A family of oxide–carbide–carbon and oxide–nitride–carbon nanocomposites

Abstract

This paper describes a powerful and versatile approach that combines the benefits of sol-gel processing with controlled phase separation to yield oxide-carbide-carbon or oxide-nitride-carbon nanocomposites.Transition metal carbides and nitrides are valuable materials for applications such as fuel cell electrocatalysis, 1 ammonia synthesis 2 and photocatalytic water splitting. 3 In these systems, the properties often depend on control of particle size, morphology and accessible surface area. 4 Sol-gel, 5 solvothermal 6 and templating 7 processes have all been used to generate carbides and nitrides with small particle size or porosity. A major hurdle that remains is the controlled and tunable synthesis of composites, particularly carbides or nitrides combined with metal oxides. 8 Such architectures are critical for applications such as noble metal-replacement in solar water splitting. 9 Combining two different ceramics in a nanocomposite presents a significant synthetic challenge. Historically, it has been achieved simply by mechanical mixing, which can seriously compromise the properties of one or both components. More effective methods use ammonolysis 10,11 or temperature programmed reduction 12 of oxide-oxide nanocomposites but have the disadvantage of using hazardous synthesis gases, or multi-step processes. Pinpointing more efficient and sustainable synthesis routes is critical to future materials design. 13 On this basis, a generalised one-pot sol-gel method to composites of two or more ceramic materials would have the key advantages of: (i) a single heating step, (ii) fine-tuning of particle size while achieving good interspersion and (iii) standard, inexpensive and non-hazardous synthesis atmospheres such as nitrogen.Recently, we have shown that abundant biopolymers such as alginate are effective precursors for synthesis of oxide, 14 nitride 15 or carbide 16 nanostructures. Biopolymers are particularly suited to control of particle size in ceramics due to their ability to bind strongly to metal cations. 17 Remarkably, it is also possible to force oxide/carbide or oxide/nitride phase separation from these homogeneous precursors by exploiting differences in stability of intermediate oxides. 18 So far, however, full separation was only achieved in one system (Fe 3 C-MgO). 19 In this paper, we demonstrate the versatility of the sol-gel route in the synthesis of a family of nanocomposites. This new general route emphasizes the potential of sol-gel methods for synthesis of multifunctional materials.Nanocomposites of TiO 2 -Fe 3 C, TiO 2 -WN, CeO 2 -Fe 3 C, CeO 2 -WN, MgO-Fe 3 C and MgO-WN were synthesized by mixing aqueous (or ethanolic) metal salts with hot gelatin solution, stirring vigorously and then drying at 80 1C. The resulting sponge-like precursors were calcined under nitrogen. Samples are denoted TF, TW, CF, CW, MF and MW. A range of molar ratios (100 : 0, 75 : 25, 50 : 50 and 0 : 100) were prepared for each sample and labelled 0, 25, 50 and 100 respectively. For example, sample TF25 was prepared with ...

The evolution of ‘sol–gel’ chemistry as a technique for materials synthesis

“…14e) with potential applications in catalysis. 124 In the case of MgO/Fe 3 C, mild acid washing can be used to remove the nanoparticles to produce carbons with trimodal (macro/meso/ micro) porosity ( Fig. 14f and g).…”

Iron-catalyzed graphitization of biomass

“…This simultaneous formation of oxide and carbide phases has previously only been achieved in sol-gel matrices. 20 The theory behind the method is that different metals require different temperatures to achieve carbothermal reduction from an oxide to a carbide. Therefore by choosing an intermediate temperature it is possible to achieve selective carbothermal reduction of one metal while leaving the other one as an oxide.…”

Mechanistic insights into the formation of porous carbons from gelatin

“…10 These can either be used directly as catalysts e.g. in methanol reforming, 11 or etched with dilute acid to produce carbons with multimodal porosity with applications in electrocatalysis. 12 The latter example was produced by mixing iron and magnesium nitrates with gelatin and heating under nitrogen to form a foam with MgO and Fe 3 C nanoparticles (Fig.…”