Tsvaygboym PhD Thesis 2007 - BW

RICE UNIVERSITY

Photochemical Studies of

Single-Walled Carbon Nanotube Ozonides and -Azoxy Ketones

by

Konstantin Tsvaygboym

A THESIS SUBMITTED
IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE

Doctor of Philosophy
APPROVED, THESIS COMMITTEE:

Paul S. Engel,
Professor of Chemistry

W. Edward Billups,
Professor of Chemistry

Michael R. Diehl,
Assistant Professor of Bioengineering

HOUSTON, TEXAS
APRIL 2007

ABSTRACT

Photochemical Studies of

Single-Walled Carbon Nanotube Ozonides and -Azoxy Ketones

by

Konstantin Tsvaygboym

This thesis contributes to two disparate problems in chemistry: studying properties of

carbon nanotube ozonides and products of their decomposition and determining behavior

of -azoxy radicals.

This work demonstrates that interaction of ozone with single-walled carbon

nanotubes (SWNT) results in formation of 1,2,3-trioxolanes (SWNTO3). Their formation

rate was found to be on the order of subseconds at room temperature for diluted SWNT -

1% aqueous SDS suspensions. SWNTO3 decayed to SWNT epoxides (SWNTO) with

release of molecular oxygen. Gas evolution measurements performed on dry ozonated

SWNT showed oxygen release to follow a simple exponential rise with rates

approximately 1.5 – 2 min-1 at r. t. The lifetime of SWNTO3, with a dissociation

activation energy of approximately 0.7 eV, depends on temperature and SWNT type. At

room temperature, it is less than two minutes for small-diameter SWNTs suspended in

water. Ozonides exhibited extreme quenching of SWNT fluorescence and substantial

bleaching of NIR absorption. The maximum number of 1,2,3-trioxolanes forming on the

surface of SWNT at any given time was found to be less than 4% of the theoretical value,

indicating a saturation point. Reaction of ozonated nanotubes with excess ozone is

limited by the SWNTO3 decomposition rate. Thinner tubes exhibited faster ozonide

decay rates resulting in greater oxidation levels over time in excess of ozone. Ozonation

with small quantities of ozone did not result in a D-band increase in the Raman spectra,

both for solid and liquid state experiments, though substantial decrease of the G band was

observed. IR absorbance kinetics of SWNT films revealed exponential intensity drift over

time with rates close to those in fluorescence and NIR absorbance techniques. Ozonated

SWNTs were found to abstract electrons from amines and thiols, thus resulting in

covalent attachment of nucleophiles to the sidewall.

The azoxy functional group greatly stabilizes an attached carbon-centered radical,

but the chemistry of such -azoxy radicals is unclear. This work reports that generation

of -azoxy radicals by irradiation of -azoxy ketones PhCO-C(Me)2-N=N(O)-R causes

ketone rearrangement to azoester compounds PhCOO-C(Me)2-N=N-R. This study

proposes a mechanism for this rearrangement.

Acknowledgments

I am grateful to my advisor, Prof. Paul S. Engel for allowing me to work on an

exciting, cutting edge project revolving around carbon nanotube ozonides. I have been

honored to work with a number of faculty, post docs, graduate and undergraduate

students, who immensely deepened my understanding of scientific principles and fostered

my teaching skills. There is no doubt some of them will become leading figures in

science, technology and business.

I would like to thank friends and relatives who were very supportive throughout my

graduate studies. Your help and advice are much appreciated.

Table of Contents

Volume I

Title Page i

Abstract iii

Acknowledgments v

Table of Contents vi

List of Symbols and Abbreviations ix

Part I

Chapter 1. Spectral and physical characteristics of reference SWNT samples 2

Introduction 3

References and Notes 12

Chapter 2. Carbon nanotube ozonides: formation rates, oxygen evolution,

decomposition rates and activation energies, determination of

saturation limits and a comparison of spectral changes in

fluorescence and UV-Vis-NIR absorption 13

Introduction, Results and Conclusions 14

Experimental Part 82


Chapter 3. Influence of SWNT ozonation on D and G bands in Raman spectra 91


vii



Chapter 4. IR studies of SWNT ozonides and of products of their reactions

with different classes of compounds 109




Chapter 5. Reaction of ozonated SWNT with electron rich nucleophiles

(amines, thiols and other) 139




Chapter 6. Trapping reactive centers on SWNTOn with electron rich

nucleophiles (amines, thiols) 177




Chapter 7. Reactions between ozonated SWNT and different classes of

compounds studied by X-ray photoelectron spectroscopy 183




viii

Part II

Chapter 1. Photorearrangement of -Azoxy Ketones and Triplet Sensitization

of Azoxy Compounds 203




Volume II

Appendix A Mathematics for regression analysis of fluorescence and NIR

absorbance data 235

Appendix B Supporting Information for Part I, Chapter 5. 1H NMR spectrum 251

Appendix C Supporting Information for Part I, Chapter 7. XPS spectra for

reactions of ozonated SWNT with different classes of compounds 253

Appendix D Supporting Information for Part II, Chapter 1. Calculated isotropic

Fermi contact couplings, computed structures, ESR, UV and NMR

spectra 323

ix

List of Symbols and Abbreviations

a.u. absorbance units

abs absorbance

ATR FT-IR attenuated total reflectance Fourier transform infrared

C60 fullerene C60

ca. Latin word for approximately

DTT dithiothreitol

ESCA electron spectroscopy for chemical analysis

em emission

ex excitation

HipCo high pressure carbon monoxide method

HOMO highest occupied molecular orbital

Imax maximum intensity

I/Imax normalized value(s)

Imax/I quenching factor, a degree of quenching, inverted normalized value(s)

absmax local absorption maximum (spectral)

emmax local emission maximum (spectral)

LUMO lowest unoccupied molecular orbital

NIR near IR

(n,m) carbon nanotube indices

O3 ozone

PM3 parametric method No. 3

x

PTFE polytetrafluoroethylene

r2 coefficient of determination, same as correlation coefficient

RBM radial breathing mode

SDS sodium dodecyl sulfate

SDBS sodium dodecyl benzyl sulfonate

SWNT single-walled nanotube

SWNTO3 product(s) of ozonation of single-walled carbon nanotube

 lifetime

TMPD N,N,N’,N’-tetramethyl-p-phenylenediamine

uL microliter(s)

Wurster reagent N,N,N’,N’-tetramethyl-p-phenylenediamine (same as TMPD)

XPS X-ray photoelectron spectroscopy (same as ESCA)

Chapter 1

Spectral and physical characteristics of reference SWNT samples

3

1.1. Introduction

Single walled carbon nanotubes (SWNTs), a graphene sheet rolled up into a tubular

shape, may turn out to be a promising material for electronics, field emission, heat

transfer, sensing, material reinforcement, imaging, medicinal and other applications.1-3

Research in the area of carbon nanotubes increased significantly in the last several years

and is highly competitive, partly due to possible commercialization of their unique

properties. This chapter provides a brief introduction to key aspects of the spectroscopic

measurements of single walled carbon nanotubes (SWNT) discussed throughout this

thesis. Spectroscopic changes of SWNT after functionalization may not have the same

behavior as would be expected for a small molecule. An interesting example of this can

be found in Chapter 4 discussing IR absorption changes of SWNT over time after

ozonation. Chapter 1 contains an interconversion table of wavelengths and wavenumbers

that will be of use in Chapter 3, describing the Raman measurements performed on

aqueous SWNT suspensions as well as for discussion of IR results. Also, SWNT

fluorescence spectra obtained with different excitation sources are shown deconvoluted.

A brief table summarizes how much each tube contributes to the observed fluorescence

intensity. Other aspects like nomenclature and 3D structure are discussed as well. The

following section provides UV and NIR absorption spectra and talks about work with

different batches from the HipCo reactor (Rice University).

1.2. Near-IR fluorescence spectra

Two lasers, 660 nm and 785 nm were used for excitation of single-walled carbon

nanotubes (SWNT), the former one utilized for the majority of the spectra presented.

Wavenumber and wavelength scales are used interchangeably in this work. Table 1

4

shows the relation of the two scales. Specific Raman shifts from the 669.9 nm excitation

source are also included.

Table 1. Interconversion of wavelengths and wavenumbers for Visible, NIR and IR
regions. Raman shifts from 669.9 nm excitation source are provided.
Range , nm , cm-1 Shift, cm-1
Visible 669.9 14928 0
700.0 14286
733.5 13633 1294 (D)
749.5 13342 1585 (G)
785.0 12739
811.0 12330 2597 (G’)
830.0 12048
NIR 900.0 11111
1000 10000
1100 9091
1200 8333
1300 7692
1400 7143
1429 7000
IR 1500 6667
1600 6250
2000 5000
2500 4000
3333 3000
5000 2000
8333 1200
9091 1100
10000 1000
11111 900
12500 800

Aqueous SWNT-SDS suspensions are known to fluoresce when excited with suitable

lasers. Spectra obtained after excitation with 660 and 785 nm lasers are shown in Figure

1. The spectra were deconvoluted and peaks of interest assigned (n,m) numbers according

to published data.4

5

7,6
Normalized Fluorescence 1.00
8,3 ex = 660 nm
0.75
7,5

0.50 10,2 9,5 10,3
8,7
0.25 11,1

0.00

0.50
10,5
Normalized Fluorescence

ex = 785 nm 11,3

8,7
9,7
0.25
7,6
6,5 10,2
8,3 7,5

0.00

10500 9500 8500 7500
-1
Optical frequency (cm )
Figure 1. Fluorescence of aqueous SWNT-SDS suspensions. Tubes of interest are
marked with (n,m) numbers. The same (n,m) tube is shown with the same color and
symbol on both graphs. Top: excited with 660 nm. Bottom: excited with 785 nm laser.
Fluorescence changes in spectra obtained with em 660 nm were examined at four

distinct wavelengths: 954, 1027, 1125 and 1250 nm. The major contributors to

fluorescence intensity at each wavelength are summarized in Table 2.

6

Table 2. Major contributors to fluorescence intensity at four distinct wavelengths.*

em, nm , cm-1 (n,m) type of tube diameter, % of total
major nm emission at em
contributors
955.6 10465 8,3 0.782 95.4
6,5 0.757 1.9
1027.6 9731 7,5 0.829 85.0
10,2 0.884 5.3
8,1 0.678 4.3
1124.6 8892 7,6 0.895 78.9
8,4 0.840 8.3
9,2 0.806 3.7
9,4 0.916 3.4
1250.1 8000 9,5 0.976 39.8
10,3 0.936 30.3
11,1 0.916 12.0
8,7 1.032 6.2
10,5 1.050 3.8
8,6 0.966 3.0
* max
Excitation source ex 660 nm.
Minor contributors were excluded from Table 2 for clarity. Tube (8,3) contributed

95% of peak intensity at 954 nm, as deduced from spectrum deconvolution.5

Analogously, 85 % of peak intensity at 1027 nm was from tube (7,5). Tube (7,6) gave

only 79% of peak intensity at 1125 nm. The peak at 1250 nm was from a combination of

tubes, none contributing more than 40 % of total intensity.

Assignment of numbers (n,m) for carbon nanotubes is summarized in Figure 2.

7

Basis vectors Chiral
angle Zigzag
0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0 13,0

1,1 2,1 3,1 4,1 5,1 6,1 7,1 8,1 m 9,1 3 10,1
= 11,1 12,1 13,1

2,2 3,2 4,2 5,2 6,2 7,2 8,2 Roll-up vector
9,2 10,2 11,2 12,2

3,3 4,3 5,3 6,3 7,3 8,3 9,3 10,3 11,3 12,3
n=8
4,4 5,4 6,4 7,4 8,4 9,4 10,4 11,4
Ar
m ch
air 5,5 6,5 7,5 8,5 9,5 10,5 11,5

6,6 7,6 8,6 9,6 10,6

7,7 8,7 9,7 10,7

8,8 9,8

Figure 2. Construction of a nanotube from a graphene sheet. Numbers n and m determine
the final position of a roll-up vector. Rolling sheet to superimpose hexagons (0,0) and
(8,3) will result in tube (8,3) with roll-up vector being perpendicular to tube direction.
Tubes of interest are emphasized with thick hexagons.
The physical structures of tubes of interest are shown in Figure 3.

(8,3) (7,5) (7,6) (9,5) (10,3)

Figure 3. Tubes (n,m) with the highest fluorescence intensity in HipCo samples for 661
nm excitation source. Each tube is shown in two projections (top and bottom).

8

It is important to note that there is no linear relationship between (n,m) tubes’

relative concentrations and their emission intensities for any given ex . This is because

SWNT fluorescence intensity is dependent on the wavelength of incident light. For

example, tubes (8,3), (7,5) and (7,6) with the highest emission intensity in the ex 660 nm

spectrum (Figure 1) are only a small fraction of a bulk sample (Figure 4).
zigzag
5,0 7,0 8,0 10,0 11,0 13,0 14,0 16,0

5,1 6,1 8,1 9,1 11,1 12,1 14,1 15,1

6,2 7,2 9,2 10,2 12,2 13,2 15,2

5,3 7,3 8,3 10,3 11,3 13,3 14,3

5,4 6,4 8,4 9,4 11,4 12,4 14,4

6,5 7,5 9,5 10,5 12,5 13,5

7,6 8,6 10,6 11,6 13,6

8,7 9,7 11,7 12,7

9,8 10,8 12,8
a rm
ch 10,9 11,9
air
11,10

Figure 4. Distribution of (n,m) species in HipCo SWNT sample calculated from emission
spectra with ex 660 and 785 nm.6 Thickness of a hexagon is linearly proportional to tube
abundance in the sample.
Relative abundances of tubes were estimated by recording two separate emission

spectra with ex 660 and 785 nm. The knowledge of (n,m) tube abundance is of great

importance for absorption studies where measurements are performed on a bulk sample.

For example, if the bulk sample has two types of species, A and B, which transform over

time, independently of each other, into species A’ and B’ with corresponding rates c and

9

d, an overall absorbance can be expressed with a first order equation

ct dt
Abs(t ) a e b e

where a and b are Arrhenius prefactors derived from tube abundances. Typical HipCo

SWNT samples are estimated to have over forty different semiconducting tubes and

about fifteen metallic tubes. This means that observed absorbance can be affected by as

many as fifty five different species in a sample. Knowing relative abundances of specific

(n,m) tubes may help interpret absorbance kinetics.

Since metallic tubes do not fluoresce, their number is only an estimate. Studies of

SWNT radial breathing modes (RBM) in Raman spectra served as a basic for the relation

of abundances of metallic and semiconducting tubes.

Discussion of the mathematics behind (n,m) tube relative abundance calculations,

based on fluorescence emission spectra, is beyond the scope of this work and is not

included.5

1.3. UV-Visible and Near-IR absorption spectra

UV-Vis absorption spectrum for SWNT (HipCo, batch 162.4, Rice University) is

provided below:

10

0.6
Absorbance (a.u.)

0.4

0.2

300 400 500 600 700
Wavelength (nm)
Figure 5. UV-Vis absorption spectrum of aqueous SWNT – SDS suspension.

Absorption peaks in the area 450-550 nm are commonly assigned to metallic tubes.

Peaks in the area 650-750 nm are commonly assigned to semiconducting tubes.

NIR absorption of SWNT is thought to be caused by a conjugated network of double

bonds. It is not clear if the conjugated acene system in SWNT can be considered truly

aromatic. Hückel molecular orbital (HMO) theory states planarity as one of the most

important prerequisites of aromaticity. Ozonation of SWNT sidewall results in significant

decrease of NIR absorption. NIR absorption spectrum of pristine SWNT is provided

below.

11

0.25
Absorbance (a.u.)

0.20

0.15

900 1000 1100 1200 1300
Wavelength (nm)
Figure 6. NIR absorption spectrum of aqueous SWNT-SDS suspension.
Note the difference in the vertical scale for the above two spectra

1.4. Other spectra

Other reference spectra of SWNT, like IR, solid and liquid Raman and ESCA will be

introduced throughout the text.

1.5. Properties of different batches of SWNT

Different batches of SWNT from the HipCo process (Rice University) were used in

this work. All batches had similar or identical spectroscopic properties. Batch 153.3 was

used for fluorescence studies of the reaction between 2-methoxyethylamine and ozonated

SWNT. Batches 162.4 and 162.8 were used for IR studies. Batch 161.1 was used for UV,

liquid Raman and fluorescence studies. The majority of SWNT samples in this work were

used as synthesized, without purification. Unless otherwise noted, tubes were pristine.

SWNT – 1 wt. % aq. SDS suspension was prepared by a standard procedure outlined in

12

the experimental part. SWNT bundles, carbonaceous matter, metal catalyst and other

impurities are thought to be removed from the final SWNT – SDS suspension. Unless

otherwise noted, all SWNT – SDS samples used in this work were prepared by the same

procedure. Typically a large stock of SWNT – SDS suspension was prepared and used

for a great number of experiments.

1.6. References and Notes

1. Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A., Carbon nanotubes - the route
toward applications. Science 2002, 297, (5582), 787-792.

2. Avouris, P., Molecular electronics with carbon nanotubes. Accounts of Chemical
Research 2002, 35, (12), 1026-1034.

3. Calvert, P., Nanotube composites - A recipe for strength. Nature 1999, 399, (6733),
210-211.

4. Weisman, R. B.; Bachilo, S. M., Dependence of optical transition energies on
structure for single-walled carbon nanotubes in aqueous suspension: An empirical
Kataura plot. Nano Letters 2003, 3, (9), 1235-1238.

5. Deconvolution performed with software package that accompanied NS1
NanoSpectralyzer (Applied NanoFluorescence LLC.).

6. Applied NanoFluorescence LLC http://www.appliednanofluorescence.com/.

13

Chapter 2

Carbon nanotube ozonides: formation rates, oxygen evolution,

decomposition rates and activation energies, determination of

saturation limits and a comparison of spectral changes in fluorescence

and UV-Vis-NIR absorption

14

2.1. Introduction

A number of publications have been dedicated specifically to ozonation of carbon

nanotubes. Recently, Chen1, 2 reported that 9 wt. % O3 in O2 bubbled through SWNT

suspension in perfluoropolyether (PFPE) at r. t. for periods ranging from 1 to 8 hours,

followed by a 30 minute purge with oxygen, resulted in SWNT shortening. Simmons et

al. 3 studied ozonation as a possible tool to selectively decrease conductivity of SWNT on

a microfabricated chip upon UV/ozone exposure. Samples exposed for one hour at r. t.

were shown to form characteristic carbonyl and ether bonds (XPS data), and SWNT

electrical resistance increased. The provided Raman spectra show D and G bands at

different times. After ten minutes of UV/ozone exposure, the G band decreased ca. five

times, but the D band did not change. The authors concluded that sidewall oxidation by

ozone and molecular oxygen resulted in - conjugated network disruption. Banerjee et

al.4-6 conducted a series of studies on ozonation of carbon nanotubes. The author noted

that Raman spectra of carbon nanotubes are strongly resonance enhanced, and as a result

signals from the functionalizing moieties are rarely seen in Raman spectra. 4, 7 In a

different study, SWNT sidewall was ozonated (ca. ~10% O3 in O2) in a methanolic

suspension (100 mg in 150 mL) at -78 C for one hour and reacted with “cleaving”

reagents (either sodium borohydride or dimethyl sulfide).5 The authors assumed

formation of ozonides, by an analogy with alkenes, pointing out that C60O3 has been

reported in the literature.8 The “cleaving” step was introduced to alter relative distribution

of products (ethers, carbonyls and esters). The authors concluded that SWNT ozonation

15

could be used as a nondestructive method of introducing oxygenated functionalities

directly onto the sidewall.

In another study6 Banerjee et al. demonstrated that after solution phase ozonolysis of

SWNT (ethanolic suspensions, 2 hours), Raman peaks corresponding to smaller diameter

tubes were relatively diminished in intensity when compared to the profile of larger

diameter tubes. The author found no chiral selectivity (i.e. dependence on tube “twist,”

Figure 3, Chapter 1) and concluded that tube curvature and -orbital misalignment are the

main reasons for the observed selectivity. A theoretical study providing activation

energies for a reaction of ozone with SWNT has been reported.9 Cai et al.10 reported

ozonation of SWNT and their assembly on top of oligo(phenylene ethynylene) self-

assembled monolayers. Oxidation produced oxygenated functional groups like carboxylic

acids, esters and quinone moieties. Depending on the degree of ozonation, the electrical

resistance was found 20 to 2000 times higher than that of pristine SWNT. Oxidation was

performed on a dry “bucky” paper with UV/O3 generator in ambient air for 25 minutes to

5 hours. Ozonated SWNT absorption in the IR region was shown to stop changing after 3

hours of ozonation. An IR peak at 1580 cm-1 was assigned to the stretching mode

(C=C) of double bonds in the nanotube backbone near functionalized carbon atoms.11

Ogrin et al.12 estimated an approximate molecular formula of SWNT ozonated for 3

hours to be C6O, i.e. every third double bond had an epoxide. None of the mentioned

publications focuses on SWNT ozonides kinetics.

A number of articles have been published on ozonation of fullerenes, a short analog

of SWNT, and their properties.13-19 Chibante and Heymann determined products of

16

ozonation of C60 in toluene solution included structures C60On, with n ranging from 1 to

6, and insoluble tan-colored precipitates.20 Bulgakov et al.21 found that epoxides C60On

(n = 1 – 6) are accumulated within the first three minutes of continuous ozonation.

Further ozone/oxygen mixture bubbling resulted in formation of ketone and ester

functional groups. Heymann et al.8 found that at 23 C ozonide C60O3 had a lifetime ca.

22 minutes in toluene, 330 minutes in a dry state and 770 min in octane.

Razumovskii et al.18, 19 reported ozonide formation rates for C60O3 (8.8 104 M-1s-1

at 0 C) and C70O3 (5 104 M-1s-1 at 22 C) in CCl4 solvent. The authors found that the

reactions obeyed a bimolecular rate law. The reactivity of C60 with ozone decreased ca.

90 times after the formation of C60O3. A similar tendency was found for C70, where the

formation of the first ozonide was 6 – 8 times faster than the subsequent ones. Fullerene

C70 was shown to uptake only 12 molecules of ozone within the first 16 minutes of

continuous O3/O2 gaseous mixture bubbling. The authors concluded that the formation of

the ozonide exerts an electronegative inductive effect on the adjacent network of

conjugated double bonds, similar to ozonation of divinylbenzene.22

Kinetics of SWNT ozonides have not been published to date. Among the reasons,

there are: different production methods resulting in different (n,m) types of SWNT in a

batch sample, a presence of a large number of different tubes in each SWNT sample,

poor solubility of SWNT in solvents, the need for efficient purification from the metal

catalyst, different purification techniques affect differently chemical and physical

properties of SWNT. Measuring kinetics on SWNT is a challenge. This chapter will

describe some interesting research findings discovered while attempting to study kinetics

17

of SWNT ozonides. Topics like deoxygenation of SWNT ozonides, NIR fluorescence

quenching degree, influence of high and low load of ozone on SWNTO3 decomposition

rates, proposed electronic transitions in SWNT and SWNTO3, decomposition rate

dependence on tube diameter, saturation limits in excess ozone, comparison of NIR

fluorescence and NIR absorption kinetics, establishing an average decomposition rate by

UV, structural changes and decomposition activation energies will be discussed.

2.2 Results and Discussion

A set of experiments was designed to measure the amount of oxygen evolving from

the surface of ozonated SWNT. No such study has been reported to date, even though a

number of articles on SWNT ozonation have been published.

Results of the experiment are summarized in Figure 1.

18

100

75
Pressure (mTorr)

50

25

0
100
1.52
A

NIR Absorbance (a.u.)
75 1.50
P22
Pressure (mTorr)

P21
1.48
50

P1 1.46
25 Pressure (P1)
Pressure (P21)
1.44
Pressure (P22)

0 NIR Abs (A)

0 2 4 6 8 10
Time (min)

Figure 1. (Top) Pressure change at r. t. due to oxygen release from 2 () and 4 mg (
and ) of ozonated dry SWNT-coated glass (upper three curves) and corresponding
system-leak references (lower two curves,  and ). (Bottom) NIR Absorbance
recovery of ozonated SWNT in solid form monitored at 1450 nm and r. t. (upper curve,
) and pressure change at r. t. due to oxygen release from 2 () and 4 mg ( and ) of
ozonated SWNT in solid state (lower three curves) after system leak correction. Curves
 and  were measured after the first and the second ozonation of the same sample
correspondingly.
Slurry of 2 or 4 mg of SWNT (as noted in Figure 1) in benzene (ca. 10 mL) was

added to the reaction vessel and was kept rotating until all the solvent was evaporated.

19

Such circular motion resulted in a thin SWNT film along the entire reaction vessel. A

vacuum line was degassed overnight, then the vessel was cooled to 5 C and 10 mL of

O3/O2 gaseous mixture (ca. 3 v/v % ozone23) was injected to the bottom of the cylinder,

the cap closed and the vessel was left at atmospheric pressure for one minute. The valve

on the vessel was opened to the vacuum system and the vessel was evacuated for 1.5 min,

after which the pump was cut off and data were acquired. Degassing for one and half

minutes was found sufficient to bring the vacuum in the entire system to below 1 mTorr.

Time t = 0 min in Figures 1 and 2 indicates the point when the pump was cut off from the

system.

100
1.52
A

NIR Absorbance (a.u.)
75 1.50
P22
Pressure (mTorr)

P21
1.48
50

P1 1.46
25 Pressure (P1)
Pressure (P21)
1.44
Pressure (P22)

0 NIR Abs (A)

0 2 4 6 8 10
Time (min)

Figure 2. Regression curves for NIR Absorbance at abs 1450 nm and for pressure
changes after SWNT ozonation in a dry state. Curves P1 () and P21 () correspond to
first ozonation of 2 and 4 mg of SWNT respectively. Curve P22 () was measured after
the sequential ozonation of 4 mg sample.
Cutting off the vacuum pump was followed by removing the ice bath and warming

the reaction vessel to r. t. with a water bath. Data points were collected until the observed

20

deoxygenation rate decreased to below the system leak rate value (ca. 0.5 mTorr/min).

The second sample (4 mg SWNT) was ozonated two times with approximately one hour

interval between oxidations.

The highest amount of oxygen evolved after gaseous ozonation of solid SWNT was

estimated as 0.72 umol within a 20 min time period at room temperature. This

corresponds to 0.2% of carbon atoms (or to 0.1% of double bonds) of SWNT (4 mg)

oxidized with ozone, assuming that all carbon soot was indeed SWNT or had a fullerene-

like structure. Weighing error of SWNT could bring an error into the calculated value. It

is possible that the number of double bonds reacted with O3 was higher, though it would

still be significantly less than the 3 – 4 %, estimated in UV studies at 260 nm. (NIR

absorbance estimation was at ca. 4 – 5 %). A possible explanation for such a low yield of

oxygen is SWNT bundling, which physically prevented large surface areas of SWNT

from reacting with gaseous ozone.

Ozonation of SWNT flakes resulted in their immediate burning. Ice bath cooling and

SWNT deposited along the glass wall of the reaction vessel were found necessary to

prevent this highly exothermic reaction from overheating.

NIR absorption was fitted with formula F1, while the pressure curves were fitted

with the 5-parameter two exponential rise formula F2 (formula selection discussed in

Appendix A):

1 y min
y bt
y min (F1)
bt n
y final ae ce

21

bt
bt n
y y0 a(1 e ) c(1 e ) (F2)

Regression results are summarized in Table 1 below.

Table 1. Regression results for pressure changes and for NIR Absorption at
*
em 1450 nm after SWNT ozonation.

Data Set Oxygen gas release NIR Absorption
Parameter F2, 4 mg (P21) F2, 4 mg (P22) F2, 2 mg (P1) F1
b, min-1 2.07 1.45 1.88 1.540
n 9.63 11.45 9 14.02
r2 0.9991 0.9987 0.9995 0.9995
ymin - - - 0.0000
*
Ozonation of SWNT film deposited on a glass surface. The formula number and SWNT
amount used for the experiment are written at the head of each column. Rates b are
expressed in [min-1]. Active constraints used in analysis were n > 9 (2 mg SWNT
pressure curve) and ymin > 0 (NIR absorption).
Points at time zero were excluded from regression because those were acquired at

5 C; all subsequent points were acquired at or near r. t. Constraints n > 9 and ymin > 0

were introduced to generate a better fit to the experimental data. Limiting n to greater

bt
n
than nine was needed to better describe the term ce , a “slow” component, for

pressure curve P1. Parameter n describes how many times the slow component is slower

than the fast one.

Approximately the same amount of ozone (O3/O2 gaseous mixture) was injected into

the reaction vessel in each experiment. The first time ozonation (P21) yielded a slightly

higher rate than the subsequent one (P22). All rates were comparable to those observed by

NIR fluorescence recovery, indicating that decomposition of a single ozonide is likely to

increase fluorescence intensity. This result means that the smallest section of SWNT

22

needed for a tube to fluorescence can be loaded with no more than one or two ozonides

on its surface, at least in an aqueous suspension.

Fluorescence studies demonstrated that 1,2,3-trioxolanes on the surface of SWNT

prevented the tube from emitting in the NIR region. If the “minimal” section of SWNT

needed for fluorescence carried several ozonides, all of them would have to decompose

before this section would gain its ability to fluoresce. If that were the case, then true

ozonide decay rates would be several times greater than those observed by fluorescence.

Observation of similar rates in vacuum deoxygenation of SWNTO3 and in fluorescence

techniques implies that decomposition of nearly every ozonide results in a fluorescence

increase.

It was found difficult to quench SWNT fluorescence completely. The highest

quenching degree (Imax/I) was less than 1000 times and tubes were shown to quickly

recover from that state. Quenching 1000 times means that 0.1% of previously emitting

“sections” of SWNT continued to fluoresce. Full fluorescence quenching was not

observed. A study was performed to investigate the fluorescence quenching degree

(Imax/I) as a function of the volume of injected O3/O2 gaseous mixture (ca. 3 v/v %

ozone). After excluding the most extreme points (i.e. the lowest intensity point after

ozonation), even with large amounts of ozone, such as 2 mL of O3/O2 gaseous mixture,

fluorescence could not be quenched more than 140 times (Figure 3).

23

Fluorescence Quenching (Imax/I)
100

80

60

40
954 nm
20 1027 nm
1125 nm
1251 nm
0

0.0 0.5 1.0 1.5 2.0
O3 / O2 mixture volume (mL)

Figure 3. Dependence of fluorescence quenching degree (Imax/I) on the amount of O3/O2
gaseous mixture (ca. 3 v/v % ozone) injected.
Figure 3 demonstrates that injection of 0.3 mL of O3/O2 gaseous mixture decreased
max
fluorescence intensity of tube (8,3) with em 954 nm approximately 6 times. In

percent values it means that only 17% of all emitting “sections” were contributing to

fluorescence. One would expect that increasing the ozone load by 20 % could nearly

completely extinguish fluorescence from the tube (8,3). Interestingly, injection of 0.5 mL

of O3/O2 mixture quenched fluorescence only 16 times, i.e. 6% of SWNT was still

emitting. Further increase of the ozone load to 1.0 mL quenched emission only 41 times,

with 2.4% of emitters still left to be quenched. To conclude, increasing ozone load from

0.3 mL to 1.0 mL, i.e. by 330%, could not extinguish the remaining 17% of emitting

sections of SWNT. This observation meant that tubes are getting oxidized with ozone in

bands and not randomly.

24

Changes in SWNT fluorescence after oxidation with ozone

1.0
Normalized Fluorescence Intensity

ex = 660 nm

0.8
Before O3
1 min
0.6 3 min
9 min

0.4

0.2

0.0
950 1050 1150 1250 1350
Wavelength (nm)
Figure 4. Addition of aqueous solution of ozone (50 uL, Abs (260 nm, 1 cm) = 1.25 a.u.)
to 0.5 mL SWNT-SDS aq. suspension. Used 660 nm laser for excitation. Fluorescence
emission quenching was followed by a slow recovery. Spectra recorded before, 1, 3 and 9
min after ozonation.
The overlaid spectra in Figure 4 show SWNT fluorescence change over time after

ozonation. The spectrum of pristine SWNT is provided for comparison.

SWNT oxidation was accomplished by an addition of a small volume of water

saturated with ozone. It was desired to prepare a saturated solution of ozone, thus

decreasing the volume of ozonated water needed for oxidation. A dilution of SWNT-SDS

suspension was a concern, since dilution could result in SWNT agglomeration, thus

leading to lower fluorescence intensity. In general, bubbling O3/O2 gaseous mixture

through the solution was of a greater benefit, since in that case there was no need to

25

worry about sample dilution. While dilution with 1% SDS decreases SWNT fluorescence

intensity, no comprehensive study was performed in this work to estimate the influence

of dilution on fluorescence. Aqueous SDS solution was not used for ozone accumulation

primarily because this surfactant is a known catalyst for conversion of ozone into

molecular oxygen.

For ozonation in the solution phase, fluorescence quenched to a different extent (20

to 100 times) had fairly close recovery rates as shown in logarithmic scale in Figure 5B.

26

1.0
A
0.8
I / Imax

0.6

0.4

0.2

0.0

100 B
Imax / I (log scale)

40
20
10

4
2
1
3 6 9 12 15
Time (min)
Figure 5. SWNT emission change at 1247 nm after ozonation. Used 661 nm laser for
excitation. Every fifth experimental point is shown with a symbol. (A) Change of
normalized fluorescence intensity (I/Imax) with addition of different amounts of ozone.
The higher level of ozone resulted in a lower fluorescence intensity (line ). The lowest
amount of ozone gave the highest intensity (line ). (B) Fluorescence quenching factor
(Imax/I) shown in logarithmic scale. Different oxidation degrees gave close decomposition
rates.

27

Inverted Normalized Fluorescence, Imax / I
A D
20 954 nm
1026 nm
15 C
150 1123 nm
1250 nm
10

5
100
0 1 2

50 B
A

0

0 5 10 15
Time (min)

Figure 6. Influence of ozone load on fluorescence quenching at different emmax.
Inverted normalized fluorescence (Imax/I) at four distinct wavelengths is shown. Four
independent experiments (A-D) are shifted along the time axis for clarity. A zoom-in for
experiment A is provided in the upper left corner. Wider tubes ( em 1250 nm) were
quenched more at low ozone loads (A-B). With higher loads all tubes were quenched to
the same degree (C-D).
Large loads of ozone, typically above 1 mL of O3/O2 gaseous mixture (ca. 3 v/v %

O3), injected into 1 mL of SWNT – SDS suspension resulted in slower decay rates. Rates

obtained from samples with quenching degree (Imax/I) below 200 were reproducible.

Rates obtained from higher levels of ozonation were difficult to reproduce even with a

thermostated cuvette. The common problem was the curve deviation from simple

exponential decay.
max
Tubes emitting at longer wavelength, em = 1250, typically with wider diameters,

were quenched to a higher degree within Imax/I range 20 to 130 (Figures 6A and B).

Higher ozonation loads resulted in all tubes getting quenched to the same degree (Figure

6C and D).

28

Inverted Normalized Fluorescence
em = 1026 nm
100
13 A less O3
H
B
C
75 9 more O3
D
L
5
50

25 2 4 6
H
L

0

0 2 4 6 8 10
Time (min)

Figure 7. Regression fit for inverted normalized fluorescence at 1026 nm (formula F4).
Each regression curve represents an independent experiment and is shown with a solid
thin line. Samples were ozonated to a different extent; curves A and B correspond to a
low ozone load, while C and D to a high load. Arrows labeled L and H point to curve
deviation caused by slowly decaying ozonides. Curves B-D were shifted along the time
axis for clarity.
max
Emission kinetics at em = 1026 nm for different ozone loads were fitted with

regression curves; the highest two points (after ozonation) on inverted normalized

fluorescence data sets were excluded from regression analysis. As described in Appendix

A equilibration periods should be excluded from ozonide decay regression.

Overall decomposition rates were found to be lower with higher ozone loads. Two

possible explanations for such a phenomenon are a) ozonides formed are either lateral or

longitudinal to tube axis (Figure 27), or b) closely situated ozonides affect decomposition

of nearby ozonides.

29

The ozonide decay rates for curves A – D (Figure 7) were calculated with the

following formula:

bt
bt n
y y final ae ce (F4)

Regression results for Figure 7 are summarized in Table 2.

Table 2. Regression results calculated with formula F4 for inverted normalized
fluorescence data recorded at 1026 nm emission wavelength.*
Curves
Parameter A, n > 10 B, n > 10 C, n > 8 C, n > 10 D, n > 8 D, n > 10
a 28.45 58.43 102.4 195.1
b, min-1 1.96 1.59 1.27 1.24 1.36 1.32
yfinal 1.71 1.48 1.10 1.10
c 1.18 2.24 4.48 6.26
n 10 10.00 8.00 8.00
r2 0.9997 0.9997 0.9977 0.9954
*
Curve one-letter symbol and a lower boundary for variable n are written at the head of
each column. Constraints used for regression were: 0 < a < 1000; 0 < b < 100; 0 < c < a;
n < 100; yfinal > 1.05. Rates b are in min-1.
For regression purposes, 'tails' on inverted data sets were truncated to increase the

weight of points related to a fast decay. Rates were calculated with 5-parameter formula

F4. The formula has fast and slow exponential terms, the slow one being n times slower

than the fast one. (For details on mathematics behind regression see Appendix A.)

Parameter n was kept greater than 10 for low ozone load curves, since formation of

slowly decaying ozonides was minimal; n was set to be greater than 8 for high load

curves, since there was a greater number of slowly decaying ozonides. Decreasing n

value to less than 8 would increase influence of the slower component on regression

curve.

30

The higher load of ozone resulted in oxidation of sites with slower decay rates. Sites

that required higher activation energy for oxidation resulted in formation of more stable

ozonides, contributing to a slower component. In other words, double bonds that were

harder to oxidize gave slower 1,2,3-trioxolane decay rates.

Curves with lower ozone load were fitted well with n > 10 (i.e. small “slow”

component). Regression curves for higher ozone load had difficulty fitting to

experimental points and n value constraint was brought down to n > 8. Even such

adjustment did not help regression curve to fit D data set (Figure 7 D), Experiment D had

the highest ozonation degree. Arrows H and L point to deviation of experimental points

from regression line. (Rates for curves C and D were also calculated with n >10

constraint; see Table 2)

The main purpose of the introduction of the slow exponential component was to

improve correlation between normalized and inverted normalized data sets. Appendix A

explains this issue in great detail. Normalized experimental curves were found to have

slowly rising tails and required an introduction of a slow component. The assumption was

made that the slow component should be n times slower than the fast one.

Rates calculated for lower loads of ozone, curves A and B, were 1.96 and 1.59

accordingly. Rates for higher loads of ozone were 1.27 and 1.36 for curves C and D

accordingly. Curve D could not be fitted as well as other three curves. Higher degree of

ozonation resulted in lower coefficients of determination, r2 (SigmaPlot® software

package, used for regression analysis, defines r2 as a coefficient of determination).

31

SWNT oxidation with solvated ozone. Influence of ozone load on NIR absorption
and fluorescence.

1.0

0.8
Normalized Intensity

A1
0.6

0.4

0.2 954 nm
F1 1026 nm
1123 nm
0.0 1250 nm

1.0

0.8 A2

0.6

0.4
F2
0.2

0.0

0 200 400 600 800 1000 1200
Time (sec)
Figure 8. Regression analysis of normalized absorbance (A1 and A2) and fluorescence
(F1 and F2) intensities of ozonated SWNT at four distinct wavelengths. SWNT sample
was oxidized with solvated ozone. Data points for absorption and fluorescence were
acquired sequentially with 1 sec delay. Points not used in regression are depicted with
dotted lines. Regression curves are shown with solid lines. Legends are the same for the
top and the bottom plots. Used 661 nm excitation source for fluorescence measurements.
Top: high load of trioxolanes, Bottom: low load of trioxolanes.

32

Curves in Figure 8 show NIR absorption and fluorescence change with introduction

of ozone into system. Intensities dropped down and then slowly recovered to sub initial

values. Formula used for regression on NIR absorption had six parameters (F1); formula

used for normalized fluorescence data sets had five parameters (F2).

1 y min
y bt
y min (F1)
bt n
y final ae ce

1
y bt (F2)
bt n
y final ae ce
In this particular experiment, water saturated with ozone was used instead of

bubbling gaseous ozone. Absorption points after reagent addition were adjusted to

compensate for dilution. Calculated ozonide decomposition rates are summarized in

Table 3.

33

Table 3. SWNT oxidation with solvated ozone. Regression results calculated with
formulas F1 and F2 for normalized NIR absorption and fluorescence data recorded at
four distinct emission wavelengths.*

Fluorescence NIR absorption
em, nm 954 1026 1123 1250 954 1026 1123 1250

High load F1 A1
b, s -1 0.0225 0.0146 0.0100 0.0061 0.0187 0.0110 0.0076 0.0058
b, min -1 1.35 0.88 0.60 0.37 1.12 0.66 0.46 0.35
n 10.57 10.00 10.00 10.00 20.00 20.00 20.00 20.00
r2 0.9995 0.9996 0.9996 0.9993 0.9652 0.9834 0.9964 0.9974

Low load F2 A2
b, s -1 0.0480 0.0428 0.0276 0.0132 - - 0.0302 0.0158
b, min -1 2.88 2.57 1.66 0.79 - - 1.81 0.95
n 15.00 15.00 12.62 10.00 - - 20.00 20.00
r2 0.9673 0.9895 0.9959 0.9985 - - 0.9822 0.9949
*
Emission or absorption wavelength is written at the head of each column. Constraints
used for regression analysis were: 0 < a < 1000; 0 < b < 100; 0 < c < a; for fluorescence
10 < n < 15; for absorption n < 20; yfinal > 1.05. Abbreviations: r2 – coefficient of
determination, n – determines how many times slow exponential term is slower than the
fast one, b – 1,2,3-trioxolane decomposition rate.
Constraint 10 < n < 15 used in fluorescence regression was needed to prevent very
bt
n
low n values, leading to a greater influence of the term ce . Such reduction of n value

led to meaningless rates b, and it was imperative to keep 'slow' component as a small

contributor to the overall intensity change. Constraint n < 20 was set for NIR absorption

regression curve. With no upper constraint for parameter n, regression on NIR absorption

data set was attempting to set abnormally large n values for nearly straight 'tails'. Greater

n value resulted in a slower second term. When n values are abnormally high, regression

results in converting the curve into a straight line, which is not the case.

34

Fluorescence rates for low ozone load were found to be at least two times faster than

those for a high load of ozone. The same was true for NIR absorption rates.

Fluorescence and absorption rates obtained from the same sample were found to be

close, but not equal. For high load of ozone, fluorescence rates were slightly higher than

those for NIR absorption. For low load of ozone fluorescence rates were slightly lower.

Observation of a close relationship between fluorescence and NIR absorption growth

rates led to the following diagram of transition states (Figure 9).

A B C
4
conduction
3

2 c2 c2 c2

1 c1 c1 c1
Energy

E11 E22 ozonide E11 ozonide
0 E11 E22
fluorescence abs NIR abs
-1 v1 v1 v1

-2 v2 v2 v2

-3
valence
-4

Density of Electronic States
Figure 9. Schematic density of electronic states for pristine and ozonated SWNTs. Thick
solid arrows depict optical excitation and emission transitions of interest; thin dashed
arrows denote nonradiative relaxation of the electron (in the conduction band) and the
hole (in the valence band) before emission. (A) Transitions of interest in pristine SWNT.
Diagram adopted from Science, 2002, 298, 2361-2366. (B) Transitions in ozonated
SWNT. Nonradiative relaxation c1 → ozonide → v1 is a major process and shown with
thick solid arrows. Fluorescence from c1 to v1 is a minor process and shown with a dotted
arrow. (C) NIR absorption of ozonated SWNT. Depletion of an electron density of band
v1 by ozonides resulted in a weaker NIR absorption v1 → c1 (v – valence band, c –
conduction band.)

35

SWNT oxidation with gaseous ozone (high load)

0.9 A

0.6

L
H
0.3
L 954 nm
F 1026 nm
1123 nm
H
0.0 L 1250 nm

0 200 400 600 800 1000 1200
Time (sec)

Figure 10. Regression analysis of normalized NIR absorption (A) and fluorescence (F)
intensities of ozonated SWNT at four distinct wavelengths. SWNT - SDS suspension was
bubbled with O3/O2 gaseous mixture (ca. 3 v/v % ozone). Data points for absorption and
fluorescence were acquired sequentially with 1 sec delay. Points not used in regression
are depicted with dotted lines. Regression curves are shown with solid lines. Each
wavelength is marked with an individual symbol. Labels L and H denote curve wobbling
above and below regression line. Used 661 nm excitation source for fluorescence
measurements. Upper curves: NIR absorption, Lower curves: fluorescence.
Regression results for Figure 10 above are summarized in Table 4.

36

Table 4. SWNT oxidation with gaseous ozone. Regression results were calculated with
formulas F1 for normalized NIR absorption and F2 for fluorescence data recorded at four
distinct emission wavelengths*

High load Fluorescence NIR absorption
em, nm 954 1026 1123 1250 954 1026 1123 1250

b, s -1 0.0211 0.0153 0.0113 0.0064 0.0180 0.0099 0.0079 0.0051
-1
b, min 1.27 0.92 0.68 0.38 1.08 0.59 0.47 0.31
n 10.00 10.00 8.00 8.00 20.00 20.00 20.00 20.00
2
r 0.9995 0.9998 0.9998 0.9979 0.9824 0.9918 0.9986 0.9987
*
Emission or absorption wavelength is written at the head of each column. Constraints
used for regression were: 0 < a < 1000; 0 < b < 100; 0 < c < a; for fluorescence 8 < n <
10; for absorption n < 20; yfinal > 1.05. Abbreviations: r2 – coefficient of determination, n
– determines how many times slow exponential term is slower than the fast one, b –
1,2,3-trioxolane decomposition rate.

Formation rates for SWNT ozonides

Formation of SWNT ozonide is schematically shown in Scheme 1 below

Scheme 1
O
O3 O O

SWNT SWNTO3

Formation rates of 1,2,3-trioxolanes were measured at 25 C by monitoring
max
absorption at ozone abs = 260 nm (Figure 11).

37

b1
Absorbance x 1000 (a.u.)

2 b2
b3

1

0

0 1 2 3 4 5
Time (sec)
Figure 11. Absorption kinetics at 25 ºC monitored at 260 nm after bubbling O3/O2
gaseous mixture through SWNT-SDS suspension. Absorption decrease represents ozone
consumption and 1,2,3-trioxolane formation rates. Curves are shifted along the vertical
axis to bring regression lines to approximate zero with t → . Curves are: ozonation of
4x diluted SWNT (line ), ozonation of 8x diluted SWNT (line ), and ozone bubbled
through preliminary ozonated 4x diluted SWNT (line ).
Each curve in Figure 11 represents a separate experiment. The same amount of gas

(0.5 mL) with the same concentration of ozone (ca. 0.5 v/v %) were used for all

injections. SDS was found to be unreactive with small amounts of ozone and its influence

on absorption was below detection limits. There is always a possibility that small percent

of impurities from SDS can affect absorption change, thus leading to misinterpretation of

kinetics results. This is thought not to be the case in the above-mentioned experiments

(Figure 11), because pre-ozonated SWNT gave a comparable ozone

consumption/trioxolanes formation rate. Additionally, all samples were bleached to levels

below original absorbance, indicating that some double bonds were no longer existing.

Based on these facts, it is believed that the measured kinetics curves are from chemical

38

reaction between ozone and SWNT and not from some unknown impurity. Dilution 4 and

8 times of stock SWNT – SDS suspension with 1 wt. % aq. SDS was necessary to acquire

sufficient data points for regression.

1,2,3-Trioxolane formation rates are summarized in Table 5.

Table 5. SWNT ozonide formation rates.
Curve b, s –1 r2 ccarbon, mg/L cdouble bond, mmol/L
4x diluted SWNT 1.44 0.8936 1.44 0.06
8x diluted SWNT 0.63 0.8498 0.72 0.03
4x dil. prelim. ozntd SWNT* 0.52 0.9475 1.44 0.06
*
- Preliminary ozonated tubes were heated to 40 C for ca. 4 hours before their use in this
experiment; b – rate.
Rate for 8 times diluted SWNT – SDS suspension was found to be at least two times

slower than the one for 4 times diluted sample. Concentration of SWNT for 4 and 8 times

diluted suspensions are summarized in Table 5. Total concentration of double bonds in 4

times diluted sample was calculated 0.06 mmol/L, yielding the reaction rate constant

2.4·104 M-1s-1, which is of the same order of magnitude as the formation rate constants of

C60O3 (8.8 104 M-1s-1 at 0 C) and C70O3 (5 104 M-1s-1 at 22 C) in CCl4 solvent.18, 19

The rate constant for 4 times diluted SWNT suspension is five orders of magnitude

slower than the diffusion rate constant 109 M-1s-1 for small organic molecules in hexane.

It is likely that water viscosity, solvation of ozone with water molecules, SDS

hydrophobic shell around SWNT, large molecular weight of the tubes and tubular

structure with large aspect ratio, all contributed to the rate decrease.

39

Establishing of a saturation limit with different amounts of ozone

Absorption changes after injections of different amounts of ozone into diluted

SWNT – SDS suspension are shown in Figure 12. SWNT - SDS suspension was diluted

eight times to decrease reaction rate between SWNT and ozone. Ozone concentrations

were approximately 0.5%, 0.6%, 0.75% and 1% in air stream (curves , , , 

accordingly). Ozone concentration was manipulated by dilution with air. A half milliliter

of O3 - air gaseous mixture was injected into 1 mL of SWNT – SDS suspension in each

case. All measurements were performed in a thermostated cuvette at 25.0 C.

saturation with ozonides
8.4
ozone consumption
Absorbance x100 (a.u.)

8.2
O3 / O2 mixture
dilution with air:
8.0
3x
4x
7.8 5x
6x

7.6
d8

0 20 40 60 80 100 120
Time (sec)
Figure 12. Dependence of SWNT absorption on the amount of injected ozone. Each
curve represents a separate experiment. Kinetics curves were monitored at 260 nm and
25.0 C. Symbols are labeled with O3/O2 mixture dilution degrees. The difference
between initial and final absorbance is marked with d8.
Curves were shifted along the time axis to set injection point to zero seconds. Three

of the four curves were multiplied by the corresponding coefficients to bring the initial

40

absorbance to the same level (difference in absorbance before adjustment was very small;

initial absorbance ranged between 0.0737 and 0.0761 a.u.)

Spikes at 0 seconds are due to needle insertion and shown with dotted lines.

Exponential decrease of absorbance right after the injection represents ozone

consumption and 1,2,3-trioxolane formation.

Higher concentrations of ozone resulted in identical downward step (value d8 in

Figure 12), indicating that tubes got saturated with ozonides. Exponential decay is

schematically divided into two sections: saturation of SWNT with ozonides (left) and

ozone consumption by „emptied‟ sections of SWNT (right). Value d8 equaled to 0.0017

a.u. was found approximately 9 times smaller than downward step d in non-diluted

SWNT suspension (Figure 13). Addition of 0.5 mL of 6x diluted O3/O2 gaseous mixture

was not sufficient to saturate SWNT with 1,2,3-trioxolanes. Rate of reaction between

SWNT and the least concentrated ozone/air gas mixture (curve ) was calculated to be

b = 0.63 s-1, corresponding to lifetime = 1.6 sec (see Figure 12 above for details.)

41

Influence of multiple ozone injections on SWNT saturation. Oxidation with 4 min

intervals

0.60
Absorbance (a.u.)

b1
b2
b3
0.57 b4
d b5
b6
b7
b8

0.54

10 20 30
Time (min)
Figure 13. Injections of 1.5 mL of O3/O2 gaseous mixture into 1.5 mL SWNT - SDS
suspension with 4 min time intervals. Suspension absorbance was monitored at 260 nm
and r. t. Upward spikes are due to needle insertion and are emphasized with arrows.
Difference in absorbance before and after the first injection is marked with letter d.
Difference in absorbance between before and after the first ozone injection was

d = 0.0154 a.u.(ca. 3%). This value is approximately 9 times larger than that for 8x

diluted SWNT suspension. Rates noted as b1 through b8 in Figure 13 are summarized in

Table 6.

Increase in rate b and decrease in the absolute value of step d (marked on Figure 13)

are believed to be associated with SWNT saturation. Experimental points in Figure 13

above, shown with circles, form simple exponential decay curves right after each

injection of ozone. This decay represents reaction of freely floating ozone with SWNT

and conversion of ozone to oxygen by collision with SDS and water molecules.

42

Table 6. Regression results for single exponential decay of absorbance after multiple
ozone injections into SWNT – SDS suspension.
Injection cycle*
1st 2nd 3rd 4th 5th 6th 7th 8th
b, min-1 16.60 10.68 7.51 4.95 3.55 3.23 2.91 2.93
, sec 3.6 5.6 8.0 12.1 16.9 18.6 20.6 20.5
2
r 0.9917 0.9971 0.9987 0.9987 0.9988 0.9994 0.9992 0.9988

* - Regression was performed with a single exponential decay formula. Variables: b –
decay rate, - lifetime, r 2– coefficient of determination.
SWNT oxidation with different amounts of ozone. Estimation of a saturation limit

by NIR fluorescence.

before O3
0.5 mL
Fluorescence Intensity (nW/nm)

0.15 1 mL 0.05
2 mL
3 mL
4 mL
5 mL 0.03
6 mL
0.10 8 mL
10 mL
1265

0.05

0.00
950 1050 1150 1250 1350
Wavelength (nm)

Figure 14. Fluorescence spectra after bubbling specific amounts of ozone through
SWNT-SDS suspensions. Each curve represents a separate experiment. One percent
aqueous SDS solution ozonated with 10 mL of O3/O2 gaseous mixture (ca. 3 v/v %
ozone) served as a baseline for all experiment. Samples were ozonated 3 days before
fluorescence acquisition. The 785 nm laser was used for excitation. Arrows point to the
saturation level. A zoom-in shows that saturation was reached with 3 mL of O3/O2
gaseous mixture (ca. 3 v/v % ozone; curve ). Tube (6,5) with emmax 977 nm was the
most difficult to oxidize.

43

SWNT suspension aliquots were bubbled with different amounts of O3/O2 gaseous

mixture (0.5 – 10 mL) and spectra overlaid (Figure 14). Gas was injected slowly in each

case. To avoid misinterpretations, spectra were recorded three days after ozonation,

which was plenty of time for ozonide decomposition and SWNT structural

rearrangements. One percent aqueous sodium dodecyl sulfate solution bubbled with

10 mL of O3/O2 gaseous mixture (ca. 3 v/v % ozone) served as a baseline for all curves in

Figure 14. Spectra overlay demonstrated that SWNT got saturated with ozone at 3 mL

O3/O2 gaseous mixture load (ca. 3 v/v % ozone). Curves for 8 and 10 mL have less

intense fluorescence than all other ones. It may be concluded that during slow O3/O2

gaseous mixture injection some of the ozonides have decomposed, thus allowing for a

greater amount of ozone to react with SWNT. Separate UV studies demonstrated that

after SWNT got saturated with 1,2,3-trioxolanes, which occurred at or below 3 mL load,

excess of ozone dissolved in aqueous media (Figure 13). This in turn provided an

additional supply of ozone for subsequent oxidation. Because collision of ozone with

SDS molecules leads to its conversion to oxygen (ozone decay rate in 1% aq. SDS is

about 0.43 min-1 at r. t.), slow addition of 8 and 10 mL provided sufficient amount of

ozone to overcome deactivation by SDS, hydroxyl and water species. Typically, injection

of 10 mL of O3/O2 gaseous mixture required more than a minute to complete.
max
Notably, tube (6,5) with em 977 nm (diameter = 0.76 nm) had the least ozonation

degree, i.e. was the most difficult to oxidize. This tube has a very small twist when

compared to other tubes in the experiment.

3D Structures of tubes of interest are shown in Figure 15.

44

(8,3) (6,5) (10,2) (11,3) (10,5)

Figure 15. Tubes (n,m) with well separated emission peaks on spectrum for 785 nm
excitation source. Each tube is shown in two projections (top and bottom). Tube number
is indicated below each pair. Tubes are drawn not to scale.
The physical and optical properties of tubes that were well separated in emission

spectrum with 785 nm excitation are summarized in Table 7.

Table 7. Summary of physical and optical properties of tubes with well separated peaks
in emission spectrum*
, cm-1
max
Tube (n,m) em , nm diameter, nm
8,3 954 10486 0.78
6,5 977 10234 0.76
10,2 1056 9468 0.88
11,3 1201 8327 1.01
10,5 1253 7982 1.05
*
- Used 785 nm excitation source.
The only tube that had difficulty getting ozonated with 10 mL of O3/O2 gaseous

mixture (ca. 3 v/v % ozone) was tube (6,5). It has the smallest “twist” out of all well

defined tubes in emission spectrum (Figure 14). It may be concluded that the tube “twist”

increases double bond reactivity with ozone. Tube (6,5) was also estimated to be the most

abundant in utilized HipCo sample (see Chapter 1 for tube abundance distribution). All

45

tubes had substantial difference in diameter and no conclusion could be made with regard

to dependence of ozonation degree on the tube diameter at 10 mL O3/O2 gaseous mixture

load (ca. 3 v/v % ozone). Particularly, tubes (8,3) and (6,5) had very close diameter, but

different oxidation degree, as evidenced by peaks at 954 and 977 nm (Figure 14).


by NIR absorption.

NIR absorption was measured on samples discussed above. For experimental details

see Figure 14 above and accompanying notes.

before O3
0.5 mL
0.25
1 mL
2 mL
3 mL
Absorbance (a.u.)

4 mL
5 mL
6 mL
0.20 8 mL
10 mL

0.15

950 1050 1150 1250
Wavelength (nm)

Figure 16. NIR absorption spectra after bubbling specific amounts of O3/O2 through
aqueous SDS ozonated with 10 mL of O3/O2 served as a baseline for all experiment.
Samples were ozonated 3 days prior to NIR absorption acquisition. Arrows point to the
saturation level, which was reached at 2 mL of O3/O2 gaseous mixture (ca. 3 v/v %
ozone; curve ). The area near 977 nm was the most difficult to oxidize and had the
least percent decrease.

46

Analogously to fluorescence spectra, the area near 977 nm had the least decrease in

absorbance as compared to percent values for all tubes. Saturation point was reached with

2 mL O3/O2 gaseous mixture (ca. 3 v/v % ozone; pointed with arrows in Figure 16). To

avoid misinterpretations, NIR absorption spectra were recorded three days after

ozonation.

Noisiness of spectra starting from 2 mL is presumed to be associated with production

of a number of nonequivalent sections of tubes. Notably, there are no peak shifts between

2 and 10 mL of O3/O2 gaseous mixture (ca. 3 v/v % ozone). This means that ozonation is

following a specific pattern, rather than a random one. An increase in the number of

peaks would be expected for random ozonation of SWNT. Despite significant decrease in

absorption (compare curves  and ), the number of peaks and their abs
max
were

preserved, thus indicating an ordered oxidation. Higher loads of ozone (within 10 mL

O3/O2 gaseous mixture) are thought to produce a greater number of „sections‟ of SWNT

ozonated with the same pattern.


by UV-Vis absorption.

UV-Vis spectrum of 1% aq. SDS was found to be unchanged in a region 235 - 800

nm after bubbling with 10 mL of O3/O2 gaseous mixture. Aqueous SDS solution purged

with 10 mL of O3/O2 gaseous mixture served as a baseline for all spectra in Figure 17.

47

0.63 before O3
0.5 mL
0.6 1 mL
0.59 2 mL
0.21
3 mL
Absorbance (a.u.)

4 mL
0.55 5 mL
6 mL
8 mL
10 mL
0.4 245 285
0.19

730

0.2

250 350 450 550 650 750
Wavelength (nm)
Figure 17. UV-Vis absorption spectra after bubbling specific amounts of ozone through
aqueous SDS ozonated with 10 mL of O3 served as a baseline for all experiment. Samples
were ozonated 3 days before UV-Vis absorption acquisition. Arrows point to the
saturation level, which was reached at or below 2 mL of O3/O2 gaseous mixture (ca. 3 v/v
% ozone; curve ).
SWNT spectra had smooth transition from 0 mL (curve ) spectrum to 10 mL one

(curve , Figure 17).

SWNT ozonation for a specific period of time. Influence of ‘saturation’ and

1,2,3-trioxolane decomposition rates on overall sidewall oxidation as monitored by

NIR fluorescence.

Ozonation of SWNT-SDS suspensions was conducted for specific periods of time,

ranging from 30 sec to 30 min. One percent aq. solution of sodium dodecyl sulfate,

bubbled with ozone for specified periods of time served as a baseline for each curve (i.e.

each curve had its own baseline). Interesting spectral changes were observed and are

summarized in Figure 18.

48

Fluorescence Intensity (nW/nm) 0.16 before O3
0.5 m
1 m
2 m
0.12 4 m
5 m
10 m
30 m

0.08

0.04

0.00
950 1050 1150 1250 1350
Wavelength (nm)

Figure 18. Influence of ozonation on SWNT fluorescence spectra. Each curve represents
a separate experiment. Symbol legends denote ozone bubbling times in minutes. Spectra
were recorded 3 days after ozonation. Ozone was bubbled through samples at room
temperature. The 785 nm laser was used for excitation. Arrows point to fluorescence
curves after 0.5 and 5 min of continuous bubbling of O3/O2 gaseous mixture (ca. 3 v/v %
ozone) through SWNT-SDS suspension. Curves for 5, 10 and 30 min and before O3 are
shown with thick lines.
Two sets of pairs of arrows in Figure 18 demonstrate location of curves after 30 sec

and 5 min of continuous bubbling of O3/O2 gaseous mixture. The gas flow rate was

approximately 26 mL/min. This means 0.5 and 5 min bubbling correspond to 13 and 130

mL of O3/O2 gaseous mixture.

The upper left arrow points to tube (6,5) which was found to be fairly robust to

ozonation with 10 mL of O3/O2 gaseous mixture (ca. 3 v/v % ozone). In terms of percent

values, intensities of all tubes except (6,5) were substantially bleached within 30 sec of

continuous bubbling. Notably, tube (8,3), with roughly the same diameter as (6,5), was

bleached more than (6,5) after 30 sec ozonation. (Tube properties are summarized in

Tsvaygboym PhD Thesis 2007 - BW

Tsvaygboym PhD Thesis 2007 - BW

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Tsvaygboym PhD Thesis 2007 - BW

Similar to Tsvaygboym PhD Thesis 2007 - BW (20)

Recently uploaded

Recently uploaded (20)

Tsvaygboym PhD Thesis 2007 - BW