Review pubs.acs.org/CR
Probing the Optical Property and Electronic Structure of TiO2 Nanomaterials for Renewable Energy Applications Mukes Kapilashrami,† Yanfeng Zhang,‡ Yi-Sheng Liu,† Anders Hagfeldt,§ and Jinghua Guo*,†,∥ †
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang, Hebei 050024, People’s Republic of China § Physical Chemistry, Department of Chemistry−Ångström Laboratory, Uppsala University, 751 20 Uppsala, Sweden ∥ Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States ‡
Author Information Corresponding Author Notes Biographies Acknowledgments References
1. INTRODUCTION The world’s energy consumption is predicted to grow by 56% by the year 2040, from 520 quadrillion Btu p.a. in 2010 to 820 quadrillion Btu/p.a., as reported by the U.S. Energy Information Administration in 2013.1 Our continuously growing energy demands have been of major concern to the scientific community, industry, and commonalty in the past few decades. Nonetheless, joint forces driven by alliances between academia, government, and industries have resulted in many technological advances for the production of green energy and decomposition of toxic elements from the environment due to the surge to address the most pressing energy-related concerns.2−9 Although renewable energy and nuclear power are today the fastest growing energy sources, with an annual growth rate of 2.5%, fossil fuels are for economic reasons still the preeminent energy source that meets ca. 80% of the world’s energy supply.1 Reflecting back on the scientific advances in the quest for clean and sustainable environmentally friendly energy, metal oxide compounds at reduced scale (e.g., thin films, nanorods, nanoparticles, and quantum dots) have established their prominence in the field as they are readily stable and cost efficient and their morphological and electrical characteristics are tunable by varying the synthesis route and conditions, i.e., properties carrying both technical and monetary importance.10−30 One such oxide compound is the IV−VI titanium dioxide (TiO2), which in nature exists in one of its three polymorphs, rutile, anatase, and brookite, with a band gap (Eg) ranging from ca. 3.0 to ca. 3.2 eV (i.e., from ca. 413 to ca. 387 nm); hence, it is mainly photosensitive in the ultraviolet (UV) wavelength range.31,32 The band gap of TiO2 does not strictly depend on its crystal phase but is also strongly controlled by its morphology and the presence of impurity elements in the host oxide lattice
CONTENTS 1. Introduction 2. Light−Matter-Related Applications Including TiO2 2.1. TiO2 as a Photocatalyst 2.2. TiO2 in Photovoltaics 3. Correlation between Phase, Surface, and Photoelectric Properties 4. Optical and Electronic Properties of TiO2 4.1. Survey through Recent and Past Theoretical Work on the Band Theory of TiO2 4.2. Evolution of the Electronic Structure upon Introduction of Impurity Elements into the Host Oxide Lattice 4.3. Scalability of the Electronic Properties of Intrinsic and Doped TiO2 4.4. Hybrid-Structure TiO2 for Enhanced Photoelectric Activity 5. Overview of Recent Progress in the Tailoring of the Optical Properties of TiO2 5.1. Introduction of Impurity Elements into the Host TiO2 Lattice 5.1.1. Metal Doping 5.1.2. Nonmetal Doping 5.1.3. Codoping 5.2. Heterostructure TiO2 5.3. Dye Sensitization for Enhanced Surface Characteristics 5.4. Correlation between the Orientation of Crystal Facets and Optical Properties 6. X-ray Spectroscopy on the Band Structure of TiO2 6.1. Soft X-ray Spectroscopy Enabling Characterization at the Atomistic Level 6.2. General Spectroscopic Features of TiO2 6.3. Multilayer Architecture and Interfacial Electronic Structure 7. Conclusions and Outlook © 2014 American Chemical Society
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considered to intensify the photoelectric effect is the prohibition of recombination of the photoexcited electrons with their associated holes.49 The photoelectrochemical property of TiO2 was demonstrated by Fujishuma and Honda already in the early 1970s.50 In this early study an electrochemical cell was constructed and comprised of TiO2 and Pt electrodes (immersed in water) interconnected through an external load. Upon irradiation of the oxide electrode, a current flow was measured from the metal electrode to the external circuit (i). By monitoring the direction of the current flow, it was concluded that oxidation occurred at the oxide electrode (ii) and reduction at the metal electrode (iii), reflecting oxygen and hydrogen evolution, respectively (as indicated in eq 1). Thus, it was suggested that water could plausibly be decomposed by visible light into oxygen and hydrogen without the application of an external bias.50
substituting for either the anions or the cations. This emphasizes the importance of tailoring the intrinsic properties and chemical stoichiometry for optimum performance in any light−matter-related application. Similar to ZnO (which is also an industrially important material with a direct band gap near the UV range of ca. 3.37 eV and an exciton binding energy of 60 meV at room temperature, which is almost twice that for GaN), TiO2 is a very versatile and well-integrated semiconductor in our everyday life, with applications ranging from cosmetics to state-of-the-art electronics.21,30,31,33−48 One important feature of TiO2 which has led to a huge surge of interest in this material (Figure 1) is its intrinsic photoelectric property (and its tunabillity), which is the main ingredient in any light−matter-related application, e.g., photoelectrochemistry.
TiO2 + 2hν → 2e− + 2p+
(i)
2p+ + H 2O → 1 2 O2 + 2H+ (ii) 2e− + 2H+ → H 2
(iii)
(1)
These findings opened new avenues for (i) production of solar hydrogen, (ii) extermination and purification of pollutants in the environment via a process known as photocatalysis using TiO2, and (iii) new means to produce electrical energy from solar energy (photovoltaics).32,51−62 All of these applications are considerably economically and environmentally important and rely on the light-induced charge that is used to either directly generate electricity or induce a chemical reaction. Another interesting and prospective wide band gap material for advanced energy applications is MnO, which is an air-stable and earth-abundant intermediate Mott−Hubbard/charge transfer (MH/CT) insulator,63 with energy band edges favorably positioned with respect to both the reduction and oxidation potentials for production of water hydrogen and reduction of CO2, respectively.64 Although intrinsic MnO has a band gap of ca. 3.8 eV (i.e., mainly responding to light in the UV wavelength range), its reduction to ca. 2.6 eV upon alloying with, e.g., Zn has previously been demonstrated by Kanan and Carter as described in ref 65. Complexes of MnO offer many interesting properties suitable for photoelectric applications and are commonly included in the tailoring of the optical properties of TiO2.66−69 Progress in the current field would not have seen the light of day if it were not for all the engineers and scientists that have contributed toward the development and optimization of novel and efficient synthesis techniques as well as the development of state-of-the-art experimental capabilities. This has enabled us to both grow samples at atomistic scale and further perform in situ reaction/analysis emanating fingerprints of matter at the same with high precision. As a matter of fact, the ability to, e.g., scale down to particles with diameters of just a few nanometers has proved to be the fuel behind the progress in conversion of solar energy into any other applicable means of energy or other environmental benefits. Ultraviolet−visible (UV−vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and the photoluminescence (PL) technique are among the most widely employed methods to probe the electronic structure and optical and chemical properties in matter related to photoelectric applications.
Figure 1. Overview of the increasing interest in TiO2 in photocatalytic-related applications over the course of 25 years as indicated by the increasing number of publications on the topic. Reprinted with permission from ref 9. Copyright 2013 InTech.
The photoelectric property simply refers to photon (light)induced excitation of electrons in the occupied valence band (VB) to the unoccupied conduction band (CB), wherein the efficiency by which charge excitation takes place depends on the energy of the incident photons with respect to the band gap of the material (Figure 2). Besides tuning the size of the band gap to enhance photosensitivity, another important aspect to be
Figure 2. Schematic presentation of the typical light-induced excitation of electrons in the valence band into the unoccupied conduction band by absorption photons with energy greater than Eg. 9663
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emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS) are typical synchrotron techniques that are now more commonly used in the field of materials science.79−82 XAS and XES measure the unoccupied and occupied electronic structure and thus the band gap through the conduction band minimum (CBM) determined from XAS and valence band maximum (VBM) determined from XES.83−89 RIXS measures the d−d excitations in transition metals and charge transfer processes in energy conversion and storage materials.90−96 In situ photon-in/photon-out soft X-ray spectroscopy allows us to estimate the band gap and charge transfer in the presence of liquids or reactive gases at a pressure up to 1 bar with both elemental and chemical-state selectivity.97−106 A detailed description of the synchrotron-based X-ray spectroscopy techniques and their importance/application in materials research for energy application follows in section 6. Clearly, synchrotron-radiation-based XPS offers better energy-resolution measurements of core levels as compared to conventional laboratory-based instruments.107−111 In addition, angle-resolved photoelectron spectroscopy (ARPES) is a powerful technique for band structure studies. However, since we are not able to include it in the scope of this review, readers are encouraged to view some of the references on ARPES listed herein.112−115 Many comprehensive review papers have previously been presented on TiO2, wherein most discussions are on how the synthesis route and conditions, as well as morphological properties of the oxide, relate to its photoelectric properties.22,29,49,61,62,116−122 However, given the continuous progress in the field and the shortfall in comprehensive compilations of studies that mainly focus on the electronic and optical properties of TiO2, we present herein a review on the optical and electronic properties of TiO2 nanomaterials to provide good insight into the importance of successful tailoring of the electronic properties of TiO2 at reduced scale as well as the various pathways for the same to enable the production of costeffective sustainable alternatives for production of green energy with high endurance. This review is comprised of four main sections and is organized as follows: the first part of this paper (sections 1 and 2) provides the reader a motivation for the need to address the most pressing energy-related concerns we face today and the available transformative technologies for efficiently harvesting solar energy. The second part of this paper (sections 3 and 4) presents a survey comprised of mainly theoretical studies on the correlation between phase, morphology, and photoelectric properties in TiO2. Significant emphasis has been given herein to detail (on the basis of theoretical prediction) the evolution in the band structure of TiO2 as a function of phase transition by virtue of introduction of impurity elements into the host oxide lattice and manipulation of its morphological properties. The third part of this paper (section 5) presents a detailed summary on recent experimental studies on the alteration of the optical and photoelectric properties of TiO2 via the manipulation of compositional and structural properties of the oxide that enables alternative pathways for the photoelectrons to propagate to reduce the probability for electron−hole recombination. Section 5 is a follow-up to the theoretical work presented on the same in section 4. Finally, the fourth part of this review details the principles of synchrotron-based X-ray spectroscopy techniques and their importance/contribution to the field of materials research and related energy applications. This section also includes a survey that presents
Although the often used laboratory-based UV−vis spectroscopy technique offers the ability to study charge transfer between individual atoms/ions and molecules independent of surrounding vibrational bands,70 the energy levels in the UV− vis spectra are commonly convoluted (as described by Himpsel et al. in ref 71) due to the occurrence of multiple charge transitions from the occupied density of states to the unoccupied density of states under the same excitation energy. This clearly makes it difficult to fully resolve the individual energy levels in the two states, especially for most of the novel materials in which the emerging functionality rises from the complex systems (Figure 3).71
Figure 3. Difference in electron excitation and absorption processes between UV−vis and synchrotron-radiation-based X-ray spectroscopy techniques, wherein the latter provides element selectivity due to the involvement of core electrons. Reprinted with permission from ref 71. Copyright 2013 Elsevier.
XPS and Raman spectroscopy are two additional important light−matter-probing techniques. While XPS is commonly employed to map the chemical composition and formation of chemical compounds in matter by recording the variation in the core-level binding energies with respect to the Fermi level (which further details the valence band region with high precision, 0.025 eV),3,72 Raman spectroscopy is used for studying chemical bonding, structural phases, and molecular interactions in matter at reduced dimensions with high sensitivity by recording inelastic scattered monochromic light arising from interaction between incident light and molecules that give rise to vibrational and/or rotational modes.73−77 PL is an optical technique that directly probes the optical properties of matter and their band gap by correlating the ability to absorb light of certain wavelengths (which brings the material into an excited state as electrons in the valence band are excited into the conduction band) and the following emission from the repopulation of the valence band by the photoelectrons (i.e., from electron−hole recombination).78 This emission carries important information on the existence of, e.g., active defect sites within the band gap that may serve as charge/light-trapping sites and is typically evaluated as a comparison between photons in and photons out. Different from (and complementary to) the UV−vis spectroscopy technique, the synchrotron-based X-ray spectroscopy technique (with element selectivity by virtue of the excitation of a core electron into the unoccupied density of states or de-excitation from a valence electron into the core level) very clearly (and specifically) details the local charge transitions, electronic structure, charge symmetry, and local chemical environment in complex materials at high precision (Figure 3). X-ray absorption spectroscopy (XAS), X-ray 9664
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the general spectroscopy features of TiO2 and the variation in the same with morphological and structural changes. We conclude this section by presenting an illustration (Figure 4) by Tachibana et al.58 that envisions a futuristic
Figure 5. Schematic presentation of the typical light-induced decomposition of water molecules wherein (i) hydrogen is produced via a reduction process at the conduction band assisted by a photoelectron and (ii) oxygen is produced via an oxidation process assisted by a photoinduced hole in the valence band of the photocatalyst.
required to generate the electron−hole pairs in the catalyst through photoexcitation.
Figure 4. Future power plant envisioned by Tachibana et al. in ref 58 for production of hydrogen gas via decomposition of water molecules by means of solar energy. Reprinted with permission from ref 58. Copyright 2012 Nature Publishing Group.
energy production plant for the production of water hydrogen through artificial photosynthesis (powered by solar and wind power) to remind ourselves of the general ambitions in addressing the current most pressing energy-related concerns.
2. LIGHT−MATTER-RELATED APPLICATIONS INCLUDING TIO2 2.1. TiO2 as a Photocatalyst
Photocatalysis refers to light-induced acceleration of a reaction in the presence of a light-sensitive catalyst and can be compared to the way plants use chlorophyll to convert water and carbon dioxide into oxygen and glucose fueled by sunlight (thereby commonly also referred to as artificial photosynthesis).58 Building further on Figure 2, Figure 5 presents a model that compiles the essential features in the process of light-induced decomposition of water molecules for the production of hydrogen and oxygen; it also describes the preferred energy band level alignment between energy band edges of the catalyst and the reactant to enable a redox reaction via charge transfer from the catalyst to the reactant. For the generation of hydrogen from water splitting, the relative position of the CBM of the catalyst needs to be located at more negative potential than the hydrogen production level, i.e., the reduction potential of H+ → H2 (ca. 0 eV vs NHE (near hydrogen electrode)), while the position of the VBM of the catalyst needs to be located at a more positive potential than the oxygen production level, i.e., the oxidation potential of H2O → O2 (ca. 1.23 eV vs NHE).37,49,123,124 Although the energy required for a complete redox cycle in this case does not exceed ca. 1.23 eV (Figure 6), it does not account for the energy
Figure 6. Compilation of energy band positions for some selected semiconductors in contact with an aqueous electrolyte at pH 1 with respect to NHE and the vacuum level as a reference, as compiled by Grätzel in ref 128. The scale on the right presents the potentials of several redox couples. Reprinted with permission from ref 128. Copyright 2001 Nature Publishing Group.
In spite of the fact that most semiconductors manifest a certain degree of photosensitivity, they are all far from being applicable as photocatalysts or in other photoelectric applications essentially (i) because of the high electron−hole recombination rate, which prevents the photoelectrons from reaching the surface (where a reaction typically takes place at the interface between the catalyst and the reactant), (ii) because some semiconductors (e.g., ZnO, CdS, and SiC) cannot withstand photocorrosion, which decreases their photocatalytic activity, (iii) because of the poor phase stability and poor tunability of the band gap (and the relative position of the energy band edges), and (iv) because of the low solubility 9665
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Other fields of application where TiO2 can be found as an active ingredient are photovoltaics (most commonly in dyesensitized solar cells (DSSCs) or quantum-dot-sensitized solar cells) and electrochromic devices.59,60,71,116,118,119,140a,141−145 DSSCs are essentially comprised of the following three main components: (i) a light absorber (an organic dye) sandwiched between (ii) a charge donor (typically a semiconductor or an electrolyte) and (iii) a charge acceptor (typically TiO 2 nanoparticles); see Figure 7. The efficient charge transfer between the three components requires careful alignment of their respective energy levels such that there is a small drop between the VBM of the charge donor and the HOMO (highest occupied molecular orbital) of the dye, as well as between the LUMO (lowest unoccupied molecular orbital) of
the light absorber and the CBM of the charge acceptor. As the photoelectrons are generated in the dye (upon light absorption), charge injection to the TiO2 takes place instantaneously (within hundreds of femtoseconds) while the oxidized dye is regenerated by accepting electrons from the charge donor (preventing the event of electron−hole recombination).29,146−149 As indicated in Figure 7, the opencircuit voltage (Voc) is maximized by minimizing the energy difference between the LUMO of the light absorber and the CBM of the TiO2 and beween the VBM of the charge donor and the HOMO of the absorber. However, if this energy difference is too small, the driving force for the charge transfer through the interface between the electron acceptor (and electron donor) and the light absorber process will be reduced and result in a higher degree of recombination losses.71 TiO2-based DSSCs are commonly preferred over other oxide-based DSSCs as they manifest the best device performance.29,150−156 One principal advantage of TiO2 nanocrystals compared to many other metal oxide compounds is that they do not have to be electronically doped to attain n-type characteristics. Instead, injection of a single electron into the conduction band of a ca. 20 nm particle of TiO2 has been found to be sufficient to alter its transport properties from an insulating to a conductive state.157 This is by far an important feature of TiO2 considering the challenge of synthesizing highquality ternary-nanostructured oxide systems with the desired optical and transport properties. Another important feature of TiO2 is the disparity between the electrons in its valence band (comprised of O 2p−Ti 3p) and those in the conduction band (comprised of Ti 3d), which abates the probability for electron−hole recombination. Such disparity between the valence band and conduction band is however also expected in both ZnO (with completely filled 3d orbitals) and Nb2O5 (with the conduction band and valence band comprised of mainly Nb 4d and O 2p, respectively).29,157,158 In striking contrast, the 3d orbitals in Fe2O3 are present both in its valence band and in its conduction band, which results in energy band edges with similar parity and therefore further enhances the probability for electron−hole recombination.158 Clearly, the ability to control the band structure and the local charge symmetry in matter is of great importance to further control the propagation of the photoelectrons from the bulk to the surface and their injection from the catalyst into the reactant. Discussions on these important materials characteristics, based on both theoretical predictions and experimental findings, are presented in the following sections.
Figure 7. Typical energy diagram for a dye-sensitized solar cell device with the preferable energy alignment between its three main components as indicated for optimized charge transfer between the same. Reprinted with permission from ref 71. Copyright 2013 Elsevier.
3. CORRELATION BETWEEN PHASE, SURFACE, AND PHOTOELECTRIC PROPERTIES Before we look into the details of the band structure of TiO2 (section 4), we bring to attention the principle properties of the rutile- and anatase phase of TiO2, and their critical impact (both individually and combined) on the performance of TiO2 in light-matter related applications. Lastly, we’ll reflect over recent reports on particle size and light induce phase stabilization. Although rutile TiO2 represents the most stable phase among its three polymorphs and has a smaller band gap than anatase TiO2, the latter is still preferred over rutile TiO2 in photocatalytic applications. Different from the rutile phase, the anatase phase has (i) a higher conduction band energy, electron Fermi level, and adsorptive affinity and (ii) a lower electron−hole recombination rate, all important characteristics that make it more applicable in photoelectric applica-
of impurity ions in the host lattice (an otherwise common route to tune the optical and electronic properties).37,49,61,125−128 A wide range of semiconductor compounds, both oxides and nonoxides, with band gaps ranging from ca. 1.4 to ca. 3.8 eV, have previously been suggested and investigated for their aptness to serve as potential catalysts in photoelectrochemical reactions, see Figure 6.116,128 With the advantage of having strong catalytic activity, chemical stability, biocompatibility, cost-effectiveness, and long levity of electron−hole pairs, TiO2 has been favored and most widely used as a catalyst material among the available semiconductors.29,37,49,117,127 However, the overall efficiency in catalytic reactions for water splitting involving TiO2 (as well as many other semiconductors) remains to be improved since poor responsivity to light in the visible wavelength range (contributing to ca. 50% of the solar radiation range) and the reoccurring electron−hole recombination impede the catalytic activity.129,130 One direct approach to reduce the energy separation between the CBM and VBM in wide band gap semiconductors, and thereby enhance their photosensitivity in the visible wavelength range, is through introduction of impurity elements, both nonmetals and transition metals, into the host semiconductor matrix. In the case of TiO2 such manipulation of the local chemical stoichiometry of the oxide has reportedly been shown to disturb the local chemical state and charge symmetry such that the oxide undergoes either a rutile → rutile + anatase → anatase or an anatase → anatase + rutile → rutile phase transition upon doping depending on the dopant characteristics, concentration, and synthesis conditions.131−139,140a 2.2. TiO2 in Photovoltaics
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anatase + rutile → rutile or rutile → rutile + anatase → anatase are commonly achieved and controlled by post thermal treatment and/or introduction of impurity ions into the host oxide lattices.28,124,131−133,164−173 In addition, morphology is yet another key property to manipulate and/to improve the phase stability in TiO2. From atomistic simulations, Zhang and Banfield have reported ca. 30% lower surface energy in anatase TiO2 nanoparticles (average particle size 16 nm) can be summarized by the following sequence: anatase → brookite → rutile. These findings correlate very well to the studies on thermally induced phase transition in TiO2 wherein coalescence of individual particles, i.e., an increase in the average particle size, contributes toward a sequential phase transition.167−172,174,175 Using a thermodynamic model based on surface free energies and tensions, Barnard and Curtiss have in ref 176 presented a study on the effects of the surface chemistry on the phase stabilization and morphology of surface-terminated (by adsorbates with different oxygen/hydrogen ratios) TiO2 nanoparticles. Figure 10 presents the predicted morphological changes in TiO2 nanoparticles upon surface treatment under acidic and alkaline conditions.176 While hydrogen-rich surface adsorbates stabilize the anatase phase, oxygen-rich adsorbates have been found to stabilize the rutile phase.176 These findings, together with those later reported on the influence of the surface crystal plane and chemistry on the overall performance of the catalyst, provide substantial knowledge to the field and open up new pathways for synthesis of highly photoactive nanostructured TiO2 upon surface modifications.177−180 Another very interesting mechanism for phase transition that should be highlighted before this section is concluded is lightinduced phase transition and stabilization.181−184 Recently Ricchi et al.181 have in their studies on light-induced phase transition in TiO2 demonstrated a sequential structural phase transition (anatase → amorphous phase → rutile) in TiO2 nanoparticles upon irradiation in the visible wavelength range using a laser beam light source (with 5 and 10 mW laser power)
Figure 8. Illustration presenting the suggested charge-trapping mechanism in a two-phase catalyst composed of a mixed phase of rutile and anatase TiO2. Proposed models question whether the photoelectrons are generated in the anatase phase and further trapped via injection into the rutile phase (preventing them from recombining with their associated holes) (a) or vice versa (b). Reprinted from ref 160. Copyright 2003 American Chemical Society.
raised is whether the enhanced photoactivity in the two-phase system root in the lower energy of the conduction band edge in the rutile phase (causing a small “dip” at the interface) that traps the photoelectrons generated in the anatase phase (thus preventing them from recombining with their associated holes) or vice versa. Although the prior scenario (Figure 8a) is the most commonly accepted explanation, the authors of ref 160 attribute (on the basis of electron paramagnetic resonance spectroscopy studies) the enhanced catalytic activity in the mixed-phase catalyst to a synergistic activation of the rutile phase by the anatase phase. The rutile phase is believed to extend the optical range within which the TiO2 responds to light (i.e., into the visible wavelength range), that in turn enhances the overall photoinduced charge density. The twophase construction further reduces the probability for electron−hole recombination as the photoelectrons generated in the rutile phase propagate toward anatase charge-trapping sites (Figure 8b).160 The importance of the ability to control the phase structure of TiO2 for improved performance in photoelectric reactions is at this point beyond doubt. Phase transitions such as anatase → 9667
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both in a vacuum and oxygen molecules, thereby controlling the charged F centers onargon atmosphere. From detailed Raman studies, the reported experimental findings in ref 181 indicate that both the irradiation energy (part a vs part b of Figure 11) as well as the ambient conditions (part a vs part c of Figure 11) may affect and accelerate a phase transition from anatase to rutile.181 Note that in all three cases the TiO2 enters an amorphous state prior to undergoing a complete transition sequence. The observed light-induced phase transition has been attributed to an athermal mechanism that induces surface defects (i.e., activated by an electron excitation process rather than thermally driven) and the interaction of these surface defects with their chemical surroundings (implying the important role surface adsorption and desorption of oxygen molecules play here).181 It has further been argued that surface-adsorbed oxygen molecules may serve as charge collectors at the conduction band of the oxide, facilitating the formation of Ti3+ and Ti4+ (as a result of the charge-trapping effect). On the other hand,
Figure 10. Predicted morphological variation in (i) anatase and (ii) rutile TiO2 nanoparticles upon surface treatment under hydrogen- and oxygen-rich conditions. Reprinted from ref 176. Copyright 2005 American Chemical Society.
Figure 11. Time evolution in the Raman spectra of TiO2 anatase nanoparticles under continuous irradiation using a laser beam light source under different ambient conditions: (a, b) spectra recorded in vacuum conditions at 10 and 5 mW (respectively) laser beam power, (c) spectra recorded in an argon atmosphere at 10 mW laser beam power, (×) anatase phase, (o) rutile phase. Reprinted from ref 181. Copyright 2013 American Chemical Society. 9668
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desorption of oxygen molecules will most likely result in recombination of the trapped electrons with their associated hole in the valence band. Therefore, controlling the ambient conditions may greatly affect the adsorption and desorption of oxygen molecules (and thereby the F centers on the particle surface), as indicated by comparing parts a and c of Figure 11.181 The difficulties with synthesizing brookite TiO2 have for a long time inhibited the research around this polymorph and its further development in photoelectrically related applications (which explains the limitation of available data on the same). However, due to the continuous search for new sustainable and efficient functional materials suitable for energy applications, researchers have been directed to also explore this third polymorph of TiO2.31,163,185−193
Figure 12. Compilation of data presenting the general difference in the predicted and experimentally confirmed band gaps in semiconductors.186,187 Reprinted with permission from ref 229. Copyright 2010 American Physical Society.
4. OPTICAL AND ELECTRONIC PROPERTIES OF TIO2 4.1. Survey through Recent and Past Theoretical Work on the Band Theory of TiO2
1.194a For instance, DFT-PBE represents an underestimation of the electronic band gap, DFT-HSE06 is a hybrid function correcting for any band gap underestimation in standard DFT, and PBE-G0W0 represents full frequency calculation accounting for the quasi particle state.194,239−241 Figure 14 presents the typical DOS spectra of (a) rutile, (b) anatase, and (c) brookite TiO2 that delineate the valence and conduction bands to be mainly comprised of O 2p and Ti 3d states, respectively, at the band edges.140b,194a,195−198,203 A molecular orbital bonding energy diagram, constructed from the individual atomic energy levels, not only provides a more clear picture for the chemical bonding that exists between the constituent atoms in matter but also serves as an excellent tool to visualize the hybridization between the different energy levels/orbitals within the cation and anion, as well as the change in the same upon doping. As previously mentioned, the difficulties with synthesizing brookite TiO2 have limited the research around this polymorph of TiO2 and thereby also impeded the successful application of brookite TiO2 in photoelectrochemical applications. For obvious reasons we hereafter focus our discussion/review on rutile and anatase TiO2. Asahi et al. have in ref 195 presented a detailed study of the chemical bonding in anatase TiO2 by decomposing its DOS spectra (Figure 15a) into Ti eg and t2g (dyz, dzx, and dxy), O pσ (in the TiO3 cluster plane), and O pπ (out-of-plane). These orbitals have further been translated into a molecular orbital bonding energy diagram (Figure 15b) that now more clearly shows that (i) the valence band in anatase TiO2 is comprised of O pπ (higher energy region), pσ and pπ (intermediate energy region), and pσ (lower energy region) and (ii) the conduction band is comprised of Ti 3d and 4s, wherein the lower energy regions of the conduction band are represented by the degenerate eg-like and 3-fold t2g-like states arising from the crystal field splitting of Ti 3d.195,242,243 Similarly, Figure 16 presents a simplified molecular orbital bonding energy diagram for rutile TiO2 by Krol203 (following earlier work by Fischer201 and Stoyanov et al.202). In a more detailed investigation on the molecular orbital energy diagram of Ti−O complexes (TiO, TiO2, and Ti2O3), Stoyanov et al.202 not only emphasize the importance of considering Ti 3d, 4sp, and O 2sp atomic orbitals for the construction of such an energy diagram, but also report the degree of occupancy in the
The optical properties for a given system are commonly defined by its band gap, which is the energy difference between the highest occupied (or VBM) and lowest unoccupied (or CBM) densities of states (DOSs) for the same, and this energy separation between the two states defines its sensitivity/ responsivity to the solar spectrum. Our current understanding of the electronic structure of TiO2 (as well as many other semiconductors) has been achieved as a result of both independent and combined theoretical (e.g., DOS and density functional theory calculations) and experimental (e.g., X-ray diffraction, X-ray spectroscopy, UV−vis spectroscopy, and Raman spectroscopy) studies.22,62,140b,141,194−228 The DOS of a system represents the number of states at each energy level available for electron occupancy (note that, different from that of secluded atoms, the density distribution in semiconductors is continuous and not discrete). The band structure of a system is defined by the nature of the constituent atoms/ions and their radius, ionic state, bonding length, and crystallographic geometry. All these features affect the local charge and chemical symmetry therein and the various energy states within which an electron may exist and propagate. Density functional theory (DFT) with the local density approximation (LDA) is the primary theory/model commonly employed to probe the electronic density distribution at the ground state, although the band gap given by the Kohn−Sham gap (EKS) typically underestimates the experimental value for the same by ca. 30−100% (Figure 12). Since the band gap is often described as the difference between the ground state and the excited state, it cannot be successfully described by the DFT due to the limitation of the derivative discontinuity and delocalization error.229−234 However, this is overcome by applying the many-body perturbation theory in addition to the DFT, which accounts for the quasi particle excitation energies (Figure 13).140b,194a,235−238 As evident from Figure 13, both rutile and brookite TiO2 have a direct band gap with Γ → Γ (CBM → VBM) transitions, while anatase TiO2 has an indirect band gap defined by 0.88Γ → M transitions. Note that the actual (relative) positions of the band edges vary depending on the function/correction employed with the DFT, e.g., DFT-PBE (Perdew−Burke− Ernzerhof), DFT-HSE06 (Heyd Scuseria and Ernzerhof hybrid function), PBE-G 0W 0 (GW approximation in the fullfrequency-dependent scheme), or HSE06-G0W0; see Table 9669
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Figure 13. Typical DFT-HSE06 band structures of TiO2 and quasi particle energies (dots) extracted from PBE-G0W0 calculations as presented by Landman et al. in ref 194a: (a) rutile, (b) anatase, and (c) brookite TiO2. G0W0 quasi particle energies are given with respect to the DFT-PBE energy scale. The HSE06 calculations have been carried out with regular 7 × 7 × 11, 9 × 9 × 3, and 3 × 5 × 5 Γ-centered k-point meshes for rutile, anatase, and brookite. GW calculations were carried out on top of DFT-PBE calculations with 192, 384, and 768 electronic bands, regular 8 × 8 × 12, 10 × 10 × 4, and 4 × 6 × 6 Γ-centered k-point meshes, and 192 frequency points for sampling the dielectric function. Reprinted with permission from ref 194a. Copyright 2012 IOP Publishing.
impurity ions into the host oxide lattice substituting for the host anions and/or cations (Figure 17). However, identifying the most appropriate impurity element is an essential step since it disturbs the local chemical environment and charge symmetry in the oxide lattice due to, e.g., deviation in the ionic radii and/ or oxidation states between the two and, depending on the impurity concentration, also the phase stability. This section presents the predicted variation in the DOS of TiO2 upon introduction of impurity ions into the host oxide lattice substituting for the host anions and/or cations and how they contribute toward improving the photosensitivity in the visible wavelength range. The objective of this section is to provide the reader an immense understanding of how impurity elements impinge on the local band structure in semiconductors (depending on their nature and concentration), which is eventually reflected in the device performance of, e.g., a photocatalyst and/or a photovoltaic cell, before we detail recent experimental reports (section 5) on the role of impurity elements in the photoelectric properties of TiO2. By metal doping, one normally refers to the substitution of Ti ions by other 3d transition-metal ions, e.g., V, Fe, Cr, Cu, Mo, and Mn.121,204,205,244−249 The crystal field splitting of the transition-metal 3d orbitals (3d → t2g and eg) gives rise to both localized (Figure 18a−g) and delocalized (Figure 18h) impurity states within the band gap of the oxide.244
Table 1. Variation in the Predicted Band Gap of Rutile, Anatase, and Brookite TiO2 Depending on the Approximation Applied to the DFT Functional: (i) DFTPBE, (ii) DFT-HSE06, (iii) PBE-G0W0, and (iv) HSE06G0W0a fundamental band gap method DFT-PBE DFT-HSE06 PBE-G0W0 HSE06-G0W0
rutile 1.88 3.39 3.46 3.73
(Γ (Γ (Γ (Γ
→ → → →
anatase Γ) Γ) Γ) Γ)
1.94 3.60 3.73 4.05
(0.89Σ → Γ) (0.89Σ → Γ) (Σ → Γ) (Σ → Γ)
brookite 1.86 3.30 3.45 3.68
(Γ (Γ (Γ (Γ
→ → → →
Γ) Γ) Γ) Γ)
a
Reprinted with permission from ref 194a. Copyright 2012 IOP Publishing.
t2g state in the different oxides of Ti to differ significantly in the different oxide complexes.202 4.2. Evolution of the Electronic Structure upon Introduction of Impurity Elements into the Host Oxide Lattice
The ability to engineer the band gap of TiO2 upholds the interest in this oxide material in light−matter-related applications. Alteration of the band gap, and thereby the optical properties, is predominantly achieved by introducing
Figure 14. Total and partial DOSs (calculated using the HSE06 exchange correlation functional) of the three polymorphs of TiO2. Reprinted with permission from ref 194a. Copyright 2012 IOP Publishing. Two separated peaks at the conduction band edge arise from the crystal field splitting of Ti 3d wave functions into t2g and eg symmetries representing π and σ Ti d hybridized with O 2p. The modest two-peak separation close to the valence band edge results from sp2-like hybridization in the planar OTi3 building blocks.194a,195,201,203 9670
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Figure 15. (a) Presentation of the total and projected [(i) Ti eg and t2g (dyz, dzx, and dxy), (ii) O pσ (in the Ti3O cluster plane), and (iii) O pπ (out of the Ti3O cluster plane] DOSs of anatase TiO2 calculated from first-principles calculations using the full potential linearized augmented plane wave method. (b) Detailed molecular orbital bonding diagram composed of the components extracted from the DOS spectra. Reprinted with permission from ref 195. Copyright 2000 American Physical Society.
Figure 16. Simplified molecular orbital diagram of rutile TiO2 summarized by Van de Krol. Reprinted with permission from ref 203. Copyright 2012 Springer.
Figure 18. Variation in the DOS (calculated using the full potential linearized augmented plane wave method based on the DFT with the generalized gradient approximation) of TiO2 upon doping with transition-metal 3d ions. Additional energy states in the band gap upon doping arise due to the t2g states in the dopants. Reprinted with permission from ref 244. Copyright 2002 Elsevier.
Figure 17. Diagram summarizing commonly employed methods to engineer the electronic structure of TiO2 for optimum photoelectric performance.
The relative energy position of these impurity states (relative to the energy band edges) depends on the electron distribution 9671
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between t2g and eg, wherein the repulsive forces between t2g and the ligand become more pronounced at reduced electron occupancy in eg. This contributes to a shift of the impurity state(s) away from the valence band (where the higher energy region is mainly represented by O 2p) toward the conduction band (where the lower energy region is mainly represented by Ti 3d).250 As these impurity states position closely below the conduction band (or above the valence band), a “tail” is formed that extends the conduction band region (or valence band region) via charge transfer between the impurity state and the energy band edge (Ti 3d or O 2p), thus reducing the overall energy separation between the CBM and VBM.244,251 Although the LDA is the most widely used first-principles method to project the ground state of elements, it falls back on projecting the band structure of strongly correlated systems, e.g., transition metals which have both degenerate states and a nearly degenerate correlation effect on the ground-state structure.252−255 This results in insufficient crystal field splitting of the transition-metal d orbital (d → t2g−eg). For instance, in the case of Cr-doped TiO2, the doped oxide compound has been described both as a metal (using standard PBE calculations) and as a half-metal (using hybrid (B3LYP) DFT calculations).253−255 A strong p−d interaction in transitionmetal oxides affects the valence band states such that the overall energy difference between the CBM and VBM is reduced (typically resulting in an underestimation of the band gap).84,256,257 This error in the calculations can however be overcome by the application of a correction, e.g., DFT +Hubbard U (DFT+U). This results in higher accuracy in the projection of multielectron systems by correcting the LDA total energy curve with respect to the electron number (U is defined as the second derivative (δ2E/δn2) of the ground-state energy).258−260 From combined LDA, LDA+U, and XPS analysis, respectively, on bulk Cu, Zn, and ZnO, Lee and Kim have in ref 260 demonstrated a better agreement in their binding energies between the XPS analysis and LDA+U results as compared to XPS analysis and LDA, as the single-electron model LDA consistently underestimated the binding energies by up to 2 eV.260 It is often more difficult to substitute the host anions by anion impurities due to the often larger disparity in charge states and ionic radii between the two. Instead, impurity anions position interstitially in the oxide lattice, which consequently influences the local charge symmetry surrounding the cations.22 However, as also highlighted by Cheng and Mao in ref 22 (and refs 434 and 437 therein), the overall ability to incorporate impurity elements (both anions and cations) is improved in nanostructured materials since the definition of the compound’s matrix often is limited to clusters of few atoms, thus manifesting higher tolerance toward structural distortion. This can directly be reflected by the often reported difficulty in doping of intrinsically n-type oxide thin films with nitrogen to achieve p-type conductivity that however becomes viable at a reduced scale. Among the investigated nonmetal dopants, nitrogen doping of TiO2 has shown great promise in enhancing the photoactivity of the oxide in the visible wavelength range.22,117,121,204,261−270 In contrast to metal doping, nitrogen doping causes changes in the electronic structure manifested as localized 2p acceptor states (so-called impurity energy levels) residing close to the valence band and not necessarily overlapping with the valence band (Figure 19a,b).132,204,205,261,265,267,271,272
Figure 19. Variation in the DOS (DFT calculations performed by the ESPRESSO-3.018 package within the plane wave pseudopotential approach within the framework of the generalized gradient approximation) of anatase TiO2 with increasing nitrogen impurity concentration: (a) intrinsic TiO2, (b) 1 atom % nitrogen-doped TiO2, (c) 2 atom % nitrogen-doped TiO2, (d) 1 atom % nitrogen-doped TiO2, and (e) 4 atom % nitrogen-doped TiO2. Reprinted from ref 273. Copyright 2011 American Chemical Society.
Such acceptor states have (among others) been reported by Yang et al. in ref 273 in which first-principles DFT calculations have been performed to study the variation in the DOS of anatase TiO2 with increasing nitrogen impurity concentration (from 0 to 4 atom %). As evident from Figure 19, the nitrogen impurities give rise to localized N 2p states closely above the VBM of the oxide, contributing to a red shift of the absorption edge that consequently results in a decrease in the band gap (from 2.2 to
