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aSo Matter Materials Branch, Materials a

Research Laboratory, Wright-Patterson Air

Placidus.Amama@wpa.af.mil; Benji.MarubUniversity of Dayton Research Institute (UD

45469, USAcUniversal Technology Corporation, Dayton,dUniversity of Central Florida, Department

Orlando, Florida 32817, USAeDepartment of Chemistry, Rice University, HfCollege of Engineering, Swansea University,

UK

Cite this: Nanoscale, 2013, 5, 2642

Received 14th November 2012Accepted 10th February 2013

DOI: 10.1039/c3nr33634d

www.rsc.org/nanoscale

2642 | Nanoscale, 2013, 5, 2642–264

Wetting behavior and activity of catalyst supports incarbon nanotube carpet growth

Placidus B. Amama,*ab Shawn A. Putnam,cd Andrew R. Barronef

and Benji Maruyama*a

A simple, reliable, and non-destructive approach based on contact

angle measurements is described for predicting the activity of cata-

lyst supports in carbon nanotube (CNT) carpet growth. The basic

component of the surface free energy of different alumina supports –

determined from the van Oss–Good–Chaudhury model and the

Young–Dupre equation – was found to correlate with the activity of

Fe catalyst during water-assisted CVD growth of CNT carpets.

Carbon nanotube (CNT) carpets have shown promise as keyplatforms in a number of important applications.1–5 Since therst report6 on the growth of single-walled CNT carpets usingethanol chemical vapor deposition (CVD), a number of high-yield CVD processes involving other carbon precursors7,8 andoxygen-containing additives9 have been advanced. One of themost inuential parameters in these high-yield CVD processesis the catalyst system, which is usually composed of a thin Felm supported on an alumina layer.9–14 However, the lack of areliable and a non-destructive tool for rapidly analyzing theactive site density of the surfaces of the catalyst supports islimiting the rational development of catalysts for improved andcontrolled CNT carpet growth. Here, we show, for the rst time,that the surface free energy components (Lifshitz–van derWaalsand Lewis acid–base) determined from the van Oss–Good–Chaudhury (VOGC) model and the Young–Dupre equation canreliably be used to predict the activity of alumina as a catalystsupport in CNT carpet growth. The observed trends in the Lewis

nd Manufacturing Directorate, Air Force

Force Base, Ohio 45433, USA. E-mail:

yama@wpa.af.mil

RI), University of Dayton, Dayton, Ohio

Ohio 45432, USA

of Mechanical & Aerospace Engineering,

ouston, Texas 77005, USA

Singleton Park, Swansea SA28PP, Wales,

6

basicity and acidity of the alumina lms provide furtherinsights into the poorly understood catalyst–support (Fe–alumina) interactions during CNT carpet growth. To furthercorroborate the interesting trend in Lewis basicity, directcharacterization of the hydroxyl enrichment on the varioussurfaces was carried out using X-ray photoelectron spectroscopy(XPS). Given that the properties of a thin lm are dependent onthe deposition process, this tool will benet efforts in opti-mizing the efficiency of CNT carpet growth, extending poten-tially to non-alumina catalyst supports.

The stability and activity of Fe catalyst during single-walledCNT (SWCNT) carpet growth is very sensitive to the properties ofthe support.12,15 For instance, Co catalyst supported on differentcrystal faces of sapphire (A-, R- and C-faces)16 or a thin Fecatalyst supported on SiO2, Al2O3, and Al2Ox

17 show differentcatalyst behaviors. In fact, our previous study revealed thatamorphous alumina lms deposited using different methodsproduces stark differences in catalyst activity and lifetime.12 Inthe larger eld of catalysis, the role of catalyst support goesbeyond physical interactions with the catalyst or simply inhib-iting catalyst coarsening. The acid–base character of the catalystsupport is known to strongly affect the catalyst activity ofvarious reactions. For CNTs, Magrez et al.8 recently reporteddramatic increase in the activity of Fe catalyst supported onhighly basic alumina supports; they achieved 1000 times longerCNT carpets. It is evident from the above studies that thecatalyst support is indeed an important component of thecatalyst structure, and dramatic improvement in the catalystactivity and control of CNT properties can be achieved by properselection or modication of the supporting layer.

The alumina samples studied were c-cut single crystalsapphire substrates [0001], and amorphous alumina lms(nominally 10 and 50 nm thick) deposited on p-type Si (100)wafers by atomic layer deposition (ALD), e-beam evaporation,and magnetron sputtering, hereaer referred to as sapphire,ALD, e-beam, and sputtered, respectively. In most studiesinvolving CNT carpet growth, the alumina layers used as cata-lyst supports are typically deposited using any of the foregoing

This journal is ª The Royal Society of Chemistry 2013

Fig. 1 (a) Schematic representation of the contact angle (q) as a force balanceequilibrium for a drop of liquid on a flat catalyst support at the solid–liquid–airtriple point. (b) Representative microdroplet image of water acquired with a high-speed camera at t ¼ 1 ms. The value of cosine q is a measure of the equilibriumbetween the energy of cohesion of the liquid molecules (horizontal arrows; left-hand side of the equation) and the energy of adhesion between the liquid andthe solid (vertical arrows; right-hand side of the equation). If the energy ofcohesion between liquid molecules is higher than the energy of adhesionbetween solid and liquid, the drop of liquid forms a non-zero contact angle;conversely, if the energy of adhesion is higher than the energy of cohesionwetting or spreading takes place. The solid arrows represent apolar or Lifshitz–vander Waals (LW) interactions while the dashed arrows represent polar or Lewisacid–base (AB) interactions.

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techniques. Now the processes involved in each of these tech-niques is considered. The substrate surface during magnetronsputtering is constantly under powerful electron bombardmentand the lms are affected by the high kinetic energy of sputteredparticles that results in continued mobility of the particles onthe substrate; in general, this process results in lms withrelatively higher defect density. In the case of e-beam deposi-tion, the particles deposited on the substrate have limitedmobility because of their comparatively lower kinetic energyand the lms formed tend to have more preformed particles ordiscontinuities in the lm structure. On the other hand, ALD isa slow, well-controlled chemical deposition process wherebyatomic layers are deposited one layer at a time and the lms arehighly uniform in thickness and relatively free of defects. It istherefore expected that alumina lms obtained from thedifferent techniques would show different properties.

Liquid contact angle measurements were performed on thealumina surfaces using a custom optical contact anglemeasurement apparatus described in detail elsewhere.18 Theexperimental procedure consists of dispensing a small drop ofthe test uid on a clean substrate, having a target volume of�100 nL. The contact angle prole is monitored with theimaging system during deposition and uid evaporationprocess; the sample stage is rotated to ensure that a cleansurface is tested in each experiment. For a partially wettingdroplet–surface system, the test uid spreads to an equilibriumcontact angle within a few hundred microseconds aer depo-sition. The droplet evaporates in a pinned contact line mode ofevaporation until a de-pinning contact angle is reached. ImageJcontact angle plugin was used to measure the de-pinningcontact angle for each experiment from both ends (le andright) of the liquid droplet in contact with the alumina surface.The contact angles formed by the drops of the different liquids[water (H2O), diiodomethane (CH2I2), and formamide (CH3NO)]are the equilibrium values calculated as the mean of measure-ments from at least seven droplet experiments (Table 1).

A schematic representation of the contact angle (q) as a forcebalance equilibrium for a sessile drop of liquid on a at catalystsupport at the solid–liquid–air triple point is presented in Fig. 1.q is actually a measure of the competing tendencies between theenergy of cohesion of the liquid molecules and the energy ofadhesion between the solid surface and the liquid droplet. TheYoung's equation relates q to the interfacial tensions (g):19

gLcos q ¼ gS � gSL (1)

Table 1 Measured mean values and standard deviations (SD) of de-pinningcontact angles (in degrees) for diiodomethane (qD), water (qW), and formamide(qF) on the different alumina surfaces

Alumina support

Contact angles (q) and mean SD (degrees)

CH2I2 (qD) H2O (qW) CH3NO (qF)

Sapphire 63.51 � 6.73 80.2 � 5.08 66.29 � 13.1ALD 28.21 � 5.75 22.79 � 7.94 28.95 � 8.01E-beam 40.63 � 2.87 19.94 � 5.4 33.23 � 9.06Sputtered 43.15 � 1.00 21.20 � 5.38 38.58 � 6.71

This journal is ª The Royal Society of Chemistry 2013

where gL, gS, and gSL represent the surface tensions of theliquid, the solid, and the solid–liquid interface, respectively.From the Wenzel relation,20 it is clear that surface topographyand geometry strongly inuences the contact angle measure-ment of a sessile liquid drop on a plane surface. As a result,meaningful comparison of contact angles and resultingsurface free energies require surface topographies that arehighly identical. Fig. 2 shows representative tapping modeAFM surface topography 3D images (500 � 500 nm2) of thedifferent alumina surfaces. The topographies are seen to havez values that are in the range of 0.2–2 nm (insets in Fig. 2)with no obvious surface heterogeneity. Also, the root-mean-square surface roughness (Rrms), dened as the standarddeviation of the z values in a 500 � 500 nm2 area, forsapphire, ALD, e-beam, and sputtered are 0.17 � 0.01, 0.15 �0.02, 0.27 � 0.02, and 0.14 � 0.01 nm, respectively. Studieshave shown that the contact angle measurement is unaffectedby surface roughness if the Rrms values are in the sub-nano-meter range.21,22 The low Rrms values of the surfaces of our

Nanoscale, 2013, 5, 2642–2646 | 2643

Fig. 2 AFM topography images of the different alumina surfaces: sapphire andamorphous alumina films deposited by ALD, e-beam, and sputtering. The insetsare the resulting histograms from the roughness analysis showing the z-heights ofthe surface features.

Table 2 Summary of the calculated surface free energy components and thetotal surface free energies (mJ m�2) of amorphous alumina films deposited by e-beam, sputtering, and ALD

Catalyst support

Surface free energy components and totalsurface free energy (mJ m�2)

gLW gAB g+ g� gTOT

Sapphire 26.7 9.9 2.1 11.9 36.7ALD 45.0 7.4 0.2 56.3 52.4E-beam 39.3 10.1 0.4 61.9 49.4Sputtered 38.0 7.6 0.2 65.7 45.6

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alumina samples give us condence that the inuence ofsurface roughness on the contact angle measurement isinsignicant, and thus the variation in the contact angleobserved (Table 1) is mainly due to the properties of the atomsand chemical groups (terminations) that exist at theinterfaces.

In the following, the van Oss–Good–Chaudhury (VOCG)model and Young–Dupre equation23,24 are used in isolating thedifferences in the surface free energies of the various aluminasupporting layers. Dupre proposed the concepts of work ofcohesion (Wcoh) and work of adhesion (Wadh) to describe therelation between the work of adhesion between solid and liquid.A thermodynamic notation, in terms of free energy (G) is used indescribing Wcoh and Wadh because g is actually dened as thefree energy per unit area, as well as force per unit length. Inorder to emphasize (DGSL):

DGSL ¼ gSL � gS � gL (2)

The Young–Dupre equation is obtained by combining eqn(1) and (2):

�DGSL ¼ gL(1 + cos q) (3)

Based on VOCG, the total surface free energy (gTOT) is sum ofLifshitz–van der Waals (LW) component and the acid–base (AB)component:

gTOT ¼ gLW + gAB (4)

The gAB component is expressed in terms of electron-accepting (g+) and electron-donating (g�) components (i.e.,Lewis acid–base interactions):

gAB ¼ 2ffiffiffiffiffiffiffiffiffiffiffiffigþg�

p(5)

2644 | Nanoscale, 2013, 5, 2642–2646

The solid–liquid interfacial energies are given as

gLWSL ¼

� ffiffiffiffiffiffiffiffiffiffiffiffigLWS

q�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffigLWL

q �2

(6)

gABSL ¼ 2

� ffiffiffiffiffiffiffiffiffiffiffiffiffigþS g�

S

qþ

ffiffiffiffiffiffiffiffiffiffiffiffiffigþL g�

L

q�

ffiffiffiffiffiffiffiffiffiffiffiffiffigþS g�

L

q�

ffiffiffiffiffiffiffiffiffiffiffiffiffig�S gþ

L

q �

(7)

Combining eqn (4), (6), (7), and (3) yields the complete formof the Young–Dupre equation:

ð1þ cos qÞgL ¼ 2

� ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigLWS gLW

L

qþ

ffiffiffiffiffiffiffiffiffiffiffiffiffigþS g�

L

qþ

ffiffiffiffiffiffiffiffiffiffiffiffiffig�S gþ

L

q �(8)

Eqn (8) is a force-balance equilibrium that occurs as a resultof a competition between liquid cohesion (le-hand side) andliquid adhesion to the surface (right-hand side); the concept isschematically shown in Fig. 1.24 From the values of contactangles of the three different liquids, two polar [water (H2O) andformamide (CH3NO)] and one nonpolar [diiodomethane(CH2I2)], with known surface free energies (gLW

L , g+L, and g�

L ), it ispossible to determine the Lifshitz–van der Waals and the acid–base components (gLW

S , g+S, and g�S ) of the alumina surfaces by

simultaneously solving the three equations. The calculatedsurface free energy components and the corresponding totalsurface free energies (mJ m�2) for the alumina supports aresummarized in Table 2.

In calculating the surface free energy components, de-pinning contact angle measurements in Table 1 were utilized toproduce 1000 datasets with a normal distribution based on thestandard deviation; the surface free energy components wereextracted from each dataset with a user dened tting routinebased on eqn (8). As shown in Table 2, for all the samples, theapolar component of the surface free energy appears to be thedominant component. The (gTOT) for sapphire is signicantlylower than those of the amorphous alumina lms, and thesurface free energy for the acidic component (g+

S) is highest insapphire. In particular, the data reveals that the surface freeenergies for the Lewis basic components (g�

S ) are signicantlyhigher than their Lewis acidic components (g+

S) and decreasesin the following order: sputtered > e-beam > ALD. The activity ofthe catalyst or the supporting layer is dened as the increase inthe average CNT carpet height with time.12 From Fig. 3, theinuence of g�

S on the carpet height aer 15 min of water-assisted CVD growth (adapted from our previous study)12 can be

This journal is ª The Royal Society of Chemistry 2013

Fig. 4 (a) The O/Al atomic ratios as a function of the alumina type; (b) the peakwidth (FWHM) of the O 1s peak as a function of the type of alumina. The aluminasupporting layers are enriched in hydroxyl groups at the surface/subsurface, andthe hydroxyl enrichment and Lewis basicity decreases in the order: sputtered > e-beam > ALD > sapphire.

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easily discerned.12 Evidently, the impact of the alumina type onthe activity of Fe catalyst during CNT carpet growth follows atrend analogous to g�

S . In particular, sputtered alumina showsthe strongest basic or electron donating character and Fe sup-ported on sputtered alumina shows the highest activity duringCNT carpet growth. These results are consistent with earlierndings by Magrez et al.8 that showed dramatic enhancementin CNT carpet growth when the catalyst support is treated with abasic solution. The relationship between the key parameters(g�, porosity, and carpet height or catalyst activity) for thedifferent alumina types summarized in Fig. 3 reveal that thecorrelation between carpet height and porosity is muchstronger than that between carpet height and g�. Our results,however, provide a broader understanding of the effects ofLewis acidity and basicity on CNT carpet growth, and howcontact angle measurements can be used to predict the activityof the catalyst support in CNT carpet growth.

In addition to contact angle measurement, XPS was used tocharacterize the hydroxyl enrichment on the surface andsubsurface to further corroborate the observed trend in theLewis basicity of the alumina supports. The increase in both theFWHM of the O 1s peak and the O/Al atomic ratio observed inFig. 4 corresponds to hydroxyl enrichment on the aluminasurface as demonstrated by van den Brand et al.25 The FWHM ofthe O 1s peak and the O/Al atomic ratio for the alumina samplesdecreases in the same order: sputtered > e-beam > ALD >sapphire. Interestingly, the hydroxyl enrichment on thealumina surfaces agrees very well with g�

S . The XPS resultscorroborate our contact angle data and further demonstrate thereliability of this tool for rational catalyst development. Inaddition, the XPS data further conrms that amorphousalumina lms and, in particular, sputter-deposited lms arenon-stoichiometric, highly enriched in hydroxyl groups andbasic in nature, supporting that these attributes are highlycorrelated with the high activity and long lifetime of Fe catalystsupported on sputtered alumina.

Fig. 3 Relationship between the Lewis basicity (g�) of alumina, porosity ofalumina, and the CNT carpet height (catalyst activity) after 15 min of water-assisted CVD growth from Fe catalyst supported on the different alumina types.The carpet heights and porosity data were obtained from ref. 12 for comparison.

This journal is ª The Royal Society of Chemistry 2013

Conclusions

In summary, the rst use of contact angle measurement as areliable tool for predicting active catalyst supports in CNTcarpet growth is demonstrated. While there are a variety oftechniques for studying properties of lms in the 1–10 nmthickness range, only contact angle measurement actuallyallows for precise characterization of the surface and interfa-cial properties of lms used for CNT carpet growth. Thesurface properties of the alumina supporting layers wereinvestigated using contact angle measurement and XPS in anattempt to demystify the catalyst structure used for CNTcarpet growth. Analysis of the contact angle measurementusing the VOGC model and the Young–Dupre equation hasrevealed that to maximize CNT carpet growth the basiccomponent of the surface free energy of alumina supportsshould be high. Results of our study so far indicate that theideal catalyst support for CNT carpet growth should possessattributes that include high stability, porosity, and hydroxylenrichment or g�.

Acknowledgements

This work was supported by the Air Force Office of ScienticResearch (AFOSR). We thank Dr Michael Clark for providingtechnical support during contact angle experiment.

Notes and references

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