Editorial pubs.acs.org/JPCC
TiO2 Nanostructures: Recent Physical Chemistry Advances
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TITANIUM DIOXIDE TiO2 has emerged as one of the most fascinating materials in the modern era. It has succeeded in capturing the attention of physical chemists, physicists, material scientists, and engineers in exploring distinctive semiconducting and catalytic properties. Inertness to chemical environment and long-term photostability have made TiO2 an important component in many practical applications and in commercial products. From drugs to doughnuts, cosmetics to catalysts, paints to pharmaceuticals, and sunscreens to solar cells, TiO2 is used as a desiccant, brightener, or reactive mediator. The U.S. Food and Drug Administration permits up to 1% TiO2 as an inactive ingredient in food products. While there are no known health effects, a recent study found 3−6 year old children are the most affected group of people that consume TiO2 particles from food products.1 Many new properties of TiO2 have been explored during the past few years. This virtual issue (http://pubs.acs. org/page/jpccck/titanium-dioxide.html) highlights a few key physical chemistry advances made with TiO2 nanostructures. Photocatalysis. TiO2 is a large bandgap semiconductor that is commonly investigated in rutile (bandgap 3.0 eV) and anatase (bandgap 3.2 eV) phases. Its response to UV light has led to the emergence of the photocatalysis research field.2−4 It has been investigated extensively for its super hydrophilicity and use in environmental remediation and solar fuel production. Bandgap excitation of TiO2 causes charge separation, followed by scavenging of electrons and holes by surface adsorbed species (Scheme 1A). Alternatively, the photocatalytic activity in the
catalytic response in the visible region. In spite of extensive efforts to dope TiO2 with C, N, S, and transition metal ions, photocatalytic activity in the visible has remained quite low. Recent perspective articles highlight issues and challenges related to photocatalysis.2−4 Solar Cells. Mesoscopic TiO2 film is a major component of dye-sensitized solar cells,5,6 organic photovoltaics,7 and quantum dot sensitized solar cells.8,9 Mesoscopic TiO2 film, which is employed as a substrate in these next-generation solar cells, assists in capturing electrons from an excited sensitizer or quantum dot and transports the electrons to the collecting electrode surface (Scheme 2). A recently developed quantitative Scheme 2. Principle of Operation of Liquid Junction Dye or Quantum Dot Sensitized Solar Cell Using Mesoscopic TiO2 Films
Scheme 1. Photocatalytic Activation Using TiO2 Nanoparticles: (A) Direct Bandgap (UV) Excitation and (B) Sensitized Charge Injection Using a Visible Light Absorber (Adapted From Reference 4)
model explains the dependence of the photovoltaic response of the dye-sensitized solar cell on the nonlinear charge recombination rate (Un = krnβ, with β < 1) and interprets the diffusion length measurements obtained from various techniques.10 Significant strides have been made to achieve efficiencies in the 6−12% range for sensitized liquid junction solar cells. Various spectroscopy studies have been conducted to probe the ultrafast charge injection from an excited sensitizer into TiO2 and charge recombination processes to identify the factors that limit energy conversion efficiency.11,12 Designing Nanostructure Architectures. TiO2 nanostructures of different shapes have been designed to tailor optical and electronic properties. While anodization of Ti foil is a popular technique to prepare TiO2 nanotubes, hydrothermal methods provide convenient routes to prepare 1- and 2dimensional nanostructures.13 The ability to design anatase nanocrystals with the most active [001] facet has enabled researchers to systematically probe the catalytic activity.14 Semiconductor−metal composite nanostructures in core− shell or coupled geometry provide a convenient way to enhance the photocatalytic activity by facilitating reduction processes.15 Electron storage, as well as localized plasmon resonance of
visible can be extended by coupling with a sensitizing dye or short bandgap semiconductor (Scheme 1B). By controlling the surface treatment and medium conditions, it is possible to tune the photocatalytic properties for desired applications. TiO2 is very effective for mineralization of contaminants from air and is used in commercial products such as self-cleaning glass and ceramic tiles. On the other hand, practical use of TiO2 for remediation of chemical contaminants from wastewater remains a challenge because of adverse catalyst poisoning effects. Challenges remain to extend the photo© 2012 American Chemical Society
Published: June 7, 2012 11849
dx.doi.org/10.1021/jp305026h | J. Phys. Chem. C 2012, 116, 11849−11851
The Journal of Physical Chemistry C
Editorial
(2) Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655− 2661, DOI: 10.1021/jz1007966. (3) Teoh, W. Y.; Scott, J. A.; Amal, R. Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors. J. Phys. Chem. Lett. 2012, 3, 629− 639, DOI: 10.1021/jz3000646. (4) Kamat, P. V. Manipulation of Charge Transfer Across Semiconductor Interface. A Criterion that Cannot be Ignored in Photocatalyst Design. J. Phys. Chem. Lett. 2012, 3, 663−672, DOI: 10.1021/jz201629p. (5) Peter, L. M. The Gratzel Cell: Where Next? J. Phys. Chem. Lett. 2011, 2, 1861−1867, DOI: 10.1021/jz200668q. (6) Miyasaka, T. Toward Printable Sensitized Mesoscopic Solar Cells: Light-Harvesting Management with Thin TiO2 Films. J. Phys. Chem. Lett. 2011, 2, 262−269, DOI: 10.1021/jz101424p. (7) Ratcliff, E. L.; Zacher, B.; Armstrong, N. R. Selective Interlayers and Contacts in Organic Photovoltaic Cells. J. Phys. Chem. Lett. 2011, 2, 1337−1350, DOI: 10.1021/jz2002259. (8) Mora-Sero, I.; Bisquert, J. Breakthroughs in the Development of Semiconductor-Sensitized Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3046−3052, DOI: 10.1021/jz100863b. (9) Braga, A.; Gimenez, S.; Concina, I.; Vomiero, A.; Mora-Sero, I. Panchromatic Sensitized Solar Cells Based on Metal Sulfide Quantum Dots Grown Directly on Nanostructured TiO2 Electrodes. J. Phys. Chem. Lett. 2011, 2, 454−460, DOI: 10.1021/jz2000112. (10) Bisquert, J.; Mora-Sero, I. Simulation of Steady-State Characteristics of Dye-Sensitized Solar Cells and the Interpretation of the Diffusion Length. J. Phys. Chem. Lett. 2010, 1, 450−456, DOI: 10.1021/jz900297b. (11) Anderson, A. Y.; Barnes, P. R. F.; Durrant, J. R.; O’Regan, B. C. Quantifying Regeneration in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2011, 115, 2439−2447, DOI: 10.1021/jp1101048. (12) Pijpers, J. J. H.; Ulbricht, R.; Derossi, S.; Reek, J. N. H.; Bonn, M. Picosecond Electron Injection Dynamics in Dye-Sensitized Oxides in the Presence of Electrolyte. J. Phys. Chem. C 2011, 115, 2578−2584, DOI: 10.1021/jp1104246. (13) Mowbray, D. J.; Martinez, J. I.; Calle-Vallejo, F.; Rossmeisl, J.; Thygesen, K. S.; Jacobsen, K. W.; Norskov, J. K. Trends in Metal Oxide Stability for Nanorods, Nanotubes, and Surfaces. J. Phys. Chem. C 2011, 115, 2244−2252, DOI: 10.1021/jp110489u. (14) Fang, W. Q.; Gong, X. Q.; Yang, H. G. On the Unusual Properties of Anatase TiO2 Exposed by Highly Reactive Facets. J. Phys. Chem. Lett. 2011, 2, 725−734, DOI: 10.1021/jz200117r. (15) Yu, J. G.; Hai, Y.; Cheng, B. Enhanced Photocatalytic HProduction Activity of TiO2 by Ni(OH)2 Cluster Modification. J. Phys. Chem. C 2011, 115, 4953−4958, DOI: 10.1021/jp111562d. (16) Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. Plasmon-Assisted Photocurrent Generation from Visible to NearInfrared Wavelength using a Au-Nanorods/TiO2 Electrode. J. Phys. Chem. Lett. 2010, 1, 2031−2036, DOI: 10.1021/jz1006675. (17) Meekins, B. H.; Kamat, P. V. Role of Water Oxidation Catalyst, IrO2 in Shuttling Photogenerated Holes Across TiO2 Interface. J. Phys. Chem. Lett. 2011, 2, 2304−2310, DOI: 10.1021/jz200852m. (18) Tang, Z. R.; Li, F.; Zhang, Y. H.; Fu, X. Z.; Xu, Y. J. Composites of Titanate Nanotube and Carbon Nanotube as Photocatalyst with High Mineralization Ratio for Gas-Phase Degradation of Volatile Aromatic Pollutant. J. Phys. Chem. C 2011, 115, 7880−7886, DOI: 10.1021/jp1115838. (19) Ng, Y. H.; Lightcap, I. V.; Goodwin, K.; Matsumura, M.; Kamat, P. V. To What Extent Do Graphene Scaffolds Improve the Photovoltaic and Photocatalytic Response of TiO2 Nanostructured Films? J. Phys. Chem. Lett. 2010, 1, 2222−2227, DOI: 10.1021/ jz100728z. (20) Bell, N. J.; Yun, H. N.; Du, A. J.; Coster, H.; Smith, S. C.; Amal, R. Understanding the Enhancement in Photoelectrochemical Properties of Photocatalytically Prepared TiO2-Reduced Graphene Oxide Composite. J. Phys. Chem. C 2011, 115, 6004−6009, DOI: 10.1021/ jp1113575.
metal nanoparticles embedded in the TiO2 core or placed in the close proximity of the TiO2 nanostructure, have been shown to improve photocatalytic and photovoltaic performances.16 To date, only limited spectroscopic studies have been conducted to establish a hole transfer process aided by an oxidation catalyst such as IrO2.17 A better understanding of interfacial charge transfer kinetics is important in designing more efficient photocatalysts for water splitting reactions.4 The development of carbon nanostructure composites is another emerging area that has led to the design of photocatalysts with tailored properties.18 In particular, graphene oxide-based TiO2 composites have been shown to enhance photocatalytic and photoelectrochemical properties.19,20 Graphene’s ability to capture and shuttle electrons provides a unique opportunity to develop catalyst mats with multifunctional characteristics (Scheme 3).21 Scheme 3. Illustration of Selective Oxidation and Reduction Processes in a TiO2−Graphene Oxide−Metal Nanoparticle Composite (Adapted from Reference 21)
Surface Studies. Photon-stimulated reactions of O2 on the [110] surface of reduced rutile TiO2 have enabled researchers to explore the diverse photochemistry of adsorbed oxygen species.22 The storage of electrons in rutile TiO2, which results from trapping at interstitial Ti species or oxygen vacancies (Ti3+ sites), dictates the overall photocatalytic properties.23 Theoretical studies using density functional theory (DFT) have been applied to probe interactions of surface-to-adsorbed species and obtain a better understanding of the surface binding aspects and reactivity.24 TiO2 has been extensively studied worldwide, resulting in more than 13 600 publications in 2010−2011 (Source: Thomson Scientific Web of Science). During this same period, the Journals of Physical Chemistry A/B/C and Letters published more than 750 papers on this topic, thus showing JPC’s preeminence in disseminating TiO2-related scientific advances. Opportunities for TiO2 now lie in more challenging areas, such as energy conversion and storage. TiO2 nanostructures have recently shown promise in designing Li-ion and Na-ion storage batteries.25 Shape- and size-controlled TiO2 nanostructures will continue to provide the base architecture to construct lightharvesting assemblies and facilitate photoinduced charge separation processes.
Prashant V. Kamat, Deputy Editor
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The Journal of Physical Chemistry Letters, University of Notre Dame, Notre Dame, Indiana 46556, United States
REFERENCES
(1) Weir, A.; Westerhoff, P.; Fabricius, L.; Hristovski, K.; von Goetz, N. Titanium Dioxide Nanoparticles in Food and Personal Care Products. Environ. Sci. Technol. 2012, 46, 2242−2250, DOI: 10.1021/ es204168d. 11850
dx.doi.org/10.1021/jp305026h | J. Phys. Chem. C 2012, 116, 11849−11851
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Editorial
(21) Kamat, P. V. Graphene-Based Nanoarchitectures. Anchoring Semiconductor and Metal Nanoparticles on a Two-Dimensional Carbon Support. J. Phys. Chem. Lett. 2010, 1, 520−527, DOI: 10.1021/ jz900265j. (22) Petrik, N. G.; Kimmel, G. A. Electron- and Hole-Mediated Reactions in UV-Irradiated O2 Adsorbed on Reduced Rutile TiO2(110). J. Phys. Chem. C 2011, 115, 152−164, DOI: 10.1021/ jp108909p. (23) Chretien, S.; Metiu, H. Electronic Structure of Partially Reduced Rutile TiO2(110) Surface: Where Are the Unpaired Electrons Located? J. Phys. Chem. C 2011, 115, 4696−4705, DOI: 10.1021/ jp111209a. (24) Deskins, N. A.; Rousseau, R.; Dupuis, M. Defining the Role of Excess Electrons in the Surface Chemistry of TiO2. J. Phys. Chem. C 2010, 114, 5891−5897, DOI: 10.1021/jp101155t. (25) Xiong, H.; Slater, M. D.; Balasubramanian, M.; Johnson, C. S.; Rajh, T. Amorphous TiO2 Nanotube Anode for Rechargeable Sodium Ion Batteries. J. Phys. Chem. Lett. 2011, 2, 2560−2565, DOI: 10.1021/ jz2012066.
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dx.doi.org/10.1021/jp305026h | J. Phys. Chem. C 2012, 116, 11849−11851
