Growth Confined by the Nitrogen Source: Synthesis of Pure Metal Nitride Nanoparticles in Mesoporous Graphitic Carbon Nitride

Abstract

The synthesis of materials in nanometer-sized confinements, or "nanoreactors", is a suitable method for the preparation of nanoparticles. As "soft" organic nanoreactors, the synthesis of nanoparticles in micelles formed from surfactants [1] and block copolymers, [2][3][4][5][6] emulsions, [7][8][9] and gels [10] has been previously described. Soft nanoreactors often allow simple purification procedures, that is, the resulting nanoparticles can be separated from the self-assembled confinement in a nondestructive fashion. On the other hand, the softness of the reactor walls sometimes leads to unpredicted forms and shapes of the resulting nanoparticles, and the method is not often applicable for desirable nanoparticle compositions, which is because of the complex reactant mixtures present during synthesis. This is especially true for metal nitrides.Other approaches have used the confinement provided by the pore system of porous solids, mainly silica or alumina. These "hard" nanoreactors or exotemplates [11] inevitably result in exactly shaped replications of nanoparticles, nanowires, or even fully replicated mesoporous frameworks, depending on the structure and pore size of the confinement. Using this approach nanoparticles of metals, [12] metal oxides, [13][14][15] sulfides, [16] and nitrides, [17] as well as carbon nanostructures [18] and nanoparticles fabricated from high-temperature engineering plastics [19] have been produced. Still, a severe disadvantage of the hard-template approach is that isolation of the wanted materials is accompanied by the destruction of the confinement, often by using harmful etching reagents such as hydrofluoric acid. Furthermore, the incorporation of precursors into the confinement has to be ensured; an adequate pore filling and, if necessary, adequate mixing and stoichiometry of the starting components inside the nanopores is hard to control and obeys the statistics of small numbers.Here we report the synthesis of metal nitride nanoparticles in a new type of nanoconfined environment, namely mesoporous graphitic carbon nitride (mpg-C 3 N 4 ).[20] This approach adds two significant improvements to reported nanostructure synthesis using exotemplates: First, the reaction-confining matrix is thermally decomposed during synthesis, making additional purification and isolation steps obsolete. Second, the medium serves as a reagent in the nanoparticle synthesis by donating an excess of nitrogen during decomposition. Appropriate metal sources in the pores of the carbon nitride are thus converted into metal nitride nanoparticles with defined nanoparticle size. Such metal nitride nanoparticles are interesting because of their potential application as abrasive materials, [21] catalysts, [22][23][24] or optoelectronic compounds. [25] Metal nitrides in general can be produced by the conversion of metals or metal oxides into the corresponding nitride using nitrogen sources, such as ammonia [26] or hydrazine [27,28] at high temperatures.Also, the use of carbon nitride as a nitrogen source was descri...

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