Current Research

Research Program on Nanotechnology for Sustainable Energy and Environment

The goal of our research is to contribute to our society through nanomaterial innovations for energy and environmental applications. Our research interests are thus in the areas of nanoparticle synthesis, assembly, and nanofabrication; energy conversion, storage, and conservation; nanostructure-based gas sensors, biosensors, and water sensors; carbon nanotubes (CNTs), graphene, and hybrid nanomaterials; pollution control; and corona discharges and plasma reacting flows (see schematic below and descriptions for six research themes).

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Theme 1: Synthesis and Assembly of Novel Hybrid Nanomaterials

Active hybrid nanocrystals-CNT/graphene structures represent a new class of nanomaterials that could potentially display not only the unique properties of nanocrystals and those of nanotubes or graphene, but also additional novel properties due to the interaction between the nanocrystal and the CNT or graphene.

These hybrid nanomaterials are promising for a wide range of applications including in nanoelectronic and optoelectronic devices, catalysis, nanomanufacturing, renewable energy harvest and storage, biomedical engineering, and environmental remediation. The availability of affordable hybrid nanostructures and their fundamental properties will thus accelerate new discoveries and inventions in nanoscience and nanotechnology. We have developed several methods to assemble nanoparticles onto a variety of semiconducting and conducting substrates, including CNTs, graphene, and nanowires, to produce hybrid nanostructures with considerable control. We are interested in investigating the fundamental properties (e.g., the interaction between nanoparticles and CNTs/graphene), manufacture, and applications of the novel hybrid nanostructures through experiments as well as density functional theory (DFT) calculations.
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Representative journal publications:

1. J. H. Chen and G. H. Lu, "Controlled Decoration of Carbon Nanotubes with Nanoparticles," Nanotechnology 17(12), 2891-2894, 2006. (Highlighted by Nanowerk Spotlight, http://www.nanowerk.com/spotlight/spotid=545.php)
2. G. H. Lu, L. Y. Zhu, P. X. Wang, J. H. Chen, D. A. Dikin, R. S. Ruoff, Y. Yu, and Z. F. Ren, "Electrostatic Force Directed Assembly of Ag Nanocrystals onto Vertically Aligned Carbon Nanotubes," Richard E. Smalley Memorial Issue of J. Phys. Chem. C., 111(48), 17919-17922, 2007.
3. L. Y. Zhu, G. H. Lu, S. Mao, J. H. Chen, D. A. Dikin, X. Q. Chen, and R. S. Ruoff, "Ripening of Silver Nanoparticles on Carbon Nanotubes," NANO 2(3), 149-156, 2007. (Featured Cover Article)
4. S. Mao, G. H. Lu, and J. H. Chen, "Coating Carbon Nanotubes with Colloidal Nanocrystals by Combining an Electrospray Technique with Directed Assembly by an Electrostatic Field," Nanotechnology 19(45), No. 455610, 2008.
5. G. H. Lu, S. Mao, S. Park, R. S. Ruoff, and J. H. Chen, "Facile, Noncovalent Decoration of Graphene Oxide Sheets with Nanocrystals," Nano Research 2(3), 192-200, 2009.
6. S. Mao, Z. H. Wen, Haejune Kim, G. H. Lu, P. Hurley, and J. H. Chen*, "A General Approach to One-Pot Fabrication of Crumpled Graphene-Based Nanohybrids for Energy Applications," ACS Nano 6(8), 7505-7513, 2012.
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Theme 2: Design and Fabrication of Chemical and Biological Sensors/Sensor Arrays

Miniaturized chemical and biological sensors that rapidly and accurately detect and differentiate trace amount of chemical or biological species are attractive for environmental monitoring, medical diagnosis, and lab-on-a-chip analytical devices. The objective of this research theme is to explore novel sensing platforms based on hybrid nanoparticle-CNT/graphene structures by taking advantage of electronic interactions between nanoparticles and CNT/graphene.

The hybrid platform allows for the room-temperature sensing of a wide range of chemical or biological species at high sensitivity, which is otherwise unattainable with either nanoparticle sensors or CNT/graphene sensors. We have demonstrated a generic room-temperature gas sensing platform based on CNT/graphene-SnO2 nanoparticle hybrids for detection of low-concentration gases (e.g., CO, H2, NO2, NH3) and a versatile biosensing/water sensing platform based on CNT/graphene-Au nanoparticles for detection of proteins, DNAs, bacteria (e.g., E. coli), and heavy metal ions. Our research focuses on detailed characterization of the sensing platforms, understanding the sensing mechanism, optimization of the fabrication process, and scale-up of the process for multi-sensor fabrication. Our approach is to combine experiments (atomic, electronic, electrical, and spectroscopic characterizations) with theoretical modeling (e.g., DFT calculations) to establish the composition-structure-processing-sensing relationship for revealing the novel sensing mechanism.
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Representative journal publications:

1. G. H. Lu, L. E. Ocola, and J. H. Chen, "Room-Temperature Gas Sensing through Electronic Transfer between Discrete Tin Oxide Nanocrystal and Multiwalled Carbon Nanotube," Advanced Materials 21(24), 2487-2491, 2009. (Featured as Frontispiece)
2. G. H. Lu, S. Park, K. H. Yu, R. S. Ruoff, L. E. Ocola, D. Rosenmann, and J. H. Chen*, "Toward Practical Gas Sensing Using Highly Reduced Graphene Oxide: A New Signal Processing Method to Circumvent Run-to-Run and Device-to-Device Variations," ACS Nano 5(2), 1154-1164, 2011.
3. G. H. Lu, L. E. Ocola, and J. H. Chen, "Gas Detection Using Low-Temperature Reduced Graphene Oxide Sheets," Applied Physics Letters 94(8), No. 083111, 2009. Selected for the March 16, 2009 issue of Virtual Journal of Nanoscale Science & Technology.
4. S. Mao, G. H. Lu, K. H. Yu, Z. Bo, and J. H. Chen*, "Specific Protein Detection using Thermally Reduced Graphene Oxide Sheet Decorated with Gold Nanoparticle-antibody Conjugates," Advanced Materials 22(32), 3521-3526, 2010.
5. G. H. Lu, K. H. Yu, L. E. Ocola, and J. H. Chen*, "Ultrafast Room-Temperature NH3 Sensing with Positively-Gated Reduced Graphene Oxide Field-Effect Transistors," Chemical Communications 47(27), 7761-7763, 2011.
6. K. H. Chen, G. H. Lu, J. B. Chang, S. Mao, K. H. Yu, S. M. Cui, and J. H. Chen*, "Rapid Hg(II) Ion Detection Using Thermally Reduced Graphene Oxide Decorated with Functionalized Gold Nanoparticles," Analytical Chemistry 84(9), 4057-4062, 2012.
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Theme 3: Novel Photovoltaic Cells based on Hybrid Nanostructures

Energy supply has arguably become one of the most important problems facing humankind. The exponential demand for energy is evidenced by dwindling fossil fuel supplies and record-high oil and gas prices due to global population growth and economic development. The current energy crisis is further exacerbated by major concerns about global warming from greenhouse gas emissions due to increasing fossil fuel consumption. Solar energy seems to be the most viable choice to meet our clean energy demand; however, the existing solar-to-electricity conversion technologies cannot compete with the conventional fossil fuel energy sources in terms of efficiency and cost. The objective of this study is to explore fabrication and characterization of quantum dot (QD) or dye-sensitized solar cells with vertically aligned CNTs or graphene coated with QDs (e.g., CdSe and Si) and dye molecules. The new solar cell architecture takes advantage of efficient electron transfer between nanocrystals and CNTs, effective charge separation at the QD-CNT interface, fast electron transport through CNTs, and the unique attributes of QDs. Moreover, the capability to coat the same CNT with QDs of multiple materials and sizes will enable us to maximize the overlap between the cell absorption and the solar spectra. Our work in this theme area includes both photoanodes and counter electrodes. We have demonstrated cost-effective counter electrodes based on graphene-TiN nanoparticle structures and vertical graphene that could potentially replace conventional Pt counter electrodes
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Representative journal publications:

1. K. H. Yu and J. H. Chen, "Enhancing Solar Cell Efficiencies through 1-D Nanostructures," Nanoscale Research Letters 4(1), 1-10, 2009.
2. Z. H. Wen, S. M. Cui, H. H. Pu, S. Mao, K. H. Yu, X. L. Feng*, and J. H. Chen*, "Metal Nitride/Graphene Nanohybrids: General Synthesis and Multifunctional Titanium Nitride/Graphene Electrocatalyst," Advanced Materials 23(45), 5445-5450, 2011.
3. K. H. Yu, G. H. Lu, Z. H. Wen, H. J. Kim, Y. Y. Qian, E. Andrew, S. Mao, and J. H. Chen*, "Hierarchical Vertically-Oriented Graphene as a Catalytic Counter Electrode in Dye-Sensitized Solar Cells," Accepted to Journal of Materials Chemistry.
4. K. H. Yu, X. Lin, G. H. Lu, Z. H. Wen, C. Yuan*, and J. H. Chen*, "Optimizing CdS Quantum Dot Sensitized Solar Cell Performance through Atomic Layer Deposition of Ultrathin TiO2 Coating," RSC Advances 2(20), 7843-7848, 2012.
5. K. H. Yu, G. H. Lu, S. Mao, H. Kim, and J. H. Chen*, "Selective Deposition of CdSe Nanoparticles on Reduced Graphene Oxide to Understand Photoinduced Charge Transfer in Hybrid Nanostructures," ACS Applied Materials & Interfaces 3(7), 2703-2709, 2011.
6. K. H. Yu, G. H. Lu, Z. Bo, S. Mao, and J. H. Chen*, "Carbon Nanotube with Chemically-bonded Graphene Leaves for Electronic and Optoelectronic Applications," The Journal of Physical Chemistry Letters 2(13), 1556-1562, 2011.
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Theme 4: High-Performance Energy Storage Devices

Energy storage devices such as lithium-ion batteries (LIBs) and electrochemical double layer (EDL) capacitors (sometimes referred to as ‘supercapacitors’ or ‘ultracapacitors’) are critical not only to stationary power generation (e.g., solar and wind energy harvesting) but also to mobile power sources (e.g., automotive applications). For instance, LIB technologies will be critical to driving the future energy applications, portable electronics, and electric/hybrid vehicles because of their high energy density with high cell voltage, low maintenance and because they are more environmentally friendly than nickel-cadmium batteries. Supercapacitors on the other hand could offer high power densities and rapid charging/discharging for instantaneous high load applications. Our research objective is to develop novel electrode materials based on hybrid nanostructures for high-performance LIBs and supercapacitors. We have demonstrated high capacity LIBs using graphene-Sn hybrids, vertically-oriented graphene, and crumpled graphene-SnO2 nanoparticle structures. We have also demonstrated supercapacitors with a high specific capacity using vertical graphene, high-porosity graphene, and crumpled graphene-Mn3O4 nanoparticle hybrids. This research is expected to result in novel methods to fabricate hybrid nanostructures at a low cost and a large scale for both LIB and supercapacitor applications and to shed lights on performance enhancement mechanisms.
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Representative journal publications:

1. S. Mao, Z. H. Wen, Haejune Kim, G. H. Lu, P. Hurley, and J. H. Chen*, "A General Approach to One-Pot Fabrication of Crumpled Graphene-Based Nanohybrids for Energy Applications," ACS Nano 6(8), 7505-7513, 2012.
2. Z. H. Wen, X. C. Wang, S. Mao, Z. Bo, H. Kim, S. M. Cui, G. H. Lu, X. L. Feng*, and J. H. Chen*, "Crumpled Nitrogen-Doped Graphene Nanosheets with Ultrahigh Pore Volume for High-performance Supercapacitor," Advanced Materials 24(41), 5610-5616, 2012.
3. H. J. Kim, Z. H. Wen, K. H. Yu, O. Mao, and J. H. Chen*, "Straightforward Fabrication of Highly Branched Graphene Nanosheet Array for Li-ion Battery Anode," Journal of Materials Chemistry 22(31), 15514-15518, 2012.
4. Z. Bo, S. Mao, K. H. Yu, S. M. Cui, and J. H. Chen*, "One-step Fabrication and Capacitive Behavior of Electrochemical Double Layer Capacitor Electrodes Using Vertically-oriented Graphene Directly Grown on Metal," Carbon 50(12), 4379-4387, 2012.
5. Z. H. Wen, S. M. Cui, H. J. Kim, S. Mao, K. H. Yu, G. H. Lu, H. H. Pu, O. Mao, and J. H. Chen*, "Binding Sn-Based Nanoparticles on Graphene as Anode of Lithium Ions Batteries," Journal of Materials Chemistry 22(8), 3300-3306, 2012.
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Theme 5: Pollution Control in Water and Air

Water is in seriously short supply in many regions of the world. Both for personal use — drinking, cleaning, cooking, and removal of waste — and large-scale use such as irrigation for agriculture, water must be available and sustainably provided to maintain quality of life. New technologies for desalinating sea water may be helpful, but small-scale technologies for local water purification may be even more effective for personal needs. Atmospheric direct current corona discharge from micro-sized objects has been widely used as an ion source in many devices, such as photocopiers, laser printers, and electronic air cleaners. However, ozone generation from these indoor corona devices poses significant health risks to indoor occupants. Our research goal is to control, degrade, and remove contaminants in water and air using nanomaterials and nanotechnology. We have developed a cost-effective route to a promising oxygen reduction reaction (ORR) catalyst, namely the nitrogen-enriched core-shell structured Fe/Fe3C-C (N-Fe/Fe3C@C) nanorods, with high activity and improved kinetics. The N-Fe/Fe3C@C was successfully used as cathode catalysts of microbial fuel cells (MFCs) and performed better than the commercial Pt/C-based cathode at the maximum power output. We have also demonstrated hybrids of CNTs-Fe nanoparticles and TiO2 nanorods-Ag nanocrystals for adsorption and photodegradation of various contaminants in water. Finally, we have demonstrated corona discharges from CNTs and vertical graphene to minimize ozone generation from indoor corona devices.
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Representative journal publications:

1. W. Smith*, S. Mao, G. H. Lu, A. Catlett, J. H. Chen*, and Y. P. Zhao, "The Effect of Ag Nanoparticle Loading on the Photocatalytic Activity of TiO2 Nanorod Arrays," Chemical Physics Letters 485(1-3), 171-175, 2010.
2. Z. H. Wen, S. Q. Ci, F. Zhang, X. L. Feng, S. M. Cui, S. Mao, S. L. Luo, and Z. He*, and J. H. Chen*, "Nitrogen-Enriched Core-Shell Structured Fe/Fe3C-C Nanorods as Advanced Catalysts for Oxygen Reduction Reaction," Advanced Materials 24(11), 1399-1404, 2012.
3. Z. Bo, K. H. Yu, G. H. Lu, S. M. Cui, S. Mao, and J. H. Chen*, "Vertically-oriented Graphene Sheets Grown on Metallic Wires for Greener Corona Discharges: Lower Power Consumption and Minimized Ozone Emission," Energy & Environmental Science 4(7), 2525-2528, 2011.
4. Z. Bo, K. H. Yu, G. H. Lu, S. Mao, J. H. Chen*, and F. G. Fan, "Nanoscale Discharge Electrode for Minimizing Ozone Emission from Indoor Corona Devices," Environmental Science & Technology 44(16), 6337-6342, 2010.
5. J. Ma*, J. H. Chen*, F. Yu, Z. W. Yuan, and L. L. Yu, "A Facile One-pot Method for Low-cost Synthesis of Magnetic Carbon Nanotubes and Their Applications for Dye Removal," New Journal of Chemistry, 36(10), 1940-1943, 2012.
6. F. Yu, J. H. Chen*, L. Chen, J. Huai, W. Y. Gong, Z. W. Yuan, J. H. Wang, and J. Ma*, "Magnetic Carbon Nanotubes Synthesis by Fenton's Reagent Method and Their Potential Application for Removal of Azo Dye from Aqueous Solution," Journal of Colloid and Interface Science 378(1), 175-183, 2012.
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Theme 6: Emerging Nanomaterials and Their Applications

Our group is interested in developing novel nanomaterials beyond what are currently available and then demonstrating their exciting applications. For instance, crumpled graphene and vertically-oriented graphene (VG) or carbon nanowall (CNW) have been produced by our group using novel methods. Crumpled graphene oxide (GO)/graphene (CG) is a new type of carbon nanostructure that has drawn growing attention due to its three-dimensional open structure and excellent stability in an aqueous solution. First of all, the CG structure exhibits significantly enhanced specific surface area and possesses excellent stability under redispersion or other material processing treatments. Additionally, the synthetic method is facile and suitable for scalable continuous manufacturing. Finally, the controlled deformation of 2D graphene nanostructures can markedly increase reactive sites and can be further used for tuning the reaction barrier and reaction energetics of graphene, which opens up new opportunities for various applications. CNWs are two-dimensional networks of vertically-aligned graphitic walls. An individual CNW can be a few layers of graphene up to several micrometers wide. Due to the high electron mobility of graphene, sharp edge and vertical alignment of CNWs, CNW materials have been demonstrated as a good field emission material. Moreover, CNWs have open edges; the theoretical surface area of CNWs is twice that of closed boundary structures, such as carbon nanotubes (CNTs) or C60. The high surface area of CNWs makes them attractive for catalyst supports. Recently, we have shown that both CG and CNWs are useful in Li-ion batteries and electrochemical capacitors.
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Representative journal publications:

1. K. H. Yu, Z. Bo, G. H. Lu, S. Mao, S. M. Cui, Y. W. Zhu, X. Q. Chen, R. S. Ruoff, and J. H. Chen*, "Growth of Carbon Nanowalls at Atmospheric Pressure for One-step Gas Sensor Fabrication," Nanoscale Research Letters 6:202, 2011.
2. Z. Bo, K. H. Yu, G. H. Lu, S. Mao, and J. H. Chen*, "Understanding Growth of Carbon Nanowalls at Atmospheric Pressure Using Normal Glow Discharge Plasma-enhanced Chemical Vapor Deposition," Carbon 49(6), 1849-1858, 2011.
3. K. H. Yu, P. X. Wang, G. H. Lu, K. H. Chen, Z. Bo, and J. H. Chen*, "Patterning Vertically Oriented Graphene Sheets for Nanodevice Applications," The Journal of Physical Chemistry Letters 2(6), 537-542, 2011.
4. K. H. Yu, G. H. Lu, Z. Bo, S. Mao, and J. H. Chen*, "Carbon Nanotube with Chemically-bonded Graphene Leaves for Electronic and Optoelectronic Applications," The Journal of Physical Chemistry Letters 2(13), 1556-1562, 2011.
5. Z. Bo, S. M. Cui, K. H. Yu, G. H. Lu, S. Mao, and J. H. Chen*, "Uniform Growth of Few-layer Graphene Vertically Standing on Cylindrical Surface at Atmospheric Pressure," Review of Scientific Instruments 82(8), 086116, 2011.
6. S. Mao, Z. H. Wen, Haejune Kim, G. H. Lu, P. Hurley, and J. H. Chen*, "A General Approach to One-Pot Fabrication of Crumpled Graphene-Based Nanohybrids for Energy Applications," ACS Nano 6(8), 7505-7513, 2012.
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