Layered perovskite nanofiber heterojunctions with tailored diameter to

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Layered perovskite nanofiber heterojunctions with tailored diameter to enhance photocatalytic water splitting performance André Bloesser, and Roland Marschall ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00265 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Energy Materials

Layered perovskite nanofiber heterojunctions with tailored diameter to enhance photocatalytic water splitting performance

André Bloesser, Roland Marschall* Institute of Physical Chemistry, Justus-Liebig University Giessen, 35392 Giessen, Germany

Keywords: Electrospinning, layered perovskite, heterojunctions, photocurrent doubling, water splitting

ABSTRACT Controlled heterojunction design was combined with diameter-tailored electrospinning to prepare mesostructured photocatalysts, leading to strongly enhanced photocatalytic water splitting rates. The (111)-layered perovskite Ba5Ta4O15 in heterojunction with Ba3Ta5O15 was for the first time prepared as nanofibers by means of electrospinning, including variation of the fiber diameter. The result is a tailored mesostructured heterojunction leading to hydrogen evolution rates from water/methanol mixtures of up to 4.4 mmol h-1 without co-catalyst, and remarkable water splitting activity after Rh-Cr2O3 decoration with hydrogen production rates up to one mmol h-1. Our studies also confirm the photocurrent doubling effect of sacrificial alcohols exhibiting α-H atoms with this heterojunction, in contrast to tert-butanol.

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Introduction Due to low efficiencies in photocatalytic hydrogen production and water splitting, contemporary researchers investigate different strategies to reduce recombination processes in such reactions. One prominent strategy is nanostructuring,1 leading to reduced diffusion pathways for charge carriers to the surface of the semiconductor, and in many cases to an increase in active surface area. However, nanostructuring can have several disadvantages, including band gap increase due to quantum confinement2 or reduced space charge layer thickness at the semiconductor surface.3 Electrospinning is a facile and reproducible method to prepare mesostructured metal oxide fibers.4 Thus, it has been applied to prepare several mesostructured semiconductors, including TiO2,5 CuO,6 and Ba5Ta4O157, for photocatalytic and photoelectrochemical applications with fibrous morphology, while avoiding the disadvantage of band gap increase. The latter material is a highly active photocatalyst for water splitting under UVlight due to its layered crystal structure,8 which showed better water splitting performance in fiber morphology than in powder form.7 Facile post-synthetic procedures are known to modify the light absorption properties of Ba5Ta4O15, e.g. by uplifting the valence band via nitrogen doping9 or attachment of gC3N4 to the semiconductor surface10, thus allowing visible light water splitting. Recently, it was shown that by adjusting the synthesis conditions and precursor ratios, barium tantalate multicomponent heterojunctions could be easily obtained.11,12 The construction of multiphase or multicomponent semiconductor heterojunctions is another very prominent strategy to improve photocatalytic activity by enhanced charge carrier separation.13,14 Heterojunctions made of Ba5Ta4O15-Ba3Ta5O15 and Ba5Ta4O15-Ba3Ta5O15-BaTa2O6 therefore showed strongly improved performance in hydrogen production and water splitting compared to phase-pure Ba5Ta4O15.12 For the two-component heterojunction, laser flash photolysis experiments revealed the mechanism of enhanced activity, being most probably due to prolonged electron lifetimes by charge injection from the .CH2OH radical (formed upon hole oxidation of methanol) into the conduction band of Ba3Ta5O15 (photocurrent doubling).15 Moreover, it was concluded that optimizing composite formation can be much more efficient for charge carrier separation


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than co-catalyst decoration.12 However, it is generally accepted that intimate contact between the different components is crucial for good charge separation.16 Several examples in literature are presenting the formation of semiconductor multicomponent heterojunctions made of binary oxides or sulfides by electrospinning.17–28 Furthermore, Jing et al. prepared a heterojunction of two ternary oxides (SrTiO3/NiFe2O4) via side-by-side spinneret setup.29 Lv et al. were the first to prepare multiphase heterojunction nanofibers in one step, resulting in BiVO4/Bi4V2O11 nanofibers.30 Here we present the first one-step preparation of barium tantalate multiphase heterojunctions by electrospinning. Moreover, the fiber diameter of the composite was also tailored for optimum hydrogen evolution performance. Nanofiber heterojunctions of Ba5Ta4O15-Ba3Ta5O15 with 100 nm thickness exhibit hydrogen evolution rates without co-catalyst of up to 4.4 mmol h-1. We also investigated the use of different sacrificial alcohols to verify the mechanism leading to enhanced hydrogen evolution. Finally, very good water splitting performance was observed with this in-situ formed heterojunction, after RhCr2O3 co-catalyst decoration31, from pure water.

Experimental Section Reagents and Materials All chemicals were of analytical grade and used as received. Barium(II) ethylhexanoate (98%, Sigma Aldrich), Tantalum(V) ethoxide (0,24 M solution in ethanol, Alfa Aesar), Polyvinylpyrrolidone (MW = 1300000, Alfa Aesar), tetrahydrofurane (technical grade, J.T.Baker), sodium hexachlororhodate (Na3RhCl3, 99.999%, Sigma Aldrich). Catalyst Preparation Barium tantalate nanofibers were synthesized via the route presented by Hildebrandt et al.7 To synthesize fibers consisting of barium tantalate composites the synthesis route for Ba5Ta4O15 fibers was slightly modified. The amount of barium ethylhexanoate precursor in the spinning solution was reduced. To ACS Paragon Plus Environment

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obtain fibers with the composition Ba5Ta4O15-Ba3Ta5O15 according to the best two-phase heterojunction reported by Marschall et al.,11 756 mg of Ba-2-ethylhexanoate were dissolved in 3 mL of THF. Then, 7 mL of a 0.24 M solution of Ta(OEt)5 in EtOH were added, and 160-400 mg of PVP were added and shaken until a clear spinning solution was obtained. Unfortunately, we were not able to prepare the three-phase composite reported by Soldat et al.12 as nanofibers yet. Electrospinning was conducted at a humidity level of 30% using a 0.8 mm cannula with a tip potential of +18 kV. The collector was placed at a distance of 15 cm and a potential of –2 kV was applied. Subsequent calcination was conducted at 900 °C for 10 hours (heating rate: 5 K min-1) to remove the spinning polymer and to crystallize the material. Characterization X-ray powder diffraction (XRPD) experiments were carried out with a PANalytical X’Pert Pro instrument using Cu Kα irradiation with a wavelength of 1.5406 Å operating at 40 mA and 40 kV. A Perkin Elmer Lambda 750 UV/VIS/NIR spectrometer was used to record diffuse reflectance spectra of the fibers. The reflected light was collected using a Praying Mantis mirror unit, and reflectance spectra were converted into absorption spectra according to the Kubelka-Munk theory. Diffuse Reflection FTIR spectra were measured using a Bruker Alpha spectrometer equipped with a DRIFT module. SEM images were recorded on a Zeiss Merlin scanning electron microscope. The samples were not subjected to sputtering prior to the SEM measurements. Images were taken at working distances ranging from 3 to 5 mm with a column voltage of 2 kV and a probe current of 80 pA. Fiber diameters were determined from SEM images using the Java based open source image processing software ImageJ. TEM images were taken with a Philips CM30 transmission electron microscope at 300 kV high tension, with samples prepared on S-160-3 Plano carbon coated copper grids. XPS spectra were recorded on a PHI VersaProbe II Scanning ESCA microprobe (Physical Electronics) with a monochromatic Al Ka X-ray source. A 50 W X-ray power was used and the pass energy of the ACS Paragon Plus Environment

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analyzer was set to 23.5 eV for detailed spectra. The C 1s signal from adventitious hydrocarbons was set to 284.8 eV in order to correct possible charging effects of the material. Nitrogen physisorption experiments were carried out with a Quantachrome Quadrasorb evo setup. The samples were heated to 260 °C for 3 hours before the actual measurements to remove all adsorbates. Surface areas were determined via Brunauer-Emmet-Teller (BET) theory. Hydrogen Production 150 mg photocatalyst were suspended in 600 mL of an aqueous methanol solution (8 vol. %), and filled into a home-made inner irradiation-type quartz reactor. A 700 W Hg mid-pressure lamp set to 71 % (500 W, Peschl UV-Consulting) was used as light source and cooled to 10 °C with a double-walled quartz mantle using a thermostat (LAUDA RP845). Gas evolution was measured online using a mass spectrometer (Hiden HPR-20 Q/C). Argon 5.0 was used as carrier gas; the continuous gas flow was controlled by Bronkhorst mass flow controller. The gas flow was set to 100 NmL min-1. Before photocatalytic reactions were initiated, the whole system, with the photocatalyst included, was flushed with Argon 5.0 for 90 minutes to remove any trace of air. Co-catalyst precursor (Na3RhCl6) addition for photodeposition of 0.0125 wt.-% co-catalyst was done through a rubber sealing (without opening the reactor). For the preparation of samples for water splitting, additionally 0.0125 wt.-% Cr2O3 was deposited onto Rh, also via photodeposition from K2CrO4 added through a rubber sealing. After the deposition procedure the photocatalyst was filtered, washed thoroughly with water, and dried overnight. The presence of the co-catalyst on the semiconductor was confirmed by transmission electron microscopy (Figure S1) Water Splitting 90 mg of Ba5Ta4O15-Ba3Ta5O15 sample decorated with Rh-Cr2O3 were immersed in pure water in the same setup described above. Irradiation was performed for 5.5 hours until reaching steady-state values.

Results and Discussion


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The reduction of the molar Ba/Ta ratio from 1.25 for Ba5Ta4O15 down to 1.06 resulted in the formation of composite fibers with the composition Ba5Ta4O15-Ba3Ta5O15. The presence of the by-phase in the fibers was confirmed via XRPD (Figure 1a). The additional reflections originating from Ba3Ta5O15 can be made out easily, especially those in the vicinity of the two major reflections of the phase-pure Ba5Ta4O15 material shown for comparison.

Figure 1. XRD pattern of pure Ba5Ta4O15 (black) and of the synthesized two-phase composites with different fiber thickness. The additional reflexes are in very good agreement with the shown reference pattern for Ba3Ta5O15 (JCPDS 83-0713, orange line pattern).

Composite fibers were synthesized with diameters ranging from 100 to 212 nm, which can be achieved by variation of polymer content in the spinning solution.32 Thus, they are quite comparable to the phase pure material regarding their behavior towards variations in the viscosity of the spinning solution. Their morphology is quite similar to that of phase-pure Ba5Ta4O15, as shown in Figure 2.


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Figure 2. SEM images of Ba5Ta4O15-Ba3Ta5O15 composite fibers with varying diameter; from left to

[F(R)*hν]0.5

right: 100, 134, 146, 212 nm

Normalized absorption / a.u.

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


3,0

3,5

4,0

4,5

5,0

5,5

Energy / eV 300

400

500

600

700

Wavelength / nm

Figure 3. Kubelka-Munk absorption spectra of barium tantalate composites Ba5Ta4O15-Ba3Ta5O15, colors are according to Figure 1. The inlay shows the corresponding Tauc plots. The shoulder and the shift of the absorption band give a clear indication for the presence of the intended by-phase.

The presence of the by-phase was confirmed by the diffuse reflectance spectra. Tauc plots of the composite fibers (Figure 3) show a distinct shoulder at a photon energy of roughly 4 eV that is caused by the lower band gap of the additional phase Ba3Ta5O15. No quantum size effect for an increased band gap is


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observed. Additionally X-ray photoelectron spectroscopy revealed the presence of Ta4+ which further hints to the existence of the reduced tetragonal tungsten bronze phase Ba3Ta5O15 (Figure S2). To perform photocatalytic water splitting with the barium tantalate heterojunction nanofibers, the materials were suspended in water/methanol mixtures and irradiated for 1.5h. All heterojunction fibers are able to generate hydrogen without any co-catalyst, steady-state rates after 1 hour are shown in Figure 4. Moreover, the photocatalytic activity is much higher compared to phase-pure Ba5Ta4O15 with cocatalyst, up to 4.4 mmol h-1.32 Interestingly, in contrast to phase-pure Ba5Ta4O15 fibers, in case of the heterojunctions the thinnest nanofibers exhibit the highest activity, as expected. Charge separation on the different phases is already eminent in all samples, thus the activity is solely determined by the diffusion length of charge carriers to the surface, which is a result of the varied fiber diameter.

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H2 evolution / mmol h-1

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6 5 4 3 2 0.0125 w% Rh without co-catalyst

1 0 100

150

200

Diameter / nm

Figure 4. Hydrogen evolution rates for Ba5Ta4O15-Ba3Ta5O15 heterojunction fibers (150 mg) strongly depending on fiber diameter.

As a result, the fibers with the smallest diameter show the highest activity, since they exhibit the lowest number of grain boundaries on the charge carrier diffusion path, taking similar crystallite sizes from XRPD into account.


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After photodeposition of same amounts of Rhodium (Rh, 0.0125 wt.-%) in a second step, the activity of all fibers is further enhanced. Interestingly, the increase in hydrogen evolution is not very high, however this is expected since it was shown by Soldat et al. that charge carrier separation by Rh decoration is less pronounced compared to heterojunction formation, and that decorating Barium tantalate heterojunctions with Rh gives comparably lower activity enhancements.12 The outstanding photocatalytic performance of barium tantalate composites in hydrogen production is attributed to a photocurrent doubling mechanism that occurs when α-hydroxy-methyl radicals (E0 = 0.736 V vs. NHE33) are formed as intermediates. The injection of an electron by the radical into the conduction band of the semiconductor Ba3Ta5O15 that was confirmed by laser flash photolysis Hydrogen evolution / mmol h-1

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3

2 H2 MeOH H2 iPrOH

1

H2 tBuOH

0

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6

t/h

Figure 5. Hydrogen evolution curves of 150 mg of Ba5Ta4O15-Ba3Ta5O15 composite fibers (150 mg) with a diameter of 100 nm and no co-catalyst deposited, utilizing different sacrificial agents

experiments 15 is verified using hydrogen evolution experiments in this work. Therefore the hydrogen evolution of the two-phase heterojunction was measured in the presence of methanol as well as 2-propanol and tert-butyl alcohol. The α-hydroxy- isopropyl radical that is formed upon the first oxidation step of 2-propanol has an even higher reduction potential (E0 = -1.056 V vs. NHE33) than the α-hydroxy-methyl radical, and thus photocurrent doubling should occur as well when ACS Paragon Plus Environment

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2-propanol was used as a hole scavenger, according to the aforementioned mechanism. Ba5Ta4O15 has a reported conduction band edge of -1.2 V vs. NHE,11 so still the α-hydroxy-isopropyl radical cannot insert an electron into the Ba5Ta4O15 phase of the heterojunction. High hydrogen evolution rates were thus expected, even if the electron transfer from 2-propanol to reduce holes in the valence band of the semiconductor is somewhat hindered compared to methanol.34 With tert-butyl alcohol no proton in αposition to the alcohol group can be abstracted, thus no comparable radical can be formed. Hence no performance benefit is expected from composite formation when tert-butyl alcohol is used as sacrificial agent, and the hydrogen evolution rate with tert-butyl alcohol should be much lower. Figure 5 shows that this is in fact the case. With methanol and 2-propanol the hydrogen evolution curves show a rapid increase after the lamp is turned on. A steady state hydrogen evolution of about 4.4 mmol h-1 was already reached after 45 minutes with both alcohols. When tert-butyl alcohol was used as sacrificial agent, the increase of the hydrogen evolution progressed much slower and a steady-state was reached not until 90 minutes after the lamp was turned on. The overall amount of hydrogen produced also was lower (maximum 3.2 mmol h-1) because of the smaller amount of electrons in the conduction band due to the absence of the current doubling. For comparison, the same reactions were performed with phase-pure Ba5Ta4O15 nanofibers, shown in Figure 6. The performance for all three sacrificial agents used is much Hydrogen evolution / mmol h-1

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3,0 H2 MeOH H2 iPrOH

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H2 tBuOH

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Figure 6. Hydrogen evolution curves of pure Ba5Ta4O15 nanofibers (150 mg) with a diameter of 112 nm and no co-catalyst deposited, utilizing different sacrificial agents.

lower compared to the composite, please be aware of the different scaling of Figures 5 and 6. The activity for hydrogen evolution decreases from methanol via iso-propanol to tert-butanol, indicating a steric effect on the kinetics of hole scavenging. Comparing the hydrogen evolution rates of phase-pure Ba5Ta4O15 fibers with Ba5Ta4O15-Ba3Ta5O15 heterojunction fibers of the same diameter, it can be seen that the rates are not even half as high as for the composites. The reduction potentials of the α-hydroxy-methyl radical and the α-hydroxy-isopropyl radical have been discussed above, thus no second electron injection into phase-pure Ba5Ta4O15 should be possible. With no photocurrent doubling possible, hydrogen evolution rates of approx. 2.2 mmol h-1 would be expected, but steady-state rates only reach 1.5 mmol h-1. Therefore, another effect leading to strongly increased activity of the Ba5Ta4O15-Ba3Ta5O15 heterojunction fibers must be evident.

Figure 7. Scheme depicting the improved charge separation from the catalytically very active layered perovskite Ba5Ta4O15 to the tetragonal tungsten bronze phase Ba3Ta5O15. In the presence of hole scavenger that forms α-hydroxy-alkyl radicals photocurrent doubling is possible.


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We believe that the in-situ growth of this heterojunction during electrospinning and calcination, leads to a very intense interfacial contact between both components. This actually does enable efficient charge separation among this heterojunction, resulting in reduced recombination probability of photoexcited charge carriers and in improved photocatalytic activity (Figure 7). This is in contradiction to earlier results found by laser flash photolysis,15 however variations in sacrificial agents to underline other possible mechanisms were not performed in that earlier study. The synthesis via electrospinning also seems to have an effect on the heterojunction formation and charge carrier separation. Barium tantalate fibers have a very low BET surface area, as reported,32 which is however comparable to that of Ba5Ta4O15-Ba3Ta5O15 heterojunction powders.12 Nevertheless, the increase in hydrogen evolution rate from pure Ba5Ta4O15 powder to the heterojunction powder was reported only by a factor of 2, while in the present study on fibers from electrospinning the increase in hydrogen evolution rate in water/methanol from pure Ba5Ta4O15 fibers to Ba5Ta4O15-Ba3Ta5O15 heterojunction fibers is by a factor of 3. 1,2

Gas evolution / mmol h-1

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Hydrogen

1,0

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Oxygen 0,4

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0,0 0

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t/h

Figure 8. Water splitting gas evolution curves of Ba5Ta4O15-Ba3Ta5O15 composite fibers (90 mg) with a diameter of 100 nm and Rh-Cr2O3 co-catalyst deposited.

Finally, we have decorated the Ba5Ta4O15-Ba3Ta5O15 heterojunction fibers with Rh-Cr2O3 co-catalysts via photodeposition for water splitting (see Experimental Section). The measurements show a typical ACS Paragon Plus Environment

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induction period that is known for this type of samples32 and is attributed to the presence of residual carbonates which are preferentially decomposed when no other hole scavenger is present. The existence of such carbonates could be confirmed by DRIFT (Figure S3) and XPS (Figure S4) measurements. After the induction period, simultaneous hydrogen and oxygen evolution can be observed, reaching steadystate rates with nearly 2:1 ratio after 5 hours (Figure 8). The hydrogen production rate from water splitting reaches one mmol h-1, equivalent to a photonic efficiency of 0.68 % (photon flux determined by ferrioxalate actinometry). Extrapolating to an equal mass of photocatalyst, the rate of hydrogen evolution from water splitting with Rh/Cr2O3-decorated Ba5Ta4O15-Ba3Ta5O15 heterojunction fibers is even higher than that of phase-pure Ba5Ta4O15 fibers generating hydrogen without co-catalyst from water/methanol (compare Figure 6). Moreover, while in the absence of sacrificial electron donors most photocatalysts lose a lot of their activity, our Rh/Cr2O3-decorated heterojunction fibers in water splitting still show 31% of the hydrogen evolution rate of Rh-decorated Ba5Ta4O15-Ba3Ta5O15 fibers from water/methanol (compare Figure 4), which is remarkable keeping the absence of photocurrent doubling for pure water splitting in mind. Recombination in such fibrous samples seems to be strongly reduced compared to powder samples, which give in water splitting only 7 % of the hydrogen production compared to Rh-decorated Ba5Ta4O15Ba3Ta5O15 heterojunction powders generating hydrogen from water/methanol.12 So far, we can only explain this remarkable behavior with an intense interfacial contact between Ba5Ta4O15 and Ba3Ta5O15 achieved by the electrospinning synthesis, which leads to very effective charge carrier generation, -separation, and very good water splitting rates. Transient absorption spectroscopy now has to provide evidence for the improved charge carrier separation assumed in this study, while we investigate to transfer this high activity via materials design to the visible light range.

Conclusion


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We have presented the first study on controlled heterojunction design in combination with diametertailored electrospinning to prepare Ba5Ta4O15 in heterojunction with Ba3Ta5O15 as nanofibers for photocatalytic hydrogen production and water splitting. Heterojunction nanofibers with 100 nm diameter show the highest hydrogen evolution rates, of up to 4.4 mmol h-1 with methanol as sacrificial agent. We showed by variation of sacrificial hole scavengers that not only photocurrent doubling, but also intense interfacial contact and improved charge carrier separation do lead to strongly enhanced photocatalytic hydrogen evolution rates. Moreover, water splitting experiments after Rh-Cr2O3 decoration resulted in a very good water splitting performance of Ba5Ta4O15-Ba3Ta5O15 fibers with photonic efficiency of 0.68 %.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: +49-641-9934592 Author Contributions The manuscript was written through contributions of all authors. Funding Sources German Research Foundation DFG (MA 5392/3-1)

ACKNOWLEDGMENT We thank Elif Kübra Özkan for TEM measurements and Dr. Joachim Sann for XPS measurements. Further we would like to thank Dr. Pascal Vöpel for his help and very fruitful discussions, and Prof. Bernd


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M. Smarsly for his support (both Justus-Liebig-University Giessen). R. M. gratefully acknowledges funding in the Emmy-Noether program (MA 5392/3-1) of the German Research Foundation DFG.

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Layered perovskite nanofiber heterojunctions with tailored diameter to

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