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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Rational Development of Cobalt β‑Ketoiminate Complexes: Alternative Precursors for Vapor-Phase Deposition of Spinel Cobalt Oxide Photoelectrodes Kai junge Puring,†,∥ Dennis Zywitzki,†,∥ Dereje H. Taffa,‡ Detlef Rogalla,§ Manuela Winter,† Michael Wark,‡ and Anjana Devi*,† †

Inorganic Materials Chemistry, Faculty of Chemistry and Biochemistry and §RUBION, Ruhr-University Bochum, 44801 Bochum, Germany ‡ Chemical Technology 1, Institute of Chemistry, Carl von Ossietzky University Oldenburg, 26129 Oldenburg, Germany S Supporting Information *

ABSTRACT: A series of six cobalt ketoiminates, of which one was previously reported but not explored as a chemical vapor deposition (CVD) precursor, namely, bis(4-(isopropylamino)pent-3-en-2-onato)cobalt(II) ([Co(ipki)2], 1), bis(4-(2methoxyethylamino)pent-3-en-2-onato)cobalt(II) ([Co(meki)2], 2), bis(4-(2-ethoxyethylamino)pent-3-en-2-onato)cobalt(II) ([Co(eeki)2], 3), bis(4-(3-methoxy-propylamino)pent-3-en-2-onato)cobalt(II) ([Co(mpki)2], 4), bis(4-(3ethoxypropylamino)pent-3-en-2-onato)cobalt(II) ([Co(epki)2], 5), and bis(4-(3-isopropoxypropylamino)pent-3-en2-onato)cobalt(II) ([Co(ippki)2], 6) were synthesized and thoroughly characterized. Single-crystal X-ray diffraction (XRD) studies on compounds 1−3 revealed a monomeric structure with distorted tetrahedral coordination geometry. Owing to the promising thermal properties, metalorganic CVD of CoOx was performed using compound 1 as a representative example. The thin films deposited on Si(100) consisted of the spinelphase Co3O4 evidenced by XRD, Rutherford backscattering spectrometry/nuclear reaction analysis, and X-ray photoelectron spectroscopy. Photoelectrochemical water-splitting capabilities of spinel CoOx films grown on fluorine-doped tin oxide (FTO) and TiO2-coated FTO revealed that the films show p-type behavior with conduction band edge being estimated to −0.9 V versus reversible hydrogen electrode. With a thin TiO2 underlayer, the CoOx films exhibit photocurrents related to proton reduction under visible light.

1. INTRODUCTION

For the fabrication of such photoelectrodes, various techniques have been employed so far.12,15−18 Generally, vapor-phase techniques, such as chemical vapor deposition (CVD), are regarded as promising methods, due to their ability for up-scaling, conformal step coverage of the resulting films, which enables deposition on nanostructured substrates and control over material properties and composition by tuning the CVD process parameters. Another advantage of vapor-phase deposited films is the well-defined film−substrate interface.19 CVD processes are strongly dependent on the employed precursor, which needs to be volatile at moderate temperatures, and should decompose in a clean manner during the deposition process. For CVD of cobalt oxides, there are a few precursors known in the literature: [CoCp(CO)2] was utilized in a metalorganic (MO) CVD process for the deposition of CoOx thin films at low pressure (1 ×10−2 mbar) under oxidizing atmosphere with deposition rates of 0.1−9.4 nm/min over a

Thin films containing cobalt oxides are of great interest for a plethora of applications, due to their high abundance, low cost, and low toxicity. They have been successfully applied in catalysis,1,2 gas sensing,3,4 and lithium ion batteries.5 Especially in the field of electrocatalysis, this material has been proven to be highly efficient for the oxygen evolution reaction (OER).6−8 With respect to photocatalysis it is noteworthy that Co3O4 is a p-type semiconductor, possessing a band gap of 2.07 eV, and has been extensively used as a cocatalyst for the OER in combination with different photoabsorber materials.9−11 Barreca et al. reported the application of Co3O4 and F-doped Co3O4 for photocatalytic production of H2.12 Li et al. reported the use of hydrothermal grown Co3O4 nanoparticles for the hydrogen evolution reaction (HER) and OER in alkaline electrolytic cells.13 However, there are few reports on the use of p-type Co3O4 for photoelectrochemical H2 generation. Ebadi et al. report the photoelectrochemical production of H2 using electrodeposited Co3O4/CuO films from neutral solution.14 © XXXX American Chemical Society

Received: January 23, 2018

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DOI: 10.1021/acs.inorgchem.8b00204 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis Route for the Preparation of Cobalt β-Ketoiminates (1−6)

deposition temperature range from 200 to 650 °C. The evaporation temperature was kept at room temperature.20 Cobalt β-diketonates were employed in CVD processes for CoOx by Jaegermann et al.,8 where [Co(acac)3] (acac = acetylacetone) was used in a low-pressure (3 mbar) setup, applying an evaporation temperature of 190 °C. Its Co(II) counterpart [Co(acac)2] was utilized in atmospheric pressure (AP) CVD, using an evaporation temperature of 130 °C.21 Furthermore, the more bulky cobalt β-diketonate [Co(thd)2] (thd = 2,2,6,6-tetramethylheptane-3,5-dionate) was employed by Barecca et al. in CVD processes at an evaporation temperature of 90 °C in a pressure range from 2 to 10 mbar.22 However, Co(II) β-diketonates are prone toward oligomerization due to their preferred sixfold coordination sphere, and hence, water and amine adducts of these compounds were synthesized. Pasko et al. developed a pulsed liquid injection CVD (PI-CVD) process for [Co(acac)2(TMEDA)] (TMEDA = 2,2,6,6-tetramethylethylenediamine) and [Co(thd) 2 (TMEDA)] precursors, but classical MOCVD was not attempted within this study.23 The introduction of fluorine-bearing ligands, as in [Co(hfac)2] (hfac = 1,1,1,5,5,5 hexafluoropentane-2,4-dionate) and the corresponding amine adduct [Co(hfac)2(TMEDA)], reduces the intermolecular interactions and results in an increased vapor pressure of the respective compounds.24 The latter compound was utilized by Barreca et al. in a plasmaenhanced (PE) CVD process to deposit the aforementioned fluorine-doped Co3O4, where the fluorine dopants originate from the precursor ligands.12 A promising and fluorine-free precursor family for transitionmetal oxides are the β-ketoiminates. These complexes, comprising of bidentate chelating, mixed nitrogen, oxygen coordinated ligands can be regarded as a compromise between the comparably unreactive β-diketonates and the highly reactive β-diketiminates. Furthermore, they can be easily modified by exchanging ligand moieties at the imino functionality, which in turn opens a broad spectrum of potential precursors for different vapor- and solution-based deposition techniques. For example, the introduction of etheric or amino side chains can lead to enhanced solubility, additional metal coordination of the etheric oxygen or amino nitrogen atoms, and a change in terms of volatility due to their polar nature.25,26 β-Ketoiminates were successfully employed for the fabrication of other transition-metal oxide thin films, such as Fe, Zn, Cu, and Ce.16,25,27−30 For Co itself only limited research toward the β-ketoiminate chemistry has been conducted so far, none of which focused on thermal properties or precursor applications. In 1965 Everett and Holm reported the first synthesis of Co β-ketoiminate complexes, including bis(4-(isopropylamino)pent-3-en-2onato)cobalt(II) ([Co(ipki)2], 1), which was also investigated in this work.31 More recently Richards and Lugo reported the synthesis of Co β-ketoiminate complexes; however, for most of them the ligands coordinated in a monodentate fashion, most

likely due to their bulky side chains and encumbered ligand spheres.32 To the best of our knowledge, only two reports of cobalt precursors for CVD, bearing mixed O/N coordinated ligands, were published, namely, [Co(saliprn)2 ] (saliprn = Nisopropylsalicylaldimine) and [Co(PyCHCOCF3)3] (Py = pyridine). While the former precursor was employed in an MOCVD process at atmospheric pressure and 550 °C evaporation temperature, the latter required an evaporation temperature of 150 °C at 1 × 10−3 mbar.33,34 Furthermore, the heteroleptic, dimeric compound [Co 2(dmamp)2 (acac) 2 ] (dmamp = 1-dimethylamino-2-methyl-2-propoxide) and its derivates were recently proposed as potential precursors for vapor-phase depositions by Chung et al.35 This class of complexes however has despite their promising thermal properties not been employed in a deposition process yet. Our research focus was to identify new Co complexes bearing the ketoiminate ligand system that can be synthesized via a straightforward route, in high yields and, in addition, to study the influence of the imino side chains on the physicochemical properties such as melting point, volatility, reactivity, and thermal stability that are relevant for vapor-phase techniques like CVD. Herein, we report the exploration of the β-ketoiminate ligand system to cobalt, and a rational approach was undertaken by systematically varying the side chains of the ligands. As a result a series of new cobalt ketoiminato complexes with varied imino side-chains, namely, bis(4-(isopropylamino)pent-3-en-2onato)cobalt(II) ([Co( i pki) 2 ], 1), bis(4-(2-methoxyethylamino)pent-3-en-2-onato)cobalt(II) ([Co(meki)2], 2), bis(4-(2-ethoxyethylamino)pent-3-en-2-onato)cobalt(II) ([Co(eeki)2 ], 3), bis(4-(3-methoxy-propylamino)pent-3-en-2onato)cobalt(II) ([Co(mpki)2], 4), bis(4-(3-ethoxypropylamino)pent-3-en-2-onato)cobalt(II) ([Co(epki)2], 5), and bis(4-(3-isopropoxypropylamino)pent-3-en-2-onato)cobalt(II) ([Co(ippki)2], 6), were synthesized by a straightforward salt metathesis reaction and thoroughly characterized with regard to their molecular structure and thermal properties. Furthermore, the suitability of this class of compounds for MOCVD of cobalt oxide thin films was also verified in terms of physicochemical properties. Owing to the low onset of volatilization, [Co(ipki)2] 1 was exemplary employed in an MOCVD process. The film deposition process was optimized on Si substrates by varying the process parameters, and the resulting films were thoroughly investigated toward their stoichiometry, crystallinity, and morphology by means of Rutherford backscattering spectrometry and nuclear reaction analysis (RBS/NRA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) analyses, and scanning electron microscopy (SEM). Subsequently representative CoOx films were deposited on fluorine-doped tin oxide (FTO) glasses and used in a heterojunction photoelectrode, and their activities toward the HER was evaluated. B

DOI: 10.1021/acs.inorgchem.8b00204 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data for Compounds 1−3 empirical formula formula weight radiation (Å) temperature (K) crystal system space group A (Å) B (Å) C (Å) α (deg) β (deg) γ (deg) volume (Å3) Z Dcalc (g cm−3) μ (mm−1) F(000) crystal size (mm3) 2Θ range for data collection (deg) reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] largest diff peak/hole/e Å−3 CCDC Nos.

[Co(ipki)2] (1)

[Co(meki)2] (2)

[Co(eeki)2] (3)

C16H28N2O2Co 339.33 Cu Kα (λ = 1.541 84) 116.30(14) orthorhombic Iba2 10.6153(3) 15.6626(5) 20.5540(9) 90 90 90 3417.4(2) 8 1.319 7.915 1448.0 0.23 × 0.18 × 0.15 8.604 to 152.762 5496 2819 [Rint = 0.0287, Rsigma = 0.0313] 2819/1/199 1.114 R1 = 0.0345, wR2 = 0.0962 R1 = 0.0381, wR2 = 0.0990 0.40/−0.34 1587793

C16H28CoN2O4 371.33 Cu Kα (λ = 1.541 84) 116.2(8) monoclinic C2/c 18.6328(1) 5.5872(2) 18.3873(1) 90 110.508(6) 90 1792.91(2) 4 1.376 7.679 788.0 0.12 × 0.09 × 0.07 10.138 to 152.194 8188 1844 [Rint = 0.1143, Rsigma = 0.0493] 1844/0/108 1.042 R1 = 0.0490, wR2 = 0.1181 R1 = 0.0534, wR2 = 0.1211 0.66/−0.71 1587794

C18H32CoN2O4 399.38 Cu Kα (λ = 1.541 84) 114.7(4) monoclinic P2/n 13.2525(4) 5.88147(2) 13.9576(5) 90 106.817(4) 90 1041.39(6) 2 1.274 6.645 426.0 0.11 × 0.07 × 0.04 8.104 to 152.986 10 729 2155 [Rint = 0.0236, Rsigma = 0.0170] 2155/0/117 1.083 R1 = 0.0271, wR2 = 0.0690 R1 = 0.0292, wR2 = 0.0699 0.23/−0.26 1587795

°C, single crystals of 2 and 3 were obtained via crystallization from saturated hexane solutions at the same temperature. The three complexes were characterized by single-crystal XRD to determine the solid-state molecular structure of the compounds. While the crystallographic data are summarized in Table 1, the molecular structures of the complexes 1−3 are presented in Figures 1−3, and selected bond lengths and angles are depicted in Table 2.

2. RESULTS AND DISCUSSION 2.1. Precursor Synthesis and Characterization. The synthesis of the cobalt β-ketoiminato complexes was performed via salt metathesis of CoCl2 with the lithium salt of the respective β-ketoimine compound (Scheme 1). The resulting cobalt β-ketoiminates (1−6) were isolated by crystallization from hexane or pentane solutions at room temperature or −30 °C in yields in the range of 55−75% as orange/red crystals or red liquids. While the isopropyl-substituted complex 1 is merely soluble in hexane and weakly soluble in tetrahydrofuran (THF) and toluene, the ether-substituted complexes (2−6) exhibit good solubility in any of these solvents. While being reasonably stable in dry air at room temperature, the ether-substituted complexes 2−6 readily hydrolyze upon exposure to water. On the contrary, 1 is reasonably stable in humid air for a couple of weeks. The paramagnetic nature of the central cobalt atom results in a strong NMR upfield shift and broadening of the respective proton signals for all synthesized complexes. In the NMR spectra of compounds 1−6 (Figure S1), no evidence for significant impurities can be found. Because of the strong broadening of the signals, a detailed discussion is not feasible. However, the observed peaks can be assigned within reasonable limitation. To describe the spectra briefly, all complexes, with the exception of 2, exhibit similar signals in the range from −14 to −32 ppm, which can be assigned to the acac backbone of the ligands. Furthermore, specific peaks for the side chain of the ligands are observed rather upfield. The specific assignment of the peaks is given in the experimental part. Of the six complexes, suitable crystals for single-crystal XRD analysis could be obtained in cases of 1−3. While single crystals of 1 were grown from a concentrated toluene solution at −30

Figure 1. Solid-state molecular structure of [Co(ipki)2] (1). Hydrogen atoms are omitted. Thermal ellipsoids are depicted at 50% probability level.

The three compounds crystallize depending on the type of side-chain alkyl (1) or ether-functionalized (2 and 3)in orthorhombic (space group: Iba2) or monoclinic (space groups: C2/c (2) and P2/n (3)) crystal systems, respectively. The crystal system as well as the space group of compound 1 are in agreement with those reported in the literature for iron and zinc analogues.36 Furthermore, compound 1 crystallizes with two distinct molecules in one unit cell, which are identical C

DOI: 10.1021/acs.inorgchem.8b00204 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

96.9(1) ° (3) than expected for a tetrahedral coordination. In contrast to its iron analogue, for compound 2 no coordination of the etheric oxygen was observed, which also explains the deviation of the space group.37 Furthermore, the other angles within the fourfold coordination generally exhibit values larger than the expected 109.5° (Table 2). According to Houser’s qualitative τ4 measurements38 all three complexes show similar degrees of distortion (0.82−0.88) from an ideal tetrahedron (τ4, tetra = 1). In case of the cobalt βketoiminates with an ether functionalized side chain 2 and 3 the ethereal oxygen O(2) is far from the coordination sphere of the Co2+. However, the ethoxyethyl-functionalized complex 3 exhibits a significantly shorter Co(1)···O(2) distance (3.627(1) Å) compared to 2 (4.236(2) Å). As expected, the Co(1)−O(1) bonds in all compounds are shorter than the Co(1)−N(1) bonds and are comparable to literature reports on other cobalt β-ketoiminates.39 The six-membered CoONC3 units of the three complexes appear in a puckered shape suggesting a decreased delocalization of π-electrons throughout this unit. The puckering is reflected by the deviations of the torsion angles of O(1), C(1), C(2), C(3), C(4), C(5), N(1), and C(6) from a planar system by 1.1°−6.8°, which is expected for a delocalized π-system (see Table S1). Moreover, the torsion angles between the above-mentioned atoms of the sixmembered ring and the cobalt center are even higher in case of 1 (10°) and 2 (15°). On the contrary, the Co2+ center of 3 is less tilted out of the plane at 6°. The fragmentation pattern of the cobalt bis(ketoiminato) compounds 1−6 was examined by electron ionization mass spectrometry (EI-MS). Complex 1 shows a molecular peak with a relative intensity of 100%, indicating a relatively high stability of this compound under EI-MS conditions. In contrast, 2−5 show peaks corresponding to an M+ species at lower relative intensities. Hence, a higher degree of fragmentation of these complexes is suggested, and therefore a lower stability under these conditions can be presumed. In case of compound 6, no molecule peak was detected, supporting high fragility of the complex under EI-MS conditions. With the exception of 1, all mass spectra (Figures S2−S7) strongly support the complete cleavage of the whole ligand under EI-MS conditions (Table S2). In the case of the short-chained derivatives 2 and 3, additional peaks corresponding to higher masses than the molecular peaks are observed, which can be assigned to a dimeric species with a cleaved ligand (2·M − L)+. The melting points of the six complexes were determined by capillary measurements (Table S3). On the one hand, a huge difference in the melting points of the alkyl-substituted compound 1 and the ether-substituted compounds 2−6 is evident. While the ether-substituted complexes already melt at temperatures up to 76 °C or are even liquid at room temperature (5), compound 1 has a much higher melting point at 171 °C. On the other hand, within the ethersubstituted complexes, the melting points of the methoxy complexes (65 °C for 2, 76 °C for compound 4) are considerably higher than those of the ethoxy complexes (48 °C for compound 3, less than room temperature (RT) for compound 5). The melting point of compound 6 is only slightly lower than that of compound 2 at 63 °C. The thermogravimetric (TG) curves (Figure 4) from the thermal analysis indicate a clean evaporation behavior exhibiting singlestep weight loss for all compounds. The onset temperature of volatilization of compound 1 is the lowest at 138 °C. In comparison, the ether-substituted complexes exhibit relatively

Figure 2. Solid-state molecular structure of [Co(meki)2] (2). Hydrogen atoms are omitted. Thermal ellipsoids are depicted at 50% probability level.

Figure 3. Solid-state molecular structure of [Co(eeki)2] (3). Hydrogen atoms are omitted. Thermal ellipsoids are depicted at 50% probability level.

Table 2. Selected Bond Lengths (Å) and Angles (deg) of Compounds [Co(ipki)2] (1), [Co(meki)2] (2), and [Co(eeki)2] (3) mean bond length

angle

1

2

3

Co(1)−O(1)

1.933(3)

1.923(2)

1.934(1)

Co(1)−N(1) O(1)−C(2) N(1)−C(4) Co(1)···O(2) N(1)−Co(1)− O(1) O(1)−Co(1)− O(1′) O(1)−Co(1)− N(1′) N(1)−Co(1)− O(1′) N(1)−Co(1)− N(1′) chelate twist τ4

1.985(3) 1.293(4) 1.323(4) 97.7(1)

1.980(2) 1.300(3) 1.320(3) 4.236(2) 95.9(1)

1.988(1) 1.294(2) 1.309(2) 3.627(1) 96.9(1)

117.5(2)

113.7(1)

107.9(1)

111.2(1)

117.8(1)

122.2(1)

111.2(1)

117.8(1)

122.2(1)

123.0(2)

117.2(1)

112.6(1)

81.3(1) 0.85

84.7(3) 0.88

79.5(1) 0.82

with experimental uncertainty. Therefore, only data for one molecule contained in a unit cell are discussed herein. The solid-state molecular structures of 1, 2, and 3 adopt in all cases monomeric structures with a Co2+ center coordinated in a distorted tetrahedral way by two nearly perpendicular bidentate β-ketoiminato ligands. The distortion of the tetrahedron is reflected by the deviations of the angles within the fourfold coordination geometry from the ideal tetrahedron (109.5°). The corresponding bite angles of the three complexes, contributing toward the tetrahedral coordination sphere, appear in a much sharper fashion at 97.7(1) ° (1), 95.9(1) ° (2), and D

DOI: 10.1021/acs.inorgchem.8b00204 Inorg. Chem. XXXX, XXX, XXX−XXX

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highest evaporation rate, as well as the highest yield and stability, among the six discussed complexes, it was chosen as a representative precursor for cobalt oxide thin films via MOCVD. 2.2. Thin-Film Fabrication and Evaluation. With compound 1 in combination with oxygen as co-reactant, cobalt oxide thin films were grown on Si(100) substrates in a broad temperature range of 400−800 °C. The as-deposited thin films were uniform and appeared dark gray, with thicknesses varying between 130 and 550 nm (Table 3), as derived from RBS/NRA measurements (see Table S8) by assuming bulk density (6.11 g cm−3). Average growth rates were estimated and found to vary between 2.2 and 9.1 nm min−1. While the lowest growth rate of 2.2 nm min−1 was observed at a deposition temperature of 400 °C, a sharp increase in growth rate (9.1 nm min−1) could be observed at a deposition temperature of 500 °C, and it remains stable until 600 °C, indicating the transition from a kinetically controlled to a diffusion-controlled growth regime between 400 and 500 °C. At higher temperatures slightly decreased growth rates of 8.8 and 8.7 nm min−1 at 700 and 800 °C, respectively, were obtained that could be due to desorption or gas-phase reactions at higher temperatures. The XRD patterns of the cobalt oxide thin films were measured to investigate their crystallinity (Figure 5). The

Figure 4. TGA and isothermal TGA traces (inset) at 130 °C of [Co(ipki)2] (1), [Co(meki)2] (2), [Co(eeki)2] (3), [Co(mpki)2] (4), [Co(epki)2] (5), and [Co(ippki)2] (6).

higher onset temperatures (10 °C−50 °C). While the onset temperature appears to rise with rising molecular mass from compound 2 (161 °C) over compound 4 (172 °C) to compound 6 (187 °C), two exceptions are notable. On the one hand, compound 3 exhibits a slightly lower onset (159 °C) than compound 2 and a considerably lower one than compound 4, despite being of similar molecular mass. A reason for this behavior could be its molecular structure, which exhibits a smaller distance from the ethereal oxygen of the side chain to the cobalt center and hence the more compact shape of the molecule. On the other hand, compound 5 only shows a slightly increased onset temperature (148 °C) than compound 1. Further investigations on the thermal stability of the six complexes were performed via isothermal TG analysis (Figure 4 (inset)) over the course of 3 h at a temperature of 130 °C. From the isothermal TGA traces a constant evaporation without decomposition can be presumed due to the linear weight loss of all complexes. Evidently, higher molecular masses result in lower vapor pressures. Therefore, compound 1 shows the highest volatility of the presented compounds, followed by 2. Compound 3, however, was determined to possess a higher evaporation rate than 4, despite having equal molecular masses. This can be explained probably by the higher polarity of the side chains in case of compound 4, which results in stronger intermolecular interactions. Because of their straightforward synthesis and excellent physicochemical properties such as high volatility, high thermal stability, clean evaporation, and low melting points, cobalt βketoiminates appear to be promising candidates as precursors for CVD of cobalt oxide thin films. Since compound 1 was found to have the lowest onset of evaporation temperature and

Figure 5. X-ray diffractograms of the CoOx thin films deposited on Si(100) as a function of deposition temperature. Asterisk denotes the Si substrate reflex.

observed reflections at 2θ = 31.2°, 2θ = 36.8°, 2θ = 38.6°, 2θ = 44.8°, 2θ = 55.6°, and 2θ = 59.4° can be assigned to (220), (311), (222), (400), (422), and (511) planes of the cubic spinel phase Co3O4 (ICSD 24210, space group Fd3m), respectively. At a deposition temperature of 400 °C, only two main reflexes, corresponding to the (220) and (311) planes of the Co3O4 phase, are observed with low intensity, which indicates low crystallinity of this thin film. When the deposition temperature is increased to 500 °C, more reflections arise, which are also present for films deposited at higher temper-

Table 3. Thickness and Composition of CoOx Thin Films Deposited on Si(100) as Determined by RBS/NRA deposition temp (°C)

thickness (nm)

C (atom %)

N (atom %)

O (atom %)

Co (atom %)

Co/O

400 500 600 700 800

130 545 545 530 520

2.9 0.5 0.3 0.8 0.4

0.9 0.3 0.1 0.1 0.1

56.7 57.2 57.7 56.7 58.2

39.4 42.0 41.8 42.4 41.3

0.70 0.74 0.72 0.75 0.71

E

DOI: 10.1021/acs.inorgchem.8b00204 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. SEM analysis of the CoOx thin films deposited on Si(100) as a function of deposition temperature.

From the combined RBS/NRA analysis, the composition of the thin films was evaluated (Table 3). The ratio of cobalt to oxygen was determined to vary between 0.70 and 0.75, which is in close agreement with the expected value for a Co3O4 phase (ideal Co/O ratio is 0.75) in the film. This formation of pure spinel phase is in accordance to the observed reflexes in XRD (Figure 5). From NRA analysis, very low levels of C or N ( FTO/TiO2 > FTO/ TiO2/Co3O4 indicating radiative recombination being greatly suppressed in FTO/TiO2/Co3O4 electrode. Further investigation is in progress to optimize the photoresponse and understand the Co3O4/ TiO2 interface in detail.

3. CONCLUSIONS A new and promising class of cobalt precursors bearing the βketoiminate ligands has been synthesized and successfully applied as precursors for the deposition of cobalt-based thin films via MOCVD. The presented results highlight that, by finetuning the skeleton of the ketoimine ligand chain, the physicochemical properties can be influenced as seen by the variation of the melting points, volatility, and decomposition temperatures, which can influence the deposition conditions for thin film growth. As a result, a new MOCVD process was developed for Co3O4 thin films under moderate process conditions with the onset temperature for the deposition of crystalline Co3O4 as low as 400 °C when [Co(ipki)2] was reacted with O2. The MOCVD process conditions led to pure and stoichiometric spinel Co3O4, and the surface morphology was characterized by faceted grains, which could play an active role for the function of Co3O4 as photoelectrodes. First studies were performed on the fabrication of photoelectrodes by depositing Co3O4 thin films on FTO and TiO2-coated FTO. The Mott−Schottky measurements confirmed the p-type behavior of the Co3O4 films. While the Co3O4/FTO electrodes revealed low photoresponse according to chopped light voltammetry, the addition of a titania underlayer resulted in drastically increased performance toward the HER. The enhanced performance is attributed to the creation of a p−n heterojunction, leading to a more efficient charge carrier separation. This study further evidences that, by a proper choice of precursor and process conditions, the structure and morphology of the CVD-grown films can be tuned, and in this particular case the Co β-ketoiminates seem highly promising for CVD related applications. Moreover, this family of precursors is a new and valuable addition to the library of Co precursors, which is small compared to other transition metals. Future efforts will be directed toward the influence of the substitution of the backbone moieties with tert-butyl or alkoxy side chains. Additional studies are warranted to exploit the advantages associated with this new process for developing photoelectrodes with higher efficiency for HER. Particularly, fine-tuning of the deposition process parameters such as temperature and thickness of both the absorber layer (Co3O4) and the under layer (TiO2) may significantly affect the photoactivities of the films. The reactivity and thermal stability of these compounds could be highly relevant for ALD applications. Thus, our future work will focus in this direction. 4. EXPERIMENTAL SECTION 4.1. Precursor Synthesis and Characterization. All manipulations of air- and moisture-sensitive compounds were performed by using standard Schlenk techniques. Storage and preparations of samples for further analysis were performed in argon-filled glove boxes (MBraun, Lab-Master). The solvents used for synthesis were purified by an MBraun solvent purification system (SPS) and stored over H

DOI: 10.1021/acs.inorgchem.8b00204 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

atures ranging from 400 to 800 °C using an inductive high-frequency heating system. A bubbler temperature of 120 °C was used for precursor evaporation, and the deposition time was set to 60 min. Thin-film XRD measurements were performed using a Bruker AXS D8 Advance diffractometer with Cu Kα radiation (1.5418 Å) in Bragg−Brentano θ-2θ geometry. Determination of the crystallite size by means of Scherer equation was conducted without the use of an external standard. The surface morphology of the films was determined by scanning electron microscopy (SEM) by using an LEO 1530 Gemini instrument (Zeiss). The composition of the films was estimated by RBS/NRA experiments performed at the RUBION facility at the Ruhr-University Bochum, using a He+ beam with the energy of 2 MeV and a beam current of 20−40 nA for RBS. A silicon surface barrier detector was placed at an angle of 160° with respect to the beam axis, and the solid angle of the detector (16 keV resolution) was 1.911 msrad. The spectra were analyzed with the software SIMNRA.56 NRA measurements were conducted by means of an incident deuteron beam of 1 MeV at an angle of 20° with a typical beam current up to 40 nA. Protons emitted by the nuclear reactions with light elements were detected by a Ni-foil (6 μm) shielded detector situated at an angle of 135°. The detector covered a solid angle of 23 msrad. The collected beam charge for the sample was 12 mC in an area with a radius of ∼1 mm. XPS was performed using an ESCALAB 250 Xi (Thermo Fisher) device with a monochromatized Al Kα radiation source (hν = 1486.6 eV). The electron binding energies of the measured elements were referenced to adventitious C 1s at 284.8 eV. High-resolution XPS core-level spectra for C, O, and Co were collected using pass energy of 10 eV and step size of 0.02 eV and analyzed using the Avantage software (version 5.951). 4.3. Evaluation of Functional Properties. Samples for photoelectrochemical (PEC) measurements were fabricated by depositing CoOx films at 600 °C on either bare fluorine-doped tin oxide (FTO) (Pilkington, NSG Tec A7, 2.2 μm, 7 Ω□−1) or TiO2 (150 nm thickness) coated FTO substrates. The TiO2 coatings were deposited on FTO at 100 °C using PEALD employing tetrakis(dimethylamido)titanium(IV) (TDMAT) as Ti precursor and oxygen feedstock gas (purity/company) for the plasma discharge. Details on the PEALD process are reported elsewhere.57,58 PEC measurements were performed with a Zahner Zennium E potentiostat controlled by Thales software (ZAHNER Elektrik). Three-electrode configuration was employed for all measurements in N2 saturated 0.1 M NaOH (pH = 12.6) or 0.1 M Na2SO4 (pH = 6.6). Platinum wire and Ag/AgCl (saturated (satd) KCl) were used as counter and reference electrode, respectively. The working electrode was a cobalt oxide coated FTO (1.5 × 2.5 cm2), deposited at 600 °C with or without TiO2 underlayer electrically contacted through a copper tape using the bare FTO surface. The illumination area (0.785 cm2) was defined with an O-ring, and light enters the cell from the electrolyte side through a quartz window. The light source is equipped with a calibrated white lightemitting diode (LED; WLC01, Zahner) controlled and powered by a second potentiostat (PP211, Zahner), and the output power was set to 100 mW cm−2. Current−voltage (I−V) curves were recorded at a rate of 10 mV s−1 from 1 to −0.7 V versus Ag/AgCl (satd KCl) in the dark and under illumination. Potential values were converted to the reference RHE using the relationship (eq 1)

(CH3)CHC(CH3)OLi] (1.20 g, 6.8 mmol). The product was isolated by crystallization from a concentrated hexane solution at −30 °C as red crystals, suitable for single-crystal XRD. Yield: 0.79 g (1.98 mmol, 58.1%) mp (capillary) [°C]: 40−45. 1H NMR (250 MHz, C6D6): δ [ppm] = −1.76 (s, br, 6H, -N(CH2)2OCH2CH3), −2.46 (s, br, 8H, -NCH2CH2OCH2CH3), −17.40 (d, br, 14H, OC(CH3)C(H)C(CH3)N), −28.81 (s, br, 4H, -NCH2CH2OCH2CH3). IR [cm−1]: 1560 (s), 1487 (s), 1390 (s), 1330 (m), 1267 (m), 1089 (s), 929 (m), 756 (s). EI MS (70 eV): m/z (%): 399 (13.7) [M+], 229 (100.0) [(M − L)+]. EA calculated (%) for C18H32CoN2O4: C 54.13, H 8.08, N 7.01; found (%): C 53.52, H 8.18, N 7.33. Synthesis of Bis(4-(methoxypropylamino)pent-3-en-2-onato)cobalt(II) [Co{CH3O(CH2)3NC(CH3)C(H)C(CH3)O}2] ([Co(mpki)2], 4). The synthesis procedure was similar to that adopted for compound 1. CoCl2 (0.44 g, 3.4 mmol) was treated with [CH3O(CH2)3NC(CH3)CHC(CH3)OLi] (1.20 g, 6.8 mmol). The product was isolated as orange crystals by crystallization from a concentrated hexane solution at −30 °C. Suitable crystals for X-ray analysis were obtained by sublimation at 100 °C in vacuo. Yield: 0.88 g (2.20 mmol, 65.0%) mp (capillary) [°C]: 72−75. 1H NMR (250 MHz, C6D6): δ [ppm] = −0.49 (s, 10H, -N(CH2)2CH2OCH3), 7.34 (s, br, 4H, -NCH2CH2CH2OCH3), −18.91 (d, br, 14H, OC(CH3)C(H)C(CH3)N), −26.93 and −31.11 (2s, br, 4H, -NCH2(CH2)2OCH3). IR [cm−1]: 1557 (s), 1490 (s), 1395 (s), 1370 (w) 1346 (m), 1103 (s),1089 (sh), 931 (m), 788 (m). EI MS (70 eV): [m/z]: 399 (22.3) [M+], 229 (100.0) [(M − L)+]. EA calculated (%) for C18H32CoN2O4: C 54.13, H 8.08, N 7.01; found (%): C 53.95, H 7.69, N 7.27. Synthesis of Bis(4-(ethoxypropylamino)pent-3-en-2-onato)cobalt(II) [Co{CH3CH2O(CH2)3NC(CH3)C(H)C(CH3)O}2] ([Co(epki)2], 5). The synthesis procedure was similar to that adopted for compound 1. CoCl2 (0.44 g, 3.4 mmol) was treated with [CH3CH2O(CH2)3NC(CH3)CHC(CH3)OLi] (1.35 g, 6.8 mmol). The residue was extracted with pentane. The target compound was isolated by removal of the solvent and subsequent drying in vacuo overnight at 80 °C to remove residual ligand yielding a blood-red viscous liquid. Yield: 0.94 g (2.20 mmol, 65.0%) 1H NMR (250 MHz, C6D6): δ [ppm] = −0.72 (s, 14H, -N(CH2)2CH2OCH2CH3), 7.59 (s, br, 4H, -NCH2CH2CH2OCH2CH3), −18.77 (d, br, 14H, OC(CH3)C(H)C(CH3)N), −26.92 and −31.00 (2s, br, 4H, -NCH2CH2CH2OCH2CH3). IR [cm−1]: 1557 (s), 1489 (s), 1390 (m), 1103 (s), 931 (m), 751 (m). EI MS (70 eV): m/z (%): 427 (15.6) [M+], 243 (100.0) [(M − L)+]. EA calculated (%) for C20H36CoN2O4: C 56.20, H 8.49, N 6.55; found (%): C 55.91, H 8.30, N 6.90. Synthesis of Bis(4-(isopropoxypropylamino)pent-3-en-2-onato)cobalt(II) [Co{(CH3)2CHO(CH2)2NC(CH3)C(H)C(CH3)O}2] ([Co(ippki)2], 6). The synthesis procedure was similar to that adopted for compound 1. CoCl2 (0.44 g, 3.4 mmol) was treated with [(CH3)2CHO(CH2)2NC(CH3)CHC(CH3)OLi] (1.20 g, 6.8 mmol). The target compound was isolated by crystallization at −30 °C. Crystals, suitable for X-ray analysis, were grown from a concentrated solution in pentane/toluene (10:1) at −30 °C. Yield: 0.92 g (2.02 mmol, 59.4%) mp (capillary) [°C]: 55−59. 1H NMR (250 MHz, C6D6): δ [ppm] = −1.20 (s, 18H, -N(CH2)2CH2OCH(CH3)2), 8.10 (s, br, 4H, -NCH2CH2CH2OCH(CH3)2), −18.68 (d, br, 14H, OC(CH3)C(H)C(CH3)N), −26.89 and −31.01 (2s, br, 4H, -NCH2(CH2)2OCH(CH3)2). IR [cm−1]: 1558 (m), 1488 (s), 1392 (s), 1104 (s), 1090 (sh), 930 (m), 788 (m). EI MS (70 eV): m/z (%): 382 (48.7) [(M − CH2OiPr)+], 257 (100.0) [(M − L)+], 214 (98.5) [(M − L − iPr)+]. EA calculated (%) for C22H40CoN2O4: C 58.01, H 8.85, N 6.15; found (%): C 56.62, H 7.83, N 6.37. 4.2. Thin-Film Deposition and Characterization. MOCVD experiments using [Co(ipki)2], (1), as precursor were performed in a custom-built CVD reactor as described elsewhere.55 Prior to film deposition, the substrates (Si(100)-wafers (1.0 × 1.0 cm2)) were rinsed with acetone and subsequently cleaned ultrasonically in isopropanol and water (analytical grade) for 30 min and then dried in a dry nitrogen stream. The reactor pressure was kept constant at 1 mbar for all depositions. Carrier gas (nitrogen 6.0 purity) and reactant gas (oxygen) were both maintained at 50 sccm during the deposition process. Cobalt oxide thin films were grown at deposition temper-

0 E RHE = EAg/AgCl + 0.197 + 0.059pH

(1)

where E is the experimentally measured potential versus Ag/AgCl, and E0Ag/AgCl (satd KCl) is the standard potential of Ag/AgCl reference electrode against the NHE (0.196 V). For electrochemical impedance spectroscopy (EIS) measurements, the potential was varied from −0.8 to +0.5 V versus Ag/AgCl with 50 mV steps and equilibrated for 30 s at each potential. Impedance data were collected using a 20 mV alternating current amplitude at 100 Hz. The flat band potential (Vfb) was calculated using the Mott−Schottky relationship (eq 2)

⎛ 1 1 kT ⎞ = ⎜Vapp − Vfb − ⎟ q ⎠ C2 qNDεoεrA2 ⎝ I

(2) DOI: 10.1021/acs.inorgchem.8b00204 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry where A is the surface area of the electrode, εo is the permittivity of free space, εr is the dielectric constant of cobalt oxide, ND is the donor density, Vapp is the applied potential, T is the temperature, and k is Boltzmann’s constant. C is the space charge capacitance, and q denotes the elementary charge. Room-temperature photoluminescence (PL) spectra were recorded with a Cary Eclipse fluorescence spectrophotometer (Varian) with excitation wavelength of 450 nm; the PL spectra were collected between 675 and 800 nm.

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(5) Jena, A.; Munichandraiah, N.; Shivashankar, S. A. Metal-organic chemical vapor-deposited cobalt oxide films as negative electrodes for thin film Li-ion battery. J. Power Sources 2015, 277, 198−204. (6) Mellsop, S. R.; Gardiner, A.; Marshall, A. T. Electrocatalytic Oxygen Evolution on Electrochemically Deposited Cobalt Oxide Films: Comparison with Thermally Deposited Films and Effect of Thermal Treatment. Electrocatalysis 2014, 5, 445−455. (7) Jeon, H. S.; Jee, M. S.; Kim, H.; Ahn, S. J.; Hwang, Y. J.; Min, B. K. Simple Chemical Solution Deposition of Co3O4 Thin Film Electrocatalyst for Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2015, 7, 24550−24555. (8) Weidler, N.; Paulus, S.; Schuch, J.; Klett, J.; Hoch, S.; Stenner, P.; Maljusch, A.; Brötz, J.; Wittich, C.; Kaiser, B.; Jaegermann, W. CoOx thin film deposited by CVD as efficient water oxidation catalyst: Change of oxidation state in XPS and its correlation to electrochemical activity. Phys. Chem. Chem. Phys. 2016, 18, 10708−10718. (9) Wang, L.; Deng, J.; Lou, Z.; Zhang, T. Cross-linked p-type Co3O4 octahedral nanoparticles in 1D n-type TiO2 nanofibers for highperformance sensing devices. J. Mater. Chem. A 2014, 2, 10022− 10028. (10) Wang, J.; Osterloh, F. E. Limiting factors for photochemical charge separation in BiVO4 /Co3O4, a highly active photocatalyst for water oxidation in sunlight. J. Mater. Chem. A 2014, 2, 9405−9411. (11) Feckl, J. M.; Dunn, H. K.; Zehetmaier, P. M.; Müller, A.; Pendlebury, S. R.; Zeller, P.; Fominykh, K.; Kondofersky, I.; Döblinger, M.; Durrant, J. R.; Scheu, C.; Peter, L.; FattakhovaRohlfing, D.; Bein, T. Ultrasmall Co3O4 Nanocrystals Strongly Enhance Solar Water Splitting on Mesoporous Hematite. Adv. Mater. Interfaces 2015, 2, 1500358. (12) Gasparotto, A.; Barreca, D.; Bekermann, D.; Devi, A.; Fischer, R. A.; Fornasiero, P.; Gombac, V.; Lebedev, O. I.; Maccato, C.; Montini, T.; van Tendeloo, G.; Tondello, E. F-Doped Co3O4 photocatalysts for sustainable H2 generation from water/ethanol. J. Am. Chem. Soc. 2011, 133, 19362−19365. (13) Li, R.; Zhou, D.; Luo, J.; Xu, W.; Li, J.; Li, S.; Cheng, P.; Yuan, D. The urchin-like sphere arrays Co3O4 as a bifunctional catalyst for hydrogen evolution reaction and oxygen evolution reaction. J. Power Sources 2017, 341, 250−256. (14) Ebadi, M.; Mat-Teridi, M. A.; Sulaiman, M. Y.; Basirun, W. J.; Asim, N.; Ludin, N. A.; Ibrahim, M. A.; Sopian, K. Electrodeposited ptype Co3O4 with high photoelectrochemical performance in aqueous medium. RSC Adv. 2015, 5, 36820−36827. (15) Carraro, G.; Maccato, C.; Gasparotto, A.; Kaunisto, K.; Sada, C.; Barreca, D. Plasma-Assisted Fabrication of Fe2O3-Co3O4 Nanomaterials as Anodes for Photoelectrochemical Water Splitting. Plasma Processes Polym. 2016, 13, 191−200. (16) Peeters, D.; Sadlo, A.; Lowjaga, K.; Mendoza Reyes, O.; Wang, L.; Mai, L.; Gebhard, M.; Rogalla, D.; Becker, H.-W.; Giner, I.; Grundmeier, G.; Mitoraj, D.; Grafen, M.; Ostendorf, A.; Beranek, R.; Devi, A. Nanostructured Fe2O3 Processing via Water-Assisted ALD and Low-Temperature CVD from a Versatile Iron Ketoiminate Precursor. Adv. Mater. Interfaces 2017, 4, 1700155. (17) Qin, W.; Wang, N.; Yao, T.; Wang, S.; Wang, H.; Cao, Y.; Liu, S. F.; Li, C. Enhancing the Performance of Amorphous-Silicon Photoanodes for Photoelectrocatalytic Water Oxidation. ChemSusChem 2015, 8, 3987−3991. (18) Deng, X.; Tüysüz, H. Cobalt-Oxide-Based Materials as Water Oxidation Catalyst: Recent Progress and Challenges. ACS Catal. 2014, 4, 3701−3714. (19) Ariffin, S. N.; Lim, H. N.; Talib, Z. A.; Pandikumar, A.; Huang, N. M. Aerosol-assisted chemical vapor deposition of metal oxide thin films for photoelectrochemical water splitting. Int. J. Hydrogen Energy 2015, 40, 2115−2131. (20) Schmid, S.; Hausbrand, R.; Jaegermann, W. Cobalt oxide thin film low pressure metal-organic chemical vapor deposition. Thin Solid Films 2014, 567, 8−13. (21) Maruyama, T. Electrochromic Properties of Cobalt Oxide Thin Films Prepared by Chemical Vapor Deposition. J. Electrochem. Soc. 1996, 143, 1383−1386.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00204. Further detail on precursor characterization, including NMR spectra, torsion angles, thermal properties, and EIMS spectra. RBS spectra, XPS survey, and C 1s core-level spectra, XRD patterns of thin films deposited on FTO, and MS plots of the TiO2 PEALD layer (PDF) Accession Codes

CCDC 1587793−1587795 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anjana Devi: 0000-0003-2142-8105 Author Contributions ∥

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the German Science Foundation (DFG-SPP-1613 and SolarH2) for funding this project (DE-790-13-1 and WA 1116/28-1). Thanks to M. Gebhard for providing PEALD-grown TiO2 layers on FTO substrates.

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REFERENCES

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Inorganic Chemistry PE-ALD process for TiO2 thin films employing a new Ti-precursor. J. Mater. Chem. C 2016, 4, 1057−1065.

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DOI: 10.1021/acs.inorgchem.8b00204 Inorg. Chem. XXXX, XXX, XXX−XXX