Статья поступила в редакцию 29.08.14. Ред. рег. № 2085
The article has entered in publishing office 29.08.14. Ed. reg. No. 2085
О НЕКОТОРЫХ ТЕОРЕТИЧЕСКИХ И ЭКСПЕРИМЕНТАЛЬНЫХ (STM, STS, HREELS/LEED, PES, ARPS, RAMAN SPECTROSCOPY) ДАННЫХ ПО СОРБЦИИ ВОДОРОДА ГРАФЕНОВЫМИ НАНОМАТЕРИАЛАМИ В СВЯЗИ С ПРОБЛЕМАМИ ЧИСТОЙ ЭНЕРГЕТИКИ
ЦНИИчермет им. И.П. Бардина, Институт металловедения и физики металлов им. Г. В. Курдюмова 105005 Москва, 2-я Бауманская ул., д. 9/23 E-mail: Yuri1939@inbox.ru
Заключение совета рецензентов: 05.09.14 Заключение совета экспертов: 10.09.14 Принято к публикации: 15.09.14
Представлены результаты термодинамического анализа ряда теоретических и экспериментальных (TDS, STM, STS, HREELS/LEED, PES, ARPS, Raman spectroscopy и др.) данных по «обратимому» гидрированию и дегидрированию некоторых графеновых (моно- и полислойных) наноструктур.
В рамках приближения формальной кинетики для реакций первого порядка определены термодинамические характеристики процессов сорбции водорода этими наноструктурами (константы скорости, энергии активации, предэкспоненци-альные факторы констант скорости).
При интерпретации полученных термодинамических характеристик использованы некоторые модели и характеристики химической сорбции водорода на базальной и краевых поверхностях графита с целью раскрытия атомных механизмов гидрирования и дегидрированию различных графеновых (моно- и полислойных) наноструктур. Рассмотрена кинетика как сорбционных процессов, в которых не лимитирует диффузионный массоперенос водорода в сорбенте, так и процессов с лимитирующей диффузионной стадией.
Рассматриваются также некоторые «открытые вопросы» и перспективы решения актуальной проблемы эффективного хранения водорода «на борту» экоавтомобиля и ряда других проблем чистой энергетики на основе использования графитовых нановолокон.
Ключевые слова: эпитаксиальные и мембранные графены; слоистые графеновые системы; гидрирование-дегидрирование; термодинамические характеристики; атомные механизмы; проблема эффективного хранения водорода «на борту» экоавтомобиля.
№ 17 (157) Международный научный журнал
ON SOME THEORETICAL AND EXPERIMENTAL (STM, STS, HREELS/LEED, PES, ARPS & RAMAN SPECTROSCOPY) DATA ON HYDROGEN SORPTION WITH GRAPHENE-LAYERS NANOMATERIALS, RELEVANCE TO THE CLEAN ENERGY APPLICATIONS
Bardin Institute for Ferrous Metallurgy, Kurdumov Institute of Metals Science and Physics 9/23 Vtoraya Baumanskaya str., Moscow, 105005, Russia E-mail: Yuri1939@inbox.ru
Referred: 05.09.14 Expertise: 10.09.14 Accepted: 15.09.14
Herein, results of thermodynamic analysis of some theoretical and experimental (TDS, STM, STS, HREELS/LEED, PES, ARPS, Raman spectroscopy and others) data on "reversible" hydrogenation and dehydrogenation of some graphene-layer-nanostructures are presented.
In the framework of the formal kinetics approximation of the first order rate reaction, some thermodynamic quantities for the reaction of hydrogen sorption (the reaction rate constant, the reaction activation energy, the per-exponential factor of the reaction rate constant) have been determined.
Some models and characteristics of hydrogen chemisorption on graphite (on the basal and edge planes) have been used for interpretation of the obtained quantities, with the aim of revealing the atomic mechanisms of hydrogenation and dehydrogenation of different graphene-layer-systems. The cases of both a non-diffusion rate limiting kinetics, and a diffusion rate limiting kinetics are considered.
Some open questions and perspectives of solving of the actual problem of the effective hydrogen on-board storage and other clean energy applications, with using the graphite nanofibers, are also considered.
Keywords: epitaxial and membrane graphenes; other graphene-layer-systems; hydrogenation-dehydrogenation; thermodynamic characteristics; atomic mechanisms, the hydrogen on-board efficient storage problem.
As is noted in a number of articles 2007 through 2014, hydrogenation of graphene-layers-systems, as a prototype of covalent chemical functionality and an effective tool to open the band gap of graphene, is of both fundamental and applied importance ([1-44] and others).
It is relevant to the current problems of thermodynamic stability and thermodynamic characteristics of the hydrogenated graphene-layers-systems ([1-18] and others), and also to the current problem of hydrogen on-board storage ([19-21] and others).
In the case of epitaxial graphene on substrates such as SiO2 and others, hydrogenation occurs only on the top basal plane of graphene, and it is not accompanied with a strong (diamond-like) distortion of the graphene network, but only with some ripples. The first experimental indication of such a specific single-side hydrogenation came from Elias et al. . The authors mentioned a possible contradiction with the theoretical results of Sofo et al. , which had down-played the possibility of a single side hydrogenation. They proposed an important facilitating role of the material ripples 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 (on the freestanding graphene membrane).
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 .
As it is supposed by many scientists, 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 . But, firstly, these two aspects are of the kinetic character, and therefore they can not influence the thermodynamic predictions [3, 24, 31]. Secondly, as is shown in the present analytical study, the above noted two aspects have not been studied in an enough degree.
As was shown in , 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% with respect to the pristine graphene. This amount of stretching (10%) is close to the limit of possible elastic deformations in
№ 17 (157) Международный научный журнал
graphene ( and others), and indeed it has been observed that some of their membranes rupture during hydrogenation. It was believed  that the stretched regions were likely to remain non-hydrogenated. They also 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 periodicity in both stretched and compressed domains .
It can be supposed that the rigidly fixed graphene membranes are related (in some degree) to the epitaxial graphenes those may be rigidly fixed by the cohesive interaction with the substrates.
As was noted in , the double-side hydrogenation of graphene is now well understood, at least from a theoretical point of view. For example, Sofo et al. 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 the 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 sp3 hybridization, accompanied by a strong (diamond-like) distortion of the graphene network [3, 22]. In contrast to graphene (a zero-gap semiconductor), graphane is an insulator with an energy gap of Eg ~ 5.4 eV [4, 23].
Only if hydrogen atoms adsorbed on one side of graphene (in graphane) are retained, we obtain graphone of C2H composition, which is a magnetic semiconductor with Eg ~ 0.5 eV and a Curie temperature of Tc ~ 300400 K .
As was noted in , neither graphone nor graphane are suitable for real practical applications, since the former has a low value of Eg, and undergoes a rapid disordering because of hydrogen migration to neighboring vacant sites even at a low temperature, and the latter cannot be prepared on a solid substrate .
It is also expedient to refer to a theoretical single-side hydrogenated graphene (SSHG) of CH composition (that is an alternative to graphane ), in which hydrogen atoms are adsorbed only one side [7, 25]. 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 (1.6 eV lower than in graphane), and it can be prepared on a solid substrate in principle. But, this quasi-two-dimensional carbon-hydrogen theoretical system is shown to have a relatively low thermal stability, which makes it difficult to use SSGG in practice [6, 7].
As was noted in , it may be inappropriate to call 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. 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. For instance, there may be the diamond-like local distortion of the graphene network, showing the signature of sp3 bonded system. However, in SSHGraphene all the carbon atoms remain in one plane, making it difficult to call it sp3 hybridized. Obviously, this is some specific sp3-like hybridization.
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