Zirconia Nanostructures - Wiley Online Library

Int. J. Appl. Ceram. Technol., 13 [1] 108–115 (2016) DOI:10.1111/ijac.12393

Zirconia Nanostructures: Novel Facile Surfactant-Free Preparation and Characterization Sahar Zinatloo-Ajabshir and Masoud Salavati-Niasari* Institute of Nano Science and Nano Technology, University of Kashan, P. O. Box. 87317-51167, Kashan, Iran

Zirconia nanostructures have been prepared via a facile precipitation route using Zirconium (IV) oxynitrate hydrate and tetraethylenepentamine (TEPA). Here, TEPA was used as novel precipitator to fabricate zirconia nanostructures. The influence of reaction time, dosage of TEPA, and solvent was also examined to control the shape and particle size. Results of this work indicate that these reaction parameters have important impact on the control of shape and grain size of the zirconia. To characterize the as-synthesized nanostructures, techniques such as X-ray diffraction (XRD), ultraviolet–visible (UV–vis) spectroscopy, energy dispersive X-ray microanalysis (EDX), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and photoluminescence (PL) spectroscopy were applied. In addition, the formation mechanism of zirconia nanostructures was discussed.

Introduction The nanomaterials have attracted extraordinary research interest due to their unique and advantageous applications in different fields.1,2 One of the nanometerscale oxides is zirconium dioxide. It is reported that zirconia is an important metal oxide because of its unique electrical and optical characteristics3–5 and also potential usages, including preparation of the transparent optical devices and production of the electrochemical capacitor electrodes,6 fabrication of fuel cells,7 and catalysts.8,9 Zirconia has monoclinic, tetragonal, and cubic crystal forms that are stable at below 1175°C, 1175–2370°C, and 2370–2680°C temperature ranges, respectively.10 So far, various routes have been presented to prepare zirconium dioxide, such as hydrothermal,11 microwave irradiation,12 sol–gel,13 thermal decomposition,14 precipitation,15,16 and sonochemical.17,18 The development of a simple and reproducible approach for preparing nanostructured zirconia is of great importance to the potential investigations of its properties. It has been shown that the particle size and shape of nanomaterials have a great impact on their characteristics.19–25 So, exploring suitable methods to prepare zirconia and controlling its morphology and particle size is important and necessary. Herein, the precipitation route is presented to synthesize zirconia nanostructures. Precipitation route is well-known as an *[email protected] © 2015 The Authors. International Journal of Applied Ceramic Technology published by Wiley Periodicals, Inc. on behalf of The American Ceramics Society. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

appropriate preparation approach for production of various nanomaterials. This route is known as facile, reproducible, and useful preparation approach and presents an advantageous way to the preparation of homogeneous nanostructures. This work is the first successful attempt for the preparation of zirconium dioxide nanostructures using tetraethylenepentamine (TEPA) as precipitator. To go further into the study, the influence of dosage of TEPA, reaction time, and solvent on the shape and particle size of zirconia is examined.

Experimental Zirconium (IV) oxynitrate hydrate (ZN), tetraethylenepentamine (TEPA), ethylenediamine (EN), ethylene glycol (EG), propylene glycol (PG), and methanol were bought from Merck Company and were utilized as received. To synthesize sample 1, in a typical experiment, an aqueous solution of TEPA (1 mmol of TEPA was dissolved in 40 mL of distilled water) added into an aqueous solution of ZN (1 mmol of ZN dissolved in 40 mL of distilled water) dropwise under magnetic stirring during 10 min. The zirconia sample was obtained after filtering, washing with distilled water for three times, drying at 80°C, and calcining of as-prepared hydroxide precipitate at 600°C during 4 h (Scheme 1). To examine the influence of TEPA on the shape of the zirconium dioxide nanostructures, a blank test was carried out by EN instead of TEPA. The influences of dosage of TEPA, solvent, and reaction time on the particle size and the shape of zirconium dioxide samples were investigated and the results listed in Table 1.

Zirconia Nanostructures

www.ceramics.org/ACT

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Table 1. Reaction Conditions for Preparation of Zirconia Nanostructures

Sample No.

Precipitator

Molar ratio (Precipitator: ZrO(NO3)2. xH2O)

1 2 3 4 5 6 7 8 9 10 11*

TEPA TEPA TEPA TEPA TEPA TEPA TEPA TEPA TEPA TEPA EN

1:1 2:1 3:1 4:1 5:1 2:1 2:1 2:1 2:1 2:1 4:1

*

Solvent H2 O H2 O H2 O H2 O H2 O H2 O H2 O H2O/Methanol (25/75) H2O/EG (25/75) H2O/PG (25/75) H2 O

Time (min) 0 0 0 0 0 20 40 0 0 0 0

Figure of SEM images 6a 6b 6c 6d 6e 6f 6g 7a 7b 7c 7d

Blank test, in the absence of TEPA.

The X-ray diffraction (XRD) patterns were collected from a diffractometer of Philips Company with X’PertPro monochromatized Cu Ka radiation (k = 1.54 A). A Hitachi s4160 Japan scanning electron microscope (SEM) was used to investigate the morphological properties of zirconium dioxide. A Perkin Elmer (LS 55) fluorescence spectrophotometer was utilized to study the room temperature photoluminescence (PL). Fourier transform infrared spectra (FT-IR) were recorded with a FTIR spectrometer (Thermo Nicolet Magna-IR 560 spectroscopy, Thermo-Nicolet, Madison, WI) in KBr pellets. The energy dispersive spectrometry (EDS) analysis was obtained on XL30, Philips microscope. A JEM2100 transmission electron microscope (TEM) with an accelerating voltage of 200 kV was utilized to study the detailed morphological properties of zirconia. UV–visible spectrum of zirconium dioxide was recorded using a Scinco UV–vis scanning spectrometer (Model S-4100).

Results and Discussion FT-IR spectra of sample 2 in the range of 500– 4000 cm1 after washing steps and after calcination are shown in Figs. 1a and 2b, respectively. The (C-H) bending vibration peak at 1383 cm1 and (C-N) stretching vibration peak at 1342 cm1 indicate the presence of TEPA (Fig. 1a). These peaks perfectly vanish after calcination at 600°C. The absorption band centered at 3400 cm1 and a weak peak at 1630 cm1 are attributable to the v(OH) stretching and bending vibrations, respectively, which indicates the presence of physisorbed water molecules linked to zirconium dioxide sample.26

Fig. 1. FT-IR spectra of sample 2 after washing (a) and after calcination (b).

The peak at 502 cm1 in Fig. 1b is attributed to Zr–O vibration.16 XRD was applied to characterize the purity, average crystallite diameter and phase of the as-prepared zirconia. Figure 2 presents the XRD patterns of the sample 2 after washing steps and after calcination. As observed in Fig. 2a, the sample prepared by the precipitation route (before calcination) seems amorphous. As shown in Fig. 2b, nearly all of the diffraction peaks may be indexed to monoclinic ZrO2 (space group P2/c, JCPDS card 01-0750). Besides, four diffraction peaks in Fig. 2b can be ascribed to tetragonal ZrO2 (JCPDS 50-1089). The volume content of

International Journal of Applied Ceramic Technology—Zinatloo-Ajabshir and Salavati-Niasari Vol. 13, No. 1, 2016

110

Fig. 3. EDS pattern of ZrO2 (sample 2).

Fig. 2. XRD patterns of sample 2 after washing (a) and after calcination (b).

monoclinic and tetragonal phases of the as-obtained ZrO2 was calculated using following equation:16 Vm ¼

1:311Xm 1 þ 0:311Xm

ð1Þ

where Vm is volume content of the monoclinic phase and that of tetragonal phase is Vt = 1Vm and Xm is given by: Xm ¼

Im ð 111Þ þ Im ð111Þ Im ð111Þ þ Im ð111Þ þ It ð101Þ

ð2Þ

where Im ð 111Þ and Im (111) are the integrated intensities (area under the peaks) of the ð 111Þ (at 2h = 28.5°) and (111) (at 2h = 31.5°) peaks for the monoclinic phase (m), and It (101) is the intensity of the (101) (at 2h = 30.5°) peak for tetragonal phase (t) of zirconium oxide. The volume content of the monoclinic phase and that of tetragonal phase of the obtained product were 77% and 23%, respectively. To determine the average crystallite size of the prepared zirconia, Scherrer equation27 was used: s¼

kk bcosh

ð3Þ

where s is the crystallite size, k is the so-called morphology factor, b is the breadth of the shown diffraction peak at its half intensity maximum, and k is the wavelength of X-ray source applied in XRD. The average crystallite size of the prepared zirconium dioxide was calculated about 28 nm.

To determine the composition of zirconia nanostructures (sample 2), the EDS analysis was employed. As seen in Fig. 3, from the EDS, it is clearly identified that the obtained zirconium dioxide is composed of Zr and O. The morphology of the zirconia in the optimum condition (sample 2) was further studied by transmission electron microscopy (TEM). As it can be observed in Fig. 4, most of the sintered together particles have a quasi-spherical shape. Moreover, the average particle size determined from the SEM image is in a relative agreement with the TEM investigations which display the particle’s size in the range of 35–70 nm. To examine the optical characteristics of the as-prepared zirconium dioxide, PL and UV–vis analyses were performed. Figure 5a presents the UV–vis absorption spectrum of as-obtained zirconia sample. It has been shown that the optical properties of the nanostructured zirconia less affected by lattice parameters trend and therefore the quality of the monoclinic ZrO2 nanocrystal structure is independent of the volume content of the tetragonal phase.28 As the volume content of the monoclinic phase in the sample was larger than that of the tetragonal phase, the optical properties of monoclinic structure in the sample 2 probably became dominant. In the UV–vis absorption spectrum, the absorption band was observed approximately at 224 nm (5.5 eV). This absorption band can be related to the optical band gap for monoclinic ZrO2 nanocrystals, which shows a blue shift of about 0.4 eV compared with the band gap amount of obtained zirconia in previous report28 that this observed blue shift is related to decline in the size of particle that bring about variation in energy surfaces of particles and enhance the amount of the band gap. Also can be seen two weak peaks at 377 nm (3.29 eV) and 320 (3.87 eV) that can be owing to the mid-gap trap states involvement including the defects of the surface and oxygen vacancies.28,29 The estimated optical band gap of the sample is lower than those of bulk ZrO2. The obtained data are in good agreement with previous report.28



111

(a)

Fig. 4. TEM image of sample 2.

The PL spectrum of the sample 2 is shown in Fig. 5b. The excitation wavelength was 325 nm. An emission band located at 438 nm may be shown in Fig. 5b. This emission band can be attributed to electron transitions in intrinsic defects of zirconia nanostructures, which is similar to the literatures.30 Scanning electron microscope was applied to consider the influences of dosage of TEPA, solvent, and reaction time on the shape of the zirconium dioxide nanostructures. As mentioned, novelty of this work compared to other performed reports is the usage of TEPA as a new precipitator to prepare of zirconium dioxide, without applying any surfactant and capping agent. TEPA played both precipitator role and capping agent role. The TEPA can precipitate ZrO(OH)2 from ZrO (NO3)2 and may hinder the aggregation of prepared ZrO(OH)2 nanoparticles as capping agent (Scheme 2). It seems that TEPA with high steric hindrance brings about nucleation to be occurred rather than the particle growth. Figures 6a–e shows the SEM images of the products prepared by applying 1, 2, 3, 4, and 5 mmol of TEPA, respectively. When the molar ratio of TEPA to ZN was 1:1, the zirconium dioxide with coalesced particles/bulk structures was formed (Fig. 6a). By increasing the molar ratio from 1:1 to 2:1, homogeneous sponge-like zirconia nanostructures were obtained (Fig. 6b). It seems that when TEPA concentration increases, due to the steric hindrance of TEPA, the chance of collision between ZrO(OH)2 nanoparticles is decreased. So, among these used dosage of TEPA, zirconium dioxide nanostructures with uniform sponge-like shape and with small grain size may be formed by applying 2 mmol of TEPA (sample 2). On the other hand, with more dosage of TEPA, irregular and not uniform

(b)

Fig. 5. UV–vis absorbance spectrum (a) and PL spectrum (b) of the sample 2.

sponge-like nanostructures may be formed that were highly aggregated in some places (Figs. 6c–e). Therefore, the molar ratio of TEPA to ZN has an efficacious impact on the control of the shape and particle size of zirconia nanostructures. To examine the reaction time influence on the shape of the zirconia, the reaction performed using (2:1) molar ratio of TEPA to ZN for 20 and 40 min. SEM images of the zirconium dioxide synthesized for 20 and 40 min were presented in Figs. 6f and g. When the reaction time was enhanced to 20 and 40 min, sponge-like zirconia nanostructures with large nanocrystallite size and irregular morphology were obtained (samples 6 and 7). Figure 7 shows SEM images of the zirconia were synthesized using 2 mmol of TEPA for 0 min with various solvents through precipitation route. As seen in Fig. 6b, when H2O was applied as solvent, homogeneous sponge-like zirconia nanostructures are obtained. The shape of as-obtained zirconium dioxide varies from homogeneous sponge-like nanostructures to dense


112

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 6. SEM images of (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, (e) sample 5, (f) sample 6, and (g) sample 7.

agglomerated particle-like structures when CH3OH, C2H6O2 (EG), and C3H8O2 (PG) are applied as solvent, as seen in Figs. 7a–c, respectively. The C2H6O2

and C3H8O2 with polar factors can cap the nuclei in all dimensions and bound them from additional volume in all directions and finally impede from prepara-



(a)

(b)

(c)

(d)

113

Fig. 7. SEM images of (a) sample 8, (b) sample 9, (c) sample 10, and (d) sample 11.

Scheme Schematic diagram of the synthesis of zirconia nanostructures.

tion of homogeneous sponge-like nanostructures (Scheme 3). To study the TEPA influence on the shape of the ZrO2 nanostructures, sample 11 (as blank sample) was

prepared using EN. Figure 7d presents the SEM image of blank sample. As shown, in the absence of TEPA, irregular particle-like structures with high agglomeration were prepared. As mentioned, by applying TEPA as


114

Scheme Schematic of a mechanism effect of TEPA for formation of zirconia nanostructures.

(a)

Conclusions

(b)

Scheme Effect of (a) EG and (b) PG on morphology.

precipitator (Fig. 6b), the probability of collision of ZrO(OH)2 nanoparticles together decreased because of steric hindrance influence of TEPA, and thus, the size of particles lessened. Thus, homogeneous sponge-like zirconia nanostructures with small grain size can be produced by applying TEPA as precipitator and capping agent. The probable formation mechanism of zirconium dioxide micro/nanostructures utilizing TEPA can be summarized as follows: H2 NðCH2 CH2 NHÞ3 CH2 CH2 NH2 þ 2H2 O ! H3 Nþ ðCH2 CH2 NHÞ3 CH2 CH2 Nþ H3 þ 2OH þ by product 2OH þ ZrOðNO3 Þ2 ! 2NO 3 þ ZrOðOHÞ2 D

ZrOðOHÞ2 ! ZrO2

In this work, we proposed a simple and low-cost procedure for production of homogeneous sponge-like zirconia nanostructures through a facile precipitation method with a novel precipitator. This study demonstrates that using of tetraethylenepentamine (TEPA) is an excellent choice for synthesis of zirconia nanostructures without using any surfactants. In the present work, TEPA was utilized as novel precipitator and capping agent for the preparation of zirconium dioxide nanostructures. The influence of dosage of TEPA, reaction time, and solvent was also examined. Results show that zirconium dioxide nanostructures with very uniform sponge-like shape and with fine grain size may be formed by applying 2 mmol of TEPA as well as water as solvent. Furthermore, the optical characteristics of synthesized zirconia samples were examined.

Acknowledgment Authors are grateful to the University of Kashan for supporting this work by Grant No (159271/240).

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