BACKGROUND AND THEORY - 晶體內部與表面摻雜釩離子對二氧化鈦光觸媒物化特性與光催化活性之影響

2-1 TiO

semiconductor photocatalysts 2-1-1 Background and material properties

In 1972, Fujishima and Honda successfully found TiO2 nanoparticles that can be used in photocatalytic reactors for degrading pollutions in water or air.²² Afterward, scientific studies on development and fabricate of photocatalysis by semiconductor was blossomed, which are listed in Table 2-1. In order to improve the efficiency of photoactivity, some surface modification methods have been proposed for the development of advanced photocatalysts, including composite semiconductors, surface sensitization, and transition metal doping.^{5, 9, 17} Nowadays, TiO2 has been widely used in industrial application such as photocatalysis²³, solar energy cell²⁴, and gas sensors.²⁵

TiO2possesses a bandgap of 3.0-3.2 eV. Figure 2-1 shows the total density of electronic states of TiO2. The conduction band (CB) and valence band (VB) of TiO2mainly consist of the Ti 3d and O 2p states, respectively. Generally, TiO2can be excited by energy in terms of heat or photon. The excited semiconductor has electrons and holes pairs within conduction and valence bands for further redox reactions, respectively. Thus, the principle of photocatalysis is the transformation of excited electrons and holes which play important roles in this system.^{17, 26}

Figure 2-1 The density of electronic states of TiO2.²⁷

Table 2-1 Milestone of TiO₂-related studies.

Year Authors The results and findings Ref.

1972 Fujishima et al.

First finding about electrochemical photolysis of water at a semiconductor electrode.

1987 Matthews et al.

Photooxidation of organic impurities in water using thin films of titanium dioxide.

1994 Choi et al. The summarization of metal-ion dopants in quantum-sized TiO2.

1994 Martin et al. Photochemical mechanism of quantum-sized vanadium-doped TiO2particles

1995 Linsebigler et al.

Mechanisms of photocatalysis of TiO₂, including surface modification method.

1999 Litter et al. The mechanism of photocatalytic systems on transition metal ions doped in TiO₂.

2002 Haber et al. Surface doping of rutile by vanadium. ¹⁸ 2003 Diebold et al. The surface science of titanium dioxide. ³¹ 2003 Weckhuysen

et al.

Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis

2004 Lee et al. Electronic surface state of TiO₂electrode doped with transition metals, studied with cluster model and DV-X alpha method

2007 Xin et al. The mechanisms of photoinduced carriers separation and recombination for Fe³⁺–TiO2photocatalysts

TiO₂ has three types of crystalline structures: anatase, rutile, and brookite. At higher temperature above 623 K, anatase starts to transform to brookite and /or rutile, and then brookite transforms to rutile. At lower temperature below 623 K, the transformation between anatase and brookite may be reversible. The activation energy of transformation from anatase to brookite (11.9 KJ mol^-1) is much lower than that from brookite to rutile (163 KJ mol^-1). This means the brookite → rutile transformation occurs under higher temperature,

transforms to brookite and then transforms to rutile.

Figure 2-2 Variation of enthalpies of anatase, brookite, and rutile as a function of particle size.³³

Table 2-2 The relationship between crystal size and stable phase³³. Crystal size Most stability phase

< 11 nm Anatase

11 ~ 35 nm Brookite

> 35 nm Rutile

Figure 2-3 shows the crystalline structures of anatase and rutile TiO₂. Each Ti⁴⁺ion is surrounded by an octahedron of six O^2- ions. In the rutile structure, each octahedron is in contact with 10 neighboring octahedrons (two sharing edge oxygen pairs and eight sharing corner oxygen atoms), while in the anatase structure each octahedron is in contact with eight neighbors (four sharing an edge and four sharing a corner). These differences in lattice structures cause different mass densities and electronic band structures between anatase and rutile.¹⁷ The octahedron in anatase is significantly distorted so that its symmetry is lower than orthorhombic. Therefore, anatase shows a higher adsorptive ability toward organic compounds²³ and lower rate of charge recombination³⁴, while rutile shows a lower bandgap energy and higher thermal stability.¹⁷

Figure 2-3 The crystal phases of TiO2.¹⁷

When the crystallite dimension of a semiconductor particle falls below a critical radius of approximately 10 nm, the band gap increases and the band edges shift to yield larger redox potentials, as shown in Figure 2-4. Thus, the size-quantized semiconductor TiO2 particles may result in increased photo-efficiencies for systems in which the rate-limiting step is charge transfer. Figure 2-5 shows a high photo-reactivity of quantum-size TiO₂ due to the lack of band bending, while both electrons and holes are readily available at the interface (or vary close). However, size-quantized TiO₂ have been found to be less photoactive than their bulk-phase cases because surface speciation and surface defect density reduce photoactivity.

Thus, the positive effects of increased over potentials (i.e., difference between E_vband E_redox) on quantum yields could be offset by unfavorable surface speciation and surface defects due to the preparation method of size-quantized semiconductor particles.²⁶

Figure 2-4 UV-Vis reflectance spectra of size-quantized TiO₂.²⁶

Figure 2-5 The formation of a space charge layer in a large and small semiconductor particle in equilibrium with a solution redox system.¹⁰

2-1-2 Principle of photocatalysis

Heterogeneous photocatalysis is one of the popular techniques for decontamination of air and wastewater, because photocatalyst can transform solar energy into chemical energy to degrade pollutants. The basic principles of heterogeneous photocatalysis can be simply summarized as follows. After the generation of charge carriers by absorbing UV-light with the energy over the band gap, the charge carriers undergo trapping, recombination, detrapping, and migration to the surface, as seen in Figure 2-6.^{14, 26, 29}

Figure 2-6 Schematic photoexcitation in a solid followed by deexcitation events.¹⁷

Figure 2-7 shows the basic transitions in a semiconductor, including intrinsic and extrinsic transitions. When the semiconductor is irradiated with UV light, photons are absorbed to create electron-hole pairs while the photon energy (hν)is equal to or larger than the bandgap energy (Eg). Ifhν isgreaterthan Eg, excess energy is dissipated as heat, as shown in Figure 2-7 (b). These processes are called intrinsic transitions or band-to-band transitions. In addition,forhν islessthan Eg, a photon will be absorbed by energy states which are created by chemical impurities or physical defects. Above performance is defined extrinsic transition, as shown in Figure 2-7 (c).³⁵

Figure 2-7 Opticalabsorption for(a)hν= Eg, (b)hν > Eg, and (c)hν < Eg.³⁵

For thermodynamic view point, the band energy positions and the redox potential of semiconductors determine the ability of charge transfer to acceptors. Adsorbed pollutants can be reduced by conduction band (CB) electrons if they have redox potential more positive than the Vfb of the CB. Besides, the pollutants can also be oxidized by valence band (VB) holes if they have reduction potential more negative than the Vfb of the VB.^{30, 36} Figure 2-8 shows the band edge positions for various semiconductors. Left axis presents the internal energy scale relative to the vacuum level and right one shows the comparison with normal hydrogen electrode (NHE). The positions are originated from the flat band potentials in a contact solution of aqueous electrolyte at pH = 1. Therefore, more pollutants can be decomposed in case the band gap is larger.

Figure 2-8 The energies for various semiconductors in aqueous electrolytes at pH = 1.¹⁷

Figure 2-9 shows the time scale of charge carrier generation, trapping, recombination, and interfacial transfer. After charge-carrier generation (~fs), recombination is mediated primarily by Ti³⁺in the first 10 ns. Valence-band holes are sequestered as long-lived TiOH⁺ after 10 ns. TiOH is reformed by recombination with conduction band electrons or oxidation of the substrate on the time scale of 100 ns. However, the electrons transfer from CB to surface is micro-seconds, so the phenomena is determine step in photocatalysis.²⁶

[Ti⁴⁺OH^˙]⁺ is the surface-trapped valence band (VB) hole (i.e., surface-bound hydroxyl radical), and [Ti³⁺OH] is the surface-trapped conduction band (CB) electron. And the arrow lengths are representative of the respective time scales.²⁶

Hoffmann et al.²⁶ and Hurum et al.³⁷ reported that the photogenerated holes recombine with surface electrons easily. So, the present of oxygen not only act as an electron acceptor, but also perform H2O2which is a direct source of hydroxyl radicals.²⁶ Besides, the hydroxyl radical (˙OH) was proposed to be the primary oxidant in the degradation of organic water contaminants.^{38, 39} Therefore, the processes of secondary reactions with activated oxygen are summarized in Figure 2-10. Figure 2-10 shows the oxidation occur by either oxidation via the surface-bound hydroxyl radical (i.e., trapped hole at the TiO2 surface) or via the other radicals (i.e., formation of superoxide radicals). And the reduction occurred by directly electrons diffusion under lower conduction states of TiO₂. Hence, it is important for the present of oxygen which plays as the primary electron acceptor.³⁹

Figure 2-10 Secondary reactions with activated oxygen species in the photoelectrochemical mechanism.²⁶

2-1-3 Photoassisted degradation of Rhodamine B

In general, the TiO2degrade many organic pollutants and to mineralize completely under UV irradiation.^{26, 40} In order to expand the wavelength range in visible light for the photocatalysts, surface sensitization of TiO2via chemical or physic adsorbed dyes molecular were used to promote the efficiency of the charge carrier separation with visible irradiation.^12,

17, 40

For example, the chemisorbed RhB is excited at wavelengths longer than 470 nm to produce singlet and triplet states (denoted here simply as RhB*ads). Subsequently, RhB*ads

injects an electron into the conduction band (or to some surface state) of TiO2with RhB being converted to the radical cation RhB^•⁺, as shown in Figure 2-11. Afterward, the electrons in the conduction band of TiO2 react with adsorbed oxidants, usually O2, to produce reactive oxygen radicals (Equation 2-3 to 2-6).¹² Figure 2-12 shows the de-ethylation reaction since the radical cation RhB^•⁺ultimately reacts with reactive oxygen radicals and/or molecular oxygen.⁴⁰ Moreover, oxygen plays an additional important role to inhibit recombination between RhB^•⁺and e^-_CB. In addition, the secondary radical processes occurred might lead to mineralization. The semiconductor TiO₂ acts as an electron-transfer mediator and the oxygen as an electron acceptor leading to efficient separation of the injected electron and the radical cation, thereby facilitating the degradation process.

Figure 2-11 Electron-transfer processes (a) for UV irradiation of TiO₂ with the self-photosensitized pathway (b) under visible light irradiation which subsequent to excitation



Figure 2-12 Formation and competitive reactions of •OH radicals during visible light irradiation of Rhodamine B.²²

2-2 Synthesis toward metal oxide 2-2-1 Sol-gel method

Sol-gel processes have been widely used to synthesize TiO2, because there are many advantages of the sol-gel processes include cheaper, low reaction temperature, uniform structure, extreme purity, selective of precursor and widely applications. The sol-gel process can be characterized by a series of distinct steps.⁴¹

Step 1: In order to stable solutions of the alkoxide or solvated metal precursor (the sol), so the precursor would under hydrolysis and condensation for couple days.

Step 2: After hydrolysis and condensation, gelation resulting from the formation of an oxide-or alcohol-bridged netwoxide-ork (the gel) by a polycondensation oxide-or polyesterification reaction that results in a dramatic increase in the viscosity of the solution. If so desired, the gel may be cast into a mold during this step.

Step 3: Aging of the gel (syneresis), during which the polycondensation reactions continue

until the gel transforms into a solid mass, accompanied by contraction of the gel network and expulsion of solvent from the gel pores. Ostwald ripening and phase transformations may occur concurrently with syneresis. The aging process of gels can exceed 7 days and is critical to the prevention of cracks in gels that have been cast.

Step 4: Drying of the gel, to remove water and other volatile liquids from the gel network.

This process is complicated due to fundamental changes in the structure of the gel, which was occurred between 100 and 180 ºC. If isolated by thermal evaporation, the resulting monolith is termed a xerogel. If the solvent is extracted under supercritical or nearsupercritical conditions, the product is an aerogel.

Step 5: Dehydration, during which surface-bound M-OH groups are removed, thereby stabilizing the gel against rehydration. This is normally achieved by calcining the monolith at temperatures up to 800°C. Besides, the calcination processes also cause crystal structure of materials to produce.

Figure 2-13 shows the reaction of sol-gel, including hydrolysis, condensation, and gelation. The hydrolysis occurs by the nuclephilic attack of the oxygen contained in water on the silicon atom as evidenced by the reaction of isotopically labeled water with TEOS that produces only unlabelled alcohol in both acid and basic catalyzed system. Besides, the polymerization to form siloxane bonds occurs by either an alcohol producing condensation reaction or a water-producing condensation reaction. Relative to different condition (i.e. pH), the typical of condensation products is monomer, dirmer, linear trimer, cyclic trimer, cyclic tetramer and higher-order rings. In basic condition, particle growth in size with decrease in number; in acid condition, the particles aggregate into three-dimensional networks and form gel. So, the structure was linear or randomly branched polymer under acid condition, while it was branched cluster under basic condition. And the final process is drying and calcination which lead to the structure of gels stable by thermal treatment.⁴¹ In addition, Figure 2-14 shows the processing steps involved in making sol-gel-derived.

M

Figure 2-13 The process of sol-gel under acid condition. (a)Hydrolysis, (b)Condensation, and (c)Gelation.⁴¹

Figure 2-14 Gel process sequence.^{42, 43}

Generally speaking, the transition metal systems are distinguished from silicates by greater chemical reactivity resulting from the lower electronegativity of the metal and its

ability to exhibit several coordination states, so that coordination expansion occurs spontaneously upon reaction with water or other nucleophilic regents.⁴⁴ Therefore, the sol-gel processes of silicon and titanium precursors were similar, silicon was used to make example in this study. Sol-gel processes appear to be a simple operation, but several variables can influence the properties of the final products. Such as, pH of the reaction medium, water: alkoxide ratio, reaction temperature, and polarity of solvent. Therefore, by varying these processing parameters, materials with different physicochemical properties can be obtained. Thus the different parameters were introduced below.

The introduction of water to the Si(OR)2 precursor initiates hydrolysis, as shown in Figure 2-15. The water:alkoxide ratio determines the sol-gel chemistry and the structural characteristics of the hydrolyzed gel. High water:alkoxide ratios in the reaction medium ensure a more complete hydrolysis of alkoxides, favoring nucleation versus particle growth.

Thus precursor solution reacts very quickly with water especially in the presence of excess of water. The rapid initial hydrolysis results a solution with a high degree of supersaturation of hydroxylated metal oxide. This leads to a high rate of nucleation and the formation of small particles or crystallites.^{44, 45}

Figure 2-15 The relationship between gelation times and H2O:alkoxide ratio.⁴⁴

Whether the hydrolysis is acid or base catalyzed also has important consequences for the

the synthesis with HCl addition, so that a turbid gel was formed instead of white precipitates.

HCl serves not only as an acid catalyst, but also as an electrolyte to prevent particle growth or agglomeration through electrostatic repulsion.⁴⁶ Besides, under basic conditions, the silica products tend to form large agglomerates that eventually cross-link.⁴⁴ Therefore, the differences between acid and base catalyzed reactions and the consequences for particle morphology are conceptually represented in Figure 2-16.

Figure 2-16 The different structure of particles depend on pH.⁴¹

The polarity of solvent affects the hydrolysis process, because the hydrolysis reaction proceeds via nuclepphilic reaction mechanism with OH^- as the nucleophile. This phenomenon facilitate further attack of the nuclephile on the silion atom which is more positive charge on the silicon atom after the hydrolysis of the first alkoxy group, because OH -is a marginally better leaving group than –OR while the condensation process can occur.

Thus, the rate of hydrolysis and condensation reaction followed sequence 1-butanol >

methanol > 1-propanol > ethanol > 2-propanol, because the reaction rate was caused by hydrogen bonding and steric effect in solvent alcohol. If the hydrogen bonding ability is the only one factor, then increasing hydrogen bonding ability of solvent decreases the mobility of water to react with TEOS while the rate of hydrolysis reaction followed order 1-butanol >

methanol > 1-propanol > ethanol > 2-propanol.^{47, 48} However, in order to slow down the rate of hydrolysis, 2-propanol is the better chose for solvent.

2-2-2 Surface sol-gel method

In addition to sol-gel method, surface sol-gel can fabricate ultra-thin films with molecular-scale. Figure 2-17 shows the process of surface sol-gel. This process is

composed of chemisorption of alkoxide, rinse, hydrolysis of the chemisorbed alkoxides, and drying.^{44, 49} The moving substrate entrains liquid in a fluid mechanical boundary layer carrying g some of the liquid toward the deposition region, where the boundary layers splits in two (see Figure 2-18). The inner layer moves upward with the substrate, while the outer layer is returned to the bath. The thickness of the deposited film is related to the position of the streamline dividing the upward- and downward-moving layers.

Therefore, there are six forces in the film deposition region govern the film thickness and position of the streamline: (1) viscous drag upward on the liquid by the moving substrate, (2) force of gravity, (3) resultant force of surface tension liquid in the concavely curved meniscus, (4) inertial force of the boundary layer liquid arriving at eh deposition region, (5) surface tension gradient and (6) the disjoining or conjoining pressure (important for films less than 1 μm thick). When the liquid viscosity and substrate speed are high enough to lower the curvature of the meniscus, then the deposited film thickness balances the viscous drag and gravity force.⁴⁴

According to surface sol-gel principle, the process could be applied to various materials surfaces irrespective of their shape, size and structure. Besides, it is applicable to a wide range of metal precursor (metal alkoxides).⁴⁹ Therefore, the reaction between the surface TiO₂ hydroxyl groups with vanadia precursor molecules is therefore the best route to obtain well-defined surface concentrations of vanadium. However, Figure 2-19 shows the structure of production was prepared with different path way.

Figure 2-17 Schematic representation of the surface sol-gel process.⁴⁹

Figure 2-18 Detail of liquid flow patterns of the containous process. U is the withdrawal speed, S is the station point, δis the boundary layer, and h is the thickness of the fluid film.⁴⁴

Figure 2-19 The production prepared by sol-gel-derived with different processes.^{42, 44}

2-3 Doping TiO

₂

with impurities

Photocatalytic activity of a particular semiconductor system for the stated purpose is measured by several factors including the stability of the semiconductor under irradiation, the efficiency of the photocatalytic process, the selectivity of the products, and the wavelength range response. Therefore, photocatalyst for a particular use can be surmounted by modifying the surface of the semiconductor. Three benefits of modifications to

photocatalytic semiconductor systems have been studied: (1) inhibiting recombination by increasing the charge separation; (2) increasing the wavelength response range (i.e. excitation of wide band gap semiconductors by visible light); and (3) changing the selectivity or yield of a particular product. For examples, impurities like transition metal ions (Fe, V)^{2-5, 50}, and non-metal ions (N, C)^7-9 were mostly used to dope into crystalline structure of TiO2 to vary some physicochemical properties of original TiO2, including microstructure, electronic structure, and photocatalysis.

In order to define the structure of vanadium doped in TiO2, the catalysts have been studied by in situ FT-Raman. Figure 2-20 and Figure 2-21 show the structure of surface and bulk doped materials. For surface doped materials, there are two kinds of forms of VOx species attached to the TiO2surface: monomeric vanadyl and polymeric vanadates.^51-53 In particular, the coordination of the surface oxygen atoms plays a key role in reactivity. All of the potentially active oxygen sites proposed to be vanadyl V=O (1030 cm^-1), bridging V-O-V (822 cm^-1), interface V-O-Ti, and surface Ti-O-Ti (638 cm^-1). For hydrogen atomic adsorption, the most reactive sites are those located at the interface between the V₂O₅and the

在文檔中晶體內部與表面摻雜釩離子對二氧化鈦光觸媒物化特性與光催化活性之影響 (頁 15-41)