Nanostructured IrOx Coatings for Efficient Oxygen Evolution Reactions in PV-EC Setup

Abstract New heteroleptic iridium compounds exhibiting high volatility and defined thermal decomposition behavior were developed and tested in plasma-enhanced chemical vapor deposition (PECVD). The iridium precursor [(COD)Ir(TFB-TFEA)] (COD = 1,5-cyclooctadiene; TFB-TFEA = N-(4,4,4-Trifluorobut-1-en-3-on)-6,6,6-trifluoroethylamin) unifies both reactivity and sufficient stability through its heteroleptic constitution to offer a step-by-step elimination of ligands to provide high compositional purity in CVD deposits. The substitution of neutral COD ligands against CO groups further increased the volatility of the precursor. PECVD experiments with unambiguously characterized Ir compounds (single crystal X-ray diffraction analysis) demonstrated their suitability for an atom-efficient (high molecule-to-precursor yield) gas phase deposition of amorphous iridium oxide (IrOx) phases. Thin films of IrOx were well suited as electrocatalyst in oxygen evolution reaction so that an efficient coupled system in combination with solar cells is viable to perform water-splitting reaction without external bias.


Introduction
Iridium oxide is one of the most efficient electrocatalysts used in oxygen evolution reaction (OER) because of its favorable combination of properties including high corrosion resistance and the low overpotential (ca. 0.3 V) required for proceeding the reaction [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Research on water splitting reaction has focused mainly on catalysis of the oxygen evolution reaction because of the sluggish kinetics compared with hydrogen evolution reaction (HER) [15,16]. Although in the natural photosystems the OER is catalyzed by a manganese oxide cluster, iridium-based materials offer promising opportunities to perform this kinetically challenging OER in the laboratories [17]. One of the key experimental findings about the relationship between activity and stability of iridium-based electrodes is the identification of transient surface states characterized by the formation of nonstoichiometric IrO x species that lead to an enhanced OER performance [5,[18][19][20]. Water splitting, coupled with wind or solar energy generators, produces hydrogen which is an attractive alternative energy source. Furthermore pure H 2 is generated at the cathode that can be used as a fuel, with O 2 as the only waste by-product, which makes water splitting an attractive route for producing an environmentally clean fuel [21][22][23]. In this context plasma-enhanced chemical vapor deposition (PECVD) processes enable reproducible synthesis of catalytically active IrO x thin films, which however requires molecular compounds with substantial vapor pressure essential to achieve iridium feedstock in the gas phase [24,25]. In the context of iridium precursor chemistry, known metal-organic compounds like metal alkoxide, carbonyl or β-diketonate complexes are used that suffer from low vapor pressure and high decomposition temperatures thereby providing little control over film growth conditions [26]. Moreover, in many cases, use of reactive gases is necessary to accelerate the precursor flow in the reactor that results in compositional gradients due to limited ad-atom mobilities. Since the volatility of precursors strongly depends on inter-and intramolecular interactions as well as the molecular weight of the compound, one of the synthetic challenges is to find strategies to decrease molecular weight and nuclearity of precursors to render them kinetically labile. We demonstrate here that application of neutral chelating ligands increases the stability of complexes and allows a careful design of heteroleptic precursors showing sufficient volatility and adequate reactivity and accessibility to modify precursor properties ( Figure 1) [27][28][29][30][31][32][33][34].
We report herein the solid-state and solution phase structure of new heteroleptic iridium(I) precursor [(CO) 2 Ir(TFB-TFEA)] (5) demonstrate its intrinsic advantages as molecular precursor for synthesis of iridium-based films via PECVD at low temperatures. Furthermore, quantitative conversion of amorphous iridium oxide films into phase-pure iridium dioxide films and electrocatalytic OER behavior in PV-EC coupled systems is described.
Compound 5 was investigated by EI mass spectrometry at an ionization energy of 70 eV and evaporation temperatures of 30°C (10 −6 mbar). The mass spectrum was dominated by the molecular ion [M] +• base peaks. Thermal behavior of the precursor investigated by thermogravimetric (TG) analysis ( Figure 3) showed a complete evaporation of compound 5 without any decomposition in the temperature range 100-200°C. This demonstrated high volatility of the new precursors compared to conventionally used Ir-precursors and an enhanced feedstock in the gas phase. This also indicated the challenges associated with high volatility that can influence the growth rate and materials yield due to intact transport of precursor to the cold trap.

Plasma-enhanced chemical vapor deposition of [(CO) 2 Ir(TFB-TFEA)]
Amorphous iridium oxide (IrO 2-x ) thin films were deposited by decomposition of [(CO) 2 Ir(TFB-TFEA)] (5) by radio frequency PECVD onto silicon, aluminum oxide, FTO, Au and Au@Cu substrates with pure O 2 gas as the reactive gas and maintaining the precursor reservoir at the room temperature. In X-ray diffraction of as deposited thin films no peaks of crystalline materials could be detected to conclude that those films are amorphous. XPS analysis showed that as deposited iridium oxide films for deposition time periods of 60 min ( Figure 4, above), whereas annealing of iridium-based films produced iridium dioxide films that exhibited hydroxide groups on the surface (Figure 4, below) [37]. Additionally disproportionation reactions of subvalent Ir(I) precursor to elemental iridium and iridium (IV) oxides could possibly occur in as-deposited films.
The successful deposition of iridium oxide films on single crystal (Si), amorphous (SiO 2 ) and polycrystalline (Al 2 O 3 ) substrates showed the universal applicability of the precursor and its chemically-controlled decomposition irrespective of the surface chemistry of the substrate material. Obtained XPS spectra revealed the formation of iridium oxide and dioxide deposits as a function of the deposition and post-treatment conditions [38]. XRD patterns of annealed IrO 2 thin films at Si substrates are depicted in supporting information. Sublimation at room temperature (25°C) using compound 5 and deposition of iridium-based coating has been reported for the first time and enriches the family of precursors due to its high volatility and defined deposition in gas phase deposition processes.

Application of iridium-based coatings in oxygen evolution
The electrochemical activity of iridium layers on different kind of substrates was evaluated in a three electrode system in alkaline media. To analyze the potential of IrO 2-x films in the electrochemical water splitting, they have been analyzed regarding they morphology by SEM ( Figure 5, additional micrographs see supporting information). IrO x @Au@Cu (deposited by PECVD) and Ir@Au (MOCVD) are representatively depicted in Figure 5.
They were deposited onto different conductive substrates (FTO, Au@Cu and Au) to obtain rough nanotopographies of iridium-based materials in the size range of ∼50 nm and below. Their electrochemical performances were comparatively analyzed. A platinum mesh was used as counter electrode while Ag|AgCl served as reference electrode. Figure 6 demonstrates high electrochemical activity of all iridium-based thin films obtained from different deposition methods (MOCVD, PECVD; Figure 6, left) with overpotential values in the range of 0.35-0.60 V (measured potentials: 1.58-1.83 V) at 10 mA/cm 2 (vs. Ag/AgCl). The lowest overpotential values of ∼1.6 mV was found for IrO x @Au@Cu (PECVD). Regarding the potential over time all samples, expect for IrO x @FTO (PECVD) and IrO x @Au (PECVD), show a stabile behavior under the applied measurement conditions (900 s in 1 M KOH) (Figure 6, right). Figure 6 reveals that films deposited by PECVD (Figure 6; right; black and blue lines) showed highest potentials lowest stability in alkaline media. Furthermore, as deposited films also showed increased current densities at lower potentials ( Figure 6; left; green, blue and light blue lines). The most active layers offering lowest overpotentials (Ir@FTO (MOCVD); IrO x @Au (PECVD); IrO x @Au@Cu  (PECVD)) were investigated in a customized two electrode setup [39,40] as OER catalyst in combination with an a-Si:H/a-Si:H/µc-Si:H (a-Si:H = hydrogenated amorphous silicon, µc-Si:H = hydrogenated microcrystalline silicon) thin film silicon solar cell that operate at V OC ≈ 2.2 V with a platinum plate as HER (Figure 7, right) [41,42]. Furthermore a heat-treated oxide film (IrO 2 @Au (MOCVD-annealed) as well as a crystalline metallic film (Ir@Au (MOCVD) [27]) were investigated towards their light induced water splitting potential. In all the experiments, bias  [40] in a two electrode setup with iridium-based thin films as OER catalyst (left). Current densities-voltage curves (J-V) measured for different PV-EC devices employing the shown solar cell + HER catalyst combination (left side) and different OER catalysts under illumination using 1 M KOH as electrolyte. Bias free water splitting is enabled at 0 V. free solar driven water splitting was possible and the generation of H 2 and O 2 was observed by eye (Figure 7, right). A current density of −3 mA/cm 2 at 0 V was observed for as-deposited Ir(0) films on FTO substrates. Other samples showed an increased activity of ∼−3.5 mA/cm 2 at 0 V.

Conclusion
A new class of heteroleptic iridium(I) complexes containing both anionic and neutral multidentate ligands demonstrated a synergistic interplay of reactivity and stability while pointing new possibilities of ligand modification and precursor design (Figure 1). Through the availability of a new class of iridium precursors for an efficient gas phase transport, the efficiency of thin film deposition processes was enhanced. The controlled deposition of metallic (Ir) and oxide (IrO 2-x ) films on various substrate materials underlines the potential of new precursor chemistries enabling low-temperature deposition of catalytically active coatings with high reproducibility. Application of the Ir-based coatings in an integrated water splitting device combining PV and PEC technology offers possibilities of splitting water by solar energy without an external bias.

Chemicals and methods
All manipulations of air and moisture sensitive materials were carried out under nitrogen using all glass assemblies. NMR spectra were recorded on a Bruker Avance II spectrometer operating at 1 H: 300.1 MHz; 13 C: 75.5 MHz; 19 F: 282.4 MHz, chemical shifts δ are given in part per million (ppm) relative to TMS ( 1 H, 13 C) and CFCl 3 ( 19 F) and coupling constants (J) are given in Hz. NMR spectra have been plotted by Bruker TopSpin 3.5 pl7 [43] software. Mass spectra were recorded on a Finnigan MAT 95 (EI, 70 eV) in m/z (intensity in %). Data collection for Xray structure elucidation was performed on a STOE IPDS II diffractometer using monochromated Mo-K α radiation (λ = 0.71071 Å). The programs used in this work are STOE's X-Area [44] and the WINGX suite of programs [45], including SIR-92 [46], SHELX & SHELXL [47] and PLATON [48] for structure solution and refinement. H-atoms were calculated geometrically and a riding model was applied during the refinement process. TG measurements were performed on a TGA/DSC1 (METTLER-TOLEDO GmbH, Germany) apparatus. , 1) was prepared by dissolving 2.9 mL (30 mmol, 1.0 equiv.) ethyl vinyl ether and 12.1 mL (150 mmol, 5.0 equiv.) pyridine were dissolved in 80 mL DCM. This solution was cooled to 0°C under stirring. Over the course of 30 min 4.3 mL (30 mmol, 3.0 equiv.) trifluoroacetic acid anhydride was added dropwise. This solution was left to stir overnight at rt. Afterwards 300 mL of 3% Na 2 CO 3(aq) were added. The mixture was extracted with EtOAc (4 × 200 mL). The organic phase was washed with 4 × 100 mL brine and the solvent was removed under reduced pressure. The crude product was purified by vacuum distillation (40°C, 10 −3 mbar) and was obtained as a yellow oil 1 that was used in further synthesis of compound 2. Yield: 69% (3.48 g). 1  [(COD)IrCl] 2 (3) was prepared following a modified procedure described by Tani et al. [49] To a stirred solution of 0.441 g (NH 4 ) 2 [IrCl 6 ] (1.0 mmol, 1.0 equiv.) in a 2:1 mixture of H 2 O and isopropyl alcohol (15 mL) 0.98 mL cycloocta-1,5-diene (COD) (8.0 mmol, 8.0 equiv.) was slowly added. The resulting solution was heated to reflux for 12 h when a red-orange solid precipitated out. After cooling to rt the reaction mixture was evaporated to dryness and the crude product was extracted with 20 mL of n-pentane. After decanting and removal of the excess solvent, a red-orange solid (3)  [(CO) 2 Ir(TFB-TFEA)] (5) 1.5 mmol (2.1 equiv.) of the sodium salt of bidentate ligand 2 (2b) was placed in a flask. Afterwards 0.470 g (0.7 mmol, 1.0 equiv.) of [(COD)IrCl] 2 (3) dissolved in 30 mL anhydrous THF were added to the sodium compound (2b). The mixture was stirred for 4 h at room temperature. Afterwards CO was vigorously bubbled through the reaction mixture for 10 min. The solvent was removed under reduced pressure and the crude product was sublimed (45°C, 10 −3 mbar) in dynamic vacuum to afford red crystals 5. Yield: 0.402 g (0.86 mmol, 86%), 1

CVD experiments
Thin film deposition was performed in a horizontal cold-wall MOCVD reactor equipped with an inductive field heating. The substrates (1.3 cm × 1.3 cm) were heated through graphite susceptor. The molecular precursor was introduced to the reactor through a glass flange by applying a dynamic vacuum (10 −3 mbar) and heating the precursor reservoir to the desired temperature. No carrier or reactive gases were used [50]. Also thin films were deposited by radio frequency PECVD onto FTO, Au and Au@Cu substrates using precursor 5 as iridium source and pure O 2 gas as additional oxygen source. The as-grown films were amorphous IrO x and were annealed for 5 h in air at 750°C to obtain crystalline IrO 2 as reported elsewhere [27].

Materials characterization
Room-temperature powder X-ray diffraction (XRD) was obtained on a STOE-STADI MP diffractometer operating in the reflection mode using Cu-K α (λ = 1.5406 Å) radiation. The microstructures of the samples were examined using fieldemission scanning electron microscopy (FE-SEM, FEI Nova NanoSEM 430, see supporting information). Water splitting measurements were carried out in a two electrode setup with a 1 M KOH electrolyte using a "Oriel LCS-100, Newport, model 94011A" to simulate the solar spectrum and data was collected with "Gamry Reference 600 Potentiostat". As current-voltage source a thin film silicon solar cell (a-Si:H/a-Si:H/µc-Si:H) equipped with platinum as hydrogen evolution catalyst has been used in an in-house produced cell that has been invented at Forschungszentrum Jülich GmbH [39,40]. As OER iridium containing films have been deposited on different kind of substrates (see above) at University of Cologne.

Supporting information available
NMR, EI mass spectra and XRD pattern are presented in SI. X-ray crystallographic data files (CCDC 1908527) in CIF format are available free of charge via the Internet.