Nan Wu

In situ synthesis of graphitic-C3N4 nanosheet hybridized N-doped TiO2 nanofibers for efficient photocatalytic H2 production and degradation

Graphitic carbon nitride nanosheets (g-C3N4 NSs) hybridized nitrogen doped titanium dioxide (N-TiO2) nanofibers (GCN/NT NFs) have been synthesized in situ via a simple electrospinning process combined with a modified heat-etching method. The prepared GCN/NT NFs were characterized by a variety of methods and their photocatalytic activities were evaluated by hydrogen (H2) production from water splitting and degradation of rhodamine B in aqueous solution. It was found that the GCN/NT NFs have a mesoporous structure, composed of g-C3N4 NSs and N-doped TiO2 crystallites. The g-C3N4 NSs synthesized after heat-etching were found to be embedded in, and covered, the hybrid NFs to form stable interfaces. The partial decomposition of g-C3N4 releases its nitrogen content which eventually gets doped into the nearby TiO2 skeleton. The GCN/NT NFs give a high photocatalytic H2 production rate of 8,931.3 μmol·h−1·g−1 in aqueous methanol solution under simulated solar light. Such a highly efficient photocatalytic performance can be ascribed to the combined effects of g-C3N4 NSs and N-doped TiO2 with enhanced light absorption intensity and improved electron transport ability. Also, the large surface area of the mesoporous NFs minimizes light reflection on the surface and provides more surface-active sites. This work highlights the potential of quasi-one dimensional hybrid materials in the field of solar energy conversion.

Hierarchically porous SiC ultrathin fibers mat with enhanced mass transport, amphipathic property and high-temperature erosion resistance

Porous silicon carbide (SiC) has attracted considerable attention as an alternative catalyst support, particularly in corrosive and high-temperature environment. Herein, we report a facile strategy to controllably fabricate macroporous, meso-microporous and macro-meso-microporous SiC ultrathin fibers (M-SFs, MM-SFs and MMM-SFs, respectively) mats with good flexibility via electrospinning combined with polymer-derived ceramics route. The formation mechanism of different porous structures has been discussed. The MMM-SFs mat is found to exhibit simultaneously hydrophilic and lipophilic behaviors. Compared with M-SFs and MM-SFs, the MMM-SFs showed higher adsorption capacity, excellent adsorption durability and particularly faster adsorption rate (mass transport) in the adsorption experiments using methylene blue dye as a model. After being treated in dilute sulphuric acid for 5 h and subsequently heated at 800 °C for 1 h, the MMM-SFs retained their long-fiber shape and intact porous structure. Such a MMM-SFs mat may be of interest in high-temperature catalyst support, biosensor and biomedicine, energy storage, gas separation, particularly in harsh environment.

Three-dimensional (3D) interconnected networks fabricated via in-situ growth of N-doped graphene/carbon nanotubes on Co-containing carbon nanofibers for enhanced oxygen reduction

The strategy of combining highly conductive frameworks with abundant active sites is desirable in the preparation of alternative catalysts to commercial Pt/C for the oxygen reduction reaction (ORR). In this study, N-doped graphene (NG) and carbon nanotubes (CNT) were grown in-situ on Co-containing carbon nanofibers (CNF) to form three-dimensional (3D) interconnected networks. The NG and CNT bound the interlaced CNF together, facilitating electron transfer and providing additional active sites. The 3D interconnected fiber networks exhibited excellent ORR catalytic behavior with an onset potential of 0.924 V (vs. reversible hydrogen electrode) and a higher current density than Pt/C beyond 0.720 V. In addition, the hybrid system exhibited superior stability and methanol tolerance to Pt/C in alkaline media. This method can be extended to the design of other 3D interconnected network architectures for energy storage and conversion applications.

Mesoporous silicon carbide nanofibers with in situ embedded carbon for co-catalyst free photocatalytic hydrogen production

Silicon carbide (SiC) has been considered a promising metal-free photocatalyst due to its unique photoelectrical properties and thermal/chemical stability. However, its performance suffers from the fast recombination of charge carriers. Herein, we report mesoporous SiC nanofibers with in situ embedded graphitic carbon (SiC NFs-Cx) synthesized via a one-step carbothermal reduction between electrospun carbon nanofibers and Si powders. In the absence of a noble metal co-catalyst, the hydrogen evolution efficiency of SiC NFs-Cx is significantly improved under both simulated solar light (180.2 μmol·g–1·h–1) and visible light irradiation (31.0 μmol·g–1·h–1) in high-pH solution. The efficient simultaneous separation of charge carriers plays a critical role in the high photocatalytic activity. The embedded carbon can swiftly transfer the photogenerated electrons and improve light absorption, whereas the additional hydroxyl anions (OH–) in highpH solution can accelerate the trapping of holes. Our results demonstrate that the production of SiC NFs-Cx, which contains exclusively earth-abundant elements, scaled up, and is environmentally friendly, has great potential for practical applications. This work may provide a new pathway for designing stable, lowcost, high efficiency, and co-catalyst-free photocatalysts.

Vertical SnO2 nanosheet@SiC nanofibers with hierarchical architecture for high-performance gas sensors

Increasing demands for detection of harmful gases in harsh environments have stimulated considerable efforts to develop a novel gas sensor with high sensitivity, superior thermal/chemical stability and fast response/recovery rate. In this paper, we report the vertical growth of ultrathin SnO2 nanosheets (SnO2 NSs) on quasi-one-dimensional SiC nanofibers (SiC NFs) forming a hierarchical architecture via a simple hydrothermal method. In comparison to pure SnO2 NSs, the SnO2 NS@SiC NF hierarchical composite shows an ultrafast response/recovery rate, high sensitivity, and simultaneously excellent reproducibility to various target gases including ethanol, methanol, hydrogen, isopropanol, acetone and xylene, even at high temperature. The response times are less than 5 s with corresponding recovery times <15 s. Furthermore, the SnO2 NS@SiC NF gas sensor shows a superior sensing selectivity and long-term stability to ethanol. The hierarchical architecture and synergetic effect of the SnO2–SiC heterojunction as well as plenty of active sites from the vertically ultrathin SnO2 NSs have critical effect on the superior sensing performance of SnO2 NS@SiC NFs. This work highlights the possibility to develop a novel high-performance gas sensor for application in harsh environments.

Electrospun interconnected Fe-N/C nanofiber networks as efficient electrocatalysts for oxygen reduction reaction in acidic media

One-dimensional electrospun nanofibers have emerged as a potential candidate for high-performance oxygen reduction reaction (ORR) catalysts. However, contact resistance among the neighbouring nanofibers hinders the electron transport. Here, we report the preparation of interconnected Fe-N/C nanofiber networks (Fe-N/C NNs) with low electrical resistance via electrospinning followed by maturing and pyrolysis. The Fe-N/C NNs show excellent ORR activity with onset and half-wave potential of 55 and 108 mV less than those of Pt/C catalyst in 0.5 M H2SO4. Intriguingly, the resulting Fe-N/C NNs exhibit 34% higher peak current density and superior durability than generic Fe-N/C ones with similar microstructure and chemical compositions. Additionally, it also displays much better durability and methanol tolerance than Pt/C catalyst. The higher electroactivity is mainly due to the more effective electron transport between the interconnected nanofibers. Thus, our findings provide a novel insight into the design of functional electrospun nanofibers for the application in energy storage and conversion fields.

N-doped graphene grown on silk cocoon-derived interconnected carbon fibers for oxygen reduction reaction and photocatalytic hydrogen production

Carbon-based metal-free catalysts are a promising substitute for the rare and expensive platinum (Pt) used in the oxygen reduction reaction. We herein report N-doped graphene (NG) that is exquisitely integrated into highly conductive frameworks, simultaneously providing more active sites and higher conductivity. The NG was in situ grown on carbon fibers derived from silk cocoon (SCCf) using a simple one-step thermal treatment. The resulting product (NG-SCCf), possessing a meso-/macroporous structure with three-dimensional (3D) interconnected networks, exhibits an onset potential that is only 0.1 V less negative than that of Pt/C and shows stability and methanol tolerance superior to those of Pt/C in alkaline media. Moreover, in the absence of Pt as co-catalyst, NG-SCCf shows a photocatalytic H2 production rate of 66.0 μmol·h–1·g–1, 4.4-fold higher than that of SCCf. This outstanding activity is intimately related to the in situ grown NG, hierarchically porous structure, and 3D interconnected networks, which not only introduce more active sites but also enable smooth electron transfer, mass transport, and effective separation of electron-hole pairs. Considering the abundance of the green raw material in combination with easy and low-cost preparation, this work contributes to the development of advanced sustainable catalysts in energy storage/conversion fields, such as electro- and photocatalysis.

Fe/Fe3C@C nanoparticles encapsulated in N-doped graphene–CNTs framework as an efficient bifunctional oxygen electrocatalyst for robust rechargeable Zn–air batteries

3d transition metals or their derivatives encapsulated in nitrogen-doped nanocarbon show promising potential in non-precious metal oxygen electrocatalysts. Herein, we describe the simple construction of a bifunctional oxygen electrocatalyst with a “framework-active sites” structure, namely Fe/Fe3C@C (Fe@C) nanoparticles encapsulated in 3D N-doped graphene and bamboo-like CNTs (Fe@C–NG/NCNTs). The Fe@C structure provides additional electrons on the carbon surface, promoting the oxygen reduction reaction (ORR) on adjacent Fe–Nx active sites. The 3D NG hybrid with a bamboo-like CNTs framework facilitates fast reactant diffusion and rapid electron transfer. The optimized sample displays excellent ORR and oxygen evolution reaction (OER) activity, with a potential difference of only 0.84 V; this places it among the best bifunctional ORR/OER electrocatalysts. Most importantly, Zn–air batteries using Fe@C–NG/NCNTs as the cathode catalyst deliver a peak power density of 101.2 mW cm−2 and a specific capacity of 682.6 mA h g−1 (energy density of 764.5 W h kg−1). After 297 continuous cycle tests (99 h), the rechargeable batteries using Fe@C–NG/NCNTs show a voltage gap increase of only 0.13 V, almost half that of Pt/C + Ir/C (0.22 V) under the same conditions. This work provides new insight into advanced electrocatalysts utilizing the structural features of host nanocarbon materials and guest active species toward energy conversion.

Facile synthesis of FeCo@NC core–shell nanospheres supported on graphene as an efficient bifunctional oxygen electrocatalyst

Electrocatalytic conversion of oxygen holds great potential for clean energy technologies, including water electrolysis, regenerative fuel cells, and rechargeable metal-air batteries. The development of highly efficient and inexpensive oxygen electrocatalysts as replacements for precious metal-based catalysts is vitally important for large-scale practical application in the future. A bifunctional oxygen electrocatalyst based on FeCo nanoparticles/N-doped carbon core–shell spheres supported on N-doped graphene sheets was prepared via one-step pyrolysis of graphitic carbon nitride and acetylacetonates. The optimized product exhibited an oxygen electrode activity of 0.87 V and excellent durability. The remarkable performance is mainly attributed to the synergetic effect arising from the FeCo nanoparticles and N-doped carbon shell. This study introduces an inexpensive and simple way to develop highly active bifunctional oxygen electrocatalysts.

A simply prepared flexible SiBOC ultrafine fiber mat with enhanced high-temperature stability and chemical resistance

Boron containing silicon-based ceramics with enhanced high-temperature stability have attracted great attention. An ultrafine and flexible silicon boron oxycarbide (SiBOC) fiber mat fabricated by electrospinning with a sol–gel system and subsequent pyrolysis is reported. The formation, composition and structure of the SiBOC fibers were studied by FT-IR, XRD, TG-MS, FE-SEM, HRTEM, XPS and MAS NMR. In comparison with the SiOC fibers, the SiBOC fibers showed better high-temperature stability. After being heated at 800 °C in air for 1 h, or immersed in 2 M NaOH and 1 M H2SO4 solutions for 24 h, the SiBOC fibers remained intact without of any detectable change. The SiBOC fibers illustrated superior amphiphilic affinity to water and xylene. Such a flexible SiBOC fiber mat may be of interest as a high-temperature catalyst support, LIB anode, capacitor, and so on.