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Single-Crystalline Thin Films for Studying Intrinsic Properties of BiFeO3SrTiO3 Solid Solution Photoelectrodes in Solar Energy Conversion Seungho Cho, Ji-Wook Jang, Wenrui Zhang, Ady Suwardi, Haiyan Wang, Dunwei Wang, and Judith L. MacManus-Driscoll Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02394 • Publication Date (Web): 03 Sep 2015 Downloaded from http://pubs.acs.org on September 9, 2015

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Chemistry of Materials

Single-Crystalline Thin Films for Studying Intrinsic Properties of BiFeO3-SrTiO3 Solid Solution Photoelectrodes in Solar Energy Conversion Seungho Cho,a,† Ji-Wook Jang,b,† Wenrui Zhang,c Ady Suwardi,a Haiyan Wang,c,d Dunwei Wang,b,* and Judith L. MacManus-Driscolla,* a

Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road,

Cambridge, CB3 0FS, United Kingdom b

Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon Street, Chestnut

Hill, Massachusetts 02467, United States c

Department of Materials Science and Engineering, Texas A&M University, College Station, Texas

77843, United States d

Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas

77843, United States †

*

S. C. and J.-W. J. contributed equally to this work. Corresponding authors: E-mail: [email protected] (D.W.); [email protected] (J.L.M.-D.)

Abstract Solid solutions have been widely investigated for solar energy conversion because of the ease to control properties (e.g., band edge positions, charge carrier transport and chemical stability). In this study, we introduce a new method to investigate intrinsic solar energy conversion properties of solid solutions through fabricating high-quality single-crystalline solid solution films by pulsed laser deposition. This ACS Paragon Plus Environment

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method rules out external factors, such as morphology, crystalline grain size, orientation, density and distribution, surface area and particle-particle or particle-conducting layer connection, that have plagued previous studies on solid solution photoelectrodes. Perovskite BiFeO3 (BFO) and SrTiO3 (STO) were chosen as “end” members of the solid solutions (i.e., (BFO)x(STO)1-x (0 ≤ x ≤ 1)). Optical and photoelectrochemical (PEC) properties of the solid solutions significantly varied with changing compositions. Among the six studied compositions, BFO:STO (3:1 molar ratio) exhibited the highest photocurrent density with the photovoltage of 1.08 V. The photoelectrode also produced stable photocurrent for 12 h. Faradaic efficiencies of H2 and O2 formation close to 100% were measured.

1. Introduction Achieving a set of desired properties on simple materials such as binary or ternary compounds is exceedingly difficult for a wide range of applications. Commonly combinations of two or more components made of simple materials have been employed to meet the needs in the field of solid-state sciences. When the mixing of two or more solid-state constituents does not change the crystal structures, the resulting material is often referred to as solid solutions. A key advantage offered by solid-solutions is that the properties can be precisely modulated in a systematic way. In principle, fine-tuning of properties over a wide range is possible through choice of “end” members and their concentrations. In addition, there is the possibility that unexpected properties may result beyond simple averaging of the properties of the end members.1 For solar energy conversion applications, solid solutions such as ZnO-GaN,1 CdS-ZnS,2 CdSCdSe,3 Fe2O3-Nb2O5,4 AgNbO3-NaNbO3,5 and Cu(Ga,In)(S,Se)26-8 have been reported for giving the possibility of creating materials with controlled band edge positions, enhanced chemical stability and better charge carrier transport. However, these solid solution photoelectrodes have been prepared by post-deposition of pre-synthesized particles on a conducting substrate or by in-situ deposition on polycrystalline transparent conducting layer (e.g., fluorine-doped tin oxide or indium-doped tin oxide)ACS Paragon Plus Environment

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coated substrates and are consequently polycrystalline in nature. Hence, studies of intrinsic solar energy conversion properties of solid solution photoelectrodes prepared by those methods are made difficult by the complications connected to their structural features such as sizes, orientations, densities and distribution of crystalline grains, specific surface areas, and particle-particle or particle-conducting layer connection. Epitaxial thin films have been used for investigating solar energy conversion properties of materials because of their highly controlled crystallographic parameters and elemental distribution.9-16 Here, we show that high-quality single-crystalline solid solution photoelectrodes are suitable to explore and systematically understand the properties of solid solution mixtures for PEC water splitting. Films are grown by pulsed laser deposition (PLD). This gives stoichiometric transfer of ablated material from multication targets for many materials.17 This unique feature is made possible by the nonequilibrium nature of the ablation process, resulting in vaporization which is not dependent on the vapor pressures of the constituent cations. In addition, PLD can also realize atomically abrupt and epitaxial interfaces between materials. Unlike other preparation methods for solid solution photoelectrodes, the intrinsic photo-energy conversion properties of solid solutions were investigated without extrinsic variabilities as mentioned above. Perovskite BiFeO3 (BFO) and SrTiO3 (STO) were chosen as model “end” members of solid solutions for the following reasons: These perovskite materials are well-known photocatalytic materials owing to their high catalytic activity and good chemical stability.13,18-20 BFO and STO feature distinct band gaps of ~2.67 eV (its optical absorption edge in the visible light region) and ~3.2 eV (its optical absorption edge in the UV region), respectively,9,21 and can form a solid solution.22,23

2. Experimental Section Film fabrication: The BiFeO3 (BFO): SrTiO3 (STO) target was prepared by thoroughly mixing stoichiometric amounts of Bi2O3, Fe2O3, and STO (molar rations of 1.05: 1.01: 2 for BFO50-STO50, 1.05: 1.01: 6 for BFO25-STO75, 1.05: 1.01: 0.67 for BFO75-STO25 and 1.05: 1.01: 0.22 for BFO90ACS Paragon Plus Environment

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STO10) followed by sintering at 800°C. Films (nominal thicknesses of ~180 nm) were grown on (001) STO substrates by pulsed laser deposition (PLD) with a KrF laser (λ = 248 nm) with a fluence of 2.25 J/cm2 and a repetition rate of 1 Hz. The oxygen pressure was fixed at 0.2 mbar. During the depositions, a substrate temperature was 650°C. SrRuO3 (SRO)-buffered STO substrates were used for photoelectrochemical (PEC) measurements. The SRO-buffered STO substrates were prepared by PLD with a substrate (STO) temperature of 650°C and an O2 pressure of 0.2 mbar using a polycrystalline SRO target. The nominal thickness of each SRO layer was 38 nm. The SRO-buffered STO substrates were post-annealed at 450°C for 1 h under 400 mbar O2 prior to deposition of the solid solution films. Charaterizations: The phase and the crystalline quality of the thin films were investigated by ω-2θ and asymmetric X-ray diffraction (XRD) on a Bruker D8 theta/theta diffractometer with Cu-Kα radiation and a graded mirror. ω-rocking curves were obtained by measuring diffracted beam intensities around the BFO:STO film (002) as a function of the angle between incident x-rays and sample surface (ω). For investigating in-plane orientation, Phi (φ) scans were obtained by 360° in-plane sample rotation around (202) peaks of the films and substrates. Reciprocal space maps (RSMs) were collected about the (-1-13) and (103) of STO substrates. ω-2θ diffraction peaks and RSM peaks were used to calculate lattice parameters of the films. Transmission electron microscopy (TEM) was performed using a JEOL 2010 microscope. For optical absorption measurement, double-side polished STO substrates were used. UVvisible absorption spectra were obtained using an Agilent 8453. Photoelectrochemical (PEC) measurements: The current-potential (I-V) curves of PEC water oxidation were obtained with a scan rate of 20 mV/s in a phosphate solution (0.5 M, pH 12) using a platinum foil counter electrode, a Hg/HgO (1 M NaOH) reference electrode and a Potentiostat/Galvanostat (CH Instruments, CHI604C). Prior to a measurement, the solution was purged with nitrogen for 30 min. The photocathodes were illuminated with AM 1.5 solar simulator (100 mW/cm2, Newport Oriel 96000) calibrated by a thermopile optical detector (Newport, Model 818P-010-12). The evolved amounts of H2 and O2 were analyzed by a gas chromatograph (HP5890) with a thermal conductivity detector and a ACS Paragon Plus Environment

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molecular sieve 5-A column. Incident-photon-to-current-conversion efficiency (IPCE) was measured using the 150 W Xe lamp and a monochromator (Jobin Yvon Flurolog3, bandpass 5 nm). The IPCE was measured at 0 V vs. RHE in the same phosphate solution. Open-circuit potential measurements were done under the same AM 1.5 solar simulator with H2 saturated condition to maintiain reversible hydrogen evolution reactions near the standard conditions. Pt was deposited by a photo-assited electrodeposition at 0 V vs. RHE in the 1 mM H2PtCl5 phosphate solution (0.5 M phosphate solution, pH 12) for 5 min under illumination with AM 1.5 solar simulator.

3. Results and Discussion Bulk BFO exhibits a rhombohedral structure with a pseudocubic unit cell (a = 3.962 Å, α = 89.4°, JCPDS # 74-2016). Bulk STO is cubic structure with a lattice parameter of 3.905 Å at room temperature (JCPDS # 35-0734). Out-of-plane lattice parameters of thin epitaxial BFO films on materials with inplane lattice parameters smaller than those of BFO are usually larger than 3.962 Å because of in-plane compressive stresses.24 To investigate lattice parameters of our films deposited by PLD, we took X-ray reciprocal space maps near the (-1-13) STO substrate peaks of the samples with different compositions (Figures 1(a-e)) The estimated compositions of the solid solution films were provided in the Supporting Information (Table 1). The (-1-13) peaks of STO substrates were around qz of ~7.68. The (-1-13) peaks of the films were located in lower qz areas (Figures 1(a-d)). As molar ratios of STO in the films increased from 0 to 1 (from Figure 1(a) to 1(e)), c-axis lattice constants of the films linearly decreased from 4.051 Å to 3.905 Å (Figure 1(f)). The corresponding unit cell volumes decreased from 61.98 Å3 to 59.55 Å3 (Figure 1(g)). If we assume that phase separations of BFO and STO occurred in the deposited films, the out-of-plane lattice parameters of BFO phases would expand because the films were compressively in-plane stressed by the underlying materials as shown in Figures 1(a-e). Consequently, it is expected that STO phases expand along the out-of-plane direction by adjacent BFO phases if phase separation occurred. However, we did not observe STO phase reflections of the films shifted downward from the (-1-13) STO substrate peaks in Figures 1(b-d). Therefore, it is concluded that the deposited ACS Paragon Plus Environment

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films are solid solutions, which is confirmed by TEM observations, as shown later. The UV-visible absorption spectra of the deposited films are shown in Figure 2(a). Pure STO film showed an absorption only in the UV region, as expected from the wide band gap STO. As the BFO content increased, the films absorbed more visible light. The band gaps of the films (estimated by Tauc plots) were tuned from 2.66 (pure BFO) to 3.2 eV (pure STO) by varying solid solution compositions (Figure 2(b) and Figure S1 in the Supporting Information). Solid solution photoelectrodes were fabricated by growing films on conducting SrRuO3 (SRO, nominal thickness ~38 nm) buffered STO substrates. The conducting layers were required for photogenerated charge carrier extraction.25 Nominal thicknesses of the solid solution films were ~180 nm. The crystalline nature of each film on a SRO-buffered STO substrate was investigated using fourcircle X-ray diffraction (XRD). Figure 3 shows ω-2θ scans for films with different compositions. The ω2θ scans revealed the high degree of crystallographic orientation in the deposited films. Only (00l) perovskite diffraction peaks were observed. Figure 4(a) shows the ω-2θ scan of the BFO75-STO25 film around the STO substrate (002) peak. The BFO:STO film and SRO film (002) reflections were observed at 45.1° and 45.9° in 2θ, respectively (45.6° in 2θ for bulk SRO(002), JCPDS # 89-5715). As the molar ratios of STO in the solid solution increased, the positions of (00l) reflections were gradually shifted to higher 2θ (Figure 3), indicating reduction of the out-of-plane lattice parameters, consistent with the observations in Figures 1(a-e). In the case of the BFO25-STO75 film (Figure S3, Supporting Information), the film peak is observed at a higher 2θ than that of the SRO peak. The X-ray reciprocal space map around STO(103) for the BFO75-STO25 film on SRO/STO substrate are presented in Figure 4(b). SRO(103) and BFO:STO film(103) peaks are observed in lower qz regions than the high intensity (103) peak of STO substrate. A reciprocal space map with increased φ by 90° is also provided in Figure S4 of the Supporting Information. The position and width of qx of SRO(103) is similar to those of STO substrate (103), which indicates the SRO layer was well strained along the in-plane direction. In-plane lattice

parameters are, within error range, equal to that of the STO substrate. Figure 4(c) shows an ω-rocking curve of the (002) diffraction peak of BFO:STO film with the full width at half-maximum of 0.067°, ACS Paragon Plus Environment

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indicating high crystallinity of the solid solution film. In addition, Figure 4(d) shows 360° φ-scans of the (202) peaks of the BFO:STO film, SRO film and STO substrate. The BFO:STO and SRO peaks have the same peak positions (4-fold symmetry) as the STO substrate indicative of cube-on-cube epitaxy on the STO (001) substrate. Therefore, the epitaxial relationship between BFO:STO, SRO and STO substrate is [001] BFO:STO // [001] SRO // [001] STO, [100] BFO:STO // [100] SRO // [100] STO and [110] BFO:STO // [110] SRO // [110] STO. Figures 4(e) and 4(f) show low magnification cross-sectional TEM and high-angle annular darkfield scanning transmission electron microscope (HAADF-STEM) images of BFO75-STO25 film on SRO/STO, respectively. The cross-sectional STEM image indicates high uniformity of composition without phase separation. The selected-area electron diffraction (SAED) pattern of BFO:STO film along the [010] zone axis confirmed the film was single-crystalline (Figure 4(g)). Figures 4(h and i) display high-resolution cross-sectional TEM images on BFO:STO film and near the interface of the BFO:STO film and the SRO film, respectively, showing the solid solution film has a single-crystalline nature and it grew on the SRO(001) in a cube-on-cube fashion (Figure 4j), consistent with the X-ray data. Thicknesses and PLD substrate temperatures were optimized with respect to PEC activities as shown in Figure S2 in the Supporting Information. The PEC water splitting properties of the photoelectrodes were then investigated. Figure 5 shows current-voltage (I-V) curves for the solid solution photoelectrodes in a phosphate buffer solution (0.5 M, pH 12), under chopped AM 1.5 illumination. The BFO film exhibited a cathodic photocurrent because Bi loss in the PLD processes leads to p-type conduction.26-28 A 10% STO incorporation led to a greatly enhanced photocurrent density of 0.2 mA/cm2 at 0 V vs. RHE. The BFO75-STO25 photoelectrode showed a further increased photocurrent density of 0.33 mA/cm2 at 0 V vs. RHE. If we define the turn-on voltage (Von) as the potential where we first observed a photocurrent under chopped light, a Von of ~1.05 V was obtained for the photocathode. Further increasing the STO content (BFO50-STO50 and BFO25-STO75) reduced the photocurrent densities. Hence the BFO75-STO25 case among our composition series exhibited the highest photocurrent density. In addition, it is noteworthy that STO incorporation suppressed the dark ACS Paragon Plus Environment

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current around 0 V vs. RHE. We propose a possible reason for the enhanced PEC performance for BFO75-STO25. In the cathodic region, BFO shows an unstable performance because the reduction potential of BFO (about 0.3 V vs. RHE) is more positive than the water reduction potential (H+/H2, 0 V),29 which is the reason why BFO showed the high dark current density at 0 V vs. RHE shown in Figure 5. By formation of a solid solution with STO containing TiO2 layers which has a negative reduction potential (about -0.8 V),29 the shift of the reduction potential to the positive direction is expected, which can lead to an enhanced stability. The BFO75-STO25 solid solution film exhibits a stable PEC performance. However, further increasing the STO content (BFO50-STO50 and BFO25STO75) reduced the photocurrent densities, most likely due to decreasing visible light absorption (Figure 2(a)). To investigate the effects of a hydrogen evolution reaction catalyst on our single-crystalline solid solution photoelectrodes, Pt was deposited on the solid solution film by a photo-assisted electrodepostion, and their PEC properties were measured. Figure 6(a) shows polarization curves of BFO75-STO25 photoelectrodes with and without Pt deposition in a phosphate buffer solution, under chopped AM 1.5 illumination. The Pt deposition yielded a 39% enhancement in the photocurrent density at 0 V vs. RHE (0.46 mA/cm2). Transient peaks of the bare photoelectrode were reduced indicating increased charge separation efficiency on the surface of the photocatalyst upon Pt cocatalyst loading.30 Incident-photon-to-current-conversion efficiency (IPCE) measurements for the films, carried out at 0 V vs. RHE in the same solution were conducted to further study the photoresponse of the single-crystalline solid solution photoelectrodes as a function of the wavelength of incident light (Figure 6(b)). In general, the IPCE behaviors of the photoelectrodes followed the UV-visible absorption spectra qualitatively (Figure 2(a)). As expected, the STO film exhibited no photoresponse under visible light irradiation because of its wide band gap (Figure 2(a)). However, by forming solid solution with BFO, the deposited films showed photocatalytic activities under visible light. In the case of the pure BFO, the IPCE was relatively low under UV and visible light wavelengths. The IPCE was the highest for the BFO75-STO25

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solid solution, and Pt deposition enhanced the IPCE further in both UV and visible light regions due to the enhanced charge separation characteristic on the surface of the photocathode. As discussed above, the onset potential of the single-crystalline solid solution photocathode (BFO75-STO25) from current–voltage measurements (Figure 5) was ~1.05 V. Nevertheless, the steadystate current measurements are not ideal for quantitative analysis because the change in turn-on voltage may be influenced either by thermodynamic or kinetic factors or both.31 Thus, open-circuit potential was measured to obtain the photovoltage of the photoelectrode, excluding kinetic factors. The photovoltage of the BFO75-STO25/Pt photoelectrode was 1.08 V (see Figure S5 in the Supporting Information), which matched with the onset potential indicating that only 0.15 V of additional bias is needed to achieve the theoretical minimum requirement for overall water splitting. In addition, a constant potential measurement was performed at 0 V vs. RHE in the same phosphate solution to examine the stability of photocurrent generated by the photoelectrodes. The current-time curves of BFO75-STO25/Pt under continuous irradiation (Figure 7(a) for 3 h with detection of gas products and Figure S6 in the Supporting Information for more than 12 h) show that the photocurrent was sustained for at least 12 h. By detecting the reaction products, H2 and O2, and by comparing the amount formed with the number of charges passed through, Faradaic efficiencies of the system close to 100% were obtained (Figure 7(b)). These results indicate that the solid solution photoelectrodes are of good stability, and that the photo-generated charges are successfully collected for water splitting, accomplishing a solar-to-hydrogen energy conversion. It is also noteworthy that the photoelectrode is stable in basic solutions (pH 12). Most of photocathodes are active and stable in the acid solution, while most of photoanodes are stable under natural or basic solution except for WO3.32-35 Thus, in the two-electrode-system (photocathode and photoanode) for unassisted water splitting, stability of photocathode under basic solution is essential. Combined with its high photovoltage, the solid solution photoelectrodes investigated in this study promise to be a good choice for a photocathode material.


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4. Conclusions In summary, we have demonstrated single-crystalline solid solution films as an ideal tool for investigating solar energy conversion properties of solid solutions. This method excludes unwanted external factors, such as morphology, crystal grain size, orientation, density and distribution, surface area and particle-particle or particle-conducting layer connection, to measure intrinsic photo-energy conversion properties of solid solutions. We chose the perovskites BFO and STO as model end member materials. Among a series of our compositions, the BFO75-STO25 photoelectrode exhibited the highest photocurrent density with the photovoltage of 1.08 V. By using epitaxial thin films, solid solutions with a variety of end member combinations (2 ≤ members) and compositions can be evaluated. Also, depending on the solid solution end members and underlying substrates being explored, instead of SRO used in this study, other materials can be used as electrically conductive layers for photogenerated charge carrier extraction such as La1-xSrxMnO3, CaRuO3, LaNiO3 or Nb-doped STO. Combinations of materials and their compositions optimized by this intrinsic property evaluation method should be adopted to other synthetic methods such as solution based syntheses realizing nanostructures and/or large scale fabrications to enhance their applicability to solar energy conversion.

Acknowledgement. This work was supported by the European Research Council (ERC) (Advanced Investigator grant ERC-2009-AdG-247276-NOVOX).

Supporting Information Available: Estimated compositions of the solid solution films (Table S1). Tauc plots of BiFeO3 (BFO), SrTiO3 (STO) and BFO:STO solid solution films (Figure S1). Polarization curves of photoelectrodes with different substrate temperatures during PLD and different nominal thicknesses (Figure S2). ω-2θ scan of BiFeO325-SrTiO375 film on SrRuO3/SrTiO3 substrate. (Figure S3). Reciprocal space map around the (103) reflections with different φ by 90° from that shown in Figure 4(b) (Figure S4). Open-circuit potential measurements (Figure S5). Current-time curve of


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BFO75-STO25 film/Pt kept at 0 V vs. RHE in the solution for more than 12 h (Figure S6). The Supporting Information is available free of charge on the ACS Publications website.


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Figure 1. Reciprocal space maps around the (-1-13) reflections of SrTiO3 (STO) substrates: (a) BiFeO3 (BFO)100-STO0 film. (b) BFO75-STO25. (c) BFO50-STO50. (d) BFO25-STO75. (e) BFO0-STO100. As the molar ratio of STO in the films increased, the unit cell volumes gradually decreased. (a) Out-ofplane lattice parameters and (b) unit cell volumes of pure BFO, pure STO and BFO:STO films on STO substrates as a function of composition.


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Figure 2. (a) Absorption UV-visible spectra of the BiFeO3 (BFO) film, the SrTiO3 (STO) film and BFO-STO films and the underlying STO substrates. (b) Band gaps of the films (estimated by Tauc plots) as a function of composition.

Figure 3. ω-2θ scans of pure BiFeO3 (BFO), pure SrTiO3 (STO) and BFO:STO films on SrRuO3 (SRO)/STO. ACS Paragon Plus Environment

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Figure 4. (a) ω-2θ scan of BiFeO3 (BFO)75-SrTiO3 (STO)25 film on SrRuO3 (SRO)/STO. (b) Reciprocal space map around the (103) reflections. (c) ω-rocking curve of the (002) diffraction peak of the BFO:STO film. (d) 360° φ-scans of the (202) peaks of the BFO:STO film, SRO film and STO substrate. (e and f) Low magnification cross-sectional TEM and high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images of BFO75-STO25 film on SRO/STO, respectively. (g) SAED pattern of BFO:STO film along the [010] zone axis. (h and i) High resolution cross-sectional TEM images on BFO:STO film and around the interface between BFO:STO film and SRO film, respectively. (j) Crystallographic model of interface between BFO:STO solid solution and SRO films.


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Figure 5. Polarization curves of BiFeO3 (BFO):SrTiO3 (STO) films on SrRuO3-buffered STO in the phosphate buffer solution (0.5 M, pH 12), under chopped AM 1.5 illumination.


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Figure 6. (a) Polarization curves of BiFeO3 (BFO)75-SrTiO3 (STO)25 films on SrRuO3/STO without and with Pt deposition in phosphate buffer solution (0.5 M, pH 12), under chopped AM 1.5 illumination. (b) Incident-photon-to-current-conversion efficiency (IPCE) measurements for the deposited films, carried out at 0 V vs. RHE in the solution.


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Figure 7. (a) Current-time curve of BiFeO3 (BFO)75-SrTiO3 (STO)25 film/Pt, kept at 0 V vs. RHE in the solution. (b) Amount of hydrogen and oxygen evolved by the photocathode, matching the number of charges measured. The theoretical lines were calculated according to Faraday’s law of electrolysis.


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Table of Contents

Single-Crystalline Thin Films for Studying Intrinsic Properties of BiFeO3-SrTiO3 Solid Solution Photoelectrodes in Solar Energy Conversion Seungho Cho, Ji-Wook Jang, Wenrui Zhang, Ady Suwardi, Haiyan Wang, Dunwei Wang, and Judith L. MacManus-Driscoll

ToC figure


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Single-Crystalline Thin Films for Studying Intrinsic ... - ACS Publications

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