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Superior Wear-Resistance of Ti3C2Tx Multilayer Coatings
No full textOwing to MXenes' tunable mechanical properties induced by their structural and chemical diversity, MXenes are believed to compete with state-of-the-art 2D nanomaterials such as graphene regarding their tribological performance. Their nanolaminate structure offers weak interlayer interactions and an easy-to-shear ability to render them excellent candidates for solid lubrication. However, the acting friction and wear mechanisms are yet to be explored. To elucidate these mechanisms, 100-nm-thick homogeneous multilayer Ti3C2Tx coatings are deposited on technologically relevant stainless steel by electro-spraying. Using ball-on-disk tribometry (Si3N4 counterbody) with acting contact pressures of about 300 MPa, their long-term friction and wear of performance under dry conditions are studied. MXene-coated specimens demonstrate a 6-fold friction reduction and an ultralow wear rate (4 x 10(-9) mm(3) N-1 m(-1)) over 100 000 sliding cycles, outperforming state-of-the-art 2D nanomaterials by at least 200% regarding their wear life. High-resolution characterization verified the formation of a beneficial tribolayer consisting of thermally/mechanically degraded MXenes and amorphous/nanocrystalline iron oxides. The transfer of this tribolayer to the counterbody transforms the initial steel/Si3N4 contact to tribolayer/tribolayer contact with low shear resistance. MXene pileups at the wear track's reversal points continuously supply the tribological contact with fresh, lubricious nanosheets, thus enabling an ultra-wear-resistant and low-friction performance
Effects of Ribbon Thickness on Structure and Soft Magnetic Properties of a High-Cu-Content FeBCuNb Nanocrystalline Alloy
No full textThe effects of ribbon thickness (t) on the structure and magnetic properties of a Fe82.3B13Cu1.7Nb3 alloy in melt-spun and annealed states have been investigated. Increasing the t from 15 to 23 mu m changes the structure of the melt-spun ribbons from a single amorphous phase to a composite with dense alpha-Fe nanograins embedded in the amorphous matrix. The grain size (D alpha-Fe) of the alpha-Fe near the free surface of the ribbon is about 6.7 nm, and it gradually decreases along the cross section toward the wheel-contacted surface. Further increasing the t to 32 mu m coarsens the D alpha-Fe near the free surface to 15.2 nm and aggravates the D alpha-Fe ramp along the cross section. After annealing, the ribbon with t = 15 mu m has relatively large alpha-Fe grains with D alpha-Fe > 30 nm, while the thicker ribbons possessing the pre-existing nanograins form a finer nanostructure with D alpha-Fe < 16 nm. The structural uniformity of the ribbon with t = 23 mu m is better than that of the ribbon with t = 32 mu m. The annealed ribbons with t = 23 and 32 mu m possess superior soft magnetic properties to the ribbon with t = 15 mu m. The ribbon with t = 23 mu m exhibits a high saturation magnetic flux density of 1.68 T, low coercivity of 9.6 A/m, and high effective permeability at 1 kHz of 15,000. The ribbon with t = 32 mu m has a slightly larger coercivity due to the lower structural uniformity. The formation mechanism of the fine nanostructure for the ribbons with suitable t has been discussed in terms of the competitive growth effect among the pre-existing alpha-Fe nanograins
Coordination-driven assembly of actinide-organic polyrotaxanes involving crown ether macrocycles
No full textResearch on synthesizing new coordination-driven polyrotaxanes as well as regulating their structural diversity contributes to the success of macrocycle-based supramolecular materials. In this work, we describe the synthesis of a new kind of actinide-organic polyrotaxane involving crown ether macrocycles for the first time. Crystal structures of two uranyl polyrotaxane compounds, UCER-1 and UCER-2, as well as a third non-polyrotaxane compound UON-1, have been determined, and the formation mechanism of actinide-organic polyrotaxanes is discussed in terms of factors affecting the assembly process. A comparison of the reaction conditions and the corresponding outcomes suggests the necessity of a host-guest pseudorotaxane linker for the successful construction of targeted actinide-organic polyrotaxanes. The coordination of different coordination atoms, especially bromide ions and uranyl ions, is analyzed in detail through theoretical calculations. As an extension of cucurbituril-based uranyl-organic polyrotaxanes, the introduction of crown ether macrocycles as a new supramolecular element brings a different kind of actinide polyrotaxane with intriguing molecular structures and supramolecular assembly behaviours, and will, we believe, expand the research scope of actinide rotaxane coordination polymers
A phosphate semiconductor-induced built-in electric field boosts electron enrichment for electrocatalytic hydrogen evolution in alkaline conditions
No full textAt the semiconductor and metal interface, a built-in electric field leading to electron enrichment can be applied in developing efficient nano-hybrid catalysts because the induced electron-rich and electron-poor counterparts can synergistically modulate the active sites and elementary reaction steps. To overcome the extra difficulty in alkaline water dissociation during the production of green hydrogen, it is expected that such a built-in electric field can be constructed to boost interfacial electron enrichment to increase the water dissociation and hydrogen evolution kinetics. Herein, an n-type BiPO4 semiconductor is integrated with metallic Ru clusters (Ru/BiPO4) to produce an intrinsically built-in electric field, which causes electron enrichment via unidirectional electron transfer from BiPO4 to Ru. The resultant Ru/BiPO4 nanocomposite demonstrates superior water splitting activity toward electrocatalytic hydrogen evolution to Ru/C without electron enrichment in alkaline solution, and even exhibits nine-fold mass activity of commercial Pt/C in a harsher medium (3 M KOH). DFT calculation demonstrates that the positively charged BiPO4 matrix significantly decreases the energy barrier of water dissociation, while the negatively charged Ru clusters with more active electronic states optimize the proton adsorption and combination kinetics. The Ru layer in close contact with the phosphate matrix accepts the greatest number of electrons and shows the optimal Delta G(H*). This work sheds new light on the advantage of the physical effect for designing advanced electrocatalysts for energy conversion and storage
Hollow Mesoporous Nanoreactors with Encaged PtSn Alloy Nanoparticles for Selective Hydrogenation of Furfural to Furfuryl Alcohol
No full textHollow mesoporous nanoreactors with encaged functional nanoparticles are promising heterogeneous catalysts due to the advantages related to their hollow cavities. In this study, we employ metal ion-bound polymer micelles to synthesize PtSn alloy nanoparticle-encaged hollow mesoporous nanoreactors (PtSn@HMSNs), which contain similar to 4 nm PtSn alloy NPs located in similar to 13 nm hollow cavities and relatively large (similar to 9 nm) mesoporous channels in silica shells. Relative to monometallic Pt@HMSNs and supported Pt1Sn0.3/SiO2, Pt1Sn0.3@HMSNs exhibit greatly enhanced activity and selectivity for hydrogenation of furfural to furfuryl alcohol. At 1.0 MPa H-2, 100 degrees C, and a furfural/Pt molar ratio of 1884:1, 97.5% of furfuryl alcohol yield was achieved in 5.0 h. The dramatically promoted catalytic performance of Pt1Sn0.3@HMSNs can be assigned to the confinement effect that the location of active NPs inside hollow cavities increases the collision rates between reactants and active NPs to promote catalytic activity, as well as the synergistic effect between Pt and Sn
Negative thermal expansion in the Ruddlesden-Popper calcium titanates
No full textMaterials exhibiting negative thermal expansion (NTE) are important for the fabrication and operation of microelectronic devices and optical systems. As an important group of Ruddlesden-Popper (RP) perovskites, calcium titanates Can+1TinO3n+1 [(CTO), n = 1, 2, ..., infinity] have layered structures and may exhibit quasi-two-dimensional (quasi-2D) NTE within their three-dimensional structural architectures. In this paper, combining density-functional-theory calculations and the self-consistent quasiharmonic approximation method, we investigate the variation of the quasi-2D character of the phonon spectra and thermal expansion in the Can+1TinO3n+1 family (n = 1-3, and infinity) with respect to n. We find that a quasi-2D NTE mechanism is active in the RP-CTOs at n of 1-3, whereas a quasi-rigid-unit mode mechanism is active at n = infinity (i.e., the perovskite phase). We find a NTE trend with layer number for the orthorhombic materials comprising the RP series, but the monoclinic polymorph is an outlier. For the orthorhombic members, we find the critical pressure for NTE increases with increasing n, but the NTE critical temperature decreases (when materials are compared at the same pressure). Additionally, the elastic moduli can be used as effective descriptors for this layer-dependent behavior of the NTE, i.e., the stiffer the RP-CTO then the lower its NTE. We also propose the integrated NTE capacity to capture the correlation between the quasi-2D NTE and n, and it monotonically decreases with increasing n
Tuning the magnetic properties of Fe3GeTe2 by doping with 3d transition-metals
No full textDoping with alien transition-metal atoms is an effective method to tune the magnetic properties of two-dimensional magnetic material Fe3GeTe2. Using first principles calculation, we investigated the magnetic properties of Fe3GeTe2 doped with 3d atoms such as V, Cr, Mn, etc. Calculation results indicate that site I is more energetically favorable than site II for all 3d atoms, except for Co-doped Fe3GeTe2. Doping of these 3d atoms leads to significant variations of the average magnetic moment of Fe-I and Fe-II atoms. The magnetic moment of Fe-I decreased substantially for all 3d atom-doped-Fe3GeTe2 systems, except for the Co-doped system. Compared to Fe-I, the magnetic moment of Fe-II is more sensitive to doping with 3d-atoms. The observed variations of the magnetic moment can be explained by the electron-transfer of the doped atoms. These results can aid future experimental studies and provide valuable information for the development of Fe3GeTe2-based spin electronic devices. (C) 2021 Elsevier B.V. All rights reserved
In Situ Anchoring Polymetallic Phosphide Nanoparticles within Porous Prussian Blue Analogue Nanocages for Boosting Oxygen Evolution Catalysis
No full textThe controllable synthesis of metal-based nano-clusters for heterogeneous catalytic reactions has received considerable attention. Nevertheless, manufacturing these architectures, while avoiding aggregation and retaining surface activity, remains challenging. Herein, for the first time we designed NiCoFe-Prussian blue analogue (PBA) nanocages as a support for in situ dispersion and anchoring of polymetallic phosphide nanoparticles (pMP-NPs). Benefiting from the porous surfaces and the synergistic effects between pMP-NPs and the cyano groups in PBA, the NiCoFe-P-NP@ NiCoFe-PBA nanocages exhibit a significantly enhanced catalytic activity for oxygen evolution reaction (OER) with an overpotential of 223 mV at 10 mA cm(-2) and a Tafel slope of 78 mV dec(-1), outperforming the NiCoFe-PBA nanocubes, NiCoFe-P nanocages, NiFe-P-NP@NiFe-PBA nanocubes, and CoFe-P-NP@CoFe-PBA nanoboxes. This work not only offers the synthesis strategy of in situ anchoring pMP-NPs on PBA nanocages but also provides a new insight into optimized Gibbs free energy of OER by regulating electron transfer from metallic phosphides to PBA substrate
Recent Advances in Transition Metal Nitride-Based Materials for Photocatalytic Applications
No full textPhotocatalysis is a promising and convenient strategy to convert solar energy into chemical energy for various fields. However, photocatalysis still suffers from low solar energy conversion efficiency. Developing state of the art photocatalysts with high efficiency and low cost is a huge challenge. Transition metal nitrides (TMNs) as a class of metallic interstitial compounds have attracted significant attention in photocatalytic applications. In fact, TMNs exhibit multifunctional properties in various photocatalytic systems. This review is the first attempt that summarizes recent research on TMNs-based materials in various photocatalytic applications. Different roles of TMNs materials in photocatalytic systems including semiconductor active components, co-catalysts, inter-band excitation, and surface plasmon resonance components are systematically discussed and summarized. The fundamentals, latest progress, and emerging opportunities for further improving the performances of TMNs-based materials for photocatalysis are also discussed. Finally, some challenges facing TMNs, and perspectives on their future that are relevant for furthering research in the area of photocatalysis are also proposed
C/SiC composite coatings with ultra-high thermal emissivity
No full textC/SiC gradient composite coatings with ultra-high emissivity have been deposited at 900 degrees C on 316L stainless steel (SS) substrates using electron beam-physical vapour deposition (EB-PVD) by directly evaporating the raw powder of alpha-Sick The structural properties of the coating were investigated in detail. The results showed that the coating was mainly composed of C and nanocrystalline SiC. Its surface appeared to be dense and rough. Moreover, the thickness of the coating was about 0.7 mu m, and good adhesional properties (no delamination) were found at the interface between the coating and the steel substrate. In addition, the thermal emissivity of the coating was analysed by FTIR spectroscopy. The result demonstrated that the coating had an ultra-high thermal emissivity