Kai-Yang Niu

One-step synthesis of fluorescent carbon nanoparticles by laser irradiation

Fluorescent carbon nanoparticles (CNPs) were synthesized by laser irradiation of a suspension of carbon powders in organic solvent. The surface modification on the CNPs was fulfilled simultaneously with the formation of the CNPs, and tunable light emission could be generated by selecting appropriate solvents. The origin of the luminescence was attributed to carboxylate ligands on the surface of the CNPs.

Morphology Control of Nanostructures via Surface Reaction of Metal Nanodroplets

We report on the controllable synthesis of diverse nanostructures using laser ablation of a metal target in a liquid medium. The nanodroplets generated by laser ablation react with the liquid and produce various nanostructures, such as hollow nanoparticles, core-shell nanoparticles, heterostructures, nanocubes, and ordered arrays. A millisecond laser with low power density is essential for obtaining such metal nanodroplets, while the target material, the reactivity of liquid medium, and the laser frequency are decisive for controlling the morphology and size of the nanostructures produced. This green and powerful technique can be extended to different material systems for obtaining various nanostructures.

Hollow Nanoparticles of Metal Oxides and Sulfides: Fast Preparation via Laser Ablation in Liquid

In this work, diverse hollow nanoparticles of metal oxides and sulfides were prepared by simply laser ablating metal targets in properly chosen liquids. The Kirkendall voiding and the selective heating with an infrared laser were shown to work as two independent mechanisms for the formation of such hollow nanoparticles in only one- or two-step synthesis approaches. One of the prepared materials, ZnS hollow nanoparticles, showed high performance in gas sensing. The simple, fast, inexpensive technique that is proposed demonstrates very promising perspectives.

Observation of growth of metal nanoparticles

An understanding of nanocrystal growth mechanisms is of significant importance for the design of novel materials. The development of liquid cells for transmission electron microscopy (TEM) has enabled direct observation of nanoparticle growth in a liquid phase. By tracking single particle growth trajectories with high spatial resolution, novel growth mechanisms have been revealed. In recent years, there has been an increasing interest in liquid cell TEM and its applications include real time imaging of nanoparticles, biological materials, liquids, and so on. This paper reviews the development of liquid cell TEM and the progress made in using such a wonderful tool to study the growth of nanoparticles (mostly metal nanoparticles). Achievements in the understanding of coalescence, shape control mechanisms, surfactant effects, etc. are highlighted. Other studies relevant to metal precipitation in liquids, such as electrochemical deposition, nanoparticle motion and electron beam effects, are also included. At the end, our perspectives on future challenges and opportunities in liquid cell TEM are provided.

In Situ Study of Lithiation and Delithiation of MoS2 Nanosheets Using Electrochemical Liquid Cell Transmission Electron Microscopy.

We report the observation of lithiation/delithiation of MoS2 nanosheets in a LiPF6/EC/DEC commercial electrolyte for the application of lithium-ion batteries using electrochemical liquid cell transmission electron microscopy (TEM). Upon discharge in a voltage range of 1.8-1.2 V, MoS2 on the Ti electrode underwent irreversible decomposition resulting in fragmentation of the MoS2 nanosheets into 5-10 nm MoS2 nanoparticles. Repeated experiments also show that some MoS2 nanosheets do not decompose upon lithiation. Instead, lithiation induced structural expansion and deformation has been observed. A solid-electrolyte interface (SEI) was formed on the anode side of the Ti electrode in contact with Li metal. The SEI layer is composed of LiF nanocrystals distributed within the entire layer with the constituent elements C, O, and F. However, no passivation film was observed on the cathode side of the Ti electrode with MoS2 nanosheets on it. Such an in situ electrochemical liquid cell TEM study sheds light on battery degradation mechanisms.

Structural and Morphological Evolution of Lead Dendrites during Electrochemical Migration

The electrochemical deposition and dissolution of lead on gold electrodes immersed in an aqueous solution of lead nitrate were studied in situ using a biasing liquid cell by transmission electron microscopy (TEM). We investigate in real time the growth mechanisms of lead dendrites as deposited on the electrodes under an applied potential. TEM images reveal that lead dendrites are developed by the fast protrusion of lead branches in the electrolyte and tip splitting. And, the fast growing tip of the dendritic branch is composed of polycrystalline nanograins and it develops into a single crystalline branch eventually. This study demonstrated unique electrochemical growth of single crystal dendrites through nucleation, aggregation, alignment and attachment of randomly oriented small grains. Additionally, we found the lead concentration in the electrolyte drastically influences the morphology of dendritic formation.

Laser Dispersion of Detonation Nanodiamonds

Detonation nanodiamonds (DNDs), which were first produced from detonation of explosives in 1960s, have recently found attractive applications, such as in bioimaging, cellular marking, and drug delivery to DNA, thanks to their excellent biocompatibility, nontoxicity, and dimensional, thermal, and chemical stability . However, it has been a challenge to deaggregate the nanodiamonds, despite many efforts during the past 40 years. In 2002 and 2003, Osawa et al. made significant progress by recognizing the microstructure of DNDs agglomerates and developed the technique of wetstirred-media milling and bead-assisted sonication to destroy the agglomerant mechanically. As a result, the nanodiamonds were dispersed in solution. This work brought DNDs as a “novel” nanomaterial into nanoscience and bionanotechnology. 6] However, the mechanical process may contaminate the nanodiamonds, and the dispersity of DNDs still needs to be improved. 8] Laser heating has been widely adopted for the synthesis of nanomaterials. Herein, we propose that selective laser heating in liquids can be an effective solution process to deaggregate DNDs. It was demonstrated that the raw DNDs are aggregates of primary nanodiamonds connected with amorphous carbon and covalent bands. On the other hand, nanodiamonds have a large band gap of about 5.5 eVand are transparent to visible or infrared light; in contrast, amorphous carbon without a band gap can absorb the light and be heated. On the basis of this understanding, we suspended the DNDs in liquid and irradiated them using an infrared laser (wavelength 1064 nm). The amorphous carbon absorbs the laser energy and is heated into hot carbon species. Subsequently, the explosion of amorphous carbon can further destroy the covalent bonds between primary nanodiamonds in DNDs. As a consequence, the primary nanodiamonds are released from DNDs and dispersed in liquid free of contamination. Once the amorphous carbon and covalent bonds are removed, the fresh surface of the primary nanodiamonds is then exposed to the liquid environment and can bond facilely with the solvent, thus giving rise to functional nanomaterials. Hu et al. reported the one-step laser synthesis of luminescent nanocomposites by in situ modification of carbon nanodots with organic molecules during nanoparticle formation. Similarly, we have produced nanodiamonds with unique properties by varying the liquid medium. We find that magnetic properties of DNDs change significantly after deaggregation, and the well-dispersed nanodiamonds coated with organic ligands give visible-light emission. Because of the inert features of nanodiamonds, these luminescent nanomaterials are expected to find useful applications in bioimaging and biosensing. Three samples were prepared to investigate the effect of different treatments on the dispersion of DNDs. Samples 1, 2, and 3 (denoted S1, S2, and S3) were the products of sonication, acid oxidization, and laser treatment of raw DNDs, respectively. The raw DNDs are severe agglomerates with amorphous carbon around the crystal nanodiamonds (Supporting Information Figure S1). After sonication treatment, the nanodiamonds in S1 were still agglomerated and coated with amorphous or graphitic carbon (Figure 1a,d). After acid oxidization, the degree of aggregation in S2 has decreased (agglomerate diameter ca. 50 nm, Figure 1b,e). However, well-dispersed nanodiamonds were only obtained in S3 after laser treatment (Figure 1c,f). TEM images indicated that the average size of the nanodiamonds is about 6.3 nm (measured from 150 particles; see inset in Figure 1c). The suspension of nanodiamonds with fine sizes (S3) was stable even after five-month storage. In contrast, S1 and S2 precipitated completely only after one month (Supporting Information Figure S2). As a control experiment, laser irradiation of acid-oxidized DNDs did not markedly improve their dispersity (see the results for S4 in Supporting Information Figure S3). The size distributions of the three samples subjected to different treatments (S1, S2, and S3) were measured using dynamic light scattering (DLS). The effective diameters of particles in S1, S2, and S3 are 1300, 167, and 9.8 nm, respectively (Figure 2a). Although there are deviations between the effective diameters determined by DLS and those observed in the TEM images (e.g., 9.8 vs. 6.3 nm for S3), the results from these measurements agree roughly and show the same trend. We further measured size distribution of DNDs after they were irradiated for different lengths of time. DLS results show that the extent of aggregation decreases gradually with the irradiation time, which suggests that the [*] K. Y. Niu, Dr. J. Yang, Prof. J. Sun, Prof. Dr. X. W. Du Tianjin Key Laboratory of Composite and Functional Materials School of Materials Science and Engineering, Tianjin University Tianjin 300072 (People’s Republic of China) Fax: (+86)22-2740-5694 E-mail: xwdu@tju.edu.cn

In Situ TEM Study of Catalytic Nanoparticle Reactions in Atmospheric Pressure Gas Environment

The understanding of solid-gas interactions has been greatly advanced over the past decade on account of the availability of high-resolution transmission electron microscopes (TEMs) equipped with differentially pumped environmental cells. The operational pressures in these differentially pumped environmental TEM (DP-ETEM) instruments are generally limited up to 20 mbar. Yet, many industrial catalytic reactions are operated at pressures equal or higher than 1 bar-50 times higher than that in the DP-ETEM. This poses limitations for in situ study of gas reactions through ETEM and advances are needed to extend in situ TEM study of gas reactions to the higher pressure range. Here, we present a first series of experiments using a gas flow membrane cell TEM holder that allows a pressure up to 4 bar. The built-in membrane heaters enable reactions at a temperature of 95-400°C with flowing reactive gases. We demonstrate that, using a conventional thermionic TEM, 2 Å atomic fringes can be resolved with the presence of 1 bar O2 gases in an environmental cell and we show real-time observation of the Kirkendall effect during oxidation of cobalt nanocatalysts.

Laser synthesis of gold/oxide nanocomposites

The technique of pulsed laser ablation in liquid has been developed to achieve one-step synthesis of diverse gold/oxide nanocomposites with uniform morphology and superior dispersibility. High-speed photography and transmission electron microscopy observations reveal that many metal nanodroplets are ejected from the target immersed in the liquid medium during the ablation process. The nanodroplets adsorb oxygen and gold atoms that are decomposed in the solution, and form nanocomposites at high temperature. The nanocomposites exhibit excellent properties, such as enhanced catalytic activity with the increasing of the cycle number, and bifunctional character with optical and magnetic effects. Since the pulsed-laser-ablation-in-liquid process is free of toxic agents, the nanocomposites have unique advantages for catalysis or biotechnology.

Galvanic Replacement Reactions of Active-Metal Nanoparticles

We present a systemic investigation of a galvanic replacement technique in which active-metal nanoparticles are used as sacrificial seeds. We found that different nanostructures can be controllably synthesized by varying the type of more noble-metal ions and liquid medium. Specifically, nano-heterostructures of noble metal (Ag, Au) or Cu nanocrystals on active-metal (Mg, Zn) cores were obtained by the reaction of active-metal nanoparticles with more noble-metal ions in ethanol; Ag nanocrystal arrays were produced by the reaction of active-metal nanoparticles with Ag(+) ions in water; spongy Au nanospheres were generated by the reaction of active-metal nanoparticles with AuCl(4)(-) ions in water; and SnO(2) nanoparticles were prepared when Sn(2+) were used as the oxidant ions. The key factors determining the product morphology are shown to be the reactivity of the liquid medium and the nature of the oxidant-reductant couple, whereas Mg and Zn nanoparticles played similar roles in achieving various nanostructures. When microsized Mg and Zn particles were used as seeds in similar reactions, the products were mainly noble-metal dendrites. The new approach proposed in this study expands the capability of the conventional nanoscale galvanic replacement method and provides new avenues to various structures, which are expected to have many potential applications in catalysis, optoelectronics, and biomedicine.