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Conversion Mechanism of Soluble Alkylamide Precursors for the Synthesis of Colloidal Nitride Nanomaterials Yang Chen, Nathan T. Landes, Daniel J. Little, and Rémi Beaulac J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06063 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018
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Conversion Mechanism of Soluble Alkylamide Precursors for the Synthesis of Colloidal Nitride Nanomaterials Yang Chen‡, Nathan T. Landes‡, Daniel J. Little, Rémi Beaulac* Department of Chemistry, Michigan State University, East Lansing MI, 48824-1322, USA Supporting Information Placeholder ABSTRACT: There are few molecular precursors that
chemically convert to nitride nanomaterials, which severely limits the development of this important class of materials. Alkylamides are soluble and stable nitride precursors that can be based on the same primary amines that are often used in colloidal nanomaterial synthesis, but their conversion involves the breaking of stable C–N bonds through a mechanism that remained unknown up to now. A critical aspect of this conversion mechanism is uncovered here, involving a prelimary step whereby alkylamides are oxidized to N-alkylimines to yield NH2amide species that are postulated to be the actual reactive precursors in the formation of indium nitride nanomaterials. Interestingly, this step also involves the concomitant reduction of indium(III) to In(0) nanodroplets, which consequently catalyze the growth of InN nanomaterials. The elucidation of the origin of the surprising reactivity of otherwise stable molecular precursors opens the door to the development of new solution-phase approaches for the synthesis of nitride materials.
metallic nitrides is the thermal decomposition of inorganic amide precursors,12-14 e.g. using indium(III) salts:
(
In NH 2
3
⎯Δ⎯ → InN + 2NH 3 ( g )
This approach essentially forms the basis for many widely-used solution-phase approaches to synthesize colloidal InN nanocrystals (NCs), where amide salts such as sodium or lithium amides are reacted with indium(III) salts in high-boiling point solvents.9-10, 15-17 Although such methods reliably yield InN NCs, the nucleation and growth processes are notoriously hard to control reliably due to the lack of solubility of the inorganic amide precursors in the non-polar solvents that are compatible with colloidal NC chemistry. Other solution-phase approaches involve ammonia gas as the nitride source,1819 but similarly suffer from heterogeneous conditions. The issue of solubility of nitrogen precursors is arguably the most critical factor that presently limits the development of solution-phase chemistry of colloidal nitride
Absorbance
(a)
Nitride semiconductors form an important class of materials in view of their potential applications. For instance, nitrides are often proposed as attractive candidates to replace oxide materials for efficient photoelectrochemical oxidative transformations,1 and gallium nitride, a prominent member of the group III-nitride semiconductors, has gained fame in the generation of efficient blue light emission.2-3 Another member of the III-V group, indium nitride also received much attention because of its narrow bandgap,4-5 high charge mobility,6 and its peculiar propensity for surface n-type charge accumulation,7-8 which leads to localized surface plasmon resonances (LSPR) in colloidal InN nanocrystals.9-10 In spite of many such desirable properties, nitrides remain among the least studied semiconductor nanomaterials. A major issue that severely impedes the development of this class of materials is the small number of synthetic approaches that reliably yield nitride materials.11 An old but direct and efficient approach to
)
50 nm
(b)
Wavelength (nm) 400 500 1000 0.6 (d) 0.4 LSPR
0.2 0.0
interband
50 nm
(c)
10 nm
(e)
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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30 50 70 2.0 1.0 2θ Energy (eV) Figure 1. (a) TEM image of InN nanorods and In(0) nanodroplets obtained after the synthesis. (b-c): TEM images of the InN nanorod, after acid-etching of the In(0). (d) Absorption spectrum of InN nanorods, after acid-treatment. (e) Powder XRD pattern of the InN nanorods, after acid treatment, and the literature pattern for bulk cubic InN (blue). 3.0
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Journal of the American Chemical Society 3 RCH2NHLi TMEDA C16H34 210 oC
(a)
In(0) + InN
Scheme 1. Synthesis of InN nanorods from lithium alkylamide (RCH2NHLi) precursors; TMEDA: N,N,N’,N’-tetramethylethylenediamine. nanomaterials: reactive nitrogen precursors are either ionic, insoluble species, or highly reactive ones. Case in point, there are only two known methods that rely on homogeneous solution-phase conditions to yield colloidal InN nanomaterials.20-21 The first one, due to Buhro et al., is based on the solution-liquid-solid (SLS) mechanism2223 and involves the in situ formation of solubilized indium azide species that readily decompose to form InN nanofibers catalyzed by metallic nanodroplets formed during the reaction.20 The second approach (Scheme 1), described by our group, was also shown to proceed by the SLS mechanism, but relying instead on the decomposition of alkylamide species that simply obtained from the deprotonation of the same alkylamine species that are often used in colloidal syntheses:24
Intensity (a.u.)
[InBr3·x RCH2NH2]
Ha
7.65
10
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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8
7.55
3.40
3.35
3.30
6 4 2 Chemical Shift (ppm)
516.55
(b)
0
(c)
510 520 (m+1)/z
RCH 2 NH 2 + n-BuLi ⎯⎯ → RCH 2 NHLi + n-BuH
(R = C17H33 for oleylamine, but other alkylamines lead to similar results).21 The rapid injection of lithium alkylamide complexes in solubilized indium halide salts at temperatures above 170 oC results in the rapid formation of metallic indium (In(0)) nanodroplets, from which InN nanorods grow (full details of the method provided in SI). Figure 1 summarizes some of the characteristic features of InN synthesized from the reaction in Scheme 1, including the prototypical BursteinMoss shifted interband transition and plasmonic resonance in the infrared, similar to those observed for colloidal InN nanomaterials synthesized from inorganic amide approaches.9-10 An intriguing aspect of the alkylamide approach, which could not be addressed at the time of the original report, is the reactivity of these simple nitride precursors, which seemingly involves the breaking of strong C—N bonds in the alkylamide species. With bond dissociation energies around 330 kJ·mol-1,25 C—N bonds are not typically wellsuited for atom-transfer reactions. In order to clarify the origin of this surprising reactivity we proceeded to analyze the byproducts of the reaction shown in Scheme 1. A representative 1H NMR spectrum of the solution after the growth of the InN NCs is shown in Fig. 2(a). A clear set of triplets appear at 3.34 and 7.61 ppm that integrate to a 2:1 ratio, respectively relative each other, a typical signature of secondary aldimines (R1HC=NR2),26-27 which are generally obtained from the condensation of primary amines with electrophilic species (typically aldehydes). Concomitantly, mass spectrometry (MS) data on the same solution (Fig. 2(b)) shows a prominent peak centered at m/z = 515.55, with the isotopic distribution
7.60
Hb
530
m = 515.5430
Figure 2. (a) 1H NMR spectrum in CDCl3 (red) of the postreaction solution. Two triplets that correspond to a generated product are emphasized in the two boxes, Ha (7.61 ppm, 1 H, J = 4.9 Hz) and Hb (3.34 ppm, 2 H, J = 7.2 Hz), along with the corresponding integration (blue). Other notable peaks belong to ferrocene (4.16 ppm; added postsynthesis for quantification), CHCl3 (7.27 ppm), and olefinic protons from excess oleylamine ligands (5.35 ppm). (b) APCI-MS spectrum of the same solution (red), along with simulated MS spectrum of the imine shown to the right which is the condensation product of two oleylamines (C18H35-NH2), along with linoleic (C18H33-) and stearic (C18H37-) oleyl-imines (see SI for details). (c) Proposed structure of the N-oleyl-oleylimine secondary aldimine byproduct, C36H69N.
that would be expected from the condensation of two oleylamide (or oleylamine) species (N-oleyl-oleylimine, C36H69N, predicted monoisotopic mass = 515.5503, structure drawn in Fig. 2(c)). Further data supporting the formation of the secondary aldimine, including 13C NMR and 1H correlation spectroscopy (COSY) NMR spectra, are given in SI. R H
R
NH
NH R
R
NH
N
R
NH2
Scheme 2. Generation of the active nitride precursor NH2- from the starting alkylamide (uncoordinated forms depicted here for simplicity).
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The observation of this aldimine species provides a crucial insight into the likely origin of the reactivity of alkylamide as nitride precursors. As shown in Scheme 2, a likely intermediate before the formation of the secondary aldimine is the corresponding primary aldimine (oleylimine), that is, the direct product of the oxidation of oleylamide. Primary aldimines are notoriously unstable species, and likely readily undergo rapid condensation with a second oleylamide equivalent to form the more stable secondary aldimine which is observed here. Importantly, this proposed path yields an amido (NH2-) equivalent which is likely coordinated to an indium center here, corresponding to a reactive species that can then easily proceed toward the formation of indium nitride by simple, low-energy, deprotonation steps. This hypothesis thus leads us to conclude that the actual nitride precursor is not the alkylamide reactant used to trigger the reaction, but rather the amido species which is formed in situ during the reaction. Critically, the overall reaction in Scheme 1 does not formally involve the net breaking of a C—N bond, the broken one used to release NH2- being compensated by a new one to form the secondary aldimine product. It is also interesting to note that the chemistry that ensues likely then resembles closely that obtained from the direct reaction of inorganic amides (such as NaNH2) with indium salts,9-10, 15-16 with the consequential differences that the alkylamide route described here generate solubilized [In-NH2] clusters in situ, and then proceeds through the SLS mechanism.23 The In(0) nanodroplet catalysts are generated by the reduction of the starting In(III) species, a step which likely involves the hydride equivalent associated with the oxidation of the alkylamide shown in Scheme 2, an hypothesis which is supported by the observation of H2 evolving from the reaction (SI), although we cannot exclude the possibility where the hydride would deprotonate acidic species (the excess oleylamine used to solubilize the indium(III) salt being the most likely
Table 1. Yields of formation of the observed products of the reaction shown in Scheme 1.
Product
Yield (mmol) a
InN b
0.17 ± 0.01
In(0)
b
0.48 ± 0.01
RCH=NCH2R
c
0.21 ± 0.06
NH3 d H2
–e
d
–e
a: using 0.5 mmol InBr ; b: measured by ICP-AE; c: measured by NMR, with ferrocene 3 internal standard; d: detected by GC/TCD; e: not quantified.
candidate) to yield H2. It is not possible at this moment to determine which one of H2 or a transient hydride species, or both, act as the reductant in the generation of In(0). A complete reaction scheme can thus be drawn as in Scheme 3, where each unit of InN would ideally require four equivalents of alkylamide (two for the formation of NH2- shown in Scheme 2, two for the subsequent deprotonation of that species) for 4/3 equivalents of indium(III) (1 equivalent used in InN, a third one to be reduced to In(0) by the hydride generated in Scheme 2), that is, the 3:1 alkylamide:indium(III) ratio which is used here. Experimentally, we observed that injections of other ratios (either larger or smaller) invariably lead to lower InN:In(0) ratios. It is also noteworthy that even for the seemingly optimal 3:1 ratio, In(0) is always formed in larger yields (Table 1) than the ideal scenario described above, where every equivalent of InN would be accompanied by at most 1/3 equivalent of In(0). This suggests that not all equivalents of the reactive NH2species that are formed are usefully converted to InN, which is readily confirmed by the observation of ammonia as a side-product of the reaction (SI). Furthermore, since the reaction of inorganic amides (MNH2, M = Li or Na) with indium(III) salts also
1/3 In0 + 1/2 H2 1/3 In3+
R1 2/3 In3+
2/3 In0 + H+
R2
H
R3
R
H 1a
H2 R
R
NH
1b
R
NH
NH2
NH 2
N H+ S1
In3+ + 2 B
R
NH3 B
InN 2 BH
BH = NH3 , H2 , R
= NH2 , H , R
NH
NH2
Scheme 2. Overall reaction scheme. 1a: alkylamide precursor oxidation; 1b: formation of the secondary imine leading to NH2- production; 2: nitride formation; R1, R2, R3: three hypothetical reduction processes concomitant with reaction 1a; S1: detrimental hypothetical side reaction. Orange: Reactants; Blue: Intermediates (not isolated); Red: Identified products. Uncoordinated forms depicted here for simplicity. ACS Paragon Plus Environment
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invariably leads to the formation of In(0), it is likely that similar side reactions also occur here. Overall, the results presented here provide important insight on the mechanism by which seemingly stable organic precursors can be converted into reactive nitride precursors. Further work is presently ongoing to investigate further the details of this mechanism, including optimization of the growth of InN nanomaterials as well as the applicability to the synthesis of other nitride materials. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Complete experimental section including methods and synthetic protocols. Supplemental experimental results (XRD, TEM, ICP-AE, GCTCD, 1H and 13C NMR). (PDF)
AUTHOR INFORMATION Corresponding Author
* E-mail:
[email protected] Author Contributions
‡These authors contributed equally. Funding Sources
No competing financial interests have been declared. This research was supported by a grant from the National Science Foundation, under award CHE-1412776.
ACKNOWLEDGMENT The authors thank Dr. Daniel Holmes for help with the NMR measurements, and support from the MSU Center for Research Excellence on Complex Materials (CORE-CM), and Profs. Milton R. Smith and Thomas W. Hamann for access to GC-TCD instrumentation.
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Table of Content (TOC) Figure alkylimine 1 R R N 2 side product alkylamide R
NH
precursor
1 NH2 2 intermediate
Indium Nitride nanorod In0 product
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