Synthesis and Characterization of Nine Metal Nitrides
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oxygen and carbon contamination, and environmentally friendly. RESULTS AND DISCUSSION
We have already discussed the electrochemical synthesis of AIN and NbN precursors in detail. [12-15] In these cases the same experimental methods were used but two distinct chemical pathways led to the precursor. The AIN precursor requires formation of NH2 at the cathode and A13+ at the anode, while the NbN precursor is formed by anodic dissolution of Nb, which subsequently reacts directly with liquid NH 3 by ammonolysis to form the precursor. The following discussion briefly reviews the AIN and NbN syntheses, which represent the two pathways open to the other metals, and then describes the synthesis and characterization of the other materials. The possible anode reactions in all the electrolysis cells are oxidation of the metal (M), the electrolyte anion (X-), the solvent (NH 3 ), or solvated electrons (e-NH 3) (eq 1-4, respectively). M --> Mn+ + ne-
()
X- ->/2X2 + e-
(2)
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Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
2NH 3 -->N 2 + 6H+ + 6e-
(3)
e-NH 3 -> e-M
(4)
In general, metal oxidation (eq 1) is the dominant anode process early in the electrolysis (< 2 h). At longer electrolysis times e-NH 3 are generated at the cathode, and subsequently oxidized at the anode (eq 4).[16] Typically after about 4 h, eNH3 oxidation becomes the dominant anode process, and we observe no further metal oxidation. Following dissolution, the chemistry of the different electrochemically generated metal cations in the electrolysis solutions varies. A13+ forms simple, stable ammonia complexes called ammoniates (eq 5).[17] For example, when AIBr 3 is dissolved in liquid NH 3 for 30 min at -78 C, and the NH 3 is allowed to evaporate, a white powder, which is probably AI(NH 3 )6Br 3, is recovered. However, if this material is calcined at 1100 C for 2 h in flowing NH 3 , only 1% of the Al originally present as AIBr 3 is converted to AIN. In contrast to this very low yield, a I h electrolysis of Al in the NH 4 Br/NH 3 electrolyte solution typically results in a 60+% conversion of metallic Al to AIN powder. A13+ + nNH 3 --> AI(NH 3 )n3+
(5)
The precursor that results from the oxidation of Al in liquid NH 3 is a white insoluble powder, which condenses to a polymer that can be conveniently expressed as [AI(NH 2)(NH)Jn. Since A1 3+ undergoes only slight ammonolysis, a source of NH2- is required to form this precursor. For the Al electrolysis, NH 2 - is generated by direct cathodic reduction of NH 3 (eq 6) or by e- solvation (e-NH) at the cathode (eq 7) followed by solvent reduction (eq 8).[18] The other possible cathodic reactions that occur in the electrolysis cell are reduction of NH 4 + and the metal cation (Mn+) (eq 9 and 10, respectively). NH 3 + e- --> NH 2 + 1/2H 2
(6)
e-M --> e-NH 3
(7)
NH 3 + e-NH 3 --> NH 2 - + 1/2H 2
(8)
NH 4+ + e
(9)
-->
NH 3 + 1/2H 2
Mn+ + ne --> M
(10)
Electrolyte reduction (eq 9) is the primary cathode reaction until the concentration of NH 4 + is reduced to a low enough level t
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