Characterization of MFe2O4 (M = Mg, Zn) Thin Films Prepared by

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

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Characterization of MFeO (M = Mg, Zn) Thin Films Prepared by Pulsed Laser Deposition for Photoelectrochemical Applications Ralph Andreas Henning, Patrick Uredat, Christopher Simon, André Bloesser, Pascal Cop, Matthias Thomas Elm, and Roland Marschall J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04635 • Publication Date (Web): 09 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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The Journal of Physical Chemistry

Characterization of MFe2O4 (M = Mg, Zn) Thin Films Prepared by Pulsed Laser Deposition for Photoelectrochemical Applications R. A. Henning,a P. Uredat,b,c C. Simon,a,d A. Bloesser,a,d P. Cop,a,c M. T. Elm,a,b,c and R. Marschall*a,d

a. Institute of Physical Chemistry, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany.

b. Center for Materials Research (LaMa), Heinrich-Buff-Ring 16, 35392 Giessen, Germany.

c. Institute of Experimental Physics I, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany.

ACS Paragon Plus Environment

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d. Physical Chemistry III, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany.

E-mail: [email protected]

ABSTRACT

Earth-abundant visible light-absorbing photoelectrodes of the spinel ferrites ZnFe2O4 and MgFe2O4 have been prepared as dense and crack-free thin films using pulsed laser deposition, to investigate the basic electronic properties of these two emerging absorber materials. X-ray diffraction and Raman spectroscopy confirm the phase purity of the prepared thin films, while magnetotransport and Hall measurements in combination with Mott-Schottky and photoelectrochemical measurements were performed to reveal the performance limiting factors of those absorbers for photoelectrochemical water oxidation.


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Our results provide new insights to improve the performance of ferrite-based photoelectrodes in the future.

Introduction Visible light absorbing photoelectrodes are focus of current research solar energy conversion.1 Besides the importance of broad visible light absorption, the fundamental material parameters of new absorber materials are difficult to investigate when the photoelectrodes are meso- or nanostructured for improved charge carrier transfer to the electrolyte. On the other hand, fundamental bulk properties are equally important to judge whether new absorber materials are promising for e.g. tandem water splitting devices. Therefore, non-porous reference electrodes are needed to determine fundamental properties like charge carrier properties or conductivity. Typical synthesis methods to prepare photoelectrodes for water splitting are dip- or spin-coating

techniques

for

sol-gel-derived

photoelectrodes,2

spray

pyrolysis,3

electrophoresis,4 particle transfer methods,5 and screen printing.6 Some of those


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techniques can lead to porous photoelectrodes or nanostructures,7 which can have a strong influence on the charge carrier transport. Since in a porous and/or mesostructured thin film the irradiation of the sample might be inhomogeneous due to internal scattering, and the electrolyte diffusion into the electrode can be insufficient, electrochemical and spectroscopic methods investigating materials’ properties can show unexpected results due to the nanostructure.8 For example, measurements of the flat band potential and space charge width can be influenced by charging effects due to a large surface. Therefore, to determine fundamental properties of new absorber materials including absorption coefficients, cheap preparation methods to synthesize efficiently dense, nonnanostructured photoelectrodes are needed, to impede the influence of e.g. surface charging and mass transport limitations during measurements. Recently, Abdi et al. showed that pulsed laser deposition (PLD), a method capable of producing compact and dense complex metal oxide films, can be used to prepare dense -SnWO4 thin film photoanodes,9 to investigate the fundamental properties of this absorber material, such as charge carrier transport properties, light absorption and chemical stability,10 without the influence of surface effects.


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Here, we chose the promising ternary ferrite absorbers ZnFe2O4 and MgFe2O4 for our study, and prepared phase-pure spinel-type thin film photoanodes by PLD. Both ferrite materials recently gained a lot of attention as alternatives to hematite (-Fe2O3) in photocatalysis and photoelectrochemistry as nanostructured and inversion-controlled materials.11–16 They both absorb large amounts of visible light with band gaps of around 2.2 eV, however several reports on nanostructured ZnFe2O4 have been shown contradicting flat band potentials.17,11 This value is however of utmost importance to judge whether an absorber is suitable for water splitting or only for one half reaction, and should be determined on dense and flat electrodes, as explained above. We present photoelectrochemical data of such PLD-derived electrodes for the first time, to the best of our knowledge. We also performed detailed analyses of the resulting dense thin films, including magnetotransport, Hall and Mott-Schottky measurements, to reveal the performance limiting factors of those absorbers. Experimental Synthetic procedures


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Dry citrate process. Following a procedure by Nakayama et al.,18 for the preparation of ZnFe2O4 (ZFO) 2.46 g (8.27 mmol) zinc nitrate hexahydrate, 6.70 g (16.58 mmol) iron(III) nitrate nonahydrate and 5.22 g (24.85 mmol) citric acid monohydrate were mixed in an agate mortar and transferred into a round flask. The mixture was slowly heated in a rotary evaporator at 850 mbar to 85 °C generating a homogeneous melt. At constant temperature of 85 °C, the pressure was reduced slowly to 50 mbar, until a foam-like solid remained in the flask, which was kept for three hours at 20 mbar and 85 °C for drying. The final product was ground in a mortar and calcined in air at 600 °C for five hours (heating rate: 5 °C/min). For the preparation of MgFe2O4 (MFO), 2.58 g (10.06 mmol) magnesium nitrate hexahydrate, 8.08 g (20.00 mmol) iron(III) nitrate nonahydrate and 6.25 g (29.75 mmol) citric acid monohydrate were used.

Pulsed Laser Deposition (PLD). After calcination, pellets have been prepared from the ZFO and MFO powders, as targets for the thin film deposition via pulsed laser deposition (PLD). Therefore, the powders have been pressed in a cylindrical form (d = 13mm) by an uniaxial press with 30 kN for 15 min. Subsequently, they were pressed by an isostatic press with 500 bar for one hour and heated in air at 500 °C for 24 h.


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Thin films were deposited in a commercial PLD chamber (Surface systems+technology GmbH & Co. KG) with a KrF excimer laser (λ = 248 nm, Lambda Physik). Thin film deposition was performed with a fluence of 2.5 J/cm² and 15.000-20.000 pulses with a frequency of 10 Hz. The substrate-target distance was 4 cm, and an oxygen partial pressure of 5∙10-3 mbar has been applied. Prior to the deposition, the chamber was pumped below 5∙10-5 mbar. Only the deposition temperature (400°C or 500 °C) and the post annealing steps differ and are indicated for each sample in the corresponding graphs. For the magnetoresistance (MR) and Hall measurements thin films were deposited on insulating (0001)-sapphire substrates (Crystec, 1x1 cm²), while for electrochemical measurements FTO coated quartz glass (2x3 cm) was used as substrates. As discussed in the experimental section, structural characterization reveals no difference in the thin films deposited on the two different substrates. The thickness of the films was determined with a profilometer (α-Stepper, KLA Tencor).

Characterization


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X-ray diffraction (XRD). The diffraction patterns of the powders and the pellets have been collected with the Empyrean Series 2 X-ray Diffraction System (PANalytical) equipped with a Cu Kα tube (wavelength = 0.154 nm) and operated with 40 kV / 40 mA. The powders and the ZFO and MFO pellets were investigated with a reflection transmission spinner (0.5 rotations/s). On the incident beam path a soller slit (0.04 rad.), a fixed mask (10 mm), a fixed anti scatter slit (2°) and a programmable divergence slit (fixed to 1°) have been utilized. On the reflected beam path an anti-scatter slit fixed to 1°, a large soller slit (0.04 rad.) and a large nickel beta-filter (0.02 mm) were used. The signals were recorded with a PIXcel3D area detector, with lower PHD (Pulse Height Discrimination) level of 49% and an upper level of 70%. Typically, the Gonio scans were performed from 10° / 20° to 60° / 90° with a step size of 0.0131° / 0.0263° and a counting time of 399 s / 197 s for ZFO / MFO. The crystal structure of the thin films deposited by PLD was analyzed by grazing incidence X-ray diffraction (GIXRD) with a thin film diffractometer (PANalytical X’Pert Pro MRD). Beforehand the sample and the geometry was calibrated by means of 2θ,  and height (z-axis). An incident angel () of 0.5° was used and a 2θ range between 15° and 90° was scanned with a step size of 0.15°/step


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and an accumulation time of 14 s/step. With this technique intense substrate reflections can be excluded and solely the thin film crystal structure is investigated.

Scanning electron microscopy (SEM). Surface and microstructure were characterized using a FE-SEM (Zeiss, Merlin) with a working distance of 4 mm and an accelerating voltage of 3-4 kV with a current of 100 pA. The SEM images were obtained with an inlense or secondary electron detector.

Raman-spectroscopy. Raman spectra were measured using a Bruker Senterra Raman microscope. Sample excitation was performed with a Nd:YAG laser (λ = 532 nm) at a laser power of 0.2 mW, a 50x magnification and a spectral resolution of 9 – 12 cm-1. In general, 40 co-additions and 20 seconds integration time were used. The obtained spectra were processed using OPUS 7.5 software (BRUKER).

Time-of-Flight

Secondary

Ion

Mass

Spectrometry

(ToF-SIMS).

ToF-SIMS

measurements were performed on the deposited thin films using a ToF-SIMS 5-100 (IonTOF GmbH). Bi(I) was used as source for the primary ions with an acceleration voltage of 25 keV and a current of 1.9 pA. For depth profiling an area of 200x200 µm² was sputtered using O2 with an energy of 1 keV and an ion current of 65 nA, while only


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100x100 µm2 of the generated crater were analysed with a resolution of 128x128 pixels2. By stopping the ablation process when the substrate signal arises, the thickness of the investigated films was obtained using the aforementioned profilometer.

Transport measurements. To reveal the transport mechanisms in ZFO and MFO thin films, magnetoresistance and Hall measurements were performed. For the transport measurements, Indium contacts were soldered onto the sample surface. The temperature dependence of the conductivity, the magnetoresistance as well as the Hall-resistance were investigated in van der Pauw geometry with a magnetic field applied perpendicular to the sample’s surface. In the temperature range from 200 K to 280 K the measurements were performed using a He-4 flow cryostat (Oxford Instruments) in a superconducting magnet system yielding external magnetic fields up to 10 T. In the temperature range between 280 K and 400 K the transport measurements were carried out using a homebuilt setup with a magnetic field of 1 T.

Photoelectrochemistry. Mott-Schottky plots and photocurrent densities were measured in 1 M NaOH electrolyte solution including 1 M Na2SO3 acting as hole scavenger (pH =


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13.2), and were performed on a Zahner Zennium potentiostat using a three electrode setup of PLD-derived ZFO or MFO working electrode, Ag/AgCl in 3 M NaCl reference electrode, and a Pt counter electrode. The studied area of the thin films was fixed to 1 cm2. Mott Schottky measurements were conducted in the dark from -0.2 VRHE to 1.4 VRHE at a 1 kHz with an amplitude of 5 mV and a scan speed of 50 mV⋅s-1. Photocurrent measurements were performed with intermittent irradiation using a white light LED (Zahner) without UV-light emission, the light intensity was 1000 W⋅m-2. The potential was varied from 2 VRHE to 0 VRHE with a sweep rate of 5 mV⋅s-1.

Results and Discussion The XRD patterns of ZFO and MFO powders prepared via dry citrate route are shown in Figure 1 a) and b), respectively. Both powders were prepared phase-pure, only the expected reflections according to the reference data (also shown) for cubic spinel-type ferrites are visible. ZFO is usually a normal spinel, while MFO crystallizes in the inverse


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spinel structure. After the target preparation for the PLD process, also no impurities can be detected (Figure 1).

Figure 1. X-ray diffraction data of powder samples after dry citrate synthesis route, after target preparation, and the corresponding reference data for a) ZFO and b) MFO.


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Raman spectra shown in Figure S1 (Supporting Information) also exhibit no impurities of e.g. hematite, but rather the expected peak scheme for ZFO12 or MFO13, respectively. GI-XRD of PLD-derived ZFO and MFO thin films on fluorine-doped tin oxide (FTO) glass are shown in Figure 2 for different deposition temperatures and post-treatment temperatures. For the thin films deposited on sapphire substrates the same GI-XRD patterns are observed revealing that the substrate has a negligible influence on the thin film orientation at the deposition parameters used. In case of ZFO, the PLD process at 500 °C results in phase pure thin films. Moreover, the further calcination for one hour in air at 600 °C adds no impurities to the sample, no by-phases can be detected, only the reflection gain in intensity indicating a more crystalline thin film can be observed. For MFO thin film, deposition at 400 °C already results in crystalline and phase pure thin films. At this temperature, ZFO could not be obtained in crystalline form (not shown). Deposition at 500 °C has no effect on the XRD patterns, while an additional treatment at 600 °C results in by-phase formation of hematite.


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Figure 2. GIXRD of PLD-derived a) ZFO and b) MFO thin films.


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SEM images of the respective thin films are shown in Figure 3. Again, no differences in the surface morphology of the thin films deposited on sapphire substrates were observed. In all cases, dense, non-porous thin films were obtained with a rough surface. No droplet formation can be observed, which underlines the suitability of our PLD process for the preparation of ferrite thin films. In some areas, where some pieces were broken out of the thin films during preparation, it can be seen that the thin films are dense and non-porous

throughout their thickness (see inset of the MFO 500). ZFO thin films seem to be slightly rougher than the MFO thin films prepared by PLD. As already mentioned for the diffraction patterns one can confirm that larger crystals are visible after the annealing step at 600 °C in air.


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Figure 3. SEM images of MFO and ZFO PLD-derived thin films. For better comparison the insets show SEM images with higher magnification. The deposition of a dense thin film can clearly be seen in the inset of the MFO 500 thin film, where a part of the thin film is broken out.

ToF-SIMS has been used to determine thickness and homogeneity of the PLD-derived spinel ferrite thin films. As shown in Figure 4, MFO-400 thin films deposited on FTO glass substrate are roughly 290 nm thick, with homogeneous distribution of all consisting elements through the film. The ZFO-500+600 films are about 380 nm thick.


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Figure 4. ToF-SIMS depth profiles of MFO-400 and ZFO-500+600 normalized on the total ion counts.

To reveal the transport mechanisms in ZFO and MFO thin films, magnetoresistance and Hall measurements were performed. Unfortunately, MFO turned out to be immeasurable due to a too high resistance of the MFO thin film. In magnetite the electronic conductivity arises from small polaron hopping of the itinerant t2g electron between the Fe2B +

and

Fe3B + sites on the B sublattice.19,20 Replacing Fe2B + with

nonmagnetic Mg2B + reduces the amount of itinerant charge carriers resulting in a high resistance as observed for the MFO thin films prepared.


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In case of ZnxFe3-xO4 the situation is slightly different. In bulk crystals, the nonmagnetic Zn2A + substitutes the Fe3A + ions on the tetrahedrally coordinated A sites of the inverse spinel structure of magnetite.21–24 However, due to charge compensation, the number of Fe2B + ions on B site is also reduced,20,25 also reducing the amount of charge carrier responsible for the electrical conductivity via electron hopping between Fe2B + and Fe3B + sites on the B sublattice.20,25,26 Furthermore, the substitution of Fe3A + with nonmagnetic Zn2A + reduces the antiferromagnetic exchange interaction 𝐽AB between the A and B sites. Consequently, the corresponding magnetic moments are not aligned perfectly antiparallel anymore resulting in a spin canting and a reduced hopping probability between Fe2 + and Fe3 + sites. Both effects result in an increase of the resistivity with increasing x in MexFe3xO4

(Me = Zn, Mg).20,27 Thus, for ZnFe2O4 a higher resistance compared to Fe3O4 is

expected due to a lack of itinerant charge carriers. The ZFO thin film prepared shows a resistivity of about 173 cm at 300 K, which is higher than for sputtered epitaxial ZFO thin films reported in literature.27 However, the lower resistivity of the ZFO film compared to the MFO thin film is probably caused by a partial disorder of Fe and Zn ion as typically observed in nanocrystalline ZFO21,28, as well as due to the existence of oxygen vacancies,


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which increases the amount of Fe2B + and thus the itinerant charge carriers.20,27,28 Fig. 5 a) shows the temperature dependence of the conductivity in an Arrhenius-representation. In the whole temperature range the ZFO thin film shows a semiconducting behaviour, which is typical for ZFO.26,29,30 As discussed above, the electrical conductivity in ferrites arises from small polaron hopping of electrons between Fe2B + and Fe3B + sites on the B sublattice. Then, the conductivity is given by:

𝜎 =

(

)

𝜎0 𝐸A exp ― , 𝑇 𝑘B𝑇

(1)

where 𝐸A is the activation energy for the small polaron hopping. However, in contrast to recent results on the transport properties in ZFO thin films,20,27,31,32 two slopes can be observed in the Arrhenius plot indicating a change of the transport mechanism at around 290 K. Plotting ln(𝜎𝑇) vs. 1/𝑇, the activation energy for both transport regimes was determined from the slope of the linear fit to the experimental data, resulting in activation energies of 173±5 meV and 198±2 meV for the temperature range between 300 K and 400 K and between 300 K and 200 K, respectively. Comparable activation energies of about 180 - 220 meV for ZFO were also found by several groups.29,33,34


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Figure 5. Transport properties of the ZFO thin film: a) Temperature dependence of the conductivity, b) of the carrier concentration, c) of the Hall-mobility, and d) magnetoresistance.


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To get further insights into the underlying transport mechanisms, the carrier concentration and the Hall mobility were determined using Hall measurements. Figs. 5 b) and c) show the temperature dependence of the carrier concentration and the Hallmobility, respectively. As for the conductivity, a change in the behaviour is observed at a transition temperature at around 290 K. For low temperatures, the ZFO thin film exhibits a constant Hall-mobility and an exponential increase of the carrier concentration, while for temperatures above 290 K the carrier concentration remains constant and the mobility increases. The increase in the carrier concentration at low 𝑇 shows an activation energy of 193±20 meV, which is in excellent agreement with the activation energy found for the conductivity in this temperature region. At high temperature above 𝑇 = 290 K, the increase in conductivity is attributed to an increase in the mobility, which reveals an activation energy of about 170±40 meV also in excellent agreement with the activation energy determined from the conductivity. A comparable behavior can also be observed for polaron transport in La0.7Cr0.3MnO single crystals35 and is well described by the model of adiabatic hopping of small polarons developed by Friedman, Emin and Holstein.36,37 According to this model, at low temperatures the electron motion takes place in a polaron-


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band with a constant polaron mobility as observed for the ZFO films at T < 290 K. At high temperatures, the polaron mobility increases exponentially as a function of temperature, while the number of contributing charge carriers remain constant as observed for T < 290 K. Thus, the observed temperature-dependence of the mobility and the carrier concentration confirms that small polaron hopping is responsible for the electronic conductivity in the ZFO thin film. Finally, Fig. 5 d) shows the magnetoresistance defined as

MR(𝐵) =

𝜌(𝐵) ― 𝜌(0) ⋅ 100% 𝜌(0)

(2)

of the ZFO thin films for temperatures between 200 K and 280 K with 𝜌(𝐵) being the resistivity with magnetic field and 𝜌(0) is the resistivity without an magnetic field. For all temperatures, a negative magnetoresistance effect is observed. As discussed in detail by Venkateshvaran et al., 20 the negative MR effect is a consequence of the spin canting as well as the reduced hopping probability between Fe2 + and Fe3 + sites caused by the substitution of Fe3A + with Zn2A + . By applying an external magnetic field the spin canting


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is

reduced,

i.e.

the

hopping

probability

increases,

resulting

in

a

negative

magnetoresistance effect, which follows a Brillouin-function 𝐵𝐽(𝑥):20,27

(

𝐵𝐽 MR ≈ ―𝛽

)

𝜇eff𝜇0𝐻 𝑘B𝑇 𝑘B𝑇

,

(3)

Here, 𝜇eff is the effective magnetic moment of the local spins, 𝐽 = 2, 𝜇0 is the vacuum permeability, and  is a constant. The MR effect can be described well using Eq. (3) as shown in Fig. 5 d) as solid lines. The effective magnetic moment 𝜇eff is about 3.2 for all temperatures, which is in good agreement with the magnetic moment of ZnxFe3-xO2 thin films (with 0 ≤ x ≤ 0.9) determined by Jin et al.27 or Venkateshvaran et al.20 from magnetization measurements. In their ZnxFe3-xO2 films the effective magnetic moment determined from MR measurements was much larger, ranging between 50 to 120 𝜇B, which was interpreted as the magnetic moment of small ferromagnetic clusters. The much smaller effective moment observed in our ZFO thin films corresponds to a magnetic moment of a single Fe2 + ion indicating that no ferromagnetically aligned clusters are present in the ZFO thin films. This is reasonable as due to the high amount of Zn2A + ions a large spin canting between the Fe2B + and Fe3B + ions is expected.


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Figure 6 shows the Mott-Schottky plot and photocurrent densities for PLD-derived ZFO500+600 as well as the MFO-500 thin film photoanode. As can be seen, the determined flat band potential of ZFO lies at 0.82 V vs. RHE (Fig. 6 a)), being below the hydrogen evolution potential. In fact, this value is in good agreement with earlier values for mesoporous ZFO thin films.12 When the Mott-Schottky measurement is performed under white light illumination (dotted line), no change in the flat band potential is observed. This indicates that no surface charge is built up upon illumination, as observed for CdS,38 an indication that ZFO is not prone to strong photocorrosion.


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Figure 6. a) Mott-Schottky plots (in dark and under illumination [dotted]) and b) photocurrent density (under WLED irradiation, no UV, in the presence of SO32- hole scavenger) of PLD-derived ZFO-500+600. c) Mott-Schottky plot (in dark and under illumination [dotted]) and d) photocurrent density (under WLED irradiation, no UV) of MFO-500.


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Also shown are the photocurrent densities in the presence of a hole scavenger (Fig. 6 b)), which are quite low compared to mesoporous ZFO thin films.12 Comparing such data with the present results on non-porous PLD-derived ZFO thin films, it can be confirmed that in case of ZFO a mesoporous morphology is necessary to have a positive influence on the achievable photocurrents. The photocurrent onset potential lies at 0.8 V vs. RHE, in agreement with the Mott-Schottky analysis, as expected. For MFO photoanodes, the flatband potential was found to be slightly different compared to ZFO, being 0.67 V vs. RHE (Figure 6 c)). Interestingly, the flat band potential shifts upon illumination towards 0.6 V vs RHE. Although being only a small shift, this might indicate some charging effects to due instabilities of MFO under illumination. The photocurrent onset potential could only be estimated due to the low signal-to-noise ratio, being approximately 0.7 V vs. RHE, in the range of the measured flat band potential. The observed photocurrents of MFO shown in Fig. 6 d) are approximately three times higher compared to ZFO (e.g. 26.1 and 7.3 µA/cm2 @ 1.5 V vs. RHE), although its transport properties were immeasurable. This is


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very surprising, but might be due to different effects. The higher photocurrent densities might, according to the flat band potential shift under illumination, exhibit contributions from photocorrosive currents. Another reason might be that under light irradiation, MFO exhibits a lower number of polaron trapping states. The photocurrent values in both cases are very low. As we confirmed above, small polaron hopping is responsible for the electronic dark conductivity in the ZFO thin films. The low photocurrents however might be due to polaron trapping states related to the formation of reduced Fe2+ under light irradiation, as recently confirmed for hematite.39 In hematite, electrons photoexcited into the conduction band do not stay there, but are trapped in such polaron states in 2 ps,40 and we assume this process to occur in MFO and ZFO, too. This has to be confirmed by according measurements in the future, but this would render the measured flatband potentials not being the conduction band minimum, but rather the polaron state. This could explain the low photocurrents of ZFO in literature due to a much more cathodic valence band position, according to the measured optical band gaps, than expected.


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Conclusions Dense ZnFe2O4 and MgFe2O4 thin film electrodes have been for the first time prepared by pulsed laser deposition (PLD) to investigate their electronic and photoelectrochemical properties without the influence of nanostructures. MgFe2O4 turned out to have very high resistance. The lower resistivity of the ZnFe2O4 thin films compared to MgFe2O4 thin films is most probably caused by a partial disorder of Fe and Zn ion as typically observed in nanocrystalline ZnFe2O4 as well as due to the existence of oxygen vacancies. Magnetotransport as well as Hall measurements confirm a transport mechanism of the ZnFe2O4 thin films, which is governed by small polaron hopping of electrons between the Fe2B + and Fe3B + ions in the B sublattice. The ZnFe2O4 thin film deposited by PLD showed much lower photocurrent densities than the MgFe2O4 thin film at comparable preparation conditions indicating a higher density of polaron trapping states in the ZnFe2O4 thin film.

ASSOCIATED CONTENT


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Supporting Information. Raman spectra are provided in the SI.

AUTHOR INFORMATION

Corresponding Author * Prof. Dr. Roland Marschall, Physical Chemistry III, University of Bayreuth [email protected]

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

Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style).

ACKNOWLEDGMENT

We like to thank Prof. B. M. Smarsly for his support. A.B. and R.M. gratefully acknowledge financial support from the AiF within the program for promoting the Industrial


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Collective Research (IGF) of the German Federal Ministry of Economic Affairs and Energy (BMWi), based on a resolution of the German Parliament (project "QuinoLight", 18904N1-5). C.S. and R.M. acknowledge funding by the German Research Foundation DFG, project 5392/7-1. R.M. acknowledges funding by the German Research Foundation DFG under the priority program SPP 1613, project MA 5392/5-1. M.T.E. thanks the German Federal Ministry of Education and Research (BMBF) for funding of the NanoMatFutur project NiKo (03XP0093).

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TOC Graphic


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Characterization of MFe2O4 (M = Mg, Zn) Thin Films Prepared by

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