The (011) Twin Structure Periodical in Monoclinic ZrO 2 Nano(cid:12)ber

The twin structure of monoclinic ZrO 2 nanoﬁber was characterized using transmission electron microscopy. The m-ZrO 2 nanoﬁber with diameter ∼ 300 nm was grown by the thermal oxidation of ZrSi 2 with gallium. The (011) twinning was observed periodically along the growth direction of m-ZrO 2 nanoﬁber. [DOI: 10.1380/ejssnt.2014.418]


INTRODUCTION
One-dimensional oxides have attracted much interest due to their excellent physical, chemical and mechanical properties as compared to those of bulk oxides [1]. As an excellent insulator, ZrO 2 provides a new option as structural materials, because it has excellent mechanical characteristics, such as high level of hardness and strength, thermal and chemical stability. Besides the wide investigations on the nano-ZrO 2 composite ceramics, the research has been focused on the growth and characterization of one-dimensional nano-ZrO 2 , which is expect to lead to the development of new applications for nano-scale devices, such as sensors and optical fibers [2][3][4][5][6][7][8][9].
On the other hand, planar defects such as stacking fault and twining are frequently observed in one-dimensional nanostructures, the characterizations of which are essential to understand the growth mechanism of onedimensional nanostructures with corresponding physical and chemical properties [10][11][12][13]. To the best of our knowledge, the twin structures have been observed in monoclinic ZrO 2 films [14] and particles [15,16]. However, it has been seldom reported on one-dimensional nano-ZrO 2 . In this paper, we report the periodical twin structure of ZrO 2 nanofiber.

II. EXPERIMENTAL
ZrO 2 nanofibers were grown by the thermal oxidation of ZrSi 2 substrates with gallium for 24 h at a temperature of 900 • C with vacuum pressure of 10 −4 Torr. The preparation procedure was described in detail previously [9]. The as-grown nanofibers were characterized using transmission electron microscopy (TEM) with selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). * Corresponding author: yangqing@xaut.edu.cn  Figure 1(a) shows the TEM image of a ZrO 2 nanofiber with diameter ∼300 nm. Figure 1(b) shows the corresponding SAED pattern which is composed of two sets of diffraction spots that have symmetrical geometrical distribution. As indexed in the SAED pattern, the common spot is the twin boundary plane (011) and the incident electron beam direction is [011]. Figure 1(c) shows the HRTEM image recorded from the edge of ZrO 2 nanofiber. As shown in the enlarged image in Fig. 1(c), all the fringes clearly observed correspond to the (100) plane. As indexed by the dotted lines, it clearly displays the mirror symmetry between the two parts of the twin which occurs periodically along the nanofiber, namely the growth direction of the ZrO 2 nanofiber is normal to the (011) twin plane. It should be mentioned that several ZrO 2 nanofibers displaying the same twin structure have been observed. According to the monoclinic structure of ZrO 2 ISSN 1348-0391 c ⃝ 2014 The Surface Science Society of Japan (http://www.sssj.org/ejssnt) e-Journal of Surface Science and Nanotechnology Volume 12 (2014) nanofiber revealed by the SAED and HRTEM characterizations, Fig. 1(d) shows the schematic illustration of crystallographic configuration of twinned m-ZrO 2 . Pure ZrO 2 exists in three phases with temperature increases: monoclinic (∼1205 • C), tetragonal (∼2377 • C), and cubic (∼2710 • C) [17]. It has been reported that the monoclinic ZrO 2 twin structure could be formed during the transformation from tetragonal to monoclinic structure [14][15][16]18]. The (011) plane was proved to act as twinning plane as transformation as well [14], which has been observed actually in transformed monoclinic ZrO 2 particles confined in mullite matrix and was considered to help to minimize the thermal strain on the particle arising due to the different thermal expansion coefficients of particle and matrix [15]. The theory calculation revealed that the twinning on the (011) plane is possible only if the twinning is accompanied by local nonhomogeneous deformation due to other mechanisms near the twin planes [18]. In this research, the growth temperature of ZrO 2 nanofiber is lower than the formation temperature of tetragonal structure. It is considered that the (011) twin structure in the as-synthesized monoclinic ZrO 2 nanofiber may be growth twin rather than transformation twin. However, the twin structure has not been reported in one-dimensional nano-ZrO 2 synthesized at low temperatures or by solution phase growth methods [2][3][4][5][6][7][8].

III. RESULTS AND DISCUSSION
Gallium can exist in a liquid state even at a wide temperature range (29.78∼2403 • C) [17], which means that the gallium can guarantee a suitable supersaturation for the nanostructure growth. The growth of ZrO 2 nanofibers is assumed to be guided by the gallium droplets, which should finally exist at the top or bottom of the nanofibers [19,20]. In this research, the vacuum pressure of 10 −4 Torr is nearly equal to the vapor pressure of gallium at 900 • C [21]. The gallium would be easily evaporated at the growth temperature of 900 • C. Therefore, the droplets were not observed at the end of ZrO 2 nanofibers during the TEM observation. It has been reported that interface between catalyst particles and their guided onedimensional nanostructures determined the shape due to the strain and lattice matching [22,23]. It was proposed that the wettability of the solid-liquid interface varies during the nanostructure growth process which would lead to the periodical formation of planar twin defects to release the stored energy [24]. The fact is that the similar phenomena have been observed in various 1D nanomaterials, such as ZnS, ZnSe, SiC, GaP, GaAs, InAs [11][12][13][24][25][26][27][28][29]. Therefore, it is assumed that the solid-liquid interface between ZrO 2 nanofiber and gallium droplet possibly played a key role in the growth of periodical twin structure which was terminated with gallium evaporated.

IV. CONCLUSION
The (011) twin structure periodical in monoclinic ZrO 2 nanofiber with diameter ∼300 nm was obtained by the thermal oxidation of ZrSi 2 with gallium. The synthesis of twinning along the nanofiber may develop a new understanding of twin formation in one-dimensional nano-scale.