An MnNCN-Derived Electrocatalyst for CuWO4 Photoanodes

Mar 19, 2018 - Surface functionalization of CuWO4 photoanodes with MnNCN increased the photocurrent from 22 to 30 μA cm–2 at 1.23 V vs RHE...
0 downloads 0 Views 15MB Size
Download PDF
Subscriber access provided by UNIV OF SCIENCES PHILADELPHIA

Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage 4

An MnNCN-Derived Electrocatalyst for CuWO Photoanodes Martin Davi, Markus Mann, Zili Ma, Felix Schrader, Andreas Drichel, Serhiy Budnyk, Anna Rokicinska, Piotr Kustrowski, Richard Dronskowski, and Adam Slabon Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00149 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9 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

Langmuir

An MnNCN-Derived Electrocatalyst for CuWO4 Photoanodes Martin Davi†, Markus Mann†, Zili Ma†, Felix Schrader†, Andreas Drichel†, Serhiy Budnyk‡, Anna Rokicinska§, Piotr Kustrowski§, Richard Dronskowski† and Adam Slabon†,* † ‡ §

Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany AC2T Research GmbH, Viktor-Kaplan-Straße 2 C, AT-2700 Wiener Neustadt, Austria Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland

ABSTRACT: CuWO4 is a photoanode candidate in neutral pH, and manganese-based oxygen evolution reaction electrocatalysts are of high interest due to their low price and low toxicity. Considering the unexplored chemistry of transition-metal carbodiimides/cyanamides for the PEC water oxidation, we investigated MnNCN as an electrocatalyst for CuWO4 under AM 1.5G illumination in potassium phosphate electrolyte (pH 7). Surface functionalization of CuWO4 photoanodes with MnNCN increased the photocurrent from 22 to 30 µA cm−2 at 1.23 V vs. RHE. Complementary structural analysis by means of XRD and XPS revealed that MnNCN forms a core-shell structure MnNCN@MnPOx in phosphate electrolyte and mimics a manganese phosphate electrocatalyst. As such, the surface chemistry of MnNCN significantly differs from previous studies on the cobalt analogue (CoNCN). A separately prepared MnNCN electrode developed a small but detectable photocurrent due to photogenerated holes inside the semiconducting carbodiimide core of the MnNCN@MnPOx structure.

INTRODUCTION Solar energy conversion based on photoelectrochemical (PEC) cell technology represents a promising alternative to energy generation based on fossil fuels.1 A PEC cell produces either solely electricity or chemical fuel, such as hydrogen by splitting water into elements.2 Since the first experimental realization of the PEC water-splitting with a photoanode-driven PEC device by Honda et al.3, intense research has been focused to semiconductor photoelectrodes. Water-splitting involves the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with the latter being more demanding, because of the necessity to transfer 4 electrons per one water molecule.4 There are several critical requirements for a semiconductor material in terms of realistic application as a photoanode: i) a suitable electronic band gap for efficient solar energy harvesting; ii) a valence band edge position more positive than 1.23 V vs reversible hydrogen electrode (RHE); iii) stability in the electrolyte at given pH; iv) comprising earth-abundant elements.5 The photocurrent of the photoanode is related to the amount of photogenerated holes reaching the semiconductorelectrolyte surface. Nanostructuring6 and fabrication of heterojunctions7 may improve charge-carrier separation, i.e., ameliorate the PEC OER efficiency. Moreover, the slow kinetics of water oxidation requires surface modification of the semiconductor with an OER catalyst.8 Such functional coatings can be made by sputtering9, electrodeposition10, drop-casting11, impregnation12 or atomic-layer deposition (ALD)13. CuWO4 is a promising photoanode component due to its favorable band gap of 2.3 eV, stability in neutral pH and a valence band edge position appropriate for water oxidation.14 This compound crystallizes with a wolframite-type structure and was synthesized for the first time in the 1960s.15 CuWO4

can be manufactured on transparent electrically conductive substrates by spin casting16, spray pyrolysis17, ALD18 and electrochemical deposition19. Although the theoretical photocurrent density of CuWO4 is approximately 10.7 mA cm−2, the highest achieved values reach merely around 0.3 mA cm−2 at 1.23 V vs RHE.20 The low electrical conductivity and light absorption of CuWO4 electrodes can be augmented by silver nanowires21 and gold nanoparticles22. The OER on CuWO4 photoanodes has been proposed to be determined by an intermediate electronic surface state.23 The reduction of the resulting surface electron concentration requires using larger potentials during the PEC water oxidation.24 This hindrance may be overcome by modifying the surface with secondary phase catalysts. Furthermore, only a manganese phosphate (MnPOx) electrocatalyst is known to increase the photocurrent of CuWO4 photoanodes.25 Manganese-based OER electrocatalysts are of high interest due to their low price and low toxicity.26 Electrodeposited thin films of manganese oxides (MnOx) containing trivalent manganese cations can reach high OER activities.27 On the contrary, thin films of an ordered MnIVO2 phase are catalytically inactive.28 During OER, MnOx thin films can undergo a plethora of structural changes depending on the applied potential.29 The electrochemical behavior of the manganese catalyst depends on its initial phase and on the support material.30 MnOx catalysts can be therefore electrochemically activated by electrochemical cycling, leading to the formation of Mn3+ cations.31 The origin of the activation has been found to rely on the formation of a catalytically active disordered MnO2 phase during anodic cycling.32 Metal carbodiimides MNCN (with M= Cu, Zn, Fe, Co, Ni and Mn, for example) consist of a transition metal and an NCN2− anion which can be present in the form of a carbodiimide or cyanamide. Carbodiimides are closely related to oxides but are

ACS Paragon Plus Environment

Langmuir 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

characterized by a higher degree of covalency.33 Only recently, this class of materials has been discovered as promising anode materials for Li-ion batteries.34 Besides materials for energy storage, MNCN-containing systems have gained attention as prospective materials for energy conversion.35,36 For instance, we demonstrated a threefold rise in photocurrent density of CuWO4 photoanodes after integration with the semiconducting cyanamide Ag2NCN.37 Another example is the OER electrocatalyst CoNCN which is structurally stable in the bulk and on the surface during water oxidation.38 Considering the OER activity of manganese-based electrocatalysts and the unexplored chemistry of carbodiimides for the PEC water oxidation, our attention has been drawn toward manganese carbodiimide.39 MnNCN has an experimental band gap of 3.4 eV, about 0.4 eV smaller than for the isoelectronic MnO.40 We were interested to see whether MnNCN may yield a synergetic effect with CuWO4 photoanodes during PEC water oxidation in neutral pH. Herein, we report for the first time on heightened photocurrents of CuWO4 thin film photoanodes after modification of their surface with MnNCN. These results emphasize the importance of considering surface segregation effects for MNCN catalysts with respect to the electrochemically active form. EXPERIMENTAL SECTION Synthesis of MnNCN. A manganese carbodiimide powder was synthesized by a solid state reaction of ZnNCN and MnCl2 (anhydrous, 99.999 wt % Sigma Aldrich).40 ZnNCN was obtained by dissolving ZnCl2 (anhydrous, 99.999 wt % Sigma Aldrich) in deionized water and adding a stoichiometric amount of H2NCN (99 wt % Sigma Aldrich). ZnNCN was precipitated as a white powder from the reaction mixture by adding 10 ml of 25 % ammonia solution. The product was filtered, washed with ethanol and dried under vacuum at 383 K. Amounts of 227.9 mg (2.162 mmol) of ZnNCN, and 326.5 mg (2.595 mmol) of MnCl2 were mixed under inert gas atmosphere inside an argon-filled glove box (H2O < 1 ppm; O2 < 1 ppm), sealed in an evacuated quartz tube and heated at 873 K for 72 h. The dark green powder was washed with water to remove MnCl2 excess and dried under vacuum. Preparation of CuWO4/MnNCN photoanodes. Thin films of CuWO4 were synthesized on fluorine-doped tin oxide (FTO) glass slides (2.2 mm thick, ~ 7 Ω/sq, Sigma Aldrich) by electrochemical deposition. Before electrodeposition, the FTO glass slides were cleaned in diluted nitric acid and sonicated in ethanol for 20 min.14 A peroxytungstate precursor was synthesized by dissolving 1.237 g (3.8 mmol) of sodium tungstate dihydrate (Na2WO4·2H2O, 99.9%, Acros Organics) in 15 mL deionized water. 2.5 ml of hydrogen peroxide (30%, Geyer Chemsolute) was added to the yellow solution and stirred for 20 min. A platinum wire was inserted into the solution followed by an addition of 25 ml deionized water and 22.5 ml isopropanol (>99.7%, Fisher Scientific). 0.583 g (2.4 mmol) of copper(II) nitrate trihydrate (Cu(NO3)2·3H2O, >99%, Sigma) was dissolved in 10 ml of water and added to the solution. The pH value of the precursor solution was changed to 1.1 by adding 5% nitric acid solution (Fluka). A FTO glass was electrically wired with a counter electrode (Pt wire) and reference electrode (1M Ag/AgCl) and submerged in the precursor solution. Electrochemical deposition was conducted with a SP-50 potentiostat (BioLogic). The thin film was formed by sweeping the potential from –0.5 V to 0.4 V vs. 1 M Ag/AgCl refer-

ence electrode for 12 cycles. The FTO glass was washed carefully with deionized water, dried in ambient atmosphere and heated at 823 K for 2 h under ambient atmosphere. The photoanode was immersed in 0.5 M HCl solution for 30 min to remove residual copper oxide and washed with water again. The product was a yellow thin film. Composite CuWO4/MnNCN electrodes were prepared by drop-casting. A CuWO4 thin-film photoelectrode was placed on a heating plate at 323 K. Simultaneously, MnNCN powder was dispersed by ultrasounds in ethanol (20 µg ml‒1) and quickly transferred on the CuWO4 surface with a micropipette. The electrode was dried under vacuum overnight. Powder X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). A STOE STADI-P diffractometer (Cu Kα1 radiation) equipped with a DECTRIS Mythen 1K detector in transmission mode was used to record powder XRD patterns. Before the measurements, thin films were mechanically removed from the substrate. The surface composition 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 aluminium source Al Kα (E = 1486.6 eV). Electron Microscopy and UV-Vis Spectroscopy. SEM and TEM images were obtained by means of a Leo Supra 35VP (Zeiss) and a Libra 200 FE TEM (Zeiss), respectively. Crystal planes of MnNCN were determined by computing the fast Fourier transform (FFT) of the lattice fringes, and the inverse FFT was taken to find an interplanar spacing. UV-vis spectra were collected using a Shimadzu UV-2600 spectrophotometer. Band gaps were determined from Tauc plots by calculation with the Kubelka-Munk function F(R) = (1 – R)2 / 2R. Photoelectrochemistry. Experiments were carried out in a PEC cell operating in a three-electrode setup. A platinum wire and a 1 M Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. All current values were recorded vs. 1 M Ag/AgCl and converted vs. RHE according to ERHE (V) = E1M Ag/AgCl + (0.059 × pH). A solar light simulator (class-AAA 94023A, Newport) with an ozone-free 450 W xenon short-arc lamp was used to illuminate the photoelectrodes with 100 mW cm−2 (AM 1.5 G) simulated visible light. 0.1 M potassium phosphate buffer (KPi) was used as electrolyte for the PEC experiments and prepared with Milli-Q water (18.3 Ω cm). Thin-film electrodes were illuminated from the back side. Potentials were swept with a scan rate of 10 mV s−1 from 0.70 V to 1.44 V vs RHE. 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 chosen wavelength was determined with a UV enhanced silicon photodetector (818-UV, Newport) from 380 nm to 650 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 is the measured wavelength (nm) and P is the intensity of monochromatic light (mW cm−2). The absorbed photon-to-current conversion efficiency (APCE) was calculated by dividing the IPCE by the light harvesting efficiency (LHE). The LHE was calculated according to LHE = 1 – 10 −A where A is the absorbance at a given wavelength.

ACS Paragon Plus Environment

Page 2 of 9

Page 3 of 9 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

Langmuir

Figure 1. Experimental (top) and simulated (bottom) powder XRD reflection patterns of CuWO4 (ICSD 16009) thin film on FTO. The thin film was mechanically removed from the FTO substrate.

and has a smaller electronic band gap than the oxide MnO.42 We measured also powder XRD patterns of an MnNCN electrode after PEC water oxidation at 1.23 V for 60 min to investigate its structural stability (Figure 2 top) (vide infra). For the measurement, an anode was prepared by drop-casting MnNCN on FTO glass. After the photoreaction, the MnNCN layer was mechanically removed and analyzed by means of XRD. The sample exhibits sharp reflection peaks, which can be indexed to MnNCN. The background signal is due to the low amount of the material and the grease which was used to prepare the powder for the measurement in transmission mode (Figure S2). The powder XRD results indicate that the MnNCN kept its structure during PEC water oxidation at 1.23 V vs RHE in phosphate electrolyte at pH 7. The composite photoanode CuWO4/MnNCN was produced by drop-casting 1.2 µg of the carbodiimide on the oxide surface. This amount of MnNCN gave the highest photocurrent in the PEC water oxidation and did not alter the surface morphology noticeably (Figure 3). The thickness of the CuWO4 thin-film was approximately (2.4 ± 0.3) µm (Figure S1 inset) while the MnNCN crystallites are in the micrometer range (Figure 3 inset).

Figure 2. Experimental (before and after PEC water oxidation at 1.23 V vs. RHE in 0.1 M KPi electrolyte at pH 7 under AM 1.5G illumination) and simulated powder XRD reflection pattern of MnNCN (ICSD 170135) thin film on FTO. The thin film was mechanically removed from the FTO.

Figure 3. SEM micrograph of CuWO4/MnNCN thin film. The inset displays a MnNCN crystallite.

RESULTS AND DISCUSSION Structural characterization. The yellow thin films of CuWO4 do not show any impurity phase in the powder XRD patterns (Figure 1). Possible reflection peaks originating from tin oxide are also not present in the XRD patterns, because the thin film was mechanically removed from the substrate before the measurement. The electrodeposited CuWO4 was obtained as a dense thin film and did not resemble any nanostructured morphology (Figure S1). Similar to the CuWO4 thin film, no side phase was present in the powder XRD patterns of the synthesized MnNCN powder (Figure 2). MnNCN is isotypical to CaNCN and consists of octahedral MnN6 units. Each nitrogen atom is coordinated to a carbon atom and to three manganese atoms. The carbodiimide anion [N=C=N]2– is isoelectronic to the oxide anion and can be regarded as a nitridic pseudo-oxide.41 MnNCN is more covalent

We determined the band gap of the CuWO4 thin film by means of UV-Vis (Figure 4). The obtained value of 2.29 eV is close to the formerly reported values within the 2.2 to 2.4 eV range.20 We also determined the band gap for the thin film after modification with MnNCN. The calculated value of 2.33 eV is still very close to the value measured before the surface modification. Since the band gap of MnNCN is 1.1 eV larger than for CuWO4 and the amount of deposited MnNCN was very low, we can safely assume the deviation of 0.04 eV to be within the error margin of the measurement. Photoelectrochemistry. Figure 5 depicts the recorded LSV curves of the pristine CuWO4, MnNCN, and CuWO4/MnNCN photoanodes in KPi electrolyte (pH 7). Both the bare CuWO4 and MnNCN electrodes developed anodic currents when sweeping the potential to the positive direction under simulated solar illumination.

ACS Paragon Plus Environment

Langmuir 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

CuWO4 anodes is mainly hindered by the photo-induced creation of an efficiency-limiting intermediate surface species at the same potential.23 We prepared an additional MnNCN electrode on FTO substrate with the identical amount of material (1.2 µg). The photocurrent generated by the composite photoanode exceeded the sum of the bare CuWO4 and MnNCN electrodes. Deposition of MnNCN crystallites on the CuWO4 thin film shifted the potential, at which significant photocurrent was generated, about 100 mV into the cathodic direction. We also measured additional LSV curves of a bare MnNCN electrode on FTO with similar amount of material above 1.4 V vs RHE (Figure S5). The current drops slightly after exceeding 1.4 V before it increases again at around 1.6 V and reaches 16 µA cm−2 at 1.8 vs RHE. The synergetic effect between CuWO4 and MnNCN is also reflected during chronoamperometry (CA) at 1.23 V vs RHE (Figure 6).

Figure 4. Tauc plots for CuWO4 and CuWO4/MnNCN thin films. The inset shows the normalized absorbance.

Figure 5. LSV of photoanodes CuWO4, MnNCN and CuWO4/MnNCN at in 0.1 M KPi electrolyte (pH 7) at a scan rate of 10 mV s−1 under AM 1.5 G illumination (100 mW cm−2). The inset displays the relative photocurrent enhancement as a function of the applied potential.

The dashed line corresponds to the current of the CuWO4 photoanode recorded without illumination and is close to zero. Although the produced photocurrent of our basic CuWO4 photoanode was meaningfully lower than for electrodes in preceding works19, our thin-film electrode is appropriate to study the surface functionalization with electrocatalysts. We tested several composite electrodes with different amounts of added MnNCN (Figure S3). The highest photocurrent was obtained for 1.2 µg of the carbodiimide. The amount of MnNCN deposited on the CuWO4 electrode was determined by means of atomic absorption spectroscopy (AAS). We calculated the relative enhancement of the photocurrent at given potential (Figure 6 inset and Figure S4) according to [I(CuWO4/MnNCN) / I(CuWO4/MnNCN)] × 100%. The catalytic effect of MnNCN on the CuWO4 photoanode reaches the highest value of 72% at 0.96 V vs RHE. Interestingly, it has been suggested only recently that the PEC OER on bare

Figure 6. CA of photoanodes CuWO4, MnNCN and CuWO4/MnNCN at 1.23 V vs. RHE. Measurements were performed in 0.1 M KPi electrolyte (pH 7) under AM 1.5 G illumination (100 mW cm−2). The CuWO4 thin film exhibited a rise in photocurrent from 22 to 30 µA cm−2 at 1.23 V vs. RHE after modification with MnNCN. The photocurrent was also apparent after a longer llumination time of 60 min (Figure S6). In addition to the large crystallite sizes, the non-uniformity of distribution represents the major limitation of the herein described method for surface functionalization with MnNCN.43 The values of enhancement and potential shift of CuWO4 after functionalization with MnNCN were very close to the study by Bard et al.25 on manganese phosphate electrocatalysts for CuWO4 electrodes. The improved PEC activity after functionalization with MnNCN crystallites was also consistent with an increase of IPCE and APCE (Figure 7). Both IPCE and APCE demonstrated an upsurge above 520 nm and increased for shorter wavelengths. Since the photocurrent of the bare MnNCN electrode was small, we manufactured another electrode with a larger amount of material to see whether MnNCN would display a photocurrent and to investigate its structural stability. Figure 8 illustrates the CA at 1.23 vs RHE recorded for 60 min.

ACS Paragon Plus Environment

Page 4 of 9

Page 5 of 9 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

Langmuir

Figure 7. IPCE of photoanodes CuWO4 and CuWO4/MnNCN at 1.23 V vs. RHE. The inset contains the APCE.

being a large band gap semiconductor. Recorded XRD patterns of the electrode after CA indicated MnNCN to maintain its structural stability as carbodiimide (vide supra). In order to investigate better the PEC OER performance of the CuWO4 photoanodes after carbodiimide modification, we analyzed the surface composition of the bare MnNCN electrode before and after CA at 1.23 V vs RHE for 60 min. The XPS measurements confirm the presence of surface MnNCN phase in the fresh sample. In the XPS Mn 2p spectra (Figure 9), the peak at 640.6 eV, attributed to photoemission from the Mn 2p3/2 level in Mn2+ species44, is observed. The chemical state of manganese changed due to partial oxidation during the PEC water oxidation that is manifested by a clear shift of the Mn 2p3/2 peak to higher binding energy (Eb = 641.2 eV). Moreover, phosphorus appears on the surface of the electrode. The P 2p3/2 peak is found at a binding energy of 133.1 eV, typical of P in phosphates (Figure S7).45,46A decrease in the N/Mn atomic ratio from 3.0 to 1.6 during the PEC water oxidation together with the final P/Mn atomic ratio of 1.3 suggest that manganese phosphate formed on the surface during the contact of the MnNCN phase with the electrolyte. The Mn 2p3/2 peak shift is identical with the one observed by Nam et al.47 for the MnPOx electrocatalyst and reveals that manganese carbodiimide formed in situ a phosphate shell during the PEC water oxidation.

Figure 8. CA of bare MnNCN (1 mg) at 1.23 V vs. RHE in 0.1 M KPi electrolyte (pH 7) under AM 1.5 G illumination (100 mW cm−2). The inset contains the CA of the FTO substrate under identical conditions. During the measurement, the illumination was partially interrupted to find if there is a dependency of the current on incident light. Upon interruption of illumination, the current decreased but could be restored to the previous level when the illumination was turned on again. Although the photocurrent was very low, its dependency on incident light would hint toward photogeneration of holes by MnNCN. We also recorded a CA of the FTO substrate for 15 min (Figure 8 inset) in order to determine its contribution of photocurrent, originating from the tin oxide layer, to the fabricated MnNCN electrode. The illumination was interrupted near the end of the measurement, and the difference between dark current and photocurrent was compared to the MnNCN electrode. The difference, i. e., photocurrent subtracted by the dark-current, was approximately 72 and 243 nA cm−2 for the FTO substrate and MnNCN electrode, respectively. The larger photocurrent of the latter should be therefore due to generation of electron-hole pairs. This is in agreement with MnNCN

Figure 9. XPS Mn 2p spectra of bare MnNCN before and after PEC water oxidation.

The catalytically active form of a MnPOx electrocatalyst is believed to be Mn3+ that is formed upon anodic cycling. The electrochemical oxidation of Mn2+ to the catalytically active Mn3+ is usually reflected by an anodic peak. Cyclic voltammetry measurements of the bare MnNCN electrode showed such anodic peak when the potential was swept toward the positive direction (Figure S8). The complementary reverse scan contained a reduction peak corresponding to the previous Mn2+ form. This electrochemical behavior is in agreement with the previously reported slightly higher oxidation state than +2 for MnPOx electrocatalyst.47 However, at present we do not have sufficient experimental data to determine: i) whether the oxidation of Mn2+ cations in MnNCN happens via a one- or two-

ACS Paragon Plus Environment

Langmuir 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

electron process and ii) if a mixed valency of manganese cations occurs. The complementary XRD patterns of the electrode, which show reflection peaks of MnNCN, indicate that the electrocatalytic active form of the material is MnNCN@MnPOx (Scheme 1). This core-shell structure is visible in the recorded TEM images of the bulk MnNCN crystallites after PEC water oxidation at 1.23 V vs RHE for 60 (Figures 10, S9‒S10). The lattice distance of 0.476 nm of the crystalline core corresponds to the (003) plane of MnNCN. The crystalline MnNCN grain is surrounded by an amorphous layer of approximately 3-4 nm.

Scheme 1. Illustration of in-situ core-shell formation of MnNCN during PEC water oxidation at pH 7 in KPi electrolyte.

This is in contrast to the core-shell formation of the MnNCN electrocatalyst. CONCLUSION We have demonstrated an augmented photocurrent of CuWO4 photoanodes from 22 to 30 µA cm−2 at 1.23 V vs. RHE after surface functionalization with MnNCN. Structural analysis based on XRD and XPS revealed MnNCN to adopt a coreshell structure in phosphate electrolyte. As such, MnNCN mimics, by core-shell formation, a manganese phosphate electrocatalyst. Consequently, the surface chemistry of MnNCN significantly differs from previous studies on the cobalt analogue. CoNCN has been described to have the same chemical composition in the bulk and on the surface. A separately prepared MnNCN electrode developed a small but detectable photocurrent due to photogenerated holes inside the semiconducting carbodiimide core of the MnNCN@MnPOx core-shell structure. This is the first study on catalytic activity of MnNCN and its integration with CuWO4 photoanodes for PEC water oxidation under neutral conditions. Taking into account that manganesebased electrocatalysts usually operate better in either acidic or alkaline conditions, MnNCN may be also applicable for OER occurring in acidic or alkaline conditions. This work highlights the potential of MnNCN-derived electrocatalysts for PEC water oxidation at neutral pH and introduces MnNCN as a non-oxidic representative to the group of manganese electrocatalysts.

ASSOCIATED CONTENT Supporting Information SEM image of unmodified CuWO4 thin film (Figure S1), blank powder XRD in transmission mode (Figure S2), LSV curves after step-wise addition of MnNCN (Figure S3), photocurrent difference between composite and bare electrode CuWO4 (Figure S4), LSV of MnNCN at pH7 (Figure S5), CA of composite electrode for 60 min (Figure S6), phosphorus XPS of MnNCN (Figure S7), CV aof MnNCN (Figure S8), cyclic voltammetry curve of MnNCN electrode (Figure S9), TEM FFT of MnNCN after PEC water oxidation (Figure S10), LSV and CA of MnNCN-modified CuWO4 electrode which was obtained by drop-casting (Figure S11). The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Figure 10. TEM micrograph of MnNCN after CA at 1.23 C vs RHE in 0.1 M KPi electrolyte (pH 7) under AM 1.5G illumination. The spacing between the planes was measured from the FFT and indexed to MnNCN.

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

ORCID Cobalt boride is known to be a comparable in situ formed active electrocatalyst for the PEC water oxidation.48 During the PEC water oxidation in NaOH electrolyte (pH 13), the Co2B transforms irreversibly to a core-shell structure Co2B/CoOOH. Our results indicate that the amended PEC efficiency of the CuWO4 photoanodes after MnNCN functionalization originates from the carbodiimide’s core-shell formation. The electrocatalytically active form of the MnNCN-derived catalyst is highly likely Mn3+. Patzke et al.38 investigated cobalt carbodiimide as an electrocatalyst at pH 7 for the OER and their study revealed CoNCN to exhibit no surface segregation.

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

ACKNOWLEDGMENT A.S. would like to thank the Fonds der Chemischen Industrie (FCI) for a Liebig habiliation fellowship. We thank Birgit Hahn for acquiring SEM images, Miriam Al-Enezy-Ulbrich and Christoph Groh for helpful discussion, Oliver Linnenberg for UV-vis measurements, Dr. Kirill Monakhov for accessibility to UV-vis spectroscopy, and Prof. Ulrich Simon for accessibility to TEM and SEM facilities. The XPS measurements were carried out with

ACS Paragon Plus Environment

Page 6 of 9

Page 7 of 9 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

Langmuir 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.02.01.0012-023/08).

REFERENCES (1) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338–344. (2) 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. (3) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37–38. (4) Sivula, K.; van de Krol, R. Semiconducting Materials for Photoelectrochemical Energy Conversion. Nat. Rev. Mater. 2016, 1, 16. (5) Nocera, D. G. The Artificial Leaf. Acc. Chem. Res. 2012, 45, 767–776. (6) Davi, M.; Peter, S.; Slabon, A. Fabrication of Hierarchically Ordered Porous Scheelite-Related Monoclinic BiVO4 Nanotubes by Electrochemical Deposition. Funct. Mater. Lett. 2016, 9, 1650036. (7) Rao, P. M.; Cai, L.; Liu, C.; Cho, I. S.; Lee, C. H.; Weisse, J. M.; Yang, P.; Zheng, X. Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano Lett. 2014, 14, 1099–1105. (8) Pihosh, Y.; Turkevych, I.; Mawatari, K.; Uemura, J.; Kazoe, Y.; Kosar, S.; Makita, K.; Sugaya, T.; Matsui, T.; Fujita, D.; Tosa, M.; Kondo, M.; Kitamori, T. Photocatalytic Generation of Hydrogen by Core-Shell WO3/BiVO4 Nanorods with Ultimate Water Splitting Efficiency. Sci. Rep. 2015, 5, 10. (9) Zhou, X. H.; Liu, R.; Sun, K.; Friedrich, D.; McDowell, M. T.; Yang, F.; Omelchenko, S. T.; Saadi, F. H.; Nielander, A. C.; Yalamanchili, S.; Papadantonakis, K. M.; Brunschwig, B. S.; Lewis, N. S. Interface Engineering of the Photoelectrochemical Performance of Ni-Oxide-Coated n-Si Photoanodes by Atomic-Layer Deposition of Ultrathin Films of Cobalt Oxide. Energy Environ. Sci. 2015, 8, 2644–2649. (10) Slabon, A.; Krumeich, F.; Wächter, F.; Nesper, R. Fabrication of Nanoporous Nickel Coatings by Template-Assisted Electrodeposition. ChemElectroChem 2014, 1, 536–538. (11) Davi, M.; Ogutu, G.; Schrader, F.; Rokicinska, A.; Kustrowski, P.; Slabon, A. Enhancing Photoelectrochemical Water Oxidation Efficiency of WO3/α-Fe2O3 Heterojunction Photoanodes by Surface Functionalization with CoPd Nanocrystals. Eur. J. Inorg. Chem. 2017, 2017, 4267-4274. (12) Davi, M.; Schrader, F.; Scholz, T.; Ma, Z.; Rokicinska, A.; Dronskowski, R.; Kustrowski, P.; Slabon, A. SrTaO2N Nanowire Photoanode Modified with a Ferrihydrite Hole-Storage Layer for Photoelectrochemical Water Oxidation. ACS Appl. Nano Mater. 2018, 1, 869-876. (13) Zhou, X. H.; Liu, R.; Sun, K.; Papadantonakis, K. M.; Brunschwig, B. S.; Lewis, N. S. 570 mV Photovoltage, Stabilized n-Si/CoOx Heterojunction Photoanodes Fabricated Using Atomic Layer Deposition. Energy Environ. Sci. 2016, 9, 892–897. (14) Yourey, J. E.; Bartlett, B. M. Electrochemical Deposition and Photoelectrochemistry of CuWO4, a Promising Photoanode for Water Oxidation. J. Mater. Chem. 2011, 21, 7651–7660. (15) Leute, V. Das Wolframtrioxid Und Seine Reaktion Mit Den Oxiden Zweiwertiger Metalle - II. Die Reaktion CuO + WO3 = CuWO4. Z. Phys. Chem. 1966, 48, 319–339. (16) Yourey, J. E.; Pyper, K. J.; Kurtz, J. B.; Bartlett, B. M. Chemical Stability of CuWO4 for Photoelectrochemical Water Oxidation. J. Phys. Chem. C 2013, 117, 8708–8718. (17) Bohra, D.; Smith, W. A. Improved Charge Separation via FeDoping of Copper Tungstate Photoanodes. Phys. Chem. Chem. Phys. 2015, 17, 9857–9866.

(18) Gao, Y.; Zandi, O. Atomic Layer Stack Deposition-Annealing Synthesis of CuWO4. J. Mater. Chem. A 2016, 4, 2826–2830. (19) Hu, D. Y.; Diao, P.; Xu, D.; Xia, M. Y.; Gu, Y.; Wu, Q. Y.; Li, C.; Yang, S. B. Copper(II) Tungstate Nanoflake Array Films: Sacrificial Template Synthesis, Hydrogen Treatment, and Their Application as Photoanodes in Solar Water Splitting. Nanoscale 2016, 8, 5892–5901. (20) Lhermitte, C. R.; Bartlett, B. M. Advancing the Chemistry of CuWO4 for Photoelectrochemical Water Oxidation. Acc. Chem. Res. 2016, 49, 1121–1129. (21) Zhang, H.; Yilmaz, P.; Ansari, J. O.; Khan, F. F.; Binions, R.; Krause, S.; Dunn, S. Incorporation of Ag Nanowires in CuWO4 for Improved Visible Light-Induced Photoanode Performance. J. Mater. Chem. A 2015, 3, 9638–9644. (22) Valenti, M.; Dolat, D.; Biskos, G.; Schmidt-Ott, A.; Smith, W. A. Enhancement of the Photoelectrochemical Performance of CuWO4 Thin Films for Solar Water Splitting by Plasmonic Nanoparticle Functionalization. J. Phys. Chem. C 2015, 119, 2096–2104. (23) Gao, Y.; Hamann, T. W. Elucidation of CuWO4 Surface States During Photoelectrochemical Water Oxidation. J. Phys. Chem. Lett. 2017, 8, 2700–2704. (24) Gao, Y.; Hamann, T. W. Quantitative Hole Collection for Photoelectrochemical Water Oxidation with CuWO4. Chem. Commun. 2017, 53, 1285–1288. (25) Nam, K. M.; Cheon, E. A.; Shin, W. J.; Bard, A. J. Improved Photoelectrochemical Water Oxidation by the WO3/CuWO4 Composite with a Manganese Phosphate Electrocatalyst. Langmuir 2015, 31, 10897–10903. (26) Huynh, M.; Ozel, T.; Liu, C.; Lau, E. C.; Nocera, D. G. Design of Template-Stabilized Active and Earth-Abundant Oxygen Evolution Catalysts in Acid. Chem. Sci. 2017, 8, 4779– 4794. (27) Gorlin, Y.; Jaramillo, T. F. A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc. 2010, 132, 13612–13614. (28) Zaharieva, I.; Chernev, P.; Risch, M.; Klingan, K.; Kohlhoff, M.; Fischer, A.; Dau, H. Electrosynthesis, Functional, and Structural Characterization of a Water-Oxidizing Manganese Oxide. Energy Environ. Sci. 2012, 5, 7081-7089. (29) Robinson, D. M.; Go, Y. B.; Mui, M.; Gardner, G.; Zhang, Z.; Mastrogiovanni, D.; Garfunkel, E.; Li, J.; Greenblatt, M.; Dismukes, G. C. Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis. J. Am. Chem. Soc. 2013, 135, 3494– 3501. (30) Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F. SI: In Situ X-Ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 8525–8534. (31) Huynh, M.; Bediako, D. K.; Nocera, D. G. A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid. J. Am. Chem. Soc. 2014, 136, 6002–6010. (32) Huynh, M.; Shi, C.; Billinge, S. J. L.; Nocera, D. G. Nature of Activated Manganese Oxide for Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 14887–14904. (33) Eguia-Barrio, A.; Castillo-Martinez, E.; Liu, X.; Dronskowski, R.; Armand, M.; Rojo, T. Carbodiimides: New Materials Applied as Anode Electrodes for Sodium and Lithium Ion Batteries. J. Mater. Chem. A 2016, 4, 1608–1611. (34) Sougrati, M. T.; Darwiche, A.; Liu, X. H.; Mahmoud, A.; Hermann, R. P.; Jouen, S.; Monconduit, L.; Dronskowski, R.; Stievano, L. Transition-Metal Carbodiimides as Molecular Negative Electrode Materials for Lithium- and Sodium-Ion Batteries with Excellent Cycling Properties. Angew. Chem. Int. Ed. 2016, 55, 5090–5095. (35) Zhao, W.; Liu, Y. F.; Liu, J. J.; Chen, P.; Chen, I. W.; Huang, F. Q.; Lin, J. H. Controllable Synthesis of Silver Cyanamide

ACS Paragon Plus Environment

Langmuir 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

(36)

(37)

(38)

(39)

(40)

(41)

(42)

(43)

(44)

(45)

(46)

(47)

(48)

as a New Semiconductor Photocatalyst under Visible-Light Irradiation. J. Mater. Chem. A 2013, 1, 7942–7948. Meng, H.; Li, X. X.; Zhang, X.; Liu, Y. F.; Xu, Y.; Han, Y. D.; Xu, J. L. Fabrication of Nanocomposites Composed of Silver Cyanamide and Titania for Improved Photocatalytic Hydrogen Generation. Dalt. Trans. 2015, 44, 19948–19955. 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. Ressnig, D.; Shalom, M.; Patscheider, J.; More, R.; Evangelisti, F.; Antonietti, M.; Patzke, G. R. Photochemical and Electrocatalytic Water Oxidation Activity of Cobalt Carbodiimide. J. Mater. Chem. A 2015, 3, 5072–5082. Liu, X. H.; Krott, M.; Müller, P.; Hu, C. H.; Lueken, H.; Dronskowski, R. Synthesis, Crystal Structure, and Properties of MnNCN, the First Carbodiimide of a Magnetic Transition Metal. Inorg. Chem. 2005, 44, 3001–3003. Boyko, T. D.; Green, R. J.; Dronskowski, R.; Moewes, A. Electronic Band Gap Reduction in Manganese Carbodiimide: MnNCN. J. Phys. Chem. C 2013, 117, 12754–12761. Liu, X.; Dronskowski, R.; Glaum, R.; Tchougréeff, A. L. Experimental and Quantum-Chemical Investigations of the UV/Vis Absorption Spectrum of Manganese Carbodiimide, MnNCN. Z. Anorg. Allg. Chem. 2010, 636, 343–348. Nelson, R.; Konze, P. M.; Dronskowski, R. First-Principles Chemical Bonding Study of Manganese Carbodiimide, MnNCN, As Compared to Manganese Oxide, MnO. J. Phys. Chem. A 2017, 121, 7778–7786. Davi, M.; Keßler, D.; Slabon, A. Electrochemical Oxidation of Methanol and Ethanol on Two-Dimensional SelfAssembled Palladium Nanocrystal Arrays. Thin Solid Films 2016, 615, 221–225. Liu, C.; Zhang, C.; Fu, H.; Nan, X.; Cao, G. Exploiting HighPerformance Anode through Tuning the Character of Chemical Bonds for Li-Ion Batteries and Capacitors. Adv. Energy Mater. 2017, 7, 1601127. Wang, G.; Cao, N.; Wang, Y. Characteristics and Corrosion Studies of Zinc–manganese Phosphate Coatings on Magnesium–lithium Alloy. RSC Adv. 2014, 4, 59772–59778. Glorieux, B.; Berjoan, R.; Matecki, M.; Kammouni, A.; Perarnau, D. XPS Analyses of Lanthanides Phosphates. Appl. Surf. Sci. 2007, 253, 3349–3359. Jin, K.; Park, J.; Lee, J.; Yang, K. D.; Pradhan, G. K.; Sim, U.; Jeong, D.; Jang, H. L.; Park, S.; Kim, D.; Sung, N. E.; Kim, S. H.; Han, S.; Nam, K. T. Hydrated Manganese(II) Phosphate (Mn3(PO4)2·3H2O) as a Water Oxidation Catalyst. J. Am. Chem. Soc. 2014, 136, 7435–7443. Masa, J.; Weide, P.; Peeters, D.; Sinev, I.; Xia, W.; Sun, Z. Y.; Somsen, C.; Muhler, M.; Schuhmann, W. Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution. Adv. Energy Mater. 2016, 6, 1502313.

ACS Paragon Plus Environment

Page 8 of 9

Page 9 of 9 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

Langmuir

ACS Paragon Plus Environment