Macroporous crystalline vanadium oxide foam

Abstract

Porous inorganic solids can be synthesized in complex forms^1,2, and here we describe a simple method to create an ultralight, macroporous, crystalline vanadium oxide foam by bubbling oxygen gas produced in situ through a viscous vanadium oxide gel. This foaming process could be extended to other metal oxides.

Main

Vanadium oxide (V₂O₅) powder (1 g, or 5.5 × 10⁻³ mol) is mixed with a solution of a long-chain alkylamine (1-hexadecylamine, C₁₆H₃₃NH₂; 1.99 g, or 8.24 × 10^–3 mol) in acetone (3 ml). A pasty material is formed, to which is added an aqueous solution of hydrogen peroxide (50 ml, 30%). As H₂O₂ decomposes, oxygen gas evolves and a voluminous bright yellow foam forms spontaneously (Fig. 1). The volume of the foam increases rapidly and reaches 2.5 litres after about 30 min. This new material is ultralight, as only 1 g V₂O₅ is needed to generate the large volume of foam.

Figure 1

Light fantastic: up to 3 litres of a low-density, bright yellow vanadium oxide foam are formed when 1 g of V₂O₅ reacts with hydrogen peroxide in the presence of hexadecylamine.

Scanning electron microscopy of the foam (Fig. 2a) reveals that it is highly porous and has an irregular, honeycomb-like morphology. Large pores a few micrometres in diameter are surrounded by vanadium oxide walls about 1 µm thick. This unusual pattern resembles the hexagonal cellular structures found in the skeletons of some diatoms and radiolarians³. The mechanical properties of this porous material are poor — the ultralight foam is brittle and can easily be crushed into a powder.

Figure 2: Structure of vanadium oxide foam.

a, Scanning electron micrograph of vanadium oxide foam, showing its honeycomb-like structure. b, The vanadium oxide walls are crystalline, as indicated by the 00l peaks of the X-ray diffraction diagram.

The vanadium oxide walls are crystalline, and X-ray diffraction reveals five sharp peaks that can be assigned to the 00l peaks of a lamellar solid with a preferred orientation (Fig. 2b). The first 001 peak gives a basal distance, d, of 33.4 Å, which is much larger than the distance between vanadium oxide planes in the crystalline oxide or in V₂O₅,1.6H₂O gels (d = 11.5 Å). This basal distance corresponds to that noted for hexadecylamine molecules intercalated in vanadium oxide gels in which the alkyl chains are perpendicular to the oxide planes⁴.

Contrary to observations of hybrid organic–inorganic vanadium oxide com pounds, the bright yellow coloration of our vanadium oxide foam indicates that all the vanadium ions are probably in their highest oxidation state (V⁵⁺). The hybrid foams contain no water, even when dried at room temperature, and they have amphiphilic alkylamines intercalated between vanadium oxide layers and are highly hydrophobic, separating readily at the surface of an aqueous solution.

Vanadium oxide reacts with hydrogen peroxide to give V₂O₅.nH₂O gels⁵ that have a layered structure composed of negatively charged vanadium oxide ribbons. These gels can intercalate a wide variety of inorganic and organic species, such as protonated long-chain alkylamines⁶. In the foaming process described here, gelation and intercalation both occur when the 1-hexadecylamine and hydrogen peroxide solutions are added to vanadium oxide powder. The intercalation of hexadecylamine between the V₂O₅ layers creates a pasty solid, while the hydrogen peroxide decomposes spontaneously in the presence of vanadium oxide. Large pores are formed by oxygen gas released through the viscous gel in the presence of surfactants: as they escape, the oxygen bubbles cause the formation of a voluminous polygonal foam, without destroying the crystalline structure of the oxide network of surfactant molecules intercalated between the layers of vanadium oxide.

Meso- and macroporous solids are usually made in the presence of organic templates which then have to be removed without damaging the inorganic shell^7,8. In the foaming method we describe here, amines are simply intercalated between vanadium oxide planes and do not act as templates for the formation of macropores. The experimental requirements are not crucial — foams can form with amine/V₂O₅ molar ratios ranging from 0.5 to 2.5, other surfactants can be used and the process can be extended to many different oxides (for example, those of molybdenum (MoO₃), titanium (TiO₂) and zirconium (ZrO₂)).