In recent years, the many important works are then published in which a chemical vapor deposition (CVD) is introduced and successfully synthesized in the nanofibers or nanorods of ZnO [1] , SnO₂ [2] , CaN [3] , TiO₂ [4] and Bi₂O₃ [5] etc. They are a type of one-dimensional (1-D) nano-structural materials exhibit novel physical properties in a number of areas [6] . They have been widely studied due to the great potential applications in electronic properties [7] , and excellent optical properties [8] . Among which the ZrO₂ nanofibers have a high aspect ratio and a high specific surface area, the nanofibers have been extensively applied in excellent thermal and chemical stability, high strength and fracture toughness, low thermal conductivity, high corrosion resistance, because of its technical importance and broad practical applications, such as thermal barrier coatings [9] , solid oxide fuel cells [10] , various sensors [11] , gate dielectrics [12] , catalysts [13] , ceramic biomaterial [14] , metallic glass [15] . Several methods have been used to synthesize ZrO₂ nanofibers, including ionic-liquid route [16] , a modified sol-gel method [6] , electrospinning [17] . However, there have been no reports on the ZrO₂ nanofibers using CVD method.

In the present study, we have obtained nanofibers that have diameter of about 100 nm with using phosphorus/water mixture as catalyst. This method produces mist of phosphorus/water mixture by ultrasonic spray pyrolysis for the synthesis of nanofibers. A possible growth mechanism of such ZrO₂ nanofibers is also discussed in the present study, e.g. the growth of such nanofibers is attributed to solid-liquid-solid (SLS) growth mechanism [18] . This is the promising method to grow ZrO₂-nanofibers.

Details of the present method have been reported previously as shown in Figure 1 [18] . A zirconia substrate was used as a substrate and the mixture of phosphorus/water or water was used as corrosive source. The reactor furnace was heated to the range of 700˚C - 1000˚C in N₂ flow, the phosphorus mixed with the water or only water was placed inside the atomization chamber of the ultrasonic spray pyrolysis machine [19] . The synthesis was in N₂ gas atmosphere. In the present study, phosphorus (Kojundo Chemical Laboratory; 99%) was used. Produced samples were analyzed using a scanning electron microscope (SEM), a transmission electron microscope (TEM), as well as energy dispersive spectrum (EDS).

Figure 2(a) shows SEM image of ZrO₂ nanofibers that were synthesized on the zirconia substrate at 900˚C for 1 h, they were uniform and their diameter was about 100 nm, disordered vertically condition. We found that the present an individual nanofiber having a diameter of about 100 nm, a black spot on the tip of nanofiber by the TEM observations which is shown in Figures 2(b) and (c). The TEM observation indicates that the present bulky bottom of nanofiber as shown in Figure 2(d), it indicated that the nanofibers of growth from the zirconia substrate. We also observed a black spot at the tip of nanofiber. Figure 2(e) shows the EDS spectrum of the tip (spot (i)) of nanofiber shows three distinct peaks of Cu, O and Zr, with the Cu peak possibly coming partially from the TEM grid. However, we did not find any phosphorus in the tip of nanofiber within the detection limits.

The short and sparse nanofibers was observed in the sample synthesized at 700˚C as shown in Figure 3(a). The yield of nanofibers can be synthesized at 800˚C, the length of nanofibers nearly did not change as shown in Figure 3(b). Straight and longer nanofibers yield were observed in the sample synthesized at 900˚C as shown in Figure 3(c). At 1000˚C, the density of nanofibers became sparse again, and the aggregation of the bottom of nanofibers originated from the surface of zirconia substrate. According to above these SEM images, we expected that the growth temperature can affect the synthesis of nanofibers. The amounts of active zirconia/phosphorus

mixture increase with increasing growth temperature. However, the aggregation of the bottom of nanofibers are prompted by higher temperature heating, as shown in Figure 3(d). It is worth pointing out that the calcination at 900˚C resulted in some substantial the ZrO₂ nanofibers morphology as compared to synthesis at 700˚C, 800˚C, 1000˚C.

Figure 4(a) shows a TEM image of nanofiber body, many little particles were adsorbed on the surface of the body, due to evaporation of the polymer and the crystallization of ZrO₂, that was composed of nanocrystalline grains <5 nm in size. The inset shows the electron diffraction pattern of selected area. The rings were indexed as the (111), (200), (220) and (311) lattice planes as shown in Figure 4(b). Figure 4(c) shows the EDS of the body (spot (ii)) of the nanofiber. The Cu, Zr and oxygen elements are detected, respectively. The Cu peak results from the Cu parts of the measurement chamber and the Zr and O peaks comes from the body of nanofiber. However, we found that the phosphorus element cannot existed in the nanofibers, and we estimated that flow N₂ gas can carry phosphorus element away from the nanofibers owing to the high temperature.

Figure 5 is a schematic illustration of the growth mechanism. It involved in the formation of the ZrO₂-nanofiber are proposed as follows. Initially, the phosphorus coating on the zirconia substrate forms a thin phosphrous film, as illustrate in Figure 5(a). When the reaction temperature increases, the zirconia substrate is oxidized in the carrier N₂ and by the water. A reaction between the Zr and P occur form a thin Zr-P eutectic liquid layer, as illustrated in Figure 5(b). Because the melting point of Zr is 1855˚C, a temperature at which the Zr vapor phase is negligible, and the Zr substrate itself serves as an Zr source without and additional external Zr source, the nanofibers should be formed via a solid-liquid-solid (SLS) mechanism. Finally, the surface of the Zr-P eutectic liquid soon becomes supersaturated, and appeared the aggregating into ZrO₂-nanoparticles condition via heating as shown in Figure 5(c). And then the nanofibers precipitate out and continue to grow from ZrO₂-nanopaticles (saturation nucleation) of this supersaturated surface as illustrate in Figure 5(d), possibly through the oxidation

reactions of Zr + 2H₂O → ZrO₂ + 2H₂.

In the present study, we successfully synthesized the nanofibers with a mixture of phosphorus/water on a Zr substrate using ultrasonic spray pyrolysis method. The ZrO₂ nanofibers were produced on the surface of the Zr

substrate, and the mixture of Zr/phosphorus eutectic liquid layer thereafter the synthesized nanoparticles on Zr substrate played a key role in determining the growth model of nanofibers. A possible mechanism for this process is the formation of the nanofibers from the surface the Zr-P liquid layer. We believed that this approach to the synthesis of nanofibers may be further used in the fabrication of fuel cells and oxygen sensors.