Sensors and Actuators B 98 (2004) 227–232

Gas adsorption effects on surface conductivityof nonstoichiometric CuO

A. Cruccolini, R. Narducci, R. Palombari∗Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy

Received 19 August 2003; received in revised form 28 September 2003; accepted 6 October 2003

Abstract

The aim of the present paper is to test the capability of nonstoichiometric CuO as a conductance sensor material. The surface conductivitychanges in CuO were measured in the presence of various donor–acceptor couples. The influence of water and oxygen was investigated aswell as NO2 and CO. NO2 had a very high sensitivity.

Kinetic schemes of surface reactions are also reported and discussed.© 2003 Elsevier B.V. All rights reserved.

Keywords: CuO semiconductor; Conductance sensor; NO2; CO

1. Introduction

The sensing mechanism of conductance sensors based onelectrically active grain boundaries of semiconductor com-pounds is ascribed to variations of majority carrier surfaceconcentration under the influence of adsorbed substances[1]. Among the semiconductor compounds used for this pur-pose, the nonstoichiometric oxides are the most importantclass.

This paper deals with the influence of some pollutinggases on grain boundary conductivity of copper oxide, whichseems to have been neglected as a single material for makingsensors[1]. Recently, it has been used either mixed withother oxides to form a composite or as a heterojunction toform a p–n diode. In these forms, it is prevalently used fordetecting CO, NO2 and H2S [2–5].

The main features of CuO as a semiconductor, althoughlithium doped, have been reported by Koffyberg and Benko[6], while its transport properties in relation to thermal treat-ments have been described by Jeong and Choi[7]. Accordingto the latter authors, copper ion vacancies can be thermallyinduced in the oxide. The vacancy charges are compensatedby the formation of an equivalent concentration of Cu3+,whose extra charge can migrate to one of the proximal Cu2+ions by an activated hopping process. The majority carri-

∗ Corresponding author. Tel.:+39-075-585-5564;fax: +39-075-585-5566.E-mail address: palomb@unipg.it (R. Palombari).

ers are therefore the holes, as confirmed by thermoelectricpower, and the material behaves like a p-type semiconductor.

The formation of copper vacancies in Cu1−xO have beenobserved at temperatures lower than 1000◦C, at which thematerial is, with a good approximation, stoichiometric. Thecopper deficiency, reported as thex values, was thermo-gravimetrically determined in the 700–900◦C range and wasfound to be between 1.3× 10−2 and 3.3× 10−2 at the low-est temperature. From such data it was estimated the holeconcentrations and, using conductivity data, their mobility.It should be noted that the reported mobility values weremiscalculated. For example, the value of 0.3 cm2 V−1 s−1 at394◦C for CuO conditioned at 900◦C does not fit the con-ductivity value of 0.03 S cm−1 and a carrier concentrationof 6.6 × 1019 cm−3. From the latter data the mobility turnsout to be 3× 10−3 cm2 V−1 s−1, a more realistic value fora hopping semiconductor.

It is also important to note that samples, conditioned atthe above temperature range, show an exstrinsic behaviorfor temperatures lower than 400◦C.

2. Experimental

2.1. Materials

Crystalline nonstoichiometric CuO samples were pre-pared by heating copper(II) oxide (Aldrich powder<5�m,>99%) to 900◦C for 4 h. The solid was then rapidlyquenched at room temperature and ground. The X-ray

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228 A. Cruccolini et al. / Sensors and Actuators B 98 (2004) 227–232

diffraction pattern of the material only showed lines fromthe crystal structure of CuO (Tenorite), while those fromCu2O were absent. This material was used for the electro-chemical measurements in the form of pellets or thin layers.

Pellets (10-mm diameter, 0.2 g) were obtained using apressure of 40 K N cm−2.

Thin layers, deposited onto a gold-plated stainless steeldisk, were prepared by mixing the oxide with water and col-loidal silica (Ludox 30 wt.%, Aldrich ) in weight proportionsof 5–12% SiO2 in the dry material. The thin layers werethen dried at 200◦C for three hours (layer 1 (12% SiO2)) orslowly heated to 300◦C and maintained at that temperaturefor 3 h (layer 2 (12% SiO2)) and (layer 2a (5% SiO2)).

The inorganic proton conductor,� zirconium phosphate(Zr(O3P-OH)2), (ZP), was prepared according to the proce-dure developed in our laboratory[8].

ZP was converted into a partial Ag form, (Zr(O3P-OH0.9Ag0.1)2) (ZP(H, Ag)), by putting 1 g of microcrystallinesolid in contact with an acqueous solution of silver acetate(C. Erba) (0.11 g in 10 ml) under stirring at room temp. TheZP(H, Ag) was then separated by centrifugation and dried at110◦C. Pellets (10 mm diameter, 0.2 g) of ZP(H, Ag) wereobtained using a pressure of 40 K N cm−2.

The Ag, AgCl electrode was prepared starting with a silverdisk (C. Erba purity 99.9%; 10 mm diameter). The disk wasfirst cleaned with diluted HNO3, then one side was coatedwith AgCl by anodic oxidation in HCl 1 M using a currentof 0.5 mA for 1 h.

2.2. Potentiometric measurements

The cell for potentiometric measurements was assembledby stacking the following components together: Ag, AgCl,ZP(H, Ag) pellet and copper oxide; the latter was preparedon the pellet in a thin layer 1. The output potential wasrecorded using a data acquisition apparatus (HP 3852A).

2.3. Conductometric measurements

The conductivity measurements on CuO, for which holesare the majority carriers, were performed using a simpledc method, because the impedance measurements on thesematerials generally show that only the resistive componentis dependent on the composition of the adsorbed gas[9].

The oxide in the form of a pellet or a thin layer, insertedinto two porous gold disks, was supplied with a constantcurrent in the 0.1–1�A range, using a potentiostat (Amel2059); the output potential was measured using the aboveapparatus.

The cells were held in a gas-tight stainless housingwith teflon insulators and seals. To obtain variable oxy-gen or water pressures, synthetic air and nitrogen werebubbled through a trap filled with water taken at 25◦Cusing known flow rates coming from flow controllers(Bronkhorst). Constant temperatures were obtained by usinga temperature-controlled oven (Lenton LTF12/50/300).

3. Results and discussion

The aim of this study was to investigate the influence ofsome polluting gases in air on the conductivity of pressedpellets or layers of nonstoichiometric CuO. The influenceof water and oxygen on surface conductivity and on thepotential of this material was tested first.

It is well-known that water and oxygen may interact onthe surface of semiconductor materials, so that variations ofthe chemisorbed concentrations of these latter causes varia-tions in surface hole concentration. The difference betweenthe surface and bulk hole concentration produces a spacecharge region. Assuming Boltzmann statistics, for interme-diate temperature conditions, the ratio between the surfaceand bulk hole concentrations is given by the following rela-tion:

ps

pb= exp

(−eΦs

kT

)(1)

where Φs is the surface potential with respect to an un-perturbed inner part characterised by flat bands, while theother symbols have the usual meanings. The extension ofthe charge space inside the grain, in which the potential,Φ,varies exponentially, is characterised by the so-called De-bye length,L, which corresponds to the distance from thesurface, for whichΦ = Φs exp(−1) [10].

The value ofL for a semiconductor of p-type is given by:

L =(

εε◦kT

e2pb

)1/2

(2)

Under the condition that the mean radius of the materialparticles is larger than the Debye length, the surface con-ductivity of the material is related toΦs as follows[1]:

σ ∝ exp

(−eΦs

kT

)(3)

When the conductivity is essentially determined by the trans-port across the grain boundaries, on the basis ofEq. (1), theconductivity is directly related to the surface hole concen-tration. The latter is controlled by charge transfer reactionswhich take place among chemisorbed species[11,12].

The following scheme can be invoked for the O2, H2Ocouple on the CuO surface.

12O2(g)

k1�k−1

Oad (4)

Oadk2�k−2

O′ad + h• (5)

H2O(g)k3�k−3

H2Oad (6)

H2Oad + h• k4�k−4

H2Oad• (7)

H2Oad• + O′

adk5−→products (8)

A. Cruccolini et al. / Sensors and Actuators B 98 (2004) 227–232 229

Fig. 1. Relative conductivity of CuO as a function of variable water pressure: (a) pressed pellet 170◦C (open circles), 200◦C (crosses); (b and c) layer2 at 170 and 200◦C, respectively.

The steady state analysis for the surface hole concentrationgives:

k2[Oad] = (k−2[O′ad] + k4[H2Oad])ps (9)

From Eq. (9), the dependence of the conductivity on waterpressure, keeping the oxygen pressure constant, is given by:

σθ=0

σ= 1 + k∗θaq (10)

whereθaq is the water coverage andk∗ encompasses the con-stant terms: (k4/k−2[O′

ad]) and surface water concentrationfor θaq = 1.

Actually the dependence ofσθ=0/σ, for a pressed pelletof CuO treated at 900◦C, as a function of water pressurePat 170 and 200◦C is shown inFig. 1a. The continuous linefitting is obtained using the Langmuir function:

θaq = kP

1 + kP(11)

The values obtained for the Langmuir constants are: 65±10 atm−1 (k∗ = 0.87 ± 0.1) and 55± 10 atm−1 (k∗ =0.93 ± 0.1), respectively. The continuous curve, which fitsthe experimental data, is calculated assuming 60 to be theconstant value. Regarding the oxide layers, the type-1 layerbehaves like the pellet, in that they are rather insensitiveto changes of humidity, while for layer 2, the influence ofwater pressure on conductivity is more marked. The Lang-muir constants found at 170 and 200◦C (Fig. 1b and c) are:67± 10 atm−1 (k∗ = 1.9 ± 0.2) and 43± 10 atm−1 (k∗ =2.7 ± 0.3), respectively.

As expected, if the oxygen pressure is varied, while thewater concentrationis is held constant, changes of conductiv-ity are observed. The dependence of conductivity on oxygenpressure using the above reaction scheme is more involvedunless approximations can be applied. Generally, it is foundthat the relation which links conductivity and pressure or

concentration of a speciesx, whose coverage is negligible(θx ∝ Px), is given by the following relationship[9,11,12]:

σ ∝ (Px)n (12)

The influence of oxygen follows the above relation-ship. Fig. 2a shows the variation of relative conductivityof layer 1 of CuO (log(σ/σ◦)) at 200◦C as a function ofthe logarithm of oxygen pressure with the water pressure at3.1× 10−2 atm. On the basis of these data, the value of theexponent turns out to ben = 0.29± 0.01 (σ◦ represents theconductivity in synthetic air at the above water pressure).

This value is confirmed by measuring the potentialchanges of the cell:

Ag|AgCl|(Ag, H)ZP|CuO, (O2, H2O) (cell A)

under the same conditions. The potential values of the cellversus the logarithm of oxygen pressure shows a linear de-pendence (Fig. 2b). The slope value of the linear fit is 25.7±1.0 mV which, on the basis ofEq. (1), corresponds to a valueof the exponentn equal to 0.27±0.01 (25.7 = nkT(ln 10)/e)which is in fair agreement with the conductivity value. Theelectrolyte (Ag, H)ZP is a solid ionic conductor for whichthe charge carriers are protons and silver ions[13]. The va-lidity of these potential measurements is conditioned by theconstancy of the interface potential CuO|HZP. Actually itwas found that, at room temperature in the presence of wa-ter, CuO can exchange protons with the electrolyte givinga constant potential. A likely electrode reaction, previouslyassumed for NiO[14,15], is the following:

CuOOH(Cu1−xO) + H+ = Cu(OH)2(Cu1−xO) + h• (13)

After the above preliminary experiments, samples of CuOtreated at 900◦C were tested with NO2, at low concentra-tion in the 0.24–4.8 ppm range, in humidified air. Some pre-liminary results are shown inFig. 3, in which the relativeconductivity of CuO in the form of pressed pellets (open cir-cles) and layer 1 (black circles) is reported versus the con-

230 A. Cruccolini et al. / Sensors and Actuators B 98 (2004) 227–232

Fig. 2. Logarithm of relative conductivity vs. logP(O2) (a), and output potential of the cell A vs. logP(O2) (b), both at a water pressure of 3.1×10−2 atm(layer 1 (12% SiO2)).

centration in ppm units in logarithm scale, at 170◦C. Theplotted values are mediated for three levels of water pres-sure: 7.8 × 10−3, 15.5 × 10−2 and 2.33× 10−2 atm, whichcorrespond to relative humidities of 25, 50, and 75%, re-spectively, at 25◦C.

NO2 is acting as an acceptor and the presence of water isfundamental, because it is the donor. The water content doesnot influence the response in the above range. On the otherhand, oxygen is competing with NO2. This explains the flat-tening of the signal at concentrations less than 1 ppm.Fig. 3also shows the behavior of a layer 1 at 200◦C, for whichthe water pressure was maintained in the above range (uptriangle). It is important to note that the experimental pointsfall on a straight line, the slope of which, within the exper-imental errors, is 1.0. In the same figure, the other seriesof experimental points tend to have the same slope. Subse-

Fig. 3. Logarithm of relative conductivity of CuO vs. logarithm of NO2 concentration (ppm): (open circles) pressed pellet at 170◦C, (black circles) layer 1at 170◦C, and (triangles) layer 1 at 200◦C. The plotted values are mediated for three levels of water pressure: 7.8×10−3, 15.5×10−2 and 2.33×10−2 atm.

quent experiments confirmed that the value of the exponentn never exceeded the limit value of 1. Taking into accountthe above kinetic scheme, in the presence of NO2, two newequations must be considered:

NO2(g)k∗

1�k∗−1

NO2 ad (14)

NO2 ad

k∗2�

k∗−2

NO′2ad+ h• (15)

The steady state analysis for surface hole concentration turnsout to be:

k2[Oad] + k∗2[NO2 ad]

= (k−2[O′ad] + k∗

−2[NO′2 ad] + k4[H2Oad])ps (16)

A. Cruccolini et al. / Sensors and Actuators B 98 (2004) 227–232 231

Fig. 4. Logarithm of relative conductivity at 170◦C vs. logarithm of NO2 concentration (ppm): (a) layer 2a and (b) layer 2. The plotted values aremediated for three levels of water pressure: 7.8 × 10−3, 15.5 × 10−2 and 2.33× 10−2 atm.

The latter equation can be approximated to:

ps = constP(NO2) (17)

under the condition that the termsk2[Oad] andk∗−2[NO′

2 ad]are negligible in comparison withk∗

2[NO2 ad] and (k−2[O′ad]+

k4[H2Oad]), respectively.The two different trends between a pressed pellet and the

silica-containing layer inFig. 3could be ascribed to differentspecific areas of the samples or to a specific influence ofsilica. It is known that silica gel surface contains water inthe form of silanol groups and physisorbed water, the latteris lost in dry conditions at 170◦C. Dehydration of silicaby the elimination of surface water and formation of bothsingle and adjacent surface hydroxyl groups take place in thetemperature range between 170◦ and 400◦C [16]. Althoughit has been reported that these processes are reversible below

Fig. 5. Logarithm of relative conductivity at 200◦C vs. logarithm of CO concentration (ppm) for layer 1. The plotted values are mediated for three levelsof water pressure: 7.8 × 10−3, 15.5 × 10−2 and 2.33× 10−2 atm.

400◦C, layers of CuO containing silica previously heatedto 300◦C (layers 2) behaved differently in the detectionof NO2, as shown inFig. 4a and b, in which the relativeconductivities refer to layers 2a and 2 with two differentconcentrations of silica: 5 and 12%, respectively. As can benoted, all the experimental points lie on a straight line whoseslope,n, is smaller than 1: 0.60 and 0.38, respectively. Theminimum detectable is shifted to a lower limit, which can beestimated on the order of 20 ppb ( assuming as the minimumdetectable is a variation of 10% conductivity).

The different behaviors of layers 1 and 2 are associatedwith the different concentrations of available water on thesurface of the oxide. It is likely that during heating to 300◦Cthe physisorbed water was lost and was not readsorbed at170◦C. On the contrary, layer 1 maintained a high water con-centration, even when anhydrous air was supplied to the cell.

232 A. Cruccolini et al. / Sensors and Actuators B 98 (2004) 227–232

Now it is important to consider the effect of CO as an inter-fering species. CO, according to its role as a donor, decreasesconductivity.Fig. 5shows the relative conductivity as a func-tion of CO concentration (ppm) (layer 1) at 200◦C with thewater pressure in the 7.8×10−3 to 2.33×10−2 atm range. Us-ing layer 1, CO was also detected at very low water pressure.

It was found that the sensitivity of layer 2 towards COat 170◦C was negligible (decrease ofσ less than 10% for90 ppm CO) with respect to that observed for layer 1, there-fore, it does not represent a serious interference in the de-tection of NO2.

4. Conclusions

The experimental data here reported show that CuO pos-sesses good characteristics for its potential use as a conduc-tance sensor material. It shows, in mixture with silica, goodsensitivity towards NO2 and CO with a high selectivity forNO2. Moreover the role of water, whose effect can be con-trolled by the state of SiO2, was discussed.

Kinetic schemes among the chemisorbed species are re-ported in order to explain the variations of conductivity.

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