Epitaxial Bi2FeCrO6 Multiferroic Thin-Film Photoanodes with Ultrathin

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Epitaxial Bi2FeCrO6 Multiferroic Thin Film Photoanodes with Ultrathin p-Type NiO layers for Improved Solar Water Oxidation Wei Huang, Catalin Harnagea, Xin Tong, Daniele Benetti, Shuhui Sun, Mohamed Chaker, Federico Rosei, and Riad Nechache ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20998 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Applied Materials & Interfaces

Epitaxial Bi2FeCrO6 Multiferroic Thin Film Photoanodes with Ultrathin pType NiO layers for Improved Solar Water Oxidation Wei Huang,† Catalin Harnagea,† Xin Tong,†,‡ Daniele Benetti,† Shuhui Sun,† Mohamed Chaker,† Federico Rosei,*,†,§ and Riad Nechache*,$

†Centre Énergie, Matériaux et Télécommunications, Institut National de la Recherche Scientifique,

1650, Boulevard Lionel-Boulet, Varennes, Québec J3X 1S2, Canada. ‡School

of Chemistry and Materials Science, Guizhou Normal University, Guiyang 550001,

People’s Republic of China. §Institute

of Fundamental and Frontier Science, University of Electronic Science and Technology

of China, Chengdu 610054, People’s Republic of China. $École

de Technologie Supérieure, 1100 Rue Notre-Dame Ouest, Montréal, Québec H3C 1K3,

Canada.

• Supporting Information

KEYWORDS: transparent conducting oxides, heterojunction photoanode, ferroelectric polarization, photoelectrochemistry, charge recombination rate.

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ABSTRACT: The photoelectric properties of multiferroic double-perovskite Bi2FeCrO6 (BFCO), such as above-bandgap photovoltages, switchable photocurrents and bulk photovoltaic effect, have recently been explored for potential applications in solar technology. Here we report the fabrication of photoelectrodes based on n−type ferroelectric (FE) semiconductor BFCO heterojunctions coated with p−type transparent conducting oxides (TCOs) by pulsed laser deposition, and their application for photoelectrochemical (PEC) water oxidation. The photocatalytic properties of the bare BFCO photoanodes can be improved by controlling the FE polarization state. However, the charge recombination as well as the limited charge transfer kinetics in the photoanode/electrolyte cause major energy loss and thus hinder the PEC performance. We show that this problem may be addressed by the deposition of an ultrathin p−type NiO layer on the photoanode to enhance the charge transport kinetics and reduce charge recombination at surface trapped states for increased surface band bending. A 4−fold enhancement of photocurrent density, up to 0.4 mA cm−2 (at +1.23 V vs. RHE), a best performance of stability over four hours and a high incident photon-to-current efficiency (~3.7%) were achieved under 1 sun illumination in such p-NiO/n-BFCO heterojunction photoanodes. These studies reveal the optimisation of PEC performance by polarization switching of BFCO, as well as the successful achievement of p−TCOs/n−FE heterojunction photoanodes able to sustain multiple hour-stable water oxidation.


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1. INTRODUCTION Photoelectrochemical (PEC) hydrogen generation from water represents a promising route for producing a clean and sustainable fuel, to address the increasing demand for renewable energy.1-3 PEC cells employ both photocathodes (p−type semiconductors) for hydrogen generation and photoanodes (n−type semiconductors) for the production of oxygen.4 Even though some semiconductor photocatalysts can promote both hydrogen and oxygen evolution reactions (i.e., HER and OER), their PEC performance in the overall process is low, mainly due to the limited thermodynamics and kinetics of photocatalytic processes for semiconductor systems.5,6 In addition, the unfavorable energy band bending either upward or downward at their surfaces is also thought to be a crucial factor that limits photocatalytic reactivity in conventional semiconductors,7,8 as schematically illustrated in Figure S1 (Supporting Information). In some artificial photocatalysts, the space charge region (SCR) and built-in electrical field in the p−n junction are the major driving forces for the separation of photogenerated charges.9-11 However, in most cases, these driving forces are severely limited by the small bandgap value of the semiconductors in use.7 Moreover, efficient semiconductor-based devices with high PEC performance require a spatial separation of electrons and holes to reduce recombination events.9 An alternative approach to overcome the limitations of conventional semiconductors is to fabricate photoelectrodes made of ferroelectric (FE) materials. FE systems are able to effectively separate charge carriers, exploiting the FE polarization induced internal electric field.12-15 Unlike conventional semiconductor materials, photovoltaic (PV) devices based on FE materials under light irradiation can achieve high open-circuit voltages with values higher than their bandgap.16,17 In contrast to the built-in electric field that forms in a p−n junction, the remnant FE polarization induced electric fields extend over the entire volume of the film or bulk material, effectively



separating electron−hole pairs and driving them towards their respective electrodes.13,18,19 This induced electrical field also modulates the chemical potential and surface band bending of FE materials based photoelectrodes, thereby allowing to tailor the PEC performance.20-25 Compared to conventional FE materials, multiferroic systems, in which magnetic order and ferroelectricity coexist, exhibit strong electron−electron interactions able to modulate the magnetization and tune their bandgap,26-28 thereby offering a unique opportunity to enhance their PEC properties. Previous work indicated that the n−type FE semiconductor BFCO is promising for high performance ferroelectric PV devices, with efficiency reaching up to 8.1% in a multilayer device.29 Li et al.26 first reported the p−type semiconductor BFCO as a photocathode for hydrogen generation. Hence it is of high interest to investigate the n−type semiconductor BFCO as photoanode for solar water oxidation, and further explore the optimization of PEC activity by manipulating its FE polarization. On the other hand, it is well known that narrow bandgap semiconductors naturally corrode or passivate when used as photoanodes in contact with aqueous electrolytes,35 thus requiring to be stabilized, usually by surface treatment and modification or by implementing a protective layer prior to their use as photoelectrodes in a PEC device.36-38 In addition, the PEC performance of intrinsic n−type semiconductor based photoanodes could be improved by adding an effective electron-blocking layer (EBL), to reduce the recombination of photogenerated charges, consequently improving the OER efficiency.39 The solution consists in designing a protective and electron-blocking layer that is simultaneously chemically stable, antioxidative, transparent and conducting.30 The most suitable materials having such properties are p−type transparent conducting oxides (TCOs), also called p−type window layers.10 Among them, the p−type semiconductor nickel oxide (NiO) has a cubic crystal structure with a wide bandgap (~3.6 eV).35,36 In addition, ultrathin NiO films exhibit high optical transparency (~80% at 10 nm and 75% at 20


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nm in the visible range36), relatively low electrical resistivity (~120 Ω cm35) and p−type hole conduction with high conduction band minimum (CBM), making them suitable as an EBL.29 Here we report the fabrication of photoelectrodes for PEC water splitting based on a p−NiO/n−BFCO heterojunction by pulsed laser deposition (PLD). Tailoring the PEC performance of bare BFCO photoanodes was performed by manipulating the internal electric field induced by the FE polarization of BFCO. Under 1 sun illumination, a four−fold enhancement of photocurrent density (~0.4 mA cm−2 at +1.23 V vs. RHE) for photoanodes coated with a p−type NiO layer was achieved in 1 M Na2SO4 (pH 6.8) electrolyte, indicating an effective suppression of charge recombination at surface trapped states. The constant current densities generated over a period of four hours under one sun at +0.5 V (vs. RHE) further confirmed the stability of device performance in Na2SO4 aqueous electrolyte. The incident photon-to-current efficiency (IPCE) indicated a higher photon-to-current conversion rate in the visible region for the 10 nm NiO/BFCO heterojunction photoanodes (e.g., ~3.7% for IPCE), compared with that in the bare BFCO devices (e.g., ~1.2% for IPCE).

2. RESULTS AND DISCUSSION 2.1. Characterization of Thin Films. Figure 1a (bottom) exhibits the X-ray diffraction (XRD) θ−2θ pattern of BFCO thin films. The observed (0 0 1), (0 0 2), (0 0 3) peaks evidence the c-axis orientation of the films with the absence of secondary phases. The (0 0 l) (l = 1, 2, 3) reflections of the SrRuO3 (SRO) thin film are buried into that of BFCO. As mentioned in our previous reports,26,29 the degree of Fe/Cr ordering (i.e., R) and the ordered domain size (i.e., D) in BFCO films are the crucial parameters for tuning its direct bandgap (Eg). The Fe/Cr ordering in BFCO was verified by using XRD asymmetrical scans around the (111) SrTiO3 (STO) reflection (Figure



1a, top). The corresponding crystal structure of BFCO (i.e., double perovskite) having different types of octahedral stacking of Fe/Cr along cubic [001] and [111] directions is shown in the inset of Figure 1a. As detailed in Supporting Information, the values of R and D of BFCO were estimated to be around 0.5% and 32 nm, respectively, demonstrating a partial long-range ordering along [111] direction. These ordered and disordered domains of the BFCO phase (denoted as oBFCO and d-BFCO, respectively) were further analyzed in XRD reciprocal space mapping (RSM) around (204) plane of (001) STO substrate, as shown in Figure 1b. The Φ-scan measurements indicate an epitaxial growth of BFCO on the SRO buffered STO substrate (Figure S2, Supporting Information). The surface topography at nanoscale of the 90 nm-thick BFCO film was imaged by atomic force microscopy (AFM) (Figure S3, Supporting Information), which revealed a smooth surface throughout the films, with root mean square roughness of ~3.2 nm over a scanned area of 5×5 µm2. Hence, the films exhibit a high crystallinity and smooth surface, both critical parameters to achieve heteroepitaxial structures with low-density charged defects (e.g. oxygen vacancies, deficiency of metallic cations), which are sites for electron-hole recombination. These features minimize their effect on device performance (including PV and PEC).29,37 As previously described, the driving force for the separation of charge carriers depends strongly on the polarization magnitude in FE materials. The wavelength dependence of the absorption coefficient α of BFCO films extracted from ellipsometry measurements is displayed in Figure 1c. The optical transition occurs mainly at 420670 nm in spectra of absorption, with a maximum coefficient α of ~2.5×105 cm−1 at 520 nm. The two threshold gaps were assessed from the (αE)2 versus photon energy plot in the right coordinate axis of Figure 1c. The first threshold gap located at ~2.78 eV is attributed to the d-BFCO phase, whereas the second optical transition area, obtained by a linear extrapolation to zero, yields


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bandgap ≈ 1.8 eV. This is thought to originate from the partially ordered BFCO domain.29 Figure 1d displays the microscopic FE hysteresis loop for BFCO films yielding a polarization up to 20 μC cm-2. To further confirm the local FE properties of the BFCO films, we performed piezoresponse force microscopy (PFM) experiments. PFM evidenced the switching of FE domains of BFCO films by applying ±8 V voltage pulses (Figure S4b, Supporting Information), with a hysteretic behavior (Figure S4a, Supporting Information). The contrast demonstrates that ferroelectricity is present in BFCO and can be reversed by applying an external voltage. Prior to the investigation of the p−NiO/n−BFCO heterojunction properties, it is important to gain insight on the p−type transparent conducting NiO thin films. For this purpose, ~10 or 20 nm-thick NiO layer was directly grown on transparent STO substrate by PLD. The optical transmission and absorbance spectra in the UV-visible range are shown in Figure 2a (left y-axis). As expected, the thicker film, with an optical transmission of ~60% in the visible range absorbed more photons than the thinner one (~70% transmission).36 This is also confirmed by optical absorbance measurements (right y-axis in Figure 2a). From the Tauc plots of (αE)2 vs. E (inset of Figure 2a), we calculated the direct bandgap of NiO thin films, which act as a window layer, finding a value of ~3.6 eV. We also investigated the electrical properties of NiO films for both thicknesses. The enhancement of electrical conduction with temperature increase from 253 to 473 K is consistent with a typical semiconducting behaviour with a conductivity at 300 K of ~0.13 and ~0.25 S cm-1 for 10 and 20 nm-thick NiO films, respectively (Figure 2b). These values indicate a 15−30 fold enhancement compared with those previously reported.35 The conductivity increase is thought to be due to the presence of Ni2+ vacancies or O2 interstitials which usually form in PLDdeposited NiO films,29,38 which was confirmed by x-ray photoelectron spectroscopy (XPS) (Figure S5, more details are discussed in Supporting Information). The hole concentration and mobility



were obtained by Hall-effect measurements which are 1.2×1018 cm-3, 0.7 cm2 V-1s-1 for 10 nm NiO and 1.5×1018 cm-3, 1.1 cm2 V-1s-1 for 20 nm NiO. To understand the energy band structure of NiO thin films, the electron affinity and ionization potential for 20 nm-thick NiO films were determined to be −1.6 eV and −5.2 eV, using ultraviolet photoemission spectroscopy (UPS) measurements (Figure S6).

2.2. Photoelectrochemical performance of devices. To assess the PEC performance of the bare BFCO thin film based photoelectrodes for oxygen generation, we measured linear sweep voltammetry (J–V) curves under chopped simulated-sunlight with intensity of 100 mW cm-2, to monitor simultaneously the dark and light current. The obtained J−V curves are displayed in Figure 3a. After subtraction of the dark current contribution, the bare BFCO photoelectrode generated an anodic photocurrent density of 0.1 mA cm−2 at an applied bias of +1.23 V (vs. RHE) in the electrolyte consisting of 1 M Na2SO4 at pH 6.8. The open-circuit potential (Voc) decreases (i.e. from 0.22 V in dark to 0.06 V vs. RHE under AM 1.5G and the open-circuit photovoltage (OCP) change comes to −0.16 V) when the electrode is illuminated (Figure 3a), suggesting a typical n−type semiconductor behavior. This could be explained by the presence of oxygen vacancies (Figure S7, Supporting Information) formed in the BFCO lattice during PLD growth under reduced oxygen partial pressure. This result is in agreement with that previously reported for n−type FE semiconductor BFCO-based PV devices.29 Under moderate oxygen pressure (10 mTorr) and laser fluence (2.1 J cm-2) conditions, the p−type BFCO was obtained with less oxygen vacancies and lattice defects, acting as a photocathode for solar-driven water reduction to hydrogen evolution.26 Therefore, the obtained n−type FE semiconductor BFCO-based photoanode for water oxidation is complementary to the previous study on BFCO used in PEC devices.


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To understand the effect of polarization orientations on photocatalytic properties, the J–V curves were recorded for a bare BFCO photoanode after having switched its polarization in opposite directions, perpendicular to the surface. To achieve this, we first deposited silver electrodes on top of the BFCO films to perform electrical poling (Figure S8, Supporting Information), then chemically removed them to carry out PEC measurements (see experimental section, Supporting Information). The polarization was oriented upward (downward), hereafter denoted as Pup (Pdown) by applying +15 V (−15 V) electrical voltage pulses during 1 µs, respectively. The positively poled (Pup) BFCO film based photoanodes exhibited a photocurrent density of 0.06 mA cm−2 at +1.23 V (vs. RHE) (Figure 3a), slightly smaller than that of the sample in its pristine state (P0). In contrast, for the negatively poled (Pdown) BFCO films, the photocurrent density increased to 0.15 mA cm−2 at the same bias, which shows a 50% and 250% enhancement compared with the sample at P0 and Pup states, respectively. Based on the aforementioned analysis, the polarization state must be adjusted to obtain multiferroic thin film based photoanodes with efficient oxygen evolution, which in turn control the migration of photogenerated electrons to the SRO electrode. We illustrate in Figures S9a−c (Supporting Information) tentative simplified energy band diagrams for our photoanode system. After the electrical poled treatment on BFCO thin films, the polarization charges on surface induce a band bending in the space charge region (SCR) with an electric field gradient.20 The band bending influence at interface cannot be neglected due to the 90 nm thickness of the films which is close to the estimated width of SCR (i.e., 20–50 nm, see Supporting Information Section II). For the BFCO film poled by negative pulses, the band diagram was modified as illustrated in Figure S9b (Supporting Information). Under illumination, the electron/hole pairs were separated by the polarization-induced driving force. The electrons drove toward the BFCO/SRO interface while the holes migrated to the BFCO/electrolyte interface. The



charge carrier’s recombination was thus limited, promoting an enhancement of photocatalytic performance at a negatively poled state. When the film was positively poled, the oppositely slanted band caused the electrons to drift into the BFCO/electrolyte interface and the holes to accumulate at the BFCO/SRO interface (Figure S9c, Supporting Information). This scenario is unfavorable for charge transport and thus the process results in a reduced PEC efficiency. So far, we have discussed the PEC properties of bare BFCO photoanodes and the tuning of their PEC performance by their FE polarization switching. In the following, we will show that the performance can be further improved by adding a p−type TCO layer. PEC measurements for NiO/BFCO/SRO heterojunction photoanodes were performed in a three-electrode configuration, reported in Figure 3b. The bias is applied between the working electrode (NiO/BFCO/SRO) and reference electrode (Ag/AgCl). An alternative NiO/BFCO/SRO heterojunction device structure for water splitting showing that light is absorbed by BFCO thin films and the photogenerated holes transfer to the NiO, at which O2 is produced, is suggested in Figure 3b. We deposited NiO films, used as electron-blocking and protective layer with two thicknesses, ~10 and 20 nm atop of the BFCO thin-films photoanodes by PLD. In the J–V curves (Figure 3c) we first observed a significant increase of the current density compared to the bare BFCO-based photoelectrodes, of approximately one order of magnitude. The device made with the thinner layer (10 nm) yields an anodic photocurrent density of 0.4 mA cm−2 at +1.23 V (vs. RHE) which represents a 2−fold enhancement compared to that of the 20 nm NiO/BFCO photoanode (~0.2 mA cm−2). This occurrence might be due to the higher optical transmission of the former (~70% in the visible range) compared to the latter (~60%) and the increase of resistance at NiO/BFCO interfacial regions40. Implementing a 10 nm-thick NiO layer promotes a four−fold increase of the photocurrent density of the photoelectrodes. The performance of such


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heterostructure photoanodes is largely superior to that of typical multiferroic perovskite BiFeO3 thin films counterparts (J ≈ 0.08 mA cm-2 at 0 V vs. Ag/AgCl measured in 0.5 M Na2SO4 under 1 sun

illumination18).

This

is

a

significant

improvement

in

PEC

performance

of

ferroelectric/multiferroic photoanodes compared to the previous studies, as shown in Table 1. Upon exposing the electrode to illumination, the change of OCP value under dark and light condition increases (i.e. from −0.13 to −0.32 V) in devices having 20 nm thin NiO layer compared to the 10 nm thick counterpart (inset of Figure 3c). This is due to the decrease of the difference between Fermi level (heterojunctions: 10 or 20 nm NiO/BFCO) and Eredox, which thus further change the OCP value. To further understand the improvement of performance of devices based on the NiO/BFCO/SRO heterojunction, we evaluated the photon-to-current conversion efficiency by using the incident photon-to-current efficiency (IPCE). To measure the IPCE, we recorded the J−V measurements under specific radiation wavelengths (Figure 3d). To avoid band-to-band absorption at ~387 nm for STO substrate37 and promote the contribution from BFCO, the measurements were varied by using different incident wavelengths from 405 to 785 nm. The calculation details of IPCE for both photoelectrode architectures are reported in Supporting Information. The results confirm the conversion from absorbed photons with different energies to photocurrents. In particular, the IPCE values are more pronounced at the range of 400-700 nm, which is in agreement with the ellipsometry results (i.e., α vs. λ in Figure 1c), confirming the high photon-to-current conversion rate in the visible region at 1.23 V vs. RHE. The higher IPCE values (i.e. the peak value is ~3.7% and ~2.7%) are recorded in 10 and 20 nm NiO/BFCO heterojunctions, respectively, compared with bare BFCO photoanodes (i.e. the peak value is ~1.2%), thus indicating an improvement of photon-to-current conversion efficiency in the heterojunction due to the efficient



charge separation/collection leading to a low recombination rate as well as a fast charge transfer at interfacial regions of NiO/BFCO/SRO heterojunction.13,29

2.3. Charge dynamics and Stability. To further confirm the suppression of charge recombination in electrolyte/NiO/BFCO heterojunction, the transient photocurrent curve was measured by chronoamperometry to investigate charge recombination (inset in Figure 4a). Fast responses for both bare BFCO and NiO/BFCO photoanodes were observed with light switching. A normalized parameter (i.e., D) was evolved from the J−t curve to determine the charge recombination rate.39 The normalized plots of lnD as a function of time were displayed in Figure 4a. The transient time constant (i.e., T) is defined as the time when lnD = −1. T was estimated to be 4.0 s for bare BFCO and 6.8 s for NiO/BFCO heterojunction, which confirms the suppression of charge recombination.40 This phenomena is also observed in time-resolved photoluminescence (TRPL) measurements for bare BFCO and NiO/BFCO heterojunction with a lifetime (τ) of ~6.4 and ~8.4 ns, respectively (cf. Figure S10). All the obtained parameters of PEC performance for BFCObased photoanodes without and with the NiO layer are summarised in Table 2. Next, we analyzed the stability and efficiency of p−NiO/n−BFCO heterojunctions during their use as photoanodes. The stability tests of fabricated photoanodes were conducted using chronoamperometry at a potential of +0.5 V (vs. RHE) in Na2SO4 electrolyte under 1 sun illumination. As shown in Figure 4b, the 10 nm NiO/BFCO heterojunction photoanode showed a slight initial decrease in the photocurrent, then stabilized at a current density of 0.27 mA cm−2 during four hours of continuous operation. Over this time, we observed an insignificant change in morphology of the NiO/BFCO photoanode surface (SEM image in inset of Figure 4b), evidencing a mechanical stability of the photoanodes. On the other hand, the photocurrent density of BFCO


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photoanodes without the NiO protective layer under long-time irradiation in aqueous electrolyte decays by 34% within the first five minutes, followed by a further 20% decrease during the next hour. The results could be explained by the presence of defects at the BFCO surface, among which oxygen vacancies (VO2) or deficiency of metallic cations (i.e. VBi3+, VFe3+, VCr3+). These are associated with the growth of PLD-deposited BFCO crystals,29 behaving as charge carrier recombination sites that dramatically affect photocurrent generation. The experimentally produced gas was analyzed using gas chromatography, recorded at 0.5 V vs RHE under 1 sun illumination. Figure S11 (Supporting Information) indicates that NiO/BFCO heterojunction photoanodes produced more amount of O2 in a certain of time, compared with the bare BFCO photoanodes, which further confirmed the improved photocatalytic performance of photoanodes by coating an ultrathin transparent conducting p-type NiO layer.

2.4. Mott-Schottky analysis and Band alignment. To further understand the charge transfer mechanism in the electrolyte/NiO/BFCO/SRO heterojunction, we performed a Mott-Schottky analysis to determine the flat band potential (Vfb) and donor density by using capacitance impedance analysis on electrode/electrolyte at frequency of 2 kHz in dark conditions, as shown in Figure 5a. The positive slope of the C−2 curve indicates that the majority carriers in BFCO thin film are electrons. Thus, for bare BFCO photoanodes, the x-axis intercept [V0 = −0.06 vs. RHE] can be used to determine Vfb, which yields a value of −0.09 V vs. RHE. The conduction band (CB) edge is located at −0.49 V (vs. RHE), and thus the valence band (VB) edge would then occur at +1.31 V vs. RHE. The calculation method is detailed in Supporting Information, and these values were further confirmed by ultraviolet photoelectron spectroscopy (UPS) measurements, as illustrated in Figure S12 (Supporting Information). In addition, from the slope of the linear fit of



the Mott−Schottky plot recorded at 2000 Hz, the carrier (electrons) density (ND) estimated is 2.4×1021 cm−3 (see details in Supporting Information Section II). As shown in Figure 5a, for NiO/BFCO photoanodes, the obtained values of Vfb is around −0.04 V (vs. RHE), which were extracted from the x-axis intercept (V0 = −0.01 V vs. RHE), and the donor density is ~7.4×1021 cm−3. The improved donor density contributed to high PEC performance of NiO/BFCO heterojunction. The work function of NiO and the as-known SRO was −4.80 eV (Figure S6) and −5.20 eV, respectively, and then the band alignment is illustrated in Figure 5b. Under one sun illumination, the photogenerated electrons at BFCO’s CB migrate to SRO, and then to Pt electrode for HER. Photogenerated holes at BFCO’s VB flow to the interlayer of NiO/BFCO, and then to the NiO surface active sites for OER. The recombination of charge carriers is effectively limited, resulting in an improvement of the PEC performance, compared to the bare BFCO based devices.

3. CONCLUSIONS AND PERSPECTIVES We demonstrated that the p−NiO/n−BFCO heterojunction is an efficient and stable photoanode for solar water oxidation. The PEC performance of bare BFCO thin films based photoanodes can be tuned by switching the FE polarization direction. The enhanced PEC performance and multihours stability were both achieved by coating the bare BFCO photoanodes with a transparent and conductive active p−type NiO layer. The 10 nm NiO/BFCO heterojunction based photoanodes exhibited a higher photocurrent density (0.40 mA cm−2 at 1.23 V vs. RHE), a best performance of stability with time and a higher photon-to-current conversion efficiency. This finding is not limited to the p−NiO/n−BFCO platform and is expected to provide an alternative way to advantageously use TCOs/FE semiconductors in photocatalysis. Further efforts are aimed at the investigation of the photocatalytic efficiency, gas generation and the kinetics of water splitting. 14 ACS Paragon Plus Environment

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4. EXPERIMENTAL SECTION BFCO films in thickness of 90 nm with a buffer layer of 15 nm-thick SRO as bottom electrode were both deposited on STO (100) substrates by PLD. Stoichiometric BFCO and SRO targets were ablated by using a KrF excimer laser with λ = 248 nm, pulse lifetime = 25 ns, power density = 1.7 J cm-2 and laser frequency = 8 Hz. The temperature and O2 pressure were about 680 °C, 6 mTorr for BFCO, and 20 mTorr for SRO films growth, respectively. ~10 and 20 nm-thick NiO films were deposited on BFCO films. The NiO target was ablated after BFCO deposition using the same PLD setup with parameters: 1.7 J cm-2, 5 Hz, 200 °C and 10 mTorr. XRD (i.e., Panalytical X’pert Pro diffractometer) was used to investigate the crystal orientation and quality as well as the value of R by using asymmetrical θ–2θ measurements around the (111) reflections. AFM and PFM measurements were performed using the same setup from App Nano. The analyser TFA 2000 was used to measure FE properties of BFCO films at 2 kHz. SEM was applied to observe the morphology of photoanodes after stability test. The TRPL transient measured at 650 nm by using a 383 nm laser source. The direct optical absorption of BFCO thin films was measured by spectroscopic ellipsometry (i.e., a VASE ellipsometer). The absorption coefficient α was obtained from the equation α = 4π k/λ, where k is extinction coefficient. Tauc’s equation calculation was determined to study the direct bandgap of BFCO by using linear slopes in (αE)2 vs. E curves. Optical transmittance and absorbance of NiO films with different thickness were measured by a Cary 5000 UV-Vis-NIR spectrophotometer. The electrical properties of NiO films at various temperatures (253−473 K) were recorded using a four-point probe technique. Hall Effect setup was used to determine the nature (type n or p), mobility and concentration of carriers in NiO films 15 ACS Paragon Plus Environment


at room temperature with a ±0.4 Tesla magnetic field applied perpendicular to the sample surface. The XPS analysis revealed the formation of valence state 2+ for Ni. The UPS was used to determine the valence band edges and Fermi level. J−V curve measurements were performed in a three-electrode configuration: a Pt counter electrode, a reference electrode (i.e., Ag/AgCl) and a working electrode (BFCO-based heterojunction with SRO bottom electrode connected with a copper wire). The samples encapsulated by insulating epoxy (except for thin film surface) acting as photoelectrode were measured in Na2SO4 (pH 6.8, 1 mol L−1) under one Sun (~100 mW cm-2). To convert the measured potentials vs. Ag/AgCl to RHE, we used the formula: VRHE = VAg/AgCl + V0Ag/AgCl + pH × 0.059, where V0Ag/AgCl = 0.198 at 25°C. The silver paste electrodes were deposited on top of the films to pole the BFCO. As finished poling treatments, we used the HNO3 solution to remove silver electrode, and then repeated the procedure for the samples underwent an opposite polarization. We applied an electrochemical impedance method at ~1 and 2 kHz frequency to make the Mott–Schottky measurements. To derive the IPCE values for heterojunction photoanodes, we performed I−V curve measurements using different band-pass optical filters (i.e., 405, 460, 505, 595, 694 and 785 nm). The distance between sunlight simulator and heterojunction photoanode was ~30 cm. The generated gas in the chamber was collected using a vacuum tight syringe and injected into a gas chromatograph, which was calibrated to quantify oxygen and hydrogen gas.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.


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The characterization of BFCO thin films, e.g., local ferroelectric properties, AFM, XPS and UPS analyses, and calculations of the flat-band potential are supplied as Supporting Information.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (F.R.) *E-mail: [email protected] (R.N.) ORCID Federico Rosei: 0000-0001-8479-6955 Riad Nechache: 0000-0002-5431-6858 Wei Huang: 0000-0003-1170-1665 Author contributions W.H. designed the devices and experiments. W.H. and C.H. investigated the ferroelectric properties. W.H. and X.T. evaluated the photoelectrochemical performance of the devices. D.B. and W.H. performed the Mott-Schottky analysis. W.H. wrote the first draft of manuscript, and R.N., F.R., M.C., S.S. revised it. All the authors discussed the results and approved the final manuscript for submission. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge financial support from the Canada Foundation for Innovation. F.R. and R.N. thank NSERC for individual Discovery grants. F.R. is grateful to the Canada Research Chairs



program, the Chiang-Jiang short term scholar award and the 1000-talent award in Sichuan province of China.


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Table 1 Recent studies on J of ferroelectric/multiferroic photoanodes for solar water oxidation. J (mA cm-2)

Year

Photoanodes

Electrolyte

2013

BiFeO3/SRO/STO

1 M Na2SO4

0.01

[24]

2017

KNbO3/Nb-STO

1 M Na2SO4

0.01

[21]

2018

BiFeO3/SRO/STO

0.5 M Na2SO4

0.08

[20]

2019

NiO/BFCO/SRO/STO

1 M Na2SO4

0.40

This work

Ref.

Table 2 PEC performance of photoanodes with/without the NiO layer under one sun. Photoanodes

J

OCP dark/light

T

τ

IPCE

(mA cm-2)

(V)

(s)

(ns)

(%)

P0

0.10

−0.16

4.0

6.4

1.2

Pup

0.06

−

−

−

−

Pdown

0.15

−

−

−

−

20 nm NiO/BFCO

0.20

−0.13

−

−

2.7

10 nm NiO/BFCO

0.40

−0.32

6.8

8.4

3.7

bare BFCO



Figure 1. BFCO thin films on (100) STO substrate: (a) (bottom) XRD θ-2θ scan, and (top) asymmetric scan around (111) plane of (100) STO substrate. Insert images refer to the corresponding crystal structure of BFCO showing different types of Fe and Cr octahedra stacking along [001] and [111] cubic directions. The stars correspond to the (00l) Kβ line and the points indicate tungsten contamination of the x-ray cathode tube. (b) RSM measurements around the (204) reflection of STO showing the spots related to the disordered (d-) and ordered (o-) BFCO phase in the film. (c) Absorption spectra and direct optical transitions of BFCO films. 10 and 20 nm-thick NiO films grown on transparent STO substrate. (d) FE polarization−electric field hysteresis loop measured at the macroscale at 2 kHz and RT.


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Figure 2. (a) UV-visible transmission and absorbance; Inset shows the corresponding direct optical transition of films, and the intercepts of dashed lines indicate the direct bandgap of NiO. (b) Electrical conductivity with temperature for NiO films.



Figure 3. Linear sweep voltammetry (LSV) curves measured in Na2SO4 under chopped AM 1.5G illumination with intensity of 100 mW cm-2: (a) J−V vs. RHE for bare BFCO-based photoanodes at P0, Pup, and Pdown. The inset shows the Voc vs. time. (b) A schematic diagram of the experimental setup used for PEC measurements, and the proposed layout of NiO/BFCO/SRO heterojunction. (c) J−V vs. RHE for the photoanodes coated with 10 and 20 nm NiO layer; The inset shows the corresponding Voc vs. time. (d) IPCE spectra for BFCO photoanodes with/without NiO layer at 1.23 V (vs. RHE).


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Figure 4. (a) Normalized plots of the photocurrent−time (J−t) dependence for bare BFCO and NiO/BFCO photoanodes. The insert image refers to the ON/OFF J−t curves of the corresponding photoanodes at 1.23 V vs RHE. D = (It − Ist)/(Iin − Ist), where It, Iin, and Ist are the time-dependent, the initial photocurrent, and the steady-state photocurrent. (b) Stability test of photoanodes with/without the 10 nm thick-NiO layer at 0.5 V vs. RHE under 1 sun illumination. Inset shows the SEM image to surface of NiO/BFCO photoanodes after stability tests of several hours.



Figure 5. (a) Mott–Schottky measurements at 0 V bias in dark. The intercept of the dashed line can be used to determine the flat band potential (Vfb) of BFCO and NiO/BFCO. (b) Band alignment for electrolyte/NiO/BFCO/SRO heterojunctions, and an active interface of the electrolyte/NiO heterojunction.


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Epitaxial Bi2FeCrO6 Multiferroic Thin-Film Photoanodes with Ultrathin

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