(Fe3+) Photoanodes for Surface Enhanced Photoelectrochemical

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Influence of Sb5+ as Double Donor on Hematite (Fe3+) Photoanodes for Surface Enhanced Photoelectrochemical Water Oxidation Alagappan Annamalai, Robin Sandström, Eduardo Gracia-Espino, Nicolas Boulanger, Jean-Francois Boily, Inge Muehlbacher, Andrey Shchukarev, and Thomas Wagberg ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02147 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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ACS Applied Materials & Interfaces

Influence of Sb5+ as Double Donor on Hematite (Fe3+) Photoanodes for Surface Enhanced Photoelectrochemical Water Oxidation Alagappan Annamalai 1, Robin Sandström 1, Eduardo Gracia-Espino 1, Nicolas Boulanger 1, Jean-François Boily 2, Inge Mühlbacher 3, Andrey Shchukarev 2 and Thomas Wågberg 1* 1 2 3

*

Department of Physics, Umeå University, SE-90187 Umeå, Sweden

Department of Chemistry, Umeå University, SE-90187 Umeå, Sweden

Polymer Competence Center Leoben GmbH PCCL, Leoben 8700, Austria

Corresponding author. E-mail address: [email protected]

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Abstract: In order to exploit the full potential of hematite (α–Fe2O3) as an efficient photoanode for water oxidation, the redox processes occurring at the Fe2O3/electrolyte interface needs to be studied in greater detail. Ex-situ doping is an excellent technique to introduce dopants onto the photoanode surface and to modify the photoanode/electrolyte interface. In this context, we selected antimony (Sb5+) as ex-situ dopant for its effective electron donor and reduced recombination effects and concurrently utilize the possibility to tune surface charge and wettability. Following the presence of Sb5+ states in Sb-doped Fe2O3 photoanodes, as confirmed by X-ray photoelectron spectroscopy, we observed a tenfold increase in carrier concentration (1.1 x 1020 cm–3 vs.1.3 x 1019 cm–3) and decreased photoanode/electrolyte charge transfer resistance (~990 Ω vs. ~3700 Ω). Furthermore, a broad range of surface characterization techniques such as FTIR, zeta potential and contact angle measurements reveal that changes in the surface hydroxyl groups following the exsitu doping also have an effect on the water splitting capability. Theoretical calculations suggest that Sb5+ can activate multiple Fe3+ ions simultaneously, in addition to increase the surface charge and enhances the electron/hole transport properties. To a greater extent, the surface doped Sb5+ determines the interfacial properties of electrochemical charge transfer leading to an efficient water oxidation mechanism.

KEYWORDS Hematite, Ex-situ doping, Fe2O3-Sb, Water Splitting, Sb5+, Fe3+, Surface Charge, Double Donors.

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1. Introduction Hematite (α–Fe2O3) is a widely studied metal oxide photoanode material for solar fuels, owing to its superior chemical stability in alkaline medium, abundance, and appropriate band gap (2.2 eV).1-2 However, α–Fe2O3 (hereafter Fe2O3 for convenience) photoanodes suffer from poor conductivity and low diffusion length or hole mobility (2–4nm) leading to a deficient photoactivity.3 Common strategies to improve its photoactivity include nanostructuring and elemental doping.4 These strategies have helped to overcome the aforementioned problems by enhancing the carrier collection and decreasing the carrier recombination at the photoanode/electrolyte interface

2, 5

, but there is still a need for a better

understanding of the doping process and its effect on the overall device performance. Elemental doping can be achieved using in-situ and ex-situ methods.5 Generally, in in-situ methods the dopants are widely distributed throughout the entire nanostructure, while ex-situ doping introduces dopants only at the surface of the host material without altering the crystal structure.5 Thus, the key challenge to achieve an adequate ex-situ doping is to prepare a uniform layer of dopant precursor over the as-synthesized β-FeOOH nanorods and subsequently achieve a homogeneous dopant incorporation. Ex-situ doping methods can be clubbed together with an activation step in which the Fe2O3 photoanodes are exposed to high temperature to accomplish an effective dopant diffusion, and thus ex-situ doping is considered a one-step method for introducing the dopants by simultaneously modifying the Fe2O3/electrolyte interface.6,7 Generally, tetravalent (4+) dopants occupy the substitutional Fe3+ sites in hematite reducing the neighboring iron atoms to Fe2+ to maintain charge neutrality.8,

9-10

These Fe2+

sites play a crucial role in the photoelectrochemical activity of Fe2O3 photoanodes because the main mechanism for electron transport involves electron hopping troughs these Fe2+ intermediates.11,12 In addition to the intentional ex-situ doping, Sn4+ can be introduced during 3



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the activation step when fluorine doped tin oxide (FTO) substrates are utilized, a common transparent conductive oxide routinely used during Fe2O3 growth13, resulting in additional injection of electrons to neighboring Fe3+ and ultimately enhancing the conductivity of Fe2O3 photoanodes. This type of co-doping has already shown an increment in conductivity of Fe2O3 photoanodes with a variety of cationic (n-type) tetravalent dopants (Sn4+ 14, Ti4+ 15, Si4+ 16

, Pt4+ 17, Zr4+

9,18

), and few pentavalent dopants (Ta5+ 9,19, P5+ 20, Nb5+

9,21,22

) where both

cases improve the overall solar water splitting performance. The main advantage of pentavalent dopants is their availability to donate two extra electrons when incorporated into Fe2O3. Of course, selecting the proper pentavalent dopant is crucial, and thus elements with similar Pauling electronegativity and ionic radius3 to Fe3+ are preferred in order to induce as low strain as possible in the lattice.23 In this aspect, the pentavalent antimony (Sb+5) ion is a promising candidate with required characteristics (ionic radii Sb5+ (0.62 nm) and Fe3+ (0.64 nm)24 and similar electronegativity25). Despite the above listed favorable attributes, Sb has not been properly explored as a dopant for Fe2O3 photoanodes for water splitting. Here, we report the synthesis of Sb-doped Fe2O3 photoanodes by an ex-situ process. An expected increase in carrier concentration and reduction in charge recombination are observed. Interestingly, the optimum ex-situ Sb-doping in Fe2O3 is manifested to be as low as 0.1 mM of precursor concentration, 50 times lower than previous studies.5-6 Such low amount of dopants increases the photoelectrochemical activity of Fe2O3 photoanodes by 70%, thanks to changes in the surface charge, surface concentration of hydroxyl groups and hydrophilicity of the Fe2O3 photoanodes. In addition, theoretical calculations suggest that superficial Sb5+ activate multiple Fe3+ simultaneously increasing the number of available active sites and enhancing the electron/hole transport.

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2. Results and Discussion 2.1. Sb-doped Fe2O3 photoanodes: Structural characteristics Sb5+ doped hematite nanorods were produced using a hydrothermal method based on previous reports13,17,26, where vertically aligned β–FeOOH nanorods with uniform lengths of ~350 nm and diameters of ~100 nm, in line with previous reports2,14, are first grown onto a FTO substrate and later an ex-situ Sb doping is performed as described in the experimental section. Figure 1 shows the morphology of Sb-doped Fe2O3 photoanodes, and a detailed structural analysis confirms that there are no significant changes when compared to pristine (non-doped) Fe2O3 nanorods, neither in morphology (Figure 1(a) and 1(b)) or crystal structure (Figure S1). Both pristine and Sb-doped Fe2O3 photoanodes display similar X-ray diffraction patterns with a predominant (110) peak (Figure S1).The absence of additional diffraction peaks or peak shifts in Sb-doped Fe2O3 suggest the presence of a thin and amorphous Sb overlayer.8 Overall the x-ray diffraction data suggest a successful incorporation of the Sb5+ dopant into Fe2O3 lattice, predominately at the outer layers, without destroying the 1-D configuration and the highly conductive (110) crystal orientation.2,27 To further investigate the morphological changes on the surface of Sb-doped Fe2O3 photoanodes, HRTEM bright-field images were obtained as shown in Figure 1(c) displaying a Fe2O3 rod with lattice spacing’s representing the (110) plane. Although the overall nanorod morphology of the Fe2O3 photoanodes were retained after ex-situ Sb-doping (see SEM images in Figure 1(a) and 1(b)), a disordered amorphous surface layer with thickness of approximately 5 nm is observed for Sb-doped Fe2O3 photoanodes, in agreement with the absence of additional crystalline observations in the XRD pattern. Similar amorphous overlayers were observed when other dopants such as Si, Ni and Ag were doped onto the Fe2O3 nanorod surface by the

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ex-situ doping method.28 The composition of such amorphous overlayers is difficult to characterize by XRD, FESEM or HRTEM; instead Raman and X-ray photoelectron spectroscopy (XPS) are used as shown in Figure S2 and Figure 2, respectively. The observed Raman peaks assigned to two A1g modes (226 cm−1 and 497 cm−1), and five Eg modes (245 cm−1, 292 cm−1, 299 cm−1, 410 cm−1 and 611 cm−1). are in a good agreement with literature data for a α–Fe2O329, while Raman peaks around 660 cm -1 in Sb-doped Fe2O3 photoanodes, not present in the pristine photoanodes are ascribed to the disorder phase induced by the dopant incorporation (Figure S2).30,31 XPS analysis demonstrates the chemical states and elemental composition of Sb5+ dopant into Fe2O3 nanostructures as shown in Figure 2. The XPS survey spectrum (Figure S3) reveals that the Sb-doped Fe2O3 photoanode surfaces are composed of Fe, Sn, O and Sb, where Sn originates from the FTO substrate during the activation process.13 Fe 2p spectra in Figure 2(b) shows two distinct peaks at 710.9 and 724.0 eV, corresponding to Fe 2p3/2 and Fe 2p1/2, respectively. The peak present at 719– 722 eV is a characteristic feature of Fe3+ ions in Fe2O3.32 The deconvoluted Sb 3d corelevel spectrum (Figure 2(a)) exhibits two peaks at 530.6 and 540.1 eV corresponding to Sb 3d5/2 and Sb 3d3/2, respectively, both associated with Sb5+.33 During ex-situ doping in Fe2O3 photoanodes, the ex-situ dopant usually diffuses into the Fe2O3 lattice and substitutes the surface Fe3+ sites28, and considering the similar ionic radius of Fe3+ (0.64 nm) and Sb5+ (0.62nm), the substitution of Fe3+ by Sb5+ is highly viable.34 It is noteworthy to mention that Sb5+ is more stable than Sb3+ when treated at high temperature under oxygen rich conditions35 (similar to our activation step) which favors the integration of Sb5+ rather than Sb3+. A peak at 530.9 eV in the O 1s core level spectra (Figure 2(a)) corresponds to O2− while the peak at 533.0 eV usually is attributed to surface adsorbed hydroxyl groups (–OH).36 The surface adsorbed hydroxyl groups are discussed in detail

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later in the manuscript. To conclude, both Raman and XPS firmly indicate successful incorporation of Sb5+ dopants at the surface of Fe2O3 photoanodes.

2.2. Electrochemical Properties of Sb-doped Fe2O3 photoanodes Figure 3 illustrates the current-potential (J–V) plots for the pristine and Sb-doped Fe2O3 photoanodes measured in a 1 M NaOH electrolyte (pH 13.8) under standard illumination with solar simulated light (100 mW/cm2). The photocurrent density for the pristine Fe2O3 photoanodes, 0.64 mA cm–2 at 1.23 VRHE (Figure 3(a)), is similar to previous reported values.10,37 For Sb-doped nanorods with an optimum Sb precursor (SbCl3) concentration of 0.1 mM in ethanol, the obtained Sb-doped Fe2O3 photoanodes exhibit a dramatic increase in photocurrent, with the highest photocurrent density reaching 1.1 mA cm-2 at 1.23 VRHE for 0.1mM of Sb precursor (Figure S4), which is 74% higher than that of pristine Fe2O3 photoanodes. Another important parameter derived from the (J–V) plots is the onset potential (VON).38,39 We obtained a VON value of 0.61 VRHE and 0.75 VRHE for the pristine and Sb-doped Fe2O3 photoanodes, respectively (Figure S5). Similarly from the Butler plots we obtained a VFB value of 0.74 VRHE and 0.87 VRHE for the pristine and Sbdoped Fe2O3 photoanodes, respectively (Figure S6). The undesired anodic shift has been previously observed5,8,40 on similar systems, probably caused by an increased number of surface trapping states originating from the introduction of dopants.39,41 Electrochemically critical parameters such as charge transport properties and carrier concentration responsible for the enhanced PEC properties was deduced by Nyquist42 and Mott-Schottky plots43 as shown in Figure 3(c) and 3(d). Electrochemical impedance spectroscopy (EIS) is employed to study the charge transport kinetics of pristine and Sb-

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doped Fe2O3 photoanodes. All the measured EIS spectra were fitted using Nova (2.0.2) software with the equivalent circuit as shown in Figure S7. The results are shown in Figure 3(c) and summarized in Table S1. According to the EIS measurements, we found that the Sbdoped Fe2O3 photoanodes exhibit a reduced charge transfer resistance at the bulk (RCT1 = 630 Ω) and at the photoanode/electrolyte interface (RCT2 = 900 Ω) when compared to pristine Fe2O3 (RCT1 = 1000 Ω, RCT2 = 3700 Ω). Additionally, other intrinsic properties were deduced by performing Mott-Schottky measurements in dark conditions to determine the carrier concentration and conductivity (Figure 3(d)). The estimated charge carrier concentration (ND) calculated from the slopes of Mott-Schottky plots for pristine and Sb-doped Fe2O3 photoanodes are 1.3x1019 and 1.1 x1020 cm–3, respectively. The one order of magnitude increase suggests that the Sb5+ act as an excellent n-type dopant for Fe2O3 photoanodes. The overall effect from the Sb-doping clear; in comparison to pristine Fe2O3 photoanodes, Sb doping improves the PEC performance significantly, mainly (but not entirely, as discussed below) attributed to an increased donor density and reduced recombination (Figure S8) the charge transport resistance at the photoanode/electrolyte interface.2 2.3. Surface characteristics: Zeta Potential & Contact Angle The increase in the conductivity and reduced recombination with the Sb5+ doping is not likely to be the sole source for the dramatic 74% increase in the photocurrent. Usually for ex-situ doping the concentration used for preparing the dopant precursor solution is a few millimolar (mM),44 whereas in most cases around 5–10 mM is used.5 However, in our specific Sb-doped Fe2O3 photoanodes we used only 0.1 mM, which is 50 times lower than the typical optimum reported dopant concentration. Since the ex-situ dopant treatment affects the surface of the Fe2O3 photoanodes drastically, we put extra emphasis on potentially important surface properties that might affect the overall water splitting efficiency. Wettability is one such widely studied surface parameter and is directly related to surface adsorbed hydroxyl 8


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groups on metal oxides.45,46 Sample wettability’s were investigated from contact angle measurements. Figure 4 shows the changes in the contact angle of DI-water droplets on pristine and Sb-doped Fe2O3 photoanodes. The pristine Fe2O3 photoanodes exhibit a hydrophilic nature with a contact angle of ~ 26°, however, after Sb-doping it showed superhydrophilic surface properties with a contact angle of ~ 0° (see Supporting info Mov file). In general, the hydroxyl group at the surface of metal oxide photoanodes governs the charge transfer across the photoanode/electrolyte interface to a greater extent, as these hydroxyl groups directly participate in the oxygen evolution reaction (OER). Therefore, knowing the concentration of such functional groups could provide insight into the enhanced PEC performance observed on Sb-doped photoanodes. Generally metal oxides like TiO2, SnO2, SiO2 and Fe2O3 have diverse types of surface adsorbed –OH– groups46 located on both metal ions and adjacent oxygen atoms, as depicted in the schematic diagram Figure S9.47 The –OH groups attached to the metal site are termed as terminal or singly bonded –OH groups, and the ones on the oxygen site are termed as bridging or doubly bonded –OH groups since oxygen is attached to two metal atoms.23,47 Changes in terminal and bridging –OH groups can be confirmed by XPS measurements. The O1s spectra of pristine and Sb-doped Fe2O3 photoanodes are shown in Figure 5 (a) and (b). The O 1s peaks of the pristine Fe2O3 photoanodes can be deconvoluted with three peaks at 530.2, 531.3 and 532.6 eV. The peak at 530.2 eV corresponds to the metal-oxygen bonds in the metal oxide framework, while the 531.3 and 532.6 eV peaks correspond to the superficial bridging and terminal –OH groups, respectively.22 Upon introducing Sb5+ dopant, two extra peaks arise at 530.6 and 540.1 eV corresponding to Sb 3d5/2 and Sb 3d3/2 of Sb5+ as explained earlier. However, no considerable changes were observed in the bridging and terminal –OH groups for the Fe2O3 photoanodes with and without Sb-dopant. To further confirm that the –OH groups are not responsible for the change in contact angle, we probed the populations of these groups by FTIR spectroscopy

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(Figure 5c). Identification of these groups was facilitated by previous FTIR band assignments made on synthetic α-Fe2O3 nanoparticles48, for which the main crystallographic faces and hydroxo group populations were known. Based on this recent work, we readily identified singly- (−OH) and doubly- (µ−OH) coordinated hydroxo groups on both pristine and Sbdoped photoanodes. Because no significant differences in the densities of these groups could be detected from our measurements, we conclude that the contrasting contact angle values cannot be explained by differences in populations of –OH groups. The surface charge of pristine and Sb-doped Fe2O3 was further investigated by zeta potential measurements, where the isoelectric point (IEP) of Sb-doped Fe2O3 nanorods (3.9) was slightly reduced in comparison with the pristine counterpart, (4.4) as shown in Figure 4 (a).49 This observation suggests that the surface of the Sb-doped Fe2O3 photoanodes carries slightly more negative charges compared to the pristine photoanodes.20,50 The observed superhydrophilic nature and the negative shift in IEP upon introduction of ex-situ Sb5+ dopants could contribute to an extent to the drastic improvement in the photocurrent by enhancing the charge separation of photoexcited electron-hole pairs, where the holes are effectively transported to the surface and electrons towards the bulk. 2.4. Sb5+ as Double Donor: Density Functional Theory From the above experimental results, we have verified that the surface Sb-doping is almost entirely responsible for the dramatic increase in the photoactivity or OER activity of Sb-doped Fe2O3 photoanodes. However, the full reason why such a low amount of dopant (50 times lower than usual) concentration induces such a huge increase in photocurrent is still unclear. Our hypothesis is that the newly incorporated Sb5+ acts as a double donor, activating more Fe2+ than any other 4+ n-type dopants. The presence of Fe2+ is associated to an enhanced electron/hole transport manifested as improved photocatalytic

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activity.11,12 In addition, the presence of Sb at the surface enhanced the chemical reactivity of the Fe atoms, thus increasing the number of active sites. Density functional theory (DFT) calculations were performed to investigate the proposed hypothesis. The spin-polarized DFT computations were carried out within the DFT+U formalism51 with an effective Hubbard correction term of 4.5 eV. A (110) hydroxylterminated hematite surface was selected as a surface model, where a single superficial iron atom was replaced by Sb, as expected for an ex-situ doping procedure, while a Sn atom was located underneath to emulate the unintentional doping from the FTO substrate. A total of four systems were investigated, pristine, Sb-, Sn- and Sb/Sn-doped (110)-Fe2O3 surfaces. After geometrical optimization, the electron distribution of the Sb/Sn-doped Fe2O3, and the results indicate that the two extra e- from Sb are shared with up to 4 neighboring Fe atoms located at the surface. Two of the nearby Fe atoms exhibit an additional charge of 0.6e-, while the other two 0.3e-, summing up 1.8e- out of the 2 available from Sb, see Figure S10 and Table S2. On the other hand, the contribution of the Sn dopant in the co-doped Sb/Sn system is only limited to a single adjacent iron atom donating 0.3e-, indicating that only one atom is being activated by Sn. These results suggest that up to five Fe atoms could be activated when Sb and Sn are present, where four are located at the surface acting as catalytic active sites6 and as plausible sites for electron hopping, while the fifth Fe atom is located underneath near the Sn dopant also contributing to the overall conductivity as described in Figure 6.11,12 In addition, the excess of charge agrees with the IEP measurements, where a more negative surface charge is observed for Sb-doped photoanodes. Similar electron redistribution was observed when only a single dopant is included, where Sb donates up to 1.5e- with four Fe atoms, while Sn only donates 0.3e- to a single adjacent Fe atom, which is in agreement with the observed trend in the PEC performance, being the Sb/Sn Fe2O3 photoanodes the most active. Note that the

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amount of charge transferred to nearby Fe atoms strongly depends on the dopant position in the crystal lattice, see Figure S10, S11 and Table S2, however the observed trend still holds.

3. Conclusion Fe2O3 photoanodes have been doped with an ultra-low optimum concentration of 0.1 mM of Sb5+ dopant resulting in a dramatic 70% increase in the overall photoelectrochemical properties. The morphology, crystallinity, and superficial hydroxyl group of the Fe2O3 photoanodes remain merely unchanged after Sb doping. Therefore, apart from the charge transport kinetics and carrier concentration, the improved photoactivity is attributed to the enhanced wettability, larger negative surface charge and the increase in catalytic active sites after the induction of Sb. This new perspective will have a major effect on all types of surface treatment carried out and the suitable choice of double donor dopants for both the electrocatalysts and photocatalysts.

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Supporting Information Information on experimental, computational details, XRD, Raman spectra, XPS, PEC optimization, estimation of VON & VFB values, equivalent circuit for EIS fitting, Bode phase diagram, and DFT calculations for both pristine and Sb-doped Fe2O3 photoanodes. This information is available free of charge via the Internet at http://pubs.acs.org The authors declare no competing financial interest.

Acknowledgment This work was supported by the Artificial Leaf Project Umeå (K&A Wallenberg foundation). T. W. acknowledges support from Vetenskapsrådet (2017-04862) and Energimyndigheten (45419-1). EGE thanks the Carl Tryggers foundation (CTS-16-161) for the financial support. We also acknowledge the facilities and technical assistance of the Umeå Core Facility Electron Microscopy (UCEM), and the Core facility for Vibrational Spectroscopy (VISP) at the Chemical Biological Centre (KBC). The theoretical simulations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the High Performance Computing Center North (HPC2N). The zeta potential measurements were performed at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET-program of the Federal Ministry for Transport, Innovation and Technology and the Federal Ministry of Science. The PCCL is funded by the Austrian Government and the State Governments of Styria, Lower Austria and Upper Austria.

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Figures and Tables

Figure 1. FESEM images of (a) pristine and (b) Sb-doped Fe2O3 photoanodes sintered at 800 ° C for 10 min. (c) HRTEM of ex-situ Sb-doped Fe2O3 photoanode identified as αFe2O3 by lattice fringes ((110) plane) and associated FFT (inset).

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Figure 2. XPS spectra recorded Sb-doped photoanodes on FTO substrates sintered at 800 °C for 10 min: (a) Fe 2p and (b) O 1s.

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Figure 3. (a) Photocurrent-potential (J-V) curves and (b) Photo-conversion efficiencies as a function of applied potential for pristine and Sb-doped Fe2O3 photoanodes. (c) Nyquist plots of pristine and Sb-doped Fe2O3 photoanodes measured at 1.23 VRHE under 1 sun illumination condition with the equivalent circuit used for fitting the experimental data in the inset and (d) Mott-Schottky plot of the pristine and Sb-doped Fe2O3 photoanodes measured under dark conditions.

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Figure 4. (a) Isoelectric point (IEP) for the pristine and Sb-doped Fe2O3 photoanodes determined using Zeta potential measurements. The values were pH 4.4 for the pristine and pH 4.0 for Sb-doped Fe2O3 photoanodes. Contact angle measurements on (b) pristine and (c) Sb-doped Fe2O3 photoanodes.

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Figure 5. O1s XPS spectra for (a) pristine and (b) Sb-doped Fe2O3 photoanodes. (c) FTIR spectra of pristine and Sb-doped Fe2O3 photoanodes, showing comparable populations of surface functional groups (-OH, m-OH). The broad O-H stretching region centered at ~3300 suggest that Fe2O3 contains non-stoichiometric hydroxo and/or aquo groups, and is therefore hydrohematite. The C-H stretching region points to comparable levels of organic impurities in both samples.

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Figure 6. Schematic representation of electron hopping mechanism with pristine and Sbdoped Fe2O3 photoanodes.

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