Experimental and numerical investigation of fractal-tree-like heat exchanger manufactured by 3D printing

© 2018 Elsevier Ltd The manufacturing difficulties of complex fractal-tree-like heat exchangers have limited their industrial applications, although many evidences have shown that they have significant advantages in heat transfer. Nevertheless, the emerging 3D printing technology has brought great opportunity for the development of complex structured device. In the present study, three-dimensional (3D) fractal-tree-like heat exchangers were designed and manufactured using 3D printing technology. Their performance was evaluated from both thermal and hydrodynamic perspectives, the flow characteristics were investigated in detail. The results show that a fractal-tree-like heat exchanger can improve hydrodynamic performance, reduce pressure drops and has great heat transfer ability. In general, the fractal-tree-like heat exchanger has a comprehensive advantage over the traditional spiral-tube exchangers as it has a higher value of coefficient of performance (COP). Furthermore, the 3D printing provides a visual, efficient, and precise approach in the present research

Synthesis of Au@Ag Core–Shell Nanocubes Containing Varying Shaped Cores and Their Localized Surface Plasmon Resonances

We have synthesized Au@Ag core–shell nanocubes containing Au cores with varying shapes and sizes through modified seed-mediated methods. Bromide ions are found to be crucial in the epitaxial growth of Ag atoms onto Au cores and in the formation of the shell’s cubic shape. The Au@Ag core–shell nanocubes exhibit very abundant and distinct localized surface plasmon resonance (LSPR) properties, which are core-shape and size-dependent. With the help of theoretical calculation, the physical origin and the resonance mode profile of each LSPR peak are identified and studied. The core–shell nanocrystals with varying shaped cores offer a new rich category for LSPR control through the plasmonic coupling effect between core and shell materials

Facile Synthesis of Surfactant-Free Au Cluster/Graphene Hybrids for High-Performance Oxygen Reduction Reaction

Non-Pt noble metal clusters like Au clusters are believed to be promising high performance catalysts for the oxygen reduction reaction (ORR) at the cathode of fuel cells, but they still suffer big problems during the catalysis reactions, such as a large amount of the capping agents being on the surface and easy occurrence of dissolution and aggregation. To overcome these obstacles, here, we present a novel and general strategy to grow ultrafine Au clusters and other metal (Pt, Pd) clusters on the reduced graphene oxide (rGO) sheets without any additional protecting molecule or reductant. Compared with the currently generally adopted nanocatalysts, including commercial Pt/C, rGO sheets, Au nanoparticle/rGO hybrids, and thiol-capped Au clusters of the same sizes, the as-synthesized Au cluster/rGO hybrids display an impressive eletrocatalytic performance toward ORR, for instance, high onset potential, superior methanol tolerance, and excellent stability

Conformation Modulated Optical Activity Enhancement in Chiral Cysteine and Au Nanorod Assemblies

Assemblies of chiral cysteine (CYS) and Au nanorods (GNRs) are constructed in two typical patterns, end-to-end and side-by-side. Impressively, side-by-side assembled GNRs with CYS show obviously stronger plasmonic circular dichrosim (CD) response compared with the end-to-end assemblies. The corresponding theoretical calculation elucidates the intrinsic relationship among geometric structure, electromagnetic interaction, and induced plasmonic CD of the assemblies. This work will significantly benefit the design and application of plasmonic nanodevices with controllable chiroptical responses

Toward Understanding of Transfer Mechanism between Electrochemiluminescent Dyes and Luminescent Quantum Dots

Electrochemiluminescence resonance energy transfer (ECL-RET) based on dye–quantum dot (QD) hybrids, is a very powerful tool for chemical sensing and probing many important biological processes. In this work, we have investigated both electrochemiluminescence (ECL) and photoluminescence (PL) properties of the hybrid system, in which tris­(2,2′-bipyridyl)­ruthenium­(II) ([Ru­(bpy)₃]²⁺)/2-(dibutylamino)­ethanol (DBAE) and QD are employed as the ECL donor and acceptor, respectively. Unexpectedly, we find that ECL of the [Ru­(bpy)₃]²⁺/DBAE system can be efficiently quenched by various types of QDs. In addition, ECL quenching in the [Ru­(bpy)₃]²⁺/DBAE system is independent of the core size and the surface charge of QDs, indicating that the quenching effect does not originate from resonance energy transfer between the [Ru­(bpy)₃]²⁺/DBAE system and QDs. Photoluminescence properties of the hybrid system under electrochemical control and electron spin resonance (ESR) measurements further reveal that a charge transfer between QDs and the radical-state DBAE is responsible for ECL quenching in the [Ru­(bpy)₃]²⁺/DBAE system. Contrary to previously published information, we propose that electron transfer, rather than energy transfer, dominates in the hybrid system under electrochemical control. We further demonstrate that such electron transfer could be switched to energy transfer by controlling the distance between [Ru­(bpy)₃]²⁺/DBAE ECL and QDs. The energy/electron transfer process between [Ru­(bpy)₃]²⁺/DBAE ECL and QDs is implemented to develop a novel platform for immune sensing

Manipulation of Collective Optical Activity in One-Dimensional Plasmonic Assembly

The manipulation of the chirality and corresponding optical activity in the visible–near-infrared (NIR) light region is significant to realize applications in the fields of chemical sensing, enantioselective separation, chiral nanocatalysis, and optical devices. We studied the plasmon-induced circular dichroism (CD) response by one-dimensional (1D) assembly of cysteine (CYS) and gold nanorods (GNRs). Typically, GNRs can form end-to-end assembly through the electrostatic attraction of CYS molecules preferentially attached on the ends of different GNRs. CD responses are observed at both the UV and visible–NIR light region in the 1D assembly, which are assigned to the CYS molecules and the GNRs, respectively. In addition, the wavelength of the CD responses can be manipulated from 550 nm to more than 900 nm through altering the aspect ratios of GNRs in 1D assembly. Anisotropic enhancement of optical activity is discovered, suggesting that the enhancement of the longitudinal localized surface plasmon resonance (LSPR) peak of GNRs in the CD response is much more apparent than that of the transverse LSPR. The CD responses of individual CYS-attached GNRs and CYS-assembled gold nanoparticles (GNPs) substantiate that the form of assembly and the shape of building blocks are significant not only for the intensity but for the line shape of the CD signals

Regulating Oxygen Vacancies for Enhanced Higher Oxygenate Synthesis via Syngas

Constructing highly efficient dual active sites for preferential formation of higher oxygenates via direct syngas conversion remains a grand challenge. Herein, we reported that the regulation of oxygen vacancy density of metal–oxide support could effectively promote the production of oxygenates. Compared with an inert SiO2-supported Co-based catalyst, the rutile TiO2-supported catalyst with abundant oxygen vacancies exhibited up to 44.7% CO conversion with the selectivity and space–time yield (STY) of the oxygenate increased to 43.4 wt % and 50 mg gcat.–1 h–1, respectively. Further studies established a nearly linear relationship between the density of the oxygen vacancy and the atomic ratio of Co2+/Co0 or the STY of oxygenated products. Characterization confirmed that the oxygen vacancies not only promote CO adsorption, dissociation, and subsequently the carburization of cobalt species to form Co2C but also create a C-rich and H-poor local microchemical environment that benefits CO associative adsorption and CO bond insertion to form oxygenates. The synergistic effect of oxygen vacancies and the Co0/Co2C interface site contributed to the observed enhanced performance for direct syngas conversion to higher oxygenates

Growth Mechanism Deconvolution of Self-Limiting Supraparticles Based on Microfluidic System

The synthesis of colloidal supraparticles (SPs) based on self-assembly of nanoscopic objects has attracted much attention in recent years. Here, we demonstrate the formation of self-limiting monodisperse gold SPs with core–shell morphology based on the building blocks of flexible nanoarms in one step. A flow-based microfluidic chip is utilized to slow down the assembly process of the intermediates, which surprisingly allows for observation of ultrathin gold nanoplates as first intermediates. Notably, these intermediate cannot be observed in traditional synthesis due to their rapid rolling-up to form the second-order nanostructure of flexible hollow nanoarms. The growth mechanism of SPs can then be deconvoluted into two seed-mediated steps. Monte Carlo simulations confirm that the self-limiting growth of binary SPs is governed by a balance between electrostatic repulsion and van der Waals attraction

Photocatalytic Properties of Graphdiyne and Graphene Modified TiO₂: From Theory to Experiment

The chemical structure and electronic properties of two-dimensional (2D) carbon-supported TiO₂, TiO₂–graphdiyne, and TiO₂–graphene composites have been studied by first-principles density functional theory. Calculation results show that TiO₂(001)–graphdiyne composites possess superior charge separation and oxidation properties, having the longest lifetimes of photoexcited carriers among all of the 2D composites containing TiO₂ of different facets. Our experimental results further proved that TiO₂(001)–graphdiyne composites could be a promising photocatalyst. For photocatalytic degradation of methylene blue, the rate constant of the TiO₂(001)–graphdiyne composite is 1.63 ± 0.15 times that of the pure TiO₂(001) and 1.27 ± 0.12 times that of the TiO₂(001)–graphene composite

Core–Shell Palladium Nanoparticle@Metal–Organic Frameworks as Multifunctional Catalysts for Cascade Reactions

Uniform core–shell Pd@IRMOF-3 nanostructures, where single Pd nanoparticle core is surrounded by amino-functionalized IRMOF-3 shell, are prepared by a facile mixed solvothermal method. When used as multifunctional catalysts, the Pd@IRMOF-3 nanocomposites exhibit high activity, enhanced selectivity, and excellent stability in the cascade reaction. Both experimental evidence and theoretical calculations reveal that the high catalytic performance of Pd@IRMOF-3 nanocomposites originates from their unique core–shell structures