Effect of preparation conditions on the ordering of structural elements in vanadium pentoxide xerogels

Abstract

Vanadium(V) oxide xerogels with an ordered laminar structure are of considerable interest in modern inorganic chemistry [1]. They combine the ease of intercalation of inorganic ions [2] or organic molecules [3] into the interlayer space and the presence of a transition metal ions with variable oxidation states, giving rise to nonlinear electrical and, in some cases, magnetic characteristics. Hybrid inorganic‐organic materials based on xerogels of vanadium(V) oxide and its nanotubular forms [4] attract considerable attention due to their unique physical and unusual physicochemical and rheological properties, which enable their use as cathode materials for lithium batteries [5], chemical sensors [6], electrochromic materials [7], and catalysts [8]. Vanadium(V) oxide xerogels can be prepared by hydrolysis of organic derivatives of vanadic acid, by polycondensation of vanadates in an aqueous solution in an acid medium, and by decomposition of peroxovanadium(V) compounds upon dissolution of the crystalline oxide in hydrogen peroxide [1, 9, 11]. The mechanisms of these processes are essentially different; therefore, it is important to elucidate the structural features of the xerogels as a function of the preparation method. Small-angle X-ray scattering, which is a standard method for measuring the kinetics of gel condensation in solutions [10], can also be used rather efficiently to study the structural features of xerogels. This study reveals the correlation between the chemical prehistory and the structural organization of samples obtained. Complex analysis of small-angle X-ray scattering (SAXS) patterns of xerogel films synthesized by different methods was carried out for the first time and possible models for description of their structure were considered. Colloid solutions of vanadium(V) oxide for the formation of xerogel films were prepared by three procedures (table). According to method A, analytical grade crystalline V 2 O 5 (0.5 g) was dissolved in 15% H 2 O 2 (30 mL) by slow addition of V 2 O 5 powder to the hydrogen peroxide solution [11]. After the oxygen evolution ceased, the solution became yellow. Heating the solution to ~40°C gave a dark red solution, which was kept under ambient conditions for 7 days until a xerogel film formed. According to method B, the gel was obtained by hydrolysis of tert -butyl metavanadate VO(OC(CH 3 ) 3 ) 3 . The ester was synthesized [11] by refluxing a mixture of calcined vanadium(V) oxide with excess tert -butyl alcohol in the presence of a desiccant (silica gel) for 7 h. The resulting mixture was separated by distillation at 81°C . Hydrolysis was carried out by mixing the ester with an equal volume of water (an excess) for the preparation of bulk samples or under ambient conditions. In method C, polycondensation of V took place in an aqueous solution of sodium vanadate on acidification. A colloid solution of V 2 O 5 · n H 2 O was obtained by