2-1. Photocatalysis
2-1-1. Principle of photocatalysis
The treatment techniques for environmental pollution with semiconductor photocatalysis have been developed rapidly in recent years. Photocatalysis involves semiconductors which utilize photons to produce electron-hole pairs and then induce oxidation or reduction of adsorbed contaminants through interfacial charge transfer. In general, semiconductors such as TiO2, ZnO2, SnO2 and WO3 have been used to degrade organic compounds under UV or visible light irradiation[33]. Usually a semiconductor has different band gap energy that can influence the optical and physicochemical properties. A large band gap range has a high reactivity, but also needs more excitation energy.
However, a small band gap range can easily recombine the electron-hole pairs to reduce photocatalyic activity.
At room temperature, an electron is bound by the nucleus; the most electrons stay in the valence band. When the energy of a photon is higher than the band gap of a semiconductor, the semiconductor will separate the electron and hole pairs (Figure 2-1).
But the electron and hole pairs still have an opportunity for recombination (Figure 2-2).
Therefore, the excited electrons reach the conduction band, those reduce surface absorbed oxygen to form superoxide radicals (O2·- ). Furthermore, the holes may react with OH- and H2O absorbed at the surface to convert to hydroxyl radicals (·OH). The redox reaction that occurred at the surface of the semiconductor can transform organic compounds into oxidized or reduced products.
Figure 2-1 Reaction diagram of photocatalysis at a semiconductor by illumination.
Figure 2-2 Schematic photoexcitation in a solid followed by excitation events.[34]
In general, photocatalysis can be observed in continues steps of oxidation-reduction.
Illumination
TiO2 + hν → e+ + h- (2-1)
Oxygen
2H2O + 4h+ → O2 + 4H+ (2-2)
Peroxide
O2 + 2H+ + 2e- → H2O2 (2-3)
Superoxide
O2 + e- → O2- (2-4)
Radical
H2O + h+ → OH• + H+ (2-5)
2-1-2. TiO
2photocatalysis
Since the discovery of photochemical water splitting using TiO2 electrode by Fujishima and Honda[35], a multitude of studies have been focused on chemical systems that implicate the absorption of photoirradiation by chemical agents, followed by reactions leading to the splitting of water into H2 and O2[36]. TiO2 has been commonly used because their physical and chemical properties are very stable, resist acidity, have high photocatalytic activity, are cheaper, easily prepared, non-toxic, and oxidizes or reduces the majority of organic pollutants[1-3]. The band gap of anatase TiO2 is about 3.2 eV, so their critical wavelength is 380 nm, in ultraviolet range.
Figure 2-3 Energies for various semiconductors in aqueous electrolytes at pH = 1.[34]
TiO2 is the n-type semiconductor. The basic structure is a titanium atom in the center, surrounded by six oxygen atoms to form an octahedral structure with 6 coordinations. A titanium atom has 22 electronics with 3d orbit electron and four oxygen atoms to form a covalent bond. TiO2 has three types of crystal phases: anatase, rutile and brookite (Figure 2-4, 2-5). Anatase and rutile phase are usually used in photocatalytic reactions, where band gaps are 3.2 eV and 3.0 eV, respectively. Moreover, anatase is the metastable structure, which will cover to rutile phase at about 400-500oC[34].
Figure 2-4 The structures of (a) brookite, (b) anatase, (c) rutile.[37]
Figure 2-5 Structure of rutile and anatase TiO2.[34]
2-2. Mesoporous materials
According to IUPAC definition, pore sizes can be divided into: microporous (< 2 nm), mesoporous (2-50 nm) and macroporous (> 50 nm). Porous materials were interesting and focused on designing materials that have extremely high surface areas and tunable pore
sizes. Recently, porous materials have been widely used in adsorption, catalysts, sensors and other areas[38-41].
Table 2-1 Pore-size regimes and representative porous inorganic materials.[42]
Pore-size regimes Definition Examples Actual size range macroporous > 500 Å glasses > 500 Å
aerogels > 100 Å pillared layered clays 10 Å, 100 Å(a)
mesoporous 20-500 Å
M41S 16-100 Å
zeolites, zeotypes < 14.2 Å microporous < 20 Å
activated carbon 6 Å (a)Bimodal pore-size distribution
2-2-1. Synthesis and templates
In 1992, Mobil Oil Company used quaternary ammonium surfactants as a template with negatively charged aluminate silicate to synthesize order mesoporous M41S material first[16]. The scheme is shown in Figure 2-6. The pore sizes of M41S were about 20-30 Å. The series of M41S can be divided into three structures based on the method of each molecular array. (1) MCM-41 has a hexagonal array of noninterconnecting cylindrical pores (Figure 2-7a), (2) the structure of cubic space group Ia3d was MCM-48 (Figure 2-7b);
and (3) the structure of MCM-50 is lamellar[16,43] (Figure 2-7c). These porous materials have some advantages such as high specific surface area (BET surface area of about 1000 m2/g), thermal stability, adjustable pores, order structures, and uniform pore size.
Figure 2-6 Liquid-crystal templating mechanism showing two possible pathways for the formation of MCM-41.[43]
Figure 2-7 Illustrations of mesoporous M41S materials: (a) MCM-41, (b) MCM-48, and (c) MCM-50.[44,45]
Subsequently, the SBA (Santa Barbara) amorphous materials were synthesized in 1994.
Syntheses of SBA structures used both organic and inorganic salts to form electrostatic
interaction forces. Furthermore, the order structure of SBA-1 was under strong acidic conditions. Cetyltriethylammonium bromide (CTEABr) and tetraethyl orthosilicate (TEOS) were used as a surfactant and silicon source for the successful synthesis of a space group Pm3n cubic structure[46] (Figure 2-8). The tunable pore size property of the SBA materials was better than that of MCM materials, and SBA materials have higher hydrothermal stability as well. As the demand for application of scientific interest, increases ideas about using surfactants as templates to prepare porous inorganic metal oxides were discussed, using materials such as Al2O3, SiO2[47], SiC, ZrO2, and TiO2[48], etc.
Figure 2-8 The Pm3n cubic phase, in which the polyhedral represent micelles.[49]
2-2-2. Mesoporous TiO
2It has been demonstrated that TiO2 has high surface area, which can enhance the photocatalytic activity by physical methods. Template method has been utilized mostly for the preparation of porous photocatalysts mostly. In 1995, mesoporous TiO2 were synthesized using phosphate as a surfactant and titanium alkoxide utilized sol-gel method[48]. At present, preparation of mesoporous TiO2 have been developed in different ways, including the sol-gel method[48], hydrothermal method[18], microwave-assisted method[3],
sonochemistry method[27], block copolymers[50], ionic liquid containing cellulose[51], phosphotungstic acid assisted sol-gel method[17], and deposition method[52]. In general, TiO2 prepared by the use of titanium chloride or alkoxide as the precursor, processed hydrolysis-condensation and recrystallization, resulted in a pore size range of 5.7-14 nm and a highly specific surface area of 395-467 m2/g[27]. Many researches focus on discussing the influence of the different synthetic parameters on the materials in the synthesized process of mesoporous TiO2. Callreja et al.[14] used the nonionic co-polymer, triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO20PPO70PEO20, P123) as the template by sol-gel synthesis. They discussed water/Ti source ratio, and the effect of solvent and surfactants concentration. The results found that the water/titanium ratio was greater than 10, and the surfactant make it difficult to form micelle to lead to the pore structure. Moreover, this study showed that using ethanol as a solvent can reduce the hydrolytic condensation to enhance the surface area. Serrano et al.[53] discussed the effect of using different acidic reagents to synthesize TiO2. Results show that sulfate and phosphate have larger interaction with Ti ion, leading to the collapsing of the pore structure.
Furthermore, nitric acid and hydrochloric acid can stabilize the pore structure to enhance photocatalytic activity. Wang et al.[54] utilized the titanium phosphate (TiPO) and CTAB to synthesize porous materials by hydrothermal method and observed under temperature can influence the structure. Results showed that when the synthesis occurred at room temperature, the outcome was a hexagonal structure, and a lamellar structure formed at 100oC. The preparations of mesoporous TiO2 are summarized in Table 2-2.
Table 2-2 Preparation of mesoporous TiO2.
Method Precursor Template Surface area
(m2/g)
Pore diameter
(nm) Ref.
Titanium isopropoxide TX-100 187-487 3.8-4.6 [30]
Sol-gel
Titanium isopropoxide P123 137-582 2.0-4.2 [14]
Ligand-assisted
templating Titanium isopropoxide Dodecyl
phosphate 90-712 - [55]
Template-free sol-gel Titanium n-propoxide Formamide 150 5 [56]
Surfactant-templated
sol-gel Titanium isopropoxide P123 200-250 4.9-6.4 [53]
Nonhydrolytic evaporation-induced self-assembly
Titanium chloride and
Titanium butoxide P123 80-135 3.5-6.1 [24]
Titanium tetrapropoxide C16(EO)10 101-215 9.1-9.9 [29]
Tetrabutyl titanate CTAB 117-205 2.2-2.8 [33]
Tetrabutyl titanate - 100-400 3-4 [28]
Hydrothermal treatment
Titanium isopropoxide - 49-126 8-10 [57]
2-2-3. The photocatalytic activity of mesoporous TiO
2Mesoporous TiO2 has a larger surface area because of its confined porous structure and high surface to volume ration, and enhanced high photocatalytic activity, because of improved access to the active sites of TiO2. Kim et al.[22] used titanium isopropoxide and diblock copolymer surfactants to synthesis mesoporous TiO2. They discussed the photocatalytic activity was increased with decrease of crystallite size, larger surface area, and smaller pore-size distribution. In comparison with commercial TiO2, Ishihara ST-01, the mesoporous TiO2 materials exhibit better photocatalytic activities in the decomposition of methylene blue. Yu et al.[12] prepared bimodal nanocrystalline mesoporous anatase TiO2 powder photocatalyst using tetrabutylorthotitanate as precursor via a hydrothermal method. The results show that optimal hydrothermal condition (180oC for 10 h) was determined. The photocatalytic activity of the TiO2 powders prepared under an optimal hydrothermal condition exceeds that of P25 by a factor of more than three times. Peng et al.[19] apply a hydrothermal method to synthesize mesoporous anatase TiO2 nanoparticles by using CTAB as a surfactant-directing agent. This literature indicated the large surface area, small crystalline size, and well-crystallized anatase mesostructure can explain the high photocatalytic activity of mesoporous TiO2 nanoparticles calcined at 400°C. Therefore, the obtained mesoporous TiO2 nanopowders exhibit higher photocatalytic activity than the commercial nonporous photocatalyst P25. Many researches have been discussed the mesoporous TiO2 enhanced photocatalytic activity are summarize in Table 2-3.
Table 2-3 Mesoporous TiO2 affects photoactivtive efficiency.
Precursor Target compound The photoactivtive efficiency Ref The MT-18 was higher than ST-01 by 5.8
times.
[22]
Methylene blue The mesoporous TiO2with high surface areas, which exhibit photocatalytic activity superior to P25.
[3]
Mordant Yellow All mesoporous samples show higher photocatalytic activity than P25.
[58]
Titanium isopropoxide
Dibenzothiophene (DBT)
The 98% removal efficiency of 300 mg/L DBT is within 2 min reaction.
[59]
2,4,6-tribromophenol (TBP)
The degradation efficiencies increased from 89.48% to 95.87% with PEG molecule was increased from 200 to 20000.
[10]
VOCs
The photocatalytic activity was about 3 times higher than P25.
Titanium butoxide [12]
Phenol The sample calcined at 800oC was removal 90% phenol than P25 (20%) within 120min.
[20]
Ti(SO4)2
Rhodamine B (RhB)
The optimum reactivity is observed at the sample calcined at 400oC, which causes 97%
RhB to be degraded after 2h irradiation.
[19]
2-3. Surface modification
The limiting factors of the reactive dynamics of photocatalysts include large surface area, in addition to the pollutants absorbed capacity on the photocatalyst surface and the surface charge transfer. Consequently, the research and technology focused on photocatalyst surface modification has received great attention in recent years. The material for modification can be divided into metals, transition metal and organic compounds.
2-3-1. Metal deposition
Adding metals (such as Pt, Pd, Au, Ag) in photocatalysts is for the purpose of separating electrons and hole pairs, inhibiting electron and hole recombination[7,60-64]. After the photocatalyst is excited, electrons are produced which effectively improving can transfer to metal particles quickly because of electric potential difference, leading to electron and hole separation and the increase of the efficiency of photodegradation.
Adding rare metals to photocatalysis will change the mechanism of the photocatalysis because of a transformation of the surface property. The results would enhance the reaction rate of photocatalysis, stabilize the yield of the specific product, and alter the final product. Li et al.[61] prepared mesoporous titania photocatalysts by embedding gold nanoparticles. This result shows that Au nanoparticles embedded within the TiO2 pore tunnel may also serve as an electron conductor, which facilitates photoelectron transfer to pore surface and further reduces the probability of charge recombination. In addition, enhanced light absorption and improved quantum efficiency were the main factors leading to improved photocatalytic activity.
2-3-2. Transition metal modification
Transition metals, such as V, Mn, Fe, Co, Ni and Cu have been extensively used as dopants to enhance the photocatalytic activity of TiO2[65-67]. These doping ions in the TiO2
structure has caused a significant absorption shift to the visible region compared to pure TiO2[68]. Adding transition metals in the photocatalyst can promote the efficiency of electron capture and thus restrain electron and hole unification. In addition, the transition metals will be forming a new independent band between with the conduction band and valence band. Therefore, it can use lower energy to excite electrons, which expand the light range. Huo et al.[69] prepared La-doped TiO2 photocatalysts via ultrasound-assisted sol-gel method, followed by supercritical treatment. This literature indicated the La-modification could increase the oxygen vacancies and surface defects in the TiO2
photocatalysts which might capture photoelectrons and thus, inhibit the recombination between photoelectrons and holes, resulting in enhanced quantum efficiency.
2-3-3. Organic modification
In recent years, bonding of TiO2 with organic moieties attracted large attention. The surface electronic structure as well as hydrophicility of TiO2 change with organic modifiers, and consequently enhance the photocatalytic activity by three ways: (1) by inhibiting charge recombination[31,32,70], (2) by exploring the wavelength response range[71,72] and (3) by changing the selectivity or yield of a particular product[31,73]. Organic modifiers are chemisorbed on the TiO2 surface via chelation or formation of covalent bonds. Ou et al.[31]
used an ascorbic acid as modifier for TiO2 and found that the modified TiO2 was more photoactive for oxidation of azo-dye than pure TiO2 both under UV and solar irradiation.
Chang et al.[8] used a non-hydrolytic sol-gel method to prepare trioctylphosphine oxide
(TOPO) modified TiO2 and compared the photodegradation ability of the TOPO-capped TiO2
with that of P25 in terms of degradation of different hydrophilic and hydrophobic of endocrine disrupting chemicals (EDCs). Relative to P25, the TOPO-capped TiO2 showed 1.4 and 3.2 times higher activity for the degradation of phenol and bisphenol A, respectively.
Chang and Chen[5] used microwave-assisted method to prepare salicylic acid modified TiO2. The modified salicylic acid extends absorption wavelength of the TiO2 to visible light (452 nm) and enhances surface charge transfer and the adsorption of reactants on the catalyst surface. Therefore the photocatalytic activity of the salicylic acid modified TiO2 was higher than commercial photocatalysts (Degussa P25) by 1.6 times. Comparelli et al.[74] prepared oleic acid (OLEA)- and tri-n-octylphosphine oxide (TOPO)-capped anatase TiO2 nanocrystal powders. They indicated that the organic-capped TiO2 exhibited lower degradation rates than the P25. The organic capping prevents dye access to the catalyst and/or limit the local density of –OH groups. If it combines surface modification and high surface area of the porous structure, it can be expected that the photocatalytic efficiency will be enhanced.
Angelome et al.[75] prepared the surface modified porous structure in two steps: synthesis of mesoporous materials, and then surface modification of the inner pore with organic compounds. However, the post adsorption was inefficient and many adsorbed modifier could detach from the surface.
2-4. Non-hydrolytic sol-gel process
As we know, the sol-gel method is a simple process for transition metal oxides (including titania) with nanoscale microstructures and provides for excellent chemical homogeneity. Sol-gel derived unique metastable structures occur at low reaction temperature. The conventional sol-gel routes are based on the hydroxylation and polycondensation of molecular precursors. Figure 2-9 expresses the conventional sol-gel
process. Hydrolysis and polymerization of the precursors occurs, usually involving which are usually inorganic metal salts or metal organic compounds such as metal alkoxides.
Finally the condensation reaction is two molecules or moieties (functional groups) combining to form one single molecule, with the loss of a small molecule[76-78].
Hydrolysis
Figure 2-9 Conventional sol-gel process.[76]
The hydrolytic sol-gel process has been studied extensively over the last 2 decades as a facile route to transition metal oxides. It was only in the last 10 years that the corresponding non-hydrolytic sol-gel (NHSG) process has been recognized as a useful route to inorganic oxides. The NHSG route has been evaluated for the synthesis of silica, titania, alumina1 and mixed or binary oxides such as aluminosilicates and silica-titania systems.
The NHSG process involves the reaction of a metal halide with an oxygen donor such as an alkoxide, ether, alcohol and so forth under nonaqueous conditions to form an inorganic oxide. Subsequently, when metal chlorides and metal alkoxides are mixed, ligand-exchange reactions occur, giving rise to a mixture of metal chloroalkoxides[79-82].
Condensation
MCln + M(OR) n → 2 MOn/2 + nRCl (2-10)
Etherolysis or Alcoholysis
≡MCl + H-O-R → ≡MOH + RCl (2-11)
≡MCl + R-O-R → ≡MOR + RCl (2-12)
MCln + n/2 R-O-R → MOn/2 + nRCl (2-13)
Ligand Exchange
MCln + M’(OR) n ↔ MCln-x(OR)x + M’Cl x (OR)n-x (2-14)
2-5. Endocrine disrupted chemicals
Recently, scientists have found that some chemicals in the environment have high potential to interfere the endocrine system, which called “endocrine disrupters” or
“endocrine disrupting chemicals” (EDCs). The Organization of Economic and Cooperate Development define EDCs as “an exogenous substance or mixture that alters the function(s) of the endocrine systems and consequently causes adverse health effect in an intact organism, or its progeny or (sub) pollution”[83]. EDCs are highly toxic and carcinogenic.
They remain in the environment for a long time due to their stability and bioaccumulation.
The effects associated with the presence of EDCs in the environment are: (1) toxic to the reproductive system and development in mammals, fishes and birds, (2) feminization of male fishes, (3) changes in the immunologic system of marine mammals, (4) causing irreversible damage to the aquatic life[84]. Moreover, the EDCs can lead some adverse effects in human health and the function of the endocrine system by binding to nuclear receptors[85]. The effects of EDCs in human beings reported so far have been: (1) low sperm count, (2) increases in the incidences of breast cancer, (3) early puberty and (4) the endometriosis[83,84]. Therefore, EDCs are a great concern because of their potential in altering the normal endocrine function and physiological status of organism.
Bisphenol-A (BPA) has been used for the production of epoxy resins, polycarbonates and polysulfones, and has been suspected as one of endocrine disruptors[86]. Recently, BPA was widely applied in various polycarbonate plastics, poly(vinylchloride) (PVC) and epoxy resins such as the inner coating of food cans, powder paints, plastic containers, dental fillings and baby bottles[87-89]. BPA is often contained in environmental water and now is attracting attention. Various methods have been developed to remove BPA from water, such as a biological method[90,91], chemical oxidation[92,93], electrochemical oxidation[94], Fenton[95], and a photocatalytic method[30,89,96-98]. One of the most promising methods is photocatalytic degradation due to its high mineralization efficiency, low toxigenicity, ideally producing carbon dioxide, water and inorganic mineral ions as end products. Wang et al.[99] carried out the photocatalysis in a horizontal circulating bed photocatalytic reactor
(HCBPR). An optimum condition for HCBPR operation was achieved as follows, initial BPA concentration at 10 mg/L, initial pH at 12.3, TiO2 dosage at 1% and temperature at 24.3oC, under which 95% TOC removal and nearly 97% BPA degradation were achieved after 6 h of UV radiation. Gao et al.[100] utilized Zr-doped TiO2 to degradation of BPA under UV irradiation and indicated that the Zr-doped TiO2 show enhanced efficiency in comparison with a pure anatase TiO2. Furthermore, nearly complete removal of TOC can be achieved. Guo et al.[101] reported two different 3D mesoporous TiO2 to degradation of BPA under UV irradiation and proposed the degradation mechanism of BPA. The mechanism involves reactions of hydroxyl radicals. In addition, photogenerated holes also can oxidize the organic molecule directly, while the electrons can react with the adsorbed molecular oxygen on the Ti(III)-surface to generate superoxide radical anion HOO•. In the TiO2 photocatalysis toward aromatic compounds, initial hydroxylation of aromatic rings by hydroxyl radicals is believed to play a dominate role in a sequential ring cleavage. The reaction pathways of BPA degradation mechanism showed in Fig 2-10.
Table 2-4 Characteristics of BPA[102]
Structure of BPA Formula MW Water solubility (mg/L at 25oC)
Figure 2-10 Proposed reaction pathways of BPA degradation.[101]