Ultrasmall Nanocrystals Research Articles

In-situ confinement of ultrasmall SnO2 nanocrystals into redox-active polyimides for high‐rate and long-cycling anode materials

SnO2 as a promising anode material possesses a high theoretical capacity and a safe lithiation potential; however, it suffers from low cycle stability and poor rate capability due to its nano-aggregation effect, structural instability, and sluggish electrode kinetics. Herein a one-pot hydrothermal process was developed to craft ultrasmall SnO2 nanocrystals uniformly encapsulated in the polyimide matrix through a facile in-situ polymerization. The polyimide phase can suppress the over growth of SnO2 nanoparticles while preventing from their re-aggregation through the space-confined effect during the material synthesis. The robust polyimide matrix can be also served as a protective layer to relieve the volume change of SnO2 nanoparticles and inhibit the cracks propagation throughout the whole electrode during the charge/discharge processes. Moreover, the carbonyl-contained polyimide is intrinsically redox-active, capable of affording acceptable ionic conduction ability. Remarkably, the as-fabricated SnO2@PI composite anode exhibits a reversible capacity as high as 897 mAh g−1 at 0.1 A g−1 and a high rate capability (479 mAh g−1 at 2.0 A g−1) with a higher retention (53%) than pure SnO2 (39%). The SnO2@PI anode also retains a capacity of 653 mAh g−1 at 0.5 A g−1 after 150 cycles. This work would offer an alternative strategy by incorporating metal oxide nanoparticles into the redox-active polymer matrix for advanced anode materials with high rate capability and long-term cyclability.

A facile construction of heterostructured ZnO/Co3O4 mesoporous spheres and superior acetone sensing performance

The rapid development of internet and internet of things brings new opportunities for the expansion of intelligent sensors, and acetone as a major disease detection indicator (i.e., diabetes) making it become extremely important clinical indicator. Herein, uniform mesoporous ZnO spheres were successfully synthesized via novel formaldehyde-assisted metal-ligand crosslinking strategy. In order to adjust the pore structure of mesoporous ZnO, various mesoporous ZnO spheres were synthesized by changing weight percentage of Zn(NO3)2·6H2O to tannic acid (TA). Moreover, highly active heterojunction mesoporous ZnO/Co3O4 has been fabricated based on as-prepared ultra-small Co3O4 nanocrystals (ca. 3 nm) and mesoporous ZnO spheres by flexible impregnation technique. Profit from nano-size effect and synergistic effect of p-n heterojunction, mesoporous ZnO/Co3O4 exhibited excellent acetone sensing performance with high selectivity, superior sensitivity and responsiveness. Typically, 5 wt% Co3O4 embedded mesoporous ZnO sphere showed prominent acetone response (ca. 46 for 50 ppm), which was about 11.5 times higher than that in pure ZnO sensing device, and it was also endowed high cyclic stability. The nanocrystals embedded hybrid material is expected to be used as promising efficient material in the field of catalysis and gas sensing.

Phonon density of states in lanthanide-based nanocrystals

We report a combined inelastic neutron and X-ray scattering study of the phonon density of states of the nano- and microcrystalline lanthanide-based materials NaY$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$ and NaGd$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$. While large (20 nm) nanocrystals display the same vibrational spectra as their microcrystalline counterparts, we find an enhanced phonon density of states at low energies, $E \leq 15\,\rm{meV}$, in ultra-small (5 nm) NaGd$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$ nanocrystals which we assign to an increased relative spectral weight of surface phonon modes. Based on our observations for ultra-small nanocrystals, we rationalize that an increase of the phonon density of states in large nanocrystals due to surface phonons is too small to be observed in the current measurements. The experimental approach described in this report constitutes the first step toward the rationalization of size effects on the modification of the absolute upconversion quantum yield of upconverting nanocrystals.

Trace detection based on quantum dots embedded in the biocarbon with multi-scale pores

The design of sensitive nanostructure for trace detection is crucial to monitoring analysis for daily life. Herein, we develop an electrochemical sensor with high sensitivity and reliability for efficient detection of minute melamine. The hierarchically porous biocarbon is obtained from hollyhock stem and strewn with CeO2-Mn3O4 quantum dots. The unique biocarbon skeleton and ultra-small nanocrystals show a huge surface area, low impedance and abundant macro-meso-micro porous structure, which lead to a highly active electrochemical performance. In cyclic voltammetric and differential pulse voltammetric test for detecting melamine molecule, the modified electrode decorated with proposed materials exhibits approximately four times more sensitive than the bare electrode in a wide range of concentration (0.12 μM–0.85 mM), and in a low detection limit of 3.66 × 10−12 mol·L−1. We expect this electrochemical probe to provide accurate information for quantitative trace measurement in environmental and biological analysis.

Bi-Sb Nanocrystals Embedded in Phosphorus as High-Performance Potassium Ion Battery Electrodes.

The development of high-performance potassium ion battery (KIB) electrodes requires a nanoengineering design aimed at optimizing the construction of active material/buffer material nanocomposites. These nanocomposites will alleviate the stress resulting from large volume changes induced by K+ ion insertion/extraction and enhance the electrical and ion conductivity. We report the synthesis of phosphorus-embedded ultrasmall bismuth-antimony nanocrystals (BixSb1-x@P, (0 ≤ x ≤ 1)) for KIB anodes via a facile solution precipitation at room temperature. BixSb1-x@P nanocomposites can enhance potassiation-depotassiation reactions with K+ ions, owing to several attributes. First, by adjusting the feed ratios of the Bi/Sb reactants, the composition of BixSb1-x nanocrystals can be systematically tuned for the best KIB anode performance. Second, extremely small (diameter ≈ 3 nm) BixSb1-x nanocrystals were obtained after cycling and were fixed firmly inside the P matrix. These nanocrystals were effective in buffering the large volume change and preventing the collapse of the electrode. Third, the P matrix served as a good medium for both electron and K+ ion transport to enable rapid charge and discharge processes. Fourth, thin and stable solid electrolyte interface (SEI) layers that formed on the surface of the cycled BixSb1-x@P electrodes resulted in low resistance of the overall battery electrode. Lastly, in situ X-ray diffraction analysis of K+ ion insertion/extraction into/from the BixSb1-x@P electrodes revealed that the potassium storage mechanism involves a simple, direct, and reversible reaction pathway: (Bi, Sb) ↔ K(Bi, Sb) ↔ K3(Bi, Sb). Therefore, electrodes with the optimized composition, i.e., Bi0.5Sb0.5@P, exhibited excellent electrochemical performance (in terms of specific capacity, rate capacities, and cycling stability) as KIB anodes. Bi0.5Sb0.5@P anodes retained specific capacities of 295.4 mA h g-1 at 500 mA g-1 and 339.1 mA h g-1 at 1 A g-1 after 800 and 550 cycles, respectively. Furthermore, a capacity of 258.5 mA h g-1 even at 6.5 A g-1 revealed the outstanding rate capability of the Sb-based KIB anodes. Proof-of-concept KIBs utilizing Bi0.5Sb0.5@P as an anode and PTCDA (perylenetetracarboxylic dianhydride) as a cathode were used to demonstrate the applicability of Bi0.5Sb0.5@P electrodes to full cells. This study shows that BixSb1-x@P nanocomposites are promising carbon-free anode materials for KIB anodes and are readily compatible with the commercial slurry-coating process applied in the battery manufacturing industry.

Electrochemically exfoliated graphene as high-performance catalyst support to promote electrocatalytic oxidation of methanol on Pt catalysts

Electrochemically exfoliated graphene (EEG) is a kind of high-quality graphene with few oxygen-containing functional groups and defects on the surface, and thereby is more suitable as catalyst support than other carbon materials such as extensively used reduced graphene oxide (rGO). However, it is difficult to grow functional materials on EEG due to its inert surface. In this work, ultra-small Pt nanocrystals (∼2.6 nm) are successfully formed on EEG and show better electrocatalytic activity towards methanol oxidation than Pt catalysts on rGO. The outstanding catalytic properties of Pt catalysts on EEG can be attributed to the fast electron transfer through EEG and high quality of Pt catalysts such as small grain size, high dispersibility and low oxidation ratio. In addition, SnO2 nanocrystals are controllably generated around Pt catalysts on EEG to raise the poison tolerance of Pt catalysts through using glycine as a linker. Owing to its outstanding properties such as high electrical conductivity and mechanical strength, EEG is expected to be widely used as a novel support for catalysts.

Synergetic enhancement of surface reactions and charge separation over holey C3N4/TiO2 2D heterojunctions

Efficient charge separation and rapid interfacial reaction kinetics are crucial factors that determine the efficiency of photocatalytic hydrogen evolution. Herein, a fascinating 2D heterojunction photocatalyst with superior photocatalytic hydrogen evolution performance – holey C3N4 nanosheets nested with TiO2 nanocrystals (denoted as HCN/TiO2) – is designed and fabricated via an in situ exfoliation and conversion strategy. The HCN/TiO2 is found to exhibit an ultrathin 2D heteroarchitecture with intimate interfacial contact, highly porous structures and ultrasmall TiO2 nanocrystals, leading to drastically improved charge carrier separation, maximized active sites and the promotion of mass transport for photocatalysis. Consequently, the HCN/TiO2 delivers an impressive hydrogen production rate of 282.3 μmol h−1 per 10 mg under AM 1.5 illumination and an apparent quantum efficiency of 13.4% at a wavelength of 420 nm due to the synergetic enhancement of surface reactions and charge separation. The present work provides a promising strategy for developing high-performance 2D heterojunctions for clean energy applications.

Scalable neutral H2O2 electrosynthesis by platinum diphosphide nanocrystals by regulating oxygen reduction reaction pathways

Despite progress in small scale electrocatalytic production of hydrogen peroxide (H2O2) using a rotating ring-disk electrode, further work is needed to develop a non-toxic, selective, and stable O2-to-H2O2 electrocatalyst for realizing continuous on-site production of neutral hydrogen peroxide. We report ultrasmall and monodisperse colloidal PtP2 nanocrystals that achieve H2O2 production at near zero-overpotential with near unity H2O2 selectivity at 0.27 V vs. RHE. Density functional theory calculations indicate that P promotes hydrogenation of OOH* to H2O2 by weakening the Pt-OOH* bond and suppressing the dissociative OOH* to O* pathway. Atomic layer deposition of Al2O3 prevents NC aggregation and enables application in a polymer electrolyte membrane fuel cell (PEMFC) with a maximum r(H2O2) of 2.26 mmol h−1 cm−2 and a current efficiency of 78.8% even at a high current density of 150 mA cm−2. Catalyst stability enables an accumulated neutral H2O2 concentration in 600 mL of 3.0 wt% (pH = 6.6).

Ultrasmall SnO2 nanocrystals with adjustable density embedded in N-doped hollow mesoporous carbon spheres as anode for Li+/Na+ batteries

SnO2 has been widely studied in lithium-ion batteries (LIBs) because of its high theoretical specific capacity and reversible alloying reaction. Herein, we reported a novel strategy of confinement growth to implant nano-sized SnO2 crystals into N-doped hollow mesoporous carbon spheres (NHMCS) to form SnO2@NHMCS composite with unique nanoscale voids. The nanocrystals (~ 5 nm) decrease the required activation energy for redox reactions, and the carbon shell of NHMCS improves the conductivity and structural stability. It is worth noting that this method can effectively control the filling degree of ultrasmall nanocrystals in NHMCS and adjust the nanosize of voids between nanocrystals and NHMCS. When SnO2@NHMCS is evaluated as an anode material for LIBs, it is proved to exhibit high reversible capacity and stable cycling performance, which is attributted to the appropriate content of active components and the ample buffer space for conversion reaction of SnO2 and alloying reaction of Sn. It also shows excellent electrochemical property as anode material for sodium-ion batteries.

Site- and Spatial-Selective Integration of Non-noble Metal Ions into Quantum Dots for Robust Hydrogen Photogeneration

Summary Artificial photosystems consisting of semiconductor quantum dots (QDs) and non-noble metal ions (e.g., Fe2+, Co2+, Ni2+) show intrinsic advantages in efficient and cost-effective hydrogen (H2) evolution. However, well-controlled integration of these metal ions into ultra-small QDs is a major challenge. Here, we present a two-step strategy to realize site- and spatial-selective integration of earth-abundant Fe2+ ions in CdSe QDs; i.e., CdSe QDs are modified with a thin and anisotropic ZnS shell, and partial Zn2+ ions are selectively substituted by Fe2+ via cation exchange. The anisotropic ZnS layer not only passivates CdSe core but also works as an ideal medium to intimately anchor Fe2+. The multifunctional CdSe/Zn1−xFexS QD exhibits extraordinary activity and stability toward H2 photogeneration. Specifically, more than 880 mL (∼39,300 μmol) H2 gas is produced from 6 mL of aqueous solution during a consecutive 172 h (>1 week) experiment, thus achieving a turnover number of >600,000 per QD.

Large-scale Ni-MOF derived Ni3S2 nanocrystals embedded in N-doped porous carbon nanoparticles for high-rate Na+ storage

Metal organic framework (MOF) has been confirmed as the promising precursor to develop the conversion-typed anode materials of sodium-ion batteries (SIBs) because of the tunable structure design and simple functional modification. Here, we prepare the ultrasmall Ni3S2 nanocrystals embedded into N-doped porous carbon nanoparticles using the scalable Ni-MOF as precursor (denoted as Ni3S2@NPC). The ultrasmall size of Ni3S2 can work for accelerated electron/ion transfer to facilitate the electrochemical reaction kinetics. Moreover, the robust conductivity network originated from N-doped porous carbon nanoparticles can not only improve the electron conductivity, but also enhance the electrode integrity and stability of the electrode/electrolyte interface. In addition, the N heteroatoms provide extra Na storage sites. Accordingly, the electrode delivers the obviously competitive capacities and high-power output with respect to the currently reported Ni3S2/C composites. This study provides a scalable and universal strategy to develop the advanced transition metal sulfides for practically feasible SIBs.

SYNTHESIS OF ULTRASMALL CD0.3ZN0.7SENANOCRYSTALS VIA MECHANOCHEMICAL ROUTE

Mechanical alloying through severe plastic deformation (SPD) was used to synthesize ultrasmall nanocrystals of (Cd0.3Zn0.7Se)with a diameter (d = 1.3 nm) after 20 hours milling time. A high-resolution electron microscope was (HRTEM) used to examine the mechanical deformation on the nanocrystals. The images reveal a bulk-like cluster structure without a distinctive regular shape. The atoms appear to be located entirely at the surface. The energy band-gap of the ultrasmall was determined to be 4.56 eV from UV-Vis absorption spectra.

Direct White Light Emission from Ultrasmall Europium Nanocrystals

Direct white light emission from ultrasmall europium nanocrystals (WLEuNCs) is reported in the present study. The one-pot synthesis of WLEuNCs was done using thiourea, oleyl amine (OlAm), oleyl alc...

Size-Selective ZnO Nanocrystals as Spectral Energy Converters for Use in Advanced Display Devices.

In this paper, we report the synthesis of size-selective zinc oxide (ZnO) nanocrystals by three different techniques namely, auto-combustion, co-precipitation and atomizer and their effective energy conversion from harmful ultra-violet (UV, 365 nm) to visible green light. All the three methods are facile and convenient for the controlled synthesis of ZnO nanocrystals with an average crystallite size ranging 12-35 nm. The structural characterization using XRD revealed that the phase of ZnO nanocrystals was wurtzite. The morphology of the ZnO nanocrystals in all the three cases was found to be spherical and uniform as determined by High Resolution Transmission Electron Microscope. The novelty of the current work is to synthesize highly luminescent ultra-small ZnO nanocrystals without any dopant and their possible use as efficient spectral converters in displays. This result is further supported by steady-state photoluminescence spectra recorded under UV (365 nm) radiation as excitation. The presence of quantum confinement effect and increased surface to volume ratio perfectly established that ZnO nanocrystals synthesized by various routes have significant impact in the fields of optoelectronics, biomedical fields and advanced active display devices.

SnO2 nano-crystals anchored on N-doped porous carbon with enhanced lithium storage properties

Sn-based oxides have been considered as promising anode candidates for lithium-ion batteries (LIBs), but their commercialization is always impeded by severe volume expansion and unsatisfactory electron conductivity. In order to overcome these disadvantages, a novel SnO2/C nanocomposites are devised and fabricated via a 3D-network composite-gel precursors derived from macromolecule polyacrylamide, in which ultrasmall SnO2 nanocrystals (<10 nm) are uniformly confined in the N-doped porous carbon skeleton matrix. To generate suitable carbon coating and inherit the N-doped porous framework, mild heat treatment at 300℃ is used to make the carbonization process moderate, which isconduciveto the formation of ultrafine SnO2 and further strengthen skeleton structure. Such a stable N-doped porous matrix allows the strengths, not only can the integrity of electrode and SnO2 nanoparticles be well maintained, but also the electronic conductivity and the quantity of active sites can be effectively promoted. As a result, this novel microstructure availably gives a reversible capability of 1011.2 mAh/g at 0.2A/g after 160 cycles and delivers a remarkable high-rate lithium storage of 357.8 mAh/g even at 12.8 A/g. Presumably, these findings may present a tactic to provide reasonable guidance for the design of metal oxide-based energy storage functional materials with huge volume variation.

Room-Temperature Defect Qubits in Ultrasmall Nanocrystals.

There is an urgent quest for room-temperature qubits in nanometer-sized, ultrasmall nanocrystals for quantum biosensing, hyperpolarization of biomolecules, and quantum information processing. Thus far, the preparation of such qubits at the nanoscale has remained futile. Here, we present a synthesis method that avoids any interaction of the solid with high-energy particles and uses self-propagated high-temperature synthesis with a subsequent electrochemical method, the no-photon exciton generation chemistry to produce room-temperature qubits in ultrasmall nanocrystals of sizes down to 3 nm with high yield. We first create the host silicon carbide (SiC) crystallites by high-temperature synthesis and then apply wet chemical etching, which results in ultrasmall SiC nanocrystals and facilitates the creation of thermally stable defect qubits in the material. We demonstrate room-temperature optically detected magnetic resonance signal of divacancy qubits with 3.5% contrast from these nanoparticles with emission wavelengths falling in the second biological window (1000–1380 nm). These results constitute the formation of nonperturbative bioagents for quantum sensing and efficient hyperpolarization.

Resistive switching memory performance in oxide hetero-nanocrystals with well-controlled interfaces

ABSTRACT For realization of new informative systems, the memristor working like synapse has drawn much attention. We developed isolated high-density Fe3O4 nanocrystals on Ge nuclei/Si with uniform and high resistive switching performance using low-temperature growth. The Fe3O4 nanocrystals on Ge nuclei had a well-controlled interface (Fe3O4/GeOx/Ge) composed of high-crystallinity Fe3O4 and high-quality GeOx layers. The nanocrystals showed uniform resistive switching characteristics (high switching probability of ~90%) and relatively high Off/On resistance ratio (~58). The high-quality interface enables electric field application to Fe3O4 and GeOx near the interface, which leads to effective positively charged oxygen vacancy movement, resulting in high-performance resistive switching. Furthermore, we successfully observed memory effect in nanocrystals with well-controlled interface. The experimental confirmation of the memory effect existence even in ultrasmall nanocrystals is significant for realizing non-volatile nanocrystal memory leading to neuromorphic devices.

From coordination polymers to nanocrystals: general and facile synthesis of ultra-small metal oxide nanocrystals.

A facile and general strategy for the large-scale synthesis of ultrasmall metal oxide nanocrystals with particle size of about 2 nm was proposed using coordination polymers (CPs) as templates.

Exploring the surface-to-volume ratio in ultrasmall nanocrystals using the optical probe of Eu3+ ion.

We demonstrate fine control of the nanocrystal size of ultrasmall Eu3+-doped Sc2O3 nanocrystals within an extremely small nanometer scale from 2.6 to 9.7 nm, thereby enabling us to thoroughly investigate the size-dependent surface-to-volume ratio in these ultrasmall NCs using an optical probe of the red-emitting Eu3+ ion for the first time.

Nanoscopically and uniformly distributed SnO2@TiO2/C composite with highly mesoporous structure and bichemical bonds for enhanced lithium ion storage performances

A superior nanoarchitecture with vast phase boundaries interconnected via chemical bonds between carbon and ultrasmall nanocrystals shows enhanced Li+ storage performances.