- Email: [email protected]
TiO2 photoanode with reconstructed interfaces for efficient photoelectrocatalytic water splitting
Journal Pre-proof A ternary nanostructured ␣-Fe2 O3 /Au/TiO2 photoanode with reconstructed interfaces for efficient photoelectrocatalytic water splitting Yanming Fu, Chung-Li Dong, Wu Zhou, Ying-Rui Lu, Yu-Cheng Huang, Ya Liu, Penghui Guo, Liang Zhao, Wu-Ching Chou, Shaohua Shen
PII:
S0926-3373(19)30953-1
DOI:
https://doi.org/10.1016/j.apcatb.2019.118206
Reference:
APCATB 118206
To appear in:
Applied Catalysis B: Environmental
Received Date:
12 June 2019
Revised Date:
30 August 2019
Accepted Date:
16 September 2019
Please cite this article as: Fu Y, Dong C-Li, Zhou W, Lu Y-Rui, Huang Y-Cheng, Liu Y, Guo P, Zhao L, Chou W-Ching, Shen S, A ternary nanostructured ␣-Fe2 O3 /Au/TiO2 photoanode with reconstructed interfaces for efficient photoelectrocatalytic water splitting, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118206
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
A
ternary
nanostructured
α-Fe2O3/Au/TiO2
photoanode with reconstructed interfaces for efficient
ro
of
photoelectrocatalytic water splitting
-p
Yanming Fua, Chung-Li Dongb, Wu Zhoua, Ying-Rui Lub, Yu-Cheng Huangb,c, Ya Liua, Penghui
a
re
Guoa, Liang Zhaoa, Wu-Ching Chouc, Shaohua Shena,*
International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow
c
Department of Physics, Tamkang University, Tamsui 25137, Taiwan
ur na
b
lP
in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China
Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
Jo
*Corresponding author.
E-mail address: [email protected] (S. Shen).
1
-p
ro
of
Graphical abstract
re
Highlights
A ternary nanostructured α-Fe2O3/Au/TiO2 photoanode is fabricated.
The photocurrent density is increased by ~4-folds for photoelectrocatalytic water splitting.
A photoelectrocatalytic mechanism related to the reconstructed interfaces is proposed.
ur na
lP
Abstract: In this study, a ternary nanostructured α-Fe2O3/Au/TiO2 film with integrating a
Jo
crystalline α-Fe2O3 core, metallic Au nanoparticles (NPs), and an amorphous TiO2 overlayer is fabricated and examined as a photoanode for photoelectrocatalytic water splitting. Under simulated solar illumination, the as-prepared photoanode exhibits a four-fold increase (1.05 mA cm-2) in photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) relative to bare α-Fe2O3. Based on systematic investigations, it is proposed that Au NPs extract photoholes from
2
the bulk of α-Fe2O3 core and then shuttle them to the outer TiO2 overlayer, and meanwhile, TiO2 overlayer efficiently captures and stores the photoholes and facilitates the hole injection into electrolyte. Thus, the remarkably improved photoelectrocatalytic water splitting performance of α-Fe2O3/Au/TiO2 photoanode is attributed to the significantly suppressed bulk and surface charge recombination due to the relayed pumping of the photogenerated charge carriers through the
of
photoanode/electrolyte interfaces reconstructed by Au NPs and TiO2 overlayer.
ro
Keywords: Hematite; Photoelectrocatalytic water splitting; Au nanoparticles; TiO2 overlayer;
-p
Reconstructed interfaces
re
1. Introduction
lP
Hematite (α-Fe2O3) frequently has been promoted as one of the most promising materials for use as the photoanode in a semiconductor-based sunlight-driven photoelectrocatalytic water
ur na
splitting device, because of its favorable optical band gap (~2.1 eV) for absorption of visible light, excellent chemical stability, earth abundance, and low cost [1-6]. Although the theoretical solarto-hydrogen (STH) conversion efficiency for a α-Fe2O3 water splitting cell can reach up to 15.5%, α-Fe2O3 photoelectrodes suffer from poor majority carrier conductivity, short hole-diffusion
Jo
lengths (2-4 nm), and sluggish kinetics for the oxygen evolution reaction (OER) at the photoelectrode/electrolyte interface. In combination, these deleterious effects severely hindering the photoelectrocatalytic water splitting performance, and yielding STH conversion efficiencies still far lower than the theoretical maximum [7-16].
3
Extensive efforts have been devoted to modification of the physicochemical and structural properties of α-Fe2O3 photoelectrodes via a variety of different approaches, including nanostructuring [6,7], elemental doping [8-12], and surface modification [13-16]. In addition, constructing heterojunctions by coupling α-Fe2O3 with a second semiconductor is also an effective approach to improving the photoelectrocatalytic water splitting performance [17,18]. The heterojunctions can facilitate charge separation by introducing an electric field at the interface
of
between two semiconductors, thereby suppressing the rate of electron-hole recombination
ro
[5,17,18]. For example, α-Fe2O3/TiO2 heterojunctions formed using chemical or physical deposition processes, have been investigated as photoanodes and have exhibited improved
-p
photoelectrocatalytic activities for water splitting [19,20]. Even so, the α-Fe2O3/TiO2
re
heterojunction presents an energy barrier of ~0.6 eV for hole injection from α-Fe2O3 into TiO2, due to misalignment of the valence band edges (VBEs), which creates a barrier at the α-Fe2O3/TiO2
lP
interface for interfacial tunneling of photogenerated holes, limiting the photoelectrocatalytic water oxidation performance [21-26]. Moreover, the kinetic overpotential for oxygen evolution reaction
ur na
(OER) at the photoanode surface increases with the thickness of the electrically insulating TiO2 overlayer, due to a large series resistance, resulting in an additional loss in photoelectrocatalytic efficiency [24,27,28]. Therefore, α-Fe2O3/TiO2 heterostructures require ultrathin TiO2 overlayers (< 4 nm), deposited by either atomic layer deposition (ALD) or other methods that allow very thin
Jo
layers with precise control over the thickness [25,26]. For instance, Jeon et al. [26] found that a 3.5 nm TiO2 overlayer on pure or hydrogen-treated α-Fe2O3 nanorod (NR) arrays noticeably increased their photocurrent densities, while a thicker TiO2 overlayer significantly reduced the photoelectrocatalytic activity. Recently, the interfacial tunneling barrier and series resistance associated with TiO2 have been eliminated by creating a “leaky” amorphous TiO2 overlayer that
4
has been deposited on narrow-band-gap semiconductors (e.g., Si [27], GaAs [27], GaP [27], CdTe [28], and BiVO4 [29]) to yield stable and efficient photoanodes for photoelectrocatalytic water splitting. This thick amorphous electronically leaky TiO2 overlayer not only imparted stability against photocorrosion, but also allowed effective transport of photoholes to the electrolyte, dramatically improving the photoelectrocatalytic water oxidation performance. The mechanism of hole transport has been ascribed variously to hopping through mid-gap states, electronic defects,
of
or impurities, in addition to other possible effects [27].
ro
Integrating α-Fe2O3 photoelectrodes with noble metal (e.g., Au and Ag) has also been
-p
recognized as an alternative strategy for improving the STH conversion efficiency [30-41]. Li et al. [40] fabricated an Au/Fe2O3 photoanode where α-Fe2O3 NR arrays were assembled into
re
plasmonic Au nanohole arrays, and the photoanode achieved approximately ten-fold increase of photocurrent density relative to that of pristine α-Fe2O3 at 0.23 V versus Ag/AgCl. Plasmon-
lP
induced resonance energy transfer (PIRET) mainly accounted for the enhancement at energies below the absorption band edge of α-Fe2O3, while the extended light absorption at energies above
ur na
the band edge was attributed to the surface plasmon polariton (SPP) mode [40]. Besides, Schottky junctions formed at the interface between α-Fe2O3 and noble metals act as electron traps, prolonging the lifetime of photogenerated electron-hole pairs and thereby promoting the beneficial
Jo
collection of charge carriers [41,42]. Hung and co-workers [41] obtained pseudocubic polyhedral α-Fe2O3 films via a hydrothermal method and then deposited Au NPs on the films. The Au/αFe2O3 photoelectrode demonstrated a 60% enhancement in photocurrent density at 0.8 V versus Ag/AgCl in comparison with bare counterpart, due to both the Schottky junctions and the surface plasmon resonance (SPR) effects [41]. Recently, researchers devised a new strategy for enhancing the bulk charge separation in heterostructure photoelectrodes by adding metallic media to the
5
heterojunction interface [43,44]. Xie’s group [43] designed a ternary nanostructured TiFe2O3/Co/CoOx photoanode, and found that the metallic Co at the interface facilitated the transfer of holes from α-Fe2O3 to the CoOx cocatalyst, enhancing the photoelectrocatalytic water oxidation efficiency. Similarly, Wu et al. [44] reported improved hole transport at the interface using metallic Au NPs in a layered CdS/Au/TiO2 photoelectrode. These results suggest that introducing Au NPs,
might improve the photoelectrocatalytic water splitting activity.
of
which are the most investigated plasmonic and conductive NPs, into the α-Fe2O3/TiO2 interface
ro
In this study, we design a ternary nanostructured heterojunction photoanode (denoted as α-
-p
Fe2O3/Au/TiO2) by sputtering Au NPs onto the surface of α-Fe2O3 nanorods (NRs), and then deposit an amorphous TiO2 overlayer using pulsed laser deposition (PLD). In contrast to bare α-
re
Fe2O3 and α-Fe2O3 solely decorated with either Au NPs or TiO2 overlayer, the α-Fe2O3/Au/TiO2 photoanode exhibits remarkably boosted photoelectrocatalytic water spliting performance. The
lP
electrochemical properties and the electronic structures of the photoanodes are systematically investigated, and the results evidence that the simutaneously suppressed bulk and surface charge
ur na
recombination contributes to the significantly improved photoelectrocatalytic water spliting performance, due to the relayed pumping of photogenerated charge carriers through the photoanode/electrolyte interfaces reconstructed by Au NPs and TiO2 overlayer, where the
Jo
photogenerated holes are effectively extracted from the α-Fe2O3 bulk, shuttled by Au NPs across the α-Fe2O3/TiO2 interface, and then promptly injected by the TiO2 overlayer into the electrolyte for photoelectrocatalytic water oxidation. The ternary nanostructure design strategy of exploiting reconstructed interfaces towards efficient photoelectrocatalytic water splitting will provide important insights into the rational design of high performance nanostructured semiconductor photo(electro)catalysts for solar energy conversion.
6
2. Results and discussion Fig. 1a displays a step-wise schematic illustration of the fabrication of the ternary nanostructured α-Fe2O3/Au/TiO2, with top-down and cross-sectional scanning electron microscopy (SEM) images of the structures at each step (Fig. 1b-1g). Details of the synthetic
of
processes are provided in Experimental section in Supplementary material and Fig. S1-S4.
ro
Briefly, the steps of the synthesis are: 1) hydrothermal growth of β-FeOOH (Fig. 1b); 2) sputtering with Au NPs to yield β-FeOOH/Au (Fig. 1e); 3) conversion of β-FeOOH/Au to α-Fe2O3/Au (Fig.
-p
1f) via annealing process; and, 4) deposition of an amorphous TiO2 overlayer onto α-Fe2O3/Au to construct α-Fe2O3/Au/TiO2 (Fig. 1g). Fig. 1c and 1d respectively show pristine α-Fe2O3 and α-
re
Fe2O3/TiO2 synthesized by the same β-FeOOH annealing and TiO2 deposition processes as are
lP
used to fabricate the α-Fe2O3/Au/TiO2, which are provided for comparison purposes. The crosssectional SEM images reveal that all samples consisted of NR arrays grown with a vertical
ur na
orientation with respect to the fluorine-doped tin oxide (FTO) substrate, with variation in the diameters and morphologies, but very similar average film thicknesses of 750 nm for β-FeOOH based films and ca. 600 nm for α-Fe2O3 based samples, respectively. It is clear that the bundled NR arrays in β-FeOOH (Fig. 1b) and β-FeOOH/Au (Fig. 1e) are similar in morphology, with
Jo
average diameters of 75 nm and rough surfaces. Sputtering introduced Au NPs unevenly distribute on the surface of NRs in β-FeOOH/Au (Fig. 1e). The diameter of the NRs is reduced to about 60 nm on average and the roughness of the films also decreases during high-temperature annealing, due to the crystal-phase transformation from β-FeOOH (Fig. 1b and 1e) to α-Fe2O3 (Fig. 1c and 1f). Notably, the annealing results in spheroidal Au NPs which homogeneously distribute on the near-surface region of α-Fe2O3/Au (Fig. 1f). After deposition of TiO2, the NRs are covered by a
7
fairly uniform TiO2 overlayer, causing an apparent increase in the diameter of NRs to ~130 nm (Fig. 1d and 1g), while the surfaces of the films appear rougher than those of α-Fe2O3 and αFe2O3/Au. Transmission electron microscopy (TEM) images of α-Fe2O3, α-Fe2O3/Au, α-Fe2O3/TiO2, and α-Fe2O3/Au/TiO2 are shown in Fig. 2. All of the films are composed of NRs. The NR diameters
of
are ~60 nm for α-Fe2O3 and α-Fe2O3/Au (Fig. 2a and 2b), and increase to ~130 nm for αFe2O3/TiO2 and α-Fe2O3/Au/TiO2 (Fig. 2c and 2d) due to deposition of the conformal TiO2
ro
overlayer. Au NPs with an average size of 15 nm are preferentially located in the near-surface
-p
region for α-Fe2O3/Au (Fig. 2b), and are distributed randomly at the interface between α-Fe2O3 and TiO2 for α-Fe2O3/Au/TiO2 (Fig. 2d). Fig. 2c and 2d indicate that the TiO2 overlayer is ca. 35
re
nm thick and is uniformly porous, with abundant nanopores. Fig. 2e-2h display the high-resolution enlargements of the corresponding areas indicated in Fig. 2a-2d. The lattice-fringe spacings of
lP
0.25 nm (Fig. 2e, inset) and 0.24 nm (Fig. 2f, inset) are well indexed to the crystal planes of hematite (110) and metallic Au (111), respectively [11-13, 39], which can be further evidenced by
ur na
X-ray diffraction (XRD) patterns (Fig. S5) and X-ray photoemission spectra (XPS) analysis (Fig. S6a-6c). Detailed investigation into the interface structure confirms the amorphous structure of the TiO2 overlayer (Fig. 2g) with Ti existing in 4+ states through selected-area electron diffraction
Jo
(SAED) pattern (inset of Fig. 2g) and Ti 2p XPS spectra (Fig. S6d) [27-29], along with spheroidal Au NPs locating at the interface of crystalline α-Fe2O3 core and amorphous TiO2 overlayer (Fig. 2h). Energy-dispersive spectroscopy (EDS) mappings (Fig. 2i and 2k) reveal that Au is concentrated into spheres and enriched on the surface of the α-Fe2O3 NRs, while Ti is uniformly distributed across the entire selected area (Fig. 2j and 2k), suggesting that the Au NPs are confined
8
to the surface of individual α-Fe2O3 NRs, while the TiO2 coverage is fairly uniform along the entire NRs. Photocurrent-density versus potential (J-V) measurements were performed in 1 M NaOH solution under chopped simulated solar illumination. Fig. 3a shows that α-Fe2O3/Au and αFe2O3/TiO2 photoanodes yield much higher photocurrent densities than pristine α-Fe2O3. At the
of
thermodynamic potential for water oxidation (1.23 VRHE), the photocurrent densities relative to bare α-Fe2O3 (~0.25 mA cm-2), are increased 1.6-fold for α-Fe2O3/Au (~0.40 mA cm-2), and 2.0-
ro
fold for α-Fe2O3/TiO2 (~0.50 mA cm-2). Significantly, α-Fe2O3/Au/TiO2 photoanode yields a
-p
maximum photocurrent density of 1.05 mA cm-2 at 1.23 VRHE, which is almost 4.2 times greater than that of bare α-Fe2O3, and is even greater than the simple sum of the photocurrent densities
re
yielded by α-Fe2O3/Au and α-Fe2O3/TiO2 photoanodes. Comparison on the potential depent photocurrent density under front-side and back-side illumination (Fig. S7) further confirms that
lP
this ternary nanostructured photoanode not only promotes the charge transfer at the photoanode/electrolyte interface but also allows a more efficient charge extraction in the bulk,
ur na
especially those charges generated close to the FTO substrate. Therefore, the remarkable improvement in performance exhibited by α-Fe2O3/Au/TiO2 photoanode cannot be attributed simply to additive effects of the Au and TiO2, and instead strongly suggests that the combination
Jo
of the Au NPs and the TiO2 overlayer to boost the photoelectrocatalytic water splitting performance. Fig. 3b shows the wavelength-dependent incident photon-to-current conversion efficiencies (IPCEs) for the various photoanode structures. Of the samples, the α-Fe2O3/Au/TiO2 photoanode achieves the largest IPCE value of 26% at 340 nm, which is larger than the sum of the IPCEs for the other two hybrid films and further supports the combination effects. All IPCE
9
profiles show a similar trend and approach zero at about 600 nm, which corresponds to the band gap of α-Fe2O3 (2.1 eV) [37,40]. The photostability of the photoanodes was assessed by measuring the photocurrent density at a constant potential (1.23 VRHE) and under simulated solar illumination over 12 hours (Fig. 3c). All of the photoanodes display excellent photostability, with photocurrent densities remaining at
of
nearly 100% of their initial values over the 12 h test. The quantities of O2 and H2 produced by the α-Fe2O3/Au/TiO2 photoanode and counter electrode were measured using gas chromatography
ro
during a 12 h electrolysis under conditions identical to those of the photostability test (Table S1).
-p
As shown in Fig. 3d, the gases are evolved at a constant rate over the course of the experiment; the rates of H2 and O2 evolution are calculated as 18.67 and 9.24 μmol cm-2 h-1, respectively,
re
yielding the stoichiometric 2:1 ratio expected for the real water splitting reaction. The Faradaic efficiencies for H2 and O2 evolution are ~93.5% and ~91.6%, and the slightly smaller Faradaic
lP
efficiency for O2 evolution is due to the relatively greater dissolution of O2 into the electrolyte [45]. Thus, nearly all photogenerated charge carriers are consumed to drive the water splitting reactions.
ur na
Besides, there are no obvious difference in the morphology and chemical composition in αFe2O3/Au/TiO2 after the photoelectrocatalytic measurement (see Fig. S8-S10), further confirming the excellent chemical stabilization of the photoanode. In combination, all of these results
Jo
demonstrate that α-Fe2O3/Au/TiO2 photoanode is highly active and stable for use in photoelectrocatalytic water splitting. To investigate the roles of Au NPs and TiO2 overlayer, spectroscopy measurements were
conducted. The UV-visible absorbance spectra are similar for all films (Fig. S11), eliminating the increased optical absorption as a possible reason for the improved photoelectrocatalytic water splitting performance. Besides, the negligible enhancement of absorption in the visible range
10
agrees well with the inset of the IPCE profiles in Fig. 3b, and suggests that the SPR effects of Au NPs are absent [37-41]. This conclusion is further confirmed by the identical-shape of the normalized IPCE profiles for the films with and without Au NPs (Fig. S12) [37,38,40], and this is also consistent with the result obtained by Xiao et al. for Au-implanted α-Fe2O3 photoanodes [46]. Unexpectedly, the intensity of the photoluminescence (PL) responses are decreased in the α-Fe2O3 films coated with TiO2, revealing that the surface passivation effect should also be ruled out (for
of
details see Fig. S13) [47,48]. To better uncover the reasons of the photoelectrocatalytic water
ro
splitting performance improvements with Au NPs and TiO2 overlayer, electrochemical techniques
-p
were implemented for further characterization.
Mott-Schottky (M-S) measurements (Fig. 4a) was performed in the dark at 1 kHz frequency
re
and the calculated flat-band potentials (Vfb) and charge carrier densities (Nd) are shown in Table S2. The dimensions of the space-charge layer (SCL) at applied potential of 1.0 VRHE are calculated
lP
based on the M-S results (Table S3). The width of SCL (WSCL) and the potential-barrier height (HSCL) at the electrode/electrolyte interface are both greater for α-Fe2O3/Au than for bare α-Fe2O3,
ur na
indicating the expanded SCL and the increased band bending [49,50]. Since photogenerated charge carriers are more effectively separated within the SCL, with band bending providing the driving force for the separation [49-51]. It is proposed that the Au NPs enlarge the volume ratio of the
Jo
SCL and increase the band bending to promote charge separation in the bulk. However, the electrode/electrolyte interfaces of samples with TiO2 overlayer are quite different from those without TiO2, thus the interfacial properties of them require further investigation. The electrochemically active surface area (ECSA) are estimated based on the double-layer capacitance (Cdl, Fig. S14) for the photoanodes [52]. As shown in Fig. 4b, Cdl values of films with TiO2 overlayer are more than two times greater than those of films without TiO2, meaning the
11
ECSAs are increased about 2-fold after TiO2 coating (the values are 3.20, 10.90, 4.33, and 12.38 cm2 for α-Fe2O3, α-Fe2O3/TiO2, α-Fe2O3/Au, and α-Fe2O3/Au/TiO2, respectively, calculation details see Fig. S14). These results indicate that the TiO2 overlayer is ion-permeable, therefore facilitates the diffusion of OH- ions from electrolyte and effectively enlarge the contact area between electrolyte and photoanodes [53,54]. Furthermore, the anodic transient photocurrent spikes originating from the accumulation of surface holes at the photoanode/electrolyte interface
of
are significantly reduced with TiO2 coating (Fig. S15), suggesting the efficient hole injection into
ro
electrolyte on TiO2 surface for catalyzing water oxidation reaction [25,43,49]. Since TiO2 is not an electrochemical co-catalyst for the OER, these combined results confirm the increased density
-p
of accessible active sites for water oxidation in TiO2 overlayer [53,55,56].
re
In order to gain more electrochemical insight into the charge transfer and separation processes, electrochemical impedance spectra (EIS), charge separation efficiency (ηsep), and charge injection
lP
efficiency (ηinj) were collected under simulated solar illumination. The Nyquist plots are fitted using two different proposed equivalent circuits (insets of Fig. 4c and 4d) based on the structure
ur na
of the photoanodes (see discussion in Fig. S16) [53-58], and the calculated parameters values are summarized in Table S4 and Table S5. The much larger space-charge layer capacitance (Cbulk) of α-Fe2O3/Au than that of bare α-Fe2O3 indicates the increased dimension of the SCL and this is consistent with the results determined from the M-S analysis (Fig. 4a). Besides, α-Fe2O3/Au
Jo
exhibits smaller bulk charge trapping resistance (Rtrap) and larger ηsep than those of pristine α-Fe2O3 (Fig. 4e), suggesting that Au NPs significantly improve the charge separation in the bulk and facilitate the migration of photoholes to the surface [48,51,53]. The photoanode/electrolyte interface capacitance (Css) is remarkably greater in α-Fe2O3/TiO2 than that of bare α-Fe2O3, which reveals that TiO2 behaves as a charge storage layer to efficiently capture and store holes from α-
12
Fe2O3 [58,59]. The charge transfer resistance across the photoanode/electrolyte interface (Rct,trap) of α-Fe2O3/TiO2 is pronouncedly smaller and the ηinj are significantly greater relative to pristine α-Fe2O3 (Fig. 4f), meaning the promoted hole injection into the electrolyte for catalyzing water oxidation reaction [53,54,56]. Compared to bare α-Fe2O3, the Rtrap and Rct,trap are both reduced dramatically in α-Fe2O3/Au/TiO2, indicating that the charge transfer processes in the bulk and across the photoanode/electrolyte interface are simultaneously promoted [56,58]. This is further
of
confirmed by the both highest ηsep and ηinj achieved by α-Fe2O3/Au/TiO2 over the whole potential
ro
range (Fig. 4e and 4f). Hence, the primary role of the Au NPs is to extract photoholes from the αFe2O3 bulk and shuttle them across the α-Fe2O3/TiO2 interface, while the TiO2 overlayer facilitates
-p
the hole injection into the electrolyte for catalyzing water oxidation reaction, working together to
re
simultaneously suppress electron-hole recombination losses in the bulk and on the surface of αFe2O3.
lP
To further elucidate the the origin of the enhancement and the underlying mechanisms determining the photoelectrocatalytic water splitting performances, deep understandings in
ur na
particular at the interfacial region are highly desired. X-ray absorption spectroscopy (XAS) was performed to investigate the electronic structure of all films. Fig. 5a shows the XAS at the Fe Ledge for all of the films, both in the dark and under solar illumination. No difference is evident for bare α-Fe2O3, suggesting nearly no photoinduced electrons occupy the empty Fe 3d states due to
Jo
the rapid electron-hole recombination. The intensity of the Fe L-edge is slightly decreased in αFe2O3/Au under illumination, implying that the Fe 3d states gain some charges and facilitate electron transfer. Under illumination, the intensity of the Fe L-edge also decreases in α-Fe2O3/TiO2, and the difference in the spectra with and without illumination is larger for α-Fe2O3/TiO2 than for α-Fe2O3/Au, indicating that the TiO2 overlayer improves charge transfer and the Fe 3d states in α-
13
Fe2O3/TiO2 gain even more electrons than those in α-Fe2O3/Au. Finally, the spectral difference is greatest when Au and TiO2 are both present, consistent with the most enhanced electron transfer in α-Fe2O3/Au/TiO2. As shown in Fig. 5b, no differences are observed for the Ti L-edge of both α-Fe2O3/TiO2 and α-Fe2O3/Au/TiO2 in the dark and under illumination, suggesting that the Ti is not active under solar illumination. Fig. 5c displays the O K-edge of all the films in the dark. The two pre-peaks (located at 530 and 531.5 eV, P1 and P2) in α-Fe2O3 and α-Fe2O3/Au are ascribed
of
to the hybridized Fe 3d-O 2p states, while the pre-peaks at higher energy (531 and 533.4 eV, P3
ro
and P4) in α-Fe2O3/TiO2 originate from the hybridized Ti 3d-O 2p states. Notably, there are three peaks (P5, P6, and P7) presented in α-Fe2O3/Au/TiO2 (Fig. 5c). Peaks P5 has similar energy
-p
position in comparison with peak P1 (in α-Fe2O3/Au) and Peak P7 has similar energy position with
re
peak P4 (in α-Fe2O3/TiO2). Besides, the energy position of peak P6 locates at the energy range between peak P2 (in α-Fe2O3/Au) and (in α-Fe2O3/TiO2). Above analytical results indicate that
lP
embedded Au NPs facilitates the cation interdiffusion and therefore assists the formation of the interface, which is also confirmed by O K-edge in later session. All of these results reveal that
ur na
decoration of α-Fe2O3 with Au and TiO2 substantially modified the electronic properties of α-Fe2O3. An enlargement of the pre-peak region in the O K-edge spectra upon illumination (Fig. 5d) is observed only for samples with TiO2, suggesting that the holes transfer to TiO2 upon solar illumination. The largest spectral difference is again observed in α-Fe2O3/Au/TiO2 for the O K-
Jo
edge spectra with and without illumination, which confirms the most efficient hole transfer from α-Fe2O3 to TiO2, together with the results for the Fe L-edge in Fig. 5a. Ti L-edge spectra (Fig. 5e) presents that the intensity of the peak located at high energy (~461 eV), originating from 3deg (3dx2-y2, 3dz2) states and associated with local atomic symmetry, is greater for α-Fe2O3/Au/TiO2 than for α-Fe2O3/TiO2. The change in the intensity of this peak may
14
be attributable to changes in empty hole states or atomic symmetry. To investigate the atomic symmetry, the spectra are further deconvoluted into three components (peaks A, B and C in the inset of Fig. 5e). The solid red (blue) represents the α-Fe2O3/Au/TiO2 (α-Fe2O3/TiO2) and the green circles indicate the fitted data. Peaks B and C correspond to 3dz2 and 3dx2-y2 states which are related to the electron density along the apical Ti-O bond and basal plane, respectively. If the atomic symmetry is distorted, the electron density is modulated and the relative intensity of the 3dz2 and
of
3dx2-y2 states varies. The ratios of peak B to peak C for α-Fe2O3/Au/TiO2 and α-Fe2O3/TiO2 are
ro
estimated from the fitted data to be 1.619 and 1.620, respectively. These almost identical ratios suggest that the local atomic structure is nearly unaffected by the embedded Au NPs, and the
-p
difference in intensity of the peak at 461 eV is attributable to a change in unoccupied electronic
re
states.
A comparison of the Ti L-edge spectra (Fig. 5e) with the Fe L-edge spectra (Fig. 5f) indicates
lP
that addition of Au NPs to the structure redistributes electron density between Fe and Ti sites, consistent with Au NPs being beneficial for electron transfer from TiO2 to α-Fe2O3. To validate
ur na
our interpretation of the electronic effects at the interface, the O K-edge spectra of αFe2O3/Au/TiO2 are compared with a fitted spectrum calculated from a linear combination of the spectra of α-Fe2O3/Au and α-Fe2O3/TiO2 (Fig. 5g). The overall spectral profile of α-Fe2O3/Au/TiO2 is similar to the fitted spectrum, but with subtle deviation, e.g. the actual peak intensity is larger
Jo
than the fit. If there were no interface formed between α-Fe2O3 and the TiO2 overlayer, the spectra for α-Fe2O3/Au/TiO2 would be nearly identical to the fit. Thus, the difference in the peak intensity between the actual and fitted spectra may reflect the importance of Au embedded between α-Fe2O3 and TiO2, and may also support that the embedded Au NPs induce unoccupied hole states in the interfacial region, allowing efficient charge transport.
15
Based on the above systematical investigations into both the electrochemical properties and the electronic structure evolution, Fig. 6 illustrates the mechanisms yielding the remarkable improvement in photoelectrocatalytic water splitting performance for the ternary nanostructured photoanode. Under illumination, photogenerated electrons and holes are excited into the conduction band (CB) and valence band (VB) of α-Fe2O3, respectively. Bare α-Fe2O3 suffers from a high rate of electron-hole recombination in the bulk and on the surface (Fig. 6a), which greatly
of
limits the photoelectrocatalytic water splitting efficiency [6]. For α-Fe2O3/Au, the intimate contact
ro
between the Au NPs and α-Fe2O3 enlarges the SCL and increases band bending, promoting charge separation in the bulk of α-Fe2O3 (Fig. 6b). Addition of the TiO2 overlayer coated on α-Fe2O3
-p
increases the density of accessible active sites and facilitates the injection of photoholes into the
re
electrolyte for photoelectrocatalytic water oxidation, due to the porous and ion-permeable scaffolds of the TiO2 overlayer (Fig. 6c). Ultimately, the enhanced photoelectrocatalytic water performance
of
α-Fe2O3/Au/TiO2
photoanode
can
be
ascribed
to
the
lP
splitting
photoanode/electrolyte interfaces reconstructed by Au NPs and TiO2 overlayer (Fig. 6d). When
ur na
Au NPs are fixed to the surface of α-Fe2O3, the intimate contact between the Au NPs and α-Fe2O3 makes Au NPs to act as hole pumps that extract photogenerated holes from the bulk of α-Fe2O3, meanwhile as hole shuttles that facilitate migration of the holes across the reconstructed interfaces. The holes are then efficiently captured and stored by the TiO2 overlayer which serves as a charge-
Jo
storage layer that promotes the injection of photoholes into the electrolyte for catalyzing water oxidation reaction. Therefore, with the help of these reconstructed interfaces, the composite αFe2O3/Au/TiO2 photoanodes realize the reduced charge carrier recombination rates at the surface as well as in the bulk, resulting in the remarkably improved photoelectrocatalytic water splitting performance. In particular, the photocurrent density is significantly increased by 4.1 folds in α-
16
Fe2O3/Au/TiO2, which is superior to most α-Fe2O3-based composite photoanodes previously reported (as shown in Table 1) and means that the reconstructed interfaces strategy will pave the way toward the design of high-performance semiconductor photoelectrodes for efficient solar energy conversion.
of
3. Conclusions
ro
In summary, ternary nanostructured α-Fe2O3 photoanodes for photoelectrocatalytic water splitting have been successfully fabricated by coating α-Fe2O3 NR arrays with metallic Au NPs
-p
and a porous TiO2 overlayer. In contrast to bare α-Fe2O3, α-Fe2O3/Au, and α-Fe2O3/TiO2, the α-
re
Fe2O3/Au/TiO2 photoanode yields a maximum photocurrent density of 1.05 mA cm-2 at 1.23 VRHE, achieves an IPCE of 26% at 340 nm, amongst the highest values reported to date for α-Fe2O3, and
lP
exhibits excellent photostability. Systematic studies suggest that the significantly improved photoelectrocatalytic water splitting performance is attributed to the simultaneously reduced bulk
ur na
and surface recombination caused by the relayed pumping of charge carriers through the photoanode/electrolyte interfaces reconstructed by Au NPs and TiO2 overlayer, where the photogenerated holes are effectively extracted from the α-Fe2O3 bulk, shuttled by Au NPs across the α-Fe2O3/TiO2 interface, and then promptly injected by the TiO2 overlayer into the electrolyte
Jo
for catalyzing water oxidation reaction. Thus, the strategy of exploiting reconstructed interfaces to simultaneously reduce bulk and surface recombination has important implications for the design of high-performance semiconductor photoelectrodes for efficient solar energy conversion.
Acknowledgments 17
This work was supported by the National Natural Science Foundation of China (No. 21875183, 51672210 and 51888103), the National Program for Support of Top-notch Young Professionals, and the Fundamental Research Funds for the Central Universities. C.L.D is grateful to MoST for financially supporting this work under contracts MoST 107-2112-M-032-004-MY3
of
and 108-2218-E-032-003-MY3.
Appendix A. Supplementary data
-p
online version, at https://doi.org/10.1016/j.apcatb.2019.xx.xxx.
ro
Supplementary material related to this article can be found, in the online version, at in the
re
References
K. Sivula, R. van de Krol, Nat. Rev. Mater. 1 (2016) 15010.
[2]
X. Wang, F. Wang, Y. Sang, H. Liu, Adv. Energy Mater. 7 (2017) 1700473-1700488.
[3]
H. Zhang, J. Ming, J. Zhao, Q. Gu, C. Xu, Z. Ding, R. Yuan, Z. Zhang, H. Lin, X. Wang, J.
lP
[1]
ur na
Long, Angew. Chem. Int. Ed. 58 (2019) 7718-7722. [4]
C. Jiang, S.J.A. Moniz, A. Wang, T. Zhang, J. Tang, Chem. Soc. Rev. 46 (2017) 4645-4660.
[5]
S. Kment, F. Riboni, S. Pausova, L. Wang, L. Wang, H. Han, Z. Hubicka, J. Krysa, P.
Jo
Schmuki, R. Zboril, Chem. Soc. Rev. 46 (2017) 3716-3769. [6]
C. Li, Z. Luo, T. Wang, J. Gong, Adv. Mater. 30 (2018) 1707502.
[7]
P. Zhang, L. Gao, X. Song, J. Sun, Adv. Mater. 27 (2015) 562-568.
[8]
Y. Zhang, S. Jiang, W. Song, P. Zhou, H. Ji, W. Ma, W. Hao, C. Chen, J. Zhao, Energy Environ. Sci. 8 (2015) 1231-1236.
18
[9]
S. Shen, J. Jiang, P. Guo, C.X. Kronawitter, S.S. Mao, L. Guo, Nano Energy 1 (2012) 732741.
[10] X. Wang, W. Gao, Z. Zhao, L. Zhao, J.P. Claverie, X. Zhang, J. Wang, H. Liu, Y. Sang, Appl. Catal. B Environ. 248 (2019) 388-393. [11] Y. Fu, C.-L. Dong, Z. Zhou, W.-Y. Lee, J. Chen, P. Guo, L. Zhao, S. Shen, Phys. Chem. Chem. Phys. 18 (2016) 3846-3853.
of
[12] Y. Fu, C.-L. Dong, W.-Y. Lee, J. Chen, P. Guo, L. Zhao, S. Shen, ChemNanoMat 2 (2016)
ro
704-711.
https://doi.org/10.1016/j.scib.2019.07.008
-p
[13] L. Mao, Y.-C. Huang, Y. Fu, C.-L. Dong, S. Shen, Science Bulletin (2019),
re
[14] I. Roger, M. Shipman, M. Symes, Nat. Rev. Chem. 1 (2017) 0003. [15] M.R. Nellist, F.A. Laskowski, F. Lin, T.J. Mills, S.W. Boettcher, Acc. Chem. Res. 49 (2016)
lP
733-740.
[16] R. Liu, Z. Zheng, J. Spurgeon, X.G. Yang, Energy Environ. Sci. 7 (2014) 2504-2517.
ur na
[17] S. Shen, S.A. Lindley, X. Chen, J.Z. Zhang, Energy Environ. Sci. 9 (2016) 2744-2775. [18] M.T. Mayer, Y. Lin, G. Yuan. D. Wang, Acc. Chem. Res. 46 (2013) 1558-1566. [19] M. Wang, M. Pyeon, Y. Gönüllü, A. Kaouk, S. Shen, L. Guo, S. Mathur, Nanoscale 7 (2015) 10094-10100.
Jo
[20] D. Barreca, G. Carraro, A. Gasparotto, C. Maccato, M.E.A. Warwick, K. Kaunisto, C. Sada, S. Turner, Y. Gönüllü, T.-P.
Ruoko, L. Borgese, E. Bontempi, G.V. Tendeloo, H.
Lemmetyinen, S. Mathur, Adv. Mater. Interfaces 2 (2015) 1500313.
[21] A.G. Scheuermann, J.D. Prange, M. Gunji, C.E.D. Chidsey, P.C. McIntyre, Energy Environ. Sci. 6 (2013) 2487-2496.
19
[22] R. Yang, Y. Ji, Q. Li, Z. Zhao, R. Zhang, L. Liang, F. Liu, Y. Chen, S. Han, X. Yu, H. Liu, Applied Catalysis B: Environmental 256 (2019) 117798. [23] H. Zhang, L. Ma, J. Ming, B. Liu, Y. Zhao, Y. Hou, Z. Ding, C. Xu, Z. Zhang, J. Long, Appl. Catal. B Environ. 243 (2019) 481-489. [24] L. Ji, H.-Y. Hsu, X. Li, K. Huang, Y. Zhang, J.C. Lee, A.J. Bard, E.T. Yu, Nat. Mater. 16 (2017) 127-131.
of
[25] X. Yang, R. Liu, C. Du, P. Dai, Z. Zheng, D. Wang, ACS Appl. Mater. Interfaces 6 (2014)
ro
12005-12011.
[26] T.H. Jeon, G.-h. Moon, H. Park, W. Choi, Nano Energy 39 (2017) 211-218.
-p
[27] S. Hu, M.R. Shaner, J.A. Beardslee, M. Lichterman, B.S. Brunschwig, N.S. Lewis, Science
re
344 (2014) 1005-1009.
[28] M.F. Lichterman, A.I. Carim, M.T. McDowell, S. Hu, H.B. Gray, B.S. Brunschwig, N.S.
lP
Lewis, Energy Environ. Sci. 7 (2014) 3334-3337.
[29] D. Eisenberg, H.S. Ahn, A.J. Bard, J. Am. Chem. Soc. 136 (2014) 14011-14014.
ur na
[30] P. Zhang, T. Wang, J. Gong, Adv. Mater. 27 (2015) 5328-5342. [31] J.B. Lee, S. Choi, J. Kim, Y.S. Nam, Nano Today 16 (2017) 61-81. [32] D. Wang, W. Wang, M.P. Knudson, G.C. Schatz, T.W. Odom, Chem. Rev. 118 (2017) 28652881.
Jo
[33] J. Zhao, B. Liu, L. Meng, S. He, R. Yuan, Y. Hou, Z. Ding, H. Lin, Z. Zhang, X. Wang, Long, Appl. Catal. B Environ. 256 (2019) 117823.
[34] L. Meng, Z. Chen, Z. Ma, S. He, Y. Hou, H. Li, R. Yuan, X. Huang, X. Wang, X. Wang, Energy Environ. Sci. 11 (2018) 294-298.
20
[35] J. Long, H. Chang, Q. Gu, J. Xu, L. Fan, S. Wang, Y. Zhou, W. Wei, L. Huang, X. Wang, P. Liu, W. Huang, Energy Environ. Sci. 7 (2014) 973-977. [36] T. Jiang, C. Jia, L. Zhang, S. He, Y. Sang, H. Li, Y. Li, X. Xu, H. Liu, Nanoscale 7 (2015) 209-217. [37] E. Thimsen, F. Le Formal, M. Grätzel, S.C. Warren, Nano Lett. 11 (2011) 35-43. [38] B. Kong, J. Tao, C. Selomulya, W. Li, J. Wei, Y. Fang, Y. Wang, G. Zheng, D. Zhao, J. Am.
of
Chem. Soc. 136 (2014) 6822-6825.
ro
[39] L. Wang, H. Hu, N.T. Nguyen, Y. Zhang, P. Schmuki, Y. Bi, Nano Energy 35 (2017) 171178.
-p
[40] J. Li, S.K. Cushing, P. Zheng, F. Meng, D. Chu, N. Wu, Nat. Commun. 4 (2013) 2651.
re
[41] W-H. Hung, C.-J. Peng, C.-R. Yang, C.-J. Li, J.-J. Shyue, P.-C. Chang, C.-M. Tseng, P.-C. Juan, Nano Energy 30 (2016) 523-530.
lP
[42] S. Bai, J. Jiang, Q. Zhang, Y. Xiong, Chem. Soc. Rev. 44 (2015) 2893-2939. [43] S. Li, Q. Zhao, D. Meng, D. Wang, T. Xie, J. Mater. Chem. A 4 (2016) 16661-16669.
ur na
[44] J. Li, S.K. Cushing, P. Zheng, T. Senty, F. Meng, A.D. Bristow, A. Manivannan, N. Wu, J. Am. Chem. Soc. 136 (2014) 8438-8449. [45] J.Y. Kim, J.-W. Jang, D.H. Youn, G. Magesh, J.S. Lee, Adv. Energy Mater. 4 (2014) 14004761.
Jo
[46] D. He, X. Song, Z. Ke, X. Xiao, C. Jiang, Sci. China Mate. 61 (2018) 878-886. [47] D. Cao, W. Luo, J. Feng, X. Zhao, Z. Li, Z. Zou, Energy Environ. Sci. 7 (2014) 752-759. [48] Y.-F. Xu, X.-D. Wang, H.-Y. Chen, D.-B. Kuang, C.-Y. Su, Adv. Funct. Mater. 26 (2016) 4414-4421.
21
[49] D.W. Kim, S.C. Riha, E.J. DeMarco, A.B.F. Martinson, O.K. Farha, J.T. Hupp, ACS Nano 8 (2014) 12199-12207. [50] L. Steier, J. Luo, M. Schreier, M.T. Mayer, T. Sajavaara, M. Grätzel, ACS Nano 9 (2015) 11775-11783. [51] C. Li, A. Li, Z. Luo, J. Zhang, X. Chang, Z. Huang, T. Wang, J. Gong, Angew. Chem. Int. Ed. 56 (2017) 4150-4155.
of
[52] C.C.L. McCrory, S. Jung, I.M. Ferrer, S.M. Chatman, J.C. Peters, T.F. Jaramillo, J. Am.
ro
Chem. Soc. 137 (2015) 4347-4357.
[53] Z. Fan, Z. Xu, S. Yan, Z. Zou, J. Mater. Chem. A 5 (2017) 8402-8407.
-p
[54] Z. Xu, H. Wang, Y. Wen, W. Li, C. Sun, Y. He, Z. Shi, L. Pei, Y. Chen, S. Yan, Z. Zou,
re
ACS Appl. Mater. Interfaces 10 (2018) 3624-3633.
[55] Y.-J. Dong, J.-F. Liao, Z.-C. Kong, Y.-F. Xu, Z.-J. Chen, H.-Y. Chen, D.-B. Kuang, D.
lP
Fenske, C.-Y. Su, Appl. Catal. B: Environ. 237 (2018) 9-17. [56] H.-J. Ahn, K.-Y. Yoon, M.-J. Kwak, J.-H. Jang, Angew. Chem. Int. Ed. 55 (2016) 9922-
ur na
9926.
[57] B. Klahr, S. Gimenez, F. Fabregat-Santiago, T. Hamann, J. Bisquert, J. Am. Chem. Soc. 134 (2012) 4294-4302.
[58] B. Klahr, S. Gimenez, F. Fabregat-Santiago, J. Bisquert, T.W. Hamann, J. Am. Chem. Soc.
Jo
134 (2012) 16693-16700.
[59] G.M. Carroll, D.K. Zhong, D.R. Gamelin, Energy Environ. Sci. 8 (2015) 577-584. [60] J. Deng, X. Lv, K. Nie, X. Lv, X. Sun, J. Zhong, ACS Catal. 7 (2017) 4062-4069. [61] H. Zhang, Y.K. Kim, H.Y. Jeong, J.S. Lee, ACS Catalysis 9 (2019) 1289-1297. [62] S.-S. Yi, B.-R. Wulan, J.-M. Yan, Q. Jiang, Adv. Funct. Mater. 29 (2019) 1801902.
22
[63] F. Le Formal, N. Tétreault, M. Cornuz, T. Moehl, M. Grätzel, K. Sivula, Chem. Sci. 2 (2011) 737-743. [64] L. Steier, I. Herraiz-Cardona, S. Gimenez, F. Fabregat-Santiago, J. Bisquert, S.D. Tilley, M. Grätzel, Adv. Funct. Mater. 24 (2014) 7681-7688. [65] S.C. Riha, B.M. Klahr, E.C. Tyo, S. Seifert, S. Vajda, M.J. Pellin, T.W. Hamann, A.B.F. Martinson, ACS Nano 7 (2013) 2396-2405.
of
[66] W. Li, S.W. Sheehan, D. He, Y. He, X. Yao, R.L. Grimm, G.W. Brudvig, D. Wang, Angew.
ro
Chem. Int. Ed. 54 (2015) 11428-11432.
[67] J. Zhang, R. García-Rodríguez, P. Cameron, S. Eslava, Energy Environ. Sci. 11 (2018)
-p
2972-2984.
re
[68] L. Wang, N.T. Nguyen, X. Huang, P. Schmuki, Y. Bi, Adv. Funct. Mater. 27 (2017) 1703527.
1805737.
lP
[69] H. Zhang, W.Y. Noh, F. Li, J.H. Kim, H.Y. Jeong, J.S. Lee, Adv. Funct. Mater. 29 (2019)
ur na
[70] P. Peerakiatkhajohn, J.-H. Yun, H. Chen, M. Lyu, T. Butburee, L. Wang, Adv. Mater. 28 (2016) 6405-6410.
Figure and Table Citation Page
Jo
Fig. 1. (a) Step-wise schematic of the synthesis of α-Fe2O3/Au/TiO2. SEM images of (b) β-FeOOH, (c) α-Fe2O3, (d) α-Fe2O3/TiO2, (e) β-FeOOH/Au, (f) α-Fe2O3/Au, and (g) α-Fe2O3/Au/TiO2. Insets are the cross-sectional SEM images. Fig. 2. TEM images of (a,e) α-Fe2O3; (b,f) α-Fe2O3/Au; (c,g) α-Fe2O3/TiO2; and (d,h) αFe2O3/Au/TiO2 with EDS mappings of (i) α-Fe2O3/Au, (j) α-Fe2O3/TiO2, and (k) α-Fe2O3/Au/TiO2. (e)-(h) are the high-resolution images of corresponding areas in (a)-(d) with further enlarged images of α-Fe2O3 and Au NP, inset in (g) is the SAED pattern of the TiO2 overlayer.
23
Fig. 3. (a) J-V curves of the photoanodes. (b) IPCE plots measured at 1.23 VRHE, the inset shows an enlargement for the wavelength range from 580 to 700 nm. (c) Chronoamperometry for illuminated samples over a period of 12 h at 1.23 VRHE. (d) Amounts of evolved gases and the calculated Faradic efficiencies using α-Fe2O3/Au/TiO2 photoanode during the photostability test. All tests are measured under front-side illumination (from the photoanode side).
of
Fig. 4. (a) M-S plots of the films. (b) Capacitive current density at 1.23 VRHE as a function of scan rate. Nyquist plots of (c) α-Fe2O3 and α-Fe2O3/Au, (d) α-Fe2O3/TiO2 and α-Fe2O3/Au/TiO2; the insets are the corresponding equivalent circuits used to fit the plots. (e) ηsep and (f) ηinj of all the photoanodes, which are calculated by adding Na2SO3 as a hole scavenger into the electrolyte (further details are given in Fig. S17).
re
-p
ro
Fig. 5. XAS spectra of (a) Fe L-edge, (b) Ti L-edge, and (c), (d) O K-edge; right part of (a) and left part of (d) are the corresponding enlarged regions marked by dashed boxes. (e) Ti L-edge and (f) Fe L-edge in the dark. Peaks in (e) are deconvoluted into three components (peaks A, B and C); the solid lines in the inset represent the tested data and the circles indicate the fitting spectra. (g) O K-edge of α-Fe2O3/Au/TiO2 (red solid line) and a fitted spectrum (open solid circle) calculated from a linear combination from the spectra of α-Fe2O3/Au and α-Fe2O3/TiO2.
lP
Fig. 6. Proposed charge transfer mechanisms of different α-Fe2O3 photoanodes for photoelectrocatalytic water splitting; (a) α-Fe2O3, (b) α-Fe2O3/Au, (c) α-Fe2O3/TiO2, and (d) αFe2O3/Au/TiO2. The SCL region are marked with dash line and labeled as W.
Jo
ur na
Table 1. Comparisons of photoanodes in this study with other α-Fe2O3-based composite films reported previously. Unless otherwise specified, all photoelectrocatalytic tests were carried out under simulated 1 sun (AM 1.5G, 100 mW cm-2) illumination and all biases were converted to RHE(VRHE).
24
of ro -p re
Jo
ur na
lP
Fig. 1. (a) Step-wise schematic of the synthesis of α-Fe2O3/Au/TiO2. SEM images of (b) β-FeOOH, (c) α-Fe2O3, (d) α-Fe2O3/TiO2, (e) β-FeOOH/Au, (f) α-Fe2O3/Au, and (g) α-Fe2O3/Au/TiO2. Insets are the cross-sectional SEM images.
25
of ro -p re
Jo
ur na
lP
Fig. 2. TEM images of (a,e) α-Fe2O3; (b,f) α-Fe2O3/Au; (c,g) α-Fe2O3/TiO2; and (d,h) αFe2O3/Au/TiO2 with EDS mappings of (i) α-Fe2O3/Au, (j) α-Fe2O3/TiO2, and (k) α-Fe2O3/Au/TiO2. (e)-(h) are the high-resolution images of corresponding areas in (a)-(d) with further enlarged images of α-Fe2O3 and Au NP, inset in (g) is the SAED pattern of the TiO2 overlayer.
26
of ro -p
Jo
ur na
lP
re
Fig. 3. (a) J-V curves of the photoanodes. (b) IPCE plots measured at 1.23 VRHE, the inset shows an enlargement for the wavelength range from 580 to 700 nm. (c) Chronoamperometry for illuminated samples over a period of 12 h at 1.23 VRHE. (d) Amounts of evolved gases and the calculated Faradic efficiencies using α-Fe2O3/Au/TiO2 photoanode during the photostability test. All tests are measured under front-side illumination (from the photoanode side).
27
of ro -p re lP ur na
Jo
Fig. 4. (a) M-S plots of the films. (b) Capacitive current density at 1.23 VRHE as a function of scan rate. Nyquist plots of (c) α-Fe2O3 and α-Fe2O3/Au, (d) α-Fe2O3/TiO2 and α-Fe2O3/Au/TiO2; the insets are the corresponding equivalent circuits used to fit the plots. (e) ηsep and (f) ηinj of all the photoanodes, which are calculated by adding Na2SO3 as a hole scavenger into the electrolyte (further details are given in Fig. S17).
28
of ro -p re lP
Jo
ur na
Fig. 5. XAS spectra of (a) Fe L-edge, (b) Ti L-edge, and (c), (d) O K-edge; right part of (a) and left part of (d) are the corresponding enlarged regions marked by dashed boxes. (e) Ti L-edge and (f) Fe L-edge in the dark. Peaks in (e) are deconvoluted into three components (peaks A, B and C); the solid lines in the inset represent the tested data and the circles indicate the fitting spectra. (g) O K-edge of α-Fe2O3/Au/TiO2 (red solid line) and a fitted spectrum (open solid circle) calculated from a linear combination from the spectra of α-Fe2O3/Au and α-Fe2O3/TiO2.
29
of ro
Jo
ur na
lP
re
-p
Fig. 6. Proposed charge transfer mechanisms of different α-Fe2O3 photoanodes for photoelectrocatalytic water splitting; (a) α-Fe2O3, (b) α-Fe2O3/Au, (c) α-Fe2O3/TiO2, and (d) αFe2O3/Au/TiO2. The SCL region are marked with dash line and labeled as W.
30
Table 1. Comparisons of photoanodes in this study with other α-Fe2O3-based composite films reported previously. Unless otherwise specified, all photoelectrocatalytic tests were carried out under simulated 1 sun (AM 1.5G, 100 mW cm-2) illumination and all biases were converted to RHE(VRHE). Photocurrent density (mA cm-2) Photoanodes Test Mechanism Ref. condition Bias Pristine Treated Fold 1 M NaOH
1.23
0.25
1.05
4.12
Reconstructed interfaces
This work
α-Fe2O3/Au
1 M NaOH
1.4
1.25
2.00
1.60
SPR effect
[37]
α-Fe2O3/Au
1 M NaOH
1.7
0.85
1.90
2.22
Fe2O3/Fe2TiO5
1 M NaOH
1.23
0.87
1.61
1.85
Fe2O3/FeNbO4
1 M NaOH
1.23
1.24
2.24
Fe2O3/Co3O4
1 M NaOH
1.23
1.20
2.00
Fe2O3/TiO2
1 M NaOH
1.40
0.77
1.01
Fe2O3/TiO2
1 M NaOH
1.43
3.10
3.00
Fe2O3/Al2O3
1 M NaOH
1.43
3.10
Fe2O3/SiOx
1 M NaOH
1.23
1.23
Fe2O3/Ga2O3
1 M NaOH
1.23
Fe2O3/Ni(OH)2
1 M NaOH
1.53
Fe2O3/CoOx Fe2O3/Ir-WOC
Heterojunction
[60]
ro
[41]
[61]
1.67
Heterojunction
[62]
1.31
Passivation
[25]
0.97
Passivation
[63]
3.20
1.03
Passivation
[63]
1.75
1.42
Passivation
[56]
0.65
0.60
0.92
Passivation
[64]
0.55
0.70
1.27
Co-catalyst
[50]
lP
re
-p
Heterojunction
0.1 M KPi
1.60
0.80
1.51
1.88
Co-catalyst
[59]
0.1 M NaOH
1.53
1.40
2.10
1.50
Co-catalyst
[65]
0.1 M KNO3
1.40
0.75
1.10
1.47
Co-catalyst
[66]
1 M NaOH
1.23
0.88
1.20
1.36
Co-catalyst
[67]
Jo
Fe2O3/CoFeOx
SPR effect
1.81
ur na
Fe2O3/Co-Pi
of
α-Fe2O3/Au/TiO2
Fe2TiO5/Fe2O3/Pt
1 M KOH
1.23
0.50
1.00
2.00
Synergistic effects
[68]
Fe2O3/FeTaO4
1 M NaOH
1.23
1.17
2.37
2.02
Synergistic effects
[69]
Fe2O3/CIO
1 M KOH
1.23
0.92
2.49
2.71
Combined effects
[48]
Fe2O3/Ag/Co-Pi
1 M NaOH
1.23
1.50
4.68
3.12
Combined effects
[70]
31
PII:
S0926-3373(19)30953-1
DOI:
https://doi.org/10.1016/j.apcatb.2019.118206
Reference:
APCATB 118206
To appear in:
Applied Catalysis B: Environmental
Received Date:
12 June 2019
Revised Date:
30 August 2019
Accepted Date:
16 September 2019
Please cite this article as: Fu Y, Dong C-Li, Zhou W, Lu Y-Rui, Huang Y-Cheng, Liu Y, Guo P, Zhao L, Chou W-Ching, Shen S, A ternary nanostructured ␣-Fe2 O3 /Au/TiO2 photoanode with reconstructed interfaces for efficient photoelectrocatalytic water splitting, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118206
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
A
ternary
nanostructured
α-Fe2O3/Au/TiO2
photoanode with reconstructed interfaces for efficient
ro
of
photoelectrocatalytic water splitting
-p
Yanming Fua, Chung-Li Dongb, Wu Zhoua, Ying-Rui Lub, Yu-Cheng Huangb,c, Ya Liua, Penghui
a
re
Guoa, Liang Zhaoa, Wu-Ching Chouc, Shaohua Shena,*
International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow
c
Department of Physics, Tamkang University, Tamsui 25137, Taiwan
ur na
b
lP
in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China
Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
Jo
*Corresponding author.
E-mail address: [email protected] (S. Shen).
1
-p
ro
of
Graphical abstract
re
Highlights
A ternary nanostructured α-Fe2O3/Au/TiO2 photoanode is fabricated.
The photocurrent density is increased by ~4-folds for photoelectrocatalytic water splitting.
A photoelectrocatalytic mechanism related to the reconstructed interfaces is proposed.
ur na
lP
Abstract: In this study, a ternary nanostructured α-Fe2O3/Au/TiO2 film with integrating a
Jo
crystalline α-Fe2O3 core, metallic Au nanoparticles (NPs), and an amorphous TiO2 overlayer is fabricated and examined as a photoanode for photoelectrocatalytic water splitting. Under simulated solar illumination, the as-prepared photoanode exhibits a four-fold increase (1.05 mA cm-2) in photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) relative to bare α-Fe2O3. Based on systematic investigations, it is proposed that Au NPs extract photoholes from
2
the bulk of α-Fe2O3 core and then shuttle them to the outer TiO2 overlayer, and meanwhile, TiO2 overlayer efficiently captures and stores the photoholes and facilitates the hole injection into electrolyte. Thus, the remarkably improved photoelectrocatalytic water splitting performance of α-Fe2O3/Au/TiO2 photoanode is attributed to the significantly suppressed bulk and surface charge recombination due to the relayed pumping of the photogenerated charge carriers through the
of
photoanode/electrolyte interfaces reconstructed by Au NPs and TiO2 overlayer.
ro
Keywords: Hematite; Photoelectrocatalytic water splitting; Au nanoparticles; TiO2 overlayer;
-p
Reconstructed interfaces
re
1. Introduction
lP
Hematite (α-Fe2O3) frequently has been promoted as one of the most promising materials for use as the photoanode in a semiconductor-based sunlight-driven photoelectrocatalytic water
ur na
splitting device, because of its favorable optical band gap (~2.1 eV) for absorption of visible light, excellent chemical stability, earth abundance, and low cost [1-6]. Although the theoretical solarto-hydrogen (STH) conversion efficiency for a α-Fe2O3 water splitting cell can reach up to 15.5%, α-Fe2O3 photoelectrodes suffer from poor majority carrier conductivity, short hole-diffusion
Jo
lengths (2-4 nm), and sluggish kinetics for the oxygen evolution reaction (OER) at the photoelectrode/electrolyte interface. In combination, these deleterious effects severely hindering the photoelectrocatalytic water splitting performance, and yielding STH conversion efficiencies still far lower than the theoretical maximum [7-16].
3
Extensive efforts have been devoted to modification of the physicochemical and structural properties of α-Fe2O3 photoelectrodes via a variety of different approaches, including nanostructuring [6,7], elemental doping [8-12], and surface modification [13-16]. In addition, constructing heterojunctions by coupling α-Fe2O3 with a second semiconductor is also an effective approach to improving the photoelectrocatalytic water splitting performance [17,18]. The heterojunctions can facilitate charge separation by introducing an electric field at the interface
of
between two semiconductors, thereby suppressing the rate of electron-hole recombination
ro
[5,17,18]. For example, α-Fe2O3/TiO2 heterojunctions formed using chemical or physical deposition processes, have been investigated as photoanodes and have exhibited improved
-p
photoelectrocatalytic activities for water splitting [19,20]. Even so, the α-Fe2O3/TiO2
re
heterojunction presents an energy barrier of ~0.6 eV for hole injection from α-Fe2O3 into TiO2, due to misalignment of the valence band edges (VBEs), which creates a barrier at the α-Fe2O3/TiO2
lP
interface for interfacial tunneling of photogenerated holes, limiting the photoelectrocatalytic water oxidation performance [21-26]. Moreover, the kinetic overpotential for oxygen evolution reaction
ur na
(OER) at the photoanode surface increases with the thickness of the electrically insulating TiO2 overlayer, due to a large series resistance, resulting in an additional loss in photoelectrocatalytic efficiency [24,27,28]. Therefore, α-Fe2O3/TiO2 heterostructures require ultrathin TiO2 overlayers (< 4 nm), deposited by either atomic layer deposition (ALD) or other methods that allow very thin
Jo
layers with precise control over the thickness [25,26]. For instance, Jeon et al. [26] found that a 3.5 nm TiO2 overlayer on pure or hydrogen-treated α-Fe2O3 nanorod (NR) arrays noticeably increased their photocurrent densities, while a thicker TiO2 overlayer significantly reduced the photoelectrocatalytic activity. Recently, the interfacial tunneling barrier and series resistance associated with TiO2 have been eliminated by creating a “leaky” amorphous TiO2 overlayer that
4
has been deposited on narrow-band-gap semiconductors (e.g., Si [27], GaAs [27], GaP [27], CdTe [28], and BiVO4 [29]) to yield stable and efficient photoanodes for photoelectrocatalytic water splitting. This thick amorphous electronically leaky TiO2 overlayer not only imparted stability against photocorrosion, but also allowed effective transport of photoholes to the electrolyte, dramatically improving the photoelectrocatalytic water oxidation performance. The mechanism of hole transport has been ascribed variously to hopping through mid-gap states, electronic defects,
of
or impurities, in addition to other possible effects [27].
ro
Integrating α-Fe2O3 photoelectrodes with noble metal (e.g., Au and Ag) has also been
-p
recognized as an alternative strategy for improving the STH conversion efficiency [30-41]. Li et al. [40] fabricated an Au/Fe2O3 photoanode where α-Fe2O3 NR arrays were assembled into
re
plasmonic Au nanohole arrays, and the photoanode achieved approximately ten-fold increase of photocurrent density relative to that of pristine α-Fe2O3 at 0.23 V versus Ag/AgCl. Plasmon-
lP
induced resonance energy transfer (PIRET) mainly accounted for the enhancement at energies below the absorption band edge of α-Fe2O3, while the extended light absorption at energies above
ur na
the band edge was attributed to the surface plasmon polariton (SPP) mode [40]. Besides, Schottky junctions formed at the interface between α-Fe2O3 and noble metals act as electron traps, prolonging the lifetime of photogenerated electron-hole pairs and thereby promoting the beneficial
Jo
collection of charge carriers [41,42]. Hung and co-workers [41] obtained pseudocubic polyhedral α-Fe2O3 films via a hydrothermal method and then deposited Au NPs on the films. The Au/αFe2O3 photoelectrode demonstrated a 60% enhancement in photocurrent density at 0.8 V versus Ag/AgCl in comparison with bare counterpart, due to both the Schottky junctions and the surface plasmon resonance (SPR) effects [41]. Recently, researchers devised a new strategy for enhancing the bulk charge separation in heterostructure photoelectrodes by adding metallic media to the
5
heterojunction interface [43,44]. Xie’s group [43] designed a ternary nanostructured TiFe2O3/Co/CoOx photoanode, and found that the metallic Co at the interface facilitated the transfer of holes from α-Fe2O3 to the CoOx cocatalyst, enhancing the photoelectrocatalytic water oxidation efficiency. Similarly, Wu et al. [44] reported improved hole transport at the interface using metallic Au NPs in a layered CdS/Au/TiO2 photoelectrode. These results suggest that introducing Au NPs,
might improve the photoelectrocatalytic water splitting activity.
of
which are the most investigated plasmonic and conductive NPs, into the α-Fe2O3/TiO2 interface
ro
In this study, we design a ternary nanostructured heterojunction photoanode (denoted as α-
-p
Fe2O3/Au/TiO2) by sputtering Au NPs onto the surface of α-Fe2O3 nanorods (NRs), and then deposit an amorphous TiO2 overlayer using pulsed laser deposition (PLD). In contrast to bare α-
re
Fe2O3 and α-Fe2O3 solely decorated with either Au NPs or TiO2 overlayer, the α-Fe2O3/Au/TiO2 photoanode exhibits remarkably boosted photoelectrocatalytic water spliting performance. The
lP
electrochemical properties and the electronic structures of the photoanodes are systematically investigated, and the results evidence that the simutaneously suppressed bulk and surface charge
ur na
recombination contributes to the significantly improved photoelectrocatalytic water spliting performance, due to the relayed pumping of photogenerated charge carriers through the photoanode/electrolyte interfaces reconstructed by Au NPs and TiO2 overlayer, where the
Jo
photogenerated holes are effectively extracted from the α-Fe2O3 bulk, shuttled by Au NPs across the α-Fe2O3/TiO2 interface, and then promptly injected by the TiO2 overlayer into the electrolyte for photoelectrocatalytic water oxidation. The ternary nanostructure design strategy of exploiting reconstructed interfaces towards efficient photoelectrocatalytic water splitting will provide important insights into the rational design of high performance nanostructured semiconductor photo(electro)catalysts for solar energy conversion.
6
2. Results and discussion Fig. 1a displays a step-wise schematic illustration of the fabrication of the ternary nanostructured α-Fe2O3/Au/TiO2, with top-down and cross-sectional scanning electron microscopy (SEM) images of the structures at each step (Fig. 1b-1g). Details of the synthetic
of
processes are provided in Experimental section in Supplementary material and Fig. S1-S4.
ro
Briefly, the steps of the synthesis are: 1) hydrothermal growth of β-FeOOH (Fig. 1b); 2) sputtering with Au NPs to yield β-FeOOH/Au (Fig. 1e); 3) conversion of β-FeOOH/Au to α-Fe2O3/Au (Fig.
-p
1f) via annealing process; and, 4) deposition of an amorphous TiO2 overlayer onto α-Fe2O3/Au to construct α-Fe2O3/Au/TiO2 (Fig. 1g). Fig. 1c and 1d respectively show pristine α-Fe2O3 and α-
re
Fe2O3/TiO2 synthesized by the same β-FeOOH annealing and TiO2 deposition processes as are
lP
used to fabricate the α-Fe2O3/Au/TiO2, which are provided for comparison purposes. The crosssectional SEM images reveal that all samples consisted of NR arrays grown with a vertical
ur na
orientation with respect to the fluorine-doped tin oxide (FTO) substrate, with variation in the diameters and morphologies, but very similar average film thicknesses of 750 nm for β-FeOOH based films and ca. 600 nm for α-Fe2O3 based samples, respectively. It is clear that the bundled NR arrays in β-FeOOH (Fig. 1b) and β-FeOOH/Au (Fig. 1e) are similar in morphology, with
Jo
average diameters of 75 nm and rough surfaces. Sputtering introduced Au NPs unevenly distribute on the surface of NRs in β-FeOOH/Au (Fig. 1e). The diameter of the NRs is reduced to about 60 nm on average and the roughness of the films also decreases during high-temperature annealing, due to the crystal-phase transformation from β-FeOOH (Fig. 1b and 1e) to α-Fe2O3 (Fig. 1c and 1f). Notably, the annealing results in spheroidal Au NPs which homogeneously distribute on the near-surface region of α-Fe2O3/Au (Fig. 1f). After deposition of TiO2, the NRs are covered by a
7
fairly uniform TiO2 overlayer, causing an apparent increase in the diameter of NRs to ~130 nm (Fig. 1d and 1g), while the surfaces of the films appear rougher than those of α-Fe2O3 and αFe2O3/Au. Transmission electron microscopy (TEM) images of α-Fe2O3, α-Fe2O3/Au, α-Fe2O3/TiO2, and α-Fe2O3/Au/TiO2 are shown in Fig. 2. All of the films are composed of NRs. The NR diameters
of
are ~60 nm for α-Fe2O3 and α-Fe2O3/Au (Fig. 2a and 2b), and increase to ~130 nm for αFe2O3/TiO2 and α-Fe2O3/Au/TiO2 (Fig. 2c and 2d) due to deposition of the conformal TiO2
ro
overlayer. Au NPs with an average size of 15 nm are preferentially located in the near-surface
-p
region for α-Fe2O3/Au (Fig. 2b), and are distributed randomly at the interface between α-Fe2O3 and TiO2 for α-Fe2O3/Au/TiO2 (Fig. 2d). Fig. 2c and 2d indicate that the TiO2 overlayer is ca. 35
re
nm thick and is uniformly porous, with abundant nanopores. Fig. 2e-2h display the high-resolution enlargements of the corresponding areas indicated in Fig. 2a-2d. The lattice-fringe spacings of
lP
0.25 nm (Fig. 2e, inset) and 0.24 nm (Fig. 2f, inset) are well indexed to the crystal planes of hematite (110) and metallic Au (111), respectively [11-13, 39], which can be further evidenced by
ur na
X-ray diffraction (XRD) patterns (Fig. S5) and X-ray photoemission spectra (XPS) analysis (Fig. S6a-6c). Detailed investigation into the interface structure confirms the amorphous structure of the TiO2 overlayer (Fig. 2g) with Ti existing in 4+ states through selected-area electron diffraction
Jo
(SAED) pattern (inset of Fig. 2g) and Ti 2p XPS spectra (Fig. S6d) [27-29], along with spheroidal Au NPs locating at the interface of crystalline α-Fe2O3 core and amorphous TiO2 overlayer (Fig. 2h). Energy-dispersive spectroscopy (EDS) mappings (Fig. 2i and 2k) reveal that Au is concentrated into spheres and enriched on the surface of the α-Fe2O3 NRs, while Ti is uniformly distributed across the entire selected area (Fig. 2j and 2k), suggesting that the Au NPs are confined
8
to the surface of individual α-Fe2O3 NRs, while the TiO2 coverage is fairly uniform along the entire NRs. Photocurrent-density versus potential (J-V) measurements were performed in 1 M NaOH solution under chopped simulated solar illumination. Fig. 3a shows that α-Fe2O3/Au and αFe2O3/TiO2 photoanodes yield much higher photocurrent densities than pristine α-Fe2O3. At the
of
thermodynamic potential for water oxidation (1.23 VRHE), the photocurrent densities relative to bare α-Fe2O3 (~0.25 mA cm-2), are increased 1.6-fold for α-Fe2O3/Au (~0.40 mA cm-2), and 2.0-
ro
fold for α-Fe2O3/TiO2 (~0.50 mA cm-2). Significantly, α-Fe2O3/Au/TiO2 photoanode yields a
-p
maximum photocurrent density of 1.05 mA cm-2 at 1.23 VRHE, which is almost 4.2 times greater than that of bare α-Fe2O3, and is even greater than the simple sum of the photocurrent densities
re
yielded by α-Fe2O3/Au and α-Fe2O3/TiO2 photoanodes. Comparison on the potential depent photocurrent density under front-side and back-side illumination (Fig. S7) further confirms that
lP
this ternary nanostructured photoanode not only promotes the charge transfer at the photoanode/electrolyte interface but also allows a more efficient charge extraction in the bulk,
ur na
especially those charges generated close to the FTO substrate. Therefore, the remarkable improvement in performance exhibited by α-Fe2O3/Au/TiO2 photoanode cannot be attributed simply to additive effects of the Au and TiO2, and instead strongly suggests that the combination
Jo
of the Au NPs and the TiO2 overlayer to boost the photoelectrocatalytic water splitting performance. Fig. 3b shows the wavelength-dependent incident photon-to-current conversion efficiencies (IPCEs) for the various photoanode structures. Of the samples, the α-Fe2O3/Au/TiO2 photoanode achieves the largest IPCE value of 26% at 340 nm, which is larger than the sum of the IPCEs for the other two hybrid films and further supports the combination effects. All IPCE
9
profiles show a similar trend and approach zero at about 600 nm, which corresponds to the band gap of α-Fe2O3 (2.1 eV) [37,40]. The photostability of the photoanodes was assessed by measuring the photocurrent density at a constant potential (1.23 VRHE) and under simulated solar illumination over 12 hours (Fig. 3c). All of the photoanodes display excellent photostability, with photocurrent densities remaining at
of
nearly 100% of their initial values over the 12 h test. The quantities of O2 and H2 produced by the α-Fe2O3/Au/TiO2 photoanode and counter electrode were measured using gas chromatography
ro
during a 12 h electrolysis under conditions identical to those of the photostability test (Table S1).
-p
As shown in Fig. 3d, the gases are evolved at a constant rate over the course of the experiment; the rates of H2 and O2 evolution are calculated as 18.67 and 9.24 μmol cm-2 h-1, respectively,
re
yielding the stoichiometric 2:1 ratio expected for the real water splitting reaction. The Faradaic efficiencies for H2 and O2 evolution are ~93.5% and ~91.6%, and the slightly smaller Faradaic
lP
efficiency for O2 evolution is due to the relatively greater dissolution of O2 into the electrolyte [45]. Thus, nearly all photogenerated charge carriers are consumed to drive the water splitting reactions.
ur na
Besides, there are no obvious difference in the morphology and chemical composition in αFe2O3/Au/TiO2 after the photoelectrocatalytic measurement (see Fig. S8-S10), further confirming the excellent chemical stabilization of the photoanode. In combination, all of these results
Jo
demonstrate that α-Fe2O3/Au/TiO2 photoanode is highly active and stable for use in photoelectrocatalytic water splitting. To investigate the roles of Au NPs and TiO2 overlayer, spectroscopy measurements were
conducted. The UV-visible absorbance spectra are similar for all films (Fig. S11), eliminating the increased optical absorption as a possible reason for the improved photoelectrocatalytic water splitting performance. Besides, the negligible enhancement of absorption in the visible range
10
agrees well with the inset of the IPCE profiles in Fig. 3b, and suggests that the SPR effects of Au NPs are absent [37-41]. This conclusion is further confirmed by the identical-shape of the normalized IPCE profiles for the films with and without Au NPs (Fig. S12) [37,38,40], and this is also consistent with the result obtained by Xiao et al. for Au-implanted α-Fe2O3 photoanodes [46]. Unexpectedly, the intensity of the photoluminescence (PL) responses are decreased in the α-Fe2O3 films coated with TiO2, revealing that the surface passivation effect should also be ruled out (for
of
details see Fig. S13) [47,48]. To better uncover the reasons of the photoelectrocatalytic water
ro
splitting performance improvements with Au NPs and TiO2 overlayer, electrochemical techniques
-p
were implemented for further characterization.
Mott-Schottky (M-S) measurements (Fig. 4a) was performed in the dark at 1 kHz frequency
re
and the calculated flat-band potentials (Vfb) and charge carrier densities (Nd) are shown in Table S2. The dimensions of the space-charge layer (SCL) at applied potential of 1.0 VRHE are calculated
lP
based on the M-S results (Table S3). The width of SCL (WSCL) and the potential-barrier height (HSCL) at the electrode/electrolyte interface are both greater for α-Fe2O3/Au than for bare α-Fe2O3,
ur na
indicating the expanded SCL and the increased band bending [49,50]. Since photogenerated charge carriers are more effectively separated within the SCL, with band bending providing the driving force for the separation [49-51]. It is proposed that the Au NPs enlarge the volume ratio of the
Jo
SCL and increase the band bending to promote charge separation in the bulk. However, the electrode/electrolyte interfaces of samples with TiO2 overlayer are quite different from those without TiO2, thus the interfacial properties of them require further investigation. The electrochemically active surface area (ECSA) are estimated based on the double-layer capacitance (Cdl, Fig. S14) for the photoanodes [52]. As shown in Fig. 4b, Cdl values of films with TiO2 overlayer are more than two times greater than those of films without TiO2, meaning the
11
ECSAs are increased about 2-fold after TiO2 coating (the values are 3.20, 10.90, 4.33, and 12.38 cm2 for α-Fe2O3, α-Fe2O3/TiO2, α-Fe2O3/Au, and α-Fe2O3/Au/TiO2, respectively, calculation details see Fig. S14). These results indicate that the TiO2 overlayer is ion-permeable, therefore facilitates the diffusion of OH- ions from electrolyte and effectively enlarge the contact area between electrolyte and photoanodes [53,54]. Furthermore, the anodic transient photocurrent spikes originating from the accumulation of surface holes at the photoanode/electrolyte interface
of
are significantly reduced with TiO2 coating (Fig. S15), suggesting the efficient hole injection into
ro
electrolyte on TiO2 surface for catalyzing water oxidation reaction [25,43,49]. Since TiO2 is not an electrochemical co-catalyst for the OER, these combined results confirm the increased density
-p
of accessible active sites for water oxidation in TiO2 overlayer [53,55,56].
re
In order to gain more electrochemical insight into the charge transfer and separation processes, electrochemical impedance spectra (EIS), charge separation efficiency (ηsep), and charge injection
lP
efficiency (ηinj) were collected under simulated solar illumination. The Nyquist plots are fitted using two different proposed equivalent circuits (insets of Fig. 4c and 4d) based on the structure
ur na
of the photoanodes (see discussion in Fig. S16) [53-58], and the calculated parameters values are summarized in Table S4 and Table S5. The much larger space-charge layer capacitance (Cbulk) of α-Fe2O3/Au than that of bare α-Fe2O3 indicates the increased dimension of the SCL and this is consistent with the results determined from the M-S analysis (Fig. 4a). Besides, α-Fe2O3/Au
Jo
exhibits smaller bulk charge trapping resistance (Rtrap) and larger ηsep than those of pristine α-Fe2O3 (Fig. 4e), suggesting that Au NPs significantly improve the charge separation in the bulk and facilitate the migration of photoholes to the surface [48,51,53]. The photoanode/electrolyte interface capacitance (Css) is remarkably greater in α-Fe2O3/TiO2 than that of bare α-Fe2O3, which reveals that TiO2 behaves as a charge storage layer to efficiently capture and store holes from α-
12
Fe2O3 [58,59]. The charge transfer resistance across the photoanode/electrolyte interface (Rct,trap) of α-Fe2O3/TiO2 is pronouncedly smaller and the ηinj are significantly greater relative to pristine α-Fe2O3 (Fig. 4f), meaning the promoted hole injection into the electrolyte for catalyzing water oxidation reaction [53,54,56]. Compared to bare α-Fe2O3, the Rtrap and Rct,trap are both reduced dramatically in α-Fe2O3/Au/TiO2, indicating that the charge transfer processes in the bulk and across the photoanode/electrolyte interface are simultaneously promoted [56,58]. This is further
of
confirmed by the both highest ηsep and ηinj achieved by α-Fe2O3/Au/TiO2 over the whole potential
ro
range (Fig. 4e and 4f). Hence, the primary role of the Au NPs is to extract photoholes from the αFe2O3 bulk and shuttle them across the α-Fe2O3/TiO2 interface, while the TiO2 overlayer facilitates
-p
the hole injection into the electrolyte for catalyzing water oxidation reaction, working together to
re
simultaneously suppress electron-hole recombination losses in the bulk and on the surface of αFe2O3.
lP
To further elucidate the the origin of the enhancement and the underlying mechanisms determining the photoelectrocatalytic water splitting performances, deep understandings in
ur na
particular at the interfacial region are highly desired. X-ray absorption spectroscopy (XAS) was performed to investigate the electronic structure of all films. Fig. 5a shows the XAS at the Fe Ledge for all of the films, both in the dark and under solar illumination. No difference is evident for bare α-Fe2O3, suggesting nearly no photoinduced electrons occupy the empty Fe 3d states due to
Jo
the rapid electron-hole recombination. The intensity of the Fe L-edge is slightly decreased in αFe2O3/Au under illumination, implying that the Fe 3d states gain some charges and facilitate electron transfer. Under illumination, the intensity of the Fe L-edge also decreases in α-Fe2O3/TiO2, and the difference in the spectra with and without illumination is larger for α-Fe2O3/TiO2 than for α-Fe2O3/Au, indicating that the TiO2 overlayer improves charge transfer and the Fe 3d states in α-
13
Fe2O3/TiO2 gain even more electrons than those in α-Fe2O3/Au. Finally, the spectral difference is greatest when Au and TiO2 are both present, consistent with the most enhanced electron transfer in α-Fe2O3/Au/TiO2. As shown in Fig. 5b, no differences are observed for the Ti L-edge of both α-Fe2O3/TiO2 and α-Fe2O3/Au/TiO2 in the dark and under illumination, suggesting that the Ti is not active under solar illumination. Fig. 5c displays the O K-edge of all the films in the dark. The two pre-peaks (located at 530 and 531.5 eV, P1 and P2) in α-Fe2O3 and α-Fe2O3/Au are ascribed
of
to the hybridized Fe 3d-O 2p states, while the pre-peaks at higher energy (531 and 533.4 eV, P3
ro
and P4) in α-Fe2O3/TiO2 originate from the hybridized Ti 3d-O 2p states. Notably, there are three peaks (P5, P6, and P7) presented in α-Fe2O3/Au/TiO2 (Fig. 5c). Peaks P5 has similar energy
-p
position in comparison with peak P1 (in α-Fe2O3/Au) and Peak P7 has similar energy position with
re
peak P4 (in α-Fe2O3/TiO2). Besides, the energy position of peak P6 locates at the energy range between peak P2 (in α-Fe2O3/Au) and (in α-Fe2O3/TiO2). Above analytical results indicate that
lP
embedded Au NPs facilitates the cation interdiffusion and therefore assists the formation of the interface, which is also confirmed by O K-edge in later session. All of these results reveal that
ur na
decoration of α-Fe2O3 with Au and TiO2 substantially modified the electronic properties of α-Fe2O3. An enlargement of the pre-peak region in the O K-edge spectra upon illumination (Fig. 5d) is observed only for samples with TiO2, suggesting that the holes transfer to TiO2 upon solar illumination. The largest spectral difference is again observed in α-Fe2O3/Au/TiO2 for the O K-
Jo
edge spectra with and without illumination, which confirms the most efficient hole transfer from α-Fe2O3 to TiO2, together with the results for the Fe L-edge in Fig. 5a. Ti L-edge spectra (Fig. 5e) presents that the intensity of the peak located at high energy (~461 eV), originating from 3deg (3dx2-y2, 3dz2) states and associated with local atomic symmetry, is greater for α-Fe2O3/Au/TiO2 than for α-Fe2O3/TiO2. The change in the intensity of this peak may
14
be attributable to changes in empty hole states or atomic symmetry. To investigate the atomic symmetry, the spectra are further deconvoluted into three components (peaks A, B and C in the inset of Fig. 5e). The solid red (blue) represents the α-Fe2O3/Au/TiO2 (α-Fe2O3/TiO2) and the green circles indicate the fitted data. Peaks B and C correspond to 3dz2 and 3dx2-y2 states which are related to the electron density along the apical Ti-O bond and basal plane, respectively. If the atomic symmetry is distorted, the electron density is modulated and the relative intensity of the 3dz2 and
of
3dx2-y2 states varies. The ratios of peak B to peak C for α-Fe2O3/Au/TiO2 and α-Fe2O3/TiO2 are
ro
estimated from the fitted data to be 1.619 and 1.620, respectively. These almost identical ratios suggest that the local atomic structure is nearly unaffected by the embedded Au NPs, and the
-p
difference in intensity of the peak at 461 eV is attributable to a change in unoccupied electronic
re
states.
A comparison of the Ti L-edge spectra (Fig. 5e) with the Fe L-edge spectra (Fig. 5f) indicates
lP
that addition of Au NPs to the structure redistributes electron density between Fe and Ti sites, consistent with Au NPs being beneficial for electron transfer from TiO2 to α-Fe2O3. To validate
ur na
our interpretation of the electronic effects at the interface, the O K-edge spectra of αFe2O3/Au/TiO2 are compared with a fitted spectrum calculated from a linear combination of the spectra of α-Fe2O3/Au and α-Fe2O3/TiO2 (Fig. 5g). The overall spectral profile of α-Fe2O3/Au/TiO2 is similar to the fitted spectrum, but with subtle deviation, e.g. the actual peak intensity is larger
Jo
than the fit. If there were no interface formed between α-Fe2O3 and the TiO2 overlayer, the spectra for α-Fe2O3/Au/TiO2 would be nearly identical to the fit. Thus, the difference in the peak intensity between the actual and fitted spectra may reflect the importance of Au embedded between α-Fe2O3 and TiO2, and may also support that the embedded Au NPs induce unoccupied hole states in the interfacial region, allowing efficient charge transport.
15
Based on the above systematical investigations into both the electrochemical properties and the electronic structure evolution, Fig. 6 illustrates the mechanisms yielding the remarkable improvement in photoelectrocatalytic water splitting performance for the ternary nanostructured photoanode. Under illumination, photogenerated electrons and holes are excited into the conduction band (CB) and valence band (VB) of α-Fe2O3, respectively. Bare α-Fe2O3 suffers from a high rate of electron-hole recombination in the bulk and on the surface (Fig. 6a), which greatly
of
limits the photoelectrocatalytic water splitting efficiency [6]. For α-Fe2O3/Au, the intimate contact
ro
between the Au NPs and α-Fe2O3 enlarges the SCL and increases band bending, promoting charge separation in the bulk of α-Fe2O3 (Fig. 6b). Addition of the TiO2 overlayer coated on α-Fe2O3
-p
increases the density of accessible active sites and facilitates the injection of photoholes into the
re
electrolyte for photoelectrocatalytic water oxidation, due to the porous and ion-permeable scaffolds of the TiO2 overlayer (Fig. 6c). Ultimately, the enhanced photoelectrocatalytic water performance
of
α-Fe2O3/Au/TiO2
photoanode
can
be
ascribed
to
the
lP
splitting
photoanode/electrolyte interfaces reconstructed by Au NPs and TiO2 overlayer (Fig. 6d). When
ur na
Au NPs are fixed to the surface of α-Fe2O3, the intimate contact between the Au NPs and α-Fe2O3 makes Au NPs to act as hole pumps that extract photogenerated holes from the bulk of α-Fe2O3, meanwhile as hole shuttles that facilitate migration of the holes across the reconstructed interfaces. The holes are then efficiently captured and stored by the TiO2 overlayer which serves as a charge-
Jo
storage layer that promotes the injection of photoholes into the electrolyte for catalyzing water oxidation reaction. Therefore, with the help of these reconstructed interfaces, the composite αFe2O3/Au/TiO2 photoanodes realize the reduced charge carrier recombination rates at the surface as well as in the bulk, resulting in the remarkably improved photoelectrocatalytic water splitting performance. In particular, the photocurrent density is significantly increased by 4.1 folds in α-
16
Fe2O3/Au/TiO2, which is superior to most α-Fe2O3-based composite photoanodes previously reported (as shown in Table 1) and means that the reconstructed interfaces strategy will pave the way toward the design of high-performance semiconductor photoelectrodes for efficient solar energy conversion.
of
3. Conclusions
ro
In summary, ternary nanostructured α-Fe2O3 photoanodes for photoelectrocatalytic water splitting have been successfully fabricated by coating α-Fe2O3 NR arrays with metallic Au NPs
-p
and a porous TiO2 overlayer. In contrast to bare α-Fe2O3, α-Fe2O3/Au, and α-Fe2O3/TiO2, the α-
re
Fe2O3/Au/TiO2 photoanode yields a maximum photocurrent density of 1.05 mA cm-2 at 1.23 VRHE, achieves an IPCE of 26% at 340 nm, amongst the highest values reported to date for α-Fe2O3, and
lP
exhibits excellent photostability. Systematic studies suggest that the significantly improved photoelectrocatalytic water splitting performance is attributed to the simultaneously reduced bulk
ur na
and surface recombination caused by the relayed pumping of charge carriers through the photoanode/electrolyte interfaces reconstructed by Au NPs and TiO2 overlayer, where the photogenerated holes are effectively extracted from the α-Fe2O3 bulk, shuttled by Au NPs across the α-Fe2O3/TiO2 interface, and then promptly injected by the TiO2 overlayer into the electrolyte
Jo
for catalyzing water oxidation reaction. Thus, the strategy of exploiting reconstructed interfaces to simultaneously reduce bulk and surface recombination has important implications for the design of high-performance semiconductor photoelectrodes for efficient solar energy conversion.
Acknowledgments 17
This work was supported by the National Natural Science Foundation of China (No. 21875183, 51672210 and 51888103), the National Program for Support of Top-notch Young Professionals, and the Fundamental Research Funds for the Central Universities. C.L.D is grateful to MoST for financially supporting this work under contracts MoST 107-2112-M-032-004-MY3
of
and 108-2218-E-032-003-MY3.
Appendix A. Supplementary data
-p
online version, at https://doi.org/10.1016/j.apcatb.2019.xx.xxx.
ro
Supplementary material related to this article can be found, in the online version, at in the
re
References
K. Sivula, R. van de Krol, Nat. Rev. Mater. 1 (2016) 15010.
[2]
X. Wang, F. Wang, Y. Sang, H. Liu, Adv. Energy Mater. 7 (2017) 1700473-1700488.
[3]
H. Zhang, J. Ming, J. Zhao, Q. Gu, C. Xu, Z. Ding, R. Yuan, Z. Zhang, H. Lin, X. Wang, J.
lP
[1]
ur na
Long, Angew. Chem. Int. Ed. 58 (2019) 7718-7722. [4]
C. Jiang, S.J.A. Moniz, A. Wang, T. Zhang, J. Tang, Chem. Soc. Rev. 46 (2017) 4645-4660.
[5]
S. Kment, F. Riboni, S. Pausova, L. Wang, L. Wang, H. Han, Z. Hubicka, J. Krysa, P.
Jo
Schmuki, R. Zboril, Chem. Soc. Rev. 46 (2017) 3716-3769. [6]
C. Li, Z. Luo, T. Wang, J. Gong, Adv. Mater. 30 (2018) 1707502.
[7]
P. Zhang, L. Gao, X. Song, J. Sun, Adv. Mater. 27 (2015) 562-568.
[8]
Y. Zhang, S. Jiang, W. Song, P. Zhou, H. Ji, W. Ma, W. Hao, C. Chen, J. Zhao, Energy Environ. Sci. 8 (2015) 1231-1236.
18
[9]
S. Shen, J. Jiang, P. Guo, C.X. Kronawitter, S.S. Mao, L. Guo, Nano Energy 1 (2012) 732741.
[10] X. Wang, W. Gao, Z. Zhao, L. Zhao, J.P. Claverie, X. Zhang, J. Wang, H. Liu, Y. Sang, Appl. Catal. B Environ. 248 (2019) 388-393. [11] Y. Fu, C.-L. Dong, Z. Zhou, W.-Y. Lee, J. Chen, P. Guo, L. Zhao, S. Shen, Phys. Chem. Chem. Phys. 18 (2016) 3846-3853.
of
[12] Y. Fu, C.-L. Dong, W.-Y. Lee, J. Chen, P. Guo, L. Zhao, S. Shen, ChemNanoMat 2 (2016)
ro
704-711.
https://doi.org/10.1016/j.scib.2019.07.008
-p
[13] L. Mao, Y.-C. Huang, Y. Fu, C.-L. Dong, S. Shen, Science Bulletin (2019),
re
[14] I. Roger, M. Shipman, M. Symes, Nat. Rev. Chem. 1 (2017) 0003. [15] M.R. Nellist, F.A. Laskowski, F. Lin, T.J. Mills, S.W. Boettcher, Acc. Chem. Res. 49 (2016)
lP
733-740.
[16] R. Liu, Z. Zheng, J. Spurgeon, X.G. Yang, Energy Environ. Sci. 7 (2014) 2504-2517.
ur na
[17] S. Shen, S.A. Lindley, X. Chen, J.Z. Zhang, Energy Environ. Sci. 9 (2016) 2744-2775. [18] M.T. Mayer, Y. Lin, G. Yuan. D. Wang, Acc. Chem. Res. 46 (2013) 1558-1566. [19] M. Wang, M. Pyeon, Y. Gönüllü, A. Kaouk, S. Shen, L. Guo, S. Mathur, Nanoscale 7 (2015) 10094-10100.
Jo
[20] D. Barreca, G. Carraro, A. Gasparotto, C. Maccato, M.E.A. Warwick, K. Kaunisto, C. Sada, S. Turner, Y. Gönüllü, T.-P.
Ruoko, L. Borgese, E. Bontempi, G.V. Tendeloo, H.
Lemmetyinen, S. Mathur, Adv. Mater. Interfaces 2 (2015) 1500313.
[21] A.G. Scheuermann, J.D. Prange, M. Gunji, C.E.D. Chidsey, P.C. McIntyre, Energy Environ. Sci. 6 (2013) 2487-2496.
19
[22] R. Yang, Y. Ji, Q. Li, Z. Zhao, R. Zhang, L. Liang, F. Liu, Y. Chen, S. Han, X. Yu, H. Liu, Applied Catalysis B: Environmental 256 (2019) 117798. [23] H. Zhang, L. Ma, J. Ming, B. Liu, Y. Zhao, Y. Hou, Z. Ding, C. Xu, Z. Zhang, J. Long, Appl. Catal. B Environ. 243 (2019) 481-489. [24] L. Ji, H.-Y. Hsu, X. Li, K. Huang, Y. Zhang, J.C. Lee, A.J. Bard, E.T. Yu, Nat. Mater. 16 (2017) 127-131.
of
[25] X. Yang, R. Liu, C. Du, P. Dai, Z. Zheng, D. Wang, ACS Appl. Mater. Interfaces 6 (2014)
ro
12005-12011.
[26] T.H. Jeon, G.-h. Moon, H. Park, W. Choi, Nano Energy 39 (2017) 211-218.
-p
[27] S. Hu, M.R. Shaner, J.A. Beardslee, M. Lichterman, B.S. Brunschwig, N.S. Lewis, Science
re
344 (2014) 1005-1009.
[28] M.F. Lichterman, A.I. Carim, M.T. McDowell, S. Hu, H.B. Gray, B.S. Brunschwig, N.S.
lP
Lewis, Energy Environ. Sci. 7 (2014) 3334-3337.
[29] D. Eisenberg, H.S. Ahn, A.J. Bard, J. Am. Chem. Soc. 136 (2014) 14011-14014.
ur na
[30] P. Zhang, T. Wang, J. Gong, Adv. Mater. 27 (2015) 5328-5342. [31] J.B. Lee, S. Choi, J. Kim, Y.S. Nam, Nano Today 16 (2017) 61-81. [32] D. Wang, W. Wang, M.P. Knudson, G.C. Schatz, T.W. Odom, Chem. Rev. 118 (2017) 28652881.
Jo
[33] J. Zhao, B. Liu, L. Meng, S. He, R. Yuan, Y. Hou, Z. Ding, H. Lin, Z. Zhang, X. Wang, Long, Appl. Catal. B Environ. 256 (2019) 117823.
[34] L. Meng, Z. Chen, Z. Ma, S. He, Y. Hou, H. Li, R. Yuan, X. Huang, X. Wang, X. Wang, Energy Environ. Sci. 11 (2018) 294-298.
20
[35] J. Long, H. Chang, Q. Gu, J. Xu, L. Fan, S. Wang, Y. Zhou, W. Wei, L. Huang, X. Wang, P. Liu, W. Huang, Energy Environ. Sci. 7 (2014) 973-977. [36] T. Jiang, C. Jia, L. Zhang, S. He, Y. Sang, H. Li, Y. Li, X. Xu, H. Liu, Nanoscale 7 (2015) 209-217. [37] E. Thimsen, F. Le Formal, M. Grätzel, S.C. Warren, Nano Lett. 11 (2011) 35-43. [38] B. Kong, J. Tao, C. Selomulya, W. Li, J. Wei, Y. Fang, Y. Wang, G. Zheng, D. Zhao, J. Am.
of
Chem. Soc. 136 (2014) 6822-6825.
ro
[39] L. Wang, H. Hu, N.T. Nguyen, Y. Zhang, P. Schmuki, Y. Bi, Nano Energy 35 (2017) 171178.
-p
[40] J. Li, S.K. Cushing, P. Zheng, F. Meng, D. Chu, N. Wu, Nat. Commun. 4 (2013) 2651.
re
[41] W-H. Hung, C.-J. Peng, C.-R. Yang, C.-J. Li, J.-J. Shyue, P.-C. Chang, C.-M. Tseng, P.-C. Juan, Nano Energy 30 (2016) 523-530.
lP
[42] S. Bai, J. Jiang, Q. Zhang, Y. Xiong, Chem. Soc. Rev. 44 (2015) 2893-2939. [43] S. Li, Q. Zhao, D. Meng, D. Wang, T. Xie, J. Mater. Chem. A 4 (2016) 16661-16669.
ur na
[44] J. Li, S.K. Cushing, P. Zheng, T. Senty, F. Meng, A.D. Bristow, A. Manivannan, N. Wu, J. Am. Chem. Soc. 136 (2014) 8438-8449. [45] J.Y. Kim, J.-W. Jang, D.H. Youn, G. Magesh, J.S. Lee, Adv. Energy Mater. 4 (2014) 14004761.
Jo
[46] D. He, X. Song, Z. Ke, X. Xiao, C. Jiang, Sci. China Mate. 61 (2018) 878-886. [47] D. Cao, W. Luo, J. Feng, X. Zhao, Z. Li, Z. Zou, Energy Environ. Sci. 7 (2014) 752-759. [48] Y.-F. Xu, X.-D. Wang, H.-Y. Chen, D.-B. Kuang, C.-Y. Su, Adv. Funct. Mater. 26 (2016) 4414-4421.
21
[49] D.W. Kim, S.C. Riha, E.J. DeMarco, A.B.F. Martinson, O.K. Farha, J.T. Hupp, ACS Nano 8 (2014) 12199-12207. [50] L. Steier, J. Luo, M. Schreier, M.T. Mayer, T. Sajavaara, M. Grätzel, ACS Nano 9 (2015) 11775-11783. [51] C. Li, A. Li, Z. Luo, J. Zhang, X. Chang, Z. Huang, T. Wang, J. Gong, Angew. Chem. Int. Ed. 56 (2017) 4150-4155.
of
[52] C.C.L. McCrory, S. Jung, I.M. Ferrer, S.M. Chatman, J.C. Peters, T.F. Jaramillo, J. Am.
ro
Chem. Soc. 137 (2015) 4347-4357.
[53] Z. Fan, Z. Xu, S. Yan, Z. Zou, J. Mater. Chem. A 5 (2017) 8402-8407.
-p
[54] Z. Xu, H. Wang, Y. Wen, W. Li, C. Sun, Y. He, Z. Shi, L. Pei, Y. Chen, S. Yan, Z. Zou,
re
ACS Appl. Mater. Interfaces 10 (2018) 3624-3633.
[55] Y.-J. Dong, J.-F. Liao, Z.-C. Kong, Y.-F. Xu, Z.-J. Chen, H.-Y. Chen, D.-B. Kuang, D.
lP
Fenske, C.-Y. Su, Appl. Catal. B: Environ. 237 (2018) 9-17. [56] H.-J. Ahn, K.-Y. Yoon, M.-J. Kwak, J.-H. Jang, Angew. Chem. Int. Ed. 55 (2016) 9922-
ur na
9926.
[57] B. Klahr, S. Gimenez, F. Fabregat-Santiago, T. Hamann, J. Bisquert, J. Am. Chem. Soc. 134 (2012) 4294-4302.
[58] B. Klahr, S. Gimenez, F. Fabregat-Santiago, J. Bisquert, T.W. Hamann, J. Am. Chem. Soc.
Jo
134 (2012) 16693-16700.
[59] G.M. Carroll, D.K. Zhong, D.R. Gamelin, Energy Environ. Sci. 8 (2015) 577-584. [60] J. Deng, X. Lv, K. Nie, X. Lv, X. Sun, J. Zhong, ACS Catal. 7 (2017) 4062-4069. [61] H. Zhang, Y.K. Kim, H.Y. Jeong, J.S. Lee, ACS Catalysis 9 (2019) 1289-1297. [62] S.-S. Yi, B.-R. Wulan, J.-M. Yan, Q. Jiang, Adv. Funct. Mater. 29 (2019) 1801902.
22
[63] F. Le Formal, N. Tétreault, M. Cornuz, T. Moehl, M. Grätzel, K. Sivula, Chem. Sci. 2 (2011) 737-743. [64] L. Steier, I. Herraiz-Cardona, S. Gimenez, F. Fabregat-Santiago, J. Bisquert, S.D. Tilley, M. Grätzel, Adv. Funct. Mater. 24 (2014) 7681-7688. [65] S.C. Riha, B.M. Klahr, E.C. Tyo, S. Seifert, S. Vajda, M.J. Pellin, T.W. Hamann, A.B.F. Martinson, ACS Nano 7 (2013) 2396-2405.
of
[66] W. Li, S.W. Sheehan, D. He, Y. He, X. Yao, R.L. Grimm, G.W. Brudvig, D. Wang, Angew.
ro
Chem. Int. Ed. 54 (2015) 11428-11432.
[67] J. Zhang, R. García-Rodríguez, P. Cameron, S. Eslava, Energy Environ. Sci. 11 (2018)
-p
2972-2984.
re
[68] L. Wang, N.T. Nguyen, X. Huang, P. Schmuki, Y. Bi, Adv. Funct. Mater. 27 (2017) 1703527.
1805737.
lP
[69] H. Zhang, W.Y. Noh, F. Li, J.H. Kim, H.Y. Jeong, J.S. Lee, Adv. Funct. Mater. 29 (2019)
ur na
[70] P. Peerakiatkhajohn, J.-H. Yun, H. Chen, M. Lyu, T. Butburee, L. Wang, Adv. Mater. 28 (2016) 6405-6410.
Figure and Table Citation Page
Jo
Fig. 1. (a) Step-wise schematic of the synthesis of α-Fe2O3/Au/TiO2. SEM images of (b) β-FeOOH, (c) α-Fe2O3, (d) α-Fe2O3/TiO2, (e) β-FeOOH/Au, (f) α-Fe2O3/Au, and (g) α-Fe2O3/Au/TiO2. Insets are the cross-sectional SEM images. Fig. 2. TEM images of (a,e) α-Fe2O3; (b,f) α-Fe2O3/Au; (c,g) α-Fe2O3/TiO2; and (d,h) αFe2O3/Au/TiO2 with EDS mappings of (i) α-Fe2O3/Au, (j) α-Fe2O3/TiO2, and (k) α-Fe2O3/Au/TiO2. (e)-(h) are the high-resolution images of corresponding areas in (a)-(d) with further enlarged images of α-Fe2O3 and Au NP, inset in (g) is the SAED pattern of the TiO2 overlayer.
23
Fig. 3. (a) J-V curves of the photoanodes. (b) IPCE plots measured at 1.23 VRHE, the inset shows an enlargement for the wavelength range from 580 to 700 nm. (c) Chronoamperometry for illuminated samples over a period of 12 h at 1.23 VRHE. (d) Amounts of evolved gases and the calculated Faradic efficiencies using α-Fe2O3/Au/TiO2 photoanode during the photostability test. All tests are measured under front-side illumination (from the photoanode side).
of
Fig. 4. (a) M-S plots of the films. (b) Capacitive current density at 1.23 VRHE as a function of scan rate. Nyquist plots of (c) α-Fe2O3 and α-Fe2O3/Au, (d) α-Fe2O3/TiO2 and α-Fe2O3/Au/TiO2; the insets are the corresponding equivalent circuits used to fit the plots. (e) ηsep and (f) ηinj of all the photoanodes, which are calculated by adding Na2SO3 as a hole scavenger into the electrolyte (further details are given in Fig. S17).
re
-p
ro
Fig. 5. XAS spectra of (a) Fe L-edge, (b) Ti L-edge, and (c), (d) O K-edge; right part of (a) and left part of (d) are the corresponding enlarged regions marked by dashed boxes. (e) Ti L-edge and (f) Fe L-edge in the dark. Peaks in (e) are deconvoluted into three components (peaks A, B and C); the solid lines in the inset represent the tested data and the circles indicate the fitting spectra. (g) O K-edge of α-Fe2O3/Au/TiO2 (red solid line) and a fitted spectrum (open solid circle) calculated from a linear combination from the spectra of α-Fe2O3/Au and α-Fe2O3/TiO2.
lP
Fig. 6. Proposed charge transfer mechanisms of different α-Fe2O3 photoanodes for photoelectrocatalytic water splitting; (a) α-Fe2O3, (b) α-Fe2O3/Au, (c) α-Fe2O3/TiO2, and (d) αFe2O3/Au/TiO2. The SCL region are marked with dash line and labeled as W.
Jo
ur na
Table 1. Comparisons of photoanodes in this study with other α-Fe2O3-based composite films reported previously. Unless otherwise specified, all photoelectrocatalytic tests were carried out under simulated 1 sun (AM 1.5G, 100 mW cm-2) illumination and all biases were converted to RHE(VRHE).
24
of ro -p re
Jo
ur na
lP
Fig. 1. (a) Step-wise schematic of the synthesis of α-Fe2O3/Au/TiO2. SEM images of (b) β-FeOOH, (c) α-Fe2O3, (d) α-Fe2O3/TiO2, (e) β-FeOOH/Au, (f) α-Fe2O3/Au, and (g) α-Fe2O3/Au/TiO2. Insets are the cross-sectional SEM images.
25
of ro -p re
Jo
ur na
lP
Fig. 2. TEM images of (a,e) α-Fe2O3; (b,f) α-Fe2O3/Au; (c,g) α-Fe2O3/TiO2; and (d,h) αFe2O3/Au/TiO2 with EDS mappings of (i) α-Fe2O3/Au, (j) α-Fe2O3/TiO2, and (k) α-Fe2O3/Au/TiO2. (e)-(h) are the high-resolution images of corresponding areas in (a)-(d) with further enlarged images of α-Fe2O3 and Au NP, inset in (g) is the SAED pattern of the TiO2 overlayer.
26
of ro -p
Jo
ur na
lP
re
Fig. 3. (a) J-V curves of the photoanodes. (b) IPCE plots measured at 1.23 VRHE, the inset shows an enlargement for the wavelength range from 580 to 700 nm. (c) Chronoamperometry for illuminated samples over a period of 12 h at 1.23 VRHE. (d) Amounts of evolved gases and the calculated Faradic efficiencies using α-Fe2O3/Au/TiO2 photoanode during the photostability test. All tests are measured under front-side illumination (from the photoanode side).
27
of ro -p re lP ur na
Jo
Fig. 4. (a) M-S plots of the films. (b) Capacitive current density at 1.23 VRHE as a function of scan rate. Nyquist plots of (c) α-Fe2O3 and α-Fe2O3/Au, (d) α-Fe2O3/TiO2 and α-Fe2O3/Au/TiO2; the insets are the corresponding equivalent circuits used to fit the plots. (e) ηsep and (f) ηinj of all the photoanodes, which are calculated by adding Na2SO3 as a hole scavenger into the electrolyte (further details are given in Fig. S17).
28
of ro -p re lP
Jo
ur na
Fig. 5. XAS spectra of (a) Fe L-edge, (b) Ti L-edge, and (c), (d) O K-edge; right part of (a) and left part of (d) are the corresponding enlarged regions marked by dashed boxes. (e) Ti L-edge and (f) Fe L-edge in the dark. Peaks in (e) are deconvoluted into three components (peaks A, B and C); the solid lines in the inset represent the tested data and the circles indicate the fitting spectra. (g) O K-edge of α-Fe2O3/Au/TiO2 (red solid line) and a fitted spectrum (open solid circle) calculated from a linear combination from the spectra of α-Fe2O3/Au and α-Fe2O3/TiO2.
29
of ro
Jo
ur na
lP
re
-p
Fig. 6. Proposed charge transfer mechanisms of different α-Fe2O3 photoanodes for photoelectrocatalytic water splitting; (a) α-Fe2O3, (b) α-Fe2O3/Au, (c) α-Fe2O3/TiO2, and (d) αFe2O3/Au/TiO2. The SCL region are marked with dash line and labeled as W.
30
Table 1. Comparisons of photoanodes in this study with other α-Fe2O3-based composite films reported previously. Unless otherwise specified, all photoelectrocatalytic tests were carried out under simulated 1 sun (AM 1.5G, 100 mW cm-2) illumination and all biases were converted to RHE(VRHE). Photocurrent density (mA cm-2) Photoanodes Test Mechanism Ref. condition Bias Pristine Treated Fold 1 M NaOH
1.23
0.25
1.05
4.12
Reconstructed interfaces
This work
α-Fe2O3/Au
1 M NaOH
1.4
1.25
2.00
1.60
SPR effect
[37]
α-Fe2O3/Au
1 M NaOH
1.7
0.85
1.90
2.22
Fe2O3/Fe2TiO5
1 M NaOH
1.23
0.87
1.61
1.85
Fe2O3/FeNbO4
1 M NaOH
1.23
1.24
2.24
Fe2O3/Co3O4
1 M NaOH
1.23
1.20
2.00
Fe2O3/TiO2
1 M NaOH
1.40
0.77
1.01
Fe2O3/TiO2
1 M NaOH
1.43
3.10
3.00
Fe2O3/Al2O3
1 M NaOH
1.43
3.10
Fe2O3/SiOx
1 M NaOH
1.23
1.23
Fe2O3/Ga2O3
1 M NaOH
1.23
Fe2O3/Ni(OH)2
1 M NaOH
1.53
Fe2O3/CoOx Fe2O3/Ir-WOC
Heterojunction
[60]
ro
[41]
[61]
1.67
Heterojunction
[62]
1.31
Passivation
[25]
0.97
Passivation
[63]
3.20
1.03
Passivation
[63]
1.75
1.42
Passivation
[56]
0.65
0.60
0.92
Passivation
[64]
0.55
0.70
1.27
Co-catalyst
[50]
lP
re
-p
Heterojunction
0.1 M KPi
1.60
0.80
1.51
1.88
Co-catalyst
[59]
0.1 M NaOH
1.53
1.40
2.10
1.50
Co-catalyst
[65]
0.1 M KNO3
1.40
0.75
1.10
1.47
Co-catalyst
[66]
1 M NaOH
1.23
0.88
1.20
1.36
Co-catalyst
[67]
Jo
Fe2O3/CoFeOx
SPR effect
1.81
ur na
Fe2O3/Co-Pi
of
α-Fe2O3/Au/TiO2
Fe2TiO5/Fe2O3/Pt
1 M KOH
1.23
0.50
1.00
2.00
Synergistic effects
[68]
Fe2O3/FeTaO4
1 M NaOH
1.23
1.17
2.37
2.02
Synergistic effects
[69]
Fe2O3/CIO
1 M KOH
1.23
0.92
2.49
2.71
Combined effects
[48]
Fe2O3/Ag/Co-Pi
1 M NaOH
1.23
1.50
4.68
3.12
Combined effects
[70]
31