Bulk and Surface Characterization Techniques of TiO2 and TiO2-Doped Oxides

C H A P T E R

3 Bulk and Surface Characterization Techniques of TiO2 and TiO2-Doped Oxides Di Lan1, Hongjing Wu1, Fabrizio Puleo2 and Leonarda F. Liotta2 1

School of Science, Northwestern Polytechnical University, Xi’an, P.R. China, 2Istituto per lo Studio dei Materiali Nanostrutturati (ISMN)-CNR, Palermo, Italy

3.1 INTRODUCTION

out in the presence of inorganic materials (such as TiO2, Mn3O4/MnCO3, SrTiO3, and SrTi12xMnxO3) used both as catalysts and photocatalysts have been critically reviewed. Differences in mechanistic aspects of the reactions and distribution of the most important intermediates and products can be due to various reasons, for example, the effects of the interaction of light with the solid surface. Photocatalysis belongs to the catalytic process, which can be divided into various steps: (1) diffusion of the reactants from the fluid phase to the surface of the catalyst, (2) adsorption of the reagent(s), (3) reaction in the adsorbed phase, (4) desorption, and (5) diffusion of the product(s) into the bulk of the fluid phase. Powdered TiO2 has been the most popular photocatalyst, due to its excellent photocatalytic activity. The particle absorbs a photon generating an electron/hole couple that recombines or induces oxidation and reduction reactions on the surface or near it.

As global population growth and economic development continue to increase, sustainable energy production and the environment are becoming the top issues and challenges facing humanity. Since the discovery of the Honda Fujishima effect [1,2], TiO2 has been widely used as a photocatalyst for solar energy conversion and environmental applications because of its high photocatalytic activity, biological and chemical stability, low cost, nontoxic nature, and long-term stability. Research indicates that the effectiveness of TiO2 as a photocatalyst depends on its structure of bulk and surface [3], so it is fundamental to clarify this information. Recently, Palmisano et al. [4] presented the differences and similarities between heterogeneous photocatalysis and thermal catalysis. Some papers where a reaction has been carried

Heterogeneous Photocatalysis DOI: https://doi.org/10.1016/B978-0-444-64015-4.00003-1

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3. BULK AND SURFACE CHARACTERIZATION TECHNIQUES OF TIO2 AND TIO2-DOPED OXIDES

So steps 2 and 3 are very crucial for this photocatalytic reaction. Considering the adsorption of the reagent(s), the physicochemical properties of the TiO2 photocatalysts should first be fully understood. In the field of photocatalysis, X-ray diffraction (XRD) is usually used to characterize the lattice parameter of the photocatalyst, including but not limited to the crystal size, crystallinity, facet parameters and crystal type, to further study the composition of crystals, the transformation of crystal phase, its temperature or light intensity dependence, and so on. Transmission electron microscopy (TEM) is usually combined with XRD to accurately investigate structural properties, such as crystal parameters (crystal size, crystal plane spacing, etc.) and bulk defects of samples. Considering the reactions in the adsorbed phase, photocatalytic reactions are actually the reactions of radicals and reactants on the surface of TiO2. Electron paramagnetic resonance (EPR) is usually applied to detect the paramagnetic species on the surface of TiO2, especially the free radicals formed under ultraviolet radiation, which are the reactive species in photocatalytic reactions. On the other hand, X-ray photoelectron spectroscopy (XPS) is successfully applied to characterize the types, distribution, chemical bonds, and chemical environment of the chemical elements on the photocatalyst surface. A detailed surface characterization of TiO2 and TiO2-based oxides is useful in view of further studies for improving their photoactivity behavior. Looking at the catalytic properties of TiO2based oxides, it is important to remember that TiO2 is a reducible oxide able to induce strong metal support interaction with noble metals, improving their catalytic properties [5]. Moreover, TiO2 is characterized by relatively high specific surface area and exhibits both Brønsted and Lewis acidity sites, which contribute to the stabilization of metallic nanoparticles. TiO2 supported noble metal catalysts have been widely investigated thanks to their high activity at low temperature in oxidation and selective oxidation reactions

(such as CO and VOCs (volatile organic compounds) oxidation, PROX (preferential oxidation) reaction) [6 9]. The occurrence of a strong metal support interaction has been claimed as responsible for the stabilization of noble metal nanoparticles strongly interacting with the TiO2 support that enhances the catalytic performances. In particular, XRD and TEM characterizations revealed the presence of nanoparticles as small as 2 3 nm, highly dispersed on the TiO2 support, while XPS analyses have shown the presence of both metallic and ionic species that provide active sites for CO/VOCs activation.

3.2 CHARACTERIZATION TECHNIQUES 3.2.1 X-Ray Diffraction XRD technology is widely used in materials characterization. It is used to identify crystalline phases of the sample, and analyze its space group, symmetry, and lattice parameters. X-rays whose wavelength ranges from 0.01 to 10 nm are part of the electromagnetic spectrum and are generated when matter is irradiated by a beam of high-energy charged particles such as electrons. In the laboratory, a filament is heated to produce electrons, which are then accelerated in a vacuum by a high electric field in the range 20 60 kV towards a metal target, which being positive is called the anode. The corresponding electric current is in the range 5 100 mA. The process is extremely inefficient with 99% of the energy of the beam being dissipated as heat in the target. X-ray photons with a short wavelength (below 0.2 nm) have the energies above 5 keV, therefore possessing the penetrating ability. Moreover, X-rays interact with matter and project the matter’s information into X-ray diffraction pattern. As a result, X-ray is widely used to image the interior of the object and is thought highly of in the field of material characterization.

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In the field of photocatalysis, XRD is usually applied to characterize the lattice parameter of the photocatalyst, including the crystal structure, crystal size, crystallinity, and facet parameters. TiO2 exists in nature in three crystalline polymorphs: anatase, rutile, and brookite. The manner of TiO6 octahedra connections determines the crystal and surface structure as well as the electronic structure and redox potentials of photoinduced charge carriers [10]. Among them, TiO2 anatase, characterized by the lowest packing density (3.8 3.9 g/cm3), shows superior photocatalytic activity to rutile and brookite. Yu et al. [11] developed a surfactanttemplated approach to synthesize phosphated mesoporous TiO2 (PMT) by incorporating phosphorus from H3PO4 directly into the framework of TiO2. The incorporation of phosphorus stabilizes the TiO2 framework and increases the surface area significantly in comparison with pure mesoporous TiO2 (MT). The resulting materials were characterized by several techniques, such as XRD, nitrogen adsorption, TEM, and XPS analyses, and the photocatalytic activity was tested in the oxidation of n-pentane.

From the wide-angle XRD patterns of MT and PMT (see Fig. 3.1), the crystalline size and its temperature dependence can be calculated from the (101) peak of anatase TiO2 phase, at 25.3 2θ. The crystalline sizes of pure MT are 9.0 and 11.4 nm when calcined at 400 and 500 C, respectively, and increase to 21.8 nm up to 600 C. Unlike MT, the crystalline sizes of PMT do not change too much with the increase of calcination temperature from 400 C to 600 C (just from 6.0 to 6.9 nm). A single broad peak is observed on low-angle XRD patterns of the PMT sample calcined at 600 C, indicating its excellent thermodynamic stability; on the other hand the mesoporous framework of the MT collapsed after treatment at 600 C due to the growth of TiO2 grains. As no peaks corresponding to titanium phosphate were observed in the XRD patterns of PMT even after calcination at 600 C, the titanium phosphate should be amorphous. Such amorphous phase proved to be crucial in the stabilization of the PMT structure. Combining XRD results with other characterizations (XPS and TEM) the authors demonstrated the stabilization mechanism of the PMT by phosphoric acid and a model of the framework of the FIGURE 3.1

(101)

Relative intensity (a.u.)

B-600 B-500 B-400 (101)

Wide-angle XRD patterns of MT and PMT calcined at different temperatures. A and B denote MT and PMT, respectively. The numbers following A and B denote the calcination temperatures. Source: Reprinted with permission from J.C. Yu, L.Z. Zhang, Z. Zheng, J.C. Zhao, Synthesis and characterization of phosphated mesoporous titanium dioxide with high photocatalytic activity, Chem. Mater. 15 (11) (2003) 2280 2286, copyright 2003 American Chemical Society.

A-600 A-500 A-400 20

30

40

50

59

60

2 theta (degree)

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calcined PMT was proposed [11]. It seems that the presence of an amorphous titanium phosphate wall between the TiO2 crystalline grains is fundamental for the stabilization of PMT, inhibiting TiO2 crystal growth during calcination. Interestingly, both MT and PMT show better photocatalytic activities than the commercial photocatalyst P25. This was attributed to their high surface area and the facile diffusion of reactants in the wormhole-like mesoporous structure. Moreover, the calcined PMT has better photocatalytic activity than the calcined MT and this was explained by the extended band gap energy, by the larger surface area, and the existence of Ti ions in a tetrahedral coordination that can provide additional surface hydroxyl groups by adsorbing water in air, thus producing hydroxyl radicals, which are powerful oxidants in degrading organics. Indeed, it is known that the adsorbed water and the oxygen of titanium ions in tetrahedral coordination can stabilize electron hole pairs, slowing down the recombination rate and increasing the photocatalytic activity [12]. Stone and Davis [3] prepared mesoporous titania and niobia molecular sieves by a ligand-assisted templating method. All materials were thoroughly characterized by XRD. In general, the photocatalytic activity of TiO2 depends on its crystal phase, particle size, and crystallinity. Among the crystalline phases of TiO2, anatase phase is generally regarded as the most active phase. While in consideration of particle size, smaller particles are usually better photocatalysts due to their high surface area-to-volume ratios. However, the usefulness of particle with diameter less than 10 nm would be limited by the blue shift in usable photons [13 15], so an optimum particle size should be controlled to enhance the photocatalytic activity. In Fig. 3.2 the XRD pattern of synthesized MT is displayed. The large peak at low angle is typical of samples synthesized in the presence

Intensity

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1

2

3

4 2θ/degrees

5

6

7

FIGURE 3.2 Low-angle X-ray diffraction pattern of assynthesized mesoporous titania. Source: Reprinted with permission from V.F. Stone, R.J. Davis, Synthesis, characterization, and photocatalytic activity of titania and niobia mesoporous molecular sieves, Chem. Mater. 10 (5) (1998) 1468 1474, copyright 1998 American Chemical Society.

of surfactant. In addition, the small peaks at higher angles have been attributed to hexagonal ordering of mesopores in these solids [16,17]. Fig. 3.3 compares the crystallinity of MT, ns-TiO2 (titanium precursor in the absence of surfactant), and Degussa P25. The titania P25 contains both phases, that is, anatase and rutile. In the case of ns-titania the prolonged calcination favored partial crystallization to anatase phase, while, in the mesoporous sample the phosphorus present likely prevented crystal growth even after calcination. The abovementioned titania samples were tested as photocatalysts in the liquid-phase oxidative dehydrogenation of 2-propanol to acetone. It was found that the quantum yield of the reaction increased as the amorphous titania and the mesoporous niobia samples were crystallized to greater extents. Indeed, the photocatalytic activity of the poorly crystallized samples was suppressed by defects that act as electron hole traps. Korosi et al. [18] prepared phosphatemodified titania (P-TiO2) samples by the sol gel method with different phosphate

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3.2 CHARACTERIZATION TECHNIQUES

Anatase (101) Rutile (110) 700ºC

P-TiO2/0.05

700ºC

P-TiO2/0.01

700ºC

TiO2

700ºC

Intensity (a.u.)

P-TiO2/0.10

FIGURE 3.3 X-ray diffraction patterns showing the presence of anatase and rutile in various titania samples. Meso-TiO2 was calcined for 1 h at 873 K. Source: Reprinted with permission from V.F. Stone, R.J. Davis, Synthesis, characterization, and photocatalytic activity of titania and niobia mesoporous molecular sieves, Chem. Mater. 10 (5) (1998) 1468 1474, copyright 1998 American Chemical Society.

contents and the photocatalytic activity was tested in the gas-phase degradation of ethanol at room temperature. The XRD characterization method was used to characterize the structural properties of the P-TiO2 samples, and the structural changes of the TiO2 sample were due to the phosphate modification. The results of XRD analysis evidence significant changes upon increasing the phosphate content of the TiO2 sample. Diffractograms of pure TiO2 and P-TiO2 samples with various phosphate contents are compared in Fig. 3.4. It is clear that the pure TiO2 almost completely converts into rutile phase from anatase when calcined at 700 C, while the dominant phase of P-TiO2 calcined at 700 C is still anatase (about 80 wt%). By increasing the phosphate content titania crystallites with smaller size are formed and samples with a high content of phosphate (10 25 wt%) are amorphous. Therefore, doping with phosphate delayed the crystallization of anatase and the transformation of anatase to rutile phase.

TiO2 20

500ºC 22

26

24

28

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2θθ º

FIGURE 3.4 XRD patterns of the pure TiO2 (calcined at 500 C and 700 C) and the phosphate-modified titania samples (calcined at 700 C). Source: Reprinted with permission from L. Korosi, S. Papp, I. Bertoti, I. Dekany, Surface and bulk composition, structure, and photocatalytic activity of phosphatemodified TiO2, Chem. Mater. 19 (19) (2007) 4811 4819, copyright 2007 American Chemical Society.

Other authors also found that the textural characteristics (anatase phase content, particle size and specific surface area) and optical properties of the titania samples are strongly influenced by the presence and content of phosphate [19]. Moreover, the surface-bound phosphate species may enhance the separation of e2/h1 and inhibit the recombination of the charge carrier. The importance of the improved charge separation was previously confirmed for TiO2capped SnO2 (SnO2@TiO2) systems from the enhanced efficiency of hole trapping monitored from the absorption peak at 360 nm [20]. As reported by Chen et al. [21], B-doped TiO2 nanoparticles were prepared by the sol gel method and characterized by XRD. XRD results showed that the doping of boron ions could efficiently inhibit the grain growth and facilitate the anatase-to-rutile

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FIGURE 3.5 XRD patterns of TiO2-RB-500 with different RB values: (a) 0, (b) 1, (c) 3, (d) 5, (e) 10, and (f) 20. Source: Reprinted with permission from D. Chen, D. Yang, Q. Wang, Z.Y. Jiang, Effects of boron doping on photocatalytic activity and microstructure of titanium dioxide nanoparticles, Ind. Eng. Chem. Res. 45 (12) (2006) 4110 4116, copyright 2006 American Chemical Society.

FIGURE 3.6 XRD patterns of B-doped TiO2 samples with RB 10 and calcined at (a) 400 C, (b) 500 C, (c) 600 C, (d) 700 C, and (e) 800 C for 1 h. Source: Reprinted with permission from D. Chen, D. Yang, Q. Wang, Z.Y. Jiang, Effects of boron doping on photocatalytic activity and microstructure of titanium dioxide nanoparticles, Ind. Eng. Chem. Res. 45 (12) (2006) 4110 4116, copyright 2006 American Chemical Society.

transformation prior to the formation of diboron trioxide phase. Fig. 3.5 shows the effect of RB (the atomic ratio of B to Ti) on the crystal structure of TiO2 nanoparticles calcined at 500 C for 1 h. With the increase of RB, the anatase peaks (A) gradually become wider, which means that the size of B-doped TiO2 nanoparticles (calculated from Scherrer formula) decreases correspondingly. These results indicate that B-doping can efficiently inhibit the crystal size and increase the surface area of TiO2. Information on the effect of calcination temperature on the structure of B-doped TiO2 samples can be obtained from Fig. 3.6. The crystallinity of anatase increased with the increase of calcination temperature. It is not difficult to infer that the boron ions may be present in the interstitial site of anatase, which can reduce the surface energy of the nanoparticles and improve the growth of anatase grains. In conclusion, the doping of boron could efficiently inhibit the grain growth and facilitate the anatase-torutile transformation before the formation of

diboron trioxide phase. The anatase phase is in favor of the visible absorption and photocatalytic activity, which has been confirmed in many references. Among TiO2-doped oxides, it is worth mentioning the system Ag TiO2. Ag TiO2 catalysts with different Ag contents were prepared via a sol gel method in the absence of light by Xin et al. [22]. Based on the XRD characterizations, it was found that the Ag doping enhanced the phase transformation of anatase to rutile as well as had a negative effect on the growth of anatase crystallite. Fig. 3.7 shows the XRD pattern of Ag TiO2 with different doping percentages calcined at 400 C for 2 h. Compared with pure TiO2, in the Ag TiO2 catalysts the transformation of anatase to rutile occurs at lower temperature, between 500 C and 700 C. This was likely due to an increased number of defects and oxygen vacancies on the surface caused by Ag doping. Appropriate calcination temperature is needed to make TiO2 evenly distributed on the surface to obtain ideal and perfect crystallinity. The

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FIGURE 3.7 The XRD patterns of Ag TiO2 with different doping ratios calcined at 400 C for 2 h. Source: Reprinted with permission from B.F. Xin, L.Q. Jing, Z.Y. Ren, B.Q. Wang, H.G. Fu, Effects of simultaneously doped and deposited ag on the photocatalytic activity and surface states of TiO2, J. Phys. Chem. B, 109 (7) (2005) 2805 2809, copyright 2005 American Chemical Society.

value of d-spacing increases with the increase of admixture, which means that the silver ion diffuses into the lattice of TiO2. In this study, Xin et al. [22] found that Ag TiO2 photocatalysts with appropriate content of Ag (Ag species concentration is from about 3 to 5 mol%) possess abundant electron traps, which could greatly enhance the activity of the photocatalysts. Some of us have recently reported that XRD diffraction is a very useful technique for the characterization of silica titania functionalized cotton textiles that have been prepared by covering the surface of the fabrics with silica as binder and then with titania. Herein, two different types of titania have been used, a mesostructured high surface area titania (TiMS) and the

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commercial P25. The XRD patterns registered on the functionalized cotton fabrics have shown that the intensity of cotton peaks decreases in the presence of titania coatings onto the surface. X-ray diffraction measurements also confirm the presence of crystalline anatase phase, as results from the most intense peak detected at about 25.3 2θ when the titania content was 10 and 20 wt% for both cotton silica TiMS and cotton silica P25 [23]. For fabrics containing TiMS, the peaks of anatase phase were broad indicating the presence of small crystallites, while sharper peaks were detected for cotton silica functionalized with P25. The self-cleaning properties of such functionalized cotton fabrics were investigated in the discoloration of anthocyanin stains. The use of mesostructured high surface area titania (TiMS) leads to a more efficient stain discoloration with respect to commercial P25 titania according with the presence of smaller crystallites detected by XRD for the former sample [23].

3.2.2 Transmission Electron Microscopy TEM is a technique in which a beam of electrons is transmitted through an ultrathin specimen. An image is formed from the interaction of the electrons transmitted through the specimen. The image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera. TEM is usually combined with XRD to accurately investigate structural parameters (crystal size, crystal plane spacing, etc.) and defects and modifications that may occur at different temperatures. This helps researchers to better understand the nature of photocatalysts and catalysts and improve their properties and performances. Liu et al. [24] think that the distribution of dopants in semiconductors can intrinsically determine the electronic structure and consequently the absorbance, redox potential, and

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charge-carrier mobility of the semiconductor photocatalysts. They prepared N-doped titania photocatalysts through the homogeneous substitution of O by N in the whole particles of layered titanates, and energy-filtered TEM was used to prove the uniform distribution of N element. The elemental maps of Ti, O, and N in the resultant photocatalysts were presented in Fig. 3.8, from which we can obtain the chemical compositions of the single titanate particle. It is very clear that the N species is uniformly presented in the Cs0.68Ti1.83O42xNx particle with very strong contrast. Homogeneous substitution of O by N resulted in extraordinary band-to-band excitation in the visible-light range up to 472 nm, the red-shift to the visible light region, and hence substantially enhances the visible-light absorption and higher mobility of photoexcited carriers.

Finally, it contributes to its high visible-light photocatalytic activity. Authors from the same group [25] have also synthesized I-doped mesoporous TiO2 (MTI) with a bicrystalline (anatase and rutile) framework by a two-step template hydrothermal synthesis route. They also prepared I-doped titania (TI) with anatase structure without the use of a block copolymer as a template. By XRD and TEM analyses it was demonstrated that the high crystallinity and the presence of a bicrystalline framework has a strong effect in improving the photodegradation of methylene blue. In detail, TEM investigation was used to observe the formation of pores with narrow size distribution and the wormhole-like interstitial pores with irregular structure. TEM images of the samples TI and MTI are shown in Fig. 3.9A D, respectively. Parts (A) and (B)

FIGURE 3.8 Energy-filtered TEM images of (A and D) general morphology, (B and E) N map, and (C and F) Ti/O/N overlapped maps of (A C) Cs0.68Ti1.83O4 and (D F) Cs0.68Ti1.83O42xNx, respectively. Source: Reprinted with permission from G. Liu, L.Z. Wang, C.H. Sun, X.X. Yan, X.W. Wang, Z.G. Chen, et al., Band-to-band visible-light photon excitation and photoactivity induced by homogeneous nitrogen doping in layered titanates, Chem. Mater. 21 (7) (2009) 1266 1274, copyright 2009 American Chemical Society.

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FIGURE 3.9 TEM images of the samples (A) TI and (C) MTI. Images (B) and (D) are the area bound by the rectangle in (A) and (B), respectively. Source: Reprinted with permission from G. Liu, Z.G. Chen, C.L. Dong, Y.N. Zhao, F. Li, G.Q. Lu, et al., Visible light photocatalyst: iodine-doped mesoporous titania with a bicrystalline framework, J. Phys. Chem. B, 110 (42) (2006) 20823 20828, copyright 2006 American Chemical Society.

evidence the heterogeneity of pore sizes, while the formation of pores with narrow size distribution occurs for MTI, as can be seen in parts (C) and (D). Combining the results of structural characterization with photocatalytic experiments, it seems that crystal size and surface area can significantly affect the photocatalysis of titania rutile. The disordered mesoporous structure facilitates the diffusion of reactants and products and enhances photocatalytic activity in the methylene blue photodegradation by promoting the presence of active sites on the surface of the photocatalyst. For N-doped and F-doped MT phases, it was demonstrated that the peculiar mesoporous structure is useful for the improvement of photocatalytic activity of such anion and nonmetal cation-doped TiO2 [26,27]. In the experiments of Zhao et al. [28], a carbon@TiO2 “dyad” structure was synthesized to promote visible-light photocatalysis. The TEM image of a microtomed sample demonstrates a sponge-like mesoporous architecture

(Fig. 3.10), the carbon matrix being deposited either on the surface or between particles, which means that the structure can be best understood as a brick-and-mortar type 3D construction. They allege that the novel dyad-type structure with an improved TiO2 hole reactivity can result in an improved photocatalytic activity over the complete spectral range. Zhang et al. [1] reported that the photocatalytic activity of TiO2 was directly related to the surface-phase structure and found that the phase junction formed between the surface anatase and rutile particles can greatly enhance the photocatalytic activity for H2 production. TEM analysis gives direct evidence for the surface anatase/rutile junction formed on rutile TiO2 particles (Fig. 3.11). In the experiments of Yu et al. [11] a wormhole-like mesoporous structure was observed by TEM in MT and PMT in both the as-prepared and in the material calcined at 400 C (Fig. 3.12). The structure of the MT collapsed at 600 C, however, the wormhole-like mesoporous structure and crystalline sizes of

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FIGURE 3.10 TEM micrographs of (A) pure TiO2 obtained upon calcination of C@TiO2 composite; (B) pure TiO2 synthesized directly via solvothermal treatment of Ti isopropoxide in the absence of furfural. Source: Reprinted with permission from L. Zhao, X.F. Chen, X.C. Wang, Y.J. Zhang, W. Wei, Y.H. Sun, et al., One-step solvothermal synthesis of a carbon@TiO2 dyade structure effectively promoting visible-light photocatalysis, Adv. Mater., 22 (30) (2010) 3317 3321, copyright 2010 John Wiley & Sons, Inc. FIGURE 3.11 HRTEM image of a TiO2 (A)/TiO2(R)-n sample. Source: Reprinted with permission from J. Zhang, Q. Xu, Z. Feng, M. Li, C. Li, Importance of the relationship between surface phases and photocatalytic activity of TiO2, Angew. Chem. Int. Ed. 47 (9) (2008) 1766 1769, copyright 2008 John Wiley & Sons, Inc.

FIGURE 3.12

TEM images of as-prepared and calcined MT and PMT. (A) and (B) denote MT and PMT, respectively. AP following (A) and (B) denote as-prepared sample. The numbers following (A) and (B) denote the calcination temperatures. Source: Reprinted with permission from J.C. Yu, L.Z. Zhang, Z. Zheng, J.C. Zhao, Synthesis and characterization of phosphated mesoporous titanium dioxide with high photocatalytic activity, Chem. Mater. 15 (11) (2003) 2280 2286, copyright 2003 American Chemical Society.

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PMT keep stable by TEM observation at 600 C, which is consistent with the XRD results. An interesting application of TEM analysis was the study of the structure and the morphology of a titanium dioxide photocatalyst (Degussa P25) that revealed a multiphasic material consisting of an amorphous phase along with anatase and rutile crystalline phases, in the approximate proportions 80/20 [29]. TEM provided evidence that some particles were a mixture of the amorphous state with either the anatase phase or with the rutile phase, while other particles, mostly anatase, were covered by a thin overlayer of rutile, as confirmed by Moire fringes. The photocatalytic activity of this form of TiO2 was higher than the activities of the single pure crystalline phases and this finding was attributed to the enhancement in the magnitude of the spacecharge potential created by contact between the different phases present in the material. Later, some authors from the same research group, Marcı` et al. [30], prepared photocatalytic activity of polycrystalline ZnO/TiO2. The TEM technique does not reveal defect structures, thus indicating that ZnO has not entered into the TiO2 anatase or rutile structures. It seems likely that ZnO exists as surface species. They think that the coupling of anatase and rutile TiO2 with ZnO on the surface seems useful to achieve a more efficient e2 h1 pair separation

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under visible light due to the defects, thus leading to higher photocatalytic activity. TEM combined with other characterization techniques, like XRD, can help in the detailed investigation of crystal structure, particle size distribution, and shape of the particles in both photocatalysis and heterogeneous catalysis. With the aim to compare, in the present chapter, the advantages of structural characterization techniques applied to photocatalytic and catalytic materials as well, it is noteworthy to cite the HRTEM studies of TiO2 nanoparticles prepared by using oil-in-water microemulsions and used as support for Au catalysts [31]. The new strategy implies the use of organometallic precursors, dissolved in nanometer-scale oil droplets (stabilized by surfactant) and dispersed in a continuous aqueous phase. As shown by HRTEM analysis the average particle size of TiO2 was 2.6 nm and the particles were globular in shape (Fig. 3.13). Such oxide retained a high surface area, small crystallite size, and mesoporosity upon calcination at 400 C and was a suitable support material for deposition of gold nanoparticles to be used for CO oxidation reaction. When HRTEM images were registered for the corresponding Au/TiO2 catalyst (see Fig. 3.14A) Au nanoparticles in the order of 10 20 nm were found (observed as darker, spherical particles) and it was possible to FIGURE 3.13 HRTEM micrograph and particle size histogram of TiO2 nanoparticles obtained in o/w microemulsions. Source: Reprinted with permission from M. Sanchez-Dominguez, L.F. Liotta, G. Di Carlo, G. Pantaleo, A.M. Venezia, C. Solans, et al., Synthesis of CeO2, ZrO2, Ce0.5Zr0.5O2, and TiO2 nanoparticles by a novel oil-in-water microemulsion reaction method and their use as catalyst support for CO oxidation, Catal. Today 158 (2010) 35 43, copyright 2010 Elsevier.

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FIGURE 3.14 HRTEM picture of (A) as-prepared Au/TiO2 catalyst and (B) after CO oxidation. The top inset shows high resolution (HR) of the support and the bottom inset shows HR of Au. The values of the corresponding d spacings and Miller indexes are indicated. Source: Reprinted with permission from M. Sanchez-Dominguez, L.F. Liotta, G. Di Carlo, G. Pantaleo, A.M. Venezia, C. Solans, et al., Synthesis of CeO2, ZrO2, Ce0.5Zr0.5O2, and TiO2 nanoparticles by a novel oil-in-water microemulsion reaction method and their use as catalyst support for CO oxidation, Catal. Today, 158 (2010) 35 43, copyright 2010 Elsevier.

detect the d spacings for crystalline Au (see ˚ for Au 2 0 0) along with d the inset d 5 2.04 A ˚ for spacings for crystalline anatase (d 5 3.52 A TiO2 anatase 101). HRTEM characterization carried out on the Au/TiO2 catalyst after CO oxidation reaction showed that it presented similar characteristics to the as-prepared sample, in terms of support particle size, crystallinity, and gold nanoparticle dispersion (Fig. 3.14B). The high dispersion of Au nanoparticles and the good stability upon catalytic testing explained the good catalytic performances registered for such catalyst in the oxidation of CO at low temperature [31]. Some of us have recently reported a new active mesoporous titania foam as size limiter for Au nanoparticles [32]. TEM characterization of such peculiar titania mesofoams (mFs) have shown the presence of open cells with homogeneous size and shape that make the material appealing for applications as supports in heterogeneous catalysis, especially for Au deposition. Indeed, the problem of particle stability is especially serious for gold, which tends to sinter when treated above 400 C, losing its catalytic activity. Fig. 3.15A displays TEM images of TiO2 nanoparticles from pristine foam, while Fig. 3.15B D display the

high-resolution electron microscopy images of TiO2 and Au/TiO2 nanoparticles. In detail, an individual TiO2 nanoparticle showing lattice fringes from (101) crystal planes of anatase phase, spaced by 0.35 nm, is visible. Such TiO2 mesofoam was fabricated from an aqueous suspension of monodisperse polystyrene beads in diluted titanium acetylacetonate. It was demonstrated that the thermal treatments of the foam controlled the opening of the cells and determined the size and the sintering of the final TiO2 particles forming the cell architecture. Such surface morphology consists of mesopores able to delimit the size of the gold nanoparticles to the mesopore size of the foam (i.e., at average size below 10 nm) even in severe reaction conditions [32].

3.2.3 Electron Paramagnetic Resonance or Electron Spin Resonance EPR or electron spin resonance (ESR) is a spectroscopic technique for studying materials with unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance, but the electron spins are excited instead of the spins of atomic nuclei. EPR can be used to study the electrons and

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69

FIGURE 3.15 TEM micrograph of TiO2 nanoparticles composing the foams. (A) Dispersion of TiO2 nanoparticles from pristine foam. (B) HRTEM image of an individual TiO2 nanoparticle, showing anatase (1 0 1) lattice fringes. (C) As prepared mesofoam/Au catalyst dispersed over TEM grid. (D) HRTEM of gold nanoparticles over aggregates of TiO2. Source: Reprinted with permission from C. Dionigi, L.F. Liotta, L. Ortolani, G. Pantaleo, T. Ivanovska, G. Ruani, New active meso-porous titania foam as size limiter for metal nanoparticles, J. Alloys Compd. 735 (2018) 1611 1619, copyright 2017 Elsevier.

holes in semiconductors and the defects into the lattices. It is also a sensitive and specific method for studying both radicals formed in chemical reactions and the reactions themselves. Organic and inorganic radicals (such as H, OH, and HO2) can be detected by EPR in electrochemical systems and in photocatalysts exposed to UV light. Furthermore, such technique can be used to study the electrical and optical properties of various paramagnetic and ferromagnetic nanomaterials. EPR has proved to be an important tool to study semiconductors, in particular in identifying and elucidating the structure of defects and impurity ions in the structure of solids [33,34]. Electrical activity and the defects of optical activity could also be detected by EPR. Konstantinova et al. [35] reported the EPR investigation of carbon-doped TiO2 in visible light photocatalysis. To characterize these C-doped materials in detail and obtain basic information

on the nature of the carbon dopants, they discussed the electronic properties by EPR spectroscopy. They prepared a series of undoped and C-doped TiO2 powders and discussed the photocatalytic activity and EPR spectra to clarify the properties of the paramagnetic centers and their changes under/without irradiation. The EPR signal arises with C doping, and the higher the carbon content in the samples, the higher the content of paramagnetic centers. So, no changes in the EPR spectra were found for undoped TiO2 samples, as the unmodified titania inactive under these irradiation conditions. On the other hand, all carbon-doped samples were highly active. The increasing of the concentration of these carbon-centered radicals in the ultraviolet or visible light regions may indicate that they are related to photocatalytic reaction. EPR spectroscopy was used by Hurum et al. [36] to probe the charge separation characteristics of a mixed-phase titania

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photocatalyst (Degussa P25). This research described the critical and active role of rutile in TiO2 photocatalyst. The fundamental nature of the enhanced photoactivity was probed by EPR. A broad signal at g 5 2.014 is assigned to surface hole trapping sites (Fig. 3.16), and the signal at g\ 5 1.990 and shoulder at gO 5 1.957 are assigned to lattice electron trapping sites in anatase TiO2. The remaining signals at g\ 5 1.975 and gO 5 1.940 are the lattice electron trapping sites in rutile TiO2. The EPR results indicate that the aggregates in Degussa P25 are composed of variable-sized rutile/anatase clusters, and the nanoclusters contained typically small rutile crystallites interwoven with anatase crystallites. The transition points between these two phases allow for rapid electron transfer from rutile to anatase, therefore, the rutile plays a key role to extend the photoactivity into visible wavelengths and creates catalytic hot spots at the rutile anatase interface to improve the photocatalytic activity. Photocatalytic reactions involve the reactions of radicals and reactants on the surface of active oxides, such as TiO2. EPR was

successfully applied to detect the paramagnetic species on the surface of TiO2, especially the free radicals formed under ultraviolet radiation. Kumar et al. [37] studied the EPR spectra of species photogenerated on anatase and rutile phases of TiO2 nanoparticles at different temperatures to investigate the character of electron and hole pairs. The stability and recombination of trapped electrons and holes from 4.2 K to room temperature have been discussed. They found that it is too hard to observe the EPR spectra at room temperature due to the short lifetime of the electron and hole radicals. While when the temperature dropped to 4.2 K, the system tended to be more orderly. Meanwhile, the stability of electron and hole signals was promoted under the UV illumination. The trapped electron remained stable below 85 K and disappeared at a higher temperature due to the adsorption of electrons by oxygen. The trapped holes were more stable up to B150 K, and the EPR signal of the calcined TiO2 sample decreased or disappeared while the temperature was increased up to 773 K. Continuing to heat up, FIGURE 3.16

EPR spectra of a P25 aqueous slurry under visible and UV illumination, in comparison to those of rutile and anatase under UV and visible illumination. Note the absence of any anatase signal under the experimental visible illumination conditions. Source: Reprinted with permission from D.C. Hurum, A.G. Agrios, K.A. Gray, T. Rajh, M.C. Thurnauer, Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO2 using EPR, J. Phys. Chem. B 107 (19) (2003) 4545 4549, copyright 2003 American Chemical Society.

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3.2.4 X-Ray Photoelectron Spectroscopy XPS, also known as electron spectroscopy for chemical analysis, was developed in the

(A)

Intensity [arbitrary units]

inner electron traps (Ti31) appeared. EPR signals show that anatase TiO2 possesses higher thermodynamic stability and photocatalytic activity than rutile TiO2. Di Valentin et al. [38] combined the EPR, XPS, and density functional theory (DFT) to characterize the paramagnetic species in N-doped anatase TiO2 powders obtained by the sol gel method. They compared the observed EPR hyperfine coupling constants with computed values for structural models of substitutional and interstitial nitrogen impurities. Finally, a new approach to solve complex problem of the interaction of nitrogen with a titanium dioxide matrix was found. Marcı` et al. have prepared polycrystalline ZnO/TiO2 solids and investigated the bulk and surface properties of such materials by using several techniques, such as TEM [30], as previously discussed. In a subsequent article [39], they studied the effect of ZnO addition to TiO2 on electron hole pair separation under light irradiation and used EPR to detect signals attributable to Zn1 species in ZnO/TiO2 (anatase) solids. When the zinc entered into anatase phase TiO2, a new signal appeared (signal B) with a g-value of 1.972. The intensity of the gvalue increased with the zinc content (Fig. 3.17b and c). Signal B was ascribed to Zn1 ions in the small ZnO nuclei, which formed on the surface of anatase, indicating the presence of ZnO on the surface of the samples. Combining the results of photoreactivity experiments reported in this paper, ESR measurements carried out on selected samples suggested that the presence of zinc was beneficial to the stabilization of the free charge carriers, at least in the supported rutile samples, and this result is in principle beneficial for the photoreactivity.

a)

b) (B)

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3300 3400 3500 Magnetic field [G]

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FIGURE 3.17

ESR spectra of (a) TA, (b) TAZnN5, and (c) TAZnN10 samples outgassed at room temperature. Source: Reprinted with permission from G. Marci, V. Augugliaro, M.J. Lopez-Munoz, C. Martin, L. Palmisano, V. Rives, et al., Preparation characterization and photocatalytic activity of polycrystalline ZnO/TiO2 systems. 2. Surface, bulk characterization and 4-nitrophenol photodegradation in liquid solid regime, J. Phys. Chem. B 105 (5) (2001) 1033 1040, copyright 2001 American Chemical Society.

1960s by Swedish scientist Kai Siegbahn, who earned the Nobel Prize in 1981. XPS is a surface-sensitive quantitative technique used to investigate surface chemistry and determine quantitative atomic composition of materials. In XPS an incident X-ray photon (of energy hf, f being the frequency) knocks an electron (with binding energy (BE)), which escapes out of the atom with a kinetic energy equal to E 5 hf 2 BE 2 Φ, where Φ is the work function of the spectrometer (of the order of a few eV). Knowing Φ, the energy of the incident photon (hf) and the kinetic energy of the expelled electron (E), it is possible to determine the BE of the electron and, thus, to identify the specific element.

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The sampling volume of XPS technique extends from the surface to a depth of approximately 5 10 nm, so such technique is unique in providing chemical state information about the composition of surface layers or thin films. In the field of photocatalysis, XPS is usually applied to characterize the nature, distribution, and chemical environment of the chemical elements on the photocatalyst surface. We will show how a thorough characterization of the surface of photocatalysts and catalysts is useful with the aim to improve their activity. Zhao et al. [28] synthesized by one-step solvothermal method a carbon@TiO2 “dyad” structure to promote visible-light 458.7

XPS

FIGURE 3.18 XPS of the C@TiO2 “dyad”-type network. Source: Reprinted with permission from L. Zhao, X.F. Chen, X.C. Wang, Y.J. Zhang, W. Wei, Y.H. Sun, et al., Onestep solvothermal synthesis of a carbon@TiO2 dyade structure effectively promoting visible-light photocatalysis, Adv. Mater. 22 (30) (2010) 3317 3321, copyright 2010 John Wiley & Sons, Inc.

284.9

C 1s

Ti 2p

photocatalysis. They showed collective polarization modes and these optical absorption transitions are feasible to sensitize TiO2, which acts as a novel dyad-type structure with improved hole reactivity and higher photocatalytic activity over the complete spectral range. XPS was used to ascertain the chemical and bonding environment of the TiO2 matrix and carbon phase (see Fig. 3.18, the survey spectrum is given in Fig. 3.19). Two weak peaks were detected at 286.3 and 288.6 eV and assigned to the oxygen bound species, C O and CQO, respectively. No C 1s peak was observed at 281 eV (Ti C bond) [40], and the chemical environments for Ti and O were not

286.3 464.4

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280

FIGURE 3.19 XPS survey spectrum of C@TiO2. Source: Reprinted with permission from L. Zhao, X.F. Chen, X.C. Wang, Y.J. Zhang, W. Wei, Y.H. Sun, et al., One-step solvothermal synthesis of a carbon@TiO2 dyade structure effectively promoting visible-light photocatalysis, Adv. Mater. 22 (30) (2010) 3317 3321, copyright 2010 John Wiley & Sons, Inc.

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changed, strongly suggesting that carbon does not enter the TiO2 phase. It was argued that a polymeric carbonaceous layer is grafted onto the surface of TiO2 via C O Ti bonds [41], with such structure favoring the desired charge transfer upon light excitation and photoactivity. Yu et al. [11] reported that doping with phosphorus, from H3PO4, can result in a stable structure of the MT framework forming high surface area PMT. The generated PMT presents very high photocatalytic activity. The resulting materials were characterized by XPS analysis. To distinguish the different chemical environments of Ti, P, and O in the two samples, MT and PMT, high-resolution XPS spectra were recorded (see Fig. 3.20). The Ti ions in MT are in an octahedral environment (one peak at 458.7 eV), while, the Ti ions in PMT are in two different chemical environments, one in octahedral coordination with oxygen and the other in a tetrahedral environment (two peaks at 458.9 and 459.7 eV) [42]. The P ions in PMT present in a P51 state (the P 2p BE of 133.8 eV) [43]. The O 1s single peak at 529.8 eV in the spectrum of MT corresponds to the Ti O bond in TiO2. In the case of PMT,

73

four peaks were detected and assigned to Ti O (at 529.6 eV), to P O (at 530.6 eV), to OH groups (at 531.6 eV), and to C O bonds (at 532.8 eV), respectively. Impregnation method was used by Hu et al. [44] to prepare surface bond-conjugated TiO2/ SiO2. Based on the results of XPS analyses, they concluded that the growth of titanium dioxide on silicon dioxide substrate seems to be achieved through the crosslinking bond of Ti O Si in the TiO2 phase. Combining XPS data with other characterizations, XRD, Fourier-transform infrared spectrophotometer, and Brunauer Emmett Teller, the structure model of TiO2/SiO2 was proposed. Ti 2p photoelectron peaks are shown in Fig. 3.21 for Ims8 and Ims30 (having different silica gel size). The binding energies of Ti 2p of such samples are 0.1 and 0.4 eV smaller than that of the pure TiO2 because of the interface formation of TiO2 SiO2. XPS results indicate that the growth of titania on the silica substrate seems to be determined by Ti O Si cross-linking bonds of the TiO2 phase over silica. The structure model of TiO2/SiO2 was confirmed to enhance the adsorption and initial rate of the reactant and related to the photodegradation of reactive FIGURE 3.20 High-resolution XPS spectra of the O 1s region taken on the surface of MT and PMT calcined at 600 C. (A) and (B) denote MT and PMT, respectively. Source: Reprinted with permission from J.C. Yu, L.Z. Zhang, Z. Zheng, J.C. Zhao, Synthesis and characterization of phosphated mesoporous titanium dioxide with high photocatalytic activity, Chem. Mater. 15 (11) (2003) 2280 2286, copyright 2003 American Chemical Society.

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1000

FIGURE 3.21

3500

900 3000 800 Ims30

2500

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CPS

CPS

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Ti 2p photoelectron peak of TiO2 loaded on silica for Ims8 and Ims30 as measured by XPS. Source: Reprinted with permission from C. Hu, Y. Wang, H. Tang, Preparation and characterization of surface bond-conjugated TiO2/ SiO2 and photocatalysis for azo dyes, Appl. Catal. B: Environ. 30 (3 4) (2001) 277 285, copyright 2001 Elsevier.

Ims8 400

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2p3/2

2p1/2

TiO2

FIGURE 3.22 X-ray photoelectron spectral details collected from TiO2 and N-TiO2 samples: (A) Ti 2p, (B) N 1s, and (C) O 1s core levels. Note the shift in Ti 2p BE after the introduction of N into the TiO2 lattice. Source: Reprinted with permission from M. Sathish, B. Viswanathan, R.P. Viswanath, C.S. Gopinath, Synthesis, characterization, electronic structure, and photocatalytic activity of nitrogen-doped TiO2 nanocatalyst, Chem. Mater. 17 (25) (2005) 6349 6353, copyright 2005 American Chemical Society.

TiO2 N-TiO2 N-TiO2 456

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dye R15 and cationic dye CBX (cationic blue X-GRL). Sathish et al. [45] reported a chemical method using TiCl3 as precursor to synthesize the nitrogen-doped TiO2 (N-TiO2) nanocatalyst with spherical shape and homogeneous size. The chemical nature of N evolved as

N Ti O in the anatase TiO2 lattice as identified by XPS. As is shown in Fig. 3.22A, Ti 2p3/2 core level appears at 459.3 and 458.5 eV for TiO2 and N-TiO2 calcined at 400 C, respectively. Lower BE of Ti 2p in N-TiO2 indicates that the electronic interaction of Ti with anions is

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totally different from that of TiO2. This suggests that TiO2 lattice is considerably modified due to N-substitution. It can also be explained by the formation of covalence between the Ti and nitrogen bond. Accordingly, the peak observed in the present study at 398.2 eV (Fig. 3.22B) was attributed to the N2 anion incorporated in the TiO2 as N Ti O structural feature. The N should be from the TiO2 lattice as N Ti O linkages. In both cases the O 1s peak appears at around 530 eV (Fig. 3.22C), indicating that the nature of oxygen is similar, however, a broadening of the signal at 531.5 eV is detectable for the N-TiO2 sample; this might due to the presence of oxygen and nitrogen from the same lattice in TiO2. In conclusion, the XPS results demonstrate that the state of N is like an anion and anion doping of TiO2 has a considerable effect on the band gap reduction, therefore, it increased the photocatalytic activity [45]. Liu et al. [46] reported the first example demonstrating visible light photoactivity of

75

N-doped anatase TiO2 sheets with dominant {001} facets. They demonstrated that such TiO2 sheets exhibit the capability for generating important photocatalysis active species of •OH radicals and splitting water into hydrogen under visible light irradiation. In Fig. 3.23, XP spectra of F 1s, N 1s, Ti 2p, and O 1s for the anatase TiO2 sheets are displayed. The BE of F 1s at 684.2 eV belongs to the typical surface Ti F species, the N 1s binding energies of 399.5 eV originate from interstitial N or O Ti N, and the other peak signal at 401.0 eV was due to adsorbed nitrogen. Compared with the pure anatase TiO2, the nitrogen-doped anatase TiO2 sheets show a BE shift toward high energy by 0.15 eV for Ti 2p and O 1s due to nitrogen doping. It is known that F doping itself cannot introduce visible light absorption but can be favorable for enhancing photocatalytic activity. In conclusion, Liu et al. [46] demonstrated that by simple one-pot method was possible to prepare N-doped TiO2 anatase phase with a large FIGURE 3.23 High-resolution X-ray photoelectron spectra (XPS) spectra of N 1s, F 1s, Ti 2p, and O 1s of the nitrogen-doped anatase TiO2 sheets (a); high-resolution XPS spectra of Ti 2p and O 1s of undoped anatase TiO2 sheets (b). Source: Reprinted with permission from G. Liu, H.G. Yang, X.W. Wang, L.N. Cheng, J. Pan, G.Q. Lu, et al., Visible light responsive nitrogen doped anatase TiO2 sheets with dominant (001) facets derived from TiN, J. Am. Chem. Soc. 131 (36) (2009) 12868 12869, copyright 2009 American Chemical Society.

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percentage of {001} facets, induced by surface Ti F bonds, able to split water into hydrogen under visible light irradiation. XPS is a valuable technique helpful in the characterization of thin films and their surface modification. Recently, some of us have reported that hybrid paper TiO2, paper Cu2O TiO2, and paper TiO2 Cu2O photocatalysts can be successfully prepared via a nonhydrolytic sol gel process [47]. To investigate the surface modification and composition, XPS characterization was carried out over three samples: the neat paper, the paper TiO2, and the paper TiO2 Cu2O. Such sample, prepared by deposition of copper from CuSO4 solution over preformed paper TiO2, was more active than the paper Cu2O TiO2 in the photodegradation of toluidine in water under exposure to simulated sunlight. The XPS survey of the neat paper, paper TiO2, paper TiO2 Cu2O, and paper Cu2O TiO2 is shown in Fig. 3.24A and B, respectively. The loading of TiO2 over the paper leads to the disappearance of Si peaks present in the paper and the appearance of the Ti 2p peak at 458.8 eV (Fig. 3.24A). Moreover, a downshift of the O 1s peak form 533 eV of the bare paper to 529.9 eV and 531.5 eV that are O 1s peak positions typical of lattice oxygen and to hydroxyl groups of TiO2, was observed (see inset Fig. 3.24A and B). The peaks due to the Cu element are well evident in the survey spectra of paper TiO2 Cu2O and paper Cu2O TiO2 (see Fig. 3.24B). The peak position of Cu 2p3/2 centered at 932.3 eV and the lack of the shakeup features are indicative of the presence of Cu2O in accordance with the reduction treatment performed with hydrazine. This is further confirmed by the absence of the multiplet structure, between 938 and 947 eV, typical of Cu21 species. In the XP spectrum of the sample, paper Cu2O TiO2, the Cu 2p3/2 peak was not visible, presumably because Cu2O nanoparticles are covered

by the TiO2 layer with thickness exceeding the analysis depth of XPS, around 8 10 nm. In conclusion this paper demonstrated that the preparation method strongly influenced the photocatalytic efficiency, the best performance being observed when the Cu2O NPs were present on top of the TiO2 layer, according with XPS investigation that was decisive in highlighting the differences. Lately, we have applied the XPS technique also for the characterization of TiO2 thin layers immobilized on cellulose paper as effective photocatalyst for 2-propanol degradation in gas phase [48]. Such paper TiO2 composites were prepared by two simple routes. The first method involved the generation in situ of a TiO2 layer over the sheet of paper, then, a hydrothermal treatment at 120 C was performed. The second approach involved the adsorption on the paper of TiO2 sol nanoparticles formed in an autoclave at 140 C. The X-ray photoelectric spectroscopy confirms that deposition of TiO2 on the surface of the paper occurs with both preparation methods. As previously reported [47], the presence of the titanium peak and the decrease of the O 1s component at 533 eV related to the hydroxo groups of the paper confirmed the efficient covering of the surface by titania, the presence of which was further confirmed by the appearance of two components at 530 and at 532 eV, attributed, respectively, to the lattice oxygen and to hydroxyl groups. Moreover, based on the relative percentages of the two O 1s components, it resulted that the paper TiO2 prepared by hydrothermal method contained a higher number of surface OH with respect to paper TiO2 by sol. The higher superficial hydroxylation of such paper TiO2 with respect to the analogue prepared by sol helped the photocatalytic efficiency in the complete oxidation of 2-propanol in CO2 studied under simulated sunlight irradiation and at room temperature.

HETEROGENEOUS PHOTOCATALYSIS

FIGURE 3.24 (A) XPS survey of paper and paper TiO2; (B) XPS survey of paper TiO2 Cu2O and paper Cu2O TiO2. In the inset the high-resolution O 2s, Ti 2p, and Cu 2p regions are reported. Source: Reprinted with permission from M. Sboui, S. Bouattour, M. Gruttadauria, L.F. Liotta, V. La Parola, S. Boufi, Hybrid paper-TiO2 coupled with a Cu2O heterojunction: an efficient photocatalyst under sun-light irradiation, RSC Adv., 6 (2016) 86918 86929, copyright 2016 The Royal Society of Chemistry.

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3.2.5 Acidity Characterization The acidity properties are also important characteristics that affect photocatalytic and catalytic activity, as will be discussed in this section. The acidity characterization of a solid material is based on making a clear distinction between Brønsted acid and Lewis acid, measuring the value and distribution of acidity and acid strength quantitatively. There are two main methods that are used generally and that will be described elsewhere: the temperatureprogrammed desorption (TPD) of ammonia and the Fourier-transform infrared spectroscopy (FT-IR) of chemisorbed probe molecules, such as pyridine [49 51].

Garcia-Lopez et al. [52] prepared and characterized binary materials composed of the oxides SiO2, TiO2, and N-doped TiO2 and the Keggin heteropolyacid, H3PW12O40 (PW12), which were used as catalysts and photocatalysts for the hydration of propene to 2propanol. The characterization results of the acidity properties were useful to explain the key role played by the PW12 in the composite materials in both the thermal and photoassisted catalytic processes. In this research, the NH3-TPD method was used to evaluate the acidic properties of the samples. The NH3-TPD experiments provided information on the total acidity of the heteropolyacids supported on TiO2. As it is reported in Fig. 3.25 the materials

FIGURE 3.25 NH3-TPD curves for the as-prepared samples: (A) (a) PW12, (b) PW12/N-TiO2, (c) PW12/TiO2, (d) PW12/ TiO2 ex A; (B) (a) PW12, (b) PW12/SiO2, (c) PW12/SiO2 ex A; (C) TiO2; and (D) SiO2. Source: Reprinted with permission from E.I. Garcia-Lopez, G. Marci, F.R. Pomilla, L.F. Liotta, B. Megna, M.C. Paganini, et al., Improved (photo)catalytic propene hydration in a gas/solid system by using heteropolyacid/oxide composites: electron paramagnetic resonance, acidity, and role of water, Eur. J. Inorg. Chem. (2017) 1900 1907, copyright 2017 John Wiley & Sons, Inc.

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show different Lewis or Brønsted acidities that account for the different catalytic and photocatalytic activities. In Fig. 3.26, the 2-propanol formation rates for the binary materials as catalysts and photocatalysts are reported per gram of PW12, which is the active catalyst/photocatalyst. All the binary materials showed higher catalytic and photocatalytic activities than the bare PW12, with the exception of PW12/TiO2 ex A, which was completely inactive. The acidity properties play, therefore, an important role in the propene hydration reaction that occurs through an acid/base mechanism involving dioxonium anions placed between PW12 anions. Strong Brønsted acidic sites are needed for the occurrence of propene hydration and their amount is directly proportional to the catalyst activity [52]. Marci et al. [53] investigated the (photo) catalytic activity of Wells Dawson (H6P2W18O62) in comparison with Keggin (H3PW12O40) heteropolyacids for 2-propanol dehydration in gas solid regime paying special attention to the acidity effect on the photocatalytic activity. In this research, two types of heteropolyacids (HPAs), that is, H3PW12O40 or H6P2W18O62, were deposited on SiO2 or TiO2 and used as photocatalysts in the reaction of 2-propanol dehydration to propene in gas solid regime.

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For comparison, bare HPAs were also investigated. The acidity of the photocatalysts was determined by NH3-TPD experiments, which provided information on the total acidity of the catalysts without distinguishing between Brønsted acid sites (typical of HPAs and SiO2) and Lewis acid sites, characteristic of defective TiO2 support. The amount of NH3 desorbed (ppm/g HPAs) gives a quantitative evaluation of the number of active sites, while the temperature of desorption is an indication of the strength of the acid sites. In Fig. 3.27, the NH3TPD profiles versus time and temperature are displayed for SiO2 supported HPAs (A), for TiO2 supported HPAs (B), and for bare SiO2 and TiO2 (C). Bare P2W18 and PW12, were inserted for comparison. A careful analysis of the TPD curves suggests, for both samples, P2W18 and P2W18/SiO2, the presence of some strong acid sites. In the case of bare and SiO2 supported PW12 the main desorption peak of ammonia was detected at 600 C confirming that the acid sites in the Keggin-type heteropolyacids are stronger than those of Dawsontype HPAs (P2W18). By looking at the NH3-TPD curves of TiO2 supported HPAs, a broad peak centered at around 450 C was observed attributed to the Lewis acid sites typical of TiO2. Moreover, peaks corresponding to the Brønsted acid sites of the HPAs were FIGURE 3.26 2-Propanol formation rate per gram of PW12 present in the various solids in the catalytic (white) and photocatalytic (gray) propene hydration reaction. Water concentration: 2 mM. Source: Reprinted with permission from E.I. Garcia-Lopez, G. Marci, F.R. Pomilla, L.F. Liotta, B. Megna, M.C. Paganini, et al., Improved (photo)catalytic propene hydration in a gas/solid system by using heteropolyacid/oxide composites: electron paramagnetic resonance, acidity, and role of water, Eur. J. Inorg. Chem. (2017) 1900 1907, copyright 2017 John Wiley & Sons, Inc.

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20

40 Time [min]

60

80

FIGURE 3.27 NH3-TPD profiles versus time and temperature for SiO2 supported HPAs (A), for TiO2 supported HPAs (B), and for bare SiO2 and TiO2 (C). Bare P2W18 and PW12, were inserted for comparison in (A) and (B). Source: Reprinted with permission from G. Marci, E.I. Garcia-Lopez, F.R. Pomilla, L.F. Liotta, L. Palmisano, Enhanced (photo)catalytic activity of Wells-Dawson (H6P2W18O62) in comparison to Keggin (H3PW12O40) heteropolyacids for 2-propanol dehydration in gas solid regime, Appl. Catal. A 528 (2016) 113 122, copyright 2016 Elsevier.

detected at 495 C for P2W18/TiO2 and at 600 C for PW12/TiO2. Summarizing from NH3-TPD results it emerges that P2W18 shows higher amount of acid sites with respect to PW12, while PW12 presents stronger acid sites with respect to P2W18. In Fig. 3.28 the reaction rate of propene formation per gram of HPA versus 2-propanol concentration are compared by using bare PW12, P2W18 (A), SiO2 and TiO2 supported HPAs (B) and (C), respectively. As previously reported [52], the catalytic dehydration of 2-propanol occurs by means of an acidbase mechanism (elimination E1) involving the dioxonium ions placed between the HPA

anions, therefore, the strength and the amounts of acid sites present in the catalysts account for their activity. The results indicate that the Wells Dawson heteropolyacid has higher catalytic and photocatalytic activity than the Keggin one particularly when supported over titania. As already mentioned, the acidic properties can be investigated also by FT-IR spectroscopy of chemisorbed probe molecules. Lebarbier et al. [54] prepared two series of WOx/TiO2 catalysts by pore volume impregnation of two different supports, amorphous TiO2 or crystallized anatase TiO2. The relationships between

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(B) 3.0

Rate of propene formation [mmol/h/gHPA–1]

Rate of propene formation [mmol/h/gHPA–1]

(A)

2.5 2.0 1.5 1.0 0.5 0.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

2-Propanol concentration [mM]

(C)

5.0

4.0

3.0

2.0

1.0

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

2-Propanol concentration [mM]

Rate of propene formation [mmol/h/gHPA–1]

10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

2-Propanol concentration [mM]

FIGURE 3.28 Reaction rate of propene formation per gram of HPA versus 2-propanol concentration for runs carried out by using: (A) PW12 (W) and bare P2W18 (◇), (B) PW12/SiO2 (W) and P2W18/SiO2 (◇), and (C) PW12/TiO2 (W) and P2W18/TiO2 (◇) as catalysts (dotted line) or as photocatalysts (full line). Flow rate of the feeding gas equal to 100 mL/ min, temperature 80 C and 0.5 g of catalyst. Source: Reprinted with permission from G. Marci, E.I. Garcia-Lopez, F.R. Pomilla, L.F. Liotta, L. Palmisano, Enhanced (photo)catalytic activity of Wells-Dawson (H6P2W18O62) in comparison to Keggin (H3PW12O40) heteropolyacids for 2-propanol dehydration in gas solid regime, Appl. Catal. A 528 (2016) 113 122, copyright 2016 Elsevier.

the acidity trend and catalytic performances were examined. The experiment of 2-propanol dehydration was used to test the catalytic activity. A direct relationship was observed between the concentration of Brønsted acid sites and the catalytic activity for 2-propanol dehydration. In the present article, the acidity was monitored by adsorption of two different probe molecules:

2,6-dimethylpyridine and CO followed by IR. Fig. 3.29 shows the variation of propene formation rate as a function of the abundance of Brønsted acid sites and the results indicate an increase in the catalytic activity with an increase in the number of Brønsted acid sites prior to the reaction. Vishwanathan et al. [55] prepared a series of TiO2 ZrO2 mixed oxides with varying

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Propene formation rate (mmol/h/g)

40

30

20

10

0

0

20 30 10 µmol/g) Conc. Bronsted acid sites (µ

FIGURE 3.29 Variation of propene formation rate determined at 403 K and D 5 60 mL/min as a function of Brønsted acid sites concentration measured after lutidine desorption at 573 K. Source: Reprinted with permission from V. Lebarbier, G. Clet, M. Houalla, Relations between structure, acidity, and activity of WOx/TiO2: influence of the initial state of the support, titanium oxyhydroxide, or titanium oxide, J. Phys. Chem. B 110 (2006) 22608 22617, copyright 2006 American Chemical Society.

molar ratio of TiO2 to ZrO2 by the coprecipitation method and the acid base properties were investigated by TPD of NH3 and CO2 and by IR spectroscopy of adsorbed pyridine. The catalytic activities were investigated in the vapor phase dehydration of methanol to dimethyl ether in a fixed-bed reactor under atmospheric pressure. A relationship between dehydration activity and the acid base properties of the TiO2 ZrO2 catalysts was observed. NH3-TPD was used to estimate the amount and strength of acid sites formed on the surface of TiO2 ZrO2 catalysts. As is shown in Figs. 3.30 and 3.31, the influence of surface acidity on methanol dehydration activity was studied by varying TiO2 content in TiO2 ZrO2 mixed oxides. The acidity increases with the increase of TiO2 content, even though the sample Ti/Zr 5 1/1 has more surface acidity than Ti/Zr 5 7/3. The CH3OH conversion is higher

FIGURE 3.30 NH3-TPD on oxides: (a) ZrO2; (b) Ti/Zr 5 1/9; (c) Ti/Zr 5 3/7; (d) Ti/Zr 5 1/1; (e) Ti/Zr 5 7/3; and (f) TiO2. Source: Reprinted with permission from V. Vishwanathan, H.S. Roh, J.W. Kim, K.W. Jun, Surface properties and catalytic activity of TiO2 ZrO2 mixed oxides in dehydration of methanol to dimethyl ether, Catal. Lett. 96 (2004) 23 28, copyright 2004 Springer International Publishing AG.

FIGURE 3.31 Influence of TiO2 content in TiO2 ZrO2 on surface acidity and CH3OH conversion (reaction conditions: T 5 280 C, catalyst 5 1.5 g, WHSV (weight hourly space velocity) 5 0.316 h21, and P 5 1 bar). Source: Reprinted with permission from V. Vishwanathan, H.S. Roh, J.W. Kim, K. W. Jun, Surface properties and catalytic activity of TiO2 ZrO2 mixed oxides in dehydration of methanol to dimethyl ether, Catal. Lett. 96 (2004) 23 28, copyright 2004 Springer International Publishing AG.

in the latter case, because strong acid sites are required for the dehydration of alcohols to form hydrocarbons [55].

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Among the different methods applied for surface acidity determination in titania doped oxides, it is worthy to mention the study by Navarrete et al. [56] who prepared sulfated titania silica gels. Three different methods have been used to study the acidity of these solids: formation of H-bonds with benzene, selective H/D exchange with deuteronbenzene, and adsorption of pyridine. They showed that the nonsulfated TiO2 SiO2 gels contain Lewis acid properties that increased by sulfation, while the Brønsted acidity remains negligible. A material exhibiting quite different surface properties can be obtained by using sulfuric acid as catalyst in the gelation step of synthesis. In that case, strong Brønsted acid sites were detected as it was shown by both pyridine adsorption and a shift of OH groups by interaction with benzene. Kozlov et al. [57] studied the effect of the acidity of TiO2 surface on its photocatalytic activity in acetone gas-phase oxidation. The activity of TiO2 was shown to depend strongly on the concentration of acid and basic sites on the surface. It was 1.23 times greater for TiO2 treated with 10 M H2SO4 solution than for the untreated sample. Such an enhancement could be explained by the increase in the number of acid sites on the catalyst surface, and this dependence may be related to changes in the adsorption energy of the reagents. Similarly, Lin et al. [58] acquired TiO2 nanotubes with strong acidity by means of impregnating TiO2 with sulfuric acid solution and calcining at 300 C, and they found that the sulfated TiO2 nanotubes were very reactive toward the esterification reaction. Anderson and Bard [59] prepared improved photocatalysts by fabricating composite materials through sol gel process. The photocatalytic properties of TiO2 and the adsorptive properties of SiO2 or Al2O3 were combined, and the basic Al2O3 adsorbed acidic species and showed improved photocatalytic decomposition of salicylic acid.

Acidity measurements from DFT based molecular dynamics have been also reported in the literature. Recently, Cheng and Sprik investigated the thermodynamics of protonation and deprotonation of the rutile TiO2 (110) water interface using a combination of DFT based molecular dynamics (DFTMD) and free energy perturbation methods [60]. Acidity constants were computed from the free energy for chaperone assisted insertion/removal of protons in fully atomistic periodic model systems treating the solid and solvent at the same level of theory. They compared the results with those derived from the MUltiSIte Complexation (MUSIC) model. The conclusion regarding the MUSIC model is that, while there is good agreement for the acidity of an adsorbed water molecule, the proton affinity of the bridging oxygen obtained in the DFTMD calculation is significantly lower than the MUSIC model value. However, despite the discrepancies, such information may be useful for further development of models for intrinsic surface acidity constants.

3.3 CONCLUSIONS Several characterization techniques of surface and bulk properties of TiO2 and TiO2based oxides have been discussed in the present chapter and the relationships between characterization results and photocatalytic/catalytic data have been analyzed. The XRD technique applied to the characterization of (photo)catalysts allows the identification of the crystalline phases and structural parameters, including the crystal symmetry, crystal size, and lattice parameters. TEM is usually combined with XRD to more accurately perform structural characterization and to observe defects in the lattice. EPR is a useful technique to study the physical source of some typical defects along with the electrical and optical properties of paramagnetic and

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ferromagnetic materials that find wide application in photocatalysis and catalysis. As it concerns the surface properties, XPS is a fundamental technique providing information about the nature, oxidation state, and distribution of the elements on the surface of materials, as powders or thin layers. Finally, we cannot neglect the study of the acidic properties of TiO2-based oxides and as well of supported heteropolyacids. Acidic properties play an important role in the mechanisms of photocatalytic and catalytic reactions and it has been proved that the (photo)catalytic activity depends on both strength and concentration of acid sites. In conclusion, an exhaustive characterization of the surface/bulk phases of TiO2 and TiO2-doped oxides can be obtained by means of the so-far mentioned techniques, which allow the researchers to understand the relationship between physicochemical properties and (photo)catalytic performances.

Acknowledgements Financial support was provided by National Natural Science Foundation of China (Nos. 50771082 and 60776822). The Project Supported by Natural Science Basic Research Plan in Shaanxi Province of China (program no. 2017JQ5116).

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