Influence of chemical synthesis on the crystallization and properties of zinc oxide

Materials Chemistry and Physics 77 (2002) 521–530

Influence of chemical synthesis on the crystallization and properties of zinc oxide - Dragˇcevi´c a , Miroslava Maljkovi´c a , Stanko Popovi´c b Svetozar Musi´c a,∗ , –Durdica b

a Division of Materials Chemistry, Ruder - Boškovi´c Institute, P.O. Box 180, 10002 Zagreb, Croatia Department of Physics, Faculty of Science, University of Zagreb, P.O. Box 331, 10002 Zagreb, Croatia

Received 3 July 2001; accepted 22 January 2002

Abstract ZnO powders were synthesized using (a) the decomposing urea process and (b) crystallization from the suspensions obtained by abrupt mixing of Zn(NO3 )2 and NH4 OH solutions. The samples were characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), microelectrophoresis and BET. Basic zinc carbonate was always produced in decomposing urea solution at 95 ◦ C in excess of urea and for times between 1 and 8 h. ZnO was produced by heating of basic zinc carbonate at 300–600 ◦ C. The FT-IR spectrum of ZnO produced at 600 ◦ C showed the superposition of three IR bands located at 565, 489 and 403 cm−1 . The ZnO powder prepared at 300 ◦ C showed the greatest specific surface area of 17.88 m2 g−1 . The precipitates obtained by abrupt adding of concentrated NH4 OH solution into Zn(NO3 )2 solution corresponded to a complex compound with the general formula Zn5 (OH)8 (NO3 )2 (H2 O)2−x (NH3 )x . Autoclaving of the aqueous suspension of this compound yielded ZnO. For these ZnO powders the relative intensity of the band at 498 cm−1 was substantially increased. On the other hand, the specific surface area of ZnO produced by autoclaving was found significantly decreased in relation to the urea process. Microelectrophoretic measurements with ZnO particles showed pHiep varying between 9.3 and 9.8. The zeta potentials measured for ZnO particles produced by the urea process were much higher than those measured for ZnO particles produced by autoclaving. TEM showed two different morphologies and different sizes of ZnO particles in dependence on the method of chemical synthesis. The difference in the FT-IR spectra of these two kinds of ZnO particles was ascribed to their microstructural properties. Microelectrophoretic and BET measurements were also related with the microstructure of ZnO particles. © 2002 Elsevier Science B.V. All rights reserved. Keywords: ZnO; Urea; Hydrothermal crystallization; XRD; FT-IR; TEM; Microelectrophoresis; BET

1. Introduction Zinc oxide (ZnO) has been used in various technologies as varistor, gas-sensor, catalyst, pigment, etc. These various applications of ZnO are due to the specific chemical, surface and microstructural properties of ZnO. The microstructural and physical properties of ZnO can be modified by introducing changes into the procedure of its chemical synthesis. ZnO-based ceramics are characterized by high nonlinear current–voltage properties, and this effect was utilized in varistors, i.e., devices for protection against abrupt increase of the voltage. In the varistor applications the microstructural properties of ZnO are the most important factor. The best varistor quality can be achieved by a high homogeneity of the packed particles which exhibit nonlinear current–voltage properties at their boundaries. On the other hand, for the ap∗ Corresponding author. Tel.: +385-1-4561-094; fax: +385-1-4680-084. E-mail address: [email protected] (S. Musi´c).

plication of ZnO as humidity sensor, the opposite properties of ZnO are required, and in this case, the material must possess a very open, porous microstructure with a controlled pore size. The sensitivity and response times of ZnO-based humidity sensors strongly depend on the porosity of this material. In the catalytic applications of ZnO the surface acid/base properties and the adsorption capacity are the most important functional parameters. For the application of ZnO as pigment, the morphology and particle size are essential parameters. In the past, synthesis of ZnO was mainly conducted with the only aim to change the properties of this compound to be utilized in various applications. Solid state decomposition of various zinc compounds and “wet” precipitation were mainly used in the chemical synthesis of ZnO. Spray pyrolysis is a simple method for the production of ZnO particles. For example, a ZnO film was deposited by spray pyrolysis of Zn(CH3 CO2 )2 solution on polycrystalline alumina substrates between 400 and 450 ◦ C [1]. Spray pyrolysis of Zn(CH3 CO2 )2 solution, containing small amounts

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of foreign cations such as bismuth, manganese, copper or cobalt ions, was used to prepare ZnO films doped with these cations [2]. Doping of ZnO films with various cations modifies the electrical and optical properties of the corresponding films. The thickness of ZnO films can be changed in dependence on the pH of Zn(CH3 CO2 )2 solution [3]. The precipitation of Zn2+ ions in the presence of decomposing urea at 90 ◦ C was investigated [4]. The pH and the time of heating had an important effect on the colloidal behavior of the particles in this process. Uniform particles of basic zinc carbonate were precipitated from the solution with decomposing urea [5]. Thus obtained material was used for the production of ZnO. Chou et al. [6] reported homogeneous precipitation of ZnO using decomposition of HMTA (hexamethylenetetramine). At elevated temperatures, the aqueous solutions of urea, as well as HMTA, undergo chemical decomposition generating OH− ions which increase the pH of the solution. Sonder et al. [7] used an urea process to produce ZnO-based varistors. Nitric acid solution of Zn(NO3 )2 and associated metal nitrates were denitrated with urea at temperatures up to 250 ◦ C and then the solid residue was additionally calcined at 675 ◦ C. Audebrand et al. [8] investigated the early stages of ZnO formation by thermal decomposition of Zn(II)-nitrate, -oxalate, -hydroxide, -carbonate or -acetate. The chemical nature of these precursors influenced the formation of ZnO. Takehana et al. [9] prepared a ZnO varistor doped with Co and Pr, using calcination of the mixture of their oxalates. The authors reported that the varistor thus obtained showed advantages in relation to the same type of the varistor obtained by the ball milling method. Fine ZnO powder was prepared by hydrolysis of zinc(II)-acetylacetonate in a methanol–water mixture [10]. Transmission electron microscopy (TEM) revealed a significant difference in the particle shape between ZnO powders prepared at 25 and 80 ◦ C. Hohenberger and Tomandl [11] reported the sol–gel procedure for the preparation of ZnO-based varistors. This procedure gave a more homogeneous distribution of the dopants compared to the commercial synthesis. Li et al. [12] synthesized acicular ZnO particles using the hydrothermal discharging gas operation after autoclaving the solution of Zn(CH3 CO2 )2 and NaNO2 at 190 ◦ C for 1 h. Spherical ZnO particles for the fabrication of ZnO varistors were obtained using an aqueous precipitation method [13]. Addition of butylamine to the precipitation system yielded smaller and more uniform particles than in the urea process. Nanosized ZnO particles were prepared by adding LiOH into the solution of Zn(CH3 CO2 )2 in ethanol [14]. The visible luminescence properties of ZnO particles depended on their surface properties which were, in turn, determined by the concentration ratio of Zn(CH3 CO2 )2 /LiOH. Trindade et al. [15] used selected organic ligands to inhibit Zn(OH)2 precipitation and to control the growth of ZnO particles with different morphologies. The morphological characteristics of the ZnO particles depended on the presence of organic lig-

ands, as well as on their chemical nature. Meulenkamp [16] prepared nanosized ZnO particles of 2–7 nm by adding LiOH to the ethanol solution of Zn(CH3 CO2 )2 . The main modification in relation to the original method by Spanhel and Anderson [17] consisted in the change of a washing procedure. Singhal et al. [18] reported the preparation of nanosized ZnO using the microemulsion method. This microemulsion was used to produce zinc oxalate which was further converted to nanosized ZnO particles at 300 ◦ C for 3 h. Morioka et al. [19] synthesized zinc basic acetate by the reaction of ZnO and Zn(CH3 CO2 )2 in water and, after heating zinc basic acetate at 600 ◦ C, ZnO crystals with thin-plate morphology were produced. The formation of thin ZnO films was also investigated due to their utilization in electronic or optoelectronic applications. For example, Natsume et al. [20] prepared c-axis oriented ZnO films by thermal decomposition of zinc(II)-acetylacetonate vapor in O2 atmosphere. Fine grained ZnO films were deposited on silica substrates using r.f. sputtering, pulsed laser deposition, and “wet” precipitation methods [21]. The solution deposited films exhibited a smaller degree of ordering and showed chemical reactivity with silica substrates thus producing Zn2 SiO4 when heated at 1000 ◦ C. In the present work, we focused on the synthesis of ZnO using (a) the decomposing urea process and (b) crystallization from the suspensions obtained by abrupt mixing of Zn(NO3 )2 and NH4 OH solutions. The aim of the present work was to compare the crystallization and properties of ZnO in dependence on the type of chemical synthesis. 2. Experimental Chemicals of analytical purity and double distilled water were used. Experimental conditions for the preparation of samples from an aqueous solution of Zn(II)-nitrate with decomposing urea are given in Table 1. The precipitates were subsequently washed with double distilled water using a Sorvall RC2-B ultraspeed centrifuge (maximum 20,000 rpm). The washed precipitates were dried at 60 ◦ C. Table 1 Experimental conditions for the preparation of the samples from an aqueous solution of Zn(II)-nitrate with decomposing urea Sample

Zn(NO3 )2 (M)

Urea (M)

Time of heating (h)

Temperature of oil bath (◦ C)

Final pH

Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Z10 Z11

0.001 0.001 0.01 0.01 0.01 0.01 0.01 0.1 0.1 0.1 0.1

0.1 0.1 0.05 0.05 0.1 0.1 0.1 0.5 1.0 1.0 1.0

6 6 2 8 1 4 8 8 1 4 8

95 110 95 95 95 95 95 95 95 95 95

8.82 9.31 5.50 5.70 5.80 5.80 7.95 6.01 5.96 6.15 7.35

S. Musi´c et al. / Materials Chemistry and Physics 77 (2002) 521–530 Table 2 Experimental conditions for the preparation of the samples in the system Zn(NO3 )2 + NH4 OH Sample

Time of ageing

Temperature of heating (◦ C)

Final pH

Number of washings

Z12 Z13 Z14 Z15 Z16 Z17 Z18 Z19 Z20 Z21 Z22 Z23

6h 3 days 7 days 21 days 15 min 40 min 45 min 50 min 1h 20 min 2h 1 day

20 20 20 20 120 120 120 120 120 160 160 160

8.63 8.63 8.58 8.55 8.66 8.64 8.52 8.67 8.57 8.45 8.55 8.60

6 7 7 7 7 7 7 7 7 7 5 6

Sample Z8 was additionally heated at 300 or 600 ◦ C for 5 h. Commercial zinc carbonate hydroxide with declared approximate composition 5ZnO·2CO3 ·4H2 O supplied by Ventron Alfa Produkte, Karlsruhe, was used as reference material. Experimental conditions for autoclaving the samples in the system Zn(NO3 )2 + NH4 OH are given in Table 2. Previously, 40 ml 1 M Zn(NO)3 and 10 ml 25% aq. NH3 solution were abruptly mixed and then strongly shaken. After a proper time of crystallization the precipitates were washed with double distilled water and then dried to remove the absorbed water. Ultrapure ZnO (99.999%) supplied by Ventron Alfa Produkte, Karlsruhe, and ZnO p.a. supplied by Kemika, Zagreb, were used as the reference material. pH measurements were carried out with a Model pHM-26 Radiometer pH meter. A combined glass electrode with an operating range up to pH ∼14, manufactured by Radiometer, was used. X-ray powder diffraction (XRD) patterns were taken at room temperature (RT) using a Model MPD1880 automatic Philips diffractometer (Cu K␣ radiation, graphite monochromator, proportional counter). Fourier transform infrared (FT-IR) spectra were recorded at RT using a Model 2000 Perkin-Elmer spectrometer. The FT-IR spectrometer was coupled with a personal computer and operated with the IRDM (IR Data Manager) program. The specimens were pressed into KBr pellets. The (BET) measurements were performed using a FlowSorb II 2300 surface area analyzer (Micromeritics, Norcross, GA). Precise electrophoretic measurements of the selected samples were carried out as a function of pH, using a PenKem S3000 automated instrument for microelectrophoresis (PenKem, Bedford Hills, NY). The measurements were performed by dispersing about 50 mg of a sample into 100 ml 10−3 M KNO3 solution using an ultrasound bath. The pH value of the suspension was regulated by the addition of 0.1 M KOH or HNO3 solution. Two independent measurements were made for each sample.

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TEM was carried out with an electron microscope manufactured by Opton, Model EM-10. For TEM characterization, a small amount of powder was dispersed in doubly distilled water using an ultrasound bath. A drop of thus prepared dispersion was dropped onto a copper grid previously coated with a thin film of organic polymer.

3. Results and discussion The precipitate, denoted as Z8, formed from an aqueous solution of Zn(NO3 )2 in the presence of urea, was analyzed with XRD and this measurement showed the formation of basic zinc carbonate (Zn5 (OH)6 (CO3 )2 ). The precipitate was subjected to thermal treatment at 300, 400 and 600 ◦ C, and after treatment at all these temperatures the formation of ZnO was observed. Fig. 1 shows characteristic parts of the XRD patterns of ZnO produced between 300 and 600 ◦ C. The characteristic part of the XRD pattern of ultrapure ZnO was also recorded for comparison. A decrease of the line broadening, i.e., an increase of crystallite size with increase of the temperature was observed. The crystallite sizes were estimated using the Scherrer formula D=

0.9λ β cos θ

(1)

Fig. 1. Characteristic parts of the XRD patterns of ultrapure ZnO and thermal decomposition products of sample Z8 at 300, 400 and 600 ◦ C. The measurements are made at RT.

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data are based on the literature sources [22,23]. The effects of chemical bonding and of the crystal structure on the positions, splitting and relative intensities of IR bands of the carbonate group are well documented [24–27]. The FT-IR spectrum of sample Z8 clearly indicates the presence of a carbonate group in this, and in other precipitates obtained by the urea process. The splitted ν 3 frequency is well visible, as well as ν 1 at 1047 cm−1 , ν 2 at 831 and ν 4 at 708 cm−1 . On the basis of the FT-IR spectra, for all precipitates obtained by the urea process, the formation of additional ZnO phase could not be confirmed in the present work. Boccuzzi et al. [28] used IR spectroscopy to investigate the surface behavior of the well-crystallized ZnO, produced

Fig. 2. FT-IR spectra of sample Z8 and its thermal decomposition products at 300 and 600 ◦ C. The spectra are recorded at RT.

where λ is the X-ray wavelength, θ the Bragg’s angle and β the pure full width of the diffraction line at half of the maximum intensity. For ZnO powders produced at 300, 400 and 600 ◦ C the corresponding crystallite sizes of D = 20(3), 26(3) and 46(6) nm were measured. The diffraction lines recorded for commercial sample 5ZnO·2CO3 ·4H2 O were broader than for sample Z8. The FT-IR spectra of sample Z8 and its decomposing products at 300 and 600 ◦ C are shown in Fig. 2. The main features of the FT-IR spectrum of sample Z8 are in accord with those recorded for commercial 5ZnO·2CO3 ·4H2 O (declared composition by Ventron Alfa Produkte). For comparison, the characteristic positions of IR bands of carbonate groups in CaCO3 polymorphs are shown in Table 3. These

Fig. 3. FT-IR spectra of: (a) ultrapure ZnO (99.999%) supplied by Ventron Alfa Produkte and (b) ZnO (p.a.) supplied by Kemika. The spectra are recorded at RT.

Table 3 Characteristic positions of the IR bands of carbonate groups in different polymorphs of CaCO3 (data are based on literature sources [22,23]) Aragonite [22]

Vaterite [23]

IR position (transverse)

IR position (longitudinal)

Mode

1085 1083 853 1466 1443 713 699

1085 1083 877 1588 1561 716 701

ν 1 (B1u ) ν 1 (B3u ) ν 2 (B3u ) ν 3 (B1u ) ν 3 (B2u ) ν 4 (B1u ) ν 4 (B2u )

IR position

Calcite [23] Mode

IR position

Mode

1089

ν1

ν1

877 ∼1450

ν2 ν3

877 1419

ν2 ν3

744

ν4

713

ν4

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by thermal decomposition of ZnCO3 in air, with pyridine as an adsorbate. The IR measurements showed that the range from 600 to 700 cm−1 was specifically sensitive to surface reactions. Also, Boccuzzi et al. [29] investigated the influence of the temperature and atmosphere, during the heating of ZnO, on the corresponding IR spectrum. Hayashi et al. [30] recorded IR transmission spectra of ZnO and compared them with the calculated spectra. The authors found that ZnO showed three distinct absorption bands located between bulk TO-phonon frequency (ωT ) and LO-phonon frequency (ωL⊥ ). In addition, these absorption peaks shifted towards lower frequencies when the permittivity of the surrounding medium was increased.

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The FT-IR spectra of sample Z8, after thermal treatment between 300 and 600 ◦ C also show the superposition of three bands located at 565, 489 and 403 cm−1 . However, these IR bands are not well resolved. Fig. 3 shows the FT-IR spectra of (a) commercial ultrapure ZnO (99.999%) supplied by Ventron Alfa Produkte and (b) ZnO (p.a.) supplied by Kemika and these spectra also show the superposition of three bands, as it is observed for the thermal decomposition products of sample Z8. Sigoli et al. [31] reported two IR bands at 565 and 420 cm−1 for spherical ZnO particles, as well as the splitting of IR band at 420 cm−1 into three peaks located at 425, 410 and 387 cm−1 for agglomerates of acicular ZnO particles. In the present work, we did not

Fig. 4. TEM photographs of: (a) sample Z8 and (b) after heating at 600 ◦ C.

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observe in any of the samples the splitting of the IR band at 420 cm−1 , as reported in that work [31]. Andrés-Vergés and Serna [32] showed that the IR spectrum of ZnO particles varied from a broad single band, over the doublet up to the three bands superposition. Also, Andrés-Vergés et al. [33] considered the IR spectrum of ZnO particles as a function of various geometrical shapes and suggested the origin of IR band at 494 cm−1 . Wiedemann et al. [34] found that the kinetics of the thermal decomposition of Zn5 (OH)6 (CO3 )2 is dependent on the experimental atmosphere (moist air, dry air, high vacuum). Electron microscopy and electron diffraction were used [35] to monitor the thermal decomposition of basic zinc carbonate, zinc oxalate, zinc formate and zinc acetate. Porosity, specific surface area and crystallite size of ZnO depended on the morphology and chemical nature of these ZnO precursors. In the case of ZnO produced from basic zinc carbonate and zinc oxalate the grains showed dimensions of the order of 10 or 25 nm, respectively. Fig. 4 shows TEM photographs of: (a) sample Z8 and (b) after heating at 600 ◦ C. Sample Z8 consists of the agglomerates of small needles corresponding to basic zinc carbonate. The particles of irregular shapes are also visible and they are probably due to the presence of an amorphous fraction. ZnO produced at 600 ◦ C consisted of spherical

Fig. 5. Characteristic parts of XRD patterns of samples Z17, Z18 and Z19 recorded at RT.

Fig. 6. FT-IR spectra of samples Z16–Z20 recorded at RT.

particles which showed tendency to form the aggregates. More precise inspection of these spherical particles showed that they are actually hexahedrons of irregular shape. The TEM photographs of the commercial ZnO samples showed the same type of the microstructure as sample Z8. Crystallization of ZnO from the precipitates obtained by an abrupt addition of concentrated NH4 OH solution into Zn(NO3 )2 solution differed in relation to the urea process. For the given experimental conditions in Table 2, a complex Zn(II)-compound precipitated at RT and with further

Fig. 7. FT-IR spectra of samples Z21, Z22 and Z23 recorded at RT.

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increase of the time of ageing it slowly transformed to ZnO. After 6 h of ageing at RT traces of ZnO in the precipitate were detected with XRD, whereas after 21 days of ageing the precipitate contained ∼1–2 mol% of ZnO. The rate of ZnO crystallization increased with increase of the temperature. Autoclaving of an aqueous suspension of complex Zn(II)-compound at 120 ◦ C for 15 min to 1 h showed the crystallization of pure ZnO phase after 50 min. Fig. 5 shows characteristic parts of the XRD patterns of samples Z17, Z18 and Z19. Samples Z17 and Z18 contained a complex Zn(II)-compound whose positions and intensities of diffraction lines are in good agreement with the data for Zn5 (OH)8 (NO3 )2 ·1.3NH3 ·0.7H2 O [36] or with the data for

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Zn5 (OH)8 (NO3 )2 ·2NH3 [37]. The authors of these works [36,37] reported that these compounds are isostructural having similar unit-cell parameters. The stoichiometry of the mentioned Zn(II)-compound varied in dependence on the experimental conditions. The FT-IR spectra of samples Z16–Z20 are shown in Fig. 6. In the spectrum of sample Z18 the transitional changes are visible, e.g., the appearance of IR band at 501 cm−1 . The FT-IR spectra of samples Z19 and Z20 correspond to ZnO. The relative intensity of the band at 498 cm−1 is significantly increased in relation to the band of the same origin as shown in Fig. 2. Autoclaving of an aqueous suspension of complex Zn(II)-compound at 160 ◦ C

Fig. 8. TEM photographs of samples: (a) Z17 and (b) Z23.

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for 20 min to 1 day yielded pure ZnO having rather sharp diffraction lines. Fig. 7 shows the FT-IR spectra of samples Z21–Z23 produced at 160 ◦ C. These spectra correspond to ZnO. The TEM photographs of samples (a) Z17 and (b) Z23 are shown in Fig. 8. All ZnO particles which crystallized from a complex Zn(II)-compound showed the morphology typical for sample Z23. The present work showed that spectral difference of ZnO particles can be related with two different experimental routes which produced two different morphologies of ZnO particles. Bénard et al. [38] showed that the substitution of H2 O by NH3 molecules in the basic zinc nitrate, Zn5 (OH)8 (NO3 )2 · 2H2 O, could be partial or complete. This substitution influenced the thermal behavior of these Zn(II)-compounds and consequently, different decomposition products were found in dependence on the stoichiometry. However, in the present experiments with autoclaving of an aqueous suspension of complex Zn(II)-compound the crystallization of ZnO is probably solution mediated and in this sense the process is different from the thermal decomposition of complex Zn(II)-compounds as described by Bénard et al. [38]. As mentioned in Section 1, the surface properties of ZnO particles are very important for many applications of this material. Therefore, we also focused on the surface characterization of ZnO particles synthesized in the present work. The zeta potential was calculated from the electrophoretic mobility, ue , by using Henry’s equation [39] ue =

2εζ f1 (Kα) 3η

(2)

where ε is the permittivity of the medium, η the viscosity, ζ the electrokinetic (zeta) potential, α the particle radius, K the reciprocal double layer thickness and f1 (Kα) is a function dependent on particle size and shape. We assumed a high ratio between the particle radius and double layer thickness, giving Kα 1 and f1 (Kα) = 3/2. In this case, the relation between the microelectrophoretic mobility (m2 V−1 s−1 ) and the zeta potential (mV) is ζ = 12.8 × 108 ue

Fig. 9. Zeta potentials measured for ultrapure ZnO, and the thermal decomposition products of sample Z8 at 300 and 600 ◦ C as a function of pH.

Fig. 10. Zeta potential measured for sample Z19 as a function of pH.

(3)

The results of microelectrophoretic measurements with selected samples are shown in Figs. 9–11. Generally, the zeta potentials of ZnO particles obtained by heating sample Z8 at 300 and 600 ◦ C are higher than that for the ZnO particles produced by the hydrothermal process. The dependence of zeta potential on pH was very similar to that of the commercial ultrapure ZnO and of the sample produced by heating sample Z8 at 600 ◦ C. pHiep of the sample produced by heating sample Z8 at 300 ◦ C was shifted from 9.3 to 9.8 as shown in Table 4. The greatest specific surface area (17.88 m2 g−1 ) was measured for ZnO particles produced at 300 ◦ C, whereas this value decreased to 10.39 m2 g−1 for the particles produced at 600 ◦ C which can be explained by the particle increase and sintering effect. Commercial ultrapure ZnO showed a specific surface area of 7.47 m2 g−1 . ZnO particles obtained by the hydrothermal procedure showed

Fig. 11. Zeta potentials measured for samples Z21, Z22 and Z23 as a function of pH.

S. Musi´c et al. / Materials Chemistry and Physics 77 (2002) 521–530 Table 4 pHiep and surface area by BET for selected samples Sample

pHiep

Surface area by BET (m2 g−1 )

Ultrapure ZnO Z8 heated at 300 ◦ C Z8 heated at 600 ◦ C Z19 Z21 Z22 Z23

9.3 9.8 9.3 9.4 9.5 9.3 9.4

7.47 17.88 10.39 4.47 1.63 1.94 1.13

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• TEM showed two different morphologies and different sizes of ZnO particles in dependence on two experimental routes applied in the present work. The differences in the FT-IR spectra of these two kinds of ZnO particles can be related with their different morphologies. The results of microelectrophoretic and BET measurements can be also related with the microstructure of ZnO particles.

Acknowledgements The authors wish to thank Dr. Neda Vdovi´c and Prof. Nikola Ljubeši´c for their help in the experimental work.

much smaller adsorption capacity, as can be concluded on the basis of zeta potential measurements. These samples also showed a smaller surface area compared to that of the ZnO powder produced by the urea process. The specific surface area rapidly decreased with increase of the temperature and the time of autoclaving. The results of microelectrophoretic and BET measurements are in accordance with the TEM observation.

4. Conclusions • The crystallization and properties of ZnO strongly depended on the type of its chemical synthesis. In the present work, two types of the chemical synthesis of ZnO were used: (a) the decomposing urea process and (b) crystallization from the suspensions obtained by abrupt mixing of Zn(NO3 )2 and NH4 OH solutions. • Basic zinc carbonate was found in all the samples precipitated in decomposing urea, in excess of urea. ZnO was produced by heating basic zinc carbonate at ≥300 ◦ C. The FT-IR spectrum of ZnO produced at 600 ◦ C showed the superposition of three IR bands located at 565, 489 and 403 cm−1 , whereas the ZnO produced at 300 ◦ C showed the greatest specific surface area by BET (17.88 m2 g−1 ). • The precipitates obtained by abrupt adding of concentrated NH4 OH solution into Zn(NO3 )2 solution corresponded to the complex compound with the general formula Zn5 (OH)8 (NO3 )2 (H2 O)2−x (NH3 )x as found by XRD. Autoclaving of this compound yielded ZnO, and the rate of conversion increased with increase of the heating temperature. For these ZnO samples, the FT-IR spectra were different in relation to the previous one, because the relative intensity of the band at 489 cm−1 was significantly increased. Also, the specific surface areas of these ZnO samples were significantly decreased in relation to the urea process. • The pHiep values of the investigated ZnO particles ranged from 9.3 to 9.8. The zeta potentials measured for ZnO particles, obtained by the urea process, indicated a much greater adsorption capacity of these particles than of those obtained by hydrothermal crystallization.

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