Porous sol gel silica films doped with crystalline NiO nanoparticles for gas sensing applications

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Publisher’s version / la version de l'éditeur: Journal of Sol-Gel Science and Technology, 40, pp. 299-308, 2006

Porous sol-gel silica films doped with nanocrystalline NiO particles for gas sensing applicationsBuso, 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 nanoparticlesfor gas sensing applications

D. Buso · M. Guglielmi · A. Martucci · C. Cantalini ·

M. L. Post · A. Hache

Published online: 22 August 2006C© 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–350◦C 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: [email protected]

C. Cantalini

Dipartimento di Chimica e Materiali, Universita 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. Hache

Departement de physique et d’astronomie, Universite 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-SiO2 thin 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.

Springer

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

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%SiO2 and up to 40%NiO-

60%SiO2 molar 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 SiO2 matrix.

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 character-

ization (UV-vis absorbance) and optical functional character-

ization with gases in transmittance mode. Si/Si3N4 substrates

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−1 range

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 CuKα Ni 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-SiO2 films 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 H2 and 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 concentrations 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

Springer

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

Fig. 1 FTIR spectra of 40%

NiO – 60% SiO2 films annealed

from 500◦C to 1000◦C. 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% SiO2 molar 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. 1 highlight 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−1 range 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−1 which is proposed to be due to LO3 stretching

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−1 is 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%SiO2 films 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 43.3◦ are 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-Kα radiation

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, International Center for Diffraction

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% SiO2 film annealed at 500◦C (solid line)

and 700◦C (dashed line)

The TEM image of Fig. 3 shows the morphology of a 40%

NiO – 60% SiO2 film heated at 500◦C. NiO round shaped par-

ticles are homogeneously distributed in the SiO2 matrix 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.

Figure 4 shows the refractive index n and absorption in-

dex k of 40%NiO-60%SiO2 film 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% SiO2 film annealed

at 500◦C. Inset: the size

distribution of the NiO crystals

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attributed to Ni2+ cations hexacoordinated with oxygen

atoms of the silica network [21, 22]. For this reason it is pos-

sible that inside the SiO2 network 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, H2 and reduction

of NO2 through thermally activated mechanisms.

A gas induced change in optical transmittance of NiO-

SiO2 films 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. 4 indicates that NiO-SiO2

films are sensitive to CO (1% vol in dry air) at all NiO con-

centrations in SiO2 ranging from 10% to 40%NiO. Moreover

Fig. 5 highlights 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 SiO2 matrix.

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.

Figure 6 reports 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. 7 the 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

SiO2 walls 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% NiO-

70% SiO2 film (annealed at 700◦C) is heated at temperatures

Springer


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 NO2 detection 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 NO2 is considered

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

SiO2 film 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. Figure 8 indicates that for

optimal NO2 detection 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

Springer


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 different 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 NO2 all 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 → CO2−

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% SiO2 film annealed

at 700◦C at temperatures

ranging from 50◦C to 300◦C.

Each temperature step has a

[air/7 ppm NO2/air] cycle

Fig. 9 Resistance changes

induced by several

concentrations of NO2, CH4,

CO and H2 for a 30% NiO –

70% SiO2 film 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% SiO2 film annealed at 700◦C vs. gas concentration for

4 gases

Temporal plots like the one of Fig. 9 are 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 H2 the 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. CH4 and CO induce

responses with T90 values in the 7 to 10 min range, while

H2 and NO2 induce 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 NO2 response RR

is defined as [RA/RG]).

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

NiO-70% SiO2 film 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–640◦C 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

SiO2 films 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 SiO2 films 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

signal.

Springer


gases. The film is characterized by a higher sensitivity to

H2 and a minimum towards CH4 and CO, while sensitivity

to NO2 stands 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 CH4 leads to RR values 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.

Table 1 summarizes 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 SiO2 matrix, 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 SiO2 films 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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Porous sol gel silica films doped with crystalline NiO nanoparticles for gas sensing applications