Chengqun Qin

Nitrogen and fluorine co-doped graphene as a high-performance anode material for lithium-ion batteries

Nitrogen and fluorine co-doped graphene (NFG) with the N and F content as high as 3.24 and 10.9 at% respectively was prepared through the hydrothermal reaction of trimethylamine tri(hydrofluoride) [(C2H5)3N·3HF] and aqueous-dispersed graphene oxide (GO) as the anode material for lithium ion batteries (LIBs). The N and F co-doping in graphene increased the disorder and defects of the framework, enlarged the space of the interlayer, wrinkled the nanosheets with many open-edge sites, and thus facilitated Li ion diffusion through the electrode compared with sole-N or F doped graphene. X-ray photoelectron spectroscopy (XPS) analysis of NFG demonstrated the presence of active pyridine and pyrrolic type N, and highly electrically conductive graphitic N and the semi-ionic C–F bond in the structure. The N and F doping content and the component types of N and F functional groups could be controlled by the hydrothermal temperature. The NFG prepared at 150 °C exhibited the best electrochemical performances when tested as the anode for LIBs, including the high coulombic efficiency in the first cycle (56.7%), superior reversible specific discharge capacity (1075 mA h g−1 at 100 mA g−1), excellent rate capabilities (305 mA h g−1 at 5 A g−1), and outstanding cycling stability (capacity retention of ∼95% at 5 A g−1 after 2000 cycles), which demonstrated that NFG was a promising candidate for anode materials of high-rate LIBs.

A high energy density azobenzene/graphene hybrid: a nano-templated platform for solar thermal storage

Effective conversion of light into heat is an emerging field showing great potential for large-scale applications, markedly driven by novel molecules and structures. Unfortunately, until now, it is still hindered by a low storage capacity and short-time storage. A nano-template for covalently attaching new azobenzene chromophores on graphene as solar thermal fuels is presented here, in which the intermolecular hydrogen bond and proximity-induced interaction, resulting from a high functionalization density and inter-planar bundling interaction, remarkably improve both the storage capacity and lifetime. This nanoscopic template exhibits a high energy density up to 112 W h kg−1 and long-term storage with a half-life of more than one month (33 days), which are also confirmed by the calculations using density functional theory, simultaneously maintaining an excellent cycling stability tuned by visible light for 50 cycles. Our work develops a promising class of solar thermal fuels with high energy density, which outperform previous nano-materials and are comparable to commercial soft-packing Li-ion batteries.

A layer-nanostructured assembly of PbS quantum dot/multiwalled carbon nanotube for a high-performance photoswitch

A layered nanostructure of a lead sulfide (PbS) quantum dot (QD)/multi-walled carbon nanotube (MWNT) hybrid was prepared by the electrostatic assembly after the phase transfer of PbS QDs from an organic to an aqueous phase. Well-crystallized PbS QDs with a narrow diameter (5.5 nm) was mono-dispersed on the sidewalls of MWNT by the electrostatic adsorption. Near-infrared absorption of PbS/MWNT nanostructures was improved and controlled by the packing density of PbS QDs. Efficient charge transfer between PbS and MWNT at the interface resulted in a remarkable quenching of photoluminescence up to 28.6% and a blue-shift of emission band by 300 nm. This feature was facilitated by band energy levels based on the intimate contact through the electrostatic interaction. Two-terminal devices using PbS/MWNT nanostructures showed an excellent on/off switching photocurrent and good stability during 20 cycles under light illumination due to electron transfer from PbS to MWNT. The photoswitch exhibited a high photo sensitivity up to 31.3% with the photocurrent of 18.3 μA under the light of 3.85 mW/cm2, which outperformed many QD/carbon-based nanocomposites. Results indicate that the electrostatic layered assembly of QD/MWNT nanostructure is an excellent platform for the fabrication of high-performance optoelectronic devices.

A supramolecular assembly of cross-linked azobenzene/polymers for a high-performance light-driven actuator

Light-driven flexible actuators based on a photo-responsive polymer draw much attention due to their great ability for rapid and reversible light-to-work transduction based on a large deformation. An azobenzene chromophore with disulfonic groups (AAZO) was noncovalently grafted on the side-chain of a cationic polymer, poly(diallyldimethylammonium chloride) (PDAC), by electrostatic interaction in a specific weight ratio. A supramolecular assembly of cross-linked AAZO/PDAC showed a reversible isomerization on irradiation by UV light followed by a reversion with a good cycling stability for 50 cycles. Light-driven actuators based on the AAZO/PDAC film exhibited a large deformation by rolling-up into a tube with double walls even when the light was off, along with a spontaneous shape recovery. This photomechanical deformation arose from different rates and degrees of structural transformation of AAZO/PDAC between the front (facing UV light) and back side with the segmental motion of polymers.

Large-Scale Synthesis of a Uniform Film of Bilayer MoS2 on Graphene for 2D Heterostructure Phototransistors

The large-scale synthesis of atomically thin, layered MoS2/graphene heterostructures is of great interest in optoelectronic devices because of their unique properties. Herein, we present a scalable synthesis method to prepare centimeter-scale, continuous, and uniform films of bilayer MoS2 using low-pressure chemical vapor deposition. This growth process was utilized to assemble a heterostructure by growing large-scale uniform films of bilayer MoS2 on graphene (G-MoS2/graphene). Atomic force microscopy, Raman spectra, and transmission electron microscopy characterization demonstrated that the large-scale bilayer MoS2 film on graphene exhibited good thickness uniformity and a polycrystalline nature. A centimeter-scale phototransistor prepared using the G-MoS2/graphene heterostructure exhibited a high responsivity of 32 mA/W with good cycling stability; this value is 1 order of magnitude higher than that of transferred MoS2 on graphene (2.5 mA/W). This feature results from efficient charge transfer at the interface enabled by intimate contact between the grown bilayer MoS2 (G-MoS2) and graphene. The ability to integrate multilayer materials into atomically thin heterostructures paves the way for fabricating multifunctional devices by controlling their layer structure.

High-energy, stable and recycled molecular solar thermal storage materials using AZO/graphene hybrids by optimizing hydrogen bonds

An important method for establishing a high-energy, stable and recycled molecular solar heat system is by designing and preparing novel photo-isomerizable molecules with a high enthalpy and a long thermal life by controlling molecular interactions. A meta- and ortho-bis-substituted azobenzene chromophore (AZO) is covalently grafted onto reduced graphene oxide (RGO) for solar thermal storage materials. High grafting degree and close-packed molecules enable intermolecular hydrogen bonds (H-bonds) for both trans-(E) and cis-(Z) isomers of AZO on the surface of nanosheets, resulting in a dramatic increase in enthalpy and lifetime. The metastable Z-form of AZO on RGO is thermally stabilized with a half-life of 52 days by steric hindrance and intermolecular H-bonds calculated using density functional theory (DFT). The AZO-RGO fuel shows a high storage capacity of 138 Wh kg(-1) by optimizing intermolecular H-bonds with a good cycling stability for 50 cycles induced by visible light at 520 nm. Our work opens up a new method for making advanced molecular solar thermal storage materials by tuning molecular interactions on a nano-template.

Surface passivation of carbon dots with ethylene glycol and their high-sensitivity to Fe3+

Hydroxyl functionalized carbon dots (H-CDs) were prepared by monoesterification of ethylene glycol. The H-CDs exhibit a narrow size distribution of 1–4 nm and enhanced photoluminescent (PL) intensity due to an increased amount of electron donor hydroxyl groups. According to fluorescence spectra, the H-CDs exhibit a high sensitivity to Fe3+ with a detection limit of 2.56 nM, which is superior to the detection limit of CDs (7.4 μM). The quenching fluorescence is primarily controlled by the formation of a chelate compound based on the complexation between Fe3+ and the hydroxyl on the surface of the H-CDs. Furthermore, we demonstrate that paper impregnated with H-CDs exhibits a high sensitivity to Fe3+ by fluorescence quenching. In the future, the modified CDs can be developed for high sensitivity fluorescent probes by optimizing the chemical structures and microstructures.

An energy-dense and thermal-stable bis-azobenzene/hybrid templated assembly for solar thermal fuel

Designing an energy-dense and thermal-stable photo-isomerizable chromophore/hybrid template is a challenge for devising a highly customizable solar-heat conversion and storage technology. This study presents the templated assembly of close-packed 2-chloro-4,6-bis(4-(phenyldiazenyl)phenoxy)-1,3,5-triazine (bis-azobenzene chromophores) covalently bound to reduced graphene oxide (RGO-bis-Azo). The steric configuration and energy of bis-Azo (trans- and cis-isomers), calculated by density functional theory, are influenced by intra- and inter-molecular steric hindrance due to high grafting density and the bundling effect. This results in a dramatic increase in enthalpy and activation energy of the isomerization. The RGO-bis-Azo hybrid combines a high energy density of 80 W h kg−1, the maximum power density of 2230 W kg−1 and a tunable heat release time from 2 min to 5520 h. These findings pave the way for developing a chromophore/hybrid template for solar thermal fuel by optimizing molecular interaction.

The light-switching conductance of an anisotropic azobenzene-based polymer close-packed on horizontally aligned carbon nanotubes

Anisotropic light-switching properties are of great importance in advanced photodetectors, light-gated transistors, and energy storage devices. However, controlling the light-switching conductance of anisotropic materials remains challenging because of the difficulties in tuning the electronic interactions and microstructure in the longitudinal and transverse directions, respectively. We present a transparent and flexible photo-responsive film of azobenzene–poly(methyl methacrylate) (Azo–PMMA) close-packed on the sidewalls of horizontally aligned carbon nanotubes (HACNTs). The alignment leads to an anisotropic electrical conductivity (σ). The aligned composite film shows a steady increase in σ in the longitudinal (σ∥) and transverse directions (σ⊥) during UV irradiation and a continuous decrease after irradiation. The light-switching conductance shows good cycling performance over 25 cycles, which is consistent with the isomerisation of the azobenzene groups in Azo–PMMA. Light-switching conductance is demonstrated in two directions, originating from the light-induced switch to a greater proportion of more conductive cis-Azo isomers, photo-doping and the film contracting during the trans-to-cis photo-isomerisation. The anisotropic Azo–PMMA/HACNT composite film with a light-switchable conductivity paves the way towards the fabrication of anisotropic photo-controllable materials by molecular design and microstructure modulation.

A sulfonimide-based alternating copolymer as a single-ion polymer electrolyte for high-performance lithium-ion batteries

For the next generation of lithium-ion batteries (LIBs), single Li-ion polymer electrolytes (SPEs) are widely considered an effective substitute to traditional dual-ion electrolytes, due to their ability to restrain the salt concentration gradient and the polarization loss in the cells. A new single-ion conductor with an alternating structure is synthesized by the simple radical copolymerization of lithium 4-styrenesulfonyl(phenyl-sulfonyl)imide and maleic anhydride. Its SPE membrane composite with poly(vinylidene fluoride-co-hexafluoropropylene) exhibits both high lithium ion conductivity (σLi+ = 2.67 mS cm−1) and transference number (tLi+ = 0.98). The full cell with the prepared SPE sandwiched between a LiFePO4 cathode and a Li4Ti5O12 anode shows good cycling stability and rate capability. These results suggest that this novel electrolyte is promising for application in next-generation LIBs.