# Diverse features of dust particles and their aggregates inferred from
experimental nanoparticles

Yuki Nakano
Institute of Low Temperature Science
Hokkaido University
Sapporo, Japan
<EMAIL_ADDRESS>
Yuki Kimura
Institute of Low Temperature Science
Hokkaido University
Sapporo, Japan
Akihiko Hashimoto
Suns & Fishermen Society,
Sapporo, Japan

###### Abstract

Nanometre- to micrometre-sized solid dust particles play a vital role in star
and planet formations. Despite of their importance, however, our understanding
of physical and chemical properties of dust particles is still provisional. We
have conducted a condensation experiment of the vapour generated from a solid
starting material having nearly cosmic proportions in elements. A laser flash
heating and subsequent cooling has produced a diverse type of nanoparticles
simultaneously. Here we introduce four types of nanoparticles as potential
dust particles in space: amorphous silicate nanoparticles (type S);
core/mantle nanoparticles with iron or hydrogenised-iron core and amorphous
silicate mantle (type IS); silicon oxycarbide nanoparticles and hydrogenised
silicon oxycarbide nanoparticles (type SiOC); and carbon nanoparticles (type
C), all produced in a single heating-cooling event. Type IS and SiOC
nanoparticles are new for potential astrophysical dust. The nanoparticles are
aggregated to a wide variety of structures, from compact, fluffy, and
networked. A simultaneous formation of nanoparticles, which are diverse in
chemistry, shape, and structure, prompts a re-evaluation of astrophysical dust
particles.

##  1 Introduction

Protoplanetary discs (PPDs) surrounding young stars, are composed of gas and
nanometre- to micrometre-sized dust particles, which are derived from
molecular clouds at source [reviewed by 1]. While the abundance of dust
particles is merely 1 % in a total mass of PPD medium, they determine the
thermal and geometrical structure of the discs and a fraction of them is used
for forming planets [e.g. 2, 3]. Studying physical and chemical properties of
the particles are crucial to understanding their evolution in PPDs and the
planetary systems.

Interstellar dust is considered a principal source of dust particles before
their evolving to solid ingredients in molecular clouds and in PPDs. It is
postulated to be mainly composed of either carbon-rich or silicate-rich
nanoparticles, depending on their C-rich or O-rich stellar origin either as
ejecta of dying-stars or as stellar winds [4]. In cold interstellar
environment, these nanoparticles are expected to have served as substrates
onto which volatile gas adsorbed and evolved into ice or more complex
molecules [e.g. 5, 6]. The theoretical picture, interstellar particles have
either silicate or carbon cores and volatile, organic and icy mantle [7, 8],
is currently in the ascendancy.

Most interstellar dust, whether it is monomineralic or layered, however, must
have been far more processed than was once believed [e.g. 9]. They are
destroyed by intense stellar radiation and interstellar shock waves, on a
timescale of <108 yrs [10, 11, 12], which is shorter than the lifetime of the
galactic interstellar dust ($\sim$5 x 108 yrs)[12]. The vaporised refractory
atoms and molecules should subsequently be condensed into dust of new
generation, viz. recycled, which would refill the interstellar space [13]. The
short timescale for the dust destruction implies that most of interstellar
dust possibly lose their primary memory acquired when they formed around
stars.

By the same token, dust particle delivered to PPDs can be regenerated one and
thus most probably differ from principal particles born around dying stars and
in stellar winds. Observations of light scattering by dust particles in PPDs,
aided by model calculations, have been applied to understand the properties of
the dust particles, which shows that the size of dust particles in PPDs is
much larger than that of interstellar dust on average probably due to a dust
growth process inside of PPDs [2]. Fraction of the dust particles might have
been reprocessed inside PPDs by vaporisation, melting, mixing, condensation,
etc., as inferred from chemical and isotopic evidence in chondritic meteorites
[14] in addition to recycling processes in interstellar environments. The
existence of presolar grains in meteorites, however, is a manifestation of
survival of original interstellar dust, despite of their abundance as low as a
few hundred ppm at most [e.g. 15].

Apart from the theoretical and observational works, experimental research has
also contributed to the study on dust particle properties, especially for
interstellar dust [e.g. 16, 17, 18, 19]. Most experimental techniques use
simultaneous evaporation and condensation of solid materials in low-pressure
ambient gases, and have been applied to various materials such as oxides,
silicates, metals, carbon, and metal carbides by way of producing analogues of
interstellar nanoparticles and measuring their optical properties [e.g. 17,
20]. The starting materials used in those studies, however, are too simple to
represent the composition of actual interstellar and PPD materials. Simple
materials are often beneficial for studying elementary kinetics, but there is
no guarantee for the nanoparticles thus produced to be real astrophysical
analogues.

We apply a laser flash heating to a starting material which is composed of a
homogenised solid charge of the Allende meteorite and of a pure H2 ambient
gas. The Allende meteorite contains all natural elements in their cosmic
proportions except for the elements which are volatile at room temperature,
and is suitable, together with hydrogen atmosphere, as a dust and gas mixture
analogous to PPD and interstellar environments. Here we show that diverse
kinds of refractory nanoparticles with characteristic chemistry, morphology
and aggregation structures condensed simultaneously from the vapour generated
by a single shot laser-evaporation and subsequent cooling.

Figure 1: Bright field TEM images of condensed nanoparticle aggregates. Top
panels show whole views of aggregates of types S and IS nanoparticles. Centre
panels are magnified images of the corresponding top panels. Bottom panels
show aggregates of types SiOC and C nanoparticles. Lower case letters
following dots indicate sub-types of the preceding NP types. The locations of
magnified images in the top panels are shown in Fig. S1.

##  2 Experimental and analytical methods

A pulverised and homogenised powder of Allende meteorite (<5 µm in grain size)
was used as a source of initial sample. The powder was pressed into
cylindrical shape and fired for 12 hours at 900 ℃ in a PO2 controlled furnace
with a H2/CO2 = 1/2 gas ratio to stabilise iron as a wüstite or FeO component
in silicates. A narrow hole (0.5 mm in diameter) was drilled to a halfway
through the central axis of the sample. Then it was scraped off to its shape,
3 x 2 mm in height and diameter, and weight, 15 mg. A Re wire (0.5 mm in
diameter) stuck with a small and thin graphite disc was inserted to the bottom
of the hole. The distance between the sample and the disc was kept at
$\sim$1.5 mm, so that the sample would neither slip off of the wire nor react
with graphite when it got completely molten.

The sample was set in the centre of a vacuum chamber [97 mm inner diameter and
310 mm height; see for details 21] . A cw-CO2 laser (wavelength 10.6 µm;
20-210 watts; NIIC model NC-30100-M) with a ZnSe lens (focal distance of 127
mm) was used to heat the sample with its hot-spot diameter of 1 mm. The sample
was heated with a power of 100 W (temperature $\sim$3000 K) for 3 seconds in a
1 atm H2 gas flow ($\sim$6 litter/min) continuously evacuated with a vacuum
pump through a feed-through. A smoke, clusters of condensed particles, rose
for a few seconds from the vicinity of the sample. A fraction of the smoke was
collected on a standard Cu-grid (3 mm in diameter) set beforehand on the upper
inner wall of the chamber, to be served with transmission electron microscopy
(TEM) after the run. Nearly ten wt. % of the original mass evaporated and
condensed as nanoparticles (NPs).

We used a TEM (JEOL JEM-2100F, operated at 200 keV and equipped with an
energy-dispersive X-ray spectroscope with a silicon drift detector (JED23000T,
JEOL Ltd.)) to characterize morphology, structure, and composition of the
collected NPs and their aggregates.

##  3 Results

###  3.1 Characterisation of experimental nanoparticles

The nanoparticles (NPs) produced in our experiment invariably exist in their
aggregate forms. We found 66 aggregates of NPs on the Cu-grid. On the basis of
EDS chemical analysis and electron diffraction, the NPs are categorised into
four types. Excluding some cases (see below), each aggregate consists of mono-
type NPs. Figures 1, S1, and S2 show aggregates that belong to each of the
four types of NPs. Diffraction patterns and typical EDS spectra for each type
of NPs are shown in Figs. 2 and S3 to S10. Table 1 summarises physical and
chemical properties of the four types of NPs and their aggregates.

Table 1: Characterisation of the collected condensed nanoparticles.

Type S are Si-, Fe-rich silicate NPs and are amorphous from their diffraction
(Fig. S3) except minor crystals. Type IS are core/mantle NPs with an iron or
iron compound core and an amorphous silicate mantle. Silicate are similar in
composition to those of type S NPs (Fig. S7). Iron is $\alpha$-Fe (Fig. 2),
and the iron compound is most probably a hydrogenised iron from the
observation to be described in Section 3.2. For type S and IS NPs we
determined the chemical composition of selected NPs by adjusting the size of
electron beam diameter to a size of individual NP and thus the obtained
composition is a bulk composition of the whole NP grain whether or not it is
core-mantle structured. The S and IS types are sub-divided into three subtypes
according to their NP morphology: spherical, fused, and skeletal (a, b, and c
in Table 1, respectively). The b and c subtypes possess nanocrystals such as
wüstite (Fig. S4). Aggregates of either S or IS type NPs tend to be fluffy or
networked in their morphology, with many branches of linearly-connected NPs
linked together at one or more ends (Figs. 1 and 2; see also Fig. S1).

We have found NPs which consistently exhibit Si, O, and C as major peaks in
EDS spectra (Fig. S9; we randomly measured many parts of a selected aggregate
and took their average composition), and named them SiOC NPs. Diffuse rings in
the electron diffraction pattern of type SiOC NPs indictate amorphous
strctures (Fig. 2). We identify them as "silicon oxycarbides" from their
typical morphology and composition akin to the reported hydrogenised silicon
oxycarbides [22] in addition to the similarity in experimental conditions
between ours and Tavakoli2015 [22]. Silicon oxycarbide refers to a carbon-
containing silicate glass wherein oxygen and carbon atoms share bonds with
silicon in an amorphous network structure [23]. Aggregates of SiOC NPs are
rather compact and massive compare to those of type S and IS NPs.

Type C NPs are nearly pure carbon (Fig. S10; we randomly measured many parts
of a selected aggregate and took their average composition). They are sub-
divided into three subtypes according to their NP morphology and crystal
structures; (a) onions (fullerene), (b) stacked sheets (graphite), and (c)
lump (poly-diamond) (Figs. 1, 2, S5 and S6; see also Table 1). Aggregates of
all the subtypes are compact in morphology (Fig. 1).

The sizes range of each type of NPs is listed in Table 1. Type S and IS are
similar in their size range. It is noted that the size of NPs decreases in the
order of subtypes a, b, and c (the width of limbs for the subtypes c) for both
type S and IS NPs and that there is a size break between the spherical
(subtype a) and the fused (subtype b) at $\sim$30 nm in NP diameter (see Table
1). The diameter of SiOC type NPs is fairly uniform $\sim$20 nm. Type C.a NPs
are aggregates of onion shaped units whose size is 20-80 nm. Basically the
size of all types of NPs is within 10-100 nm (the largest being a limb length
of subtype S.c or IS.c NPs) except for type C.b (stacked sheets of graphite)
and C.c (poly-diamonds).

There are some exceptions to the mono-NP-type aggregation. Nearly 25 % of type
S and IS aggregates are partially mixed, but such a mixture always occurs in
units of separate branches, viz. each branch consists of the same type of NPs.
In one instance, an aggregate of type SiOC NPs is attached to a single branch
of type IS NPs. Type C NP aggregates are exclusively isolated. These
observations suggest that the type S and IS NPs initially formed a two-
dimensional, chain-like aggregate of the same NP type and that the chains were
linked together later. It must be the latter occasion when mixing of different
types of NPs occurred. Type SiOC and C NPs, on the other hand, obviously did
not form chain-like structures but agglomerated randomly.

Figure 2: Left panels are bright field TEM images of type IS, C, and SiOC NPs.
Right panels show the electron diffraction patterns of the selected areas
(marked in red) shown on the left images, indicating $\alpha$-iron [24],
hexagonal-diamond [25] and amorphous from top to bottom. Miller indices are
also shown for the $\alpha$-iron and the hexagonal-diamond.

###  3.2 Hydrogenisation of Fe-metal - possibility

During the EDS analysis of type IS NPs, the thickness of both silicate mantle
and iron core were significantly reduced after a long exposure to electron
beam irradiation while collecting X-ray counts. The diametre of the metal core
became smaller by 4 to 20% (Fig. 3). Such an observation is explained if the
core of type IS NPs was hydrogenised beforehand since hydrogen is easy to
escape by electron bombardment. The change in core diameter by 4 to 20% means
a reduction of the core volume by 12 to 50%. According to the experimental
works [26, 27, 28], Fe atoms and clusters react with hydrogen molecules and
form FemHn clusters. Based on a density functional theory, [29] showed that a
single gas-phase Fe atom can bind up to ten hydrogen atoms and that a large
Fe-H cluster such as Fe9H56 is also stable. At present we do not know how many
hydrogen atoms per each Fe atom are contained in the type IS NPs. Considering
the smallest size of H atoms and the volume shrinkage up to 50%, however, the
number of hydrogen per Fe should be much larger than unity at least in some of
type IS NPs. The shrinkage of silicate mantle (also type S NPs) by electron
bombardment during the TEM observation also suggests that silicates were
highly hydrogenised, perhaps as Si-H or Si-OH bonds.

Figure 3: Bright field TEM images of an aggregate of type IS NPs. (a) shows
NPs before a prolonged electron beam irradiation for EDS analysis of the area.
(b) shows the same NPs after the EDS analysis. Observe that the sizes of both
metal cores and silicate mantles of the same NPs diminished after the EDS
analysis.

##  4 Discussion

###  4.1 Formation of diverse nanoparticles

Our experimental NPs condensed from the vapour of a molten sample in the 1 atm
ambient gas of pure H2. According to classical nucleation theory, nucleation
of condensed phases from supersaturated vapour proceeds through the formation
of gas-phase clusters by mutual collisions of vapour molecules [30]. The
ambient gas would confine vapour molecules near the evaporation source, here
the molten sample. In our experimental condition, the mean free path,
approximately 100 nm, could bring a supersaturation of the vapour in the
immediate vicinity of a heated solid sample.

Even in a single flash heating event, four types of NPs, viz. S, IS, C, and
SiOC were produced in our experiment. According to the classical nucleation
theory, each type of gas-phase clusters are produced in a supersaturated
vapour around a molten sample (viz. the vapour source), and the clusters could
grow through more collisions of the same type of vapour molecules. Then, we
would need the reason for the simultaneous formation of many types of NPs. We
might consider two possibilities. (1) The condition of the supersaturated
vapour such as composition and density is spatio-temporally heterogeneous. (2)
In a homogeneous evaporating gas, NPs of the type that become supersaturated
early form before other types, and a time-dependent condensation sequence
occurs between different types of NPs.

We point out the mode of occurrence of NPs in their aggregates; all aggregates
are basically composed of the same type of NPs no matter what the type of NPs
is. We have described in Section 3.1 that some branches of type IS NP
aggregates are occasionally being replaced with branches of type S NP
aggregates, but they never mix with each other in individual NP wise. If
various types of NPs coexisted nearby in a parcel of the ambient gas, they
would collide each other and form aggregates with an assortment of various
NPs. In case of the first possibility, a microscopic convection induced by the
laser heating would have isolated gas parcels each with unique chemistry,
density, and/or temperature, in which mono-type NPs condensed and stuck
together. In case of the second possibility, not only the nucleation and
growth of one type of nanoparticles precedes those of other types of
nanoparticles, but also the mutual sticking of the earliest type nanoparticles
must prevent late types of nanoparticles from sticking onto the former. It
means that the ticking probabilities between different types of nanoparticles
must be significantly small.

###  4.2 Formation of type IS nanoparticles

Formation of the type IS NPs that contain iron core and amorphous silicate
mantle is particularly curious because its iron core seems to form prior to
its silicate mantle although a homogeneous nucleation of a metallic iron is
highly inefficient [31]. In order to nucleate iron first, classical nucleation
theory tells, a very high density of iron vapour in the supersaturated vapour
and/or a significant rise in iron nucleation rate is required as compared to
those of silicates. Such conditions seem unlikely in our experiment. A
recently suggested non-classical nucleation process [e.g. 32] has been applied
to postulate a nucleation of Ti-C system NP [33]. A similar explanation may be
conceivable for metal-silicate NPs like ours. Precursory nuclei of silicate
may form through homogeneous nucleation and serve as substrate not only for
more silicate vapour but also for iron vapour to stick on. At some point,
fusion of the numerous nuclei could lead to a single NP that is composed of an
iron core covered with a silicate mantle. Such a schematic scenario for the
formation of type IS NPs needs a quantitative evaluation.

A hindering factor of Fe metal nucleation is a large binding energy of Fe
atoms released during the formation of a dimer, that prompts its dissociation
into atoms again [31]. If monomers such as FeHn (n $\geq$1) were available,
however, formation of dimers and oligomers would be easier because the binding
energy can be released by Fe-H vibrations. Therefore, it is possible that FeHn
forms clusters and (FeHn)m nucleates. As described in Section 3.2, our
experiment, performed in pure 1 atm hydrogen, has a reasonable probability to
have formed hydrogenised iron.

In interstellar environment the ambient gas (H2 and H) density is very low.
Considering the number ratio of H to Fe in the mass balance between the
ambient gas and the vapour, however, our experiment is not so different from
the interstellar environment, or even smaller from the following analysis.
Since the vapour pressure of the Allende meteorite rock (the staring material)
at $\sim$3000 K (the experimental condition) is nearly 0.3 atm [21], the
experimental gas/vapour density ratio is $\sim$30 by taking into account the
difference in temperature, 300 K (ambient H2) vs. 3000 K (vapour), while the
gas/dust mass ratio in interstellar medium is $\sim$100 including ice in the
dust. Allende meteorite rock dose not contain ice. Thus, a hydrogenisation of
Fe (and also Si) is not unlikely in interstellar medium. In fact, FeH and FeHn
(n>1) molecules are found in solar and other star spectra [e.g. 26, 34] and in
exoplanet atmospheres [35]. In interstellar space and PPD, iron particles
could have formed via iron dimers with the help of hydrogen in its initial
stage. Which nucleates faster in interstellar space, (hydrogenised) iron or
silicate, however, still needs investigation and leaves the possibility of
type IS NPs in space on hold.

###  4.3 Comparison with interstellar dust

Interstellar silicate dust imposes broad absorption/emission bands on
starlight features at 10 and 18 µm, attributable to the Si-O stretching and
O-Si-O bending vibrations [e.g. 9]. The experimental NPs of types S and IS, as
well as type SiOC, are candidates for the interstellar silicate dust because
not only types S and IS but the SiOC type also has Si-O bonds. For
interstellar carbonaceous dust, spectroscopic evidence indicates that the
predominant phase is amorphous carbon [reviewed by 36], but type C NPs
produced in our experiment show diamond, fullerene, and graphitic structures
although all of the allotropic forms of C are found in space [36].

The spectral feature of starlight alone is not enough to determine the
composition of the interstellar silicate. Minerals found in actual primitive
meteorites and a theoretical sequence of mineral condensation from the gas
having a cosmic abundance of elements have been considered in order to
speculate a more detailed composition [37, 4]. The optical properties of
conceivable analogue minerals have been measured in laboratory and compared
with the spectra features of observed starlight [e.g. 17]. Our experimental
NPs also wait for spectroscopic studies. As Mathis1989 [38] suggested, the
interstellar dust may exist in the form of fluffy aggregates rather than
separate nanoparticles. The morphology and optical properties of dust
aggregates have been investigated by experimental and modelling works [e.g.
39, 40, 41]. Since our experimental NPs exhibit a wide variety of aggregate
structures, it is imperative to study the effect that such various aggregate
structures have on the spectral shapes.

The size distribution of interstellar dust has been estimated by combining the
spectral feature of starlight and modelling works [e.g. 42, 43]. The size
range of amorphous carbon and silicate grains is 0.5 to 500 nm in diameter,
depending on the assumption about their shapes [43]. All types of our
experimental NPs are within the estimated size range, but their majorities are
less than 70 nm (Table 1). The shape of the interstellar dust can be evaluated
by the polarization of starlight, and it has been predicted that interstellar
grain is elongated [e.g. 44]. Our type S and type IS NPs show both spherical
(subtype a) and skeletal (subtype c) shapes and appear to vary continuously
from spherical to elongated (Table 1 and Fig. 1).

As described, it is likely that the core of type IS NPs are variably
hydrogenised. If type IS NPs are major dust in interstellar medium, it may
resolve a long-standing paradox that the interstellar medium is highly
depleted in gaseous iron and yet solid iron particles are not positively
identified [e.g. 45, 46]. If the hydrogenised iron had been transferred to the
solar system during the formation of planets, their cores would have been rich
in hydrogen [47, 48, 49]. It may also give an easy answer to the origin of
terrestrial water, because the D/H ratio of the terrestrial water, $\sim$10-4
[50], is close to that of the interstellar gas, $\sim$10-5 [51], rather than
to that of the interstellar ice, $\sim$2 x 10-2 [52, fig. 10]. A simple
arithmetic tells that nearly 99 % of the terrestrial hydrogen would have
inherited the interstellar gas if it had been derived only from those gas and
ice.

Based on our experimental study that uses a realistic chemical composition,
both for dust and gas, of interstellar and PPD medium, we point out the
followings. First, the composition of interstellar and PPD dust particles
probably has a wide range as shown by the diverse types of nanoparticles
generated in our experiment, and are not necessarily stoichiometric such as
inferred from meteoritic minerals and from the equilibrium condensation
calculation. Second, the shape of nanoparticles and the structure of their
aggregates can be variable even if nanoparticles are chemically identical,
such as shown by our experimental type S and IS NPs. The spectroscopic
features of interstellar and PPD dust and analogue materials to compare with
should be evaluated in light of nanoparticles which are much more diverse in
chemistry, shape, and structure. Third, it is likely that interstellar and PPD
dust particles are "unexpected amalgam" of elements, which otherwise are not
easy to combine or coexist, such as the hydrogenised iron core (Fe and H
combined) in type IS NPs, the (hydrogenised-) silicon oxycarbide (Si, O, C,
and H combined) in type SiOC NPs, and type IS NPs (where metal and silicate
intimately coexist, not as a mechanical, chance mixture). Such a feature may
partly originate in atomic and molecular interactions in
angstrom$\sim$nanometre-scaled clusters and particles, which are different and
unpredictable from those in macro-scaled bulk matter. Although the ambient gas
was a pure H2 in our experiment, a use of a more realistic (interstellar-like)
gas, which also contains He, Ar, and CO and many other volatiles such as H2O,
CO2, N2, NH3, H2S, etc. that are mainly ice components at low temperature,
will produce more diverse types of NPs by reaction with the vapour generated
from a cosmic solid material or Allende meteorite.

##  5 Conclusion

A single laser flash heating in a pure H2 gas of a solid starting material
having nearly cosmic proportions in elements generated four types of
nanoparticles simultaneously, which are diverse in chemistry, size, shape, and
aggregation structure. Mechanisms of simultaneous nucleation of diverse
nanoparticles are considered, but micro-scaled fluid dynamics as well as
multi-component effects on nucleation and cluster physics need to be more
clarified. The incorporation of hydrogen atoms into the iron core in type IS
nanoparticles should have a profound consequence on the fate of astrophysical
and planetary irons and the origin of planetary water. Spectroscopic studies
of nanoparticles and aggregates as we have produced are yet to come in future.
Meanwhile, astronomical survey should consider a wider range in chemical and
structural diversity of nanoparticles in interstellar environment and in PPDs.

## Acknowledgements

We acknowledge staffs of Institute of Low Temperature Science and Department
of Cosmosciences, Planets & Space group, Hokkaido University for their
technical supports. We have greatly benefited from discussion with Dr. H.
Hidaka, Dr. M. Tsuge, and Dr. T. Yamazaki.

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