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The invention refers to water treatment technology sector and is intended for use in photocatalysis processes in visible light, for example, in water treatment reactors.

Visible-light photocatalysis is a green, reagent-free and zero-energy technology for energy harvesting and environmental remediation [1]. Photocatalysis is based on semiconductor oxides absorbing light with incident photon energy matching or exceeding the semiconductor's bandgap [2]. Absorbed photons excite electrons to the conduction band (CB) and leave an electron hole in the valence band (VB), thus creating photogenerated electron-hole pairs. In combination with ambient water, the electron-hole pairs trigger the formation of H2 and 02 [3] or reactive oxygen species (ROS) with strong oxidation capacity for the degradation of organic substances [4]. Semiconductor photocatalysis has several disadvantages. First, the most excellent photocatalytic material, T1O2, does not absorb visible light and can only be excited by ultraviolet radiation [5]. Incorporating dopants, such as nitrogen [6], sulfur [7], carbon [8] or transition metals [9], into T1O2 can add visible-light activity, but the utilized synthesis methods generally have low yield, high cost, or high ecological impact. Additionally, the resulting photocatalytic activities may be limited [10]. Narrow band gap visible-light-absorbing semiconductors (WO3, Fe203, B1VO4, etc.) have been demonstrated as promising candidates for photocatalysis [11-13], but nevertheless most have limited photocatalytic efficiency due to the fast recombination of photogenerated charge carriers. Some narrow band gap photocatalysts, for example, Ag20 and CU2O, are especially active using visible light but are not stable [14,15] and suffer from photocorrosion. One of the most effective strategies to decrease the overall recombination and to improve the photocatalytic efficiency or stability is to increase the spatial separation of photogenerated charge carriers by coupling semiconductor oxides with metal and/or other semiconductors to form two- or three-component systems [16,17]. In the most common system two semiconductors are coupled with mismatched band edges, generating a potential slope at the interface, which causes electrons to migrate to the component with the more-positive CB edge and causes holes to transfer to the material with the more- negative VB edge. The main drawback of such a system is a decrease of its (overall) redox potential [18]. The most promising photocatalytic materials are all-solid semiconductor systems with a Z-scheme photogenerated charge-transfer mechanism [19]. Z-scheme systems have been reported for water splitting [20], dye degradation [21] and CO2 conversion [22]. In Z-scheme systems, semiconductors with mismatched band edges are coupled via ohmic contact to position the CB and VB potentials of one semiconductor more negative than those of the other semiconductor [18]. Ohmic contact in a Z-scheme system triggers the recombination of electrons and electron holes with lower reduction or oxidation potential, thus leaving more reducing electrons and more oxidative holes intact In this case a particularly large amount of photogenerated holes and electrons is preserved for oxidation-reduction reactions (usually the absolutely largest majority of photoinduced electrons and holes recombine, emitting energy in the form of heat) on the surface of photocatalyst, and the oxidation-reduction potential of these charge carriers in this photocatalyst system (Z-scheme) is preserved as high as possible for the visible-light-active narrow band gap semiconductor system.

The main obstacles for Z-scheme practical applications are complicated (non-industrializable) multistep synthesis methods, small yields and expensive reagents. Moreover, often Z-scheme photocatalyst synthesis techniques are not green, but photocatalysis technology can be fully considered as green if the green synthesis principles have been followed. Additionally, many involved materials are rare or toxic.

The closest known p- and n-type narrow band gap photocatalyst system is FeiC /CuiO [23] (selected as a prototype). CmO compound used in this system is unstable [24], and a complicated solvothermal method is used for synthesis of heterostructure [23].

The objective of this invention is to develop a new photocatalyst system with Z-scheme photoinduced charge transfer mechanism from chemical elements widely found in nature and their semiconductor compounds with narrow band gap, using industrially applicable aqueous synthesis methods.

The objective of the invention is achieved by creating Z-scheme semiconductor photocatalyst system based on hematite Fe203 and brownmillerite Ca2Fe205 with excellent charge separation (reduced recombination), excellent visible-light harvesting ability and high redox potential. Both Fe203 and Ca2Fe205 consist from earth abundant elements and are narrow band gap semiconductors with band gap energy approximately 2 eV. Moreover, hematite is n-type semiconductor, but brownmillerite is p-type semiconductor, thus providing ohmic contact and avoiding additional synthesis steps for depostion of electronic mediators between two semiconductors in Z-scheme. Hematite and brownmillerite also exhibit proper band gap positions as described below. The system was made using an aqueous synthesis to maintain green chemistry principles.

The Fe203/Ca2Fe205 system of photocatalyst compounds has the following advantages: (i) semiconductors Fe203 and Ca2Fe205 have narrow band gap energy; therefore, they absorb visible light— photocatalytic reactions are possible in sunlight. Using of sunlight serves as a basis for zero energy photocatalysis processes; (ii) Fe203/Ca2Fe205 systems of photocatalyst compounds contain chemical elements widely found in nature - Fe, Ca and O; (iii) Fe203 and Ca2Fe205 semiconductor compounds included in the system are stable in photocatalysis reactions; (iv) Fe203/Ca2Fe205 system of photocatalyst compounds can be generated by using industrialized aqueous-based chemical methods.

The Fe203/Ca2Fe205 system of photocatalyst compounds can be used in photocatalysis processes in visible light: (i) for water purification; (ii) disinfection; (iii) air purification; (iv) sterile surfaces; (v) water splitting; (vi) obtaining chemical compounds from ambient environment C02. Coatings of Fe203/Ca2Fe205 systems of photocatalyst compounds can serve as antibacterial or air purifying surfaces, which operate with indoor lighting.

Fe203/Ca2Fe205 system of photocatalyst compounds can be used both for generation of powder products and in coatings. Powder materials can be used for production of photocatalysis reactors or improvement of already existing photocatalyst reactors,

allowing using visible light as the source of light, instead of UV radiation. Sources of visible light radiation consume less energy and are considerably cheaper.

The method for generating Fe203/Ca2Fe2()5 systems of photocatalyst compounds is characteristic with the feature, that iron containing amorphous nano-dimensional sediment suspension is impregnated with Ca by adding Ca containing salt aqueous solution. In next step suspensions are thermally processed: (i) dried at the temperature up to 100°C— sufficient temperature to dry water, continuing the drying process until it is completely dry; (ii) thermally treated at the temperature up to 1100°C for 1 hour. Thermal treatment at higher temperatures than 1100°C can cause reduction of compounds, evaporation of any element or formation of other compounds. Longer thermal treatment process reduces the specific surface, which will reduce the activity of compounds.

Examples for implementation of the invention

Example 1 : powder product is synthesized initially with equal volume proportions: mixing 0.1 M Fe(NC>3)3 9H20 water solution with 0.5 M hexamethylenetetramine solution, obtaining Fe containing amorphous sediments. Sediments are filtered and washed with water. After washing 1 M Ca(N03)2 aqueous solution is filtered through Fe sediment layer (concentrated suspension). Sediments after filtering are dried at the temperature of 60°C for 1 hour and are thermally treated at the temperature of 820°C for 20 minutes.

Example 2: during the process of coating synthesis, Fe amorphous layer is placed on a conductive substrate (working electrode) surface by using electrochemical deposition from 0,02 M FeCl2 aqueous solution by using Pt wire as an auxiliary electrode and applying the external circuit potential 1.2V. The obtained layer is immersed in Ca(N03)2 aqueous solution for 1 minute. The coating is dried at the temperature of 60°C for 1 hour and is thermally treated at the temperature of 820°C for 20 minutes.

Sediments for production of powder and coatings are amorphous and dimensions of their separate particles are smaller than 100 nm.


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