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Balanced Excitation between Two Semiconductors in Bulk Heterojunction Z-Scheme System for Overall Water Splitting Nagarajan Srinivasan, Etsuo Sakai, and Masahiro Miyauchi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00267 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016
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ACS Catalysis
Balanced Excitation between Two Semiconductors in Bulk Heterojunction Z-Scheme System for Overall Water Splitting Nagarajan Srinivasan a, Etsuo Sakai a and Masahiro Miyauchi a * a
Department of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ABSTRACT: Heterogeneous bilayered semiconductor thin films were fabricated using p-type CaFe2O4 and n-type BiVO4 for photocatalytic overall water splitting. The interface of the semiconductor layers was modified with Co3O4 and Pt cocatalyst. The resulting a bulk heterojunction film, which was composed of Co3O4/BiVO4/Pt/CaFe2O4/Pt and was fabricated on a substrate, produced both oxygen and hydrogen under visible-light irradiation in the absence of applied bias potential or sacrificial agents. Further, a balanced absorbed photon number between CaFe2O4 and BiVO4 was shown to be critical factor for achieving overall water splitting, thereby generation of hydrogen and oxygen molecules in 2:1 stoichiometric ratio. These results demonstrate that the interfacial design and management of absorbed photon number in heterogeneous photocatalysts are essential factors for constructing Z-scheme systems that are capable of overall water splitting.
KEYWORDS: heterogeneous photocatalysts, water splitting, Z-scheme, solar energy conversion, visible light
Photoinduced water splitting is a promising renewable energy production technology for the large-scale production of hydrogen from water without the associated emission of carbon dioxide.1,2 Although several visible-light-responsive photocatalysts, including WO3,3 Fe2O3,4 TaON,5 and metal chalcogenide,6 have been constructed, most visible-light-driven photocatalysts developed to date require a sacrificial electron donor or acceptor added to the aqueous solution to split water molecules completely.7,8 To mimic natural photosynthetic systems found in plant leaves, a photocatalytic material that accepts electrons directly from water to simultaneously produce hydrogen and oxygen is needed. Overall water splitting has been achieved using solid solutions such as GaN-ZnO 9 or graphitic carbon nitride,10 but few systems capable of water splitting under visible-light irradiation have been reported. Another potential strategy to achieve complete water splitting is the use of a two-photon excitation process involving two different semiconductors, known as "Z-scheme" systems.11 Several Z-scheme systems have been constructed using composite catalysts in powder form, such as MgTa2O6-xNy/TaON,12 SrTiO3:Rh/BiVO4,13 and RuO2–TaON/Pt–TaON.14 Sayama et al also reported the construction of a dispersed powder Z-scheme WO3/SrTiO3 system using iodine as a shuttle redox mediator.15 In addition to powder-based systems, two-electrode photoelectrochemical systems, which consist of separate photo-anodes and -cathodes, are being actively developed.16 apart from the photocatalytic powder-based and two-electrode heterojunction systems, the construction of a bilayered wireless heterojunction film that functions as an artificial leaf is highly desirable as it would not yet been satisfactorily reported.17 In systems with a bilayered single-electrode structure, the balance of photon number between the two semiconductors is critical, because Z-scheme systems utilize two photons to generate an electron-hole pair,
while a second electron-hole pair recombines at the semiconductor interface. Herein, we constructed a bilayered bulk heterojunction semiconductor film consisting of visible-light active p-type CaFe2O4 (CFO) 16, 18 and n-type BiVO4 (BVO) semiconductors 19, 20 for the overall water splitting reaction. The porosity and thickness of the layered films were optimized and the cocatalyst layers were modified at the interface of the semiconductor layers to maximize the catalytic activity under visible-light irradiation in the absence of a bias potential. In addition, the effect of the excitation balance between CFO and BVO on the photocatalytic water splitting property of the system was examined. The structural requirements of the bilayered film and photon irradiation conditions needed to achieve complete overall water splitting were also investigated. Figure 1 shows cross-sectional SEM images of the optimized thin film structure of the CFO/BVO heterojunction semiconductor. The CFO and BVO layers were coated onto a substrate by screen printing the dispersed paste of semiconductor particles and annealing in air. FTO-coated glass was used as the substrate to allow measurement of the photo-electrochemical properties of the single-component films of BVO or CFO (their photoelectrochemical properties are described in the Supporting Information). However, the layered heterojunction system had a wireless configuration and therefore did not require an FTO layer. The BVO layer functions as a photoanode, and nanoparticles of Co3O4 were introduced between the FTO substrate and BVO layer as cocatalysts for promoting water oxidation. In contrast, the CFO layer works as a photocathode, and a thin Pt layer, which served as a recombination layer to enhance the efficiency of the Z-scheme process, was introduced between the CFO and BVO layers by DC magnetron sputtering. In addition, small Pt nanoparticles were deposited and distributed over the surface of the CFO layer by the photo-
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reduction of platinum chloride under visible-light irradiation to allow the excitation of only CFO at wavelengths (λ) >550 nm. The screen printing method used in the construction of this system yielded highly porous CFO and BVO electrodes, which promote the diffusion of protons and hydroxyl ions from the surface into the film during the wireless water splitting reaction. In addition to facilitating ion diffusion, the porous bulk-heterojunction structure markedly increases the interface area between the photoanode and photocathode, thereby promoting the efficient recombination of electron-hole pairs at the interface. The absorbed photon number is strongly influenced by the thickness of each semiconductor layer. When the film is thicker than light penetration depth, photons would not reach to the underlying layer. Thus, we made an effort to coat thinner film to excite both semiconductors. In the present study, a few particles of BVO or CFO were deposited along cross sectional direction. EDX pint point analysis was also shown in our supporting information (Figure SI2), and all components, i. e. Co3O4, BVO, Pt, CFO were indispensable to achieve complete overall water splitting. Details of the semiconductor synthesis, thin film fabrication, and evaluation procedures are described in the Supporting Information (Figure SI1).
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were estimated to be 1.73 and 2.37 eV, respectively, which are consistent with those previously reported.21,22 From the MottSchottky plots, it was determined that CFO was a p-type semiconductor and had a flat band potential of 1.40 V (vs normal hydrogen electrode (NHE) at pH =12), whereas BVO was an n-type semiconductor with a flat band potential of 0.303 V (vs NHE). Based on the results of these optical and electrochemical analyses, a band diagram was constructed (Figure SI4, Supporting Information). Notably, the conduction band of CFO was determined to be higher than the hydrogen production potential, and the valence band of BVO is deeper than the oxygen production potential. We confirmed that the Co3O4/BVO and Pt/CFO films generated oxygen and hydrogen, respectively, under visible-light irradiation in the presence of the sacrificial agents of NaI and NaIO3, respectively (Figure SI5). From current-potential curves of the CFO and BVO electrodes under chopped visible-light irradiation (Figure SI6), it was demonstrated that CFO behaved as a p-type photocathode, whereas BVO exhibited properties of an n-type photoanode. The calculated open circuit photopotentials for the CFO and BVO electrodes were 0.476 V and -0.629 V (vs Ag/AgCl), respectively. The difference between these two open circuit photopotentials drives spontaneous redox reactions to generate both hydrogen and oxygen through complete water splitting 16.
Figure 2. Band gap values of CFO (a) and BVO (b) were determined from optical absorption spectra using the Kubelka-Munk function. Mott-Schottky plots of flat band potential measurements for CFO (c) and BVO (d) electrodes. Figure 1. Cross-sectional SEM image of CFO/BVO heterojunction semiconductor film. (a) Reflection image and (b) EDX mapping image. We first investigated the structural, optical, and photoelectrochemical properties of CFO and BVO films deposited on FTO. XRD and SEM analyses confirmed that the CFO and BVO particles were crystalline and were several micrometers in diameter (Figure SI3, Supporting Information). The optical absorption spectra of CFO and BVO powder and electrochemical Mott-Schottky plots of CFO and BVO electrodes are shown in Figure 2. The band gap values of CFO and BVO
We next investigated the photocatalytic water splitting properties of the constructed heterojunctioned-layered films under visible-light irradiation in the absence of an applied bias potential and sacrificial agent. To control the absorbed photon number of each semiconductor, the films were irradiated with visible light from several different directions, as illustrated in Figure 3 (a). The absorbed photon number in the CFO and BVO layers were determined using a spectrum-radiometer, as described in the Supporting Information. Under photon irradiation from the CFO side of the layered film (Figure 3 b),
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ACS Catalysis
oxygen production was limited. Similarly, irradiation from the BVO side resulted in limited hydrogen production (Figure 3c). Under these conditions, the number of absorbed photons differed by one to two orders of magnitude between the two semiconductors. The excess photoexcited charge carriers in one side of semiconductors undergo recombination through band to band transient process or charge-up in the semiconductor itself. In contrast, the simultaneous photon irradiation of the CFO and BVO layers resulted in photocatalytic overall water splitting (Figure 3 d). Notably, the amount of generated hydrogen was twice that of oxygen, indicating 2:1 stoichiometric water splitting. As Z-scheme processes involve the absorption and recombination of two photons at the CFO and BVO interface, the balanced excitation between photoanodes and photocathode is critical to achieve complete overall water splitting. The thickness of each semiconductor is also an important factor in the design of efficient Z-scheme photocatalytic systems. CFO and BVO are indirect gap semiconductors, and the visible-light penetration depth of these materials was estimated to be a few micrometers.23
was used to generate the thinnest films as possible. Using this approach, balanced excitation between the two semiconductors was achieved by controlling the photon irradiation conditions. These results suggest that balanced excitation would also be achieved using a reflective or scattering technique, such as a mirror system. Further, bulk heterojunction structure and cocatalyst modification at the BVO and CFO interface were also shown to be important for overall water splitting in the absence of a sacrificial agent or bias potential. The present findings are not limited to CFO and BVO semiconductors, but are also applicable to other Z-scheme combinations of various semiconductors. In summary, we demonstrated that a bilayered CFO/BVO heterojunction film functions as a Z-scheme system for the overall water splitting reaction. A balanced absorbed photon number between the semiconductors is critical for achieving complete water splitting. The bulk heterojunction structure and management of absorbed photon number is applicable to other photocatalytic semiconductor heterojunction combinations and represents a strategic approach for the development of simple wireless artificial leaf-like devices.
ASSOCIATED CONTENT Supporting Information Experimental procedures and supplementary data are presented. This material is available free of charge via the Internet at http://pubs.acs.org.”
AUTHOR INFORMATION Corresponding Author
Phone: +81 3 5734 2527. Fax: +81 3 5734 3368. E-mail:
[email protected] ACKNOWLEDGMENT We thank Mr. Y. Orai and Mr. T. Chibiki for their help with the FE-SEM analysis at Hitachi High-Technologies Corp. Research Center, Japan. This work is supported by JST, Japan. We also acknowledge Mr. Greg Newton for a critical reading of the manuscript.
Figure 3. (a) Schematic illustration depicting the direction of visible light used to irradiate the CFO/BVO heterojunction semiconductor film in the plots shown in (b), (c) and (d). (b-d) Plots showing the amounts of hydrogen and oxygen generated with respect to light irradiation direction as a function of irradiation time. The experiment was conducted in 0.1 M NaOH under irradiation with a 150-W Xe lamp equipped with a 400-nm cutoff filter for 5 h. In the present experiment, although we practically used two lamps to excite both semiconductors with a few micrometers scale thickness, single side irradiation would be feasible if the film thickness was less than several tens nanometers. In case of the ultrathin film, single light irradiation from BVO side is appropriate, since the band gap of BVO (2.4 eV, λ= 517 nm) is wider than that of CFO (1.8 eV, λ= 689 nm), and photons above the wavelength of 517 nm can pass through the BVO layer, which can excite CFO layer. However, when the total thickness is several tens nanometers, the total photon number becomes limiting. By considering these factors, here, the screen printing of highly crystallized semiconductor particles
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