9.24.3
The In luence o Reac ion
Condi ions on he P ope ies o
G aphene Oxide
Mi osla Huskić, Dejan Kepić, Duška Kleu , Mi an Moze ič, Alenka Vesel, Alojz Anžlo a ,
Danica Bajuk Bogdano ić and S e lana Jo ano ić
A icle
h ps://doi.o g/10.3390/nano14030281
Ci a ion: Huski´c, M.; Kepi´c, D.; Kleu ,
D.; Moze iˇc, M.; Vesel, A.; Anžlo a ,
A.; Bogdano i´c, D.B.; Jo ano i´c, S.
The In luence o Reac ion Condi ions
on he P ope ies o G aphene Oxide.
Nanoma e ials 2024,14, 281. h ps://
doi.o g/10.3390/nano14030281
Academic Edi o : Ma co Cannas
Recei ed: 19 Decembe 2023
Re ised: 22 Janua y 2024
Accep ed: 24 Janua y 2024
Published: 30 Janua y 2024
Copy igh : © 2024 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
nanoma e ials
A icle
The In luence o Reac ion Condi ions on he P ope ies o
G aphene Oxide
Mi osla Huski´c 1,*, Dejan Kepi´c 2, Duška Kleu 2, Mi an Moze iˇc 3, Alenka Vesel 3, Alojz Anžlo a 4,
Danica Bajuk Bogdano i´c 5and S e lana Jo ano i´c 2
1Facul y o Polyme Technology, 2380 Slo enj G adec, Slo enia
2Vinˇca Ins i u e o Nuclea Sciences, Na ional Ins i u e o he Republic o Se bia, Uni e si y o Belg ade,
11000 Belg ade, Se bia; [email p o ec ed] (S.J.)
3Jože S e an Ins i u e, 1000 Ljubljana, Slo enia; [email p o ec ed] (M.M.); [email p o ec ed] (A.V.)
4Na ional Ins i u e o Chemis y, 1000 Ljubljana, Slo enia; [email p o ec ed]
5Facul y o Physical Chemis y, Uni e si y o Belg ade, 11158 Belg ade, Se bia; danabb@ h.bg.ac. s
*Co espondence: mi osla [email p o ec ed]
Abs ac : The p esen s udy ocuses on co ela ions be ween h ee pa ame e s: (1) g aphi e pa icle
size, (2) he a io o g aphi e o oxidizing agen (KMnO
4
), and (3) he a io o g aphi e o acid
(H
2
SO
4
and H
3
PO
4
), wi h he eac ion yield, s uc u e, and p ope ies o g aphene oxide (GO). The
co ela ions a e a challenge, as hese h ee pa ame e s can ha dly be sepa a ed om each o he due o
he a ia ions in he iscosi y o he sys em. The la ge he g aphi e pa icles, he highe he iscosi y
o GO. Dec easing he a io o g aphi e o KMnO
4
om 1:4 o 1:6 gene ally leads o a highe deg ee
o oxida ion and a highe eac ion yield. Howe e , he di e ences a e e y small. Inc easing he
g aphi e- o-acid- olume a io om 1 g/60 mL o 1 g/80 mL, excep o he smalles pa icles, educed
he deg ee o oxida ion and sligh ly educed he eac ion yield. Howe e , he eac ion yield mainly
depends on he ex en o pu i ica ion o GO by wa e , no on he eac ion condi ions. The la ge
di e ences in he he mal decomposi ion o GO a e mainly due o he bulk pa icle size and less o
o he pa ame e s.
Keywo ds: g aphene oxide; syn hesis; he mal cha ac e iza ion; Raman spec oscopy; XPS; DLS;
TGA; UV-VIS; XRD
1. In oduc ion
G aphene is a one-a om- hick shee o hexagonally packed ca bon a oms ha ex-
hibi s se e al in e es ing p ope ies, such as a high speci ic su ace a ea (2630 m
2
/g), high
elas ic modulus (1 TPa), and excellen elec ical (6000 S/cm) and he mal conduc i i y
(
3000–5000 W/mK
) [
1
,
2
]. These p ope ies make g aphene a good candida e as a nano ille
o he ab ica ion o high-s eng h, high- he mal, and elec ically conduc i e polyme
nanocomposi es. The e a e se e al di ec me hods o p oduce g aphene, such as mechani-
cal clea age o g aphi e, chemical apo deposi ion, epi axial g ow h [
1
,
2
], unzipping o
ca bon nano ubes [
3
], and elec ochemical ex olia ion [
4
]. Howe e , hese me hods a e no
sui able o mass p oduc ion, which is equi ed o he manu ac u ing o conduc i e poly-
me composi es. La ge quan i ies o g aphene can be p oduced by he chemical o he mal
educ ion o g aphene oxide (GO). GO is no only an in e es ing ma e ial as a p ecu so o
g aphene bu can also be used as a ein o cing ille in polyme nanocomposi es. GO has
he e o e been he subjec o in ensi e esea ch o se e al yea s [5–9].
The exac s uc u e o GO is s ill no clea , bu we do know ha he e a e hyd oxyl,
epoxy, ca bonyl, and ca boxyl g oups bound o g aphene shee s [
1
,
6
,
10
]. Howe e , i is
wo h no ing ha he e should be a dis inc ion be ween g aphene oxide and g aphi e oxide
(G O). Du ing he oxida ion o g aphi e, GO is o med since i is dispe sed in acid on a
molecula le el. This migh be a gued due o he laye ed s uc u e o he slu y, as will be
Nanoma e ials 2024,14, 281. h ps://doi.o g/10.3390/nano14030281 h ps://www.mdpi.com/jou nal/nanoma e ials
Nanoma e ials 2024,14, 281 2 o 21
shown la e . Howe e , he laye ed s uc u e is a consequence o in e cala ed wa e (o he
o he pola sol en ) be ween he GO shee s. Adding mo e wa e o he slu y will inc ease
he in e laye hickness and inally ex olia e he GO shee s. On he o he hand, once i is
d ied, he shee s s ick oge he , and G O is o med. The di e ence be ween GO and G O
is some imes a ibu ed o he numbe o laye s (<10 is GO; >10 is G O) [
6
]. G O can be
dispe sed in wa e bu no ully dissol ed. In ou expe ience, G O p ecipi a es om wa e
dispe sion wi hin a ew days. The e o e, he majo i y o cha ac e iza ions o GO ound in
he li e a u e a e based on G O.
GO is ela i ely easily syn hesized by he oxida ion o g aphi e using a ious me hods
like Humme s’, B odie, and S audenmaie , which a e being modi ied and imp o ed [
11
,
12
].
Addi ionally, GO has also been syn hesized mechanochemically [
13
] and elec ochemically [
14
].
The e a e se e al e iew a icles co e ing he chemical syn hesis and modi ica ions o
GO [
6
,
8
,
15
–
17
]. Howe e , i is e y o en ha d o compa e he esul s o di e en au ho s
who use almos iden ical ma e ials. The eason is ha e y di e en a ios o g aphi e o
oxidan and acid we e used, a a ious eac ion imes, a oom o inc eased empe a u es.
Some imes e en ul asound was used o p omo e he eac ion [
18
]. Va ious me hods migh
p oduce GO wi h di e en chemical s uc u es, which esul s in di e en p ope ies and
eac i i y. Fo example, i has been obse ed ha GO p oduced using Humme s’ me hod
has lowe eac i i y han GO p oduced using B odie’s me hod [19].
The p esen s udy is ocused on he in luence o h ee pa ame e s, namely g aphi e
pa icle size, g aphi e- o-oxidan (KMnO
4
) a io, and g aphi e- o-acid a io, on he s uc u e
and p ope ies o GO p epa ed using he so-called modi ied Humme s’ me hod o Tou ’s
me hod [
12
]. The in luence o g aphi e pa icle size on GO pa icle size has al eady been in-
es iga ed, bu he esul s epo ed by di e en g oups a y conside ably and a e he e o e
no conclusi e. Fo example, Zhou and Liu p oduced GO pa icles up o 200
µ
m in size
and epo ed ha he size o GO is somehow smalle bu compa able in size o he pa en
g aphi e pa icles [
20
]. Chen e al. used sie ed g aphi e and ob ained la ge GO pa icles
wi h la ge g aphi e pa icles. Howe e , hese au ho s used a ious eac ion condi ions
( ime o eac ion and g aphi e/acid a io) o he a ious g aphi e sizes [
21
]. O he au ho s
claim ha he use o la ge-size na u al g aphi e lakes as p ecu so s does no necessa ily
p oduce la ge GO shee s [22].
The li e a u e su ey e ealed ha a ious a ios o g aphi e o KMnO
4
we e used
o syn hesize GO; howe e , no sys ema ic s udy o he in luence o he a io on he inal
p ope ies o GO was pe o med, so he exac co ela ion is ye o be epo ed.
This s udy comp ehensi ely conside s h ee di e en syn he ic ac o s and add esses
hei e ec s on GO yield and i s s uc u al p ope ies. Thanks o he mul i ac o ial app oach,
i b ings new da a ega ding he e ec o he iscosi y o he eac ion mix u es and c i ically
discusses he issues in he syn he ic and he mal cha ac e iza ion p oblems in GO. Se e al
o he p oblems associa ed wi h GO syn hesis and cha ac e iza ion will be disclosed and
discussed in his a icle, and he di e ences will be explained.
2. Ma e ials and Me hods
2.1. Ma e ials
Expanded g aphi e Sig a he m
®
GFG 130 (a e age pa icle size: 130
µ
m) is he p oduc
o SGL Ca bon GmbH, Mei ingen, Ge many. I was ac iona ed in o h ee phases by si ing
i h ough 45-
µ
m and 100-
µ
m sie es. The g aphi e lakes ob ained by si ing h ough a
45-
µ
m sie e we e designa ed as G45; lakes ha emained on a 45-
µ
m sie e and passed a
100-
µ
m sie e we e designa ed as G45+; and lakes ha did no pass a 100-
µ
m sie e we e
designa ed as G100+.
H
2
SO
4
(95–97%), H
3
PO
4
(85%), H
2
O
2
(30%), KMnO
4
, and me hanol (MeOH) we e
pu chased om Me ck KGaA, Da ms ad , Ge many and used as ecei ed.
Nanoma e ials 2024,14, 281 3 o 21
2.2. Syn hesis o G aphene Oxide (GO)
GO was p epa ed acco ding o he modi ied Humme s’ o Tou ’s me hod [
12
]. A o al
o 1 g o g aphi e lakes was added o a 9:1 mix u e o concen a ed H
2
SO
4
/H
3
PO
4
acids
(60 mL o 80 mL) in a 250-mL beake equipped wi h an o e head poly e a luo oe hylene
(PTFE) s i e . KMnO
4
(4 g o 6 g) was added in a 1 g po ion pe hou , and he eac ion
p oceeded a oom empe a u e o 20 h. A he end o he eac ion, ice (
≈
0.2 kg) was
added, ollowed by 5 mL o 30% H
2
O
2
. The p oduc was di ided in o six cen i uge ubes
(50 mL), cen i uged a 9000 pm o 15 min, and decan ed. The p oduc was successi ely
washed wi h wa e , 10% HCl ( wice), and MeOH ( h ee imes). This p ocedu e was used
o ob ain as much p is ine GO as possible since i was ound ha pu i ica ion wi h wa e
leads o a change in GO s uc u e [
10
]. Each ime, he cen i uge ube was illed up o
48 mL
, mixed ho oughly o edispe se he pa icles, and cen i uged/decan ed again. The
pu i ied GO samples we e ei he d ied o kep as MeOH dispe sion (≈8 w .% o GO) and
s o ed in a idge. The GO samples we e designa ed acco ding o hei g aphi e size (i.e.,
GO45, GO45+, and GO100+) and g aphi e:KMnO4:acid a io ( o example, GO45 1:4:60).
2.3. Cha ac e iza ion
The size o he g aphene oxide pa icles in deionized (DI) H
2
O was measu ed by dy-
namic ligh sca e ing (DLS) using a Mal e n Ze asize Nano-ZS. Sca e ing was measu ed
a an angle o 173
◦
. The concen a ion o GO in he deionized wa e was 0.05 mg/mL. The
GO pa icle size o he d ied as well as slu y samples was de e mined. The ca bon black
and H2O e ac i e indices o 1.470 and 1.3317, espec i ely, we e used in calcula ions.
The mo phologies o hese samples we e e alua ed h ough a omic o ce mic oscopy
(AFM) on a Quesan mic oscope (Agou a Hills, CA, USA). ope a ing in apping mode
in he ai a oom empe a u e. The samples we e dispe sed in wa e , d op cas on a Si
subs a e, and imaged a e d ying. Silicon ips (Nano and Mo e, Gmbh, We zla , Ge many)
wi h a cons an o ce o 40 N/m we e used o AFM imaging. The images we e analyzed
using Gwyddion so wa e ( e sion 2.44).
The chemical composi ion o he GO pa icles was in es iga ed by X- ay pho oelec on
spec oscopy (XPS) using a model TFA XPS om Physical Elec onics (Chanhassen, MN,
USA). The GO samples we e exci ed wi h monoch oma ic Al K
α
1,2 adia ion a
1486.6 eV
.
Su ey-scan spec a we e measu ed a a pass ene gy o 187 eV and an ene gy s ep o
0.4 eV. High- esolu ion spec a o ca bon we e measu ed a a pass ene gy o 29.35 eV
and an ene gy s ep o 0.125 eV. The measu ed spec a we e analyzed using Mul iPak
8.1c so wa e supplied by he XPS manu ac u e . Ca bon C1s spec a we e i ed using a
Gaussian–Lo en zian unc ion and Shi ley backg ound sub ac ion.
The he mal s abili y o he GO samples was de e mined using a Me le Toledo
(G ei ensee, Swi ze land) TGA/DSC 1 ins umen . The samples o »3 mg we e hea ed om
40 ◦C o 800 ◦C a a hea ing a e o 5 K/min in a ni ogen low a e o 20 mL/min.
The cha ac e iza ion o he GO pa icles by X- ay di ac ion (XRD) was pe o med wi h
he X- ay powde di ac ome e PANaly ical X’Pe PRO MPD (CuK
α1 adia ion = 1.5406 Å
)
in 0.033
◦
s eps om 2
θ
= 2.0
◦
o 60
◦
. The sample o GO slu y in me hanol was co e ed wi h
a Kap on oil o p e en he e apo a ion o he me hanol. O he samples we e acuum-d ied
a 40
◦
C o 24 h p io o XRD cha ac e iza ion. The basal spacings we e calcula ed acco ding
o B agg’s law.
Raman spec a we e eco ded on a DXR Raman mic oscope (The mo Fishe Scien-
i ic, Wal ham, MA, USA). Each spec um was ob ained a oom empe a u e by using a
532-nm
exci a ion line wi h a powe o 5 mW. The spec al esolu ion was 1 cm
−1
, and he
acquisi ion ime was 10 ×10 s.
The UV-VIS abso p ion spec a we e eco ded wi h a LLG-uniSPEC 2 spec opho ome-
e . The samples dispe sed in wa e we e eco ded in qua z cu e es a oom empe a u e.
The s uc u al p ope ies o GO we e in es iga ed using ATR-FTIR spec oscopy. The
measu emen s we e pe o med on a The mo Scien i ic Nicole iS20 (Wal ham, MA, USA)
FTIR spec ome e equipped wi h a single e lec ion diamond c ys al. All measu emen s
Nanoma e ials 2024,14, 281 4 o 21
we e conduc ed a oom empe a u e. The FTIR spec a o di e en GO samples we e
eco ded in he ange o 400 o 4000 cm
−1
a 64 scans pe spec um wi h a esolu ion o
2 cm−1.
Wa e con ac angle (WCA) measu emen s we e ca ied ou by using he sessile d op
me hod on he The a Li e con ac angle me e (Biolin Scien i ic, S ockholm, Sweden). To
ob ain hin ilms o GO samples sui able o measu ing he WCA, 100
µ
L o he GO
sample dispe sed in wa e was d op cas on eshly cleaned mic oscope slides and d ied a
60 ◦C
. This p ocedu e was epea ed se e al imes un il homogeneous ilms wi h a a he
smoo h su ace we e p epa ed. To acqui e he WCA, a 6-
µ
L d ople o deionized wa e
was ca e ully placed on he samples using a mic osy inge. All WCA measu emen s we e
pe o med a ambien condi ions (25
◦
C). The da a we e analyzed using OneA ension
so wa e ( e sion 4.0.3).
3. Resul s and Discussion
One o he i s obse a ions du ing his wo k was ha no only he a io o eac an s is
impo an bu also he amoun o eagen s. In he ini ial phase o he syn hesis o g aphene
oxide pa icles, he g aphi e lakes became mo e hyd ophilic due o he oxida ion in he
mix u e o acids and po assium pe mangana e, esul ing in swelling and inc eased iscosi y.
The la ge he g aphi e pa icles, he mo e iscous he pas e. In ou p e ious expe imen s,
10 g o g aphi e (GFG 130) was oxidized in 500 mL o he acid mix u e ( a io: 1:50) [
9
].
Mixing was no e y e ec i e, as he eac ion mix u e was e y iscous a he end o
he eac ion. Howe e , using only 1 g o g aphi e and a smalle beake , as in he cu en
expe imen s, i was no possible o ca y ou a eac ion using he same g aphi e- o-acid
a io (1:50). As soon as he iscosi y inc eased, he p oduc s uck o he glass wall and he
s i e only mixed he ai . The e o e, he g aphi e: acid a io was inc eased o 1:60 and 1:80
in hese expe imen s.
3.1. De e mina ion o Reac ion Yield
Since he exac chemical s uc u e o GO is no known, he eac ion yield is usually
de e mined as he weigh o GO di ided by he weigh o he g aphi e used in he eac ion.
The quan i a i e sepa a ion o GO om he eac ion mix u e is necessa y o de e mine
he eac ion yield. The e a e wo me hods equen ly epo ed o he sepa a ion and
pu i ica ion o GO, namely cen i uga ion and il a ion. Howe e , hese me hods a e
usually inadequa ely desc ibed in he li e a u e. Fo example, Rou ke e al. used a modi ied
Humme s’ me hod and pu i ied GO by cen i uga ion, disca ding he supe na an , and
esuspending he esidue a leas en imes [
23
]. Un o una ely, Rou ke e al. did no speci y
which liquid was used o hei pu i ica ion. I wa e was used en imes, he yield o GO
would ha e been e y low, as some GO is always soluble in wa e and los .
Chen e al. pu i ied GO by dialysis, ollowed by cen i uga ion. The yield hey
ob ained was 92–96% [
11
]. A simila yield (»100%) was ob ained by Daud e al. [
24
], and
110% by Be a e al. [
18
] A e y low yield (30%) was ob ained by Xu e al. Howe e , unlike
he o he s, Xu e al. disca ded he sedimen a e cen i uga ion and only used he soluble
ac ion o GO pa icles [25].
Hussein e al. p epa ed GO acco ding o he S audenmaie me hod and washed i i s
wi h 5% HCl, hen wi h wa e , and he slu y was acuum- il e ed [
26
]. This seems simple,
bu , acco ding o ou expe ience, he il a ion should be explained in de ail, especially
wha kind o il e was used. The il e ing o GO/wa e is no i ial, and he il e ing ime
is also an impo an pa ame e . In ou expe imen s, we ied se e al il e pape s and glass
il e s wi h e y li le success; only e y small amoun s o GO can be il e ed be o e he
po es become clogged. The bes il e seems o be he Isopo e
TM
polyca bona e memb ane,
which enables he p oduc ion o hin ilms; howe e , il e ing la ge quan i ies is also e y
p oblema ic (we only es ed memb anes wi h 0.22 µm po e size and 47 mm in diame e ).
Inc easing he a io o g aphi e o KMnO
4
om 1:4 o 1:6 inc eased he eac ion yield,
as shown in Table 1. Howe e , he e ec is small, e en o he smalles pa icles (GO45).
Nanoma e ials 2024,14, 281 5 o 21
Ob iously, he 1:4 a io enables enough oxidan o almos op imal yield. This is consis en
wi h p e ious indings [10].
Table 1. The yield o GO, calcula ed as a weigh o GO di ided by he weigh o g aphi e used in he
eac ion, depending on he g aphi e pa icle size (<45
µ
m, 45–100
µ
m, and >100
µ
m), g aphi e- o-
KMnO4 a io (1:4 o 1:6), and quan i y o acids (60 mL o 80 mL).
Yield
(%)
Yield
(%)
Yield
(%)
GO45 1:4:60 210 GO45+ 1:4:60 223 GO100+ 1:4:60 223
GO45 1:4:80 217 GO45+ 1:4:80 218 GO100+ 1:4:80 223
GO45 1:6:60 226 GO45+ 1:6:60 226 GO100+ 1:6:60 227
GO45 1:6:80 228 GO45+ 1:6:80 222 GO100+ 1:6:80 226
Table 1also shows he yield o he cases in which he GO syn hesis p ocedu e was
pe o med using wo di e en acid amoun s (60 mL and 80 mL). The in luence o he
a io o acid o g aphi e on he yield is no s aigh o wa d. Fo he smalles pa icles, a
ce ain inc ease in yield was obse ed wi h inc easing a io, while a dec ease was obse ed
o la ge g aphi e pa icles. Due o he lowe concen a ion o KMnO
4
a a highe a io,
which could dec ease he eac ion a e, he dec ease in yield was somehow expec ed. The
unexpec ed inc ease o he smalles pa icles can be ela ed o he highe mobili y o he
eagen s in a much less iscous solu ion. The iscosi y o he GO slu y a he end o he
syn hesis inc eased wi h inc easing g aphi e pa icle size. All he p oduc s p epa ed in
60 mL
o acid we e pas y a he end o he eac ion. The p oduc s p epa ed om G45 in
80 mL
we e iscous liquids, while he p oduc s p epa ed om G45+ o G100+ we e s ill
pas y. The e o e, he mobili y o he eagen s only s ongly inc eased o G45, which migh
lead o a sligh ly inc eased yield.
Howe e , all hese di e ences in yields migh be a consequence o he di e en
solubili ies o he p oduc s due o di e en chemical s uc u es and di e en pa icle sizes,
as well as due o small and unin ended di e ences du ing he pu i ica ion p ocess. We
obse ed ha a pa o GO is always soluble in wa e and o ms a sa u a ed solu ion.
The es o GO is dispe sed and can be sepa a ed by cen i uga ion. The e o e, ou esul s
summa ized in Table 1do no depend so much on he eac ion condi ions ( a io o eac an s)
bu a he on he pu i ica ion me hod. I mo e wa e is used in he pu i ica ion, a lowe
yield is obse ed.
To p o e his, we ook a la ge ba ch o GO/me hanol slu y om p e ious s udies and
pe o med a pu i ica ion wi h demine alized wa e and cen i uga ion. App oxima ely 2.5 g
o slu y (11% GO in me hanol) was weighed in o h ee cen i uge ubes, and app oxima ely
35 g o wa e was added. The cen i uging ime was 2 h a 9000 pm. The soluble pa was
disca ded, and wa e (app oxima ely 25 g, as he swelling o GO in wa e is g ea e han in
me hanol) was added o ubes #2 and #3, mixed ho oughly, and cen i uged again. This
p ocess was epea ed once mo e o ube #3. All he samples we e d ied by lyophiliza ion
o 48 h and hen in acuo a 40
◦
C o 8 h. The loss o GO was 15%, 25%, and 58% a e he
i s , second, and hi d pu i ica ion s eps, espec i ely.
3.2. XPS Analysis o GO
The eac ion yield in ou expe imen s is e y high compa ed o he p e iously pub-
lished da a. This is a consequence o he ac ha we only pe o med one wash using wa e .
E en when 10% HCl is used o washing, GO is much less soluble, less swellable, and
easie o sepa a e by cen i uga ion. Al hough i is desi able o ha e a good yield, his
aises he ques ion o he pu i y o GO pu i ied by ou me hod. Figu e 1shows he XPS
su ey spec a o GO100+ 1:6:80 as an example.
Nanoma e ials 2024,14, 281 6 o 21
The eac ion yield in ou expe imen s is e y high compa ed o he p e iously pub-
lished da a. This is a consequence o he ac ha we only pe o med one wash using wa-
e . E en when 10% HCl is used o washing, GO is much less soluble, less swellable, and
easie o sepa a e by cen i uga ion. Al hough i is desi able o ha e a good yield, his
aises he ques ion o he pu i y o GO pu i ied by ou me hod. Figu e 1 shows he XPS
su ey spec a o GO100+ 1:6:80 as an example.
Figu e 1. The XPS su ey spec a o GO100+ 1:6:80.
Only he peaks o ca bon and oxygen a e clea ly isible, which indica es ha he con-
cen a ion o impu i ies, excep o sul u and chlo ine, is below he de ec ion limi . The e-
o e, he XPS esul s show ha he GO we p epa ed is jus as pu e as epo ed by o he s.
Ob iously, he acids and sal s we e soluble in 10% HCl and/o alcohol, and he es o he
acids we e mos ly emo ed using me hanol. The p esence o chlo ine (0.2–0.3 a .%) and
sul u (1.5–2.0 a .%) was con i med, and simila quan i ies we e equen ly men ioned in
o he publica ions abou he syn hesis o GO [10,21,27].
Due o a ela i ely small di e ence in he chemical s uc u e o GO, he XPS analysis
was only pe o med o he GO syn hesized om he smalles and la ges g aphi e pa i-
cles. The high- esolu ion C1s spec a o hese samples a e shown in Figu e 2 and explained
la e in he ex . Table 2 ep esen s he esul s o he p opo ions o a ious bonds ob ained
by i ing he high- esolu ion C1s spec a (Figu e S1 in he Supplemen a y Ma e ials). The
C1s spec a we e i ed using ou componen s co esponding o C-C and C=C (sp2), C-O
(epoxy o hyd oxyl), C=O, and COOH. The majo i y o oxygen is ound in he epoxy o
hyd oxyl g oups.
Figu e 1. The XPS su ey spec a o GO100+ 1:6:80.
Only he peaks o ca bon and oxygen a e clea ly isible, which indica es ha he
concen a ion o impu i ies, excep o sul u and chlo ine, is below he de ec ion limi .
The e o e, he XPS esul s show ha he GO we p epa ed is jus as pu e as epo ed by
o he s. Ob iously, he acids and sal s we e soluble in 10% HCl and/o alcohol, and he es
o he acids we e mos ly emo ed using me hanol. The p esence o chlo ine (0.2–0.3 a .%)
and sul u (1.5–2.0 a .%) was con i med, and simila quan i ies we e equen ly men ioned
in o he publica ions abou he syn hesis o GO [10,21,27].
Due o a ela i ely small di e ence in he chemical s uc u e o GO, he XPS analysis
was only pe o med o he GO syn hesized om he smalles and la ges g aphi e pa icles.
The high- esolu ion C1s spec a o hese samples a e shown in Figu e 2and explained la e
in he ex . Table 2 ep esen s he esul s o he p opo ions o a ious bonds ob ained by
i ing he high- esolu ion C1s spec a (Figu e S1 in he Supplemen a y Ma e ials). The
C1s spec a we e i ed using ou componen s co esponding o C-C and C=C (sp
2
), C-O
(epoxy o hyd oxyl), C=O, and COOH. The majo i y o oxygen is ound in he epoxy o
hyd oxyl g oups.
Table 2. The p opo ions o a ious ca bon bonds oge he wi h hei oxygen- o-ca bon a omic a ios.
C-C/C=C
(%)
C-O
(%)
C=O
(%)
COOH
(%) O/C
GO45 1:4:60 43.7 44.7 8.1 3.5 0.52
GO45 1:4:80 38.7 47.4 9.8 4.1 0.58
GO45 1:6:60 35.5 50.2 10.4 3.8 0.62
GO45 1:6:80 38.2 45.1 11.9 4.8 0.58
GO100+ 1:4:60 43.6 44.0 7.9 4.5 0.55
GO100+ 1:4:80 47.8 45.0 3.5 3.7 0.44
GO100+ 1:6:60 42.5 46.1 7.4 4.0 0.54
GO100+ 1:6:80 44.7 45.8 5.9 3.6 0.50
Nanoma e ials 2024,14, 281 7 o 21
Figu e 2. The high- esolu ion C1s spec a o (a) small pa icles, GO45, and (b) la ge pa icles,
GO100+.
Table 2. The p opo ions o a ious ca bon bonds oge he wi h hei oxygen- o-ca bon a omic
a ios.
C-C/C=C
(%)
C-O
(%)
C=O
(%)
COOH
(%) O/C
GO45 1:4:60 43.7 44.7 8.1 3.5 0.52
GO45 1:4:80 38.7 47.4 9.8 4.1 0.58
GO45 1:6:60 35.5 50.2 10.4 3.8 0.62
GO45 1:6:80 38.2 45.1 11.9 4.8 0.58
GO100+ 1:4:60 43.6 44.0 7.9 4.5 0.55
GO100+ 1:4:80 47.8 45.0 3.5 3.7 0.44
GO100+ 1:6:60 42.5 46.1 7.4 4.0 0.54
GO100+ 1:6:80 44.7 45.8 5.9 3.6 0.50
3.2.1. The In luence o he G aphi e Pa icle Size
Acco ding o he O/C a ios in Table 2, he smalle pa icles we e mo e oxidized han
he la ge ones. The one excep ion is GO45 1:4:60, which seems o be oxidized o a com-
pa able le el as he la ge pa icles. The a e age pe cen age o C-C bonds was 39.0% and
44.7% o GO45 and GO100+, espec i ely. The a e age oxygen- o-ca bon a io (O/C) was
also highe o GO45 (0.58) han o GO100+ (0.51). Acco ding o hese esul s, he eac ion
yield should be highe wi h he smalle pa icles (because o hei highe oxygen con en ,
and hus he mass should be g ea e ), bu his was no he case (Table 1). I seems ha he
smalle pa icles we e mo e soluble in wa e due o a highe deg ee o oxida ion and
smalle size, so hey we e los du ing he pu i ica ion p ocess. This may explain a de ia-
ion in GO45 1:4:60. Small and s ongly oxidized pa icles could ha e been u he b oken
down by mixing a iscous solu ion, became mo e soluble, and we e emo ed by washing.
The concen a ion o ca bonyl bonds is highe o he smalle pa icles, which is in
ag eemen wi h p e ious esul s [9]. The ca bonyl bonds a e o med a he edge a oms
and on he edges o acancy de ec s in he basal plane. Since he smalle pa icles exhibi
a la ge a io be ween he edge and plane ca bon a oms han he la ge ones, a la ge
numbe o he ca bonyl g oups is expec ed o he smalle pa icles. The edge a oms o
g aphene oxide a e also e mina ed by ca boxyl (COOH) g oups. The concen a ion o
hese g oups is app oxima ely 4% o bo h ypes o GO ma e ial. He e, i has o be s essed
Figu e 2. The high- esolu ion C1s spec a o (a) small pa icles, GO45, and (b) la ge pa icles, GO100+.
3.2.1. The In luence o he G aphi e Pa icle Size
Acco ding o he O/C a ios in Table 2, he smalle pa icles we e mo e oxidized
han he la ge ones. The one excep ion is GO45 1:4:60, which seems o be oxidized o a
compa able le el as he la ge pa icles. The a e age pe cen age o C-C bonds was 39.0% and
44.7% o GO45 and GO100+, espec i ely. The a e age oxygen- o-ca bon a io (O/C) was
also highe o GO45 (0.58) han o GO100+ (0.51). Acco ding o hese esul s, he eac ion
yield should be highe wi h he smalle pa icles (because o hei highe oxygen con en ,
and hus he mass should be g ea e ), bu his was no he case (Table 1). I seems ha
he smalle pa icles we e mo e soluble in wa e due o a highe deg ee o oxida ion and
smalle size, so hey we e los du ing he pu i ica ion p ocess. This may explain a de ia ion
in GO45 1:4:60. Small and s ongly oxidized pa icles could ha e been u he b oken down
by mixing a iscous solu ion, became mo e soluble, and we e emo ed by washing.
The concen a ion o ca bonyl bonds is highe o he smalle pa icles, which is in
ag eemen wi h p e ious esul s [
9
]. The ca bonyl bonds a e o med a he edge a oms
and on he edges o acancy de ec s in he basal plane. Since he smalle pa icles exhibi a
la ge a io be ween he edge and plane ca bon a oms han he la ge ones, a la ge numbe
o he ca bonyl g oups is expec ed o he smalle pa icles. The edge a oms o g aphene
oxide a e also e mina ed by ca boxyl (COOH) g oups. The concen a ion o hese g oups
is app oxima ely 4% o bo h ypes o GO ma e ial. He e, i has o be s essed ha he C=O
and COOH g oups pa ially o e lap in he C1s spec a, so he exac concen a ion o he
ca boxyl g oup is di icul o de e mine. Inc easing he acid- o-g aphi e a io om 60 o 80
inc eased he concen a ion o C=O and COOH o GO45, while he opposi e e ec was
obse ed o GO100+.
The C-O bonds can be ei he in he epoxy o hyd oxyl unc ional g oups, bu un o u-
na ely, he XPS echnique does no dis inguish be ween hese g oups since he di e ences
in he chemical shi a e ma ginal.
3.2.2. The In luence o he Ra io o Reac an s
Inc easing he a io o g aphi e o oxidan (KMnO
4
) om 1:4 o 1:6 dec eased he
amoun o C-C/C=C bonds, indica ing mo e e icien oxida ion, bu he di e ences a e
a he small. The amoun o C=O and COOH inc eases wi h inc easing oxidizing agen
and acid con en o he small pa icles, while he opposi e end is obse ed o he
la ge pa icles.
Nanoma e ials 2024,14, 281 8 o 21
3.3. Cha ac e iza ion o GO Using UV-VIS, FTIR, and Raman Spec oscopy
The s uc u e o he ob ained ma e ials was analyzed using UV-VIS, FTIR, and Raman
spec oscopy. The UV-VIS abso p ion spec a (Figu e 3) ha e one dominan peak a
~235 nm
and a shoulde a ~300 nm, con i ming ha all he samples ha e simila s uc u es. The
peak a 235 nm o igina es om he
π
-
π
* ansi ion o he a oma ic C=C bonds, while
he shoulde a ~300 nm o igina es om he n-
π
* ansi ion o he C=O bonds [
12
]. The
shoulde is p esen in all GO45 and GO100+ samples and indica es he p esence o oxygen
unc ional g oups in he s uc u e o oxidized g aphene, which is in good ag eemen wi h
XPS analysis.
‐
ffi ‐ ‐
ff
‐ ‐
ff ‐
‐
‐
‐
‐
π‐π
‐π
‐
‐
− −
− − ‐
‐ −
− ‐
−
‐ ‐
Figu e 3. The UV-VIS spec a o he ob ained GO samples: samples o GO p oduced om GO45 (a)
and GO samples p oduced s a ing om GO100+ (b).
The p esence o unc ional g oups in he GO samples was analyzed using FTIR spec-
oscopy, and he esul s a e shown in Figu e 4. All GO samples (GO45 1:40:60 (a), GO45
1:6:60 (b), and GO100+ 1:4:80 (c)) showed he same bands in hei FTIR spec a, while no
band was de ec ed in g aphi e (Figu e 4d). The band assigned o he s e ching ib a ions o
he ca bonyl g oups was obse ed a 1725 cm
−1
. The band a 1626 cm
−1
o igina es om he
ib a ions o he C=C bonds. The bands a 3420 cm
−1
and 1048 cm
−1
o igina e om C-O-C
and he s e ching and bending ib a ions o he OH g oups. Weak bands a
2930 cm−1
and 2849 cm
−1
a e he esul o s e ching ib a ions o he H-C bonds, and he band a
860 cm
−1
is associa ed wi h he ib a ions o he epoxide g oups. These analyses demon-
s a e he success ul chemical oxida ion o g aphi e and he binding o oxygen-con aining
unc ional g oups.
The Raman spec a o GO (Figu e 5) a e composed o wo dominan ea u es, he G
and he D peaks, which appea a ~1580 and ~1350 cm−1, espec i ely [28]. The G peak is
a ibu ed o he ib a ion o sp
2
-bonded ca bon a oms in a wo-dimensional hexagonal
la ice and is he cha ac e is ic Raman peak o all ca bonaceous ma e ials a anged in
hexagons. On he o he hand, he D peak is ei he due o he s uc u al de ec s [
29
,
30
] o he
edges o he nanoshee s [
31
]. The in ensi y a io o hese wo bands (I
D
/I
G
a io) can se e as
an indica i e pa ame e o es ima e he quali y o g aphene-like ma e ials [
30
]. The alues
o he I
D
/I
G
a io be ween 0.88 and 0.95 indica ed de ec s in he sp
2
ca bon s uc u e o
g aphene. All GO samples ha e compa able I
D
/I
G
a io alues (Table 3). Howe e , sligh ly
lowe alues ha e been calcula ed o he GO ob ained om GO100+, which is a esul o
lowe oxida ion.
Nanoma e ials 2024,14, 281 15 o 21
c ys al la ice du ing d ying. Inc easing he G:KMnO
4
a io om 1:4 o 1:6 inc eased he
c ys alli e size o GO45 and GO100+, while he opposi e e ec was obse ed o GO45+.
3.7. The mog a ime ic Analysis o GO
The e a e many epo s on he TGA o GO, bu only a ew men ion ha GO can
decompose explosi ely a a high hea ing a e [
38
,
39
]. In ou ini ial expe imen s, explosi e
decomposi ion was obse ed a a hea ing a e o 10
◦
C/min and a sample mass o
10 mg
.
The explosion is accompanied by he deposi ion o ine black powde h oughou he
in e io o he TGA ins umen . In he TGA he mog am, he explosion can be ecognized
as an immedia e loss o weigh , ollowed by a sligh inc ease in weigh (inse in Figu e 10)
due o he se ling o he powde back in o he c ucible. Some imes he explosion is no
ecognized as such, and he e o e inco ec mass loss da a has been published [40].
The weigh loss o GO, du ing hea ing, p oceeds h ough ou no well-sepa a ed
s eps, al hough he ou h s ep is mino and does no end a a empe a u e as high as 800
°C. The decomposi ion cu es a e qui e simila , and he e o e only h ee a e shown as an
example in Figu e 10. The esul s o he weigh loss and peak decomposi ion a e empe -
a u e du ing hea ing GO in he ni ogen a mosphe e a e summa ized in Table 7. An ini ial
mass loss (a empe a u es up o abou 120 °C) is usually a ibu ed o he elease o ab-
so bed wa e , bu could pa ially o e lap wi h he onse o he decomposi ion a he highe
empe a u es. Du ing he second s ep, apid decomposi ion is obse ed, and he weigh
is educed by app oxima ely 30%. The maximum decomposi ion a e is obse ed in a em-
pe a u e ange o 159–173 °C.
Figu e 10. TGA cu es o g aphene oxides syn hesized unde he same condi ions using g aphi e
p ecu so s o a ious sizes.
Since no co ela ion be ween he peak decomposi ion empe a u e and he s uc u e
o GO could be de i ed, a TGA analysis was pe o med on a la ge ba ch o an olde sam-
ple, syn hesized om g aphi e o 130 μm (GO130), which was s o ed as a me hanol slu y
in a e ige a o . Pa o he me hanol slu y was d ied i s in he ai and hen in a acuum
a 40 °C. Ano he pa was u he pu i ied using wa e , as explained in he De e mina ion
o Reac ion Yield sec ion. Dense and ha d ma e ial was ob ained a e d ying in he ai . A
small piece (4.5 mg) o his sample was i s cu and analyzed by TGA. Ano he piece was
c ushed in o smalle pa icles o powde using aga e mo a p io o analysis. The samples
o GO130, which we e addi ionally pu i ied by wa e and d ied by lyophiliza ion, had he
o m o hin-walled oam. The TGA was pe o med using bo h he as-syn hesized and
pu i ied samples.
The he mal p ope ies di e a lo om he o he samples desc ibed in his a icle.
The TGA he mog ams and hei i s de i a i es, which ep esen he decomposi ion a e
o he GO130 samples, a e shown in Figu es 11 and 12, espec i ely. The impo an di e -
ences we e obse ed in he i s and second decomposi ion s eps, while he end (abo e
230 °C) is p ac ically he same.
Figu e 10. TGA cu es o g aphene oxides syn hesized unde he same condi ions using g aphi e
p ecu so s o a ious sizes.
To a oid an explosion du ing TGA cha ac e iza ion, he hea ing a e was educed o
5◦C/min and he sample mass o abou 4 mg.
The weigh loss o GO, du ing hea ing, p oceeds h ough ou no well-sepa a ed
s eps, al hough he ou h s ep is mino and does no end a a empe a u e as high as
800
◦
C. The decomposi ion cu es a e qui e simila , and he e o e only h ee a e shown
as an example in Figu e 10. The esul s o he weigh loss and peak decomposi ion a e
empe a u e du ing hea ing GO in he ni ogen a mosphe e a e summa ized in Table 7. An
ini ial mass loss (a empe a u es up o abou 120
◦
C) is usually a ibu ed o he elease
o abso bed wa e , bu could pa ially o e lap wi h he onse o he decomposi ion a he
highe empe a u es. Du ing he second s ep, apid decomposi ion is obse ed, and he
weigh is educed by app oxima ely 30%. The maximum decomposi ion a e is obse ed in
a empe a u e ange o 159–173 ◦C.
Nanoma e ials 2024,14, 281 16 o 21
Table 7. Weigh loss and peak decomposi ion a e empe a u e du ing hea ing GO in a ni ogen
a mosphe e we e de e mined by TGA.
Weigh loss Peak
1s s . (%) 2nd s . (%) 3 d s . (%) 4 h s . (%) (◦C)
GO45 1:4:60 8.8 28.9 11.7 6.2 172
GO45 1:4:80 8.5 30.3 12.7 4.6 162
GO45 1:6:60 8.1 32.7 13.5 3.4 170
GO45 1:6:80 7.9 29.8 12.8 4.0 159
GO45+ 1:4:60 7.7 31.8 13.4 4.4 168
GO45+ 1:4:80 9.3 30.9 13.5 6.0 173
GO45+ 1:6:60 7.7 31.3 12.0 4.7 170
GO45+ 1:6:80 7.8 29.9 11.6 3.8 173
GO100+ 1:4:60 7.4 30.0 13.1 6.5 163
GO100+ 1:4:80 8.0 30.4 13.6 3.0 164
GO100+ 1:6:60 7.5 31.3 12.0 3.7 170
GO100+ 1:6:80 7.8 31.1 14.5 4.4 166
Since no co ela ion be ween he peak decomposi ion empe a u e and he s uc u e
o GO could be de i ed, a TGA analysis was pe o med on a la ge ba ch o an olde sample,
syn hesized om g aphi e o 130
µ
m (GO130), which was s o ed as a me hanol slu y in a
e ige a o . Pa o he me hanol slu y was d ied i s in he ai and hen in a acuum a
40
◦
C. Ano he pa was u he pu i ied using wa e , as explained in he De e mina ion
o Reac ion Yield sec ion. Dense and ha d ma e ial was ob ained a e d ying in he ai .
A small piece (4.5 mg) o his sample was i s cu and analyzed by TGA. Ano he piece
was c ushed in o smalle pa icles o powde using aga e mo a p io o analysis. The
samples o GO130, which we e addi ionally pu i ied by wa e and d ied by lyophiliza ion,
had he o m o hin-walled oam. The TGA was pe o med using bo h he as-syn hesized
and pu i ied samples.
The he mal p ope ies di e a lo om he o he samples desc ibed in his a icle. The
TGA he mog ams and hei i s de i a i es, which ep esen he decomposi ion a e o
he GO130 samples, a e shown in Figu es 11 and 12, espec i ely. The impo an di e ences
we e obse ed in he i s and second decomposi ion s eps, while he end (abo e 230
◦
C) is
p ac ically he same.
pe a u e o he wa e -dissol ed sample was highe han he empe a u e o GO130-1x pu-
i ied, despi e he hicke ilm, we can conclude ha he decomposi ion kine ics depend
on bo h he pu i y and he pa icle size o he measu ed sample. The sligh ly lowe de-
composi ion empe a u e obse ed o he GO130-3x om me hanol could be due o a
small solubili y and an inc ease in pa icle size as an e ec o me hanol.
Figu e 11. Weigh loss o GO syn hesized om g aphi e wi h 130 mm pa icle size, d ied in he ai
(pa icle), c ushed in o powde , pu i ied using wa e (1–3 imes), and d ied by lyophiliza ion. Inse :
Sligh inc ease in weigh a e he explosi e decomposi ion.
Figu e 12. The i s de i a i e o decomposi ion a e o GO130 deg ada ion.
Wi h he inc easing de elopmen o new de ices and echnologies based on g a-
phene [41–43], he need o new, high-yield me hods o he syn hesis o g aphene is g ea
nowadays. Table 8 summa izes he me hods and GO p ope ies. I is no iceable ha he
p oduced GO lakes show de ec s a a simila le el as in o he s udies, while he lake size
is in he same ange, indica ing ha he selec ed p ocedu e is app op ia e o he p epa-
a ion o la ge GO lakes wi h mild de ec s.
Figu e 11. Weigh loss o GO syn hesized om g aphi e wi h 130 mm pa icle size, d ied in he ai
(pa icle), c ushed in o powde , pu i ied using wa e (1–3 imes), and d ied by lyophiliza ion. Inse :
Sligh inc ease in weigh a e he explosi e decomposi ion.
Nanoma e ials 2024,14, 281 17 o 21
pe a u e o he wa e -dissol ed sample was highe han he empe a u e o GO130-1x pu-
i ied, despi e he hicke ilm, we can conclude ha he decomposi ion kine ics depend
on bo h he pu i y and he pa icle size o he measu ed sample. The sligh ly lowe de-
composi ion empe a u e obse ed o he GO130-3x om me hanol could be due o a
small solubili y and an inc ease in pa icle size as an e ec o me hanol.
Figu e 11. Weigh loss o GO syn hesized om g aphi e wi h 130 mm pa icle size, d ied in he ai
(pa icle), c ushed in o powde , pu i ied using wa e (1–3 imes), and d ied by lyophiliza ion. Inse :
Sligh inc ease in weigh a e he explosi e decomposi ion.
Figu e 12. The i s de i a i e o decomposi ion a e o GO130 deg ada ion.
Wi h he inc easing de elopmen o new de ices and echnologies based on g a-
phene [41–43], he need o new, high-yield me hods o he syn hesis o g aphene is g ea
nowadays. Table 8 summa izes he me hods and GO p ope ies. I is no iceable ha he
p oduced GO lakes show de ec s a a simila le el as in o he s udies, while he lake size
is in he same ange, indica ing ha he selec ed p ocedu e is app op ia e o he p epa-
a ion o la ge GO lakes wi h mild de ec s.
Figu e 12. The i s de i a i e o decomposi ion a e o GO130 deg ada ion.
F om he i s s ep o decomposi ion (up o 140
◦
C), we can conclude ha no only
wa e e apo a ion bu also he mal decomposi ion akes place. I only wa e is eleased in
he i s s ep, he mass loss a e would be as e o he powde ed sample due o a la ge
speci ic su ace a ea. Howe e , we see ha he decomposi ion o he powde ed sample is
sligh ly slowe in he i s s ep. The same is obse ed o he 1
×
pu i ied sample. Howe e ,
he weigh loss in he i s s ep is signi ican ly as e o he 2
×
and 3
×
pu i ied samples.
A 100
◦
C, 15% o he mass was al eady los . As all he pu i ied samples we e d ied a
he same ime, we assume ha his is no due o he highe mois u e con en in hese wo
samples bu o as e decomposi ion.
La ge di e ences we e also obse ed in he second decomposi ion s ep. The single
piece exploded unde he hea ing a e o 5
◦
C/min a 187.9
◦
C, while as bu s eady
decomposi ion was obse ed o he powde ed sample, and he maximum decomposi ion
a e was obse ed a 193
◦
C ( he signal o he decomposi ion a e o he la ge sample in
Figu e 11 is la ge and has, he e o e, been cu o cla i y easons). F om his, we can conclude
ha he pa icle size o he d ied GO (o a he G O) s ongly in luences decomposi ion
kine ics. The decomposi ion eac ion is as and exo he mic. The eleased hea can no be
dissipa ed om he la ge pa icle as enough o main ain he p og ammed empe a u e.
The sample is o e hea ed, and he inc eased empe a u e o he sample u he inc eases he
a e o decomposi ion un il i ac ually explodes. The o e hea ing can be easily ecognized
as a loop on he hea low– empe a u e diag am measu ed using he DSC senso (Figu e S2
in he Supplemen a y Ma e ials). The smalle he pa icles, he easie he hea is dissipa ed,
he lowe he eac ion a e, and he peak shi s o a highe empe a u e.
Hea dissipa ion is be e , and he peak o decomposi ion shi s o a highe empe a u e
wi h each pu i ica ion s ep, as can be seen in Figu e 12. The concen a ion o GO in
wa e be o e lyophiliza ion was 1.5%, 1.2%, and 1.0% a e he i s , second, and hi d
pu i ica ion s eps, espec i ely. The e o e, he wall hickness o he GO oam dec eased
wi h each pu i ica ion s ep, which could lead o decomposi ion a highe empe a u es, as
explained abo e. Howe e , each pu i ica ion s ep could also emo e a pa o he p oduc
o impu i ies ha could ca alyze decomposi ion. The e o e, pa o GO130-3x pu i ied was
dissol ed in wa e again, and he second pa was imme sed in me hanol. D y GO no longe
dissol es in me hanol, and e en swelling is no isually ecognizable. The sample emained
in a powde y o m. A e d ying in he ai and in a acuum, a TGA was pe o med on
bo h samples. The peak decomposi ion empe a u e o he samples p epa ed in his way
dec eased om 220.6
◦
C o 218.1
◦
C (me hanol) and om 200.5
◦
C o
212.4 ◦C
o wa e -
dissol ed samples o di e en hicknesses. Since he peak decomposi ion empe a u e o
he wa e -dissol ed sample was highe han he empe a u e o GO130-1x pu i ied, despi e
he hicke ilm, we can conclude ha he decomposi ion kine ics depend on bo h he
pu i y and he pa icle size o he measu ed sample. The sligh ly lowe decomposi ion
Nanoma e ials 2024,14, 281 18 o 21
empe a u e obse ed o he GO130-3x om me hanol could be due o a small solubili y
and an inc ease in pa icle size as an e ec o me hanol.
Wi h he inc easing de elopmen o new de ices and echnologies based on
g aphene [
41
–
43
], he need o new, high-yield me hods o he syn hesis o g aphene
is g ea nowadays. Table 8summa izes he me hods and GO p ope ies. I is no iceable
ha he p oduced GO lakes show de ec s a a simila le el as in o he s udies, while he
lake size is in he same ange, indica ing ha he selec ed p ocedu e is app op ia e o he
p epa a ion o la ge GO lakes wi h mild de ec s.
Table 8. Compa ison o GO p ope ies p oduced in di e en syn he ic p ocedu es.
Me hod ID/IGShee Size (µm) TGA WL 500 ◦C (%) XRD (◦) Re .
Elec ochemical ollowed by oxida ion wi h KMnO41.24 ≈10 ≈80 8.88 [44]
Humme s’ me hod 0.66–0.90 0.98 61.55 10.50 [45]
Imp o ed Humme s’ me hod wi h bo ic acid 0.92–1.04 / 46.22 10.10 [46]
Modi ied Humme s’ 1.0 ≈10 / / [47]
Modi ied Humme s’ 0.83 / / 10.44 [48]
Tou me hod 1.77 / / 10.44 [49]
Elec ochemical syn hesis 0.41–1.71 / / 9–12 [50]
Elec ochemical syn hesis 0.04 Up o 12 [51]
Modi ied Humme s’ 0.92 Up o 5.25 / 9.91 This s udy
4. Conclusions
The goal o he p esen wo k was o de e mine he in luence o g aphi e pa icle size,
he a io o g aphi e o oxidan (KMnO
4
), and he a io o g aphi e o acid (H
2
SO
4
and
H
3
PO
4
) on he s uc u e and p ope ies o he syn hesized GO. In his way, we wan ed o
ind he explana ion o some la ge de ia ions in he esul s obse ed in he li e a u e.
The esul s epo ed in his pape show ha hese h ee pa ame e s can ha dly be
sepa a ed due o he a ia ions in he iscosi y o he sys em. Du ing oxida ion, he g aphi e
lakes became mo e hyd ophilic, which led o swelling in he acid and inc eased he iscosi y.
The la ge he g aphi e pa icles, he mo e iscous he pas e. Due o he high iscosi y, he
pa icles can be b oken by mixing, which a ec s hei size. This could be an impo an eason
why he size o he GO pa icles made om la ge g aphi e lakes is no much la ge han he
size o he GO pa icles made om smalle g aphi e lakes. Howe e , he esul s show ha
he pa icle size o he d ied GO is 50% la ge han be o e d ying. This esul can be explained
by he o ma ion o a chemical bond be ween neighbo ing GO shee s du ing d ying, which
p e en s comple e ex olia ion in wa e o e en swelling in me hanol.
Acco ding o he li e a u e sou ces, he eac ion yield a ies a lo . Ou esul s show
ha he main eason o his is he pu i ica ion me hod. The mo e wa e is used, he lowe
he GO yield. In e es ingly enough, when we compa ed he amoun o sul u in ou GO
samples wi h he o he s ha used mo e igo ous pu i ica ion and mo e wa e , qui e simila
alues we e ob ained. This means ha excessi e pu i ica ion leads o he excessi e was e
o good and aluable GO and no necessa ily o a pu e GO.
Inc easing he a io o g aphi e o KMnO
4
om 1:4 o 1:6 gene ally led o an inc ease
in he deg ee o oxida ion and he eac ion yield. Howe e , he di e ences a e e y small.
Inc easing he amoun o acids om 60 mL o 80 mL led o a lowe deg ee o oxida ion and
a sligh ly lowe eac ion yield, excep o he smalles pa icles.
The measu emen o he wa e con ac angle showed no co ela ion be ween he
we abili y and he deg ee o oxida ion. The highes wa e con ac angle o 34.3
◦
was
measu ed o he GO100+ 1:4:80 sample, which had he lowes O/C a io. The a ia ions
in he con ac angles o he o he samples could be ela ed o he lack o uni o mi y o he
ilms, which was in luenced by he deposi ion me hod o by a ia ions in he oxida ion
s a e. Epoxy and C=O g oups con ibu e o oxida ion bu a e no as pola as hyd oxy
Nanoma e ials 2024,14, 281 19 o 21
and -COOH. Un o una ely, XPS analysis canno dis inguish be ween hem, so he ac ual
s uc u e canno be deduced om high- esolu ion XPS C1s spec a.
The c ys alli e size o GO is la ges (11.2–14.4 nm) o he smalles pa icles (GO45)
and smalles (9.4–10.5 nm) o he la ges pa icles (GO100+). This can be explained by he
ac ha he smalle shee s i mo e easily in o a c ys al la ice du ing d ying. I dec eases
wi h inc easing acidi y (dilu ion) and inc eases wi h he inc easing a io o g aphi e o he
oxidizing agen om 1:4 o 1:6.
The he mal decomposi ion o GO p oceeds in ou no -well-sepa a ed s eps. The
second s ep is he as es and can e en be explosi e, especially when la ge samples and/o
high hea ing a es a e used. The empe a u e o he maximum decomposi ion a e o his
s ep depends no only on he syn hesis and pu i ica ion condi ions bu p ima ily on he
size o he pa icles. I he GO pa icle is hick, he eleased hea canno be dissipa ed
as enough o main ain he p og ammed empe a u e. The sample will o e hea , and
he inc eased empe a u e will u he s imula e he decomposi ion a e un il i explodes.
Al hough he u he pu i ica ion o he GO (up o h ee imes) wi h wa e inc eased he
decomposi ion empe a u e by up o 22
◦
C, i was shown ha he inc ease was mainly due
o he hinne pa icles ob ained a e d ying by lyophiliza ion.
The syn hesis o GO is ime-consuming and ecologically un iendly. The esul s
p esen ed in his a icle gi e some guidelines o op imiza ion, especially in e ms o
inc easing eac ion yield, which migh be e en inc eased i less wa e and mo e me hanol
a e used. The p ope ies o GO a e changing o e ime, which is a opic ha has no been
ackled in he li e a u e. How he pa icle size changes o e ime when GO is d ied o i
s o ed in he o m o a slu y (wa e o me hanol) a e in e es ing ques ions ha s ill need
o be answe ed. In his a icle, we ha e gi en some explana ions o he di e en he mal
s abili ies o GO, bu i would be good o in es iga e in mo e de ail he in luence o he
composi ion, he p esence o o he elemen s (Cl, S, e c.), and he ime ha has passed since
he syn hesis o he samples on he he mal s abili y o GO.
Supplemen a y Ma e ials: The ollowing suppo ing in o ma ion can be downloaded a : h ps:
//www.mdpi.com/a icle/10.3390/nano14030281/s1, Figu e S1: Indi idual high- esolu ion C1s
spec a o he in es iga ed samples wi h subcomponen s posi ioned a 284.8, 287.0, 288.0, and
289.0 eV
.
Figu e S2: Hea low signal o TGA-DSC measu emen .
Au ho Con ibu ions: M.H. pe o med he syn hesis, XRD, and he mal cha ac e iza ion. D.K.
(Dejan Kepi´c) and S.J. we e esponsible o Raman and UV spec oscopy and con ac angle measu ing.
D.K. (Duška Kleu ) pe o med AFM measu emen s. M.M. and A.V. pe o med XPS measu emen s.
A.A. pe o med pa icle size measu emen s by DLS. D.B.B. conduc ed FTIR measu emen s. All
au ho s con ibu ed o he w i ing o he a icle. All au ho s ha e ead and ag eed o he published
e sion o he manusc ip .
Funding: This esea ch was unded by he Eu opean Union’s Ho izon Eu ope Coo dina ion and
Suppo Ac ions p og am unde g an ag eemen No. 101079151—p ojec G InShield. The au ho s
also acknowledge he inancial suppo om he Slo enian Resea ch Agency ( esea ch co e unding
No. P2-0145) and he Minis y o Science, Technological De elopmen , and Inno a ion o he Republic
o Se bia (g an numbe s 451-03-47/2023-01/200017 and 451-03-47/2023-01/200146).
Da a A ailabili y S a emen : Da a ob ained wi hin he G inshield p ojec will be published on
Zenodo a e he a icle is accep ed o publica ion.
Con lic s o In e es : Au ho Mi osla Huski´c is employed by he company Facul y o Polyme
Technology. The emaining au ho s decla e ha he esea ch was conduc ed in he absence o any
comme cial o inancial ela ionships ha could be cons ued as a po en ial con lic o in e es .
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