Article pubs.acs.org/JPCC
Fine-Tuning Pulse Reverse Electrodeposition for Enhanced Photoelectrochemical Water Oxidation Performance of α‑Fe2O3 Photoanodes Pravin S. Shinde,†,‡ Alagappan Annamalai,† Jae Young Kim,‡ Sun Hee Choi,§ Jae Sung Lee,*,‡ and Jum Suk Jang*,† †
Division of Biotechnology, Advanced Institute of Environmental and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University (Iksan Campus), Iksan 570-752, Republic of Korea ‡ School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 689-798, Republic of Korea § Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea S Supporting Information *
ABSTRACT: High-quality hematite (α-Fe2O3) photoanodes were synthesized from a sulfate electrolyte bath by the pulse reverse electrodeposition (PRED) method. The influence of PRED parameters (viz. duty cycle, pulse period, and deposition time) was systematically investigated on the structural, optical, morphological, and photoelectrochemical properties of the films. The optimized parameters of pulse duty cycle, pulse period, and the deposition time were 20%, 10 ms, and 45 s, respectively. The granular and compact nanocrystalline morphology of the α-Fe2O3 was found to alter according to the process parameters. The α-Fe2O3 electrodes (film thickness ∼200 nm) prepared by annealing at 550 °C for 4 h followed by 800 °C for 15 min exhibited an optimum photocurrent density of 504 μA cm−2 measured at 1.23 V vs RHE in 1 M NaOH electrolyte under 100 mW cm−2 light illumination.
1. INTRODUCTION Hematite (α-Fe2O3) has emerged as a promising photocatalyst for solar water splitting due to its favorable optical band gap (∼2.2 eV), excellent chemical stability in a wide pH range (pH > 3), earth abundance, and low cost.1−10 The band gap gives a theoretical upper limit of solar-to-hydrogen efficiency of 16.8% and a photocurrent of 12.6 mA cm−2 under the standard AM 1.5G 1 sun illumination.4 Therefore, enormous efforts have been focused on improving the performance of α-Fe2O3 photoanodes in photoelectrochemical (PEC) water splitting by employing various strategies, such as nanostructure engineering via different synthesis methods, doping, or forming a heterojunction.11 The preparation of α-Fe2O3 nanowires,6,12 nanotubes,13 nanopetals,11 nanonets,14 and dendritic nanocauliflowers15 are examples of successful attempts that achieved improvement in the light harvesting and charge carrier separation to enhance the PEC performance. Several physical and chemical methods have been developed to obtain such nanostructures of α-Fe2O3. Among these, pulse reverse electrodeposition (PRED)11 has been relatively less employed despite having several potential advantages as it serves a potential strategy to tailor both the chemical composition and the structure of materials including metallic films and alloys. It perturbs the electrocrystallization process to change the © XXXX American Chemical Society
morphology, texture, and properties of the electrodeposits. In PRED, a periodic waveform consisting of a cathodic pulse and anodic pulse is applied to deposit the desired material. The mechanism of PRED is simply based on the adsorption or desorption phenomena at the electrode−electrolyte interface. The nucleation and growth of the deposited material depends on the adsorption−desorption rate of the species in the electrolyte, controlled by PRED. Although a simple method, PRED requires careful attention for optimization of the parameters such as cathodic current density, anodic current density, duty cycle, pulse period, and the deposition time. It can be employed to obtain nanostructures with greater control over the grain size and hardness through variation of such process parameters. The schematic of potential pulse reverse waveform for electrodeposition of iron oxide is shown in Scheme 1. The use of PRED for synthesizing α-Fe2O3 electrodes is rather a new approach, and the influence of the PRED process parameters for the performance improvement has not yet been investigated. In a previous communication,11 α-Fe2O3 photoanodes were prepared by the PRED method using fixed Received: October 3, 2014 Revised: February 18, 2015
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DOI: 10.1021/jp5100186 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Scheme 1. Potential Pulse Reverse Waveform for Electrodeposition of Iron (Oxide)
parameter condition on a large area substrate (1 × 5 cm2). The photocurrent per unit area for smaller areas (≤1 cm2) was not reported, although it has become a common practice in the materials research field. It is well-known that the increase in substrate area, especially for FTO, causes an increase in the total sheet resistance. The nucleation and growth rate of the electrodeposits are also affected by such increased internal resistance, thereby affecting the overall photocurrent density of the photoanodes. Moreover, the photoactivity of such photoanodes was investigated only for low temperature calcination (500 °C), and the study of photoactivity at high-temperature annealing conditions has not been investigated previously. In this work, for first time, we addressed the influence of the pulse reverse electrodeposition parameters such as pulse time (with fixed pulse voltage and fixed duty cycle) and film deposition time on the evolution of nanostructured α-Fe2O3 morphology and its resulting photoelectrochemical performance. To the best of our knowledge, characterization of the α-Fe2O3 material fabricated by tuning the PRED conditions has not been previously reported.
color and reflected the light. Films were rinsed several times in deionized water and dried using nitrogen stream. The two-step annealing method was followed to obtain highly photoactive photoanodes.16 The as-grown films were heated to 550 °C at the rate of 5 °C min−1 and baked there for 4 h. Finally, the films were annealed at 800 °C briefly for 15 min and subsequently transferred to a forced-convection oven preheated at 100 °C for 10 min to avoid breaking of the electrodes and the supporting ceramic crucibles. The sintered films appeared reddish or dark brown in color, depending on the film thickness. 2.2. PEC Measurements of α-Fe2O3 films. The photoactivity of the synthesized α-Fe2O3 photoanodes was measured in a PEC cell consisting of a three-arm glass compartment with a circular quartz window for light illumination. The PEC cell comprised Fe2O3/FTO photoanode as the working electrode, Pt wire as the counter electrode, Ag/AgCl (saturated with KCl) as the reference electrode, and 1 M NaOH as an electrolyte. All potentials were measured relative to the Ag/AgCl electrode and were converted to the reversible hydrogen electrode (RHE) scale using the Nernst eq 1.15 ° VRHE = VAg/AgCl + 0.059pH + V Ag/AgCl
2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. Pulse reverse electrodeposition was carried out in a two-electrode configuration cell. Transparent conducting glass (fluorine doped tin oxide, FTO, 10−15 Ω cm−1) substrate was used as a working electrode, and convoluted platinum wire as a counter electrode. The distance between the two electrodes was ∼1 cm. Prior to deposition of iron oxide films, FTO substrates (1 cm × 2.5 cm) were successively cleaned by ultrasonic degreasing in acetone, ethanol, and deionized water (each for 10 min) then dried using a nitrogen stream. The electrolyte consisted of 6 g of ferrous sulfate, 0.15 g of ascorbic acid, 0.05 g of amidosulfonic acid, and 1.5 g of boric acid in 0.1 L of deionized water (pH = 5.71). The pH of the resulting electrolyte solution was 2.6. The chemicals were purchased from Alfa Aesar and Kanto Chemicals and were used without further treatment. Potential control was provided by means of a function generator. The PRED of Fe2O3 films was carried out by keeping the amplitude (peak-to-peak voltage) of the square-wave pulse fixed at 10 V (−6/+4 V). The other deposition parameters such as duty cycle (20%), pulse period (frequency), and the total deposition time were varied systematically. At least three electrodes of each condition were prepared. As-grown films were dark grayish in
(1)
where VRHE was the converted potential vs RHE, VAg/AgCl ° = 0.1976 V at 25 °C, and VAg/AgCl was the experimental potential against the Ag/AgCl electrode. The simulated light illumination of 1 sun (100 mW cm−2) was provided using a solar simulator (Abet Technologies). The photocurrent−voltage measurement, electrochemical impedance spectroscopy (EIS), and Mott− Schottky studies were performed using a portable potentiostat (Ivium, Netherland) equipped with an electrochemical interface and impedance analyzer facility. The EIS study was performed under the 1 sun illumination at 0.235 V vs Ag/AgCl (i.e., 1.23 VRHE). Mott−Schottky measurement was performed under dark condition with a dc applied potential window of −0.6 to 0.4 V vs Ag/AgCl at the ac frequency of 0.5 kHz. The amplitude of the ac potential was 10 mV for both EIS and Mott−Schottky measurements. The experimental EIS data were validated using the Kramers−Kronig transform test and then fitted to a suitable equivalent circuit model using the ZView (Scribner Associates Inc.) program. 2.3. Characterization. Structural analysis was performed using a PANalytical X’pert Pro MPD diffractometer equipped with a Cu Kα radiation source (wavelength Kα1 = 1.540 598 Å and Kα2 = 1.544 426 Å) operated at 40 kV and 30 mA. The XB
DOI: 10.1021/jp5100186 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
0.1, 1, 10, 100, 1000, and 5000 ms. Figure 1a shows the current density−voltage (J−V) curves of α-Fe2O3 photoanodes
ray diffraction patterns were recorded in the range of 20°−80° with a step size of 0.0167° s−1 and a dwell time of 50.165 s. The phase determination was done using PANalytical X’pert Highscore Plus software with the help of standard diffraction databases (ICDD). The chemical state and elemental quantification in the freshly synthesized iron oxide samples was performed using X-ray photoelectron spectroscopy (XPS). XPS analysis was performed on a Thermo Scientific XPS spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV). Wide survey spectra (binding energy, BE: 1200−0 eV) were recorded for the samples using the X-ray spot size of 400 μm at room temperature with an analyzer pass energy of 200 eV and energy step size of 1 eV. High-resolution spectra in the region of interest were acquired at the pass energy of 50 eV and step size of 0.1 eV. The calibration of the binding energy of the spectra was made by referring the maximum of the adventitious C 1s carbon peak at 284.8 eV. The processing of XPS data involving the deconvolutions of peaks with Shirley background was performed with the help of an XPS Peak-fit program. The spectra were decomposed with a least-squares fitting routine provided within the software. A symmetric Gaussian−Lorentzian product function was used to approximate the line shapes of the fitting components. Raw experimental data were used for analysis with no preliminary smoothing. The electronic and geometrical local structures were examined by X-ray absorption fine structure (XAFS). The X-ray absorption spectra for Fe K-edges (E0 = 7112 eV) were taken in a fluorescence mode on 7D beamline of Pohang Accelerator Laboratory (PLS-II, 3.0 GeV). The incident beam was detuned by 30% in order to minimize the higher harmonics from Si(111) crystals of double crystal monochromators. The incident beam was monitored with a He-filled IC spec ionization chamber, and the fluorescence signal from the sample was measured with a PIPS (passivated implanted planar silicon) detector attached to a He-flowing sample chamber. The obtained data were analyzed with Athena in the Ifeffit suite of software programs.17 The morphology of the α-Fe2O3 films was examined on a field emission scanning electron microscope (FESEM) (SUPRA 40VP, Carl Zeiss, Germany) equipped with an X-ray energy dispersive spectrometer (EDX). Prior to FESEM examination, the α-Fe2O3 electrodes were sputtercoated with osmium. The ImageJ program was used to analyze the FESEM images for measurement of the lateral dimensions of the grains. UV−vis absorption study in the wavelength range of 350−800 nm was performed using a dual-beam spectrophotometer (Shimadzu, UV-2600 series). The absorbance spectra were recorded with a baseline correction using FTO substrate as a reference.
Figure 1. (a) Current density−voltage (J−V) curves of α-Fe2O3 photoanodes prepared using different pulse periods. (b) Photocurrent density measured at 1.23 VRHE for the different pulse periods.
prepared by two-step annealing process with the different pulse periods. The optimum photocurrent density (Jph) of 474 μA cm−2 at 1.23 VRHE was observed for the pulse period of 10 ms. Thus, 10 ms was chosen as the optimized value of pulse period and was kept constant during further experiments for optimization of the process parameters. The variation of Jph measured at 1.23 VRHE for different pulse periods is shown in Figure 1b. The α-Fe2O3 photoanode synthesized with deposition time of 30 s and annealed at low temperature (550 °C for 4 h) also demonstrated photoactivity with Jph of 116 μA cm −2 at 1.23 V RHE (Figure S1, Supporting Information). Next, the pulse voltage (−6/+4 V), pulse period (10 ms), and film deposition time (30 s) were kept constant while the duty cycle was varied. The duty cycle variation was limited because the films were inhomogeneous, the film quality was poor, and the resulting photocurrent was also poor except for the 20% duty cycle. The α-Fe2O3 has a short hole diffusion length (2−4 nm) and a light penetration depth (1/α) of around 118 nm at the wavelength of 550 nm. To ensure optimum absorption (>90%) of the incident light, the film thickness must be greater than 2.3 times the value of 1/α.18 The films should have an enough thickness that would offer sufficient amounts of light absorption, but not too thick that would hold electrons too long in the film and expose them to charge recombination. Hence, the film thickness effect on photocurrent response was studied by varying the PRED time. The pulse period (10 ms), the duty cycle (20%), and the pulse voltage (−6/+4 V) were kept constant during PRED time variation. The thinner films of α-Fe2O3 (with PRED time
