SrTaO2N Nanowire Photoanode Modified with a Ferrihydrite Hole

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SrTaON Nanowire Photoanode Modified with a Ferrihydrite Hole-Storage Layer for Photoelectrochemical Water Oxidation Martin Davi, Felix Schrader, Tanja Scholz, Zili Ma, Anna Rokicinska, Richard Dronskowski, Piotr Kustrowski, and Adam Slabon ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00269 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Nano Materials

SrTaO2N Nanowire Photoanode Modified with a Ferrihydrite HoleStorage Layer for Photoelectrochemical Water Oxidation Martin Davi†, Felix Schrader†, Tanja Scholz†, Zili Ma†, Anna Rokicinska‡, Richard Dronskowski†, Piotr Kustrowski‡, and Adam Slabon†,* † ‡

Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland

ABSTRACT: The perovskite-related oxynitride SrTaO2N is a prospective photoanode candidate with favorable band-edge positions. We have synthesized SrTaO2N nanowires with roughened surfaces by a hydrothermal process on a tantalum substrate and nitridation at 1273 K under flowing ammonia and hydrogen. The nanowires were coated with a ferrihydrite hole-storage layer for photoelectrochemical water oxidation under AM 1.5G illumination in 0.1 M NaOH electrolyte (pH 13). The nanowires exhibit an electronic band gap of 2.32 eV and the photocorrosion can be decreased with ferrihydrite coating. In the presence of Na2SO3 as a hole scavenger the nanowires do not yield higher photocurrent. This indicates that the limiting factors for hole extraction are processes occurring in the bulk instead at the semiconductor-electrolyte interface. Our work is the first trial of a SrTaO2N photoanode based on nanowires and may also be promising for other quaternary oxynitride semiconductors to achieve a one-dimensional morphology. KEYWORDS: nanowires, oxynitrides, water-splitting, SrTaO2N, photoanode, photoelectrochemistry

INTRODUCTION The declining demand for traditional energy generation based on nuclear and fossil fuels in parts of the world has led to a high interest for solar fuel generation. Energy conversion based on photoelectrochemical (PEC) cells is regarded as a realistic alternative for energy generation in the near future.1 Depending on the semiconductor material and electrolyte, PEC cell technology can be applied for generation of electricity or chemicals.2 The latter solution is especially promising for artificial photosynthesis and requires a two-electron process for the hydrogen evolution reaction (HER) and a more demanding four-electron process for the oxygen evolution reaction (OER).3 The first report of a PEC cell producing chemical fuel, defined as a photoelectrosynthetic cell, was a photoanode-driven device based on a TiO2 semiconductor electrode and a metallic cathode.4 It is imperative that for an efficient solar-to-hydrogen conversion, the band gap has to be narrower than the value of 3.2 eV in the case of TiO2. This has triggered intense research into semiconductor oxides for photoanodes with narrow electronic band gaps and structural stability. Semiconductor materials based on chalcogenides5, nitrides6, cyanamides7, and oxynitrides8 have been identified as potential photoelectrodes, too. Tantalum-based nitrides, such as Ta3N59, and oxynitrides have received high attention as photoelectrodes due to their favorable band gaps for PEC operation.10 The integration of powderous oxynitrides into an photoelectrode can be accomplished by the particle transfer method11 and electrophoretic deposition (EPD)12. However, those assembled thin films still reach photocurrent densities significantly below their theoretically values. This originates mainly from low electrical conductivity in the bulk and low charge carrier transport between the agglomerated oxynitride particles.13 One may overcome this problem, however, by means of

a necking procedure based on impregnation and additional annealing.14 Besides the abovementioned bottleneck in electrode preparation, oxynitrides require protection of their surfaces to prevent photocorrosion during PEC operation, in the form of passivating layers, including TiOx15, Al2O316, MnOx17, Ta2O518, NiOx19 and ferrihydrite20. These thin overlayers can be formed by sputtering21, electrochemical deposition22, atomic layer deposition (ALD)23, impregnation from solution24 and transfer of self-assembled nanocrystal arrays25. Coating of semiconductors with sequential different overlayers may enable to construct a potential gradient for ameliorated hole extraction.26 Within this context, a ferrihydrite overlayer has appeared to be an efficient hole-storage layer for nitride photoanodes.27 Li et al. demonstrated a strategy to extract almost all photogenerated holes for PEC water oxidation from a Ta3N5 photoanode by coating with hole-storage layers, consisting of ferrihydrite and nickel hydroxide, for charge transfer mediation to molecular catalysts.28 Beyond tantalum nitrides and oxynitrides, quaternary oxynitrides in perovskite-related structure types29 are currently intensively investigated as prospective photoanode candidates.30 SrTaO2N displays a band gap of 2.1 eV and its edge positions of the valence and conduction band are theoretically suitable for overall water splitting.31 SrTaO2N photoanodes have been already successfully coupled to a cobalt phosphate (Co-Pi) electrocatalyst to improve stability and photocurrent.32 Although loading with iridium33- and cobalt34-based catalysts is known to enhance significantly photocurrents of TaON photoanodes, the poor electrical conductivity and bulk recombination have to be addressed first in the case of SrTaO2N.35 Semiconducting materials of strongly anisotropic morphology, e.g., nanotubes36 or nanowires37, can harvest light efficiently while ensuring simultaneously high charge-carrier

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conductivity. Most nitrides and oxynitrides are usually obtained in the form of microcrystals by nitridation of a precursor oxide phase. Ta3N5 nanowire photoanodes can be manufactured by drop-casting of nanowire powder on a conductive substrate.38 Another approach is the conversion of a flat tantalum substrate into anisotropic structures by anodization and its nitridation.39 This method cannot be applied directly to quaternary oxynitrides, because the second metal component is not incorporated in the substrate. The perovskite-related quaternary oxynitrides are therefore usually obtained as a microcrystalline powder. Cubic shapes can be produced as powder if the synthesis is carried out in a metal salt flux.40 Domen et al. have recently reported nanostructured BaTaO2N thin films in the form of nanosheets on a tantalum substrate.41 The oxynitride nanostructure was grown hydrothermally followed by nitridation and maintained the nanosheet morphology.42 We were interested to grow one-dimensional nanostructures of SrTaO2N as potential photoanodes. Herein, we report on the fabrication of oxynitride SrTaO2N nanowires and their surface functionalization with a hole-storage layer in the form of ferrihydrite (Fh) (Scheme 1).

Scheme 1. Fabrication process for SrTaO2N/Fh nanowire photoanode. Tantalum is used as substrate (1) for the growth of oxynitride nanowires (2). The obtained SrTaO2N nanowires are subsequently coated with a hole-storage layer of Fh (3).

EXPERIMENTAL SECTION Preparation of Ta/SrTaO2N nanowire photoanode. The nanowires were grown, by a modified method for BaTaO2N thin films reported by Domen41 et al., on a tantalum substrate using a hydrothermal method and subsequent nitridation43. Tantalum foil (99.99 wt.-%, Smart Elements) was cut into a piece of 20 mm x 10 mm and washed with nitric acid, deionized water and ethanol. The tantalum foil was placed vertically in a 20 mL autoclave and filled with 14 mL of Milli-Q water (18.3 Ω cm). 5.3 g (0.02 mmol) of Sr(OH)2 8H2O (99.995 wt.%, Sigma) was added to the autoclave and Ar was bubbled through the solution for 10 min. The autoclave was tightly sealed and heated at 473 K for 20 h. After cooling to room temperature, the tantalum foil was removed from the solution, rinsed with deionized water and dried. The tantalum foil was annealed at 923 K for 15 min under ambient atmosphere. After cooling down, the foil was put horizontally on an alumina boat and nitridated at 1273 K for 3 h under a constant flow of mixed NH3 (15 mL min−1) in H2 (5 mL min−1).44 The color of the thin film changed from white to orange. The thin film was inserted for 20 s in a 0.1 M HNO3 solution and washed with deionized water.45 The real ratio of strontium and tantalum was determined by means of Atomic Absorption Spectroscopy (AAS). Surface modification with Fh. The oxynitride photoelectrode was immersed for 2 min in an aqueous solution of 0.05 M Fe(NO3)3 and 0.8 M NaNO3, dried under ambient atmosphere, and heated at 373 K for 10 min. After cooling down, the photoanode was washed with ethanol and dried under vacuum.

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Powder X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). Powder XRD patterns were recorded on a STOE STADI-P diffractometer (Cu Kα1 radiation) equipped with a DECTRIS Mythen 1K detector in transmission mode. Before the measurement the thin film was mechanically removed from the tantalum substrate. The surface composition of the oxynitride thin film was determined by using a Prevac photoelectron spectrometer equipped with a hemispherical analyzer (VG SCIENTA R3000) and a low energy flood gun (FS40A-PS). The spectra were recorded using a monochromatized aluminum source Al Kα (E = 1486.6 eV). UV-Vis Spectroscopy and Electron Microscopy. UV-vis spectra were collected on a Shimadzu UV-2600 spectrophotometer. The band gap was calculated assuming a direct allowed transition according to (αhν)2 = hν − Eg where α is the absorption coefficient, hν is the photon energy and Eg is the band gap. (αhν)2 was plotted as a function of hν and the band gap was determined by extrapolating the linear region to zero. SEM and TEM images were acquired on a Leo Supra 35VP SMT(Zeiss) and a Zeiss Libra 200 FE TEM, respectively. Energy dispersive X-ray (EDX) spectroscopy was performed with an XFlash 5030 detector from Bruker. Photoelectrochemistry. Experiments were carried out in an PEC cell (WAT Venture) operating in a three-electrode setup. Platinum wire and 1 M Ag/AgCl electrode were used as a counter electrode and reference electrode, respectively. Electrochemical data were recorded with a potentiostat (SP-150, BioLogic) operating with the EC Lab Software package. All current values of the oxynitride photoanodes were recorded vs. 1 M Ag/AgCl and converted vs. reversible hydrogen electrode (RHE) according to ERHE (V) = E1M Ag/AgCl + (0.059 V × pH). The photoanodes were illuminated with 100 mW cm−2 (AM 1.5 G) simulated visible light which was generated by a solar light simulator (class-AAA 94023A, Newport) with an ozonefree 450 W xenon short-arc lamp. 0.1 M NaOH was used as electrolyte for all PEC experiments and diluted with Milli-Q water (18.3 Ω cm). Potentials were swept with a scan rate of 10 mV s−1 toward the positive direction from 0.70 V to 1.44 V vs RHE. Mott-Schottky measurements were performed in 0.1 M NaOH solution at 10 Hz and analyzed with the EC Lab Software.32 For determination of the incident photon-to-current conversion efficiency (IPCE), monochromatic light was created by a 300 W arc xenon light source. The light intensity at given wavelength was determined with a UV enhanced silicon photodetector (818-UV, Newport) from 380-700 nm. The IPCE was calculated according to IPCE (%) = (I × 1240 V nm) / (λ × P) × 100; where I is the photocurrent density (mA cm−2), λ - the measured wavelength (nm) and P - the intensity of monochromatic light (mW cm−2). RESULTS AND DISCUSSION Structural characterization. SrTaO2N crystallizes in a tetragonal perovskite-related pseudo-cubic structure type (space group I4/mcm), the quantum-chemical reason for the lowered symmetry lying in the different covalency of the Ta-O and TaN bonds, a typical oxynitride phenomenon.46 The tantalum cations form TaO4N2 octahedra and the nitrogen atoms can either occupy two opposite or vicinal coordination sites.47 The crystallographic positions O/N can be either fully ordered48 or statistically disordered49. The most stable configuration of the octahedron has been reported to be a cis configuration of nitrogen atoms.50


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Figure 1. Experimental and simulated powder XRD patterns of SrTaO2N nanowire (ICSD 95373) thin films modified with Fh. For the Ta/SrTaO2N/Fh thin film, the oxynitride was mechanically removed from the tantalum substrate. For comparison, the simulated powder XRD pattern of Ta3N5 was added. The inset shows the crystal structure of SrTaO2N. Strontium and oxygen/nitrogen atoms are drawn in blue and red color, respectively. TaO6-xNx octahedra are drawn in green color.

Figure 2. SEM micrograph of SrTaO2N/Fh nanowire photoanode on tantalum substrate.

The recorded powder XRD patterns confirm that the sample consists of the tetragonal SrTaO2N phase (Figure 1). Prior to the measurements, the oxynitride thin films were mechanically removed from the tantalum substrate. The relative strontiumtantalum molar ratio was determined to be 1 : 1.04 by means of AAS. This is in agreement with the XRD results, and the slightly higher tantalum content may be due to the preparation of the sample, because the nanowires were removed from the tantalum substrate. The morphology of the SrTaO2N thin film grown hydrothermally on tantalum substrate is anisotropic (Figure S1). The nanowires display approximately a length of 3 µm (Figure S2) and a thickness of 30-60 nm. Surface functionalization of the nanowires with Fh did not alter the surface morphology (vide infra). Figure 2 shows that the resulting SrTaO2N/Fh nanowires are strongly roughened and partially

Figure 3. (a) Elemental distribution across two parallel SrTaO2N/Fh nanowires. The grey line illustrates the direction of the EDX line scan. (b) SEM micrograph of SrTaO2N/Fh nanowires in higher magnification.

interconnected as a result of nitridation at elevated temperatures of 1273 K. Nevertheless, the anisotropic nanowire morphology is maintained. Since the small amount of ferrihydrite was not detectable in XRD, we detached the nanowires with a razor blade from the tantalum substrate and transferred them on a TEM grid in order to analyze the chemical composition by means of EDX (Figure 3). A linescan across two nanowires SrTaO2N/Fh aligned parallel confirmed the presence of strontium, tantalum, nitrogen and iron. Although the amount of iron is low, it can be clearly detected in the EDX linescan. As a consequence of the low annealing temperature of 373 K after iron deposition on the oxynitride nanowires, the measured iron content in the EDX linescan corresponds most likely to surface coverage. Complementary TEM images of the nanowires scratched from the tantalum surface are presented in Figure 4. The well-defined reflection peaks of the selected area electron diffraction (SAED) patterns reveal the SrTaO2N nanowires to be highly crystalline (Figure 4 c and d). Although the nanowires are not single-crystalline, the reflection peaks can be indexed to the crystal structure of SrTaO2N (Figure S3). The band gap of the nanowires was determined by means of UV-vis spectroscopy (Figure 5). Figure S4 shows the absorption coefficient of SrTaO2N nanowires as a function of wavelength. It is known that the electronic band gap may be affected by the coordination configuration of the TaO4N2 octahedra in the crystal structure.51 The calculated band gap of 2.32 eV is consistent with the previously reported values (2.3–2.6 eV).52 The type of anionic ordering in SrTaO2N has been reported to be dependent on the substrate at which the oxynitride is grown and to have an impact on its optical properties such as the band gap. We analyzed the surface of the SrTaO2N nanowire photoanode by means of XPS. The corresponding high-resolution XPS spectra for the selected regions are presented in Figure 6. In the Fe 2p spectrum, the Fe 2p3/2 and Fe 2p1/2 peaks centered at 711.2 eV and 724.8 eV, respectively, reveal multiplet structures typical of high-spin Fe3+ in iron oxyhydroxides.53 Moreover, in the O 1s spectrum beside the component at 530.2 eV, attributed to lattice O2−, another distinct peak appears at 532.5 eV. This line assigned to OH− species clearly confirms the presence of iron oxyhydroxide phase in the studied material.54



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Figure 4. TEM images of SrTaO2N/Fh nanowires (a and b) and spot (c) where SAED patterns (d) were recorded.

Figure 5. Tauc plot for direct allowed transition for SrTaO2N nanowires.

In the range of binding energies (Eb) between 394 eV and 412 eV, the photoemission from the Ta 4p and N 1s core levels is detected. The peaks at relatively low Eb of 395.9 eV and 399.7 eV, assigned to N3− species in lattice and interstitial sites, respectively55, confirm the incorporation of nitrogen atoms into the Ta−O bonds. On the other hand, at higher values of Eb the Ta 4p3/2 signals are observed. In addition to the peak at 403.7 eV, which corresponds to Ta−O bondings, another distinct peak at extremely high Eb of 407.2 eV is present. Such high value of Eb suggests the photoemission from Ta5+ ions located in the neighborhood of strongly electronegative species.

Figure 6. XPS Fe 2p, O 1s, Ta 4p and N 1s spectra of Ta/SrTaO2N/Fh nanowire photoanode.

PEC water oxidation. The SrTaO2N/Fh nanowire photoanode was analyzed in NaOH electrolyte at pH 13 by linear square voltammetry (LSV) (Figure 7) and cycling voltammetry (Figure S5). Various time of the iron precursor deposition was studied by coating a series of nanowire electrodes (Figure S6).


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Figure 7. LSV of Ta/SrTaO2N/Fh nanowire photoanode and Fh anode. Measurements were performed in 0.1 M NaOH electrolyte (pH 13) at a scan rate of 10 mV s−1 under AM 1.5 G illumination (100 mW cm−2).

The bare Fh electrode did not develop any photocurrent which is in agreement with the studies by Li et al.27 on ferrihydritemodified Ta3N5 photoelectrodes. PEC water oxidation on nitride and oxynitride photoanodes requires protective coating in order to prevent the semiconductor from photocorrosion, i.e. loss of nitrogen from the crystal structure being equivalent with a loss of the narrow electronic band gap. The SrTaO2N/Fh nanowire photoanode produced an anodic photocurrent when the potential was swept toward the positive direction. Upon interruption of illumination, the photocurrent dropped to a value close to zero but maintained in the anodic region. This behavior has been frequently found for oxynitride photoanodes prepared on tantalum substrate.41 This is most likely due to charge carrier recombination processes at the semiconductor surface. The ferrihydrite acts as a protecting layer to prevent photocorrosion of the oxynitride. Upon illumination the nanowire photoanode exhibited a sharp increase in current density followed by a fast decay to a stable anodic current. We carried out supplementary experiments on microcrystalline SrTaO2N electrodes and observed similar tendency in the PEC performance (Figures S7-S11). During CA at 1.23 V vs RHE the nanowire photoanode showed a sharp upsurge in photocurrent that subsequently decreased and settled at 35 µA cm−2 (Figure 8). Upon interruption of illumination, the current dropped below zero, hinting toward charge recombination processes occurring at the surface. This behavior is known for ferrihydrite-functionalized electrodes and may be a consequence of stored photogenerated holes in the overlayer. Without surface protection, the oxynitride displayed higher dark current and lower current originating from illumination (Figure S12). However, after having analyzed the PEC efficiency with respect to LSV, CA and IPCE, we performed an additional CA for 2 h during which the nanowire photoanode suffered significant decay in photocurrent (Figure S13). An EDX line scan recorded on a single nanowire after PEC water oxidation showed a significantly decreased amount of nitrogen (Figure S14). This is an indication that the decrease in activity originates from photocorrosion, a common behavior for oxynitride photoelectrodes.

Figure 8. CA of Ta/SrTaO2N/Fh nanowire photoanode and Fh anode. Measurements were performed in 0.1 M NaOH electrolyte (pH 13) under AM 1.5 G illumination (100 mW cm−2).

Figure 9. LSV of Ta/SrTaO2N/Fh nanowire photoanodes with and without a hole scavenger in the form of Na2SO3. Measurements were performed in 0.1 M NaOH electrolyte (pH 13) at a scan rate of 10 mV s−1 under AM 1.5 G illumination (100 mW cm−2).

Since the Fh coating was obtained by impregnation, ameliorated surface coverage and thus enhanced corrosion protection may be achieved for Fh deposition by means of ALD. We also investigated the PEC oxidation performance of the nanowire photoanode in the presence of Na2SO3 as a hole scavenger (Figure 9). The fast oxidation of the latter allows to determine the possible photocurrent if all holes at the surface are used. We did not detect any significant difference in the photocurrent density if the hole scavenger was present in the electrolyte. This indicates that major limitations in the achieved photocurrent stem from low charge carrier conductivity or recombination in the bulk, instead of recombination



processes occurring at the semiconductor-electrolyte interface. One should take into account that the nanowires were not subject to any post-annealing necking treatment14 and were not modified with catalytic layers56. Corresponding IPCE measurements on the SrTaO2N/Fh nanowire photoanode showed the development of a photocurrent at 600 nm that reached a stable value of 1.8% below 540 nm (Figure S15). The band edge positions of the nanowires were determined by means of Mott-Schottky measurements (Figure 10). The flatband potential exhibited a value of −0.2 V vs RHE which is in agreement to previous studies on SrTaO2N.32,52 The positive value of the slope confirms that the

nanowires are n-type semiconductors. The inset of Figure 10 indicates a scheme of the estimated band edge position of the SrTaO2N nanowires based on the determined electronic band gap (vide supra).

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TaO2N grown on tantalum substrate, which can harvest a higher fraction of incident light due to a smaller band gap, developed a photocurrent of 0.75 mA cm−2 after deposition of a electrocatalyst.41 The critical advance of our work is the demonstration of a synthetic procedure for oxynitride nanowires that may be also applicable to other quaternary oxynitride compounds.

ASSOCIATED CONTENT Supporting Information SEM image of SrTaO2N nanowires before Fh coating (Figure S1), cross section of nanowires (Figure S2), assigned TEM SAED pattern of SrTaO2N nanowires (Figure S3), absorption coefficient (Figure S4), cyclic voltammetry of nanowire photoanode (Figure S5), LSV curves with different amount of Fh coating (Figure S6), SEM images of c microcrystals on FTO glass (Figures S7-S8), LSV curves of SrTaO2N nanowires and microcrystals (Figure S9), CA of SrTaO2N (microcrystals) photoanode with and without a hole scavenger (Figure 10), LSV of ball-millled SrTaO2N microcrystals (Figure 11), CA of nanowire photoanode with and without Na2SO3 (Figure S12), CA of nanowire photoanode for 2 h (Figure S13), TEM EDX line scan on a nanowire after PEC water oxidation (Figure S14), IPCE of nanowire photoanode (Figure S15). The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * (A.S.) Email: [email protected]. Telephone; +49 (0) 241 809 2075

ORCID Figure 10. Mott-Schottky plot of SrTaO2N/Fh nanowire photoanode. The inset shows band edge positions of the SrTaO2N nanowires.

Richard Dronskowski: 0000-0002-1925-9624 Adam Slabon: 0000-0002-4452-1831

Author Contributions The manuscript was written through contributions of all authors.

CONCLUSION Our work is the first report on the fabrication of a nanowire photoanode SrTaO2N for PEC water oxidation. In contrast to previously reported nanostructured thin films of BaTaO2N on tantalum substrate, the herein reported strontium analogue is characterized by a distinct anisotropic morphology. Surface modification with ferrihydrite as a hole-storage layer improved the stability of the photocurrents of the oxynitride photoanodes. Although the SrTaO2N/Fh nanowire photoanode may be functionalized with an electrocatalyst, such as Co-Pi, in order to improve PEC efficiency, we believe that two major issues need to be addressed first: i) post-annealing and necking treatment to improve electrical conductivity, and ii) sequential deposition of hole-storage layers (TiOx, Fh, NiOx) for enhanced hole extraction. For example, the currently bestperforming SrTaO2N-based photoanode reported by Li et al. reached 1.1 mA cm−2 at 1.23 V vs RHE after a TiCl4-necking treatment, additional annealing under hydrogen and coating with a Co-Pi electrocatalyst.32 These additional and sophisticated modifications may explain the lower performance of our nanowires than for the SrTaO2N/Co-Pi photoanode. For comparison, chemically related nanostructured thin films of Ba-

ACKNOWLEDGMENT A.S. would like to thank the Fonds der Chemischen Industrie (FCI) for a Liebig habiliation fellowship. We thank George Ogutu for helpful discussions, Birgit Hahn and Dr. Dirk Oliver Schmidt for acquiring SEM images, Marek Drozdek for XPS measurements, Prof. Ulrich Simon for accessibility to TEM and SEM facilities. TEM measurements were performed at the Gemeinschaftslabor für Elektronenmikroskopie (GFE) of the RWTH Aachen University, Ahornstraße 55, 52074 Aachen. The XPS measurements were carried out with the equipment purchased thanks to financial support of the European Regional Development Fund in the framework of the Polish Innovation Operational Program (Contract No. POIG.02.01.00-12-023/08). This project was funded by the Excellence Initiative of the German federal and state governments.

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Fig 6. XPS. 110x297mm (96 x 96 DPI)


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SrTaO2N Nanowire Photoanode Modified with a Ferrihydrite Hole

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