Engineering the Surface/Interface Structures of Titanium Dioxide Micro and Nano Architectures towards Environmental and Electrochemical Applications

Abstract

Titanium dioxide (TiO₂) materials have been intensively studied in the past years because of many varied applications. This mini review article focuses on TiO₂ micro and nano architectures with the prevalent crystal structures (anatase, rutile, brookite, and TiO₂(B)), and summarizes the major advances in the surface and interface engineering and applications in environmental and electrochemical applications. We analyze the advantages of surface/interface engineered TiO₂ micro and nano structures, and present the principles and growth mechanisms of TiO₂ nanostructures via different strategies, with an emphasis on rational control of the surface and interface structures. We further discuss the applications of TiO₂ micro and nano architectures in photocatalysis, lithium/sodium ion batteries, and Li-S batteries. Throughout the discussion, the relationship between the device performance and the surface/interface structures of TiO₂ micro and nano structures will be highlighted. Then, we discuss the phase transitions of TiO₂ nanostructures and possible strategies of improving the phase stability. The review concludes with a perspective on the current challenges and future research directions.

Keywords: Li–S batteries; crystal structure; lithium/sodium ion batteries; phase stability; photocatalysis; surface/interface structure; titanium dioxide.

Figure 1

Crystal structures of typical TiO₂ polymorphs: (a) rutile; (b) brookite; (c) anatase; and (d) TiO₂(B). Gray and red spheres are Ti⁴⁺ and O²⁻ ions, respectively.

Figure 2

Engineering the surface/interface structures in TiO₂ materials via one step approach. (a) Cross section and (b) front view scanning electron microscopy (SEM) images of amorphous TiO₂ nanotube arrays fabricated by anodic oxidation. Reproduced with permission from [47], Copyright Nature Publishing Group, 2010.

Figure 3

Post treatment route to tune the surface/interface structures in TiO₂ materials. (a) A photo comparing unmodified white and disorder-engineered black TiO₂ nanocrystals; (b,c) High-resolution transmission electron microscopy (HRTEM) images of TiO₂ nanocrystals before and after hydrogenation, respectively. In (c), a short dashed curve is applied to outline a portion of the interface between the crystalline core and the disordered outer layer (marked by white arrows) of black TiO₂; (d,e) X-ray Diffraction (XRD) and Raman spectra of the white and black TiO₂ nanocrystals (reprinted from [49] with permission, Copyright American Association for the Advancement of Science, 2011). (f) Schematic and (g–l) electron microscopy images of mesoporous single-crystal nucleation and growth within a mesoporous template. (g) Pristine silica template made up of quasi-close-packed silica beads; (h) non-porous truncated bipyramidal TiO₂ crystal; (i) template-nucleated variant of the crystal type shown in (h); (j) replication of the mesoscale pore structure within the templated region; (k,l) fully mesoporous TiO₂ crystals grown by seeded nucleation in the bulk of the silica template. (Reproduced with permission from [44], Copyright Nature Publishing Group, 2013).

Figure 4

Theoretical calculation guides the modification of surface/interface structures. (a–f) Slab models and calculated surface energies of anatase TiO₂ (001) and (101) surfaces. (a,b) Unrelaxed, clean (001) and (101) surfaces; (c,d) Unrelaxed (001) and (101) surfaces surrounded by adsorbate X atoms; (e) Calculated energies of the (001) and (101) surfaces surrounded by X atoms; and, (f) Plots of the optimized value of B/A and percentage of {001} facets for anatase single crystals with various adsorbate atoms X. Here, the parameters of A and B are the lengths of the side of the bipyramid and the side of the square {001} “truncation” facets (see the geometric model). The value of B/A describes the area ratio of reactive {001} facets to the total surface. (g,h) SEM images and statistical data for the size and truncation degree of anatase single crystals (Reproduced with permission from [43], Copyright Nature Publishing Group, 2008).

Figure 5

(a) Different stages in heterogeneous photocatalysis (Reproduced with permission from [63], Copyright The Royal Society of Chemistry, 2016); surface/interface engineered TiO₂ structures for photocatalytic improvement: (b) crystallographic plane tuning (Reproduced with permission from [64], Copyright American Chemical Society, 2014), (c) defects engineering (Reproduced with permission from [65], Copyright Elsevier B.V., 2016), and (d) creating interfaces in TiO₂ nanostructures (Reproduced with permission from [66], Copyright Elsevier B.V., 2017).

Figure 6

Typical TiO₂ anodes and their lithium storage properties: (a) three-dimensional (3D) anatase TiO₂ hollow microspheres assembled with high-energy {001} facets (reprinted from [105] with permission, Copyright The Royal Society of Chemistry, 2012); (b) Rutile TiO₂ nanoparticles with quantum pits (reprinted from [50] with permission, Copyright The Royal Society of Chemistry, 2016); (c) Brookite TiO₂ nanocrystalline (reprinted from [105] with permission, Copyright The Electrochemical Society, 2007); (d) bunchy hierarchical TiO₂(B) structure assembled by porous nanosheets (reprinted from [119] with permission, Copyright Elsevier Ltd., 2017); and (e) Ultrathin anatase TiO₂ nanosheets embedded with TiO₂(B) nanodomains (Reproduced with permission from [125], Copyright John Wiley & Sons, 2015).

Figure 7

The interaction between sulfur or lithium polysulphides and electrodes. (a) On reduction of S₈ on a carbon host, Li₂S_X desorb from the surface and undergo solution-mediated reactions leading to broadly distributed precipitation of Li₂S; (b) On reduction of S₈ on the metallic polar Ti₄O₇, Li₂S_X adsorb on the surface and are reduced to Li₂S via surface-mediated reduction at the interface (reprinted from [135] with permission, Copyright Nature Publishing Group, 2014); Adsorption configuration of (c,d) Li–S* and (e,f) Li₂S on the (c,e) anatase-TiO₂ (101) surface and (d,f) rutile-TiO₂ (110) surface (Reproduced with permission from [136], Copyright The Royal Society of Chemistry, 2016).

Figure 8

Schematic illustration of the synthesis process and electrochemical properties of TiO@C-HS/S composites. (a) Nyquist plots before cycling from 1 MHz to 100 mHz; (b) the second-cycle galvanostatic charge/discharge voltage profiles at 0.1 C; (c) cycle performances at 0.1 C; (d) rate capabilities; and (e) the potential differences between the charge and discharge plateaus at various current densities of the TiO@C-HS/S, titanium dioxide@carbon hollow nanospheres/S composite (TiO₂@C-HS/S), carbon coated conductive TiO_2-x nanoparticles/S composite (TiO_2-x@C-NP/S), pure carbon hollow spheres/S composite (C-HS/S) and TiO₂ nanoparticles/S composite (TiO₂-NP/S) electrodes. (f) Voltage profiles at various current densities from 0.1 to 2 C and (g) prolonged cycle life and Coulombic efficiency at 0.2 and 0.5C of the TiO@C-HS/S electrode. (h) Areal capacities and (i) voltage profiles at various current densities from 0.335 (0.05 C) to 1.34 mA·cm⁻² (0.2 C) of the TiO@C-HS/S electrode with high sulfur mass loading of 4.0 mg·cm⁻² (reprinted from [40] with permission, Copyright Nature Publishing Group, 2016).

Figure 9

Typical transmission electron microscopy (TEM) images of the as-prepared and annealed TiO₂ nanowires with diameters of (a) 20; (b) 50; and (c) 80 nm. The insets show corresponding selected area electron diffraction (SAED) patterns (Reproduced with permission from [16], Copyright Springer, 2012).

Figure 10

Nucleation and growth kinetics of nanocrystalline anatase to rutile. Annealing time dependence of the size of the rutile in the (a) nanowire and (c) free-state powders at different temperatures; Annealing temperature variations of the nucleation rate (NR) and the growth saturation rate t_E⁻¹ for rutile in the (b) nanowire and (d) free-state powders, respectively (Reproduced with permission from [16], Copyright Springer, 2012).

Figure 11

Atomic evolution of the (1 × n) reconstructions on anatase TiO₂ (001) surface. (a) Sequential HRTEM images of the dynamic structural evolution, viewed from [010] direction, with the red arrows indicating the unstable states; (b) The statistical diagram of the locations of the TiO_x rows with green and red lines indicating the stable states and the unstable states; (c) Side view of the proposed model for the unstable two-row state with the TiO_x row shown as ball-and-stick (Ti, gray; O, red) on the TiO₂ stick framework. The green and red arrows indicate the stable single-row and instable double-row structures, respectively; (d,e) Experimental HRTEM image compared with the simulated image based on the model in (c). (Reproduced with permission from [149], American Chemical Society, 2016).