Synthesis of Metastable, Wurzite-Based Zinc Oxide–Cobalt(II) OxideSolid Solutions by Spray Pyrolysis

Vikram Jayaram, J. Rajkumar, and B. Sirisha Rani

Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India

Aqueous solutions of acetates and nitrates of zinc and co-balt have been spray decomposed to study the productionof extended solid solutions in the ZnO–CoO system. Exami-nation of the products of a variety of synthesis conditionsindicates that up to 70% CoO may be retained in the solidsolution in the wurzite phase, even though a comparison ofthe equilibrium solubility in the phase diagram might beexpected to favor the formation of a rock-salt-based solidsolution.

I. Introduction

THE solubility in zinc oxide (ZnO, wurzite (denoted as‘‘W’’)) of oxides that crystallize in the rock-salt (denoted

as ‘‘R’’) structure is a subject of fundamental and practicalinterest. Under equilibrium conditions, the solubility limit at atemperature of 800°C is∼0.9% for NiO and 6.5% for CoO.1,2

These values can be extrapolated to∼0.1% and 0.5%, respec-tively, at a temperature of 500°C.1,2 The corresponding solu-bility for MgO is <5% at a temperature of 900°C,3 which isthe lowest temperature for which data are available. However,the solubility of ZnO can be substantial in these three rock-salt oxides and attains such values as 25% in MgO at a tem-perature of 900°C,2,3 and 22% in CoO1 and 30% in NiO4 at atemperature of 800°C. Even at a temperature of 500°C, ZnOdissolves to the extent of∼10% in CoO and∼25% in NiO. Theimportance of valence states must be emphasized at this stage.In the case of CoO and NiO, partial oxidation of the cation to3+ can be initially accommodated in the rock-salt structurewith the introduction of compensating vacancies, (e.g.,Cox

3+Co2+2−(3x/2)O). However, in the case of cobalt, extensive

oxidation leads to the stabilization of spinel (Co3O4) at atmo-spheric pressure below a temperature of∼800°C. If sufficientlyreducing conditions are used, the divalent state may be re-tained, which leads to the previously described solubility lim-its. It is anticipated that the equilibrium solubility of the triva-lent ions in ZnO in the W form would be lower than that for thedivalent ions.

Interestingly, it is possible to convert ZnO at high pressure tothe R structure.5 Conversely, it is possible to synthesize CoO inthe W and sphalerite (‘‘Sh’’) forms via the pyrolysis of acetateand other organic precursors at temperatures of 200°–300°C.6,7

The excess energy of the W form over the R forms of CoO andNiO has been estimated from thermochemical data onspinels.8–10 More recently, direct thermochemical measure-ments have been made to estimate the enthalpy of transforma-tion from Sh to R in CoO, in which the role of the residualvolatile species has been considered.11 Lattice binding-energycalculations on CoO7 indicate that the trend in stability is R >

W > Sh. In ZnO, the stability at ambient pressure is W > R,whereas an Sh modification has also been reported but neversubsequently confirmed.12

From a practical standpoint, the solid-solution content ofZnO is one of the factors that control its varistor behavior (inparticular, the conductivity of the grain interior, which be-comes pertinent beyond the voltage at which the grain bound-aries have broken down electrically). In sensors for moleculessuch as H2O, CO, etc., the point-defect population, which de-termines electrical conductivity, also is strongly affected bysolute additions. ZnO forms excess singly charged zinc inter-stitials that are compensated by electrons, whereas CoO andNiO are cation-deficientp-type semiconductors. Thus, there isconsiderable interest in the production of extended solid solu-tions, particularly of the W form, which does not accept muchsolute under equilibrium conditions at low temperatures.

Of the various oxides in the second period that crystallize inthe R structure, MgO, being the most strongly ionic, is ex-pected to maximize its coordination of anions and is, therefore,the least likely to enter into tetrahedral coordination. At theother extreme lies CoO, which has been synthesized with four-fold coordination via decomposition of organic precursors un-der inert or reducing conditions below temperatures of∼300°C.The present work explores the possibility of preparing meta-stable solid solutions in the ZnO–CoO system via the spraydecomposition of precursors. This technique has been provento be versatile, along with gel decomposition, for the prepara-tion of molecularly homogeneous precursors at low tempera-tures at which nonequilibrium structures may be retained.13

II. Experimental Procedure

Aqueous precursor solutions of zinc nitrate, zinc acetate, andcobalt acetate (tetrahydrate) were prepared to yield final oxidecompositions of 40, 70, and 90 mol% CoO, in addition to pureCoO and ZnO. Salt concentrations were typically 100 g/L.Solutions were sprayed under air and nitrogen onto a hot Tef-lon substrate that was maintained at a temperature of 200°C.The above-described combination of precursors and ambientspraying explores the reducing and oxidizing conditions thatwere expected to influence the valence state of cobalt and,consequently, its propensity for dissolving in ZnO. Sprayingunder nitrogen was performed in a specially designed arrange-ment in which a horizontal glass nozzle delivered the solution,which was sprayed by using nitrogen from a vertical tube anddirected toward the substrate, which was heated via a sandbath. The entire setup was maintained at a slight overpressureof nitrogen to minimize air intake. Thermocouples were used tomeasure the temperature of the substrate surface, and sprayingwas stopped when the temperature dropped by more than 10°C.

The sprayed powder was characterized by using thermogra-vimetry (TGA) (Model TG 171, Cahn Instruments, Cerritos,CA) in commercial-purity nitrogen. Heat treatments to studythe phase evolution were conducted in a tube furnace for 30min at temperatures of 300°C, 500°C, and 800°C under lowvacuum (0.2 mbar (20 Pa)), high vacuum (10−5 mbar (10−3

Pa)), and a gettered high vacuum of 10−5 mbar in which cobalt-metal powder was placed in boats on either side of the sample.

C. G. Levi—contributing editor

Manuscript No. 190355. Received February 24, 1998; approved November 2, 1998.Supported by the INDO-US programme through NSF-INT and Grant No. 9633039

and by the Dept. of Science and Technology, Government of India.

J. Am. Ceram. Soc., 82 [2] 473–76 (1999)Journal

473

In all cases, the tube was flushed with nitrogen before evacu-ation. The entire variety of precursors, spraying atmosphere,and ambient heat treatment was explored only for the 40%composition. Based on the results of these studies, the 70%,90%, and 100% CoO compositions were synthesized under themost-reducing combination, i.e., acetates sprayed in nitrogenand heat treated under a gettered high vacuum. X-ray diffrac-tometry (XRD), using CuKa radiation, was performed to iden-tify the phases that were present. Peak broadening and identi-fication of overlapping peaks were established by using stepscans at intervals of 0.01°, with an acquisition time of 2 s.

III. Results

TGA on the sprayed powders reveals that, following a sharploss of 5%–15% up to a temperature of 100°C, which repre-sents the loss of adsorbed moisture during storage, the majorportion of the weight loss is completed by 250°C (Fig. 1). Theinterpretation of the data is slightly complicated by the con-current weight gain due to the oxidation of divalent cobalt tothe trivalent state in the commercial-purity nitrogen. However,given that pure CoO has a theoretical maximum weight gain of7% during full conversion to Co3O4 and that the major portionof this oxidation is accomplished by∼300°C, it seems reason-able to conclude that weight changes after 300°C are not sub-stantial in either the 40% or the 70% sprayed powder. In par-ticular, of the acetates that were sprayed in nitrogen, whichrepresent the ideal combination (which will be explained later),the 40% powder shows no weight change after 300°C, whereasthe 70% powder shows a small increase, presumably due tooxidation to Co3+. The loss in weight at temperatures of 750°–800°C corresponds to the reduction of Co3+ to Co2+, whichleads to the destabilization of spinel (‘‘S’’) in favor of rock saltin the equilibrium phase diagram.3,8 The greater loss in curves‘‘A’’ and ‘‘B’’ in Fig. 1 probably results from the higher frac-tion of Co3+ in samples that have been made with nitrates.

The ability to obtain wurtzite-based solid solutions isstrongly dependent on the precursor and the ambient duringspraying and heat treatment, as illustrated in Figs. 2–4 forZnO–40 mol% CoO. When mixed nitrate–acetate solutions aresprayed in air (Fig. 2), the as-sprayed powder displays broad Wpeaks. However, rock salt (R) is not evident; prominent S peakscould be buried beneath the background of the W peaks. At atemperature of 500°C, distinct spinel Co3O4 peaks appear andthe proportion of S to W decreases continuously with the oxy-

gen partial pressure until it disappears entirely under a getteredhigh vacuum in which R-form CoO appears. The W+R com-bination of phases also persists at 800°C. When acetate–nitrateprecursors are sprayed in nitrogen, the distribution of phases isbroadly similar, as shown in Fig. 3. The presence of cobaltpowder during heat treatment prevents the formation of spinel.However, even in the absence of such a getter, the relativeintensities of S peaks are lower than when spraying is per-formed in air. The pyrolysis of acetate precursors (Fig. 4) innitrogen, followed by low-vacuum heat treatment, leads tospinel formation. However, heat treatment under gettered con-ditions at temperatures of 300°C (not shown) and 500°C re-veals a single-phase W solid solution. Phase separation occursat 800°C to yield R.

The above-described sequence reveals that decompositionunder oxidizing conditions (which is due to the nitrate-containing precursors as well as the atmosphere during spray-ing/heat treatment) produces Co3+ that is not dissolved in Wand which, therefore, leads to the formation of a separate phaseat a temperature of 500°C (i.e., S in oxidizing conditions and Rin the presence of a getter). (Note that if Co3+ were fullydissolved in W after spraying, then heat treatment under agettered high vacuum would have produced a single-phase Wsolution upon reduction.) When non-oxidizing conditions areused, from the precursor to the ambient heat treatment, it ispossible to produce single phase W-based solid solutions ofCoO in ZnO.

The sequence of phases that are observed in the 70% com-position is shown in Fig. 5. The as-sprayed powder is single-phase W and remains so up to a temperature of 300°C. At atemperature of 500°C, the precipitation of CoO leads to a broadR peak. The 90% composition shows a mixture of R and W,even upon spraying, whereas pure cobalt acetate yields R (Fig.6). This last result is in contrast to the product of the pyrolysisof the pure salt (anhydrous or hydrated), which has been re-ported to yield W at temperatures <320°C.6,7

Typical grain sizes of the single-phase W phase, as deducedfrom the width of the XRD peak, are 15–20 nm in the as-sprayed condition. The grain size does not change appreciablywhen a solid solution is maintained at a temperature of 500°C.However, there is an increase by a factor of 2, as observed from

Fig. 1. TG analysis of the sprayed powders (40 mol% CoO in sprayatmosphere/precursor environments of air/nitrate–acetate (curve‘‘A’’), nitrogen-gas/nitrate–acetate (curve ‘‘B’’), and nitrogen-gas/acetate–acetate (curve ‘‘C’’); the TG analysis of 70 mol% CoO in aspray atmosphere/precursor environment of nitrogen-gas/acetate–acetate is shown as curve ‘‘D’’).

Fig. 2. XRD pattern of ZnO–40% CoO sprayed in air from nitrate–acetate precursors (as-sprayed (pattern ‘‘(A)’’), 500°C under lowvacuum (pattern ‘‘(B)’’), 500°C under high vacuum without a getter(pattern ‘‘(C)’’), 500°C under high vacuum with a cobalt getter (pat-tern ‘‘(D)’’), and 800°C under high vacuum with a cobalt getter (pat-tern ‘‘(E)’’)). Wurzite, rock salt, and spinel are represented by thesymbols ‘‘W,’’ ‘‘R,’’ and ‘‘S,’’ respectively; the shoulder on the wur-zite peak at 18° indicates rock salt (111).

474 Communications of the American Ceramic Society Vol. 82, No. 2

the X-ray peak width, when cobalt precipitates out as spinel(Fig. 4).

IV. DiscussionThe foundation of the present work lies in the small reported

enthalpy difference between the tetrahedral (W or Sh) and oc-

tahedral forms of CoO. The experimental results vindicate theassumption that the solubility of CoO in the W form of ZnOcan be substantially extended, in this case to∼70%. A sche-matic of the free-energy variation (DG) with composition at atemperature of 500°C for the two competing phases is shown inFig. 7(a), whereas the phase diagram according to Bateset al.1

is reproduced in Fig. 7(b). TheDG values for the pure com-ponents are reported estimates from high-pressure data forZnO8 and from thermodynamic data on spinels or calorimetryfor CoO.8 Given the substantial equilibrium solubility of ZnOin R (10%), compared to that of CoO in W (<0.5%), it isreasonable to conclude that the intersection of the free-energycurves would lie at a CoO content of less than∼60%. (Note thatthe schematic in Fig. 7(a) is not consistent with the reportedideality of the W phase and the small positive deviation fromideality in R. However, these activity relationships are notcompatible with the solubilities, given the free-energy differ-

Fig. 3. XRD pattern of ZnO–40% CoO, sprayed in nitrogen fromnitrate–acetate precursors (as-sprayed (pattern ‘‘(A)’’), 500°C underlow vacuum (pattern ‘‘(B)’’), 500°C under high vacuum with a cobaltgetter (pattern ‘‘(C)’’), and 800°C under high vacuum with a cobaltgetter (pattern ‘‘(D)’’)). Symbols are as given in Fig. 2.

Fig. 4. XRD pattern of ZnO–40% CoO, sprayed in nitrogen fromacetate–acetate precursors (as-sprayed (pattern ‘‘(A)’’), 500°C underlow vacuum (pattern ‘‘(B)’’), 500°C under high vacuum with a cobaltgetter (pattern ‘‘(C)’’), and 800°C under high vacuum with a cobaltgetter (pattern ‘‘(D)’’)). Symbols are as given in Fig. 2.

Fig. 5. XRD pattern of ZnO–70% CoO, sprayed in nitrogen fromacetate–acetate precursors (as-sprayed (pattern ‘‘(A)’’), 300°C underhigh vacuum with a cobalt getter (pattern ‘‘(B)’’), and 500°C underhigh vacuum with a cobalt getter (pattern ‘‘(C)’’)). Symbols are asgiven in Fig. 2.

Fig. 6. XRD pattern of ZnO–90% CoO sprayed in nitrogen fromacetate–acetate precursors (pattern ‘‘(A)’’) and CoO sprayed in nitro-gen from an acetate precursor (pattern ‘‘(B)’’). Symbols are as givenin Fig. 2.

February 1999 Communications of the American Ceramic Society 475

ence that has been estimated between R and W for the endcompounds.) Thus, the nucleation of W that is experimentallyobserved extends to compositions in which R is probably ther-modynamically more stable. Indeed, the preference for zinc toremain in tetrahedral coordination as W in this synthesis routecan be determined from the fact that the 90% CoO composi-tion, which is close to the extrapolated equilibrium solubility of∼10% ZnO and which could, therefore, be expected to crystal-lize as single-phase R, is, in fact, phase separated at a tempera-ture of 200°C as a mixture of R+W.

In the past, the nucleation of metastable phases has beenrationalized by various arguments, including the preference forsimple structures and phases that, because of the strain-energyeffect on the nucleation barriers, have a lower molar-volumemismatch with the precursor. Examples of this phenomenoninclude the formation of fluorite-based phases in yttria14 andlead zirconate titanate,15 the formation of rock salt in MgO–Al2O3 solutions,16 and the preference for theg-phase over thea-phase in alumina. In the present case, both competing struc-tures are simple prototypes of octahedral and tetrahedral coor-dination in oxides. It seems likely that the dominance of W and,in particular, the absence of a single-phase R field in the pres-ent experiments reflects the strong preference for zinc to beproduced in tetrahedral coordination. The possibility of mixedcoordination may also favor W. For example, it has beenshown from the XRD intensities of quenched solid solutions ofZnO–MgO that Zn substitutes in octahedral sites for Mg in theR phase, whereas Mg appears in tetrahedral as well as octahe-

dral interstices in the W structure.17 However, an octahedralcoordination of cations should increase the relative intensity of0002 compared to 1010, whereas the present results indicatethat I0002/I1010 actually decreases by∼25%–30% in the 40 and70 mol% compositions.

Residual volatile species are sometimes suspected to stabi-lize metastable phases such as the case of OH− in g-alumina.Although such a possibility cannot be entirely excluded in thepresent results, TGA indicates that weight losses due to volatilespecies at temperatures greater than∼400°C are negligible,whereas drop calorimetry of CoO that has been formed fromthe acetate at a temperature of 320°C11 suggests that the oxideis probably free of other groups and that residual undecom-posed material is present as separate phases of carbon or aceticacid. The appearance of R in pure CoO, which is contrary toearlier reports of W and sphalerite formed from the solid ac-etates,6–8 is very probably due to the octahedral coordination ofwater molecules that prevails around the cobalt ion in solution.Indeed, this octahedral preference seems to induce phase sepa-ration at CoO contents of >70%. If precursor solutions could besynthesized in which the ligand between cobalt and the solventis tetrahedral, then it should be feasible to extend the produc-tion of W all the way across the composition range. Thus,phase selection in ZnO–CoO seems to be driven more by co-ordination preferences that are derived from the precursor thanby simple extrapolations that are based on equilibrium solubility.

V. Conclusions

Wurtzite-based solid solutions up to 70% CoO have beensynthesized in the ZnO–CoO system. This extended solubilityrequires that cobalt ions remain divalent and that heat treat-ments are performed in the nearby presence of free cobaltmetal. Solutions of 40% CoO are stable up to a temperature of500°C, whereas partitioning to yield a cobalt-rich rock-saltsolid solution occurs at lower temperatures as the CoO contentincreases.

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Fig. 7. (a) Schematic curves of free energy versus composition forwurzite (‘‘W’’) and rock salt (‘‘R’’). (b) Phase diagram for the ZnO–CoO system ((×) temperatures and bulk compositions of hydrothermalruns and (s) microprobe results). (From Bateset al.1)

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