Yongcai Qiu

High-Rate, Ultralong Cycle-Life Lithium/Sulfur Batteries Enabled by Nitrogen-Doped Graphene

Nitrogen-doped graphene (NG) is a promising conductive matrix material for fabricating high-performance Li/S batteries. Here we report a simple, low-cost, and scalable method to prepare an additive-free nanocomposite cathode in which sulfur nanoparticles are wrapped inside the NG sheets (S@NG). We show that the Li/S@NG can deliver high specific discharge capacities at high rates, that is, ∼ 1167 mAh g(-1) at 0.2 C, ∼ 1058 mAh g(-1) at 0.5 C, ∼ 971 mAh g(-1) at 1 C, ∼ 802 mAh g(-1) at 2 C, and ∼ 606 mAh g(-1) at 5 C. The cells also demonstrate an ultralong cycle life exceeding 2000 cycles and an extremely low capacity-decay rate (0.028% per cycle), which is among the best performance demonstrated so far for Li/S cells. Furthermore, the S@NG cathode can be cycled with an excellent Coulombic efficiency of above 97% after 2000 cycles. With a high active S content (60%) in the total electrode weight, the S@NG cathode could provide a specific energy that is competitive to the state-of-the-art Li-ion cells even after 2000 cycles. The X-ray spectroscopic analysis and ab initio calculation results indicate that the excellent performance can be attributed to the well-restored C-C lattice and the unique lithium polysulfide binding capability of the N functional groups in the NG sheets. The results indicate that the S@NG nanocomposite based Li/S cells have a great potential to replace the current Li-ion batteries.

All-solid-state hybrid solar cells based on a new organometal halide perovskite sensitizer and one-dimensional TiO2 nanowire arrays

A novel organometal halide perovskite (CH3NH3PbI2Br) is synthesized and used as a visible light absorber to sensitize one-dimensional (1D) TiO2 nanowire arrays (NWAs) for all-solid-state hybrid solar cells. It achieved a power conversion efficiency (PCE) of 4.87% and an open circuit voltage (Voc) of 0.82 V, both higher than those of its analogue CH3NH3PbI3.

Secondary branching and nitrogen doping of ZnO nanotetrapods: Building a highly active network for photoelectrochemical water splitting

A photoanode based on ZnO nanotetrapods, which feature good vectorial electron transport and network forming ability, has been developed for efficient photoelectrochemical water splitting. Two strategies have been validated in significantly enhancing light harvesting. The first was demonstrated through a newly developed branch-growth method to achieve secondary and even higher generation branching of the nanotetrapods. Nitrogen-doping represents the second strategy. The pristine ZnO nanotetrapod anode yielded a photocurrent density higher than those of the corresponding nanowire devices reported so far. This photocurrent density was significantly increased for the new photoanode architecture based on the secondary branched ZnO nanotetrapods. After N-doping, the photocurrent density enjoyed an even more dramatic enhancement to 0.99 mA/cm(2) at +0.31 V vs Ag/AgCl. The photocurrent enhancement is attributed to the greatly increased roughness factor for boosting light harvesting associated with the ZnO nanotetrapod branching, and the increased visible light absorption due to the N-doping induced band gap narrowing of ZnO.

Synthesis of Size-Tunable Anatase TiO2 Nanospindles and Their Assembly into Anatase@Titanium Oxynitride/Titanium Nitride−Graphene Nanocomposites for Rechargeable Lithium Ion Batteries with High Cycling Performance

This paper embarks upon three levels of undertaking ranging from nanomaterials synthesis to assembly and functionalization. First, we have prepared size-tunable anatase TiO(2) nanospindles via a hydrothermal process by using tubular titanates as self-sacrificing precursors. Second, we have densely dispersed the TiO(2) nanospindles onto functional graphene oxides (GO) via a spontaneous self-assembly process. After annealing of the TiO(2)/GO hybrid nanocomposite in an NH(3) gas flow, the TiO(2) surface was effectively nitridated and the GO was reduced to graphene sheets (GS) in order to further fortify the electronic functionality of the nanocomposite. Third, the anatase@oxynitride/titanium nitride-GS (TiO(2)@TiO(x)N(y)/TiN-GS) hybrid nanocomposite was studied as an anode material for lithium-ion batteries (LIBs), showing excellent rate capability and cycling performance compared to the pure TiO(2) nanospindles. Our systematic studies have revealed that the TiO(2)@TiO(x)N(y)/TiN-GS nanocomposites with graphene nanosheets covered with the TiO(2)@TiO(x)N(y)/TiN nanospindles on both sides provide a promising solution to the problems of poor electron transport and severe aggregation of TiO(2) nanoparticles by enhancing both electron transport through the conductive matrix and Li-ion accessibility to the active material from the liquid electrolyte. More generally, the size-tunable TiO(2) nanospindles with their unique (101) outer surface planes provide an archetype for the in depth investigation of their surface-specific and size-dependent physicochemical properties.

Efficient Photoelectrochemical Water Splitting with Ultrathin films of Hematite on Three-Dimensional Nanophotonic Structures

Photoelectrochemical (PEC) solar water splitting represents a clean and sustainable approach for hydrogen (H2) production and substantial research are being performed to improve the conversion efficiency. Hematite (α-Fe2O3) is considered as a promising candidate for PEC water splitting due to its chemical stability, appropriate band structure, and abundance. However, PEC performance based on hematite is hindered by the short hole diffusion length that put a constraint on the active layer thickness and its light absorption capability. In this work, we have designed and fabricated novel PEC device structure with ultrathin hematite film deposited on three-dimensional nanophotonic structure. In this fashion, the nanophotonic structures can largely improve the light absorption in the ultrathin active materials. In addition, they also provide large surface area to accommodate the slow surface water oxidation process. As the result, high current density of 3.05 mA cm(-2) at 1.23 V with respect to the reversible hydrogen electrode (RHE) has been achieved on such nanophotonic structure, which is about three times of that for a planar photoelectrode. More importantly, our systematic analysis with experiments and modeling revealed that the design of high performance PEC devices needs to consider not only total optical absorption, but also the absorption profile in the active material, in addition to electrode surface area and carrier collection.

High performance supercapacitors based on highly conductive nitrogen-doped graphene sheets

Thermal nitridation of reduced graphene oxide sheets yields highly conductive (∼1000-3000 S m(-1)) N-doped graphene sheets, as a result of the restoration of the graphene network by the formation of C-N bonded groups and N-doping. Even without carbon additives, supercapacitors made of the N-doped graphene electrodes can deliver remarkable energy and power when operated at higher voltages, in the range of 0-4 V.

Double-layered photoanodes from variable-size anatase TiO2 nanospindles: a candidate for high-efficiency dye-sensitized solar cells.

nanospindlesofdifferentsizestoconstruct a double-layered photoanode for dye-sensitizedsolar cells (DSSC). One layer made of larger nanospindlesenhances light scattering, and the other consisting of smallernanospindles increases the roughness factor for efficient dyeadsorption. The two-layer structure with variable-size, single-crystalline TiO

A novel nanostructured spinel ZnCo2O4 electrode material: morphology conserved transformation from a hexagonal shaped nanodisk precursor and application in lithium ion batteries

In this paper, we report a successful synthesis of porous ZnCo2O4 nanoflakes by a morphology-conserved and pyrolysis-induced transformation of novel hexagonally shaped, highly ordered, and inorganic–organic–inorganic layered hybrid nanodisks. It is shown that the hexagonal hybrid nanodisks are constructed from organic molecule (ethylene glycol)-directed assembly of inorganic bilayers. The assembly mechanism has been established by a number of structural and spectroscopic techniques. The porous ZnCo2O4 nanoflakes have also been tested as a lithium ion battery electrode, showing high capacity and high cyclability.

A double layered photoanode made of highly crystalline TiO2 nanooctahedra and agglutinated mesoporous TiO2 microspheres for high efficiency dye sensitized solar cells

We report the development of a novel double layered photoanode for dye sensitized solar cells made of highly crystalline TiO2 octahedral nanocrystals and agglutinated mesoporous TiO2 microspheres. The underlayer of nanooctahedra serves as a transparent photoanode for copious and strong dye adsorption on the smooth (101) surfaces and for facilitated electron transport. Although the nanooctahedra are extremely small, our synthetic route has ensured a well-faceted crystalline shape with sharp edges and smooth surfaces, resulting in a 7.61% power conversion efficiency, much higher than that of P25 (5.76%). Separately, the overlayer of hierarchical TiO2 mesoporous microspheres plays the multiple roles of efficient light scattering, dye absorption and electrolyte permeation. Especially noteworthy is the agglutination of the microspheres through our 3D necking process, which has yielded an electron diffusion coefficient five times that of the P25 network and four times that of the nanooctahedra network. This is a significant breakthrough in DSSCs, which ensures that the photogenerated electrons in the overlayer can be effectively transported through such highway-like paths and ultimately collected at the FTO electrode. Therefore, in this double layered photoanode we have taken into consideration a number of disparate factors aimed at enhancing the overall DSSC performance. Drawing on the judicious combination of materials synthesis and engineering of nano-architectures and interfaces, solar cells based on this double layered structure have achieved 8.72% power conversion efficiency even with simple device fabrication procedures, showing promise as a new photoanode design for high efficiency dye sensitized solar cells.

Morphology-conserved transformation: synthesis of hierarchical mesoporous nanostructures of Mn2O3 and the nanostructural effects on Li-ion insertion/deinsertion properties

By means of morphology-conserved transformation, we have synthesized hierarchically structured Mn2O3 nanomaterials with different morphologies and pore structures. The key step of this method consists of the formation of a precursor containing the target materials interlaced with the judiciously chosen polyol-based organic molecules, which are subsequently knocked out to generate the final nanomaterials. In the present work, two kinds of precursor morphologies, oval-shaped and straw-sheaf-shaped, have been selectively prepared by hydrothermal treatment of different functional polyol molecules (oval-shape with fructose and straw-sheaf-shape with β-cyclodextrin) and potassium permanganate. Thermal decomposition of the precursors resulted in the formation of mesoporous Mn2O3 maintaining the original morphologies, as revealed by extensive characterization. These novel hierarchical nanostructures with different pore sizes/structures prompted us to examine their potential as anode materials for lithium ion batteries (LIBs). The electrochemical results with reference to LIBs show that both of our mesoporous Mn2O3 nanomaterials deliver high reversible capacities and excellent cycling stabilities at a current density of 200 mA g−1 compared to the commercial Mn2O3 nanoparticles. Moreover, the straw-sheaf-shaped Mn2O3 exhibits a higher specific capacity and a better cycling performance than the oval-shaped one, due to the relatively higher surface area and the peculiar nanostrip structure resulting in the reduced length for lithium ion diffusion. Morphology-conserved transformation yields two kinds of hierarchical mesoporous Mn2O3 nanomaterials with high capacities and cycling stabilities for lithium ion batteries.