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Present Perspectives of Advanced Characterization Techniques in TiO2-based Photocatalysts Chengzhi Luo, Xiaohui Ren, Zhigao Dai, Yupeng Zhang, Xiang Qi, and Chunxu Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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ACS Applied Materials & Interfaces

Present Perspectives of Advanced Characterization Techniques in TiO2-based Photocatalysts Chengzhi Luo†,‡,||, Xiaohui Ren§,||, Zhigao Dai†,||, Yupeng Zhang*,‡, , , Xiang Qi*,§, , ⊥

⊥

Chunxu Pan*,†

†

School of Physics and Technology, and MOE Key Laboratory of Artificial Micro-

and Nano-structures, Wuhan University, Wuhan 430072, China. ‡

College of Electronic Science and Technology, Shenzhen University, Shenzhen

518060, China. §

Laboratory for Quantum Engineering and Micro-Nano Energy Technology and

Faculty of Materials and Optoelectronic Physics, Xiangtan University, Xiangtan 411105, China. ⊥

Department of Materials Science and Engineering, Monash University, Wellington

Road, Clayton, Victoria 3800, Australia

||

These authors contributed equally to this work.

*Address correspondence to: [email protected] (Y. Zhang), [email protected] (X. Qi), [email protected] (C. Pan)

ACS Paragon Plus Environment


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ABSTRACT TiO2 is the most investigated photocatalyst because of its non-toxicity, low cost, chemical stability, and strong photooxidative ability. Due to the morphology and structure dependent photocatalytic properties of TiO2, accurate characterization of the crystal and electronic structures of TiO2-based materials and their performance during the photocatalytic process is crucial not only for understanding the photocatalytic mechanism but also for providing experimental guidelines as well as a theoretical framework for the synthesis of high performance photocatalysts. In this review, we focused on the advanced characterization techniques that are utilized in the studies on the TiO2 structures and photocatalytic performance of TiO2 and TiO2-based materials. It is therefore anticipated that this review can provide a novel perspective to understand the fundamental aspects of photocatalysis and inspire the development of new photocatalysts with superior performances.

KEYWORDS: TiO2, photocatalysts, characterization, Space and time domain, Spectroscopy


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■ INTRODUCTION During the past decades, photocatalysis has been identified as a sustainable and environmentally-friendly method for the degradation of pollutants and hydrogen generation.1-9 This artificial photochemical process has the potential to solve many serious environmental and energy challenges, which are receiving increasing global concern.10-14 Due to its non-toxicity, low cost, chemical stability, and strong photooxidative ability, titanium dioxide (TiO2) has become the most promising semiconductor in photocatalysis.15-17 Studies have showed that both morphology and microstructure can affect the photocatalytic efficiency of TiO2.18-21 Therefore, significant advances have been made to enhance the photocatalytic performance of TiO2 and TiO2-based photocatalysts through methods such as, surface modification, structure optimization, doping, and preparing composites with other photosensitive materials.22-24 Moreover, many fundamental issues concerning the structure-property relationship, light-matter interaction, and photocatalyst-pollutant interaction are also essential for the performance optimization of photocatalyts.25-28 Therefore, accurate characterization of the crystal and electronic structures, and the corresponding performance in TiO2-based materials during the photocatalytic process is crucial not only for understanding the photocatalytic mechanism but also for providing experimental guidelines as well as a theoretical framework for the synthesis of high performance photocatalysts. Recently, various advanced characterization techniques have been used to provide high-resolution images of the atomic structure of materials as well as direct observation of the photocatalytic process in the space and time domains, respectively.29-33 For instance, X-ray diffraction (XRD) has become a fundamental tool in investigating the relationship between crystal phase and the photocatalytic ability of TiO2. High-resolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy (STM) provide insights into the effect of microstructures on photocatalytic properties of TiO234-35 as well as the distribution of reaction sites, helping to track the chemical reactions on the surface of photocatalysts (especially for transition-metal oxides).36-37 In addition, analytical spectroscopy



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techniques such as, Raman spectroscopy (Raman), Fourier Transform Infrared spectroscopy (FTIR) and Photoluminescence (PL), provide multi-scale information of TiO2 in photocatalysis, which indirectly describe the photocatalytic reaction process and illustrate the mechanism of photocatalysis.38-42 In addition, time-resolved spectroscopies, such as time-resolved PL, time-resolved Infrared spectroscopy (IR), positron annihilation lifetime spectroscopy (PALS), two-photon photoemission (2PPE), and transient absorption spectroscopy (TAS), are utilized to quantitatively study the diffusion/transfer mechanism of photo-induced carriers in TiO2 based photocatalysts.43-46 Thorough investigation of the photocatalytic process and mechanism using these newly developed characterization techniques will provide detailed experimental evidence for the structure-property relationship, light-matter interaction, and photocatalyst-pollutant interaction in photocatalysis. Herein, this review mainly focuses on the advanced characterization techniques that are used to study the structures and photocatalytic performances of TiO2 and TiO2-based materials. We believe that this review can give us a novel perspective to contribute to the fundamental studies of photocatalysis and inspire the development of new photocatalysts with superior performances. ■ BRIEF INTRODUCTION TO TIO2 STRUCTURE AND PROPERTIES TiO2 has three important crystalline phases, anatase, rutile, and brookite. In all three forms, titanium (Ti4+) atoms are surrounded by six O2- ions, forming the TiO6 octahedron, as shown in Figure 1a.47 The octahedron in rutile TiO2 is irregular, showing slightly orthorhombic distortion, whereas the anatase TiO2 shows obvious orthorhombic distortion, resulting in lower symmetry than the orthorhombic system. For brookite, both edges and corners are shared to give an orthorhombic structure. These polymorphs have different structures that lead to different photocatalytic activities. In general, rutile and anatase TiO2 are widely used in photocatalysis while brookite TiO2 receives limited research interest. Therefore, this present review mainly focuses on rutile and anatase TiO2. The properties of rutile and anatase TiO2 are summarized in Table 1.48-54 As is known, the band gap of anatase TiO2 is about 3.2 eV, while it is 3.0 eV for rutile TiO2.50 Due to its lower packing density (3.8-3.9 g/cm3),


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anatase TiO2 exhibits superior photoactivity compared to rutile TiO2.48 The binding energies are also different for the rutile and anatase TiO2, as shown in Figure 1b. For rutile, the electron energy at the conduction band minimum (CBM) and hole carrier energy in the valence band maximum (VBM) are 0.22 and 0.39 eV, respectively, which are much higher compared to anatase.55 Figure 1c shows the typical density of states (DOS) of anatase and rutile TiO2, in which the conduction and valence bands are both composed of Ti 3d and O 2p states.55-56 However, the crystal momentums in the Brillouin zone are totally different. The same k-vectors for electrons and holes provide a direct gap at Γ→Γ transitions for rutile TiO2 while anatase TiO2 has an indirect gap between Γ → M.57-58 Due to the fast radiative recombination in direct band gap materials, the photocarrier diffusion length/recombination lifetime in anatase TiO2 are much longer than that in rutile TiO2. In addition, the photo-induced carriers of anatase TiO2 have the lowest average effective masses, meaning that the photocarriers of anatase TiO2 have faster migration from the interior to surface, as well as a lower recombination rate.59 On the basis of the effective electron mass and carrier relaxation time, the initial carrier recombination time in anatase TiO2 can be estimated as ~100 ps, and the diffusion length is as large as ~24 nm.60 As the recombination rate is much higher than the separation rate,61 it is important to regulate the optical and electronic structures of TiO2. Doping pure TiO2 with other elements is a useful approach to modify the band structure and increase the lifetime of photo-induced carriers. Crystal facet engineering of TiO2 is another important way to tune the surface properties in order to improve the photocatalytic activity of TiO2. Generally, in different phases of TiO2, the dominant facets are different. The dominant facets for rutile TiO2 are {110}, {100}, and {101}, while {101} and {001} are the dominant facets for anatase TiO2.62 The {110} facet of rutile TiO2 has been widely studied because of its lowest energy. For anatase TiO2, scientists have demonstrated that the {001} facet exhibits remarkable photocatalytic activity,63 because the {001} facet possesses a large number of under-bonded Ti atoms and large Ti-O-Ti bond angles.64



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In short, various strategies have been developed to enhance the photocatalytic performance of TiO2,65-70 which have extended the light absorption region to the entire spectrum, enhanced the optical absorption and light collecting efficiency, and improved the separation efficiency of carriers. However, there are still a few important questions about the excited state properties of TiO2-based photocatalysts, i.e., what is the intrinsic mechanism that causes the recombination of photo-induced carriers? How do the structures and chemical compositions affect the dynamic properties of the photo-induced carriers? How do the photo-induced carriers diffuse/transfer over the interface in TiO2-based photocatalysts? Therefore, it is of fundamental importance to elucidate the physical mechanism of generation, separation and transfer of the photo-induced carriers in TiO2-based photocatalysts using advanced characterization techniques in order to understand the working principle, and for the further performance optimization of TiO2-based photocatalysts. ■ ADVANCED CHARACTERIZATION TECHNIQUES FROM SPACE DOMAIN XRD. Since W. H. Bragg and V. L. Bragg used XRD in 1912 to characterize the structural properties of the NaCl crystal, this technique has become a popular tool to identify the atomic and molecular structures of a crystal. XRD measurement is based on the manner in which the crystalline atoms cause a beam of incident X-ray to diffract into many specific directions. The XRD technique is highly useful to determine the structural information such as crystallite size, size distribution, morphology, crystal structure, and crystallinity of TiO2 and TiO2-based photocatalysts, which can reveal the relationships between the structure and photocatalytic performance of TiO2-based photocatalysts. As mentioned in Section 2, the crystal phase in TiO2 is important for surface adsorption as well as photo-induced carrier recombination during the photocatalytic process. Previous studies have shown that both anatase-rutile and anatase-brookite mixtures of a certain percentage exhibit higher photocatalytic activity than pure anatase, owing to the synergetic effect.71-72 It is thus desirable to unravel the mystery


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of the synergetic effect by quantitatively measuring the phase proportions in the mixed phase TiO2 crystal. For this purpose, XRD is an effective approach, as it allows the anatase-to-rutile ratio to be determined by the peak intensities of anatase and rutile phases.73-75 Based on many XRD studies, the synergetic effect in photocatalysis is found to be distinct when the well-defined anatase content ranges from 40% to 80%, and the optimum mixture is found to be 60% anatase and 40% rutile. This method can be also applied to the anatase-brookite TiO2 system.73 XRD is also an effective method to determine the orientations and the exposed crystal facets of TiO2. Recently, anatase and rutile TiO2 with tailored crystal facets have received great attention, because different facets with different surface atomic structures exhibit distinct photocatalytic abilities. For instance, recent researches reveal that the {001} facets (Figure 2a) of anatase TiO2 demonstrate higher photocatalytic activity than the thermodynamically stable {101} facets.76-79 In this regard, it is important to quantitatively investigate the percentage of the exposed facets. As an example, the (001) facets of anatase TiO2 can be determined by XRD based on the structural information of anatase TiO2 and the full width at the half-maximum of (004) and (200) diffraction peaks. The thickness and side length of anatase TiO2 crystal can be calculated from XRD peaks, thus the percentage of the exposed {001} facets in anatase TiO2 can be roughly calculated (Figure 2b and 2c).80-81 The calculation of the thickness and side length is based on the Scherrer Formula: D=Kλ/Bcosθ, where K, λ, B, θ are the Scherrer constant, incident radiation wavelength, full width at half-maximum and diffraction angle, respectively. Theoretically, the shape of the TiO2 nanosheets is supposed to be a standard cuboid. According to the geometrical relationships between thickness in [001] direction and the length in the [100] direction, the surface area of the exposed {001} facets and the total area of the TiO2 nanosheets can be calculated. Moreover, the effects of dopants on the crystallinity and phase transformation of TiO2 have also been investigated using XRD, since the presence of dopants (such as Nb,82 Ce,83 Li84) lower the degree of crystallinity and also induce phase transformation if the dopants are preferentially present in one of the phases (Figure



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2c). For example, doping Ce in rutile TiO2 rapidly decreases the intensity of signature peaks of the rutile phase,83 while a new peak corresponding to the (121) crystal plane of brookite appears simultaneously, which indicates that a small amount of dopant exerts a great influence on the photocatalytic properties of TiO2. "In-situ" XRD is a powerful semi-quantitative tool to study crystallization behavior by changing the external conditions. The use of "in-situ" technique allows for the direct monitoring of the evolution of phase composition and crystallization. Lü et al.82 prepared Nb-TiO2 nanoparticles for "in-situ" high-pressure XRD. Their study revealed that the increase in pressure gradually induced a red shift of the 2θ value and broadened the peak width of all diffraction peaks, indicating that the pressure promoted the growth of baddeleyite phase. It was also observed that the phase transition was completed at the pressure of 25 GPa and remained stable until the pressure was up to 40 GPa. Shen et al.85 used "in-situ" XRD to study the complex phase transformation in anatase TiO2 and found that the phase transition was a particle size-dependent non-equilibrium process. Similarly, the crystal growth, structural change and dynamic morphological evolution of the F-doped TiO2 from the initial intermediate NH4TiOF3 to HTiOF3 and TiOF2 were verified through "in-situ" temperature-dependent XRD.86 Moreover, "in-situ" synchrotron powder XRD technique can provide the hydrothermal/solvothermal crystallization information of TiO2. Extent of crystallization as well as quantitative information on the crystallite mean size and size distribution of anatase TiO2 can be obtained from the crystallization curves of "in-situ" PXRD (Figure 2d).87 XRD measurement is a feasible tool to investigate crystal structure and crystallographic phases. Its many advantages provide us a valid method to analyze the phase transformation, phase content, and changes in orientation and lattice spacing in TiO2-based photocatalysts.

TEM and EELS Characterizations from Atomic Scale. TEM (or HRTEM) is generally used for direct observation of the grain size, crystal structure, and interface


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in nanomaterials.88-90 As is known, the catalytic behavior of TiO2 is strongly dependent on its microstructures (e.g., phase, crystallinity, grain size, specific surface area, pore size). Regarding the detailed investigation of the microstructures, TEM has been used for identifying microstructures, revealing the nature of defects and monitoring the kinematics progress at each stage of photocatalysis. Many researches have shown that TEM observation provides a powerful approach for revealing the microstructural evolution and morphology transformation of TiO2 nanocrystals during the preparation processes, such as hydrothermal treatment,91 micro-arc oxidation,92 ion implantation,93 thermal annealing (Figure 3a),94 etc. For example, Liu et al. investigated the influence of Ti ion implantation on the growth process and the formation mechanism of TiO2 nanofilms by TEM at different accelerating voltages.93 They found that a thick TiO2 nanofilm was formed on the surface of SiO2 for the annealed samples implanted at a lower accelerating voltage of 20 kV. When the accelerating voltage was higher than 50 kV, most of the TiO2 nanocrystals were embedded in the substrate with thin layers of TiO2 on their surfaces. The SAED patterns showed a phase change from anatase to rutile after the TiO2 nanoparticles were embedded into substrate.93 In order to gain further insights into the relationship between microstructures and photocatalytic property, it is necessary to observe TiO2 at the atomic level during the photocatalytic reaction. Zhang et al. directly observed lattice distortion of TiO2 during the degradation process by using HRTEM (Figure 3b).95 It was noticed that the lattice distortion of TiO2 occurred after the sample degraded under UV-vis light irradiation. When the degraded sample was placed in the ambient environment with solar illumination for about 30 days, the lattice distortion of the TiO2 recovered. These results revealed that the degradation process can induce lattice distortion of TiO2, which in turn plays a key role during the photocatalytic process. Further in-situ observation of TiO2 during light irradiation in water vapor was conducted by Zhang et al. to explore the photocatalytic splitting of water.96 Different from previous TEM researches, they employed atomic resolution environmental TEM, in which TiO2 could be observed in the presence of reactant, water and light illumination. They found that the initially crystalline surface of TiO2



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was transformed into a disordered layer in the presence of light illumination and water vapor, as shown in Figure 3c. When TiO2 was subjected to light illumination without water vapor, the TiO2 surface remained unchanged, which indicates that water is a key factor for inducing crystal structure transformation on the TiO2 surface. These TEM observations provide a direct evidence for further understanding the photocatalytic mechanism on an atomic scale. In addition to the observation of atomic structure, TEM (or HRTEM) equipped with electron energy-loss spectroscopy (EELS) can provide various types of information, including chemical compositions, lattice and crystal structure, band-gap, oxidation state, etc., to provide more comprehensive results. For example, spatially resolved EELS on an aberration corrected HRTEM can be utilized to investigate the oxidation state of the titanium atoms in the center and surface of the TiO2 crystal.96 EELS measurements can provide the evidence for charge transfer from the TiO2 surface to the O2 molecule based on disappearance of the vacancy loss feature.97 Liberti et al. combined valence EELS with scanning TEM to obtain the dielectric response of anatase nanoplatelets,98 and they were able to characterize the complex dielectric properties with superior spatial (

Present Perspectives of Advanced Characterization Techniques in

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