The design and construction of Ag 2 S heterostructures capable of significantly separating photocarriers is a significant challenge in photocatalysis. In this study, CdS/Ag 2 S heterojunction composite photocatalyst is fabricated through a simple one-step hydrothermal method. The composite structure is characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and UV-visible diffuse reflection spectroscopy. Under visible light irradiation, the photocatalytic activity of the sample is evaluated by degradation of methyl orange. The photoelectric chemical performance of the CdS nanoparticles modified by Ag 2 S is closely related to the ratio of Cd/Ag cation. The experimental results show that the sample ACS-2 (CdS-Ag 2 S = 1:1) composite has higher charge transfer/separation efficiency and longer carrier life. The degradation efficiency reaches 74.4% within 30 min, and the degradation rate constant is 0.047 min −1 . After four cycles, the degradation efficiency of the catalyst only decreases slightly, indicating that the CdS/Ag 2 S catalyst has good stability and repeatability. Meanwhile, the study of the photocatalytic mechanism shows that •O 1 − is the primary active substance in the catalytic process, and h + and •OH play a secondary role. This work provides a great significance for constructing efficient Ag 2 S-based heterojunction photocatalyst systems.
The built-in electric field of heterojunction can effectively promote carrier separation and transfer. While, its interface orientation is often random, leading to lattice mismatch and high resistance, thus limiting the efficiency of interfacial charge transfer. Herein, the lattice-matched heterojunction (CdS-Ag 2 S) was constructed by ion-exchange epitaxial growth . The results of surface photovoltage spectroscopy (SPV), transient photovoltage spectroscopy (TPV), and time-resolved photoluminescence (TRPL) show that the lattice-matched heterojunction has higher charge separation efficiency and longer photogenerated carrier lifetime than that of lattice-mismatched one. The lattice-matched CdS-Ag 2 S has a high built-in electric field (BIEF) value of 103.42 and a bulk-charge separation (BCS) efficiency of 68.71%, which is about three times higher than that of the lattice-mismatched heterojunction (CdS-Ag 2 S-M). In addition, the photodegradation efficiency of CdS-Ag 2 S towards norfloxacin (NOR) was also 3.4 times higher than that of CdS-Ag 2 S-M. The above results and density functional theory (DFT) calculations indicate that improving the lattice matching at the heterojunction is beneficial for establishing a high-intensity built-in electric field and effectively promoting bulk-charge separation efficiency, thus achieving excellent photocatalytic performance. This work provides an essential reference for the research of high-performance heterojunction photocatalysts.
ABSTRACT The production of renewable fossil fuels such as CH 4 and CO by photocatalytic CO 2 reduction has attracted more and more attention. However, single photocatalyst is less efficient for photocatalytic reduction of CO 2 due to the fast recombination of photogenerated electron pairs. Herein, we successfully prepare CdS-Ag 2 S composite by assembling the Ag 2 S QDs cocatalyst on the surface of CdS nanosheet-assembled flower through oil-bath solvothermal method . This composite is prepared through a simple self-assembly strategy using cadmium chloride, ammonia and thiourea as precursors of the CdS nanosheet-assembled flower and silver nitrate and 3-mercaptopropionic acid as the precursors of Ag 2 S QDs. The average diameter of Ag 2 S QDs is apparently 6.0 nm. The light absorption edge of the composite is at around 560 nm, with the corresponding band gap at 2.14 eV. The CdS-Ag 2 S QDs composite with 5 wt% Ag 2 S QDs loaded achieves CO evolution rate of 16.6 μmol•g −1 •h −1 without noble-metal cocatalysts. This strengthened photocatalytic performance and photocatalytic stability were attributed to the energy band broadening of Ag 2 S QDs caused by quantum size effect and the large specific surface area due to the assembled flower. The mechanism underlying the enhanced photocatalytic CO 2 reduction activity is further proposed. This study demonstrates that semiconductor-based quantum dots are strong candidates for excellent cocatalysts in photocatalysis .
The separation of photo-generated carriers in heterojunctions plays a crucial role in regulating the built-in electric field. The design of materials with directional migration characteristics inside the bulk phase and at the interface between the two phases is of great significance for studying photocatalytic hydrogen evolution. The P-CdS/Ag 2 S composite photocatalytic system prepared in this study was doped with P element by heating CdS and NaH 2 PO 2 in a muffle furnace, resulting in the directed migration of photo-generated charges in the prepared sample driven by an internal electric field. By in-situ deposition of Ag 2 S on the surface of P-CdS, a Janus heterojunction was prepared to enhance the transfer of charge carriers at the phase interface. The hydrogen evolution rate of P-CdS/Ag 2 S is 12.44 mmol·h −1 ·g −1 , which is 30 times higher than that of the previously prepared pure CdS sample (0.411 mmol·h −1 ·g −1 ). This significant improvement in photocatalytic performance is attributed to the doping of P element and the in-situ loading of Ag 2 S co-catalyst, which together promote the effective separation and transport of photogenerated charge carriers, thereby enhancing the photocatalytic activity of the material. This work is of great significance for exploring the charge-directed transfer problem of sulfide photocatalysts in the field of photocatalytic hydrogen evolution. Graphical
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