Precise control of TiO₂ overlayer on hematite nanorod arrays by ALD for the photoelectrochemical water splitting

Abstract

The short lifetime of electron–hole pairs and high electron–hole recombination rate at surface states significantly limit the practical applications of hematite (α-Fe₂O₃) photoanodes in photoelectrochemical (PEC) water splitting. Surface modification with a TiO₂ overlayer has been demonstrated to be an efficient way to improve the PEC performance. However, a fine control of the TiO₂ overlayer and a deep understanding of the impact of the TiO₂ overlayer with variable thickness on the PEC performance, to the best of our knowledge, has yet to be done. Here, a conformal ultrathin TiO₂ overlayer is successfully deposited on hydrothermal grown one-dimensional hematite nanorod arrays by atomic layer deposition. The morphology and thickness of the TiO₂ overlayer can be precisely controlled. The effect of the thickness of the TiO₂ overlayer on the overall water splitting efficiency of hematite photoanodes under visible and UV light has been systematically investigated. The charge excitation and transfer mechanism at the semiconductor–electrolyte interface has also been studied.

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1. Introduction

To solve the energy crisis, it has become urgent to develop novel technologies that can efficiently utilize solar energy and convert sunlight into chemical fuels.^1,2 Photoelectrochemical (PEC) water splitting using semiconductors has been extensively studied for solar hydrogen generation.^3–5 Hematite (α-Fe₂O₃) is a promising n-type photoanode material because of its favorable optical band gap (2.0–2.2 eV), chemical stability, earth abundance and low cost.^6,7 One-dimensional (1D) hematite nanostructures such as nanorods, nanowires, nanosheets and nanotubes can enhance charge collection efficiency in PEC water splitting due to their unique morphology and large surface to volume ratio.^8–11 Nevertheless, the water splitting efficiency of hematite is severely limited by the very short lifetime of electron–hole pairs, short hole diffusion length (2–4 nm), poor charge carrier mobility and the high charge recombination rate at surface states.^12,13

Numerous efforts have been made to overcome these issues and improve the PEC efficiency of hematite, such as element doping, constructing heterojunction and surface modification.^14–17 Among these strategies, surface modification of semiconductor photoanodes has proven to be an effective strategy to improve the charge separation and enhance the efficiency of water oxidation of hematite photoanodes. TiO₂ has attracted considerable interest as a photoanode for water splitting due to its good chemical stability, outstanding optoelectronic properties and nontoxicity.^18–20 Recently, many studies have focused on the modification of TiO₂ overlayer on hematite for PEC water splitting applications. For instance, Yang et al. showed that an ultrathin TiO₂ layer grown on hematite could lead to better charge separation and a 100 mV cathodic shift of the turn on potential because of the enhanced charge separation.¹² Feng et al. displayed an effective strategy in promoting the charge transfer by decorating TiO₂ at the grain boundaries of hematite.²¹ However, these reports mainly demonstrated an improved PEC performance of hematite photoanodes by surface treatment with TiO₂. The precise control over the morphology and thickness of the TiO₂ overlayer remains challenging. Furthermore, the impact of the thickness of the TiO₂ overlayer on the PEC performance and the charge transfer mechanisms at the semiconductor–electrolyte interface, to the best of our knowledge, have yet to be investigated.

A TiO₂ overlayer with optimized thickness can increase electronic conductivity and suppress surface charge carrier recombination.^18,19,22 So far, a variety of techniques have been used to deposit a TiO₂ overlayer, including spin coating, atomic layer deposition (ALD), sputtering, chemical bath deposition and dip coating.^21,23–26 ALD is a thin film deposition technique based on self-terminating surface reactions, which allows the conformal and homogeneous coating on high aspect ratio substrates, with precise control of thin film thickness at the Ångström scale.^27–29 Progress has already been made in the formation of homogeneous TiO₂ overlayer on hematite by ALD.^27,30,31 Nevertheless, achieving a conformal, pinhole-free and high quality ultrathin TiO₂ film is still challenging due to limited understanding of the TiO₂ growth at early stages.

In this work, TiO₂ continuous ultrathin films with variable thickness are successfully deposited onto 1D hematite nanorod arrays by ALD. The morphology and the thickness of the TiO₂ overlayer can be accurately controlled. Fe₂O₃ can be excited to generate electron–hole charge carriers under both visible and UV light owing to its low band gap, while TiO₂ can only be excited under UV light because of its large band gap (3.2 eV for anatase).^32,33 To understand the role of a TiO₂ overlayer and the underlying charge transfer mechanism in PEC water splitting, the PEC performances of as-synthesized TiO₂/Fe₂O₃ under visible and UV light as a function of the TiO₂ thickness were systematically investigated. The morphology and thickness of the resulting films were characterized by high-resolution transmission electron microscopy (HRTEM). The energy level alignment was analyzed by X-ray photoelectron spectroscopy (XPS). This work provides a promising strategy to elucidate the role of a TiO₂ overlayer and the charge transfer mechanisms in PEC water splitting of hematite, which can be further applied to other nanostructured hematite photoanodes.

2. Experimental section

2.1 Materials

Titanium tetrachloride (TiCl₄, 99%) was purchased from ABCR GmbH. Ozone (O₃) was provided by a BMT803N ozone delivery system using pure oxygen at a pressure of 0.5 bar, and nitrogen (99.99%) was purchased from Air Liquide. Iron(III) chloride hexahydrate (FeCl₃·6H₂O) and urea (CH₄N₂O) were purchased from Carl Roth and Sigma-Aldrich, respectively. VWR GmbH supplied absolute ethanol and acetone for synthesis. Water with a resistivity of 18.2 MΩ cm⁻¹ was used in all syntheses. 1 M KOH standard aqueous solution (pH = 13.6) was supplied by Carl Roth. All other chemicals and reagents were of analytical grades. SnO₂ : F transparent conducting glass (FTO, TEC8, thickness 2.2 mm, resistance 6.70 ± 0.27 Ω square⁻¹) was purchased from Ossila. Silicon wafers (B014002) were provided by Siegert wafer GmbH and used after cleaning with piranha solution.

2.2 Preparation of Fe₂O₃ nanorods

Hematite (α-Fe₂O₃) nanorod arrays were grown on FTO substrates through an easy hydrothermal method followed by an annealing procedure in air flow. Prior to the synthesis, an accurate cleaning of the FTO-glass is required, including ultrasonication with absolute acetone, absolute ethanol, and ultrapure water (MilliQ) for 10 minutes, respectively. 1.8 mmol FeCl₃·6H₂O and 4.2 mmol CH₄N₂O were dissolved in 15 mL of water and stirred at room temperature for 20 min. Subsequently, the solution was transferred into a Teflon-lined stainless-steel autoclave with the FTO substrates placed with the conductive side facing down and then heated at 100 °C in the oven for 12 h. The product of β-FeOOH forming a uniform yellow film on the FTO substrates was thoroughly washed with absolute ethanol and water and dried with nitrogen gas. After a two-step annealing treatment of β-FeOOH in a muffle furnace (250 °C for 30 min and 500 °C for 30 min with a heating rate of 10 °C min⁻¹), α-Fe₂O₃, hematite samples were obtained.

2.3 Preparation of TiO₂/Fe₂O₃ heterostructures

TiO₂ was deposited on hematite and Si-wafers using a PLAY 2018-01 (CTECHnano) thermal ALD system. Before starting the ALD process, the ALD system was evacuated, and the temperature of the ALD chamber was well stabilized. The baseline pressure was maintained at 7.3 × 10⁻¹ mbar with a 40 sccm of nitrogen flow. TiCl₄ and H₂O were used as the metal precursor and the oxygen source, respectively. Both the precursors were kept in stainless steel canisters at room temperature. The temperatures of the manifold and the ALD chamber were maintained at 100 °C and 120 °C, respectively. TiCl₄ and H₂O were introduced into the ALD chamber in a sequence using nitrogen as purging and carrier gas. Typically, pulse time, exposure time and purge time are 0.5 s, 50 s and 30 s for TiCl₄ and 0.15 s, 40 s and 30 s for H₂O, respectively. The samples were deposited with 10, 20, 40, 80 and 150 ALD cycles and named as TiO₂-10/Fe₂O₃, TiO₂-20/Fe₂O₃, TiO₂-40/Fe₂O₃, TiO₂-80/Fe₂O₃, and TiO₂-150/Fe₂O₃, respectively. After TiO₂ coating, the hematite samples were annealed at 450 °C for 2 h in air.

2.4 Photoelectrochemical measurements

PEC measurements were carried out on a Bio-Logic VMP3 potentiostat in a homemade three-electrode electrochemical system with a 0.385 cm² working area. Platinum sheet (1 × 1 cm²) and Hg/HgO (1.0 mol L⁻¹ NaOH) were employed as counter and reference electrodes, respectively. 1 M KOH (pH = 13.6) was used as electrolyte. The electrolyte solution was degassed with Ar flow for 20 min before the PEC measurement to remove the dissolved oxygen. LEDs nominal wavelengths (λ = 430 nm, M430L4 Spectrum) and (λ = 365 nm M365L4 Spectrum) were used as the light source. The light power density for LED illumination under 430 nm and 365 nm were 57 mW cm⁻² and 26 mW cm⁻², respectively. The LEDs power was measured with a Compact Power and Energy Meter (PM100D, ThorLabs). Linear sweep voltammetry (LSV) curves were measured in the potential window −0.1 V to 0.6 V bias versus Hg/HgO with a scan rate of 10 mVs⁻¹. Electrochemical impedance spectroscopy (EIS) measurements were carried out in a frequency range from 100 kHz to 0.1 Hz with an amplitude of 5 mV.

2.5 Characterizations

Spectroscopic ellipsometry (SENpro Sentech) was utilized to measure the relative thickness of the TiO₂ thin film on Si-wafers with a wavelength range from 370 to 1000 nm under a 70° incidence angle. Field-emission scanning electron microscopy (FESEM) was performed with Zeiss SUPRA 40 instruments. All micrographs were acquired at a 5 kV accelerating voltage. Transmission electron microscopy (TEM) was performed on a Philips CM 200 equipped with a LaB₆ filament. High-resolution transmission electron microscopy (HR-TEM), high-angle annular dark field scanning transmission electron microscopy and energy dispersive X-ray analysis elemental mapping were performed on a FEI Talos F200S scanning/transmission electron microscope operated at 200 kV. Raman spectrometer (Thermo Scientific) equipped with an ANDOR CCD camera and excited by a solid-state laser operating at 2.33 eV (532 nm) was used to measure Raman scattering. The spectra were recorded in the range 100–800 cm⁻¹ with an acquisition of 16 s. Before measurement, the Raman spectra were calibrated using the 520 cm⁻¹ phonon mode of Si. UV-vis-NIR diffuse reflectance spectra were measured with a PerkinElmer Lambda 1050+ UV-vis-NIR spectrophotometer in the range of 200–800 nm. XPS measurements were performed using a JEOL JPS-9030 setup with a base pressure of 2 × 10⁻⁹ mbar, employing the K_α-radiation of a non-monochromated Al X-ray source (hν = 1486.6 eV) for excitation and a hemispherical analyzer to detect the kinetic energy of the emitted electrons. A pass energy of 50 eV was used for the survey scans and a pass energy of 10 eV for the narrow scans. The binding energy scale was calibrated by measuring a sputter-cleaned polycrystalline gold foil and setting the Au 4f_7/2 peak to 84.00 eV. The core level spectra were fitted with CasaXPS, using Voigt-functions.³⁴

3. Results and discussions

3.1 Structural characterizations

Fig. 1a represents the synthesis procedure of ALD-grown TiO₂ film on hematite nanorods (TiO₂/α-Fe₂O₃) with FTO glass as a substrate. In detail, vertically aligned β-FeOOH nanorods were prepared by hydrothermal method and subsequently converted to hematite nanorods via post annealing treatment. Afterward, the TiO₂ film with various thicknesses was deposited on hematite nanorods by ALD.

Fig. 1 (a) Schematic view of the synthesis process for TiO₂/α-Fe₂O₃ hierarchical nanostructure. (b) SEM images of pristine hematite nanorods, (c) SEM images of 40ALD cycles of TiO₂ deposited on pristine hematite, (d) Raman-active modes of pristine hematite and TiO₂ deposited on hematite.

The phase composition and structure of β-FeOOH and α-Fe₂O₃ were characterized by XRD (Fig. S1, ESI†). The diffraction peak at 35.2° of 2θ can be assigned to the (211) reflection of tetragonal FeOOH (JCPDS 34-1266, red line). The diffraction peaks located at 35.9° and 64.5° can be ascribed to the (110) and (300) reflections of α-Fe₂O₃, respectively (COD 96-901-5504, black line). In addition, the reflections located at 26.6°, 33.9°, 38.0°, 51.8°, 61.9° and 66.0° and marked with asterisks correspond to the (110), (101), (200), (211), (130) and (301) reflections of the cassiterite phase of SnO₂, respectively. Both samples show the characteristic signals of SnO₂, which come from the FTO substrate.

The morphology and microstructure of β-FeOOH and hematite nanorods were thoroughly investigated by TEM (Fig. S2, ESI†). The typical solid β-FeOOH nanorods are uniform, with diameters ranging from 40 to 80 nm (Fig. S2a and b, ESI†). The main rings in the selected area electron diffraction (SAED) pattern correspond to the (200), (310) and (211) planes for tetragonal FeOOH (Fig. S2c, ESI†). Fig. S2d and e of the ESI† illustrate TEM images of pristine hematite nanorods. The concentric rings in the SAED corresponding to the (102), (104), (110) and (113) planes can be indexed to hematite, indicating the polycrystalline structure of Fe₂O₃ (Fig. S2f, ESI†).

The morphology and homogeneity of the pristine and TiO₂-coated hematite samples were examined by FESEM. Fig. 1b displays that the pristine hematite nanorods are vertically aligned on the FTO. The diameter of the nanorods is around 60 to 80 nm. The SEM images of TiO₂-40/Fe₂O₃ demonstrate a well-preserved hematite morphology (Fig. 1c). However, it is difficult to distinguish the TiO₂ film because of its low thickness. The composition of the hematite before and after ALD-TiO₂ coating was confirmed by Raman spectra (Fig. 1d). The Raman peaks at 220, 241, 286, 404, 494 and 606 cm⁻¹ are assigned to the hematite structure. The LO peak center at 658 cm⁻¹ is detected, which is attributed to the disorder-induced breaking of Raman symmetry properties.^14,35 After TiO₂ deposition, an intense Raman peak located at 144 cm⁻¹ is observed, which can be indexed to the E_g mode of anatase TiO₂.¹⁹ The Raman peaks of the Fe₂O₃ nanorods are not affected by ALD of TiO₂.

The HRTEM images of TiO₂-coated Fe₂O₃ nanorods with a different number of ALD cycles (from 10 to 150 cycles) is shown in Fig. S3, ESI.† At 10 ALD cycles, a continuous TiO₂ film coverage cannot be clearly observed due to the small amount of TiO₂ coating (Fig. S3a, ESI†). After 20 ALD cycles, Fe₂O₃ nanorods are homogeneously and uniformly coated with an ultrathin TiO₂ film (Fig. S3b–e, ESI,† red dotted lines). The thicknesses of the TiO₂ on Fe₂O₃ nanorods increase with the further increase of the number of ALD cycles. Fig. S3f of ESI† shows the lattice fringes of TiO₂-150/Fe₂O₃ match well with the interplanar distance of the TiO₂ anatase structure. The calculated lattice spacing of 0.35 nm can be assigned to the interplanar distance of the (011) plane of TiO₂.

The HAADF-STEM images and the corresponding elemental mappings for all the TiO₂-coated hematite samples are shown in Fig. 2. Fig. 2a shows small TiO₂ nuclei randomly deposited on hematite nanorods after 10 ALD cycles. As the deposition proceeds, small nuclei grow up, and small nanocrystallites coalesce together to form a very thin, continuous TiO₂ layer. The HAADF-STEM images of TiO₂-20/Fe₂O₃, TiO₂-40/Fe₂O₃, TiO₂-80/Fe₂O₃ and TiO₂-150/Fe₂O₃ show a conformal and homogeneous TiO₂ deposition on the Fe₂O₃ nanorods, where the phases of TiO₂ can be easily identified because of their different Z-contrast. The average thickness of the TiO₂ film is calculated as 1.7, 3.5, 6.8, and 13.2 nm for 20, 40, 80 and 150 ALD cycles, respectively (Fig. 2b–e). This indicates that the shell thickness of the TiO₂ coatings versus the number of ALD cycles shows good linearity with a slope corresponding to a growth per cycle (GPC) of 0.88 Å. The thickness of the TiO₂ overlayer from TEM images is well-controlled and shows the same trend as the ellipsometry data on Si wafers (Table S1, ESI†). The corresponding elemental maps further elucidate a homogenous distribution of Ti and O elements on the Fe₂O₃ nanorods. The EDX spectra for the pristine Fe₂O₃ and TiO₂-20/Fe₂O₃, further indicates the presence of both Fe and Ti elements (Fig. S4, ESI†). The atomic/weight percentage of all the samples with different ALD cycles are summarized in Table S2 of ESI.†

Fig. 2 HAADF-STEM and corresponding EDX elemental maps for (a) TiO₂-10/Fe₂O₃, (b) TiO₂-20/Fe₂O₃, (c) TiO₂-40/Fe₂O₃, (d) TiO₂-80/Fe₂O₃ and (e) TiO₂-150/Fe₂O₃.

The optical properties of the pristine TiO₂ on FTO substrate, pristine hematite and TiO₂-coated hematite were measured by UV-vis-NIR diffuse reflectance spectroscopy (Fig. S5, ESI†). As expected, TiO₂ shows a high absorbance in the UV region, while Fe₂O₃ and TiO₂/Fe₂O₃ show a high absorbance in the visible region (Fig. S5a, ESI†). Fig. S5b–d in the ESI† shows the estimated optical band gap, which can be measured from the plot of (F(R)·hν)^1/2versus hν.³⁶ The band gaps obtained from Tauc-plots for pristine TiO₂, pristine Fe₂O₃ and TiO₂/Fe₂O₃ are around 3.25, 2.07 and 2.02 eV, respectively, that match well with the reported values.^4,10 The slight change of band gap between Fe₂O₃ and TiO₂/Fe₂O₃ indicates that the doping of Ti in hematite is negligible.

The surface composition and chemical states of the samples were investigated by high-resolution XPS (Fig. S6, ESI†). The survey spectra of pristine TiO₂, pristine hematite and hematite coated with 20 TiO₂ ALD cycles are shown in Fig. S6a, ESI,† confirming the presence of all expected elements. Fig. S6b of ESI† shows the Fe 2p peaks of the pristine Fe₂O₃ and TiO₂-coated Fe₂O₃. The iron peaks are still well visible for the TiO₂-20/Fe₂O₃ sample, indicating that the TiO₂ layer is sufficiently thin, which is in good agreement with TEM measurements. The two peaks located at binding energies of 711.9 and 725.1 eV can be assigned to Fe 2p_3/2 and Fe 2p_1/2 of hematite, respectively, indicating the oxidation state III of iron.^37,38 XPS spectrum of O 1s peak for hematite samples is centered at 530.2 eV, which is consistent with literature values (Fig. S6c, ESI†).^21,39

To determine the energy level offset at the α-Fe₂O₃/TiO₂ heterostructures, the Kraut's method was used, as a direct measurement of the valence band offsets is typically difficult due to overlapping valence band signals.^40,41 Assuming the energy difference between the valence band and the core levels is the same for the pristine materials and the heterostructure, the valence band offset in the heterostructure can be calculated with the following equation:

The corresponding core level and valence spectra are shown in Fig. 3a–d.

While the binding energy of the core levels was determined from the peak position after fitting, the valence band onsets were determined from a linear extrapolation of the leading edges. This results in a valence band offset of 0.83 eV, with the valence band of α-Fe₂O₃ being closer to the Fermi-level. Considering the band gaps determined by absorption spectroscopy (Fig. S5, ESI†), we find that the α-Fe₂O₃/TiO₂ interface forms a straddling gap (type I heterostructure) with a conduction band offset of 0.35 eV (Fig. 3e).

Fig. 3 (a) Fe 3p core level spectra of Fe₂O₃ (black) and TiO₂-20/Fe₂O₃ (red). Shirley backgrounds have been removed. (b) Valence band spectrum of Fe₂O₃ with a linear extrapolation of the valence band onset. (c) Ti 2p core level spectra of TiO₂ (black) and TiO₂-20/Fe₂O₃ (red). Shirley backgrounds have been removed. (d) Valence band spectrum of TiO₂ with a linear extrapolation of the valence band onset. (e) Schematic energy level diagram of TiO₂/Fe₂O₃.

3.2 Photoelectrochemical activities

The PEC performance was measured on pristine hematite and hematite modified with various thicknesses of the TiO₂ overlayer (Fig. 4). Fig. 4a shows the photocurrent–potential (J–V) curves of hematite photoanodes under visible light. Compared to pristine hematite, the photoanodes of TiO₂-coated hematite show an obvious lower overpotential for water oxidation reaction and a cathodic shift of the onset potential. Furthermore, the photocurrent density shows a significant enhancement by deposition of the TiO₂ overlayer.

Fig. 4 (a) Photocurrent versus potential plots of a different number of ALD cycles for TiO₂ coated hematite photoanodes under visible light irradiation. (b) Time-based photocurrent density at 0.5 V bias vs. Hg/HgO with the light off and on for 10 s. (c) Nyquist plots of pristine Fe₂O₃ and TiO₂-20/Fe₂O₃ photoanodes measured under light illumination. (d) OCV curves of pristine Fe₂O₃ and TiO₂-20/Fe₂O₃ photoanodes with the light on for 20 s.

To further study the impact of the TiO₂ overlayer on hematite photoanodes on the PEC performance, the measurement of the photocurrent response under visible light at 0.5 V bias vs. Hg/HgO (1.4 V vs. RHE) was carried out on all the samples (Fig. 4b). As mentioned above, Fe₂O₃ can be excited to produce photogenerated electron–hole pairs under visible light due to the low band gap, while TiO₂ shows almost no photocurrent response because of its large band gap. Therefore, the photocurrent response is attributed only to Fe₂O₃ under visible light. The photocurrent density of pristine hematite is 0.01 mA cm⁻². The low photocurrent response is due to the high recombination rate of photogenerated charge carriers. By the deposition of the TiO₂ overlayer, all the composite samples display improved photocurrent densities, which can be attributed to the passivation effect of TiO₂.^18,42 It is well known that the passivation layer is normally very thin (<100 nm), which can prevent parasitic light absorption and charge-transfer inhibition.⁴³ TiO₂ passivates the surface states, reduces the recombination rate of the photogenerated electron–hole pairs, facilitates photogenerated hole transfer and improves the photocurrent response.²² The photocurrent densities of TiO₂ coated hematite with 10, 20, 40, 80 and 150 ALD cycles are 0.06, 0.27, 0.13, 0.06 and 0.02 mA cm⁻², respectively. The highest photocurrent response is recorded for TiO₂-20/Fe₂O₃ (∼1.7 nm), showing a photocurrent density ca. 27 times higher than that obtained for the pristine hematite. At this optimized thickness, the TiO₂ overlayer passivates surface recombination states of hematite and the photogenerated holes can be transferred from hematite to electrolyte. With a further increase of the TiO₂ thickness, the TiO₂ overlayer can hinder the hole transfer efficiency and block light from entering the inner hematite layer, leading to a dramatic decrease in photocurrent response.

During visible light illumination, photogenerated holes in pristine hematite travel to the semiconductor/liquid junction (SCLJ) and accumulate there, owing to the existence of surface trap states. This leads to the appearance of large anodic transient peaks, which decay until they finally reach an equilibrium between charge carrier transfer and recombination at the interface.⁴⁴ On the other hand, cathodic transient peaks overshoot and decay back to zero under dark conditions, which are attributed to the electrons diffusing from the external circuit and recombining with holes accumulated at the SCLJ.^12,22 However, the transient photocurrent spike decreases obviously after the deposition of TiO₂. Moreover, the feature was highly repeatable for numerous ALD cycles. This result further demonstrates the passivation effect of the TiO₂ overlayer. Therefore, Fe₂O₃ essentially acts as a light absorption layer, absorbing the visible light and producing photogenerated electron–hole pairs.

EIS was measured under light illumination to study the interfacial kinetics of the charge transfer process. Fig. 4c displays the Nyquist plots of pristine Fe₂O₃ and TiO₂-20/Fe₂O₃ that are simulated using the equivalent circuit (EC) shown in Fig. 4c inset. The equivalent circuit elements include the series resistance in the PEC cell (R_s), the charge transfer resistance (R_ct) at the Fe₂O₃/electrolyte interface and a constant phase element (CPE1).^39,45 The fitting parameters for the equivalent circuit elements are shown in Table 1. Compared to pristine hematite, TiO₂-20/Fe₂O₃ has a lower charge transfer resistance than pristine hematite, which indicates that the charge transfer across hematite/electrolyte interface is significantly facilitated due to the passivation effect of TiO₂. The EIS result confirms that TiO₂/Fe₂O₃ heterostructure can significantly enhance the charge transfer and separation and improve the PEC water splitting performance.

Table 1 Fitted parameters for the Nyquist plots of pristine Fe₂O₃ and TiO₂-20/Fe₂O₃

	R s (Ω cm^2)	R ct (Ω cm^2)
Pristine-Fe2O3	7.5	247.3
TiO2-20/Fe2O3	13.9	1807.0

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The open circuit voltage (OCV) of pristine hematite and hematite coated with 20 TiO₂ ALD cycles were measured in the dark and illumination conditions to study the behavior of the photogenerated carriers (Fig. 4d). Under illumination, the value of pristine Fe₂O₃ moves to a negative potential, while the potential of TiO₂-20/Fe₂O₃ shifts more negative than pristine Fe₂O₃. This suggests that both samples are n-type semiconductors, in line with the XPS results.^46–48 Under illumination, both samples exhibit a similar trend and create a similar number of photogenerated electrons, which can accumulate on the surface of photoanodes, leading to a potential negative shift and a sudden increase in the OCV. Meanwhile, the recombination of photogenerated electrons and holes occurs, which leads to a decrease of the OCV until a relatively steady state is reached. It can be seen that the potential change for Fe₂O₃ and TiO₂-20/Fe₂O₃ is comparable. However, compared to pristine Fe₂O₃, the photoanode of TiO₂-20/Fe₂O₃ is more stable due to the passivation effect of TiO₂ and the low photogenerated carrier recombination dynamics. After irradiation for 20 seconds, the light is turned off, and the OCV continues to decline due to the high recombination of photoelectron–hole pairs. The photovoltages of the pristine Fe₂O₃ and TiO₂-20/Fe₂O₃ photoanodes are 14.7 mV and 36.0 mV, respectively, as obtained from the difference between the OCV under dark and light conditions. Compared to pristine Fe₂O₃, the photovoltage obtained with TiO₂-20/Fe₂O₃ is much higher due to the fact that more photogenerated electrons are collected and a more efficient separation of the photogenerated electron–hole pairs.

To study the photo-response properties further, the incident photon to converted electron (IPCE) and stability test of pristine and TiO₂-coated hematite photoanodes were measured under visible light. The IPCE values have been calculated according to the equation:

where J_photo is the photocurrent density (mA cm⁻²) under visible light, λ is the wavelength of the incident light (nm), and P_light is the light power density (mW cm⁻²).^21,49

A maximum IPCE of 1.35% at 430 nm is achieved by TiO₂-20/Fe₂O₃, which is much higher than the corresponding value of 0.05% for pristine Fe₂O₃ photoanodes (Fig. S7a, ESI†). The significant improvement of photoconversion efficiency indicates higher utilization efficiency of photoinduced carriers for the TiO₂-coated hematite photoanodes, which is consistent with the J–V measurements (Fig. 4a).

Applied bias photon-to-current efficiency (ABPE) was calculated from the photocurrent–potential plots according to the following equation:

where I (mA cm⁻²) is the measured photocurrent density, V_bias (V vs. RHE) is the applied potential, and P_light is the incident illumination power density.

The ABPE was plotted as a function of the applied potential. The TiO₂-20/Fe₂O₃ photoanode achieves the maximum ABPE value of 0.01% at 1.02 V, which is significantly higher than that of 0.004% at 0.98 V for pristine hematite electrode (Fig. S7b, ESI†). Clearly, the improvement of PEC water splitting performance can be attributed to the enhanced charge separation caused by the passivation effect of TiO₂.

Fig. S8 in ESI† shows the current retention as a function of the time of pristine hematite and hematite coated with 20 TiO₂ ALD cycles under continuously visible light irradiation for 60 min. A significant improvement of the photocurrents of pristine hematite electrode upon TiO₂ deposition as well as a good stability can be obtained. The morphology and the composition of the synthesized photocatalyst after stability test are well maintained, which are shown in Fig. S9 and S10 in ESI.†

Fig. S11 in ESI† shows the Mott–Schottky plots of the pristine Fe₂O₃ and TiO₂-20/Fe₂O₃. The slopes determined from the Mott–Schottky plots are used to estimate the carrier densities. The positive slopes display the n-type semiconductor nature of the pristine Fe₂O₃ and TiO₂-20/Fe₂O₃. Moreover, the plot of TiO₂-20/Fe₂O₃ depicts a suppressed slope compared to pristine Fe₂O₃, suggesting a higher carrier concentration in the heterostructure than in the pristine Fe₂O₃. The as-calculated carrier densities for pristine Fe₂O₃ and TiO₂-20/Fe₂O₃ are 1.6 × 10¹⁹ cm⁻³ and 4.4 × 10¹⁹ cm⁻³, respectively.

Contrary to the visible light system, both Fe₂O₃ and TiO₂ participate in the photocurrent response under UV light irradiation due to the suitable band gap of Fe₂O₃ and TiO₂. As a result, the photocurrent response is not only attributed to Fe₂O₃ but is also related to the TiO₂ overlayer. To verify the role of TiO₂ overlayer in the PEC performance of hematite, the photocurrent response for hematite and TiO₂-coated hematite samples were evaluated under UV light (Fig. S12, ESI†).

The experimental results show that the photocurrent response varies significantly with the thickness of the TiO₂ overlayer. Compared to pristine hematite, the TiO₂ overlayer results in a remarkable enhancement of photocurrent response. Before 80 ALD cycles, the variation trend of the photocurrent response of hematite samples with TiO₂ thickness under UV irradiation was similar to that under visible light irradiation. The photocurrent densities of pristine hematite and TiO₂ coated hematite with 10, 20, 40 and 80 ALD cycles are 0.03, 0.07, 0.41, 0.22 and 0.09 mA cm⁻², respectively. The optimized TiO₂-20/Fe₂O₃ (ca. 1.7 nm) shows a photocurrent response of 0.41 mA cm⁻², which is 14 times higher than the photocurrent response of the pristine hematite. TiO₂ thicknesses up to 1.7–6.8 nm (20 ALD cycles to 80 ALD cycles) lead to a decrease in photocurrent response. At this stage, the TiO₂ overlayer mainly acts as a passivation layer and Fe₂O₃ acts as a light absorption layer. However, the photocurrent response increases with a further increase of the thickness of the TiO₂ overlayer (i.e., from 6.8 to 13.2 nm, 80 ALD to 150 ALD), in contrast to what was observed under visible light. The photocurrent density of TiO₂-150/Fe₂O₃ is 0.14 mA cm⁻². This change of photocurrent response at a higher thickness suggests that the photocurrent response is not dominated by Fe₂O₃ anymore, but is restricted to the TiO₂ overlayer.

Fig. 5 compares the photocurrent response of hematite samples coated with different thicknesses of the TiO₂ overlayer under different illumination. The experimental results reveal a substantial variation of the photocurrent response as a function of the thickness of the TiO₂ overlayer. The highest photocurrent response exhibited by the TiO₂-20/Fe₂O₃ photoanodes is recorded under both visible and UV light. After 20ALD cycles, the photocurrent response decreases with a further increase of the TiO₂ thickness under visible light, while it first decreases and then increases under UV light.

Fig. 5 Current densities of hematite photoanodes with different ALD TiO₂ cycles at 0.5 V bias vs. Hg/HgO, 1 M KOH, under visible light and UV light irradiation, respectively.

Under visible light, the TiO₂ overlayer acts as a passivation layer. The photogenerated holes from Fe₂O₃ can cross the passivation layer primarily by tunneling, which gradually vanishes within a few nanometers.^43,50,51 Because of this mechanism, the TiO₂ overlayer must be of the optimal thickness. It should not be too thin to passivate the surface recombination states enough, and not too thick to allow the photogenerated holes transfer between Fe₂O₃ and electrolyte to participate in the water oxidation. Under UV light, when the TiO₂ thickness is ≤6.8 nm, it does not absorb enough UV light, and therefore only few photogenerated charge pairs are produced in the TiO₂. In this case, TiO₂ mainly acts as a passivation layer, and the photocurrent response is dominated by Fe₂O₃. Therefore, the photocurrent response under UV light follows the same trend as under visible light. However, when the thickness of TiO₂ is larger than the optimal 1.7 nm (20 ALD cycles), it hinders the transfer of the photogenerated holes. Therefore, the photogenerated holes in Fe₂O₃ cannot efficiently cross the TiO₂ overlayer. Conversely, when the TiO₂ overlayer is thick enough, it absorbs more UV light and produces more photogenerated charge pairs that participate in water oxidation. The contribution of the TiO₂ overlayer for photocurrent response becomes dominant at 150 ALD cycles (13.2 nm). At this stage, TiO₂ mainly behaves as the active layer, and the contribution of Fe₂O₃ becomes negligeable. This demonstrates that the photoresponse under UV illumination is largely related to the thickness of the TiO₂ overlayer.

3.3 Charge transfer mechanism

Fig. 6 displays the schematic diagram of the charge separation and transport mechanism under visible and UV light illumination. Under visible light, Fe₂O₃ with a suitable band gap can produce photogenerated electrons in the conduction band and photogenerated holes in the valence band. The photogenerated electrons driven by the bias voltage will transfer through the external circuit (Fig. 6a). The photogenerated holes from Fe₂O₃ cannot be transferred to the valence band of TiO₂ owing to an unfavorable valence band edge alignment of Fe₂O₃ and TiO₂. Therefore, TiO₂ behaves as a passivation layer and Fe₂O₃ acts as a light absorption layer. In the case of UV light illumination, both Fe₂O₃ and TiO₂ have suitable band gaps that can be excited to produce electron–hole pairs. The photogenerated electrons of TiO₂ migrate to the conduction band of Fe₂O₃, which lies at a more positive potential than that of TiO₂. Subsequently, the electrons in Fe₂O₃ flow into the external circuit. The photoinduced holes in Fe₂O₃ can pass through the TiO₂ thin layer by tunneling effect for the oxidation reaction. The photogenerated holes from TiO₂ can react directly with the electrolyte. It can also transfer to the valence band of Fe₂O₃ and then cross the TiO₂ thin layer to participate in the water oxidation reaction (Fig. 6b). This indicates that the photocurrent response is not always governed by the Fe₂O₃. The TiO₂ overlayer also plays an essential role in producing photocurrent response under UV light. As we have discussed above, the impact of the TiO₂ overlayer is largely dependent on the film thickness.

Fig. 6 Schematic illustration of the charge-transfer pathways in the TiO₂/Fe₂O₃ system under visible light and UV light.

4. Conclusion

TiO₂/Fe₂O₃ heterostructures have been synthesized by a hydrothermal method and atomic layer deposition. The as-synthesized samples show a homogeneous and conformal coating of TiO₂ onto the hematite nanorods. The morphology and thickness of a TiO₂ overlayer can be precisely controlled. The pristine hematite and hematite coated with various thicknesses of TiO₂ are studied for their photocurrent response under visible and UV light. Compared to pristine hematite, the TiO₂ coating leads to a significant improvement in photocurrent response. The photocurrent response of the TiO₂/Fe₂O₃ heterostructures is strictly related to the thickness of the TiO₂ overlayer. The best photoanode response is observed with a coating of 1.7 nm (TiO₂-20/Fe₂O₃) under both visible and UV light. Our results show that the photocurrent density of TiO₂-20/Fe₂O₃ reaches 0.27 mA cm⁻² at 0.5 V vs. Hg/HgO under visible light, which is 27 times higher than that of pristine α-Fe₂O₃. Under visible light, the TiO₂ overlayer acts as a passivation layer facilitating the photogenerated holes transfer and reducing the surface recombination rate of electron–hole pairs. Fe₂O₃ is the major contributor to the photogeneration of electron–hole pairs. Under UV light, the TiO₂ overlayer behaves mainly as a passivation layer at lower thicknesses (≤6.8 nm). At higher thickness, TiO₂ acts as a light absorption layer, while the contribution of Fe₂O₃ is reduced. This work not only provides deep insights into the impact of a TiO₂ overlayer on the PEC water splitting performance of hematite, but can be also valuable and promising for the design and development of high-performance photoanodes.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

J. Wang and L. Liccardo contributed equally to this work. J. Wang acknowledges the fellowship from the China Scholarship Council (CSC). C. Erdmann is acknowledged for the TEM measurement. C. X. Shen is acknowledged for Raman measurement. E. Wierzbicka would like to acknowledge the Polish National Agency For Academic Exchange, Polish Returns Programme (project no. BPN/PPO/2021/1/00002) for the financial support of the project.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3se01633a

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