Fe3O4

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Electronically-Coupled Phase Boundaries in #-Fe2O3/Fe3O4 Nanocomposite Photoanodes for Enhanced Water Oxidation Jennifer Leduc, Yakup Gönüllü, Pedram Ghamgosar, Shujie You, Johanne Mouzon, Heechae Choi, Alberto Vomiero, Matthias Grosch, and Sanjay Mathur ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01936 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Applied Nano Materials

Electronically-Coupled Phase Boundaries in -Fe2O3/Fe3O4 Nanocomposite Photoanodes for Enhanced Water Oxidation Jennifer Leduca,ǂ, Yakup Goenuelluea,ǂ, Pedram Ghamgosarb, Shujie Youb, Johanne Mouzonb, Heechae Choia, Alberto Vomierob, Matthias Groscha, Sanjay Mathura,*

a

University of Cologne, Institute of Inorganic Chemistry, University of Cologne, 50939,

Cologne, Germany. b

Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå

University of Technology, 97187, Luleå, Sweden. ǂ both

authors contributed equally

* Corresponding author. Email: [email protected] Keywords: solar water splitting, valence dynamics, magnetite, Raman, single-source CVD, heterostructures

Abstract: Photoelectrochemical (PEC) water splitting reactions are promising for sustainable hydrogen production from renewable sources. We report here, the preparation of -Fe2O3/Fe3O4 composite films via a single-step chemical vapor deposition of [Fe(OtBu)3]2 and their use as efficient photoanode materials in PEC setups. Film thickness and phase segregation was controlled by varying the deposition time and corroborated through cross-section Raman spectroscopy and scanning electron microscopy. The highest water oxidation activity (0.48 mA/cm2 at 1.23 V vs RHE) using intermittent AM 1.5 G (100 mW/cm2) standard illumination was found for hybrid films with a thickness of 11 µm. This phenomenon is attributed to an improved electron transport resulting from a higher magnetite content towards the substrate interface and an increased light absorption due to the hematite layer mainly located at the top surface of the film. The observed high efficiency of -Fe2O3/Fe3O4 nanocomposite photoanodes is attributed to the close proximity and establishment of 3D interfaces between the weakly ferro- (Fe2O3) and ferrimagnetic (Fe3O4) oxides, which in view of their differential chemical constitution and valence states of Fe ions (Fe2+/Fe3+) can enhance the charge separation and thus the overall electrical conductivity of the layer. 1 ACS Paragon Plus Environment

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1.

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Introduction

Hematite (-Fe2O3) has been intensively studied as anode material for photoelectrochemical (PEC) water splitting applications due to its high stability, non-toxicity, abundance, suitable band gap (~2.1 eV) and high theoretical maximum solar-to hydrogen (STH) conversion efficiency (15%) which makes it a promising candidate for cost-effective PEC hydrogen production.1-2 However, due to critical limitations (low carrier mobility, short carrier lifetime, high density of surface states, slow kinetics for oxygen evolution), the values of experimentally observed maximum STH efficiency are still far away from reaching the theoretical value.3-4 Therefore, several attempts were undertaken, among those nanostructuring, doping, formation of composite materials and usage of co-catalysts, to circumvent these limitations. Nanostructured hematite electrodes in the form of nanoparticles5, nanowires6, nanotubes7, nanoflowers8 or nanocones9 have been generated to overcome the drawback of a small hole diffusion length (2-4 nm) as well as a short excited-state lifetime (3-10 ps).4,

10

Doping

represents a well-established method for the enhancement of PEC performance as it can be used to increase the carrier concentration and to fine-tune the band gap energies and band positions. 11

Thus, the incorporation of various dopants (e.g. Ti12, Zr13, Si14, Sn15, Ni16, Ta17 or Cr18) into

the hematite lattice was investigated and correlated with the water splitting efficiency. The effect of Sn doping of hematite films deposited on F:SnO2 substrates (FTO) was examined by Wang et al. who found that Sn ions alter the nucleation and growth of the hematite film and hence influence the morphology, physical properties and PEC performance of the material. The resulting enhancement in PEC performance was explained by an increase in charge carrier diffusion and transfer due to the creation of oxygen vacancies (reduction of Fe3+ to Fe2+).19 Investigations on the formation of oxygen vacancies and their influence on the PEC performance of hematite were performed by several groups.20-25 Thereby, oxygen vacancies were either introduced via high temperature treatment of hematite films in a reducing atmosphere26 or under low oxygen partial pressure22. In general, oxygen vacancies were reported to act as shallow donor dopants in hematite increasing the overall majority carrier electron concentration and consequently the conductivity.21 More detailed mechanistic studies were performed using transient absorption spectroscopy by Forster et al., who found that oxygen vacancies block slow surface hole-bulk electron recombination pathways.27 However, the introduction of a too high amount of oxygen vacancies was reported to result in a reduction of PEC performance, due to a partial transition into magnetite (Fe3O4).21 In our recent work, we have examined the effect of directed post-deposition plasma-chemical reduction of hematite.28 Through variation of the process temperature during hydrogen plasma treatment, different 2 ACS Paragon Plus Environment

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fractions of Fe3O4 (mixed-valent Fe+2/Fe+3) in the resulting Fe3O4/-Fe2O3 nanocomposites could be achieved. For temperatures lower than 300 °C and higher than 500 °C, either pristine -Fe2O3 or Fe3O4 could be selectively generated. Notably for this work, even though Fe3O4 had been reported to be photo-inactive29, the mixed Fe3O4:-Fe2O3 photoanodes showed a better PEC performance (photocurrent density of 3.5 mA/cm² at 1.8 V vs. RHE (reversible hydrogen electrode) and onset potential of 1.28 V vs. RHE) than the respective pristine hematite samples. This enhancement in PEC response was ascribed to a reduced band gap energy and higher carrier density resulting from the generation of additional charge carriers upon reduction of Fe3+.29-31 Recently, it has been shown for iron oxide nanoparticles that the phase formation is dependent on the synthesis and/or processing temperature.32 Herein, we report on a simple method to significantly improve the water oxidation performance of -Fe2O3 photoanodes by the directed growth of -Fe2O3/Fe3O4 hybrid film structures via thermal CVD of [Fe(OtBu)3]2. The film thickness as well as the local distribution of magnetite in the composite material increase the charge carrier generation and collection without decreasing the carrier transport and can simply be controlled by varying the deposition time. The interplay of phase composition, allocation and film thickness was investigated in detail using (cross-section) Raman spectroscopy and correlated with the water oxidation efficiency.

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2.

Experimental Part

2.1

Material Synthesis

The gas phase deposition of Fe2O3 layers was performed using [Fe(OtBu)3]2 as precursor in a horizontal cold-wall CVD reactor.33-34 The precursor temperature was kept constant at 100 °C to ensure a homogenous gas flow. The precursor flux was guided to an inductively heated (500 °C) substrate using low pressure (~ 3x10-6 mbar). The deposition time was varied between 5-45 min in order to scan for morphological and crystallographic changes. Iron oxide layers were deposited onto conductive F:SnO2 (FTO) substrates (Sigma Aldrich, ~8 Ω/sq) for photoelectrochemical measurements and silicon substrates for XRD analysis. The substrates were cleaned before CVD experiments by ultrasonication in water and isopropanol. A post deposition heat treatment was conducted at 500 °C for 5 h in air (scheme 1).

Scheme 1: Illustration of the fabrication and chemical composition of as deposited and annealed iron oxide samples prepared via CVD (time-dependent).

2.2

Materials Characterization

To investigate the microstructural properties of the film, a Nikon Eclipse MA200 was used. This light optical microscope is equipped with Nikon's DS-U2 Camera Control Unit and NISElements tracer. The film morphology was analyzed by field emission-scanning electron microscopy (FE-SEM) using a SUPRA 40VP instrument (Zeiss) operated at 10.0 kV, while cross-sections of the film was investigated using a Magellan 400 (the FEI company) at 3kV. AFM topography measurements were performed using a Park Systems XE-100 in true noncontact mode with a cantilever (PPP-NCHR-20, Nanosensors). Energy dispersive X-ray spectroscopy (EDS) was carried out in parallel to SEM observations. The powder X-ray diffraction (XRD, STOE-STADI MP) patterns were measured in reflection mode using Cu Kα (λ = 0.15406 nm) radiation. Decomposition analysis was conducted using a quadrupole-mass spectrometer QMG 220 M3 Prisma Plus (Pfeiffer). As ion source electron impact (EI) with 4 ACS Paragon Plus Environment

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70 eV was used. Signal intensity was increased with a secondary electron multiplier with 940 V (detection limit 10-15 A). Scan stair mode from m/z= 0 – 100 with a filter time of 200 ms/amu was used. X-ray photoelectron spectroscopy (XPS) measurements were carried out at a pressure of 10-9 mbar under Al Kα excitation (1486.6 eV) with an ESCA M-Probe spectrometer (Surface Science Instruments SSI). Correction to the binding energies was done in reference to the C1ssignal (284.8 eV). Spectral corrections and composition calculations were done with CasaXPS. Raman spectra were recorded using a Senterra Raman spectrometer with 1200 groove/mm grating under 532 nm and/or 785 nm laser excitation. Electrochemical measurements were performed in a flat three-electrode electrochemical cell using a saturated calomel electrode (SCE) as the reference, a Pt wire as the counter electrode and 1 M NaOH as the electrolyte (pH = 13.6). Linear sweep voltammetry (10 mV/s) was carried out in a potential range from -1 to 1 V vs. SCE using a potentiostat (PAR, Versa state IV) in the dark and under simulated solar illumination (Xe lamp, 150 W, Oriel with a Schott KG-3 filter). Potentials with respect to the reversible hydrogen electrode (RHE) scale were calculated using the Nernst equation (1). 𝐸𝑅𝐻𝐸 = 𝐸𝑆𝐶𝐸 + 𝐸0𝑆𝐶𝐸 +0.059𝑝𝐻 (1)

2.3

Density Functional Theory Computations

To analyze the enhanced photocatalytic activity of heterojunctioned iron oxides, density functional theory (DFT) calculations were performed using the Vienna ab initio package (VASP).35-36 Interactions between valence and core electrons were described through the projector augmented wave (PAW) method.37 A generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was employed for the planewave basis expansion.38-39 A kinetic energy cut-off of 400 eV was used. Brillouin zones were sampled with a gamma-centered 2 x 2 x 1 k-point grid in the supercell of Fe3O4(001)/Fe2O3(110) junction which consists of 96 atoms (40 Fe and 56 O atoms).40 The Methfessel-Paxton smearing scheme was applied with a smearing width of 0.1 eV.41 The energy convergence criteria in the selfconsistent field was set to 10-5 eV. All geometry structures were fully relaxed until HellmanFeynman forces achieved a range of 0.02 eVÅ ―1. To avoid neighboring periodic images, the vacuum region in the z direction was set to a value larger than 20 Å.



3.

Results and Discussion

Using a precursor charge of 300 mg, a non-linear film growth from 1.5 m (5 min) to 11 µm (45 min) was observed (figure S1a). The 5 to 20 min depositions exhibit dark red to black colors with a band gap value of ~2.15 eV (Supporting information, figure S2), whereas the 30 and 45 min samples appear opaque and grey. Figures S1b-c display top view SEM images of 5 and 45 min depositions on FTO. The surfaces of both samples are rough as determined by AFM analysis (Ra(5min)=20 nm; Ra(45min)=168 nm). 45 min growth time resulted in the formation of micrometer sized large grains, whereas for shorter deposition times the grain sizes came out significantly smaller and the surfaces were much smoother. To evaluate the surface and bulk structure of the films, SEM was carried out on freshly cleaved cross sections of both the 5 and 45 min depositions (figure 1). The bulk layers were composed of densely intergrown grains and exhibited a microstructure typical for competitive growth. The size of the largest grains on the top of the film was 5.6 m), oxidation of the magnetite grains via post-annealing in air (500 °C, 5 h) did not proceed to completion resulting in Fe3O4/-Fe2O3 nanocomposite films showing a compositional gradient with a higher magnetite content towards the bottom of the layer. This gradient supports carrier separation and transport as it allows for faster electron diffusion towards the backside of the electrode through iron redox cycling. Intriguingly, even after annealing for 48 h in air the magnetite phase did not oxidize pointing towards the stability of the nanocomposite structure.

Figure 3: XRD patterns of composite films deposited at different deposition times (5, 10, 20, 30 and 45 min). Due to the overlap of FTO substrate reflexes with the iron oxide reflexes, XRD results are shown for samples deposited onto silicon substrates only to investigate their crystallinity. XRD patterns for depositions onto FTO show similar results (see figure S5) as confirmed by Raman analysis (figure 4).

The local distribution of magnetite throughout the film was studied by Raman spectroscopy performed using different laser excitations (532 and 785 nm) and thus different penetration depths (figure 4). Whereas the 532 nm laser is more sensitive to surface features (figure 4b), the 785 nm laser penetrated deeper and revealed the bulk composition (figure 4a). Only hematite signals (220, 241, 287, 405, 496, 606 cm-1) were detected in the planar surface of all samples using 532 nm excitation.46 Apart from hematite, only the signal from the FTO substrate (broad band at ~1380 cm-1) was detected in the bulk of the sample obtained after 5 min 9 ACS Paragon Plus Environment


deposition time with 785 nm excitation. In contrast, the 30 and 45 min depositions exhibited hematite at the surface and both hematite and magnetite (660 cm-1) in the bulk. a) FeOx _FTO, ex= 785 nm

b) FeOx _FTO, ex= 532 nm 45min 30min 20min 10min 5min

++ +

+

O

+: hematite + + +

O: magnetite

+

+

*: FTO

+ +

Intensity

O

Intensity

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+ 200

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* + 400

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-1

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Raman shift (cm )

Raman shift (cm )

Figure 4: Raman spectra of iron oxide films using a) 785 nm and b) 532 nm laser excitations.

To investigate the phase distribution from the substrate to the film surface, Raman spectra were recorded on cross-sectional films (figure 5). The line scans showed that magnetite was the dominant phase towards the film-substrate interface, whereas a higher amount of hematite was detected at the surface. In the bulk, a dispersed 3D heterojunction is formed with a homogeneous spatial distribution of magnetite and hematite phases.

Figure 5: Raman spectra of CVD prepared iron oxide films using a) 785 nm and b) 532 nm laser excitations for the 45 min deposition. The spectra were recorded on the cross section (inset in a)) of a freshly fractured sample on a silicon substrate; the red arrow in the inset of a) and in the main panels a) and b) from the bottom to the top refers to the line scan direction from the substrate to the surface of the sample.


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The PEC performance of -Fe2O3/Fe3O4 composite films was evaluated by recording current– voltage curves in 1 M NaOH solution using intermittent AM 1.5 G (100 mW/cm2) standard illumination (figure 6a). The onset potential of the samples and their photocurrent density at 1.23 V vs RHE for different deposition times (figure 6b) showed no significant change in the onset potential of the samples. In contrast, Shinde et al. observed either an anodic or a cathodic shift depending on the annealing conditions, when they replaced Fe3+ with Sn4+ at the surface of Fe2O3 photoanodes.47 The 5 min deposition exhibited a photocurrent density of ~0.28 mA/cm² (at 1.23 V vs. RHE), which is comparably very high for a pristine hematite layer with a thickness of ~1.5 µm so far (table S3). An increase in thickness without changing the chemical composition (10 min, ~4 µm) resulted in a lower PEC activity, due to the poor hole diffusion length in pristine hematite (2-4 nm).4 The opposite trend was observed when the as deposited magnetite layers were only partly oxidized to give -Fe2O3/Fe3O4 hybrid film structures. Although magnetite is considered as photoinactive,29 an increase in deposition time and thus thickness and magnetite content resulted in an enhanced photocurrent density of 0.48 mA/cm2 (at 1.23 V vs. RHE) for the 45 min sample.

Figure 6: a) Photocurrent density-voltage curves of FeOx layers and b) photocurrent density and onset potential of the FeOx layers at 1.23 V vs RHE depending on the deposition time.

This improvement can be attributed to two effects, an increase in electrical conductivity and a reduction of bulk defects.48 The increased conductivity can be attributed to the higher amount of charge carriers in magnetite, effectively the delocalized electrons of the Fe+2/Fe+3 couple, which contribute mainly to the charge transport in magnetite.49-50 The hybrid structure facilitates the charge collection upon light absorption and the charge separation due to the distribution of the phases throughout the film. Lower concentrations of magnetite are present at the surface, which is beneficial for the light absorption of the photoactive hematite layer. Additionally, the concentration gradient of magnetite supports the electron transfer due to band 11 ACS Paragon Plus Environment


bending at the heterojunctions. The higher magnetite content towards the bottom of the layer facilitates the electron transport to the counter electrode by collecting from the top α-Fe2O3. In addition, the very dense layer of intergrown grains reduces the amount of bulk defects resulting in a decreased recombination rate.31

Figure 7: Raman spectra using 785 nm and 532 nm laser excitations of iron oxide films deposited on FTO with subsequent H2 (a and b) or O2 (c and d) plasma treatments for deposition times of 5 and 45 min, respectively.

In order to corroborate this hypothesis, we have performed post-deposition oxygen and hydrogen plasma treatments to test the PEC performances of phase pure α-Fe2O3 and Fe3O4 layers with film thicknesses of 1.5 µm and 11 µm. Using oxygen plasma, the surface of the 45 min sample was mostly oxidized to -Fe2O3 as confirmed by the absence of the Raman peaks for the magnetite phase (figure 7c-d) and the decreased magnetite peaks in the XRD pattern (figure S5).

Figure 8: Current density-voltage curves of hydrogen and oxygen plasma treated and reference FeOx layers deposited for a) 45 min and b) 5 min.


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As expected, the photocurrent density of the 11 µm -Fe2O3 layer decreased to a value of 0.06 mA/cm² (at 1.23 V vs. RHE) resulting from the higher charge carrier recombination rate (figure 8a).4 Even though the phase composition of the 5 min sample was not influenced by the oxygen plasma (figures S6a-b), the PEC performance also decreased which was ascribed to the reduced crystallinity of the layer and thus higher amount of bulk defects (figure 8b). A decrease in photocurrent density and an anodic onset potential shift upon oxygen plasma treatment was likewise observed by Hu et al. and was ascribed to the filling of prevalent oxygen vacancies resulting in a decrease of surface states and hence impeded water adsorption.20 Hydrogen plasma treatment of both the 5 and 45 min samples resulted in the formation of magnetite at the film surfaces (figure 7 and figure S6). However, dark measurements in alkaline solution (figure S7) showed that these photoanodes were not stable when a potential of higher than 0.3 V was applied. From these experiments, it can be concluded that the formation of hybrid -Fe2O3/Fe3O4 film structures as well as the phase distribution throughout the layer are of importance to give reasonable PEC performances for iron oxide photoanodes with a film thickness of higher than 1.5 µm. The enhanced photocatalytic activity of iron oxide by the formation of a Fe3O4/Fe2O3 junction was further analyzed by DFT calculations (figure 9). The electron potential was higher (by 1.4 V) on the Fe2O3 side, allowing the photoexcited hole (electron) carrier to spontaneously drift to the Fe2O3 (Fe3O4) phase (figure 9a). The calculated differential charge densities are expressed with yellow and green isosurfaces in figure 9a for electron accumulation and depletion, respectively. Interestingly, a substantial amount of charge transfer across the interface was not observed. However, instead, an interface polarization was induced at the Fe3O4(001)/Fe2O3(001) interface with an electron depletion and accumulation on the Fe3O4 and Fe2O3 sides, respectively. Due to the internal built-in potential by the interface polarization, the conduction band minimum (CBM) and valence band maximum (VBM) of the Fe3O4 phase are positioned lower than those of Fe2O3 after forming a junction (figure 9b). The band gap of Fe3O4 was reported to amount to ~0.1 eV. As demonstrated in figure S9, the spin-minority states exhibit a narrow band gap, which is consistent with the experimental report.51 However, the spin-majority states have a band gap of ~ 2.3 eV. Hence, the band offset driven electron hole separation between Fe2O3 and Fe3O4 is expected to occur via spin-majority states of Fe3O4. The switched relative band positions between the two semiconductor phases observed in this calculation are comparable to those in the Fe2O3/ZnO system.52 The Fe3O4/Fe2O3 interface can 13 ACS Paragon Plus Environment


enhance photocatalytic activity of the whole system as the interface built-in potential separates photoexcited electron-hole pairs (electrons to Fe3O4 and holes to Fe2O3) and elongate the lifetime of the photoexcited carriers. Meanwhile, the Fe3O4 phase also becomes capable of splitting water as both the CBM and VBM are lowered due to the interface polarization induced by the Fe2O3 phase. Hence, even partial phase transition to Fe3O4 does not degrade the water splitting performance as long as the redox-interface with Fe2O3 is well kept and the thickness of Fe3O4 is properly controlled, since the internal built-in interface potential effect decays with distance.

Figure 9: a) Calculated electrostatic potential and supercell model of Fe3O4/Fe2O3 heterojunction with calculated differential charge density, and b) schematics of band edge levels before and after junction formation between Fe3O4 and Fe2O3. The crystallographic orientations of Fe2O3 and Fe3O4 in the Fe2O3(001)-Fe3O4(001) supercell were chosen based on experimental reports and/or low surface energy surface orientations.53-54 As the calculated ionization potential of 6.6 eV was in good agreement with the reported value of 6.9 eV,55 the validity of the band edge level calculation was confirmed. The vacuum layer was put over the Fe2O3-Fe3O4 junction supercell to obtain the energy levels of the Fermi energy.


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4.

Conclusions

To summarize, mixed -Fe2O3/Fe3O4 heterostructures generated via CVD of [Fe(OtBu)3]2 and subsequent thermal oxidation showed improved photoactivity for water oxidation. The -Fe2O3/Fe3O4 anode yielded a photocurrent density of 0.48 mA/cm2 at 1.23 V (vs RHE) despite a film thickness of 11 µm. The enhancement in photoactivity is attributed to the improved electron transport resulting from the higher magnetite content towards the bottom of the layer and the increased light absorption of the hematite layer mainly located at the top of the film. To the best of knowledge, this is the first report on efficient hematite-based water oxidation catalysts with a layer thickness of higher than 10 µm. Acknowledgements Authors are thankful to the University of Cologne for providing the infrastructural support. J.L. is thankful to Fonds der chemischen Industrie for a PhD fellowship. The financial support in the framework of the DFG priority programs (SPP 1613; “Fuels Produced Regeneratively Through Light-Driven Water Splitting: Clarification of the Elemental Processes Involved and Prospects for Implementation in Technological Concepts” and SPP 1959 "Manipulation of matter

controlled

by

electric

and

magnetic

field:

Towards novel synthesis and processing routes of inorganic materials") and the Framework Program of the European Commission (FP7) that funded the European Project SOLAROGENIX (www.solarogenix.eu) are gratefully acknowledged. A.V. acknowledges Knut & Alice Wallenberg Foundation, the Swedish Foundations Consolidator Fellowship, LTU Labfund program and Kempe Foundation for partial funding. A.V. acknowledges the European Union’s Horizon 2020 research and innovation program under grant agreement No 654002 for financial support. S.J. acknowledges Kempe Foundation for a postdoctoral fellowship. HC and SM acknowledge the DAAD and the German Federal Ministry of Education and Research (BMBF) to support this research through MOPGA-GRI Initiative.

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32. Bora, D. K.; Braun, A.; Erat, S.; Safonova, O.; Graule, T.; Constable, E. C. Evolution of Structural Properties of Iron Oxide Nano Particles during Temperature Treatment from 250 °C–900 °C: X-Ray Diffraction and Fe K-Shell Pre-Edge X-Ray Absorption Study. Curr. Appl.Phys. 2012, 12, 817-825. 33. Mathur, S.; Veith, M.; Sivakov, V.; Shen, H.; Huch, V.; Hartmann, U.; Gao, H. B. Phase-Selective Deposition and Microstructure Control in Iron Oxide Films Obtained by Single-Source CVD. Chem. Vap. Deposition 2002, 8, 277-283. 34. Mathur, S.; Veith, M.; Sivakov, V.; Shen, H.; Gao, H. B. Composition, Morphology and Particle Size Control in Nanocrystalline Iron Oxide Films Grown by Single-Source CVD. J. Phys. IV 2001, 11, 487-494. 35. Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comp. Mater. Sci. 1996, 6, 15-50. 36. Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 37. Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. 38. Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces - Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671-6687. 39. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 40. Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. 41. Methfessel, M.; Paxton, A. T. High-Precision Sampling for Brillouin-Zone Integration in Metals. Phys. Rev. B 1989, 40, 3616-3621. 42. Fiz, R.; Appel, L.; Gutierrez-Pardo, A.; Ramirez-Rico, J.; Mathur, S. Electrochemical Energy Storage Applications of CVD Grown Niobium Oxide Thin Films. Acs Appl. Mater. Interfaces 2016, 8, 21423-21430. 43. Li, X. Y.; Lin, E. H.; Zhang, C. Y.; Li, S. B. Preparation of gamma-Fe2O3-Fe2O3 Ultrafine Powders by Laser VaporPhase Reaction. J. Mater. Sci. Lett. 1995, 14, 1335-1337. 44. Fujii, T.; de Groot, F. M. F.; Sawatzky, G. A.; Voogt, F. C.; Hibma, T.; Okada, K. In Situ XPS Analysis of Various Iron Oxide Films Grown by NO2-Assisted Molecular-Beam Epitaxy. Phys. Rev. B 1999, 59, 3195-3202. 45. Zhang, X. Q.; Klaver, P.; van Santen, R.; van de Sanden, M. C. M.; Bieberle-Hutter, A. Oxygen Evolution at Hematite Surfaces: The Impact of Structure and Oxygen Vacancies on Lowering the Overpotential. J. Phys. Chem. C 2016, 120, 1820118208. 46. Chernyshova, I. V.; Hochella, M. F.; Madden, A. S. Size-Dependent Structural Transformations of Hematite Nanoparticles. 1. Phase Transition. Phys. Chem. Chem. Phys. 2007, 9, 1736-1750. 47. Shinde, P. S.; Choi, S. H.; Kim, Y.; Ryu, J.; Jang, J. S. Onset Potential Behavior in alpha-Fe2O3 Photoanodes: The Influence of Surface and Diffusion Sn Doping on the Surface States. Phys. Chem. Chem. Phys. 2016, 18, 2495-509. 48. Pyeon, M.; Ruoko, T.; Leduc, J.; Gönüllü, Y.; Deo, M.; Tkachenko, N. V.; Mathur, S. Critical Role and Modification of Surface States in Hematite Films for Enhancing Oxygen Evolution Activity. J. Mater. Res. 2018, 33, 455-466. 49. Wu, N.-L.; Wang, S.-Y.; Han, C.-Y.; Wu, D.-S.; Shiue, L.-R. Electrochemical Capacitor of Magnetite in Aqueous Electrolytes. J. Power Sources 2003, 113, 173-178. 50. Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y. Sn-Doped Hematite Nanostructures for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 2119-2125. 51. Leonov, I.; Yaresko, A. N.; Antonov, V. N.; Anisimov, V. I. Electronic Structure of Charge-Ordered Fe3O4 from Calculated Optical, Magneto-Optical Kerr Effect, and O K-Edge X-Ray Absorption Spectra. Phys. Rev. B 2006, 74, 165117. 52. Zhang, J.; Liu, X. H.; Wang, L. W.; Yang, T. L.; Guo, X. Z.; Wu, S. H.; Wang, S. R.; Zhang, S. M. Synthesis and Gas Sensing Properties Of alpha-Fe2O3@ZnO Core-Shell Nanospindles. Nanotechnology 2011, 22, 18. 53. Pauling, L.; Hendricks, S. The Crystal Structures of Hematite and Corundum. J. Am. Chem. Soc. 1925, 47, 781-790. 54. Novotny, Z.; Mulakaluri, N.; Edes, Z.; Schmid, M.; Pentcheva, R.; Diebold, U.; Parkinson, G. S. Probing the Surface Phase Diagram of Fe3O4(001) towards the Fe-Rich Limit: Evidence for Progressive Reduction of the Surface. Phys. Rev. B 2013, 87, 195410. 55. Jiang, C.; Moniz, S. J. A.; Wang, A.; Zhang T.; Tang J. Photoelectrochemical Devices for Solar Water Splitting Materials and Challenges. Chem. Soc. Rev. 2017, 46, 4645-4660.



Cross section and top view SEM micrographs of CVD prepared iron oxide films deposited on FTO for a-c) 5 min and d-f) 45 min. Images c) and f) display the morphology of the top layer (100-350 nm) for both deposition times. 338x190mm (300 x 300 DPI)

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Color-surface mass spectrum recorded from 0-100 amu. The precursor [Fe(OtBu)3]2 was sublimed at a temperature of 88 °C and a pressure of 5x10-4 mbar and was fragmented at room temperature (0-300 s, white line) and at 500 °C (>370 s). The red line marks the point where the precursor flow was increased. 157x125mm (150 x 150 DPI)



XRD patterns of composite films deposited at different deposition times (5, 10, 20, 30 and 45 min). 81x62mm (220 x 220 DPI)


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Raman spectra of iron oxide films using a) 785 nm and b) 532 nm laser excitations. 1016x508mm (120 x 120 DPI)



Raman spectra of CVD prepared iron oxide films using a) 785 nm and b) 532 nm laser excitations for the 45 min deposition. The spectra were recorded on the cross section (inset in a)) of a freshly fractured sample on a silicon substrate; the red arrow in the inset of a) and in the main panels a) and b) from the bottom to the top refers to the line scan direction from the substrate to the surface of the sample. 145x72mm (220 x 220 DPI)


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a) Photocurrent density-voltage curves of FeOx layers and b) photocurrent density and onset potential of the FeOx layers at 1.23 V vs RHE depending on the deposition time. 159x64mm (220 x 220 DPI)



Raman spectra using 785 nm and 532 nm laser excitations of iron oxide films deposited on FTO with subsequent H2 (a and b) or O2 (c and d) plasma treatments for deposition times of 5 and 45 min, respectively. 158x81mm (150 x 150 DPI)


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Current density-voltage curves of hydrogen and oxygen plasma treated and reference FeOx layers deposited for a) 45 min and b) 5 min. 141x68mm (220 x 220 DPI)



338x190mm (300 x 300 DPI)


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a) Calculated electrostatic potential and supercell model of Fe3O4/Fe2O3 heterojunction with calculated differential charge density, and b) schematics of band edge levels before and after junction formation between Fe3O4 and Fe2O3. 94x117mm (220 x 220 DPI)



Illustration of the fabrication and chemical composition of as deposited and annealed iron oxide samples prepared via CVD (time-dependent). 339x87mm (150 x 150 DPI)


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Fe3O4

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