Showing papers by "Yonggang Yao published in 2023"

Programmable Nucleation and Growth of Ultrathin Tellurium Nanowires via a Pulsed Physical Vapor Deposition Design

Abstract: Physical vapor deposition (PVD) methods have been widely employed for high‐quality crystal growth and thin‐film deposition in semiconductor electronics. However, the fabrication of emerging low dimensional nanostructures is hitherto challenging in conventional PVD systems due to their large thermal mass and near‐continuous operation which hinder flexible control of the nucleation and growth events. Herein, a pulsed PVD method is reported that features finely controllable temperature and heating time (down to milliseconds), which enables programming of the vapor supersaturation and decoupling of nucleation and growth events. Take tellurium as an example, the pulsed PVD allows transient source vaporization (≈1000 °C, 30 ms) for burst nucleation, followed by relatively low‐temperature volatilization (≈600 °C, 5 min) for steady‐state growth with well‐suppressed random nucleation. As a result, uniform and high‐density tellurium nanowires are obtained at the ultrathin thickness of sub‐10 nm and length >10 µm, which is in sharp contrast to the randomly formed nanostructures in conventional PVD. When used in the field‐effect transistor, the thin tellurium nanowires display a high on‐off ratio of >104 and hole mobility of ≈40 cm2 V−1 s−1, showing the potential for high‐performance electronics. Pulsed PVD therefore enables to flexibly program and finely tailor the nucleation and growth events during vapor phase deposition, which are otherwise impossible in conventional PVD.

Transient and general synthesis of high-density and ultrasmall nanoparticles on two-dimensional porous carbon via coordinated carbothermal shock

Abstract: Carbon-supported nanoparticles are indispensable to enabling new energy technologies such as metal-air batteries and catalytic water splitting. However, achieving ultrasmall and high-density nanoparticles (optimal catalysts) faces fundamental challenges of their strong tendency toward coarsening and agglomeration. Herein, we report a general and efficient synthesis of high-density and ultrasmall nanoparticles uniformly dispersed on two-dimensional porous carbon. This is achieved through direct carbothermal shock pyrolysis of metal-ligand precursors in just ~100 ms, the fastest among reported syntheses. Our results show that the in situ metal-ligand coordination (e.g., N → Co2+) and local ordering during millisecond-scale pyrolysis play a crucial role in kinetically dominated fabrication and stabilization of high-density nanoparticles on two-dimensional porous carbon films. The as-obtained samples exhibit excellent activity and stability as bifunctional catalysts in oxygen redox reactions. Considering the huge flexibility in coordinated precursors design, diversified single and multielement nanoparticles (M = Fe, Co, Ni, Cu, Cr, Mn, Ag, etc) were generally fabricated, even in systems well beyond traditional crystalline coordination chemistry. Our method allows for the transient and general synthesis of well-dispersed nanoparticles with great simplicity and versatility for various application schemes.

Transient and dry recycling of battery materials with negligible carbon footprint and roll-to-roll scalability

Abstract: Battery recycling becomes increasingly important due to the ubiquitous application of Li-ion batteries that challenges both critical material supply and environmental sustainability. Compared with pyro- and hydro-metallurgy, direct recycling recovers...

Three-dimensional atomic positions and local chemical order of medium- and high-entropy alloys

Abstract: Medium- and high-entropy alloys (M/HEAs) mix multiple principal elements with near-equiatomic composition and represent a paradigm-shift strategy for designing new materials for metallurgy, catalysis, and other fields. One of the core hypotheses of M/HEAs is lattice distortion. However, experimentally determining the 3D local lattice distortion in M/HEAs remains a challenge. Additionally, the presumed random elemental mixing in M/HEAs has been questioned by atomistic simulations, energy dispersive x-ray spectroscopy (EDS), and electron diffraction, which suggest the existence of local chemical order in M/HEAs. However, the 3D local chemical order has eluded direct experimental observation since the EDS elemental maps integrate the composition of atomic columns along the zone axes, and the diffuse reflections/streaks in electron diffraction of M/HEAs may originate from planar defects. Here, we determine the 3D atomic positions of M/HEA nanocrystals using atomic electron tomography, and quantitatively characterize the local lattice distortion, strain tensor, twin boundaries, dislocation cores, and chemical short-range order (CSRO) with unprecedented 3D detail. We find that the local lattice distortion and strain tensor in the HEAs are larger and more heterogeneous than in the MEAs. We observe CSRO-mediated twinning in the MEAs. that is, twinning occurs in energetically unfavoured CSRO regions but not in energetically favoured CSRO ones. This observation confirms the atomistic simulation results of the bulk CrCoNi MEA and represents the first experimental evidence of correlating local chemical order with structural defects in any material system. We expect that this work will not only expand our fundamental understanding of this important class of materials, but also could provide the foundation for tailoring M/HEA properties through lattice distortion and local chemical order.

Transient and in situ Growth of Nanostructured SiC on Carbon Fibers toward Highly Durable Catalysis

Abstract: Carbon is ubiquitously used as catalyst supports in various clean energy technologies, particularly emerging electrocatalysis, yet it often suffers slow oxidation and corrosion along with performance degradation. The harmonious combination of refractory silicon carbide (SiC, chemically inert) with carbon is alluring but often a great challenge, particularly to achieve desirable nanostructures and strong interfaces. Herein, a shockwave-type transient heating is designed (> 1750 °C for 1 s per pulse) for controllable growth of conformal SiC coating and massive SiC nanowires on carbon fibers (denoted as CF/SiC-NW), which serves as a high surface area and durable support for catalysis under harsh environments. The transient heating in SiO vapor triggers in situ transformation of the carbon surface into a seamless SiC protective layer, while the following fast cooling is essential for the growth of numerous self-assembled SiC nanowires. The CF/SiC-NW exhibits excellent structural stability in the air at high temperatures, in concentrated acidic/alkaline solutions after electrochemical stressing for 2000 cycles, and in oxygen evolution reaction after 10 h of continuous operation. This strategy enables delicate structure control in refractory carbides and is also general for various carbon/carbide functional materials (e.g., C/TiC, C/WC) for electro- or electrified catalysis under harsh conditions.

Selective Hydrodeoxygenation of Lignin‐Derived Vanillin via Hetero‐Structured High‐Entropy Alloy/Oxide Catalysts

Abstract: The chemoselective hydrodeoxygenation of natural lignocellulosic materials plays a crucial role in converting biomass into value-added chemicals. Yet their complex molecular structures often require multiple active sites synergy for effective activation and achieving high chemoselectivity. Herein, it is reported that a high-entropy alloy (HEA) on high-entropy oxide (HEO) hetero-structured catalyst for highly active, chemoselective, and robust vanillin hydrodeoxygenation. The heterogenous HEA/HEO catalysts were prepared by thermal reduction of senary HEOs (NiZnCuFeAlZrOx), where exsolvable metals (e.g., Ni, Zn, Cu) in situ emerged and formed randomly dispersed HEA nanoparticles anchoring on the HEO matrix. This catalyst exhibits excellent catalytic performance: 100% conversion of vanillin and 95% selectivity toward high-value 2-methyl-4 methoxy phenol at low temperature of 120 °C, which were attributed to the synergistic effect among HEO matrix (with abundant oxygen vacancies), anchored HEA nanoparticles (having excellent hydrogenolysis capability), and their intimate hetero-interfaces (showing strong electron transferring effect). Therefore, our work reported the successful construction of HEA/HEO heterogeneous catalysts and their superior multifunctionality in biomass conversion, which could shed light on catalyst design for many important reactions that are complex and require multifunctional active sites.