GaN nanoparticles, 3–8 nm in diameter, are prepared by a microwave-assisted reaction of GaCl3 and KNH2 in ionic liquids. Instantaneously after the liquid-phase synthesis, the β-GaN nanoparticles are single-crystalline. The band gap is blue-shifted by 0.6 eV in comparison to bulk-GaN indicating quantum confinement effects. The GaN nanoparticles show intense green emission with a quantum yield of 55 ± 3%

Feldmann, Claus

Gaiser, Hannah F.

Gerthsen, Dagmar

Popescu, Radian

English

KITopen

2312 | Chem. Commun., 2020, 56, 2312--2315 This journal is©The Royal Society of Chemistry 2020Cite this:Chem. Commun., 2020,56, 2312Ionic-liquid-based synthesis of GaNnanoparticles†Hannah F. Gaiser,a Radian Popescu,b Dagmar Gerthsenb and Claus Feldmann *aGaN nanoparticles, 3–8 nm in diameter, are prepared by amicrowave-assisted reaction of GaCl3 and KNH2 in ionic liquids.Instantaneously after the liquid-phase synthesis, the b-GaN nano-particles are single-crystalline. The band gap is blue-shifted by0.6 eV in comparison to bulk-GaN indicating quantum confinementeffects. The GaN nanoparticles show intense green emission with aquantum yield of 55  3%.Since the discovery of blue- and UV-light emitting diodes(LEDs), GaN has become one of the most important semi-conductor materials.1 Here, a technological breakthrough forGaN thin films was achieved by gas-phase deposition, whichguarantees both high purity and high crystallinity.2 The synthesisof nanoparticulate GaN is an even greater challenge compared tothin films, since the risk of contamination is even higher due tothe large surface area of the nanoparticles and the associatedhigh reactivity. In addition, nanoparticles are typically excludedfrom crystallization at high temperature (Z400 1C) as they thennaturally show uncontrolled growth and aggregation. High-qualityGaN nanoparticles are nevertheless highly requested for opto-electronics (e.g., LEDs, lasers)3 and catalysis (e.g., photocatalysis,electrocatalysis).4In order to achieve high purity, high crystallinity anduniform particle size, GaN nanoparticles are mostly preparedusing gas phase methods such as laser ablation deposition,chemical vapour deposition, thermal pyrolysis, or plasma pyrolysis.5Moreover, direct nitridation of gallium metal,6 ammonolysisof oxide-containing starting materials (e.g., Ga2O3, GaO(OH),Ga(NO3)3, CO(NH2)2)7 or the thermal pyrolysis of nitrogen-rich molecular precursors (e.g., Ga(N3)3, Ga2(NMe2)6)8 in thegas phase and under supercritical conditions were applied.The resulting GaN nanoparticles often suffer from broad sizedistribution and aggregation, low crystallinity and/or impuritiesthat deteriorate the semiconductor properties due to undesiredenergy levels within the band gap.8b,9 High temperatures (4400 1C)and powder post-sintering are often required for crystallizationand to obtain fluorescent GaN nanoparticles.6–8 For liquid-phasesynthesis of GaN nanoparticles thermolysis of Ga(NH2)3 turned outto be promising and was performed in trioctylamine/hexadecyl-amine at 360 1C or by the reaction of GaCl3 and LiN(SiMe3)2with stearic acid as a stabilizer in octadecene at 280 1C to obtaina-GaN.10 Moreover, liquid-ammonia-based microemulsions werereported.11 After liquid-phase synthesis, the emission intensity istypically limited and only visible under laser excitation10a or atlow temperatures.10b,11Aiming at the liquid-phase synthesis of GaN nanoparticles,we here suggest an ionic-liquid-based strategy,‡ which resultsin well-crystallized b-GaN nanoparticles with a diameter of3–8 nm showing intense green emission with a quantum yieldof 55%. The synthesis strategy comprises two steps (Fig. 1). First ofall, GaCl3 was dissolved in liquid NH3 at –35 1C. Upon addition ofKNH2 as a base, a colourless suspension of Ga(NH2)3 was formed(Fig. 1a). After evaporation of all NH3 at room temperature, theFig. 1 Scheme illustrating the liquid-phase synthesis of b-GaN nanoparticles:(a) reaction of GaCl3 and KNH2 in liquid ammonia with precipitationof colourless Ga(NH2)3, (b) the almost transparent GaN suspension inionic liquid after microwave heating, and (c) the b-GaN suspension afterwashing and redispersion in acetonitrile (0.2 mg mL1).a Institut für Anorganische Chemie, Karlsruhe Institute of Technology (KIT),Engesserstrasse 15, 76131 Karlsruhe, Germany. E-mail: claus.feldmann@kit.edub Laboratorium für Elektronenmikroskopie, Karlsruhe Institute of Technology (KIT),Engesserstrasse 7, 76131 Karlsruhe, Germany. E-mail: dagmar.gerthsen@kit.edu† Electronic supplementary information (ESI) available: analytical equipment.See DOI: 10.1039/c9cc09133eReceived 23rd November 2019,Accepted 13th January 2020DOI: 10.1039/c9cc09133ersc.li/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 13 January 2020. Downloaded on 3/9/2020 11:11:29 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View IssueThis journal is©The Royal Society of Chemistry 2020 Chem. Commun., 2020, 56, 2312--2315 | 2313colourless solid was dispersed in [Bu3MeN][NTf2] as the ionicliquid (Bu3MeN: tributylmethylammonium, NTf2: bis(trifluoro-methane)sulfonimide). In a second step, the resulting suspen-sion of Ga(NH2)3 in the ionic liquid was heated in a microwaveoven for 1 hour to 300 1C (Fig. 1b). Finally, the colourless GaNnanoparticles were separated by centrifugation and purified bysequential redispersion/centrifugation in/from methanol andacetonitrile (Fig. 1c).The synthesis strategy is characterized by GaCl3 as a most simplestarting material and the strict exclusion of contaminants causinghydrolysis (i.e. OH/H2O in starting materials, stabilizers, solvents,atmosphere). KNH2 is used as a base and amide source to obtainGa(NH2)3. Microwave heating guarantees extremely fast heating andfast ammonolysis, especially at the beginning of the reaction. Theionic liquid stabilizes the GaN nanoparticles against uncontrolledgrowth and agglomeration. The high temperature (300 1C) in theaprotic ionic liquid supports the completion of the ammonolysisand evaporation of all NH3. Ionic liquids, in this regard, were yetonly used as inert electrolytes to gate semiconducting GaN thinfilms.5f,12 For the synthesis of GaN, however, ionic liquids arehere used for the first time. Finally, it needs to be noticed thatthe Ga(NH2)3 intermediate is not nanosized (Fig. 1a). The rapidmicrowave-assisted decomposition with a significant loss ofvolume, however, supports the formation of GaN nanoparticles.Particle size, particle shape and size distribution of theas-prepared GaN nanoparticles were determined by electronmicroscopy (Fig. 2). High-angle annular dark-field (HAADF)scanning transmission electron microscopy (STEM) overviewimages and transmission electron microscopy (TEM) imagesshow uniform nanoparticles with diameters of 3–8 nm (Fig. 2aand b). Moreover, high-resolution (HR)TEM images indicateparallel lattice fringes (Fig. 2c) that evidence the single-crystallinecharacter of the as-prepared GaN. According to the observedlattice fringe distance of 2.2 Å, the structure of the nanoparticlesis compatible with cubic sphalerite-type b-GaN (bulk-b-GaN withd200 = 2.18 Å).13 The crystal structure of the b-GaN nanoparticles isalso confirmed by two-dimensional (2D) Fourier transformation(FT) analysis (Fig. 2d), which is in accordance with the calculateddiffraction pattern of bulk b-GaN in the [001] zone axis (Fm%3mwith a = 3.985 Å).13Beside HRTEM and FT analysis, the presence and purity ofthe as-prepared GaN are confirmed by infrared spectroscopy(FT-IR) and energy-dispersive X-ray spectroscopy (EDXS). FT-IRspectra show the strongest absorption at 600–400 cm1, whichrelates to the lattice vibration of crystalline GaN (Fig. 3a).14 Theweaker but similar shaped vibration at 1100–900 cm1 can beascribed to a combination vibration. Further low-intensityvibrations at 2970–2880 cm1 (n(C–H)) and 1400–1200 cm1(n(SQO)) relate to surface-adhered ionic liquid. The quantificationof EDX spectra, recorded from bundles of GaN nanoparticles(Fig. 4), results in Ga and N contents of 54  4 and 46  4 at%,which is in fair agreement with the expectation (Ga and Nwith 50 at% each). According to EDXS element mappings, theas-prepared GaN shows uniform distribution of the elements(Ga and N) over the nanoparticle bundles (Fig. 4).X-ray powder diffraction (XRD) analysis of the as-preparedGaN nanoparticles generally shows low scattering intensity.However, the broad peak centred at 351 of 2-Theta matcheswith the highest intensity peak of bulk-b-GaN (Fig. 3b). The lowFig. 2 Particle size, particle shape, and particle size distribution of the as-prepared GaN nanoparticles: (a) overview HAADF STEM image nano-particles, (b) detailed TEM image of single-crystalline nanoparticles withlattice fringes, (c) HRTEM image with area for the 2D-FT analysis (redframe), (d) 2D-FT of the single-crystalline nanoparticle in (c) and calculateddiffraction pattern with Miller indices for bulk-b-GaN in the [001] zone axis(white circle indicates zero-order beam, ZB).Fig. 3 FT-IR spectrum (a) and XRD (b) of the as-prepared GaN (FT-IRspectrum of the ionic liquid [Bu3MeN][NTf2] and XRD of bulk-b-GaN (ICDD088-2364) as references).Fig. 4 HAADF STEM image of GaN nanoparticle bundles with EDXSelemental maps of Ga (red: based on Ga-Ka1 line) and N (blue: based onN-Ka line).Communication ChemCommOpen Access Article. Published on 13 January 2020. Downloaded on 3/9/2020 11:11:29 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Online2314 | Chem. Commun., 2020, 56, 2312--2315 This journal is©The Royal Society of Chemistry 2020intensity and broad peak width, in fact, are to be expected takingthe particle size of 3–8 nm into account. In regard of the slowsolid-state diffusion in nitrides, all in all, the good crystallinity ofGaN as confirmed by HRTEM, FT and XRD is surprising in viewof the here applied liquid-phase synthesis with a maximumtemperature of 300 1C. Typically, completion of the ammonolysisand nitride crystallization require temperatures of Z400 1C.6–8The optical properties of the as-prepared GaN nanoparticleswere examined with UV-Vis spectroscopy (Fig. 5) and photo-luminescence (PL) spectroscopy (Fig. 6). In this regard, UV-Visspectra show a comparably steep absorption at 300–400 nm(Fig. 5a). By plotting (F(R)hn)2 versus (hn) in a Tauc plot, a bandgap of 3.9 eV can be determined (Fig. 5b). The Tauc plot – asexpected – indicates the nanomaterial to exhibit a direct bandgap. In comparison to bulk-b-GaN (3.3 eV),8a,b,15 the band gapof the as-prepared nanoparticles is significantly blue-shifted by0.6 eV, which can be ascribed to the small particle size, andwhich points to the presence of quantum-confinement effects.Excitation spectra of the as-prepared GaN nanoparticlesshow strong absorption below 325 nm as well as a shoulderat 325–400 nm (Fig. 6a). Here, the strong absorption can berelated to the band transition, whereas the shoulder indicatesadditional energy levels within the band gap. The strong bandtransition and the steep increase in absorption neverthelesspoint to the crystallinity of the as-prepared GaN, whereas thedefect levels could relate from surface states. The width of theoptical absorption observed in UV-Vis and excitation spectra(Fig. 5a and 6a) are narrower as compared to GaN nanoparticlesthat were post-synthesis sintered at Z400 1C as powdersamples.6–8Emission spectra of the as-prepared GaN nanoparticlesindicate strong fluorescence at 400 to 700 nm with its maximumat 523 nm (Fig. 6a). Direct conduction band-to-valence band emis-sion is only observed with subordinate intensity at 325–400 nm.Consequently, the most relevant emission is located at lower energythan the band emission and can be related to donor–acceptor-typeemission. The respective energy levels with wide dispersion mayresult from surface states as well as from gallium and/or nitrogenvacancies in the lattice. Qualitatively, the strong fluorescence isverified upon illumination using a UV lamp (lexc: 254 nm) thatresults in a bright green emission (Fig. 6b). The strong emission wasquantified by determining the absolute quantum yield follow-ing the method given by Friend et al.,16 which results in aremarkable quantum yield of 55  3%. To our surprise, data onthe fluorescence efficiency and quantum yield of liquid-phase-made GaN nanoparticles were not reported in the literatureuntil now. A quantum yield of 55  3% for GaN nanoparticles –to the best of our knowledge – is achieved for the first time. Thisalso impressively shows the effectiveness of the ionic-liquid-based synthesis strategy.In conclusion, a synthesis strategy comprising the formationof a Ga(NH2)3 intermediate in liquid ammonia followed bymicrowave-assisted heating in ionic liquids turned out to besuccessful to obtain GaN nanoparticles. Subsequent to liquid-phase synthesis, the as-prepared b-GaN nanoparticles are well-crystallized and exhibit particle diameters of 3–8 nm. The opticalproperties are characterized by a blue-shifted band gap of 3.9 eV,which is 0.6 eV above the band gap of bulk-b-GaN. This findingcan be related to quantum-confinement effects due to the smallparticle size. The GaN nanoparticles show strong absorptionbelow 325 nm, which is in agreement with valence-band-to-conduction-band transition. Intense donor–acceptor-type greenemission is observed at 400–700 nm (peaking at 523 nm) with anunprecedented quantum yield of 55  3%. Based on theseresults, the synthesis strategy will definitely be interesting alsoto prepare further metal nitrides.The authors thank the Deutsche Forschungsgemeinschaft(DFG) for funding (NanoNitrid: FE911/10-1, GE 841/28-1).Conflicts of interestThere are no conflicts to declare.Notes and references‡ Experimental: All experiments and purification procedures wereperformed under inert gas (argon), using standard Schlenk techniquesor glove boxes. This also includes all centrifugation and washingprocedures. Moreover, sample preparation and sample transfer foranalytical characterization were strictly performed under inert condi-tions, e.g. by using specific transfer modules. Acetonitrile (Sigma-Aldrich, 99.5%) was refluxed over CaH2 onto P4O10 and degassed bythree freeze–pump–thaw cycles. KNH2 was synthesized by reactingpotassium (Riedel-de-Haën, 99%) in liquid ammonia (Air Liquide,99.98%) at –78 1C using Fe2O3 as a catalyst, followed by filtering anddrying under vacuum. Gallium(III) chloride (ABCR, 99.999%) was used aspurchased. [Bu3MeN][NTf2] was prepared according to literature proce-dures and dried for several days under reduced pressure (o103 mbar) at230 1C before use.17 b-GaN nanoparticles: 88 mg of GaCl3 were dispersedin 15 mL of liquid ammonia at –35 1C to obtain a colourless solution.Fig. 5 Optical properties of the as-prepared GaN nanoparticles: (a) UV-Vis spectrum and (b) Tauc plot.Fig. 6 Fluorescence properties of the as-prepared GaN nanoparticles:(a) excitation spectrum (lem: 523 nm) and emission spectrum (lexc: 274 nm);(b) GaN nanoparticle powder in daylight and under excitation (lexc: 254 nm).ChemComm CommunicationOpen Access Article. Published on 13 January 2020. Downloaded on 3/9/2020 11:11:29 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineThis journal is©The Royal Society of Chemistry 2020 Chem. Commun., 2020, 56, 2312--2315 | 2315Thereafter, 83 mg KNH2 were added, which instantaneously results inthe precipitation of colourless Ga(NH2)3. This mixture was stirred for anadditional 10 minutes and then naturally warmed to room temperature.After evaporation of all NH3, a voluminous, fluffy and colourless powderremained. This powder was dispersed in 15 mL of [Bu3MeN][NTf2] asionic liquid. Subsequent to intense mixing, the suspension was rapidlyheated (120 seconds) with a laboratory microwave oven (CEM MARS6)to 300 1C. This temperature was maintained for an additional 60 min.After natural cooling to room temperature, the resulting almost transparentand colourless suspension was diluted with 15 mL of acetonitrile. Theas-prepared GaN nanoparticles were finally purified by centrifugationand redispersion in/from methanol and acetonitrile. The nanoparticlescan be easily redispersed in acetonitrile or dried at room temperatureunder vacuum to obtain GaN powder samples.1 S. Nakamura, Science, 1998, 281, 956.2 S. Nakamura, Angew. Chem., Int. Ed., 2015, 54, 7770.3 (a) H. Alnoor, G. Pozina, V. Khranovskyy, X. Liu, D. Landolo, M. Willanderand O. Nur, J. Appl. Phys., 2016, 119, 165702; (b) Y. C. Yao, J. M. Hwang,Z. P. Yang, J. Y. Haung, C. C. Lin, W. C. Shen, C. Y. Chou, M. T. Wang,C. Y. Huang, C. Y. Chen, M. T. Tsai, T. N. Lin, J. L. Shen and Y.-J. Lee,Sci. Rep., 2016, 6, 22659.4 (a) Z. Wang, J. Han, Z. Li, M. Li, H. Wang, X. Zong and C. Li, Adv.Energy Mater., 2016, 6, 1600864; (b) B. AlOtaibi, S. Fan, D. Wang, J. Yeand Z. Mi, ACS Catal., 2015, 5, 5342; (c) S. Fan, B. AlOtaibi, S. Y. Woo,Y. Wang, G. A. Botton and Z. Mi, Nano Lett., 2015, 15, 2721.5 (a) Y. H. Ji, R. Z. Wang, X. Y. Feng, Y. F. Zhang and H. Yan, J. Phys.Chem. C, 2017, 121, 24804; (b) Z. S. Schiaber, G. Calabrese, X. Kong,A. Trampert, B. Jenichen, J. H. Dias da Silva, L. Geelhaar, O. Brandtand S. Fernandez-Garrido, Nano Lett., 2017, 17, 63; (c) A. Demirel,T. Oztas, C. Kursungoz, I. Yilmaz and B. Ortac, J. Nanopart. Res.,2016, 18, 1; (d) Y. Xu, B. Yao and Q. Cui, RSC Adv., 2016, 6, 7521;(e) B. Liu, B. Yang, F. Yuan, Q. Liu, D. Shi, C. Jiang, J. Zhang,T. Staedler and X. Jiang, Nano Lett., 2015, 15, 7837; ( f ) Z. Y. Chen.,H. T. Yuan, X. Q. Wang, N. Ma, Y. W. Zhang, H. Shimotani, Z. X. Qin,B. Shen and Y. Iwasa, Appl. Phys. Lett., 2013, 103, 253508.6 (a) G. A. Sapunov, A. D. Bolshakov, V. V. Fedorov, A. M. Mozharov,D. A. Kirilenko, A. A. Sitnikova and I. S. Mukhin, J. Phys., 2018,1038, 012053; (b) C. Collado, G. Goglio, G. Demazeau, A. S. Barriere,L. Hirsch and M. Leroux, Mater. Res. Bull., 2002, 37, 841.7 (a) M. Kumar, V. P. Singh, S. Dubey, Y. Suh and S. H. Park, Opt.Mater., 2018, 75, 61; (b) W. Lei, M. G. Willinger, M. Antonietti andC. Giordano, Chem. – Eur. J., 2015, 21, 18976; (c) N. F. Zhuang,X. Wang, F. Fei, C. C. Liu, L. Wei, Y. F. Zhang, X. L. Hu and J. Z. Chen,J. Nanopart. Res., 2013, 15, 1458.8 (a) B. Giroire, S. Marre, A. Garcia, T. Cardinal and C. Aymonier,React. Chem. Eng., 2016, 1, 151; (b) B. Mazumder and A. L. Hector,J. Mater. Chem., 2009, 19, 4673; (c) A. C. Frank and R. A. Fischer, Adv.Mater., 1998, 10, 961.9 A. Patsha, S. Amirthapandian, R. Pandian and S. Dhara, J. Mater.Chem. C, 2013, 1, 8086.10 (a) Y. C. Choi, H. Kim, C. Lee, J. Son, H. Baik, S. Park, J. Kim andK. S. Jeong, Chem. Mater., 2019, 31, 5370; (b) O. I. Micic, S. P. Ahrenkiel,D. Bertram and A. J. Nozik, Appl. Phys. Lett., 1999, 75, 478.11 F. Gyger, P. Bockstaller, H. Gröger, D. Gerthsen and C. Feldmann,Chem. Commun., 2014, 50, 2939.12 (a) A. Lahiri, N. Borisenko, A. Borodin and F. Endres, Chem.Commun., 2014, 50, 10438; (b) Q.-P. Li and B. Yan, RSC Adv., 2012,2, 10840.13 A. F. Wright and J. S. Nelson, Phys. Rev. B: Condens. Matter Mater.Phys., 1995, 51, 7866.14 (a) G. Pan, M. E. Kordesch and P. G. van Patten, Chem. Mater., 2006,18, 3915; (b) A. S. Barker and M. Ilegems, Phys. Rev., 1973, 7, 743.15 H. Morkoc, S. Strite, G. B. Gao, M. E. Lin, B. Sverdlov and M. Burns,J. Appl. Phys., 1994, 76, 1363.16 J. C. de Mello, H. F. Wittmann and R. H. Friend, Adv. Mater., 1997,9, 230.17 T. Welton, Chem. Rev., 1999, 99, 2071.Communication ChemCommOpen Access Article. Published on 13 January 2020. Downloaded on 3/9/2020 11:11:29 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Online

Ionic-liquid-based synthesis of GaN nanoparticles

https://publikationen.bibliothek.kit.edu/1000117608/62617987

Ionic-liquid-based synthesis of GaN nanoparticles

Abstract

Similar works

Full text

Available Versions

KITopen