nanomaterials

Article Thermal Plasma Synthesis of Crystalline Nitride Nanopowder from Gallium Hydrate and Melamine

Tae-Hee Kim 1, Sooseok Choi 2,* and Dong-Wha Park 1,*

1 Department of Chemistry and Chemical Engineering and Regional Innovation Center for Environmental Technology of Thermal Plasma (RIC-ETTP), Inha University, 100 Inha-ro, Nam-gu, Incheon 22212, Korea; [email protected] 2 Department of Nuclear and Energy Engineering, Jeju National University, 102 Jejudaehak-ro, Jeju 63243, Korea * Correspondence: [email protected] (S.C.); [email protected] (D.-W.P.); Tel.: +82-64-754-3644 (S.C.); +82-32-860-7468 (D.-W.P.)

Academic Editors: Krasimir Vasilev and Melanie Ramiasa Received: 30 December 2015; Accepted: 10 February 2016; Published: 24 February 2016

Abstract: (GaN) nanopowder used as a blue fluorescent material was synthesized by using a direct current (DC) non-transferred arc plasma. Gallium nitrate hydrate (Ga(NO3)3¨xH2O) was used as a raw material and NH3 gas was used as a nitridation source. Additionally, melamine (C3H6N6) powder was injected into the plasma flame to prevent the oxidation of gallium to gallium oxide (Ga2O3). Argon thermal plasma was applied to synthesize GaN nanopowder. The synthesized GaN nanopowder by thermal plasma has low crystallinity and purity. It was improved to relatively high crystallinity and purity by annealing. The crystallinity is enhanced by the thermal treatment and the purity was increased by the elimination of residual C3H6N6. The combined process of thermal plasma and annealing was appropriate for synthesizing crystalline GaN nanopowder. The annealing process after the plasma synthesis of GaN nanopowder eliminated residual contamination and enhanced the crystallinity of GaN nanopowder. As a result, crystalline GaN nanopowder which has an average particle size of 30 nm was synthesized by the combination of thermal plasma treatment and annealing.

Keywords: thermal plasma; annealing; gallium nitride; gallium nitrate hydrate; melamine; nanopowder

1. Introduction Gallium nitride (GaN) has been used as a binary III–V direct band-gap semiconductor material in light-emitting diodes since the 1990s. GaN is a blue fluorescence material used in LEDs. Lighting devices create various colors by combining red (R), green (G), and blue (B) [1]. In order to prepare a white light source, blue fluorescence is mixed with other light sources such as yellow, red, or green light. Such white LEDs are taking the place of traditional incandescent and fluorescent lights. GaN has a large band gap energy of 3.4 eV at room temperature and a high thermal conductivity of 130 W/m¨K[2–4]. It is possible to use these materials in optoelectronic devices which have wide band gap with energies from the visible to the deep ultraviolet region. Gallium nitride is a very hard material that has a strong atomic bonding as a wurtzite crystal structure. It can be used for applications in optoelectronic, high-power, high-frequency, and high temperature devices. For example, GaN can be applied as the substrate which makes violet laser diodes at 405 nm without the requirement of nonlinear optical frequency-doubling. It is usually applied by deposition on silicon carbide (SiC) and

Nanomaterials 2016, 6, 38; doi:10.3390/nano6030038 www.mdpi.com/journal/nanomaterials Nanomaterials 2016, 6, 38 2 of 15

sapphire (Al2O3) plates. During the deposition of GaN onto plates, the mismatching of GaN lattice structure could occur, and doping to n-type and p-type materials with silicon or oxygen elements has appeared [5,6]. This mismatch disturbs the crystal growth and leads to defects of crystal GaN due to increased tensile stress. In order to produce an excellent GaN device, crystalline GaN should be grown uniformly on the plate. In the present industry, gallium nitride is often grown on foreign substrates on thin films by MOVPE (metal organic vapor phase epitaxy) and MOCVD (metal organic chemical vapor deposition). However, the thermal expansion coefficient of GaN is considerably higher than that of silicon or sapphire. These different properties invite a crack of epitaxial films or wafer bowing in the GaN growth process on the substrate. Therefore, it is difficult to produce bulk single crystals. GaN powder can be produced by a novel hot mechanical alloying process. It requires a lengthy process to yield the powder by this method and the purity of the product is not sufficient [7]. In the methods for synthesis of GaN powder, GaN can be synthesized from molten gallium metal with ˝ a stream of reactive NH3 gas in a furnace. Gallium metal is maintained at a molten state at 30 C. However, it is difficult to vaporize at a low temperature due to its high vaporization temperature of 2400 ˝C. Although this method is simple and economic, it is not suitable to produce quality GaN powder [2,8]. The ammonothermal reduction nitridation method can produce GaN powder by reacting ˝ gallium oxide with NH3 in the temperature range between 600 and 1100 C. However, incomplete nitridation of the oxide easily occurs. In other words, the purity of synthesized GaN is fairly low. (GaP) and (GaAs) are possible alternative materials for Gallium oxide in the temperature range of 1000–1100 ˝C[2,9]. Carbothermal reduction nitridation is applied to synthesize GaN nanorods and nanowires. Carbon is employed as a catalyst for crystal growth [3]. Therefore, an additional decarbonization process is necessary to eliminate the residual carbon. Liquid precursors can be used by an aerosol-assisted vapor phase synthesis method. It is completed at a relatively low temperature. However, this method has a multitude of steps such as preparation of gallium solid compounds, oxidation of gallium precursor, and nitridation chemistry. Therefore, it takes an extended synthesis time to produce GaN powder [10]. Arc plasma has been regarded as an effective method to obtain nanometer-sized GaN. This method requires a very short time compared with other synthesis methods [11,12]. However, gallium lump is an expensive precursor and its evaporation requires an extended time in the arc plasma region. Generally, conventional synthesis methods have several limitations for the preparation of nano-sized particles and complete nitridation. GaN has commonly been grown in the industrial field by the epitaxial method on the substrate. However, it is expected that the uniform deposition of GaN is achievable using the nanoparticle printing method rather than the conventional chemical vapor deposition (CVD) method [6]. Therefore, using the new method would mean that the fine particles with a high crystallinity could be printed on a substrate. It was expected that this method would prevent the mismatch of lattice and irregular growth. In this work, thermal plasma which is able to produce GaN fine particles was applied as substitute technology of the conventional CVD method. Gallium nitrate hydrate (Ga(NO3)3¨xH2O) was used as the raw material instead of Ga or Ga2O3 which have been used in previous studies to synthesize GaN [2–4,10,13–20]. However, the raw material itself has abundant oxygen elements. Therefore, melamine (C3H6N6) was additionally injected into the thermal plasma jet to prevent the oxidation of decomposed Ga into Ga2O3 [21,22]. In order to improve the crystallinity of synthesized GaN, products from the thermal plasma were subjected to annealing using a vacuum furnace. The crystallinity and purity of synthesized GaN nanopowder were investigated before and after annealing.

2. Experimental Setup

GaN nanopowder was synthesized from Ga(NO3)3¨xH2O (99.9% purity, Alfa Aeser Inc., Boston, MA, USA) and C3H6N6 (99% purity, Ald rich Inc., St. Louis, MO, USA) using the non-transferred DC arc plasma. The schematic diagram of the thermal plasma system is indicated in Figure1. The system consists of a DC power supply (YC-500TSPT5, Technoserve, Toyohashi, Japan), a plasma Nanomaterials 2016, 6, 38 3 of 15 torch (SPG-30N2S, Technoserve, Japan), a powder feeder (ME-14C, SHINKO Electric Co Nagoya, Japan) for the injection of the precursor, a chamber, and a crucible for the precursor. The thermal plasmaNanomaterials jet was generated 2016, 6, 38 in the plasma torch by Ar or Ar–N2, which formed gases under atmospheric3 of 15 pressure. The thermal plasma jet was ejected from a 6 mm diameter nozzle by the thermal expansion of plasma,generated forming in the gas plasma by an torch electric by Ar arc or channelAr–N2, which which formed was connected gases under between atmospheric a conical pressure. cathode The and thermal plasma jet was ejected from a 6 mm diameter nozzle by the thermal expansion of plasma, the anode nozzle. Ga(NO3)3¨xH2O powder was used as the raw material for the synthesis of GaN forming gas by an electric arc channel which was connected between a conical cathode and the anode nanopowder. The injected precursor powder was hundreds of micrometers in size, and its morphology nozzle. Ga(NO3)3∙xH2O powder was used as the raw material for the synthesis of GaN nanopowder. is not specific. The precursor was a pellet which had a diameter of 45 mm and a thickness of 10 mm The injected precursor powder was hundreds of micrometers in size, and its morphology is not createdspecific. by a The hydraulic precursor press. was a Thepellet precursor which had pellet a diameter was of placed 45 mm on and a a tungsten thickness crucibleof 10 mm whichcreated was fixedby a hydraulic water-cooling press. holder.The precursor As a result, pellet thewas precursor placed on wasa tungsten rapidly crucible melted which and evaporatedwas fixed by bya the confrontingwater-cooling thermal holder. plasma As jet, a as result, shown the in Figure precursor1. The was vaporization rapidly melted temperature and evaporated of Ga(NO by3 ) the3¨xH 2O ˝ is lowerconfronting than 100 thermalC. Therefore, plasma it is jet a, more as showneconomical in Figure precursor 1. The than vGaaporization or Ga2O 3 whichtemperature were usedof as precursorsGa(NO to3)3 synthesize∙xH2O is lower GaN than in previous100 °C. Therefore work [23, it] is. However, a more economical it contains precursor excessive than nitrogen, Ga or Ga oxygen,2O3 which were used as precursors to synthesize GaN in previous work [23]. However, it contains and hydrogen elements. The complete dissociation of the oxygen from Ga(NO3)3¨xH2O is difficult excessive nitrogen, oxygen, and hydrogen elements. The complete dissociation of the oxygen from due to the high latent heat of H2O. For this reason, it is normal that evaporated Ga(NO3)3¨xH2O is Ga(NO3)3∙xH2O is difficult due to the high latent heat of H2O. For this reason, it is normal that oxidized to Ga2O3 by its inherent oxygen elements. Therefore, melamine (C3H6N6) powder was used evaporated Ga(NO3)3∙xH2O is oxidized to Ga2O3 by its inherent oxygen elements. Therefore, to prevent the production of gallium oxide. The injected C3H6N6 was converted to carbon, nitrogen, melamine (C3H6N6) powder was used to prevent the production of gallium oxide. The injected hydrogen, and other molecules by decomposition at high temperatures in the thermal plasma jet. C3H6N6 was converted to carbon, nitrogen, hydrogen, and other molecules by decomposition at high Thesetemperatures byproduct molecules in the thermal consume plasma the jet. oxygenThese byproduct elements molecules of the Ga(NO consume3)3 ¨thexH 2oxygenO precursor. elements Injected of melaminethe Ga(NO powder3)3∙x consistsH2O precursor. of nonspecific-shaped Injected melamine particles powder under consists 500 of nm. nonspecific In addition,-shaped ammonia particles (NH 3) gas wasunder diagonally 500 nm. injected In addition, into the ammonia pellet of (NH Ga(NO3) gas3 )3 was¨xH 2 diagonallyO from the injected upper side into of the the pellet chamber of to nitrideGa(NO the Ga3)3∙ element.xH2O from the upper side of the chamber to nitride the Ga element.

Plasma generating gas DC power supply

Reactive NH3 gas Powder feeder Plasma torch

Plasma jet

Exhaust Pellet

NH3 Ar N2 Cooling water Figure 1. Experimental apparatus for the synthesis of GaN (gallium nitride) nanopowder by DC Figure 1. Experimental apparatus for the synthesis of GaN (gallium nitride) nanopowder by DC (direct (direct current) non-transferred thermal plasma. current) non-transferred thermal plasma. The detailed operating conditions are indicated in Table 1. In the first case, the condition of ThePlasma detailed 1, the operatingGa(NO3)3∙x conditionsH2O pellet was are indicatedreacted with in only Table NH1. In3 gas the and first without case, theC3H condition6N6. The thermal of Plasma 1, theplasma Ga(NO jet3)3 was¨xH 2generatedO pellet wasby mixed reacted argon with and only nitrogen NH3 gasgases. and The without high thermal C3H6N conductive6. The thermal nitrogen plasma jet waswas generated injected as by a plasma mixed forming argon and gas to nitrogen help the gases. nitridation The or high evaporation thermal as conductive the input power nitrogen was was injectedincreased. as a plasma The plasma forming input gaspower to was help 12.6 the kW nitridation at the fixed orcurrent evaporation of 300 A and as the average input powervoltage was of 42 V. In the other cases of Plasma 2, 3, 4, and 5, the Ga(NO3)3∙xH2O pellet was evaporated by pure increased. The plasma input power was 12.6 kW at the fixed current of 300 A and the average voltage Ar thermal plasma together with C3H6N6. In these cases, the input power for the generation of the of 42 V.thermal In the plasma other casesjet was of 8.4 Plasma kW with 2, the 3, 4, current and 5, fixed the Ga(NOat 300 A3 and)3¨x Han2 Oaverage pellet voltage was evaporated of 28 V. Argon by pure Ar thermalis a monoatomic plasma together molecule with and nitrogen C3H6N 6is. Ina diatomic these cases, molecule. the input In order power to generate for the thermal generation plasma of the thermalfrom plasma nitrogen jet wasgas, more 8.4 kW energy with is the req currentuired compared fixed at 300to argon A and gas. an Accordingly, average voltage electric of 28resistance V. Argon is a monoatomicfor arc generation molecule between and nitrogenthe cathode is and a diatomic the anode molecule. is increased. In orderTherefore, to generate argon thermal thermal plasma plasma from nitrogenhas a lower gas, average more voltage energy with is required lower resistance compared than to argon argon–nitrogen gas. Accordingly, thermal plasma electric at the resistance fixed for arccurrent. generation Input betweenpower of the8.4 cathodekW was sufficient and the anodeto vaporize is increased. the Ga(NO Therefore,3)3∙xH2O pellet argon due thermal to its low plasma has avaporization lower average temperature voltage withof 80 lower°C. Synthesized resistance nanoparticles than argon–nitrogen were attached thermal on the plasmainner surface at the of fixed Nanomaterials 2016, 6, 38 4 of 15

current. Input power of 8.4 kW was sufficient to vaporize the Ga(NO3)3¨xH2O pellet due to its low vaporizationNanomaterials temperature 2016, 6, 38 of 80 ˝C. Synthesized nanoparticles were attached on the inner surface4 of 15 of the reactor which was chilled by cooling water. The product was collected by scraping with a thin film the reactor which was chilled by cooling water. The product was collected by scraping with a thin for thefilm post for processingthe post processing and analysis. and analysis.

TableTable 1. Operating 1. Operating conditions conditions for for thethe synthesissynthesis of of GaN GaN powder powder by bythermal thermal plasma. plasma.

Plasma ExperimentExperiment No. No. PlasmaPlasma 1 1 Plasma 2 PlasmaPlasma 3 3 PlasmaPlasma 4 4 PlasmaPlasma 5 5 2 Weight of Weight of 6 g Ga(NO3)3¨xH2O 6 g Ga(NO3)3∙xH2O Condition of 15 g 15 g Weight of 15 g 15 g 15 g precursorsCondition - 15 g (powder 8.8 g (powder CWeight3H6N6 of C3H6N6 - (pellet) (powder 8.8 g (powder of (pellet) injection) injection) injection) injection) precursors Molar ratio of Molar ratio of Ga(NO3)3¨xH2O - 1:6 1:6 1:3 1:6 Ga(NO3)3∙xH2O and - 1:6 1:6 1:3 1:6 and C3H6N6 C3H6N6 Flow rate of 13 L/min Ar + plasma forming 13 L/min Ar Flow rate of plasma2 L/min13 NL/min2 Ar gas 13 L/min Ar Condition of forming gas + 2 L/min N2 plasma Flow rate of 10 L/min 3 L/min 3 L/min 3 L/min - generating reactive NH3 gas Condition Flow rate of reactive Flow rate of 10 L/min 3 L/min 3 L/min 3 L/min - of plasma NH3 gas - - 3 L/min N 3 L/min N 3 L/min N carrier gas 2 2 2 generating PlasmaFlow rate input of carrier gas12.6 kW - - 3 L/min N2 8.4 kW3 L/min N2 3 L/min N2 power (300 A, 42 V) (300 A, 28 V) 12.6 kW 8.4 kW Plasma input power (300 A, 42 V) (300 A, 28 V) C3H6N6 powder was used in the three different procedures in Plasma 2, 3, 4, and 5. A detailed illustration of C H N injection methods is indicated in Figure2a–d. C H N was mixed with C3H6N6 powder3 6 6 was used in the three different procedures in Plasma 2, 3,3 4,6 and6 5. A detailed Ga(NOillustr3)3¨ationxH2O, of they C3H were6N6 injection pressed methods as the pellet is indicated precursor in in Figure Plasma 2a– 2.d. C C33HH66N66 waspowder mixed was with injected into theGa(NO high3)3 temperature∙xH2O, they were of the pressed thermal as plasmathe pellet jet precursor as powder in Plasma through 2. theC3H anode6N6 powder electrode was ininjected Plasmas 3, 4, andinto 5. the The high molar temperature ratio of Ga(NO of the thermal3)3¨xH 2plasmaO and jet C3 asH 6powderN6 was through controlled the anode at 1:6 electrode and 1:3 in Plasma Plasmass 3, 4, and 5.3, 4, In and these 5. The cases, molar NH ratio3 gas of Ga(NO was injected3)3∙xH2O into and theC3H pellet6N6 was as controlled a nitridation at 1:6 and source 1:3 in by Plasma the probes 3, 4, as in and 5. In these cases, NH3 gas was injected into the pellet as a nitridation source by the probe as in Figure1. Ga(NO 3)3¨xH2O was reacted with only C3H6N6 powder and without NH3 gas in Plasma 5, Figure 1. Ga(NO3)3∙xH2O was reacted with only C3H6N6 powder and without NH3 gas in Plasma 5, in in order to check the possibility of nitridation by the nitrogen element of C3H6N6. The products from order to check the possibility of nitridation by the nitrogen element of C3H6N6. The products from thermal plasma were annealed in a vacuum furnace (SH-TMFGF-50, Samheung Inc., Sejong, Korea) to thermal plasma were annealed in a vacuum furnace (SH-TMFGF-50, Samheung Inc., Sejong, Korea) to eliminate contamination and to enhance the crystallinity. It was carried out at 850 ˝C for three hours. eliminate contamination and to enhance the crystallinity. It was carried out at 850 °C for three hours.

a) Torch b) Torch NH3 gas NH3 gas

Ga(NO3)3∙xH2O Ga(NO3)3∙xH2O +

C3H6N6 (pellet) (pellet)

Plasma reactor Plasma reactor

Torch Torch C H N powder c) C3H6N6 powder d) 3 6 6 NH3 gas

Ga(NO3)3∙xH2O Ga(NO3)3∙xH2O

(pellet) (pellet)

Plasma reactor Plasma reactor

Figure 2. Detailed illustration of C3H6N6 injection methods in Plasma 1, 2, 3, 4, and 5; (a) Plasma 1; (b) Figure 2. Detailed illustration of C3H6N6 injection methods in Plasma 1, 2, 3, 4, and 5; (a) Plasma 1; Plasma 2; (c) Plasmas 3 and 4; (d) Plasma 5. (b) Plasma 2; (c) Plasmas 3 and 4; (d) Plasma 5. Nanomaterials 2016, 6, 38 5 of 15 Nanomaterials 2016, 6, 38 5 of 15

A TGA analysis resultresult ofof Ga(NOGa(NO33))3∙¨xH2OO as as raw raw materials materials according according to temperature is indicated in Figure3 3.. Analysis Analysis was was conducted conducted from from 40 40˝ °CC toto 800800 ˝°CC withwith anan increaseincrease inin incrementsincrements of of 10 10˝ °CC/min/min under a nitrogennitrogen atmosphere.atmosphere. An obvious mass reduction of approximately 67% occurred between ˝ ˝ 6060 °CC and 18 1800 °CC as as is is shown shown in in Figure Figure 3.3. Ga(NO Ga(NO3)33)∙3x¨HxH2O2 Owas was converted converted to toGa Ga2O23O, H3,H2O,2 andO, and N2O N3 2gasO3 gaswith with increasing increasing temp temperatures,eratures, which which is in accordance is in accordance with previous with previous studies studies [23]. Therefore, [23]. Therefore, it can be it canassumed be assumed that the that final the product final product of Ga2O of3 with Ga2O a3 masswith reduction a mass reduction of 78% would of 78% be would produced be produced at 800 °C at. ˝ 800As a C.result, As a the result, estimated the estimated value of value x in ofGa(NOx in Ga(NO3)3∙xH23O)3 ¨wasxH2 Odetermined was determined to be 32.1 to be based 32.1 basedon the on mass the ˝ ˝ massreduction reduction from from12.45 12.45mg at mg 40 at°C 40 to 2.80C to mg 2.80 at mg 800 at °C 800. Accordingly,C. Accordingly, Ga(NO Ga(NO3)3∙xH3)23O¨x isH 2referredO is referred to as toGa(NO as Ga(NO3)3∙32H3)32O¨32H in this2O in work. this work.

100

80

60

Mass (%) 40

20

100 200 300 400 500 600 700 800 Sample temperature (C)

3 3 2 FigureFigure 3.3. TGATGA (thermogravimetric(thermogravimetric analysis)analysis) ofof Ga(NOGa(NO3))3¨∙xH2OO raw raw material. material.

The crystallinity of the synthesized powder was observed by XRD (X-ray diffraction, DMAX The crystallinity of the synthesized powder was observed by XRD (X-ray diffraction, DMAX 2500, 2500, Rigaku Co., Akishima, Japan) with Cu Kα source. Morphology and particle size were analyzed Rigaku Co., Akishima, Japan) with Cu Kα source. Morphology and particle size were analyzed by by FE-SEM (Field-emission scanning electron microscopy, S-4300, Hitachi Co., Tokyo, Japan) and FE- FE-SEM (Field-emission scanning electron microscopy, S-4300, Hitachi Co., Tokyo, Japan) and FE-TEM TEM (Field-emission transmission electron microscopy, JEM-2100F, Jeol, Japan). In addition, the (Field-emission transmission electron microscopy, JEM-2100F, Jeol, Japan). In addition, the elemental elemental composition of particles was confirmed by an EDS (Energy dispersive spectroscopy) with composition of particles was confirmed by an EDS (Energy dispersive spectroscopy) with SEM and SEM and TEM. The mean particles size was calculated from the variation of the hundreds of different TEM. The mean particles size was calculated from the variation of the hundreds of different synthesized synthesized particles in the FE-SEM images. TGA (thermogravimetric analysis, Diamond TG-DTA particles in the FE-SEM images. TGA (thermogravimetric analysis, Diamond TG-DTA Lab system, Lab system, Perkin Elmer, Waltham, MA, USA) was applied to investigate the atomic ratio of Perkin Elmer, Waltham, MA, USA) was applied to investigate the atomic ratio of Ga(NO3)3¨xH2O as Ga(NO3)3∙xH2O as raw materials and the thermal behavior of particles synthesized by the thermal raw materials and the thermal behavior of particles synthesized by the thermal plasma and annealing plasma and annealing procedure. Chemical bonding and nanostructure of particles synthesized by procedure. Chemical bonding and nanostructure of particles synthesized by the thermal plasma and the thermal plasma and vacuum furnace method were analyzed by XPS (X-ray photoelectron vacuum furnace method were analyzed by XPS (X-ray photoelectron spectroscopy, K-Alpha, Thermo spectroscopy, K-Alpha, Thermo scientific Inc., Waltham, MA, USA). scientific Inc., Waltham, MA, USA).

3. Results Results and and Discussion Discussion Figure4 a4ashows shows the thethermodynamic thermodynamic equilibrium equilibrium composition composition of of melamine melamine at at changes changes in in temperature fromfrom 500500 toto 6000 K. The calculation was conducted by a commercial software of FactSage (Ver.(Ver. 6.4, 6.4, CRCT>T, CRCT>T, Canada Canada and Germany). and Germany The) purpose. The purpose of thermodynamic of thermodynamic equilibrium equilibrium calculation wascalculation to expect was a preferable to expect chemical a preferable reaction chemical and stable reaction chemical and species stable inchemical a given species temperature in a range. given temperature range. C3H6N6 containing carbon, hydrogen, and nitrogen elements can be decomposed C3H6N6 containing carbon, hydrogen, and nitrogen elements can be decomposed and dissociated atand about dissociated 350 ˝C. at The about melamine 350 °C. The is converted melamine to is ,converted hydrocarbons,to cyanides, hydrocarbons, and carbon inand the carbon high in the high temperature plasma zone. These decomposed or dissociated C3H6N6 byproducts can react temperature plasma zone. These decomposed or dissociated C3H6N6 byproducts can react with the with the oxygen of a Ga(NO3)3∙32H2O precursor. Furthermore, the various exothermic gases such as oxygen of a Ga(NO3)3¨32H2O precursor. Furthermore, the various exothermic gases such as CO, CO2, CO, CO2, H2, N2, NO2, and the releasing of heat generated by the oxygen capture reaction with H2,N2, NO2, and the releasing of heat generated by the oxygen capture reaction with Ga(NO3)3¨32H2O Ga(NO3)3∙32H2O and C3H6N6 promote the synthesis of GaN nanopowder. and C3H6N6 promote the synthesis of GaN nanopowder. Nanomaterials 2016, 6, 38 6 of 15 Nanomaterials 2016, 6, 38 6 of 15 a) C H N (melamine) b) 3 6 6 CH4 + O2 → CO2 + 2H2

1500000 HCN + O2 → CO2 + 0.5N2 + 0.5H2

C2H2 + O2 → 2CO + H2

C2H + O2 → 2CO + 0.5H2 N2 (g) 1000000 C2N + O2 → 2CO + 0.5N2

CN + O2 → CO + 0.5N2 H(g) Ga + 1.5O2 → Ga2O3 500000 C + O → CO H (g) 2 2 ] 2 Temperature [K]

mol 0 [ 6000 5000 4000 3000 2000 1000

C(g) C3(g) -500000 CH4 (g) C2N(g) C2H(g) N (g) HCN(g) CN(g) C2H2(g) C (g)

2 free energy Gibbs [J] -1000000 △ Temperature Thermodynamic equilibrium composition equilibrium Thermodynamic [K] -1500000 c) 1000000 Ga + HCN → GaN + C + 0.5H2

Ga + C2N → GaN + 2C 750000 Ga + CN → GaN + C

500000

250000 Temperature [K] 0 6000 5000 4000 3000 2000 1000 -250000

-500000 Gibbs free free energy Gibbs [J]

△ -750000 -1000000

FigureFigure 4. 4. ThermodynamicThermodynamic equilibrium equilibrium calculation calculation at at changes changes in in temperature; temperature; ( (aa)) thermodynamic thermodynamic equilibriumequilibrium composition composition of of m melamine;elamine; (b (b) )change change in in Gibbs Gibbs free free energy energy during during the the oxygen oxygen capture capture

reactionreaction of of C C33HH66NN6 6fromfrom Ga(NO Ga(NO3)33)∙x3¨HxH2O2;O; and and (c () cchange) change in in Gibbs Gibbs free free energy energy of of nitridation nitridation reaction reaction byby decomposed decomposed C C33HH6N6N6.6 .

TheThe change change in in Gibbs Gibbs free free energy energy during during oxygen oxygen capture capture reactions reactions with with byproducts byproducts converted converted by by decomposition of the C3H6N6 which was injected into the thermal plasma jet are shown in Figure 4b. decomposition of the C3H6N6 which was injected into the thermal plasma jet are shown in Figure4b. TheThe graph graph consists consists of of seven seven dotted dotted lines lines and and one one solid solid line. line. The The dotted dotted lines lines indicate indicate the theoxidation oxidation by byproducts generated from C3H6N6 decomposition, while the solid line indicates the oxidation of by byproducts generated from C3H6N6 decomposition, while the solid line indicates the oxidation gallium to Ga2O3. Oxidation reactions of hydrocarbons, cyanides, and carbon generated by the of gallium to Ga2O3. Oxidation reactions of hydrocarbons, cyanides, and carbon generated by the decomposition of C3H6N6 are thermodynamically spontaneous throughout the whole temperature decomposition of C3H6N6 are thermodynamically spontaneous throughout the whole temperature range because Gibbs free energies are less than zero. Conversely, Ga2O3 can be produced range because Gibbs free energies are less than zero. Conversely, Ga2O3 can be produced spontaneously spontaneouslyunder 3100 K. under However, 3100 oxidation K. However, of byproducts oxidation of is byproducts still predominant is still predominant at temperatures at temperatures above 750 K above 750 K owing to a greater amount of negative Gibbs free energy. Therefore, C3H6N6 powder is owing to a greater amount of negative Gibbs free energy. Therefore, C3H6N6 powder is suitable to suitable to capture oxygen molecules of Ga(NO3)3∙32H2O before they oxidize with gallium, even capture oxygen molecules of Ga(NO3)3¨32H2O before they oxidize with gallium, even though various though various byproducts could be produced from the decomposition of C3H6N6. For this reason, byproducts could be produced from the decomposition of C3H6N6. For this reason, C3H6N6 powder Cwas3H6N injected6 powder into thewas thermal injected plasma into jet the to thermal facilitate plasma the synthesis jet to of facilitate GaN nanopowder. the synthesis Moreover, of GaN the nanopowder. Moreover, the TGA analysis and thermodynamic equilibrium calculation revealed that TGA analysis and thermodynamic equilibrium calculation revealed that six moles of C3H6N6 powder six moles of C3H6N6 powder were needed, while only one mole of Ga(NO3)3∙32H2O was required to were needed, while only one mole of Ga(NO3)3¨32H2O was required to sufficiently convert oxygen sufficiently convert oxygen from Ga(NO3)3∙32H2O into carbon oxides, nitrogen oxides, and from Ga(NO3)3¨32H2O into carbon oxides, nitrogen oxides, and hydrogen oxides. hydrogen oxides. Nitridation by the nitrogen element of C3H6N6 is considered in Figure4c. The thermodynamic Nitridation by the nitrogen element of C3H6N6 is considered in Figure 4c. The thermodynamic equilibrium is calculated for the nitridation reaction by HCN, C2N, and CN molecules. equilibriumThese components is calculated containing for the nitrogen nitridation elements reaction were reacted by HCN, with C2 theN, Ga and element CN molecules. at temperatures These componentsunder 3000 K.containing Although nitrogen∆G ofthe elements threenitridation were reacted reactions with the have Ga negativeelement at values, temperatures they are under lower 3000 K. Although ∆G of the three nitridation reactions have negative values, they are lower than those than those of the oxidation reaction in Figure4b. ∆G values for the oxidation reaction of HCN, C2N, ofand the CN oxidation at about reaction 3000 K arein Figure under 4 500,000b. ∆G values J, as shown for the in oxidation Figure4b. reaction Those values of HCN, for theirC2N, nitridationand CN at about 3000 K are under 500,000 J, as shown in Figure 4b. Those values for their nitridation reaction at approximately 3000 K are nearly zero. In other words, oxidation reaction is absolutely superior Nanomaterials 2016, 6, 38 7 of 15

Nanomaterialsreaction at 2016 approximately, 6, 38 3000 K are nearly zero. In other words, oxidation reaction is absolutely7 of 15 superior compared to the nitridation reaction at all temperature ranges. Therefore, the byproducts, whichcompared are convertedto the nitridation from the reaction decomposition at all temperature of C3H6N ranges.6, usually Therefore, react with the the byproducts, oxygen elements which are of convertedthe Ga(NO 3 from)3¨32H the2O rawdecomposition material. of C3H6N6, usually react with the oxygen elements of the Ga(NOXRD3)3∙32H patterns2O raw of material. products synthesized in Plasma 1 and 2 conditions are shown in Figure5a,b, respectively.XRD patterns For Plasma of products 1, only synthesized a Ga(NO3)3¨ 32Hin Plasma2O pellet 1 and was 2 used conditio withn 10s are L/min shown NH in3 to Figure synthesize 5a,b, respectivelyGaN nanopowder. For Plasma without 1, only C3H a6 NGa(NO6. The3)3 plasma∙32H2O jetpellet was was generated used with by 10 argon L/min and NH nitrogen3 to synthesize mixed GaNgas in nanopowder a conventional without synthesis C3H6 processN6. The ofplasma nitride jet materials was generated by thermal by argon plasma. and Allnitrogen peaks mixed shown gas in inFigure a conventional5a correspond synthesis to Ga process2O3. The of results nitride revealed materials that by thermal Ga(NO3 plasma.)3¨32H2 OAll is peaks not nitrided shown in solely Figure by 5NHa correspond3 gas, which to is Ga a2 typicalO3. The nitrogen results revealed source for that the Ga(NO nitridation3)3∙32H reaction.2O is not Ionized nitrided nitrogen solely gasby NH which3 gas, is producedwhich is a bytypical the generating nitrogen source thermal for plasma the nitridation jet was not reaction. suitable Ionized for the nitrogen nitridation gas of which Ga(NO is3 produced)3¨32H2O. byGa(NO the 3 generating)3¨32H2O has thermal a low evaporation plasma jet temperature was not suitable and is rapidly for the vaporized nitridation in the of Ga(NOhigh temperature3)3∙32H2O. Ga(NOenvironment3)3∙32H of2O the has thermal a low evaporation plasma jet. temperature However, nitridation and is rapidly of Ga vaporized did not occur,in the high and temperature oxidation of environmentGa was completed of theby thermal the presence plasma of jet. abundant However, oxygen nitridation elements of Ga from did the not precursor occur, and itself. oxidation NH3 gas of Gausually was decomposescompleted by into the N presence2 and H2 ofgases abundant at high oxygen temperatures. elements As from shown the inprecursor the following itself. chemicalNH3 gas usuallyreactions, decomposes the oxidation into of N Ga2 and dominates H2 gases over at high the reactiontemperatures. with N As2 and shown NH in3 to the GaN. following chemical reactions, the oxidation of Ga dominates over the reaction with N2 and NH3 to GaN. Ga ` 0.5 N2 Ñ GaN ∆H298K : ´109.6 kJ (1) Ga + 0.5 N2 → GaN ∆H298K: −109.6 kJ (1) Ga ` NH Ñ GaN ` 1.5 H ∆H : ´63.7 kJ (2) Ga + NH3 →3 GaN + 1.5 H2 2 298K∆H298K: −63.7 kJ (2)

2 Ga ` 1.5 O2 Ñ Ga2O3 ∆H298K : ´1089.1 kJ (3) 2 Ga + 1.5 O2 → Ga2O3 ∆H298K: −1089.1 kJ (3)

a) Ga2O3 b) * GaO(OH) CH6N4O NH4NO

* C

Intensity (a.u.) * *

Intensity (a.u.)

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 Two Theta (deg) 2 Theta (deg)

FigFigureure 5. XRD ( (X-rayX-ray diffraction diffraction)) pattern of product synthesized by thermal plasma; ( a) Plasma 1 by Ga(NO3)3∙32H2O and NH3 without C3H6N6; (b) Plasma 2 by Ga(NO3)3∙32H2O and C3H6N6 pellet with Ga(NO3)3¨32H2O and NH3 without C3H6N6;(b) Plasma 2 by Ga(NO3)3¨32H2O and C3H6N6 pellet 3 withNH gas. NH3 gas.

These findings indicate that the nitridation of Ga(NO3)3∙32H2O is difficult without a substance These findings indicate that the nitridation of Ga(NO3)3¨32H2O is difficult without a substance that can consume oxygen as a reductant. Therefore, C3H6N6 powder was used as a reductant to that can consume oxygen as a reductant. Therefore, C3H6N6 powder was used as a reductant to oxidize with the abundant oxygen elements of the Ga(NO3)3∙32H2O precursor. oxidize with the abundant oxygen elements of the Ga(NO3)3¨32H2O precursor. C3H6N6 powder was used as a pellet by compressing it with Ga(NO3)3∙32H2O as a precursor in C3H6N6 powder was used as a pellet by compressing it with Ga(NO3)3¨32H2O as a precursor Plasma 2. The pellet consisting of Ga(NO3)3∙32H2O and C3H6N6 was vaporized by a thermal plasma in Plasma 2. The pellet consisting of Ga(NO3)3¨32H2O and C3H6N6 was vaporized by a thermal jet. NH3 gas was injected into the pellet at 3 L/min. The XRD pattern of synthesized product at Plasma plasma jet. NH gas was injected into the pellet at 3 L/min. The XRD pattern of synthesized product 2 conditions is 3 indicated in Figure 5b. The peaks are analyzed as gallium hydroxide (GaO(OH)), at Plasma 2 conditions is indicated in Figure5b. The peaks are analyzed as gallium hydroxide carbohydrazide (CH6N4O), and ammonium nitrate (NH4NO). The vaporized Ga elements were (GaO(OH)), carbohydrazide (CH6N4O), and ammonium nitrate (NH4NO). The vaporized Ga elements oxidized, and C3H6N6 was not applied in sufficient amounts to act as a reductant. Unlike the were oxidized, and C3H6N6 was not applied in sufficient amounts to act as a reductant. Unlike the experiment involving Ga(NO3)3∙32H2O and NH3 gas without C3H6N6, GaO(OH) was produced. experiment involving Ga(NO ) ¨32H O and NH gas without C H N , GaO(OH) was produced. Products including C-H-O bonding3 3 or2 ammonium3 ions were produced3 6 6 in the experiment using a Products including C-H-O bonding or ammonium ions were produced in the experiment using a pellet mixed with C3H6N6 and Ga(NO3)3∙32H2O. It was assumed that C3H6N6 could not be used to pellet mixed with C3H6N6 and Ga(NO3)3¨32H2O. It was assumed that C3H6N6 could not be used to capture oxygen when it was mixed with Ga(NO3)3∙32H2O in a pellet. Although C3H6N6 affects the product, the effect is not sufficient to nitrate the gallium. Ga(NO3)3∙32H2O was vaporized above 80 °C. The vaporizing temperature of C3H6N6 is 345 °C . The difference of vaporization temperatures for the two raw materials should be considered to prevent oxidation of the Ga element. The useful reductants Nanomaterials 2016, 6, 38 8 of 15

capture oxygen when it was mixed with Ga(NO3)3¨32H2O in a pellet. Although C3H6N6 affects the ˝ product, the effect is not sufficient to nitrate the gallium. Ga(NO3)3¨32H2O was vaporized above 80 C. ˝ TheNanomaterials vaporizing 2016, temperature6, 38 of C3H6N6 is 345 C. The difference of vaporization temperatures for8 of the 15 two raw materials should be considered to prevent oxidation of the Ga element. The useful reductants generated by vaporization ofof CC33H6NN66 havehave to to react react with with the the gallium gallium element element before before oxidation. In In order order to vaporize C 3HH66NN66 early,early, it it was was injected injected into into the the higher higher temperature temperature region region of of the the thermal thermal plasma. plasma. C3HH66NN66 powderpowder was was fed fed solely solely into into a a high high temperature temperature thermal thermal pla plasmasma jet jet through through two nozzles inside the anode electrode toto enableenable moremore activeactive decomposition.decomposition. Ga(NOGa(NO33))33¨∙32H32H2OO has has an an evaporation evaporation temperature below 10 1000 °C˝C,, and rapidly vaporized when it contactedcontacted the high temperature thermal plasma jet. The molar ratioratio ofof Ga(NOGa(NO33))33¨∙32H32H2OO and C 3HH66NN66 waswas controlled controlled at at 1:6 1:6 and and 1:3 1:3 in in Plasma Plasma 3 and 4 4 conditions. conditions. Initially, Initially, C3 H C63NH66Npowder6 powder was was injected injected at a 1:6 at molar a 1:6 ratio molar according ratio according to the calculation to the calculationfrom the TGA from analysis the TGA of Ga(NOanalysis3) of3¨32H Ga(NO2O raw3)3∙32H materials.2O raw materials. This molar This ratio molar was sufficientratio was tosufficient oxidize toC3 oxidizeH6N6 from C3H the6N6 oxygenfrom the element oxygen of element the Ga(NO of the3) 3Ga(NO¨32H2O.3)3 The∙32H graph2O. The in graph Figure in6 showsFigure XRD6 shows patterns XRD pofatterns the product of the synthesized product synthesized in Plasma 3.in ThePlasma main 3. peaks The main correspond peaks tocorrespond C3H6N6 and to C other3H6N peaks6 and reflectother peaksCxHyN reflectz bound CxH byproductsyNz bound byproducts such as melam, such melem as melam, and melem melon. and C3H melon.6N6 can C be3H6 convertedN6 can be converted into other intoderivatives other derivatives by thermal by condensation thermal condensation [24–27]. The [24 synthesized–27]. The productsynthesi waszed subjectedproduct was to annealing subjected in to a annealingvacuum furnace in a vacuum to enhance furnace the GaN to enhance crystallinity the GaN and crystallinity eliminate the and residual eliminate C3H6 theN6. residual The XRD C pattern3H6N6. Theof annealed XRD pattern nanopowder of annealed is indicated nanopowder in Figure is6. indicated Distinct peaks in Figure of GaN 6. were Distinct observed peaks after of GaN annealing were ˝ observedat 850 C forafter three annealing hours. at Weak 850 graphite°C for three and hours. carbon Weak nitride graphite (C3N4) peaksand carbon were alsonitride observed. (C3N4) Itpeaks was wereestimated also that observed. the synthesized It was estimated GaN nanopowder that the synthesized was not accurately GaN nanopowder detected in the was XRD not pattern accurately due detectedto its low in crystallinity the XRD pattern and low due quantity. to its low crystallinity and low quantity.

GaN

C3H6N6 C2H4N4

Synthesized product

Intensity (a.u.)

Annealing at 850℃, 3h graphite C3N4

10 20 30 40 50 60 70 80 90 Two Theta (deg)

FigFigureure 6. XRD patterns ofof productsproducts synthesizedsynthesized fromfrom a a Ga(NO Ga(NO3)33)3¨∙32H32H22O pelletpellet andand CC33HH66N6 powder injection with NH 3 gasgas by by the the thermal thermal plasma process in Plasma 3.

This result was reliable as confirmedconfirmed by the Ga–NGa–N chemical bonding by XPS result resultss shown in Figure 77.. TheThe Ga2p,Ga2p, N1s,N1s, andand C1sC1s orbitalorbital graphsgraphs areare indicatedindicated inin FigureFigure7 7a,b.a,b. In In the the Ga2p Ga2p graphs graphs of of Figure 7a, Ga–N bonding peaks of Ga2p(1/2) and Ga2p(3/2) were identically observed at 1143.0 eV and Figure7a, Ga–N bonding peaks of Ga2p (1/2) and Ga2p(3/2) were identically observed at 1143.0 eV 1116.2 eV [26,28–31]. Although peak intensities of Ga2p(1/2) and Ga2p(3/2) in Figure 7a are weak, the and 1116.2 eV [26,28–31]. Although peak intensities of Ga2p(1/2) and Ga2p(3/2) in Figure7a are weak, twothe twopeaks peaks could could be accurately be accurately observed observed in the in Ga2p the Ga2p graph. graph. This Thisresult result demonstrated demonstrated that GaN that GaNwas wasdefinitely definitely synthesized synthesized by the by thermal the thermal plasma, plasma, but was but not was detected not detected in the XRD in the pattern XRD pattern due to its due low to crystallinity.its low crystallinity. The N=C The sp2 N=C binding sp2 peak binding is high peak (398.7 is high eV) (398.7 in the eV)N1s in graphs the N1s of Figure graphs 7 of. This Figure peak7. was caused by residual C3H6N6 and its derivatives after synthesis using thermal plasma [32]. An This peak was caused by residual C3H6N6 and its derivatives after synthesis using thermal plasma [32]. N–Ga bonding peak was observed at 397.8 eV. Amino functional groups having (–NHx) were An N–Ga bonding peak was observed at 397.8 eV. Amino functional groups having (–NHx) were indicated at 400.2 eV by residual C3H6N6 and it derivatives. In the C1s graph, C=N and C–C binding indicated at 400.2 eV by residual C3H6N6 and it derivatives. In the C1s graph, C=N and C–C binding peaks were present at 287.5 and 284.3 eV, respectively, which was attributed to the remaining C3H6N6. Additionally, the intensity of the C=N bonding peak was higher than that of the C-C bonding peak [32]. Nanomaterials 2016, 6, 38 9 of 15

peaks were present at 287.5 and 284.3 eV, respectively, which was attributed to the remaining C3H6N6. Additionally,Nanomaterials 2016 the, 6,intensity 38 of the C=N bonding peak was higher than that of the C-C bonding peak9 of [32 15].

a) Ga2p Unannealed sample N1s Unannealed sample C1s Unannealed sample

398.7 eV (N=C sp2) 287.5 eV 284.3 eV (C=N) 397.8 eV (C–C) (N–Ga)

400.2 eV (–NHx)

Arb. Unit (counts/s)Arb. Unit (counts/s)Arb. Unit

Arb. Unit (counts/s)Arb. Unit

1150 1140 1130 1120 1110 410 408 406 404 402 400 398 396 394 298 296 294 292 290 288 286 284 282 280 Binding Energy (eV) Binding Energy (eV) Binding Energy (eV)

b) Ga2p Annealed sample N1s Annealed sample C1s Annealed sample

399.1 eV (N-Ga)

396.4 eV 287.6 eV 400.4 eV (N–C sp2) (C–N sp3) (–NHx)

Arb. Unit (counts/s)Arb. Unit (counts/s)Arb. Unit

Arb. Unit (counts/s)Arb. Unit

1150 1140 1130 1120 1110 410 408 406 404 402 400 398 396 394 298 296 294 292 290 288 286 284 282 280

Binding Energy (eV) Binding Energy (eV) Binding Energy (eV)

Figure 7. XPSXPS (X (X-ray-ray photoelectron photoelectron spectroscopy spectroscopy analysis analysis)) of of nanopowder synthesized from a Ga(NO ) ¨32H O pellet and C H N powder injection with NH gas by the thermal plasma process in Ga(NO3)33∙32H22O pellet and C33H6N6 6 6powder injection with NH33 gas by the thermal plasma process in a b Plasma 3 3;; ( a) before and (b) after annealing.annealing.

Figure 77bb showsshows XPSXPS analysisanalysis resultsresults ofof annealedannealed nanopowdernanopowder afterafter thermalthermal plasmaplasma synthesissynthesis in Plasma 3. XRD and TEM analysis revealed that the GaN synthesized by the thermal plasma had enhanced crystallinity after annealing. After After annealing annealing at at 85 8500 °C˝C under rough vacuum pressure for three hours, the peak intensities of Ga Ga–N–N bonding were increased to levels which were significantly significantly higher than beforebefore thethe annealingannealing step. step. Ga–N Ga–N bonding bonding peaks peaks of of Ga2p Ga2p(1/2)(1/2) andand Ga2pGa2p(3/2)(3/2) werewere observed observed at 1143.5 eV and 1116.7 eV, respectively [[26,2826,28–31]. However, the N1s graphs all show different peak trendstrends,, as is shown in Figure7 7a.a. FollowingFollowing annealingannealing ofof thethe productproduct synthesizedsynthesized inin PlasmaPlasma 3,3, thethe peaks appeared at 400.4 and 396.4 eV in N1s,N1s, which can bebe observedobserved in FigureFigure7 7b.b. ThisThis peakpeak waswas attributed to nitrogen in the –NH–NHx and N–CN–C sp2 bond fromfrom CC33N4 generated from the conversion of residual CC33HH66N6 [[33,34].33,34]. AA deconvoluteddeconvoluted N–Ga N–Ga bonding bonding peak peak was was observed observed at 399.1 at 399.1 eV [ 27eV]. [27]. This peakThis waspeak shiftedwas shift compareded compared to pre-annealing to pre-annealing due todue the to sufficient the sufficient levels levels of nitrogen of nitrogen of synthesized of synthesized GaN GaNby decomposition by decomposition of residual of residual C3H6 CN36H.6 InN6 the. In C1sthe graphC1s graph of Figure of Figure7b, a 7 C–Nb, a C sp3–N bonding sp3 bonding peak peak was producedwas produced at 287.6 at 287.6 eV. C–C eV. bondingC–C bonding peaks peaks are deconvoluted are deconvoluted at 289.8 at 289.8 and 284.8 and 28 eV4.8 [33 eV,34 [33,34]. ]. The results of TGA analysis of product synthesized under the Plasma 3 operating conditions are indicated inin FigureFigure8 .8 The. The temperature temperature of of the the crucible crucible containing containing the synthesizedthe synthesized product product powder powder was wasincreased increased from from 40 ˝ C40 to°C 850 to 85˝C0 °C at 10at 1˝0C/min °C/min intervals. intervals. The The atmosphere atmospherewas was filledfilled withwith a nitrogen gas. A total mass reduction of up to 98% of weight occurred. The “ “asas prepared product product”” after TGA analysis was considered to consist of GaNGaN andand aa smallsmall amountamount ofof CC33NN44.. As As reported in previous ˝ studies [[2626,,33,3433,34],], CC33N4 cancan be be synt synthesizedhesized at high temperaturestemperatures (>600(>600 °CC)) byby slowlyslowly heatingheating CC33HH66N66 ˝ ˝ alone. The The mass mass reduction reduction curve curve had had a high a high gradient gradient from from about about 300 °C 300 to 65C0 to°C. 650 The residualC. The residual C3H6N6 andC3H derivatives6N6 and derivatives undergo thermal undergo degradation thermal degradation and decomposition and decomposition at temperatures at temperatures in excess of in 30 excess0 °C. Basedof 300 ˝ onC. Basedthe mass on the maintenance mass maintenance at above at 62 above0 °C, 620 the˝ C, temperature the temperature for annealing for annealing to enhance to enhance the crystallinitythe crystallinity of synthesized of synthesized GaN GaN nanopowder nanopowder was was set set at at85 8500 °C˝.C. Since Since the the melting melting point point of of GaN GaN is above 2 2500500 °C˝C,, synthesized GaN nan nanopowderopowder was not melted or vaporized by the annealing. Nanomaterials 2016, 6, 38 10 of 15 NanomaterialsNanomaterials 2016 2016, ,6 6, ,3 388 1010 of of 15 15

100 Synthesized product 100 Synthesized product

80 80

60 60

40 40

Mass (%)

Mass (%)

20 20

0 0 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 Sample temperature (C) Sample temperature (C)

Figure 8. TGA analysis of product synthesized from a Ga(NO3)3∙32H2O pellet and C3H6N6 powder FigureFigure 8. TGA analysisanalysis ofof productproduct synthesizedsynthesized fromfrom aa Ga(NO Ga(NO3)3)33¨∙3232HH22O pellet andand CC33H6N66 powderpowder injection with NH3 gas by the thermal plasma process in Plasma 3. injectioninjection with with NH NH33 gasgas by by the the thermal thermal plasma plasma process process in in Plasma Plasma 3. 3.

Figure 9a,b shows FE-TEM images of the synthesized product before and after annealing in FigureFigure 99a,ba,b shows shows FE-TEMFE-TEM images images of of thethe synthesizedsynthesized product product beforebefore and and after after annealing annealing inin Plasma 3 conditions. Small particles under 100 nm and spherical large particles of about 200 nm were PlasmaPlasma 3 3 conditions. conditions. Small Small particles particles under under 100 100 nm nm and and spherical spherical large large particles particles of of about about 200 200 nm nm were were mixed in Figure 9a. After annealing, the large spherical particles were no longer present, and only mixedmixed in in Figure Figure 99a.a. AfterAfter annealing,annealing, the large sphericalspherical particles were no longerlonger present,present, andand onlyonly nano-sized round particles which had an average particle size of 21.63 nm remained, as is shown in nanonano-sized-sized round round particles particles which which had had an an average average particle particle size size of of 21.63 21.63 nm nm remained, remained, as as is is shown shown in in Figure 9b. It is believed that the large spherical particles are the residual C3H6N6 precursor. Figure 9c FigureFigure 99b.b. ItIt isis believedbelieved thatthat thethe largelarge sphericalspherical particlesparticles arearethe theresidual residual C C3H6NN6 precursor.precursor. Figure Figure 99cc shows TEM-EDS results before and after annealing in Plasma 3. The Ga3 element6 6 was detected as showsshows TEM-EDSTEM-EDS resultsresults before before and and after after annealing annealing in Plasmain Plasma 3. The 3. The Ga elementGa element was detectedwas detected as about as about 2.5% of weight before annealing. It was revealed that melamine powder remained in micro- about2.5% of2.5 weight% of weight before before annealing. annealing. It was It revealedwas revealed that melaminethat melamine powder powder remained remained in micro-sized in micro- sized particles. The content of Ga elements was increased dramatically from 2.45% to 71.6% of weight sizedparticles. particles. The content The content of Ga of elements Ga elements was increasedwas increased dramatically dramatically from from 2.45% 2.45 to 71.6%% to 71.6 of% weight of weight after after annealing. This analysis demonstrates that the purification was completed and residual afterannealing. annealing. This analysis This analysis demonstrates demonstrates that the that purification the purification was completed was completed and residual and melamine residual melamine powder was eliminated by the heat treatment of the vacuum furnace. As shown by the melaminepowder was powder eliminated was eliminated by the heat by treatment the heat of treatment the vacuum of furnace.the vacuum As shownfurnace. by As the shown XRD patterns by the XRD patterns in Figure 6b, a small amount of carbon and carbon nitride was generated after XRDin Figure patterns6b, a small in Figure amount 6b, of a carbon small andamount carbon of nitride carbon was andgenerated carbon nitride after annealing was generated because after the annealing because the treatment was done under vacuum conditions. The vaporized melamine anntreatmentealing wasbecause done the under treatment vacuum was conditions. done under The vacuum vaporized conditions. melamine The converted vaporized as a byproduct melamine converted as a byproduct with gallium nitride nanoparticles. convertedwith gallium as a nitride byproduct nanoparticles. with gallium nitride nanoparticles.

a)a) bb))

0.20.2㎛㎛ 5050 nm nm

Figure 9. Cont. FigFigureure 9 9.. ContCont.. Nanomaterials 2016, 6, 38 11 of 15 NanomaterialsNanomaterials 20162016,, 66,, 3388 1111 ofof 1515

Ga(NO3)3∙xH2O : C3H6N6 (mol) = 1:6 (annealed at 850℃) c) Ga(NO3)3∙xH2O : C3H6N6 (mol) = 1:6 (annealed at 850℃) c) C C O O N N Ga Ga

(unannealed) (unannealed)

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Accumulated chemical composition (%) Accumulated chemical composition (%)

FigFigFigureureure 9 9.9.. FEFEFE-TEM--TEMTEM (f(f (field-emissionieldield--emissionemission scanning scanning scanning electron electron electron microscopy microscopy microscopy))) andand and EDS EDS EDS (( (energyeenergynergy dispersivedispersive spectroscopy) results of product synthesized from a Ga(NO3)3∙32H2O pellet and C3H6N6 powder spectroscopy)spectroscopy) results results of of product synthesized fromfrom a a Ga(NOGa(NO33))33¨∙32H32H2OO pellet and C33H6NN66 powderpowder injection with NH3 gas by the thermal plasma process in Plasma 3; FE-TEM images of (a) before and injectioninjection with with NH NH33 gasgas by by the the thermal thermal plasma plasma process process in in Plasma Plasma 3; 3; FE FE-TEM-TEM images images of of ( (aa)) before before and and (((bb))) afterafter after annealingannealing annealing;;; (( cc)) TEMTEM TEM-EDS--EDSEDS results.results.

FEFEFE-SEM--SEMSEM imagesimages images andand and sizesize size distributiondistribution distribution ofof synthesizedsynthesized of synthesized GaNGaN GaNnanopowdernanopowder nanopowder areare indicatedindicated are indicated inin FigureFigure in 10a,b. The large residual C3H6N6 particles were removed, but the uniform GaN nanoparticles 10Figurea,b. The 10a,b. large The residuallarge residual C3H6N C36 Hparticles6N6 particles were were removed, removed, but but the the uniform uniform GaN GaN nanoparticles nanoparticles remainedremainedremained,,, asas as isis is shownshown shown inin in FigureFigure Figure 1010 10aa.a.. TheThe The particleparticle particle sizesize size distributiondistribution distribution waswas was arrarr arrangedangedanged byby bymeasurementmeasurement measurement ofof each particle in the FE-SEM frame, as is seen in Figure 10b. The size distribution of synthesized eachof each particle particle in inthe the FE FE-SEM-SEM frame frame,, as as is is seen seen in in Figure Figure 1 100bb.. The The size size distribution distribution of of synthesized synthesized nanoparticles was analyzed by FE-SEM images. Hundreds of nanoparticles were measured with nanoparticlesnanoparticles was was analyzed analyzed by by FE FE-SEM-SEM images images.. Hundreds Hundreds of of nanoparticles nanoparticles were were measured measured with with particles sizes varying from 10 to 60 nm. The mean particle size was 29.8 nm. The uniform-sized particlesparticles sizes sizes varying varying fro fromm 10 10 to 60 nm. The The mean mean particle particle size size was 29.8 nm. The The uniform uniform-sized-sized particle GaN nanopowder was synthesized by thermal plasma and an additional annealing process. particleparticle GaN GaN nanopowder nanopowder was was synthesized synthesized by by thermal thermal plasma plasma and an additional annealing process.

40 PlasmaPlasma bb)) 40 a) 35 a) Ga(NO3)3∙xH2O : C3H6N6 (mol) = 1:6 35 Ga(NO3)3∙xH2O : C3H6N6 (mol) = 1:6 30 30

25 25

20 20

15 15

10

Frequency (%) Frequency 10

Frequency (%) Frequency 5 5

0 0 10 20 30 40 50 60 10 20 30 40 50 60 ParticleParticle sizesize (nm)(nm) Figure 10. (a) FE-SEM image and (b) size distribution of synthesized GaN nanopowder after FigFigureure 10. ((aa)) FE-SEMFE-SEM image image and and (b) size (b) distributionsize distribution of synthesized of synthesized GaN nanopowder GaN nanopowder after annealing after annealing in Plasma 3. annealingin Plasma in 3. Plasma 3.

3 6 6 ToTo reducereduce residualresidual CC3HH6NN6 presentpresent inin thethe productproduct generatedgenerated byby thethe thermalthermal plasma,plasma, thethe ratioratio ofof To3 reduce3 2 residual3 C36H6N6 present in the product generated by the thermal plasma, the ratio of Ga(NGa(NOO3))3∙32H∙32H2OO andand CC3HH6NN6 waswas decreaseddecreased toto 1:31:3 underunder thethe operatingoperating conditionsconditions usedused toto generategenerate Ga(NO ) ¨32H O and C H N was decreased to 1:3 under the operating conditions used to generate PlasmaPlasma 34.4.3 XRDXRD 2patternspatterns ofof3 productproduct6 6 synthesizedsynthesized underunder thesethese conditionsconditions areare shownshown inin FigureFigure 1111aa.. TheThe Plasma 4. XRD patterns of product synthesized under these conditions are shown in Figure 11a. upperupper pattern pattern is is for for synthesized synthesized nanopowder nanopowder by by thermal thermal plasma plasma while while the the bottom bottom pattern pattern is is for for annealedThe upper nanopowder pattern is for at 850 synthesized °C for three nanopowder hours. In the by upper thermal XRD plasma pattern, while the themain bottom peak of pattern C3H6N is6 annealed nanopowder at 850 °C for˝ three hours. In the upper XRD pattern, the main peak of C3H6N6 wasfor annealedobserved nanopowderat a weak 26°, at while 850 Cmain for threepeaks hours. of GaN In were the upperfound XRDat 32°, pattern, 34°, and the 38°. main However, peak of was observed at a weak 26°, while main˝ peaks of GaN were found at 32°, 34°, and ˝38°. ˝However,˝ GaC3H2O63N peaks6 was appeared observed after at a annealing weak 26 in, whilethe vacuum main peaksfurnace of at GaN 850 °C were for foundthree hours. at 32 These, 34 , findings and 38 . Ga2O3 peaks appeared after annealing in the vacuum furnace at 850 °C for three hours.˝ These findings However, Ga2O3 peaks appeared after annealing in the vacuum3 3 2 furnace3 at6 8506 C for three hours. demonstratedemonstrate thatthat thethe oxygenoxygen capturecapture reactionreaction ofof Ga(NOGa(NO3))3∙32H∙32H2OO byby CC3HH6NN6 coucouldld havehave negativenegative 3 6 6 effectseffects asas thethe amountamount ofof CC3HH6NN6 decreases.decreases. Nanomaterials 2016, 6, 38 12 of 15

These findings demonstrate that the oxygen capture reaction of Ga(NO3)3¨32H2O by C3H6N6 could haveNanomaterials negative 2016 effects, 6, 38 as the amount of C3H6N6 decreases. 12 of 15

a) GaN Ga2O3 b) C3H6N6

Synthesized product

Intensity (a.u.)

Annealing at 850 ℃, 3 h 10 20 30 40 50 60 70 80 90 50 nm Two Theta (deg) Figure 11. (a) XRD patterns before and after annealing and (b) FE-TEM image of unannealed product Figure 11. (a) XRD patterns before and after annealing and (b) FE-TEM image of unannealed product from a Ga(NO3)3∙32H2O pellet and C3H6N6 powder injection with NH3 gas by the thermal plasma from a Ga(NO3)3¨32H2O pellet and C3H6N6 powder injection with NH3 gas by the thermal plasma process in Plasma 4. process in Plasma 4.

Figure 11b shows FE-TEM images of synthesized nanopowder by thermal plasma, when the Figure 11b shows FE-TEM images of synthesized nanopowder by thermal plasma, when the molar ratio of Ga(NO3)3∙32H2O and C3H6N6 of 1:3 is adopted in Plasma 4 conditions. The product molarsynthesized ratio ofin Ga(NO Plasma3 )43 ¨32Hprim2arilyO and consisted C3H6N6 ofof fine 1:3 isparticles adopted which in Plasma had an 4 conditions. average particle The product size of synthesized9.25 nm. In the in PlasmaEDS result 4 primarily in Table consisted2, the content of fine of particlesthe Ga element which is had higher, an average about 63% particle of weight, size of 9.25than nm. that In of the the EDS synthesized result in Table nanopowder2, the content in Plasma of the Ga3 without element additional is higher, about annealing. 63% of H weight,owever, than the thatoxygen of the content synthesized is high nanopowderat 23% of weight in Plasma due to 3 incomplete without additional nitridation annealing. and generated However, gallium the oxygen oxide. content is high at 23% of weight due to incomplete nitridation and generated gallium oxide. The degree The degree of Ga oxidation increased as the molar ratio of injected C3H6N6 decreased. Therefore, of Ga oxidation increased as the molar ratio of injected C H N decreased. Therefore, injection injection of excessive C3H6N6 is effective for preventing the oxidization3 6 6 of Ga. Moreover, C3H6N6 aids ofin the excessive complete C3H synthesi6N6 is effectivezing of GaN for preventingnanopowder. the However oxidization, annealing of Ga. Moreover,is required Cto3 Heliminate6N6 aids the in the complete synthesizing of GaN nanopowder. However, annealing is required to eliminate the remaining C3H6N6. remaining C3H6N6.

Table 2. Elemental composition of product synthesized from a Ga(NO3)3∙32H2O pellet and C3H6N6 Table 2. Elemental composition of product synthesized from a Ga(NO ) ¨32H O pellet and C H N powder injection with NH3 gas by the thermal plasma process in Plasma3 34 (all results2 in % of weight)3 6 6. powder injection with NH3 gas by the thermal plasma process in Plasma 4 (all results in % of weight). Spectrum C N O Ga Total SpectrumSpectrum 1 8.70 C3.51 N23.04 O Ga64.76 Total 100.00 SpectrumSpectrum 2 112.60 8.704.78 3.5123.34 23.04 64.7659.28 100.00100.00 SpectrumSpectrum 3 29.05 12.601.73 4.7823.40 23.34 59.2865.82 100.00100.00 Mean Spectrum 310.11 9.053.34 1.7323.26 23.40 65.8263.28 100.00100.00 Mean 10.11 3.34 23.26 63.28 100.00

C3H6N6 was used as a reductant to prevent oxidation of gallium. However, it also consists of an abundantC3H6 Nsource6 was of used nitrogen. as a reductant Therefore, to a preventnitridation oxidation reaction of using gallium. only However,C3H6N6 powder, it also consistswithout ofthe an addition abundant of NH source3 is needed of nitrogen. to confirm Therefore, the complete a nitridation synthesis reaction of GaN using nanopowder. only C3H6 NIn6 previouspowder, withoutexperiments, the addition NH3 gas of was NH 3 injectedis needed into to the confirm Ga(NO the3)3∙32H complete2O pellet synthesis as a nitridation of GaN nanopowder. source. A InGa(NO previous3)3∙32H experiments,2O pellet wasNH3 reactedgas was injectedwith only into C the3H6 Ga(NON6 powder3)3¨32H without2O pellet NH as3 a nitridationgas in Plasma source. 5 Aconditions. Ga(NO3) 3C¨32H3H6N2O6 powder pellet was was reactedinjected with through only two C3H nozzles6N6 powder of an anode without electrode NH3 gas as in Plasma 3. 5 conditions.The molar ratio C3H 6 ofN 6 Ga(NOpowder3)3∙32H was2 injectedO and C through3H6N6 was two set nozzles at 1:6. of The an anode XRD pattern electrode of as synthesized in Plasma 3.nanopowder The molar ratioby reacting of Ga(NO Ga(NO3)3¨32H3)3∙32H2O and2O and C3H C63NH66Nwas6 without set at 1:6. NH The3 gas XRD is indicated pattern ofin synthesized Figure 12a. nanopowderThe intensities by of reacting all peaks Ga(NO were3 )weak.3¨32H 2TheO and observed C3H6N peaks6 without corresponded NH3 gas is with indicated gallium in Figureoxide and12a. Themelamine. intensities When of the all NH peaks3 gas were was weak. injected The into observed the reactor peaks at the corresponded same molar ratio with of gallium Ga(NO oxide3)3∙32H and2O melamine.and C3H6N When6 in Plasma the NH 3,3 onlygas was residual injected C3H into6N6 the peaks reactor were at detected the same in molar the XRD ratio ofpattern Ga(NO in3 )Figure3¨32H2 O6. andLow Ccrystallinity3H6N6 in Plasma GaN was 3, only synthesized. residual C However,3H6N6 peaks Ga2O were3 peaks detected were indetected the XRD with pattern residual in Figure C3H6N6.6 in Plasma 5 of Figure 12a. It was revealed that the nitridation reaction did not occur with the nitrogen element of C3H6N6 powder. Figure 12b shows XPS analysis results of synthesized nanopowder in Plasma 5. Ga-N bonding peaks as Ga2p(1/2) and Ga2p(3/2) were identically observed at 1143.0 eV and 1116.2 eV [81,83–86]. However, two peaks were observed at 1145.9 eV and 1119.3 eV. Analysis of these peaks confirmed Ga–O chemical bonding. An increase of binding energy from Ga2p electrons Nanomaterials 2016, 6, 38 13 of 15

Low crystallinity GaN was synthesized. However, Ga2O3 peaks were detected with residual C3H6N6 in Plasma 5 of Figure 12a. It was revealed that the nitridation reaction did not occur with the nitrogen element of C3H6N6 powder. Figure 12b shows XPS analysis results of synthesized nanopowder in Plasma 5. Ga-N bonding peaks as Ga2p(1/2) and Ga2p(3/2) were identically observed at 1143.0 eV and 1116.2Nanomaterials eV. However,2016, 6, 38 two peaks were observed at 1145.9 eV and 1119.3 eV. Analysis of these13 peaks of 15 confirmed Ga–O chemical bonding. An increase of binding energy from Ga2p electrons was observed wasdue toobserved Ga–O chemical due to Gabonding.–O chemical The nitrogen bonding. element The nitrogen has a less element electronegative has a less property electronegative than the propertyoxygen element. than the Therefore, oxygen element. the binding Therefore, energy the of binding Ga–O chemical energy bondingof Ga–O chemical is higher thanbonding that is of higher Ga–N thanchemical that bondingof Ga–N [chemical30]. bonding [30].

Ga O a) 2 3 b) Ga2p C3H6N6

Intensity (a.u.) Intensity

Arb. Unit (counts/s)Arb. Unit

1150 1140 1130 1120 1110 10 20 30 40 50 60 70 80 90

Two Theta (deg) Binding Energy (eV)

Figure 12. (a) XRD patterns and (b) XPS analysis of products synthesized from a Ga(NO3)3∙32H2O Figure 12. (a) XRD patterns and (b) XPS analysis of products synthesized from a Ga(NO3)3¨32H2O 3 6 6 3 pellet and C3H6NN6 powderpowder injection injection without without NH NH3 gasgas by by the the thermal thermal plasma plasma process process in in Plasma Plasma 5. 5.

C3H6N6 was decomposed and converted to cyanides, hydrocarbons, and carbon in the high C3H6N6 was decomposed and converted to cyanides, hydrocarbons, and carbon in the high temperature of the thermal plasma jet. Among these, only cyanides have nitrogen elements. However, temperature of the thermal plasma jet. Among these, only cyanides have nitrogen elements. the nitridation reaction by molecules is an inferior chemical reaction compared with However, the nitridation reaction by cyanide molecules is an inferior chemical reaction compared with oxidation. It was examined using the thermodynamic equilibrium calculation in Figure 4c. oxidation. It was examined using the thermodynamic equilibrium calculation in Figure4c. Overall, these findings indicate that GaN nanopowder was synthesized from Ga(NO3)3∙32H2O Overall, these findings indicate that GaN nanopowder was synthesized from Ga(NO3)3¨32H2O and C3H6N6 by thermal plasma, despite low crystallinity. Its crystallinity can be enhanced by and C3H6N6 by thermal plasma, despite low crystallinity. Its crystallinity can be enhanced by annealing annealing in a vacuum furnace. Moreover, if the annealing can be completed at atmospheric pressure in a vacuum furnace. Moreover, if the annealing can be completed at atmospheric pressure in an inert in an inert atmosphere, not by rough vacuum, production of carbon dioxide or carbon nitride can be atmosphere, not by rough vacuum, production of carbon dioxide or carbon nitride can be controlled. controlled. 4. Conclusions 4. Conclusions GaN nanopowder was synthesized by a thermal plasma process. Ga(NO3)3¨xH2O was used GaN nanopowder was synthesized by a thermal plasma process. Ga(NO3)3∙xH2O was used as as the raw material. At first, it was discovered that Ga(NO3)3¨xH2O was not nitrided by a solely the raw material. At first, it was discovered that Ga(NO3)3∙xH2O was not nitrided by a solely conventional NH3 nitridation source and that it is converted into Ga2O3. It required a reductant to conventional NH3 nitridation source and that it is converted into Ga2O3. It required a reductant to prevent oxidation to Ga2O3 instead of GaN. Therefore, C3H6N6 powder was injected into the high preventtemperature oxidation region to of Ga the2O thermal3 instead plasma of GaN. jet throughTherefore, an C anode3H6N6 electrode. powder was The molarinjected ratio into of the injected high temperature region of the thermal plasma jet through an anode electrode. The molar ratio of injected Ga(NO3)3¨xH2O and C3H6N6 was controlled at 1:6 and 1:3. GaN nanopowder with low crystallinity Ga(NO3)3∙xH2O and C3H6N6 was controlled at 1:6 and 1:3. GaN nanopowder with low crystallinity and residual C3H6N6 was synthesized by the thermal plasma process. Crystallinity of the synthesized andGaN residual nanopowder C3H6N was6 was further synthesized enhanced by afterthe thermal annealing plasma at 850 process.˝C for three Crystallinity hours in of a vacuumthe synthesized furnace. TheGaN size nanopowder of synthesized was further GaN nanopowder enhanced after is distributedannealing at from 850 °C 10 for to 60three nm, hours and meanin a vacuum particle furnace size is. The size of synthesized GaN nanopowder is distributed from 10 to 60 nm, and mean particle size is calculated to be 29.8 nm. In order to confirm the nitridation of Ga(NO3)3¨xH2O by C3H6N6 reductant, calculated to be 29.8 nm. In order to confirm the nitridation of Ga(NO3)3∙xH2O by C3H6N6 reductant, the Ga(NO3)3¨xH2O was reacted with C3H6N6 without NH3 gas, and Ga(NO3)3¨xH2O precursor was 3 3 2 3 6 6 3 3 3 2 theoxidized Ga(NO to Ga) ∙x2HO3O. Therefore,was reacted GaN with nanopowder C H N without was successfully NH gas, and synthesized Ga(NO ) from∙xH Ga(NOO precursor3)3¨32H was2O oxidizedand C3H 6toN Ga6 powders2O3. Therefore, with NHGaN3 gasnanopowder by a thermal was successfully plasma process. synthesized Furthermore, from Ga(NO the additional3)3∙32H2O andannealing C3H6N step6 powders was required with NH to enhance3 gas by its a thermalcrystallinity. plasma It was process. speculated Furthermore that the, the production additional of annealingcarbon dioxide step orwas carbon required nitride to enhance during the its annealingcrystallinity. step It could was speculated be controlled that as the anneal production in an inert of carbon dioxide or carbon nitride during the annealing step could be controlled as anneal in an inert atmosphere. The synthesis of GaN crystalline nanopowder is difficult to achieve using conventional production methods. This research demonstrates the potential to synthesize GaN nanopowder through a thermal plasma process, from raw materials comprising abundant oxygen elements.

Acknowledgments: This research was supported by the World Class 300 Project (10043264, Development of the electrode materials for high efficiency (21%) and low-cost c-Si solar cells) funded by the Ministry of Knowledge Economy of the Republic of Korea, and by the Basic Science Research Program through the National Research Nanomaterials 2016, 6, 38 14 of 15 atmosphere. The synthesis of GaN crystalline nanopowder is difficult to achieve using conventional production methods. This research demonstrates the potential to synthesize GaN nanopowder through a thermal plasma process, from raw materials comprising abundant oxygen elements.

Acknowledgments: This research was supported by the World Class 300 Project (10043264, Development of the electrode materials for high efficiency (21%) and low-cost c-Si solar cells) funded by the Ministry of Knowledge Economy of the Republic of Korea, and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education of the Republic of Korea (No. NRF-2010-0020077). Author Contributions: T.-H.K. and S.C. conceived and designed the experiments; T.-H.K. performed the experiments; T.-H.K. and D.-W.P. analyzed the data; All the three authors wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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