- Email: [email protected]
Fabrication of superior α-Fe2O3 nanorod photoanodes through ex-situ Sn-doping for solar water splitting
Solar Energy Materials & Solar Cells 144 (2016) 247–255
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Fabrication of superior α-Fe2O3 nanorod photoanodes through ex-situ Sn-doping for solar water splitting Alagappan Annamalai a, Pravin S. Shinde a, Tae Hwa Jeon b, Hyun Hwi Lee c, Hyun Gyu Kim d, Wonyong Choi b, Jum Suk Jang a,n a Division of Biotechnology, Advanced Institute of Environmental and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 570-752, Republic of Korea b School of Environmental Science and Engineering, POSTECH, Pohang 790-784, Republic of Korea c Pohang Accelerator Laboratory, POSTECH, Pohang 790-784, Republic of Korea d Korea Basic Science Institute, Busan Center, Busan 609-735, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 2 August 2015 Accepted 13 September 2015
Doping transition metals into 1-D nanostructures is of crucial importance for their application in photovoltaics and photoelectrochemical (PEC) systems; performance enhancements arise from both dopant incorporation and the 1-D nanostructures. Both in-situ and ex-situ doping methods have been demonstrated for 1-D hematite (α-Fe2O3) nanostructures, with tin (Sn) as the dopant, for photoelectrochemical water oxidation. In-situ Sn-doped hematite photoanodes adopted a morphology consisting of nanocorals with the (104) plane as the preferred direction of crystal growth. As an alternative solution, ex-situ doping not only preserves the vertically-aligned nanorod morphology but also sustains the preferred orientation of the (110) axis, which is favorable for high conductivity in pristine hematite photoanodes. In-situ Sn-doping was carried out by the same method: Sn precursors were added and dissolved in ethanol during the hydrothermal synthesis. Ex-situ doping was carried out in two stages (during pre-deposition and during high temperature sintering). During pre-deposition, a defined amount of the Sn precursor was introduced near the surface region of the 1-D nanostructure, and the Sn content was controlled by changing the concentration of the precursor solution. In subsequent high temperature sintering (800 °C), the dopant atoms diffused into the hematite lattice to attain the desired doping profile. We found that ex-situ Sn-doping resulted in a 60% increase in the photocurrent while in-situ Sndoping yielded an increase of only 20% in the photocurrent, as compared with pristine hematite photoanodes, at 1.4 V vs. RHE. The improvement in the photocurrent was caused by a combination of Sn dopants in the hematite, which act as electron donors by increasing the donor density, and better surface charge transfer kinetics, thereby enhancing the overall device performance. & 2015 Elsevier B.V. All rights reserved.
Keywords: β-FeOOH Ex-situ Doping Hematite α-Fe2O3 Intrinsic Sn doping
1. Introduction Photoelectrochemical (PEC) water oxidation is a promising method to convert solar energy into chemical energy that is stored in the form of molecular hydrogen and oxygen [1,2]. Beginning with the first report by Fujishima and Honda [1] in 1972, which used TiO2 as a photoanode, various metal oxide semiconductors [3] have been extensively studied as photoanodes for water oxidation. Among different metal oxide semiconductors, hematite (αFe2O3) is a promising photoanode candidate for photoelectrochemical water splitting due to its favorable band gap (2.2 eV) and extraordinary chemical stability [3,4]. Hematite has a n
Corresponding author. Tel.: þ 82 63 850 0846. E-mail address: [email protected] (J.S. Jang).
http://dx.doi.org/10.1016/j.solmat.2015.09.016 0927-0248/& 2015 Elsevier B.V. All rights reserved.
maximum theoretical solar-to-hydrogen (STH) efficiency of 15%, which is key for practical applications. However, hematite has some limiting factors such as the short-diffusion length of its holes and its extremely poor conductivity [3]. Nanostructuring of hematite photoanodes has significantly improved the photogenerated carrier collection, while elemental doping has improved the conducting properties of hematite photoanodes by increasing the donor density [5,6]. 1-D nanostructures of hematite have shown significantly improved photocurrent densities compared to those of the bulk and other nanostructures [6,7]. To date, hematite nanostructures have been synthesized using a variety of techniques including sol–gel processing [8], electrodeposition [9,10], spray pyrolysis [11], and hydrothermal synthesis [12]. In particular, hydrothermal synthesis is simple, cost effective, and provides good control over the composition and morphology [13,14]. Elemental doping plays a critical role in improving the
248
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
electrical conductivity and in enhancing the PEC performance of hematite photoanodes [5,15]. Dopants, such as Si [16,17], Ti [17– 21], Sn [6,15,17,22], Zr [18], Ge [17,23], Mn [14,17], Al [24], and Pt [9,12,25], have been investigated for use in hematite photoanodes. Sn has an advantage over other dopants because Sn ions have a similar ionic radius and Pauling electronegativity compared to Fe ions. They have been reported to improve the electronic properties of hematite [26]. The addition of a surface electrocatalyst (e.g., Co– Pi, Au, Pt, and RuO2) on hematite photoanodes has also been proposed to improve PEC device performance [27,28]. In general, elemental doping in hematite photoanodes can be classified as either in-situ doping or ex-situ doping [15]. In most of the reports that use hydrothermal synthesis, dopants are incorporated into the hematite nanostructures during material synthesis (in-situ doping) [6,9,12,14,29]. In-situ doping is a simple process that uses a relatively low sintering temperature, allows for flexibility in the choice of dopants, and requires no additional doping steps [30]. However, changes in the morphology and crystallinity are observed for in-situ doped hematite photoanodes [6,15]. In contrast, the ex-situ process begins with the final product of the hydrothermal synthesis and is followed by a Sn surface treatment and high temperature annealing to incorporate the elemental Sn dopant into the hematite nanostructure through a solid-state reaction [18]. In the case of in-situ doping-based hematite photoanodes, cationic substitution has only been reported for goethite (α-FeOOH) [31] and not for akaganeites (β-FeOOH) [32–34]. Holm et al. [34] synthesized β-FeOOH in the presence of Cr, Mn, Co, Ni, and Zn (50 mol%) and reported only small incorporations of foreign cations (despite using concentrations of dopants that were equimolar with Fe). Thus, in-situ doping based on β-FeOOH is neither reliable nor reproducible due to the intrinsic properties of β-FeOOH, as has been noted in previous reports [33]. Another limitation of in-situ doping is related to the fact that when the metal ion concentration is increased, only a part of the metal ions can be doped into the structure of β-FeOOH. This results in the formation of undesired metal oxides during the hydrothermal synthesis [26]. In order to realize the advantages from both types of processes, and to compare the microstructural evolution of in-situ and exsitu methods, it is essential to fabricate 1-D hematite nanostructures using Sn as the dopant. Some groups have reported Sndoping in hematite photoanodes through ex-situ methods [18,22,35,36]. However, to the best of our knowledge, nobody has reported the superior performance of ex-situ Sn-doping over insitu Sn-doping in hematite photoanodes. Herein, we explain the importance and advantages of an ex-situ method for Sn-doping into 1-D hematite nanostructures. Additionally, our recommended ex-situ doping method can be successfully applied to other types of 1-D nanostructured metal oxide photoanodes with various elemental dopants without compromising the morphology and/or crystallinity of the parent metal oxide photoanode. Enhanced performance in hematite photoanodes was observed when the nanostructures were doped by subjecting them to high temperature annealing in order to encourage dopant incorporation into the surface of the hematite nanostructures. Ex-situ Sn-doped hematite nanorod photoanodes exhibit enhanced electron transport. This superior performance is caused by the fact that the direct chargediffusion pathways reduce the number of grain boundaries and defects, which act as major charge recombination sites. Using an optimized Sn concentration (5 mM), we achieved a photocurrent of 1.6 mA/cm2 at 1.4 V vs. RHE under illumination in ex-situ Sndoped hematite photoanodes. This value is 60% higher compared to pristine hematite photoanodes.
2. Experimental 1-D hematite nanostructures on FTO (F:SnO2) glass were prepared by a simple hydrothermal method based on a previous report [37]. In a typical experiment, a piece of cleaned FTO glass (1 cm 2.5 cm) was placed within a 20 ml vial containing a solution (10 ml) consisting of 0.4 g FeCl3 6H2O and 0.85 g NaNO3 (at pH 1.5, adjusted by HCl) [38]. A hydrothermal reaction was conducted at 100 °C for 6 h. After cooling to room temperature, the FTO glass was rinsed several times with distilled water and then dried at 60 °C. The formed β-FeOOH is strongly adhered to the FTO glass substrate even after ultrasonication for 10 min. Annealing at 550 °C for 4 h was carried out to cause a phase transition from βFeOOH to pure α-Fe2O3 [38]. In-situ Sn-doped hematite photoanodes were prepared by adding 1 ml of an ethanol–SnCl4 solution (10 mg/ml), which was added to the hydrothermal precursor solution as reported previously [6]. The extrinsic Sn-doping was carried out by a simple dipping method in order to treat the hematite photoanodes with the Sn precursor solution. Varied concentrations of SnCl4 were dissolved in ethanol to prepare the Sn precursor solution. After dipping the photoanodes in the Sn precursor, they were allowed to air dry. Then, the surface-treated samples were subjected to high temperature sintering (Fig. S2) [39]. This high temperature sintering (800 °C for 10 min) is believed to be important for the activation of hematite photoanodes by enhancing the electron transfer between hematite and the conductive substrates [4]. X-ray diffraction (XRD) patterns of all samples were collected using an X-ray diffractometer (Rigaku RINT 2500) with CuKα radiation. The crystallinity and preferential ordering of the hematite nanostructure samples were obtained by measuring the X-ray diffraction profiles with conventional two theta scans and two-dimensional X-ray diffraction (2D XRD) patterns. These were performed at the 5A X-ray scattering beamline for materials science at the Pohang Light Source II (PLS-II) in Korea. The X-ray diffraction patterns were measured using a point detector mounted on a diffractometer, whereas the 2D XRD data were collected using a two-dimensional (2D) image plate detector in a quasi-symmetric reflection mode at Qz 2.4 Å 1 with a sample-todetector distance of 306 mm. The probing X-ray energy was 11.57 keV (1.0716 Å) for the scattering measurements. The surface morphologies of the samples were analyzed using field emission scanning electron microscopy (FESEM, JEOL JSM 700F). X-ray photoelectron spectroscopy (XPS) was performed using an X-ray photoelectron spectrometer (Kratos XSAM 800pci) using Mg KR lines (1253.6 eV) as an excitation source. A high resolution transmission electron microscope (HRTEM) with electron energy loss spectroscopy (EELS) mapping capabilities was used to study the elemental mapping in Sn-doped hematite photoanodes. All photoelectrochemical measurements were carried out in an electrolyte of 1 M NaOH (pH¼13.8) using an Ivium CompactStat potentiostat with a platinum coil as the counter electrode and Ag/AgCl as the reference electrode. Photocurrent–potential (J–V) curves were swept at 50 mV s 1 from 0.7 to þ0.7 V vs. Ag/AgCl. To measure the incident photon-to-current conversion effciencies (IPCEs), a 300 W xenon lamp (Newport, 6258) was coupled to a grating monochromator (Newport, 74125). This was operated in a wavelength range from 330 to 600 nm, and the incident light intensity was measured with a UV silicon detector (Newport, 71675). The photoelectrode was biased at þ0.6 or þ1 V (vs. Ag/AgCl) during all IPCE measurements. Impedance spectroscopy measurements were performed using an impedance analyzer (Ivumstat). The impedance spectra were measured over a frequency range of 1 10 2–3 106 Hz at 25 °C under open circuit conditions with an amplitude of 10 mV and under a bias illumination of 100 mW cm 2.
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
3. Results and discussion Fig. 1 depicts the FESEM images comparing pristine, in-situ, and ex-situ Sn-doped hematite photoanodes. This figure shows very similar nanorod morphology for pristine and ex-situ Sn-doped hematite photoanodes; alternatively, clustered nanocorals for the in-situ Sn-doped hematite photoanodes were observed. For pristine hematite photoanodes sintered at 800 °C, nanorods of 30– 50 nm in diameter and 400 nm in length, which were aligned roughly vertically to the FTO substrates, were observed. This has also been reported elsewhere [12]. In-situ Sn-doped photoanodes have a tiny nanocoral-like clustered morphology, which is completely different from the pristine hematite photoanodes with a length of 500 nm. The distinct morphology of in-situ Sn-doped photoanodes, from aligned nanorods to clusters, originates from the inclusion of ethanol and dopant SnCl4 in the precursor solution that is utilized for Sn incorporation [6]. Alternatively, ex-situ Sndoped photoanodes show very similar morphologies to those of pristine hematite photoanodes (vertically-aligned nanorods) with no additional deposits or differences in morphology (that can be distinguished by FESEM) after Sn-doping. This is the case because the surface treatment of the dopant precursor, which is followed by high temperature sintering, effectively induces the diffusion of Sn dopants into the host hematite photoanodes without significantly altering the aligned hematite nanorod morphology. HRTEM micrographs of both the in-situ and ex-situ Sn-doped hematite photoanodes are shown in Fig. 2. In-situ Sn-doped hematite shows nanocoral-shaped particles on the FTO substrates. The coral-like nanostructures in in-situ Sn-doped hematite photoanodes have many grain boundaries, which may have detrimental effects on the electron transport properties of in-situ Sndoped hematite photoanodes. However, the ex-situ Sn-doped hematite photoanodes retain the 1-D nanorod morphology of pristine hematite photoanodes. The elemental mapping images (Fig. 2a and b) show that Fe, O, and Sn were all uniformly distributed in both the nanocoral-shaped in-situ and nanorod-shaped ex-situ Sn-doped hematite photoanodes. However, elemental analyses by TEM and EELS reveal that the coral-like in-situ Sndoped hematite photoanodes showed a higher Sn content (0.92 wt%) compared to that of ex-situ Sn-doped hematite photoanodes (0.4 wt%). This further supports the XPS data.
249
Fig. 3 shows the XRD patterns of pristine, in-situ, and ex-situ Sn-doped hematite nanostructures. Since the (104) phase of αFe2O3 coincides with the FTO lattice parameters (Fig. S1), we chose synchrotron X-ray diffraction (XRD) over conventional XRD. With the exception of the FTO substrate peaks, all of the other peaks can be indexed to hematite (JCPDS card 33-0664). The absence of βFeOOH diffraction peaks indicates the complete conversion of iron oxyhydroxides into hematite [12]. When sintered at 550 °C for 4 h, the as-synthesized, yellow-colored β-FeOOH is converted into redcolored α-Fe2O3. The XRD peaks of the hematite photoanodes sintered at 800 °C show similar features to those of the hematite films sintered at 550 °C, with the exception of sharper diffraction peaks from FTO (Fig. S2). No diffraction peaks of Sn or other impurity phases were observed in any of the samples. Both the pristine and ex-situ Sn-doped hematite photoanodes displayed similar XRD patterns with a predominant (110) diffraction peak. Alternatively, the in-situ Sn-doped hematite photoanodes displayed a stronger (104) diffraction peak, as shown in Fig. 3. The (110) reflection was observed for 1-D hematite nanostructures, which indicates a strong preferential orientation of the [110] axis vertical to the substrate, as shown in Fig. 1a and c. Hematite with [110] orientation has been reported to have an anisotropic conductivity that is four orders of magnitude higher and better facilitates charge collection of photo-excited charge carriers along the 1-D nanostructures [6,7,40,41]. In the case of in-situ Sn-doped photoanodes, a stronger (104) diffraction peak and a smaller (110) diffraction peak were observed. Similar (104) diffraction peaks were reported for in-situ Si-doped [42] and Sn-doped [6] hematite photoanodes, which were induced by either the presence of the dopants during synthesis or by changes in the morphology, as shown in Fig. 1b. To clarify the preferential ordering of hematite nanostructures, 2D XRD was performed. These data are depicted in Fig. 4a–c. Fig. 4d and e shows circular line cuts passing through the hematite (110) and (104) peaks. In Fig. 4d, the maximum intensities of the pristine and ex-situ Sn-doped samples at 0° represent the preferential ordering of hematite (110); alternatively, the featureless intensity distribution of the in-situ sample implies little ordering or a powder-like distribution. The angular spread of the (110) peak was 50° for both the pristine and ex-situ doped samples (taken from the full-width at half-maximum of the circular line cuts). Thus, ex-
Fig. 1. FESEM images of (a) pristine hematite photoanodes, (b) in-situ Sn-doped hematite photoanodes, (c) ex-situ Sn-doped hematite photoanodes, and insets with their respective cross-sectional images sintered at 800 °C.
250
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
Fig. 2. HRTEM images of (a) in-situ Sn-doped and (b) ex-situ Sn-doped hematite photoanodes. Their respective elemental mappings are included (Fe, O, and Sn).
Fig. 3. X-ray diffraction patterns and digital pictures of pristine, in-situ, and ex-situ Sn-doped hematite photoanodes annealed at 800 °C.
situ Sn-doping incorporates the Sn dopants into the hematite photoanodes without changing the morphologies or the [110] orientation; this [110] orientation has been suggested as the preferential direction for electron transport in hematite [41]. In order to confirm the presence of Sn dopant atoms after doping of the hematite nanostructures, XPS analyses were performed on pristine, in-situ, and ex-situ Sn-doped hematite nanostructures, as shown in Fig. 5. The XPS spectra (Fig. 5b) for pristine, in-situ, and ex-situ Sn-doped samples show Fe(2p) peaks at 724.2 and 711.2 eV. These peaks are consistent with the Fe3 þ ions in hematite [43]. The major O(1s) peak, appearing at 530.2 eV for pristine and Sn-doped samples, was also consistent with that of hematite. Alternatively, the O1s peak at 531.7 eV represents the surface hydroxyl groups [43,44]. Detailed analysis of the Sn3d peaks for Sn-doped hematite photoanodes shows the presence of two intense peaks at 487 (Sn3d5/2) and 495.4 eV (Sn 3d3/2). These confirm the presence of Sn4 þ dopants in both in-situ and ex-situ doped samples [22,45]. The Sn4 þ peaks observed from pristine hematite photoanodes come from the Sn diffusion from FTO substrates when the samples are sintered at 800 °C (to activate the hematite photoanodes) [6]. The relatively higher concentration of Sn dopants found in in-situ and ex-situ Sn-doped hematite photoanodes (compared to pristine photoanodes) supports the assumption that a significant amount of Sn dopants diffuse into the hematite lattice. It is clearly evident from Fig. 5d that a higher amount of Sn is incorporated into the hematite lattice for in-situ Sn-doped photoanodes compared to
ex-situ Sn-doped photoanodes [6,22]. It should be noted that the origin of Sn dopants in the case of ex-situ doped samples is Sn atoms that diffuse into the surface of the hematite nanorods upon sintering at 800 °C. Alternatively, in the case of in-situ doped samples, the Sn dopants were incorporated into the hematite lattice during the synthesis stage. Photoelectrochemical measurements were performed in a three-electrode electrochemical cell using pristine and doped 1-D hematite nanostructures on FTO as the working electrode. These cells used a platinum coil as the counter electrode and Ag/AgCl as the reference electrode. Fig. 6a depicts the photocurrent responses of pristine, in-situ, and ex-situ Sn-doped hematite photoanodes under dark and illuminated conditions. Among the various surface treatment conditions that we attempted, in-situ Sn-doping with 1.0 ml of ethanol–SnCl4 and ex-situ Sn-doping with 5 mM SnCl4 (Fig. S4) showed the greatest photocurrent enhancement. In-situ Sn-doping was carried out with 1.0 ml of an ethanol–SnCl4 precursor, which was added to the hydrothermal solution during the synthesis, as previously reported [6]. In the case of the ex-situ doping, increasing the Sn4 þ concentration above 5 mM in the treatment solution can result in an increase in the particle size, which disrupts the surface morphology of the hematite photoanodes by reducing the surface area; this would result in poor photoelectrochemical performance. The best performance was achieved by ex-situ Sn-doped hematite photoanodes treated with a solution containing 5 mM SnCl4. Upon illumination, the pristine hematite photoanodes show a photocurrent of 1.03 mA/cm2 at 1.4 V vs. RHE. In comparison to pristine hematite photoanodes, in-situ Sn-doping shows a photocurrent of 1.21 mA/cm2 (a 20% increase); a much higher photocurrent of 1.63 mA/cm2 (a 60% increase) is exhibited for ex-situ Sndoped hematite photoanodes. However, both the in-situ and exsitu Sn-doped hematite photoanodes showed a positive shift in the onset potential from 0.8 to 1.0 V vs. RHE, as compared to the bare photoanodes annealed at 800 °C. A similar anodic onset potential shift has been observed previously for in-situ Sn-doped [6] and Ptdoped [25] hematite photoanodes. This anodic shift in the onset potential indicates that the water oxidation kinetics are limited, which is possibly due to the increase in the surface trapping states that is caused by the increased surface area after surface treatment [46,47]. Since Sn-doping is carried out in solution, the Sn precursor prepared in ethanol could easily penetrate through the hematite nanostructures. During annealing, a thin layer of SnO2 is formed on the surface of the hematite nanostructures. Annealing at a
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
251
Fig. 4. (a)–(c) 2D XRD images of the pristine (black dot), ex-situ (blue dot), and in-situ (red dot) Sn-doped hematite photoanodes annealed at 800 °C. (d)–(e) The circular line cuts passing through the hematite (110) and (104) peaks, where zero degrees corresponds to the direction normal to the surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
temperature of 800 °C for 10 min is sufficient to gradually incorporate Sn into the hematite photoanodes, both from the surface treatment [24] as well as from the FTO substrates [6]. The Sn dopants in the hematite photoanodes can serve as electron donors by drastically increasing the donor density, which leads to an enhancement in the conducting properties of the Sn-doped hematite photoanodes [15,48]. Additional incident photon-to-current-efficiency (IPCE) measurements were carried out for pristine, in-situ, and ex-situ Sndoped hematite photoanodes at 1.4 V vs. RHE, as shown in Fig. 6b. In comparison to pristine and in-situ Sn-doped photoanodes, exsitu Sn-doped photoanodes showed higher IPCE values over the whole visible spectrum. These results were consistent with the differences observed in the photocurrent densities in pristine, insitu, and ex-situ Sn-doped hematite photoanodes. Thus, the photocurrent of ex-situ Sn-doped hematite at 1.4 V vs. RHE was increased by 60% (as compared to that of the pristine hematite photoanodes). This simple solution-based ex-situ Sn-doping was found to be highly reproducible compared to that of in-situ doping. Ex-situ Sn-doped hematite photoanodes still showed superior enhancement in the photocurrent compared to in-situ Sn- doped and pristine hematite photoanodes despite an anodic onset shift; this suggests that the photocurrent enhancement caused by Sndoping may be further enhanced by placing an appropriate oxygen evolving catalyst on the photoanode surface to improve the surface catalytic activity. We further analyzed the pristine and Sndoped hematite photochemical response (Fig. 7a) and stability of the photoanodes (Fig. 7b) at 1.4 V vs. RHE under standard
illumination conditions over a period of 1200 s. There is no observable degradation, indicating the excellent chemical and structural stability of the ex-situ Sn-doped photoanodes compared to the pristine and in-situ Sn-doped hematite photoanodes for long-term PEC conversion. Using electrochemical impedance spectroscopy (EIS), the internal resistances and the electron transport kinetics of pristine, in-situ, and ex-situ Sn-doped hematite photoanodes were studied. Fig. 8 depicts the Nyquist plots of pristine and doped hematite photoanodes, from electrochemical impedance under standard illumination conditions, which shows two semicircles for each type of photoanode. The semicircles at lower frequencies are related to the electron transfer resistance, and the ones at higher frequencies describe the tendency of recombination at the hematite/electrolyte interface [44,49]. From Fig. 8, the arc decreased with ex-situ Sn-doping and increased with in-situ Sn-doping (as compared to pristine hematite photoanodes). This decrease indicates that hematite photoanodes with 1-D nanostructures result in pronounced electron transport properties [25]. In the case of in-situ Sn-doped photoanodes, the 1-D nanostructure of the pristine hematite photoanodes is changed into nanocorals, which have a larger surface area; however, this structure also produces greater amounts of electron recombination and a larger electron transport resistance due to the increased number of grain boundaries. The in-situ Sn-doping enhances the photocurrent compared to pristine photoanodes but compromises the electronic properties due to the presence of the inferior (104) crystalline phase of in-situ Sn-doped nanocorals compared to the (110) phase of pristine hematite photoanodes. The increased electron recombination is caused by the
252
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
Fig. 5. X-ray photoelectron spectra of (a) full scan, (b) Fe 2p, (c) O 1s, and (d) Sn 3d recorded from pristine, in-situ, and ex-situ Sn-doped hematite nanostructures annealed at 800 °C.
Fig. 6. (a) Photocurrent–potential (J–V) curves and (b) incident photon-to-current-efficiency (IPCE) spectra for PEC water oxidation reaction with pristine, in-situ, and ex-situ Sn-doped hematite photoanodes under standard illumination conditions.
electron hopping mechanism that is observed in nanoparticle-based PECs. However, ex-situ Sn-doping has several advantages over in-situ doping because it can preserve the highly conductive (110) crystalline phase and the 1-D nanostructures, while also improving the charge transport and decreasing electron recombination (surface passivation induced from ex-situ surface treatment) [22,24] by providing fast pathways for electrons. The effect of the ex-situ Sn-doping on the hematite photoanodes can increase the conductivity and reduce the recombination of charges in the hematite photoanodes, resulting in an enhancement of the photocurrent. This suggests that the 1-D nanostructured ex-situ Sn-doped hematite photoanodes have a longer electron lifetime than the nanocoral-shaped in-situ Sn-doped hematite photoanodes. The EIS results (Table S1) are in good agreement with the results of the photocurrent measurements and the IPCEs of in-situ and ex-situ Sn-doped hematite photoanodes. To further understand the role of in-situ and ex-situ Sn-doping, Mott– Schottky measurements were also performed (Fig. 9).
The positive slopes in Fig. 9 indicate that the pristine, in-situ, and ex-situ Sn-doped hematite photoanodes are all n-type semiconductors. By extrapolating the x-intercepts of the linear region in the Mott– Schottky plots, the VFB values of the pristine, in-situ, and ex-situ Sndoped hematite photoanodes were found to be 0.62, 0.66, and 0.71 V vs. RHE, respectively. These VFB potentials are consistent with previous reports for pristine and doped hematite photoanodes [4]. The slopes derived from the Mott–Schottky analysis were also used to further estimate the donor densities of pristine, in-situ, and ex-situ Sndoped hematite photoanodes; these values were determined to be 1.26 1019, 6.32 1019, and 2.49 1020 cm 3, respectively. The above data provides concrete evidence and supports the idea that extrinsic Sn-dopants act as electron donors and that the donor density increased substantially after both in-situ and ex-situ doping. The hydrothermal method is an ideal method for the synthesis of 1-D hematite nano-structures and offers effective control over the size and shape of nanostructures at relatively low reaction temperatures and short reaction times (Scheme 1). An iron (III) chloride
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
253
Fig. 7. (a) Photochemical response and (b) photochemical stability for PEC water oxidation reactions with pristine, in-situ, and ex-situ Sn-doped hematite photoanodes at 1.4 V vs. RHE under standard illumination conditions.
Fig. 8. Nyquist plots of pristine, in-situ, and ex-situ Sn-doped hematite photoanodes at 1.23 V vs. RHE under standard illumination conditions. The inset in the Nyquist plot represents the equivalent circuit for EIS.
Fig. 9. Mott–Schottky plots for PEC water oxidation reactions with pristine, in-situ, and ex-situ Sn-doped hematite photoanodes.
(FeCl3) solution acts as the iron source, while NaNO3 acts as the structure-directing agent for the formation of 1-D α-Fe2O3 nanostructures; during this reaction, an intermediate phase of β-FeOOH is produced prior to α-Fe2O3 synthesis [37]. The polymorphs of iron oxyhydroxide consist of α-FeOOH (goethite), β-FeOOH (akaganeite), and γ-FeOOH (lepidocrocite) [33]. Among the aforementioned iron oxyhydroxides, β-FeOOH has been widely synthesized as an intermediate oxide for the synthesis of α-Fe2O3. The synthesis of β-FeOOH in solutions containing urea and Al(III), Cr(III), and Cu(II) ions has been studied by Garcia et al. [50]. It has been reported that the resultant βFeOOH possessed only subtle differences in its crystallographic and hyperfine properties. Significantly, recent studies [5,33] show that the dopant incorporation observed in in-situ doped β-FeOOH samples is inconsistent and irreproducible, which makes in-situ doping an
unreliable method for hydrothermal doping. An in-situ Sn-doped hematite nanocoral was reported by Yichuan Ling et al. [6]; this was made by adding SnCl4 dissolved in ethanol as a tin source during the hydrothermal synthesis. These hematite nanocorals had a different morphology compared to that of the bare hematite photoanodes, which was caused by the presence of the ethanol solution and the Sndopant (SnCl4). According to previous reports, cationic substitution for element doping is favorable only for α-FeOOH and not for β-FeOOH [33]. Therefore, using the hydrothermal method to prepare in-situ Sndoped β-FeOOH is problematic. We believe that the morphology and crystallinity of the in-situ Sn-doped hematite is strongly influenced by the presence of ethanol and the Sn dopant. As a result of the influence of the Sn dopant, the formed Sndoped nanocoral hematite is not consistent and has different thicknesses at different regions within the same sample (ranging from 150 to 300 nm, as shown in Fig. S5). Additionally, the crystal domain sizes calculated from the (110) peak widths were found to be 80 nm for pristine and ex-situ Sn-doped samples but only 40 nm for in-situ Sn-doped photoanodes. To verify the superiority of ex-situ Sn-doping, the photocurrent for both the in-situ and ex-situ Sn-doped hematite photoanodes were measured by reproducibility testing. The ex-situ doped samples showed better consistency than the in-situ doped samples, as shown in Fig. S6. Since non-uniform photoanodes can significantly lower the photocurrent and the overall device performance, it is important to produce uniform and reproducible Sn-doped hematite photoanodes to ensure better device performance. The aforementioned problems in in-situ doping of β-FeOOH can be easily overcome by simple surface treatment of the dopants (e.g., by dipping or dropcasting) [15]. Moreover, the ex-situ Sn-doped hematite photoanodes yielded a greater increase in the photocurrent compared to in-situ doped photoanodes, as shown in Fig. 4. It would be highly beneficial to have a simple surface treatment, followed by high temperature sintering, to induce extrinsic elemental doping of hematite photoanodes without altering the morphologies. This simple surface treatment is performed only on the surface of hematite and not on β-FeOOH; this is done to avoid doping and reproducibility issues. The ex-situ method is simple and complementary to doping strategies that do not depend on the method of hematite synthesis and may be easily applied to any type of photoanode.
4. Conclusions In the case of in-situ Sn-doping, Sn dopants are incorporated into a hematite lattice during the hydrothermal synthesis, which eventually changed the crystal orientation from (110) to (104) and the hematite nanostructure from nanorods to nanocorals. We
254
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
Scheme 1. Schematic representation of the syntheses methods adopted for pristine, in-situ, and ex-situ Sn-doped hematite photoanodes.
demonstrated a simple and more effective ex-situ doping method for doping hematite photoanodes (compared to in-situ doping). The problems faced in in-situ doping of β-FeOOH have been overcome using this particular ex-situ doping method. Sn surface treatment, followed by high temperature annealing, introduced Sn dopants into the hematite nanostructure. The ex-situ Sn-doping method allows for complete control over the amount of Sn-doping, which can be varied by changing the concentration of the Sn precursor, while also preserving the vertically-aligned nanorod morphology and forming highly conductive (110) crystals for hematite photoanodes. Compared to in-situ Sn-doping, ex-situ Sndoping shows much lower charge transport resistance values, implying the superior electron transport kinetics in the 1-D nanostructure-based hematite photoanode (as demonstrated by EIS measurements). This ex-situ doped Sn dopants act as electron donors and improve the conductivity of hematite photoanodes. This methodology can also be extended to other types of semiconductor photoanodes, irrespective of their morphology and synthesis methods. This simple ex-situ doping treatment shows great potential to be applied to the production of hematite nanostructures with different morphologies. Acknowledgments This research was supported by the Basic Science Research Programs through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A6A3A04038530), as well as Korea Ministry of Environment (MOE) as Public Technology Program based on Environmental policy.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2015.09.016.
References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] M. Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338–344.
[3] Z.S. Li, W.J. Luo, M.L. Zhang, J.Y. Feng, Z.G. Zou, Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook, Energy Environ. Sci. 6 (2013) 347–370. [4] K. Sivula, F. Le Formal, M. Gratzel, Solar water splitting: progress using hematite (α-Fe2O3) photoelectrodes, Chemsuschem 4 (2011) 432–449. [5] C. Liu, N.P. Dasgupta, P.D. Yang, Semiconductor nanowires for artificial photosynthesis, Chem. Mater. 26 (2014) 415–422. [6] Y.C. Ling, G.M. Wang, D.A. Wheeler, J.Z. Zhang, Y. Li, Sn-doped hematite nanostructures for photoelectrochemical water splitting, Nano Lett. 11 (2011) 2119–2125. [7] A. Kay, I. Cesar, M. Gratzel, New benchmark for water photooxidation by nanostructured α-Fe2O3 films, J. Am. Chem. Soc. 128 (2006) 15714–15721. [8] K. Sivula, R. Zboril, F. Le Formal, R. Robert, A. Weidenkaff, J. Tucek, J. Frydrych, M. Gratzel, Photoelectrochemical water splitting with mesoporous hematite prepared by a solution-based colloidal approach, J. Am. Chem. Soc. 132 (2010) 7436–7444. [9] Y.S. Hu, A. Kleiman-Shwarsctein, A.J. Forman, D. Hazen, J.N. Park, E. W. McFarland, Pt-doped α-Fe2O3 thin films active for photoelectrochemical water splitting, Chem. Mater. 20 (2008) 3803–3805. [10] P.S. Shinde, A. Annamalai, J.Y. Kim, S.H. Choi, J.S. Lee, J.S. Jang, Fine-tuning pulse reverse electrodeposition for enhanced photoelectrochemical water oxidation performance of α-Fe2O3 photoanodes, J. Phys. Chem. C 119 (2015) 5281–5292. [11] L.F. Xi, P.S. Bassi, S.Y. Chiam, W.F. Mak, P.D. Tran, J. Barber, J.S.C. Loo, L.H. Wong, Surface treatment of hematite photoanodes with zinc acetate for water oxidation, Nanoscale 4 (2012) 4430–4433. [12] J.Y. Kim, G. Magesh, D.H. Youn, J.W. Jang, J. Kubota, K. Domen, J.S. Lee, Singlecrystalline, wormlike hematite photoanodes for efficient solar water splitting, Sci. Rep. 3 (2681) (2013) 1–8. [13] A. Annamalai, D. Carvalho, K.C. Wilson, M.-J. Lee, Properties of hydrothermally synthesized Zn2SnO4 nanoparticles using Na2CO3 as a novel mineralizer, Mater. Charact. 61 (2010) 873–881. [14] Gurudayal, S.Y. Chiam, M.H. Kumar, P.S. Bassi, H.L. Seng, J. Barber, L.H. Wong, Improving the efficiency of hematite nanorods for photoelectrochemical water splitting by doping with manganese, ACS Appl. Mater. Interface 6 (2014) 5852–5859. [15] Y. Ling, Y. Li, Review of Sn-doped hematite nanostructures for photoelectrochemical water splitting, Part. Part. Syst. Charact. 31 (2014) 1113–1121. [16] M.A. Lukowski, S. Jin, Improved synthesis and electrical properties of Si-doped α-Fe2O3 nanowires, J. Phys. Chem. C 115 (2011) 12388–12395. [17] J. Liu, C.H. Liang, H.M. Zhang, Z.F. Tian, S.Y. Zhang, General strategy for doping impurities (Ge, Si, Mn, Sn, Ti) in hematite nanocrystals, J. Phys. Chem. C 116 (2012) 4986–4992. [18] R. Franking, L.S. Li, M.A. Lukowski, F. Meng, Y.Z. Tan, R.J. Hamers, S. Jin, Facile post-growth doping of nanostructured hematite photoanodes for enhanced photoelectrochemical water oxidation, Energy Environ. Sci. 6 (2013) 500–512. [19] D.P. Cao, W.J. Luo, J.Y. Feng, X. Zhao, Z.S. Li, Z.G. Zou, Cathodic shift of onset potential for water oxidation on a Ti4 þ doped Fe2O3 photoanode by suppressing the back reaction, Energy Environ. Sci. 7 (2014) 752–759. [20] S.H. Shen, C.X. Kronawitter, D.A. Wheeler, P.H. Guo, S.A. Lindley, J.G. Jiang, J. Z. Zhang, L.J. Guo, S.S. Mao, Physical and photoelectrochemical characterization of Ti-doped hematite photoanodes prepared by solution growth, J. Mater. Chem. A 1 (2013) 14498–14506.
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255 [21] O. Zandi, B.M. Klahr, T.W. Hamann, Highly photoactive Ti-doped α-Fe2O3 thin film electrodes: resurrection of the dead layer, Energy Environ. Sci. 6 (2013) 634–642. [22] L.F. Xi, S.Y. Chiam, W.F. Mak, P.D. Tran, J. Barber, S.C.J. Loo, L.H. Wong, A novel strategy for surface treatment on hematite photoanode for efficient water oxidation, Chem. Sci. 4 (2013) 164–169. [23] J. Liu, C.H. Liang, G.P. Xu, Z.F. Tian, G.S. Shao, L.D. Zhang, Ge-doped hematite nanosheets with tunable doping level, structure and improved photoelectrochemical performance, Nano Energy 2 (2013) 328–336. [24] R.L. Spray, K.J. McDonald, K.S. Choi, Enhancing photoresponse of nanoparticulate α-Fe2O3 electrodes by surface composition tuning, J. Phys. Chem. C 115 (2011) 3497–3506. [25] G. Rahman, O.S. Joo, Electrodeposited nanostructured α-Fe2O3 thin films for solar water splitting: influence of Pt doping on photoelectrochemical performance, Mater. Chem. Phys. 141 (2013) 589. [26] W.W. Wang, J.L. Yao, Effect of metal ions (Sn and Zn) on the thermal property of akaganeite nanorods, Mater. Res. Bull. 47 (2012) 1762–1767. [27] A. Watanabe, H. Kozuka, Photoanodic properties of sol–gel-derived Fe2O3 thin films containing dispersed gold and silver particles, J. Phys. Chem. B 107 (2003) 12713–12720. [28] D.K. Zhong, M. Cornuz, K. Sivula, M. Graetzel, D.R. Gamelin, Photo-assisted electrodeposition of cobalt-phosphate (Co–Pi) catalyst on hematite photoanodes for solar water oxidation, Energy Environ. Sci. 4 (2011) 1759–1764. [29] X.F. Sun, C. Hu, X.X. Hu, J.H. Qu, M. Yang, Characterization and adsorption performance of Zr-doped akaganeite for efficient arsenic removal, J. Chem. Technol. Biotechnol 88 (2013) 629–635. [30] L.L. Cai, I.S. Cho, M. Logar, A. Mehta, J.J. He, C.H. Lee, P.M. Rao, Y.Z. Feng, J. Wilcox, F.B. Prinz, X.L. Zheng, Sol-flame synthesis of cobalt-doped TiO2 nanowires with enhanced electrocatalytic activity for oxygen evolution reaction, Phys. Chem. Chem. Phys. 16 (2014) 12299–12306. [31] A.L. Larralde, C.P. Ramos, B. Arcondo, A.E. Tufo, C. Saragovi, E.E. Sileo, Structural properties and hyperfine characterization of Sn-substituted goethites, Mater. Chem. Phys. 133 (2012) 735–740. [32] R.M. Cornell, U. Schwertmann, The Iron Oxides : Structure, Properties, Reactions, Occurrences, and Uses, Wiley-VCH, Weinheim, 2003 2nd, completely rev. and extended ed.. [33] A. Tufo, K. García, C. Barrero, E. Sileo, Structural and hyperfine properties of Mn and Co-incorporated akaganeites, Hyperfine Interact. 224 (2014) 239–250. [34] N.G. Holm, Substitution selectivity of some transition elements (Cr, Mn, Co, Ni, Cu, Zn) during formation of β-FeOOH, GFF Stockh. 107 (1985) 297–300. [35] X. Zong, S. Thaweesak, H.Y. Xu, Z. Xing, J. Zou, G.Q. Lu, L.Z. Wang, A scalable colloidal approach to prepare hematite films for efficient solar water splitting, Phys. Chem. Chem. Phys. 15 (2013) 12314–12321. [36] D. Wang, X.-T. Zhang, P.-P. Sun, S. Lu, L.-L. Wang, Y.-A. Wei, Y.-C. Liu, Enhanced photoelectrochemical water splitting on hematite thin film with layer-by-
[37]
[38]
[39]
[40] [41]
[42]
[43] [44]
[45]
[46]
[47]
[48]
[49]
[50]
255
layer deposited ultrathin TiO2 underlayer, Int. J. Hydrog. Energy 39 (2014) 16212–16219. L. Vayssieres, N. Beermann, S.E. Lindquist, A. Hagfeldt, Controlled aqueous chemical growth of oriented three-dimensional crystalline nanorod arrays: application to iron(III) oxides, Chem. Mater. 13 (2001) 233–235. A. Annamalai, P.S. Shinde, A. Subramanian, J.Y. Kim, J.H. Kim, S.H. Choi, J.S. Lee, J.S. Jang, Bifunctional TiO2 underlayer for α-Fe2O3 nanorod based photoelectrochemical cells: enhanced interface and Ti4 þ doping, J. Mater. Chem. A 3 (2015) 5007–5013. A. Annamalai, A. Subramanian, U. Kang, H. Park, S.H. Choi, J.S. Jang, Activation of hematite photoanodes for solar water splitting: effect of FTO deformation, J. Phys. Chem. C 119 (2015) 3810–3817. N. Iordanova, M. Dupuis, K.M. Rosso, Charge transport in metal oxides: a theoretical study of hematite α-Fe2O3, J. Chem. Phys. 122 (2005). A.M. Xavier, F.F. Ferreira, F.L. Souza, Morphological and structural evolution from akaganeite to hematite of nanorods monitored by ex situ synchrotron Xray powder diffraction, RSC Adv. 4 (2014) 17753–17759. I. Cesar, A. Kay, J.A.G. Martinez, M. Gratzel, Translucent thin film Fe2O3 photoanodes for efficient water splitting by sunlight: nanostructure-directing effect of Si-doping, J. Am. Chem. Soc. 128 (2006) 4582–4583. N.S. Mcintyre, D.G. Zetaruk, X-Ray photoelectron spectroscopic studies of ironoxides, Anal. Chem. 49 (1977) 1521–1529. J.Y. Kim, J.W. Jang, D.H. Youn, G. Magesh, J.S. Lee, A stable and efficient hematite photoanode in a neutral electrolyte for solar water splitting: towards stability engineering, Adv. Energy Mater. 4 (2014) 1–7 1400476. L. Wang, C.Y. Lee, A. Mazare, K. Lee, J. Muller, E. Spiecker, P. Schmuki, Enhancing the water splitting efficiency of Sn-doped hematite nanoflakes by flame annealing, Chem.-Eur. J. 20 (2014) 77–82. J. Brillet, M. Gratzel, K. Sivula, Decoupling feature size and functionality in solution-processed, porous hematite electrodes for solar water splitting, Nano Lett. 10 (2010) 4155–4160. F. Le Formal, M. Gratzel, K. Sivula, Controlling photoactivity in ultrathin hematite films for solar water-splitting, Adv. Funct. Mater. 20 (2010) 1099–1107. G.M. Wang, Y.C. Ling, D.A. Wheeler, K.E.N. George, K. Horsley, C. Heske, J. Z. Zhang, Y. Li, Facile synthesis of highly photoactive α-Fe2O3-based films for water oxidation, Nano Lett. 11 (2011) 3503–3509. T. Lopes, L. Andrade, H.A. Ribeiro, A. Mendes, Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy, Int. J. Hydrog. Energy 35 (2010) 11601–11608. K.E. Garcia, C.A. Barrero, A.L. Morales, J.M. Greneche, Characterization of akaganeite synthesized in presence of Al3 þ , Cr3 þ , and Cu2 þ ions and urea, Mater. Chem. Phys. 112 (2008) 120–126.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Fabrication of superior α-Fe2O3 nanorod photoanodes through ex-situ Sn-doping for solar water splitting Alagappan Annamalai a, Pravin S. Shinde a, Tae Hwa Jeon b, Hyun Hwi Lee c, Hyun Gyu Kim d, Wonyong Choi b, Jum Suk Jang a,n a Division of Biotechnology, Advanced Institute of Environmental and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 570-752, Republic of Korea b School of Environmental Science and Engineering, POSTECH, Pohang 790-784, Republic of Korea c Pohang Accelerator Laboratory, POSTECH, Pohang 790-784, Republic of Korea d Korea Basic Science Institute, Busan Center, Busan 609-735, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 2 August 2015 Accepted 13 September 2015
Doping transition metals into 1-D nanostructures is of crucial importance for their application in photovoltaics and photoelectrochemical (PEC) systems; performance enhancements arise from both dopant incorporation and the 1-D nanostructures. Both in-situ and ex-situ doping methods have been demonstrated for 1-D hematite (α-Fe2O3) nanostructures, with tin (Sn) as the dopant, for photoelectrochemical water oxidation. In-situ Sn-doped hematite photoanodes adopted a morphology consisting of nanocorals with the (104) plane as the preferred direction of crystal growth. As an alternative solution, ex-situ doping not only preserves the vertically-aligned nanorod morphology but also sustains the preferred orientation of the (110) axis, which is favorable for high conductivity in pristine hematite photoanodes. In-situ Sn-doping was carried out by the same method: Sn precursors were added and dissolved in ethanol during the hydrothermal synthesis. Ex-situ doping was carried out in two stages (during pre-deposition and during high temperature sintering). During pre-deposition, a defined amount of the Sn precursor was introduced near the surface region of the 1-D nanostructure, and the Sn content was controlled by changing the concentration of the precursor solution. In subsequent high temperature sintering (800 °C), the dopant atoms diffused into the hematite lattice to attain the desired doping profile. We found that ex-situ Sn-doping resulted in a 60% increase in the photocurrent while in-situ Sndoping yielded an increase of only 20% in the photocurrent, as compared with pristine hematite photoanodes, at 1.4 V vs. RHE. The improvement in the photocurrent was caused by a combination of Sn dopants in the hematite, which act as electron donors by increasing the donor density, and better surface charge transfer kinetics, thereby enhancing the overall device performance. & 2015 Elsevier B.V. All rights reserved.
Keywords: β-FeOOH Ex-situ Doping Hematite α-Fe2O3 Intrinsic Sn doping
1. Introduction Photoelectrochemical (PEC) water oxidation is a promising method to convert solar energy into chemical energy that is stored in the form of molecular hydrogen and oxygen [1,2]. Beginning with the first report by Fujishima and Honda [1] in 1972, which used TiO2 as a photoanode, various metal oxide semiconductors [3] have been extensively studied as photoanodes for water oxidation. Among different metal oxide semiconductors, hematite (αFe2O3) is a promising photoanode candidate for photoelectrochemical water splitting due to its favorable band gap (2.2 eV) and extraordinary chemical stability [3,4]. Hematite has a n
Corresponding author. Tel.: þ 82 63 850 0846. E-mail address: [email protected] (J.S. Jang).
http://dx.doi.org/10.1016/j.solmat.2015.09.016 0927-0248/& 2015 Elsevier B.V. All rights reserved.
maximum theoretical solar-to-hydrogen (STH) efficiency of 15%, which is key for practical applications. However, hematite has some limiting factors such as the short-diffusion length of its holes and its extremely poor conductivity [3]. Nanostructuring of hematite photoanodes has significantly improved the photogenerated carrier collection, while elemental doping has improved the conducting properties of hematite photoanodes by increasing the donor density [5,6]. 1-D nanostructures of hematite have shown significantly improved photocurrent densities compared to those of the bulk and other nanostructures [6,7]. To date, hematite nanostructures have been synthesized using a variety of techniques including sol–gel processing [8], electrodeposition [9,10], spray pyrolysis [11], and hydrothermal synthesis [12]. In particular, hydrothermal synthesis is simple, cost effective, and provides good control over the composition and morphology [13,14]. Elemental doping plays a critical role in improving the
248
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
electrical conductivity and in enhancing the PEC performance of hematite photoanodes [5,15]. Dopants, such as Si [16,17], Ti [17– 21], Sn [6,15,17,22], Zr [18], Ge [17,23], Mn [14,17], Al [24], and Pt [9,12,25], have been investigated for use in hematite photoanodes. Sn has an advantage over other dopants because Sn ions have a similar ionic radius and Pauling electronegativity compared to Fe ions. They have been reported to improve the electronic properties of hematite [26]. The addition of a surface electrocatalyst (e.g., Co– Pi, Au, Pt, and RuO2) on hematite photoanodes has also been proposed to improve PEC device performance [27,28]. In general, elemental doping in hematite photoanodes can be classified as either in-situ doping or ex-situ doping [15]. In most of the reports that use hydrothermal synthesis, dopants are incorporated into the hematite nanostructures during material synthesis (in-situ doping) [6,9,12,14,29]. In-situ doping is a simple process that uses a relatively low sintering temperature, allows for flexibility in the choice of dopants, and requires no additional doping steps [30]. However, changes in the morphology and crystallinity are observed for in-situ doped hematite photoanodes [6,15]. In contrast, the ex-situ process begins with the final product of the hydrothermal synthesis and is followed by a Sn surface treatment and high temperature annealing to incorporate the elemental Sn dopant into the hematite nanostructure through a solid-state reaction [18]. In the case of in-situ doping-based hematite photoanodes, cationic substitution has only been reported for goethite (α-FeOOH) [31] and not for akaganeites (β-FeOOH) [32–34]. Holm et al. [34] synthesized β-FeOOH in the presence of Cr, Mn, Co, Ni, and Zn (50 mol%) and reported only small incorporations of foreign cations (despite using concentrations of dopants that were equimolar with Fe). Thus, in-situ doping based on β-FeOOH is neither reliable nor reproducible due to the intrinsic properties of β-FeOOH, as has been noted in previous reports [33]. Another limitation of in-situ doping is related to the fact that when the metal ion concentration is increased, only a part of the metal ions can be doped into the structure of β-FeOOH. This results in the formation of undesired metal oxides during the hydrothermal synthesis [26]. In order to realize the advantages from both types of processes, and to compare the microstructural evolution of in-situ and exsitu methods, it is essential to fabricate 1-D hematite nanostructures using Sn as the dopant. Some groups have reported Sndoping in hematite photoanodes through ex-situ methods [18,22,35,36]. However, to the best of our knowledge, nobody has reported the superior performance of ex-situ Sn-doping over insitu Sn-doping in hematite photoanodes. Herein, we explain the importance and advantages of an ex-situ method for Sn-doping into 1-D hematite nanostructures. Additionally, our recommended ex-situ doping method can be successfully applied to other types of 1-D nanostructured metal oxide photoanodes with various elemental dopants without compromising the morphology and/or crystallinity of the parent metal oxide photoanode. Enhanced performance in hematite photoanodes was observed when the nanostructures were doped by subjecting them to high temperature annealing in order to encourage dopant incorporation into the surface of the hematite nanostructures. Ex-situ Sn-doped hematite nanorod photoanodes exhibit enhanced electron transport. This superior performance is caused by the fact that the direct chargediffusion pathways reduce the number of grain boundaries and defects, which act as major charge recombination sites. Using an optimized Sn concentration (5 mM), we achieved a photocurrent of 1.6 mA/cm2 at 1.4 V vs. RHE under illumination in ex-situ Sndoped hematite photoanodes. This value is 60% higher compared to pristine hematite photoanodes.
2. Experimental 1-D hematite nanostructures on FTO (F:SnO2) glass were prepared by a simple hydrothermal method based on a previous report [37]. In a typical experiment, a piece of cleaned FTO glass (1 cm 2.5 cm) was placed within a 20 ml vial containing a solution (10 ml) consisting of 0.4 g FeCl3 6H2O and 0.85 g NaNO3 (at pH 1.5, adjusted by HCl) [38]. A hydrothermal reaction was conducted at 100 °C for 6 h. After cooling to room temperature, the FTO glass was rinsed several times with distilled water and then dried at 60 °C. The formed β-FeOOH is strongly adhered to the FTO glass substrate even after ultrasonication for 10 min. Annealing at 550 °C for 4 h was carried out to cause a phase transition from βFeOOH to pure α-Fe2O3 [38]. In-situ Sn-doped hematite photoanodes were prepared by adding 1 ml of an ethanol–SnCl4 solution (10 mg/ml), which was added to the hydrothermal precursor solution as reported previously [6]. The extrinsic Sn-doping was carried out by a simple dipping method in order to treat the hematite photoanodes with the Sn precursor solution. Varied concentrations of SnCl4 were dissolved in ethanol to prepare the Sn precursor solution. After dipping the photoanodes in the Sn precursor, they were allowed to air dry. Then, the surface-treated samples were subjected to high temperature sintering (Fig. S2) [39]. This high temperature sintering (800 °C for 10 min) is believed to be important for the activation of hematite photoanodes by enhancing the electron transfer between hematite and the conductive substrates [4]. X-ray diffraction (XRD) patterns of all samples were collected using an X-ray diffractometer (Rigaku RINT 2500) with CuKα radiation. The crystallinity and preferential ordering of the hematite nanostructure samples were obtained by measuring the X-ray diffraction profiles with conventional two theta scans and two-dimensional X-ray diffraction (2D XRD) patterns. These were performed at the 5A X-ray scattering beamline for materials science at the Pohang Light Source II (PLS-II) in Korea. The X-ray diffraction patterns were measured using a point detector mounted on a diffractometer, whereas the 2D XRD data were collected using a two-dimensional (2D) image plate detector in a quasi-symmetric reflection mode at Qz 2.4 Å 1 with a sample-todetector distance of 306 mm. The probing X-ray energy was 11.57 keV (1.0716 Å) for the scattering measurements. The surface morphologies of the samples were analyzed using field emission scanning electron microscopy (FESEM, JEOL JSM 700F). X-ray photoelectron spectroscopy (XPS) was performed using an X-ray photoelectron spectrometer (Kratos XSAM 800pci) using Mg KR lines (1253.6 eV) as an excitation source. A high resolution transmission electron microscope (HRTEM) with electron energy loss spectroscopy (EELS) mapping capabilities was used to study the elemental mapping in Sn-doped hematite photoanodes. All photoelectrochemical measurements were carried out in an electrolyte of 1 M NaOH (pH¼13.8) using an Ivium CompactStat potentiostat with a platinum coil as the counter electrode and Ag/AgCl as the reference electrode. Photocurrent–potential (J–V) curves were swept at 50 mV s 1 from 0.7 to þ0.7 V vs. Ag/AgCl. To measure the incident photon-to-current conversion effciencies (IPCEs), a 300 W xenon lamp (Newport, 6258) was coupled to a grating monochromator (Newport, 74125). This was operated in a wavelength range from 330 to 600 nm, and the incident light intensity was measured with a UV silicon detector (Newport, 71675). The photoelectrode was biased at þ0.6 or þ1 V (vs. Ag/AgCl) during all IPCE measurements. Impedance spectroscopy measurements were performed using an impedance analyzer (Ivumstat). The impedance spectra were measured over a frequency range of 1 10 2–3 106 Hz at 25 °C under open circuit conditions with an amplitude of 10 mV and under a bias illumination of 100 mW cm 2.
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
3. Results and discussion Fig. 1 depicts the FESEM images comparing pristine, in-situ, and ex-situ Sn-doped hematite photoanodes. This figure shows very similar nanorod morphology for pristine and ex-situ Sn-doped hematite photoanodes; alternatively, clustered nanocorals for the in-situ Sn-doped hematite photoanodes were observed. For pristine hematite photoanodes sintered at 800 °C, nanorods of 30– 50 nm in diameter and 400 nm in length, which were aligned roughly vertically to the FTO substrates, were observed. This has also been reported elsewhere [12]. In-situ Sn-doped photoanodes have a tiny nanocoral-like clustered morphology, which is completely different from the pristine hematite photoanodes with a length of 500 nm. The distinct morphology of in-situ Sn-doped photoanodes, from aligned nanorods to clusters, originates from the inclusion of ethanol and dopant SnCl4 in the precursor solution that is utilized for Sn incorporation [6]. Alternatively, ex-situ Sndoped photoanodes show very similar morphologies to those of pristine hematite photoanodes (vertically-aligned nanorods) with no additional deposits or differences in morphology (that can be distinguished by FESEM) after Sn-doping. This is the case because the surface treatment of the dopant precursor, which is followed by high temperature sintering, effectively induces the diffusion of Sn dopants into the host hematite photoanodes without significantly altering the aligned hematite nanorod morphology. HRTEM micrographs of both the in-situ and ex-situ Sn-doped hematite photoanodes are shown in Fig. 2. In-situ Sn-doped hematite shows nanocoral-shaped particles on the FTO substrates. The coral-like nanostructures in in-situ Sn-doped hematite photoanodes have many grain boundaries, which may have detrimental effects on the electron transport properties of in-situ Sndoped hematite photoanodes. However, the ex-situ Sn-doped hematite photoanodes retain the 1-D nanorod morphology of pristine hematite photoanodes. The elemental mapping images (Fig. 2a and b) show that Fe, O, and Sn were all uniformly distributed in both the nanocoral-shaped in-situ and nanorod-shaped ex-situ Sn-doped hematite photoanodes. However, elemental analyses by TEM and EELS reveal that the coral-like in-situ Sndoped hematite photoanodes showed a higher Sn content (0.92 wt%) compared to that of ex-situ Sn-doped hematite photoanodes (0.4 wt%). This further supports the XPS data.
249
Fig. 3 shows the XRD patterns of pristine, in-situ, and ex-situ Sn-doped hematite nanostructures. Since the (104) phase of αFe2O3 coincides with the FTO lattice parameters (Fig. S1), we chose synchrotron X-ray diffraction (XRD) over conventional XRD. With the exception of the FTO substrate peaks, all of the other peaks can be indexed to hematite (JCPDS card 33-0664). The absence of βFeOOH diffraction peaks indicates the complete conversion of iron oxyhydroxides into hematite [12]. When sintered at 550 °C for 4 h, the as-synthesized, yellow-colored β-FeOOH is converted into redcolored α-Fe2O3. The XRD peaks of the hematite photoanodes sintered at 800 °C show similar features to those of the hematite films sintered at 550 °C, with the exception of sharper diffraction peaks from FTO (Fig. S2). No diffraction peaks of Sn or other impurity phases were observed in any of the samples. Both the pristine and ex-situ Sn-doped hematite photoanodes displayed similar XRD patterns with a predominant (110) diffraction peak. Alternatively, the in-situ Sn-doped hematite photoanodes displayed a stronger (104) diffraction peak, as shown in Fig. 3. The (110) reflection was observed for 1-D hematite nanostructures, which indicates a strong preferential orientation of the [110] axis vertical to the substrate, as shown in Fig. 1a and c. Hematite with [110] orientation has been reported to have an anisotropic conductivity that is four orders of magnitude higher and better facilitates charge collection of photo-excited charge carriers along the 1-D nanostructures [6,7,40,41]. In the case of in-situ Sn-doped photoanodes, a stronger (104) diffraction peak and a smaller (110) diffraction peak were observed. Similar (104) diffraction peaks were reported for in-situ Si-doped [42] and Sn-doped [6] hematite photoanodes, which were induced by either the presence of the dopants during synthesis or by changes in the morphology, as shown in Fig. 1b. To clarify the preferential ordering of hematite nanostructures, 2D XRD was performed. These data are depicted in Fig. 4a–c. Fig. 4d and e shows circular line cuts passing through the hematite (110) and (104) peaks. In Fig. 4d, the maximum intensities of the pristine and ex-situ Sn-doped samples at 0° represent the preferential ordering of hematite (110); alternatively, the featureless intensity distribution of the in-situ sample implies little ordering or a powder-like distribution. The angular spread of the (110) peak was 50° for both the pristine and ex-situ doped samples (taken from the full-width at half-maximum of the circular line cuts). Thus, ex-
Fig. 1. FESEM images of (a) pristine hematite photoanodes, (b) in-situ Sn-doped hematite photoanodes, (c) ex-situ Sn-doped hematite photoanodes, and insets with their respective cross-sectional images sintered at 800 °C.
250
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
Fig. 2. HRTEM images of (a) in-situ Sn-doped and (b) ex-situ Sn-doped hematite photoanodes. Their respective elemental mappings are included (Fe, O, and Sn).
Fig. 3. X-ray diffraction patterns and digital pictures of pristine, in-situ, and ex-situ Sn-doped hematite photoanodes annealed at 800 °C.
situ Sn-doping incorporates the Sn dopants into the hematite photoanodes without changing the morphologies or the [110] orientation; this [110] orientation has been suggested as the preferential direction for electron transport in hematite [41]. In order to confirm the presence of Sn dopant atoms after doping of the hematite nanostructures, XPS analyses were performed on pristine, in-situ, and ex-situ Sn-doped hematite nanostructures, as shown in Fig. 5. The XPS spectra (Fig. 5b) for pristine, in-situ, and ex-situ Sn-doped samples show Fe(2p) peaks at 724.2 and 711.2 eV. These peaks are consistent with the Fe3 þ ions in hematite [43]. The major O(1s) peak, appearing at 530.2 eV for pristine and Sn-doped samples, was also consistent with that of hematite. Alternatively, the O1s peak at 531.7 eV represents the surface hydroxyl groups [43,44]. Detailed analysis of the Sn3d peaks for Sn-doped hematite photoanodes shows the presence of two intense peaks at 487 (Sn3d5/2) and 495.4 eV (Sn 3d3/2). These confirm the presence of Sn4 þ dopants in both in-situ and ex-situ doped samples [22,45]. The Sn4 þ peaks observed from pristine hematite photoanodes come from the Sn diffusion from FTO substrates when the samples are sintered at 800 °C (to activate the hematite photoanodes) [6]. The relatively higher concentration of Sn dopants found in in-situ and ex-situ Sn-doped hematite photoanodes (compared to pristine photoanodes) supports the assumption that a significant amount of Sn dopants diffuse into the hematite lattice. It is clearly evident from Fig. 5d that a higher amount of Sn is incorporated into the hematite lattice for in-situ Sn-doped photoanodes compared to
ex-situ Sn-doped photoanodes [6,22]. It should be noted that the origin of Sn dopants in the case of ex-situ doped samples is Sn atoms that diffuse into the surface of the hematite nanorods upon sintering at 800 °C. Alternatively, in the case of in-situ doped samples, the Sn dopants were incorporated into the hematite lattice during the synthesis stage. Photoelectrochemical measurements were performed in a three-electrode electrochemical cell using pristine and doped 1-D hematite nanostructures on FTO as the working electrode. These cells used a platinum coil as the counter electrode and Ag/AgCl as the reference electrode. Fig. 6a depicts the photocurrent responses of pristine, in-situ, and ex-situ Sn-doped hematite photoanodes under dark and illuminated conditions. Among the various surface treatment conditions that we attempted, in-situ Sn-doping with 1.0 ml of ethanol–SnCl4 and ex-situ Sn-doping with 5 mM SnCl4 (Fig. S4) showed the greatest photocurrent enhancement. In-situ Sn-doping was carried out with 1.0 ml of an ethanol–SnCl4 precursor, which was added to the hydrothermal solution during the synthesis, as previously reported [6]. In the case of the ex-situ doping, increasing the Sn4 þ concentration above 5 mM in the treatment solution can result in an increase in the particle size, which disrupts the surface morphology of the hematite photoanodes by reducing the surface area; this would result in poor photoelectrochemical performance. The best performance was achieved by ex-situ Sn-doped hematite photoanodes treated with a solution containing 5 mM SnCl4. Upon illumination, the pristine hematite photoanodes show a photocurrent of 1.03 mA/cm2 at 1.4 V vs. RHE. In comparison to pristine hematite photoanodes, in-situ Sn-doping shows a photocurrent of 1.21 mA/cm2 (a 20% increase); a much higher photocurrent of 1.63 mA/cm2 (a 60% increase) is exhibited for ex-situ Sndoped hematite photoanodes. However, both the in-situ and exsitu Sn-doped hematite photoanodes showed a positive shift in the onset potential from 0.8 to 1.0 V vs. RHE, as compared to the bare photoanodes annealed at 800 °C. A similar anodic onset potential shift has been observed previously for in-situ Sn-doped [6] and Ptdoped [25] hematite photoanodes. This anodic shift in the onset potential indicates that the water oxidation kinetics are limited, which is possibly due to the increase in the surface trapping states that is caused by the increased surface area after surface treatment [46,47]. Since Sn-doping is carried out in solution, the Sn precursor prepared in ethanol could easily penetrate through the hematite nanostructures. During annealing, a thin layer of SnO2 is formed on the surface of the hematite nanostructures. Annealing at a
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
251
Fig. 4. (a)–(c) 2D XRD images of the pristine (black dot), ex-situ (blue dot), and in-situ (red dot) Sn-doped hematite photoanodes annealed at 800 °C. (d)–(e) The circular line cuts passing through the hematite (110) and (104) peaks, where zero degrees corresponds to the direction normal to the surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
temperature of 800 °C for 10 min is sufficient to gradually incorporate Sn into the hematite photoanodes, both from the surface treatment [24] as well as from the FTO substrates [6]. The Sn dopants in the hematite photoanodes can serve as electron donors by drastically increasing the donor density, which leads to an enhancement in the conducting properties of the Sn-doped hematite photoanodes [15,48]. Additional incident photon-to-current-efficiency (IPCE) measurements were carried out for pristine, in-situ, and ex-situ Sndoped hematite photoanodes at 1.4 V vs. RHE, as shown in Fig. 6b. In comparison to pristine and in-situ Sn-doped photoanodes, exsitu Sn-doped photoanodes showed higher IPCE values over the whole visible spectrum. These results were consistent with the differences observed in the photocurrent densities in pristine, insitu, and ex-situ Sn-doped hematite photoanodes. Thus, the photocurrent of ex-situ Sn-doped hematite at 1.4 V vs. RHE was increased by 60% (as compared to that of the pristine hematite photoanodes). This simple solution-based ex-situ Sn-doping was found to be highly reproducible compared to that of in-situ doping. Ex-situ Sn-doped hematite photoanodes still showed superior enhancement in the photocurrent compared to in-situ Sn- doped and pristine hematite photoanodes despite an anodic onset shift; this suggests that the photocurrent enhancement caused by Sndoping may be further enhanced by placing an appropriate oxygen evolving catalyst on the photoanode surface to improve the surface catalytic activity. We further analyzed the pristine and Sndoped hematite photochemical response (Fig. 7a) and stability of the photoanodes (Fig. 7b) at 1.4 V vs. RHE under standard
illumination conditions over a period of 1200 s. There is no observable degradation, indicating the excellent chemical and structural stability of the ex-situ Sn-doped photoanodes compared to the pristine and in-situ Sn-doped hematite photoanodes for long-term PEC conversion. Using electrochemical impedance spectroscopy (EIS), the internal resistances and the electron transport kinetics of pristine, in-situ, and ex-situ Sn-doped hematite photoanodes were studied. Fig. 8 depicts the Nyquist plots of pristine and doped hematite photoanodes, from electrochemical impedance under standard illumination conditions, which shows two semicircles for each type of photoanode. The semicircles at lower frequencies are related to the electron transfer resistance, and the ones at higher frequencies describe the tendency of recombination at the hematite/electrolyte interface [44,49]. From Fig. 8, the arc decreased with ex-situ Sn-doping and increased with in-situ Sn-doping (as compared to pristine hematite photoanodes). This decrease indicates that hematite photoanodes with 1-D nanostructures result in pronounced electron transport properties [25]. In the case of in-situ Sn-doped photoanodes, the 1-D nanostructure of the pristine hematite photoanodes is changed into nanocorals, which have a larger surface area; however, this structure also produces greater amounts of electron recombination and a larger electron transport resistance due to the increased number of grain boundaries. The in-situ Sn-doping enhances the photocurrent compared to pristine photoanodes but compromises the electronic properties due to the presence of the inferior (104) crystalline phase of in-situ Sn-doped nanocorals compared to the (110) phase of pristine hematite photoanodes. The increased electron recombination is caused by the
252
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
Fig. 5. X-ray photoelectron spectra of (a) full scan, (b) Fe 2p, (c) O 1s, and (d) Sn 3d recorded from pristine, in-situ, and ex-situ Sn-doped hematite nanostructures annealed at 800 °C.
Fig. 6. (a) Photocurrent–potential (J–V) curves and (b) incident photon-to-current-efficiency (IPCE) spectra for PEC water oxidation reaction with pristine, in-situ, and ex-situ Sn-doped hematite photoanodes under standard illumination conditions.
electron hopping mechanism that is observed in nanoparticle-based PECs. However, ex-situ Sn-doping has several advantages over in-situ doping because it can preserve the highly conductive (110) crystalline phase and the 1-D nanostructures, while also improving the charge transport and decreasing electron recombination (surface passivation induced from ex-situ surface treatment) [22,24] by providing fast pathways for electrons. The effect of the ex-situ Sn-doping on the hematite photoanodes can increase the conductivity and reduce the recombination of charges in the hematite photoanodes, resulting in an enhancement of the photocurrent. This suggests that the 1-D nanostructured ex-situ Sn-doped hematite photoanodes have a longer electron lifetime than the nanocoral-shaped in-situ Sn-doped hematite photoanodes. The EIS results (Table S1) are in good agreement with the results of the photocurrent measurements and the IPCEs of in-situ and ex-situ Sn-doped hematite photoanodes. To further understand the role of in-situ and ex-situ Sn-doping, Mott– Schottky measurements were also performed (Fig. 9).
The positive slopes in Fig. 9 indicate that the pristine, in-situ, and ex-situ Sn-doped hematite photoanodes are all n-type semiconductors. By extrapolating the x-intercepts of the linear region in the Mott– Schottky plots, the VFB values of the pristine, in-situ, and ex-situ Sndoped hematite photoanodes were found to be 0.62, 0.66, and 0.71 V vs. RHE, respectively. These VFB potentials are consistent with previous reports for pristine and doped hematite photoanodes [4]. The slopes derived from the Mott–Schottky analysis were also used to further estimate the donor densities of pristine, in-situ, and ex-situ Sndoped hematite photoanodes; these values were determined to be 1.26 1019, 6.32 1019, and 2.49 1020 cm 3, respectively. The above data provides concrete evidence and supports the idea that extrinsic Sn-dopants act as electron donors and that the donor density increased substantially after both in-situ and ex-situ doping. The hydrothermal method is an ideal method for the synthesis of 1-D hematite nano-structures and offers effective control over the size and shape of nanostructures at relatively low reaction temperatures and short reaction times (Scheme 1). An iron (III) chloride
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
253
Fig. 7. (a) Photochemical response and (b) photochemical stability for PEC water oxidation reactions with pristine, in-situ, and ex-situ Sn-doped hematite photoanodes at 1.4 V vs. RHE under standard illumination conditions.
Fig. 8. Nyquist plots of pristine, in-situ, and ex-situ Sn-doped hematite photoanodes at 1.23 V vs. RHE under standard illumination conditions. The inset in the Nyquist plot represents the equivalent circuit for EIS.
Fig. 9. Mott–Schottky plots for PEC water oxidation reactions with pristine, in-situ, and ex-situ Sn-doped hematite photoanodes.
(FeCl3) solution acts as the iron source, while NaNO3 acts as the structure-directing agent for the formation of 1-D α-Fe2O3 nanostructures; during this reaction, an intermediate phase of β-FeOOH is produced prior to α-Fe2O3 synthesis [37]. The polymorphs of iron oxyhydroxide consist of α-FeOOH (goethite), β-FeOOH (akaganeite), and γ-FeOOH (lepidocrocite) [33]. Among the aforementioned iron oxyhydroxides, β-FeOOH has been widely synthesized as an intermediate oxide for the synthesis of α-Fe2O3. The synthesis of β-FeOOH in solutions containing urea and Al(III), Cr(III), and Cu(II) ions has been studied by Garcia et al. [50]. It has been reported that the resultant βFeOOH possessed only subtle differences in its crystallographic and hyperfine properties. Significantly, recent studies [5,33] show that the dopant incorporation observed in in-situ doped β-FeOOH samples is inconsistent and irreproducible, which makes in-situ doping an
unreliable method for hydrothermal doping. An in-situ Sn-doped hematite nanocoral was reported by Yichuan Ling et al. [6]; this was made by adding SnCl4 dissolved in ethanol as a tin source during the hydrothermal synthesis. These hematite nanocorals had a different morphology compared to that of the bare hematite photoanodes, which was caused by the presence of the ethanol solution and the Sndopant (SnCl4). According to previous reports, cationic substitution for element doping is favorable only for α-FeOOH and not for β-FeOOH [33]. Therefore, using the hydrothermal method to prepare in-situ Sndoped β-FeOOH is problematic. We believe that the morphology and crystallinity of the in-situ Sn-doped hematite is strongly influenced by the presence of ethanol and the Sn dopant. As a result of the influence of the Sn dopant, the formed Sndoped nanocoral hematite is not consistent and has different thicknesses at different regions within the same sample (ranging from 150 to 300 nm, as shown in Fig. S5). Additionally, the crystal domain sizes calculated from the (110) peak widths were found to be 80 nm for pristine and ex-situ Sn-doped samples but only 40 nm for in-situ Sn-doped photoanodes. To verify the superiority of ex-situ Sn-doping, the photocurrent for both the in-situ and ex-situ Sn-doped hematite photoanodes were measured by reproducibility testing. The ex-situ doped samples showed better consistency than the in-situ doped samples, as shown in Fig. S6. Since non-uniform photoanodes can significantly lower the photocurrent and the overall device performance, it is important to produce uniform and reproducible Sn-doped hematite photoanodes to ensure better device performance. The aforementioned problems in in-situ doping of β-FeOOH can be easily overcome by simple surface treatment of the dopants (e.g., by dipping or dropcasting) [15]. Moreover, the ex-situ Sn-doped hematite photoanodes yielded a greater increase in the photocurrent compared to in-situ doped photoanodes, as shown in Fig. 4. It would be highly beneficial to have a simple surface treatment, followed by high temperature sintering, to induce extrinsic elemental doping of hematite photoanodes without altering the morphologies. This simple surface treatment is performed only on the surface of hematite and not on β-FeOOH; this is done to avoid doping and reproducibility issues. The ex-situ method is simple and complementary to doping strategies that do not depend on the method of hematite synthesis and may be easily applied to any type of photoanode.
4. Conclusions In the case of in-situ Sn-doping, Sn dopants are incorporated into a hematite lattice during the hydrothermal synthesis, which eventually changed the crystal orientation from (110) to (104) and the hematite nanostructure from nanorods to nanocorals. We
254
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255
Scheme 1. Schematic representation of the syntheses methods adopted for pristine, in-situ, and ex-situ Sn-doped hematite photoanodes.
demonstrated a simple and more effective ex-situ doping method for doping hematite photoanodes (compared to in-situ doping). The problems faced in in-situ doping of β-FeOOH have been overcome using this particular ex-situ doping method. Sn surface treatment, followed by high temperature annealing, introduced Sn dopants into the hematite nanostructure. The ex-situ Sn-doping method allows for complete control over the amount of Sn-doping, which can be varied by changing the concentration of the Sn precursor, while also preserving the vertically-aligned nanorod morphology and forming highly conductive (110) crystals for hematite photoanodes. Compared to in-situ Sn-doping, ex-situ Sndoping shows much lower charge transport resistance values, implying the superior electron transport kinetics in the 1-D nanostructure-based hematite photoanode (as demonstrated by EIS measurements). This ex-situ doped Sn dopants act as electron donors and improve the conductivity of hematite photoanodes. This methodology can also be extended to other types of semiconductor photoanodes, irrespective of their morphology and synthesis methods. This simple ex-situ doping treatment shows great potential to be applied to the production of hematite nanostructures with different morphologies. Acknowledgments This research was supported by the Basic Science Research Programs through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A6A3A04038530), as well as Korea Ministry of Environment (MOE) as Public Technology Program based on Environmental policy.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2015.09.016.
References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] M. Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338–344.
[3] Z.S. Li, W.J. Luo, M.L. Zhang, J.Y. Feng, Z.G. Zou, Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook, Energy Environ. Sci. 6 (2013) 347–370. [4] K. Sivula, F. Le Formal, M. Gratzel, Solar water splitting: progress using hematite (α-Fe2O3) photoelectrodes, Chemsuschem 4 (2011) 432–449. [5] C. Liu, N.P. Dasgupta, P.D. Yang, Semiconductor nanowires for artificial photosynthesis, Chem. Mater. 26 (2014) 415–422. [6] Y.C. Ling, G.M. Wang, D.A. Wheeler, J.Z. Zhang, Y. Li, Sn-doped hematite nanostructures for photoelectrochemical water splitting, Nano Lett. 11 (2011) 2119–2125. [7] A. Kay, I. Cesar, M. Gratzel, New benchmark for water photooxidation by nanostructured α-Fe2O3 films, J. Am. Chem. Soc. 128 (2006) 15714–15721. [8] K. Sivula, R. Zboril, F. Le Formal, R. Robert, A. Weidenkaff, J. Tucek, J. Frydrych, M. Gratzel, Photoelectrochemical water splitting with mesoporous hematite prepared by a solution-based colloidal approach, J. Am. Chem. Soc. 132 (2010) 7436–7444. [9] Y.S. Hu, A. Kleiman-Shwarsctein, A.J. Forman, D. Hazen, J.N. Park, E. W. McFarland, Pt-doped α-Fe2O3 thin films active for photoelectrochemical water splitting, Chem. Mater. 20 (2008) 3803–3805. [10] P.S. Shinde, A. Annamalai, J.Y. Kim, S.H. Choi, J.S. Lee, J.S. Jang, Fine-tuning pulse reverse electrodeposition for enhanced photoelectrochemical water oxidation performance of α-Fe2O3 photoanodes, J. Phys. Chem. C 119 (2015) 5281–5292. [11] L.F. Xi, P.S. Bassi, S.Y. Chiam, W.F. Mak, P.D. Tran, J. Barber, J.S.C. Loo, L.H. Wong, Surface treatment of hematite photoanodes with zinc acetate for water oxidation, Nanoscale 4 (2012) 4430–4433. [12] J.Y. Kim, G. Magesh, D.H. Youn, J.W. Jang, J. Kubota, K. Domen, J.S. Lee, Singlecrystalline, wormlike hematite photoanodes for efficient solar water splitting, Sci. Rep. 3 (2681) (2013) 1–8. [13] A. Annamalai, D. Carvalho, K.C. Wilson, M.-J. Lee, Properties of hydrothermally synthesized Zn2SnO4 nanoparticles using Na2CO3 as a novel mineralizer, Mater. Charact. 61 (2010) 873–881. [14] Gurudayal, S.Y. Chiam, M.H. Kumar, P.S. Bassi, H.L. Seng, J. Barber, L.H. Wong, Improving the efficiency of hematite nanorods for photoelectrochemical water splitting by doping with manganese, ACS Appl. Mater. Interface 6 (2014) 5852–5859. [15] Y. Ling, Y. Li, Review of Sn-doped hematite nanostructures for photoelectrochemical water splitting, Part. Part. Syst. Charact. 31 (2014) 1113–1121. [16] M.A. Lukowski, S. Jin, Improved synthesis and electrical properties of Si-doped α-Fe2O3 nanowires, J. Phys. Chem. C 115 (2011) 12388–12395. [17] J. Liu, C.H. Liang, H.M. Zhang, Z.F. Tian, S.Y. Zhang, General strategy for doping impurities (Ge, Si, Mn, Sn, Ti) in hematite nanocrystals, J. Phys. Chem. C 116 (2012) 4986–4992. [18] R. Franking, L.S. Li, M.A. Lukowski, F. Meng, Y.Z. Tan, R.J. Hamers, S. Jin, Facile post-growth doping of nanostructured hematite photoanodes for enhanced photoelectrochemical water oxidation, Energy Environ. Sci. 6 (2013) 500–512. [19] D.P. Cao, W.J. Luo, J.Y. Feng, X. Zhao, Z.S. Li, Z.G. Zou, Cathodic shift of onset potential for water oxidation on a Ti4 þ doped Fe2O3 photoanode by suppressing the back reaction, Energy Environ. Sci. 7 (2014) 752–759. [20] S.H. Shen, C.X. Kronawitter, D.A. Wheeler, P.H. Guo, S.A. Lindley, J.G. Jiang, J. Z. Zhang, L.J. Guo, S.S. Mao, Physical and photoelectrochemical characterization of Ti-doped hematite photoanodes prepared by solution growth, J. Mater. Chem. A 1 (2013) 14498–14506.
A. Annamalai et al. / Solar Energy Materials & Solar Cells 144 (2016) 247–255 [21] O. Zandi, B.M. Klahr, T.W. Hamann, Highly photoactive Ti-doped α-Fe2O3 thin film electrodes: resurrection of the dead layer, Energy Environ. Sci. 6 (2013) 634–642. [22] L.F. Xi, S.Y. Chiam, W.F. Mak, P.D. Tran, J. Barber, S.C.J. Loo, L.H. Wong, A novel strategy for surface treatment on hematite photoanode for efficient water oxidation, Chem. Sci. 4 (2013) 164–169. [23] J. Liu, C.H. Liang, G.P. Xu, Z.F. Tian, G.S. Shao, L.D. Zhang, Ge-doped hematite nanosheets with tunable doping level, structure and improved photoelectrochemical performance, Nano Energy 2 (2013) 328–336. [24] R.L. Spray, K.J. McDonald, K.S. Choi, Enhancing photoresponse of nanoparticulate α-Fe2O3 electrodes by surface composition tuning, J. Phys. Chem. C 115 (2011) 3497–3506. [25] G. Rahman, O.S. Joo, Electrodeposited nanostructured α-Fe2O3 thin films for solar water splitting: influence of Pt doping on photoelectrochemical performance, Mater. Chem. Phys. 141 (2013) 589. [26] W.W. Wang, J.L. Yao, Effect of metal ions (Sn and Zn) on the thermal property of akaganeite nanorods, Mater. Res. Bull. 47 (2012) 1762–1767. [27] A. Watanabe, H. Kozuka, Photoanodic properties of sol–gel-derived Fe2O3 thin films containing dispersed gold and silver particles, J. Phys. Chem. B 107 (2003) 12713–12720. [28] D.K. Zhong, M. Cornuz, K. Sivula, M. Graetzel, D.R. Gamelin, Photo-assisted electrodeposition of cobalt-phosphate (Co–Pi) catalyst on hematite photoanodes for solar water oxidation, Energy Environ. Sci. 4 (2011) 1759–1764. [29] X.F. Sun, C. Hu, X.X. Hu, J.H. Qu, M. Yang, Characterization and adsorption performance of Zr-doped akaganeite for efficient arsenic removal, J. Chem. Technol. Biotechnol 88 (2013) 629–635. [30] L.L. Cai, I.S. Cho, M. Logar, A. Mehta, J.J. He, C.H. Lee, P.M. Rao, Y.Z. Feng, J. Wilcox, F.B. Prinz, X.L. Zheng, Sol-flame synthesis of cobalt-doped TiO2 nanowires with enhanced electrocatalytic activity for oxygen evolution reaction, Phys. Chem. Chem. Phys. 16 (2014) 12299–12306. [31] A.L. Larralde, C.P. Ramos, B. Arcondo, A.E. Tufo, C. Saragovi, E.E. Sileo, Structural properties and hyperfine characterization of Sn-substituted goethites, Mater. Chem. Phys. 133 (2012) 735–740. [32] R.M. Cornell, U. Schwertmann, The Iron Oxides : Structure, Properties, Reactions, Occurrences, and Uses, Wiley-VCH, Weinheim, 2003 2nd, completely rev. and extended ed.. [33] A. Tufo, K. García, C. Barrero, E. Sileo, Structural and hyperfine properties of Mn and Co-incorporated akaganeites, Hyperfine Interact. 224 (2014) 239–250. [34] N.G. Holm, Substitution selectivity of some transition elements (Cr, Mn, Co, Ni, Cu, Zn) during formation of β-FeOOH, GFF Stockh. 107 (1985) 297–300. [35] X. Zong, S. Thaweesak, H.Y. Xu, Z. Xing, J. Zou, G.Q. Lu, L.Z. Wang, A scalable colloidal approach to prepare hematite films for efficient solar water splitting, Phys. Chem. Chem. Phys. 15 (2013) 12314–12321. [36] D. Wang, X.-T. Zhang, P.-P. Sun, S. Lu, L.-L. Wang, Y.-A. Wei, Y.-C. Liu, Enhanced photoelectrochemical water splitting on hematite thin film with layer-by-
[37]
[38]
[39]
[40] [41]
[42]
[43] [44]
[45]
[46]
[47]
[48]
[49]
[50]
255
layer deposited ultrathin TiO2 underlayer, Int. J. Hydrog. Energy 39 (2014) 16212–16219. L. Vayssieres, N. Beermann, S.E. Lindquist, A. Hagfeldt, Controlled aqueous chemical growth of oriented three-dimensional crystalline nanorod arrays: application to iron(III) oxides, Chem. Mater. 13 (2001) 233–235. A. Annamalai, P.S. Shinde, A. Subramanian, J.Y. Kim, J.H. Kim, S.H. Choi, J.S. Lee, J.S. Jang, Bifunctional TiO2 underlayer for α-Fe2O3 nanorod based photoelectrochemical cells: enhanced interface and Ti4 þ doping, J. Mater. Chem. A 3 (2015) 5007–5013. A. Annamalai, A. Subramanian, U. Kang, H. Park, S.H. Choi, J.S. Jang, Activation of hematite photoanodes for solar water splitting: effect of FTO deformation, J. Phys. Chem. C 119 (2015) 3810–3817. N. Iordanova, M. Dupuis, K.M. Rosso, Charge transport in metal oxides: a theoretical study of hematite α-Fe2O3, J. Chem. Phys. 122 (2005). A.M. Xavier, F.F. Ferreira, F.L. Souza, Morphological and structural evolution from akaganeite to hematite of nanorods monitored by ex situ synchrotron Xray powder diffraction, RSC Adv. 4 (2014) 17753–17759. I. Cesar, A. Kay, J.A.G. Martinez, M. Gratzel, Translucent thin film Fe2O3 photoanodes for efficient water splitting by sunlight: nanostructure-directing effect of Si-doping, J. Am. Chem. Soc. 128 (2006) 4582–4583. N.S. Mcintyre, D.G. Zetaruk, X-Ray photoelectron spectroscopic studies of ironoxides, Anal. Chem. 49 (1977) 1521–1529. J.Y. Kim, J.W. Jang, D.H. Youn, G. Magesh, J.S. Lee, A stable and efficient hematite photoanode in a neutral electrolyte for solar water splitting: towards stability engineering, Adv. Energy Mater. 4 (2014) 1–7 1400476. L. Wang, C.Y. Lee, A. Mazare, K. Lee, J. Muller, E. Spiecker, P. Schmuki, Enhancing the water splitting efficiency of Sn-doped hematite nanoflakes by flame annealing, Chem.-Eur. J. 20 (2014) 77–82. J. Brillet, M. Gratzel, K. Sivula, Decoupling feature size and functionality in solution-processed, porous hematite electrodes for solar water splitting, Nano Lett. 10 (2010) 4155–4160. F. Le Formal, M. Gratzel, K. Sivula, Controlling photoactivity in ultrathin hematite films for solar water-splitting, Adv. Funct. Mater. 20 (2010) 1099–1107. G.M. Wang, Y.C. Ling, D.A. Wheeler, K.E.N. George, K. Horsley, C. Heske, J. Z. Zhang, Y. Li, Facile synthesis of highly photoactive α-Fe2O3-based films for water oxidation, Nano Lett. 11 (2011) 3503–3509. T. Lopes, L. Andrade, H.A. Ribeiro, A. Mendes, Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy, Int. J. Hydrog. Energy 35 (2010) 11601–11608. K.E. Garcia, C.A. Barrero, A.L. Morales, J.M. Greneche, Characterization of akaganeite synthesized in presence of Al3 þ , Cr3 þ , and Cu2 þ ions and urea, Mater. Chem. Phys. 112 (2008) 120–126.