Formation and Morphology of Zn2Ti3O8 Powders Using Hydrothermal Process without Dispersant Agent or Mineralizer

Synthesis of Zn2Ti3O8 powders for attenuating UVA using TiCl4, Zn(NO3)2·6H2O and NH4OH as precursor materials by hydrothermal process has been investigated. The X-ray diffractometry (XRD) results show the phases of ZnO, anatase TiO2 and Zn2Ti3O8 coexisted when the zinc titanate powders were calcined at 600 °C for 1 h. When calcined at 900 °C for 1 h, the XRD results reveal the existence of ZnO, Zn2TiO4, rutile TiO2 and ZnTiO3. Scanning electron microscope (SEM) observations show extensive large agglomeration in the samples. Transmission electron microscope (TEM) and electron diffraction (ED) examination results indicate that ZnTiO3 crystallites formed with a size of about 5 nm on the matrix of plate-like ZnO when calcined at 700 °C for 1 h. The calcination samples have acceptable absorbance at a wavelength of 400 nm, indicating that the zinc titanate precursor powders calcined at 700 °C for 1 h can be used as an UVA-attenuating agent.


Introduction
Ultraviolet (UV) radiation that reaches the earth and damages skin can be divided into three key wavelengths: (i) UVC (32-280 nm), (ii) UVB (280-320 nm) and (iii) UVA (320-400 nm). UVA radiation is a major culprit in photoaging and skin cancers. Moreover, UVB, which primarily reaches the top-most layer of skin, is thought to be responsible for acute photodamage, including sunburn and some non-melanoma skin cancers [1]. Therefore, protection against harmful UVA and UVB radiation is very important. Sheath [2] pointed out that sunscreens used for the protection of human skin against the harmful effects of solar radiation must contain UV-absorbing substances.
Fine particles of various metal oxides, such ZnO and TiO 2 , are extensively used as agents to attenuate (scatter and/or absorb) UV radiation, and have many attractive characteristics, such as a long history of topical use, broad spectrum absorption, high photostability and low irritancy [3]. However, an extensive literature search found that the use of ZnO-TiO 2 as a sunscreen for cosmetic applications has not been fully investigated.
Dulin and Rase [4] first established the basic phase diagram of the ZnO-TiO 2 system, and reported the temperature and composition ranges of stability for zinc metatitanate (ZnTiO 3 ) and zinc orthotitanate (Zn 2 TiO 4 ). Only Zn 2 Ti 3 O 8 , ZnTiO 3 and Zn 2 TiO 4 have been confirmed to exist in ZnO-TiO 2 systems by previous researchers [4][5][6][7]. The compound of Zn 2 Ti 3 O 8 has a cubic structure with a lattice constant of a o = 0.8390(5) nm, and has been observed to be a low-temperature form of ZnTiO 3 that exists at temperatures below 820 C [7]. ZnTiO 3 has a rhombohedral structure with lattice constants of a o = 0.5078(2) and c o = 1.3920(1) nm [5]. When heated between 965 and 1010 C, ZnTiO 3 decomposes and forms Zn 2 TiO 4 and rutile TiO 2 [6]. The compound of Zn 2 TiO 4 has a face-centered cubic crystal structure with a lattice constant of a o = 0.8460(2) nm [8].
Zinc titanates, such as ZnTiO 3 and Zn 2 TiO 4 , are attractive as sorbents for removing sulfur from hot coal gasification products [9,10], pigments [11], and gas sensors for ethanol, NO and CO [12]. Due to the recent progress of microwave applications in the area of mobile telephones and satellite communications, these substances can also be used as dielectric resonators and fitters [13,14]. Furthermore, Chang et al. [15,16] also found that doped and undoped ZnTiO 3 have a V-type resistivity-temperature characteristic and possess typical positive thermal coefficient resistivity (PTCR) properties when above the transition point. However, the use of Zn 2 Ti 3 O 8 or ZnTiO 3 as UV-attenuating agents has not been reported.
The chemistry and microstructure are important factors for applications of zinc titanate powders. Hence, various methods have been adopted for the preparation of ZnTiO 3 powders, including conventional solid state reaction [5] and sol-gel processes [16,17]. In addition, Zn 2 TiO 4 powders have been obtained by solid-state reaction [8], and the ball mill method [18]. However, the solid-state reaction processes have some drawbacks, such as high reaction temperature, large particle size and limited degree of chemical homogeneity. On the other hand, Reddy et al. [19] pointed out that a single phase of Zn 2 Ti 3 O 8 is produced when zinc titanyl oxalate hydrate decomposes at 650 C for several hours. However, until now, no information is available on the synthesis and characterization of Zn 2 Ti 3 O 8 powders by a hydrothermal process without the addition of a dispersant agent.
In the present study, high purity TiCl 4 and Zn(NO 3 ) 2 •6H 2 O have been used for the synthesis of Zn 2 Ti 3 O 8 crystallite powders by a hydrothermal process without the addition of either a dispersant agent or mineralizer. The main purpose of the present investigation was to examine the formation and morphology of Zn 2 Ti 3 O 8 nanocrystallite powders. In addition, this study (i) investigated the thermal behavior of zinc titanate precursor powders, (ii) evaluated the phase transition of zinc titanate precursor powders, and (iii) observed the morphology of zinc titanate precursor powders after calcination at various temperatures for 1 h.

Sample Preparation
The Zn 2 Ti 3 O 8 nanocrystallite powders were prepared by a hydrothermal process without the addition of a dispersant agent. The starting materials were prepared in a aqueous solution with reagent-grade titanium tetrachloride solution (TiCl 4  solution under an air atmosphere. The pH of the mixture was then raised to 7.0 by using NH 4 OH aqueous solution and stirring the resulting solution for 2 h at room temperature. Subsequently, the solution was kept in an autoclave at 150 C for 1 h. After cooling, the precipitates obtained were filtered, and washed thoroughly three times with a large amount of deionized water and ethanol (purity ≥ 99.85%, supplied by J. J. Baker, USA) to remove Cl -. The final precipitates were dried at -55 C in a vacuum and the white zinc titanate precursor powders were thus obtained.

Sample Characterization
Differential thermal analysis (DTA, Perkin-Elmer 7 Series Thermal Analysis System, Boston, MA, USA) was conducted on 50 mg zinc titanate precursor powders at a heating rate of 10 C/min in air with a reference material of Al 2 O 3 . The calcination temperature was determined from the DTA result.
The crystalline phase was identified using an X-ray diffractometer (XRD, Rigaku D-Max/IIIV, Tokyo, Japan) with Cu K α radiation and Ni filter, operated at 30 kV, 20 mA and a scanning rate of 0.25°/min.
The morphology of the zinc titanate precursor powders calcined at various temperatures for 1 h were observed with a scanning electron microscope (SEM, Hitachi, S-3000N, Japan) and transmission electron microscope (TEM, Hitachi model HF-2000, Tokyo, Japan). The crystal structure of the post-calcined powders was determined by selected area electron diffraction (SAED) analysis. The TEM samples were prepared by dispersing the post-calcined powders in an ultrasonic bath and then collected on a copper grid.

Thermal Behavior of the Zinc Titanate Precursor Powders
The DTA curve of the zinc titanate precursor powders, produced without the addition of either a dispersant agent or mineralizer, and which was heated from 25 to 1000 C in static air at a heating rate of 10 C/min, is shown in Figure 1. There are four endothermic peaks at 140, 250, 800 and 940 C in the DTA curve. The endothermic peak at 140 C is due to the dehydration of the zinc titanate precursor powders. The second endothermic peak, at 250 C, is attributed to the decomposition of NH 2 -into N 2 and H 2 [20]. The third endothermic peak, at 800 C, is caused by the decomposition of Zn 2 Ti 3 O 8 into ZnTiO 3 and rutile TiO 2 . The fourth endothermic peak, at 940 C, is due to the ZnTiO 3 decomposing, which leads to the formation of Zn 2 TiO 4 and rutile TiO 2 . Moreover, Figure 1 also shows two relatively small broad exothermic peaks at around 558 and 689 C. The first exothermic peak, at 558 C, is due to the anatase TiO 2 accompanied by Zn 2 Ti 3 O 8 formation. The second exothermic peak at 689 C is caused by the ZnTiO 3 accompanied with rutile TiO 2 formation.  Figure 2 shows the XRD patterns of the zinc titanate precursor powders prepared without a dispersant agent or mineralizer and calcined at various temperatures for 1 h. The XRD pattern of the freeze dried precursor powders before calcination is shown in Figure 2(a), which reveals that the precursor powders still maintained the amorphous state. Figure 2 Figure 2(d) shows the XRD pattern of zinc titanate precursor calcined at 900 C for 1 h. It reveals that the crystallized phase was composed of ZnO, Zn 2 TiO 4 , rutile TiO 2 and ZnTiO 3 , but the anatase TiO 2 disappeared.

Phase Transition of Zinc Titanate Precursor Powders Calcined at Various Temperatures for 1 h
Moreover, from Figure 2(c) and (d), it is seen that there is a significantly higher intensity value for the ZnO (100) reflection (I 100 ). Golón et al. [21] have pointed out that for hydrothermal treatment systems, samples reveal an apparent preferential orientation growth in the (100) direction, leading to a significant I 100 value. In fact, zinc and oxygen atoms are arranged alternatively along the c-axis, and thus as is well established, this inherent asymmetry along the c-axis results in the anisotropic growth of ZnO crystallites.
On the other hand, Bartram and Slepetys [5] pointed out that with a sample prepared at the mole ratio of ZnO:TiO 2 = 2:1 and calcined at 700 and 800 C for various times, the phase of defect-spinel type Zn 2 Ti 3 O 8 with a trace amount of uncombined TiO 2 is produced. This is caused by the four Ti ions that are missing from the 16-point positions of the spinel-type structure arrangement, resulting in a defective spinel-type structure. Mrá zek et al. [22] reported that the TiO 2 and Zn 2 TiO 4 are decomposed from prepared Zn x Ti y O z powders for various ratios of ZnO/TiO 2 . TiO 2 exists at temperatures of 400-600 C prepared by sol-gel method [23].
In Figure 2(b) and (c), it can be seen that although the intensity of Zn 2 Ti 3 O 8 increases with the calcination temperature, a small fraction of Zn 2 Ti 3 O 8 decomposes and leads to the formation of the ZnTiO 3 and rutile TiO 2 . This reaction can be expressed as follows: Yang and Swisher [24] also pointed out that Zn 2 Ti 3 O 8 is a thermodynamically stable compound up to temperatures between 700 and 800 C. Just above this temperature, ZnTiO 3 is more stable than the compound of Zn 2 Ti 3 O 8 . Furthermore, Yamaguchi et al. [6] also proposed using an amorphous material prepared by the simultaneous hydrolysis of zinc acetylautonate and titanium isopropoxide for synthesis of the ZnTiO 3 powders. The XRD result shows the reflection peaks of the compound corresponding to Zn 2 Ti 3 O 8 appeared at 600 C [5] and the intensity of the reflection peaks increased rapidly up to 760 C. No other compounds and free species, except for the hexagonal form of ZnTiO 3 , are observed up to the decomposition temperature at 965 C. These results suggest that the compound so far denoted as Zn 2 Ti 3 O 8 is a low temperature form of ZnTiO 3 .
On the other hand, the phase of Zn 2 TiO 4 formed by the thermal decomposition of Zn 2 Ti 3 O 8 in the range of 650-900 C has been reported by previous studies [5,25]. Figure 2(c)

Microstructure of the Zinc Titanate Precursor Powders Calcined at Various Temperatures for 1 h
The SEM microstructure of the zinc titanate precursor powders calcined at various temperatures for 1 h are shown in Figure 3. Figure 3(a) shows the morphology of the freeze-dried zinc titanate precursor powders without a dispersant agent or mineralizer agglomerates to the size of about 140 ± 70 μm. The SEM micrographs in Figure 3(b) and (d) shows the zinc titanate precursor powders calcined at 600, 700 and 900 C for 1 h, respectively. It can be seen that the agglomerated size of the particles increases as the calcination temperature rises from 600 to 900 C. When calcined at 900 C for 1 h, the size increases from 140 ± 70 μm to 270 ± 170 μm. Since the zinc titanate precursor powders were prepared through the wet-chemical routes, during this process, i.e., drying and/or subsequent steps, agglomeration can occur. During calcination, the most common type of agglomeration in the conventional powders was due to solid bonds that formed between the particles.
The bright field (BF) and dark field (DF) TEM micrographs and the corresponding electron diffraction (ED) patterns of the freeze dried zinc titanate precursor powders calcined at 700 C for 1 h are shown in Figure 4. Figure 4(a) shows the BF image, in which fine particles with size of about 5 nm and a larger particle with a length of 200 nm and width of 100 nm are observed. Aubert et al. [26] also reported the particle of TiO 2 is about 5 nm. Figure 4(a) shows that the larger particles, of ZnO which cause the contact area of ZnO with anatase TiO 2 to decrease, led to a decrease in the reaction of ZnO with anatase TiO 2 , meaning that insufficient Zn 2 Ti 3 O 8 was produced. Figure 4(b),(c) shows the DF images of the fine and larger particles in Figure 4(a). In addition, Figure 4(d),(e) shows the ED patterns of the particles in Figure 3 Figure 6 shows the relation between transmittance and wavelength range between 300 and 800 nm for freeze dried zinc precursor powders calcined at 700 C for 1 h. It is found that the calcined sample has an acceptable transmittance at the wavelength of 400 nm. This result indicates that zinc titanate precursor powders calcined at 700 C for 1 h can be used as an UVA-attenuating agent. Figure 6. Relation between the absorbed and wavelength between 300 and 800 nm for ZnTiO 3 precipitates calcined at 900 C for 1 h.

Conclusions
Zn 2 Ti 3 O 8 powders prepared by a hydrothermal method without a dispersant agent or mineralizer for use in UVA-attenuating applications have been investigated using DTA, XRD, SEM, TEM, ED and UV/VIS. The results are summarized as follows: (1) When the zinc titanate precursor powders were prepared at pH = 7 and calcined at 600 C for 1 h, the XRD results show that the phases of ZnO, anatase TiO 2 and Zn 2 Ti 3 O 8 coexisted in the sample. However, when calcined at 900 C for 1 h, the XRD result reveals the existence of Zn 2 TiO 4 , rutile TiO 2 , and ZnO.