Structural and Electromagnetic Signatures of Anatase and Rutile NTs and Sheets in Three Different Water Models under Different Temperature Conditions

Abstract

Experimental studies of TiO₂ nanotubes have been conducted for nearly three decades and have revealed the remarkable advantages of this material. Research based on computer simulations is much rarer, with research using density functional theory (DFT) being the most significant in this field. It should be noted, however, that this approach has significant limitations when studying the macroscopic properties of nanostructures such as nanosheets and nanotubes. An alternative with great potential has emerged: classical molecular dynamics simulations (MD). MD Simulations offer the possibility to study macroscopic properties such as the density of phonon states (PDOS), power spectra, infrared spectrum, water absorption and others. From this point of view, the present study focuses on the distinction between the phases of anatase and rutile TiO₂. The LAMMPS package is used to study both the structural properties by applying the radial distribution function (RDF) and the electromagnetic properties of these phases. Our efforts are focused on exploring the effect of temperature on the vibrational properties of TiO₂ anatase nanotubes and an in-depth analysis of how the phononic softening phenomenon affects TiO₂ nanostructures to improve the fundamental understanding in different dimensions and morphological configurations. A careful evaluation of the stability of TiO₂ nanolamines and nanotubes at different temperatures is performed, as well as the adsorption of water on the nanosurface of TiO₂, using three different water models.

Keywords: LAMMPS MD; Matsui and Akaogi; PDOS; antase; infrared spectra; low frequency; nanosheets; nanotubes; power spectra; radial distribution function (RDF); rutile; soft phonon effect.

Figure 1

Radial distribution functions (RDFs). (a) Anatase nanosheet and rutile nanosheet pairs of Ti-O. (b) Anatase natube and rutile nanotube pairs of Ti-O. (c) Anatase nanosheet and rutile nanosheet pairs of g(r O-O). (d) Anatase nanotube and rutile nanotube pairs of O-O. (e) Anatase nanosheet and rutile nanosheet pairs of Ti-Ti. (f) Anatase nanotube and rutile nanotube pairs of Ti-Ti.

Figure 2

Phonon density of states of anatase and rutile nanostructures obtained from the Fourier transform of the velocity autocorrelation. (a) Total PDOS of anatase nanosheet and rutile nanosheet. (b) Total DOS of anatase nanotube and rutile nanotube; PDOS of anatase nanosheet and rutile nanosheet (c) x-axis (e) y-axis, (g) z-axis; PDOS of anatase nanotube and rutile nanotube (d) x-axis (f) y-axis, (h) z-axis.

Figure 3

Power spectral density of each axis of a rutile nanosheet.

Figure 4

Anatase NTs density of states through calculations of the power spectrum for the whole TiO${}_2$ moiety (blue), Ti (red), and oxygen (orange) atoms.

Figure 5

Phonon density of states of anatase and rutile nanosheets at different temperatures.

Figure 6

Average total energy of anatase nanotubes of varying radius.

Figure 7

Analyze the anatase TNTs density of states using power spectrum analysis for tubes with various radii (normalized).

Figure 8

Computational infrared spectrum of Anatase NT with radius of 10 Å and length 31 Å in different temperatures.

Figure 9

Spectrum of Anatasa NT in different water models and vacuum.

Figure 10

(a) Number of water molecules confined (3 Å) next to anatase NT in different water models, (b) water molecules confined (3 Å) next to anatase NT in CVFF water model, (c) water molecules confined (3 Å) next to anatase NT in Tip3P-FW water model, (d) water molecules confined (3 Å) next to anatase NT in COMPASS water model.

Figure 11

Initial structure of nanotube (a) in rutile phase, crystal in orientation 001 (b) in anatase phase, crystal in orientation 001. During the production dynamics of nanotube, at T = 300 K (c) in rutile phase, 18 nm × 73 nm, (d) in anatase phase, 29 nm × 72 nm. During the production dynamics of nanotube in water, at T = 300 K (e) in rutile phase, radius = 12 nm, length = 20 nm, at the (f) in anatase phase, radius = 12 nm, length = 20 nm.