Enhanced photoelectrocatalytic performance of α-Fe2O3 thin films by surface plasmon resonance of Au nanoparticles coupled with surface passivation by atom layer deposition of Al2O3
© Liu et al. 2015
Received: 20 August 2015
Accepted: 15 September 2015
Published: 29 September 2015
The short lifetime of photogenerated charge carriers of hematite (α-Fe2O3) thin films strongly hindered the PEC performances. Herein, α-Fe2O3 thin films with surface nanowire were synthesized by electrodeposition and post annealing method for photoelectrocatalytic (PEC) water splitting. The thickness of the α-Fe2O3 films can be precisely controlled by adjusting the duration of the electrodeposition. The Au nanoparticles (NPs) and Al2O3 shell by atom layer deposition were further introduced to modify the photoelectrodes. Different constructions were made with different deposition orders of Au and Al2O3 on Fe2O3 films. The Fe2O3-Au-Al2O3 construction shows the best PEC performance with 1.78 times enhancement by localized surface plasmon resonance (LSPR) of NPs in conjunction with surface passivation of Al2O3 shells. Numerical simulation was carried out to investigate the promotion mechanisms. The high PEC performance for Fe2O3-Au-Al2O3 construction electrode could be attributed to the Al2O3 intensified LSPR, effective surface passivation by Al2O3 coating, and the efficient charge transfer due to the Fe2O3-Au Schottky junctions.
Solar water splitting has received great attention because of the potential of the mass production of green and renewable fuel [1–3]. The visible-light-active photocatalysts is envisioned to be successful for this application . α-Fe2O3 (hematite) with low band gap (Eg = ∼2.2 eV), natural abundance, low cost, and excellent chemical stability is one of the promising metal oxide semiconductor materials for this application . It has been theoretically predicted that a semiconductor with this band gap can achieve a solar-to-hydrogen efficiency of 16.8 % . However, the reported efficiencies of α-Fe2O3 are notoriously lower than the predicted value, mainly due to the short lifetime of photogenerated charge carriers (<10 ps) [7–9], whereas the absorption depth of 2.2 eV photons (near the band gap) in hematite (118 nm) is much larger than the diffusion distance (2 ~ 4 nm) . In this regard, very thin α-Fe2O3 films should be used for facilitating the carriers transport and collection. It is significantly crucial and challenging to make the thin film electrode possess efficient absorption for effective photoelectrocatalytic (PEC) water splitting.
Surface plasmon is an efficient method to localize photon absorption at the semiconductor surface through incorporation of plasmonic metal nanoparticles on the semiconductor electrode [11, 12]. Au is an attractive plasmonic metal for PEC water splitting , which cannot only interact with the incident light in visible and infrared region but also act as an electron trap facilitating electron–hole separation by forming local Schottky junctions. The plasmon resonance frequency and intensity depends on the geometry and distribution engineering of nanoparticles and also the dielectric property of the surrounding medium.
Surface passivation of semiconductors could efficiently reduce the surface charge recombination in semiconductor technology, which is of significant importance on enhanced performances. The atomic layer deposition (ALD) is a common and easy surface modification method that has been employed in solar cells [14, 15], water splitting [16, 17], and solar fuel production [18, 19]. ALD is a stepwise and conformal coating technique with precisely controlled composition and thickness with a few nanometers. Our recent work demonstrated that 0.8 -fold enhancement of photocurrent was achieved by coating TiO2 nanotubes with Al2O3 .
Therefore, it is accordingly hopeful to construct novel structured α-Fe2O3 electrode with high solar-to-hydrogen efficiency by integrating the surface plasmon resonance and surface passivation on α-Fe2O3. In this paper, α-Fe2O3 thin films with surface nanowire were realized by an electrodeposition and post thermally annealing process. Further, Au NPs and Al2O3 thin layers were loaded on the surface of the α-Fe2O3 to explore the PEC performance of α-Fe2O3. Different constructions were achieved with different deposition orders of Au and Al2O3. Numerical simulations by finite difference time domain (FDTD) method were employed to investigate the effect of Au and Al2O3 coating on α-Fe2O3 electrodes.
Synthesis of α-Fe2O3
Fluorine-modified tin oxide (FTO)-coated glasses were immersed in isopropanol with saturated KOH solution for 24 h to remove absorbed organics, followed by ultrasonically cleaned in acetone, ethanol, and distilled water successively for 25 min. Fe films were prepared by constant current (20 mA • cm−2) electrodeposition on the FTO for 120, 180, and 360 s. The deposition solution consists of 48 g ferrous sulfate (FeSO4 · 7H2O, ≥99.0 %, Greagent), 1.2 g ascorbic acid (C6H8O6, ≥99.7 %, Greagent), 0.4 g amidosulfonic acid (H2NSO3H, ≥99.0 %, Greagent), 12 g boric acid (H3BO3, ≥99.5 %, Greagent), and 800 mL distilled water. Electrodeposition was carried out in a standard three-electrode configuration consisting of a Pt foil counter electrode, an Ag/AgCl reference electrode (saturated by 3 M KCl), and a FTO working electrode. After electrodeposition, the α-Fe2O3 films will be formed by post annealing process in the muffle furnace immediately at 150 °C for 2 h then up to 520 °C for 4 h with a heating rate of 2 °C•min-1.
Loading of Au nanoparticles
The Au films were deposited on the Fe2O3 electrodes using an ion sputtering equipment (DENTON VACUUM/ DESK V HP) with the current of 30 mA • cm−2 for different sputtered time (10, 15, 25, 35 s). The vacuum degree during ion sputtering was lower than 0.099 Torr. The Fe2O3 electrode modified with nanoparticles (NPs) was denoted as Fe2O3-Au. Then Fe2O3-Au electrode was annealed in ambient air at 300 °C for 1 h in a rapid thermal annealing furnace to form Au spheres on the surface of Fe2O3 [20, 21].
Conformal coating of Al2O3
Al2O3 shells were conformally coated onto the Fe2O3 or Fe2O3-Au by ALD processes performed with SUNALETMR-200. Al2O3 shells were deposited at 200 °C for 25 cycles using Al(CH3)3 and H2O as precursors with a growth rate of ~1 Å•cycle-1. Thus, a series of composite nanostructures based on Fe2O3 films, i.e., Fe2O3, Fe2O3-Al2O3, Fe2O3-Au, Fe2O3-Au-Al2O3, and Fe2O3-Al2O3-Au had been constructed.
The morphology and crystalline structure of the electrodes was characterized by field-emission scanning electron microscope (FESEM, Hitachi S4800) and X-ray diffractometer (XRD, Bruker D8 Discover diffractometer), respectively.
The PEC water splitting performances of the Fe2O3 based electrodes were evaluated by AUTOLAB (PGSTAT302N/FRA2) using a three-electrode setup with the Fe2O3-based films (1 cm2) as working electrode, Ag/AgCl (3 M KCl) electrode as reference electrode, and a platinum foil as counter electrode following our previous work . Used as the supporting electrolyte was 1 M KOH solution. The Al2O3 layer deposited by ALD method is stable in the KOH solution . The photocurrent was measured at an applied potential of 0.4 V vs Ag/AgCl under chopped light irradiation with a Xe lamp (PLS-SXE300UV) coupled with an AM 1.5G filter. The light was achieved irradiated to the backside of the electrodes (FTO side, backside illumination). The electrochemical impedance spectroscopy measurements were performed in dark at open circuit potential over a frequency ranging from 100000 to 0.1 Hz with amplitude of 10 mV. The Mott-Schottky plots were obtained at a fixed frequency of 1 kHz.
Results and discussion
The LSV curves of the α-Fe2O3 electrodes with different Au sputtering durations are shown in Fig. 4b. The Fe2O3-Au (15 s) shows the best PEC performance among these electrodes above 0.1 V vs Ag/AgCl. It may be resulted from the particle size and the distribution variations for the different Fe2O3-Au samples, which changes the resonance frequency and intensity of Au SPR peaks . As the nanoparticles are smaller than 3 nm, the SPR could not be motivated due to the quantum confinement effect . The SPR begins to work as the nanoparticles increase to be larger than 4 nm, and the resonance will be red shifted with the size increase of metal nanoparticles. The spectra overlap between plasmonic Au nanoparticles, and Fe2O3 may happen on the proper Au nanoparticles morphology. Additionally, the metal nanoparticles absorb photons from an area much larger than their geometric cross section . The photon absorption by Fe2O3 itself may be decreased with the excessive size increase of metal particles. According to the experimental results, 15 s was thus chosen as the sputtering time for the following Au deposition.
Le Formal et al.  demonstrated an enhancement of photocurrent in comparison with the Fe2O3 electrode by employing Al2O3 passivation coating. The Al2O3 surrounding also results in an increase of dielectric circumstance of metal nanoparticles, which could strengthen the localized electromagnetic field with tunable resonance frequency [30, 31]. Therefore, a combination of NPs and Al2O3 coating is of special interest in PEC system. For comparison, the modified Fe2O3 electrodes with both single step coating (i.e., Fe2O3-Al2O3 and Fe2O3-Au) and sequential coating (Fe2O3-Au-Al2O3 and Fe2O3-Al2O3-Au) processes are characterized. Figure 4c shows that the photocurrent of the pristine Fe2O3-Au electrodes is obviously higher than others at 0.0 V vs Ag/AgCl, which could be attributed to the increased open circuit potential by the Au modification. The hematite films are n-type photoanodes and a positive applied bias potential will increase the photocurrent generation as the Fermi level moves to assist charge separation and facilitate water splitting . The conduction band of α-Fe2O3 is located slightly below the level needed for hydrogen production, and its valence band is well-suited for oxygen production. Therefore, an external bias is typically required for water splitting when using α-Fe2O3 materials as photoanodes [26, 33]. The photocurrent of the samples increases unceasingly at a higher potential, and the photocurrent of the Fe2O3-Au-Al2O3 electrode increases to 1.78-fold compared to that of the pristine Fe2O3 at the potential of 0.4 V. The trend of photocurrent responses measured at an applied potential of 0.4 V in Fig. 4d mirrors that of the LSV plots, which follows the order of Fe2O3-Au-Al2O3 > Fe2O3-Al2O3-Au > Fe2O3-Au > Fe2O3-Al2O3 > Fe2O3.
Where e 0 is the electron charge, ε is the dielectric constant of Fe2O3 (ε = 80) [34, 35], ε 0 is the permittivity of vacuum, N d is the donor density, and V is the applied bias at the electrode. The positive slopes indicate the n-type behaviors of both pristine and modified samples. The calculated electron density of electrodes is shown in the Fig. 5b. The electron density of Fe2O3-Au-Al2O3 is 4.61 × 10 cm−3 , which is 120 times of that of pristine Fe2O3 electrode. Then, the interfacial properties between the electrolyte and electrodes are further characterized by electrochemical impedance spectroscopy (EIS) under dark condition (Fig. 5c). The arc of Nyquist plot is characteristic of charge transportation resistance. The diameter of the arc for the Fe2O3-Au-Al2O3 electrode is the smallest one, indicating that the resistance of the charge transportation is significantly decreased. The diameter of the arc for these electrodes follows the order of Fe2O3-Au-Al2O3 < Fe2O3-Al2O3-Au ≈ Fe2O3-Al2O3 < Fe2O3-Au < Fe2O3. The small arc diameter of Al2O3 coated Fe2O3-based electrode indicates the facilitated charge transportation enhanced by the decreased surface recombination from Al2O3 passivation.
The thin film α-Fe2O3 electrodes with surface nanowire were successfully obtained by electrodeposition and post thermal annealing process for PEC water-splitting application. The LSPR of Au NPs in conjunction with surface passivation by Al2O3 shells was further introduced for the enhanced PEC performance of α-Fe2O3 photoelectrodes. Among the different configurations, the Fe2O3-Au-Al2O3 construction shows the best PEC performance, attributing to the Al2O3 intensified LSPR, effective surface passivation by Al2O3 surface coating, and the rapid charge carriers transfer due to the Schottky junctions at the interface of metal and semiconductor. These results can not only contribute fundamentally to the mechanism studies of the SPR-based photocatalysis but also open a new avenue for the design strategies of high-performance photocatalysts for solar-to-fuel energy conversion.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61171043, 51377085, 21477072, 21503261) and the Project Funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.
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