Synthesis and Characterization of Polypyrrole-Coated Anthracene Microparticles: A New Synthetic Mimic for Polyaromatic Hydrocarbon-Based Cosmic Dust.

Polyaromatic hydrocarbons (PAHs) are found throughout the universe. The ubiquity of these organic molecules means that they are of considerable interest in the context of cosmic dust, which typically travels at hypervelocities (>1 km s-1) within our solar system. However, studying such fast-moving micrometer-sized particles in laboratory-based experiments requires suitable synthetic mimics. Herein, we use ball-milling to produce microparticles of anthracene, which is the simplest member of the PAH family. Size control can be achieved by varying the milling time in the presence of a suitable anionic commercial polymeric dispersant (Morwet D-425). These anthracene microparticles are then coated with a thin overlayer of polypyrrole (PPy), which is an air-stable organic conducting polymer. The uncoated and PPy-coated anthracene microparticles are characterized in terms of their particle size, surface morphology, and chemical structure using optical microscopy, scanning electron microscopy, laser diffraction, aqueous electrophoresis, FT-IR spectroscopy, Raman microscopy, and X-ray photoelectron spectroscopy (XPS). Moreover, such microparticles can be accelerated up to hypervelocities using a light gas gun. Finally, studies of impact craters indicate carbon debris, so they are expected to serve as the first synthetic mimic for PAH-based cosmic dust.

Introduction

Polyaromatic hydrocarbons (PAHs) such as anthracene, phenanthrene, perylene, and pyrene are naturally occurring molecules found in coal and petroleum deposits, oil shale, hydrothermal vents, and volcanic ash.[1] They are also present in cigarette smoke and automotive exhaust gas and can be generated during wood burning. As such, PAHs are considered to be long-lived organic pollutants whose persistence in the environment is linked to their strong resistance to oxidative and photochemical degradation. However, it is this chemical stability that has attracted growing interest from the space science community, not least because infrared emission spectroscopy studies have confirmed the ubiquitous presence of PAHs throughout the universe.[2−5] Indeed, Greenberg et al. have postulated that photoprocessing of organic dust mantles within the interstellar medium during solar irradiation may be a possible mechanism for generating PAHs.[6] More recently, PAHs have been detected in Martian meteorites,[7] in interplanetary dust,[2,8] in the upper atmosphere of Titan,[9,10] and within comets.[11,12] They have even been implicated as an important component in the emergence of early life on Earth–the so-called “Aromatic World” hypothesis.[13,14] Moreover, one of the objectives of the ExoMars rover mission is to search for such molecules up to 2 m below the surface of the Martian soil.[15]

In principle, laboratory-based high-energy impact experiments can shed considerable light on the behavior of various types of micrometeorites, which typically travel at hypervelocities (>1 km s–1) in outer space.[16] This is because when such fast-moving micrometeorites strike a metal target, they are almost instantaneously converted into molecular and/or atomic fragments by the high-energy impact.[17] This enables their chemical composition to be inferred by time-of-flight mass spectrometric analysis of the ionic plasma.[18−21] Indeed, this was the fundamental detection mechanism for the Cosmic Dust Analyzer (CDA) instrument deployed by the CASSINI spacecraft when orbiting Saturn and its moons.[22] For example, this CDA detector has detected both low- and high-mass aromatic compounds within the plumes of water ejected from the interior of the Saturnian satellite Enceladus.[23,24]

In practice, it is not trivial to design appropriate synthetic mimics that can be accelerated up to the hypervelocity regime that typically characterizes the behavior of such “cosmic dust”. Nevertheless, Armes and co-workers have prepared a series of (sub)micrometer-sized particles that have proven to be useful mimics for either carbonaceous- or silicate-rich micrometeorites.[16] This has been achieved by coating either polystyrene or silica particles with an ultrathin overlayer of polypyrrole (PPy).[25,26] This relatively air-stable organic conducting polymer ensures that such synthetic particles can acquire sufficient surface charge to enable their electrostatic acceleration up to the hypervelocity regime using a high-field van de Graaff accelerator.[27] In addition, certain naturally occurring mineral grains such as olivine, pyroxene, and pyrrhotite have been coated with PPy and successfully accelerated for hypervelocity impact experiments.[28−30] Moreover, by coating appropriate latex particles with PPy,[31] it has been possible to demonstrate via laboratory experiments that impact ionization time-of-flight spectra can be used to distinguish between at least some types of aromatic and aliphatic organic microparticles.[20,32] However, it has not yet been possible to examine PAH-based particles in this context. One of the simplest members of the PAH family is anthracene (see Figure ). This planar fused molecule forms large organic crystals with very limited solubility in common organic solvents and is essentially insoluble in water. Herein, we report the convenient preparation of micrometer-sized anthracene particles via ball-milling in the presence of a commercial dispersant (Morwet D-425) and demonstrate that the controlled deposition of PPy from aqueous solution can be used to coat such microparticles with good precision. These new microparticles are expected to become useful synthetic PAH mimics for laboratory-based hypervelocity experiments using either a light gas gun[27,33,34] or a van de Graaff accelerator.[17,35]

Figure 1

Chemical structures for (a) anthracene, (b) polypyrrole (PPy), and (c) Morwet D-425 dispersant. [N.B. The chemical structure shown for PPy is the typical idealized structure reported in the literature; in reality, the conjugated backbone also contains unpaired electrons (radicals) as well as delocalized cationic charge].

Experimental Section

Materials

Iron(III) chloride hexahydrate (97%) was purchased from Alfa Aesar (UK). (NH4)2S2O8 (APS), anthracene (97%), and pyrrole were each purchased from Sigma-Aldrich (UK). Pyrrole was purified by alumina chromatography (basic alumina, Sigma-Aldrich UK) prior to use. Silicone SAG1572 (Momentive, Germany) was used as an antifoaming agent, while Morwet D-425 (Nouryon, Sweden; molecular weight range = 1 000 to 5 000 g mol–1) was used as a dispersant. Deionized water was obtained from an Elga Medica DV25 water purification unit. Finally, 1.0 mm ceramic beads (zirconium aluminum oxide) were obtained from Sigmund-Lindner (Germany).

Synthesis of PPy Bulk Powder

FeCl3·6H2O (9.10 g) was dissolved in deionized water (100 mL) in a 125 mL glass bottle, and this orange-brown aqueous solution was stirred using a magnetic stirrer bar. The pyrrole monomer (1.0 mL) was added to this reaction solution and allowed to polymerize for 12 h at 20 °C. The resulting black precipitate was vacuum-filtered using a Buchner funnel and washed first with deionized water and then methanol. The purified moist black powder was placed on a petri dish and dried in a 50 °C oven overnight. The approximate isolated yield of PPy was 0.87 g (73% based on the pyrrole monomer).

Preparation of Anthracene Microparticles

(a) By IKA Ultra-Turrax tube drive

Anthracene (2.28 g, 20% w/w), Morwet D-425 dispersant (0.2850 g, 2.5% w/w), silicone antifoam (0.114 g, 1.0% w/w), and deionized water (8.72 g, 76.5% w/w) were mixed in a 30 mL tube with approximately 15 g of 1 mm ceramic beads. The tube was then attached to the IKA Ultra-Turrax tube drive and milled at 6 000 rpm for 90 to 120 min until the target particle size was achieved. The ceramic beads were then removed by filtration to obtain a white free-flowing aqueous dispersion.

(b) By Retsch Planetary Ball Mill

Anthracene (5.00 g, 20% w/w), Morwet D-425 (0.6250 g, 2.5% w/w) dispersant, silicon antifoam (0.250 g, 1.0% w/w), and deionized water (19.125 g, 76.5% w/w) were added to a 50 mL ball-milling jar together with 10 g of 2 mm beads. This mixture was milled at 250 rpm using a Retsch Planetary Ball Mill PM 100 for a rotation time of 15 min and a break time of 10 min at 250 rpm. Once the target particle size was achieved, the ceramic beads were removed by filtration to obtain a white free-flowing dispersion.

Purification of Aqueous Anthracene Dispersions to Remove Excess Dispersant

Aqueous anthracene dispersions were subjected to three centrifugation–redispersion cycles (6 000 rpm, 10 min per cycle). Each cycle required careful decantation of the aqueous supernatant and redispersion of the sediment using fresh deionized water. The purified aqueous dispersions were either then freeze-dried to recover anthracene microparticles in the form of a fine white powder or coated with an ultrathin overlayer of PPy prior to further characterization studies.

Synthesis of PPy-Coated Anthracene Crystals

The following protocol was used to coat 0.50 g of 4 μm anthracene microparticles with a target PPy overlayer of 20 nm and is representative. A 20% w/w aqueous dispersion of anthracene microparticles (2.50 g), pyrrole (37.0 μL; equivalent to a mass loading of 3.5%), and deionized water (25.0 mL) were added to a 120 mL glass bottle to give a low-viscosity dispersion. FeCl3·6H2O (0.34 g) was dissolved in deionized water (5.0 mL), and the final aqueous dispersion was stirred for 24 h at 20 °C. The resulting black dispersion was purified by three centrifugation–dispersion cycles (6 000 rpm, 10 min) to remove excess inorganic salts and any unreacted pyrrole and then freeze-dried overnight to recover a fine black powder. To target other PPy overlayer thicknesses, the masses of anthracene and water were kept constant and the amounts of the pyrrole monomer and FeCl3 oxidant were varied accordingly (always employing a fixed oxidant/monomer molar ratio of 2.33).

Table summarizes the various target and actual PPy mass loadings required to achieve a desired nominal overlayer thickness. Such mass loadings depend on the mean diameter of the anthracene microparticles and the solid-state densities of the anthracene (1.25 g cm–3) and PPy (1.46 g cm–3), which were determined by helium pycnometry. Similar calculations have been previously reported by Lascelles and Armes, who assumed a core–shell morphology to derive a simple equation for coating experiments involving well-defined spherical polystyrene latex particles (see Supporting Information).[25]

Table 1

Summary of the Target PPy Overlayer Thicknesses, Target PPy Mass Loadings, Nitrogen Microanalyses, and Actual PPy Mass Loadings (Calculated by Nitrogen Microanalyses) for the Two Types of Anthracene Microparticles Prepared in this Study

sample description	anthracene microparticle diametera (μm)	target PPy overlayer thickness (nm)	target PPy mass loading (%)	nitrogen microanalysisb (%)	Calculated PPY mass loading from nitrogen microanalysis (%)
PPy bulk powder	N/A	N/A	N/A	15.90	N/A
uncoated anthracene	4	N/A	N/A	0.0	N/A
PPy-coated anthracene	4	10	1.8	0.0	0.0
PPy-coated anthracene	4	20	3.5	0.43	2.7 ± 1.9
PPy-coated anthracene	4	30	5.0	0.96	6.0 ± 1.9
PPy-coated anthracene	2	20	6.7	1.36	8.6 ± 1.9

As determined by laser diffraction studies.

Nitrogen microanalysis has an error of ± 0.30%.

In the present case, the core comprises anthracene, while the shell is composed of PPy. In practice, the anthracene microparticles do not have a well-defined spherical morphology (see later). Thus, the target PPy overlayer thicknesses are calculated for “sphere-equivalent” anthracene microparticles. It is implicitly assumed that (i) all of the pyrrole is converted into PPy and (ii) all of the PPy is deposited onto the surface of the anthracene microparticles. In principle, the actual PPy mass loading can be calculated by nitrogen microanalysis by comparing the nitrogen content of the PPy-coated anthracene microparticles to that of PPy bulk powder prepared in the absence of any anthracene microparticles.[25]

Characterization Techniques

Helium Pycnometry

The solid-state densities of anthracene and PPy bulk powder were determined to be 1.25 and 1.46 g cm−3, respectively, using a Micromeritics AccuPyc 1330 instrument operating at 20 °C.

Optical Microscopy

Optical images were recorded using a Cole–Palmer compound optical microscope equipped with a LCD tablet display and a Moticam BTW digital camera. This technique was used to estimate the mean number–average diameter of the anthracene microparticles (approximately 100 particles counted per sample).

Particle Size Analysis by Laser Diffraction

Uncoated and PPy-coated anthracene microparticles were analyzed using a Malvern Mastersizer 3000 laser diffraction instrument equipped with a Hydro EV sample dispersion unit, a He–Ne laser (λ = 633 nm), and a solid-state blue laser (λ = 466 nm). The stirring rate was set at 2000 rpm, and data from five measurements were averaged. The standard operating procedure parameters were asssumed non-spherical particles with an absorption index of 0.01. The refractive index for anthracene was taken to be 1.5948 (see http://www.chemspider.com/Chemical-Structure.8111.html).

Solution Densitometry

The solution densities of a series of aqueous solutions of Morwet D-425 (ranging from 0.05 to 3.00% w/w) were determined using an Anton Paar DMA 4500 M density meter at 20 °C. Subsequently, ball-milled anthracene dispersions with mean diameters of either 2 or 4 μm were centrifuged at 10 000 rpm for 10 min and the solution densities of their respective supernatants were measured. This information was used to calculate the amount of Morwet D-425 that was adsorbed onto the surface of the anthracene microparticles.

Scanning Electron Microscopy

Images were obtained using an Inspect-F instrument operating at an accelerating voltage of 5 kV. Each powder was dispersed and dried onto a thin glass layer before being sputter-coated with a 5 nm overlayer of gold to prevent sample charging.

FT-IR Spectroscopy

FT-IR spectra were recorded for the uncoated anthracene microparticles, PPy bulk powder, and PPy-coated anthracene microparticles using a Thermo Scientific Nicolet iS10 spectrometer equipped with a Diamond ATR Golden Gate accessory. The spectral resolution was 4 cm–1 and 32 scans were averaged per spectrum.

Raman Microscopy

Raman spectra were recorded using a HORIBA LabRam-HR spectrometer equipped with an infrared laser (λ = 785 nm, 3 mW). This wavelength was selected to minimize the well-known problem of fluorescence associated with the Raman spectra of polymers.[36] Given the well-known strongly absorbing nature of highly conjugated polymers such as PPy,[37] either 1 or 10% filters were employed to attenuate the laser power in order to avoid sample degradation. The spectrometer utilized a 600 mm–1 grating with a spectral resolution of approximately 2 cm–1 and an Olympus BX41 microscope equipped with a ×100 objective lens, which provided a spatial resolution of 1–2 μm. The spectra were obtained from individual anthracene microparticles, with typically five spectra being averaged per sample.

Aqueous Electrophoresis

Zeta potential versus pH curves were constructed using a Malvern Zetasizer NanoZS instrument operating at 20 °C. Measurements were conducted on dilute (0.5% w/w) aqueous dispersions in the presence of 1 mM KCl as the background electrolyte, with the pH being adjusted using either NaOH or HCl. Zeta potentials were calculated from an average of three measurements via the Henry equation using the Smoluchkowski approximation.

X-ray Photoelectron Spectroscopy

Uncoated and PPy-coated anthracene microparticles and PPy bulk powder were analyzed in turn by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Supra X-ray photoelectron spectrometer. Step sizes of 0.5 and 0.05 eV were used to record the survey spectra and high resolution coreline spectra, respectively. In each case, powders were placed on an indium foil and spectra were recorded from at least two separate areas.

Light Gas Gun Experiments and Impact Crater Analysis

A two-stage light gas gun[27] was used to fire PPy-coated anthracene microparticles (4 μm diameter; coated with a nominal PPy overlayer thickness of 30 nm) at an aluminum foil target with a mean thickness of 110 μm. The target chamber was evacuated to less than 0.5 mbar while firing the gun, and the shot speed was determined to be 1.87 km s–1 (±0.5%). After this shot, the aluminum foil target was examined using a Hitachi S4700-N FEG-SEM instrument equipped with a Bruker Xflash EDX detector to identify impact craters caused by the impinging microparticles.

Results and Discussion

It is well known that ball-milling can be used to prepare micron-sized particles of molecular organic crystals. For example, this technology is widely used to prepare aqueous suspensions of agrochemical actives.[38,39] We envisaged that the same approach should be applicable to PAHs such as anthracene, which form relatively large crystals. Accordingly, ball milling was used to prepare aqueous dispersions of anthracene microparticles using a commercially available anionic polymeric dispersant (Morwet D-425) to prevent aggregation of the microparticles via a steric stabilization mechanism.[40,41] Two commercial ball-milling devices were examined: an IKA Ultra-Turrax tube drive was used to ball-mill relatively small quantities of material, while a planetary ball mill enabled larger (multigram) quantities to be processed. A substantial reduction in the mean size of the anthracene microparticles was achieved in both cases. Figure a shows the laser diffraction size distributions obtained for the initial anthracene crystals and the anthracene microparticles obtained after grinding an aqueous suspension of anthracene for 30–90 min in the presence of Morwet D-425 dispersant using the tube drive. This particle sizing technique reports a “sphere-equivalent” volume–average diameter, d(0.5), which is defined such that 50% of the particles fall below this size. The as-supplied anthracene crystals have an initial volume–average diameter of 302 ± 65 μm, but this was reduced to 7 ± 2 μm within a milling time of 30 min. Longer milling times (60–90 min) produced reasonably uniform microparticles with a volume–average diameter of 4 ± 2 μm, but thereafter, there was little or no further reduction in mean particle size.

Figure 2

Laser diffraction particle size distribution curves obtained for the reduction in anthracene particle size via ball milling in the presence of Morwet D-425 dispersant using (a) the IKA Ultra-Turrax tube drive and (b) retsch planetary ball mill.

Even smaller microparticles could be achieved using the planetary ball mill (see Figures b and S1). However, the latter technique usually gave broader size distributions owing to the formation of a population of relatively fine particles. It is noteworthy that mean anthracene microparticle diameters of 2–4 μm are comparable with that expected for PAH-based microparticles in space. For example, the NASA Stardust mission collected similar-sized cometary dust within its aerogel targets during a cometary fly-by at 6.1 km s–1[42,43] Moreover, the plumes of water erupting from the interior of the Jovian satellite Europa and the Saturnian satellite Enceladus are also believed to contain microparticles within this size range.[44,45]

The aqueous dispersion of anthracene microparticles was purified to remove excess non-adsorbed dispersants. This was achieved by three centrifugation–redispersion cycles, with careful decantation of each supernatant prior to redispersion of the sedimented microparticles. Laser diffraction studies indicated no change in the mean anthracene particle size after washing, suggesting a colloidally stable dispersion (Figure S2). Solution densitometry was used to monitor the concentration of the Morwet D-425 dispersant in the aqueous phase before and after ball-milling (see Figure S3). These measurements indicated that the 4 μm anthracene microparticles contained approximately 0.50% Morwet D-425 by mass. As expected, the 2 μm anthracene microparticles contained a higher Morwet D-425 content (1.50% by mass).

PPy is readily prepared in aqueous solution using mild chemical oxidants such as FeCl3. It is usually obtained in the form of an insoluble macroscopic precipitate that has a distinctive globular morphology (see Figure S4), but it is well known that PPy can be deposited onto various types of colloidal particles with good control over its coating thickness.[25,37,46] This aqueous deposition process works rather well for hydrophobic substrates such as polystyrene latex but is less suitable for hydrophilic substrates such as silica, for which chemical modification of the surface is usually required.[25,47−49] However, for the highly hydrophobic anthracene microparticles reported herein, the aqueous deposition of PPy was expected to be straightforward.

Accordingly, the pyrrole monomer and FeCl3·6H2O oxidant were added to an aqueous dispersion of anthracene microparticles, and the pyrrole polymerization was allowed to proceed for 24 h at 20 °C. The PPy-coated anthracene microparticles were then subjected to three centrifugation–redispersion cycles to remove the spent oxidant and any unreacted pyrrole prior to freeze-drying overnight (Figure ). Some of this purified aqueous dispersion was retained for particle size analysis.

Figure 3

Schematic representation of the aqueous deposition of an ultrathin PPy overlayer onto the surface of ball-milled Morwet-stabilized anthracene microparticles.

The mean particle size of the purified PPy-coated anthracene microparticles was assessed using laser diffraction (Figure ). The dried black powder proved to be relatively hydrophobic, so a wetting agent, Aerosol OT-B (0.01% w/w based on PPy-coated anthracene microparticles), was required to disperse it. A volume–average diameter of approximately 6 ± 3 μm was determined, which was significantly larger than what was expected, given that a PPy overlayer of just 20 nm was targeted. This indicates incipient flocculation of the PPy-coated anthracene microparticles during drying, which is not unexpected in view of the relatively high Hamaker constant for this conducting polymer.[50] However, ultrasonication for 3 min just prior to laser diffraction analysis enables the microparticle floccs to be broken up. This protocol produces a volume–average diameter of 4.3 ± 2.1 μm, which is comparable to that of the original uncoated Morwet-stabilized anthracene microparticles (Figure ).

Figure 4

Laser diffraction particle size distribution curves recorded for an aqueous suspension of uncoated anthracene microparticles (red) and PPy-coated anthracene microparticles before (black) and after (blue) ultrasonication.

Optical microscopy images of the milled anthracene microparticles obtained using Morwet D-425 were in good agreement with the laser diffraction measurements (Figure a,c). The visual appearance of the PPy-coated anthracene microparticles did not differ significantly from that of the uncoated anthracene microparticles (Figure b,d). This is attributed to the rather low conducting polymer mass loadings and hence relatively thin overlayers (Table ). It is perhaps worth emphasizing that the PPy grain size is around 5–10 nm,[51] so a PPy thickness of 10 nm is essentially the thinnest coating that can be targeted to achieve a contiguous overlayer, which is required for the efficient accumulation of surface charge in van de Graaff accelerator experiments.[16−21]

Figure 5

Optical microscopy images recorded for (a) uncoated and (b) PPy-coated 4 μm anthracene microparticles (PPy overlayer thickness = 20 nm) and (c) uncoated and (d) PPy-coated 2 μm anthracene microparticles (PPy overlayer thickness = 20 nm).

Scanning electron microscopy (SEM) studies indicate that the milled anthracene microparticles were somewhat ill-defined in terms of their size and morphology (Figure a,b). Nevertheless, the mean microparticle dimensions are consistent with those indicated by optical microscopy and laser diffraction studies. There is also some evidence for an unusual perforated, porous surface morphology. Targeting a PPy loading of 3.5% by mass (mean coating thickness = 20 nm) produced a relatively uniform overlayer (Figure c). The deposited PPy overlayer has a distinct globular morphology that resembles that of PPy bulk powder (see Figure S4 in the Supporting Information), albeit with finer features. The same PPy morphology was observed on smaller 2 μm anthracene microparticles, which were coated with a target overlayer of 20 nm (Figure S1). A similar surface morphology has been observed for other PPy-coated particles.[37] Moreover, the surface voids observed for the uncoated anthracene microparticles were no longer visible for the PPy-coated anthracene microparticles (Figure d).

Figure 6

SEM images recorded for (a,b) uncoated 4 μm anthracene microparticles and (c,d) PPy-coated 4 μm anthracene microparticles (PPy mass loading = 3.5%, corresponding to a mean coating thickness of 20 nm).

The nitrogen content of PPy bulk powder is 15.9% by mass (see Table ). However, the highest target PPy mass loading for the anthracene microparticles (which have zero nitrogen content) was 6.7%, which means that their nitrogen contents should be of the order of 1.0% by mass. Given that the generally accepted accuracy for nitrogen microanalysis is typically ±0.30%, this technique is clearly not very reliable for compositional analysis of this particular system. Nevertheless, we conducted nitrogen microanalyses of PPy bulk powder and the four examples of PPy-coated anthracene microparticles (see Table ). Within the admittedly large experimental uncertainty, the PPy mass loadings calculated from the nitrogen microanalyses are consistent with those targeted, with higher PPy loadings being obtained when coating finer anthracene microparticles (2 vs 4 μm diameter).

Transmission mode FT-IR spectra recorded for PPy bulk powder, milled anthracene microparticles prepared using the Morwet D-425 dispersant, and a series of PPy-coated anthracene microparticles are shown in Figure . The latter three samples exhibit strong bands at 1550, 1180, and 1030 cm–1, corresponding to the PPy overlayer, which became more intense as thicker PPy overlayers are targeted.

Figure 7

FT-IR spectra recorded for (a) PPy bulk powder; (b) uncoated milled 4 μm anthracene microparticles prepared using the Morwet D-425 dispersant; (c) 4 μm anthracene microparticles coated with a nominal 10 nm PPy overlayer; (d) 4 μm anthracene microparticles coated with a nominal 20 nm PPy overlayer; and (e) 4 μm anthracene microparticles coated with a nominal 30 nm PPy overlayer.

Raman spectra recorded for PPy bulk powder, pure anthracene, and PPy-coated anthracene microparticles are shown in Figure . The Raman spectrum for pure anthracene exhibited strong sharp bands at 121, 395, 752, 1401, and 1556 cm–1, which correspond well to those reported in the literature.[52] The PPy bulk powder reference spectrum exhibited strong broad bands at 925, 1066, 1237, 1370, and 1592 cm–1, which are in good agreement with our previously reported observations.[47−51]

Figure 8

Raman spectra recorded for (a) pure anthracene crystals, (b) PPy-coated 4 μm anthracene microparticles (target PPy overlayer thickness = 10 nm), (c) PPy-coated 4 μm anthracene microparticles (target PPy overlayer thickness = 20 nm), and (d) PPy bulk powder. The five most intense Raman lines in the pure anthracene crystals are at 121, 395, 752, 1401, and 1556 cm–1 (see vertical dashed lines), which is in good agreement with the literature.[52] The two spectra obtained for the PPy-coated anthracene microparticles are both dominated by signals from the conducting polymer component owing to a resonance Raman effect.

The Raman spectrum recorded for the anthracene microparticles coated with a nominal 10 nm PPy overlayer is strikingly similar to that recorded for the PPy bulk powder, with relatively weak bands attributable to the underlying anthracene being observed at 121, 395, 752, and 1401 cm–1.[53−57] There is also some evidence for the 1592 cm–1 band as a rather weak shoulder on a strong PPy band. The first four bands are also present for the anthracene microparticles coated with a nominal 20 nm PPy overlayer, although they are all somewhat attenuated (compare Figure b,c). These observations are rather remarkable, given that the 10 nm PPy-coated anthracene microparticles contain more than 98% anthracene by mass (see Table ). Similar observations have been previously reported for PPy-coated latex particles and have been explained in terms of a resonance Raman effect. This leads to efficient absorption of the incident laser light by the conducting polymer overlayer, which causes obscuration of the underlying substrate.[30,41,51] It is perhaps worth emphasizing that such attenuation is much weaker for infrared radiation (see Figure ). In summary, these observations provide good evidence for a relatively uniform, rather than patchy, PPy overlayer at the surface of the anthracene microparticles. Such a core–shell morphology is highly desirable if such synthetic mimics are to be useful in the context of space science applications. For light gas gun experiments, it means that the PPy overlayer can serve as a sacrificial layer with a strong spectroscopic signature, which makes such microparticles potentially useful when assessing the likely extent of thermal ablation suffered by PAH dust grains during their capture within aerogel targets at hypervelocities of 1–6 km s–1[33] Similarly, a contiguous PPy overlayer should enable the efficient accumulation of surface charge, which is a prerequisite for acceleration up to the hypervelocity regime when using a van de Graaff instrument.[17,27]

Zeta potential versus pH curves were determined for both the milled anthracene microparticles and the PPy-coated anthracene microparticles via aqueous electrophoresis (see Figure ). The former microparticles exhibited negative zeta potentials (approximately −40 to −50 mV) regardless of the solution pH, which is consistent with the surface presence of the anionic Morwet D-425 dispersant. In contrast, the PPy-coated anthracene microparticles exhibited an isoelectric point at around pH 7.7 and acquired cationic character at low pH (e.g., −30 mV at pH 3). These observations are consistent with the deposition of an electrically conductive PPy overlayer at the surface of the anthracene microparticles.

Figure 9

Zeta potential versus pH curves recorded for (a) 4 μm anthracene microparticles prepared using the anionic Morwet D-425 dispersant and (b) PPy-coated 4 μm anthracene microparticles (nominal overlayer thickness = 20 nm).

The milled 4 μm anthracene microparticles prepared using the Morwet D-425 dispersant, PPy bulk powder, and a series of PPy-coated anthracene microparticles were studied using XPS. In addition to the expected strong C1s signal, the survey spectrum recorded for the milled anthracene microparticles also contained Si2s and Si2p signals (Figure ). These features are attributed to the silicone-based antifoam agent, which is not fully removed after the centrifugation–redispersion wash cycles. A weak S2p signal was also discernible, which is assigned to the anionic sulfonate groups of the Morwet D-425 dispersant (see Figure c). The survey spectra recorded for this commercial dispersant and the as-received (unmilled) anthracene crystals are provided in the Supporting Information (see Figure S5).

Figure 10

X-ray photoelectron survey spectra recorded for (a) uncoated milled 4 μm anthracene microparticles prepared using the Morwet D-425 dispersant; (b) PPy-coated 4 μm anthracene microparticles (nominal overlayer thickness = 10 nm); (c) PPy-coated 4 μm anthracene microparticles (nominal overlayer thickness = 20 nm); (d) PPy-coated 4 μm anthracene microparticles (nominal PPy overlayer thickness = 30 nm); and (e) PPy bulk powder.

As expected, the survey spectrum obtained for PPy bulk powder contains both N1s and Cl2p signals.[58] These signals are also detected in the spectra recorded for the series of three PPy-coated anthracene microparticles (nominal PPy overlayer thicknesses = 10, 20 and 30 nm). Comparing the relative intensities of the O1s and N1s signals for each of these three spectra, it is clear that the latter signal becomes progressively stronger as thicker PPy overlayers are targeted. The Cl/N atomic ratio calculated for the PPy bulk powder spectrum is 0.23, which is consistent with the chloride-doped, electrically conductive form of this organic polymer.[58,59] Similar Cl/N atomic ratios (0.22–0.30) were determined for the PPy overlayers deposited at the surface of the anthracene microparticles. Moreover, the S2p and Si2s/Si2p signals assigned to the Morwet D-425 dispersant and the silicone defoamer, respectively, gradually become attenuated as higher mass PPy loadings are targeted. It is also noteworthy that the survey spectra recorded for the PPy bulk powder and the PPy-coated anthracene microparticles (nominal PPy overlayer thickness = 30 nm) are strikingly similar. Such observations are understandable given the highly surface-specific nature of XPS analysis, which has a typical sampling depth of not more than 10 nm.[60] Finally, C1s core-line spectra for selected samples are shown in Figure S6. Interestingly, the C1s core-line spectrum recorded for the uncoated anthracene microparticles is significantly shifted in its binding energy compared to those recorded for PPy alone and the three examples of PPy-coated anthracene microparticles. This provides good evidence that the former sample is electrically insulating, whereas the latter four samples are electrically conductive, as expected.[60]

A preliminary hypervelocity experiment was conducted using a two-stage light gas gun[27] to fire PPy-coated 4 μm anthracene microparticles (nominal PPy overlayer thickness = 30 nm) at 1.87 km s−1 into an aluminum foil target (mean foil thickness = 110 μm). Subsequent SEM studies of this target revealed impact craters created by the impinging microparticles (see Figures a and S7). These craters are relatively shallow, with barely raised lips at their edges. There is no visible sign of significant projectile fragments or residue lining the craters. Nevertheless, a faint ring of carbonaceous debris is discernible when the same crater is subjected to X-ray elemental mapping analysis (see Figures b and S7). Some larger carbon-rich fragments are also visible that may also originate from the impinging projectile. However, we cannot be certain that the latter debris does not originate from contamination by the sabot employed in this light gas gun experiment.

Figure 11

(a) SEM image of an impact crater formed by firing PPy-coated anthracene microparticles (4 μm diameter; coated with a nominal PPy overlayer thickness of 30 nm) at an aluminum foil target (mean thickness = 110 μm) at 1.87 km s–1 using a two-stage light gas gun. (b) Corresponding elemental carbon image obtained when using X-ray elemental mapping to examine the same impact crater. Carbonaceous debris is discernible as a faint crater ring, with some larger fragments also being visible (see main text for further details).

These observations can be compared to earlier attempts to simulate what would happen if organic microparticles entrained in the water plumes emitted from icy satellites of Jupiter and Saturn (i.e., Europa and Enceladus, respectively) were to be intercepted by a passing spacecraft. For example, New et al.[61] fired poly(methyl methacrylate) microparticles of 4, 6, and 10 μm diameter at five different metal targets (including aluminum) at hypervelocities ranging from 0.5 to 3 km s–1. Below the hypervelocity regime (<1 km s–1), the impinging microparticles either adhered or rebounded. At hypervelocities of around 1 km s–1, imprints were left in the surface of the metal target, with carbon residues being detected by X-ray elemental mapping analysis. At 2–3 km s–1, craters lined with partially melted residues were obtained. Up to and including impacts at 2 km s–1, Raman microscopy studies confirmed the presence of PMMA fragments, but at higher speeds, no such debris was found. This is consistent with the prior work by Burchell and co-workers,[33] who found that PPy-coated polystyrene microparticles of 20 μm diameter survived intact when fired at aerogel targets at up to 2 km s–1 but underwent extensive thermal ablation when impinging at higher hypervelocities. Separately, Burchell and Harriss[62] recently reported that firing polystyrene and PMMA microparticles at aluminum targets at 5 km s–1 produced impact craters that were solely lined with carbonaceous residues–there were no identifiable polystyrene or PMMA residues. The preliminary data reported herein extend the growing number of hypervelocity impact studies involving organic microparticles to include PAH-rich projectiles. Moreover, they suggest that the chemical nature of the projectile is an important factor, so the earlier observations made for PMMA microparticles cannot be assumed to be valid for all types of organic microparticles. Thus, given the ubiquity of PAH throughout the universe, it is clearly important to develop suitable synthetic mimics to understand the behavior of this type of organic cosmic dust.

Conclusions

The simplest member of the PAH family, anthracene, can be conveniently prepared in the form of microparticles by ball-milling macroscopic organic crystals using a commercial polymeric dispersant (Morwet D-425). These precursor microparticles were then coated with PPy, an air-stable electrically conductive polymer. Both the original uncoated microparticles and the PPy-coated microparticles were characterized by optical microscopy, laser diffraction, aqueous electrophoresis, SEM, vibrational spectroscopy, and XPS. These techniques are consistent with the presence of an ultrathin contiguous overlayer of PPy on the surface of the anthracene microparticles. Such microparticles are expected to be useful synthetic mimics for PAH-rich cosmic dust, which is found throughout the universe. Finally, we demonstrate that a light gas gun[27,33,34] can be used to accelerate such microparticles up to the hypervelocity regime. Moreover, we note that the electrically conductive nature of the PPy overlayer should enable the efficient accumulation of surface charge[17,27,35] and hence provide access to higher hypervelocities using a van de Graaff accelerator. In such experiments, this overlayer is also likely to be a useful sacrificial layer for assessing the extent of thermal ablation of such microparticles during their capture within aerogel targets.[33]