Comparative optical study of colloidal anatase titania nanorods and atomically thin wires

Abstract

We present results of a comparative study of colloidal anatase titanium oxide nanorods and extremely thin atomic wires of systematically decreasing (2.6 nm down to 0.5 nm) diameter in terms of their optical absorption as well as steady-state and time-resolved photoluminescence. Steady-state photoluminescence spectra of the titania samples show three well-distinguished spectral components, which are ascribed to excitonic emission (4.26 AE 0.2 eV), as well as radiative recombination of trapped holes with electrons from the conduction band (4.04 AE 0.4 eV) and radiative recombination of trapped electrons with holes in the valence band (3.50 AE 0.2 eV) in nanocrystalline anatase TiO 2 . Time-resolved photoluminescence measurements point out the existence of different emissive species responsible for the appearance of high-energetic and low-energetic emission peaks of TiO 2 atomic wires and nanorods.Titanium dioxide is an extremely popular, environmentally benign and low cost material with a variety of potential applications ranging from water splitting 1 and sodium ion batteries 2 to photoelectrochemical solar cells, 3,4 to name a few. Synthesis, properties and applications of nanostructured TiO 2 have been a focus of extensive research in the last two decades, 5 with a particular emphasis on the charge recombination and transport.6,7 Recent advances of physical chemistry of TiO 2 nanostructures and their perspectives for photocatalysis and photovoltaics have been summarized in a recent Editorial by Kamat.8 The anatase phase is stabilized over rutile in nanocrystalline TiO 2 , 9 and there have been continuous efforts in developing synthetic routes leading to high-quality anatase TiO 2 nanoparticles of small sizes 10,11 and controllable dimensions, in particular rod-shaped nanocrystals.12,13 The available palette of colloidally synthesized, anisotropic TiO 2 nanostructures has been recently extended towards extremely thin anatase wires with diameters reaching the atomic limit of a few angstroms. 14,15Size and doping effects on the absorption and emission properties of colloidal TiO 2 nanoparticles have been studied in detail by Serpone and co-workers, 16,17 and have been interpreted in terms of the co-existence of the band-edge 18 (or self-trapped exciton 19) recombination and the recombination involving midgap energy levels originating from oxygen vacancies (F-type color centres).20,21 Recent studies by Knorr, McHale and co-workers 22-24addressed the origin of the trap-related emission in nanocrystalline TiO 2 , classifying it into contributions from two spatially isolated trap-state distributions, resulting in recombination of trapped electrons or holes with oppositely charged mobile charge carriers in the valence or conduction band, respectively. Yoshihara et al. employed transient absorption spectroscopy to study the localization of trapped electrons and holes in nanocrystalline TiO 2 , which have been found to be at the surface of the particle while free electrons were distributed in the bulk. 25 It has been poi...

Chiral and Luminescent TiO2 Nanoparticles

“…The TiO 2 NPs lifetime which was fit using a biexponential, showing a lifetime of 6.30 ± 0.029 and 13.62 ± 0.079 ns with a relative amplitude of 79.7% and 20.3%. Though sub‐nanosecond lifetimes for TiO 2 [16a,19], have been reported, these are associated with higher energy luminescence, while our emission is also of a much lower energy of 2.81 eV. It has therefore been assigned as a shallow trap state emission as reported in literature,[16b] and which is commonly found to display much longer luminescence lifetimes …”

“…The quantum yield of the TiO 2 nanoparticles was determined to be 3.5% in THF solution when excited at 261 nm and was determined by the relative approach, using quinine sulfate dihydrate dye in 0.105 m HClO 4 in water as our standard which has a quantum yield of 60.9% . This quantum yield compares favorably with examples in literature measured at room temperature which range generally from 1.6 to 0.25 and even as low as 0.05%, though a single report does exist claiming quantum yields up to 20% …”

Electronic structure and hot carrier relaxation in ⟨001⟩ anatase TiO2nanowire

“…Email: Dmitri.Kilin@usd.edu mal deposition method [14]. The optical absorption as well as the steady-state and time-resolved photoluminescence of anatase TiO 2 nanorods and extremely thin atomic NWs were compared by Rogach et al [15]. Although theoretical studies of TiO 2 bulk and surfaces have been extensively carried out [16][17][18][19][20][21][22], there is relatively little study on anatase TiO 2 NWs.…”

Lyotropic ‘hairy’ TiO2 nanorods

“…38 Nonhydrolytic solvothermal reactions have also been used to synthesise oleic acid-capped, solution-processable TiO 2 nanorods. 39,40 The authors are not aware of any photodimerisable and crosslinkable ligands used to prepare such solutionprocessable TiO 2 nanorods.…”