Jing-Hua Tian

Improved seedless hydrothermal synthesis of dense and ultralong ZnO nanowires

Seedless hydrothermal synthesis has been improved by introducing an adequate content of ammonia into the nutrient solution, allowing the fabrication of dense and ultralong ZnO nanowire arrays over large areas on a substrate. The presence of ammonia in the nutrient solution facilitates the high density nucleation of ZnO on the substrate which is critical for the nanowire growth. In order to achieve an optimal growth, the growth conditions have been studied systematically as a function of ammonia content, growth temperature and incubation time. The effect of polyethyleneimine (PEI) has also been studied but shown to be of no benefit to the nucleation of ZnO. Ultradense and ultralong ZnO nanowires could be obtained under optimal growth conditions, showing no fused structure at the foot of the nanowire arrays. Due to different reaction kinetics, four growth regimes could be attributed, including the first fast growth, equilibrium phase, second fast growth and final erosion. Combining this simple method with optical lithography, ZnO nanowires could be grown selectively on patterned areas. In addition, the as-grown ZnO nanowires could be used for the fabrication of a piezoelectric nanogenerator. Compared to the device of ZnO nanowires made by other methods, a more than twice voltage output has been obtained, thereby proving an improved performance of our growth method.

A facile synthesis of CoFe2O4/biocarbon nanocomposites as efficient bi-functional electrocatalysts for the oxygen reduction and oxygen evolution reaction

Efficient electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial for improving the performance of metal–air batteries. In this study, CoFe2O4/biocarbon (CFO/BC) nanocomposites have been synthesized via a facile biosynthesis method by using yeast cells as carbon sources and structural templates. The as-prepared CFO/BC nanocomposites possess a hierarchical structure with a high surface area (79.84 m2 g−1). The rotating ring-disk electrode (RRDE) and rotating disk electrode (RDE) measurements revealed that CFO/BC nanocomposites exhibit excellent catalytic activity for both the ORR and OER. The onset potential of CFO/BC for the ORR is −0.14 V (vs. Ag/AgCl), which is higher than that of CoFe2O4 (−0.29 V) and that of biocarbon (−0.25 V), respectively. Meanwhile, the CFO/BC nanocomposites show much higher activity for the OER as compared to CoFe2O4 and biocarbon. The chronoamperometric tests show that the CFO/BC catalyst shows high durability for both the ORR and OER, outperforming the commercial Pt/C (20 wt% Pt on Vulcan XC-72, Johnson Matthey). The high electrocatalytic activity and durability of the CFO/BC nanocomposite are mainly attributed to the strong coupling between CoFe2O4 nanoparticles and biocarbon as well as the hierarchical structure of CFO/BC.

Silicon-nanoparticles isolated by in situ grown polycrystalline graphene hollow spheres for enhanced lithium-ion storage

A silicon-based anode material offers extremely high lithium storage capacity, but suffers from severe volume expansion during lithiation, which causes a drastic capacity decay. Embedding and isolating Si nanoparticles (SiNPs) into sealed amorphous carbon hollow spheres with sufficient voids is a promising strategy to accommodate the volume effect of Si. However, the created voids significantly compromise the volumetric energy density. Conversely, if insufficient voids are introduced, the inferior mechanical property of the amorphous carbon turns into the decisive factor destroying the structural integrity of the composites. Graphene is more suitable as a protective shell material due to its excellent mechanical strength. However, there still remains a formidable challenge of constructing closed graphene hollow spheres owing to their unique two-dimensional structure. Herein, we first propose a bottom-up route to controllably synthesize a polycrystalline graphene hollow sphere isolated SiNP nanocomposite (Si@void@graphene) through an in situ pyrolysis and metal-catalyzed graphitization reaction, in which glucose and metal sulfate with strictly controlled content and ratio are employed as the carbon source and catalyst precursor, respectively. The obtained graphene hollow spheres with superb mechanical properties offer a permanent structural and electrical environment for the inner SiNPs even insufficient voids are introduced while maintaining a reasonable volumetric energy density. When the void space is designed based on the assumption that Si has only 250% volume change, the Si@void@graphene electrode exhibits high reversible capacity, superior rate capability and much prolonged cycle life as compared to those of the Si@void@amorphous carbon electrode.

One-pot synthesis of monodispersed porous CoFe2O4 nanospheres on graphene as an efficient electrocatalyst for oxygen reduction and evolution reactions

Monodispersed porous spinel-type cobalt ferrite oxide (CoFe2O4) nanospheres (CFO-ns) directly grown on reduced graphene oxide (rGO) sheets are fabricated by a one-pot solvothermal method. With this special structure of CFO-ns and the covalent coupling between CFO-ns and rGO in the CFO-ns/rGO hybrid, more active sites are exposed and the transport of O2 and electrolyte is faciliated when the CFO-ns/rGO hybrid is employed as an electrocatalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The CFO-ns/rGO hybrid demonstrates high catalytic activity for both the ORR and OER. It shows a more positive onset potential of −0.11 V (vs. Ag/AgCl) for the ORR, which is 50 mV higher than that of CFO-ns + rGO physical mixture (−0.16 V). Meanwhile, the onset potential of CFO-ns/rGO hybrid (0.56 V) for the OER is 40 mV lower than that of CFO-ns + rGO mixture (0.60 V). The high activity of the CFO-ns/rGO hybrid is attributed to the special structure of CFO-ns, the covalent coupling between CFO-ns and rGO as well as the suppressed agglomeration of CFO-ns and restacking of rGO in the hybrid. Moreover, the covalent coupling between CFO-ns and rGO endows the hybrid with excellent electrochemical stabilities for both the ORR and OER.

One-pot synthesis of boron-doped ordered mesoporous carbons as efficient electrocatalysts for the oxygen reduction reaction

Boron-doped ordered mesoporous carbons (B-OMCs) with a tunable and high level of doping content (>1 wt%) have been synthesized via a one-pot solvent evaporation induced self-assembly (EISA) process. The as-prepared B-OMCs show a highly ordered 2D hexagonal mesostructure with an average pore size of 3.4–4.0 nm, which could facilitate an efficient mass transport of O2 and electrolyte during the oxygen reduciton reaction (ORR) process. The electrochemical investigations demonstrate that B-doping could significantly enhance the electrocatalytic activity of the carbon materials for the ORR in alkaline media. Specifically, the B-OMCs with a boron doping content of 1.17 wt% show the highest electrocatalytic activity and best long-term durability for ORR as compared to the non-doped OMCs and the B-OMCs with other doping contents. Combined with various physical characetrizations including X-ray diffraction, small angle X-ray scattering, N2 physisorption, Raman spectroscopy and X-ray photoelectron spectroscopy, the enhanced catalytic performance of the B-OMCs could be ascribed to the synergistic effects of the ordered mesostructure, specific surface area and moderate boron doping. This work not only helps the fundamental understanding of the correlation between the catalytic performance and the morphology, structure of the OMCs and the doping extent of heteroatoms, but also shows the promising potential applications of the B-OMCs as efficient, low-cost catalysts in metal-air batteries and fuel cells.

Unexpected current-voltage characteristics of mechanically modulated atomic contacts with the presence of molecular junctions in an electrochemically assisted - MCBJ

In this article, we report on the characterization of various molecular junctions’ current–voltage characteristics (I–V curves) evolution under mechanical modulations, by employing a novel electrochemically assisted-mechanically controllable break junction (EC-MCBJ) method. For 1,4-benzenedithiol, the I–V curves measured at constant electrode pair separation show excellent reproducibility, indicating the feasibility of our EC-MCBJ method for fabricating molecular junctions. For ferrocene-bisvinylphenylmethyl dithiol (Fc-VPM), an anomalous type of I–V curve was observed by the particular control over the stepping motor. This phenomenon is rationalized assuming a model of atomic contact evolution with the presence of molecular junctions. To test this hypothesized model, a molecule with a longer length, 1,3-butadiyne-linked dinuclear ruthenium(II) complex (Ru-1), was implemented, and the I–V curve evolution was investigated under similar circumstances. Compared with Fc-VPM, the observed I–V curves show close analogy and minor differences, and both of them fit the hypothesized model well.

Anisotropic wet etched silicon substrates for reoriented and selective growth of ZnO nanowires and enhanced hydrophobicity.

Herein we report the fabrication of ZnO nanowires on anisotropic wet etched silicon substrates by selective hydrothermal growth. <100> oriented silicon wafers were first patterned by anisotropic wet etch with a KOH solution, resulting in V-shaped stripes of different periods. Then, a thin layer of gold was deposited and annealed to promote the hydrothermal growth of ZnO nanowires. It was found that the growth rate of ZnO nanowires on <111> surfaces was much higher than that on <100> surfaces. As a first application of such micro- and nanostructured surfaces, we show enhanced wetting properties by measuring the contact angle of water droplets on the samples obtained under different patterning and growth conditions. Our results also demonstrated the possibility of tuning the contact angle of the sample in the range between 115° and 155°, by changing either the pattern of the silicon template or the hydrothermal growth conditions.

Phosphorus and cobalt co-doped reduced graphene oxide bifunctional electrocatalyst for oxygen reduction and evolution reactions

Phosphorus (P) and cobalt (Co) co-doped reduced graphene oxide (P-Co-rGO) has been developed and studied through a facile electrostatic assembly followed by a pyrolysis process. The prepared P-Co-rGO catalyst shows a great enhancement in the electrocatalytic activity and stability towards the oxygen reduction reaction (ORR) in alkaline solution, characterized with a positive onset potential of 0.89 V (vs. RHE), a negative shifting of only about 12.8 mV of the half-wave potential and the closest diffusion limiting current density (−5.5 mA cm−2) as compared to those of the commercial Pt/C (20 wt%). More interestingly, the prepared P-Co-rGO also exhibits excellent catalytic activity and stability for the oxygen evolution reaction (OER), with a low potential of 1.62 V (vs. RHE) at the current density of 10 mA cm−2 and a maximum current density of almost 30 mA cm−2 at 1.66 V (vs. RHE). Specifically, the prepared P-Co-rGO shows much higher activity and stability than the mono-doped reduced graphene oxide either with P or Co, respectively. This could be ascribed to the modification of the charge and spin densities and the edge and defect effects of the rGO after the co-doping of P and Co, thus resulting in a remarkable enhancement of the electrocatalytic properties for both the ORR and OER.

MnCo2O4 decorated Magnéli phase titanium oxide as a carbon-free cathode for Li–O2 batteries

Advanced cathode catalysts are crucial to the promotion of aprotic Li–O2 batteries for practical applications. Carbon is usually used as a cathode catalyst, but it reacts with the discharge products (Li2O2, LiO2) to form an insulating layer of lithium carbonate and prevents further reaction. To resolve this issue, the development of non-carbon cathode catalysts is of great demand. Herein, for the first time, we designed and fabricated a MnCo2O4 (MCO) spinel oxide decorated Magneli phase Ti4O7 as a carbon-free cathode for Li–O2 batteries. The sub-stoichiometric Ti4O7 oxide serves as an electronic conductive network. The MCO spinel oxide along with the synergistic effect between Ti4O7 and MCO facilitate the kinetics of both oxygen reduction and decomposition of Li2O2. Furthermore, uniform anchoring of MCO nanoparticles on Ti4O7 surface provides a stable lithium peroxide–cathode interface during the discharge/charge process. The resulting Ti4O7/MCO hybrid proves to be a highly effective cathode catalyst. The discharge/charge voltage gap of the Ti4O7/MCO hybrid is about 0.75 V, which is significantly lower than that of pure carbon, C + MCO and pristine Ti4O7 cathode. A high specific capacity (5400 mA h g−1 at 100 mA g−1) and excellent cycling performance (100 cycles at a capacity of 500 mA h g−1 under 200 mA g−1) were obtained for this hybrid. The high catalytic activity and durability renders the Ti4O7/MCO hybrid a highly promising carbon-free cathode for Li–O2 batteries.

Exogenous substances regulate silkworm fat body protein synthesis through MAPK and PI3K/Akt signaling pathways

Insect fat body is an important intermediate metabolic organ that plays an important role in protein metabolism and detoxification. In order to study the effects of TiO2 NPs and phoxim on fat body protein synthesis through MAPK and PI3K/Akt signaling pathways in silkworms, we determined the effects of TiO2 NPs and phoxim, alone and in combination, on fat body protein content of silkworms, analyzed the gene expression profile of the fat body, and verified the expression of characteristic genes. We found that TiO2 NPs and phoxim alone increased the total protein content of the fat body, and up-regulated MAPK and PI3K/Akt signaling pathway genes. TiO2 NPs up-regulated the expression of two growth and development-related genes-insulin-like peptide and neuropeptide receptor B-by 5.17 and 3.89-fold, respectively. Phoxim up-regulated the expression of detoxification genes-P450, GST, and CarE2. Pretreatment with TiO2 NPs could reduce phoxim-increased total protein content and up-regulated MAPK and PI3K/Akt signaling pathway genes and detoxification genes; the activities of detoxification enzymes were consistent with the gene expression pattern. Our results showed that MAPK and PI3K/Akt signaling pathways both regulate fat body protein synthesis in silkworms, but the target proteins induced to express were different under different inducing factors. Our finding may provide a reference for investigating the mechanism of protein synthesis regulation through MAPK and PI3K/Akt signaling pathways.