Room temperature ammonia sensors based on zinc oxide and functionalized graphite and multi-walled carbon nanotubes

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Sensors and Actuators B 152 (2011) 144–154

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

oom temperature ammonia sensors based on zinc oxide and functionalizedraphite and multi-walled carbon nanotubes

ean-Marc Tulliania,∗, Alessio Cavalieri a, Simone Mussob,c, Eloisa Sardellad, Francesco Geobaldoa

Materials Science and Chemical Engineering Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, ItalyDepartment of Physics, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, ItalyDepartment of Civil and Environmental Engineering, MIT, Massachusetts Avenue 77, Cambridge, MA 02139, USAInstitute of Inorganic Methodologies and Plasmas IMIP-CNR; c/o University of Bari, via Orabona 4, 70126 Bari, Italy

r t i c l e i n f o

rticle history:eceived 13 October 2009eceived in revised form 9 November 2010ccepted 26 November 2010vailable online 3 December 2010

a b s t r a c t

In this work, different techniques are proposed to realize ammonia (NH3) sensors working at room tem-perature and a preliminary electrical characterization under water vapor and in NH3 atmospheres ispresented. Three families of ceramic planar sensors based on a zinc oxide (ZnO) layer overlapped byscreen-printed Pd-doped carboxyl groups functionalized multi-walled carbon nanotubes (Pd-COOH-

eywords:creen-printingmmonia sensorarbon nanotubesraphite

MWCNTs) or by blocks of vertically aligned MWCNTs or by graphite as such and functionalized withfluorinated or nitrogenous functional groups were studied.

These sensors were almost insensitive to humidity, while all of them gave a good response in NH3

atmosphere, starting from about 45 ppm in the case of zinc oxide with fluorinated or nitrogenous MWC-NTs and graphite or 50 ppm for Pd-COOH-MWCNTs sensors. These results are not actually as good asthose reported in the literature, but this preliminary work proposes simpler and cheaper processes to

m te

unctionalization realize NH3 sensor for roo

. Introduction

In the last years several studies have been carried out on metalxides for gas sensors applications and a particular attention haseen dedicated to the materials able to detect ammonia, such asnO, SnO2, WO3, TiO2, etc., since ammonia is a toxic gas that isound in chemical industries, fertilizers environmental monitoringnd also food decomposition.

As regard food freshness monitoring, many works showed thathe determination of biogenic amines, such as histamine, tyra-

ine, putrescine and cadaverine, caused by biological activity inermented foods and beverages [1–4] could give a possible evalua-ion of foods quality. Biogenic amines are reported in fishes andsh products, cheeses, meats and meat products and they can

nduce toxicological effects on human health if they are presentn too high quantities (about 100 mg/kg [2]). The detection ofhese compounds is possible by using mainly chromatographic

ethods [5–13], spectrometry [14,15] and electrophoresis [16],

hich are able to detect down to some nanograms (ng) of these

roups.In alternative to these accurate but expensive devices, a pre-

iminary study of ceramic sensors able to detect NH3 at room

∗ Corresponding author. Tel.: +39 11 564 47 00.E-mail address: [email protected] (J.-M. Tulliani).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.11.057

mperature applications.© 2010 Elsevier B.V. All rights reserved.

temperature is proposed in this paper, as NH3 is the simplest aminocompound.

Most of the works on NH3 sensors [17–29] have been focusedon high temperature applications (up to 300 ◦C), while the state ofthe art related to room temperature NH3 ceramic sensors is limitedto few works, as those reported in Refs. [30–36].

In particular, the sensors that were developed in this studywere composed of two layers and could be classified into threefamilies whose difference consisted in the nature of the secondlayer. For all the sensors, a ZnO layer acting both as a protec-tive film for the metallic electrodes and as an ammonia sensinglayer was screen-printed, while the second layer was produced byscreen-printing commercial carbon nanotubes functionalized withcarboxylic groups for the first family, or by glueing multi-walledcarbon nanotubes as such and functionalized with nitrogenousand fluorinated groups for the second family or, finally, by glueinggraphite as such and functionalized with nitrogenous and fluori-nated groups for the third family.

COOH-MWCNTs were chosen because it is known that ammoniachemisorbs onto carboxylic structures via the formation presum-ably of ammonium carboxylates, onto which most of the NH3

molecules can weakly bind [37]. Moreover, their dispersion into aliquid phase, when preparing screen-printing ink, is also facilitatedrespect to non functionalized ones.

The nitrogenous and fluorinated functionalizations used in ouranalysis have been already investigated in different works [38–40],

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Fig. 1. SEM micrograph of the screen-printed ZnO layer.

ut there is no reference dealing with room temperature NH3 sen-ors based on them or MWCNTs. The proposed architecture of ourensors is similar to the one described in Refs. [32,33] with the dif-erence that the layers were either glued or screen-printed so, inhis work, simpler, faster and cheaper methods for the depositionf sensing materials were used.

Functionalization procedures performed by means of wet chem-stry approaches reported in the literature are time consumingfrom 24 h to 3 days) and require the use of not negligible amountsf toxic chemicals, like acids and organic solvents (e.g. N,N-imethyl formamide, tetrahydrofuran, etc.) [39]. Plasma assistedunctionalizations of materials are generally carried out by meansf cold plasmas: environmental friendly, solvent-free and timefficient processes whose ability is to work at room-temperaturehus modifying also thermolable materials [41–45]. Therefore,

lasma-induced functionalizations of materials is believed to beompetitive compared to a wet chemical one. In this work,lasma functionalizations by means of N2/H2 and CF4/O2 fedRF, 13.56 MHz) discharges were aimed to produce N- and F-unctionalized MWCNTs and graphite, in less than 10 min, to be

ig. 2. Architectures of ZnO with (a) screen-printed Pd-doped COOH-MWCNTs; (b) glueds such, F- and N-graphite.

uators B 152 (2011) 144–154 145

integrated into a sensing system. As well known, functionalizationof materials by means of plasma processes fed with N-containingprecursors allow to graft different nitrogenous chemical groupson material surfaces like as an example imine, amine, amide. Thelast ones are a result of reactions that generally occur both duringmaterial exposure to plasma phase and after air storage (post-oxidations).

On the other hand –CFx groups (3 ≥ x ≥ 1) are expected to begrafted on materials exposed to plasmas fed with F-containingprecursors. Oxygen mixed to CF4 increases density of F atoms inthe plasma phase due to the abstraction of CFx radicals from theplasma phase: CFx radicals react with oxygen atoms and excitedoxygen molecules. As a consequence a grafting of F atoms insteadof a plasma polymerization promoted by CFx radicals is expected[46,47].

2. Materials and methods

The sensors were prepared as follows, according to differentsteps: in a first moment, interdigitated gold electrodes (ESL EUROPE8835 (520 ◦C)) were screen-printed onto �-Al2O3 planar substrates(Coors Tek, USA, ADS-96 R, 96% alumina, 0.85 cm × 5 cm) by usinga rubber squeegee and a 270 mesh steel screen. After dryingovernight, these substrates were fired at 520 ◦C for 18 min witha 2 ◦C/min heating ramp to optimize the electrical conductivity ofthe electrodes, according to the ink’s manufacturer recommenda-tions. Then, a suitable amount of ethyleneglycolmonobutyalether(Emflow, Emca Remex, USA) in which poly-(vinyl butyral-co-vinylalcohol-co-vinyl acetate) (PVB, Aldrich, USA), acting as the binder,was mixed with 2 g of ZnO powder (Advanced Nanomaterials VPAdNano ZnO 20, Degussa, Germany or AnalytiCals, Carlo Erba, Italy)to produce a first ink which was manually screen-printed, througha 270 mesh steel screen, onto the gold electrodes. The formed thick-films were porous (Fig. 1) with thicknesses of about 30–40 �m andareas of about 1 cm2.

The first family of the proposed ammonia sensors was entirelyproduced by means of screen-printing technique: the carbon nan-otubes (COOH-MWCNTs 95% purity, Nanocyl 3101 grade, Belgium)were first dispersed in an Emflow solution and sonicated for 2 h bymeans of an ultrasonic probe to disentangle them, prior to PVB addi-

as such, F- and N-VAMWCNTs sensors with interdigitated Au electrodes; (c) glued

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ion. In order to amplify the electrical response, according to Ref.32] and on the basis of preliminary results not shown here, the sus-ension containing COOH-MWCNTs was doped with 2 wt% of Pd,btained by precipitation of a PdCl2 powder [48] (Palladium(II)-hloride, ReagentPlus 99%, Aldrich, USA), which was added beforeonication.

Once the first ZnO layer was dried, Pd-doped COOH-MWCNTsere deposited, forming a rectangular area of about 0.4 cm2. The

esulting sensor is shown in Fig. 2a.The second typology is referred to the sensors composed by zinc

xide and blocks of vertically aligned MWCNTs (VAMWCNTs) asuch and functionalized with nitrogenous and fluorinated groupsFig. 2b). Two millimetres thick carpets of VAMWCNTs were pre-ared by thermal CVD on silicon substrates (1 0 0) starting fromboiling mixture of camphor and ferrocene as described in the

iterature [49,50].The third proposed typology of NH3 sensors was based on

raphite layers glued with the ZnO ink: in particular, graphite asuch, N-containing and fluorinated graphite with thicknesses ofmm, areas of ca. 9 mm2 and masses of about 5 mg were mountednto the screen-printed layer of ZnO (Fig. 2c). A foil 1 mm thick ofsotropic ultrafine grain (<5 �m) graphite (99.95 wt% purity) (Good-ellow, UK) was used as carbonaceous substrate of reference forensor assembly.

Both amination and fluorination of VAMWCNTs (second typol-gy of sensors) and graphite (third typology of sensors) haveeen achieved by plasma functionalization performed in home-ade RF (13.56 MHz) plasma reactors well described elsewhere

51,52].Experimental conditions of plasma processes investigated in

his work are reported below:

1) Functionalization with N-containing groups has been carriedout by exposing both the graphite foils and VAMWCNTs sam-ples to plasma produced first in N2 (20 sccm, 300 mTorr) for5 min at 50 W, subsequently in H2 (10 sccm, 400 mTorr) at 10 Was input power for 30 s.

2) Fluorination on graphite and VAMWCNTs has been realizedexposing the samples to a plasma fed with a mixture of CF4(10 sccm) and O2 (5 sccm) at 200 mTorr of pressure and 50 Was input power, for 1 min.

X-ray photoelectron spectroscopy (XPS) measurements wereerformed with a Theta Probe Thermo VG Scientific instrumentbase pressure 1 × 10−9 mbar) equipped with a monochromaticlK� radiation (h�: 1486.6 eV) operating at 300 W. The analysesere carried out at the 52◦ Take-off angles (T.O.As.) by means

f a 400 �m wide X-ray spot. Samples were neutralized for thelectrostatic charging by means of a flood gun (Mod. 822-06 FG)perating at 400 �A emission current, 40 V extraction voltage at× 10−7 mbar to correct differential or non-uniform charging. Theigh resolution spectra were shifted to their correct position byaking C1s spectrum centred at 285.0 eV as reference. A best fittingf C1s spectra [53,54] in different components was performed tossess the presence of different chemical functionalities on investi-ated materials. Seven components for C1s spectra were identifiedentred at: 285.0 ± 0.3 eV B.E. (C0, sp2 and sp3 hybridized carbon);85.8 ± 0.3 eV B.E. (C1, Secondary shifted carboxyl, C–COOH, and/ormine C–N); 286.6 ± 0.2 eV B.E. (C2, –C–OH (R); C N; C–C N;

N); 288.0 ± 0.2 eV B.E. (C3, –C O, –CONHxR3−x and –C–CF);89.3 ± 0.2 eV B.E. (C4, –COOH (R)); 290.5 ± 1 eV B.E. (C5, Shake up 1

nd –CF); 292.5 ± 1 eV B.E. (C6, Shake up 2 and –CF2); 294.3 ± 0.8 eV.E. (C7, Shake up 3 and –CF3).

Specific surface areas (S.S.A.) of as such VAMWCNTs andraphite were determined using the Brunauer, Emmett, TellerB.E.T.) method on a ASAP 2010 Micromeritics instrument. Prior to

uators B 152 (2011) 144–154

N2 adsorption, the samples were placed in the cell under vacuumat 573 K for 12 h.

The electrical characterizations of the sensors were carried outby using a laboratory apparatus for their testing made of a ther-mostated chamber, operating at 27 ◦C, in which relative humidity(RH) could be varied between 0% and 96% and NH3 concentrationbetween 0 and about 75 ppm. In this system, compressed air wasseparated into two fluxes: one was dehydrated over a chromatog-raphy alumina bed, while the second one was directed through twowater bubblers (three if measurements were performed under NH3atmosphere), generating, respectively, a dry and a humid flow. Twoprecision microvalves allowed to recombine the two fluxes into oneby means of a mixer and to adjust the RH content while keepingconstant the testing conditions: a flow rate of 0.05 L/s. The ammo-nia flux was obtained by diluting an ammonium hydroxide solution(Fluka, USA) in deionized water (ratio 1:20) into a drechsel throughwhich a known air flow was allowed to bubble.

An external alternating voltage (V = 3.6 V at 1 kHz) was appliedon every tested sensor acting as a variable resistance of the electri-cal circuit described above. The sensor resistance was determinedby a calibration curve drawn substituting the sensors, in the circuit,by known resistances. The laboratory apparatus for sensors testingwas calibrated such that to ensure a constant air flow during electri-cal measurements and relative humidity (RH) was varied by steps,each one of 3 min.

RH values were measured by means of a commercial humid-ity and temperature probe (Delta Ohm DO9406, Italy, accuracy:±2.5% in the 5–90% RH range), while the corresponding ppm ofNH3 concentration was estimated by chemical computation fromvapor pressure of ammonia at 27 ◦C diluted into water in a ratio1:20 [55]. These values were then transformed in mole fractionsin order to determine the amount of ammonia in 1 L of air and toobtain the ppm corresponding value from the volume of 1 mol ofammonia at 27 ◦C.

All the sensors were first tested in humid atmosphere, then inNH3 one and their electrical characterizations were evaluated bycomparing the gas response with the RH and the ammonia concen-tration. In particular the sensor response, expressed in %, (SR(%))was defined as the relative variation of the starting resistance com-paring it with the resistance measured in gas atmosphere:

SR(%) = 100 × |R0 − Rg|R0

(1)

where R0 is the original resistance in the presence of air flow andRg is the resistance after NH3 exposure until equilibrium, i.e. atsaturation of the active surfaces.

3. Results and discussion

ZnO particle size distribution was investigated by means of alaser granulometre (Fritsch Analysette 22, Germany) prior to thepreparation of the screen-printing ink: after 10 min of ultrasoni-cation, the mean diameter of the Degussa ZnO powder was about9.70 �m and the diameters corresponding to 10% and 90% of theparticle size distributions were, respectively, 1.80 and 21.00 �m;on the contrary, the Carlo Erba ZnO powder had a mean diame-ter of 2.90 �m and the diameters corresponding to 10% and 90% ofthe powder size distribution were equal to, respectively, 0.9 and7 �m. Though the Degussa powder was rather agglomerated, allthese values were compatible with the openings of the steel screen

used during the screen-printing process. The SEM images (Fig. 3a–c)showed that the VAMWCNTs are curled and entangled, probablybecause of the not-negligible amount of lattice defects producedduring the deposition of the CNTs blocks. Diameters ranging from20 to 100 nm were observed.

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ent m

acdbtet(s

Fig. 3. SEM micrographs of VAMWCNTs at differ

In Fig. 4 Raman spectra of the pristine microcrystalline graphitend CNTs are compared. The exhibited spectra are composed of twoharacteristic peaks: the 1344 cm−1 peak (the so called D band)ue to the disorder-induced phonon mode (breathing mode, A1gand) and the 1591 cm−1 peak (the so called G band) assignedo the Raman-allowed phonon mode (E -band). The two samples

2gxhibited a very similar structural ordering as it has been evaluatedhrough the ID/IG ratio. In fact, whereas the intensity of the G-bandIG) does not depend on the lattice defect density, the D-band inten-ity (ID) decreases as defect density decreases [56–59]. Since the

Fig. 4. Raman spectra of pristine graphite (grey) and CNTs (black).

agnifications: (a) 115,000×, (b) 2500×, (c) 100×.

two graphitic materials were very different from a morphologicalpoint of view, but they proved to have a similar crystalline order,we believed it could be interesting to compare the results obtainedby testing both. Furthermore, although the signal/noise ratio of thespectra of the plasma treated samples (not reported here) changedfrom sample to sample, they all exhibited the same crystalline qual-ity of the pristine ones, as confirmed by a negligible degradation ofthe ID/IG ratio.

ZnO nanometric powder (Degussa) was finally chosen to real-ize the sensors, because the electrical response in NH3 atmosphereof the sensor made with this material gave a higher variation inelectrical resistance, over 60 ppm of NH3 (Fig. 5).

It is known in the literature that ZnO acts as a n-type semicon-ductor which, at room temperature, tends to adsorb ions of oxygen(O2

−) from the atmosphere so reacting with reducing gases, as NH3.In particular it is possible to summarize the adsorption mechanismsconsidering the following reactions [60–62]:

12

O2(gas) + ne− → On−ads (2)

where On−ads is adsorbed oxygen (n = 0, 1, 2) and e− is electronic

charge. The reaction (2) means that the oxygen species tend tocapture electrons from the material, leading to an increase in holeconcentration and a decrease in electron concentration [63–65]. So,in NH3 atmosphere:

NH3(gas) = NH3ads (3)

2NH3ads + 3O− → N2 + 3H2O + 3e− (4)

2NH3ads + (7Oads)2− → 2NO2(gas) + 3H2O(gas) + 14e− (5)

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706050400

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20

40

60

80

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Fig. 7. XPS spectra of (a) native VAMWCNTs (b) F-VAMWCNTs and (c) N-VAMWCNTs. Inset: C1s spectra acquired on (a) native VAMWCNTs (b) F-VAMWCNTsand (c) N-VAMWCNTs. Cx (0 ≤ x ≤ 7) represents the position of the C1s compo-

ig. 5. Comparison of electrical responses of ZnO layer based on: Degussa nanomet-ic powder under water vapor (�) and in NH3 atmosphere (�), Carlo Erba powdernder water vapor (�) and in NH3 atmosphere (�).

The relation (5) means that when the sensor is exposed toH3 gas, the electrons trapped by the adsorptive states will be

eleased, leading to a decrease in sensor resistance, as experimen-ally observed.

By operating with a second layer, the electrical responses tend tomprove: in Fig. 6, a comparison of the electrical characterizationsf ZnO with Pd-doped COOH-MWCNTs under water vapor and inH3 atmosphere is presented. The response started to vary both inumid atmosphere (starting from 70 RH%) and in NH3 one, from0 ppm of NH3, however, the starting electrical resistance of thisensor changed between the two measurements, because capillaryondensation that induced a change in effective dielectric constantccurred, as described in Ref. [36].

For this reason, as the repeatability was not assured withd-doped COOH-MWCNTs layer, tests were performed by usinghree different kinds of carbon nanotubes (as such, F- and N-AMWCNTs), as well as three different types of graphite (as such,

-and N-graphite).

Plasma treatments fed with N2 give rise to functionalities suchs amine, imine, cyano and amide groups directly grafted onhe material exposed to plasma phase or as a result of post-

706050400

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40

60

80

100

ammonia concentration (ppm)

Se

nso

r re

sp

on

se

(%

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RH (%)

ig. 6. Comparison of electrical responses of: ZnO with Pd-doped COOH-MWCNTsnder water vapor (�) and in NH3 atmosphere (�).

nents. The percentages of different functional groups acquired on N-VAMWCNTsare: 65% C0 (sp2 and sp3 hybridized carbon); 18% C1 (Secondary shifted carboxyl,C–COOH, and/or amine C–N); 9% C2 (–C–OH (R); C N; C–C N; C N); 6% C3 (–C O,–CONHxR3−x); 2% C4 (–COOH (R)).

oxidation of material surface after its exposure to air environment.To have a certain selectivity in the plasma grafting procedure andto maximize the amount of NH2 groups on the plasma treatedsurface a combination of plasma processes is generally used. Inthis work a post-treatment by means of H2 fed plasma afterN-functionalization, has been performed aimed to convert N-containing functionalities in amino groups. As a general behaviourplasma treatments fed with H2 promote conversion of the N-functionalities in amino groups due to the presence of H atomsavailable for homogeneous and heterogeneous reactions [66].

Fluorine-containing functionalities, on the other hand, can besuccessfully grafted on a material by means of plasma assistedtreatment performed by means of a glow discharge fed with CF4and O2. Addition of oxygen, a scavenger of CFx radicals, enhancesformation of F atoms which can render plasma treatment muchmore competitive than plasma deposition of F-containing polymersduring plasma processing [67].

The presence of F and N containing functional groups onVAMWCNTs has been proved by their characterization by means ofX-ray photoelectron spectroscopy (XPS). Fig. 7 shows the spectra

acquired on native and plasma modified VAMWCNTs. The pres-ence of N1s and F1s peaks confirms the successful functionalizationof VAMWCNTs. Amounts of F around 10% was observed on fluori-nated VAMWCNTs while 9% of N was detected on N-functionalizedsubstrates.

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When C1s spectra acquired on native and plasma treated sur-aces are compared (inset of Fig. 7), it is clear that differentunctionalities including oxygenated ones are present on surfaceshatever the investigated plasma treatment. This is attested by the

roading presence of the C1s peak at higher binding energies in casef N-VAMWCNTs and F-VAMWCNTs respect to native CNTs.

It is shown that amine groups are partially lost from the sur-ace during exposure to air by oxidation reactions that convertmines to amides. A broadening of the C1s around 288 ± 0.2 eVC3) attests for the presence of amides also on the surface. On-VAMWCNTs a chemical composition of different functional-

ties was observed: 65% C0 (sp2 and sp3 hybridized carbon);8% C1 (Secondary shifted carboxyl, C–COOH, and/or amine–N); 9% C2 (–C–OH (R); C N; C–C N; C N); 6% C3 (–C O,CONHxR3−x); 2% C4 (–COOH (R)). The presence of amidesn N-VAMWCNTs was confirmed by the position of N1s peakFig. 8) at 399.8 eV (amides) instead of 399.2–399.3 eV (amine)Fig. 8). Finally, Fig. 8 shows O1s peaks acquired on VAMWC-Ts. The peak O1s acquired on N-containing surfaces is centredt 532.4 eV due to the presence of –COOH (R), C–OH (R) and

C–N; groups.On the other hand O1s spectrum acquired on F-containing sur-

aces is centred at 534.2 eV attesting for the presence of oxygenond to fluorine containing functionalities. Thus plasma treatmentsre able to functionalize MWCNTs without affecting their mor-hology as proved by comparing SEM images before and afterreatment.

In the case of ZnO and VAMWCNTs-based sensors, Fig. 9a showshat all the sensors gave an evident response in NH3 atmospheretarting from 45 ppm for N- and F-VAMWCNTs and 51 ppm for asuch nanotubes. All these sensors showed a little variation of resis-ance in humid atmosphere, for high RH values (>90%). The initialesistance of the sensors was about 570 k�.

The sensitivity is defined by the gradient of the linear fit ofxperimental data, in the field in which the sensors start to giveresponse to ammonia. The fluorinated VAMWCNTs-based sensor

howed a sensor response (Fig. 9c and e) equal to:

R(%) = 1.295 × CNH3 + 14.742 (6)

here SR(%) means sensor response and CNH3 is the ammo-ia concentration from which it was possible to extrapolate theensitivity coefficient m = 1.295 ± 0.214, that can be expresseds �R/�ppm = −8.45 ± 1.40 k�/ppm; while the N-containingAMWCNTs-based sensor gave a calibration curve described by:

R(%) = 0.959 × CNH3 + 37.312 (7)

haracterized by a sensitivity coefficient m = 0.959 ± 0.174, corre-ponding to �R/�ppm = −5.95 ± 1.07 k�/ppm.

Their stability in NH3 atmosphere was evaluated by alternat-ng the sensors to exposure cycles in NH3 atmospheres (at 51, 58nd 64 ppm) and dry air and the curves are reported in Fig. 9b (F-NTs) and d (N-CNTs). Sensor responses were almost superposed,lthough considered the fluctuation caused by the manual opera-ion of the gas microvalves.

It is known that the electronic properties of CNTs can beontrolled by chemical modifications of outer surface or by theresence, in the surrounding atmosphere or inside poorly degassedanotubes, of minute quantities of O2. In particular, the conduc-ivity type of the CNT can be changed from p-type to n-type bydsorption of O2 [68]. Ammonia is a reducing species, which tends

o donate electrons to the nanotubes, as a consequence, a decreasen the sensor resistance is observed by overlapping VAMWCNTs onZnO layer. The presence of some exposed metal catalyst (Fe) to

ensing gas, due to plasma treatment, is also probably responsibleor the CNTs response to NH3 [69].

Fig. 8. High resolution spectra (N1s, F1s and O1s) acquired on N-VAMWCNTs (black)and F-VAMWCNTs (grey) materials. An overlapping of O1s spectra has been reportedto compare peak positions.

Amine groups, as well as amide ones, introduced on the sur-face cause an enhancement of charge density in the Single-walledcarbon nanotubes (SWCNTs) and hence increased the amount ofelectron transfer between SWCNTs and gas molecules [70]. Thisbehaviour can then explain why nitrogenous CNTs were more effi-cient in ammonia detection respect to the non functionalized ones.

Moreover, it is also known in the literature that one of the waysto increase adsorption of ammonia on carbonaceous adsorbentsis the introduction of surface acidic groups [71]. Carbon–oxygencomplexes show an acidic nature and an acidic adsorbent hasa strong interaction with a basic adsorbate due to the Lewis

acid–base interaction [72]. Thus, oxyfluorinated carbon fibresshow a polar nature (or acidic nature, because of the presenceof groups such as C–F, C–O, C O, and O–C O), resulting in theammonia-removal capacity due to the polar–polar interaction (oracid–base interaction) between the fibres and the ammonia gas

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ig. 9. (a) Comparison of electrical responses of: ZnO with VAMWCNTs as such unapor (�) and in NH3 atmosphere (�), ZnO with N-VAMWCNTs under water vapotability and (e) sensitivity of N-VAMWCNTs.

72]. As the presence of oxygen atoms onto fluorinated VAMWC-Ts has been evidenced by XPS measurements (Figs. 7 and 8),

heir response in ammonia atmosphere can be explained also inerms of electrons transfer between NH3 molecules and fluorinatedroups.

Fig. 10a shows that in ammonia atmosphere, both the responsesf fluorinated and N-graphite-based sensors started to increase

ater vapor (�) and in NH3 atmosphere (�), ZnO with F-VAMWCNTs under waterand in NH3 atmosphere (�); (b) stability and (c) sensitivity of F-VAMWCNTs; (d)

at almost 44 ppm of NH3, while ZnO with graphite as such didnot give any evident response variation. Also these materials

evidenced a slight variation in resistance under water vapor,for high RH values (over 90%). In this case, the starting elec-trical resistance of these sensors did not change significantlybetween the measurements under water vapor and NH3 (about600 k�).

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F r wat( n NH3

s

b

S

c

ig. 10. (a) Comparison of electrical responses of: ZnO with graphite as such unde�) and in NH3 atmosphere (�), ZnO with N-graphite under water vapor (©) and iensitivity of N-graphite.

Fig. 10c shows that the fluorinated-based sensor operating

etween 44 and 59 ppm has the following calibration curve:

R(%) = 5.619 × CNH3 − 223.740 (8)

In particular this sensor exhibits a sensitivity m = 5.619 ± 0.696,orresponding to �R/�ppm = −36.41 ± 4.51 k�/ppm·

er vapor (�) and in NH3 atmosphere (�), ZnO with F-graphite under water vaporatmosphere (�); (b) stability and (c) sensitivity of F-graphite; (d) stability and (e)

Fig. 10e refers to N-based sensor, characterized by a calibration

curve equal to:

SR(%) = 4.635 × CNH3 − 169.180 (9)

and its sensitivity was calculated to be m = 4.635 ± 0.453, thatmeans �R/�ppm = −30.40 ± 2.97 k�/ppm.

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Table 1Sensors response times in NH3 atmosphere.

Compositions Response time (s) Recovery time (s)

40–45 ppm 45–51 ppm 58–61 ppm 61–64 ppm

ZnO nanometric as such – – – 350 220ZnO non-nanometric as such – – – 410 750ZnO + Pd-doped COOH-MWCNTs 510 400 350 – 510ZnO + native graphite 510 400 400 – 420ZnO + F-graphite 260 180 140 – 430ZnO + N-graphite 270 120 120 – 330

a

tV

steawtaetimimibho

s5wbo

totrwitlsesedct

oanrss

ZnO + native VAMWCNTs – 210ZnO + F-VAMWCNTs – 170ZnO + N-VAMWCNTs – 200

The F-graphite-based sensors exhibited a better sensitivity thanny other type of sensors.

The lack of response with as such-based sensor can be ascribedo the material purity (less metal catalyst content respect toAMWCNTs).

Due to the fact that Raman spectra (Fig. 4) showed that a veryimilar structural ordering is observed within the VAMWCNTs andhe graphite samples, as well as that XPS results evidenced the pres-nce of oxygen atoms onto their surfaces, it can be assumed that thecidic nature of the sensing material should explain its response, asell as that the functionalizations favoured the amount of electron

ransfer between ammonia molecules and graphite in the same ways what happens with CNTs. On as such materials, the S.S.A. werequal to about 8 m2/g for graphite and 60 m2/g for VAMWCNTs andhe response to ammonia was rather limited for non functional-zed graphite, respect to VAMWCNTs (Figs. 9a and 10a). As already

entioned, the sensitivity to NH3 of VAMWCNTs can be explainedn terms of acidic sites because of the presence of oxygen and

etallic impurities, therefore, the higher sensitivity of functional-zed graphite-based materials respect to VAMWCNTs ones can note explained in terms of a higher S.S.A. but rather because of theigher amount of the functionalized species fixed onto the surfacef graphite-based sensing materials.

Their stability was evaluated as with VAMWCNTs based sen-ors, by alternating exposure cycles in NH3 atmospheres (at 45, 51,8 ppm) and dry air, and are given in Fig. 10b and d. This analysisas carried out three times: all the sensors exhibited a good sta-

ility, due to the fact that the curves referred to the different cyclesverlapped between them.

Another important information about the sensors concernedheir response times (the time taken by a sensor to achieve 90%f the total resistance change in the case of gas adsorption) andheir recovery times (the time necessary to reach 90% of the totalesistance change in the case of gas desorption): the response timesere calculated from the corresponding ammonia concentration

n which the sensors started to exhibit a decrease in resistance:hese values were determined by considering three steps, each oneasting 30 min and during which sensors responses’ increased andettled down to a constant value. On the other hand, the recov-ry times were calculated from the last step to the initial one,o decreasing NH3 concentration in the atmosphere. The recov-ry time measurements lasted 15 min for all the trials carried outuring which sensors responses’ decreased and settled down to aonstant value. All the values of the response times and recoveryimes are reported in Table 1.

For ZnO as such-based sensors, response times were calculatednly when the NH3 concentration was changed from 60 to 65 ppm,

s for lower NH3 concentrations their responses did not change sig-ificantly: in particular Degussa ZnO based sensor showed a loweresponse time (350 s) than that of the Carlo Erba ZnO based sen-or (410 s). On the contrary the response times of the sensor withcreen-printed Pd-doped COOH-MWCNTs were evaluated also for

150 150 360130 130 380160 130 410

lower NH3 concentrations and even in this case the response timeswere quite high (510 s) from 40 to 45 ppm, while for higher con-centrations response times decreased (400 s from 45 to 50 ppmand 350 s from 55 to 60 ppm). The response time of ZnO + as suchVAMWCNTs sensor is approximately half that of ZnO + as suchgraphite one, this behaviour can be reasonably correlated to thedifference of their respective S.S.A. Table 1 also denotes that F- orN- functionalizations are much more effective than the increase inspecific surface area and this has already been observed on acti-vated carbon fibres produced by oxyfluoration, where, despite thedecrease in S.S.A., the ammonia-removal efficiency was enhancedby the functionalization [72]. Also on activated carbon modifiedby several acid solutions, the NH3 adsorption is enhanced by theamounts of acidic groups and is not dependent of the S.S.A. and ofpore volume [73].

In general, for all the typologies of sensors, the response timesglobally decreased with the increase of the NH3 concentrationand the lowest response times were given by the sensor with N-containing graphite.

To conclude, response and recovery times were not as highas those reported in the literature [74,75], indicating that NH3molecules are weakly attached to the sensing materials, or pre-viously adsorbed ammonia molecules. These results indicate thatphysisorption is responsible for the sensing action.

The change in response times’ values is due to the relaxationcharacteristics of the resistance during the transition from a NH3concentration (C1) to another one (C2) [76]: in case of transitionfrom C1 to C2 (C2 > C1) and the resistance for these concentrationsare respectively R1 and R2, the response time is strictly related toR1 and R2 from the following law:

�R = �R0 exp(−t

�

)(10)

where � is the response time of the relaxation process,�R0 = R1 − R2, �R = R(t) − R2, having defined R(t) as the resistancemeasured at a moment t.

As a conclusion, the functionalization of graphite and ofVAMWCNTs enhanced the sensors responses in terms of resistancedecrease and, generally, also decreased their response times.

Finally, the NH3 concentration detection of the sensors devel-oped in this study was similar, even if not as low as, to what reportedin Refs. [32,33], but we used simpler, cheaper and faster techniquesto prepare them.

4. Conclusions

Different materials for ammonia detection at room temperaturehave been successfully prepared by screen-printing and glueing:the only sensor able to detect humidity was based on ZnO with Pd-doped COOH-MWCNTs, while all the others were almost insensitiveto water.

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In NH3 atmosphere all the sensors exhibited a strong decreasen electrical resistance at room temperature, corresponding to anncrease of the sensor response, and among them, those based onnO and F-functionalized graphite gave better results for NH3 con-entration as low as 44 ppm. This detection limit was rather closeo the results given in the literature (e.g. 10 ppm for [33]).

In any case the sensors composed by functionalized graphiter VAMWCNTs gave better results than the one obtained withd-doped COOH-MWCNTs, which in the future works will bebandoned, as capacitive phenomena have been emphasized, notssuring the repeatability of the measurements. The fact that theAMWCNTs sensing layer worked better than the screen-printedne could be explained in terms of a higher number of availablearbon nanotubes.

cknowledgement

The authors wish to thank Dr. Elisa Ambrosio, IIT, Italian Institutef Technology, Politecnico di Torino, Italy, for B.E.T. measurements.

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Biographies

Jean-Marc Tulliani is currently an Associate Professor of Materials Science and Tech-nology at Politecnico di Torino, Italy. He received his Ph.D. in Materials Engineeringat the Politecnico of Torino in 1997. His primary area of interest is ceramic materialsprocessing for gas and humidity sensors including both fundamental understandingand technological aspects. He is the author/co-author of more than 60 papers, mostof them published on international scientific journals and of two patents on sensordevices.

Alessio Cavalieri obtained his degree in Physical Engineering in September 2007 atPolitecnico di Torino. From January 2008 he is a Ph.D. student at Materials Scienceand Chemical Engineering Department of Politecnico di Torino; his research work isfocused on materials employed for sensing applications, mainly ammonia and watervapor.

Simone Musso received his degree in Applied Chemistry in March 2000 atUniversity of Torino. In 2001–2002 he focused on the synthesis, functional-ization and characterization of new conductive polymers for polymeric fuelcells applications (University of Torino, Department of Macromolecular Chem-istry). In December 2002, as a Ph.D. student he began his research on carbonnanotubes (CNTs) in the Department of Physics, at the Politecnico di Torino,developing a new chemical vapor deposition technique for the synthesis of mil-limetre thick mats of CNTs. He worked until September 2009 as a post-doctoralresearcher at the Politecnico di Torino in the field of carbon nanotube researchand development, studying the deposition processes of carbon nanomaterials(nanocrystalline graphite, CNTs, nanoporous carbon, carbon fibres) and theircharacterization, properties and potential applications. Currently, he is a post-doctoral associate at MIT (Boston, USA) to develop high performance cement-rubbercomposites.

Eloisa Sardella received her degree in Chemistry in 1999 and her Ph.D. in Chem-ical Sciences at the University of Bari, Italy, in 2003. From January 1st 2008she was researcher at the Institute of Inorganic Methodologies and Plasmas-National Research Council (IMIP-CNR). Her research is mainly focused on plasmamodification of 2D and 3D materials and chemical physical characterization ofsurfaces. She is a co-author of 30 articles in international peer-reviewed journalsand scientific books. In 2007 she received the “Best Paper Award” at the 18thInternational Symposium on Plasma Chemistry (ISPC-18, Kyoto 2007) as youngresearcher.

Francesco Geobaldo is an Associate Professor at the Materials Science ad Chem-ical Engineering Department of Politecnico di Torino. He obtained his Master

Degree in Industrial Chemistry in 1990 and the Ph.D. in Chemical Science in1995. His current area of research includes the synthesis and characterizationof several materials with special attention devoted to porous silicon multi-layered structures for photonic applications. He is the author and co-authorof more than 100 scientific papers published on peer-reviewed internationaljournals.