Tingbing Cao

Processing of graphene for electrochemical application: noncovalently functionalize graphene sheets with water-soluble electroactive methylene green.

To explore graphene applications in various fields, the processability of graphene becomes one of the important key issues, particularly with the increasing availability of synthetic graphene approaches, because the direct dispersion of hydrophobic graphene in water is prone to forming agglomerates irreversibly. Here, a facile method is proposed to increase the dispersity of graphene through noncovalent functionalization graphene with a water-soluble aromatic electroactive dye, methylene green (MG), during chemical reduction of graphene oxide (GO) with hydrazine. Atomic force microscopic and UV-vis spectrophotometric results demonstrate that chemically reduced graphene (CRG) functionalized with MG (CRG-MG) is well-dispersed into water through the coulomb repulsion between MG-adsorbed CRG sheets. The electrochemical properties of the formed CRG-MG are investigated, and the results demonstrate that CRG-MG confined onto a glassy carbon (GC) electrode has lower charge-transfer resistance and better electrocatalytic activity toward the oxidation of NADH, in relation to pristine CRG (i.e., without MG functionalization). This method not only offers a facile approach to dispersing graphene in water but also is envisaged to be useful for investigations on graphene-based electrochemistry.

Directing silicon-graphene self-assembly as a core/shell anode for high-performance lithium-ion batteries.

There is great interest in utilization of silicon-containing nanostructures as anode materials for lithium-ion batteries but usually limited by manufacturing cost, their intrinsic low electric conductivity, and large volume changes during cycling. Here we present a facile process to fabricate graphene-wrapped silicon nanowires (GNS@Si NWs) directed by electrostatic self-assembly. The highly conductive and mechanical flexible graphene could partially accommodate the large volume change associated with the conversion reaction and also contributed to the enhanced electronic conductivity. The as-prepared GNS@Si NWs delivered a reversible capacity of 1648 mAh·g(-1) with an initial Coulombic efficiency as high as 80%. Moreover, capacity remained 1335 mAh·g(-1) after 80 cycles at a current of 200 mA·g(-1), showing significantly improved electrochemical performance in terms of rate capability and cycling performance.

Facile patterning of reduced graphene oxide film into microelectrode array for highly sensitive sensing.

In this study, we develop a new technique to fabricate a reduced graphene oxide (rGO)-based microelectrode array (MEA) with low-cost soft lithography. To prepare patterned rGO, a polydimethylsiloxane (PDMS) mold with an array of microwells on its surface is fabricated using soft lithography, and GO is assembled on an indium tin oxide (ITO) electrode with a layer-by-layer method. The rGO pattern is formed by closely contacting the assembled GO film onto the ITO electrode with the PDMS mold filled with hydrazine solution in the microwells to selectively reduce the localized GO into the rGO. The MEA with patterned rGO as the microelectrode is characterized with Kelvin probe force microscopy (KFM), atomic force microscopy (AFM), and cyclic voltammetry (CV) with ferricyanide in aqueous solution as the redox probe. The KFM and AFM results demonstrate that each rGO pattern prepared under the present conditions is 3 μm in diameter, which is close to that of the PDMS mold we use. The CV results show that the rGO patterned onto the ITO exhibits a sigmoid-shaped voltammogram up to 200 mVs(-1) with a microampere level current response, suggesting that the rGO-based electrode fabricated with soft lithography behalves like a MEA. To demonstrate the potential electroanalytical application of the rGO-based MEA, prussian blue (PB) is electrodeposited onto the rGO-based MEA to form the PB/rGO-based MEA. Electrochemical studies on the formed PB/rGO-based MEA reveal that MEA shows a lower detection limit and a larger current density for the detection of H(2)O(2), as compared with the macroscopic rGO electrode. The method demonstrated here provides a simple and low-cost strategy for the fabrication of graphene-based MEA that are useful for electroanalytical applications.

Transfer Printing of Metal Nanoparticles with Controllable Dimensions, Placement, and Reproducible Surface-Enhanced Raman Scattering Effects

This paper describes the fabrication of single, multiple strand, and three-dimensional patterning of metal nanoparticles by nanotransfer edge printing (nTEP), a method comprising nanoparticle self-assembly, nanotransfer printing (nTP), and edge lithography. In the process proposed here, 20 nm Au nanoparticles (AuNPs) are deterministically arranged in precise placement by manipulating a topographically patterned poly(dimethylsiloxane) (PDMS) stamp, and Ag nanoparticles are conjugated with AuNP patterns to construct surface-enhanced Raman scattering (SERS)-active substrate to detect trace amounts (10(-13) mol/L) of biological molecules such as thrombin with enhancement up to 10(10). The simple, convenient, and inexpensive procedure has extended nTP and nTEP from using evaporated thin metal film to using self-assembled nanoparticles, and may stretch to other organic and inorganic species to find broad applications in many areas.

Selective Discharge of Electrostatic Charges on Electrets Using a Patterned Hydrogel Stamp

Electrets, materials that can permanently store charge, have a long history in engineering and condensed-matter physics. Recently, the field has drawn growing attention from the field of chemistry, especially after the hot debate between two renowned chemists, George Whitesides and Allen Bard, on the mechanism of contact electrification of dielectrics. Although the argument of charge transfer on tribocharging, either electrons or ions, is controversial and has not yet reached a consensus, the thorough investigation of the electrostatic phenomenon from both sides will still pave the way for other chemists and stimulate the exploitations for new application of electrets. Patterning of electrostatic charges, as widely used in conventional xerography for many years, is attracting considerable interest because it is associated with strong electrostatic fields to control the behavior of nanoscale electronic and mechanical devices, guide the assembly of nanomaterials, or modulate the properties of biological systems. High-resolution charge patterns can be produced either by injection of electrons into materials using electron beam lithography, conducting atomic force microscopy, and electrical microcontact printing, or by printing of positive or negative ionic charges with electrohydrodynamic jetting. 17] One common ground for these advanced techniques is to selectively charge the electrets either by the inputting of electrons or ions, or by inducing and maintaining macroscopic electric dipoles by using a strong electric field. In our previous study, we proposed a much different approach in patterning of charge that involves the selective discharge of the electrostatic charges. We used a topographically patterned poly(dimethylsiloxane) (PDMS) stamp to transfer and print heat energy onto uniformly charged electrets, and the heat can neutralize the charges or release the dipoles through a thermally stimulated discharge or depolarization (TSD) process to form patterns of charges. Hot microcontact printing (mCP), a simple and inexpensive procedure, is capable of patterning of electrostatic charges on bulk and thin-film electrets without needing to be supported on conductive substrates. Since many dielectrics can be charged through a simple contact electrification process that does not involve electric field, the selective discharge of electrostatic charges should be an enhanced way to achieve high-resolution charge patterns on electrets. Herein, we propose another charge patterning approach based on the concept of selective discharge of electrostatic charges. Different from the academic TSD process, this new strategy stems from the practical observation that electrostatic charges are more stable in dry winter than in sultry summer conditions because humid air helps to dissipate electrostatic charges by keeping surfaces moist and increasing the conductivity. Herein, we use a topographically patterned hydrogel stamp to expose electrets purposely and selectively to water for discharging, hence leaving the electrostatic charges in the noncontacted area to achieve high-resolution charge patterns; by applying “single electrode” electrochemical reductions in the charged area, we can also obtain a variety of metallic microor nanostructures on electrets. Figure 1a,b shows the procedure to fabricate the hydrogel stamp. Firstly, we prepared a PDMS stamp with topography preliminarily determined by photolithography. Then, agarose was molded against the PDMS relief structure to yield a stamp with an inverted bas-relief. Agarose has good mechanical stability combined with fast internal diffusion caused by a tunable water content of 20–98%; hence, hydrogel stamps have been widely used for the etching of metals, printing of bacteria, and fabrication of polymer microstructures by soaking of appropriate solutions. Herein, the agarose hydrogel stamp was chosen for transferring and printing water onto electrets for the selective discharge of electrostatic charges. Figure 1c,d shows a typical process of charging electrets: A thin layer (100 nm) of poly(methylmethacrylate) (PMMA) film supported on a silicon wafer is uniformly charged under an electric field of 10 kVcm 1 by using aluminum foil as an electrode to apply electric potential. After applying pulsed voltage for 20 seconds, the PMMA electrets are uniformly charged, and the mechanism of this charging process is dominated to be the transport of electrons as explained by Jacobs and Whitesides. When uniformly charged electrets are contacted with the patterned hydrogel stamp as shown in Figure 1e, the electrostatic charges (either electrons or ions) on the contacted part are removed through the diffusion from hydrogel stamp, whereas the charges on the untouched area remain, which results in high-resolution charge patterns. The patterns of charge, mainly electrons on PMMA electrets by this charging method, can reduce metal ions through electrostatic electro[*] X. Ma, D. Zhao, M. Xue, Prof. T. Cao Department of Chemistry, Renmin University of China Beijing 100872 (China) Fax: (+ 86)10-6251-6444 E-mail: tcao@chem.ruc.edu.cn

Patterning of Electrostatic Charge on Electrets Using Hot Microcontact Printing

Electrets are materials that have a permanent electric field maintained either by trapping net electrostatic charge (spacecharge electrets) or by holding macroscopic electric dipole moment (dipolar electrets). Dipolar electrets are fabricated by slow cooling of a dielectric material from above its glass transition temperature in the presence of a strong electric field, and space-charge electrets result from adding charge to the surface or the bulk of a material by exposing it to an electron beam or ion beam, corona charging, triboelectric charging, and so forth. Besides some classic applications of electrets in electrophotography, electrostatic powder coating and electrostatic spray painting, electrostatic filters, electret microphones, and radiation dosimeters, the emerging unconventional applications of electrets reveal some startling new discoveries: by peeling pressure-sensitive adhesive tape in vacuum, Carama et al. have found an intense nanosecond Xray pulse caused by tribocharging of insulators. Liu and Bard have reported the electrostatic electrochemistry of electrets by dipping tribocharged dielectrics into solution to induce chemical reaction, such as changes in the pH value and chemiluminescence. Whitesides et al. have thoroughly investigated the phenomenon of contact electrification of ionic electrets arising from the separation of ions at interfaces and have developed some powerful tools to measure the charge on tribocharged electrets before exploring their applications in electrostatic self-assembly of macroscopic crystals. Patterns of electrostatic charge are widely used in conventional xerography with over 100 mm resolution, and they can also serve as templates to induce the self-assembly and patterning of nanoparticles, 11] DNA, proteins, and other building blocks with sub-micrometer or even nanoscale resolution. Some serial techniques can embed charge (either electronic or ionic) into dielectric materials with patterns at resolutions smaller than 100 nm, such as electron beam lithography, focused ion beam lithography, and scanning probe lithography. However, the expensive infrastructure required and the extremely slow writing rates of these techniques severely hinder the further applications of the nanoscale charge patterns. One parallel technique, electric microcontact printing (e-mCP), employing a topographically patterned poly(dimethylsiloxane) (PDMS) stamp coated with a thin metal film, can embed charge with sub-micrometer resolution in only a few seconds. This new form of microcontact printing, which belongs to the family of soft lithography techniques developed by Whitesides and co-workers, is a low-cost technique for charge printing that combines the high spatial resolution of sophisticated forms of photolithography with capabilities not present in other approaches, that is, single-step patterning of large areas (over 1 cm) and nonplanar surfaces. Advanced nanoxerography will enable the fabrication of a whole range of novel devices, including single-nanocomponent transistors, light-emitting diodes, lasers, sensors, passive photonic networks, or nanoparticlebased media for data storage. 20] In our previous study, we fabricated ultrathin metallic film along the sidewall of a PDMS stamp by incorporating nanotransfer printing (nTP) techniques with edge lithography, and printed nanoscale charge patterns on PMMA film. Recently, we have used metal-coated PDMS stamps as parallel microelectrodes to selectively thermo-cross-link polymer thin films on silicon substrates by applying an appropriate current between the electrodes, which induces localized heat for patterning of various materials. Although metal-coated PDMS stamps show valuable characteristics for the patterning of electrostatic charge or soft matter, one common flaw still prevents them from broader application: electrets, precursors, polymers, selfassembly monolayers (SAMs), and other materials need to be mounted onto conductive substrates (i.e. silicon or indium tin oxide glass) as the opposite electrode to complete the patterning process. Herein, we propose a novel and simple idea for parallel patterning. By heating PDMS stamps and subsequently employing “hot” microcontact printing, a rich number of polymers and SAMs can be thermochemically patterned without needing to be supported on conductive substrates. Figure 1A schematically illustrates the general procedure for hot microcontact printing (mCP). A topographically patterned PDMS stamp is heated on a hot plate; immediately after being removed from the heat source, the hot stamp is conformally contacted with certain polymers or SAMs to finish the hot mCP process. PDMS is not a good thermal conductor; hence it can maintains the heat for a relatively long time to accomplish the mCP procedure. Unlike classical mCP, which uses silanes, thiols, organic or inorganic species, biomolecules, and all sorts of materials as inks, the hot mCP [*] D. Zhao, L. Duan, M. Xue, W. Ni, Prof. T. Cao Department of Chemistry, Renmin University of China Beijing 100872 (China) Fax: (+ 86)10-6251-6444 E-mail: tcao@chem.ruc.edu.cn

Fabrication of gold-directed conducting polymer nanoarrays for high-performance gas sensor.

Gold-directed polypyrrole (PPy) nanoarrays are fabricated by hydrogel-assisted nanotransfer edge printing (HnTEP) and electrochemical polymerization. Gold nanoarrays are fabricated through the HnTEP method, which involves metal deposition, hydrogel etching, and nanotransfer edge printing. By utilizing the well-positioned gold nanostructures, PPy nanoarrays with smooth morphology and controllable dimensions are fabricated through in situ electrochemical polymerization, the results of which are characterized by scanning electron microscopy and atomic force microscopy. A gas sensor based on PPy nanoarrays results in excellent sensing capabilities towards NH₃ detection, especially the sensitivity and fast response. This method appears to be general and may aid in the future design and implementation of other active materials which can also be manipulated by the same procedure and serve as functional components for chemical sensing, optoelectronics, biodetection, and other applications.

Facile Fabrication of Metallic Nanostructures by Tunable Cracking and Transfer Printing

You crack me up: A topographically patterned PDMS stamp was coated with thin metal film and swelled under organic vapor to induce the tunable cracking of the brittle film into metallic nanostructures (see SEM images, scale bars 1 μm). UV/Vis spectra, OLED efficiency, and SERS spectra demonstrate the fine controllability of the metallic nanostructures, the well-ordered and highly regulable surface plasmons, and the facile fabrication process.

The self-assembly and patterning of thin polymer films on pyroelectric substrates driven by electrohydrodynamic instability

We use hot PDMS to parallelly transfer and print heat energy onto lithium niobate substrates to induce a local pyroelectric effect. The heterogeneous electrostatic charges built from hot μCP can guide the self-assembly of different thin polymer films and develop them into micropattern.

Self-assembly of single-walled carbon nanotube based on diazoresin

Abstract The self-assembly film of single-walled carbon nanotube (SWCNT) based on diazoresin (DR) were prepared on CaF 2 , mica substrates and Pt tip. After oxidation, the ends of SWCNT were opened and turned into carboxylic group, which can react with diazonium group via Coulombic attraction. The film formed was photosensitive. After UV irradiation, the ionic bond between carboxylic group and diazonium group would turn into covalent bond following the decomposition of the diazonium group. The self-assembly layer and the bond conversion between the layers of the film have been verified with infrared (IR) spectroscopy, UV-Vis spectroscopy and transmission electron microscope (TEM).