Effect of Morphology on Ultrafast Carrier Dynamics in Asymmetric Gold–Iron Oxide Plasmonic Heterodimers

Understanding how nanoscale interfaces affect electrical and optical properties of multifunctional nanocrystal heterostructures is of paramount importance for their technological application. In this context, we investigated the ultrafast carrier dynamics of rodlike gold–iron oxide nanocrystal heterodimers, in a spectral region close to the surface plasmon resonance frequency, by means of broad-band transient absorption spectroscopy. We found that the electron–phonon relaxation time is independent of the morphology of the iron oxide domain. Moreover, we revealed a transient shift in the surface plasmon resonance frequency, which can be related to charge transfer at the interface between gold and iron oxide

Plasmon Bleaching Dynamics in Colloidal Gold–Iron Oxide Nanocrystal Heterodimers

Colloidal nanocrystal heterodimers composed of a plasmonic and a magnetic domain have been widely studied as potential materials for various applications in nanomedicine, biology, and photocatalysis. One of the most popular nanocrystal heterodimers is represented by a structure made of a Au domain and a iron oxide domain joined together. Understanding the nature of the interface between the two domains in such type of dimer and how this influences the energy relaxation processes is a key issue. Here, we present the first broad-band transient absorption study on gold/iron oxide nanocrystal heterodimers that explains how the energy relaxation is affected by the presence of such interface. We found faster electron–electron and electron–phonon relaxation times for the gold “nested” in the iron oxide domain in the heterodimers with respect to gold “only” nanocrystals, that is, free-standing gold nanocrystals in solution. We relate this effect to the decreased electron screening caused by spill-out of the gold electron distribution at gold/iron oxide interface

Redox Centers Evolution in Phospho-Olivine Type (LiFe_0.5Mn_0.5 PO₄) Nanoplatelets with Uniform Cation Distribution

In phospho-olivine type structures with mixed cations (LiM1M2PO₄), the octahedral M1<i> </i>and M2 sites that dictate the degree of intersites order/disorder play a key role in determining their electrochemical redox potentials. In the case of LiFe_<i>x</i>Mn_1–<i>x</i>PO₄, for example, in micrometer-sized particles synthesized via hydrothermal route, two separate redox centers corresponding to Fe²⁺/Fe³⁺ (3.5 V vs Li/Li⁺) and Mn²⁺/Mn³⁺ (4.1 V vs Li/Li⁺), due to the collective Mn–O–Fe interactions in the olivine lattice, are commonly observed in the electrochemical measurements. These two redox processes are directly reflected as two distinct peak potentials in cyclic voltammetry (CV) and equivalently as two voltage plateaus in their standard charge/discharge characteristics (in Li ion batteries). On the contrary, we observed a single broad peak in CV from LiFe_0.5Mn_0.5PO₄ platelet-shaped (∼10 nm thick) nanocrystals that we are reporting in this work. Structural and compositional analysis showed that in these nanoplatelets the cations (Fe, Mn) are rather homogeneously distributed in the lattice, which is apparently the reason for a synergetic effect on the redox potentials, in contrast to LiFe_0.5Mn_0.5PO₄ samples obtained via hydrothermal routes. After a typical carbon-coating process in a reducing atmosphere (Ar/H₂), these LiFe_0.5Mn_0.5PO₄ nanoplatelets undergo a rearrangement of their cations into Mn-rich and Fe-rich domains. Only after such cation rearrangement (via segregation) in the nanocrystals, the redox processes evolved at two distinct potentials, corresponding to the standard Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox centers. Our experimental findings provide new insight into mixed-cation olivine structures in which the degree of cations mixing in the olivine lattice directly influences the redox potentials, which in turn determine their charge/discharge characteristics

Alloyed Copper Chalcogenide Nanoplatelets <i>via</i> Partial Cation Exchange Reactions

We report the synthesis of alloyed quaternary and quinary nanocrystals based on copper chalcogenides, namely, copper zinc selenide–sulfide (CZSeS), copper tin selenide–sulfide (CTSeS), and copper zinc tin selenide–sulfide (CZTSeS) nanoplatelets (NPLs) (∼20 nm wide) with tunable chemical composition. Our synthesis scheme consisted of two facile steps: <i>i.e.</i>, the preparation of copper selenide–sulfide (Cu_2–<i>x</i>Se_<i>y</i>S_1–<i>y</i>) platelet shaped nanocrystals <i>via</i> the colloidal route, followed by an <i>in situ</i> cation exchange reaction. During the latter step, the cation exchange proceeded through a partial replacement of copper ions by zinc or/and tin cations, yielding homogeneously alloyed nanocrystals with platelet shape. Overall, the chemical composition of the alloyed nanocrystals can easily be controlled by the amount of precursors that contain cations of interest (<i>e.g.</i>, Zn, Sn) to be incorporated/alloyed. We have also optimized the reaction conditions that allow a complete preservation of the size, morphology, and crystal structure as that of the starting Cu_2–<i>x</i>Se_<i>y</i>S_1–<i>y</i> NPLs. The alloyed NPLs were characterized by optical spectroscopy (UV–vis–NIR) and cyclic voltammetry (CV), which demonstrated tunability of their light absorption characteristics as well as their electrochemical band gaps

Colloidal Synthesis of Cuprite (Cu₂O) Octahedral Nanocrystals and Their Electrochemical Lithiation

We report a facile colloidal route to prepare octahedral-shaped cuprite (Cu₂O) nanocrystals (NCs) of ∼40 nm in size that exploits a new reduction pathway, i.e., the controlled reduction of a cupric ion by acetylacetonate directly to cuprite. Detailed structural, morphological, and chemical analyses were carried on the cuprite NCs. We also tested their electrochemical lithiation, using a combination of techniques (cyclic voltammetry, galvanostatic, and impedance spectroscopy), in view of their potential application as anodes for Li ion batteries. Along with these characterizations, the morphological, structural, and chemical analyses (via high-resolution electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy) of the cycled Cu₂O NCs (in the lithiated stage, after ∼50 cycles) demonstrate their partial conversion upon cycling. At this stage, most of the NCs had lost their octahedral shape and had evolved into multidomain particles and were eventually fragmented. Overall, the shape changes (upon cycling) did not appear to be concerted for all the NCs in the sample, suggesting that different subsets of NCs were characterized by different lithiation kinetics. We emphasize that a profound understanding of the lithiation reaction with NCs defined by a specific crystal habit is still essential to optimize nanoscale conversion reactions

Size-Tunable, Hexagonal Plate-like Cu₃P and Janus-like Cu–Cu₃P Nanocrystals

We describe two synthesis approaches to colloidal Cu₃P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu₃P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu₃P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu₃P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu₃P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu₃P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu₃P particles do not display an electrochemical behavior that would make them suitable for applications in batteries

Size-Tunable, Hexagonal Plate-like Cu₃P and Janus-like Cu–Cu₃P Nanocrystals

We describe two synthesis approaches to colloidal Cu₃P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu₃P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu₃P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu₃P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu₃P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu₃P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu₃P particles do not display an electrochemical behavior that would make them suitable for applications in batteries

Size-Tunable, Hexagonal Plate-like Cu₃P and Janus-like Cu–Cu₃P Nanocrystals

We describe two synthesis approaches to colloidal Cu₃P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu₃P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu₃P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu₃P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu₃P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu₃P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu₃P particles do not display an electrochemical behavior that would make them suitable for applications in batteries

From Binary Cu₂S to Ternary Cu–In–S and Quaternary Cu–In–Zn–S Nanocrystals with Tunable Composition <i>via</i> Partial Cation Exchange

We present an approach for the synthesis of ternary copper indium sulfide (CIS) and quaternary copper indium zinc sulfide (CIZS) nanocrystals (NCs) by means of partial cation exchange with In³⁺ and Zn²⁺. The approach consists of a sequential three-step synthesis: first, binary Cu₂S NCs were synthesized, followed by the homogeneous incorporation of In³⁺ by an <i>in situ</i> partial cation-exchange reaction, leading to CIS NCs. In the last step, a second partial exchange was performed where Zn²⁺ partially replaced the Cu⁺ and In³⁺ cations at the surface, creating a ZnS-rich shell with the preservation of the size and shape. By careful tuning reaction parameters (growth and exchange times as well as the initial Cu⁺:In³⁺:Zn²⁺ ratios), control over both the size and composition was achieved. This led to a broad tuning of photoluminescence of the final CIZS NCs, ranging from 880 to 1030 nm without altering the NCs size. Cytotoxicity tests confirmed the biocompatibility of the synthesized CIZS NCs, which opens up opportunities for their application as near-infrared fluorescent markers in the biomedical field

Lightweight Carbon–Metal-Based Fabric Anode for Lithium-Ion Batteries

Lithium-ion battery electrodes are typically manufactured via slurry casting, which involves mixing active material particles, conductive carbon, and a polymeric binder in a solvent, followed by casting and drying the coating on current collectors (Al or Cu). These electrodes are functional but still limited in terms of pore network percolation, electronic connectivity, and mechanical stability, leading to poor electron/ion conductivities and mechanical integrity upon cycling, which result in battery degradation. To address this, we fabricate trichome-like carbon–iron fabrics via a combination of electrospinning and pyrolysis. Compared with slurry cast Fe2O3 and graphite-based electrodes, the carbon–iron fabric (CMF) electrode provides enhanced high-rate capacity (10C and above) and stability, for both half cell and full cell testing (the latter with a standard lithium nickel manganese oxide (LNMO) cathode). Further, the CMFs are free-standing and lightweight; therefore, future investigation may include scaling this as an anode material for pouch cells and 18,650 cylindrical batteries