Subvalent Iridium Precursors for Atom-Efficient Chemical Vapor Deposition of Ir and IrO2 Thin Films

Article abstract of DOI:10.1021/acs.organomet.7b00275

A new heteroleptic Ir(I) compound exhibiting high volatility and defined thermal decomposition under CVD conditions is reported. The new iridium precursor [(COD)Ir(ThTFP)] (COD = cyclooctadiene, ThTFP = (Z)-3,3,3-trifluoro-1-(thiazol-2-yl)prop-1-en-2-olate) unifies both reactivity and sufficient stability through its heteroleptic constitution to provide a precise control over compositional purity in CVD deposits. The solution integrity of the monomeric Ir(I) complex was investigated by 1D and 2D NMR spectroscopy and EI mass spectrometry, whereas the molecular structure was confirmed by single-crystal diffraction. CVD experiments demonstrated the suitability of the iridium compound for an atom-efficient (high molecule-to-precursor yield) gas-phase deposition of nanocrystalline iridium films that could be converted into crystalline iridium dioxide upon heat treatment to demonstrate their electrocatalytic potential in the oxygen evolution reaction.

Full text of DOI:10.1021/acs.organomet.7b00275

Article
pubs.acs.org/Organometallics
Subvalent Iridium Precursors for Atom-Efficient Chemical Vapor
Deposition of Ir and IrO2 Thin Films
Lasse Jurgensen, Michael Frank, Myeongwhun Pyeon, Lisa Czympiel, and Sanjay Mathur*
̈
Institute of Inorganic Chemistry, University of Cologne Greinstrasse 6, D-50939 Cologne, Germany
S
* Supporting Information
ABSTRACT: A new heteroleptic Ir(I) compound exhibiting high volatility
and defined thermal decomposition under CVD conditions is reported. The
new iridium precursor [(COD)Ir(ThTFP)] (COD = cyclooctadiene,
ThTFP = (Z)-3,3,3-trifluoro-1-(thiazol-2-yl)prop-1-en-2-olate) unifies both
reactivity and sufficient stability through its heteroleptic constitution to
provide a precise control over compositional purity in CVD deposits. The
solution integrity of the monomeric Ir(I) complex was investigated by 1D
and 2D NMR spectroscopy and EI mass spectrometry, whereas the
molecular structure was confirmed by single-crystal diffraction. CVD
experiments demonstrated the suitability of the iridium compound for an
atom-efficient (high molecule-to-precursor yield) gas-phase deposition of
nanocrystalline iridium films that could be converted into crystalline iridium
dioxide upon heat treatment to demonstrate their electrocatalytic potential
in the oxygen evolution reaction.
Scheme 1. Janus-Headed Precursors To Combine Structural
Stability with Electronic Flexibility
INTRODUCTION
■
Iridium is one of the less earth-abundant elements; however, its
physical and chemical properties are of growing interest,1−3
especially due to the applications of iridium and iridium dioxide
thin films in catalytic oxygen evolution reactions (OER) and
optoelectronic applications.4−15 The superior hardness and
interesting conducting properties of iridium and iridium dioxide
demand new methods to produce iridium-based thin films in an
economical and ecological way.16−18 In this context, the
chemical vapor deposition (CVD) process offers a promising
solution, which however requires molecular compounds with
substantial vapor pressure essential to achieve iridium feedstock
in the gas phase.19−21 Typically, known metal−organic
compounds such as metal alkoxide, metal carbonyl, or β-
diketonate complexes are used that suffer from low vapor
pressure and decomposition temperatures, thereby providing
little control over film growth conditions.17,22−25 Moreover, in
many cases, the use of reactive gases is necessary to accelerate
the precursor in the gas phase.17,25 The necessary volatility of
precursors strongly depends on inter- and intramolecular
interactions as well as the molecular weight of the compound.
To decrease molecular weight and increase volatility, small
ligands such as methyl groups are desired for the synthesis of
precursors. Because of their low steric demand, these precursors
are kinetically labile. Application of neutral chelating ligands
should increase the stability of the complexes and allow a
careful design of heteroleptic precursors showing sufficient
volatility and adequate reactivity (Scheme 1).26−30
molecular precursor for the phase-selective synthesis of iridium-
based films via CVD. Furthermore, quantitative conversion of
nanocrystalline iridium films into phase-pure iridium dioxide
films and their electrocatalytic behavior is described.
RESULTS AND DISCUSSION
■
Precursor Synthesis. The bidentate ligand HThTFP (1)
was prepared by reacting 2-methylthiazole with trifluoroacetic
acid anhydride in the presence of pyridine under elimination of
pyridinium trifluoroacetate. The heteroleptic iridium(I) pre-
cursor [(COD)Ir(ThTFP)] (3) was synthesized via a salt
metathesis reaction (Scheme 2) by reacting dimeric [(COD)-
IrCl]2 (2) and the sodium salt of 1 (NaThTFP, 1b).31
We report herein the solid-state and solution-phase structure
of the new heteroleptic iridium(I) precursor [(COD)Ir-
(ThTFP)] (3) and demonstrate its intrinsic advantages as a
Received: April 14, 2017
© XXXX American Chemical Society
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Scheme 2. Synthetic Route to Bidentate Ligand HThTFP (1) and Heteroleptic Iridium(I) Precursor [(COD)Ir(ThTFP)] (3)
The heteroleptic iridium(I) complex 3 was isolated as an air-
stable red-orange solid by sublimation under dynamic vacuum
(110 °C, 10−3 mbar). The sublimation temperature of 3 is
significantly lower than that for Ir(acac)3 (180 °C, 10−3
mbar),25 which is conventionally used as a precursor in the
CVD of Ir(0) films. The higher volatility of [(COD)Ir-
(ThTFP)] is attributed to the lower molecular weight and a
lesser ionic character, as verified by the bond lengths
determined through crystal structure analysis (see below).
Structural Characterization of [(COD)Ir(ThTFP)] (3). 1H
1
NMR and H,1H NOESY spectra (Figure 1; for additional
NMR spectra see the Supporting Information) display two
doublets at δ 7.21 ppm (3J = 3.9 Hz) and δ 6.97 ppm (3J = 3.9
Hz) corresponding to aromatic protons 1-H and 2-H. The
vinylic proton 4-H in 3 appeared at δ 6.28 ppm as a sharp
singlet. The unsaturated protons of COD (7-H) were observed
as multiplets at δ 4.35 and 3.70 ppm, whereas the saturated part
of COD exhibits multiplets shifted more upfield at δ 2.31 and
1
1.76 ppm. The H,1H NOESY spectrum (Figure 1) exhibits
through-space coupling of 1-H and unsaturated protons of
COD (7-H) (Figure 1, red circles), where the intensities of
each signal indicate the exact ligand arrangement around the
central iridium atom.
Single-crystal X-ray diffraction analysis of the heteroleptic
iridium(I) complex 3 showed a distorted-square-planar
coordination around iridium, as observed in other iridium(I)
compounds.32,33 The distorted geometry originated from the
angle between the COD ligand and nitrogen atom N1, COD−
Ir01−N1 96.153
0.417°, which is much larger than the
COD−Ir01−O1 angle of 86.883 0.344° due to the higher
steric profile of COD and hydrogen bonding between the
enolic oxygen of ThTFP and hydrogen of the COD ligand
(distance 2.627 0.021 Å). This effect is augmented by the
steric repulsion between the unsaturated parts of both ThTFP
and COD ligands.
Compound 3 was investigated by EI mass spectrometry at an
ionization energy of 70 eV and an evaporation temperature of
100 °C (10−6 mbar). The mass spectrum was dominated by the
molecular ion [M]•+ at m/z 495 with an intensity of 100%. The
thermal behavior of the precursor investigated by thermogravi-
metric (TG) analysis (Figure 2, left) showed a distinctive two-
stage decomposition starting at 200 °C. The TG curve showed
a first mass loss of 45 wt % which corresponded well to the loss
of ThTFP. Further heating of the sample resulted in the
removal of COD, passing a plateau (Figure 2, 600−700 °C)
due to the increased stability of the COD-Ir species in the gas
phase, yielding pure iridium metal at 40 wt % (theoretical 39 wt
%) at 1000 °C. The Janus-type precursor concept described in
Scheme 1 was confirmed by a distinctive two-stage thermal
1
1
Figure 1. (top) H NMR and H,1H NOESY spectra (δ in ppm) of
heteroleptic Ir(I) compound 3 (1H (blue), 13C (black), 19F (green)),
measured in CDCl3. (bottom) Molecular structure of [(COD)Ir-
(ThTFP)] (3). Thermal ellipsoids are drawn at the 50% probability
level.
decomposition. The powder XRD data of the obtained metallic
residue (Figure 2, right) revealed characteristic diffraction peaks
B
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Figure 2. Thermogravimetric curve of heteroleptic Ir(I) compound 3 (left, red line) and obtained XRD pattern of the metallic residue (right).
(ICSD: 46-1044) for metallic iridium and confirmed the
residue to be nanocrystalline iridium with an average size of 93
4 nm (Scherer equation).
deposition time has more influence on the crystallinity of the
coatings than the precursor temperature. Additionally, no
disproportionation reactions of subvalent Ir(I) precursor to
elemental iridium and iridium(IV) or iridium(III) oxides were
noticed in XRD patterns, that indicated a defined deposition
process.
Chemical Vapor Deposition of [(COD)Ir(ThTFP)]. Phase-
pure elemental iridium thin films were deposited on silicon and
aluminum oxide substrates by decomposition of [(COD)Ir-
(ThTFP)] (3) in a cold-wall reactor operating in the
temperature range 700−800 °C with the precursor reservoir
maintained at 110, 120, and 130 °C, whereas there is no
sufficient precursor flow at 110 °C precursor temperature. XRD
analysis showed that pure iridium phase (ICSD: 46-1044) was
obtained at 800 °C for deposition times of 60 min (Figure 3),
whereas the CVD deposits were mostly amorphous at lower
substrate temperatures (Figure 3). The scanning electron
micrographs unambiguously showed a homogeneous coating of
amorphous iridium. Increasing the deposition time from 60 to
120 and 240 min enhanced the crystallinity even at 700 °C to
provide nanocrystalline iridium(0) coatings. Therefore, the
The successful deposition of iridium(0) films on single-
crystal (Si), amorphous (SiO2), and polycrystalline (Al2O3)
substrates showed the universal applicability of the precursor
and its chemically controlled decomposition irrespective of the
surface chemistry of the substrate material (Figure 4; see XRD
in the Supporting Information). The obtained micrographs
depict homogeneous coatings of iridium nanoparticles all over
the substrates (Figure 4, top left) with average diameters
smaller than 30 nm. While the morphology of polycrystalline
Al2O3 substrate is more inhomogeneous, nevertheless it was
possible to observe a homogeneous coating all over the
polycrystalline substrate. Cross-section images of the deposited
films (see the Supporting Information) confirm a homogeneous
coating throughout the whole substrate. With a precursor
temperature of 120 °C and a substrate temperature of 700 °C a
film thickness of 30 nm was achieved after a deposition time of
60 min. Further investigations concerning growth rate and
influence of carrier or reactive gases on the deposition of
iridium films are currently underway.
The annealing of iridium(0)-containing films on Si substrates
for 5 h at 700 °C under ambient conditions induced the
formation of phase-pure iridium dioxide films that were
confirmed by X-ray diffractometry (ICSD: 15-0870, see the
Supporting Information).
Application of Iridium-Containing Thin Films in
Oxygen Evolution Application. The electrochemical activity
of iridium layers was evaluated by a three-electrode system as
depicted in Figure 5. To determine the potential of the
synthesized Ir/IrO2 films in photoelectrochemical water
splitting, they were deposited onto crystalline hematite (α-
Fe2O3) and titanium oxide (TiO2; see the Supporting
Information) films deposited by a plasma-enhanced vapor
deposition (PE-CVD) and their electrochemical performances
were compared. Figure 5 (right) exhibits J−V curves of the
prepared photoelectrodes. The current densities of both Ir and
IrO2 overlayered photoelectrodes under illumination are almost
the same as that in darkness, although a slight increase in
Figure 3. XRD patterns of iridium films deposited on Si substrates at
different deposition parameters.
C
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Figure 4. SEM images of iridium(0) films on Si (top), Al2O3 (bottom, left) and glass (bottom, right), deposited at different CVD parameters.
Figure 5. Schematic overview of a PEC cell (left) and dark and photocurrent densities JC obtained from the prepared α-Fe2O3-containing
photoelectrodes as a function of applied potential V (vs RHE) (right).
current density is observed under illumination (see the inset of
Figure 5 (right)), which indicates that the overlayers of Ir and
IrO2 barely improve the photoactivity of hematite photo-
electrodes. However, the onset potential, at which the current
density starts rising, under dark conditions shifts in the cathodic
(negative) direction gradually when the IrO2 (∼1.22 V vs
RHE) and Ir (∼0.75 V vs RHE) layers were deposited in
comparison to the hematite photoelectrode (∼1.40 V vs RHE).
The onset potential values were determined by reading the
potential at 0.02 mA/cm2, although there are several methods
to determine the onset potential.34,35 This cathodic onset
potential shift of dark current can be attributed to improved
electrocatalytic activity.36 Ir and IrO2 films synthesized by a
CVD process were stable under the measurement conditions
D
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of ThTFP (1) was added to the solution. After removal of the excess
solvent, an off-white solid (1b) was obtained. Yield: 0.216 g (1.0
and seem to be promising for use as (photo)electrochemical
catalysts. Further experiments involving TiO2 as a photo-
catalytic layer and electrodes without photocatalyst (Ir/IrO2 on
FTO) have been performed and are currently underway (see
the Supporting Information).37
1
mmol, >99%). H NMR (300.1 MHz, 298 K, DMSO-d6): δ 7.38 (d,
3
3JH,H = 3.3 Hz, 1H, 1-H); 6.86 (d, JH,H = 3.3 Hz, 1H, 2-H); 5.82 (s,
1H, 4-H). 19F NMR (282.4 MHz, 298 K, CDCl3): δ −72.4 (1JF,C = 293
2
Hz, JF,C = 36 Hz).
[(COD)IrCl]2 (2). 2 was prepared following a modified procedure
described by Tani et al.31 To a stirred solution of 0.441 g of
(NH4)2[IrCl6] (1.0 mmol, 1.0 equiv) in a 2:1 mixture of H2O and
isopropyl alcohol (15 mL) was slowly added 0.98 mL of cycloocta-1,5-
diene (COD) (8.0 mmol, 8.0 equiv). The resulting solution was
heated to reflux for 12 h, upon which a red-orange solid precipitated
out. After it was cooled to room temperature, 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-
CONCLUSION
■
Heteroleptic iridium(I) complexes containing both anionic and
neutral multidentate ligands demonstrated a synergistic inter-
play of reactivity and stability (Scheme 1) essential for
development of new precursors for gas-phase methods. This
combination of less polar Ir−C bonds and coordinative Ir//
N∧O units enabled an atom-efficient and distinctive two-step
decomposition mechanism. The coordination of bidentate β-
heteroaryl-alkenolate and COD ligands on the iridium center
produced an Ir(I) precursor with high volatility originating
from a careful ligand design. The deposition of Ir(0) films on
various substrate materials without the need for additional
reactant gases underlines the potential of this precursor class for
CVD and ALD of metallic thin films. The presented synthetic
and CVD data open new possibilities in the vapor-phase
synthesis of iridium-based materials facilitating the application
of such films, for example, as electro- or photocatalysts in the
OER.
1
orange solid (2) was obtained. Yield: 0.470 g (0.70 mmol, 70%). H
NMR (300.1 MHz, 298 K, CDCl3): δ 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 APT (75.5 MHz,
298 K, CDCl3): δ 62.4 (C1); 31.9 (C2). Anal. Calcd for C16H24Cl2Ir2:
C, 28.61; H, 3.60. Found: C, 29.12; H, 2.98.
[(COD)Ir(ThTFP)] (3). 3 was prepared by placing 1.5 mmol (2.1
equiv) of the sodium salt of compound 1 (1b) in a flask. Afterward
0.470 g (0.7 mmol, 1.0 equiv) of [(COD)IrCl]2 (2) dissolved in 30
mL of anhydrous THF was added to the sodium compound (1b). The
mixture was stirred for 4 h at room temperature. After removal of the
solvent under reduced pressure, the crude product was sublimed under
dynamic vacuum (110 °C, 10−3 mbar) to afford a yellow-orange solid
1
(3). Yield: 0.662 g (1.34 mmol, 96%), H NMR (300.1 MHz, 298 K,
3
3
EXPERIMENTAL SECTION
CDCl3): δ 7.22 (d, JH,H = 3.9 Hz, 1H, 1-H); 6.97 (d, JH,H = 3.9 Hz,
1H, 2-H); 6.28 (s, 1H, 4-H); 4.36−4.34 (m, 2H, 7-H); 3.72−3.66 (m,
2H, 7-H); 2.39−2.22 (m, 4H, 8-H); 1.83−1.70 (m, 4H, 8-H). 13C
APT (75.5 MHz, 298 K, CDCl3): δ 165.4 (C3); 155.5 (C5); 139.5
(C1); 122.1 (C6); 114.6 (C2); 90.8 (C4); 68.6 (C7); 57.0 (C7); 32.4
(C8); 29.8 (C8). 19F NMR (282.4 MHz, 298 K, CDCl3): δ −72.9
■
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 H 300.1 MHz, 13C 75.5 MHz, and 19F
1
282.4 MHz; chemical shifts are given in parts 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
3.238 software. Mass spectra were recorded on a Finnigan MAT 95
instrument (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-Area39 and the
WINGX suite of programs,40 including SIR-92,41 SHELX, and
SHELXTL42 and PLATON43 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.
Precursor Synthesis. (Z)-3,3,3-Trifluoro-1-(thiazol-2-yl)prop-1-
en-2-ol (1; HThTFP). 1 was prepared by modifying the procedure
described by Kawase et al.44 A 2.97 g portion (30 mmol, 1.0 equiv) of
2-methylthiazole and 12.1 mL (150 mmol, 5.0 equiv) of pyridine were
dissolved in 80 mL of toluene and cooled to 0 °C, and then 12.7 mL of
trifluoroacetic acid anhydride (90 mmol, 3.0 equiv) was added
dropwise. The mixture was stirred for 18 h at room temperature.
Afterward 300 mL of 3% Na2CO3(aq) was added slowly and the
mixture was extracted with EtOAc (4 × 200 mL). The combined
organic phases were washed with brine solution (300 mL) and dried
over MgSO4, and the solvent was removed under reduced pressure.
The crude product was purified by sublimation under dynamic vacuum
(55 °C, 10−3 mbar) to obtain a yellowish solid (1). Yield: 63% (3.71
2
(1JF,C = 280 Hz, JF,C = 33 Hz). EI-MS (70 eV, 100 °C): m/z
(intensity) 495 (100%) [M]+,18 (66%) [H2O]+. Anal. Calcd for
C14H15F3NOSIr: C, 34.00; H, 3.06; N, 2.83; S, 6.48. Found: C, 34.92;
H, 1.98; N, 3.01; S, 6.74.
CVD Experiments. Thin film deposition was performed in a
horizontal cold-wall CVD reactor equipped with inductive field
heating. The substrates (1 cm × 1 cm) were heated through a 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.45 Hematite films were deposited by radio
frequency plasma enhanced chemical vapor deposition (PE-CVD)
onto FTO substrates using Fe(CO)5 as the iron source and pure O2
gas as the oxygen source, as reported elsewhere.46−48 TiO2 films were
deposited via PECVD onto FTO substrates by using Ti(OiPr)4 as a
precursor. The as-grown films were amorphous and were annealed for
2 h in air at 750 °C to obtain crystalline hematite films (α-Fe2O3) and
500 °C for TiO2.
Materials Characterization. Room-temperature powder X-ray
diffraction (XRD) was carried out 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). PEC measurements were carried out
in a three-electrode electrochemical cell with a 1.0 M NaOH
electrolyte using a potentiostat (PAR Model Versa state IV, USA)
and a 150 W xenon lamp (Oriel), which was equipped with an AM 1.5
filter to simulate the solar spectrum.
1
g). H NMR (300.1 MHz, 298 K, DMSO-d6): δ 13.20 (s, 1H, OH);
3
3
7.62 (d, JH,H = 4.0 Hz, 1H, 1-H); 7.13 (d, JH,H = 4.0 Hz, 1H, 2-H);
6.22 (s, 1H, 4-H). 19F NMR (282.4 MHz, 298 K, CDCl3): δ −74.3
(1JF,C = 291 Hz, 2JF,C = 38 Hz). EI-MS (70 eV, 36 °C): m/z (intensity)
195 (68%) [M]+, 126 (100%) [M − CF3]+, 98 (28%) [C4H4NS]+, 58
(20%) [C2H3S]+. Anal. Calcd for C6H4F3NOS: C, 39.93; H, 2.07; N,
7.18; S, 16.43. Found: C, 37.12; H, 1.98; N, 8.01; S, 15.74.
ASSOCIATED CONTENT
■
S
* Supporting Information
Sodium (Z)-3,3,3-Trifluoro-1-(thiazol-2-yl)prop-1-en-2-olate (1b;
NaThTFP). 1b was prepared by reacting with NaOEt. Therefore, 0.023
g (1.0 mmol, 1.0 equiv) of sodium was added to 10 mL of ethanol.
After the mixture was stirred for 30 min, 0.195 g (1.0 mmol, 1.0 equiv)
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00275.
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NMR, EI mass spectra, an additional XRD pattern, SEM
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data for this paper. These data can be obtained free of charge
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ORCID
Lasse Jurgensen: 0000-0002-8385-9886
̈
Sanjay Mathur: 0000-0003-2765-2693
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the University of Cologne
and the Regional Research Cluster Sustainable Chemical
Synthesis for financial support. We thank the German Science
Foundation (DFG) for financial support in the frame of the
priority programs “Manipulation of matter controlled by
electric and magnetic fields: Towards novel synthesis and
processing routes of inorganic materials” (SPP 1959), “Fuels
Produced Regeneratively Through Light-Driven Water Split-
ting: Clarification of the Elemental Processes Involved and
Prospects for Implementation in Technology Concepts” (SPP
1613) and the University of Cologne for providing infra-
structure and financial assistance.
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DOI: 10.1021/acs.organomet.7b00275
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DOI: 10.1021/acs.organomet.7b00275
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