The design, fabrication, and photocatalytic utility of nanostructured semiconductors: focus on TiO2-based nanostructures

Abstract

Recent advances in basic fabrication techniques of TiO2-based nanomaterials such as nanoparticles, nanowires, nanoplatelets, and both physical- and solution-based techniques have been adopted by various research groups around the world. Our research focus has been mainly on various deposition parameters used for fabricating nanostructured materials, including TiO2-organic/inorganic nanocomposite materials. Technically, TiO2 shows relatively high reactivity under ultraviolet light, the energy of which exceeds the band gap of TiO2. The development of photocatalysts exhibiting high reactivity under visible light allows the main part of the solar spectrum to be used. Visible light-activated TiO2 could be prepared by doping or sensitizing. As far as doping of TiO2 is concerned, in obtaining tailored material with improved properties, metal and nonmetal doping has been performed in the context of improved photoactivity. Nonmetal doping seems to be more promising than metal doping. TiO2 represents an effective photocatalyst for water and air purification and for self-cleaning surfaces. Additionally, it can be used as an antibacterial agent because of its strong oxidation activity and superhydrophilicity. Therefore, applications of TiO2 in terms of photocatalytic activities are discussed here. The basic mechanisms of the photoactivities of TiO2 and nanostructures are considered alongside band structure engineering and surface modification in nanostructured TiO2 in the context of doping. The article reviews the basic structural, optical, and electrical properties of TiO2, followed by detailed fabrication techniques of 0-, 1-, and quasi-2-dimensional TiO2 nanomaterials. Applications and future directions of nanostructured TiO2 are considered in the context of various photoinduced phenomena such as hydrogen production, electricity generation via dye-sensitized solar cells, photokilling and self-cleaning effect, photo-oxidation of organic pollutant, wastewater management, and organic synthesis.

Keywords: TiO2 nanostructure; TiO2 self-cleaning; TiO2-assisted photoactivity; TiO2-based dye-sensitized solar cells; doping in TiO2; fabrication techniques; organic synthesis; solar hydrogen.

Figure 1

Road map to nanotechnology. Reproduced with permission from the US National Nanotechnology Initiative report.

Figure 2

Classifications of metal oxides.

Figure 3

Crystallographic unit cell structure of TiO₂ with A) rutile and B) anatase structures. Copyright © 2003, Cangiani. Adapted with permission from Cangiani G. Ab Initio Study of the Properties of TiO₂ Rutile and Anatase Polytypes. Lausanne, France: Faculté des sciences de base, Ecole polytechnique fédérale de Lausanne EPFL; 2003.

Figure 4

Arrangement of TiO₆ octahedra in relation to the unit cells in A) rutile and B) anatase. Only one chain is shown for each structure. Highlighted bonds are the O–O bonds. Copyright © 2003, Cangiani. Adapted with permission from Cangiani G. Ab Initio Study of the Properties of TiO₂ Rutile and Anatase Polytypes. Lausanne, France: Faculté des sciences de base, Ecole polytechnique fédérale de Lausanne EPFL; 2003.

Figure 5

Schematic design of a VLS system.

Figure 6

Mechanism of vapor–liquid–solid growth of TiO₂ nanowires.

Figure 7

Schematic representation of various de-excitation pathways for photogenerated electron and holes in a TiO₂ particle (adapted and redrawn). Kamat PV. Meeting the clean energy demand: nanostructure architectures for solar energy conversion. J Phys Chem C. 2007;111(7): 2834–2860. Abbreviations: CB, conduction band; VB, valance band; hν, photon energy of frequency ν.

Figure 8

Various electron transfer and energy transfer processes of de-excitation of photogenerated e⁻−h⁺ pairs (adapted and redrawn). Kamat PV. Meeting the clean energy demand: nanostructure architectures for solar energy conversion. J Phys Chem C. 2007;111(7): 2834–2860.

Figure 9

Schematic representation of TiO2/SWCNT and TiO2/MWCNT nanocomposite structures, (b)proposed model for reduction in photogenerated electro-hole recombination in TiO2/CNT nanocomposites (adapted and redrawn). Schnitzler DC, Zarbin AJG. Organic/inorganic hybrid materials formed from TiO2 nanoparticles and polyaniline. J Braz Chem Soc. 2004; 15(3):378–384. Abbreviations: CNT, carbon nanotubes; MW, multiwall; SW, single wall.

Figure 10

Schematic representation of principle of operation and energy level scheme of the dye-sensitized nanocrystalline solar cell (adapted and redrawn). Luo H, Takata T, Lee Y, Zhao J, Domen K, Yan Y. Photocatalytic activity enhancing for titanium dioxide by co-doping with bromine and chlorine. Chem Mater. 2004;16(5):846–849. Note: The diagram is not drawn to the scale.