Why is tio2 n type

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Point defects are caused by missing or misplaced ion or atom in the crystal lattice i. Formula of a compound is same as the formula of unit cell. This type of defect is typically observed in highly ionic compounds, highly coordinated compounds, and where there is only a small difference in sizes of cations and anions of which the compound lattice is composed.

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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.

Degradation performance using TiO 2 -based heterostructures under different experimental conditions. Kinetic constants of photodegradation estimated by the Langmuir-Hinshelwood model according to a first-order reaction. The use of such concentration levels enables the determination of the kinetic constants in the photocatalytic degradation of the pollutants in water.

Even when for the results shown in Table 1 , direct comparisons are difficult to be established, it seems clear that tailored heterostructures formed by TiO 2 and low band gap semiconductors are efficient to achieve high photocatalytic degradation yields under visible light irradiation when azo dyes are used as target pollutants. On the other hand, very low photocatalytic efficiency is observed for refractory industrial pollutants, such as nitrophenols and chlorophenols.

Some environmentally relevant pollutants, which are commonly found in surface water sources, are efficiently removed by the visible light-driven photocatalysis process. Also, high loads of pharmaceutically active substances, bisfenol A and the widely used herbicide 2,4-D are efficiently removed from water under visible light by the TiO 2 based heterostructures. From these results, the use of these materials in advanced oxidation processes for ternary drinking water treatment sounds like a plausible option, keeping in mind that the high efficiency showed in these lab-scale studies can be affected by the complexity of the liquid matrix.

For the studies reported in Table 1 , the occurrence of synergistic effects was observed when the photocatalytic performance of the heterostructures and their single components was compared.

For some cases, the increment in the degradation rate and degradation yields was found in the order of 1. When the photocatalytic performance of thin films is assessed, a clear decrease in the degradation rate of organic pollutants is observed.

This is because of the decrease in the number of active sites exposed to the aqueous matrix due to the immobilization of the photocatalyst on a substrate.

Conversely, in other study [ 67 ], the complete degradation of methyl orange was achieved in 8 h of visible light irradiation by using BiOCl-TiO 2 thin films. This study reveals the importance of the charge carrier transference in the immobilized photocatalyst material.

When this factor is taken into account, the reactivity of the thin film surface increases, leading to a higher photocatalytic activity and overcoming the mass transference hindrance. In this sense, the arrangement of the heterostructure components is of high relevance since some approaches may favor the transfer of photo-holes or photoelectrons to the surface of the thin films. In this sense, Monfort et al.