One-dimensional GaN nanomaterials transformed from one-dimensional Ga2O3 and Ga nanomaterials

One-dimensional (1D) GaN nanomaterials exhibiting various morphologies and atomic structures were prepared via ammoniation of either Ga2O3 nanoribbons, Ga2O3 nanorods or Ga nanowires filled into carbon nanotubes (CNTs). The 1D GaN nanomaterials transformed from Ga2O3 nanoribbons consisted of numerous GaN nanoplatelets having the close-packed plane, i.e. (0002)2H or (111)3C parallel to the axes of starting nanoribbons. The 1D GaN nanomaterials converted from Ga2O3 nanorods were polycrystalline rods covered with GaN nanoparticles along the axes. The 1D GaN nanomaterials prepared from Ga nanowires filled into CNTs displayed two dominant morphologies: (i) single crystalline GaN nanocolumns coated by CNTs, and (ii) pure single crystalline GaN nanowires. The cross-sectional shape of GaN nanowires were analyzed through the transmission electron microscopy (TEM) images. Formation mechanism of all-mentioned 1D GaN nanomaterials is then thoroughly discussed.

electron conditions. GaN nanoribbons consisting of platelets were fabricated in a conventional furnace with a horizontal quartz tube. Ga and Ga 2 O 3 powders were homogeneously mixed in a weight ratio 1:1. The mixture was placed in a BN crucible and covered by a BN plate with drilled mm-size channels. Bundles of CNTs were placed on the BN plate. The synthesis process was performed in two steps. First, the furnace was washed with a N 2 gas (3 l/min) over ~2 hours under gradual heating to 900°C. Second, a mixture of NH 3 (0.3 l/min) and N 2 (1 l/min) was substituted for the original N 2 gas flow while the temperature was gradually increased to 970°C over 10 min and then kept constant during 30 min. Then, the whole quartz tube was taken out from the furnace and cooled to the room temperature. We found that the surface of the starting CNTs was covered with a thin layer of yellow-colored material.
Firstly, Ga 2 O 3 nanorods and Ga-filled CNTs were synthesized in a vertical radio-frequency furnace by mixture of Ga 2 O 3 and pure amorphous active carbon. (see also Refs. [19,20]) The furnace was heated at 1360°C over 1-2 h. Ga-tipped Ga 2 O 3 nanorods were found to be deposited on the outer surface of the C fiber coat where the temperature was estimated to be approximately 1000°C. Meanwhile, Ga-filled CNTs were collected from the inner surface of the outlet pipe of the furnace where the temperature was estimated to be ~800°C. Then, place Ga 2 O 3 nanorods and Ga-filled CNTs into the conventional furnace for ammoniation at 970°C (described above), respectively. Finally all the resultant materials were collected and studied by a 300 kV field emission analytical highresolution transmission electron microscope (HRTEM, JEM-3000 F) equipped with an X-ray energy dispersive spectrometer (EDS). nanoribbons are crystallized on the surface of CNTs, and, while a N 2 gas having impurities of O 2 , is introduced into the furnace, the chemical reactions proceed as follows: where a Ga 2 O vapor is generated in line with the reaction described by Han et al [3].
The vapor cannot be oxidized further after it reaches the surface of CNTs, because the CNTs serve as a shelter for the O 2 impurity in N 2 gas; (ii) Ga 2 O 3 nanoribbons are transformed into GaN nanoribbons while NH 3 is introduced into the furnace; the involved reaction may be written as follows, where GaN platelets may initially nucleate on the side-surface of a Ga 2 O 3 nanoribbon and the subsequent growth of GaN platelets propagates along the whole nanoribbon.
To confirm the growth mechanism of the yellow-colored found to be GaN. There were nanoribbons covered with nano-platelets. A nanoribbon is shown in Fig. 1(c), its axis is parallel to the (0002) plane, as shown in Fig. 1(d), which is the [112 0] zone axis diffraction pattern. The similarity between the morphologies in Fig. 1(a) and Fig. 1(c) suggests that the proposed growth mode for the nanoribbons depicted in Fig. 1(a) is correct. The reasonability of the proposed growth mode may require other evidence additive to the fact of similar morphologies of the two kinds of the GaN nanoribbons.
Detailed HRTEM investigation may become effective.    CNTs. However, such explanation may find a better proof if a pure GaN nanorod not covered with a CNT may be found in the ammonia-treated materials. In fact, such material was found, as shown in Fig. 8(a). This rod has an axis perpendicular to the (11 00) plane, as displayed in Fig. 8(b) and (c). The most intriguing fact is that new contrast fringes appear in Fig. 8(a). It is believed that the fringes are originated from the thickness difference [21]. According to this, we can conjecture that the cross-section of the rod may have two kinds of possible shapes shown in Fig. 9. The above-presented analysis made us possible to estimate the thickness and the cross-section shape using the observed contrast fringes. It is noted though that a more definite confirmation using a slice method of cross-section is probably needed [22].
The formation of a pure GaN nanorod can also be explained due to ammoniation of Ga in CNTs. When a metallic Ga with a density of ~5.90 g/cm 3 is gradually transformed to GaN with a density of ~5.90 g/cm 3 , the volume increased by 20% as the molecular weight increased by 20%. This extra volume part gradually grows and leads to a resultant pure GaN nanorod whose cross-section may become non-round as compared to a round shape of the initial Ga nanorod.
To sum up, the presented transformation scheme consists of two steps and relies on conversion of a given 1D material to another 1D material in a given gaseous atmosphere. This approach has been used to synthesize ZnS [23] and GaN nanotubes [24,25]. It is believed here that the process is an important practical method to synthesize novel 1D material and to fabricate complicated arrays of those with nanoparticles. We