Lei Zhai

University of Central Florida, Orlando, Florida, United States

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Publications (66)315.62 Total impact

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    ABSTRACT: Supercapacitors have drawn considerable attention in recent years due to their high specific power, long cycle life, and ability to bridge the power/energy gap between conventional capacitors and batteries/fuel cells. Nanostructured electrode materials have demonstrated superior electrochemical properties in producing high-performance supercapacitors. In this review article, we describe the recent progress and advances in designing nanostructured supercapacitor electrode materials based on various dimensions ranging from zero to three. We highlight the effect of nanostructures on the properties of supercapacitors including specific capacitance, rate capability and cycle stability, which may serve as a guideline for the next generation of supercapacitor electrode design.
    Energy & Environmental Science 12/2014; · 15.49 Impact Factor
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    ABSTRACT: Asymmetric supercapacitors (ASCs) have played a leading role in realizing energy storage devices with high energy and power densities. While both anode and cathode materials are important for high performance ASCs, more research effort has been devoted to developing cathode materials because the energy source of an ASC is mostly attributed to the cathode. However, the development of anode materials is essential in order to achieve high power density as well as stable long-term cycle life of ASCs. In this study, functionalized graphene aerogel (GA) decorated with palladium (Pd) nanoparticles is used as an efficient ASC anode material. The high surface area (328 m2 g−1) and low electrical resistivity (50 times lower than one without Pd) of the GA composite grants a high specific capacitance (175.8 F g−1 at 5 mV s−1), excellent rate capability (48.3% retention after a 10 fold increase of scan rate), and remarkable reversibility. ASCs assembled from manganese dioxide (cathode) and GA composite (anode) show stable extended cell voltage, fast charge-discharge capability, excellent cycle stability (89.6% retention after 3,000 cycles), and high energy and power densities (average of 13.9 Wh kg−1 and 13.3 kW kg−1). These results demonstrate the great potential of the GA composite as an efficient anode material for high performance energy storage devices.
    Nano Energy 11/2014; 11:611-620. · 10.21 Impact Factor
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    ABSTRACT: Photorefractive polymer composites have gained considerable attention due to their fascinating applications like 3D displays and 3D Telepresence. In this report, the performance of a novel PR polymer composite doped with graphene is studied. The addition of graphene laminates to a photorefractive composite results in up to threefold enhancement of space charge (SC) field build-up time. From our optical and electrical measurements, the faster build-up time is attributed to larger charge generation resulting from electronic interaction between graphene and the 7-DCST chromophores.
    J. Mater. Chem. C. 07/2014;
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    ABSTRACT: The assembling behavior of four thiophene-containing conjugated polymers, regioregular poly(3-hexythiophene) (rr-P3HT), poly(3,3-didodecylquaterthiophene) (PQT-12), poly(2,5-bis(3- tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-14), and poly(2,5-bis(3-tetradecylthiophen-2-yl)thiophen-2-yl)thiophen-2-ylthiazolo[5,4-d]thiazole) (PTzQT-14), on carbon nanotubes was investigated through microscopic studies of nanowire formation and theoretical simulation. It is found that polymer backbone rigidity and shape influence the attachment mode on carbon nanotubes. rr-P3HT and PQT-12 have a zigzag backbone structure that allows a thermodynamically stable coaxial attachment on CNTs, providing an ordered growth front for the nanowire formation. In contrast, fused rings in PTzQT-14 and PBTTT-14 create a stair-step like backbone structure that causes a kinetically controlled wrapping conformation on CNTs, generating a twisted growth front that hinders the nanowire formation. In addition, the rigidity of polymer backbone influences the wrapping mode. Polymers with more flexible backbones (i.e., PBTTT-14) would take a dense irregular wrapping mode on CNTs. The CNT diameter plays an important role in the nanowire formation when CPs attach to the CNT in the wrapping mode. Larger nanotubes with smaller surface curvature provides a less twisted polymer growth front, allowing the formation of CPNWs.
    Macromolecules 12/2013; 47(2). · 5.93 Impact Factor
  • Lei Zhai
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    ABSTRACT: Stimuli-responsive polymer films undergo interesting structural and property changes upon external stimuli. Their applications have extended from smart coatings to controlled drug release, smart windows, self-repair and other fields. This tutorial review summarizes non-covalent bonding, reversible reactions and responsive molecules that have played important roles in creating stimuli-responsive systems, and presents the recent development of three types of responsive polymer systems: layer-by-layer polymer multilayer films, polymer brushes, and self-repairing polymer films, with a discussion of their response mechanism. Future research efforts include comprehensive understanding of the response mechanism, producing polymer systems with controlled response properties regarding single or multiple external signals, combining polymer film fabrication with nanotechnology, improving the stability of polymer films on substrates, and evaluating the toxicity of the degradation products.
    Chemical Society Reviews 06/2013; · 30.43 Impact Factor
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    ABSTRACT: The association of cellular toxicity with the physiochemical properties of graphene-based materials is largely unexplored. A fundamental understanding of this relationship is essential to engineer graphene-based nanomaterials for biomedical applications. Here, an in vitro toxicological assessment of graphene oxide (GO) and reduced graphene oxide (RGO) and in correlation with their physiochemical properties is reported. GO is found to be more toxic than RGO of same size. GO and RGO induce significant increases in both intercellular reactive oxygen species (ROS) levels and messenger RNA (mRNA) levels of heme oxygenase 1 (HO1) and thioredoxin reductase (TrxR). Moreover, a significant amount of DNA damage is observed in GO treated cells, but not in RGO treated cells. Such observations support the hypothesis that oxidative stress mediates the cellular toxicity of GO. Interestingly, oxidative stress induced cytotoxicity reduces with a decreasing extent of oxygen functional group density on the RGO surface. It is concluded that although size of the GO sheet plays a role, the functional group density on the GO sheet is one of the key components in mediating cellular cytotoxicity. By controlling the GO reduction and maintaining the solubility, it is possible to minimize the toxicity of GO and unravel its wide range of biomedical applications.
    Particle and Particle Systems Characterization 02/2013; 30(2). · 0.54 Impact Factor
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    ABSTRACT: We report the synthesis of a soluble, amorphous, and adhesive electrolyte based on poly(ethylene glycol). A high molecular weight poly(PEGMA-co-MMA-co-IBVE) random copolymer with the lithium ion conductivity of 4.8 × 10− 5 S/cm at room temperature was synthesized through a facile statistical copolymerization of poly(ethylene glycol) methyl ether methacrylates (PEGMA), methyl methacrylate (MMA), and isobutyl vinyl ether (IBVE). The polymer composition, thermal properties, adhesion, and electrochemical properties are presented. Such a copolymer has the adhesion strength to permanently hold 800 times its own weight, and high solubility in water and organic solvents for easy material processing. Its unique and versatile properties belong to a class of multifunctional soft matter electrolytes.
    Solid State Ionics 02/2013; 232:37–43. · 2.11 Impact Factor
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    ABSTRACT: The electrochemical oxidation of methanol and ethanol in acidic media was studied using electrodes composed of multi-walled carbon nanotubes (MWCNTs) decorated with Pt, Ru and ceria nanoparticles. Polystyrene sulfonate (PSS) was used to disperse the MWCNTs in water and provide nucleation sites for the growth of catalyst nanoparticles. Composite electrodes were characterized for structural and electrochemical properties and all electrodes modified with Ru displayed greater catalytic ability for alcohol oxidation than those without Ru. In addition, the inclusion of ceria seemed to increase the catalytic ability in every sample suggesting a synergistic effect between Pt, Ru and ceria for the oxidation of methanol and ethanol. The catalytic effect of Pt and Ru concentration was studied by holding Ru concentrations constant and increasing the concentration of Pt. The same concentration of ceria was used for all modified electrodes. The results of this study show that the electrode prepared from 3:1 Pt:Ru solutions with ceria showed the highest peak current density for methanol oxidation (at 0.6 V vs. Ag/AgCl/Cl– which was nearly 20 times greater than that for an unmodified Pt electrode. Similar results were seen for ethanol oxidation on the same electrode which resulted in peak current densities greater than 20 times those for the unmodified Pt electrode at 0.8 V versus Ag/AgCl/Cl–.
    Materials Express 01/2013; 3(1). · 1.72 Impact Factor
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    ABSTRACT: We report the first fully compressed Li4Ti5O12 electrode designed by an aqueous process. An adhesive, elastomeric, and lithium ion conductive PEG-based copolymer is used as a binder for the aqueous fabrication thick, flexible, and densely packed Li4Ti5O12 (LTO) electrodes. Self-adherent cathode films exceeding 200 μm in thickness and withholding high active mass loadings of 28 mg/cm2 deliver 4.2 mAh/cm2 at C/2 rate. Structurally defect-free electrodes are fabricated by casting aqueous cathode slurries onto nickel foam, dried, and hard-calendared at 10 tons/cm2. As a multifunctional material, the binder is synthesized by the copolymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA), methyl methacrylate (MMA), and isobutyl vinyl ether (IBVE) in optimal proportions. Furthermore, coordinating the binder with lithium salt is necessary for the electrode to function.
    Electrochimica Acta 01/2013; 88:536–542. · 4.09 Impact Factor
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    ABSTRACT: Structural order of conjugated polymers at different length scales direct the optoelectronic properties of the corresponding materials, thus it is of critical importance to understand and control conjugated polymer morphology for successful application of these materials in organic optoelectronics. Herein, with the aim of probing the dependence of single chain folding properties on the chemical structure and rigidness of the polymer backbones, single molecule fluorescence spectroscopy was applied to four thiophene-based conjugated polymers. These include regioregular poly (3-hexylthiophene) (RR-P3HT), poly(2,5-bis(3-tetradecylthiophen-2-yl)-thieno[3,2-b]thiophene) (PBTTT-14), poly(2,5-bis(3-tetradecylthiophen-2-yl)thiophene-2-yl)thiophen-2-ylthiazolo[5,4-d]thiazole) (PTzQT-12) and poly(3,3-didodecyl-quaterthiophene)] (PQT-12). Previous work has shown that RR-P3HT and PBTTT-14 polymer chains fold in their nanostructures, while PQT-12 and PTzQT-12 do not fold in their nanostructures. At the single molecule level, it was found that RR-P3HT single chains almost exclusively fold into loosely and strongly aggregated conformations, analogous to the folding properties in nanostructures. PQT-12 displays significant chain folding as well, but only into loosely aggregated conformations, showing an absence of strongly aggregated polymer chains. PBTTT-14 exhibits a significant fraction of rigid polymer chain. The findings made for single molecules of PQT-12 and PBTTT-14 are thus in contrast with the observations made in their corresponding nanostructures. PTzQT-12 appears to be the most rigid and planar conjugated polymer of these four polymers. However, while the presumably non-folding polymers PQT-12 and PTzQT-12 exhibit less folding than RR-P3HT, there is still a significant occurrence of chain folding for these polymers at the single molecule level. These results suggest that the folding properties of conjugated polymers can be influenced by the architecture of the polymer backbones, however, other factors such as intermolecular stacking interactions, solvent environment and side chain interactions in corresponding materials should also be taken into account to predict conjugated polymer material morphology.
    The Journal of Physical Chemistry B 12/2012; · 3.38 Impact Factor
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    ABSTRACT: Electronic transport of regioregular poly(3-hexylthiophene)-block-poly styrene (rr-P3HT-b-PS) copolymer in field effect transistor (FET) geometry with different surface treatment and different temperature is investigated. The devices show p type behavior with a maximum saturation mobility of 6 × 10−3 cm2/V s and current on/off ratio of 2.6 × 104 in an OTS treated sample at room temperature, which is lower compared to the controlled P3HT sample of same molecular weight fabricated with the same surface treatment. The mobility measured at different temperatures (300–150 K) show thermally activated hopping type transport mechanism with gate bias dependent activation energy of 100–270 meV which is higher compared to the reported value of pristine P3HT FET. The higher activation energy in hopping behavior and lower mobility in this block copolymer is caused by insulating PS segments.
    Synthetic Metals 10/2012; 162(s 17–18):1531–1536. · 2.22 Impact Factor
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    ABSTRACT: Multi-walled carbon nanotube (MWCNT)/MnO2 supercapacitor electrodes containing MnO2 nanoflakes in the MWCNT network are fabricated through the oxidation of manganese acetate with poly(4-styrenesulfonic acid) (PSS) dispersed MWCNTs. The structural evolution of the electrodes under charge/discharge (reduction/oxidation) cycles and its impact on the electrodes’ electrochemical properties are evaluated. Structural evolution involves the dissolution of MnO2 upon reduction, the diffusion of the reduced Mn species from the MWCNT network toward the electrolyte solution, and the deposition of MnO2 on the electrode surface upon oxidation. Electrode structural changes, including the electrode dissolution and the growth of the MnO2 crystals, are scan rate dependent and have deteriorating effect on the electrode's electrochemical properties including the specific capacitance and cyclic stability
    Electrochimica Acta 01/2012; · 4.09 Impact Factor
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    ABSTRACT: Polymer-derived amorphous SiBCN ceramics are synthesized through a simple dehydrocoupling and hydroboration reaction of an oligosilazane containing amine and vinyl groups and BH3·Me2S, followed by pyrolysis. Two types of ceramics, denoted as Si2B1 and Si4B1, are produced from preceramic polymers with Si/B ratios of 2/1 and 4/1, respectively. The structural evolution of these ceramics with respect to the pyrolysis temperature and boron concentration is investigated using solid-state NMR, Raman, and EPR spectroscopy. Solid-state NMR suggests the presence of three major components in the ceramics: (i) hexagonal boron nitride (h-BN), (ii) turbostratic boron nitride (t-BN), and (iii) BN2C groups. Increasing pyrolysis temperature leads to the transformation of BN2C groups into BN3 and “free” carbon. A thermodynamic model is proposed to explain such transformation. Raman spectroscopy measurements reveal that the concentration of the “free” carbon cluster decreases with increasing pyrolysis temperature, and Si4B1 contains more “free” carbon cluster than Si2B1. EPR studies reveal that the carbon (C)-dangling bond content also decreases with increasing pyrolysis temperature. It appears that the complete decomposition of the metastable BN2C groups to the BN3 groups and the “free” carbon affects the crystallization of SiBCN, which leads to Si4B1 ceramics crystallized at 1500 °C, whereas Si2B1 ceramics crystallized at 1600 °C.
    The Journal of Physical Chemistry C 11/2011; 115(50). · 4.84 Impact Factor
  • ACS Applied Materials & Interfaces 06/2011; · 5.90 Impact Factor
  • Jianhua Zou, Binh Tran, Lei Zhai
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    ABSTRACT: A convenient approach to fabricate metal (i.e. gold, platinum, and palladium) nanoparticles on highly dispersed pristine carbon nanotubes (CNTs) was developed using a conjugated block copolymer of poly(3-hexylthiophene)-b-poly(vinylpyrrolidone) (P3HT-b-PVP). P3HT-b-PVP not only provides a stable dispersion of pristine CNTs through the π–π interactions between P3HT block and CNTs, but also introduces PVP groups on CNT surfaces to induce the heterogeneous nucleation of metal nanoparticles and protect them from aggregating. The density of metal nanoparticles on CNT surfaces was controlled by the metal salt/CNT feed ratio. The simple processing procedure, versatility in synthesizing various metal nanoparticles, high metal nanoparticle loading capacity, and excellent dispersibility and processability of the product make this approach a promising method to fabricate metal nanoparticles on CNTs.
    International Journal of Smart and Nano Materials. 06/2011; 2(2):92-100.
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    ABSTRACT: Combining the structural characterization of solution crystals fabricated from thiophene-based conjugated polymers with different molecular structures and a theoretical investigation of the polymer conformational transformability leads to an interesting discovery of the relationship between the molecular structures and their crystallization behaviors. The chain folding or nonfolding behavior of thiophene-based conjugated polymers in crystallization, an important factor to shape polymer crystals, is determined by their molecular structures, and can be estimated by the inter-ring rotation energy barriers of the polymer backbones. A quantitative theoretical calculation is proposed to evaluate the inter-ring rotation energy barriers, and the values are correlated with the experimentally observed chain folding or nonfolding behavior. The higher percentage of type I inter-ring σ bond (CH3 and H are at 3 and 3′ position of adjacent aromatic rings, respectively) or the lower average rotation barrier in polymer backbones creates higher capability of polymer conformational transformation and higher tendency of chain folding. Our study provides a valid prediction of the crystallization behavior of thiophene-based conjugated polymers through a theoretical evaluation of conjugated polymer molecular structures, and offers an essential understand of the structure–property relationship of conjugated polymers.
    Polymer 05/2011; 52(10):2302-2309. · 3.77 Impact Factor
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    ABSTRACT: In order to address the challenges and restrictions given by a traditional classroom lecture environment, the top-down and bottom-up nanotechnology teaching modules were developed, implemented and evaluated. Then based on the hypothesis that instructors could further develop students' interest in this emerging area through the introduction of the teaching modules and a career module, an early stage evaluation of the effectiveness of the modules in selected engineering courses was conducted. The data suggested that adoption of modular lectures in regular engineering courses influenced attitude towards nanotechnology - overall, the teaching modules did a better job of piquing student's interest (albeit in the short term) in the subject, but there were also positive gains in interest in nanotechnology as a career. There was some evidence that the hands-on demonstration teaching modules with visual elements and the career module were more effective than traditional lecture presentations in the classroom.
    European Journal of Engineering Education. 05/2011; 36(2):199-210.
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    ABSTRACT: Graphene, a two dimensional monoatomic thick building block of a carbon allotrope, has emerged as an exotic material of the 21st century, and received world-wide attention due to its exceptional charge transport, thermal, optical, and mechanical properties. Graphene and its derivatives are being studied in nearly every field of science and engineering. Recent progress has shown that the graphene-based materials can have a profound impact on electronic and optoelectronic devices, chemical sensors, nanocomposites and energy storage. The aim of this review article is to provide a comprehensive scientific progress of graphene to date and evaluate its future perspective. Various synthesis processes of single layer graphene, graphene nanoribbons, chemically derived graphene, and graphene-based polymer and nano particle composites are reviewed. Their structural, thermal, optical, and electrical properties were also discussed along with their potential applications. The article concludes with a brief discussion on the impact of graphene and related materials on the environment, its toxicological effects and its future prospects in this rapidly emerging field.
    Progress in Materials Science 04/2011; 56(8):1178–1271. · 25.87 Impact Factor
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    ABSTRACT: We show that the low-temperature electron transport properties of chemically functionalized graphene can be explained as sequential tunneling of charges through a two-dimensional array of graphene quantum dots (GQDs). Below 15 K, a total suppression of current due to Coulomb blockade through a GQD array was observed. Temperature-dependent current-gate voltage characteristics show Coulomb oscillations with energy scales of 6.2–10 meV corresponding to GQD sizes of 5–8 nm, while resistance data exhibit an Efros-Shklovskii variable range hopping arising from structural- and size-induced disorder.
    Physical Review B 03/2011; 83(11):115323. · 3.66 Impact Factor
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    ABSTRACT: We fabricated organic field effect transistors (OFETs) by directly growing poly (3-hexylthiophne) (P3HT) crystalline nanowires on solution processed aligned array single walled carbon nanotubes (SWNT) interdigitated electrodes by exploiting strong π-π interaction for both efficient charge injection and transport. We also compared the device properties of OFETs using SWNT electrodes with control OFETs of P3HT nanowires deposited on gold electrodes. Electron transport measurements on 28 devices showed that, compared to the OFETs with gold electrodes, the OFETs with SWNT electrodes have better mobility and better current on-off ratio with a maximum of 0.13 cm(2)/(V s) and 3.1 × 10(5), respectively. The improved device characteristics with SWNT electrodes were also demonstrated by the improved charge injection and the absence of short channel effect, which was dominant in gold electrode OFETs. The enhancement of the device performance can be attributed to the improved interfacial contact between SWNT electrodes and the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one-dimensional crystalline nanowire structure.
    ACS Applied Materials & Interfaces 03/2011; 3(4):1180-5. · 5.90 Impact Factor

Publication Stats

961 Citations
315.62 Total Impact Points

Institutions

  • 2007–2013
    • University of Central Florida
      • • NanoScience Technology Center
      • • Department of Chemistry
      Orlando, Florida, United States
  • 2001–2004
    • Carnegie Mellon University
      • Department of Chemistry
      Pittsburgh, Pennsylvania, United States