFeO3. In addition, no secondary phases such as iron or bismuth oxides were observed, implying high purity ofthe final material.

XPS spectra ofthe (a) Bi 4fline and (b) Fe2p lineofthe BiFeO3. In the Bi 4fspectrum, the 4f7/2 peakis located at 159 eV, indicating that Biis in the 3+ oxidation state. In the Fe 2p core scan, the 2p3/2 peak is positioned at 710.5 eV, with an additional distinctive shake-up satellite peak observed at 719 eV. These features indicate that the oxidation state ofFe in the deposited BFO films is Fe3+.

Linear sweep voltammogram ofbare hematite under chopped light illumination by using three- electrode configuration setup, employing Ag/AgCl electrode and a platinum wire as reference and counter electrodes, respectively.

Photocurrent of self biased photoelectrochemical cell to confirm photo-induced electron transfer. Illumination ofboth halfcells results in a decrease ofthe photocurrent, which restored by switching the light on again. This result implies that serial coupling oflight excitation steps in hematite and BiFeO3 ensure efficient electron transfer without external bias.

Photochemical NADH regeneration performed by self biased photoelectrochemical cell with various buffer solution pH in the anode compartment. The concentration of regenerated NADH is increased with buffer solution pH in the anode compartment, in contrast to negligible increment with increasing external bias. These results indicate that buffer solution pH in the anode compartment is the key to overc

Photochemical NADH regeneration performed by self biased photoelectrochemical cell with various external bias. The concentration ofregenerated NADH is increased with buffer solution pH in the anode compartment, in contrast to negligible increment with increasing external bias. These results indicate that buffer solution pH in the anode compartment is the key to overcome thermodynamic barrier for w

Time profiles of photochemically transforming CO2 into formate coupled with cofactor regeneration. Amount of0.92 mM formate were obtained after three hours ofreaction. Note that concentration ofproduced formate 1S increased with time, because ofcontinuous CO2 injection during the whole reaction.