Rapid assessment of sputtered nanoparticle ionic liquid combinations

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Rapid assessment of sputtered nanoparticle ionic liquid combinations Hajo Meyer, Michael Meischein, and Alfred Ludwig ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.8b00017 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Combinatorial Science

Rapid assessment of sputtered nanoparticle ionic liquid combinations Hajo Meyer, Michael Meischein, Alfred Ludwig Werkstoffe der Mikrotechnik, Institut für Werkstoffe, Fakultät für Maschinenbau, RuhrUniversität Bochum, Universitätsstr.150, D-44801 Bochum Corresponding author: [email protected] KEYWORDS: silver nanoparticles, ionic liquids, sputter deposition, combinatorial methods, high throughput characterization

ABSTRACT: A high-throughput method is presented for the efficient assessment of the formation and stability of nanoparticle suspensions in ionic liquids which differ in their cations and anions. As a proof of principle, Ag was sputtered on a cavity array filled with 9 different ionic liquids. Not all nanoparticle ionic liquid combinations form a stable suspension with separated nanoparticles. Directly after synthesis, the formation of non-agglomerated nanoparticle suspensions with sizes from 4 nm to 9 nm is observed by transmission electron microscopy as well as different time dependencies of the suspension stabilities. Only 3 out of the tested 9 nanoparticle ionic liquid suspensions show long term stability: Stable suspension of spherical nanoparticles

are

formed

(perfluoroethylsulfonyl)imide

in

the

ionic

liquids

[Bmim][(Pf)2N],

1-butyl-3-methylimidazolium 1-butyl-3-methylimidazolium

bisbis-

ACS Paragon Plus Environment

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ACS Combinatorial Science 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(trifluoromethylsulfonyl)imide

[Bmim][(Tf)2N],

and

1-butyl-1-methylpyrrolidinum

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bis-

(trifluoromethylsulfonyl)imide [BmPyr][(Tf)2N].

Introduction: Metal nanoparticles (NPs) are of interest for a multitude of applications.1,2,3 NPs can be synthesized e.g. by wet chemical synthesis including the reduction or decomposition of metal precursors4,5,6,7,8,9 with the drawback of byproducts from the reactions.10 The synthesis of multinary NPs is challenging and different concepts for the rapid synthesis of NP libraries have been introduced.11,12,13 Sputter deposition of NPs in ionic liquids (ILs) has several advantages like the possibility of preparing metal NPs with tunable size distributions which are free from surfactants. By combinatorial co-sputtering NP libraries can be synthesized, without the need of any chemicals except the liquid IL “substrate”. However, it was reported that the type of the used IL has an influence on the formation of NPs by sputtering into ILs concerning the resulting NP sizes and their distribution.10,14 Recently, the formation process of Ag NPs by sputter deposition in functionalized ILs was reported and showed an influence of the functional groups on the NP size and location.15 ILs are salts which are liquid at temperatures < 100°C, they have negligible vapor pressure16,17 which allows their use in (ultra)high vacuum environments. Furthermore, their chemical and physical capabilities are used as green chemistry solvents18, in catalysis19 and industry.20 Taking all suitable cations and anions for IL synthesis into account, there are approximately 1018 possible IL combinations21, enabling the design of ILs for many requirements.


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The sputtering technique using ILs as substrate for NP synthesis contributes to the preparation of tailored NP systems for different tasks due to the adaptable material composition, especially for multinary materials systems, and size distributions of the NP. A magnetron sputter process with elemental metal targets and Ar as sputtering gas enables the synthesis of NP composed out of the used target material. Using two or more targets simultaneously, alloy NPs with tunable compositions can be fabricated; in comparison with the wet chemical approach without limits related to the chemical reactivity of the used metal precursors chemicals. The size and distribution of the NP are affected by the used IL, but the full process of the NP formation and the quality of the affecting abilities of the ILs are not completely understood.22,23 Due to these circumstances, a simple experiment for determining the influence of the IL concerning NP size, distribution and stability would enable deeper insights into this research area. However, until now investigations published in literature focused on the synthesis of metal NP in a single IL. In order to receive efficiently results of the IL influence on the NP formation with very good comparability, we propose to sputter metal(s) in several ILs at the same time. With our approach and a custom-build cavity holder (figure 1), it is possible to use nine different ILs (9 x 4 cavities) in a single sputter deposition process. Even more or less cavities could be used using a different shadow mask. A pin holder keeps the mask in place and prevents it from covering cavities unintentionally. The holder contains a maximum amount of 68 cavities (see suppl. inf.) which are available without a shadow mask. Ag was chosen as an exemplary material, due to its insensitivity to oxidation and its good visibility in the transmission electron microscope (TEM). The sputter deposition of Ag in nine different ILs generates significant information on the NP formation and stability of the NP-IL suspensions. The identical process parameters due to the simultaneous deposition in all ILs and the identical preparation and


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cleaning treatment guarantee insights independent from alternating conditions, which could occur when the NP-IL combinations would be prepared in subsequent experiments. Thus, the observed characteristics of the NP depend only on the variation of the ILs, generating new information concerning the NP formation and stabilization ability of the particular NP-IL combination.

Fig. 1: Schematic overview of the cavity holder (100 mm diameter) on which a metal mask is applied to open 36 cavities for the different ILs. Four cavities each are filled with the same IL. Next to the cavities, wafer pieces for thin film analysis can be placed. The base cation and anion and the variations of the counter-ions are shown on the left and the right. Photographs of the cavity wafer filled with 9 different ILs: (A) before and (B) after sputter deposition of Ag. The shadowed areas of two patterned Si/SiO2 wafer pieces can be seen in the upper section of (B).


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Methods and materials For sputtering into IL the following ILs were purchased from Iolitec (Germany): (1)1-butyl-3methylimidazolium

bis-(perfluoroethylsulfonyl)imide

[Bmim][(Pf)2N],

(2)1-butyl-3-

methylimidazolium

bis-(trifluoromethylsulfonyl)imide

[Bmim][(Tf)2N],

(3)1-butyl-3-

methylimidazolium

dicyanamide

hexafluorophosphate

[Bmim][(CN)2N],

[Bmim][PF6],

(4)1-butyl-3-methylimidazolium

(5)1-butyl-3-methylimidazolium

tetrafluoroborat

[Bmim][BF4],

(6)1-etyl-3-methylimidazolium


[Emim][(Tf)2N],

(7)1-hexyl-3-methylimidazolium


[Hmim][(Tf)2N], (8)1-butylpyridinium bis-(trifluoromethylsulfonyl)imide [BuPy][(Tf)2N] and (9)1-butyl-1-methylpyrrolidinum


[BmPyr][(Tf)2N].

The

received ILs were stored under Ar atmosphere and used with no further purification. The purities of all received ILs are listed in table 1.

Table 1: Purities of the used ILs by certificate analysis from Iolitec. IL

purity

halides

water

1 [Bmim][(Pf)2N]

> 98% < 250 ppm

60 ppm

2 [Bmim][(Tf)2N]

> 99% < 100 ppm

51 ppm

3 [Bmim][(CN)2N]

> 98% < 2 %

1402 ppm

4 [Bmim][PF6]

> 99% < 100 ppm

150 ppm

5 [Bmim][BF4]

> 99% < 100 ppm

109 ppm

6 [Emim][(Tf)2N]

> 99% < 100 ppm

59 ppm

7 [Hmim][(Tf)2N]

> 99% < 50 ppm

64 ppm

8 [BuPy][(Tf)2N]

> 99% < 100 ppm

61 ppm


5


9 [BmPyr][(Tf)2N]

> 99% < 100 ppm

Page 6 of 22

80 ppm

Sputter depositions were performed in an AJA POLARIS-5 vacuum chamber with 1.5” magnetron sputter cathodes and a DC-XS 1500 multiple sputter source DC power supply. Ar was used as process gas (purity 99.999%, Praxair). The different ILs (1-9) and pieces of a patterned Si/SiO2 wafer (2 cm × 3 cm, photolithographically structured with a photoresist lift-off cross pattern for film thickness determination) were placed on the cavity holder. Pumping the vacuum chamber overnight results in a starting vacuum of 1.7×10-4 Pa. For sputtering, a 38.1 mm diameter × 4.775 mm thickness Ag target was used (99.99% purity, EvoChem). After ignition at an Ar partial pressure of 1.333 Pa, 20 W sputter power and a 2 min preclean phase with closed shutter, the pressure was reduced to 0.5 Pa. During this procedure a thin film is deposited on the shutter surface, which is part of the conditioning of the Ag target. The main deposition for synthesizing Ag NPs in IL was performed at 30 W for 15 min with a rotation of the cavity holder of 30 rotations per minute with opened shutter. A cathode tilt of 5 mm, which leads to an angle of 16° of the cathode normal respective to the normal of the cavity holder, was used for deposition. Together with the rotation of the substrate, this leads to a homogenous deposition across the substrate. However, a cathode tilt is not necessary in general. The resulting Ag film thickness on the wafer pieces is 240 nm with a deposition rate of 0.28 nm/s. A cavity holder (Fig. 1) was designed for the deposition of metals into the IL with a volume of 40 µL per cavity. Using a mask covering the unfilled cavities of the holder, 9 x 4 cavities are available. For cleaning, the holder was treated in an ultrasonic bath 20 minutes each in acetone and isopropanol. The cavities were filled in a glove box under Ar atmosphere and were then transferred directly into the sputter chamber. After deposition, the holder was transferred directly into Ar atmosphere and the IL was collected and stored under Ar.


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TEM analyses of the NP were performed with a FEI Tecnai F20 S/TEM. For preparation of TEM samples, a holey carbon-coated Au grid (200 mesh, Plano GmbH) was prepared by dropping 2.5 µl IL on the carbon coated side and left for adhesion of the NP for 3 h. The grid was washed with acetonitrile dropwise for 1 h under inert conditions and was then stored under Ar. For simultaneously preparing 5 TEM grids, a special holder was designed (see supporting information). The size of the NPs in dispersion was determined by automated particle counting using Gatan DigitalMicrograph software.

Results and discussion: After sputtering Ag in the ILs, the NP-IL combinations show different colors ranging from slightly yellow/green to orange and red. All ILs show a stable and clear Ag NP suspension directly after the deposition process. However, after a few minutes the ILs (3) [Bmim][(CN)2N] and (5) [Bmim][BF4] started to sedimentate the NPs. TEM images shows the appearance of agglomerates all over the TEM grid for ILs 3 and 5 (Fig. 2). Due to the necessary preparation process, the TEM images in Fig. 2 show the status after 3 h.


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Fig. 2: Overview of TEM images of Ag NPs as sputtered in ILs 1-9 immobilized on carboncoated Au grids and washed with acetonitrile. (1) [Bmim][(Pf)2N], (2) [Bmim][(Tf)2N], (3) [Bmim][(CN)2N], (4) [Bmim][PF6], (5) [Bmim][BF4], (6) [Emim][(Tf)2N], (7) [Hmim][(Tf)2N], (8) [BuPy][(Tf)2N], (9) [BmPyr][(Tf)2N].


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All NP suspensions except 3 and 5 are stable within the first days. For the ILs 1, 2 and 4 with the base cation 1-butyl-3-methylimidazolium [Bmim] a median diameter of (7 ± 4) nm for (1) [Bmim][(Pf)2N], (8 ± 4) nm for (2) [Bmim][(Tf)2N] and (8 ± 4) nm for [Bmim][PF6] (6) was calculated. The ILs 6-9 with the base anion bis-(trifluoromethylsulfonyl)imide [(Tf)2N] gave (6 ± 3) nm for (6) [Emim][(Tf)2N], (7 ± 3) nm for (7) [Hmim][(Tf)2N], (4 ± 3) nm for (8) [BuPy][(Tf)2N] and (9 ± 8) nm for (9) [BmPyr][(Tf)2N]. All determined median diameters show the same range for imidazolium-based ILs, regardless of changing the anion or cation. The largest difference in diameter showed the ILs 8 and 9 with the smallest and biggest size. The size of the observed NPs ranges from 4 nm (8) to 9 nm (9). For the ILs 1, 3, 7, 8 and 9 no references with Ag NPs have been found. However, Ag NPs have been produced in various ILs and with different techniques like sputtering24,25, electrodeposition26 and use of precursors.27 Increasing the size of the Ag NPs, when using sputtering, can be achieved by increasing the discharge current. This leads to a higher amount of surface atoms/clusters which will be physically ejected from the Ag target which leads to bigger NPs stabilized in the IL.24 Janiak reported in 2008 a size-dependent formation of Ag NPs in the ILs [Bmim][PF6] (2.8 ± 0.8) nm and [Bmim][BF4] (4.4 ± 1.3) nm using the hydrogen reduction method and soluble Ag salts.27 The reported NPs are smaller in diameter compared to 4, due to the different synthesis. A size dependence with the anion volume was reported.27 A bigger anion volume leads to larger Ag NPs according to DLVO (Derjaguin, Landau, Verwey, Overbeek) theory which describes the first anionic inner shell for electrostatic stabilization of Ag NPs. The Ag NP diameter is decreasing, for the base cation [Bmim] and different anions, from the biggest anionic volume of [(Pf)2N] (Ag-NP = (7 ± 4) nm) to [(Tf)2N] (Ag-NP = (8 ± 4) nm) to the smallest [PF6] (Ag-NP =


9


Page 10 of 22

(6 ± 3) nm). The recognition of a trend with a few nanometer differences is quite daring but cannot be ruled out as it was described for the formation of Co NPs as well.13 Other parameters like density, viscosity, or conductivity might play an additional role for the stabilization of Ag NPs in ILs. Changing the cation, for the base anion [(Tf)2N], leads to more obvious differences in NP diameter. The difference in the alkyl chain length is greatest between IL 6 and 7, but no change in NP diameter is observed. Obvious differences in these ILs is the heterocyclic part from 9 (pyrrolidinium) with a NP diameter of (9 ± 8) nm to 2, 6 and 7 (imidazolium) with (8 ± 4) nm or either with a NP diameter of (7 ± 3) nm to 8 (pyridinium) with the smallest observed NP diameter of (4 ± 3) nm. The IL with the six-membered aromatic ring (8) resulted in the smallest size of NPs followed by the five-membered aromatic ring (2,6,7) and the non-aromatic fivemembered heterocycle resulted in the biggest NPs. The formation of NPs might be additionally influenced by acidic hydrogen sites, π– π interactions, hydrogen bonding and the formation of Nheterocyclic carbenes in the IL.13 However, we observe a smaller size diameter with less carbon double bonds. The colors of colloidal Ag suspensions are due to the plasmon absorbance of the electrons of the NPs. The spectral absorbance of Ag NPs depends on size, shape and agglomeration of the nanoparticles.28 As deposited, the color of the NP-IL combination 4 [Bmim][PF6] compares perfectly with the color of the sputtered IL (yellow) in literature, as well as the described size distribution of (6 ± 1.5) nm by Torimoto.29 Sputtering Ag into [Emim][(Tf)2N] was reported, however, no characterization of the NPs has been done.30


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Table 2: Median diameter and standard deviation (2σ) of sputtered Ag NPs in ILs. A minimum of 160 particles was counted. Agglomerated NP-IL combinations 3 and 5 have not been evaluated. IL

diameter / nm

standard deviation 2σ / nm

1 [Bmim][(Pf)2N]

7

4

2 [Bmim][(Tf)2N]

8

4

3 [Bmim][(CN)2N]

-

-

4 [Bmim][PF6]

6

3

5 [Bmim][BF4]

-

-

6 [Emim][(Tf)2N]

7

3

7 [Hmim][(Tf)2N]

7

3

8 [BuPy][(Tf)2N]

4

3

9 [BmPyr][(Tf)2N]

9

8

The growing process of Au NPs in the IL [Bmim][PF6] by sputter deposition was used to control the size of the formed NPs for optical properties.31 The time-dependent stability of vapordeposited Au and Cu NPs in various ILs at room temperature has been reported for [Bmim][(Tf)2N], [Bmim][PF6] and [Bmim][BF4]. The ripening process was observed at room temperature and for heated samples and resulted in bigger and agglomerated NPs and was faster for the heated samples. However, the resulting rank of stabilizing anions ([(Tf)2N]

Rapid assessment of sputtered nanoparticle ionic liquid combinations

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