Tuning Intercrystalline Void-like Defects in Nanowire Clusters to TiO₂ Quantum Wires with Enhanced Photocatalytic Performance

Abstract

Enthalpically driven dopant induced defects generated from the interaction of bulk titanium oxide and structure directed agents composed of charge separated ions have important consequences in directing interfacial energies in organometallic semiconductors. Such external factors imposed by the chemical environment at contrasting surfaces generated at bulk catalytic interfaces are capable of introducing lattice defects at the structural and morphological level. Here, we demonstrate that post-modification of primary defect sites of TiO 2 nanowires from the bulk state in a group reactive ionic liquid (IL) environment has the potential to structurally redirect defect states altering both the order of dimensionality and electronic properties. This is demonstrated by the fabrication of surface modified quantum wires (SMoQWs) from zero-dimensional nanoclustered nanowires (NCNWs) formed under moderate temperature annealing and ambient pressure. This approach demonstrates that structural distortions that exist within crystal lattices of the NCNWs can generate new functionalities at non-equilibrium sites by modulating the mixed Ti 3+ /Ti 4+ valence signature. The concentration of displaced oxygen molecules and their consumption have a direct impact on vacancy growth patterns, increasing the Ti 3+ /Ti 4+ ratio at crystal sites when mediated by nitrogen rich species. Evidence shows that the growth confinement of TiO 2 is locked in a one-dimensional geometrical configuration formed by a complex of caged Tiporphyrin clusters bridged by polyphenylquinoxiline linkers formed via O−Ti−N bonding. The importance of charge carrier separation and charge mobility was demonstrated by spatial reordering of functionalized TiO 2 quantum wires via dye adsorption (N719). SMoQWs demonstrate superior photocatalytic degradation properties to the NCNWs, enhancing their utility in DSSC device applications.

Improving Photocatalytic Performance Using Nanopillars and Micropillars

“…Additionally, altering the band gap structure at boundary defect sites through chemical intervention techniques, such as imidazolium type ionic liquids, allow for more efficient charge transfer routes and an increased oxygen evolution for semiconductor nanowires. Song et al [ 14 ] explored the case of TiO 2 in which the imidazolium ionic liquid semiconductor (liquid–solid) interface is developed as a tuning surface for altering shape and size of TiO 2 nanocrystals into aggregated clusters—this alteration tunes the defect sites at boundary regions and allows for the development of efficient photocatalytic materials. Surface-modified quantum wires (SMoQWs) were observed for photocatalysis as well as the zero-dimensional nanoclustered nanowires (NCNWs) from which they were fabricated.…”

“…( a ) Centers of nucleation form and ( b ) assemble into larger particles by the process of coalescence driven by Ostwald ripening and undergo ( c ) multidirectional growth into NCNWs and ( d ) removal of high-energy weakly bonded particles to form ( e ) nanopillars of SMoQWs. (Adapted from [ 14 ] Figure 1, with permissions from ACS).…”