Dual Templating Synthesis of Mesoporous Titanium Nitride Microspheres

Adv. Mater. 2009, 21, 1–5 2009 WILEY-VCH Verlag Gmb The synthesis of new porousmaterials remains an exciting area of research, in part because of their potential use in diverse applications. Materials having hierarchically porous nanostructures (such as bimodal mesoporous, meso–macroporous, or mesoporous–hollow materials) are particularly intriguing because they can uncouple overall mass transport from the chemical and physical properties of the finer pore structure. The general synthetic methodology for hierarchically porous nanostructures has relied heavily on the use of multiple sacrificial templates (amphiphilic micellar scaffolds, block copolymers, colloidal silica, or polymer spheres) as components of a preliminary nanocomposite. Thus, post-treatments (chemical etching or thermal heat-treatment) must be used to render the various nanostructured porosities in the final products. Despite the usefulness of such templates, they inherently require the use of prestructured templates (which are relatively expensive) and the necessity of post-processing removal of the template (which is generally cumbersome, hazardous, or incomplete). There remains, therefore, an urgent need for new synthetic methods that do not require an externally added sacrificial template. In addition, there have been few attempts to prepare the hierarchically porous nanostructures of non-oxide materials such asmetal nitrides and carbides (TiN, Si3N4, F orMo2C). Metal nitrides and carbides have substantial advantages in hightemperature catalysis, including higher melting points, lower sintering tendencies, and greater chemical inertness under non-oxidizing conditions. We have therefore developed a new and general route to the preparation of hierarchically porous nanostructures of refractory nitrides that avoids the use of external sacrificial templates. Here, we report an ultrasonic spray pyrolysis (USP) preparation of hierarchically nanostructured titanium nitride (TiN) using an in situ dual templating from an initial soft liquid-core template and second from the resulting hard binary-oxide shell. In our new synthetic approach, multimodal porous nanostructures (mesoporous and hollow macroporous) are obtained without the use of any prestructured templates or any additional template-removing procedures. TiN is a hard, electrically conductive, and wearand corrosion-resistive material that has been used in the microelectronic industry as a conductive diffusion barrier. Recent reports have found its potential use in various other applications, such as catalysis, hydrogen storage, supercapacitors, pH sensors, electroanalysis, etc. In general, TiN powder is prepared by firing Ti metal or TiO2 under ammonia gas as a nitrogen source. [5b,7] Several attempts have been made toward the preparation of nanostructured TiN, but none have succeeded in creating a simple and facile synthetic route to hierarchically porous nanostructures of TiN. Recently, ultrasonic spray pyrolysis (USP) has achieved significant prominence for the synthesis of various nanostructured materials, including metal sulfides, oxides, and carbons. Because of its unique operation conditions (continuous production of sub-micrometer microreactors, that is, isolated droplets in a hot-gas stream), USP stands out from other various synthetic routes (which are all essentially batch reactions in macroscale reactors) in the preparation of nanocomposite materials. The success of recent USP syntheses in preparing hollow or porous materials has generally still relied on sacrificial-template materials (colloidal silica or polymer spheres) as one of the components of the nanocomposites, which then require post-treatment to produce the final porous structures. There have been some efforts to address this issue, such as the use of water-soluble metal salts (NaCl or LiCl) as the second component of nanocomposites. In order to generate porous TiN, we decided to use zinc titanate (Zn2TiO4) as a precursor from which we discovered we could remove Zn during nitration with ammonia, as outlined in Figure 1. We therefore used USP of an aqueous solution containing a simple Zn salt (zinc nitrate hexahydrate, Zn(NO3)2 6H2O) and a soluble Ti(IV) complex ([NH4]2 [Ti(OH)2(OCH(CH3)CO2)2]) to obtain zinc titanate. This Ti(IV) complex was specifically chosen because it does not readily hydrolyze to TiO2; indeed, it is stable at room temperature approximately between pH 2 and pH 10. To our surprise, the products so obtained were not solid particles, but rather hollow spheres of zinc titanate (Zn2TiO4), as shown in Figure 2a and b (see Fig. S1 in Supporting Information (SI) for more electron microscopy images of Zn2TiO4). The formation of the hollow spheres was further confirmed by energy-dispersive X-ray spectroscopy (EDS) line analysis and elemental mapping analysis, respectively (Figs. S2 and S3 in SI). The final TiN powder obtained after the nitration of hollow Zn2TiO4 spheres exhibited a dramatic change in morphology and