Effect of calcination temperature on the

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Effect of calcination temperature on the physicochemical properties of zinc oxide nanoparticles synthesized by coprecipitation To cite this article: Khairul Basyar Baharudin et al 2018 Mater. Res. Express 5 125018

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Mater. Res. Express 5 (2018) 125018

https://doi.org/10.1088/2053-1591/aae243

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15 July 2018 REVISED

10 September 2018

Effect of calcination temperature on the physicochemical properties of zinc oxide nanoparticles synthesized by coprecipitation

ACCEPTED FOR PUBLICATION

Khairul Basyar Baharudin1,2, Nurulhuda Abdullah3 and Darfizzi Derawi1

18 September 2018

1

PUBLISHED

26 September 2018

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Laboratory for Biolubricant, Biofuels and Bioenergy Research, Centre for Advanced Materials and Renewable Resources, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Faculty Of Biotechnology and Biomolecular Science, University Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Technology & Engineering Division, Malaysian Rubber Board, 47000, Sungai Buloh, Malaysia

E-mail: [email protected] and darfi[email protected] Keywords: zinc oxalate, coprecipitation, calcination temperature, ZnO nanoparticles

Abstract In this study, an intermediate zinc oxalate phase was synthesized by the coprecipitation of an oxalate from a nearly saturated solution of zinc acetate and a 2-propanol solution of oxalic acid. A highly crystalline pure phase of ZnO nanoparticles was successfully obtained after calcination at 400 °C– 600 °C for 4 h. A unimodal, narrow size distribution of ZnO nanoparticles with average particle sizes of 3, 28, and 25 nm was observed at 400 °C, 500 °C, and 600 °C, respectively. Moreover, field-emission scanning electron microscopy and transmission electron microscopy images revealed different surface structures and morphologies for ZnO nanoparticles obtained at different calcination temperatures. With the increase in the calcination temperature from 400 °C to 600 °C, the specific surface area of ZnO nanoparticles significantly decreased from 22.9 to 2.6 m2 g−1.

1. Introduction Zinc oxide (ZnO) nanoparticles constitute one of the important inorganic materials that have been widely applied in chemical and materials sciences. ZnO exhibits a non-centro symmetric wurtzite structure with a polar surface, with a wide band gap of 3.37 eV and a large excitation binding energy of ∼60 meV, rendering it a superior II–IV semiconductor material [1–3]. The physicochemical properties of ZnO nanoparticles are controlled by the synthesis method and parameters associated with the processing conditions. To obtain ZnO nanoparticles with tailormade chemical, optical, chemical properties, and catalytic activity suitable for specific applications, their particle size and morphology, as well as high purity, must be controlled [3–5]. The large surface area-to-volume ratio in several nanomaterials is one of the interesting properties that can open new avenues in surface-based science applications such as catalysis. Numerous techniques and methods have been developed by the researcher in order to synthesize the ZnO nanoparticles system with the best performance such as coprecipitation [6], chemical bath deposition [7], electrochemical techniques [8], microwave assisted [9], hydrothermal [10], solvothermal [11] and etc. Among these methods, coprecipitation carried out at a moderately low temperature is a simple, potentially scalable method, with advantages of large-scale production, flexible process, less time consumption, and low cost production [6, 12, 13]. Thus, coprecipitation only requires a simple equipment setup and use of inexpensive precursor material. Coprecipitation involves the simultaneous precipitation of metal ions in the expected stoichiometric proportion, followed by centrifugation, washing, and drying. The resultant material is then calcined at a certain temperature to obtain the pure final product. Recent studies have reported that nanopowders obtained by this technique are highly crystalline, homogenous materials with a small particle size [14–17]. In this study, ZnO nanoparticles were prepared by coprecipitation. Economical oxalic acid was used as the precipitation agent for this low temperature solution route because of the facile decomposition of the solid solution formed by the oxalates [18–20]. The reaction between zinc acetate and oxalic acid produces an © 2018 IOP Publishing Ltd


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intermediate metal oxalate phase. The decomposition of this intermediate phase affords a highly crystalline hexagonal wurtzite structure of pure ZnO nanoparticles. Furthermore, the effect of different calcination temperatures on the physicochemical properties of the resulting ZnO materials was discussed.

2. Methodology 2.1. Synthesis of ZnO nanoparticles In this study, analytical-grade reagents were used without further purification. First, zinc acetate (ZnAc) was dissolved in ethanol to form a 0.1 M solution (solution A). Second, oxalic acid was dissolved in a 1.5:1 ratio of 2-propanol: water, affording a 0.5 M solution (solution B). Next, both solutions were soaked in an ice bath for several minutes. Under ice bath conditions, solution B was added into solution A. The as-formed uniform, stable milky suspension was stirred for 30 min and sonicated for 15 min before centrifugation. The resulting precipitate was washed several times with ethanol and dried in an electrical oven at 70 °C for 18 h. The fine white powder obtained after grinding was then subjected to calcination in a muffle furnace at ramp rate of 5 °C min−1 to the desired temperatures (300 °C, 400 °C, 500 °C, and 600 °C) and maintained for 4 h. 2.2. Characterization of ZnO nanoparticles XRD diffraction patterns of the prepared sample were recorded on a Shimadzu XRD-6000 powder diffractometer at CuKα (λ=1.54 Å), operating at 40 kV and 20 mA for phase confirmation. The particle size distribution of the resulting material was analyzed on a Sympatec Nanophox analyzer (Germany). Essentially, photon cross-correlation spectroscopy (PCCS) was used to measure the particle size in the nanometer (nm) size regime. ZnO nanoparticles were dispersed in an ethanol solution by ultrasonication before performing the particle size distribution measurements of the ZnO suspension. Thermogravimetric analysis (TGA) and differential thermal gravimetric (DTG) analysis curves were recorded on a Mettler-Toledo TGA thermal analyzer (Star System). The thermal analysis of the sample was carried out in an alumina pan up to 1000 °C at a heating rate of 10 °C under flowing nitrogen at a flow rate of 50 ml min−1. Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer 1725X FTIR spectrophotometer using the KBr pellet technique. For investigating the surface properties of the resulting material, surface area and pore size were estimated on a model BELSORP mini II (BEL Japan) by nitrogen gas adsorption–desorption measurements conducted at 77 K. Furthermore, field-emission scanning electron microscopy (FESEM; JEOL model JSM 6700F) images were recorded to observe the particle surface morphology. Moreover, transmission electron microscopy (TEM; Hitachi model H-7100) images were recorded to observe the morphology of the ZnO nanoparticle powder. For TEM observations, dry powder was sonicated in ethanol for several minutes, followed by dipping the copper grid in the ethanolic powder solution and allowing it to dry at room temperature before TEM imaging.

3. Results and discussion 3.1. Structural analysis A crystalline solid with an intermediate phase of zinc oxalate dihydrate was directly obtained by coprecipitation. The powder x-ray diffraction (PXRD) patterns (figure 1) of as-synthesized α-ZnC2O4·2H2O showed peaks indexed to the monoclinic phase (JCPDS No 24-1029). The decomposition of zinc oxalate dihydrate was facilitated by calcination at 300 °C–600 °C. Anhydrous zinc oxalate, Zn(C2O4), was formed at 300 °C, which later formed well-crystallized ZnO nanoparticles after decomposition at 400 °C. The diffraction patterns of the resultant ZnO nanoparticle sample completely matched those of ZnO with a hexagonal wurtzite structure (JCPDS No 01-075-0576). With the increase in the calcination temperature to 500 °C and 600 °C, ZnO nanoparticles with better crystallinity were apparently formed, with similar PXRD patterns, albeit with a higher intensity and a narrower peak compared with the ZnO nanoparticles formed at 400 °C. The broader peak observed in the PXRD patterns of ZnO formed at 400 °C revealed a smaller size for the ZnO particles compared with those obtained by calcination at 500 °C and 600 °C. Other peaks corresponding to the unknown phase were not detected within the detection limit of x-ray diffraction at 400 °C, 500 °C, and 600 °C, indicative of complete decomposition. Hence, the preparation of a relatively pure phase of ZnO nanoparticles is feasible using the technique. In this study, coprecipitation was carried out in an ice bath to minimize the loss of cations during centrifugation because at low temperatures (0 °C–2 °C), the solubility of metal oxalates decreases. Typically, metal oxalate is insoluble in pure alcohol, but its solubility in a mixed solvent of alcohol–water is expected to be lower than that in pure water. 2-Propanol, which exhibits a low dipole moment, is expected to provide low solubility for oxalate. This lower solubility renders advantages in terms of preserving the initial stoichiometry, thereby optimizing the reaction between the saturated solution of acetate and 2-propanol solution of oxalic acid 2


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Figure 1. PXRD patterns of ZnO synthesized by coprecipitation via the direct calcination of zinc oxalate dihydrate at 300 °C–600 °C.

Figure 2. TGA–DTG thermogram of the zinc oxalate dihydrate precursor.

[21–23]. The high-temperature decomposition of zinc oxalates releases H2O and CO2 and effectively produces ZnO molecules. Furthermore, sufficient heating is needed to induce the formation of well-crystallized ZnO nanoparticles [24]. 3.2. Thermal studies The TGA–DTG thermogram as a function of temperature (figure 2) revealed two major weight losses for the zinc oxalate dihydrate precursor, α-ZnC2O4·2H2O. The first weight loss corresponding to the loss of two water molecules was observed at ∼150 °C, indicating the conversion of zinc oxalate dihydrate into anhydrous zinc oxalate (Zn(C2O4)). The conversion of Zn(C2O4) to ZnO nanoparticles was thought to occur at ∼390 °C, which was reflected by the second weight loss observed at this temperature. The conversion of Zn(C2O4) to ZnO was clearly detected by the PXRD patterns shown in figure 1. Further decomposition was not observed at above 425 °C as additional weight loss was not noted beyond this temperature, clearly verifying that this thermally stable residue corresponds to highly purified ZnO nanoparticles. Further characterization by FTIR analysis was conducted to verify this result. 3.3. FTIR spectroscopy An absorption band was observed at ∼400 cm−1 in the FTIR spectrum of the resulting ZnO nanoparticles, verifying the presence of ZnO (figure 3). Sharp bands corresponding to the carboxylate species were observed at ∼1631 and 1326 cm−1, indicative of the formation of zinc oxalate dihydrate. The formation of highly pure ZnO 3


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Figure 3. FTIR spectra of ZnO synthesized by coprecipitation by the calcination of zinc oxalate dihydrate at 300 °C–600 °C for 4 h.

nanoparticles was verified by the presence of the ZnO peak and the absence of other stretching peak above 500 cm−1, peaks mainly corresponding to acetate and oxalates for the sample prepared at a temperature of greater than 400 °C. 3.4. Particle size distribution Figure 4 shows the particle size distributions of the synthesized ZnO nanoparticles. The effect of the calcination temperature on the particle size distribution was analyzed by PCCS as the measured particle sizes were not specific to primary particles, but the sizes can be extended to secondary particles or even tertiary particles. Results revealed unimodal, narrow size distributions of ZnO nanoparticles with an average particle size of 3 nm upon heating at 400 °C. With increasing temperature, the average particle size of ZnO progressively increased clearly, especially for the sample heated at 500 °C (average size of 28 nm) and for that heated at 600 °C (average size of 25 nm). Effects of broadening on the particle size distribution of ZnO nanoparticles are proportional to the increase of the calcination temperature. Hence, inferring the role of the calcination temperature leads to less uniform ZnO nanoparticles, affecting the broadness of the particle size distribution. 3.5. Particle morphology A good correlation between the particle size distribution was observed and confirmed by FESEM. ZnO nanoparticles comprised primary nanoparticles with diameters of 1–3 nm. The compact structures of monodisperse spherical particles that agglomerate were clearly observed (figure 5). These spherical structures comprised primary particles of an extremely small size, aggregating to form secondary or even tertiary particles. 4


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Figure 4. Particle size distributions of ZnO synthesized by coprecipitation followed by calcination of the zinc oxalate dihydrate at 400 °C, 500 °C, and 600 °C for 4 h.

The use of 2-propanol as an organic solvent in this technique is thought to play a crucial role in controlling the nucleation and crystal orientation, leading to a small particle size and spherical morphology of as-prepared ZnO nanoparticles [21, 25]. Aggregation between particles during calcination leads to the reduction of the free surface with the secondary elimination of the grain boundary area via grain growth, thereby disturbing and decreasing the crystal surface energy. With increasing calcination temperature, the pore volume decreases, leading to compact shrinkage [26]. In addition, further analysis revealed the effect of the aggregation of isotropic particles on the final shape of the formed ZnO particles. Dispersive forces and electrostatic interparticle attraction are the main factors controlling aggregation. Isotropic aggregation typically leads to the formation of spherical particles. Such aggregation occurs in the isoelectric point region [27]. The above-mentioned facts can be verified by the fact that irregular particles correspond to the presence of the amorphous fraction at 600 °C, and the particles slightly merge and form large secondary or tertiary particles. Secondary or tertiary spherical particles at 400 °C and 500 °C adhered to each other in random orientation, related to weak van der Waals forces. Notably, TEM analysis also supported the result shown in figure 6. Agglomeration in the form of aggregated, compact structures of spherical particles was further verified by the TEM results. On the other hand, this result corresponded to the poor correlation between the TEM estimated sizes and particle size distribution analysis. 5


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Figure 5. FESEM micrographs of ZnO nanoparticles synthesized by coprecipitation by the calcination of zinc oxalate dihydrate at 400 °C (a), 500 °C (b), and 600 °C (c) for 4 h.

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Figure 6. TEM image of ZnO nanoparticles synthesized by coprecipitation by the calcination of zinc oxalate dihydrate at 400 °C (a), 500 °C (b), and 600 °C (c) for 4 h.

The occurrence of large particles in TEM images was related to the aggregation of small or primary particles, affording large compact secondary or even tertiary agglomerated particles as verified by FESEM analysis.

3.6. Surface area analysis From the BET method, the specific surface area of ZnO nanoparticles synthesized at 400 °C was 22.9 m2 g−1 compared with 10.7 and 2.6 m2 g−1 for ZnO samples calcined at 500 °C and 600 °C, respectively. The increased 7


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Figure 7. BJH pore size distribution for ZnO synthesized by coprecipitation followed by calcination at (a) 400 °C, (b) 500 °C, and (c) 600 °C for 4 h.

particle size due to the crystallization and collapse of the mesoporous structure resulting from the agglomeration and aggregation of ZnO particles are the main factors leading to the reduction of the specific surface area. The BJH desorption branch clearly revealed a trimodal pore size distribution for the ZnO nanoparticles calcined at 400 °C (figure 7). The first broad distribution was observed at ∼3 nm, with the second broad distribution at ∼15 nm. Both these distributions corresponded to the formation of the interparticle pores caused by the agglomeration of primary ZnO particles. The third broad distribution was observed at ∼30 nm and extended to 100 nm, and the largest pores corresponded to the interparticle pores of secondary particles or the tertiary agglomeration of spherical particles of ZnO. Moreover, the third broad pore distribution was observed for ZnO nanoparticles synthesized at 500 °C, albeit at a lower pore volume. The BJH desorption branch for the pore size distribution of ZnO nanoparticles synthesized at 600 °C revealed two modes of pore size distribution, with an extremely small pore volume at ∼40 and 60 nm. This BJH pore size distribution pattern clearly verified the collapse of the mesoporous structure with increasing calcination temperature. Figure 8 shows the nitrogen adsorption–desorption isotherms of ZnO nanoparticles. According to the BET classification, ZnO nanoparticles exhibited Type IV isotherms, indicative of mesoporous and macroporous materials, with pore openings of greater than 2 and 50 nm, respectively [28]. ZnO nanoparticles synthesized at 400 °C revealed the adsorption of the adsorbent, reaching an optimum value at ∼120 cm3 g−1 at STP. With increasing calcination temperature, the adsorbate uptake was affected, with the optimum uptake drastically decreasing to 25 and 5 cm3 g−1 at STP for calcination at 500 °C and 600 °C, respectively. The reduction trend in the adsorbed volume of the adsorption–desorption isotherms at different calcination temperatures indicated a low uptake of nitrogen with increasing calcination temperature, leading to the decrease in the pore volume due to the aggregation of ZnO nanoparticles. Figure 8 also shows the H3 hysteresis loops of the adsorption– desorption isotherms. With increasing calcination temperature, a narrow hysteresis loop was observed, indicative of slightly different pore sizes and texture of the resulting material, as confirmed by the BJH pore size distribution shown in figure 6. 8


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Figure 8. Nitrogen adsorption–desorption isotherms of ZnO synthesized by coprecipitation by the calcination of zinc oxalate dihydrate at 400 °C, 500 °C, and 600 °C for 4 h.

4. Conclusion ZnO nanoparticles were obtained by the decomposition of a precursor, zinc oxalate dihydrate, which was prepared by coprecipitation. Highly crystalline pure phase ZnO nanoparticles were prepared by decomposition at 400 °C. With the increase in the calcination temperature from 400 °C to 500 °C and 600 °C, the average 9


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particle size increased, and the specific surface area decreased because of the compact agglomeration of ZnO nanoparticles.

Acknowledgments The authors are grateful for financial support through the Newton-Ungku Omar Fund, NA150407 provided by the Royal Society, UK (60888) and Academy of Science Malaysia/MIGHT (ST-2016–006). This work was also supported by the Universiti Kebangsaan Malaysia through the Collaborative Matching Fund (DPK-2017-011). We gratefully acknowledge the research facilities provided by Aston University, Universiti Kebangsaan Malaysia and Universiti Putra Malaysia. We would also thank to the Ministry of Education for the PhD Scholarship (Hadiah Latihan Persekutuan) provided.

ORCID iDs Darfizzi Derawi

https://orcid.org/0000-0003-4967-8122

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