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Effect of multiwalled carbon nanotube functionalization on the gas sensing propertiesof carbon nanotube–titanium dioxide hybrid materials

M. Sánchez⁎, M.E. RincónCentro de Investigación en Energía-Universidad Nacional Autónoma de México, Privada Xochicalco S/N. Col. Centro, Temixco, MOR, 62580, México

a b s t r a c ta r t i c l e i n f o

Article history:Received 16 June 2011Accepted 23 September 2011Available online 5 October 2011

Keywords:Carbon nanotubesTitanium dioxideGas sensorAmmoniaCapacitorTitania

The effect of multiwalled carbon nanotube functionalization on the sensing properties of carbon nanotube–titanium dioxide hybrid materials during ammonia exposure was investigated. Strongly adherent filmswere evaluated by impedance spectroscopy, finding highly sensitive and completely reversible capacitancevalues when the materials were exposed to ammonia vapors at room temperature. The abundance of oxygen-ated functional groups caused by functionalization of carbon nanotubes in strong acid solutions correlateswith the formation of carbon nanotube–titania hybrids with synergistic sensing properties, such as fasteradsorption/desorption cycles, lower impedance values, and high sensitivity for ammonia. X-ray diffractionand atomic force microscopy studies showed that oxygenated functional groups on the carbon surface actas nucleation points for titania growth, resulting in thinner films with smaller crystallite size for the titaniaphase than those obtained with untreated carbon nanotubes. The better integration between both compo-nents produced films with unique sensing properties.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Carbon nanotubes (CNTs) and nanosized titanium dioxide (TiO2)have been extensively studied for gas sensing applications [1–12],and some road maps foresee that in the short term these materialswill have an even more predominant role [13,14]. As an active sens-ing layer, nanosized TiO2 has been used as nanowires [2,15], nano-tubes [4,5,15], and nanoparticles [1,3,6,15–24], showing fastadsorption/desorption cycles and enhanced sensor response [1–3,6,15,16,19]. In these materials, research is focused on their high re-sistivity and cross sensitivity. In particular, TiO2 metal doping hasbeen used to overcome these issues, in addition to prevent coarseningof nanosized grains, and delay the anatase to rutile phase transition athigh temperatures [17,18,21–24]. Similarly, CNTs have shown inter-esting properties for use as sensors such as high conductivity, largeaspect ratio, large surface area, and high sensitivity [7–12,25–27].However the use of CNTs as ammonia or nitrogen dioxide sensors re-quires long recovery times due to the strong interaction of these gaseswith the CNT surface [7,8,25–27].

In order to overcome the drawbacks of metallic oxides and carbonnanotubes, and looking for novel properties, studies on sensors basedon CNT/TiO2 composites and CNT–TiO2 hybrid materials have beenreported [28–35]. Screen printed films based on multiwalled carbonnanotubes (MWCNTs) and TiO2 were used as room temperature

sensors for acetone and ammonia [30], demonstrating the importanceof poorly coordinated Ti sites for defining the direction and magni-tude of charge transfer between the active sensing layer and the ad-sorbate [31]. More recently, thin compact films of MWCNT–TiO2,prepared by sol gel and dip coating techniques were tested as resis-tive ammonia sensors, showing a different sensing mechanism thanthe one observed in screen printed films [32]. Evidently, the weakor strong interaction between MWCNTs and TiO2 depends on thepreparation method, and will have an impact on the sensing proper-ties of the compoundmaterial explaining the conflicting data found inthe literature, where responsive [32] and not responsive [33] mate-rials were obtained.

To study the source of interaction in MWCNT–TiO2 in more detail,the effect of MWCNT functionalization on the gas sensing propertiesof MWCNT–TiO2 films was studied by impedance spectroscopy (IS).Changes in capacitance and resistance indicated that functionaliza-tion of CNTs causes a better integration between both componentsresulting in materials with unique sensing properties.

2. Experimental

2.1. Film preparation techniques

Commercial MWCNTs (Nanostructured & Amorphous Inc.,95 wt.% CNT, outer diameter b10 nm, length: 5–15 μm) wererefluxed in aqueous solutions of sulfuric and nitric acids (JT-Baker) at 100 °C for 6 h to remove amorphous carbon and to pro-mote the formation of oxygenated functional groups grafted to

Diamond & Related Materials 21 (2012) 1–6

⁎ Corresponding author. Tel.: +52 5556 229836; fax: +52 5556 229742.E-mail address: msanchez@profesores.valles.udg.mx (M. Sánchez).

0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.diamond.2011.09.010

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the CNT surface. The functionalization was performed varying theconcentration of nitric acid (2.5, 7.5 and 12.5 M), and using afixed concentration of sulfuric acid (0.5 M). With exception ofnon-treated CNTs, all the others nanotubes were subsequentlyrefluxed for 5 h in 3 M hydrochloric acid (JT-Baker) solution at100 °C. Titania films were deposited from a sol gel bath containing82 mL of 2-propanol (Sigma-Aldrich), 0.5 mL of concentratedhydrochloric acid, and 8 mL of titanium isopropoxide (Sigma-Aldrich).

Films of MWCNT–TiO2 were prepared in a trilayer system on indi-um tin oxide substrates (ITO, Delta Technologies, Rs=13–15 Ω,1 in.×1.5 in.). First, a titania layer of 10 immersions was depositedby dip-coating, at 30 mm/min dipping/withdrawing speed. Filmswere annealed in air at 400 °C for 5 min after each immersion andat 500 °C in air after the 10th immersion for 1 h. The second layer(CNT) was deposited by spray of aqueous CNT inks prepared with tri-ton X-100 (CNT/triton ratio of 1/6 wt.). The two layer system wasannealed in air for 20 min at 300 °C to dry and to burn out impurities.Finally, a second titania layer of 10 immersions was deposited by dip-coating and the whole system was annealed at 400 °C for 30 min. Thehybrid materials containing functionalized carbon nanotubes were la-beled as CNT-2.5, CNT-7.5, and CNT-12.5 according to the concentra-tion of nitric acid used in the functionalization, while those

containing non-treated carbon nanotubes were labeled as CNT-P(from pristine).

2.2. Characterization

An Alpha Step profilometer (Tencor Instruments) was used tomeasure film thickness. Fourier Transform Infrared (FTIR) spectrawere obtained in a Bruker FTIR-5000 spectrophotometer in therange from 400 to 4000 cm−1. X-ray diffraction (XRD) studies werecarried out in a Rigaku Dmax 2200 diffractometer with CuKα radia-tion (λ=1.5405 Å), using the Bragg–Brentano configuration in the2θ range 10–70°; the JADE (Materials Data, Inc.) software wasemployed for the analysis of chemical composition and the Debye–Scherrer equation [36] was used for crystallite size estimation. Sur-face topography was studied by atomic force microscopy (AFM) in aNanosurf Easyscan unit (Nanosurf AG, Switzerland), using the soft-ware Gwyddion (Czech Metrology Institute) for image processing.

2.3. Sensing experiments

The measurement system for the gas sensing experiments was de-scribed elsewhere [30,31]. Basically, the sensor configuration (seeFig. 1) consists of a two electrode cell with the counter electrode con-nected to silver lines printed on the MWCNT–TiO2 film surface, andthe working electrode connected to the ITO substrate. Impedancespectroscopy was performed with an Autolab PGSTAT302N poten-tiostat/galvanostat unit (Eco Chemie), at room temperature (27 °C),in a closed cell kept in the dark. Measurements were done at open cir-cuit potential, with a perturbation potential of 5 mV, in the rangefrom 1 MHz to 10 Hz. Frequency sweeps were taken continuously sothat the time needed to apply the same frequency between two con-secutive sweeps was ~1 min. Dry air (5 L/min) was used to establishthe baseline, followed by measurements in ammonia/nitrogen (N2)

a b

Fig. 1. Sensor configuration: (a) top view; (b) side view.

Inte

nsi

ty (

a.u

.)

2θ (o)20 25 30 35 40 45 50

A(1

01)

A(0

04)

R(2

10)

A(2

00)

1

2

3

4

Fig. 2. XRD diffractograms of MWCNT–TiO2 films, 1: CNT-P; 2: CNT-2.5; 3: CNT-7.5; 4:CNT-12.5. A: anatase, R: rutile.

Table 1Crystallite size computed from XRD characterization usingDebye–Scherrer equation.

Film Crystallite size (nm)

CNT-P 59CNT-2.5 47CNT-7.5 23

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(0.01–0.4 vol.% ammonia in 150 mL/min N2). After 10–15 min, N2

flow was stopped and air was injected again to recover the baseline.Complex non-linear least squares fitting and pseudocapacitance esti-mation [37] were done using the Zsimpwin software (Princeton Ap-plied Research). Sensor response (S) was calculated from Eq. (1):

S %ð Þ ¼ Y0;NH3−Y0;air

� �=Y0;air

h i� 100 ð1Þ

where Y0,air and Y0,NH3 are base admittances in air and ammonia,respectively.

3. Results and discussion

3.1. Structural characterization

Thickness of MWCNT–TiO2 films shows a strong correlation withthe degree of functionalization of CNTs, yielding CNT-P and CNT-12.5 as the thickest (~1.5 μm) and thinnest (~0.5 μm) films, respec-tively, whereas the thickness of carbon-free TiO2 films was ~0.3 μm.XRD patterns of the films (Fig. 2) show anatase (JCPDS 21–1272) asthe principal crystalline phase, and a sizable decrease in intensityand broadening of its main peaks [(101), (004,) and (200) crystallineplanes] as the acid treatment becomes more intense. Apparently, therole of functional groups attached to carbon nanotubes is to facilitatethe nucleation of titania crystallites, increasing its number and de-creasing its size, as it is shown in Table 1.

FTIR spectra of MWCNTs with and without acid treatment are pre-sented in Fig. 3. Absorptions at 3744 and 3620 cm−1 correspond tothe stretching of aromatic C\H and O\H bonds respectively, while1741 cm−1 adsorption is related to the stretching of C_O bonded toa carboxylic group, 1620 and 677 cm−1 correspond to vibrationsand combinations of free water, 1514 cm−1 to the stretching of singleC\C coupled with a double bond (C_C), and 1395 cm−1 to the bend-ing of carboxyl groups [38]. It is clear from this figure that as the in-tensity of the acid treatment increases so does the absorptionsignals related to oxygenated functional groups.

The effect of CNT functionalization on the topography of the threelayer films is evident from the sequence of AFM images shown inFig. 4, where film height (vertical scale) decreases as the acid treat-ment intensifies. The average height and roughness of these films(Fig. 5) show similar tendencies than those observed for film thick-ness and crystal size, indicating that the effect of functionalizationon the overall film microstructure is a combination of improvedCNT dispersion and generation of more TiO2 nucleation sites.

Fig. 3. FTIR results of MWCNTs as a function of HNO3 concentration, 1: nontreated; 2:2.5 M; 3: 7.5 M; 4: 12.5 M.

d

c

a

b

Fig. 4. AFM images of the hybridfilms: (a) CNT-P; (b) CNT-2.5; (c) CNT-7.5; (d) CNT-12.5.

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3.2. Impedance spectroscopy

IS curves of CNT-P and CNT-12.5 films are shown in Figs. 6 and 7respectively. Figs. 6(a) and 7(a) correspond to anisotropic Cole–Colegraphs used to magnify the difference between the curve in air andthose obtained during the transition from air to 0.4 vol.% of ammonia.For CNT-P, the impedance in air is described by one semicircle athigher frequencies, while the transition from air to ammonia requirestwo semicircles. During the transition, the high frequency semicircledecreases in size up to negligible values once a steady state is reached(i.e. when air is excluded from the system). Isotropic Cole–Cole [Fig. 6(b)] and Bode [Fig. 6(c)] graphs depicting the response in air and0.4 vol.% ammonia/N2, confirmed a substantial difference in the char-acteristic frequency in both environments, appearing in the kHzrange in air, and in the Hz range in ammonia. The good reversibilityof CNT-P can be appreciated from the similitude of the air-baselinebefore and after ammonia sensing. For CNT-12.5, Fig. 7(a) shows dis-torted semicircles with the maximum at almost the same frequencyin both air and ammonia, and no transition regime (i.e. two semicir-cles evolving into one). The impedance values are ten times lowerfor CNT-12.5 than for CNT-P, and it cannot be accounted for by thick-ness differences. The high reversibility of this sensor can be appreciat-ed in the Cole–Cole plot of Fig. 7(b), where the responses in air beforeand after ammonia adsorption are overlapped. Moreover, Fig. 7(c)confirms the similarity in the characteristic frequencies in air and inammonia (~10 kHz). The sensible difference in characteristic fre-quencies of CNT-P and CNT-12.5 is related to the times required for

adsorption/desorption cycles (desorption transition not shown),~8 min for CNT-P and ~2 min for CNT-12.5 [Figs. 6(a) and 7(a)]. ForCNT-12.5, the decrease in impedance when sensing an electrondonor molecule such as ammonia indicates the n-type conductivityof the material in spite of the use of CNTs (p-type conductivity) andin contrast with the behavior of CNT-P.

Ave

rag

e h

eig

ht

(µm

)

0

1

2

3

ITO TiO2CNT-P

CNT-2.5CNT-7.5

CNT-12.5

ITO TiO2CNT-P

CNT-2.5CNT-7.5

CNT-12.5

Ro

ug

hn

ess

(µm

)

0

0.2

0.4

0.6

a

b

Fig. 5. Statistical analysis of AFM images: (a) average height; (b) roughness.

-Zi (

koh

ms)

0

1

2

Zr (kohms)0 1 2 3 4

NH3 0.4%

Air

Time

-Zi (

koh

ms)

0

1

2

3

4

Zr (kohms)0 1 2 3 4

Zi (

koh

ms)

0

1

2

Frequency (Hz)101 102 103 104 105 106

a

b

c

Fig. 6. IS results of CNT-P films sensing 0.4 vol.% of ammonia: (a) transition from air toammonia/N2, every curve was taken with a difference of 1 min; (b–c) response in airand at 0.4 vol.% of ammonia, (b) Cole–Cole graph, (c) Bode graph. White square: mea-surement in air before exposure to ammonia, triangle: measurement in 0.4 vol.% of am-monia, black square: measurement in air after exposure to ammonia.

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Complex non-linear least squares fitting of the experimentalresults was accomplished by using the equivalent circuit (EQC)presented in Fig. 8, a resistance R0 in series with two sub-circuitsRQ (i.e. a resistor R in parallel with a constant phase element Q).

For CNT-P, the higher frequency sub-circuit fits the response in air,while the lower frequency sub-circuit fits the response inammonia/N2, and both sub-circuits are required during the transitionfrom air to ammonia/N2. In contrast, CNT-12.5 requires only the highfrequency sub-circuit to fit the experimental results in both air andammonia/N2. The values of the circuit elements are presented inTable 2, with the relaxation times (τ) calculated from the Eq. (2).

τ ¼ RC ð2Þ

Values of capacitance in polycrystalline materials are orders ofmagnitude lower in the grain bulk (10−12 F), than in grain bound-aries (10−11 to 10−8 F), or at the film/electrode interface (10−7 to10−5 F) [39,40]. Therefore, the pseudocapacitance values presentedin Table 2 suggest that the high frequency sub-circuit of CNT-P andCNT-12.5 could be assigned to events taking place at the grain bound-aries of the films. Functionalization of carbon nanotubes promotes theformation of more integrated hybrid materials due to the formation ofester-like linkages between TiO2 and CNTs [41], and the completecoverage of CNT by a thin titania film. This intimate contact causes aunique response (semicircle) in both, air and ammonia, in contrastto the response of more segregated systems like CNT-P. Additionally,the decrease of R1 (the high frequency sub-circuit resistor) from~103 Ω for CNT-P to ~102 Ω for CNT-12.5 confirms the superior dis-persion and doping of CNTs caused by the acid treatment. For CNT-Psensors, CNTs are the active surface in the segregated system and un-dergo dedoping when exposed to ammonia [32], given that R2 (thelow frequency sub-circuit resistor) increases from a no measurablevalue up to ~10 kΩ. As the aggressiveness of the acid treatment in-creases the individual characteristics of TiO2 and CNTs are graduallylost. The intimate contact between CNTs and titania nanoparticles re-duces the size and contribution of the low frequency sub-circuit (i.e.the straight adsorption on CNTs). The relaxation times presented inTable 2 indicate that the introduction of functional groups removesthe slow processes in both air and ammonia sensing.

The dynamical behavior of the best sensor (CNT-12.5) during am-monia adsorption/desorption cycles is presented in Fig. 9. Here, thebase admittance (Y0,1) of the constant phase element Q1 is plottedversus time. Fast adsorption and complete desorption are observedfor this sensor at room temperature, without any additional aid to

Zi (

koh

ms)

0

0.1

0.2

Frequency (Hz)101 102 103 104 105 106

c

-Zi (

koh

ms)

-Zi (

koh

ms)

0

0.1

0.2

0.3

Zr (kohms)

Zr (kohms)

0 0.2 0.4 0.6 0.8

a

NH3 0.4%

Air

Time

0

0.2

0.4

0.6

0.8

0 0.2 0.4 0.6 0.8

b

Fig. 7. IS results of CNT-12.5 films sensing 0.4 vol.% of ammonia: (a) transition from airto ammonia/N2, every curve was taken with a difference of 1 min; (b–c) response inair and at 0.4 vol.% of ammonia, (b) Cole–Cole graph, (c) Bode graph. White square:measurement in air before exposure to ammonia, triangle: measurement in 0.4 vol.%of ammonia, black square: measurement in air after exposure to ammonia.

Fig. 8. Electrical equivalent circuit used to fit the experimental impedance data.

Table 2Values of the EQC elements for the best fit of the experimental data.

Parameter CNT-P CNT-12.5

Air 0.4 vol.% NH3 Air 0.4 vol.% NH3

R0 (Ω) 96 128 75 67R1 (Ω) 3150 582 617Y0,1 (Ssn1) 1.0×10−7 6.6×10−8 1.5×10−6

n1 0.82 0.89 0.70C1 (F) 1.7×10−8 1.9×10−8 7.5×10−8

τ1 (μs) 54 11 46R2 (Ω) 9610Y0,2 (Ssn2) 2.9×10−5

n2 0.62C2 (F) 1.3×10−5

τ2 (s) 0.10

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promote ammonia desorption (i.e., heating, high flow of inert gas, ul-traviolet light). The sensitivity of CNT-12.5, defined as the variation ofS with respect to ammonia concentration, was evaluated in the con-centration range from 0.01 to 0.3 vol.% and it was found of ~0.6(Fig. 10). Additionally, the response (~103%) of this material is supe-rior to the values of ~101% reported for sensors based on CNTs [7–12].

4. Conclusions

It has been shown that strong functionalization of MWCNTs is akey step in the formation of highly adherent, reversible, and sensitiveMWCNT–TiO2 ammonia sensors working at room temperature. Theabundance of oxygen-containing functional groups on the carbonnanotube surface promotes the nucleation of titania particles andthe interaction at the molecular level. Films containing functionalizedCNTs show a substantial reduction on the response and recoverytimes, most likely due to the promotion of a uniform coverage ofCNTs by the titania phase, as well as the elimination of amorphouscarbon and other impurities. Those films based on untreated CNTsshow the presence of slow processes, which indicate the straight ad-sorption of ammonia on CNTs.

Acknowledgments

Financial support from Dirección General de Asuntos del PersonalAcadémico-Universidad Nacional Autónoma de México (DGAPA-UNAM) IN104309-3, Proyecto Universitario de NanotecnologíaAmbiental (PUNTA-UNAM), and Consejo Nacional de Ciencia y Tecno-logía (CONACYT-México) (49100), is gratefully acknowledged, aswell as the fellowship (M. Sánchez) provided by CONACYT-Mexico.We thank R. Moran Elvira, M.L. Ramon Garcia, and G. AlvaradoTenorio for technical assistance.

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Y0

(Ssn

)

10-7

10-6

NH

3 (%)

0

0.2

0.4

Time (min)20 40 600 10 30 50 70

Fig. 9. Dynamical behavior of the base admittance of CNT-12.5 during ammonia sens-ing at room temperature. Symbols correspond to the sensor response and the dashedline to variations in NH3 concentration.

S(%

) x1

000

0.0

0.5

1.0

1.5

2.0

NH3 (%)0 0.1 0.2 0.3

S(%)=0.57[NH3]R2=0.97

Fig. 10. Sensor response of CNT-12.5 as a function of ammonia concentration.

6 M. Sánchez, M.E. Rincón / Diamond & Related Materials 21 (2012) 1–6