The sample synthesis process was started by sputtering of Cu into IL for the NP formation, followed by the Cu precipitation process using HDA as capping agent and the extraction of the precipitated Cu NPs from the NP/IL suspension.
2.1 Up-scaled sputter synthesis of Cu NP in IL
As liquid “substrate” for the sputter deposition of the NPs, the IL 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide [Bmim][(Tf)2N] was used. This IL was purchased from Iolitec (Heilbronn, Germany) with purity > 99%, halides content < 100 ppm and water content 51 ppm. Without further purification the IL was stored and processed under Ar atmosphere in a glovebox (water and oxygen content both < 0.5 ppm). A commercial co-sputter system (AJA POLARIS-5 from AJA INTERNATIONAL, Inc., North Scituate, MA, USA) with 1.5-inch diameter magnetron sputter cathodes and multiple sputter source DC power supplies (DC-XS 1500 from AJA INTERNATIONAL, Inc., North Scituate, MA, USA) was used for all sputter depositions. The process gas was Ar (purity 99.9999%, Praxair, Düsseldorf, Germany). A 38.1 mm diameter 3.175 mm thick Cu target (purity 99.99%, EvoChem, Offenbach am Main, Germany) was used.
For the up-scaled deposition, 50 mL of the IL [Bmim][(Tf)2N] were filled into a clean petri dish (110 mm inner diameter 20 mm height) inside the glovebox. Prior to the deposition, the IL was evacuated inside the sputter chamber for 96 h in order to remove remaining water and oxygen from the transport out of the glovebox into the sputter chamber. The chamber pressure was Pa before the deposition. For plasma ignition, the chamber pressure was set to 1.33 Pa. After plasma ignition the target was pre-cleaned for 120 s at 20 W sputter power with a closed shutter in front of the target, a rotation of the substrate plate of 30 rotations per minute and a successive reduction of the Ar pressure to the deposition pressure of 0.5 Pa.
Pre-cleaning of the target was performed for removing possible oxide layers (Meyer et al. 2018), followed by adjusting the deposition power to 30 W (405 V, 75 mA) and opening the shutter in front of the cathode for the specific deposition time. To reach the total deposition time of 24 h, the deposition was conducted over three days with 9 h, 9 h and 6 h sputtering time per day. Between the depositions the sputter chamber was continuously pumped when the sputter gas line was closed after each deposition to achieve that the vacuum never exceeded the sputter pressure during the deposition processes. The cathode was tilted by an angle of 18° between the normal of the IL surface and the target normal. The distance between the IL surface and the cathode was decreased from 11 cm (standard) to 9 cm by placing another pair of petri dishes below the IL filled dish. A homogeneous deposition was achieved on the substrate due to rotation and cathode tilt. After the deposition, right after opening the chamber, 300 of the sputtered Cu NP/IL suspension were separated from the IL dish and transferred into the glovebox for storage and TEM sample preparation of the untreated IL. The remaining Cu IL volume was filled into a glass bottle and kept in air since for the capping procedure acetonitrile was used for washing the IL and thus a transfer into the glovebox would not be possible.
Subsequent to the deposition in IL, the sputter rate was determined using the identical sputter system configuration and deposition parameters (except deposition time) as for the IL deposition. The rate was determined using Si/SiO2 pieces photolithographically structured with a photoresist lift-off cross pattern for film thickness measurements, also placed on a closed pair of petri dishes. The deposition time was 30 minutes to guarantee a sufficient film thickness. After deposition, the pieces were stored for 4 h in technical acetone to remove the lift-off photoresist and cleaned afterwards with technical acetone (purity 99.5%) and isopropanol (purity 99.7%). The film thicknesses was measured with a profilometer on 27 measurement points revealing a Cu deposition rate of 0.143 nm/s for this chamber configuration, which is approximately 80% higher than the usual Cu rate (0.08 nm/s) used in our standard scientific chamber configuration (Meischein et al. 2019). For the synthesis parameters of the latter Cu NPs obtained from a scientific sputter deposition (used for comparison issues) see Meischein et al. (Meischein et al. 2019).
2.2 Capping of Cu NPs using HDA as capping agent
Acetonitrile (purity 99.5% from Fisher Scientific UK, Loughborough, England) was added to the Cu/IL suspension until both liquid phases had the same height in the bottle. Then, HDA (purity > 95.0%, from TCI Deutschland GmbH, Eschborn, Germany) was continuously added to the mixture with increasing single portions for slowly starting the NP capping and precipitation process (see Fig. 1). After each addition of capping agent (HDA), the IL acetonitrile mixture was stirred at 1500 rpm for 48 h. With increasing amount of HDA, the strength of the color of the remaining Cu IL decreases (Fig. 1(c)). A color change from brown to green of IL and capped NPs occurs due to the reaction of Cu with oxygen and water since all addition and stirring cycles were performed in air. For extracting all Cu NPs from the IL and obtaining a transparent IL phase above the NP phase, a total amount of 80 g HDA had to be used.
2.3 Extraction of HDA-capped Cu NPs
Extraction of the HDA-capped Cu NPs from the IL/acetonitrile mixture was achieved by centrifugation. The whole amount of IL/acetonitrile mixture was filled into centrifuge tubes (total volume 50 mL) so that each tube was filled with 35 mL. The rest of the volume was filled with fresh acetonitrile. After that, the tubes were shaken so that both liquid phases were mixed. The centrifugation was conducted using a Sigma 4-16S centrifuge (from Sigma Laborzentrifugen GmbH, Osterode am Harz, Germany) operated at 8000 rotations per minute, corresponding to a relative centrifugal force (rcf) of 10375 g. After the first centrifugation cycle of 2 minutes, the mixture of slight yellow/green colored IL/acetonitrile mixture above the capped Cu NPs at the bottom of the tubes was removed with a syringe and filled back into the container with the IL/acetonitrile mixture. This was done to ensure that the NP material, which was not capped by the HDA, was not lost during extraction. The tubes were refilled with new acetonitrile and were shaken again as long as the capped NPs were re-dissolved homogeneously in the tube volume. The centrifugation process was conducted again for the second to forth cycle. After a total of 4 cycles with consecutive removal and disposal of the remaining (clear) IL/acetonitrile mixture above the capped NPs, the remaining wet Cu NP powder from each centrifugation tube was mixed into acetonitrile and left in an open container overnight so that the acetonitrile could evaporate. The remaining HDA-capped and dried Cu NP powder was weighed for determining the extracted NP amount using the measured Cu metal content evaluated by ICP-MS.
2.4 TEM sample preparation
For the preparation of TEM samples holey carbon-coated Au grids (200 mesh, Plano GmbH, Wetzlar, Germany) were used. For the untreated as-sputtered Cu IL, 2.5 suspension was dropped on the carbon-coated grid side and left at this side for adhesion of the NPs for 2.5 h. In order to prevent possible grid contamination originating from interaction of the electron beam with the IL during TEM analysis, the grids were washed dropwise with dried acetonitrile for 1 h under inert conditions (see supporting information of Meyer et al. (Meyer et al. 2018)) and then stored in Ar atmosphere.
For the TEM investigations of the HDA-capped Cu NPs a spatula tip of the centrifuged powder was filled under air in an Eppendorf cap with toluene (purity 99.5 %, from VWR International GmbH, Darmstadt, Germany) and treated in an ultrasonic bath for 2 minutes. After the treatment in the ultrasonic bath, the powder was completely dissolved in toluene, resulting in a homogeneous green color of the beforehand colorless liquid, proving the successful re-dispersion of the HDA-capped Cu NPs into an apolar solvent. A few drops of the obtained mixture were filled into a new Eppendorf cap and were mixed with more toluene. This dilution was necessary to reduce the amount of HDA remaining on the TEM grid. A few drops of the obtained final solution were dropped on the carbon-coated side of the TEM grid and left for evaporation of the toluene inside a fume hood. Conventional TEM and high-resolution TEM (HRTEM) studies were performed using a FEI Tecnai F20 S/TEM instrument operated at 200 kV. For the analysis of the Cu NP mean diameter, at least 253 NPs were evaluated manually.
2.5 ICP-MS measurements
For ICP-MS measurements an amount of two times 50 of the Cu IL and an amount of 13.5 mg of the dried Cu NP powder obtained from the centrifugations were separated in suitable Teflon containers. To ensure a correct volume of IL separated into the containers, each container with the respective IL inside was additionally weighed. For the Cu IL, weighs of 54.7 mg in the first loaded and 69.6 mg in the second loaded container were measured. The differences between first and second loaded container may be due to the dry pipette tip during the first loading since the viscosity of the IL prevents the complete loading and unloading of the adjusted IL volume in the used Eppendorf pipette tip. The separated samples were diluted each with 4 mL of 69% concentrated phosphoric acid of the “ROTIPURAN® Supra” line. These mixtures were chemically digested in a Multiwave Pro microwave digestion device with 8-slot container holder 8NXF100 (Anton Paar GmbH, Graz, Austria). The digestion occurred at maximum temperature 240°C and maximum pressure 60 bar to ensure a complete transfer of the investigated material into solvable nitrides which could be measured in the ICP-MS. The resulting solutions were further diluted with ultrapure water (conductivity 0.055 /cm) to a total volume of 10 mL per sample. From this stem-solutions, 1:100- and 1:1000-dissolutions were produced and experienced an acidification with 2% phosphoric acid prior to the measurements in an iCAP RQ ICP-MS device (from Thermo Fisher Scientific Inc., Waltham, Massachusetts, USA). The ICP-MS measurements were conducted in the KED-mode to decrease disturbances from molecule ions.