ON THERMODYNAMIC CHARACTERISTICS OF HYDROGENATED GRAPHENE-BASED NANOSTRUCTURES, RELEVANCE TO THE PROBLEM OF THE HYDROGEN STORAGE IN FUEL-CELL-POWERED ECOLOGICAL VEHICLES Текст научной статьи по специальности «Физика»

Статья поступила в редакцию 15.05.14. Ред. рег. № 2000

The article has entered in publishing office 15.05.14. Ed. reg. No. 2000

УДК 541.67:541.142

О ТЕРМОДИНАМИЧЕСКИХ ХАРАКТЕРИСТИКАХ ГИДРИРОВАННЫХ МОНО- И ПОЛИГРАФЕНОВЫХ НАНОСТРУКТУР В СВЯЗИ С ПРОБЛЕМОЙ ХРАНЕНИЯ ВОДОРОДА В ЭКО-АВТОМОБИЛЯХ С ТОПЛИВНЫМИ ЭЛЕМЕНТАМИ

Ю. С. Нечаев

ФГУП «ЦНИИчермет им. И.П. Бардина», Институт металловедения и физики металлов им. Г.В. Курдюмова, 2-ая Бауманская ул., 9/23, 105005 Москва, Российская Федерация

Заключение совета рецензентов 22.05.14 Заключение совета экспертов 05.06.14 Принято к публикации 11.06.14

Рассматриваются аналитические результаты определения характеристик и механизмов термодинамической стабильности и соответствующих термодинамических характеристик ряда гидрированных моно- и полислойных графеновых наноструктур, а именно:

1) гидрированный (с обеих сторон) графен состава CH (теоретический графан и экспериментальный графан); 2) теоретический гидрированный (с одной из сторон) графен состава CH; 3) теоретический гидрированный (с одной из сторон) графен состава C2H (графон); 4) экспериметнтальные гидрированные эпитаксиальный графен, двухслойный эпитаксиальный графен и многослойный эпитаксиальный графен (на SiO2 или другой подложке); 5) экспериментальные и теоретические гидрированные углеродные однослойные нанотрубки и экспериментальный гидрированный фуллерен C60H36; 6) экспериментальные графеновые поверхностные «наноблистеры», гидрированные с их внутренней стороны (до графанового состава) и содержащие «интерколированный» газообразный молекулярный водород высокого давления, образующиеся на поверхности высоко ориентированного пиролитического графита (HOPG) или эпитаксиального графена при их обработке атомарным газообразным водородом; 7) экспериментальные гидрированные (до графанового состава) графитовые нановолокна с «интерколированными» в них нанообластями твердого (или жидкого) молекулярного водорода с высокой плотностью, что связано с разработкой прорывной нанотехнологии хранения водорода в эко-автомобилях с топливными элементами и другими проблемами водородной энергетики.

Ключевые слова: гидрированные моно- и полиграфеновые наноструктуры, термодинамическая стабильность, проблема эффективного хранения водорода в эко-автомобилях с топливными элементами.

ON THERMODYNAMIC CHARACTERISTICS OF HYDROGENATED GRAPHENE-BASED NANOSTRUCTURES, RELEVANCE TO THE PROBLEM OF THE HYDROGEN STORAGE IN FUEL-CELL-POWERED ECOLOGICAL VEHICLES

Yu.S. Nechaev

Bardin Institute for Ferrous Metallurgy Kurdjumov Institute of Metals Science and Physics Vtoraya Baumanskaya St., 9/23, Moscow 105005, RUSSIA Yuri1939@inbox.ru

Referred 22.05.14 Expertise 05.06.14 Accepted 11.06.14

The present analytical study is devoted to the current problem of the thermodynamic stability, and related thermodynamic characteristics of the following graphene layers systems: 1) double-side hydrogenated graphene of composition CH (theoretical graphane) and experimental graphane; 2) theoretical single-side hydrogenated graphene of composition CH; 3) theoretical singleside hydrogenated graphene of composition C2H (graphone); 4) experimental hydrogenated epitaxial graphene, bilayer graphene and a few layer graphene on SiO2 or other substrates; 5) experimental and theoretical single-external side hydrogenated singlewalled carbon nanotubes, and experimental hydrofullerene C60H36; 6) experimental single-internal side hydrogenated (up to C2H or CH composition) graphene nanoblisters with intercalated high pressure H2 gas inside them, formed on a surface of highly oriented pyrolytic graphite or epitaxial graphene under the atomic hydrogen treatment; and 7) experimental hydrogenated graphite nanofibers - multigraphene with intercalated solid H2 nanoregions of high density inside them, relevant to solving the current problem of the hydrogen storage in fuel-cell-powered ecological vehicles and other clean energy applications.

Keywords: hydrogenated graphene mono- and multylayer systems (nanostructures), the thermodynamic stability, the problem of the hydrogen storage in fuel-cell-powered ecological vehicles

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1. Introduction

As has been noted in a number of articles of 20072014, the hydrogenation of graphene (a single layer of carbon atoms arranged in a honeycomb lattice ([1, 2] and others)), as a prototype of covalent chemical functionalization and an effective tool to open the band gap of graphene, is of fundamental importance.

It is relevance to the actual problem of the hydrogen on-board storage, and also to the actual related problem of the thermodynamic stability and thermodynamic characteristics of the following systems:

1) double-side hydrogenated graphene (theoretical graphane of composition CH [3, 4] and experimental graphane [5]);

2)theoretical single-side hydrogenated graphene of composition CH (SSHG, [6-8]);

3)theoretical single-side hydrogenated graphene of composition C2H (graphone, [9]);

4)experimental hydrogenated epitaxial graphene, bigraphene and a few layer graphene on SiO2 or other substrates ([5] and others);

5)experimental and theoretical single-external-side hydrogenated single-walled carbon nanotubes of composition about C2H and experimental hydrofullerene C60H36 ([10-13], analytical review [14]);

6)experimental single-internal-side hydrogenated graphene nanoblisters - single-side graphane* nanoblisters possessing of a very high Young's modulus and corresponding binding energy (with intercalated into them H2 gas of a high pressure) formed on surface of highly oriented pyrolytic graphite (HOPG) or epitaxial graphene under the definite atomic hydrogen treatment ([15-21]);

7)experimental hydrogenated graphite nanofibers -multigraphane* nanofibers [18-21] possessing of a very high Young's modulus and corresponding binding energy (with intercalated into them solid H2 of a high density, that is relevance to the problem of the hydrogen on-board storage (Supplement 1).

In this analytical review, results are presented of the thermodynamic analysis and comparison of some theoretical and experimental data (especially, from the most cited works [3, 5] and from the near non-cited ones [18-21]).

As was noted in [8], the double-side hydrogenation of graphene is now well understood, at least from theoretical point of view. For example, Sofo et al. [3] predicted theoretically a new insulating material of CH composition called graphane (double-side hydrogenated graphene), in which each hydrogen atom adsorbs on top of a carbon atom from both sides (so that hydrogen atoms adsorbed in different carbon sublattices are on different sides of the monolayer plane). The formation of graphane was attributed to the efficient strain relaxation for the sp3 hybridization, accompaning with a strong (diamond-like or other) distortion of the graphene network [3, 22]. In contrast to graphene (zero-gap semiconductor), graphane is an insulator with an energy gap£g «5.4 eV [23,4].

If only hydrogen atoms adsorbed on one side of graphene (in graphane) are retained, we obtain graphone

of C2H composition, which is a magnetic semiconductor with £g » 0.5 eV and a Curie temperature Tc ~ 300-400 K [24].

As was noted in [6], neither graphone nor graphane are suitable for real practical applications, since the former has a low value of Eg and undergoes rapid disordering because of hydrogen migration to neighboring vacant sites even at a low temperature [9] and the latter cannot be prepared on a solid substrate.

Single-side hydrogenated graphene (SSHG) of CH composition [7, 25] is an alternative: in contrast to graphane, hydrogen atoms are adsorbed only one side; in contrast to graphone, they are adsorbed on all carbon atoms rather than on every second carbon atom. The value of Eg in SSHG is sufficiently high (by 1.6 e V lower than in graphane [7, 6]), and it can be prepared on a solid substrate in principle [6]. But, this quasi-two-dimensional carbon-hydrogen theoretical system is shown [6] to have a relatively low thermal stability, which makes it difficult to use SSGG in practice.

As was noted in [7], it may be inappropriate to call ; e ~ the covalently bonded SSHG system sp3 hybridized, since the characteristic bond angle of 109.5° is not present anywhere, i.e., there is no diamond-like strong distortion of the graphene network, rather than in graphane [3]. Generally in the case of a few hydrogen atoms interacting with graphene or even for graphane, the underlining carbon atoms are displaced from their locations (i.e., there may be the diamond-like local | distortion of the graphene network), showing the o signature of sp3 bonded system. However, in SSHGrapene all the carbon atoms remain in one plane, s making it difficult to call it sp3 hybridized; obviously, this is some specific .s/x-likc hybridization. Such model is taken into account when the further consideration (in ; this analytical study) of works [10-21].

In a number of works considered in [25], it was | shown that hydrogen chemisorption corrugates the graphene sheet (in fullerene, carbon nanotube [26], graphite [27] and graphene [28]), and transforms it from | a semimetal into a semiconductor [3, 5]; this can even induce magnetic moments [29-31].

It is worth to repeat that the prediction [3] for the double-side hydrogenated graphene (a free-standing membrane) was partially (in terms of [8]) confirmed by Elias et al. [5]. They [5] demonstrated that graphene can react with atomic hydrogen, which transforms this highly conductive zero-overlap semimetal into an insulator of a high thermal stsbility, and the double-side hydrogenation of graphene is reversible. The authors [5] themselves expressed some doubts, relevance to the complete adequacy of the experimental graphane [5] to the theoretical one [3]. They [5] supposed, alternatively, that the produced by them experimental graphane (a free-standing membrane) may have "a more complex hydrogen bonding than the one suggested by theory" [3], and that the latter may be as the "until-now-theoretical material".

In the case of the epitaxial graphene (on substrate such as SiO2 [5] or others), hydrogenation occurs only on the top basal plane of graphene, and obviously, it is

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not accompanied with a strong (diamand-like or other) distortion of the graphene network, but only with some its rippling. The first experimental indication of such a specific single-side hydrogenation came from Elias et al. [5]. The authors [5] mentioned a possible contradiction with the theoretical results of Sofo et al. [3], which had down-played the possibility of a single side hydrogenation. They [5] supposed an important facilitating role of the material rippling (for hydrogenation of graphene on SiO2) and believed that such a single-side hydrogenated epitaxial graphene can be a disordered material, similar to graphene oxide, rather than a new graphene-based crystal - the experimental graphane produced by them. In other words, the two different materials have been produced in work [5].

On the other hand, it is expedient to note that changes in Raman spectra of graphene caused by hydrogenation were rather similar (with respect to locations of D, G, D', 2D and (D+D') peaks) both for the epitaxial graphene on SiO2 and for the free-standing graphene membrane (Fig. 3 in [5]).

As is supposed by many scientists (for instance, [5, 8]), such a single side hydrogenation of epitaxial graphene occurs because the diffusion of hydrogen along the graphene-SiO2 interface is negligible and perfect graphene is impermeable to any atom and molecule [32]. But these two aspects are of the kinetic character, and therefore they can not influence on the thermodynamic predictions [3, 31, 24].

Authors of [8] noted that their test calculations (relevance to [32] results) show that the barrier for the penetration of a hydrogen atom through the six-membered ring of graphene is larger than 2.0 eV. Thus, they [8] believe (a private communication from H.G. Xiang and M.-H. Whangbo) "that it is almost impossible for a hydrogen atom passes through the six-membered ring of graphene at room temperature".

In the present analytical review, a real possibility is considered of a situation when a hydrogen atom can pass through the graphene network at room temperature. This is the case of existence of relevant defects in graphene, for instance, as grain boundaries and/or vacancies [3342]. It is related to the further consideration (in this analytical study) of data [15-21] mentioned above.

Previous theoretical studies [31, 24] suggest that single-side hydrogenation of ideal (perfect) graphene would be thermodynamically unstable. Thus (in terms of [8]), it remains a puzzle why the single-side hydrogenation of epitaxial graphene is possible (and even reversible, according to [5] data), and why the hydrogenated species are stable at room temperatures [5, 43].

This puzzle situation is also considered in the present analytical review. And the main aim of this study is to show a real possibility (at least, from the thermodynamic point of view) a of existence of hydrogenated graphene-based nanostructures -multigraphanes* [18-21] possessing of a very high Young's modulus and corresponding binding energy, and also to show a real possibility of intercalation in such multigraphanes* of

solid molecular hydrogen under definite hydrogenation conditions, that is relevance to the actual problem of the hydrogen storage in fuel-cell-powered ecological vehicles [18-21] (Supplement 1).

2. Analysis and comparison of data [3-5, 25]

2.1. Considearation of data [3, 25] on theoretical

graphanes (CH)

In work [3], the stability of graphane (a fully saturated extended two-dimentional hydrocarbon derived from a single graphene sheet with formula CH) has been predicted on the basis of first-principles total-energy calculations. All of the carbon atoms are in sp3 hybridization forming a hexagonal network (a strongly (diamond-like or other) distorted graphene network) and the hydrogen atoms are bonded to carbon on both sides of the plane in an alternative manner. It has been found [3] that graphane can have two favorable conformations: a chair-like (diamond-like) conformer and a boat-like (zigzag-like) one.

The diamond-like conformer (Fig. 1 in [3] or Fig. 4D in [5]) is more stable, rather than the zigzag-like one. It was concluded [3] from results of calculations of the binding energy (AHbmd(graphane[3])) - the difference between the total energy of the isolated atoms and the total energy of the compounds, Table 1 in [3]) and the standard energy of formation (AH^feraphanep]), Fig. 2 in [3]) of the compounds (CH(graphane[3])) from crystalline graphite (C(graphite)) and gaseous molecular hydrogen (H2(gas)) at (as was noted in [3]) standard pressure (usually, 0.1 MPa) and temperature (usually, 298 K) conditions.

FIG. 1. (Color online) Structure of graphane in the chair conformation. The carbon atoms are shown in gray and the hydrogen atoms in white. The figure shows the hexagonal network with

FIG. 1ft from [3].

For the diamond-like graphane, the former quantity is A#bmd.(graphane[3]) = 6.56 eV/atom (Table 1 in [3]), and the latter one is AH = AH0E98(g raphane[3]) - 0.15 eV/atom (Fig. 2 in [3]). The latter quantity corresponds to the following reaction:

C(graphite)+ ^H2(gas)^ CH(graphane[3]> (AH1), (1)

where AH is the standard energy (enthalpy) change for this reaction.

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By using the theoretical [3] quantity AH^graphanep]), one [18-21] can evaluate (in the framework of the thermodynamic method of cyclic processes, for instance, [44]), a value of the energy of formation (AH2) of graphane (CH(graphane[3])) from graphene (C(graphene)) and gaseous atomic hydrogen (H(gas)). For this, it is necessary to include in the consideration the folloing three additional reactions:

C(graphene)+ H(gas) * CH(graphane[3]), (AH), (2)

C(graphene) * C(graphite), (AH3), (3)

H(gas) * '/2 H2(gas), (AH4), (4)

where AH2, AH3 and AH4 are the standard energy (enthalpy) changes.

Reaction (2) can be presented as a sum of reactions (3), (4) and (1); hence (in the framework of the thermodynamic method of cyclic processes [44]):

AH2 = (AH3+AH4+AHi). (5)

Substituting in Eq. (5) the known experimental values ([45] and/or other related Handbooks) of AH4 = -2.26 eV/atom and AH3 « -0.05 eV/atom [45, 25], and also the theoretical [3] value of \//, = -0.15 3B/atom one [1821] can obtain a desired value of AH2 = -2.5 ±0.1 eV/atom. The quantity of -AH2 (according to [18-21]) characterizes the breake-down energy of C-H sp3 bond in graphane [3], relevance to breaking away of one hydrogen atom from the material, which is AH(C-

H)graphane[3] = -AH2 = 2.5 ± 0.1 eV.

For evaluating of the above mentioned value of AH3, one can use, for instance, experimental data [45] on the grapliite sublimation energy at 298 K (AHsubUgmphie)[45] = 7.41 ± 0.05 eV/atom ) and theoretical data [25] on the binding cohesive energy at about 0 K (in the sense [1821] of the breake-down energy of 1.5 C-C sp2 bonds, relevance to breaking away of one carbon atom from material) for graphene (AHcohes.(graphene)[25] = 7.40 eV/atom, Fig. 1 in [25]). Hence, neglecting the temperature dependence of these quantities (in the interval of 0-298 K), one [18-21] can obtain the value of AH3 ~ -0.05 eV/atom.

AHcohes(graphene[25]) quantity characterizes the breake-down energy of 1.5 C-C sp2 bond in graphene [25], relevance to breaking away of one hydrogen atom from the material. Hence, one [18-21] can evaluate the breake-down energy of C-C sp2 bonds in graphene, which is AH(C-C)graphene[25] = 4.93 eV. This theoretical [25] quantity coincides with the similar empirical quantities (obtained in [18-21] from A/fsubUgraphltel[45]) for C-C sp2 bonds in graphene and grapliite, which are A/f(C-c)graPhene[45,i8-2i] ~ A/f(c-c>graphite[45,i8-2i] = 4.94 ± 0.03 eV. The similar empirical quantity for C-C sp3 bonds in diamond (obtained in [18-21] from the diamond sublimation energy AHsubl.(diamond)[45]) is AH(C-C)diamond[45,18-21] = 3.69 ±

0.02 eV.

It is expedient to note that in [25], chemisorption of hydrogen on graphene was studied using atomistic simulations with the second generation of reactive

empirical bond order Brenner inter-atomic potential. According to data [25], the cohesive energy of graphane (CH) in the ground state (in the sense [18-21], it may be the breake-down energy of one C-H sp3 bond and 1.5 CC sp3 bonds, relevance to breaking away of one hydrogen atom and one neighbouring carbon atom from the material) is AHCohes.(graphane[25]) = 5.03 eV/atom(C) (Fig. 1 in [25]). It results [25] in the hydrogen binding energy (in the sense [18-21] of the breake-down energy of C-H sp3 bond), which is AH(C-H)graphane[25] = 1.50 eV/atom(H) (Table 1 in [25]).

The theoretical AHbmd(graphane[3]) quantity [3] characterizes (in therms of [18-21]) the breake-down energy of one C-H sp3 bond and 1.5 C-C sp3 bonds (Fig. 1 in [3] or Fig. 4D in [5]). Hence, by using the above mentioned values of AHbmd.(graphane[3]) and AH(C-H)graphane[3], one [18-21] can evaluate the breake-down energy of C-C sp3 bonds in the theoretical graphane [3], which is AH(C-C)graphane[3] = 2.7 eV. And by using the above noted theoretical [25] values of AHcohes.(graphane[25]) and AH(C-H)graphane[25], one can evaluate (in the similar manner the breake-down energy of C-C sp3 bonds in the theoretical

graphane [25], which is AH(

(C-C)graphane[25]

= 2.35 eV.

Compearing of the obtained in such a way [18-21]

values of AH(

(C-C)graphane[3],

AH(

(C-C)graphane[25]

AH(

(C-

C)graphene[25^ AH(C-C)graphene[45,18-21], AH(C-C)graphite[45,18-21] and

AH(

(C-C)diamond[45,18-21]

shows that the elastic and intrinsic

strength properties (and particularly, a Young's modulus) of the theoretical [3, 25] graphanes can be much less than those for perfect (i.e., without defects) graphene [1, 25], perfect graphite [45] or perfect diamond [45].

2.2. Considearation of data on hydrogen thermal desorption from theoretical [3, 4] and experimental [5] graphanes

In [4], the process of hydrogen thermal desorption from graphane (in the sence of [3]) had been studied using the method of molecular dynamics. The temperature dependence (for T = 1300-3000 K) of the time (zb oi) of hydrogen desorption onset (i.e., the time of the removal of ~1 % (-AC) of the initial hydrogen concentration C0 » 0.5 (in atomic fractions), -AC/C0 ~ 0.01) from the C54H7,54+i8, claster (wth 18 hydrogen passivating atoms at the edges to saturate the dangling bonds of s/x'-hybridi/cd carbon atoms) was calculated [4]. The corresponding activation energy (/•.'„ = 2.46 ± 0.17 eV) and the corresponding near-temperature-independent frequency factor (A = (2.1 ± 0.5)-1017 s"1) were also calculated [4].

The process of hydrogen desorption (at T = 13003000 K) was described [4] in terms of the following standard Arrhenius relationship:

1/to.oi[4] =A exp (-£; ' A-B7), (6)

where kB is the Boltzann constant.

The authors [4] supposed that their results do not contradict the experimental data [5], according to which (Fig. 3A in [5]) the near-complete desorption of

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hydrogen (-AC/C0 ~ 0.9) from a graphane membrane was acliieved by annealing it in argon at T = 723 K for 24 hours (i.e., ii, 9(membr [5]i723K = 8.64-104 s). By using Eq. (6), the authors [4] evaluated the quantity of ^b.oi(graphane[4]) for T= 300 K (-1-1024 s) and for T = 600 K (-2-103 s); but they noted that the above two values of ib.oi(graphane[4]) should be considered as rough estimates. In such a manner (Eq. (6)), one can evaluate the quantity of zb.oi(graphane[4])723K ~ 0.7 s for T = 723 K, wliich is much less (by five orders) of r0.9(membr.[5]l723K.

In the framework of the formal kinetics approximation of the first order rate reaction [46], a characteristic quantity for the reaction of hydrogen desorption is t0 63 - the time of the removal of -63 % (AC) of the initial hydrogen concentration C0 (i.e., -AC/C0 « 0.63) from the hydrogenated graphene. Such a first order rate reaction (desorption) can be described by the following equations [46, 14]:

d CI dt=-KC,(1) (C / C„) = exp (- K t) = exp (-1 / (8) K = (l/ro.«) = Ko cxp (-AHdes / A-b T), (9)

where K = {\It063) is the reaction (desorption) rate constant; AHdes. is the reaction (desorption) activation energy; Kb is the per-exponential (or frequency) factor of the reaction rate constant.

For the case of a non-diffusion-rate-limiting kinetics, the quantity of K0 is the related vibrational frequency (K0 = i'), and Eq. (9) corresponds to the Polanyi-Wigner one [14].

By substituting in Eq. (8) the quantities of I =

zb.oi(graphane[4])723K and (C / C0) = 0.99, one can evaluate the desired quantity zb63(graphane[4])723K ~ 70 s. Hence, using Eq. (9) results in the analytical quantity of Aan [4] = 21015 s"1. Analogically, one can evaluate the desired quantity zb63(membr [5])723K ~ 3.8-104 s, which differs from zb 63(graphane[4]i723K only by about three orders.

By substituting in Eq. (9) the quantity of K =

£,membr.[5])723K = l/ib.63(membr.[5] )723K and SUppOSing that AHdes. = AHdes AHo -H(graphane[3]) -AH2[3], one

can evaluate (in such an approximation) the desired

quantity ii;,(membr.[5]l[3] = Mmembr.[5]l[3] ~ 7-1012 S_1 (for tile

experimental grapliane membranes [5]).

The obtained quantity of Mmembr [5]>[3] is 'css by one and half orders of the vibrational frequency rRD[s = 2.51014 s"1 corresponding to the D Raman peak (1342 cm"1) for hydrogenated graphene membrane and epitaxial graphene on SiO2 (Fig. 3 in [5]), the activation of which in the hydrogenated samples authors [5] attribute to breaking of the translation symmetry of o-o sp1 bonds after formation of C-H sp3 bonds. The obtained quantity of v(membr.[5]i[3] is less by one order of the vibrational frequency tl[i<i-;.r;i.s|r = 8.7-1013 s"1 corresponding to an additional HREELS peak (Fig. 3 in [47]) arising from O-H sp3 stretching appears at 369 meV after a partial hydrogenation of the epitaxial graphene. The authors [47] supposed that this peak can be assigned to the vertical o-H bonding, giving direct

evidence for hydrogen attachment on the epitaxial graphene surface.

Taking into account rRD[s and tl[i<i-;.r;i.s| r quantities, and substituting in Eq.(9) quantities of K = £,membr [5]>723K

= 1/Zb.63(membr.[5])723K and Kq « ^0(membr.[5]l[47] ~ HiREELS[47]. one can evaluate A/fdes.(membr.[5])[47] = A/fC-H(membr.[5])[47] ~ 2.66 eV. The obtained (in such approximation) value of AHO-H(membr. [5])[47] coincides (within the errors) with the experimental [l3] value of the breake-down energy of CH .s/x-likc bonds in hydrofullerene C6oH36 (AHc. H(C60H36[13],= 2.64 ± 0.01 eV).

The above analysis of the related data shows that for the experimental [5] graphene membranes (hydrogenated up to the near-saturation) can be used the following thermodesorption characteristics, relevance to Eq. (9), of the empirical character: A.//,, rilorili[ s| ,| i | = AHc.

H(membr.[5]l[47,13] ~

H(membr.[5])[47,13]

- 2.6 ± 0.1 eV, ^0(membr.[5]l[47,13] ~ VC-. The above analysis also

5-10

shows that it is the case of a non-diffusion-rate-limiting kinetics, when Eq. (9) corresponds to the Polanyi-Wigner one [14]. And certainly, these tentative results could be directly confirned (and/or modified) by

receiving and treatment (within Eqs. (8, 9)) of experimental data on ib63[5] at several annealing temperatures.

The above noted fact that the empirical quantity zb.63(membr.[5]723Ki is much larger (by about 3 ordes), rather than the theoretical One ( Tb.63(graphane[4]723K))-is consistent with the mentioned in [5] alternative possibility that the experimental graphane membrane [5] (a free-standing membrane) may have a more complex hydrogen bonding than the one suggested by theory [3]. It may be true and for the further theoretical development [4].

2.3. Considearation of a thermodynamic probability of existence of hydrogenated graphenes - graphanes* [18-21] possessing of a very high binding energy

In connection with the above consideration, it seems expedient to consider a thermodynamic probability (possibility) of existence of hydrogenated graphene -

graphane* (CH?l(l|;i|l(IMe.|iX_2i |) possessing of the values of AH(c_Hlgraphaneni8-2i] » 2.6 eV [3-5, 12, 13, 18-21] and A//,c-Clgraphane*[i8-2i] ~ 4 9 eV [18-21]. It corresponds to a very high binding (cohesive) energy (\/Ai,„,i1ol(l|,h(,ne-|ix-2i | , ~ 10 eV/atom) in comparison with those (above consided) for theroretical graphanes [3, 4, 25]. The quantity of AH(c-c)graphane*[i8-2i] is related to the graphene and/or graphite situation. For showing of such a thermodynamic probability, it is necessary to include in the consideration two more additional reactions [18, 19]:

C(gas) * C(graphene> (AH1b), (1b) C(gas)+ H(gas) * CH(graphane*[18-21]), (AHn), (11)

where AH1b and AH11 are the standard energy (enthalpy) changes.

Reaction (11) can be presented as a sum of reaction (2), relevance to graphane* (2*), and reaction (10);

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hence (in the framework of the thermodynamic method of cyclic processes [44]):

AHn = (AH2* + AH10). (12)

Substituting in Eq. (12) the above consded values of

AH2. ~ AH2~ -Aifc-H(membr.[5]i[47,i3] = -2.6 ± 0.1 eV/atom and AHw ~ -A/fcohes(graphene|[25] ® -A/fcohes (graphene|[45j8_2i] ® -AHsubUgrapllltel[45] = -7.41 ± 0.05 eV/atom, one [18-21] can obtain a desired value of AHu = -10.0 ± 0.1 eV/atom. The quantity of -AH11 corresponds to the binding (cohesive) energy of graphane* (-AH11 =

AHbind.(graphane*[18-21]) = AHcohes.(graphane*[18-21]). According to

[18-21], the quantity of -AH11 characterizes the breake-down energy of one (C-H) sp3-like bond and 1.5 (C-C) sp3-like bonds, relevance to breaking away of one hydrogen atom and one neighbouring carbon atom from the material*, i.e.,

AH11 = -AHbind.(graphane*[18-21]) = -AH(C-H)graphane*[18-21] -

1.5 AH(c -C)graphane*[18-21]. (13)

Hence, substituting in Eq. (13) the above noted AH(C-H)graphane*[18-21] value, one [18-21] can evaluate the desired quantity A/f(c-ClgraPhane*[i8-2i] ~ 4.9 eV, which coincides (within the errors) with the analogical quantities for perfect graphene and perfect graphite.

The same value of AHiC.C] ~ 4.9 eV can be evaluate (in a similar manner [18-21]), for instance, for hydrogenated (up to composition C2H) single-walled graphite nanotubes* [10-12] and hydrofullerene* C60H36 [13].

Compearing of the obtained in such a way [18-21]

values of AH(C-C)graphane*[18-21], AH(C-C)nanotubes*[18-21], AH(C-C)hydrofullerene*[18-21], AH(C-C)graphene[25], AH(C-C)graphene[45,18-21] and AH(C-C)graphite[45,18-21] shows that the elastic and

intrinsic strength properties (and particularly, a Young's modulus (E)) of graphane*-like nanostructures can be close to those for graphene.

In this connection, it is relevant to note that a unique experimental value from work [48] (mentioned in [5] as Ref. S3) of a Young's modulus of graphene is Egraphene[48] = 1.0 terapascals.

As was noted in [5] (p.p. 1-S - 2-S), when a hydrogenated graphene membrane had no free boundaries (a rigidly fixed membrane), in the expanded regions of it the lattice was stretched isotropically by nearly 10 % (i.e., the elastic deformation degree £fixmembr [5] ~ 0.1) with respect to pristine graphene (Fig. SIB in [5]). This amount of stretching (s ~ 0.1) is close to the limit of possible elastic deformations in graphene (Ref. S3 = [48]), and, indeed, they [5] observed some of their membranes to rupture during hydrogenation. They [5] believed that the stretched regions are likely to remain non-hydrogenated. They [5] found that instead of exhibiting random stretching, hydrogenated graphene membranes normally split into domain-like regions of the size of the order of 1 |im. and that the annealing of such membranes led to complete recovery of the periodicy in both stretched and compressed domains.

By using the experimental value [5] of the degree of elastic deformation (6'|L,mem|:i||s » 0.1) of the hydrogenated fixed graphene membranes and the experimental value [48] of a Young's modulus of graphene (Egraphene[48] = 10 TPa), one [18-21] can evaluate (within Hooke's law approximation) the stretching stress value (<Tflx.membr.[5] » (fife.membr.is] £graphene[48]) »0.1 TPa) in the expanded regions (domains or grains) of the material [5].

This analytical result is consistent with the analytical results of the further consideration (in this study) of the related data from [15-21], relevance to a possibility of existence of hydrogenated graphane*-like nanostructures possessing of a Young's modulus value close to the graphene one (Egraph ane* [18-21] » -£graPhene[48] ~~ 1-0 TPa).

2.4. Considearation of data [5] on hydrogen desorption in the hydrogenated mono- and bi-layer epitaxial graphene samples

In [5], both the graphene membrane samles (considered above) and the epitaxial graphene and bi- g graphene samples (on substrate Si02) were exposed to a '"Vjj^ cold hydrogen dc plasma (for 2 hours to reach the saturation in the measured characteristics). It was used a low-pressure (0.1 mbar) hydrogen-argon mixture (10 % H2), and obviously that the apparent partial pressure of

Raman shift, cm Raman shift, cm

.O

Fig. 3. Changes in Raman spectra of graphene caused by hydrogenation. The spectra are normalized to have a similar intensity of the G peak. (A) Graphene on Si02. (B) Free-standing graphene. Red, blue, and green curves (top to bottom) correspond to pristine, hydrogenated, and annealed samples, respectively. Graphene mas hydrogenated for -2 hours, and the spectra were measured with a Renishaw spectrometer at wavelength 514 nm and low power to avoid damage to the graphene during measurements. (Left inset) Comparison between the evolution of D and D' peaks for single- and double-sided exposure to atomic hydrogen. Shown is a partially hydrogenated state achieved after 1 hour of simultaneous exposure of graphene on Si02 (blue curve) and of a membrane (black curve). (Right inset) TEM image of one of our membranes that partially covers the aperture 50 , m in diameter.

Fig. 3ft from [5].

Raman spectra for hydrogenated and subsequently annealed graphene membranes (Fig. 3B in [5]) were rather similar to those for epitaxial graphene samples (Fig. 3A in [5]), but with some notable differences. If hydrogenated simultaneously (for 1 hour) and before reaching the saturation (a partial hydrogenation), the D peak area for a membrane was by a factor of about two greater than the area for graphene on a substrate (Fig. 3A, inset in [5]), which indicates the formation of twice as many C-H sp3 bonds in the membrane. As has been also noted in [5], this result agrees with the general

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expectation that atomic hydrogen attaches to both sides of membranes.

Moreover [5], the D peak area became up about three times greater than the G peak area after prolonged exposures (for 2 hours, a near-complete hydrogenation) of membranes to atomic hydrogen (Fig. 3B in [5]).

As is also seen from Fig. 3 in [5], the integrated intensity area of the D peak in Fig. 3B corresponding to the adsorbed hydrogen saturation concentration in the graphene membranes is larger by factor of about 3 of the area of the D peak in Fig. 3A corresponding the hydrogen concentration in the epitaxial graphene samples.

It may be related to some partial hydrogenation (for instance, localized in some defect nanoregions [33-42, 49]) of the epitaxial graphene samples even after the prolonged (for 3 hours) exposures, i.e. after reaching of their near-saturation.

It is expedient to note that in [5], the absolute values of the adsorbed hydrogen concentration (C0) were considered, neither for the hydrogenated graphene membranes, nor for the hydrogenated epitaxial graphene samples.

According to a private communication from D.C. Elias, a near-complete desorption of hydrogen (-AC/C0 -0.95) from a hydrogenated epitaxial graphene on a substrate SiO2 (Fig. 3B in [5]) was achieved by annealing it in (90% Ar/10% H2) mixture at T = 573 K for 2 hours (i.e., ib.95(epitax.[5]i573K = 7.2-103 s). Hence, by using Eq. (8), one can evaluate the quantity of i:i 63(ePitax [5])573K = 2.4-103 s, wliich is by about six orders less of the evaluated quantity of ro.63(membr.[5]>573K = 1.5-109 s.

It can be taken into account that changes in Raman spectra of graphene [5] caused by hydrogenation were rather similar (with respect to locations of D, G, D', 2D and (D+D') peaks) both for the epitaxial graphene on Si02 and for the free-standing graphene membrane (Fig. 3 in [5]). Hence, one can suppose that K,,e|)ll(ix|s|, = vc.

: K

0(membr.[5])[47,13] ~~ Иг-Н(тетЬг.[5])[47ЛЗ]

5-10 s"

H(epitax.[5] ) :

1. Then, by substituting in Eq.(9) quantities of К =

K

(epitax.[5])573K

= 1/Гг

0.63(epitax.[5])573K

and K

0

£o(membr.[5])[47,i3]. one can evaluate AHdes .(epitax.[5]) AHc-H(epitax.[5]) ~ 2.0 eV. Here, the case is supposed of a non-diffusion-rate-limiting kinetics, when Eq. (9) corresponds to the Polanyi-Wigner one [14].

certainly, these tentative thermodynamic characteristics of the hydrogenated epitaxial graphene on a substrate SiO2 [5] could be directly confirned and/or modified by receiving and treatment (within Eqs. (8, 9)) of experimental data on ib.63(ePitax.[5]) at several annealing temperatures.

And it is easy to show that: 1) these analytical results are not consistent with the mass spectrometry data (Fig. S5 in [5]) on thermal desorption of hydrogen from a specially prepared single-side graphane; 2) they can not be described in the framework of the theoretical models and ctaracteristics of thermal stability of single-side hydrogenated graphene [6] or graphone [9].

According to the further considerations (in this

study), it may be a hydrogen desorption case of a diffusion-rate-limiting kinetics, when K0 ^ v, and Eq. (9) does not correspond to the Polanyi-Wigner one [14].

Figure S5. Desorption of hydrogen from single-sided graphane. The measurements were done by using a leak detector tuned to sense molecular hydrogen. The sample was heated to 300°C (the heater was switched on at i - 10s). Control samples (exposed to pure argon plasma) exhibited much weaker and featureless response (<5 < 0 s mbar L/s), which is attributed to desorption of water at heated surfaces and subtracted from the shown data (water molecules are ionized in the

Figure S51T from [5].

By using the method [14] of treatment of thermal desorption spectra, relevance to the mass spectrometry data (Fig. S5 in [5]) on thermal desorption of hydrogen from a specially prepared single-side graphane (under heating from, obviously, the room temperature to 573 K for 6 minutes), one can obtain the following results: 1) the total integrated area of the thermal desorption spectra corresponds to -2-10"8 g of desorbed hydrogen; 2) it can be approximated by three therodesorption (TDS) peaks (# 1 , # 2 and # 3); 3) TDS peak # 1 (-30 % of the total area, 7m(i;, , ~ 370 K) can be characterized by the activation energy itiDs-peak # i[5] = 0.6 ± 0.3 eV and by the per-exponential factor of the reaction rate constan k, Jl TI ,s_ peak # i [5]) - 2-107 s"1; 4) TDS peak # 2 (-15 % of the total area, Tmax#2 = 445 K) can be characterized by the activation energy itiDs-peak # 2[5] = 0.6 ± 0.3 eV and by the per-exponential factor of the reaction rate constan k, Jl TI ,s_ peak # 2[5]) - MO® s"1; 5) TDS peak # 3 (-55 % of the total area, Tmax#3 = 540 K) can be characterized by the activation energy itiDs-peak # 3[5] = 0.23 ± 0.05 eV and by the per-exponential factor of the reaction rate constan k,,'[ i)s-|,e„i: # 3[5]> - 2.4 s"1. These analytical results show that all the three above noted thermal desorption (TDS) processes (# 1TDS[5], # 2TDS[5] and # 3TDS[5]) may related to a hydrogen desorption case of a diffusion-rate-limiting kinetics, when in Eq. 9 the quantity of K„ « (Aiapp. / L2) and the quantity of A/fdes = (/l|;i|;i. where A>aPP is the perexponent factor of the apparent diffusion coefficient (^app. = Doapp.exp(-Qapp./kBT)), L is the characteristic diffusional size (length), and Qapp. is the apparent diffusion activation energy [14].

TDS process # 3TDS[5] may be related to TDS process (or peak) I in [14, 18-21], for which the apparent

diffusion activation energy is 0{

app.I ;

: 0.2 eV » E-

TDS-peak #

3[5]

and D0

Oapp.I

3-10"3 cm2/s. Hence, one can evaluate the

quantity of iTDS.peak # 3[5] ® (^Oapp.I / -^'«TDS-peak # 3[5]l) »

3.5-10"3 cm, wliich may be related to the linear size of the graphene specimens.

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Thus, TDS process # 3TDS[5] may be related to chemisorption models "H" and/or "G" (Fig. 1) corresponding to TDS process (or peak) I in [14, 18-21].

TDS processes # 1TDS[5] and # 2TDS[5] may be related (in some extent) to chemisorption models "H" and/or "G" (Fig. 1). Model "H" corresponds to TDS process (or peak) II in [14, 18-21], for which the apparent diffusion activation energy Oapp.n ~ 1.2 eV (tliat is comparatively close to itiDs-peak # i,2[5> Table 1) and Afcpp.ii « 2-103 cnr/s.

Obviously, chemisorption models "H" and/or "G" (Fig. 1) can be applied only for the defect nanoregions in the epitaxial graphene flakes [5] (for instance, vacancies, grain boundaries (domains) and/or triple junctions (nodes) of the grain-boundary network [33-42, 49]), where the dangling carbon bonds can occur.

It is expedient to note that the above consinered (in Items 2.1-2.3) chemisorption of atomic hydrogen on graphene membranes [3-5] may be related to model "F*" [14, 18-21], which is relevance to chemisorption of a single hydrogen atom on one of carbon atoms possessing of 3 not occupied (by hydrogen) nearest carbons (but not two hydrogen atoms on two carbons, as in model "F"). Model "F*" is characterized [14, 18-21] by the quantity \//ii;_i[ ,■■ illx-21 ~ 2.5 eV that coincides with the similar quantities for graphanes [3-5] (Table 1).

o

Table 1. Some analytical (an.) results of Item 2.

Quantity => Material U AH(C-H) (eV) A^bind.) (eV) AH(c-c) (eV) A-Hdes.) (eV) ^0(des.) (s-1)

Graphane [3] (2.5±0.1)an. 6.56 (2.7)an. (2.5)an.

Graphane [25] 1.50 5.03 (2.35V (1.5)an.

Graphane [4] Graphane [4]an. 2.46±0.17 2.46±0.17 2.46±0.17 2.46±0.17 (2.1±0.5)-1017 2.0-1015

Graphane membr. [5]an. 2.5 ±0.1 2.6 ± 0.1 2.5 ±0.1 2.6 ± 0.1 7-1012 5-1013

Graphane epitax. [5]an. 1.84 1.94 1.84 1.94 7-1012 5-1013

Graphane ep. TDS #1 [5]an. 0.6 ± 0.3 2-107

Graphane ep. TDS #2 [5]an. 0.6 ± 0.3 1-106

Graphane ep. TDS #3 [5]an. 0.23 ± 0.05 2.4

Graphene [25] 7.40 (4.93)an.

Graphane* [18-21] 2.6 9.95 4.9 2.6

Graphite [45, 1821] 7.41 ± 0.05 4.94 ± 0.03

G H

Fig. 1. Schematics (used in [14, 18-21]) of some theoretical models (ab initio molecular orbital calculations [50]) of chemisorption of atomic hydrogen on graphite on the basal and edge planes.

In work [5], the same hydrogenation procedures of the 2 hours long expositions have been applied also to the third system - to bilayer epitaxial graphene on SiO2/Si wafer. Bilayer samples showed little change in their charge carrier mobility and a small D Raman peak, as compared to sinle-layer epitaxial graphene on SiO2/Si wafer exposed to the same hydrogenation procedures (Fig. S7 in [5]). The authors [5] believed that higher rigidity of bilayers suppressed their rippling, thus reducing the probability of hydrogen adsorption.

The further consideration (in this study) of some ">An" other known experimental data on hydrogenation and thermal stability characteristics of mono-layer, bi-layer and three-layer epitaxial graphene systems shows an important role also of some defects in graphene networks [33-42, 49], relevance to the probability of hydrogen adsorption and the permeability of graphene networks for atomic hydrogen.

Some analytical results of Item 2 are presented in Table 1.

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Diamond [45, 18-21] 7.38 ± 0.04 3.69 ± 0.02

Hydrofullerene СбоНз6 [13] 2.64 ± 0.01

Hydr. SSCNs (as C2H, [12]) 2.5 ± 0.2

3. Analysis and comparison of data [51-54] 3.1. Analysis of the Raman spectroscopy data [51] on thermal desorption of hydrogen from hydrogenated graphene flakes

In [51], it was reported the hydrogenation of single and bilayer graphene flakes by an argon-hydrogen plasma produced in a reactive ion etching (RIE) system. It was analysed two cases: one where the graphene flakes are electrically insulated from the chamber electrodes (by the SiO2 substrate) and one where the flake is in electric contact with the source electrode (a graphene device). Electronic transport measurements in combination with Raman spectroscopy were used to link the electric mean free path to the optically extracted defect concentration, which is related to the defect distance (Ldef.). It was shown that under the chosen plasma conditions the process does not introduce considerable damage to the graphene sheet and that a rather partial hydrogenation (CH < 0.05%) occurs primarily due to the hydrogen ions from the plasma and

not due to fragmentation of water adsorbates on the graphene surface by highly accelerated plasma electrons. To quantify the level of hydrogenation, it was used the integrated intensity ratio (/D//G) of Raman bands. The hydrogen coverage (CH) determined from the defect distance (Zdef) does not exceed ~ 0.05 %.

It was perform [51] the heating of the hydrogenated single graphene flakes (on the SiO2 substrate) in a nitrogen environment on a hot-plate, with temperature ranging from 348 to 548 K, each time (At) for 1 min. As can be seen in Fig. 2c in [51], heating results in a decrease of the integrated intensity ratio (/D//G) of Raman bands. Within a formal kinetics approach [14, 46], the averaged kinetic data for samples of 20, 40 and 10 min exposure can be treated by using Eq. (7) transformed to a more suitable form (7'): K ~ -((A('/M)/C). where At = 60 s, AC and C are determined from Fig. 2 (c ) in [51].

FIG. 2ÍÍ from [51].

It results in finding 5 values of the reaction (desorption) rate constant (K) for 5 temperatures (T = 348, 398, 448, 498 and 548 K). Their temperature dependence is described by Eq. (9). Hence, the desired quantities are determined (Table 2): the reaction (desorption) activation energy A/f(des >[51] = 0.11 ± 0.07 eV, and the per-exponential factor of the reaction rate

constant £oides.i[5i] ~ 015 s"1. Hence, the desorption time, for instance, at 553 K is r0.63(des.>[5i]553K ~ 70 s (Table 2).

The obtained quantities of A/f(des >[51] and A.'0(des.l[51] are close (within the errors) to those for TDS process # 3 (Table 1). These two desorption processes may be related to TDS process (or peak) I in [14, 18-21], for which the apparent diffusion activation energy is (_>l|;i|;i| » 0.2 eV « -ElDS-peak # 3[5] ~ A/f(des. )[51]-

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By taken into account the facts that the RIE exposure regime [51] lias a form of (ID/IG) ~ Zdef "2 (for (Id/Iq) < 2.5), Zdef « 11 - 17 nm (Fig. 4 in [51]) and CH < 5-10"4, one can suppose that the hydrogen adsorption centers in the single graphene flakes (on the SiO2 substrate) are related some point nanodefects (for instace, vacancies and/or triple junctions (nodes) of the grain-boundary network [33-42, 49]) of diameter i/def » const. In such a model, the quantity CH can be sutisfactory described as:

! «H (4fcf.)2 / (¿def.)2

(14)

number of hydrogen atoms

CH

where nu « const, is adsorbed by a center.

As was also found, after the Ar/H2 plasma exposure the (/D//G) ratio for bilayer graphene device is large than that for single graphene device (inset in Fig. 4 in [51]). As was noted [51], this observation is in contradiction to the Raman ratios after exposure of graphene to atomic hydrogen [5] (Item 2.4) and when other defects are introduced.

3.2. Analysis of the STM and STS data [52] on reversible hydrogenation of epitaxial graphene and graphite surfaces

In [52], the effect of hydrogenation on the topography and electronic properties of graphene (grown by CVD on top of a nickel surface) and graphite (HOPG) surfaces were studied by scanning tunneling microscopy (STM) and spectroscopy (STS). The surfaces were chemically modified using 40 min Ar/H2 plasma (of the power 3 W) treatment (Figure 1 in [52]). It has been determined that the hydrogen chemisorption on the surface of graphite/graphene opens on average an energy bandgap of 0.4 eV around the Fermi level. Although the plasma treatment modifies the surface topography in an irreversible way (Figure 1 in [52]), the change in the electronic properties can be reversed by moderate thermal annealing (10 min at 553 K) and the samples can be hydrogenated again to yieled a similar, but slightly reduced, semiconducting behavior after the second hydrogenation.

HOPG

(a) Freshly cleaved

(b) 40 min. Ar/H2 plasma (c) 10 min. at 280°C

r

CVD Graphene

(d) As received

(e) 40 min. Ar/H2 plasma (f) 10 min. at 280°C

-1.5 -1 -0.5 0 0.5 1 1 5 Bias voltage (V)

(h)

-1 -05 0 05 1 Bias voltage (V)

Figure l. a-f) Topography images acquired in the constant-current STM mode: a-c) HOPG, d-f) graphene grown by CVD on top of a nickel surface at different steps of the hydrogenation/dehydrogenation process. a,d) Topography of the surface before the hydrogen plasma treatment. For the HOPG, the typical triangular lattice can be resolved all over the surface. For the CVD graphene, a Moiré pattern, due to the lattice mismatch between the graphene and the nickel lattices, superimposed onto the honeycomb lattice Is observed. b,e) After AO min of H2/Ar plasma treatment, the roughness of the surfaces increases. The surfaces are covered with bright spots where the atomic resolution is lost or strongly distorted. c,f) Graphene surface after 10 min of moderate annealing; the topography of both the HOPG and CVD graphene surfaces does not fully recover Its original crystallinity. g) Current-voltage traces measured for a CVD graphene sample In several regions with pristine atomic resolution, such as the one marked with the red square in (e). h) The same as (g) but measured in several bright regions, such as the one marked with the blue circle in (e), where the atomic resolution is distorted.

Figure 1ft from [52].

These data [52] show that the time of desorption from both the epitaxial graphene/Ni samples and HOPG samples of about 99 % of hydrogen under 553 K annealing is za99(des,>[52]553k ~ 6-102 s. Hence, by using Eq. (8), one can evaluate the quantity iodides,i[52]553k ~ 130 s (Table 2), which is close (within the errors) to the

similar quantity r0.63(des,>[5i]553K ~ 70 s (Table 2) for the epitaxial graphene flakes [51] considered in the previous Item 3.1.

Figure 3 in [53] is related to graphene/Ni samples (Figure 1 in [52]).

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Figure 3. (a) and (b) are topography images, acquired in the constant current STM mode, of graphene grown by CVD on top of a nickel surface before and after hydrogenation. (c) Several current vs. voltage traces measured for a CVD graphene sample in regions with pristine atomic resolution like the one marked with the red circle in (b). (d) Same as (c) but measured in several bright regions, like the one marked with the blue square in (b), where the atomic resolution is distorted. Adapted from Ref. [8].

Ref. [8] in [53] corresponds to Ref. [52] in this study.

Figure 3ft from [53].

As is noted in [53], Figure 3 shows an 9 x 9 nnr topographic image acquired in the constant current STM mode of the surface of graphene grown on nickel by CVD. Before the plasma treatment, the CVD graphene exhibites a Moiré pattern superimposed to the honeycomb lattice of graphene (Figure 3(a) in [53]). This is due to the lattice parameter mismatch between the graphene and the nickel surfaces and it is the characteristic of most of epitaxial graphene samples.

On the other hand [53], for the hydrogenated CVD graphene, the expected structural changes are twofold. First, the chemisorption of hydrogen atoms will change the sp2 hybridization of carbon atoms to tetragonal sp3 hybridization, modifying the surface geometry. Second, the impacts of heavy Ar ions, present in the plasma, could also modify the surface by inducing geometrical displacement of carbon atoms (rippling graphene surface) or creating vacancies and other defects (for instance, grain or domain boundaries [33-42, 49]). Figure 3(b) in [53] shows the topography image of the

surface CVD graphene after the extended (40 min) plasma treatment. The nano-order-corrugation increases after the treatment and there are brighter nano-regions (of about 1 nm in height and several nm in diameter) in which the atomic resolution is lost or strongly distorted. It has been also found [53] that these bright nano-regions present a semiconducting behaviour while the rest of the surface remains conducting (Figure 3(c)-(d) in [53]).

It is reasonable to assume that the most of the chemisorbed hydrogen is localized into these bright nano-regions, which have a blister-like form. Moreover, it is also reasonable to assume that the monolayer (single) graphene flakes on the Ni substrate are permeable to atomic hydrogen only in these defect nanoregions. This problem [8, 32] has been formulated in Item 1 (Introduction).

A similar model may be valid and relevance for the HOPG samples (Figure 1 in [52].

Figures 1, 2 in [53] are related to graphene/SiO2 samples [53, 54].

Figure 1. (a) Optical image of the coarse tip positioning on a few-layers graphene flake, (b) AFM topography image of the interface between the few-layers graphene flake and the Si02 substrate. Areas with different number of layers (labelled as>10L, 6L, 4Land 1L) are found, (c) Topographic line profile acquired along the dotted line in (b), showing the interface between the Si02 substrate and a monolayer graphene region, (d) STM topography image of the region marked by the dashed rectangle in (b). Adapted from Ref. [7]. Ref. [7] in [53] corresponds to Ref. [54] in this study. Figure 1ft from [53].

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20 40

Distance (nm)

Figure 2. (a) and (b) show the local tunnelling decay constant maps measured on a multilayer and a single-layer region, respectively, (c) Radial autocorrelation function of the local tunnelling decay image in (b). Adapted from Ref. [7]. Ref. [7] in [53] corresponds to Ref. [54] in this study. Figure 2ft from [53].

It has been found [53] that when graphene is deposited on a SiO2 surface, the charged impurities present in the graphene/substrate interface produce strong inhomogeneities of the electronic properties of graphene. On the other hand, as has been shown [53], how homogeneous graphene grown by CVD can be altered by chemical modification of its surface (by the chemisoption of hydrogen). It strongly depresses the local conductance at low bias indicating the opening of a bandgap in graphene.

The charge inhomogeneities (defects) of epitaxial hydrogenated graphene/ SiO2 samples do not show long range ordering, and the mean spacing between them is Zdef = 20 mn (Figure 2 in [53]). It is reasonable to assume [53] that the charge inhomogeneities (defects) are located at the interface between the SiO2 layer (300 mn thick) and the graphene flake.

A similar quantity (Zdef «11-17 mn (Fig. 4 in [51])) for the hydrogen adsorption centers in the single graphene flakes on the SiO2 substrate has been considered in Item 3.1.

3.3. Analysis of the HREELS/LEED data [55] on thermal desorption of hydrogen from hydrogenated graphene on SiC

In [55], hydrogenation of deuterium-intercalated quasi-free-standing monolayer graphene on SiC(0001) was obtained and studied with low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). While the carbon honeycomb structure remains intact, it has been shown that a significant band gap opens in the hydrogenated material. Vibrational spectroscopy evidences for hydrogen chemisorption on the quasi-free-standing graphene was provided and its thermal stability was studied (Fig. 3 in [55]).

30--

20--

ь

Quasi tree standing grapnene

-■30---

Buffer Layer

Hydrogenated quasi free standing

, ° ° О °

20--

10-

-O O-O-O

E0=5eV Specular

J_i_I_i_I_i_I_i_L

As-prepared 300 500 700 900 1100 1300 samples Annealing temperature (K)

FIG, 3, (Color online) Evolution of the HREELS elastic peak FWHM of SiC-D/QFMLG-H upon annealing, The annealing temperature uncertainty is estimated to be ±5%, Error bars represent the =cr variation of FWHM measured across the entire surface of several samples,

FIG. 3ft from [55].

Deuterium intercalation, transforming the buffer layer in quasi-free-standing monolayer graphene (denoted as SiC-D/QFMLG), was performed with a D atom exposure of ~5-1017 cm"2 at a surface temperature of 950 K. Finaly, hydrogenation up to saturation of quasi-free-standing monolayer graphene was performed at room temperature with a H atom exposure > 3-1015 cm"2. The latter sample is denoted as SiC-D/QFMLG-H to stress the different isotopes used, first, to prepare quasi-free-standing monolayer graphene (the D-intercalation step), and second, to hydrogenate it.

According to a private communication from R. Bisson, the temperature indicated at each point in Fig. 3 in [54] corresponds to successive temperature ramp (not linear) of 5 minutes, i.e. the 5th point at 600K on figure 3 has been obtained after 4 previous HREELS/LEED measurements and 3 previous ramps of 5 minutes to lower temperatures.

Within a formal kinetics approach (for the first order reactions) [14, 46], one can treat the above noted points at T1 = 543 K, 611 К and 686 K, by using Eq. (8) transformed to a more suitable form (8'): К, ~ -(ln(CI(\„)li). where t = 300 s, and the corresponding quantities C0i and C are determined from Fig. 3 in [54].

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It results in finding values of the reaction (hydrogen desorption from SiC-D/QFMLG-H samples) rate constant (£i) for 3 temperatures (T = 543, 611 and 686 K). Their temperature dependence is described by Eq. (9). Hence, the desired quantities are determined (Table 2): the reaction (hydrogen desorption) activation energy

АЯЛ

= 0.7 ± 0.2 eV,

1 des.(SiC-D/QFMLG-H )[55]

exponential factor of the reaction rate constant k, 9-102 s"1.

and the per-

0des.(SiC-

D/QFMLG-H )[55] '

The obtained quantity A/fdes (SlC-D/QFMLG-H)[55] is close (within the errors) to the similar ones (^TDs-peak # i[5] and ^TDS-peak # 2[5]) for TDS processes # 1 and # 2 (Item 2.4, Table 1). But the obtained quantity ^odes.(&c-D/QFMLG-H)[55] differs by several orders from the similar ones (K0des.(TDS-

peak # 1[5]) and ^0des.(TDS-peak # 2[5])) for TDS processes # 1

and # 2 (Item 2.4, Table 1). Nevertheless, these three desorption processes may be (in some extent) related to chemisorption models "H" and/or "G" (Fig. 1). Model "H" corresponds to TDS process II in [14, 18-21], for which the apparent diffusion activation energy is Oapp n ~ 1.2 eV.

In the same way, one can treat the points (from Fig. 3 in [54]) at T = 1010, 1120 and 1200 K, those are related to the intercalated deuterium desorption from SiC-D/QFMLG samples. It results in finding the desired quantities (Table 2): the reaction (deuterium desorption) activation energy AHdes.(SlC-D/QFMLGi[55] = 2.0 ± 0.6 eV, and the per-exponential factor of the reaction rate constant A'0des.(Sic-D/QFMLGi[55] ~ 1-106 s"1. It may be

related [55] to hydrogen chemisorbed on the silicon atoms of the SiC substrate below the graphen plane. . Formaly, this desorption process (obviously, of a diffusion-limiting character) may be described similarly to TDS process (peak) III in [14, 18-21], and (within such a model) the apparent diffusion activation energy may be close to the breake-down energy of the Si-H bonds. As was concluded in [55], the exact intercalation mechanism (hydrogen diffusion through the anchored graphene lattice, at defect or at boundary of the anchored graphene layer) remains an open question.

It is reasonable to assume that the quasi-free-standing monolayer graphene on the SiC-D substrate is permeable to atomic hydrogen (at room temperature) in some defect nanoregions (probably, in vacancies and/or triple junctions (nodes) of the grain-boundary network [33-42, 49]).

It is expedient to note that the HREELS data [55] on bending and stretching vibration C-H frequencies in SiC-D/QFMLG-H samples (153 meV (3.7-1013 s1) and 331 meV (8.0-1013 s"1), respectively) are consistent with the considered in Item 2.2 related HREELS data [47] for the epitaxial graphene.

The obtained characteristics (Table 2) of the desorption processes [51, 52, 55] show that these processes may be of a diffusion-rate-controlling character [14].

Table 2. Some analytical results of Items 3.1-3.3

Quantity => Material U AH(des.) (eV) K0(des.) ( s-1) ^0.63(des.)553K (s)

Graphene flakes/Si02 [51] 0.11 ± 0.07 0.15 70

Graphene/Ni [52] HOPG [52] 130 130

(SiC-D/QFMLG-H) [55] 0.7 ± 0.2 9102

(SiC-D/QFMLG) [55] 2.0 ±0.6 l-lO6

3.4. Analysis of the Raman spectroscopy data [56] on thermal desorption of hydrogen from hydrogenated graphene layers on SiO2 substrate

In [56], graphene layers on SiO2/Si substrate have been chemically decorated by radio frequency hydrogen plasma (the power of 5-15 W, the pressure of 1 Tor) treatment for 1 min. As has been shown, hydrogen coverage investigation by Raman spectroscopy and micro-x-ray photoelectron spectroscopy characterization demonstrates that the hydrogenation of single layer graphene on SiO2/Si substrate is much less feasible than that of bilayer and multilayer graphene; both the hydrogenation and dehydrogenation processes of the graphene layers are controlled by the corresponding energy barriers, which show significant dependence on the number of layers.

The results [56] on bilayer graphene/SiO2/Si are in contradiction to the results [5] (Item 2.4) on a negligible hydrogenation of bilayer epitaxial graphene on SiO2/Si wafer, when obviously other defects are produced.

Within a formal kinetics approach [14, 46], the kinetic data from Fig. 6 (a) in [56] for single layer graphene samples (1LG-5W and 1LG-15W ones) can be treated. It is used Eq. (7) transformed to a more suitable form {!'): K ~ -((A('/At)/C). where At = 1800 s, AC and C are determined from Fig. 6 (a ) in [56].

It results in findings for 1LG-15W samples 3 values

of the I[56] reaction rate constant K

I[56](1LG-15W)

for 3

temperatures (T = 373, 398 and 423 K), and 3 values of

the II[56] reaction rate constant K

II[55](1 LG-15W)

for 3

temperatures (T = 523, 573 and 623 K). Hence, by using Eq. (9), the desired quantities for 1LG-15W samples are determined (Table 3): the I[56] reaction activation energy

AH

des.I[56](1LG-15W)

= 0.6 ± 0.2 eV, the per-exponential

factor of the I[55] reaction rate constant K

2-10

I[55]

the II,

[56]

reaction

AH,

des.II[56](1LG-15W)

= 0.19 ± 0.07

0des.I[56](lLG-15W) ~

activation energy eV, and the per-

exponential factor of the II[56] reaction rate constant

K

0des.II[56](lLG-15W) '

310"2 s\

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-1

s

c

d <■«

ra

Figure 6. (a) The evolution of the D and G band intensity ratio l/0/JG)with annealing temperatures of 1 LG hydrogenated by 5 and 1 5 W. 1 Torr hydrogen plasma for 1 min; lb: the evolution of A(V^i) with annealing temperatures of 1 LG hydrogenated by 5 and 1 5 W. 1 Torr h ydrogen plasma for 1 min; (c) the evolution of the D and G band intensity ratio (fe/fa) with annealing temperatures of 2LG hydrogenated by 5 and 1 5 W. 1 Torr hydrogen plasma for 1 min; |d> the evolution of AI/q/'o) with annealing temperatures of 2LG hydrogenated by 5 and 1 5W, 1 Torr hydrogen plasma for 1 min. The asterisk (**> denotes the as-treated sample by H, plasma.

Figure 6ft from [56].

It also results in findings for 1LG-5W samples 4 values of the I[56] reaction rate constant ^I[56](iLG-5w) for 4 temperatures (T = 348, 373, 398 and 423 K), and 2 values of the II[56] reaction rate constant ^II[56](1LG-5W) for 2 temperatures (T = 523 and 573 K). Hence, by using Eq. (9), one can evaluate the desired quantities for 1LG-5W specimens (Table 3): the I[56] reaction activation energy A/ides i[56](1lg-5w> = 0.15 ± 0.04 eV, the per-exponential factor of the I[56] reaction rate constant ^'odes.i[56]( 1 lg-5w i « 210"2 s"1, the II[56] reaction activation energy A/fdeS.ii[56](iLG-5W) = 0.31 ± 0.07 eV, and the per-exponential factor of the II[56] reaction rate constant

K

0des.II[56](1LG-5W)

0.5 s-1.

AH

des.II[56](2LG-15W)

= 0.9 ± 0.3 eV, the per-exponential

factor of the II[56] reaction rate constant K<

d-103 s"1.

0des.II[56](2LG-15W)

A similar treatment of the kinetic data from Fig. 6 (c) in [56] for bilayer graphene 2LG-5W samples results in the findings of 4 values of the I[56] reaction rate constant ^I[56](2LG-5W) for 4 temperatures (T = 348, 373, 398 and 423 K), and 3 values of the II[56] reaction rate constant -Kn[56](2LG-5W) for 3 temperatures (T = 573, 623 and 673 K). Their temperature dependence is described by Eq. (9). Hence, one can evaluate the desired quantities (Table 3): the I,

AH

des.I[56](2LG-5W)

reaction activation energy = 0.50 ± 0.15 eV, the per-exponential

[56]

factor of the I[56] reaction rate constant K

1[56]

2T03 s"1, the II,

[56]

A similar treatment of the kinetic data from Fig. 6 (c) in [56] for bilayer graphene 2LG-15W samples results in the findings of 4 values of the II[56] reaction rate constant -Kn[56](2LG-15W) for 4 temperatures (T = 623, 673, 723 and 773 K). Hence, by using Eq. (9), the desired quantities are found (Table 3): the II[56] reaction activation energy

0des.I[56](2LG-5W)

reaction activation energy A/fdeSii[56](2LG-5W) = 0.40 ± 0.15 eV, and the per-exponential factor of the II[56] reaction rate constant

£odes.II[56](2LG-5W) ~ 1 S .

The obtained characteristics (Table 3) of the desorption processes I[56] and II[56] show that these processes may be of a diffusion-rate-controlling character [14].

Table 3. Some analytical (an.) results of Items 3.4, 3.5, 3.6 and 3.7

Quantities => Samples U AH(des.)I (eV) K0(des.)I (s-1) AH(des.)II (eV) K0(des.)II (s-1)

1LG-15W (graphene) [56] 0.6 ± 0.2 2-104 0.19 ± 0.07 310-2

2LG-15W (bi-graph.) [56] 0.9 ±0.3 MO3

1LG-5W (graphene) [56] 0.15 ± 0.04 2-10-2 0.31 ±0.07 5-10"1

2LG-5W (bi-graph.) [56] 0.50 ±0.15 2T03 0.40 ±0.15 1

HOPG [57],TDS-peaks I,II 0.6 ±0.2 1.5-104 1.0 ±0.3 2-106

Graphene/SiC [17] 3.6 2-1014

HOPG [59], TDS-peaks I,II HOPG [59], TDS-peak I 2.4 [59] (2.4 ± 0.5)an. (21010)an. 4.1 [59]

GNF [61,62] ,TDS-peaksI,II (2.4 ± 0.5)an

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3.5. Analysis of TDS and STM data [57] on HOPG treated by deuterium

In [57], results are present of scanning tunneling microscopy (STM) study of graphite (HOPG) treated by atomic deuterium, which reveal the existence of two distinct hydrogen dimer states on graphite basal planes (FIG. 1 and FIG. 2 (b), from [57]). The density functional theory calculations allow [57] to identify the atomic structure of these states and to determine their recombination and desorption pathways. As has been supposed [57], direct recombination is only possible from one of the two dimer states. As has been concluded [57], this results in increased stability of one dimer species and explains the puzzling double peak structure observed in temperature programmed desorption (TPD or TDS) spectra for hydrogen on graphite (FIG. 2 (a), from [57]).

FIG. I (color), (a) STM image (103 X I 14 Â2) of dimer structures of hydrogen atoms on the graphite surface after a I min deposition at room temperature. Imaging parameters: V, = 884 mV, /, = 160 pA. Examples of dimer type A and B are marked. Black arrows indicate the (2Ï 10) directions and white arrows indicate the orientation of the dimers 30° off. (b) Close up of dimer A structure in top white circle in image (a), (c) Close up of dimer B structure in lower white circle in image (a).

FIG. lft from [57].

FIG. 2 (color), (a) A mass 4 amu, i.e., D;,TPD spectrum from the HOPG surface after a 2 min D atom dose (ramp rate: 2 K/s below 450 K, I K/s above). The arrow indicates the maximum temperature of the thermal anneal performed before recording the STM image in (b) STM image (103 X 1 14 A2) of dimer structures of hydrogen atoms on the graphite surface after a I min deposition at room temperature and subsequent anneal to

525 K (ramp rate: I K/s, 30 s dwell at maximum temperature).

shows a higher resolution STM image of dimer structures of hydrogen atoms on the graphite surface after a 6 min deposition at room temperature and subsequent anneal to 550 K. Imaging

FIG. 2ft from [57].

By using of the described in [14] method of TPD

(TDS) peak treatment (for the first order reactions), relevance to TPD (TDS) peak I[57] (-65 % of the total area, 7m(l,|[S- ~ 473 K) in FIG. 2(a) in [57], one can obtain values of the reaction I[57] rate constant (/J^- = A bides ii[57]) for several temperatures (for instance, T = 458, 482 and 496 K). Their temperature dependence can be described by Eq. (9). Hence, the desired quantities are defined (Table 3): the reaction (desorption) I[57] activation energy A/A,|es il[s- = 0.6 ± 0.2 eV, and the per-exponential factor of the reaction I[57] rate constant A"0(des.)I[57] ~ 1.5-104 S 1.

In a similar way, relevance to TPD (TDS) peak II[57] (-35 % of the total area, 7:1Ililv.||[ — - 588 K)) in Fig. 2a in [57], one can obtain values of the reaction II[57] rate constant (/mi|s- = zb63(des >n[57]) for several temperatures (for instance, T = 561 and 607 K). Hence, the desired quantities are defined (Table 3): the reaction (desorption)

Щэт] activation energy AH(

(des.)II[S7]

= 1.0 ±0.3 eV, and the

per-exponential factor of the reaction II[57] rate constant

A"0(des.)II[57] ~ 2-10 S .

The obtained characteristics (Table 3) of the desorption processes I[57] and II[57] show that these processes, very probably, are of a diffusion-ratecontrolling character [14]. In a diffusion-rate-controlling case, these processes can not be described by using the Polanyi-Wigner equation (as it has been done in [57]).

The observed in [57] "nano-dimer states" or "nano-protrusions" (FIG. 1 and FIG. 2(b) in [57]) may be related to defect nanoregions, very probably, as grain (domain) boundaries [49] and/or triple and other junctions (nodes) of the grain-boundary network in the HOPG samples. Some defect nanoregions at the grain boundary network (hydrogen adsorption centres #I[57], mainly, the "dimmer B" structures) can be related to TPD (TDS) peak I[57], the others (hydrogen adsorption centres #II[57], mainly, the "dimmer A" structures) - to TPD (TDS) peak II[57].

In Figures 1(a) and 2(b) in [57], one can imagine some grain boundary network (with the grain size of

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about 2-5 nm) decorated (in some nano-regions at grain boundaries) by some bright nano-protrusions.

Similar "nano-protrusions" are observed and in graphene/SiC systems (Figures 1 and 2 in [58] and Figures 1 and 2 in [17]).

In Figures 1(a) and 2(b) in [58], one can also imagine some grain boundary network [33-42, 49] (with the grain size of about 2-5 nm) decorated (in some nano-regions at grain boundaries) by some bright nano-protrusions.

In [58], it was studied hydrogenation, by a beam of atomic deuterium 1012-1013 cmV1 (corresponding to PD « 10-4 Pa) at 1600 K and the time of exposure to 5-90 s, single graphene on SiC-substrate. It was observed the formation of graphene blisters intercalated with hydrogen in them (Figure 1 and 2 in [58]), similar to those observed on graphite [57] and grafene/SiO2 [17].

Blisters [58] disappeared after keeping the samples in vacuum at 1073 K (~ 15 min). Hence, by using Eq.

t * f*

>

i 0t%l

¿V1. ^

«B #§

• IV '

» I '

--a »■ —3!

Figure 1. (a) Scanning tunneling microscopy image of hydrogenated graphene. The bright protrusions visible in the image are atomic hydrogen adsórbate structures identified as A = ortho-dimers, B = para-dimers, C = elongated dimers, D = monomers (imaging parameters: V, = -0.245 V, /, = -0.26 nA). Inset in (a): Schematic of the A ortho- and B para-dimer configuration on the graphene lattice, (b) Same image as in (a) with inverted color scheme, giving emphasis to preferential hydrogen adsorption along the 6 x 6 modulation on the SiC (0001)-(1 x 1) surface. Hydrogen dose at Ttum = 1600 K, t = 5 s, F = 10'2-1013 atoms/cm2 s.

fm ^WJ * i yhPyj*

* • + : 'Í

r rSfoii -2- B

Figure 2. (a) STM image of the graphene surface after extended hydrogen exposure. The bright protrusions in the image are identified as atomic hydrogen clusters (imaging parameters: V, = -0.36 V, /, = -0.32 nA). Hydrogen dose at 7" = 1600 K, t = 90 s, F = 101!-10!3 atoms/cm2 s. (b) Large graphene area recovered from hydrogenation by annealing to 800 °C (imaging parameters: V, = -0.38 V, /, = -0.410 nA).

(8), one can evaluate the quantity of zb.63(des.j[58]io73K ~ 5 min. which coincides (within the errors) with the similar quantity of T^ides.i[59]io73K ~ 7 min evaluated for grapheme/SiC samples [17] (Item 3.6, Table 3).

A near-complete such decoration of the grain boundary network [33-42, 49] can be imagined in Figure 1(b) in [17]. And, as is seen in Figure 2 in [17], such decoration nano-regions (obviously, at the grain boundaries [33-42, 49]) have a blister-like cross-section of height of about 1.7 nm and width of 10 nm order.

According to the thermodynamic analysis (presented in Item 3.7, Eq. (15)), such blister-like decoration nano-regions (at the grain boundaries [33-42, 49]) may contain the intercalated gaseous molecular hydrogen at a high pressure.

Fij 1.irM:magesa>JECKdatV=-1 Vand!=500 pAof a" inonDtejErcaphene. b: afttrasraaJ hyiirage^oqiosure, and:, after a Ij^ehj'dro^neioosure.cl Selected pert ofthe I£ED pattern ciliectEC at i=107 cV Snin monolajtr pphene, e: rfr a smatl hjtl^ea expesire. and f' after a ji^e hjirpgin expos-j^.

rig. 2. SIM images of a; an island mated by the hydrogen exposure (V=-1V, 1=500 pA), b) line profile across the island, cj a dehydrogenated sample shown (v'5 X 6vT;830' smuure fan the buffer lay« (V=-2», 1=100 pA', and di lint profile across the i 5v3x &i J(H0' araaure.

Figures 1ft and 2ft from [17]. 3.6. Analysis of PES and ARPES data [17] on dehydrogenation of graphene/SiC samples

Atomic hydrogen exposures (at a pressure PH ~ 1T0" 4 Pa and temperature T = 973 K) on a monolayer graphene grown on the SiC(0001) surface are shown [17] to result in hydrogen intercalation. Shown [17] that the hydrogen intercalation induces a transformation of the monolayer graphene and the carbon buffer layer to bi-layer graphene without a buffer layer. The STM, LEED, and core-level photoelectron spectroscopy (PES) measurements reveal that hydrogen atoms can go underneath the graphene and the carbon buffer layer. This transforms the buffer layer into a second second graphene layer [17]. Hydrogen exposure (15 min) results

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initially in the formation of bi-layer graphene (blisterlike) islands of height of ~ 0.17 nm and a linear size of ~ 20-40 nm, covering about 40% of the sample (Figures 1b, 1e, 2a and 2b in [17]). With larger (additional 15 min) atomic hydrogen exposures, the islands grow in size and merge until the surface is fully covered with bi-layer graphene (Figure lc. If, 2c and 2d in [17]). A (V 3 x V 3) R30° periodicity is observed on the bi-layer areas (Figure 3 in [17]). Angle resolved photoelectron spectroscopy (ARPES) and energy filtred X-ray photoelectron emission microscopy (XPEEM) investigations of the electron band structure confirm that after hydrogénation the single 7i-band characteristic of monolayer graphene is replaced by two ti-bands that represent bi-layer graphene (Figure 6 in [17]).

Annealing an intercalated sample, representing bi-layer graphene, to a temperature of 1123 K, or higher, re-establishes the monolayer graphene with a buffer layer on SiC(0001).

The dehydrogenation was performed by subsequently annealing (for a few minutes duration) the hydrogenated samples at different temperatures, from 1023 to 1273 K. After each annealing step, the depletion of hydrogen was probed by PES and ARPES (Figures 4 and 5 in [17]). From these data, one can determine, by using Eqs. (8, 9), the tentative quantities: rondes. >[i 7] (at 1023 K and 1123 K), AH(de,1[17] » 3.6 eV and A0(des.1[17] « 2-1014 s"1 (Table 3). These results can be interpreted within the model (noted in [17]) of the interaction of hydrogen and silicon atoms at the graphene-SiC interface resulting in Si-C bonds at the intercalated islands. Obviously, that the quantities of A0(des.)[17] and AH(des.)[17] correspond to the Polanyi-Wigner equation [14], relevance for the Si-C bonds [17].

Figures 4ft and 5ft from [17].

3.7. Analysis of TDS and STM data [15, 59] on HOPG treated by hydrogen

Atomic hydrogen accumulation in HOPG samples and etching their surface on hydrogen thermal desorption (TD) have been studied in [15] using a scanning tunneling microscope (STM) and atomic force microscope (AFM). STM investigations [15] revealed that the surface morphology of untreated reference HOPG samples was found to be atomically flat (Fig. 2

(a)), with a typical periodic structure of graphite (Fig. 2

(b)). Atomic hydrogen exposure (treatment) of the reference HOPG samples (30-125 min at atomic hydrogen pressure PH - 10"4 Pa and near-room temperature (-300 K)) to different atomic hydrogen doses (D) has drastically changed the initially flat HOPG surface into a rough surface, covered with nanoblisters of the average radius -25 mn and the average height -4 mn (Figs. 2 (c) and 2 (d)).

Fij 4. Monraüzed C Is cort icvel spectre oi a - moooiayii pêphem beixe and after bj<iiogra£tiui snd substq jeii! aaeiling at 750.850. S50. and ICOO :C. b] Faly iijiliogenattd jijteie along with mala« jrapnene Won ¡ijimpatm H* spectra »nt attjuti at a pitfm tue® ti 500 eV.

Fig. 5. Normalized Si 2p care level spectra at' monolayer graphene before and after hydrogénation and subsequent annealing at 750, S50, 950, and 1000CC. The spectra were acquired at a photon energy of 140 eV.

Fig. 2. (from [15], with the permission). STM images of the untreated HOPG sample (under ambient conditions) taken from areas of (a) 60.8 x60.8 nm and (b) 10.9x10.9 nm (high resolution image of the square in image (a)). (c). AFM image (area of 1x1 nm) of the HOPG sample subjected to atomic hydrogen dose (D) of 1.81016 H0/cm2. (d) Surface height profile obtained from the AFM image reported in (c). The STM tunnel Vbias and current are 50-100 mV and 1-1.5 mA, respectively.

Thermal desorption (TD) of hydrogen has been found on heating of the HOPG samples under mass spectrometer control. As is shown in Fig. 3 (a), with the increase of the total hydrogen doses (D) to which HOPG samples have been exposed the desorbed hydrogen amounts (Q) increase and the percentage of D retained in samples (Q) approaches towards a saturation stage. After TD, no nanoblisters were visible on the HOPG surface, the graphite surface was atomically flat, and covered with some etch-pits of nearly circular shapes, one or two layers thick (Fig. 3 (b)). This implies that after release of the captured hydrogen gas, the blisters become empty of hydrogen and the HOPG surface restores back a flat

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surface morphology under action of the corresponding forces.

According to [15], nanoblisters found on the HOPG surface after atomic hydrogen exposure are simply monolayer graphite (graphene) blisters, containing hydrogen gas in molecular form (Fig. 4). As is suggested in [15] (Fig. 4), atomic hydrogen intercalates between layers in the graphite net through holes in graphene hexagons (due to the small diameter of atomic hydrogen compared to the hole size) and is then converted to a H2 gas form which is captured inside the graphene blisters (due to the relatively large kinetic diameter of hydrogen molecules).

But such interpretation [15] is in contradiction with the noted in Item 1 (Introduction) results [8, 32] "that it is almost impossible for a hydrogen atom passes through the six-membered ring of graphene at room temperature".

It is reasonable to assume (as it has been done in the previous Items) that in HOPG [15] samples atomic hydrogen passes into the graphite near-surface closed nanoregions (the graphene nanoblisters) through defects (probably, mainly, through tripple junctions of the grain and/or subgrain boundary network) in the surface graphene layer. And it is expedient to note that in Fig. 3(b), one can imagine some grain boundary network decorated by the etch-pits.

Fig. 4. (from [15], with the permission). Model showing the hydrogen accumulation (intercalation) in HOPG, with forming blister-like nanostructures. (a) Pre-atomic hydrogen interaction step. (b) H2, captured inside graphene blisters, after the interaction step. Sizes are not drawn exactly in scale.

Found [15] that the average blister has radius ~25 nm and height ~4 nm. Approximating the nanoblister to be a semi-ellipse form results in blister area (Sb ~ 2.0-10-11 cm2) and its volume (Vb ~ 8.4-10-19 cm3) [15]. The amount of retained hydrogen in this sample is Q ~ 2.8-1014 H2/cm2 and the number of hydrogen molecules captured inside the blister is n ~ (Q Sb) ~ 5.5•lO3 [15]. Hence, (within the ideal gas approximation, with accuracy of order of the magnitude) the internal pressure of molecular hydrogen in a single nanoblister at near-room temperature (T ~ 300 K) is P*H2 ~ {A-B (O ,S'h) T / J b} ~ 108 Pa.

The hydrogen molecular gas density in the blisters in [15] (at T » 300 K and P*H2 ~ MO8 Pa) can be estimated as p ~ {(<9 MH2 SbVF'b} ~ 0.045 g/cm3, where MH2 is the hydrogen molecule mass. It agrees with data [60] (considered in [18-21]) on the hydrogen (protium) isotherm of 300 K.

These results [15] can be quantitativelly described [18-21] (with accuracy of order of the magnitudes), within the thermodynamic approach [44, 46], by using the law of mass action for the reaction of 2H

H

2(gas_in_blisters)-

as follows [18]:

(gas)

(P*H2 /P h2) P*H2 AV )] / R T},

(Ph /P0h)2 exp{[Ahdis (15)

TAX

dis

Fig. 3. (from [15], with the permission). (a) Hydrogen storage efficiency of HOPG samples, desorbed molecular hydrogen (Q) versus dose (D) of atomic hydrogen exposure. (b) STM image for 600x600 nm area of the HOPG sample subjected to atomic hydrogen dose of 1.81016 H0/cm2, followed by hydrogen thermal desorption.

where P*H2 ~ 1-10 Pa is related to the backpressure (caused by /J*n2) - the so called surface pressure [44], !'u ~ MO-4 Pa is the atomic hydrogen pressure corresponding to the atomic flux [15], P0H2 = P0H = 1 Pa is the standard pressure [44, 46], AHdis = 448 kJ/mol(H2) is the experimental value [45] of the dissociation energy (enthalpy) of one mole of gaseous hydrogen (at room temperatures), ASdis = 98.5 J/K-mol is the dissociation entropy [45], AV ~ (Sb rb NA / n) is the apparent volume change, rb ~ 30 nm is the radius of curvature of nanoblisters (at the nanoblister edge, Fig. 4(b)), NA is the Avogadro number, R is the gas constant, T ~ 300 K; the quantity of (P*H2AV) is related to the isobaric-isothermic work of the nanoblister surface increase when intercalation of 1 mole of H2.

The value of the tensile stresses ab (caused by P*H2) in the graphene nanoblister "walls" of a thickness db and a radius of curvature rb can be evaluated from the thermodynamic equation (condition) of the thermal-elastic equilibrium [44, 18], which is also related to Eq. (15):

oi^OPVrb/2 dh)* (fib £1,), (16)

where sh is a degree of elastic deformation of the graphene nanoblister walls, and Eb is a Young's modulus of the graphene nanoblister walls.

Substituting in the first part of Eq. (16) the quantities 0fPV ~ MO8 Pa, rb ~ 30 nm and dh ~ 0.15 mn results in the value of crb[15] ~ M01" Pa.

The degree of elastic deformation of the graphene nanoblister walls, apparently reaches fib^j ~ 0.1 (Fig. 4(b)). Hence, within a Hooke's law approximation (the

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© Scientific Technical Centre «TATA», 2014

second part of Eq. (16)), one can estimate (with accuracy of one-two orders of the magnitude) the value of a Young's modulus of the graphene nanoblister walls [15]: /•.!;,[i s « (Ob[i5] /£b[i5]) «0.1 TPa. It is close (within the errors) to the experimental value [48] of a Young's modulus of graphene (£,graphene[48] = 10 TPa). And it is considered in Item 2.3, relevance to the experimental value [5] of the degree of elastic deformation (£flx membr [5] ~ 0.1) of the hydrogenated fixed graphene membranes and the assumed stretching stress value (crflx membr.[5] » (^fix.membr.[5] £graPhene[48]) ~ 0.1 TPa) in the expanded regions (domains or grains) of the material.

The experimental data [15, 59] (Figure 2 in [59]) on the thermal desorption of hydrogen from graphene nanoblisters in pyrolytic graphite (Figs. 2-4) can be approximated by three thermodesorption (TDS) peaks (# f[59] with T^^g] ~ 1123 K, # II[59] with ^^[SS] ~ 1523 K, and # III[59] with 7m(l, ni|sy » 1273 K). But their treatment, with using the mentioned above (in previous Items) method [14], is difficult due to some uncertainty, relevance to the zero level of the Jdes quantity (in Figure 2 in [59]). Nevertheless, TDS peak # I[59] can be characterized by the activation desorption energy A/A,ieS I ||v) = 2.4 ± 0.5 eV and by the per-exponential factor of the reaction rate constan K, „, los Table 3).

2-1010s_1 (

htf*"[i4,18-21] = (2.5 ± 0.3) eV (~C?app.in) that coincides with the similar quantities for graphanes (Table 1).

Finally, it is reasonable to assume that the inner surfaces ("walls") in the graphene nanoblisters in HOPG [15, 59] are hydrogenated, and that the graphene "walls" situation is related to some hydrogenated graphenes (Table 1). Obviously, such hydrogenation of the inner graphene surfaces in the nanoblisters [15, 59] occurs under action of the gaseous molecular hydrogen of a high pressure (P*H2) intercalated into the stressed (expanded) hydrogenated graphene nanoblister "walls" possessing of a high Young's modulus.

As is considered in the next Item, a similar (in some extent) situation may occur in hydrogenated graphite nanfibers (GNFs).

4. A possibility [18-21] of intercalation of solid H2 into hydrogenated graphite nanofibers [61, 62], relevance to the hydrogen on-board storage problem

Such a real possibility (considered in [18-21]) is based on the following facts.

1) As is followed from data in Figs. 5-6 (from [60]), a solid molecular hydrogen (or deuterium) of density of Pi ,2 = 0.3-0.5 g/cm3(H2) can exist at 300 K and the external pressure of P = 30-50 GPa.

Figure 2ÍÍ from [59].

Analysis [18-21] has shown that TDS peak I[59] is related to TDS peak (process) III in [14, 18-21], for which the apparent diffusion activation energy Oappin = (2.6 ± 0.3) eV and Aiapp.m ~ 3 10"3 cnr/s.

Hence, one can obtain (with accuracy of one-two orders of the magnitude) a reasonable value of the diffusion characteristic size of ¿TDs-Peak#i[59] ~ (A>appin / K,,,ics ' [|v)|)12 ~ 4 mn, which is related to the separation distance between the graphene nanoblisters (Fig. 4(b)) or (within the errors) to the separation distance between etch-pits (Fig. 3(b)) in the HOPG specimens [15, 59].

Thus, TDS peak (process) I[59| is related to TDS peak (process) III in [14, 18-21], which is related to model "F*" [14, 18-21] (Fig. 1) considered in Item 2.4. Model "F*" is characterized [14, 18-21] by the quantity AH(C-

Fig. 5. (from [60], with the permission). Isentropes (at entropies S/R = 10, 12 and 14, in units of the gas constant R) and isotherms (at T = 300 K) of molecular and atomic deuterium. The symbols show the experimental data, and curves fit calculated dependences. The density (p) of protium was increased by a factor of two (for the scale reasons). Thickened portion of the curve is an experimental isotherm of solid form of molecular hydrogen (H2). The red additional symbol corresponds to a value of the twinned density p « 1 g/cm3 of solid H2 (at 7" « 300 K) and a near-megabar value of the external compression pressure P~ 50 GPa [18].

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6000 T,K 5000

4000

3000

2000

1000

' Liquid I

S/R

Km

100

200

300

400

S00

600

Fig. 6. (from [60], with the permission). Phase diagram,

adiabats, and isentropes of deuterium calculated with the equation of state: 1 and 2 are a single and a

doubled adiabat, • - the experimental data, 3 - melting curve,

thickened portion of the curve - the experimental data. The red additional symbol

corresponds to a value of temperature 7"» 300 Kand a near-megabar value of the external

compression pressure P» 50 GPa [18].

2) As is followed from data in Figs. 2-4 (from [15]) and Eqs. (15-16) (from [18]), the external surface pressure of P = P*m = 30-50 GPa at T~ 300 K may be provided (at the expence of the association energy of atomic hydrogen (TASdis -AHdis)) into some closed hydrogenated (in gaseous atomic hydrogen with the corresponding pressure PH) graphene nanostructures possessing of a high Young's modulus (£graphene ^ 1 TPa).

3) As has been shown in [18], the treatment of the obtained in [61, 62] data (Fig. 7 from [61], Supplement 2) on hydrogenation of graphite nanofibers (GNFs) results in the experimental value of the hydrogen density Ai2 = (0.5 ± 0.2) g(H2)/cm3(H2) (or Am-c-system, « 0.2 g(H2)/cm3(H2-C-system)) of the intercalated high-purity reversible hydrogen (-17 mass.% H2) (Fig. 8) corresponding, according to data from Figs. 5-6, to the state of solid molecular hydrogen at the surface pressure of P = P*H2[6i] - 50 GPa and T~ 300 K.

Fig. 7. Micrographs [61] of hydrogenated graphite nanofibers (GNFs) ater release from them (at -300 K for -10 min [61, 62]) of intercalated (obviously, reversible) high-purity

hydrogen (-17 mass.% - the gravimetrical reversible hydrogen capacity [61]). It occurs under a sharp decrease of the external (locking-like) pressure of H2 (from -10 MPa to ~1 MPa, at -300 K [61, 62], Supplement 2). The arrows in the picture indicate some of the slit-like closed nanopores of the lens shape (between hydrogenated graphite (graphene) nanoregions), where the intercalated reversible high-purity hydrogen was localized. Dehydrogenation of the hydrogenated graphite (graphene) nanoregions, relevance to the covalent bonded "nonreversible" hydrogen in them, occurs during thermodesorption annealing at elevated temperatures. Two TDS peaks (I[6i,62], II [6i,62]) were observed [61, 62]; peak I[61,62] is related to TDS peak III in [14,18-21], (Table 3). It allows us to realize a multi-cycle process [61] of charging-decharging of the same samples, relevance to the intercalated reversible hydrogen, with the permanently hydrogenated graphite (graphene) nanoregions.

4) Substituting in Eq. (16) the quantities of P*

H2[61] ~

5-1011'Pa, £b[6i] - 0.1 (Fig. 7), the largest possible value of Eb[61] - 1012 Pa, the largest possible value of the tensile stresses (<rb[6i] - 10n Pa) in the edge graphene "walls" (of a thikness of c/|:i|i,i and a radius of curvature of rb[6i]) of the slit-like closed nanopores of the lens shape (Fig. 7), one [18] can obtain the quantity of (rb[61] / 4>[6i]) - 4. It is reasonable to assume rb[61] = 20 mn (Fig. 7); hence, a reasonable value follows of db[6j] - 5 mn. A similar result can be obtained [18], supposing the quantity of /•.'ii|r,i - 1011 Pa (as it is for the hydrogenated graphene nanoblisters inHOPG [15], Item 3.7).

5) As has been noted in [18-21], a definite residual plastic deformation of the hydrogenated graphite (graphene) nanoregions is observed in Fig. 7. Such plastic deformation of the nanoregins during hydrogenation of GNFs may be accompanied with some mass transfer resulting in such thickness (db[6i]) of the walls [61].

6) A very important role of the spillover effect [63 -69], relevance to hydrogenation of GNFs [61, 62], has been shown [18-21].

7) Some relevant data presented in Fig. 8 and Fig. 9 point to a breake-through character of results [18-21], relevance for solving of the actual problem [71, 73] of the hydrogen on-board storage in fuel-cell-powered ecological vehicles (Supplement 1).

International Scientific Journal for Alternative Energy and Ecology № 10 (150) 2014

© Scientific Technical Centre «TATA», 201 4

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Fig. 8. (from [70]) It is shown (in the face of known achievements) U.S. DOE targets [71], relevance to gravimetric and volumetric hydrogen on-board storage densities for 2010 (6.0 mass.% H2, 45 kg(H2)/m3(system)) and for 2015 (9.0 mass.% H2, 81 kg(H2)/m3(systems)). The red additional symbol [18] is related to the solid hydrogen intercalated into the hydrogenated GNFs [18, 61] (Fig. 7).

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10000

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Fig. 9. (from [72]). It is shown the known data on volumetric and gravimetric energy densities for different energy carriers. The red additional symbol [18, 61] is related to the solid hydrogen intercalated into the hydrogenated GNFs [18, 61] (Fig. 7).

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5. Conclusions

1) Consideration of some the most cited works [3-9, 17, 25, 51-59, 62] and the near non-cited ones [18-21, 61] on the thermodynamic stability of a number of hydrogenated graphene layers systems has shown the expediency of the further related (mainly, experimental) studies for determination of a complete and compatible set of the thermodynamic characteristics of such systems.

2) Particularly, it has been confirmed that the experimental graphane (a free-standing membrane [5]) may have "a more complex hydrogen bonding than the one suggested by theory [3]".

3) It has been also shown a breake-through character (Figs. 8-9) of analytical results [18-21] on solid

hydrogen intercalated in hydrogenated graphite nanofibers (Fig. 7), relevance for solving of the actual problem [71, 73] of the hydrogen on-board storage in fuel-cell-powered ecological vehicles. It shows the expediency of the constructive open discussion, relevance for promotion of the further developments.

Acknowledgments The author is greatful to H.G. Xiang, M.-H. Whangbo, D.C. Elias, C. Casiraghi, A. Eckmann, N. Tombros, B.J. Van Wees, A. Castellanos-Gomez, R. Bisson, T. Yu, L. Hornekaer and Z. Waqar for helpful discussing, valuable completing and/or reading of the related parts of the present analytical results.

This work has been supported by the RFBR (Project #14-08-91376 CT).

Supplement 1 The hydrogen on-boarf storage problem

Hydrogen is currently one of the most promising —green" fuels, owing to the fact that its gravimetric (mass) energy density of 142 MJ/kg (39.5 kWh/kg) (Fig. 9. (from [72])) exceeds that of petroleum (oil) by a factor of three and that the product of its combustion is water vapor. On the other hand, the volumetric (volume) energy density of molecular gaseous hydrogen at 1 bar pressure is lower by several orders than that of oil, but it can exceed the oil quantity at megabar pressures. In light of this, the issue of finding systems and materials for a compact and energy efficient hydrogen storage assumes a primary importance.

As is noted in a number of studies, hydrogen-based fuel cells are promising solutions for the efficient and clean delivery of electricity. Since hydrogen is an energy carrier, a key step for the development of a reliable hydrogen-based technology requires solving the issue of efficient storage and transport of hydrogen. During the last few decades several proposals based on the design of advanced materials such as metal hydrides and carbon structures have been made to overcome the limitations of the conventional solution of compressing or liquefying of hydrogen in tanks. Nevertheless, none of the proposed systems, with the exception of the nobody reproduced extraordinary experimental data [61, 62] (Supplement 2) and the related analytical data [18-21], are currently offering the required performances for the on-board hydrogen storage in fuel-cell-powered electrical (ecological) vehicles. The performances are usually formulated in terms of hydrogen storage gravimetric (mass) and volumetric (volume) capacities (Fig. 9 from [72]) and control of adsorption/desorption processes, particularly, relevance to so called "reversibility" of the stored hydrogen. Therefore the problem of hydrogen efficient storage remains so far unsolved and it continues to represent a significant bottleneck to the advancement and proliferation of fuel cell and hydrogen technologies.

As has been noted in [74, 75], "...to realize a compact and energy efficient hydrogen storage is a key technology", ."breakthroughs in hydrogen densities are

strongly required", and it is necessary "to find a breakthrough technology". As is also shown (Fig. 3, [74]), the present time is a very suitable (in the plan of the market entry prognosis) for such a developments.

In this connection, it's also expedient to note about a number of communications of 2013-2014 (in Internet) on the nowadays market situation, relevance to fuel-cell electrical vehicles (FCVs) and hydrogen charging stations, for instance:

1.Hyundai's fuel-cell vehicle could be a massive success

The Motley Fool

Oct. 24, 2013: Hyundai isn't the only manufacturer in the race for FCVs. Toyota Motors, Daimler's MercedesBenz, BMW, and a number of other car companies have spent billions in fuel-cell technology, and are all competing to see who can be the first to market with a consumer-friendly FCV.

2.Hydrogen and Fuel Cells: GM - Honda Collaboration on Next-Generation Fuel Cell Technologies (03.07.2013). (http://www.netinform. net/h2/Aktuelles_Detail.aspx?ID =3285) Goal is commercially feasible fuel cell and hydrogen storage in 2020 time frame. On July 2, 2013 General Motors (NYSE: GM) and Honda (NYSE: HMC) announced a long-term, definitive master agreement to co-develop next-generation fuel cell system and hydrogen storage technologies, aiming for the 2020 time frame. The collaboration expects to succeed by sharing expertise, economies of scale and common sourcing strategies. Source: www.gm.com

3.Ballard signs long-term engineering services contract to advance Volkswagen AG Fuel Cell Automotive Research Program Ballard Power Mar. 7, 2013: Ballard Power Systems has announced signing of an agreement with Volkswagen Group for a major Engineering Services contract to advance development of fuel cells for use in powering demonstration cars in Volkswagen's fuel cell automotive research program. The contract term is for 4 years, with

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an option for an extension. The expected contract value is in the range of $60-100 million.

4.Hydrogen fuel research may benefit from shift in auto industry Hydrogen Fuel News Feb. 7, 2013: Ford, Daimler, and Nissan recently joined forces to make progress in the field of hydrogen fuel cell technology. Each of the companies has an ambitious goal in mind in terms of hydrogen transportation, but each has also been faces with challenges that threaten to derail these goals. The automakers decided to team up in order to overcome some of these challenges and introduce new standards to hydrogen fuel cell technology as a whole.

5.U.S. Department of Energy launches new hydrogen fuel initiative Hydrogen Fuel News May 23, 2013: Hydrogen fuel has become a major focus for the global auto industry and this focus is likely to transform the transportation sector around the world. As automakers put more emphasis on clean transportation, global markets are beginning to respond by supporting the promotion of hydrogen fuel in the transportation sector. Much of this support comes in the form of governments working to establish a working hydrogen fuel infrastructure that will be capable of supporting a new generation of fuel cell vehicles.

6. As is noted in "CHFCA Weekly Fuel" of January 2014,

Toyota unveils zero-emissions hydrogen fuel-cell 'Car Of The Future' for sale next year Think Progress

Toyota announced the launch of a hydrogen-powered fuel-cell car in the U.S. next year at the annual Consumer Electronics Show (CES) in Las Vegas. The car, which resembles the popular Corolla, is yet to be named, but like the birth of a royal child it's the pedigree that counts — and Toyota is the largest auto manufacturer in the world.

7. As is noted in "CHFCA Weekly Fuel" of January 2014,

Toyota touts hydrogen fuel vehicles despite criticism (The Detroit News)

A top U.S. Toyota Motor Corp. executive strongly stood by its focus on hydrogen fuel cell vehicles, defending their safety and dismissing criticism from the top executives at Tesla Motors, Nissan Motor Co. and others. "I realize there is no shortage of naysayers regarding the viability of this technology and the infrastructure to support it," said Bob Carter, senior vice president for automotive operations, Toyota Motor Sales USA Inc.

In connection with this, it's also expedient to take into account the long-term corporation (partnership) of Shell and General Motors Companies, relevance to hydrogen charging stations and fuel cell electrical vehicles, for instance:

1) July 2011: "The hydrogen infrastructure for automobiles is economically viable and do-able," said Larry Burns, Ph.D., General Motors Vice President, Research & Development and Strategic Planning. ... Shell Hydrogen has been developing hydrogen and fuel cell businesses since 1999.

www.minichamps.ru/2011/general-motors-shell-fuel-up-on-hydrogen-in-los/angeles..

2) It was installed in November 2007 by Shell and General Motors to provide a venue for demonstrations to federal lawmakers and officials. Hydrogen fuel cells have long been touted as the next great energy revolution.

www.thelivingmoon.com/41pegasus/02files/Alternate_F uel_Shell_Oil_Hydrogen. html

3) November 2004: The hydrogen-dispensing pump is the first installed at a public gas station in the country, according to officials from Shell Hydrogen and General Motors Corp., who will team up today to ... Virtually all the major auto manufacturers have prototype vehicles that run on fuel cells and are refining the technology. www.washingtonpost.com/wp-dyn/articles/A38168-2004Nov9.html

4) Washington, D.C., March 5, 2003 - General Motors Corp. and Shell Hydrogen are combining resources to help make hydrogen fuel cell vehicles a commercially viable reality, the two announced today. "The partnership brings together two leaders in hydrogen energy and transportation to take a coordinated, comprehensive approach," said Donald Huberts, chief executive officer of Shell Hydrogen. "By combining GM's expertise in vehicle technology with Shell's leadership in refueling technologies, the initiative represents an important step forward in the commercialization of hydrogen fuel cell vehicles." http://www.wec.org/news/shell-gm-partner-to-make-hydrogen-fuel-cell-vehicles-a-reality

5) Shell predicts the end of the gas guzzler by 2070 The New Zealand Herald (2014)

Shell has released a report predicting the end of petrol-powered cars will be in 2070. The oil giant have compiled a 46-page report, using the progress in automotive fuel technology and economic scenarios as a basis for their prediction of all petrol cars becoming a thing of the past by 2070.

Международный научный журнал «Альтернативная энергетика и экология» № 10 (150) 2014 © Научно-технический центр «TATA», 2014

KYUSHU UNIVERSITY

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Supplement 2 The Northeastern University (NU) group [62] hydrogen storage activity

As was noted in [20] (2002) by Maeland, whose two works were cited in [4] (Ref. [1] (1978) and Ref. [4] (1981) in it), at the 1996 fall meeting of the U.S. Materials Research Society (MRS), held in Boston, Massacusetts, Rodriguez and Baker of Northeastern University (NU), Boston, presented a paper in which they claimed the development of a "super" hydrogen storage nanomaterial. The material, graphite nanofibers (GNFs), discovered by Baker back in 1972 was claimed to be capable of storing up to 30 liters of gaseous hydrogen (H2) per gram of nanofibers of the herringbone-like structure (Ref. [50] (1996) and Ref. [51] (1997) in [20]), i.e. about 73 mass. % (of the graphite-hydrogen system mass) corresponding to formula CH32. The graphite nanofibers (GNFs) were preparing by reacting hydrocarbons with carbon monoxide on catalytic particles of bi- or tri- metallic nickel or iron. Hydrogen uptakes were determined by exposing, in a system of known volume, a purified GNF bundle (batch-like) sample to molecular gaseous hydrogen at room temperature, and observing the drop in

pressure over a 24 hour period from an initial value of 11.2 MPa [4] (Ref. [61] (1998) in [20]). Three of the herringbone-like structure samples, according to [4], take up hydrogen to give the hydrogen adsorption data of 62 ± 5 mass. % (but not 73 mass. %, as it was declared in the earlier talks (1996, 1997)), two of the platelet-like structure samples take up hydrogen to give the hydrogen adsorption data of 46 and 54 mass. %, and the tubular-like structure sample - 11 mass. % . Four of the herringbone-like structure hydrogenated samples, according to [4], released the most of the stored hydrogen (at room temperature for 5-10 min) to give the reversible hydrogen desorption data of 48 ± 5 mass. %; the rest part of the stored hydrogen was released at higher temperatures (under temperature-programmed desorption examination) to give the irreversible hydrogen desorption data of 14 ± 5 mass. %.

The results reported by Rodriguez and Baker immediately caused controversy. Michal Heben of the National Renewable Laboratory in Denver, Colorado, U.S.A., pointed out that the highest ratio of hydrogen to carbon found in Nature is 4/1 (CH4) and corresponds to 25 mass. % and expressed skepticism (Ref. [51] (1997)

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© Scientific Technical Centre «TATA», 2014

in [20]) of the results and attempts to verify the results of Rodriguez and Baker.

Ahn et al. (Ref. [52] (1998) in [20]) measured hydrogen adsorption and desorption at 300K on graphite nanofibers (GNFs) and reported that the absolute level of hydrogen desorption from these materials were typically less than 0.025 H/C (0.2 mass. %) which is comparable to other forms of carbon.

Jarvi et al. (Ref. [53] (1999) in [20]) reported very low hydrogen storage capacity at 303K, comparable to activated carbon, for graphite nanofibers (GNFs) prepared by catalytic decomposition of ethylene over nickel, iron, copper/nickel and alumina/magnesia catalysts and concluded that they were unlikely storage materials for hydrogen. However, they left the door open by stating that subtle processing effect might convert inactive materials into effective hydrogen sorbents (as has been recently shown in analytical studies [2, 3], and is considered in the present analytical study).

As was also noted in [20], the announcement at the MRS meeting did not escape the automakers and Daimler-Chrysler began an evaluation study with the NU group of these "super" hydrogen storage materials. Later, however, Daimler-Chrysler ended their participation in the study. Then, Ford Motor Company was supporting the NU group (Ref. [54] (1999) in [20]). The NU group was also supported by the DOE in U.S.A. (Ref. [55] (1998) in [20]), but the support was terminated presumably because of the un-willingness of Rodriguez and Baker to share their GNF samples with other DOE laboratories for examination. More detailed description of the situation was done by Jennifer Babson, a freelance writer in Boston, who interviewed (in November 1997) Rodriguez and Baker; see two 10/25/97 articles from the Economist on hydrogen fuel.

This un-willingness to submit samples to other investigations had continued to fuel the controversy and prompted Dr. Gary Sandrock, a well known expert in the field of hydrogen storage materials, to publically call on Rodriguez to submit samples to others for a "Real-Word Test" of her nanofiber materials (Ref. [56] (1998) in [20]). It, however, had not been done (so far as we know) up to 2005 [21, 22], despite the fact that Rodriguez and Baker were issued two related U.S. patents of 1997 ("Storage of hydrogen in layered nanostructures") [6] and 2000 ("Method for introducing hydrogen into layered nanostructures") [7] and had thus secured protection for their process.

As can be shown, the negative test-results [21, 22], with respect to [4-12] data (i.e., both for Rodriguez-Baker et al. data [4-8], and for Gupta et al. ones [9-12]), could be caused by using in [21, 22] the non-adequate GNF samples (including samples supplied for this test by Rodriguez and Baker themselves).

The work of the NU group had been presented in a number of talks (Refs. [58-60] (1999) in [20]) and in two yearly cited (up to nowadays) articles in the Journal of Physical Chemistry of 1998 [4] and 1999 [5].

Article [4] has been cited (from 1998) 168 times in Scopus; the most recent citing is in two articles of 2013 [23, 24]. Article [5] has been cited (from 1999) 196 times in Scopus; the most recent citing is in two articles of 2013 [25, 26].

Unlike [4, 5] (1998, 1999), the Gupta et al. papers [9-11, 12] (2000-2004, 2006) have not been discussed and/or cited so much, despite of the situation that, as far as we know, only experimental results [9-12] confirm (and reproduce, in an essential degree) the extraordinary experimental data [4-8].

And as far as we know, both authors [4-8] (19982000, 2005) and authors [9-12] (2000-2004, 2006) have never crossed out their extraordinary experimental results. Nevertheless, authors [4] (1998) had done some corrections (and/or modifications) in [5] (1999) of their original adsorption-desorption data. And Baker (in [8] (2005)) had modified the reversible hydrogen adsorption-desorption data [4] (for the herringbone-like structure GNF samples) up to value of "40% by mass of molecular hydrogen per gram of carbon" that corresponds to 29 mass. % of hydrogen (of carbon-hydrogen system mass), instead of 48 ±5 mass. % quantity declared in [4].

In article of Lueking et al. [27] (2004), where Rodriguez and Baker were co-authors, it was noted the quantity of -67 mass. %, relevance to [4, 6] data (for the herringbone-like structure GNF samples), and the quantity up to 40 %, relevance to [5, 7] data (for the herringbone-like structure GNF samples). The graphite nanofibers (GNFs) possessing a herringbone-like structure and a high degree of defects (dislocations) were found [27] to exhibit the best performance for hydrogen storage resulted in 3.8 mass. % release after exposure at 69 bar and room temperature (for 10 h). This result is in contrary with data [4-8]. But, as was stressed in [27], the "herringbone" graphite nanofibers (GNFs) used in that investigation were produced from a different catalyst formulation than that used in [4] and [5]. Furthermore, as was also stressed in [27], the hydrogenation adsorption/desorption protocol followed there was not the same as that used in [4] and [5].

It is necessary to emphasize that the extraordinary experimental results [4-12] have not been reproduced by other research teams worldwide [20-22, 27-30]. But the rather known authors [4-12] (see, for instance, information about them in Scopus and/or ScienceDirect.com Internet programs) have not definitely crossed out their data (as far as we know). Some of these works [4, 5] have been yearly cited (up to nowadays). On the other hand, the physics of such results [4-12] has been developed (in an essential degree) in analytical studies [2, 3, 31-33]. Therefore, there are serious reasons to assume (following to [34, 35]) that experimental works [4-12] have contained some "know-how" methodological and/or technological elements. It is under a consideration in the present analytical study.

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