Preformed 2 nm Ag Clusters Deposited into Ionic Liquids: Stabilization

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Preformed 2 nm Ag Clusters Deposited Into Ionic Liquids: Stabilization by Cation-Cluster Interaction David Christopher Engemann, Stefanie Roese, and Heinz Hövel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12120 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on March 1, 2016

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The Journal of Physical Chemistry

Preformed 2 nm Ag Clusters Deposited Into Ionic Liquids: Stabilization by Cation-Cluster Interaction David Christopher Engemann, Stefanie Roese, and Heinz Hövel

∗

Fakultät Physik / DELTA, Technische Universität Dortmund, 44227 Dortmund, Germany

E-mail: [email protected]

1

ACS Paragon Plus Environment


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Abstract Recently the formation of nanoparticles by sputter deposition of metal atoms onto the surface of room temperature ionic liquids (RTIL) was reported, however the growth and stabilization mechanism within the ionic liquid are still in discussion. Here we present another approach by depositing Ag clusters with a diameter of 2 nm preformed in a supersonic nozzle expansion into an ionic liquid. Thus, the properties and size distribution of the clusters is well known before deposition. The mixture of the clusters with the ionic liquid is investigated in-situ and ex-situ with UV/Vis measurements and X-ray absorption near-edge structure (XANES) spectroscopy at the Ag L2 edge. The plasmon resonances of the Ag clusters show that up to 10 µg/ml the clusters stay separated in the RTIL and suggest an interaction process between the cations in the liquid and the surfaces of the clusters which is conrmed by a shift of the absorption edge in the XANES measurements. For higher cluster concentration and on longer time scale the stabilization ability of ionic liquids can be investigated.

1 Introduction Metal clusters with sizes of several nanometres can be used in future technologies such as biosensors

1

in catalysis

2

or in medicine.

3

Especially the properties of silver clusters open a

wide eld of applications due to their optical activity and antimicrobial eect. various cluster preparation methods.

4

There are

The wet chemistry production is attractive for the

production of large amounts of clusters with well dened size distributions. Room temperature ionic liquids (RTILs) like 1-Butyl-3-methylimidazolium hexauorophosphate (BMIM PF6 , see Fig.

1) are used as solvents because of their extraordinary properties such as

good solubility properties, high thermal stability and negligible vapour pressure.

57

Several

publications on dierent wet chemistry production methods can be found in literature; for example Shankar et al., decanethiol.

8

who have synthesized silver nanocrystals with ligand shells of do-

The samples have been investigated with various techniques such as TEM,

2


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UV/Vis absorption spectroscopy, pXRD and XPS. Silver clusters in ionic liquids have for instance been produced by synthesis

9

or by sputtering deposition.

10

In both cases, the cluster

growth processes and stability are inuenced by the RTIL itself and by additional chemicals like residues of precursors.

11,12

In the present experiments, the clusters were not grown in

the RTIL but preformed as free clusters in vacuum by a super sonic nozzle expansion before they were deposited into a RTIL. The properties of the clusters were investigated in-situ and ex-situ by UV-Vis spectroscopy of the cluster plasmon resonance. Additional information on the electronic structure of the clusters could be derived by X-ray absorption near-edge structure (XANES) spectroscopy at the Ag L 2 edge.

2 Experimental Setup Two batches of the room temperature ionic liquid 1-Butyl-3-methylimidazolium hexauorophosphate (BMIM PF 6 , see Fig.

1) were purchased from Carl Roth GmbH + Co.

(RTIL1 : batch no. 20896717, with a purity purity

≥

≥

KG

99 %, RTIL2 : batch no. 323203553, with a

99 %). In the vacuum chamber of the experiment water degassed from the RTIL,

which usually took several hours at room temperature. The cluster deposition was started after the process was nished. For a long time ionic liquids themselves were known to have no detectable vapor pressure. PF6 ab initio to

13

1 · 10−10 mbar

Paulechka et al. (RT) and to

estimated the vapor pressure of BMIM

2 · 10−4 mbar (T = 400 K), 14

nevertheless this

calculation should only be an order-of-magnitude estimation. Ag clusters were produced by a supersonic expansion ( T

pAg =600 mbar, dnozzle =0.5 mm)

p = 10−4 mbar)

15

15

pAr =4500 mbar,

The source chamber

p=10-7 mbar,

Ar pressure during

by a heated skimmer.

Using Ar as carrier gas those clusters have a diameter of 2 TEM studies.

2300 K,

using a thermal cluster apparatus.

is connected to the experimental chamber (base pressure operation

≈

± 0.6

nm as characterised in

Within the limits of the respective instrumental accuracy the same cluster

3



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size distribution was measured using TEM and AFM.

16,17

Page 4 of 21

As both, cluster diameter using

TEM and cluster height using AFM, were measured on arc-produced C-foil (amorphous carbon) the clusters on this substrate resemble a spherical shape and therefore the size can be also used for the same clusters embedded isotropically into the ionic liquid. For experiments with the cluster apparatus which we used as the basis of our design a cluster velocity of about 1.4

· 103 m/s,

corresponding to a kinetic energy of about 300 eV/cluster was measured using

time-of-ight analysis for cluster ions atoms) were reported.

20,21

18,19

and cluster sizes of similar magnitude (50...1000

The Ag cluster ux is measured using a quartz crystal microbal-

ance and typically amounted for the experiments presented here between

∼ 0.01

- 0.1 nm/s

on a deposition spot with 30 mm diameter at a distance of about 1.2 m behind the cluster source corresponding to about 4 - 44

µg/min.

At this position the cluster beam hits a mixer (Fig. 2), which consists of a tub, lled with the ionic liquid, and a roller. While the roller with a diameter of 6 cm and 6 cm width is rotating with 1 rotation per second, its surface is covered with a thin lm of the ionic liquid and the clusters can be deposited into this lm. After each rotation, the clusters are mixed into the reservoir at the bottom of the tub. Two fused silica windows enable optical measurements through the reservoir during the deposition process.

The integration time

for each measurement was large compared with the time of one rotation and the reference spectrum was recorded before the deposition started. The optical measurements were performed using an Avantes AvaLight-DH-S-BAL light source.

This combined light source consists of a deep ultra violet deuterium lamp and a

balanced halogen lamp and covers a wavelength range of

λ = 190 nm - 2500 nm.

The spectra

were recorded using an Avantes AvaSpec-2048x14 spectrometer with a detectable spectral range from

λ

= 200 nm - 1100 nm with a resolution of

∆λ

= 1.4 nm. The combined lamps

have characteristic lines especially at 1.89 eV, 2.26 eV, 2.55 eV and 2.85 eV. Small shifts of these lines between recording the reference and sample extinction spectra can cause small artefacts at these positions which will be ignored below. The refractive index

4


n

of the RTIL

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at the Na-D line (589 nm) and its dispersion were measured using an Abbe refractometer (ZEISS). The refractive index

nexp

at 400 nm (3.1 eV) close to the position of the plasmon

was then calculated by extrapolation using the dispersion. The XANES experiments were carried out at the beamline ID26 at the European Synchrotron Radiation Facility at the Ag L 2 absorption edge. A Si(111) monochromator (intrinsic resolution of 1.4

· 10-4 )

yields a photon ux of

> 1013

photons per second. The mea-

surements were performed using partial uorescence yield mode with an energy resolution of

∆E

= 0.1 eV.

The results of the experiments are grouped in 4 sections (2.1-2.4) with dierent batches of the RTIL, temperatures of the RTIL during deposition, deposition amounts and measurement techniques. The UV-Vis measurements were performed during (in-situ) and after (ex-situ) the deposition of the clusters into the RTIL. The XANES experiments were performed exsitu after the deposition of the clusters. A summary of the experiment parameters is given by Table 1.

Table 1: The performed experiments dier in the batch of the RTIL, measurement technique and deposition amount.

2.1

exp. sec.

RTIL

measurement

Fig.

2.1

1

UV-Vis ex-situ

3

2.2

2

UV-Vis in-situ

4

2.2

2

UV-Vis ex-situ

5

2.3

2

UV-Vis in-situ

6

2.3

2

UV-Vis ex-situ

7

2.4

2

XANES (clusters)

8

2.4

1

XANES (crystals)

8

Deposition in

RTIL1 ,

cluster density

µg/ml 10.0 µg/ml 10.0 µg/ml 8.7 µg/ml 8.7 µg/ml 24.0 µg/ml 118.5 µg/ml 5.9

room temperature

In a rst experiment, using RTIL1, the colour of the RTIL changed from transparent into yellow during the deposition (150 s), with increasing deposition amount, due to the absorption by the cluster plasmon. UV-Vis extinction measurements of the cluster plasmon were performed ex-situ after depositing

mAg =123 µg

into a volume of

5


VRTIL =21 ml

(5.9

µg/ml).


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The spectra are shown in Fig. 3. The rst spectrum was recorded about three hours after the deposition was completed, followed by measurements in variable time steps up to about eight hours after deposition. The maximum extinction decreased and the full width at half maximum (FWHM) of the plasmons increased with increasing time. The FWHM is measured by the double distance between the peak position and the low energetic ank of the plasmon at half maximum because the high energetic ank is inuenced by the Ag interband transition.

22

3 h 15 min

after the deposition the FWHM was 0.6 eV. The increase of the FWHM is attributed to the formation of agglomerates.

23

However, the energetic position of the plasmon maximum stays

approximately stable at 2.9 eV.

2.2

Deposition in

RTIL2 ,

room temperature

After those rst observations, a second experiment with RTIL2 was performed and the cluster plasmon extinction was measured in-situ during the deposition of the clusters using two fused silica windows, which were added to the mixer setup (compare Fig. 2). In order to investigate the cluster plasmon during deposition with each spectrum taking about 1 min integration time the deposition rate was decreased to 0.6 nm/s for this and the following depositions. Otherwise an extinction

lg(Iref /I)>2

with possible saturation eects due to the

longer optical path length (60 mm instead of 10 mm in Fig. 3) would have occured within a few minutes. The resulting spectra are shown in Fig. 4. The overall extinction of the clusters increased with time due to the increasing amount of clusters during the deposition. At the end of the deposition (14 min) an amount of 150 (10

µg/ml).

µg

cluster material was deposited into 15 ml RTIL

The peak position again stays approximately stable at 2.9 eV. The measured

cluster spectra are compared with the calculated spectrum of a single cluster with a diameter of

2R=2 nm.

Christy

24

The calculation was done using the dielectric function from Johnson and

for silver bulk material corrected for chemical interface damping.

6


15

A damping

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constant of

A=1.3

describing the limitation of electron mean free path

medium with a refraction index of

nmedium =1.67

25

in an embedding

was used to generate a best match of the

calculated spectrum with the measured spectra. However, the refraction index at an energy of 2.9 eV of this ionic liquid was measured as

nexp =1.418 .

For a refraction index of the embedding medium of 1.418 the calculated plasmon

resonance maximum is expected at an energy of 3.13 eV. This deviation may be explained by an interaction between the cluster surface and the surrounding ions. The physisorption of methylimidazolium-based cations was observed for metallic electrodes at zero potential.

26,27

The formation of a cation layer around the clusters, which inuences the refraction index of the liquid next to the clusters, seems probable. This shifts the plasmon resonance which can be described by an alteration of the refraction index surrounding the cluster.

More

generally, however, the details of the cluster/liquid interface modify the energetic position and width of the cluster plasmon, which can be employed for surface analysis by clusterplasmon spectroscopy . In Fig.

28,29

4 (left) the spectrum recorded for a deposition time of 2 min is compared to

the calculated spectrum. In Fig. 4 (right) the spectra were normalised to equal maximum extinction at 2.9 eV. The spectra have similar shapes and are in agreement with the calculated spectrum up to a deposition time of six minutes, corresponding to a cluster plasmon extinction of about 1 at an optical path length of 60 mm. Therefore these spectra can be considered as caused by separated clusters. The FWHM of the separated cluster plasmons is (1.10

±

0.02) eV. For comparison, the plasmon FWHM of Ag clusters with a diameter of

2 nm in a SiO2 matrix is 1.00 eV.

15

After a deposition time of about six minutes, the cluster

plasmon spectra started to change their shape, with an increase of the extinction of the low energetic ank at 2 eV. The FWHM increase for the highest amount of cluster material indicates the formation of agglomerates. However, already for separate clusters at deposition times < 6 min the FWHM is larger than for the deposition of 5.9

µg/ml

in RTIL1 after 3 h

15 min. The dierent batches of RTIL probably had a noticeable eect on the FWHM of

7



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Page 8 of 21

the cluster plasmons and the velocity of the agglomeration process.

Additionally, at the

end of the deposition process, with increasing density of the clusters and the corresponding decrease of next neighbour distances, a second peak P 2 emerges at 3.05 eV. Two hours after deposition the sample was extracted from the experimental chamber and investigated ex-situ. The UV-Vis spectra (Fig. 5) dier clearly from the spectra recorded during the deposition and from those of clusters in RTIL1 recorded after deposition. The shapes of the spectra suggest an increase of agglomerated clusters and the presence of rst coalesced clusters. Especially the high extinction at an energetic range between 1.5 eV and 2.5 eV suggests complicated chain-like agglomerates.

23

The intensity of peak P 2 increased

compared to Fig. 4 (left). Further changes on the time scale of several hours up to a few days can be assigned to the precipitation of coalesced cluster material, indicated by the overall decrease of the plasmon intensity with increased time.

2.3

Deposition in

RTIL2 ,

80

◦

C

◦ In a control experiment, RTIL2 was heated to about 80 C during the deposition process to further decrease water impurities and lower the viscosity of the ionic liquid. of up to 130

µg

was deposited into 15 ml of RTIL2 (8.6

µg/ml).

30

An amount

The results of the in-situ

measured cluster extinctions are shown in Fig. 6. Compared with the previously performed experiment, the peak position shifted to 3.0 eV and the FWHM of 1.03 eV for separated clusters was slightly decreased. The comparison of the normalised cluster plasmon spectra (normalised to the maximum extinction at 3.1 eV) shows a good agreement of the plasmon shapes after dierent times steps of deposition. The deviations for energies 10

µg/ml cluster agglom-

eration is observed, with increased absorption at low energies, together with an additional spectral feature at approximately 3.2 eV, which can tentatively be interpreted as due to a local optical response at the cluster contacts inside the agglomerates. For deposition densities < 10

µg/ml

the clusters stayed separated at least during the in-situ experiment and

by comparison of measured and calculated cluster plasmon spectra we were able to use the method of surface analysis by cluster-plasmon spectroscopy

28,29

to detect the formation of

a cation surface layer at the cluster/liquid interface which is perturbed at higher temperature. The formation of a cation layer is also corroborated by XANES studies showing a shift of the absorption edge to higher energy.

Acknowledgement The transfer of the thermal cluster apparatus to the TU Dortmund in 2008 initiated by U. Kreibig is gratefully acknowledged. We are grateful to Kristina Kvashnina and Pieter Glatzel at ESRF for providing assistance in using beamline ID26 at the European Synchrotron

12


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Radiation Facility (ESRF), Grenoble, France for the Ag L 2 XANES measurements. Samples were precharacterized by XANES measurements at the Ag L 3 edge that were previously performed at BL8, DELTA, Dortmund, Germany. The help of Ralph Wagner is gratefully acknowledged. We thank ESRF and DELTA for providing synchrotron radiation and NRW Forschungsschule "Forschen mit Synchrotronstrahlung in den Nano- und Biowissenschaften" for nancial support.

We also would like to thank Monika Meuris at the Department of

Biochemical and Chemical Engineering TU Dortmund for performing the TEM experiments for the Ag clusters in RTIL.

Figure 1: Schematic of the investigated ionic liquid 1-Butyl-3-methylimidazolium hexauorophosphate (BMIM PF 6 ), chemical formula: C 8 H15 F6 N2 P.

Figure 2: The mixer for the deposition process has two fused silica windows which enable UV-Vis in-situ measurements (light path indicated with arrows). The roller is driven from outside the experimental chamber.

13



03:15:00 03:30:00 03:42:00 03:55:00 04:18:00 04:39:00 04:52:00 05:43:00 06:43:00 07:51:00

0.7 0.6

extinction

0.5 0.4 0.3 0.2 0.1 0

1.5

2

2.5

3

3.5

energy [eV] Figure 3: Cluster plasmon extinction spectra of 5.9

µg/ml

Ag clusters in RTIL1 recorded

ex-situ up to several hours after the deposition ended, optical path length 10 mm.

1.6 00:14:00 00:12:00 00:10:00 00:06:00 00:04:00 00:03:00 00:02:00 00:01:00 theory

↓P2 normalised extinction

1.4 1.2

extinction

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

Page 14 of 21

1 0.8 0.6 0.4 0.2 0

1.5

2

2.5

3

3.5

1.5

energy [eV] Figure 4:

2

2.5

3

3.5

energy [eV]

Cluster plasmon extinction spectra of Ag clusters with increasing deposition

amount in RTIL2 recorded in-situ during the deposition, optical path length 60 mm. The normalised spectra are plotted in the right gure.

14


Page 15 of 21

P

0.35

↓

2

0.3

extinction

0.25 0.2

0.15 0.1

00d, 00d, 03d, 05d, 25d,

0.05 0

01:46:00 20:01:00 23:16:00 23:47:00 23:38:00

1.5

2

2.5

3

3.5

energy [eV] Figure 5: Cluster plasmon extinction spectra of 10

µg/ml

Ag clusters in RTIL2 recorded

ex-situ hours up to several days after the deposition ended, optical path length 10 mm.

00:10:00 00:08:00 00:07:00 00:06:00 00:05:00 00:04:00 00:03:00 00:02:00 00:01:00 theory

normalised extinction

1.2 1

extinction

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


0.8 0.6 0.4 0.2 0

1.5

2

2.5

3

3.5

1.5

energy [eV]

2

2.5

3

3.5

energy [eV]

Figure 6: Cluster plasmon extinction spectra Ag clusters with increasing deposition amount in RTIL2 recorded in-situ during the deposition, optical path length 60 mm. The normalised spectra are plotted in the right gure.

15



0.16 0.14 ↓ 0.12

extinction

P2

0.1

0.08 0.06 0.04

3 d, RT 4 d, RT 15 d, RT 15 d, 4◦ C

0.02 0

1.5

2

2.5

3

3.5

energy [eV] Figure 7: Cluster plasmon extinction spectra of of 8.6

µg/ml Ag clusters in

RTIL2 recorded

ex-situ hours up to several days after the deposition ended, optical path length 10 mm. The artefact at 1.5 - 1.8 eV is due to problems with the reference measurement.

fluorescence signal

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

Page 16 of 21

Ag reference Ag crystals Ag clusters 3.520

3.525

3.530

3.535

energy [keV] Figure 8: XANES Ag L 2 edge of agglomerated clusters (Ag clusters) and coalesced clusters (Ag crystals) dispersed in RTIL compared with an Ag lm reference.

16


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Figure 9: Photos of the sample with agglomerated Ag clusters (24

µg/ml)

in RTIL before

(left) and after (right) X-ray exposition in the XANES experiment.

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2015, 17, 21072

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Graphical TOC Entry Ag clusters

roller extinction

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


UV/Vis in-situ

energy

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Preformed 2 nm Ag Clusters Deposited into Ionic Liquids: Stabilization

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