Fabrication of Thickness-Controlled Hematite Thin Films via

Sep 29, 2016 - Nanoparticulate thin films of hematite with well-controlled thicknesses were prepared on transparent conductive substrates by a simple ...
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Fabrication of Thickness-Controlled Hematite Thin Films via Electrophoretic Deposition and Subsequent Heat Treatment of Pyridine-Capped Maghemite Nanoparticles Brionna L. Bennett,† Elijah C. Wyatt,‡ and Clifton T. Harris*,† †

Department of Chemistry, Physics, and Geology, Winthrop University, Sims Building, Rock Hill, South Carolina 29733, United States ‡ Department of Biology, Winthrop University, Dalton Building, Rock Hill, South Carolina 29733, United States S Supporting Information *

ABSTRACT: Nanoparticulate thin films of hematite with well-controlled thicknesses were prepared on transparent conductive substrates by a simple and tunable electrophoretic deposition method. Atomic force and scanning electron microscopy images reveal adherent, uniform, crack-free deposits comprised of clustered nanospheres with average diameters of 47 ± 18 nm. An asymptotic relationship is observed between film thickness and deposition time for fixed voltages and particle concentrations, which enables facile reproducibility. The mild conditions of this method make it highly suitable for incorporation of hematite into multilayer thin-film assemblies that may comprise more sensitive materials (e.g., quantum dots).

1. INTRODUCTION In recent years, there has been a resurgence of interest in thin films of hematite (α-Fe2O3) regarding applications in photovoltatic (PV) and photoelectrochemical water-splitting devices.1−28 α-Fe2O3 is an n-type semiconductor having an indirect band gap energy of approximately 2.2 eV, making it an excellent choice as a visible light absorber. Its band potentials and photochemical stability across a wide pH range make it particularly useful for photocatalytic water oxidation.14,17,18,29,30 Nanoparticulate α-Fe2O3 offers additional attributes over the bulk counterpart such as increased catalytic surface area and diminished recombination rates via minimization of minority charge carrier diffusion lengths, although the comparatively poor charge transport properties of even nano α-Fe2O3 have resulted in reported incident photon-to-charge efficiencies that fall far below the theoretical maximum of 12.9%.2,6,8,9,15 Exponential drops in the absorbance cross section of α-Fe2O3 particles as the energies of incident photons approach that of the band gap have also contributed to the challenges of boosting efficiencies in α-Fe2O3-based systems.9 Furthermore, a major limitation regarding the use of α-Fe2O3 as a catalyst for water-splitting is its relatively positive conduction band potential, which renders the hydrogen evolution reaction virtually impossible without a substantial amount of applied bias.6,8,15,31 For this reason, α-Fe2O3 may be best employed as a component of a multilayered thin-film assembly, wherein it is paired in tandem with a suitable hydrogen evolution catalyst to facilitate complete water splitting.32 In consideration of all of these facets, synthetic methods for α-Fe2O3 thin films to be © XXXX American Chemical Society

employed in multijunction PV devices should adhere to the following guidelines: (1) the synthesis should yield spherical particles with nanoscale dimensions; (2) the conditions under which α-Fe2O3 is incorporated into a multijunction assembly should be reasonably mild to ensure compatibility with a multitude of water-reduction catalysts as well as avoid damage to underlying materials; and (3) the deposition method should allow for precise control of film thickness. Commonly employed methods for the fabrication of α-Fe2O3 thin films such as spray pyrolysis,33−35 anodic21 and cathodic3,5 electrodeposition, chemical vapor deposition,30 and sol−gel-based methods14 all require either strongly oxidizing conditions or extreme pH values which are unsuitable for many non-oxide materials. In this work, we report a three-step electrophoretic deposition (EPD) method of uniform, crack-free, transparent, thickness-controlled α-Fe2O3 thin films. In the first step, pyridine-capped magnetite (Fe3O4) nanoparticles are synthesized in aqueous media at modest pH and temperature. Under these conditions, surface oxidation of Fe3O4 by free nitrate ions occurs rapidly, leading to the formation of maghemite (γFe2O3) nanoparticles. In the second step, the pyridine-capped γ-Fe2O3 nanoparticles are magnetically decanted, washed, transferred into neat pyridine, and dispersed. Finally, the Received: June 22, 2016 Revised: August 18, 2016 Accepted: September 29, 2016

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DOI: 10.1021/acs.iecr.6b02394 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Schematic of the electrophoretic deposition method.

2.3. Fabrication of α-Fe2O3 Thin Films via EPD of γFe2O3. The deposition bath was prepared by diluting an aliquot of the nanoparticle/pyridine solution to 20% (v/v) with acetone. The FTO to be coated (working electrode) was connected to the negative terminal of a 500 V (Tel-Atomic) power source. A clean FTO was also used as the counter electrode. The conductive faces of the electrodes were aligned parallel and face-to-face with a separation distance of 1 cm, and the bath was very gently stirred. A 2.5 cm2 portion of the working electrode was exposed to the deposition bath. A moderate potential difference of 50 V was applied between the FTOs for varying periods of time to obtain the desired thicknesses. No material was deposited on the counter electrode, indicating that the pyridine-capped particles were only positively charged. This is consistent with the particlecharging tendencies observed in other pyridine-based EPD systems.36 The coated FTOs were annealed in air at 450 °C for 30 min. The dark orange films became deep red, indicating the conversion of γ-Fe2O3 to α-Fe2O3. The annealed films show excellent adhesion, uniformity, and transparency.

particles are electrophoretically deposited at a moderate voltage with various time intervals onto SnO2:F (FTO) transparent conductive substrates and annealed to yield thin films of αFe2O3. The film thickness is directly related to the time interval of deposition. This study provides a fast, reliable, low-cost method for obtaining α-Fe2O3 thin films of reproducible thicknesses as well as a simple and widely compatible route for incorporation of α-Fe2O3 into heterogeneous, multijunction PV devices.

2. EXPERIMENTAL SECTION 2.1. Materials. Iron(II) chloride tetrahydrate, iron(III) nitrate nonahydrate, pyridine (ACS grade), and acetone (ACS grade) were purchased from VWR and used as received. Rectangular-cut FTO substrates (4.5 × 1 cm, 15Ω) were purchased from Hartford Glass Inc. The FTO substrates were cleaned by sonication in a water/acetone/detergent mixture for 30 min, followed by rinsing with deionized water and drying under a pressurized air stream prior to use. 2.2. Synthesis of Pyridine-Capped γ-Fe2O3 Nanoparticles. Fe(NO3)3·9H2O (1mmol) was added to 20 mL of deionized water, and the solution was heated to 70 °C and simultaneously purged with N2 (g). Then, 0.50 mmol of FeCl2· 4H2O was added to the solution. Immediately after, 5 mL of degassed pyridine (2.48 M) was rapidly added to the bath under vigorous stirring. The solution immediately became nonuniform, turbid, and dark brown in color, then gradually orange. The suspension was mixed for an additional 5 min to allow the reaction to complete. The final pH of the bath was ∼6. The particles were easily separated from aqueous solution by either centrifugation or magnetic decantation. The particles were washed several times with acetone and dispersed in 30 mL of pyridine by sonication without drying. The dark orange suspension can sit for weeks without flocculation.

3. RESULTS AND DISCUSSION 3.1. Characterization of Particles. A schematic describing the full, three-step deposition method is shown in Figure 1. The synthesis of γ-Fe2O3 begins with the initial formation of Fe3O4 via coprecipitation of Fe2+ and Fe3+. Almost immediately, Fe3O4 is oxidized to γ-Fe2O3, evidenced by a distinct color shift from dark brown to orange. The proposed oxidation mechanism, which involves conversion of the Fe2+ centers in Fe3O4 to Fe3+ by excess NO3−, is shown below.37,38 NO−3 + 2Fe 2 + + 3H 2O → NO−2 + 2FeOOH + 4H+ (1)

This hypothesis is supported by the fact that the final pH of the aqueous solution is far below what would be expected based B

DOI: 10.1021/acs.iecr.6b02394 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. Left: Powder X-ray diffaction data pre- and postannealing. Reference data is also included. Right: a visual correspondence of γ-Fe2O3 and α-Fe2O3 deposits.

moderate voltages to quickly deposit thick coatings of γFe2O3 onto FTO. This suggests that a significant degree of positive charge is localized on the surfaces of the particles. We propose that proton transfer from pyridinium ions to surfacebound hydroxyls on the surfaces of the γ-Fe2O3 nanoparticles during the initial formation of γ-Fe2O3 in aqueous solution causes the positive surface charge, as described in Figure 3.

on the concentration of pyridine remaining after the initial extraction of OH− from solution during the coprecipitation reaction (2Fe3+ + Fe2+ + 8OH− → Fe3O4 + 4H2O). When accounting for the additional protons generated by Reaction 1, coupled with the removal of free pyridine from solution via chemisorption to the surfaces of the γ-Fe2O3 particles, the slightly acidic pH of the solution is rationalized. Dynamic light scattering experiments were conducted by NanoComposix, Inc. (San Diego, CA) to determine the average size of the nanoparticles. The particles were found to have an average diameter of 47 ± 18 nm with a moderate size distribution (see Supporting Information, Figure S1). The formation of γ-Fe2O3 and its subsequent conversion to α-Fe2O3 was confirmed using an X-ray diffractometer (XRD, Rigaku MiniFlex 600, Cu Kα radiation). A sample of the orange precipitate was loaded onto a glass slide and air-dried. The diffraction pattern obtained from this sample, shown in Figure 2, corresponds to γ-Fe2O3 (PDF 01-076-3169). Following the initial scan, the glass slide containing the γ-Fe2O3 sample was removed from the XRD, transferred to a furnace, and annealed at 450 °C for 30 min. The sample was cooled, loaded back into the XRD, and rescanned. The diffraction pattern of the annealed, dark red product reveals a markedly different diffraction pattern and a substantially higher degree of crystallinity compared to those of the unannealed sample. As expected, the pattern was found to match that of α-Fe2O3 (PDF 00-001-1053). Images of the γ-Fe2O3 and α-Fe2O3 films are also provided in Figure 2. Although crystalline γ-Fe2O3 and α-Fe2O3 are known to have similar cubic crystal structures, there are several preliminary pieces of evidence in addition to XRD that confirm the initial formation of γ-Fe2O3, such as the strong magnetic behavior of the particles, the color of the pyridine suspension, and the observation of a very large shift in the band gap of the material following heat treatment (∼2−2.18 eV). Furthermore, there are no reports in literature of direct aqueous synthesis of α-Fe2O3 nanoparticles at low temperatures, as most reported methods require hydrothermal treatment. The annealed films are also transparent, as confirmed by transmittance spectra (see Supporting Information, Figure S2). 3.2. Electrophoretic Deposition of Iron Oxide Thin Films. 3.2.1. Proposed Mechanism for Particle Charging. Electrophoretic deposition can be carried out at low to

Figure 3. Proposed ligand binding and surface charging mechanism. X− = counteranion.

Functionalization of metal oxide surfaces by amphoteric hydroxyls is well-known.39 The remaining anions in the aqueous reaction bath (e.g., Cl−, OH−) are able to physisorb to the surface to establish electroneutrality. During EPD, the anions are pulled from the particle surface toward the counter electrode. 3.2.2. ζ-Potential Measurement. ζ-Potential analysis (conducted by NanoComposix, Inc.) was performed on the particulate dispersion in pyridine. The average ζ-potential of the γ-Fe2O3 surface was found to be approximately 39 mV (see Supporting Information, Figure S3). The ability of the particles to retain a positive surface charge in pyridine, a weak base, suggests that proton transfer from the particle surface to pyridine is either slow or unfavorable and further supports the proposed mechanism. 3.2.3. Spectrophotochemical Analysis of EPD Working Solution. UV−vis absorbance data was collected for the deposition bath (see Section 2.3) before and after a 40 s EPD (Jasco V-660 dual beam spectrophometer), as shown in Figure 4. The spectral features remain unchanged after deposition, which confirms that no chemical or structural changes occur during the process. The concentration of the colloidal dispersion decreases by 34% during this deposition C

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charging effects. The samples were then loaded vertically into a large double slotted set screw vise holder (Ted Pella Inc., #16312, Ø32 × 10 mm) to obtain cross-sectional images. SEM reveals that the films are comprised of stacked clusters of spherical particles, and a direct correlation between deposition time and film thickness is shown. This is discussed in further detail in Section 3.6. 3.4. Thin-Film Surface Analysis via Atomic Force Microscopy and Top-Down SEM. The uniformity and roughness of the surface of the α-Fe2O3 thin films was evaluated using a Bruker Multimode 8 atomic force microscope. Friction and height measurements were performed in contact mode using a silicon tip, a scan rate of 5 Hz, and a scan area of 12 × 12 μm. Images were captured using Nanoscope V software and are presented in Figure 6. The data show small deviations in both surface height and friction across the scan area, suggesting a relatively uniform surface with an appreciable roughness. Top-down SEM images were also collected to coincide with the AFM data. The images reveal that the FTO substrate is completely coated with nanoparticles as no cracks or exposed FTO are observed. Slight peaks and valleys are seen throughout the surface, consistent with the AFM results. 3.5. Evaluation of Film Adhesion. A destructive force test was used to quantify the adhesive force between the annealed α-Fe2O3 film and the substrate. To perform the test, the film was coated with an ultrastrength epoxy (Loctite, EA E-120HP), a piece of wire mesh was pressed firmly into the epoxy, and then a second coat of epoxy was added to the top of the mesh. After curing, the mesh was connected to a Pasco CI-6537 force sensor. The FTO was laid on a flat surface and held firmly in place while upward force was applied to the mesh. The minimum applied force required to break the adhesion between the film and the substrate was determined to be ∼9.6 N/cm2. Thus, the adhesion of the α-Fe2O3 thin film is excellent (see Supporting Information, Figure S4).

Figure 4. Absorbance spectra of γ-Fe2O3 working solution before and after 40 s EPD at 50 V. Dashed lines indicate values of the absorbance at 500 nm (0.350 and 0.231, respectively). Inset: tauc plot of α-Fe2O3 thin film.

interval, which can provide insight on the degree of packing within the film. Furthermore, extrapolation of the absorbance onset to the x-axis reveals that the dispersed particles have a band gap energy of ∼2 eV, in agreement with literature values for γ-Fe2O3.33,40 After the film was annealed to produce αFe2O3, the band gap was found to increase to 2.18 eV, as calculated from the Tauc plot shown in the inset in Figure 4. This value is also in agreement with literature.1 3.3. Analysis of Thin-Film Cross Sections via Scanning Electron Microscopy. SEM images (JEOL JSM-6010PLUS/LA, 8 kV AV) are shown for α-Fe2O3 films prepared with select deposition times (10, 20, 40, and 80 s) in Figure 5. Prior to being scanned, the coated FTOs were cut using an LKB-7800 Knife Maker. The cross sections of the α-Fe2O3 films were sputtered with gold using an ISI sputter-coater to minimize

Figure 5. Cross-sectional SEM images of α-Fe2O3 films electrophoretically deposited on FTO at 50 V at select time durations (top left to bottom right: 10, 20, 40, and 80 s.). D

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concentration in the deposition bath result in the plateau shown in the curve.41 The thickness−time data for the specified parameters (voltage, concentration, and electrode surface area) were fitted to an asymptotic function of the form: T = a − bc t

where T is the film thickness in micrometers, t is the deposition time in seconds, and the computed constants (a, b, and c) are fitting parameters for the curve.

4. CONCLUSIONS We demonstrated the synthesis and phase transfer of pyridinecapped γ-Fe2O3 nanoparticles and fabrication of uniform, adherent, transparent α-Fe2O3 thin films of controllable thicknesses following heat treatment at 450 °C. Because the deposition conditions are compatible with a wide variety of photocatalytic materials, incorporation of α-Fe2O3 thin films into multicomponent photocatalytic devices is quite simple. Furthermore, the proposed method utilizes low-cost precursors, low temperatures, and low to moderate voltages while offering quick deposition times and a high degree of precision for control of the film thickness.

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ASSOCIATED CONTENT

* Supporting Information

Figure 6. AFM and top-down SEM images for the α-Fe2O3 thin film obtained from 40 s electrophoretic deposition. Top left: surface friction measurement. Top right: surface height profile. Bottom: topdown SEM image.

S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02394. DLS particle size distribution curve, transmittance spectra of the annealed α-Fe2O3 film, ζ-potential distribution curve, and adhesive force test (PDF)

3.6. Evaluation of the Thickness−Time Relationship. The film thickness is easily controlled through variation of the electrophoretic deposition time, assuming that all other experimental conditions are constant. A plot of film thickness versus deposition time is shown in Figure 7. The data reveals an asymptotic dependence of film thickness on deposition time. As is expected, the deposition rate is near-linear at constant potential for short durations. At longer durations, decreases in the electric field strength along the film surface coupled with thermodynamic factors relating to the depletion of the particle

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +1-803-323-4929. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge Dr. Maria Gelabert (Winthrop University) for assistance with XRD, Dr. Clifton Calloway (Winthrop University) for assistance with the EPD apparatus, Tyra Douglas (University of Alabama) for assistance with AFM, and NIH SC-INBRE (8 P20 GM103499) and Winthrop Research Council for financial support.

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REFERENCES

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DOI: 10.1021/acs.iecr.6b02394 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX