Xuan Pan

Comparing Graphene-TiO2 Nanowire and Graphene-TiO2 Nanoparticle Composite Photocatalysts

We demonstrate that uniform dispersion of TiO(2) on graphene is critical for the photocatalytic effect of the composite. The hydrothermal method was employed to synthesize TiO(2) nanowires (NW) and then fabricate graphene-TiO(2) nanowire nanocomposite (GNW). Graphene oxide (GO) reduction to graphene and hybridization between TiO(2) NWs and graphene by forming chemical bonding was achieved in a one-step hydrothermal process. Graphene-TiO(2) nanoparticle (NP) nanocomposite (GNP) was also synthesized. Photocatalytic performance and related properties of NP, NW, GNP, and GNW were comparatively studied. It was found that by incorporation of graphene, GNP and GNW have higher performance than their counterparts. More importantly, it was found that NWs, in comparison with NPs, have more uniform dispersion on graphene with less agglomeration, resulting in more direct contact between TiO(2) and graphene, and hence further improved electron-hole pairs (EHPs) separation and transportation. The adsorbability of GNW is also found to be higher than GNP. The result reveals that the relative photocatalytic activity of GNW is much higher than GNP and pure NWs or NPs.

TiO2 nanotubes infiltrated with nanoparticles for dye sensitized solar cells

We present a detailed study of the infiltration of titanium dioxide (TiO(2)) nanotubes (NTs) with TiO(2) nanoparticles (NPs) for dye sensitized solar cells (DSSCs). The aim is to combine the merits of the NPs high dye loading and high light harvesting capability with the NTs straight carrier transport path and high electron collection efficiency to improve the DSSC performance. On infiltrating NTs with TiCl(4) solution followed by hydrothermal synthesis, 10 nm size NPs were observed to form a conformal and dense layer on the NT walls. Compared with the bare NT structure, dye loading of this mixed NT and NP structure is more than doubled. The overall photon conversion efficiencies of the fabricated DSSCs are improved by 152%, 107%, and 49% for 8, 13, and 20 µm long NTs, respectively. Electron transport and recombination parameters were extracted based on electrochemical impedance spectroscopy measurements. Although a slight reduction of electron lifetime was observed in the mixed structures due to enhanced recombination with a larger surface area, the diffusion length is still significantly longer than the NT length used, suggesting that most electrons are collected. In addition to dye loading and hence photocurrent increment, the photovoltage and filling factor were also improved in the mixed structure due to a low serial resistance, leading to the enhancement of the overall efficiency.

Quercetin-nanostructured lipid carriers: characteristics and anti-breast cancer activities in vitro.

Quercetin (Q), a common dietary flavonoid, has gained research attention in cancer chemo-prevention, but its low level of aqueous solubility, stability, cellular bioavailability has limited its application. We have synthesized biocompatible and biodegradable Q-nanostructured lipid carriers (Q-NLC) using a novel phase inversion-based process method. The average size of Q-NLC was 32 nm in diameter. Q-NLC had good chemical and physical stability, and showed a sustained release pattern. The encapsulation efficiency and loading capacity of Q-NLC were 95% and 11%, respectively. The aqueous solubility of Q was dramatically improved by at least 1000 folds. The results from Raman spectroscopy, powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) demonstrated that Q presented in NLC as an encapsulated molecule form. As compared to native Q, Q-NLC dramatically increased cytotoxicity in a dose-dependent manner (1-50 μM) and induced apoptosis at 20 μM in MCF-7 and MDA-MB-231 breast cancer cells. The enhanced cytotoxicity and apoptosis were parallel to increased Q uptake by those cancer cells. Void NLC did not change the viability and apoptosis of those cancer cells as compared to phosphate buffered saline. In conclusion, Q-NLC dramatically enhanced the anti-cancer activities of Q, which were associated with enhanced Q solubility and stability, and increased Q content in those cancer cells. Q-NLC have a potential for chemo-preventive use in breast cancer.

Influence of defects on structural and electrical properties of VO2 thin films

We present the structural and electrical properties of (011) preferred polycrystalline (Poly) and multidomain (020) epitaxial (Epi) VO2 thin films grown at different temperature (Ts) and on different substrates with variable defects. These defects cause variation in strain, metal-insulator transition (MIT) temperature (TMIT), activation energy (ΔEa), and charge carrier type in insulating phase. Both the Poly- and Epi-VO2 behave n-type conductivity when grown at relative low TS. As TS increases, defects related acceptor density increases to alter conductivity from n- to p-type in the Poly-VO2, while in the Epi-VO2 donor density increases to maintain n-type conductivity. Moreover, the strain along monoclinic am axis dramatically reverses from tensile to compressive in both the Poly- (848 K < TS < 873 K) and Epi-VO2 (873 K < TS < 898 K), and eventually approaches to a constant in the Poly-VO2 (TS ≥ 898 K) in particular. TMIT decreases with increasing the carrier density independent of the conductive type in ...

Anticancer activities of (−)-epigallocatechin-3-gallate encapsulated nanoliposomes in MCF7 breast cancer cells

Abstract The chemopreventive actions exerted by green tea are thought to be due to its major polyphenol, (−)-epigallocatechin-3-gallate (EGCG). However, the low level of stability and bioavailability in the body makes administering EGCG at chemopreventive doses unrealistic. We synthesized EGCG encapsulated chitosan-coated nanoliposomes (CSLIPO-EGCG), and observed their antiproliferative and proapoptotic effect in MCF7 breast cancer cells. CSLIPO-EGCG significantly enhanced EGCG stability, improved sustained release, increased intracellular EGCG content in MCF7 cells, induced apoptosis of MCF7 cells, and inhibited MCF7 cell proliferation compared to native EGCG and void CSLIPO. The CSLIPO-EGCG retained its antiproliferative and proapoptotic effectiveness at 10 μM or lower, at which native EGCG does not have any beneficial effects. This study portends a potential breakthrough in the prevention or even treatment of breast cancer by using biocompatible and biodegradable CSLIPO-EGCG with enhanced chemopreventive efficacy and minimized immunogenicity and side-effects.

Vertically aligned VO2(B) nanobelt forest and its three-dimensional structure on oriented graphene for energy storage

Assembling two-dimensional (2D) nanomaterials into an ordered forest structure that provides an easily accessible large surface area and/or chemically active 2D edges will present numerous opportunities, including as electrodes for energy storage. We report a densely packed vertically aligned VO2(B) nanobelt (NB) based forest structure synthesized by a solvothermal method using a vertically oriented graphene (VOG) network as the underlying support. We further expanded this forest structure into a folded three-dimensional (3D) forest structure using VOG-covered metallic foam as the scaffold. To demonstrate its potential, this free-standing 3D ordered structure built from 2D nanomaterials was directly used as the electrode for lithium-ion batteries. Excellent performance was confirmed by a stable discharge capacity of 178 mA h g−1 at a current density of 10 A g−1 (or a rate of 59 C) and 100 mA h g−1 at 27 A g−1 (300 C), contributed by both lithium ion intercalation into the crystal lattice and surface-related pseudocapacitance. A high cycling stability over 2000 cycles under a high current density was also demonstrated. We expect that our method can be expanded to synthesize 2D sheet based forest structures of other layered oxides and hydroxides by using VOG as a versatile platform for numerous applications such as energy storage and catalytic energy conversion.

Tuning the properties of VO2 thin films through growth temperature for infrared and terahertz modulation applications

Results are reported on tuning the electrical and optical properties of sputter-deposited vanadium dioxide (VO2) thin films through control of substrate growth temperature (Ts). As Ts increases from 550 to 700 °C, the morphology changes from granular to smooth film and finally to rough film. X-ray diffraction shows the presence of VO2 along with additional weak features related to the presence of non-stoichiometric phases. Electrical measurements show the phase transition to change from abrupt to gradual as both the below- and above-transition resistivities vary with Ts. The transition and hysteresis dependences observed in electrical resistivity are similarly observed in infrared transmission. Terahertz transmission measurements show that high conductivity above the phase transition is more important in achieving high modulation depth than obtaining high resistivity below the transition. We attribute changes in the electrical and optical properties to the formation of V and O vacancies, which result in diverse valence states from the ideal V4+ of VO2. Low Ts produces material with V5+ states resulting in higher resistivity in both the insulating and metallic phases. Alternatively, high Ts introduces material with V3+ states leading to lower resistivity in the insulating phase but slightly higher resistivity in the metallic phase.

Electrocatalytic properties of a vertically oriented graphene film and its application as a catalytic counter electrode for dye-sensitized solar cells

Vertically oriented graphene (VOG), with graphene sheets perpendicular to the substrate, exhibits meritorious electrocatalytic properties due to the fully exposed graphene (or graphitic) edges and other plasma introduced defects, even without any heteroatom functionalization. We report the growth and electrocatalytic properties of VOG by plasma-enhanced chemical vapor deposition using methane and nitrogen as source gases. Nitrogen was adopted as plasma ancillary gas to obtain a higher plasma temperature that promotes VOG growth with better electrocatalytic performance, as comparative studies demonstrated. Electron transfer dynamics measurement using a Fe(CN)63−/4− redox couple found reduction and oxidation peak-to-peak separation as low as 70 mV, indicating rapid electron-transfer kinetics. The electrocatalytic activity of the VOG electrode on an I−/I3− redox couple was found approaching that of platinum. Using VOG as the counter electrode for dye-sensitized solar cells (DSSCs), a promising power conversion efficiency of 7.63% was demonstrated. Electrochemical impedance spectroscopic study further discloses the impact of VOG properties on DSSC performance.

Hydrogen-doping stabilized metallic VO2 (R) thin films and their application to suppress Fabry-Perot resonances in the terahertz regime

We demonstrate that catalyst-assisted hydrogen spillover doping of VO2 thin films significantly alters the metal-insulator transition characteristics and stabilizes the metallic rutile phase at room temperature. With hydrogen inserted into the VO2 lattice, high resolution X-ray diffraction reveals expansion of the V-V chain separation when compared to the VO2(R) phase. The donated free electrons, possibly from O-H bond formation, stabilize the VO2(R) to low temperatures. By controlling the amount of dopants to obtain mixed insulating and metallic phases, VO2 resistivity can be continuously tuned until a critical condition is achieved that suppresses Fabry-Perot resonances. Our results demonstrate that hydrogen spillover is an effective technique to tune the electrical and optical properties of VO2 thin films.