Binjie Zheng

Self-Assembled Coral-like Hierarchical Architecture Constructed by NiSe2 Nanocrystals with Comparable Hydrogen-Evolution Performance of Precious Platinum Catalyst

For the first time, self-assembled coral-like hierarchical architecture constructed by NiSe2 nanocrystals has been synthesized via a facile one-pot DMF-solvothermal method. Compared with hydrothermally synthesized NiSe2 (H-NiSe2), the DMF-solvothermally synthesized nanocrystalline NiSe2 (DNC-NiSe2) exhibits superior performance of hydrogen evolution reaction (HER): it has a very low onset overpotential of ∼136 mV (vs RHE), a very high cathode current density of 40 mA/cm2 at ∼200 mV (vs RHE), and an excellent long-term stability; most importantly, it delivers an ultrasmall Tafel slope of 29.4 mV dec-1, which is the lowest ever reported for NiSe2-based catalysts, and even lower than that of precious platinum (Pt) catalyst (30.8 mV dec-1). The superior HER performance of DNC-NiSe2 is attributed to the unique self-assembled coral-like network, which is a benefit to form abundant active sites and facilitates the charge transportation due to the inherent high conductivity of NiSe2 nanocrystals. The DNC-NiSe2 is promising to be a viable alternative to precious metal catalysts for hydrogen evolution.

Vertically oriented few-layered HfS2 nanosheets: growth mechanism and optical properties

For the first time, large-area, vertically oriented few-layered hafnium disulfide (V-) nanosheets have been grown by chemical vapor deposition. The individual nanosheets are well [001] oriented, with highly crystalline quality. Far different from conventional van der Waals epitaxial growth mechanism for two-dimensional transition metal dichalcogenides, a novel dangling-bond-assisted self-seeding growth mechanism is proposed to describe the growth of V- nanosheets: difficult migration of adatoms on substrate surface results in seeds growing perpendicularly to the substrate; V- nanosheets inherit the growth direction of seeds; V- nanosheets further expand in the in-plane direction with time evolution. Moreover, the V- nanosheets show strong and broadened photons absorption from near infrared to ultraviolet; the V--based photodetector exhibits an ultrafast photoresponse time of 24 ms, and a high photosensitivity ca. 103 for 405 nm laser.

Nanocrystalline Co0.85Se Anchored on Graphene Nanosheets as a Highly Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction

For the first time, a porous and conductive Co0.85Se/graphene network (CSGN), constructed by Co0.85Se nanocrystals being tightly connected with each other and homogeneously anchored on few-layered graphene nanosheets, has been synthesized by a facile one-pot solvothermal method. Compared to unhybridized Co0.85Se, CSGN exhibits much faster kinetics and better electrocatalytic behavior for hydrogen evolution reaction (HER). The HER mechanism of CSGN is improved to Volmer-Tafel combination, instead of Volmer-Heyrovsky combination, for Co0.85Se. CSGN has a very low Tafel slope of 34.4 mV/dec, which is much lower than that of unhybridized Co0.85Se (41.8 mV/dec) and is the lowest ever reported for Co0.85Se-based electrocatalysts. CSGN delivers a current density of 55 mA/cm2 at 250 mV overpotential, much larger than that of Co0.85Se (33 mA/cm2). Furthermore, CSGN shows superior electrocatalytic stability even after 1500 cycles. The excellent HER performance of CSGN is attributed to the unique porous and conductive network, which can not only guarantee interconnected conductive paths in the whole electrode but also provide abundant catalytic active sites, thereby facilitating charge transportation between the electrocatalyst and electrolyte. This work provides insight into rational design and low-cost synthesis of nonprecious transition-metal chalcogenide-based electrocatalysts with high efficiency and excellent stability for HER.

Self-assembled pearl-bracelet-like CoSe2–SnSe2/CNT hollow architecture as highly efficient electrocatalysts for hydrogen evolution reaction

For the first time, self-assembled pearl-bracelet-like CoSe2–SnSe2/CNT hollow nanoboxes (CSCB), as a novel non-precious electrocatalyst for hydrogen evolution reaction (HER), have been synthesized by a facile aqueous reaction and selenization processes. Compared to bare CoSe2–SnSe2 nanoboxes (CSB), the CSCB hybrid shows much faster kinetics and far better HER performance: the HER mechanism of CSCB is improved to be a Volmer–Tafel combination, instead of Volmer–Heyrovsky combination (as in case of CSB); the Tafel slope of CSCB dramatically decreases from 74.5 mV dec−1 (for CSB) to 32.4 mV dec−1, which nearly approaches to that of the commercial Pt/C electrocatalyst; the CSCB hybrid delivers a very large current density of 35 mA cm−2 at −250 mV vs. RHE, over 85 times larger than that of CSB (0.4 mA cm−2). Furthermore, CSCB shows an excellent long-term electrocatalytic stability even after 1500 cycles. The superior HER performances of CSCB can be attributed to its well-designed pearl-bracelet-like hollow architecture with high conductivity and rich porosity: the CNT chains with a good mechanical strength and high conductivity construct a highly conductive network for fast charge transportation during HER; the hollow CoSe2–SnSe2 nanoboxes with large internal voids and rich nano-/mesopores provide abundant catalytic active sites and facilitate ion-diffusion. This study provides insight into a rational design and low-cost synthesis of the non-precious transition-metal chalcogenide-based electrocatalysts with high efficiency and stability for HER.

Ultrafast ammonia-driven, microwave-assisted synthesis of nitrogen-doped graphene quantum dots and their optical properties

Abstract For the first time, a facile, ultrafast, ammonia-driven microwave-assisted synthesis of high-quality nitrogen-doped graphene quantum dots (NGQDs) at room temperature and atmospheric pressure is presented. This one-step method is very cheap, environment friendly, and suitable for large-scale production. The as-synthesized NGQDs consisting of one to three graphene monolayers exhibit highly crystalline quality with an average size of 5.3 nm. A new fluorescence (FL) emission peak at 390 nm is observed, which might be attributed to the doped nitrogen atoms into the GQDs. An interesting red-shift is observed by comparing the FL excitation spectra to the UV-visible absorption spectra. Based on the optical properties, the detailed Jablonski diagram representing the energy level structure of NGQDs is derived.

Observation of tunable electrical bandgap in large-area twisted bilayer graphene synthesized by chemical vapor deposition.

Although there are already many efforts to investigate the electronic structures of twisted bilayer graphene, a definitive conclusion has not yet been reached. In particular, it is still a controversial issue whether a tunable electrical (or transport) bandgap exists in twisted bilayer graphene film until now. Herein, for the first time, it has been demonstrated that a tunable electrical bandgap can be opened in the twisted bilayer graphene by the combination effect of twist and vertical electrical fields. In addition, we have also developed a facile chemical vapor deposition method to synthesize large-area twisted bilayer graphene by introducing decaborane as the cocatalyst for decomposing methane molecules. The growth mechanism is demonstrated to be a defined-seeding and self-limiting process. This work is expected to be beneficial to the fundamental understanding of both the growth mechanism for bilayer graphene on Cu foil and more significantly, the electronic structures of twisted bilayer graphene.

Flexible terahertz modulator based on coplanar-gate graphene field-effect transistor structure

The terahertz (THz) modulators, as an essential component of the THz system, have been developed by many efforts until now. However, the development of flexible THz modulators is hindered due to the lack of flexible THz modulating materials. Herein, for the first time to the best of our knowledge, we demonstrated the feasibility of flexible THz modulators based on the coplanar-gate field-effect transistor (FET) structure of ion-gel/graphene/polyethylene terephthalate. The THz transmittance through this THz graphene modulator can be well controlled with a modulation depth up to 22% by tuning the carrier concentration of graphene via electrical gating. Furthermore, because of the integration of high flexibilities of graphene, ion-gel, and polyethylene terephthalate (PET), the proposed THz graphene modulator shows superior flexible performance, where the modulation properties can be maintained almost unchanged, not only under bending deformations, but also before and after bending 1000 times. In addition, due to the unique structure of ion-gel/graphene/PET, the flexible THz graphene modulator has a low insertion loss (1.2 dB). Therefore, this Letter is expected to be beneficial for the potential applications, ranging from the traditional compact THz system to a new flexible THz technology.

In situ synthesis of hierarchical MoSe2–CoSe2 nanotubes as an efficient electrocatalyst for the hydrogen evolution reaction in both acidic and alkaline media

Hierarchical MoSe2–CoSe2 nanotubes (MS–CS NTs) are in situ converted from the CoMoO4 nanowires (NWs) via a facile hydrothermal selenization method. As an electrocatalyst, MS–CS NTs show highly efficient and stable performance for the hydrogen evolution reaction (HER). In acidic (or alkaline) medium, MS–CS NTs demonstrate superior HER performance with a very low onset overpotential of 148 (or 127) mV vs. RHE, a low overpotential of 206 (or 237) mV vs. RHE at −10 mA cm−2, a very large current density of 84.6 (or 23.5) mA cm−2 at −300 mV (vs. RHE), a small Tafel slope of 45 (or 89) mV dec−1 and remarkable long-term cycling stability. The outstanding HER activity of MS–CS NTs in both media is attributed to their unique hierarchical and nanoporous architecture constructed with the homogeneous distribution of few-layered MoSe2 nanosheets and CoSe2 nanoparticles, which can not only efficaciously suppress the aggregation of MoSe2 nanosheets, but also generate more active sites or edges to take part in the HER. In addition, the uniform dispersion of highly conductive CoSe2 nanoparticles in few-layered MoSe2 nanosheets efficiently promotes the electron transfer from the electrode to the active sites on MoSe2 nanosheets, further improving the electrocatalytic properties.

Scalable synthesis of porous hollow CoSe2–MoSe2/carbon microspheres for highly efficient hydrogen evolution reaction in acidic and alkaline media

For the first time, porous microspheres of bimetallic CoSe2–MoSe2 hybrids with reduced graphene oxide and amorphous carbon (CS-MS/rGO-C) are synthesized through a facile, low-cost and scalable spray drying approach and a subsequent selenization process. The CS-MS/rGO-C electrocatalyst delivers excellent performance for the hydrogen evolution reaction (HER) in both acidic and alkaline media: in acidic solution, CS-MS/rGO-C has a very low onset potential of −142 mV vs. RHE and a low Tafel slope of 51.3 mV dec−1, and the overpotential at −10 mA cm−2 is as low as −195 mV vs. RHE; in alkaline media, the onset potential of CS-MS/rGO-C is −125 mV vs. RHE, and the Tafel slope is 83.2 mV dec−1, and the overpotential at −10 mA cm−2 is −215 mV vs. RHE. Moreover, it has outstanding long-term stability in both media even after 1000 cycles. The outstanding HER performance of CS-MS/rGO-C can be attributed to its bi-metallic composition, porous rGO-C microsphere structure, and conductive rGO-C skeleton, which can not only provide abundant active sites, but also guarantee high conductivity for CS-MS, thus facilitating the charge transfer. This work provides new insights into scalable synthesis of low-cost electrocatalysts with high efficiency and long-term stability, which can be extended to large-scale production of other advanced electrocatalysts to replace precious Pt-based catalysts for hydrogen evolution.

Growth and properties of large-area sulfur-doped graphene films

Heteroatom doping can effectively tune the structure and properties of graphene. Theoretical calculations indicate that sulfur doping can effectively modify the band structure and further modulate the carrier transport properties of graphene. However, it is still a big challenge to synthesize large-area sulfur-doped graphene (SG) films with a high sulfur doping concentration and reasonable electrical properties since sulfur has a much larger atomic radius than carbon. In this study, the solid organic source thianthrene (C12H8S2) is employed as both a carbon source and sulfur dopant to grow large-area, few-layered SG films via chemical vapor deposition (CVD). The results show that the doping concentration, doping configuration and electrical properties can be effectively tuned via the hydrogen flux. The sulfur doping concentration is as high as 4.01 at% and the maximal mobility of SG can reach up to 270 cm2 V−1 s−1, which are the highest ever reported for sulfur-doped graphene.