An All Oxide

Subscriber access provided by Iowa State University | Library

C: Energy Conversion and Storage; Energy and Charge Transport 2

3

2

An All Oxide #-FeO/H:TiO Heterojunction Photoanode: A Platform for Stable and Enhanced Photoelectrochemical Performance through Favourable Band Edge Alignment Nisha Kodan, Khushboo Agarwal, and Bodh Raj Mehta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10794 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

An all oxide α-Fe2O3/H:TiO2 heterojunction photoanode: A platform for stable and enhanced photoelectrochemical performance through favourable band edge alignment Nisha Kodan1, Khushboo Agarwal1 and B. R. Mehta1* 1

Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

Abstract To improve the photoelectrochemical performance of hematite thin film photoanode, we report a novel heterostructure based on Fe2O3/TiO2 and hydrogenated Fe2O3/TiO2 (Fe2O3/H:TiO2) for faster charge transfer owing to passivation of surface states in Fe2O3 via TiO2 overlayer and favourable band alignment using hydrogen annealing of TiO2 overlayer. The valence band offset, band gap and work-function values have been measured using X-ray photoelectron spectroscopy (XPS), optical absorption, Kelvin probe force microscopy (KPFM) to construct the energy band diagram of the heterostructure photoanodes. The results confirm the upshift in the valence band edge of TiO2 over Fe2O3 after hydrogen treatment of TiO2 overlayer which leads to formation of type-II band edge alignment in Fe2O3/H:TiO2 heterojunction and improved PEC performance compared to that of the Fe2O3/TiO2, pristine Fe2O3 and TiO2 thin film photoelectrodes. The well straddled and improved band alignment in Fe2O3/H:TiO2 heterostructure gives rise to substantial enhancement in photocurrent density, up to 3.36 mA/cm2 at 1.23 V (vs RHE) with a low onset potential of 0.1 V (vs RHE), under AM1.5 illumination condition. The observed photocurrent density in Fe2O3/H:TiO2 heterostructure is 15-fold higher than bare Fe2O3 (0.22 mA/cm2) photoanode. This work shows how a simple bilayer junction and its hydrogen treatment can be used to enhance the PEC response of heterojunctions and offers valuable insights for the further development of all oxide heterojunctions. 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

1. Introduction Generation of clean solar hydrogen by photoelectrochemical (PEC) water splitting for harnessing solar energy represents a promising but challenging route to attain sustainable energy production1. The PEC water splitting for generation of solar hydrogen is a viable method for the practical application of the most abundant renewable energy source2. It can be recognised as a reasonably good solution to the ever-growing energy challenge with negligible impact on the environment. The reformation of solar energy into the elementary form of fuel (H2) can be perceived by means of semiconductor materials as working electrodes in a PEC water splitting cell3,4. A crucial criterion for PEC water splitting is to look for a suitable photoelectrode material that satisfies the challenging requirements of band edge (conduction band, CB and valence band, VB) alignment, high visible light absorption, and better charge generation and separation; for solar hydrogen production along with stability against photocorrosion. Among the various metal oxide candidates, hematite (α-Fe2O3) is a most promising n-type semiconductor photoanode owing to large abundance of its constituent elements, good stability in chemical environment, non-toxicity, cost effective and appropriate energy level for water oxidation and high visible light absorption 5. Inspite of these advantages the efficiency of hematite falls off due to a number of factors such as slow hole kinetics of the oxygen evolution reaction, short diﬀusion length, high probability of charge trapping in surface states and positive conduction band edge to the hydrogen redox potential6,7. In addition, surface recombination is also a serious issue which limits water oxidation efficiencies8,9,10. Therefore, in order to enhance light absorption, suppress charge carrier recombination and improve charge transport, the most commonly used strategies are nanostructure engineering11,12,13,14, elemental doping15,16,17,18 and formation of nano-heterostructure19,20,21,22,23,24,8,25.

Additionally, the

involvement of catalysts (OECs), for eg. Ni-, Ir-, Co- and Ru-based ones19,26,27,16,9,28 for surface treatment of the hematite layer have been utilized to improve the photocurrent and reduce the 2 ACS Paragon Plus Environment

Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


overpotential, respectively. Among the various material modification strategies, formation of heterojunction photoelectrodes helps in upgrading the system efficiency, by creating potential gradients in the photoelectrode which facilitate charge separation which improves the carrier transfer rate at the semiconductor−liquid interface29,30. There have been several studies focussed on the coupling of Fe2O3 thin films with various metal oxides such as Al2O3, Ga2O3, TiO2, etc.31,26, 8,21,14. Barreca et al. reported the photocurrent density ~2.0 mAcm−2 (at 1.23 V vs RHE in 1 M NaOH) in Fe2O3–TiO2 photoanodes where α-Fe2O3 nanostructures were fabricated plasma enhanced-chemical vapour deposition with subsequent annealing at 650°C1 and TiO2 overlayer using atomic layer deposition. Several efforts have been made to investigate the role of Ti incorporation into hematite photoelectrodes11,32,33,34,35,18 and Fe2O3–TiO2 bilayer5,1 and composite systems36, the hydrogen treated Fe2O3-TiO2 (Fe2O3/H:TiO2) have been rarely investigated. We emphasis on the PEC behaviour of hydrogen treated over layers and untreated Fe2O3-TiO2 nanostructures compared to pristine Fe2O3, TiO2 and hydrogen treated TiO2 (H:TiO2). The actual interest of using titanium dioxide as a top layer in a heterostructure with Fe2O3 is because of its stability against corrosion and high water splitting ability. In the present investigation, we outline a technique for the improvement in Fe2O3 photoanode charge transfer via band realignment which gives rise to significant enhancement of the PEC behaviour in an all oxide heterojunction, by a simple hydrogen treatment process. A special attention is given to understand the effect of charge transfer process in the heterojunction before and after hydrogen treatment process. The prepared Fe2O3/TiO2 and Fe2O3/H:TiO2 heterojunction thin films photoelectrochemical performance is compared to pristine Fe2O3, TiO2 and H:TiO2 thin film photoelectrodes.

3 ACS Paragon Plus Environment


2. Experimental Section Preparation of Fe2O3 Thin Films In the present work, n-type (100) Si wafer and indium doped tin oxide (ITO) coated glass were used as the substrates to deposit thin films of Fe2O3 via rf sputtering of high purity (99.99%) iron oxide (Fe2O3) target. Initially, the chamber pressure of approximately 2 × 10-6 Torr was obtained via a combination of diffusion and rotary pumps. The Fe2O3 thin films were deposited at optimized rf-power of 80W, 20 sccm argon as sputtering gas at room temperature and a deposition pressure of 2.5 × 10-2 Torr. The as-deposited Fe2O3 samples were air annealed at 450°C for 4 hrs to achieve catalytic hematite as an appropriate phase. Preparation of TiO2 Thin Films A simple and cost-effective sol–gel spin coating method was utilized to deposit TiO2 thin films using titanium tetra isopropoxide (TTIP), diethanolamine (DEA) and ethanol as precursor solution37. Typically, 30 ml of clear colloidal solution was prepared by mixing 3 ml TTIP into 25 ml ethanol with a successive addition of 2 ml DAE. The gel was prepared from the solution by stirring continuously for 3 hours at RT. A fix and small quantity approximate 200 μL of the gel was then poured onto already prepared Fe2O3 thin films placed in the spin coater using spin rate of 3000 rpm for 30s. The coated films were dried at 70-80°C on a hot plate and further cooled to RT prior to another spin-coat cycle. Finally, the samples were annealed at 450°C for 2 hrs to achieve the required anatase phase of TiO2. Preparation of Fe2O3/TiO2 and Fe2O3/H:TiO2 Heterostructure Thin Films For fabricating the heterostructures, pristine Fe2O3 thin films were coated with TiO2 via spin coating for different cycles (1-3 cycles) to vary thickness of TiO2 overlayers. After coating the sample were annealed at 450°C to obtain the desired phase of TiO2. For the vacuum hydrogen treatment, the annealing chamber was pumped down to 5.0 × 10-6 Torr and 30 sccm 5% H2 balanced Ar gas was inlet into the chamber while keeping the pressure and temperature at 2 × 4 ACS Paragon Plus Environment

Page 4 of 29

Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10-2 Torr and 300°C, respectively. The pristine hematite thin film coated with TiO2 overlayer via sol-gel process heated in air at 450 °C were annealed in vacuum in presence of 5% H2 balanced Ar gas for 10 hours to obtain hydrogenated Fe2O3/TiO2 heterostructure (Fe2O3/H:TiO2). Schematic diagram screening all steps used for sample preparation is shown in Fig S1.

3. Characterizations The X-ray diffraction (XRD) pattern were recorded using Rigaku Ultima IV X-ray diffractometer with a step size of 0.04° and a scan rate of 2° per minute using Cu Kα (λ = 1.54 Å) radiation and Raman measurements have been performed using He−Cd laser with 325 nm wavelength using Horiba Scientific LabRAM HR Evolution Raman spectrometer. The X-ray photoelectron spectroscopy (XPS) measurements were performed using PHI Quantera SXM apparatus system. The Al Kα line (1486.6 eV) was used as the monochromatic X-ray excitation source. The analyser collects excited photoelectrons from the small area on the sample (~4.0 mm2). The obtained spectra were calibrated using amorphous carbon (C1s peak at 284.4 eV) present in the sample and data analysis was done with XPS peak fit software by Shirley background subtraction. Low-resolution survey spectra were acquired between binding energies (B.E.) of 1–1100 eV. High-resolution scans were obtained for the individual XPS features of interest. High resolution spectra were collected at a pass-energy of 69 eV, providing an overall instrumental peak broadening of 0.5 eV. XPS depth profile were recorded using Ar+ ions of 1.5 kV at 45° as an incidence angle. The surface topography/roughness and work function were obtained using field emission scanning electron microscope (FESEM), atomic force microscopy (AFM) and kelvin probe force microscopy (KPFM) using field-emission SEM (Quanta™ 3D FEG) and Digital Instruments (Nanoscope IIIA) equipped with Pt-Ir coated Si cantilever having resonance frequency of 60 kHz and radius of curvature of 30 nm (SCM-PIT from Bruker, Inc., USA), respectively. In order to understand the optical properties, 5 ACS Paragon Plus Environment


Page 6 of 29

the absorbance spectra were recorded by UV-visible spectrophotometer (Perkin-Elmer Lambda 35) in 300-800 nm range of wavelength and the coefficient of absorption (α) was obtained by using the relation, α = 2.303(A)/d, where d and A denotes the total thickness and measured absorbance of the film, respectively. The photoelectrochemical measurements were performed in a three-electrode PEC cell comprising the bare Fe2O3, TiO2, H:TiO2, Fe2O3/TiO2, Fe2O3/H:TiO2 thin films as the working electrodes (approximate exposed area ~ 0.45 cm2), Ag/AgCl (3M KCl) as a standard reference electrode and a platinum mesh as the counter electrode.1 M NaOH aqueous solution was used as an electrolyte for all PEC measurements. The photoelectrochemical cell was operated via AutoLab PEC workstation having nova software system (X-Pot Potentiostat). Linear sweep voltammetry (LSV) scans in dark and light condition and Mott-Schottky measurement under dark were conducted in the potential range 1.0 to +1.0 V (vs Ag/AgCl) with a scan rate of 10 mV/s where 280 W xenon lamp equipped with AM1.5 filter (illumination intensity = 100 mW/cm2) was used as a light source. The electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10 mHz to 100 kHz with an AC signal amplitude of 10 mV under 0 V vs Ag/AgCl. Mott−Schottky (MS) measurements have been carried out at AC frequency of 1kHz. The theoretical redox potential to obtain hydrogen from hydrogen ions, i.e. reversible hydrogen electrode (RHE) potential can be obtained from Ag/AgCl reference electrode using the equation38: ᵒ 𝑉(𝑅𝐻𝐸) = 0.059 × 𝑝𝐻 + 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙

………….. (1)

Where, pH is the pH value of the aqueous electrolyte used for PEC measurements, ᵒ 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 and 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 are the standard potential of Ag/AgCl reference electrode at RT (0.197

V) and potential applied on working electrode w.r.t Ag/AgCl.


Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. Results and discussion Figure 1(a) represents XRD patterns of Fe2O3, Fe2O3/TiO2 and Fe2O3/H:TiO2 thin films. The observed diffraction pattern represents hematite phase of α-Fe2O3 (JCPDS No. 33-0664) and anatase phase of TiO2 (JCPDS No. 21-1272). Absence of other impurity diffraction peak confirms the phase purity of the films. In Fe2O3/TiO2 sample, the diffraction peaks at 2θ value of 25.31° can be labelled as (101) plane of the anatase phase TiO2, whereas at 33.14° and 54.08° are assigned to (104) and (116) plane of hematite phase of Fe2O3, respectively. After hydrogen treatment of Fe2O3/TiO2 thin films, no additional peak has been observed. The intensity of the peak decreases gradually in Fe2O3/H:TiO2 thin films which may be due to post deposition annealing of the heterostructure thin films. All observed diffraction peaks of Fe2O3/TiO2 and Fe2O3/H:TiO2 thin film heterostructures can be assigned to the hematite and anatase phases of Fe2O3 and TiO2,respectively, without any other peaks related to complex phases of Fe–Ti were observed in the pattern, which is in accordance with the results reported by Zhao et al39.

Figure 1. (a) XRD pattern and (b) Raman Spectra of Fe2O3, Fe2O3/TiO2, and Fe2O3/H:TiO2 thin film samples. The letter ‘F’ shows the peaks from hematite α-Fe2O3 and ‘T’ represents peaks from anatase TiO2. The star symbol (*) indicates the substrate diffraction peaks. The crystal structure of the pristine and heterostructure thin films have been further confirmed with Raman analysis as shown in Figure 1(b). The obtained Raman active modes at 223(A1g), 7 ACS Paragon Plus Environment


243(Eg), 293(Eg), 408(Eg), 611(Eg), 638, and 1315 cm-1 are attributed to the hematite phase of Fe2O3 whereas modes at 144(Eg), 511(B1g), and 638(Eg) cm-1 confirm the formation of anatase phase of TiO2 in Fe2O3/TiO2 and Fe2O3/H:TiO2 heterostructure thin films. The observed Raman spectra of heterostructure thin films show peaks at 144, 511 and 638 cm-1 respectively which are related to symmetric vibration modes (B1g + 2Eg) of tetragonal anatase phase of TiO2. The slight red shift of ~3 cm-1 is observed in Raman mode at 243 cm-1 in Fe2O3/TiO2 and Fe2O3/H:TiO2 w.r.t Fe2O3 thin films. This red shift is related to the symmetry of molecules and chemical bonds, any change in bond length of molecules due to any internal stress or external effects like annealing gives rise to the red shift in Raman peaks40.

Figure 2. (a) The cross-sectional FESEM micrograph of Fe2O3 and (b) the heterostructure Fe2O3/TiO2 thin film samples. The surface morphology and microstructure information of prepared samples have been investigated by cross-sectional FESEM measurement. As seen in Fig. 2(a), the FESEM image of pristine Fe2O3 film shows uniformly distributed grains having thickness ~300−400 nm with interparticle pores. The cross-sectional micrograph indicating the formation of a double layer structure with TiO2 overlayer of ~100 nm uniformly covering the underlying Fe2O3 thin film as shown in Figure 2(b). Morphology of the films was also analysed by atomic force microscopy (AFM) measurements, which shows uniform topography with very small roughness values (Fig. S2). The estimated root mean square (RMS) roughness is ~1-6 nm.


Page 8 of 29

Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe2O3/TiO2 heterostructure was investigated by X-ray photoelectron spectroscopy (XPS) concentration depth profiles. An abrupt interface appears between the Fe2O3 and TiO2 thin film as shown in Fig. 3(a), indicating that the double layer films consists of oxygen, iron and titanium, respectively. The signals from Ti 2p, Fe 2p and O 1s clearly depicts the element configuration of Fe2O3/TiO2 thin films. As expected, O 1s showed a constant concentration throughout the film whereas Ti 2p decreases in concentration while Fe 2p increases as we move from the top surface of the film to the bottom surface which shows that there is a uniform coating of TiO2 over Fe2O3 thin films to form heterostructure Fe2O3/TiO2 heterojunction. In Ti 2p depth profile spectra, the binding energy peaks at approximately 458.6 and 464.4 eV with ~5.8 eV peak splitting between Ti 2P3/2 and Ti 2P1/2 core levels, confirming Ti4+ state of anatase phase of TiO241,42 as shown in figure 3(b). There is slight negligible shift in peak positions (458.6 eV to 458.4 eV for Ti 2p3/2; 464.4eV to 463.9 eV for Ti 2p1/2) towards lower binding energy values as we move from top surface to interface owing to some chemical changes at the interface due to bombardment of Ar+ ions and as expected no peak of Ti 2p was observed from bottom surface which indicated that after moving across interface, we basically have contribution from Fe2O3 bottom layer. Similarly, no peak of Fe 2p doublet was observed on top surface, whereas as we move from interface to bottom surface, peaks corresponding to Fe 2p start to appear as shown in figure 3(c). As one moves to bottom surface, peaks at 709.3 eV (Fe2p3/2) and 722.9 eV (Fe2p1/2) with satellite peak at 715.4 eV (with spin orbit splitting factor of 13.6 eV) attributed to Fe3+ in α-Fe2O3

43-46

. Moreover, observed O1s spectrum is

deconvoluted in two peaks at ~530 eV and 531.9 eV which corresponds to contribution form lattice oxygen and surface hydroxyl groups presents at the surface. At interface and bottom surface, O1s spectra shows contribution only from lattice oxygen with one peak positioned at 529.9 (~530 eV) as shown in figure 3 (d). The depth profile analysis of bilayer Fe2O3/TiO2 heterostructure depicts no significant change in peak positions as one move from top surface



to bottom surface which shows that there are no mixing or chemical changes at the interface indicating chemical integrity of the two layers is intact at the interface. Survey spectra of Fe2O3/TiO2 heterojunction spectrum and high-resolution core level XPS spectra of Fe 2p, Ti 2p and O1s are shown in Fig. S3 (a-d).

Figure 3. (a) Depth profile and elemental signals of (b) Ti 2p, (c) Fe 2p and (d) O1s, using XPS, at “top”, “interface” and “bottom” of the Fe2O3/TiO2 heterostructure thin film photoanodes. Figure 4 (a) displays the optical absorption spectra of Fe2O3, Fe2O3/TiO2 and Fe2O3/H:TiO2 samples recorded in wavelength ranges from 300-800 nm. A sharp and noticeable rise in absorption at wavelengths lower than ∼550 nm could be assigned to the expected band gap of crystalline TiO2 and Fe2O3. In Fe2O3/H:TiO2 system, the optical absorption slightly increases at wavelengths larger than ∼600 nm, which is higher than both Fe2O3 and Fe2O3/TiO2. This 10 ACS Paragon Plus Environment

Page 10 of 29

Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


high absorption is due to the hydrogenation of TiO2, which is well- known to produce black TiO241,42,43,44.

Figure 4. (a) Absorbance spectra measured using UV-Visible spectrometry technique, (b) valence band spectra using VB XPS measurement, (c) work function obtained via KPFM measurement and (d) Band edge diagram for Fe2O3, TiO2 and H:TiO2 samples. The Tauc’s plot considering direct transition ((αhυ)2 vs. hυ) are used to estimate the optical bandgap of the films, see Figure S4. The optical absorption coefficient (α) can be calculated from absorbance data. The bandgap value is estimated by extrapolating the linear portion of the Tauc’s plots to α=0. Furthermore, the valence band XPS measurement is used to obtain the valence band maxima (VBM) for the samples and valence band offset values were found to be 2.12eV,1.92eV and 1.42 eV below the zero-potential energy point for Fe2O3, TiO2 and H:TiO2 samples, respectively (Figure 4(b)). Finally, the work function is obtained by measuring the 11 ACS Paragon Plus Environment


surface potential of Fe2O3, TiO2 and H:TiO2 thin films in dark condition by KPFM technique. The observed surface potential profiles for Fe2O3, TiO2 and H:TiO2 samples are plotted and shown in Figure 4(c). In KPFM measurement, surface mode is used to measure the surface potential difference (VCPD) between the Fe2O3, TiO2 and H:TiO2 surfaces and the AFM tip. The work function value of the thin film samples (Φsample) were determined using the relation: Φsample=Φtip-VCPD, where Φtip denotes the work function of the tip. Using the surface potential images, the surface potential peaks were observed at 20 mV,140 mV and 440 mV. The work function values calculated for Fe2O3, TiO2 and H:TiO2 samples are 5.08 eV, 4.96 eV and 4.66 eV respectively (Figure 4(c)). The band edge diagram w.r.t Fermi energy level (EF) is drawn for Fe2O3, TiO2 and H:TiO2 samples (Figure 4 (d)) using the experimentally observed values of band gap, valence band off-set and work function. For Fe2O3, the valence band off-set, work function and the band gap the values are 2.12 eV, 5.08 eV and 2.6 eV, the corresponding values for TiO2 and H:TiO2 are 1.92 eV, 4.96 eV and 3.2 eV and 1.42eV, 4.66eV and 2.9 eV, respectively. The higher bandgap (2.6 eV) of Fe2O3 thin films (crystallite size ~15nm) post annealed at high temperature 450ᵒC has been also reported in literature and explained due to the strong quantum confinement or re-arrangement of the electrons near the fermi level45. As shown in Figure 4(d), the valence band edge of Fe2O3 is much below (~1.55 eV) as compared to valence band edges of TiO2 and H:TiO2 which could result better water oxidation in Fe2O3 but the short hole diffusion length inhibits the oxidation process and results in poor PEC performance of bare Fe2O3 photoelectrodes whereas conduction band edges of TiO2 and H:TiO2 are above Fe2O3 band edge, results in better water reduction mechanism. Since, photogenerated holes in Fe2O3 are of short diffusion length so unable to reach to surface to take part in water oxidation, if fraction of generated holes reached the surface then will trapped and recombine with electrons in surface states. The conduction edge of Fe2O3 lies below the water redox potential of water so electrons will not be able to reach surface for reduction of water


Page 12 of 29

Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


unless provided sufficient external bias to overcome the barrier. On the other hand, utilizing these experimental values we have plotted the energy band diagram of Fe2O3/TiO2 and Fe2O3/H:TiO2 photoanodes, Fe2O3/H:TiO2 photoanode straddle the water redox potentials, therefore, it is energetically favorable for the electrons and holes to reach the hydrogen (H2/H2O) and oxygen (H2O/O2) evolution level, respectively, to drive solar hydrogen production reaction as shown in figure 5(d), TiO2 passivation suppress the surface recombination and also the higher surface potential in heterostructure photoanodes separate the generated charge. For the pristine TiO2, the observed VB offset, work function, and the band gap values are 1.92 eV, 4.96, 3.2 eV and the corresponding values for H:TiO2 are 1.42 eV, 4.66eV 2.9 eV respectively. As shown in Figure S5(c), the valence band maxima of H:TiO2 is shifted upward by ~0.5 eV in comparison to pristine TiO2, while the CB remains nearly unchanged w.r.t vacuum level (reference energy level). A comprehensive experimental study on hydrogenated TiO2 thin films prepared via rf magnetron sputtering have been reported earlier46. The change in the band diagram resulting in a well aligned band edge positions of Fe2O3/H:TiO2 w.r.t. Fe2O3 is an important factor for the efficient PEC performance. Heterostructures of Fe2O3 with TiO2 shows type I band alignment also reported earlier using transient absorption spectroscopy measurement1. This band alignment shows that the valence band edge of TiO2 is below Fe2O3 which energetically impede hole transport from Fe2O3 to TiO2. Therefore, photoexcited holes in Fe2O3 are unable to oxidize water at TiO2 surface as shown in band edge diagram of fig. 5(b). There are reports on improvement in PEC response at high applied external biases in Fe2O3/TiO2 heterojunctions which provide sufficient energy to the carriers to overcome the barrier. However, as annealing in hydrogen ambient induces lattice disorder. Therefore, our measurements propose a favourable band edge realignment of the hydrogen-treated TiO2 (H:TiO2) and Fe2O3 (see Fig. 5(d)).



Figure 5. Energy band diagram of Fe2O3/TiO2 before contact (a), after contact (b), Fe2O3/H:TiO2 before contact (c) and after contact (d). The realignment of band edge after hydrogen treatment improves photocurrent densities as photogenerated electron and holes can be easily moved to respective surfaces due to favourable position of Fe2O3 and H:TiO2 conduction and valence band edges. Since, TiO2 valence band edge shifted above Fe2O3 after hydrogen treatment, holes generated in Fe2O3 can easily migrate to TiO2 surface to oxidise water and conduction band of TiO2 is above Fe2O3 therefore, electrons generated in TiO2 transfers to ITO substrate via Fe2O3, and can eventually drifted to the external circuit for reducing hydrogen ions into water at the counter electrode, suppressing unfavourable effects due to recombination. Further valuable contribution to the


Page 14 of 29

Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


actual performances is associated to the fact that TiO2 overlayers can prevent hematite photo corrosion in a wide pH range. Photoelectrochemical Measurements The a three electrode PEC cell have been used to perform the photocurrent density (Jph) measurements to investigate the PEC performance of photoanodes. Figure 6(a) shows the PEC performance in terms of photocurrent density for pure TiO2, H:TiO2, Fe2O3, Fe2O3/TiO2 and Fe2O3/H:TiO2 heterostructure photoanodes using 1 M NaOH aqueous solution (pH = 13.6) as electrolyte where Xenon arc lamp equipped with AM1.5 filter was used as a light source (output intensity 100mW/cm2).

Figure 6. (a) The photocurrent density and (b) the photoconversion efficiency of the TiO2, H:TiO2, Fe2O3, Fe2O3/TiO2 and Fe2O3/H:TiO2 thin film samples measured under illumination. The photocurrent density (Jph) values of the pristine TiO2, H:TiO2 and Fe2O3 photoanodes are low (∼0.01, 1.1 and 0.22 mA/cm2 at 1.23 V vs RHE) and have onset potential values at 1.03 V and 1.12 V vs RHE, respectively. A lower onset potential (0.1 V) and higher photocurrent of ∼2.10 mA/cm2 at 1.23 V vs RHE was observed in the Fe2O3/TiO2 heterostructure. The observed value of Jph for the Fe2O3/TiO2 thin film heterostructure system is comparable to reported photocurrent values (~2 mA/cm2)1,5. In addition, hydrogen annealing in Fe2O3/H:TiO2 heterostructure increases Jph to 3.36 mA/cm2 without any shift in onset potential, which is a



Page 16 of 29

large photocurrent density for a hydrogen treated metal oxide heterostructure of Fe2O3 and TiO2. The stable photocurrent of the Fe2O3/H:TiO2 photoanode reaches to 3.56 mA/cm2 at 1.6 V vs RHE, which is apparently higher than single system utilised as visible light photon absorber40. Additionally, the photoconversion efficiency (η) under illumination was calculated for all photoanodes constituting the PEC cell using the expression47: 1.23−𝑉𝑎𝑝𝑝

𝜂 (%) = 𝐽𝑝ℎ (

𝐼𝑖𝑛

) × 100

..…… (2)

where Jph denoted the photocurrent density, Iin is the intensity of the incident light, and Vapp = Vm − Voc, where Vm and Voc are the potentials of the working electrode (V vs RHE) measured under illumination and in open circuit conditions in the same electrolyte. The proposed band alignment results in a heterostructure, where external electrical bias will always be required to separate and transfer charge carriers to produce H2 at the counter electrode. It may also be observed that no photocurrent is observed at zero bias. The photoconversion efficiency (η) as a function of the applied potential (in RHE) is plotted and shown in Figure 6(b). The Fe2O3/TiO2 and Fe2O3/H:TiO2 photoanodes exhibit an optimal η values of 1.13% and 1.68% at 0.6 V vs RHE respectively, while a very small values of 0.04% 0.003% and 0.55% are observed for the pristine Fe2O3, TiO2 and H:TiO2 photoanodes. This could be attributed to the different synthesis methods have been used to fabricate TiO2 thin films which may lead to different surface properties and catalytic response.


Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (a) Mott-Schottky measurement and (b) EIS spectra (Nyquist plots) of Fe2O3, Fe2O3/TiO2, and Fe2O3/H:TiO2 thin film samples measured in 1 M NaOH electrolyte. Further, the Mott-Schottky measurements were carried out on Fe2O3, Fe2O3/TiO2 and Fe2O3/H:TiO2 at applied frequency of 1 kHz under dark conditions in order to examine the effect of hydrogen annealing on intrinsic electrical properties. The flat band potential (VFB) and the carrier density (ND) values were estimated using the Mott-Schottky equation48: 𝐶 −2 =

2 𝑞𝜖𝜊 𝜖𝑠 𝑁𝐷

[𝑉 − 𝑉𝐹𝐵 −

𝐾𝑇 𝑞

]

…………… (3)

The observed values of flat band potential w. r. t RHE potential scale shifts to more negative (cathodic) side from 0.9 V for Fe2O3 to 0.2 V for Fe2O3/TiO2 heterostructure photoanode. Whereas, hydrogen annealing of top TiO2 layer reduced the flat band potential to 0.1 V vs RHE. Additionally, the electrochemical impedance spectroscopy (EIS) measurements were conducted under illuminatio on all three samples (Figure 7(b)) to study the charge carrier dynamics at the photoanode/electrolyte interface. From observed Nyquist plots, it is clear that the diameter of the semi-circular arcs (which shows the equivalent charge transfer resistances) is much smaller for Fe2O3/TiO2 and Fe2O3/H:TiO2 as compared to pristine Fe2O3 photoanode. The smaller diameters or charge transfer resistance (values shown in Table S1) in Fe2O3/TiO2 and Fe2O3/H:TiO2 heterostructure photoanodes indicate the strongly improved charge transport properties in the heterostructure samples as compared to pristine Fe2O3 sample. 17 ACS Paragon Plus Environment


Figure 8. Transient photocurrent density response of TiO2, H:TiO2, Fe2O3, Fe2O3/TiO2 and Fe2O3/H:TiO2 thin film samples measured in 1 M NaOH electrolyte. The inset of Figure 8 shows few cycles (20 s each) of TiO2, H:TiO2, Fe2O3 samples transient photocurrent response. Further, for investigating the long-term stability of the photoanodes for PEC applications, the samples are tested for its stability by chronoamperometry in 1 M NaOH under chopped (Figure 8). After Switching ON the light, a spike is observed in the photocurrent density due to the fast upshot upon excitation, afterward the photocurrent swiftly returned to a saturated and stable state within 10-20 seconds as shown in inset of Figure 8. Fig. S7 shows the total current under illumination of all photoanodes TiO2, H:TiO2, Fe2O3, Fe2O3/TiO2 and Fe2O3/H:TiO2 over a time of 300 min to confirm the stability. It shows that under illumination, the photocurrent of the photoanodes get saturate after ~25 sec and then remains steady up to 300 minutes. The photocurrent was ~0.01, 1.1, 0.22, 2.1, and 3.3 mA/cm2 for TiO2, H:TiO2, Fe2O3, Fe2O3/TiO2, and Fe2O3/H:TiO2, respectively. The attained good stability and high efficiency indicate that the Fe2O3/H:TiO2 heterostructure photoanode possess the original structure after prolonged PEC water splitting performance and the efficiency further improves with sustained stability after hydrogen treatment. In other way, very high photocurrents observed in Fe2O3/TiO2 and Fe2O3/H:TiO2 photoanodes with decreased onset potential shows the benefit provided by Fe2O3/TiO2 heterojunctions in imparting favourable photocatalytic improvements.


Page 18 of 29

Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. IPCE spectra of TiO2, Fe2O3, H:TiO2, Fe2O3/TiO2 and Fe2O3/H:TiO2 thin film photoanodes measured in 1 M NaOH electrolyte. To examine the photocurrent as a function of wavelength of incident light, the incident photon to current conversion efficiency (IPCE) measurements have also been carried out in the visible region of spectrum using 1 M NaOH as a electrolyte solution (Figure 9). The observed results are consistent with their corresponding J−V curves (Figure 6a). The pristine TiO2 is photoactive only in ultraviolet region, thus, shows negligible IPCE in visible region, whereas pristine Fe2O3 shows IPCE below 500 nm as it starts absorption below this wavelength though its value is small (5%) because of fast carrier recombination. Post hydrogen treatment results in improved IPCE in visible region for TiO2 thin film photoanodes as there are defect states created above the VBM. The heterostructure show improved photoresponse in the visible light region (for wavelength

An All Oxide

Recommend Documents