# Sub-2 nm particle measurement in high-temperature aerosol reactors: a review

Pratim Biswas, Yang Wang, Michel Attoui

► To cite this version:

Pratim Biswas, Yang Wang, Michel Attoui. Sub-2 nm particle measurement in high-temperature aerosol reactors: a review. Current Opinion in Chemical Engineering, 2018, 21, pp.60-66. 10.1016/j.coche.2018.03.004. hal-04568533

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Sub-2 nm Particle Measurement in High-temperature Aerosol Reactors: A Review

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Pratim Biswas,<sup>a*</sup> Yang Wang,<sup>a#</sup> and Michel Attoui<sup>b</sup>

<sup>a</sup> Aerosol and Air Quality Research Laboratory
Department of Energy, Environmental & Chemical Engineering
Washington University in St. Louis
St. Louis, MO-63130, USA

<sup>b</sup> LISA, UMR CNRS 7583
Université Paris Est Creteil & Université Paris Diderot
Institut Pierre Simon Laplace
61 av du Général de Gaulle
Créteil Cedex, 94010, France.

Revised Manuscript Submitted to :
Current Opinion in Chemical Engineering
March 2, 2018

* To whom correspondence should be addressed:

Tel: +1-314-935-5548; Fax: +1-314-935-5464

E-mail address: pbiswas@wustl.edu

# Present address: Environmental and Climate Sciences Department, Brookhaven National
Laboratory, Upton, NY-11973, USA
46 **Abstract:**

47 Aerosol science and technology enable continual advances in material synthesis and atmospheric
48 pollutant control. Among these advances, one important frontier is characterizing the initial
49 stages of particle formation by real-time measurement of particles below 2 nm in size. Sub-2 nm
50 particles play important roles by acting as seeds for particle growth, ultimately determining the
51 final properties of the generated particles. Tailoring nanoparticle properties requires a thorough
52 understanding and precise control of the particle formation processes, which in turn requires
53 characterizing nanoparticle formation from the initial stages. This article reviews current aerosol
54 instruments measuring the size, concentration, and chemical composition of sub-2 nm particles.
55 Recent measurements of sub-2 nm particles generated from different types of high-temperature
56 aerosol reactors are also summarized.
## 57 Introduction

The monumental agreement at the 21st Conference of the Parties (COP-21) demonstrates that all countries, regardless of their development status, are committed to deep cuts in greenhouse gas emissions. To accomplish this, there is a need to make rapid progress in developing new fossil fuel combustion technologies and manufacturing advanced materials for sustainable energy utilization. Aerosol science and technology play a central role in both areas. The combustion of fossil fuels is accompanied with the inadvertent generation of ultrafine particles, both in a primary manner at the source [1] and in a secondary manner with precursors from these sources [2]. Control of airborne particulate matter (PM) is a critical issue in developing countries, and is a complicated task to manage resulting in very high ambient levels of PM. On the other hand, aerosols synthesized in the gas phase have received tremendous attention during the past few decades [3]. Their mechanical, thermal, acoustic, electrical, and optical characteristics arise from their large specific surface area and quantum effects [4,5]. In order to control the aerosol emission from fossil fuel combustion and tailor the properties of synthesized aerosols, a thorough understanding of aerosol formation and growth dynamics at high temperatures is needed.

Classical theories of aerosol dynamics predict that most nanoparticles develop in a “bottom-up” approach, starting with precursor reactions and moving sequentially through supersaturated vapor nucleation, vapor condensation, coagulation, and sintering. Among these processes, incipient particle formation and growth, especially in the size range near or below 2 nm, still remain mostly mysterious due to the limitations of instrumentation. Due to this reason, one important frontier of nanotechnology is characterizing the initial stages of particle formation by real time measurement of particles below 2 nm in size.
Conventional aerosol instruments face difficulties in characterizing particles below 2 nm. For example, the latest generation of condensation particle counters cannot detect particles smaller than 2.5 nm, so the initial stages of particle formation, such as nucleation and condensation, cannot be observed. The high diffusivity of sub-2 nm particles also causes diffusion broadening of the instrument's transfer function, resulting in errors in their measurement [6,7]. Furthermore, for sub-2 nm particles, their chemical properties, such as electron and proton affinities, start to play crucial roles during clustering and growth, meaning that these processes cannot be fully characterized by particle physics alone [8]. These challenges led to the design of advanced aerosol instruments that help us measure the initial stages of aerosol formation. This article intends to provide a state-of-the-art overview of advanced instruments and studies measuring incipient clusters and particles generated from high-temperature aerosol reactors. These aerosol reactors include (1) flame aerosol reactors (FLARs), (2) furnace aerosol reactors (FUARs), (3) glowing wire generators (GWGs), and (4) spark discharge generators (SDGs). These research findings greatly help instruct industrial material synthesis and control of particles from primary sources. A summary and outlook of these measurements have been made regarding the understanding of particle formation in aerosol reactors.

## Instruments for measuring sub-2 nm particles

Aerosols can be characterized in many aspects, among which size, concentration, and chemical composition are most important. In order to characterize sub-2 nm particles, conventional aerosol instruments need to be modified and upgraded.

## Differential mobility analyzers
Differential mobility analyzers (DMAs) are widely utilized for measuring particle mobility sizes online. They act as an electric filter, allowing only particles with certain electrical mobilities, i.e., mobility sizes, to pass through and be further monitored by particle counters (Fig. 1a and 1b).

The diffusion broadening of the DMA transfer function is a significant issue when we measure sub-2 nm particles, since the diffusion coefficient is inversely proportional to the square of particle diameter in the free molecular regime [9]. As shown in Fig. 1c, when a Long DMA (Model 3081, TSI Inc.) and a Nano DMA (Model 3085, TSI Inc.) were used to classify 3 nm particles, the transfer functions become broadened in different degrees [10]. Because of the longer residence time of particles in the Long DMA, particles with sizes between 2.54 and 3.88 nm can all pass through the classification zone of the DMA when the DMA was set to classify 3 nm particles. There are two methods for reducing the broadening effect of DMA transfer functions. One remedy is to increase the sheath-to-aerosol flow ratio to inherently enhance the measurement resolution. The other is to reduce the residence time of particles in the DMA.

Based on these concepts, much effort has been made to develop high-resolution DMAs (HRDMAs) that are optimized for sub-2 nm particle measurement [11,12]. These HRDMAs can accommodate sheath flow rates higher than 800 lpm without developing turbulent flow, and the length of the classification zone is significantly reduced to decrease the particle residence time.

120

## Particle counters

Particle concentrations are normally measured with an electrometer or a condensation particle counter (CPC). An aerosol electrometer measures the current created by the collection of charged aerosols on a metal filter. It is ideal for the measurement of sub-2 nm charged particles, since these particles normally carry one charge only, and the metal filter can reach almost 100%

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efficiency in particle collection. But an electrometer measures charged particles only, while the charging of sub-2 nm particles is highly dependent on the particle chemical composition [8]. On the other hand, the lowest detection limit of an electrometer is in the range of fA (femtoampere, $10^{-15}$ A). For charged aerosols with a concentration of $1 \text{ cm}^{-3}$, to create a response above this detection limit, the flow rate of aerosols going through the electrometer should be at least 600 lpm, which is incredibly high to handle with. CPCs have a low detection limit (down to $0.1 \text{ cm}^{-3}$) and measure particle concentration regardless of the charging state. A CPC grows a sampled particle, by condensing the vapor of a working fluid on the surface of the particle, until it has an optically detectable size (~ 1 µm) (Fig. 1d). The smallest particle that can be condensationally grown is determined by the Kelvin equation, which dictates that the commonly used butanol-based CPCs (bCPCs) have a smallest detectable size of 2.5 nm. Although changing the bCPC's working condition could further lower the detectable size, the improvement is not significant [13]. A detailed calculation using the Kelvin equation and the data of more than 800 organic compounds suggests that diethylene glycol (DEG) is the best candidate working fluid for activating sub-2 nm particle growth [14]. However, the DEG-grown particles are not large enough to be detected by the optical components in the CPC. Hence, a two-stage CPC was developed, where the first stage uses DEG to grow sub-2 nm particles to a moderate size, and the second stage uses butanol to measure the concentration of the grown particles (Fig. 1e) [15]. This design led to the manufacture of the two newest types of CPCs, the particle size magnifier (PSM, Airmodus Oy) and the Nano Enhancer (Model 3777, TSI Inc.). In addition, neutral particle size distributions can be measured with the two-stage CPCs by altering the saturation ratio in the condenser, alleviating the interference of particle charging [16].
149 **Mass spectrometers**

150 The aerosol mass spectrometer (AMS, Aerodyne Inc.) is well established for measuring the
151 chemical compositions of aerosols with sizes above 40 nm. To resolve the participating
152 compounds during particle formation in the initial stages, the chemical composition
153 measurements of atmospheric ions/clusters must be improved. Molecular beam mass
154 spectrometry (MBMS) has often been used in studying ion generation in flames, but these
155 studies were typically conducted in low pressure and fuel-rich conditions due to the strict
156 requirements of the setup [17]. The newly developed atmospheric pressure interface time-of-
157 flight mass spectrometer (APi-TOF, Tofwerk AG) can measure ambient ions in the mass/charge
158 range up to 2000 Th [18]. It has a mass accuracy higher than 0.002% and a mass resolving power
159 of 3000 Th/Th. The APi-TOF provides a nearly universal interface for atmospheric pressure
160 sampling, and this key feature is widely utilized in laboratory and field studies of sub-2 nm
161 particle formation.

162

163 **Measuring sub-2 nm particles generated from high-temperature aerosol reactors**

164 The combination of the three mentioned enhanced particle detectors (HRDMA, PSM-bCPC, and
165 APi-TOF) has served as a powerful tool in understanding atmospheric particle formation below 2
166 nm [19,20] and the properties of protein and large organic molecules [21,22]. Few studies have
167 reported the measurement of sub-2 nm particles with these instruments in aerosol reactors (Table
168 1). Because aerosol reactors generate particles through intense chemical reactions, the
169 mechanism of particle formation and the resulting observations are largely different from
170 atmospheric studies. Existing measurements of sub-2 nm particles generated from aerosol
171 reactors are reviewed as follows.
172

173 Flame aerosol reactor

174 Until now, most of progress in measuring sub-2 nm particles from flame aerosol reactors
175 (FLARs) has been made in the study of nascent soot particles and functional oxide nanoparticles.

176

177 In order to clarify the mechanism of soot formation from the gas to particle phase, many studies
178 have reported the measurement of ultrafine ($d_p$ < 100 nm) particle size distributions using DMAs
179 [23,24]. The first endeavor of probing sub-2 nm soot particles were made by Sgro et al. through a
180 series of experiments using HRDMAs to unveil the charging characteristics of nascent soot
181 particles generated from ethylene flames [25,26]. Incipient particle size distributions (>0.5 nm)
182 were measured under a series of equivalence ratios, showing that the first particles observed in
183 flames have a size of 1 nm, consistent with previous measurements using other techniques. The
184 charge distribution of the nascent soot particles was found to be very close to the Boltzmann
185 charge distribution [26]. Recent measurements of nascent soot in flames using a half-mini DMA
186 shed light on the challenges and potential artifacts affecting studies on soot inception by DMA
187 techniques [27,28].

188

189 The physical and chemical formation pathway of inorganic oxide nanoparticles during
190 combustion was studied extensively by Wang et al. [7,29-31]. An HRDMA was used to
191 investigate the formation of $TiO_2$ clusters during flame synthesis. When the synthesis precursor
192 was introduced into the FLAR, discrete $TiO_2$ ion peaks were observed, indicating a discrete size
193 distribution in the initial stages of particle formation [7]. The incipient particle physical size
194 distribution measured by the atomic force microscopy matched quantitatively well with the
mobility size [32]. Particle formation during flame synthesis was also investigated using the tandem differential mobility analysis-mass spectrometry (DMA-MS) technique [30]. A HRDMA and an APi-TOF were used in series to measure the size and mass of the sub-2 nm particles simultaneously in flames, with and without the addition of synthesis precursors (Fig. 2a).

Measurements in a blank flame detected a large number of sub-2 nm clusters generated from chemical ionization reactions, and determined that nitrate ions dominated in the negative ions.

The blank flame-generated ions played an important role during particle synthesis, since the APi-TOF observed the appearance of nitrate ions in particles containing synthesized materials.

Coupled with a charged particle remover (CPR), the PSM-bCPC measured the neutral particle size distributions during combustion, and the neutral particle fraction in the flame was found to be significantly lower than predicted by classical theory (Fig. 2b) [29]. The influence of flame-generated ions on particle formation was also confirmed by the simulation of simultaneous particle charging and coagulation at various temperatures [31]. These findings provide new perspectives on the synthesis of functional nanomaterials by changing the ion characteristics in flame environments.

## Furnace aerosol reactor

Furnace aerosol reactors (FUARs) are widely used in the synthesis of functional nanomaterials because of the controllable furnace temperatures [33,34]. Through rapidly quenching particle growth by introducing clean diluting gas at high flow rates, FUARs can generate sub-2 nm particles by heating a material. Measurements of the early stages of particle formation in FUARs showed that the sub-2 nm particle size distribution evolution can be used to calculate precursor reaction kinetics (Fig. 2c and 2d) [35]. The mass spectropic analysis of FUAR-generated sub-2
218 nm particles proved the difficulty in generating incipient particles with pure compositions
219 [36,37]. The heated substances either transformed into other species due to thermal
decomposition, or became attached with organic impurities due to the lower activation energy
220
221 needed for heterogeneous condensation.

222

223 **Glowing wire generator**

224 The glowing wire generator (GWG), where a material is evaporated from the surface of a heated
225 wire and subsequently quenched by a gas stream, has been commonly applied to produce
226 metallic particles [38]. The generated particles are typically in the range of a few angstroms to a
227 few nanometers, giving them unexpected behaviors [39]. The invention of instruments for
228 measuring sub-2 nm particles made it feasible to accurately measure the size and concentration
229 of these particles, where the mechanisms of metal evaporation and ion attachment were further
230 elucidated. The GWG is a convenient source of sub-2 nm particles, since it produces self-
231 charged particles with a single charge and repeatable mass spectrums [36]. The study of the
232 GWGs, in return, directly accelerated the development of instruments for detecting incipient
233 particles. Sub-2 nm particles generated from GWGs have been used to calibrate the transfer
234 functions and transmission efficiencies of HRDMAs and characterize the detection efficiency of
235 sub-2 nm particles when using two-stage CPCs under various conditions [36,40].

236

237 **Spark discharge generator**

238 Spark discharge generator (SDGs) are one type of versatile and energy-efficient devices to
239 produce nanoparticle aerosols in the entire nanometer range (1–100 nm), and beyond [41]. A
240 SDG generates nanoparticles by charging a shunted capacitor with a high voltage, where the
241 discharged spark evaporates the materials of the electrodes and forms aerosols [42]. The SDGs
242 reduce the energy consumption of nanoparticle synthesis, compared to FUARs and GWGs, since
243 the heating and cooling times are significantly decreased. Existing measurements with a
244 HRDMA showed that sub-2 nm particles with discrete sizes can be generated from SDGs
245 synthesizing silver nanoparticles [43]. By using the collision cross section values and calculating
246 the corresponding mobility values, these incipient particles were highly probable to be silver
247 clusters.

248

249 **Summary and future directions**

250 Characterizing incipient particle formation in aerosol reactors can fill the knowledge gap
251 between chemical reactions and particle growing dynamics, benefiting our understanding of
252 high-temperature particle synthesis, primary particle emission, and control technologies. The
253 discrete size distributions, special chemical compositions, and unique charging characteristics
254 demonstrate that the initial stages of particle formation cannot be easily described by classical
255 theories of particle dynamics or predicted by experimental findings of particles with larger sizes.

256 The detailed particle formation pathway may be strongly dependent on the choice of high-
257 temperature aerosol reactors; however, it can be investigated with similar set of experimental
258 skills. Current measurement of sub-2 nm particles can still be improved in the design of *in situ*
259 sampling methods, because the dilution sampling should not only reduce the aerosols to a
260 measurable concentration, but also quench all the ongoing reactions and particle growth.

261 Although much work has been done with particles of larger sizes [24,44], existing dilution
262 sampling systems (Fig. 3) has not been fully evaluated in the measurement of sub-3 nm particles
263 generated at high temperature environments. On the other hand, with the invention of enhanced
particle detectors for measuring sub-2 nm particles, detailed characterization of incipient
particles in various types of aerosol reactors should be conducted to elucidate the physical (size
and charging characteristics) and chemical (composition) formation pathways of high
temperature aerosols. Experimental conditions that are systematically different from existing
measurements will greatly help us understand the particle formation in various sources.

269

270 Acknowledgements

We gratefully thank support provided by NSF Grant CBET 1705864 for this work.

272

273 Additional information:

The authors declare no conflict of interest.
275 **References:**

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277 been highlighted as:

278 * of special interest

279 ** of outstanding interest

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415 47. Fang J, Wang Y, Kangasluoma J, Attoui M, Junninen H, Kulmala M, Petäjä T, Biswas P:
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423 Particle Detection with Commercial TSI 3772 and Airmodus A20 Fine
424 Condensation Particle Counters. Aerosol Sci. Technol.** 2015, **49**:674-681.

425 50. Feng J, Huang L, Ludvigsson L, Messing ME, Maisser A, Biskos G, Schmidt-Ott A:
426 **General approach to the evolution of singlet nanoparticles from a rapidly quenched
427 point source. J. Phys. Chem. C** 2015, **120**:621-630.

429

430
<table><caption>431 Table 1 List of work measuring sub-2 nm particles generated from aerosol reactors</caption><thead><tr><th>Reactor Type</th><th>Particle Type</th><th>Aerosol Instruments</th><th>References</th></tr></thead><tbody><tr><td rowspan="4">Flame Aerosol Reactor (FLAR)</td><td>Soot</td><td>HRDMA</td><td>[25-28]</td></tr><tr><td></td><td>Nano DMA, butanol-based CPC</td><td>[45]</td></tr><tr><td>TiO<sub>2</sub></td><td>HRDMA</td><td>[7,32]</td></tr><tr><td></td><td>HRDMA, APi-TOF, PSM-bCPC</td><td>[29,46]</td></tr><tr><td rowspan="2">Furnace Aerosol Reactor (FUAR)</td><td>TiO<sub>2</sub>, SiO<sub>2</sub></td><td>HRDMA, APi-TOF</td><td>[30,47]</td></tr><tr><td>Engine emitted particles</td><td>PSM-bCPC, Nano DMA</td><td>[48]</td></tr><tr><td rowspan="3">Glowing Wire Generator (GWG)</td><td>TiO<sub>2</sub></td><td>HRDMA</td><td>[35]</td></tr><tr><td>(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>, NaCl, Ag</td><td>HRDMA, APi-TOF, PSM-bCPC</td><td>[36]</td></tr><tr><td>AlCl<sub>3</sub>, MnCl<sub>2</sub>, ZnCl<sub>2</sub></td><td>PSM-bCPC, APi-TOF</td><td>[37]</td></tr><tr><td rowspan="2">Spark Discharge Generator (SDG)</td><td>WO<sub>x</sub></td><td>HRDMA, APi-TOF, PSM-bCPC</td><td>[36,40]</td></tr><tr><td>WO<sub>x</sub></td><td>HRDMA, PSM-bCPC</td><td>[49]</td></tr><tr><td></td><td>Ag</td><td>HRDMA</td><td>[42]</td></tr><tr><td></td><td>Au</td><td>DMA</td><td>[50]</td></tr></tbody></table>

432

433
434 **Figure Captions**

435 Fig. 1. a) Schematic diagram of a TSI “Nano” DMA (Taken from the TSI SMPS Model 3080
436 manual); b) the trajectory of particles classified by the DMA; c) transfer functions of a DMA
437 under the non-diffusing condition, a TSI “Nano” DMA, and a TSI “Long” DMA, when
438 classifying 3 nm particles (Reprinted with permission from Stolzenburg and McMurray, 2008;
439 Copyright 2008 Taylor & Francis); d) and e) the working principles of a single stage and a two-
440 stage CPC.

441 Fig. 2 (a) Contour plot showing the abundance of negatively charged particles during flame
442 synthesis conditions as a function of size and m/z using TTIP as synthesis precursor. The mass-
443 size relationships assuming that particles were spherical are displayed as short-dashed lines. The
444 fit to Kilpatrick's mass-mobility relationship is displayed as long-dashed lines (Reprinted with
445 permission from Wang et al., 2017b; Copyright 2016 Elsevier); (b) Neutral fractions of flame-
446 generated sub-2 nm particles generated with different precursor (TTIP) feed rates (Reprinted
447 with permission from Wang et al., 2017a; Copyright 2017 Elsevier). (c) Size distributions of
448 incipient particles generated in a FUAR synthesizing TiO₂ as a function of reaction temperatures;
449 (d) First order reaction rate constants of TTIP thermal decomposition calculated based on the
450 evolution of particle size distributions shown in (c). (Reprinted with permission from Wang et
451 al., 2015; Copyright 2015 Springer).

452 Fig. 3 Common designs of dilution samplers for measuring aerosols at high temperatures: (a)
453 hole-in-a-tube (HiaT) sampler and (b) straight tube (ST) sampler.
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>d Conventional CPCs</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>e Two-stage CPCs</figcaption></figure>

<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>
a HiaT Sampler

<figure><img src="image_11.png" /></figure>

b ST Sampler

<figure><img src="image_12.png" /></figure>
## Particle formation & Growth in initial stages (< 2 nm)

<figure><img src="image_13.png" /></figure>

Reaction

<figure><img src="image_14.png" /></figure>

Nucleation

**Knowledge gap:**

- Chemistry dependence

- Fast reaction rate

- Difficulty in sampling

- Brownian diffusion

- Limitation in instrumentation

## Further growth (> 2 nm)

<figure><img src="image_15.png" /></figure>

Condensation

<figure><img src="image_16.png" /></figure>

Coagulation

<figure><img src="image_17.png" /></figure>

Sintering

## Characterization in high-temperature aerosol reactors

- Size, Concentration, and Chemical Composition Analysis

## Energy and Environmental Applications

Pollution control

<figure><img src="image_18.png" /></figure>

Nanoparticle synthesis

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>