NANO LETTERS
Facile Azidothermal Metathesis Route to Gallium Nitride Nanoparticles
2002 Vol. 2, No. 8 899-902
Jianjun Wang, Luke Grocholl, and Edward G. Gillan* Department of Chemistry and the Optical Science and Technology Center, UniVersity of Iowa, Iowa City, Iowa 52242-1294 Received June 5, 2002; Revised Manuscript Received June 20, 2002
ABSTRACT This report describes a straightforward, metathesis (exchange) reaction between gallium chloride and sodium azide that produces gallium nitride nanoparticles below 210 °C. Slowly heating these two reagents together circumvents rapid, exothermic reactions, which can decompose the nitride product. The resulting GaN powders are nanocrystalline and crystallize to the hexagonal phase upon annealing. Well-formed nanoparticles (ca. 50 nm) are clearly resolved in annealed samples, while as-synthesized particles sizes are near 10 nm.
Nanoparticles and nanocrystallites of bulk inorganic solids have been shown to exhibit size dependent properties, such as lower melting points, higher energy gaps, and nonthermodynamic structures.1 Nanoparticle systems ranging from oxide insulators2 to chalcogenide semiconductors1,3 have been developed over the past decade. Many nanoparticle syntheses are low-temperature (1000 °C). It has been observed that since GaN is thermally unstable, it decomposes during exothermic metathesis reactions. For example, sealed tube reactions between GaI3 with Li3N produce gallium metal,12 and hot-wire initiated metathesis reactions require multiple nitrogen sources (i.e., Li3N with NH4Cl and LiNH2) for successful GaN synthesis.13 Li3N has also been utilized, with some success, in solvothermal GaN reactions.14 As an alternative precursor, sodium azide (NaN3) is an air-stable salt with very high nitrogen content that decomposes near 350 °C to nitrogen gas and sodium metal. A recent melt-based GaN crystal growth method utilized NaN3 as a sodium and nitrogen source with molten gallium.15 The rapid exothermic reaction between GaI3 and NaN3 produces gallium metal and nitrogen; however, the addition of an elemental iodine “heat sink” enhances GaN powder formation.16 A molecular gallium triazide, Ga(N3)3, was recently isolated and studied in detail by Fisher and co-workers.17 While Ga(N3)3 is explosive and difficult to handle, it can be converted to nanocrystalline GaN under thermal or solvothermal conditions.17a,b We recently demonstrated that nonaqueous solvothermal reactions between GaCl3 and NaN3 produce amorphous/nanocrystalline GaN that crystallizes to GaN nanostructures upon annealing.18 In the present work we describe the solVent-free reaction of GaCl3 with NaN3 to produce gallium azide intermediates that are slowly decomposed in a nonexplosive manner to produce GaN nanoparticles (ca. 10 nm). The yellow-orange products crystallize upon annealing to faceted nanoparticles near 50 nm in size and occasionally form elongated nanostructures. This study takes advantage of the low GaCl3 melting point (78 °C) and utilizes it as a liquid precursor that reacts with NaN3. These two precursors were heated together in a stirred
Figure 1. Powder X-ray diffraction of GaN products: (a) assynthesized with NaCl peaks marked with an asterisk (*), (b) after glycerol/ethanol wash, (c) after washing and annealing at 1000 °C, and (d) a hexagonal GaN standard pattern (JCPDS #76-0703).
Parr reactor. The temperature was maintained near 80 °C for 1 day to facilitate the reaction of molten GaCl3 with NaN3 and limit GaCl3 transport to the cooler regions of the reactor. A short-lived internal temperature rise of about 10 °C occurs near the GaCl3 melting point, indicating the exothermic formation of a gallium azide intermediate and the thermodynamically stable NaCl byproduct (∆Hf ) -386 kJ/mol). The temperature was slowly raised further, and gas evolution was apparent by 170-180 °C. The reactor temperature was slowly raised to 210 °C and held there until gas evolution ceased (see S1 Supporting Information for complete experimental details). Safety note: Metal azides and undecomposed azide intermediates may be thermally unstable, shock sensitiVe, and should be treated as potentially explosiVe. In addition, metathesis reactions with low melting precursors can rapidly ignite; for example, stirring GaCl3 with Na3P produces a rapid and exothermic reaction.19 The overall thermal reaction between gallium chloride and sodium azide is described by eq 1, where polymeric gallium azide intermediates are produced and then slowly decomposed in situ to GaN products. GaCl3 + 3NaN3 f [Ga(N3)3]n + 3NaCl f GaN + 3NaCl + 4N2 (1) Based on the final product distribution in eq 1, the isolated product mass for the as-synthesized GaN product was 98% of theoretical and the evolved gas pressure (175 psi) was equal to that predicted for N2 evolution. Gas-phase IR of the evolved gases showed no absorption peaks, consistent with nitrogen gas. The sodium chloride byproduct and unreacted starting materials were removed from the assynthesized product with a 1-hour glycerol/ethanol wash. The washed GaN powder was isolated (72% of theoretical) by decanting off the cloudy yellow glycerol solution, followed by an ethanol rinse. A second solid GaN fraction (28% of theoretical) settled out of the glycerol/ethanol filtrate after sitting overnight (see S1 Supporting Information for more 900
Figure 2. Transmission electron microscopy on washed GaN that settled out of a glycerol/ethanol solution overnight. Electron diffraction spots and rings correspond to NaCl, except as noted.
details). The total isolated GaN yield was essentially quantitative based on eq 1. Both the as-synthesized and washed GaN products are yellow-orange in color. After washing, residual azide (N3-) IR bands near 2100 cm-1 disappear and a few weak bands corresponding to glycerol surface coatings are observed (see S2 Supporting Information). The characteristic broad GaN absorption band near 600 cm-1 is unaffected by the wash process. A sharp peak at 1400 cm-1 is attributed to -Nd N- bonds that may form on the particle surface to tie up dangling bonds. These bonds are likely reactive, and this band decreases with longer solvent exposure. The NaCl byproduct is clearly resolved in the powder X-ray diffraction (XRD) pattern of as-synthesized GaN, but is nearly absent from washed GaN (Figure 1a,b). The XRD patterns of washed GaN powder show broad diffraction in the GaN regions, similar to products from our previous solvothermal work.18 The GaN crystallite size in the washed product is below 3 nm, as estimated from XRD peak broadening. The washed GaN powder that settled out of solution overnight has the same XRD pattern as shown in Figure 1b. The thermal stability of washed GaN powder was examined by thermogravimetric-differential analysis (TG-DTA) under flowing argon (see S3 Supporting Information). This Nano Lett., Vol. 2, No. 8, 2002
Figure 3. Transmission and scanning electron microscopy of annealed GaN materials: (a,b) from stirred reactions, (c) after selective carbon coating removal, (d) from heterogeneous nonstirred reactions. All diffraction spots in 3c (inset) correspond to hexagonal (wurtzite) GaN.
product showed no significant events other than a 3% weight loss near 350 °C consistent with the loss or decomposition of surface attached glycerol and a 7% loss due to residual NaCl evaporation at 800 °C. This suggests that a more accurate GaN yield after washing is closer to 90%. The gradual weight loss from 400 to 800 °C is attributed to pyrolysis of surface-bonded organic species. The washed GaN powder was annealed in a sealed, evacuated silica tube at 1000 °C. Any residual NaCl transported to cooler regions of the tube and the resulting black solid retains a GaN IR signature (see S2 Supporting Information) and shows significantly improved hexagonal GaN crystallinity (Figure 1c). The annealed GaN crystallite size calculated from XRD line broadening analysis is 4050 nm. The black carbon coating on the annealed nanoparticles is likely the result of surface decomposition of residual glycerol groups. This coating should form by 600 °C and may serve to limit GaN interparticle diffusion and growth. A TGA-DTA oxidation test (20% O2 flow) on the annealed GaN powder shows that the particles have a 4-5 wt % carbon coating and GaN oxidation occurs above 750 °C. The observed weight gain in the 750-1000 °C region corresponds to a GaN1.17 composition. Some surface oxidation likely Nano Lett., Vol. 2, No. 8, 2002
occurs prior to this major GaN to Ga2O3 oxidation event, and the observed weight gain is also consistent with a (GaN)0.76(Ga2O3)0.12 composition at 750 °C (see S3, Supporting Information). The crystalline GaN nanoparticle core in annealed samples can be safely reexposed by gentle, brief oxidation in air (T < 600 °C) to burn off the carbon coating, which leaves behind light-yellow crystalline GaN powder. No Ga2O3 is observed in this carbon-cleaned powder by IR or XRD. Transmission electron microscopy (TEM) shows that washed GaN that settled from solution overnight consists of small 10 nm particles that pack into dense arrays and exhibit some interparticle connectivity and fusion (Figure 2). In some cases, electron diffraction (Figure 2 bottom, inset) shows a diffuse ring for the most intense GaN (101) reflection, in addition to NaCl spotted rings. Possibly a few particles have NaCl crystallite cores with GaN coatings. The washed GaN powder consists of larger aggregates that show similar nanoparticle features. After annealing, scanning electron microscopy (SEM) and TEM reveal that the GaN product predominantly consists of faceted nanoparticles nearly 50 nm in size (Figures 3a and 3b), consistent with their XRD crystallite size. The annealed GaN particles before and after 901
This study shows that a careful liquid-solid metathesis reaction between GaCl3 and NaN3 can be performed without a rapid exothermic decomposition event to produce nanocrystalline/amorphous 10 nm GaN particles at low temperatures (210 °C). Once separated from the salt byproduct, the nanoscale GaN particles coalesce and crystallize on annealing to form 50 nm faceted crystallites. We are currently exploring the application of this synthetic methodology to other metal nitrides and transition-metal doped GaN systems. Acknowledgment. The authors gratefully acknowledge funding for this work from the Office of Naval Research (N00014-99-1-0953), the Research Corporation (Research Innovation Award, E.G.G.), and the University of Iowa. Supporting Information Available: Complete experimental details, IR spectra, and TG-DTA data for isolated GaN products. This material is available free of charge via the Internet at http://pubs.acs.org. References
Figure 4. UV-vis absorption spectra (top) for particle suspensions of (a) washed GaN that settled from solution overnight, (b) annealed GaN, and (c) annealed GaN with surface carbon removed. Photoluminescence spectra (bottom, 300 nm excitation) for (a) washed GaN that settled from solution overnight and (b) annealed GaN with surface carbon removed. The spectra have been scaled and offset for clarity and solvent spectra are included for reference.
carbon coating removal appear identical by SEM; however, TEM shows evidence for elongated particle growth that may be a result of nanoparticle fusion after the carbon coating is removed (Figure 3c). The advantage of selectively removing the carbon surface coating is that it increases chances of observing optical phenomena associated with annealed GaN nanoparticles. The optical properties of washed and annealed GaN nanoparticle suspensions are shown in Figure 4. The annealed sample has a broad UV-vis absorption at 345 nm (3.6 eV) that is slightly blue-shifted relative to bulk GaN (365 nm, 3.4 eV). The washed sample also has absorption in this region, but it is overshadowed by a broad, red-shifted tail that may be a consequence of its orange color. The suspensions of washed and annealed GaN both show a broad luminescent feature centered near 355 nm, consistent with GaN emission. This contrasts with our previous GaN solvothermal reaction products where surface solvent coordination strongly dominated luminescence.18 During the gallium azide formation and decomposition process it is crucial to stir the two precursors to achieve product homogeneity. Without stirring, the product is heterogeneous in appearance (a black and yellow mixture) and annealed materials show regions of gallium metal and hollow crystalline GaN tweezer-like structures (Figure 3d). These GaN nanostructures may initially form around a gallium center that melts away during annealing. 902
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