Redox supercapacitor performance of nanocrystalline molybdenum nitrides obtained by ammonolysis of chloride- and amide-derived precursors
Introduction
The most studied nitrides of molybdenum are the rocksalt-type γ-MoNx, with x typically between 0.39 and 0.54 (hence often labelled as γ-Mo2N) and disordered nitrogen vacancies [1], and the hexagonal phases δ1-MoN (WC type, P-6m2), δ2-MoN (NiAs-type, P63/mmc) and δ3-MoN (FeS-type, P63mc) with compositions close to stoichiometry (Fig 1) [2], [3]. The α-phase is molybdenum metal with small amounts of dissolved nitrogen, whereas β-Mo2N is tetragonally distorted rocksalt-type with ordered stacking faults, and requires careful control of conditions for its synthesis [4], [5], and Mo5N6 has been obtained by ammonolysis of molybdenum sulfide [6].
Much of the interest in molybdenum nitrides in recent years stems from their catalytic activity for ammonia synthesis [4], NO decomposition [7], alkane hydrogenolysis and dehydrogenation [8], hydrodesulphurisation [9] and hydrodenitrogenation [10]. They have been investigated for their corrosion resistance [11], superconductivity [12], wear resistance [13], diffusion resistance [14] and electrocatalytic activity in the hydrogen evolution reaction [15]. The charge storage capability of metal nitrides in electrochemical capacitor (“supercapacitor”) electrodes has grown in importance recently due to reports of high capacities in some materials [16], e.g. with nanocrystalline VN Choi et al. found a specific capacity of 1340 F g−1 at 2 mV s−1 and 554 F g−1 at 100 mV s−1 in aqueous KOH electrolyte [17]. Mo2N/MoN films produced by ammonolysis of MoO3 were first investigated as supercapacitor electrode materials by Finello, Roberson and Conway in the 1990s [18], [19], [20]. These compounds were found to be stable in aqueous H2SO4 and to exhibit redox behaviour similar to that of RuO2, but the smaller electrochemical window of ∼0.6 V compared with the ∼1.4 V of RuO2 was considered to limit their capability. Chen et al. showed that an unidentified polycrystalline composite material formed by ammonolysis of a molybdenum/tantalum oxide mixture had higher capacity than molybdenum nitride produced under the same conditions [21]. Later the same group used temperature programmed reduction of MoO3 to produce 16 nm γ-MoN crystallites and found both a high specific capacitance (172 F g−1) and an increased potential window (1.1 V) [22]. Choi et al. reacted MoCl5 in chloroform with ammonia then heated the precipitate in ammonia to make γ-MoN with ∼110 F g−1 capacity at 2 mV s−1 but poorer performance at faster rates and ∼40% loss of capacity over 500 cycles [23]. Recently Lee et al. used ammonolysis of single crystal MoO3 nanowires to make mesoporous Mo3N2 wires which exhibited higher capacities than the same material made from commercial MoO3 and maintained a capacity over 200 F g−1 even at a charge/discharge rate of 200 mV s−1 [24].
Synthesis of molybdenum nitrides commonly involves heating a suitable precursor material in ammonia including the metal [25] and MoCl5 [2], [26], [27], whereas carefully controlled ammonolysis of MoO3 [5], [28] and various other oxide-containing materials [29], [30] have been used to obtain high surface area materials suitable for catalysis. Vapour phase deposition methods for films are also well developed [14], [31], and high pressure has been used in a number of cases to obtain stoichiometry or more crystalline products [3], [32]. Whilst the catalysis interest has led to significant reports of high surface area and thus nanocrystalline materials, there are only a small number of reports of the synthesis of anisotropic molybdenum nitride structures [33]. Nanorods have been obtained via the topotactic transformation of MoO3 nanorods [4], [24], templated growth in porous alumina [29], solvothermal synthesis from MoCl5 and LiNH2 [34] and in composites with silicon nitride using sol–gel methods [35]. Nanotubes have been obtained by topotactic ammonolysis of hydrothermally prepared molybdenum sulfide nanotubes [36], and by atomic layer deposition onto the internal walls of anodised aluminium oxide followed by dissolution of the template [37].
In this paper we report solution phase ammonolysis reactions of MoCl5 and [Mo(NMe2)4] to form polymeric precursors, and their decomposition in ammonia at high temperature to produce molybdenum nitrides in nanocrystalline, nanorod and nanotube form. The surprising finding is that the two precursors result in very different molybdenum nitride phase behaviours. Our general interest in such reactions stems from the potential to access higher metal oxidation states [38], [39], [40] and to make controlled material morphologies [41]. Herein we targeted small particle molybdenum nitride samples that may act as good charge storage materials and tested their performance as electrodes in aqueous electrochemical capacitors. One set of materials exhibited charge storage behaviour consistent with a mainly double layer based mechanism whereas the other set had significant redox properties.
Section snippets
Experimental
All reagents and products were handled under nitrogen using glove box or Schlenk line methods. MoCl5 (99.6%), nBuLi (1.6 M in hexanes) and Me2NH (99+ %) were purchased from Aldrich and used as supplied. Solvents were dried by distillation from sodium/benzophenone. Liquid ammonia was distilled from Na/ammonia solution, and gaseous ammonia was dried by passing through a column of pre-dried 4 Å molecular sieves. Mo(NMe2)4 was prepared according to a literature procedure [42] and checked for purity
Results and discussion
Solution phase ammonolysis of molybdenum chloride or dimethylamide is expected to lead to polymeric precipitates containing amide and imide groups due to transamination of the metal centres followed by condensation reactions between metal centres:MoCl5 + a NH3 → [MoClx(NH2)y(NH)z]n + b NH4Cl[Mo(NMe2)4] + a NH3 → [Mo(NMe2)x(NH2)y(NH)z]n + b HNMe2
The reactions of MoCl5 with NH3 in the solid state have previously been found to suffer from melting of the MoCl5 which results in large, low surface
Conclusions
Reactions of MoCl5 or Mo(NMe2)4 with ammonia followed by pyrolysis of the resultant polymer in ammonia at various temperatures result in γ- or δ1- molybdenum nitrides. Materials produced from Mo(NMe2)4 at moderate temperatures contain a high proportion of nanotubes and a distortion to the γ-Mo2N rocksalt-type structure is observed. These materials show good electrochemical charge storage capability in aqueous H2SO4 and K2SO4 electrolytes. Cyclic voltammograms of MoCl5-derived materials exhibit
Acknowledgements
The authors thank The University of Southampton for a PG Scholarship to SIUS.
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