Study of the Structure and Mechanism of Formation through Self-Assembly of Mesostructured Vanadium Oxide

The structure and mechanism of formation of a novel hexagonal mesostructured vanadium oxide−surfactant composite (HMVO), synthesized by acid-catalyzed hydrolysis of ethanolic cetyltrimethylammonium vanadate (CTAV) solution, has been studied by a variety of spectroscopic techniques. The poorly crystalline CTAV precursor used in the synthesis has been characterized and is shown to consist of pyrovanadate-like anions in which one oxygen atom is shared between vanadate tetrahedra. The HMVO formed from this precursor appears to feature a distorted octahedral vanadium coordination similar to that found in extended structures such as the two-dimensional vanadium pentoxide gel network. Information on the solution speciation during the synthesis is monitored by 51V NMR spectroscopy. Prior to acid addition, the ethanolic solution of CTAV shows a single narrow resonance at −556.7 ppm, assigned to a tetrahedral diester anion [VO2(OEt)2]-, undergoing fast reorientational averaging of the quadrupole interaction in solution. This behavior indicates that mesophases do not exist in solution prior to the initiation of acid-catalyzed polymerization. Progressive addition of aqueous acid results in a monotonic decrease in the diester concentration with concomitant formation of minor quantities of higher vanadate oligomers and an increasing proportion of mesophase not directly detectable by solution 51V NMR. The water in the aqueous acid plays an important role as an initiator in the hydrolysis of the diester, producing the higher oligomers, as well as a postulated transitory intermediate, which condenses rapidly through the action of the stronger acid, HCl, and directed by the surfactant to give the mesostructured material. The presence of the surfactant is seen to be crucial for inhibiting the formation of anionic decavanadate clusters, and VO2 + species, which are normally associated with the acidification of aqueous vanadate solutions. These observations are consistent with a formation mechanism in which the vanadium mesophase is generated by cooperative formation of micellar entities and higher vanadate oligomers.