In-situ characterization of metal nanoparticles and their organic coatings using laser vaporization aerosol mass spectrometry

Transcription

1 Nano Research DOI /s Nano Res 1 In-situ characterization of metal nanoparticles and their organic coatings using laser vaporization aerosol mass spectrometry Patrik T. Nilsson 1 ( ), Axel C. Eriksson 2, Linus Ludvigsson 3, Maria E. Messing 3,4, Erik Z. Nordin 1, Anders Gudmundsson 1, Bengt O. Meuller 3, Knut Deppert 3, Edward C. Fortner 5, Timothy B. Onasch 5, and Joakim H. Pagels 1 Nano Res., Just Accepted Manuscript DOI /s on August 10, 2015 Tsinghua University Press 2015 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 Graphical table of content This work introduces laser vaporization aerosol mass spectrometry as an in-situ technique to characterize metal nanoparticles. In-situ techniques are advantageous to ex-situ techniques such that the aerosol particles do not have to be deposited on substrates and extracted from the production line, which may alter the characteristics of the particle surface. 1

3 In-situ Characterization of Metal Nanoparticles and their Organic Coatings Using Laser Vaporization Aerosol Mass Spectrometry Patrik T. Nilsson 1*, Axel C. Eriksson 2, Linus Ludvigsson 3, Maria E. Messing 3,4, Erik Z. Nordin 1, Anders Gudmundsson 1, Bengt O. Meuller 3, Knut Deppert 3, Edward C. Fortner 5, Timothy B. Onasch 5 and Joakim H. Pagels 1 1 Ergonomics and Aerosol Technology, Lund University, P.O. Box 118, SE-22100, Lund, Sweden 2 Nuclear Physics, Lund University, P.O. Box 118 SE 22100, Lund Sweden 3 Solid State Physics, Lund University, P.O. Box 118, SE-22100, Lund, Sweden 4 Synchrotron Radiation Research, Lund University, P.O. Box 118, SE-22100, Lund, Sweden 5 Aerodyne Research Inc., MA , Billerica, USA *Corresponding author patrik.nilsson@design.lth.se Keywords: metal, aerosol, organic surface coating, contamination, morphology, alloy, spark discharge 2

4 Abstract The development of methods to produce nanoparticles with unique properties via the aerosol route is rapidly growing. Typical characterization techniques extract the particles from the synthesizing process for subsequent off-line analysis, which may alter the characteristics of the particles. In this work we use laser vaporization aerosol mass spectrometry (LV-AMS) using 70 ev electron ionization for real-time, in-situ nanoparticle characterization. Particle characteristics were studied while varying the aerosol synthesizing method, degree of sintering and by controlled condensation of organic material to simulate surface coating/functionalization. The LV-AMS was used to characterize several types of metal nanoparticles (Ag, Au, Pd, PdAg, Fe, Ni, and Cu). The degree of oxidation of Fe and Ni nanoparticles was found to increase with increased sintering temperature while the amounts of surface organic impurities on metal particles decrease with increased sintering temperature. For aggregated metal particles the amount of organic impurities were found to be similar to a monolayer. By comparing different equivalent diameter measurements, we show that the LV- AMS can be used in tandem with a differential mobility analyzer to determine the compactness of synthesized metal particles, both during sintering and during addition of material for surface functionalization. Materials supplied in the particle production line, downstream the particle generators, were found to reach the generators as contaminants. The possibilities of such in-situ observations are important since it allows rapid responses to undesired behavior within a particle production process. We demonstrate the utility of realtime, in-situ aerosol mass spectrometric measurements to characterize metal nanoparticles, including chemical composition, contamination, and shape obtained directly from the synthesizing process line, with the potential of effective optimization of process operating parameters. 3

5 1. Introduction In the last decade, the field of nanotechnology has grown immensely and the potential to create materials with novel characteristics have opened up almost unlimited numbers of applications for nanomaterials and nanoparticles [1, 2]. With a growing demand, a need for new production methods with higher throughput and high reliability exists. Aerosol production methods are attractive since they can be performed in a continuous manner and particles with high purity and tailored morphologies can be synthesized [3]. Aerosol production, or aerosol synthesizing, methods are based on the formation of particles from either a gas, liquid or a solid material. The optimal synthesizing method depends on the desired characteristics of the particles and the desired quantities needed. After synthesis, metal nanoparticles are typically modified through heat treatment (e.g. sintering to more compact shapes), surface functionalization via chemical reactions and/or condensation of additional material. Different rates of after-treatments of the synthesized particles can be used to alter the particle characteristics and methods to study such particle treatments are necessary [3]. Currently aerosol synthesis approaches are hampered by a lack of development of instruments that can be used to analyze and characterize the synthesized particles in-situ and in real-time [4]. The state-of-the-art analysis of the particles today includes offline techniques such as electron microscopy, X-ray diffraction [5] and X-ray photoelectron spectroscopy [6]. A drawback with such techniques is that the aerosol particles under most circumstances need to be extracted from the production line and collected on different types of substrates before analysis. This may alter the particle characteristics (e.g. morphology, surface properties, oxidation and impurities) during collection, storage or analysis. This is important since impurities that may exist as complete or partial coating on the particles, can play a crucial role 4

6 in the ultimate intended efficacy. To avoid this problem, a solution is to analyze the particles directly in the aerosol phase and in the production line. Thus, a real-time instrument that simultaneously quantifies and determines the chemical composition, size, and morphology of particles can be considered as an optimal aerosol analytical instrument. Aerosol mass spectrometers constitute a group of techniques where gasborne particles are sampled directly from the aerosol phase into a vacuum chamber followed by vaporization, ionization and detection with mass spectrometry. The first such techniques relied upon combined laser vaporization and desorption. However, such approaches are limited in terms of quantification [7]. Electron ionization aerosol mass spectrometry (EI-AMS, or just shortly AMS) was developed to study atmospheric aerosols [8, 9], but its applications have recently expanded [10-12]. With the AMS one can determine the chemical composition of sampled particles as well as particle size distributions and total particle mass concentrations of non-refractory species (species that flash vaporize at 600 C) [13] including some volatile metal species [14]. Recently, further developments of the AMS have given the possibility to also detect refractory species such as carbon black and metal containing particles [15, 16]. This instrument is commonly referred to as the soot particle aerosol mass spectrometer (SP-AMS) and uses intracavity IR laser vaporization followed by 70 ev electron ionization. The sensitivity of the SP-AMS to metals internally mixed (existing in the same particles) in trace amounts in black carbon particles has been reported [17]. In such approaches the black carbon core acts as the vaporizer (heated up to 4000 K before vaporization). These approaches are important for atmospheric applications, but have only limited use for engineered nanoparticles. 5

7 In this work, we aim to explore the potential application of the SP-AMS technique as a tool to investigate important characteristics of synthesized metal nanoparticles. Therefore, we choose to denote the instrument as laser-vaporization aerosol mass spectrometer (LV-AMS), which reflects the wider applications of the technique. We show that the LV-AMS can be used to determine important chemical and physical characteristics such as alloy composition, degree of oxidation, type and amount of impurities, size and morphology of metal particles in realtime. We show that the LV-AMS can be used to characterize and compare different metal nanoparticle synthesis techniques and how post-production sintering and condensation of functionalization material may affect metal nanoparticles. In-situ analysis techniques, like the LV-AMS, are important for developing and understanding new synthesizing methods and to optimize the quality of production and to control production processes. 2. Methods 2.1 Particle generation setups Metal particles were synthesized with two different types of techniques: (1) spark discharge generators (SDG) and (2) high temperature furnaces (HTF). The general experimental setup, including particle synthesis, size selection, sintering, organic surface coating and instruments for particle characterization, is shown in Figure 1. The metal particles generated with the SDG setups were Ag, Au, Cu, Fe, Ni, Pd and PdAg nanoalloys. The HTF was used to generate Au nanoparticles. A spark discharge generator consists of a particle generation chamber that houses two metal electrodes, separated by a few millimeters. When a high voltage is applied to the electrodes, a spark is formed. The high temperature generated by the spark instantly evaporates metal material from the electrodes and particles are subsequently formed through nucleation, 6

8 condensation and coagulation of the vapors. The formed particles are aggregates with a typical primary particle size of 5-7 nm and average geometric mean diameters (GMD) of nm for the aggregates. The metal composition of the two electrodes and their separation distance can be changed to vary the chemical composition, size, and morphology of the metal nanoparticles generated. During this study, two different SDG instruments were used: (i) SDGP (GFG 1000, Palas) and (ii) SDGC (in-house built system) [18]. The HTF technique consisted of a tube furnace with a graphite or ceramic tube. The desired base material for the particles were placed in a crucible inside the tube furnace and heated to a desired temperature. The particles formed by the HTF are aggregates with similar primary particle sizes as formed with the SDG. The average GMD for HTF generated aggregates was nm. The chemical composition, size, and morphology of the generated metal nanoparticles are dependent on the type and amount of material being evaporated, the furnace temperature and the carrier-gas flow rate. After metal particle generation, either using SDG or HTF, the particles were size selected using a differential mobility analyzer (DMA), and passed through a sintering furnace. The furnace was used to heat up the aggregated particles to a temperature where they sinter, e.g. the aggregates collapse to near-spherical shapes. DMA s were used both upstream and downstream of the sintering furnace to select particles of specific mobility sizes and measure the changes in mobility size during sintering. Downstream the metal particle generator setup, depicted in Figure 1, an aerotaxy system used for nanowire growth on the produced metal particles was usually connected. This aerotaxy system, described in detail elsewhere [19], was not connected as a part of the generator setup during the measurements presented in this paper. The influence of the aerotaxy system on the metal generator setup was however studied by examining if material, supplied in the aerotaxy 7

9 system, in fact could reach and contaminate the metal particle generators. This was done such that after the aerotaxy system had been disengaged from the metal particle generators maintenance and cleaning of the HTF was deliberately not performed. The nanowires produced in the aerotaxy system consisted of GaAs and were created by supplying trimethylgallium (Ga(CH3)3) and arsine (AsH3) into the system. For a subset of experiments, a condenser tube (1/2 ID Pyrex glass tube) was used after the sintering furnace to condense organic surface coatings on the metal nanoparticles to study its effects on the particles chemical composition, size, and shape. The condenser tube was temperature controlled up to 150 C by using two temperature zones governed by heating tape wrapped on the outside of the glass tube. A non-refractory liquid organic model substance, dioctyl sebacete (DOS), was placed in a small aluminum container inside the condenser tube. Dual temperature zones enabled better condensation control and the elimination of selfnucleated DOS particles. Three different types of Ag nanoparticles were used for DOS condensation: Aggregates, semi-sintered and sintered, all with a a single particle mass of ~4 fg. The semi-sintered Ag particles were partly sintered at a temperature of 200 C and the sintered Ag particles were heated to a temperature of 900 C. 2.2 Instrumentation In-situ characterization of airborne nanoparticles was performed with a LV-AMS (Aerodyne Inc.). This instrument is the same as the High Resolution Time of Flight-AMS (HR-ToF-AMS) with the addition of an intracavity Nd:YAG laser (λ: 1064 nm) as a vaporization technique [15]. In atmospheric applications, this system is referred to as a Soot Particle AMS (SP-AMS). In AMS instruments, aerosol particles are focused to a tight particle beam into vacuum using an aerodynamic lens [8, 9]. In the conventional AMS, the focused particles are accelerated across a vacuum chamber and impacted on a heated tungsten vaporizer (W), typically set at 8

10 600 C. Upon flash vaporization, the molecules are ionized by electron ionization (70 ev) and the formed ions are extracted into a time of flight mass spectrometer. Electron ionization at 70 ev is standard in an AMS. At this energy level the electron ionization cross sections, including those for carbon and organic molecules, are close to maximum. Ionization efficiencies for metal components are also maximized close to 70 ev [20]. A drawback by using electron ionization is that organic molecules are fragmented and parent molecules are troublesome to identify if the sampled aerosol consist of multiple unknown organic species. Since no separation of molecules prior to ionization can be performed the chemical identification of organic molecules with the AMS typically implicate that different molecular classes can be identified and quantified, rather than parent organic molecules. Studies have shown that the AMS gives mass spectra that are very similar to spectra found in databases, such as the NIST database, for molecules that are resistant to thermal decomposition and fragmentation [21]. Some molecules however, e.g. aliphatic organic molecules, may fragment due to the thermal vaporization preceding the ionization. For these types of molecules the mass spectra from the AMS show the same ion fragments as those found in NIST mass spectra but in different relative intensities [22]. Soft ionization techniques that uses less energy than electron ionization and that may lead to less or no molecular fragmentation are being investigated to be included in the AMS. However, such techniques show lower ionization cross sections and are less versatile compared to electron ionization [9]. With the W vaporizer, non-refractory species, which evaporate below 600 C, can be detected. The laser in the LV-AMS is positioned in the vaporization zone, perpendicular to the particle beam. The laser vaporizer makes it possible to vaporize and detect refractory species when they are internally mixed with material that sufficiently absorbs laser-light at 1064 nm, such as soot, metal and selected metal oxide particles. By keeping the vaporization and ionization 9

11 separate, the number of formed ions will be directly proportional to the vaporized particle mass which allows quantitative measurements of particle mass concentrations. The LV-AMS was operated in two different modes; the size-resolved particle time of flight (PToF) mode and the size integrated mass spectrum (MS) mode. In the size-resolved mode, the particle beam is modulated by a continuously rotating (140 Hz) mechanical chopper. The chopper has a small slit (2%) that allows the particle beam to pass through once per chopper cycle. By timing the mechanical chopper cycle and ion detection in the mass spectrometer, the vacuum aerodynamic diameter (dva) and mass size distributions of the sampled particles can be determined. This is possible since the particle time of flight (~3 ms) is significantly longer than the ion time of flight (~50 µs) in the mass spectrometer [23]. When the chopper is alternated between open and closed (i.e., blocking particle beam) positions, an average background corrected difference mass spectrum (MS-mode) of the sampled particles can be achieved. Compared to the PToF mode, MS mode gives no size resolved data but it has a higher sensitivity. Data analysis was performed with SQUIRREL (Sequential Igor Data Retrieval, version 1.56D) and PIKA (Peak Integration by Key Analysis, version 1.15D). Detection limits of the LV-AMS (3σ) were determined by particle-filtered measurements of the sample aerosol. The LV-AMS technique will provide quantitative measurements after mass-specific ionization efficiencies, mie, (in ions per pg) for different metals and metal oxides are determined. This depends on the relative ionization efficiency of the metal relative to the calibration aerosol (RIEs). The calibration aerosol may be nitrate [15] or regal black, a commercial carbon black [17]. The RIEs is determined from the absorption cross sections for 70 ev ionization of metal and the calibrate particles. In addition, the collection efficiencies (CE) for the different vaporizer configurations need to be known. The overall CE is a function 10

12 of particle transmission through the sampling lens (EL) and vaporizer-specific issues (EB, ES and EV), CE = E L E B E S E V (1) EL is affected by losses in the sampling inlet at vacuum aerodynamic sizes above about 400 nm, which will be of increasing importance with increased sintering of particles. The standard aerodynamic lens used in the LV-AMS allows penetration of particles smaller than 1000 nm. The possibility to sample particles of larger aerodynamic diameters exists if a lens that allows penetration of particles up to nm is implemented [24]. For the heated tungsten vaporizer, the CE will be governed by incomplete vaporization due to particle bounce (EB) off the vaporizer surface [25]. The CE is expected to trend downward below unity for more compact metal nanoparticles (i.e., lower surface area for constant inertia) while non-refractory liquid coatings commonly decrease bounce. For the laser vaporizer, the CE may be lower than unity due to overlap mismatches between the sampled particle beam and the laser beam (ES) in the vaporization and detection chamber [26]. ES will trend upward toward unity for more spherical metal nanoparticles (i.e. lower particle beam divergence due to symmetric particle shapes), which occurs with increased sintering temperatures. When both vaporizers are in use, we expect to maximize the vaporizer-specific CE factors (i.e., EB and ES close to unity), especially as they trend in opposite directions with sintering. For metal and metal oxide particles, incomplete vaporization due to limitations in the absorption cross section at 1064 nm of the material (EV), may also become a limiting factor. This work represents one of the first direct applications of the LV-AMS technique to characterize the size, mass, and chemical composition of metal nanoparticles. We focus on demonstrating the potential benefits of this technique for in-situ characterization of metal 11

13 nanoparticles; therefore, we assume that the CE is unity. A CE of unity is expected to be a reasonable approximation for organic surface coatings, as both vaporizers contributes to this signal. But it would most likely lead to underestimations of metal concentrations, as the metals are vaporized by the laser vaporizer only. Therefore metal signals are reported in Hz only and further quantification will be a topic for future studies. An Aerosol Particle Mass Analyzer (APM) (Model 3600, Kanomax Inc., Japan) combined with a Differential Mobility Analyzer (DMA) was used to determine the particle mass (mp) and effective densities (ρeff) of the particles [27, 28]. In subsets of experiments a centrifugal particle mass analyzer (CPMA) [29] replaced the APM to quantify the amount of DOS coatings present on Ag particles. The APM, or CPMA, was used downstream a DMA which allowed determination of the particle mass for one mobility particle size at a time. By knowing the particle mass and mobility diameter (dm), and by using the same nomenclature as DeCarlo et al. (2004), the effective density ρeff(i) can be calculated from: ρ eff(i) = m p π 6 d m 3 (2) A second, closely related effective density ρeff(iii) can be determined from serial determination of dm and the vacuum aerodynamic diameter, dva, from the LV-AMS: ρ eff(iii) = d va d m ρ 0 (3) where ρ0 is unit density. Effective densities determined by different methods may differ in relation to each other, depending on particle size and the dynamic shape factor of the particles. Effective densities determined from mass-mobility relationships ( eff(i)) and vacuum aerodynamic size-mobility ( eff(iii)) are highly correlated (i.e., trend similarly with particle size and shape), and their values typically differ by less than 30% [30]. However, little experimental information is available on such differences. 12

14 The total particle number concentrations were determined with an in-house built electrometer and a Condensation Particle Counter (CPC). Two CPCs were alternatively used, a CPC 3010 (TSI Inc) at a flow rate of 1 lpm and a CPC 3022 (TSI Inc.) at a flow rate of 0.3 lpm. Particle mobility size distributions were determined with a SMPS (DMA 3071 and CPC 3010, TSI Inc.). Filter samples were collected for off-line analysis of metals with particle induced X-ray emissions analysis (PIXE). The PIXE samples were collected on 37 mm polycarbonate filters (0.4 µm pore size, Nuclepore) held in plastic three pieces filter cassettes. The flow rate through the filters was 1 lpm. The organic mass fraction (mfoa) on (or in) the engineered nanoparticles was determined by quantifying the mass concentration of the organic aerosol (COA) from the LV-AMS (as described in Onasch et al. (2012)), the total particle mass concentration based on the number concentration from the CPC (N) and the total mass per particle (mp = metal + impurity) determined from either the APM/CPMA or the mobility and vacuum aerodynamic diameters. The mfoa was calculated by: m foa = C OA 1 N m p = I OA 1 (4) RIE OA CE mie NO3 Q N m p where IOA is the sum of the signal in Hz for all organic ion fragments, RIEOA is the relative ionization efficiency of organic aerosol (1.4) relative to nitrate. mieno3 is the measured ionization efficiency of NO3 (700 ions/pg) and Q is the volumetric flow rate (1.47 cm 3 /s) of the LV-AMS. We conducted these measurements for both LV-AMS vaporizer configurations (dual vaporizers and heated tungsten vaporizer alone). 3. Results and discussion 3.1 Chemical analysis using high resolution aerosol mass spectrometry 13

15 With a growing use of high resolution mass spectrometers, the concept of mass defects has been increasingly used [31]. The mass defect is defined as the deviation of a compound s exact mass to its nominal mass. This is used in high resolution mass spectrometry to differentiate between different detected ions with the same nominal mass to charge ratio (see example in Figure 2a). In Figure 2a the 107 Ag isotope can be separated from the main organic ion (C8H11 + ) that in this example is found at the same nominal m/z. This gives the possibility to characterize any non-refractory particulate matter (i.e. impurities) that may exist on metal particles. Impurities, such as organic ions, located at the same nominal m/z can often also be separated from each other since their mass defect depends on the atomic composition of the ions. For example, the addition of H atoms (1.008 u) changes the mass defect in a positive direction while the addition of O atoms ( u) to an ion changes the mass defect in a negative direction. As shown in Figure 2b, metals have a large negative mass defect which typically makes them easy to identify in high resolution mass spectra. Therefore, chemical information on both metal nanoparticles and any potential impurities can be achieved with high resolution mass spectrometry. 3.2 Aerosol mass spectra of metal nanoparticles The separate mass spectra achieved when sampling synthesized metal nanoparticles of four different materials are shown in Figure 3. Each mass spectrum shows ions representing the different metal and metal oxide isotopes (red) and organic impurities (green). To further strengthen the identification of metal ions, the isotopic abundances for the metal isotopes can be compared to literature values. Such comparison can also be used to evaluate the data analysis and the mathematical fitting procedures. In Figure S1, the isotopic abundances of Pd 14

16 particles can be found. The errors between the calculated isotopic abundances, also considering those for Ag and Fe ions, and the values from the literature were all within 1 %. Pd (Fig. 3a) and Ag (Fig. 3b) particles were generated with the SDGC system at sintering temperatures of 500 C and 250 C respectively, which render semi-sintered particles. The mass spectra of Fe particles, that were generated with the same SDG system and sintered at 800 C is shown in Figure 3c. The Fe mass spectrum is dominated by the most abundant isotope, 56 Fe. However, along with the other Fe isotopes, iron oxide fragments could be detected. Note that even if iron oxide were found for each Fe isotope (i.e. 56 FeO, 54 FeO, 57 FeO and 58 FeO) only 56 FeO is visible in Figure 3c due to the signal strengths and the scale of the vertical axis. The detection of oxidized ions means that the Fe particles become partly oxidized either during synthesis or downstream processing (in this case sintering at 800 C) as described in more detail below. Au particles were generated with the HTF (Fig. 3d). Cu particles were also synthesized but were not detected with the LV-AMS. A plausible reason for this is that Cu is easily oxidized and copper oxide does not absorb enough energy at the used vaporization wavelength (1064 nm). 3.3 Mass spectra of organic and inorganic impurities The LV-AMS was used to explore how particle synthesis method, sintering temperature and particle material affected the composition of impurities in the particle phase. As can be seen in Figure 3, the organic impurities on Fe particles differ from the organics found on both Ag and Pd particles, even though the three particle types were synthesized with the same method and setup. The organic ion fragments detected on, or in, Ag and Pd particles were typical for aliphatic hydrocarbons while a higher fraction of oxygen containing fragments was found for Fe particles. Explanations for this result were not studied but possible hypotheses are presence 15

17 of traces of oxygen in the carrier gas, oxides on the Fe electrodes, the different sintering temperatures used and/or differences in surface properties of the particles that lead to adsorption of components with different polarities. During the sampling of Fe particles, the dominating organic fragment was m/z=45, identified as C2H5O +. A high-resolution mass spectrum of organics on aggregated Pd particles is shown in Figure 4a. With no sintering (20 C), the organic mass spectrum is dominated by aliphatic fragments (CxHy) with oxygen to carbon ratio (O/C) of The calculation of O/C is based on the improved ambient method for calculating O/C ratios as presented by Canagaratna et al. [32]. The highest contribution is from m/z 41, 43, 55 and 57, which are common fragments from alkanes and alkenes. With the LV-AMS, we observed that the oxygen to carbon (O/C) ratio decreased when a temperature greater than 100 C was reached which means that the relative contribution of oxygenated ions, such as CO2 + and CHO +, decreased. This is shown in Figure 4b. When comparing Au aggregated particles, synthesized with the SDGP and the HTF during the same day, the mass spectra of the organic impurities show large differences (Figure 5a and b) with dominating contribution at m/z 74 for the SDGP and m/z 85 and 126 for the HTF. The organics present on the Au particles formed in the SDGP appears to have a higher content of oxygenated species while the organic species on the HTF generated particles mainly consist of aliphatic components. Messing et al. [33] concluded that the choice of synthesizing method had little effect on the properties of nanowires for which the Au particles were used as seed particles. In the above mentioned study, the Au particles were sintered at high temperature. This meant that the possible effect that organic coatings may have on the end results was minimized since the presence of organic coating on the metal particles was minimized. If no, or low sintering temperatures are used our results indicate that the synthesized particles may have different surface properties depending on differences in the composition of impurities. 16

18 Prior to the measurements with the LV-AMS the HTF had, along with the sintering furnace, been used to generate Au seed particles for GaAs nanowire growth in an aerotaxy system. After the aerotaxy system had been disengaged from the HTF (and the sintering furnace) maintenance of the HTF was deliberately not performed. This was done in order to find out if any traces of the materials, supplied into the aerotaxy system, could reach the HTF, even though the HTF was connected upstream the aerotaxy system. The mass spectrum of Au aggregates (Fig. 5a), synthesized with the HTF after that the aerotaxy system had been disengaged, showed content of both Ga and As components with AsO + (m/z ) and As3O4 + (m/z ) being the main fragments. This presence of Ga and As species inside the HTF shows a need for maintenance of not only downstream equipment but also upstream equipment in particle generator setups. Contaminations of upstream equipment may lead to undesired behavior of the synthesized particles since the chemical content and surface properties of the particles may be altered. A possibility to detect contaminations during continuous production of nanoparticles in real-time is important in order to keep, or enhance, the production efficiency of the process. In-situ characterization of the nanoparticles is a way to rapidly detect undesired behaviors within the synthesizing process and equipment. In summary, the LV-AMS allows us to show that generator types, particle materials, sintering temperatures and the history of the generators all may affect the composition of the impurities in the particle phase. The measurements were carried out by directly sampling the particles from the aerosol-phase in the generator into the LV-AMS, thus uncertainties related to storage of collected material on substrates were avoided. The aerodynamic lens inlet allows us to concentrate the particle phase by about a factor of 10 7 relative to the gas-phase, thus most gasphase artefacts (less than ~10 ppm) can be safely neglected. In addition, vapor pressures of organic components commonly found in the particle phase are low enough that volatilization during transport in the vacuum chamber can be neglected. 17

19 3.4 Quantification of organic impurity coatings on metal nanoparticles Figure 6 shows that the amount of organic impurities on synthesized Pd particles decreases upon passage through increased temperatures in the sintering furnace. The organic mass fraction decreases from 4-7% at sintering temperatures below 200 C down to less than 1% at a sintering temperature of 500 C. The mass fractions of impurities were similar for the other investigated metals. Measurements were also performed at a sintering temperature of 650 C. At this temperature, the organic signal strength reached the detection limit of the LV-AMS (3σ) and is therefore not presented. The detection limit for organic aerosol was 54 Hz, which corresponds to an organic aerosol mass concentration of 0.04 µg/m 3 if an RIEOA of 1.4 and CE of 1 is assumed. It is important to note that the impurity fraction on Pd particles does not drop to zero at temperatures where one would have expected most organic components to be evaporated (500 C). This observation was found for the other materials as well. This suggests that the metal nanoparticles are strong absorbers of organics and/or that the organics are volatilized in the sintering furnace, followed by re-condensation of a fraction of the organics upon exit. A 0.5 nm monolayer of organics with unit density around a spherical Pd primary particle with a diameter of 5 nm gives an organic mass fraction of about 6%. By assuming that the full surface area of all primary particles in the aggregate is accessible for uptake of organic impurities these 6 % can be compared with our experimental observations of 4-7% for nonsintered particles. Thus, based on our results, we hypothesize that a mono-layer of organics may be present in many applications of nanoparticles. As the particles are sintered, the accessible surface area decreases, which may be part of the explanation of the decreasing impurity mass fractions upon sintering. 18

20 The importance of the properties of the monolayer surrounding aerosol particles investigated here, both in technical applications (such as catalysis and nanowire/tube growth) and for particle inhalation, remains to be investigated in detail. In this respect, it can be noted that it has recently been shown that the layer of biomolecules that binds to nanoparticles upon deposition in body fluids has been suggested to be a key property for the particle interactions with different target organs and intra-cellular trafficking, often more important than the properties of the bare metal nanoparticle surfaces [34]. The in-situ sampling capability of the LV-AMS is of high importance when investigating organic surface coatings and impurities since these may be altered during sampling, storage or during analysis using conventional off-line techniques. Conventional techniques to determine organic surface coatings from collected samples include thermogravimetry as a screening method [35]. Detailed chemical analysis methods commonly used for organic surface coatings of nanomaterials include Thermal desorption (TD) or Accelerated Solvent Extraction (ASE) followed by GC-MS, MALDI-TOF-MS or Electrospray ionization MS. 3.5 Detection of metal nanoparticle oxidation The LV-AMS can be used to monitor changes in the degree of oxidation by measuring the signal strength of metal oxide and pure metal ions. In this work, metal oxide ion signals were observed for synthesized Fe and Ni particles with the LV-AMS, whereas particles of Ag, Pd and Au showed no signal of ions containing both metals and oxygen. The degree of oxidation of both Fe and Ni particles increases with increasing sintering furnace temperature (Figure 7). An increase of the sintering temperature from 300 to 800 C doubles the signal of FeO + ions compared to the signal of the Fe metal ions. An even bigger effect is seen for Ni particles, where an increase from 400 to 700 C increases the nickel oxide signal fraction six times 19

21 compared to the Ni metal ions. The highest sintering temperature used for Pd particles was 650 C. No PdO + fragments were found in the mass spectra at this temperature. The degree of oxidation depends on factors such as pre-existing metal oxide on the electrodes, oxygen penetrating into the synthesizing setup, traces of oxygen in the carrier gas or possibly oxidized organic components on the setup interior walls. Further studies of the origin of the oxygen were not conducted in this work. The likely explanation is that small and unidentified penetrations of air leaked into the synthesizing systems. With the LV-AMS, we observed that the amount of oxygen present in the synthesizing setup and the temperature used was sufficient to create oxides of Fe and Ni, but not for Pd and Ag. The degree of particle oxidation is important for the particle surface properties and may be both a desired and undesired feature when particles are being synthesized. Segregation of active constituents to the surface of bimetallic nanoparticles, for example, has been given much attention as a way to increase the effectiveness of catalysts [36]. One way to achieve this is by introducing a reactive gas and applying heat to the synthesized particles. Blomberg et al. [36] studied the segregation of Pd in mixed PdAg particles by introducing traces of O2 into the system. At elevated temperatures, surface oxides were formed and Pd was found to segregate to the particle surface due to the favored formation of PdO compared to AgO. The LV-AMS can be used to track such degree of metal nanoparticle oxidation in-situ as a function of sintering temperature, which makes it possible to find optimal operating conditions depending on the purpose of the synthesized particles. However, many metal oxides cannot be vaporized by the Nd:YAG laser due to their small absorption cross-sections at 1064 nm. We found no detectable signal for Cu nanoparticles which is most likely due to that the Cu particles were oxidized and CuO has a negligible absorption cross section at 1064 nm. In this work we showed that FeO could be detected by the LV-AMS. Whether vaporization of iron oxide was achieved by laser absorption of the oxide itself or of pure Fe, 20

22 which in turn vaporized the oxide, needs further investigation. Metal oxides may also occur externally mixed from black carbon (not in the same particle) in combustion emissions (oxides of Zn, Ce, V etc) and wind blown dust emissions (iron oxides), thus further studies of the suitability of the LV-AMS for the applications towards metal oxides are needed. 3.6 Detection of alloy nanoparticles The spark discharge generator makes it possible to synthesize bimetallic particles with an alloyed structure by using one electrode of each component. While synthesizing PdAg particles, the LV-AMS signal ratio of Pd to Ag was determined as a function of mobility diameter. The SDGC was continuously operated at the same settings and different sized particles were selected using the DMA. The results in Figure 8 show that the Pd to Ag mass ratio is independent of particle size. In repeated measurements, the LV-AMS, with assumptions of the same mass-specific ionization efficiencies for both metals, measured 3 times lower Pd to Ag ratios compared to the reference method, PIXE (Table 1). The full quantification with the LV-AMS lies beyond the scope of this work. A hypothesis regarding the apparent underestimation of Pd signal strength would be that different relative ionization efficiencies apply for Pd and Ag. This hypothesis is supported by the fact that the ratio of the PIXE/LV-AMS measurements was constant, even though the alloy ratio differed by a factor of approximately two between the two different SDG instruments. Pd has a higher boiling point and a lower vapor pressure than Ag, thus it is also possible that Pd is incompletely vaporized from these particles, which may explain the lower sensitivity to Pd compared to Ag. In any case, these results points towards a potential of the LV-AMS technique to provide a method for in-situ monitoring of the relative composition of alloys and mixed component nanoparticles. 21

23 3.7 Effect of sintering and organic coatings on equivalent particle sizes and densities A key property of catalysts is the active surface available for heterogeneous reactions to take place. A large surface-to-volume ratio is desirable, which makes nanoparticles optimal as catalysts. The highest surface-to-volume ratio is achieved for complex aggregates built up by coagulation of small primary particles. The primary particles may be fused together by bridging and its structure determines the particle shape and density. Synthesized metal nanoparticle size, physical structure, and shape can be characterized in-situ in the aerosol phase by a tandem measurement of mobility diameter, dm, with the DMA, followed by the vacuum aerodynamic diameter, dva, with the LV-AMS [37]. Sintering is a way to control the complexity of the overall particle shape and to eventually achieve spherical shapes. As the sintering temperature increases, the bridging between the primary particles within an aggregate gets more and more prominent and the particles become denser. This is illustrated in Figure 9a. In these experiments the selected mobility diameter of the processed particles was kept constant (100 nm). The vacuum aerodynamic diameter on the other hand increased with increased sintering temperature. The relationship between dva and dm (eq. 3) is used to determine the effective density of the particles (Fig. 9b). To achieve near spherical shapes of Pd particles a sintering temperature of at least 800 C needs to be applied. In this set of experiments the highest temperature was 500 C, thus the temperature was not high enough to get near-spherical shapes and a particle effective density close to the inherent bulk density of Pd (12.02 g/cm 3 ). The measurement of particle effective densities, from the combination of the LV-AMS and DMA measurements, provides a mechanism to monitor the shape of the particles and to ensure that they possess the desired morphology. The possibility to measure two different 22

24 equivalent diameters is also valuable if the synthesized metal particles are to be used for nanowire and nanotube growth. Depending on the sphericity of the seed particles, the achieved nanowires and nanotubes may get different properties. In Figure 9b, two types of effective densities are shown, determined by two different methods. The most explicit method to determine effective density is the DMA-APM measurement (ρeff(i)) as it involves a direct measurement of particle mass. The effective density derived from DMA-LVAMS (ρeff(iii)) measurements shows slightly higher values compared to ρeff(i). Figure 10 shows that condensation of liquid organic compounds for surface functionalization/coating affects ρeff(iii) and the measured equivalent diameters of the nanoparticles as well. We used controlled condensation of a commonly used low volatility organic liquid (DOS) on Ag nanoparticles of three different structures (aggregates, semisintered and sintered) as a model for surface coating/functionalization. All three types of Ag particles had the same initial mass (4 fg). While an increased sintering temperature leads to an increased effective density, condensation of organic coatings often leads to a decrease in effective density. This is shown in Figure 10 for the sintered particles. The inherent material density of the condensing organic component is much lower than the material density of the metal containing nanoparticle, thereby lowering the particle effective density (Fig. 10a). However, for the highly fractal aggregates, condensation of organic surface coatings initially leads to an increase in effective density due to a decrease in dm (Fig. 10b), even though the initial effective density is similar to the bulk density of DOS. This is most likely due to surface-tension enforced restructuring of the aggregate, leading to a more compact shape. This results in a lower drag force and reduced equivalent mobility diameter as has previously been observed for atmospheric soot particles 23

25 when coated with sulphuric acid [38]. The partly compacted aggregates are thereafter filled with the organic coating before full sphericity is achieved. 3.8 Limitations and recommendations for future research For full quantification of metal nanoparticles with the LV-AMS the collection efficiency as well as the relative ionization efficiency of the metals relative to the calibration aerosol needs to be known. The collection efficiency factors that depend on losses in the inlet and focusing of the particle beam are more complex for metal particles compared to atmospheric particles. Lens penetration (EL) may be low for fully sintered particles (high density leads to high vacuum aerodynamic diameter). For aggregate particles the focusing of the particle beam (ES) may deteriorate. Future studies need to focus on EV, the fraction of the incoming particle that is vaporized when passing through the laser beam. The metals commonly have a lower absorption cross section at 1064 nm compared to black carbon for which the instrument was primarily designed. On the other hand the vaporization temperature is lower for the metals compared to black carbon, such that they may require less energy for vaporization. Carbone et al. (2015) compared theoretically predicted and experimentally measured relative ionization efficiencies at 70 ev (relative to C3 carbon clusters from regal black aerosol) for metals when internally mixed in the same particles as carbon black. The ratio between measured and theoretical RIE was for seven different metals (excluding metals where thermal surface ionization occurred). For the metal particles in this study the highest obtained ratio of measured to theoretical RIE defined in the same way was 0.5. This occurred for Ag agglomerates coated with liquid DOS. However, since not only the RIE but also the collection efficiencies vary 24

26 between the different metal particles and the calibrate particles (regal black), the comparison is complex and require further study. 4. Conclusions In this work, we have demonstrated that the LV-AMS can be used to characterize metal nanoparticles in the aerosol phase. Such in-situ characterization presents a way to minimize sampling artifacts which can arise when particles are extracted from the aerosol phase and deposited on surfaces or substrates for subsequent analysis. We have shown that the LV-AMS can be used for analysis of several key particle properties that determine the efficacy of metal particle synthesizing methods. These particle properties include particle chemical content (including analysis of impurities and metal oxides), particle mass-weighted size distributions and particle compactness. The chemical content and composition of impurities, present on metal nanoparticles, were found to be different depending on which synthesizing method that was used. Such differences may have an effect on the intended usage of the particles if aftertreatment, like sintering, is not carried out. We found that contaminations, originating from downstream equipment, were present in the High Temperature Furnace. If in-situ characterization techniques are implemented into particle production processes such undesired behaviors can be detected and handled in a way that would not compromise the efficiency of the production. With a growing demand for new aerosol nanoparticle synthesizing methods, it is important to develop new techniques that can be used for in-situ characterization of the produced nanoparticles. In-situ characterization, like the LV-AMS technique, can be used for quality assurance and can help understanding nanoparticle formation processes. An interesting type of nanoparticles for future studies is nanowires (of metal or semi-conductors). Acknowledgements 25

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30 Table 1. Determined ratio of Pd and Ag in PdAg particles. Three separate measurements were made. Measurements 1 and 2 were done with SDGP while the third was done with SDGC. No filters for PIXE analysis were sampled during the first measurements when the LV-AMS was used to sample particles of different dm. Generation method SDGP SDGP SDGC PIXE NA LV-AMS PIXE/LVAMS

31 Figure 1. Schematic of the particle generation and transformation setup and the instruments used for particle characterization. The instruments connected with dashed arrows were used for selected particle types. 30

32 Figure 2. a) Example of high resolution data, where metal ions ( 107 Ag + in this case) are clearly separated from an organic ion fragment (impurity). b) Mass defect vs m/z plot showing the most abundant metal isotopes, of the metals used in this work, and molecular fragments fitted to the high resolution mass spectra. Note that the figure shows examples of mass defects. All the possible ion fragments that are shown were not necessarily identified in any of the measurements. The ion fragments are divided into different families depending on their atomic composition. 31

33 Figure 3. Mass spectra of four different synthesized nanoparticles. Metal ions are shown in red and fragments representing organic impurities are shown in green. a) 100 nm semisintered Pd particles (SDGC) b) 100 nm semi-sintered Ag particles (SDGC) c) 110 nm sintered Fe particles (SDGC) and d) 40 nm sintered Au particles (HTF). The diameters are mobility diameters. 32

34 Figure 4. a) High resolution organic mass spectrum for impurities on 100 nm Pd aggregated particles (sampled at 20 C). The ion fragments are divided into different families depending on their atomic composition. The organic mass fraction is dominated by CxHy + fragments. b) Ratio of oxygen to carbon (O/C) as a function of sintering temperature. The error bars show standard error of means. 33

35 Figure 5. a) Unit mass resolution spectrum of impurities on 100 nm Au aggregated particles generated with the HTF. b) Unit mass resolution spectrum of impurities for 100 nm Au aggregated particles generated with the SDGP. 34

36 Organic mass frasction Laser + Tungsten Tungsten Sintering temperature ( C) Figure 6. Ratio of the organic mass fraction to the total particle mass for 100 nm (dm) Pd particles as a function of sintering temperature. Results are shown for the two vaporizer configurations with the assumption that CE=1. 35

37 Figure 7. Oxide signal mass fractions of particles from three different metals as a function of sintering temperature. The oxide signal fraction is determined by the MO/(MO+M) ratio where MO is the summed signal of metal oxide ions and M is the summed signal of pure metal ions, assuming that the relative ionization efficiencies and collection efficiencies for metal and metal oxide ions are the same. 36