Template synthesis of Zn2TiO4 and Zn2Ti3O8 nanorods by hydrothermal-calcination combined processes

Accepted Manuscript Template synthesis of Zn2TiO4 and Zn2Ti3O8 nanorods by hydrothermal-calcination combined processes Jirapong Arin, Somchai Thongtem, Anukorn Phuruangrat, Titipun Thongtem PII: DOI: Reference:

S0167-577X(17)30177-5 http://dx.doi.org/10.1016/j.matlet.2017.01.142 MLBLUE 22098

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

30 November 2016 24 January 2017 28 January 2017

Please cite this article as: J. Arin, S. Thongtem, A. Phuruangrat, T. Thongtem, Template synthesis of Zn2TiO4 and Zn2Ti3O8 nanorods by hydrothermal-calcination combined processes, Materials Letters (2017), doi: http:// dx.doi.org/10.1016/j.matlet.2017.01.142

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Template synthesis of Zn 2TiO4 and Zn2Ti3O8 nanorods by hydrothermalcalcination combined processes

Jirapong Arina,b, Somchai Thongtemc,d, Anukorn Phuruangrate,* and Titipun Thongtema, * a

Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand

b

The Graduate School, Chiang Mai University, Chiang Mai 50200, Thailand c

Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand

d

Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand e

Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand *Corresponding authors: [email protected] (A. Phuruangrat) Tel. +66 (0)74 288374, Fax. +66 (0)74 288395 and [email protected] (T. Thongtem) Tel. +66 (0)53 943344, Fax. +66 (0)53 892277

Abstract ZnO nanorods coated with TiO2 nanoparticles of 2:1 and 2:3 mole ratios Zn:Ti were synthesized by 120 oC and 12 h hydrothermal reaction. Upon calcination the nanocomposites at 750 oC for 5 h, they were transformed into Zn2TiO 4 and Zn2Ti3O8 nanorods. The XRD results revealed the presence of zinc orthotitanate (Zn2TiO4) and zinc polytitanate (Zn2Ti3O8) by calcination of the precursors with 2:1 and 2:3

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mole ratios Zn:Ti. SEM and TEM analyses show TiO2 nanoparticles coated on ZnO nanorods. Upon calcination at high temperature, TiO2 nanoparticles fused and incorporated in ZnO nanorods to form zinc orthotitanate and zinc polytitanate nanorods. Their formation mechanism was proposed and discussed in this research.

Keywords: Zn2TiO4; Zn2Ti3O 8; X-ray diffraction; Electron microscopy

1. Introduction Zinc titanate is a promising material for gas detection, paint pigment, catalyst, etc [1−3]. Zn2TiO4 was reported as a material with some exceptional electrical properties for microwave resonators [4]. Zinc titanate particles ranging from nanometers to micrometers are expected to enhance some performance because they have large surface-to-volume ratio and symmetry breaking on top [5−8]. Thus, preparation and characterization of nanoscale zinc titanate have been received much attention [9−11]. In this work, ZnO nanorods were hydrothermally synthesized under basic condition to certify the presence of [Zn(OH)4]2−, which were thermally decomposed to form ZnO nuclei. The solution containing sufficient OH− enhances nucleation and growth but H+ leads to the opposite results [12]. For the solution containing high NH4OH content, OH− and NH3 were the important roles in nanorod growing in a specified direction [13]. TiO2 synthesized under basic condition was composed of very fine nanoparticles and that synthesized under acidic condition tended to form other morphology with lower surface area to volume ratio [14]. Zn2TiO4 and Zn2Ti3O8 nanorods were synthesized by calcination of ZnO nanorods coated with TiO2 nanoparticles. Formation mechanism of Zn2TiO4 and Zn2Ti 3O8 nanorods was also discussed in this report.

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2. Experiment 0.200 mol Zn(NO3)2·6H2O and 0.015 mol La(NO3)3·6H2O were dissolved in 100 ml de-ionized water. NH4OH solution was slowly added until the pH was 10. Then 0.100 mol and 0.300 mol C4K2O 9Ti·2H2O in 100 ml de-ionized water each were slowly added to the transparency solutions with keeping the pH at 10 throughout the process. These solutions were hydrothermally processed at 120 oC for 12 h. The samples were calcined at 750 oC for 5 h for further characterization.

3. Results and discussion XRD pattern (Fig. 1a) of the nanocomposites synthesized by hydrothermal method corresponds with wurtzite ZnO and anatase TiO2 (JCPDS nos. 05-0664 and 04-0477) [15]. All diffraction peaks are quite sharp and reveal the compositional homogeneity powder. The formation mechanism of ZnO-TiO2 nanocomposites was explained as follows. In C4K2O9Ti·2H2O aqueous solution, titanium ions exist in the solution as the TiO2+ ions. C4K2O9Ti·2H2O → 2K+ + TiO2+ + 2C2O42− + 2H2O

(1)

When the pH was increased by NH4OH adding, titanium hydroxide complexes precipitated [16,17]. 3NH4OH ↔ 3NH4+ + 3OH−

(2)

2NH4+ + C2O42−(aq) → (NH4)2C2O4

(3)

TiO2+ + NH4+ + 3OH− → Ti(OH)4 + NH3

(4)

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Fig. 1 XRD patterns of (a) ZnO-TiO2 nanocomposites of 2:1 mole ratio Zn:Ti hydrothermally synthesized at 120

o

C

for 12 h, (b) Zn2TiO4 (ZnO-TiO2

nanocomposites of 2:1 mole ratio Zn:Ti with 750 oC and 5 h calcination) and (c) Zn2Ti3O8 (ZnO-TiO2 nanocomposites of 2:3 mole ratio Zn:Ti with 750 oC and 5 h calcination).

Titanium (IV) hydroxide was easily hydrolyzed to form anatase TiO2 by hydrothermal heating. Ti(OH)4 → TiO2 + 2H2O

(5)

Concurrently, ZnO nuclei formed and grew to be nanorods in alkaline solution. At the pH range of 7−9.5, some white solid was obtained. In the solution containing a lot of OH−, Zn2+ ions were precipitated very quickly, resulting in fast contribution of ZnO nanorod growth [12].

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Zn2+ + 2OH− → Zn(OH)2

(6)

The existence of ammonia in the solution can solve the problem. The precipitates started to dissolve and the solution became transparent again, explained by excessive NH3 by crystal-field splitting. NH4OH can stabilize Zn2+ through reversible reaction by consuming and decomposing of zinc-ammonia complexes, maintaining a stable content of Zn2+ in the solution [12]. Upon increasing temperature, the heterogeneous growth was promoted while the homogeneous nucleation in the bulk solution was suppressed. Thus the growth existed for a long time without replenishing the reagents. Zn(OH)2 + 4NH3 → [Zn(NH3)4]2+ + 2OH− During

hydrothermal

processing,

[Zn(NH3)4]2+

(7) was

transformed

into

[Zn(OH)4]2− by excessive OH− with the thermal evaporation of NH3 [18]. [Zn(NH3)4]2+ + 4OH− → [Zn(OH)4]2− + 4NH3

(8)

[Zn(OH)4]2− further decomposed at high temperature inside the hydrothermal bomb. Under this environment, Zn2+ content in the solution was stable. An expectation of local rise in pH may assist initiation of secondary nucleated branches of nanorods with advanced growth [13]. This explains the change in ZnO nanorods dominated growth surface and branching with NH4OH addition to the growth solution. Obviously, the nanorods branched and grew, including the depletion of Zn ions in the solution containing ammonia [13]. [Zn(OH)4]2− → ZnO + H2O + 2OH−

(9)

Moreover, the increase content of precursors provides a lot of ZnO nuclei and dense nanorods [12]. The solution environment containing sufficient OH− enhances nucleation and growth but H+ shows the opposite result.

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ZnO-TiO2 nanocomposites were further calcined at 750 oC for 5 h. XRD pattern of the product with 2:1 mole ratio Zn:Ti after calcination (Fig. 1b) revealed the presence of all synthesized Zn2TiO4 powder (JCPDS no. 19-1483) [15]. For the 2:3 mole ratio Zn:Ti, the diffraction peaks (Fig. 1c) were specified as the (220), (311), (222), (400), (421), (422), (511), (440) and (533) planes of cubic Zn2Ti3O8 phase (JCPDS no. 38-0500) [15]. The product morphologies were investigated by SEM (Fig. 2). Before calcination, ZnO-TiO2 nanocomposites of both 2:1 and 2:3 mole ratios Zn:Ti show TiO2 nanoparticles coated on ZnO nanorods. Upon calcination at high temperature, Zn2TiO4 and Zn2Ti3O8 appeared as nanorods. The formation of zinc titanate was proposed by the Wagner’s cationic counter diffusion reaction model [19]. At high temperature calcination, Ti4+ and Zn2+ diffused across ZnO layer at different rates (v) with v Ti4+ > v Zn2+ [20]. TiO2 nanoparticles fused and incorporated in ZnO nanorods first. Thus Zn2TiO4 and Zn2Ti3O8 were synthesized and their shapes remained as nanorods.

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Fig. 2 SEM images of (a) ZnO-TiO2 nanocomposites of 2:1 mole ratio Zn:Ti hydrothermally synthesized at 120 oC for 12 h, (b) ZnO-TiO2 nanocomposites of 2:3 mole ratio Zn:Ti hydrothermally synthesized at 120 oC for 12 h, (c) Zn2TiO4 (ZnOTiO2 nanocomposites of (a) with 750 oC and 5 h calcination) and (d) Zn2Ti3O (ZnOTiO2 nanocomposites of (b) with 750 oC and 5 h calcination).

To further understand wurtzite ZnO-anatase TiO2 nanostructure, the composites were also characterized by TEM (Fig. 3a). The ZnO-TiO2 composites were composed of very tiny nanoparticles of anatase (20−40 nm) coated on ZnO nanorods. The SAED pattern (inset of Fig. 3a) was indexed to anatase TiO2 and ZnO. Zn2TiO4 nanorods with different diameters (50−100 nm) are shown in Fig. 3b. The SAED pattern of Zn2TiO4 corresponds to the (112), (103), (220) and (321) planes, in accordance with the XRD analysis. TiO2 islands as a mobile phase can incorporate in ZnO lattice (v Ti4+ > vZn2+) [20]. Thus Zn2TiO4 spinel was synthesized by

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high temperature calcination. Zn2Ti3O8 (Fig. 3c) has nanorod shape with 50−100 nm diameter. The zinc titanate was synthesized from the calcined ZnO-anatase TiO2 with different mole ratios through the incorporation of TiO2 nanoparticles in ZnO nanorods.

Fig. 3 TEM images and SAED patterns of (a) ZnO-anatase TiO2 of 2:1 mole ratio Zn:Ti hydrothermally synthesized at 120

o

C for 12 h, (b) Zn2TiO4 (ZnO-TiO2

nanocomposites of 2:1 mole ratio Zn:Ti with hydrothermal-calcination combined processes) and (c) Zn2Ti3O8 (ZnO-TiO2 nanocomposites of 2:3 mole ratio Zn:Ti with hydrothermal-calcination combined processes).

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During calcination at high temperature, TiO2 has a strong driving force under melting-point depression effect. TiO2 incorporated in ZnO nanorod lattice as a solid at the early stage because the diffusion rate of Ti4+ is faster than that of Zn2+ [20] (Fig. 4). In the end, zinc titanate was produced and controlled by the stoichiometry of ZnO and TiO2. Zn2TiO4 and Zn2Ti 3O8 nanorods were obtained by calcination the ZnO-TiO2 nanocomposites of 2:1 and 2:3 mole ratios Zn:Ti at 750 oC for 5 h.

Fig. 4 Schematic formation of zinc titanate from TiO2 nanoparticles coated on ZnO nanorods.

4. Conclusions Zinc orthotitanate (Zn2TiO4) and zinc polytitanate (Zn2Ti3O8) nanorods were synthesized via hydrothermal-calcination combined processes by the reaction of ZnO nanorods as a template coated with anatase TiO2 nanoparticles. ZnO nanorods were successfully synthesized by specific growth direction of nanorods with NH4OH inducing in basic condition. TiO2 nanoparticles with high surface area favored the

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basic condition and were synthesized on ZnO nanorods. In this research, the zinc titanate nanorods were developed via high surface area to volume ratio TiO2 nanoparticles incorporated in lattice of ZnO nanorods. Subsequently, they were transformed into Zn2TiO4 nanorods by calcining of ZnO-TiO2 with 2:1 mole ratio Zn:Ti and Zn2Ti3O8 nanorods by calcining of ZnO-TiO2 with 2:3 mole ratio Zn:Ti.

Acknowledgments: We wish to thank the Thailand's Office of the Higher Education Commission for providing financial support through the Research Professional Development Project under the Science Achievement Scholarship of Thailand, and Chiang Mai University through the Center of Excellence in Materials Science.

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Highlights

• Precursors can be used as a model for producing Zn2TiO4 and Zn2Ti3O8 nanorods. • They were successfully synthesized by hydrothermal-calcination combined processes. • Their formation mechanism was explained according to the experimental results.