This review highlights the synthesis and optical properties of carbon quantum dots and their application in metal ion sensing.

In recent years, metal–organic frameworks (MOFs) have been extensively investigated for diverse heterogeneous catalysis due to their diversity of structures and outstanding physical and chemical properties. Currently, most related work focuses on employing MOFs as porous substrate materials to fabricate confined nanoparticle or heteroatom-doped electrocatalysts which have to be annealed at high temperature before application. However, the annealing process would destroy the structure completely and lose the intrinsic active sites in MOFs framework. Herein, a simple solvothermal process is used to synthesize a series of Fe/Ni bimetallic MOFs. The as-prepared MOFs are applied directly as highly efficient oxygen evolution reaction (OER) electrocatalysts with no post-annealing treatment. The bimetallic FeNi-MOFs show higher OER activity than single metal MOFs and commercial precious RuO2 catalysts. With the optimized FeNi-MOF as the catalyst, the OER current densities of 50 and 100 mA/cm2 can be achieved at the overpotentials of only 270 and 287 mV, respectively. Meanwhile, a small Tafel slope of 49 mV/dec was obtained. Moreover, this catalyst shows high electrochemical stability in strong basic solution. This work demonstrates that through structural optimization, bimetallic and multimetallic MOFs may have promising potentials as advanced catalysts for electrochemical energy conversion.

In this study, two-dimensional (2D) and three-dimensional (3D) freestanding reduced graphene oxide-supported CuO composites (CuO-rGO) were synthesized via simple and cost-efficient hydrothermal and filtration strategies. The structural characterizations clearly showed that highly porous 3D graphene aerogel-supported CuO microcrystals (3D CuO-GA) have been successfully synthesized, and the CuO microcrystals are uniformly assembled in the 3D GA. Meanwhile, paper-like 2D reduced graphene oxide-supported CuO nanocrystals (2D CuO-rGO-P) have also been prepared by a filtration process. It was found that the products prepared from different precursors and methods exhibited different sensing performances for HO detection. The electrochemical measurements demonstrated that the 3D CuO-GA has high electrocatalytic activity for the HO reduction and excellent sensing performance for the electrochemical detection of HO with a detection limit of 0.37 μM and a linear detection range from 1.0 μM to 1.47 mM. Meanwhile, the 2D CuO-rGO-P structure also showed good electrochemical sensing performance toward HO detection with a much wider linear response over the concentration range from 5.0 μM to 10.56 mM. Compared to the previously reported sensing materials, the as-obtained 2D and 3D CuO-rGO materials exhibited higher electrochemical sensing properties toward the detection of HO with high sensitivity and selectivity. The 2D and 3D CuO-rGO composites also exhibited high sensing performance for the real-time detection of HO in human serum. The present study indicates that 2D and 3D graphene-CuO composites have promising applications in the fabrication of nonenzymatic electrochemical sensing devices.

Energy-efficient, low-cost, and highly durable catalysts for the electrochemical hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) are extremely important for related sustainable energy systems. In the present work, hierarchical coassembled cobalt molybdenum sulfide nanosheets deposited on carbon cloth (CC) were synthesized as catalysts for hydrogen evolution and urea oxidation. By adjusting the doping amount of Mo, 2D nanosheets with different morphologies and compositions (Co x Mo y S-CC) can be obtained. The as-prepared nanosheet materials with abundant active sites exhibit superior properties on the electrochemical HER and UOR in alkaline medium. Significantly, the Mo-doping concentration and composition of the formed nanosheets have large effects on the electrocatalytic activity. The fabricated nanosheets with optimal Mo doping (Co3Mo1S-CC) illustrate the best catalytic properties for the HER in N2-saturated 1.0 M KOH. A small overpotential (85 mV) is needed to meet the current density of 10 mA/cm2. This study indicates that the doping of an appropriate amount of molybdenum into CoS2 nanosheets can efficiently improve the catalytic performance. Also, the nanosheet catalyst exhibits an extremely high electrocatalytic activity for the UOR, and the electrochemical results indicate that a relatively low cell voltage of 1.50 V is needed to obtain the current density of 10 mA/cm2. The present work demonstrates the potential application of CoMoS nanosheets in the energy electrocatalysis area and the insights into performance-boosting through heteroatom doping and optimization of the composition and structure.

Metal oxide semiconductors (MOS) with different nanostructures have been widely used as gas sensing materials due to the tunable interface structures and properties. However, further improvement of the sensing sensitivity and selectivity is still challenging in this area. Constructing appropriate heterogeneous interface structures and oxygen vacancies is one of the important strategies to tune the sensing properties of MOS. In the present study, interfacial heterostructures in PdxW18O49 nanowires (PdxW18O49 NWs) were fabricated and manipulated by doping different Pd contents through a simple hydrothermal process. Relevant characterization proved that the structure and composition of the one-dimensional (1D) nanomaterial can be effectively changed by Pd doping. It was found that the oxygen vacancy concentration increases first with the increase of Pd content, and when the Pd content increases to 7.18% (Pd7.18%W18O49 NWs), the oxygen vacancy content reaches the maximum (52.5%). If the Pd content continues to increase, the oxygen vacancy ratio decreases. The gas sensing investigations illustrated that the PdxW18O49 NWs exhibited enhanced sensing properties than pure W18O49 NWs toward acetone. Among the as-prepared catalysts, the Pd7.18%W18O49 NWs showed the best sensing response and the fastest response-recovery speeds (5 and 10 s, respectively) at a working temperature of 175 °C. In addition, this 1D nanostructure with fabricated heterostructures also delivers a good sensing selectivity and a wide detection range from 100 ppb to 300 ppm, with maintaining excellent performance in the presence of high concentrations of ethanol and carbon dioxide. The excellent gas sensing behavior could be attributed to the generated oxygen vacancies and the heterostructures upon Pd doping. This study offers a novel strategy for the design of high-performance gas sensors for ppb-level acetone sensing.

Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

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