Recent advances in hybrid Cu2O-based heterogeneous nanostructures

Construction of Ultrathin BiVO4-Au-Cu2O Nanosheets with Multiple Charge Transfer Paths for Effective Visible-Light-Driven Photocatalytic Degradation of Tetracycline

In this study, unique BiVO4-Au-Cu2O nanosheets (NSs) are well designed and multiple charge transfer paths are consequently constructed. The X-ray photoelectron spectroscopy measurement during a light off-on-off cycle and redox capability tests of the photo-generated charge carriers confirmed the formation of Z-scheme heterojunction, which can facilitate the charge carrier separation and transfer and maintain the original strong redox potentials of the respective component in the heterojunction. The ultrathin 2D structure of the BiVO4 NSs provided sufficient surface area for the photocatalytic reaction. The local surface plasmon resonance (LSPR) effect of the electron mediator, Au NPs, enhanced the light absorption and promoted the excitation of hot electrons. The multiple charge transfer paths effectively promoted the separation and transfer of the charge carrier. The synergism of the abovementioned properties endowed the BiVO4-Au-Cu2O NSs with satisfactory photocatalytic activity in the degradation of tetracycline (Tc) with a removal rate of ≈80% within 30 min under visible light irradiation. The degradation products during the photocatalysis are confirmed by using ultra-high performance liquid chromatography-mass spectrometry and the plausible degradation pathways of Tc are consequently proposed. This work paves a strategy for developing highly efficient visible-light-driven photocatalysts with multiple charge transfer paths for removing organic contaminants in water.

Multifunctional modification polyester with Au@Cu2O-ZnO ternary heterojunction fabricated by in situ polymerization

In situ polymerization has been proven to be an effective method to introduce functional materials into polymers. In this work, a nano-heterojunction material was prepared successfully and evenly dispersed in PET by in situ polymerization methods to yield multifunctionally modified PET. The modified PET fibers showed excellent antibacterial activity and strong moisture absorption and perspiration, which could efficiently expel moisture from humans. Significantly, these prepared PET textiles demonstrate a strong safety without any cytotoxicity. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the uniform dispersion of heterojunctions and well-defined truncated octahedra including nano-gold rods. A series of characterizations including FTIR, XPS, XRD and DSC showed that the nano-heterojunction participates in the reaction during polymerization. It is interesting that the SEM images of the modified PET fiber presented an intriguing organ fold structure, which makes a significant contribution to moisture absorption and perspiration. The formation mechanism is discussed preliminarily.

Surface and interface engineering of BiOCl nanomaterials and their photocatalytic applications

BiOCl materials have received much attention because of their unique optical and electrical properties. Still, their unsatisfactory catalytic performance has been troubling researchers, limiting the application of BiOCl-based photocatalysts. Therefore, many researchers have studied the adjustment of BiOCl-based materials to enhance photocatalytic efficiency. This review focuses on surface and interface engineering strategies for boosting the photocatalytic performance of BiOCl-based nanomaterials, including forming oxygen vacancy defects, constructing metal/BiOCl, and the fabrication of semiconductor/BiOCl nanocomposites. The photocatalytic applications of the above composites are also concluded in photodegradation of aqueous pollutants, photocatalytic NO removal, photo-induced H2 production, and CO2 reduction. Special emphasis has been given to the modification methods of BiOCl and photocatalytic mechanisms to provide a more detailed understanding for researchers in the fields of energy conversion and materials sciences.

Waste remediation: Low-temperature synthesis of hybrid Cu(OH)2/CuO and CuO nanostructures from spent printed circuit boards and their dye degradation studies

The demand for environmentally friendly and sustainable resource utilization techniques for recycling waste printed circuit boards is significant due to their status as valuable secondary resources, containing high-purity copper and precious metals. In this context, Cu(OH)2/CuO and CuO nanostructures were fabricated using alkaline precipitation and low-temperature aging methods using the strip solution originated from laboratory-scale spent mobile phone printed circuit board recovery process. XRD, FTIR, FESEM-EDX, and TEM were utilized to characterize the as-recovered nanoproducts. A hybrid structure of Cu(OH)2/CuO was formed at 70°, and monoclinic CuO phase was formed at 80 °C aging time. The results show that Cu(OH)2/CuO nanoflakes have an average crystallite size of 24.06 nm and a particle width of 22 ± 3 nm. Cu(OH)2/CuO nanoflakes formed at 70 °C aging temperature and 24-h residence time have finer crystallite and particle sizes than CuO-ridged nanospheres formed at 80 °C aging temperature. The optical band gap energy of Cu(OH)2/CuO and CuO nanostructures formed was found to be 2.28 eV and 2.22 eV, respectively. The hybrid Cu(OH)2/CuO nanostructure photocatalyzed the decomposed 97.28% rhodamine blue using a visible light source, whereas the CuO nanostructure degraded only 14.64% rhodamine blue dye under similar conditions. A surfactant-less hybrid structure is developed without the use of any chemical precursor. Thus, a high value-added product is produced using one waste material to remove another waste in wastewater treatment.

Atomic-Scale Engineering of CuOx-Au Interfaces over AuCu Single-Nanoparticles

A face-centered tetragonal (fct) AuCu particle with a size of 7.1 nm and an Au/Cu molar ratio of 1/1 was coated by a silica shell of 6 nm thickness. Segregation of Cu atoms from the metal particle under an oxidative atmosphere precisely mediated the CuO_x-Au interfacial structure by simply varying the temperature. As raising the temperature from 473 to 773 K, more Cu atoms emigrated from the AuCu particle and were oxidized into CuO_x layers that grew up to 0.8 nm in thickness. Simultaneously, the size of the Au-rich particle lowered moderately while the crystalline structure transformed from the fct phase into the face-centered cubic (fcc) phase. The CuO_x-Au interface shifted from the CuO_x monolayer bound to Au single-atoms to Au@CuO_x core-shell geometry, while the catalytic activity for CO oxidation at 433 K decreased dramatically. Moreover, a sharp loss in activity was observed as the crystal-phase transition occurred. This change in catalytic performance was ascribed to the geometrical configuration at the interfacial sites: the synergetic effect between the fct-AuCu particle and CuO_x monolayer contributed to the much higher activity, whereas the fcc-AuCu/Au particle weakened its interaction with the thicker CuO_x layer and thus decreased the activity.

Oscillatory Reaction Activity of Single Cuprous Oxide Microparticles with NO2

Here, we report on using dark-field microscopy (DFM) as a simple and low-cost imaging platform to visually resolve the kinetics of single cuprous oxide (Cu₂O) microparticles for NO₂ removal in a real-time manner. Unexpectedly, we find that the redox reaction between Cu₂O microparticles and NO₂ is oscillating with the reaction time. Specifically, the oscillatory behavior of single Cu₂O microparticles for NO₂ reduction shows a large particle-to-particle variability, which is also dependent upon the NO₂ pressure and Cu₂O facets. A combined DFM imaging, spectroscopic, scanning electron microscopy, and density functional theory study uncovers that Cu₂O is gradually transformed to copper nitrate hydroxide [Cu₂(NO₃)(OH)₃], and this oscillatory reaction is attributed to the cyclic formation and structural collapse of Cu₂(NO₃)(OH)₃. The present findings open an alternative avenue for probing structure-performance relationships, which are anticipated to benefit the creation of functional materials for air purification.

Influence of Electrode Structure on Performance of Laser Direct Writing Cu-PI Flexible Humidity Sensor

Electrode structure is an essential factor affecting the performance of flexible humidity sensors. In this study, Cu and Cu2 + 1O electrodes were printed by the one-step method using laser direct writing technology to reduce the nano–CuO ink on flexible substrate PI and to be used for a humidity sensor. The resistance of the humidity sensors with nine various electrode structures was measured under the relative humidity (RH) of 16–78%. It was observed that all sensors showed good humidity sensing characteristics, and the sensitivity of the copper-based humidity sensor was not affected by the electrode structure under low humidity conditions but was significant under high humidity conditions. The sensor with the length of 1960 μm and the width of 120 μm shows the lowest sensitivity of 180.2 KΩ/%RH under 35% RH, and the sensor with the length of 2430 μm and the width of 180 μm shows the highest sensitivity of 1744 kΩ/%RH under 65% RH. It is expected that the results can provide an assessment of the performance improvement of the flexible humidity sensor and a reference for the research and development of intelligent wearable devices.

Double profound enhancements of Cu2O nano-octahedrons connected by intertwined Ag nanovines for elevating SERS activity toward ultrasensitive pesticide detection

Recently, hybrid plasmonic metal/semiconductor-based surface-enhanced Raman scattering (SERS) has attracted ever-increasing attention due to its combined characteristics of electromagnetic (EM) enhancement and chemical (CM) enhancement, holding great potential for trace molecular detection. Herein, we demonstrate an interesting heterostructure by linking Cu₂O nano-octahedrons with intertwined Ag nanovines (NVs). The obtained Ag NVs/Cu₂O heterostructures exhibit excellent SERS activity, which is about 2.7 and 7.0 times higher than that of monodispersed Ag or Au nanoparticles (NPs) modified Cu₂O. The intertwined Ag NVs among adjacent Cu₂O octahedrons serve as efficient electron transport channels, which can obviously promote the separation of electrons and holes, reduce the recombination of photogenerated carriers, and then improve the CM enhancement effect. Meanwhile, the accumulated electrons on plasmonic NVs can effectively optimize the collective oscillation of electrons and further improve the EM enhancement. The optimal SERS substrate possesses fascinating multifunctional SERS properties, including ultra-low detection limit (CV, 10^-14 M), excellent anti-interference capability and selectivity. Finally, the established nanosensor can be effectively applied for the quantitative detection of pesticide thiram molecules in soil and biological samples, with low detection limits of 0.48 ng g^-1 and 10^-7 M, respectively. The proposed work demonstrates a high-performance SERS heterostructure with both improved CM enhancement and enhanced EM effect by linking adjacent Cu₂O nano-octahedrons with Ag NVs, which is particularly suitable for ultrasensitive residual pesticide detection in real-world environment.

Growth mechanism and microstructures of Cu2O/PVP spherulites

Cu₂O spherulites are solvothermaly fabricated by using Cu(NO₃)₂ as the starting material and polyvinylpyrrolidone (PVP) as a multifunctional growth agent. The specimens at different growth stages are investigated by using X-ray diffraction, electron microscopy, energy dispersive X-ray spectroscopy, soft X-ray emission spectroscopy and infrared spectroscopy. The formation mechanism of Cu₂O spherulites is proposed accordingly. Hierarchically, the spherulites are composed of needle-like submicron-rods lying along the radial orientations. The submicron-rods are constructed by piling up of small Cu₂O/PVP spheres. The embedded Cu₂O nanocrystallites can generate a dipolar field in each along the [100] direction. They deposit at the surface of a negatively charged PVP-containing spherical core, and self-oriented along the radial directions. Therefore, all the Cu₂O nanocrystallites would have their positively charged (100) facet facing to the core and their negatively charged (1̄00) facet turning towards to the spherulite surface, leading to a negatively charged surface of spherulites. Unlike randomly oriented nanocrystallites embedded in polymer microspheres, the spherulites would not undergo surface recrystallisation into a single crystal shell due to the restricted potential of local shift and rotation of the nanocrystallites by the Coulomb force from the core. This work provides new perspective towards the formation of spherulites and their structural properties.

Cu₂O/PVP spherulites are synthesised using Cu(NO₃)₂ as precursor, PVP as a reductant/capping-agent and DMF as the solvent. The interaction between dipoles of Cu₂O nanocrystals and a negatively charged core plays an important role in their formation.

Metal–organic framework (MOF)-derived hollow hybrid Cu2O/Cu/Au for non-enzymatic H2O2 sensing

Hollow Cu2O/Cu/Au is synthesized using hollow Cu2O/Cu derived from hollow Cu-MOF-74 as a self-sacrificial template with a uniform dispersion of Au particles. It integrates high sensitivity and wide detection range for H2O2 non-enzymatic sensing.

High-sensitivity photo-electrochemical heterostructure of the cuprous oxide-metal organic framework for a dioctyl phthalate molecularly imprinted sensor

This work designed a novel dioctyl phthalate (DOP) photoelectrochemical (PEC) sensor based on a molecularly imprinted polymer (MIP) modified Cu₃(BTC)₂@Cu₂O heterostructure. In this work, a metal organic framework (MOF), Cu₃(BTC)₂, was coated on Cu₂O through a simple immersion method to form a Cu₃(BTC)₂@Cu₂O heterostructure. The heterostructure exhibited strong light adsorption ability, good stability and enhanced photocurrent under visible light irradiation. Using the Cu₃(BTC)₂@Cu₂O heterostructure as the photoelectric converter, a PEC sensor was constructed by imprinting DOP on the heterostructure. Under the optimal experimental conditions, the PEC sensor showed a wide linear range from 25.0 pM-0.1 μM and a low detection limit of 9.15 pM. This method with good sensitivity, stability, selectivity and reproducibility in actual sample analyses showed promising applications of the MOF-based heterostructure in photoelectrochemical analysis fields.

Construction of MOF-shell porous materials and performance studies in the selective adsorption and separation of benzene pollutants

Metastable Cu2O is an attractive material for the architectural design of integrated nanomaterials. In this context, Cu2O was used as the sacrificial agent to form the core-shell structure of Cu2O@HKUST-1 by in situ growth technology. The MOFs with BOPs adsorption property were gathered together by a Cu2O etching method, and the hollow structure of the HKUST-1 shell material with fast BOP adsorption was successfully constructed. The adsorption experiments showed that the HKUST-1 shell has a good adsorption effect on nitrobenzene pollutants in wastewater. The investigation of various factors affecting the adsorption, thermodynamic and kinetic equations was carried out. The adsorption equilibrium was reached within 30 min, and the maximum adsorption capacity was 94.67 mg g-1 at 298 K. The adsorption capacity of nitrobenzene by the HKUST-1 shell is in good agreement with the Freundlich model and the second-order kinetic model. The possible mechanism of adsorption of nitrobenzene by the HKUST-1 shell was discussed. The experimental results suggested that Cu-BTC materials have potential applications for wastewater treatment involving benzene pollutants.

Janus Nanoparticles: From Fabrication to (Bio)Applications

Janus nanoparticles (JNPs) refer to the integration of two or more chemically discrepant composites into one structure system. Studies into JNPs have been of significant interest due to their interesting characteristics stemming from their asymmetric structures, which can integrate different functional properties and perform more synergetic functions simultaneously. Herein, we present recent progress of Janus particles, comprehensively detailing fabrication strategies and applications. First, the classification of JNPs is divided into three blocks, consisting of polymeric composites, inorganic composites, and hybrid polymeric/inorganic JNPs composites. Then, the fabrication strategies are alternately summarized, examining self-assembly strategy, phase separation strategy, seed-mediated polymerization, microfluidic preparation strategy, nucleation growth methods, and masking methods. Finally, various intriguing applications of JNPs are presented, including solid surfactants agents, micro/nanomotors, and biomedical applications such as biosensing, controlled drug delivery, bioimaging, cancer therapy, and combined theranostics. Furthermore, challenges and future works in this field are provided.

Cu2O nanocubes-grafted highly dense Au nanoparticles with modulated electronic structures for improving peroxidase catalytic performances

Based on the intermediate states of metal ions in metal oxide nanomaterials (NMs) that acted as the primary active species, the design of high-performance nanozymes has greatly stimulated current research in diverse biomedical applications. Herein, Cu₂O nanocubes-grafted highly dense Au nanoparticles (NPs) was developed as an appealing nanozyme for H₂O₂ colorimetric sensor and antioxidant detections. The obtained Au/Cu₂O heterostructures show efficient electron-transfer from metallic NPs to Cu₂O nanocubes owing to the difference of Fermi energy between two components. The modulated electronic structure of Au/Cu₂O hybrids enables them to possess enhanced peroxidase catalytic activity for the oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB) in the presence of H₂O₂, which is about 32% higher than that of pristine Cu₂O nanocubes. Then, an excellent H₂O₂ colorimetric sensor was established by using Au/Cu₂O heterostructures with a low limit of detection (LOD) of 0.054 μM, which is much lower than the H₂O₂ allowance level of US FDA regulations (ca.15 μM, 0.05 wt%). The obtained Au/Cu₂O nanoproducts exhibit pronounced long-time stability with 95% peroxidase activity maintained after keeping them for 30 days, while residual 64.5% via Cu₂O nanocubes. Furthermore, we assessed the anti-oxidative behavior of three natural antioxidants (tannic acid, gallic acid, tartaric acid) with the LODs as low as 0.039, 0.16 and 1.55 μM, respectively, and the antioxidant capacity in the following order: tannic acid > gallic acid > tartaric acid. Therefore, it is believed that the as-prepared Au/Cu₂O nanozymes have promising potential applications in fields of biomedicine and food safety.

Conjugated Acetylenic Polymers Grafted Cuprous Oxide as an Efficient Z-Scheme Heterojunction for Photoelectrochemical Water Reduction

As attractive materials for photoeletrochemical hydrogen evolution reaction (PEC HER), conjugated polymers (e.g., conjugated acetylenic polymers [CAPs]) still show poor PEC HER performance due to the associated serious recombination of photogenerated electrons and holes. Herein, taking advantage of the in situ conversion of nanocopper into Cu₂ O on copper cellulose paper during catalyzing of the Glaser coupling reaction, a general strategy for the construction of a CAPs/Cu₂ O Z-scheme heterojunction for PEC water reduction is demonstrated. The as-fabricated poly(2,5-diethynylthieno[3,2-b]thiophene) (pDET)/Cu₂ O Z-scheme heterojunction exhibits a carrier separation efficiency of 16.1% at 0.3 V versus reversible hydrogen electrode (RHE), which is 6.7 and 1.4-times higher respectively than those for pDET and Cu₂ O under AM 1.5G irradiation (100 mW cm^-2 ) in the 0.1 m Na₂ SO₄ aqueous solution. Consequently, the photocurrent of the pDET/Cu₂ O Z-scheme heterojunction reaches ≈520 µA cm^-2 at 0.3 V versus RHE, which is much higher than pDET (≈80 µA cm^-2 ), Cu₂ O (≈100 µA cm^-2 ), and the state-of-the-art cocatalyst-free organic or organic-semiconductor-based heterojunctions/homojunctions photocathodes (1-370 µA cm^-2 ). This work advances the design of polymer-based Z-scheme heterojunctions and high-performance organic photoelectrodes.

Transparent Conductive p-Type Cuprous Oxide Films in Vis-NIR Region Prepared by Ion-Beam Assisted DC Reactive Sputtering

Cu2O thin film has been widely studied due to its intrinsic p-type conductivity. It can be used as p-type transparent conductive electrode or hole transport layer in various potential applications. However, its intrinsic p-type conductivity is very limited, which needs to be optimized by introducing acceptor defects. In this work, the electrical properties of the Cu2O films was improved through introducing interstitial oxygen in the films those were deposited via direct current sputtering assisted by oxygen ion beam. The results show that with oxygen ion beam current increase, the carrier concentration effectively improves. However, with more interstitial oxygen introduced, the film’s crystallinity significantly reduces, as well as the carrier mobility decreases. Meanwhile, all of the Cu2O films present moderate transmittance in the visible region (400–800 nm), but ideal transmittance in the near infrared (NIR) light region (800–2500 nm). When compared with the strong reflection of the n-type transparent conductive film to the near infrared light, the Cu2O film is transparent conductive in NIR region, which expands its application in the fabrication of NIR electrical devices.

Facet-energy inspired metal oxide extended hexapods decorated with graphene quantum dots: sensitive detection of bisphenol A in live cells

The development of crystal-facet metal oxide heterostructures has been of great interest owing to their rational design and multifunctional properties at the nanoscale level. Herein, we report a facile solution-based method for the synthesis of single-crystal Cu₂O nanostructures (i.e. Cu₂O-CuO) as a core. Graphene quantum dots (GQDs) with varying concentrations are fabricated on the surface of Cu₂O extended hexapods (EHPs) in ethanol solution at room temperature via self-assembly, where copper acts as a sacrificial model and a stabilizer as well. The Cu₂O crystals displayed a good sensing activity toward BPA oxidation owing to their high energy facets, dangling bonds and great proportion of surface copper atoms. Structural, morphological, chemical and vibrational investigations were performed in detail, presenting high crystallinity of hybrid nanocomposites and Cu₂O-CuO heterojunction positions along with the growth of GQDs on the core of Cu₂O-CuO crystals. The electrochemical sensing performance of the as-fabricated Cu₂O-CuO@GQD EHPs was monitored for the determination of bisphenol A (BPA) as an early diagnostic marker and environmental contaminant. The synergistic effects of the boosted surface area, exposed Cu {111} crystallographic planes and mixed copper valences enhance redox reaction kinetics by increasing the electron shuttling rate at the electrode-analyte junction. Benefitting from the improved electrocatalytic activity for BPA oxidation, the electrochemical sensor displayed the lowest limit of detection (≤1 nM), good chemical stability, a broad linear range (2 nM-11 mM), and high sensitivity (636 μA mM^-1 cm^-2). The Cu₂O-CuO@GQD EHP-based sensing platform was used for BPA detection in water and human serum samples. We have also constructed a pioneering electrochemical sensing platform for BPA detection in live cells, which might be used as a marker for early disease diagnosis.

Enhanced electrochemiluminescence of luminol based on Cu2O-Au heterostructure enabled multiple-amplification strategy

Herein, a credible construction strategy to improve electrochemiluminescence (ECL) of luminol was developed based on Cu₂O-Au heterostructures. Summarily, gold nanoparticles (AuNPs) were anchored on surface of Cu₂O nanocube (Cu₂O@AuNPs) by spontaneous reduction reaction. Then, luminol molecules were concentrated on Cu₂O@AuNPs using L-Cysteine (Cys) as covalent linkage to build the composite emitter (Cu₂O@AuNPs-Cys-luminol). The enhancement mechanism was realized by following aspects: (I) Cu₂O@AuNPs worked as electrocatalyst for glucose to generate coreactant of H₂O₂ in situ, avoiding the instability of direct addition of H₂O₂. (II) luminol molecules were firmly attached on Cu₂O@AuNPs to achieve centralized and strong luminescence at low consumption. (III) Cys acted as an intramolecular coreactant and directly linked to luminol to increase luminous efficiency. To validate the effectiveness, a sandwiched immunoassay was built using concanavalinA (ConA) as analyte. Electroreduced graphene film as substrate provided phenoxy-derivatized dextran (DexP) with abundant binding sites and improved conductivity. To improve the specificity, DexP was used to identify ConA via the specific carbohydrate-ConA interaction. Then, Cu₂O@AuNPs-Cys-luminol was modified on electrode as ECL signal indicator. The ECL immunosensor achieved determination of ConA with low detection limit of 2.9 × 10^-5 ng/mL and excellent stability of continuous potential scan for 8 cycles. Experimental results demonstrated that the proposed construction strategy made considerable progress in ECL efficiency and stability of luminol. The creational pattern of construction strategy achieves high detection capabilities to ConA and expands the applicability of luminol in ECL system. It is expected to have more potential application value in immunoassay with universality.

An Mn2+-mediated construction of rhombicuboctahedral Cu2O nanocrystals enclosed by jagged surfaces for enhanced enzyme-free glucose sensing

A rhombicuboctahedral (26-facet) Cu₂O single crystal, with well-developed {100}, {110} and {111} crystallographic planes, has attracted considerable attention due to its faceted synergistic effects in catalysis, sensing, and energy conversion.

Nanostructured MoO3 for Efficient Energy and Environmental Catalysis

This paper mainly focuses on the application of nanostructured MoO₃ materials in both energy and environmental catalysis fields. MoO₃ has wide tunability in bandgap, a unique semiconducting structure, and multiple valence states. Due to the natural advantage, it can be used as a high-activity metal oxide catalyst, can serve as an excellent support material, and provide opportunities to replace noble metal catalysts, thus having broad application prospects in catalysis. Herein, we comprehensively summarize the crystal structure and properties of nanostructured MoO₃ and highlight the recent significant research advancements in energy and environmental catalysis. Several current challenges and perspective research directions based on nanostructured MoO₃ are also discussed.

Surface-enhanced Raman scattering studies of Cu/Cu2O Core-shell NPs obtained by laser ablation

In order to perform SERS (surface-enhanced Raman scattering) measurements, spherical Cu/Cu₂O core-shell NPs with a rather rough rugged surface and well-defined crystallographic structures were fabricated using nanosecond Ce: Nd YAG pulsed laser ablation in liquid (PLAL). Raman, Fourier transform infrared (FTIR) spectroscopy and TEM imaging of the prepared NPs reveal the existence of additional minority CuO phase, not determined earlier through XRD patterns. The SERS activity of Cu/Cu₂O core-shell NPs substrates was investigated by using crystal violet (CV) and methylene blue (MB) as the analyte molecules under 532 nm excitation wavelength irradiation. The effect of localized surface plasmon resonance (LSPR) from Cu core contributing to the electromagnetic enhancement and Cu2O shell with a rough surface which itself contributes to chemical enhancement with adsorbed analyte molecule is due to a high overall SERS enhancement. The intensities of the totally and non-totally symmetric modes were used to calculate the degree of charge-transfer. The results demonstrate that the LSPR enhancement dominates charge-transfer resonance contribution in SERS of Cu/Cu2O-CV and Cu/Cu2O -MB systems. The reproducibility of the prepared SERS substrates was investigated and the SERS signals intensity variation was <28%.

Passivating Surface States on Water Splitting Cuprous Oxide Photocatalyst with Bismuth Decoration

To enhance the visible light photocatalystic activity of Cu 2 O(100) surface, we performed first-principles calculations on the structural, electronic and optical properties of a bismuth (Bi)-decorated Cu 2 O(100) surface (Bi@Cu 2 O(100)). It is shown that the Bi prefer to be loaded to the hollow sites among four surface oxygen atoms and tend to individual dispersion instead of aggregating on the surface due to the lowest formation energy and larger distance between two Bi atoms at the surface than the Bi clusters; the coverage of around 0.25 monolayer Bi atoms can effectively eliminate the surface states and modify the band edges to satisfy the angular momentum selection rules for light excited transition of electrons, and the loaded Bi atoms contribute to the separation of photogenerated electron-holes. The relative positions between the band edges and the redox potentials are suitable for photocatalytic hydrogen production from the redox water, and moreover, the optical absorption spectrum indicates a positive response of the Bi 0 . 25 @Cu 2 O(100) to visible light, implying that the Bi 0 . 25 @Cu 2 O(100) is a promising visible light photocatalyst.

High-index faceted metal oxide micro-/nanostructures: a review on their characterization, synthesis and applications

Exposed high-index facets with a high density of low-coordinated atoms (including edges, steps and kinks) can provide more high-active sites for chemical reactions. Therefore, great progress has made in the facet-dependent application of various high-index faceted micro-/nanostructures in the past decades. Previous review papers have mainly highlighted the advances in high-index faceted noble metal nanocrystals. However, to date, there is no specialized review paper on high-index faceted metal oxides and their facet-dependent applications. Thus, in this review, the existing high-index faceted metal oxide micro-/nanostructures, including Cu2O, TiO2, Fe2O3, ZnO, SnO2 and BiVO4, are reviewed based on their characterization, synthesis engineering and facet-dependent applications in the fields of catalysis, sensors, lithium-ion batteries and carbon monoxide oxidation. Also, several challenges and perspectives are presented. Hopefully, this review article will be a useful guideline and resource for researchers currently concentrating on high-index faceted metal oxides to design and synthesize novel micro-/nanostructures for overcoming the practical environment-, biology- and energy-related problems.

Cu-Doped Porous Carbon Derived from Heavy Metal-Contaminated Sewage Sludge for High-Performance Supercapacitor Electrode Materials

In this paper, we report a complete solution for enhanced sludge treatment involving the removal of toxic metal (Cu(II)) from waste waters, subsequent pyrolytic conversion of these sludge to Cu-doped porous carbon, and their application in energy storage systems. The morphology, composition, and pore structure of the resultant Cu-doped porous carbon could be readily modulated by varying the flocculation capacity of Cu(II). The results demonstrated that it exhibited outstanding performance for supercapacitor electrode applications. The Cu(II) removal efficiency has been evaluated and compared to the possible energy benefits. The flocculant dosage up to 200 mg·L⁻¹ was an equilibrium point existing between environmental impact and energy, at which more than 99% Cu(II) removal efficiency was achieved, while the resulting annealed product showed a high specific capacity (389.9·F·g⁻¹ at 1·A·g⁻¹) and good cycling stability (4% loss after 2500 cycles) as an electrode material for supercapacitors.

Au@Cu Nanoarrays with Uniform Long-Range Ordered Structure: Synthesis and SERS Applications

The nanostructures with uniform long-range ordered structure are of crucial importance for performance standardization of high-quality surface-enhanced Raman scattering (SERS) spectra. In this paper, we described the fabrication and SERS properties of Au decorated Cu (Au@Cu) nanoarrays. The Cu nanoarrays with uniform long-range ordered structure were first synthesized by in-situ electrochemistry assembly on insulated substrate. The Cu nanoarrays can reach a size of centimeters with strictly periodic nano-microstructure, which is beneficial for the production and performance standardization of SERS substrates. Then Au nanoparticals were decorated on the Cu nanoarrays by galvanic reaction without any capping agent. The obtained Au@Cu nanoarrays exhibit excellent SERS activity for 4-Mercaptopyridine, and the sensitivity limit is as low as 10⁻⁸ M. Therefore, this facile route provides a useful platform for the fabrication of SERS substrates based on nano ordered arrays.

Photocatalytic reduction of carbon dioxide over Cu/TiO2 photocatalysts

The photocatalytic reduction of CO₂ with H₂O was investigated using Cu/TiO₂ photocatalysts in aqueous solution. For this purpose, Cu/TiO₂ photocatalysts (with 0.2, 0.9, 2, 4, and 6 wt.% of Cu) have been synthesized via sol-gel method. The photocatalysts were extensively characterized by means of inductively coupled plasma optical emission spectrometry (ICP-OES), N₂ physisorption (BET), XRD, UV-vis DRS, FT-IR, Raman spectroscopy, TEM-EDX, and photoelectrochemical measurements. The as-prepared photocatalysts contain anatase as a major crystalline phase with a crystallite size around 13 nm. By increasing the amount of Cu, specific surface area and band gap energy decreased in addition to the formation of large agglomeration of CuO. Results revealed that the photocatalytic reduction of CO₂ decreased in the presence of Cu/TiO₂ in comparison to pure TiO₂, which might be associated to the formation of CuO phase acting as a recombination center of generated electron-hole pair. Decreasing of photoactivity can also be connected with a very low position of conduction band of photocatalysts with high Cu content, which makes H₂ production necessary for CO₂ reduction more difficult.

Fabrication of Integrated Cu2O@HKUST-1@Au Nanocatalysts via Galvanic Replacements toward Alcohols Oxidation Application

Metastable Cu₂O is an attractive material for the architectural design of integrated nanomaterials. In this context, the in situ growth of microporous metal-organic frameworks (MOFs) on Cu₂O nanocrystals with well-defined shapes (such as octahedron, truncated octahedron, and corner-truncated cube) was conducted under ambient conditions by using sacrificial Cu₂O nanocrystals as a copper ion source for the construction of HKUST-1. It was found that the growth rate of HKUST-1 on the (111) facet of octahedral Cu₂O was much faster than that on the (100) facet of cubic Cu₂O. A subsequent etching process on the core-shell-structured Cu₂O@HKUST-1 to remove Cu₂O results in hollow HKUST-1 with hierarchical pores, where the shapes of cavities are tailored depending on the original Cu₂O crystals. Importantly, it was found that the integrated Cu₂O@HKUST-1@Au nanocatalysts could be fabricated by galvanic replacement reaction between Cu₂O and AuCl₄^-, wherein the copper(I) ions from Cu₂O etching diffuse through the microporous MOF shell and reach to the external surface to reduce AuCl₄^- ions, thereby forming Au nanoparticles exclusively on the MOF external surface. Furthermore, the catalytic applications of Cu₂O@HKUST-1@Au toward liquid-phase oxidation of both aromatic and aliphatic alcohols were studied. Notably, 100% selectivities of aldehyde products were achieved without any further oxidation byproducts such as acid and ester.

Highly visible-light-responsive Cu2O/rGO decorated with Fe3O4@SiO2 nanoparticles as a magnetically recyclable photocatalyst

The rhombic dodecahedral cuprous oxide-reduced graphene oxide/core-shell Fe₃O₄@SiO₂ composites (denoted as rCu₂O-rGO/Fe₃O₄@SiO₂) are successfully synthesized facilely via a wet-chemical route. The resulting rCu₂O-rGO/Fe₃O₄@SiO₂ combines the unique structure of Cu₂O, electronic characteristics of reduced graphene oxide (rGO) and magnetic property of Fe₃O₄@SiO₂ to be an effective and recoverable photocatalyst for the degradation of methyl orange (MO). The obtained results show that rCu₂O-rGO/Fe₃O₄@SiO₂ is capable of completely degrading MO in the presence of a very low catalyst concentration (0.125 g l^-1) within a short time (60 min) under visible light compared to the reported catalysts. The observations may be due to the distinctive interfacial structures of rhombic dodecahedral Cu₂O nanoparticles connected to rGO sheets that can enhance the separation of photogenerated electron-hole pairs, stabilize the Cu₂O and increase MO adsorption, as evidenced by a variety of spectroscopic analyses (transmission electron microscopy, x-ray photoelectron spectroscopy and photoluminescence). More importantly, these efficient photocatalysts can easily be recovered under a magnetic field and remain highly photoactive towards the degradation of MO after cyclic tests, and may be promising photocatalysts for practical applications in the solar-energy purification of wastewater.

Facet-Dependent Cuprous Oxide Nanocrystals Decorated with Graphene as Durable Photocatalysts under Visible Light

Three morphologies (octahedral, hierarchical and rhombic dodecahedral) of crystal Cu₂O with different facets ({111}, {111}/{110}, and {110}) incorporating graphene sheets (denoted as o-Cu₂O-G, h-Cu₂O-G and r-Cu₂O-G, respectively) have been fabricated by using simple solution-phase techniques. Among these photocatalysts, the r-Cu₂O-G possesses the best photocatalytic performance of 98% removal efficiency of methyl orange (MO) with outstanding kinetics for 120 min of visible light irradiation. This enhancement is mainly due to the dangling “Cu” atoms in the highly active {110} facets, resulting in the increased adsorption of negatively charged MO. More importantly, the unique interfacial structures of Cu₂O rhombic dodecahedra connected to graphene nanosheets can not only decrease the recombination of electron-hole pairs but also stabilize the crystal structure of Cu₂O, as verified by a series of spectroscopic analyses (e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM)). The effective photocatalysts developed in this work could be applied to the efficient decolorization of negatively charged organic dyes by employing solar energy.

A Comparison of ACQ, AIE and AEE-Based Polymers Loaded on Polyurethane Foams as Sensors for Explosives Detection

An aggregation-caused quenching (ACQ)-active polymer (PF), an aggregation-induced emission (AIE)-active polymer (PFTPE) and an aggregation-enhanced emission (AEE)-active polymer (PTTPE) were synthesized by tetraphenylethane (TPE), fluorene and thiophene moieties. Polyurethane (PU) foams modified by PF, PFTPE and PTTPE, namely PU-PF, PU-PFTPE and PU-PTTPE, using ultrasonication-assisted method have been prepared. A comparative study of PU-PF, PU-PFTPE and PU-PTTPE for detection explosives had been performed, and significant fluorescence quenching was observed with the introduction of PA solutions. The as-prepared PU-PF, PU-PFTPE and PU-PTTPE sensors exhibited a superior sensitivity for PA solutions with different concentrations. Remarkably, PU-PF gave a quenching efficiency of 96.2%, higher than 93.5% for PU-PFTPE and 86.7% for PU-PTTPE at a PA concentration of 180 µg·mL⁻¹ in methanol, which was attributed to the effective energy transfer from the fluorophore (PF) to the nitro explosive (PA). This suggested that some ACQ polymers, applied to detect explosives, could afford better performances than AIE or AEE polymers through modification of structures and selection of adequate carriers. At the same time, these chemical sensors can be recycled many times.

Self-Assembled Ag-Cu₂O Nanocomposite Films at Air-Liquid Interfaces for Surface-Enhanced Raman Scattering and Electrochemical Detection of H₂O₂

We employ a facile and novel route to synthesize multifunctional Ag-Cu₂O nanocomposite films through the self-assembly of nanoparticles at an air-liquid interface. In the ethanol-water phase, AgNO₃ and Cu(NO₃)₂ were reduced to Ag-Cu₂O nanoparticles by NaBH₄ in the presence of cinnamic acid. The Ag-Cu₂O nanoparticles were immediately trapped at the air-liquid interface to form two-dimensional nanocomposite films after the reduction reaction was finished. The morphology of the nanocomposite films could be controlled by the systematic regulation of experimental parameters. It was found that the prepared nanocomposite films serving as the substrates exhibited strong surface-enhanced Raman scattering (SERS) activity. 4-aminothiophenol (4-ATP) molecules were used as the test probes to examine the SERS sensitivity of the nanocomposite films. Moreover, the nanocomposite films synthesized by our method showed enhanced electrocatalytic activity towards hydrogen peroxide (H₂O₂) and therefore could be utilized to fabricate a non-enzymatic electrochemical H₂O₂ sensor.

Oxygen-deficient photostable Cu2O for enhanced visible light photocatalytic activity

Oxygen vacancies in inorganic semiconductors play an important role in reducing electron-hole recombination, which may have important implications in photocatalysis. Cuprous oxide (Cu2O), a visible light active p-type semiconductor, is a promising photocatalyst. However, the synthesis of photostable Cu2O enriched with oxygen defects remains a challenge. We report a simple method for the gram-scale synthesis of highly photostable Cu2O nanoparticles by the hydrolysis of a Cu(i)-triethylamine [Cu(i)-TEA] complex at low temperature. The oxygen vacancies in these Cu2O nanoparticles led to a significant increase in the lifetimes of photogenerated charge carriers upon excitation with visible light. This, in combination with a suitable energy band structure, allowed Cu2O nanoparticles to exhibit outstanding photoactivity in visible light through the generation of electron-mediated hydroxyl (OH˙) radicals. This study highlights the significance of oxygen defects in enhancing the photocatalytic performance of promising semiconductor photocatalysts.

SERS study of surface plasmon resonance induced carrier movement in Au@Cu2O core-shell nanoparticles

A plasmon induced carrier movement enhanced mechanism of surface-enhanced Raman scattering (SERS) was investigated using a charge-transfer (CT) enhancement mechanism. Here, we designed a strategy to study SERS in Au@Cu₂O nanoshell nanoparticles with different shell thicknesses. Among the plasmonically coupled nanostructures, Au spheres with Cu₂O shells have been of special interest due to their ultrastrong electromagnetic fields and controllable carrier transfer properties, which are useful for SERS. Au@Cu₂O nanoshell nanoparticles (NPs) with shell thicknesses of 48-56nm are synthesized that exhibit high SERS activity. This high activity originates from plasmonic-induced carrier transfer from Au@Cu₂O to 4-mercaptobenzoic acid (MBA). The CT transition from the valence band (VB) of Cu₂O to the second excited π-π* transition of MBA, and is of b₂ electronic symmetry, which was enhanced significantly. The Herzberg-Teller selection rules were employed to predict the observed enhanced b₂ symmetry modes. The system constructed in this study combines the long-range electromagnetic effect of Au NPs, localized surface plasmon resonance (LSPR) of the Au@Cu₂O nanoshell, and the CT contribution to assist in understanding the SERS mechanism based on LSPR-induced carrier movement in metal/semiconductor nanocomposites.

Colloidal zinc oxide-copper(I) oxide nanocatalysts for selective aqueous photocatalytic carbon dioxide conversion into methane

Developing catalytic systems with high efficiency and selectivity is a fundamental issue for photochemical carbon dioxide conversion. In particular, rigorous control of the structure and morphology of photocatalysts is decisive for catalytic performance. Here, we report the synthesis of zinc oxide-copper(I) oxide hybrid nanoparticles as colloidal forms bearing copper(I) oxide nanocubes bound to zinc oxide spherical cores. The zinc oxide-copper(I) oxide nanoparticles behave as photocatalysts for the direct conversion of carbon dioxide to methane in an aqueous medium, under ambient pressure and temperature. The catalysts produce methane with an activity of 1080 μmol g_cat⁻¹ h⁻¹, a quantum yield of 1.5% and a selectivity for methane of >99%. The catalytic ability of the zinc oxide-copper(I) oxide hybrid catalyst is attributed to excellent band alignment of the zinc-oxide and copper(I) oxide domains, few surface defects which reduce defect-induced charge recombination and enhance electron transfer to the reagents, and a high-surface area colloidal morphology.

Photocatalytic reduction and oxidation reactions, involving multiple electrons and operating in tandem, are extremely challenging to achieve. Here, with a hybrid structure of ZnO and Cu₂O, the authors report photocatalytic carbon dioxide reduction to methane with >99% selectivity using electrons from water.