FC250 Nano- and microtechnologies chapter 2. Carbon Nanotubes
Lenka Zajíčková
Přírodovědecká fakulta & CEITEC, Masarykova univerzita, Brno
lenkaz@physics.muni.cz
spring semester 2017
Central European Institute of Technology ^ U*I# i
BRNO | CZECH REPUBLIC ^*SL*f^
FC250 Nano- and microtechnologies:
arbon Nanotubes
• 2.1 Structure of Carbon Nanotubes
• 2.2 Synthesis of Carbon Nanotubes
• 2.3 Growth Mechanism of Carbon Nanotubes
• 2.4 Properties of CNTs
• 2.5 Carbon Nanotube-Based Nano-Objects
• 2.6 Potential and Current Applications
FC250 Nano- and microtechnologies:
2.1 Structure of Carbon Nanotubes
ka Zajíčková
3/50
2.1 Structure of Carbon Nanotubes
FC250 Nano- and microtechnologies:
2.1 Structure of Carbon Nanotubes
Lenka Zajíčková
4/50
sp2-C valence orbitals:
Potential energy
1 sp2 hybridization 111
77   2Px 2^ 2Pz
■} ■> ~> p sp1  sp-  sp1 r
sp-
sp'
unhybridized p orbital
0
sp" orbitals
Bonds:
► one valence electron in pure p state creates 7r-bond by sideways overlapp of atomic orbitals
► other three electrons in sp2 hybrid orbitals (combination of 1 /3s and 2/3p) create cr-bonds by head-to-head overlapps
- trigonal planar bonding directions with angles of 120(
sp'
0
sideways overlap
0
head to head
overlap
0
sideways overlap
C
a
a
2
SP'
sigma bond
pi bond
FC250 Nano- and microtechnologies:
2.1 Structure of Carbon Nanotubes
Lenka Zajíčková
5/50
Various ways to roll graphene into tubes are mathematically defined by the vector of helicity
(chirality vector) Ch, _L to the tube axis, and the angle of helicity (chiral angle) 0
OA= Ch = na^ + ma2
aV3 _   a _ where a = 2.46 Á and
a2
COSc9 =
aVŠ ^ a ^ -x--y
2n+ m 2\Jn2 + m2 + nm
(a) armchair m = n, i. e.      || to any C=C bonds -here (5,5)
(b) zigzag m = 0, i. e. C/, J_ to any C=C bonds -here (9,0)
(c) m / n chiral (n,m) - here (10,5)
Nanotube tips contain C atoms also in pentagonal rings (hemi-fullerene) - 6 x 5 = 30 atoms    (for ideal SWCNTs) chemical reactivity will be highly favored at the tube tips (at pentagonal rings)
FC250 Nano- and microtechnologies:
.1 Structure of Carbon Nanotubes
ka Zajíčková
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Single-wall nanotubes - diameter, sp2C bond deformation
Diameter D is related to the vector of chirality Ch
where the C=C bond length is elongated by the curvature imposed
1.41 A ^ 1.44 A
(graphite)     L=L (Qo)
The C=C bond angles are not plannar    hybridization is not pure sp2 (some degree of sp3) - same effect as in fullerene C60 molecule (radius 0.35 nm, bonds have 10% sp3 character).
•-XTca-
Bond deformation induces variable overlapping of energy bands (unique and versatile electronic behaviour), perhaps also a bit more reactive than planar graphene?
D =
Čh I     aCc \/3(n2 + m2 + nm)
7T
7T
FC250 Nano- and microtechnologies:                    2.1 Structure of Carbon Nanotubes	Lenka Zajíčková	7/50
		
Single-wall nanotubes - ropes		
Fig.    High-resolution transmission electron microscopy images of a SWNT rope, (a) Longitudinal view. An isolated single SWNT also appears at the top of the image, (b) Cross-sectional view
Fig, Image of two neighboring chiral SWNTs within a SWNT bundle as seen using high-resolution scanning tunneling microscopy
FC250 Nano- and microtechnologies:
.1 Structure of Carbon Nanotubes
ka Zajíčková
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Multi-wall nanotubes
Fi g. 3. & H igh-reso lut io u transm iss ion e Lection m icroscop) image {Lougitudi na I view\ of n concentric muLtiwall carboi nanotube (c-MWNT) prepared using an elecuic ait. 'J'Jie insert shows a sketch of tJie Russian doll-Like arrange mem of graphenes
Fig. 3.6a,b Sonne of l lie earliest high-resolution trans missIon electron microscopy images of a herringbone {and bamboo) muitiwall uanotulw {bh-MWNT. longitudinal view) prepared by CO dlí pro-port Ionat Ion on he-Co catalyst, (a) As-grown. 71le nanotube surface is made of free grapJtene edges, (b) After 2900 "C beat treatment. Both the IwrriiigboLie and the bamboo textures have become obvious. Graplieue edges from the surface Jiavc buckled Willi Uicli1 neighbors {nrn>wl closing off access to the LutergrapJiene space
Fig. 3.7a,b Transmission elecUou microscopy images from bamboo mult [wall nanotubes {longitudinal views).
(a) Low magnification of a foaiiül>i>ivlieiTiiigL>oiie multiwall nanotube {bh-MWNT) showing tlie nearly periodic nature of die texture, which occurs very frequently {from [3.1 ft]);
(b) High-resolution image of a bamboo-concentric multi-wall iianotube{bc-M WNT) {modified from [3.19])
Fig. 3.S Sketchentpiaiiiing the various parameter's obtaLied from high-resolution {lattice fringe mode) trans miss ion electron microscopy used to quantify nanotexture: L\ is tlie average length of perfect {distortion-free) graphenes of coliereut areas; N is tlie number of pi led-up giaphenes in coJiereut {distoilion-free) areas; L2 is tlie average length of continuous though distorted grapJieues within grapJtene stacks; is tlie average distortion angle. L\ and jV are related to tlie La and L1: values obtained from X-ray diffraction
FC250 Nano- and microtechnologies:
.2 Synthesis of Carbon Nanotubes
Lenka Zajíčková
9/50
2.2 Synthesis of Carbon Nanotubes
► Various methods - need of controlling purity, structural quality and chirality (for SWCNTs)
► How to understand the mechanism of nanotube nucleation and growth? - still controversial subject
► Not only carbon involved: aside from c-MWCNTs derived from electric arc (catalyst-free), nanotubes are frequently found to be associated with the catalyst crystals from which they are formed
► 2.2.1 Solid Carbon Source - Based Synthesis of CNTs
► 2.2.2 Gaseous Carbon Source-Based Production Techniques
FC250 Nano- and microtechnologies:
2.2 Synthesis of Carbon Nanotubes
Lenka Zajíčková
10/50
2.2.1 Solid carbon source - based synthesis of CNTs
four solid carbon source - based techniques
► laser ablation
► dc electric arc
► three-phase ac arc plasma
► solar energy
=>- high temperature (1000 K < T < 6000 K) is common feature but morphologies of carbon nanostructures and the SWCNTs yields can differ notably
Before CNTs, production of fullerenes:
► laser vaporization of graphite -1 st method for C60
► electric arc - 1st method for production of fullerenes in relatively large quantities
Methods that helped to discover CNTs:
► 1991, lijima, catalyst-free formation of c-MWCNTs by electric arc
► 1993, lijima+lchihashi and independently Bethune, attempts to fill MWCNTs with metal =>- transition metals introduced as part of electrode    accidentally discovered SWCNTs
FC250 Nano- and micro-technologies:
.2 Synthesis of Carbon Nanotubes
ka Zajíčková
11/50
Laser ablation - experimental devices
Furnace
During the interaction between the laser beam and the material, numerous phenomena occur at the same time and/or follow each other within the certain time period. It is sensitive to
► characteristics of laser beam,
► incoming power density (fluence),
► nature of target,
► surrounding environment.
Two types are currently used
► pulsed laser
► laser operating in continous mode (generally smaller fluence)
Furnace
Graphite target
Fig. 3.9 Sketch of an early laser vaporization apparatus
Graphite pellet with catalyst is placed in the middle of a quartz tube filled with inert gas and placed in oven (1200 °C).
Pellet is vaporized, carbon species swept by a flow and deposited as soot in different regions (conical water-cooled copper collector, tueb walls, backside of the pellet)
FC250 Nano- and microtechnologies:
.2 Synthesis of Carbon Nanotubes
Lenka Zajíčková
12/50
Pump
W
Fimiace
I Graphite I taraet
Optical pyrometer
yšž^lj Filter
Silica pipe
Fig. 3.9 Sketch of an early laser vaporization apparatus
some improvements:
► second pulsed laser -evaporation of coarse aggregates
► peller on revolving system for uniform heating
► two pellets (graphite and catalyst separately)
Continuous C02 laser
Water
cooled
chamber
Gas injector
continuous C02 laser (10.6 /xm), power 100-1600 W, control of pellet T - pyrometer, gas is heated by contact with pellet - external furnace unnecessary, products taken by gas flow and collected on filter
FC250 Nano- and micro-technologies:
.2 Synthesis of Carbon Nanotubes
Lenka Zajíčková
13/50
► without catalyst: soot contains mainly c-MWCNTs, length up to 300 nm, quantity and quality depend on oven T (best at 1200 °C, lower T yields deffective MWCNTs).
► with catalyst (transition metal like Ni, Co), a few percent or less: SWCNTs, yield depends on catalyst and increases with T, uniform diameter, rope-like crystals with diameter 5-20nm (see fig)
Fig. 3.11 Low-magnification TEM images of a typical raw SWNT material obtained using the laser vaporization technique. The fibrous structures are SWNT bundles, and the dark particles are remnants of the catalyst. Raw SWNT materials obtained from an electric arc exhibit similar features
FC250 Nano- and microtechnologies: 2.2 Synthesis of Carbon Nanotubes Lenka Zajíčková 14/50
Electric arc - experimental devices
Window
~U Gas inlet
Cathode
Anode
Motor
Window
uu
Vacuum
Fig. 3.12 Sketch of an electric arc reactor
► two graphite rods (few mm in diameter) are the electrodes between which the potential is applied
► anode - graphite with catalyst (Fe, Ni, Co, Y etc.) for SWCNTs growth
► inert atmosphere (Ar, He)
plasma in carbon and catalyst vapors mixed with inert gas
Two types of anode
► graphite anode with a coaxial hole, several cm in length, into which mixture of catalyst and graphite powder is placed (more popular)
► graphite anode within which catalysts are homogeneously dispersed
FC250 Nano- and micro-technologies: 2.2 Synthesis of Carbon Nanotubes Lenka Zajíčková 15/50
Electric arc - experimental devices
zone 1 -
zone 2
zone 2
FIGURE Schematic of MWNT growth in a DC arc. Positively charged graphitic splinters spallated from the anode partially vaporize on the way to the tops of the columns (zone 1). Intense carbon flux above the columns is due to enhanced thermionic emission from zone 1, which produces a high degree of carbon vapor ionization. Carbon density above the columns is further increased by circular convection of helium. Nanotubes grow in dense gas near the cathode surface from low-molecular carbon species and precipitate in zone 1 and zone 2 with their axes oriented preferentially parallel to the surface. This orientation is held constant in as-produced deposit core in both zone 2 and the nanotube braid of the column [1].
SWCNTs deposited in different regions
► collaret (formed around cathode)
► weblike deposit above the cathode
► soot all around reactor walls and the bottom
MWCNTs are formed in a hard cathode deposit under the cathode whether catalysts are used or not
The formation of collaret and web is not systematic and depends on exp. conditions. Cathode deposit and soot are obtained consistenly.
Products do not consist solely of CNTs (see table on next page): carbon NPs, fullerene-like structures, poorly organized polyaromatic carbons, nearly amorphous carbon nanofibers, multiwall shells, single-wall nanocapsules, amorphous carbon ...
FC250 Nano- and micro-technologies:
2.2 Synthesis of Carbon Nanotubes
ka Zajíčková
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Table 3,1 Different carbon morphologies obtained by changing the type of anode, the type of catalyst and tlie pressure in a series of arc discharge experiments [electrode gap = 1 mm)
Catalyst (atom%) Arc conditions	O.oNi+O.iCo (homogeneous anode) i»~ (SOkPa 7~80A	O.tSNi+O.óCo (homogeneous anode) P ~ 40 tPa J~80A	05M+«.5Co P ~ 60 kPa 7~ 80 A	4.2Ni-t-LY i>~ SOkPa J~8*A
Soot	- MWNT + MWS -POPAC or Cn - catalysis 0   3-35 nm • NANF + catalysts • AC particles -I- catalysts • [DWNT], [SWNT], ropes or isolated. + POPAC	« POPAC and AC particles + catalvsts 0~2-2Onm • NANF + catalysts <p ~ 5-20mn + MWS • [SHWJ] ^ —1-1.4 mn, distorted or damaged, isolated or ropes + Cn	• AC: and POPAC particles + catalysts tf>'-3-35nm • NANT + catalysts <p -- 4-15nm • [SWNT] <p ~ 1,2 nni, isolated or ropes	• POPAC and AC + particles + catalysts ip < 30 nm > SWNT <p ~ 1.4 nm. clean 4- Cn, short with   tips, [damaged], isolated or ropes ip < 25 nm • [SWNC] particles
Web • [MWNT].. DWNT,
02.7-4-5,7nm SWNT
01.2-1.Snm, isolated or ropes
0 ■=: 1 5 nun
+ POPAC ± Cu
• AC particles 4- catalysis 0~3^H)nm + MWS
• [NANF]
Collaret     ■ POPAC and SWNC
particles
• Catalysts
0 — 3-250nm_ «: 50mn+ MWS . SWNT 01-1,2 nm. [opened], distorted, isolated or ropes 0 < 15 nm, + Cn
• [AC] particles
Cathode      • POPAC and SWNC deposit particles
• Catalvsts 0 ~ 5-300 run MWS
■ MWNT0 <: 50 nm
• [SWNT] <p-~ 1.6run clean + Cn, isolated or ropes
None
None
• AC and POPAC
particles + catalysts 0 ■-- 3-25 nm -SWNT £~ 1-1.4nm clean + Cn, [isolated] or ropes p < 25 urn
• Catalysts
0 -5-50nm + MWS. ■ [SWNC]
• POPAC and SWNC particles + Cn
• Catalvsts
0 - 20-100nm + MWS
• Catalysts
6 - 3-170mr. I MWS
• AC or POPAC particles + catalysts
0 ~ 3-50nm
• SWNT 0 - 1.4 run clean + Cn isolated or rapes 0 < 20 nm
p MWS. catalyst-free >MWNT0 < 35 nm i POPAC and PSWNC particles
i [SJFNT\, isolated ex ropes
i [Catalysis] 0~3-3Onm
SWNT, 0-1.4 mil isolated or ropes 0 < 20nm + AC POPAC and AC particles+catalysis 0 - 3 - 10 - 40 nm + MWS
■ SWNT
<p ~ 1.4-2.5 nm. clean + Cn, [damaged], isolated or ropes 0   30 nm
• POPAC or AC particles + catalysts 0~3-3Onm
*[MWS] + catalysts or catalyst-free.
.SWNT
0 ~ 1 4-4 1 rain
clean + Cn, short with tips, isolated or ropes 0 < 20 nm.
• POPAC or AC particles 4- catalysts 0~3-3Onrn
• MWS + catalysts 0 < 40 nm or catalyst-free
• [MWNT]
Abundant - Present - [Rare]
Glossary: AC: amorphous carbon; POPAC: poorly organized polyaromalic carbon; Cn: fullerene-like structure, inchtding C50; NANF: nearly amorphous nanofiben MWS: multiwall shell; SWNT: single-wall nanotube; DWNT: doublfi-wall nanotube, MWNT: multiwall nanotube; SWNC: single-wall nanocapsule.
SWCNTs favoring conditions: He 600 mbar, arc current 80 A„ electrode gap 1 mm, Ni/Y 50-70% SWCNTs in collaret, 50% in web and soot MWCNTs in cathode deposit
FC250 Nano-and microtechnologies: 2.2 Synthesis of Carbon Nanotubes Lenka Zajíčková 17/50
Three-phase AC arc plasma
0-600 V AC       Current transducer transformer with adjusted
t v
R=10i5
Electrode 2
i2
' Electrode 1
^^Eleci
Electrode 3
i3 Oscilloscope
TTTT
a, b, c "
Semi-industrial technology: electric arc is established between three graphite electrodes, three-phase AC power supply operating at 600 Hz, current 250-400 A.
Carbon precursors, gaseous, liquid or solid, are injected at the desired (variable) position into the plasma zone.
Free parameters: arc current, flow rate and nature of plasma gas (N2, Ar, H2, He etc.), carbon precursor, injection and extraction positions
T. M. Gruenberger et al., Fullerenes, Nanotubes and Carbon Nanostructures 12 (3), 571 (2004)
FC250 Nano- and microtechnologies: 2.2 Synthesis of Carbon Nanotubes Lenka Zajíčková 18/50
► based on the sublimation of a mixture of graphite powder and catalysts placed in a crucible in an inert gas
► high temperature of about 4000K causes both the carbon and the catalysts to vaporize; the vapors are then dragged by the neutral gas and condense onto the cold walls of the thermal screen
► target is a graphite rod that includes pellets containing the catalysts, which is surrounded by a graphite tube
Fig. 3.1i«- c Sketch of a solar energy reactor in use in the PROMES-CXRS Laboratory, Odeilho (France), (a) Gathering of sun rays, focused at F, (b) Side view of the experimental set-up at the focus, of the 1 MW solar furnace: (c) top view of the target graphite rod_
► with Ni/Co, and at low pressure, the sample collected contains mainly MWNTs with bamboo texture, carbon shells, and some bundles of SWNTs
► at higher pressures, only bundles of SWNTs are obtained,with fewer carbon shells; relatively long bundles of SWNTs are observed with Ni/Y and at a high pressure; the diameters of the SWNTs range from 1 to 2 nm
► can be scaled up from 0.1-0.2g/h to 10 g/h and then to 100 g/h productivity using existing solar furnaces; experiments performed on a medium scale produced about 10 g/h of SWNT-rich material using various mixtures of catalysts (Ni/Co, Ni/Y, Ni/Ce)
FC250 Nano- and microtechnologies:	2.2 Synthesis of Carbon Nanotubes	Lenka Zajíčková	19/50
			
2.2.2 Gaseous Cart	)on Source-Based Proc	Juction Techniques	
Catalytic decomposition of a carbon-containing source on small metallic particles or clusters - catalytic CVD involves
► either an heterogeneous process if a solid substrate is involved (supported catalyst)
► or an homogeneous process if everything takes place in the gas phase (floating catalyst)
Both, homogeneous and heterogeneous, processes appear very sensitive to the nature and the structure of the catalyst used, as well as to the operating conditions:
► metals generally used for these reactions are transition metals, such as Fe, Co and Ni
► rather low-temperature process compared to arc discharge and laser ablation methods, with the formation of carbon nanotubes typically occurring between 600 °C and 1000 °C
Quality of CNTs:
► CNTs prepared by CCVD methods are generally much longer (a few tens to hundreds of micrometers) than those obtained by arc discharge (a few micrometers); depending on the experimental conditions, it is possible to grow dense arrays of nanotubes
MWNTs from CCVD contain more structural defects than MWNTs from arc discharge, due to the lower temperature of the reaction, which does not allow any structural rearrangements
► CCVD SWNTs generally gather into bundles of smaller diameter (a few tens of nm) than their arc discharge and laser ablation counterparts. Specifically, when using fluidized bed reactor, CCVD provides reasonable good perspectives on large-scale and low-cost processes.
FC250 Nano- and micro-technologies:
.2 Synthesis of Carbon Nanotubes
ka Zajíčková
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CCVD heterogeneous process - supported catalys
Although CCVD formation mechanisms for SWNTs and MWNTs can be quite different
it is agreed that CNTs form on very small metal particles, typically nm range size two steps for CCVD heterogenous process:
preparation of catalytic particles - reduction of transition metal compounds (salts, oxides) by H2 prior to the CNTs growth (important parameters: temperature, treatment duration, gas composition and flow rate, catalyst nature and size)
at a given temperature, depending mainly on the nature of the catalyst and the carbon-containing gas, the catalytic decomposition will take place at the surfaces of the metal particles, followed by mass transport of the freshly produced carbon by surface or volume diffusion until the carbon concentration reaches the solubility limit, and the precipitation starts.
a)
Supported catalyst or solid solution
Catalytical metal particles
CNTs
(1) (2)
Fig. 3.15 (a) Formation of nanotubes via the CCVD-based impregnation technique. (1) Formation of catalytic metal particles by reduction of a precursor; (2) Catalytic decomposition of a carbon-containing gas, leading to the growth of carbon nanotubes; (3) Removal of the catalyst to recover the nanotubes (b) Example of a bundle of double-wall nanotubes (DWNTs) prepared this way
FC250 Nano- and microtechnologies: 2.2 Synthesis of Carbon Nanotubes Lenka Zajíčková 21 / 50
Preparation of Catalysts
Floating catalyst method uses organometalic volatile compounds such as ferrocene Fe(C5H5)2 or iron pentacarbonyl Fe(CO)s.
Supported catalyst method uses Fe, Ni, Co, ... and their alloys in the form of
► thin film (vacuum evaporation, magnetron sputtering):
The key factor is to perform restructuring into active catalyst nanoparticles (NPs): heating in N2, H2, NH3 or plasma treatment.
n III [illl Mlfl
I
m
11......rJ^iid
■inn
Process depends on time, gas, thickness of the film and its morphology. Importance of material under the catalyst. Particles can coalesce during continuous heating, other material phases can be formed ...
► wet chemistry impregnation of substrate - impregnation of a substrate with a solution of salt (nitrate, chloride, acetate) of desired transition metal catalyst
► solid solution-based catalysts - preparation of solid solution of an oxide of the chosen catalytic metal in a chemically inert and thermally stable host oxide
► direct deposition of NPs - electrochemical deposition, PECVD
Controlling metal particle size is key issue, coalescence has to be avoided (various supports like Al203, Si02, zeolites, MgO).
methane or other hydrocarbon CVD
passing a gaseous flow containing a given proportion of a hydrocarbon (mainly CH4, C2H2, C2H4, or C6H6, usually as a mixture with either H2 or an inert gas such as Ar) over small transition metal particles (Fe, Co, Ni) in a furnace
► reaction is chemically defined as catalysis-enhanced thermal cracking
OxHy -+ xC + y/2H2
► Synthesis of carbon nanofibres (< 10 nm) in 70-80ties. Galuszka et al. Carbon 22 (1984), Tibbetts et al. Carbon 25 (1987) Synthesis of MWCNTs mostly with C2H2 and C2H4 from 90ties . Synthesis of bulk amount of single-walled nanotubes (SWNTs) using methane CVD. Dai's group: Kong et al. Nature 395 (1998), Kong et al. Chem. Phys. Lett 292 (1998)
catalyst (Fe, Ni, Co ...)
furnace 500-1000 V.
FC250 Nano- and micro-technologies: 2.2 Synthesis of Carbon Nanotubes Lenka Zajíčková 23 / 50
Supported catalyst - other gases
carbon monoxide CVD
► Mass production of SWCNTs by high-pressure decomposition of CO (HiPCO) on metal NPs. Smalley's group: Dai et al. Chem. Phys. Lett. (1996) 471, Hafner et al. Chem. Phys. Lett. (1998) 195
alcohol CVD
► CO can be used instead of hydrocarbons; the reaction is then chemically defined as catalysis-enhanced disproportionation (the so-called the Boudouard equilibrium)
► 2CO^C + C02
► High-purity long SWCNTs using alcohols (CH3OH, C2H5OH) - OH radical formed at high temperature from alcohol can remove amorphous carbon.
Maruyama et al. Chem. Phys. Lett. (2002) 229
supergrowth - water-assisted CVD
► CVD often limited by low activity and short life time of catalyst. Dramatic improvement obtained by addition of water vapor - root growth of SWCNTs forest. Although aligned, the SWCNTs occupy only 3.6 % of the total volume, i.e. growth is not diffusion limited. Hata et al. Science (2004) 1362: 10 min growth using C2H4
FC250 Nano- and microtechnologies: 2.2 Synthesis of Carbon Nanotubes Lenka Zajíčková 24 / 50
Example of the CCVD involving thin film catalysts from M. Jung et al. Diamond and Related Materials 10 (2001) 1235
► Substrate: Si with Si02 film (1 /iim)
► Catalyst: Ni film prepared by DC magnetron sputtering (2-7 nm)
► NPs preparation: annealing at 800 °Cfor 15 min in H2 =>- size and number density dependening on film thickness
Fiu;. I. SEM microsLrucLnre of Ni par Lides on SlO^/Sl substrate formed by anneal im; of spuLLer deposited N"i film of (a} J.4 nm and (b) ft.fi nm in Lhk'kne^s.
FC250 Nano- and microtechnologies:
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CCVD Involving Thin Film Catalysts
M. Jung et al. Diamond and Related Materials 10 (2001) 1235
Fig. 2. Schema!it of horizontal flow reactor tor carbon nanoLube
studies of H2/N2/C2H2 mixtures:
	
	
	
	
L	:.....■ ..
	
Flj;. j. SEM Tnii:[iisLrin:Ltints uf [be carbon ileptRiils iilz 20 min wilh 2.4 vul.% CiH2 at fiSfi°CaflÉí preLrealmera fůr 1 h in the environ-möÜS of various I ^/(n,, +11,} frati ions: (a} I l_,/(K , + 11 ,} = I); (b) Hí/ÍTÍj + HíJ-Oů; <c> Hí/(I^+Hj)-0-83; and (jD evcnj + H2)-lJQ.
► NPs preparation (contin.): pre-treatment directly in quartz tube, 700-1000 °C heating for 1 hour in various gas environments such as H2, N2 , H2/N2, H2/Ar, H2/NH3
► CNTs growth: C2H2 added to previous gases (concentration varied from 2.4 to 20 vol.%), 20 min deposition
H2 prevents passivation of catalyst
mixture influences decomposition kinetics of C2H2 (no growth for 850 °C for H2/(N2+H2)=0.35 but ok for pure H2)
Piu;. 5. SEM micnBilruCLures of Lbe deposits fit: 3) min wilh 2.4 V6L% CiHi af Icr preLreaLmenL for 1 h (a) al 75ITC when Hj/GAi + Hi>= 0.35 and (b)aL 9SD°C when Hi/(Nj + H2) - 14
Fig.íi. TEM mitnüiLrutLure of CNTsynthesized wiLh2.4 vül^CjHj after pielrealment tor 1 h aL fi50ůCwhenH1/(N1 + H1)= 1.0.
FC250 Nano- and microtechnologies:
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M. Jungetal. Diamond and Related Materials 10 (2001) 1235
Fig. 7. (a) SEM mimtalrueLures ůf the CNTs synthesized for 20 min with^.l vúl.% C1ll1 after prctreatment for 1 h inNII, environment, (b) 5EM micros! met ures of I he CNTü synthesized for 20 min with 13 j0 voi.% Ci Hj a fie r prel realmen L fur I h in K II.. environment, (c) S CM mirrostruelures of Lhe deposits synthesized fór 20 min with 7.0 voi.% C1H1 after pretrealment for 1 h in the environment of mixture of N, and 11^ environment Illz/(N_, + II 2> = 0.75j. (d)TEM micros! met Lire of the CNTof rig. 7b.
► improvements of CNTs growth for admixed NH3
► size and density of catalytic NPs did not change but nitrogen is incorporated in Ni particles
FC250 Nano- and micro-technologies:	2.2 Synthesis of Carbon Nanotubes	Lenka Zajíčková	27/50
			
CCVD Involving"	rhin Film Catalysts		
pre-heating of the substrate in Ar/H2 plasma, average T = 700 °C
Ar/H2 - 1500/570 seem, ŕ = 30 s, Ar/H2 - 1500/430 scem, ŕ = 30 s
Fe 10 nm Ar/H2/CH4 - 1500/430/42 scem, ŕ = 30 s
Fe 15 nm
O. Jašek et al. J. Phys. Chem. Solids 68 (2007) 738
FC250 Nano- and micro-technologies:                    2.2 Synthesis of Carbon Nanotubes	Lenka Zajíčková	28/50
		
CCVD Involving Impregnated Catalysts		
Process generally involves four different and successive steps:
1. impregnation of the support by a solution of a salt (nitrate, chloride) of the chosen metal catalyst;
2. drying and calcination of the supported catalyst to get the oxide of the catalytic metal;
3. reduction in a H2 containing atmosphere to make the catalytic metal particles,
4. the decomposition of a carbon-containing gas over the freshly prepared metal particles that leads to nanotube growth
FC250 Nano- and microtechnologies:                    2.2 Synthesis of Carbon Nanotubes	Lenka Zajíčková	29/50
		
CCVD Involving Impregnated Catalysts		
Example of CCVD involving impregnated catalysts from Ph. Mauron etal. Carbon 40 (2002) 1339
► Step 1 - deposition of iron(lll) nitrate, Fe(N03)3, film:
Fe(N03)3/ethanol solutions of different concentrations (7.5-60 mmol/l) spin-coated on Si substrate. The film thickness can be changed by varying the concentrations of Fe(N03)3 solution.
► Step 2 - heating prior to CNTs growth: quartz glass tube
evacuated to 10-4 Pa, temperature increased to 650-750 °C (8 min to reach the temperature, kept constant for 10 min), constant flux of N2 (98 seem) introduced to get
p = 5 x 104 Pa. Iron oxide clusters were formed by heating the Fe(N03)3-coated substrate according to the reaction:
4Fe(N03)3      2Fe2Os + 12N02 + 302
•  .: * '•   >-*    - V. t 9* %     * • t        ór« ♦	\: ■ -.v - r: -.• - -. " •"      v •.
ťc)>      ■•• ..v *% » ■ • ■ • * »». _ • • •  . • * .. •.. • *..»•». « . lLim.': ; ■ . - .	»*** 1               '          V v*. ■ "■'■'■"***»*
Fuj. 1. SEM inuigeí íbiitk stirttereJ e-lettruní;) uf Fe,J01 dusters furmed on HÍliaui ^Ľh'-lnilij'-. J'ht i-uhi-l nilc ■■ were c<i;ilud nilh ircuilKIi
^ I spin-coating of FetNO^;,-ý Ľtľiatiíil solution
2)
I
heating
FeíNO^rfilm Si
FcjO,-clusters Si
Fij;. I. [Je-pusition of nanotubes in three steps: ill spin-eu^tinj; of iron! ID) nilnlie solution on the silitun sufastnite: \1) forrmtion of Fe-,0H elusteis by heuting to 650-750 °C: {3) pyralysis of acetylene- nt 650 -750 CC.
FC250 Nano- and microtechnologies:
2.2 Synthesis of Carbon Nanotubes
Lenka Zajíčková
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CCVD Involving Impregnated Catalysts
Ph. M auro n et al. Carbon 40 (2002) 1339
► Step 3 - synthesis of CNTs by adding a constant flux of acetylene (2 seem) to the N2 carrier gas during 30 min.
	
2 iuli	
Fi£. i. S liM irnuiges fsetundiiry elections) uf rainutube fibs mi i silicon subsmte. The samples were- tut iind are shuwn frum the sioe. The Hindu bes wot synthesized by pyiu lysis uf ueetylene uver un imnfDI) nitrate tuuteu' silitun substrjle. fjj Film thitJcness d^i'i \Lm, di juneter u f the runulubes H-2& ran, tuntenlratiun uf the FefNO.,), sulutiun f = 7.5 mmul/l, synthesis tempeniture JT=7(J(J Xl. \ b) b-'ilm thitJcness d*=5 \i.m, diameter uf the njnutubes 5fJ-25 nm, tuntentriitiun uf the FefNO,), sulutiun c = 15 mmul/l, synthesis temperature r=70C °C-
FC250 Nano- and microtechnologies:
2.2 Synthesis of Carbon Nanotubes
Lenka Zajíčková
31 /50
CCVD Involving Solid Solution-Based Catalysts
... uses solid solution of an oxide of the chosen catalytic metal in a chemically inert and thermally stable host oxide
► solid solution of two metal oxides is formed when ions of one metal mix with ions of the other metal, e.g. Fe203 can be prepared in solid solution in Al203 —► AI2_2xFe2x03
► use of a solid solution allows a perfectly homogeneous dispersion of one oxide in the other to be obtained
Preparation methods:
► coprecipitation of mixed oxyalates
► combustion synthesis
Homogeneous dispersion of catalytic oxide    very small catalytic metal NPs
► heating of solid solution from room to 850-1050 °C in H2/CH4 (18 mol% of CH4)
► the product depends upon nature of both, the transition metal (or alloy) and inert oxide (matrix)
► nanotubes obtained are typically gathered into small-diameter bundles (less than 15 nm) with lengths of up to 100/im; nanotubes are mainly SWNTs and DWNTs, with diameters of between 1 and 3 nm
► to remove host oxides aggressive treatments (KOH, NaOH for Al203; HF for Si02) are required; these treatments have no effect, however, on other impurities such as other forms of carbon (a-C, graphitized carbon particles and shells ...).
► oxidizing treatments (air oxidation, use of strong oxidants such as HN03, KMn04, H202) are thus required to remove most unwanted forms of carbon, but they result in a low final yield
FC250 Nano- and microtechnologies: 2.2 Synthesis of Carbon Nanotubes Lenka Zajíčková 32 / 50
CCVD with Floating Catalyst - Homogeneous Processes
Supported catalyst - reactors are using static phase placed within the gas flow or fine powder suspended by gas phase, so called fluidised bed process
Floating catalyst - differs because it uses only gaseous species and does not require the presence of any solid phase in the reactor
► basic principle of this technique, similar to the other CCVD processes, is to decompose a carbon source (ethylene, xylene, benzene, carbon monoxide, and so on) on nanosized transition metal (generally Fe, Co, or Ni) particles in order to obtain carbon nanotubes
► catalytic particles are formed directly in the reactor, however, and are not introduced before the reaction, as occurs in supported CCVD for instance
FC250 Nano- and micro-technologies:
2.2 Synthesis of Carbon Nanotubes
CCVD with Floating Catalyst
► typical reactor used in this technique is a quartz tube placed in an oven into which the gaseous feedstock, containing the metal precursor, the carbon source, some hydrogen and a vector gas (N2,Ar, or He)
► first zone of the reactor is kept at a lower temperature, and the second zone, where the formation of tubes occurs, is heated to 700-1200 °C
► metal precursor is generally a metal-organic compound, such as a zerovalent carbonyl compound like Fe(CO)s, or a metallocene such as ferrocene, nickelocene or cobaltocene
FC250 Nano- and micro-technologies:	2.2 Synthesis of Carbon Nanotubes	Lenka Zajíčková	34/50
			
CCVD with Floating <	Catalyst III		
► metal-organic compound decomposes in the first zone of the reactor, generating nanosized metallic particles that can catalyze nanotube formation; in the second part of the reactor, the carbon source is decomposed to atomic carbon, which is then responsible for the formation of nanotubes
► quite flexible and SWNTs, DWNTs and MWNTs have been obtained, in proportions depending on the carbon feedstock gas
► main drawback of this type of process is again that it is difficult to control the size of the metal NPs, and thus nanotube formation is often accompanied by the production of undesired carbon forms (amorphous carbon or polyaromatic carbon phases found as various phases or as coatings)
► same kind of parameters have to be controlled as for heterogeneous processes in order to finely tune this process and selectively obtain the desired morphology and structure of the nanotubes formed, such as: the choice of the carbon source; the reaction temperature; the residence time; the composition of the incoming gaseous feedstock, with particular attention paid to the role played by the proportion of hydrogen, and the ratio of the metallorganic precursor to the carbon source
FC250 Nano- and micro-technologies:
1.2 Synthesis of Carbon Nanotubes
ka Zajíčková
ance
it
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PECVD in hard competition with CVD:
Mostly MWNTs because of plentiful supply of carbon (presence of reactive radicals and unsaturated hydrocarbons), ion damage and non-optimized catalyst preparation
Deposition of SWNTs |B|
► low pressure PECVD: Kato et al. Chem. Phys. Lett. (2003) 422, Kato et al. Nanotechnology (2006), 2223, Iwasaki at al., J. Phys. Chem. B. Lett. (2005), 19556
► atmospheric pressure PECVD: Nozaki at al. Carbon (2007) 364
What PECVD can offer over CVD?
Presence of inherent electric field influencing the growth orientation of CNTs: widely accepted argument for PECVD
W^'v'ÄfSiX Chhowalla et al., J.
^«L-iraa^^wi \       APP>- Phys-90' (2001)'5308
► Lower process temperature: conventional argument for PECVD over CVD but CNT growth does not require pyrolysis of precursor
Hoffman et al., Appl. Phys. Lett. (2003), 135 - carbon nanfibres at 120 °C by dc-PECVD
► Preferential growth of semiconducting SWNTs. Li et al., Nano Lett. (2004), 317
FC250 Nano- and microtechnologies:
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Lenka Zajíčková
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► f = 2.45 GHz
► P = 100-400 W
► dual gas flow:
► central channel: Ar (500-1500 seem)
► side channel: CH4l H2, Ar + Fe(CO)5
dual gas flow flowmeters
MW Generator
quarlz
stretcher -plunger N
ferrite circulator
c
water load
waveguide
gradual transition
O
900 800 700 600
waveguide & matching
2
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B
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-		controllers opened	
		discharge off	
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20
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FC250 Nano- and micro-technologies:
2.2 Synthesis of Carbon Nanotubes
ka Zajíčková
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Visualization of Torch Dynamics
MW power 210 W, Ar 660 seem
side view     front view
1st appearance of visible    after 29 s plasma channel (front view)
Ar/Fe(CO)5 670/1 seem
145sccm
Ar/H2/CH4
total video time 30s,
camera resolution 1000 frames/s,
video resolution 10 frames/s
FC250 Nano- and microtechnologies:
1.2 Synthesis of Carbon Nanotubes
ka Zajíčková
38/50
SiO
► Ar/H2/CH4 gas feed - 1500/430/42 seem,
► catalytic film - Fe 5 nm, average T = 700 °C
► non-uniform deposition because of gradients of substrate T
center
average diameter 38 nm
edge
average diameter 18nm
SEI       5.0kV    X20.000     1,um      WD 9.4i
B c
8
B c
8
10 12 14 16 18 20 22 24 26 28 30
diameter [nm]
25 35 45 55 65 75
diameter [nm]
L. Zajíčková et al. Plasma Process. Polym. 2007, 4, S245-S249
FC250 Nano- and micro-technologies:
1.2 Synthesis of Carbon Nanotubes
ka Zajíčková
39/50
growth on Si without barrier layer - some applications require electrical contact to CNTs
► 1500/430/42 seem
► 700 °C, 45 s
► 15nm Fe
alignment of CNTs (people) by crowding effect
SEI       5.0kV    X1.000     10řim     WD 8.0mm
CNTs deposited without barrier layer exhibit similar density and quality as with Si02 barrier
5.0kV X20.000
L. Zajíčková et al. Pure Appl. Chem. 82(2010) 1259-1272
FC250 Nano- and micro-technologies:                    2.2 Synthesis of Carbon Nanotubes				ka Zajíčková                                 40 / 50
	>mpia.inn			
► only other method aside from the electric arc that is able to synthesize CNTs without any catalyst
► principle is to deposit the solid carbon coating obtained from the CVD method onto the walls of a porous substrate whose pores are arranged in parallel channels
► feedstock is a hydrocarbon; substrate can be alumina or zeolite for instance, while the whole system is heated to a temperature that cracks the hydrocarbon selected as the carbon source
► both MWNTs (exclusively concentric type) and SWNTs can be obtained; the smallest SWNTs have diameters ~ 0.4 nm, lengths are directly determined by the channel lengths; in other words by the thickness of the substrate plate
Anodic aluminium oxide film
Carbon deposition on die pore wall
Carbon tubes
(Propylene, 800 °C)
lOOum
Il0-500um
HF washing
Fig. 3.16 Principle of the templating technique used in the catalyst-free formation of single-walled or concentric-type multiwalled carbon nanotubes
FC250 Nano- and microtechnologies: 2.3 Growth Mechanism of Carbon Nanotubes Lenka Zajíčková 41 / 50
2.3 Growth Mechanism of Carbon Nanotubes
► Catalyst-free growth (templating method, electric arc - here driving force is related to electric field, precursors species C2 as in fullerene production)
► Catalytically Activated Growth
► low T conditions - typically in CCVD: VLS mechanism (1. adsorption + decomposition, 2. dissolution + diffusion 3. precipitation); catalyst as crystallized solid (texture defined by crystal) or liquid droplet (energy minimization make concentric product); c-MWCNTs, h-MWCNTs, platelet nanofibers (formed for large particles > 100 nm); tip-growth for large NPs or absence of substrate x base growth
► high T conditions - solid carbon-source: carbon source and catalyst is atomized, at some distance from atomization zone - carbon metal alloy droplets + secondary carbon species (C2 and higher like corannulene: central pentagon surrounded by five hexagons) adjacent pentagons are not stable     hexagons; in the zones with low T solubility threshold of C in metal decreases - combination of VLS and adatom mechanism
FC250 Nano- and microtechnologies: 2.4 Properties of CNTs Lenka Zajíčková 42 / 50
Mechanical Properties
► Strongest and stiffest material yet discovered (tensile strength and elastic modulus)
► In 2000 a MWNT tested to have a tensile strength of 63 GPa
► Translates into ability to hold 6500 kg on a cable with cross-section of 1 mm2
► Specific strength of up to 48,000 kN-m-kg-1 (high carbon steel's is 154 kN-m-kg-1)
► Not so strong under compression
FC250 Nano- and micro-technologies:
2.4 Properties of CNTs
ka Zajíčková
43/50
► Multi-walled coaxial nanotubes show striking telescopic properties
► Shells slide with almost no friction
► Molecular nanotechnology
► World's smallest rotational motor created already
FC250 Nano- and microtechnologies: 2.4 Properties of CNTs Lenka Zajíčková 44 / 50
► Indices (m, n) tune the gap
► n = m - nanotube is metallic
► n = -m - nanotube is semiconducting with a very small gap
► otherwise - moderate semiconductor
FC250 Nano- and microtechnologies: 2.4 Properties of CNTs
Thermal Properties
► Very good thermal conductivity along a tube Good insulator laterally to the tube axis
► Three orders difference
► One order better on-axis conductivity than copper
FC250 Nano- and microtechnologies:                    2.5 Carbon Nanotube-Based Nano-Objects							Len	ka Zajickova                                 46 / 50
								
2.5 Carbon	N	anotube-Based		N	ano-0	bjects		
► 2.5.1 Extreme CNTs and Related Geometries
► 2.5.2 Functionalized Nanotubes
► 2.5.3 Heteronanotubes (replacement of some C atoms by other elements, e.g. N, B)
► 2.5.4 Hybrid CNTs (inner cavities filled with foreign atoms, molecules, compounds or crystals)
FC250 Nano- and microtechnologies:
2.5 Carbon Nanotube-Based Nano-Objects
Lenka Zajíčková
47/50
2.5.1 Extreme Carbon Nanotubes and Other Related Geometries
► The longest carbon nanotube (18.5 cm) reported in 2009. Grown on Si using improved CVD
► The shortest CNT is the organic compound cycloparaphenylene synthesized in early 2009
► The shortest CNT synthesized is armchair (2,2) with diameter of 3 A
► The thinnest free-standing SWNT is about 4.3 Ain diameter
► Nanotorus - Theoretically described, many unique properties described
► Nanobud - Combination of fullerene and NT allotropes. Useful properties of both, improving mechanical properties
FC250 Nano- and microtechnologies:                    2.6 Potential and Current Applications	Lenka Zajíčková	48/50
		
Structural Applications		
► Because of superior mechanical properties, many structures have been proposed
► From clothes, sports gear, combat jackets up to space elevators
FC250 Nano- and micro-technologies:
2.6 Potential and Current Applications
Lenka Zajíčková
49/50
Electrical Circuits
► Nanotube-based transistors have been made
► Semiconducting nanotube required (n / at?)
► Capable of digital switching using a single electron
FC250 Nano- and microtechnologies:
2.6 Potential and Current Applications
Lenka Zajíčková
50/50
Other Applications
► Solar Cells - Nanotubes with buckyballs containing a polymer As paper batteries - supercapacitors
► Nanoscale electric motors
► Radio receiver consisting of a single NT demonstrated
► Demonstrated that sheet on NTs can work like a speaker - thermoacoustically
► Nano-tips for SPM
► Emitters for electron microscopy