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

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.


Background
Solar water splitting has received great attention because of the potential of the mass production of green and renewable fuel [1][2][3]. The visible-light-active photocatalysts is envisioned to be successful for this application [4]. α-Fe 2 O 3 (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 [5]. It has been theoretically predicted that a semiconductor with this band gap can achieve a solar-to-hydrogen efficiency of 16.8 % [6]. However, the reported efficiencies of α-Fe 2 O 3 are notoriously lower than the predicted value, mainly due to the short lifetime of photogenerated charge carriers (<10 ps) [7][8][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) [10]. In this regard, very thin α-Fe 2 O 3 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 [13], 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 TiO 2 nanotubes with Al 2 O 3 [17].
Therefore, it is accordingly hopeful to construct novel structured α-Fe 2 O 3 electrode with high solar-tohydrogen efficiency by integrating the surface plasmon resonance and surface passivation on α-Fe 2 O 3 . In this paper, α-Fe 2 O 3 thin films with surface nanowire were realized by an electrodeposition and post thermally annealing process. Further, Au NPs and Al 2 O 3 thin layers were loaded on the surface of the α-Fe 2 O 3 to explore the PEC performance of α-Fe 2 O 3 . Different constructions were achieved with different deposition orders of Au and Al 2 O 3 . Numerical simulations by finite difference time domain (FDTD) method were employed to investigate the effect of Au and Al 2 O 3 coating on α-Fe 2 O 3 electrodes.

Synthesis of α-Fe 2 O 3
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 (FeSO 4 · 7H 2 O, ≥99.0 %, Greagent), 1.2 g ascorbic acid (C 6 H 8 O 6 , ≥99.7 %, Greagent), 0.4 g amidosulfonic acid (H 2 NSO 3 H, ≥99.0 %, Greagent), 12 g boric acid (H 3 BO 3 , ≥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-Fe 2 O 3 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 Fe 2 O 3 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

Characterization
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 Fe 2 O 3 based electrodes were evaluated by AUTOLAB (PGSTAT302N/FRA2) using a three-electrode setup with the Fe 2 O 3 -based films (1 cm 2 ) as working electrode, Ag/AgCl (3 M KCl) electrode as reference electrode, and a platinum foil as counter electrode following our previous work [22]. Used as the supporting electrolyte was 1 M KOH solution. The Al 2 O 3 layer deposited by ALD method is stable in the KOH solution [23]. 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.  Figure 2 shows the top and cross-sectional view of α-Fe 2 O 3 on the FTO. The thickness of α-Fe 2 O 3 film was increased from 180 to 270 nm with electrodeposition time increase from 120 to 360 s (Fig. 2b, c). The Fe film formation by electrodeposition is a chemical equilibrium between the chemical dissolution of Fe in the acid deposition solution and the deposition of Fe on the FTO, which is accomplished only as the deposition rate is higher than the surface dissolution rate [24]. The nanowire arrays with diameter of~40 nm were found to exist uniformly on the surface of oxide thin film. The formation of surface nanowires is ascribed to the vapor-solid oxidation approach on Fe films, which has been reported to be a useful method for vertical growth of α-Fe 2 O 3 nanowires or nanorods [25]. Figure 3 shows the surface morphology of Fe 2 O 3 -Au electrodes with different Au sputtering time. It was found that the diameter of Au NPs increases with the sputtering time. The Au NPs sputtered for 10 s are not homogeneous distributed with diameter ranging from 2 to 20 nm (see Fig. 3a). The sample with Au sputtered for 15 s shows the best homogeneous distribution of NPs with diameter of 10 ± 1 nm (see Fig. 3b). Some NPs on the nanowires can also be observed. The Fe 2 O 3 -Au with Au sputtered for 25 s shows the Au diameters with the range of 7-18 nm, while the diameter of sample with Au sputtered for 35 s is up to 40 nm which can be clearly observed in Fig. 3d.

Results and discussion
The linear sweeps voltammetry (LSV) curves of the pristine Fe 2 O 3 electrodes with varying deposition durations are shown in Fig. 4a. The photocurrent of the Fe 2 O 3 electrode with deposition time for 180 s is the highest of all above −0.15 V vs Ag/AgCl. When the deposition time is 120 s, the thickness of film is~180 nm (seen in Fig. 2b) which is too thin to absorb sufficient light. As the deposition time increases to 360 s, the  photocurrent is even lower, as seen from the Fig. 4a. The short hole diffusion length (∼2-4 nm) [9] allows only holes created close to the electrolyte interface to oxidize water. Since the light penetration length in α-Fe 2 O 3 is of the order of 100 nm [26], most holes created in the bulk will recombine with electrons before reaching the surface as the thickness of Fe 2 O 3 film increases to 270 nm (see Fig. 3d). In this regard, the following α-Fe 2 O 3 photoelectrodes are studied based on 180 s deposition unless otherwise stated.
The LSV curves of the α-Fe 2 O 3 electrodes with different Au sputtering durations are shown in Fig. 4b. The   2 O 3 -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 Fe 2 O 3 -Au samples, which changes the resonance frequency and intensity of Au SPR peaks [27]. As the nanoparticles are smaller than 3 nm, the SPR could not be motivated due to the quantum confinement effect [28]. 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 Fe 2 O 3 may happen on the proper Au nanoparticles morphology. Additionally, the metal nanoparticles absorb photons from an area much larger than their geometric cross section [29]. The photon absorption by Fe 2 O 3 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. [23] demonstrated an enhancement of photocurrent in comparison with the Fe 2 O 3 electrode by employing Al 2 O 3 passivation coating. The Al 2 O 3 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  Figure 4c shows that the photocurrent of the pristine Fe 2 O 3 -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 [32]. The conduction band of α-Fe 2 O 3 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-Fe 2 O 3 materials as photoanodes [26,33]. The photocurrent of the samples increases unceasingly at a higher potential, and the photocurrent of the The Mott-Schottky and Nyquist plots are used to determine the carrier density, capacitance, and impedance of the electrodes, and the results are shown in Fig. 5. Carrier density can be calculated from the slope of Mott-Schottky plots. The equation is shown as follows: Where e 0 is the electron charge, ε is the dielectric constant of Fe 2 O 3 (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 ntype behaviors of both pristine and modified samples. The calculated electron density of electrodes is shown in the Fig. 5b. The electron density of Fe 2 O 3 -Au-Al 2 O 3 is 4.61 × 10 cm −3 [17], which is 120 times of that of pristine Fe 2 O 3 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 Fe 2 O 3 -Au-Al 2 O 3 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 The FDTD simulations were performed to calculate electric field distribution across the interfaces in different electrodes under 574 nm as shown in Fig. 6. The color index represents the magnitude of electric field intensity normalized with that of the light propagating in free space. The electric field intensity of pristine Fe 2 O 3 and Fe 2 O 3 -Al 2 O 3 electrodes is very weak under 574 nm, and there is no change in color on the interface (not shown here). As coupling the Au plasmonic structure and Al 2 O 3 coating on Fe 2 O 3 electrode, high-electric field intensity can be always found in the area where the Au is in contact with semiconductor and Al 2 O 3 . It is generally accepted that there are three non-mutually exclusive energy-transfer mechanisms in plasmonic-photocatalyst systems, involving the SPR-induced charge injection from metal to semiconductor, near-field electromagnetic, and scattering mechanism [29]. The scattering effect can be safely ruled out in our study because it usually occurs in plasmonic metal nanostructures with the diameter larger than 50 nm. SPR-induced charge injection and near-field electromagnetic mechanism may co-contribute on our Fe 2 O 3 -Au systems. In near-field electromagnetic mechanism, the excited Au increases the intensity of local electric field, which will penetrate into Fe 2 O 3 and amplify the local light intensity. For the Fe 2 O 3 -Al 2 O 3 -Au electrode, the intensified electromagnetic field is largely blocked by the Al 2 O 3 spacer layer. This suggests that the near-field electromagnetic enhancement mechanism can hardly contribute the enhanced PEC performance with the presence of dielectric spacer layer, which is in accordance with our previous work [16]. Despite this, the Fe 2 O 3 -Al 2 O 3 -Au electrode still shows the higher photocurrent than Fe 2 O 3 -Al 2 O 3 electrode, which could benefit from the SPR-mediated hot-electron injection process. The Fe 2 O 3 -Au-Al 2 O 3 electrode shows the strongest LSPR electric field, biggest radiation areas, and deepest penetration depth into the Fe 2 O 3 (Fig. 6c) , as compared with the other configurations. This may be mainly resulted from the fact that the Al 2 O 3 coating increases the refractive index of the surrounding medium from 1.33 (in water) to 1.76 (in Al 2 O 3 ) [36,37], which could intensify the plasmon resonance. Both near-field electromagnetic effect and SPRinduced charge injection from metal to semiconductor  could contribute the intensified electric field and the enhanced PEC performances. The rate of electron-hole formation is proportional to the local intensity of the electric field and radiation areas [38]. It means that more pairs of electron-hole are generated in the Fe 2 O 3 semiconductor, which is in accordance with the much increased ND value in the Mott-Schottky analysis (see Fig. 5). Consequently, most of the photogenerated charges created by the plasmon excitation contribute to the surface catalysis for water splitting. From the perspective of the SPR-mediated hotelectron injection mechanism, the intensified electromagnetic field will facilitate more energetic electrons on Au nanoparticles. Electrons photoexcited by the Au NPs will pass over the Schottky barrier and migrate to the conduction band of Fe 2 O 3 . Schottky barrier at the interface also helps the transferred hot electrons accumulate in the Fe 2 O 3 conduction band, preventing them from traveling back to the Au NPs.

Conclusions
The thin film α-Fe 2 O 3 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 Al 2 O 3 shells was further introduced for the enhanced PEC performance of α-Fe 2 O 3 photoelectrodes. Among the different configurations, the Fe 2 O 3 -Au-Al 2 O 3 construction shows the best PEC performance, attributing to the Al 2 O 3 intensified LSPR, effective surface passivation by Al 2 O 3 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.