Hierarchical S-scheme titanium dioxide@cobalt-nickel based metal-organic framework nanotube photocatalyst for selective carbon dioxide photoreduction to methane.

Efficient photocatalysts are of great importance for the photochemical conversion of CO2 into fuels. Herein, S-scheme titanium dioxide@cobalt-nickel based metal-organic framework (TiO2@CoNi-MOF) heterojunction photocatalysts with high surface area and porosity are designed and fabricated by a multi-step controllable strategy. The photocatalytic activity of the composites can be optimized by adjusting the loading content of CoNi-MOF in TiO2@CoNi-MOF and molar ratios of Co2+ and Ni2+ in CoNi-MOF. The optimized hybrid photocatalyst showed a much higher CO2 photoreduction activity than the control single-component samples (TiO2 and CoNi-MOF) with a high CH4 yield (41.65 µmol g-1 h-1) and selectivity (93.2%). The accelerated charge carrier separation induced by the S-scheme heterojunction significantly promoted the photocatalytic performance of TiO2@CoNi-MOF NTs. Meanwhile, the introduction of bimetallic CoNi-MOF nanosheets significantly resulted in the increase of active sites, CO2 adsorbability, visible-light utilization, and CH4 selectivity. Moreover, the S-scheme photoinduced charge transfer model of the TiO2@CoNi-MOF NTs photocatalyst was confirmed by photoluminescence spectroscopy, free radical trapping tests, and work function calculated from Kelvin probe. The work aims to design and fabricate heterojunction photocatalysts with high efficiency for solar fuel production.

Efficient charge transfer in cadmium sulfide quantum dot-decorated hierarchical zinc sulfide-coated tin disulfide cages for carbon dioxide photoreduction.

Constructing hybrid photocatalysts with advanced structures and controllable compositions is a promising way to improve CO2 photoreduction performance. In this work, SnS2 nanosheets are grown on ZnS polyhedron cages to fabricate hierarchical ZnS@SnS2 double-shelled heterostructured cages. This design integrates ZnS cages and SnS2 nanosheets into a stable heterostructured hybrid catalyst with a hierarchical double-shelled cage-like architecture, possessing abundant active sites, quick charge separation/migration, and high CO2 adsorption capacity. Benefiting from these advantages, the optimized hierarchical ZnS@SnS2 heterostructured cages exhibit significant gas-phase CO2 photoreduction activity with a CO generation rate of 95.38 µmol g-1h-1 and 72.4% CO selectivity, which are greatly improved in comparison with those of pure ZnS cages and nanosheet-assembled SnS2 particles. Furthermore, charge carrier separation efficiency and visible light harvesting ability are further improved by constructing a ZnS@SnS2/CdS type-I/type-II complex heterostructured system through surface decoration of CdS quantum dots. The optimized ZnS@SnS2/CdS hybrid exhibits a CO generation rate of 155.57 µmol g-1h-1 and an excellent selectivity of 80.4%. This work is conducive to the design and manufacture of advanced hybrids for solar energy utilization and photocatalytic reactions.

Fabrication of size-controlled hierarchical ZnS@ZnIn2S4 heterostructured cages for enhanced gas-phase CO2 photoreduction.

Designing and constructing advanced heterojunction architectures are desirable for boosting CO2 photoreduction performance of semiconductor photocatalysts. Herein, we have prepared hierarchical ZnS@ZnIn2S4 core-shell cages with controlled particle sizes using sequential synthesis of Zeolitic imidazolate (ZIF-8) polyhedrons, ZnS cages, and ZnIn2S4 nanosheets on the ZnS polyhedron cages. ZIF-8 polyhedrons are firstly synthesized by a liquid-phase approach. The subsequent sulfidation of the ZIF-8 polyhedrons results in the formation of ZnS polyhedron cages, which act as substrates for fabricating ZnS@ZnIn2S4 core-shell cages by growing ZnIn2S4 nanosheets. The size of ZnS cages can be tuned to optimize CO2 photoreduction performance of hierarchical ZnS@ZnIn2S4 core-shell cages. The synergy of the unique hierarchical core-shell cage-like structure and heterojunction composition endows the hybrid catalyst high incident light utilization, abundant active sites, and effective separation of photoexcited charge carriers. Benefiting from these advantages, the optimized hierarchical ZnS@ZnIn2S4 core-shell cages exhibit enhanced performance for CO2 photoreduction with the CO yield of 87.43 µmol h-1g-1 and 84.3% selectivity, which are much superior to those of single ZnIn2S4 or ZnS. Upon Au decoration, the CO2 photoreduction performance of ZnS@ZnIn2S4 core-shell cages is further enhanced because of the Schottky junctions and surface plasmon resonance effect.

Achieving cadmium selenide-decorated zinc ferrite@titanium dioxide hollow core/shell nanospheres with improved light trapping and charge generation for photocatalytic hydrogen generation.

We report the rational design and fabrication of magnetically separable zinc ferrite@titanium dioxide (ZnFe2O4@TiO2) hollow core/shell nanospheres as photocatalysts for efficient H2 evolution by loading the TiO2 shell layer on the prepared ZnFe2O4 hollow nanospheres using the kinetics-controlled coating method. Meanwhile, the incident light absorption, photogenerated charge transfer and separation and photocatalytic hydrogen evolution activity were remarkably improved by well anchoring cadmium selenide (CdSe) quantum dots on the ZnFe2O4@TiO2 hollow core/shell nanospheres. This unique design integrates the structural and functional merits of the ZnFe2O4, TiO2, and CdSe quantum dots into porous hollow nanospheres with the double-shell heterostructure. This design significantly accelerates the separation and transport of photogenerated charge carriers, enhances the light absorption, and offers more active sites for the photocatalytic H2 evolution reaction. Benefitting from the unique structural and component merits, the optimized magnetically separable ZnFe2O4@TiO2/CdSe hollow core/shell nanospheres exhibit excellent photocatalytic hydrogen evolution performance with a high H2 generation rate (266.0 µmol h-1·g-1) and high stability.

Carbothermal synthesis of ordered mesoporous carbon-supported nano zero-valent iron with enhanced stability and activity for hexavalent chromium reduction.

Composites of nano zero-valent iron (nZVI) and ordered mesoporous carbon (OMC) are prepared by using simultaneous carbothermal reduction methods. The reactivity and stability of nZVI are expected to be enhanced by embedding it in the ordered pore channels. The structure characteristics of nZVI/OMC and the removal pathway for hexavalent chromium (Cr(VI)) by nZVI/OMC are investigated. Results show that nZVI/OMC with a surface area of 715.16 m(2) g(-1) is obtained at 900 °C. nZVI with particle sizes of 20-30 nm is uniformly embedded in the OMC skeleton. The stability of nZVI is enhanced by surrounding it with a broad carbon layer and a little γ-Fe is derived from the passivation of α-Fe. Detection of ferric state (Fe 2p3/2, around 711.2eV) species confirms that part of the nZVI on the outer surface is inevitably oxidized by O2, even when unused. The removal efficiency of Cr(VI) (50 mg L(-1)) by nZVI/OMC is near 99% within 10 min through reduction (dominant mechanism) and adsorption. nZVI/OMC has the advantage in removal efficiency and reusability in comparison to nZVI/C, OMC and nZVI. This study suggests that nZVI/OMC has the potential for remediation of heavy metal pollution in water.