Photoelectrochemistry

DOI: 10.1002/ange.200504454

An Organic Photoelectrode Working in the WaterPhase: Visible-Light-Induced Dioxygen Evolutionby a Perylene Derivative/Cobalt PhthalocyanineBilayer**

Toshiyuki Abe,* Keiji Nagai,* Satoko Kabutomori,Masao Kaneko, Akio Tajiri, and Takayoshi Norimatsu

Photochemical conversion is one of the most promisingrenewable energy resources.[1] Great efforts have been madeto create a photochemical energy conversion system eversince Fujishima and Honda reported the photoelectrochem-ical water splitting by the UV-responsive TiO2 electrode.

[2] Toincrease the efficiency of solar energy conversion, it isdesirable to create a photoenergy conversion system that isresponsive to visible light across the entire solar spectralregion. Inorganic semiconductors have been used as effectivephotocatalysts for the photolysis water into O2 and H2 withvisible light at wavelengths under around 500 nm.[3] However,there have been no examples to date of organic photocatalystsas these are susceptible to oxidative decomposition.Photoelectrochemistry is a promising method to inves-

tigate and establish photoenergy conversion systems. Highlyordered molecule-based photoelectrodes have been fabri-cated by means of self-assembled monolayers (SAMs) andLangmuir–Blodgett (LB) films.[4] We recently reported thatthe organic bilayer composed of 3,4,9,10-perylenetetracar-boxylic acid bisbenzimidazole (PTCBI, an n-type semicon-ductor) and 29H,31H-phthalocyanine (H2Pc, a p-type semi-conductor) can act as a photoanode owing to the photo-physical character of the bilayer, whereby oxidation of anaqueous thiol was efficiently induced at the H2Pc/waterinterface.[5] Although it has been postulated that organicbilayers are not sufficiently stable for application in water,[6]

we have shown the PTCBI/H2Pc bilayer to function as a stablephotoanode in the water phase. However, SAMs, LB films,and the PTCBI/H2Pc bilayer have not been able to be coupledwith electron donation from water (or OH� ions) even thoughthe oxidation of water molecules (or OH� ions) into O2 is amost important process in the production of H2 in a waterphotolysis system. Herein we show that an organic bilayer ofPTCBI and cobalt(ii) phthalocyanine (CoPc, (Figure 1)) canact as a photoanode that is capable of evolving O2 at theCoPc/water interface. This bilayer could form the basis of astable and efficient photoelectrochemical water-splittingsystem.

The photoelectrochemical characteristics of an ITO/PTCBI/CoPc electrode (ITO= indium–tin oxide) that hadbeen soaked in water were investigated first in the presence ofa sacrificial electron donor (triethylamine, TEA). The cyclicvoltammogram (CV; Figure 2a) showed a large photoanodiccurrent on the order of mAcm�2 at about 0 V (vs. Ag/AgCl),which is attributable to TEA oxidation. However, no anodiccurrent was observed in the dark; moreover, there was almostno photoelectrochemical response at the ITO coated with asingle layer of CoPc (denoted as ITO/CoPc). The bilayercomposed of perylene and phthalocyanine is known tofunction as a photovoltaic cell in the dry state;[7, 8] moreover,an electrochemical photovoltaic cell consisting of a single-layered perylene photoanode and a Zn–phthalocyaninephotocathode in the liquid phase, which also contains aredox couple, has also been constructed.[9] In a separate

Figure 1. Chemical structures of the perylene derivative PTCBI andcobalt phthalocyanine (CoPc), and schematic illustration of the appara-tus for photoelectrochemical measurements.[*] Prof. T. Abe, S. Kabutomori, Prof. A. Tajiri

Department of Materials Science and TechnologyHirosaki UniversityHirosaki 036-8561 (Japan)Fax: (+81)172-39-3580E-mail: tabe@cc.hirosaki-u.ac.jp

Dr. K. Nagai, Prof. T. NorimatsuInstitute of Laser Engineering (ILE)Osaka University, Suita 565-0871 (Japan)Fax: (+81)6-6877-4799E-mail: knagai@ile.osaka-u.ac.jp

Prof. M. KanekoFaculty of ScienceIbaraki UniversityMito 310-8512 (Japan)

[**] This work was partially supported by Grant-in-Aid for ScientificResearch (No. 15750110 (T.A.)) and for Scientific Research inPriority Areas (Fundamental Science and Technology of Photo-functional Interfaces, No. 417 (T.A., K.N., and M.K.)) from theMinistry of Education, Culture, Sports, Science, and Technology(MEXT), Japan.

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experiment, we also observed a photovoltage of approxi-mately 0.5 V in a dry cell of the PTCBI/CoPc bilayer; thus, itappears that a p–n junction is formed at the interface of thisbilayer. Therefore, the generation of a photoanodic current atthe CoPc/water interface (shown in Figure 2a) may becoupled with hole conduction by the CoPc layer. CVs werealso measured with different directions of the incidentirradiation, and this showed that only a small photocurrentis generated when the cell is irradiated on the CoPc side. Thedependence of the CV on the irradiation direction suggeststhat the photoanodic current generation cannot be induced byabsorption of the whole layer of PTCBI/CoPc. The origin ofthe photocurrent generated at the ITO/PTCBI/CoPc elec-trode is discussed below. The time course of the photocurrentat the photoanode was measured in the presence of TEA(Figure 2b). It shows that the present organic bilayer canexhibit a stable photoelectrochemical response as a photo-electrode even in the water phase.In the absence of TEA in the water phase, the photo-

anodic current is coupled with electron donation from theconversion of OH� into O2; however, a noticeable photo-electrochemical response could not be seen at either ITO/CoPc or ITO/PTCBI/H2Pc (Figure 3). Potentiostatic electrol-ysis at the ITO/PTCBI/CoPc electrode was conducted inalkaline water free of the sacrificial reagent (pH 11). Table 1shows the results of the potentiostatic electrolysis. Thestoichiometric photoelectrochemical splitting of water intoO2 and H2 took place at an applied bias potential lower thanthe formal potential (ca. + 0.38 V vs. Ag/AgCl for the O2/OH� couple); that is, O2 was generated by highly efficientphotocatalysis by CoPc at the photoanode and H2 was yielded

concurrently at the counter electrode. In determining theturnover number (TON) for the photocatalytic ability ofCoPc for O2 evolution, the number of CoPc moleculesexposed on the electrode surface (area 1 cm2) was calculatedby considering the size of a CoPc molecule.[10] The TON wasincreased by applying a higher bias potential to the bilayer.This increase is attributed to the increase of photogeneratedcarriers coupled with O2 evolution at the CoPc/water inter-face. Moreover, the amount of O2 generated increased withelectrolysis time for a constant applied potential of + 0.4 Vatthe bilayer (Figure 4), which shows that continual waterphotolysis involving O2 evolution takes place at the CoPc/water interface. The slight deviation from a linear relationshipbetween the amount of generated O2 with time in Figure 4may be ascribed to adsorption of O2 to the CoPc surface or itsdissolution into the water phase, especially in the initial stageof the electrolysis. Potentiostatic electrolysis was also carriedout in > 95% H2

18O (pH 11).[11a] Mass spectrometry afterelectrolysis confirmed the formation of 18O18O from H2

18O(there was almost no formation of 16O18O (< 1% w.r.t.18O18O) and 16O16O (< 1% w.r.t. 18O18O)),[11b] which showsthat the evolved O2 originates from water. To our knowledge,this is the first example of visible-light-induced O2 evolutionby CoPc. Figure 5 presents a schematic energy diagram for

Figure 2. a) Typical CVs at both ITO/PTCBI/CoPc and ITO/CoPc underillumination. Conditions: aqueous NaOH electrolyte with TEA(1.0E10�3 molL�1; pH�11); film thickness: bilayer 600 nm (PTCBI)and 190 nm (CoPc); CoPc single layer 120 nm; scan rate 20 mVs�1;reference electrode Ag/AgCl. b) Time course of photoanodic current atITO/PTCBI/CoPc, irradiated from the PTCBI side. Conditions: aqueousNaOH electrolyte with TEA (1.0E10�3 molL�1; pH�11); film thick-ness: 600 nm (PTCBI)/190 nm (CoPc); applied potential +0.3 V.

Figure 3. Typical CVs at ITO/PTCBI/CoPc, ITO/CoPc, and ITO/PTCBI/H2Pc in an aqueous NaOH solution (pH�11). Conditions: filmthickness: PTCBI/CoPc bilayer 350 nm (PTCBI)/190 nm (CoPc); CoPcsingle layer 70 nm; PTCBI/H2Pc bilayer 470 nm (PTCBI)/130 nm(H2Pc); scan rate 20 mVs�1; reference electrode Ag/AgCl.

Table 1: Data for the photoelectrochemical photolysis of water at a ITO/PTCBI/CoPc electrode.[a]

Run Bias Potential[V vs. Ag/AgCl]

t [h] O2 [mL] H2 [mL] TON [h�1]

1 +0.3 3 9.65 20.8 2.0E103

2 +0.4 3 17.0 32.6 3.5E103

3[b] +0.4 3 0 0

[a] Conditions: film thickness 160 nm (PTCBI)/90 nm (CoPc); alkaline(NaOH) electrolyte (pH 11). [b] The potentiostatic electrolysis wascarried out in the dark.

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the O2 evolution at the CoPc/water interface. In view of theenergy levels for ITO,[12] PTCBI,[12] CoPc,[13] and the O2/OH

�

redox couple, the oxidative power photogenerated at theCoPc/water interface is high enough to evolve O2 from OH

� .Although the energy level of the valence band (EVB) for H2Pcis close to that of CoPc,[13a] no evolution of O2 has beenconfirmed at the photoanode of the PTCBI/H2Pc bilayer.

[5] Inthe present bilayer, an inner-sphere interaction between theOH� and CoIII ions can be expected, especially at the pHemployed,[13b] and this may be a crucial factor in enabling O2evolution.To investigate the origin of the photocurrent generated at

the CoPc/water interface, action spectra for the photocurrentwere measured with respect to both the direction of theincident light and the thickness of the film; these arecompared with the absorption spectra of PTCBI, CoPc, andthe bilayer in Figure 6. First, the PTCBI side was irradiatedwith monochromatic light (I and II in Figure 6a). A photo-current was found to be generated for a thin PTCBI layer (I)over the wavelength range employed, and the resulting actionspectrum resembled the absorption spectrum of PTCBI(Figure 6b). The action spectrum was also measured for a

thick PTCBI layer (II), and the photocurrent was larger thanfor I. However, the action spectrum for irradiation from theCoPc side of I, (I’; Figure 6a) differed noticeably from that ofI. At wavelengths longer than 550 nm, where the absorptionof CoPc (Figure 6b) is large, only a small photocurrent wasgenerated, which suggests that absorption by CoPc does notresult in photocurrent generation. The action spectra shownin Figure 6a do not correspond with the absorption spectrumof the PTCBI/CoPc bilayer (Figure 6c). Thus, the photo-current is not generated by visible-light absorption by thewhole PTCBI/CoPc bilayer, but rather by absorption by onlythe PTCBI layer, in accord with the case for the PTCBI/H2Pcphotoanode.[5]

The role of CoPc is of note in comparing the presentphotoanodic characteristics with the conventional ones.Firstly, oxidative power can be generated at the CoPc/waterinterface through p-type photoconduction by CoPc; that is,the photoelectrode characteristic of the PTCBI/CoPc bilayerdiffers from that of the CoPc single layer, in which photo-reduction can be induced at the solid/liquid interface.[14]

Secondly, the hole-doped CoPc (CoIIIPc) can efficiently

Figure 4. Time course of the amount of O2 evolved at ITO/PTCBI/CoPc in an aqueous NaOH solution (pH�11). Conditions: appliedpotential +0.4 V; film thickness: 160 nm (PTCBI)/90 nm (CoPc);irradiated from the PTCBI side.

Figure 5. Schematic energy diagram for O2 evolution at the CoPc/water interface. EVB and ECB denote the energy levels of the valence andconduction bands, respectively.

Figure 6. a) Action spectra for photocurrent generated at ITO/PTCBI/CoPc (* for I, * for I’, and ~ for II). Conditions: film thickness: I andI’ 55 nm (PTCBI)/105 nm (CoPc); II 605 nm (PTCBI)/190 nm (CoPc);applied potential +0.3 V; aqueous NaOH electrolyte containing TEA(5E10�3 molL�1; pH�11). b) Absorption spectra of PTCBI and CoPc.Film thickness: PTCBI 60 nm; CoPc 190 nm. c) Absorption spectra ofPTCBI/CoPc bilayer for I, I’, and II. IPCE= incident photon-to-currentconversion efficiency.

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activate OH� for O2 evolution through an inner-sphereinteraction; this differs from the PTCBI/H2Pc bilayer, whichdoes not lead to evolution of O2 at the solid/liquid interface.The organic PTCBI/CoPc bilayer acts as a stable photo-catalyst for multiple-electron oxidation that is responsive tovisible light over a wide range of wavelengths and couldconstitute an essential part of an artificial photosyntheticsystem.

Experimental SectionPTCBI was prepared and purified by a previously describedmethod.[15] CoPc was purchased from Tokyo Kasei Kogyo Co., Ltd.ITO-coated glass (sheet resistance 13Wcm�2; transmittance 85%;thickness of the coated indium–tin oxide 110 nm) was obtained fromthe Nippon Sheet Glass Co., Ltd. The PTCBI/CoPc bilayer wasprepared by vapor deposition (degree of the vacuum < 1D 10�3 Pa;deposition speed 0.05 nms�1) and consisted of PTCBI coated on anITO glass plate with CoPc coated on top of the PTCBI layer (denotedas ITO/PTCBI/CoPc). The temperature at the ITO plate was notcontrolled during the vapor deposition. Absorption spectra weremeasured with a Hitachi U-2010 spectrophotometer. The resultingabsorption spectra of both PTCBI and CoPc were identical to thosereported earlier,[7,16] and their absorption coefficients indicated thethickness of the film employed (the aggregation structure of the CoPcsingle layer can also be identified from the absorption spectrum; thatis, the polymorph of CoPc is assignable to the alpha phase, which isalso supported by an earlier study[16]). Since the additivity of theabsorption coefficients is considered to be held on the visibleabsorption spectrum of the bilayer, the thickness of each layer wasestimated by the solving simultaneous equations for the absorbance attwo distinct wavelengths.

An electrochemical glass cell was equipped with the modifiedITO working electrode (effective area 1 cm2), a spiral Pt counterelectrode, and an Ag/AgCl (in saturated KCl electrolyte) referenceelectrode. The complete photoelectrochemical study was carried outin alkaline water (pH 11) under an Ar atmosphere by using apotentiostat (Hokuto Denko, HA-301) with a function generator(Hokuto Denko, HB-104), a coulomb meter (Hokuto Denko, HF-201), and an X–Y recorder (GRAPHTEC, WX-4000) under illumi-nation. A 150-W halogen lamp (light intensity ca. 70 mWcm�2) wasused as a light source under typical conditions. The procedure of thephotoelectrochemical measurement is shown schematically inFigure 1. The action spectra for the photocurrent were measured byusing a halogen lamp (300 W) as the light source in combination witha monochromator (SOMAOPTICS, LTD., S-10). The light intensitywas measured by using a power meter (type 2A, OPHIR JAPAN,LTD.). The IPCE value was calculated with Equation (1), in which I is

IPCE ð%Þ ¼ ð½I=e�=½W=e�Þ 100 ð1Þ

the photocurrent density (measured in Acm�2), e is the elementaryelectric charge (in C),W is the light intensity (in Wcm�2), and e is thephoton energy. In the present study, the effect of the reflection of theincident light from the glass surface was not considered (i.e., the lightintensity was not corrected).

The production of O2 and H2 was analyzed with a thermalconductivity detector (TCD) gas chromatograph (Shimadzu, GC-8A)with a 5-J molecular sieve column and Ar carrier gas. The gaseousproducts were quantified by a chromatogram analyzer (Shimadzu, ACHTUNGTRENNUNGC-R8A). The Supporting Information contains details of the quantifi-cation of O2 and shows that there was no contamination of O2 from airin the present work.

Received: December 15, 2005Published online: March 20, 2006

.Keywords: dioxygen evolution · photocatalysis ·photoelectrochemistry · phthalocyanines · semiconductors

[1] a) M. Kaneko, I. Okura in Photocatalysis (Eds.: M. Kaneko, I.Okura), Kohdansha-Springer, Tokyo, 2002, pp. 1 – 5; b) T. Abe,M. Kaneko, Prog. Polym. Sci. 2003, 28, 1441 – 1488.

[2] A. Fujishima, K. Honda, Nature 1972, 238, 37 – 40.[3] a) Z. Zou, J. Ye, K. Sayama, H. Arakawa,Nature 2001, 414, 625 –627; b) S. U. M. Khan, M. Al-Shahry, W. B. Ingler, Jr., Science2002, 297, 2243 – 2245.

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[11] a) For the 18O-isotopic labeling study, > 95% 18O-labeled water(H2

18O) was purchased from Cambridge Isotope Laboratories,Inc. Potentiostatic electrolysis was run by applying a biaspotential of + 0.4 V to ITO/PTCBI/CoPc under illuminationfor 3 h in a He atmosphere. The gaseous products were analyzedby a mass spectrograph (Shimadzu, QP2010) operated in theelectron-ionization mode; b) In a control experiment in whichan electrochemical cell was allowed to stand for 3 h in the darkunder a He atmosphere, no 18O18O was detected.

[12] A. Yakimov, S. R. Forrest,Appl. Phys. Lett. 2002, 80, 1667 – 1669.[13] a) R. O. Loutfy, Y. C. Cheng, J. Chem. Phys. 1980, 73, 2902 –

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