A Quaternary Core-Shell Oxynitride Nanowire Photoanode Containing

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A Quaternary Core-Shell Oxynitride Nanowire Photoanode Containing a Hole-Extraction Gradient for Photoelectrochemical Water Oxidation Zili Ma, Thomas Thersleff, Arno Görne, Niklas Cordes, Yanbing Liu, Simon Jakobi, Anna Rokicinska, Zebulon Schichtl, Robert H Coridan, Piotr Ku#trowski, Wolfgang Schnick, Richard Dronskowski, and Adam Slabon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Applied Materials & Interfaces

A Quaternary Core-Shell Oxynitride Nanowire Photoanode Containing a Hole-Extraction Gradient for Photoelectrochemical Water Oxidation Zili Ma†, Thomas Thersleff‡, Arno L. Görne†,‡, Niklas Cordes⊥, Yanbing Liu†, Simon Jakobi†, Anna Rokicinska∥, Zebulon G. Schichtl§, Robert H. Coridan§, Piotr Kustrowski∥, Wolfgang Schnick⊥, Richard Dronskowski†, and Adam Slabon‡,* †Chair

of Solid-State and Quantum Chemistry, Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany ∥Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland ⊥Department of Chemistry, University of Munich (LMU), Butenandtstraße 5-13 (D), 81377 Munich, Germany §Department

of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, United States

‡Department

of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16 C, 106 91 Stockholm, Sweden KEYWORDS: oxynitrides, nanowires, photoelectrochemical water splitting, perovskite, electrocatalysis, hole storage layer ABSTRACT: A nanowire photoanode SrTaO2N, a semiconductor suitable for overall water-splitting with a bandgap of 2.3 eV, was coated with functional overlayers to yield a core-shell structure while maintaining its one-dimensional morphology. The nanowires were grown hydrothermally on tantalum, and the perovskite-related oxynitride structure was obtained by nitridation. Three functional overlayers have been deposited on the nanowires to enhance the efficiency of photoelectrochemical (PEC) water oxidation. The deposition of TiOx protects the oxynitride from photocorrosion and suppresses charge-carrier recombination at the surface. Ni(OH)x acts a hole-storage layer and decreases the dark-current contribution. This leads to a significantly improved extraction of photogenerated holes to the electrode-electrolyte surface. The photocurrents can be increased by deposition of a cobalt phosphate (CoPi) layer as a co-catalyst. The heterojunction nanowire photoanode generates a current density of 0.27 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE) under simulated sunlight (AM 1.5G). Simultaneously, the dark-current contribution, a common problem for oxynitride photoanodes grown on metallic substrates, is almost completely minimized. This is the first report of a quaternary oxynitride nanowire photoanode in core-shell geometry containing functional overlayers for synergetic hole extraction and an electrocatalyst.

INTRODUCTION Converting solar energy into storable chemical fuels is a highly challenging goal for green energy generation.1–5 PEC water splitting on semiconductor devices to produce hydrogen is such environment-friendly solution. Water splitting encompasses the hydrogen evolution reaction and the oxygen evolution reaction. Since the latter includes the transfer of four electrons, the oxidation of water is the efficiency-limiting half reaction and requires photoanodes that can drive oxidative processes upon absorption of photons.6 The first investigated photoelectrode was a TiO2 photoanode, but despite its stability under operating conditions, the large band gap of 3.2 eV limits its absorption of visible light.7 Therefore, it is indispensable to develop a stable semiconducting material with a narrower band gap to utilize capably visible light, which represents almost half of the available solar spectrum. Enormous attempts have been made to identify new prospective semiconductor materials for PEC water-splitting devices. Among them, n-type oxide-based semiconductors, such as CuWO4,8 WO3,9 BiVO4,10 α-Fe2O3,11 and ZnO,12 have been extensively investigated for photoanodes. Recently, several nitrides and perovskite-type oxynitrides (e.g.,

SrTaO2N13, BaTaO2N14, LaTaON215, TaON16, LaTiO2N17, Ta3N518) with optical absorption above 550 nm have received increasing attention19. Some of these (oxy)nitrides can theoretically perform overall water-splitting, i.e., without an external potential, due to the beneficial positions of their valence band edge (VBE) and conduction band edge (CBE).20 The tantalum-based SrTaO2N falls into this category with a CBE of −0.2 V and a VBE of +2.1 vs. RHE.21 Various strategies have been attempted to advance the performance of oxynitride photoelectrodes.22 Besides nanostructuring23, doping of the semiconductor material has proved successful to advance the photocatalytic performance, such as for Na-doped SrTaO2N20 and Ca-doped BaTaO2N24 photoanodes. Effort has been also made to build heterojunction structures to achieve larger photocurrents.16 The formation of interfaces can yield heightened chargecarrier separation if the band edges are matched.25 Pan et al. showed that a Sr2Ta2O7−xNx/SrTaO2N heterojunction can ameliorate the photocurrents of SrTaO2N electrodes.26 Another example is the heterojunction photoanode C3N4/TaON which shows further improvement upon deposition of CoOx nanoparticles.23 Cobalt-based coatings are powerful catalysts

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for water oxidation, and their deposition on the electrode surface is a general strategy.27 Similar to their oxidic counterparts, oxynitride thin films suffer from a large electric resistance, leading to an impeded charge-carrier separation. Thermal treatment of oxynitrides electrodes under an oxygen-deficient atmosphere can reduce the resistance.28 Post-necking treatment is another possibility to improve charge transport across agglomerated particles forming the photoelectrode. 15 All the strategies mentioned above are focused on three main problems involved in the PEC water oxidation: i) the effective production of electron-hole pairs related to the absorption, ii) the separation and migration instead of recombination of the photogenerated charges, and iii) the surface reaction initiated by the photogenerated charges.29 In addition to these issues, the photocorrosion of oxynitrides, i.e., loss of nitrogen anions from the structure, must be inhibited. This can be accomplished by depositing thin coatings on the semiconductor surface. Ferrihydrite21, Al2O330, and TiOx31 thin layers are some examples employed as protection layers. Integration of sequentially different functional overlayers and photoelectrodes can synergistically augment the photocurrents. Li et al.32 showed that TiOx deposited on Ta3N5 can decrease electron-hole recombination, and further coating with nickel hydroxide and ferrihydrite can store holes. In comparison to flat electrodes, semiconducting nanowire thin films exhibit both high light harvesting and charge-carrier separation. As a consequence of the one-dimensional structure, the holes have short diffusion pathways across the nanowire, and electrons can be transferred along the nanowire to the counter electrode. Although one-dimensional structures of Ta3N5 and TaON have already been prepared by a variety of synthetic methods, there is up to now only our recent report on a quaternary oxynitride nanowire photoanode such as SrTaO2N.21 Here, we present the first successful attempt to create a quaternary oxynitride nanowire photoanode with functional overlayers to yield a hole extraction gradient. The photogenerated holes from the oxynitride core can be efficiently transferred through overlayers to the outer shell consisting of a catalyst for water oxidation. The obtained heterojunction photoanode is analyzed by means of powder Xray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Scanning (SEM) and Transmission Electron Microscopy (TEM) including Electron Energy-Loss Spectroscopy (EELS) and Energy-Dispersive X-Ray Spectroscopy (EDX), and (photo)electrochemical methods. Chemical bonding of SrTaO2N is discussed at the density functional theory (DFT) level. EXPERIMENTAL SECTION Fabrication of SrTaO2N nanowire thin films. A hydrothermal synthesis followed by nitridation was employed to grow SrTaO2N nanowires on tantalum substrates according to our previous work.21 Tantalum foil (99.99 wt %) was purchased from Smart Elements. The tantalum foil (0.5 mm × 10 mm × 20 mm) was washed with nitric acid solution, deionized water, and ethanol for 15 min, respectively. 5.3 g (0.02 mol) of Sr(OH)2·8H2O (99.995 wt %, Sigma-Aldrich) and 14 ml Milli-Q water (18.3 Ω cm) were added into a 20 ml stainless steel autoclave and tantalum foil was placed vertically inside. Argon was bubbled through the solution for 10 min prior sealing and heating at 473 K for 20 h.

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Scheme 1. Schematic illustration of the fabrication procedure for SrTaO2N/TiOx/Ni(OH)x/CoPi core–shell nanowire photoanode. SrTaO2N (STON) nanowires are fabricated on tantalum substrate by hydrothermal growth and nitridation. The nanowires are subsequently coated with three different overlayers.

The white film on the tantalum plate was washed with water, dried and heated at 923 K for 15 min. The conversion to the oxynitride was performed by nitridation under flow of NH3 (15 mL min−1) and H2 (5 mL min−1) at 1273 K for 3 h. Subsequently, the sample was placed in a 0.1 M HNO3 solution for 20 s and washed with water. Electrodeposition of TiOx blocking layer. An amorphous TiOx layer was formed on SrTaO2N nanowires by means of a modified electrodeposition route.33 2 ml 20 % TiCl3 solution in HCl (abcr GmbH) were diluted by 40 ml deionized water. The pH value was adjusted to 2.45±0.03 by slowly adding dropwise 0.6 M NaHCO3 solution. The solution was used as electrolyte to electrodeposit a TiOx layer at 0.07 V vs. 1 M Ag/AgCl for 15 s. Then, the electrode SrTaO2N/TiOx was washed gently with deionized water and dried in air. The FTO/TiOx electrodes were obtained according to the same procedure but with electrodeposition time of 5 min to ensure a sufficient thickness. Atomic Layer Deposition (ALD) Growth of TiOx layer. Deposition was performed at 423 K using tetrakis(dimethylamido)titanium (TDMAT, 99 wt %; Strem, Inc.) and H2O as substrates. The flow rate of the nitrogen carrier gas was set to 20 sccm. The temperatures of the TDMAT and water precursor cylinders were kept at 343 K and room temperature, respectively. A deposition cycle was started with a 100 ms pulse of TDMAT, a 15 s purge, a 15 ms pulse of water, and a final purge for 25 s. For the deposition of 4 and 10 nm of TiOx, 120 and 300 cycles were performed, repectively.12 Electrodeposition of Ni(OH)x hole-storage layer. Ni(OH)x was deposited from 25 mM NiSO4 electrolyte at pH 6.9 (adjusted by adding NaOH) at a potential of 1.17 V vs. 1 M Ag/AgCl electrode for 2 mins. Deposition of CoPi layer. Photo-assisted electrodeposition of CoPi onto the SrTaO2N/TiOx/Ni(OH)x electrode was carried out under 1 sun (AM 1.5G) illumination at a constant potential of 0.2 V vs. 1 M Ag/AgCl for 15 s in a electrolyte containing 5 mM Co(NO3)2 and 0.1 M potassium phosphate (KPi) buffer at pH 7.


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Figure 2. SEM micrograph of SrTaO2N/TiOx/Ni(OH)x/CoPi nanowire photoanode on tantalum substrate. Figure 1. Experimental and simulated powder XRD patterns of SrTaO2N nanowire (ICSD 95373) thin films and the simulated powder XRD pattern of Ta (ICSD 96594). The electrode was subsequently washed with deionized water. Scheme 1 shows the complete preparation procedure for the SrTaO2N/TiOx/Ni(OH)x/CoPi photoanode. Making of FTO/LaNbON2 and FTO/SrNbO2N photoanodes. LaNbON2 powder was prepared by the previously described ammonothermal method.34-35 La (99.9%, Sigma–Aldrich) and Nb (99.8 %, 325 mesh, Sigma-Aldrich) were used as metal sources. Electrophoretic deposition (EPD) was used to fabricate the FTO/LaNbON2 photoanodes.21 Similar synthesis was applied for SrNbO2N. Computational details. All DFT calculations were carried out with the Vienna Ab initio Simulation Package (VASP)36–39, using the PBE exchange-correlation functional40 and PAW pseudo-potentials41-42 with an energy cutoff of 500 eV for the plane-waves. For all electronic-structure calculations, a convergence criterion of at least 10–5 eV was used. The atomic positions were optimized until the Hellmann–Feynman forces fell below 5×10–3 eV Å–1. DOS and crystal orbital Hamilton population (COHP) curves were projected with LOBSTER.43– 46

XRD, XPS and UV-Vis Spectroscopy. For XRD measurements, the thin film was mechanically scratched off from the tantalum substrate (Figure 1). The patterns were recorded on a STOE STADI-P diffractometer (Cu Kα1 radiation) equipped with a DECTRIS Mythen 1K detector in transmission mode. A Prevac photoelectron spectrometer equipped with a hemispherical analyzer (VG SCIENTA R3000) and a low-energy flood gun (FS40A-PS) was employed to determine the surface composition of the thin films. The spectra were recorded using a monochromatized aluminum source Al Kα (E = 1486.6 eV). 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. (αhν)2 was plotted as a function

of hν, and the band gap was determined by extrapolating the linear region to zero.

Electron Microscopy. SEM images were acquired on a Leo Supra 35VP SMT (Zeiss) (Figure 2). The EELS / EDX data presented in Figure 3 were acquired on a double-aberration corrected Themis Z TEM fabricated by FEI company (part of Thermal Fisher Scientific) and operated at 300 kV. Probe aberrations were corrected up to fourth order prior to the experiment, and a probe current of 2 nA was used, yielding a useable spatial resolution of approximately 1.2 Å. The probe was scanned over the region of interest at a rate of approximately 400 nm/s, dwelling for just under 10 ms at each pixel position. Both the EDX and the EELS signals were simultaneously acquired using the Velox 2.6 software by FEI company. EDX spectra were recorded with a FEI ChemiSTEM detector in the Super XG2 geometry, while the EEL spectra were generated in the magnetic prism of a Quantum GIF 965 spectrometer by Gatan Inc. This spectrometer is equipped with dual-EELS capability, and both the low-loss and core-loss EELS regions were simultaneously recorded. An energy dispersion of 1 eV / channel with an offset of 200 eV was used for the core-loss region, allowing for the capture of all energy edges ranging from C-K at 284 eV to P-K at 2146 eV. The electron optics of the microscope were configured to use a convergence semi-angle of 17.9 mrad and a collection angle of 30 mrad. Following data acquisition, the core-loss EELS data were treated using Multilinear Single-Value Decomposition (MLSVD)47-49, as implemented in the TensorLab package for Matlab50. The data were transformed to an approximately Gaussian noise space by weighting them prior to the decomposition51, and centered about the mean observation. The dataset used to produce the maps shown in Figure 3b was generated by truncating the spectral mode of the MLSVD to the nine components of highest variance. The two spatial modes were left uncompressed. The reconstructed model was subtracted from the raw data to ensure that the signal subspace was adequately approximated by the data compression, and inspection of the scree plot and loading curves was used to estimate the rank of the reconstruction. The EELS maps in Figure 3b were generated using the EELS Analysis package in Digital Micrograph. Hartree-Slater crosssections were fitted to the reconstructed core-loss data, and the low-loss spectrum image was used to correct for the effects of plural scattering52. This yields quantitative inelastic scattering



cross sections for thicknesses up to approximately 1.4 mean free paths (corresponding to a total thickness of approximately 150 nm). It should be noted that weak evidence for the presence of P, Ni, and Co was observed in the EELS maps, and that these elements were localized to the shell region (Figure S1). However, we were unable to find compelling evidence for the presence of these edges in the raw data. While it is possible that the maps reveal a slight change in the pre-edge background slope caused by the ionization edges under investigation, and that this change was detected by MLSVD, this interpretation is somewhat ambiguous and more work would be needed to draw a conclusion. The absolute thickness in Figure 3d is a calculated value derived from the ratio of elastic to inelastic scattering, which was captured in the low-loss EELS acquisition. This yields thickness in units of the mean free path for inelastic scattering of electrons with a kinetic energy of 300 keV. As the mean free path is a material dependent property, the average atomic mass at each pixel position extracted from the EELS maps was used to convert this to absolute thickness values. PEC measurements. PEC experiments were performed in a three electrode PEC cell (WAT Venture) with an SP-150 (BioLogic) potentiostat coupled with the EC-lab electrochemical software. The as-prepared film on Ta substrate, a 1 M Ag/AgCl electrode, and a platinum wire were used as the working, reference, and counter electrode, respectively. 0.1 M NaOH aqueous solution was used as electrolyte for the PEC measurements (pH 13). The measured potentials vs. 1 M Ag/AgCl have been converted to the RHE scale according to the Nernst equation (ERHE = EθAg/AgCl + 0.059 pH + EAg/AgCl). Electrochemical data were recorded with a scan rate of 10 mV/s toward the positive direction from 0.70 to 1.44 V versus RHE. The simulated solar illumination was obtained from an ozone-free 450 W xenon short-arc lamp (class AAA 94023A, Newport) equipped with an AM 1.5G filter and convex lens. RESULTS AND DISCUSSION Structural Characterization. Nitridation of metal oxide precursors at elevated temperatures can yield (oxy)nitrides, e.g. SrTaO2N derived from Sr2Ta2O7.53 Figure 1 shows the powder XRD patterns of the nanowires grown on the tantalum substrate. In order to avoid reflection peaks from binary tantalum nitride, the nanowires were mechanically removed from the substrate. The diffractogram reveals that the deposit contains SrTaO2N (space group I4/mcm). The morphology of the SrTaO2N nanowires after modification with three overlayers in the form of TiOx, Ni(OH)x and CoPi was analyzed by SEM (Figure 2). Although three serial coatings steps have been deposited, the one-dimensional structure is maintained. The nanowires are partially sintered as a consequence of the treatment at high nitridation temperature of 1273 K. The nanowire thin film displays a thickness of several µm although the nanowires are not perpendicular orientated (Figure S2). The determined band gap of 2.25 eV for the SrTaO2N/TiOx/Ni(OH)x/CoPi the eV is consistent with previous reports on SrTaO2N (Figure S3). Since the resolution of the SEM images was insufficient to detect the overlayers, we carried out a complementary TEM analysis on the nanowires. Figure 3a shows a High Angle

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Annular Dark Field (HAADF) STEM micrograph of the STON nanowire. This image can be interpreted in terms of mass-thickness contrast where regions of higher average atomic number or larger thickness appear brighter. From this exact region, both EELS and EDX datasets were simultaneously acquired. The EELS data were used to produce the elemental abundance maps in Figure 3b following the procedure outlined in the methods section. These reveal the relative concentrations of C, N, O, Ti, Sr, and Ta within the region defined in Figure 3a. Of particular interest is the Ti map. The Ti signal is only observed in the outer edges of the nanowire in this 2D projection. This can be explained by a higher concentration of Ti atoms along the edges due to the experimental geometry of the 2D projection, which would be consistent with a core-shell structure. The lack of a Ti signal in the core region is due to the low relative concentration of Ti, causing the Ti-L2,3 edges used for the mapping to fall below the experimental noise level due to plural scattering. The compositional line profile from the magenta lines in figures 3a and 3b is presented in Figure 3c. The relative concentrations of each of the six presented elements is plotted along with the absolute thickness (see Figure 3d). The enrichment in both the Ti and O signals in the shell region is evident, as is their correlation. The O present in the underlying C film leads to an offset of the percent composition that increases the relative amount in the nanowire region. By subtracting this offset, we can estimate the stoichiometry of the nanowire to be close to the expected ratio of Sr:Ta:O:N 1:1:2:1. The nanowire morphology and thickness is nicely visualized in the form of a relief map, presented in Figure 3d. In combination with the HAADF micrograph, we observe that the nanowire appears to consist of multiple smaller particles or grains that agglomerate within the wire. Inspection of the EELS maps reveals that the O concentration appears to follow this particle morphology, as does the surface morphology. While this may indicate that the TiOx shell encompasses all of these individual particles, future work would be needed to draw this conclusion. We note that this visualization is not a true 3D representation of the wire, but is merely intended to emphasize the morphology. To prove that the Ti visible at the edges of this 2D projection actually constitute a shell structure encapsulating the nanowire particles, we use the simultaneously-acquired EDX data presented in Figure 3e. Here, we use the Ti map from the EELS analysis to create two spatial masks (inset) which are used to average out the EDX spectra in the shell and core regions, respectively. The resulting averaged spectra are presented in the graph. It is clear to see that the Ti signal is present in both regions. The Ti:N ratio in the shell region is approximately 0.20, while it falls to 0.02 in the core region. Taken together with the EELS maps, we conclude that the shell of the nanowire must be Ti-rich, and that this encapsulates the entire wire. We interpret this as strong evidence in favor of the presence of the hypothesized coreshell structure. We furthermore use these data to estimate that the thickness of the TiOx encapsulating layer must be between 1 – 3 nm. The presence of Co and Ni is below the detection limit for spatial mapping in EELS, but is visible in the corresponding EDX analysis (Figure 3e).


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Figure 3. (a) HAADF micrograph of a SrTaO2N/TiOx/Ni(OH)x/CoPi nanowire. (b) EELS maps of C, N, O, Ti, Sr, and Ta from the same region as (a). (c) Line profiles for each of the elements as well as the absolute thickness extracted from the magenta lines in (a) and (b). (d) Thickness of this region after conversion to units of nanometers, visualized as a relief map. (e) Mean EDX spectra from two different spatial regions of the nanowire. The light gray spectra comes from the shell while the dark red comes from the core. The exact regions used to extract these spectra are denoted in the EDX signal mask (inset). The gray labels correspond to impurities in the TEM that are unrelated to the nanowire. It should be noted that the presence of Co and Ni is below the detection limit for spatial mapping in EELS, but is visible in the corresponding EDX spectra. The surface composition of the SrTaO2N nanowire photoanode was investigated by XPS. Figure 4 presents the corresponding high-resolution spectra for the selected regions. Ta 4p and N 1s core levels were detected in the range of binding energies (Eb) between 394 and 412eV (Figure 4a); such low Eb of N 1s is characteristic for metal (oxy)nitrides and is in agreement with previous reports.21 The Ta 4p1/2 peak was observed at Eb of 465.3 eV and overlaps with Ti 2p1/2 (Figure 4b). As such, the peak is a superposition of Ta 4p1/2 and Ti 2p1/2 photoemission. The Ti 2p3/2 peak at lower Eb of 459.3 eV is characteristic for TiO2 although being shifted

toward higher binding energy.54-55 The O 1s spectrum splits into two peaks at 530.7 eV and 532.4 eV (Figure 4c). The main peak at 530.7 eV can be attributed to lattice O2−, whereas the minor peak at 532.4 eV corresponds to hydroxyl56 and oxynitride57 species. However, nickel is practically not visible, i.e. indicating a small surface concentration. The peak at 781.3 eV can be assigned to Co 2p3/2 and its high Eb value indicates Co-P instead Co-O bonding (Figure 4d).



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Figure 5. Projected COHP curves of SrTaO2N vs. BaTaO2N. Bonding interactions to the right, antibonding ones to the left, and non-bonding ones around zero. The antibonding Ta-N states in SrTaO2N (missing in BaTaO2N) are marked by an arrow. Ta–N bonds shown in green, Ta–O bonds in blue (overlap dark green).

We calculated the projected COHP for SrTaO2N and the structurally related BaTaO2N in order to analyze the bonding situation in quaternary oxynitrides (Figure 5).58 The structure of the two compounds is only to some extent different: Ta5+ cations form TaO4N6 octahedra, but in the case of Sr, these octahedra are slightly tilted against each other. For the larger barium cation, this twisting is not observed; a behavior easily reproduced within DFT. This difference allows for stronger bonding in SrTaO2N, as studied with a COHP analysis. Both the Ta–O and Ta–N bonds are shorter, and the integrated “bond-weighted” density-of-states, the ICOHP value, indicates stronger bonding. Looking at the COHP curve, a different feature can be noticed; only for SrTaO2N, one finds occupied antibonding levels for the Ta-N bonds. Although both compounds show antibonding interactions in the Ta-O bonds (a typical phenomenon caused by the more covalently bonding N atoms), they are not as close to the Fermi level as the new levels for the Ta-N bonds in SrTaO2N. Again, the Ta–N bond is still stronger than in BaTaO2N since the integrated value over all levels up to εF is more favorable (–4.4 eV vs. –4.2 eV, thus more bonding energy). Nonetheless, the Ta‒N interactions in SrTaO2N are not optimized; the bonding would be stronger if there were fewer electrons to occupy these states.

Figure 4. XPS Ta 4p, N 1s (a), Ti 2p (b), O 1s (c), and Co 2p (d) spectra of SrTaO2N/TiOx/Ni(OH)x/CoPi nanowire photoanode. The inset shows the crystal structure of SrTaO2N. The TaO4N2 octahedra are drawn in green. Strontium and oxygen/nitrogen atoms are drawn in blue and red, respectively.

PEC Water Oxidation. We chose amorphous TiOx as the initial overlayer for the protection of the oxynitride from photocorrosion. Similar to a ferrihydrite coating on SrTaO2N, depositing TiOx is known to stabilize photocurrents of Ta3N5 electrodes. We measured a Mott–Schottky (MS) plot of a bare FTO/TiOx electrode in 0.1 M NaOH aqueous solution (pH 13) at an AC amplitude of 10 mV and frequency of 1000 Hz (Figure S4). The non-linear behavior indicates several electronic states in the band gap, in accordance with the XPS results, and is characteristic for a blocking layer.32


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Linear square voltammetry (LSV) experiments were carried out to evaluate the effect of particular coatings on the PEC performance. Depositing the layer TiOx on the nanowires did not alter the curvature of the LSV under chopped illumination noticeably (Figure S5). Upon turning on the light, the strong increase of photocurrent is followed by an exponential decrease until the current stabilizes at a certain value. The dark-current is quite common in case of oxynitride photoanodes prepared on tantalum substrate and is caused by processes at the semiconductor surface.21,59 Electrochemical deposition of Ni(OH)x on the surface of Ta3N5 is known to store photogenerated holes and prevent photocorrosion by forming passivation layers. The comparison of the LSV curves for the bare and Ni(OH)x-coated SrTaO2N electrode shows dark-current decline being close to zero after anchoring Ni(OH)x on the surface (Figure S6). It should be noted that an instantaneous spike in the signals for each cycle, caused by the light switching on and off, were observed for SrTaO2N and SrTaO2N/Ni(OH)x photoelectrodes. This originates from the recombination of photogenerated holes at the photoelectrode surface with conduction band electrons.60 Since the TiOx or Ni(OH)x did not improve the photocurrent during PEC water oxidation, we chose CoPi as the electrocatalyst. The latter can be photoelectrodeposited on the active sites of the semiconductors. The CoPi-modified bare SrTaO2N nanowire photoanode was compared with the bare SrTaO2N (Figure S7). Although the dark-current contribution deteriorated, its value did not reach zero which indicates photocorrosion. Considering the aforementioned features in a rational design, we integrated the blocking layer TiOx, the passivation layer Ni(OH)x and the co-catalyst layer CoPi step by step to modify the SrTaO2N nanowire to construct core-shell nanostructured photoanodes.

Figure 6. LSV of SrTaO2N nanowire photoanodes with TiOx, Ni(OH)x and CoPi overlayers. Measurements were performed in 0.1 M NaOH electrolyte (pH 13) at a scan rate of 10 mV s−1 under AM 1.5G illumination (100 mW cm−2). The measurements were performed on the same electrode.

Figure 7. Chronoamperometry of SrTaO2N nanowire photoanode modified with TiOx, Ni(OH)x and CoPi overlayers. Measurements were performed in 0.1 M NaOH electrolyte (pH 13) at 1.23 V vs. RHE under AM 1.5G illumination (100 mW cm−2). After the integration of the catalytic CoPi layer onto the SrTaO2N/TiOx/Ni(OH)x photoanode, the photocurrent during LSV significantly increased to 0.27 mA cm−2 at 1.23 V vs. RHE (Figure 6). Concurrently, the dark-current decreases to the same level as for the SrTaO2N/Ni(OH)x photoanode. In other words, water oxidation kinetics could be augmented while maintaining the surface passivation. An interesting feature between the core‒shell photoanodes SrTaO2N/TiOx/Ni(OH)x/CoPi and SrTaO2N/Ni(OH)x/CoPi could be observed in their LSV curves (Figure S8). The photocurrent of the SrTaO2N/Ni(OH)x/CoPi photoanode, without the TiOx blocking layer, was only around 0.1 mA cm−2 at 1.23 V vs. RHE. This demonstrates the necessity of the intermediate TiOx layer, between the oxynitride core and the consecutive shells, in order to achieve a synergetic effect for amended charge separation. The TiOx layer suppresses the recombination of photogenerated electron-hole pairs, resulting in a higher concentration of holes available for oxidative processes.32 The influence of the TiOx thickness on the photocurrent of the nanowires was determined by comparing the impregnation method with depositing 4 and 10 nm by means of ALD (Figures S9-S10). Increasing the TiOx thickness decreases both the dark current and photocurrent contribution. Subsequently, we used photoanodes containing TiOx layers (4 and 10 nm) prepared by ALD for investigating the effect of the amount of Ni(OH)x deposition on the photocurrent (Figure S11). Extending the deposition time over 2 min yields only a minor further decrease in the dark current contribution. Chronoamperometry (CA) for the SrTaO2N nanowire photoanode with and without the overlayers were measured at 1.23 V vs. RHE (Figure 7). Coating the nanowires only with amorphous TiOx leads to a dark-current contribution of almost 50 % to the total current of 0.075 mA cm−2. The dark-current can be completely suppressed if applying the Ni(OH)x coating at the cost of a current drop below 0.015 mA cm−2. Coating with the electrocatalytic layer in the form of CoPi gives a photocurrent of 0.25 mA cm−2 while keeping the dark-current contribution close to zero.



Figure 8. Stability test of SrTaO2N/TiOx/Ni(OH)x/CoPi nanowire photoanode. Measurements were performed in 0.1 M NaOH electrolyte (pH 13) at 1.23 V vs. RHE under AM 1.5G illumination (100mW cm−2). The photocurrent was interrupted four times. Prolonged water oxidation at 1.23 V vs. RHE showed the stability of the photocurrent and negligible dark current contribution of the oxynitride core-shell photoanode (Figure 8). Table 1 compares the performance of the nanowire photoanode with previously published quaternary oxynitride photoanodes synthesized hydrothermally on metallic substrates. The list contains additionally a report of a SrTaO2N photoanode on FTO substrate. We tested whether the developed sequence is a general methodology to enhance photocurrent of oxynitride electrodes, too. We produced two photoanodes based on powder samples of LaNbON2 and SrNbO2N by means of electrophoretic deposition. During CA at 1.23 V vs. RHE for these electrodes under identical conditions, the same sequence could be applied for LaNbON2 but not for SrNbO2N (Figures S12-S13). This suggests the overlayer sequence to be dependent on material and its morphology.

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CONCLUSION We have demonstrated the first quaternary oxynitride coreshell nanowire SrTaO2N/TiOx/Ni(OH)x/CoPi photoanode. The rational design contains three functional overlayers: TiOx for reducing surface charge-carrier recombination and protection from photocorrosion; Ni(OH)x for hole storage; and a CoPi electrocatalyst for water oxidation. The coating sequence shows a synergetic hole extraction from the oxynitride core to the catalyst surface. The nanowire photoanode develops a photocurrent of 0.27 mA cm−2 at 1.23 V vs. RHE in the PEC water oxidation and exhibits a dark-current contribution close to zero. Our results suggest that there is no general coating sequence to be applied to oxynitride photoanodes. Bonding analysis at the DFT level reveals the Ta‒N and Ta‒O bonding to be stronger for SrTaO2N than for BaTaO2N but the Ta‒N bonding is not optimized and would be even stronger if there were fewer electrons to occupy these levels. Our work highlights the core-shell concept for the development of oxynitride nanowire photoanodes, i.e. to extract holes to the surface while simultaneously enabling efficient electric transport along the nanowire. ASSOCIATED CONTENT Supporting Information. EDS maps from the region investigated in Figure 3a (Figure S1). SEM image of the crosssection (Figure S2) of the nanowire photoanode. Tauc plot for SrTaO2N/TiOx/Ni(OH)x/CoPi (Figure S3). Mott–Schottky (MS) plot of the TiOx/FTO electrode (Figure S4). LSV of SrTaO2N/TiOx (Figure S5). LSV of SrTaO2N/Ni(OH)x (Figure S6). LSV of SrTaO2N/CoPi (Figure S7). LSV of SrTaO2N/Ni(OH)x/CoPi (Figure S8). SEM image SrTaO2N coated with 10 nm of TiOx by ALD (Figure S9). LSVs of SrTaO2N coated with TiOx by ALD and impregnation (Figure S10). LSV of SrTaO2N, modified with TiOx overlayers by ALD and different amounts of Ni(OH)x (Figure S11). CA of FTO/LaNbON2/TiOx/Ni(OH)x/CoPi (Figure S12). CA of FTO/SrNbO2N/TiOx/Ni(OH)x/CoPi (Figure S13). AUTHOR INFORMATION

Table 1. Comparison to previously published quaternary oxynitride photoanodes grown on metallic substrates. Current densities relate to values obtained during CA. Reference

Compound

Substrate

Current density at 1.23 V vs. RHE

This work

SrTaO2N

Ta

0.25

CoPi

21

SrTaO2N

Ta

0.03

None

59

BaTaO2N

Ta

0.75

CoPi

61

SrTaO2N

FTO-glass

0.3

CoPi

62

LaTiO2N

Ti

*

IrO2

Catalyst

*Comparison not possible due to different illumination parameters.

Corresponding Author *E-mail: [email protected] ORCID Zili Ma: 0000-0001-7975-9201 Thomas Thersleff: 0000-0002-0999-3569 Simon Jakobi: 0000-0002-9166-2231 Robert H. Coridan: 0000-0003-1916-4446 Wolfgang Schnick: 0000-0003-4571-8035 Richard Dronskowski: 0000-0002-1925-9624 Adam Slabon: 0000-0002-4452-1831 Author Contributions Z.M. and A.S. designed the experiments. Z.M. performed most synthetic and all electrochemical experiments on SrTaO2N nanowires. T.T. carried out the HRTEM EELS and EDX experiments. A.L.G. and R.D. calculated and analyzed


Page 9 of 11 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


the electronic band structures. N.C. and Y.B.L. produced ANbO2N photoanodes and S.J. performed TEM ED analysis. A.R. and P.K. performed XPS experiments. Z.B.S. and R.H.C. carried out the ALD experiments. P.K., W.S., R.D. and A.S. started the research. A.S. guided the work and wrote with Z.M. and T.T. the manuscript. All authors discussed the results and commented on the manuscript. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Z. M. would like to thank the China Scholarship Council for a Ph.D. scholarship. We thank Professor Ulrich Simon for access to electron microscopy facilities. A.S. and W.S. would like to thank the Fonds der Chemischen Industrie (FCI) for financial support. The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) within the research group "Chemistry and Technology of the Ammonothermal Synthesis of Nitrides" (FOR 1600), project SCHN377/16-2. S.J. acknowledge financial support from the research training group “mobilEM”, funded by the German Research Foundation. The XPS measurements were carried out with the equipment purchased with the 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). A part of STEM measurements was performed at the Gemeinschaftslabor für Elektronenmikroskopie (GFE) of the RWTH Aachen University, Ahornstraße 55, 52074 Aachen. T. T. acknowledges support from the Electron Microscopy Center (EMC) at Stockholm University as well as funding from the Swedish Research Council (Project No. 2016-05113). This work was financially supported by Stockholm University with a Start Up Grant for A.S. We thank Birgit Hahn for acquiring SEM images. REFERENCES (1) Bae, D.; Seger, B.; Vesborg, P. C. K.; Hansen, O.; Chorkendorff, I. Strategies for Stable Water Splitting: via Protected Photoelectrodes. Chem. Soc. Rev. 2017, 46, 1933–1954. (2) Nocera, D. G. Solar Fuels and Solar Chemicals Industry. Acc. Chem. Res. 2017, 50, 616–619. (3) Nielander, A. C.; Shaner, M. R.; Papadantonakis, K. M.; Francis, S. A.; Lewis, N. S. A Taxonomy for Solar Fuels Generators. Energy Environ. Sci. 2015, 8, 16–25. (4) Liu, C.; Gallagher, J. J.; Sakimoto, K. K.; Nichols, E. M.; Chang, C. J.; Chang, M. C. Y.; Yang, P. Nanowire–Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals. Nano Lett. 2015, 15, 3634–3639. (5) Davi, M.; Drichel, A.; Mann, M.; Scholz, T.; Schrader, F.; Rokicinska, A.; Kustrowski, P.; Dronskowski, R.; Slabon, A. Enhanced Photoelectrochemical Water Oxidation Efficiency of CuWO4 Photoanodes by Surface Modification with Ag2NCN. J. Phys. Chem. C 2017, 121, 26265–26274. (6) Ran, J.; Zhang, J.; Yu, J.; Jaroniec, M.; Qiao, S. Z. Earth–Abundant Cocatalysts for Semiconductor-Based Photocatalytic Water Splitting. Chem. Soc. Rev. 2014, 43, 7787–7812. (7) Cao, T.; Li, Y.; Wang, C.; Shao, C.; Liu, Y. A Facile in Situ Hydrothermal Method to SrTiO3/TiO2 Nanofiber Heterostructures with High Photocatalytic Activity. Langmuir 2011, 27, 2946–2952. (8) Ma, Z.; Linnenberg, O.; Rokicinska, A.; Kustrowski, P.; Slabon, A. Augmenting the Photocurrent of CuWO4 Photoanodes by Heat Treatment in the Nitrogen Atmosphere. J. Phys. Chem. C 2018, 122, 19281–19288. (9) Ma, M.; Zhang, K.; Li, P.; Jung, M. S.; Jeong, M. J.; Park, J. H. Dual Oxygen and Tungsten Vacancies on a WO3 Photoanode for Enhanced Water Oxidation. Angew. Chemie Int. Ed. 2016, 55, 11819–11823. (10) Han, H. S.; Shin, S.; Kim, D. H.; Park, I. J.; Kim, J. S.; Huang, P. S.; Lee, J. K.; Cho, I. S.; Zheng, X. Boosting the Solar Water Oxidation

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A Quaternary Core-Shell Oxynitride Nanowire Photoanode Containing

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