nanomaterials Article

Thermal Plasma Synthesis of Crystalline Gallium Nitride Nanopowder from Gallium Nitrate Hydrate and Melamine Tae-Hee Kim 1 , Sooseok Choi 2, * and Dong-Wha Park 1, * 1

2

*

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] 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: Gallium nitride (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 ¨xH2 O) was used as a raw material and NH3 gas was used as a nitridation source. Additionally, melamine (C3 H6 N6 ) powder was injected into the plasma flame to prevent the oxidation of gallium to gallium oxide (Ga2 O3 ). 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 C3 H6 N6 . 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; melamine; nanopowder

annealing;

gallium

nitride;

gallium

nitrate

hydrate;

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 (Al2 O3 ) 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. Gallium phosphide (GaP) and Gallium arsenide (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 ¨xH2 O) was used as the raw material instead of Ga or Ga2 O3 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 (C3 H6 N6 ) was additionally injected into the thermal plasma jet to prevent the oxidation of decomposed Ga into Ga2 O3 [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 ¨xH2 O (99.9% purity, Alfa Aeser Inc., Boston, MA, USA) and C3 H6 N6 (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 Figure 1. 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 plasma jet was generated Nanomaterials 2016, 6, 38 in the plasma torch by Ar or Ar–N2 , which formed gases under atmospheric 3 of 15 pressure. The thermal plasma jet was ejected from a 6 mm diameter nozzle by the thermal expansion generated in thegas plasma by Ar orchannel Ar–N2, which gases under atmospheric pressure. The and of plasma, forming by antorch electric arc whichformed was connected between a conical cathode thermal plasma jet was ejected from a 6 mm diameter nozzle by the thermal expansion of plasma, the anode nozzle. Ga(NO3 )3 ¨xH2 O 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 created by a The hydraulic press. was of placed specific. precursor was aThe pelletprecursor which hadpellet a diameter 45 mmon anda atungsten thickness crucible of 10 mm which created was fixed by by aa hydraulic water-cooling holder. As a result, the precursor was rapidly melted and evaporated press. The precursor pellet was placed on a tungsten crucible which was fixed byby a the confronting thermalholder. plasmaAs jet,aasresult, shownthe in Figure 1. The temperature of Ga(NO water-cooling precursor wasvaporization rapidly melted and evaporated by3 )the 3 ¨xH2 O confronting plasma as shown in Figure 1. The temperature of as is lower than 100 ˝thermal C. Therefore, it isjet, a more economical precursor than vaporization Ga or Ga2 O3 which were used Ga(NO ∙xH2O is lower than 100 °C. Therefore, it is a more economical precursor than Ga or Ga 2O3 precursors to3)3synthesize GaN in previous work [23] However, it contains excessive nitrogen, oxygen, . which were used as precursors to synthesize GaNofinthe previous . However, it contains and hydrogen elements. The complete dissociation oxygenwork from[23] Ga(NO 3 )3 ¨xH2 O is difficult excessive nitrogen, oxygen, and hydrogen elements. The complete dissociation of the oxygen) from due to the high latent heat of H2 O. For this reason, it is normal that evaporated Ga(NO 3 3 ¨xH2 O is Ga(NO3)3∙xH2O is difficult due to the high latent heat of H2O. For this reason, it is normal that oxidized to Ga2 O3 by its inherent oxygen elements. Therefore, melamine (C3 H6 N6 ) 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 C3 H6 N6 was converted to carbon, nitrogen, melamine (C3H6N6) powder was used to prevent the production of gallium oxide. The injected hydrogen, other molecules by decomposition at high temperatures in the thermal plasma jet. C3H6Nand 6 was converted to carbon, nitrogen, hydrogen, and other molecules by decomposition at high Thesetemperatures byproduct molecules consume elementsmolecules of the Ga(NO O precursor. Injected 3 )3 ¨xH in the thermal plasmathe jet. oxygen These byproduct consume the 2oxygen elements of melamine powder consists of nonspecific-shaped particles under 500ofnm. In addition, ammonia (NH3 ) the Ga(NO 3)3∙xH 2O precursor. Injected melamine powder consists nonspecific-shaped particles under 500 nm. In addition, ammonia (NH 3 ) gas was diagonally injected into the pellet of gas was diagonally injected into the pellet of Ga(NO3 )3 ¨xH2 O from the upper side of the chamber to Ga(NO 3)3∙xH 2O from the upper side of the chamber to nitride the Ga element. nitride the Ga element. Plasma generating gas DC power supply Powder feeder

Reactive NH3 gas Plasma torch

Plasma jet Exhaust

Ar

Pellet

NH3

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 Plasma 1, the operating Ga(NO3)3∙xH 2O pellet was reacted with gas and without C3Hcondition 6N6. The thermal The detailed conditions are indicated in only TableNH 1. 3In the first case, the of Plasma was generated by mixed argon and nitrogen gases. The high thermal conductive nitrogen 1, the plasma Ga(NOjet ) O pellet was reacted with only NH gas and without C H N . The thermal plasma ¨xH 3 3 2 3 3 6 6 was injected asby a plasma gas to help thegases. nitridation evaporation the input power was was jet was generated mixed forming argon and nitrogen The or high thermal as conductive nitrogen increased. The plasma input power was 12.6 kW at the fixed current of 300 A and the average voltage injected as a plasma forming gas to help the nitridation or evaporation as the input power 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. In theplasma other cases Plasma 2, the 3, 4,current and 5,fixed the Ga(NO )3 ¨xH pelletvoltage was evaporated by pure thermal jet wasof8.4 kW with at 300 A3 and an2 O average of 28 V. Argon Ar thermal plasma together with C H N . In these cases, the input power for the generation of the 3 6 6 is a monoatomic molecule and nitrogen is a diatomic molecule. In order to generate thermal plasma thermal plasma jet was 8.4 kW with the current fixed at 300 A and an average voltage of 28 V. Argon from nitrogen gas, more energy is required compared to argon gas. Accordingly, electric resistance is a monoatomic molecule and nitrogen is and a diatomic molecule. In order to generate thermal plasma for arc generation between the cathode the anode is increased. Therefore, argon thermal plasma has a lower average lower resistance thantoargon–nitrogen thermal plasma at theresistance fixed from nitrogen gas, morevoltage energywith is required compared argon gas. Accordingly, electric Inputbetween power of the 8.4 cathode kW was sufficient to vaporize the Ga(NO 3)3∙xH2O pellet to its low for arccurrent. generation and the anode is increased. Therefore, argondue thermal plasma vaporization temperature of 80 °C. Synthesized nanoparticles were attached on the inner surface has a lower average voltage with lower resistance than argon–nitrogen thermal plasma at theoffixed

Nanomaterials 2016, 6, 38

4 of 15

current. Input power of 8.4 kW was sufficient to vaporize the Ga(NO3 )3 ¨xH2 O pellet due to its low Nanomaterials 2016, 6, 38 4 of 15 vaporization temperature of 80 ˝ C. Synthesized nanoparticles were attached on the inner surface 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 the post and analysis. film for processing the post processing and analysis. Table 1. Operating conditions ofGaN GaNpowder powder thermal plasma. Table 1. Operating conditionsfor forthe the synthesis synthesis of byby thermal plasma. Experiment No. Experiment No. Weight of Weight of Ga(NO3 )3 ¨xH2 O Ga(NO3)3∙xH2O

Plasma PlasmaPlasma 1 1 Plasma 2 2

Plasma3 3 Plasma

Plasma Plasma 5 Plasma 4 4 Plasma 5

6g 6g

1515gg (powder (powder injection) injection)

Condition of Weight of 15 g Condition 15 g precursors C (pellet) Weight 3 H6 N6 of C3H6N6 of (pellet) precursors Molar ratio of Molar ratio of Ga(NO3 )3 ¨xH2 O 1:6 Ga(NO 3∙xH2O and 1:6 and C3 H63)N 6 C3H6N6 Flow rate of 13 L/min Ar + plasma forming 13 N L/min Ar 2 L/min Flow rate of plasma 2 gas Condition of + 2 L/min N2 forming gas Flow rate of plasma 10 L/min 3 L/min reactive NH3 gas generating Flow rate of reactive Condition 10 L/min 3 L/min NHof of plasma Flow rate 3 gas carrier gas generating Flow rate of carrier gas Plasma input 12.6 kW -

8.8 g 8.8 g

1:6 1:6

1:3

15 g 15 g (powder (powder injection) injection)

1:3

1:6

1:6

13 L/min Ar 13 L/min Ar 3 L/min

3 L/min 3 L/min N2

3 L/min

3 L/min 3 L/min N2

-

3 L/min N2

3 L/min N2 8.4 kW 3 L/min N2 3 L/min N2 (300 A, 28 V) 8.4 kW (300 A, 28 V)

(300 A, 42 V) 12.6 kW Plasma input power (300 A, 42 V) power

C3 H6 N6 powder was used in the three different procedures in Plasma 2, 3, 4, and 5. A detailed illustration C3 H6 N6 was injection methods indicated in Figure 2a–d. 2,C3,3 H N6 was mixed with C3Hof 6N6 powder used in the threeisdifferent procedures in Plasma 4,6and 5. A detailed Ga(NO ) O, they were pressed as the pellet precursor in Plasma 2. C H N powder was injected ¨xH 3 3 2 of C3H6N6 injection methods is indicated in Figure 2a–d. C33H66N66 was mixed with illustration into the high3)3temperature of the thermal as powder through2.the electrode Plasmas 3, Ga(NO ∙xH2O, they were pressed as plasma the pelletjetprecursor in Plasma C3Hanode 6N6 powder was in injected 4, andinto 5. the Thehigh molar ratio of Ga(NO ) ¨xH O and C H N was controlled at 1:6 and 1:3 in Plasmas 3, 4, temperature of the thermal plasma jet as powder through the anode electrode Plasmas 3 3 2 3 6 6 3, 4, and 5. The molar ratio of Ga(NO 3 ) 3 ∙xH 2 O and C 3 H 6 N 6 was controlled at 1:6 and 1:3 in Plasmas 3, 4, and 5. In these cases, NH3 gas was injected into the pellet as a nitridation source by the probe as in and In these cases, NH3 gas was injected into the pellet as a nitridation source by the probe as in Figure 1. 5.Ga(NO 3 )3 ¨xH2 O was reacted with only C3 H6 N6 powder and without NH3 gas in Plasma 5, Figure 1. Ga(NO 3∙xH2O was reacted with only C3H6N6 powder and without NH3 gas in Plasma 5, in in order to check the3)possibility of nitridation by the nitrogen element of C3 H6 N6 . 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. Torch

a)

NH3 gas

Ga(NO3)3∙xH2O

Plasma reactor

Torch

NH3 gas

Ga(NO3)3∙xH2O + C3H6N6

(pellet)

c)

Torch

b)

Plasma reactor

C3H6N6 powder NH3 gas

Ga(NO3)3∙xH2O

(pellet)

Torch

d)

C3H6N6 powder

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 C3 H6 N6 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 Nanomaterials 2016, 6, 38

5 of 15 5 of 15

analysis result result of of Ga(NO Ga(NO33)33∙xH ¨xH22O A TGA analysis O as as raw raw materials materials according according to temperature is indicated Figure 3. 3. Analysis Analysis was was conducted conductedfrom from40 40˝°C to800 800˝°C with an an increase increasein inincrements incrementsof of10 10˝°C/min in Figure C to C with C/min under a nitrogen nitrogen atmosphere. atmosphere. An obvious mass reduction of approximately 67% occurred between ˝ C as 2O was converted toto GaGa 2O23O ,H N2O 60 ˝°C C and 180 °C asisisshown shownin inFigure Figure3.3.Ga(NO Ga(NO3)33)∙xH was converted H2and O, and N32gas O3 3 ¨xH 2O 3 ,2O, withwith increasing temperatures, which is in accordance with previous studies [23]. Therefore, it can be gas increasing temperatures, which is in accordance with previous studies [23]. Therefore, it assumed that thethat final product of Ga2Oof3 with of 78% would produced at 800 °C. can be assumed the final product Ga2 Oa3 mass with reduction a mass reduction of 78% be would be produced at As a˝ C. result, estimated value of x in of Ga(NO 3)3∙xH23O was2 O determined to be 32.1 on theon mass 800 As a the result, the estimated value x in Ga(NO )3 ¨xH was determined to bebased 32.1 based the ˝ C. Accordingly, reduction from from 12.45 12.45 mg atmg 40 at °C40 to˝2.80 800at°C. Ga(NO 3)3∙xH is 2referred to as mass reduction C to mg 2.80atmg 800Accordingly, Ga(NO ¨xH O is referred 3 )23O Ga(NO 3)3∙32H in this work. to as Ga(NO ¨32H this work. 3 )32O 2 O in

100

Mass (%)

80

60

40

20 100

200

300

400

500

600

700

800

Sample temperature (C)

Figure3.3. TGA TGA (thermogravimetric (thermogravimetricanalysis) analysis)of ofGa(NO Ga(NO33))33¨xH ∙xH22O Figure O 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 FEFE-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 ¨xH2 O 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 3. Resultsand andDiscussion Discussion Figure 4a 4a shows shows the the thermodynamic thermodynamic equilibrium equilibrium composition composition of of melamine melamine at at changes changes in in Figure temperature from 500 to 6000 K. The calculation was conducted by a commercial software of FactSage temperature from 500 to 6000 K. The calculation was conducted by a commercial software of FactSage (Ver.6.4, 6.4,CRCT>T, CRCT>T, Canada and Germany). The purpose of thermodynamic (Ver. Canada and Germany). The purpose of thermodynamic equilibriumequilibrium calculation calculation was to expect a preferable chemical reaction and stable chemical in a range. given was to expect a preferable chemical reaction and stable chemical species in a given species temperature temperature range. C 3H6N6 containing carbon, hydrogen, and nitrogen elements can be decomposed C3 H6 N6 containing carbon, hydrogen, and nitrogen elements can be decomposed and dissociated and dissociated about 350 °C. The melaminetois cyanides, convertedhydrocarbons, to cyanides, hydrocarbons, and carbon at about 350 ˝ C.atThe melamine is converted and carbon in the high in the high temperature plasma zone. These decomposed or dissociated C 3H6N6 byproducts can react temperature plasma zone. These decomposed or dissociated C3 H6 N6 byproducts can react with the with theofoxygen of a) Ga(NO 3)3∙32H2O precursor. Furthermore, the various exothermic gases such as oxygen a Ga(NO 3 3 ¨32H2 O precursor. Furthermore, the various exothermic gases such as CO, CO2 , CO, CO 2, 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 ¨32H2 O Ga(NO 3)3∙32H2O and C3H6N6 promote the synthesis of GaN nanopowder. and C H N promote the synthesis of GaN nanopowder. 3

6

6

Nanomaterials 2016, 6, 38

6 of 15

Nanomaterials 2016, 6, 38

6 of 15

b)

C3H6N6 (melamine)

CH4 + O2 → CO2 + 2H2 HCN + O2 → CO2 + 0.5N2 + 0.5H2

1500000

C2H2 + O2 → 2CO + H2 C2H + O2 → 2CO + 0.5H2

N2 (g) H(g) H2 (g)

CH4 (g)

C2N + O2 → 2CO + 0.5N2

1000000

C(g) C3(g) C2N(g) C2H(g) CN(g) HCN(g) C2H2(g) C2(g)

N (g)

Temperature [K]

△Gibbs free energy [J]

Thermodynamic equilibrium composition [mol]

a)

CN + O2 → CO + 0.5N2 Ga + 1.5O2 → Ga2O3

500000

C + O2 → CO2

Temperature [K]

0 6000

5000

4000

3000

2000

1000

-500000

-1000000

-1500000

c) 1000000

Ga + C2N

750000

△Gibbs free energy [J]

→ GaN + C + 0.5H2 → GaN + 2C → GaN + C

Ga + HCN Ga + CN

500000 250000

Temperature [K]

0 6000

5000

4000

3000

2000

1000

-250000 -500000 -750000 -1000000

Figure Figure 4. 4. Thermodynamic Thermodynamicequilibrium equilibriumcalculation calculationatatchanges changes in in temperature; temperature; (a) (a) thermodynamic thermodynamic equilibrium composition of melamine; (b) change in Gibbs free energy during the equilibrium composition of melamine; (b) change in Gibbs free energy during the oxygen oxygen capture capture reaction 2O;O; and (c) change in Gibbs free energy of nitridation reaction reaction of of C C33H H66NN6 6from fromGa(NO Ga(NO3)33)∙xH ¨xH and (c) change in Gibbs free energy of nitridation reaction 3 2 by 6. . bydecomposed decomposedCC33HH6N 6N 6

The Thechange changein inGibbs Gibbsfree freeenergy energyduring duringoxygen oxygencapture capturereactions reactionswith withbyproducts byproductsconverted convertedby by decomposition of the C 3H6N6 which was injected into the thermal plasma jet are shown in Figure 4b. decomposition of the C3 H6 N6 which was injected into the thermal plasma jet are shown in Figure 4b. The and one solid line. The dotted lines indicate the the oxidation by Thegraph graphconsists consistsofofseven sevendotted dottedlines lines and one solid line. The dotted lines indicate oxidation byproducts generated from C 3H6N6 decomposition, while the solid line indicates the oxidation of by byproducts generated from C3 H6 N6 decomposition, while the solid line indicates the oxidation gallium to to GaGa 2O3. Oxidation reactions of hydrocarbons, cyanides, and carbon generated by the of gallium 2 O3 . Oxidation reactions of hydrocarbons, cyanides, and carbon generated by the decomposition 6 are thermodynamically spontaneous throughout the whole temperature decompositionof of C C33H H6N 6 N6 are thermodynamically spontaneous throughout the whole temperature range because Gibbs free energies arethan less than zero. Conversely, Gaproduced 2O3 can be produced range because Gibbs free energies are less zero. Conversely, Ga2 O3 can be spontaneously spontaneously 3100oxidation K. However, oxidation ofisbyproducts is still predominant at temperatures under 3100 K. under However, of byproducts still predominant at temperatures above 750 K above owing amount to a greater amount Gibbs of negative Gibbs free energy.CTherefore, C3H6Nis6 powder owing750 to aKgreater of negative free energy. Therefore, H N powder suitable is to 3 6 6 suitable to capture oxygen molecules of Ga(NO 3)3∙32H2O before they oxidize with gallium, even capture oxygen molecules of Ga(NO3 )3 ¨32H2 O before they oxidize with gallium, even though various though various byproducts could be the produced from theofdecomposition of Creason, 3H6N6. For this reason, byproducts could be produced from decomposition C3 H6 N6 . For this C3 H6 N6 powder Cwas 3H6N6 powder was injected into the thermal plasma jet to facilitate the synthesis of GaN injected into the thermal plasma jet to facilitate the synthesis of GaN nanopowder. Moreover, the nanopowder. the TGA analysis and thermodynamic equilibrium calculation revealed that TGA analysis Moreover, and thermodynamic equilibrium calculation revealed that six moles of C3 H 6 N6 powder six moles of C 3H6N6 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 ¨32H2 O was required to sufficiently convert oxygen sufficiently oxygen from Ga(NO3)3∙32H2O into carbon oxides, nitrogen oxides, and from Ga(NOconvert 3 )3 ¨32H2 O into carbon oxides, nitrogen oxides, and hydrogen oxides. hydrogen oxides.by the nitrogen element of C H N is considered in Figure 4c. The thermodynamic Nitridation 3 6 6 Nitridation the nitrogen of C3H6N6reaction is considered in Figure 4c. The equilibrium is by calculated for element the nitridation by HCN, C2 N, and thermodynamic CN molecules. equilibrium is calculated for the nitridation reaction by HCN, C 2N, and CN molecules. These These components containing nitrogen elements were reacted with the Ga element at temperatures components containing elements were reacted with the Ga negative element at temperatures under 3000 K. Althoughnitrogen ∆G of the three nitridation reactions have values, they areunder 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 Figure 4b. ∆G values for the oxidation reaction of HCN, C2 N, of the oxidation reaction in Figure 4b. ∆G values for the oxidation reaction of HCN, C 2N, and CN at and CN at about 3000 K are under 500,000 J, as shown in Figure 4b. Those values for their nitridation 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

reaction at 2016, approximately Nanomaterials 6, 38

3000 K are nearly zero. In other words, oxidation reaction is absolutely 7 of 15 superior compared to the nitridation reaction at all temperature ranges. Therefore, the byproducts, which are converted from the decomposition of C3 H6 Nranges. react with oxygen elements of compared to the nitridation reaction at all temperature Therefore, thethe byproducts, which are 6 , usually the Ga(NO3from )3 ¨32Hthe O raw material. converted decomposition of C 3 H 6 N 6 , usually react with the oxygen elements of the 2 XRD patterns of material. products synthesized in Plasma 1 and 2 conditions are shown in Figure 5a,b, Ga(NO 3)3∙32H 2O raw respectively. For Plasma 1, only synthesized a Ga(NO3 )3 ¨32H was 2used with 10are L/min NHin synthesize XRD patterns of products in Plasma 1 and conditions shown Figure 5a,b, 2 O pellet 3 to GaN nanopowder without C H N . The plasma jet was generated by argon and nitrogen mixed respectively. For Plasma 1, only 3)3∙32H2O pellet was used with 10 L/min NH3 to synthesize 3 a6 Ga(NO 6 gas innanopowder a conventional synthesis nitride by thermal plasma. peaksmixed showngas in GaN without C3H6process N6. The of plasma jetmaterials was generated by argon and All nitrogen Figure 5a correspond to Gaprocess Ga(NO3plasma. )3 ¨32H2 O not nitrided solely by in a conventional synthesis ofresults nitriderevealed materialsthat by thermal Allispeaks shown in Figure 2 O3 . The NHcorrespond a 2typical source forthat theGa(NO nitridation reaction. Ionized nitrogen which is 5a toisGa O3. Thenitrogen results revealed 3)3∙32H 2O is not nitrided solely gas by NH 3 gas, 3 gas, which produced the generating thermal jet was not suitableIonized for the nitrogen nitridation Ga(NO )3 ¨32H2 O. which is aby typical nitrogen source forplasma the nitridation reaction. gasofwhich is3produced Ga(NO )3 ¨32H2 O hasthermal a low evaporation and is rapidly in the high temperature by the 3generating plasma jet temperature was not suitable for thevaporized nitridation of Ga(NO 3)3∙32H2O. environment plasma jet.temperature However, nitridation of Ga did not occur, and temperature oxidation of Ga(NO 3)3∙32Hof 2Othe hasthermal a low evaporation and is rapidly vaporized in the high Ga was completed the presence abundant oxygen elements from itself. NH3 gas environment of theby thermal plasmaofjet. However, nitridation of Ga didthe notprecursor occur, and oxidation of usually and H gases at high temperatures. As shown in the following chemical Ga was decomposes completed byinto theNpresence of abundant oxygen elements from the precursor itself. NH 3 gas 2 2 reactions, the oxidation dominates the reaction with NAs NHin GaN. usually decomposes intoofNGa 2 and H2 gasesover at high temperatures. shown the following chemical 2 and 3 to 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 NH GaN ` 1.5 : ´63.7 3Ñ Ga 3 → GaN + 1.5 H2 H2 ∆H298K ∆H298K : −63.7kJkJ

(2) (2)

Ga+`1.51.5 : ´1089.1 2 Ñ Ga2 O3 ∆H298K 22Ga OO 2 → Ga2O3 ∆H298K : −1089.1kJkJ

(3) (3)

a)

b)

Ga2O3

* GaO(OH) CH N O 6

4

Intensity (a.u.)

Intensity (a.u.)

NH4NO

10

20

30

40

50

60

Two Theta (deg)

70

80

90

*

C

*

10

20

*

30

40

50

60

70

80

90

2 Theta (deg)

Figure 5. XRD Figure 5. XRD (X-ray (X-ray diffraction) diffraction) pattern pattern of of product product synthesized synthesized by by thermal thermal plasma; plasma; (a) (a) Plasma Plasma 11 by by Ga(NO 3 ) 3 ∙32H 2 O and NH 3 without C 3 H 6 N 6 ; (b) Plasma 2 by Ga(NO 3 ) 3 ∙32H 2 O and C 3 H 6 N 6 with Ga(NO3 )3 ¨32H2 O and NH3 without C3 H6 N6 ; (b) Plasma 2 by Ga(NO3 )3 ¨32H2 O and C3 Hpellet N pellet 6 6 NH3 gas. with 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 ¨32H2 O 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, C3 H6 N6 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 ¨32H2 O precursor. C3H6N6 powder was used as a pellet by compressing it with Ga(NO3)3∙32H2O as a precursor in C3 H6 N6 powder was used as a pellet by compressing it with Ga(NO3 )3 ¨32H2 O 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 ¨32H2 O and C3 H6 N6 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. NH3 gas was injected into the pellet at 3 L/min. The XRD pattern of synthesized product 2 conditions is indicated in Figure 5b. The peaks are analyzed as gallium hydroxide (GaO(OH)), at Plasma 2 conditions is indicated in Figure 5b. The peaks are analyzed as gallium hydroxide carbohydrazide (CH6N4O), and ammonium nitrate (NH4NO). The vaporized Ga elements were (GaO(OH)), carbohydrazide (CH6 N4 O), and ammonium nitrate (NH4 NO). The vaporized Ga elements oxidized, and C3H6N6 was not applied in sufficient amounts to act as a reductant. Unlike the were oxidized, and C3 H6 N6 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(NO3 )3 ¨32H2 O and NH3 gas without C3 H6 N6 , GaO(OH) was produced. Products including C-H-O bonding or ammonium ions were produced 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 C3 H6 N6 and Ga(NO3 )3 ¨32H2 O. It was assumed that C3 H6 N6 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 ¨32H2 O in a pellet. Although C3 H6 N6 affects the product, the effect is not sufficient to nitrate the gallium. Ga(NO3 )3 ¨32H2 O was vaporized above 80 ˝ C. The vaporizing of C3 H6 N6 is 345 ˝ C. The difference of vaporization temperatures for Nanomaterials 2016, temperature 6, 38 8 ofthe 15 two raw materials should be considered to prevent oxidation of the Ga element. The useful reductants generated by vaporization vaporization of of C C33H66N N66have haveto toreact reactwith withthe thegallium gallium element element before before oxidation. In In order order to vaporize C33H H66N N66early, early,ititwas wasinjected injectedinto intothe thehigher higher temperature temperature region region of of the the thermal thermal plasma. plasma. C33H H66N N66powder powderwas wasfed fedsolely solelyinto intoaahigh hightemperature temperature thermal thermal plasma plasma jet jet through through two nozzles inside the anode electrode electrode to to enable enable more more active active decomposition. decomposition.Ga(NO Ga(NO33))33¨32H ∙32H22O O has has an an evaporation evaporation ˝ temperature below 100 °C, C, and rapidly vaporized when it contacted the high temperature thermal molar ratio ratio of of Ga(NO Ga(NO33))33¨32H ∙32H22O plasma jet. The molar O and C33H H66N N66was wascontrolled controlledat at1:6 1:6 and and 1:3 1:3 in in Plasma Plasma 3 and 44conditions. conditions.Initially, Initially, C63N H66N 6 powder injected a 1:6ratio molar ratio according to the C3 H powder was was injected at a 1:6atmolar according to the calculation calculation from the TGA analysis3 )of Ga(NO 3 ) 3 ∙32H 2 O raw materials. This molar ratio was sufficient ¨32H from the TGA analysis of Ga(NO O raw materials. This molar ratio was sufficient to oxidize 3 2 to C3Hthe 6N6 oxygen from theelement oxygenofelement of the 3)3The ∙32Hgraph 2O. The 6 shows XRD ¨32H2 O. C3oxidize H6 N6 from the Ga(NO in graph Figurein6 Figure shows XRD patterns 3 )3Ga(NO patterns of thesynthesized product synthesized in The Plasma The main peaks to correspond to C 3H6N 6 and reflect other of the product in Plasma 3. main3.peaks correspond C3 H6 N6 and other peaks peaks reflect C x H y N z bound byproducts such as melam, melem and melon. C 3 H 6 N 6 can be converted C x Hy N bound byproducts such as melam, melem and melon. C H N can be converted into other z 3 6 6 into other derivatives by thermal condensation [24–27]. The synthesized product was subjected derivatives by thermal condensation [24–27]. The synthesized product was subjected to annealing intoa annealing in a vacuum furnace to enhance the GaN eliminate 3H6N6. vacuum furnace to enhance the GaN crystallinity and crystallinity eliminate theand residual C3 H6the N6 . residual The XRDCpattern The XRD pattern of annealed nanopowder in Figure 6. were Distinct peaksafter of GaN were of annealed nanopowder is indicated in Figureis6.indicated Distinct peaks of GaN observed annealing ˝ observed after annealing 850 graphite °C for three and carbon nitride (C3N4) It peaks at 850 C for three hours. at Weak and hours. carbon Weak nitridegraphite (C3 N4 ) peaks were also observed. was were also that observed. It was estimated that the synthesized GaN nanopowder was notpattern accurately estimated the synthesized GaN nanopowder was not accurately detected in the XRD due detected in the XRD pattern due to its low crystallinity and low quantity. to its low crystallinity and low quantity.

Intensity (a.u.)

GaN C3H6N6 C2H4N4

Synthesized product

Annealing at 850℃, 3h graphite C3N4

10

20

30

40

50

60

70

80

90

Two Theta (deg)

Figure 6. XRD patterns patterns of of products productssynthesized synthesizedfrom fromaaGa(NO Ga(NO ∙32H22O O pellet pellet and andCC33H H66N66 powder 3 )33)3¨32H injection with NH33 gas gas by by the the thermal thermal plasma plasma process process in Plasma 3.

This confirmed by the Ga–N Ga–N chemical chemical bonding shown in This result result was was reliable reliable as as confirmed by the bonding by by XPS XPS results results shown in Figure 7. The Ga2p, N1s, and C1s orbital graphs are indicated in Figure 7a,b. In the Ga2p graphs Figure 7. The Ga2p, N1s, and C1s orbital graphs are indicated in Figure 7a,b. In the Ga2p graphs of of Figure and Ga2p(3/2) were identically observed at 1143.0 eV and Figure 7a, 7a, Ga–N Ga–N bonding bondingpeaks peaksof ofGa2p Ga2p(1/2) (1/2) and Ga2p(3/2) were identically observed at 1143.0 eV 1116.2 eV [26,28–31]. Although peakpeak intensities of Ga2p (1/2) and Ga2p(3/2) in Figure 7a are weak, the and 1116.2 eV [26,28–31]. Although intensities of Ga2p (1/2) and Ga2p(3/2) in Figure 7a are weak, two peaks could be accurately observed in the Ga2p graph. This This result demonstrated that GaN was the two peaks could be accurately observed in the Ga2p graph. result demonstrated that GaN definitely synthesized by the thermal plasma, but was not detected in the XRD pattern due to its low was definitely synthesized by the thermal plasma, but was not detected in the XRD pattern due to crystallinity. The N=CThe sp2N=C binding is high (398.7 eV)(398.7 in theeV) N1singraphs of Figure peak its low crystallinity. sp2 peak binding peak is high the N1s graphs7.ofThis Figure 7. was peak caused bycaused residual C3H6N6 C and its derivatives after synthesis using using thermal plasma [32]. [32]. An This was by residual H N and its derivatives after synthesis thermal plasma 3 6 6 N–Ga bonding peak waswas observed at at 397.8 (–NHx)) were An N–Ga bonding peak observed 397.8eV. eV.Amino Aminofunctional functional groups groups having having (–NH x were indicated at 400.2 eV by residual C 3H6N6 and it derivatives. In the C1s graph, C=N and C–C binding indicated at 400.2 eV by residual C3 H6 N6 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 C3 H6 N6 . Nanomaterials 2016, 38 15 Additionally, the6,intensity of the C=N bonding peak was higher than that of the C-C bonding peak9 of [32].

Arb. Unit (counts/s) 1140

1130

1120

1110

398.7 eV (N=C sp2) 397.8 eV (N–Ga)

400.2 eV (–NHx )

410

408

Binding Energy (eV) Ga2p

406

1130

1120

398

396

394

298

296

1110

399.1 eV (N-Ga) 396.4 eV (N–C sp2)

400.4 eV (–NHx )

410

408

406

404

402

400

398

Binding Energy (eV)

294

292

290

288

284.3 eV (C–C)

286

284

282

280

Binding Energy (eV)

Annealed sample

Arb. Unit (counts/s) 1140

400

N1s

Annealed sample

Binding Energy (eV)

402

287.5 eV (C=N)

Binding Energy (eV)

Arb. Unit (counts/s) 1150

404

Unannealed sample

C1s

396

394

Annealed sample

C1s

Arb. Unit (counts/s)

1150

b)

Unannealed sample

N1s

Arb. Unit (counts/s)

Unannealed sample

Ga2p

Arb. Unit (counts/s)

a)

287.6 eV (C–N sp3)

298

296

294

292

290

288

286

284

282

280

Binding Energy (eV)

Figure 7. XPS (X-ray (X-ray photoelectron photoelectron spectroscopy spectroscopy analysis) analysis) of of nanopowder nanopowder synthesized synthesized from from aa Figure 7. XPS Ga(NO ) ¨32H O pellet and C H N powder injection with NH gas by the thermal plasma process in 6 6 6powder injection with NH33gas by the thermal plasma process in Ga(NO33)33∙32H22O pellet and C33H6N Plasma 3; (a) before and (b) after annealing. Plasma 3; (a) before and (b) after annealing.

Figure 7b shows XPS analysis results of annealed nanopowder after thermal plasma synthesis in Plasma 3. XRD and TEM analysis revealed that the GaN synthesized by the thermal plasma had ˝ C under crystallinity after after annealing. annealing. After After annealing annealing at at 850 850 °C under rough vacuum vacuum pressure for enhanced crystallinity three hours, the peak intensities of Ga–N bonding were increased to levels which were significantly were observed observed higher than than before before the theannealing annealingstep. step.Ga–N Ga–Nbonding bondingpeaks peaksofofGa2p Ga2p (1/2) and andGa2p Ga2p(3/2) (3/2) were (1/2) at 1143.5 eV and 1116.7 eV, respectively [26,28–31]. However, the N1s graphs all show different peak Following annealing annealing of the product synthesized synthesized in Plasma 3, the trends, as is shown in Figure 7a. Following peaks appeared at 400.4 and 396.4 eV in N1s, which can be observed be observed in Figure Figure 7b. 7b. This peak was generated from the conversion of attributed to nitrogen in the –NH –NHxx and N–C sp2 bond from from C C33N44 generated peak was observed at 399.1 eV [27]. This peak residual CC33H H66N66 [33,34]. A A deconvoluted deconvolutedN–Ga N–Gabonding bonding peak was observed at 399.1 eV [27]. This was compared to pre-annealing due todue thetosufficient levelslevels of nitrogen of synthesized GaN peakshifted was shifted compared to pre-annealing the sufficient of nitrogen of synthesized by decomposition of residual C3 H6C N36H. 6In of Figure 7b, a7b, C–N sp3 bonding peakpeak was GaN by decomposition of residual N6the . InC1s the graph C1s graph of Figure a C–N sp3 bonding produced at 287.6 eV. C–C peakspeaks are deconvoluted at 289.8 and 284.8 eV [33,34]. was produced at 287.6 eV. bonding C–C bonding are deconvoluted at 289.8 and 284.8 eV [33,34]. The results of TGA analysis of product synthesized under the Plasma 3 operating conditions are thethe crucible containing thethe synthesized product powder was indicated in in Figure Figure8.8.The Thetemperature temperatureofof crucible containing synthesized product powder ˝ C °C ˝ C/min increased fromfrom 40 ˝ C at 10 intervals. was increased 40to °C850 to 850 at 10 °C/min intervals.The Theatmosphere atmospherewas wasfilled filled with with a nitrogen reduction of up to 98% of weight occurred. The The “as prepared product” after TGA gas. A total mass reduction As reported in previous analysis was considered to consist of GaN GaN and and aa small small amount amount of of CC33N N44. As ˝ can be synthesized at high C) by studies [26,33,34], C C33N44 can high temperatures temperatures(>600 (>600 °C) by slowly slowlyheating heatingCC33H H66N N66 ˝ C to alone. The curve hadhad a high gradient fromfrom aboutabout 300 °C to 650 °C.650 The˝residual C3H6N6 Themass massreduction reduction curve a high gradient 300 C. The residual and undergo thermal and decomposition at temperatures in excess of °C. C3 Hderivatives undergodegradation thermal degradation and decomposition at temperatures in300 excess 6 N6 and derivatives ˝ ˝ Based the mass maintenance at aboveat620 °C,620 the C, temperature for annealing to enhance the of 300 on C. Based on the mass maintenance above the temperature for annealing to enhance ˝ C.Since crystallinity of synthesized GaN nanopowder was setset atat 850 the crystallinity of synthesized GaN nanopowder was 850°C. Sincethe themelting meltingpoint pointof of GaN GaN is ˝ C, synthesized GaN nanopowder was not melted or vaporized by the annealing. above 2500 °C,

Nanomaterials 2016, 6, 38 Nanomaterials2016, 2016,6,6,38 38 Nanomaterials

10 of 15 10of of15 15 10

Synthesizedproduct product Synthesized

100 100

Mass Mass(%) (%)

80 80

60 60

40 40

20 20

0 0

100 100

200 200

300 300

400 400

500 500

600 600

700 700

800 800

Sample temperature temperature (C (C)) Sample Figure 8. 8. TGA TGA analysis analysisof ofproduct productsynthesized synthesizedfrom fromaaaGa(NO Ga(NO 3)3∙32H2O pellet andCCC33H 3H6N6 powder powder Figure Figure of product synthesized from Ga(NO ∙32H22O pelletand and 3 )3)33¨32H 66N66 powder injectionwith withNH NH333gas gas by the thermal plasmaprocess processin inPlasma Plasma3. 3. injection with NH gasby bythe thethermal thermalplasma plasma process in Plasma 3. injection

Figure 9a,b shows shows FE-TEM images images of the the synthesized product product before and and after annealing annealing in Figure Figure 9a,b 9a,b shows FE-TEM FE-TEM images of of the synthesized synthesized product before before and after after annealing in in Plasma 3 conditions. Small particles under 100 nm and spherical large particles of about 200 nmwere were Plasma Plasma 33 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 mixed in Figure Figure 9a. After After annealing, the the large spherical spherical particles were were no longer longer present, and and only mixed mixed in in Figure 9a. 9a. After annealing, annealing, the large large spherical particles particles were no no longer present, present, and only only nano-sized round particles which had an average particle size of 21.63 nm remained, as is shown in nano-sized round particles which had an average particle size of 21.63 nm remained, as is shown nano-sized round particles which had an average particle size of 21.63 nm remained, as is shown in in Figure 9b. ItItis is believedthat that the large large spherical particles particles are arethe the residual residual CC33H H6N66precursor. precursor. Figure9c 9c Figure Figure 9b. 9b. It is believed believed that the the large spherical spherical particles are the residual C3 H66N N6 precursor. Figure Figure 9c shows TEM-EDS results before and after annealing in Plasma 3. The Ga element was detected as shows TEM-EDS results resultsbefore beforeand andafter after annealing in Plasma 3. The Ga element was detected as shows TEM-EDS annealing in Plasma 3. The Ga element was detected as about about 2.5% of weight before annealing. It was revealed that melamine powder remained in microabout 2.5% of weight annealing. was revealed that melamine powder remained in micro2.5% of weight beforebefore annealing. It wasItrevealed that melamine powder remained in micro-sized sized particles. The content of Ga elements was increased dramatically from 2.45% to 71.6% ofweight weight sized particles. The content Ga elements was increased dramatically to 71.6% of particles. The content of Gaof elements was increased dramatically from from 2.45%2.45% to 71.6% of weight after after annealing. This analysis demonstrates that the purification was completed and residual after annealing. This analysis demonstrates that the purification was completed and residual annealing. This analysis demonstrates that the purification was completed and residual melamine melamine powder powder was was eliminated by by the the heat heat treatment treatment of of the the vacuum vacuum furnace. furnace. As shown shown by by the the melamine powder was eliminated eliminated by the heat treatment of the vacuum furnace. As shown byAs the XRD patterns XRD patterns patterns in in Figure Figure 6b, 6b, aa small small amount of of carbon and and carbon carbon nitride nitride was was generated generated after after XRD in Figure 6b, a small amount of carbonamount and carbon carbon nitride was generated after annealing because the annealing because the treatment was done under vacuum conditions. The vaporized melamine annealing because treatment was done under conditions. vaporized melamine treatment was donethe under vacuum conditions. The vacuum vaporized melamine The converted as a byproduct converted as as aa byproduct byproduct with with gallium galliumnitride nitride nanoparticles. nanoparticles. converted with gallium nitride nanoparticles.

a) a)

b) b)

0.2㎛ 0.2㎛

50nm nm 50 Figure 9.Cont. Cont. Figure Figure 9. 9. Cont.

Nanomaterials 2016, 6, 38 Nanomaterials 2016, 2016, 6, 6, 38 38 Nanomaterials

11 of 15 11 of of 15 15 11

c)

Ga(NO ) ∙xH O : C H N (mol) = 1:6 (annealed at 850℃) Ga(NO 33)33∙xH22O : C33H66N66(mol) = 1:6 (annealed at 850℃) C C O O N N Ga Ga

(unannealed) (unannealed)

0 0

10 10

20 20

30 30

40 40

50 50

60 60

70 70

80 80

90 90

100 100

Accumulated chemical composition (%) Accumulated chemical composition (%)

Figure FE-TEM (field-emission scanning electron microscopy) and EDS (energy dispersive Figure 9. 9. FE-TEM FE-TEM(field-emission (field-emissionscanning scanningelectron electron microscopy) microscopy) and and EDS EDS (energy (energy dispersive dispersive Figure spectroscopy) results of product synthesized from a Ga(NO 3 ) 3 ∙32H 2 O pellet and C 3 H 6 N powder spectroscopy) results results of product product synthesized and C H66N666 powder powder spectroscopy) synthesized from from aa Ga(NO Ga(NO33))33¨32H ∙32H22O pellet and C33H injection with NH gas by the thermal plasma process in Plasma 3; FE-TEM images of (a) before and injectionwith withNH NH333gas gasby bythe thethermal thermalplasma plasmaprocess processin inPlasma Plasma3; 3;FE-TEM FE-TEMimages imagesof of(a) (a)before beforeand and injection (b) after annealing; (c) TEM-EDS results. (b) after annealing; (c) TEM-EDS results.

FE-SEM images and size distribution of synthesized synthesized GaN GaN nanopowder are indicated indicated in Figure Figure FE-SEM size distribution of GaN nanopowder are in FE-SEMimages imagesand and size distribution of synthesized nanopowder are indicated in 10a,b. The large residual C 3H6N6 particles were removed, but the uniform GaN nanoparticles 10a,b. The large residual C 3 H 6 N 6 particles were removed, but the uniform GaN nanoparticles Figure 10a,b. The large residual C3 H6 N6 particles were removed, but the uniform GaN nanoparticles remained, as is shown in Figure 10a. The particle size distribution was arranged byby measurement of remained, particle size distribution was arranged by measurement of remained,as asisisshown shownininFigure Figure10a. 10a.The The particle size distribution was arranged measurement each particle in the FE-SEM frame, as is seen in Figure 10b. The size distribution of synthesized each particle in in thethe FE-SEM frame, asas is isseen of each particle FE-SEM frame, seenininFigure Figure10b. 10b.The Thesize sizedistribution distributionof of synthesized synthesized nanoparticles was was analyzed analyzed by by FE-SEM FE-SEM images. images. Hundreds Hundreds of of nanoparticles nanoparticles were were measured measured with with nanoparticles nanoparticles was analyzed by FE-SEM images. Hundreds of nanoparticles were measured with particles sizes varying from 10 to 60 nm. The mean particle size was 29.8 nm. The uniform-sized particles particles sizes sizes varying varying from from 10 10 to to 60 nm. The The mean mean particle particle size size was was 29.8 nm. The The uniform-sized uniform-sized particle GaN GaN nanopowder nanopowder was was synthesized synthesized by by thermal thermal plasma plasma and and an an additional additional annealing annealing process. process. particle particle GaN nanopowder was synthesized by thermal plasma

a)

Plasma Plasma Ga(NO ) ∙xH O : C H N (mol) 1:6 3 3 2 3 6 6 Ga(NO3)3∙xH 2O : C 3H6N6 (mol) == 1:6

b)

40 40 35 35

Frequency Frequency(%) (%)

30 30 25 25 20 20 15 15 10 10 5 5 0 0

10 10

20 20

30 30

40 40

50 50

60 60

Particle size size (nm) (nm) Particle

Figure 10. 10. (a) (a) FE-SEM FE-SEM image image and and (b) (b) size size distribution distribution of of synthesized synthesized GaN GaN nanopowder nanopowder after after Figure Figure 10. (a) FE-SEM image and (b) size distribution of synthesized GaN nanopowder after annealing annealing in Plasma 3. annealing in Plasma in 3. Plasma 3.

To reduce reduce residual residual C C33H H66N N66 present present in in the the product product generated generated by by the the thermal thermal plasma, plasma, the the ratio ratio of of To To reduce residual C H N present in the product generated by the thermal plasma, the ratio of 3 6 6 Ga(NO33))33∙32H ∙32H22O O and and C C33H H66N N66 was was decreased decreased to to 1:3 1:3 under under the the operating operating conditions conditions used used to to generate generate Ga(NO Ga(NO ) ¨32H O and C H N was decreased to 1:3 under the operating conditions used to generate 3 product 6 6 Plasma 34. 4.3XRD XRD 2patterns patterns of of synthesized under under these these conditions conditions are are shown shown in in Figure Figure 11a. 11a. The The Plasma product synthesized Plasma 4. XRD patterns of product synthesized under these conditions are shown in Figure upper 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 11a. for upper for The upper pattern is for synthesized nanopowder by thermal plasma while the bottom pattern is annealed nanopowder at 850 °C for three hours. In the upper XRD pattern, the main peak of C 3H6N6 annealed nanopowder at 850 °C for˝ three hours. In the upper XRD pattern, the main peak of C3H6N6 for annealed at while 850 Cmain for three Inwere the upper pattern, the38°. main peak of was observednanopowder at aa weak weak 26°, 26°, peakshours. of GaN GaN foundXRD at 32°, 32°, 34°, and and However, was observed at while main peaks of were found at 34°, ˝ , while ˝38°. ˝However, C H N was observed at a weak 26 main peaks of GaN were found at 32 , 34 , and 38˝ . 3 6 6 Ga22O O33 peaks peaks appeared appeared after after annealing annealing in in the the vacuum vacuum furnace furnace at at 850 850 °C °C for for three three hours. hours. These These findings findings Ga ˝ However, Gathat appeared afterreaction annealing in the vacuum furnace at 850 C for three hours. 2 O3 peaks demonstrate the oxygen oxygen capture of Ga(NO Ga(NO 3)3∙32H2O by C3H6N6 could have negative demonstrate that the capture reaction of 3)3∙32H2O by C3H6N6 could have negative effects as as the the amount amount of of C C33H H66N N66 decreases. decreases. effects

Nanomaterials 2016, 6, 38

12 of 15

These findings demonstrate that the oxygen capture reaction of Ga(NO3 )3 ¨32H2 O by C3 H6 N6 could have negative effects Nanomaterials 2016, 6, 38 as the amount of C3 H6 N6 decreases. 12 of 15

a) Intensity (a.u.)

GaN Ga2O3 C3H6N6

b)

Synthesized product

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 ¨32H2 O pellet and C3 H6 N6 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 shows3)FE-TEM images of synthesized nanopowder by thermal plasma, when the molar ratio 11b of Ga(NO 3∙32H2O and C3H6N6 of 1:3 is adopted in Plasma 4 conditions. The product molar ratio of Ga(NO ) O C3 H6 N6 of of fine 1:3 isparticles adoptedwhich in Plasma 4 conditions. The product ¨32H 2 and synthesized in Plasma3 43 primarily consisted had an average particle size of synthesized in Plasma 4 primarily consisted of fine particles which had an average particle size of 9.25 nm. In the EDS result in Table 2, the content of the Ga element is higher, about 63% of weight, 9.25 nm. In the EDS result in Table 2, the content of the Ga element is higher, about 63% of weight, than than that of the synthesized nanopowder in Plasma 3 without additional annealing. However, the that of the synthesized in Plasma without additional annealing. However, the oxygen oxygen content is high nanopowder at 23% of weight due to3incomplete nitridation and generated gallium 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 the molar ratio of injected C3 H6 N6 decreased. Therefore, injection of excessive C3H6Nas 6 is effective for preventing the oxidization of Ga. Moreover, C3Hinjection 6N6 aids of excessive C H N is effective for preventing the oxidization of Ga. Moreover, C H N aidsthe in 3 6 6 3 6 6 in the complete synthesizing of GaN nanopowder. However, annealing is required to eliminate the complete synthesizing of GaN nanopowder. However, annealing is required to eliminate the remaining C3H6N6. remaining C3 H6 N6 . Table 2. Elemental composition of product synthesized from a Ga(NO3)3∙32H2O pellet and C3H6N6 Table 2. injection Elemental composition of the product synthesized from ainGa(NO and 3 )34¨32H 2 O pellet 3 H6 N6 powder with NH3 gas by thermal plasma process Plasma (all results in % of C weight). powder injection with NH3 gas by the thermal plasma process in Plasma 4 (all results in % of weight).

Spectrum C C SpectrumSpectrum 1 8.70 SpectrumSpectrum 2 12.60 1 8.70 2 9.05 12.60 SpectrumSpectrum 3 Mean Spectrum 310.11 9.05 Mean

10.11

N 3.51 N 4.78 3.51 1.73 4.78 3.34 1.73 3.34

O O 23.04 23.34 23.04 23.34 23.40 23.40 23.26

23.26

Ga

Total

Ga 64.76

Total 100.00

59.28 64.76 59.28 65.82 65.82 63.28 63.28

100.00100.00 100.00100.00 100.00100.00 100.00

C3H6N6 was used as a reductant to prevent oxidation of gallium. However, it also consists of an C3 H6 N used as a reductant oxidation gallium. it also consists abundant source nitrogen. Therefore,toa prevent nitridation reactionofusing only However, C3H6N6 powder, without 6 wasof of an abundant source of nitrogen. Therefore, a nitridation reaction using only C H N powder, the addition of NH3 is needed to confirm the complete synthesis of GaN nanopowder. 3 6 In6 previous without the addition NH3injected is needed confirm the complete synthesis of GaN nanopowder. experiments, NH3 gasofwas intotothe Ga(NO 3)3∙32H 2O pellet as a nitridation source. A In previous experiments, NH3 reacted gas was injected intoCthe ) ¨32H O pellet as a nitridation source. Ga(NO 3)3∙32H 2O pellet was with only 3H6Ga(NO N6 powder without NH 3 gas in Plasma 5 3 3 2 A Ga(NO3 )3C¨32H pellet was onlytwo C3 Hnozzles without NH3 gas Plasma 3. 5 conditions. 3H6N 6 powder wasreacted injectedwith through of an anode electrode as in Plasma 2O 6 N6 powder conditions. C3 H6of N6Ga(NO powder was 2injected twoset nozzles an anode electrode in Plasma The molar ratio 3)3∙32H O and Cthrough 3H6N6 was at 1:6.ofThe XRD pattern ofassynthesized 3. The molar ratio of Ga(NO C3 HC63N was set at 1:6. XRD pattern of nanopowder by reacting Ga(NO 3)3∙32H 2O and H66N 6 without NHThe 3 gas is indicated insynthesized Figure 12a. 3 )3 ¨32H 2 O and nanopowder byofreacting Ga(NO O and C3 H6 Npeaks NH3 gas iswith indicated in Figure 12a. The intensities all peaks were3 )weak. observed corresponded gallium oxide and 3 ¨32H2The 6 without The intensities of the all NH peaks were The observed peaks corresponded with of gallium and melamine. When 3 gas wasweak. injected into the reactor at the same molar ratio Ga(NOoxide 3)3∙32H 2O melamine. When the NH gas was injected into the reactor at the same molar ratio of Ga(NO ) ¨32H and C3H6N6 in Plasma 3,3 only residual C3H6N6 peaks were detected in the XRD pattern in3 Figure 6. 3 2O and 3, only residual CHowever, were detected the XRD pattern in Figure LowCcrystallinity GaN was synthesized. Ga2O 3 peaks were in detected with residual C3H6N6.6 3 H6 N6 in Plasma 3 H6 N6 peaks 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, Ga2 O3 peaks were detected with residual C3 H6 N6 in Plasma 5 of Figure 12a. It was revealed that the nitridation reaction did not occur with the nitrogen element of C3 H6 N6 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. However, peaks Nanomaterials 2016, 6, 38 two peaks were observed at 1145.9 eV and 1119.3 eV. Analysis of these 13 of 15 confirmed Ga–O chemical bonding. An increase of binding energy from Ga2p electrons was observed was observed due to Ga–O chemical bonding. The nitrogen has a less electronegative due to Ga–O chemical bonding. The nitrogen element has a lesselement electronegative property than the property than the oxygen element. Therefore, the binding energy of Ga–O chemical bonding is oxygen element. Therefore, the binding energy of Ga–O chemical bonding is higher than that ofhigher Ga–N than that bonding of Ga–N [30]. chemical bonding [30]. chemical

a)

b)

Ga2p

Intensity (a.u.)

Arb. Unit (counts/s)

Ga2O3 C3H6N6

10

20

30

40

50

60

Two Theta (deg)

70

80

90

1150

1140

1130

1120

1110

Binding Energy (eV)

Figure 12. (a) (a) XRD XRD patterns patterns and and (b) (b) XPS XPSanalysis analysisof ofproducts productssynthesized synthesizedfrom fromaaGa(NO Ga(NO3 )33)3¨32H ∙32H22O O Figure 12. pellet and and C C33H pellet H66N N66powder powderinjection injectionwithout withoutNH NH33gas gasby bythe thethermal thermalplasma plasmaprocess processin inPlasma Plasma5.5.

C3H6N6 was decomposed and converted to cyanides, hydrocarbons, and carbon in the high C3 H6 N6 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 cyanide 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 Figure 4c. 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 ¨32H2 O and C3H6N6 by thermal plasma, despite low crystallinity. Its crystallinity can be enhanced by and C3 H6 N6 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 ¨xH2 O was used GaN by a thermal plasma3 )process. Ga(NO3)3∙xH2O was used as as the rawnanopowder material. Atwas first,synthesized it was discovered that Ga(NO 3 ¨xH2 O was not nitrided by a solely the raw material. At first, it source was discovered Ga(NO3)3into ∙xH2Ga O 2was not nitrided by a solely conventional NH3 nitridation and that itthat is converted O3 . It required a reductant to conventional NH 3 nitridation source and that it is converted into Ga2O3. It required a reductant to prevent oxidation to Ga2 O3 instead of GaN. Therefore, C3 H6 N6 powder was injected into the high prevent oxidation 3 instead of GaN. Therefore, C3H6N6 powder was injected into the high temperature regiontoofGa the2Othermal plasma jet through an anode electrode. The molar ratio of injected temperature ofCthe thermal plasma jet through an1:3. anode electrode. The molar ratiocrystallinity of injected Ga(NO3 )3 ¨xHregion O and H N was controlled at 1:6 and GaN nanopowder with low 2 3 6 6 Ga(NO 3)3∙xH2O and C3H6N6 was controlled at 1:6 and 1:3. GaN nanopowder with low crystallinity and residual C3 H6 N6 was synthesized by the thermal plasma process. Crystallinity of the synthesized and C3H6N 6 was synthesized by the thermal plasma process. Crystallinity the synthesized GaNresidual nanopowder was further enhanced after annealing at 850 ˝ C for three hours in of a vacuum furnace. GaN nanopowder was further enhanced after annealing at 850 °C for three hours in a vacuum furnace. The size of synthesized GaN nanopowder is distributed from 10 to 60 nm, and mean particle size is The size oftosynthesized nanopowder distributed from 10 to 60) nm, and mean particle size is calculated be 29.8 nm.GaN In order to confirmisthe nitridation of Ga(NO 3 3 ¨xH2 O by C3 H6 N6 reductant, calculated be 29.8 nm. In order to confirm the nitridation of Ga(NO3)3∙xH2O by C3H6N6 reductant, the Ga(NOto 3 )3 ¨xH2 O was reacted with C3 H6 N6 without NH3 gas, and Ga(NO3 )3 ¨xH2 O precursor was the Ga(NO 3 ) 3 ∙xH was reacted with C3H6N6 without NH3 gas, and Ga(NO3)from 3∙xH2Ga(NO O precursor was oxidized to Ga2 O23O . Therefore, GaN nanopowder was successfully synthesized 3 )3 ¨32H2 O oxidized 2O3. Therefore, GaN nanopowder was successfully synthesized from Ga(NO3)3∙32H2O and C3 H6toNGa 6 powders with NH3 gas by a thermal plasma process. Furthermore, the additional and C3H6Nstep 6 powders with NH 3 gas by its a thermal plasma process. Furthermore, additional annealing was required to enhance crystallinity. It was speculated that the the production of annealing step was required to enhance its crystallinity. It was speculated that the production of carbon dioxide or carbon nitride during the annealing step could be controlled as anneal in an inert 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.

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.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19.

Morkoc, H.; Mohammad, S.N. High-luminosity blue and blue-green gallium nitride light-emitting diodes. Science 1995, 267, 51–55. [CrossRef] [PubMed] Balkas, C.M.; Davis, R.F. Synthesis routes and characterization of high-purity, single-phase gallium nitride powders. J. Am. Ceram. Soc. 1996, 79, 2309–2312. [CrossRef] Han, W.; Fan, S.; Li, Q.; Hu, Y. Synthesis of gallium nitride nanorods through a carbon nanotube-confined reaction. Science 1997, 277, 1287–1289. [CrossRef] Cheng, G.S.; Zhang, L.D.; Zhu, Y.; Fei, G.T.; Li, L. Large-scale synthesis of single crystalline gallium nitride nanowires. Appl. Phys. Lett. 1999, 75, 2455–2457. [CrossRef] Mohammad, S.N.; Salvador, A.A.; Morkoc, H. Emerging gallium nitride based devices. Proc. IEEE 1995, 83, 1306–1355. [CrossRef] Kim, H.M.; Kang, T.W.; Chung, K.S. Nanoscale ultraviolet-light-emitting diodes using wide-bandgap gallium nitride nanorods. Adv. Mater. 2003, 15, 567–569. [CrossRef] Millet, P.; Calka, A.; Williams, J.S.; Vantenaar, G.J.H. Formation of gallium nitride by a novel hot mechanical alloying process. Appl. Phys. Lett. 1993, 63. [CrossRef] Wu, H.; Hunting, J.; Uheda, K.; Lepak, L.; Konkapaka, P.; DiSalvo, F.J.; Spencer, M.G. Rapid synthesis of gallium nitride powder. J. Cryst. Growth 2005, 279, 303–310. [CrossRef] Lorenz, M.R.; Binkowski, B.B. Preparation, stability, and luminescence of gallium nitride. J. Electrochem. Soc. 1962, 109, 24–26. [CrossRef] Wood, G.L.; Pruss, E.A.; Paine, R.T. Aerosol-assisted vapor phase synthesis of gallium nitride powder. Chem. Mater. 2001, 13, 12–14. [CrossRef] Li, H.D.; Yang, H.B.; Yu, S.; Zou, G.T.; Li, Y.D.; Liu, S.Y.; Yang, S.R. Synthesis of ultrafine gallium nitride powder by the direct current arc plasma method. Appl. Phys. Lett. 1996, 69, 1285–1287. [CrossRef] Li, H.D.; Yang, H.B.; Zou, G.T.; Yu, S.; Lu, J.S.; Qu, S.C.; Wu, Y. Formation and photoluminescence spectrum of w-GaN powder. J. Cryst. Growth 1997, 171, 307–310. [CrossRef] Chen, C.C.; Yeh, C.C.; Chen, C.H.; Yu, M.Y.; Liu, H.L.; Wu, J.J.; Chen, K.H.; Chen, L.C.; Peng, J.Y.; Chen, Y.F. Catalytic growth and characterization of gallium nitride nanowires. J. Am. Chem. Soc. 2001, 123, 2791–2798. [CrossRef] [PubMed] Jung, W.S. Reaction intermediate(s) in the conversion of β-gallium oxide to gallium nitride under a flow of ammonia. Mater. Lett. 2002, 57, 110–114. [CrossRef] Jung, W.S.; Min, B.K. Synthesis of gallium nitride powders and nanowires from gallium oxyhydroxide under a flow of ammonia. Mater. Lett. 2004, 58, 3058–3062. [CrossRef] Jung, W.S. Synthesis and characterization of GaN powder by the cyanonitridation of gallium oxide powder. Ceram. Int. 2012, 38, 5741–5746. [CrossRef] Melnikov, P.; Nascimento, V.A.; Zanoni Consolo, L.Z. Thermal decomposition of gallium nitrate hydrate and modeling of thermolysis products. J. Therm. Anal. Calorim. 2012, 107, 1117–1121. [CrossRef] Ogi, T.; Kaihatsu, Y.; Iskandar, F.; Tanabe, E.; Okuyama, K. Synthesis of nanocrystalline GaN from Ga2 O3 nanoparticles derived from salt-assisted spray pyrolysis. Adv. Powder Technol. 2009, 20, 29–34. [CrossRef] Di Lello, B.C.; Moura, F.J.; Solorzano, I.G. Synthesis and characterization of GaN using gas-solid reactions. Mater. Sci. Eng. B 2002, 93, 219–223. [CrossRef]

Nanomaterials 2016, 6, 38

20. 21. 22. 23. 24.

25. 26.

27. 28.

29. 30. 31.

32. 33. 34.

15 of 15

Jung, W.S. Preparation of gallium nitride powders and nanowires from a gallium(III) nitrate salt in flowing ammonia. Bull. Korean Chem. Soc. 2004, 25, 51–54. Zhao, H.; Lei, M.; Chen, X.; Tang, W. Facile route to metal nitrides through melamine and metal oxides. J. Mater. Chem. 2006, 16, 4407–4412. [CrossRef] Jung, W.S. Use of melamine in the nitridation of aluminium oxide to aluminium nitride. J. Ceram. Soc. Jpn. 2012, 119, 968–971. [CrossRef] Berbenni, V.; Milanese, C.; Bruni, G.; Marini, A. Thermal decomposition of gallium nitrate hydrate Ga(NO3 )3 ¨32H2 O. J. Therm. Anal. Calorim. 2005, 82, 401–407. [CrossRef] Ono, S.; Funato, T.; Inoue, Y.; Munechika, T.; Yoshimura, T.; Morita, H.; Rengakuji, S.I.; Shimasaki, C. Determination of melamine derivatives, melame, meleme, ammeline and ammelide by high-performance cation-exchange chromatography. J. Chromatogr. A 1998, 815, 197–204. [CrossRef] Costa, L.; Camino, G. Thermal behavior of melamine. J. Therm. Anal. 1988, 34, 423–429. [CrossRef] Yan, H.; Chen, Y.; Xu, S. Synthesis of graphitic carbon nitride by directly heating sulfuric acid treated melamine for enhanced photocatalytic H2 production from water under visible light. Int. J. Hydrog. Energy 2012, 37, 125–133. [CrossRef] Xiao, H.D.; Ma, H.L.; Xue, C.S.; Hu, W.R.; Ma, J.; Zong, F.J.; Zhang, X.J.; Ji, F. Synthesis and structural properties of GaN particles from GaO2 H powders. Diam. Relat. Mater. 2005, 14, 1730–1734. [CrossRef] Wolter, S.D.; Luther, B.P.; Waltemyer, D.L.; Onneby, C.; Mohney, S.E.; Molnar, F.J. X-ray photoelectron spectroscopy and x-ray diffraction study of the thermal oxide on gallium nitride. Appl. Phys. Lett. 1997, 70, 2156–2158. [CrossRef] Yang, Y.; Ma, H.; Hao, X.; Ma, J.; Xue, C.; Zhuang, H. Preparation and properties of GaN films on Si(111) substrates. Sci. China Ser. G 2003, 46, 173–177. [CrossRef] Pal, S.; Mahapatra, R.; Ray, S.K.; Chakraborty, B.R.; Shivaprasad, S.M.; Lahiri, S.K.; Bose, D.N. Microwave plasma oxidation of gallium nitride. Thin Solid Film. 2003, 425, 20–23. [CrossRef] Xiao, H.D.; Ma, H.L.; Xue, C.S.; Zhuang, H.Z.; Ma, J.; Zong, F.J.; Zhang, X.J. Synthesis and structural properties of beta-gallium oxide particles from gallium nitride powder. Mater. Chem. Phys. 2007, 101, 99–102. [CrossRef] Feng, D.; Zhou, Z.; Bo, M. An investigation of the thermal degradation of melamine phosphonate by XPS and thermal analysis techniques. Polym. Degrad. Stable 1995, 50, 65–70. [CrossRef] Yao, L.D.; Li, F.Y.; Li, J.X.; Jin, C.Q.; Yu, R.C. Study of the products of melamine (C3 H6 N6 ) treated at high pressure and high temperature. Phys. Status Solidi (a) 2005, 202, 2679–2685. [CrossRef] Li, X.; Zhang, J.; Shen, L.; Ma, Y.; Lei, W.; Cui, Q.; Zou, G. Preparation and characterization of graphite carbon nitride through pyrolysis of melamine. Appl. Phys. A 2009, 94, 387–392. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons by Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).