The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide
The kinetics of the sorption and the subsequent desorption of ⁴He by the starting graphite oxide (GtO) and the thermally reduced graphene oxide samples (TRGO, T reduction = 200, 300, 500, 700 and 900 °C) have been investigated in the temperature interval 1.5–20 K. The effect of the annealing tempera...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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| Цитувати: | The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide / A.V. Dolbin, M.V. Khlistuck, V.B. Esel’son, V.G. Gavrilko, N.A. Vinnikov, R.M. Basnukaeva, A.I. Prokhvatilov, I.V. Legchenkova, V.V. Meleshko, W.K. Maser A.M. Benito // Физика низких температур. — 2017. — Т. 43, № 3. — С. 471-478. — Бібліогр.: 39 назв. — англ. |
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irk-123456789-1294222018-01-20T03:03:21Z The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide Dolbin, A.V. Khlistuck, M.V. Esel’son, V.B. Gavrilko, V.G. Vinnikov, N.A. Basnukaeva, R.M. Prokhvatilov, A.I. Legchenkova, I.V. Meleshko, V.V. Maser, A.M. Benito, W.K. Наноструктуры при низких температурах The kinetics of the sorption and the subsequent desorption of ⁴He by the starting graphite oxide (GtO) and the thermally reduced graphene oxide samples (TRGO, T reduction = 200, 300, 500, 700 and 900 °C) have been investigated in the temperature interval 1.5–20 K. The effect of the annealing temperature on the structural characteristics of the samples was examined by the x-ray diffraction (XRD) technique. On lowering the temperature from 20 to 11–12 K, the time of ⁴He sorption increased for all the samples, which is typically observed under the condition of thermally activated diffusion. Below 5 K the characteristic times of ⁴He sorption by the GtO and TRGO-200 samples were only weakly dependent on temperature, suggesting the dominance of the tunnel mechanism. In the same region (T < 5 K) the characteristic times of the TRGOs reduced at higher temperatures (300, 500, 700 and 900 °C) were growing with lowering temperature, presumably due to the defects generated in the carbon planes on removing the oxygen functional groups (oFGs). The estimates of the activation energy ( Ea) of ⁴He diffusion show that in the TRGO-200 sample the Ea value is 2.9 times lower as compared to the parent GtO, which is accounted for by GtO exfoliation due to evaporation of the water intercalated in the interlayer space of carbon. The nonmonotonic dependences Ea( T) for the GtO samples treated above 200 °C are determined by a competition between two processes—the recovery of the graphite carbon structure, which increases the activation energy, and the generation of defects, which decreases the activation energy by opening additional surface areas and ways for sorption. The dependence of the activation energy on T reduction correlates well with the contents of the crystalline phase in GtO varying with a rise of the annealing temperature. 2017 Article The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide / A.V. Dolbin, M.V. Khlistuck, V.B. Esel’son, V.G. Gavrilko, N.A. Vinnikov, R.M. Basnukaeva, A.I. Prokhvatilov, I.V. Legchenkova, V.V. Meleshko, W.K. Maser A.M. Benito // Физика низких температур. — 2017. — Т. 43, № 3. — С. 471-478. — Бібліогр.: 39 назв. — англ. 0132-6414 PACS: 68.43.Jk, 68.43.Mn http://dspace.nbuv.gov.ua/handle/123456789/129422 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
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English |
| topic |
Наноструктуры при низких температурах Наноструктуры при низких температурах |
| spellingShingle |
Наноструктуры при низких температурах Наноструктуры при низких температурах Dolbin, A.V. Khlistuck, M.V. Esel’son, V.B. Gavrilko, V.G. Vinnikov, N.A. Basnukaeva, R.M. Prokhvatilov, A.I. Legchenkova, I.V. Meleshko, V.V. Maser, A.M. Benito, W.K. The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide Физика низких температур |
| description |
The kinetics of the sorption and the subsequent desorption of ⁴He by the starting graphite oxide (GtO) and the thermally reduced graphene oxide samples (TRGO, T reduction = 200, 300, 500, 700 and 900 °C) have been investigated in the temperature interval 1.5–20 K. The effect of the annealing temperature on the structural characteristics of the samples was examined by the x-ray diffraction (XRD) technique. On lowering the temperature from 20 to 11–12 K, the time of ⁴He sorption increased for all the samples, which is typically observed under the condition of thermally activated diffusion. Below 5 K the characteristic times of ⁴He sorption by the GtO and TRGO-200 samples were only weakly dependent on temperature, suggesting the dominance of the tunnel mechanism. In the same region (T < 5 K) the characteristic times of the TRGOs reduced at higher temperatures (300, 500, 700 and 900 °C) were growing with lowering temperature, presumably due to the defects generated in the carbon planes on removing the oxygen functional groups (oFGs). The estimates of the activation energy ( Ea) of ⁴He diffusion show that in the TRGO-200 sample the Ea value is 2.9 times lower as compared to the parent GtO, which is accounted for by GtO exfoliation due to evaporation of the water intercalated in the interlayer space of carbon. The nonmonotonic dependences Ea( T) for the GtO samples treated above 200 °C are determined by a competition between two processes—the recovery of the graphite carbon structure, which increases the activation energy, and the generation of defects, which decreases the activation energy by opening additional surface areas and ways for sorption. The dependence of the activation energy on T reduction correlates well with the contents of the crystalline phase in GtO varying with a rise of the annealing temperature. |
| format |
Article |
| author |
Dolbin, A.V. Khlistuck, M.V. Esel’son, V.B. Gavrilko, V.G. Vinnikov, N.A. Basnukaeva, R.M. Prokhvatilov, A.I. Legchenkova, I.V. Meleshko, V.V. Maser, A.M. Benito, W.K. |
| author_facet |
Dolbin, A.V. Khlistuck, M.V. Esel’son, V.B. Gavrilko, V.G. Vinnikov, N.A. Basnukaeva, R.M. Prokhvatilov, A.I. Legchenkova, I.V. Meleshko, V.V. Maser, A.M. Benito, W.K. |
| author_sort |
Dolbin, A.V. |
| title |
The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide |
| title_short |
The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide |
| title_full |
The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide |
| title_fullStr |
The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide |
| title_full_unstemmed |
The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide |
| title_sort |
effect of the thermal reduction on the kinetics of low-temperature ⁴he sorption and the structural characteristics of graphene oxide |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2017 |
| topic_facet |
Наноструктуры при низких температурах |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/129422 |
| citation_txt |
The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide / A.V. Dolbin, M.V. Khlistuck, V.B. Esel’son, V.G. Gavrilko, N.A. Vinnikov, R.M. Basnukaeva, A.I. Prokhvatilov, I.V. Legchenkova, V.V. Meleshko, W.K. Maser A.M. Benito // Физика низких температур. — 2017. — Т. 43, № 3. — С. 471-478. — Бібліогр.: 39 назв. — англ. |
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Физика низких температур |
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 3, pp. 471–478
The effect of the thermal reduction on the kinetics
of low-temperature 4He sorption and the structural
characteristics of graphene oxide
A.V. Dolbin, M.V. Khlistuck, V.B. Esel’son, V.G. Gavrilko, N.A. Vinnikov,
R.M. Basnukaeva, A.I. Prokhvatilov, I.V. Legchenkova, and V.V. Meleshko
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine
47 Nauky Ave., Kharkiv 61103, Ukraine
E-mail: dolbin@ilt.kharkov.ua
W.K. Maser and A.M. Benito
Instituto de Carboquímica, 4, ICB-CSIC Miguel Luesma Castán, 4 E-50018 Zaragoza, Spain
Received October 5, 2016, published online January 24, 2017
The kinetics of the sorption and the subsequent desorption of 4He by the starting graphite oxide (GtO) and
the thermally reduced graphene oxide samples (TRGO, Treduction = 200, 300, 500, 700 and 900 °C) have been in-
vestigated in the temperature interval 1.5–20 K. The effect of the annealing temperature on the structural charac-
teristics of the samples was examined by the x-ray diffraction (XRD) technique. On lowering the temperature
from 20 to 11–12 K, the time of 4He sorption increased for all the samples, which is typically observed under
the condition of thermally activated diffusion. Below 5 K the characteristic times of 4He sorption by the GtO and
TRGO-200 samples were only weakly dependent on temperature, suggesting the dominance of the tunnel mech-
anism. In the same region (T < 5 K) the characteristic times of the TRGOs reduced at higher temperatures (300,
500, 700 and 900 °C) were growing with lowering temperature, presumably due to the defects generated in
the carbon planes on removing the oxygen functional groups (oFGs). The estimates of the activation energy (Ea)
of 4He diffusion show that in the TRGO-200 sample the Ea value is 2.9 times lower as compared to the parent
GtO, which is accounted for by GtO exfoliation due to evaporation of the water intercalated in the interlayer
space of carbon. The nonmonotonic dependences Ea(T) for the GtO samples treated above 200 °C are deter-
mined by a competition between two processes — the recovery of the graphite carbon structure, which increases
the activation energy, and the generation of defects, which decreases the activation energy by opening additional
surface areas and ways for sorption. The dependence of the activation energy on Treduction correlates well with
the contents of the crystalline phase in GtO varying with a rise of the annealing temperature.
PACS: 68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation;
68.43.Mn Adsorption kinetics.
Keywords: graphene oxide, thermal reduction, kinetics of 4He sorption, structural characteristics.
1. Introduction
Graphene is a two-dimensional system of carbon atoms
in which three electrons of each C atom form strong hyb-
ridized sp2 bonds in the plane that build a honeycomb struc-
ture; the fourth π-electron is smeared below and above
the carbon layer. The π-electrons are very important as
they form π-electron bonds and largely determine the pro-
perties of multilayered graphenes and graphene oxides [1,2].
Owing to its two-dimensional structure and electron hy-
bridization, graphene possesses unique properties, such as
ballistic electrical conductivity and high intrinsic carrier
mobility (200 000 cm2/(V·s) [3,4], high thermal conductiv-
ity ~ 5000 W/(m·K) [5], extraordinarily high mechanical
characteristics (Young’s modulus of ~1.0 TPa [6]) and
the quantum Hall effect [7]. Besides, graphene has a large
© A.V. Dolbin, M.V. Khlistuck, V.B. Esel’son, V.G. Gavrilko, N.A. Vinnikov, R.M. Basnukaeva, A.I. Prokhvatilov, I.V. Legchenkova,
V.V. Meleshko, W.K. Maser, and A.M. Benito, 2017
mailto:dolbin@ilt.kharkov.ua
A.V. Dolbin et al.
specific surface area (2630 m2/g) and can be used as a high-
ly efficient sorbent. Graphene can serve as a basis for other
allotropic carbon modifications: it can be rolled up to form
0D fullerenes, or twisted to produce 1D carbon nanotubes,
or packed as 3D graphite [2]. At present there exist a large
variety of mechanical [8], physical [9] and chemical [10]
methods for obtaining graphene. The modified Hummers
method [11,12] is most advantageous for cost-effective
large-scale production of graphene oxide (GO) through
oxidation-induced exfoliation of graphite. However, the re-
sulting graphene sheets are greatly overloaded at both sides
with oxygen inherent in the added hydroxyl, epoxy, car-
boxyl and other oxygen functional groups (oFGs) [13,14].
The cFGs contents in GO can be decreased considerably
through chemical and thermal treatment [15,16]. GO holds
much promise as a stock for large-scale production of
graphene-based materials and their widespread applica-
tions in advanced industrial technologies, such as photo-
voltaic cells, capacitors, sensors and transparent conduc-
tive electrodes [2,17–19]. Owing to their advantageous
characteristics, such as low density, chemical stability di-
versity of structural forms, easy-to-modify porous struc-
ture, surface susceptible to a variety of treatment tech-
niques and relatively simple technologies of industrial-
scale production, graphene-based materials hold consider-
able potential for development of gas storage technologies,
adsorption/desorption of impurity particles among them.
Previously we investigated the sorption and the subse-
quent desorption of 4He, H2, Ne, N2, CH4 and Kr by glu-
cose- and hydrazine-reduced GO [20]. It was found that in
the temperature interval 2–290 K, the temperature depend-
ences of the diffusion coefficients of light impurities (hyd-
rogen and helium) were controlled by a competition be-
tween the thermally activated and tunnel mechanisms of
diffusion. The contribution of the tunnel process is domi-
nant at low temperatures, which makes the diffusion coef-
ficients practically independent of temperature. However,
the tunnel effects were less pronounced for heavier impuri-
ties (N2, CH4 and Kr). Tunnel diffusion of light impurities
at low temperatures was also observed in other carbon
nanostructures, specifically in carbon nanotubes [21,22]
and fullerite C60 [23–25]. This led us to assume that the
kinetics of gas sorption by carbon nanostructures was sig-
nificantly influenced by the potential relief of their surfac-
es. Furthermore, it was found that oFGs also had a signifi-
cant effect on the sorption properties of graphene oxide [20]:
the sorption capacity of GO increased three- to six-fold
after removing oFGs through hydrazine reduction. The re-
sult suggests that the removal of oFGs via hydrazine reduc-
tion unblocks the interlayer space in GO and allows the gas
impurities to penetrate inside through the defects on the
graphene surface.
The condition of the carbon surface of GO depends
strongly on the type, quantity and distribution of oFGs, the
degree and the method of their removal (GO reduction) as
well as on the quantity and the character of defects gener-
ated by the oFGs removal. Investigation of the reduction
effect on the sorption characteristics of GO will help us to
trace the structural evolution during GO reduction and to
find the ways of modifying the RGO (reduced GO) proper-
ties. There exist two basic approaches — chemical and
thermal — that can ensure a high degree of reduction.
At present thermally reduced graphene (TRGO) is pre-
pared from graphite oxide (GtO) [26,27] which is in turn
obtained from graphite using various chemical oxidizing
agents [11,28,29]. The thermal reduction of graphene at-
tracts interest because, on the one hand, it offers the advant-
age of cost-effective large-scale production of graphene and,
on the other hand, the thermally reduced graphene is free
from the chemical residuals unremovable in GO powders
and films [27,30]. Note that on chemical reduction some of
the GO sheets in solutions contain chemically strong bases
(for example, hydrazine) and are therefore unsuitable for
biological and medical applications [22,31]. However,
thermal reduction is, a complex process involving a ther-
mally activated multistep removal of intercalated H2O mo-
lecules and oxide functional carboxyl (COOH), hydroxyl
(C–OH) and epoxy (C=O) groups, including interlayer
epoxy (C–O–C) and single-bonded (C=O) ones at the car-
bon plane surface. The functional groups are located both
on the graphene surface and at the plane edges. The reduc-
tion process is aimed at removing oFGs and obtaining
a carbon structure with the desired properties. It should be
noted that oxidizing treatment generates numerous defects
and surface ruptures in GO. In the course of GO reduction
the number of imperfections increases since the removal of
oFGs often strips the carbon atoms off the graphene plane.
Besides, the C–Ox type sp3-bonded oxide groups can be
present inside the defects of GO and GtO. The interlayer
spacings are considerably larger in GO than in GtO due to
the intercalated H2O molecules and the presence of various
oxide groups (9–12 Å depending on the method of GO
preparation and the amount of the intercalated water against
7 Å for GtO [11,32]).
Previously the sorbed quantities of 4He, H2, Ne, N2 and
Kr were investigated as a function of the thermal reduction
temperature of GO [33,34]. The largest quantities of these
impurities were sorbed by the samples reduced at 300 and
900 °C. It was assumed that the sorption capacity of the
samples reduced at 300 °C grew higher because of the dis-
ordering of the layered GO structure inflicted by water
deintercalation. The higher sorption capacity of the sample
heated to 900 °C was attributed to the generation of nu-
merous defects at the carbon surfaces on removing the
oFGs, which allowed the gas impurities to penetrate into
the space between the folds and sheets of the graphene
structure [33].
Here we have investigated the effect of the reduction
temperature upon the kinetics of the low-temperature 4He
sorption and the structural characteristics of graphene oxide.
472 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 3
The effect of the thermal reduction on the kinetics of low-temperature 4He sorption
2. Samples and their characterization by the x-ray
diffraction method
The starting graphite oxide (GtO) was prepared from
graphite powder (Sigma–Aldrich) using the modified Hum-
mers method and vigorous oxidizing agents (NaNO3,
H2SO4 and KMnO4) [11,35]. The resulting product was
then thermally treated under Ar atmosphere to prepare
thermally reduced graphene oxide (TRGO). Five samples
were obtained by heating the starting GtO material at 200,
300, 500, 700 and 900 °C, respectively [33]. The GtO and
reduced TRGO samples were powders with an average grain
size about 10 μm, the mass of each sample was ~40 mg.
The effect of the annealing temperatures upon the struc-
tural characteristics of GtO, was investigated by the x-ray
diffraction (XRD) method in Cu-Kα radiation using a
DRON-3 diffractometer. The obtained XRD patterns (Fig. 1)
are similar qualitatively to those in [33] but for a few dis-
tinctions (a detailed analysis and a comparison of the XRD
patterns with other data were rather difficult because of the
lack of quantitative data processing in [33]).
It is seen that oxidation causes serious damage to the
graphite structure (Fig. 1, T = 20 °C).The intensive sharp
peaks (002) in the region 2Θ = 26° produced by the basal
planes of the hexagonal close-packed lattice of pure graph-
ite disappear. Instead, the starting GtO exhibits a broad-
ened asymmetric medium-intensity line. The considerable
asymmetry of the reflection most likely betokens the pres-
ence of another carbon phase. The analysis of the reflec-
tions shows that the obtained diffractogram can be de-
scribed quite adequately by a sum of the intensities of the
two lines (Fig. 2) corresponding to diffraction from two
nanocarbon phases having significantly different degrees
of crystallinity. The small-angle reflection has the structural
parameters that are typical for amorphous states. The angu-
lar smearing of the line runs to several degrees, which
points to severe distortions of the structure in the short-
range order region of the basal planes causing numerous
static interlayer displacements ∆d00l in this phase as com-
pared to the mean interlayer spacing in pure graphite.
The second phase of GtO has a higher degree of crystal-
linity. Like pure graphite, it has a hexagonal close-packed
(HCP) structure. But the diffraction reflections from this
phase are broader than those from graphite single crystals,
being however almost three times narrower in comparison
with the disordered latent-crystalline (amorphous) modifi-
cation (Fig. 2). These facts suggest that the crystalline
phase of the starting graphite is not perfect structurally.
The XRD results on the effect of high temperature an-
nealing upon the structural characteristics of GtO and
TRGO are shown in Figs. 1 and 3–7. The general outline in
Fig. 1 suggests that a rise of the annealing temperature
causes appreciable transformations in the structure of
Fig. 1. The XRD patterns of GtO powder at different annealing
temperatures (indicated at the end of each XRD pattern). The
XRD patterns taken in Cu-Kα radiation contain (111) and (200)
reflections from a copper substrate.
Fig. 2. The separated intensity contributions of XRD from the
(002) planes of the crystalline GtO and “amorphous” GO phases
at room temperature (20 °C).
Fig. 3. The averaged interplane spacings d002 in the crystalline
() and amorphous (•) phases of the starting GtO and TRGO
at different annealing temperatures. Comparison with the data
of [15] ().
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 3 473
A.V. Dolbin et al.
the samples, which results in reduction of graphite. The
diffraction reflections from the basal planes (002) of both
phases shift towards the larger angle region and their width
and asymmetry decrease. It was surprising to observe a
distinct narrow reflection (Fig. 1) at Tanneal = 500 °C in the
angular region 2Θ = 36°, which had nothing to do with
either crystalline or amorphous graphite. Among the known
carbon forms, only cubic and hexagonal diamonds can
produce reflections at these angles. It is then reasonable to
assume that the annealing of GtO at T ≥ 500 °C causes sp3
hybridization of carbon in individual regions of the GtO
(graphene) crystallites. However, this assumption did not
agree with the subsequent results: the line in question was
absent at Tanneal = 700 °C but reappeared at T = 900 °C
(Fig. 1). Thus, the nature of the coherent diffraction at 36°
remains uncertain and calls for further investigation by
XRD and other methods, including spectroscopy.
The quantitative changes in the structural parameters
(interplanar spacing d002, integral intensity and diffraction
line half-width) of both phases are illustrated in Figs. 3–6.
As the annealing temperature rises, the parameter magni-
tudes decrease for both the phases, then approach each
other (Figs. 3 and 6) and at T > 500 °C reach the values
typical for pure graphite. The structural characteristics of
the latent crystalline (amorphous) phase are particularly
sensitive to the annealing temperature. Note that heating
to T = 900 °C does not ensure complete reduction of gra-
phite. Even at this high temperature the samples still con-
tain an appreciable quantity of the amorphous phase form-
ed at the starting stage of graphite oxidation. Moreover, as
is evident from our results and the data of [15], the tempera-
ture dependence of the structural parameters (interplanar
spacing d200 (Fig. 3) and the XRD reflection half-widths
(Fig. 5)) suffers inversion at T > 500 °C presumably due to
the thermal defects formed and accumulated in the carbon
sublattice. However, this process has only a minor effect
on the intensity of diffraction from the crystalline phase
and its quantity increases monotonically, as against the
amorphous phase, in the whole interval of annealing tem-
peratures (Fig. 7).
Fig. 4. The effect of annealing temperatures on the integral inten-
sity of XRD reflections (002) from the crystalline (▲) and amor-
phous (•) phases of GtO and GO.
Fig. 6. The effect of the GtO annealing temperatures on the dif-
ference between the interplanar spacings d002 in the amorphous
and crystalline phases.
Fig. 5. Variations of the maxima half-widths of scattering diffrac-
tion from the (002) planes of the crystalline () and amorphous
(•) phases of graphite oxide at different annealing temperatures.
Symbols are XRD data of [15].
Fig. 7. The growth of the crystalline GtO phase with increasing
annealing temperature characterized by a ratio of integral intensi-
ties Icryst/Iamorph.
474 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 3
The effect of the thermal reduction on the kinetics of low-temperature 4He sorption
It is known that thermal treatment is one of the efficient
methods of obtaining reduced graphene oxide. We believe
that the amorphous phase observed at high annealing tem-
peratures contains a great quantity of multilayered graphene,
or rather TRGO, flakes. The assumption is consistent with
our results (see above) on the structural parameters of the
latent crystalline (amorphous) phase which in turn agree
well qualitatively and quantitatively with literature XRD
data [15,16,33] (Figs. 3–8). At T > 900 °C the carbon
phase is almost completely reduced, the oxigen contents
being below 3% (see Fig. 8).
Besides, our XRD patterns exhibit a weak feature in the
region 2Θ = 10°, which can be interpreted as x-ray diffrac-
tion by the quasi-two-dimensional carbon graphene phase.
This value (10°) corresponds to (001) of graphene oxide
with the intercalated water, and in our case, perhaps is due
to the presence of trazes of nonreduced graphene oxide.
Unfortunately, the growing background of the initial x-ray
beam in this region of diffraction angles makes it difficult
to characterize this phase quantitatively.
3. Experimental desorption technique
The kinetics of sorption and desorption of 4He gas by
the starting graphite oxide and thermally reduced graphene
oxide was investigated through measuring the time —
pressure dependence of the gas contacting the sample in
a closed vessel. The experimental technique and equipment
are detailed in [23,36,37]. Prior to investigation each sam-
ple was placed into a measuring cell and kept in vacuum
no lower than 10–4 Torr for a week to remove gas impuri-
ties. The cell was washed three times with dry nitrogen for
a more efficient removal of water vapor. The impurity con-
tents in 4He gas were no more than 0.002%. The sample was
saturated with the 4He impurity under a pressure of ~1 Torr.
The lowest temperature of the experiment (up to 1.5 K)
was dictated by the setup design. In the whole range of the
experimental temperatures the 4He pressure in the measur-
ing cell was maintained 2.5–3 times lower than the satura-
tion vapor pressure for this impurity. The temperature con-
ditions prevented condensation of 4He vapor on the sample
surface and the cell walls. As the impurity was adsorbed,
new portions of 4He gas were added. The gas supply was
cut off on reaching the equilibrium pressure (10–2 Torr) in
the cell.
At each step of saturation and desorption measurement
the sample was kept at a pre-assigned invariant tempera-
ture. The pressure variations of the gas in the closed vessel
with the sample were measured during saturation/desorp-
tion using Baratron MKS capacitance pressure transducers,
the error being no more than 0.05%. When the process of
sorption was completed, the cell was hermetically sealed,
and the pressure variations were registered in the process
of the impurity desorption from the powder on its stepwise
heating. The gas impurity released on heating was taken in
portions to an evacuated calibrated vessel. The gas extrac-
tion from the samples was continued until the gas pressure
over the samples decreased to 10–2 Torr. Then the process
of desorption was repeated at the next temperature point.
4. Results and discussion
The kinetics of the helium sorption and the subsequent
desorption by the starting graphite oxide and graphene oxide
thermally reduced at different reduction temperatures
has been investigated in the temperature interval 1.5–20 K.
The obtained time dependences of the 4He pressure varia-
tions in the cell with the sample in the course of sorp-
tion/desorption are described well by the exponential one–
parameter function (τ) (see as an example the results at
T = 14 K, Fig. 9):
(1 exp ( / ))P A t∆ = − − τ . (1)
At a constant temperature the characteristic times of sorp-
tion/desorption measured on the same sample coincide with-
in the experimental error. The temperature dependences of
Fig. 8. The effect of annealing temperatures on the elemental GO
composition according to the XPS data of [16] (,) and [33]
(, ).
Fig. 9. The pressure variations in the course of 4He desorption
from the GtO sample (symbols) and their description with
the exponential function, Eq. (1) (line).
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 3 475
A.V. Dolbin et al.
the characteristic times of 4He sorption/desorption are il-
lustrated in Fig. 10 for GtO (Fig. 10(a)) and TRGOs
(Figs. 10(a), (b)). Note that in the whole interval of the
temperatures used in the experiment the measurement error
induced by the proper time taken by the gas phase to reach
the thermal equilibrium in the measuring gas system (therm-
alization time) was at least an order of magnitude lower for
all the samples than the measured characteristic times.
The times of 4He sorption increased for all samples when
the temperature was lowered from 20 K to about 11–12 K
(see Fig. 10). This behavior indicates that in this tempera-
ture interval the sorption is mainly controlled by thermally
activated diffusion of 4He atoms. On a further drop of tem-
perature the sorption times started to decrease and at
T < 5 K the characteristic times of 4He sorption by the GtO
and TRGO-200 samples were only little dependent on
temperature (Fig. 10(a)). The observed features suggest
that below 5 K the sorption/desorption rate is determined
by the dominant process of 4He atom tunneling between
the carbon planes of GO. The nonmonotonic behavior of
the temperature dependences of the characteristic times of
the 4He sorption by the GtO and TRGO-200 samples is
most likely accounted for by a competition between ther-
mally activated diffusion dominant at T > 12 K and the
tunneling process prevailing at low temperatures. Similar
effects were also observed while investigating the gas
sorption by fullerite C60, single-walled carbon nanotubes
[21,22] and chemically reduced graphene [20]. The ten-
dency for a growth of the characteristic times of 4He sorp-
tion with the thermal treatment temperature observed for
the TFGO-200 and TRGO-300 samples, as against the
starting GtO (Fig. 10(a)), is most likely related to the struc-
tural and morphological changes occurring as the water
intercalated in the interlayer spacings of carbon is evapo-
rated in the course of thermal treatment [33].
Below 5 K the characteristic times of GO samples
thermally reduced at higher temperatures (TRGO-300,
TRGO-500, TRGO-700 and TRGO-900) increased with
lowering temperature (Fig. 10). This can be due to the
growing quantity of defects in the carbon planes on ther-
mally stimulated removal of oxygen functional groups [33].
They form additional potential barriers impeding diffusion
at the defect locations and diminishing the probability of
tunneling.
The obtained τ values were used to estimate the coef-
ficients of helium diffusion into GtO and thermally re-
duced GO:
2
4
D ≈
τ
, (2)
where is the mean grain size of the GtO and TFGO pow-
ders (~10 μm); τ is the characteristic diffusion time. Since
the 4He atoms occupied the GO grains mainly along the
carbon planes, the proportionality coefficient (the denomi-
nator in Eq. (2)) of diffusion close to the 2D case was tak-
en to be equal to ~4.
The activation energy Ea of 4He diffusion in GO was
estimated by plotting the temperature dependence of the
diffusion coefficients in the ln D vs. 1 / T coordinates (see
Fig. 11, typical example for GtO and TRGO-200). Ea was
found through a linear approximation of the experimental
Fig. 10. The temperature dependences of the characteristic times of 4He sorption by (a) GtO, TRGO-200, TRGO-300 and (b) TRGO-
500, TRGO-700, TRGO-900 samples.
Fig. 11. The coefficients of 4He diffusion in GtO () and TRGO-
200 () samples. The lines mark the linear portions of the exper-
imental dependence corresponding to Eq. (3).
476 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 3
The effect of the thermal reduction on the kinetics of low-temperature 4He sorption
data in the ln D vs. 1 / T coordinates, Eq. (3). The proce-
dure was performed for the thermally activated portion of
the curve at each reduction temperature
0 exp a
B
ED D
k T
= −
, (3)
where D0 is the entropy factor depending on the frequency
of collisions between the matrix and impurity molecules,
kB is the Boltzmann constant. The obtained Ea values for
GtO and TRGO samples are shown in Fig. 12.
The anomalous behavior of the diffusion coefficients of
4He below 5 K observed for the TRGO-200 sample (such
behavior has typical for other thermally reduced samples)
may be caused by a transition of the impurity (4He) atoms
to the state of a two-dimensional quantum liquid [38].
According to XRD (see this study, Sec. 2) and Raman
spectroscopy [33] data, heating to T = 200 °C causes inten-
sive evaporation of the water intercalated in the interplanar
space of carbon and “exfoliation” of the GO sheets into
individual flakes [15]. The number of the interlayer cavi-
ties decreases and the influence of the other cavity wall on
the 4He atoms diminishes significantly. As a result, the
activation energy of 4He diffusion in the TRGO-200 sam-
ple drops sharply as against the starting GtO (Fig. 12).
Heating to higher temperatures triggers several processes
influencing the kinetics of helium sorption in the thermally
reduced samples. Firstly, with oFGs removed, the neigh-
boring graphene flakes “stick” together again under the
Van der Waals force [15,39]. Besides, heating encourages
relaxation of mechanical stresses and smoothes down folds
and ripples [33]. These processes are favourable to the re-
duction of the starting layered structure of graphene oxide
and raise the activation energy (see Fig. 12, Treduction = 300
and 500 °C). On the other hand, the removal of gFGs al-
lows the carbon atoms depart from the planes and form
defects, which opens additional surface areas and ways
for sorption and lowers the activation energy (Fig. 12,
TRGO-700). The reduction of the layered structure and
graphitization were the dominant processes in the TRGO-
900 sample. The heating of GtO samples above 200 °C
activates two competing processes controlling the tempera-
ture dependences of the diffusion coefficients of helium,
namely, the reduction of the carbon structure of graphite
(Fig. 7) enhancing the activation energy and the formation
of defects suppressing the activation energy. As a result,
the dependence of the activation energy of helium diffu-
sion on the temperature of the thermal reduction of gra-
phene oxide exhibits a nonmonotonic behavior (Fig. 12),
which agrees well with the XRD data (see Figs. 6 and 7).
Conclusions
The effect of the reduction temperature on the kinetics
of low temperature 4He sorption and the structural charac-
teristics of graphene oxid has been investigated in the tem-
perature interval 1.5–20 K. The time dependences of pres-
sure variations on sorption/desorption of 4He are well
described by the exponential one-parameter function. The
times of 4He sorption increased for all the samples as the
temperature lowered from 20 K to 11–12 K, which is typi-
cal for thermally activated diffusion. Below 5 K the char-
acteristic times of 4He sorption by the GtO and TRGO-200
samples were only slightly dependent on temperature,
which is indicative of the dominance of tunnel diffusion
over the thermally activated mechanism. The characteristic
times of the graphene oxide samples reduced at higher
temperatures (TRGO-300, TRGO-500, TRGO-700 and
TRGO-900) grew at T < 5 K with lowering temperature
presumably because the removal of oxygen functional
groups produced additional defects in the carbon planes and
hence increased the number of diffusion-impeding potential
barriers in the imperfect areas. The anomalous behavior of
the diffusion coefficients of 4He below 5 K observed for the
TRGO-300, TRGO-500, TRGO-700 and TRGO-900 sam-
ples can be caused by transformation of the impurity 4He
atoms into the state of a two-dimensional quantum liquid.
The activation energies Ea of 4He diffusion have been
estimated for the starting and thermally treated GtO sam-
ples. In the TRGO-200 the activation energy of 4He diffu-
sion decreases 2.9 times as against the starting graphite
oxide due to the exfoliation of the GtO sheets caused by
evaporation of the water intercalated in the interlayer space.
The thermal treatment of GtO samples above 200 °C trig-
gers two competing processes accounting for the nonmo-
notonic behavior of the activation energy as a function of
the thermal treatment temperature, namely, the recovery of
the carbon structure enhancing the activation energy and
the formation of defects suppressing the activation energy
through opening additional surface areas and ways for
sorption. The dependence of the activation energy on the
thermal treatment temperature correlates well with the
amount of the crystalline GtO phase varying with increas-
ing annealing temperature.
Fig. 12. The dependence of the activation energy of 4He diffu-
sion (●) and growth of the crystalline TRGO phase (■) upon
the temperature of thermal reduction of graphene oxide.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 3 477
A.V. Dolbin et al.
Acknowledgment
Financial support from Spanish Ministry MINECO and
the European Regional Development Fund (project
ENE2013-48816-C5-5-R), the Regional Government of
Aragon and the European Social Fund DGA-ESF (project
T66) and Targeted Comprehensive Fundamental Research
Program of NASU (project 6/16-Н) is gratefully acknow-
ledged.
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1. Introduction
2. Samples and their characterization by the x-ray diffraction method
3. Experimental desorption technique
4. Results and discussion
Conclusions
Acknowledgment
|