The direct release of untreated wastewaters from various industries and households results in the release of toxic pollutants to the aquatic environment. Advanced oxidation processes (AOP) have gained wide attention owing to the prospect of complete mineralization of nonbiodegradable organic substances to environmentally innocuous products by chemical oxidation. In particular, heterogeneous photocatalysis has been demonstrated to have tremendous promise in water purification and treatment of several pollutant materials that include naturally occurring toxins, pesticides, and other deleterious contaminants. In this work, we have reviewed the different removal techniques that have been employed for water purification. In particular, the application of TiO2-SiO2 binary mixed oxide materials for wastewater treatment is explained herein, and it is evident from the literature survey that these mixed oxide materials have enhanced abilities to remove a wide variety of pollutants.
The energy demand is expected to be greater than 25 TW by the year 2050. This increase is expected to pose undue burden on natural resources and create challenges for sustaining our environment and quality of human life. In addition to energy, the demand for clean water is also expected to rise rapidly due to increasing global population. In addition, with the expeditious pace of industrialization, the disposal of industrial effluents poses threats to the environment and is becoming the biggest concern for the sustainable development of human society. Wastewater reclamation and recycling are essential goals to protect the global ecosystem and improve the quality of the environment. Several methods have been utilized for the removal of pollutants from contaminated water sources [
Although TiO2 has several important properties, such as ease of synthesis, excellent photostability, nontoxicity, and valence bands that are located at high positive potentials, there are several drawbacks that impair the performance of TiO2 in photocatalytic processes. The absorbance of TiO2 is limited to the UV region, and, thus, only a small fraction of the solar spectrum is utilized. The fast recombination of the photoinduced electron-hole pairs impedes the efficiency of the overall photocatalysis reaction. In addition, the relatively low surface area of TiO2 limits the number of adsorptive sites of the target pollutant molecule. To overcome these aforementioned challenges, researchers have developed TiO2 based mixed oxide materials that can provide large number of adsorptive sites by dispersion of TiO2 species into a porous support with large surface area. Silica has been widely employed as a robust and stable mesoporous support for immobilizing photoactive TiO2 species. The TiO2-SiO2 mixed oxide photocatalysts have shown significantly enhanced activities compared to pure TiO2 for a number of photocatalytic reactions for environmental remediation. The improved photocatalytic performance over TiO2-SiO2 mixed oxide materials can be accredited to the presence of highly dispersed TiO2 species in the SiO2 support, the better adsorption of the pollutant, and the presence of Ti-O-Si bonds that favor the activation of the organic pollutant [
In this review, we will first discuss the different types of aqueous pollutants followed by a discussion of selected removal techniques and their basic principles. Then, a brief overview of the synthesis methods of titania-silica (TiO2-SiO2) mixed oxides is presented. This is followed by an extensive description of available characterization techniques of periodic and aperiodic titania-silica catalysts. Following this, the heterogeneous photocatalytic degradation of several pollutants, in particular, organic materials in aqueous phase, is discussed. Finally, the factors that influence the degradation reactions are critically reviewed.
Clean water is the most important and indispensable resource that maintains the demands for the daily activities of every aspect of human society, such as drinking, cleansing, industrial manufacture, and farm irrigation. However, the squandering of clean water at discretion and careless handling of wastewater to aquatic systems from households and industries severely contaminate the quality of natural aquatic environments. In general, the sources that result in water pollution can be classified as point and nonpoint sources [
The most commonly observed hazardous wastes that threaten the global aquatic system can be divided into four groups according to the classification by the EPA: (i) hazardous wastes from nonspecific industrial processes, (ii) hazardous wastes from specific industrial sources, (iii) commercial chemical products, and (iv) toxic wastes. The EPA estimates that the above-mentioned pollutants have been increasingly detected during the past few decades in rivers, lakes, and oceans. In the following part, we will discuss the some important inorganic and organic wastes in water.
The contamination of water by nitrates poses a threat to the health of humans and other animals. Nitrate is found to be extremely toxic at high concentrations in water. It has been postulated to be the origin of methemoglobinemia in infants and it can cause toxic effects on livestock. Although phosphorus is not as toxic as nitrate in water, it can stimulate the growth of algae in water along with nitrate pollutants. Their excessive discharge to the surface water sources can lead to severe eutrophication in surface water sources. Eutrophication is the most widespread water contamination in global aquatic systems and it brings about numerous negative aftermaths to the environment and ecosystem. The most commonly observed consequence of eutrophication is that it causes the multiplication of algae and aquatic weeds, which give bad odor and taste of water in the aquatic systems, and prevents the use of such polluted systems as sources for clean water for industry, agriculture, and humans. In addition, eutrophication can induce the growth of phytoplankton and zooplankton in various aquatic environments. Besides, eutrophication is considered to be the cause for the disappearance of coral reefs and the extinction of several fish species.
Cyanides are another important class of anionic pollutants. Cyanides, which can be generated from either anthropogenic or natural sources, commonly exist in the form of cyanide salts, such as sodium cyanide and potassium cyanide, or in gaseous phase as hydrogen cyanide. Cyanides can be discharged into aquatic system from carelessly treated industry sewage, coal gasification, electroplating operations, and incomplete combustion of fuels. Cyanides cause severe threat to human life as the cyanide anion (
Organic pollutants are toxic molecular compounds and can cause significant diseases in humans, when exposed to high concentration levels. These organic compounds originate from a variety of industrial products such as detergents, petroleum hydrocarbons, plastics, organic solvents, pesticides, and dyes, and they can be found in diverse environments. In addition, these organic pollutants are a threat to wildlife and human, due to long-term deleterious effect and chemical complexity. In particular, thousands of persistent organic pollutants (POPs) are a family of chemicals consisting of a diverse group of organic substances, which are toxic, bioaccumulative, and prone to long range of transport [
Aliphatic organic compounds are mainly runoffs from the surface and are particularly seen in urban areas. In addition, the petroleum oils and the byproducts from the combustion of oil also are mostly aliphatic compounds. A variety of aliphatic organic compounds, that include alkenes, alkynes, dichlorodifluoromethane, 1,2-dichloroethane, 2-propanol, and tetramethylammonium ions, have been reported as toxic pollutants from aquatic environment mainly from surface runoffs. Polycyclic aromatic hydrocarbons (PAH) are another type of organic substances released to the environment from the incomplete combustion of organic substances including wood, carbon, and oil. They are neutral, nonpolar organic molecules consisting of two or more fused benzene rings and reported as a priority pollutant by the EPA [
Surfactants are among the most versatile group of organic compounds utilized in industrial, household, personal care, and health products [
Phenol and phenolic compounds byproducts formed from many industrial processes, such as the manufacturing of herbicides, plastics, polymer precursors, photographic developers, dyes, drugs, and pulp and paper industry [
Continuous increase of pollutants in water bodies has necessitated the need to develop cost-effective methods for their removal. Destroying the pollutants to benign chemicals and/or removing these pollutants from contaminated water is imperative for a green environment. There are numerous treatment processes that have been applied for pollutant removal from wastewater, such as electrochemical oxidation [
Electrochemical oxidation is an efficient and economic method, suitable when the wastewater contains nonbiodegradable organic pollutants. This method poses several advantages since it does not require auxiliary chemicals, high pressures, or high temperatures. In addition, owing to its versatility and cost-effectiveness, electrochemical techniques have gained great attention for the removal of pollutants. The process of electrochemical oxidation mechanism is mainly based on the generation of the hydroxyl radicals at the electrode surface.
Two different types of mechanisms have been elaborated for electrochemical oxidation, such as direct and indirect oxidation methods [
In the indirect electrochemical oxidation, the organics are treated in the bulk solution by oxidants, such as
The pilot plant arrangement and illustration of electrochemical cell (reprinted with permission from [
The effectiveness of this electrode was evaluated by monitoring the concentration of BPA. Chemical oxygen demand (COD) removal of 78.3% was obtained, when 0.05 M NaCl was used as the electrolyte at an initial pH of 5 and a current density of 12 mA/cm2. In a different study, the effect of different types of supporting electrolytes in the degradation of phenol using BDD electrode was studied by Alencar de Souza and coworkers [
In the 1990s, biological processes were used for the removal of heavy metal due to the reactive ability of microorganisms with a variety of pollutants that include organic and inorganic species. It was recognized that the microorganisms influence the mobility of the metal by modifying the chemical and physical characteristics of the metals [
Combined biological and chemical degradation methods were carried out to evaluate the effectiveness of mature municipal landfill leachate in laboratory scale by Di Iaconi and coworkers. The biological treatment was followed by chemical oxidation for further removal of COD [
Experimental setup for combined chemical and biological degradation:
It was found that O3 and O3/UV oxidation treatment was able to achieve 90 and 100% removal of the pesticide deltamethrin, in a period of 210 min. Utilization of ozone with UV irradiation was found to enhance the degradation of pesticides. It has been well documented elsewhere that the rate of pesticide removal mainly depends on both the chemical nature of the pesticides being treated [
In a very recent study, wastewater from a pharmaceutical formulation facility in Israel was treated with a biological activated-sludge system followed by ozonation [
Adsorption is an effective and well-known process and has been widely explored as an alternate technique compared with the other waste removal methods due to the lower cost, flexibility and simplicity of design, and ease of operation. Moreover, adsorption does not result in production of any harmful substances. Discharge of several types of pollutants that include household wastes, phenolic wastes [
Dyes have been identified as a major contaminant in wastewater. Many industries, such as textile, leather, paper, plastics, food, and cosmetics, use several dyes as coloring materials [
Adsorption is a method that is capable of removing nondegradable waste pollutants. There are several adsorbents that include clay minerals [
First, a brief discussion about the principle and equations related to some of the adsorption isotherms is made. Generally, the equilibrium adsorption capacity
A wide variety of equilibrium isotherm models that include Langmuir, Freundlich, Dubinin-Radushkevich, Temkin, Flory-Huggins, and Hill isotherms that have been classified under two parameter isotherms, and Redlich-Peterson, Sips, Toth, Koble-Corrigan, Khan, and Radke-Prausnitz isotherms grouped under three parameter categories [
The Langmuir isotherm has been applied to a variety of pollutant sorption processes, which involve homogeneous surfaces and negligible interaction between the adsorbed molecules. In addition, monolayer adsorption on the adsorptive site is the main assumption, in the Langmuir adsorption process, and the saturated monolayer adsorption capacity can be obtained using the following formula [
Freundlich expressed an empirical equation, known as the Freundlich adsorption isotherm, in 1906. The relationship between the concentration of the solute in equilibrium,
The Redlich-Peterson equation is another popular model and has three different parameters commonly named as
A large number of materials have been employed as adsorbents for removal of pollutants and they include natural materials, such as clays, coal, fly ash, zeolites and other siliceous materials, biomaterials, agricultural and industrial solid wastes, activated carbon, oxides, and mixed oxide materials.
Natural materials are well known as adsorbents from the beginning of recorded human development. In particular, clays have been utilized as an adsorbent and as an ion-exchange material for the removal of ions and organics due to their low cost, natural abundance, high adsorption capacity, and ion-exchangeable property [
Fly ash is another type of natural material, which is relatively abundant and inexpensive and is currently being explored as an adsorbent for the removal of various organic pollutants that are present in wastewater, such as phenolic compounds, pesticides, and dyes. It has been documented elsewhere that these contaminants can be effectively removed by using fly ash as an adsorbent [
Biomaterials are another type of natural materials, which are mainly used for adsorption and degradation for water treatment applications. A few publications have discussed the adsorption of hazardous and toxic pollutants using biological materials [
Activated carbon is another important, efficient, and commercially available material that consists of a wide variety of pores in it. Even though the adsorption process proceeds through a sequence of diffusion steps from the bulk phase into the mesopores followed by diffusion into the micropores, the major adsorption sites on activated carbon were reported to be located in the micropores [
In a recent literature, Prola et al. used multiwalled carbon nanotubes (MWCNT) and powder activated carbon (PAC) for the adsorption of direct blue 53 dye from aqueous solution [
Oxides constitute an important class of adsorbents, and, in this regard, zeolites [
AOP is an oxidation technique, which typically uses ambient conditions (room temperature and atmospheric pressure). Several AOP techniques such as ozonation, H2O2 photolysis, Fenton process, photo-Fenton process, and heterogeneous photocatalysis have been explored for the elimination of pollutants, particularly from water sources. These AOP techniques destroy the pollutants by chemical oxidation or reduction. In particular, AOP relies on the production of hydroxyl radicals
Ozone is unstable in water and the chemical properties of ozone rely on the experimental conditions. The molecular ozone can react as a dipole, electrophile, or nucleophile due to the two different resonance structures. In addition, depending on the pH, temperature, and concentration of organic and inorganic compounds in water, the half-life of ozone varies from a few seconds up to a few minutes. Ozone is a powerful oxidant and it can oxidize a large number of organic and inorganic materials. Ozone reacts either directly or indirectly with aqueous compounds. In the direct reaction, the molecular ozone directly reacts with the compounds, whereas the
The direct reactions are very slow and solute selective, whereas the indirect radical reactions are fast and nonselective. Additionally, the direct reactions are dominant in acidic solutions, while the indirect reactions occur mostly at basic pH values. It was reported elsewhere that catalytic ozonation significantly enhanced the rate of oxidation more than noncatalytic ozonation [
Einaga and coworkers carried out catalytic oxidation of benzene with ozone over several support materials, such as Al2O3, SiO2, TiO2, and ZrO2, and they found that the surface area of the catalysts is one of the important factors that affect the reaction. In addition, they carried out oxidation of benzene with ozone over several Mn ion-exchanged zeolite catalysts, such as Mn-Y, Mn-b, Mn-MOR, and Mn-ZSM-5 to investigate the effect of catalyst support on the reaction [
H2O2 treatment in AOP also involves the formation of
There are a large number of organic pollutants, such as phenols and phenolic compounds, benzene and substituted benzene, salicylic acid, proline, pyridine, and dyes [
Fenton reaction is a homogeneous catalytic oxidation process that uses a mixture of hydrogen peroxide (H2O2) and ferrous ions. Due to its simplicity and the availability of chemicals, this oxidation is considered as an advanced technique for waste removal. In acidic environment, H2O2 and Fe2+ ions in the contaminated solution produce hydroxyl radicals expressed as follows:
There are several reports that have examined the applications of Fenton and photo-Fenton reactions. For example, Gutowska et al. studied the degradation mechanism of Reactive Orange 113 in aqueous solution by using H2O2/Fe2+ [
Heterogeneous photocatalysis is a process, which embraces a large variety of reactions such as oxidation, dehydrogenation, metal deposition, organic synthesis, water splitting, photoreduction, hydrogen transfer, isotopic exchange, disinfection, anticancer therapy, water detoxification, and gaseous pollutant removal [
Recombination of electrons and holes within a semiconductor particle in the presence of acceptor (A) and donor (D) (reprinted with permission from [
In this photocatalysis process, the contaminant molecule gradually breaks down by its reaction with highly reactive oxidative species (ROS), such as
Different removal techniques and its advantage(s) and disadvantage(s).
Removal techniques | Advantage(s) | Disadvantage(s) |
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Electrochemical oxidation | Does not require auxiliary chemicals, high pressures, or high temperatures. | Low selectivity and low reaction rates. |
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Biological process | Ecologically favorable process. | High capital and operational cost. |
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Adsorption | Cost-effective method. |
Merely removes the pollutants from one phase (aqueous) to another (solid matrix). |
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Advanced oxidation processes (AOP) | ||
(i) Ozonation | Powerful oxidation technique oxidizes a large number of organic and inorganic materials. | More complex technology and requires high capital/operational cost. |
(ii) UV | An effective method that typically does not leave any byproducts which are harmful to the environment. | Less effective if the wastewater has high amounts of particulates which can absorb UV light. |
(iii) UV/H2O2 | An effective technique in the oxidation and mineralization of most organic pollutants. |
Less effective, when the wastewater has high absorbance. |
(iv) O3/UV/H2O2 | Most effective process due to the fast generation of |
Needs to compete with high turbidity, solid particles, and heavy metal ions in the aqueous stream. |
(v) Fenton reaction | Simple process. |
Production of sludge iron waste and handling the waste pose logistical problems. |
(vi) Photo-Fenton reaction | Reduction of sludge iron waste compared to original Fenton reaction. |
Needs a controlled pH medium for better performance. |
(vii) Heterogeneous photocatalysis | Long-term stability at high temperature. |
Could form byproducts that can be harmful to the environment. |
Frank and Bard have reported the heterogeneous photocatalytic oxidation of cyanide and sulfite in the presence of several semiconductors that include TiO2, ZnO, CdS, Fe2O3, and WO3, in aqueous medium under sunlight [
The removal of inorganic anions that include cyanide [
Even though TiO2 has been utilized as a photocatalyst for the degradation of a number of pollutants, its efficiency towards degradation is partly limited owing to its poor adsorptive property. In order to improve the efficiency of titania, researchers have prepared mixed oxide materials that can provide greater number of adsorptive sites; furthermore, by generating highly porous structures with large surface areas, effective dispersion of titania can also be achieved [
As discussed in the previous section, the presence of various inorganic and organic hazardous pollutants in the aquatic system poses a huge threat to public hygiene and human health. Hence, effective and economic means of remediating the polluted water sources need to be developed. Several conventional methods for removal of hazardous materials from wastewater, such as physical adsorption, condensation, biofiltration, and catalytic destruction [
The following sections will briefly provide an overview of the synthesis of TiO2-SiO2 mixed oxides and an extensive discussion about the applications of TiO2-SiO2 mixed oxide materials towards the degradation of aquatic pollutants will also be carried out. A variety of synthetic methods have been reported for the preparation of periodic mesoporous materials, which are materials with uniform, regular, and well-arranged pores, and aperiodic TiO2-SiO2 mixed oxide materials that have randomly arranged pores. Thus, some of the main preparation methods followed by the structural characterization of titania-silica are presented here. Finally, the photocatalytic activity of these TiO2-SiO2 materials will be elaborated in great depth. Consequently, the factors that influence the photocatalytic activity such as the structural properties of the photocatalysts, pollution type and concentration, and pH will also be reviewed.
Among numerous semiconductor materials, there has been considerable interest in the use of TiO2 as a photocatalyst to degrade a variety of pollutants in water [
In order to promote AOP in practical applications, TiO2 based photocatalysts with large specific surface area and porosity that is conducive to adsorption of aquatic pollutants need to be developed. A common strategy is to incorporate TiO2 into periodic mesoporous support materials such as SiO2. Periodic mesoporous SiO2 materials possess high surface area, tunable pore size, and large pore volume that facilitate good dispersion of TiO2 and adsorption of pollutant molecules as well.
In recent years, TiO2 has been incorporated into highly ordered mesoporous siliceous materials such as SBA-15, MCM-41, and MCM-48. These periodic mesoporous siliceous supports with very high surface area and long-range ordered array of mesopores are recognized to be robust and stable supports for immobilizing TiO2 species. In addition, the large surface area of mesoporous SiO2 enables the high dispersion of TiO2 species. With the above-mentioned merits, TiO2 containing periodic mesoporous materials are expected to achieve markedly enhanced efficiencies for the photocatalytic degradation of aqueous wastes in comparison with nonsupported TiO2.
In this section, the commonly conducted synthetic methods for the preparation of TiO2 containing periodic SiO2 mesoporous materials will be discussed. In addition, typical characterization techniques along with examples in which TiO2 containing periodic SiO2 mesoporous materials are used as photocatalysts for degradation of aqueous pollutants will also be covered.
SBA-15 is an important mesoporous material that is prepared by the using of triblock copolymers as structure directing agents. Because of its relatively large pore size (>6 nm) and thick pore walls, it has been used as a support to disperse TiO2.
The synthesis of SBA-15 mesoporous materials has been established and well documented [
MCM-41 and MCM-48 belong to the M41S series of mesoporous materials that were originally developed by Mobil research scientists. These mesoporous materials have been widely employed as supports to disperse TiO2.
The important techniques to characterize the titania containing mesoporous materials are discussed in this section.
Figure
Small-angle XRD patterns of TiO2/SBA-15 samples (reprinted with permission from [
Apart from the low-angle XRD, long-range XRD analysis was also carried out to investigate the nature of the titania species in the Ti-SBA-15 mesoporous materials. Figure
Wide-angle XRD patterns of TiO2/SBA-15 mesoporous samples with different loadings of TiO2: (a) bare SBA-15, (b) 7% TiO2, (c) 12% TiO2, (d) 18% TiO2, (e) 22% TiO2, (f) 26% TiO2, and (g) 31% TiO2 (reprinted with permission from [
Similar observations can also be found from the powder XRD analysis for titania incorporated into MCM-41 and MCM-48 mesoporous materials. Kasahara et al. performed powder XRD analysis for titania incorporated MCM-41 mesoporous materials [
Figure
(a) N2 adsorption-desorption isotherm: (a) RH-MCM-41, (b) 10% TiO2/RH-MCM-41, (c) 20% TiO2/RH-MCM-41, (d) 40% TiO2/RH-MCM-41, (e) 60% TiO2/RHMCM-41, and (f) bare TiO2. (b) Pore size distribution in RH-MCM-41 and TiO2/RH-MCM-4 (reprinted with permission from [
All the isotherms indicate type IV classification that is typical for mesoporous materials. In addition, a steep inflection that occurs at relative pressure values from 0.2 to 0.4 is an indication of the periodic array of the mesopores in the samples. Figure
Diffuse reflectance UV-Vis spectra of Ti-SBA-(x) materials with varying titania loadings corresponding to (a) Ti-SBA-
FT-IR spectra of Ti-MCM-41 with different Si/Ti ratios (reprinted with permission from [
Raman spectra of TiO2/SBA-15 mesoporous materials with different loadings of TiO2 and pure TiO2 samples (reprinted with permission from [
O 1s core-level spectra of TiO2/SBA-15 mesoporous samples (reprinted with permission from [
TEM images of (a) SBA-15 along [
SEM images of (a) Rod-SBA-15, (b)
In this section, we will discuss the photocatalytic reactions explored using titania incorporated into periodic mesoporous materials.
In addition to the structural properties of the photocatalysts, the experimental conditions also had a dramatic influence on the photocatalytic decolorization of MB. Suraja et al. investigated the effect of the reaction time, concentration of MB, and the pH environment over the photocatalysis process [
Besides the hexagonal SBA-15 mesoporous materials, TiO2 containing cubic MCM-48 mesoporous materials have also been studied for MB photodegradation. Liou and Lai developed TiO2/MCM-48 composite materials and evaluated their performance for photocatalytic decomposition of MB [
Rhodamine dyes (R6G and B (RhB)) are also commonly seen as pollutants in the aquatic environment and they can be decomposed by titania incorporated into periodic mesoporous materials. De Witte et al. tested the photocatalytic performance of titania loaded SBA-15 for the photodecomposition of R6G [
Li et al. investigated the photocatalytic properties for methyl orange (MO) degradation over the TiO2/SBA-15 mesoporous materials with different morphologies [
Anandan conducted the photocatalytic degradation of MO by using titania incorporated MCM-41 mesoporous materials and the role of peroxomonosulphate (PMS), peroxodisulphate (PDS), and H2O2 as electron accepters was examined in the photocatalytic reaction [
Ding et al. observed that Ti-SBA-15 exhibited enhanced photocatalytic activity for the degradation of indigo carmine (IC) than anatase titania [
Bhattacharyya et al. applied Ti-MCM-41 materials for the photocatalytic decoloration of Orange II dye [
Adams et al. designed TiO2/SBA-15 catalysts in different morphologies (thin film and powder form) to study their activity for the photodegradation of 2,4-dichlorophenol (2,4-DCP) [
Do et al. developed a set of Ti-MCM-41 samples with variable TiO2 loading for the photodegradation of 4-nitrophenol (4-NP) [
Artkla assessed the photocatalytic degradation of polyphenols (gallic acid) by using TiO2/MCM-41 catalysts [
Artkla et al. assessed the activity for photocatalytic degradation of tetramethylammonium (TMA) ions over TiO2/MCM-41 mesoporous materials [
Aperiodic titania-silica binary mixed oxides are commonly synthesized using the sol-gel method. Aperiodic mesoporous materials do not possess uniform and periodic arrangement of pores. Several reported methods of preparation that include coprecipitation, impregnation, hydrothermal, flame hydrolysis, and chemical vapor deposition (CVD) have been implemented to tune the structural features of the mixed oxides by optimizing the synthetic procedures. In a recent review article, we have successfully discussed the different synthetic methodologies to prepare TiO2-SiO2 mixed oxide materials [
The sol-gel synthesis of TiO2-SiO2 materials usually involves the hydrolysis and condensation of titanium and silicon alkoxide precursors that lead to the formation of polymeric gels. By changing the experimental conditions, that include the amounts and types of precursors and solvents, water, pH, and temperature, the structural properties of the materials can be tailored. Furthermore, the porosities of the materials can be tuned by using an appropriate drying method. In a typical synthesis of xerogels materials, appropriate amounts of silica precursors and titania precursors are mixed with an alcoholic solvent, that include ethanol, methanol, or isopropanol, under vigorous stirring at ambient conditions. The sol is allowed to gel at ambient conditions and the solvent is allowed to evaporate. The dry powder obtained is then calcined. Generation of homogeneous gel is critical due to the unequal hydrolysis and condensation rates of the two different titanium and silicon alkoxide precursors. The unequal hydrolysis rates of these two alkoxide precursors can be explained by the differences in the partial charge of Si (+ 0.32) and Ti (+ 0.61). Due to their higher partial charge, titanium alkoxides undergo more rapid hydrolysis than silicon alkoxides, and this may lead to inhomogeneous distribution of titania in the mixed oxide materials. Thus, prehydrolysis of silicon alkoxides is necessary [
Preparation of TiO2/SiO2 nanomaterials was attained from the hydrolysis and polycondensation of tetrabutyl orthotitanate and tetraethyl orthosilicate via a sol-gel process by Cheng et al. [
Homogeneous hydrolysis and condensation can also be achieved as discussed thereafter. The silicon alkoxide is hydrolyzed in an alcohol solution in the presence of concentrated acid, for example, HNO3, HCl, or H2SO4, and water with vigorous stirring to create silanol groups [
The hydrothermal treatment has been utilized by several researchers to achieve complete hydrolysis and condensation of the silica and titania precursors in a short period of time [
Different synthetic recipes have been attempted for the preparation of TiO2-SiO2 materials in order to attain desirable structural properties that are responsible for enhanced photocatalytic activity. Use of appropriate solvents, acids, or bases, and effective metal precursors in the synthesis procedure leads to the formation of materials with enhanced porosities and surface area. TiO2-SiO2 xerogel materials with different pore sizes and surface areas have been prepared with a constant 1 : 4 molar composition of Ti : Si by simply changing the amounts of water, acids, or bases and hydrolysis procedure [
Avendaño and coworkers developed an experimental strategy to obtain mesoporous ZrO2-SiO2 and TiO2-SiO2 mixed oxides via the sol-gel process [
It has been postulated that, by improving the hydrophobic nature of titania-silica, one can enhance the adsorption of organic compounds on them compared to bare TiO2. Larsen and coworkers tried to enhance the hydrophobicity of titania-silica materials using noncyclic silicon precursors [
TiO2/SiO2 composite nanoparticles were prepared directly from the acidic precursor solutions of TiOSO4 and TEOS by hydrolysis under hydrothermal conditions by Hirano et al. [
Li and coworkers synthesized silica-modified TiO2 by utilizing hydrothermal methods [
Another approach to significantly enhance the porosities and surface area is preparation of aerogels under supercritical conditions. In this method, the wet gel prepared by the same procedures explained above is slowly heated to critical temperature (Tc) and critical pressure (Pc) of the solvent employed. The solvent is then removed once the appropriate supercritical conditions of temperature and pressure are attained. Removal of solvent under such conditions prevents the formation of both the liquid-vapor meniscus and capillary pressure in the pores. Thus, pore collapse is prevented and the final materials possess relatively large pores.
Dutoit et al. synthesized mesoporous titania-silica aerogels with highly dispersed titania by a sol-gel process [
Deng et al. synthesised TiO2-SiO2 aerogel materials with different TiO2 contents [
Kim et al. synthesized titania-silica binary aerogel with 1 : 1 molar ratio of Ti to Si by first modifying tetrabutylorthotitanate (TBOT) with acetylacetone in methanol solution followed by the addition of TEOS [
The influence of the preparation method on the physicochemical properties of titania-silica aerogel materials was assessed by Brodzik and coworkers [
In summary, the synthetic routes for preparation of the binary aperiodic mixed oxide materials significantly influence the structural properties of the materials, which ultimately affect their photocatalytic activity. The following section will provide a brief discussion of several characterization techniques.
In most of these TiO2-SiO2 mixed oxide materials the X-ray patterns show a broad amorphous peak due to the presence of silica. However, with an increase in the calcination temperature and/or use of cosolvents, the crystallinity can be modulated as depicted in Figure
The wide-angle XRD patterns of TiO2-SiO2 materials: (a) prepared in the ratio of solvent: cosolvent = 1 : 1, in which the cosolvents for TS01, TS02, TS03, TS04, TS05, and TS06 are ethanol, hexane, toluene,
The FT-IR spectra of (a) calcined mesoporous silica, (b) titania modified silica with Ti/Si = 10, (c) Ti/Si = 2.5, and (d) Ti/Si = 5 (reprinted with permission from [
The nitrogen physisorption patterns of a series of materials with different Ti/Si amount are illustrated in Figure
Nitrogen physisorption properties of TiO2-SiO2 xerogel materials: (a) pure titania (X-Ti-01) and pure silica (X-Si-06) and (b) TiO2-SiO2 mixed oxides with Ti/Si ratio of 1 : 1 (X-Ti-02), 1 : 2 (X-Ti-03), 1 : 3 (X-Ti-04), and 1 : 4 (X-Ti-05) prepared in ethanol-toluene solvent system (reprinted with permission from [
On the other hand, the mixed oxide xerogels with higher silica contents, such as 1 : 2, 1 : 3, and 1 : 4 ratios of Ti : Si (X-TiSi-03, X-TiSi-04, and X-TiSi-05), exhibit H3 type loops that are not leveled off at relative pressure that are very close to that of saturation vapor pressure. The pore size distribution plots for the corresponding mixed oxide materials are shown in Figure
Pore size distribution of TiO2-SiO2 xerogel materials: (a) pure titania (X-Ti-01) and pure silica (X-Si-06) and (b) TiO2-SiO2 mixed oxides with Ti/Si ratio of 1 : 1 (X-Ti-02), 1 : 2 (X-Ti-03), 1 : 3 (X-Ti-04), and 1 : 4 (X-Ti-05) prepared in ethanol-toluene solvent system (reprinted with permission from [
(a) UV-Vis DRS spectra of the mixed oxide materials MAT01, MAT02 along with Degussa P25, Acros anatase, and Alfa rutile materials and (b) the corresponding Kubelka-Munk function plot (reprinted with permission from [
As the content of Si in the mixed oxides increases, the UV absorption edge shifts to higher energies. Furthermore, a blue shift was observed in the titania-silica mixed oxides prepared using different cosolvents and this shift was explained by changes in the crystallite size of the anatase TiO2 [
Typical scanning electron micrographs of (a) silica particles and (b) titania coated silica particles (reprinted with permission from [
The SEM image illustrates that the silica particles have spherical shape and smooth surface. The coating was carried out by the gradual addition of titania sol into the aqueous dispersion of silica which resulted in the attachment of small titania particles on the silica surface. The SEM image illustrates that, after coating, there is a loss in the smoothness of the silica surface.
TEM images of TiO2 nanoparticles prepared with different silica content and solvent composition: (a) pure TiO2 (ethanol to water = 4 : 1), (b) pure TiO2 (ethanol to water = 1 : 8), (c) 0.2 SiO2-0.8 TiO2 (ethanol to water = 4 : 1), and (d) high resolution TEM (HRTEM) image of (c) (reprinted with permission from [
Ti (2p) peaks of TiO2-SiO2 mixed oxides prepared with different Ti : Si ratios: (a) Ti : Si = 1 : 10, (b) Ti : Si = 3 : 7, (c) Ti : Si = 7 : 5, and (d) Ti : Si = 7 : 3 (reprinted with permission from [
In a different study, the O 1s peak was found to be shifted to the lower binding energy with an increase in the TiO2 content (Figure
X-Ray photoelectron spectra of the O 1s level for Ti-Si binary oxides. The Ti weight % was in the range of 0.4–80 in the mixed oxide (reprinted with permission from [
Semiconductor binary mixed oxides have emerged as promising materials for photocatalytic degradation of pollutants and photosplitting of water and, among them, TiO2-SiO2 has been quite well studied.
Various types of aquatic pollutants that include inorganic ions, aliphatic hydrocarbons, such as alkanes and alkenes and their substituted derivatives, benzyl compounds, dyes, phenolic compounds, and pesticides were experimented using advanced oxidation process. This section will review the various photocatalytic reactions that have utilized aperiodic TiO2-SiO2 materials.
Removal of cyanide ions using TiO2-SiO2 aerogel as photocatalysts has been examined in a separate study by Ismail et al. [
Photocatalytic degradation of tetramethylammonium (TMA) ions in water was studied with pure TiO2 and silica-loaded TiO2 [
Catalytic reactions that include epoxidation of olefins and selective oxidation of saturated hydrocarbons have been carried out by utilizing amorphous microporous titania-silica mixed oxide materials by Klein and coworkers [
Decomposition of benzyl trimethyl ammonium chloride (BTMA) and propionic acid was carried out over SiO2 loaded TiO2 under UV irradiation. Propionic acid showed lower degradation rate due to the presence of more negative charge on the surface of TiO2-SiO2 at pH 6 [
The utilization of TiO2-SiO2 aerogels for the photocatalytic degradation of phenol pollutant was investigated by Deng and coworkers and it was noticed that the aerogels contained anatase microcrystallites after supercritical drying in ethanol [
Malinowska and coworkers chose three different phenol
The application of photocatalysts with different Ti/Si ratios for the photodecomposition of R6G has been investigated and it has been noted that a Ti/Si ratio of 30/70 produces a catalyst about three times more active than commercially available Degussa P25 due to the enhanced adsorption of the dye. However, it has been noticed that the presence of larger amounts of SiO2 decreases the activity. In a following study, Anderson and Bard have utilized two different binary mixed oxides TiO2/SiO2 and TiO2/Al2O3 for the photodecomposition of salicylic acid and phenol [
Schematic representation of the TiO2/SiO2 and TiO2/Al2O3 photocatalysts with no interaction between the TiO2 and SiO2 or Al2O3 phases (reprinted with permission from [
It was noticed by Cheng and coworkers that the addition of silica in the titania-silica mixed oxides increases the photoactivity by suppressing the phase transformation of titania from anatase to rutile, and this has been also reported by several other researchers [
Photodegradation of two different dyes, reactive 15 (R15) and cationic blue X-GRL (CBX), was carried out over different surface bond conjugated TiO2/SiO2 materials prepared by impregnation method [
The photocatalytic degradation of a RhB with TiO2 [
In another recent study, a series of aperiodic titania-silica photocatalysts was prepared in ethanolic solutions of polar aprotic cosolvents such as ethyl acetate (EtOAc), acetonitrile (ACN), acetone (ACT), and N,N-dimethylformamide (DMF) using sol-gel procedure by our group [
Degradation of acid orange 7 (AO7) using TiO2/SiO2 composites, which were prepared by coating the SiO2 surface with nano-TiO2 by hydrolysis of TiCl4, was examined by Cetinkaya and coworkers [
It is noted that the photocatalytic activity of TiO2-SiO2 mixed oxides is closely related to their structural properties, such as crystallinity and crystallite size of titania, crystal composition, surface area, particle size distribution, porosity, bandgap, surface hydroxyl density, dispersion of TiO2, and Ti-O-Si linkages. The use of a high surface area silica support provides good dispersion of titania. Also, the silica support increases the hydrophobic nature of the mixed oxide. This helps to adsorb a wide variety of organic pollutants and concentrate them close to the reactive TiO2 centre [
The porous properties such as pore volume and pore size of the mixed oxide materials influence the adsorption of the pollutant. In particular, larger pore sized materials provide better transport (molecular trafficking) of the pollutants and the product(s), in and out from the active sites, and contribute to enhanced degradation. It is generally observed that the rate of degradation increases with an increase in the pollutant concentration to a certain level. However, further increase in the pollutant concentration results in a decrease in the degradation rate. Among the various ROS, •OH radical is an important species in the degradation processes and the rate of degradation depends on both the probability of the formation of •OH radicals on the catalyst surface and the reactivity of •OH radicals with the pollutants. Thus, the enhancement in activity will rely on the probability of the reaction between the pollutant and these oxidizing species. However, the degradation efficiency decreases beyond a particular level of pollutant concentration. This is due to the inhibition of •OH radical generation via the coverage of active sites on the catalysts by the pollutant molecules/ions. In addition, another reason that was reported for the low activity particularly for dye pollutants was the UV-screening effect by the dyes. At high dye concentrations, a significant amount of UV light may be absorbed by the dye molecules rather than by the semiconductor particles and this reduces the efficiency of the catalytic reaction by reducing the generation of •OH radical and superoxide radical (
The characteristic features of the pollutant in the wastewater differ with the variation in the solution pH [
A comparative table is provided in the supplementary section (in the Supplementary Material available online at
This review has attempted to cover a wide range of wastewater effluent removal techniques for water treatment. The reader can get an idea about the various types of removal methods and the basic principles behind each technique. Heterogeneous photocatalytic oxidation (HPO) has garnered extensive attention due to its effective removal of toxic compounds from waste effluents. The HPO process employs several oxide and mixed oxide catalysts, mainly for water purification. Among these materials, TiO2-SiO2 mixed oxides have been found to be more active than the other mixed oxides for the degradation of organics. In this work, we have covered the degradation of organics by utilizing TiO2-SiO2 binary mixed oxide materials and the factors that influence the degradation have been discussed. Furthermore, this review briefly explains the synthetic procedures and the main characterization techniques of the two main types of TiO2-SiO2 binary mixed oxides, periodic and aperiodic oxides. Even though these binary mixed oxides show better activity than pure TiO2 materials in most instances, the utilization of these TiO2-SiO2 mixed oxides is limited for the mineralization of selected pollutants.
The authors declare no conflict of interests.
Thanks are due to the National Science Foundation Grants NSF-CHE-0722632 and NSF-EPS-0903804 and the Department of Energy Grant DE-EE0000270.