ISO/TS 10868:2017

TECHNICAL ISO/TS
SPECIFICATION 10868
Second edition
2017-05
Nanotechnologies — Characterization
of single-wall carbon nanotubes using
ultraviolet-visible-near infrared (UV-
Vis-NIR) absorption spectroscopy
Nanotechnologies — Caractérisation des nanotubes à simple couche
de carbone par utilisation de la spectroscopie d’absorption UV-Vis-NIR
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
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ii © ISO 2017 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 2
4 Principle . 2
4.1 General . 2
4.2 UV-Vis-NIR absorption spectroscopy . 2
4.3 Optical absorption peaks of SWCNTs in the UV-Vis-NIR region . 2
4.4 Relation between SWCNT diameter and optical absorption peaks . 4
4.5 Derivation of the purity indicator from optical absorption peak areas . 4
4.6 Derivation of ratio of metallic SWCNTs from optical absorption peak areas . 6
5 UV-Vis-NIR spectrometer . 6
6 Sample preparation method . 6
6.1 General . 6
6.2 Preparation of D O dispersion for measurement of mean diameter and the ratio of
metallic SWCNTs . 7
6.3 Preparation of solid film dispersion for measurement of the mean diameter and
the ratio of metallic SWCNTs . 7
6.4 Preparation of DMF dispersion for determination of the purity indicator . 8
7 Optical measurement procedures and conditions . 8
8 Data analysis and results interpretations . 9
8.1 Data analysis for characterization of SWCNT diameter. 9
8.2 Data analysis for determination of the purity indicator . 9
8.3 Data analysis for characterization of the ratio of metallic SWCNTs . 9
9 Measurement uncertainties . 9
10 Test report .10
Annex A (informative) Case study for derivation of the relation between optical absorption
peaks of SWCNTs and their mean diameter .11
Annex B (informative) Case study for determination of the purity indicator .16
Bibliography .19
Foreword
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This document was prepared by Technical Committee ISO/TC 229, Nanotechnologies.
This second edition cancels and replaces the first edition (ISO/TS 10868:2011), which has been
technically revised.
iv © ISO 2017 – All rights reserved

TECHNICAL SPECIFICATION ISO/TS 10868:2017(E)
Nanotechnologies — Characterization of single-wall carbon
nanotubes using ultraviolet-visible-near infrared (UV-Vis-
NIR) absorption spectroscopy
1 Scope
This document provides guidelines for the characterization of compounds containing single-wall
carbon nanotubes (SWCNTs) by using optical absorption spectroscopy.
The aim of this document is to describe a measurement method to characterize the diameter, the purity,
and the ratio of metallic SWCNTs to the total SWCNT content in the sample.
The analysis of the nanotube diameter is applicable for the diameter range from 1 nm to 2 nm.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/TS 80004-4, Nanotechnologies — Vocabulary — Part 4: Nanostructured materials
3 Terms, definitions and abbreviated terms
For the purposes of this document, the terms and definitions given in ISO/TS 80004-4 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1 Terms and definitions
3.1.1
purity indicator
optically defined indicator of the ratio of the mass fraction of SWCNTs to the total carbonaceous content
in a sample
Note 1 to entry: Purity indicator is NOT “purity” itself which is defined as the percentage of mass of SWCNTs to
the total mass of the sample. This guideline cannot evaluate this general purity because absorption spectroscopy
cannot detect metallic impurities that are generally contained in any SWCNT sample. In order to characterize
metal impurity content, there is a different Technical Specification on thermogravimetric analysis. Metallic
impurity is defined as catalytic metal particle and does not include metallic carbon nanotube. See ISO TS 11308.
3.1.2
ratio of metallic SWCNTs
optically defined compositional ratio of metallic SWCNTs to the total SWCNTs contained in the sample
3.2 Abbreviated terms
For the purposes of this document, the following abbreviated terms apply.
CMC Sodium carboxymethylcellulose
DMF Dimethylformamide
DOS Density of states
NIR Near infrared
NMP N-Methyl-2-Pyrrolidone
SC Sodium cholate
DOC Sodium Deoxycholate
SDS Sodium dodecyl sulfate
SDBS Sodium dodecylbenzene sulfonate
SWCNT Single-wall carbon nanotube
TEM Transmission electron microscope
UV Ultraviolet
VHS van Hove singularity
Vis Visible
4 Principle
4.1 General
All SWCNT samples contain both semiconducting and metallic SWCNTs, together with impurities
consisting of carbon and other elements unless the samples have been altered after production. UV-
Vis-NIR absorption spectroscopy can be used for the measurement of interband optical transitions
specific to SWCNTs. The analysis of these optical transitions provides qualitative and semiquantitative
information important for the characterization of SWCNT samples, such as mean diameter, purity, and
the ratio of metallic SWCNTs to the total SWCNT content.
4.2 UV-Vis-NIR absorption spectroscopy
The intensity of light passing at a specified wavelength, λ, through a specimen (I) is measured and it
is compared to the intensity of light before it passes through the specimen (I ). The ratio I/I is called
0 0
a transmittance. The absorbance, A, is expressed as −log (I/I ). The plot of the absorbance against
wavelength for a particular compound is referred to as an absorption spectrum.
NOTE The relationship between transmittance and absorbance is only rigorously correct when reflectance
is negligible and there is no scattering.
4.3 Optical absorption peaks of SWCNTs in the UV-Vis-NIR region
The shape of the electronic DOS of semiconducting and metallic SWCNTs shown in Figure 1 is a series
of spikes that are referred to as VHS. The peaks observed in the optical absorption spectra of SWCNTs
are attributed to the electronic transitions between these VHSs as shown by arrows in Figure 1. S
and S are used as the symbols of the absorption due to the first and second interband transitions of
2 © ISO 2017 – All rights reserved

semiconducting SWCNTs, respectively [see Figure 1 a)]. M means the absorption arising from the first
interband transition of metallic SWCNTs [see Figure 1 b)].
a) Electronic DOS of semiconducting SWCNTs b) Electronic DOS of metallic SWCNTs
Key
X energy (eV)
Y electronic DOS (arbitrary unit)
S first interband optical transition attributed to semiconducting SWCNTs
S second interband optical transition attributed to semiconducting SWCNTs
M first interband optical transition attributed to metallic SWCNTs
NOTE 1 Arrows represent interband transitions that result in optical absorption.
NOTE 2 See Reference [2].
Figure 1 — Electronic DOS diagram of SWCNTs near the Fermi level
To interpret the absorption spectra of SWCNTs, band structures calculated using the zone-folding
method are frequently used. The electronic structure of an SWCNT is generally given by that of a two-
[2]
dimensional graphite sheet expressed by the tight binding approximation as shown in Formula (1) :
12/
 
  ka ka
   
3ka
yy
x 2
E =± γ 14± cosc  os +4cos  (1)
   
2D
     
22 2
 
     
 
where
is the two dimensional energy dispersion relation for a single graphene sheet;
E
2D
[3]
a is the lattice parameter ;
are the components of the reciprocal unit vector;
k and k
x y
γ is the overlap integral.
4.4 Relation between SWCNT diameter and optical absorption peaks
Within a simple tight-binding theory, in which the electronic band structure is assumed to arise from
a pure p-orbital at each conjugated carbon atom, the low-energy band gap transitions take a simple
analytical form. The energy gaps corresponding to the electron transitions are given by Formula (2) to
Formula (4):
2aγ
Eg()S = (2)
d
4aγ
Eg()S = (3)
d
6aγ
Eg()M = (4)
d
where
E (S ),
g 11
E (S ), are the energy gaps corresponding to the transitions of S , S and M , respectively;
g 22 11 22 11
E (M )
g 11
[4]
d is the diameter of SWCNTs .
Formula (2) to Formula (4) show a simple relationship between the diameter and the optical transition
energies (and thus the peak wavelengths). This allows the estimation of the mean diameter of a SWCNT
sample by the analysis of the absorption spectra originating from the optical transitions between VHSs.
Formula (2) to Formula (4) can give information related to the diameter within some limitations. One of
the limitations is that the analysed peak(s) needs to be clearly resolved.
4.5 Derivation of the purity indicator from optical absorption peak areas
As mentioned in 4.3, there are the specific absorptions of SWCNTs originating from interband transition
between VHSs. These absorption peaks are typically observed in the Vis-NIR region. On the other hand,
[5]
in the UV region, most SWCNT samples present optical absorption with the peak at 200 nm to 300 nm .
This absorption is attributed to the collective excitations of π electron systems (π -plasmons) and can
[5]
also be observed in most graphitic compounds . Therefore, the π -plasmon absorption observed in
most SWCNT samples is due to both SWCNTs and carbonaceous impurities. The π -plasmon absorption
is extremely broad and is superposed on the above-mentioned specific absorption of SWCNTs as a
featureless background extending to the Vis-NIR and IR region. To summarize, the absorption spectrum
of SWCNT samples in the Vis-NIR region is composed of the interband transitions of semiconducting
and metallic SWCNTs and π -plasmon absorbance (see Figure 2).
4 © ISO 2017 – All rights reserved

Key
X photon energy (eV)
Y absorbance (absorbance unit)
NOTE The relative contribution from each component is arbitrary and also has differing chiral angle
distributions.
[6]
Figure 2 — Typical UV-Vis-NIR absorption spectrum of an SWCNT sample
In Figure 2, the absorption from S and M gives rise to the absorption peak areas, AA(S ) and
nn 11 nn
AA(M ), and that of π-plasmon as AA(π ). In addition, the total absorption [AA(S )+ AA(π) or
11 nn
AA(M )+AA(π )] is designated as AA (see Annex B). As long as samples of concern have similar mean
11 t
diameters and diameter distributions, the relative magnitude of AA(S ) [or AA(M )] to AA can be used
nn 11 t
[7][8]
as an indicator of purity, P (S ) or P (M ) , which is given by Formula 5:
i nn i 11
PP()SM or ()=AA( SMor )/AA (5)
ii
nn 11 nn 11 t
Formula 5 gives information related to purity within some limitations. One of the limitations is that
the analysed peak(s) needs to be clearly resolved. Another is that samples need to have almost similar
mean diameters and distributions as determined by the locations of the peak positions.
NOTE Surfactants and/or dispersing agents could also add complexity to the spectra.
4.6 Derivation of ratio of metallic SWCNTs from optical absorption peak areas
On the basis of the analogy of 4.5, an analysis of the area under the peak for semiconducting and metallic
SWCNTs provides an indicator of the ratio of metallic SWCNTs to the total SWCNTs, which is given by
Formula (6):
AA()M
R = (6)
Metal
AA()SM+AA()
11 11
Furthermore, Formula (6) can be converted into Formula (7) for R as the function of AA(S ) and
Metal 22
AA(M ):
AA()M
R = (7)
Metal
12,(AA SM)(+AA )
22 11
Use of Formula (7) is frequently more favourable than use of Formula (6) because AA(S ) is sensitive to
[9]
the charge transfer .
R does not literally represent the ratio of metallic SWCNTs, because integrated molar extinction
Metal
coefficients in the M and S regions (or their relative magnitude) are not completely clarified.
11 11
In the case of the SWCNT sample with the diameter distribution of 1,1 nm to 1,3 nm, Formula (6)
and Formula (7) provide the actual ratio of metallic SWCNTs because these coefficients have been
[10]
determined to be equal experimentally .
R nonetheless, can be utilized as an indicator of the ratio of metallic SWCNTs in the comparison
Metal,
of different samples within some limitations. One of the limitations is that all the peaks involved need
to be clearly resolved. Another is that samples need to have similar mean diameters and distributions.
NOTE Most UV-Vis-NIR absorption spectra of SWCNTs show separate groups of peaks each of which can
be assigned to optical transitions in the metallic or semiconducting components. At the present stage, however,
determining their compositional ratio by spectral analysis is not possible, because of experimental difficulties
such as the unavailability of their extinction coefficients and ambiguity in background subtraction. A qualitative
comparison could still be made as to the relative abundance of each component using a certain standard sample.
For example, some SWCNT samples are known to have the ratio of 0,33, as theoretically predicted under the
[11]
assumption of equal synthetic probability , or the ratio of 1 in the sample treated by the special separation
[10]
process , which can be used as a reference.
5 UV-Vis-NIR spectrometer
A calibrated standard spectrophotometer covering a broad, ultraviolet to NIR wavelength range shall be
used. The long wavelength limit shall be 3 000 nm or longer to cover SWCNT diameter up to 2,5 nm. The
spectrophotometer shall be turned on 1 h prior to the measurement to allow the baseline to stabilize.
6 Sample preparation method
6.1 General
Because all the SWCNT samples are generally produced as powder or solid aggregates, they shall be
processed into a form that enables optical absorption measurements. Homogeneous, non-scattering
and stable dispersion of SWCNTs in liquid or solid media is best suited for this purpose, the preparation
of which requires a solvent and a dispersant. As they have their own optical absorption that can disturb
a spectral measurement of SWCNT, solvents and dispersants shall be properly chosen as follows.
For measurement of the mean diameter and ratio of metallic SWCNTs, the dispersing method using
water or heavy water (D O) and water-soluble surfactants shall be used because of high dispersing
...