The sorption of⁴He by graphene oxide powders thermally reduced at T = 200, 300, 500, 700, 900 ºC has been

investigated in the interval 1.5–290 K. The measured dependence of the quantity of sorbed helium upon the reduction

temperature shows up as a nonmonotonic curve. The highest quantities of helium were sorbed by the

samples reduced at T = 300 and 900 ºC. It is assumed that the thermal reduction of graphite oxide by heating it to

300 ºC causes evaporation of the water intercalated in the spacings of the carbon layers, this results in exfoliation

of the graphene planes, which enhances the sorptive capacity. Heating the samples to 900 ºC generates numerous

defects in the carbon planes, as a result, the interlayer spacings become accessible for sorption, which enhances

the sorptive capacity

Basnukaeva, R.M.

Benito, A.M.

Dolbin, A.V.

Esel'son, V.B.

Gavrilko, V.G.

Khlistyuck, M.V.

Maluenda, I.

Maser, W.K.

Vinnikov, N.A.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 1, pp. 75–78 The effect of the temperature of graphene oxide reduction on low-temperature sorption of 4He A.V. Dolbin, M.V. Khlistyuck, V.B. Esel'son, V.G. Gavrilko, N.A.Vinnikov,  and R.M. Basnukaeva B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine 47 Lenin Ave., Kharkov 61103, Ukraine E-mail: dolbin@ilt.kharkov.ua I. Maluenda, W.K. Maser, and A.M. Benito Instituto de Carboquímica  ICB-CSIC, 4 Miguel Luesma Castán, E-50018 Zaragoza, Spain Received July 16, 2015, published online November 23, 2015 The sorption of 4He by graphene oxide powders thermally reduced at T = 200, 300, 500, 700, 900 ºC has beeninvestigated in the interval 1.5–290 K. The measured dependence of the quantity of sorbed helium upon the re-duction temperature shows up as a nonmonotonic curve. The highest quantities of helium were sorbed by the samples reduced at T = 300 and 900 ºC. It is assumed that the thermal reduction of graphite oxide by heating it to 300 ºC causes evaporation of the water intercalated in the spacings of the carbon layers, this results in exfoliation of the graphene planes, which enhances the sorptive capacity. Heating the samples to 900 ºC generates numerous defects in the carbon planes, as a result, the interlayer spacings become accessible for sorption, which enhances the sorptive capacity. PACS: 65.80.+n Thermal properties of small particles, nanocrystals, and nanotubes. Keywords: graphene oxide, thermal reduction, 4He sorption.Introduction The recent two decades have witnessed a significant advance in nanotechnologies which have become a new strategic trend in various fields of industry. Materials sci-ence owes its drastic changes to the advent of novel nanostructural forms of some substances, for example, allotropic carbon modifications that have been a particular concern of the scientific community. One of these is graphene, a new form of carbon which has been recently an object of the most extensive research. Graphene is a two-dimensional single-layer carbon structure. Its surface consists of regular hexagons of side 1.42 Å [1] with sp2-hybridized carbon atoms at each ver-tex. This type of structure is a constituent of crystalline graphite in which graphene planes are about 3.35 Å apart from one another [1]. The advent of a relatively simple technique of separat-ing graphene as an individual sample and new macroscopi-cally available graphene-based materials (graphene oxide, graphene oxide paper) have drawn special attention to pro-duction, investigation and application of graphene. This is due to the wide-range industrial potentialities of graphene allowed by its unique physical and chemical properties: high electric [2] and thermal conductivity [3], the depend-ence of the electronic characteristics on various-origin added radicals at the graphene surface [4], controllable energy gap width [5], quantum Hall effect [6], extraordi-narily high-charge carrier mobility [7] and high elasticity. These properties promise attractive applications of graphene as a basis for novel nanomaterials possessing improved mechanical, electric and thermal physical char-acteristics or as highly efficient gas [8,9] and biological [10,11] sensors. Graphene oxide consists of undamaged graphite do-mains with inclusions of sp3-hybridized carbon atomswhich contain functional hydroxyl and epoxy groups on the upper and lower surfaces of each graphene sheet. There are also inclusions of sp2-hybridized carbon-containingcarboxyl and carbonyl groups concentrated mainly at the edges of a graphene sheet. Graphene oxide, like graphite, has a layer structure (normally several carbon layers). The © A.V. Dolbin, M.V. Khlistyuck, V.B. Esel'son, V.G. Gavrilko, N.A.Vinnikov, R.M. Basnukaeva, I. Maluenda, W.K. Maser, and A.M. Benito, 2016 A.V. Dolbin et al. interlayer spacing varies within 6–8 Å [12,13] depending on the employed preparation technique and the oxidation level of graphene oxide. The carbon layers in graphene oxide are deformed because of the sp2 → sp3 transitions of the carbon atoms. Graphene oxide has a rule abundant to-pological structure defects and ruptures. The layers of graphene oxide are bonded rather weakly. At present thermal reduction is one of the most prac-ticed techniques of preparing graphene-type materials on a commercial scale. Thermally reduced graphene oxide (TRGO) is prepared from graphene oxide (GO) [14,15]. In turn, GO is obtained from graphite using various chemical oxidizing agents [13,16]. Thermal reduction of graphene is an intriguing technology: on the one hand, it holds much promise of cost cuttings on mass TRGO production; on the other hand, a lack of chemical oxidizers is not improbable and can be a serious challenge. Besides, it is impossible to remove completely the traces of chemical oxidants from GO powders [15,17,18], which may impede their special biological and medical applications. It is also important that residual oxide groups and surface defects inevitable on reducing graphene oxide affect significantly the structure of graphene planes. In this context reduced graphene oxide and graphene can hardly be considered as totally identical. Thermal reduction of graphene oxide is rather a complex process involving thermally activated multi-stage removal of the intercalated water molecules and oxide groups, such as carboxyl (–COOH), carboxyl and partially-hydroxyl (C–OH and O–H), epoxy (C–O–C), surface single-bonded oxygen (C–O) and carbonyl (C=O) groups. The presence of water and O2-containing groups has a significant effect on the physical properties of graphene oxide, including its sorptive characteristics. Owing to their large specific surface area [19,20], graphene oxide and reduced graphene oxide are used as highly efficient sorbents. It is therefore topical to investigate the sorptive characteristics of GO and TRGO in a wide temperature range, particularly in regard of helium whose properties at low temperatures are largely due to quantum effects [21,22]. Experimental technique and results The sorptive characteristics of graphene oxide in respect of 4He have been investigated as a function of the tempera-ture of thermal graphene oxide reduction. The starting graphite oxide (GtO) was obtained from graphite powder using the modified Hammers method. Its subsequent reduc-tion and exfoliation were carried out through thermal heat-ing of GO in the atmosphere of argon. Five samples were treated and each of them was heated to a certain highest temperature (200, 300, 500, 700 and 900 ºC). The sorptive properties of the thermally reduced GO samples were inves-tigated on a laboratory test bench (its design and operation are detailed elsewhere [21,22]). The temperature of the ex-periment was 1.5–290 K. Prior to testing, the powder sam-ples were evacuated for five days at room temperature in the measuring cell of the test bench, which was aimed at remov-ing possible gas impurities and moisture. Graphite oxide and the thermally reduced samples of graphene oxide were saturated with helium under the pres-sure ~ 1 Torr. The lowest temperature of the experiment was dictated by the 4He gas pressure in the measuring sys-tem. In the course of saturation the pressure was main-tained 2.5–3 times lower than the equilibrium pressure of saturated 4He vapor at a particular temperature. As helium was sorbed, additional portions of 4He were fed to the cell. This saturation procedure secured the system against 4He vapor condensation and the formation of a film on the sur-faces of the powder grains and the cell walls.  The supply of the 4He gas was cut off when the equilib-rium pressure 10–2 Torr was reached in the cell. The cell was then hermetically sealed and the changes in the pres-sure were registered during the 4He desorption on stepwise heating of the sell. The 4He gas released on heating was taken to an evacuated calibrated vessel whose internal pressure was measured using two capacitive MKS-627 pressure transducers which allowed pressure measurement in the range 10–3–1000 Torr, the error being ± 1·10–4 Torr. Then the measurement procedure was repeated at the next temperature point. The temperature dependences of the 4He quantities desorbed from GtO and TRGO samples are illustrated in Fig. 1. The desorption was monitored over the interval T = = 9–900 K, however the sorbed helium was nearly fully desorbed from the samples on the heating to 24 K. The dependences of the total quantity of desorbed 4He upon the temperature of the sample reduction is shown in Fig. 2 and Table 1. In Figs. 1, 2 and in Table 1 the quantity of the de-sorbed 4He atoms is normalized to the total quantity of C atoms in the samples. Note that the total quantities of the sorbed and desorbed 4He were equal within the experimental error. The depend-ence of the total quantity of sorbed 4He upon the tempera-ture of the sample reduction is described by a non-monotonic curve. The largest quantities of the sorbed 4He Table 1. The total 4He concentrations in the GtO and TRGO samples at different reduction temperatures (quantity of desorbed He atoms NHe per total quantity of C atoms NC in the samples) Sample GtO TRGO-200 TRGO-300 TRGO-500 TRGO-700 TRGO-900 NHe/NC 0.00054 0.0063 0.022 0.018 0.0044 0.027 wt.% 0.018 0.211 0.736 0.392 0.146 0.901  76 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 1 The effect of the temperature of graphene oxide reduction on low-temperature sorption of 4He were observed on the samples reduced at 300 and 900 ºC (Fig. 2). The increased sorptive capacity of the sample heated to 300 ºC may be attributed to the disordering of the layer structure in graphite oxide by evaporation of the in-tercalated water. On heating to 700 ºC the O2-containing groups disappear, defects are produced and the spacings of the graphene layers decrease, which deteriorates the sorptive capacity of the samples. The thermal treatment at a higher temperature (900 ºC) can destroy the residual O2-containing groups and produce a great number of defects [23]. As a result, the interlayer spacing in GO become accessible from sorption, which increases the sorptive capacity of the sample. The growing number of defects in the carbon planes at higher reduction temperatures show us another pronounced desorption max-imum at T ~ 10 K (Fig. 1(b), TRGO-500, TRGO-700 and TRGO-900 samples). Conclusions The desorption spectra characterizing the sorptive ca-pacity of the samples of thermally reduced graphene oxide for helium have been taken in the interval T = 1.5–24 K using the method of thermoprogrammed desorption. The dependence of quantity of desorbed 4He upon the tempera-ture of the sample reduction has been plotted on the basis of the experimental results. It is assumed that the thermal reduction of graphite oxide by heating it to 300 ºC in the Ar atmosphere causes evaporation of the water intercalated in the spacings of the carbon layers. This results in exfolia-tion of the graphene planes, which enhances the sorptive capacity. The thermal reduction at 700 ºC suppresses sorp-tive capacity of graphene oxide because heating to 700 ºC destroys O2-containing groups and reduces in the interlayer spacings in graphene. Heating the samples to 900 ºC gen-erates numerous defects in the carbon planes. As a result, the interlayer spacings become accessible for sorption, which enhances the sorptive capacity. Financial support from Spanish Ministry MINECO and the European Regional Development Fund (project ENE2013-48816-C5-5-R), the Regional Government of Ara-gon and the European Social Fund DGA-ESF (project T66) and Targeted Comprehensive Fundamental Research Pro-gram of NASU (project 6/15-Н) is gratefully acknowledged.  1. D.D.L. Chung, J. Mater. Sci. 37, 1475 (2002). 2. C. Lee, X. Wei, J.W. Kysar, and J. Hone, Science 321, 385 (2008). 3. A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, and C.N. Lau, Nano Lett. 8, 902 (2008). 4. X. Huang, X. Qi, F. Boey, and H. Zhang, Chem. Soc. Rev. 41, 666 (2012). 5. F. Guinea, A.H. Castro Neto, and N.M.R. Peres, Phys. Rev. B 73, 245426 (2006). 6. K.S. Novoselov, Z. Jiang, Y. Zhang, S.V. Morozov, H.L. Stormer, U. Zeitler, J.C. Maan, G.S. Boebinger, P. Kim, and A.K. Geim, Science 315, 1379 (2007). 7. K.I. Bolotin, K.J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim, and H.L. Stormer, Solid State Commun. 146, 351 (2008). Fig. 1. (Color online) The temperature dependences of the rela-tive 4He quantities desorbed from the samples: (a) GtO (■), TRGO reduced at T = 200 ºC (∆), 300 ºC (); (b) TRGO re-duced at T = 500 ºC (▼), 700 ºC (∇ ) and 900 ºC (). 0 100 200 300 400 500 600 700 800 900 100000,010,020,03T, °CNNHeC/Fig. 2. The dependence of the relative quantity of sorbed helium on the temperature of the sample reduction. Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 1 77 A.V. Dolbin et al. 8. F. Schedin, A.K. Geim, S.V. Movozov, E.W. Hill, P. Blake, M.I. Katsnelson, and K.S. Novoselov, Nature Materials 6, 652 (2007). 9. B. Huang, Z.Y. Li, Z.R. Liu, G. Zhou, S.G. Hao, J. Wu, B.L. Gu, and W.H. Duan, J. Phys. Chem. C 112, 13442 (2008). 10. B.G. Choi, H. Park, M.H. Yang, Y.M. Jung, J.Y. Park, S.Y. Lee, W.H. Hong, and T.J. Park, Nanoscale 2, 2692 (2010). 11. K.J. Huang, D.J. Niu, J.Y. Sun, C.H. Han, Z.W. Wu, Y.L. Li, and X.Q. 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The effect of the temperature of graphene oxide reduction on low-temperature sorption of ⁴He

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The effect of the temperature of graphene oxide reduction on low-temperature sorption of ⁴He

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