Increased Photocurrent of CuWO4 Photoanodes by Modification with the Oxide Carbodiimide Sn2O(NCN)

aChair of Solid-State and Quantum Chemistry, Institute of Inorganic Chemistry, RWTH Aachen University, 52056 Aachen, Germany bSection of Solid State and Theoretical Inorganic Chemistry, Institute of Inorganic Chemistry, University of Tübingen, 72076 Tübingen, Germany cFaculty of Chemistry, Jagiellonian University, 30-387 Krakow, Poland dDepartment of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden eHoffmann Institute of Advanced Materials, Shenzhen Polytechnic, 7098 Liuxian Blvd, Shenzhen, China *Corresponding author. E-mail: adam.slabon@mmk.su.se


Introduction
One of the critical technical problems facing humanity is the development of a long-term sustainable energy economy. 1 This especially includes clean and renewable energy generation; a task that can be theoretically fulfilled by photoelectrochemical (PEC) water-splitting to yield hydrogen. 2 Water-splitting consists of two half reactions: the transfer of two electrons for the hydrogen evolution reaction (HER) and four electrons for the oxygen evolution reaction (OER). 3 The photo-generated charge and holes separate and migrate through the semiconductor in opposite directions to generate a photocurrent. Since the initial report on solar-driven water-splitting on a TiO 2 photo-electrode in 1972, 4 metal oxide semiconductors have been intensively investigated, and BiVO 4 has emerged as the benchmark oxide photoanode. 5 There are only few known oxidic photoanode candidates that display a smaller band gap than BiVO 4 , such as CuWO 4 with an electronic band gap in the range of 2.2-2.4 eV. 6 There are several critical characteristics for a semiconductor material in terms of realistic application as a water-splitting photoelectrode: (i) suitable conduction and/or valence band edge positions; (ii) stability under operating conditions; (iii) efficient charge carrier separation and transport along and across the thin-film electrode; (iv) low-cost and earth-abundant elements; and (v) sufficient light absorption. 5-7 Additionally, the surface of the light absorber has to be modified in most cases with a suitable catalyst to overcome poor reaction kinetics, i.e. to drive the HER and/or OER. 8 Copper tungstate is one of the few promising photoanode materials with a suitable band gap, a valence band edge (VBE) more positive than 1.23 V vs. RHE to permit water oxidation, stability under neutral conditions and composed of abundant elements. 9 It can be manufactured on differential electrically conductive substrates by electrochemical deposition, 10 atomic layer deposition (ALD), 11 spray pyrolysis, 12 and spin-casting. 13 Several strategies have been reported to improve the perform- † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c9dt04752b a Chair of Solid-State and Quantum Chemistry, Institute of Inorganic Chemistry, RWTH Aachen University, 52056 Aachen, Germany b ance of CuWO 4 by doping with zinc, 14 molybdenum 15 or iron, 12 incorporating with silver nanowires, 16 functionalizing with gold nanoparticles. 17 Charge transport is also facilitated by post-synthetic hydrogen or nitrogen treatment of CuWO 4 due to the formation of oxygen vacancies. 10,18 Recently, we have reported that surface modification of CuWO 4 with Ag 2 (NCN) 8a or Mn(NCN) 8b displays synergetic effects between its constituents, which improve PEC water oxidation efficiency.
Carbodiimides have received attention as novel materials for photochemical energy conversion in addition to their application as electrode materials for Li-ion batteries. 19 This has been mainly motivated by their suitable band gap values for solar light harvesting and a beneficial VBE position for PEC water oxidation. 20 They are related to oxides but are characterized by a higher degree of covalency. 21 The carbodiimide anion ( − NvCvN − ) is considered as a pseudo-chalcogenide anion and lies between oxide and sulfide anions in view of the HSAB concept. 22 The oxide carbodiimides are as such mixedanion compounds. The oxide carbodiimide representative Sn 2 O(NCN) was obtained recently by Meyer et al. as crystalline powder with an optical band gap of approximately 2.0 eV. 23 The title compound is closely related to tin(II) oxide (SnO); a semiconductor with a band gap of 2.8 eV as calculated on the basis of electronic band structure calculations. 24 In this work, we report on the photochemical properties of Sn 2 O(NCN) and the fabrication of heterojunction CuWO 4 /Sn 2 O (NCN) thin film photoanodes. We show that Sn 2 O(NCN) undergoes structural changes during PEC water oxidation and can augment the photocurrent of CuWO 4 photoanodes.

Synthesis of Sn 2 O(NCN)
Li 2 (NCN) was prepared as previously reported by Meyer. 23,25 Equimolar amounts of Li 2 (NCN), Na 2 O and SnCl 2 (Sigma Aldrich 99.999%, ultra dry) were mixed and ground in an agate mortar under argon. Samples of 250 mg were sealed into silica tubes under vacuum and each mixture was heated in a furnace to temperatures ranging from 450 to 500°C. The samples were kept for 12 h before cooling to room temperature. The ampoules were opened in air, the product was washed with deionized water and dried in oven at 80°C for 4 h. Sn 2 O(NCN) was obtained as a red powder.

Synthesis of CuWO 4 thin films
CuWO 4 thin films were produced on conductive fluorinedoped tin oxide (FTO) glass (2.0 mm thick, Sigma-Aldrich), based on a synthesis by Bartlett. 9a Before the electrochemical synthesis, the FTO glass was cleaned in diluted nitric acid (Sigma), acetone and ethanol, respectively. 1.26 g (3.8 mmol) sodium tungstate dihydrate (Na 2 WO 4 ·2H 2 O, 99.9%, Acros Organics) was dissolved in 15 mL deionized water by stirring, and 1 mL hydrogen peroxide (30%, Geyer Chemsolute) was added to the tungstate precursor solution. The solution was stirred for 20 min at room temperature. 25 mL deionized water and 25 mL isopropanol (>99.7%, Fisher Scientific) were added to the solution. A solution of 0.73 g (2.7 mmol) copper(II) nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O, >99%, Sigma) in 10 mL deionized water was added to the tungsten precursor solution. The pH value was adjusted to 1.2 by adding nitric acid and the solution was used for electrochemical deposition on FTO glass. The electrochemical deposition was performed in a three-electrode setup with platinum wire and 1 M Ag/AgCl (WAT Venture) as a counter electrode and a reference electrode, respectively. The electrochemical deposition was carried out by a Gamry potentiostat and the Gamry framework software package. The potential was swept in the range from −0.9 to +0.2 V vs. 1 M Ag/AgCl for 12 cycles at the scan rate of 50 mV s −1 . The working electrode was disconnected in the electrical circuit, washed with deionized water and dried at room temperature under vacuum. The working electrode was heated at 450°C for 2 h under ambient atmosphere. The excess of copper oxides was etched by immersing the working electrode into 0.5 M HCl for acidic treatment. The electrode was subsequently annealed one more time at 450°C for 30 min under ambient atmosphere.

Preparation of Sn 2 O(NCN) and CuWO 4 /Sn 2 O(NCN) photoanodes
Sn 2 O(NCN) powder was dispersed in ethanol (120 μg mL −1 ) by ultrasounds. FTO, a bare graphite (for XPS) and CuWO 4 thin film electrode were placed on a heating plate at 50°C. The Sn 2 O(NCN) dispersion was drop-casted on the surfaces of the corresponding thin film electrodes.

Structural characterization
Powder XRD patterns were recorded in transmission mode on a STOE STADI-P diffractometer (Cu K α1 radiation) operating with a DECTRIS Mythen 1K detector. For the analysis of the photoanodes by XRD, the samples were mechanically removed from the photoanodes in advance. SEM images of Sn 2 O(NCN) powder were recorded by a Leo Supra 35VP SMT (Zeiss).
A Themis Z TEM (Thermo Fisher) equipped with a SuperX energy dispersive X-ray (EDX) detector operated at 300 kV in the scanning TEM mode was used for determination of the chemical composition of Sn 2 O(NCN) particles, which were subject to chronoamperometry at 1.23 V vs. RHE. Prior to the analysis, the particles were mechanically removed from the pure Sn 2 O(NCN) electrode. XPS spectra were collected by a hemispherical VG SCIENTA R3000 analyzer using a monochromatized aluminum source Al K α (E = 1486.6 eV) at constant pass energy of 100 eV. The binding energies were referenced to the Au 4f core level (E b = 84.0 eV). The composition and chemical surrounding of the sample surface were determined on the basis of the areas and binding energies of Na 1s, K 2p, P 2p, O 1s, N 1s, C 1s and Sn 3d photoelectron peaks. The fitting of high resolution spectra was obtained by using the Casa XPS software. UV-Vis spectra were recorded on a Shimadzu UV-2600 spectrophotometer. Measurements were recorded in absorbance and reflectance mode. The Tauc plots were calculated by Kubelka-Munk function F(R) = (1 − R) 2 /2R to determine the electronic band gap.

Photoelectrochemistry
The experiments were carried out in an electrochemical cell operating in a three-electrode setup. The photoanode was used as a working electrode. Platinum wire and a 1 M Ag/AgCl electrode were used as a counter electrode and a reference electrode, respectively. All current values of the electrodes were recorded vs. 1 M Ag/AgCl reference electrode and converted vs.

Results and discussion
Structure characterization of the bulk  23 The carbodiimide anion is closely related to the oxide anion and can be regarded as an N-based pseudo-oxide. 26 For structural characterization, we tested the photoanodes for PEC water oxidation (vide infra) and removed subsequently a part of the thin film for XRD and XPS characterization. The powder XRD patterns of a pure Sn 2 O(NCN) photoanode indicate that the compound maintains its structural bulk stability after PEC water oxidation in phosphate electrolyte at pH 7.0 (Fig. 2). The corresponding powder XRD patterns of the composite CuWO 4 /Sn 2 O(NCN) photoanodes do not exhibit reflection peaks of the oxide carbodiimide due to the low amount of the latter (Fig. 3). The XRD patterns before and after PEC operation remain unchanged and match the simulated patterns of the copper tungstate.

Electronic structure
The electronic band gaps of CuWO 4 23 This would render both compounds as potential photoanode candidates for water oxidation.   Fig. S1. † Similar to SnO 2 , the oxide carbodiimide would be theoretically suited for overall water-splitting. 24,27 Photoelectrochemistry For the investigation of heterojunction photoanodes CuWO 4 / Sn 2 O(NCN), two additional electrodes of the individual components were produced. All measurements were performed in phosphate electrolyte at pH 7.0. The summarized LSV curves for all three electrodes, recorded with a scan rate at 10 mV s −1 under backlight AM 1.5G illumination (100 mW cm −2 ), are depicted in Fig. 6. The CuWO 4 photoanode developed an anodic current that reached 32 μA cm −2 at 1.23 V vs. RHE.
Upon functionalizing the surface with Sn 2 O(NCN) particles by drop-casting, the photocurrent showed an upsurge which was reached when adding 36 µg (Fig. S2 †). A bare Sn 2 O(NCN) electrode with the same amount of material as for the composite photoanode CuWO 4 /Sn 2 O(NCN) developed only a small photocurrent. The photocurrent produced by the heterojunction electrode, being equal 59 μA cm −2 at 1.23 V vs. RHE, exceeds the sum of its individual components and exhibits as such a synergistic effect. This trend is more visible during chronoamperometry (CA) with interrupted illumination (Fig. 7). The photocurrent remains relatively stable for a tested period of 2 hours (Fig. S3 †).
Since the photocurrent of the oxide carbodiimide was very small, we created another Sn 2 O(NCN) photoanode by electrophoretic deposition with a higher amount of material. Fig. 8 illustrates the results of CA for this Sn 2 O(NCN) photoanode and FTO glass, as a reference measurement, at 1.23 V vs. RHE in the same electrolyte under illumination. It can be clearly seen that the photocurrent of Sn 2 O(NCN) outperforms the substrate. As such one can rule out that the photoactivity of Sn 2 O(NCN) may be due to surface oxidation toward a tin oxide phase.    In order to elucidate the origin of increased photocurrents after Sn 2 O(NCN) functionalization, we determined the hole collection efficiency η hc . The oxidation of a hole scavenger, such as the sulfite anion, allows to determine the number of surface-reaching holes because this oxidation reaction is much faster than the sluggish oxidation of water. This allows to estimate that each hole reaching the semiconductor-electrolyte interface will be used for an oxidative reaction. The comparison of the photocurrents for sulfite oxidation ( J Na 2 SO 3 ) and water oxidation ( J H 2 O ) allows to calculate η hc = (J H 2 O /J Na 2 SO 3 ). For the CuWO 4 thin films, the η hc value significantly increases upon functionalization with Sn 2 O(NCN) at higher potentials (Fig. 9). This indicates that the reactivity of the surface increases upon functionalization with the oxide carbodiimide.
To investigate the key factors of improved PEC performance, we analyzed the surface of bare Sn 2 O(NCN) photoanodes after the PEC experiment by XPS. The electrode was prepared on graphite instead of FTO substrate to avoid interference with Sn stemming from FTO. Except the presence of O, C, N and Sn, XPS reveals the presence of Na, K and P which originate from the K/NaP i electrolyte. In Fig. 10, the high-resolution XPS Sn 3d spectrum, with the Sn 3d 5/2 and Sn 3d 3/2 peaks distanced at the splitting energy of 8.4 eV, is shown. The latter one is partially overlapped with the Na KLL Auger signal. The relative high values of binding energies found for both components (487.5 eV for Sn 3d 5/2 and 495.9 eV for Sn 3d 3/2 ) obviously suggest that Sn 2 O(NCN) formed a phosphate-type shell on the surface being exposed to the phosphate electrolyte, i.e. a core-shell structure Sn 2 O(NCN)@SnPO x . This indicates that the catalytically active form is a tin phosphate shell while the core contains the semiconducting Sn 2 O(NCN). Similar positions of the Sn 3d signals have been observed recently for tin phosphate (SnPO x ) in relation to SnO 2 , which exhibited the Sn 3d 5/2 and Sn 3d 3/2 components at 486.0 eV and 494.4 eV, respectively. 28 Different from the behavior of Co (NCN) electrocatalysts, 29 the electrochemical activation is similar to the behavior of Mn(NCN) electrocatalysts 8b as previously reported. Tin phosphate is known as a heterogeneous catalyst for efficient dehydration of glucose into 5-hydroxymethylfurfural in ionic liquid according to the literature    report. The fourfold coordinated Sn 4+ sites from tin phosphate are identified as the active species. 28 The presence of phosphorous was also confirmed by complementary TEM EDX analysis, which was performed on Sn 2 O (NCN) particles that were mechanically removed from a FTO/ Sn 2 O(NCN) thin film electrode after 30 min of CA at 1.23 V vs. RHE. The high-angle annular dark field (HAADF) images in Fig. 11 shows agglomerated particles that were scanned by means of EDX. Besides phosphorous, the presence of tin, oxygen, carbon and nitrogen could be confirmed, too.
The augmented charge carrier separation for the heterojunction can be understood by analyzing the energy band diagram of both semiconductors (Fig. 12). 8a The energetically higher position of the CBE for Sn 2 O(NCN) in comparison to CuWO 4 enables injection of electrons into the conduction band of the latter. At the same time, the photogenerated holes can diffuse from the VBE of CuWO 4 to Sn 2 O(NCN). This results in decreased recombination of photogenerated electrons and holes. In addition to the improved hole collection efficiency, being the consequence of the tin phosphate shell, the PEC water oxidation efficiency could be further increased by 10% when depositing cobalt phosphate as co-catalyst on the surface of the heterojunction photoanode (Fig. S4 †).

Conclusions
We have investigated the photochemical properties of Sn 2 O (NCN) and showed that this n-type semiconductor can be successfully coupled to CuWO 4 thin film photoanodes. Sn 2 O (NCN) exhibits a flat-band potential of approx. −0.03 V and as such, the position of the valence band edge would be suitable for photochemical water oxidation. During PEC water oxidation in phosphate buffer electrolyte Sn 2 O(NCN) undergoes an in situ transformation to a core-shell structure; maintaining a semiconducting core while forming an electrocatalyticallyactive SnPO x shell. The obtained composite CuWO 4 /Sn 2 O (NCN)@SnPO x photoanodes display a synergetic effect between its constituents during the PEC water oxidation, which shows up in an upsurge of photocurrent from 32 μA cm −2 to 59 μA cm −2 at 1.23 V vs. RHE at pH 7.0 under simulated AM 1.5G illumination. This is due to improved charge carrier separation and augmented hole collection efficiency. Our study demonstrates that mixed-anion compounds containing an oxidic and carbodiimide anion are potential materials for photochemical oxidation reactions, while at the same time, the surface can be electrochemically activated into a catalytically-active form.

Conflicts of interest
There are no conflicts to declare.