(Invited) Synthesis of Photoactive Hexagonal-Shape GaN Nanoplate Using Solid Nitrogen Source in Molten Salt
Authors: Sugiura Takashi , Purwiandono Gani , Manseki Kazuhiro 
Published: December 2020

Gallium nitride (GaN), is a group III nitride semiconductor. Thin films of GaN have beenwidely adopted for both commercial and fundamental research, such as in light-emitting diodes (LEDs) and UV light water-splitting. Much interest has also been paid to the creation of nanostructures of GaN including the morphology of wires, rods, sheets, seeds and hollows. The nitridation reactions are often conducted using gas flow techniques for growing GaN crystals. In this process, a nitrogen-rich gas, such as ammonia, is usually used as a source of nitrogen. We have reported a new technique to synthesize low cost GaN powder under moderate condition. Li3N or LiNH2 were used as a high reactive solid nitrogen source to improve the reaction rate and lowering the reaction temperature. Crystalline wurtzite GaN powder with several hundred nanometer size was successfully synthesized. But the reaction yield was not so high because its solid phase reaction. Molten salts are often used as reaction media to increase the reaction rate and the crystallinity of ceramics. In this study, the synthesis of GaN nanoplate has been established by using LiCl as the molten salt at relatively low temperatures. Also, we anticipated that the formation of hexagonal-shape GaN nanoplates instead random-shaped crystals could enhance the charge transport in the photoelectrochemical applications. Here, we present the effect of the molten salt on the crystal growth of the GaN microstructure and the photoelectrochemical properties of the GaN electrodes.
GaCl3 was reacted with LiNH2 at temperature of 400 - 650oC, LiCl was used as molten salts. LiCl has a melting point of 614°C. The graphite crucible with the starting materials (GaCl3, LiNH2) and molten salts (LiCl) was put into a sealed stainless reaction vessel. The reaction temperature was at 400 - 650°C. The products were washed with water and HCl solution after cooling.
Figure 1 shows SEM photographs of GaN samples synthesized at (a) 550oC, (b) 600oC, (c) 650oC, (d) 700oC and (e) 800oC. The hexagonal-plate morphologies were observed at 600°C, 650°C, and 700°C, GaN formed with the particle size smaller than 500 nm. These GaN nanoplates are termed as 2D HS-GaN. Notably, by increasing temperature up to 700°C, particle sizes increased to be more than 300 nm and the crystal shape changed from granular to hexagonal. However, at 800°C, the crystal size decrease and the particles adopted a random plate-like shape. It is consider that thermal decomposition occurred at 800°C. The thickness of the 2D HS-GaN nanoplates varied from 50 to 80 nm by increasing the temperature from 600°C to 700°C. It is most likely that the LiCl assisted synthesis facilitated a more homogeneous reaction than that of the solid phase synthesis at 550°C. In the molten salt, the mobility of the reactants GaCl3 and LiNH2 is higher than that in the solid phase. The increased mobility causes the reactants to be uniformly diffused within a short period. Thus, at temperatures, more than 600°C, GaCl3 and LiNH2 in the molten salt allowed the reaction to proceed at an atomic level. The photoelectrochemical characterization revealed that the hexagonal structure gave a higher anodic photocurrent by a factor of up to 2 compared to the random-shaped GaN obtained at a higher temperature. To our belief, the photo-electrochemical properties of 2D HS-GaN electrode indicate that it has significant potential for photo-catalytic water splitting.

Figure 1