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Sn:TiO2 nanorod photoanode via bulk and surface dual modifications
Nano Energy 59 (2019) 33–40
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Communication
Synergistically enhanced charge separation in BiFeO3/Sn:TiO2 nanorod photoanode via bulk and surface dual modifications
T
Jing Huanga,1, Yang Wanga,b,1, Xueqin Liua, , Yinchang Lia, Xiaoqin Hua, Bing Hea, Zhu Shua, ⁎ ⁎ Zhen Lia, , Yanli Zhaob, ⁎
a
Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, Hubei 430074, PR China b Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore, Singapore
ARTICLE INFO
ABSTRACT
Keywords: Charge separation Ferroelectric materials Photoanode Photoelectrochemistry TiO2 nanorod arrays
Charge separation is regarded as a vital factor determining the photoelectrochemical (PEC) performance of TiO2 photoanode. Herein, for the first time, the synergistic effect between Sn doping and ferroelectric BiFeO3 (BFO) coating in BFO/Sn:TiO2 composite photoanode for enhanced PEC performance is reported. The Sn doping leads to enhanced charge carrier density due to efficient charge separation. After the deposition of ferroelectric BFO thin film, the charge-separation efficiency (ηsep) is further enhanced because of spontaneous polarization of the BFO layer. More importantly, the PEC performance could be further improved by positive polarization of the BFO/Sn:TiO2 composite photoanode, achieving remarkable photocurrent density (Jph) of 1.76 mA cm−2 at 1.23 V vs. reversible hydrogen electrode and high stability. This work indicates that the dual modification (i.e. Sn doping in bulk and ferroelectric BFO thin film deposition on the surface) holds a great promise in improving the PEC performance of photoanodes by promoting charge separation, which can be extended to other common photoanode materials, such as Fe2O3 and BiVO4.
1. Introduction Photoelectrochemical (PEC) water splitting has been recognized as one of the most efficient methods for the harvest of clean and sustainable energy [1]. The search for robust and efficient photoanode materials has attracted wide scientific and technological interest, since the first report about PEC energy conversion in 1972 [2]. Owing to its superior photostability, suitable band-edge positions and abundance, titanium dioxide (TiO2) has been regarded as one of the most promising photoanode materials for hydrogen production [3,4]. However, the poor charge separation [5] currently limits its applications for ultimate industry-level solar to hydrogen production. Various strategies have been explored to tackle this challenge, for instance, bulk modifications by doping TiO2 with metal (Fe, Nb, Sn, etc) [6–8]. or nonmetal (C, H, N, S, etc) [9–12]. elements to improve the charge transport properties as well as induce red shift in the bandgap absorption [13], surface coupling TiO2 with other semiconductors to tune electronic bandgap and enhance the charge separation efficiency [14], and decorating TiO2 with water oxidation electrocatalysts to increase the surface reaction
kinetics [15]. Normally, these strategies could be divided into two categories, i.e., bulk (such as element doping) and surface (such as deposition of cocatalysts and other semiconductors) modifications. Single modification of a photoanode is hard to attain satisfying charge separation effect due to the limited scope of separation effect. Thus, bulk and surface dual modification approach of photoanodes has been applied to the improvement of PEC performance [10,16,17]. For instance, Choi and coworkers reported the deposition of TaOxNy thin film on the surface of N-doped TiO2 nanotubes, showing significantly improved PEC water splitting efficiency because of the simultaneous introduction of N-doping and TaOxNy semiconductor [10]. Therefore, combined bulk and surface modification on a photoanode is an excellent strategy for the improvement of photoanode PEC performance. The electric field in the depletion region at the interface of electrode/electrolyte provides the driving force for the charge separation of a photoanode [18]. Ferroelectric materials with non-centrosymmetric structures can produce internal spontaneous polarization or depolarization field, since positive and negative charges have different centers of symmetry [19]. An internal electric field is built within ferroelectric
Corresponding authors. E-mail addresses: [email protected] (X. Liu), [email protected] (Z. Li), [email protected] (Y. Zhao). 1 These authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.nanoen.2019.02.025 Received 2 December 2018; Received in revised form 2 February 2019; Accepted 9 February 2019 Available online 11 February 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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materials on account of the polarization, which can force the separation of photogenerated charges [20,21]. As a result, the PEC performance of a heterojunction formed by coupling ferroelectric materials with semiconductors could be significantly enhanced via the control of depletion region width by the ferroelectric polarization. Various studies have focused on the combination of ferroelectric polarization and semiconductors, such as TiO2-SrTiO3 core-shell nanowires [22], TiO2BaTiO3 core-shell nanowires [23] and BiFeO3 anchored TiO2 nanotube arrays [24], resulting in high efficiency in separating photo-generated charges of composite photoanodes. As a typical ferroelectric material, BiFeO3 (BFO) is a vital narrow bandgap semiconductor [25] that can transfer photo-generated holes to the interface of photoanode and electrolyte. Although the introduction of the BFO ferroelectric material is an efficient surface modification method to the charge separation of a photoanode [24], only the charge separation near the interface of photoanode and BFO can be enhanced [21]. Element doping is a typical bulk modification approach for photoanodes, which can effectively separate electron-hole pairs of inner photoanode by adjusting the band structure of semiconductors [8]. Thus, it is desirable to improve the PEC performance of photoanodes via bulk and surface dual modification, i.e., element doping in the bulk and ferroelectric material deposition on the surface.
Sn:TiO2 NRs. The energy dispersive X-ray (EDX) elemental analysis (Fig. S1) further confirms the existence of Sn, Ti, Fe, Bi and O, and the Sn/Ti ratio is about 1:15. The phase and structure of the obtained TiO2, Sn:TiO2 and BFO/ Sn:TiO2 samples were then investigated using powder X-ray diffraction (XRD). As shown in Fig. S2, the pristine TiO2 NRs present well-defined diffraction peaks of rutile TiO2 (JCPDS No. 21-1276), and Sn doping does not significantly affect the crystal structure of TiO2. For BFO/ Sn:TiO2, the diffraction peaks at 22.5°, 31.9°, 32.1°, 39.6°, 45.9° and 68.9° are corresponding to (012), (104), (110), (203), (024) and (214) planes of rhombohedral structure BFO with R3c space group (JCPDS No. 71-2494) respectively, and no secondary phase is identified [26]. The composition and chemical state of BFO/Sn:TiO2 were further examined by X-ray photoelectron spectroscopy (XPS). The overall XPS spectrum of BFO/Sn:TiO2 exhibits the presence of Sn, Ti, Bi, Fe and O elements (Fig. S3a). As shown in Fig. S3b, the binding energies of the Sn 3d5/2 and Sn 3d3/2 are located at 486.7 and 495.3 eV respectively, confirming that Sn exists as Sn4+ in BFO/Sn:TiO2 [27]. In Fig. S3c, the peaks of Fe 2p3/2 and Fe 2p1/2 located at 710.0 and 723.9 eV are assigned to the trivalent oxidation state of Fe in BFO [28]. Moreover, two tiny satellite peaks are observed at the binding energies of 712.0 and 717.5 eV, further indicating the presence of Fe3+ valence state [29]. The Bi 4f XPS spectrum (Fig. S3d) presents two strong peaks at 158.7 and 164.0 eV, ascribed to Bi 4f7/2 and Bi 4f5/2 respectively, which are in agreement with the XPS spectrum of Bi3+ in BFO [30]. The O 1 s XPS spectrum in Fig. S3e exhibits that the two different peaks centered at 529.5 and 531.5 eV could be assigned to the lattice oxygen bound to metals (i.e., Ti and Fe) and the chemisorbed oxygen, respectively [31]. The binding energy peaks of Ti 2p3/2 and Ti 2p1/2 with a splitting energy of 5.8 eV (Fig. S3f) are located at 458.1 and 463.9 eV respectively, which are consistent with the values of Ti4+ [32]. To understand the optical properties of TiO2, Sn:TiO2 and BFO/ Sn:TiO2 photoanodes, the UV–Vis absorption spectra of different samples were provided in Fig. S4a. The spectrum of bare TiO2 NRs shows the characteristic absorption edge at around 420 nm due to the large bandgap of TiO2 (3.0 eV for rutile). After Sn doping, the Sn:TiO2 NRs exhibit a similar light absorption property, demonstrating that the bandgap of the TiO2 NRs is not changed via the element doping and the enhanced PEC performance of Sn:TiO2 NRs may not be caused by the light absorption process. However, due to the narrow bandgap of BFO (Eg = 2.2 eV) [25], the BFO/Sn:TiO2 NRs display a red shift in the bandgap transition and slightly enhanced absorption in the visible-light region. Based on the plots of (αhv)2 vs. photon energy shown in Fig. S4b, the bandgap values (Eg) of TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs are about 3.09, 3.06 and 2.84 eV, respectively. Different PEC measurements were carried out using TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs as photoanodes to investigate their PEC properties. Fig. 2a shows the linear sweep voltammogram (LSV) curves, in which bare TiO2 NR photoanode exhibits small photocurrent density (Jph) of only 0.54 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (VRHE) due to the limited visible light absorption and low quantum efficiency of TiO2 [33]. The Sn:TiO2 photoanode presents an increased Jph value, which is 0.84 mA cm−2 at 1.23 VRHE, corresponding to 48% improvement over the pristine TiO2 NRs. The enhanced Jph of Sn:TiO2 NRs may be derived from the increased charge carrier density induced by the improved charge separation efficiency of TiO2. For comparison, the LSV of BFO film and BFO/TiO2 NRs was also tested (Fig. S5). As expected, pure BFO film exhibits poor PEC performance because of the rapid recombination of photocharges. On the other hand, BFO/TiO2 NRs show obviously increased Jph (1.01 mA cm−2 at 1.23 VRHE) as compared with that of pristine TiO2 NRs, indicating positive effect of spontaneous polarization of BFO on the PEC performance of TiO2 NRs. After the coating of ferroelectric BFO thin film on the surface of Sn:TiO2 NRs, the resulted BFO/Sn:TiO2 NR photoanode yields the largest photocurrent density of 1.47 mA cm−2 at 1.23 VRHE because of the efficient charge separation, which is 2.72 and 1.85 times higher than that of
2. Results and discussions Here, for the first time, we reported dual modified TiO2 nanorod arrays (NRs) via the Sn-doping in the bulk and the coating of ferroelectric BFO on the surface, simultaneously promoting the charge separation of TiO2 NRs from inside to the interface. More importantly, the PEC property of BFO/Sn:TiO2 NRs was further enhanced by the positive polarization with a long stability due to the existence of the depolarization field. To investigate the synergistic mechanism between Sndoping and ferroelectric BFO coating, the band structure evolution of TiO2 was also studied in detail. The synthetic route of BFO/Sn:TiO2 NRs is illustrated in Fig. 1a. Sn:TiO2 NRs were firstly grown on the fluorinedoped tin oxide (FTO) substrate via a hydrothermal method with the mixture of titanium isopropoxide and SnCl4·4H2O (Sn4+/Ti4+ = 15–45%), and then annealed in air at 450 °C for 0.5 h. Subsequently, Sn:TiO2 NRs were coated with a thin BFO film by a simple sol-gel method, followed by annealing at 550 °C in air for 5 min (see the Supporting information for details). The morphology and chemical composition evolution of TiO2, Sn:TiO2, and BFO/Sn:TiO2 were investigated using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and element mapping analyses. As shown in Fig. 1b, the pristine TiO2 NRs grown on FTO substrate are highly dense and uniform with an average diameter of ~50 nm. The vertically aligned and highly dense TiO2 NRs facilitate the electrons transfer and light absorption [18]. After the doping of Sn, no obvious change in the morphology was observed between Sn:TiO2 and TiO2 NRs (Fig. 1c). For the BFO/Sn:TiO2 NRs, Sn:TiO2 is coated by the BFO thin film homogeneously (Fig. 1d). Because the size of BFO nanoparticles is extremely small, they cannot be clearly distinguished by SEM. The cross-sectional FESEM images of BFO/Sn:TiO2 NRs (Fig. 1e) confirm that almost all NRs are vertically aligned on the FTO substrate with an average length of ~1 µm. Fig. 1f presents a low-magnification TEM image of a BFO/Sn:TiO2 NR with the diameter of ~50 nm, which is consistent with the SEM result. As shown in Fig. 1g, the measured d-spacing of 0.32 nm belongs to (110) plane of rutile TiO2. A fairly uniform BFO shell with the thickness of 2–6 nm covering on the entire Sn:TiO2 core is evidenced from high resolution TEM (HRTEM) image. The lattice spacing of the shell was measured to be 0.39 nm, corresponding to the (012) plane of rhombohedral BFO. The scanning TEM (STEM) and corresponding element mapping images also show a typical core-shell heterostructure (Fig. 1h). Moreover, uniform and distinct signals from Sn, Ti, Fe, Bi and O were observed, indicating a full and homogeneous coating of the BFO thin film onto 34
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Fig. 1. (a) Schematic representation for the growth mechanism of BFO/Sn:TiO2 NRs. (b-d) Top-view SEM images of TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs, respectively. (e) Side-view SEM image of BFO/Sn:TiO2 NRs. (f) TEM and (g) HRTEM images of a single as-synthesized BFO/Sn:TiO2 NR. (h) STEM image and element mapping of a BFO/Sn:TiO2 NR.
electrode using 35% Sn4+ solution concentration, indicating that the Sn doping into TiO2 at a suitable level could effectively enhance the PEC activity of TiO2, while excessive incorporation of Sn leads to a decrease in the photoconversion efficiency. The Jph of the BFO/Sn:TiO2 NRs is obviously affected by the thickness of the BFO shell, which can be tuned by repeating the spin-coating process (1–3 times). As shown in Fig. S6b, the Jph of the BFO/Sn:TiO2 prepared by one time spin-coating is significantly higher than other samples prepared by repeating the coating process of 2 or 3 times. This deterioration is likely the result of
pristine TiO2 NR and Sn:TiO2 NR photoanodes, respectively. In addition, the onset potential of BFO/Sn:TiO2 exhibits a cathodic shift from 0.24 to 0.18 VRHE as compared to that of pristine TiO2. In order to investigate the effect of Sn doping concentration on the PEC performance of the photoanode, the LSV curves of a series of Sn:TiO2 samples prepared using different concentrations of Sn4+ solutions (Sn/Ti = 15–45%) were tested (Fig. S6a). Obviously, the doping content of Sn shows a remarkable influence on the Jph of Sn:TiO2 photoanode. The optimal PEC performance was achieved for the
Fig. 2. PEC performance of TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs: (a) LSV measurements. (b) Transient Jph vs. time plots at an applied potential of 1.23 VRHE. (c) ABPE curves. (d) IPCE curves. The PEC experiments were carried out in 1 M NaOH electrolyte under AM 1.5 G (100 mW cm−2) illumination. 35
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increased hole transport resistance caused by over-loading of BFO. Since ferroelectric BFO is a semiconductor material having certain light absorption capability, thick BFO film may compete with underlying TiO2 for the light absorption. The representative transient photocurrent (I-t) measurements of obtained photoanodes were conducted at external potential of 1.23 VRHE under chopped illumination (Fig. S7a). A significant enhancement in the Jph of Sn:TiO2 NR photoanode is achieved as compared to the pure TiO2 NR photoelectrode. Furthermore, the BFO/Sn:TiO2 NR electrode demonstrates the highest photocurrent density, which is consistent with the LSV results. The applied bias photon-to-current efficiency (ABPE) of different photoanodes was calculated (Fig. 2b) to quantitatively evaluate the PEC performance of TiO2, Sn:TiO2 and BFO/ Sn:TiO2 NRs. The ABPE was determined by the following equation [34]:
After the Sn species were doped into TiO2, the O 2p VB edge of TiO2 has a great shift toward the vacuum level, attributing to the introduction of O 2p/Sn 5s hybrid orbitals in the VB of TiO2, yielding a new VB edge on the surface of TiO2. The result clearly manifests that the incorporation of Sn species remarkably changes the electronic structure of TiO2 via the Ti-O-Sn linkage, which is in accordance with previous studies [36–38]. By combining the optical bandgaps of TiO2 NRs (3.09 eV) and Sn:TiO2 NRs (3.06 eV) obtained from the UV–Vis spectra, the conduction band minimum (CBM) is inferred to occur at − 0.85 and − 1.13 eV for TiO2 and Sn:TiO2 NRs, respectively. Therefore, the band structure alignments of TiO2 and Sn:TiO2 are well resolved and schematically given in Fig. S11. Notably, the CBM of Sn:TiO2 NRs is more negative than that of pristine TiO2 NRs, which increases the driving force for the electron injection from TiO2 to the substrate (FTO) and thus facilitates the separation of photoinduced electrons and holes [39]. The photoinduced electrons and holes can recombine by emitting photons, which is the reason for the photoluminescence (PL). Thus, the PL measurement is an efficient method to the characterization of charge separation from a photoanode [18]. The separation behavior of photoinduced carriers for TiO2, Sn:TiO2 and BFO/Sn:TiO2 was demonstrated via the PL measurement. As shown in Fig. 3d, the pristine TiO2 shows two typical intensive fluorescence emission peaks at about 409 and 460 nm [40]. After the introduction of Sn doping and ferroelectric BFO, the fluorescence signals decrease sharply, indicating that the fluorescenceassociated recombination in BFO/Sn:TiO2 is greatly suppressed. Electrochemical impedance spectroscopy (EIS) measurements were conducted to further clarify the charge separation and transport behavior of the as-prepared electrodes. As shown in Fig. 3e, all three samples (i.e., TiO2, Sn:TiO2 and BFO/Sn:TiO2) present semicircles in the EIS Nyquist curves, the diameter of which demonstrates the electron transfer resistance (Rct) [41,42]. Based on the fitting results (inset figure in Fig. 3e and Table S1), BFO/Sn:TiO2 photoanode exhibits the Rct value of 408 Ω, which is much smaller than that of TiO2 (1326 Ω) and Sn:TiO2 (1250 Ω), indicating facile charge transport of BFO/Sn:TiO2 electrode. These results confirm that more effective separation and faster migration of photoinduced charges on the BFO/Sn:TiO2 NRs are responsible for the enhanced PEC activity. The photogenerated charge separation behavior of different samples was further investigated by measuring the surface photovoltage spectrum (SPS) [43]. The surface photovoltage (SPV) originates from the surface potential change under the light irradiation, whenever photoinduced charge carriers are generated in bulk. For a n-type semiconductor, a positive SPV signal indicates the transfer of photogenerated holes to the illumination side of the electrode [44]. For the SPS measurements here, the incident light irradiates from the surface of the electrode. Consequently, the positive SPV signals of all electrodes (Fig. 3f) demonstrate the accumulation of photoinduced holes at the surface of photoanodes [45]. In general, strong SPV response suggests that high concentration of photogenerated charge carriers migrates to the surface [46]. As compared with TiO2 (8 μV) and Sn:TiO2 (11 μV), much higher SPV signal was observed for BFO/Sn:TiO2 (18 μV), indicating that more photogenerated holes migrate to the surface of BFO/ Sn:TiO2 before the recombination. The SPS results further confirm the enhanced charge separation of TiO2 NRs on account of the synergistic effect from the Sn doping and ferroelectric BFO film coating. In order to further improve the PEC performance of BFO/Sn:TiO2 photoanode via the utilization of ferroelectric property of the BFO film, the BFO/Sn:TiO2 photoanode was polarized in positive direction under an external bias (+2 V) before the LSV test, and a negative polarization (−2 V) was also conducted as the reference. As shown in Fig. 4a, the LSV curves of BFO/Sn:TiO2 photoanode with positive and negative polarization undoubtedly indicate the crucial effect of the polarization state of the ferroelectric BFO on the PEC performance of the photoanode. Specifically, after positive poling (i.e., the orientation of major ferroelectric domains in the BFO film is from the photoanode surface to
= Jph (1.23–|V |)/I0 where Jph is the photocurrent density, V is the potential vs. RHE, and I0 is the power density of incident light (100 mW cm−2). The maximum photoconversion efficiency for TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs is 0.21% at 0.58 V, 0.39% at 0.51 V, and 0.72% at 0.53 V respectively, indicating that the conversion efficiency could be significantly enhanced by the Sn doping and the coating of ferroelectric BFO. To further evaluate the PEC water oxidation performance of the as-prepared electrodes, the incident photon-to-current conversion efficiency (IPCE) was determined under monochromatic light irradiation at an applied potential of 1.23 VRHE (Fig. S7b). At around 375 nm, a maximum IPCE of 82% was achieved for the BFO/Sn:TiO2 photoanode, which is much higher than that of pristine TiO2 (48%) and Sn:TiO2 (63%) at around the same wavelength, indicating that the synergistic effect of the Sn doping and ferroelectric BFO thin film coating effectively enhances the charge harvesting during the PEC process. To confirm the synergistic effect of Sn doping and ferroelectric BFO coating on the charge separation of as-prepared photoanodes, the charge injecting efficiency (ηinj) and charge separation efficiency (ηsep) of different electrodes were calculated (Figs. S8 and S9 for calculation details). As shown in Fig. 3a, only a slight increase in the ηinj was observed after the Sn doping and BFO thin film coating, suggesting that the surface reactivity was not the determining factor for the enhanced PEC performance. For the charge separation efficiency (Fig. 3b), however, Sn:TiO2 NRs exhibit the value of 63% at 1.23 VRHE, which is 1.54 times higher than that of the pristine TiO2 NRs (41% at 1.23 VRHE). Moreover, the BFO/Sn:TiO2 NR photoanode presents the highest ηsep of 74% at 1.23 VRHE. The remarkable increasement in ηsep matches well with the Jph variation shown in Fig. 2a, demonstrating that the improved PEC performance of BFO/Sn:TiO2 NRs is attributed to efficient charge separation by the dual modification of Sn doping and ferroelectric BFO thin film coating. In order to study the water splitting capacity, the evolved H2 and O2 gases using TiO2, Sn:TiO2 and BFO/Sn:TiO2 photoanodes were detected by gas chromatography every 30 min (Fig. S10). The ratio between H2 and O2 gases produced from the photoanodes and Pt counter electrode is close to 2:1, indicating the complete water splitting. The Faradic efficiency of TiO2, Sn:TiO2 and BFO/Sn:TiO2 photoanodes was calculated to be 93.1%, 90.7% and 92.1%, respectively. The high Faradic efficiency suggests that most of the photogenerated holes are used for the water oxidation [35]. In addition, the BFO/Sn:TiO2 photoanode shows an average O2 production of 12.6 μmol h−1 cm−2, which is 0.77 and 1.68 times higher than that of Sn:TiO2 and TiO2 respectively, demonstrating that the dual modifications of Sn doping and BFO coating in BFO/Sn:TiO2 indeed improve the water splitting capacity. Valence band maximum (VBM) of both TiO2 and Sn:TiO2 was measured to investigate the mechanism for the enhanced charge separation of TiO2 via the Sn doping (Fig. 3c). The VBM estimated for the pristine TiO2 and Sn:TiO2 is 2.24 and 1.93 eV, respectively. The negative shift of VBM could be regarded as the introduction of Sn 5 s orbital to form hybridized one located at the valence band (VB) level in TiO2. 36
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Fig. 3. (a) Charge injection efficiency and (b) charge separation efficiency of TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs. (c) Valence band XPS spectra of TiO2 and Sn:TiO2 NRs. (d) PL spectra, (e) EIS and (f) SPS for TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs.
the electrolyte), the Jph (1.76 mA cm−2) of BFO/Sn:TiO2 photoanode is significantly improved as compared to the unpoled sample. This is because that the spontaneous polarization in the as-prepared ferroelectric BFO is not fully aligned throughout the whole shell along the positive polarization direction before being poled under an external electric field, and the whole BFO shell exhibits a weak polar state [22]. In this case, the BFO film has an inferior effect on the charge separation near the BFO/Sn:TiO2 interface. After the positive polarization, the wellaligned dipoles in the BFO film have a substantial effect on the charge separation, illustrative of a markedly enhanced PEC performance of the BFO/Sn:TiO2 photoanode [47]. On the other hand, a significant decay for the Jph of BFO/Sn:TiO2 photoanode after negative polarization was observed due to reverse alignment direction of ferroelectric domains in the BFO film. Representative polarization-electric (P-E) hysteresis loop of the BFO film with an obvious remnant polarization (Pr) of 0.6 μC cm−2 (Fig. S12) indicates its ferroelectric property. To confirm the substantial effect of the BFO remnant polarization on the charge separation of the photoanode, the stability of the BFO/Sn:TiO2 photoanode after positive polarization was investigated at 1.23 VRHE for 3 h. As shown in Fig. 4b,
the Jph of BFO/Sn:TiO2 only decreases from initial 1.76–1.63 mA cm−2, corresponding to 4% decay. The high stability indicates that the remnant polarization of the BFO film could be maintained for a long time, which is important to sustain the enhanced PEC performance induced by the positive polarization. To gain more insight in the effect of positive polarization on the charge separation of BFO/Sn:TiO2 photoanode, the ηinj and ηsep of BFO/ Sn:TiO2 photoanode with positive and negative polarization were measured (Fig. 4c,d). As compared to the BFO/Sn:TiO2 without the polarization (74% at 1.23 VRHE), ηsep reaches 87% after positive polarization, which is 2 times higher than that of pristine TiO2 NRs (41% in Fig. 3b). In contrast, these three different photoanodes exhibit almost identical ηinj, indicating that the polarization of the BFO shell has a little effect on the surface oxidation reaction of BFO/Sn:TiO2 photoanode. Inspired by this result, further deposition of oxygen evolution catalysts on the surface of BFO/Sn:TiO2 may significantly increase its PEC performance. According to above investigation results, a mechanism (Fig. 5) based on the band structure evolution was proposed to elucidate the enhanced charge separation of BFO/Sn:TiO2 photoanode. The Sn 37
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Fig. 4. (a) J-V curves of BFO/Sn:TiO2 NRs with no poling, positive poling and negative poling. (b) Time courses for the photocurrent density of BFO/Sn:TiO2 NRs after the positive poling. (c) Charge injection efficiency and (d) charge separation efficiency of BFO/Sn:TiO2 NRs with no poling, positive poling and negative poling.
charge carrier density was calculated to be 1.2 × 1019 and 3.8 × 1019 cm−3 for TiO2 and Sn:TiO2 NRs, respectively. Obviously, the charge carrier density of TiO2 NRs is effectively increased via the Sn doping, which is consistent to the previous report [8]. To equilibrate the Fermi levels between n-type TiO2 and electrolyte, the electrons transfer from TiO2 to electrolyte when the photoanode is immersed in an electrolyte, leading to upward band bending and formation of a depletion layer on the surface of TiO2. The built-in electric field in the depletion layer provides the driving force for the separation of photoinduced charges under solar light illumination and facilitates the transfer of holes toward the electrolyte [48,49]. Typically, the amplification of the band bending leads to the enhanced charge separation. Applying an external electric field is an effective approach to tune the magnitude of the depletion region in semiconductors [18]. After the coating of the BFO film, the electric field with a direction from electrode to electrolyte induced by the spontaneous polarization (PS) of ferroelectric BFO further increases the magnitude of the band bending (Fig. 5b), resulting in easy transfer of holes to the interface of electrode/ electrolyte, supported by the results of EIS and SPV (Fig. 3e,f). In order to confirm spontaneous polarization of the BFO film, piezoresponse force microscopy (PFM) amplitude and phase images of BiFeO3/Sn:TiO2 NRs were characterized by PFM (Fig. S14). Yellow and purple areas in the PFM phase image with strong contrast even in the absence of bias voltage indicate upward-downward oriented domains (Fig. S14d), verifying the existence of spontaneous polarization across the BFO film. After electric poling at + 10 V supplied by the PFM tip, most of the domains were aligned upwards (Fig. S14e). Obvious switching of the domain direction was observed after further poling at an opposite electric field (Fig. S14f), confirming the ferroelectric nature of the BFO film. The intensity of an external electric field bought by ferroelectric materials could be improved significantly by the positive poling, because the remnant polarization (Pr) intensity is much higher than that of spontaneous polarization. As a result, the magnitude of band bending from the BFO/Sn:TiO2 photoanode exhibits obvious increasing after the
Fig. 5. Schematic electronic band diagram of (a) Sn:TiO2 and (b-d) BFO/ Sn:TiO2 with (b) no poling BFO, (c) positive poling BFO, and (d) negative poling BFO.
doping is an effective method to improve the charge separation as verified by the results of PL and ηsep, leading to the increased charge carrier density (Fig. 5a). In order to further validate the effect of Sn doping on the improved PEC performance of TiO2 NRs, Mott-Schottky plots of TiO2 and Sn:TiO2 NRs were obtained (Fig. S13), from which the 38
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positive poling as compared to the case without poling (Fig. 5c). Furthermore, the negative polarization creates a reverse direction (from electrolyte to electrode) of external electric field near the semiconductor surface, leading to the reduced magnitude of band bending (Fig. 5d), confirmed by the ηsep of BFO/Sn:TiO2 photoanode with positive or negative polarization (Fig. 4d). It should be noted that the width of the depletion layer is also a decisive factor to the charge separation, and the widening of the layer could lead to the improved charge separation. The calculated width of the depletion layer for BFO/ Sn:TiO2 with no/positive/negative poling (Fig. S13) is 8, 7, and 10 nm, respectively. The results mean that the positive poling decreases the width of the depletion layer, leading to smaller number of photons being absorbed in the depletion region. Based on significantly increased PEC performance of BFO/Sn:TiO2 after positive polarization, it was concluded that the enhanced charge separation within the depletion region induced by the amplification of band bending magnitude could compensate the negative effect resulted by the narrowing of the depletion region, which is consistent with the previous report [50]. In addition, Sn-doping, BFO coating or positive polarization has a negligible effect on the charge injection efficiency of the photoanode, meaning that the band bending is not the decisive factor for surface reaction kinetics of the photoanode, and the PEC performance of the BFO/Sn:TiO2 photoanode could be further enhanced via the deposition of oxygen evolution cocatalysts.
[6] C. Wang, Z. Chen, H. Jin, C. Cao, J. Li, Z. Mi, J. Mater. Chem. A 2 (2014) 17820–17827. [7] Y. Alivov, V. Singh, Y. Ding, L.J. Cerkovnik, P. Nagpal, Nanoscale 6 (2014) 10839–10849. [8] M. Xu, P. Da, H. Wu, D. Zhao, G. Zheng, Nano Lett. 12 (2012) 1503–1508. [9] A. Nakada, S. Nishioka, J.J.M. Vequizo, K. Muraoka, T. Kanazawa, A. Yamakata, S. Nozawa, H. Kumagai, S.-i. Adachi, O. Ishitani, K. Maeda, J. Mater. Chem. A 5 (2017) 11710–11719. [10] H.-i. Kim, D. Monllor-Satoca, W. Kim, W. Choi, Energy Environ. Sci. 8 (2015) 247–257. [11] M. Wang, J. Ioccozia, L. Sun, C. Lin, Z. Lin, Energy Environ. Sci. 7 (2014) 2182–2202. [12] B. Roose, S. Pathak, U. Steiner, Chem. Soc. Rev. 44 (2015) 8326–8349. [13] M. Reza Gholipour, C.T. Dinh, F. Beland, T.O. Do, Nanoscale 7 (2015) 8187–8208. [14] X. Liu, Y. Wang, X. Cui, M. Zhang, B. Wang, M. Rager, Z. Shu, Y. Yang, Z. Li, Z. Lin, J. Mater. Chem. A 7 (2019) 165–171. [15] G. Ai, H. Li, S. Liu, R. Mo, J. Zhong, Adv. Funct. Mater. 25 (2015) 5706–5713. [16] K.R. Yoon, J.W. Ko, D.-Y. Youn, C.B. Park, I.-D. Kim, Green Chem. 18 (2016) 944–950. [17] M.P. Suryawanshi, U.V. Ghorpade, S.W. Shin, M.G. Gang, X. Wang, H. Park, S.H. Kang, J.H. Kim, ACS Catal. 7 (2017) 8077–8089. [18] Z. Zhang, J.T. Yates Jr, Chem. Rev. 112 (2012) 5520–5551. [19] L. Li, P.A. Salvador, G.S. Rohrer, Nanoscale 6 (2014) 24–42. [20] D. Cao, Z. Wang, Nasori, L. Wen, Y. Mi, Y. Lei, Angew. Chem. Int. Ed. 53 (2014) 11027–11031. [21] L. Yang, Y. Xiong, W. Guo, M. Zhou, K. Song, P. Xiao, G. Cao, Nano Energy 44 (2018) 63–72. [22] F. Wu, Y. Yu, H. Yang, L.N. German, Z. Li, J. Chen, W. Yang, L. Huang, W. Shi, L. Wang, X. Wang, Adv. Mater. 29 (2017) 1701432. [23] W. Yang, Y. Yu, M.B. Starr, X. Yin, Z. Li, A. Kvit, S. Wang, P. Zhao, X. Wang, Nano Lett. 15 (2015) 7574–7580. [24] A. Sarkar, A.K. Singh, D. Sarkar, G.G. Khan, K. Mandal, ACS Sustain. Chem. Eng. 3 (2015) 2254–2263. [25] Z. Li, Z. Hou, W. Song, X. Liu, W. Cao, X. Shao, M. Cao, Nanoscale 8 (2016) 10415–10424. [26] Y. Ren, F. Nan, L. You, Y. Zhou, Y. Wang, J. Wang, X. Su, M. Shen, L. Fang, Small 13 (2017) 1603457. [27] G.D. Park, Y.C. Kang, Nano Res. 11 (2018) 1301–1312. [28] P. Tang, H. Xie, C. Ros, L. Han, M. Biset-Peiró, Y. He, W. Kramer, A.P. Rodríguez, E. Saucedo, J.R. Galán-Mascarós, T. Andreu, J.R. Morante, J. Arbiol, Energy Environ. Sci. 10 (2017) 2124–2136. [29] W. Eerenstein, F.D. Morrison, J. Dho, M.G. Blamire, J.F. Scott, N.D. Mathur, Science 307 (2005) 1203. [30] L.T. Quynh, C.N. Van, Y. Bitla, J. Chen, T. Do, W. Tzeng, S. Liao, K. Tsai, Y. Chen, C. Wu, C. Lai, C. Luo, Y. Hsu, Y. Chu, Adv. Energy Mater. 6 (2016) 1600686. [31] X. Chen, S. Shen, L. Guo, S.S. Mao, Chem. Rev. 110 (2010) 6503–6570. [32] C. Chen, Y. Wei, G. Yuan, Q. Liu, R. Lu, X. Huang, Y. Cao, P. Zhu, Adv. Funct. Mater. 27 (2017) 1701575. [33] F. Lei, L. Zhang, Y. Sun, L. Liang, K. Liu, J. Xu, Q. Zhang, B. Pan, Y. Luo, Y. Xie, Angew. Chem. Int. Ed. 54 (2015) 9266–9270. [34] Y. Hou, F. Zuo, A. Dagg, P. Feng, Nano Lett. 12 (2012) 6464–6473. [35] S. Wang, T. He, J. Yun, Y. Hu, M. Xiao, A. Du, L. Wang, Adv. Funct. Mater. 28 (2018) 1802685. [36] H. Zhuang, Y. Zhang, Z. Chu, J. Long, X. An, H. Zhang, H. Lin, Z. Zhang, X. Wang, Phys. Chem. Chem. Phys. 18 (2016) 9636–9644. [37] F. Wang, J. Ho, Y. Jiang, R. Amal, ACS Appl. Mater. Interfaces 7 (2015) 23941–23948. [38] H. Zhuang, Q. Gu, J. Long, H. Lin, H. Lin, X. Wang, RSC Adv. 4 (2014) 34315–34324. [39] S. Guo, Z. Deng, M. Li, B. Jiang, C. Tian, Q. Pan, H. Fu, Angew. Chem. Int. Ed. 126 (2016) 1862–1866. [40] F. Ning, M. Shao, S. Xu, Y. Fu, R. Zhang, M. Wei, D.G. Evans, X. Duan, Energy Environ. Sci. 9 (2016) 2633–2643. [41] M. Zhu, Z. Sun, M. Fujitsuka, T. Majima, Angew. Chem. Int. Ed. 57 (2018) 2160–2164. [42] Y. Zhou, L. Zhang, L. Lin, B.R. Wygant, Y. Liu, Y. Zhu, Y. Zheng, C.B. Mullins, Y. Zhao, X. Zhang, G. Yu, Nano Lett. 17 (2017) 8012–8017. [43] L. Jing, W. Zhou, G. Tian, H. Fu, Chem. Soc. Rev. 42 (2013) 9509–9549. [44] M.A. Melo Jr., Z. Wu, B.A. Nail, A.T. De Denko, A.F. Nogueira, F.E. Osterloh, Nano Lett. 18 (2018) 805–810. [45] M. Xie, X. Fu, L. Jing, P. Luan, Y. Feng, H. Fu, Adv. Energy Mater. 4 (2014) 1300995. [46] Z. Zhao, E.J. Willard, H. Li, Z. Wu, R.H.R. Castro, F.E. Osterloh, J. Mater. Chem. A 6 (2018) 16170–16176. [47] H.S. Kim, S.K. Kim, B.J. Kim, K.S. Shin, M.K. Gupta, H.S. Jung, S.W. Kim, N.G. Park, J. Phys. Chem. Lett. 6 (2015) 1729–1735. [48] S. Bai, J. Jiang, Q. Zhang, Y. Xiong, Chem. Soc. Rev. 44 (2015) 2893–2939. [49] C. Jiang, S.J.A. Moniz, A. Wang, T. Zhang, J. Tang, Chem. Soc. Rev. 46 (2017) 4645–4660. [50] A.J. Abel, A.M. Patel, S.Y. Smolin, B. Opasanont, J.B. Baxter, J. Mater. Chem. A 4 (2016) 6495–6504.
3. Conclusions In summary, ferroelectric BFO coated Sn-doped TiO2 NR photoanode has been fabricated for enhanced PEC water splitting. On account of synergistic effect from the Sn doping and positive polarization of BFO, the obtained BFO/Sn:TiO2 photoanode exhibits remarkable photocurrent of 1.76 mA cm−2 at 1.23 VRHE with an excellent stability. Based on the band structure evolution of TiO2, Sn-doping can improve the charge carrier density, while positive polarization of BFO contribute to the increased magnitude of TiO2 band bending, resulting in remarkably enhanced charge separation. In addition, the deposition of oxygen evolution cocatalysts may further enhance the water splitting efficiency of the BFO/Sn:TiO2 photoanode. The proposed surface and bulk dual modification of TiO2 provides a model for the design of other high performance photoanodes. Acknowledgements The work was supported by the National Natural Science Foundation of China (41502030), the Natural Science Foundation of Hubei Province of China (2017CFB190), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG170638), and the Open Foundation of Engineering Research Center of Nano-Geomaterials of Ministry of Education (NGM2017KF002 and NGM2018KF017). This research is also supported by the Singapore Academic Research Fund (RG5/16, RG11/17 and RG114/17). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2019.02.025. References [1] [2] [3] [4]
X. Lu, S. Xie, H. Yang, Y. Tong, H. Ji, Chem. Soc. Rev. 43 (2014) 7581–7593. A. Fujishima, K. Honda, Nature 238 (1972) 37–38. Y. Wang, X. Liu, C. Zheng, Y. Li, S. Jia, Z. Li, Y. Zhao, Adv. Sci. 5 (2018) 1700844. Y. Wang, X. Liu, J. Liu, B. Han, X. Hu, F. Yang, Z. Xu, Y. Li, S. Jia, Z. Li, Y. Zhao, Angew. Chem. Int. Ed. 57 (2018) 5765–5771. [5] Q. Peng, B. Kalanyan, P.G. Hoertz, A. Miller, D.H. Kim, K. Hanson, L. Alibabaei, J. Liu, T.J. Meyer, G.N. Parsons, J.T. Glass, Nano Lett. 13 (2013) 1481–1488.
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J. Huang, et al. Jing Huang received her B.E. degree from Wuhan Textile University, China in 2017. Currently, she is pursuing her M.S. degree under the supervision of Prof. Zhen Li from Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, China. Her current research is focused on the synthesis of ferroelectric-TiO2based hybrid materials and their applications in photoelectrochemical water-splitting field.
Bing He received her B.E. degree from Henan University of Technology, China in 2017. Currently, she is pursuing his M.S. degree under the supervision of Prof. Zhen Li from Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, China. Her current research is focused on the fabrication of nanomaterials and the photoelectrochemical properties of semiconductors.
Yang Wang received his B.E. degree in the College of Mechanical and Electronic Engineering at China University of Petroleum, Qingdao, China in 2012. He obtained his Ph.D. degree from China University of Geosciences in 2018, under the supervision of Prof. Zhen Li. He has spent two years in Prof. Yanli Zhao's group at Nanyang Technological University as a visiting PhD student. His current research is focused on photoelectrochemical properties of grapheneTiO2 hybrid semiconductors and efficient electron transfer materials in photoelectrochemical cells.
Zhu Shu is an Associate Professor in the Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). He received his Ph.D. degree in Environmental Science and Engineering from China University of Geosciences (Wuhan) in 2011, with one year (2010) in Instituto de Tecnología Cerámica, Jaume I University, Spain as a visiting student. He carried out his postdoctoral research in the group of Prof. Jianlin Shi in Shanghai Institute of Ceramics, Chinese Academy of Sciences during 2011–2013. His research interests are green catalysts for solar energy conversion and environmental remediation.
Xueqin Liu is currently an Associate Professor in the Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). He received his Ph.D. degree in Materials Science and Engineering from China University of Geosciences (Wuhan) in 2016. He has spent two years in the group of Prof. Zhiqun Lin at Georgia Institute of Technology as a visiting PhD student. His research interests include the structure modification of catalytic materials and their applications in solar energy conversion and environment remediation.
Zhen Li is a Professor in the Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). She received her Ph.D. degree in mineral-petrological materials science from China University of Geosciences (Wuhan) in 2004. Her research is devoted to the development and application of graphite materials, the growth and application of multifunctional nanocrystals, comprehensive utilization of non-metallic minerals, and the preparation of functional nanocomposites.
Yinchang Li received his B.E. degree from China University of Geosciences, Wuhan, China in 2013. Currently, he is pursuing his Ph.D. degree under the supervision of Prof. Zhen Li from Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, China. His research focused on the fabrication and modification of nanomaterials, especially in the electrocatalysis water-splitting field.
Yanli Zhao is a Professor at Nanyang Technological University, Singapore. He received his B.Sc. degree in chemistry from Nankai University and Ph.D. degree in physical chemistry under the supervision of Professor Yu Liu. He was a Postdoctoral scholar with Professor Sir Fraser Stoddart at the University of California, Los Angeles and subsequently at Northwestern University. His current research focuses on the development of integrated systems for diagnostics and therapeutics, as well as porous materials for gas storage and photocatalysis.
Xiaoqin Hu received her B.Sc. degree in the School of Material Science and Technology at Jiangxi University of Science and Technology, Jiangxi, China in 2013. Currently, she is pursuing his Ph.D. degree under the supervision of Prof. Zhen Li from Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, China. Her current research is focused on photoelectrochemical properties of Fe2O3 semiconductors in photoelectrochemical cells.
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Communication
Synergistically enhanced charge separation in BiFeO3/Sn:TiO2 nanorod photoanode via bulk and surface dual modifications
T
Jing Huanga,1, Yang Wanga,b,1, Xueqin Liua, , Yinchang Lia, Xiaoqin Hua, Bing Hea, Zhu Shua, ⁎ ⁎ Zhen Lia, , Yanli Zhaob, ⁎
a
Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, Hubei 430074, PR China b Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore, Singapore
ARTICLE INFO
ABSTRACT
Keywords: Charge separation Ferroelectric materials Photoanode Photoelectrochemistry TiO2 nanorod arrays
Charge separation is regarded as a vital factor determining the photoelectrochemical (PEC) performance of TiO2 photoanode. Herein, for the first time, the synergistic effect between Sn doping and ferroelectric BiFeO3 (BFO) coating in BFO/Sn:TiO2 composite photoanode for enhanced PEC performance is reported. The Sn doping leads to enhanced charge carrier density due to efficient charge separation. After the deposition of ferroelectric BFO thin film, the charge-separation efficiency (ηsep) is further enhanced because of spontaneous polarization of the BFO layer. More importantly, the PEC performance could be further improved by positive polarization of the BFO/Sn:TiO2 composite photoanode, achieving remarkable photocurrent density (Jph) of 1.76 mA cm−2 at 1.23 V vs. reversible hydrogen electrode and high stability. This work indicates that the dual modification (i.e. Sn doping in bulk and ferroelectric BFO thin film deposition on the surface) holds a great promise in improving the PEC performance of photoanodes by promoting charge separation, which can be extended to other common photoanode materials, such as Fe2O3 and BiVO4.
1. Introduction Photoelectrochemical (PEC) water splitting has been recognized as one of the most efficient methods for the harvest of clean and sustainable energy [1]. The search for robust and efficient photoanode materials has attracted wide scientific and technological interest, since the first report about PEC energy conversion in 1972 [2]. Owing to its superior photostability, suitable band-edge positions and abundance, titanium dioxide (TiO2) has been regarded as one of the most promising photoanode materials for hydrogen production [3,4]. However, the poor charge separation [5] currently limits its applications for ultimate industry-level solar to hydrogen production. Various strategies have been explored to tackle this challenge, for instance, bulk modifications by doping TiO2 with metal (Fe, Nb, Sn, etc) [6–8]. or nonmetal (C, H, N, S, etc) [9–12]. elements to improve the charge transport properties as well as induce red shift in the bandgap absorption [13], surface coupling TiO2 with other semiconductors to tune electronic bandgap and enhance the charge separation efficiency [14], and decorating TiO2 with water oxidation electrocatalysts to increase the surface reaction
kinetics [15]. Normally, these strategies could be divided into two categories, i.e., bulk (such as element doping) and surface (such as deposition of cocatalysts and other semiconductors) modifications. Single modification of a photoanode is hard to attain satisfying charge separation effect due to the limited scope of separation effect. Thus, bulk and surface dual modification approach of photoanodes has been applied to the improvement of PEC performance [10,16,17]. For instance, Choi and coworkers reported the deposition of TaOxNy thin film on the surface of N-doped TiO2 nanotubes, showing significantly improved PEC water splitting efficiency because of the simultaneous introduction of N-doping and TaOxNy semiconductor [10]. Therefore, combined bulk and surface modification on a photoanode is an excellent strategy for the improvement of photoanode PEC performance. The electric field in the depletion region at the interface of electrode/electrolyte provides the driving force for the charge separation of a photoanode [18]. Ferroelectric materials with non-centrosymmetric structures can produce internal spontaneous polarization or depolarization field, since positive and negative charges have different centers of symmetry [19]. An internal electric field is built within ferroelectric
Corresponding authors. E-mail addresses: [email protected] (X. Liu), [email protected] (Z. Li), [email protected] (Y. Zhao). 1 These authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.nanoen.2019.02.025 Received 2 December 2018; Received in revised form 2 February 2019; Accepted 9 February 2019 Available online 11 February 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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materials on account of the polarization, which can force the separation of photogenerated charges [20,21]. As a result, the PEC performance of a heterojunction formed by coupling ferroelectric materials with semiconductors could be significantly enhanced via the control of depletion region width by the ferroelectric polarization. Various studies have focused on the combination of ferroelectric polarization and semiconductors, such as TiO2-SrTiO3 core-shell nanowires [22], TiO2BaTiO3 core-shell nanowires [23] and BiFeO3 anchored TiO2 nanotube arrays [24], resulting in high efficiency in separating photo-generated charges of composite photoanodes. As a typical ferroelectric material, BiFeO3 (BFO) is a vital narrow bandgap semiconductor [25] that can transfer photo-generated holes to the interface of photoanode and electrolyte. Although the introduction of the BFO ferroelectric material is an efficient surface modification method to the charge separation of a photoanode [24], only the charge separation near the interface of photoanode and BFO can be enhanced [21]. Element doping is a typical bulk modification approach for photoanodes, which can effectively separate electron-hole pairs of inner photoanode by adjusting the band structure of semiconductors [8]. Thus, it is desirable to improve the PEC performance of photoanodes via bulk and surface dual modification, i.e., element doping in the bulk and ferroelectric material deposition on the surface.
Sn:TiO2 NRs. The energy dispersive X-ray (EDX) elemental analysis (Fig. S1) further confirms the existence of Sn, Ti, Fe, Bi and O, and the Sn/Ti ratio is about 1:15. The phase and structure of the obtained TiO2, Sn:TiO2 and BFO/ Sn:TiO2 samples were then investigated using powder X-ray diffraction (XRD). As shown in Fig. S2, the pristine TiO2 NRs present well-defined diffraction peaks of rutile TiO2 (JCPDS No. 21-1276), and Sn doping does not significantly affect the crystal structure of TiO2. For BFO/ Sn:TiO2, the diffraction peaks at 22.5°, 31.9°, 32.1°, 39.6°, 45.9° and 68.9° are corresponding to (012), (104), (110), (203), (024) and (214) planes of rhombohedral structure BFO with R3c space group (JCPDS No. 71-2494) respectively, and no secondary phase is identified [26]. The composition and chemical state of BFO/Sn:TiO2 were further examined by X-ray photoelectron spectroscopy (XPS). The overall XPS spectrum of BFO/Sn:TiO2 exhibits the presence of Sn, Ti, Bi, Fe and O elements (Fig. S3a). As shown in Fig. S3b, the binding energies of the Sn 3d5/2 and Sn 3d3/2 are located at 486.7 and 495.3 eV respectively, confirming that Sn exists as Sn4+ in BFO/Sn:TiO2 [27]. In Fig. S3c, the peaks of Fe 2p3/2 and Fe 2p1/2 located at 710.0 and 723.9 eV are assigned to the trivalent oxidation state of Fe in BFO [28]. Moreover, two tiny satellite peaks are observed at the binding energies of 712.0 and 717.5 eV, further indicating the presence of Fe3+ valence state [29]. The Bi 4f XPS spectrum (Fig. S3d) presents two strong peaks at 158.7 and 164.0 eV, ascribed to Bi 4f7/2 and Bi 4f5/2 respectively, which are in agreement with the XPS spectrum of Bi3+ in BFO [30]. The O 1 s XPS spectrum in Fig. S3e exhibits that the two different peaks centered at 529.5 and 531.5 eV could be assigned to the lattice oxygen bound to metals (i.e., Ti and Fe) and the chemisorbed oxygen, respectively [31]. The binding energy peaks of Ti 2p3/2 and Ti 2p1/2 with a splitting energy of 5.8 eV (Fig. S3f) are located at 458.1 and 463.9 eV respectively, which are consistent with the values of Ti4+ [32]. To understand the optical properties of TiO2, Sn:TiO2 and BFO/ Sn:TiO2 photoanodes, the UV–Vis absorption spectra of different samples were provided in Fig. S4a. The spectrum of bare TiO2 NRs shows the characteristic absorption edge at around 420 nm due to the large bandgap of TiO2 (3.0 eV for rutile). After Sn doping, the Sn:TiO2 NRs exhibit a similar light absorption property, demonstrating that the bandgap of the TiO2 NRs is not changed via the element doping and the enhanced PEC performance of Sn:TiO2 NRs may not be caused by the light absorption process. However, due to the narrow bandgap of BFO (Eg = 2.2 eV) [25], the BFO/Sn:TiO2 NRs display a red shift in the bandgap transition and slightly enhanced absorption in the visible-light region. Based on the plots of (αhv)2 vs. photon energy shown in Fig. S4b, the bandgap values (Eg) of TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs are about 3.09, 3.06 and 2.84 eV, respectively. Different PEC measurements were carried out using TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs as photoanodes to investigate their PEC properties. Fig. 2a shows the linear sweep voltammogram (LSV) curves, in which bare TiO2 NR photoanode exhibits small photocurrent density (Jph) of only 0.54 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (VRHE) due to the limited visible light absorption and low quantum efficiency of TiO2 [33]. The Sn:TiO2 photoanode presents an increased Jph value, which is 0.84 mA cm−2 at 1.23 VRHE, corresponding to 48% improvement over the pristine TiO2 NRs. The enhanced Jph of Sn:TiO2 NRs may be derived from the increased charge carrier density induced by the improved charge separation efficiency of TiO2. For comparison, the LSV of BFO film and BFO/TiO2 NRs was also tested (Fig. S5). As expected, pure BFO film exhibits poor PEC performance because of the rapid recombination of photocharges. On the other hand, BFO/TiO2 NRs show obviously increased Jph (1.01 mA cm−2 at 1.23 VRHE) as compared with that of pristine TiO2 NRs, indicating positive effect of spontaneous polarization of BFO on the PEC performance of TiO2 NRs. After the coating of ferroelectric BFO thin film on the surface of Sn:TiO2 NRs, the resulted BFO/Sn:TiO2 NR photoanode yields the largest photocurrent density of 1.47 mA cm−2 at 1.23 VRHE because of the efficient charge separation, which is 2.72 and 1.85 times higher than that of
2. Results and discussions Here, for the first time, we reported dual modified TiO2 nanorod arrays (NRs) via the Sn-doping in the bulk and the coating of ferroelectric BFO on the surface, simultaneously promoting the charge separation of TiO2 NRs from inside to the interface. More importantly, the PEC property of BFO/Sn:TiO2 NRs was further enhanced by the positive polarization with a long stability due to the existence of the depolarization field. To investigate the synergistic mechanism between Sndoping and ferroelectric BFO coating, the band structure evolution of TiO2 was also studied in detail. The synthetic route of BFO/Sn:TiO2 NRs is illustrated in Fig. 1a. Sn:TiO2 NRs were firstly grown on the fluorinedoped tin oxide (FTO) substrate via a hydrothermal method with the mixture of titanium isopropoxide and SnCl4·4H2O (Sn4+/Ti4+ = 15–45%), and then annealed in air at 450 °C for 0.5 h. Subsequently, Sn:TiO2 NRs were coated with a thin BFO film by a simple sol-gel method, followed by annealing at 550 °C in air for 5 min (see the Supporting information for details). The morphology and chemical composition evolution of TiO2, Sn:TiO2, and BFO/Sn:TiO2 were investigated using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and element mapping analyses. As shown in Fig. 1b, the pristine TiO2 NRs grown on FTO substrate are highly dense and uniform with an average diameter of ~50 nm. The vertically aligned and highly dense TiO2 NRs facilitate the electrons transfer and light absorption [18]. After the doping of Sn, no obvious change in the morphology was observed between Sn:TiO2 and TiO2 NRs (Fig. 1c). For the BFO/Sn:TiO2 NRs, Sn:TiO2 is coated by the BFO thin film homogeneously (Fig. 1d). Because the size of BFO nanoparticles is extremely small, they cannot be clearly distinguished by SEM. The cross-sectional FESEM images of BFO/Sn:TiO2 NRs (Fig. 1e) confirm that almost all NRs are vertically aligned on the FTO substrate with an average length of ~1 µm. Fig. 1f presents a low-magnification TEM image of a BFO/Sn:TiO2 NR with the diameter of ~50 nm, which is consistent with the SEM result. As shown in Fig. 1g, the measured d-spacing of 0.32 nm belongs to (110) plane of rutile TiO2. A fairly uniform BFO shell with the thickness of 2–6 nm covering on the entire Sn:TiO2 core is evidenced from high resolution TEM (HRTEM) image. The lattice spacing of the shell was measured to be 0.39 nm, corresponding to the (012) plane of rhombohedral BFO. The scanning TEM (STEM) and corresponding element mapping images also show a typical core-shell heterostructure (Fig. 1h). Moreover, uniform and distinct signals from Sn, Ti, Fe, Bi and O were observed, indicating a full and homogeneous coating of the BFO thin film onto 34
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Fig. 1. (a) Schematic representation for the growth mechanism of BFO/Sn:TiO2 NRs. (b-d) Top-view SEM images of TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs, respectively. (e) Side-view SEM image of BFO/Sn:TiO2 NRs. (f) TEM and (g) HRTEM images of a single as-synthesized BFO/Sn:TiO2 NR. (h) STEM image and element mapping of a BFO/Sn:TiO2 NR.
electrode using 35% Sn4+ solution concentration, indicating that the Sn doping into TiO2 at a suitable level could effectively enhance the PEC activity of TiO2, while excessive incorporation of Sn leads to a decrease in the photoconversion efficiency. The Jph of the BFO/Sn:TiO2 NRs is obviously affected by the thickness of the BFO shell, which can be tuned by repeating the spin-coating process (1–3 times). As shown in Fig. S6b, the Jph of the BFO/Sn:TiO2 prepared by one time spin-coating is significantly higher than other samples prepared by repeating the coating process of 2 or 3 times. This deterioration is likely the result of
pristine TiO2 NR and Sn:TiO2 NR photoanodes, respectively. In addition, the onset potential of BFO/Sn:TiO2 exhibits a cathodic shift from 0.24 to 0.18 VRHE as compared to that of pristine TiO2. In order to investigate the effect of Sn doping concentration on the PEC performance of the photoanode, the LSV curves of a series of Sn:TiO2 samples prepared using different concentrations of Sn4+ solutions (Sn/Ti = 15–45%) were tested (Fig. S6a). Obviously, the doping content of Sn shows a remarkable influence on the Jph of Sn:TiO2 photoanode. The optimal PEC performance was achieved for the
Fig. 2. PEC performance of TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs: (a) LSV measurements. (b) Transient Jph vs. time plots at an applied potential of 1.23 VRHE. (c) ABPE curves. (d) IPCE curves. The PEC experiments were carried out in 1 M NaOH electrolyte under AM 1.5 G (100 mW cm−2) illumination. 35
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increased hole transport resistance caused by over-loading of BFO. Since ferroelectric BFO is a semiconductor material having certain light absorption capability, thick BFO film may compete with underlying TiO2 for the light absorption. The representative transient photocurrent (I-t) measurements of obtained photoanodes were conducted at external potential of 1.23 VRHE under chopped illumination (Fig. S7a). A significant enhancement in the Jph of Sn:TiO2 NR photoanode is achieved as compared to the pure TiO2 NR photoelectrode. Furthermore, the BFO/Sn:TiO2 NR electrode demonstrates the highest photocurrent density, which is consistent with the LSV results. The applied bias photon-to-current efficiency (ABPE) of different photoanodes was calculated (Fig. 2b) to quantitatively evaluate the PEC performance of TiO2, Sn:TiO2 and BFO/ Sn:TiO2 NRs. The ABPE was determined by the following equation [34]:
After the Sn species were doped into TiO2, the O 2p VB edge of TiO2 has a great shift toward the vacuum level, attributing to the introduction of O 2p/Sn 5s hybrid orbitals in the VB of TiO2, yielding a new VB edge on the surface of TiO2. The result clearly manifests that the incorporation of Sn species remarkably changes the electronic structure of TiO2 via the Ti-O-Sn linkage, which is in accordance with previous studies [36–38]. By combining the optical bandgaps of TiO2 NRs (3.09 eV) and Sn:TiO2 NRs (3.06 eV) obtained from the UV–Vis spectra, the conduction band minimum (CBM) is inferred to occur at − 0.85 and − 1.13 eV for TiO2 and Sn:TiO2 NRs, respectively. Therefore, the band structure alignments of TiO2 and Sn:TiO2 are well resolved and schematically given in Fig. S11. Notably, the CBM of Sn:TiO2 NRs is more negative than that of pristine TiO2 NRs, which increases the driving force for the electron injection from TiO2 to the substrate (FTO) and thus facilitates the separation of photoinduced electrons and holes [39]. The photoinduced electrons and holes can recombine by emitting photons, which is the reason for the photoluminescence (PL). Thus, the PL measurement is an efficient method to the characterization of charge separation from a photoanode [18]. The separation behavior of photoinduced carriers for TiO2, Sn:TiO2 and BFO/Sn:TiO2 was demonstrated via the PL measurement. As shown in Fig. 3d, the pristine TiO2 shows two typical intensive fluorescence emission peaks at about 409 and 460 nm [40]. After the introduction of Sn doping and ferroelectric BFO, the fluorescence signals decrease sharply, indicating that the fluorescenceassociated recombination in BFO/Sn:TiO2 is greatly suppressed. Electrochemical impedance spectroscopy (EIS) measurements were conducted to further clarify the charge separation and transport behavior of the as-prepared electrodes. As shown in Fig. 3e, all three samples (i.e., TiO2, Sn:TiO2 and BFO/Sn:TiO2) present semicircles in the EIS Nyquist curves, the diameter of which demonstrates the electron transfer resistance (Rct) [41,42]. Based on the fitting results (inset figure in Fig. 3e and Table S1), BFO/Sn:TiO2 photoanode exhibits the Rct value of 408 Ω, which is much smaller than that of TiO2 (1326 Ω) and Sn:TiO2 (1250 Ω), indicating facile charge transport of BFO/Sn:TiO2 electrode. These results confirm that more effective separation and faster migration of photoinduced charges on the BFO/Sn:TiO2 NRs are responsible for the enhanced PEC activity. The photogenerated charge separation behavior of different samples was further investigated by measuring the surface photovoltage spectrum (SPS) [43]. The surface photovoltage (SPV) originates from the surface potential change under the light irradiation, whenever photoinduced charge carriers are generated in bulk. For a n-type semiconductor, a positive SPV signal indicates the transfer of photogenerated holes to the illumination side of the electrode [44]. For the SPS measurements here, the incident light irradiates from the surface of the electrode. Consequently, the positive SPV signals of all electrodes (Fig. 3f) demonstrate the accumulation of photoinduced holes at the surface of photoanodes [45]. In general, strong SPV response suggests that high concentration of photogenerated charge carriers migrates to the surface [46]. As compared with TiO2 (8 μV) and Sn:TiO2 (11 μV), much higher SPV signal was observed for BFO/Sn:TiO2 (18 μV), indicating that more photogenerated holes migrate to the surface of BFO/ Sn:TiO2 before the recombination. The SPS results further confirm the enhanced charge separation of TiO2 NRs on account of the synergistic effect from the Sn doping and ferroelectric BFO film coating. In order to further improve the PEC performance of BFO/Sn:TiO2 photoanode via the utilization of ferroelectric property of the BFO film, the BFO/Sn:TiO2 photoanode was polarized in positive direction under an external bias (+2 V) before the LSV test, and a negative polarization (−2 V) was also conducted as the reference. As shown in Fig. 4a, the LSV curves of BFO/Sn:TiO2 photoanode with positive and negative polarization undoubtedly indicate the crucial effect of the polarization state of the ferroelectric BFO on the PEC performance of the photoanode. Specifically, after positive poling (i.e., the orientation of major ferroelectric domains in the BFO film is from the photoanode surface to
= Jph (1.23–|V |)/I0 where Jph is the photocurrent density, V is the potential vs. RHE, and I0 is the power density of incident light (100 mW cm−2). The maximum photoconversion efficiency for TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs is 0.21% at 0.58 V, 0.39% at 0.51 V, and 0.72% at 0.53 V respectively, indicating that the conversion efficiency could be significantly enhanced by the Sn doping and the coating of ferroelectric BFO. To further evaluate the PEC water oxidation performance of the as-prepared electrodes, the incident photon-to-current conversion efficiency (IPCE) was determined under monochromatic light irradiation at an applied potential of 1.23 VRHE (Fig. S7b). At around 375 nm, a maximum IPCE of 82% was achieved for the BFO/Sn:TiO2 photoanode, which is much higher than that of pristine TiO2 (48%) and Sn:TiO2 (63%) at around the same wavelength, indicating that the synergistic effect of the Sn doping and ferroelectric BFO thin film coating effectively enhances the charge harvesting during the PEC process. To confirm the synergistic effect of Sn doping and ferroelectric BFO coating on the charge separation of as-prepared photoanodes, the charge injecting efficiency (ηinj) and charge separation efficiency (ηsep) of different electrodes were calculated (Figs. S8 and S9 for calculation details). As shown in Fig. 3a, only a slight increase in the ηinj was observed after the Sn doping and BFO thin film coating, suggesting that the surface reactivity was not the determining factor for the enhanced PEC performance. For the charge separation efficiency (Fig. 3b), however, Sn:TiO2 NRs exhibit the value of 63% at 1.23 VRHE, which is 1.54 times higher than that of the pristine TiO2 NRs (41% at 1.23 VRHE). Moreover, the BFO/Sn:TiO2 NR photoanode presents the highest ηsep of 74% at 1.23 VRHE. The remarkable increasement in ηsep matches well with the Jph variation shown in Fig. 2a, demonstrating that the improved PEC performance of BFO/Sn:TiO2 NRs is attributed to efficient charge separation by the dual modification of Sn doping and ferroelectric BFO thin film coating. In order to study the water splitting capacity, the evolved H2 and O2 gases using TiO2, Sn:TiO2 and BFO/Sn:TiO2 photoanodes were detected by gas chromatography every 30 min (Fig. S10). The ratio between H2 and O2 gases produced from the photoanodes and Pt counter electrode is close to 2:1, indicating the complete water splitting. The Faradic efficiency of TiO2, Sn:TiO2 and BFO/Sn:TiO2 photoanodes was calculated to be 93.1%, 90.7% and 92.1%, respectively. The high Faradic efficiency suggests that most of the photogenerated holes are used for the water oxidation [35]. In addition, the BFO/Sn:TiO2 photoanode shows an average O2 production of 12.6 μmol h−1 cm−2, which is 0.77 and 1.68 times higher than that of Sn:TiO2 and TiO2 respectively, demonstrating that the dual modifications of Sn doping and BFO coating in BFO/Sn:TiO2 indeed improve the water splitting capacity. Valence band maximum (VBM) of both TiO2 and Sn:TiO2 was measured to investigate the mechanism for the enhanced charge separation of TiO2 via the Sn doping (Fig. 3c). The VBM estimated for the pristine TiO2 and Sn:TiO2 is 2.24 and 1.93 eV, respectively. The negative shift of VBM could be regarded as the introduction of Sn 5 s orbital to form hybridized one located at the valence band (VB) level in TiO2. 36
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Fig. 3. (a) Charge injection efficiency and (b) charge separation efficiency of TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs. (c) Valence band XPS spectra of TiO2 and Sn:TiO2 NRs. (d) PL spectra, (e) EIS and (f) SPS for TiO2, Sn:TiO2 and BFO/Sn:TiO2 NRs.
the electrolyte), the Jph (1.76 mA cm−2) of BFO/Sn:TiO2 photoanode is significantly improved as compared to the unpoled sample. This is because that the spontaneous polarization in the as-prepared ferroelectric BFO is not fully aligned throughout the whole shell along the positive polarization direction before being poled under an external electric field, and the whole BFO shell exhibits a weak polar state [22]. In this case, the BFO film has an inferior effect on the charge separation near the BFO/Sn:TiO2 interface. After the positive polarization, the wellaligned dipoles in the BFO film have a substantial effect on the charge separation, illustrative of a markedly enhanced PEC performance of the BFO/Sn:TiO2 photoanode [47]. On the other hand, a significant decay for the Jph of BFO/Sn:TiO2 photoanode after negative polarization was observed due to reverse alignment direction of ferroelectric domains in the BFO film. Representative polarization-electric (P-E) hysteresis loop of the BFO film with an obvious remnant polarization (Pr) of 0.6 μC cm−2 (Fig. S12) indicates its ferroelectric property. To confirm the substantial effect of the BFO remnant polarization on the charge separation of the photoanode, the stability of the BFO/Sn:TiO2 photoanode after positive polarization was investigated at 1.23 VRHE for 3 h. As shown in Fig. 4b,
the Jph of BFO/Sn:TiO2 only decreases from initial 1.76–1.63 mA cm−2, corresponding to 4% decay. The high stability indicates that the remnant polarization of the BFO film could be maintained for a long time, which is important to sustain the enhanced PEC performance induced by the positive polarization. To gain more insight in the effect of positive polarization on the charge separation of BFO/Sn:TiO2 photoanode, the ηinj and ηsep of BFO/ Sn:TiO2 photoanode with positive and negative polarization were measured (Fig. 4c,d). As compared to the BFO/Sn:TiO2 without the polarization (74% at 1.23 VRHE), ηsep reaches 87% after positive polarization, which is 2 times higher than that of pristine TiO2 NRs (41% in Fig. 3b). In contrast, these three different photoanodes exhibit almost identical ηinj, indicating that the polarization of the BFO shell has a little effect on the surface oxidation reaction of BFO/Sn:TiO2 photoanode. Inspired by this result, further deposition of oxygen evolution catalysts on the surface of BFO/Sn:TiO2 may significantly increase its PEC performance. According to above investigation results, a mechanism (Fig. 5) based on the band structure evolution was proposed to elucidate the enhanced charge separation of BFO/Sn:TiO2 photoanode. The Sn 37
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Fig. 4. (a) J-V curves of BFO/Sn:TiO2 NRs with no poling, positive poling and negative poling. (b) Time courses for the photocurrent density of BFO/Sn:TiO2 NRs after the positive poling. (c) Charge injection efficiency and (d) charge separation efficiency of BFO/Sn:TiO2 NRs with no poling, positive poling and negative poling.
charge carrier density was calculated to be 1.2 × 1019 and 3.8 × 1019 cm−3 for TiO2 and Sn:TiO2 NRs, respectively. Obviously, the charge carrier density of TiO2 NRs is effectively increased via the Sn doping, which is consistent to the previous report [8]. To equilibrate the Fermi levels between n-type TiO2 and electrolyte, the electrons transfer from TiO2 to electrolyte when the photoanode is immersed in an electrolyte, leading to upward band bending and formation of a depletion layer on the surface of TiO2. The built-in electric field in the depletion layer provides the driving force for the separation of photoinduced charges under solar light illumination and facilitates the transfer of holes toward the electrolyte [48,49]. Typically, the amplification of the band bending leads to the enhanced charge separation. Applying an external electric field is an effective approach to tune the magnitude of the depletion region in semiconductors [18]. After the coating of the BFO film, the electric field with a direction from electrode to electrolyte induced by the spontaneous polarization (PS) of ferroelectric BFO further increases the magnitude of the band bending (Fig. 5b), resulting in easy transfer of holes to the interface of electrode/ electrolyte, supported by the results of EIS and SPV (Fig. 3e,f). In order to confirm spontaneous polarization of the BFO film, piezoresponse force microscopy (PFM) amplitude and phase images of BiFeO3/Sn:TiO2 NRs were characterized by PFM (Fig. S14). Yellow and purple areas in the PFM phase image with strong contrast even in the absence of bias voltage indicate upward-downward oriented domains (Fig. S14d), verifying the existence of spontaneous polarization across the BFO film. After electric poling at + 10 V supplied by the PFM tip, most of the domains were aligned upwards (Fig. S14e). Obvious switching of the domain direction was observed after further poling at an opposite electric field (Fig. S14f), confirming the ferroelectric nature of the BFO film. The intensity of an external electric field bought by ferroelectric materials could be improved significantly by the positive poling, because the remnant polarization (Pr) intensity is much higher than that of spontaneous polarization. As a result, the magnitude of band bending from the BFO/Sn:TiO2 photoanode exhibits obvious increasing after the
Fig. 5. Schematic electronic band diagram of (a) Sn:TiO2 and (b-d) BFO/ Sn:TiO2 with (b) no poling BFO, (c) positive poling BFO, and (d) negative poling BFO.
doping is an effective method to improve the charge separation as verified by the results of PL and ηsep, leading to the increased charge carrier density (Fig. 5a). In order to further validate the effect of Sn doping on the improved PEC performance of TiO2 NRs, Mott-Schottky plots of TiO2 and Sn:TiO2 NRs were obtained (Fig. S13), from which the 38
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positive poling as compared to the case without poling (Fig. 5c). Furthermore, the negative polarization creates a reverse direction (from electrolyte to electrode) of external electric field near the semiconductor surface, leading to the reduced magnitude of band bending (Fig. 5d), confirmed by the ηsep of BFO/Sn:TiO2 photoanode with positive or negative polarization (Fig. 4d). It should be noted that the width of the depletion layer is also a decisive factor to the charge separation, and the widening of the layer could lead to the improved charge separation. The calculated width of the depletion layer for BFO/ Sn:TiO2 with no/positive/negative poling (Fig. S13) is 8, 7, and 10 nm, respectively. The results mean that the positive poling decreases the width of the depletion layer, leading to smaller number of photons being absorbed in the depletion region. Based on significantly increased PEC performance of BFO/Sn:TiO2 after positive polarization, it was concluded that the enhanced charge separation within the depletion region induced by the amplification of band bending magnitude could compensate the negative effect resulted by the narrowing of the depletion region, which is consistent with the previous report [50]. In addition, Sn-doping, BFO coating or positive polarization has a negligible effect on the charge injection efficiency of the photoanode, meaning that the band bending is not the decisive factor for surface reaction kinetics of the photoanode, and the PEC performance of the BFO/Sn:TiO2 photoanode could be further enhanced via the deposition of oxygen evolution cocatalysts.
[6] C. Wang, Z. Chen, H. Jin, C. Cao, J. Li, Z. Mi, J. Mater. Chem. A 2 (2014) 17820–17827. [7] Y. Alivov, V. Singh, Y. Ding, L.J. Cerkovnik, P. Nagpal, Nanoscale 6 (2014) 10839–10849. [8] M. Xu, P. Da, H. Wu, D. Zhao, G. Zheng, Nano Lett. 12 (2012) 1503–1508. [9] A. Nakada, S. Nishioka, J.J.M. Vequizo, K. Muraoka, T. Kanazawa, A. Yamakata, S. Nozawa, H. Kumagai, S.-i. Adachi, O. Ishitani, K. Maeda, J. Mater. Chem. A 5 (2017) 11710–11719. [10] H.-i. Kim, D. Monllor-Satoca, W. Kim, W. Choi, Energy Environ. Sci. 8 (2015) 247–257. [11] M. Wang, J. Ioccozia, L. Sun, C. Lin, Z. Lin, Energy Environ. Sci. 7 (2014) 2182–2202. [12] B. Roose, S. Pathak, U. Steiner, Chem. Soc. Rev. 44 (2015) 8326–8349. [13] M. Reza Gholipour, C.T. Dinh, F. Beland, T.O. Do, Nanoscale 7 (2015) 8187–8208. [14] X. Liu, Y. Wang, X. Cui, M. Zhang, B. Wang, M. Rager, Z. Shu, Y. Yang, Z. Li, Z. Lin, J. Mater. Chem. A 7 (2019) 165–171. [15] G. Ai, H. Li, S. Liu, R. Mo, J. Zhong, Adv. Funct. Mater. 25 (2015) 5706–5713. [16] K.R. Yoon, J.W. Ko, D.-Y. Youn, C.B. Park, I.-D. Kim, Green Chem. 18 (2016) 944–950. [17] M.P. Suryawanshi, U.V. Ghorpade, S.W. Shin, M.G. Gang, X. Wang, H. Park, S.H. Kang, J.H. Kim, ACS Catal. 7 (2017) 8077–8089. [18] Z. Zhang, J.T. Yates Jr, Chem. Rev. 112 (2012) 5520–5551. [19] L. Li, P.A. Salvador, G.S. Rohrer, Nanoscale 6 (2014) 24–42. [20] D. Cao, Z. Wang, Nasori, L. Wen, Y. Mi, Y. Lei, Angew. Chem. Int. Ed. 53 (2014) 11027–11031. [21] L. Yang, Y. Xiong, W. Guo, M. Zhou, K. Song, P. Xiao, G. Cao, Nano Energy 44 (2018) 63–72. [22] F. Wu, Y. Yu, H. Yang, L.N. German, Z. Li, J. Chen, W. Yang, L. Huang, W. Shi, L. Wang, X. Wang, Adv. Mater. 29 (2017) 1701432. [23] W. Yang, Y. Yu, M.B. Starr, X. Yin, Z. Li, A. Kvit, S. Wang, P. Zhao, X. Wang, Nano Lett. 15 (2015) 7574–7580. [24] A. Sarkar, A.K. Singh, D. Sarkar, G.G. Khan, K. Mandal, ACS Sustain. Chem. Eng. 3 (2015) 2254–2263. [25] Z. Li, Z. Hou, W. Song, X. Liu, W. Cao, X. Shao, M. Cao, Nanoscale 8 (2016) 10415–10424. [26] Y. Ren, F. Nan, L. You, Y. Zhou, Y. Wang, J. Wang, X. Su, M. Shen, L. Fang, Small 13 (2017) 1603457. [27] G.D. Park, Y.C. Kang, Nano Res. 11 (2018) 1301–1312. [28] P. Tang, H. Xie, C. Ros, L. Han, M. Biset-Peiró, Y. He, W. Kramer, A.P. Rodríguez, E. Saucedo, J.R. Galán-Mascarós, T. Andreu, J.R. Morante, J. Arbiol, Energy Environ. Sci. 10 (2017) 2124–2136. [29] W. Eerenstein, F.D. Morrison, J. Dho, M.G. Blamire, J.F. Scott, N.D. Mathur, Science 307 (2005) 1203. [30] L.T. Quynh, C.N. Van, Y. Bitla, J. Chen, T. Do, W. Tzeng, S. Liao, K. Tsai, Y. Chen, C. Wu, C. Lai, C. Luo, Y. Hsu, Y. Chu, Adv. Energy Mater. 6 (2016) 1600686. [31] X. Chen, S. Shen, L. Guo, S.S. Mao, Chem. Rev. 110 (2010) 6503–6570. [32] C. Chen, Y. Wei, G. Yuan, Q. Liu, R. Lu, X. Huang, Y. Cao, P. Zhu, Adv. Funct. Mater. 27 (2017) 1701575. [33] F. Lei, L. Zhang, Y. Sun, L. Liang, K. Liu, J. Xu, Q. Zhang, B. Pan, Y. Luo, Y. Xie, Angew. Chem. Int. Ed. 54 (2015) 9266–9270. [34] Y. Hou, F. Zuo, A. Dagg, P. Feng, Nano Lett. 12 (2012) 6464–6473. [35] S. Wang, T. He, J. Yun, Y. Hu, M. Xiao, A. Du, L. Wang, Adv. Funct. Mater. 28 (2018) 1802685. [36] H. Zhuang, Y. Zhang, Z. Chu, J. Long, X. An, H. Zhang, H. Lin, Z. Zhang, X. Wang, Phys. Chem. Chem. Phys. 18 (2016) 9636–9644. [37] F. Wang, J. Ho, Y. Jiang, R. Amal, ACS Appl. Mater. Interfaces 7 (2015) 23941–23948. [38] H. Zhuang, Q. Gu, J. Long, H. Lin, H. Lin, X. Wang, RSC Adv. 4 (2014) 34315–34324. [39] S. Guo, Z. Deng, M. Li, B. Jiang, C. Tian, Q. Pan, H. Fu, Angew. Chem. Int. Ed. 126 (2016) 1862–1866. [40] F. Ning, M. Shao, S. Xu, Y. Fu, R. Zhang, M. Wei, D.G. Evans, X. Duan, Energy Environ. Sci. 9 (2016) 2633–2643. [41] M. Zhu, Z. Sun, M. Fujitsuka, T. Majima, Angew. Chem. Int. Ed. 57 (2018) 2160–2164. [42] Y. Zhou, L. Zhang, L. Lin, B.R. Wygant, Y. Liu, Y. Zhu, Y. Zheng, C.B. Mullins, Y. Zhao, X. Zhang, G. Yu, Nano Lett. 17 (2017) 8012–8017. [43] L. Jing, W. Zhou, G. Tian, H. Fu, Chem. Soc. Rev. 42 (2013) 9509–9549. [44] M.A. Melo Jr., Z. Wu, B.A. Nail, A.T. De Denko, A.F. Nogueira, F.E. Osterloh, Nano Lett. 18 (2018) 805–810. [45] M. Xie, X. Fu, L. Jing, P. Luan, Y. Feng, H. Fu, Adv. Energy Mater. 4 (2014) 1300995. [46] Z. Zhao, E.J. Willard, H. Li, Z. Wu, R.H.R. Castro, F.E. Osterloh, J. Mater. Chem. A 6 (2018) 16170–16176. [47] H.S. Kim, S.K. Kim, B.J. Kim, K.S. Shin, M.K. Gupta, H.S. Jung, S.W. Kim, N.G. Park, J. Phys. Chem. Lett. 6 (2015) 1729–1735. [48] S. Bai, J. Jiang, Q. Zhang, Y. Xiong, Chem. Soc. Rev. 44 (2015) 2893–2939. [49] C. Jiang, S.J.A. Moniz, A. Wang, T. Zhang, J. Tang, Chem. Soc. Rev. 46 (2017) 4645–4660. [50] A.J. Abel, A.M. Patel, S.Y. Smolin, B. Opasanont, J.B. Baxter, J. Mater. Chem. A 4 (2016) 6495–6504.
3. Conclusions In summary, ferroelectric BFO coated Sn-doped TiO2 NR photoanode has been fabricated for enhanced PEC water splitting. On account of synergistic effect from the Sn doping and positive polarization of BFO, the obtained BFO/Sn:TiO2 photoanode exhibits remarkable photocurrent of 1.76 mA cm−2 at 1.23 VRHE with an excellent stability. Based on the band structure evolution of TiO2, Sn-doping can improve the charge carrier density, while positive polarization of BFO contribute to the increased magnitude of TiO2 band bending, resulting in remarkably enhanced charge separation. In addition, the deposition of oxygen evolution cocatalysts may further enhance the water splitting efficiency of the BFO/Sn:TiO2 photoanode. The proposed surface and bulk dual modification of TiO2 provides a model for the design of other high performance photoanodes. Acknowledgements The work was supported by the National Natural Science Foundation of China (41502030), the Natural Science Foundation of Hubei Province of China (2017CFB190), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG170638), and the Open Foundation of Engineering Research Center of Nano-Geomaterials of Ministry of Education (NGM2017KF002 and NGM2018KF017). This research is also supported by the Singapore Academic Research Fund (RG5/16, RG11/17 and RG114/17). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2019.02.025. References [1] [2] [3] [4]
X. Lu, S. Xie, H. Yang, Y. Tong, H. Ji, Chem. Soc. Rev. 43 (2014) 7581–7593. A. Fujishima, K. Honda, Nature 238 (1972) 37–38. Y. Wang, X. Liu, C. Zheng, Y. Li, S. Jia, Z. Li, Y. Zhao, Adv. Sci. 5 (2018) 1700844. Y. Wang, X. Liu, J. Liu, B. Han, X. Hu, F. Yang, Z. Xu, Y. Li, S. Jia, Z. Li, Y. Zhao, Angew. Chem. Int. Ed. 57 (2018) 5765–5771. [5] Q. Peng, B. Kalanyan, P.G. Hoertz, A. Miller, D.H. Kim, K. Hanson, L. Alibabaei, J. Liu, T.J. Meyer, G.N. Parsons, J.T. Glass, Nano Lett. 13 (2013) 1481–1488.
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J. Huang, et al. Jing Huang received her B.E. degree from Wuhan Textile University, China in 2017. Currently, she is pursuing her M.S. degree under the supervision of Prof. Zhen Li from Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, China. Her current research is focused on the synthesis of ferroelectric-TiO2based hybrid materials and their applications in photoelectrochemical water-splitting field.
Bing He received her B.E. degree from Henan University of Technology, China in 2017. Currently, she is pursuing his M.S. degree under the supervision of Prof. Zhen Li from Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, China. Her current research is focused on the fabrication of nanomaterials and the photoelectrochemical properties of semiconductors.
Yang Wang received his B.E. degree in the College of Mechanical and Electronic Engineering at China University of Petroleum, Qingdao, China in 2012. He obtained his Ph.D. degree from China University of Geosciences in 2018, under the supervision of Prof. Zhen Li. He has spent two years in Prof. Yanli Zhao's group at Nanyang Technological University as a visiting PhD student. His current research is focused on photoelectrochemical properties of grapheneTiO2 hybrid semiconductors and efficient electron transfer materials in photoelectrochemical cells.
Zhu Shu is an Associate Professor in the Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). He received his Ph.D. degree in Environmental Science and Engineering from China University of Geosciences (Wuhan) in 2011, with one year (2010) in Instituto de Tecnología Cerámica, Jaume I University, Spain as a visiting student. He carried out his postdoctoral research in the group of Prof. Jianlin Shi in Shanghai Institute of Ceramics, Chinese Academy of Sciences during 2011–2013. His research interests are green catalysts for solar energy conversion and environmental remediation.
Xueqin Liu is currently an Associate Professor in the Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). He received his Ph.D. degree in Materials Science and Engineering from China University of Geosciences (Wuhan) in 2016. He has spent two years in the group of Prof. Zhiqun Lin at Georgia Institute of Technology as a visiting PhD student. His research interests include the structure modification of catalytic materials and their applications in solar energy conversion and environment remediation.
Zhen Li is a Professor in the Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). She received her Ph.D. degree in mineral-petrological materials science from China University of Geosciences (Wuhan) in 2004. Her research is devoted to the development and application of graphite materials, the growth and application of multifunctional nanocrystals, comprehensive utilization of non-metallic minerals, and the preparation of functional nanocomposites.
Yinchang Li received his B.E. degree from China University of Geosciences, Wuhan, China in 2013. Currently, he is pursuing his Ph.D. degree under the supervision of Prof. Zhen Li from Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, China. His research focused on the fabrication and modification of nanomaterials, especially in the electrocatalysis water-splitting field.
Yanli Zhao is a Professor at Nanyang Technological University, Singapore. He received his B.Sc. degree in chemistry from Nankai University and Ph.D. degree in physical chemistry under the supervision of Professor Yu Liu. He was a Postdoctoral scholar with Professor Sir Fraser Stoddart at the University of California, Los Angeles and subsequently at Northwestern University. His current research focuses on the development of integrated systems for diagnostics and therapeutics, as well as porous materials for gas storage and photocatalysis.
Xiaoqin Hu received her B.Sc. degree in the School of Material Science and Technology at Jiangxi University of Science and Technology, Jiangxi, China in 2013. Currently, she is pursuing his Ph.D. degree under the supervision of Prof. Zhen Li from Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, China. Her current research is focused on photoelectrochemical properties of Fe2O3 semiconductors in photoelectrochemical cells.
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