Tuning orientation of doped hematite photoanodes for enhanced photoelectrochemical water oxidation

Tuning orientation of doped hematite photoanodes for enhanced photoelectrochemical water oxidation

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Solar Energy Materials and Solar Cells xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

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Tuning orientation of doped hematite photoanodes for enhanced photoelectrochemical water oxidation ⁎

Song Lia, , Jiajia Caib, Yinglei Liua, Meiqi Gaoa, Feng Caoa, Gaowu Qina, a b



Key Laboratory for Anisotropy and Texture of Materials (MoE), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China School of Energy and Environment, Anhui University of Technology, Maanshan, Anhui 243002, China

A R T I C L E I N F O

A B S T R A C T

Keywords: α-Fe2O3 Crystal orientation Sn doping Photoelectrochemistry Water splitting

The limited conductivity in hematite has been currently considered as a critical bottleneck in improving the efficiency of solar-driven water oxidation. Here we shed light on the improvement of carrier conductivity by combing orientation control and transition metal doping. To rule out the influence of surface facets of hematite when adjusting the film orientation, the hematite photoanode films were fabricated by assembling the spherical particles with single crystalline nature. The doped hematite particles were aligned with (001) plane normal to the substrate by a magnetic field (MF) during the drop-casting process. A considerable improvement in photocurrent density was resulted from the orientation control, which is due to the increased carrier density from Sn doping and efficient charge transportation in the (001) plane observed through electrochemical impedance spectra.

1. Introduction Photoelectrochemical (PEC) conversion of water or CO2 to produce hydrogen or carbon-based fuels is a promising technique to capture and store the intermittent solar energy in a sustainable way [1]. The halfreaction of oxygen evolution on photoanode surface provides the electrons that are required to reduce water or CO2, and the efficiency of the whole reaction is limited by the oxygen evolution reaction (OER) due to its slow kinetics [2]. The development of highly-efficient photoanodes composing of earth-abundant elements is a critical challenge and therefore received extensive research efforts. Hematite (α-Fe2O3), with a proper band gap of 2.1 eV allowing the light absorption of incident solar irradiation below 590 nm, is one of the most promising photoanode material candidates [3,4]. However, the reported PEC conversion efficiency of hematite as photoanode are generally much lower than the predicted value. The short hole diffusion length and sluggish reaction kinetics on the surface are the main causes. In the past few years, it has been demonstrated that surface modification using cocatalysts [5–9] or by thermal treatment [10] could effectively shift the photocurrent onset potential by minimizing the overpotential, though fully understandings is still unachieved. As such, the photocurrent density of α-Fe2O3 is largely confined by the fraction of photogenerated holes that reach the surface. Improving the carrier conductance is therefore important for enhancing the hematite PEC performance. Doping strategies are widely adopted to improve the carrier



transport in the α-Fe2O3 bulk. Transition metals such as Ti [11,12], Zr [13] Sn [14], and Pt [15] and nonmetal elements [16] have been proven effective for the enhancement of carrier transport by increasing free carrier concentration [17] or the reactivation of the dead layer close to the conducting substrate [11]. Considering that the charge conductance in (001) plane is 104 higher than along the orthogonal directions [18,19], an alternative approach to manipulate the conductivity of α-Fe2O3 is tuning the film orientations. By controlling the synthesis conditions, hematite films with preferred orientation have been obtained and the enhanced PEC activity were correlated with the texture effect [20–22]. Unfortunately, adjusting the orientation distribution of the films generally leads to different exposed surface planes. And the dependence of catalytic reactivity of oxide nanoparticles on the exposing facets has been widely observed [23,24]. The concurrence of the two factors brings difficulty to distinguish contributions of surface plane on surface energetics [25] and orientations on charge transport in the bulk. Previously, we demonstrated the role of film orientations on its photoelectrochemical performance by building the films using hematite particles of the same surface structure [26]. However, the photocurrent densities of the aligned hematite photoanodes are still relatively small due to the low electrical conductivity nature of the undoped α-Fe2O3. In this work, we show that photoelectrochemical activity of hematite films could be enhanced by applying both the orientation strategy and doping method. α-Fe2O3 films were prepared by assembling single-

Corresponding authors. E-mail addresses: [email protected] (S. Li), [email protected] (G. Qin).

https://doi.org/10.1016/j.solmat.2017.12.028 Received 12 April 2017; Received in revised form 13 December 2017; Accepted 19 December 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

Please cite this article as: LI, S., Solar Energy Materials and Solar Cells (2017), https://doi.org/10.1016/j.solmat.2017.12.028

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crystalline α-Fe2O3 particles which were prepared with the hydrothermal method. Due to the anisotropy in magnetic susceptibility of hematite, the orientation of the films were successfully manipulated by a strong magnetic field. The effects of Sn doping and (001) preferred orientation on the PEC performance of α-Fe2O3 were discussed, and the correlation between the photocurrent density and conductivity of the film were characterized by the electrochemical impedance spectra (EIS) in detail.

The working electrode area was about 0.2 cm2 for electrochemical measurements. Oxygen was detected using a dissolved oxygen analyzer (3-Star, Thermo Scientific) during the chronoamperometric PEC measurement at 1.4 VRHE. The electrolyte was stirred vigorously to assist the diffusion of produced dioxygen. Impedance spectra measurements were performed using 5 mV amplitude perturbation of between 10 kHz and 0.1 Hz. The light source was a 500 W xenon lamp (CEL-HX, ozone free) through AM 1.5 filter with the intensity of 100 mW cm−2.

2. Experimental

3. Results and discussions

2.1. Materials and methods

Fig. 1 shows the top-view morphology of (Fe0.99Sn0.01)2O3 films prepared by drop-casting followed by heat-treatment at 500 °C and 800 °C. It can be found that the films are consisted of oval particles with major and minor axis of 133 nm and 103 nm, respectively. The surface of particles became smooth and the size regrew a little larger (Fig. S2) when the films were calcined at higher temperature of 800 °C. It is worth noting that no noticeable morphology change was observed in the samples prepared under 0 T and 10 T, as shown in Fig. 1. In addition, the Sn content in the particles was about 1 at% from the EDS measurement (Fig. S3). About 5 layers of hematite particles were assembled into the film with thickness about 481 ± 24 nm (Fig. S4). The (Fe0.99Sn0.01)2O3 particles are further characterized using TEM, as shown in Fig. 2. The size distribution of oval-like (Fe0.99Sn0.01)2O3 particles are identical to the observation in the SEM. The SAED pattern with sharp diffraction spots assigned to α-Fe2O3 reveals the single crystalline feature of the (Fe0.99Sn0.01)2O3 particles (Fig. 2b). The elemental mapping of (Fe0.99Sn0.01)2O3 in Fig. 2c confirms that the Fe and Sn are uniformly distributed in the particles, indicating the uniformly Sn doping. Though the magnetic response of hematite is weak [28], the single crystalline nature of the these particles provides foundations for magnetic alignment when the strength of the field is high enough to suppress the thermal energy. For photoelectrochemical cell using n-type semiconductor as anode, hole-electron pairs are generated by incident photons. As shown in Scheme 1, holes generated within the (L+WSC ) length to the solidelectrolyte interface win the chance of reaching the surface due to the interface electric field. These surface holes participate in water oxidation and release oxygen molecules and the cogenerated electrons move to the counter electrode to produce hydrogen under an appropriate

Single crystalline α-Fe2O3 particles were synthesized using hydrothermal method that we previously reported [23] with slight modifications. The mixture of 0.1 M FeCl3, 1 wt% PEG 20 K, and 1 wt% NaOH solution was stirred for 30 min before being transferred into a Teflon-lined autoclave for 5 h reaction at 180 °C. The resultant precipitates were washed with acetone and ultrapure water. Sn-doped hematite ((Fe0.99Sn0.01)2O3) particles was prepared by adding 1 mM SnCl2 into the precursor solution. The hematite particles were assembled onto FTO substrate via drop-casting process during which a magnetic field was applied to assist the orientation control of the film [27], as schematically shown in Fig. s1. The hematite films were annealed under 500 °C for 2 h and then under 800 °C for 10 min. 2.2. Characterization The hematite films were characterized using FE-SEM (Zeiss EVO18) equipped with EDS, TEM (JEM-2100F), XRD (Rigaku D/max 2500 using Cu Kα radiation) to acquire the chemical and structural information. The UV–vis spectra of samples were recorded on a Lambda 750S UV/Vis Spectrometer with an integrating sphere detector, using a bare FTO substrate as reference. The electrochemical tests were performed in deaerated 1.0 M NaOH electrolyte with a conventional threeelectrode on a ZAHNER potentiostat. Saturated Hg/HgO and platinum wire were used as the reference and counter electrode respectively. Hematite electrode was fabricated by establishing contact between the FTO layer and copper wire using silver conductive paint. Exposed areas of the back contact and edges were carefully sealed with epoxy resin.

Fig. 1. SEM images of α-Fe2O3 films prepared under magnetic field 0 T (a, c) and10 T (b, d) by drop casting method and followed by heat treatment at 500 °C (a, b) and 800 °C (c, d).

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Fig. 2. (a) TEM image and (b) corresponding selected area electron diffraction pattern on a single particle for α(Fe0.99Sn0.01)2O3, (c) elemental mapping images of α(Fe0.99Sn0.01)2O3 particles.

external bias (VB) [29] since the conduction band edge of hematite lies below the H2 evolution potential

4OH−+4h+ → 2H2 O+O2 4H2 O + 4e−→4OH− + 2H2 Applying an anodic potential could enhance the separation of holeelectron pairs in the depletion region. A higher current density is thus expected due to faster water splitting reaction and decreased back-reaction. In this configuration, the photocurrent density of a photoanode film is determined by two factors, i.e. the hole diffusion length (L) which is related to the conductivity and surface energetics. The former factor controls the hole density reaching the surface [30,31] and the latter decides transfer efficiency at the interface [32]. For hematite, the (L+WSC ) length is much shorter than light penetration depth. Therefore, any technique that can increase the conductivity may improve the photoelectrochemical performance of hematite, for example, doping or orientation control. Fig. 3a clearly shows the activation of hematite by the tin dopants which yields much higher photocurrent densities than the pure sample.

Scheme 1. Energy diagram of hematite photoanode for photoelectrochemical water splitting. An external bias is required for H2 production at the cathode since the conduction band of hematite lies below the H2 evolution potential.

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Fig. 3. J-V curves of undoped and Sn-doped hematite films fabricated under zero magnetic field and annealed at 500 °C. The plots were measured in 1 M NaOH electrolyte (a) without and (b) with 0.5 M H2O2.

Dioxygen detection shows that the Faradaic efficiency of photocurrents over Sn-doped hematite photoanode approaches 100%, which has been reported [33]. Though there still is room for improving the current density, tin doping increased the photocurrent to the level comparable with many reported literatures by transporting more holes to the semiconductor-electrolyte interface. To further demonstrate the effect of tin doping on charge transportation, H2O2 was used as a hole scavenger because it can effectively suppress the surface recombination [34]. The photocurrent density of the doped sample increased to 0.5 mA/cm2 at 1.23 V vs RHE, suggesting that the hole flux is increased by Sn doping. The flat-band potentials (VFB ) of both hematite samples were determined from Mott-Schottky plots (Fig. S6) to be around 0.3 VRHE, which is close to the literature values [35]. Due to the unity efficiency of H2O2 for collecting surface holes [36,37], the anodic photocurrent begins at a potential ~0.6 VRHE which is almost the theoretical onset potential for hematite photoanode. In contrast, for hematite in aqueous alkaline electrolyte without scavengers, surface modification is generally required to obtain such a low onset potential. For example, a low photocurrent onset of 0.62 VRHE has been reported for ALD grown hematite electrode modified with NiFeOx co-catalyst [8]. The carrier density of hematite increased from 3.2×1019/cm2 to 6.3 × 1020/cm2 after doping (Fig. S6), providing evidence that dopant increased hole diffusion length (L ) is responsible for the larger hole densities on the hematite surface. In addition, enhanced stabilities of hematite are expected after doping as a result of high photocurrent density. The chronoamperometric measurements, as shown in Fig. S9, shows that the stabilities of the two sample are acceptable. During the process of assembling single crystalline (Fe0.99Sn0.01)2O3 particles into film via drop-casting method, high magnetic fields were applied to assist the alignment of these particles. It is believed that mild Sn doping does not change the spin configurations of the Fe ions in the hematite lattice, meaning the (001) planes α-(Fe0.99Sn0.01)2O3 tend to rotate toward the MF direction. X-ray diffraction spectra of films were measured to understand the correlation between MF and film orientation. All XRD patterns (Fig. S5) can be indexed to the rhombohedral αFe2O3 (JPCDS, 33–664) and SnO2 from the substrate. With increasing the MF from 0 T to 5 T and then 10 T, the intensity of (110) increased while (104) decreased. Because of the extinction of the (001) plane in the XRD pattern, we define γ = I(110)/(I(104)+I(110)) as a measure for evaluating the degree of preferred orientation of the films [38,39]. Fig. 4 shows the dependence of γ on MF, from which one can see that more hematite particles are rotated to the position with their (001) planes perpendicular to the substrate under a high magnetic field. Fig. 5 shows the orientation dependent PEC performance of α(Fe0.99Sn0.01)2O3. Higher photocurrent density could be obtained with more particle aligned by the external magnetic field. For example, the photocurrent density increased from 0.44 mA/cm2 to 0.67 mA/cm2, and then to 0.74 mA/cm2 at 1.23 VRHE for the 800 °C sample. Since the

Fig. 4. Correlation between the γ value of hematite films and the strength of the magnetic field in which the film was fabricated. γ defines the ratio between the diffraction peak area of (110) and sum of (110) and (104) planes.

composing hematite particles exhibit identical morphology and similar light absorption, we deduce the orientation dependent enhancement for two reasons. Firstly, due to the anisotropic charge mobility in hematite crystals, the overall charge mobility was enhanced for more (001) of α(Fe0.99Sn0.01)2O3 particles were manipulated perpendicular to the substrate. As demonstrated in Scheme 1, the longer hole diffusion length (L ) due to favorable orientation increases the hole flux drifting to the photoelectrode surface [40]. Second, the adjacent particles with close orientation relationship form low-angle grain boundary which is favorable for the charge transport [41]. In addition, the 800 °C calcined film displayed higher photocurrent density than the 500 °C calcined samples, indicating that the contact between particles is enhanced by high temperature treatment. The enhancement in PEC activity of oxide semiconductors can be attributed to improved charge transportation in the bulk or speeding up of reactions on the surface. For hematite, the Fermi level is pinned by the surface states which are due to specific adsorption of OH radicals, thus the open circuit potentials (OCPs) can reflect the surface chemistry [32]. To rule out the influence of surface, OCPs of doped hematite films fabricated under magnetic field were measured. As shown in Fig. S8, all the samples obtained under different magnetic field exhibited similar OCPs under illumination and in the dark which are around 0.7 VRHE and 0.9 VRHE, respectively. The resulted photovoltages are about 0.20 V in α-(Fe0.99Sn0.01)2O3 films with different γ, which agrees well with the similar onset potentials near 0.8 VRHE. Therefore, it is concluded that the orientation has little effect on the surface states of the (Fe0.99Sn0.01)2O3 films. Obviously, the photocurrent density enhancement only correlate with the (001) orientation. 4

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Fig. 6. Nyquist plots of tin doped α-Fe2O3 films under zero (a), 5 T (b) and 10 T (c) magnetic field measured at open circuit voltage. The solid dots represented the experimental data, and the dash lines represented fitting data. The inset (A-1, B-1) showed equivalent circuit to simulate the Nyquist plots, and the resistance RH were shown in inset (A-2, B-2). The samples annealed at 500 °C and 800 °C designated as A and B, and measured in 1 M NaOH (pH = 13.6).

4. Conclusions Fig. 5. J-V curves of (Fe0.99Sn0.01)2O3 films under different magnetic fields. The samples were annealed at 500 °C (a) and 800 °C (b). The photoelectrochemical properties were measured in 1 M NaOH (pH = 13.6).

In summary, we show that the (001) orientation modulation and chemical doping can be coupled to enhance the PEC performance of hematite for water oxidation. Using a high magnetic field could effectively align the hydrothermally synthesized single crystalline α(Fe0.99Sn0.01)2O3 particles with their favorable charge transport directions normal to the substrate. The PEC measurement showed the photocurrent density of hematite enhanced after Sn doping and can be further increased by modulating the (001) orientation, finally reached ca. 0.74 mA/cm2 at 1.23 VRHE. This enhancement was proved to depend on the increased carrier density from Sn doping and efficient charge transportation in the (001) plane observed through electrochemical impedance spectra.

To further elucidate the influence of orientations on the charge transport, the film resistance was measured with electrochemical impedance spectra (EIS) at OCPs in the dark for no net current was expected. Fig. 6 shows the Nyquist plots of tin doped hematite films with different (001) orientation. All Nyquist dots fitted well with the equivalent circuit (the insets of Fig. 6A-1, and B-1), in which the Re represents the series resistance, the RH and CPEH characterize the film resistance and the pseudo capacitance. As illustrated in Fig. 6, the film resistance, RH, decreases with (001) orientation increasing. It suggests that the decreased film resistance depends on the (001) preferred orientation modulation, which tunes the charge transport, increase the low-angle grain boundary, as well as shorten the charge transport path. The improved crystallization after higher temperature treatment results in further decreased film resistance. The conductivity of semiconducting films, σ, is determined by the carrier concentration, n , and mobility μ according to σ = neμ . Moreover, μ depends exponentially on the activation energy for charge transport, ΔG* [42]. In this work, both n and μ were changed to influence the σ. First, Sn doping increases n by introducing extra electrons to hematite lattice. Second, larger μ is resulted from (001) orientation modulation by decreasing the ΔG*, of which the minimum value is supposed to be 0.11 eV in (001) plane of Fe2O3 [34,37]. The σ is therefore increased with (001) orientation. As a result, the film resistance decreases after Sn doping, and is further reduced by (001) orientation modulation. It is demonstrated that controlling the crystallographic orientation and chemical doping could be combined to tune the photoelectrochemical performance of hematite for water oxidation.

Acknowledgements The work was supported by The National Key Research and Development Program of China under 2016YFB0701100 and National Natural Science Foundation of China (51771047, 51525101). S Li thanks the Fundamental Research Funds for the Central Universities (N160208001) and the Provincial Education Department of Liaoning (LJQ2014026) for financial supports. The authors are grateful to an anonymous reviewer for the constructive comments to improve the present manuscript. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2017.12.028 References [1] K. Sivula, R. van de Krol, Semiconducting materials for photoelectrochemical energy conversion, Nat. Rev. Mater. 1 (2016) 15010.

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