Ci a ion: Memisoglu, G.; Mu ugesan,
R.C.; Zubia, J.; Rozhin, A.G.
G aphene Nanocomposi e
Memb anes: Fab ica ion and Wa e
T ea men Applica ions. Memb anes
2023,13, 145. h ps://doi.o g/
10.3390/memb anes13020145
Academic Edi o : Leona d Tijing
Recei ed: 31 Decembe 2022
Re ised: 14 Janua y 2023
Accep ed: 18 Janua y 2023
Published: 22 Janua y 2023
Copy igh : © 2023 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/).
memb anes
Re iew
G aphene Nanocomposi e Memb anes: Fab ica ion and Wa e
T ea men Applica ions
Go kem Memisoglu 1,2,* , Ragha an Chinnambedu Mu ugesan 3, Joseba Zubia 1and Aleksey G. Rozhin 3
1Depa men o Communica ions Enginee ing, Escuela de Ingenie ía de Bilbao, Uni e si y o he Basque
Coun y (UPV/EHU), E-48013 Bilbao, Spain
2Depa men o Elec onics Technology, Is iklal Uni e si y, Kah amanma as 46300, Tü kiye
3As on Ins i u e o Pho onic Technologies, As on Uni e si y, Bi mingham B4 7ET, UK
*Co espondence: [email p o ec ed]
Abs ac :
G aphene, a wo-dimensional hexagonal honeycomb ca bon s uc u e, is widely used in
memb ane echnologies hanks o i s unique op ical, elec ical, mechanical, he mal, chemical and
pho oelec ic p ope ies. The ligh weigh , mechanical s eng h, an i-bac e ial e ec , and pollu ion-
adso p ion p ope ies o g aphene memb anes a e aluable in wa e ea men s udies. Inco po a ion
o nanopa icles like ca bon nano ubes (CNTs) and me al oxide in o he g aphene il e ing nanocom-
posi e memb ane s uc u e can p o ide an imp o ed pho oca alysis p ocess in a wa e ea men
sys em. Wi h he apid de elopmen o g aphene nanocomposi es and g aphene nanocomposi e
memb ane-based acous ically suppo ed il e ing sys ems, including CNTs and isible-ligh ac i e
me al oxide pho oca alys , i is necessa y o de elop he esea ches o sus ainable and en i onmen-
ally iendly applica ions ha can lead o new and g oundb eaking wa e ea men sys ems. In his
e iew, cha ac e is ic p ope ies o g aphene and g aphene nanocomposi es a e examined, a ious
me hods o he syn hesis and dispe sion p ocesses o g aphene, CNTs, me al oxide and polyme
nanocomposi es and memb ane ab ica ion and cha ac e iza ion echniques a e discussed in de ails
wi h using li e a u e epo s and ou labo a o y expe imen al esul s. Recen memb ane de elop-
men s in wa e ea men applica ions and g aphene-based memb anes a e e iewed, and he cu en
challenges and u u e p ospec s o memb ane echnology a e discussed.
Keywo ds:
g aphene nanocomposi e memb ane il e ; ca bon nano ubes; pho oca aly ic memb ane;
hyb id unc ion memb ane; memb ane ab ica ion; wa e ea men
1. In oduc ion
The en i onmen we li e in is basically composed o wa e , soil and ai , and sus ain-
able li e can be main ained as long as his en i onmen can be kep away om pollu an s.
In his con ex , one o he ac o s ha can di ec ly a ec i ali y and sus ainabili y is wa e
pollu ion [
1
–
6
]. Wa e is o i al impo ance o he su i al o li ing hings, and wa e
pollu ion can easily occu wi h issues such as he discha ge o was ewa e con aining
ha m ul componen s in o basins wi hou adequa e ea men . F esh wa e esou ces on
ea h a e limi ed, and i is ex emely impo an o use wa e wi hou was ing i , as well o
suppo en i onmen ally iendly wa e ea men applica ions [
7
–
10
]. Failu e o ensu e
wa e sus ainabili y can c ea e many consequences such as d ough -dese i ica ion (sho -
age o esh wa e ), poo quali y wa e (pollu ed wa e ), dec ease in o es a ion (dec ease
in oxygen a io in he wo ld), dec ease in ag icul u e (g ow h o unheal hy p oduc s), o
inc ease in ch onic diseases (due o a sho age o esh wa e ) [1–9].
In o de o p e en hese p oblems, i is impo an o use wa e ea men applica ions
and o con ol he quali y o na u al, d inking, o a i icial wa e sou ces and basins [
11
–
44
].
In wa e ea men sys ems, memb anes a e equen ly used as a quali y con ol
medium combined wi h he il e ing unc ion. They a e echnical ba ie s o selec i ely
Memb anes 2023,13, 145. h ps://doi.o g/10.3390/memb anes13020145 h ps://www.mdpi.com/jou nal/memb anes
Memb anes 2023,13, 145 2 o 32
pe meable phases ha allow wa e o pass h ough bu p e en he passage o unwan ed
subs ances in wa e ea men [
8
,
11
–
18
,
30
–
47
]. New app oaches and esea ch a e being
ca ied ou on wa e ea men in an en i onmen ally iendly way using less ene gy, ewe
chemicals, and ewe a i icial ligh sou ces. In pa icula , he applicabili y o memb ane
echnologies, which can p o ide excellen sepa a ion e iciency, modula s uc u e, and low
chemical sludge ou pu , has been p o en o help in wa e ea men [11–46].
Memb anes a e echnical ba ie s ha wo k like cell walls in he li ing body, il e ing
ou pa icles like i uses o sal s om wa e [
11
,
12
,
21
–
27
,
42
]. They can keep he il e ed
subs ances on hei su ace a e il e ing. In gene al, memb anes wi h a po ous and
pe meable s uc u e a e used in sys ems such as il a ion-based wa e quali y con ol,
d inking o was e (domes ic o indus ial) wa e ea men , gas sepa a ion, dispe sed solid
(such as mac omolecules o mic oo ganisms) sepa a ion om wa e , ai quali y con ol,
desalina ion, blood o u ine dialysis in he en i onmen al moni o ing o biomedical ield,
so a as hey mee sus ainabili y c i e ia [
11
–
36
,
41
–
44
,
48
]. Memb anes ha o m a ba ie
acco ding o he ype o con aminan o e ad an ages such as high e luen quali y, less use
o chemicals, low olume, and small oo p in [
11
–
29
,
35
–
38
]. Gene al con aminan emo al
me hods a e su ace adso p ion, biodeg ada ion, memb ane il e ing, and pho oca aly ic
deg ada ion. Memb ane il e ing and pho oca alysis p ocesses in a wa e ea men sys em
ha e di e en mechanisms wi h hei own speci ic dynamics. Pho oca alysis can p o ide
mic obial disin ec ion, p e en apid ouling, and po e clogging o he memb ane, while
il e ing p ocesses like nano il a ion, ul a il a ion, e e se osmosis, o o wa d osmosis
can lead o memb ane ouling ha can also be a ec ed by he memb ane po e p ope ies
like po e mo phology and po e size o hyd ophobici y [29,47,49,50].
In exis ing indus ial memb ane-based wa e ea men sys ems using polyme mem-
b anes made o ma e ials such as cellulose ace a e, polyamide, polyac yloni ile, polyp opy-
lene, poly inylidene luo ide (PVDF), polye he sul one o polyca bona e (PC), il a ion
ope a es unde high p essu e (ene gy-dependency, d i ing o ces like powe ul il a ion
pumps) wi h pe iodic main enance (demand o memb ane cleaning, memb ane eplace-
men s, and main enance due o he apid memb ane ouling ha inc eases he cos ) and
p e ea men p ocess equi emen s, whils he pho oca alysis using chemical ca alyze
ma e ials is gene ally ac i a ed unde ul a iole ligh [
9
,
27
,
30
,
51
]. Such equi emen s
can inc ease he o al cos s o he wa e ea men p ocess [
8
,
33
]. So, al hough memb ane
echnology has been used as a sus ainable solu ion o wa e ea men echnology, i s ill
has limi s. Fo memb ane echnologies o become a sus ainable and cos -e ec i e solu ions,
i is impo an o de elop inno a i e memb ane ma e ials, nanocomposi es and p ocesses
in pho o-ca alysis and memb ane il a ion [8,33].
G aphene s ands ou wi h i s high-pe o mance and cos -e ec i e p oduc ion and p o-
cessing me hods, as well as being a g een and sus ainable ma e ial which canno exis alone
in na u e as a single laye . The aw ma e ial o g aphene is he g aphi e c ys al ha can be
ound in coal beds o me amo phic ocks like schis and gneiss. Un il he 2004 disco e y,
many esea che s ied o ob ain g aphene ia physical and chemical me hods [
47
,
52
]. In
2004, No oselo and Geim ex olia ed g aphi e and ob ained g aphene, a 2-dimensional,
semime al ma e ial wi h a hexagonal honeycomb ca bon la ice s uc u e, which has unique
mechanical s eng h, he mal conduc i i y, and op ical, elec ical, chemical and pho o-
elec ic p ope ies [
47
,
52
]. Figu e 1shows he chemical s uc u es o a g aphene laye , a
g aphene oxide which is he oxide o m o g aphene and a educed g aphene oxide.
G aphene-based memb anes a e one o he mos impo an unc ional memb ane
ma e ials and o g ea in e es in wa e ea men applica ions, hanks o i s excellen la ice
s uc u e an i- ouling and an imic obial ac i i ies, and i s con amina ion esis ances [
11
–
26
,
28
,
29
,
32
,
36
,
53
–
57
]. As an example o he an imic obial e ec o g aphene, a bac e ial cell
can be damaged and inac i a ed by he sha p edges o g aphene [
55
], o bac e ial cell walls
can gain an inhibi o y e ec by he educed g aphene oxide [
54
]. Hu e al. in es iga ed he
an ibac e ial ac i i y o g aphene’s oxide o m. They s a ed ha he g aphene shee s ha e
sha p edges, which can cause memb ane s ess (memb ane s ess is he p ima y cause o
Memb anes 2023,13, 145 3 o 32
cell dea h), dis up memb ane in eg i y o mic obes (such as bac e ia), and cause ibonucleic
acid leakage, which can lead o he an ibac e ial ac i i y o g aphene [
55
]. Musico e al.,
in es iga ed memb ane il e s wi h and wi hou g aphene, and hey ound ha an ibac e ial
p ope ies we e imp o ed in he memb anes wi h he g aphene ma e ial [
29
]. Kuma e al.
in es iga ed he an ibac e ial ac i i y o g aphene’s educed oxide o m. They ound
ha g aphene exe ed an inhibi o y e ec agains bac e ia (g am-posi i e bac e ia and
g am-nega i e bac e ia) [54].
Figu e 1. Fo ms o g aphene, g aphene oxide and educed g aphene oxide.
G aphene can be a ached, coa ed o sealed on o a nano- o mic o-po ous poly-
me subs a es like PVDF, nylon, PC o mixed cellulose es e wi h po e sizes be ween
a ew en nanome e s o mic ome e s o inc ease he mechanical du abili y o he mem-
b ane s uc u e. Addi ionally, he mos p e e ed g aphene-based composi e ma e ial
combina ion in wa e ea men memb ane echnology is g aphene wi h polyme s like
polyamide, PVDF, poly e a luo oe hylene (PTFE), cellulose ace a e o PC. The eason o
his p e e ence is he mechanical, chemical, he mal and op ical du abili y o he poly-
me medium [
15
,
23
,
25
,
28
,
36
,
46
]. G aphene oxide [
16
–
18
,
20
,
23
,
24
,
29
,
32
,
36
,
58
] and educed
g aphene [
12
,
13
,
22
,
26
,
43
,
54
,
59
] a e he main de i a i es o g aphene, and hey ca y a
signi ican sha e o g aphene p ope ies. Oxida ion can educe he g aphene’s agg ega ion
endency, and oxide o m o g aphene has bo h hyd ophobic (g aphene po ion) and hy-
d ophilic (like –COOH ca boxyl g oup, o –OH hyd oxyl g oup) edges ha a e shown in
Figu e 1. The di e ence be ween he oxide and educed oxide o ms o g aphene is mainly
in hei unc ional g oups. In g aphene oxide, he e a e oxygena ed unc ional g oups,
while in educed g aphene oxide he e a e ew oxygen-con aining unc ional g oups. Ac-
co ding o he dispe sibili y p ope ies in aqueous media, g aphene oxide shows high
dispe sibili y, while educed g aphene oxide dispe sibili y is low. Al hough he educed
g aphene oxide has simila cha ac e is ics o g aphene, i is in e io in quali y o g aphene
in e ms o he p esence o s uc u al de ec s. G aphene oxide can be p epa ed by modi ied
Humme s me hod, while i s educ ion can c ea e educed g aphene oxide by low-cos
he mal, chemical, o elec ochemical me hods [
60
]. Thanks o i s amphiphilic na u e,
g aphene oxide can bind o wa e -insoluble pa icles (by hyd ophobic in e ac ion,
π
-
π
s acking, o non-co alen bonding) [
61
]. This binding abili y o insoluble pa icles, such as
impu i ies, makes g aphene an e ec i e ma e ial o wa e ea men applica ions.
I is known om he li e a u e ha by nanopa icle doping in o memb anes, pe me-
abili y, high empe a u e s abili y, sepa a ion pe o mance, and selec i i y o he memb ane
can be imp o ed [
45
,
62
,
63
]. The e o e, nanopa icle doped g aphene nanocomposi e mem-
b anes ha e in e es o us. Nanopa icles such as ca bon nano ubes (CNTs) o me al oxide
nanopa icles’ doping o he g aphene can modi y he su ace mo phology and imp o e
he con aminan selec i i y and wa e lux o con aminan emo al [
23
–
25
,
29
,
36
,
40
,
45
].
The e o e, g aphene:CNTs nanocomposi es as wa e ea men memb anes a e p omising
o he ul a il a ion and ouling de ec ion applica ions [15,23,43–45].
Bo h g aphene and CNTs a e o ms o ca bon ma e ials. Like g aphene, CNTs, which
a e ubes made o g aphene laye (s) in me allic o semiconduc o cylinde s [
23
,
27
,
30
,
59
,
64
,
65
],
Memb anes 2023,13, 145 4 o 32
show an imic obial mechanism ha may a y depending on physical and chemical e ec s
and bac e ial oxida ion due o hei elec onic s uc u es [56].
Figu e 2shows a hexagon s uc u e o ca bon, g aphi e c ys al, g aphene laye ,
and CNTs.
Figu e 2. Fo ms o ca bon ma e ials; 3-D g aphi e, 2-D g aphene and 1-D nano ube.
Cha ac e is ic p ope ies o g aphene and CNT memb ane ma e ials a e summa ized
in Table 1. Beside g aphene, CNTs also ha e excep ional, mechanical, he mal, elec ical,
elec onic, and op ical p ope ies [
56
]. Due o hei unique physical p ope ies, CNTs a e
widely explo ed in many applica ions like ma e ials science (addi i e ma e ials, compos-
i es), pho o ol aics, elecommunica ion, senso s, and bio-medical applica ions [66–68].
Table 1. Cha ac e is ic p ope ies o g aphene and CNTs [67,69–73].
Key Fea u es/
Ad an ages
G aphene CNTs
Ex olia ed G aphene G aphene Oxide Reduced G aphene Oxide Single Wall Mul i Wall
Chemical
s uc u e
Elas ic
modulus (TPa) ~1 >1 >1 ~1.4 0.3–1
Speci ic su ace
a ea (m2/g) 341–392 759 ±198 669 ±113 400–900 200–400
The mal
s abili y in
ai (◦C)
600–800 600–800 600–800 600–800 600–800
Cha ac e iza ion
Easy Easy Easy Easy Di icul
Bulk o
massi e
p oduc ion
Rela i ely di icul Easy Easy Di icul Easy
G aphene nanocomposi es wi h CNTs can exhibi be e an i ouling p ope y and po e
s uc u es han he p is ine g aphene memb anes. The e o e, using CNTs in he s uc u e
o g aphene composi e is impo an [
15
,
23
,
43
–
45
]. Basically, CNTs a e classi ied in o wo
di e en classes based on he numbe o olled-up g aphene shee s, namely single walled
CNTs (SWCNTs) and mul iwalled CNTs (MWCNTs) [
56
,
67
]. The basic equi emen o he
syn hesis o CNTs a e a ca bon sou ce, ca alys , and adequa e ene gy.
The u iliza ion o ansi ion me al ca alys leads o o ma ion o SWCNTs, whils he
absence o ca alys esul s in he o ma ion o MWCNTs. Th ee common me hods, namely,
elec ic a c discha ge, lase abla ion, and chemical apo deposi ion (CVD), a e gene ally
used o syn hesize CNTs, and a e b ie ly desc ibed in he CNTs syn hesis sec ion. The SWC-
NTs and MWCNTs can be di e en ia ed using ansmission elec on mic oscope (TEM) and
Raman spec oscopy s udies. SWCNTs exhibi s unique chi ali y-dependen pho olumines-
cence p ope ies in he nea in a ed (NIR) egions, which gi e his class o ma e ial g ea
po en ial o pho oluminescence imaging and Raman/pho oacous ic imaging. Mo eo e ,
Memb anes 2023,13, 145 5 o 32
he s ong nonlinea op ical abso p ion o SWCNTs makes hem highly p omising as a
sa u able abso be in high-powe lase gene a ion. MWCNTs lack op ical p ope ies due
o hei de ec s; howe e , hey ha e been widely employed in many applica ions, such as
ield emission de ices, adio equency in e e ence (RFI) shields, elec ochemical capaci o
o ein o cing addi i e ma e ials [
66
–
68
]. Gene ally, p is ine CNTs ha e di icul ies in being
dispe sed in wa e o o he common sol en s due o hei hyd ophobic na u e as well as
Van-de -Walls a ac ion [
22
,
24
]. The e o e, di e en unc ionaliza ion, using non-co alen
and co alen app oaches, ha e been de eloped o enhance he dispe sibili y/solubili y
o CNTs. Fo ins ance, in o de o dispe se he hyd ophobic CNTs in wa e , unc ional
g oups a e o med on hei su aces by using acid and dispe sion o CNTs in wa e can be
acili a ed [
22
,
24
]. The nonco alen unc ionaliza ion in ol es physiso p ion o su ac an s,
such as sodium dodecyl benzene sul ona e (SDBS) o sodium dodecyl sul a e (SDS) on
he sidewall o he SWCNTs leads ex olia ion o CNTs wi h enhanced dispe sibili y. The
non-co alen app oach is bene icial in e ms o e aining in insic p ope ies. Howe e , des-
o p ion due o aging leads o uns able dispe sion o CNTs, using co alen unc ionaliza ion
o ca boxylic acid ia su ace oxida ion using s ong acids (sul u ic acid (H
2
SO
4
)/ni ic acid
(HNO
3
)), which leads o s able dispe sion wi h high dispe sibili y o solubili y. Howe e ,
he dis up ion o sp
2
la ice leads o enhanced de ec s and a comple e loss o in insic
p ope ies [56,66–68].
Me al oxides like i anium dioxide (TiO
2
) o zinc oxide (ZnO) pho oca aly ic ma e ials
a e in aluable in wa e ea men applica ions. Me al oxide semiconduc o s, such as
ZnO and TiO
2
, ha e been ex ensi ely used because hey a e abundan , en i onmen ally
iendly, and ha e high elec on mobili y [
21
,
22
,
32
,
34
,
59
,
64
,
74
–
76
]. Nanocomposi es wi h
G aphene:CNTs:me al oxide show be e cha ge ca ie li e ime, mobili y, an imic obial
ac i i y, and con amina ion esis ance [
59
,
64
,
74
,
77
–
79
], which a e impo an pa ame e s
o he pho oca aly ic p ocesses in applica ions like isible ligh ac i e il a ion, dye
deg ada ion, and ouling de ec ion. G aphene nanocomposi e memb anes including CNTs
and me al oxides a e o g ea in e es in pho oca alysis and il a ion memb ane wa e
ea men applica ions, hanks o hei an imic obial ac i i ies and con aminan esis ances.
The combina ion o g aphene, CNTs, and me al oxide semiconduc o s is expec ed o
imp o e he mobili y o pho o-gene a ed elec on hole pai s, and he eby enhance he
cha ge ca ie ’s li e ime, and p esen s a po en ial nanocomposi e s uc u e [
59
,
64
,
74
,
77
–
79
]
ha is impo an o he pho oca alysis p ocess in wa e ea men . Beside his, ex e nal
suppo s like acous ic signals can imp o e he memb ane pe o mance and li espan in
e ms o p o ec ing he memb ane om con amina ion and help emo e con amina ion
in wa e ea men [
33
,
80
–
85
]. The e o e, he p oduc ion o biocompa ible, sel -cleaning,
sus ainable and an ibac e ial memb anes made o g aphene, CNTs, and isible ligh ac i e,
me al oxide based, acous ically suppo ed pho oca aly ic il a ion memb anes o use in
wa e ea men could be one o he mos impo an issues o be in es iga ed.
The aim o his s udy is o: e iew he exis ing g aphene and g aphene nanocomposi es
o wa e ea men sys ems in e ms o ma e ial and memb ane p ope ies, as well as syn-
hesis and cha ac e iza ion me hods; e iew cu en p oblem analysis o memb ane-based
wa e ea men sys ems; examine solu ions o inc ease he memb ane li espan and pe o -
mance while dec easing he o al cos ; and o discuss he u u e expec a ions/p ospec s
ega ding hese sys ems.
Ma e ial syn hesis, g aphene nanocomposi e memb ane p epa a ion p ocesses, and
cha ac e iza ions a e gi en in Sec ions 2and 3. In Sec ion 4, G aphene nanocomposi e
memb ane applica ions in wa e ea men applica ions a e summa ized. Sec ion 5p esen s
he cu en challenges and he u u e p ospec s o memb ane echnology. Conclusions a e
gi en in Sec ion 6.
Memb anes 2023,13, 145 6 o 32
2. P epa a ion o G aphene Nanocomposi e Memb anes
2.1. G aphene
G aphene nanocomposi es s and ou wi h hei g een and sus ainable ma e ial compo-
si ion, as well as being sui able o high-pe o mance and cos -e ec i e p oduc ion and
p ocessing me hods. The e a e a ious me hods o p epa e g aphene memb ane.
Some o he me hods used in he syn hesis o g aphene a e: CVD (Figu e 3g); chemical
ex olia ion (liquid phase ex olia ion ha can be ob ained in a de ice like sonica ion-machine
o shea mixe ) (Figu e 3d); physical ex olia ion (mechanical ex olia ion using s icky ape)
o g aphi e, pho o-ex olia ion, p ecipi a ion, chemical syn hesis, spin coa ing o educed
g aphene oxide e c. (Figu e 3a); anodic bonding (Figu e 3b), pho o-ex olia ion (Figu e 3c);
g aphene g ow h on silicon ca bide (SiC) (Figu e 3e); g aphene p ecipi a ion om me al
(Figu e 3 ); molecula beam epi axy (Figu e 3h); chemical syn hesis (Figu e 3i); dip-coa ing
(Figu e 3j); od coa ing (Figu e 3k); and ink-je p in ing (Figu e 3l) [
86
–
93
]. In hese me hods,
ma e ials like g aphi e, ca bon, and g aphene-ink can be used as he sou ce ma e ial o he
esul ing g aphene [
86
]. In Figu e 3a–i, gene al g aphene syn hesis me hods a e shown,
and Figu e 3j–l show g aphene p epa a ion by g aphene-ink sou ce.
Figu e 3.
G aphene syn hesis me hods; (
a
) mechanical ex olia ion, (
b
) anodic bonding, (
c
) pho o
ex olia ion, (
d
) liquid phase ex olia ion, (
e
) g ow h on silicon ca bide, (
) p ecipi a ion om me al,
(g) chemical
apo deposi ion, (
h
) molecula beam epi axy, (
i
) chemical syn hesis, (
j
) dip cas ing,
(k) od
coa ing, and (
l
) ink-je p in ing. Sub igu es ha e been adap ed/ ep oduced om e e ence [
86
]
wi h pe mission om Else ie .
Some g aphene syn hesis me hods a e desc ibed in de ail below.
Memb anes 2023,13, 145 7 o 32
CVD is sui able o mass p oduc ion. G aphene can be g own on a hea ed ca aly ic
me al oil subs a e (like nickel o cuppe ) ia a chemical eac ion o gas-phase p ecu so s
( om gaseous ca bon sou ces) in he CVD me hod [
87
–
90
] a high empe a u es. An
illus a ion o CVD is shown in Figu e 3g. The basic s eps o his me hod a e as ollows:
he gaseous ca bon sou ce ( eac an gas) is anspo ed in o he eac o , and i s adhe es
o he hea ed ca aly ic me al oil su ace by adso p ion. A e his, he g aphene laye is
o med on he me al su ace by ca aly ic decomposi ion, cooling, and eo ganizing o he
su ace ca bon a oms o g aphene. Many pa ame e s such as he empe a u e, a mosphe ic
p essu e le el, cooling a e, concen a ion o he ca bon sou ce, and solubili y o ca bon
ha e impac on he g aphene quali y, such as he numbe o laye s, laye dimension, and
su ace oughness o he p oduced g aphene ilm.
In mechanical (physical) ex olia ion (also called he apping me hod o mic omechan-
ical clea age) o g aphi e, he Van de Waals a ac ion o ces be ween g aphene laye s
a e o e come and he laye s a e peeled o om he g aphi e, and g aphene is ob ained.
Mechanical ex olia ion o g aphi e o g aphene laye (s) ha e been in es iga ed in de ail
in he li e a u e [
47
,
52
,
86
,
94
]. In 2004, physicis s No oselo and Geim a he Uni e si y
o Manches e achie ed ex olia ion o he g aphi e mine al by sepa a ing su ace in o
laye s by physical ex olia ion, and ob ained he i s wo-dimensional g aphene plane wi h
mechanical ex olia ion me hod [
52
]. This g een syn hesis me hod is widely used in he
manu ac u e o low-wid h g aphene laye s. An example o mechanical ex olia ion p ocess
is shown in Figu es 3a and 4whe e na u al g aphi e lakes a e used as he sou ce ma e ial
( om HQ-G aphene Inc., G oningen, The Ne he lands) o be ex olia ed by a ious adhesi e
apes om Ni o Denko Inc., Osaka, Japan, Sco ch- ape and 3M Inc., Sain Paul, MN, USA
(Figu e 4). A e he g aphi e ex olia ion p ocesses, many laye s o g aphene a e ans e ed
on o he a ge (by he use o mic omanipula o s) om he adhesi e ape (by we - ans e
whe e ace one: deionized wa e mix u e was used as he in e ace medium). G aphene
(on glass subs a e) is isible unde he op ical mic oscope as shown in Figu e 4. Ano he
g een coa ing me hod is dip cas ing (Figu e 3j), which is use ul o g aphene composi e
memb anes. A simple and g een dip-coa ing me hod was used o p epa e g aphene oxide
composi e memb anes by Lou e al. [95], and g aphene on polyme by Khan e al. [17].
Figu e 4.
Na u al g aphi e lakes’ mechanical (physical) ex olia ion o g aphene many-laye s by
adhesi e apes (pic u e and mic oscope image a e aken in Uni e sidad del Pais Vasco- Euskal
He iko Unibe si a ea—UPV/EHU labo a o ies, Bilbao, Spain).
Liquid phase ex olia ion, also called chemical ex olia ion, is ano he me hod o p epa e
g aphene laye (s), can be seen in Figu e 3d. In he p oduc ion o g aphene om g aphi e
by chemical ex olia ion, he oxida ion o g aphi e is ensu ed, and unc ional g oups con-
aining oxygen make g aphi e oxide which is hyd ophilic and dispe sible in wa e [
96
–
98
].
Chemical ex olia ion o g aphi e is usually done in he p esence o oxidan s ( o educe he
in e ac ion be ween g aphene laye s).
Memb anes 2023,13, 145 8 o 32
In chemical ex olia ion, chemicals like s ong acids like sul u ic acid, po assium pe -
mangana e o sodium ni a e can be used as oxidan s. Alkali me als a e also used o
ex olia e g aphi e and dispe se he g aphene in a liquid medium [94].
Elec ochemical ex olia ion is ano he me hod o p epa e g aphene. In elec ochemical
ex olia ion, a wo king elec ode (can be g aphi e od o cylinde ) and a coun e elec ode
(can be g aphi e od o cylinde ) can be imme sed in elec oly e solu ions, wi h di ec
cu en (DC) applied o he elec odes (wo king and coun e ) o he ex olia ion o g aphi e
o g aphene laye (s) [99].
G aphene can also be syn hesized by su ace p ecipi a ion o ca bon in some ansi ion
me als as desc ibed in [
87
,
91
,
100
]. Fu he mo e, silicon deso p ion om silicon ca bide
single-c ys al su aces can gi e a mul ilaye g aphene s uc u e as desc ibed in [
92
,
93
].
Addi ionally, o he unc ionaliza ion o g aphene su ace o in he educ ion p ocesses
o g aphene and i s composi es, low-cos and biocompa ible ma e ials can be used, and
a e called ‘g een unc ionaliza ion o ‘g een educ ion’ [
75
]. In g een educ ion p ocesses,
which is a kind o hyd o he mal me hod, biological o biologically compa ible sou ces like
plan ex ac s can be used as he bio- educ an s [75].
I is a challenging issue o be able o p oduce g aphene using a sus ainable and
en i onmen ally iendly solu ion, which can be applied a a la ge scale, wi h high quali y
and a low cos . The e o e, u he de elopmen o hese g een p oduc ion solu ions, such
as ex olia ion o dip-coa ing in g een g aphene p oduc ion, is e y impo an . I should
also be no ed ha g aphene is sensi i e o i s subs a e o amewo k-subs a e ma e ial
wi h which i is in con ac . Thus, he ma e ial o he g aphene subs a e is a challenge. Fo
p ac ical and eal ime applica ions, i is di icul o ind a sui able subs a e ha can be
moun ed unde g aphene. The mos sui able subs a e ma e ials o g aphene a e SiO
2
and
silicon ca bide [75,101].
2.2. CNTs and G aphene Nanocomposi es wi h CNTs
CNTs can be unc ionalized and doped o he g aphene s uc u e. CNTs can be
p epa ed by h ee s anda d me hods, iz., a c discha ge syn hesis, lase abla ion, and
chemical apo deposi ion [
67
,
102
]. These h ee me hods a e shown in Figu e 5. Figu e 5a
shows he elec ic a c discha ge me hod. A c discha ge, in which high powe elec ic a c is
gene a ed be ween he high pu i y g aphi e elec odes (sepa a ed by he dis ance ~1–2 mm)
in a chambe illed wi h an ine gas like helium o a gon. The apo iza ion o g aphi e
elec ode by he passage o DC ac oss he elec odes a a p essu e high p essu e (abou
100 To
) leads o he o ma ion CNTs a he chambe wall o a he ca hode. Bo h SWCNTs
and MWCNTs can be syn hesized by his me hod.
Doping o a me al ca alys —such as i on (Fe), cobal (Co), nickel (Ni) o molybdenum
(Mo)—on bo h elec odes leads o he o ma ion o SWCNTs. By op imizing p essu e,
cu en , ine gas ype, empe a u e, and sys em geome y, he quali y and quan i y o
CNTs will be con olled.
Lase abla ion is o en p e e ed o syn hesize high quali y SWCNTs wi h an app o-
p ia e me al ca alys as used in he a c discha ge me hod. In he lase abla ion me hod (as
shown in Figu e 5b), he lase beam (mos ly ca bon dioxide (CO
2
) and neodymium-doped
y ium aluminum ga ne ; Nd:Y
3
Al
5
O
12
(Nd-YAG) lase sou ce) apo izes a mix u e o
g aphi e and me al ca alys a ge in a closed ho izon al ube unde ine gas a mosphe e
a high empe a u e wi h con olled p essu e. CNTs a e deposi ed on a cooled su ace
o he ho u nace. The g aphi e a ge wi hou ca alys leads o he o ma ion o MWC-
NTs [67,102].
The CVD me hod (shown in Figu e 5c) is conside ed o be a po en ial al e na i e o a c
discha ge and lase abla ion o p epa e CNTs due o i s syn hesis empe a u e ha can be
educed o below 800
◦
C. Mo eo e , he CVD me hod p o ides a g ea deg ee o eedom
in con ol o pu i y, yield, nano ube o ien a ion, leng h, diame e , and densi y.
Du ing CVD, he ca bon eeds ock, ypically in he o m o hyd oca bon gas was
decomposed a a speci ied empe a u e in o ca bon gas deposi ed on ansi ion me al
Memb anes 2023,13, 145 9 o 32
ca alys s such as Fe, Co, o Ni in he wa e cooling end o he qua z ube. Hyd oca bon
sou ces such as me hane (CH
4
), ace ylene (C
2
H
2
), e hanol (C
2
H
5
OH), lique ied pe oleum
gas (LPG), and ca bon monoxide (CO) a e used o he syn hesis o CNTs [
67
]. Ca aly ic
CVD g ow h o CNTs can be achie ed ei he he mal o plasma enhanced echnique.
Mo eo e , o he echniques like wa e assis ed-CVD, oxygen assis ed CVD, ho ilamen -
CVD (HFCVD), mic owa e plasma CVD (MPECVD), and adio equency CVD (RF-CVD)
can be used o he syn hesis o CNTs [67,102].
Figu e 5.
An illus a ion o CNT syn hesis me hods: (
a
) elec ic a c discha ge, (
b
) lase abla ion, and
(c) chemical apo deposi ion me hod.
The combina ion o g aphene and CNTs is p omising o wa e ea men applica ions.
The addi ion o CNTs o he g aphene s uc u e can imp o e he po e s uc u e o memb ane
su aces, hyd ophilici y, and an i ouling p ope ies o he ul a il a ion and elec ochemical
il e memb anes [15,23,43–45,103].
Syn hesis o nanocomposi es wi h g aphene and CNTs can be comple ed using he
ollowing p ocess: (i) g aphene p epa a ion by, e.g., mechanical ex olia ion o g aphi e,
Memb anes 2023,13, 145 16 o 32
Akba i e al. ob ained 71 L m
−2
h
−1
ba
−1
wa e pe meabili y wi h high ejec ion (o e
95%) o a ious dyes om he la ge-a ea g aphene-based nano il a ion memb anes ha
we e p epa ed by g a u e p in ing p ocess [
36
]. Jha e al. p epa ed nano- il e memb anes
(by cas ing) wi h 95.21
µ
m hickness and 0.0267
µ
m po e-size, made o educed g aphene
oxide o emo e i on [
12
]. P epa ed memb anes had high me al con aminan (i on) ejec ion
(95.77%). Gao e al. used g aphene oxide wi h polyme ma e ials in he memb ane
composi ion, and ecei ed 68.21 L m
−2
h
−1
ba
−1
wa e pe meabili y and mo e han 97%
ejec ion o dye like me hylene blue and congo ed [
21
]. Yada e al. p epa ed g aphene
based nanocomposi e memb ane whe e CNTs and PVA we e used as a nano-space and as
an adhesi e, espec i ely [
45
]. They coa ed (by acuum il a ion) he nanocomposi e on a
hyd ophilic mixed cellulose es e suppo (po e size 0.22
µ
m) o ob ain a highly o de ed
lamina ed s uc u e. They ob ained 94.2% sodium sulpha e ejec ion and 85.86% sodium
chlo ide ejec ion wi h a high wa e pe mea e a e o 14.2–13.45 L m
−2
h
−1
a 5 ba p essu e
le el. In addi ion, Liu H. e al. s udied ees anding ul a hin memb anes made o educed
g aphene oxide o be used in wa e ea men [
23
]. The hickness ange was be ween
20 nm
and 200 nm in hei s udy. Fu he mo e, ecen ly, Liu T. e al. pe o med a heo e ical
s udy on a g aphene-based memb ane il e o discuss i s mechanical p ope ies and wa e
pe mea ion [
20
]. In hei wo k, g aphene s ips a e wo en in o he il e memb ane. In his
way, hey sugges ed ha he mechanical p ope ies o he g aphene-based memb ane we e
inc eased signi ican ly. Mo eo e , one o he newly de eloped composi e memb anes in
wa e ea men is g aphene oxide/MXene. Mxene is a 2D ma e ial ha can be ca bide o
ni ide based. Wi h he addi ion o MXene o g aphene oxide, nanocapilla y channels we e
o med in he composi e, and i was epo ed ha he wa e pe meabili y o he g aphene
oxide/MXene composi e memb ane inc eased [112].
Thanks o he g aphene’s ligh weigh , hyd ophobici y, an i-bac e ial e ec , and con-
aminan adso p ion capabili ies [
15
,
64
,
74
], he in oduc ion o g aphene in o a pho oca al-
ysis sys em can enhance he pho oac i i y by o ming syne gic con ac wi h me al oxide
ca alys like TiO
2
o ZnO [
14
,
32
,
34
,
65
,
74
,
113
]. The e iciency o he pho oca alysis p ocess—
which is a pho ochemical eac ion ha occu s wi h he ini ia ion o ee adical mechanisms
when he pho on in e ac s wi h he ma e ial— a ies acco ding o he p ope y o he used
ma e ial. In me al oxide pho oca alys s, TiO
2
is he mos commonly used biocompa ible
(used in mos oo hpas es and pha maceu icals) me al oxide pho oca alys wi h i s s ong
oxida ion powe , ouling esis ance, an ibac e ial unc ion, and low-cos , along wi h s ong
abso p ion in ul a iole [
9
,
27
,
113
] ha can be shi ed o he longe wa eleng hs [
9
,
27
,
114
].
Jiang e al. and Fan e al. epo ed ha g aphene can imp o e he pho oca aly ic ac i i y o
TiO
2
unde isible ligh in bulk- o m [
77
,
79
], o in a laye -by-laye o m whe e TiO
2
was
coa ed on g aphene laye [59,77,79].
While a high amoun o ene gy is consumed by using a ligh sou ce in ul a iole -ac i e
pho oca alysis applica ion wi hin he scope o disin ec ion wi h pho oca aly ic deg ada-
ion, ene gy e iciency is p o ided by using dayligh in isible ligh ac i e pho oca aly-
sis. The e o e, he applica ion o isible ligh -ac i a ed pho oca alysis is mo e ad an a-
geous han ul a iole -ligh ac i a ed pho oca alysis which equi es high-ene gy ul a iole
sou ces [9,27,113].
Chemical doping o TiO
2
can shi i s abso p ion om ul a iole o he isible
egion [9,27,78,104,113–115]
. Fo ins ance, gold, sil e , o ni ogen doping o he TiO
2
can shi i s abso p ion egion om ul a iole o he isible [
9
,
27
,
114
], and can disin ec
biological con aminan s such as bac e ia and i uses [27].
Visible ligh ac i e, biocompa ible, an i ouling, sel -cleaning coa ings wi h TiO
2
and
g aphene-based il a ion memb anes o be used in wa e ea men ha e been epo ed
in li e a u e [
78
,
104
,
114
,
115
]. A hanasekou e al. (2015) p epa ed g aphene-based wa e
ea men memb anes wi h he ni ogen doped TiO
2
[
114
]. While he mos ob ious ad an-
age o Ag-doped TiO
2
(Ag-TiO
2
) pho oca alys s is ha e en a e y low le els o isible
ligh , hey can be su icien o inac i a e biological con aminan s/pollu an s [
9
,
27
,
113
],
ni ogen doped TiO
2
can gi e mo e eliabili y o he wa e ea men sys em emaining
Memb anes 2023,13, 145 17 o 32
su icien wi hou addi ional me al nanopa icles. An addi ional ad an age is ha ni ogen
is abundan in ou a mosphe e [114].
G aphene, CNTs, and me al oxide composi es show enhanced pho oca aly ic ac i i y
o he emo al o dye and/o me al [15,59,64,74]. Veci is e al. p oposed an an imic obial
mechanism o CNTs which may a y depending on physical and chemical e ec s as well
as bac e ial oxida ion due o he elec onic s uc u e [
56
]. CNTs o polyme s can p e en
es acking o g aphene laye s [59].
Bellamkonda e al. desc ibed he es acking p e en ion o g aphene as CNTs c ea e an
addi ional elec on anspo laye , and he elec on hole ecombina ion a e educes in he
g aphene:CNTs:me al oxide nanocomposi e [74].
G aphene and CNTs oge he ha e high con aminan adso p ion capaci y wi h mo e
po osi y and la ge su ace a ea o p o ide mo e con ac be ween me al oxide and con am-
inan [
59
,
74
]. In he nanocomposi es composed o g aphene and CNTs ha e addi ional
elec on anspo channels in blend and can accep pho oexci ed elec ons, hey can educe
ecombina ion by imp o ing elec on-hole li e ime in pho oca alysis o c ea e eac i e
oxygen species [
59
,
74
]. Bo h CNTs and g aphene a e good elec on accep o s o TiO
2
me al
oxide [
74
]. To in es iga e he pho oca aly ic ac i i y o a dye deg ada ion, me al oxide was
used as a pho oca alys in he CNTs and g aphene s uc u e by Huang e al. [59].
Wa e pe meabili y and memb ane pe o mance imp o emen ha e been in es iga ed
by he use o acous ic echnology in wa e ea men sys ems [
33
,
80
,
81
,
116
,
117
]. I was
epo ed ha he acous ic s imuli can p o ec he memb ane om ouling and help emo e
con amina ion [33,80–85].
In a pa en in en ed by Ga alas (assignee is NASA), an acous ic ac ua o was used
wi h mic opo ous il e ing memb ane made o CNTs. The acous ic ac ua o , which allows
he acous ic ib a ions o sp ead o e he memb ane and con ains a PVDF ilm laye in
con ac wi h he il e ing memb ane, was used o suppo he wa e lux h ough he
memb ane by s imula ing i wi h acous ic ib a ions [80].
In a s udy by Fung e al., acous ic echnology was used o inc ease he pe o mance o
he sys em in e ms o elimina ing he cake-laye (o gel-laye ) ha inc eases he ouling
on he memb ane. In hei s udy, memb ane cake-laye was b oken up and emo ed
wi hin 100 ms wi h acous ic suppo [
81
]. The e o e, i is necessa y o con ibu e o he
de elopmen o sus ainable and en i onmen ally iendly wa e ea men applica ions by
e alua ing il e ing sys ems suppo ed by acous ic echnology, including memb anes made
o g aphene-based nanocomposi e ma e ials, CNTs, and isible ligh ac i a ed me al oxide
pho oca alys , which has been pa ially in es iga ed in wa e ea men s udies.
3. Cha ac e iza ions and Cha ac e is ic P ope ies o G aphene Nanocomposi es
3.1. Cha ac e iza ions o Ma e ials and Nanocomposi es
Some good ools o cha ac e ize g aphene laye (s) and CNTs a e he scanning elec-
on mic oscopy (SEM), a omic o ce mic oscope (AFM), ansmission elec on mic oscope
(TEM), and Raman spec oscopy. Mo phological analysis, agg ega ion s a e, de ec s such
as adhesion de ec in he CNTs wall, amo phous ca bon amoun in a CNT sample, di-
ame e , and leng h and hickness can be de e mined by elec on mic oscopy o a omic
o ce mic oscopy.
An example o an SEM image o single-laye g aphene p epa ed by CVD on sili-
con/silicon dioxide subs a e is shown in Figu e 7a whe e he scale ba is 100 nm. W inkles
and accumula ions a e p esence in he single-laye g aphene sample. Figu e 7b shows
an AFM image o a g aphene many-laye sample wi h 10
µ
m scale-ba [
118
]. I can be
obse ed ha he e a e lakes o a ious wid hs in he many-laye sample.
TEM analysis can show he axial de ec s o a sample. Figu e 7c,d show he TEM
analysis o SWCNT and MWCNT samples, espec i ely. In TEM images o he samples, no
de ec s a e p esen in he SWCNT sample (in Figu e 7c), whe eas li le de ec concen a ion
was ound in he MWCNT sample (in Figu e 7d) as desc ibed in [119].
Memb anes 2023,13, 145 18 o 32
Figu e 7.
(
a
) Single-laye g aphene’s SEM image (wi h 100 nm scale-ba ), (
b
) many-laye g aphene’s
AFM image (wi h 10
µ
m scale-ba ), and TEM images o (
c
) SWCNTs and (
d
) MWCNTs. Sub igu es
(
c
,
d
) ha e been adap ed om [
119
] wi h pe mission om John Wiley and Sons. Sub igu e (
b
) has been
adap ed om [
118
] wi h pe mission om MDPI. (SEM and AFM expe imen s in sub igu e (
a
,
b
) a e
pe o med in Uni e sidad del Pais Vasco/Euskal He iko Unibe si a ea—UPV/EHU labo a o ies,
Bilbao, Spain).
Sample images o he g aphene nanocomposi e memb anes a e gi en in Figu e 8. A
TEM image o g aphene oxide/CNTs/TiO
2
laye , a SEM image o g aphene oxide/TiO
2
memb ane, an AFM image o g aphene oxide laye , an op ical mic oscope image o a
g aphene oxide memb ane, and a pho og aph o a lexible g aphene/CNTs memb ane on
PTFE a e shown in Figu e 8a–e, espec i ely.
Raman spec oscopy is a use ul ool o in es iga ing he cha ac e is ic peaks o a
sample as well as ob aining in o ma ion abou he quali y o con amina ion le el o he
sample. Homogenei y and quali y o a g aphene ilm on a subs a e like SiO
2
, Si o Ni,
can be e alua ed by Raman spec oscopy. Fu he mo e, cha ac e is ic peaks o a g aphene
sample can be in es iga ed by Raman spec oscopy. An example o a Raman spec um
in Figu e 9a shows he cha ac e is ic G-peak, D-band, and 2D-band o g aphene. In his
spec um, a 1348 cm
−1
he D-band, a 1599 cm
−1
he G-peak, and a 2693 cm
−1
he 2D-
band a e isible o a single-laye g aphene sample unde 532 nm lase exci a ion sou ce.
Laye numbe , de o ma ion le el o diso de le el o g aphene sample can be in e p e ed
by e alua ing he in ensi ies o he cha ac e is ic g aphene peak and bands (like G, D, 2D)
in Raman spec oscopy. De o ma ion and de ec le el o he g aphene can be analyzed
by in es iga ing he D-band and G-peak in ensi y a io (I
D
/I
G
). Fo ins ance, i he I
D
/I
G
a io is high, de o ma ion and de ec le el o g aphene can be low in a sample [
118
,
120
].
Figu e 9b shows he g aphene many-laye s wi h a ious hicknesses wi hou wa enumbe
shi in G-peak alue (1582 cm
−1
) in he Raman spec um o many-laye g aphene samples
(in his igu e, he hinnes sample is sample-a, whils he hickes sample is sample-d in
Figu e 9d). Calcula ed I
D
/I
G
alue o he samples a e he highes (1.19) o he hinnes
sample (sample-a in Figu e 9b), and he lowes (0.04) o he hickes sample (sample-d
Memb anes 2023,13, 145 19 o 32
in Figu e 9b) which means ha he de o ma ion and de ec le el a e low in he hinnes
sample [118].
Figu e 8.
(
a
) TEM image o g aphene oxide/CNTs/TiO
2
laye [
59
], (
b
) SEM image o g aphene
oxide/CNTs memb ane [
37
], (
c
,
d
) AFM and op ical mic oscope images o g aphene oxide memb ane
(pe o med in Uni e sidad del Pais Vasco/Euskal He iko Unibe si a ea—UPV/EHU labo a o ies,
Bilbao, Spain), (
e
) a pho og aph o a g aphene/CNTs memb ane on PTFE [
39
]. Sub igu es (
a
,
b
,
e
)
ha e been adap ed/ ep oduced om e e ences [
37
,
39
,
59
] wi h pe mission om Else ie , MDPI, and
he Royal Socie y o Chemis y.
Raman spec oscopy can also help o de e mine he impu i y le el o a CNTs sample.
Fo example, acco ding o he cha ac e is ic Raman peak and band in es iga ions o a CNTs
sample, absence o he D-band can be a ibu ed o a pu e sample, o he adial b ea hing
mode (RBM) band, which is sensi i e o he diame e and can dis inguish he CNTs as
single walled o mul i walled, can be clea ly isible unde he 785 nm ed lase sou ce
exci a ion spec a ha can show signals om semiconduc ing ubes (514 nm g een lase
shows he p esence o me allic ubes), o quali y o a CNTs sample can be de e mined by
he D-band and G-peak in ensi y a ios [66–68,119,121–123].
Raman spec um examples o SWCNT and MWCNT a e shown in Figu e 9c,d. Cha -
ac e is ic G-peak, D-band and 2D-bands o CNTs a e p esen a 1575 cm
−1
, 1340 cm
−1
and 2680 cm
−1
, espec i ely. The cha ac e is ic adial b ea hing modes (RBMs) in Raman
spec a can only be obse ed o he SWCNTs [
66
–
68
]. The RBM, which ep esen s he
mode in which all ca bon a oms in CNTs mo e simul aneously in he adial di ec ion, is
in isible in he MWCNT, is isible only in he SWCNT, and is seen a 142 cm
−1
, as can be
seen in Figu e 9c,d [122–124].
Memb anes 2023,13, 145 20 o 32
Figu e 9.
Raman spec um o (
a
) single-laye g aphene, (
b
) many-laye g aphene, (
c
) SWCNTs
and (
d
) MWCNTs. Sub igu es (
c
,
d
) ha e been adap ed om [
123
] wi h pe mission om Sp inge
Na u e. Sub igu e (
b
) has been adap ed om [
118
] wi h pe mission om MDPI. (Raman Spec oscopy
expe imen s a e pe o med in Uni e sidad del Pais Vasco/Euskal He iko Unibe si a ea—UPV/EHU
labo a o ies, Bilbao, Spain. In Raman analysis in Figu e (a), λexc = 532 nm).
3.2. Cha ac e is ic P ope ies o Nanocomposi e Memb anes in Te ms o Wa e T ea men
Sus ainabili y, biocompa ibili y, and en i onmen ally iendliness a e impo an p op-
e ies in nanocomposi e memb anes when using hem in biological pu poses like wa e
ea men [
62
,
63
,
125
–
129
]. Nanocomposi e memb anes con aining new il a ion ma e ials,
nano ille s, polyme s, o ino ganic oxide ma e ials can ha e he mal s abili y, high su ace
hyd ophilici y, high wa e pe meabili y, o high s eng h depending on he ype o ma e ials
used in hei s uc u es [62,63,125–129].
In wa e ea men , he memb ane il a ion me hod is mo e ad an ageous han he
adi ional il a ion me hods as i consumes less ene gy and is he e o e mo e economical,
and memb ane de elopmen s udies a e e y impo an o sol e he p oblems ela ed
o he need o clean wa e [
15
,
42
,
126
]. In addi ion o he memb ane il e ing p ocess,
he pho oca aly ic unc ion combines il a ion memb anes, which a e also called hyb id
unc ion memb anes, and which pe o m be e in ma e s such as o ganic con aminan (like
dye) deg ada ion/ emo al compa ed o p is ine il a ion. These ha e been in es iga ed in
he li e a u e [
50
,
78
,
115
]. Fo an e ec i e o ganic con aminan deg ada ion, semiconduc o
ma e ial p ope ies like band gap, su ace a ea, pa icle size, po osi y, and c ys al s uc u e
a e some impo an pa ame e s [
130
]. In pho oca aly ic memb anes, pho oca alysis s a s
wi h he ac i a ion o a pho oca alys in he memb ane s uc u e ia he ligh abso p ion.
Ligh abso p ion, which can be al e ed by ecombina ion o cha ge ca ie s gene a ed by
pho ogene a ion, can cause a change in he numbe o pho oca alys ligh ac i e si es and
limi he cha ge ca ie [131].
A e he bandgap ene gy (Eg) o he semiconduc o is exceeded by he ligh abso p-
ion, h ough he edox eac ion in he semiconduc o , he elec on is exci ed om he
Memb anes 2023,13, 145 21 o 32
alence band (VB) o he conduc ion band (CB), o ming elec on hole pai s, and eac ions
o elec on ans e happen (like adical o ma ion), which inalizes wi h con aminan
deso p ion o he compounds like mine al acids, and hen ans e s back o he inle wa e
sample. An example o such a p ocess can be seen in Figu e 10.
Figu e 10.
Schema ic examples o a memb ane s uc u e, wa e pe mea ion p ocess, and a pho oca al-
ysis p ocess.
In s udies on nanocomposi e pho oca alysis, hanks o p o iding addi ional elec on
anspo channels in blend, accep ing pho oexci ed elec ons, and educing ecombi-
na ion by imp o ing elec on-hole li e ime in pho oca alysis o c ea e eac i e oxygen
species, g aphene and CNTs a e good elec on accep o s o he TiO
2
pho oca alys [
59
,
74
].
Figu e 10
depic s a gene al lab-scale memb ane s uc u e, wa e pe mea ion p ocess, and
pho oca alysis p ocess illus a ions.
Pho oca aly ic wa e ea men memb anes can be made o unc ionalized composi es
like g aphene, CNTs, me al oxides, and polyme s. Func ionaliza ion is a chemical su ace
ea men and a basic echnique ha adds new p ope ies/capabili ies o he ma e ial. The
use o unc ionalized memb ane ma e ials and addi i es in he memb ane s uc u e can im-
p o e memb ane pe o mance o educe memb ane ouling in e ms o wa e pe meabili y,
selec i i y, and high pollu an ejec ion [
16
,
17
,
21
,
29
]. Memb ane ma e ial unc ionaliza ion
can be in he o m o a achmen o unc ional g oups o he su ace o he ma e ial laye .
In he unc ionaliza ion o he g aphene laye s, unc ional g oups (like nanopa icles o
polyme s) can be a ached o he su ace o he g aphene laye by co alen bonding [
94
,
104
].
Nanopa icle doping can imp o e some p ope ies o memb anes such as he pe me-
abili y, s abili y (a high empe a u e), and sepa a ion pe o mance, and he selec i i y ade-
o o he memb ane can be elimina ed by nanopa icle doping [
45
,
62
,
63
]. CNTs o me al
Memb anes 2023,13, 145 22 o 32
oxide nanopa icles can be added o he g aphene nanocomposi e memb ane s uc u es o
change he memb ane’s mo phology, and o modi y he su ace p ope ies o imp o e he
mechanical p ope ies, wa e lux, and con aminan selec i i y/ emo al h ough he mem-
b ane [
23
–
25
,
29
,
36
,
40
,
45
]. Howe e , i he numbe o nanopa icles in he nanocomposi e is
no adjus ed, he memb ane unc ions like wa e lux may dec eases. Fo example, Yada
e al. ound ha inc easing CNT concen a ion in a g aphene nanocomposi e memb ane
dec eases he memb ane’s con aminan ejec ion a e and wa e lux [
45
]. On he o he
hand, one o he main p oblems wi h he nanopa icle: polyme ype composi e memb ane
is ha nanopa icles o m a ine dispe sion a ound he polyme s uc u es [
127
]. In addi ion
o he composi es, memb anes made o p is ine ma e ials ha e also been in es iga ed o
hei ees anding o ms, such as g aphene ees anding memb anes [
13
,
46
,
102
]. Al hough
he p is ine g aphene laye s show high mechanical s eng h, ees anding g aphene mem-
b anes can su e om some s uc u al p oblems such as s abili y, con inuous s eng h, and
applica ion/p ocess di icul ies like applicabili y o la ge a eas o sepa a ion in an aqueous
medium [23,46,102].
In nanocomposi e il e ing memb anes, po e-clogging is a c ucial issue o be o e come.
Clogging o he memb ane po es inc eases bo h he apid decline in memb ane pe o mance
and he ouling endency o he memb anes [
8
,
9
,
11
–
18
,
30
–
44
,
46
,
47
,
49
,
50
]. Fu he mo e,
memb anes’ wa e pe meabili y, undesi ed ma e ial ejec ion, lux eco e y, long- e m
iabili y, eusabili y, and con aminan e en ion capabili ies a e impo an pa ame e s o
he wa e ea men applica ions.
Pe mea ion can be analyzed by a dead-end s i ed cell- il a ion uni (in a chambe
illed wi h ni ogen gas). The wa e pe meabili y a e o g aphene memb ane can be
inc eased wi h inc easing applied p essu e and lux. The wa e pe meabili y (pu e wa e
lux) Mo a il a ion memb ane can be de ined as ollows [36,44];
M=V
A∆P∆ (1)
whe e Vis he pe mea ed wa e olume, A(m
2
) is he memb ane’s e ec i e a ea,
∆P
(ba ) is
he p essu e di e ence and
∆
(hou ) is he pe mea e ime. Pe meabili y is in L m
−2
h
−1
ba
−1
.
The ejec ion (sepa a ion) pe cen age Ro a memb ane can be de ined as [44];
R= 1−Cpe mea e
C eed !×100% (2)
whe e
Cpe mea e
is he pe mea e solu ion concen a ion and
C eed
is he eed solu ion con-
cen a ion. Fu he mo e, he memb ane’s lux eco e y is as ollows [36,44];
Flux eco e y (%) = Jw,i
Jw,1 ×100 (3)
whe e Jw,1 is he ini ial wa e lux be o e he i s cycle and Jw,iis he lux a e cycle i.
4. Wa e T ea men Applica ions o G aphene Nanocomposi e Memb anes
Ea h’s p ecious eshwa e esou ce can be con amina ed by ag ochemicals, pha -
maceu ical de i a i es, hea y me als, and endoc ine dis up o s being somehow mixed
in o he wa e esou ces [
10
,
63
,
127
–
129
]. The p oduc ion o memb ane ma e ials ha a e
esis an o ag ochemicals, pha maceu ical de i a i es, hea y me als, endoc ine dis up o s,
hea , and oxida ion a e in aluable in memb ane de elopmen . G aphene nanocomposi e
memb anes ha e ecei ed g ea a en ion hanks o p ope ies such as ouling esis ance,
an imic obial ac i i y, and imp o ed memb ane li e ime. Se e al nanome e s hick laye s
made o g aphene a e o g ea in e es because o g aphene’s unique physical, chemical,
op ical, and elec oop ical p ope ies [12–16,18,21,29,32,37,40–42,44,45,47,52,64,74].
Memb anes 2023,13, 145 23 o 32
Ul a il a ion o mic o il a ion and o wa d osmosis o e e se osmosis echniques
(using hollow o spi al shapes memb anes) a e he mos used ones in cu en wa e ea -
men memb ane echnologies, which gene ally ope a e unde high p essu e and ul a iole
ligh [9,21,65].
Polyme -based memb anes, which a e adi ionally used in comme cial sys ems, acil-
i a e he con inuous ope a ion o he memb ane in wa e pe meabili y p ocesses. Howe e ,
wha eally plays he ole o selec i e pe meabili y a e g aphene-based nanocomposi e
memb ane componen s. G aphene nanocomposi e memb anes can be mo e use ul o cos -
e ec i e wa e ea men , including wa e desalina ion, il a ion, pu i ica ion, dye o me al
deg ada ion in wa e ul a il a ion, pu i ica ion, con amina ion de ec ion o wa e sepa a-
ion applica ions [
15
,
21
,
74
]. G aphene memb anes ha e be e hyd ophilici y, an i ouling
p ope ies, po e s uc u es, and su ace oughness compa ed o he p is ine polyme mem-
b anes. Fu he mo e, g aphene memb anes’ an ibac e ial ac i i y, hyd ophilici y, wa e
lux, and ouling p ope ies can be imp o ed when hey a e used in a modi ied composi e
memb ane s uc u e wi h ma e ials such CNTs, polyme , o me al oxides [15,74].
Modi ica ion and unc ionaliza ion o memb ane su ace impa s new p ope ies o he
memb ane, and can inc ease he u ili y o memb anes in sepa a ion p ocesses by imp o ing
he an i ouling p ope ies and s abili y o he memb anes [17,18,30,35,37].
In addi ion o modi ica ion and unc ionaliza ion, he use o sound wa es in memb ane-
based wa e ea men sys ems o issues such as imp o ing he wa e low h ough he
memb ane, p e en ing con amina ion o he memb ane, and impu i y accumula ion on he
memb ane su ace, as well as p olonging i s li e, is an en i onmen ally iendly applica ion
ha does no equi e he use o any chemicals [26,82,116,132].
Sound wa es in he 20–20 kHz equency ange (acous ic) and mo e han 20 kHz
equency (ul asonic) can suppo wa e low and il e ing capabili ies o il e ing mem-
b anes [
26
,
82
,
116
,
132
–
134
]. Fu he mo e, ul asonic equency ca ego ies a e he powe
ul asonic equency anging om 20 kHz o 100 kHz, he high- equency ul asonic om
100 kHz o 1 MHz, and he diagnos ic ul asonic om 1 MHz o 500 MHz. In indus ial clean-
ing p ocesses like memb ane su ace cleaning o pollu ion p e en ion, 20 kHz o
500 kHz
a e o en used. Thanks o he d ag o ce ac ing on he memb ane su ace wi h acous ic o
ul asonic suppo , impu i ies on he memb ane su ace can be emo ed [
116
,
133
]. Acous ic
o ul asonic equency sou ces include acous ic chips, piezoelec ic chips, and ul asonic
ho ns [26,82,116,132,133].
Figu e 11 shows a se -up o an ul a il a ion memb ane ha is suppo ed by ul a-
sound which was p oduced by a ansduce whose imme sed ip is no in con ac wi h
he memb ane [
116
]. In he ul asound-assis ed memb ane sys em, ca i a ion caused by
ul asound wa es is s a ed as he main ac o ha acili a es he passage o liquid ma e ial
h ough he memb ane [
116
]. Fu he mo e, high ul asound in ensi y helps o clean he
memb ane su ace, keeping he memb ane clean by pushing he con aminan s away om
he memb ane su ace [
116
,
134
]. Howe e , i has been epo ed ha in hese applica ions,
especially a ul a-high equencies, bubble o ma ion on he liquid and memb ane su -
ace may ad e sely a ec il a ion, and memb ane damage due o ib a ion should be
p e en ed [26,82,132].
Wa e pollu ion and pollu an s—which a e one o he oday’s ongoing en i onmen al
p oblems, a e likely o be encoun e ed by he memb ane in wa e ea men p ocesses
and a e s ill being s udied o emo e hem om wa e —can be classi ied as o ganic and
ino ganic. O ganic pollu an s in a wa e sample can be dyes o phenol de i a i es, while
ino ganic pollu an s can be ino ganic sal s o oxic hea y me als. G aphene nanocomposi es
can adso b and emo e bo h he o ganic and ino ganic pollu an s om wa e [135].
Memb ane adso p ion echnology is an economical and apid me hod o emo e
bo h o ganic and ino ganic con aminan s. Howe e , i should also be no ed ha mos
o he epo s in he li e a u e a e he esul s o expe imen al in es iga ions o pollu ion
s uc u es wi h p ede ined p ope ies in he labo a o y en i onmen . Howe e , in he eal
Memb anes 2023,13, 145 24 o 32
wo ld, wa e esou ces and wa e in he was e class can ha e pollu ion s uc u es wi h e y
di e en componen s [135–137].
Figu e 11.
Expe imen al se up example o an ul a il a ion o suspended solu ions wi h ul asound.
Figu e has been adap ed/ ep oduced om e e ence [116] wi h pe mission om Else ie .
E ec s o G aphene Nanocomposi e Memb anes P oduced by G een Me hods on Wa e
T ea men Applica ions
En i onmen ally iendly p ocesses in wa e ea men applica ions ha e d awn a -
en ion. The mos en i onmen ally iendly wa e ea men applica ions a e likely o be
sys ems ha do no con ain ha m ul chemicals o consume high ene gy, and a e com-
bined wi h g een ma e ials, g een p oduc ion me hods, and p ocesses o a sus ainable
en i onmen [74,75,138–141].
G een ma e ials, g een unc ionaliza ion, g een educ ions, and g een p oduc ion
me hods mean ha he ma e ial o p ocess a e en i onmen ally iendly and sus ainable.
Fo ins ance, TiO
2
pho oca alys is known as a g een ma e ial [
76
], and mechanical ex olia-
ion is a g een p oduc ion me hod o g aphene wi hou any need o ene gy o haza dous
chemicals. The sou ce ma e ial o g aphene is g een g aphi e c ys al ha can be ound in
na u e. Fu he mo e, using g een ma e ials like plan s in unc ionaliza ion o g aphene
su ace o educ ion o a ma e ial like g aphene oxide, o using isible ligh ac i e ni ogen
doped TiO
2
pho oca alys in wa e ea men , a e some examples o he biocompa ible
g een p ocesses [
74
,
75
]. G aphene nanocomposi es a e one o he g een ma e ials and ha e
sus ainable ma e ial composi ions. They can be p oduced by g een p oduc ion and p o-
cessing me hods. G een memb ane p oduc ion me hod examples can include mechanical
ex olia ion, dip-coa ing, blade-coa ing, od-coa ing, and phase in e sion. The ad an ages
o g een memb ane p oduc ion me hods a e hei low ene gy consump ions, easy and
low-cos p ocesses [141].
G een p oduc ion me hod can e e o inno a i e sys em design, equipmen and
p ocess me hods. Reducing p oduc ion cos , ene gy consump ion, and equipmen size and
cos a e examples o imp o emen s o a g eene p oduc ion me hod. Memb ane-based
echnologies a e open o imp o emen s in hei g een p oduc ion me hods [142].
Memb anes, which a e widely used in memb ane-based wa e ea men echnologies,
a e gene ally accep ed as g een wa e ea men echnology [
75
,
138
,
140
]. Howe e , wi h he
widesp ead use o memb anes p epa ed wi h g een p oduc ion me hods in wa e ea men
echnologies, bo h he o al p oduc ion cos will dec ease and sus ainable de elopmen will
be suppo ed wi hou ha ming na u e and li ing hings.
Today, ins ead o using p is ine polyme memb anes in wa e ea men , i is necessa y
o de elop, p oduce by g een p oduc ion me hods, and use nanos uc u ed memb ane ma e-
ials which ha e highe e iciencies and ene gy sa ings han he polyme memb anes [
108
].
Acco ding o he esea ches ca ied ou o da e, i can be seen ha he p omising ech-
Memb anes 2023,13, 145 25 o 32
nologies a e he g een echnologies based on hei ad an ages. The e o e, bo h he g een
ma e ials and he g een p oduc ion me hod de elopmen s a e c ucial o he u u e o
sus ainable wa e ea men . Howe e , mo e esea ch needs o be done on his issue o
g een echnologies o become es ablished in wa e ea men .
5. Challenges and Fu u e P ospec s o Memb ane Technology
The de elopmen o sus ainable, en i onmen ally iendly, cos -e ec i e, high-
pe o mance, and long-las ing se ice li e wa e ea men sys ems is c i ical o ackling
global wa e quali y challenges, and a ious s udies ha e been unde aken o educe en i-
onmen al oo p in s and cos while inc easing ouling esis ance, an imic obial ac i i y,
and memb ane li e ime [
8
,
9
,
12
,
14
–
47
,
50
,
52
,
54
–
56
,
59
,
64
,
65
,
74
,
105
,
110
]. High cos and ech-
nical issues a e he challenges o he cu en wa e ea men memb ane echnology [
8
,
33
].
In mos o he wa e ea men sys ems using memb anes, cos can be inc eased while using
il e ing p ocesses like nano il a ion, ul a il a ion, e e se osmosis, and o wa d osmosis
p ocesses, and du ing pho oca alysis p ocesses he e can be s iking cos s o ma e ials
(memb ane, ca alys ), ene gy equi emen s ( o he pumps a high p essu e-assis ed ones,
and ul a iole sou ce equi emen o he pho oca alysis), and pe iodic main enance (de-
mand o he memb ane cleaning o memb ane eplacemen s) [
8
,
33
]. The e o e, he e a e
s ill challenges o o e come o a sus ainable and cos -e ec i e memb ane echnology in
wa e ea men echnology.
A ew o he ways o o e come hese challenges include he de elopmen o pho o-
ca alysis sys ems and he use o inno a i e memb ane ma e ials, composi es, and p ocesses
in il a ion [
8
,
33
,
64
]. Pho oca alysis p ocesses wi h isible ligh ac i e ma e ials a e e-
po ed o be low-cos and long-las ing wa e ea men s uc u es [
64
]. The e o e, one o he
mos impo an issues o be in es iga ed in wa e ea men may be il a ion memb anes
based on g aphene o he p oduc ion o isible ligh ac i e pho oca alysis, biocompa ibili y,
sel -cleaning, and an ibac e ial e ec s. Bo h memb ane eplacemen /main enance and
ope a ion cos s o he exis ing sys ems can be emedied by de eloping no el isible ligh
ac i e an i- ouling memb anes wi h acous ic ope a ion, leading o wa e lux suppo and
ex ended se ice li e [19,26,27,30,80,81,116].
High p essu e usage as a d i ing o ce in memb ane-based wa e il e sys em is
ano he challenge ha needs o be o e come.os -e ec i e wa e ea men sys ems a e
being s udied by esea che s, as a e examples o a wa e ea men sys em ha will no
consume ene gy wi hou also applying p essu e, o a wa e ea men sys em ha will
consume less ene gy wi h low p essu e applica ions [9,20,27,30,93].
Besides he cos issues, he e a e also echnical challenges wi h wa e ea men , which
can be explained as ollows:
Syn hesis o high-quali y g aphene is s ill unde de elopmen and is conside ed a
key ask in mos publica ions [
48
,
143
]. In addi ion, i is seen ha such sus ainable and
high-pe o mance ma e ials a e being de eloped wi h mo e emphasis on p oduc ion wi h
g een p oduc ion me hods such as chemical ex olia ion [
143
]. Fu he mo e, al hough he
g aphene p oduc ion me hod, such as liquid phase ex olia ion, also called elec ochemical
ex olia ion, is an en i onmen ally iendly me hod, he e a e s ill challenges associa ed
wi h la ge a ea and high quali y memb ane p oduc ion [
141
,
143
]. In addi ion, CVD has
been used o p epa e la ge a ea and high-quali y memb anes. Howe e , CVD also has
challenges such as ene gy consump ion and gas ela ionship ha may be ha m ul o heal h.
I is no , he e o e, g een.
The issue o p e en ing he po e clogging o memb anes a e use is one o he chal-
lenges ha needs o be sol ed in memb ane echnology. The challenge o memb ane
ouling (like bio ouling, his means ei he mic o ouling o mac o ouling) has been sough
o be o e come by de eloping an i ouling memb anes. Memb ane po es can be illed
easily by bio-pollu an s (biological mic obes such bac e ia (mic o ouling), o wa m oys e
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The s a emen s, opinions and da a con ained in all publica ions a e solely hose o he indi idual
au ho (s) and con ibu o (s) and no o MDPI and/o he edi o (s). MDPI and/o he edi o (s) disclaim esponsibili y o any inju y o
people o p ope y esul ing om any ideas, me hods, ins uc ions o p oduc s e e ed o in he con en .
