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In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/materialsA Page 1 of 26 Journal of Materials Chemistry A Journal Name RSC Publishing ARTICLE A Review of One-dimensional TiO 2 Nanostructured CCC ite this: DOI: 10.1039/x0xx00000x Materials for Environmental and Energy Applications Mingzheng Ge, a Chunyan Cao,a Jianying Huang, a,b Shuhui Li, a Zhong Chen, c Ke- Received 00th January 2012, a,b d a,b Qin Zhang, S.S. Al-Deyab, and Yuekun Lai* Manuscript Accepted 00th January 2012 DOI: 10.1039/x0xx00000x One-dimensional TiO 2 (1D TiO 2) nanomaterials with unique structural and functional properties have been extensively used in various fields including photocatalytic degradation of pollutants, photocatalytic www.rsc.org/ CO 2 reduction into energy fuels, water splitting, solar cells, supercapacitors and lithium batteries. In the past a few decades, 1D TiO 2 nanostructured materials with well-controlled size and morphology has been designed and synthesized. Compared to 0D and 2D nanostructures, more attention has been paid to 1D TiO 2 nanostructures due to its high aspect ratio, large specific surface area, excellent electronic or ionic charge transport property. In this review, we present the crystal structure of TiO 2 and the latest development on the fabrication of 1D TiO 2 nanostructured materials. Besides, we will look into some Accepted critical engineering strategies that give rise to the excellent properties of 1D TiO 2 nanostructures such as improved enlargement of surface area, light absorption and efficient separation of electron/holes that A benefit their potential applications. Moreover, their corresponding environmental and energy applications are described and discussed. With the fast development of the current economy and technology, more and more effort will be put into endowing TiO 2-based materials with advanced functionalities and other promising applications. 1. Introduction photocatalysis, hydrogen production, solar cells, adsorbents and sensors due to large surface area and a broadened band gap.25-28 The fast development of global economy and industrialization in the However, it also showed unavoidable disadvantages such as fast past century has also brought about serious problems to the world's Chemistry recombination of electron and holes, slow charge carriers transfer population. Our rapacious consumption of fossil resources at the and high recycling cost. 29, 30 The synthesis of 2D nanostructured exponentially increasing rate has led to global warming and climate materials was comparably complex and required harsh experimental change. On the other hand, the extensive use of these energy sources 31-33 1-3 conditions. Recently, 1D nanostructure has been extensively has also caused much pollution on the environment. This has studied due to its distinctive advantages. Generally, a 1D aroused serious concerns over the need for renewable sources of nanostructure had a high aspect ratio with the diameter ranging from clean energy and degradation of pollutants. For example, upon 1 to 100 nm, and it can be in the tube-, rod-, wire-, fiber- or belt- exposure to sunlight, solar energy can be utilized by photocatalysts shaped. Therefore, 1D TiO 2 nanostructured materials inherited all to degrade organic pollutants, reduce CO 2 into renewable 4-6 the typical features of TiO nanoparticles and displayed a large Materials hydrocarbon solar fuels and produce hydrogen by water splitting. 2 specific surface area which is easy for photogenerated carriers to Besides, supercapacitors, solar cells and lithium ion batteries are 7-9 transfer along the axial direction. In addition, the relative easiness in of useful technology to overcome the shortage of nature resources. its production adds into its advantages. Meanwhile, the relatively These all strategies are promising to solve the environment large specific surface area and chemical stability make it an ideal contamination problems and energy crisis. building block for assembling various surface heterostructures, allowing 1D TiO nanostructured materials to be widely used in Titanium dioxide (TiO 2), since discovered for water photolysis 2 34-37 by Fujishima and Honda in 1972,10 has been paid much attention and photocatalysis, water splitting and nanodevices. However, the 11, 12 widely used in photocatalytic degradation of pollutants, wide application of 1D TiO 2 nanostructured materials in some fields 13,14 15, was limited due to several critical issues. Associated with the wide Journal photocatalytic CO 2 reduction into energy fuels, water splitting, 16 solar cells,17, 18 supercapacitors,19, 20 biomedical devices 21, 22 and band gap, TiO 2 (anatase: 3.2 eV, rutile: 3.0 eV) occupies only 3-5% lithium ion batteries.23, 24 Most initial investigations were focused on of the total solar spectrum. Besides, fast recombination of photogenerated electron-hole pairs also leads to decreased efficiency 0D TiO 2 nanoparticles and it showed excellent performances in This journal is © The Royal Society of Chemistry 2013 J. Name ., 2013, 00 , 1-3 | 1 Journal of Materials Chemistry A Page 2 of 26 ARTICLE Journal Name in the photocatalytic activity. What’s more, a relatively low specific No obvious peak was observed in the XRD pattern of the TiO 2 surface area on a smooth surface and a single crystal phase may pose nanotube arrays (TiO 2 NTAs) before annealing. Conversion of the 38-41 certain constraints for it performaces. Fortunately, 1D TiO 2 amorphous TiO 2 NTAs into anatase began at around 250 °C. The surface heterostructure can be designed and fabricated by relative intensity of the anatase peaks increased with annealing introducing second phases with special morphology (nanoparticles, temperature raising from 250 to 650 °C. When annealed at 650 °C, 42, 43 nanorods, nanowires) or doping with other materials (metals, the TiO 2 NTAs consisted of an anatase and rutile mixed phase, non-metals, semiconductors) 44-46 to overcome the above-mentioned indicating that a part of the anatase phase changed into a rutile phase problems. As a result, light absorption can be broadened from the as the temperature increased. At 850 °C, the peak intensity of rutile UV light to visible light and surface area be increased at the same increased, while the anatase decreased. Besides, the TiO 2 NTAs with time. appropriate ratio of anatase and rutile showed the best photoelectric properties. Mi et al. also proved the mixed phase of anatase and In recent years, great progress has been made in the synthesis, rutile showed higher photocatalytic performance than pure one- 52 modification and applications of 1D TiO 2 nanostructured materials phase crystal structure. Tay et al. synthesized two-phase and other semiconductor oxides. In this review, we first provided a anatase/brookite TiO 2 nanostructures via a simple hydrothermal 53 detailed discussion on the crystal structure of TiO2 and the latest method. In comparison with highly crystalline pure brookite and Manuscript development on the fabrication of 1D TiO 2 nanostructured materials. P25, the two-phase anatase/brookite TiO 2 showed higher hydrogen Then, we will look into some critical structure engineering strategies production activity. that give 1D TiO 2 nanostructured materials excellent properties. Lastly, the environmental and energy applications of 1D TiO 2 Besides anatase, rutile and brookite, there exists another nanostructured materials are summarized. Special focus has been crystal phase of pure TiO 2, TiO 2(B) (Figure 1d), which is usually given to their use as photocatalyst, supercapacitors, solar cells and synthesized by hydrothermal method. Although having the same lithium battery electrode. chemical formula TiO 2, TiO 2(B) has a more open framework structure, a relatively low density and a larger specific capacity than other polymorphs. This has led to growing applications in the fields Accepted of energy conversions, such as photocatalysts, solar cells and 2. Crystal structure and synthesis of 1D TiO 2 lithuium batteries. 54-57 Its space group C2/m composed of only two A nanomaterials TiO 6 octahedra, with adjacent sheets joined together by edge- 2.1 Crystal structure of TiO 2 sharing. Zheng et al. denoted TiO 2(B) as the an intermediate product in the calcination of titanate, following the transformation sequence TiO exists naturally mainly in three crystalline phases: anatase 2 of protonated titanate → TiO (B) → anatase TiO .58 Besides, (tetragonal, space group I41/amd), rutile (tetragonal, space group 2 2 M Ti O (M = H, Li, Na, K etc.) 59-62 is another formula crystal P42/mnm), and brookite (orthorhombic, space group Pbca) (Figure x y x/2+2y structure of nanotubular products from the alkaline hydrothermal 1a-c). In all polymorphs, titanium cations are six-fold coordinated to treatment of TiO . This structure is the intermediate product of TiO oxygen anions, forming distorted TiO octahedra. Anatase have 2 2 6 and its nanobelts, nanowires and nanorods structures tend to have corner sharing with TiO 6 octahedra and rutile joined by sharing the good crystallinity after ion exchange or calcination. Chemistry octahedral edges, while brookite have TiO 6 octahedra joined by sharing both the octahedral corner and the octahedral edges.