Nanocrystalline Nickel, Iron, and Manganese Nitrides
to late transition-metal nitrides (MxN x > 1) are generally
accomplished using metal-ammonia complexes or metal
amine precursors that are converted to crystalline metal
nitrides via elevated temperature reactions with gaseous or
supercritical ammonia, including Co3N,5 Ni3N,5,6 and
Cu3N.5,7 Thin films of several metal-rich nitrides, most
commonly Cu3N, have also been produced by metal or metal
precursor reactions with nitrogen plasmas or ammonia.8 Iron
nitrides have also been synthesized from reactive iron
nanoparticles and ammonia below 400 °C.9
Thermally stable nanocrystalline metal nitrides have been
synthesized from solid-state metathesis (SSM) reactions using
reactive solid nitrogen sources such as Li3N, Mg3N2, and
NaN3.10 The SSM approach has wide applicability and
flexibility, but the highly exothermic nature of such rapidly
propagating systems makes it difficult to isolate thermally
sensitive (metastable) nitrides. Since SSM reactions often
reach very high transient temperatures (∼1300 °C), they
usually produce thermodynamically stable metal nitrides. We
recently demonstrated that non-aqueous superheated toluene
is a viable reaction medium for the synthesis of GaN at 260
°C via decomposition of energetic metal azide precursors
derived from SSM-style reactions.11 The solvent moderates
energetic decomposition processes, and the azide precursor
intermediates convert at temperatures below 300 °C to nitride
structures, including thermally metastable nanocrystalline
InN12 and Cu3N13 powders, which decompose below 500
°C. Selected other precedents for the solvothermal synthesis
of thermally stable metal nitrides using reactive Li3N or NaN3
precursors are nanocrystalline TiN powders from superheated
benzene at 380 °C14 and nanocrystalline GaN in benzene at
280 °C.15 Recent studies using solvothermal formation and
decomposition of M-NH2 intermediates formed from SSM
precursor reactions demonstrated the direct synthesis of
crystalline TaN nanoparticulate materials.16
Increasing the materials chemist’s synthetic toolkit with
new and flexible solvothermal approaches to crystalline
transition metal nitrides will facilitate further studies on
synthesis of new metastable nanoparticulate metal nitride
stoichiometries, including metal-doped semiconducting ni-
trides. This current study demonstrates that several thermally
metastable nanocrystalline mid to late transition metal nitrides
are accessible in superheated toluene solvents from in situ
produced energetic metal azide intermediates. The synthesis
and characterization of Ni3N, Fe2N, and Mn-N materials
are described below.
Experimental Section
Solvothermal Reaction Considerations. The synthetic approach
and precautions used in the present studies are similar to those of
our previously published metal azide to metal nitride solvothermal
chemistry.12,13 Toluene (C6H5CH3, Fisher Scientific, certified) used
for the solvothermal reactions was dried over sodium and distilled
under N2. Methanol (Fisher, 99.9%, anhydrous H2O <0.01%) was
used as received and degassed with N2 gas prior to use. The general
synthetic method involves reactions of a metal halide (MXn, M )
Ni, Fe, Mn; X ) Br, Cl) with NaN3 that were balanced so the
molar ratio of metal halide to NaN3 used is 1 to 2 for dihalide (1
to 3 for trihalide) reactions to ensure that all halide is ideally
sequestered as NaX. The anhydrous metal halide and NaN3 solids
were separately ground to fine powders with a mortar and pestle
in an argon-filled glovebox (Vacuum Atmospheres MO-40M) and
loaded into a high-temperature, high-pressure stainless steel reactor
(125 mL, Parr Instruments Model 4752, 3000 psi limit). The reactor
with the powdered reagent mixture and a Teflon-coated stirbar was
capped with a septum, removed from the glovebox, and partially
filled with N2 degassed toluene (∼85 mL or 77% of the reactor
volume) using inert atmosphere Schlenk techniques. The reactor
was then mated with its high-pressure head, sealed under a flow of
nitrogen gas, and placed in a beaker-shaped Glas-Col heating mantle
where it was slowly heated with constant stirring using an external
magnetic stirrer. The reported reaction temperatures were measured
by an internal reactor thermocouple that was submerged in the
solvent and about one inch from the bottom of the reactor. Slow
heating ramps were chosen to avoid rapid exothermic degradation
of metal azide intermediates, which could lead to product decom-
position. Onset and degree of azide decomposition was monitored
by gas evolution rates using the reactor’s analog pressure gauge.
Increases in reaction temperature were made after the vessel reached
a pressure plateau, which usually occurred within a few hours of
raising the temperature. Temperature ramp rates were adjusted based
on pressure changes due to nitrogen gas evolution. The maximum
reaction temperature was determined as the point where gas
evolution ceased, which indicated that the azide precursor had
completely decomposed.
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Safety Note. Caution! Metal azides are often thermally unstable
and shock sensitiVe. Care should be taken wheneVer working with
reactions that produce azides as products or intermediates.
Reactions should be performed on small scales in high-pressure
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