Editorial pubs.acs.org/CR
Introduction: Titanium Dioxide (TiO2) Nanomaterials
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optical, and electronic properties, the property modifications, and the applications of TiO2 nanotubes in optoelectronic, electrical, and biomedical fields.13 Two-dimensional TiO2 nanosheets are reviewed by Wang and Sasaki.14 They summarize the synthesis, characterization, and properties of TiO2 nanosheets, discuss the strategies for designing complex structures based on these nanosheets, and finally present an overview of the various photochemical, electrochemical, dielectric, catalytic, and biomedical applications.14 Three-dimensional materials are next, and Fattakhova-Rohlfing, Zaleska, and Bein present a thorough review of the synthesis of different 3-D porous TiO 2 nanostructures,15 where they detail the synthetic approaches for fabricating various nanostructured porous films, porous spheres, hierarchical hollow spheres, porous fibers, and 3D structures.15 An important theme in the recent literature on nano- and microcrystalline TiO2 has been the synthesis of TiO2 crystals with tailored facets. This is the subject of the review article by Liu et al.,16 which summarizes the basic synthetic strategies for obtaining anatase, rutile, and brookite TiO2 crystals with specific facets and describes the unique structural, electronic, adsorption, and diffusion properties of the different facets.16 The next five articles examine theoretical and experimental studies on the fundamental physical and chemical properties of various TiO2 nanomaterials. As a reference, bulk materials are in some cases considered as well. In the first article of this group, Zhang and Banfield discuss the structural, mechanical, and thermodynamic properties of TiO2 nanomaterials,17 including different crystalline phases, lattice contraction/expansion and dislocations, and the phase stability of TiO2 nanoparticles.17 Next, Coppens, Chen, and Trzop present a review of the structures and properties of polyoxotitanate TiO2 nanoclusters, including the attachment of chromophores, and the frameworks formed by these nanoclusters.18 The following review, by Kapilashrami et al., is on the optical and electronic properties of TiO2 nanoparticles,19 and includes discussions of the effects of doping and heterostructure formation, the correlation between the crystal facets and the optical properties of TiO2 nanomaterials, and the utilization of X-ray spectroscopies.19 In the fourth article, De Angelis et al. present an overview of theoretical studies on TiO2 bulk, surfaces, and nanomaterials.20 They discuss the electronic properties of bulk anatase, the structure and reactivity of anatase surfaces, and the modeling of bare and sensitized TiO2 nanoparticles, nanosheets, and nanotubes.20 In the last article of this group, Bourikas, Kordulis, and Lycourghiotis summarize the surface and liquid−solid interface chemistry of anatase and rutile TiO2 and the deposition of supported catalysts.21 The following three articles review the property modifications brought about by doping, compositing, and self-structural alteration. Doping of TiO2 with nonmetal atoms, especially nitrogen, has attracted significant interest in the past decade as a promising way to reduce the TiO2 absorption threshold from the
itanium dioxide (TiO2) nanomaterials are known for their numerous and diverse applications, which range from common products, such as sunscreens, to advanced devices, such as photovoltaic cells, and include, among others, a series of environmental and biomedical applications, such as photocatalytic degradation of pollutants, water purification, biosensing, and drug delivery. The importance and variety of these applications have spurred enormous interest and substantial advances in the fabrication, characterization, and fundamental understanding of TiO2 nanomaterials in the past decades.1−9 The primary aim of this thematic issue is to present a comprehensive and up-to-date overview of TiO2 nanomaterials, which can act both as an introduction for newcomers in this field and as a valuable reference for experienced researchers at the forefront of this area of research. Although a large number of review articles on this topic have already been published in the last years,2−9 we believe this thematic issue is unique in its scope. With as many as 21 critical summaries of the most recent experimental and theoretical findings by leading experts in the field, this issue provides an exhaustive picture of TiO2 nanomaterials, ranging from their synthesis to properties, property modifications, and applications. It seems reasonable to expect that it will stimulate new ideas and developments in this and related areas of research as well. Synthetic methods have a central role in the development of new materials. Accordingly, the first seven articles of this issue present an overview of the progress in the synthesis of TiO2 nanomaterials. These articles are organized based mainly on the dimensions (0 to 3 dimensions) of the nanomaterials, where zero dimension refers to nearly spherical nanoparticles, one dimension is for nanowires, nanorods, nanobelts, and nanotubes, two dimensions for nanosheets, and three dimensions for porous nanostructures. Combined together, these seven review articles provide a comprehensive picture of the synthesis of 0-, 1-, 2-, and 3-dimensional TiO2 nanomaterials and their more complex nanostructures. In the first article, Sang, Zhao, and Burda present a general overview of zero-dimensional TiO2 nanoparticles, ranging from synthetic methods and property characterization to charge separation characteristics.10 In the following article, Cargnello, Gordon, and Murray focus on the solution-phase synthesis of TiO2 nanoparticles.11 They lay out the general synthesis principles and discuss the impact of various titanium precursors and the commonly seen aqueous, nonaqueous, and templated approaches for making TiO2 nanoparticles.11 Turning next to one-dimensional TiO2 structures, Wang et al. consider nanowires, nanorods, and nanobelts.12 They summarize the various growth mechanisms, such as vapor−liquid−solid, oriented attachment, and surface reaction-limited growth, review the different solution and vapor-based synthetic methods, and present a brief overview of the properties and applications of these nanostructures, e.g. in photovoltaics and electrical energy storage.12 In the following article, Lee, Mazare, and Schmuki examine another important example of one-dimensional structures, i.e., TiO2 nanotubes.13 They discuss the growth techniques, the assembly into ordered arrays, the structural, © 2014 American Chemical Society
Special Issue: 2014 Titanium Dioxide Nanomaterials Published: October 8, 2014 9281
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Chemical Reviews
Editorial
UV to the visible region. In the first articles of this group, Asahi et al. present an overview on nitrogen-doped TiO2 nanomaterials,22 which summarizes the design strategies for visible-light absorption, the synthesis and properties of nitrogen-doped TiO2 nanomaterials, and their applications in textile, interior, air, and water purification.22 In the following article, Dahl, Liu, and Yin discuss another hot topic, notably property modification of TiO2 nanomaterials by formation of composites with other materials, e.g. metals, metal oxides, nonoxide semiconductors, and nanostructured carbon materials.23 One of the advantages of these composites is the formation of heterojunctions between TiO2 and the other materials which favor separation of photoexcited carriers.23 In the third article of this group, Liu and Chen summarize recent work on the property changes induced by so-called self-structural modifications,2 which include effects due to the surface, lattice strain, defects, nanoscale size, synergistic effects between mixed phases, and lattice deformations in amorphous and hydrogenated phases.24 They emphasize the dramatic property changes in the hydrogenated phases and the related physical and chemical characteristics.24 The last six articles of this thematic issue are devoted to various applications of TiO2 nanomaterials, from the more established ones, such as photocatalysis, to the more recent ones, such as biosensing. First, Schneider et al. examine the environmental applications of TiO2 based photocatalysis.25 They summarize time-resolved analysis of the photocatalytic process, the synthesis and properties of doped TiO2 nanoparticles, visible-light responsive thin films, and photoinduced surface wettability changes.25 Next, Ma et al. focus on the application of TiO2 nanomaterials to photocatalytic fuel generation.26 They describe the different aspects of photocatalytic hydrogen generation from water splitting and biomass reforming, summarize the progress in photocatalytic reduction of CO2 into fuels on various TiO2 nanomaterials, along with the impacts of reaction conditions and the reaction mechanisms.26 In the following article, Liu et al. present an overview of bioinspired TiO2 nanomaterials with special wettability.27 They start with fundamental theories and next describe superhydrophilic, superhydrophobic. and superoleophobic TiO2 surfaces and their applications as antibacterial, anticorrosion, antifogging, biomedical, self-cleaning, water condensation, etc., surfaces.27 The photovoltaic applications of TiO2 nanomaterials are the subject of the following review, by Bai et al.,28 which includes discussions of the fundamental principles, organic−inorganic interactions, electron transport and recombination, and surface treatments in various types of solar cells.28 In the following paper, Bai and Zhou review the use of TiO2 nanomaterials for sensor applications, notably gas sensors, chemical oxygen demand (COD) sensors, and biosensors.29 Finally, in the last article of this thematic issue, Rajh et al. provide an overview of the relatively new area of biomedical applications of TiO 2 nanomaterials. 30 Discussed topics include the importance of surface sites, nanoparticle redox active centers, reactive oxygen species, phototoxicity, sensitization of TiO2, TiO2−DNA hybrid, TiO2−protein hybrid, drug delivery, and imaging guided therapy.30 In summary, with 21 outstanding review articles from leading experts, this issue provides an exhaustive overview of the state-ofthe-art research on TiO2 nanomaterials that can be useful both as an introduction for newcomers and as a reference for experienced researchers in this field. We hope that this issue will inspire further exciting developments in TiO2 nanomaterials as well.
Department of Chemistry, University of MissouriKansas City
Annabella Selloni
Department of Chemistry, Princeton University
AUTHOR INFORMATION Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
REFERENCES (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. Rev. 1995, 95, 735. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (4) Thompson, T. L.; Yates, J. T., Jr. Chem. Rev. 2006, 106, 4428. (5) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891. (6) Chen, H.; Nanayakkara, C. E.; Grassian, V. H. Chem. Rev. 2012, 112, 5919. (7) Zhang, Z.; Yates, J. T., Jr. Chem. Rev. 2012, 112, 5520. (8) Henderson, M. A.; Lyubinetsky, I. Chem. Rev. 2013, 113, 4428. (9) Pang, C. L.; Lindsay, R.; Thornton, G. Chem. Rev. 2013, 113, 3887. (10) Sang, L.; Zhao, Y.; Burda, C. Chem. Rev. 2014, dx.doi.org/10. 1021/cr400629p. (11) Cargnello, M.; Gordon, T. R.; Murray, C. B. Chem. Rev. 2014, dx. doi.org/10.1021/cr500170p. (12) Wang, X.; Li, Z.; Shi, J.; Yu, Y. Chem. Rev. 2014, dx.doi.org/10. 1021/cr400633s. (13) Lee, K.; Mazare, A.; Schmuki, P. Chem. Rev. 2014, dx.doi.org/10. 1021/cr500061m. (14) Wang, L.; Sasaki, T. Chem. Rev. 2014, dx.doi.org/10.1021/ cr400627u. (15) Fattakhova-Rohlfing, D.; Zaleska, A.; Bein, T. Chem. Rev. 2014, dx.doi.org/10.1021/cr500201c. (16) Liu, G.; Yang, H. G.; Pan, J.; Yang, Y. Q.; Lu, G. Q. (Max); Cheng, H.-M. Chem. Rev. 2014, dx.doi.org/10.1021/cr400621z. (17) Zhang, H.; Banfield, J. F. Chem. Rev. 2014, dx.doi.org/10.1021/ cr500072j. (18) Coppens, P.; Chen, Y.; Trzop, E. Chem. Rev. 2014, dx.doi.org/10. 1021/cr400724e. (19) Kapilashrami, M.; Zhang, Y.; Liu, Y.-S.; Hagfeldt, A.; Guo, J. Chem. Rev. 2014, dx.doi.org/10.1021/cr5000893. (20) De Angelis, F.; Di Valentin, C.; Fantacci, S.; Vittadini, A.; Selloni, A. Chem. Rev. 2014, dx.doi.org/10.1021/cr500055q. (21) Bourikas, K.; Kordulis, C.; Lycourghiotis, A. Chem. Rev. 2014, dx. doi.org/10.1021/cr300230q. (22) Asahi, R.; Morikawa, T.; Irie, H.; Ohwaki, T. Chem. Rev. 2014, dx. doi.org/10.1021/cr5000738. (23) Dahl, M.; Liu, Y.; Yin, Y. Chem. Rev. 2014, dx.doi.org/10.1021/ cr400634p. (24) Liu, L.; Chen, X. Chem. Rev. 2014, dx.doi.org/10.1021/ cr400624r. (25) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Chem. Rev. 2014, dx.doi.org/10.1021/ cr5001892. (26) Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Chem. Rev. 2014, dx.doi.org/10.1021/cr500008u. (27) Liu, K.; Cao, M.; Fujishima, A.; Jiang, L. Chem. Rev. 2014, dx.doi. org/10.1021/cr4006796. (28) Bai, Y.; Mora-Seró, I.; De Angelis, F.; Bisquert, J.; Wang, P. Chem. Rev. 2014, dx.doi.org/10.1021/cr400606n. (29) Bai, J.; Zhou, B. Chem. Rev. 2014, dx.doi.org/10.1021/cr400625j. (30) Rajh, T.; Dimitrijevic, N. M.; Bissonnette, M.; Koritarov, T.; Konda, V. Chem. Rev. 2014, dx.doi.org/10.1021/cr500029g.
Xiaobo Chen*
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