Gold nanoparticles grown inside carbon nanotubes: synthesis and electrical transport measurements

13 Pages·2014·3.4 MB·English

Seguraetal.NanoscaleResearchLetters2014,9:207
http://www.nanoscalereslett.com/content/9/1/207
NANO EXPRESS Open Access
Gold nanoparticles grown inside carbon
nanotubes: synthesis and electrical transport
measurements
Rodrigo A Segura1*, Claudia Contreras1, Ricardo Henriquez2, Patricio Häberle2, José Javier S Acuña3, Alvaro Adrian4,
Pedro Alvarez4 and Samuel A Hevia4
Abstract
The hybrid structurescomposed of goldnanoparticles and carbonnanotubes were prepared using porous
alumina membranes as templates. Carbon nanotubes were synthesized inside thepores of these templates bythe
non-catalytic decomposition ofacetylene. The inner cavityof thesupported tubes was used as nanoreactors to
grow goldparticlesby impregnationwith a gold salt, followed by a calcination-reduction process. The samples
were characterizedby transmissionelectron microscopy and X-rayenergy dispersion spectroscopy techniques.
The resulting hybrid products are mainly encapsulated goldnanoparticles with different shapes and dimensions
depending onthe concentration of thegold precursor and the impregnationprocedure.Inorder to understand
theelectronic transport mechanismsinthesenanostructures, their conductance was measured as a function of
temperature. The samples exhibit a ‘non-metallic’temperature dependencewhere thedominantelectron transport
mechanism is 1D hopping. Depending onthe impregnation procedure, theinclusion of gold nanoparticles inside
theCNTs can introduce significant changes inthe structure of the tubesand the mechanisms for electronic
transport. The electrical resistance of thesehybrid structures was monitored under different gas atmospheres at
ambient pressure.Using this hybrid nanostructures, small amounts ofacetylene and hydrogen were detected
withan increased sensibility compared with pristine carbonnanotubes. Although the sensitivity of thesehybrid
nanostructures is ratherlow comparedto alternativesensing elements,their response is remarkably fast under
changing gas atmospheres.
Keywords: Carbon nanotubes; Au-CNT hybrid; Electric transport; Gas sensing
PACS: 81.07.-b; 81.15.Gh; 81.07.De
Background instance, by capillary forces [2]. Another method is
Currently,theuseofnanostructuredtemplates ormoulds throughtheuseofpolymerstamps.Theyarefabricatedby
has become a preferred way to build ordered structures casting on lithographically generated rigid moulds [3] or
organizedoverareasofhundredsofsquaremicrometerin made using self-assembled copolymers deposited on flat
size.Bydepositing/castingthedesiredmaterialsinsidethe substrates[4,5].Anotherstrategytogeneratethetemplate
templates, large arrays can be made efficiently and material is the use of anodized aluminum oxide
economically [1]. One of the simplest and most membranes (AAOs). This type of membrane is usually
widely used materials for this purpose is opaline. It prepared by the anodization of aluminum foils or thin
consistsofspheres ofglass,minerals,orplasticstacked in films to obtain a honeycomb arrangement of pores
close-packed arrays. These arrays can either be produced perpendicular to the exposed surface [6-8]. This material
naturally or artificially by induced self-assembly, for hasbeenusedtobuildmetal-insulator-metalnanocapacitor
arrays for energy storage [9] and also to design highly
*Correspondence:[email protected] specificand sensitivedetectorsfor moleculesof biological
1InstitutodeQuímicayBioquímica,FacultaddeCiencias,Universidadde origin such as troponin, a protein marker for individuals
Valparaíso,Av.GranBretaña1111,Valparaíso2340000,Chile
withahigherriskofacutemyocardialinfarction[10].
Fulllistofauthorinformationisavailableattheendofthearticle
©2014Seguraetal.;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons
AttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproduction
inanymedium,providedtheoriginalworkisproperlycredited.
Seguraetal.NanoscaleResearchLetters2014,9:207 Page2of13
http://www.nanoscalereslett.com/content/9/1/207
Carbon nanotubes (CNTs) can be considered as an extremes,ifthemembraneisself-supportedandthebarrier
alternative nanoscale material with multiple applications layer has been removed. Since the tubes' outside walls are
in electronic and biological detection devices [11,12]. initially completely covered by the AAO template, we
AAO membranes have been widely used to prepare can very easily access selectively the inside of the
CNTs using metal nanoparticles (MNPs) at the bottom tubesbymoleculesormetalprecursorsinliquidsolutions,
of the pores and decomposing different carbon sources, at while the outside wall remains free of any molecules or
elevated temperatures [13-17]. In these cases, the MNPs particles. We have used this principle to synthesize
catalyzethecrackingofthegaseoushydrocarbonsandalso palladium and titanium dioxide nanostructures inside
incorporate C atoms into their structures. The subse- nanotubes [38]. Other authors have used closely related
quent precipitation of a tubular structure happens procedurestoobtainplatinum,nickelhydroxide,iron,and
once NPs have reached C supersaturation [18]. The permalloynanostructures[39-43].
diameter of the resulting CNTs is directly linked to In this report we have employed AAO membranes to
the nanoparticle size [16] and synthesis temperature. synthesize supported CNTs arrays without the need to
Within certain limits, their lengths correlate well with use metal catalysts. Taking advantage of the protection
the synthesis time [17]. providedbythenanotubesbythehollowaluminacylinders,
Another approach to synthesize CNTs with AAO we have used these CNTs as nanoreactors to grow gold
templatesisthetemperature-activatedpolycondensationof nanostructures selectively inside them. The nanotubes can
alkenes or alkyne derivatives. In this process, hydrocarbon subsequently be extracted from the AAO template to
unitspolymerize to formmultiwall graphitic sheets, which obtain hybrid peapod-like Au-CNTcomposites. Since our
follow the shape of the AAO membrane. The physical interestisevaluatingthecollectivebehaviorofthesehybrid
dimensions of the resulting products are determined nanostructures, interdigitated electrodes have been used to
by the shape of the pores. After the synthesis process is measure the conductance temperature dependence.
completed, the alumina mould can be dissolved and the Additionally, changes in the electrical resistance of these
CNTs released from its matrix. Using this method, it is structures were verified under different atmospheric
then possible to prepare straight, segmented, and also conditions in order to test the use of the new material as
branched CNTs but with a crystalline structure poorer activeelementsinsensordevices.
thanthosegrownbycatalysis[19-22].
Several groups have successfully synthesized hybrid Methods
nanostructures composed of gold nanoparticles (AuNPs) SynthesisofCNTsandAu-CNThybridnanostructures
attached to the outer surface of CNTs. They havemostly For the CNT synthesis, the catalytic decomposition of
used covalent linkage through bifunctional molecules acetylene was carried out in a chemical vapor deposition
[23-25],whileothershavepreparedhybridsonlybytaking apparatus (CVD), consisting of a horizontal tube furnace
advantage of the intermolecular interaction between the and a set of gas flow lines [44]. In a typical synthesis,
ligand molecules, usually long carbonated molecular performed at atmospheric pressure, a piece of alumina
chains bound to the AuNP surface and attached to the membrane (approximately 2×5 cm2) was heated at a
CNTssidewalls[26-28].Othermetalshavealsobeenused rate of 20°C/min under an O stream (100 sccm) until
2
to synthesize hybrids with CNTs. For example, AgNPs reaching the desired synthesis temperature, (650°C).
have been electrocrystallized onto functional MWCNT Then,O wasreplaced byAr(100 sccm), and thesystem
2
surfaces [29]. Magnetic iron [30], cobalt [31], and nickel was kept under these conditions for 5 min. Acetylene
[32] NPs have also been linked to CNTs to form (25 sccm) was later added for 10 min into the furnace.
hybrids structures. The use of these hybrids in mag- ThehydrocarbondecomposesandtheCNTsgrowinside
netic storage as well as in nuclear magnetic resonance the porous AAO substrate to produce at the end a
as contrast agents for imaging and diagnosis has been CNT-AAO composite. The sample generated by this
considered [33]. Other metals such as Pd [34], Pt procedurewaslabeledasCNT_(AAO/650°C).
[35], Rh [36], and Ru [37] have also been incorpo- FortheAu-CNThybridsynthesis,theCNT-AAOcom-
rated into CNTs mainly with the purpose of using posite membranes were impregnated with a HAuCl /2-
4
them as catalysts or gas sensors. propanol solution by dip-coating or drop-casting. Both
Despite the large number of contributions regarding methods were used in order to introduce quite different
the synthesis of carbon nanotube-metal nanoparticle amounts of gold inside the CNTs. In the dip-coating
hybrid systems, only a few authors report the selective procedure, a piece of membrane was completely
synthesis of metal nanocrystals inside CNTs. Using CVD, immersed in a diluted gold solution (0.001 M) for 24 h.
our group has synthesized CNTs by decomposition of This sample was labeled as Au-CNT-A. To prepare a
acetylene on self-supported and silicon-supported AAO sample by drop-casting, 40 μL of a concentrated gold
membranes [38]. These nanotubes are open at both solution (1 M) was directly dropped on each side of
Seguraetal.NanoscaleResearchLetters2014,9:207 Page3of13
http://www.nanoscalereslett.com/content/9/1/207
approximately1×1cm2pieceoftheCNT-AAOmembrane. Transportmeasurementsasafunctionoftemperature
ThissamplewaslabeledasAu-CNT-B. A 10-K closed cycle refrigerator system, from Janis
After impregnation, the pieces of membrane were Research Company (Wilmington, MA, USA), was used
placed in a tube furnace for calcination-reduction together with a Keithley electrometer model 6517B
process. First, the membranes were dried at 150°C in an (Keithley Instruments Inc., Cleveland, OH, USA) in order
Arstream(100sccm)for30min.ThenanO /Armixture to measure the current-voltage (I-V) curves as a function
2
was added into the furnace and the temperature was of temperature. The I-V curves were recorded in the
raised up to 350°C for 1 h. Oxygen was later replaced by absence of light and in high vacuum environment
hydrogen (100 sccm), and the temperature was increased (<10−6 Torr). A drop of CNTs and Au-CNTs dispersions
again up to 450°C for 1 h. The system was then cooled (2-propanol) was deposited onto interdigitated micro-
downtoroomtemperature(RT)inanArflow. electrodes (IME) composed of platinum fingers (5 μm
thickness×15 μm gap) embedded in a ceramic chip.
PurificationandcharacterizationofCNTsandAu-CNT The resistance of IME-deposited CNTs and Au-CNTs
hybridnanostructures is several orders of magnitude larger than the total
To release the CNTs (with or without AuNPs), the mem- resistance of the wires and electrodes; therefore, the
branes are immersed in a 5% aqueous NaOH solution for errors introduced by using a two-probe measurement
24 h. This procedure dissolves the AAO. In addition, if are negligible in this case. The conductance at zero
ultrasonic dispersion is used (15 min at the beginning, bias voltage was obtained from the I-V curves while
15 min after 12 h, and 15 min at the end of the 24-h the temperature was ranged from 10 to 300 K.
period), the dissolution of the aluminas occur, since they
have never been exposed to temperatures beyond the Roomtemperaturetransportmeasurementsindifferent
hardening phase transition. The CNTs and hybrids atmospheres
were purified by using a repetitive centrifugation The electrical resistance of the CNT-covered IME chips
process(threetimes),decantingthesupernatantandusing was measured at room temperature in the presence
deionized H O and 2-propanol to disperse them. The of different gas mixtures. The IME chips were loaded
2
samplesweresubsequentlydriedat150°Cfor1hinAr. into a vacuum chamber fitted with inlets for different
Conventional transmission electron microscopy (TEM) gases. The concentration of the gases in each test is
and high-resolution TEM measurements were performed described below. The resistance was measured using a
on the purified samples. For this purpose, small amounts Keithley6487picoammeter.Somesamplesweremeasured
of the purified and dried products were dispersed in withalternatingcurrent (AC),and lock-in amplifiers were
2-propanol in an ultrasonic bath (5 min). A drop of usedtoacquirethevoltages.Theresultsofthesemeasure-
the dispersed sample was left to dry out over commercial ments indicate that the changes in resistance are indeed
holey carbon-coated Cu grids. Bright field micrographs dominatedbytheCNTs'response.
were taken using a JEOL JEM 1200EX (JEOL Ltd.,Tokyo,
Japan) operating at 120 kV acceleration voltage, with Results and discussion
apointresolutionofapproximately4Å.Forhigh-resolution As already mentioned, for the synthesis of gold
transmissionelectronmicroscopy(HRTEM)measurements, nanostructures inside CNT, a solution of HAuCl in
4
we used a JEOL JEM 2100 operated at 200 kV, with a 2-propanolwasused toimpregnatetheCNT-AAOmem-
point-to-point resolution of approximately 0.19 Å and branes.Drop-castinganddip-coatingwerebothappliedto
equipped with an energy dispersive X-ray spectrometer impregnate the chloroauric solution in the membranes.
(EDS) detector (Noran Instrument System, Middleton, After impregnation, the CNT-AAO membranes were
WI, USA). The micrographs were captured using a CCD calcinated (350°C) in an O /Ar mixture and reduced
2
camera Gatan MSC 794 (Gatan Inc., Pleasanton, CA, (450°C) in a H /Ar atmosphere. The alumina template
2
USA).DuringtheEDSmeasurements,ananometerprobe wasfinallyremovedwithaNaOHsolution,leavingbehind
was used (approximately 10 nm in diameter) allowing nanotubesfilledwithgoldnanoparticles.
the qualitative identification of both Au and C in the Figure 1 shows TEM images of the synthesized
samples. Scanning electron microscopy (SEM) was CNTs and the products obtained by reducing gold
also used to characterize CNTs and the Au-CNT films. ions inside the nanotubes after the dissolution of the
SEM analysis was carried out using a LEO SEM model AAO membrane. Figure 1a shows a TEM micrograph
1420VP (Carl Zeiss AG, Oberkochen, Germany; Leica of CNTs_(AAO/650°C) grown by decomposition of
Microsystems, Heerbrugg, Switzerland) operated between acetylene for 10 min. These CNTs exhibit a uniform
10 and 20 kV. Raman spectroscopy was performed diameter and uniform wall thickness with both ends
using a LabRam010 spectrometer (Horiba, Kyoto, Japan) open.Asexplainedinourpreviousreport,itispossibleto
witha633-nmlaserexcitation. control the wall thickness, hence the inner diameter of
Seguraetal.NanoscaleResearchLetters2014,9:207 Page4of13
http://www.nanoscalereslett.com/content/9/1/207
ionic concentration in the CNTs' cavities is rather
(a) CNTs low (1 mM); hence, small gold nanoparticles were
formed (2- to 10-nm mean diameter). Figure 1c shows
the Au-CNT hybrid nanostructures prepared by the
drop-coating method. In this case the nanoparticles
have grown to a size close to 40 nm with evident
facets in their geometrical structure, suggesting the
formation of nanocrystals, as shown in the insert of
Figure 1c. In this latter case, the gold ions were
introduced by dropping a concentrated gold solution
(1 M) directly onto the membrane. Larger agglomerates of
gold precursor salt can be formed inside the tubes
after a drying process, implying that larger nanoparticles
can be formed after the calcination-reduction process;
nevertheless, the maximum size of these agglomerates is
determinedbytheinnerdiameterofthetube.
(b)
Figures 2 and 3 show a set of HRTEM images for
the hybrid nanostructures prepared by dip-coating
(Au-CNT-A) and drop-casting (Au-CNT-B), respectively.
In Figure 2, Au-CNT-A, it is possible to note that the
nanoparticlesacquiredifferentsizesandshapes.Adetailed
examination revealed that these Au nanoparticles have
indeedaface-centeredcubicstructureanddominantfacets
consistent with the (111) orientation of the crystal planes
(2.35 Å interlayer spacing) [45]. Particularly, Figure 2c
exhibits a fivefold twinned structure suggesting a
110000 nnmm decahedral shape [46,47]. In this last figure, we have
inserted a view of a decahedral polyhedron to compare
Au-CNTs-A
similarities with the NPs shapes in the HRTEM image.
From Figures 2d and 3a,b, it is possible to verify that the
(c)
AuNPs are attached to the inner wall of the nanotubes.
These AuNPs are surrounded by a C onion-like shells,
well attached to the CNT inner walls, as it has been
220000 nnmm verified previously [48]. These NPs, grown inside CNTs,
can acquire the surrounding carbon layers by a relatively
low-temperature activation process. Figure 3d shows an
improved view ofthe structuralorder ofthe nanocrystals.
In the same figure, the interlayer spacing of the encapsu-
5500 nnmm lated AuNPs has been highlighted, and again the (111)
crystalplaneisthedominantfacetorientation.
Fromtheseimages(Figures1,2,3),itisthenclearthat
gold nanostructures can be grown selectively inside the
Au-CNTs-B
CNTs and attached to the inner walls. In this particular
synthesis procedure, ions have the unique possibility of
Figure1TEMimagesofpureCNTsandAu-CNThybrids.
diffusing inside the CNTs through the open ends. After a
(a)PureCNTspreparedusingtheAAOtemplate.(b)Au-CNThybrids
preparedbydip-coatingmethod.(c)Au-CNThybridspreparedby calcination-reduction process, the gold salt agglomerates
drop-casting.Importantdeformationsareindicatedbyredarrows. into zerovalent gold nanostructures inside the nanotubes.
Our results indicate the lateral extent of the particles can
beeitherlimitedbyconcentrationoftheAuprecursorsor
CNTs, by varying the exposure time to the hydrocarbon by the tube's inner diameter when this concentration
source [38]. In this contribution, we have used a 10-min is high enough. We have also noted that during the
synthesis time, which means the wall thickness is close to formation of largernanoparticle(Au-CNT-B),partofthe
7 nm. Figure 1b shows the Au-CNT hybrid nanostruc- CNT wall shrinks around it, causing important deforma-
tures prepared by dip-coating method. In this case the tions as we indicated by arrows in the Figure 1c. In some
Seguraetal.NanoscaleResearchLetters2014,9:207 Page5of13
http://www.nanoscalereslett.com/content/9/1/207
(a) (b)
(c) (d)
Figure2HRTEMimagesofthehybridnanostructurespreparedbydip-coating(Au-CNT-A).(a-d)Individualgoldnanoparticles.(a)An
onion-likecarbonshellsurroundingtheAuNP.(b,c)Theinterplanarspacing,consistentwithAufcc,ishighlightwithredlines.Theinsertin
(c)showstheshapeofadecahedralobjecttoallowcomparisonwiththeHRTEMimage.
cases, those particles appear to be outside the tubes, semi-quantitativemethod,itprovidesaclearconfirmation
but closer observations indicate they are actually that gold has been incorporated to the CNTs. Since the
encapsulatedbytheCNTwall.Inaddition,thealuminais EDS signal from small nanoparticles is very low, the
onlyremovedafterthenanostructuresareformed,making detection time for these NPs was increased; this explains
very unlikely that the particles could be bound to the the emergence of a copper signal, probably from the
CNTs'externalwall. copper grid used to support the samples. Other elements
Lee et al. have reported the synthesis of nanotubes suchasironandcobalt(duetoTEMsampleholders)have
decorated with gold nanoparticles also by using AAO alsobeendetected.
templates [49]. In this report, they have first prepared To explore the electronic transport mechanisms and
the AuNPs inside the AAO pores by impregnation of a properties of these hybrid nanostructures, after being
gold dissolution and a thermal treatment. Then, they released, they were deposited on IME chips. Figure 5a
impregnate the Au-loaded AAO membrane with sucrose shows an optical image of the IME chip. Figure 5b,c
and subsequently a carbonization process was done in shows a typical SEM image of an IME chip with
order to obtain bamboo-like carbon nanotubes filled CNTs. In all the samples considered in this study, the
with AuNPs. Their results show a scarce homogeneity in CNTs and Au-CNTs hybrids were randomly oriented
the physical distribution along the tube with a relatively on the surface, forming a network of tubes between
wideparticlesizedistribution. the electrodes.
In order to corroborate the presence of gold in these The first electrical measurement was oriented to
hybrid structures, we have performed energy dispersive obtain the temperature dependence of the sample
X-ray analysis with a 200-kV electron beam. Figure 4 conductance (G), at zero bias voltage, in high vacuum
shows typical EDS spectra for the samples prepared conditions, from 10 to 300 K. The conductance as a
by dip-coating Figure 4a and drop-casting Figure 4b. function of temperature for sample CNTs_(AAO/650°C),
Also, this figure displays tables with the weight and Au-CNTs-A, and Au-CNTs-B exhibit a non-metallic
atomic percentage (%) for carbon and gold atoms in temperature dependence. Their conductivity can be
the hybrid samples. Even though the EDS analysis is a explained using the variable range hopping (VRH) model
Seguraetal.NanoscaleResearchLetters2014,9:207 Page6of13
http://www.nanoscalereslett.com/content/9/1/207
(a) (b)
CNT-wall
(c) (d)
Figure3HRTEMimagesofthehybridnanostructurespreparedbydrop-casting(Au-CNT-B).(a-c)ThesurroundingCshellandthe
AuNP-CNTinterfacecanbeobserved.(d)Interlayerspacingof0.235nmisconsistentwithfcc(111)planesinAu.
in which charge carriers move by phonon-assisted hop- obtained for T are similar inall three samples, the inclu-
0
pingbetweenlocalizedstates[50].Therefore,theconduct- sion of gold nanoparticles implies a larger value for T .
0
ance at zero electric field can be obtained by Mott's law This is consistent with the fact that forming the gold
[51]asfollows: nanoparticles by drop-casting (T ≈5.0×103 K) produces
0
noticeable modifications to the tubular structure of the
G¼G expð−T =TÞ1=ðdþ1Þ; ð1Þ CNTs compared to those generated through dip-coating
0 0
(T ≈4.4×103 K). As an example, several locations in
0
where d is the dimensionality and T =α3/k n(E ) (the which these changes occurhavebeen indicated byarrows
0 B f
characteristic activation temperature). The parameter α in Figure 1c. Figure 6 indicates that the inclusion of gold
is related to the decay of the localized electronic wave nanoparticles by drop-casting modifies the electronic
function, n(E ) is the density of localized one-electron transport below 60 K (see the curve with red open circle
f
statesattheFermilevel,andk ,theBoltzmannconstant. markers in Figure 6). In this low temperature range, only
B
Thebestfitobtainedforourdatawasford=1,consistent sampleAu-CNTs-Bexhibitthe1Dhoppingprocess,while
with a dominant 1D electronic transport mechanism in the other two show a residual metallic behavior, inferred
our samples. Figure 6 shows a plot of the natural from the tendency to display a constant conductance. In
logarithmofGasafunctionofT−1/2;theexperimentaldata the case of sample Au-CNTs-B, the residual metallic
shows a linear dependence for almost the complete behavior of the conductance is almost non-existent and
temperature range. By fitting the function in Equation 1, theVRHmodelcanbeextendedtoverylowtemperatures
with d=1, to the average data curve from sample CNTs_ to account for the observed behavior. This result is
(AAO/650°C), a value of T ≈4.4×103 K is obtained. For consistent with the fact that the walls of the Au-CNT-B
0
samples CNTs-A and Au-CNTs-B, the values of T tubes are completely distorted by the presence of
0
from the fit of the average data were≈4.4×103 K AuNPs, as detected byTEM(Figure1c),andcausingthe
and≈5.0×103 K, respectively. These results are in suppressionofthemetallicconduction.
agreement with Wang et al.'s report [52], in which a 1D At this point, it is important to note that the transport
dependence within the VRH model is found for CNTs measurements were performed using interdigitated
prepared using alumina templates. Although the values microelectrodes,implyingthatconductionoccursthrough
Seguraetal.NanoscaleResearchLetters2014,9:207 Page7of13
http://www.nanoscalereslett.com/content/9/1/207
thermal treatment. Figure 7c shows the average Raman
spectra obtained from the corresponding samples.
FromtherelativeintensitiesoftheGandDresonances,itis
possibleto conclude that the spectrum from CNTs-2900 K
is consistent with a carbon sample with a high degree of
graphitization [53-55], whereas the CNTs_(AAO/650°C)
exhibits a structure with a considerable amount of
amorphous carbon. Since the dominant electronic
transport mechanism in amorphous carbon films [56] is
based in a 3D hopping mechanism, it is not surprising
that 1D hopping is the dominant electronic transport
mechanism in sample CNTs_(AAO/650°C) as previously
discussed.
Alternatively, the high degree of graphitization of
the multiwalled tubes contained in the CNTs-2900 K
sample, together with their large diameters, implies that
these tubes should display a metallic behavior. Figure 8
shows the conductance's temperature dependence of
samplesCNTs-2900KandCNTs_(AAO/650°C).Thefirst
remarkable discrepancy between both samples is the
huge difference in their electrical conductance, both
in magnitude and temperature dependence. Since
both samples are built up from the same tubes, prior
to annealing, this difference in conductance can be
attributed mainly to modifications of the tubes' intrinsic
electricalproperties.Hence,theobservedhoppingtransport
mechanisminsampleCNTs_(AAO/650°C)comesfromthe
CNTs themselves and not only from the way they are dis-
persed on the substrate. On the other hand, the conduct-
anceinsampleCNTs-2900Kincreasestonearlylinearasa
functionoftemperature.Thisnon-metallictemperaturede-
pendencecouldthenbeattributedtothejunctionsbetween
CNTs. In order to explain the peculiar behavior of this
Figure4EDSanalysisofthehybridnanostructuresprepared
sample, we can consider a 2-pathway model to describe its
by(a)dip-coatingand(b)drop-casting.
conductance[57].Oneofthem isdominatedbytheintrin-
sicmetallictransport(G )withintheMWCNTs,whilethe
M
other one is mainly due to the hopping mechanism (G )
H
ameshofCNTsbetweentheelectrodefingers(Figure5c). betweenthetubes.Underthisassumption,theconductance
Consequently, the interconnections between the CNTs at low temperatures is given by the expression G=G +
M
needtobeincluded inanymodel putforwardtodescribe G ,whereG canbeconsideredasroughlyindependentof
H M
the conductance in this system. To verify this issue, we temperatureandG ashavingthesamefunctionalformas
H
preparedanadditionalsample,labeledasCNTs-2900K.It shown in Equation 1. The best fit for the free parameters
containsCNTswithahighdegreeofgraphitization.These (see Figure 8), considering 3D hopping, gave the following
tubes were synthesized in the same way described in result: G ≈3.3×10−3 Ω−1, G ≈3.3×10−2 Ω−1 and T ≈
M 0 0
Section 2.1, but after their removal from the porous 3.8×104 K. These values agree well with those obtained
membrane,theywereannealedinaninertatmosphereup fromexfoliatedgraphiteinasimilarexperiment[57].
to 2,900 K. Under this treatment, the tubes' shape and The electrical transport measurements were also
dimensions were conserved; however, the graphitization performed under variable pressure conditions and room
of their walls was dramatically increased. Figure 7a,b temperature. The purpose of this second set of measure-
shows respectively HRTEM micrographs of the CNT's ments was to determine the effects of the different atmo-
wall as grown and after the annealing treatment. The spheres in the electronic transport parameters of these
inserts in Figure 7a,b show the selected area electron samples. Figure 9 shows the sample resistance of
diffraction (SAED) patterns of these samples, consistent CNTs_(AAO/650°C) subjected to several pressure cycles
with a higher degree of crystallinity of the CNTs after the of the different gases. In zone (1), vacuum/air cycles were
Seguraetal.NanoscaleResearchLetters2014,9:207 Page8of13
http://www.nanoscalereslett.com/content/9/1/207
(a) (b)
(c)
Figure5ImagesoftheIMEchipandAu-CNTsamplesdepositedoverIMEchip.(a)OpticalimageoftheIMEchip.(b,c)Representative
SEMimagesofAu-CNTsamplesdepositedoverIMEchip.
-6
Au-CNTs-A
-8
CNTs_(AAO/650OC)
-10
Au-CNTs-B
-12
-14
)
G
( -16
n
l
-18
-20
-22
-24
-26
0.05 0.10 0.15 0.20 0.25 0.30
-1/2 -1/2
T (K )
Figure6Plotsofln(G)forthesamplesCNTs_(AAO/650°C),Au-CNTs-A,andAu-CNTs-BasafunctionofT−1/2.Inadditiontothemeasured
data(opensymbols),illustrativeerrorbarshavebeenincludedforeachsample.
Seguraetal.NanoscaleResearchLetters2014,9:207 Page9of13
http://www.nanoscalereslett.com/content/9/1/207
were considered. The mixture started with 2 sccm of
(a) C2H2 until it reached 10 sccm by increasing 2 sccm in
each cycle while keeping constant the total gas flow
CNTs_(AAO/650°C) at 100 sccm. These nominal amounts of acetylene in
the incoming mixture have been transformed, taking
into account the volume of the vessel used as detection
chamber (close to 200 cc) and the amount of gas feed
during the half minute, to actual concentration near the
sensor surface. Consistently, the amounts of acetylene
near the sensor were varied from 5,000 ppm, for 2 sccm
nominal concentration to 25,000 ppm for 10 sccm. The
electrical resistance of the chips was recorded as a
function of time and later the data was transformed
(b)
to ‘sensitivity’ defined as the variation of resistance
CNTs-2900K due to the gas mixture (ΔR=Ri-R0) normalized by the
resistance of the baseline (R , pure Ar in this case) in
0
percentage, S (%) [58]. The resulting data of this experi-
ment is presented in Figure 10. These plots show the
sensitivity (%) of CNTs (Figure 10a), Au-CNT-A
(Figure10b),andAu-CNT-B(Figure10c)forC H detec-
2 2
tion. Apart from the final concentration of the analyte,
thereareseveralfactsthatcanbeextractedfromFigure10.
First,thesensingmaterialalwaysdetectedacetyleneinthe
workingrangeatroomtemperature.AlthoughpureCNTs
(c)
) are able to detect C H , with a maximum sensitivity of
s 2 2
nit 0.37% for Au-CNT-(A and B), the maximum sensitivity
u valuewasclose0.90%.Thesenumbersindicatearelatively

b. G high increase in the sensitivity to hydrocarbons for the
r
a gold-loaded CNTs. No significant differences were found
( CNTs_(AAO/650°C)
between the dip-coated and the drop-casted samples.
y
sit Another important fact is that sensitivity rises linearly
n D with the analyte concentration for all samples which can
e
t beseeninthegraphofFigure10d.Inthisfigure,wehave
n
I plottedthemaximumsensitivityasafunctionofacetylene
CNTs-2900K
concentrationinpartspermillion,whichiswelldescribed
1000 1200 1400 1600 1800 byalinearfittothedata.TheRvaluesofthesefitarevery
Raman shift (cm-1) close to 1. Within this linear range, these materials could
be used for the determination of an unknown concentra-
Figure7HRTEMimages,SAEDpatterns,andaverageRaman
spectrafrompurifiedandannealedCNTs.(a,b)Representative tion of this particular gas. Additionally, these samples
HRTEMmicrographsoftubewallsofthesamplesCNTs_(AAO/650°C) display a rapid response and recovery times to variations
andCNTs-2900K,respectively.Theinsertsin(a)and(b)arethe inthegasmixture.
diffractionpatternstakenintherespectivemicrograph.(c)The
Penzaetal.havestudiedthesensingpropertiesofCNTs
averageRamanspectraobtainedfromseveralmeasurementson
decorated with gold particles [59]. They sputtered thin
differentlocationsonthesamples.
goldlayersoverthickCNTfilmswithvacuum-evaporated
Au-Cr leads. This report shows that the substantial
performed.Inzone(2),airwasreplacedbyargon.Inzone improvements in the gas (NO , NH , and H S) sensing
2 3 2
(3), the chamber was pumped out. Zone (4) corresponds properties of CNTs are indeed induced by gold. Their
tothevacuum/Arcycles. results are consistent with a high sensitivity at 200°C;
The resistance changes observed between the different nevertheless, this material, in most cases, has larger
sampling zones suggest that these materials could be detection and recovery times.
usedaschemiresistor gassensors.Thisconcept hasbeen In addition, we have performed a similar set of
verified by running several cycles of alternating gas measurements using hydrogen as the analyte gas. The
mixtures.Forexample,cyclesofAr(100sccm×2min)as sensitivity (%) plots of H vstimefor CNTs and Au-CNT
2
baselinegas,followedbyamixtureofAr/C H (×0.5min) hybridsamplesarepresentedinFigure11.Allsamplesare
2 2
Seguraetal.NanoscaleResearchLetters2014,9:207 Page10of13
http://www.nanoscalereslett.com/content/9/1/207
Figure8TemperaturedependenceoftheconductanceforpurifiedandannealedCNTs.Temperaturedependenceoftheconductance
(G)measuredatzerobiasvoltageforthesamplesCNTs-2900K(greenopencircles)andCNTs_(AAO/650°C)(blacksquares).Theredlinesarethe
fittothecorrespondingmodels;seetextforfurtherdetails.
lesssensitivetoH thanacetylene.PureCNTsdisplayvery
2
little sensitivity. In the case of Au-CNT samples, no
significant signal was detected for low H concentration
2
(5,000to10,000ppm), and the linearityof thesignal with
concentration is not as good as in the case of C H ,
2 2
(Figure 11d). Sadek et al. have electrocrystallized AuNPs
on nitrogen-doped CNTs and use them in hydrogen
detection[60].Intheirsample,platinum metalleadswere
sputtered directly onto the film to improve the electrical
contacts. The high sensitivity values obtained in this
report could be explained as due to the large number of
gold clusters interacting with hydrogen molecules and
causing charge transfer to the CNT network. This charge
transferis expected toinduce substantial modifications in
theelectricaltransportparameters.
In both cases, CNT and Au-CNT hybrids, increased
resistance under acetylene or hydrogen exposure has been
detected.Thisbehavioristypicalofp-typesemiconductors
Figure9ChangesinresistanceofCNT_(AAO/650°C)sample
depositedonIMEchipduetodifferentenvironmental exposedtoelectrondonorgases,sincethesespeciesinduce
conditions.Inzone(1),vacuum/aircycleswereperformed(vacuum a reduction of the density of holes [59]. Particularly for
levelisclose68kΩ).Inzone(2),airwasreplacedbyargon.Inzone Au-CNT hybrid nanostructures, the sensing mechanism
(3),thechamberwaspumped,andinzone(4),vacuum/Arcycles
could be explained by two process: (1) the adsorption of
wereperformed.
gases in the side walls of CNTs, with the simultaneous