Development Of Ag₃PO₄-based Composite Photocatalysts And Their Degradation Property Toward The Organic Pollutants In Water

Semiconductor photocatalytic technology is an environmentally friendly technology, which appears in the 20 th century, it has gained great development, and research of visible light-responsive photocatalyst is the hot topic in recent years. The Ag₃PO₄ is a major breakthrough in the field of visible light-responsive photocatalyst’s development, and its photogenerated holes possesses high photocatalytic oxidation. However, the photocorrosion of Ag₃PO₄ caused by its sparingly soluble in aqueous solution and electronic structure features, seriously decreases its photocatalytic activity and stability. To enhance the resistance of Ag₃PO₄ to photocorrosion and further improve its photocatalytic activity, as well as optimize the application of Ag₃PO₄-based photocatalysts in the field of organic pollutants in wastewater treatment, we makes Ag₃PO₄ combine with the semiconductor with appropriate energy band structure or carriers with good electrical conductivity based on the photogenerated electron-hole mobility conversion principle in the composites, to construct a series of Ag₃PO₄-based photocatalysts（Ag₃PO₄-GO, magnetic micro/nano structures Ag₃PO₄/Zn Fe₂O₄, magnetic core-shell structure CMSs@Zn Fe₂O₄/Ag₃PO₄, magnetic ternary composites RGO/Zn Fe₂O₄/Ag₃PO₄, and Z-Scheme Cu Bi₂O₄/Ag₃PO₄）. The characteristics of the as-prepared materials, such as crystal structure, morphology, light absorption properties and fluorescence properties were detected systematically through XRD, XPS, EDS, SEM, TEM, UV-Vis, and PL etc., and the preparation method of catalysts was optimized, further to achieve the regulation and synthesis of catalysts. Then the asprepared photocatalysts was used for the degradation of organic pollutants（such as 2,4-DCP and diclofenac）, the degradation mechanisms of organics in different photocatalysts systems were explored, and the photocatalytic mechanism and anti-corrosion mechanism of different types of Ag₃PO₄-based composite was also revealed. The results will provide theoretical and experimental evidence for the wastewater treatment with photocatalytic technology. The main results are as follows:1. Preparation of Ag₃PO₄-GO composite and its photocatalytic performance and mechanism toward 2,4-DCP degradation under visible light irradiationGraphite oxide（GO） was prepared based on the Hummers method. And then the insitu precipitation method was used to prepare the Ag₃PO₄-GO. When GO dispersion was introduced into the synthetic system, the diameter of Ag₃PO₄ gradually decreased with GO content increasing, the specific surface area was increased, the absorption intensity over the entire spectral range was also improved, and since the metastasis and capture of GO to the photo-generated electrons, the photocatalytic activity and stability of the photocatalyst was enhanced. When the GO content is 5%, the degradation rate of 2,4-DCP in Ag₃PO₄-GO photocatalytic system is 0.0600 min-1, which is 5.21 times of pure Ag₃PO₄. All the OH·, h+and O2·- contribute to the photodegradation of 2,4-DCP through the free radical capture experiments, and the order is O2·- > h+ > OH·. And the main photodegradation intermediates of 2,4-DCP is 4-chlorophenol（4-CP）, hydroquinone（HQ）, benzoquinone（BZQ）, 2-chlorohydroquinone and hydroxyhydroquinone（HHQ）.2. Preparation of magnetic micro/nano structural Ag₃PO₄/Zn Fe₂O₄ and its photocatalytic performance toward 2,4-DCP degradationTo improve the separation and recovery efficiency of the catalyst, we introduced Zn Fe₂O₄ into the photocatalyst system, trying to use its own magnetic property to improve the separation and recovery efficiency of the composite photocatalyst; Moreover, it will also take advantage of the matching and binding of energy band between the semiconductor Zn Fe₂O₄ and Ag₃PO₄ to alter the migration of the photogenerated electrons-holes, thereby improving the photocatalytic activity and light anti-corrosion of the catalyst. Based on this, we used the solvothermal and in-situ precipitation method to synthesize the magnetic microand nanostructures Ag₃PO₄/Zn Fe₂O₄, and studied the organic additives（hexadecyl trimethyl ammonium bromide（CTAB）, sodium diethyldithiocarbamate（DDTC）, and DL-malic acid（DLMA）） on the structure and catalytic activity/stability of Ag₃PO₄/Zn Fe₂O₄. The results indicate that all the composite photocatalyst exhibited higher photocatalytic activity and light anti-corrosive than the pure Ag₃PO₄, and the catalysts can be magnetically separated and recovered for reuse. When the DDTC was used as organic additive, and the mass ratio of Ag₃PO₄ and Zn Fe₂O₄ in the composite is 9:1, the photodegradation efficiency of 2,4-DCP gained the highest value and the photodegradation rate is 0.0155 min-1, which is 5.74 times of pure Zn Fe₂O₄, and 1.89 times of pure Ag₃PO₄.3. Construction of magnetic core-shell structural CMSs@Zn Fe₂O₄/Ag₃PO₄ and its photocatalytic performance and mechanism toward 2,4-DCP degradation under visible light irradiationTo further enhance the composite photocat alysts’ activity after the combination of Zn Fe₂O₄ and Ag₃PO₄, we use the solvothermal（water） and in-situ precipitation method to introduce the carbon microspheres（CMSs） into the catalysts to synthesize the core-shell structural CMSs@Zn Fe₂O₄/Ag₃PO₄. The results indicate that the composition of the composite played important role in the composite’s optical absorption property, fluorescent property, and catalytic activity. When the composition of the composite is 2:1:3, the photodegradation efficiency of 2,4-DCP is 94.39%, and the photodegradation rate is 0.0184 min-1, which is 7.36 times of pure Zn Fe₂O₄, 2.45 times of pure Ag₃PO₄, 4 times of CMSs@Zn Fe₂O₄（2:1）, 1.19 times of Ag₃PO₄/Zn Fe₂O₄（DDTC, 9:1） prepared in the previous chapter. Moreover, the degradation efficiency of 2,4-DCP is still up to 89.29% after four times recycling. In the photocatalytic reaction process, h+, OH· and O2·- were all involved in the degradation of 2,4-DCP, and its contribution order is OH·> O2·-> h+.4. Preparation of magnetic ternary composites RGO/Zn Fe₂O₄/Ag₃PO₄ and its photocatalytic performance and mechanism toward 2,4-DCP degradation under visible light irradiationThe RGO with high specific surface area and electron transfer capability was introduced into the catalysts to construct the magnetic ternary composites RGO/Zn Fe₂O₄/Ag₃PO₄, and its photocatalytic performance and mechanism toward 2,4-DCP degradation under visible light irradiation was also explored. The results indicate that compared with the pure Ag₃PO₄, the obtained RGO/Zn Fe₂O₄/Ag₃PO₄ showed enhanced absorbance over the entire spectral range; But with the content of Ag₃PO₄ in the composite increases, the absorbance intensity of the composite gradually weakened. The introduce of RGO increased the specific surface area and the adsorption efficiency of the composite. The main active species for 2,4-DCP degradation is h+, the contribution of e- is mainly in in the dechlorination process, which is the first stage for 2,4-DCP photodegradation; while the contribution of OH·is mainly in the later degradation stage of 2,4-DCP. The contribution of O2·- is relatively weak. Furthernore, 2,4-DCP was converted into small molecule acid including maleic acid, glyoxylic acid, glycolic acid, oxalic acid, acetic acid, and formic acid, in the photocatalytic system of RGO/Zn Fe₂O₄/Ag₃PO₄.5. Construction of Z-Scheme Cu Bi₂O₄/Ag₃PO₄ and its photocatalytic performance and mechanism toward diclofenac degradation under visible light irradiationThe Z-scheme photocatalysts, mimicking the natural photosynthesis process, are the combination of the appropriate energy band structure of semiconductors, the interface formed between these semiconductors become to be the recombination center for the photogenerated electron-hole, which makes the semiconductor with higher position of conduction band exhibit stronger reducing ability, and the semiconductor with lower position of valence band exhibit stronger oxidation ability, thereby enhancing the photocatalytic activity of the composite photocatalyst. We attempt to use the hydrothermal synthesis-in situ precipitation method to combine the Ag₃PO₄ with high photocatalytic oxidation ability and Cu Bi₂O₄ with high photocatalytic reduction ability to build the composite photocatalyst Cu Bi₂O₄/Ag₃PO₄, and its photocatalytic activity toward diclofenac was investigated. The results indicate that p H is the key point that influencing the morphology structure of Cu Bi₂O₄. And when the solution p H is 12.98, the obtained Cu Bi₂O₄ displays poor aggregation and well-distributed micro-sphere structures, whose surfaces are comprised of nanometer quadrilateral plates with the length of ²50 nm and the width of ²00 nm, and the gap exists in the adjacent nanometer quadrilateral plates. When the nano-sized Ag₃PO₄ were precipitated on the surface of Cu Bi₂O₄, the gap of the nanosheets on the Cu Bi₂O₄ surface can also be occupied. The obtained Cu Bi₂O₄/Ag₃PO₄ shows strong absorbance over the entire visible spectral range, which is stronger than the intensity of pure Ag₃PO₄. Under the same conditions, the photocatalytic activity and stability of Cu Bi₂O₄/Ag₃PO₄ toward diclofenac is much higher than that in pure Ag₃PO₄ system. After 120 min of reaction, the photolysis mineralization rate of diclofenac is 14.36%, and the mineralization rate of diclofenac in Cu Bi₂O₄, Ag₃PO₄ and Cu Bi₂O₄/Ag₃PO₄ photocatalytic system was 35.43%, 49.62% and 57.33%, respectively. In addition, the active species involved in the degradation of diclofenac is different within the different photocatalytic reaction systems. That is in the Cu Bi₂O₄ photocatalytic system, the contribution order of active specie was h+ > O2·- > OH·; in the Ag₃PO₄ photocatalytic system, the main active species were OH·and h+; and in the Cu Bi₂O₄/Ag₃PO₄ photocatalytic system, the main active species were O2·- and OH·. Moreover, the higher photocatalytic activity of Cu Bi₂O₄/Ag₃PO₄ can be explained by the Z-scheme theory, i.e. the Ag0 formed in the contact interface between Cu Bi₂O₄ and Ag₃PO₄ acts as the recombination center for photogenerated electron from the combination band of Ag₃PO₄ and photogenerated hole from the valence band of Cu Bi₂O₄.