In Situ Generation and Stabilization of Accessible Cu/Cu2O Heterojunctions inside Organic Frameworks for Highly Efficient Catalysis

Heterostructural metal/metal oxides are the very promising substituents of noble‐metal catalysts; however, generation and further stabilization of accessible metal/metal oxide heterojunctions are very difficult. A strategy to encapsulate and stabilize Cu/Cu₂O nanojunctions in porous organic frameworks in situ is developed by tuning the acrylate contents in copper‐based metal–organic frameworks (Cu‐MOFs) and the pyrolytic conditions. The acrylate groups play important roles on improving the polymerization degree of organic frameworks and generating and stabilizing highly dispersed and accessible Cu/Cu₂O heteronanojunctions. As a result, pyrolysis of the MOF ZJU‐199, consisting of three acrylates per ligand, generates abundant heterostructural Cu/Cu₂O discrete domains inside porous organic matrices at 350 °C, demonstrating excellent catalytic properties in liquid‐phase hydrogenation of furfural into furfuryl alcohol, which are much superior to the non‐noble metal‐based catalysts.     

Enhancement of oxygen reduction at Co-porphyrin catalyst by supporting onto hybrid multi-layered film of polypyrrole and polyoxometalate-modified gold nanoparticles

Three-dimensional multi-layered films (on glassy carbon) composed of networks of polyoxometallate (PMo₁₂O₄₀ ³⁻)-modified gold nanoparticles linked together through the alternately deposited ultra-thin layers of polypyrrole have served as active supports for Co-porphyrin catalytic centers. The hybrid organic-inorganic films (supports) have been prepared by using the layer-by-layer approach. The fact that polyanionic (phosphomolybdate) adsorbates on gold nanoparticles are attracted by positively charged sites of conducting polymer (polypyrrole) structures leads to the stabilizing effect and facilitates distribution of Au nanostructures. The systems have been characterized using scanning electron microscopy, as well as with chronoamperometric and voltammetric techniques. By supporting Co-porphyrin centers onto the hybrid film of the polymer-linked phosphomolybdate-stabilized gold nanoparticles, significant electrocatalytic enhancement effects (namely voltammetric current increases) have been observed during the electroreduction of oxygen in acid medium relative to a standard response of the simple porphyrin deposit on glassy carbon measured under analogous conditions. Among important issues is the high activity of the hybrid film (support) itself toward the reductive decomposition of hydrogen peroxide to water. When it comes to performance of the Co-porphyrin-containing system, it is reasonable to expect that the O₂ reduction process is initiated at Co-porphyrin catalytic sites (two-electron reduction to H₂O₂) and continued (two-electron reduction to H₂O) at the hybrid film containing gold nanoparticles dispersed within the highly porous cauliflower-like structures of polypyrrole multi-layers. While the gold networks facilitate charge distribution within the hybrid electrocatalytic film, non-covalent π-π interactions of porphyrin rings with polypyrrole interlayers and charge transfers between negatively charged (PMo₁₂O₄₀ ³⁻ modified) gold nanoparticles and positively charged nitrogen sites of polypyrrole could also cause synergism.

     

Intrinsic Peroxidase‐like Activity of Porous CuO Micro‐/nanostructures with Clean Surface

Porous CuO micro‐/nanostructures with clean surface, prepared through Cu₂(OH)₂CO₃ precursor followed by calcination in air, were proven to be an effective peroxidase mimic. They can quickly catalyze oxidation of the peroxidase substrate 3,3′,5,5′‐tetramethylbenzidine (TMB) in the presence of H₂O₂, producing a blue color. The obtained porous CuO micro‐/nanostructure have potential application in wastewater treatment. The apparent steady‐state kinetic parameter was studied with TMB as the substrate. In addition, the potential application of the porous CuO in wastewater treatment was demonstrated with phenol‐containing water as an example. Such investigation not only confirms the intrinsic peroxidase‐like activity of micro‐/nanostructured CuO, but also suggests its potential application in wastewater treatment.     

Reactive Template Synthesis of Polypyrrole Nanotubes for Fabricating Metal/Conducting Polymer Nanocomposites

Unique nanocomposites of polypyrrole/Au and polypyrrole/Pt hybrid nanotubes are synthesized employing polypyrrole (PPy) nanotubes as an advanced support by solution reduction. The conducting polymer PPy nanotubes are fabricated by using pre‐prepared MnO₂ nanowires as the reactive templates. MnO₂ nanowires induce the 1D polymerization of pyrrole monomers and the simultaneous dissolution of the templates affords the hollow tube‐like structure. The loading content of metal nanoparticles in the nanocomposites could be adjusted by simply changing the amount of metal precursors. This work provides an efficient approach to fabricate an important kind of metal/conducting polymer hybrid nanotubes that are potentially useful for electrocatalyst and sensor materials.     

Metal‐Free Electrocatalyst for Oxygen Reduction: Synthesis‐Controlled Density of Catalytic Centers and Impact on ORR

This work demonstrates a rapid and scalable route for the preparation of N‐doped carbon spheres of 80–120 nm via pyrolysis of polypyrrole as the only carbon and nitrogen source. The resulting porous catalyst has a nitrogen doping level of 6–8 at%. Electrochemical studies show that N‐doped C is very active toward oxygen reduction in alkaline electrolyte and the mechanism of ORR process is controlled by the surface concentration of catalytic active sites that promote either a direct four‐electron or two‐electron process. An interesting observation is that we can generate precursors for the N‐doped carbon with desirable particle size, shape and with the preferential structure (linear polypyrrole from the α? α coupling during slow polymerization or cross‐linked polypyrrole from α? β coupling during fast polymerization) that promotes the formation of favorable catalytic sites for O₂ reduction. The XPS analysis in conjunction with RDE voltammetry highlights the effect of polymer precursor synthesis on the chemical structure and a resulting electrochemical activity of the N‐doped carbon materials.     

Silica‐Protection‐Assisted Encapsulation of Cu2O Nanocubes into a Metal–Organic Framework (ZIF‐8) To Provide a Composite Catalyst

The integration of metal/metal oxide nanoparticles (NPs) into metal–organic frameworks (MOFs) to form composite materials has attracted great interest due to the broad range of applications. However, to date, it has not been possible to encapsulate metastable NPs with high catalytic activity into MOFs, due to their instability during the preparation process. For the first time, we have successfully developed a template protection–sacrifice (TPS) method to encapsulate metastable NPs such as Cu₂O into MOFs. SiO₂ was used as both a protective shell for Cu₂O nanocubes and a sacrificial template for forming a yolk–shell structure. The obtained Cu₂O@ZIF‐8 composite exhibits excellent cycle stability in the catalytic hydrogenation of 4‐nitrophenol with high activity. This is the first report of a Cu₂O@MOF‐type composite material. The TPS method provides an efficient strategy for encapsulating unstable active metal/metal oxide NPs into MOFs or maybe other porous materials.     

Click approach to the three‐component synthesis of novel β‐hydroxy‐1,2,3‐triazoles catalysed by new (Cu/Cu2O) nanostructure as a ligand‐free,green and regioselective nanocatalyst in water

Copper nanostructures were produced as an effective and regioselective catalyst for the synthesis of 1,2,3‐triazoles from a wide range of raw materials, such as sodium azide, epoxides and terminal alkynes, in water via a one‐pot three‐component click reaction. The new heterogeneous catalyst was prepared by a simple ball mill reduction of CuO with NaBH₄ using a ball‐to‐powder weight ratio of 50:1 under air atmosphere at room temperature. The catalyst was fully characterized using scanning electron microscopy, energy‐dispersive X‐ray analysis, Fourier transform infrared spectroscopy and X‐ray diffraction. The copper nanostructures catalysed both ring opening and triazole cyclization steps. Products were obtained in high yields and short reaction times. The reactions were performed at ambient temperature in water as a green solvent. The Cu/Cu₂O nanostructures revealed high reusability and high stability via a simple recycling process.     

Formation of Copper Oxide Nanotextures on Porous Calcium Carbonate Templates for Water Treatment

The necessity of providing clean water sources increases the demand to develop catalytic systems for water treatment. Good pollutants adsorbers are a key ingredient, and CuO is one of the candidate materials for this task. Among the different approaches for CuO synthesis, precipitation out of aqueous solutions is a leading candidate due to the facile synthesis, high yield, sustainability, and the reported shape control by adjustment of the counter anions. We harness this effect to investigate the formation of copper oxide-based 3D structures. Specifically, the counter anion (chloride, nitrate, and acetate) affects the formation of copper-based hydroxides and the final structure following their conversion into copper oxide nanostructures over porous templates. The formation of a 3D structure is obtained when copper chloride or nitrate reacts with a Sorites scaffold (marine-based calcium carbonate template) without external hydroxide addition. The transformation into copper oxides occurs after calcination or reduction of the obtained Cu₂(OH)₃X (X = Cl⁻ or NO₃⁻) while preserving the porous morphology. Finally, the formed Sorites@CuO structure is examined for water treatment to remove heavy metal cations and degrade organic contaminant molecules.     

Continuous Tuning of Au–Cu2O Janus Nanostructures for Efficient Charge Separation

In photocatalysis, the Schottky barrier in metal–semiconductor hybrids is known to promote charge separation, but a core–shell structure always leads to a charge build-up and eventually shuts off the photocurrent. Here, we show that Au–Cu₂O hybrid nanostructures can be continuously tuned, particularly when the Cu₂O domains are single-crystalline. This is in contrast to the conventional systems, where the hybrid configuration is mainly determined by the choice of materials. The distal separation of the Au–Cu₂O domains in Janus nanostructures leads to enhanced charge separation and a large improvement of the photocurrent. The activity of the Au–Cu₂O Janus structures is 5 times higher than that of the core–shell structure, and 10 times higher than that of the neat Cu₂O nanocubes. The continuous structural tuning allows to study the structure–property relationship and an optimization of the photocatalytic performance.     

Ni3FeN‐Supported Fe3Pt Intermetallic Nanoalloy as a High‐Performance Bifunctional Catalyst for Metal–Air Batteries

Electrocatalysts for both the oxygen reduction and evolution reactions (ORR and OER) are vital for the performances of rechargeable metal–air batteries. Herein, we report an advanced bifunctional oxygen electrocatalyst consisting of porous metallic nickel‐iron nitride (Ni₃FeN) supporting ordered Fe₃Pt intermetallic nanoalloy. In this hybrid catalyst, the bimetallic nitride Ni₃FeN mainly contributes to the high activity for the OER while the ordered Fe₃Pt nanoalloy contributes to the excellent activity for the ORR. Robust Ni₃FeN‐supported Fe₃Pt catalysts show superior catalytic performance to the state‐of‐the‐art ORR catalyst (Pt/C) and OER catalyst (Ir/C). The Fe₃Pt/Ni₃FeN bifunctional catalyst enables Zn–air batteries to achieve a long‐term cycling performance of over 480 h at 10 mA cm⁻² with high efficiency. The extraordinarily high performance of the Fe₃Pt/Ni₃FeN bifunctional catalyst makes it a very promising air cathode in alkaline electrolyte.     

Porous Mn2O3: A Low‐Cost Electrocatalyst for Oxygen Reduction Reaction in Alkaline Media with Comparable Activity to Pt/C

Preparing nonprecious metal catalysts with high activity in the oxygen reduction reaction (ORR) can promote the development of energy conversion devices. Support‐free porous Mn₂O₃ was synthesized by a facile aerosol‐spray‐assisted approach (ASAA) and subsequent thermal treatment, and exhibited ORR activity that is comparable to commercial Pt/C The catalyst also exhibits notably higher activity than other Mn‐based oxides, such as Mn₃O₄ and MnO₂. The rotating ring disk electrode (RRDE) study indicates a typical 4‐electron ORR pathway on Mn₂O₃. Furthermore, the porous Mn₂O₃ demonstrates considerable stability and a good methanol tolerance in alkaline media. In light of the low cost and high earth abundance of Mn, the highly active Mn₂O₃ is a promising candidate to be used as a cathode material in metal–air batteries and alkaline fuel cells.     

Carnation‐like CuO Hierarchical Nanostructures Assembled by Porous Nanosheets for Nonenzymatic Glucose Sensing

Carnation‐like CuO hierarchical nanostructures assembled by ultrathin porous nanosheets were successfully fabricated via a facile solvothermal route followed with heat treatment. As‐prepared CuO nanostructures exhibited excellent catalytic activity toward glucose oxidation in the absence of any enzymes. Under the optimized conditions, the CuO‐based enzymeless glucose sensor showed high sensitivity of 3.15 mA mM^?1 cm^?2, low limit of detection (98 nM, S/N=3), good reproducibility, excellent selectivity and long‐time stability. The superb nonenzymatic glucose sensing performance of the CuO hierarchical nanostructures was attributed to the highly catalytically active sites at the edges and basal planes of the CuO nanosheets, facile transportation of analytes through the abundant mesopores and macropores, robust and stable hierarchical structure. Moreover, the CuO‐based enzymeless glucose sensor showed high accuracy and reliability in comparison with clinical glucometer for quantitative determination of glucose in human blood serum samples.     

Copper oxide nanostructures: Controlled synthesis and their catalytic performance

Oval-plate-like, sphere-like, bundle-like and plate-like copper oxide (CuO) nanostructures were prepared by hydrothermal method using Cu(CH₃COO)₂·H₂O and NaOH as the reagents in the absence of any surfactants or templates. The morphology and structure of CuO nanostructures could be easily tailored by adjusting the amount of NaOH. The catalytic activity of the as-prepared CuO nanostructures was demonstrated by catalytic oxidation of methylene blue (MB) in presence of hydrogen peroxide (H₂O₂). The oval-plate-like CuO exhibited better catalytic activity and which was mainly attributed to the larger specific surface area.     

The composite catalysts of Cu/CuxO nanoparticles supported on the carbon fibers were prepared for styrene oxidation reaction

In this paper a novel simple method for preparing two different catalysts with various‐valences copper was reported. Carbon nanofibers supported copper‐cuprous oxide nanoparticles (Cu‐Cu₂O NPs/CNFs) and copper oxide nanoparticles (CuO NPs/CNFs) through electrospinning, adsorption and reduction in the high‐pressure hydrogenation and the high‐temperature calcination methods. These catalysts were investigated by a series of characterizations and were applied in reaction in nitrogen atmosphere, which had a good catalytic activity and selectivity of benzaldehyde for the reaction. Above all, the new study has been certified clearly, in which Cu‐Cu₂O NPs/CNFs and CuO NPs/CNFs composite catalysts enhanced the generation of benzaldehydeand the excellent catalytic properties were exhibited.     

Tuning Interfacial Cu‐O Atomic Structures for Enhanced Catalytic Applications

Cu₂O/CuO_x (x=0, 1) nanocomposites with well‐defined morphologies have been widely applied in catalytic reactions. However, people still understand less about tuning interfacial Cu‐O atomic structures for enhanced catalytic applications, and a special review on this topic has not been reported so far. Herein, we summarize our understanding on tuning interfacial Cu‐O atomic structures based on the literature, including the formation as well as evolution mechanism of Cu‐O interfaces in Cu₂O/CuO and Cu₂O/Cu systems, and the improved performances in the fields of CO oxidation, NO_x oxidation, photoelectrocatalysis, water gas shift reaction, photodegradation of organic dyes, hydrogen evolution, and photoreduction of CO₂. Finally, we briefly propose several potential research directions.     

A photoelectrochemical study of passive copper in alkaline solutions

The photoelectrochemical behaviour of copper covered with a passivating Cu₂O layer has been studied in alkaline solution. Cu₂O shows the characteristics of a p-type semiconductor with a band gap of 2.3 eV and a flatband potential of −0.28 V (SHE). Its photocurrent spectrum shows the characteristics of the absorption spectrum of Cu₂O films. Several redox systems have been tested, including a CuO layer of the duplex film formed at sufficiently positive potentials. The cathodic photocurrent leads to a reduction of the CuO overlayer to Cu₂O rather than to a self-reduction of Cu₂O to Cu. For the duplex film a decrease of the band gap and an increase of the flatband potential is found, suggesting a participation of CuO in the generation of photoelectrons.     

An Isolated Zinc–Cobalt Atomic Pair for Highly Active and Durable Oxygen Reduction

A competitive complexation strategy has been developed to construct a novel electrocatalyst with Zn‐Co atomic pairs coordinated on N doped carbon support (Zn/CoN‐C). Such architecture offers enhanced binding ability of O₂, significantly elongates the O?O length (from 1.23 Å to 1.42 Å), and thus facilitates the cleavage of O?O bond, showing a theoretical overpotential of 0.335 V during ORR process. As a result, the Zn/CoN‐C catalyst exhibits outstanding ORR performance in both alkaline and acid conditions with a half‐wave potential of 0.861 and 0.796 V respectively. The in situ XANES analysis suggests Co as the active center during the ORR. The assembled zinc–air battery with Zn/CoN‐C as cathode catalyst presents a maximum power density of 230 mW cm^?2 along with excellent operation durability. The excellent catalytic activity in acid is also verified by H₂/O₂ fuel cell tests (peak power density of 705 mW cm^?2).     

Highly Branched Metal Alloy Networks with Superior Activities for the Methanol Oxidation Reaction

Three‐dimensional (3D) interconnected metal alloy nanostructures possess superior catalytic performance owing to their advantageous characteristics, including improved catalytic activity, corrosion resistance, and stability. Hierarchically structured Ni‐Cu alloys composed of 3D network‐like microscopic branches with nanoscopic dendritic feelers on each branch were crafted by a facile and efficient hydrogen evolution‐assisted electrodeposition approach. They were subsequently exploited for methanol electrooxidation in alkaline media. Among three hierarchically structured Ni‐Cu alloys with different Ni/Cu ratios (Ni_0.25Cu_0.75, Ni_0.50Cu_0.50, and Ni_0.75Cu_0.25), the Ni_0.75Cu_0.25 electrode exhibited the fastest electrochemical response and highest electrocatalytic activity toward methanol oxidation. The markedly enhanced performance of Ni_0.75Cu_0.25 eletrocatalyst can be attributed to its alloyed structure with the proper Ni/Cu ratio and a large number of active sites on the surface of hierarchical structures.     

Cube‐in‐Cube Hollow Cu9S5 Nanostructures with Enhanced Photocatalytic Activities in Solar H2 Evolution

Hydrogen produced from water under solar energy is an ideal clean energy source, and the efficiency of hydrogen production usually depends on the catalytic systems based on new compounds and/or a unique nanostructure. Herein, well‐defined cube‐in‐cube hollow Cu₉S₅ nanostructures have been successfully prepared with Cu₂O nanocubes and CS₂ as precursors, and single‐shell hollow Cu₉S₅ nanocubes could be obtained by replacing CS₂ with Na₂S. The formation mechanism of cube‐in‐cube hollow nanostructures has been proposed based on the Kirkendell effect and an outward self‐assembly process. Further studies revealed that the cube‐in‐cube hollow Cu₉S₅ nanostructures exhibited better photocatalytic activity toward solar H₂ evolution and would be a promising photocatalyst in the solar hydrogen industry.     

Preparation of ultrafine Cu1.5Mn1.5O4 spinel nanoparticles and its application in p-nitrophenol reduction

In this work, ultrafine Cu_1.5Mn_1.5O₄ spinel nanoparticles were successfully synthesized by a sol–gel method combined with two complexing agents, which was firstly employed in the reductive transformation from p-nitrophenol into p-aminophenol. The effect of calcination temperature on the crystal phase and microstructure of Cu_1.5Mn_1.5O₄ nanoparticles was investigated in this article. It was found that Cu_1.5Mn_1.5O₄ nanoparticles with pure spinel phase can be obtained at 500 °C with the help of EDTA acid–citric acid complexing agents. Below 500 °C, there exists some Mn₂O₃ impure phase. SEM characterization indicated that the particle size of the spinel Cu_1.5Mn_1.5O₄ rapidly increases above 600 °C. The catalytic experimental results show that the Cu_1.5Mn_1.5O₄ nanoparticles prepared at 500 °C exhibit the highest catalytic activity which is even superior to some precious metal catalysts. With the calcination temperature increasing, the catalytic activity of Cu_1.5Mn_1.5O₄ spinel nanoparticles gradually degrades which can be ascribed to the particle size growth of Cu_1.5Mn_1.5O₄. It can also be observed that all the oxide samples, namely CuO, Mn₂O₃ and Cu_1.5Mn_1.5O₄, possess certain catalytic ability for the transformation from p-nitrophenol into p-aminophenol. However, the catalytic activity of Cu_1.5Mn_1.5O₄ spinel nanoparticles is obviously higher than CuO and Mn₂O₃. Especially, Mn₂O₃ alone has very poor catalytic activity in the reduction of p-nitrophenol.