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Enhanced Photoelectrochemical Water Oxidation Efficiency of CuWO Photoanodes by Surface Modification with AgNCN 4

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Martin Davi, Andreas Drichel, Markus Mann, Tanja Scholz, Felix Schrader, Anna Rokicinska, Piotr Kustrowski, Richard Dronskowski, and Adam Slabon J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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The Journal of Physical Chemistry

Enhanced Photoelectrochemical Water Oxidation Efficiency of CuWO4 Photoanodes by Surface Modification with Ag2NCN Martin Davi,† Andreas Drichel,† Markus Mann,† Tanja Scholz,† Felix Schrader,† Anna Rokicinska,‡ Piotr Kustrowski,‡ Richard Dronskowski† and Adam Slabon*,† †

Institute of Inorganic Chemistry, RWTH Aachen, Landoltweg 1, D-52074 Aachen, Germany

‡

Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland

ABSTRACT: We investigated CuWO4 as a promising photoanode and its surface functionalization with Ag2NCN. The composite photoanode displays a synergetic effect between its constituents during photoelectrochemical (PEC) water oxidation. After placing Ag2NCN particles on the CuWO4 thin film, the photocurrent density increases from 15 µA cm−2 to 45 µA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE) in phosphate buffer electrolyte at pH 7 under simulated AM1.5 G illumination. The different positions of the band edges of CuWO4 and Ag2NCN favor charge carrier separation at their interface. Upon illumination, photogenerated electrons of both semiconductors are transferred to the conduction band and can migrate to the counterelectrode. The higher position of the conduction band edge of Ag2NCN allows its electrons to be injected into the conduction band of CuWO4. Simultaneously, holes from the cyanamide are blocked, because the valence band edge position of CuWO4 is positioned lower than for Ag2NCN. This results in more efficient charge separation and hole collection efficiencies. Herein, we emphasize the potential of carbodiimides and cyanamides in the design of photoelectrodes beyond oxide and nitride semiconductors.

drogen treatment of CuWO4 due the formation of oxygen vacancies.19 Heterojunctions between CuWO4 and a second semiconductor, such as CuO20, BiVO421, and WO322 show synergetic effects improving photocurrents. The latter is especially frequently used as an electronic conductor in core-shell structures.23 In case of WO3/CuWO4 photoanodes, the different position of the corresponding band edges favor charge separation that corresponds to higher photocurrents.24 Another approach is the functionalization of semiconductor surfaces with electrocatalysts to ameliorate the PEC efficiency.25 Electrocatalysts can be anchored on surfaces of photoelectrodes by several techniques, such as electrodepositon26, drop-casting27, coating of nanoparticle arrays28, and sputtering29. In case of CuWO4, only a manganese phosphate electrocatalyst is known to increment PEC efficiency.30 When placed on the CuWO4 surface, the onset potential is shifted cathodically by 100 mV during PEC water oxidation.31 Furthermore, PEC efficiency of CuWO4 can be augmented with silver nanowires incorporated in the bulk32 or gold nanoparticles on the surface33 due to their high electrical conductivity and plasmonic resonance, respectively. Oxide semiconductors are today still intensively investigated as photoelectrodes due to their stability and relatively low band gaps.34 The anionic charge of 2- makes oxides chemically closely related to entities consisting of trimeric nitrogen-carbon-nitrogen anions, which carry the identical negative charge.35 These can occur whether in the form of a carbodiimide (N=C=N)2− or cyanamide (N-C≡N)2− anion.36 Metal carbodiimide/cyanamide compounds MNCN (M = Metal) demonstrate frequently similar crystal chemistry with respect to oxides, although the former are characterized by a small distortion due to the larger NCN-anion.37 Besides their crystal chemistry and intriguing magnetic properties, MNCN

INTRODUCTION Solar fuel generation in the form of hydrogen derived directly from water represents an environmentally friendly technology to obtain clean energy.1 Solar power may be converted to chemical fuel by electrolyzers powered by photovoltaic generators or directly by PEC cells, i.e. devices having at least one solid/electrolyte junction.2 Opposite to a regenerative PEC cell that produces only electricity, a PEC cell that creates chemical fuel at the junction is defined as a photoelectrosynthetic cell.3 That device was realized in the 1970s in a system consisting of a TiO2 semiconductor photoanode and metallic counterelectrode.4 Water-splitting requires the transfer of two electrons for the hydrogen evolution reaction (HER) and four electrons for the oxygen evolution reaction (OER).5 Semiconductor electrodes can be manufactured by electrochemical deposition6, hydrothermal methods7, microwave plasma8, physical/chemical vapor deposition9 and/or colloidal methods10. CuWO4 is semiconductor with a band gap of 2.25 eV and was synthesized for the first time in 1966 by Leute11. The compound crystallizes with a distorted wolframite-type structure.12 It took more than four decades until this ternary oxide was discovered as a photoanode candidate with a theoretical photocurrent density of 10.7 mA cm−2.13 The material has been synthesized in various morphologies, such as dense films14, nanoporous films15, and nanoplatelets16. Today, the achieved photocurrents of CuWO4 are still low in comparison to other oxide semiconductor photoanodes, such as BiVO4 or hematite.17 Several attempts have been reported to overcome the poor performance of CuWO4. For instance, iron can be applied as dopant to increase the low bulk electronic conductivity.18 Charge transport can be also facilitated by post-synthetic hy-

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compounds have emerged recently as materials for energy storage and conversion. For example, the carbodiimides MNCN (M=Cu, Fe, Co) have been discovered as cathode materials for Li-ion batteries.38 CoNCN has been also identified as an OER catalyst in photochemical systems.39 In case of silver, only the cyanamide form was synthesized in 1958 by Yoffe.40 Ag2NCN crystallizes in a monoclinic space group with carbon-nitrogen distances of 1.19 Å and 1.26 Å.41 Thermal decomposition of this material yields nanoporous metallic silver foam.42 The optical band gap of Ag2NCN has recently drawn attention towards application for solar fuel generation systems.43 Liu et al. reported heterostructures TiO2/Ag2NCN that transform in situ to TiO2/Agnanoparticles during (electroless) photochemical water reduction and elevate the hydrogen evolution rate.44 For the counterreaction of water oxidation, there is only one report on PEC water oxidation based on Ag2NCN photoanodes in sodium sulfate electrolyte at pH 6.45 We report on the integration of Ag2NCN with CuWO4 into heterojunction thin film photoanodes. The photoanode has an interface between the oxide and its isoelectronic NCN counterpart. The photoanode exhibits improved efficiency during PEC water oxidation in potassium phosphate (KPi) electrolyte at pH 7.

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Synthesis of Ag2NCN. The synthesis was carried out at room temperature and under ambient atmosphere. 4.43 g (26.08 mmol) silver nitrate (AgNO3, 99.9 %, ABCR Chemicals) and 0.61 g (14.52 mmol) cyanamide (H2NCN, 98.89 %, AlzChem) were dissolved in 30 ml of deionized water.41 Ag2NCN was precipitated as a yellow powder from the reaction mixture by adding 10 ml of 25 % ammonia solution. The product was centrifuged at 6000 rpm for 10 min, washed with deionized water and dried at 373 K under vacuum for 24 hours. Preparation of CuWO4/Ag2NCN and Ag2NCN photoanodes. Ag2NCN powder was dispersed by ultrasound in ethanol (125 µg ml-1). The CuWO4 thin film electrode was placed on a heating plate at 323K and the Ag2NCN dispersion was drop-casted on the surface. The amount of silver was determined by atomic absorption spectroscopy. Electron microscopy and powder X-ray diffraction (XRD). SEM images of photoelectrodes and Ag2NCN powder were recorded on a Leo Supra 35VP SMT (Zeiss). TEM images were recorded with a Zeiss Libra 200 FE TEM and energydispersive X-Ray (EDX) spectroscopy performed with an XFlash 5030 detector from Bruker. Powder XRD patterns were recorded in transmission mode on a STOE STADI-P diffractometer (Cu Kα1 radiation) operating with a DECTRIS Mythen 1K detector. For powder XRD analysis of CuWO4 photoelectrode, the yellow thin film was mechanically removed from the FTO substrate in order to avoid diffraction peaks originating from SnO2. X-ray photoelectron spectroscopy (XPS) and UV-Vis spectroscopy. The surface composition of photoanodes was determined by using a Prevac photoelectron spectrometer equipped with a hemispherical analyzer (VG SCIENTA R3000) and a low energy electron flood gun (FS40A-PS). The spectra were taken using a monochromatized aluminum source Al Kα (E=1486.6 eV). UV-Vis spectra were recorded using a Shimadzu UV-2600 spectrophotometer. Measurements were recorded in reflectance, transmittance and absorbance mode. Tauc plots were calculated for the determination of band gaps by using the Kubelka-Munk function F(R)=(1-R)2/2R. Photoelectrochemistry. Electrochemical experiments were carried out in a PEC cell (WAT Venture) operating in a threeelectrode setup. The illuminated area of the working electrode was 0.79 cm2. 1 M Ag/AgCl electrode (WAT Venture) and platinum wire were used as reference electrode and counter electrode, respectively. Potentials were recorded vs. 1 M Ag/AgCl and converted vs. RHE according to ERHE (volt) = E1M Ag/AgCl + (0.059 x pH).47 Working electrodes were illuminated from the backside, i.e. non-conductive side of the FTO glass, with 100 mW cm−2 (AM 1.5 G) simulated visible light. A solar light simulator (class-AAA 94023A, Newport) with an ozone-free 450 W xenon short-arc lamp was used as light source and its irradiance measured by a spectroradiometer (model ILT950, Quantum Design). Potassium phosphate (KPi) buffer at pH 7 was used as electrolyte for all measurements.48 During linear square voltammetry (LSV) potentials were swept with a scan rate of 10 mV s−1 towards the positive direction from 0.60 V to 1.44 V vs RHE. Incident photon-to-current conversion efficiency (IPCE) was calculated according to IPCE (%) = (i x 1240 V nm) / (λ x P) × 100; i: photocurrent density (mA cm−2), λ: measured wavelength (nm), P: intensity of monochromatic light (mW cm−2). Monochromatic light was created by a 300 W arc xenon light source. Measurements were performed with

EXPERIMENTAL SECTION Synthesis of CuWO4 thin films. Conductive fluorine-doped tin oxide (FTO) glass slides (2.2 mm thick, ~ 7 Ω/sq, Sigma Aldrich) were used as substrate for CuWO4 thin-films. Photoelectrodes were prepared according to a changed electrochemical synthetic route described by Bartlett13 and Diao46. FTO glass slides were cleaned in diluted hydrochloric acid (Sigma) and sonicated in acetone for 15 min. 1.73 g (5.2 mmol) of sodium tungstate dihydrate (Na2WO4·2H2O, 99.9%, Acros Organics) was dissolved in 15 ml deionized water and 2.5 ml hydrogen peroxide (30%, Geyer Chemsolute) were added to the tungstate precursor solution. The solution was stirred for 20 min at room temperature and a platinum wire was inserted. 25 ml deionized water and 22.5 ml isopropanol (>99.7%, Fisher Scientific) were added to the solution. A separate solution of 0.73 g (3.0 mmol) copper(II) nitrate trihydrate (Cu(NO3)2·3H2O, >99%, Sigma) in 10 ml of deionized water was prepared and added to the solution with the tungsten precursor. Subsequently, the pH value was lowered to 1.1 by adding diluted nitric acid (Sigma). This solution was used for electrochemical deposition of the copper-tungsten matrix on FTO glass in a three-electrode setup with platinum wire and 1 M Ag/AgCl (WAT Venture) as counter electrode and reference electrode, respectively. Electrodeposition was carried out with a SP-150 potentiostat (BioLogic) and the EC-Lab® software package. The potential was swept in the potential range from −0.5 V to +0.5 V vs. 1 M Ag/AgCl reference electrode for 12 cycles at a scan rate of 50 mV s−1. The working electrode was disconnected from the electrical circuit, washed with deionized water and dried at room temperature under vacuum. The FTO glass was heated at 723 K for 2 h under ambient atmosphere. After cooling down to room temperature, excess of copper oxide was etched by immersing the photoelectrode into 0.5 M HCl for 20 min. The product is a yellow thin-film. After copper oxide etching, the electrode was annealed at 723 K for 10 min under ambient atmosphere.


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Figure 2. Experimental and calculated powder XRD patterns of Ag2NCN before and after PEC water oxidation.

Figure 1. Experimental and calculated powder XRD patterns of bare CuWO4 (ICSD 16009) and Ag2NCN (ICSD 411091) modified thin films. For sample preparation, these were mechanically removed from the FTO surface to avoid diffraction peaks of tin oxide.

an Atlas 0931 potentiostat (Atlas Sollich). Light intensity was determined with a low-power UV enhanced silicon photodetector (818-UV, NewPort) from 380 to 600 nm. Absorbed photon-to-current conversion efficiency (APCE) was determined by dividing the IPCE by the light harvesting efficiency (LHE) at each wavelength. The LHE was calculated according to LHE = 1-10-A (A: absorbance at certain wavelength). Computational methods. Electronic structures of CuWO4 and Ag2NCN were calculated by iterating 2 x 2 x 2 supercells and the unit cell, respectively, towards self-consistency using the Vienna ab initio simulation package (VASP) at the density functional theory (DFT) level.49 We used the exchangecorrelation functional from Perdew-Burke-Ernzerhof (PBE) in the generalized gradient approximation (GGA).50 Projectoraugmented-wave (PAW) potentials were applied for core- and valence-electron separation.51 The k-space integration was done on a grid of 3 x 2 x 3 for CuWO4 and 6 x 6 x 6 for Ag2NCN. The kinetic energy cutoff of plane-wave expansion was set to 500 eV. Stress tensors, forces, atomic positions, unit cell shapes and volumes were allowed to relax during optimization. The convergence criterion for all electronic structures was set to 10-5 meV. Atom-projected densities of states (pDOS) and pCOHPs were obtained from the LOBSTER software package 2.0.0 for electronic band structure reconstruction.52

Figure 3. SEM micrograph of Ag2NCN powder. The inset illustrates the crystal structure of Ag2NCN: silver, nitrogen and carbon are indicated as red, green and white spheres, respectively.

Modification of the surface with small amounts of Ag2NCN (vide infra) were below the detection limit of XRD (Figure 1, top). Since the thin films were scratched from the FTO substrate, the diffraction patterns did not contain diffraction peaks originating from tin oxide. The powder XRD patterns of the obtained silver cyanamide revealed that the material is a single phase as well (Figure 2). We carried out stability tests of Ag2NCN powder, which was fabricated as a photoanode. After PEC water oxidation, the powder was scratched from the surface for the recording of powder XRD diffraction patterns. No difference in the diffraction patterns before and after PEC water oxidation could be observed, which is an indication for structural stability under operation conditions during water oxidation. The sharp diffraction peaks of the Ag2NCN thin film relate to crystallite sizes in the micrometer range. This is also evident from the recorded SEM micrographs (Figure 3), since the sample is characterized by mainly large particles accompanied by smaller entities. The assembled heterojunction photoanode CuWO4/Ag2NCN consisted of an approximately 2 µm thick

RESULTS AND DISCUSSION Structural Characterization. CuWO4 thin films were synthesized on FTO substrate in a two-step process. First, a copper oxide and tungsten oxide matrix was formed by electrochemical deposition and subsequently annealed to form CuWO4. Excess copper oxide was removed by insertion of the photoelectrode in an acidic solution. The diffraction peaks in the powder XRD patterns of the thin film could be indexed to CuWO4 (Figure 1). The powder was single phase and did not include an impurity phase of binary tungsten oxides.



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Figure 4. SEM micrograph of the heterojunction photoanode CuWO4/Ag2NCN. The inset depicts the cross-section of the bare CuWO4 thin film.

Figure 6. XPS Ag 3d spectra of Ag2NCN and CuWO4 /Ag2NCN photoanodes.

consistent with the stoichiometry of CuWO4. The silver signal clearly increases in the area of the rod whereas both the copper and tungsten signal decrease. The oxidation state of silver on the surface of the fabricated photoanodes was studied by XPS. In the collected Ag 3d spectra two spin-orbit components at 367.6-367.8 eV (Ag 3d5/2) and 373.6-373.8 eV (Ag 3d3/2) can be distinguished (Figure 6). The positions of both components at relatively low binding energy suggest that the external surface of the studied photoanodes is covered by a thin layer of metallic silver. This phase is not detected in XRD before and after PEC water oxidation due to its very low content. The similar effect of segregation of metallic Ag on Ag2NCN was previously observed in Ag2NCN-anatase composites.44 The electronic band gaps were determined by UV-Vis spectroscopy on powder samples and are consistent with previously measured values (Figure 7).31 CuWO4 and Ag2NCN display almost identical band gaps of 2.21 eV and 2.24 eV, respectively. PEC water oxidation. CuWO4 thin film photoanodes were examined by means of LSV and chronoamperometry (CA) at 1.23 V vs. RHE for PEC water oxidation (Figure 8). Without illumination, the detected current drops to zero due to the lack of photogenerated holes in the valence band. We observed that further thermal treatment at 723 K for 10 min, after having etched the excess copper oxides, raised the photocurrent from 9 µA cm−2 to 18 µA cm−2. During LSV the CuWO4 thin film electrode developed an anodic current with an onset potential of approximately 0.85 V vs. RHE. We modified the surface of the oxide photoanode with the cyanamide by drop-casting stepwise a dispersion of the latter on the surface. All experiments were conducted on one single CuWO4 thin film photoelectrode to ensure more precise comparison of current

Figure 5. TEM micrograph of the photoanode CuWO4/Ag2NCN and elemental distribution at the heterojunction. The white line represents the direction of the EDX line scan.

CuWO4 thin film and deposited Ag2NCN particles on the surface. Figure 4 illustrates the overview on the composite electrode and the cross-sectional view of the CuWO4 electrode after synthesis (inset). The amount of Ag2NCN on the CuWO4 thin film was too small to obtain a SEM image of the interface between both constituents. For its visualization, we recorded TEM images of the CuWO4/Ag2NCN thin film that was scratched from the FTO surface. The TEM image in Figure 5 shows an Ag2NCN crystallite being in contact with intergrown particles of the CuWO4 thin film. The Ag2NCN particles can be identified due to their regular shapes. The elemental composition of silver, copper and tungsten was confirmed by EDX spectroscopy. The spectra were recorded along the CuWO4/Ag2NCN interface (Figure 5, horizontal white line). The similar intensities of copper and tungsten signal are


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Figure 7. Absorption spectra for CuWO4 and Ag2NCN (Inset: Tauc plots). Figure 9. LSV of photoanodes CuWO4, Ag2NCN and CuWO4/Ag2NCN (12.5 µg or 37.5 µg added amount of Ag2NCN to CuWO4). Measurements were performed in 0.1 M KPi electrolyte (pH 7) at a scan rate of 10 mV s−1 under backlight AM 1.5 G illumination (100 mW cm−2) (full lines) and without illumination (dashed line).

Figure 8. LSV of CuWO4 photoanodes: without (black) and with (red) a surplus thermal treatment after synthesis. Measurements were performed in 0.1 M KPi electrolyte (pH 7) at a scan rate of 10 mV s−1 under backlight AM 1.5 G illumination (100 mW cm−2) (full lines) and without illumination (dashed lines).

Figure 10. CA of photoanodes: bare CuWO4, bare Ag2NCN and CuWO4/Ag2NCN (12.5 µg or 37.5 µg added amount of Ag2NCN to CuWO4). Measurements were performed in 0.1 M KPi electrolyte (pH 7) at a scan rate of 10 mV s−1 under interrupted backlight AM 1.5 G illumination (100 mW cm−2).

densities during PEC water oxidation after addition of Ag2NCN particles. After drop-casting 12.5 µg of cyanamide, the photoanode yielded an upsurge in photocurrent during LSV in comparison to the bare oxide electrode (Figure 9). The rise in photocurrent saturated when approximately 37.5 µg of Ag2NCN was drop-casted on the surface. Without illumination the semiconductor thin film did not develop any photocurrent due to the lack of holes in the valence bands. We prepared also a complementary electrode containing only 37.5 µg of Ag2NCN. The preparation procedure of the bare Ag2NCN electrode was identical as for the heterojunction photoanode. During LSV the photocurrent of the bare cyanamide electrode was close to zero. Coupling CuWO4 and Ag2NCN leads consequently to a synergetic effect, because achieved photocurrent densities exceed the sum of its individual constituents in the whole potential range. The ameliorated photocurrent cannot be accounted to larger thickness of the light-absorber layer of the composite photoanode, because the

bare Ag2NCN did not contribute enough current density under illumination. The altered surface chemistry of the photoanode after cyanamide modification is also evident during PEC water oxidation at 1.23 V vs. RHE (Figure 10). The photocurrent density of the bare CuWO4 photoanode dropped to zero if the illumination was intermitted. After modification with 37.5 µg of Ag2NCN the photocurrent increased above 50 µA cm−2. Besides greater PEC efficiency for the CuWO4/Ag2NCN photoanode, the shape of its photocurrent curves differs significantly from the bare CuWO4 photoanode. When incident light is turned on, the sharp photocurrent upsurge is followed by a rapid exponential decay, which settles at 45 µA cm−2. This originates from electron-hole recombination processes being in competition with PEC water oxidation by holes at the



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Figure 11. CA of Ag2NCN photoanode with 2 mg of material in 0.1 M KPi electrolyte (pH 7) under interrupted backlight AM 1.5 G illumination (100 mW cm−2).

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Figure 12. SEM micrograph of a photoanode Ag2NCN (2 mg of material) after PEC water oxidation in 0.1 M KPi electrolyte (pH 7) under backlight AM 1.5 G illumination (100 mW cm−2) for one hour. The inset shows an image of the photoanode after the measurement and the photocurrent density. We interrupted the illumination at the start and close to the end of the CA in order to prove that the current originates from the incident light.

electrode/electrolyte interface.53 Further addition of suitable thin catalyst layers may therefore stabilize the photocurrents.54,55 In opposite to the bare CuWO4 photoanode, interrupting the irradiation of CuWO4/Ag2NCN photoanode did not result in a zero-current but exposed a small negative current. This behavior of semiconductor electrodes is likely to occur if charge carrier recombination processes are still present at the electrode surface. Similar to the anodic spikes under illumination, the cathodic overshoot decays rapidly and settles to a stable value. The bare Ag2NCN electrode containing 37.5 µg of material revealed very small current close to zero. We prepared a complementary electrode with 2 mg of Ag2NCN to investigate whether photocurrent may be detected if a higher amount of material would be deposited on the FTO substrate. CA at 1.23 V vs. RHE under illumination shows that Ag2NCN develops a small anodic photocurrent that decays if the light source is interrupted (Figure 11). Both CuWO4 and Ag2NCN are n-type semiconductors that can oxidize water under illumination. Huang et al. investigated the cyanamide as photoanode material in Na2SO4 electrolyte at pH 6 under illumination with a 300 W Xe lamp equipped with a filter of λ ≥ 420 nm.45 However, no direct comparison with our cyanamide photoanodes is possible, because we used different PEC parameters, such as pH value, electrolyte composition, light source, and intensity of the incident light. Nevertheless, both Ag2NCN photoanodes display small photocurrent densities below 10 µA cm−2. Figure 12 shows the SEM images of the prepared Ag2NCN photoanode after one hour of illumination at 1.23 V vs RHE (Figures 12, S1-S2). The rectangular shape of the Ag2NCN crystallites remains unchanged in comparison to the SEM images recorded before PEC water oxidation. The photoanode kept also its yellow color (Figure 12, inset) which is consistent with the collected powder XRD patterns of the thin film after chronoamperometry (Figure 2), indicating structural stability of the cyanamide. The increased PEC efficiency of the CuWO4/Ag2NCN photoanode is also visible in the IPCE at 1.23 V vs. RHE (Figure 13). IPCE values of the pristine Ag2NCN are close to zero due to its very low current. Both the bare and Ag2NCN-modified CuWO4 photoanode are characterized by similar dependency of IPCE and reach their maximal values with decreasing wavelength. Similar tendency in the

Figure 13. IPCE at 1.23 V vs. RHE of CuWO4, Ag2NCN and CuWO4/Ag2NCN with 37.5 µg of cyanamide under backlight AM 1.5 G illumination (100 mW cm−2). The inset shows the APCE of the photoanodes CuWO4 (red), Ag2NCN (green), and CuWO4/Ag2NCN (black).

APCE values can be observed for the CuWO4 and heterojunction CuWO4/Ag2NCN photoanodes (Figure 13, inset). This is in agreement with the almost identical electronic band gaps of Ag2NCN and CuWO4. The photocurrent during PEC water oxidation is defined as JH2O = Jmax x ηsep x ηabs x ηhc, where Jmax is the theoretical maximum photocurrent of CuWO4 equal 10.7 mA cm-2 and ηabs is the absorption efficiency. ηabs was calculated by integrating the product of the LHE and incident spectral irradiance (Figure S4) from 300 to 520 nm. The charge separation efficiency ηsep is decreased by recombination in the bulk. The efficiency of hole collection at the surface of a photoelectrode is defined as the hole collection efficiency ηhc. For determination of the values of ηsep and ηhc for the CuWO4 and CuWO4/Ag2NCN photoanodes, PEC oxidation of 0.1 M Na2SO3 was measured as a function of applied potential for each photoanode. The fast oxidation of a hole scavenger


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Figure 14. Charge separation efficiencies ηsep of CuWO4 and CuWO4/Ag2NCN photoanodes.

Figure 16. pCOHP curves of N-C, N(1)-Ag, and N(2)-Ag interactions in Ag2NCN acquired from LOBSTER calculations. Bonding and antibonding states are indicated on the x-scale to the right and left, respectively. Bond lengths and integrated pCOHP values are noted in each graph.

than the experimentally determined value of 2.28 eV45, the typical DFT underestimation.56 For CuWO4, the calculated band gap of about 1.80 eV (Figure S2) is smaller than the experimentally determined value of 2.25 eV31, too. The pCOHP partitions the electronic band structure energy into orbital-pair interactions in order to quantify the strengths of the chemical bonds. The positive value (i.e., negative – pCOHP) of the Ag‒N interaction close to the upper valence band indicate that the transfer of photogenerated electrons from the valence band stems from antibonding N(2)-Ag and N(1)-Ag levels. In case of CuWO4, an electron is excited after illumination from an antibonding combination between copper 3d and oxygen 2p orbitals to mixed levels between tungsten 5d orbitals and oxygen 2p orbitals (Figure 16b). It should be noted that the position of the Fermi level obtained from DFT is below the expected optical band gap. This is in agreement with previous theoretical studies on CuWO4.57 Despite the quantitatively inaccurate position of the Fermi level, our calculations allow to qualitatively derive the bonding situation of CuWO4. The heightened activity of the CuWO4 photoanode after modification with Ag2NCN should be a consequence of the heterojunction formation between these two semiconductors. It is known that interface formation of two semiconductors with different position of the valence and conduction band edges can result in improved charge separation.58 This corresponds to a sophisticated concentration of holes at the

Figure 15. Hole collection efficiencies ηhc of CuWO4 and CuWO4/Ag2NCN photoanodes. The inset shows the photocurrent densities during water and sulfite oxidation for photoanodes CuWO4 (red) and CuWO4/Ag2NCN (black).

such as sulfite allows to determine the photocurrent JNa2SO3 that can be reached if all photogenerated holes at the surface are used. The charge separation and hole colletion efficiencies can be subsequently calculated according to ηsep = JNa2SO3 / (Jmax x ηabs) and ηhc = (JH2O / JNa2SO3). Functionalizing the surface of CuWO4 with Ag2NCN particles increases significantly ηsep (Figure 14), indicating improved charge carrier separation due to formation of a heterojunction CuWO4/Ag2NCN. The corresponding ηhc value of the latter one is nearly equal to the unmodified CuWO4 electrode at lower potentials and increases at higher applied potentials (Figure 15). At 1.23 V vs RHE, the photoanodes CuWO4/Ag2NCN and CuWO4 reach ηhc values of 36 % and 23 %, respectively. The photoactivity of Ag2NCN has been suggested to originate due to excitation of electrons from nitrogen 2p orbitals into silver 5s orbitals. We calculated the density of states (DOS) (Figure S1) and pCOHP between the Ag cation and the NCN anion to analyze the type of bonding (Figure 16a). The calculated band gap for Ag2NCN, about 1.45 eV, is smaller



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the heterojunction photoanode meaningfully exceeds the sum of its single constituents. Depositing Ag2NCN particles on the CuWO4 thin film yields an increased photocurrent density of 45 µA cm−2, in comparison to 15 µA cm−2 before modification, at 1.23 V vs. RHE in phosphate buffer electrolyte at pH 7 under simulated AM1.5 G illumination. The different positions of the band edges of CuWO4 and Ag2NCN favor charge carrier separation at their interface. Upon illumination, photogenerated electrons of both semiconductors are transferred to the conduction band and can migrate to the FTO surface. The calculated pCOHP indicate that the transfer of photogenerated electrons stems from antibonding N(2)-Ag and N(1)-Ag levels for Ag2NCN and an antibonding combination between copper 3d and oxygen 2p orbitals for CuWO4. Since the conduction band edge of Ag2NCN is positioned higher, its electrons can be injected into the conduction band of CuWO4. Simultaneously, holes from the cyanamide are blocked, because the valence band edge position of the oxide is positioned lower than for the cyanamide. This reflects in more efficient charge separation of electron-hole pairs and improved hole collection efficiency. Our study shows the high potential of carbodiimides and cyanamides with respect to the design of photoelectrodes beyond oxide and nitride semiconductors.

Scheme 1. Energy band diagram of heterojunction photoanode. CuWO4/Ag2NCN. In this configuration, electron-hole pairs are formed in the oxide and cyanamide constituents. Valence and conduction band edges are plotted on RHE scale.

ASSOCIATED CONTENT Supporting Information

electrode/electrolyte interface and would give thus higher photocurrent densities. We have converted the valence and conduction band edge positions for CuWO4 and Ag2NCN vs. RHE according to the values reported by Bartlett13 and Huang45, respectively (Scheme 1). The electronic band gaps of CuWO4 (2.25 eV) and Ag2NCN (2.28 eV) are almost identical and their conduction band edges of +0.4 V and +0.2 V mismatch the potential of water reduction. In contrary, the valence band edge positions of +2.65 eV for CuWO4 and +2.48 eV for Ag2NCN are more positive than the thermodynamic potential for water oxidation. As such, the different positions of the band edges of the composite photoanode favor charge carrier separation at the oxide/cyanamide interface. Upon illumination, photogenerated electrons of both semiconductors are transferred to the conduction band and can migrate to the FTO surface. Since the conduction band edge of Ag2NCN is positioned higher, its electrons can be injected into the conduction band of CuWO4. Simultaneously, holes from the cyanamide are blocked, i.e. have a lower probability, to migrate through the CuWO4 to the FTO substrate, because the valence band edge position of the oxide is positioned lower than for the cyanamide. The outcome is more efficient charge separation of electron-hole pairs and thus larger photocurrent. That is consistent with the obtained higher value of ηsep for the heterojunction photoanode CuWO4/Ag2NCN in comparison to unmodified CuWO4 (vide supra).

Additional SEM images and XRD pattern of Ag2NCN photoanode after PEC water oxidation (Figures S1-3), spectral output of solar simulator (Figure S4), and calculated pDOS for Ag2NCN and CuWO4 (Figures S5 and S6).

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

Author Contributions All authors contributed to this work. A.S. wrote the manuscript with contributions from all authors.

Funding Sources This work was financially supported by the Fonds der Chemischen Industrie and WAT Venture.

ACKNOWLEDGMENT A.S. would like to thank the Fonds der Chemischen Industrie (FCI) for a Liebig habilitation fellowship. We thank Birgit Hahn for recording SEM images, Björn Faßbänder for chemical analysis, Marek Drozdek for collecting XPS spectra, Oliver Linnenberg and Dr. Kirill Monakhov for UV-Vis measurements, and Prof. Ulrich Simon for accessibility to TEM facilities. The XPS measurements were carried out with the equipment purchased thanks to 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).

CONCLUSION We have found that the surface modification of photoanodes CuWO4 with Ag2NCN enhances the PEC efficiency. The composite photoanode displays a synergetic effect between its constituents during PEC water oxidation. The photocurrent of

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Synergetic effects between CuWO4 and Ag2NCN during PEC water oxidation


Enhanced Photoelectrochemical Water Oxidation ... - ACS Publications

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