Centralized water treatment has dominated in developed urban areas over the past century, although increasing challenges with this model demand a shift to a more decentralized approach wherein advanced oxidation processes (AOPs) can be appealing treatment options. Efforts to overcome the fundamental obstacles that have thus far limited the practical use of traditional AOPs, such as reducing their chemical and energy input demands, target the utilization of heterogeneous catalysts. Specifically, recent advances in nanotechnology have stimulated extensive research investigating engineered nanomaterial (ENM) applications to AOPs. In this Perspective, we critically evaluate previously studied ENM catalysts and the next-generation treatment technologies they seek to enable. Opportunities for improvement exist at the intersection of materials science and treatment process engineering, as future research should aim to enhance catalyst properties while considering the unique roadblocks to practical ENM implementation in water treatment.

Challenges and prospects of advanced oxidation water treatment processes using catalytic nanomaterials.

Metal-organic frameworks (MOFs) have been attracted considerable attention in the field of energy generation and environmental remediation. However, the functionalization and diversification of MOFs are still challenging and imperative for the development of highly active MOF-based materials. In this article, a series of heterostructured ZnIn2S4@NH2-MIL-125(Ti) nanocomposites with different NH2-MIL-125(Ti) contents were fabricated via a facile solvothermal method. The photocatalytic activities of the obtained samples were evaluated by the photocatalytic H2 production under visible-light illumination (λ > 420 nm). The results showed that the ZnIn2S4 nanosheets were highly dispersed on the surface of NH2-MIL-125(Ti). The ZnIn2S4@NH2-MIL-125(Ti) photocatalysts displayed higher photocatalytic activity than the pristine components for H2 production. The optimal content of NH2-MIL-125(Ti) was about 40 wt% and the corresponding photocatalytic H2 production rate was 2204.2 μmol·h−1·g−1 (with an apparent quantum efficiency of 4.3% at 420 nm), which was 6.5 times higher than that of pure ZnIn2S4. The enhanced photocatalytic activity of ZnIn2S4@NH2-MIL-125(Ti) composites should be attributed to the well-matched band structure and intimate contact interfaces between ZnIn2S4 and NH2-MIL-125(Ti), which led to the effective transfer and separation of the photogenerated charge carriers. Moreover, the ZnIn2S4@NH2-MIL-125(Ti) nanocomposites showed excellent stability during the photocatalytic reactions under visible light. Therefore, these kinds of MOF-based composites have great potentiality in energy conversion fields.

Construction of heterostructured ZnIn2S4@NH2-MIL-125(Ti) nanocomposites for visible-light-driven H2 production

Photocatalysis is a promising and ideal technology to utilize solar energy for energy regeneration and environmental remediation. Photocatalysis based on metal-organic frameworks (MOFs) has attracted extensive attention because of their excellent properties such as tunable structure and optical properties. This review principally summarizes the progress of various modification strategies, including decoration of organic linker or metal center, combination with semiconductors and metal nanoparticles loading. applied to different MOFs (mainly UiOs, MILs, and ZIFs), and their specific applications were summarized as well. In addition, the existing problems and the development prospect of MOFs for photocatalysis were also presented.

Modified metal-organic frameworks as photocatalysts

Both pristine metal-organic frameworks (MOFs) and graphitic carbon nitride (g-C3N4) displayed outstanding photocatalytic performances toward H2 production, CO2 reduction, Cr(VI) reduction and organic pollutants degradation. To further enhance their photocatalytic performances under visible light or sunlight irradiation, MOFs and g-C3N4 were combined to construct g-C3N4/MOF heterojunctions with the purpose of overcoming their individual disadvantages like fast recombination of pohotogenerated electron-hole pairs. The fabrication methods, characterizations, photocatalytic performances and the corresponding mechanism of some typical g-C3N4/MOF composites were highlighted. Also, the prospective and challenges of this research field were declared.

Powerful combination of MOFs and C3N4 for enhanced photocatalytic performance

Applications of water-stable metal-organic frameworks in the removal of water pollutants: A review.

To selectively remove heavy metal from dye solution, inspired by the unique pore structure of ZIF-8, we developed a synthetic strategy for rapid construction of ZnO@ZIF-8 heterostructure photocatalyst for selective reduction of Cr(VI) between Cr(VI) and methylene blue (MB). In particular, ZnO@ZIF-8 core-shell heterostructures were prepared by in situ ZIF-8 crystal growth using ZnO colloidal spheres as template and zinc source within 8-60 min. The shell of the resulting ZnO@ZIF-8 core-shell heterostructure with a uniform thickness of around 30 nm is composed of ZIF-8 crystal polyhedrons. The concentration of organic ligand 2-methylimidazole (Hmim) was found to be crucial for the formation of ZnO@ZIF-8 core-shell heterostructures. Different structures, ZnO@ZIF-8 core-shell spheres and separate ZIF-8 polyhedrons could be formed by altering Hmim concentration, which significantly influences the balance between rate of Zn(2+) release from ZnO and coordinate rate. Importantly, such ZnO@ZIF-8 core-shell heterostructures exhibit size-selective photocatalysis properties due to selective adsorption and permeation effect of ZIF-8 shell. The as-synthesized ZnO@ZIF-8 heterostructures exhibited enhanced selective reduction of Cr(VI) between Cr(VI) and MB, which may find application in the dye industry. This work not only provides a general route for rapid fabrication of such core-shell heterostructures but also illustrates a strategy for selectively enhanced photocatalysis performance by utilizing adsorption and size selectivity of ZIF-8 shell.

Rapid Construction of ZnO@ZIF-8 Heterostructures with Size-Selective Photocatalysis Properties

Nanostructured Zr(IV) metal organic frameworks (MOFs-808) with high stability are synthesized by using trimesic acid (BTC), a highly electron-deficient and polar molecule, as an organic ligand in order to produce efficient adsorbents for hydrogen gas. The MOFs-808 are characterized by XRD, FT-IR, SEM, TEM, specific surface area measurements and thermogravimetric analysis, and the conditions (e.g., the reactant molar ratio, the amount of solvent, the reaction temperature and the reaction time) for synthesis of MOFs-808 are optimized. The MOF-808 obtained under optimized preparation conditions has higher crystallinity, a larger specific surface area, and relatively high stability both in acidic solution and at temperatures up to 300 °C. The hydrogen storage capacity of the optimized MOF-808 under 4 MPa reaches 7.31 wt% at 77 K, which is close to the ultimate target defined for hydrogen storage. Our results indicate that the presence of the electron-deficient ligand on MOFs-808 favors the adsorption of hydrogen gas.

Xu Yongfei

Papers

Rapid Construction of ZnO@ZIF-8 Heterostructures with Size-Selective Photocatalysis Properties

Optimized synthesis of Zr(IV) metal organic frameworks (MOFs-808) for efficient hydrogen storage

Structure induced selective adsorption performance of ZIF-8 nanocrystals in water

Hydrothermal preparation of hierarchical ZIF-L nanostructures for enhanced CO2 capture.

Hydrogen storage properties of the novel crosslinked UiO-66-(OH)2