Photoactive Zinc Ferrites Fabricated via Conventional CVD Approach

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Photoactive zinc ferrites fabricated via conventional CVD approach Daniel Peeters, Dereje Hailu Taffa, Marissa Marie Kerrigan, Andreas Ney, Niels Jöns, Detlef Rogalla, Stefan Cwik, Hans-Werner Becker, Markus Grafen, Andreas Ostendorf, Charles H. Winter, Sumit Chakraborty, Michael Wark, and Anjana Devi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02233 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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Photoactive zinc ferrites fabricated via conventional CVD approach AUTHOR NAMES Daniel Peeters,a Dereje H. Taffa,b Marissa M. Kerrigan,c Andreas Ney,d Niels Jöns,e Detlef Rogalla,f Stefan Cwik,a Hans-Werner Becker,f Markus Grafen,g Andreas Ostendorf,g Charles H. Winter,c Sumit Chakraborty,e Michael Wark,b Anjana Devia* AUTHOR ADDRESS a) Inorganic Materials Chemistry, Ruhr-University Bochum, Universitätsstraße 150, 44801 Bochum, Germany. * E-Mail: [email protected] b) Institute of Chemistry, Chemical Technology 1, Carl von Ossietzky University Oldenburg, Carl-von-Ossietzky Straße 9-11, 26129 Oldenburg, Germany. c) Department of Chemistry, Wayne State University Detroit, 5101 Cass Avenue, Detroit, Michigan 48202, United States. d) Institute of Semiconductor & Solid State Physics, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria. e) Department of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Universitätsstraße 150, 44801 Bochum, Germany. f) RUBION, Ruhr-University Bochum, Universitätsstraße 150, 44801 Bochum, Germany. g) Chair of Applied Laser Technologies, Ruhr-University Bochum, Universitätsstraße 150, 44801 Bochum, Germany. KEYWORDS. Photoelectrochemical water splitting; zinc ferrite; ternary oxides; chemical vapor deposition; band gap ABSTRACT: Owing to its narrow band gap and promising magnetic and photocatalytic properties, thin films of zinc ferrite (ZFO, ZnFe2O4) are appealing for fabrication of devices in magnetic recording media and photoelectrochemical cells. Herein we report for the first time the fabrication of photactive zinc ferrites via a, solvent free, conventional CVD approach and the resulting ZFO layers show promise as a photocatalyst in PEC water-splitting. For large scale applications, chemical vapor deposition (CVD) routes are appealing for thin film deposition, however very little is known about ZFO synthesis following CVD processes. The challenge in precisely controlling the composition for multicomponent material systems, such as ZFO, via conventional thermal CVD is an issue which is caused mainly by the mismatch in thermal properties of the precursors. The approach of using two different classes of precursors for zinc and iron with a close match in thermal windows led to the formation of polycrystalline spinel type ZFO. Under the optimized process conditions, it was possible to fabricate solely ZFO in the desired phase. This work demonstrates the potential of employing CVD to obtain photoactive ternary material systems in the right composition. For the first time, the application of CVD grown ZFO films for photoelectrochemical applications is being demonstrated, showing a direct band gap of 2.3 eV and exhibiting activity for visible light driven photoelectrochemical water splitting.

INTRODUCTION. Transition metal ferrites have attracted the attention of the research society for more than a century, mainly due to their interesting magnetic properties. The spinel type materials of the general formula MFe2O4 (M = Mn, Fe, Co, Ni, Cu, Zn or mixtures of these) are applied in the field of electrical and electromagnetic engineering, e.g. in magnetic recording media, high frequency applications, and magnetic core memories.1 Since the turn of the millennium, dwindling oil supplies and

projected climate changes have resulted in increased efforts for the development of new technology for energy storage and energy conversion. Consequently, ferrites have been explored for the suitability as an anode material in lithium-ion batteries.2 Quite recently, transition metal ferrites have been considered as photoanode materials in photoelectrochemical (PEC) water splitting.3 This has led to not only the pursuit of identifying new materials for photocatalysis, but also to identify the role of po-

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tential dopants in n-type iron oxide. Zinc ferrite, ZnFe2O4 or ZFO, is an n-type semiconductor with a direct band gap of 1.8 to 2.1 eV4-6 and suitable band gap positions to allow the reduction and oxidation of water without external bias.7 One can consider ferrites as a solid solution of iron(III) oxide with a transition metal oxide of the form MO and the coexistence of these two can potentiate properties of both materials. Indeed, it has been reported that the presence of iron oxide on zinc oxide enhances the charge carrier separation and vice versa.8 However, the focus of this report was only on the effects in photocatalytic reactions, rather than to identify in which form the co-catalyst is present. Usually moderate postdeposition annealing procedures rely on temperatures which are sufficiently high to enable a solid solution of the two oxides. There has not been a clear explanation to whether the enhanced photocatalytic effect is due to the coexistence of iron oxide and zinc oxide or due to the presence of zinc ferrite on top of one of the oxides. Noteworthy studies performed wherein Miao et al. and McDonald et al. have shown beneficial effects due to the presence of thin layers of ZnFe2O4 on iron oxide7-8 and Sheik et al. have shown similar effects when deposited on zinc oxide.9 Furthermore, the presence of defined interfaces between the materials as well as the microstructure of the system allow for device optimization.10-12 In a different way, Dom et al. proved beneficial effects of hematite on the activity of zinc ferrite.13 In addition, zinc ferrites show beneficial effects when applied as co-catalyst on CdS or CaFe2O4.14-15 Common synthesis procedures for ZFO can be generally divided into solution phase approaches,16-17 solid state reactions1 or vapor phase techniques18 and the usual processing of the material is in the form of films, nanoparticles or conventional powders. When it comes to the application of ZFO as a photoanode in catalytic reactions, deposition of the material on a conducting and transparent substrate and a sufficient electrical contact to the conducting layer is indispensable. Therefore, chemical and physical routes which allow the direct deposition on the electrode are preferred. Physical vapor phase routes such as pulsed laser deposition (PLD)19 and magnetron sputtering20 or chemical vapor phase routes like chemical vapor deposition (CVD) and spray pyrolysis typically offer the advantage of direct synthesis of the active material on the electrodes and allow precise stoichiometry, morphology and crystallinity control on large area coatings. CVD possesses several advantages compared to high vacuum physical vapor routes and is an industrially established process. To the best of our knowledge, reports on ZFO deposited by conventional CVD are lacking, with only an early report on CVD of ZFO, where [M(acac)x] (M = Zn, Ni, Fe; acac = 2,4pentanedionato) is applied to synthesize nickel zinc ferrite for magnetic applications.21 However, solvent assisted methods, such as liquid injection CVD (LICVD) or aerosol assisted CVD (AACVD), are reported based on [M(thd)x] (M = Zn, Ni, Fe; thd = 2,2,6,6-tetramethyl-3,5heptanedionato) dissolved in tetrahydrofuran or the bimetallic precursor [Fe2(acac)4(dmaeH)2][ZnCl4] (dmaeH = N,N-dimethylaminoethanol) dissolved in various organic

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solvents.5, 22-23 In times of dwindling oil reserves and the goal to manufacture green fuels with cheap inorganic materials, an overall synthesis approach independent of oil based organic solvents is favored. In this context Dom et al. analyzed the performance of zinc ferrite in photocatalysis and found that solvent-independent physical routes outperform solution-based chemical routes.24 Herein we present a solution-free synthesis approach for zinc ferrite via conventional chemical vapor deposition employing metal organic precursors namely the ketoester of iron [Fe(tbaoac)3] and ketominate of zinc [Zn(mpki)2]. Both these precursors have comparable thermal properties in terms of sublimation and decomposition thus enabling the formation of ternary metal oxides in the right stoichiometry. This is the first ever report where ZFO deposited via conventional thermal CVD has been successfully applied as photoanode in photoelectrochemical water splitting. Based on the thorough characterization of the ZFO layers using complementary analytical tools, it was possible to gain insights of the materials behavior during the application as photoanode. EXPERIMENTAL. The precursors [Fe(tert-butyl acetoacetate)3], [Fe(tbaoac)3], and [Zn(Nmethoxypropylketoiminate)2], [Zn(mpki)2] employed for the CVD of ZFO were synthesized in the laboratory according to literature25-26 and characterized to verify their formation and purity. They were then subjected to detailed thermal analysis. Thermogravimetric (TG) experiments were performed on a Seiko TG/DTA 6200/SII at ambient pressure (sample mass ≈ 10 mg), with a heating rate of 5 K min–1 (N2 flow rate = 300 mL min–1) in a temperature range of 30 – 550 °C. In order to estimate the evaporation rate, isothermal experiments at 120 °C were carried out for an extended period of time (~ 3hrs). Zinc ferrite thin films were deposited on 1.5 x 1.5 cm² Si(100) (Siegert Consulting) substrates in a custom built, horizontal cold-wall, low pressure CVD reactor. The substrates were cleaned prior to film deposition by a sequence of rinsing with 2-propanol and 10 minutes sonication in water in order to remove surface contaminations. The precursors [Fe(tbaoac)3], and [Zn(mpki)2] were vaporized at 120 °C and delivered to the substrate by a nitrogen gas flow (25 sccm for each precursor). Standard process conditions during the CVD process development on Si(100) substrates were set to a pressure of 1 mbar, duration of 1 hour per deposition, an oxygen gas flow of 50 sccm and the substrate temperature was varied between 400 °C and 800 °C. The as-deposited zinc ferrite thin films were analyzed by X-ray diffraction (XRD) in the θ - 2θ geometry on a Bruker D8 Advanced Diffractometer [Cu Kα radiation (1.5418 Å)] with a position sensitive detector. Surface morphology and surface coverage of the deposits were investigated by an environmental scanning electron microscope (ESEM, FEI ESEM Dual Beam™ Quanta 3D FEG). Transmission electron microscopy (TEM) measurements were performed with a JEOL 2100FS-TEM operating at 200 kV. ZFO sample scratches were dispersed


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under ultrasonication in Ethanol and mounted on copper grids by drop casting. High-resolution-TEM images were taken for structural determination through lattice plane spacing and Energy-dispersive X-ray spectroscopy (EDX Oxford ICNA Energy TEM250 with SDD-Detector XMax80) measurements to give sight specific compositional information. Integral magnetic measurements were performed using a commercial superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS XL5) applying the magnetic field in the film plane. Magnetism/hysteresis (M(H)) curves were recorded at 300 K and 2 K, respectively. M(T) curves were measured while warming from 2 K to 300 K in a field of 10 mT, after the sample was either cooled from 300 K to 2 K in a 5-T field (field-heated; FH) or after demagnetizing the sample in an oscillatory field at 300 K (zero-field cooled; ZFC). Subsequently a M(T) curve was measured while cooling down to 2 K in 10 mT (field-cooled; FC). Raman spectroscopy was performed using a confocal Raman microscope (InVia, Renishaw plc, GB). A frequency doubled Nd:YAG laser with a wavelength of 532 nm and a maximum output power of 500 mW was used for excitation. Electron probe microanalysis (EPMA) on ZFO films grown on Si(100) was applied to determine the ratios of zinc and iron. A Cameca SX Five FE field emission electron microprobe equipped with five wavelength dispersive spectrometers was applied and on every sample 30 measurement points were selected for each acceleration voltage. The raw data were corrected using the X-Phi procedure of Merlet.27 The composition of the thin films was then determined using the Cameca software 'Layer quant', which is based on the 'X-Film' method developed by C. Merlet.28 Rutherford backscattering spectroscopy (RBS) measurements were carried out using a 2.0 MeV He beam (intensity = 20-40 nA) at RUBION DTL on ZFO films grown on Si(100). The backscattered particles were measured at an angle of 160° by a Si detector with a resolution of 16 keV. The resulting spectra were analyzed with the help of the SimNRA program.29 A deuteron beam with the energy of 1 MeV was guided onto the samples tilted by an angle of 7° for nuclear reaction analysis (NRA) measurements. Protons emitted by nuclear reactions with light elements were detected at an angle of 135° with respect to the beam axis. The detector covered a solid angle of 23 msrad, and was shielded by a 6 μm Ni-foil to eliminate elastically scattered deuterons. Typical beam currents on the ZFO deposits were close to 40 nA in an area of ~ 1 mm in diameter, while the collected beam charge for a sample was 12 μC. X-ray photoelectron spectroscopy (XPS) was carried out using the ESCALAB 250 Xi (Thermo Fisher, UK) with a monochromatized Al Kα radiation source (hν = 1486.6 eV). The electron binding energies of the measured elements were referenced to adventitious C 1s of hydrocarbon contaminations at 284.8 eV. High resolution XPS core level spectra for C, O, Fe, and Zn were collected and analyzed using the Avantage software (version 5.951).UVVis absorption measurements of ZFO films deposited on FTO substrates were performed using Cary 4000 UV-Vis spectrophotometer (VARIAN) equipped with an integrating sphere using a spectral range of 200-800 nm.

Photoelectrochemical measurements were performed with a Zennium potentiostat controlled by Thales software (ZAHNER Elektrik). Three electrode configuration was employed for all measurements in 1 M NaOH (pH=13.6). Platinum wire and Ag/AgCl (sat. KCl) used as counter electrode and reference electrode, respectively. The working electrode was a zinc ferrite coated FTO (1.5 x 2.5 cm²; deposited at 600 °C) electrically contacted through a copper tape using the bare FTO surface and the illumination area (0.785 cm2) was defined with an O-ring. Light enters the cell from the electrolyte side through a quartz window. The light source is equipped with a calibrated white LED (WLC01, Zahner) controlled and powered by a second potentiostat (PP211, Zahner) and the output power was set to 100 mW cm-2. Current-voltage (IV) curves were recorded at a rate of 10 mV/s from -0.5 V to +1.0 V vs. Ag/AgCl (sat.KCl) in the dark and under illumination. Potential values were converted to the reference reversible normal hydrogen electrode (NHE) using the relationship (eq. 1).

o E NHE = E + E Ag / AgCl + 0.59 pH

(1)

where E is the experimentally measured potential vs Ag/AgCl and EoAg/AgCl is the standard potential of Ag/AgCl reference electrode against the NHE (0.196 V). Incident photon to current conversion efficiency (IPCE) measurements were carried out using a white LED (TLS, Zahner) coupled with an USB controlled monochromator between 430-730 nm. During the measurement a constant background light source is maintained and a small light excitation (ca. 10%) was superimposed at 0.2 Hz. The data were collected every 10 nm and simulated curves were obtained using the CIMPS software. For electrochemical impedance (EIS) measurements the potential was varied from -0.8 to +0.5V vs. Ag/AgCl with 50 mV steps and equilibrated for 30 s at each potential. Impedance data were collected using 10 mV ac amplitude at 1 kHz. The flat band potential (Vfb) was calculated using Mott-Schottky relationship (eq 2).

 2 kT  1  V − V fb −  2 = 2  app q  C  qN Dε oε r A 

  

(2)

where A is the surface area of the electrode, εo is the permittivity of free space and εr is dielectric constant of zinc ferrite, ND is the donor density, Vapp is the applied potential, T is the temperature and k is the Boltzmann’s constant. RESULTS AND DISCUSSION. CVD Process Optimization. The challenge in the CVD of ternary compounds is to find precursors for the respective elements with similar thermal properties. Firstly, the processes for the synthesis of the respective binary oxides should be understood, in order to know the compatibility of both precursors in terms of deposition temperature, pressure and gas feeding rates in order to optimize the process for the ternary metal oxide system. Based on the processes for zinc oxides and iron oxides, which have been previously developed by our group, we have chosen


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zinc (N-methoxypropyl)ketoiminate, [Zn(mpki)2], and iron tertbutyl acetoacetate, [Fe(tbaoac)3]. In these binary processes ZnO and α-Fe2O3 were synthesized under very similar conditions.30-31

Figure 1. TG analysis of [Zn(mpki)2] and [Fe(tbaoac)3] under constant N2 flow and isothermal TG of both compounds at 120 °C (inset).

In order to verify the similar vaporization behavior, thermogravimetry and isothermal gravimetry experiments were performed. Both compounds show a one-step mass loss with residual masses of 22 % and 2 % for the iron and zinc precursor, respectively. The onset of volatilization (1 % mass loss) is derived from Figure 1, being 145 °C for [Zn(mpki)2] and 144 °C for [Fe(tbaoac)3], respectively. As can be seen from the isothermal TG (Figure 1 inset) both compounds can be sublimed with constant rates at typical evaporation temperatures of 120 °C without decomposition. The evaporation rates of the compounds at 120 °C and atmospheric pressure are 0.012 µg/min and 0.015 µg/min for [Zn(mpki)2] and [Fe(tbaoac)3], respectively, and as such highly compatible. Due to the slightly different reactor set up used for the ternary oxides, at first depositions of the binary oxides were conducted at two different set reactor pressures (10 mbar and 1 mbar) for 60 minutes, at 600 °C. At 10 mbar no visible deposition of iron oxide was observed, whereas at 1 mbar ZnO and Fe2O3 were obtained, respectively. The overall gas flow (carrier gas and reactive gas) was kept constant, when combining two binary processes to a ternary. Consequently, optimized growth parameters for the growth of ZFO were set to 1 mbar, 50 sccm oxygen flow, 25 sccm nitrogen flow for each precursor, 60 minutes deposition and an evaporation temperature of 120 °C. The successful synthesis of ZFO thin films explored was verified by a comprehensive materials characterization. In particular, crystallinity, stoichiometry and morphology of the deposits are of high importance as well for various functional applications, especially in PEC water splitting of iron oxide containing photoanodes.32 For this purpose a complementary characterization approach including XRD, Raman spectroscopy, RBS/NRA, EPMA, XPS, EDX, and SEM/TEM has been adopted to investigate the samples.

Figure 2. X-ray diffraction patterns of zinc ferrite deposited on Si(100) at displayed deposition temperatures and reference patterns taken from ICSD database: No. 162843 for ZnO, No. 183974 for Fe3O4 and No. 72031 for ZnFe2O4.

Figure 2 displays the X-ray diffraction patterns of CVD grown materials based on the precursors [Zn(mpki)2], [Fe(tbaoac)3] and oxygen at 400 – 800 °C. At low temperatures (400 °C) the deposits were found to be amorphous in nature. Starting from 500 °C, three reflections at 29.9°, 35.3° and 42.8° corresponding to (220), (311) and (400) orientation in spinel-type ferrites were observed. The only distinctive reflection not also present in magnetite (Fe3O4) at 36.9° corresponding to (222) orientation of ZFO appears at 600 °C together with a reflection at 53.1° (442). Notably, a zinc oxide diffraction corresponding to (101) orientation is significantly different (36.3°) to the ZFO diffraction. With increasing deposition temperature no further reflections were observed, but a decrease in the full-width at half maximum of the reflection occurs. Accordingly the crystallite size, calculated with the Scherrer equation based on the most intensive reflection at 35.3°, increases from 22 nm at 500 °C to 42 nm at 800 °C. It should be noted that in conventional CVD processes of ternary iron oxides crystallographic phase purity is very difficult to achieve and is usually accompanied by contributions from the binary oxides.33-36


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ACS Sustainable Chemistry & Engineering very thin and with no evidence of pronounced surface features.

Figure 3. Raman spectra of zinc ferrite thin films on Si(100) at displayed temperatures. Beside the characteristic bands, bands of the underlying silicon substrate are present.

Raman spectroscopy was performed to evaluate the samples with regard to the presence of zinc ferrite and, equally important, the absence of other characteristic iron oxide bands (Figure 3). All the thin films deposited at temperature equal or above 500 °C do indicate the presence of spinel type ferrites. However, the Raman bands at 353, 442, 645 and 1125 cm-1 are assigned to ZnFe2O4, whereas the bands located at 305 and 674 cm-1 are attributed to FeFe2O4.23, 37-39 The allowed Raman modes of zinc ferrite and magnetite materials are almost identical and therefore a definite assignment remains ambiguous.39 For deposition temperatures below 700 °C an additional very broad and intense Raman band appears at 1380 cm-1, which has been assigned to the presence of maghemite and hematite.39-40 While this band is alongside with the aforementioned bands at deposition temperatures of 500 and 600 °C, it gets very dominant at 400 °C and the bands attributed to zinc ferrite are diminished. For the sample deposited at 600 °C beside bands attributed to ZFO additional characteristic peaks of hematite are present. The intensity of these peaks is, however small in comparison to samples deposited in the absence of the zinc precursor (Figure S1, SI). Nevertheless, as a matter of fact, the CVD of [Fe(tbaoac)3] in the absence of a zinc precursor lead to α-Fe2O3 irrespective of the applied deposition temperature.30 The characteristic bands of crystalline hematite are, however, only present in the 600 °C sample. Therefore it is concluded that the presence of zinc(II) cations is responsible for the of the growth of spinel structured deposits and is therefore indicative of the successful formation of crystalline zinc ferrite at deposition temperatures above 400 °C, although contributions of maghemite or hematite are present in samples deposited at 500 and 600 °C, respectively. The morphology of potential photoanode materials is of high importance due to the influence on the number of catalytic reaction sites and higher interface area with the electrolyte; surfaces with higher roughness provide more sites. To this regard top view and cross sectional SEM micrographs of films deposited on Si(100) (Figure 4) and FTO (Figure S2, SI) were recorded. The deposits at 400 °C (not shown) are inhomogeneous,

Figure 4. SEM micrographs of zinc ferrite thin films deposited at 500 – 800 °C on Si(100). The scale bar is valid for top view and cross-sectional images, respectively.

At 500 and 600 °C smooth, non-compact and continuous films are observed. Small and weakly defined crystallites are visible in the cross-sectional views. The morphological appearance of the samples is nearly similar for both temperatures, as well as the thickness (ca. 490 nm). Upon increase of the deposition temperature to 700 °C and above, the crystallinity is significantly enhanced and


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the films get denser becoming obvious by a reduction in thickness to ca 365 nm. The surface roughness is gradually increasing with temperature and bigger crystallites are observed from cross sectional analysis, which is in line with the XRD Scherrer analysis.

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ture of (antiferromagnetic) bulk ZnFe2O4.41 At room temperature only paramagnetic behavior is observed (see Fig. S4). These kind of magnetic properties indicate a sample with low A/B disorder within the spinel structure and a stoichiometry close to ideal ZnFe2O4. 42

Figure 6. RBS spectra of zinc ferrite thin films deposited at 400 – 800 °C on Si(100).

Figure 5. High resolution TEM micrograph of a ZFO sample deposited at 600 °C on FTO.

Compositional analysis lends a deeper insight into the materials nature and in this regard the elemental ratios in the bulk were calculated via RBS/NRA in combination with EPMA. In general, the combination of RBS and NRA allows detection of light and heavy elements simultaneously, thereby the presence of the expected elements zinc, iron and oxygen, along with minor carbon impurities, was determined. Since the atomic numbers of iron and zinc are not significantly different, the RBS signals of these elements overlap and as a consequence the estimation of the atomic ratio is not straightforward. On the other hand the characteristic bands of zinc and iron in EPMA are significantly different. With the given ratios of zinc and iron by EPMA the fitting of the respective elements in RBS is possible and accordingly the integrals of iron and zinc were simulated within the boundary conditions for the oxygen amount given by NRA and metal content from EPMA.

HR-TEM measurements (Figure 5) show that ZFO films are crystalline and the lattice spacing is around 2.57 Å which corresponds to the distance between (311) crystal planes of the normal spinel of zinc ferrite (ICSD database: No. 72031). The EDX spectra (Figure S3, Table S2, SI) show the presence of Zn, Fe and O in the ratio that are in agreement with the RBS measurements. The Si signal originates from residual glass scratches during sample preparation. Preliminary magnetic analysis via SQUID measurements reveal a narrow hysteresis at low temperature superimposed with paramagnetic behavior (Figure S4). Temperature dependent measurements reveal a bifurcation between FC and ZFC curves (inset of Figure S4) which indicates a low (ferrimagnetic) order temperature of around 20 K which is close to the known Néel temperaTable 1. Elemental atomic concentrations and ratios taken from EPMA, NRA and RBS measurements and thickness of the deposits obtained from SEM cross sectional analysis and calculation from RBS. Depositiontemperature

Fe/Zn (at. %) (EPMA)

(Zn + Fe)/O (RBS/NRA)

C (at.%)

N (at.%)

O (at.%)

Thickness (SEM)

Thickness

400 °C

3.0

0.63

23.2

4.7

44.1

Not analyzed

210 nm

500 °C

1.4

0.80

7.3

0.2

51.4

470 ± 25 nm

470 nm

600 °C

2.5

0.82

4.4

0.2

52.5

510 ± 40 nm

490 nm

700 °C

1.5

0.77

1.9

0.1

55.5

370 ± 40 nm

360 nm

800 °C

1.9

0.78

0.7

0

55.6

360 ± 40 nm

340 nm


(RBS)

6

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Figure 7. Core level spectra of a) Zn2p, b) Fe2p,c) ZnLMM and d) O1s of ZFO deposits on FTO synthesized at 600 °C.

The metals (zinc + iron) to oxygen ratios of samples deposited between 400 – 800 °C are presented in Table 1. Within the limits of the accuracy of the measurement technique samples deposited at 500 °C and higher, possess the expected metal to oxygen ratio of 0.75 which matches with the expected values for ZnFe2O4. However, with the exception of the 800 °C sample, all other deposits indicate a non-stoichiometric ratio of the metals. As can be seen in Figure 6, the zinc ferrite deposits show an inhomogeneous in depth distribution of the metals apparent by an increase of the metal peak at lower channel numbers (interface to silicon). This indicates a slightly iron rich interface and in turn a slightly zinc rich surface. In more detail, the XPS survey spectrum of the ZFO photoanode (deposition temperature 600 °C on FTO) is dominated by the Zn2p, Fe2p and O1s (Figure 7 a, b and d) binding energy peaks along with the respective auger transitions Fe LMM, Zn LMM, C KVV and O KLL (figure S5, SI). The Fe2p binding energy (BE) doublet is characterized by strong shake-ups at approximately 8.1 eV higher binding energies compared to the Fe2p3/2 (710.9 eV) and Fe2p1/2 peaks. This suggests that all the iron exists as Fe3+ and rules out the presence of Fe2+.Eventhough Fe 3+ ions are octahedrally coordinated both in ZFO and αFe2O3, the Fe2p3/2 in ZFO appear higher in BE due the difference in crystal structure. The energy separation of the doublet is 14.1 eV in good agreement with values reported in the literature for zinc ferrite.43 The O1s peak can be deconvoluted into three peaks centered at 529.8 eV (lattice O), 531.3 eV (hydroxide or O vacancies) and 532.0 eV (C-O species).44 If the peak centered at 531.3 eV is due to the oxygen vacancies one expects the presence of Fe2+ in the Fe2p region45 which is not the case. Additionally,

the C1s peak (figure S7, SI) shows two oxygenated C species at 286.2 eV (C-O) and 288.2 eV (C=O), respectively. Based on these evidences the two high BE peaks in the O1s occur more likely due to hydroxides and carbonate species. The Zn2p3/2 peak is detected at 1021.7 eV and by taking into account the kinetic energy of the Zn L3M45M45Auger transition (Figure 7c) at 989.3 eV and the resulting Auger parameter the presence of ZnFe2O4 is proven from the position of these three values in a Wagner chemical state plot.46 The elemental ratios at the surface reveal a zinc rich surface, which could lower the functional performance of a photoanode under illumination with visible light due to the wide band gap of ZnO.47 Nevertheless, the concentration of Zn at the surface is very low, owing to the absence of characteristic reflections and bands in XRD and Raman spectra respectively. Excess zinc oxide at the surface is, however, easily leached in basic media.17 XPS investigations on ZFO deposits on FTO substrates after photoelectrochemical investigations in sodium hydroxide solution at pH 13.6 confirmed the successful removal of excess ZnO, whereas the ZFO deposit stays intact. This is especially supported by the zinc Auger parameter, which confirms the presence of ZFO on FTO before and after PEC tests (supporting information). FUNCTIONAL PROPERTIES. Having an optical bandgap similar to α-Fe2O3, ZnFe2O4 is a promising photo absorber which offers some attractive additional features. Both the conduction band and the valence band edges are lower in value than in α-Fe2O3 and, as a result, ZnFe2O4 as photoanode requires less overvoltage to split water.17, 23, 48 As most ferrites are not effective enough in separating the electron-hole pairs they are also used in composite photo


7

ACS Sustainable Chemistry & Engineering electrodes. Particularly, ZnFe2O4 coupled with other well investigated semiconductors such as α-Fe2O3,8 TiO2,49 CaFe2O4,14 proved to enhance the efficiency. The improved photoresponse is mainly attributed to (i) extended light absorption to the visible region and (ii) enhanced charge separation and collection efficiencies. The CVD grown ZFO films of different thickness were probed as photoanode material to assess the potential of these films for PEC water splitting. Figure 8 presents the current versus potential scans on the ZFO electrodes in 1 M NaOH solution from -0.4 to 1.0 V vs. Ag/AgCl (saturated KCl) with a scanning rate of 10 mV s-1 under white light irradiation. As shown in Figure 8, regardless of the deposition time the dark I-V curves for all samples show similar behavior and start to increase above 1.70 V vs. RHE. Under illumination, a photocurrent onset of 1.1 V is observed for 340 nm thick samples and 1.3 V vs. RHE is observed for thinner and thicker samples, respectively. These onset potentials are much higher than the flat band potentials of ZFO films reported (0.64 V vs. RHE).50

a)

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E (V) vs. Ag/AgCl Figure 8. Current-voltage curves of measurements of ZnFe2O4 films deposited for 20, 40 and 60 min (corresponding thicknesses are given in the graphs) in 1 M NaOH under a) front- and b) backside illumination.

The large over potential is believed to be due to the slow water oxidation kinetics leading to accumulation of

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holes until sufficient band bending is achieved for holes to transfer across the interface. Under front side illumination, the highest photocurrent is observed for the thinnest sample (170 nm) and the photocurrent drops as the films are getting thicker. The observed photocurrent trend is explained as the electrons and holes generated inside/near the surface of a thinner film have higher chances of overcoming recombination and reaching the interfaces, in effect leading to higher photocurrent. However, the magnitude of the photocurrent remains low as the fraction of absorbed light in thin films is naturally low (see the UV-Vis absorption in SI, figure S8). The band gap of the materials was determined via Tauc plots,51 from which a direct, allowed optical band gap of 2.3 eV was determined irrespective of the thickness. The observed band gap is slightly higher than those reported in the literature.6 Under back side illumination the photocurrent of the thinnest sample is very similar compared to front side illumination. Both thicker samples studied (340 nm and 510 nm) show, however, increased photocurrent under backside illumination. Note that under this condition the photocurrent of the 340 nm sample surpasses that of the thickest sample. Such illumination direction dependent photocurrent has been reported for compact Fe2O3 and back side illumination mostly lead to lower photocurrent as the high energy photons are not contributing to the measured photocurrent due to recombination.52-53 However, for films exhibiting certain degree of porosity, backside illumination is beneficial as the short wavelength photons can be collected effectively.54 Thus the samples with thicker deposits likely exhibit minor degree of porosity. In general, the photocurrents recorded on these films are smaller than values reported,14, 17, 23 but for the photocurrent recorded on the ZFO film (340 nm) the measured 85 µA cm-2 at 1.6 V vs. RHE are higher than a reported value of 24 µA cm-2 for ZFO films prepared by solution precursor plasma spray (SPPS).13 Furthermore, thickness and deposition temperature optimization is expected to increase the photocurrent density. IPCE experiments (Figure 9a) using both front and backside illumination were performed on the sample deposited for 40 min (thickness: 340 nm) to see the photocurrent response at different wavelengths. The backside illumination still results in a slightly higher IPCE value compared to front side illumination which gives additional evidence that the thicker films possess some porosity. An IPCE value of 5.2% at 450 nm was achieved at a bias potential of 1.23 V vs. RHE. The flat band potential of the ZFO films was determined from impedance data collected at 1 kHz and fitted to the Mott-Schottky equation (figure S9, SI). Vfb of 0.5−0.6 V vs. RHE were obtained from the ﬁt. The Vfb can also be estimated from the onset of the photocurrent in the presence of a kinetically reversible redox couple. We performed the chopped light voltammetry experiment in 1M NaOH containing 0.1 M [Fe(CN)6 ]4-/3- (Figure 9b). The photocurrent onset was between 0.6 -0.7 V vs. RHE being close to the value determined from Mott-Schottky plot. The value obtained using both methods is fairly in agreement with reported Vfb for other ZnFe2O4 films.50 However, the value is 100-150 mV more negative than the Vfb


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reported by Tahir et al. for aerosol-assisted CVD (AACVD) ZnFe2O4 films.23

% IPCE@ 1.23 V vs. RHE

a) front side back side

6

4

450

500

Supporting Information. Raman of hematite, SEM cross section of ZFO grown on FTO, XPS details, UV-Vis of ZFO grown on FTO and respective Tauc plots, Mott-Schottky plot. “This material is available free of charge via the Internet at http://pubs.acs.org.”

550

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AUTHOR INFORMATION

E (V) vs. RHE 0.35 0.30

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript

0.20 0.15

Corresponding Author * E-Mail: [email protected], Phone: ++49 234 32 24150, Fax: ++49 234 32 14174.

1 M NaOH -4 1 M NaOH + 0.1 M [Fe(CN)6]

0.25 2

ferrites in detail for PEC related applications. In this particular case, the goal is to further enhance the efficiency of ZFO and therefore, future work should focus on improving the crystallinity of the ZFO layers and nanostructuring; possibly obtainable by choosing conductive substrates with sufficient stability at high temperatures, which are however not yet available. The magnetic measurements of the CVD grown ZFO indicate very low A/B disorder within the spinel structure and motivate us to explore the magnetic properties of the material in more detail in future.

ASSOCIATED CONTENT

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onset

Funding Sources

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E (V) vs. Ag/AgCl Figure 9. a) IPCE spectra of a 340 nm thick ZnFe2O4 film deposited for 40 min showing the effect of illumination direction and b) Chopped light voltammetry in 1 M NaOH / 40.1 M [Fe(CN)6] showing the onset of the photocurrent.

CONCLUSION. Photoactive zinc ferrites were for the first time successfully fabricated by conventional CVD via the rational selection of iron and zinc precursors, namely [Fe(tbaoac)3] and [Zn(mpki)2]. Upon process optimization, crystallographic phase pure zinc ferrite films were obtained and thereby excluding any other inclusions of other iron oxide polymorphs. Phase pure and nearly stoichiometric films were synthesized as verified by EPMA, RBS/NRA, Raman, EDX, and XPS analysis. The photoelectrochemical measurements were performed in detail, wherein the optical band gap clearly indicated the suitability of ZFO as photocatalyst in PEC water-splitting. Subsequently the influence of the thin film thickness under front- and backside illumination was investigated. A medium thickness of 340 nm was found to give best results with an IPCE value of 5.2 % at 450 nm. These results are highly encouraging and pave a way to explore

AD and DP thank the DFG-SPP1613 (DE-790-12-1) for financial support. Research stay of DP at Wayne State University and Research stay of MMK at Ruhr-University Bochum was supported by the Ruhr University Research School PLUS, funded by Germany’s Excellence Initiative [DFG GSC 98/3]. Research at the University Oldenburg (DT and MW) was funded by the DFG (WA 1116/28-1) within the Priority Program SPP 1613 (Solar H2). CHW and MMK gratefully acknowledge the U.S. National Science Foundation (Grant No. CHE-1212574) for partial support.

Notes The authors declare no competing financial interest.

ABBREVIATIONS AACVD, aerosol assisted CVD; CVD, chemical vapor deposition; EPMA, electron probe microanalysis; FTO, fluorine doped tin oxide; IPCE, incident photon to current conversion efficiency; [Fe(tbaoac)3], iron tris(tertbutyl acetoacetate); NRA, nuclear reaction analysis; PEC, photelectrochemical; RBS, Rutherford backscattering spectroscopy, RHE, reversible hydrogen electrode; SEM, scanning electron microscopy; SQUID, superconducting quantum interference device; TEM, Transmission Electron Microscopy; TG, thermogravimetry; UV-Vis, Ultraviolet-visible; XPS, X-ray photon spectroscopy, XRD, X-ray diffraction; ZFO, zinc ferrite; [Zn(mpki)2],zinc (N-methoxypropyl)ketoiminate.

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Combining Mesoporosity and Ti-Doping in Hematite Films for Water Splitting. J. Phys. Chem. C 2015, 119, 1642-1650.


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PHOTOACTIVE ZINC FERRITES FABRICATED VIA CONVENTIONAL CVD APPROACH a

b

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Daniel Peeters, Dereje H. Taffa, Marissa M. Kerrigan, Niels Jöns, Detlef Rogalla, Stefan Cwik, Hans-Werner e f f c d b Becker, Markus Grafen, Andreas Ostendorf, Charles H. Winter, Sumit Chakraborty, Michael Wark, Anjana Dea vi *

Synopsis: The ternary oxide ZnFe2O4 was synthesized via a solvent free and sustainable vapor phase deposition technique capable to split water into hydrogen and oxygen.


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Photoactive Zinc Ferrites Fabricated via Conventional CVD Approach

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