Preparation And Photocatalytic Properties Of T-ZnOw And WO₃·H₂O-based Heterojunction Photocatalysts

Photocatalysis has been recognized as an effective strategy to cope with the current energy crisis and environmental pollution problems.It can transform the clean solar energy into chemical energy,thus showing unique advantages in dealing with increasingly serious water pollutants.Praticularly,one of the key points for the application of the photocatalysis technology is the development and application of high-efficient and stable visible light responsive photocatalysts.Most single-semiconductor photocatalysts have narrow spectral response range and high recombination rate of photogenerated carriers,and their photocatalytic degradation performance of water pollutants is strongly inhibited.With the purpose of enhancing the photocatalytic degradation performance of semiconductors,one effective stragegy,i.e.,the construction of heterojunction photocatalysts has been developed.The three-dimensional structural tetrapod-like Zn O whiskers（T-Zn Ow）are readily available photocatalytic materials,which show good photodegradation activity under UV light.However,it cannot be excited by visible light.Tungsten trioxide monohydrate（WO₃·H₂O）has some visible light responsiveness and is easy to be prepared,but its photogenerated carriers are easy to recombine.Taking commercial T-Zn Ow photocatalyst with wide-bandgap and WO₃·H₂O photocatalyst with narrow-bandgap as research objects,we prepared a series of T-Zn Ow and WO₃·H₂O-based heterojunction composites with visible light response and studied their visible light photocatalytic degradation properties of organic pollutants（dyes and antibiotics）.Besides,the carriers transport mechanism in the heterojunction photocatalyst was analyzed,and the photocatalytic activity enhancement mechanism was proposed.The main research contents are as follows:（1）Ag₃PO₄/T-Zn Ow photocatalysts were prepared by depositing Ag₃PO₄ particles on the surface of T-Zn Ow via a simple in-situ precipitation method using T-Zn Ow as the substrate.The photocatalytic activity was evaluated by photocatalytic degradation of rhodamine B（Rh B）under visible light（λ>420 nm）.The photocatalytic activities of Ag₃PO₄/T-Zn Ow heterojunction photocatalysts were higher than that of single T-Zn Ow.The degradation efficiency of Rh B（10 mg/L）over the Ag₃PO₄/T-Zn Ow-2（the molar percentage of Ag₃PO₄ to T-Zn Ow is 10%）photocatalyst was 92.9%,and its reaction rate constant was 3.6 times that of T-Zn Ow.The degradation efficiency of Rh B over Ag₃PO₄/T-Zn Ow-2 was 77.8%after five cycles of degradation experiments,which showed higher photocatalytic stability than Ag₃PO₄.The visible light response range of T-Zn Ow was broadened after the introduction of Ag₃PO₄,thus enhancing the visible light degradation activity of T-Zn Ow.（2）The p-n type Bi OI/T-Zn Ow heterojunction photocatalysts were prepared by in-situ precipitation with Bi OI being loaded on the surface of T-Zn Ow.Bi OI/T-Zn Ow photocatalysts gave high degradation efficiencies for both Rh B and colorless oxytetracycline（OTC）.The BZ-10（the molar ratio of Bi to Zn is 1:10）composite showed the highest photocatalytic activity,with 97.1%and 88.0%degradation of Rh B and OTC（both are 20 mg/L）after 75 min and 240 min of visible light irradiation,respectively,which were higher than those of T-Zn Ow and Bi OI.In addition,the degradation efficiency of OTC over the BZ-10 photocatalyst after four photodegradation cycles reached 80.7%,which showed a good reusable capacity.The improvement of the activity of the Bi OI/T-Zn Ow photocatalyst is due to the fact that the loading of Bi OI enhances the visible light responsiveness of T-Zn Ow,and the formation of p-n heterojunction effectively promotes the transfer of carriers in the system,making it easy for the transfer of electrons from the conduction band of Bi OI to the conduction band of T-Zn Ow,with holes remaining in the valence band of Bi OI.（3）WO₃·H₂O nanosheets were prepared by a simple hydrothermal method and used as a substrate to construct the type II Ag₃PO₄/WO₃·H₂O heterojunction photocatalysts.The composite photocatalysts exhibited good photodegradation activity for methylene blue（MB）.The 3Ag/1W（the molar ratio of Ag₃PO₄ to WO₃·H₂O is 3:1）photocatalyst showed higher photodegradation activities for MB（10 mg/L）and OTC（20 mg/L）than Ag₃PO₄ and WO₃·H₂O.After illuminating the MB solution for 30 min,the degradation efficiency of MB solution over 3Ag/1W reached 98.9%.Free radical trapping experiments showed that h⁺and·O₂^-were the main active species involved in MB degradation,and h⁺was the most critical active species.The matched energy band positions between Ag₃PO₄ and WO₃·H₂O enable the efficient separation of light-induced electron-hole pairs,thereby enhancing the photodegradation activity of the Ag₃PO₄/WO₃·H₂O composite.（4）The novel Ag/Ag I/WO₃·H₂O composites were prepared by hydrothermal method and one-step photoreduction method using WO₃·H₂O nanosheets as the substrate.The composite photocatalyst showed higher photodegradation activity for 10 mg/L of tetracycline hydrochloride（TCH）than Ag/Ag I and WO₃·H₂O.The reaction rate constant of the AAW-2（the molar ratio of Ag to WO₃·H₂O is 0.8:1）photocatalyst is 24.6 times and 1.6 times that of WO₃·H₂O and Ag/Ag I,respectively.The corresponding analysis of PL（fluorescence spectrum）and photocurrent response tests indicated that AAW-2 had higher separation efficiency of carriers than WO₃·H₂O and Ag/Ag I.The results of radical trapping experiments showed that h⁺,·O₂^-and·OH were all active species for TCH photodegradation.The enhanced photocatalytic activity originates from the Z-type heterojunction composed of Ag,Ag I and WO₃·H₂O,which enables efficient separation of carriers.（5）The Ag I/Ag₃PO₄/WO₃·H₂O ternary heterojunction photocatalysts were constructed by introducing Ag I nanoparticles on the surface of Ag₃PO₄ with the prepared Ag₃PO₄/WO₃·H₂O composite as the matrix.The photodegradation activities of the composite photocatalysats for Rh B and TCH（both are 20 mg/L）were higher than those of Ag₃PO₄/WO₃·H₂O,Ag I/WO₃·H₂O and single photocatalysts.After visible light irradiation for12 min and 24 min,the degradation efficiencies of Rh B and TCH over the S2-50%（the molar percentage of Ag I to Ag₃PO₄ is 50%）photocatalyst were 98.0%and 85.3%,respectively.After degrading Rh B for four times by S2-50%,the degradation efficiency of Rh B still reached 93.2%,which was significantly higher than that of the Ag₃PO₄/WO₃·H₂O photocatalyst,indicating its good recyclability.The enhanced photocatalytic activity of the Ag I/Ag₃PO₄/WO₃·H₂O composite photocatalyst is mainly due to the transport path of the photogenerated carriers in the formed Z-type/traditional type II heterojunction,which realizes the effective spatial separation of electrons and holes and retains the strong reducibility of electrons in the Ag I conduction band.