Raman Spectroscopy of Single Light-Absorbing Carbonaceous

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Raman Spectroscopy of Single Light-Absorbing Carbonaceous Particles Levitated in Air Using an Annular Laser Beam Masaru Uraoka, Keisuke Maegawa, and Shoji Ishizaka Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03455 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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Analytical Chemistry

Raman Spectroscopy of Single Light-Absorbing Carbonaceous Particles Levitated in Air Using an Annular Laser Beam Masaru Uraoka, Keisuke Maegawa, and Shoji Ishizaka* Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan ABSTRACT: A laser trapping technique is a powerful means to investigate the physical and chemical properties of single aerosol particles in a non-contact manner. However, optical trapping of strongly light-absorbing particles such as black carbon or soot is quite difficult because the repulsive force caused by heat is orders of magnitude larger than the attractive force of radiation pressure. In this study, a laser trapping and Raman microspectroscopy system using an annular laser beam was constructed to achieve non-contact levitation of single light-absorbing particles in air. Single acetylene carbon black or candle soot particles were arbitrarily selected with a glass capillary connected to a three-axis oil hydraulic micromanipulator and introduced into a minute space surrounded by a repulsive force at the focal point of an objective lens. Using the developed system, we achieved optical levitation of micrometer-sized carbonaceous particles and observation of their Raman spectra in air. Furthermore, we demonstrated in situ observations of changes in the morphology and chemical composition of optically trapped carbonaceous particles in air, which were induced by heterogeneous oxidation reactions with ozone and hydroxyl radicals.

Soot generated by incomplete combustion of carbon-based fuels influences the Earth's climate through both direct and indirect mechanisms. Since soot can efficiently absorb sunlight, it acts as a heat source in the atmosphere. On the other hand, soot can act as cloud condensation nuclei (CCN) and decrease the temperature of the atmosphere. Due to these two conflicting roles, the net effect of soot particles on the global climate is complex.1–5 Soot is generally composed of elemental carbon, amorphous carbon, and polycyclic aromatic hydrocarbons, and its surface is inherently hydrophobic.5–7 Therefore, the CCN activity of freshly emitted soot particles is extremely low.8 During transport in the atmosphere, soot particles are subject to many complex physical and chemical processes, which modify their morphology, chemical composition, and hygroscopic properties.9 It has been reported that heterogeneous reactions of soot with gaseous oxidants play a crucial role in converting the soot surface from hydrophobic to hydrophilic.8,10–12 Despite the importance of fundamental knowledge about the chemical and physical properties that influence the CCN activity of soot, our current understanding of the ageing processes of soot in the atmosphere remains limited. Experimental studies of heterogeneous oxidation reactions on soot have hitherto been conducted by using aerosol chambers or flow tube reactors.10,13–18 Although these experimental approaches are quite useful for evaluating the reaction dynamics and water uptake behavior of soot particles, the experimental results are ensemble averages for the various types of particles. Since the chemical species in the gas phase were usually observed with a mass spectrometer in the experiments, no information was provided about temporal changes in size, morphology, and chemical composition at the single-particle level. Recently, a single particle soot photometer (SP2) coupled with a humidified tandem differential mobility analyzer has been developed and used to evaluate the hygroscopicity of single

black carbon (BC) particles.2,6,9 The SP2 is based on the laserinduced incandescence of single BC particles.19 Refractory BC particles are irradiated with a 1064 nm laser beam and heated to their incandescent temperature. Since the particles are vaporized during incandescence, the SP2 represents a destructive analysis, and it is not capable of measuring timedependent changes in the chemical composition and morphology of an individual soot particle during oxidation with gaseous oxidants.1 On the other hand, Raman spectroscopy is a powerful means for in situ observation of oxidation reactions proceeding on soot surfaces, because it is sensitive not only to changes in the crystal structures of graphite but also to the formation of carbonyl groups responsible for the hygroscopicity of soot particles.20–25 For example, using Raman and infrared spectroscopy with a water sorption analyzer, Liu et al. reported that the heterogeneous oxidation reaction enhances the hygroscopicity of soot due to the formation of oxygen-containing surface species.22 In their experiment, soot particles deposited on a solid substrate were investigated before and after exposure to O3. However, it has been pointed out that the deliquescence and efflorescence processes of aerosol particles collected on solid substrates are strongly affected by the hydrophilic or hydrophobic nature of the substrates.26 Therefore, to achieve further advances in the evaluation and interpretation of cloud droplet formation induced by soot particles in the atmosphere, it is worth developing a novel experimental approach that is capable of characterizing individual soot particles levitated in air. A laser trapping technique is a fundamental basis for studying aerosol droplets, since a metastable liquid state such as supercooled or supersaturated water droplets can be stably observed without contact with solid substrates.1,27–31 Therefore, the laser trapping technique coupled with Raman spectroscopy is a promising experimental approach for evaluating the

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reaction dynamics and water uptake behavior of soot particles in the atmosphere. However, a conventional laser trapping technique based on radiation pressure is not applicable to the optical trapping of light-absorbing carbonaceous particles because the repulsive force (photophoretic force) caused by heat is orders of magnitude larger than the attractive force of radiation pressure.32–36 To date, several methods for the optical trapping of light-absorbing particles in air using the photophoretic force have been developed, in which counterpropagating laser beams,35,37,38 a speckled coherent beam,39,40 an optical bottle beam,41,42 a hollow laser beam,43,44 or a slowly diverging vortex beam36 were used as trapping light sources. However, the number of reports on the Raman spectroscopy of light-absorbing particles trapped in air remains limited.38,45 Pan et al. developed a method for obtaining Raman spectra of single light-absorbing particles trapped in air using two counter-propagating circular hollow laser beams.38 Recently, an alternative method for measuring Raman spectra of single light-absorbing particles in air was proposed by Ling et al.45 They employed a single Gaussian beam as the trapping light source, which is easy to align compared with counterpropagating double laser beams. Although the mechanism is not fully elucidated, it has been reported that a single focused Gaussian beam can be used to trap and transport lightabsorbing particles in air.45–47 It should be noted that the position of the particles trapped by a single focused Gaussian beam was extremely sensitive to air perturbations, and the particles moved randomly along the beam axis even though the laser power was constant.45 The large fluctuation in the trapping position makes it difficult to obtain Raman spectra of the trapped particles. To stabilize the trapping position, Ling et al. used a position-sensitive detector and locking circuit to control the laser power and succeeded in measuring Raman spectra of single carbonaceous particles trapped by a single focused Gaussian beam.45 To obtain Raman spectra of individual soot particles in air during heterogeneous oxidation reactions, the particles should be trapped in a fixed position for a long period even in an air flow caused by the introduction of gaseous oxidants into a chamber. In order to realize stable trapping of light-absorbing particles in air, it is necessary to form a “light-cage,” which is a minute space surrounded by laser light.48 Since a light-absorbing particle in the light-cage experiences repulsive forces from all directions, the particle will be stably trapped in the light-cage. In this study, we constructed a new laser trapping and Raman spectroscopy system capable of trapping single carbonaceous particles in air, in which an annular laser beam was employed as a trapping light source. Using the developed system, we achieved non-contact levitation and obtained Raman spectra of single carbonaceous particles in air. To the best of our knowledge, this is the first report on optical trapping and Raman spectroscopy of light-absorbing carbonaceous particles in air using a single annular laser beam. Furthermore, we demonstrate in situ observation of heterogeneous oxidation reactions of optically trapped soot particles with O3 and hydroxyl (OH) radicals. The heterogeneous oxidation reaction mechanisms of soot will be discussed on the basis of the curve fitting of the Raman spectra.

EXPERIMENTAL SECTION Commercially available acetylene carbon black (Strem Chemicals, Inc., avg. particle size 0.042 µm) and soot collected onto a stainless-steel plate held above the burning

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flame of a candle (Nippon Kodo Co., Ltd.) were used for the experiments. The elemental composition of the candle soot was determined by elemental analysis: C 93.00%, H 0.57%, and N 0.07%. All samples were used without further purification.

Figure 1. Schematic illustration of laser trapping and Raman spectroscopy system (A), and the beam cross section at the focal plane (B).

A schematic illustration of the laser trapping and Raman spectroscopy system designed for light-absorbing carbonaceous particles is shown in Fig. 1(A). The beam diameter of a Gaussian laser beam from a CW-Nd:YVO4 laser (532 nm, Coherent, Verdi V2) was reduced with a pair of convex lenses, and it was converted into an annular beam with an axicon lens (EKSMA Optics, cone angle 170°). In order to block the remaining light at the central region of the beam, a photomask having a circular pattern (d = 2.8 mm) of chromium fabricated on a glass plate was inserted into the optical axis. The beam was collimated with a pair of convex lenses and focused with a dry objective lens (Olympus LUCPLFLN 60×, N.A. = 0.70). The diameter of the beam cross section at the focal plane was about 40 µm, as shown in Fig. 1(B). The power of the laser beam passed through the objective lens was measured with a photodiode sensor (Ophir Optronics Solutions Ltd., PD300-SH) and adjusted with a circular variable neutral density filter. An inverted optical microscope (Olympus, IX71) equipped with a three-axis oil hydraulic micromanipulator (NARISHIGE, MMO-203) was used for the experiments. A glass capillary with a tip diameter of ~1 µm was made with a dual-stage glass micropipette puller (NARISHIGE, PC-10) and manipulated three-dimensionally with the micromanipulator to inject carbonaceous particles into a light-cage (i.e., a space surrounded by repulsive force) at the focal point. Scattered light from the optically trapped particles was collected with the same objective lens. After passing through a single-notch filter (Semrock, NF03-532E25, optical density > 6) to remove Rayleigh scattered light, Raman scattered light was focused onto the entrance slit of a polychromator (SOLAR TII, MS3504i, 1200 grooves/mm) and analyzed with a cooled EMCCD detector (ANDOR, Newton DU970N-BV), with a spectral resolution of 0.04 nm. Bright-field images under the microscope were observed with a CCD camera (TOSHIBA TELI, CS9301-03).


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Figure 2. The procedure for laser trapping of single carbonaceous particles in air, (A) an arbitrarily selected particle was attached to the tip of a glass capillary, (B) the particle was lifted to the focal position of the objective lens, (C) the laser beam was turned on to surround the particle with laser light, (D) the particle was trapped in the light-cage. The green circles in the images represent the position of the cross section of the laser beam.

To induce heterogeneous oxidation reactions, O3 and OH radicals were generated by irradiating 185 and 254 nm emission from a low-pressure mercury lamp (Hamamatsu Photonics Inc., L937-02) in humid air (RH ~95%) and introduced into the chamber. Humid air was generated with the equipment described in the previous paper.28 All the experiments were conducted at room temperature (23 ± 1ºC).

RESULTS AND DISCUSSION Laser trapping and Raman spectroscopy of single lightabsorbing carbonaceous particles in air. The procedure for laser trapping of single carbonaceous particles in air is shown in Fig. 2. Acetylene carbon black or candle soot particles were deposited on a glass cover slip at the bottom of a chamber set on the stage of an inverted optical microscope (Fig. 2(A)). An arbitrarily selected particle was attached to the tip of a glass capillary and lifted to the focal position of the objective lens (Fig. 2(B)). The laser beam was turned on to surround the particle with laser light (Fig. 2(C)). When the glass capillary was removed from the focal position, the particle was pushed by a repulsive force and trapped in the light-cage (Fig. 2(D)). A series of snapshots taken during the trapping of an acetylene carbon black particle are shown in the lower part of Fig. 2: a full movie (Video S1) is available in the supporting information. While the particle moved about in the light-cage, it was not able to escape from it (Movie S1). The particle should experience repulsive forces from all directions. Thus, the particle was trapped in the minute space surrounded by the potential of the repulsive force. The power of the laser beam required to remove the particle from the tip of the glass capillary was about 27 mW. After trapping the particle in air, the power of the laser beam was reduced, and it was possible to keep the particle in the air even at 0.5 mW. By using this technique, not only acetylene carbon black particles but also candle soot particles could be levitated in air for several hours. Since the trapping laser beam can be used simultaneously as an excitation light source for Raman spectroscopy, in situ characterization of the single particles levitated in air can be achieved by means of Raman spectroscopy. Fig. 3 shows a typical Raman spectrum obtained from an optically trapped

acetylene carbon black particle in air. Two intense peaks were observed at around 1350 and 1590 cm−1, which are characteristic Raman bands of carbonaceous materials known as D (Defect) and G (Graphite) bands, respectively. The Dband corresponds to a graphitic lattice vibrational mode with A1g symmetry. The G-band is assigned to the E2g symmetry mode of an ideal graphitic lattice. As demonstrated in Figs 2 and 3, optical levitation of micrometer-sized carbonaceous particles in air and in situ observations of Raman spectra were achieved using the developed system.

Intensity

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1000

1200

1400

1600

1800

2000

Raman shift / cm-1 Figure 3. Raman spectrum of an optically trapped acetylene carbon black particle in air.

In situ observations of heterogeneous oxidation reactions of optically trapped soot particles in air. According to the procedure described above, a candle soot particle was trapped in air. After trapping the particle, the glass capillary was withdrawn from the reaction chamber, and the top of the chamber was covered with a glass cover slip. To study the heterogeneous oxidation reactions of soot, gaseous oxidants were introduced into the chamber. Changes in the morphology of the optically trapped soot particle after exposure to gaseous oxidants are shown in Fig. 4. It is worth noting that the size of soot gradually decreased as the reaction time increased. The diameter of the particles before the reaction was about 20 µm



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Table 1. Spectral parameters for the Raman bands of soot before and 30 min after starting the oxidation reaction Reaction time 0 min

30 min

Band

Position / cm-1

FWHMa / cm-1

Relative areab

G

1593

75

0.22

D1

1360

196

0.62

D2

1620

42

0.04

D3

1500

44

0.05

D4

1180

373

0.07

G

1593

64

0.17

D1

1360

156

0.61

D2

1620

43

0.11

D3

1500

73

0.06

D4

1180

308

0.04

Carbonyl

1805

42

0.01

a. Full width at half maximum. b. Each peak area was divided by the sum of all peak areas.

(Fig. 4(A)); it was approximately 10 µm 30 min after the introduction of oxidants (Fig. 4(C)). No further decrease in particle size was observed even when the reaction time was prolonged over 30 min. No significant change in the particle size was observed in the absence of oxidants, indicating that a heterogeneous oxidation reaction gave rise to the morphological change in the optically trapped soot particle. In this experiment, humid air (RH ~ 95%) was irradiated with 185 and 254 nm light from a low-pressure mercury lamp to generate the gaseous oxidants. Ozone is generated by photoexcitation of oxygen with 185 nm light. Since O3 has a very strong absorption at 254 nm, OH radicals are produced by photochemical reaction of O3 in the presence of water vapor.49 In this experiment, O3 and OH radicals would be the main gaseous oxidants. Vlasenko et al. reported that formic acid and formaldehyde were detected by using a mass spectrometer in the study of heterogeneous oxidation of hexane soot with OH radicals.14 Therefore, the decrease in the volume of the optically trapped soot particle suggests the removal of volatile organic compounds, i.e., formic acid and/or formaldehyde, due to heterogeneous oxidation with OH radicals.

that the band widths of the Raman peaks are broad compared to those of acetylene carbon black (Fig. 3), indicating higher chemical heterogeneity in soot. Since the Raman scattering of graphite crystals is sensitive to the degree of structural disorder of graphite lattices, the shapes and intensities of the Raman peaks

(A)

0 min D1 G D2

Intensity

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D3 D4

(B)

30 min

1805

Figure 4. Changes in the morphology of the optically trapped soot particle during heterogeneous oxidation reaction: 0 (A), 15 (B), and 30 min (C) exposure to gaseous oxidants.

To investigate the detailed mechanisms of heterogeneous oxidation reactions, we conducted Raman spectroscopy on the soot particle shown in Fig. 4. The Raman spectra of the soot particle measured before and 30 min after the start of the oxidation reaction are shown in Fig. 5(A) and (B), respectively. As shown in Fig. 5, two intense peaks were observed at around 1360 and 1600 cm−1. It should be noted

1000

1200

1400 1600

1800

2000

-1

Raman shift / cm

Figure 5. Raman spectra of the optically trapped soot particle before (A) and 30 min (B) after the start of the oxidation reaction. The red lines represent the best fit of the experimental spectra according to the sum of the components listed in Table 1.


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of carbonaceous materials are related to the size, morphology, and chemical composition of crystalline domains. It has been reported that the Raman spectra of soot can be analyzed according to the sum of several Raman bands.20–24 Sadezky et al. proposed that a combination of four Lorentzian bands and one Gaussian band was best suited for analysis of Raman spectra of soot and related carbonaceous materials.20 Therefore, we analyzed the Raman spectra in the same manner. For the unoxidized soot particle, the Raman spectra were reasonably fitted by a combination consisting of four Lorentzian bands (G, D1, D2, and D4) and one Gaussian band (D3) as judged by the χ2 value (0.990). The fitting result is indicated by the red line in Fig. 5(A), and the peak wavenumber, full width at half maximum (FWHM), and relative area of each band are summarized in Table 1. The G band is assigned to the ideal graphitic lattice stretching mode with E2g symmetry. The peak wavenumber of the G band of the candle soot particle was observed at 1593 cm−1, falling within the range 1571 ~ 1598 cm−1 reported for various types of soot.20 The D1, D2, D3, and D4 bands were observed at around 1360, 1620, 1500, and 1180 cm−1, respectively. The attributions of the D1–D4 bands have been reported as follows.20 The Lorentzian-shaped D1 band corresponds to a vibrational mode with A1g symmetry, which has been suggested to arise from the edge of a graphene layer. The Lorentzian-shaped D2 band is assigned to a lattice vibration with E2g symmetry involving graphene layers at the surface of a graphitic crystal. The Gaussian-shaped D3 band is associated with the amorphous carbon content of soot. The Lorentzianshaped D4 band is assigned to vibrations of disordered graphitic lattices with A1g symmetry, polyenes, and ionic impurities. After 30 min from the start of the oxidation reaction, it should be noted that a broad peak appeared at around 1805 cm−1 as shown in Fig. 5(B). To describe the Raman spectrum as a sum of several peaks, it was necessary to add one Gaussian peak at 1805 cm−1 in addition to the five bands mentioned above. The χ2 value (0.995) for the fitting indicates that the Raman spectra were reasonably fitted with a combination of the six bands listed in Table 1. Liu et al. reported that Raman spectra of soot aged with O3 comprised seven bands, which were the five bands (G, D1, D2, D3, and D4) and an additional two Gaussian bands at 1060 and 1750 cm−1.22 Since carbonyl groups would be produced on soot by heterogeneous reaction with O3, the bands at 1060 and 1750 cm−1 were assigned to the stretching vibrations of C-O and C=O groups, respectively. Based on the similarity with the peak maximum of soot aged with O3, we concluded that the band at 1805 cm−1 in Fig. 5(B) might be attributed to the C=O stretching vibration of carbonyl groups. On the other hand, the stretching vibration of C-O groups could not be observed in the present study. Furthermore, the overall peak maximum at ~1600 cm−1 slightly shifted to higher wavenumber by about 10 cm−1 after oxidation, as shown in Fig. 5(B). This corresponds to the fact that the relative area of the G band decreased and that of the D2 band increased after oxidation, as listed in Table 1. The G and D2 bands are assigned to the E2g modes of the bulk and surface layers of the graphite crystals, respectively. Therefore, Sze et al. suggested that the peak area ratio of the D2 band relative to the G band should be inversely proportional to the thickness of the graphitic domains in soot.23 An increase in the relative intensity of the D2 band indicates a decrease in the average crystal size of graphite

crystals due to the oxidation reactions. In contrast to the results shown in Fig. 5, no significant change in the D2/G peak area ratio was observed in the Raman spectra of soot upon oxidation with O3 alone.22 This implies that the oxidation reactions of graphite crystals were mainly induced by OH radicals. In a previous study on the oxidation of soot using mass spectrometry, it was assumed that the graphitic domain of soot was inert to oxidation by either OH radicals or ozone.13 However, our experimental results suggest that the graphitic domain underwent oxidation with OH radicals. In order to confirm the reactivity of the graphitic domain with OH radicals, experiments on the heterogeneous oxidation of acetylene carbon black were conducted under the same experimental conditions (Supporting information: Fig. S1 and Table S1). In the case of acetylene carbon black, the Raman spectra were reasonably fitted by the sum of the G, D1, and D2 bands, as shown in Fig. S1. This means that acetylene carbon black is mainly composed of graphite crystals. An increase in the D2/G peak area ratio was also observed in the Raman spectra of acetylene carbon black after 30 min oxidation. Therefore, we concluded that the graphitic domain underwent oxidation with OH radicals.

CONCLUSIONS As demonstrated in this study, we achieved the optical levitation of single light-absorbing carbonaceous particles in air. In a previous study on the optical manipulation of lightabsorbing colloidal particles in water, a focused laser beam was repeatedly circularly scanned under an optical microscope to produce a potential space surrounded by repulsive forces.48 Such an experimental strategy was successfully applied to the optical trapping of light-absorbing particles in air. In this study, we constructed a laser trapping and Raman spectroscopy system in which a continuous annular beam was employed as a trapping light source instead of a circularly scanning laser beam. Using the developed system, we demonstrated the non-contact levitation of micrometer-sized soot particles in air and Raman spectroscopy of the optically trapped particles. To the best of our knowledge, this is the first report on the optical trapping of single carbonaceous particles using a single annular laser beam. Furthermore, we achieved in situ observation of the heterogeneous oxidation of optically trapped carbonaceous particles with O3 and OH radicals. Although the relative intensity ratio of the C=O stretching vibration was very small, the formation of the aldehyde group, which is very important for controlling the hygroscopic properties of soot, could be detected based on the curve fitting of the Raman spectra. Further detailed elucidation of molecular and crystalline structures and their relation to wateruptake properties will be required to clarify the roles of aged soot particles as CCN in the atmosphere. Comprehensive and systematic studies on soot aged by gaseous oxidants under controlled RH conditions will subsequently be undertaken in our research. Non-contact studies on the aging processes of carbonaceous aerosols at the individual particle scale will be indispensable in achieving a better understanding of heterogeneous atmospheric processes. We are convinced that annular laser beam trapping combined with Raman microspectroscopy will be a powerful means to study aerosol chemistry.

ASSOCIATED CONTENT Supporting Information



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The Supporting Information is available free of charge on the ACS Publications website. Figure S1: Raman spectra of the optically trapped acetylene carbon black particle before and 30 min after starting the oxidation reaction, Table S1: Spectral parameters for the Raman bands of acetylene carbon black (PDF) Video S1: Laser trapping of an acetylene carbon black particle in air (MPG)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was partially supported by JSPS International Joint Research Program and Grand-in-Aid for Scientiﬁc Research (C) (No. 25410144) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. The authors would like to thank Enago (www.enago.jp) for the English language review.

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Raman Spectroscopy of Single Light-Absorbing Carbonaceous

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