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Low-temperature synthesis of large-area graphene films by chemical vapor deposition (CVD) method has attracted extensive research interests owing to their extraordinary potential applications. Current reports on the low-temperature graphene growth usually involve catalytic metal substrates, which exceedingly impedes the direct use of graphene in nanoelectronic fabrications. In this project, we design a space-confined growth chamber utilizing two SiO2/Si chips. The bottom chip loading pre-transferred graphene flakes accommodates the growth of second monolayer graphene, with a tellurium (Te) vapor assisted growth process. Polymethylmethacrylate (PMMA) residues introduced from the transferred graphene substrate is employed as the solid carbon source. Further, the numerical simulation results prove that the high concentration of active carbons in the confined space plays a vital role in this growth process. Density functional theory (DFT) calculations reveal that, Te atoms prefer to bond with the edge carbons of small graphene flakes and serve as an effective catalyst, which helps to grow small graphene flakes by reducing their energy of formation. The successful growth can provide the high-quality monolayer graphene grains at the low temperature (∼350 °C). This facile strategy promises the mass production of graphene at mild conditions and low costs, which contributes to the green chemistry in future.

The further improvement of sodium ion batteries requires the elucidation of the mechanisms pertaining to reversibility, which allows the novel design of the electrode structure. Here, through a hydrogel-embedding method, we are able to confine the growth of few-layer SnS2 nanosheets between a nitrogen- and sulfur-doped carbon nanotube (NS-CNT) and amorphous carbon. The obtained carbon-sandwiched SnS2 nanosheets demonstrate excellent sodium storage properties. In operando small-angle X-ray scattering combined with the ex situ X-ray absorption near edge spectra reveal that the redox reactions between SnS2/NS-CNT and the sodium ion are highly reversible. On the contrary, the nanostructure evolution is found to be irreversible, in which the SnS2 nanosheets collapse, followed by the regeneration of SnS2 nanoparticles. This work provides operando insights into the chemical environment evolution and structure change of SnS2-based anodes, elucidating its reversible reaction mechanism, and illustrates the significance of engineered carbon support in ensuring the electrode structure stability.

To enhance interlayer lithium diffusion, we engineer electrodes consisting of epitaxially grown ReSe2 nanosheets by chemical vapor deposition, supported on three-dimensional (3D) graphene foam, taking advantage of its weak van der Waals coupling and anisotropic crystal structure. We further demonstrate its excellent performance as the anode for lithium-ion battery and catalyst for hydrogen evolution reaction (HER). Density functional theory calculation reveals that ReSe2 exhibits a low energy barrier for lithium (Li) interlayer diffusion because of negligible interlayer coupling and anisotropic structure with low symmetry that creates additional adsorption sites and leads to a reduced diffusion barrier. Benefitting from these properties, the 3D ReSe2/graphene foam electrode displays excellent cycling and rate performance with 99.6% capacity retention after 350 cycles and a capacity of 327 mA h g-1 at the current density of 1000 mA g-1. Additionally, it has exhibited a high activity for HER, in which an exchange current density of 277.8 μA cm-2 is obtained and only an overpotential of 106 mV is required to achieve a current density of -10 mA cm-2. Our work provides a fundamental understanding of the interlayer diffusion of Li in transition-metal dichalcogenide (TMD) materials and acts as a new tool for designing a TMD-based catalyst.

For lithium-sulfur batteries (LSBs), the dissolution of lithium polysulfide and the consequent "shuttle effect" remain major obstacles for their practical applications. In this study, we designed a new cathode material comprising MoSe2/graphene to selectively adsorb polysulfides on the selenium edges and thus to mitigate their dissolution. More specifically, few-layered MoSe2 was first grown on nitrogen-doped reduced graphene oxide (N-rGO) using the chemical vapor deposition method and then infiltrated with sulfur as the cathode for LSBs. An initial capacity of 1028 mA h g-1 was achieved for S/MoSe2/N-rGO at 0.2 C, higher than 981 and 405.1 mA h g-1 for pure graphene and sulfur, respectively, along with enhanced cycling durability and rate capability. Moreover, the density functional theory simulation, in addition to the experimental adsorption test, X-ray photoelectron spectroscopy analysis, and transmission electron microscopy technique, reveals the dual roles that MoSe2 plays in improving the performance of LSBs by functioning as the binding sites for lithium polysulfides and as the platform that enables fast Li-ion diffusion by reducing its diffusion barrier. The reported finding suggests that the transition-metal selenides could be an efficient alternative material as the cathode for LSBs.

The rhenium-based transition metal dichalcogenides (TMDs), as new members in the TMDs family, have raised great interests recently. Due to the anisotropic structure and unique photoelectric properties, they have potential applications for electrochemical energy conversion and storage. In this work, we performed density functional theory (DFT) calculations on pristine 1T′-ReSe2 toward hydrogen evolution reaction (HER). The results indicated that the Gibbs free energy of the 1T′-ReSe2 edge site for HER could be as small as 0.01 eV, superior to other reported TMDs. Experimentally, we developed a strategy to fabricate sub-5 nm sized 1T′-ReSe2 nanoflakes on carbon nanotubes. Such a small size for the nanoflakes brought abundant edge exposure, which boosted the catalytic activity in the HER. Specifically, the 1T′-ReSe2 nanoflakes needed only 23 and 60 mV overpotentials to achieve −1 and −10 mA cm−2 current densities, along with a low Tafel slope of 37 mV dec−1 and a high exchange current density of 0.3 mA cm−2. The edge-rich and layered 1T′-ReSe2 was also explored as an anode for sodium ion battery. The in operando X-ray absorption near edge structure (XANES) technique was applied to investigate the TMD behavior in real-time during the sodiation/desodiation process. The in situ results revealed that the nanosized 1T′-ReSe2 is electrchemically reversible during discharge/charge cycles. The electrochemical test results demonstrated that 1T′-ReSe2 could be a promising anode material for alkaline batteries.

Conventional absorbents for hemoperfusions suffer from low efficiency and slow absorption with numerous side effects. In this research, we developed cellulose acetate (CA) functionalized graphene oxide (GO) beads (∼1.5-2 mm) that can be used for direct hemoperfusion, aiming at the treatment of kidney dysfunction. The CA-functionalized GO bead facilitates adsorption of toxins with high biocompatibility and high-efficiency of hemoperfusion while maintaining high retention for red blood cell, white blood cells, and platelets. Our in vitro results show that the toxin concentration for creatinine reduced from 0.21 to 0.12 μM (p < 0.005), uric acid from 0.31 to 0.15 mM (p < 0.005), and bilirubin from 0.36 to 0.09 mM (p < 0.005), restoring to normal levels within 2 h. Our in vivo study on rats (Sprague-Dawley, n = 30) showed that the concentration for creatinine reduced from 83.23 to 54.87 μmol L-1 (p < 0.0001) and uric acid from 93.4 to 54.14 μmol L-1 (p < 0.0001), restoring to normal levels within 30 min. Results from molecular dynamics (MD) simulations using free-energy calculations reveal that the presence of CA on GO increases the surface area for adsorption and enhances penetration of toxins in the binding cavities because of the increased electrostatic and van der Waals force (vdW) interactions. These results provide critical insight to fabricate graphene-based beads for hemoperfusion and to have the potential for the treatment of blood-related disease.

We developed a general platform for the fabrication of transition metal oxide nanoparticles supported by a graphene foam (GF) by first coating it with a methacrylated gelatin (GelMA) hydrogel, which served as a 3D matrix for nanoparticle dispersion. The engineered GelMA/GF matrix was hydrophilic with good mechanical strength and high conductivity, therefore providing a good platform for the dispersion of a variety of metal/oxide precursors. Due to this platform, well-dispersed Co3O4 nanoparticles with the smallest size of 3 nm assembled on the nitrogen-doped graphene foam (Co3O4/NGF). The crystalline transformation from a CoCl2[H2O]2 precursor to Co3O4 was revealed by in operando X-ray diffraction and absorption techniques. After applying Co3O4/NGF as a free-standing electrocatalyst for water splitting, the nanoparticles of size 3 nm exhibited optimal catalytic activity in alkaline media; the corresponding cell could promote water splitting at a current density of 10 mA cm-2 with only 1.63 V and exhibited excellent stability in a 25 h long-term operation. Our results demonstrate that the GelMA hydrogel-coated 3D graphene foam can be a promising platform for the design and fabrication of graphene-based multifunctional materials.

With the increasing demand for wearable and implantable electronics, structured sensors with exceptional performance in sensitivity, stretchability, biocompatibility, and adhesion to the biological surfaces have become essential. Here, we report a novel graphene strain sensor, whose structure contains a carefully selected graphene foam (GF) and polyacrylamide/calcium-alginate (PAM/CA) double network hydrogel coupled with chitosan, can meet the above requirements to the maximum extent. Taking the advantage of the excellent electrical properties of graphene and the mechanical properties of hydrogel, this strain sensor can measure an exceptionally wide range of strain up to 500%, which allows us to monitor and distinguish almost all human body motions, with a gauge factor of ∼1800, which is up to 3 orders of magnitude larger than the 0.1–1000 value reported previously. The device also shows high stability and excellent durability for 1000 cycles. Most importantly, the chitosan-bridge and skin-like elastic modulus of 8–90 kPa allow conformal adhesion between the skin and hydrogel, with the adhesion energy of ∼200 J m−2. Moreover, this structured sensor is highly biocompatible and can be implanted in vivo, and has further potential in clinical applications.

The edge site of two-dimensional (2D) transition metal dichalcogenides (TMDs) is active towards the hydrogen evolution reaction (HER). Herein, a feasible synthesis of sub-100 nm molybdenum/tungsten diselenide [Mo(1−x)WxSe2] crystals is described. The abundant edge exposure and heteroatom-doping synergistically boost the catalysis of HER by this material. In this work, sub-100 nm Mo(1−x)WxSe2 single crystals were grown on a nitrogen-doped multiwall carbon nanotube before being applied as HER electrocatalysts. At x = 0.13 ± 0.02, the Mo(1−x)WxSe2 shows optimal HER catalytic performance with low overpotentials (70 and 129 mV) required to achieve current densities of −1 and −10 mA cm−2, respectively, along with a Tafel slope of 53.6 mV dec−1 and an exchange current density of 49.5 μA cm−2. Density functional theory (DFT) calculations indicate that the Gibbs free energy of the HER process at the edge site of the crystals reaches a minimum value of 0.06 eV, which is lower than when the reaction is catalysed on Pt active sites. This study provides a general approach to increasing the edge proportion of the catalyst material and activating the terrace of the 2D materials for catalysis, which may be of benefit to the design and fabrication of other TMDs-based compounds.