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Plasma Assisted Reduction of Graphene Oxide Films
Sri Hari Bharath Vinoth Kumar *, Ruslan Muydinov and Bernd Szyszka


Citation: Vinoth Kumar, S.H.B.;
Muydinov, R.; Szyszka, B. Plasma
Assisted Reduction of Graphene
Oxide Films. Nanomaterials 2021,11,
382. https://doi.org/10.3390/
nano11020382
Academic Editor: Guqiao Ding
Received: 8 January 2021
Accepted: 28 January 2021
Published: 3 February 2021
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Attribution (CC BY) license (https://
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4.0/).
Institute of High-Frequency and Semiconductor System Technologies, Technische Universität Berlin, HFT 5-2,
Einsteinufer 25, 10587 Berlin, Germany; [email protected] (R.M.); [email protected] (B.S.)
*Correspondence: [email protected]
Abstract:
The past decade has seen enormous efforts in the investigation and development of reduced
graphene oxide (GO) and its applications. Reduced graphene oxide (rGO) derived from GO is known
to have relatively inferior electronic characteristics when compared to pristine graphene. Yet, it has its
significance attributed to high-yield production from inexpensive graphite, ease of fabrication with
solution processing, and thus a high potential for large-scale applications and commercialization.
Amongst several available approaches for GO reduction, the mature use of plasma technologies
is noteworthy. Plasma technologies credited with unique merits are well established in the field
of nanotechnology and find applications across several fields. The use of plasma techniques for
GO development could speed up the pathway to commercialization. In this report, we review the
state-of-the-art status of plasma techniques used for the reduction of GO-films. The strength of
various techniques is highlighted with a summary of the main findings in the literature. An analysis
is included through the prism of chemistry and plasma physics.
Keywords: graphene oxide; plasma treatment; reduction
1. Introduction
The term “graphene” was coined by Boehm et al. in 1985, which refers to a two-
dimensional single layer of carbon atoms in a honeycomb lattice [
1
]. A. Geim and K.
Novoselov exfoliated graphene for the first time in the year 2004, which consequently
earned them a Physics Nobel prize in 2010. Even before its discovery and eventually gaining
the “wonder material” nickname [
2
], graphene was known to scientists and used in theo-
retical studies dating back to 1947 [
3
–
9
]. Following the discovery, graphene has gained a lot
of attention from the scientific community across various disciplines
(Figure 1a
). This can
be credited to its remarkable electrical, optical, thermal, and mechanical
properties [9–11].
Additionally, it possesses complete non-permeability to all standard gases [
12
] and the
ability to be chemically functionalized [13,14].
Figure 1b presents schematic illustrations of the common production methods of
graphene. A detailed account of various production and processing techniques of graphene
and related materials can be found in the literature [
15
–
18
]. Methods such as mechanical
exfoliation [
19
,
20
], epitaxial synthesis [
20
,
21
], and bottom-up synthesis from structurally
defined organic precursors [
20
] restrict the use of graphene to fundamental research and
niche applications, owing to limited scalability and high production costs. Graphene layers
can be also obtained by chemical vapor deposition (CVD), a well-established technique in
the industry [
22
,
23
]. The downside to this technique is that it requires suitable substrates
(which are limited), a high temperature, and vacuum environment. Additionally, it involves
the laborious transfer of the grown layers onto desired application substrates [
23
]. In liquid-
phase exfoliation (LPE), pristine or expanded graphite particles (thermally expanded
graphite intercalation compounds) are first dispersed in a solvent to weaken van der Waals
attraction between the graphene layers. High-quality graphene sheets are then obtained by
following ultrasonication [
24
], electric field [
25
], shearing [
26
], and microfluidization [
27
]
to induce exfoliation of graphite layers. Chemical additives (surfactants) are often needed
Nanomaterials 2021,11, 382. https://doi.org/10.3390/nano11020382 https://www.mdpi.com/journal/nanomaterials
Nanomaterials 2021,11, 382 2 of 37
to keep the suspensions stable for a long period, and the removal of solvent may cause
restacking of the graphene platelets due to van der Waal’s forces [
17
]. Some solvents, such
as the N-methyl-pyrrolidone (NMP), are toxic and expensive with a high boiling point
mandating special treatment and handling [28].
Nanomaterials2021,11,xFORPEERREVIEW2of38
andmicrofluidization[27]toinduceexfoliationofgraphitelayers.Chemicaladditives
(surfactants)areoftenneededtokeepthesuspensionsstableforalongperiod,andthe
removalofsolventmaycauserestackingofthegrapheneplateletsduetovanderWaal’s
forces[17].Somesolvents,suchastheN‐methyl‐pyrrolidone(NMP),aretoxicandexpen‐
sivewithahighboilingpointmandatingspecialtreatmentandhandling[28].
Figure1.(a)Theyearlynumberofpublicationsrelatedtographenetopic(left‐sideaxis).Dataex‐
tractedfromtheWebofScienceTM(ClarivateAnalytics),searchedwiththe“graphene”keywordin
the“topic”field(dateofretrieval:September14,2020).Yearlyaccumulatedgraphene‐relatedpa‐
tents(right‐sideaxis),dataextractedfrom[29].(b)Schematicillustrationofsomeofthemaingra‐
pheneproductiontechniquesrepresentedintermsofyieldandquality.Theevaluationofthetech‐
nique’sproductionprocessisrepresentedinapentagonwithgraphenecrystallinity(G),purity
(P),layernumbercontrollability(L),cost(C,alowvalueisrelatedtothehighcostofproduction),
andscalability(S).Numbers1,2,and3indicatelow,medium,andhigh,respectively.Reproduced
fromChenetal.[30]withpermissionfromTheRoyalSocietyofChemistry.
Comparedtoothertechniques,thereductiontechniquesofGOyieldrelativelylower
productqualitybutprovidesomeinterestingcharacteristics.GOitselfishighlyhydro‐
philicandcanformstablemonolayersinaqueouscolloids[31,32].GOandrGOcanbe
chemicallyfunctionalizedthroughcovalentandnon‐covalentbondstoenhancetheir
Figure 1.
(
a
) The yearly number of publications related to graphene topic (left-side axis). Data
extracted from the Web of Science
TM
(Clarivate Analytics), searched with the “graphene” keyword
in the “topic” field (date of retrieval: September 14, 2020). Yearly accumulated graphene-related
patents (right-side axis), data extracted from [
29
]. (
b
) Schematic illustration of some of the main
graphene production techniques represented in terms of yield and quality. The evaluation of the
technique’s production process is represented in a pentagon with graphene crystallinity (G), purity
(P), layer number controllability (L), cost (C, a low value is related to the high cost of production),
and scalability (S). Numbers 1, 2, and 3 indicate low, medium, and high, respectively. Reproduced
from Chen et al. [30] with permission from The Royal Society of Chemistry.
Compared to other techniques, the reduction techniques of GO yield relatively lower
product quality but provide some interesting characteristics. GO itself is highly hydrophilic
and can form stable monolayers in aqueous colloids [
31
,
32
]. GO and rGO can be chemically
functionalized through covalent and non-covalent bonds to enhance their properties and
Nanomaterials 2021,11, 382 3 of 37
functionalities. In the non-covalent approach, they can be modified with metals, metal
oxides, and polymers through non-covalent interactions like van der Waal’s forces,
π
–
π
stacking, hydrogen bonding, hydrophobic interactions, and ionic crosslinking [
33
]. In
applications such as sensors, energy storage, electrochemical systems, catalysis, etc., the
superior properties arise from the reactivity of the intrinsic defects and dangling bonds in
GO [34].
Numerous review-articles on synthesis, structure and properties, fabrication tech-
niques, chemical modifications, and applications of GO are available [
13
,
16
,
35
–
41
]. On
the reduction front, specific methods (such as chemical, thermal, eco-friendly, microwave
methods, etc.), as well as an overview of several reduction approaches have been exam-
ined [
42
–
54
]. Amongst these, some reviews have only outlined the plasma method along
with other techniques [
49
,
51
]. The use of plasma for GO modification and functionaliza-
tion also has been reported [
14
,
55
]. This article aims to review the application of plasma
exclusively from the perspective of GO reduction, presenting an up-to-date analysis. The
primary objective here is to elucidate the reduction of GO-films with various plasma and
to highlight the potential of plasma technologies in this topic. GO in the form of mono-
layers, thin-films, and paper (a few
µ
m-thick interlocked layered-structure consisting of
micrometer-sized graphene crystals [
56
,
57
]) is covered here apart from GO-composites [
58
]
and powders [
59
,
60
]. The fundamentals, principles, configurations, and applications of
plasma are not covered, as they are available in literatures [
61
–
67
]. A table is included in
the Appendix A(Table A1) with a non-exhaustive list of relevant publications for easy ref-
erencing. It incorporates various plasma generation techniques and active gases employed,
briefing the important experimental parameters, application, and results.
2. GO/rGO: Properties, Reduction Methods, and Characterization
In this chapter, the structure, properties, and applications of GO/rGO are briefly
discussed first. It is then followed by a short overview of common reduction techniques
highlighting the advantages of plasma methods. Finally, the basic characterization tech-
niques needed to evaluate the reduction degree in rGO-films are introduced.
2.1. Structure and Properties of GO/rGO
GO is strictly a single-layered material that is obtained by exfoliation of oxidized
graphite [
68
]. Oxidized graphite depicts a berthollide layered solid produced by treating
graphite with strong oxidants where the graphite surface and edges undergo covalent
chemical oxidation [
68
]. This technique dates back to 1859 when the chemist Benjamin
Brodie performed a similar treatment to elucidate the structure of graphite oxide [
69
]. As a
result of chemical exfoliation, GO incorporates many oxygen-containing functional groups
where domains of sp2- and sp3-hybridized carbon atoms exist [70].
Until today, some ambiguity persists on the precise chemical structure of GO, and
several different models describe the same [
52
,
71
,
72
]. The most widely accepted model is
the one proposed in 1996 by Lerf and Klinowski (LK) [
73
,
74
], originally describing graphite
oxide. In the past two decades, various researchers have claimed additional structural
changes [
75
,
76
]. In a recent review, Brisebois et al. [
76
] presented a representative structure
of GO based on historic and modern models including recently suggested adjustments
in the literature (Figure 2a). As shown in Figure 2a, features A–E account for the LK
model. The monolayer surface gets its nearly flat carbon-grid from double bonds (A),
aromatic entities (B), and epoxide groups (C). Hydroxyl group-containing carbon results in
wrinkling of the monolayer. A large number of oxygen-containing groups (C, D, D’, and
D”) lie above and below the carbon-grid. Hydroxyl groups (D’) and carboxylic groups (E)
terminate the structure of GO. Features such as F, G, H, and I are aspects of the Dékány
model [
76
]. According to Brisebois et al. [
76
], the complete chemistry of GO is not yet fully
understood, and the general LK model should be updated with recently made observations
such as carbon vacancies (M), sulfate esters (N), carbon radicals (O), 1,3 butadiene systems
(P), and C–H bonds (Q).
Nanomaterials 2021,11, 382 4 of 37
The mainly present oxygen-functional groups in GO are epoxides (C-O-C), hydroxyls
(-OH
), carboxylic (-COOH), and ketone (C=O) ones [
77
]. The presence of polar groups
makes GO hydrophilic and facilitates its exfoliation in aqueous media [
78
]. The incorpo-
rated functional groups cause an increased interlayer distance of >0.625 nm in GO from
0.335 nm in graphite [
79
,
80
]. GO has a heterogeneous chemical and electronic structure,
and the presence of oxygen-containing groups make it an insulator [
35
,
81
]. The sp
2
net-
work of carbon atoms, and thus the electrical conductivity, can be substantially restored by
various reduction strategies. Figure 2b presents high-resolution imaging of rGO monolayer
reduced with H
2
-plasma [
82
]. A large portion of crystallized graphene regions with hexag-
onal lattice (light gray) is observed. The average graphene-like regions here range from
3–6 nm covering 60% of the surface. Carbonaceous adsorbates and trapped contaminants
are also observed (dark gray). Other visible features include topological defects (blue and
green), individual ad-atoms or substitutions (red), and holes (yellow). Such heterogeneous
structure gives rise to properties that are different from pristine graphene. Some of the
important properties of GO/rGO are summarized against CVD-graphene in Table 1.
Nanomaterials2021,11,xFORPEERREVIEW4of38
understood,andthegeneralLKmodelshouldbeupdatedwithrecentlymadeobserva‐
tionssuchascarbonvacancies(M),sulfateesters(N),carbonradicals(O),1,3butadiene
systems(P),andC–Hbonds(Q).
Themainlypresentoxygen‐functionalgroupsinGOareepoxides(C‐O‐C),hydroxyls
(‐OH),carboxylic(‐COOH),andketone(C=O)ones[77].Thepresenceofpolargroups
makesGOhydrophilicandfacilitatesitsexfoliationinaqueousmedia[78].Theincorpo‐
ratedfunctionalgroupscauseanincreasedinterlayerdistanceof>0.625nminGOfrom
0.335nmingraphite[79,80].GOhasaheterogeneouschemicalandelectronicstructure,
andthepresenceofoxygen‐containinggroupsmakeitaninsulator[35,81].Thesp2net‐
workofcarbonatoms,andthustheelectricalconductivity,canbesubstantiallyrestored
byvariousreductionstrategies.Figure2bpresentshigh‐resolutionimagingofrGOmon‐
olayerreducedwithH2‐plasma[82].Alargeportionofcrystallizedgrapheneregionswith
hexagonallattice(lightgray)isobserved.Theaveragegraphene‐likeregionshererange
from3–6nmcovering60%ofthesurface.Carbonaceousadsorbatesandtrappedcontam‐
inantsarealsoobserved(darkgray).Othervisiblefeaturesincludetopologicaldefects
(blueandgreen),individualad‐atomsorsubstitutions(red),andholes(yellow).Suchhet‐
erogeneousstructuregivesrisetopropertiesthataredifferentfrompristinegraphene.
SomeoftheimportantpropertiesofGO/rGOaresummarizedagainstCVD‐graphenein
Table1.
Figure2.(a)AhistoricalstructuralaccountofaGO‐flakewithC/Oatomicratioof~2.Reproduced
fromBriseboisetal.[76]withpermissionfromTheRoyalSocietyofChemistry.Theatomicresolu‐
tion,aberration‐correctedTEMimageofanH2‐plasmareducedmonolayerGO:(b)Originalimage
and(c)withcolorshighlightingdifferentfeatures.Lightgray:defect‐freecrystallinegraphene
area.Darkgray:contaminatedregions.Blue:disorderedsingle‐layercarbonnetwork,orextended
topologicaldefects,suggestedasremnantsoftheoxidation‐reductionprocess.Red:individualad‐
atomsorsubstitutions.Green:isolatedtopologicaldefects,thatis,singlebondrotationsordisloca‐
tionscores.Yellow:holesandtheiredgereconstructions.ReproducedfromGomez‐Navarroetal.
[82].Copyright©2010,AmericanChemicalSociety.
Figure 2.
(
a
) A historical structural account of a GO-flake with C/O atomic ratio of ~2. Reproduced
from Brisebois et al. [
76
] with permission from The Royal Society of Chemistry. The atomic resolution,
aberration-corrected TEM image of an H
2
-plasma reduced monolayer GO: (
b
) Original image and
(
c
) with colors highlighting different features. Light gray: defect-free crystalline graphene area.
Dark gray: contaminated regions. Blue: disordered single-layer carbon network, or extended
topological defects, suggested as remnants of the oxidation-reduction process. Red: individual
ad-atoms or substitutions. Green: isolated topological defects, that is, single bond rotations or
dislocations cores. Yellow: holes and their edge reconstructions. Reprinted with permission from
Gomez-Navarro et al. [82]. Copyright © 2010, American Chemical Society.
Nanomaterials 2021,11, 382 5 of 37
Table 1.
Summary of physical properties of monolayer CVD-graphene, various-GO, and various-rGO.
Properties listed are atomic C/O ratio (R
C/O
), optical transmittance (T) at 550 nm for monolayers,
electrical conductivity (
σ
), bandgap (E
g
), thermal conductivity (
κ
) at room-temperature, in-plane
Young’s modulus (E), and intrinsic strength (τc).
Property CVD-Graphene Various-GO Various-rGO
RC/O - 0.6–2.38[47,83,84] 1.48–12 [52,85,86] *
T (%, @ 550 nm) 97.7 [87] >97.5 [88] ~97.5 [88,89]
σ(S/cm) ~104[90,91]~10−3–103[92,93]†
Eg(eV) 0 0–3.5 [94,95]†
κ(W/m·K) 300–5300 [96,97]8.8–625 [98,99]†,‡ 46–2600 [100,101]
E (GPa) 1000 [102]290–430 [103]†,‡
τc(GPa) 130 [102]28–48 [103]†,‡
* a wide range of R
C/O
have been reported for rGO [
51
], in certain cases exceeding >100 [
104
,
105
].
†
range obtained
as a function of oxidation/reduction degree. ‡theoretical studies.
The opportunity for tailoring the optoelectronic properties of GO arises from the abil-
ity to manipulate its shape, size, and the fraction of sp
2
/sp
3
hybridized carbon domains by
controlled reduction [
35
,
106
]. Finally, GO and r-GO serve as a tunable platform for several
applications. Table 2summarizes some of the features and properties of GO/rGO exploited
in a wide range of science and technology topics. In optoelectronics, under the field of solar-
cells alone, rGO has contributed to the progress of perovskite, perovskite-silicon tandem,
dye-sensitized, and organic technologies [
107
–
109
]. Graphene materials have not been
widely utilized for CIGSe as in the case of other solar cell technologies. Nevertheless, CVD–
graphene has been incorporated into CIGSe, which includes a demonstration of flexible
solar cell [
110
–
112
]. It is emphasized that GO/rGO can serve as hole-selective contacts and
intermediate tunnel junction layer in monolithic CIGSe–Perovskite tandem solar cell appli-
cations, which is yet to be reported [
113
]. Factors such as precursor material-form (powder,
dispersion, films, paper, etc.), quality, properties, and application of rGO can influence the
choice of reduction method adopted, which is discussed in the following section.
Table 2. A summary of some of the properties and features of GO/rGO with their relevant applications.
Features/Properties Applications/Technologies Reference
Large specific area; lightweight; high conductivity; hetero-atom
doping; micro-structuring; composite material formation
Electrochemical storage (batteries and capacitors)
[16,114]
Large specific area; tunable electronic structure; hetero-atom
doping; structural modification and functionalization
Electrocatalysts for electrochemical energy
conversion reactions (water splitting; CO2, N2,
and O2reduction reaction)
[115]
Nanocapillaries; ease of making atomically thin layers; good
mechanical properties
Membranes (selective ion-, vapor-, gas-,
water-transport; proton exchange; desalination) [116]
Biocompatibility; functionalization; physiochemical
properties; fluorescence Pharmaceutical, biomedical, and biosensing [106,117]
Tunable electronic properties; optical transparency;
mechanical flexibility Flexible-, thin-film, and opto-electronics [106,118]
Non-linear optics (saturable absorption; reverse saturable
absorption; two-photon absorption) Mode-locking; Q-switching; optical limiters [106]
Seebeck coefficient; electrical conductivity; thermal conductivity
Thermoelectric devices [106]
Advanced mechanical and structural properties in composites
Mechanical and rheological (cement composites;
green plastics; composites for military
and aerospace)
[118]
2.2. Reduction Methods for GO
Table 3lists and summarizes commonly employed reduction methods and some
of their important features along with those of the plasma method. Generally, the re-
duction strategies use either a reductant (chemical, microbial, solvothermal) and/or a
Nanomaterials 2021,11, 382 6 of 37
thermal, electrical (voltage-induced), radiative (photocatalytic, microwave, plasma), and
electrochemical impact [
49
,
119
]. Chemical reagents are either used in liquid- or gas-phase
for reduction [
32
,
120
]. Typically used reagents are fairly hazardous: hydrazine [
94
], hy-
drazine hydrate [
121
], sodium borohydride [
122
], sodium hydrosulfite [
122
], and hydro-
halic acids [
121
]. Hydrazine is known to be one of the most powerful reducing agents.
However, high toxicity and environmental hazard make it unpopular [
123
]. By comparing
various reducing agents, a study suggested that a reducing agent in combination with acid
is beneficial in terms of preserving the rGO surface quality [
124
]. The processing time in
the chemical techniques is relatively longer. In certain cases, the reduction process can
consume a day to a week’s time [94,120].
Table 3. An overview of the common reduction methods with some of their important features.
Reduction Method Features Reference
Chemical
simple and scalable approach; commonly used reducing agents are
toxic/hazardous; rGO yields have lower surface area and electrical
conductivity; prolonged reduction duration
[94,125]
Thermal
simple approach; defects are created in the lattice with the removal
of carbon; high-temperature process not for suitable sensitive
substrates; substantial energy consumption
[125,126]
Electrochemical
rGO yields have good structural quality and electrical conductivity;
non-hazardous process, large-scale production is challenging [125]
Microwave-assisted
microwave absorption depends on the oxidation degree of GO;
reducing atmosphere are needed to improve quality of yield; high
temperatures attained limit substrate selection
[50,127–129]
Plasma requires special equipment; versatile and offers industrial-level
scalability; relatively short reaction period; effective in restoration
of lattice defects
[49,93,130]
The solvothermal method can yield a stable dispersion of r-GO without the use of
additional reductants [
131
]. However, the C/O atomic ratio and electrical conductivity
obtained are inferior to the chemical methods [
42
,
52
]. Microbes such as Shewanella [
132
],
E. coli [
133
],yeast [
134
],and Azotobacter chroococcum [
135
] can also reduce GO in the forms
of dispersion or film. Though such biological agents are attractive with low-negative
environmental impact, they are limited in terms of the need for sensitive culture procedure
and prolonged reaction time [49].
High-temperature annealing (up to 1100
◦
C) aids the removal of oxygenated groups
and significantly improves the conductivity of GO-films [
136
]. This approach is not appli-
cable for temperature-sensitive substrates, like glass or flexible ones. When the preheating
and cooling of chambers with active/inert gases or vacuum environment are considered,
a substantial amount of energy is consumed, leading to poor energy efficiency. Another
significant drawback of the thermal method is the creation of carbon vacancies and other
structural defects in the GO plane due to the active diffusion of epoxide groups already at
200
◦
C [
137
,
138
]. Thermogravimetric analysis reveals up to ~70% final mass loss caused
by the release of CO and CO
2
gases [
137
]. The photocatalytic reduction of GO heavily
relies on the presence of photoactive materials under UV radiation. This makes it suitable
only for hybrid nanocomposites [
46
]. Electrochemical reduction offers a faster, and safer
route compared to the previously noticed methods. However, it is not viable for large-scale
production, and its degree of reduction is incomparably lower than in the case of the
chemical or thermal method [139].
Microwave-assisted reduction of GO can be realized by three routes: (i) chemical
reduction, (ii) thermal reduction, and (iii) simultaneous exfoliation and reduction [
127
].
Microwaves are effectively absorbed by
π
-electrons that cause very rapid warming of
GO (several hundreds of degrees in few seconds) that results in the breaking of weakest
Nanomaterials 2021,11, 382 7 of 37
bonds [
127
]. According to this principle, r-GO domains with sp
2
carbon network and free
π
-electrons heat faster than GO-ones that minimize the efficiency of this method [
128
,
129
].
When chemically reduced or simultaneously exfoliated and reduced, the quality of r-GO is
low and oxygen content is as high as in the case of the conventional thermal method [
54
,
127
].
Laser irradiation can also induce local heating in light-absorbing domains of GO and thus
be utilized for reduction. The local temperatures can reach 1400 K in this case [
140
]. In the
case of GO films the heat absorbed dissipates to underlying layers and substrate, yielding
additional issues. The photothermal reduction (such as the laser and flash techniques) can
provide moderate to high atomic C/O ratio (~10 and ~15, respectively) but creates pores,
cracks, and voids in GO-films, which could limit its applications [141].
Compared to the previously discussed methods, plasma-assisted reduction techniques
are much more attractive for films due to the following reasons. Plasma processes are
established and well-controlled, which offers ease of operation also on the industrial scale.
Despite relatively expensive equipment, the versatility of plasma processes today forms a
vital part of production in various technological fields [
67
]. Different power generation
techniques extended by a wide range of operating pressures including the atmospheric one,
as well as the applicability of various active gases, make this approach multidimensional.
It opens the way to control the energy of the acting species and tune the chemical footprint
of plasma. Moreover, the thermal impact of plasma and the depth of penetrating damage
can be also restricted. For attaining graphene-like properties in rGO, significant restoration
of the graphitic structure by defect repair is essential. Amongst the available techniques
to realize this, the plasma method is one of them, others being the thermally assisted
CVD method (>1073 K) and sequential chemical reduction followed by high-temperature
graphitization (~2073 K) [
48
,
142
]. From environmental, health, and safety aspects, plasma
technology has remarkable advantages over chemical and thermal processes [
143
]. Owing
to their advantages, plasmas have demonstrated their attractiveness in the synthesis of
graphene and related materials [130,144–146].
2.3. Characterization of rGO
The effectiveness of GO reduction is widely assessed based on (i) surface atomic
carbon/oxygen ratio (R
C/O
) and (ii) electrical properties: hole/electron mobility (
µ
), sheet
resistance (R
SH
), and conductivity (
σ
). Detailed reports on various GO characterization
techniques generally utilized are available in the literature [18,147,148]. When comparing
the electrical parameters of various r-GO films, one should consider that monolayers,
bi-layers, and tri-layers of r-GO may not differ proportionally. For instance, according to
Sinitskii et al. [
149
], the corresponding conductivities of reduced GO-nanoribbons were
found to be 35, 115, and 210 S/cm. In certain cases, such a difference between mono-
layers and bi-layers was attributed to the interaction between r-GO and the underlying
substrate [
150
,
151
]. The R
C/O
values determined by a surface-sensitive X-ray Photoelectron
Spectroscopy (XPS) indicate a degree of reduction. In the case of the layers thicker than
10 nm, XPS is unable to validate the reduction degree in the bulk [152,153].
It is worth noticing characterization of graphene films by Raman spectroscopy, as it
provides vital insights. Graphitic materials have Raman features at ~1584 cm
−1
(G-band),
~2700 cm
−1
(G’-or 2D-band), and ~1350 cm
−1
(D-band). The G-band arises from the first-
order scattering of E
2g
phonons of the sp
2
carbon atoms in the ring structure, while the
D-band appears from the breathing mode of sp
2
carbon atoms due to defects [
154
,
155
].
The G’-band, unlike the D-band, is not induced by defects and is more prominent in
graphene [
155
]. In graphene, the integrated intensity ratio of G’- and G-band is used
to determine the number of layers [
22
,
156
]. The G’-band intensity declines, and its full
width at half maximum (FWHM) broadens with increasing density of defects [
157
]. The
integrated intensity ratio of D-band and G-band (I
D
/I
G
) is widely used for characterizing
the defects’ quantity in graphene and related materials. The Tuinstra–Koenig empirical
relation [
158
] based on the I
D
/I
G
is used to calculate the in-plane sp
2
carbon crystallite size
(L
a
)
.
The average distance between point defects (L
D
) can also be derived from the I
D
/I
G
Nanomaterials 2021,11, 382 8 of 37
value and the FWHM of the G-band [
124
,
157
,
159
]. Wróblewska et al. [
160
] highlighted
the difficulty in comparison of materials with widely varied I
D
/I
G
ratios reported in the
literature. For instance, inhomogeneity of GO/r-GO may cause a difference in values
measured at distances of a dozen of
µ
m. Additionally, the ratio in question also depends
on the laser wavelength used [
160
]. To reduce uncertainty, a statistical approach (Raman
mapping) instead of using single-point measurement should be taken [
124
,
160
]. This is
unfortunately not the case in every investigation.
Fourier-transform infrared spectroscopy (FTIR) is also a useful tool to investigate
the effectiveness of GO-reduction. The presence of various oxygen-containing groups
can be recognized in GO/rGO, and thus their removal can be examined [
161
,
162
]. The
configurations that can be identified with FTIR are [163]:
•
epoxide (C-O-C): 1230–1320 cm
−1
, asymmetric stretching; ~850 cm
−1
bending motion,
•sp2-hybridized C=C: 1500–1600 cm−1, in-plane vibrations,
•
carboxyl (COOH): 1650–1750 cm
−1
(including C-OH vibrations at 3530 and 1080 cm
−1
),
•ketonic species (C=O): 1600–1650 and 1750–1850cm−1,and
•
hydroxyl (namely phenol, C-OH): 3050–3800 and 1070 cm
−1
) with all C-OH vibrations
from COOH and H2O.
3. Plasma-Assisted Reduction of GO
Chemist Irving Langmuir coined the term “plasma” in 1928 [
164
], which often de-
notes the fourth state of matter (see Figure 3). In the visible universe, more than 99% of
constituents are expected to be in a plasma state (center of active stars, corona flares and
sunspots, magnetospheres of the earth, comet-tails, inter-stellar and inter-galactic media,
and in the accretion disks around black holes) as opposed to the condensed matter (solids,
liquids, and gases) in the form of comets, planets, and cold stars [
165
–
167
]. Plasma is the
ionized form of gases containing energetic ions, free electrons, highly reactive radicals,
and photons. The extent of ionization can range from very low values (ionized fraction in
the order of 10
−4
–10
−6
) up to full ionization [
65
]. If all species in plasma have the same
temperature (or energy), one deals with an equilibrium plasma state. In non-equilibrium
plasmas, electrons have higher temperatures than the remaining species. Besides that,
laboratory plasma can be divided into high-temperature plasma (or fusion plasma, e.g., in
tokamaks, z-pinch system, etc.) and low-temperature plasma (or gas discharge) [
65
]. The
latter is relevant for us, and according to Szabóet al. [
168
], it can be further classified as
follows:
•operating pressure:
o low-pressure plasma
o atmospheric pressure plasma
•temperature:
o low-temperature plasma (Tgas < 2000 K)
o high-temperature plasma (Tgas > 2000 K)
•thermodynamics:
o thermal plasma/equilibrium plasma (Telectron ≈Tion ≈Tgas)
o non-thermal plasma/non-equilibrium plasma (Telectron Tion ≈Tgas)
•type of coupling:
o inductive coupling
o capacitive coupling
•plasma generation:
o microwave discharge (300 MHz ≤f≤300 GHz)
o radiofrequency (RF) discharge (ideally 13.56 MHz):
o direct current (DC) discharge
o dielectric barrier discharge (DBD)
Nanomaterials 2021,11, 382 9 of 37
o corona discharge
o electric arc
o hollow cathode discharge
o electron beam discharge (EB)
o plasma torch
o alternating current.
Nanomaterials2021,11,xFORPEERREVIEW9of38
theorderof10−4–10−6)uptofullionization[65].Ifallspeciesinplasmahavethesame
temperature(orenergy),onedealswithanequilibriumplasmastate.Innon‐equilibrium
plasmas,electronshavehighertemperaturesthantheremainingspecies.Besidesthat,la‐
boratoryplasmacanbedividedintohigh‐temperatureplasma(orfusionplasma,e.g.,in
tokamaks,z‐pinchsystem,etc.)andlow‐temperatureplasma(orgasdischarge)[65].The
latterisrelevantforus,andaccordingtoSzabóetal.[168],itcanbefurtherclassifiedas
follows:
operatingpressure:
o low‐pressureplasma
o atmosphericpressureplasma
temperature:
o low‐temperatureplasma(Tgas<2000K)
o high‐temperatureplasma(Tgas>2000K)
thermodynamics:
o thermalplasma/equilibriumplasma(Telectron≈Tion≈Tgas)
o non‐thermalplasma/non‐equilibriumplasma(Telectron≫Tion≈Tgas)
typeofcoupling:
o inductivecoupling
o capacitivecoupling
plasmageneration:
o microwavedischarge(300MHz≤f≤300GHz)
o radiofrequency(RF)discharge(ideally13.56MHz):
o directcurrent(DC)discharge
o dielectricbarrierdischarge(DBD)
o coronadischarge
o electricarc
o hollowcathodedischarge
o electronbeamdischarge(EB)
o plasmatorch
o alternatingcurrent.
Figure3.Schematicofatypicalphasediagramdepictingcorrespondingionizedstatesofmatter.
ReproducedfromAdamovichetal.[67]licensedunderCCBY3.0.
Figure 3.
Schematic of a typical phase diagram depicting corresponding ionized states of matter.
Reproduced from Adamovich et al. [
67
]. Copyright
©
2017 IOP Publishing Ltd, licensed under CC
BY 3.0.
Based on the operating pressure, the low-temperature plasmas can be broadly classi-
fied into low pressure and atmospheric pressure plasma. Traditional sources of atmospheric
plasma include the transferred arcs, plasma torches, corona discharges, dielectric barrier
discharges (DBD), and atmospheric-pressure plasma jet (APPJ) [
63
]. The classical arc
torches (ones with local thermal equilibrium) are characterized with high gas temperatures
and have been used in applications such as welding, cutting, spraying, etc., where heat
is required [
169
]. Relatively modern low-powered homogeneous arc plasma, generating
less heat, is well implanted in the production lines of automobiles, textiles, and packaging,
etc. [
169
]. Corona discharges are spatially non-uniform and are formed on sharp-points,
edges, or on thin-wires where the electric field is very large [
170
]. As the active volume is
limited, they are not well suited for the industrial production of large quantities of chemical
species [61].
The DBD was developed to overcome disadvantages of the corona discharge [
169
].
Amongst the atmospheric plasmas, the DBD is better suited for the applications needing
volume plasma chemistry, as it caters to large volume excitation with energetic electrons for
excitation of atomic and molecular species breaking chemical bonds [
61
]. The APPJ plasma
shares a similar plasma density as the low-pressure plasma but with lower breakdown
voltage and electron temperatures than the rest of the plasmas. However, the population
of electrons is considered high enough to dissociate many molecules including O
2
and
N
2
[
63
]. The main disadvantage of the APPJs is the small area that can be treated or coated,
which can be circumvented with approaches such as scanning of surface area, using an
array of APPJs, and rotating arc root plasma jet process [
171
]. In a recent review [
14
],
Nanomaterials 2021,11, 382 10 of 37
the APPJ-plasma was emphasized to be a promising candidate for large-scale roll-to-roll
functionalization of graphene and GO.
The low-pressure plasma emerges from the field of material processing and is a
key player in the semiconductor industry [
172
,
173
]. Low-pressure plasma treatment of
electronic devices, printed circuit boards, and semiconductors are state of the art. Uniform
treatment of oxidation-sensitive and three-dimensional objects can be carried out, including
cavities that can be processed in large chambers (up to 12,000 L in volume) [
174
]. They
feature some distinctive benefits: (1) uniform glow over large areas, (2) high concentration
of reactive species (able to etch or deposit at the rate of up to 10
µ
m/min), (3) lower
breakdown voltages, (4) stable operating window, and (5) sufficient electron temperatures
to dissociate molecules with lower gas temperature [
63
]. On the downside, the vacuum
systems are relatively expensive in assembly and maintenance. Furthermore, the processing
is limited by batching and transferring materials in and out of the vacuum system. Some of
the plasma parameters of low-pressure and atmospheric-pressure plasma are summarized
in Table 4.
Table 4. Characteristics of various plasma sources.
Plasma Source Breakdown Voltage
(kV) [63]
Plasma Density
(cm−3) [63]
Electron
Temperature (eV) †
Low-pressure
discharge 0.2–0.8 108–1013 0.1–10 [172]
Arc and plasma torch 10–50 1016–1019 2–7 [63]
Corona 10–50 109–1013 5‡[61]
DBD 5–25 1012–1015 1–10 [61]
Plasma jet 0.05–0.2 1011–1012 1–2 [61]
†1 eV ≈11,604 K;‡variable.
The plasma-assisted GO-reduction can be regarded analogically to the plasma etching
process in a way [
66
]. It should selectively remove oxygen-containing groups, leaving
the carbon network unaffected. On a solid surface exposed to the plasma, two processes
track simultaneously: (i) deposition of material and (ii) ablation leading to its removal.
Both are determined by the discharge gas and conditions [
175
]. The ablation of the treated
surface can involve sputtering, chemical etching, ion-enhanced energetic etching, and ion-
enhanced protective etching [
66
,
175
]. In the case of the plasma-assisted chemical etching,
the plasma species are excited and become chemically more reactive. In case of the ion-
induced etching, plasma activates surface atoms that increase their ability to release under
certain pressure and chemical conditions [
176
,
177
]. Admixing the inhibitor species into the
gas phase results in an isotropic inert coverage of the treated surface, preventing further
etching. In sputtering, a bombardment of the surface by the ions with sufficient kinetic
energy can break chemical bonds in the solid and eject atoms into a gas phase. Plasma
process is quite complex and dynamic where several processes occur simultaneously being
subjected to the plasma generation conditions.
The first use of a plasma process for GO reduction was reported in the year 2007 [
150
],
two years following the first report on GO solution processing [
31
]. Since then, in the last
13 years, several kinds of plasma-assisted reduction processes were developed. To name
some ways plasma generation is utilized: radio-frequency (RF) plasma [
150
,
162
,
178
–
197
],
low-pressure direct current (DC) plasma [
198
–
201
], micro-DC plasma [
202
], atmospheric
pressure glow discharge (AGD) plasma [
56
], electron beam (EB) plasma [
203
], active screen
(AS) plasma [
204
], atmospheric pressure plasma jet (APPJ) [
205
] and
µ
-APPJ [
206
], and
dielectric barrier discharge (DBD) plasma [
207
,
208
]. In the following sections, the reduction
processes classified according to the discharge gas will be discussed in detail.
3.1. Inert-Gas (He and Ar) Plasma
Zhou et al. [
209
] used a 60 W (AC) DBD plasma with several discharge gases (Ar, H
2
,
and CO
2
) for simultaneous exfoliation and reduction of GO powder. The plasma treat-
Nanomaterials 2021,11, 382 11 of 37
ment mechanism suggested dictates similar for the GO-films. According to this work, the
alternating electric field distorts polar bonds in oxygen-containing groups. Furthermore,
high-energy electrons and ions of plasma bombard GO surface, rupturing the bonds of
oxygen-containing groups within nanoseconds. Jin et al. [
210
] have shown by first-principle
calculations that provision of an electron to the hydroxyl group favors its desorption from
the GO surface. The plasmas in general have a high density of electrons (see Table 3); there-
fore, desorption of hydroxyl groups must be a frequent event. Zhou et al. demonstrated
that the deoxygenation of GO was strongly influenced by the discharge gas (see Figure 4).
When Ar was employed, deoxygenation occurred primarily through the bombardment of
energetic ions and electrons, unlike the H
2
-plasma, which could also provide chemically
reactive plasma species (H, H
+
, H
2+
, and H
3+
[
211
]). Other authors have reported that a
combination of inert gases and a reactive gas can be more effective in reduction compared
to pure inert or reactive gas plasma; this will be discussed in the following sections.
Nanomaterials2021,11,xFORPEERREVIEW12of38
Figure4.(a)AschematicrepresentationoftheDBDplasmaexfoliationofGO.(b)RC/Oasafunc‐
tionoftreatmenttimewithfromH2,Ar,andCO2DBDplasma.(c)FTIRspectraofGOandgra‐
phenepreparedbyDBDplasmawithdifferenttypeofworkinggases.ReproducedfromZhouet
al.[209]withpermissionfromtheRoyalSocietyofChemistry.
Cardinalietal.[188]useda25WRFplasmawithArforsimultaneousthinningand
reductionofthebulkGOplatelets.Startingfromthethicknessof~600nm,theauthors
etchedsamplesfor40mindownto5–6nmthickmulti‐layeredfilmwithovertwoorders
lowersurfaceelectricalresistivity.Althoughaninertgasplasmadoesnotprovidechem‐
icallyreactivespeciesthatcanactasreductant,thebombardmentbyenergeticinertions
andelectronshaveproventobeadequatefordeoxygenationinsomereports.Boetal.
[200]usedaninstantaneous2‐satmospheric‐pressureglowdischargeprocesswithhelium
forpreparingrGO‐paper.Theplasma‐treatedrGO‐paper(σ:59S/cm;RC/O:7.6)wasonpar
withtheonereducedchemicallybyhydrazinehydrate(σ:65S/cm;RC/O:8.5).Theauthors
attributedinstantaneousdeoxygenationwiththeirplasmatothesynergyofhigh‐density
electronsandheating.Theelectrondensityandtheneutralgastemperatureweredeter‐
minedtobe1.03×1016cm−3and~800K,respectively.Herewith,nodamagetothegraphitic
structurewasfoundbyRamanspectroscopy.Ther‐GOlayersreducedbyinert‐gas
plasmahavebeendemonstratedinsupercapacitors[200]andH2O2chemicalsensors[162].
Kimetal.[212]accomplishedselectivelyetchinganatomic‐layerofgraphenewithout
damagingtheunderneathlayersusinganinductively‐coupledplasma‐typeionbeamsys‐
tem(seeFigure5a).Thecyclicetchingprocessusedconsistedofchemicaladsorptionof
low‐energyoxygen‐ions:O2+andO+(0–20eV)followedbyphysicaldesorptionofoxidized
speciesbyAr+‐ions(11.2eV).Tocontroltheenergyofions,theauthorsappliedfloated
andgroundedgridswithanaxialmagneticfield.Thisapproachhelpedtooptimizethe
processbasedonionenergydistributionforvariouspowerandgas‐flowsinthesystem
(seeFigure5b–e).Thiscyclicetch‐processexploitedthefactthatthebindingenergyofthe
surfaceC‐atomsdecreasesfrom~6.1to~3.9eVwithchemisorptionofoxygenions.Atthe
sametime,theC–Cbindingenergyintheunderneathlayerremainsalmostunchanged
(~0.1eV).ExactlythisfactincombinationwithcontrolledenergyofAr+‐ionshasallowed
etchingofthetoplayerselectively.Thisexperienceopensthewaytocombinethechemical
andenergeticimpactsofplasmaspeciesforeffectiveandcontrolledGOreduction.
Figure 4.
(
a
) A schematic representation of the DBD plasma exfoliation of GO. (
b
) R
C/O
as a function
of treatment time with from H
2
, Ar, and CO
2
DBD plasma. (
c
) FTIR spectra of GO and graphene
prepared by DBD plasma with different type of working gases. Reproduced from Zhou et al. [
209
]
with permission from the Royal Society of Chemistry.
Cardinali et al. [
188
] used a 25 W RF plasma with Ar for simultaneous thinning
and reduction of the bulk GO platelets. Starting from the thickness of ~600 nm, the
authors etched samples for 40 min down to 5–6 nm thick multi-layered film with over
two orders lower surface electrical resistivity. Although an inert gas plasma does not
provide chemically reactive species that can act as reductant, the bombardment by energetic
inert ions and electrons have proven to be adequate for deoxygenation in some reports.
Bo et al. [
200
] used an instantaneous 2-s atmospheric-pressure glow discharge process with
helium for preparing rGO-paper. The plasma-treated rGO-paper (
σ
: 59 S/cm; R
C/O
: 7.6)
was on par with the one reduced chemically by hydrazine hydrate (
σ
: 65 S/cm; R
C/O
: 8.5).
The authors attributed instantaneous deoxygenation with their plasma to the synergy of
high-density electrons and heating. The electron density and the neutral gas temperature
were determined to be 1.03
×
10
16
cm
−3
and ~800 K, respectively. Herewith, no damage
to the graphitic structure was found by Raman spectroscopy. The r-GO layers reduced
by inert-gas plasma have been demonstrated in supercapacitors [
200
] and H
2
O
2
chemical
sensors [162].
Kim et al. [
212
] accomplished selectively etching an atomic-layer of graphene without
damaging the underneath layers using an inductively-coupled plasma-type ion beam
system (see Figure 5a). The cyclic etching process used consisted of chemical adsorption
of low-energy oxygen-ions: O
2+
and O
+
(0–20 eV) followed by physical desorption of
Nanomaterials 2021,11, 382 12 of 37
oxidized species by Ar
+
-ions (11.2 eV). To control the energy of ions, the authors applied
floated and grounded grids with an axial magnetic field. This approach helped to optimize
the process based on ion energy distribution for various power and gas-flows in the system
(see Figure 5b–e). This cyclic etch-process exploited the fact that the binding energy of the
surface C-atoms decreases from ~6.1 to ~3.9 eV with chemisorption of oxygen ions. At the
same time, the C–C binding energy in the underneath layer remains almost unchanged
(~0.1 eV). Exactly this fact in combination with controlled energy of Ar
+
-ions has allowed
etching of the top layer selectively. This experience opens the way to combine the chemical
and energetic impacts of plasma species for effective and controlled GO reduction.
Nanomaterials2021,11,xFORPEERREVIEW13of38
Figure5.(a)Schematicofatwo‐gridICP‐typeionbeamsystemwithaxialmagneticfieldusedfor
atomiclayeretchingofgraphenewithaquadrupolemassspectrometer(QMS,forionenergy/flux
measurementoftheionbeam).(b)VariousRFpowersat70sccmofArgasflowrateand(c)the
correspondingpeakenergiesandfluxfortheirAr
+
‐ionenergydistributions.(d)VariousArflow
ratesat500WRFpowermeasuredbyanionenergyanalyzerintheQMSand(e)arethecorre‐
spondingpeakenergiesandfluxesfortheirAr
+
‐ionenergydistributions.ReproducedfromKimet
al.[212]licensedunderCCBY4.0.
3.2.HydrogenPlasma
Ahydrogengasdischargecanconstitutefreeelectrons,neutrals(molecularH
2
,
atomicH),andchargedions(H
+
,H
−
,
H
2+
,H
3+
)interactingthroughasetofnumerousreac‐
tions[213].AbombardmentbyH,H
+
,H
2+
,andH
3+
specieswithenergiesvaryingfrom10
eVtofewhundredofeVisknowntoresultintheetchingofgraphene[214–216].The
impactofhydrogenplasmaonGOissimilartotheetchingeffectandremovalofoxygen‐
containinggroups[193,194,208].Amoleculardynamicsstudy[215]ongraphenesug‐
gestedthatsurfacereactionstronglyvarieswithincidentatomicHenergy:(i)atomicH
withfewtenthsofeVcanadsorbonthebasalplaneofsurface‐cleangraphene,(ii)Hen‐
ergies0.025–0.3canselectivelyetchedgeswithoutdamagingthebasalplane,(iii)Hener‐
giesintherange0.3–10eVhydrogenatesbasalplanewithoutirreversiblydamaginggra‐
phene(hydrogenationisreversible),(iv)Henergiesintherange10–100eVaresuitable
forpatterningmulti‐layergraphene,and(iv)10eVH
+
ionscanetchgraphenevertically
andthehydrogenplasmacontainingmoremolecular(H
2+
andH
3+
)thanatomic(H
+
)ions
mayinducelesssubsurfacedamageinmulti‐layeredgraphene.Suchdetailedstudiesare
yettobereportedforGOmaterials.
Figure 5.
(
a
) Schematic of a two-grid ICP-type ion beam system with axial magnetic field used for
atomic layer etching of graphene with a quadrupole mass spectrometer (QMS, for ion energy/flux
measurement of the ion beam). (
b
) Various RF powers at 70 sccm of Ar gas flow rate and (
c
) the
corresponding peak energies and flux for their Ar
+
-ion energy distributions. (
d
) Various Ar flow rates
at 500 W RF power measured by an ion energy analyzer in the QMS and (
e
) are the corresponding
peak energies and fluxes for their Ar
+
-ion energy distributions. Reproduced from Kim et al. [
212
].
Copyright © 2017 Authors, licensed under CC BY 4.0.
3.2. Hydrogen Plasma
A hydrogen gas discharge can constitute free electrons, neutrals (molecular H
2
, atomic
H), and charged ions (H
+
, H
−
, H
2+
, H
3+
) interacting through a set of numerous reac-
tions [
213
]. A bombardment by H, H
+
, H
2+
, and H
3+
species with energies varying from
10 eV to few hundred of eV is known to result in the etching of graphene [
214
–
216
]. The
impact of hydrogen plasma on GO is similar to the etching effect and removal of oxygen-
containing groups [
193
,
194
,
208
]. A molecular dynamics study [
215
] on graphene suggested
that surface reaction strongly varies with incident atomic H energy: (i) atomic H with
Nanomaterials 2021,11, 382 13 of 37
few tenths of eV can adsorb on the basal plane of surface-clean graphene, (ii) H energies
0.025–0.3 can selectively etch edges without damaging the basal plane, (iii) H energies in
the range 0.3–10 eV hydrogenates basal plane without irreversibly damaging graphene
(hydrogenation is reversible), (iv) H energies in the range 10–100 eV are suitable for pat-
terning multi-layer graphene, and (iv) 10 eV H
+
ions can etch graphene vertically and the
hydrogen plasma containing more molecular (H
2+
and H
3+
) than atomic (H
+
) ions may
induce less subsurface damage in multi-layered graphene. Such detailed studies are yet to
be reported for GO materials.
Kim et al. [
193
] applied optical emission spectroscopy (OES) to determine the optimum
process point for GO-reduction to avoid the degradation of electrical characteristics with
excess plasma exposure. The emission corresponding to the oxygen radicals released from
GO was used as an indicator of the reduction progress (see Figure 6a–c). As observed in
Figure 6c, at point C (~18 s after the start of the reduction process), the intensity of the OES
oxygen-line begins to decline, indicating the end of reduction. A crucial application of the
OES lies in the determination of the excited states of species in the plasma. Li et al. [
190
]
studied with this method the effect of variable plasma power and different gas mixtures
(Ar/H
2
) on the reduction of GO. The emission corresponding to atomic hydrogen (in OES
spectra, Figure 6d–g) was found to increase with increasing discharge power and reached
an overall maximum for the H
2
/Ar ratio 2:1. The inclusion of Ar assisted in the enhanced
dissociation of H
2
due to the penning ionization. The GO (R
C/O
: 1.1) on reduction with
a pure Ar and H
2
plasma yielded rGO with R
C/O
of 1.2 and 1.7, respectively. However,
the rGO obtained with a more populous H
2
/Ar plasma (2:1) resulted in a R
C/O
up to 6.9.
Furthermore, the electrochemical performance of the fabricated rGO was demonstrated as
an electrode (in KOH aqueous electrolyte), achieving a specific capacitance of 185.2 F/g.
The performance was higher than several graphene-based electrodes in literature.
3.3. Methane Plasma
To obtain graphene-like quality with GO as precursors, numerous investigations into
reduction with healing (or repair) of defects has been carried out. By incorporating C-
atoms into structural defects of GO, the sp
2
-hybridized graphene domains are restored.
Various strategies employed include thermal-CVD [
92
,
217
–
219
] and high-temperature
graphitization [
142
,
220
]. In a recent review, De Silva et al. [
48
] addressed the defect repair
of GO mostly with thermal methods. This section is devoted to the plasma-enhanced
CVD approach, which is more attractive in terms of large-scale applications than the
others mentioned.
Methane plasma is a popular choice for the defect repair and reduction of GO. Pure
discharge of CH
4
[
93
], as well as in combination with Ar [
180
,
185
,
203
] or H
2
[
179
,
184
,
189
,
207
,
221
], has been utilized. Cheng et al. [
93
] treated GO-monolayers on Si/SiO
2
substrates
with CH
4
-plasma (100 W, RF, ~575
◦
C, 10 min) that resulted in a decrease of the I
D
/I
G
ratio
from 1.03 to 0.53, indicating healing of defects and increasing conversion of sp
2
C-atoms.
They reported one of the highest conductivities (1590 S/cm) for rGO. Baraket et al. [
203
]
used plasma generated in a CH
4
(0–20%)/Ar gas mixture by electron-beam, resulting in the
energy of electrons and dissociated ions <0.5 eV and <3 eV, respectively. By varying plasma
duty factor, methane concentration, gas pressure, and treatment time, they showed that
oxygen concentration in rGO can be controlled in the range 5–43 at.%. Correlating Raman,
XPS, and AFM investigations, the authors concluded that the defects originating from
deoxygenation were healed by the CH
n
species. Thus, amorphous carbon was selectively
rather than uniformly deposited on rGO. In a mixed gas discharge, the compositional
ratio of gases is one of the important parameters to optimize. Yang et al. studied the
effect of RF plasma in CH
4
/Ar mixture (100 W, no substrate heating) on GO-monolayers.
They determined that the methane-rich environment (CH
4/
Ar—2:1) was the most effective
for reduction.
Using the CH
4
+ H
2
plasma can be beneficial for the following reasons: (i) the amount
of reactive species is surplus for reduction of oxygen-functional groups compared to a
Nanomaterials 2021,11, 382 14 of 37
pure CH
4
plasma [
221
]; (ii) many defects and distortions of a graphitic network are bound
and eliminated by the hydrogen active species and serve at once as active sites for carbon
species from plasma to react [
179
,
184
], and (iii) the etching nature of H
2
plasma restricts
deposition of sp
3
amorphous carbon from radicals of dissociated CH
4
molecules [
179
,
189
].
The substrate temperature, CH
4
/H
2
ratio, other plasma conditions, and treatment runtime
sensitively influence the relative rates of etching and carbonization (defect restoration).
Bodik et al. [
207
] used an instantaneous DBSCD plasma reduction (100 W/cm
3
equimolec-
ular CH
4
/H
2
ratio, atmospheric pressure, 5 s) of the unheated GO films, and observed that
this treatment was effective in the removal of oxygen-functional groups but not in sp
2
-C
restoration. Nevertheless, such a plasma process can promote application on temperature-
sensitive flexible substrates. Chiang et al. [
221
] performed a reduction of GO-nanoribbons
films with an RF CH
4
/H
2
plasma at 230
◦
C. Impressively, even at this moderate tempera-
ture, the removal of all kinds of oxygen-containing groups (C-O, C=O, and COOH) down
to < 1–1.2 at.% surface concentration was determined by XPS analysis. The I
D
/I
G
ratio
remained however quite high: 0.83.
Nanomaterials2021,11,xFORPEERREVIEW14of38
Kimetal.[193]appliedopticalemissionspectroscopy(OES)todeterminetheopti‐
mumprocesspointforGO‐reductiontoavoidthedegradationofelectricalcharacteristics
withexcessplasmaexposure.Theemissioncorrespondingtotheoxygenradicalsreleased
fromGOwasusedasanindicatorofthereductionprogress(seeFigure6a–c).Asobserved
inFigure6c,atpointC(~18safterthestartofthereductionprocess),theintensityofthe
OESoxygen‐linebeginstodecline,indicatingtheendofreduction.Acrucialapplication
oftheOESliesinthedeterminationoftheexcitedstatesofspeciesintheplasma.Lietal.
[190]studiedwiththismethodtheeffectofvariableplasmapoweranddifferentgasmix‐
tures(Ar/H2)onthereductionofGO.Theemissioncorrespondingtoatomichydrogen(in
OESspectra,Figure6d–g)wasfoundtoincreasewithincreasingdischargepowerand
reachedanoverallmaximumfortheH2/Arratio2:1.TheinclusionofArassistedinthe
enhanceddissociationofH2duetothepenningionization.TheGO(RC/O:1.1)onreduction
withapureArandH2plasmayieldedrGOwithRC/Oof1.2and1.7,respectively.However,
therGOobtainedwithamorepopulousH2/Arplasma(2:1)resultedinaRC/Oupto6.9.
Furthermore,theelectrochemicalperformanceofthefabricatedrGOwasdemonstrated
asanelectrode(inKOHaqueouselectrolyte),achievingaspecificcapacitanceof185.2F/g.
Theperformancewashigherthanseveralgraphene‐basedelectrodesinliterature.
Figure6.EmissionspectraofH2dischargewithoutGO(a)andwithGOsample(b)intheplasma
treatmentchamber,indicatingtheactivespecies.(c)TheOESsignalfromthe844.6nmoxideline
(shownin(b))duringthereductionofGO.A–Ein(c)arevariousprocesspointswheretherGO
samplepropertieswereinvestigatedinliterature[193].ReproducedfromKimetal.[193]licensed
underCCBY3.0.EmissionspectraofH2/ArplasmaasafunctionoftheratioofH2toAr(d,e)and
dischargepower(f,g).LinescorrespondingtoArexcitedstatesandatomichydrogen(HαandHβ)
areindicated.ReproducedfromLietal.[190].Copyright©2014AmericanChemicalSociety.
Figure 6.
Emission spectra of H
2
discharge without GO (
a
) and with GO sample (
b
) in the plasma
treatment chamber, indicating the active species. (
c
) The OES signal from the 844.6 nm oxide line
(shown in (
b
)) during the reduction of GO. A–E in (
c
) are various process points where the rGO
sample properties were investigated in literature [
193
]. Reproduced from Kim et al. [
193
]. Copyright
©
2013 Authors, licensed under CC BY 3.0. Emission spectra of H
2
/Ar plasma as a function of the
ratio of H
2
to Ar (
d
,
e
) and discharge power (
f
,
g
). Lines corresponding to Ar excited states and atomic
hydrogen (H
α
and H
β
) are indicated. Reprinted with permission from Li et al. [
190
]. Copyright
©
2014 American Chemical Society.
Nanomaterials 2021,11, 382 15 of 37
The work of Zhu et al. [
179
] sheds more light on the influence of temperature in defect
healing and its mechanism. Figure 7a presents a schematic where the CH
x(x < 4)
species
(from dissociated CH
4
) and H
y
(y < 2) species (from dissociated H
2
) are simultaneously
involved in the healing and etching process in GO, respectively. As one can see in
Figure 7b
,
the etching by hydrogen plasma interchanges with growing by methane plasma at ~760
◦
C.
After plasma treatment at 800
◦
C for 40 s, nearly graphene-like films were obtained from
GO-sheets. Additionally, Zhu et al. [
179
] performed density functional theory (DFT) cal-
culations to elucidate different repairing events during thermal annealing in the plasma
environment. Migration paths and their associated energy barriers for repairing a car-
bon mono-vacancy were presented for various derivatives of dissociated CH
4
molecule
(
see Figure 8
). As shown in Figure 8, the CH
2
species possessed a relatively small energy
barrier (0.32 eV) compared to CH
3
(0.69 eV), CH
4
(1.26 eV), and C
2
H
4
(2.07 eV) species,
indicating that the former species dominate in the repair of the carbon mono-vacancies. The
authors also suggested that a CH radical could directly fill a mono-vacancy, but a hydrogen
atom should simultaneously be taken away by another radical. To our knowledge, there
are no other fundamental reports discussing plasma-assisted reduction and repair of GO
yet. However, conventional thermal and chemical reduction methods have been relatively
well studied and understood [126,210,222–225].
Figure 7.
(
a
) Schematic representation of rGO recovery in a methane plasma. (
b
) Variation in the
rGO film optical transmittance (at 550 nm) and charge mobility before (T
1
and
µ1
) and after (T
2
and
µ2
) a 1-min methane + hydrogen plasma treatment from 700 to 900
◦
C. The regions of T
2−
T
1
> 0
and T
2−
T
1
< 0 are denoted as “etching” and “growth”, respectively. Reprinted from Zhu et al. [
179
],
Copyright © 2017, with permission from Elsevier.
An interesting question to be understood is whether the reduction and defect healing
can be extended to the multi-layered GO, namely, deeper layers that are not directly ex-
posed to plasma. Obata et al. [
184
,
189
] demonstrated RF CH
4
/Ar plasma (10 W, 550
◦
C)
reduced GO-monolayers with electron mobility of 460–900 cm
2
/V
·
s and contributed de-
tailed experimental investigations. On the downside, the process needed a remote Cu-film
in the vicinity of GO (up to 8 mm distance) to catalyze the reduction and restoration. The
authors characterized the plasma effect on mono-, bi-, and tri-layered GO by Raman spec-
troscopy [
184
]. As the number of layers increased, the spectra deviated from graphene-like
ones and resembled like rGO (larger I
D
/I
G
, smaller I
2D
/I
G
, wider G-band). This indicated
that while the topmost surface changed to restored graphene the deeper layers changed to
rGO only. The latter transformation was supposedly driven by the heat of plasma, while
the penetration of the dissociated CH
4
species was apparently restricted to the top surface
layer. Obata et al. [
184
] attributed this to the selective permeability of graphene and GO,
which is in turn dependent on the kinetic diameters of the involved species [
226
,
227
]. The
defect repair by plasma method thus has a limitation to GO-monolayers [48].
Nanomaterials 2021,11, 382 16 of 37
Nanomaterials2021,11,xFORPEERREVIEW16of38
barrier(0.32eV)comparedtoCH3(0.69eV),CH4(1.26eV),andC2H4(2.07eV)species,
indicatingthattheformerspeciesdominateintherepairofthecarbonmono‐vacancies.
TheauthorsalsosuggestedthataCHradicalcoulddirectlyfillamono‐vacancy,buta
hydrogenatomshouldsimultaneouslybetakenawaybyanotherradical.Toour
knowledge,therearenootherfundamentalreportsdiscussingplasma‐assistedreduction
andrepairofGOyet.However,conventionalthermalandchemicalreductionmethods
havebeenrelativelywellstudiedandunderstood[126,210,222–225].
Figure7.(a)SchematicrepresentationofrGOrecoveryinamethaneplasma.(b)Variationinthe
rGOfilmopticaltransmittance(at550nm)andchargemobilitybefore(T1andμ1)andafter(T2and
μ2)a1‐minmethane+hydrogenplasmatreatmentfrom700to900°C.TheregionsofT2−T1>0
andT2−T1<0aredenotedas“etching”and“growth”,respectively.ReproducedfromZhuetal.
[179]withpermissionfromElsevierLtd.
Figure8.Migrationpathanditsassociatedenergychangeforrepairingthegraphenewithmono‐
vacancyafterreactionwith(A)CH2,(B)CH3,(C)CH4,and(D)CH2CH2.Theredandcyanballs
representthecarbonandhydrogenatoms.AllenergyvaluesareineV.ReproducedfromZhuet
al.[179]withpermissionfromElsevierLtd.
Figure 8.
Migration path and its associated energy change for repairing the graphene with mono-
vacancy after reaction with (
A
) CH
2
, (
B
) CH
3
, (
C
) CH
4
, and (
D
) CH
2
CH
2
. The red and cyan balls
represent the carbon and hydrogen atoms. All energy values are in eV. Reprinted from Zhu et al. [
179
],
Copyright © 2017, with permission from Elsevier.
3.4. Nitrogen and Ammonia Plasma
Using the N
2
or NH
3
plasma treatment is a way to dope graphene by nitrogen [
14
,
228
,
229
].
Amongst various doping techniques available, the plasma-assisted methods yield higher
N-atomic concentrations [
230
]. In the case of GO, plasma treatment not only results in
N-doping, but also provokes simultaneous deoxygenation/reduction. The N-atom in-
corporated into the graphitic network may be pyridine-like, pyrrole-like, or graphite-
/quaternary-like (see Figure 9a) [
231
,
232
]. The doping of graphene and GO by het-
eroatoms (boron, nitrogen, phosphorus, sulfur, halogens, etc.) was thoroughly discussed
by Wang et al. [
233
]. Briefly, the quaternary-N defects donate to the graphene lattice
~0.5 free electrons per site [
234
,
235
]. In contrast, the pyridinic-N and pyrrolic-N are ac-
ceptor defects [
235
]. However, on hydrogenation, the pyridinic-N doping transforms
from p- to n-type [
235
]. The pyridinic-N and pyrrolic-N sites were suggested to promote
electrocatalytic properties [
236
,
237
]. N-doped GO materials also have potential for use
in fuel-cells [
238
], water-splitting [
239
], batteries [
240
], supercapacitors [
241
,
242
], and
perovskite solar cells [243].
The dissociation energy of N
2
is higher than NH
3
; therefore, N
2
plasma ensures
less density of active species than a NH
3
plasma for the same given conditions [
244
].
Additionally, a N
2
gas discharge constitutes excited species such as N, N
+
, N
2+
, etc. [
245
],
whereas an NH
3
plasma can constitute H, N, N
2
, NH, NH
2
, N
2
H
2
, N
2
H, NH
2+
, NH
3+
,
and NH
4+
excited species [
246
–
249
]. Well-known reducing agent hydrazine (N
2
H
4
) has
also been observed at certain conditions in NH
3
plasma [
250
,
251
]. Thus, more active and
reductive NH
3
plasma should be more effective than a pure N
2
plasma. Charged radicals
N
x
H
y
(from dissociated NH
3
, mostly N
2
H radicals) were suggested to reduce isolated
epoxide groups in GO [
252
], while -NH
2
groups were suggested to substitute hydroxyl
groups [
210
]. Mohai et al. [
181
] subjected GO-films to N
2
and NH
3
plasma, biasing the
sample’s potential to accelerate ions towards the GO-surface. The authors found by XPS
Nanomaterials 2021,11, 382 17 of 37
that the NH
3
plasma effectively reduced the amount of oxygen on the GO surface. The
penetration depth of bombarding ions was also computed in this work that revealed the
following: (a) biased substrate potential makes penetration depth of N-ions greater than
that of H-ions; (b) N-atoms from NH
3
could penetrate deeper than those from N
2
for a
given biasing energy (see Figure 9b).
Nanomaterials2021,11,xFORPEERREVIEW18of38
Figure9.(a)Illustrationofcommonlydopednitrogenspeciesingraphiticcarbonwiththecorre‐
spondingXPSbindingenergies.ReproducedfromZhangetal.[253]licensedunderCCBY‐NC
4.0.(b)ThepenetrationdepthofNandHionsintographeneoxideproducedfromNH3andN2,
calculatedasafunctionoftheappliedbias(filledmarks)andionenergy(openmarks).Repro‐
ducedfromMohaietal.[181]withpermissionfromJohnWileyandSons,Ltd.
TheuseofanN2plasmacanbeeffectivewhenH2gasisadmixed.Zhouetal.[187]
usedthiscombination(N2/H2:40/10sccm,1400W,ICP(~370kHz))alongwithnegative‐
voltagesamplebiasing(−35V)todirectandacceleratethepositiveionstowardsthesam‐
plesurface.TheOESdata(Figure10a)confirmedtheexistenceofseveralradicalsfromthe
dissociationofN2andH2moleculeswheretheN2+speciesdominated.Theelectrontem‐
perature(Te)inplasmaincreasedfrom0.55eVwhennobiasvoltagewasusedto0.79eV
intheoppositecase.TheincreaseinTeinturnactivatedmoreN2molecules.The•NH
radicalwasattributedtotheremovalofisolatedepoxidegroups,whilethereactiveH‐
ionswasattributedtoremovalofhydroxylgroups.Usingbiasvoltagealsoincreasedthe
shareofgraphitic‐Nsitesfrom16.88%to21.11%andpyridinic‐Nfrom27.55%to38.70%
(Figure10b,c)inthetotalN‐content.
Figure10.(a)Opticalemissionlinesfrom200to800nmofN2andH2plasmawith1400Winput
poweratthepressureof1.6Pa(ICP,RF—370kHz).XPSC1sspectra(b)andN1sspectraofGO(c)
ofreducedGOsampleswithoutbias(N‐RGO‐0V‐5min)andwitha−35Vbias(N‐RGO‐35V‐5
min).ReproducedfromZhouetal.[187].Copyright©2019AmericanChemicalSociety.
Figure 9.
(
a
) Illustration of commonly doped nitrogen species in graphitic carbon with the corre-
sponding XPS binding energies. Reprinted from Zhang et al. [
253
].
©
The Authors, some rights
reserved; exclusive licensee AAAS. Distributed under a Creative Commons Attribution NonCom-
mercial License 4.0 (CC BY-NC). (
b
) The penetration depth of N and H ions into graphene oxide
produced from NH
3
and N
2
, calculated as a function of the applied bias (filled marks) and ion energy
(open marks). Reproduced with permission from Mohai et al. [
181
]. Copyright
©
2018 John Wiley &
Sons, Ltd.
The use of an N
2
plasma can be effective when H
2
gas is admixed. Zhou et al. [
187
]
used this combination (N
2
/H
2
: 40/10 sccm, 1400 W, ICP (~370 kHz)) along with negative-
voltage sample biasing (
−
35 V) to direct and accelerate the positive ions towards the
sample surface. The OES data (Figure 10a) confirmed the existence of several radicals
from the dissociation of N
2
and H
2
molecules where the N
2+
species dominated. The
electron temperature (T
e
) in plasma increased from 0.55 eV when no bias voltage was used
to 0.79 eV in the opposite case. The increase in T
e
in turn activated more N
2
molecules. The
•
NH radical was attributed to the removal of isolated epoxide groups, while the reactive
H-ions was attributed to removal of hydroxyl groups. Using bias voltage also increased the
share of graphitic-N sites from 16.88% to 21.11% and pyridinic-N from 27.55% to 38.70%
(Figure 10b,c) in the total N-content.
The share of pyridinic, pyrrolic, and graphitic sites in the N-doped rGO varies from report
to report based on experimental conditions. A majority of the considered reviews reported that
the share of pyrrolic-N was the maximum according to XPS [
187
,
192
,
195
,
197
,
254
]. Graphitic-N
possessed a high share when processed at higher temperatures [
186
]. There has been
an instance where the graphitic-N share was the highest even after treatment at room
temperature [
198
]. Calculation of formation energies suggested that the substitutional N-
doping (graphitic-N) is the most favorable among other configurations [
255
]. In a presence
of carbon vacancy, however, the pyridine-N becomes energetically favorable. Another study
pointed out the preference of graphitic-N to appear at carbon atoms near other defects [
256
].
This study also concluded that pyridine- and pyrrolic-like configurations mostly realized
on mono- and di-vacancies. The N-configurations were observed to vary in graphene
depending on N
2+
-ion beam energy and irradiation time [
257
]. Similarly, sputtering
GO with 220 eV N
2+-
ions for a variable time resulted in different N-configurations and
deoxygenation extents [
258
]. More research must be done in this context for applications
where specific N-configuration on GO needs to be tailored.
Nanomaterials 2021,11, 382 18 of 37
Nanomaterials2021,11,xFORPEERREVIEW18of38
Figure9.(a)Illustrationofcommonlydopednitrogenspeciesingraphiticcarbonwiththecorre‐
spondingXPSbindingenergies.ReproducedfromZhangetal.[253]licensedunderCCBY‐NC
4.0.(b)ThepenetrationdepthofNandHionsintographeneoxideproducedfromNH3andN2,
calculatedasafunctionoftheappliedbias(filledmarks)andionenergy(openmarks).Repro‐
ducedfromMohaietal.[181]withpermissionfromJohnWileyandSons,Ltd.
TheuseofanN2plasmacanbeeffectivewhenH2gasisadmixed.Zhouetal.[187]
usedthiscombination(N2/H2:40/10sccm,1400W,ICP(~370kHz))alongwithnegative‐
voltagesamplebiasing(−35V)todirectandacceleratethepositiveionstowardsthesam‐
plesurface.TheOESdata(Figure10a)confirmedtheexistenceofseveralradicalsfromthe
dissociationofN2andH2moleculeswheretheN2+speciesdominated.Theelectrontem‐
perature(Te)inplasmaincreasedfrom0.55eVwhennobiasvoltagewasusedto0.79eV
intheoppositecase.TheincreaseinTeinturnactivatedmoreN2molecules.The•NH
radicalwasattributedtotheremovalofisolatedepoxidegroups,whilethereactiveH‐
ionswasattributedtoremovalofhydroxylgroups.Usingbiasvoltagealsoincreasedthe
shareofgraphitic‐Nsitesfrom16.88%to21.11%andpyridinic‐Nfrom27.55%to38.70%
(Figure10b,c)inthetotalN‐content.
Figure10.(a)Opticalemissionlinesfrom200to800nmofN2andH2plasmawith1400Winput
poweratthepressureof1.6Pa(ICP,RF—370kHz).XPSC1sspectra(b)andN1sspectraofGO(c)
ofreducedGOsampleswithoutbias(N‐RGO‐0V‐5min)andwitha−35Vbias(N‐RGO‐35V‐5
min).ReproducedfromZhouetal.[187].Copyright©2019AmericanChemicalSociety.
Figure 10.
(
a
) Optical emission lines from 200 to 800 nm of N
2
and H
2
plasma with 1400 W input
power at the pressure of 1.6 Pa (ICP, RF—370 kHz). XPS C1s spectra (
b
) and N 1s spectra of GO (
c
) of
reduced GO samples without bias (N-RGO-0V-5min) and with a
−
35 V bias (N-RGO-35V-5 min).
Reprinted with permission from Zhou et al. [187]. Copyright © 2019 American Chemical Society.
Lee et al. [
197
] carried out reduction and nitridation of the 10 nm thick GO-films
in the bulk and sheath regions of an NH
3
RF plasma (1000 W/m
2
power density). The
sheath region is characterized by a much stronger electric field than in the glow discharge
region that provides domination of ion bombardment [
197
]. Chemical reactions dominate
in-turn in the glowing volume. The XPS investigation revealed that even when placed in a
sheath, the GO sample partially lost oxygen and gathered nitrogen, but not as effectively
as when treated in a chemically active glow. The samples placed in the sheath were not
conductive, which was expected to result from ion bombardment breaking the graphitic
network. The authors proposed the following reactions by radicals or ions to be responsible
for the reduction of GO in a glowing region of plasma:
NH2+H+Cg-O-Cg→Cg-NH2+ Cg-OH (1)
NH + H2+ Cg-O-Cg→Cg-NH-Cg+ H2O (2)
NH2+H+Cg-OH →Cg-NH2+ H2O (3)
NH2+H+Cg-COOH →Cg-CO-NH2+ H2O (4)
NH2+H+Cg-COOH →Cg-NH2+ H2O+CO+H2O (5)
where C
g
is a carbon atom on the regular graphene site. Equations (1) and (2) explain
the reduction of epoxide groups, Equation (3) corresponds to a reduction of the hydroxyl
group, and Equations (4) and (5) correspond to that of carboxyl groups on the GO-film.
3.5. Acetylene Plasma
An acetylene discharge is also a source of carbon species assisting the healing of GO.
To our knowledge, there are only two reports on this topic [
186
,
259
]. In both cases, the
Nanomaterials 2021,11, 382 19 of 37
GO films were annealed in a vacuum (up to 6 h long) before plasma treatment. Such
pretreatment could have caused deoxygenation and formation of carbon vacancies, which
are active sites for repair. In one of those works, Ooi et al. [
259
] successfully replaced
indium–tin—oxide (ITO) with C
2
H
2
/H
2
plasma reduced rGO for an application in a mem-
ory device. Before plasma treatment (20 W, 700
◦
C, 2 min), the GO was thermally annealed
at 700
◦
C for 6 h. Thus, the obtained ~5.12 nm thick rGO film exhibited ~90 cm
2
/V
·
s
and sheet resistance of ~510
Ω
/sq. From the reported thickness and sheet resistance, we
estimate the conductivity to be ~3830 S/cm, which is one of the highest reported values
for rGO.
Similarly, Haniff et al. [
186
] prepared rGO on eight-inch Si/SiO
2
wafer using C
2
H
2
/NH
3
plasma (20 W, 700
◦
C, 3 min) for photodetector application. Before plasma treatment, the
films were also annealed in a vacuum (700
◦
C, 30 min). Since NH
3
was added to the dis-
charge gas, the films became N-doped. From the point of the mechanism, it was suggested
that active hydrogen species react with oxygen groups at first and eliminate them creating
active sites (e.g., point defects and vacancies) for the following N-doping. Simultaneously,
the dissociated C
x
H
x
(x < 2) and NH
x
(x < 3) radicals interact with active sites restoring
graphitic plane with C- and N-atoms. The photodetectors obtained in this work revealed a
photoresponsivity of 0.68 AW
−1
at 1.0 V, outperforming graphene-based photoconductor
by ~2 orders of magnitude. Although impressive applications have been demonstrated,
acetylene (and other carbonaceous gases) has not been studied to the same extent as
methane for plasma-assisted defect healing in GO. Theoretical and experimental research
is needed to identify which mechanisms and carbonaceous gas discharge are chemically
advantageous for low-temperature defect healing. Gong et al. [260] have investigated the
defect repair of GO in the presence of water, methanol, and ethanol and concluded that
ethanol can contribute better to reduction with increased defect repair in GO.
3.6. Air Plasma
GO-reduction using air as the discharge gas in APPJ-plasma has been demon-
strated [
205
,
261
]. The APPJ-plasma is created either with a noble gas or air, and the reactive
species are generated within the plasma (e.g., He + O
2
admixture) or its interaction with
the surrounding (air: mostly N
2
, O
2
, and H
2
O vapor) [
262
]. Thus, various atoms, radicals,
ions, and excited molecular species such as: O, O
3
,
1
O
2, •
OH, N
2
, N
2+
, O
2•−
/
•
OOH,
•
NO, ONOO
−
, OONOO
−
, H
2
O
2
, NO
2−
, and NO
3−
are produced [
262
–
264
] (an example,
Figure 11
). In an air plasma, active oxygen species stipulate its highly oxidative impact,
which is used to remove ad- and chemisorbed organics from the treated surface [
265
–
267
].
An RF air-plasma (7 W, 49 mL/min, 30 s) has been used to eliminate polymer
residues from the surface of transferred CVD-graphene without damaging the graphitic
network [
268
]. Huang et al. [
261
] treated the surface of the CVD-graphene monolayer with
an APPJ-He-plasma (10.7 kV and 14 mA at the electrode, 100
µ
s short pulses at 1.5 kHz
frequency) in ambient air. They observed that as the treatment time progressed from 0–60 s,
the concentration of oxygen-containing groups increased from ~10% to ~40%. At longer
treatment (>60 s), GO (oxidized CVD-graphene) started losing oxygen together with carbon.
The authors suggested that the epoxide groups are unstable under APPJ conditions due to
the intensive bombardment by electrons. At C-O-C dissociation, highly reactive radicals
like (C-O
•
) are formed. After activation of epoxide, the transient surface atomic oxygen is
captured by surrounding oxygen, forming a di-oxygen molecule as product. The authors
suggested that for graphene with high C-O-C coverage, the rate of reduction exceeded that
of oxidation.
Alotaibi et al. [
205
] performed a room-temperature reduction process on GO-films
with a scanning atmospheric plasma. Air was used as discharge gas, and a high potential
of ~10 kV was applied between the electrodes (300 W, 20 kHz power supply) to generate
an arc discharge. The reduced GO realized by the APPJ process was demonstrated in a
supercapacitor with a volumetric capacity of 536.55 F/cm
3
at a current density of 1 A/g.
By combining the plasma jet with software-controlled scanning devices, they were able to
Nanomaterials 2021,11, 382 20 of 37
demonstrate rapid and controlled GO reduction on several substrates such as glass, plastic,
and textile with various shapes and size patterns.
According to the authors, negatively charged oxygen ions participate in bond-breaking,
displacement of atoms on the surface, and charge accumulation. Through inelastic col-
lisions, the ions with low kinetic energy bombard the GO-surface, causing rupture of
oxygen-containing groups and release of oxygen ions. As a result, the ions O
2−
and O
2−
transfer free electrons to the GO-surface and charge it. The formation of nitric oxide (NO) in
plasma was also suggested. It can take part in reaction with such groups as -COOH, -CO-,
and -OH, converting itself into NO
2
. On a free-standing 25
µ
m thick GO film, the reduction
process in question produced the exposed to plasma rGO surface with sheet-resistance of
186
Ω
/sq in case of 1 min treatment and 160
Ω
/sq in case of 2 min treatment. The back rGO
side also revealed some reduction extent: R
sh
= 680
Ω
/sq (1 min) and 560
Ω
/sq (2 min),
indicating the penetration effect of plasma. To exclude the heating effect of the plasma
beam, the authors performed a control experiment by annealing GO in a convection oven
for 2–20 min. Fourier transformed infrared spectroscopy analysis revealed that a min of
20-min thermal annealing was required to achieve a comparable reduction degree with a
10 s plasma reduction. Thus, a major impact of plasma by heating was ruled out.
Nanomaterials2021,11,xFORPEERREVIEW21of38
theplasmabeam,theauthorsperformedacontrolexperimentbyannealingGOinacon‐
vectionovenfor2–20min.Fouriertransformedinfraredspectroscopyanalysisrevealed
thataminof20‐minthermalannealingwasrequiredtoachieveacomparablereduction
degreewitha10splasmareduction.Thus,amajorimpactofplasmabyheatingwasruled
out.
Figure11.(a)SchematicofanAPPJ‐plasma(20W,RF)ignitedwithHe(10m/s)andlaunched
throughambientairontothesample.(b)Theopticalemissionspectraofthecorresponding
plasma.ReproducedfromPtasinskaetal.[269]withpermissionfromtheRoyalSocietyofChemis‐
try.
4.SummaryandConclusions
Theplasmaenvironmentwithfree‐electronsandreactivespeciesreducesGO.Inthe
caseofacontrolledpotentialdifferencebetweenplasmaandGOsurface,energeticions
bombardthesurface,distortingandbreakingvariouschemicalbonds.Oxygen‐containing
groupsescapeuponfurtherbombardmentofplasmaspeciesthatfinallyresultsinGO
reduction.Plasma,especiallyinitsequilibriumstate,andplasmairradiationgenerateheat
inatreatedsolid.Inthatsense,themechanismofGOreductionbyplasmainvolvesdy‐
namiceventstypicalforestablishedthermalreduction.However,plasmaprocessesoffer
additionalparameterstotune:pressureandchemistryofthedischargegas,typeandfre‐
quencyofpowergeneration,andexacttreatmenttime.Thesetofdifferenttypesofspecies
(ions,excitedatomsormolecules,andelectrons)andtheirrelativeenergyandchemical
activitydefinesthepenetrationdepthandthechemicalimpact.
Withinthescopeofreviewedliteraturehere,threemainmissionsofplasmareduc‐
tioncanbedistinguished:(I)reductionforimprovedelectricalconductivity(andhigher
RC/Othaninitially),whichisthegeneralcase,(II)reductionwithsimultaneousdoping
Figure 11.
(
a
) Schematic of an APPJ-plasma (20 W, RF) ignited with He (10 m/s) and launched
through ambient air on to the sample. (
b
) The optical emission spectra of the corresponding plasma.
Reproduced from Ptasinska et al. [269] with permission from the Royal Society of Chemistry.
Nanomaterials 2021,11, 382 21 of 37
4. Summary and Conclusions
The plasma environment with free-electrons and reactive species reduces GO. In the
case of a controlled potential difference between plasma and GO surface, energetic ions
bombard the surface, distorting and breaking various chemical bonds. Oxygen-containing
groups escape upon further bombardment of plasma species that finally results in GO
reduction. Plasma, especially in its equilibrium state, and plasma irradiation generate
heat in a treated solid. In that sense, the mechanism of GO reduction by plasma involves
dynamic events typical for established thermal reduction. However, plasma processes
offer additional parameters to tune: pressure and chemistry of the discharge gas, type
and frequency of power generation, and exact treatment time. The set of different types
of species (ions, excited atoms or molecules, and electrons) and their relative energy and
chemical activity defines the penetration depth and the chemical impact.
Within the scope of reviewed literature here, three main missions of plasma reduction
can be distinguished: (I) reduction for improved electrical conductivity (and higher R
C/O
than initially), which is the general case, (II) reduction with simultaneous doping (mostly
by nitrogen), and (III) restoration of graphitic network with mostly sp
2
-carbon hybridiza-
tion that means healing of lattice defects. In the general case, all plasma treatments, i.e.,
based on inert gases (Ar and He) or reactive ones (H
2
, N
2
, NH
3
, CH
4
, and C
2
H
2
) or a
combination of them, serve the purpose of reduction. For the second task, plasma needs
to contain dopant species (dissociated N
2
or NH
3
) for insertion into the carbon lattice
along with reduction. The third task requires the presence of carbon species in plasma for
assisting in the healing of GO-defects and increasing sp
2
-carbon domain size. The corre-
sponding discharge processes are based on a carbonaceous gas (like CH
4
and C
2
H
2
) and
moderate- to high-substrate temperature (230–800
◦
C). The defect restoration is however
limited to monolayers that are nonetheless attractive for the realization of graphene-like
electrical properties. Using this approach has already demonstrated conductivities in the
order of ~10
3
(S/cm) and electron mobility ~900 cm
2
/V.s. To compare, a monolayer film
of CVD-graphene has electrical conductivity in the order of 10
4
S/cm [
270
,
271
] and an
R
C/O
of 20 [
272
]. Certain plasma reduced rGO have attained conductivities in the same
magnitude as that of certain highest reported for other techniques such as chemical as-
sisted thermal annealing (acetylene, 1000
◦
C, 2 h, 1425 S/cm) [
273
], joule-heating (>2000 K,
1500 S/cm) [
274
], and two-step treatments: thermal annealing (Ar, 1000 K, 1 h) and joule
heating (3000 K, 6300 S/cm), and chemical reduction (hydrazine monohydrate) and graphi-
tization (2700
◦
C, 5770 S/cm) [
220
]. These techniques compared to the plasma methods
are inferior for large-scale applications. In the case of reduction at a lower temperature
regime, which is particularly interesting for flexible applications, the rGO obtained from
the plasma method has demonstrated to be superior to those obtained from the thermal
method and is comparable to ones obtained from the chemical method [56,162,202].
In certain cases, a mixture of gases can enhance the reduction efficiency of plasma. For
instance, in a mixture of inert and reactive gases, active species of the former enhance the
dissociation degree of molecules of the reactive gas through penning ionization. Another
example: the dissociated hydrogen species (from hydrogen) carry out etching of GO,
while the carbon-containing active species (from methane) perform carbon deposition. To
optimize such plasma processes, it is crucial to find the right combination of the discharge
regime and the ratio of constituent gases for the best cooperative effect. Finally, it is
challenging to formulate a single optimal strategy for plasma-assisted reduction of GO
just based on the reviewed literature. This arises from the variations (thickness, initial
R
C/O
, the density of defects) of GO-precursors used, different plasma instrumentation and
conditions, diversity of measurements, etc. Nevertheless, we collected in this work the
factual material, which is crucial for understanding the principles of plasma reduction of
graphene oxide.
Nanomaterials 2021,11, 382 22 of 37
5. Outlook
As Eigler et al. [
124
] pointed out, the quality or defectiveness of precursor-GO material
influenced by the preparation conditions limits the potential of reduction. The use of large-
sized (>10
µ
m) GO flakes is beneficial over smaller ones due to reduced defect density and
improved electrical, thermal, and mechanical properties [
54
,
275
]. Additionally, theoretical
studies have shown that the proportion between hydroxyl- and epoxide-groups and the
total oxygen coverage of initial GO affect the outcome of a reduction process [
222
,
276
].
Therefore, improved control of the state-of-the-art GO syntheses and further processing
techniques will certainly improve the effectiveness of the subsequent plasma reduction
process. Independently, a basic understanding of the reaction mechanisms in plasma
and therefore control of its energetic and chemical footprint will allow mastering the GO
reduction. This is valid and even more essential for the simultaneous reduction and doping,
as well as for the defect healing with sp2-carbon restoration.
A vast majority of the reports are based on low-pressure plasma processes. Break-
throughs with low-temperature and atmospheric-pressure plasma for the same purpose
will not just present an opportunity to lower associated costs but importantly facilitate
integration with roll-to-roll processing. Low-temperature plasma reduction also will enable
novel and niche applications (e.g., in monolithic CIGSe–Perovskite tandem solar cell, as
mentioned earlier). Graphene materials are emerging in commercial products, breaking
their confinement to academic and research prototypes [
277
]. Among the graphene-related
materials, GO has its significance. According to a 2019 report [
278
], GO occupies 38% of the
market fraction (graphene nanoflakes—52%; CVD-films 10%) with the global production
capacity (~784 tons/year in 2017) displaying an increasing trend over the past decade. The
global reserve of recoverable graphite exceeds 800 million metric tons as per the 2020 U.S.
geological survey [
279
]. The abundance of raw material reserves along with low-cost and
scalable production is encouraging to explore GO/rGO based applications. The amount of
scientific research dedicated to plasma technologies has been tremendous over the past
decades. However, plasma for GO-reduction has not gained as much substantial atten-
tion as the chemical and thermal methods. In today’s material processing world, plasma
technology is at the forefront. If backed by persistent research, it is a strong contender for
facilitating large-scale applications of rGO.
Author Contributions:
Conceptualization and writing—original draft preparation: S.H.B.V.K. and
R.M.; writing—review and editing: S.H.B.V.K., R.M. and B.S.; supervision and funding acquisition:
B.S. and R.M. All authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the German Federal Ministry for Economic Affairs and Energy
(BMWi) under contract number 0324095H (speedCIGS). The authors acknowledge support by the
German Research Foundation and the Open Access Publication Fund of Technische Universität Berlin.
Conflicts of Interest: The authors declare no conflict of interest.
Nanomaterials 2021,11, 382 23 of 37
Abbreviations
2D two dimensional
σconductivity
µ-APPJ micro atmospheric pressure plasma jet
A ampere
AC alternating current
AGD atmospheric-pressure glow discharge plasma
APPJ atmospheric pressure plasma jet
AS active screen
CCP capacitively coupled plasma
CIGSe Cu(In, Ga)(Se, S)2solar cell
CVD chemical vapor deposition
DBD dielectric barrier discharge
DC direct current
FET field effect transistor
GO graphene oxide
hr hour
ICP inductively coupled plasma
kΩkilo-Ohm
min minute
mTorr milli-Torr
MΩmega-Ohm
MW microwave
RF radio frequency
rGO reduced graphene oxide
s second
sq square
V volt
V/V volume/volume
W watt
Nanomaterials 2021,11, 382 24 of 37
Appendix A
Table A1.
Overview of various plasma used for the reduction of GO. Temperature in the “Parameters” column denotes auxiliary heated substrate temperature, when a temperature is not specified no
external heating was used. In the “Characteristic changes” column, time indicated in brackets correspond to the reduction duration.
Serial
No.
Year &
Reference
Plasma Characteristic Changes Application
Discharge Gas Generation Time Parameters
1 2007 [150] H2– 5 s 30 W, 0.8 mbar σ: 0.05–2 S/cm, ambipolar
charge transport FET
2 2010 [203] 10% CH4+ Ar Electron beam 30 s
−
2 kV pulse at linear hollow
cathode with 150 G
Helmholtz coil, 90 mTorr
Controlled reduction with atomic
oxygen varied between 43% and 5% –
3 2011 [188] Ar RF 5–40 min 25 W, 0.02 mbar, 20 sccm
>2 orders lower surface resistivity.
Etching effect with
prolonged duration
–
4 2012 [93] CH4RF 10 min 100 W, 0.20 Torr, ~575 ◦Cσ: 1590 S/cm, ID/IG: ~0.53 –
5 2012 [202] Ar/H2(1:1) DC 30 min
~2 kV, 10 mA discharge
current, atm. pressure,
150 ◦C
Sheet resistance 47.7 kΩ/sq (~6 µm),
C/O ratio 6.95 –
6 2013 [192] NH3RF 3 min 200W, 500 mTorr, 150 ◦C,
400 sccm
~6% N-doped r-GO,
σ
: 7.4 S/cm, C/O
ratio ~9.4 ID/IG: 0.98 (GO:1.09) –
7 2013 [193] H2RF <30 s 200W, 20 mTorr, 150 ◦C,
300 sccm
σ: 2 S/cm (18 s) and C/O ratio of ~14
(30 s), >18 s exposure causes trade-off
between σ& reduction
–
8 2013 [280] H2– <10 min 2 W, 1 Torr
Reduction in sheet resistance by
3~4 orders of magnitude that could be
restricted to uppermost layers on
short exposure
FET
9 2013 [198] NH3DC 1–20 min 10–30 W, ~1 Pa
C/O ratio: ~ 6.66, N/C ratio: 15%,
work-function change from 4.4 to
3.4 eV, σ: 1–80 S/cm (5 min)
–
10 2013 [191](a) NH3(b) H2+ 10%
Ar -(a) 4.5–10 min
(b) 8.5 min 160 W, 8 sccm
(a) µe: 5.41 cm2/V.s (8.5 min) and µh:
2.1 cm2/V.s (5.5 min) (b) σ: 630 S/cm
for (8.5 min)
FET
11 2014 [194] H2RF 40 s 10 W, 0.3 mbar, 20 sccm
C/O ratio ~7.9, I
D
/I
G
: 0.81 (GO: 0.94),
71% (15%) response at 1500 ppm in
N2(in air)
CO2gas-sensor
Nanomaterials 2021,11, 382 25 of 37
Table A1. Cont.
Serial
No.
Year &
Reference
Plasma Characteristic Changes Application
Discharge Gas Generation Time Parameters
12 2014 [199] H2DC 10 s–5 min
15 & 30 W, working pressure
~50 Pa, 50–120 ◦C
σ: 0.2–31 S/cm & µh: 0.1–6 cm2/V.s
(15 W, 50 ◦C, 30 s) FET
13 2014 [195] NH3RF 30 min 10 W, 100 mTorr σ: 1666 S/m, C/O ratio ~4.16 and
N/C ratio 9.3%, I
D
/I
G
: 1.84 (GO: 2.22)
–
14 2014 [56] He AGD 2 s
−10 kV, discharge current
~1.5–1.9 mA, atmospheric
pressure (positive-column
plasma)
σ: 5900 S/m, C/O ratio 7.6, surface
area: 371 m2/g, specific capacitance
161.6 F/g (1 A/g)
Supercapacitor
15 2014 [190] H2/Ar (various ratio) RF 3–10 min 20–100 W, 4.7 Pa
C/O ratio: 9.6 (100 W, H2/Ar-2:1, 5
min), specific capacitance 185.2 F/g
(100 mV/s) for 70 W, H
2
/Ar-2:1, 5min
(C/O ratio 4.2)
Supercapacitor
16 2015 [200] Ar DC 4 min
~10 kV at cathode, 240 mTorr
C/O ratio ~7.9, ID/IG: 0.85, specific
capacitance of 190 F/g (10 mV/s) Supercapacitor
17 2015 [196] H2RF 30 min 60 W σ: 3.1 S/cm, µ: 37.5 cm2/V.s –
18 2015 [204] 25% N2+ H2AS 60 min
4 mbar in a traditional 40 kW
plasma nitriding unit,
100–200 ◦C
Doping of N from gas discharge and
Fe, Cr, and Mo elements from steel
mesh, reduction in resistance from
12.6 MΩto 50 kΩ(200 ◦C)
–
19 2015 [197] NH3RF 1–40 min 1 kW/m2,500 mTorr C/O ratio ~2.7 and N/C ratio ~17%
(30 min), σ: 80 S/m (30 min) –
20 2016 [221] H2+ CH4RF 5–30 min 100 W, 240 ◦C
All oxygen containing groups were
removed except ~1.2%, ID/IG: 0.83,
RSH: 15 k Ω/sq
–
21 2016 [178] (a) Ar (b) N2RF 10 min
100 W, 0.3 Pa, sample biased
at 0 V and sample bias from
−50 to −300 V
N2-plasma was more effective in
reduction (at all sample bias) with
relatively higher ion
penetration depth
–
22 2017 [207]H2, CH4,&H2/CH4
(1:1) DBD 1–10 s 1.9 W/cm2, each gas-flow at
0.2 mL/min
~40% sp2—Carbon restored.
Oxidation level in GO reduced from
~50% to ~10% (H2/CH4)
–
Nanomaterials 2021,11, 382 26 of 37
Table A1. Cont.
Serial
No.
Year &
Reference
Plasma Characteristic Changes Application
Discharge Gas Generation Time Parameters
23 2017 [179] 10% CH4+ H2RF ~1 min
Thermal annealing (Ar,
1000
◦
C, 30 min) followed by
plasma treatment: 200 W,
0.34 Torr, 700–900 ◦C
improvement in µfrom
0.01–1 cm2/V.s to 50 cm2/V.s., ID/IG:
0.56 (GO:1.04)
–
24 2017 [259] 20% H2+ C2H2- 2 min
Thermal annealing (vacuum,
700 ◦C, 7 hrs.) followed by
plasma treatment: 20 W,
50 sccm total flow
µhall: ~90 cm2/V.s, RSH: 510 Ω/sq
(5 nm), C/O ratio 10.9, ID/IG: 1.33
(GO:0.97)
Memory device
25 2017 [180] 5% CH4+ Ar RF 1–20 min 700 W/m2, 500 mTorr transmittance from 91.9% to ~94% at
600 nm & 8.13 MΩ/sq (10 min) –
26 2018 [181] (a) NH3(b) N2RF 10 min 100 W, 0.3 Pa, sample biased
at 0 V and −50 to −350 V
Both plasmas incorporated N into GO,
NH3was more effective in reduction
of O-groups
–
27 2018 [208] H2DBD 1–64 s 40 W/cm3,atm. pressure Resistance reduced to 2 MΩ/sq, C/O
ratio of ~4.76 (16 s) –
28 2018 [205] Air RF 10–120 s
300 W, 20 kHz, ~10 kV (APPJ
plasma)
RSH: 186 Ω/sq (25µm), vol.
capacitance 536.55 F/cm3(1 A/g) Supercapacitor
29 2018 [184] H2/CH4(1:1) RF 10–120 min
10 W, 9.7–9.8 Pa, H2& CH4:
each at 35 sccm flow,
550–650 ◦C with Cu catalyst
σ: 930 S/cm, µe: 480 cm2/V.s,
ID/IG: ~0.4 FET
30 2018 [183] H2RF 30 min 20–60 W Rise in optical bandgap energy with
increase in O-vacancy –
31 2018 [201] H2DC 1 min 15 W, ~0.3 Torr
Threshold of device reduced from 2.6
to 1.8 V, On/Off ratio improved from
~80 to ~103
Memory device
32 2018 [185] CH4/Ar (1:2 to 2:1) RF 1–10 min 100 W, 50 mTorr, total gas
flow 30 sccm
C/O ratio improved from 2.2 to 10.6 &
σ: 264 S/m (5 min) –
Nanomaterials 2021,11, 382 27 of 37
Table A1. Cont.
Serial
No.
Year &
Reference
Plasma Characteristic Changes Application
Discharge Gas Generation Time Parameters
33 2019 [189] H2/CH4(35:1 to 1:35) RF ~0.16–9 hrs
10 W, 9.7–9.8 Pa, total flow
350 sccm, 550 ◦C with Cu
catalyst
Moderate restored r-GO in ~15 min
for CH4rich condition, high
crystallinity for H2rich condition: µe:
900 cm2/V.s, ID/IG: 0.17
–
34 2019 [186] 20% C2H2+ NH3- 3 min
annealing at 700 ◦C (30 min,
10−4mbar); followed by 3
min plasma: 20 W, 2 mbar,
50 sccm total flow
C/O ratio 8.34, µe: 60–80 cm2/V.s,
photoresponsivity of 0.68 A/W at 1 V Photodetector
35 2019 [187] 20% H2+ N2RF 5–80 min
1400 W at ~370 kHz, 50 sccm
total flow, sample with 0 and
−35 V bias
DC bias contributed to efficient
reduction and recovery of GO (lowest
ID/IGratio). Resistance change
between ~425–570 kΩfor
0–1.5% strain
Stress sensor
36 2019 [162] Ar RF 30–120 min 300 W, 650 mTorr
resistance of 2 k
Ω
/sq (120 min), mean
sensor sensitivity of 277 ±
80 µA/mM.cm2; Pt reference
5.2 ±0.51 µA/mM.cm2
H2O2sensor
37 2020 [206]
1% He + N
2
& He + H
2RF 5 s–3 min 4 W, flow: 2000 sccm N2&
16 sccm He (µAPPJ plasma)
Sheet resistance: ~4 MΩ/sq (He +
N2), ~0.24 MΩ/sq (He + H2); GO
~138 MΩ/sq
–
Nanomaterials 2021,11, 382 28 of 37
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