Article pubs.acs.org/crystal
Synthesis of Wurtzite-Type InN Crystals by Low-Temperature Nitridation of LiInO2 Using NaNH2 Flux Akira Miura,* Takahiro Takei, and Nobuhiro Kumada Center for Crystal Science and Technology, University of Yamanashi, 7 Miyamae, Kofu, Yamanashi 400-8511, Japan ABSTRACT: Nitridation of oxides above 500 °C is a conventional route for the synthesis of various oxynitrides and nitrides. However, this technique has the disadvantages of requiring a high temperature to break the strong metal and oxygen bonds and of low utilization efficiency of nitrogen sources such as toxic ammonia. Here, we show a simple lowtemperature synthesis of InN nanocrystals via the nitridation of LiInO2 by NaNH2 flux at 240 °C in a Teflon-lined autoclave. The NaNH2 flux converted irregular-shaped LiInO2 powder to 50−300 nm InN nanocrystals with well-developed facets. The product crystallized in the hexagonal wurtzite structure with lattice parameters of a = 3.545 Å and c = 5.717 Å. Lithium in the starting oxide was found to be essential for this low-temperature nitridation.
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GaN. In contrast to the usefulness of Na flux for the growth of GaN crystals,14 suitable fluxes for InN crystals are not extensively known. Such fluxes could be the key for the crystal growth of bulk InN, which would improve InN-based devices by the homoepitaxial growth of InN film. In this study, we present a facile low-temperature nitridation of LiInO2 using NaNH2 flux at 240 °C in a Teflon-lined autoclave. The use of this new nitridation reaction allows for the synthesis of highly crystalline submicrometer-sized InN crystals from the oxide without using an ammonia flow, high temperature, and expensive starting materials and apparatuses.
INTRODUCTION Advances in the field of nitride chemistry, including the exploration of new materials, applications, and synthesis techniques, have been significant in the recent decades.1 Oxides are important precursors in the synthesis of nitrides and oxynitrides, as they are inexpensive and stable and can have many different compositions. However, the nitridation of oxides usually requires a high temperature (>500 °C) and long treatment time in order to break the strong bonds between metal and oxygen.2 In addition, the reaction requires a gas flow of toxic ammonia or nitrogen. As a result, synthesis from oxides has the disadvantages of high energy consumption and low utilization efficiency of nitrogen sources. The other nitridation technique employing oxides is a mechanochemical method, but it uses expensive nitrides (Li3N or Mg3N2) as starting materials and sophisticated apparatuses.3 While there have been significant developments in techniques for the low-temperature synthesis of nitrides from metals,4 intermetallics,5 sulfides,6 halides,7 and metal−organic complexes,8 the approach using oxides needs further improvement to exploit the significant advantages of the oxide precursors described above. III−V semiconductors, such as AlN and GaN, are important materials for optoelectronic devices. Among them, hexagonal wurtzite-type InN has a narrow band gap of approximately 0.7 eV.9 A larger band gap of ca. 1.8 eV is also observed when a substantial number of carriers is incorporated.10 One of the synthetic routes to indium nitrides/oxynitrides is nitridation of In2O3 in an ammonia flow at 500−800 °C.11 The morphologies of the synthesized powders are nanowires or nanopowder aggregates (ca. 50−200 nm), and well-developed hexagonal facets are rarely seen.11a−c For the synthesis of InN crystals, low-temperature routes have been developed by using sulfides, halides, or organic compounds.6b,7b−d,12 The mechanochemical method for synthesizing InN has not been successful.13 Moreover, the solution growth of InN crystals is relatively underdeveloped when compared with the extensively studied © 2012 American Chemical Society
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EXPERIMENTAL SECTION
The following powders were used without further purification: Li2CO3 (Kanto Kagaku, >99%), In2O3 (Kanto Kagaku, 99.99%), and NaNH2 (Aldrich, >95%). Caution: Sodium amide powder (NaNH2) is highly sensitive to air and moisture and is flammable, and it can cause burns. It should be handled in a glovebox. LiInO2 was synthesized by a conventional solid state reaction at 750 °C between Li2CO3 and In2O3 in air. In a nitrogen-filled glovebox, the synthesized LiInO2 (0.15 g) and NaNH2 (1.0 g) powders were placed into a Teflon-lined steel autoclave with an inner volume of approximately 56 cm3. Then, the autoclave was tightly closed and kept in an oven that operated at 240 °C for 36 h. After cooling, the autoclave was opened under normal atmosphere and distilled water was added. The black product was filtered and washed with distilled water. Then, the In(OH)3 byproduct was removed by washing with 1 M oxalic acid aqueous solution and rinsed with distilled water. The product was dried overnight at room temperature and stored in a desiccator over silica gel. Crystal structure was examined by powder X-ray diffraction (XRD) using a RINT-2000 (Rigaku; Cu Kα radiation). The lattice parameters were calculated by the d values of the indexed peaks in the 2θ range of Received: May 29, 2012 Revised: June 27, 2012 Published: July 26, 2012 4545
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Crystal Growth & Design
Article
20−130°. Sample morphologies were investigated by field-emission scanning electron microscopy (FE-SEM) (JEOL; JSM-6500) operating at an accelerating voltage of 15 kV. Ethanol suspensions of the samples were dispersed on aluminum foil for SEM observation. Elemental analysis was performed by energy-dispersive X-ray spectroscopy (EDX) equipped with a scanning electron microscope. Lithium content was analyzed by atomic absorption spectroscopy (AAS; Shimazu; AA-6800). A laser Raman spectrometer in backscattering geometry was used to study the structural features. The 488nm laser line of a Spectra-Physics Cyan Scientific CW laser was used.
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RESULTS AND DISCUSSION Figure 1 exhibits the XRD patterns of LiInO2 before and after the reaction with NaNH2 at 240 °C. The XRD pattern of the
Figure 2. SEM images of (a) LiInO2 and (b−d) InN crystals at different magnifications (from ×10k to ×100k). (e) Typical EDX spectrum of InN crystals.
content measured by AAS was ca. 0.4 wt %, indicating the incorporation of lithium into the InN crystals. Figure 3 shows the typical Raman spectrum of the InN crystals after washing with oxalic acid. The peaks at 489, 539,
Figure 1. XRD patterns of (a) LiInO2, (b) the sample obtained by heating with NaNH2 at 240 °C and subsequent washing with water, and (c) the sample after washing with 1 M oxalic acid. ICSD patterns of LiInO2 (#40665), In(OH)3 (#35636), and InN (#109463) were used for the assignments.
starting material was indexed as LiInO2. After the reaction and subsequent washing with water, the patterns were identified as hexagonal InN and cubic In(OH)3. Further washing with oxalic acid aqueous solution removed the peaks due to the In(OH)3 phase, resulting in single-phase InN. The lattice parameters of InN were a = 3.545(1) Å and c = 5.717(1) Å, which were close to the reported values (ICDS #109463: a = 3.5377 Å and c = 5.7037 Å). The sharp diffraction peaks indicated the high crystallinity of the InN phase. Figure 2 shows the SEM images of LiInO2 and the product after the washing with oxalic acid solution, which was identified as single phase InN by XRD. The LiInO2 powder formed irregular shapes about 1 μm in size (Figure 2a). On the other hand, the InN crystal products after washing showed crystal aggregates that were 50−300 nm in size (Figure 2b−d). Pyramidal crystals with hexagonal facets were found. Such welldeveloped facets of InN crystals produced by the vapor-solid reaction of In2O3 with NH3 are rarely seen.11a−c The changes in the size and morphology exclude the possibility of topotactic reaction. The EDX spectrum showed nitrogen and indium signals, in addition to an aluminum signal due to the underlying aluminum foil (Figure 2e). Little or no sodium or oxygen peaks were detected. We cannot preclude the possibility of oxygen incorporation and nitrogen deficiency because EDS is not very sensitive to oxygen and nitrogen quantities. The lithium
Figure 3. Raman spectrum of InN crystals. The tic marks represent the positions of phonon modes.14.
and 581 cm−1 corresponded to the phonon vibration frequencies of the E1(TO), B2(high), and A1(LO) modes, respectively.15 This agreement is further confirmation of the presence of wurtzite InN in the product. To study the influence of the lithium content in the starting materials on the final product, we carried out the same experiment using In2O3 instead of LiInO2. The product obtained after washing with water was grayish white, and no InN phase was identified by XRD; only peaks for In(OH)3 were found. Thus, the lithium in LiInO2 plays an important role in the nitridation. The disappearance of the LiInO2 phase and the morphological change from an irregular to a hexagonal pyramidal shape imply the decomposition of LiInO2 and subsequent crystallization processes of InN. Here, we discuss the possible conversion mechanism from LiInO2 to InN. First, NaNH2 melts at 240 °C, since its melting temperature is ca. 210 °C. At this stage, the decomposition of LiInO2 is expected. As described above, the presence of lithium is essential for the 4546
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synthesis of InN. Thus, the next nucleation and growth processes likely involve moisture-sensitive lithium species, such as Li3N, LiNH2, Li3In(NH2)6, and Li3InN2.16 The relatively strong bonding between lithium and nitrogen may be useful for such nitridation processes when compared with the weak bonding between sodium and nitrogen.4b,17 Formation of hydroxides, such as In(OH)3, NaOH, and LiOH, also occurs at some point. The fact that InN is grown with hydroxides in the closed system is an interesting aspect of this technique. The nitridation, therefore, can be formulated as follows: LiInO2 + NaNH 2 = InN + NaOH + LiOH
However, the formation mechanism for InN may be more complex, and in situ measurement may reveal further details of the reactions in the future.
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CONCLUSION The reaction of LiInO2 with NaNH2 afforded highly crystalline InN nanocrystals at 240 °C, a much lower temperature when compared with the temperature for the conventional solid− vapor reaction of In2O3 with NH3 gas. This reaction has the advantages of requiring inexpensive starting materials, simple apparatus, and low temperature, and of having a high utilization efficiency of its nitrogen source. Synthesized InN crystals had well-developed facets, unlike the crystals synthesized by the high-temperature reaction of In2O3 with NH3. The wurtzitetype structure was confirmed by both XRD and Raman spectroscopy, but further challenges with respect to lithium incorporation remain. This work shows promise for the lowtemperature nitridation of oxides and the potential of NaNH2 flux for the growth of InN crystals.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.M. acknowledges Prof. Keisuke Arimoto for helping Raman spectroscopic measurement. REFERENCES
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