Effect of nitriding/nanostructuration of few layergraphene supported iron-based particles; catalystin graphene etching and carbon nanofilamentgrowth†
Walid Baaziz,a Georgian Melinte,b Ovidiu Ersen,b Cuong Pham-Huua andIzabela Janowska*a
Stable, highly faceted and dispersed iron nitride particles supported on few layer graphene are obtained by
ammonia decomposition on iron-based particles at the temperature commonly used for the synthesis of
N-doped CNTs and graphene etching. The TEM/EELS analysis reveals nitrogen diffusion in a bulk of the
particles. The resulting facet FeNx catalyst exhibits high activity in the etching of graphene, which is assisted
by catalyst reorganization. Ammonia decomposition is used for the first time for graphene etching, while
the highly faceted catalyst has an impact on the etched channels structures. According to the shape of the
active planes of the catalyst, the etching results in sharp ‘‘V’’ channel ends and often ‘‘step-like’’ edges.
The FeNx morphology proves previously reported triangularisation of arches in highly N-doped carbon
nanotubes. The conditioning of the catalyst by its shaping and nitrogen incorporation is investigated
additionally in the carbon nanostructure formation, for decomposition of ethane. The herringbone CNFs,
‘‘hollow’’ bamboo-like CNFs/CNTs or CNTs are effectively observed.
Introduction
Design of carbon nanostructures (CNTs, CNFs, graphene) withspecific chemical/physical properties in view of particularapplications is of high interest. Since the CCVD method allowshigh yield synthesis of carbon nanostructures, a tailoring ofthe final structures by controlling the operating conditionsconstitutes a large area of research, due to the impact of manyparameters such as catalyst, support, carbon precursor. . .1–3 Ingeneral, Ni is considered as an efficient catalyst for the produc-tion of CNFs, while Fe for the formation of CNTs.4 Howevereven for a given metal, the family of CNFs and CNTs is largeand diverse as long as the shape and diameter of catalyst vary.This is related to different local kinetics of carbon diffusion/precipitation and orientation of formed carbon layers – parallelto the metal catalyst surface.5,6 The round shaped, graphitesupported Ni catalyst particles lead to the formation of CNTs,
whereas the sharp edge ones lead to fishbone CNFs.7 A desir-able structural conditioning of the catalyst remains difficult toachieve, control and observe due to the easy shape reconstruc-tion of the particles.8,9
On the other hand, chemical doping of CNTs with heteroa-toms, such as N, results in bamboo-like structures;3,10 whereinfor high N content, the orientation of graphitic walls is close tothe ‘‘fishbone’’ CNFs11 and arches reveal a triangular cross-section12 – suggesting the presence of a faceted catalyst. N-dopedCNTs demonstrate high performance particularly in catalysis.13–16
The most common way to synthesize nanostructures with Nincorporated into the carbon matrix is ‘‘in situ’’ doping/CVDgrowth on Fe with ammonia as the nitrogen precursor.17–19 Itinvolves ammonia decomposition,20 which is also an importantreaction in energy production; first, by nitridation21 – activationof metals (alternative to Pt in the ORR reaction),22–25 second, forthe production of COx-free H2 for PEMFCs.26–28 The nitriding ofiron or iron oxide particles also aims at materials with highmagnetic performance.29,30
The high reactivity of Fe with carbon also renders this metal,similarly as Ni, as a useful catalyst for etching of graphene,allowing access to planar carbon nanostructures with specificproperties related to their planarity, defined size and shape(quantum dots, nanorribons).31–33
Contrary to the carbon nanofilament growth, the impact ofcatalyst shape in the etching of graphene was never investigated.
a Institut de Chimie et Procedes pour l’Energie, l’Environnement et la Sante
(ICPEES), UMR 7515 CNRS, Universite de Strasbourg, ECPM, France.
E-mail: [email protected]; Tel: +33 (0) 3 68 85 26 33b Institut de Physique et Chimie des Materiaux de Strasbourg (IPCMS),
UMR 7504 CNRS, France
† Electronic supplementary information (ESI) available: Additional data dealingwith the crystallographic structure and reorganization of FeNx particles (XRD,HR-TEM, TEM), complementary description of carbon nanotube growth. See DOI:10.1039/c4cp01887g
In the present work we report on the nitridation and faceting offew layer graphene (FLG) supported iron based nanoparticles (Fe3O4)with ammonia at the temperature commonly applied for the etchingof graphene and CVD synthesis/N-doping of carbon nanostructures.The morphology of the treated iron-based particles is investigated bythe TEM/EELS technique. The impact of the nanostructuration byfaceting/nitrogen incorporation of/into the catalyst is investigatedin the etching of graphene; and, by decomposition of ethane inthe growth of carbon nanostructures.
Results and discussionMorphology of ammonia treated iron based particles
The FLG supported spherical Fe3O4 particles with an averagediameter of (8 � 2 nm),34 (Fig. 1A), are treated by NH3/Ar at800 1C for 2 h. Despite the high temperature, the treatmentresults in coalescence of the particles with a maximum finalsize of 50 nm, while an amount of particles remains in theirinitial size (Fig. 1B and 5). The etching of FLG is additionallysystematically observed (Fig. 1B).
According to the TEM analysis the overall dispersion ofparticles remains very high, while the spherical shape ofparticles changes into polygonal (hexagonal, rectangular and,punctually triangular) with more or less visible sharp edges. TEManalysis depicts the existence of three main types of particles; i.e.core–shell in majority, homogeneous and semi-core–shell particleslocated at the edges of graphene (ESI†). The strong variation in themorphology of the particles is the result of partial coalescencewhich depends on localization on the FLG surface, distancebetween particles, ramping and local temperature. A similarmorphology distribution was observed in our previous study forthe iron based particles after treatment with hydrogen,although the particles remained spherical.35
The element mapping performed, by the HR-TEM/EELStechnique, on the core shell particles of B50 and 35 nm,(Fig. 2) reveals that oxygen appears in the same region as iron– suggesting the existence of iron oxide species (Fig. 2A); or onlyin the shell (Fig. 2B). The detected oxygen can partially or totallyoriginate from the oxygenation once the particle is exposedto air36 (highly reductive conditions) and induces observeddisorder in the shell structure. The analysis reveals the presenceof nitrogen, and contrary to oxygen the nitrogen appears exclusively
in the core of particles with average diameters of ca. 25 and 22 nm,respectively (Fig. 2D).
Here, a detection of carbon by TEM/EELS is difficult due to thecarbon background from the support, although a very thin carbonlayer at the surface can be detected for the core–shell particlessuspended over transparent etched channels (Fig. 3). This indicatesvery low dissolution of carbon in iron particles; this carbon couldoriginate from the FLG support or from CH4 – formed during theFLG etching. The low dissolution of carbon from FLG was firstlinked to the high graphitization degree of FLG.
The second group of particles exhibits highly defined shape withsharp edges and more homogenous redistribution of elements,including nitrogen (Fig. 4). Rarely, these particles exhibit very weaksignals from nitrogen (oxygen) which could suggest quasi pureiron composition (Fig. 4B). This might be an effect of lower localammonia concentration during the treatment,37 or the analyticalnoise insufficiency due to high distribution of N within the particles.
The presence of nitrogen in Fe particles reveals adsorption andbulk diffusion of nitrogen during the ammonia decomposition:
2xFe + 2NH3 - 2FexN + 3H2 m (1)
Since the XRD pattern confirms the formation of FeNx
(ESI†), a detailed crystallographic structure will not be definedin this work due to its diversity, which can be related to the
Fig. 1 The TEM images of the Fe3O4–FLG hybrid before (A) and afterammonia treatment (B).
Fig. 2 Representative TEM micrographs and elemental mapping (TEM/EELS) of iron based core–shell particles with a nitrogen rich core and anoxygen contained core–shell (A) or shell (B).
existence of different morphology/composition of particles. Thepreliminary data from electron diffraction analysis suggest thecontribution of Fe16N2 (ESI†), which is in agreement withearlier reported single phase a00-Fe16N2 – observed for ammoniatreated spherical core–shell nanoparticles.30
The presence or absence of oxygen-rich disordered shells inthe here formed particles could be linked to different oxidation
potentials of nitrides and pure iron, and to the formation of therelated passive layer. The oxidation potential of nitrides alsoincreases with the percentage of N.38
Stability of the FeNx–FLG system
The shape retransformation of particles, when NH3/Ar is switchedto Ar during the cooling step, does not happen. Curiously, thespherical particles can be punctually observed, but the FeNx core ofparticles is highly faceted. The highly stable and well dispersedFeNx phase containing particles is a combined effect of nitro-gen diffusion, faceting and interaction with the support.The additional experiments reveal that the faceting is lesspronounced when the particles are treated under H2/Ar gas(Fig. 5); and in turn, the faceted but highly coalesced bigparticles are observed when NH3/Ar treatment is applied forthe particles supported on Al2O3 (Fig. 6).
The relatively high dispersion of the particles treated withammonia is prevented by faceting of particles, where a highercontent of atoms is exposed in the basal plane to interact withthe support. The additional stabilizing factors are the edges ofgraphene. Due to the faceting of particles, dispersion of parti-cles remains quite high over the FLG surface despite treatmentin a highly reductive atmosphere (2 h at 800 1C) and etching of thesupport. We have observed a similar stabilization of metal on theFLG support by faceting of particles and the edges of graphene inour previous study for in situ TEM heated Pt particles, but only upto 700 1C.39,40
FLG etching
Apart from nitrogen, decomposition of ammonia (1) produceshydrogen. The presence of the latter can lead to gasification of
Fig. 3 TEM and carbon/nitrogen elemental micrographs of core–shell nanoparticles obtained after ammonia treatment.
Fig. 4 TEM micrographs and C/Fe/N mapping of two homogenousparticles (A) and (B).
Fig. 5 TEM micrographs showing dispersion of NH3/Ar (A) and H2/Ar (B)treated Fe3O4 particles supported on FLG.
carbon to methane (2) and, in the case of the highly graphitizedcarbon support, the etching of the support by Fe and theformation of channels in a direction defined by the crystallo-graphic orientation of graphene.
Csolid þH2 �!Fe
CH4 " (2)
Such etching of the FLG support is presently observed. Ammoniadecomposition over iron based particles causes nitridation of metaland etches the graphene surface. Here, due to the faceted catalyst,the ‘‘ends’’ of the graphene channels are sharp. Fig. 7 presents thecarbon maps of the particles described above, i.e. of the core–shellparticle from Fig. 2A (Fig. 7A) and of homogenous particles fromFig. 4A (Fig. 7B). It is evident that the shape of etched channel endscorresponds to the edge shape of the homogenous particles or to thefaceted core of core–shell particles with disordered shells. Moreover,the edge ends of the channels always depict a ‘‘V’’ shape, with apexeslocalized symmetrically or not (Fig. 7A and B).
Statistical TEM/EELS analyses display the highest concentrationof nitrogen in the active part of the particle: near the ‘‘V’’ ends ofchannels, which indicates the high catalytic activity of the FeNx
phase and reorganization of the catalyst with displacement of FeNx
phase/core (Fig. S2A, ESI†).The edges of the etched channels often reveal ‘‘step-like’’
shape (Fig. 8), however many edges are straight lines as long asthe particle does not turn, which suggests the equally activefacets of the catalyst particle (ESI†). The direction of cutting,which starts from the graphene edges, follows crystallographicdirections of graphene, similarly to the case of HOPG etchedwith Ni nanoparticles (ESI†).32 Fig. 8A shows a 2D-projection
TEM image of the etched channel by the active core–shellparticle with hardly exhibited facets. The 3D mode analysis(Fig. 8B) performed on the same zone at the graphene surfacelevel, in a longitudinal xy direction, clearly evidences threesharp edges in the particle.
The observed ‘‘step-like’’ edge shape of the channel suggestsa 2D rotation and ‘‘creep’’ movement of the particle, which mayresult from competition between the most active facet and thepreferential orientation of etching – defined by the graphenelattice. The final turning of particles might be caused by chemicalchanges of the graphene surface, e.g. higher concentration oflocal defects. The latter is also responsible for the formationof the observed large void in the graphene surface due to theuncontrolled gasification reaction.
Carbon nanofilament structure
The sharp edges of the FeNx catalyst not only have an impact on thestructurization of graphene channels. The polygonal and punctuallytriangle shape of FeNx core/particles (ESI†), observed here, togetherwith the ‘‘V’’ shape of the channels ‘‘ends’’, confirms a contributionof the FeNx phase in the earlier reported triangularisation ofnitrogen rich arches in bamboo-like CNTs.12
Fig. 6 TEM micrographs showing extremely low dispersion of NH3/Artreated Fe3O4 particles supported on Al2O3.
Fig. 7 The carbon maps corresponding to the: (A) core–shell particle fromFig. 2A and (B) to the homogenous particle from Fig. 4A. ‘‘V’’ shape of etchedchannel ends with symmetrically (A) and non-symmetrically localized apex (B).
Fig. 8 Micrographs of the active iron nitride particle with hardly exhibitedfacets in carbon etching, by TEM projection (A) and visible facets by thelongitudinal xy direction at the graphene surface level analysis (B). Thestep-like edges of the channel are visible on the (A, B) micrographs.
Fig. 9 (A) Herringbone-type CNFs grown on the H2/Ar faceted catalyst.(B) ‘‘Hollow’’ bamboo-like CNFs/CNTs grown on the NH3/Ar faceted catalyst.
Presently, the impact of catalyst shape and nitrogen incor-poration on the growth of carbon nanofilaments is investigatedby performing decomposition of ethane. Decomposition of thehydrocarbon is preluded by the faceting (and reduction) step ofthe FLG supported Fe3O4 with H2/Ar, at first, and NH3/Ar in thesecond experiment.
C2H6 decomposition on the H2/Ar faceted catalyst results in thegrowth of CNFs with a specific herringbone-type structure, in whichgraphene sheets are packed perpendicularly to the longitudinal fiberaxis. This suggests the same quasi diffusion/precipitation of carbonat base/slanting planes of the catalyst (Fig. 9A). A similar structurewas observed exclusively for the small Ni particles (B15 nm), as asecondary product of acetylene decomposition.5
Despite the highly faceted catalyst, decomposition of ethane oniron nitride results in ‘‘hollow’’ bamboo-like CNF/CNT structureswith arches, and conical or parallel to longitudinal axis orientationof external walls (Fig. 9B). (The impact of catalyst diameter onthe formation of ‘‘hollow’’ structures can be omitted here, ascuriously, the diameter of ammonia treated particles is slightly lower(20–35 nm) than the one of hydrogen treated particles (35–45 nm)).According to the literature, the ‘‘hollow’’ structure suggests thatcarbon diffusion hardly happens in the bulk (or basal plane). Thisindicates the contribution of FeNx species, where the introduction ofnitrogen causes the increase of the bulk density.3
We also suggest that the observed high activity of the FeNx phasein the etching of graphitized carbon may be considered in thegrowth of doped carbon nanostructures. Depending on the appliedconditions during the growth of N-doped carbon nanostructures,such etching of the precipitated carbon layers might occur.
The conical shape of the walls, including external ones, inbamboo-like CNFs depicts faster precipitation of carbon at theslanting planes of the catalyst compared to the herringboneCNFs originated from the H2/Ar treated particles. This indicatesthat the sharp edge faceted catalyst acts.5
In some bamboo-like filaments, however, the orientation of theexternal walls is parallel to the longitudinal axis, which uncoversrather CNT morphology and suggests different faceting/nitrogencontent in the bulk and at the surface of catalyst (the parallelexternal walls were also found for CNTs with triangular arches12).We have assigned this carbon structure to the punctually observedspherical particles with a highly faceted FeNx core, detected even bystandard TEM analysis (Fig. 10). For this reason we call thembamboo-like CNTs and not CNFs.
To complete the investigation on the catalyst–grown nano-filament structure relation, decomposition of ethane is performedadditionally on the spherical iron catalyst reduced at 400 1C(not faceted). Adequately, it results in the formation of typicalhollow carbon nanotube structures (ESI†).
Experimental
Ammonia treatment of Fe3O4/FLG was performed in a tubularquartz reactor at 800 1C for 2 h, using the gas mixture ofNH3 : Ar (50 : 50 cc). The cooling step was performed in a NH3 : Ar(50 : 50 cc) mixture or pure Ar.
The synthesis of carbon nanostructures (CNFs, bamboo-like CNFs/CNTs and CNTs) was performed by ethane decom-position at 800 1C for 1 h in the presence of H2/Ar withan ethane : H2 : Ar ratio of 100 : 10 : 100 cc, respectively. Thesynthesis of bamboo-like CNFs and CNFs was preluded byfaceting of the FLG-supported Fe3O4 particles with NH3/Ar(above) and H2/Ar (same procedure) respectively. The synthesisof CNTs was preluded by the 2 h reduction step with H2/Ar,50 : 50 cc at 400 1C.
For the TEM analyses, a drop of suspension containingthe FLG (Al2O3) supported iron-based nanoaggregates wasdeposited on a TEM grid covered by a holey carbon membrane.The energy filtered images of the tilt series were obtained usinga JEOL 2100F (FEG) TEM/STEM electron microscope operatingat 200 kV, equipped with a TRIDIEM post-column imaging filterobtained from the Gatan Company. The images acquired on a2048 � 2048 pixel cooled CCD detector were hardware-binnedto 512 � 512 pixels. The three-window method was used toacquire EFTEM images with a monotonic dependence of thechemical signal as a function of concentration.
Conclusions
The new important aspects linked to the conditioning of theiron based catalyst by nitridation/faceting, and its reactivitywith carbon are reported:
– Decomposition of ammonia with the formed active FeNx
catalyst may be a way to etch graphene with required nano-structuration of graphene tranches and potential decoration ofetches by nitrogen;
Fig. 10 TEM micrographs of hollow bamboo-like CNFs (A) and CNTs (B) and related to the CNTs growth catalyst particle (C).
– An observed bulk diffusion of nitrogen into the iron-basedparticles and interaction of the particles with FLG condition theparticles’ faceting and stability;
– The punctually observed facet triangle FeNx catalystexplains a triangularization of arches in highly N-doped CNTs;
– The influence of the shape/nitridation on the variablecarbon layers orientation and carbon diffusion/precipitationin the grown carbon nanofilaments is presented for the firsttime within a given catalytic system.
– The FeNx morphological investigations contribute to theknowledge on the catalyst for ammonia decomposition, animportant reaction in energy production.
Acknowledgements
The authors would like to acknowledge the project FreeCats(NMP3-SL-2012-280658) for financial support.
Notes and references
1 Z. X. Yu, D. Chen, B. Totda and A. Holmen, J. Phys. Chem. B,2005, 109, 6096.
2 Z. X. Yu, D. Chen, M. Rønning, B. Totdal, T. Vralstad,E. Ochoa-Fernandez and A. Holmen, Appl. Catal., A, 2008,338, 147.
3 J. P. Tessonnier and D. S. Su, ChemSusChem, 2011, 4, 824.4 I. Kvande, Z. Yu, T. Zhao, M. Rønning, A. Holmen and
D. Chen, Chem. Sustainable Dev., 2006, 14, 583.5 G.-B. Zheng, K. Kouda, H. Sano, Y. Uchiyama, Y.-F. Shi and
H.-J. Quan, Carbon, 2004, 42, 635.6 I. Martin-Gullon, J. Vera, J. A. Conesa, J. L. Gonzalez and
C. Merino, Carbon, 2006, 44, 1572.7 A. Rinaldi, J.-P. Tessonnier, M. E. Schuster, R. Blume,
F. Girgsdies, Q. Zhang, T. Jacob, S. B. Abd Hamid, D. S. Suand R. Schlogl, Angew. Chem., Int. Ed., 2011, 50, 3313.
8 A. R. Harutyunyan, G. T. Chen, M. Paronyan, E. M. Pigos,O. A. Kuznetsov, K. Hewaparakrama, S. M. Kim, D. Zakharov,E. A. Stach and G. U. Sumanasekera, Science, 2009, 326, 116.
9 T. Yamada, A. Maigne, M. Yudasaka, K. Mizuno,D. N. Futaba, M. Yumura, S. Iijima and K. Hata, Nano Lett.,2008, 8, 4288.
10 S. Hofmann, G. Csanyi, A. C. Ferrari, M. C. Payne andJ. Robertson, Phys. Rev. Lett., 2005, 95, 036101.
11 J. W. Jang, C. E. Lee, S. C. Lyu, T. J. Lee and C. J. Lee, Appl.Phys. Lett., 2004, 84, 2877.
12 I. Florea, O. Ersen, R. Arenal, D. Ihiawakrim, C. Messaoudi,K. Chizari, I. Janowska and C. Pham-Huu, J. Am. Chem. Soc.,2012, 134(23), 9672.
13 L. F. Mabena, S. S. Ray, S. D. Mhlanga and N. J. Coville, Appl.Nanosci., 2011, 1, 67.
14 Y. Ma, L. Sun, W. Huang, L. Zhang, J. Zhao, Q. Fan andW. Huang, J. Phys. Chem. C, 2011, 115, 24592.
15 L. Feng, Y. Yan, Y. Chen and L. Wang, Energy Environ. Sci.,2011, 4, 1892.
16 K. Chizari, A. Deneuve, O. Ersen, I. Florea, Y. Liu, D. Edouard,I. Janowska, D. Begin and C. Pham-Huu, ChemSusChem,2012, 5, 102.
17 Y. Shao, J. Sui, G. Yin and Y. Gao, Appl. Catal., B, 2008,79, 89.
18 J. Liu, S. Webster and D. L. Carroll, J. Phys. Chem. B, 2005,109, 15769.
19 K. Y. Chun, H. S. Lee and C. J. Lee, Carbon, 2009, 47, 169.20 Y. Ohtsuka, C. Xu, D. Kong and N. Tsubouchi, Fuel, 2004,
83, 685.21 W. P. Tong, N. R. Tao, Z. B. Wang, J. Lu and K. Lu, Science,
2003, 299, 686.22 F. Charreteur, F. Jaouen, S. Ruggeri and J.-P. Dodelet,
Electrochim. Acta, 2008, 53, 2925.23 H. Meng, N. Larouche, M. Lefevre, F. Jaouen, B. Stansfield
and J. P. Dodelet, Electrochim. Acta, 2010, 55, 6450.24 U. I. Kramm, I. Herrmann-Geppert, P. Bogdanoff and
S. J. Fiechter, J. Phys. Chem. C, 2011, 115, 23417.25 F. Jaouen, F. Charreteur and J.-P. Dodelet, J. Electrochem.
Soc., 2006, 153, 689.26 F. Schuth, R. Palkovits, R. Schlogl and D. S. Su, Energy
Environ. Sci., 2012, 5, 6278.27 J. Zhang, M. Comotti, F. Schuth, R. Schlogl and D. S. Su,
Chem. Commun., 2007, 1916.28 X. Duan, G. Qian, X. Zhou, Z. Sui, D. Chen and W. Yuan,
Appl. Catal., B, 2011, 101, 189.29 C. Schliehe, J. Yuan, S. Glatzel, K. Siemensmeyer, K. Kiefer
and C. Giordano, Chem. Mater., 2012, 24, 2716.30 T. Ogi, A. B. D. Nandiyanto, Y. Kisakibaru, T. Iwaki,
K. Nakamura and K. Okuyama, J. Appl. Phys., 2013, 113, 164301.31 S. S. Datta, D. R. Strachan, S. M. Khamis and
A. T. C. Johnson, Nano Lett., 2008, 8(7), 1912.32 L. Ci, Z. Xu, L. Wang, W. Gao, F. Ding, K. F. Kelly,
B. I. Yakobson and P. M. Ajayan, Nano Res., 2008, 1, 116.33 C. W. Keep, S. Terry and M. Wells, J. Catal., 1980, 66,
451–462.34 W. Baaziz, L. Truong Phuoc, C. D. Viet, G. Melinte,
I. Janowska, V. Papaefthimiou, O. Ersen, S. Zafeiratos,D. Begin, S. Begin-Colin and C. Pham-Huu, J. Mater. Chem.A, 2014, 2, 2690.
35 V. Papaefthimiou, I. Florea, W. Baaziz, I. Janowska,W. H. Doh, D. Begin, R. Blume, A. Knop-Gericke, O. Ersen,C. Pham-Huu and S. Zafeiratos, J. Phys. Chem. C, 2013,117(39), 20313.
36 J. Torres, C. C. Perry, S. J. Bransfield and D. H. Fairbrother,J. Phys. Chem. B, 2003, 107, 5558.
37 C. Schliehe, J. Yuan, S. Glatzel, K. Siemensmeyer, K. Kieferand C. Giordano, Chem. Mater., 2012, 24, 2716.
38 M. Jurcik-Rajman and S. Veprek, Surf. Sci., 1987,189–190, 221.
39 I. Janowska, M.-S. Moldovan, O. Ersen, H. Bulou, K. Chizari,M.-J. Ledoux and C. Pham Huu, Nano Res., 2011, 4(5), 511.
40 M.-S. Moldovan, H. Bulou, Y. J. Dappe, I. Janowska,D. Begin, C. Pham-Huu and O. Ersen, J. Phys. Chem. C,2012, 116(16), 9274.
