ASTM E3220-20

Standard Guide for Characterization of Graphene Flakes

SIGNIFICANCE AND USE
4.1 The remarkable structural, physical and chemical properties of graphene — particularly its mechanical strength, high electronic mobility, lightness, and transparency (single layer or a few layers) — have generated worldwide research and industrial production efforts aimed at developing practical applications. Various industrially scalable production methods have been developed, including bottom-up approaches that grow graphene from small molecules (with or without a substrate), and top-down methods that start with graphite and exfoliate it by mechanical, chemical or electrochemical methods to produce nanoscale product such as graphene flakes. Two common exfoliation methods are: (1) oxidation of graphite to graphene oxide (GO) followed by additional processing to form reduced graphene oxide (r-GO) (2) and, (2) liquid phase exfoliation of graphite (3). The exfoliation methods, as well as substrate-less bottom-up approaches, produce materials in the form of flakes that can be dispersed in various solvents, making them suitable for applications requiring solution processing. Although there are many commercial “graphene” materials available on the market, the quality of these products is highly variable (4). There are many challenges in assessing the physical properties of the materials. In this guide we discuss how Raman spectroscopy (Raman) and X-ray photoelectron spectroscopy (XPS), as well as atomic force microscopy (AFM) can be used to characterize materials consisting of flakes of graphene and related materials (that is, few layer graphene (FLG), GO, r-GO). Illustrative examples are provided showing how these methods can be used to identify the type of material present and to extract important parameters including lateral flake size, average flake thickness, ratio of intensities of the D and G modes (ID/IG) in the Raman spectrum and carbon to oxygen ratio. Specifically, when encountering an “unknown” material or product purporting to be “graphene,” it is essent...
SCOPE
1.1 This standard will provide guidance on the measurement approaches for assessment of lateral flake size, average flake thickness, Raman intensity ratio of the D to G bands, and carbon/oxygen ratio for graphene and related products. The techniques included here are atomic force microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. Examples will be given for each type of measurement.  
1.2 This guide is intended to serve as an example for manufacturers, producers, analysts, and others with an interest in graphene and related products such as graphene oxide and reduced graphene oxide. This Standard Guide is not intended to be a comprehensive overview of all possible characterization methods.  
1.3 This guide does not include all sample preparation procedures for all possible materials and applications. The user must validate the appropriateness for their particular application.  
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Mar-2020
• ICS: 59.100.20 - Carbon materials
• Technical Committee: E56 - Nanotechnology
• Drafting Committee: E56.02 - Physical and Chemical Characterization

Relations

• Refers: ASTM E2530-06 - Standard Practice for Calibrating the Z-Magnification of an Atomic Force Microscope at Subnanometer Displacement Levels Using Si(111) Monatomic Steps (Withdrawn 2015)
• Effective Date: 01-Nov-2006

Overview

ASTM E3220-20: Standard Guide for Characterization of Graphene Flakes provides critical guidance for measuring and assessing key physical and chemical parameters of graphene materials. Developed by ASTM International, this guide addresses the needs of manufacturers, analysts, and researchers involved in the production and application of graphene, graphene oxide (GO), and reduced graphene oxide (r-GO). The standard emphasizes practical approaches for characterizing graphene flakes using advanced analytical techniques, focusing on parameters such as lateral flake size, average flake thickness, Raman D/G band intensity ratio, and carbon-to-oxygen (C/O) ratio.

Key Topics

• Characterization Methods: The guide details three primary analytical techniques:
  • Atomic Force Microscopy (AFM): For measuring flake size and thickness.
  • Raman Spectroscopy: For analyzing structural integrity and defect density through the D/G band intensity ratio.
  • X-ray Photoelectron Spectroscopy (XPS): For qualifying chemical composition, particularly the carbon-to-oxygen ratio.
• Measurement Parameters: Clear definitions and guidance on how to determine:
  • Lateral flake size (Feret diameter)
  • Average flake thickness
  • Intensity ratio of the Raman D and G bands (ID/IG)
  • Carbon/oxygen ratio (C/O)
• Sample Requirements: Recommendations for sample preparation, ensuring representative and contamination-free results.
• Reporting Best Practices: Suggestions for comprehensive data reporting, including material description, preparation methods, calibration, instrumentation, replicates, and summary statistics.
• Limitations: The guide is not exhaustive of all characterization techniques and does not specify all sample preparation methods for every use case.

Applications

The characterization of graphene flakes is vital for quality control, research, and commercial applications where the material's physical and chemical properties directly influence performance. Use cases include:

• Materials Development: Ensuring consistency in industrial-scale graphene production by benchmarking lateral flake size and thickness.
• Quality Assurance: Identifying and quantifying defects, layer number, and chemical modifications to optimize electrical conductivity, optical transparency, and chemical reactivity.
• Product Validation: Verifying that products marketed as "graphene," "graphene oxide," or "reduced graphene oxide" meet specific, measurable criteria, fostering trust in supply chains.
• Solution Processing Applications: Facilitating the use of well-characterized graphene flakes in dispersions for electronics, coatings, composites, and energy storage devices.

By following ASTM E3220-20, stakeholders can consistently evaluate graphene materials, making informed decisions for product development, manufacturing, and research.

Related Standards

ASTM E3220-20 aligns with and references several international standards and best practices for nanomaterial characterization:

• ISO 80004-13: Nanotechnologies - Vocabulary - Part 13: Graphene and Related Two-Dimensional (2D) Materials
• ISO/TR 18196: Nanotechnologies - Measurement Technique Matrix for Characterization of Nano-Objects
• ISO 13322-1: Particle Size Analysis - Image Analysis Methods
• ISO 18115-2: Surface Chemical Analysis - Vocabulary - Scanning-Probe Microscopy Terms
• ISO 18116: Guidelines for Preparation and Mounting of Specimens for Surface Chemical Analysis
• ISO 18554: Electron Spectroscopies - Procedures for Identifying, Estimating, and Correcting for Degradation by X-Rays in XPS
• ASTM E2530: Practice for Calibrating the Z-Magnification of an Atomic Force Microscope
• ISO/IEC Guide 99: International Vocabulary of Metrology

These related documents provide complementary terminology, methods, and calibration procedures to extend the effective use of ASTM E3220-20.

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Keywords: graphene characterization, graphene flakes, ASTM E3220-20, graphene oxide, reduced graphene oxide, Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, lateral flake size, carbon oxygen ratio, nanomaterial standards, quality assurance, graphene applications.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E3220 − 20
Standard Guide for
Characterization of Graphene Flakes
This standard is issued under the fixed designation E3220; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 Thisstandardwillprovideguidanceonthemeasurement 2.1 ASTM Standards:
approaches for assessment of lateral flake size, average flake E2530Practice for Calibrating the Z-Magnification of an
thickness, Raman intensity ratio of the D to G bands, and AtomicForceMicroscopeatSubnanometerDisplacement
carbon/oxygen ratio for graphene and related products. The Levels Using Si(111) Monatomic Steps (Withdrawn
techniques included here are atomic force microscopy, Raman 2015)
spectroscopy and X-ray photoelectron spectroscopy. Examples 2.2 ISO Standards:
will be given for each type of measurement. ISO13067:2011MicrobeamAnalysis—ElectronBackscat-
ter Diffraction — Measurement of Average Grain Size
1.2 This guide is intended to serve as an example for
ISO 13322-1:2014Particle SizeAnalysis — ImageAnalysis
manufacturers, producers, analysts, and others with an interest
Methods — Part 1: Static Image Analysis Methods
in graphene and related products such as graphene oxide and
ISO 18115-2:2013Surface Chemical Analysis — Vocabu-
reduced graphene oxide. This Standard Guide is not intended
lary—Part2:TermsUsedinScanning-ProbeMicroscopy
tobeacomprehensiveoverviewofallpossiblecharacterization
ISO 18116:2005Surface Chemical Analysis — Guidelines
methods.
for Preparation and Mounting of Specimens for Analysis
1.3 This guide does not include all sample preparation
ISO/TR 18196:2016 Nanotechnologies — Measurement
proceduresforallpossiblematerialsandapplications.Theuser
Technique Matrix for the Characterization of Nano-
must validate the appropriateness for their particular applica-
Objects
tion.
ISO 18554:2016Surface Chemical Analysis — Electron
Spectroscopies — Procedures for Identifying, Estimating,
1.4 The values stated in SI units are to be regarded as
and Correcting for Unintended Degradation by X-Rays in
standard. No other units of measurement are included in this
standard. a Material Undergoing Analysis by X-Ray Photoelectron
Spectroscopy
1.5 This standard does not purport to address all of the
ISO 80004-1:2015Nanotechnologies — Vocabulary — Part
safety concerns, if any, associated with its use. It is the
1: Core Terms
responsibility of the user of this standard to establish appro-
ISO 80004-13:2017Nanotechnologies — Vocabulary —
priate safety, health, and environmental practices and deter-
Part 13: Graphene and Related Two-Dimensional (2D)
mine the applicability of regulatory limitations prior to use.
Materials
1.6 This international standard was developed in accor-
ISO/IEC Guide 9:2010International Vocabulary of Metrol-
dance with internationally recognized principles on standard-
ogy—BasicandGeneralConceptsandAssociatedTerms
ization established in the Decision on Principles for the
(VIM)
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
1 3
This guide is under the jurisdiction of ASTM Committee E56 on Nanotech- The last approved version of this historical standard is referenced on
nology and is the direct responsibility of Subcommittee E56.02 on Physical and www.astm.org.
Chemical Characterization. Available from International Organization for Standardization (ISO), ISO
Current edition approved April 1, 2020. Published May 2020. DOI: 10.1520/ Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,
E3220-20. Geneva, Switzerland, http://www.iso.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E3220 − 20
3. Terminology 4. Significance and Use
3.1 Definitions: 4.1 The remarkable structural, physical and chemical prop-
3.1.1 Feret diameter, n—perpendicular distance between erties of graphene — particularly its mechanical strength, high
two parallel lines drawn in a given direction tangential to the electronicmobility,lightness,andtransparency(singlelayeror
perimeter of an object on opposite sides of the object. a few layers) — have generated worldwide research and
ISO 13067:2011 industrial production efforts aimed at developing practical
applications. Various industrially scalable production methods
3.1.2 fewlayergraphene,n—two-dimensionalmaterialcon-
have been developed, including bottom-up approaches that
sisting of three to ten well-defined stacked graphene layers.
grow graphene from small molecules (with or without a
ISO 80004-13:2017
substrate), and top-down methods that start with graphite and
3.1.3 graphene, n—single layer of carbon atoms with each
exfoliate it by mechanical, chemical or electrochemical meth-
atom bound to three neighbours in a honeycomb structure.
odstoproducenanoscaleproductsuchasgrapheneflakes.Two
ISO 80004-13:2017
common exfoliation methods are: (1) oxidation of graphite to
3.1.3.1 Discussion—Note the three Notes in the ISO docu-
graphene oxide (GO) followed by additional processing to
ment.
form reduced graphene oxide (r-GO) (2) and, (2) liquid phase
3.1.4 graphene oxide, n—chemically modified graphene
exfoliation of graphite (3).The exfoliation methods, as well as
prepared by oxidation and exfoliation of graphite, causing
substrate-less bottom-up approaches, produce materials in the
extensive oxidative modification of the basal plane.
formofflakesthatcanbedispersedinvarioussolvents,making
ISO 80004-13:2017
them suitable for applications requiring solution processing.
Although there are many commercial “graphene” materials
3.1.5 measurand, n—quantity intended to be measured or a
quantity that is being determined by measurement. available on the market, the quality of these products is highly
variable (4). There are many challenges in assessing the
ISO/IEC Guide 99
physical properties of the materials. In this guide we discuss
3.1.6 nanoscale, n—rangefromapproximately1to100nm.
how Raman spectroscopy (Raman) and X-ray photoelectron
ISO 80004-1:2015
spectroscopy (XPS), as well as atomic force microscopy
3.1.7 reduced graphene oxide, n—reduced oxygen content
(AFM) can be used to characterize materials consisting of
form of graphene oxide. ISO 80004-13:2017
flakes of graphene and related materials (that is, few layer
3.2 Definitions of Terms Specific to This Standard:
graphene (FLG), GO, r-GO). Illustrative examples are pro-
3.2.1 average flake thickness, n—average height of the
vided showing how these methods can be used to identify the
flake, which is determined by measuring the average cross
type of material present and to extract important parameters
section profile of the flake (measuring the position on the
including lateral flake size, average flake thickness, ratio of
substratenexttotheflakeandthestepintopographyduetothe
intensities of the D and G modes (I /I ) in the Raman
D G
flake).
spectrum and carbon to oxygen ratio. Specifically, when
encountering an “unknown” material or product purporting to
3.2.2 exfoliated graphene and related material, n—products
be “graphene,” it is essential to quantify the thickness and
producedbyexfoliationthatmaybepresentineitherapowder
lateral flake size distributions by AFM, to assess the level of
or liquid dispersion form.
defects in the flakes using the ratio of intensities of the D and
3.2.3 flake, n—graphene or layers of graphene related ma-
G bands in the Raman spectrum, and to determine the level of
terials.
oxidation of the material (C/O ratio) using XPS. These
3.2.4 lateral flake size, n—dimension determined as the
measurandsareimportantforqualitativeassessmentofthetype
Feret diameter.
of material present, as well as quantitative measures of the
3.2.4.1 Discussion—Note, it is sometimes referred to as
quality of the flakes which can be correlated with properties
Feret’s statistical diameter (1) or caliper diameter.
relevant to applications based on conductivity, optical
3.2.5 peak intensity, n—the maximum value of the intensity
transparency, and chemical reactivity.
for a Raman peak or X-ray photoelectron spectroscopy peak
4.2 Itshouldbenotedthatthesematerialsandproductsmay
afterthebaselinehasbeensubtractedandpeakfittinghasbeen
exist in either a powder or dispersion (in liquid) form. Other
performed.
techniques and measurements (ISO/TR 18196:2016) such as
3.3 Acronyms:
X-raydiffraction(XRD),opticalmicroscopy,scanningelectron
3.3.1 CVD—chemical vapor deposition microscopy (SEM), transmission electron microscopy (TEM)
and surface area measurement, can also be used for character-
3.3.2 FLG—few layer graphene
izationofgrapheneandrelatedproductsbutdiscussionofthese
3.3.3 FWHM—full width at half maximum
methods is beyond the scope of this guide.
3.3.4 GO—graphene oxide
5. Techniques
3.3.5 r-GO—reduced graphene oxide
5.1 Raman Spectroscopy:
5.1.1 General Considerations:
5.1.1.1 Ramanspectroscopyisapowerfultechniqueusedto
The boldface numbers in parentheses refer to a list of references at the end of
this standard. characterize graphene related materials in the form of powders
E3220 − 20
or dispersions. Powders can be pressed into pellets or attached giveninRef. (11).Thehighdefectdensityregime(typicallyL
d
to a substrate using an adhesive film, whereas dispersions can < 5 nm) can be identified by an increase in the FWHM of the
be used to produce a film on a substrate. Raman spectra of D band, which remains constant in the low defect density
grapheneconsistofthreemainfeatures,twoin-planemodes(D regime.IncalculatingtheI /I ratioitisimportanttonotethat
D G
peak intensities (after background subtraction) are typically
andG)andthesecond-orderovertoneoftheDmode(2D).The
frequency, intensity and line-shape of these modes all provide used,ratherthanintegratedpeakareas.Thisisduetoproximity
of the D' band to the G band which can make it difficult to
information on flake structure and can be used to distinguish
monolayer graphene from FLG, GO or graphite (5-8). The separate the contributions of these two modes. I /I values
D G
-1
calculatedfromthespectrashowninFig.X1.1areindicatedin
G-band for graphene is typically observed at ≈1580 cm ,
the figure. Edges have also been shown to contribute to the
independent of excitation energy, whereas the D and 2D peaks
D-band intensity (13), so small flake sizes can give rise to a
exhibitdispersion,sothattheirpositionwillvarydependingon
large D band intensity even if the interior of the flakes do not
the excitation laser used. The D peak is not visible in pristine
have many defects.
graphene due to selection rules but is allowed in the presence
of defects, leading to its use as an indicator of structural
5.2 X-Ray Photoelectron Spectroscopy:
quality. The 2D mode is always allowed and is particularly
5.2.1 General Considerations:
prominent in high quality monolayer samples where its inten-
5.2.1.1 XPS provides information on the chemical compo-
sity is typically larger than that of the G mode.
sitionofasamplebymeasuringtheintensityofphotoelectrons
5.1.1.2 Raman spectra for several types of graphene related
ejected from the sample as a function of kinetic energy (E ).
k
materials are shown in Fig. X1.1 with the main peaks identi-
Many of the observed peaks can be assigned to emission from
fied. A spectrum from a continuous monolayer film grown by
particular core levels of elements present in the sample,
CVD is shown as an example of the ideal case of graphene.
allowing these elements, if present in sufficient quantity to be
Similar spectra have been reported from isolated monolayer
identifiedandquantified.Formaterialsproducedbyexfoliating
flakes produced by micromechanical cleavage (9). In the CVD
graphite the ratio of carbon to oxygen is a key parameter in
graphene spectra the D peak is barely visible and the 2D mode
determining the suitability of the product for various applica-
-1
isseentoconsistofasingleLorenztianlineshapeat2640cm
tions.Furthermore,XPScanbeusedtomonitorthepresenceof
-1
(withFWHMof≈40cm )andexhibitingmorethantwicethe
contaminants that may be introduced in the processing of the
intensity of the G peak.
material (4).
-1
5.1.1.3 Itisnotedthat2DFWHMsof20−40cm havebeen
5.2.1.2 XPS is typically performed in a high vacuum envi-
-8
reported in the literature with narrower line widths indicating
ronment at pressures below5×10 mbar. Samples can be in
higher quality samples (10). The spectra for the FLG powder
the form of powders or thin films prepared from dispersions.
and FLG ink are rather similar. The D band is clearly visible,
For thin film samples, the film should be thick enough (>10
indicative of defects in the flakes or small flake size as edge
nm)toavoidcontributionsfromthesubstrate.Itisimportantto
sites, or both, can also contribute to the D peak.An additional
avoid contamination during sample preparation and transfer
-1
defectrelatedpeak(D')isalsoobservedat≈1620cm .The2D
into the measurement chamber (see ISO 18116:2005). Care
peak is broadened and peak intensity is similar to that of the G
should also be taken to minimize sample damage during
band. The position of the 2D band is shifted to higher
measurement due to irradiation by the X-ray beam (see ISO
-1
wavenumbers but is still lower than the 2685 cm band
18554:2016).
observed for graphite. This suggests that the flakes in these
5.2.2 Survey Scans:
materials are less than ≈6 layers, and the Raman spectra of
5.2.2.1 Survey scans are spectra acquired over a wide range
thickerflakesareindistinguishablefromgraphite.Forther-GO
of energies, including those representative of carbon and
film, all features in the spectrum are broadened, the D band is
oxygen photoelectron peaks and often employ a large pass
furtherincreasedinintensityandislargerthantheGpeak,and
energy yielding favorable signal:noise. Representative survey
the2Dpeakisweakened.TheGpeakisalsoseentobeshifted
scans for three samples (a film consisting of GO flakes as well
to higher frequency as a result of increasing D' intensity
as powders of FLG and r-GO) are shown in Fig. X2.1. The
coupled with the broadening which makes these peaks difficult
most notable difference between the spectra is the oxygen
to separate.
signal, which is barely visible for the FLG sample but is the
5.1.2 Calculation and Significance of I /I :
D G largest peak for the GO sample, reflecting the different oxygen
5.1.2.1 The ratio of the D and G peak intensities (I /I )isa content in these samples. Small peaks associated with impuri-
D G
useful parameter for quantifying the structural quality of ties can also be identified in the GO and r-GO samples when
graphene. Experiments using ion bombardment to create de- evaluating survey scans. These contaminants may arise from
fectsingraphenehaveshownthatatlowdefectdensities,I /I reagents commonly used in the production of graphene oxide.
D G
increases with defect density before reaching a maximum and The intensities of the labeled peaks in Fig. X2.1, after
then decreases with defect density in the high density regime appropriate background subtraction, can be used for quantita-
(11, 12). The value of I /I at the maximum, defining the tive analysis of the atomic composition of the sample. This
D G
cross-overbetweenthetworegimes,dependsonthesizeofthe requires application of relative sensitivity factors for the
defects (and varies depending on the type of ion used in the various core levels, which are typically dependent on the
bombardment).An equation relating the measured I /I to the particularspectrometerusedforthemeasurements.Theatomic
D G
distance between defects (L ) and the size of the defects is composition of the three different samples (presented in
d
E3220 − 20
percentages) is summarized in the table inset of Fig. X2.1. ment of both the lateral dimension and the layer heights
Each entry represents the average of three different locations (thickness), AFM imaging is recommended to measure the
on the sample, with the uncertainties corresponding to the lateral flake size and height (average flake thickness) distribu-
standard deviation. tions of graphene and related material (for example, GO and
5.2.3 High Resolution Scans and Calculation of C/O Ratio: r-GO).Additionally,allsizemeasurementscouldbeautomated
5.2.3.1 Whilethesurveyscansallowforrapididentification using processing software (for example, open source software
andquantificationofelementspresentinthesample,additional ImageJ (17) or Gwyddion (18)) to reduce the influence of the
information on the chemical state of the elements can also be bias from different analysts.
determined by examining the detailed structure of the peaks
5.3.1.4 For preparation of samples used inAFM imaging to
obtained from high-resolution spectra. Such spectra typically determine the dimensions of graphene and related materials, a
are acquired over a smaller, element specific energy window
dispersion is required. If the material was originally provided
usingalowerpassenergy.Thedecreasedpassenergyimproves as a powder, it requires dispersing in a suitable solvent such as
the energy resolution at the cost of reducing signal intensity.
water, isopropanol or N-methylpyrrolidone (NMP) (19, 10).
Fig. X2.1 shows a high-resolution scan of the C1s region for More importantly, a statistically meaningful number of indi-
three different types of powder samples: GO, r-GO and FLG.
vidual flakes (that is, flakes are isolated from each other on a
Apparent in Fig. X2.1 are the different shapes exhibited by the
substrate) should be measured, in order to ensure the quanti-
C1s peak for the different samples. For FLG, a single peak
fication analysis to be performed is representative. Typically,
centered at ≈284 eV is observed, with an asymmetric tail
thenumberoftobecountedfortheevaluationofastatistically
extending to higher binding energies. This type of line shape representative sample depends on the width of the size distri-
(Doniach-Sunjic) is also observed for graphite samples. For
bution. For narrow size distribution (that is, the geometric
r-GO a shoulder is observed at higher binding energy. For GO standard deviation σ <1.5, see ISO 13322-1:2014), 300 flakes
g
a clear second peak at ≈286 eV is observed with a weak
aresufficient,whereasforsampleswithawidesizedistribution
shoulder at still higher binding energy. The second C1s peak (for example, bimodal distributions or σ > 1.5), at least 700
g
and shoulder for GO are assigned to carbon atoms coordinated
flakes have to be measured.
to oxygen (2).
5.3.1.5 Usually, mica and silicon (or silicon oxide) can be
5.2.3.2 The C/O ratio is calculated by integrating the areas
selected as substrates and spin-coating or drop casting ap-
under the C1s and O1s (not shown) peaks after background
proaches can be used to produce samples for AFM measure-
subtraction. As expected, the C/O ratios are significantly
ments. It should be noted that ultrasonication steps (for
different for these different materials ranging from 2.4 for GO
example, bath sonication or probe sonication) are usually used
to85fortheFLGsample.WhiletheoreticallyperfectFLGwill
todispersethegrapheneandrelatedmaterial,inordertoobtain
onlycontaincarbon,inrealityFLGpowdersmayexhibitsome
a more stable dispersion. However, a sonication step with
oxygen related defects. However, at the small O1s signals on
different sonication energies may introduce lateral flake size
these samples (≤1 % atomic composition), adventitious con-
changes and therefore affect the characterization results of the
tamination of oxygen containing species may also contribute.
intrinsic properties of the products. For example, in the 0–20
One difficulty in determining the C/O ratio can arise if the
MJ/g range of sonication energies, the average hydrodynamic
sample exhibits oxygen containing impurities such as oxides,
diameter of commercial GO flakes varied from≈2000 down to
carbonatesandhydroxides.High-resolutionspectraofboththe
≈170 nm (20).
impurity (that is, sulfur, manganese, sodium, calcium) and the
5.3.2 Quantification of Lateral Flake Size:
O1s regions will usually reveal whether this is an issue. These
5.3.2.1 AFM scanning measurement should be performed
types of impurities usually result in a substantial shift of the
under ambient conditions, with the in
...