Sol-gel: The sol-gel method consists of the acidic or basic hydrolysis of an organometallic precursor, followed by a slow polymerization.
The obtained material is dried, allowing the decomposition and elimination of all the organic components present in the gel. The photocatalytic activity of the material was tested via the degradation of malathion using natural sunlight.
Yang et al. Water hydrothermal and several organic compounds such as ethylene glycol and glycerol solvothermal may be used as solvent during the reaction. The solvothermal method is one of the most common preparation techniques for heterostructures, since the variation of pressure and temperature parameters allows the formation of a wide diversity of crystal morphologies.
The products obtained are usually well dispersed in form and size. Also, some additives and templates may be added into de reaction mixture to favor a desired morphology or crystallite size. Xu et al. The modification of TiO 2 with CdS nanoparticles increased the absorption of visible light irradiation at nm. Zhu et al. TiO 2 MoO 3 core shell structures were synthesized by Li et al. In a first step, TiO 2 HS were synthesized by a sol—gel method, using carbon nanospheres as a template.
These heterostructures were tested for the photocatalytic degradation of Rhodamine B under visible and sunlight irradiation, showing promising results. Impregnation: This method consists of the saturation of one specific support—in this case TiO 2 —with a solution containing the desired precursor, usually a metal salt; this allows the metal ions to fill the support pores. Then, the material is dried and exposed to a thermal treatment.
Then, the composite was formed by suspending both oxides in methanol in order to achieve the impregnation assisted by ultrasonication. Lastly, the solvent was evaporated using a rotary evaporator. In another work, Maeda et al. Peng et al. UV light irradiation: This method consists of the reduction and precipitation of one or more soluble precursors over the surface of TiO 2 , which acts as a support. The presence of UV light irradiation allows the photo-formation of electrons in the support, which are responsible of reducing the chemical species in the solution.
These materials showed good efficiency in the degradation of methylene blue and 4-chlorophenol. Electrosynthesis: The electrosynthesis method consists of the use of electrochemical cells to produce the desired material. For this, three electrodes were used, namely a Pt wire counter electrode , a saturated calomel electrode reference electrode and the TNTs working electrode. The three electrodes were submerged in an electrolyte solution containing CdCl 2 and SeO 2. Well-dispersed CdSe nanoparticles deposited on TiO 2 were obtained.
Mechanical mixing: This is one of the simplest synthesis methods, which involves the direct mixing of the heterostructure precursors. Manual mixing usually results in long reaction times and low homogeneity of the products. In certain cases, binding agents may be added to the mixture in order to increase the stability of the heterostructure. Shifu et al. The precise amounts of TiO 2 and WO 3 powders were mixed in an agate ball milling tank. The two oxides were mixed for 12 h at rpm. The coupled photocatalyst showed a redshift in its light absorption compared to pure TiO 2.
Precipitation and co-precipitation: Both precipitation and co-precipitation methods consist of the formation of an insoluble material, starting from one or several solutions containing the soluble precursors.
Usually, an increase in the pH value of the solution helps in the formation of insoluble hydroxides, allowing the precipitation. Yu et al. Lastly, UV irradiation was applied to achieve a partial reduction of the formed AgCl nanoparticles into metallic Ag. Thin films: One of the main burdens of using powder photocatalysts is the need of including a separation step for the effluent in order to reuse the photocatalyst in further cycles.
This step can become difficult and very expensive, making the photocatalytic process less viable in a plant scale approach. A feasible solution is the immobilization of the material in a suitable support, such as glass, quartz or polymer. Some synthetic routes for obtaining photocatalysts as thin films are described below. Dip-coating: This is one of the most used methods for the synthesis of thin films, which consists of submerging, at a constant rate, the substrate in a solution containing the precursor of the semiconductor.
After a certain dwell time, the substrate is pulled out of the solution. Lastly, the solvent is dried, and a thermal treatment can be applied to eliminate organic residuals and induce crystallization of the semiconductor in the film. Spin Coating: This process consists of putting a small amount of a solution containing the precursor of the thin film material on the surface of the substrate. Then, the substrate is rotated at high speed, eliminating the excess solution and leaving a uniform film once the solvent is dried.
Sputtering: In this route, ionized atoms e. This collision causes that some atoms are ejected from the surface of the electrode. Subsequently, the ejected atoms are condensed on the surface of the substrate anode , forming the thin film. Chemical vapor deposition: This method uses volatile precursors at high temperature. The gaseous species react forming intermediates which are diffused and adsorbed on the surface of the substrate.
Further reactions can take place on the surface on the substrate. The coupling of TiO 2 with low band gap semiconductors leads to the activation of the photocatalyst material under visible light irradiation, as established earlier, resulting in turn in the generation of materials with high efficiency and stability. An important number of studies have reported the photocatalytic performance of these heterostructures, showing high conversion rates of organic and inorganic pollutants in water.
Some of these results are shown in Table 1. As observed in the table, conversion of azo dyes molecules is the most used way to assess the photocatalytic activity of the synthesized materials. This is due to the easy analytical determination of such molecules in water—most of them for UV—vis spectroscopy—in comparison with uncolored molecules—such as phenols—organochlorinated compounds and pharmaceutical substances.
However, as was recently pointed out, using azo dyes molecules in the evaluation of the photocatalytic performance of semiconductors may result in an artifact because of the sensitization of the semiconductors by the adsorbed organic molecules [ 62 ]. It is worth noting how the degradation rate constant is mostly determined using the pseudo first-order approach, forgetting the multiple phase conditions.
In very few studies, other models—such as the Langmuir-Hinshelwood approximation—have been used [ 63 ]. Degradation yields is the most reported parameter in this kind of experiments. Very few studies follow the content of the total organic carbon throughout the process, ignoring with this the mineralization yield of the pollutants.
This may lead to a miscalculation of the risk that treated water pose on the exposed organisms, since some of the photodegradation by-products may be more toxic or recalcitrant than the parent compound. Examples of this are benzoquinone, which degradation requires more energy than phenol and triclosan, which degrades into a low toxicity dioxin. Thus, the density of the crystal decreases due to cationic vacancies produced. When AgCl is doped with CdCl2 a cation vacancy defect is created, i.
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