Recently, considerable efforts have been made to satisfy the future requirements of electrochemical energy storage using novel functional electrode materials. Binary transition metal oxides (BTMOs) possess multiple oxidation states that enable multiple redox reactions, showing higher supercapacitive properties than single component metal oxides. In this work, a facile hydrothermal method is provided for the synthesis of wall-like hierarchical metal oxide MMoO₄ (M = Ni, Co) nanosheet arrays, which are directly grown on flexible carbon cloth for use as advanced binder-free electrodes for supercapacitors. By virtue of their intriguing structure, the resulted active material nanosheets with a high specific surface area can provide a large electroactive region, which could facilitate easy accession of electrolyte ions and fast charge transport, resulting in an enhanced electrochemical performance. Separately, the as-synthesized MMoO₄ (M = Ni, Co) samples have exhibited superior specific capacitances (1483 F g⁻¹ of NiMoO₄ and 452 F g⁻¹ of CoMoO₄ at a current density of 2 A g⁻¹), as well as excellent cycling stability (93.1% capacitance retention of NiMoO₄ and 95.9% capacitance retention of CoMoO₄ after 2000 cycles). The results show that the binder-free electrodes constructed by deposition of MMoO₄ (M = Ni, Co) nanosheets on carbon cloth are promising candidates for the application of supercapacitors.

2D photocatalytic TiO ₂ /MoS ₂ hybrid nanosheets (HNs) have been prepared via a facile hydrothermal process. X-ray diffraction patterns and Raman spectra are carried out and confirm a well crystalized anatase and 2H-MoS ₂ hybridization. Additional morphological and microstructural tests verify a distinct MoS ₂ framework, indicating the relatively stability of the MoS ₂ nanosheet platform with a high specific surface area. UV–vis spectra and electrochemical impedance spectra exhibit an enhanced light absorption ability and conductivity of TiO ₂ /MoS ₂ compared to that of just TiO ₂ . Photoelectrochemical (PEC) tests also demonstrate the photocurrent of 20 : 1 TiO ₂ /MoS ₂ HNs is greatly improved compared to that of as-prepared TiO ₂ . The saturation current density is about 33 µ A cm ⁻² when the applied potential is 0.2 V, which is nearly twice that of pure TiO ₂ and four times as high as 5 : 1 TiO ₂ /MoS ₂ HNs and 1 : 1 TiO ₂ /MoS ₂ HNs. Besides that, the duration test exhibits no detectable distinction after processing 25 cycles. The improved photocatalytic activities are perhaps derived from the high conductivity and the increased active sites for the introduction of co-catalytic MoS ₂ nanosheets as well as the positive synergetic effect between the TiO ₂ and MoS ₂ . This work demonstrates that the as-prepared TiO ₂ /MoS ₂ HNs may have a great potential application in PEC hydrogen production.

Photocatalytic CO₂ reduction to valuable hydrocarbon solar fuel is of great significance but still challenging. Strong CO₂ enrichment ability and easily adjustable structures make metal-organic frameworks (MOFs) potential photocatalysts for CO₂ conversion. Even though pure MOFs have the potential for photoreduction of CO₂, the efficiency is still quite low due to rapid photogenerated electron–hole recombination and other drawbacks. In this work, graphene quantum dots (GQDs) were in situ encapsulated into highly stable MOFs via a solvothermal method for this challenging task. The GQDs@PCN-222 with encapsulated GQDs showed similar Powder X-ray Diffraction (PXRD) patterns to PCN-222, indicating the retained structure. The porous structure was also retained with a Brunauer–Emmett–Teller (BET) surface area of 2066 m²/g. After incorporation of GQDs, the shape of GQDs@PCN-222 particles remained, as revealed by the scanning electron microscope (SEM). As most of the GQDs were covered by thick PCN-222, it was hard to observe those GQDs using a transmission electron microscope (TEM) and a high-resolution transmission electron microscope (HRTEM) directly, the treatment of digested GQDs@PCN-222 particles by immersion in a 1 mM aqueous KOH solution can make the incorporated GQDs visible in TEM and HRTEM. The linker, deep purple porphyrins, make MOFs a highly visible light harvester up to 800 nm. The introduction of GQDs inside PCN-222 can effectively promote the spatial separation of the photogenerated electron–hole pairs during the photocatalytic process, which was proved by the transient photocurrent plot and photoluminescence emission spectra. Compared with pure PCN-222, the obtained GQDs@PCN-222 displayed dramatically enhanced CO production derived from CO₂ photoreduction with 147.8 μmol/g/h in a 10 h period under visible light irradiation with triethanolamine (TEOA) as a sacrificial agent. This study demonstrated that the combination of GQDs and high light absorption MOFs provides a new platform for photocatalytic CO₂ reduction.

Cr(VI) is one of the most toxic heavy metals, posing multiple threats to humans and ecosystems. Photoreduction of toxic Cr(VI) to para-toxic Cr(III) is one of the most effective ways to remove heavy metal chromium but is still challenging. Herein, Ti₃C₂/UiO-66-NH₂ composites with different ratio of Ti₃C₂ were synthesized via an in situ solvothermal process and used for the enhanced photocatalytic removal of Cr(VI) for the first time. The UiO-66-NH₂ nanoparticles were dispersed on the surface and slits of accordion-like Ti₃C₂ homogeneously. A strong interfacial interaction between Ti₃C₂ and UiO-66-NH₂ was formed, which was indicated by the XPS. The Fermi level of the MXene cocatalyst is lower than UiO-66-NH₂; thus, Ti₃C₂ can serve as the electron sink and accumulate photogenerated electrons from UiO-66-NH₂ on its surface. A depletion layer was also formed due to the different Fermi levels of UiO-66-NH₂ and Ti₃C₂, which prevents electrons from flowing back to UiO-66-NH. The strong interfacial interaction and formed depletion layer are beneficial for the following charge transfer from UiO-66-NH₂ to Ti₃C₂ after light irradiation and for suppressing the photogenerated charge recombination. With suitable band positions and enhanced charge separation ability, Ti₃C₂/UiO-66-NH₂ composites exhibited better photoreduction efficiency of Cr₂O₇²⁻ than pure Ti₃C₂ and UiO-66-NH₂, with optimized samples reaching 100% in 40 min. The photoreduction kinetics of Cr₂O₇²⁻ by 2-T/U was also studied, with a photoreduction rate of 0.0871 min⁻¹, which is about 2.6 times higher than that of the pure UiO-66-NH. This research provides a new type of efficient and environmentally friendly photocatalysts for the photoreduction of Cr₂O₇²⁻.