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Journal of Sol-Gel Science and Technology, 40, pp. 299-308, 2006

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Porous sol-gel silica films doped with nanocrystalline NiO particles for

gas sensing applications

Buso, D.; Guglielmi, M.; Martucci, A.; Cantalini, C.; Post, M. L.; Haché, A.

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J Sol-Gel Sci Techn (2006) 40:299–308 DOI 10.1007/s10971-006-8958-6

Porous sol gel silica films doped with crystalline NiO nanoparticles

for gas sensing applications

D. Buso · M. Guglielmi · A. Martucci · C. Cantalini · M. L. Post · A. Hach´e

Published online: 22 August 2006

C

Springer Science + Business Media, LLC 2006

Abstract A reliable sol gel route to synthesize NiO doped

SiO2 films with different NiO content is here described.

The films showed detectable and reversible changes in both optical and electrical properties when exposed to some reducing/oxidizing gaseous species at temperatures in the 250◦C–350C range. A functional characterization protocol

has been designed and some of the sensing properties of the materials have been investigated for detecting NO2,

CH4, CO and H2. An optical transmittance increase up to

2% has been detected for 1% CO in dry air atmospheres, while relative resistance response (RR = Rgas/Rair) values

up to 4.97 for 850 ppm H2/air mixtures have been registered

for conductometric gas sensing. Films at all NiO molar concentrations in the 10% NiO - 40% range showed an optical response to the target gas, while only 30% and 40% NiO films provided a detectable gas induced resistance change.

D. Buso · M. Guglielmi · A. Martucci ()

Dipartimento di Ingegneria Meccanica – Settore Materiali, via Marzolo 9,

I35131 Padova, Italy

e-mail: alex.martucci@unipd.it C. Cantalini

Dipartimento di Chimica e Materiali, Universit`a dell’Aquila, I67040 Monteluco di Roio, L’Aquila, Italy

M. L. Post

Institute for Chemical Process and Environmental Technology, National Research Council of Canada,

1200 Montreal Road, Ottawa, Ontario K1A 0R6 Canada A. Hach´e

D´epartement de physique et d’astronomie, Universit´e de Moncton, Moncton,

N.-B, E1A 3E9, Canada

Keywords Porous film . Nanoparticles . Optical gas

sensing . Electrical gas sensing

Introduction

Materials that show a gas induced detectable and reversible change in some of their physical properties are extensively investigated for industrial applications in order to advance some of their functional features, such as increased sensi-tivity, selectivity and stability. Homogeneous and continu-ous NiO films have been extensively studied, together with Co3O4 and MnO3 films, for their reversible decreases in

visible-NIR absorption induced by CO [1], for potential use as optochemical sensors. Optical gas sensors show higher re-sistivity to electromagnetic noise, less danger of fire ignition, compatibility with optical fibers and the potential of multi-gas detection and recognition by using differences in the intensity, wavelength, phase and polarization of the output light signals [2]. Sol-gel products provide some of the most suitable materials for gas sensing applications because of their very high specific surface area values (up to 600 m2/g).

Doping them with a catalytically active material such as NiO would provide a highly active, specific surface gas sensor. The higher gas permeability of sol-gel doped matrices com-pared to dense NiO films obtained by laser pulsed deposition (PLD) has been reported in a previous work [3].

The aim of this work is to describe a simple sol-gel route to obtain NiO-SiO2thin films capable of detection of some

re-ducing/oxidizing gaseous agents at low concentrations in dry air mixtures. The nanocomposite film here reported showed both optochemical and electrochemical gas sensing proper-ties. A comparison between the two gas sensing functional-ities is also reported.

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Experimental section

The sol-gel procedure that has been optimized to obtain ho-mogeneous SiO2-NiO nanocomposite films comprises, as a

first step, the synthesis of two separate solutions in EtOH sol-vent, one containing the silica precursors (called the “matrix solution”) and one containing the NiO precursor (called the “doping solution”). The SiO2 network is obtained by

pro-moting hydrolysis and polycondensation of typical sol-gel reactants such tetraethylorthosilicate (TEOS) and methyl-triethoxysilane (MTES) in an acidic environment provided by aqueous HCl (1N). Many film compositions have been synthesized, by mainly varying the dopant concentration to obtain products with 10%NiO-90%SiO2and up to

40%NiO-60%SiO2molar compositions.

A typical “matrix solution” composition is TEOS:MTES: H2O:HCl:EtOH = 1:1:4:0.02:4 (molar ratios). A suitable

precursor of NiO is the nickel chloride hexahydrate (NiCl2∗6H2O), used in the “doping solution”

accord-ing to the desired Si:Ni ratio. 3-(2-aminoethylamino)-propyltrimethoxy silane (AEAPTMS) is added to the NiCl2∗6H2O solution in order to complex the metal cations

and disperse them homogeneously in the final SiO2matrix.

The amine to Ni molar ratio is fixed at 1:1, in order that each amine molecule complexes with its double aminic group one Ni2+ cation in solution. The EtOH volume in the “doping solution” is designed to reach a nominal oxide concentration in the overall solution (“matrix” + “dopant”) of 50 g/L.

The solutions are then mixed to obtain the final batch used for film deposition. Dip-coating and Spinning techniques are the deposition methods commonly adopted in the second step of the overall synthetic process. The choice of the substrate depends on the kind of characterization the film is planned to undergo. Silicon (100) substrates are commonly used for X-ray diffraction, FTIR, SIMS and RBS measurements, while glass/quartz substrates are useful for linear optical ization (UV-vis absorbance) and optical functional character-ization with gases in transmittance mode. Si/Si3N4substrates

provided with Pt interdigital electrodes of the same features as the ones described in literature [4] are used as substrates for electric functional characterization of films with gases. Dip coating of the films is performed at 30% RH using with-drawal speeds in the range of 100–120 cm/min, followed by thermal annealing for 30 min at a constant temperature selected in the range 100–1000◦C. Film thicknesses were typically 300–500 nm.

All samples undergo both morphological and functional characterization. The SiO2 matrix structure is studied via

FTIR measurements performed in the 400–4000 cm−1range

using a Perkin-Elmer 2000 System instrument.

Film structure is characterized by X-ray diffraction (XRD) using a Philips PW 1740 diffractometer equipped with glancing-incidence X-ray optics. The analysis is performed

using CuKαNi filtered radiation at 40 kV and 40 mA. The

av-erage crystallite size is calculated from the Scherrer equation after fitting the experimental profiles of the XRD scans.

TEM measurements are performed using scratched frag-ments of the films which are deposited on a carbon coated 300 mesh copper grid and imaged with a Philips CM20 STEM system operating at 200 kV.

Transmission and reflection ellipsometric measurements [5] on NiO-SiO2films were performed to obtain the

refrac-tive index (both real and imaginary part) and thickness of the nanocomposite films.

The optical response of films induced by reducing gas species is monitored by means of a custom built heater mounted in a controlled gas flow chamber. The design of the apparatus permits an unimpeded radiation transmission through the whole assembly. A Varian Cary1E spectropho-tometer is used to detect transmission data in the 400–800 nm wavelength range with the films heated at selected values from room temperature to 350◦C. Gas flow is

automati-cally controlled in order to get continuous CO flows in-side the measurement chamber at concentrations in the 10– 10.000 ppm range in dry air. The substrate size for these measurements was approximately 10 mm × 20 mm and the incident spectrophotometer beam was normal to the film sur-face and covering a 6 mm × 1.5 mm section area.

Gas induced change of the electric resistance of the films is recorded using an automated system. An MKS147 mass flow controller mixes dry air with diluted H2and CO (NO2,

CH4) mixtures (1000 ppm in air) to get controlled gas flows

at 10–1000 ppm target gas concentrations. Measurements are conducted at sample temperatures of 25–350◦C with

differ-ent gas concdiffer-entrations and the film resistance is monitored by a volt-amperometric technique using a Keithley 2001 multimeter.

Results and discussion

NiO is a p-type semiconductor with a wide band gap of 4.2 eV and studied for its interesting properties in gas sensing (CO, NO and H2) [6,7] applications. NiO is known to reversibly

change both its optical transmittance and electrical resistance while promoting red-ox reactions [1,8,9]. Sol-gel matrices offer the notable advantage of a high specific surface area (up to 600 m2/g) due to their extremely well developed inner porosity network. This is suitable in gas sensing because the target gas penetrates easily inside the film to reach the active sites for the red-ox reaction, and the reaction products would quickly leave the reaction sites improving the overall kinetic rate.

TEOS and MTES have been used as sol-gel precursors in order to obtain thicker films. MTES in the gel composition is known to avoid film cracks after thermal annealing at 500◦C

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J Sol-Gel Sci Techn (2006) 40:299–308 301

Fig. 1 FTIR spectra of 40% NiO – 60% SiO2films annealed

from 500◦C to 1000C. Thermal

evolution of the matrix structure is highlighted

of 2 µm thick films [12]. Film porosity can also be tailored by varying the TEOS/MTES ratio in order to get the desired value suitable for the sensing application. NiCl2∗6H2O is a

suitable NiO precursor because its high solubility in EtOH facilitates the high NiO concentrations which are necessary to provide films with up to 40% NiO – 60% SiO2molar

com-position. The AEAPTMS aminic groups complex the Ni2+ cations in the “doping solution” while the silanic tail ensures a homogeneous dispersion of the cations inside the silica matrix during condensation. As reported by B. Breitscheidel et al. [10], the metal complexes form in solution through the reaction between the bifunctional ligand (AEAPTMS) and the metal salts, and crystals precipitate upon heating after removing of the organic part. As reported by Piccaluga et al. [11], metal oxide nanoparticles in the sol-gel matrix proba-bly form in the cavities of the matrix, substituting for a part of the adsorbed water.

The FTIR spectra shown in Fig.1highlight the structural evolution of the SiO2 matrix after annealing at increasing

temperatures ranging from 500◦C to 1000◦C. The main peak in the 1080–1100 cm−1range is the typical anti-symmetric

stretching mode (TO3) signal associated with Si–O–Si bonds

in the SiO2 matrix [12]. The peak position shifts towards

higher wavenumber with increasing annealing temperature, directly demonstrating the progressive formation of Si–O bonds in the matrix core and matrix densification [18]. This is further confirmed by the presence of the (TO2)

symmetri-cal stretching (or bending [13]) motion signal associated to oxygen atoms in Si–O–Si bonds detectable at 830 cm−1[12]. Gas sensing functionality requires the film to retain a residual porosity after thermal treatment. There is a broad shoulder at 1200 cm−1which is proposed to be due to LO3stretching

modes in Si–O–Si bonds. There is literature agreement [9]

that this stretching mode is enhanced at larger porosities be-cause of the scattering of the IR radiation with the pore walls, and consequent activation of the LO modes [12]. This consid-eration leads to the conclusion that the films here described are still characterized by a residual porosity after thermal annealing even at 1000◦C. In ref [14] it is demonstrated that

films annealed at 900◦C are still capable of gas detection,

but the detection rate is very slow compared that found in films treated in the 500–700◦C temperature range. The

rea-sonable conclusion that can be proposed is that these films still possess some residual porosity even after 1000◦C an-nealing, but that the level of porosity suitable for gas sensing applications is obtained only for films whose heat treatment does not exceed 700◦C.

The broad band between 3800 and 2800 cm−1is mainly

related to the overlap of O–H vibration modes in residual silanol groups (Si–OH) that remain in the matrix and did not complete the condensation process and to chemisorbed water molecules. The main contribution of these two species to the entire band is demonstrated by monitoring the intensity of peaks at 960 cm−1 and the shoulder at 1650 cm−1, each

associated to O–H stretching vibrations in Si–OH groups [15,16] and H2O molecules [16] respectively. These signals

completely disappear after annealing at temperatures higher than 800◦C.

XRD spectra of 40%NiO-60%SiO2films are reported in

Fig.2, showing the progression of crystalline NiO particles formation inside the matrix core with increasing annealing temperature. Peaks at 2 = 37.3◦and 43.3are due to (111)

and (200) planes of cubic NiO crystalline structure [17]. The peak widths have been evaluated in order to determine the crystallite dimension through the Scherrer correlation, with resulting values increasing from 5–6 nm for 600◦C annealed

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Fig. 2 X-ray diffraction (XRD) pattern obtained with Cu-Kαradiation

showing thermal evolution of NiO crystals in a 40% NiO – 60% SiO2

film through annealing at the temperatures reported

films to 16 nm for films annealed at 1000◦C. NiO crystals

form also after annealing at 500◦C, as demonstrated by TEM

images as shown in a later section, with a mean particle size of 2.4 nm.

After annealing at a temperatures of 1000◦C a second

crystalline phase is detected, as indicated by new diffraction peaks at 2 = 35.35◦and 36.54. They belong to the (131)

and (112) planes of nickel silicate Ni2SiO4(powder

diffrac-tion file no. 83–1740, Internadiffrac-tional Center for Diffracdiffrac-tion Data, Newton Square, PA), whose formation is promoted at high temperatures. A direct consequence is the decrease of the diffraction peaks intensity of the NiO phase.

Fig. 4 Real and imaginary parts of refractive index obtained by

ellip-sometry of 40% NiO – 60% SiO2film annealed at 500◦C (solid line)

and 700◦C (dashed line)

The TEM image of Fig.3shows the morphology of a 40% NiO – 60% SiO2film heated at 500◦C. NiO round shaped

par-ticles are homogeneously distributed in the SiO2matrix core.

The size distribution diagram (inset) indicates a mean parti-cle size of 2.4 nm with 0.8 nm of standard deviation (SD).

Earlier work using SIMS measurements [18] highlighted a constant compositional profile through the whole film thick-ness, while RBS/ERDA [14, 19] and EDAX [20] compo-sitional calculations demonstrated that the nominal molar composition of the starting sols is well retained in all the films structure after the annealing process.

Figure4shows the refractive index n and absorption in-dex k of 40%NiO-60%SiO2film annealed at 500 and 700◦C.

The presence of an absorption band around 420 nm can be

Fig. 3 TEM image of 40% NiO – 60% SiO2film annealed

at 500◦C. Inset: the size

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J Sol-Gel Sci Techn (2006) 40:299–308 303

attributed to Ni2+ cations hexacoordinated with oxygen atoms of the silica network [21,22]. For this reason it is pos-sible that inside the SiO2network some of the Ni2+cations

remain in their ionic form, not entering in the NiO lattice. For both the films the measured refractive index n (Fig.4) and thickness (450 and 350 nm for films annealed at 500 and 700◦C, respectively) are suitable for the realization of

optical waveguides at the wavelength range used for CO detection.

The composite film samples show a detectable and reversible change in both their linear optical (i.e. trans-mittance) and electrical properties (i.e. conductometric resistivity) if exposed to a wide variety of reducing/oxidizing gases. NiO can catalyse oxidation of CO, H2and reduction

of NO2through thermally activated mechanisms.

A gas induced change in optical transmittance of NiO-SiO2films has been intensively monitored, particularly for

CO oxidation [14,19,20]. The possibility to optically detect CO in air makes these materials suitable for gas sensing ap-plications in environments where combustion processes are involved. Transmittance measurements are commonly per-formed through a custom made flow chamber that permits a controlled conditioning of the atmosphere surrounding the sample and a localized heating of the film. CO oxidation is thermally induced at temperatures above 150◦C, with

opti-mum operative conditions at 330◦C [14]. Films show a clear and reversible transmittance increase in the whole visible region when exposed to CO. Delta transmittance data (T defined as T1%CO−Tair) of Fig.4indicates that NiO-SiO2

films are sensitive to CO (1% vol in dry air) at all NiO con-centrations in SiO2ranging from 10% to 40%NiO. Moreover

Fig.5highlights that higher NiO content leads to higher val-ues of transmittance change (0.69%, 0.93%, 1.04%, 1.28% for 10%, 20%, 30% and 40% NiO content, respectively), for films with comparable thickness (300–400 nm) and same heat treatment (500◦C). This could be associated with a

Fig. 5 transmittance (T1%CO – Tair) measured in NiO-SiO2 films

with NiO content ranging from 10% to 40% molar ratio with respect to SiO2. Transmittance measurements performed at 330◦C

higher number of active sites for the gas reaction due to a higher density of NiO particles in the SiO2matrix.

The mechanism of CO induced optical transmittance change is considered to be related to a decrease in the ac-tivated oxygen concentration on the NiO particles surface which occurs when CO is oxidised to CO2. The consumption

of activated oxygen (O2−) on the NiO surface [23] during the oxidation process of CO decreases the hole density in the valence band of NiO lattice atoms [24]. This mechanism is known to be activated at temperatures higher than 100◦C

[25].

When optical transmittance vs. time is registered at a fixed wavelength corresponding to the maximum value of its variation, a step like plot is obtained if CO is in-troduced in the test chamber at varying concentration. Figure6reports the temporal evolution of optical transmit-tance monitored for films at all compositions (10% to 40% NiO) when different compositions of CO/dry air mixtures are injected into the measurement chamber. Steps (a), (b), (c), (d) and (e) refers to transmittance values registered at a fixed wavelength when dry air, 10 ppm, 100 ppm, 1000 ppm and 10.000 ppm of CO in dry air, respectively, flow inside the testing chamber. Detection of CO concentrations down to 10 ppm of CO is observed, demonstrating good sensitivity of the material.

Figure 7 shows the delta transmittance detected for a 40%NiO-60%SiO2 film annealed at 700◦C and deposited

on one side, or on both sides of the glass substrate. As is clear from the Fig.7the delta transmittance measured is al-most double in the case of the film deposited on both sides, this giving further confirmation that the number of active sites for gas reaction may directly affect the increase in the optical transmittance difference.

A gas induced change in electric resistance of NiO-SiO2

composite films is also observed and was monitored in dif-ferent gas exposure and temperature conditions. A testing protocol has been developed to perform the functional char-acterization of this material in the conductometric detection of several different gases, mostly reducing (CH4, H2, CO)

and oxidizing (NO2) agents. Unlike the optical functional

characterization, a detectable electrical response has been observed only in samples above a minimum content of NiO around 30%. All the films with lower concentrations of NiO did not show any appreciable change in electric resistance even if exposed to high gas concentrations. Conductometric gas sensing is dependent upon the mobility of free charge car-riers inside the composite film, and the electrical insulating SiO2walls between NiO particles provide an energy barrier

that charge carriers must overcome to provide a detectable material conductivity change.

Figure 8 shows an example of the film conductometric response with NO2 as the analyte species. A 30%

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Fig. 6 Optical transmittance measured at fixed wavelength and different CO concentrations (10, 100, 1000, 10.000 ppm CO) in dry air mixtures for NiO-SiO2

films with different

concentration of NiO and heated at 500◦C. Measurement

performed at λ = 630 nm and operative temperature of 330◦C

increasing from 50◦C to 300◦C and the temporal evolution of its electrical resistance is registered. The measurement made at each temperature step is a three stage cycle [air/7 ppm NO2

in air/air]. As the resistance value decreases with increasing testing temperature the semiconductive nature of the material is demonstrated, and at the same time the operative temper-ature suitable for NO2detection is determined. It is evident

from Fig. 8 that in each temperature step NO2 induces a

decrease in film electric resistance (i.e. an increase of film conductivity), this being related to a change in charge carriers density in NiO particles. NiO is known to be a p-type semi-conductor, its electric conductivity being related to mobility and density of positive charged holes, and NO2is considered

as an oxidizing agent. It is clear from Fig.8that NO2 must

induce an increase in charge carriers number in NiO. The operative temperature is evaluated considering two main parameters at each temperature step: the gas induced dynamic response (i.e. the response rate) and the recovery of the baseline after the gas ejection out of the testing cham-ber. The response rate is qualitatively evaluated by observing the shape of the resistance response line: the more squared the signal step the higher the response rate. As the differ-ence in shape of the resistance drop at each temperature is recorded, it is important to qualitatively determine at which

Fig. 7 Delta transmittance for 1%CO induced in a 40% NiO – 60% SiO2film annealed at 700◦C and deposited on only one or on both sides

of a silica glass substrate

temperature the response rate is highest. The recovery of the baseline is normally faster at higher temperature, as it is directly related to diffusion of the target gas out of the film through its porous network. Figure8indicates that for optimal NO2detection an operative temperature of 250◦C is

the most suitable.

Once the operative temperature is determined, the resis-tance response vs. time is monitored by exposing the film

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J Sol-Gel Sci Techn (2006) 40:299–308 305

samples to different gas concentrations in dry air, simulating real operative conditions of the sensors.

Figure 9 reports the temporal conductometric response of a 30% NiO-70% SiO2 film annealed at 700◦C

in-duced by several concentrations of NO2, CH4, CO and

H2. The operative temperatures of the sensor are

differ-ent according to the differdiffer-ent target gas and are reported in the figure. 250◦C is the best thermal condition for

NO2 detection, as seen before, 300◦C is found to be the

most suitable for CO and H2 sensing and 250◦C for CH4

detection.

Unlike NO2all the other gases behave as reducing agents,

inducing a decrease in electrical conductivity of the film.

Again this observation finds explanation in the oxidative nature of these gases, that leads to a decrease in positive charged carriers on the NiO particles. The accepted mech-anism of this process is the so called “gas-sensitive charge carrier density change”, that in the case of CO gas comprises the formation of carbonate-like species CO2−3 at the NiO

sur-face. This leads to a decrease in the positive holes density according to [24] CO + 2O2−+2V2+O →CO 2− 3 +2V + O

where V2+O is a divalent oxygen vacancy and V2+O is a mono-valent oxygen vacancy.

Fig. 8 Time evolution of electrical resistance of a 30% NiO – 70% SiO2film annealed

at 700◦C at temperatures

ranging from 50◦C to 300C.

Each temperature step has a [air/7 ppm NO2/air] cycle

Fig. 9 Resistance changes induced by several

concentrations of NO2, CH4,

CO and H2for a 30% NiO –

70% SiO2film annealed at

700◦C. Operative temperatures

used for the measurements are reported in the figure for each gas

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Fig. 10 Logarithmic plot of relative response (RR=Rgas/Rair) of a

30% NiO – 70% SiO2film annealed at 700◦C vs. gas concentration for

4 gases

Temporal plots like the one of Fig.9are useful to deter-mine some of the parameters usually adopted in the func-tional description of industrial sensors. One of them is the so called “sensitivity” of the sensor, i.e. the minimum gas concentration range that can be reversibly detected at its op-erative temperature. For NO2 1.4 ppm concentration in dry

air can be traced, while for CO and H2the smallest

concen-tration detected lie in the tenths of ppm range. For CH4 it

was not possible to control gaseous fluxes under 100 ppm in

dry air, but the possibility to detect lower concentrations is not excluded.

The response rate towards all gases was also investigated through calculation of the T90 value, defined as defined amount of time required for reaching 90% of the equilib-rium signal after a gas concentration variation: small T90 values equate to faster sensor response. CH4and CO induce

responses with T90 values in the 7 to 10 min range, while H2and NO2induce a slower resistance change in the order

of 15–29 min. Differences in T90 can be useful in multi-gas sensing through electronic manipulation of the sensor output signal when different T90 values are associated to specific gaseous species. Best baseline recovery is observed for H2

detection, likely a consequence of the small dimension of H2

molecules that permit an easier diffusion through the silica pores.

If we define the films Relative Response as the ratio

RR = [RG/RA] of sensor resistance in gas (RG) to that in air

(RA), by plotting RR vs. gas concentration logarithmically,

the response of film is linear (note that for NO2response RR

is defined as [RA/RG]).

These data are reported in Fig. 10where RR of a 30%

NiO-70% SiO2film annealed at 700◦C is evaluated for

ex-posure to the same gas species as above. The slope of the curves can be related to sensors sensitivity, so it is possible to define a sensitivity scale of the film towards the different

Table 1 Comparison between functional parameters of NiO-based sensors reported in literature

Physical

Operating Gas Response detection

Material Target gas temperature concentration Sensing element timea parameter Reference

NiOx H2 NO2 – 100 ppm – –

Thin films on silicon substrates by MBE

– Electrical

conductivity [26]

CO 1000 ppm –

NiO H2 450◦C 9% vol Thin films on silica glass

in vacuum

<1 min Electrical conductivity

[27]

NiO (with nobel metals)

H2

CH4

300◦C–640C 10000 ppm Paste with water on

tubular structures

– Electrical

resistivity [28]

NiO NO2 320◦C 0.04 ppm Sputter coating – Electrical

resistivity [29] NiO in SiO2 (current work) H2 CO 300◦C 300◦C <17 ppm 10 ppm

SiO2films containing NiO

nanocrystals via sol-gel

<1 sec Electrical resistivity

Current

CH4 350◦C 100 ppm

NO2 250◦C <1.4 ppm

NiO CO 175◦C 1% vol Pyrolisis of Ni

alkylcarboxilates coated glass plate

5 min Optical

absorbance [2]

NiO CO 250◦C 1% vol Pyrolisis of Ni octanoate

coated glass plate

– Optical absorbance [8] NiO in SiO2 (current work) CO 250–330◦C 10 ppm SiO

2films containing NiO

nanocrystals via sol-gel

2 sec Optical

Transmit-tance

Current

aThe “response time” is to be intended here as the amount of time required from the initial contact with the gas to the sensors processing of the

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J Sol-Gel Sci Techn (2006) 40:299–308 307

gases. The film is characterized by a higher sensitivity to H2 and a minimum towards CH4 and CO, while sensitivity

to NO2stands between these two extremes. Considering the

values of the relative response registered at the same gas concentration, i.e. the range 100–1000 ppm (data for NO2

not available), it is pointed out that H2 generates values of

RR ranging from about 2 to 4.96, CO generates values that

go from 1.094 to 1.2 and CH4leads to RRvalues in the 1 to

1.077 range, demonstrating again the better performance of the sensor in H2 detection as compared to the other gases.

Considering that the film used in the tests is the same, higher response rates for the same number of gas molecules that en-ter the film matrix means a higher number of charge carriers involved in the catalytic reaction. From this point of view, two factors can be isolated to explain differences in relative response values: the diffusion rates of the target gases inside the film and of the red/ox reaction products out of it, and the kinetics of the reaction itself. These factors together pro-vide the overall number of available active sites in which the gas molecules react according to a dynamic equilibrium be-tween reaction rate and number of gas molecules that reach the active reaction sites.

Table1summarizes the main features of NiO based sen-sors. Among the different sensing elements only the one here described consists of NiO nanoparticles embedded in a porous SiO2matrix, while the other sensors are based on

NiO thin films.

Concerning the electrochemical gas sensors all the op-erating temperatures are generally in the 250–450◦C range.

The detected minimum gas concentrations of our material are among the lowest reported, although values measured in other references may not refer to the lowest gas detection limit but just to testing operative concentrations. Moreover the measured response time of our nanocomposite film is among the lowest reported.

Concerning the optochemical gas sensors all the operative temperatures fall in the same range of the electrochemical gas sensors. This is consistent with the thermally induced catalytic activity of NiO towards the reported gasses [25]. Both minimum detected gas concentrations and response time are among the best registered.

Conclusions

The sol-gel technique has been successfully utilized in the preparation of thin and porous SiO2films doped with

homo-geneously dispersed NiO nanocrystals at several molar con-centrations. Through morphological characterization it was possible to describe such composite material as a porous sil-ica network with round shaped NiO particles of mean dimen-sion 2.4–16 nm dispersed inside its core structure. The films showed reversible and detectable changes in physical

prop-erties such as optical transmittance and electrical resistance when exposed to reducing/oxidizing agents and temporal evolution of gas induced changes in both optical and elec-tric properties has been demonstrated. Such characteristics make these materials suitable for gas sensing applications. Variations in the optical transmittance of the samples in-duced by reducing gaseous environments were registered for all NiO concentrations synthesized, while for an acceptable conductometric response a minimum of 30% molar content in NiO must be achieved, which is indicative that a mini-mum distance between conducive particles on (NiO) must be maintained in the matrix to facilitate electrical conductivity. An annealing temperature higher than 700◦C promotes a

densification of the silica matrix core that hinders a rapid gas diffusion throughout the pores network, with a consequent degradation of the sensing capability of the material.

Functional characterization towards NO2, CH4, CO and

H2 shows that a high sensitivity of the samples toward H2

detection down to tenths of ppm in dry air, and with good reversibility is possible.

Acknowledgments This work has been developed in the framework of

a program between CNR and MIUR (Legge 16/10/2000 fondo FISR).

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Figure

Figure 4 shows the refractive index n and absorption in- in-dex k of 40%NiO-60%SiO 2 film annealed at 500 and 700 ◦ C.
Fig. 5  transmittance (T 1%CO – T air ) measured in NiO-SiO 2 films with NiO content ranging from 10% to 40% molar ratio with respect to SiO 2
Fig. 6 Optical transmittance measured at fixed wavelength and different CO concentrations (10, 100, 1000, 10.000 ppm CO) in dry air mixtures for NiO-SiO 2 films with different
Figure 9 reports the temporal conductometric response of a 30% NiO-70% SiO 2 film annealed at 700 ◦ C  in-duced by several concentrations of NO 2 , CH 4 , CO and H 2
+2

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