Cobalt and Cobalt Oxide Nanoparticles in - ACS Publications

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Ionic-Liquid-Modified Hybrid Materials Prepared by Physical Vapor Codeposition: Cobalt and Cobalt Oxide Nanoparticles in [C1C2Im][OTf] Monitored by In Situ IR Spectroscopy Sascha Mehl,† Tanja Bauer,† Olaf Brummel,† Kaija Pohako-Esko,‡ Peter Schulz,‡ Peter Wasserscheid,‡,§ and Jörg Libuda*,†,§ †

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany ‡ Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany § Erlangen Catalysis Resource Center and Interdisciplinary Center Interface-Controlled Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany ABSTRACT: The synthesis of ionic-liquid-modified nanomaterials has attracted much attention recently. In this study we explore the potential to prepare such systems in an ultraclean fashion by physical vapor codeposition (PVCD). We codeposit metallic cobalt and the room-temperature ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C1C2Im][OTf] simultaneously onto a Pd(111) surface at 100 K. This process is performed under ultrahigh-vacuum (UHV) conditions in the presence of CO, or in the presence of O2 and CO. We use time-resolved (TR) and temperature-programmed (TP) infrared reflection absorption spectroscopy (IRAS) to investigate the formation and stability of the IL-modified Co deposits in situ during the PVD-based synthesis. CO is used as a probe molecule to monitor the growth. After initial growth of flat Co films on Pd(111), multilayers of Co nanoparticles (NPs) are formed. Characteristic shifts and intensity changes are observed in the vibrational bands of both CO and the IL, which originate from the electric field at the IL/Co interface (Stark effect) and from specific adsorption of the [OTf]− anion. These observations indicate that the Co aggregates are stabilized by mixed adsorbate shells consisting of CO and [OTf]−. The CO coverage on the Co particle decreases with increasing temperature, but some CO is preserved up to the desorption temperature of the IL (370 K). Further, the IL shell suppresses the oxidation of the Co NPs if oxygen is introduced in the PVCD process. Only chemisorbed oxygen is formed at oxygen partial pressures that swiftly lead to formation of Co3O4 in the absence of the IL (5 × 10−6 mbar O2). This chemisorbed oxygen is found to destabilize the CO ligand shell. The oxidation of Co is not suppressed if IL and Co are deposited sequentially under otherwise identical conditions. In this case we observe the formation of fully oxidized cobalt oxide particles.

1. INTRODUCTION Materials modified by ionic liquids (ILs) have attracted lots of attention, with potential applications in heterogeneous catalysis,1 homogeneous catalysis,2 electrocatalysis,3,4 energy conversion,5,6 and even molecular electronics.7 Many of these applications benefit from the unique properties of ionic liquids, which combine high structural diversity, low melting temperatures, low volatility, and high thermal stability.3 Moreover, certain classes of ILs are considered as environmentally friendly and can be referred to as “green solvents”.8 A unique feature of ILs is the tunability of physicochemical properties such as their solvation behavior, polarity, or miscibility, which results from their structural diversity and the large number of anion−cation combinations. In many cases, the intriguing functionalities of IL-modified nanomaterials arise from their interface properties. This not only holds true for application but also is an important aspect of materials synthesis with ILs, where ILs can be used as structure© XXXX American Chemical Society

directing agents. Many examples can be found in the recent literature on synthesis routes for nanomaterials that involve ILs, for instance, for the preparation of metal nanoparticles (NPs), nanoalloys, and metal oxides.9−11 However, most of these synthetic routes have been developed in an empirical trial-anderror approach. From the microscopic point of view, the shape control in synthesis arises from the structure sensitivity of interactions between the IL and the growing nanoparticles. Aiming at a knowledge-driven materials design, an in-depth understanding would be required of the chemistry at the NP− IL interfacial. Ideally, we would need to develop spectroscopic routes that allow us to investigate such interfaces at all stages of nanoparticle nucleation and growth. Here in situ methods are Received: June 21, 2016 Revised: July 26, 2016

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DOI: 10.1021/acs.langmuir.6b02303 Langmuir XXXX, XXX, XXX−XXX

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In this work we go one step further and probe the reactivity of NPs formed by this method toward oxygen. Our aim is to follow the formation of partially oxidized nanomaterials in situ during PVD-based synthesis. We focus on cobalt oxide, which has attracted much interest recently due to its catalytic activity with the potential to replace noble metals.31,32 In addition cobalt oxides have a large application potential in other fields such as water splitting and/or sensor technology. 11,31 Interestingly, catalytic reactions involving cobalt oxides are in general highly structure-sensitive.33,34 The same holds for the interaction of cobalt oxides with organic molecules, as we have recently shown for CoO and Co3O4 films.35 Such a structure dependence suggests that there is also a large potential to control growth processes by structure-directing agents. Indeed, differently shaped cobalt oxide NPs were prepared, such as nanotubes,36 nanocubes,37 and other nanostructures.10,32,38 To complement such synthetic work in the future, in this work we explore new methodic and spectroscopic approaches to monitor the growth of Co and cobalt oxide NPs in situ and under ultraclean conditions.

the key that provides experimental access to nanoparticles growing in an atomically clean environment. Such in situ studies of atomically clean IL/solid interfaces are possible using a surface science approach. In IL surface science we start from ultraclean, atomically well-defined single crystals to prepare IL/solid interfaces at different levels of complexity.12 In our previous work we have explored IL films on metal surfaces,13−15 on thin oxide films,16−18 and on oxide-supported metal particles.16,19 Typically, these IL films are prepared by physical vapor deposition (PVD). Model interfaces prepared in this way can then be investigated by a large spectrum of surface science methods, e.g., photoelectron spectroscopies,12,20,21 vibrational spectroscopies,14−17,19,22,23 or scanning probe microscopies.24−26 These studies have provided insight into the adsorption, structure formation, and reaction of ILs on surface at an unprecedented level of detail. In a recent study we investigated the adsorption of the room-temperature IL [C1C2Im][OTf] (1-ethyl-3-methylimidazolium trifluoromethanesulfonate) on Pd(111).15 We could show that the [OTf]− anions adsorb specifically on the metal surface, binding via the SO3 group in an upright standing geometry. At higher coverage partially oriented multilayers are formed. By means of infrared refection absorption spectroscopy (IRAS) and temperatureprogrammed desorption (TPD), we could also prove that the preadsorbed CO is preserved during codeposition of [C1C2Im][OTf], leading to a compression of the CO layer. CO occupies hollow and bridge sites, and the compression leads to a decrease of the adsorption energy. Spectroscopically, such CO−IL coadsorbate layers are characterized by a pronounced red-shift in the CO stretching frequency. The latter results from the interfacial electric field generated by the IL layer (Stark effect) and, possibly, from ligand effects due to specifically adsorbed [OTf]− anions. Such studies on planar surfaces provide also the basis for an in-depth understanding of the interaction of ILs with metal nanoparticles (NPs). The synthesis of NPs in ILs has attracted a lot of attention, motivated, for instance, by applications in catalysis.9,10,14,27 Besides the wet chemical methods to prepare such systems, an interesting approach to prepare ligand-free metal aggregates uses PVD of metals into an IL phase. Dupont and others have explored this preparation approach in detail and suggested models to explain the stabilization mechanisms and the properties in these systems.28−30 To explore the interfacial chemistry between the growing nanoparticles and the IL, however, in situ spectroscopy is required. With this goal in mind, we recently developed a new in situ IR experiment that allows us to study IL−NP interactions during particle synthesis by PVCD. We deposited the IL and the metal simultaneously onto an inert substrate under UHV conditions. Additionally we can add CO, which served as a ligand and, simultaneously, as a structure-sensitive probe molecule.14 Recently, we applied this approach to the growth of Pd NPs in [C1C2Im][OTf] and could show that Pd NPs of different sizes can be stabilized in the IL matrix.14 In addition we could show that the interaction mechanism between [C1C2Im][OTf] and the growing Pd NPs is very similar to that observed for the same IL on planar Pd(111).15 A mixed adsorbate shell is formed that consists of specifically adsorbed [OTf]− anions and coadsorbed CO. The IL compresses the CO layer and facilitates desorption, yet mixed coadsorbate shells were found to exist well above the melting point of the IL (i.e., up to ∼350 K).

2. EXPERIMENTAL DETAILS All experiments were performed in an UHV IR-spectroscopy system with a base pressure of 2 × 10−10 mbar. As we described previously, the UHV chamber is equipped with various gas dosers and evaporators to carry out PVD.39 During PVD the growing film can be exposed to reactant gases while IR spectra are acquired in a time-resolved fashion. LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy) and temperature-programmed adsorption (TPD) quadrupole mass spectrometry (QMS) were used to characterize the samples and gases. IR spectra were acquired with a vacuum Fouriertransform infrared (FTIR) spectrometer (Bruker VERTEX 80v) equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector. For the present experiments, a Pd(111) single crystal was used as IR-reflective support. Its surface was cleaned in UHV by several cycles of Ar+ ion bombardment and annealing15 (typically 30 min at p(Ar+) = 5 × 10−5 mbar (Linde, 6.0), 1 keV, ion current 10 μA, subsequent annealing at 1 × 10−8 mbar O2 (Linde, 5.0) for 10 min at 900 K). Afterward the Pd(111) single crystal was cooled to 300 K in O2 atmosphere at the same pressure. Subsequently, residual oxygen was removed by brief heating to 900 K in UHV. The cleanliness of the sample was verified by LEED/AES. PVD of [C1C2Im][OTf] was performed using a home-built thermal IL evaporator with a ceramic crucible. The deposition rate was estimated from the time-resolved IR spectra, which allowed for differentiating between monolayer (ML) and multilayer adsorption; for details, see our previous study on Pd(111).16 For the PVD experiments we used different IL deposition rates between 0.01 and 1 ML/min. From previous work we assume that 1 ML of [C1C2Im][OTf] corresponds to an average IL layer thickness of ∼7 Å.15 The deposition rate of Co was calibrated with a quartz crystal microbalance as described in previous publications.39 We used values between 0.03 and 1.6 ML/min with 1 ML corresponding to an average Co layer thickness of 2 Å. For low-temperature deposition, the Pd(111) crystal was cooled to 100 K using a liquid nitrogen cryostat. The sample was heated by radiation from a W filament and by electron bombardment. The sample temperature was measured using a type K thermocouple spot-welded to the single crystal. Except for the pulse experiments, all TR-IRA spectra were recorded at a partial pressure of 1 × 10−6 mbar CO (Westfalen, Minican). To reduce contamination, CO was precleaned with a liquid nitrogen cooling trap before the gas reservoir. Before every IRAS experiment, a reference spectrum was recorded with a spectral resolution of 2 cm−1 and with 10 min acquisition time. TR- and TP-IR spectra were collected with 2 cm−1 resolution at an acquisition time of 1 min per spectrum. During the pulse experiment the Co/Pd(111) system was exposed to 12 exponentially increasing CO doses using a computer-controlled dosing device, followed by acquisition of the IR B


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Langmuir spectrum in a fully automatized sequence (LabVIEW, National Instruments interface). For the heating ramps, the temperature was controlled by a LabVIEW (National Instruments) proportional− integral−derivative (PID) controller and set to a heating rate of 2 K/ min. Synthesis of [1-ethyl-3-methylimidazolium][trifluoromethanesulfonate]([C2C1Im][OTf]): 74.720 g (0.419 mol, 1.01 equiv) of ethyl trifluoromethanesulfonate (Aldrich) was dropwise added to 34.195 g (0.417 mol) of freshly distilled N-methylimidazole (Aldrich) under argon atmosphere at 0 °C. After addition the reaction mixture was allowed to warm to room temperature and stirred overnight. The reaction was monitored by 1H NMR spectroscopy (Jeol ECX 400 MHz). The product was obtained in quantitative yield as a clear colorless liquid. The product was dried by vacuum at about 10−2 mbar and 60 °C for 16 h. The water content after drying was determined by Karl Fischer titration (Metrohm, 756 KF Coulometer) and was found to be below 500 ppm. 1H NMR (d6-DMSO, 400 MHz): δ 1.41 (t, 3H, J = 7.2 Hz), 3.84 (s, 3H), 4.18 (q, 2H, J = 7.2), 7.67 (s, 1H), 7.76 (s, 1H), 9.08 (s, 1H).

stepwise CO exposure using a remote-controlled doser (see Figure 2). For a better understanding we have added an overview (see Table 1) of all deposition experiments we did in this paper.

3. RESULTS AND DISCUSSION 3.1. Adsorption of CO on Thin Co Films on Pd(111) Monitored by TR-IRAS. Before studying the influence of the IL, we first investigated the pristine Co/Pd(111) as a reference system. We performed two different adsorption experiments starting from a clean Pd(111) surface at 100 K. First we carried out a coadsorption experiment in which we deposited metallic Co by PVD for 60 min at a deposition rate of 0.2 ML/min in CO (1 × 10−6 mbar) (see Figure 1). During deposition, IR spectra were recorded at a rate of 1 spectrum per min. In a second experiment, we predeposited 2 ML (4 Å) of Co on Pd(111) at 100 K. Subsequently IR spectra were recorded after

Figure 2. IR spectra of the CO stretching frequency region adsorbed for stepwise adsorption of CO at 100 K on a 2 ML Co film (4 Å) prepared by PVD on Pd(111).

In the beginning of the codeposition experiment (Figure 1a), we obtained a spectrum with three features located at 1940, 2068, and 2106 cm−1. The most prominent peak at 2106 cm−1 belongs to the stretching vibration νs(C−O) of on-top CO on Pd(111).40 The broad feature around 1940 cm−1 can be assigned to CO adsorbed at bridge and hollow sites on Pd(111).40−42 Finally, the shoulder at 2068 cm−1 can be attributed to CO adsorbed to Co.43−49 Features in this region are found for CO adsorption on Co NPs but also for CO on Co single crystals at elevated pressure.50,51 Previously, Carlsson et al. attributed this feature to a carbonyl-like surface species, i.e., a Co center coordinated to more than one CO. On the basis of isotopic exchange studies, the authors suggested that Co(CO)n species with n = 3 are formed. With increasing deposition time, the peak at 2068 cm−1 becomes the most prominent band and develops a shoulder at 1992 cm−1. This spectrum closely resembles the one observed for Co NP on alumina previously reported by Carlsson et al.45 Accordingly we attribute the dominating band to on-top CO in the surface carbonyl. The shoulder at 1992 cm−1 is assigned to a second regular on-top CO species on Co. Note that the intensity of the bands does not reflect the abundance of the features, as dynamic dipole coupling is expected to lead to strong intensity transfer to the high-frequency feature at 2068 cm−1.45 In addition the dynamic dipole moment of on-top CO is much larger than that of CO at higher coordinated sites. With ongoing Co deposition, all Pd-related features are slowly attenuated. Whereas the attenuation is fast during the first 10 min of deposition, the change becomes much slower at a later stage. Interestingly, the CO intensity shows a slow and linear increase with Co deposition in the multilayer region (see Figure 1b). We assign this effect to a small amount of bulk

Figure 1. (a) Time-resolved IR spectra taken during PVD of 12 ML Co with a rate of 0.2 ML/min in CO atmosphere (1 × 10−6 mbar) onto Pd(111) at 100 K; (b) integral intensity of the IR bands in the CO region as a function of deposition time; (c) peak position as a function of deposition time. C


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Langmuir Table 1. Overview about Codeposition Experiments Performed in This Work sample

deposition experiment

substrate

[C1C2Im][OTf] layer thickness/ML

Co layer thickness/ML

p(O2)/mbar

A B C D E F

codeposition codeposition sequential codeposition codeposition sequential

Pd(111) Pd(111) IL/Pd(111) Pd(111) Pd(111) IL/Pd(111)

1 10 12 10 10 10

12 100 2 100 100 75

without O2 without O2 without O2 5 × 10−7 5 × 10−6 5 × 10−6

cobalt carbonyls formed during deposition. We will discuss this point in more detail in section 3.2. Next we discuss the evolution of the CO bands with coverage on a predeposited 4 Å Co film (2 ML) on Pd(111) at 100 K (see Figure 2). Using an automated CO doser, IR spectra were acquired at exponentially increasing doses from 0.01 to 25 L. At lowest exposure we observed a very weak feature at 1977 cm−1. Wasniowska et al. investigated the growth of Co on Pd(111) and showed that Co forms two-dimensional dendritic doublelayer islands at low temperature.52 We assigned the feature at 1977 cm−1 to on-top CO on these double-layer islands.49 With increasing exposure, the CO density on the surface causes a blue-shift due to 1997 cm−1 (3 L CO) as a result of dipole coupling. Simultaneously new bands appear around 2100 and 1840 cm−1, which are attributed to on-top CO on Pd(111) and to hollow CO on Co islands, respectively. In the limit of large exposure, these bands develop into peaks at 2119, 2055, 1850, and 1930 cm−1. Following the above discussion, the band at 2119 cm−1 is assigned to on-top CO on Pd(111), the band at 2055 cm−1 is assigned to on-top CO in the surface carbonyl on Co, and the weak bands at 1850 and 1930 cm−1 can be attributed to 3-fold-hollow and bridge-bonded CO both on the Co islands and on the Pd(111) substrate. 3.2. PVCD of [C1C2Im][OTf] and Co on Pd(111) in CO Atmosphere. To probe the influence of the IL on preadsorbed CO, we deposited 1 ML [C1C2Im][OTf] onto a CO-precovered Co film (4 Å) on Pd(111) at 100 K. IR spectra were recorded during IL deposition (see Figure 3). We observed that the IR bands associated with the [C1C2Im][OTf] grew linearly with time, indicating a constant IL deposition rate. In fact all new bands observed in Figure 3a can be attributed to molecular [C1C2Im][OTf] ,and no indication for decomposition was observed. We analyzed the IR spectrum of [C1C2Im][OTf] in detail.14 In the spectral region from 1350 to 1000 cm−1, we observed the stretching modes of [OTf]−. Here, the most intense features are the symmetric stretching mode of SO3, νs(SO3), at 1036 cm−1; the symmetric stretching mode of CF3, νs(CF3), at 1232 cm−1; the antisymmetric stretching mode of SO3, νas(SO3), at 1285 cm−1; and the antisymmetric stretching mode of CF3, νas(CF3), at 1175 cm−1. At low temperature the spectrum was more complex due to anion− cation interactions and the crystal field in the solid state. Upon melting, the IR spectrum transformed into the simpler spectrum of the liquid IL (see our previous publication14 for details). In the spectral region between 1600 and 1350 cm−1, several weaker bands were observed, some of which are associated with the [C1C2Im]+ cation. The most prominent one is located at 1576 cm−1 and assigned to a ring-stretching mode of [C1C2Im]+. Above 2900 cm−1, CH modes appeared that will not be discussed further in the present study. Focusing on the CO region, the IL deposition has a strong effect on the CO bands. Whereas the on-top CO band at 2119

p(CO)/mbar 1 1 1 1 1 1

× × × × × ×

10−6 10−6 10−6 10−6 10−6 10−6

sample temperature/K 100 100 100 100 100 100

Figure 3. (a) IR spectra taken during deposition of 1 ML (7 Å) of [C1C2Im][OTf] onto a 2 ML Co film (4 Å) on Pd(111) precovered with CO at 100 K (total IL deposition time = 17 min); (b) comparison of the CO region after 1 and 17 min of deposition; (c) peak position of the CO band as a function of deposition time.

cm−1 vanished quickly during the initial stages of deposition, the on-top CO band from the Co islands at 2050 cm−1 was preserved. However, it showed a pronounced red-shift by 33 cm−1 to 2017 cm−1 with increasing IL exposure. Previously, we observed similar shifts for the coadsortion of [C1C2Im][OTf] and CO on Pd(111). The effect originates mainly from the Stark effect induced by the interfacial electric field at the IL/ metal interface.15,53,54 Very similar to Pd(111), the symmetric [OTf]− modes at 1036 and 1232 cm−1 dominate the spectrum at low IL coverage. This observation indicates that [OTf]− adsorbs on Co in an upright standing geometry. Additional spectral shifts in the νs(SO3) region (see section 3.3) indicate that the [OTf]− interacts with the Co surface via the SO3 group. Next, we performed simultaneous codeposition experiments with Co and [C1C2Im][OTf] in CO atmosphere. In the first experiment (sample A), we codeposited 7 Å IL (1 ML) and 24 Å Co (12 ML) in CO (10−6 mbar) onto Pd(111) at 100 K (deposition time = 60 min). The IR spectra taken during deposition are shown in Figure 4. At low Co coverage the dominating band is the Co surface carbonyl at 2066 cm−1, which shifts to lower wavenumbers with increasing deposition time due to the coadsorbed IL. In addition we observe the on-top CO on Pd(111) at 2099 cm−1 and bridge-bonded and hollow CO on Pd(111) and Co around 1900 cm−1 (both red-shifted due to coadsorbed IL). With D


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Figure 4. (a) IR spectra recorded during PVCD of 1 ML [C1C2Im][OTf] (7 Å) and 12 ML Co (24 Å) in CO (10−6 mbar) at 100 K (deposition time = 60 min); (b) selected spectra after 1, 17, 40, and 60 min of PVCD.

Figure 5. IR spectra of the CO stretching frequency region taken during deposition of (a) 1 ML (7 Å) [C1C2Im][OTf] and 12 ML (24 Å) Co (sample A) and (b) 10 ML (70 Å) [C1C2Im][OTf] and 200 Å (100 ML) Co (sample B); (c) total band intensity in the CO region as a function of deposition time; (d) CO peak as a function of deposition time for sample A; (e) CO peak position as a function of deposition time for sample B (CO background pressure = 10−6 mbar; deposition time = 60 min; sample temperature = 100 K).

increasing deposition time all Pd-related bands disappeared and two new bands appeared at 2011 and 1840 cm−1. Both new bands grew linearly with deposition time (see Figure 5). In a second experiment (sample B), we increased the deposition rate of both IL and Co and deposited 70 Å IL (10 ML) and 200 Å (100 ML) Co in 60 min, again at a CO pressure of 10−6 mbar (see Figure 5). Again we observed two linearly growing broad bands but at slightly higher wavenumbers (2024 and 1850 cm−1), and the former with a 3.5× larger intensity as compared to sample A. The linearly increasing intensity as a function of deposition time indicates the formation of a homogeneous multilayer of partially CO-covered Co aggregates. These aggregates grew once the Pd substrate was fully covered by Co. In comparison to the IL-free experiment in Figure 2, the intensity of the CO bands was reduced in the presence of codeposited IL. We attribute this effect to the coadsorption of IL at the Co NPs, which reduced the CO coverage. Unfortunately, it is not possible to determine the morphology and size of the growing multilayer film of NPs with standard surface science experiments. Some structural information may be derived from the spectroscopic data, however. The IR spectra in Figure 5 suggest the formation of larger aggregates rather than small carbonyl species. Several Co carbonyls have been characterized by IR spectroscopy, such as Co(CO)4, Co2(CO)8,46−48 and Co4(CO)12.46 Matrix isolation IR spectra of Co(CO)4 show two IR bands around 2030 and 2010 cm−1; the IR spectrum of Co4(CO)12 comprises strong terminal CO bands between 2050 and 2060 cm−1 and a weaker bridge peak at 1870 cm−1, and the IR spectrum of Co2(CO)8 with terminal CO bands between 2020 and 2070 cm−1 and bridging CO bands around 1860 cm−1 is particularly complex due to the coexistence of at least three isomers. In general these

molecular carbonyl compounds show much sharper bands the IL-modified Co NPs in this work are larger in size and expose a variety of structures.49 It is noteworthy that the spectral range of the on-top and bridging CO is red-shifted by ∼30 cm−1 in comparison to the molecular carbonyls. We attribute this shift to the coadsorption of the IL in agreement with the codeposition experiments in Figure 3. A rough estimate of the aggregate size may be obtained from the intensity development in Figure 5c. Assuming growth of a Co double layer (see above), the Pd surface is covered after deposition of 4 Å Co. If we assign the corresponding CO signal to 1 ML equivalent (MLE) and assume similar CO density on all Co surfaces, we estimate that 3−4 MLE of additional CO are observed after deposition of additional 16 Å (8 ML) Co (50 min total deposition time; see Figure 5c). If we further assume random orientation of CO in the NP phase, this corresponds to a total of 5−7 MLE of additionally adsorbed CO on the 8 ML Co. This estimate yields a Co dispersion of 65−85% corresponding to a Co particle size of 1.2 ± 0.3 nm. However, this number should be considered as a rough estimate only. Our previous studies on the formation of Pd NPs in [C1C2Im][OTf] suggested that different NP sizes could be prepared depending on the deposition conditions.14 A similar behavior is found in the present case if we compare samples A and B. An 8-fold increase in the Co deposition rate for sample B yields a 4-fold increase in the CO signal only, indicating that the dispersion decreases for sample B and, correspondingly, the average particle size increases. 3.3. Co PVD in CO Atmosphere onto Frozen [C1C2Im][OTf] Films. To obtain better spectroscopic insight into the interaction between the Co particles and the IL, we performed a sequential codeposition experiment (sample C). First we deposited a liquid [C1C2Im][OTf] film (12 ML) onto clean E


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bridge/hollow CO on Co, respectively. In Figure 6e the total intensity on the CO stretching region is displayed as a function of the Co coverage. In sharp contrast to the codeposition experiments in section 3.2, we observed decreasing slope, indicating the growth of three-dimensional Co islands on the frozen IL film. As the CO frequencies are identical to those observed on the IL-free Co (see Figure 1), we conclude that the vacuum interface of the Co islands remains unaffected by the IL, i.e., the IL does not migrate onto the top of the Co islands at this temperature. An interesting phenomenon is observed in the spectral region of the IL bands (Figure 6b and d). Note that these are difference spectra with the reference taken after IL deposition. In addition to the positive bands, which indicate attenuation of IL bands due to Co deposition, we observed a negative absorption feature at 1029 cm−1. Previously a similar band for adsorption of [C1C2Im][OTf] in Pd(111) could be assigned to the symmetric SO3 stretching mode (νs(SO3)) of the [OTf]− that is interacting with the metal surface.15 Accordingly, we attributed the band at 1029 cm−1 to [OTf]− ions adsorbed to the growing Co NPs. As the CO-covered “upper” part of the Co NPs was unaffected by the IL (see above), we concluded that this signal stems from the interface between the growing Co NPs and the frozen IL layer. Thus, it is possible to differentiate by IR spectroscopy between the Co/IL and the Co/vacuum interface. 3.4. Influence of O2 on PCVD of [C1C2Im][OTf] and Co on CO Atmosphere. Finally, we investigate the influence of O2 on the formation of the Co NPs formed by PVCD of Co and [C1C2Im][OTf] in CO atmosphere. To this end we carried out a series of codeposition experiments in the presence of additional O2 (200 Å Co, 70 Å [C1C2Im][OTf], 1 × 10−6 mbar CO, deposition time = 60 min). We used two O2 partial pressures, namely, 5 × 10−7 mbar (sample D) and 5 × 10−6 mbar (sample E). Again the deposition process was followed in situ by TR-IRAS, and the corresponding spectra are shown in Figure 7b and c. For sample D (p(O2) = 5 × 10−7 mbar), we observed a similar behavior of the CO band as previously found in the absence of O2 (sample C). The dominating feature is the ontop CO band, and in addition, a weak feature around 1850 cm−1 was observed that was attributed to higher coordinated CO. Interestingly, the on-top CO band (2047 cm−1) was found to be blue-shifted by 23 cm−1 in comparison to O2-free Co deposition. For sample E (p(O2) = 5 × 10−6 mbar), an even larger blue-shift by 43 cm−1 to 2067 cm−1 was found. In addition, the CO band intensity was found to decrease dramatically. Previously we have studied the adsorption of CO on different cobalt oxide films by IRAS. Adsorption of CO on cobalt oxide gives rise to several bands in the range between 2140 and 2175 cm−1.39 Even if we take into account a red-shift of 30 cm−1 to 40 cm−1 due to the presence of coadsorbed IL, the CO band positions are clearly below the values found for CO on cobalt oxides. Therefore, we attribute the blue-shift to coadsorbed chemisorbed oxygen on the Co NPs. Shifts in this range are typical for electronegative coadsorbates like oxygen and chlorine, which lead to reduced π-backdonation. Reactive deposition in O2 at pressures up to 5 × 10−6 mbar does not lead to the formation of oxide particles but to the coadsorption of chemisorbed oxygen on Co only. The oxygen coverage increases with increasing oxygen partial pressure during deposition.

Pd(111) at a sample temperature of 300 K. Subsequently the sample was cooled to 100 K, yielding a frozen IL film. Next we deposited 2 ML Co in CO (1 × 10−6 mbar) onto this frozen IL film. All preparation steps were followed in situ by TR-IRAS. Upon deposition of the IL at 300 K (see Figure 6a), we

Figure 6. Sequential PVD of [C1C2Im][OTf] and Co in CO atmosphere: (a) PVD of [C1C2Im][OTf] onto Pd(111) at 300 K; (b) subsequent cooling to 100 K followed by PVD of Co (2 ML, 4 Å) in CO (1 × 10−6 mbar); (c) CO stretching region; (d) region of the symmetric SO3 stretching mode; (e) band intensity in the CO stretching region; (f) peak position in the CO stretching region.

observed five main bands at 1042 cm−1 (νs(SO3)), 1174 cm−1 (νas(CF3)), 1228 cm−1 (νs(CF3)), 1288 cm−1 (νas(SO3)), and 1576 cm−1 (νs(CC)) (see ref 15 for details). The spectrum is characteristic for [C1C2Im][OTf] in the liquid state.14 Subsequently the IL film was cooled to 100 K and Co was deposited in CO atmosphere (Figure 6c and d). During deposition we observed the appearance of three features in the CO stretching frequency region that blue-shift with increasing Co coverage to 2065, 2006, and 1860 cm−1 (very broad and weak band). As discussed in section 3.1, these features can be attributed to the surface carbonyl on Co, to on-top CO, and to F


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experiments were performed in the same fashion: a reference spectrum was recorded after deposition at 100 K and, subsequently, the films were heated at a rate of 2 K/min while IR spectra were recorded at a rate of 1 spectrum/min. The results are summarized in Figure 8, where positive peaks

Figure 7. Comparison of the IR spectra in the CO stretching region recorded during PVD of Co: (a) PVCD of Co and [C1C2Im][OTf] in CO ([C1C2Im][OTf]: 10 ML, 70 Å; Co 100 ML, 200 Å, 60 min, pCO = 1 × 10−6 mbar); (b) PVCD of Co and [C1C2Im][OTf] in CO/O2 ([C1C2Im][OTf]: 10 ML, 70 Å; Co 100 ML, 200 Å, 60 min, pCO = 1 × 10−6 mbar, pO2 = 5 × 10−7 mbar); (c) PVCD of Co and [C1C2Im][OTf] in CO/O2 ([C1C2Im][OTf]: 10 ML, 70 Å; Co 100 ML, 200 Å, 60 min, pCO = 1 × 10−6 mbar, pO2 = 5 × 10−6 mbar); (d) PVD of Co in CO/O2 onto a predeposited frozen [C1C2Im][OTf] film at 100 K ([C1C2Im][OTf]: 10 ML, 70 Å; Co 75 ML, 150 Å, 45 min, pCO = 1 × 10−6 mbar, pO2 = 5 × 10−6 mbar); (e) IR band intensity in the CO region.

Figure 8. Temperature-programmed IR spectra recorded during annealing of the samples prepared by PVCD (samples B, D, and E) and sequential deposition (sample F). See text for details.

(red) represent decreasing absorption (or the loss of the corresponding species) and negative peaks (blue) represent increasing absorption (or the formation of the corresponding species). We focus on the spectral range of CO. At 150 K (T1 in Figure 8), we observed formation peaks of decreasing intensity at 2043, 2053, and 2067 cm−1 for the metallic Co samples B, D, and E, respectively. In sharp contrast we observed a loss feature at 2055 cm−1 for the cobalt oxide particles of sample F. As no additional CO was provided during the temperature ramp, the increasing intensity of the CO bands must arise from a restructuring of the CO-covered Co NPs. A possible explanation involves sintering. The formation of larger or more compact particles would lead to a decrease of the Co surface and, consequently, to a compression of the CO layer. As a result, bridge-bonded CO would be concerted to on-top sites, where the molecule has a much higher dynamic dipole moment. Indeed weak loss features are seen in the region of the bridging CO. Also the blue-shift of the peak position (comparing Figures 5 and 7) would be consistent with such a scenario. T2 and T4 indicate the temperature range in which the CO desorbs from the Co NPs, and T3 is the average desorption temperature. We observed that the onset temperature for CO desorption (T2) decreased from 270 K (sample B) to 210 K (sample C) with increasing oxygen coverage. In parallel the temperature at which the CO desorption was completed (T4) decreased from 395 to 320 K. In principle the decreasing CO desorption temperature with increasing O coverage could also be due to CO oxidation with preadsorbed oxygen. In the latter case, however, an even larger red-shift (release of O coadsorbate effect and CO coverage effect) would be expected during the temperature ramp. The results suggested that CO mainly desorbed molecularly and the adsorption strength of CO decreaseD with increasing O coverage.

To explore whether the Co oxidation is indeed prevented by the IL shell, we performed a sequential deposition experiment similar to the one described in section 3.3. An IL multilayer (10 ML) was deposited at 300 K and subsequently cooled to 100 K. Next, a Co film (150 Å, deposition time = 45 min) was deposited by PVD in the presence of CO (1 × 10−6 mbar) and O2 (5 × 10−6 mbar). The O2 partial pressure was in the range that is typically used to prepare Co3O4 films in UHV studies.39 The corresponding IR spectra (Figure 7d) showed a sharp peak at 2140 cm−1 shifting to 2155 cm−1 with deposition time. This band position is in excellent agreement with the value of 2158 cm−1 previously observed for CO on Co3O4(111) at 100 K.39 We suggest that fully oxidized Co3O4 nanoparticles are formed by the sequential deposition procedure supported on an IL ice multilayer. Similarly as for O2-free deposition, the intensity behavior of the CO band suggests the growth of threedimensional oxide particles (see Figure 7e). A comparison between the codeposition experiments (samples D and E) and the sequential deposition experiments (sample F) shows clearly that the presence of the IL shell prevents the growing Co NPs from oxidation. The effect may be associated with the hindrance of oxygen dissociation on Co nanoparticles covered by the strongly adsorbing CO/IL shell. 3.5. Thermal Behavior of the Co NPs Prepared by PVCD of Co and [C1C2Im][OTf]. Finally, the thermal behavior of selected samples was investigated by temperature-programmed IRAS. For all samples in this study, the TP-IRAS G


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conditions leads to the formation of Co3O4 nanoparticles. The suppressed oxidation is associated with the hindrance of oxygen dissociation by the adsorbed IL shell. (6) Upon annealing, CO desorbs from the mixed CO IL ligand shell on the Co NPs in a temperature range from 250 to 390 K. With increasing concentration of coadsorbed oxygen, the CO desorption temperatures decreases. On the Co3O4 particles prepared by sequential PVD, CO adsorbs weakly and desorbs already at temperatures below 200 K, i.e., below the melting transition of the IL.

For the sequential deposition experiment (sample F), we also show the spectral region of the [OTf]− indicating the phase transitions in the IL film. At temperatures up to 245 K, changes in the IL bands are associated restructuring processes in the frozen IL. The sudden change of the band intensities at 245 K is associated with melting of the IL in excellent correspondence with the bulk melting temperature found by Choudhury and others at 247.5 K.55 Above 370 K the liquid [C1C2Im][OTf] multilayer starts to desorb. Some weak bands appearing at even higher temperature are attributed to decomposition of residual IL fragments (e.g., to the band at 2206 cm−1 associated with CN fragments from [C1C2Im]+). For the Co3O4 NPs (sample F), the CO desorption behavior is very different. Here, the CO desorbs from the Co oxide NPs at temperatures below 200 K (see our previous publication39 for details). Consequently the CO desorbs before the IL melts and the Co3O4 NP gets fully covered by the IL layer. As a result, no positive CO signal is observed in this case.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49 9131 8527. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft within the framework of SPP 1708 “Material Synthesis near Room Temperature”. The authors acknowledge further financial support by the Deutsche Forschungsgemeinschaft (DFG) within the project “COMCAT”, within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative and by the Clariant AG. Further travel support by the COST Action CM1104 “Reducible oxide chemistry, structure and functions” is gratefully acknowledged.

4. CONCLUSION In this work we explored the preparation of IL-modified cobaltbased nanomaterials by PVCD of the room-temperature IL [C1C2Im][OTf] and metallic Co onto Pd(111) (a) under UHV conditions, (b) in CO atmosphere, and (c) in a mixed CO and O2 atmosphere. The formation of the Co NPs and their interaction with the IL was monitored in situ by time-resolved and temperature-programmed IRAS. Specifically we found that: (1) On predeposited Co films on Pd(111), CO occupies hollow (1850 cm−1), bridge-bonded (1930 cm−1), and on-top sites (1980−2020 cm−1 on Co, 2100−2120 cm−1 on Pd). At high CO exposure, a band at 2050−2070 cm−1 indicates formation of a high coverage phase on Co, associated with the formation of surface carbonyl species. The same CO species are also observed upon PVD of Co onto Pd(111) at 100 K in CO atmosphere (10−6 mbar), before a bulk Co carbonyl species starts to grow at larger Co exposure. (2) Deposition of [C1C2Im][OTf] onto a CO-covered Co film on Pd(111) gives rise to a characteristic red-shift of the CO bands by 30−40 cm−1. The effect is associated with the coadsorption of the IL generating an additional interfacial electric field (Stark effect). The [OTf]− anions adsorb specifically to the Co surface and adopt an upright standing orientation in the first layer. (3) PVCD of Co and [C1C2Im][OTf] in CO atmosphere on Pd(111) at 100 K leads to the growth of a Co layer on Pd followed by the growth of Co NPs, stabilized by a mixed adsorbate shell consisting of the IL and CO. Dispersion and size of the NPs varies with the deposition parameters. At low deposition rates, very small Co NPs in the range of 1 nm can be formed. (4) Sequential PVD of Co in a CO atmosphere onto a frozen [C1C2Im][OTf] film at 100 K leads to the growth of threedimensional Co NPs supported on the IL ice. Clear spectroscopic signatures are found that allow us to differentiate between the parts of the Co NP in contact with the IL and those covered with CO. Above the melting point, the Co NPs fully covered the IL. (5) The presence of the IL during PVCD suppresses the oxidation of the growing Co particles. PVCD of Co and [C1C2Im][OTf] in CO (10−6 mbar) in O2 atmosphere (at pressures up to 5 × 10−6 mbar) does not lead to oxide formation but to adsorption of chemisorbed oxygen on the Co NPs. Deposition of unprotected Co under otherwise identical

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REFERENCES

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