Effect of Size and Structure on the Ground-State and Excited-State Electronic Structure of TiO₂ Nanoparticles

Abstract

Abstract.We investigated the influence of size and structure on the electronic structure of 0.5-3.2 nm diameter TiO 2 nanoparticles in both vacuum and water using density functional theory calculations. Specifically, we tracked the optical and electronic energy gap of a set of (TiO 2 ) n nanoparticles ranging from small non-bulk-like clusters with n =4, 8 and 16, to larger nanoparticles derived from the anatase bulk crystal with n = 35 and 84. As the difference between these two energy gaps (the exciton binding energy) becomes negligible in the bulk, this magnitude provides an indicator of the bulk-like character of the electronic structure of the nanoparticles under study. Extrapolating our results to larger sizes, we obtain a rough estimate of the nanoparticle size at which the electronic structure will begin to be effectively bulk-like. Our results generally confirmed that the electronic structure of nanoparticle ground state and excited state has a more pronounced structure dependency than size dependency within 0.5-1.5 nm size.We also showed that thermodynamic preference for the photocatalytic species is the first S 1 exciton. This S 1 exciton is stable in vacuum but may evolve to free charge carriers upon structural relaxation in an aqueous environment for 0.5-1.5 nm size particles studied in the present article. An analysis of ionization potentials and electron affinities relative to the standard reduction potential for the water splitting half-reactions revealed the importance of considering the structural relaxation in the excited states and the presence of water for assessing the thermodynamic conditions for photocatalytic water splitting.

The ratio law of the structure evolution and stability for TinOm (n = 3–18, m = 1–2n) clusters

“…Owing to its technological importance, numerous experimental and theoretical investigations have been carried out on isolated titanium oxide clusters to correlate their structures and properties with their bulk counterparts. [6][7][8][9][10][11][12][13][14][15][16][17][18] Yu and Freas produced isolated positive titanium/oxygen clusters by sputtering titanium dioxide powder and titanium foil (exposed to oxygen) with an energetic xenon atom beam (~8 keV). [7] The form of titanium/oxygen cluster ions from their results is [TinO2n-δ] + , where n equals 1 to 8 and δ ranges from 0 to 4.…”

“…At present, theoretical studies on titanium oxide mainly aimed at (TiO2)n (n≤24) [8,9,11,12,[14][15][16] and experimental researches focused on TinOm (n=1-7, m=2n-3, 2m+1) [7,13,14]. A clear, systemic and an in-depth understanding for the geometries of TinOm particles have not yet emerged.…”

Properties of hydrated TiO2and SiO2nanoclusters: dependence on size, temperature and water vapour pressure

“…11 A number of recent theoretical studies have used top-down bulk cut nanoparticles containing 10s -1000s of atoms in order to investigate the electronic properties of nano-TiO2 photocatalysts 12,13,14,15 and their interactions with water. 16,17,18,19,20,21 Although such calculations can provide important new insights, in most cases the nanoparticles used are still relatively small idealised model systems compared to experimental systems where larger and more complex nanoparticles are typically used. Detailed spectroscopic experiments probing the interaction of water with small oxide (i.e.…”

The elusive photocatalytic water splitting reaction using sunlight on suspended nanoparticles: is there a way forward?

“…Since in photocatalytic reactions charge carriers' lifetime is an intrinsic part of the reaction rate, TD-DFT studies are very much needed, yet at present because of demanding computational resources, they are seldom conducted, mostly on clusters. [64][65][66] Another technique that is gaining momentum for measurement of charge carriers is pump-probe transient absorption spectroscopy (TAS). [67][68][69] While this technique is well developed for molecular studies as well as nanoparticles, it is not as well studied for single crystals and well defined thin films (epitaxy).…”