Kai-yang Niu - Academia.edu (original) (raw)

Papers by Kai-yang Niu

Research paper thumbnail of Revealing dynamic processes of materials in liquids using liquid cell transmission electron microscopy

Journal of visualized experiments : JoVE, 2012

The recent development for in situ transmission electron microscopy, which allows imaging through... more The recent development for in situ transmission electron microscopy, which allows imaging through liquids with high spatial resolution, has attracted significant interests across the research fields of materials science, physics, chemistry and biology. The key enabling technology is a liquid cell. We fabricate liquid cells with thin viewing windows through a sequential microfabrication process, including silicon nitride membrane deposition, photolithographic patterning, wafer etching, cell bonding, etc. A liquid cell with the dimensions of a regular TEM grid can fit in any standard TEM sample holder. About 100 nanoliters reaction solution is loaded into the reservoirs and about 30 picoliters liquid is drawn into the viewing windows by capillary force. Subsequently, the cell is sealed and loaded into a microscope for in situ imaging. Inside the TEM, the electron beam goes through the thin liquid layer sandwiched between two silicon nitride membranes. Dynamic processes of nanoparticle...

Research paper thumbnail of One-step synthesis of MgO hollow nanospheres with blue emission

Research paper thumbnail of Revealing Bismuth Oxide Hollow Nanoparticle Formation by the Kirkendall Effect

Nano Letters, 2013

We study the formation of bismuth oxide hollow nanoparticles by the Kirkendall effect using liqui... more We study the formation of bismuth oxide hollow nanoparticles by the Kirkendall effect using liquid cell transmission electron microscopy (TEM). Rich dynamics of bismuth diffusion through the bismuth oxide shell have been captured in situ. The diffusion coefficient of bismuth through bismuth oxide shell is 3−4 orders of magnitude higher than that of bulk. Observation reveals that defects, temperature, sizes of the particles, and so forth can affect the diffusion of reactive species and modify the kinetics of the hollowing process.

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

Microscopy and Microanalysis, 2013

The understanding of solid-gas interactions has been greatly advanced over the past decade on acc... more 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.

Research paper thumbnail of Visualization of the Coalescence of Bismuth Nanoparticles

Microscopy and Microanalysis, 2014

Coalescence is a significant pathway for the growth of nanostructures. Here we studied the coales... more Coalescence is a significant pathway for the growth of nanostructures. Here we studied the coalescence of Bi nanoparticles in situ by liquid cell transmission electron microscopy (TEM). The growth of Bi nanoparticles was initiated from a bismuth neodecanoate precursor solution by electron beam irradiation inside a liquid cell under the TEM. A significant number of coalescence events occurred from the as-grown Bi nanodots. Both symmetric coalescence of two equal-sized nanoparticles and asymmetric coalescence of two or more unequalsized nanoparticles were analyzed along their growth trajectories. Our observation suggests that two mass transport mechanisms, i.e., surface diffusion and grain boundary diffusion, are responsible for the shape evolution of nanoparticles after a coalescence event.

Research paper thumbnail of Synthesis of Si–C nanostructures by laser ablation of silicon target in n-heptane vapor

Materials Letters, 2009

ABSTRACT Controllable synthesis of Si–C nanostructures was realized in a laser ablation system by... more ABSTRACT Controllable synthesis of Si–C nanostructures was realized in a laser ablation system by adopting solid silicon target and n-heptane vapor as starting materials. Ultrafine SiC nanocrystals and graphite-coated SiC nanocrystals were synthesized with the laser frequency of 1Hz and 20Hz, respectively. According to the real-time observation on the plasma evolvement, we proposed a formation mechanism of Si–C nanostructure related to vapor-phase reaction. Our method can be extended to other material systems for diverse novel nanostructures.

Research paper thumbnail of ZnO–Au hybrid nanocrystals via a high temperature reflux route

Materials Letters, 2009

... Xue Han a , Win-Jing Qin a , Jing Sun a , Jing Yang a , Kai-Yang Niu a , Hong-Li Wang b and X... more ... Xue Han a , Win-Jing Qin a , Jing Sun a , Jing Yang a , Kai-Yang Niu a , Hong-Li Wang b and Xi-Wen Du a ... to a high temperature (the boiling point of solvent), a short burst of nucleation of the secondary component could be expected according to Lamer nucleation-growth ...

Research paper thumbnail of Buckling of a Thin Face Layer on Winkler Foundation with Debonds

Journal of Sandwich Structures and Materials, 1999

Research paper thumbnail of One-step synthesis of fluorescent carbon nanoparticles by laser irradiation

Journal of Materials Chemistry, 2009

J. Mater. Chem., 2009, 19, 484-488 DOI:10.1039/B812943F (Paper). One-step synthesis of fluorescen... more J. Mater. Chem., 2009, 19, 484-488 DOI:10.1039/B812943F (Paper). One-step synthesis of fluorescent carbon nanoparticles by laser irradiation†. Sheng-Liang Huab, Kai-Yang Niua, Jing Suna, Jing Yanga, Nai-Qin Zhaoa and ...

Research paper thumbnail of Surfactant-assisted synthesis of ZnO–Au nanostructure with good dispersibility

Journal of Alloys and Compounds, 2009

The control on the size and morphology of hybrid nanocrystals is essential for the fundamental re... more The control on the size and morphology of hybrid nanocrystals is essential for the fundamental research and practical applications of these multimaterial nanocrystals with elaborate structures. In our work, the ZnO-Au hybrid nanocrystals were prepared by using an aqueous solution method, where hexadecyl trimethyl ammonium bromide, a surfactant, was proved to play an important role in obtaining fine hybrid nanocrystals with good dispersibility. Compared to pure ZnO, the hybrid nanocrystals emitted strong visible light arising from the defects at the ZnO/Au interface.

Research paper thumbnail of Galvanic Replacement Reactions of Active‐Metal Nanoparticles

Research paper thumbnail of Observation of growth of metal nanoparticles

Chemical Communications, 2013

An understanding of nanocrystal growth mechanisms is of significant importance for the design of ... more 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.

Research paper thumbnail of Laser Dispersion of Detonation Nanodiamonds

Angewandte Chemie International Edition, 2011

Research paper thumbnail of A spongy nickel-organic CO 2 reduction photocatalyst for nearly 100% selective CO production

Solar-driven photocatalytic conversion of CO 2 into fuels has attracted a lot of interest; howeve... more Solar-driven photocatalytic conversion of CO 2 into fuels has attracted a lot of interest; however, developing active catalysts that can selectively convert CO 2 to fuels with desirable reaction products remains a grand challenge. For instance, complete suppression of the competing H 2 evolution during photocatalytic CO 2-to-CO conversion has not been achieved before. We design and synthesize a spongy nickel-organic heterogeneous photocatalyst via a photochemical route. The catalyst has a crystalline network architecture with a high concentration of defects. It is highly active in converting CO 2 to CO, with a production rate of ~1.6 × 10 4 mmol hour −1 g −1. No measurable H 2 is generated during the reaction, leading to nearly 100% selective CO production over H 2 evolution. When the spongy Ni-organic catalyst is enriched with Rh or Ag nanocrystals, the controlled photocatalytic CO 2 reduction reactions generate formic acid and acetic acid. Achieving such a spongy nickel-organic photocatalyst is a critical step toward practical production of high-value multicarbon fuels using solar energy.

Research paper thumbnail of Revealing dynamic processes of materials in liquids using liquid cell transmission electron microscopy

Journal of visualized experiments : JoVE, 2012

The recent development for in situ transmission electron microscopy, which allows imaging through... more The recent development for in situ transmission electron microscopy, which allows imaging through liquids with high spatial resolution, has attracted significant interests across the research fields of materials science, physics, chemistry and biology. The key enabling technology is a liquid cell. We fabricate liquid cells with thin viewing windows through a sequential microfabrication process, including silicon nitride membrane deposition, photolithographic patterning, wafer etching, cell bonding, etc. A liquid cell with the dimensions of a regular TEM grid can fit in any standard TEM sample holder. About 100 nanoliters reaction solution is loaded into the reservoirs and about 30 picoliters liquid is drawn into the viewing windows by capillary force. Subsequently, the cell is sealed and loaded into a microscope for in situ imaging. Inside the TEM, the electron beam goes through the thin liquid layer sandwiched between two silicon nitride membranes. Dynamic processes of nanoparticle...

Research paper thumbnail of One-step synthesis of MgO hollow nanospheres with blue emission

Research paper thumbnail of Revealing Bismuth Oxide Hollow Nanoparticle Formation by the Kirkendall Effect

Nano Letters, 2013

We study the formation of bismuth oxide hollow nanoparticles by the Kirkendall effect using liqui... more We study the formation of bismuth oxide hollow nanoparticles by the Kirkendall effect using liquid cell transmission electron microscopy (TEM). Rich dynamics of bismuth diffusion through the bismuth oxide shell have been captured in situ. The diffusion coefficient of bismuth through bismuth oxide shell is 3−4 orders of magnitude higher than that of bulk. Observation reveals that defects, temperature, sizes of the particles, and so forth can affect the diffusion of reactive species and modify the kinetics of the hollowing process.

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

Microscopy and Microanalysis, 2013

The understanding of solid-gas interactions has been greatly advanced over the past decade on acc... more 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.

Research paper thumbnail of Visualization of the Coalescence of Bismuth Nanoparticles

Microscopy and Microanalysis, 2014

Coalescence is a significant pathway for the growth of nanostructures. Here we studied the coales... more Coalescence is a significant pathway for the growth of nanostructures. Here we studied the coalescence of Bi nanoparticles in situ by liquid cell transmission electron microscopy (TEM). The growth of Bi nanoparticles was initiated from a bismuth neodecanoate precursor solution by electron beam irradiation inside a liquid cell under the TEM. A significant number of coalescence events occurred from the as-grown Bi nanodots. Both symmetric coalescence of two equal-sized nanoparticles and asymmetric coalescence of two or more unequalsized nanoparticles were analyzed along their growth trajectories. Our observation suggests that two mass transport mechanisms, i.e., surface diffusion and grain boundary diffusion, are responsible for the shape evolution of nanoparticles after a coalescence event.

Research paper thumbnail of Synthesis of Si–C nanostructures by laser ablation of silicon target in n-heptane vapor

Materials Letters, 2009

ABSTRACT Controllable synthesis of Si–C nanostructures was realized in a laser ablation system by... more ABSTRACT Controllable synthesis of Si–C nanostructures was realized in a laser ablation system by adopting solid silicon target and n-heptane vapor as starting materials. Ultrafine SiC nanocrystals and graphite-coated SiC nanocrystals were synthesized with the laser frequency of 1Hz and 20Hz, respectively. According to the real-time observation on the plasma evolvement, we proposed a formation mechanism of Si–C nanostructure related to vapor-phase reaction. Our method can be extended to other material systems for diverse novel nanostructures.

Research paper thumbnail of ZnO–Au hybrid nanocrystals via a high temperature reflux route

Materials Letters, 2009

... Xue Han a , Win-Jing Qin a , Jing Sun a , Jing Yang a , Kai-Yang Niu a , Hong-Li Wang b and X... more ... Xue Han a , Win-Jing Qin a , Jing Sun a , Jing Yang a , Kai-Yang Niu a , Hong-Li Wang b and Xi-Wen Du a ... to a high temperature (the boiling point of solvent), a short burst of nucleation of the secondary component could be expected according to Lamer nucleation-growth ...

Research paper thumbnail of Buckling of a Thin Face Layer on Winkler Foundation with Debonds

Journal of Sandwich Structures and Materials, 1999

Research paper thumbnail of One-step synthesis of fluorescent carbon nanoparticles by laser irradiation

Journal of Materials Chemistry, 2009

J. Mater. Chem., 2009, 19, 484-488 DOI:10.1039/B812943F (Paper). One-step synthesis of fluorescen... more J. Mater. Chem., 2009, 19, 484-488 DOI:10.1039/B812943F (Paper). One-step synthesis of fluorescent carbon nanoparticles by laser irradiation†. Sheng-Liang Huab, Kai-Yang Niua, Jing Suna, Jing Yanga, Nai-Qin Zhaoa and ...

Research paper thumbnail of Surfactant-assisted synthesis of ZnO–Au nanostructure with good dispersibility

Journal of Alloys and Compounds, 2009

The control on the size and morphology of hybrid nanocrystals is essential for the fundamental re... more The control on the size and morphology of hybrid nanocrystals is essential for the fundamental research and practical applications of these multimaterial nanocrystals with elaborate structures. In our work, the ZnO-Au hybrid nanocrystals were prepared by using an aqueous solution method, where hexadecyl trimethyl ammonium bromide, a surfactant, was proved to play an important role in obtaining fine hybrid nanocrystals with good dispersibility. Compared to pure ZnO, the hybrid nanocrystals emitted strong visible light arising from the defects at the ZnO/Au interface.

Research paper thumbnail of Galvanic Replacement Reactions of Active‐Metal Nanoparticles

Research paper thumbnail of Observation of growth of metal nanoparticles

Chemical Communications, 2013

An understanding of nanocrystal growth mechanisms is of significant importance for the design of ... more 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.

Research paper thumbnail of Laser Dispersion of Detonation Nanodiamonds

Angewandte Chemie International Edition, 2011

Research paper thumbnail of A spongy nickel-organic CO 2 reduction photocatalyst for nearly 100% selective CO production

Solar-driven photocatalytic conversion of CO 2 into fuels has attracted a lot of interest; howeve... more Solar-driven photocatalytic conversion of CO 2 into fuels has attracted a lot of interest; however, developing active catalysts that can selectively convert CO 2 to fuels with desirable reaction products remains a grand challenge. For instance, complete suppression of the competing H 2 evolution during photocatalytic CO 2-to-CO conversion has not been achieved before. We design and synthesize a spongy nickel-organic heterogeneous photocatalyst via a photochemical route. The catalyst has a crystalline network architecture with a high concentration of defects. It is highly active in converting CO 2 to CO, with a production rate of ~1.6 × 10 4 mmol hour −1 g −1. No measurable H 2 is generated during the reaction, leading to nearly 100% selective CO production over H 2 evolution. When the spongy Ni-organic catalyst is enriched with Rh or Ag nanocrystals, the controlled photocatalytic CO 2 reduction reactions generate formic acid and acetic acid. Achieving such a spongy nickel-organic photocatalyst is a critical step toward practical production of high-value multicarbon fuels using solar energy.