Daobin Liu

Oxide Defect Engineering Enables to Couple Solar Energy into Oxygen Activation

Modern development of chemical manufacturing requires a substantial reduction in energy consumption and catalyst cost. Sunlight-driven chemical transformation by metal oxides holds great promise for this goal; however, it remains a grand challenge to efficiently couple solar energy into many catalytic reactions. Here we report that defect engineering on oxide catalyst can serve as a versatile approach to bridge light harvesting with surface reactions by ensuring species chemisorption. The chemisorption not only spatially enables the transfer of photoexcited electrons to reaction species, but also alters the form of active species to lower the photon energy requirement for reactions. In a proof of concept, oxygen molecules are activated into superoxide radicals on defect-rich tungsten oxide through visible-near-infrared illumination to trigger organic aerobic couplings of amines to corresponding imines. The excellent efficiency and durability for such a highly important process in chemical transformation can otherwise be virtually impossible to attain by counterpart materials.

Gram‐Scale Aqueous Synthesis of Stable Few‐Layered 1T‐MoS2: Applications for Visible‐Light‐Driven Photocatalytic Hydrogen Evolution

Most recently, much attention has been devoted to 1T phase MoS2 because of its distinctive phase-engineering nature and promising applications in catalysts, electronics, and energy storage devices. While alkali metal intercalation and exfoliation methods have been well developed to realize unstable 1T-MoS2 , but the aqueous synthesis for producing stable metallic phase remains big challenging. Herein, a new synthetic protocol is developed to mass-produce colloidal metallic 1T-MoS2 layers highly stabilized by intercalated ammonium ions (abbreviated as N-MoS2). In combination with density functional calculations, the X-ray diffraction pattern and Raman spectra elucidate the excellent stability of metallic phase. As clearly depicted by high-angle annular dark-field imaging in an aberration-corrected scanning transmission electron microscope and extended X-ray absorption fine structure, the N-MoS2 exhibits a distorted octahedral structure with a 2a0 × a0 basal plane superlattice and 2.72 Å Mo-Mo bond length. In a proof-of-concept demonstration for the obtained materials applications, highly efficient photocatalytic activity is achieved by simply hybridizing metallic N-MoS2 with semiconducting CdS nanorods due to the synergistic effect. As a direct outcome, this CdS:N-MoS2 hybrid shows giant enhancement of hydrogen evolution rate, which is almost 21-fold higher than pure CdS and threefold higher than corresponding annealed CdS:2H-MoS2.

In situ Integration of a Metallic 1T-MoS2/CdS Heterostructure as a Means to Promote Visible-Light-Driven Photocatalytic Hydrogen Evolution

The replacement of expensive noble‐metals cocatalysts with inexpensive, earth‐abundant, metallic nonmetal materials in most semiconductor‐based photocatalytic systems is highly desirable. Herein, we report the fabrication of stable 1T‐MoS2 slabs in situ grown on CdS nanorods (namely, 1T‐MoS2@CdS) by using a solvothermal method. As demonstrated by ultrafast transient absorption spectroscopy, in combination with steady‐state and time‐resolved photoluminescence, the synergistic effects resulting from formation of the intimate nanojunction between the interfaces and effective electron transport in the metallic phase of 1T‐MoS2 largely contribute to boosting the photocatalytic activity of CdS. Notably, the heterostructure with an optimum loading of 0.2 wt % 1T‐MoS2 exhibits an almost 39‐fold enhancement in the photocatalytic activity relative to that exhibited by bare CdS. This work represents a step towards the in situ realization of a 1T‐phase MoS2‐based heterostructure as a promising cocatalyst with high performance and low cost.

Stable metallic 1T-WS2 ultrathin nanosheets as a promising agent for near-infrared photothermal ablation cancer therapy

In this study, we present the preparation of stable 1T-WS2 ultrathin nanosheets with NH4+ intercalation using a bottom-up hydrothermal method and the potential application of this material in light-induced photothermal cancer therapy. Our results revealed that nanosheets with a size of 150 nm were highly hydrophilic and exhibited strong light absorption and excellent photostability in the broad near-infrared wavelength region. The in vitro experimental results indicated good biocompatibility of the nanosheets. More notably, our in vivo antitumor experiments illustrated that light-induced photothermal ablation originating from irradiation of the 1T-WS2 nanosheets with an 808 nm laser could efficiently kill tumor cells; these effects were obtained not only at the cellular level but also in the living organs of mice. This result may lead to new applications of two-dimensional layered materials in novel photothermal therapies and other photothermal related fields.

Unsaturated-sulfur-rich MoS2 nanosheets decorated on free-standing SWNT film: Synthesis, characterization and electrocatalytic application

Herein, we report a bottom-up solvothermal route to synthesize a flexible, highly efficient MoS2@SWNT electrocatalyst for hydrogen evolution reactions (HER). Characterization revealed that branch-like MoS2 nanosheets containing sulfurrich sites were in situ uniformly dispersed on free-standing single-walled carbon nanotube (SWNT) film, which could expose more unsaturated sulfur atoms, allowing excellent electrical contact with active sites. The flexible catalyst exhibited excellent HER performance with a low overpotential (~150 mV at 10 mA/cm2) and small Tafel slope (41 mV/dec). To further explain the improved performance, the local electronic structure was investigated by X-ray absorption near-edge structure (XANES) analysis, proving the presence of unsaturated sulfur atoms and strong electronic coupling between MoS2 and SWNT. This study provides an in-situ synthetic route to create new multifunctional flexible hybridized catalysts and useful insights into the relationships among the catalyst microstructure, electronic structure, and properties.

Stable 1T-MoSe2 and Carbon Nanotube Hybridized Flexible Film: Binder-Free and High-Performance Li-Ion Anode

Two-dimensional stable metallic 1T-MoSe2 with expanded interlayer spacing of 10.0 Å in situ grown on SWCNTs film is fabricated via a one-step solvothermal method. Combined with X-ray absorption near-edge structures, our characterization reveals that such 1T-MoSe2 and single-walled carbon nanotubes (abbreviated as 1T-MoSe2/SWCNTs) hybridized structure can provide strong electrical and chemical coupling between 1T-MoSe2 nanosheets and SWCNT film in a form of C-O-Mo bonding, which significantly benefits a high-efficiency electron/ion transport pathway and structural stability, thus directly enabling high-performance lithium storage properties. In particular, as a flexible and binder-free Li-ion anode, the 1T-MoSe2/SWCNTs electrode exhibits excellent rate capacity, which delivers a capacity of 630 mAh/g at 3000 mA/g. Meanwhile, the strong C-O-Mo bonding of 1T-MoSe2/SWCNTs accommodates volume alteration during the repeated charge/discharge process, which gives rise to 89% capacity retention and a capacity of 971 mAh/g at 300 mA/g after 100 cycles. This synthetic route of a multifunctional MoSe2/SWCNTs hybrid might be extended to fabricate other 2D layer-based flexible and light electrodes for various applications such as electronics, optics, and catalysts.

All-Carbon Ultrafast Supercapacitor by Integrating Multidimensional Nanocarbons.

Ultrafast and high capacity all-carbon supercapacitors with 3D porous aerogel electrode are realized by combining carbon nanostructures of various dimensionalities, including 0D carbon onions, 1D carbon nanotubes, and 2D graphene oxide. The synergistic effects from the different forms of nanocarbons render this hybrid outstanding capacitance with excellent stability, even at ultrafast charge-discharge rates.

Metallic 1T-WS2 nanoribbons as highly conductive electrodes for supercapacitors

Layered tungsten disulfide (WS2) has attracted great attention because of its high potential for electrochemical energy applications. However, the semiconducting nature of WS2 with a 2H phase largely hinders its electrochemical performance due to poor electronic conductivity. In this study, we have successfully synthesized a metallic 1T-WS2 nanoribbon with stable ammonia-ion intercalation as a highly conductive electrode for high-performance supercapacitors. The specific capacitance using the metallic 1T-WS2 electrode exhibits significant enhancement upto the value of 2813 μF cm−2. This value is 12 times higher compared to semiconducting 2H-WS2. Moreover, the 1T-WS2 electrode has good stability even under high current scans, which is ascribed to the stable ammonia-ion interaction. The correlation between the 1T-WS2 structure and its electrochemical performance has also been discussed.

Maneuvering charge polarization and transport in 2H-MoS2 for enhanced electrocatalytic hydrogen evolution reaction

Semiconducting 2H-MoS2 with high chemical stability is a promising alternative to the existing electrocatalysts for the hydrogen evolution reaction (HER); however, the HER performance largely suffers from the limited number of active S sites and low mobility for charge transport. In this work, we demonstrate that the limitations of 2H-MoS2 for the HER can be overcome by forming hybrid structures with metallic nanowires. Taking the integration with Pd as a proofof- concept, we show with solid experimental evidence that the one-dimensional structure of metallic nanowires facilitates electron transport to active S sites, while the interfacial charge polarization between MoS2 and Pd increases the electron density of the S sites for improved activity. As a result, the hybrid structure exhibits a current density of 122 mA·cm-2 at -300 mV versus RHE and a Tafel slope of 44 mV·decade-1 with excellent durability, well exceeding the performances of bare 2H-MoS2 and metallic 1T-MoS2. This work provides insights into electrocatalyst design based on charge transport and polarization, which can be extended to other hybrid structures.

High-strength graphene composite films by molecular level couplings for flexible supercapacitors with high volumetric capacitance

A critical challenge in fabricating the electrodes of flexible supercapacitors is to optimize their electrochemical performance without deteriorating their mechanical properties. We report here a strategy to prepare freestanding reduced graphene oxide@polyvinyl alcohol (rGO@PVA) composite films by synchronously reducing and assembling GO sheets with PVA molecules on a metal surface. Such rGO@PVA composite films realize the controllable assembly of rGO sheets and PVA in an ordered layered structure as well as the molecular level couplings between rGO sheets and PVA molecules. As a result, the rGO@PVA composite films display extremely high strength and Youngs modulus. After introducing H2SO4, the PVA/H2SO4 electrolyte layer between rGO sheets can form fast ion transport channel at the molecular level in the composite films. Therefore, the composite films deliver high volumetric capacity (206.8 F cm−3), excellent energy density (7.18 mW h cm−3) and power density (2.92 W cm−3). More importantly, the supercapacitors based on the composite films show stable electrochemical performance under different stresses and bending states, even when the supercapacitors were bent to 180°. The high flexibility and electrochemical performance of such supercapacitors will enable a broad field of energy-storage devices to be compatible with flexible and wearable electronics.