Flexible Heteroepitaxy Photoelectrode for Photo-electrochemical

Subscriber access provided by STEPHEN F AUSTIN STATE UNIV

Article

Flexible Heteroepitaxy Photoelectrode for Photoelectrochemical Water Splitting Le Thi Quynh, Chien Nguyen Van, W. Y. Tzeng, Chun-Wei Huang, Yu-Hong Lai, Jhih-Wei Chen, Kai-An Tsai, Chung Lin Wu, Wen-Wei Wu, Chih Wei Luo, Yung-Jung Hsu, and Ying-Hao Chu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00645 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Flexible Heteroepitaxy Photoelectrode for Photoelectrochemical Water Splitting Le Thi Quynh,1 Chien Nguyen Van,1 W. Y. Tzeng,2 Chun-Wei Huang,1 Yu-Hong Lai,1 Jhih-Wei Chen,3 Kai-An Tsai,1 Chung Lin Wu,3 Wen-Wei Wu,1 Chih Wei Luo,2 Yung-Jung Hsu,1 and Ying-Hao Chu1,2,4* 1

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010,

Taiwan 2

Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan

3

Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan

4

Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040,

Taiwan

* Corresponding Author: [email protected] ABSTRACT MICAtronics represents a new research direction by taking the advantages of good mechanical flexibility and optical transparency of muscovite mica, leading to a new platform to use oxide heteroepitaxy for novel transparent soft technology. Here we report a model flexible photoelectrode for photoelectrochemical water splitting based on Fe2O3/ZnO/Mica heteroepitaxy. The heteroepitaxy was confirmed by a combination of X-ray diffraction and transmission electron microscopy. The PEC performance of this flexible and semitransparent photoelectrode under various bending states was investigated. We found that the photocurrent of the heteroepitaxial system was enhanced by three times relative to pure ZnO and Fe2O3 under visible-light irradiation, and the heteroepitaxial photoelectrodes retain its photocurrent after continuous bending in cycling (>3000 cycles) with a smallest bending radius of 3.5 mm. The energy band alignment and charge dynamics under light excitation were characterized to understand the mechanism of the enhanced PEC performance. This study provides a new

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

platform for design and fabrication of flexible transparent photoelectrode in the harvest of solar energy based on oxide heteroepitaxy. Key works: ZnO, Fe2O3, heteroepitaxy, flexible, photoelectrochemical water splitting INTRODUCTION Flexible electronics with excellent portability, bendability, stretch ability, being lightweight and human-friendly interfaces have attracted great efforts in development of nextgeneration electronics with the integration of multiple functionalities. Consequently, the concept of flexible devices has been developed in the fields of display, sensor, biomedical, information storage, supercapacitors, batteries, nano-generators, integrated energy-storage systems, and solar cells

1-6

. Currently, the dream of producing clean and renewable energy by harnessing solar

energy has triggered researchers’ attempts to develop numerous technologies. One promising route that has attracted great attention is the conversion of solar power to chemical energy based on the solar-driven photoelectrochemical (PEC) water splitting.7-9 Continuous efforts are being put forward to realize high-efficient energy conversion, low-cost, light-weight, and yet flexible components. In order to open new opportunities in development of solar-driven electrolysis, the investigation of novel flexible PEC devices are highly on demand. However, researchers commonly designed and investigated the photoelectrodes deposited on rigid and heavyweight substrates such as fluorine-doped tin oxide/glass or doped-silicon substrates10-13. Recently many efforts have been made in fabrication of flexible photoelectrodes, which could offer the promise approach for new architecture design of PEC devices6,14-22. However, the recent used flexible substrates such as polyethylene terephthalate, carbon cloth, and exfoliated graphene are not suitable for the growth of high-quality photocatalysts at high temperature. The photoelectrodes


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with high crystalline quality, less grain boundaries, and epitaxial heterojunction are favorable for efficient electron transfer, reduction of carrier recombination, as well as could serve as an excellent system to understand the original characteristics of photoelectrode materials. Therefore, it is desirable to develop high crystalline flexible photoelectrodes for PEC water splitting with superior pliancy, durability, and chemical and thermal stabilities. Muscovite Mica, a layered oxide, has attracted increasing attention due to its high transparency, thermal and chemical stability, and pliability.23 In addition, unlike the conventional flexible substrates, Mica has a crystalline structure with high melting point (> 1150 K) allowing the epitaxial growth of popular inorganic materials such as metal oxides or transition-metal chalcogenides.24-26 Therefore, a new platform called “MICAtronics” has been formed to fabricate epitaxial films on muscovite for next-generation electronics.2,27,28 Such a platform can provide a new route to use heteroepitaxial photoelectrodes for PEC to meet the requirements of transparent soft technology. In this study, we developed a flexible photoelectrode based on the metal oxide heteroepitaxy. Due to its abundance in nature, low cost, and high electron mobility, ZnO is widely used in photodetectors, gas sensor, solar cells, optoelectronics, nanogenerators, photocatalysts, and photoanode for water splitting.29-33 Meanwhile, Fe2O3 is useful in a wide range of applications including advanced magnetic applications, biomedicine and biotechnology, and photocatalytic splitting of water due to its efficient light absorption, chemical stability in aqueous environments, abundance, biocompatible, and nontoxic nature.34-37 Thus, a combination of Fe2O3 and ZnO could benefit from their superior characteristics. The crystal structure, optical properties, photoelectrochemical features as well as mechanical flexibility and durability of Fe2O3/ZnO heteroepitaxial photoelectrode were investigated. The Fe2O3/ZnO photoelectrode not only exhibits remarkable PEC enhancement, but also presents good flexibility and durability.



This enhancement is attributed to the interaction of photogenerated charge carrier at crystallineinterfaces. Such a result opens a new avenue to use oxide heteroepitaxy for the applications in PEC water splitting. RESULTS AND DUSCUSSION Structure Characterization and Determination of Epitaxial Relationship

Figure 1. (a) XRD θ-2θ scans of the Fe2O3/ZnO heterostructure on sapphire and Mica. (b), (c) In-plane ϕ scans of γ-Fe2O3 {311}, ZnO {101}, sapphire {024} and Mica {202} reflections. (d) Cross–sectional TEM image taken along [010]Mica zone axis of the Fe2O3/ZnO heterostructure. (e), (f) high-resolution TEM images of the ZnO/Mica and Fe2O3/ZnO interface with the FastFourier transform (FFT) patterns of the labeled areas shown in the insets. (g) Structural schematic of the Fe2O3/ZnO/Mica heteroepitaxy. The crystal structure, crystallinity, and epitaxial relationship of the Fe2O3/ZnO heterostructures grown on both muscovite and sapphire substrates were examined by X-ray


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


diffraction (XRD) and transmission electron microscopy (TEM). The θ-2θ scans (Figure 1a) of the Fe2O3/ZnO heterostructures display only cubic γ-Fe2O3 (lll) and hexagonal ZnO (00l) diffraction peaks, indicating the epitaxial nature of thin films without secondary phases. The estimated out-of-plane lattice constants of Fe2O3 (c=8.41 Å) and ZnO (c=5.19 Å) in the heterostructure suggest that the ZnO phase is almost relaxed while the Fe2O3 phase has a compressive strain of 0.96%. To unveil the detailed in-plane structural relationships, we analyzed the ϕ-scans of ZnO {101}, Fe2O3 {311}, Mica {202}, and sapphire {024} reflections as shown in Figure 1b and c. The observation of six-fold symmetry at 600 intervals of ZnO {101} and Fe2O3 {311} grown on both substrates indicates that there are three sets of structural domains separated by a 600 rotation along c-axis of the films. The orientation relationships can be

determined

as

(111)Fe2O3//(001)ZnO//(001)Mica

[01-1]Fe2O3//[010]ZnO//010]Mica,

(111)Fe2O3//(001)ZnO//(001)sapphire [010]ZnO// [-100]sapphire, and [01-1]Fe2O3 300 misoriented with sapphire[-100]. The magnetic hysteresis loops using a superconducting quantum interference device (SQUID) magnetometer were measured to investigate the magnetic properties of γ-Fe2O3. As shown in Figure S1, the magnetic hysteresis loops along the in-plane of direction show a typical ferrimagnetic characteristic with a saturation magnetization of ~100 emu/cm3 and a high external magnetic field response. The magnetic properties of γ-Fe2O3 grown on both Mica and Sapphire substrates are similar. It is noted that the γ-Fe2O3 is the second most common Fe2O3 polymorph in nature with widespread use in the magnetic recording industry, biotechnology, and solar photocatalytic applications. Furthermore, the detailed microstructure of the heteroepitaxial Fe2O3/ZnO/Mica was investigated by using TEM. Figure 1d presents the low-magnification cross-sectional TEM image of the Fe2O3/ZnO heterostructure taken along the zone axis of [010]Mica, revealing the clear interfaces of Fe2O3/ZnO and ZnO/Mica. Figure 1e and f show the



high-resolution TEM images together with the selected area diffraction patterns of Fe2O3, ZnO, and Mica, in which the reciprocal lattices are clearly indexed. The epitaxial relationship of Fe2O3 (111) [01-1]//ZnO (001) [010]//Mica (001) [010] was obtained. Similarly, the microstructural details of Fe2O3/ZnO/sapphire were also investigated as shown in Supporting Information Figure S2. The consistency of epitaxial relationships with the XRD results is further confirmed. The sharp interfaces of Fe2O3/ZnO films grown on both Mica and sapphire indicate high quality of the heterostructure. Based on the XRD and TEM results, the quality of Fe2O3/ZnO heteroepitaxy on Mica and sapphire are nearly the same, suggesting high quality oxide heteroepitaxy can be fabricated on flexible Mica substrate. A schematic of the Fe2O3/ZnO heterostructure grown on Mica is constructed and shown in Figure 1g. PEC Performance


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Linear-sweep voltammograms of pure Fe2O3 and ZnO, and the Fe2O3/ZnO electrodes (a) visible light illumination and (b) under simulated solar light irradiation (c) UV-visible transmission of the Fe2O3/ZnO heterostructure and the inset shows the photograph of flexible Fe2O3/ZnO/Mica. (d) IPCE spectra of ZnO, Fe2O3, and the Fe2O3/ZnO heterostructure. After the investigation on structural characteristics, the PEC performances of the films grown on rigid sapphire and flexible Mica substrate were investigated. For the PEC measurement, an epitaxial Al (2%) doped ZnO (AZO) conductive layer was first deposited on the substrates. The AZO has a same crystal structure with the ZnO and good lattice match to Fe2O3, Mica, and sapphire. Figure 2a presents the typical linear-sweep I-V curves for pure ZnO, Fe2O3, and the



Page 8 of 26

Fe2O3/ZnO electrodes under visible-light irradiation (λ>400 nm). The optimal thickness of ZnO and Fe2O3 layers for PEC performance are determined to be ~240 nm and ~20 nm, respectively, as presented in Figure S3. It can be observed that the PEC activity of the Fe2O3/ZnO heterostructure is much higher than that of pure Fe2O3 and ZnO films. At 1.0 V vs. Ag/AgCl, the photocurrents of the pure ZnO and Fe2O3 photoanodes are 15 µA cm-2 and 24 µA cm-2, respectively, and it increases to 98 µA cm-2 for the Fe2O3/ZnO heterostructure. Figure 2b shows the I-V curves of the samples under simulated sunlight irradiation. At 1.0 V vs. Ag/AgCl, the photocurrents increase to 245 µA cm-2 and 80.5 µA cm-2 for the pure ZnO and Fe2O3 films, respectively, to 356.5-387 µA cm-2 for the Fe2O3/ZnO heterostructure. Such a photocurrent enhancement can be attributed to the high interfacial quality, suitable band configuration, associated with the enhanced optical absorption. The optical properties of the film electrodes were investigated using UV-vis spectroscopy. The band gaps of ZnO and Fe2O3 layers determined from the absorption spectra are 3.28 eV and 2.24 eV, respectively, as shown in Figure S4. In comparison with pure ZnO, the absorption edge of the Fe2O3/ZnO heterostructure shifted to longer wavelengths suggesting that the absorption of the Fe2O3/ZnO heterostructure was significantly enhanced in the visible light region due to the smaller band gap of Fe2O3. In addition, Figure 2c presents the spectra of optical transmittance of the pure ZnO and Fe2O3/ZnO samples. In the visible light region, the transmittances of the Fe2O3/ZnO heterostructure displays over 60%, with the oscillatory character attributed from the interference effects. The inset of Figure 2c shows the optical images of the bending Fe2O3/ZnO/Mica,

indicating

the

flexible

and

semitransparent

characteristics

of

the

heterostructure film. To further depict the task of Fe2O3 in enhancing the photoactivity of Fe2O3/ZnO, we conducted the incident photon-to-electron conversion efficiency (IPCE) spectra


Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for pure ZnO, Fe2O3, and the Fe2O3/ZnO photoelectrodes. As shown in Figure 2d, the IPCE spectra exhibit similar behaviors to those of absorption spectra. The Fe2O3/ZnO electrode exhibited significantly enhanced photoactivity in both the UV and visible regions. The observed photocurrent in the UV region is dominated by the photoactivity of both ZnO and Fe2O3 layers, while the enhancement of photoactivity in the visible region can be mainly attributed to the Fe2O3 layer. The Kinetics of Charge Transfer in Fe2O3/ZnO Heterostructures

Figure 3. The Mott-Schottky plots of (a) pure ZnO and (b) pure Fe2O3 in the dark and under AM 1.5G illumination. Schematic representation of energy band alignment for Fe2O3/ZnO heterojunction (c) under AM 1.5G illumination, (d) in thermal equilibrium calculated based on XPS, and (e) under the condition of an external positive bias. (f) The Nyquist plots of pure ZnO, Fe2O3, and the Fe2O3/ZnO photoelectrodes under light illumination. (g) Steady-state PL spectra of pure ZnO and the Fe2O3/ZnO heterostructure. The inset shows the time-resolved PL spectra of pure ZnO and the Fe2O3/ZnO heterostructure.



In order to understand the role of Fe2O3/ZnO heterojunction in boosting the PEC performance, it is important to determine the energy diagram of the heterostructure. The relative band positions of each material in Fe2O3/ZnO heterostructure were characterized by X-ray photoelectron spectroscopy (XPS) and electrochemical impendence spectroscopy (EIS). As shown in Figure 3a and b, the Mott-Schottky plots extracted from the EIS technique possess positive slopes, indicating that both ZnO and Fe2O3 photoelectrodes are n-type materials. The carrier densities of pure ZnO and Fe2O3 samples at the excitation state (under AM 1.5G illumination) were calculated according to the Mott–Schottky equation (see Supporting Information for details) are 1.6x1020 and 6.6x1019 cm-3, respectively. The high carrier density of ZnO suggests that ZnO is a good charge transfer material. The flat-band potentials of the pure photoanodes were estimated by taking the x-intercept of a linear fit to the Mott-Schottky plot as a function of applied potential. Figure 3a and b show the flat band potentials of pure ZnO and Fe2O3 samples in the dark and under AM 1.5G illumination are -0.7, -0.55, -0.64 and -0.5 V vs. Ag/AgCl, respectively. With an assumption that the flat-band potentials of both n-type ZnO and Fe2O3 semiconductors lie at same potential with that of their conduction band (CB) edges,38 the energy diagrams for the heterostructure under illumination and in the dark were constructed as shown in Figure 3c and Figure S5. In addition, as presented in Figure S6, the energy differences between the valance band (VB) maximum and their corresponding Fermi level of ZnO and Fe2O3 are 2.3 and 1.3 eV, respectively. These values are larger than half of the band gap of ZnO (~3.28eV) and Fe2O3 (2.24 eV), again confirming the behavior of n-type semiconductors. The VB offset (EVBO) and CB offset (ECBO) of the ZnO-Fe2O3 heterojunction calculated based on XPS data (see Supporting Information for details) are 0.45 eV and 0.59 eV, respectively. Based on these information, the energy band alignment of Fe2O3/ZnO heterojunction at the


Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


thermodynamic equilibrium was constructed as shown in Figure 3d. According to the energy band alignment (Figure 3c and 3d), the holes at lower VB energy of ZnO can be easily transferred to VB of Fe2O3 and then injected to the electrolyte for oxidation reaction, leading to a suppression of recombination effects within ZnO layer. Meanwhile, the higher CB energy of ZnO could prevent electron transfer from CB of ZnO to that of Fe2O3, that may reduce PEC response. However, the photocurrent enhancement observed in Figure 2a and b suggests an improvement of charge carrier separation efficiency and light absorption due to the intimate contact of ZnO and Fe2O3. When the heterostructure was illuminated by visible light excitation (λ>400 nm), the photon energy was mainly absorbed by Fe2O3 layer, and thus the excited holes and electrons mainly are generated within Fe2O3 layer attributed to the narrow band gap. There are very limited photogenerated electrons and holes in ZnO, resulting in its low photocurrent (Figure 2a) due to its large band gap. The photocurrent enhancement of the heterostructure under visible light illumination could be resulted from the high energy excitation electrons in CB of Fe2O3,39 which may have energy level even higher than that of ZnO and thus could inject to ZnO and migrate through the external electric circuit to reduce water at the cathode. Figure 3e illustrated a proposed energy band diagram under illumination and external potential biases, in which a strong band bending could be formed at Fe2O3-electrolyte interfaces.40,41 Under external driving force, which increases with the anodic potential biases, CB energy of Fe2O3 could be bent upward and shifted up to higher energy level respect to CB edge of ZnO. As a result, photoexcited electrons are migrated from Fe2O3 to ZnO and then transferred to the counter electrode under the external anodic driving force, while the photogenerated holes could transfer from lower energy potential of ZnO conduction band to the higher energy potential of Fe2O3. This proposed band alignment could contribute to facile the separation of photoexcited carriers,



suppressing their recombination effects for the photocurrent enhancement of Fe2O3/ZnO photoelectrode at high applied bias (Figure 2b). Furthermore, Figure 3f represents the Nyquist plots, an EIS technique, for the photoelectrodes under an external bias of 0.6 V (vs. Ag/AgCl) and under simulated solar light illumination. The smaller arc in the Nyquist plot of the Fe2O3/ZnO electrodes, which could be attributed to the band bending at Fe2O3/ZnO heterojunction, corresponds to lower resistances of charge transfer across the electrode/electrolyte interface to promote the PEC performance. Charge Relaxation in The Fe2O3/ZnO Heterostructure Figure 3g shows the steady-state PL spectrum of pure ZnO film. The result shows a remarkable near-band-edge emission peak located at ~378 nm, which is in good consistence with the UV-vis absorbance spectra. Indeed, the Fe2O3/ZnO heterostructure displays a significantly depressed PL emission as compared to the pure ZnO film, indicating therefore a reduction of electron–hole recombination in ZnO. Accordingly, the aforementioned PL quenching effect in the heterostructure suggests that the photoexcited charges transfer across the heterointerface resulting in an efficient separation of electrons and holes in the heterojunction. Moreover, the time-resolved PL measurement was conducted to determine carrier lifetime (inset of Figure 3f). The fastened PL decay kinetics was observed for the Fe2O3/ZnO heterostructure. The intensityaverage emission lifetime (τ) of pure ZnO/sapphire, ZnO/Mica, Fe2O3/ZnO/sapphire, and Fe2O3/ZnO/Mica heterostructures are calculated as 8.55, 3.63, 6.92, and 3.04 ns, respectively (Supporting Information Table S1). The shortened emission lifetime in the Fe2O3/ZnO heterostructures together with a decay of PL emission is a confirmation of the charge migration between Fe2O3 and ZnO layers at the heterointerface.


Page 12 of 26

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Typical ∆/ curves of pure Fe2 O3, pure ZnO, and Fe2O3/ZnO heterostructure. Solid lines are the fitting of a third-order exponential decay function. The inset shows ∆/ of the Fe2O3/ZnO heterostructure in semi-logarithmic scale, in which three decay processes can be clearly distinguished. Solid lines are a guide to the eyes. (b) The relaxation processes of photoexcited carriers in pure ZnO and pure Fe2O3. Dashed lines indicate the trapped states. (c) The relaxation processes of photoexcited carriers in Fe2O3/ZnO heterostructure. Figure 4a shows the typical time-resolved transient reflectivity change (∆/) spectra of pure ZnO layer, pure Fe2O3 layer, and the Fe2O3/ZnO heterostructure. The ∆/ spectra in Figure 4a can be fitted well by a third-order exponential decay function, i.e., ∆/

+

+ + . The fitting results are represented in Table S2. In the case of pure ZnO layer, the photoexcited carriers could relax from the conduction band (or high-energy trapped states) to low-energy trapped states with the time constant of 3.6 ps after the excitation, as shown in Figure 4b.42 Meanwhile, the photoexcited carriers (electrons) may also recombine with holes directly within 48 ps. Finally, the photoexcited carriers in low-energy trapped states would further recombine with the holes in hole trapped states with time constant of >700 ps. Similar three-step relaxation process was also observed in pure Fe2O3 as shown in Figure 4b.43,44 After pumping, the photoexcited carriers were trapped by the trapped states within 1.0 ps or



recombine with holes with time constant of 16 ps. Then, the photoexcited carriers in trapped states would further diminish (with time constant of 188 ps) through the recombination with holes, which is much faster than that in pure ZnO. Interestingly, the carrier dynamics in the Fe2O3/ZnO heterostructures is dramatically different from those in pure ZnO and Fe2O3 samples. Due to the smaller bandgap of Fe2O3 (~2.24 eV) comparing to ZnO (~3.28 eV), the photoexcited carriers are mainly generated in the Fe2O3 layer of the Fe2O3/ZnO heterostructures. This argument can be supported by the shorter relaxation time of =1.4 ps close to that ( =1.0 ps) in the pure Fe2O3 layer. Then, they would quickly relax to the conduction band and the trapped states in ZnO, which suppresses the relaxation channel from the conduction band to the trapped states in Fe2O3 (the dashed arrows in Figure 4c). Therefore, the photoexcited carriers in the ZnO layer would further recombine with holes in a time constant of =44 ps, which is almost the same with that ( =48 ps) in pure ZnO. Some of the photoexcited carriers in the Fe2O3 layer may relax within 301 ps through the recombination with holes in the hole trapped states. This indicates that the recombination processes in Fe2O3 layer of Fe2O3/ZnO heterostructures are significantly reduced to keep more holes for water splitting and enhance the PEC performance as shown in Figure 3d. Moreover, the ratio of A2 to A3 in Fe2O3/ZnO heterostructures is close to that in pure ZnO as shown in Table S-2. This means that the photoexcited carriers generated in the Fe2O3 layer indeed transfer to ZnO layer and further relax in the ZnO layer, demonstrating the intimately coupling between Fe2O3 and ZnO in the Fe2O3/ZnO heterostructure. PEC Performance under Various Bending Conditions


Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. The PEC performance of Fe2O3/ZnO/AZO/Mica as functions of (a) bending radius and (b) bending cycles. The mechanical flexibility of the Fe2O3/ZnO/AZO/Mica photoelectrode satisfies the demands of flexible catalytic and photocatalytic applications under practical operating conditions. The operational stability and mechanical durability of Fe2O3/ZnO/AZO/Mica were tested under two different bending conditions as depicted in the inset of Figure 5. The Fe2O3/ZnO heterostructure is compressively bent in the flex-in condition while it is under tensile strain in the flex-out condition. Figure 5a shows the change of photocurrent collected at 1.0 V vs. Ag/AgCl as a function of bending radius under simulated sunlight irradiation. The detailed I-V curves and I-t curves present the PEC performances of the Fe2O3/ZnO heterostructure under bending states are shown in Figure S7. It is clear that the photocurrent under the flex-out mode is excellently retained even down to R= 3.5 mm bending radius, while there is a small reduction in the flex-in mode. Moreover, it is interesting to see the durability of photocurrent change of the Fe2O3/ZnO films after being bent to 5 mm radius, as shown in Figure 5b. The Fe2O3/ZnO photoelectrode maintained its PEC performance up to 3000 bending cycles in both modes. This behavior suggests that the microstructure of the Fe2O3/ZnO/AZO/Mica and thus the PEC properties were



not altered under bending, which is attributed to high quality of oxide heteroepitaxy and flexible features of Mica substrate. Furthermore, the long-term PEC stability of the Fe2O3/ZnO photoelectrode in both the unbent and bent (with a bending radius of 5 mm) states was conducted at a constant applied potential of 0.6 V vs. Ag/AgCl for 9 hours as shown in Figure S8. It was found that the Fe2O3 layer improves the stability of ZnO photoanode. Under the same experimental conditions, the photocurrent density of ZnO decreased continuously for 1 hour of irradiation, whereas the Fe2O3/ZnO heterostructure did not show any significant decay of the photocurrent for the first 2 hours. The decline in photocurrent could be mainly assigned to the corrosion of ZnO layer. It is believed that the top Fe2O3 layer prevents the contact between ZnO layer and electrolyte, and thus resists electrochemical corrosion of the ZnO layer resulting in a superior stability of the Fe2O3/ZnO electrode in the Na2SO4 electrolyte. These results demonstrate that the Fe2O3/ZnO heterostructure photoelectrode grown on Mica substrate not only retains the superior PEC performance but also exhibits excellent mechanical flexibility. CONCLUSION In conclusion, the heteroepitaxial Fe2O3/ZnO system was fabricated on Mica substrates with high crystallinity. The Fe2O3/ZnO heterostructures exhibit a remarkable PEC activity as well as the mechanical flexibility and cyclability. The photocurrent density of the Fe2O3/ZnO heterostructures increased ~550% and 325% compared to pure ZnO and Fe2O3 layers, respectively, in the visible-light regime. The origin of the enhanced photoactivity was studied via the available evidence of PL, EIS, and the ultrafast time-resolved measurements. It was clearly demonstrated that the coupling of ZnO and Fe2O3 in the heterostructures not only can reduce the electron-hole recombination upon the charge transfer at interface but also improve the surface


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reaction kinetics. Our study demonstrates a new pathway by using oxide heteroepitaxy to create a flexible transparent PEC cell for water splitting. METHODS Sample fabrication. The Fe2O3/ZnO heteroepitaxial and AZO films were deposited on freshly cleaved Mica and sapphire substrates via pulsed laser deposition with KrF excimer laser (λ = 248 nm, Coherent) operated at laser fluence of 1 J/cm2 using commercial Fe2O3, ZnO, and AZO targets. The AZO conductive layer was deposited at a substrate temperature of 400 0C in 0.1 mTorr oxygen pressure. The ZnO and Fe2O3 layers were deposited at respective growth temperatures of 600oC and 680 oC, and oxygen pressures of 100 mTorr and 7 mTorr. Bending Tests. Predesigned Teflon molds of fixed bending radii ranging from 3.5 to 20 mm were used to induce the compressive and tensile bending states for mechanical flexibility test. Fe2O3/ZnO/Mica samples were attached to these molds using epoxy resin. To investigate the mechanical durability of the photoelectrode on Mica substrate samples, a computer-aided homebuilt bending setup was used to control the movement as well as the number of bending cycles. The samples with initial length L were compressed or stretched under an external force applied through the bending stage to change it to L − dL. The samples were bent from one side with the aid of a motor, whereas the other end was fixed. The system can set arbitrary bending radii as well as perform bending cycles. Characterization. A Bruker D8 x-ray diffractometer equipped Cu Kα1 radiation (λ=1.5406A0) was employed to examine the crystal quality and epitaxial relationship of the samples. The details of the microstructure were investigated by using spherical-aberration corrected transmission electron microscope (Cs-TEM, JEOL ARM 200F). The XPS spectra were measured on a thermo Scientific K-Alpha system equipped with a monochromatized Al Kα X-



ray source of 1486.6 eV. Optical properties were obtained at room temperature by using a Hitachi U-3900H spectrophotometer. The time-resolved PL was recorded in a customized single photon counting system. A sub-nanosecond pulsed diode laser (with λ=320 nm, PicoQuant, PLD 320) was used for excitation. The signal collected at the PL emission of ZnO (λem=380 nm) was analyzed with biexponential decay mode to generate two decay components and an intensityaverage emission lifetime. Ultrafast time-resolved photoinduced transient reflectivity change (∆R/R) spectroscopy was performed by a dual-color pump-probe system to reveal the dynamics of photoexcitation carriers in the Fe2O3/ZnO heterostructure. The light source is a commercial mode-locked Ti:sapphire pulsed laser (repetition rate: 5.2 MHz, pulse width: 70 fs, and the center wavelength: 800 nm). The pump beam was 400 nm with the photon energy of 3.1 eV and fluence of 231 µJ/cm2. The time-resolved photoinduced transient reflectivity changes was measured by a probe beam with the photon energy of 1.55 eV (800 nm) and fluence of 7.7 µJ/cm2. PEC measurements. All of the PEC measurements were performed on an Autolab PGSTAT204 electrochemical workstation using a three-electrode cell configuration, which consisted of a Pt counter electrode, an Ag/AgCl reference electrode, the working electrode with an area of 0.25 cm2, and the 0.5 M Na2SO4 electrolyte. An AM 1.5G solar simulator (Newport, LCS-100, 94011A) was used to give a one-sun irradiance of 100 mW/cm2 on the electrode surface. The linear-sweep voltammogram (I–V curves) was recorded with a scan rate of 10 mVs-1 and the amperometric analysis (I–t curves) was conducted at 0.6 V vs. Ag/ AgCl. The IPCE spectra were measured under illumination of monochromatic light from a 150 W xenon lamp coupled with a 0.2 m monochromator (Horiba, Tunable PowerArc, 1200 gr mm1 , dispersion = 4 nm). The EIS measurements were conducted under AM 1.5G illumination, and a small AC signal of 10 mV


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was applied to the cell over the frequency range from 100 kHz to 1 Hz. The Mott–Schottky analysis was performed at a fixed frequency of 1 kHz. Acknowledgement This work was supported by the Ministry of Science and Technology, Taiwan (MOST 106-2628E-009-001-MY2 and MOST 106-2119-M-009-011-MY3). Supporting Information Available: Additional data and text are described in the Supporting Information.



References (1)

Zhao, D.; Chen, C.; Zhang, Q.; Chen, W.; Liu, S.; Wang, Q.; Liu, Y.; Li, J.; Yu, H. High Performance, Flexible, Solid-State Supercapacitors Based on A Renewable and Biodegradable Mesoporous Cellulose Membrane. Adv. Energy Mater. 2017, 1700739, 1– 9.

(2)

Kumar, M. H.; Yantara, N.; Dharani, S.; Graetzel, M.; Mhaisalkar, S.; Boix, P. P.; Mathews, N. Flexible, Low-Temperature, Solution Processed ZnO-Based Perovskite Solid State Solar Cells. Chem. Commun. 2013, 49, 11089.

(3)

Yoon, J.; Baca, A. J.; Park, S.; Elvikis, P.; Geddes, J. B.; Li, L.; Kim, R. H.; Xiao, J.; Wang, S.; Kim, T. H.; Motala, M. J.; Ahn, B. Y.; Duoss, E. B.; Lewis, J. A.; Nuzzo, R. G.; Ferreira, P. M.; Huang, Y.; Rockett, A.; Rogers, J. A. Ultrathin Silicon Solar Microcells for Semitransparent, Mechanically Flexible and Microconcentrator Module Designs. Nat. Mater. 2008, 7, 907– 915.

(4)

Liu, W.; Song, M. S.; Kong, B.; Cui, Y. Flexible and Stretchable Energy Storage: Recent Advances and Future Perspectives. Adv. Mater. 2017, 29, 1603436.

(5)

Li, L.; Wu, Z.; Yuan, S.; Zhang, X. B. Advances and Challenges for Flexible Energy Storage and Conversion Devices and Systems Energy Environ. Sci 2014, 7, 2101– 2122.

(6)

Fan, F. R.; Tang, W.; Wang, Z. L. Flexible Nanogenerators for Energy Harvesting and Self-Powered Electronics. Adv. Mater. 2016, 28, 4283– 4305.

(7)

Navarro Yerga, R. M.; Consuelo Álvarez Galván, M.; Del Valle. F.; Villoria de la Mano. J. A.; Fierro. J. L. G.; Water Splitting on Semiconductor Catalysts under Visible- Light Irradiation. ChemSusChem 2009, 2, 471– 485.

(8)

Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem.


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Soc. Rev. 2009, 38, 253– 278. (9)

Marschall, R. Semiconductor Composites: Strategies for Enhancing Charge Carrier Separation to Improve Photocatalytic Activity. Adv. Funct. Mater. 2014, 24, 2421– 2440.

(10)

Ji, L.; McDaniel, M. D.; Wang, S.; Posadas, A. B.; Li, X.; Huang, H.; Lee, J. C.; Demkov, A. A.; Bard, A. J.; Ekerdt, J. G.; Yu, E. T. A Silicon-Based Photocathode for Water Reduction with An Epitaxial SrTiO3 Protection Layer and A Nanostructured Catalyst. Nat. Nanotechnol. 2014, 10, 84– 90.

(11)

Bassi, P. S.; Antony, R. P.; Boix, P. P.; Fanga, Y.; Barhera, J.; Wong, L. H. Crystalline Fe2O3/Fe2TiO5 Heterojunction Nanorods with Efficient Charge Separation and Hole Injection as Photoanode for Solar Water Oxidation. Nano Energy 2016, 22, 310- 318.

(12)

Hsu, Y. K.; Chen, Y. C.; Lin, Y. G. Novel ZnO/Fe2O3 Core−Shell Nanowires for Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2015, 7, 14157- 14162.

(13)

Li, M. Y.; Yang, Y.; Ling, Y. C.; Qiu, W. T.; Wang, F. X.; Liu, T. Y.; Song, Y.; Liu, X. X.; Fang, P. P.; Tong, Y. X.; Li, Y. Morphology and Doping Engineering of Sn-Doped Hematite Nanowire Photoanodes. Nano Letter 2017, 17, 2490- 2495.

(14)

Sivula, K.; Yu, X.; Prevot, M. S.; Guijar, N.; Sivula, K. Self-Assembled 2D WSe2 Thin Films for Photoelectrochemical Hydrogen Production. Nat. Commun. 2015, 6,7596.

(15)

Yuan, S.; Mu, J.; Mao, R.; Li, Y.; Zhang, Q.; Wang, H. All-Nanoparticle Self-assembly ZnO/TiO2 Heterojunction Thin Films with Remarkably Enhanced Photoelectrochemical Activity ACS .Appl. Mater. Interfaces 2014, 6, 5719- 5725.

(16)

Chao, J.; Wang, Z.; Xu, X.; Xiang, Q.; Song W.; Chen, G.; Hu, J.; Chen, D. Tin Sulfide Nanoribbons as High Performance Photoelectrochemical Cells, Flexible Photodetectors and Visible-Light-Driven Photocatalysts. RSC Adv. 2013, 3, 2746– 2753.



(17)

Miao, J. W.; Xiao, F. X.; Yang, H. B.; Khoo, S. Y.; Chen, J. Z.; Fan, Z. X.; Hsu, Y. Y.; Chen, H. M.; Zhang, H.; Liu, B. Hierarchical Ni-Mo-S Nanosheets on Carbon Fiber Cloth: A Flexible Electrode for Efficient Hydrogen Generation in Neutral Electrolyte. Sci. Adv. 2015, 1, e1500259.

(18)

Xu X.; Zhou G.; Dong X.; Hu J. Interface Band Engineering Charge Transfer for 3D MoS2 Photoanode to Boost Photoelectrochemical Water Splitting. ACS Sustainable Chem. Eng. 2017, 5, 3829- 3836.

(19)

Yu, H. L.; Yu, X. B.; Chen, Y. J.; Zhang, S.; Gao, P.; Li, C. Y. A Strategy to Synergistically Increase The Number of Active Edge Sites and The Conductivity of MoS2 Nanosheets for Hydrogen Evolution. Nanoscale 2015, 7, 8731– 8738.

(20)

Hou, Y.; Wen, Z.; Cui, S.; Feng, X.; Chen, J. Strongly Coupled Ternary Hybrid Aerogels of N-deficient Porous Graphitic-C3N4 Nanosheets/N-Doped Graphene/NiFe-Layered Double Hydroxide for Solar-Driven Photoelectrochemical Water Oxidation. Nano Lett. 2016, 16, 2268– 2277.

(21)

Zhang, Y.; Yan, M.; Ge, S.; Ma, C.; Yu, J.; Song, X. An Enhanced Photoelectrochemical Platform: Graphite-like Carbon Nitride Nanosheet-Functionalized ZnO Nanotubes. J. Mater. Chem. B 2016, 4, 4980– 4987.

(22)

Hou, Y.; Qiu, M.; Zhang, T.; Ma, J.; Liu, S.; Zhuang, X.; Yuan, C.; Feng, X. Effcient Electrochemical and Photoelectrochemical Water Splitting by a 3D Nanostructured Carbon Supported on Flexible Exfoliated Graphene Foil. Adv. Mater. 2017, 29, 1– 7.

(23)

Ueno K.; Saiki, K.; Shimada, T.; Koma, A. Epitaxial Growth of Transition Metal Dichalcogenides on Cleaved Faces of Mica. J. Vac. Sci. Techno. 1990, 8, 68- 72.


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24)

Barlow, S. G.; Manning, D. A. C.; Barlow , S. G.; Manning, D. A. C. Influence of Time and Temperature on Reactions and Transformations of Muscovite Mica. Br. Ceram. Trans. 1999, 98, 122- 126.

(25)

Steinberg, S.; Ducker, W.; Vigil, G.; Hyukjin, C.; Frank, C.; Tseng, M. Z.; Clarke, D. R.; Israelachvili, J. N. Van der Waals Epitaxial Growth of α-Alumina Nanocrystals on Mica. Science 1993, 260, 656−659.

(26)

Wang, K.; Liu, Y.; Wang, W.; Meyer, N.; Bao, L. H.; He, L.; Lang, M. R.; Chen, Z. G.; Che, X. Y.; Post, K.; Zou, J.; Basov, D. N.; Wang, K. L.; Xiu, F. High-Quality Bi2Te3 Thin Films Grown on Mica Substrates for Potential Optoelectronic Applications. Appl. Phys. Lett. 2013, 103, 031605.

(27)

Amrillah, T.; Bitla, Y.; Shin, K.; Yang, T.; Hsieh, Y. H.; Chiou, Y. Y.; Liu, H. J.; Do, T. H.; Su, D.; Chen, Y. C.; Jen, S. U.; Chen, L. Q.; Kim, K. H.; Juang, J. Y.; Chu, Y. H. Flexible Multiferroic Bulk Heterojunction with Giant Magnetoelectric Coupling via van der Waals Epitaxy. ACS Nano 2017, 11, 6122– 6130.

(28)

Ke, S.; Chen, C.; Fu, N.; Zhou, H.; Ye, M.; Lin, P.; Yuan, W.; Zeng, X.; Chen, L.; Huang H. Transparent Indium Tin Oxide Electrodes on Muscovite Mica for High-TemperatureProcessed Flexible Optoelectronic Devices. ACS Appl. Mater. Interfaces 2016, 8, 28406– 28411.

(29)

Khan, I.; Qurashi, A.; Berdiyorov, G.; Iqbal, N.; Fuji, K.; Yamani, Z. H. Single-step Strategy for The Fabrication of GaON/ZnO Nanoarchitectured Photoanode Their Experimental and Computational Photoelectrochemical Water Splitting. Nano Energy 2018, 44, 23- 33.



(30)

Shao, M. F.; Ning, F. Y.; Wei, M.; Evans, D. G.; Duan, X. Hierarchical Nanowire Arrays Based on ZnO Core−Layered Double Hydroxide Shell for Largely Enhanced Photoelectrochemical Water Splitting. Adv. Funct. Mater. 2014, 24, 580– 586.

(31)

Özgür, Ü.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Doǧan, S.; Avrutin, V.; Cho, S. J.; Morko̧, H. A. Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 1– 103.

(32)

Li, J. M.; Cheng, H. Y.; Chiu Y.H.; Hsu, Y. J. ZnO–Au–SnO2 Z-scheme Photoanodes for Remarkable Photoelectrochemical Water Splitting. Nanoscale 2016, 8, 15720– 15729.

(33)

Ozgur, U.; Hofstetter, D.; Morkoc, H. ZnO Devices and Applications: A Review of Current Status and Future Prospects. Proc. IEEE 2010, 98, 1255- 1268 .

(34)

Kim, D. W.; Riha, S. C.; DeMarco, E. J.; Martinson, A. B. ZF.; Farha, O. K.; Hupp, J. T. Greenlighting Photoelectrochemical Oxidation of Water by Iron Oxide. ACS Nano 2014, 8, 12199- 12207.

(35)

Shen, S.; Lindley, S. A.; Chen, X.; Zhang, J. Z. Hematite Heterostructures for Photoelectrochemical Water Splitting: Rational Materials Design and Charge Carrier Dynamics. Energy Environ. Sci. 2016, 9, 2744- 2775.

(36)

Tang, P. Y; Xie, H. B; Ros, C.; Han, L. J; Biset-Peiro, M.; He, Y. M; Kramer, W.; Rodriguez, A. P.; Saucedor, E.; Mascaros, J. R. G.; Andreu, T.; Morante R. R.; Arbiol J. Enhanced Photoelectrochemical Water Splitting of Hematite Multilayer Nanowire Photoanodes by Tuning The Surface State via Bottom-up Interfacial Engineering. Energy Environ. Sci. 2017, 10, 2117- 2123.

(37)

Steier, L.; Herraiz-Cardona, I.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Tilley, S. D.; Grätzel, M. Understanding the Role of Underlayers and Overlayers in Thin Film


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Hematite Photoanodes. Adv. Funct. Mater. 2014, 24, 7681. (38)

Van, C. N.; Do, T. H.; Chen, J. W.; Tzeng, W. Y.; Tsai, K. A.; Song, H. L.; Liu, H. J.; Lin, Y. C.; Chen, Y. C.; Wu, C. L.; Luo, C. W.; Chou, W. C.; Huang, R.; Hsu, Y. J.; Chu, Y. H. WO3 Mesocrystal-assisted Photoelectrochemical Activity of BiVO4. NPG Asia Materials 2017, 9,1- 8.

(39)

Luan. P.; Xie, M. Z.; Liu. D; Fu, X. D; Jing, L. Q. Effective Charge Separation in The Rutile TiO2 Nanorod coupled α-Fe2O3 with Exceptionally High Visible Activities, Scientific reports 2014, 4, 6180.

(40)

Barreca, D.; Carraro, G.; Gasparotto, A.; Maccato, C.; Warwick, M. E. A.; Kaunisto, K.; Sada, C.; Turner, S.; Gönüllü, Y.; Ruoko, T.P.; Borgese, L.; Bontempi, E.; Tendeloo,G. V.; Lemmetyinen, H.; Mathur, S. Fe2O3–TiO2 Nano-heterostructure Photoanodes for Highly Efficient Solar Water Oxidation. Adv. Mater. Interfaces 2015, 2, 1500313.

(41)

Barroso, M.; Pendlebury, S. R.; Cowan, A. J.; Durrant, J. R. Charge Carrier Trapping, Recombination and Transfer in Hematite (α-Fe2O3) Water Splitting Photoanodes, Chem. Sci. 2013, 4, 2724.

(42)

Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. Ultrafast Relaxation Dynamics of Charge Carriers Relaxation in ZnO Nanocrystalline Thin Film. Chem. Phys. Lett. 2004, 387, 176– 181.

(43)

Barroso, M.; Pendlebury, S.; Cowan, A.; Durrant, J. Charge Carrier Trapping, Recombination and Transfer in Hematite (α-Fe2O3) Water Splitting Photoanodes. Chem. Sci. 2013, 4, 2724.

(44)

Pendlebury, S. R.; Wang, X.; Le Formal, F.; Cornuz, M.; Kafizas, A.; Tilley, S. D.;



Grätzel, M.; Durrant, J. R. Ultrafast Charge Carrier Recombination and Trapping in Hematite Photoanodes under Applied Bias. J. Am. Chem. Soc. 2014, 136, 9854– 9857.

Table of Contents graphic:


Page 26 of 26

Flexible Heteroepitaxy Photoelectrode for Photo-electrochemical

Recommend Documents