A water splitting system using an organo-photocathode and titanium dioxide photoanode capable of bias-free H 2 and O 2 evolution †

This study examined a water-splitting system comprising a TiO 2 photoanode and an organo-photocathode consisting of a p–n bilayer. Stoichiometric decomposition of water into H 2 and O 2 successfully occurred at bias voltages lower than the theoretical value ( i.e. 1.23 V). Compared to the conventional TiO 2 and Pt systems, the proposed water-splitting system demonstrated water splitting without any externally applied bias.

For establishing a sustainable society, solar hydrogen has been considered to be a potentially clean energy substitute for fossil fuels.][3][4][5][6][7][8][9][10][11] Since the discovery of photoelectrochemical water splitting by Honda and Fujishima, 12 TiO 2 has been recognised as one of the most efficient photoanodes for the evolution of O 2 from water.4][15][16][17] This is because of the close potential of the conduction band (corresponding to the magnitude of reducing power) to the reduction potential of H + into H 2 .
9][20][21][22][23] In some instances, photocatalytic reactions via organic p-n bilayers respond to the entire visiblelight energy for the oxidative decomposition of organoamine, 19,20 alcohol, 21 aldehyde 21 and hydrazine. 22,23In addition, when an organobilayer of phthalocyanine (H 2 Pc, p type) and fullerene (C 60 , n type) was employed as a photocathode in a three-electrode system, 24 the C 60 surface modified by Pt induced the reduction of H + into H 2 at applied potentials that were more positive than the formal potential of H + /H 2 .The detailed characteristics of C 60 for producing H 2 were clarified based on two types of in situ spectroelectrochemistry (i.e.VIS-NIR and Raman). 24Recently, more efficient H 2 production was found to occur by the replacement of H 2 Pc by zinc phthalocyanine (ZnPc, p type). 25 In the present study, a photoelectrochemical water-splitting system was studied, where TiO 2 (photoanode) and a ZnPc/C 60 bilayer modified by Pt (photocathode, vide supra) were simultaneously employed for water oxidation and H + reduction, respectively (Scheme 1).The stoichiometric decomposition of water into H 2 and O 2 was found to occur upon application of bias voltages o1.23 V.Moreover, this study demonstrated that photocatalytic water decomposition (i.e.bias-free water splitting) occurs successfully in a system featuring an organo-photocathode, making it distinct from a conventional TiO 2 -Pt system (vide supra).
The ZnPc/C 60 bilayer was prepared by vapour deposition (pressure, o1.0 Â 10 À3 Pa; deposition speed, ca.0.03 nm s À1 ) using an indium-tin-oxide (ITO)-coated glass plate as the base material. 25The organic p-n bilayer was composed of ZnPc coated on ITO and C 60 coated on top of the ZnPc layer; moreover, a Pt co-catalyst was photocathodically deposited onto the C 60 surface of the bilayer (the resulting device is abbreviated as ITO/ZnPc/C 60 -Pt).A disk-like form of TiO 2 (rutile) was utilised as the photoanode, which was reductively treated under a hydrogen flow at 773 K for 2 h prior to use.In order to achieve an ohmic contact, Ga-In was applied to the back side of the TiO 2 .As represented in Scheme 1, a cell made up of twin compartments separated by a salt bridge was utilised for the water-splitting studies.All studies were performed under an Ar atmosphere in an aqueous H 3 PO 4 solution (pH = 2).Other experimental details are provided in the ESI.† The voltammetric characteristics of photoelectrodes employed were examined in a three-electrode system (see Scheme S1 and Fig. S1 (ESI †)).When using TiO 2 as a photoanode, a photocurrent due to the oxidation of H 2 O was observed at potentials more positive than 0 V (vs.Ag/AgCl (sat.)).The voltammogram measured for ITO/ZnPc/C 60 -Pt is shown in Fig. S1 (ESI †). 25 The CV data revealed that both the photoanodic and photocathodic currents are generated in the same potential window.In other words, the water-splitting system of TiO 2 and ITO/ZnPc/C 60 -Pt may produce H 2 and O 2 without any bias voltage.
A water-splitting study was conducted with and without applied bias voltages (Scheme 1).The stoichiometric decomposition of water into H 2 and O 2 was found to occur at bias voltages less than 1.23 V (the theoretical voltage for water splitting).Fig. 1(a) shows the relationship between the amounts of H 2 and O 2 evolved and the respective applied voltages.The H 2 and O 2 amounts increased when a large bias was applied to the system; however, the evolved amounts gently increased, particularly at voltages larger than 0.25 V.The bias-free H 2 and O 2 evolution occurred stoichiometrically (cf.detailed data are also shown in the following figures).Based on the results shown in Fig. 1(a), the light to hydrogen conversion efficiency (Z, ESI †) was estimated with respect to the applied voltages.
As shown in Fig. 1(b), efficient water splitting occurred at a small bias voltage of 0.25 V with ca.0.1%.The results of Fig. 1(a) and (b) can be interpreted as follows.When a bias voltage is applied to the system, efficient charge separation can occur along with an efficient charge transfer between the photoanode and the photocathode, which can involve the suppression of carrier recombination.In such an occasion, the amounts of H 2 and O 2 may essentially increase.However, it appeared that a large bias of 40.25 V cannot cause the proportional formation of H 2 and O 2 due to the presence of carriers sufficient to induce water splitting at those voltages.For example, the band bending for water oxidation at TiO 2 can sufficiently occur to be almost independent of carrier formation with respect to applied voltages.Therefore, the application of a large bias voltage to a water splitting system can only lead to a gentle enhancement of kinetics for H 2 and O 2 formation, thus resulting in decreasing efficiency (Z).][28] A prolonged study was conducted to test the durability of the present system.The linear relationships between the amounts of H 2 and O 2 evolved and the irradiation time, demonstrating the stable and durable performance for water splitting with and without bias voltages, are shown in Fig. 2.
Control experiments conducted in the presence of methanol (electron donor) or Ag + (electron acceptor) were compared with a typical result from the present system, as shown in Table 1.Irrespective of the presence of methanol, the amount of H 2 evolved was relatively constant (Table 1, entries 1 and 2); however, in the presence of an Ag + acceptor, the amount of    evolved O 2 increased (Table 1, entries 1 and 3).Therefore, these results may suggest that the kinetics of the present system (Table 1, entry 1) is dominated by H 2 formation.The evolution of H 2 and O 2 in the conventional TiO 2 -Pt system was examined for use as a reference system.Similar to previous results (vide supra), Fig. S2 (ESI †) demonstrates that the TiO 2 -Pt system needs an applied bias voltage for achieving water splitting.A comparison between the present and conventional systems is represented in Table 2.The amounts of H 2 and O 2 originating from water splitting are greater in the TiO 2 -ITO/ZnPc/C 60 -Pt system compared to the TiO 2 -Pt system.These results indicate that rate-limiting H 2 evolution becomes more efficient when employing ITO/ZnPc/C 60 -Pt instead of a Pt counter as the cathode.
The individual reactivity of the photoelectrodes employed in TiO 2 -ITO/ZnPc/C 60 -Pt has been previously clarified: O 2 and H 2 evolution can occur by the oxidising and reducing power produced at the TiO 2 photoanode [13][14][15][16][17] and the ITO/ZnPc/C 60 -Pt photocathode, 24,25 respectively.Distinct from the conventional TiO 2 -Pt system, the electrons photogenerated at TiO 2 cannot directly participate in the reduction of H + into H 2 ; however, they can contribute to the regeneration of the pristine ZnPc through electron transfer via an external circuit (Scheme 1).Action spectra for photocurrents generated at TiO 2 and ITO/ZnPc/C 60 -Pt are shown in Fig. S3 (ESI †), confirming that TiO 2 (rutile) can induce O 2 evolution based on bandgap excitation (3.06 eV in rutile TiO 2 ). 29,30n addition, we have previously reported that the entire visiblelight energy range is available for H 2 evolution occurring at an ITO/ZnPc/C 60 -Pt electrode. 25n summary, the photoelectrochemical splitting of water was demonstrated in a TiO 2 -ITO/ZnPc/C 60 -Pt system featuring an organo-photocathode with a TiO 2 photoanode, and bias-free decomposition of water into H 2 and O 2 was also found to occur.Based on the resulting action spectral characteristics (ESI †), the full region of visible light energy (i.e.o750 nm in wavelength) was available for H 2 evolution at the organo-photocathode.This implies that efficient water splitting can occur when simultaneously utilising a photoanode, responsive to visible light, for O 2 evolution.Thus, this work exhibited the effectiveness of using an organic p-n bilayer for a water-splitting system.The application of two types of materials for water splitting has merits in terms of the harvesting of solar energy and individual generation of oxidising and reducing power for O 2 and H 2 evolution, respectively.2][33][34] Novel and efficient photoelectrodes for H 2 and O 2 evolution need to be developed for both organic and inorganic semiconductors so that a breakthrough in establishing a practical system for water splitting can be found.
This work was partly supported by a grant from the Murata Science Foundation (T.A.) and the Cooperative Research Program of ''Network Joint Research Center for Materials and Devices'' and a Grant-in-Aid for Scientific Research on Innovative Areas (T. A. and H. K.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Fig. 2
Fig. 2 Relationships between the H 2 and O 2 amounts and irradiation time ((a) 0.7 V; (b) 0 V, i.e. no-biased condition).Experimental conditions were the same as those used in Fig. 1, except that the irradiation time was changed.

System H 2
evolved/mL O 2 evolved/mL Note Entry 1 b 171 76.2No control system Entry 2 c 176 -In the presence of methanol Entry 3 d -101 In the presence of Ag + a Bias voltage of 0.7 V was applied to the system under experimental conditions similar to those in Fig. 1. b Data obtained from Fig. 1. c A methanol solution (methanol/aqueous H 3 PO 4 solution (v/v) = 1:1, pH = 2) was used.d An aqueous solution of AgNO 3 (5 mM, pH = 2) was employed.

Table 2
Comparison of water splitting data in the present system to the TiO 2 -Pt system a Bias voltage of 0.7 V was applied to the system, and other experimental conditions were similar to those of Fig.1.bDataobtainedfromFig.1.cInstead of ITO/ZnPc/C 60 -Pt, a Pt wire was employed as the cathode.Data listed in this table can also be seen in Fig.S2(ESI). a