Chem. Mater. 2009, 21, 3701–3709 3701 DOI:10.1021/cm803099k
Photoactivity of Transparent Nanocrystalline Fe2O3 Electrodes Prepared via Anodic Electrodeposition Ryan L. Spray and Kyoung-Shin Choi* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received November 13, 2008. Revised Manuscript Received June 17, 2009
Highly transparent nanocrystalline R-Fe2O3 films were prepared via anodic electrodeposition using a slightly acidic aqueous medium (pH 4.1). The deposition mechanism involved oxidation of Fe2þ ions to Fe3þ ions followed by precipitation of Fe3þ ions as amorphous γ-FeOOH films. The as-deposited films were then converted to transparent nanocrystalline R-Fe2O3 films by annealing at 520 °C in air. The as-deposited and annealed films were characterized by Raman and UV-vis spectroscopy, X-ray diffraction, and scanning electron microscopy. The photoactivity of the asdeposited and annealed films was studied by measuring short-circuit photocurrents in a 60:40 solution of propylene carbonate:acetonitrile containing 0.5 M tetrabutylammonium iodide and 0.04 M iodine. Both films show n-type behavior generating anodic photocurrent. R-Fe2O3 films with various thicknesses were prepared to study the effect of film thickness on photon absorption and photocurrent. The short-circuit photocurrent of R-Fe2O3 films increased gradually as the film thickness increased to 400-500 nm because of the corresponding increase in photon absorption and surface area of the films. However, when the film thickness exceeded 400-500 nm, aggregation of Fe2O3 particles at the film/substrate interface became severe. This increased recombination losses near the collector electrode, and with no significant gain in photon absorption, led to an overall decrease in photocurrent. R-Fe2O3 films were also prepared by annealing FeOOH films produced via anodic deposition in a neutral medium (pH 7.5). The film prepared from the neutral medium was two times thicker and possessed a surface roughness factor two times higher than the film prepared from the acidic medium when the two films contained the same amount of R-Fe2O3. Comparing photocurrent of these films allowed for better understanding the effect of electrode structures (i.e., surface area, film thickness) on photocurrent generation in R-Fe2O3 electrodes with poor charge transport properties. Introduction Ferric oxide (R-Fe2O3, hematite) is an n-type semiconductor highly desirable for use in solar energy conversion because its bandgap (Eg =∼2.2 eV) allows for utilizing a significant portion of the solar energy spectrum.1-11 The *To whom correspondence should be addressed. E-mail: kchoi1@purdue. edu. Tel: (765) 494-0049. Fax: (765) 494-0239.
(1) Wilhelm, S. M.; Yun, K. S.; Ballengeer, L. W.; Hackerman, N. J. Electrochem. Soc. 1979, 126, 419–424. (2) Turner, J. E.; Hendewerk, M.; Parmeter, J.; Neiman, D.; Somorjai, G. A. J. Electrochem. Soc. 1984, :: 131, 17c77–1783. (3) Bjoerksten, U.; Moser, J.; Graetzel, M. Chem. Mater. 1994, 6, 858– 63. :: (4) Duret, A.; Gratzel, M. :: J. Phys. Chem. B 2005, 109, 17184–17191. (5) Kay, A.; Cesar, I.; Gratzel, M. J. Am. Chem. Soc. 2006, 128, 15714– 15721. (6) Alexander, B. D.; Kulesza, P. J.; Rutkowska, I.; Solarska, R.; Augustynski, J. J. Mater. Chem. 2008, 18, 2298–2303. (7) Sartoretti, C. J.; Alexander, B. D.; Solarska, R.; Rutkowska, I. A.; Augustynski, J.; Cerny, R. J. Phys. Chem. B 2005, 109, 13685– 13692. (8) Sartoretti, C. J.; Ulmann, M.; Alexander, B. D.; Augustynski, J.; Weidenkaff, A. Chem. Phys. Lett. 2003, 376, 194–200. (9) Beermann, N.; Vayssieres, L.; Lindquist, S.-E.; Hagfeldt, A. J. Electrochecm. Soc. 2000, 147, 2456–2461. (10) Lindgren, T.; Wang, H.; Beermann, N.; Vayssieres, L.; Hagfeldt, A.; Lindquist, S.-E. Sol. Energy Mater Sol. Cells 2002, 71, 231–243. (11) Prakasam, H. E.; Varghese, O. M.; Paulose, M.; Mor, G. K.; Grimes, C. A. Nanotechnology 2006, 17, 4285–4291. r 2009 American Chemical Society
low cost and environmentally benign nature of iron makes developing R-Fe2O3-based photoelectrochemical cells even more attractive. R-Fe2O3 is also one of the few materials that is resistant to photocorrosion and has a good chemical stability in neutral and basic aqueous media, making it a viable candidate to photoelectrolyze water to generate oxygen and hydrogen. Its conduction band, however, is located slightly below the level needed for hydrogen production, whereas its valence band is well-suited for oxygen production. Therefore, forming tandem cells, photoelectrochemical diodes, or biphotoelectrodes with other materials of suitable band positions is necessary to use R-Fe2O3 for photoelectrolysis of water without using an external bias.2,12-14 In this context, producing R-Fe2O3 as a thin-film type electrode is highly desirable as this form allows for facile construction of photoelectrochemical cells with multicomponent and multijunction photoelectrode architectures. (12) Ingler, W. B.; Khan, S. U. M. Electrochem. Solid-State Lett. 2006, 9, G144–G146. (13) Wang, H.; Deutsch, T.; Turner, J. A. J. Electrochem. Soc. 2008, 155, F91–F96. (14) Aroutiounian, V. M.; Arakelyan, V. M.; Shahnazaryan, G. E.; Stepanyan, G. M.; Turner, J. A.; Khaselev, O. Int. J. Hydrogen Energy 2002, 27, 33–38.
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The most commonly used methods to prepare R-Fe2O3 photoelectrodes include spray pyrolysis,4,7,8,12,15-18 chemical vapor deposition,5,19,20 sol-gel method,21 thermal oxidation,22,23 and anodization of iron metal.24 Electrodeposition has also been demonstrated as a viable method to prepare Fe2O3 (or related compounds such as FeOOH) as film-type electrodes.25-33 Having deposition potential and current as additional synthesis parameters, this solution-based method can be particularly beneficial for tuning compositions and morphologies of deposits,29,30 which is reported to be crucial for improving the intrinsically poor charge transport properties of R-Fe2O3.7,8,34-36 Both cathodic and anodic electrodeposition routes have been reported to prepare R-Fe2O3 films. The most typical cathodic deposition method is based on electrochemical generation of OH- ions by reduction of H2O2 in a solution containing Fe3þ ions.25,26 The resulting increase in the local pH near the working electrodes reduces the solubility of Fe3þ ions in the solution, causing the precipitation of Fe3þ ions as FeOOH films, which can be converted to photoactive R-Fe2O3 films by annealing. Recently, highly photoactive R-Fe2O3 electrodes doped with Pt, Mo, and Cr were prepared by McFarland and coworkers via this route.29,30 Anodic deposition method, on the other hand, involves oxidation of Fe2þ ions to Fe3þ ions followed by precipitation of Fe3þ ions as FeOOH films which was first reported by Cohen and co-workers.31-33 The neutral plating solu(15) Itoh, K.; Bockris, J. O’M. J. Appl. Phys. 1984, 56, 874–876. (16) Itoh, K.; Bockris, J. O’M. J. Electrochem. Soc. 1984, 131, 1266– 1271. (17) Dareedwards, M. P.; Goodenough, J. B.; Hamnett, A.; Trevellick, P. R. J. Chem. Soc., Faraday Trans. I 1983, 79, 2027–2041. (18) Khan, S. U. M.; Akikusa, J. J. Phys. Chem. B 1999, 103, 7184–7189. (19) Kenneth, L. H.; Allen, J. B. J. Electrochem. Soc. 1976, 123, 1024– 1026. (20) Kenneth, L. H.; Allen, J. B. J. Electrochem. Soc. 1977, 124, 215– 224. (21) Miyake, H.; Kozuka, H. J. Phys. Chem. B 2005, 109, 17951–17956. (22) Fu, Y. Y.; Wang, R. M.; Xu, J.; Chen, J.; Yan, Y.; Narlikar, A. V.; Zhang, H. Chem. Phys. Lett. 2003, 379, 373–379. (23) Takagi, R. J. Phys. Soc. Jpn. 1957, 12, 1212–1218. (24) Prakasam, H. E.; Varghese, O. K.; Paulose, M.; Mor, G. K.; Grimes, C. A. Nanotechnology 2006, 17, 4285–4291. (25) Schrebler, R.; Llewelyn, C.; Vera, F.; Cury, P.; Munoz, E.; del Rio, R.; Gomez Meier, H.; Cordova, R.; Dalchiele, E. A. Electrochem. Solid-State Lett. 2007, 10, D95–D99. (26) Schrebler, R.; Bello, K.; Vera, F.; Cury, P.; Munoz, E.; del Rio, R.; Gomez Meier, H.; Cordova, R.; Dalchiele, E. A. Electrochem. Solid-State Lett. 2006, 9, C110–C113. (27) Zotti, G.; Schiavon, G.; Zecchin, S.; Casellato, U. J. Electrochem. Soc. 1998, 145, 385–389. (28) Kulkarni, S. S.; Lokhande, C. D. Mater. Chem. Phys. 2003, 82, 151–156. (29) Hu, Y.-S.; Kleiman-Shwarsctein, A.; Forman, A. J.; Hazen, D.; Park, J.-N.; McFarland, E. W. Chem. Mater. 2008, 20, 3803–3805. (30) Kleiman-Shwarsctein, A.; Hu, Y.-S.; Forman, A. J.; Stucky, G. D.; McFarland, E. W. J. Phys. Chem. C. 2008, 112, 15900–15907. (31) Leibenguth, J. L.; Cohen, M. J. Electrochem. Soc. 1972, 119, 987– 91. (32) Markovac, V.; Cohen, M. J. Electrochem. Soc. 1967, 114, 678–81. (33) Markovac, V.; Cohen, M. J. Electrochem. Soc. 1967, 114, 674–678. (34) Shinar, R.; Kennedy, J. H. Sol. Energy Mater. 1982, 6, 323–35. (35) Aroutiounian, V. M.; Arakelyan, V. M.; Shahnazaryan, G. E.; Stepanyan, G. M.; Turner, J. A.; Khaselev, O. J. Hydrogen Energy 2002, 27, 33–38. (36) Aroutiounian, V. M.; Arakelyan, V. M.; Shahnazaryan, G. E.; Hovhannisyan, H. R.; Turner, J. A. Solar Energy 2007, 81, 1369– 1376.
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tion (pH 6.6-8.0) they used had a limited solubility of Fe2þ ions. Therefore, complexing agents (e.g., ammonium sulfate/ammonium, boric acid/ammonium borate) were added to stabilize Fe2þ ions in solutions. Cohen and co-workers did not investigate the photoactivity of RFe2O3 films obtained by annealing their as-deposited FeOOH films because the focus of their studies was examination of deposition kinetics and compositions of the as-deposited phase. In this study, we report a new anodic deposition condition to prepare R-Fe2O3 films using a slightly acidic plating solution (pH 4.1). In this pH, Fe2þ ions are soluble without the addition of complexing agents, which simplified the compositions of the plating solution. Deposition in this medium produced amorphous γ-FeOOH films that can be converted to transparent nanocrystalline RFe2O3 films by annealing. Photoactivities of the as-deposited and annealed samples with varying thicknesses were investigated. R-Fe2O3 films were also prepared using Cohen’s neutral media (pH 7.5) in order to compare morphologies and photoactivities of the resulting RFe2O3 films with those prepared from the acidic medium developed in this study. Comparing photocurrent of electrodes containing comparable amounts of Fe2O3 with different morphologies provided a unique opportunity to examine the effect of electrode architecture on photocurrent generation in R-Fe2O3 films. Establishing various deposition conditions to prepare Fe2O3 films in terms of deposition potential (i.e., cathodic vs anodic) and pH (i.e., acidic vs basic) of the medium will also be useful when Fe2O3 needs to be deposited on a semiconducting layer that is stable only for a limited range of deposition potentials and pH when preparing multijunction electrodes. This study will provide a good foundation to prepare R-Fe2O3 photoelectrodes via anodic electrodeposition from which various conditions to modify compositions and morphologies can be developed. Experimental Section Synthesis of Fe2O3 Electrodes Using a Slightly Acidic Plating Solution (pH 4.1). Electrodeposition was carried out using an aqueous solution containing 0.02 M FeCl2 3 5H2O (ACS, 99þ% purity, Aldrich). DI water further purified with a Barnstead purification system (resistivity g18.2 M Ω) was used to prepare all solutions used in this study. The pH of a freshly prepared plating solution was 4.1. Deposition was carried out potentiostatically (constant potential deposition) at 75 °C using a VMP2 Multichannel Potentiostat (Princeton Applied Research). A standard three-electrode setup in an undivided cell was used. Fluorine-doped tin oxide (FTO) (8-12 Ω resistance) was used as the working electrode while platinum foil was used as the counter electrode. The reference electrode was an Ag/AgCl electrode in 4 M KCl solution, against which all the potentials reported herein were measured. Optimum deposition potential of E=1.2 V (average deposition current density of ∼0.38 mA/cm2) was found to produce highly transparent and uniform films. Increasing deposition potential or temperature promoted oxygen evolution on the working electrode and reduced the uniformity of the films. Decreasing deposition potential or temperature reduced the deposition rate, which required a
Article longer deposition time to achieve a certain thickness and offered no other advantages. After each deposition, the resulting film was thoroughly rinsed with deionized water, and dried with a gentle stream of nitrogen gas. The color of the as-deposited films varied from light yellow to dark orange depending on deposition time (132 min). To obtain crystalline R-Fe2O3 electrodes, as-deposited films were annealed in atmosphere at 520 °C for 30 min after heating at a rate of 2 °C/min. Electrodeposition of Iron Oxide Films Using a Neutral Plating Solution (pH 7.5). Electrodeposition was carried out using an aqueous solution containing 0.02 M FeCl2 3 5H2O (same as above) and 3 M NH4Cl (Mallinckrodt). The pH of the solution was adjusted to 7.5 by adding NaOH (JT Baker). Before dissolving these chemicals, the aqueous medium was deaerated by purging with Ar(g) for 30 min. Films were deposited anodically at 0.3 V while maintaining an Ar atmosphere above the beaker. The resulting as-deposited yellow films were annealed in the same conditions as the films deposited from acidic media to produce crystalline R-Fe2O3 electrodes. Quantitative Analysis of Deposits. The amount of FeOOH deposited was spectroscopically determined by dissolving the deposits in concentrated HCl and measuring UV-vis spectra of the resulting solutions. FeCl4- species present in solution generate a well-defined absorption band centered at λ=362 nm, the absorbance of which was used to determine the concentration of Fe3þ ions using Beer’s law (see the Supporting Information).37 The absorption coefficient at λ = 362 nm was determined by measuring absorbance of standard solutions containing known amounts of Fe3þ ions (0.1 mM, 0.25 mM, 0.5 mM, and 0.75 mM) that were prepared by dissolving FeCl3 in concentrated HCl (see the Supporting Information). UV-vis spectra were recorded using a Cary 300 UV-vis spectrophotometer in dual-beam mode. Pure concentrated HCl was used in the reference cell. Surface Roughness Factor. For evaluation of surface roughness factors (i.e., ratio of real surface area of Fe2O3 to the 2D geometric area of the substrate), a dye adsorption procedure was adopted using the azo-dye Orange II (Fluka).5,38 Films were soaked in a 1.5 mM dye solution (pH adjusted to 3.5 by adding HCl) for 1 h. Under this condition, azo-dye Orange II is known to adsorb on nanocrystalline Fe2O3 surface forming a monolayer. The films were then rinsed with several mL of diluted HCl solution (pH 3.5), and dried with a gentle stream of nitrogen. The amount of adsorbed dye molecules was determined using Beer’s law by desorbing them in 3.0 mL of 1 M NaOH (ACS, Mallinckrodt) solution and measuring the absorbance of the dye molecules at 516 nm. The absorption coefficient at 516 nm was determined using standard solutions containing a known amount of dye. Characterization. X-ray diffraction (XRD) patterns were recorded on a Scintag X2 diffractometer (Cu KR radiation). Scanning electron microscopy (SEM) images were taken using a field emission scanning electron microscope (FEI Nova NanoSEM) operated at 3 kV. The iron oxide films were coated with Pt using a thermal evaporator before imaging to minimize charging problems. UV-vis spectra of the films were measured using a Cary 300 UV-vis spectrophotometer in dual-beam transmittance mode, with a bare FTO substrate as a reference. The films on transparent FTO substrate were affixed directly to (37) Liu, W.; Etschmann, B.; Brugger, J.; Spiccia, L.; Foran, G.; McInnes, B. Chem. Geol. 2006, 231, 326–349. (38) Bandara, J.; Mielczarski, J. A.; Kiwi, J. Langmuir 1999, 15, 7670– 7679.
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Figure 1. XRD patterns of (a) as-deposited and (b) annealed films. Peaks from the FTO substrate are indicated by *. The inset shows photographs of these films.
the sample stage, perpendicular to the source beam. Raman spectroscopy was carried out on samples scraped from the FTO electrodes using a 785 nm diode laser (SDL-8630) with a CCD detector (LN/CCD-1024 EHRB, Princeton Instruments) having a measured resolution better than 4 cm-1. Short-Circuit Photocurrent Measurements. Photocurrent was measured using a nonaqueous electrolyte consisting of 0.5 M tetrabutylammonium iodide (99%, Aldrich) and 0.04 M iodine (Mallinckrodt) in a 60:40 solution of propylene carbonate (99%, Alfa Aesar): acetonitrile (ACS, Mallinckrodt). The backside of the photoelectrodes were set to face an optical fiber leading from a 300 W xenon arc lamp (Oriel) and illumination was achieved through the FTO substrate (back-side illumination). The light intensity at the end of optical fiber was 3 W/cm2. Photocurrent was measured using a platinum foil as the counter electrode with no externally applied bias between the working and the counter electrode (short-circuit or zero-bias photocurrent) using the VMP2 potentiostat.
Results and Discussions The anodic deposition condition used in this study involves two steps. The first step is oxidation of Fe2þ ions to Fe3þ ions (eq 1), and the second step is precipitation of Fe3þ ions as ferric oxyhydroxide due to the limited solubility of Fe3þ ions in the plating solution ([Fe3þ] ≈ 110-7 at pH 4.1) (eq 2).39 Fe2þ f Fe3þ þe -
E o ¼ 0:771 V
ð1Þ
Fe3þ þ2H2 O f FeOOHþ3Hþ log½Fe3þ ¼ 4:84 -3pH
ð2Þ
In comparison, cathodic deposition of ferric oxyhydroxide that Schrebler et al. reported does not involve changes in oxidation state of Fe ions. Instead, it begins with a solution containing Fe3þ ions and decreases the solubility of Fe3þ ions by reducing H2O2 to generate OH- ion (39) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd ed.; National Association of Corrosion Engineers: Houston, 1974.
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Figure 2. SEM images of (a) as-deposited and (b) annealed films.
Figure 4. (a) UV-vis absorption spectra and (b) short-circuit photocurrents of γ-FeOOH (dashed) and R-Fe2O3 films (solid).
Figure 3. Raman spectra of (a) a bare FTO substrate, (b) an as-deposited film, and (c) an annealed film. Peaks expected for γ-FeOOH and R-Fe2O3 are labeled with asterisks and circles, respectively.
(eq 3), which induces precipitation of FeOOH on the working electrodes (eq 4).25,26,39 H2 O2 þHþ þ2e - f OH - þH2 O Eo ¼ 1:362 -0:0293pH
ð3Þ
FeF2þ þ3OH - f FeOOHþF - þH2 O 3þ
ð4Þ -
To enhance the stability of Fe ions in solution, F ions are used as a complexing agent to form the FeF2þ species. Formation of FeF2þ species also ensures the reduction of Fe3þ ions to be more difficult than the reduction of H2O2.26 Without the presence of F- ions, reduction of Fe3þ ions would occur predominantly, preventing deposition of FeOOH.26 The as-deposited films obtained by anodic deposition were transparent and showed uniform yellow-orange color (Figure 1 inset). These films were amorphous but became crystalline upon annealing at 520 °C for 30 min in the air. The X-ray diffraction (XRD) pattern generated by the annealed sample corresponds to that of R-Fe2O3 (JCPDS 86-0550) (Figure 1). The annealed films remained transparent but exhibited a deeper red color
(Figure 1 inset). Both as-deposited and annealed films show excellent adhesion and uniformity with no cracking. SEM studies show that the as-deposited film is composed of particles with no distinctive shapes while the annealed film contains round or oval particles with sizes ranging from approximately 10-100 nm (Figure 2). This evident morphology change, which does not seem due to simple aggregation during annealing, suggests that the as-deposited and annealed sample may contain different phases. To identify the phase of amorphous as-deposited films, Raman spectroscopy was employed. There are several polymorphs of Fe(III) oxide or oxyhydroxide (e.g., RFe2O3, γ-Fe2O3, R-FeOOH, γ-FeOOH), but each phase exhibits distinctive Raman shifts, making it possible to distinguish them by Raman study when they are X-ray amorphous.36 The Raman spectra of as-deposited and annealed films along with that of the FTO substrate used as the working electrode are shown in Figure 3. For the as-deposited film, the peaks observed at 258 and 383 cm-1 match the most and second-most intense peaks for γFeOOH.40 For the annealed film, peaks at 226, 295, 412, and 612 cm-1 all correspond to peaks for R-Fe2O3.41 Neither spectra show a peak at 660 cm-1 that can indicate the presence of Fe3O4. Fe3O4 often appears as an impurity when Fe2O3 is prepared via spray pyrolysis or cathodic :: (40) Dunnwald, J.; Otto, A. Corros. Sci. 1989, 29, 1167–1176. (41) de Faria, D. L. A.; Ven^ancio Silva, S.; de Oliveira, M. T. J. Raman Spectrosc. 1997, 28, 873–878.
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Figure 5. Cross-section SEM images of films deposited for (a) 8 and (b) 16 min. The circled region in (b) shows the aggregation of Fe2O3 particles near the FTO surface.
electrodeposition.7,8,29 This indicates that the anodic deposition successfully avoids the deposition of oxides containing mixed valencies (Fe2þ, Fe3þ). UV-vis spectra show that both the as-deposited and annealed films have bandgaps of approximately 2.1 eV (Figure 4a). The typical literature value for the bandgap of γ-FeOOH is 2.1 eV, and that of R-Fe2O3 ranges from 1.9 to 2.2 eV depending on the synthesis and measurement methods.42 Although the γ-FeOOH and R-Fe2O3 films used for the UV-vis measurement have the same thickness of deposits, the R-Fe2O3 film shows more intense absorbance, indicating its higher absorption coefficient. This explains the color difference of the as-deposited and annealed samples shown in Figure 1 inset. The photoactivity of the as-deposited and annealed samples was investigated by measuring short-circuit photocurrent using a 60:40 solution of propylene carbonate: acetonitrile containing 0.5 M tetrabutylammonium iodide and 0.04 M iodine as an electrolyte. (Figure 4b). (As mentioned earlier, the conduction band position of RFe2O3 is not appropriate to photoelectrolyze water, and cannot produce appreciable photocurrent in aqueous media without applying an external bias.) Both as-deposited and annealed samples show n-type behavior generating anodic photocurrent; the photogenerated holes are used at the electrode/electrolyte interface to oxidize iodide ions while the photoexcited electrons are transferred to the Pt counter electrode to reduce triiodide ions. The amount of photocurrent generated by an as-deposited film (γ-FeOOH) was negligible (
