A Facile Electrochemical Reduction Method for Improving

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A Facile Electrochemical Reduction Method for Improving Photocatalytic Performance of #-Fe2O3 Photoanode for Solar Water Splitting Jue Wang, Joseph L. Waters, Patrick Kung, Seongsin Margaret Kim, John T Kelly, Louis Edward McNamara, Nathan I Hammer, Barry C Pemberton, Russell H. Schmehl, Arunava Gupta, and Shanlin Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11057 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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ACS Applied Materials & Interfaces

A Facile Electrochemical Reduction Method for Improving Photocatalytic Performance of α-Fe2O3 Photoanode for Solar Water Splitting Jue Wang,1,4 Joseph L. Waters,2,4 Patrick Kung,2,4 Seongsin M. Kim,2,4 John T. Kelly,5 Louis E. McNamara,5 Nathan I. Hammer,5 Barry C. Pemberton,6 Russell H. Schmehl,6 Arunava Gupta,1,3,4 and Shanlin Pan1,4* 1. Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487 2. Department of Electrical and Computer Engineering, The University of Alabama, Tuscaloosa, Alabama 35487 3. Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, AL, USA, 35487-0203 4. Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL, USA, 35487-0209 5. Department of Chemistry and Biochemistry, University of Mississippi, Oxford, Mississippi 38655 6. Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States *Corresponding Author: [email protected]; Tel: 1-205-348-6381

Abstract. Electrochemical reduction method is used for the first time to significantly improve the photoelectrochemical performance of α-Fe2O3 photoanode prepared on FTO substrates by spin coating aqueous solution of Fe(NO3)3 followed by thermal annealing in air. Photocurrent density of α-Fe2O3 thin film photoanode can be enhanced 25 times by partially reducing the oxide film to form more conductive Fe3O4 (magnetite). Fe3O4 helps facilitate efficient charge transport and collection from the top α-Fe2O3 layer upon light absorption and charge separation to yield enhanced photocurrent density. The optimal enhancement can be obtained for < 50 nm films because of the short charge transport distance for the α-Fe2O3 layer. Thick α-Fe2O3 films 1 ACS Paragon Plus Environment


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require more charge and overpotential than thinner films to achieve limited enhancement because of the incomplete reduction of the resistive oxide film and sluggish charge transport over a longer distance to oxidize water. Electrochemical reduction of α-Fe2O3 in unbuffered pH neutral solution yield much higher but unstable photocurrent enhancement because of the increase in local pH value accompanied with proton reduction at a hematite surface. Keywords: hematite, magnetite, water splitting, solar energy, electrochemical reduction,

INTRODUCTION To meet the increasing energy demand while minimizing the detrimental environmental impacts of CO2 generated from the fossil fuels, it is vital to explore and develop alternative and clean energy sources. Solar energy holds the promise for addressing this global challenge by supplying adequate clean renewable energy on the condition that it can be efficiently harvested and converted to clean fuels or electricity. Photoelectrochemical (PEC) reactions can convert light energy to chemical energy at the surface of a semiconductor electrode material to produce hydrogen from water. For example, PEC water splitting at the surface of TiO2 can be obtained under sunlight although the power efficiency is limited by the poor visible light absorption.1 A vigorous search has been ongoing to identify semiconductor materials that can meet the critical requirements for practical applications of PEC water splitting.2,3,4 These primary requirements are low cost, durability in aqueous environment, environmental-friendliness, sufficient narrow band gaps for efficient solar absorption, suitable conduction and valence band edges that straddle the water reduction and oxidization potentials, and fast kinetics for splitting water molecules with photo-generated carriers.5 No single material has been able to meet all these requirements to date. Earth abundant hematite α-Fe2O3 has particularly shown interesting photoelectrochemical 2 ACS Paragon Plus Environment

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properties for water splitting because of its low cost, non-toxicity, stability in aqueous solutions, and a band gap of around 2.0 eV suitable for absorbing visible light.6 But the performance of αFe2O3 is largely hindered by a small hole diffusion length of 20 nm,7 which is principally related to the low electrical conductivity of α-Fe2O3 (~10-14 Ω-1cm-1).8 This issue can be partially addressed by increasing the carrier concentration through chemical doping. For example, doping α-Fe2O3 with Pt, Sn, and Ti, acting either as electron donors or electron acceptors, has been investigated to increase the carrier density of α-Fe2O3.9,10,11 Despite these efforts, the efficiency enhancement is still very limited, far from the theoretical water splitting efficiency of 16.8% predicted for α-Fe2O3.12 Recently, flame reduction method,13 acid treatment,14 and plasma treatment15 have been applied to enhance the conductivity of α-Fe2O3 while facilitating charge separation toward efficient PEC water splitting. Meanwhile, three-dimensional (3D) graphene inverse opal conducting substrates16 and 3D Fluorine doped Tin Oxide (FTO) substrates17 have been fabricated for α-Fe2O3 to provide efficient electron transfer pathways and thereby reduce the electron-hole recombination. In addition, TiO2 has been used to modify the α-Fe2O3 layer to accelerate the charge transfer across the α-Fe2O3/electrolyte interface by reducing the charge recombination at α-Fe2O3/FTO interface18,19 and nanocomposites have been developed to improve the PEC performance of α-Fe2O3.20,21 Besides the above mentioned methods to increase the electrical conductivity of α-Fe2O3 and assist its charge separation, one noteworthy fact is that the electrical conductivity of another well-known form of iron oxide Fe3O4 is ~104 Ω-1cm-1, which is 18 orders of magnitude higher than that of α-Fe2O3,22 should also be able to considerably enhance the electrical conductivity of α-Fe2O3. Although Fe3O4 is not photoactive, the small loss of photoactive α-Fe2O3 can be easily 3 ACS Paragon Plus Environment


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compensated for with the significantly enhanced conductivity. Improved transport of photogenerated electrons are expected with such improved conductivity while enhancing the charge separation involved in water splitting. In order to incorporate Fe3O4 to hematite film, one can simply electrochemically reduce a thin film of α-Fe2O3, which is normally obtained at a high annealing temperature of at least 550 °C for optimal photocatalytic activity.23 It has been reported that α-Fe2O3 can be electrochemically reduced to Fe through applying a reduction potential in aqueous solution with the intermediate formation of Fe3O4.24,25,26 It should be noted that such electrochemical reduction method has been introduced lately to enhance the PEC performance of WO3, TiO2 (rutile and anatase), BiVO4, and ZnO for water oxidation by creating oxygen vacancies.27 Here, we report a simple electrochemical way to significantly raise the conductivity of α-Fe2O3 without introducing doping ions. A facile room-temperature electrochemical reduction method is used to improve the PEC performance of α-Fe2O3 by electrochemically reducing αFe2O3 to Fe3O4 at α-Fe2O3/FTO interface and generating a hydroxide ion modified α-Fe2O3 surface. The enhancement mechanism of the PEC performance of α-Fe2O3 is investigated.

RESULTS AND DISCUSSIONS Figure 1 shows the cyclic voltammetry (CV) of a 28 nm α-Fe2O3 film in 0.1 M NaNO3. An evident reduction peak appears at -0.8 V is corresponding to the reduction of α-Fe2O3 through a solid-state transformation.26 The only minor oxidation peak around -0.68 V indicates an irreversible reduction of the α-Fe2O3 film. The control sample of bare FTO without α-Fe2O3 thin film only shows a reduction current starting around -0.6 V, corresponding to the reduction of water, and no other redox peaks were observed. It should be noted that a thin layer of TiO2 was 4 ACS Paragon Plus Environment

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applied on FTO substrates to decrease the roughness of the substrate while increasing the adhesion of α-Fe2O3. There is no significant change in the CV profile with the TiO2 interlayer modification. CV of the FTO substrate in 0.01 M Fe(NO3)3 also displays a reduction peak around -0.9 V. The different peak position for the reduction of α-Fe2O3 is attributed to the overpotential variance on different substrates. The reduction product of α-Fe2O3 is confirmed to be Fe3O4 as to be discussed later. To study the effect of reduction potential on the PEC performance of α-Fe2O3 films, photocurrent density-potential curves of α-Fe2O3 films with various thicknesses (10 nm, 28 nm, 62 nm, and 147 nm) were recorded each time after applying -0.2 V, -0.5 V, -0.75 V, -1.0 V, and 1.5 V on the same sample for 3 s in 0.1 M NaNO3. The results of 10 nm and 147 nm α-Fe2O3 films are displayed in Figure 2, with the outcomes of 28 nm and 62 nm α-Fe2O3 films being presented in Figure S1. The absorbance spectra of α-Fe2O3 thin films shown in the supporting information Figure S2 are used to estimate the film thickness. The film thicknesses are calculated by using the absorption coefficient of hematite at 500 nm obtained from a relative thick film whose thickness is measured under Scanning Electron Microscope (SEM).28 The light was chopped during the measurement with a frequency around 1 Hz. Hence the height of steps in Figure 2 indexes the magnitude of the photocurrent density generated by α-Fe2O3 films. Before the electrochemical reduction, the photocurrent density from the α-Fe2O3 film is small, which is reasonable considering the measurements are made in a neutral solution which is not favorable for the oxidation of water. After applying the reduction potential, enhancement of the photocurrent density of α-Fe2O3 film can be obtained depending on the thicknesses of α-Fe2O3 films and the potentials applied. The magnitude of the photocurrent density obtained around + 0.75 V is listed in Table 1. To better compare the effect of the reduction potential on the PEC 5 ACS Paragon Plus Environment


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performance of α-Fe2O3 films, the enhancement factor of the photocurrent density of α-Fe2O3 films around + 0.75 V after applying various reduction potentials is plotted in Figure 3. The enhancement factor is defined as the photocurrent density of α-Fe2O3 films after applying reduction potentials divided by the photocurrent density of these films before applying the reduction potentials. Figure 3 shows little photocurrent density enhancement when - 0.2 V is applied for 3 s for all the samples. After reduction at - 0.5 V, the photocurrent density of the 10 nm α-Fe2O3 film increases slightly while the thicker films are not affected. Photocurrent density of all α-Fe2O3 films starts to increase when the film is reduced at a more negative potential of 0.75 V. Photocurrent density of α-Fe2O3 films thinner than 60 nm is dramatically enhanced after treatment with a reduction potential of -1.0 V and - 1.5 V. Approximately 25 and 10 times enhancement for 10 nm and 28 nm films are obtained after applying - 1.5 V for 3 s, respectively. Overall, the thinner the α-Fe2O3 film is, the larger the photocurrent density enhancement is. The results are consistent with CV in Figure 1, which illustrates that a potential more negative than the onset potential of reducing α-Fe2O3 to Fe3O4 is required to generate an enhancement. The effect of the reduction time on the PEC performance of α-Fe2O3 films is displayed in Figure S3. The film was reduced at -1.5 V for 1 s, 3 s, 5 s, 10 s, 20 s, 30 s, 60 s, 150 s, 300 s, and 600 s on 28 nm and 147 nm α-Fe2O3 films in 0.1 M NaNO3. The photocurrent enhancement factors obtained from Figure S3 are plotted in Figure 4. It is clear that the 28 nm α-Fe2O3 film is much more sensitive to the reduction time than the 147 nm film. The photocurrent density keeps increasing until the electrochemical reduction time reaches 20 s. Further reduction yields decrease in photocurrent because hematite is fully converted to Fe3O4. The photocurrent density further drops to nearly zero when the photoactive α-Fe2O3 is entirely converted to nonphotoactive Fe3O4 and then to Fe. A similar trend is also observed for the 147 nm film, but it 6 ACS Paragon Plus Environment

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took a longer time of around 60 s to reach a maximum enhancement to obtain sufficient reduction transformation of α-Fe2O3 to Fe3O4. To understand the enhanced PEC performance of α-Fe2O3 after the electrochemical reduction, the morphologies of a 28 nm α-Fe2O3 film before and right after applying -1.5 V for 10 s in 0.1 M NaNO3 were characterized by SEM imaging in Figure 5. No detectable morphology change was observed, indicating the absence of surface reconstruction that could have affected the PEC performance of α-Fe2O3 films. Meanwhile, Transmission Electron Microscope (TEM) specimens were prepared using a dual beam focused ion beam system following a standard lift out procedure that included the deposition of a Pt protective layer, thinning down to 100 nm and specimen transfer to a five-post Cu lift-out grid. Scanning Transmission Electron Microscope (STEM) and High-resolution Transmission Electron Microscopy (HRTEM) images of the cross section of a 28 nm α-Fe2O3 film after applying -1.5 V for 20 s are displayed in Figure 6A and 6B respectively. In the STEM image of Figure 6A, a layer around 30 nm thick is found between the Pt protection layer and the FTO substrate, which agrees with the value estimated from the absorbance coefficient. For the HRTEM imaging, we expect to see lattices from α-Fe2O3 while none from Fe3O4 since Fe3O4 is obtained under room temperature while α-Fe2O3 undergoes an annealing treatment at 600 oC. According to the HRTEM image of Figure 6B, neither the lattice spacing corresponding to α-Fe2O3 or Fe3O4 is observed in the iron oxide layer after the electrochemical reduction. Only lattices from the Pt protection layer are found as displayed in the inset of Figure 6B. This suggests that the iron oxide has a poor crystallinity after the electrochemical reduction. However, according to the report of Allanore and coworkers,25 porous Fe3O4 was observed between α-Fe2O3 and substrates after αFe2O3 was electrochemically reduced in a much thicker α-Fe2O3 area. Meanwhile, in a similar 7 ACS Paragon Plus Environment


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situation of electrochemically reducing relatively thick Fe3O4 films to Fe films, it was found that the reduction of Fe3O4 started from the Fe3O4/substrate interface. Therefore, we believe our αFe2O3 film here is too thin for us to distinguish Fe3O4 from α-Fe2O3. Mott-Schottky plots of a 28 nm α-Fe2O3 film before and after applying -1.5 V for 20 s and 600 s are displayed in Figure 7. These measurements were carried out in 0.1 M NaNO3 at 100 Hz in the dark. Based on Figure 7, the flat band potential of the α-Fe2O3 electrode does not change much. The carrier density, which is obtained from the slope of the plot,29 increases from 0.1541 × 1022 cm-3 to 1.003 × 1022 cm-3 after reducing the film at -1.5 V for 20 s. When applying the reduction potential for 600 s, the carrier density further increases to 1.483 × 1022 cm-3. These results indicate a 6.5 times conductivity enhancement of α-Fe2O3 photoanode after 20 s reduction, partially explaining the improved PEC performance. It also confirms that our hypothesis of improving the electrical conductivity of α-Fe2O3 through the electrochemical reduction method is viable. In addition, transient dynamics of electron hole recombination for a 62 nm α-Fe2O3 film and a partially reduced 62 nm α-Fe2O3 film which were held at -1.5 V for 20 s in 0.1 M NaNO3 were investigated in the absence of a solvent or applied bias. The results are displayed in Figure S4 and Figure S5, respectively. The spectra in Figure S4 resemble those of α-Fe2O3 photoanodes at a variety of bias potentials between 0.8 and 1.6 V (vs. NHE) as reported by Ruoko and coworkers.30 The spectra have a distinct absorption maximum around 575 nm with a second, weaker absorption to the blue, approximately 520 nm. This group attributes decay on this time scale to recombination of Fermi level electrons with valence band holes of the α-Fe2O3. Durant and coworkers have showed earlier that transient absorption spectral changes for α-Fe2O3 at a variety of applied potentials correlate well with current decays associated with electron-hole 8 ACS Paragon Plus Environment

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recombination.31 In addition, fs to µs transient absorption measurements by Lian indicate that the fraction of transients (electron/hole pairs) remaining after 5 ns is no more than 10% of the total, with nearly complete decay of the transient species by 6 µs.32 Evaluation of the kinetics of the decays was carried out using Kohlrausch-Williams-Watts stretched exponential fits, yielding weighted lifetimes of 1 ± 0.2 µs for α-Fe2O3 film and approximately half that for the partially reduced α-Fe2O3 film. While definitive evidence is lacking at this juncture, one potential explanation for the faster decay in the partially reduced α-Fe2O3 film is that holes in the α-Fe2O3 film are injected into the reduced regions of the film because of the improved film conductivity. This is currently being investigating in greater detail. For exploring the photo-induced charge transfer processes in α-Fe2O3 photoanode before and after the electrochemical reduction, we performed Electrochemical Impedance Spectroscopy (EIS) at +0.7 V in 0.1 M NaNO3 before and after applying -1.5 V on a 28 nm α-Fe2O3 film for 20 s and 600 s. The results are displayed by Nyquist plots (Figure 8). The fitting results using the equivalent circuit shown in inset figure of Figure 8 are listed in Table 2. After the electrochemical reduction, there is little difference in the solution resistance RΩ because there is no significant change either in the bulk electrolyte or the distance between the working electrode and reference electrode. Cd increases by a factor of 3, which is attributed to the generation of OH- ions near the α-Fe2O3 surface during the reduction of α-Fe2O3. The charge transfer resistance Rct represents the charge transfer kinetic processes in the α-Fe2O3 photoanode. When the α-Fe2O3 photoanode is illuminated with an AM 1.5 light source, the Rct becomes much smaller than the Rct in the dark because α-Fe2O3 is photoactive. After applying -1.5 V for 20 s, Rct decreases to 14.7 % of the sample prior to reduction treatment. This indicates more conductive hematite film was obtained after the reduction reactions and electrons are more 9 ACS Paragon Plus Environment


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readily transported to the current collector for improved PEC performance. When the reduction time extends to 600 s, Rct drops to 2.1% of the Rct before the reduction, indicating an even more favorable charge transfer process after the electrochemical reduction. During the reduction of α-Fe2O3 accompanied with proton reduction, hydroxide ions are generated at the surface of α-Fe2O3 simultaneously24 to positively contribute to the PEC enhancement because of the decreased thermodynamic potential at a higher local pH near the photoelectrode. To separate the enhancement from OH- and the improved conductivity of αFe2O3, a 10 nm α-Fe2O3 film was subjected to -0.2 V, -0.5 V, -0.75 V, -1.0 V, and -1.5 V for 3 s in 0.1 M NaNO3, which is the same treatment as illustrated in Figure 2A. The only difference is that after applying -1.5 V for 3 s, the α-Fe2O3 film was rinsed with fresh 0.1 M NaNO3 three times and then the electrolyte in the electrochemical cell was replaced with fresh 0.1 M NaNO3. As illustrated in Figure 9A, the photocurrent enhancement factor with fresh electrolyte is around 3.5 compared to the sample before being reduced, which is less than the factor of 25 obtained in Figure 2A when the electrolyte was not replaced with a fresh one. This suggests that the OHgenerated at the surface of α-Fe2O3 film significantly enhances the PEC performance of α-Fe2O3. The production of OH- is confirmed by the X-ray Photoelectron Spectroscopy (XPS) spectra of a 147 nm α-Fe2O3 film before and after being reduced at -1.5 V for 600 s in 0.1 M NaOH (Figure S2). In Figure S2, O1s band from O-Fe in α-Fe2O3 centers at 529.4 eV before the reduction. It shifts to higher binding energy at 530.6 eV after the reduction because of the formation of OFe(OH) which centers at 531.2 eV.33 It should be noted that O1s band from O-Fe for a 28 nm αFe2O3 film after being reduced at -1.5 V for 10 s in 0.1 M NaNO3 did not shift at all, suggesting that hydroxide ions generated from the electrochemical production do not bond tightly to the αFe2O3 surface. It indicates that a long reduction time is needed to produce a large amount of 10 ACS Paragon Plus Environment

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hydroxide ions and then some of them will bond to the α-Fe2O3 surface. To minimize the effect of OH-, -1.5 V was applied on a 28 nm α-Fe2O3 film for various amounts of time in phosphate buffered solution (PBS) with a pH of 7.4, which is close to pH of NaNO3 solution. The photocurrent density is presented in Figure 9B. The maximum enhancement factor of 4.2 is obtained after 1 s reduction. In the PBS solution, the effect of OH- is minimized because OHgenerated during the reduction will be balanced by the buffer solution. This idea is further confirmed in measurements in 0.1 M NaOH electrolyte (Figure 9C) in which the amount of generated OH- is negligible compared to OH- in the bulk solution. To further study the stability α-Fe2O3 film after being electrochemically reduced and exclude the effect of OH-, a 10 nm αFe2O3 film was held at +0.75 V and its photocurrent was recorded density over time (Figure 9D) after applying -0.2 V, -0.5 V, -0.75 V, -1.0 V, and -1.5 V on this film for 3 s in 0.1 M NaNO3. The sequence of applying the potential is the same as Figure 2A. After holding the potential at +0.75 V, the photocurrent density decreases quickly in the first 100 s because of immediate oxidation of surface OH- groups. After 500 s, the photocurrent density is stable and still twice that of the same α-Fe2O3 film before applying the reduction potential. Therefore, Fe3O4 produced by the electrochemical reduction method does improve the PEC performance of α-Fe2O3 besides the benefits from the OH- generated in the reduction process. In order to explore the reduction product of α-Fe2O3 and understand where the reduction of α-Fe2O3 begins, Raman spectra of 28 nm, 62 nm, and 147 nm thick α-Fe2O3 films after being reduced at -1.5 V for various amounts of time in 0.1 M NaOH were recorded. Raman spectra characterizing the overall composition of the samples are displayed in Figure 10. The CV of hematite film in 0.1 M NaOH (Figure S7) has a similar profile as the one in 0.1 M NaNO3, indicating similar chemical reactions in these two electrolytes. The color change of a 28 nm α11 ACS Paragon Plus Environment


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Fe2O3 film from brown to light grey and a 147 nm α-Fe2O3 film from red-brown to dark-grey was observed as shown in Figure S8. A similar color change is obtained for 28 nm film after reducing for 600 s in 0.1 M NaNO3, but it took much longer to completely reduce a 147 nm film in 0.1 M NaNO3 to observe a significant color change. The assignments of the Raman peaks are listed in Table 3.34 Before applying a reduction potential, Raman peaks from α-Fe2O3 are conspicuous for all samples, showing two A1g modes (229 and 498 cm-1) and five Eg modes (248, 295, 301, 415, and 615 cm-1). Meanwhile, a broad peak corresponding to a two-magnon scattering from the interaction of two magnons created on antiparallel close spin sites also appears around 1322 cm-1. There is a tiny peak around 665 cm-1 corresponding to a trace amount of Fe3O4 which is possibly formed during the fabrication of α-Fe2O3 film. After applying the reduction potential -1.5 V for 1 s, for the 28 nm films, the peak intensity indexed to α-Fe2O3 immediately decreases while there is no intensity drop for 62 nm and 147 nm samples. After a reduction time of 5 s, the Raman peaks of α-Fe2O3 disappear for the 28 nm film, but the peaks remain clear for the 62 nm and 147 nm films. When the reduction time extends to 60 s, the peak at 665 cm-1 assigned to A1g modes of Fe3O4 becomes very strong for the 62 nm film. At this stage, peaks for α-Fe2O3 still dominate the spectra for the 147 nm film. When the reduction time reaches 150 s, the peaks for α-Fe2O3 disappear for the 62 nm film while for 147 nm film, the Fe3O4 peak dominates accompanying the peaks from α-Fe2O3. After applying the reduction potential for 300 s and longer, only the peak at 665 cm-1 from Fe3O4 is present. Raman spectroscopy is a surface sensitive technique. It takes a much longer time for Raman peaks from α-Fe2O3 to disappear in the thicker films. Meanwhile, there is no increase of Fe3O4 peaks when the reduction time extends at the beginning until Raman peaks from α-Fe2O3 tend to disappear.26 Therefore, the reduction of α-Fe2O3 to Fe3O4 is supposed to begin at the interface of α12 ACS Paragon Plus Environment

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Fe2O3/substrates. Otherwise the Raman peaks of 147 nm α-Fe2O3 film would decrease quickly and peaks from Fe3O4 should increase dramatically if the reduction of α-Fe2O3 to Fe3O4 is initiated from the top surface of α-Fe2O3. To explain the increase in the background current of α-Fe2O3 films in Figure 4, Figure 11 displays a zoom-in optical image associated with Raman spectra collected from two different spots of a and b. This figure shows that a heterogeneity is present in the films after electrochemical reduction in 0.1 M NaOH with much of the α-Fe2O3 aggregated into particles less than 100 microns in diameter. This might be due to the heterogeneities in the local electrical conductivity of FTO substrate that can conduct electrical potential differently at different sites to yield heterogeneities in chemical compositions. It is also highly possible that the catalytic activity could be tied to the formation of these microstructures. Detailed spatial distribution of the local PEC activities are currently under investigation for our reduced α-Fe2O3 films. In addition, the conversion between α-Fe2O3 and Fe3O4 is reversible. Fe3O4 electrochemically reduced from α-Fe2O3 can be reoxidized to α-Fe2O3 after being annealed at 600 oC in air for 6 hours, which is presented in Figure S9. The absorbance of a 147 nm α-Fe2O3 before and after reduction is significantly different (Figure S10). The sample after reduction shows broad absorbance in the visible region and the peak around 390 nm indexed to α-Fe2O3 disappears. This is consistent with the dark color of Fe3O4. Therefore, we conclude that the enhanced PEC efficiency of α-Fe2O3 is attributed to the improved electrical conductivity of the hematite film due to the presence of Fe3O4 and the generation of OH- near/on the surface of α-Fe2O3 as shown in Figure 12 with reduction reaction of 3 Fe2O3+ H2O + 2 e- → 2 Fe3O4 + 2 OH-.

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CONCLUSIONS Electrochemical reduction process was used to enhance the PEC performance of α-Fe2O3 photoanode for solar water splitting. The enhancement was found to be highly sensitive to film thickness, reduction time, and reduction potential. Optimal enhancement can be obtained with < 50 nm film with partial reduction in unbuffered pH neutral solution. The enhancement is attributed to the increase of the carrier density resulting from the electrochemical reduction of αFe2O3 to Fe3O4 and increase in local pH during the reduction of α-Fe2O3 accompanied with proton reduction. The reduction of α-Fe2O3 to Fe3O4 occurs close to the interface between αFe2O3 and the substrate. The more conductive Fe3O4 was found to help facilitate efficient charge transport and collection from the top α-Fe2O3 layer upon light absorption and charge separation, yielding enhanced photocurrent density.

METHOD AND MATERIALS Fabrication and Electrochemical reduction of α-Fe2O3 film. α-Fe2O3 film was prepared using a method reported in a previous publication.28 1.64 M, 0.82 M, 0.33 M and 0.16 M iron (III) nitrate (ACROS) aqueous solutions were spin-coated onto TiO2 modified FTO substrates. The prepared samples were heated at 600 °C in air for 3 hours to form α-Fe2O3 layer on FTO substrates. The concentration of the Fe(NO3)3 precursor was varied to adjust the thickness of α-Fe2O3 layer. Various reduction potentials were applied using an Electrochemical Station CHI760C (CH Instruments, Inc., Austin, TX). A sample of TiO2-coated FTO substrate prepared in the same way as α-Fe2O3 thin film was adopted as a control to study the substrate effect. CV of FTO substrate in 0.01 M Fe(NO3)3 was also carried out to confirm the reduction of α-Fe2O3.


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Characterization. Absorbance spectra were obtained using a Varian Cary 50 UV-Vis spectrophotometer. XPS results were measured using a Kratos AXIS 165 Multi-technique Electron Spectrometer. A JOEL 7000 FE SEM was used to characterize the morphology of the samples. STEM and HRTEM images were acquired from a FEI Tecnai F-20 Transmission Electron Microscope. Raman experiments were carried out using a 633 nm laser with a power of 2.5 mW and a collection time of 120 s. MicroRaman spectra were acquired using a Horiba Scientific LabRAM HR Evolution Raman Spectroscopy system. PEC Measurements. Photocurrent density of the α-Fe2O3 electrodes was obtained using an Electrochemical Station CHI760C. The light source was an Oriel AM 1.5 filtered xenon arc lamp (Newport) equipped with a solar simulator filter with an intensity of 100 mW/cm2. The light was illuminated from the top of α-Fe2O3 films. All the measurements were carried out using a threeelectrode system, with samples as the working electrode, and a Pt wire as the counter electrode. Unless otherwise specified, all potentials in this paper are against the reference electrode SCE. CV was measured with a step of 0.05 V/s. EIS was carried out at +0.7 V with a frequency range of 0.1-100,000 Hz and amplitude of 0.01 V in 0.1 M NaNO3. Mott-Schottky measurements were recorded in the dark at a frequency of 100 Hz in 0.1 M NaNO3 using a 10 mV amplitude sinusoidal perturbation.

ACKNOWLEDGMENTS. We acknowledge NSF for supporting this work under award OIA1539035 and CHE-1508192 (SP). NH, LM, and JK also acknowledge the NSF under Award CHE-1532079. SP, AG and PK acknowledge the support of UA RGC level 2 award for high resolution TEM analysis. We thank Central Analytical Facility (CAF) of The University of Alabama for the equipment support. 15 ACS Paragon Plus Environment


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Supporting Information. Photocurrent potential curves α-Fe2O3 films with various thicknesses after cathodic; absorbance spectra of α-Fe2O3 films with various thicknesses; PEC performance of α-Fe2O3 film and dependence of reduction time; transient absorption spectrum and kinetic decays of photogenerated states in α-Fe2O3 film. XPS spectra of α-Fe2O3 film before and after being; CV of a α-Fe2O3 film in 0.1 M NaOH; Photographs of α-Fe2O3 films before and after reduction treatment; Raman spectra α-Fe2O3 film after reduction treatment; Absorbance spectra of α-Fe2O3 film before and after reduction.

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(19) Wang, J.; Feng, B.; Su, J.; Guo, L. Enhanced Bulk and Interfacial Charge Transfer Dynamics for Efficient Photoelectrochemical Water Splitting: The Case of Hematite Nanorod Arrays. ACS Appl Mater Interfaces 2016. (20) Warwick, M. E. A.; Barreca, D.; Bontempi, E.; Carraro, G.; Gasparotto, A.; Maccato, C.; Kaunisto, K.; Ruoko, T. -.; Lemmetyinen, H.; Sada, C.; Gonullu, Y.; Mathur, S. PtFunctionalized Fe2O3 Photoanodes for Solar Water Splitting: The Role of Hematite NanoOrganization and the Platinum Redox State. Phys. Chem. Chem. Phys. 2015, 17, 1289912907. (21) Carraro, G.; Maccato, C.; Gasparotto, A.; Warwick, M. E. A.; Sada, C.; Turner, S.; Bazzo, A.; Andreu, T.; Pliekhova, O.; Korte, D.; Lavrenčič Štangar, U.; Van Tendeloo, G.; Morante, J. R.; Barreca, D. Hematite-Based Nanocomposites for Light-Activated Applications: Synergistic Role of TiO2 and Au Introduction. Sol. Energy Mater. Sol. Cells 2017, 159, 456-466. (22) Tsuda, N.; Nasu, K.; Fujimori, A.; Siratori. K., Eds.; In Electronic Conduction in Oxides; Springer: Berlin ; New York , 2000. (23) Ling, Y.; Wang, G.; Reddy, J.; Wang, C.; Zhang, J. Z.; Li, Y. The Influence of Oxygen Content on the Thermal Activation of Hematite Nanowires. Angew. Chem. , Int. Ed. 2012, 51, 4074-4079. (24) Allanore, A.; Lavelaine, H.; Valentin, G.; Birat, J. P.; Lapicque, F. Iron Metal Production by Bulk Electrolysis of Iron Ore Particles in Aqueous Media. J. Electrochem. Soc. 2008, 155, E125-E129. (25) Allanore, A.; Lavelaine, H.; Valentin, G.; Birat, J. P.; Delcroix, P.; Lapicque, F. Observation and Modeling of the Reduction of Hematite Particles to Metal in Alkaline Solution by Electrolysis. Electrochim. Acta 2010, 55, 4007-4013. (26) He, Z.; Gudavarthy, R. V.; Koza, J. A.; Switzer, J. A. Room-Temperature Electrochemical Reduction of Epitaxial Magnetite Films to Epitaxial Iron Films. J. Am. Chem. Soc. 2011, 133, 12358-12361. (27) Wang, G.; Yang, Y.; Ling, Y.; Wang, H.; Lu, X.; Pu, Y.; Zhang, J. Z.; Tong, Y.; Li, Y. An Electrochemical Method to Enhance the Performance of Metal Oxides for Photoelectrochemical Water Oxidation. J. Mater. Chem. A 2016, 4, 2849-2855. (28) Wang, J.; Pan, S.; Chen, M.; Dixon, D. A. Gold Nanorod-Enhanced Light Absorption and Photoelectrochemical Performance of α-Fe2O3 Thin-Film Electrode for Solar Water Splitting. J. Phys. Chem. C 2013, 117, 22060-22068. (29) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods: Fundamentals and Applications; JOHN WILEY & SONS, INC: New York, 2001. 18 ACS Paragon Plus Environment

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(30) Ruoko, T.; Kaunisto, K.; Bärtsch, M.; Pohjola, J.; Hiltunen, A.; Niederberger, M.; Tkachenko, N. V.; Lemmetyinen, H. Subpicosecond to Second Time-Scale Charge Carrier Kinetics in Hematite-Titania Nanocomposite Photoanodes. J. Phys. Chem. Lett. 2015, 6, 2859-2864. (31) Pendlebury, S. R.; Cowan, A. J.; Barroso, M.; Sivula, K.; Ye, J.; Grätzel, M.; Klug, D. R.; Tang, J.; Durrant, J. R. Correlating Long-Lived Photogenerated Hole Populations with Photocurrent Densities in Hematite Water Oxidation Photoanodes. Energy Environ. Sci. 2012, 5, 6304-6312. (32) Huang, Z.; Lin, Y.; Xiang, X.; Rodríguez-Córdoba, W.; McDonald, K. J.; Hagen, K. S.; Choi, K.; Brunschwig, B. S.; Musaev, D. G.; Hill, C. L.; Wang, D.; Lian, T. In Situ Probe of Photocarrier Dynamics in Water-Splitting Hematite (α-Fe2O3) Electrodes. Energy Environ. Sci. 2012, 5, 8923-8926. (33) Jeon, T. H.; Choi, W.; Park, H. Photoelectrochemical and Photocatalytic Behaviors of Hematite-Decorated Titania Nanotube Arrays: Energy Level Mismatch Versus Surface Specific Reactivity. J. Phys. Chem. C 2011, 115, 7134-7142. (34) Shebanova, O. N.; Lazor, P. Raman Spectroscopic Study of Magnetite (FeFe2O4): A New Assignment for the Vibrational Spectrum. J. Solid State Chem. 2003, 174, 424-430.



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Figure Captions Figure 1. CVs of TiO2 coated FTO in 0.1 M NaNO3, a 28 nm α-Fe2O3 film in 0.1 M NaNO3, and bare FTO in 0.1 M Fe(NO3)3. Scanning rate is 0.05 V/s. Figure 2. Photocurrent density-potential curves of (A) 10 nm and (B) 147 nm) α-Fe2O3 films after being reduced at -0.2 V, -0.5 V, -0.75 V, -1.0 V, and -1.5 V for 3 s in 0.1 M NaNO3, respectively. Figure 3. Enhancement factor of photocurrent density of α-Fe2O3 films around +0.75 V after being reduced at various reduction potentials. Figure 4. Photocurrent enhancement of a 28 nm α-Fe2O3 film and a 147 nm α-Fe2O3 film after being reduced at -1.5 V for various periods of time. Figure 5. SEM images of the morphology of a 28 nm α-Fe2O3 film (A) before and (B) after reduction treatment at -1.5 V for 10 s in 0.1 M NaNO3. Higher magnification images are shown as insets. Figure 6. (A) STEM and (B) HRTEM images of the cross section of a 28 nm α-Fe2O3 film after reduction treatment at -1.5 V for 20 s. Figure 7. Mott-Schottky plot of a 28 nm α-Fe2O3 film before and after reduction treatment at 1.5 V for 20 s and 600 s in 0.1 M NaNO3. Figure 8. EIS spectra of a 28 nm α-Fe2O3 film before and after reduction treatment at -1.5 V for 20 s and 600 s in 0.1 M NaNO3. Figure 9. Photocurrent density of (A) a 10 nm α-Fe2O3 film reduced in 0.1 M NaNO3 followed by rinsing with fresh 0.1M NaNO3 solution and the photocurrent was taken in fresh electrolyte of 0.1 M NaNO3, (B) 28 nm α-Fe2O3 film in PBS buffer, (C) 28 nm α-Fe2O3 film in 0.1 M NaOH, and (D) photocurrent stability test of a 10 nm reduced α-Fe2O3 film in 0.1 M NaNO3. Figure 10. Raman spectra of α-Fe2O3 films with various thicknesses after applying -1.5 V for various amounts of time in 0.1 M NaOH (A) 28 nm, (B) 62 nm, and (C) 147 nm. Figure 11. Zoom-in optical image of a reduced 147 nm α-Fe2O3 film associated with Raman spectra collected from two spots at different locations: a and b. Figure 12. Schematic of improved PEC performance of α-Fe2O3 electrode after electrochemical reduction on FTO substrate.


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TABLES Table 1. Photocurrent density dependence on reduction potential and α-Fe2O3 film thickness. These photocurrent data were obtained from Figure 2 and 3 at electrode potential of +0.75 V (vs. SCE). Table 2. EIS fitting results of a 28 nm α-Fe2O3 film before and after applying -1.5 V for 20 s and 600 s in 0.1 M NaNO3. Table 3. Assigned Raman peaks of pristine hematite and reduced film.



Figure 1

TiO2 coated FTO in 0.1 M NaNO3 ~28 nm α-Fe2O3 film on TiO2 coated FTO in 0.1 M NaNO3 FTO in 0.01 M Fe(NO3)3

0.4

0.2 Fe3O4

α-Fe2O3

Back w

ard

0.3 Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1 Forward 0.0 0.2

0.0

-0.2 -0.4 -0.6 -0.8 Potential (V vs. SCE)

-1.0


-1.2

-1.4

Page 23 of 37

Figure 2 (A)

(B) ~147 nm α-Fe2O3 film

~10 nm α-Fe2O3 film 2

Applying -1.5 V for 3 s


Applying -0.75 V for 3 s Applying -0.5 V for 3 s

Current density, 1 mA/cm


2

Current density, 0.1 mA/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60






Before the reduction


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.0

0.8

0.1

0.2

0.3

0.4

0.5

0.6

Potential (V vs. SCE)

Potential (V vs. SCE)


0.7

0.8


Figure 3

Photocurrent density enhancement factor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

25 20

~10 nm α-Fe2O3 film ~28 nm α-Fe2O3 film ~62 nm α-Fe2O3 film ~147 nm α-Fe2O3 film

15 10 5 0 Initial

-0.2 V

-0.5 V

-0.75 V

-1.0 V

Reduction potential (vs. SCE)


-1.5 V

Page 25 of 37

Figure 4

Photocurrent density enhancement factor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


~28 nm α-Fe2O3 film ~147 nm α-Fe2O3 film 9 60 s

6 20 s

3

30 s

10 s

(B) 5s

0

1s Initial

3s 150 s 300 s

Electrochemical reduction time


600 s


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5 (A)

(B)


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Figure 6 (A)

(B)



Figure 7

Before the reduction After 20 s reduction After 600 s reduction

2.0

4 -2

1.5

1.0

-2

-9

C x 10 , cm F

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5

2

y = 1.145x + 0.3806, r =0.9963 21 3 Carrier density=1.541x10 /cm 2 y = 0.1759x + 0.0675, r =0.9995 22 3 Carrier density=1.003x10 /cm

0.0

2

y = 0.1188x + 0.0424, r =0.9988 22 3 Carrier density=1.483x10 /cm

-0.6

-0.4

-0.2

0.0

0.2

0.4

Potential (vs. SCE)


0.6

0.8

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Figure 8



Figure 9 (A)

(B)

~10 nm α-Fe2O3 film in 0.1 M NaNO3 After replacing the electrolyte with a fresh electrolyte






2

2

~28 nm α-Fe2O3 film in PBS (pH=7.4)

20 s reduction 10 s reduction 5 s reduction 3 s reduction

1 s reduction Before the reduction


0.0

0.1

0.2 0.3 0.4 0.5 0.6 Potential (V vs. SCE)

0.7

0.8

0.0

0.1

0.2 0.3 0.4 0.5 0.6 Potential (V vs. SCE)

(C)

(D)

2

~28 nm α-Fe2O3 film in 0.1 M NaOH

Current density, 1 mA /cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 s reduction 10 s reduction 5 s reduction 3 s reduction 1 s reduction Before the reduction

0.0

0.1

0.2

0.3 0.4 0.5 0.6 Potential (V vs. SCE)

0.7

0.8


0.7

0.8

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Figure 10 1000

Intensity

(A)

Reduction time

Before Reduction 1s 3s 5s 10 s 20 s 30 s 60 s 150 s 300 s 600 s

400

800 1200 -1 Wavenumber (cm )

1600

Fe3O4

1000

Intensity

(B)

Reduction time


400

800

1200

1600 -1

Wavenumber (cm )

665 nm

1000

Fe3O4

Intensity

(C)


Reduction time

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

800

1200

1600

-1

Wavenumber (cm )



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 12



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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Table 1 Estimated αFe2O3 film thickness

Initial photocurrent density (mA/cm2)

-0.2 V (mA/cm2)

-0.5 V (mA/cm2)

-0.75 V (mA/cm2)

-1.0 V (mA/cm2)

-1.5 V (mA/cm2)

10 nm

0.00523

0.00655

0.00900

0.02869

0.06349

0.13037

28 nm

0.01905

0.01938

0.02321

0.05137

0.10376

0.18283

62 nm

0.04629

0.04794

0.06268

0.10997

0.18100

0.16060

147 nm

0.06548

0.06597

0.07381

0.11527

0.20474

0.17681


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Table 2

Parameters

Before the reduction in dark

Error (%)

Before the reduction under light

Error (%)

After 20 s reduction under light

Error (%)

After 600 s reduction under light

Error (%)

Rs (Ω)

1.166 × 103

1

9.956 × 102

2

9.984 × 102

3

9.359 × 102

1

C (F)

4.947 × 10-6

1

6.739 × 10-6

3

2.105 ×10-5

6

4.733 ×10-5

4

Rct (Ω)

6.801 × 105

11

8.196 × 104

6

1.197 × 104

7

1.719 × 103

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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Table 3

Oxides

Raman shift (cm-1)

Peak Assignment

α-Fe2O3

229, 498

A1g modes

248, 295, 301, 415, 615

Eg modes

1322

second- scattering

665

A1g modes

Fe3O4


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Table of Content (TOC)


A Facile Electrochemical Reduction Method for Improving

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