NANO LETTERS
Unusual Carbon-Based Nanofibers and Chains among Diesel-Emitted Particles
2003 Vol. 3, No. 1 63-64
A. Evelyn, S. Mannick, and P. A. Sermon* Nano-engineered Solids and Surface ReactiVity, Laboratory, Department of Chemistry, UniVersity of Surrey, Guildford, GU2 7XH, U.K. Received September 20, 2002; Revised Manuscript Received October 24, 2002
ABSTRACT Within the host of diesel-emitted particles, electron microscopy has detected carbon-based tubes, nanofibers, and chains, suggesting that this interface between nanotechnology and atmospheric pollution should be explored further.
Particulate matter (PM) in the atmosphere includes dieselemitted particles (DEPs),1 whose concentrations continue to be lowered by legislation and technology.2 Chronic inhalation of DEPS can alter immunological responses in the lungs, increasing susceptibility to pathogens.3 Some believe that DEP sizes, aspect ratios, and surface compositions affect their deposition, biopersistence, and any contribution to respiratory disorders.4 Four years ago, there was a brief report that needle-shaped particulates of C120N80O1Cl3 atomic composition were emitted from an Otto engine.5 This seemed to deserve further attention, the initial results of which are now briefly described.
Figure 1. Size distribution measured three times on water-dispersed DEPs (open symbols; from a Rover 218 TD operating at 1000 rpm for 40 min after it had run for 0.2 Gm without a trap or catalyst) obtained by passing the whole engine exhaust through 2 dm3 of distilled H2O. Filled symbols represent the particle size distribution found in the engine oil after dispersion in n-hexane.
To minimize aggregation, primary water-insoluble DEPs were bubble-stripped from the diesel engine exhaust under conditions indicated in the legend to Figure 1. We appreciate * Corresponding author. E-mail:
[email protected]. 10.1021/nl025803u CCC: $25.00 Published on Web 12/02/2002
© 2003 American Chemical Society
Figure 2. TEM images of carbon-based (a) nanofibers and (b) a larger tube found in DEPs. Scale bars denote 100 nm.
that the collection efficiency will have depended on the bubble size distribution, contact times, and gas hold-up. The size of these particles was investigated by dynamic light scattering (DLS; Malvern 3000HS after filtering to 1.20 µm), as was the size of particles in the engine oil after this had been n-hexane diluted. DLS in Figure 1 suggested that the most frequent size of the primary DEP particles and those in the engine oil was 200-250 nm, but the relationship between exhaust-borne
Figure 3. SEM of chainlike structures in solid DEP agglomerates. Scale bars are given below the images.
DEPs and oil-dispersed particles is a complex one. Transmission electron microscopy (TEM; Philips CM200 and 400T using holey-carbon grids) of the water-dispersed DEPs found, in addition to aggregates of primary spherical particles (about a 65-nm diameter), carbon-based fibers (see Figure 2a) of 15-28-nm diameter, 0.8-1.0-µm length, and aspect ratio ∼30-60 and larger tubes (see Figure 2b). EDX found no elements other than carbon. It is well recognized that carbon nanotubes6 (CNTs) can be produced in arcs, flames, and catalysis. The present results, illustrating the presence of related structures in DEPs, are unexpected and may relate to the formation of carbon nano-onions7 in H2O or CNTs in organic liquids.7 Their aspect ratio (partly in the range seen for lung-retained asbestos8) and potential strength9 may be of concern if they are widely emitted into the atmosphere by a variety of engines, but this has yet to be proved. DLS may have only seen the length dimension of the nanofibers, or prefiltering may have removed these. Scanning electron microscopy (Hitachi S3200N; see Figure 3a and b) of the DEPs after agglomeration revealed unusual chainlike structures 0.8-1.2 µm in length. These may result from (i) the self-assembly of DEPs10 around the carbon nanofibers in Figure 2a or (ii) the inter-twining of the nanofibers into ropes.11 XPS (VG Scientific; Al source) was clearly more sensitive than EDX and on the DEPs in Figure 3 found N1s (406.5 and 401.0 eV), O1s (533.2 eV), and S2p3/2 (168.6 eV) in addition to C1s (285.0 eV). XPS showed an atomic composition slightly higher in N, O, and S than seen12 previously by XPS (i.e., C80O18N2S0.2). The peak at 401.8 eV may correspond to surface nitroso or nitrosamine groups, that at 407 eV to surface NO3-,12 at 533 eV to C-OH,12 and at 169 eV to surface SO42-.12 The NOx surface chemistry of the nanofibers may need to be investigated because it could aggravate any physical impact and help clarify the mechanisms of DEP formation. If just one in a billion primary DEPs are of the form seen in Figure 2a, then breathing air containing 50 µg of DEPs per m3 could involve inhaling one nanofiber per minute.13 Hence, although a new era of nanomaterials based on CNTs is undoubtedly emerging, in the environmental context of diesel engine emissions, especially in urban locations,14
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carbon nanofibers are unlikely to be beneficial. Nevertheless, the present work says nothing about the relationship of diesel exhaust particles to respiratory disease.4 We assume that these nanofibers are vapor-grown (possibly after catalytic initiation15 or via conversion of tubes in Figure 2b16), whereas the nanochains are aggregation-derived. In the future, we will want to know precisely how such particles form in a diesel engine and the role of surface NOx in this and whether they are eliminated by catalytic traps and, if not, what other control technology needs to be employed. If a solution is needed, it will certainly come from cooperation between the engine design, nanotechnology, and the environmental and medical communities. For the moment, current DEP emission legislation is mass-based,13 and electron microscopy17 complements traditional methods of DEP analysis.13 Acknowledgment. The help of M. Kaszuba (Malvern Instruments) and S. Redpath/G. Gibbs (MSSU at the University of Surrey) is gratefully acknowledged. References (1) Schulz, H.; De Melo, G. B.; Ousmanov, F. Combust. Flame 1999, 118, 179. (2) Bosteels, D.; Searles, R. A. Platinum Met. ReV. 2002, 46, 27. (3) Saito, Y.; et al. Exp. Lung Res. 2002, 28, 493. (4) Pandya, R. J.; et al. EnViron. Health Perpect. 2002, 110(Suppl. 1), 103. (5) Ortner, H. M.; et al. Analyst 1998, 123, 833. (6) Iijima, S. Nature (London) 1991, 354, 56. Laplaze, D.; et al. Carbon 2002, 40, 1621. (7) Sano, N.; et al. Nature (London) 2001, 414, 506. Zhang, Y.; et al. J. Mater. Res. 2002, 17, 2457. (8) Yamada, H.; et al. EnViron. Health Perpect. 1997, 105, 504. Rogers, R. A.; et al. EnViron. Health Perpect. 1999, 107, 367. (9) Zhao, Q. Z.; Nardelli, M. B.; Bernhole, J. Phys. ReV. B 2002, 65, 144105. Salvetat-Delmotte, J. P.; Rubio, A. Carbon 2002, 40, 1729. (10) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science (Washington, D.C.) 2002, 297, 237. (11) Bandyopadhyaya, R.; et al. Nano Lett. 2002, 2, 25. (12) Albers, P. W.; et al. Phys. Chem. Chem. Phys. 2000, 2, 1051. (13) Kittelson, D. B. J. Aerosol Sci. 1998, 29, 575. (14) Kinney, P. L.; et al. EnViron. Health Perspect. 2000, 108, 213. (15) Kibria, A. K. M. F.; et al. Carbon 2002, 40, 1241. (16) Mukhopadhyay, K.; Dwivedi, C. D.; Mathur, G. N. Carbon 2002, 40, 1373. (17) Lambin, P.; et al. Carbon 2002, 40, 1635.
NL025803U
Nano Lett., Vol. 3, No. 1, 2003
