2-1. Photocatalysis
2-1-1. Principle of photocatalysis
Since 1972, Fujishima and Honda discovered the phenomena of photocatalytic splitting of water on semiconducting TiO2 e1ectrodes under UV light[16], more attention has been focused on this material as a practical photocatalyst. Semiconductors such as TiO2, ZrO2, SnO2, ZnO, CdS and WO3 have been applied in energy and environmental fields for photon-to-electricity conversion, water splitting, hydrogen storage, photocatalysis and sensing[17]. Unlike metals, which have a continuum of electronic states, semiconductors exhibit a void energy region (band gap) that extends from the top of the filled valence band (VB) to the bottom of the vacant conduction band (CB)[18]. Figure 2-1 illustrates the excitation of an electron from the VB to the CB initiated by the absorption of photons with energy equal to or greater than the band gap of the semiconductor. The separated electron and hole could migrate to the surface (pathway A and B) or undergo recombination in the volume and at the surface (pathway C and D)[19].
The detailed mechanisms of the photocatalytic oxidation or reduction reactions on semiconductors by UV illumination are shown in Figure 2-2 and presented below stepwise[20]: Photocatalysis involves the generation of electron-hole pairs by UV absorption and the charge carriers can migrate rapidly to the surfaces of catalyst where they are oxidized or reduced with suitable substrates (Step 1). The trapped hole can react with the chemisorbed OH group or the H2O molecular on the surface to produce OH radicals (Step 2) or accept electrons from adsorbed organic compounds to convert them directly to radicals (Step 3). In addition, oxygen or other oxidants including CO2 can act as an efficient electron scavenger to
form superoxide radical (Step 4) or other reduced radicals (Step 5). The fundamental processes of photocatalysis involving with TiO2 can be summarized as following:[21]
Charge carrier generation:
Figure 2-1 Schematic photoexcitation in a solid followed by excitation events.[19]
Figure 2-2 Mechanism of photocatalysis on semiconductors exposed to light irradiation.[20]
Contact between an n-type semiconductors such as TiO2 and metals generally involves a redistribution of electric charges and the formation of a Schottky barrier as shown in Figure 2-3. The Schottky barrier formed at the metal-semiconductor interface leads the metal and the semiconductor exhibiting excess negative charges and positive charges, respectively.
Separation of charge carriers from the barrier region can serve as an efficient electron trap to prevent the electron-hole recombination on photocatalysts. The height of the barrier (b is given by:
x m
b
E
(2-9)where b is the work function and Ex is the electron affinity. Figure 2-4 illustrates the properties of Schottky barrier formed at a metal-semiconductor junction. After migration of the photoexcited electron to the surface, electron trapping suppresses the recombination.
Figure 2-3 Schematic of a Schottky barrier.[19]
Figure 2-4 Schottky barriers on the surface of metal-semiconductor particle.[19]
2-1-2. Material properties of TiO
2Semiconductors participate in a variety of photocatalytic reactions including oxidative degradation of organics, reduction of metal ions and evolution of hydrogen from water to remedy the problem of chemical waste and energy renewal. TiO2 is the most investigated semiconductor which has large potential in photocatalysis, solar cells, sensor and photochromism because of its low cost, commercial availability, nontoxicity, chemical stability, ease of handling, high photocorrosion resistance and suitable optical/electronic qualities. This ability of TiO2 is related to its optical properties. TiO2 possesses a wide band gap (3.0 eV for the rutile phase and 3.2 eV for the anatase phase, as shown in Figure 2-5) that absorbs photons in the ultraviolet region, thus limiting its application under visible light.
Figure 2-5 Energies of various semiconductors in aqueous electrolytes at pH= 1.[22]
In addition, the photocatalytic activity of TiO2 is highly dependent on its porosity, surface area, bulk structure, particle size, crystal phase and crystallinity. TiO2 has three polymorphs: anatase, rutile and brokite, and anatase is the most active phase[17]. Figure 2-6 shows the unit cell structure of rutile and anatase TiO2 which are commonly used in photocatalysis. The basic building block consists of a titanium atom surrounded by a more or less distorted octahedron of six oxygenatoms[23]. These differences in lattice structures cause different mass densities and electronic band structures between the two crystals of TiO2.
Figure 2-6 Bulk structure of rutile and anatase TiO2.[24]
2-2. Sol-gel method
The sol-gel processing of inorganic ceramic materials refers to the hydrolysis and condensation of alkoxide-based precursors have been extremely investigated since the earliest study of Ebelman et al.[25]. TiO2 have been synthesized with the sol-gel method via an acid-catalyzed hydrolysis with titania precursor followed by condensation[26]. Sol-gel process offers a facile and available method for synthesis of nanoparticle or thin film that are either unitary or hybrid metal oxides, offers many advantages including excellent control and
selective of precursor solutions, easily modification of composition, customizable microstructure, relatively low reaction temperatures, the practicability of coating deposition on substrates, and simple and inexpensive equipment. The processing procedures can be characterized by a series of distinct steps (Figure 2-7):[27-28]
Step 1: Mixing. Stabilized solutions of the alkoxide or solvated metal precursor (the sol), and hydrolysis and condensation reactions were initiated by mixing with precursor and water.
Step 2: Gelation. At gelation, the formation of an oxide or alcohol bridged network (the gel) by a polycondensation or polyesterification reaction was increased in the viscosity increases sharply of the solution.
Step 3: Aging. Aging of the gel (syneresis), the polycondensation reactions continue until the gel transforms into a solid mass, follow by contracting of the gel and expulsion of solvent from the gel pores. The aging process of gels must develop to the prevention of cracks during drying because of Ostwald ripening and phase transformations may occur concurrently with syneresis.
Step 4: Dring. This process is complicated due to fundamental changes in the structure of the gel during drying the water or solvent is removed from the gel pore network.
If isolated by thermal evaporation, the resulting product is termed a xerogel. If the liquid is dried under hypercritical conditions an aerogel has been prepared with lower density.
Step 5: Dehydration or stabilization. The removal of surface-bound (M-OH) groups are removed from the pore network by calcinations at temperature up to 800 °C results in a stabilizing the gel against rehydration.
Step 6: Densification. Heating the porous gel at high temperatures (> 800 °C) causes densification and decomposition to occur. The pores of the gel network are collapsed and remaining organic species are volatilized.
Hydrolysis:
Figure 2-7 Gel-glass process sequence.[27]
2-3. Mesoporous materials
Since the amazing discovery of zeolite with tailored pore structures and high surface areas, a variety of ordered porous materials have many applications in the areas of adsorption and catalysis[29]. According to the classical definition made by IUPAC, the porous structures can be divided into three categories: microporous (d < 2 nm), mesoporous (2 nm <
d < 50 nm) and macroporous materials (d > 50 nm), based on their pore diameter. Some examples are listed in Table 2-1
Table 2-1 Examples of porous materials and showing the pore size domains.[30]
Pore size regimes Definition Example Actual size range
macroporous d > 500 Å glasses > 500 Å
mesoporous 20 Å < d < 500 Å
aerogels > 100 Å pillared layered clays 10 Å, 100 Å (a)
M41S 16-100 Å
microporous d < 20 Å
zeolites, zeotypes < 14.2 Å activated carbon Å
(a) Bimodal pore size distribution
2-3-1. Mechanisms and templates
In 1992, the synthesis of mesoporous molecular sieves as aluminosilicate M41S was discovered by scientist in Mobil Oil Corporation. The synthesis involves the co-condensation of an anionic species with cationic surfactants (S+I－)[3]. MCM-41, the remarkable one of the members of M41S, is prepared with a cationic surfactant, cetyltrimethylammonium (C16TMA+), as a template (as shown in Figure 2-8)[31]. It possesses a highly ordered hexagonal arrangement of uniform pores whose dimensions can be confirmed with a varying channel (15-100 Å) and high surface area (> 1000 m2g-1).
Other related phases such as MCM-48 and MCM-50 have a cubic and lamellar mesostructure, respectively (Table 2-2, Figure 2-9).
Figure 2-8 Two possible pathways for the liquid-crystal templating mechanism of MCM-41.[32]
Table 2-2 Mesophases of silicate molecular sieves and synthesis parameters.[33]
Name Mesophase Space group Parameter
MCM-41 hexagonal P6m [surfactant] / [Si] < 1 MCM-48 cubic I 3a d [surfactant] / [Si] < 1-1.5 MCM-50 lamellar P 2 [surfactant] / [Si] < 1.2-2
Figure 2-9 Illustrations of mesoporous M41S materials: (a) MCM-41, (b) MCM-48, and (c) MCM-50.[32]
(a) (b) (c)
Nonionic polymeric templates are later used in the synthesis of mesoporous silicas and other mesoporous oxides (as shown in Figure 2-10)[34]. The most useful groups of the surfactants are the triblock copolymers including poly(alkylene oxide)x-poly(propylene oxide)y-poly(ethylene oxide)x, (PEO)x(PPO)y(PEO)x, (trade name: Pluronic)[35-36]. These block co-polymers show excellent abilities on tailoring varied porous structure (Table 2-3), non-toxicity, specific interfacial character, commercial availability, biodegradability and low cost. The preparations of well-ordered hexagonal mesoporous material (SBA-15, Santa B Arbara No. 15) are achieved by using the amphiphilic block copolymers as structure-directing agents[37]. Compared to M41S and other silicates, SBA-15 exhibits high thermal stability which is contributed by tunable large pore sizes (50-300 Å) and thick wall (31-64 Å). Moreover, it allows more remarkable applications for the preparation of mesoporous oxides, such as Al2O3, TiO2, ZrO2 HfO2, Nb2O5, Ta2O5, WO3 and SnO2[38-39], as well as a variety of mixture (e.g., SiAlO3.5, SiTiO4, Al2TiO5, ZrTiO4 and ZrW2O8)[40].
Figure 2-10 Schematic view of the mesoporous oxide prepared with polymeric template.[41]
Table 2-3 Mesophases and synthesis parameters of triblock copolymers.[37]
Name Mesophase Space group Example
EO/PO < 0.07
hexagonal (a) P6m
EO5PO70EO5
lamellar (b)
EO/PO =0.07-1.5 hexagonal P6m EO20PO70EO20
EO/PO > 1.5 cubic I 3 m m EO80PO30O80
(a) At low concentrations (0.5-1 wt %) and (b) higher concentrations (2-5 wt %)
2-3-2. Mesoporous TiO
2Recently, mesostructural metal oxides, which have high specific surface areas and pore volumes, as well as narrow pore size distributions where offer more active sites for catalytic reaction to occur, have attracted much attention. For the photocatalytic applications of TiO2, anatase is necessary since this phase shows high photocatalytic activity. Unfortunately, synthesis of mesoporous TiO2 is much more complicated compared to silica because titania precursor shows a high reactivity toward hydrolysis and condensation which leads a distorted structure. The preparation of mesoporous TiO2 powders and films using sol-gel method[42-45], hydrothermal method[46-48], microwave method[49], sonochemical method[50-51]
and evaporation induced self-assembly (EISA) method[52-54] have been extensively investigated. The first study of hexagonal arranged mesoporous TiO2 prepared via a modified sol-gel method in the presence of alkyl phosphate surfactant as template was developed by Antonelli et al.[38]. Afterwards, they used dodecylamines as the template to prepare phosphorus-free mesostructured TiO2[55]. Yoshitake et al.[56] also used amine surfactant and improved by a chemical vapor deposition (CVD) treatment with titania
precursor to stabilize the structure. Trong[57] used acetylacetone to control the condensation of TiO2 and simply prepared lamellar and hexagonal mesoporous TiO2 in the presence of cetyltrimethylammonium chloride (C16TMA+Cl－). Pure titania and silica incorporated titania mesoporous materials have been successfully synthesized by Zheng et al.[58], who used urea as a template. Because ionic surfactants present strong interactions with inorganic walls, it is challenging to remove the surfactants from the metal oxides using extraction.
And, the collapse of the inorganic structure may occur when the calcination is employed for surfactant removal. Thus, nonionic block copolymer appeared to be an excellent candidate for the formation of weak hydrogen bond with inorganic framework and surfactant composites. Uses of tripolymeric template to direct organization of mesoporous TiO2 with worm-like distorted or hexagonal ordered structure are shown in Figure 2-11.
Figure 2-11 Mechanistic schemes for mesoporous TiO2 with triblock copolymer.[59]
The nonionic block copolymers like (PEO)x(PPO)y(PEO)x have the additional advantage of relatively thick inorganic pore wall, improving the thermal stability of the material.
Stucky et al.[9] used amphiphilic poly(alkylene oxide) block copolymers as structure-directing agents and TiCl4 as the titania source to prepare mesostructral TiO2. Calleja et al.[42]
reported the synthesis of mesoporous TiO2 with highest specific surface areas (> 300 m2g-1) of using the Pluronic P123 and titanium isopropoxide as the initial reaction agents to prepare a mesoporous TiO2. Recently, evaporation induced self-assembly (EISA) has been investigated for the production of mesoporpous TiO2[52]. EISA process controls and synchronizes the aggregation of micelles with the condensation of the inorganic framework, giving rise to well-defined porous structure. A succinct summary of some important works in this field are presented in Table 2-4.
However, pure TiO2 materials usually have poor thermal stability and relatively low quantum efficiency, which strongly restricts its applications in photocatalysis. Combination with others metal oxides are the alternative approach for property tuning to enhance activity due to structural and electronic modification[60]. TiO2-SiO2 materials have been extensively used at first as catalysts and supports for a wide variety of reactions[61]. These mixed materials are not only taken advantage of both photocatalysis and mechanical stability, but also generation of new acid sites. Thus, a great deal of TiO2-based binary metal oxides such as TiO2-Al2O3, TiO2-SnO2, TiO2-ZrO2, TiO2-WO3 and TiO2-P2O5 have been reported, among them, TiO2-ZrO2 is one of the most promising photocatalyst for tunable composition, abundant phases and more attractive photocatalysis properties. In these works, Zr4+ ions were mainly doped in the surface of TiO2, the specific redox potential, higher surface area, stronger surface acidity and creation of surface defects are proposed as the reason for the improvement in the photocatalytic performance[62-64]. Furthermore, the first synthesis of mesoporous Zr-TiOy using triblock copolymers as templates with hexagonal structure was
prepared by Stucky et al.[9]. Since then many efforts have been devoted to the fabrication of mesoporous TiO2-ZrO2 materials and some dramatically applications have been achieved[65-67]. Recently, Yuan et al.[68] published an efficient approach to fabricate ordered mesoporous TiO2-ZrO2 composites through evaporation induced self-assembly (EISA) process by using amphiphilic triblock copolymer F127 and P123 as structure-directing agents.
Overview of all mesoporous TiO2-ZrO2, it is a challenge to avoid entrance of Zr4+ ions into bulk inside, which may facilitate the recombination of electron-hole pairs then reducing the photocatalytic activity.
Table 2-4 Preparation of mesoporous TiO2
Method Ti precursor Surfactant Surface area (m2/g)
Pore size (nm)
Ref
Sol-Gel
Tantalum ethoxide Octadecylamine Over 500 2.0-4.0 [55]
Titanium isopropoxide Pluronic P123 166-381 6.3-2.8 [42]
Titanium isopropoxide Pluronic P123 205 4.4 [43]
Titanium isopropoxide Triton X-100 187-487 4.6-3.8 [45]
Titanium ethoxide Pluronic P123 134-204 8.0-5.5 [44]
Titanium isopropoxide Pluronic P123 210-260 5.6-5.2 [69]
Titanium isopropoxide CTAB 123.8 12.6 [70]
Hydrothermal
Titanium isopropoxide Pluronic P123 98.7-152.3 8.01-6.19 [71]
Titanium n-butoxide - 186.7-295.2 7.23-4.74 [46]
Titanium sulfate PEG 200 172.4-234.1 9.94-6.31 [72]
Titanium sulfate CTAB 317.5 2.5 [48]
Titanium isopropoxide Pluronic P123 87-295 10.1-6.9 [73]
Microwave Titanium isopropoxide Tetradecylamine 243-622 0.32-0.27 [49]
Sonochemical
Titanium isopropoxide CTAB 853 1.5 [50]
Titanium isopropoxide Pluronic P123 112-128 6.7-9.3 [51]
EISA
Titanium isopropoxide CTAB 260-384 2.5-1.9 [52]
Titanium n-butoxide Pluronic P123 115-151 14.0-8.3 [74]
Titanium isopropoxide CTAB 573 2.5 [54]
2-3-3. Evaporation induced self-assembly (EISA) process
Self-assembly (SA) can be generally defined as the spontaneous and reversible organization of molecular materials through non-covalent interactions (e.g. hydrogen bonding, Van der Waals forces, electrostatic forces, π-π interactions) with no external intervention.
Typically examples of SA in materials include the formation of molecular crystals, colloids, lipid bilayer and molecular polymers with periodic assemblies[75]. Above the critical micelle concentration (CMC) in liquid phase, the amphiphilic surfactant was assembled into micelles, spherical or cylindrical structures that the hydrophilic parts of the surfactant in contact with solution while the hydrophobic parts within the interior of micelle (Figure 2-12).
Further increases the concentration of surfactant result in the self-organization of micelles into well-ordered hexagonal, cubic, or lamellar mesostructures[8].
Mann et al.[76] successful developed a versatile approaches to the synthesis of organized inorganic materials, which present arrays of pores of tailored dimensions and a great variety of shapes. This method was composed of four steps, including the (1) self-assembled templates (transcriptive synthesis), (2) cooperative assemblies of surfactant and inorganic block (synergistic synthesis), (3) spatially restricted reaction fields (morphosynthesis), and (4) combinations of these approaches (integrative synthesis) into sol-gel chemistry. For example, using cetyltrimethylammonium bromide as template (CTAB), Sanchez et al.[52]
demonstrated the formation of titania nanobuilding blocks (NBB), which are self-assembled within a liquid-crystal-like mesostructure around the micelles (Figure 2-13).
Figure 2-12 Schematic phase diagrams for the surfactant in solution.[77]
Figure 2-13 Formation pathways of TiO2/CTAB hybrid nanobuilding blocks.[52]
2-4. Surface modification
TiO2 is regarded as the most efficient and environmental friendly photocatalyst.
However, its large band gap (~3.2 eV) limits TiO2 only active in the ultraviolet region which is lower than 10% of the overall solar intensity. Rapid recombination of the photoexcited electron-hole pairs at the surface also inhibits the quantum efficiency. To improve the photocatalytic activity, surface modification of TiO2 is employed. So far, three benefits of the modifications to photocatalytic activity have been studied: (1) inhibiting recombination of electron and hole by increasing the charge separation, (2) increasing the wavelength response range and (3) changing the selectivity or yield of a particular product.
In recent years, the modification of TiO2 with transition metals[78-80] (V, Cr, Mn, Fe, Co, Ni and Cu) and noble metals[81-82] (Au, Ag, Pt, Pd, Rh and Ru) for improved photocatalytic performance have been widely studied. The metal doped semiconductor exhibits a particular variation in the Fermi level to create the Schottky barrier. Kamat et al.[83]
observed a greater photocatalytic reduction efficiency and higher photocurrent generation of Au/TiO2 nanocomposites by shifting the Fermi level. Since Haruta et al.[84] developed a dramatic preparation method of golden titania catalysts by deposition-precipitation (DP) with NaOH as precipitating agent. The Au/TiO2 materials have received particular attention owing to its peculiar properties which is sensitive to gold concentration, pH value, temperature of the solution and calcination temperature. Neither Au nor the TiO2 support is catalytically active for CO oxidation at low temperature, but Au/TiO2 system shows a synergetic effect for the reaction[85]. In the literature, these catalysts have been completely evaluated that the activity for CO oxidation is strongly dependent on the size of gold particles.
Schüth et al.[86] investigated the influence of the synthesis condition on different supported gold catalysts for CO oxidation. The high catalytic activity of Au-doped metal oxide
catalysts depends strongly on the pH value during precipitation between 8 and 9. For different supported materials used, the increasing catalytic activity of gold catalysts was obtained by optimization of the isoelectric point of the support lies between 6 and 9.
Grunwaldt et al.[87] was presented the differences between these catalysts by gold colloids about 2 nm size on TiO2 and ZrO2 in aqueous solutions. Although the particle size on different supports was comparable, the Au/TiO2 catalyst showed significantly higher activity than the Au/ZrO2 catalyst corroborating that the support plays a key role in CO oxidation.
Recently, Petit et al.[88] revealed that the TiO2-ZrO2 was better than TiO2 or ZrO2 of CO oxidation as a result of a relatively high BET surface area, high surface acidity, high thermal stability and great mechanical strength.
2-5. Photocatalytic reduction of CO
2According to the Intergovernmental Panel on Climate Change (IPCC) assessment report in 2001, the global average surface temperature has increased by about 0.6 °C over the 20 th century, and most of the warming observed over the past 50 years is attributable to human activities. Emissions of greenhouse gases (GHGs) such as CO2, CH4, N2O, HFCs, PFCs, and SF6 are the primary cause of global warming that continue to change in the climate system and atmospheric composition throughout the 21st century. The primary contributor of human activities is carbon dioxide (CO2) emissions from fossil fuel combustion. Since the beginning of the age of industrialization, the atmospheric concentration of CO2 was increased about 35%, it was approximately more than 130 times greater than the quantity emitted by volcanoes, amounting to about 27 billion tonnes per year. Currently, a great amount of technologies have been developed to reduce CO2 by three approaches: (1) efficient use of carbon-based energy sources, (2) use of alternative or carbon-free energy sources, and
(3) use of a post-treatment carbon-capture technology and storage of the captured CO2[89]. The capture system refers to the removal of CO2 from industrial flue gas by chemical or physical adsorption[90], cryogenic processes[91] and membrane separation process[92-93]. The captured CO2 can be stored in deep ocean and aquifer, or injected into geological formations like depleted oil and gas wells for enhanced recovery of fossil fuel products. Furthermore, more attractive researches use CO2 as a raw material for chemical method, photochemistry, reforming, electrochemical and biological transformation. However, the production of CO2-free fuel by direct conversion into supply energy is still a challenge. Because CO2 is a relatively inert and stable compound, the Gibbs free energy (△G) indicated that the equilibriums are highly unfavorable to the expected product (Table 2-5).
(3) use of a post-treatment carbon-capture technology and storage of the captured CO2[89]. The capture system refers to the removal of CO2 from industrial flue gas by chemical or physical adsorption[90], cryogenic processes[91] and membrane separation process[92-93]. The captured CO2 can be stored in deep ocean and aquifer, or injected into geological formations like depleted oil and gas wells for enhanced recovery of fossil fuel products. Furthermore, more attractive researches use CO2 as a raw material for chemical method, photochemistry, reforming, electrochemical and biological transformation. However, the production of CO2-free fuel by direct conversion into supply energy is still a challenge. Because CO2 is a relatively inert and stable compound, the Gibbs free energy (△G) indicated that the equilibriums are highly unfavorable to the expected product (Table 2-5).