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Environmental Processes

Molecular Adsorption Mechanism of Elemental Carbon Particles on Leaf Surface Lei Wang, Huili Gong, Nian Peng, and Jin Z. Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06088 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Environmental Science & Technology

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Molecular Adsorption Mechanism of Elemental Carbon

2

Particles on Leaf Surface

3

Lei Wangƚ, Huili Gong*ƚ, Nian Pengƚ, Jin Z. Zhang*ǂ

4 5

ƚ

6

100048, China

7

ǂ

8

95064, USA

9

S Supporting Information ○

College of Resource Environment and Tourism, Capital Normal University, Beijing

Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 1

ACS Paragon Plus Environment


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ABSTRACT: Plant leaves can effectively capture and retain particulate matter

29

(PM), improving air quality and human health. However, little is known about

30

the adsorption mechanism of PM on leaf surface. Black carbon (BC) has great

31

adverse impact on climate and environment. Four types of elemental carbon

32

(EC) particles, carbon black as a simple model for BC, graphite, reduced

33

graphene oxide and graphene oxide, and C36H74/C44H88O2, as model

34

compounds for epicuticular wax, were chosen to study their interaction and its

35

impact at the molecular level using Powder X-ray diffraction and vibrational

36

spectroscopy (Infrared and Raman). The results indicate that EC particles and

37

wax can form C-H…π type hydrogen bonding with charge transfer from carbon

38

to wax, and therefore strong attraction is expected between them due to the

39

cooperativity of hydrogen bonding and London dispersion from instantaneous

40

dipoles. In reality, once settled on the leaf surface, especially without wax

41

ultrastructures, BC with extremely large surface-to-volume ratio will likely stick

42

and stay. On the other hand, BC particles can lead to phase transition of

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epicuticular wax from crystalline to amorphous structures by creating packing

44

disorder and end-gauche defects of wax molecular chain, potentially causing

45

water loss and thereby damage of plants.

46

Graphical abstract

47 2


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■ INTRODUCTION

49

Particulate matter (PM) air pollution is currently the most serious health

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issue in urban areas throughout the world1-4. Plants play an important role in

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removing PM from the atmosphere mainly depending on their leaves5 and

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have been widely used as natural air filters6. The total amount of PM2.5 (PM

53

with aerodynamic diameters ≤ 2.5 µm) removed annually by trees was

54

estimated approximately from 4.7 tons to 64.5 tons in ten U.S. cities7. PM on

55

leaf surface is mostly PM2.5 and PM1 (PM with aerodynamic diameters ≤ 1 µm)

56

accounting for 90%–99% and 80%–92% of the total number of PM on leaf

57

surface, respectively8. A great quantity of PM2.5 and PM1 still remain on leaf

58

surface under strong windy and rainy conditions8. However, little is known

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about the underlying fundamental mechanism9-11. Filling the knowledge gap

60

will help to elucidate the migration path and biogeochemical cycle of PM

61

species and the potential influence of PM accumulation on leaf surface to

62

plants.

63

Carbonaceous aerosols account for a large fraction of PM, approximately

64

10-40% of PM10 (PM with aerodynamic diameters ≤ 10 µm) and approximately

65

30-60% of PM2.512. Carbonaceous PM contains organic carbon (dominant

66

fraction), elemental carbon (EC, small fraction, 2-9% of PM2.5), usually

67

synonymous with black carbon (BC), and carbonate minerals (insignificant

68

fraction)13. BC is generated from the incomplete combustion of fossil fuels,

69

biofuels and biomass14. It is identified as an impure form of near-elemental

70

carbon with a graphite-like sp2 hybridized structure and comprises a class of

71

matter with extremely complex morphology and surface chemical composition,

72

depending on the type of fuel, combustion condition and life cycle in the

73

atmosphere. Due to its ultra-fine particle size, carrying carcinogenic polycyclic

74

aromatic hydrocarbons (PAHs), strong light-absorbing character, and

75

transportability over long distance, BC has enormously adverse impact on

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human health, air quality and climate change15-17. One study showed that 30% 3



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of atmospheric BC deposition to the forest canopy was retained on the leaves

78

in leaf growth season, however, which hardly triggered any concern18. To date,

79

the estimates of BC production exceed the accountable inventory and loss

80

rates in the global cycle19, 20. Carbon black (CB) produced industrially has been

81

widely used as an important material in automobile tires, battery electrodes

82

and pigments21. Although airborne BC particles carry more complicated

83

chemical compounds deriving from combustion process (e.g., PAHs) and

84

ageing process (secondary aerosol components e.g., sulfates, nitrates, and

85

organics) than CB22, 23, CB is still the most conveniently simplified model used

86

to repeatable control experiments. BC derived from diesel generator is very

87

similar to CB24. Other EC particles likewise with a graphite-like sp2 hybridized

88

structure, such as graphene, fullerene, carbon nanotubes and nanowires, have

89

increased tremendously in the environment due to their wider application in

90

industry25.

91

Leaf surface is composed of a cuticle, which has a vital physiological

92

function acting as a barrier between leaf interior tissue and external

93

environment, especially to prevent cell water loss and serve as the first line of

94

defense against UV radiation, PM, phyllospheric microoganisms and insect

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herbivores26. The cuticle is covered by epicuticular wax with a considerable

96

ultrastructural and chemical diversity, which is an interface between plants and

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their environment27. The chemical composition of epicuticular wax generally is

98

a mixture of numerous n-alkanes with a hydrocarbon backbone with 21 to >40

99

carbon atoms and their derivatives with one or two functional groups27. Certain

100

plant species have a high content of cyclic compounds such as triterpenoids or

101

flavonoids in their epicuticular wax, e.g. a desert plant, Rhazya stricta Decne.28.

102

However, crystal structures of all crystalline n-alkanes and their derivatives are

103

highly in common, i.e., the chains pack as straight rods by van der Walls

104

bonding between CH2 groups in layers29. Therefore, the interface of

105

epicuticular waxes exposed to air is the plane composed of methyl groups at 4


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30

106

the end of the chain of wax molecules

. Furthermore, the chemical

107

compositions of waxes are not a major factor in species differences of PM

108

adsorption capacity of leaf surface. The ultrastructures of wax crystals

109

contribute significantly to the PM adsorption of leaf surface by influencing

110

contact area between PM and leaf surface31, which can be easily and quickly

111

eroded by wind and rain32, 33.

112

Epicuticular wax consists of crystalline domains resembling a pure

113

crystalline n-alkane and amorphous zones comprised of chain ends, functional

114

groups, short-chain aliphatics and non-aliphatic compounds, and water

115

molecules only pass through the latter26. PM can facilitate ‘wax degradation’,

116

causing a decrease of the former and an increase of the latter. Then, this

117

reduces the drought tolerance of plants, especially conifers keeping PM over a

118

long period of time, which may be one of the causes of forest decline mainly

119

observed in Central and Northern Europe and the Eastern North America34.

120

The motivation of this study is to answer the following two questions. First,

121

how does EC particles and epicuticular wax interact at the molecular level,

122

since plant leaves exhibit a strong ability to capture PM? Second, what impact

123

does the interaction have on epicuticular wax or plants potentially? To address

124

these questions, four types of EC particles, carbon black as a simple model for

125

BC, graphite, reduced graphene oxide (RGO) and graphene oxide (GO) were

126

chosen. n-alkanes and esters are common components and sometimes

127

present in high content in the epicuticular wax, e.g. Gypsophila acutifolia Fisch.

128

with 70% n-alkanes35, and Typha angustifolia L. with 66% esters36. Hence,

129

hexatriacontane (C36H74) and docosanoic acid, docosyl ester (C44H88O2), were

130

chosen as model compounds for epicuticular wax. X-ray diffraction (XRD) is a

131

powerful technique for studying crystal structure37. Vibrational spectroscopy

132

(Infrared and Raman) is the primary tool to measure molecular vibration,

133

crystalline and conformational states in the crystal of n-alkanes and their

134

derivatives38-40. In addition, Raman spectroscopy is one of the most crucial 5



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methods to characterize carbon materials41. Together, XRD and vibrational

136

spectroscopy allow us to study chain packing (crystal structure), conformation

137

(trans and gauche) and vibration (bond stretching and bending) of wax or EC

138

molecules at their contact interface to reveal the interaction between wax and

139

EC and its environmental impact at the molecular level. The results have

140

significant implications in understanding the adsorption mechanism of EC

141

particles on leaf surface, the dynamic change of PM on leaf surface, the

142

erosion of epicuticular wax over time and the crystallization behaviors of

143

EC/wax nanocomposites.

144 145

■ MATERIALS AND METHODS

146

Chemicals and Reagents. Docosanoic acid, docosyl ester (C44H88O2,

147

Wako Chemicals USA 50990005), carbon black (acetylene, Alfa Aesar

148

4552730) and graphite (powder, Fisher 037487) were purchased from Fisher

149

Scientific USA. Hexatriacontane (C36H74, Aldrich H12552, with a stated

150

purity >98%), reduced graphene oxide (Aldrich 805424, Carbon >75% and

151

Nitrogen 98.5%) was

154

purchased from Fisher Scientific USA. Ultrapure water was obtained from a

155

Millipore-Milli Q system.

156

Characterization.

Differential

Scanning

Calorimetry

(DSC)

157

measurements to two waxes C36H74 and C44H88O2 were carried out on a

158

Q2000 (TA Instruments), calibrated by indium and sapphire standards.

159

Samples (3-5 mg) were placed in a standard TZero aluminum pan and sealed,

160

while a sealed empty aluminum pan was used as a reference. The purge gas

161

nitrogen flow was 50.0 mL min-1. The scanning rate was 2 °C min-1 in both

162

heating (20-100 °C) and cooling modes (100-0 °C). The transition

163

temperatures and enthalpy values were calculated using TA Universal Analysis 6


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2000 software. The results are shown in Figure S1. Four EC samples (carbon

165

black, graphite, reduced graphene oxide and graphene oxide) were

166

bath-sonicated and dispersed in acetone. The sample solutions were

167

deposited on silicon wafers and were tested after acetone completely

168

evaporated. SEM micrographs of carbon particles were taken under 5.00 kV

169

voltage and 6.66 pA current condition using an FEI Quanta 3D Dualbeam

170

microscope. The results are shown in Figure S2.

171

Powder X-ray Diffraction (XRD) Experiments. For one mixture, wax and

172

EC samples were put in 20 mL glass vials in a 10:1 mass proportion and mixed

173

with Vortex mixer in two minutes. XRD was performed on a Rigaku SmartLab

174

X-ray diffractometer with Cu Kα (1.54 Å) radiation. All samples were analyzed

175

from 1.2° to 30° (2θ) with a step size of 0.01° and scan rate of 1°min−1. 0-D

176

detector and a K-beta filter were used. The voltage is 40 kV and the amperage

177

is 44 mA.

178

Fourier Transform Infrared Spectroscopy (FTIR) Experiments. The

179

mixtures of wax and EC samples with a 30:1 mass proportion were dispersed

180

in hexane and bath-sonicated for 2 min. The sample solutions were deposited

181

on KBr pellets. The samples were tested after hexane completely evaporated.

182

Infrared spectra were recorded with a Perkin-Elmer Spectrum One FT-IR

183

Spectrometer at a resolution of 2 cm-1 and with a cumulated time of 10 and a

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scan range of 4000-450 cm-1.

185

Raman Spectroscopy Experiments. The sample solutions above were

186

deposited on glass slices. The samples were tested after hexane completely

187

evaporated. Raman spectra were obtained using a Renishaw Raman

188

microscope with a 632.8 nm, 1.4 mW Helium-Neon laser and a ×20 objective.

189

Spectra were collected for 20 s and 10 times over the range of 200-4000 cm-1.

190

The 30:1 ratio is a suitable proportion because excess EC can result in a

191

decrease or disappearance of some wax bands of FTIR and Raman spectra.

192

In general, heterogeneity of the samples exists, particularly apparent in the 7



193

Raman measurements. In this study, each spectrum was measured multiple

194

times to ensure reproducibility of the results.

195 196

■ RESULTS AND DISCUSSION

197

Powder X-ray Diffraction (XRD).

198

XRD patterns of C36H74, C44H88O2 and their mixtures with four types of EC

199

particles show two sets of diffraction peaks, ‘long spacing’ and ‘short spacing’

200

(Figure 1). C36H74 displays a series of strong high order (00l) ‘long spacing’

201

peaks, originating from the region of lower scattering density in the gap

202

between the molecular layers. C44H88O2 has a low intensity and interval

203

extinction of (00l) peaks, due to a destructive interference caused by oxygen

204

atoms37. For a known aliphatic compound, the (001)- or (002)-spacing can be

205

used to determine the tilt angle of chains and the crystal structure. The

206

intensity of the ‘long spacing’ peaks is sensitive to the order of crystal

207

structure42. The ‘disorder’ of wax crystal structure is embodied in the degree of

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disordering of molecular chain packing as well as the number of nonplanar

209

conformers, which is the end-gauche conformer caused by EC particles based

210

on the FTIR and Raman results39. Two strong ‘short spacing’ peaks at lattice

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spacings of 4.13 Å originating from the (010) reflection and of 3.7 Å from the

212

(200) reflection are characteristic of both monoclinic and orthorhombic

213

structures.

214

C36H74 can exist as orthorhombic structures (Oǀ, Oǁ) or monoclinic

215

structures (M011, M201)43. Oǀ has the chains almost perpendicular to packing

216

planes. Oǁ contains two M011 layers, one rotating alternately through 180°

217

about an axis normal to the ab plane for another. M011 and M201 have the

218

chains tilted to the packing plane and the angles α and γ are 90°. The Miller

219

indices refer to the subcell lattice plane parallel to the plane in which the end

220

groups are found. The methylene packing is nearly the same for both O and M

221

and differs in the relative displacement of adjacent chains in the direction along

222

the chain axes, none for O and two C-C units for M. Their relative stability is 8


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determined largely by differences in the end group packing. In this study,

224

C36H74 has M011 structure with tilted chains, based on its (001)-spacing of

225

approximately 43 Å, close to 42.3 Å of M011, and its FTIR spectra43.

226

The full width at half maximum (FWHM) of a XRD peak characterizes the

227

grain size with an inverse relationship. The mixtures of CB-C36 and G-C36 show

228

broader FWHM at d-values of 4.13 Å and 3.76 Å than pure C36H74, which

229

indicates that the grain size of C36H74 decreased, that is, the order of C36H74

230

chain packing was partly broken by CB or G particles. The d-values of the

231

(001)-reflection of CB-C36 and G-C36 are the same as that of C36H74, but the

232

peak intensities substantially decrease due to CB or G present. The significant

233

decrease of the intensity of ‘long spacing’ peaks is attributed to the packing

234

disorder and end-gauche defects of C36H74 chain due to CB or G present.

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The mixtures of RGO-C36 and GO-C36 show that the thickness of the

236

molecular layer is near 47.61 Å of Oǀ structure44. The sharper peaks in the

237

whole pattern indicate that RGO and GO result in more ordered chain packing

238

of C36H74. The reflection intensity for the ‘long spacing’ of RGO-C36 barely

239

changed, except that the intensity of the (002) peak sharply increased. For

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GO-C36, the reflection intensity of the ‘long spacing’ peaks decreased except

241

for the (002) peak. The end-gauche defects could cause the surface voids in

242

the packing planes of C36H74 with Oǀ structure, leading to the increase in the

243

intensity of (002)-reflection peaks42.

244

The esters assume a form with vertical chains (A form) or one with tilted

245

chains (B form, β form for ethyl esters). For C44H88O2 studied in this work, two

246

strong peaks at lattice spacings of 4.13 Å from the (010) reflection and 3.7 Å

247

from (200) reflection as well as the (002)-spacing of 52.3 Å indicate that it

248

exists in the β form with approximately 62.5° angle of tilt45. The peaks of

249

d-values of 4.6 Å and 3.8 Å are the characteristics peaks from (010) and (100)

250

reflections of triclinic structure (β' form)44, 45. Therefore, C44H88O2 consists of a

251

mixture of two crystal structures, β and β'46. In comparison with C44H88O2, the 9



252

mixtures of CB-C44 and G-C44 show broader FWHM at the d-values of 4.13 Å

253

and 3.7 Å and the same ‘long spacing’ with decreased intensity, identical to

254

CB-C36 and G-C36, which indicates that the crystal structure of C44H88O2 was

255

also partly broken due to CB or G particles present.

256

The mixtures of RGO-C44 and GO-C44 display sharper peaks and larger

257

‘long spacing’ corresponding to 69.2° tilted angle. Also, the results suggest that

258

GO-C44 has the highest order of molecular arrangement, even better than pure

259

C44H88O2. RGO and GO lead to more ordered chain packing of

260

C36H74/C44H88O2, which is consistent with a previous report that graphene

261

leads to a more ordered lipid monolayer47. However, they also results in the

262

end-gauche conformation of the wax molecular chains. Unlike n-alkanes, the

263

phases of esters are more stable due to the intermolecular O…H hydrogen

264

bond, thus their ‘long spacing’ is lower sensitive to impurities45. This may be

265

the reason that RGO and GO only cause C44H88O2 to have a larger tilt angle of

266

chains rather than in the A form with vertical chains.

267

The above results suggest that EC causes changes in the crystal structure

268

of C36H74/C44H88O2 when EC particles are in contact and interact with the

269

methyl groups on the surface of wax crystals, which largely determines the

270

stability of the wax molecular chains. CB and graphite result in a decrease in

271

crystallinity of C36H74/C44H88O2 by means of packing disorder and end-gauche

272

defects of the wax molecular chains, whereas RGO and GO make the chain

273

packing of C36H74/C44H88O2 more ordered but cause end-gauche defects of the

274

wax molecular chains. The gauche defects can shift the transition lines in the

275

phase diagram, i.e. facilitating phase transition48. Therefore, EC particles are

276

expected to decrease the stability of wax crystals.

277

In reality, BC, accounting for the overwhelming majority of EC, could cause

278

an increase in temperature of epicuticular wax on which it settles due to its

279

strong solar light absorption ability, thereby reinforcing its destructive effect on

280

the crystal structure of epicuticular wax. Hence, BC particles may be one of the 10


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reasons behind the natural erosion of epicuticular wax crystals. Because of the

282

expansion of the amorphous regions, only through which water molecules can

283

pass, plants will lose more water and are more easily subjected to drought

284

stress. However, the impact of RGO and GO on the epicuticular wax in a real

285

environment needs to be further studied.

11



0

5

10

15

20

25

30

5

10

2θ

286 287 288 289 290 291 292 293

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20

25

30

2θ

Figure 1. Powder X-ray diffraction (XRD) patterns of C36H74 (C36), C44H88O2 (C44) and their mixtures with carbon black (CB), graphite (G), reduced graphene oxide (RGO) and graphene oxide (GO). Red curves are the XRD patterns of elemental carbon particles. For C36, the d-values of 4.13 Å and 3.75 Å and the ‘long spacing’ values indicate monoclinic structure (M011) or orthorhombic structure (Oǀ). For C44, the d-values of 4.13 Å and 3.70 Å and the (002)-spacing indicate β form, while the peaks of d-values of 4.6 Å and 3.8 Å indicate triclinic structure (β' form). The theoretical (002)-spacing of C44H88O2 in the A form is 59.03 Å45. 12


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294 295

Fourier Transform Infrared Spectra (FTIR).

296

FTIR spectra of C36H74, C44H88O2 and their mixtures with four types of EC

297

particles are shown in Figure 2, with the spectra of pure EC samples presented

298

in Figure S3. The four types of EC particles have strong infrared absorption in

299

the whole measurement range, which results in the up-shifted baselines for

300

their mixtures with waxes (Figure S3, Figures 2a/b). Detailed assignments of

301

the main bands of C36H74 and C44H88O2 are given in Table S1.

302

In Figures 2a/b, all the peaks of wax molecules in the mixtures are

303

broadened compared to that of the corresponding pure wax. Also, relative to

304

the intensity of the strongest band, νas(CH2), the CH3 vibrations of both C36H74

305

and C44H88O2 in the mixtures are distinctly enhanced in intensity, including

306

νas(CH3), νs(CH3) and δsciss(CH3) (red labels), compared to the pure wax

307

samples. For C44H88O2, the bands in the γwag(CH2) and ν(C-C) regions are

308

obviously enhanced (Figure 2c). And, the bands at 1343 cm-1, 1180 cm-1, 952

309

cm-1, 904 cm-1 and 884 cm-1 are enhanced and the new bands at 1136 cm-1

310

and 986 cm-1 appeared as compared to the pure sample. These are

311

characteristic bands of end-gauche conformer49, 50. The enhancement can be

312

attributed to surface enhanced infrared absorption (SEIRA)51, which,

313

interestingly, does not happen for the mixtures with C36H74, as explained in

314

more detail in Supporting Information.

315

The enlarged absorption bands of C-H stretching vibrations and CH2

316

rocking vibrations of C36H74 /C44H88O2 and their mixtures with CB are shown in

317

Figure 2d. The band near 2920 cm-1 arises from the asymmetric stretching

318

vibration of the CH2 groups, νas(CH2), which has significant overlap with

319

vibration of the CH3 groups. The band near 2850 cm-1 arises predominantly

320

from the symmetric stretching vibration of the CH2 groups, νs(CH2)38. The

321

bands of C-H stretching vibrations in the mixtures become broader and shifted

322

noticeably. In contrast, there is no peak shift in other vibrational modes

323

including the CH2 rocking vibrations. 13



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324

The area ratio of the peaks at 720 cm-1 and 730 cm-1 is used as a measure

325

for aliphatic crystallinity38. The 720 cm-1 band is nearly pure CH2 rocking, which

326

is less dependent on temperature, while the 730 cm-1 band is more CH2

327

twisting in character that is dependent on temperature. It can be seen from

328

Table S2 that the crystallinity of the mixtures decreased, which indicated that

329

the packing order of wax molecules was reduced to some extent by EC

330

particles. Moreover, according to the ratio, the crystallinity of C36H74/EC

331

mixtures decreased more than that of the C44H88O2/EC mixtures. Among the

332

EC particles, CB and G have a more destructive effect on the packing order of

333

wax chains than RGO and GO. The trend of variation in crystallinity from FTIR

334

is consistent with that from XRD. b

c

d

Absorbance (a.u.)

a

3000

335

2900 2800 Wavenumber (cm-1)

700

Figure 2. FTIR spectra of C36H74 (C36), C44H88O2 (C44) and their mixtures with carbon 14


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336 337 338 339 340 341

black (CB), reduced graphene oxide (RGO), graphene oxide (GO) and graphite (G). a, FTIR spectra of C36 and its mixtures with four elemental carbon (EC) particles. b, FTIR spectra of C44 and its mixtures with four EC particles. The characteristic bands of C36 and C44 were labeled in a, and corresponding vibrations were assigned in b. The bands of methyl vibrations labeled in red distinctly enhanced in a and b. c, FTIR spectra of C44 and its mixtures with carbon particles in the conformationally sensitive region. The bands at

342 343 344 345 346 347 348 349

1343 cm-1 [γwag (CH2)], 1180 cm-1 [δrock(CH3), δrock(CH2)], 1136 cm-1 [δrock(CH3), ν(C-C), δrock(CH2), δsciss(C-C)], 986 cm-1 [δrock(CH3), ν(C-C), γwag (CH2), γtwist(CH2)], 952 cm-1 [δrock(CH3), ν(C-C)], 904 cm-1 [δrock(CH3), ν(C-C)] and 884 cm-1 [δrock(CH3), ν(C-C)] result from end-gauche conformer. Arrows and shades indicate the enhanced and new bands in the spectra of the mixtures. The vertical lines are added to aid visualizations of the spectral peaks. d, The enlarged detail absorption bands of C-H stretching vibrations and CH2 rocking vibrations of C36H74 or C44H88O2 and their mixtures with CB. Their intensities are normalized between waxes and mixtures and between the two ranges in the same

350 351

figure. ν, stretching; δ, in-plane bending; γ, out-of-plane bending; as, asymmetric; s, symmetric.

352 353

Raman Spectra.

354

The Raman spectra of C36H74/C44H88O2 as well as their mixtures with four

355

EC samples are shown in Figure 3. The assignments of the major bands of

356

C36H74 and C44H88O2 are given in Table S1. The G peak (~1590 cm-1) is often

357

assigned to ‘in plane’ displacement of the carbon atoms strongly coupled in the

358

hexagonal sheets52. A broad band around 1500-1550 cm-1 is associated with

359

amorphous sp2-bonded forms of carbon. The D peak (~1330 cm-1) and D′ peak

360

(~1620 cm-1) are attributed to interstitial defects. D′ peak is a shoulder peak of

361

D and is sometimes hard to recognize. The G′ peak (~2660 cm-1) corresponds

362

to double inelastic scattering of the in-plane transverse optical phonons near

363

the K point53. The G′ peaks of the mixtures are hardly determined accurately

364

because both C36H74 and C44H88O2 have a broad weak peak at the same

365

position.

366

In Figure 3a, the G, D, D′ and G′ peaks of CB in its mixtures with C36H74 or

367

C44H88O2 almost all blue shift about 6 cm-1 relative to the peaks of pure CB.

368

Similarly, the mixtures of G, RGO and GO with C36H74 or C44H88O2 have a 3-9

369

cm-1 blue shift for the G, D and D′ peaks relative to the peaks of EC particles

370

(Figure 3b). The intensity ratio of ID/IG is often used as a measure of defect 15



371

density. The ID/IG ratio of the mixtures is all larger than that of the

372

corresponding pristine EC particles.

373

The Raman bands of C36H74 or C44H88O2 and their mixtures with CB in the

374

1000-1500 cm-1 and C-H stretching regions with enlarged scale are shown in

375

Figure 3c. The bands of CB-C36 and CB-C44 have a 2-3 cm-1 red-shift in the

376

1000-1500 cm-1 region compared to pure wax samples, but the corresponding

377

C-H stretching bands have no shift. The bands of ν(C-C) at 1063 cm-1 and

378

1133 cm-1 are attributed to the all-trans chain. The decrease of their relative

379

intensity in CB-C36 and CB-C44 indicates that the number of trans bonds

380

decreased and that of gauche bonds increased in the mixtures54. The line

381

shape in the C-H stretching region can be attributed to Fermi resonance (FR)

382

interaction between the fundamental C-H stretching mode and a high density

383

of overtones of the CH2 scissoring fundamentals. The line shape of the CH2

384

symmetric stretching fundamental, νs(CH2), consists of three components: a

385

narrow band near 2850 cm-1 conventionally assigned to νs(CH2) with only

386

about 40% of the total intensity, a broad band peaked at 2898 cm-1 and a

387

shoulder band near 2930 cm-1 55. In Figure 3c, the bands at 2897 cm-1 for

388

CB-C36 and at 2898 cm-1 for CB-C44 are evidently enhanced, while the νs(CH2)

389

at 2847 cm-1, νs(CH2)/νs(CH3) bands at 2929 cm-1 and νas(CH3) band at 2956

390

cm-1 all show increased intensity and broadening.

16


Page 16 of 24

Page 17 of 24


a

b

Samples

D band cm-1

G band cm-1

D′ band cm-1

G′ band cm-1

ID/IG

CB

1326

1587

1609

2645

1.6

CB-C36

1332

1593

1615

2.0

CB-C44

1332

1593

1615

2.4

1616

G

1331

1578

G-C36

1335

1582

2670

3.7 6.5

G-C44

1334

1581

7.5

RGO

1330

1594

2.1

RGO-C36

1335

1599

2.8

RGO-C44

1337

1601

2.7

GO

1328

1574

1601

GO-C36

1333

1582

1606

2652

2.6 2.8

GO-C44

1334

1583

1607

3.1

Intensity (a.u.)

c

1000

1100

1200

1300

1400

1500

2900

3000

Raman shift (cm-1)

391 392 393 394 395 396 397 398 399 400 401 402

Figure 3. Raman spectra of carbon black (CB), graphite (G), reduced graphene oxide (RGO), graphene oxide (GO), C36H74 (C36), C44H88O2 (C44) and their mixtures with CB with 632.8 nm excitation. a, The spectra in the 800-3000 cm-1 region. b, The band positions of CB, G, RGO, GO and their mixtures with C36 and C44, and the area ratio of ID/IG. c, The detail bands of C36/C44 and their mixtures with CB in the 1000-1500 cm-1 and the C-H stretching regions with enlarged scales. Their intensities are normalized between waxes and mixtures and between the two ranges in the same figure, in order to quantitatively compare the shifts and intensity. CB resulted in a 2-3 cm-1 red-shift of bands of both C36H74 and C44H88O2 in the 1000-1500 cm-1 region. ν, stretching; δ, in-plane bending; γ, out-of-plane bending; as, asymmetric; s, symmetric; FT, Fermi resonance.

17



403

Interaction between EC and Wax from FTIR and Raman Spectra.

404 405 406

Figure 4. Schematic illustration of the mechanism of the interaction between EC and epicuticular wax.

407

The results of FTIR and Raman indicate that the EC particles and the wax

408

molecules can form C-H…π type hydrogen bonding with charge transfer from

409

carbon to wax as illustrated in Figure 456, 57. For EC particles in the mixtures,

410

the blue-shift of all the characteristic Raman peaks G, D, D′ and G′ indicates

411

hole doping (p-type), while the increased ID/IG ratio implies increased defects,

412

which is consistent with EC particles as an electron-donor58. For waxes in the

413

mixtures, upon formation of hydrogen bonding between the CH3 with the EC

414

particles as an electron-acceptor, vibrations associated with the CH3 become

415

stronger with broadened and shifted bands in the FTIR and Raman spectra.

416

Besides the CH3 vibrations, vibrations associated with CH2 and C-C also

417

showed obvious shift in peak positions. This suggests that the formation of

418

hydrogen bonding causes the electron density of the entire wax molecules to

419

redistribute. The shifts differ between the FTIR and Raman bands possibly due

420

to their different selections rules. Since the degree of band shift correlates with

421

the strength of hydrogen bonding, the results suggest that C44H88O2 has

422

stronger hydrogen bonding with the EC particles than C36H74. 18


Page 18 of 24

Page 19 of 24


423

In addition to hydrogen bonding, another, perhaps even more dominant

424

attractive force between the wax and EC particles is the London dispersion

425

(LD), an attractive interaction resulting from instantaneous dipoles, which is

426

the major attractive contribution in nonpolar or weakly polar molecules (as

427

illustrated in Figure 4)59. The long chain wax molecules studied here are

428

nonpolar or without permanent dipole moment, but are expected to have large

429

instantaneous dipole moments due to the large number of atoms and electrons

430

per molecule. Likewise, EC particles have delocalized π-orbital electrons, and

431

have large instantaneous dipole moments or large polarizability as well. The

432

large size of the EC particles also leads to large LD as larger sized species are

433

generally more polarizable. Moreover, since LD has a R-6 dependence and if

434

hydrogen bonding brings the wax and EC particles close, strong attraction is

435

expected between them due to the cooperativity of hydrogen bonding and

436

LD60.

437 438

Environmental Implications.

439

Under natural condition, leaf surface, covered by a continuous thin film of

440

epicuticular wax, superimposed ultrastructures27, adsorbs various PM, gas and

441

even water. However, there still exists bare thin film of epicuticular wax with the

442

interface composed of methyl groups exposed to air like the model compounds

443

used in this study, especial during leaf expansion. EC particles in the

444

atmosphere are mostly BC particles with complex surface chemical

445

compositions and coating types classified into embedded (heavily coated),

446

partly coated, inclusions and bare61. BC particles with the last three coating

447

types contain the same graphite-like surface as EC particles studied in this

448

work. Therefore, in reality, small BC particles can get in contact with the bare

449

thin film of epicuticular wax through their graphite-like surface. With the

450

cooperativity of LD and hydrogen bonding, once settled on the leaf surface,

451

especially

without

wax

ultrastructures,

BC

19


with

extremely

large


452

surface-to-volume ratio will likely stick and stay. Moreover, soot from vehicle

453

emission is similar to CB in size and structure, especially at the nascent state

454

of formation, thus it can effectively accumulate on leaf surface of plants on the

455

sides of city streets. In addition, the mechanistic insight gained from the study

456

may be generally applicable to carbon species with π-conjugated system, such

457

as PAHs, brown carbon containing humic-like structure and volatile and

458

semi-volatile organic compounds and leaf surface. Further research is needed

459

to determine their adsorption on leaf surface.

460

Both PM and epicuticular wax have an extremely complex chemical

461

compositions and structures and undergo a series of physical and chemical

462

changes over time in a real environment. There exist several molecular

463

mechanisms on their interaction accounting for the adsorption of PM on leaf

464

surface. The mechanism based on our findings is only one of them.

465

Nevertheless, this study serves as an important starting point for a full

466

understanding of the adsorption of PM on leaf surface and paves the way for

467

further research in the future.

468 469

■ ASSOCIATED CONTENT

470

S ○ Supporting Information

471

The Supporting Information is available free of charge on the ACS Publications

472

website at DOI: .

473

Additional figures, tables, and information as noted in the text. (PDF)

474 475

■ AUTHOR INFORMATION

476

Corresponding Authors

477

* E-mail: [email protected]

478

* E-mail: [email protected]

479

ORCID

480

Lei Wang: 0000-0003-3276-8973 20


Page 20 of 24

Page 21 of 24


481

Huili Gong: 0000-0001-9829-1608

482

Nian Peng: 0000-0002-2847-9928

483

Jin Z. Zhang: 0000-0003-3437-912X

484

Notes

485

The authors declare no competing financial interest.

486 487

■ ACKNOWLEDGEMENTS

488

This research is funded by the National Natural Science Foundation of China

489

(NSFC, No. 41571457, 41201488) and the Beijing Natural Science Foundation

490

(BNSF, No. 8133051). JZZ is grateful for Delta Dental Health Associates for

491

financial Support. We acknowledge Dr. Jesse Hauser for X-ray spectroscopic

492

data collection and Dr. Tom Yuzvinsky for SEM image acquisition.

493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517

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