Nano-Scaled Lanthanum Hexaboride (LaB6) – Control of Properties in Dependence on Type of Manufacturing

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ScienceDirect Materials Today: Proceedings 7 (2019) 835–843

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NanoFIS 2017

Nano-Scaled Lanthanum Hexaboride (LaB6) – Control of Properties in Dependence on Type of Manufacturing Volkan Yavuza, Rodrigue Ngoumenib, Karin Petera,*, Jonas Rosea, Peter Sindlhauserb, Martin Möllera,c a

DWI-Leibniz-Institut für Interaktive Materialien e.V., Forckenbeckstrasse 50, Aachen 52074, Germany b Sindlhauser Materials GmbH, Daimlerstrasse 68, Kempten 87437, Germany c ITMC-Institute of Technical and Macromolecular Chemistry, Worringerweg 1, Aachen 52074, Germany

Abstract This paper presents the influence of fabrication methods on the optical and photo-thermal properties of nano-LaB6. The nano particles (NPs) were manufactured via continuously operated ball milling or induction plasma technology. Whereas different grinding processes for LaB6 were also discussed using ethylene glycol (EG) and ZrO2 grinding media in previous works, the scaled-up plasma technology presents a new possibility to gain NPs with high yields and narrow size distribution. In our work, NPs < 100 nm are achieved by grinding experiments using ethanol, 1-methoxy-2-propanol and ethylene glycol. Furthermore, the change of grinding parameters was investigated intensively. Compared to milled NPs, nano-LaB6 in high purity are gained by plasma technology and shows differences in color, morphology (UHR-FESEM), absorption behavior and crystallite size (X-Ray). Acrylate terminated starPEG (poly ethylene glycol) was used as a high cross-linked network after in-situ UV polymerization to stabilize NPs homogenously. We also set the focus on photo-thermal conversion properties of LaB6 dispersions in ethylene glycol, i.e. transformation of the absorbed photon energy into heat, and temperature distribution around the laser spot which are characterized by an IR camera. © 2019 Elsevier Ltd. All rights reserved. Selection and/or peer-review under responsibility of NanoFIS 2017 - Functional Integrated nano Systems. Keywords: LaB6 nano particles; top-down; plasma LaB6; ball milling LaB6; LaB6 stabilization; photo-thermal conversion; SPR

* Corresponding author. Tel.: +49-241-802-3340. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or peer-review under responsibility of NanoFIS 2017 - Functional Integrated nano Systems.

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1. Introduction LaB6 is usually known as a ceramic material showing a very low work function of around 2.5 eV [1]. This makes it an ideal candidate as thermionic emitter, i.e. as long-life cathodes [2, 3], electron source in high resolution electron microscopes [4, 5], or in lithography devices [6, 7]. For those types of application LaB6 was produced by standard sintering techniques to get tips with a radius of 3 to 30 µm [4] or by electrochemical etching method to achieve 3 µm high arrays [8]. LaB6 shows metallic like properties, high chemical stability, high melting point (> 2300 °C), high hardness and low vapor pressure at high temperatures [9, 10] which partially comes from intramolecular covalently bonded boron atoms [11]. If LaB6 gets nano-scaled, for example by low temperature synthesis in an autoclave [12], by molten salt route [13], through carbo-thermal and boron carbide reduction [14], synthesis in radiofrequency thermal plasma reactors [15], or by grinding experiments [16-21] it exhibits localized surface plasmon resonance (LSPR) in the NIR range. Closer observations on grinding experiments using 50 – 300 µm sized ZrO2 balls in a batch type mill or paint shaker mill have shown that LaB6 is contaminated by the grinding media, which is an important point for our work. Chen et al. [16] have shown that 100 nm sized milled LaB6 can be stabilized in dodecyl benzene sulfonic acid (DBS) during the process, but they are lesser stable in polyethylene glycol (PEG 2000) and polyethylene imine. The tunability of the NIR absorbtion range by changing the size and shape of the NPs is described theoretically by DDA (discrete dipole approximation) [22] or deduced from dielectric functions [23]. Compared to other plasmonic materials like gold (Au) [24] and silver (Ag) [25] LaB6 is much cheaper and therefore new application fields are opened. Recently, researchers found out that LaB6 can be embedded in polymers like PVB laminates [18], PMMA laminates [26] or coated on PET [19, 27] to use it as solar control filters [28] in the range of 750 to 1200 nm of the solar spectrum. In order to enhance stability and biocompatibility LaB6 can be coated with functionalized silica shells [29]. Subsequently, these particles can be applied in living cells for NIR photo-thermal therapy to treat cancer cells [30] or ablate bacteria [31]. Furthermore LaB6/SiO2/Au can used as catalyst for the reduction of 4-nitrophenol to 4-aminophenol [32]. The transformation from absorbed NIR light into heat of milled NPs in ethylene glycol was investigated, i.e. by Chen et al. [16] using a polystyrene cell with a thermocouple after exposition with an 808 nm diode laser (820 mW; irradiation area: 0.3 cm²). Temperatures up to 80 °C are measured for 0.4 wt% dispersion after irradiation for 10 min. Another research group used glass cuvettes to observe the photo-thermal properties of grinded LaB6 for biomedical applications. They noticed temperature differences up to 32 °C with 12 g/m² LaB6 using an laser with a wavelength range of 380 – 780 nm [33]. One further possibility to measure the temperature is to use a thermal camera published by Ling et al. [34]. Temperatures up to 50 °C were measured to melt poly caprolactone based micro needles for anticancer release. Note, that there are no investigations of the photo-thermal properties of highly purified LaB6 NPs < 80 nm with an IR camera. In the present work differences of the photo-thermal conversion properties and optical properties between milled LaB6 NPs and ones produced by induction plasma technology were investigated. In contrast to previous grinding experiments with LaB6 counted above, we used a continuously operated dispersion mill to keep NPs in motion and changed the dispersion media from typically used ethylene glycol to ethanol and 1-methoxy-2-propanol. Additionally, we decreased the milling time and the amount of ZrO2 balls and subsequently, the amount of ZrO2 in the product was determined. The absorption range was tuned by mixing both types of NPs in different amounts. Instability indices of LaB6 in different dispersion media were measured and we have shown that LaB6 can be stabilized in acrylate functionalized starPEG. A high resolution IR camera from FLIR was used to detect the temperature immediately in few seconds during laser irradiation. High temperatures were measured at low concentrations and less laser power compared to previously described systems. 2. Materials and Methods Before characterizing the hydrodynamic diameter of NPs by Dynamic Light Scattering (Zetasizer Nano Series, Malvern Instruments), the instability indices (Dispersion Analyser - Lumisizer), the absorption behavior (Spectrophotometer V-780, Jasco), the photo-thermal conversion properties and recording electron microscope images (FESEM, UHR-FESEM; Hitachi) of the NPs, all samples were sonicated for 15 min with a tip. Instability indices were measured at 2000 rpm at 20 °C.

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The grinding experiments were performed with a continuous operated laboratory mill MiniSerie from Netzsch in an argon atmosphere. Different sized yttrium stabilized ZrO2 grinding media (400, 200 and 50 µm) from Netzsch were washed with ethanol before usage. The initial LaB6 powder (3-15 µm) and LaB6 plasma powder (Ar atmosphere, 10-80 nm) were provided from Sindlhauser Materials. The induction plasma technology was used from Sindlhausuer Materials to synthesis the plasma-powder which is also patented with the notification number 10 2017 122 313.4. Ethanol (abs.), 1-methoxy-2-propanol (>99.5%) and ethylene glycol (>99.5%) from Sigma Aldrich were used without further purification. The initial concentration of the violet LaB6 dispersion before milling was 0.1 wt% with a volume of 200 ml. To prevent sedimentation at the beginning of the milling process a maximum pump speed of 200 rpm was chosen. X-Ray diffraction (Rayons X) was used to determine the crystallite size and the crystal structure with the Empyrean setup from PANalytical. A Cu X-ray tube (line source of 12×0.04 mm2) provided CuKa radiation with l=0.1542 nm. A geometry with a parallel incident beam was used. A divergence slit of 1/16o was set to illuminate part of the parabolic graded multilayer system (Göbel mirror).The latter converts 0.8o from the divergent beam into an almost parallel beam (divergence ≤55 mdeg). Source and detector moved in the vertical direction around a horizontal sample fixed at the center of a phi-chi-z stage. The scattered signal was recorded by a pixel detector (256×256 pixels of 55 µm) as a function of the scattering angle 2θ. The detector was used in a scanning geometry that allowed all rows to be used simultaneously. To reduce the background, the divergent beam perpendicular to the scattering plane was controlled by a mask of 4 mm restricting the width of the beam at the sample position to about 10 mm. In addition, the perpendicular divergence was restricted by soller slits to angles ≤2.3o.The resolution of the total setup is 26 mdeg. Before stabilizing different amounts of LaB6 NPs (0.66 mg / 3.5 mL and 2 mg / 3.5 mL) in ethanol containing starPEG dispersions were sonicated for 10 min respectively. Afterwards a polymer solution of acrylate terminated starPEG (1 mg / 1 ml) and Irgacure 2959 (1 mol %) was prepared in ethanol. Subsequently, mixtures contenting 10 % and 50 % polymer solution with different amounts of LaB6 NPs were prepared and sonicated for additional 1 min. The mixtures were applied on TEM grids and polymerized in 20 min using a UV lamp (λ = 256 nm). The achievable temperature through photo-thermal conversion was recorded with an A615sc FLIR camera during continuously irradiation with a MDL-N-808nm-8W laser (Fig. 1) generating a laser spot around 2mm. Therefore, the camera was installed over the sample with a minimum distance of 15 cm and the laser was placed at an angle of 45°. The dispersion was prepared in a petri dish. The irradiation time for each measured point was at least 10 s until a constant temperature value was displayed.

Fig. 1. Scheme of the experimental setup to measure the temperature during the irradiation process of LaB6 dispersions.

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3. Results and Discussion 3.1. Top-Down Synthesis: Ball Milling and Induction Plasma Technology Grinding experiments of micro-scaled LaB6 are performed using 400 µm sized ZrO2 beads (if not otherwise stated) under various grinding parameters listed in Table 1. Sample A is defined as a greenish LaB6/ethanol dispersion gained under standard grinding parameters. First, we found out that, the smaller the agitator speed, the greater the hydrodynamic diameter of the particles due to the less specific energy input at 2100 rpm [35] (sample B). Based on standard parameters increasing milling time from 2 to 4 h has no strong impact to the particle size (sample C). Note that the efficiency of a specific sized grinding media is limited until a certain hydrodynamic diameter of the ground material is achieved. An increase of volume of grinding media from 10 to 50 mL and, therefore, the higher probability to hit a particle during the process results in smaller sized NPs and more homogenous dispersions (sample D). However, the hydrodynamic diameter decreased to 111 nm with prolongation of milling time after pregrinding using smaller sized ZrO2 beads of 50 µm for another 1 h (sample E). But, in both cases the amount of ZrO2 abrasion increases by a factor of around 2 and 3.2, which is determined gravimetrically and observed as white sediment in the flask. STEM images and EDX analysis confirmed the abrasion as a film-like material on LaB6 surfaces (Fig. 2a, 2b). Low-contrast parts mostly correspond to very fine ZrO2 powder which is mapped red. Nevertheless, the contamination amount is lesser than the values noticed from Chen et al. by EDX or more than described by Takeda et al. through inductively coupled plasma spectrometry. Nearly the same particles sizes around 100 nm are measured by Chen et al. [16] and Takeda et al. [19] after they have milled 900 and 120 min using 300, 200, 100 or 50 µm sized grinding media. In contrast to our process, both groups applied more volume of balls (5 times higher). In case of Takeda et al. it is also possible to achieve even smaller particles than 120 nm by increasing milling time up to 90 hours. We found out that it is also possible to use 5 times lesser amount of 200 µm sized ZrO2 beads to produce smaller particles than 120 nm in 2 h (sample F). Highly purified LaB6 NPs gained by plasma deposition are significantly different in absorption behavior, in shape and in the photo-thermal properties compared to milled LaB6. In this paper, it is defined as plasma-powder. First, the plasma-powder has a bluish color which is structured spherical and partially cubic (Fig. 3a, 3b), while milled NPs are randomly structured. The particles of plasma-powder are smaller than 80 nm in diameter which explains the strong shift of the major absorption peak to shorter wavelengths from 1032 to 808 nm at the same concentration (Fig. 4a) [19, 22, 23]. The small shoulder peak at 650 nm of milled NPs could probably result from either an interband transition or it is another plasmon resonance induced by LaO, as described in [23]. Interestingly this peak is not detected for the plasma-powder. The absorption area of the plasma-powder is also become smaller due to the narrow particle size distribution, but it can be tuned through mixing milled NPs (<100 nm) with a defined volume of plasma-powder (Fig. 4b). Small quantities are already sufficient to increase the absorption, especially the range between 750 and 850 nm. The extinction coefficient of the plasma-powder of around 7130 L/mol-1*cm-1 at least is 2.3 times greater than the one of milled NPs and can be compared with molar extinction coefficients measured for 43 and 120 nm sized NPs in [19]. By using an integrating sphere we found absorption of 80 % of the light for 0.01wt% LaB6 dispersion in ethanol, while the remaining 20 % is associated with scattering effects. The scattering effect percentage increases with increasing the hydrodynamic diameter of NPs which is the case of milled NPs (35 % scattering). Furthermore, crystallite sizes of 130, 28 and 20 nm respectively were calculated for the initial powder, milled powder and plasma-powder using X-Ray diffraction and the Scherrer equation (Fig. 4c). The bigger the full width at half maximum (FWHM) of the reflexes the smaller the crystallite sizes. Comparable crystallite sizes of 20 and 30 nm were measured by Takeda et al. after milling for at least 10 h. Here, we want to underline again that the highly purified plasma-powder is shown remarkable properties from any point of view compared to milled NPs. Large amounts of this plasma-powder can be manufactured in an argon atmosphere and directly stored in a glovebox.

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Table 1. Milling experiments of 0.1wt% LaB6 in ethanol performed with 400 µm sized ZrO2 beads, ZrO2 milling chamber and ZrO2 agitator (A) at standard grinding parameters (B) decreasing the agitator speed by half (C) increasing milling time from 2 h to 4 h (D) increasing volume of beads from 10 mL to 50 mL (E) 2 h with 400 µm and additional 1 h with 50 µm sized ZrO2 beads (F) replacing 400 µm by 200 µm sized ZrO2 beads. Sample

Milling time [h]

VBeads [mL]

øh [nm]

Agitator speed [rpm]

LaB6 / ZrO2

A

2

10

145 ± 40

4200

1 / 1.4

B

2

10

192 ± 50

2100

1 / 1.1

C

4

10

167 ± 52

4200

1 / 3.8

D

2

50

115 ± 42

4200

1 / 4.4

E

2+1

10, 10

111 ± 36

4200

1 / 1.8

F

2

10

112 ± 85

4200

1/2

Fig. 2. (a) UHR-FESEM image of sample A; (b) EDX mapping image of (a).

Fig. 3. (a) Bluish plasma powder; (b) UHR-FESEM image of (a).

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Fig. 4. Comparison of the plasma-powder and milled LaB6 (a) Absorbance spectra of plasma-powder and milled LaB6 at 1.3*10-4 mol/L; (b) absorbance spectra of mixed plasma-powder and milled NPs in ethylene glycol; (c) diffractrograms of all, initial, milled and plasma-powder.

3.2. Stabilization of Nano-Scaled LaB6 in Dispersion and starPEG Instability indices of LaB6 dispersions are measured to evaluate the stability of LaB6 dispersions for the first time. Initial powder (violet) which are not milled during the experiment sediments immediately after the process and are removed before further analysis. In general, it is observed that without any additional stabilizing agents, milled NPs in ethanol and isopropyl slowly agglomerate and also sediment partially due to the new generated high active surfaces. The agglomerated particles show a size of around 300 nm after 2 h. The same behavior is strongly observed for the plasma powder at concentrations ≥ 0.01 wt%, which is previously dispersed by ultrasound. The change of dispersion media from butanol, hexanol or ethanol to the higher dense polar 1-methoxy-2-propanol and ethylene glycol results in more stable dispersions with a bimodal particle size distribution of 30 ± 6 (32 %) nm and 112 ± 50 nm (68 %). Instability indices < 0.1 were measured for stable dispersions (Fig. 5a). In pure aqueous media the plasma-powder are oxidizing which is detected by color change of the dispersion from green to full transparency. Additionally, we found out that nano-scaled LaB6 (milled and plasma-powder) can be very well stabilized in acrylate terminated starPEG after UV-polymerization to a high cross-linked network (Fig. 5b, 5c). The polymer shows a structure like a non-ionic surfactant because it consists of 20 % of hydrophobic propylene oxide and 80 % hydrophilic ethylene oxide. Interestingly we observed a linear arrangement of LaB6 NPs at higher polymer concentrations (50 %; Fig. 5d).

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Fig. 5. (a) Instability indices of milled LaB6 in different dispersion media; (b) starPEG (50 %) / milled NPs (0.19 mg/mL); (c) starPEG (10 %) / plasma LaB6 (0.86 mg/ml); (d) starPEG (50 %) / plasma LaB6 (0.86 mg/ml).

3.3. Photo-Thermal Conversion The transformation of the absorbed photon energy into heat is measured with the setup shown in Fig. 1. The emission wavelength of the laser perfectly fits with the plasma powder due to the absorption wavelength of 808 nm. When the surface of plasmonic NPs is irradiated, photo-excited conductive free electrons starts to oscillate collectively. This results in amplified absorption and scattering followed by the transfer of the kinetic energy into the lattice through electron-phonon interactions and relaxation takes places by phonon-phonon interactions within few picoseconds. This physical phenomenon was described especially for gold (Au) NPs ≤ 30 nm [36, 37]. In our case, spatially and time resolved temperature measurements in the millisecond range are gained by using an FLIR camera which is fixed over the sample. It is to note that the IR camera converts invisible IR radiation, which comes from the surface of the material after irradiation by the laser, into a visualized thermal image. Therefore the StefanBoltzmann law is respected and we assumed an emissivity of 0.95 which is also used for a black body [38, 39]. Considering the 0.01 wt% plasma LaB6 / ethylene glycol dispersion, a little increase of laser power results in strong linear increase of temperature (Fig. 6a). Due to the relatively low boiling point of ethylene glycol the maximum temperature of 140 °C is measured at a laser power of 500 mW. The temperature distribution around the laser spot are mapped in Fig. 6b. A linear increase of the temperature in dependence on laser power is also observed for the 0.001 wt% plasma LaB6 / EG dispersion, because the absorption follows the Lambert-Beer law. In contrast, the

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higher concentrated dispersion containing 0.1 wt% plasma LaB6 shows a nonlinear behavior [16]. As we mentioned, high concentrated LaB6 in dispersions without stabilizing agents tends to agglomerate to particles of around 300 nm in diameter and sediments. This size effect also influences the photo-thermal conversion for a 0.1 wt% plasmapowder dispersion, which results in lower temperatures than 0.01wt% LaB6. In contrast to the 0.01 wt% plasma LaB6 dispersion, for milled NPs / EG dispersions a higher laser power is needed to achieve the same temperature of 140 °C. The larger hydrodynamic diameter and the shape [11, 40] thus the manufacturing process of LaB6 NPs affects the absorption strength, range and maximum, which corresponds to the temperature [41]. For example, the LSPR of spherical gold NPs differs clearly from Au nanorods, due to the oscillation of free electrons perpendicular and additionally along to the long axis of the rods [42].

Fig. 6. (a) Temperature measurements in dependence on laser power; (b) temperature distribution around the laser spot at around 0.5 mW for 0.01 wt% plasma LaB6 / EG.

4. Conclusion In summary, we have shown in milling experiments that NPs in a mean size of 110 nm in ethanol, 1-methoxy-2propanol and ethylene glycol without changing the basic properties of LaB6 can be manufactured, but we have to respect the ZrO2 contamination. In contrast to milled NPs, high quality and high purified nano-LaB6 < 80 nm is gained by a plasma process. While grinding experiments are performed at ambient temperatures, the plasma method is a high temperature process which results in change of basic properties, especially the shift of the absorption maximum from 1032 to 808 nm. Stable dispersions are gained and characterized by low instability indices < 0.1 using polar dispersion media. A very well homogenous distribution of LaB6 NPs is achieved by embedding milled and plasma LaB6 in acrylate terminated starPEG. The photo-thermal conversion of ethylene glycol based dispersions is observed successfully using an IR camera from FLIR. Temperatures up to 140 °C can be achieved with plasma LaB6 using very low laser power. Acknowledgment The authors would like to thank the German Federal Ministry of Education and Research (BMBF) for funding the research project 03XP0051, within the funding program “KMU innovative: Nanotechnology”. This work was performed in part at the Center for Chemical Polymer Technology CPT, which was supported by the EU and the federal state of North Rhine-Westphalia (grant EFRE 30 00 883 02).

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