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

Article abstract of DOI:10.1515/zpch-2019-1450

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 (IrO_x) phases. Thin films of IrO_x 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.

Full text of DOI:10.1515/zpch-2019-1450

Z. Phys. Chem. 2019; aop
Lasse Jürgensen, Michael Frank, David Graf, Isabel Gessner,
Thomas Fischer, Katharina Welter, Wolfram Jägermann and
Sanjay Mathur*
Nanostructured IrOx Coatings for Eflcient
Oxygen Evolution Reactions in PV-EC Setup
https://doi.org/10.1515/zpch-2019-1450
Received April 16, 2019; accepted November 13, 2019
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 precur-
sor [(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 suf-
ficient 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 character-
ized 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 cou-
pled system in combination with solar cells is viable to perform water-splitting
reaction without external bias.
Keywords: advanced materials; CVD; iridium oxide; iridium precursor; nanos-
tructured films.
1 Introduction
Iridium oxide is one of the most efficient electrocatalysts used in oxygen evolu-
tion reaction (OER) because of its favorable combination of properties including
*Corresponding author: Sanjay Mathur, Institute of Inorganic Chemistry, University of Cologne,
Greinstrasse 6, D-50939 Cologne, Germany, e-mail: sanjay.mathur@uni-koeln.de
Lasse Jürgensen, Michael Frank, David Graf, Isabel Gessner and Thomas Fischer: Institute of
Inorganic Chemistry, University of Cologne, Greinstrasse 6, D-50939 Cologne, Germany
Katharina Welter: Forschungszentrum Jülich GmbH, D-52425 Juelich, Germany
Wolfram Jägermann: Technical University Darmstadt, D-64287 Darmstadt, Germany
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high corrosion resistance and the low overpotential (ca. 0.3 V) required for pro-
ceeding the reaction [1–14]. 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 IrOx species that lead to an enhanced OER performance
[5, 18–20]. Water splitting, coupled with wind or solar energy generators, pro-
duces hydrogen which is an attractive alternative energy source. Furthermore pure
H2 is generated at the cathode that can be used as a fuel, with O2 as the only
waste by-product, which makes water splitting an attractive route for producing
an environmentally clean fuel [21–23]. In this context plasma-enhanced chemical
vapor deposition (PECVD) processes enable reproducible synthesis of catalyti-
cally active IrOx 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 com-
pounds like metal alkoxide, carbonyl or β-diketonate complexes are used that
suffer from low vapor pressure and high decomposition temperatures thereby pro-
viding little control over film growth conditions [26]. Moreover, in many cases,
use of reactive gases is necessary to accelerate the precursor flow in the reac-
tor 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 syn-
thetic challenges is to find strategies to decrease molecular weight and nucle-
arity 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)
[27–34].
We report herein the solid-state and solution phase structure of new het-
eroleptic iridium(I) precursor [(CO)2Ir(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 irid-
ium oxide films into phase-pure iridium dioxide films and electrocatalytic OER
behavior in PV-EC coupled systems is described.
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Nanostructured IrOx Coatings for Eflcient Oxygen Evolution
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Fig. 1: Synthesis of Janus-headed iridium precursors by ligand modification to manipulate
volatility and electronic properties of molecular complexes.
2 Results and discussion
2.1 Precursor synthesis
The bidentate ligand 2 was prepared by reacting ethyl vinyl ether with trifluo-
roacetic acid anhydride in presence of pyridine under elimination of pyridinium
trifluoroacetate to get 1-Ethoxy-4,4,4-trifluorobut-1-en-3-on (ETFB 1). Addition of
the fluorinated amine in toluene led to N-(4,4,4-Trifluorobut-1-en-3-on)-6,6,6-
trifluoroethylamin (HTFB-TFEA 2) under elimination of EtOH. The heteroleptic
iridium(I) precursor [(CO)2Ir(TFB-TFEA)] (5) was synthesized via salt metathe-
sis reaction by reacting dimeric [(COD)IrCl]2 (4) and the sodium salts of 2 (2b,
Scheme 1) [27, 35]. Afterwards CO was passed through the reaction mixture to
obtain precursor 5.
Scheme 1: Synthetic route to heteroleptic monomeric Ir(I) compound (5) from a dimeric
compound (3).
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L. Jürgensen et al.
The heteroleptic iridium(I) complex (5) was isolated as air-stable crystalline
red solid by sublimation in dynamic vacuum. The sublimation temperature of 5
(45 °C, 10−3 mbar) could be significantly lowered compared to similar compounds
such as [(COD)Ir(ThTFP)] (110 °C, 10−3 mbar) [27], [Ir(acac)3] and [(COD)Ir(MeCp)]
(180 °C, 100 °C, 10−3 mbar) [26, 27] which are conventionally used as precursors
in the CVD of iridium-based films. The higher volatility of [(COD)Ir(TFB-TFEA)]
compounds is attributed to the lower molecular weight and higher covalent char-
acter verified by the bond lengths determined through single crystal structure
analysis.
2.2 Structural characterization of [(CO)2Ir(TFB-TFEA)]
1H NMR spectra (see Supporting Information) of compound (5) displayed two dou-
blets for vinylic protons of the TFB-TFEA ligand at δ = 7.91 ppm (3J = 6.9 Hz)
and δ = 6.04 ppm (3J = 6.9 Hz). CH2 protons of TFB-TFEA compounds appeared
at δ = 4.66 ppm as a quartet due to coupling to C8-F3 group. Furthermore, no
COD signals could be detected for compound (5) but carbonyl ligands could be
detected in the 13C DEPT spectrum at δ = 171.6 ppm and δ = 169.5 ppm (C1/2).
19F NMR spectra (see Supporting Information) of compound (5) revealed that the
compound is pure and no secondary impurities coexist.
Single crystal X-ray diffraction analysis of heteroleptic iridium(I) complex (5)
showed a distorted square planar coordination around iridium, as observed in
other iridium(I) compounds such as [(COD)Ir(ThTFP)] (Figure 2) [27, 36]. The dis-
torted geometry of compound (5) originated from the angle between C1 and nitro-
gen atom N1 (C1–Ir01–N1) (95.4 ± 0.2)° that is larger than the C2–Ir01–O3 angle
Fig. 2: Molecular structures of [(CO)2Ir(TFB-TFEA)] (5). Thermal ellipsoids are drawn at 50%
probability level.
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Nanostructured IrOx Coatings for Eflcient Oxygen Evolution
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Fig. 3: Thermogravimetric analysis of heteroleptic Ir(I) compound (5).
with (87.5 ± 0.2)° and C1–Ir01–C2 angle (87.4 ± 0.2)° due to higher steric profile
of –CH2CF3 group present at the amine.
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 behav-
ior 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 feed-
stock 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.
2.3 Plasma-enhanced chemical vapor deposition of
[(CO)2Ir(TFB-TFEA)]
Amorphous iridium oxide (IrO2–x) thin films were deposited by decomposition
of [(CO)2Ir(TFB-TFEA)] (5) by radio frequency PECVD onto silicon, aluminum
oxide, FTO, Au and Au@Cu substrates with pure O2 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
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L. Jürgensen et al.
Fig. 4: High resolution Ir 4f (left) and O 1s (right) XPS spectra of as deposited iridium oxide films
(above) and annealed iridium dioxide films (below) deposited on Si substrates.
(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 ele-
mental iridium and iridium (IV) oxides could possibly occur in as-deposited
films.
The successful deposition of iridium oxide films on single crystal (Si), amor-
phous (SiO2) and polycrystalline (Al2O3) substrates showed the universal appli-
cability of the precursor and its chemically-controlled decomposition irrespec-
tive 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 IrO2
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 precur-
sors due to its high volatility and defined deposition in gas phase deposition
processes.
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2.4 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 IrO2–x films in the electrochemical water splitting, they have been analyzed
regarding they morphology by SEM (Figure 5, additional micrographs see sup-
porting information). IrOx@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 (mea-
sured potentials: 1.58–1.83 V) at 10 mA/cm2 (vs. Ag/AgCl). The lowest overpo-
tential values of ∼1.6 mV was found for IrOx@Au@Cu (PECVD). Regarding the
potential over time all samples, expect for IrOx@FTO (PECVD) and IrOx@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 offer-
ing lowest overpotentials (Ir@FTO (MOCVD); IrOx@Au (PECVD); IrOx@Au@Cu
Fig. 5: SEM micrographs of as deposited Ir-based films by PECVD (left) and MOCVD (right) on
different kind of Au-based substrates.
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Fig. 6: Current densities J of different prepared electrodes against applied potential E (left) and
time dependent potentials E of iridium-based substrates at different deposition condition at
10 mA/cm2 (1 M KOH, right).
(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 VOC ≈ 2.2 V with a platinum plate as HER (Figure 7,
right) [41, 42]. Furthermore a heat-treated oxide film (IrO2@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
Fig. 7: Schematic overview of a-Si:H/a-Si:H/µc-Si:H (a-Si:H = hydrogenated amorphous sili-
con, µc-Si:H = hydrogenated microcrystalline silicon) thin film silicon solar cell with Pt as HER
catalyst [40] in a two electrode setup with iridium-based thin films as OER catalyst (left). Cur-
rent 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.
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Nanostructured IrOx Coatings for Eflcient Oxygen Evolution
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free solar driven water splitting was possible and the generation of H2 and O2
was observed by eye (Figure 7, right). A current density of −3 mA/cm2 at 0 V was
observed for as-deposited Ir(0) films on FTO substrates. Other samples showed an
increased activity of ∼−3.5 mA/cm2 at 0 V.
3 Conclusion
A new class of heteroleptic iridium(I) complexes containing both anionic and neu-
tral 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 pro-
cesses was enhanced. The controlled deposition of metallic (Ir) and oxide (IrO2–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.
4 Experimental
4.1 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 1H: 300.1 MHz; 13C: 75.5 MHz; 19F: 282.4 MHz,
chemical shifts δ are given in part per million (ppm) relative to TMS (1H, 13C)
and CFCl3 (19F) 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 X-
ray structure elucidation was performed on a STOE IPDS II diffractometer using
monochromated Mo–Kα 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 refine-
ment. 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.
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4.2 Precursor synthesis
1-Ethoxy-4,4,4-trifluorobut-1-en-3-on (ETFB, 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% Na2CO3(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 puri-
fied 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). 1H NMR
(300.1 MHz, 298 K, CDCl3) δ [ppm]: 7.84 (d, 1H, 2-H, 3JH,H = 12 Hz), 5.80 (d, 1H, 3-
H, 3JH,H = 12 Hz), 4.06 (q, 2H, 6-H), 1.39 (t, 3H, 7-H). 19F NMR (282.4 MHz, 298 K,
CDCl3) δ [ppm]: −78.8 (1JF,C = 284 Hz, 2JF,C = 32 Hz).
N-(4,4,4-Trifluorobut-1-en-3-on)-6,6,6-trifluoroethylamin (HTFB-TFEA,
2) 5.04 g (30 mmol, 1.0 equiv.) of compound 1 was dissolved in 80 ml toluene and
was cooled to 0 °C under stirring. Over the course of 30 min 2.4 mL (30 mmol, 1.0
equiv.) 2,2,2-trifluoroethylamine was added dropwise. This solution was left to stir
for 1 h. A solid was crystallized from the reaction mixture, washed with n-pentane
and recrystallized in a 1:1 toluene/n-pentane mixture, yielding a colorless solid 2.
Yield: 84% (5.57 g). 1H NMR (300.1 MHz, 298 K, CDCl3) δ [ppm]: 9.99 (s, 1H, NH);
7.05 (dd, 3JH,H = 8 Hz, 1H, 4-H); 5.54 (d,3JH,H = 8 Hz, 1H, 2-H); 3.82 (m, 2H, 5-H).
19F NMR (282.4 MHz, 298 K, CDCl3) δ [ppm]: −73.2 (6-F); −77.4 (4-F). Elemental
Analysis calc. for C6H4F6NO: C 34.44, H 3.30, N 5.74 found: C 34.12, H 2.88, N 6.51.
Sodium salt of bidentate ligand (NaTFB-TFEA (2b)) was prepared by react-
ing with NaOEt. Therefore 0.023 g (1.0 mmol, 1.0 equiv.) of sodium were added
to 10 mL of ethanol. After stirring for 30 min 1.0 mmol (1.0 equiv.) of bidentate
ligand (2) were added to the solution. After removal of the excess solvent, an
off-white solid (2b) was obtained. Yield: (1.0 mmol, >99%). 1H NMR (300.1 MHz,
298 K, CDCl3) δ [ppm]: 7.57 (m, 1H, 4-H); 5.12 (d,3JH,H = 5 Hz, 1H, 3-H); 3.74 (m, 1H,
5-H). 19F NMR (282.4 MHz, 298 K, CDCl3) δ [ppm]: −72.8 (6-F); −76.7 (4-F).
[(COD)IrCl]2 (3) was prepared following a modified procedure described by
Tani et al. [49] To a stirred solution of 0.441 g (NH4)2[IrCl6] (1.0 mmol, 1.0 equiv.)
in a 2:1 mixture of H2O 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,
1
a red-orange solid (3) was obtained. Yield: 0.470 g (0.70 mmol, 70%), H NMR
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(300.1 MHz, 298 K, CDCl3) δ [ppm]: 4.24-4.23 (m, 4H, 1-H); 2.29–2.24 (m, 4H, 2-
H); 1.57–1.49 (m, 4H, 2-H). 13C NMR (75.5 MHz, 298 K, CDCl3) δ [ppm]: 62.4 (C1);
31.9 (C2). Elemental Analysis calc. for C16H24Cl2Ir2: C 28.61, H 3.60 found: C 29.12,
H 2.98.
[(CO)2Ir(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%), 1H NMR (300.1 MHz, 298 K, CDCl3) δ [ppm]: 7.91 (d, 3JH,H = 8 Hz, 1H, 6-H);
6.04 (d,3JH,H = 8 Hz, 1H, 5-H); 4.66 (m, 2H, 7-H). 13C NMR (75.5 MHz, 298 K, CDCl3)
δ [ppm]: 171.6 (C1); 169.5 (C2); 163.8 (C6); 95.4 (C5); 68.7 (C7). 19F NMR (282.4 MHz,
298 K, CDCl3) δ [ppm]: −70.9 (6-F); −73.3 (4-F). EI-MS (70 eV, 30 °C): m/z (inten-
sity) = 468 (100%) [M]+•, 412 (64%) [M-2 CO]+•, 18 (66%) [H2O]+•. Elemental
Analysis calc. for C8H4F6NO3Ir: C 20.52, H 0.86, N 2.99 found: C 20.73, H 1.07, N 3.17.
4.3 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
O2 gas as additional oxygen source. The as-grown films were amorphous IrOx
and were annealed for 5 h in air at 750 °C to obtain crystalline IrO2 as reported
elsewhere [27].
4.4 Materials characterization
Room-temperature powder X-ray diffraction (XRD) was obtained on a STOE-STADI
MP diffractometer operating in the reflection mode using Cu–Kα (λ = 1.5406 Å)
radiation. The microstructures of the samples were examined using field-
emission 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
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94011A” to simulate the solar spectrum and data was collected with “Gamry Refer-
ence 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 Forschungszen-
trum Jülich GmbH [39, 40]. As OER iridium containing films have been deposited
on different kind of substrates (see above) at University of Cologne.
5 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.
Acknowledgment: The authors gratefully acknowledge the University of Cologne
and the Regional Research Cluster Sustainable Chemical Synthesis 2.0 for finan-
cial support. We thank the German Science Foundation (DFG) for financial sup-
port in the frame of the priority program (SPP 1613) PECULIAR and the Univer-
sity of Cologne for providing infrastructure and financial assistance. The authors
declare no competing financial interest.
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