Real surface determination of Co3O4 by Zn2+ adsorption. A comparison between X-ray diffraction, cyclic voltammetry and adsorption methods

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Electrochimica Acta, Vol. 43, No. 8, pp. 893±898, 1998 # 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0013±4686/98 $19.00 + 0.00 S0013-4686(97)00219-3

Real surface determination of Co3O4 by Zn2+ adsorption. A comparison between X-ray di€raction, cyclic voltammetry and adsorption methods P. Nkeng,a S. Marlier,a J. F. Koenig,a P. Chartier,a G. Poillerata and J.-L. Gautierb a

Laboratoire d'Electrochimie et de Chimie Physique du Corps Solide, URA au CNRS 405, Faculte de Chimie, Universite Louis Pasteur, 1-4 rue Blaise Pascal, F-67000 Strasbourg, France b

Laboratorio de Electroquimica, Facultad de Quimica y Biologia, Universidad de Santiago de Chile, Casilla 40 correo 33, Santiago 2, Chili (Received 22 January 1997; in revised form 28 April 1997)

AbstractÐThe real surface areas of Co3O4 ®lms have been measured by Zn2+ adsorption. Co3O4 ®lms have been prepared by spray pyrolysis. The results are compared with crystallite sizes obtained by X-ray di€raction measurements and roughness factors calculated from cyclic voltammograms. Discrepancies between the three methods are discussed and examined. # 1997 Elsevier Science Ltd. All rights reserved. Key words: real surface area, spinels, metallic oxide.

1. INTRODUCTION

2. EXPERIMENTAL

One of the most important problems encountered in electrocatalysis is the determination of the surface area on which the reactions occur. An evaluation of this area is essential for better understanding of the role and the importance of the electrode material. This also allows the comparison of di€erent materials and di€erent preparation methods for the same material. Trasatti and Petrii [1] have described intensively the problems encountered for such a determination of real surface area. This work concerns the real surface area determination of cobalt spinel ®lms, Co3O4 by Zn2+ ion adsorption onto the surface of the oxide following the works of Kosawa [2, 3] and Savinell et al. [4]. This work and some of our previous works [5] have described that the Zn2+ adsorption method gives rather inexplicable results. Making some comparison between this method and some others, we try in this work to understand better the real surface problem in the case of the Co3O4 oxide.

The oxide ®lms have been prepared by spray pyrolysis and characterized by X-ray di€raction, scanning electron microscopy and cyclic voltammetry.

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Preparation of the cobalt spinel oxides The cobalt spinel oxide ®lms have been prepared by spray pyrolysis on glass substrates, following a procedure described elsewhere [6]. The spray gun was powered with pressured air (2±3 atm.) and with a ¯ow rate of 3±4 cm3 mnÿ1 of a 0.3 M Co(NO3)2
6H2O (Merck p.a.) aqueous solution. A rotating movement (60 rev mnÿ1) of the nozzle allowed us to obtain a better homogeneity on a larger surface. The substrate was held at 3508C, this temperature being sucient to decompose the nitrates contained in the droplets of the mist. Characterization methods X-ray di€raction. The di€ractogram of Co3O4, prepared directly on glass, displayed in Fig. 1, shows that a well crystallized phase with a spinel structure has been obtained. The cubic cell par-

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Fig. 1. X-ray di€ractogram of Co3O4 prepared by spraying a Co(NO)3)2 solution on a heated glass substrate.

ameter was computed to be 0.8085 2 0.0003 nm, which is in good agreement with the literature [7]. Scanning electron microscopy (SEM). The morphology of the oxides surface is shown in Fig. 2. A regular repartition of compact spheres (of 1 mm maximum diameter) is noticeable on an homogeneous surface. Real surface determination by Zn2+ adsorption The oxide ®lm was immersed in a ZnO±NH4Cl solution and the Zn2+ ions were supposed to cover

all the surface of the oxide. Therefore, it would be possible to estimate the surface area by determination of the number of adsorbed ions considering the surface occupied by one ion. The amount of adsorbed zinc ion was determined by EDTA titration. The samples were placed in a polyethylene adsorption cell and immersed during 20 h in 10 ml of a solution of 0.5 M NH4Cl (Chemika 09700) and 0.001 M ZnO (Chemika 96479). A rapid change of Zn2+ concentration occurred during the ®rst 3 h of immersion as noted by Savinell et al. [4] with RuO2

Fig. 2. SEM microphotographs of a Co3O4 ®lm obtained by spray pyrolysis.

Co3O4 determination by Zn2+ adsorption and IrO2. Preliminary experiments have shown that, after 3 h, there was no noticeable change of Zn2+ concentration, thus indicating that an equilibrium had been reached. After 20 h in the solution, the samples were removed and the remaining solution was titrated in order to determine the amount of adsorbed Zn2+, using EDTA 0.25  10ÿ3 mol dmÿ3 solution and Eriochrome Black T as indicator. Assuming an area of 0.17 nm2 per adsorbed Zn2+ ion [8], the real surface area, de®ned as the surface

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of the Co3O4 actually covered by Zn2+ cations, was then calculated [2].

3. RESULTS AND DISCUSSION X-ray di€raction The Co3O4 average crystallite diameter has been calculated by means of the Scherrer's equation [9]:

Fig. 3. X ray di€ractograms. Details of the (311) peak: (a) prepared by spray pyrolysis at 3508C; (b) same preparation followed by annealing at 8008C (to obtain large crystals).

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Table 1. Number of moles adsorbed per surface unity and resultant roughness factors

Substrate number 0 1 2 3 4

Number of moles of Zn2+ in the initial solution (m mol)

Number of moles of Zn2+ in the ®nal solution (m mol)

10.7 10.7 10.7 10.7 10.7

10.2 8.9 8.6 7.5 6.5

d ˆ kl=B cos A0 ;

Number of moles adsorbed per surface unit (m mol cmÿ2)

0.50 1.78 2.19 3.25 4.25

…1†

where k is a constant (0.9), l is the wavelength, A0 is the re¯ection angle corresponding to the maximum height of the peak (in 2y radian units) and B is the relative peak broadening estimated according to: B2 ˆ B2m ÿ B2s ;

Number of moles adsorbed (m mol)

…2†

where Bm is the experimental peak width (in radians unit) and Bs is the peak broadening, due to the instrumental set, which can be determined from a large crystallite di€raction peak. We have used as a standard a Co3O4 powder heated to 8008C, which exhibits di€raction peaks occurring at the same angles. A Gaussian line shape with the equation:   …A ÿ A0 †2 ‡ y0 y ˆ h exp ÿ …3† s2 has been used to approximate the experimental curves, where h is related to the height of the diffraction peak, A the re¯ection angle and y0 a constant used to compensate the background of the di€ractogram. s is related to the width Bm of the Gaussian at half-height through the equation: p Bm ˆ 2s ln 2 …4† The crystallite size determination was based on the (311) peak which exhibits the maximum height [Fig. 3 (a)]. Figure 3 (b) corresponds to larger crystallites and allows to determine the peak broadening due to the instrument. From Fig. 3 (a) and 3 (b) and ®tting the parameters of the Gaussian to the measured curves, an average diameter of the crystallite was deduced: d = 15 nm.

0.07 0.25 0.29 0.43 0.57

Roughness factor 70 260 300 440 580

Zn2+ adsorption For the determination of the real surface from Zn2+ adsorption, four samples with various oxide loading have been prepared. The roughness factors, Rf corresponding to the ratio between the real and the geometric areas, were calculated using the following formula: Rf ˆ nNa S

…5†

in which n is the number of moles of zinc adsorbed per geometric surface unit, Na the Avogadro number and S the surface occupied by one zinc ion, taken as 0.17 nm2. It must be mentioned that this last value originates from the O'Grady's work [8] and is supposed to adjust the surface area obtained by Zn2+ adsorption to BET results. However, according to Kosawa and Takai [3], this is not a general rule for all the oxide families. The number of moles adsorbed per surface unit and the resultant roughness factors are given in Table 1. The entry 0 corresponds to a glass substrate without deposit (blank). The values of roughness factor found are in a range from 260 to 580 for the substrates with oxides. It should be noted that the adsorption on the glass substrate without oxide ®lm is small but not negligible. The amount of adsorbed zinc ions has been obtained by subtracting the whole amount of ions adsorbed by the glass substrate from the total amount of ions adsorbed. The real correction applied should be between 50% of the glass plate surface, if the Zn2+ ions adsorb only on the face which is not covered by the oxide, and 100%, if all the glass surface is accessible to Zn2+. Making a 100% correction should introduce a relative error lower than 15%,

Table 2. Corrected roughness factor and electrochemicaly active surface area by unity of mass (EASAM) Substrate number 1 2 3 4

Number of moles adsorbed (m mol) Corrected 1.28 1.69 2.75 3.75

Corrected roughness factor 190 230 370 500

tf (mm) 4.0 4.7 7.4 11.1

a (mmÿ1) 45 49 50 45

Dp (mm) 0.09 0.08 0.08 0.09

EASAM (cm2 mgÿ1) 110 125 125 110

Co3O4 determination by Zn2+ adsorption which seems acceptable in this kind of measurements. We make as an hypothesis that the oxide ®lm is very porous and thus we substract the 5.0  10ÿ7 (corresponding to the total surface of the glass plate) to the results obtained for the oxide samples (see Tables 1 and 2). On the basis of Savinell's studies [4] it has been supposed that the zinc ions do not only adsorb onto the surface of the ®lm, but also di€use into the ®lm and adsorb on the surface of each oxide grain constituting the polycrystalline material. In the Savinell model, oxide particles were assimilated to spheres. If we consider N spherical particles of diameter Dp, packed in a ¯at ®lm (thickness tf, geometrical surface Sg and pore fraction e) of a total volume VT=tf Sg, the real volume of the N spheres is Vs=(1 ÿ e) VT=N D3p p/6 and the ratio a of the spheres surface SS=NpD2p, divided by the total volume VT is a = 6 (1 ÿ e)/Dp and is the speci®c surface area per unit of volume of the ®lm. The roughness factor Rf is the ratio between the surface of the sphere and the geometrical surface Sg. So Rf ˆ atf ˆ 6…1 ÿ e†tf =Dp :

…6†

If SZn is the surface corresponding to Zn2+ adsorption, Rf=SZn/Sg=EASAM Sgm/Sg, where m is the loading (mass/surface) and EASAM (electrochemicaly active surface area per mass of oxide) is the accessible surface to Zn2+ ions per mass of oxide. Introducing r as the oxide density (mass/volume): Sg m ˆ …1 ÿ e†VT r ˆ …1 ÿ e†Sg tf r:

…7†

The roughness factor estimated from experimental data can be expressed as: Rf ˆ EASAM…1 ÿ e†r tf with EASAM ˆ Ss =m ˆ

…8†

NpD2r 6 6 : ˆ rDP m

The thickness of the ®lm, tf, was estimated according to tf ˆ

m : r…1 ÿ e†

…9†

The values of the molar amounts of absorbed zinc ions corrected of the adsorption by the substrate (vide supra) are given in Table 2. These experimental results show that this model is justi®ed as terms such as the diameter Dp=6(1 ÿ e)tf/Rf and the accessible surface to zinc ions per mass of oxide (EASAM = 6/rDp), which are constant according to the theory, are found to be e€ectively independant of the loading. In Fig. 4 the roughness, represented as a function of the loading, shows a linear relation between these two variables, as suggested by the model. The average crystallite diameter obtained by X-ray di€raction (=15 nm) and Dp obtained from zinc ion adsorption (=80 nm) are signi®cantly di€erent. Nevertheless, taking into

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Fig. 4. Roughness factors (from Zn2+ adsorption) as a function of loading.

account that each grain of Co3O4 may be constituted of di€erent crystalline orientations (polycristalline grains) this di€erence is understandable. Cyclic voltammetry Cyclic voltammetry has been performed with similar Co3O4 ®lms in our previous works [10±12] on glass supports previously covered by CdO to give electrical conductivity to the electrode. The electrolyte was KOH 1 mol dmÿ3. The current was recorded in a 50 mV range where no faradaic reaction occurs. The slope of the i = f (scan rate) curve gives the capacity of the oxide/solution interface. Roughness factors have been calculated from these capacities taking the values from Levine and Smith [13] as reference for the capacity of the unit surface area of an oxide surface (C = 60 mF cmÿ2). The roughness factor obtained [14] for samples which have been prepared by the same method (spray pyrolysis) are presented in Table 3. Comparison between the three methods The three methods give very di€erent results, which is not surprising because each method leads to di€erent parameters which characterize the surface area: XRD gives the average size of the monocrystalline domains, Zn2+ adsorption leads to the surface of the oxide grains and the CV roughness corresponds to the change of the double-layer charge during one potential scan. Nevertheless, the two latter methods lead to a linear increase of the surface area with the thickness, which is in agreement with an important porosity of the ®lm. The Table 3. Roughness factor obtained by cyclic voltammetry Thickness of the ®lm, e (mm) Roughness factor

e R5 3±5

5ReR 10 10 Re 5±8 8±10

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ratio between the roughness factors obtained by Zn2+ adsorption on roughness factors obtained by cyclic voltammetry was near to 50±60. This ®gure is too large to be explained by the uncertainties on the di€erent parameters used. Geometric considerations between the surface of a ®lm (as it can be determined by cyclic voltammetry) and the whole surface developed by the packing of uniformed size spherical particles, which can be taken as a model for the ®lm in the case of the equilibrium adsorption of Zn2+, are not able also to explain the found di€erence. It appears that the two methods do not measure the same parameter, but rather two representations of the surface area which are proportional. CONCLUSION A linear relation between the roughness factor and the thickness of the sprayed Co3O4 ®lms has been established, thus proving the high porosity of the ®lm. Roughness factors higher than those obtained by cyclic voltammetry have been explained by the model proposed by Savinell et al. [4], assuming that ions do not only adsorb on the ®lm surface but also penetrate the ®lm and adsorb on the surface of each grain of the polycrystalline material. The di€erence between grain diameter obtained by X-ray di€raction and the same parameter resulting from zinc ion adsorption leads us to con®rm that the grains are polycrystalline. The discrepancies between the results obtained by Zn2+ adsorption and cyclic voltammetry could not be simply explained. It means that further studies on the real surface problem are required, for example, the determination of the area of the real surface by STM

(scanning tunneling microscopy) and porosity measurements which are in progress in our lab. ACKNOWLEDGEMENTS This work has been carried out in the frame of the cooperation between France and Chile (ECOS/ CONYCITprogramme). REFERENCES 1. S. Trasatti and O. Petrii, J. Pure Appl. Chem. 63, 711 (1991). 2. A. Kozawa, J. Inorg. Nucl. Chem. 21, 315 (1961). 3. A. Kozawa and T. Takai, Surface Electrochemistry: Advance Methods and Concepts (Edited by T. Takamura and A. Kozawa). Japan Scienti®c Society Press, Japan (1978). 4. R. F. Savinell, R. L. Zeller III and J. A. Adams, J. Electrochem. Soc. 137, 489 (1990). 5. A. Restovic, G. Poillerat, P. Chartier and J-L. Gautier, Electrochim. Acta 40, 2669 (1995). 6. M. Hamdani, J. F. Koenig and P. Chartier, J. Appl. Electrochem. 18, 561 (1988). 7. JCPDS: ICDD ®les: no 9±418. 8. W. O'Grady, C. Iwakura, J. Huang and E. Yeager, in Electrocatalysis (Edited by M. W. Breiter), p. 226 The Electrochemical Society, Princeton, NJ (1974). 9. H. P. Klug and L. E. Alexander, X-ray Di€raction Procedures. Wiley, New York (1962). 10. M. Hamdani, J. F. Koenig and P. Chartier, J. Appl. Electrochem 18, 568 (1988). 11. R. N. Singh, J. F. Koenig, G. Poillerat and P. Chartier, J. Electrochem. Soc. 137, 1408 (1990). 12. R. N. Singh, G. Poillerat, J. F. Koenig and P. Chartier, J. Electroanal. Chem. 314, 241 (1991). 13. S. Levine and A. L. Smith, Discuss. Faraday Soc. 52, 290 (1971). 14. P. Nkeng, J. F. Koenig, P. Chartier and G. Poillerat, J. Electroanal. Chem. 402, 81 (1996).