TiO2 Nanoparticles as Functional Building Blocks - Chemical Reviews

May 20, 2014 - Since then, she has worked at Beijing University of Technology and Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Mi...
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TiO2 Nanoparticles as Functional Building Blocks Lixia Sang,† Yixin Zhao,‡ and Clemens Burda*,§ †

Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education and Key Laboratory of Heat Transfer and Energy Conversion, Beijing Municipality, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China ‡ School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China § Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States 7.2.1. Improving Charge Separation with Metal NPs 7.2.2. Core−Shell Metal@TiO2 7.2.3. Increasing UV and Visible Light Absorption with Plasmonics NPs 7.3. TiO2−Semiconductor Nanoheterostructures 8. TiO2 NPs as Charge Separation Centers 8.1. Photoinduced Charge Separation in TiO2 NPs 8.2. Charge Extraction for Photocatalytic Redox Reactions 8.2.1. Charge Extraction for Photocatalytic Purification of Water and Air 8.2.2. Charge Extraction for Solar Water Splitting 8.2.3. Charge Extraction for Photocatalytic Reduction of CO2 8.3. Charge Separation and Injection in Grätzeltype Solar Cells 9. Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction: The Development of the TiO2 Nanoparticle Research Field 2. Structural Evolution of Molecule−Cluster−Nanoparticles 3. Crystal Phases and Phase Transformation 4. Synthesis and Characterization of TiO2 NPs 4.1. Novel Aspects to the Synthesis Methods of TiO2 NPs 4.1.1. Chemical Synthesis of Sub-2-nm TiO2 NPs 4.1.2. Low-Temperature Synthesis of Sub-5nm TiO2 Nanoparticles 4.1.3. Nonaqueous Preparation of TiO2 NPs 4.1.4. Controlling Shapes and Crystal Facets of TiO2 4.2. Characterization Approaches 4.2.1. Structural Properties via XRD and Raman Spectroscopy 4.2.2. Optoelectronic Properties via UV-DRS, XPS, EPR, IR, and PL Spectroscopy 4.2.3. Laser Spectroscopic Characterization of Charge Carrier Dynamics 5. TiO2 NPs as Growth Centers: Evolution of TiO2Based Nanomaterials 5.1. One-Dimensional (1-D) Growth 5.2. TiO2 with Higher Organized Architectures 5.3. Breaking One-Dimensional TiO2 Arrays into Zero-Dimensional TiO2 NPs 6. Chemical and Physical Modifications of TiO2 NPs 6.1. TiO2 NPs as Hosts for Metal and Nonmetal Dopants 6.2. Codoping 6.3. Photonic Crystals Constructed from TiO2 NPs 6.4. Ti3+ Self-Doped TiO2 7. TiO2 NP Heteronanostructures 7.1. Mixed Phase TiO2 7.2. TiO2−Metal Nanostructures

© 2014 American Chemical Society

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1. INTRODUCTION: THE DEVELOPMENT OF THE TIO2 NANOPARTICLE RESEARCH FIELD TiO2 has been commercially manufactured by the millions of tons to be widely utilized as pigment, paint additive, and sunscreen to name a few uses, due to its photostability and light dispersion, yet simultaneously strong UV light filtering. TiO2 has also been one of most investigated engineering materials during recent decades, especially in the arena of energy and environmental applications. The momentum of this research and its historical development has been significantly impacted by two milestone research reports in 1972. In 1972, Fujishima and Honda published the finding of photocatalytic splitting of water on a TiO2 electrode under ultraviolet (UV) light.1 The discovery of this phenomenon spurred a tremendous amount of

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Special Issue: 2014 Titanium Dioxide Nanomaterials Received: November 5, 2013 Published: May 20, 2014 9283

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an unusual scientific explosion of this filed since the beginning of this millennium. Accordingly, we focused this Review on work published past the year 2000 and particularly on work within the last 5 years. Astonishing progress has been reported since 2007 warranting a fresh look at the field as a whole. Hereby we attempted to emphasize newer research topics. We start with the structural evolution of TiO2 NPs (section 2), current knowledge about TiO2 phases and phase tranformations at the nanoscale (section 3), and novel insights into the preparation and characterization of TiO2 NPs (section 4). The second half of this review (sections 5−8) will summarize modifications and functionalizations recently undertaken on TiO2 nanocrystals. We review how TiO2 NPs have been used to prepare larger, more complex stuctures (section 5), how chemical and physical modifications were used to enhance the properties of TiO2 (section 6), and the recent use of TiO2 in heteronanostructures (section 7). In section 8 we highlight the use of TiO2 NPs as charge separation centers. A brief outlook in section 9 concludes this review, keeping in mind that more in-depth treatments to more focused topics follow in this Thematic Issue on TiO2.

research related to TiO2 photocatalysis including solar fuels and environmental remediation. Second, and also in 1972, Tributsch demonstrated the idea of a dye-sensitized solar cell (DSSC), fabricating a chlorophyll-sensitized zinc oxide (ZnO) electrode to convert visible light radiation into an electric current by charge injection from the excited dye molecules into the wide band gap metal oxide.2 Although in this first DSSC concept report the author used ZnO, not TiO2, as the wide band gap semiconductor, TiO2 was soon to become the most popular wide band gap semiconductor material for later DSSCs, mainly due to its better photostability. These two reports mark the start of a remarkable development toward the promise of utilizing TiO2 as a photoactive catalyst for solar fuels, photovoltaics, and environmental remediation. These three applications are still now the main research areas for TiO2. In all these applications, TiO2 has been used mainly in three important functions: the first is to work as a photochemical energy conversion material due to their electronic structure in the case of photocatalyts; the second is to function as the photosensitizer substrate due to its surface area and surface stability in photosynthesis applications; and the third is to serve as the electron transport scaffold in photovoltaic applications due to its electrical properties. The research findings derived from these investigations have then been utilized for other advanced applications such as sensing, biomedicine, and electronics.3−11 Future advanced uses of TiO2 clearly depend on the development of more complex materials with advanced functionalities. Materials that are purpose-built to perform specific tasks are the targets of such current research. For example, for photocatalysis TiO2 has the advantage of good photocatalytic activity and photostability and low cost. However, one limitation of TiO2 as a photocatalyst is its wide band gap, making TiO2 only sensitive to the UV light which covers less than 5% of the solar spectrum. To overcome this intrinsic disadvantage, two strategies have been pursued. The first is to broaden the active spectrum of TiO2 by a variety of chemical modifications that adjust its electronic structure. The other strategy, aimed at improving TiO2 utility, is to create high surface area TiO2. In photocatalysis, the photocatalytic conversion scales with the surface area. For the DSSC application, an ideal TiO2 photoelectrode should have a large enough surface area to load sufficient sensitizer dye onto the TiO2 anode for efficient solar energy conversion. The breakthrough in DSSC efficiency was reported by Grätzel and co-workers in 1991, reporting work in which the mesoporous TiO2 NP photoelectrode replaced a bulk TiO2 photolectrode.3 Since then, a tremendous number of variations of nanostructured TiO2 electrodes have been developed to achieve high efficiency DSSCs. In search of ever-improving performance for different applications, nanostructured TiO2 became one of the most investigated solid-state materials of the past 10 years. Zero-dimensional (0-D) TiO2 NPs are the most basic nanostructures of TiO2 suitable for large-scale production. They can be utilized as starting points for more complex materials with more specified and enhanced performance parameters. In the following Review, we show why TiO2 NPs are central for the development of future designer materials involving titania, how they are currently grown and incorporated, and how material properties are affected by TiO2 NPs. While the start of this development was initiated by milestones of research in 19721,2 and 1991,3 there has been

2. STRUCTURAL EVOLUTION OF MOLECULE−CLUSTER−NANOPARTICLES Compared to the bulk crystalline phases of TiO2, nanosized particles are smaller and often composed of strained lattices due to the large surface-to-volume ratio. In addition, mixed phases and interfaces incorporating a number of defects are quite common. Clusters, on the other hand, are composed of far fewer atoms, in a size range that can actually be computed and understood atom by atom. Global minima structures for the discrete (TiO2)n clusters were predicted as stable or low-energy metastable structures.12 Numerous experimental and theoretical studies investigated isolated titanium oxide clusters to correlate their structures and properties with those of the bulk phases.12−16 The TinO2n and TinO2n+1 clusters were found to be the most stable neutral clusters, while TinO2n‑1 and TinO2n‑2 clusters were formed by fragmentation.17 Titanium oxide cluster cations, TinO2n‑m+ (n = 1−8; m = 0−4), were observed by sputtering titanium foil exposed to oxygen, and for n = 1−7 and m = 1−3 by sputtering titanium dioxide powder.18 As initial parameters, the experimental and computational data used for TiO2 molecules are a Ti−O bond length of ∼1.62 Å and a O− Ti−O angle of ∼110°. The structural motifs of the (TiO2)n nanoparticles are found to substantially differ from those of the bulk TiO2 (Figure 1).19 The average Ti−O bonding length in these clusters is smaller than in the bulk. They have a more compact structure because of the high surface-to-bulk ratio. They are not molecular structures anymore and still lack the strict periodicity of the bulk crystals. The changes in the coordination of the atoms, the size and shape of the cluster, and the amount of (TiO2) units all bring about sensitive changes in the electronic structure and the energy gap.20 A spectral blue-shift and effective band gap broadening are observed when the size of the semiconductor particles becomes smaller than the exciton radius of ∼1 nm, resulting in a confined bound state of the hole and electron.21,22 Through density functional theory (DFT) calculation of (TiO2)n clusters (Figure 2), one finds a mixture of O(2p) and Ti(3d) atomic orbitals for all frontier molecular orbitals of the smaller clusters. However, in the larger clusters, there is a clear spatial separation, whereby the HOMO and HOMO-1 are composed of O(2p) orbitals and the LUMO and LUMO+1 are 9284

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dramatically from an extended-semiconducting structure to nanometer-sized small clusters of titanium oxide.25

Figure 1. Relaxed geometries of (TiO2)n clusters (a) (TiO2), (b) (TiO2)2, (c) (TiO2)3, (d) (TiO2)4, (e) (TiO2)5, (f) (TiO2)6, (g) (TiO2)7, (h) (TiO2)10, and (i) (TiO2)13. The purple spheres represent the Ti atoms while the red spheres represent the O atoms.19 Reproduced with permission from ref 19. Copyright 2010 American Chemical Society.

Figure 3. Illustration of the electronic structure changes in titanium oxide when moving from semiconducting bulk TiO2 to isolated Ti− Oxide clusters.25 Reproduced with permission from ref 25. Copyright 2012 Royal Society of Chemistry.

Qu et al.26 have studied the electronic structure and stability of defect-free (TiO2)n NPs with n = 10−16 (similar to and slightly larger in diameter than the 1 nm exciton Bohr radius of titania). Even-n (TiO2)n clusters consist of only 4-coordinated Ti(4) and 2-coordinated O(2−) atoms with compact covalent networks that are more stable, while odd-n clusters tend to form more ionic structures with additional Ti(6), Ti(5), O(3), and O(4) atoms. These theoretical studies suggested that (TiO2)n clusters with odd and small n values would help to engineer the material with visible light photoactivity because the most stable clusters with odd n exhibit small vertical excitation energies. TiO2 heterofullerenes using Ti2O4 units as the basic building blocks have been proposed due to their structural stability and appropriate growth properties.19 These defect-free nanocages display unique frequency modes at ∼827−854 cm−1 and a larger HOMO−LUMO energy gap than bulk TiO2 materials.27 During the synthesis of titanium dioxide nanoparticles, hydrolysis causes the individual TiO2+ or Ti(OH)22+ ions to polymerize via olation and oxolation to (TiO2)n− chains.20 These chains tend to agglomerate and polymerize into small particles of several nanometers. Zhai et al.28 have probed the electronic structure and band gap evolution of titanium dioxide clusters, (TiO2)n− (n = 1−10), using photoelectron spectroscopy (PES) (Figure 4). Electron detachment energies between the ground state of the neutral and its first excited state (band gap) have been shown to be strongly size-dependent for n < 6, and to rapidly approach the bulk limit for n > 7. The broad PES features observed have been attributed to the localized nature of the extra electron in the (TiO2)n− clusters. The extra electron localized in a trivalent Ti of (TiO2)n− (n > 1) clusters creates a single Ti3+ site, which can provide clusters with molecular properties suited for mechanistic studies of TiO2 surface defects and photocatalytic activity.

Figure 2. Calculated frontier molecular orbitals for (TiO2)n with n = 6 (a, b), n = 7 (c, d), n = 10 (e, f), and n = 13 (g, h) clusters. Purple and red spheres represent the titanium and oxygen atoms. Superimposed and semitransparent, HOMO-1 and HOMO orbitals are colored blue and orange, respectively, whereas, in the other panels, green and yellow are chosen for the LUMO and the LUMO+1 orbitals, respectively. In comparison, the HOMO and LUMO orbitals for bulk rutile (not shown) are uniformly distributed over the material, with the HOMO residing solely on the oxygens (2p) and the LUMO on the titanium (3d).19 Reproduced with permission from ref 19. Copyright 2010 American Chemical Society.

composed of Ti(3d) orbitals. Thus, direct excitation from the HOMO to the LUMO would have small oscillation strength. Furthermore, Shevlin et al.19 conclude that the formations of single valent oxygen atoms in subnanometer TiO2 clusters are responsible for the narrowing of the energy gap. For the ground-state structures of the (TiO2)n (n = 1−4) clusters, a common feature among the calculated energy minima is the presence of two bridge oxygen atoms between each pair of adjacent titanium centers.23 In fact, the structures of these small clusters are closer to that of the anatase phase. This observation is consistent with the experimental and theoretical results that the anatase phase is more stable than the rutile phase when the particle size is below ∼14 nm.24 The calculated adiabatic energy gaps are ∼2.2 eV for the monomer, 2.3−3.0 eV for the lowlying structures of the dimer, 2.4−2.6 eV for the trimer, and 2.1−3.5 eV for the tetramer23 Most of these gaps are below the band gap of the bulk material. Therefore, properly designing the particle size and structure is crucial to adjust and optimize the optical and photocatalytic activity of TiO2 nanoparticles. Figure 3 highlights that the electronic properties change

3. CRYSTAL PHASES AND PHASE TRANSFORMATION Titanium dioxide can exist in one of three bulk crystalline forms, rutile, anatase, and brookite, all of which can be described in terms of distorted TiO6 octahedra with different symmetries or arrangements. The anatase structure consists of edge-sharing TiO6 octahedra, while the rutile and the brookite frameworks exhibit both corner and edge-sharing configurations (Figure 5).29 The different characteristics of the Ti−O 9285

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Figure 4. Photoelectron spectra (left) of (TiO2)n− (n = 5, 10) at 193 nm (6.424 eV) and 157 nm (7.866 eV). The 193 nm spectra are shown as insets. Observed energy gaps (right) of (TiO2)n− (n = 1−10), as measured from the adiabatic detachment energy (ADE) difference between the Xand A-bands. The TiO2 bulk limits (rutile, 3.0 eV; anatase, 3.2 eV) are shown as horizontal dashed lines.28 Reproduced with permission from ref 28. Copyright 2007 American Chemical Society.

variations has been used to determine the temperature at the onset of the nucleation process through the whole crystallization process.40 Zhang41,42 investigated the mechanism of the size-dependent outer/inner phase transformation in ultrafine TiO2 particles with narrowed size distribution. It is found that particle size is the critical parameter determining the onset transition temperature and nucleation behavior. From the proposed model (Figure 6), the phase transformations take Figure 5. Representations of the TiO2 anatase, rutile, and brookite forms.29 Anatase (tetragonal, a = 3.785 Å, c = 9.513 Å), rutile (tetragonal, a = 4.593 Å, c = 2.959 Å), and brookite (orthorhombic, a = 9.181 Å, b = 5.455 Å, c = 5.142 Å). Reproduced with permission from ref 29. Copyright 2010 American Chemical Society.

bonds play an important role in the structural and electronic features of the different phases.30 Brookite TiO2 is a more exotic titania polymorph with a layered structure. Diebold et al.31 have comprehensively reviewed the bulk properties of TiO2 with different crystal structures. Phase transformations between different phases of TiO2 have been extensively studied from both scientific and technological points of view.32−34 Rutile is the only stable phase in the bulk form, while bulk brookite and bulk anatase are metastable and transform irreversibly to rutile upon heating.35 However, different phase stability can be expected at the nanoscale structures. At temperatures ranging between 325 and 750 °C, anatase is the most stable phase at particle sizes under 11 nm, brookite is the most stable phase between 11 and 35 nm, and rutile is the most stable phase at all particle sizes above 35 nm.24 The different phase stability in TiO2 nanoparticles is related to their physical environment and the interaction between TiO2 and H2O. Molecular dynamic simulations by Koparde35 show that rutile is the most stable phase for smaller TiO2 NPs at higher temperatures in vacuum. During synthesis in liquid media, the interaction between amorphous and anatase phases can destabilize anatase crystals and favor anatase-to-rutile transformation as the aging of the solution progresses.36 Phase stability and transformation in TiO2 NPs in aqueous solutions can be guided by surface energy.37,38 DFT calculations on different phase TiO2 NPs explain that the stability reversal in the nanoparticle relative to the bulk phase is due to the lower surface energies in anatase compared to that in rutile at the nanoscale.39 Surfaces, edges, and vertices induce a much higher energetic penalty in rutile than in anatase, which are relatively more abundant and can stabilize anatase at the nanoscale. In the nucleation and growth process of TiO2 NPs, conclusive evidence of local and intraparticle ordering

Figure 6. Proposed scheme for the phase transformations of TiO2 with particle size (a) smaller than 10 nm, (b) in the range 10−60 nm, and (c) larger than 60 nm (blue, anatase; red, rutile).41Reproduced with permission from ref 41. Copyright 2006 American Chemical Society.

place at lower temperatures for smaller particles ( 0.44 J/m2 for (101). Due to their high surface-free energy, (001) facets of anatase are generally considered to be more reactive than (101) facets. However, (001) facets usually diminish rapidly during a crystal growth process. Recently, an important breakthrough in the preparation of anatase TiO2 crystals with exposed (001) facets was reported by Lu and co-workers.102 They demonstrated that the (001) facets can be stabilized by the use of hydrofluoric acid as a shape-controlling agent. In Xia’s report,103 TiO2 NCs with truncated tetragonal bipyramidal shape and 9.6% of the surface being exposed by (001) facets were synthesized in high yields by controlling the hydrolysis rate of the sol−gel precursor and hydrothermal treatment. Low pH values tend to eliminate the (001) facets by forming sharp corners while high pH values favor the formation of a rodlike morphology through an oriented attachment mechanism. Changing the relative ratio of titanium precursor and HF during hydrothermal synthesis can lead to different degrees of truncation.104 Highly uniform anatase TiO2 NCs with tailorable morphology, preferentially exposing the (001) facet, can be prepared through a seeded growth technique by using titanium(IV) fluoride (TiF4), which can in situ release hydrofluoric acid (HF) during reaction (Figure 10).105 Highly crystalline TiO2 truncated tetragonal bipyramidal nanocrystals enclosed by (001) and (101) facets were fabricated via a microemulsion method employing fluorine

ions as morphology controlling agents. Chainlike 1-D TiO2 nanostructures built from nanobipyramids were generated via the mechanism of oriented attachment at higher temperatures.106 Also, the percentage of reactive (001) facets of TiO2 NCs can be tuned by the amount of water present in rapid microwave-assisted hydrothermal synthesis. With an increasing amount of water in the synthesis, the shapes of TiO2 NCs change from nanosheets to truncated octahedral bipyramids.107 Yang et al.108reported a new solvothermal method using 2propanol as a synergistic capping agent and reaction medium together with HF to synthesize high-quality anatase TiO2 single-crystal nanosheets with 64% of the (001) facets. Importantly, the percentage of highly reactive (001) facets in rectangular TiO2 nanosheets is very high (up to 89%) with optimal adjustment of the amount of hydrofluoric acid and reaction temperature during the hydrothermal synthesis.109 Such TiO2 nanosheets show excellent photocatalytic efficiency, far exceeding that of commercially available Degussa P25. Unlike anatase and rutile, it is relatively difficult to prepare high-quality brookite TiO2 NCs. Nevertheless, single phase brookite TiO2 NCs were obtained by a one-step hydrothermal treatment of an aqueous titanium complex solution.110 Also, high-quality anisotropically shaped brookite TiO2 NCs can be synthesized using a surfactant-assisted nonaqueous strategy.57 Compared with studies on controlling shapes and crystal facets of anatase and rutile TiO2, very few groups have addressed the crystal facets of brookite TiO2. On the basis of first-principle studies, the (001) facet of brookite TiO2 holds the smallest surface formation energy of 0.62 eV, while (100) exhibits a larger value of 0.88 eV. This calculation reveals that brookite crystals prefer growth along the [001] direction to produce nonspherical morphology.111 Brookite TiO2 with many kinds of morphologies, such as nanorods,52 nanofibers,55 nanotubes,53 and nanoflowers,59,112 have been reported in the literature. For the formation of TiO2 nanoflowers, brookite particles grow to spindle-like shapes, due to the small surface energy, and then these spindle-like particles can assemble into flower-like morphology.59 The preparation conditions of brookite TiO2 including inorganic salts, organic substances, pH value, reaction time, and temperature were investigated.55,113,114 Among them, sodium ions (Na+) play a key role in the formation of brookite TiO2, which may promote the brookite nucleation. Recently, it was reported that anions with different molecular structures are used as absorbents to control the growth direction of TiO2 as shown in Figure 11, resulting in rutile and brookite TiO2 NCs 9289

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crystal grain size according to the Scherrer equation. Crystallite size is determined by measuring the broadening of a particular peak in a diffraction pattern associated with a particular planar reflection within the crystal unit cells. It is inversely related to the full width at half-maximum (fwhm) of an individual peak. The narrower the peak is, the larger the crystallite size will be. However, crystallites of less than 3−4 nm cannot be determined reliably due to the detection limit of XRD. This limitation can be solved by means of Raman spectroscopy.117,118 Similarly, as the size of TiO2 nanomaterials decreases, the featured Raman scattering peaks become broader (Figure 13).119 In general, the XRD peaks at 25.3°, 14.2°, and 27.4° are identified as the characteristic diffraction peaks for anatase, brookite, and rutile TiO2, respectively. From comparing the irreducible representation of the light scattering modes with the crystal phase symmetry, the three phases of anatase, brookite, and rutile have 6(3Eg+ 2B1g + A1g), 36(9A1g + 9B1g + 9B2g + 9B3g), and 4(A1g + B1g + B2g + Eg) Raman active modes, respectively.120 Brookite, either natural or synthetic, shows strong Raman peaks at 128 (A1g), 153 (A1g), 247 (A1g), 322 (B1g), 366 (B2g), and 636 (A1g) cm−1. Anatase exhibits characteristic Raman scattering at 146 (Eg), 396(B1g), 515(A1g), and 641 (Eg) cm−1, while rutile shows typical scattering at 143(Eg), 235 (two-phonon scattering), 447(Eg), and 612 (A1g) cm−1. Besides the extensive use in phase identification of TiO2, Raman spectroscopy serves also as an efficient tool for probing oxygen deficiency of the TiO2 lattice. It is widely observed that increased contents of oxygen vacancies in the crystal structure lead to higher wavenumbers of the anatase Eg mode (146 cm−1) while the wavenumbers of the rutile Eg mode (447 cm−1) are lowered by oxygen vacancies.121 4.2.2. Optoelectronic Properties via UV-DRS, XPS, EPR, IR, and PL Spectroscopy. Fujishima98 reviewed the electronic properties of TiO2 and recognized that the creation of Ti3+ sites is responsible for the electronic conductivity. No evidence for ionic conduction is found in TiO2. Convincing evidence has revealed that the source of electronic conductivity in rutile is Ti3+ that is associated with oxygen vacancies Ov.98 The activation energy for the electronic conductivity is found to be 1.75 eV for unsintered rutile powder and 1.7 eV for sintered rutile powder. There are large differences in the electronic conductivities of rutile versus anatase thin films after reduction by heating in vacuum, which is considered to be due to the different dielectric coefficient and effective electron mass.122

Figure 11. Scheme of anion-assisted crystal form and crystal facet control of TiO2 NCs.115 Reproduced with permission from ref 115. Copyright 2013 American Chemical Society.

as well as anatase TiO2 NCs with different facets (101), (001), and (100). From Figure 12, it can be seen that the reduction ability of different anatase crystal facets can be ranked as (101) > (001) > (100), while the oxidation ability of different facets can be ranked as (101) ≈ (001) ≈ (100). That is, the results based on specific reactions should be analyzed when discussing the crystal-facet-dependent catalytic activities of TiO2 NCs.115 4.2. Characterization Approaches

For nearly all studies on TiO2 NPs as functional building blocks, electron microscopic techniques, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) are the first and foremost characterization approaches to identify the morphology of TiO2 nanostructures and the grain size of TiO2 NPs. As shown in section 4.1, high-resolution TEM (HRTEM) is one of the most powerful and versatile techniques to identify the crystal facets and shapes of TiO2 NCs. With the assistance of energy-dispersive X-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS) complementary techniques, it allows one to reach atomic resolution of crystal lattices and to obtain chemical and electronic information at the subnanometer scale.116 While TEM has made enormous contributions to nanoscience, section 4.2 is focused on the characterization approaches to structural properties, optoelectronic properties, as well as charge carrier dynamics in TiO2 nanostructures. 4.2.1. Structural Properties via XRD and Raman Spectroscopy. XRD is essential in the determination of the crystal structure and the crystallinity, and in estimating the

Figure 12. Photocatalytic performances of TiO2 NCs with different crystal forms and crystal facets in (A) reduction of nitrobenzene to aniline after reaction for 30 min and (B) oxidation of benzyl alcohol to benzaldehyde after reaction for 4 h.115 Rutile TiO2 nanorods (TiO2-R), octahedral anatase TiO2 NCs (TiO2-A-O), truncated octahedral TiO2 NCs (TiO2-A-TO), TiO2 nanosheets (TiO2-A-NS), long-rod shaped anatase TiO2 NCs (TiO2-ALR), short-rod shaped TiO2 NCs (TiO2-A-SR). Reproduced with permission from ref 115. Copyright 2013 American Chemical Society. 9290

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Figure 13. XRD (A) and Raman spectra(B) of TiO2 NPs at various sizes.119 Reproduced with permission from ref 119. Copyright 2012 American Chemical Society.

Figure 14. (a) UV−vis diffusion reflectance spectra and (b) energy dependence of (F(R)*hν)n for brookite TiO2 flowers. Corresponding data for rutile and anatase TiO2 are also shown for comparison.59 Reproduced with permission from ref 59. Copyright 2009 American Chemical Society.

Figure 15. (A) Scheme of UV-induced charge separation in TiO2. Electrons from the valence band can either be trapped (a) by defect states, which are located close to the conduction band (shallow traps), or (b) in the conduction band where they produce absorption in the IR range.135 (B) EPR spectra recorded when nanorutile is irradiated in vacuo at 4 K with broadband radiation. (a) Initial spectrum in the dark (the signal at 325 mT is due to a defect in the quartz sample tube), (b) during irradiation, (c) 15 min after switching off the light, and (d) computer simulation of the Ti3+ signal.133 Reproduced with permission from refs 135 and 133. Copyright 2005 and 2012 Elsevier.

properties. The high-quality brookite flowers presented by Hu and his co-workers59 show a direct transition with a band gap energy of 3.4 ± 0.1 eV, which is larger than those of its two other polymorphs, that is, a direct band gap of 3.0 ± 0.1 eV for rutile and an indirect band gap of 3.2 ± 0.1 eV for anatase (Figure 14). UV−vis spectroscopy of oxygen-deficient TiO2 systems demonstrates the existence of prominent absorption bands in the visible region.98 On the basis of recently demonstrated experimental observations, it is deduced that the spectral features of visible-light-active TiO2 photocatalysts originate from F-type color centers associated with oxygen vacancies and Ti-related color centers.128 X-ray photoelectron spectroscopy (XPS) is a surfacesensitive spectroscopy to measure binding energies for the

The electronic structures of TiO2 have been theoretically investigated using DFT methods.123,124 But the UV−vis spectrum is considered as the most reliable technique to measure the band gaps of TiO2 NPs. In the UV−vis spectrum of TiO2, the absorption peaks at 220−260 and 330−400 nm correspond to the tetrahedral and octahedral coordinate Ti species, respectively.125 It is beneficial to investigate the change in coordination number during the growth of a crystal. The octahedral metal centers are the basic structure unit of both anatase and rutile. As is well-known, the optical properties of TiO2 depend strongly on the type of material (e.g., single crystal, powder) and the synthesis conditions (e.g., calcination atmosphere).126 Zallen127 assigned the natural brookite crystal as an indirect-gap semiconductor by analyzing its optical 9291

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Figure 16. (A) Model for trap state photoluminescence in anatase (left) and rutile (right). Wavy and straight lines indicate nonradiative and radiative transitions, respectively. The PL of anatase is considered to be a combination of both type 1 and type 2 PL involving spatially separated hole and electron traps, respectively, while the rutile PL at ∼840 nm is type 1.139(B) PL development for TiO2 during irradiation in vacuum. The baseline spectrum, obtained using a clean tungsten grid at the sample position, is shown by the blue line.140 Reproduced with permission from refs 139 and 140. Copyright 2008 and 2012 American Chemical Society.

light-emitting properties of the TiO2 allotropes upon UV or Xray excitation (Figure 16A). The total photoluminescence (PL) of anatase involves spatially separated trapped electrons and trapped holes, which are about 0.7−1.6 and 1.8−2.5 eV below the conduction band edge, respectively. The commonly observed luminescence from anatase (nanocrystals, in most cases) is in the visible green region (∼550 nm), and the luminescence is highly sensitive to the surface properties. Rutile, on the other hand, has no visible luminescence but shows a near-IR (NIR) emission (∼800 nm) and is less surface sensitive. Knorr et al.138 found that TiO2 nanocrystalline films containing a small amount of rutile show solvent-dependent relative PL intensities of the anatase and rutile that reveal carrier transport between the two phases. Ellis et al.139 suggested that the variation in PL intensity by the adsorption of charge-donating molecules on an n-type semiconductor could alter the surface band structure of the semiconductor by reducing the depletion width as donor molecules are adsorbed. Yates et al.140 found that the PL intensity of TiO2 at 529.5 nm (2.34 eV) increased with irradiation time as the number of photon-accessible defects increased (Figure 16B). They further concluded that alteration of the surface potential of TiO2 by UV light or adsorbed electron-donor/acceptor molecules results in a change in the depth of the active PL region and in the intensity of the observed PL as a result of the bandbending effect.140 4.2.3. Laser Spectroscopic Characterization of Charge Carrier Dynamics. Time-resolved laser spectroscopic measurements provide a direct and versatile approach for probing the electronic dynamics of nanoparticle-based systems and lead to a better understanding of the rates of carrier injection and competing relaxation pathways at the nanoparticle interface. The optical properties of nanomaterials can readily be studied on time scales of tens of femtoseconds and longer. This allows the monitoring of the electron and hole dynamic processes in real time as they occur. More than 30 years ago, laser-induced photoelectrochemical effects at the TiO2 semiconductor−electrolyte interface could be measured by time-resolved flash techniques.141 Currently, transient absorption (TA) spectroscopy has been used extensively to investigate the dynamics and mechanisms of

individual participating elements. The photoelectrons ejected from Ti ions with Al Kα irradiation have a mean free path of 1− 2 nm, which means that one probes surface atoms on the nanoparticle and about 2−3 atomic layers below the surface. This technique can identify specific oxidation states of the atoms and determine their chemical environments. The valence band (VB) XPS spectra for TiO2 related peaks near 22−24 eV are generally associated with the O 2s region. The region between 3 and 9 eV, attributed to the O 2p orbitals in pure TiO2, is very sensitive to the Ti−Ti and Ti−O distances.129 The surface species and charge-transfer process of TiO2 and modified TiO2 NPs can be identified by the combination of the core level and the VB XPS data.130−132 Electron paramagnetic resonance (EPR) and infrared spectroscopy (IR) are powerful techniques to explore the electronic structure of excited TiO2 NPs.133,134 Localized carriers such as holes trapped at oxygen anions (O−) and electrons trapped at coordinatively unsaturated cations (Ti3+ formation) are accessible to EPR spectroscopy (Figure 15). During continuous UV irradiation, photogenerated electrons in TiO2 NPs get trapped at localized sites, forming paramagnetic Ti3+ centers.133 Similarly, EPR-detectable holes also form upon photogeneration of active O− anions in lattice O2− dianions.135 In contrast, delocalized electrons in the conduction band are EPR silent but can be traced by their IR absorption. Panayotov et al.136 have demonstrated that IR radiation can excite the fraction of electrons that are trapped at shallow donor levels 0.12−0.3 eV below the conduction band minimum. The trapped electrons have a broad IR absorption with a maximum characteristic of the donor level energy. The free conduction band electrons exhibit a broad featureless absorbance that increases exponentially across the entire mid-IR range.136 Since anatase and brookite TiO2 are both indirect-type semiconductors, band gap emission from recombination of conduction band electrons with valence band holes is extremely weak at room temperature. However, localized defect states have been reported to exhibit radiative recombination of trapped electrons and holes.137 The energy of the emitted photon depends thereby on the band structure and defect-site energies in the material. The difference in the crystal structures between anatase and rutile leads to interesting differences in 9292

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Figure 17. (A) Transient absorption bands for trapped holes, trapped electrons, and bulk electrons in TiO2 nanocrystalline film.147 (B) Diagram of a time-resolved IR absorption spectrometer for kinetic measurements of photocatalysts.142 (C) Scheme to explain the trapping and relaxation dynamics of electrons and holes in TiO2 excited at 266 and 355 nm.147 (D) Primary reaction steps in DSSCs.143 Reproduced with permission from refs 142, 143, and 147. Copyright 2009 and 2004 American Chemical Society. Copyright 2003 Elsevier.

conduction-band electrons within the lattice and within pico- to nanoseconds, a deep trap level filling process with time constants of up to microseconds, and much slower decay processes corresponding to the interparticulate carrier transport and deep trapped electron−hole recombinations at time scales less than a microsecond.149 At the dye−TiO2 nanoparticle interfaces, forward electron transfer occurs within femtoseconds to hundreds of picoseconds. The often observed inhomogeneous electron-transfer rates can be interpreted as the interaction between an ensemble of dye molecules with a diverse range of TiO2 NP environments.150 The benefit of transient kinetics measurements is the opportunity to obtain absolute or relative efficiencies of photoinduced processes, such as the electron injection in DSSCs (Figure 17D), which can be evaluated from the quantitative analysis of transient lifetimes. It is found that, in the gold−TiO2 NP system, the electron injection is completed within 50 fs and the electron injection yield reaches 20−50%. The charge recombination decay within 1.5 ns is nonexponential and is strongly dependent on the TiO2 particle diameter.151 On the basis of TA spectroscopy studies, transient absorption anisotropy measurements are developed and used for mechanistic characterization of photoinduced reactions at nanostructured TiO2 surfaces.152 An effective diffusion constant for self-exchange hole transfer is quantified on a minutes time scale, while the anisotropy itself decays on a micro- to millisecond time scale under most experimental conditions.152 Thus, transient absorption anisotropy provides the first direct evidence for lateral self-exchange hole transfer across semiconductor nanocrystallites after excited-state injection. Similar to TA spectroscopy derived from transmission experiments, femtosecond time-resolved diffuse reflectance (TRDR) spectroscopy and time-resolved microwave conductivity (TRMC) were employed under weak excitation conditions to clarify the charge separation and trapping

photoinduced charge transfer occurring in TiO 2 NPs structures,142,143 involving adsorption dynamics of molecules on the TiO2 surface, the driving force for the interfacial electron-transfer reactions, and the electronic interaction between TiO 2 and adsorbates.136,144−146 The transient absorptions of nanocrystalline TiO2 are assigned to three kinds of charge carriers: trapped holes absorbing at 500 nm, trapped electrons absorbing at 800 nm, and bulk electrons with an increasing absorption in the IR region toward longer wavelengths (Figure 17A).147 The absorption of injected electrons in TiO2-based DSSCs appears in the IR wavelength range.144 So, sensitive absorption spectrometers with wide spectral ranges (∼400−3000 nm) are most useful for such studies (Figure 17B).142 For femtosecond (fs) time-resolved TA spectroscopy, the observed dynamics depend on many factors, including some instrumental ones such as the pump light properties. Usually Ti-sapphire lasers in combination with optical parametric amplifiers are in use. Pulse intensity and wavelength, even the repetition rate of the excitation laser pulse train, have a direct effect on the observed relaxation dynamics. The detector materials (Si, InGaAs, or MCT photodetector) are often chosen according to the desired observation wavelength range. As an example, it has been reported that the rate of the charge recombination in nanocrystalline TiO2 films is very sensitive to laser excitation intensity Iex.148 In weak 355 nm excitation, the generated charge carrier density is low enough that the second-order electron−hole recombination processes could be ignored.147 Photoexcited holes are trapped within 100 fs at sites near the surface of the TiO2 NPs. Electrons are first trapped at shallow sites near the surface, equilibrated by migrating over a nanoparticle, and then relax into deeper trapping sites in the bulk with a common time constant of ∼500 ps (Figure 17C).147 The photoinduced carrier relaxation for nanocrystalline TiO2 films consists typically of three kinetic phases, that is, a rapid decay for 9293

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Figure 18. Basis of kinetic models used to fit time-dependent photoluminescence decay data. (A) Simple exponential model with a unique and welldefined barrier height ΔG⧧. (B) Albery model assumes a Gaussian distribution of barrier heights. (C) KWW model is comparable to the Albery model but with an asymmetric distribution. (D) Power-law model has a most-barrier probability that falls off exponentially with increasing energy.170Reproduced with permission from ref 170. Copyright 2012 American Chemical Society.

from the UV-irradiated TiO2 surface to the gas phase was successfully detected.172 It is found that the diffusion time of OH• radicals varies with the type of TiO2 powder and the heat treatments of these powders.172 In summary, in both photocatalytic and photoelectrochemical reaction systems based on TiO2 NPs, controlling the charge carrier dynamics, including hot carrier relaxation, trapping, interfacial carrier transfer, and recombination, is essential for successful energy conversion. Femtosecond laser spectroscopy can provide direct insight into charge carrier dynamics at the nanoscale and provide a basis upon which to fine-tune these optoelectronic systems with a temporal resolution of 100 fs (10−13 s) and better.168 Thus, femtosecond time-resolved laser spectroscopy is key and will pave the way to optimize the energy conversion of novel and complex functional TiO2 nanoarchitectures that are being developed at this point.168

dynamics in TiO2 NPs systems. TRDR spectroscopy has advantages of measuring turbid or powdery samples with strong scattering or bad light transmission. This technique has been utilized to investigate the charge carrier dynamics of differently doped TiO2 NPs and their photocatalysis.153−156 In TRMC, the mobility of photogenerated charge carriers can be probed by the interaction of the mobile charge carriers with microwave radiation. This technique investigates processes of carrier trapping and recombination, exciton annihilation, and quenching in TiO2 NPs. TRMC is particularly useful for studying the properties of TiO2-based solar cells and in dynamics related to photoelectrochemistry.157−167 Time-resolved photoluminescence (TRPL) spectroscopy, another popular femtosecond laser-based technique, can also provide direct insight into the charge carrier dynamics of nanomaterials. TA and PL techniques are used to probe different kinetic pathways in semiconductor NCs. It is necessary to observe the emitting signal in PL measurements. TA pump−probe spectroscopy only measures a low fraction of a few percent PL, while most of the TA signal is from nonradiative pathways, measured as excited state transient absorption.168 TRPL is often used to clarify the dynamics of interfacial charge-transfer emission in photosensitized TiO2 NPs. For example, time-resolved fluorescence and fluorescence anisotropy of molecule−TiO2 nanostructures can be characterized to describe the nature of the charge-transfer excitation using a femtosecond fluorescence upconversion setup.169 It is measured that interfacial charge-transfer emission lifetimes ranged from