Professor Zheng received her Ph.D. in Mechanical & Aerospace Engineering from Princeton University (2006), B.S. in Thermal Engineering from Tsinghua University (2000). Prior to joining Stanford in 2007, Professor Zheng did her postdoctoral work in the Department of Chemistry and Chemical Biology at Harvard University. Professor Zheng is a member of MRS, ACS and combustion institute. Professor Zheng received the TR35 Award from the MIT Technology Review (2013), one of the 100 Leading Global Thinkers by the Foreign Policy Magazine (2013), 3M Nontenured Faculty Grant Award (2013), the Presidential Early Career Award (PECASE) from the white house (2009), Young Investigator Awards from the ONR (2008), DARPA (2008), Terman Fellowship from Stanford (2007), and Bernard Lewis Fellowship from the Combustion Institute (2004).

Administrative Appointments

  • Professor, Mechanical Engineering (2020 - Present)
  • Associate Professor, Mechanical Engineering (2014 - 2020)
  • Assistant Professor, Mechanical Engineering (2007 - 2014)

Honors & Awards

  • Presidential Early Career Award for Scientists and Engineers, Presidential Early Career Awards (2009)
  • Young Investigator Program, ONR (2008)
  • Young Faculty Award, DARPA (2008)
  • Terman Faculty Award, Stanford University (2007)
  • Bernard Lewis Fellowship, The Combustion Institute (2004)
  • Amelia Earhart Fellowship, Zonta International Foundation (2003)
  • One of the Pioneers on the TR35 Global list, MIT Technology Review (2013)
  • 3M Nontenured Faculty Grant Award, 3M (2013)

Professional Education

  • BS, Tsinghua University, Thermal Engineering (2000)
  • PhD, Princeton, Mechanical and Aerospace Engineering (2006)

2023-24 Courses

Stanford Advisees

All Publications

  • Upcycling plastic wastes into value-added products via electrocatalysis and photoelectrocatalysis JOURNAL OF ENERGY CHEMISTRY Kim, S., Kong, D., Zheng, X., Park, J. 2024; 91: 522-541
  • Bulk-Heterojunction Electrocatalysts in Confined Geometry Boosting Stable, Acid/Alkaline-Universal Water Electrolysis ADVANCED ENERGY MATERIALS Jang, G., Kim, S., Choi, J., Park, J., An, S., Baek, J., Li, Y., Liu, T., Kim, E., Lee, J., Wang, H., Kim, M., Cho, H., Zheng, X., Yoo, J., Seo, K., Park, J. 2024
  • Enhanced energy delivery of direct-write fabricated reactive materials with energetic graphene oxide COMBUSTION AND FLAME Wang, H., Jiang, Y., Wang, Y., Baek, J., Zheng, X., Zachariah, M. R. 2024; 260
  • Tailoring the mechanical and combustion performance of B/HTPB composite solid fuel with covalent interfaces COMPOSITES SCIENCE AND TECHNOLOGY Jiang, Y., Leem, J., Robinson, A. M., Wu, S., Huynh, A. H., Ka, D., Zhao, R., Xia, Y., Zheng, X. 2024; 245
  • Hyperbolic Polaritonic Rulers Based on van der Waals α-MoO3 Waveguides and Resonators. ACS nano Yu, S. J., Yao, H., Hu, G., Jiang, Y., Zheng, X., Fan, S., Heinz, T. F., Fan, J. A. 2023


    Low-dimensional, strongly anisotropic nanomaterials can support hyperbolic phonon polaritons, which feature strong light-matter interactions that can enhance their capabilities in sensing and metrology tasks. In this work, we report hyperbolic polaritonic rulers, based on microscale α-phase molybdenum trioxide (α-MoO3) waveguides and resonators suspended over an ultraflat gold substrate, which exhibit near-field polaritonic characteristics that are exceptionally sensitive to device geometry. Using scanning near-field optical microscopy, we show that these systems support strongly confined image polariton modes that exhibit ideal antisymmetric gap polariton dispersion, which is highly sensitive to air gap dimensions and can be described and predicted using a simple analytic model. Dielectric constants used for modeling are accurately extracted using near-field optical measurements of α-MoO3 waveguides in contact with the gold substrate. We also find that for nanoscale resonators supporting in-plane Fabry-Perot modes, the mode order strongly depends on the air gap dimension in a manner that enables a simple readout of the gap dimension with nanometer precision.

    View details for DOI 10.1021/acsnano.3c08735

    View details for PubMedID 37948673

  • Synergistic effects of mixing and strain in high entropy spinel oxides for oxygen evolution reaction. Nature communications Baek, J., Hossain, M. D., Mukherjee, P., Lee, J., Winther, K. T., Leem, J., Jiang, Y., Chueh, W. C., Bajdich, M., Zheng, X. 2023; 14 (1): 5936


    Developing stable and efficient electrocatalysts is vital for boosting oxygen evolution reaction (OER) rates in sustainable hydrogen production. High-entropy oxides (HEOs) consist of five or more metal cations, providing opportunities to tune their catalytic properties toward high OER efficiency. This work combines theoretical and experimental studies to scrutinize the OER activity and stability for spinel-type HEOs. Density functional theory confirms that randomly mixed metal sites show thermodynamic stability, with intermediate adsorption energies displaying wider distributions due to mixing-induced equatorial strain in active metal-oxygen bonds. The rapid sol-flame method is employed to synthesize HEO, comprising five 3d-transition metal cations, which exhibits superior OER activity and durability under alkaline conditions, outperforming lower-entropy oxides, even with partial surface oxidations. The study highlights that the enhanced activity of HEO is primarily attributed to the mixing of multiple elements, leading to strain effects near the active site, as well as surface composition and coverage.

    View details for DOI 10.1038/s41467-023-41359-7

    View details for PubMedID 37741823

    View details for PubMedCentralID PMC10517924

  • Exfoliated Magnesium Diboride (MgB2) Nanosheets as Solid Fuels. Nano letters Jiang, Y., Ka, D., Huynh, A. H., Baek, J., Ning, R., Yu, S. J., Zheng, X. 2023


    Magnesium diboride (MgB2) has been explored as an alternative fuel to boron (B) due to its high energy density and the additive effect of magnesium (Mg) to promote B combustion. However, the primary oxidation of MgB2 does not occur unless it decomposes at a high temperature (830 °C), which makes ignition difficult and the reaction slow. Recently, two-dimensional (2D) exfoliated MgB2 nanosheets have attracted increasing attention due to their unique properties and potential applications in various fields. In this study, we investigate the potential of 2D exfoliated MgB2 nanosheets as solid fuels for overcoming the challenges of MgB2 combustion. We analyzed their oxidation behavior and energetic performance through material characterization and combustion tests under slow- and fast-heating conditions and compared their performance with those of bulk MgB2, B nanoparticles, and a B/Mg nanoparticle mixture. This study highlights the potential of MgB2 nanosheets as promising solid fuels with superior energetic properties.

    View details for DOI 10.1021/acs.nanolett.3c01910

    View details for PubMedID 37656036

  • Recent advances in defect-engineered molybdenum sulfides for catalytic applications. Materials horizons Zhao, Y., Zheng, X., Gao, P., Li, H. 2023


    Electrochemical energy conversion and storage driven by renewable energy sources is drawing ever-increasing interest owing to the needs of sustainable development. Progress in the related electrochemical reactions relies on highly active and cost-effective catalysts to accelerate the sluggish kinetics. A substantial number of catalysts have been exploited recently, thanks to the advances in materials science and engineering. In particular, molybdenum sulfide (MoSx) furnishes a classic platform for studying catalytic mechanisms, improving catalytic performance and developing novel catalytic reactions. Herein, the recent theoretical and experimental progress of defective MoSx for catalytic applications is reviewed. This article begins with a brief description of the structure and basic catalytic applications of MoS2. The employment of defective two-dimensional and non-two-dimensional MoSx catalysts in the hydrogen evolution reaction (HER) is then reviewed, with a focus on the combination of theoretical and experimental tools for the rational design of defects and understanding of the reaction mechanisms. Afterward, the applications of defective MoSx as catalysts for the N2 reduction reaction, the CO2 reduction reaction, metal-sulfur batteries, metal-oxygen/air batteries, and the industrial hydrodesulfurization reaction are discussed, with a special emphasis on the synergy of multiple defects in achieving performance breakthroughs. Finally, the perspectives on the challenges and opportunities of defective MoSx for catalysis are presented.

    View details for DOI 10.1039/d3mh00462g

    View details for PubMedID 37466487

  • Enhanced H2O2 Upcycling into Hydroxyl Radicals with GO/Ni:FeOOH-Coated Silicon Nanowire Photocatalysts for Wastewater Treatment. Nano letters Ning, R., Kim, S., Sun, E., Jiang, Y., Baek, J., Li, Y., Robinson, A., Vallez, L., Zheng, X. 2023


    There remains continued interest in improving the advanced water oxidation process [e.g., ultraviolet (UV)/hydrogen peroxide (H2O2)] for more efficient and environmentally friendly wastewater treatment. Here, we report the design, fabrication, and performance of graphene oxide (GO, on top)/nickel-doped iron oxyhydroxide (Ni:FeOOH, shell)/silicon nanowires (SiNWs, core) as a new multifunctional photocatalyst for the degradation of common pollutants like polystyrene and methylene blue through enhancing the hydroxyl radical (•OH) production rate of the UV/H2O2 system. The photocatalyst combines the advantages of a large surface area and light absorption characteristics of SiNWs with heterogeneous photo-Fenton active Ni:FeOOH and photocatalytically active/charge separator GO. In addition, the built-in electric field of GO/Ni:FeOOH/SiNWs facilitates the charge separation of electrons to GO and holes to Ni:FeOOH, thus boosting the photocatalytic performance. Our photocatalyst increases the •OH yield by 5.7 times compared with that of a blank H2O2 solution sample and also extends the light absorption spectrum to include visible light irradiation.

    View details for DOI 10.1021/acs.nanolett.3c00696

    View details for PubMedID 37459426

  • Organic Upgrading through Photoelectrochemical Reactions: Toward Higher Profits. Small methods Liu, T. K., Jang, G. Y., Kim, S., Zhang, K., Zheng, X., Park, J. H. 2023: e2300315


    Aqueous photoelectrochemical (PEC) cells have long been considered a promising technology to convert solar energy into hydrogen. However, the solar-to-H2 (STH) efficiency and cost-effectiveness of PEC water splitting are significantly limited by sluggish oxygen evolution reaction (OER) kinetics and the low economic value of the produced O2 , hindering the practical commercialization of PEC cells. Recently, organic upgrading PEC reactions, especially for alternative OERs, have received tremendous attention, which improves not only the STH efficiency but also the economic effectiveness of the overall reaction. In this review, PEC reaction fundamentals and reactant-product cost analysis of organic upgrading reactions are briefly reviewed, recent advances made in organic upgrading reactions, which are categorized by their reactant substrates, such as methanol, ethanol, glycol, glycerol, and complex hydrocarbons, are then summarized and discussed. Finally, the current status, further outlooks, and challenges toward industrial applications are discussed.

    View details for DOI 10.1002/smtd.202300315

    View details for PubMedID 37382404

  • Data-Driven Approach to Tailoring Mechanical Properties of a Soft Material ADVANCED FUNCTIONAL MATERIALS Leem, J., Jiang, Y., Robinson, A., Xia, Y., Zheng, X. 2023
  • Machine Learning Assisted Analysis of Electrochemical H2O2 Production ACS APPLIED ENERGY MATERIALS Leem, J., Vallez, L., Gill, T., Zheng, X. 2023; 6 (7): 3953-3959
  • Do we need perfect mixing between fuel and oxidizer to maximize the energy release rate of energetic nanocomposites? APPLIED PHYSICS LETTERS Wang, H., Jiang, Y., Wang, Y., Kline, D. J., Zheng, X., Zachariah, M. R. 2023; 122 (1)

    View details for DOI 10.1063/5.0133995

    View details for Web of Science ID 000909857500002

  • Hydrogen-substituted graphdiyne-assisted ultrafast sparking synthesis of metastable nanomaterials. Nature nanotechnology Zheng, X., Gao, X., Vila, R. A., Jiang, Y., Wang, J., Xu, R., Zhang, R., Xiao, X., Zhang, P., Greenburg, L. C., Yang, Y., Xin, H. L., Zheng, X., Cui, Y. 2022


    Metastable nanomaterials, such as single-atom and high-entropy systems, with exciting physical and chemical properties are increasingly important for next-generation technologies. Here, we developed a hydrogen-substituted graphdiyne-assisted ultrafast sparking synthesis (GAUSS) platform for the preparation of metastable nanomaterials. The GAUSS platform can reach an ultra-high reaction temperature of 3,286K within 8ms, a rate exceeding 105Ks-1. Controlling the composition and chemistry of the hydrogen-substituted graphdiyne aerogel framework, the reaction temperature can be tuned from 1,640 K to 3,286K. We demonstrate the versatility of the GAUSS platform with the successful synthesis of single atoms, high-entropy alloys and high-entropy oxides. Electrochemical measurements and density functional theory show that single atoms synthesized by GAUSS enhance the lithium-sulfur redox reaction kinetics in all-solid-state lithium-sulfur batteries. Our design of the GAUSS platform offers a powerful way to synthesize a variety of metastable nanomaterials.

    View details for DOI 10.1038/s41565-022-01272-4

    View details for PubMedID 36585516

  • Author Correction: Discovery of LaAlO3 as an efficient catalyst for two-electron water electrolysis towards hydrogen peroxide. Nature communications Baek, J., Jin, Q., Johnson, N. S., Jiang, Y., Ning, R., Mehta, A., Siahrostami, S., Zheng, X. 2022; 13 (1): 7685

    View details for DOI 10.1038/s41467-022-35478-w

    View details for PubMedID 36509777

  • Discovery of LaAlO3 as an efficient catalyst for two-electron water electrolysis towards hydrogen peroxide. Nature communications Baek, J., Jin, Q., Johnson, N. S., Jiang, Y., Ning, R., Mehta, A., Siahrostami, S., Zheng, X. 2022; 13 (1): 7256


    Electrochemical two-electron water oxidation reaction (2e-WOR) has drawn significant attention as a promising process to achieve the continuous on-site production of hydrogen peroxide (H2O2). However, compared to the cathodic H2O2 generation, the anodic 2e-WOR is more challenging to establish catalysts due to the severe oxidizing environment. In this study, we combine density functional theory (DFT) calculations with experiments to discover a stable and efficient perovskite catalyst for the anodic 2e-WOR. Our theoretical screening efforts identify LaAlO3 perovskite as a stable, active, and selective candidate for catalyzing 2e-WOR. Our experimental results verify that LaAlO3 achieves an overpotential of 510mV at 10mAcm-2 in 4M K2CO3/KHCO3, lower than those of many reported metal oxide catalysts. In addition, LaAlO3 maintains a stable H2O2 Faradaic efficiency with only a 3% decrease after 3h at 2.7V vs. RHE. This computation-experiment synergistic approach introduces another effective direction to discover promising catalysts for the harsh anodic 2e-WOR towards H2O2.

    View details for DOI 10.1038/s41467-022-34884-4

    View details for PubMedID 36433962

  • Crystal Reconstruction of Mo:BiVO4: Improved Charge Transport for Efficient Solar Water Splitting ADVANCED FUNCTIONAL MATERIALS Jeong, Y., Seo, D., Baek, J., Kang, M., Kim, B., Kim, S., Zheng, X., Cho, I. 2022
  • Perfluoroalkyl-Functionalized Graphene Oxide as a Multifunctional Additive for Promoting the Energetic Performance of Aluminum. ACS nano Jiang, Y., Wang, H., Baek, J., Ka, D., Huynh, A. H., Wang, Y., Zachariah, M. R., Zheng, X. 2022


    Aluminum (Al) is a widely used metal fuel for energetic applications ranging from space propulsion and exploration, and materials processing, to power generation for nano- and microdevices due to its high energy density and earth abundance. Recently, the ignition and combustion performance of Al particles were found to be improved by graphene-based additives, such as graphene oxide (GO) and graphene fluoride (GF), as their reactions provide heat to accelerate Al oxidation, gas to reduce particle agglomeration, and fluorine-containing species to remove Al2O3. However, GF is not only expensive but also hydrophobic with poor mixing compatibility with Al particles. Herein, we report a multifunctional graphene-based additive for Al combustion, i.e., perfluoroalkyl-functionalized graphene oxide (CFGO), which integrates the benefits of GO and GF in one material. We compared the effects of CFGO to GO and GF on the ignition and combustion properties of nAl particles using thermogravimetric analysis, differential scanning calorimetry, temperature-jump ignition), Xe flash ignition, and constant-volume combustion test. These experiments confirm that CFGO generates fluorine-containing species, heat, and gases, which collectively lower the ignition threshold, augment the energy release rate, and reduce the combustion product agglomeration of nanosized Al particles, outperforming both GO and GF as additives. This work shows the great potential of using multifunctionalized graphene as an integrated additive for enhancing the ignition and combustion of metals.

    View details for DOI 10.1021/acsnano.2c05271

    View details for PubMedID 36099637

  • Ignition and combustion of Perfluoroalkyl-functionalized aluminum nanoparticles and nanothermite COMBUSTION AND FLAME Jiang, Y., Wang, Y., Baek, J., Wang, H., Gottfried, J. L., Wu, C., Shi, X., Zachariah, M. R., Zheng, X. 2022; 242
  • Efficient and Stable Acidic Water Oxidation Enabled by Low-Concentration, High-Valence Iridium Sites ACS ENERGY LETTERS Shi, X., Peng, H., Hersbach, T. P., Jiang, Y., Zeng, Y., Baek, J., Winther, K. T., Sokaras, D., Zheng, X., Bajdich, M. 2022
  • Effect of Fluoroalkylsilane Surface Functionalization on Boron Combustion. ACS applied materials & interfaces Baek, J., Jiang, Y., Demko, A. R., Jimenez-Thomas, A. R., Vallez, L., Ka, D., Xia, Y., Zheng, X. 2022


    Boron has been regarded as a promising high-energy fuel due to its high volumetric and gravimetric heating values. However, it remains challenging for boron to attain its theoretical heat of combustion because of the existence of its native boron oxide layer and its high melting and boiling temperatures that delay ignition and inhibit complete combustion. Boron combustion is known to be enhanced by physically adding fluorine-containing chemicals, such as fluoropolymer or metal fluorides, to remove surface boron oxides. Herein, we chemically functionalize the surface of boron particles with three different fluoroalkylsilanes: FPTS-B (F3-B), FOTS-B (F13-B), and FDTS-B (F17-B). We evaluated the ignition and combustion properties of those three functionalized boron particles as well as pristine ones. The boron particles functionalized with the longest fluorocarbon chain (F17) exhibit the most powerful energetic performance, the highest heat of combustion, and the strongest BO2 emission among all samples. These results suggest that the surface functionalization with fluoroalkylsilanes is an efficient strategy to enhance boron ignition and combustion.

    View details for DOI 10.1021/acsami.2c00347

    View details for PubMedID 35467848

  • Local Structure of Sulfur Vacancies on the Basal Plane of Monolayer MoS2. ACS nano Garcia-Esparza, A. T., Park, S., Abroshan, H., Paredes Mellone, O. A., Vinson, J., Abraham, B., Kim, T. R., Nordlund, D., Gallo, A., Alonso-Mori, R., Zheng, X., Sokaras, D. 2022


    The nature of the S-vacancy is central to controlling the electronic properties of monolayer MoS2. Understanding the geometric and electronic structures of the S-vacancy on the basal plane of monolayer MoS2 remains elusive. Here, operando S K-edge X-ray absorption spectroscopy shows the formation of clustered S-vacancies on the basal plane of monolayer MoS2 under reaction conditions (H2 atmosphere, 100-600 °C). First-principles calculations predict spectral fingerprints consistent with the experimental results. The Mo K-edge extended X-ray absorption fine structure shows the local structure as coordinatively unsaturated Mo with 4.1 ± 0.4 S atoms as nearest neighbors (above 400 °C in an H2 atmosphere). Conversely, the 6-fold Mo-Mo coordination in the crystal remains unchanged. Electrochemistry confirms similar active sites for hydrogen evolution. The identity of the S-vacancy defect on the basal plane of monolayer MoS2 is herein elucidated for applications in optoelectronics and catalysis.

    View details for DOI 10.1021/acsnano.2c01388

    View details for PubMedID 35380038

  • Ultrahigh-Quality Infrared Polaritonic Resonators Based on Bottom-Up-Synthesized van der Waals Nanoribbons. ACS nano Yu, S., Jiang, Y., Roberts, J. A., Huber, M. A., Yao, H., Shi, X., Bechtel, H. A., Gilbert Corder, S. N., Heinz, T. F., Zheng, X., Fan, J. A. 1800


    van der Waals nanomaterials supporting phonon polariton quasiparticles possess extraordinary light confinement capabilities, making them ideal systems for molecular sensing, thermal emission, and subwavelength imaging applications, but they require defect-free crystallinity and nanostructured form factors to fully showcase these capabilities. We introduce bottom-up-synthesized alpha-MoO3 structures as nanoscale phonon polaritonic systems that feature tailorable morphologies and crystal qualities consistent with bulk single crystals. alpha-MoO3 nanoribbons serve as low-loss hyperbolic Fabry-Perot nanoresonators, and we experimentally map hyperbolic resonances over four Reststrahlen bands spanning the far- and mid-infrared spectral range, including resonance modes beyond the 10th order. The measured quality factors are the highest from phonon polaritonic van der Waals structures to date. We anticipate that bottom-up-synthesized polaritonic van der Waals nanostructures will serve as an enabling high-performance and low-loss platform for infrared optical and optoelectronic applications.

    View details for DOI 10.1021/acsnano.1c10489

    View details for PubMedID 35041379

  • Ultrahigh-quality van der Waals hyperbolic polariton resonators Yu, S., Jiang, Y., Roberts, J. A., Huber, M. A., Yao, H., Shi, X., Bechtel, H. A., Corder, S. G., Heinz, T. F., Zheng, X., Fan, J. A., Chang-Hasnain, C. J., Fan, J. A., Zhou, W. SPIE-INT SOC OPTICAL ENGINEERING. 2022

    View details for DOI 10.1117/12.2612301

    View details for Web of Science ID 000836330700010

  • Enhancing Electrochemical Water Oxidation toward H2O2 via Carbonaceous Electrolyte Engineering ACS APPLIED ENERGY MATERIALS Gill, T., Vallez, L., Zheng, X. 2021; 4 (11): 12429-12435
  • Probing boron thermite energy release at rapid heating rates COMBUSTION AND FLAME Gottfried, J. L., Wainwright, E. R., Huang, S., Jiang, Y., Zheng, X. 2021; 231
  • The Role of Bicarbonate-Based Electrolytes in H2O2 Production through Two-Electron Water Oxidation ACS ENERGY LETTERS Gill, T., Vallez, L., Zheng, X. 2021; 6 (8): 2854-2862
  • High thermoelectric figure of merit of porous Si nanowires from 300 to 700K. Nature communications Yang, L., Huh, D., Ning, R., Rapp, V., Zeng, Y., Liu, Y., Ju, S., Tao, Y., Jiang, Y., Beak, J., Leem, J., Kaur, S., Lee, H., Zheng, X., Prasher, R. S. 2021; 12 (1): 3926


    Thermoelectrics operating at high temperature can cost-effectively convert waste heat and compete with other zero-carbon technologies. Among different high-temperature thermoelectrics materials, silicon nanowires possess the combined attributes of cost effectiveness and mature manufacturing infrastructures. Despite significant breakthroughs in silicon nanowires based thermoelectrics for waste heat conversion, the figure of merit (ZT) or operating temperature has remained low. Here, we report the synthesis of large-area, wafer-scale arrays of porous silicon nanowires with ultra-thin Si crystallite size of ~4nm. Concurrent measurements of thermal conductivity (kappa), electrical conductivity (sigma), and Seebeck coefficient (S) on the same nanowire show a ZT of 0.71 at 700K, which is more than ~18 times higher than bulk Si. This ZT value is more than two times higher than any nanostructured Si-based thermoelectrics reported in the literature at 700K. Experimental data and theoretical modeling demonstrate that this work has the potential to achieve a ZT of ~1 at 1000K.

    View details for DOI 10.1038/s41467-021-24208-3

    View details for PubMedID 34168136

  • Enhancing Mechanical and Combustion Performance of Boron/Polymer Composites via Boron Particle Functionalization. ACS applied materials & interfaces Jiang, Y., Dincer Yilmaz, N. E., Barker, K. P., Baek, J., Xia, Y., Zheng, X. 2021


    High-speed air-breathing propulsion systems, such as solid fuel ramjets (SFRJ), are important for space exploration and national security. The development of SFRJ requires high-performance solid fuels with excellent mechanical and combustion properties. One of the current solid fuel candidates is composed of high-energy particles (e.g., boron (B)) and polymeric binder (e.g., hydroxyl-terminated polybutadiene (HTPB)). However, the opposite polarities of the boron surface and HTPB lead to poor B particle dispersion and distribution within HTPB. Herein, we demonstrate that the surface functionalization of B particles with nonpolar oleoyl chloride greatly improves the dispersion and distribution of B particles within HTPB. The improved particle dispersion is quantitatively visualized through X-ray computed tomography imaging, and the particle/matrix interaction is evaluated by dynamic mechanical analysis. The surface-functionalized B particles can be uniformly dispersed up to 40 wt % in HTPB, the highest mass loading reported to date. The surface-functionalized B (40 wt %)/HTPB composite exhibits a 63.3% higher Young's modulus, 87.5% higher tensile strength, 16.2% higher toughness, and 16.8% higher heat of combustion than pristine B (40 wt %)/HTPB. The surface functionalization of B particles provides an effective strategy for improving the efficacy and safety of B/HTPB solid fuels for future high-speed air-breathing vehicles.

    View details for DOI 10.1021/acsami.1c06727

    View details for PubMedID 34110148

  • Operando Study of Thermal Oxidation of Monolayer MoS2. Advanced science (Weinheim, Baden-Wurttemberg, Germany) Park, S., Garcia-Esparza, A. T., Abroshan, H., Abraham, B., Vinson, J., Gallo, A., Nordlund, D., Park, J., Kim, T. R., Vallez, L., Alonso-Mori, R., Sokaras, D., Zheng, X. 2021; 8 (9): 2002768


    Monolayer MoS2 is a promising semiconductor to overcome the physical dimension limits of microelectronic devices. Understanding the thermochemical stability of MoS2 is essential since these devices generate heat and are susceptible to oxidative environments. Herein, the promoting effect of molybdenum oxides (MoO x ) particles on the thermal oxidation of MoS2 monolayers is shown by employing operando X-ray absorption spectroscopy, ex situ scanning electron microscopy and X-ray photoelectron spectroscopy. The study demonstrates that chemical vapor deposition-grown MoS2 monolayers contain intrinsic MoO x and are quickly oxidized at 100 °C (3 vol% O2/He), in contrast to previously reported oxidation thresholds (e.g., 250 °C, t ≤ 1 h in the air). Otherwise, removing MoO x increases the thermal oxidation onset temperature of monolayer MoS2 to 300 °C. These results indicate that MoO x promote oxidation. An oxide-free lattice is critical to the long-term stability of monolayer MoS2 in state-of-the-art 2D electronic, optical, and catalytic applications.

    View details for DOI 10.1002/advs.202002768

    View details for PubMedID 33977043

    View details for PubMedCentralID PMC8097340

  • Operando Study of Thermal Oxidation of Monolayer MoS2 ADVANCED SCIENCE Park, S., Garcia-Esparza, A. T., Abroshan, H., Abraham, B., Vinson, J., Gallo, A., Nordlund, D., Park, J., Kim, T., Vallez, L., Alonso-Mori, R., Sokaras, D., Zheng, X. 2021
  • Electrochemical Synthesis of H2O2 by Two-Electron Water Oxidation Reaction CHEM Shi, X., Back, S., Gill, T., Siahrostami, S., Zheng, X. 2021; 7 (1): 38–63
  • Ultrahigh Doping of Graphene Using Flame-Deposited MoO3 IEEE ELECTRON DEVICE LETTERS Vaziri, S., Chen, V., Cai, L., Jiang, Y., Chen, M. E., Grady, R. W., Zheng, X., Pop, E. 2020; 41 (10): 1592–95
  • Enhancing combustion performance of nano-Al/PVDF composites with beta-PVDF COMBUSTION AND FLAME Huang, S., Hong, S., Su, Y., Jiang, Y., Fukushima, S., Gill, T., Yilmaz, N., Tiwari, S., Nomura, K., Kalia, R. K., Nakano, A., Shimojo, F., Vashishta, P., Chen, M., Zheng, X. 2020; 219: 467–77
  • Comparing Methods for Quantifying Electrochemically Accumulated H2O2 CHEMISTRY OF MATERIALS Gill, T., Zheng, X. 2020; 32 (15): 6285–94
  • Effect of Adventitious Carbon on Pit Formation of Monolayer MoS2. Advanced materials (Deerfield Beach, Fla.) Park, S., Siahrostami, S., Park, J., Mostaghimi, A. H., Kim, T. R., Vallez, L., Gill, T. M., Park, W., Goodson, K. E., Sinclair, R., Zheng, X. 2020: e2003020


    Forming pits on molybdenum disulfide (MoS2 ) monolayers is desirable for (opto)electrical, catalytic, and biological applications. Thermal oxidation is a potentially scalable method to generate pits on monolayer MoS2 , and pits are assumed to preferentially form around undercoordinated sites, such as sulfur vacancies. However, studies on thermal oxidation of MoS2 monolayers have not considered the effect of adventitious carbon (C) that is ubiquitous and interacts with oxygen at elevated temperatures. Herein, the effect of adventitious C on the pit formation on MoS2 monolayers during thermal oxidation is studied. The in situ environmental transmission electron microscopy measurements herein show that pit formation is preferentially initiated at the interface between adventitious C nanoparticles and MoS2 , rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS2 interface favors the sequential adsorption of oxygen atoms with facile kinetics. These results illustrate the important role of adventitious C on pit formation on monolayer MoS2 .

    View details for DOI 10.1002/adma.202003020

    View details for PubMedID 32743836

  • On-demand production of hydrogen by reacting porous silicon nanowires with water NANO RESEARCH Ning, R., Jiang, Y., Zeng, Y., Gong, H., Zhao, J., Weisse, J., Shi, X., Gill, T. M., Zheng, X. 2020
  • Synergistically Chemical and Thermal Coupling between Graphene Oxide and Graphene Fluoride for Enhancing Aluminum Combustion. ACS applied materials & interfaces Jiang, Y. n., Deng, S. n., Hong, S. n., Tiwari, S. n., Chen, H. n., Nomura, K. I., Kalia, R. K., Nakano, A. n., Vashishta, P. n., Zachariah, M. R., Zheng, X. n. 2020


    Metal combustion reaction is highly exothermic and is used in energetic applications, such as propulsion, pyrotechnics, powering micro- and nano-devices, and nanomaterials synthesis. Aluminum (Al) is attracting great interest in those applications because of its high energy density, earth abundance, and low toxicity. Nevertheless, Al combustion is hard to initiate and progresses slowly and incompletely. On the other hand, ultrathin carbon nanomaterials, such as graphene, graphene oxide (GO), and graphene fluoride (GF), can also undergo exothermic reactions. Herein, we demonstrate that the mixture of GO and GF significantly improves the performance of Al combustion as interactions between GO and GF provide heat and radicals to accelerate Al oxidation. Our experiments and reactive molecular dynamics simulation reveal that GO and GF have strong chemical and thermal couplings through radical reactions and heat released from their oxidation reactions. GO facilitates the dissociation of GF, and GF accelerates the disproportionation and oxidation of GO. When the mixture of GO and GF is added to micron-sized Al particles, their synergistic couplings generate reactive oxidative species, such as CF x and CF x O y , and heat, which greatly accelerates Al combustion. This work demonstrates a new area of using synergistic couplings between ultrathin carbon nanomaterials to accelerate metal combustion and potentially oxidation reactions of other materials.

    View details for DOI 10.1021/acsami.9b20397

    View details for PubMedID 31950820

  • Experimental effective metal oxides to enhance boron combustion COMBUSTION AND FLAME Huang, S., Deng, S., Jiang, Y., Zheng, X. 2019; 205: 278–85
  • ZnO As an Active and Selective Catalyst for Electrochemical Water Oxidation to Hydrogen Peroxide ACS CATALYSIS Kelly, S. R., Shi, X., Back, S., Vallez, L., Park, S., Siahrostami, S., Zheng, X., Norskov, J. K. 2019; 9 (5): 4593–99
  • Modified Micro-Emulsion Synthesis of Highly Dispersed Al/PVDF Composites with Enhanced Combustion Properties ADVANCED ENGINEERING MATERIALS Huang, S., Pan, M., Deng, S., Jiang, Y., Zhao, J., Levy-Wendt, B., Tang, S. Y., Zheng, X. 2019; 21 (5)
  • Enhancing Electrocatalytic Water Splitting by Strain Engineering ADVANCED MATERIALS You, B., Tang, M. T., Tsai, C., Abild-Pedersen, F., Zheng, X., Li, H. 2019; 31 (17)
  • A Zn: BiVO4/ Mo: BiVO4 homojunction as an efficient photoanode for photoelectrochemical water splitting JOURNAL OF MATERIALS CHEMISTRY A Lee, J., Baek, J., Gill, T., Shi, X., Lee, S., Cho, I., Jung, H., Zheng, X. 2019; 7 (15): 9019–24

    View details for DOI 10.1039/c9ta00205g

    View details for Web of Science ID 000465152700032

  • Boosting the solar water oxidation performance of a BiVO4 photoanode by crystallographic orientation control (vol 11, pg 1299, 2018) ENERGY & ENVIRONMENTAL SCIENCE Han, H., Shin, S., Kim, D., Park, I., Kim, J., Huang, P., Lee, J., Cho, I., Zheng, X. 2019; 12 (4): 1427

    View details for DOI 10.1039/c9ee90017a

    View details for Web of Science ID 000465275800023

  • Rapid Flame-Annealed CuFe2O4 as Efficient Photocathode for Photoelectrochemical Hydrogen Production ACS SUSTAINABLE CHEMISTRY & ENGINEERING Park, S., Baek, J., Zhang, L., Lee, J., Stone, K. H., Cho, I., Guo, J., Jung, H., Zheng, X. 2019; 7 (6): 5867–74
  • Epitaxial growth of WO3 nanoneedles achieved using a facile flame surface treatment process engineering of hole transport and water oxidation reactivity (vol 6, pg 19542, 2018) JOURNAL OF MATERIALS CHEMISTRY A Shi, X., Cai, L., Choi, I., Ma, M., Zhang, K., Zhao, J., Kim, J., Kim, J., Zheng, X., Park, J. 2019; 7 (10): 5832

    View details for DOI 10.1039/c9ta90049g

    View details for Web of Science ID 000463824300076

  • Selective and Efficient Gd-Doped BiVO4 Photoanode for Two-Electron Water Oxidation to H2O2 ACS ENERGY LETTERS Baek, J., Gill, T., Abroshan, H., Park, S., Shi, X., Norskoy, J., Jung, H., Siahrostami, S., Zheng, X. 2019; 4 (3): 720–28
  • Enhancing Electrocatalytic Water Splitting by Strain Engineering. Advanced materials (Deerfield Beach, Fla.) You, B., Tang, M. T., Tsai, C., Abild-Pedersen, F., Zheng, X., Li, H. 2019: e1807001


    Electrochemical water splitting driven by sustainable energy such as solar, wind, and tide is attracting ever-increasing attention for sustainable production of clean hydrogen fuel from water. Leveraging these advances requires efficient and earth-abundant electrocatalysts to accelerate the kinetically sluggish hydrogen and oxygen evolution reactions (HER and OER). A large number of advanced water-splitting electrocatalysts have been developed through recent understanding of the electrochemical nature and engineering approaches. Specifically, strain engineering offers a novel route to promote the electrocatalytic HER/OER performances for efficient water splitting. Herein, the recent theoretical and experimental progress on applying strain to enhance heterogeneous electrocatalysts for both HER and OER are reviewed and future opportunities are discussed. A brief introduction of the fundamentals of water-splitting reactions, and the rationalization for utilizing mechanical strain to tune an electrocatalyst is given, followed by a discussion of the recent advances on strain-promoted HER and OER, with special emphasis given to combined theoretical and experimental approaches for determining the optimal straining effect for water electrolysis, along with experimental approaches for creating and characterizing strain in nanocatalysts, particularly emerging 2D nanomaterials. Finally, a vision for a future sustainable hydrogen fuel community based on strain-promoted water electrolysis is proposed.

    View details for PubMedID 30773741

  • CaSnO3: An Electrocatalyst for Two-Electron Water Oxidation Reaction to Form H2O2 ACS ENERGY LETTERS Park, S., Abroshan, H., Shi, X., Jung, H., Siahrostami, S., Zheng, X. 2019; 4 (1): 352–57
  • Enhancing Catalytic Activity of MoS2 Basal Plane S-Vacancy by Co Cluster Addition ACS ENERGY LETTERS Park, S., Park, J., Abroshan, H., Zhang, L., Kim, J., Zhang, J., Guo, J., Siahrostami, S., Zheng, X. 2018; 3 (11): 2685–93
  • Epitaxial growth of WO3 nanoneedles achieved using a facile flame surface treatment process engineering of hole transport and water oxidation reactivity JOURNAL OF MATERIALS CHEMISTRY A Shi, X., Cai, L., Choi, I., Ma, M., Zhang, K., Zhao, J., Kim, J., Kim, J., Zheng, X., Park, J. 2018; 6 (40): 19542–46

    View details for DOI 10.1039/c8ta04081h

    View details for Web of Science ID 000448413100025

  • Resolving Hysteresis in Perovskite Solar Cells with Rapid Flame-Processed Cobalt-Doped TiO2 ADVANCED ENERGY MATERIALS Kim, J., Chai, S., Ji, Y., Levy-Wendt, B., Kim, S., Yi, Y., Heinz, T. F., Norskov, J. K., Park, J., Zheng, X. 2018; 8 (29)
  • Tuning the morphological, ignition and combustion properties of micron-Al/CuO thermites through different synthesis approaches COMBUSTION AND FLAME Deng, S., Jiang, Y., Huang, S., Shi, X., Zhao, J., Zheng, X. 2018; 195: 303–10
  • Activating basal plane of MoS2 for hydrogen evolution reaction through sulfur vacancy, doping and strain Zheng, X. AMER CHEMICAL SOC. 2018
  • Light-Driven BiVO4-C Fuel Cell with Simultaneous Production of H2O2 ADVANCED ENERGY MATERIALS Shi, X., Zhang, Y., Siahrostami, S., Zheng, X. 2018; 8 (23)
  • Rapid flame doping of Co to WS2 for efficient hydrogen evolution ENERGY & ENVIRONMENTAL SCIENCE Shi, X., Fields, M., Park, J., McEnaney, J. M., Yan, H., Zhang, Y., Tsai, C., Jaramillo, T. F., Sinclair, R., Norskov, J. K., Zheng, X. 2018; 11 (8): 2270–77

    View details for DOI 10.1039/c8ee01111g

    View details for Web of Science ID 000442262900035

  • Wafer-recyclable, environment-friendly transfer printing for large-scale thin-film nanoelectronics PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Wie, D., Zhang, Y., Kim, M., Kim, B., Park, S., Kim, Y., Irazoqui, P. P., Zheng, X., Xu, B., Lee, C. 2018; 115 (31): E7236–E7244


    Transfer printing of thin-film nanoelectronics from their fabrication wafer commonly requires chemical etching on the sacrifice of wafer but is also limited by defects with a low yield. Here, we introduce a wafer-recyclable, environment-friendly transfer printing process that enables the wafer-scale separation of high-performance thin-film nanoelectronics from their fabrication wafer in a defect-free manner that enables multiple reuses of the wafer. The interfacial delamination is enabled through a controllable cracking phenomenon in a water environment at room temperature. The physically liberated thin-film nanoelectronics can be then pasted onto arbitrary places of interest, thereby endowing the particular surface with desirable add-on electronic features. Systematic experimental, theoretical, and computational studies reveal the underlying mechanics mechanism and guide manufacturability for the transfer printing process in terms of scalability, controllability, and reproducibility.

    View details for PubMedID 30012591

  • Flame-Engraved Nickel-Iron Layered Double Hydroxide Nanosheets for Boosting Oxygen Evolution Reactivity SMALL METHODS Zhou, D., Xiong, X., Cai, Z., Han, N., Jia, Y., Xie, Q., Duan, X., Xie, T., Zheng, X., Sun, X., Duan, X. 2018; 2 (7)
  • Enabling silicon photoanodes for efficient solar water splitting by electroless-deposited nickel NANO RESEARCH Zhao, J., Gill, T., Zheng, X. 2018; 11 (6): 3499–3508
  • Boosting the solar water oxidation performance of a BiVO4 photoanode by crystallographic orientation control ENERGY & ENVIRONMENTAL SCIENCE Han, H., Shin, S., Kim, D., Park, I., Kim, J., Huang, P., Lee, J., Cho, I., Zheng, X. 2018; 11 (5): 1299–1306

    View details for DOI 10.1039/c8ee00125a

    View details for Web of Science ID 000432599100017

  • Rapid Formation of a Disordered Layer on Monoclinic BiVO4: Co-Catalyst-Free Photoelectrochemical Solar Water Splitting CHEMSUSCHEM Kim, J., Cho, Y., Jeong, M., Levy-Wendt, B., Shin, D., Yi, Y., Wang, D., Zheng, X., Park, J. 2018; 11 (5): 933–40


    A surface disordered layer is a plausible approach to improve the photoelectrochemical performance of TiO2 . However, the formation of a crystalline disordered layer in BiVO4 and its effectiveness towards photoelectrochemical water splitting has remained a big challenge. Here, we report a rapid solution process (within 5 s) that is able to form a disordered layer of a few nanometers thick on the surface of BiVO4 nanoparticles using a specific solution with a controllable reducing power. The disordered layer on BiVO4 alleviates charge recombination at the electrode-electrolyte interface and reduces the onset potential greatly, which in turn results in a photocurrent density of approximately 2.3 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE). This value is 2.1 times higher than that of bare BiVO4 . The enhanced photoactivity is attributed to the increased charge separation and transfer efficiencies, which resolve the intrinsic drawbacks of bare BiVO4 such as the short hole diffusion length of around 100 nm and poor surface oxygen evolution reactivity.

    View details for PubMedID 29274301

  • Thermoplasmonic Ignition of Metal Nanoparticles NANO LETTERS Mutlu, M., Kang, J., Raza, S., Schoen, D., Zheng, X., Kik, P. G., Brongersma, M. L. 2018; 18 (3): 1699–1706


    Explosives, propellants, and pyrotechnics are energetic materials that can store and quickly release tremendous amounts of chemical energy. Aluminum (Al) is a particularly important fuel in many applications because of its high energy density, which can be released in a highly exothermic oxidation process. The diffusive oxidation mechanism (DOM) and melt-dispersion mechanism (MDM) explain the ways powders of Al nanoparticles (NPs) can burn, but little is known about the possible use of plasmonic resonances in NPs to manipulate photoignition. This is complicated by the inhomogeneous nature of powders and very fast heating and burning rates. Here, we generate Al NPs with well-defined sizes, shapes, and spacings by electron beam lithography and demonstrate that their plasmonic resonances can be exploited to heat and ignite them with a laser. By combining simulations with thermal-emission, electron-, and optical-microscopy studies, we reveal how an improved control over NP ignition can be attained.

    View details for PubMedID 29356548

  • Enhancing Mo:BiVO4 Solar Water Splitting with Patterned Au Nanospheres by Plasmon-Induced Energy Transfer ADVANCED ENERGY MATERIALS Kim, J., Shi, X., Jeong, M., Park, J., Han, H., Kim, S., Guo, Y., Heinz, T. F., Fan, S., Lee, C., Park, J., Zheng, X. 2018; 8 (5)
  • Conformal Electroless Nickel Plating on Silicon Wafers, Convex & Concave Pyramids, and Ultralong Nanowires. ACS applied materials & interfaces Gill, T. n., Zhao, J. n., Berenschot, E. J., Tas, N. n., Zheng, X. n. 2018


    Nickel (Ni) plating has garnered great commercial interest, as it provides excellent hardness, corrosion resistance, and electrical conductivity. Though Ni plating on conducting substrates is commonly employed via electrodeposition, plating on semiconductors and insulators often necessitates electroless approaches. Corresponding plating theory for deposition on planar substrates was developed as early as 1946, but for substrates with micro and nanoscale features, very little is known of the relationships between plating conditions, Ni deposition quality, and substrate morphology. Herein, we describe the general theory of the mechanisms of electroless Ni deposition on semiconducting silicon (Si) substrates, detailing plating bath failures and establishing relationships between critical plating bath parameters and the deposited Ni film quality. Through this theory, we develop two different plating recipes: galvanic displacement (GD) and autocatalytic deposition (ACD). Neither recipe requires pretreatment of the Si substrate and both methods are capable of depositing uniform Ni films on planar Si substrates and convex Si pyramids. In comparison, ACD has better tunability than GD, and it provides more conformal Ni coating on complex and high-aspect ratio Si structures, such as inverse fractal Si pyramids and ultralong Si nanowires. Our methodology and theoretical analyses can be leveraged to develop electroless plating processes for other metals and metal alloys and to generally provide direction for the adaptation of electroless deposition to modern applications.

    View details for PubMedID 29882649

  • Energetic Performance of Optically Activated Aluminum/Graphene Oxide Composites. ACS nano Jiang, Y. n., Deng, S. n., Hong, S. n., Zhao, J. n., Huang, S. n., Wu, C. C., Gottfried, J. L., Nomura, K. I., Li, Y. n., Tiwari, S. n., Kalia, R. K., Vashishta, P. n., Nakano, A. n., Zheng, X. n. 2018


    Optical ignition of solid energetic materials, which can rapidly release heat, gas, and thrust, is still challenging due to the limited light absorption and high ignition energy of typical energetic materials ( e.g., aluminum, Al). Here, we demonstrated that the optical ignition and combustion properties of micron-sized Al particles were greatly enhanced by adding only 20 wt % of graphene oxide (GO). These enhancements are attributed to the optically activated disproportionation and oxidation reactions of GO, which release heat to initiate the oxidization of Al by air and generate gaseous products to reduce the agglomeration of the composites and promote the pressure rise during combustion. More importantly, compared to conventional additives such as metal oxides nanoparticles ( e.g., WO3 and Bi2O3), GO has much lower density and therefore could improve energetic properties without sacrificing Al content. The results from Xe flash ignition and laser-based excitation experiments demonstrate that GO is an efficient additive to improve the energetic performance of micron-sized Al particles, enabling micron-sized Al to be ignited by optical activation and promoting the combustion of Al in air.

    View details for DOI 10.1021/acsnano.8b06217

    View details for PubMedID 30335365

  • Sub-Thermionic Steep Switching in Hole-Doped WSe2 Transistors McClellan, C. J., Yalon, E., Cai, L., Suryavanshi, S., Zheng, X., Pop, E., IEEE IEEE. 2018
  • Ultrafast Flame Annealing of TiO2 Paste for Fabricating Dye-Sensitized and Perovskite Solar Cells with Enhanced Efficiency SMALL Kim, J., Chai, S., Cho, Y., Cai, L., Kim, S., Park, S., Park, J., Zheng, X. 2017; 13 (42)
  • Ultrafast Flame Annealing of TiO2 Paste for Fabricating Dye-Sensitized and Perovskite Solar Cells with Enhanced Efficiency. Small (Weinheim an der Bergstrasse, Germany) Kim, J. K., Chai, S. U., Cho, Y., Cai, L., Kim, S. J., Park, S., Park, J. H., Zheng, X. 2017; 13 (42)


    Mesoporous TiO2 nanoparticle (NP) films are broadly used as electrodes in photoelectrochemical cells, dye-sensitized solar cells (DSSCs), and perovskite solar cells (PSCs). State-of-the-art mesoporous TiO2 NP films for these solar cells are fabricated by annealing TiO2 paste-coated fluorine-doped tin oxide glass in a box furnace at 500 °C for ≈30 min. Here, the use of a nontraditional reactor, i.e., flame, is reported for the high throughput and ultrafast annealing of TiO2 paste (≈1 min). This flame-annealing method, compared to conventional furnace annealing, exhibits three distinct benefits. First, flame removes polymeric binders in the initial TiO2 paste more completely because of its high temperature (≈1000 °C). Second, flame induces strong interconnections between TiO2 nanoparticles without affecting the underlying transparent conducting oxide substrate. Third, the flame-induced carbothermic reduction on the TiO2 surface facilitates charge injection from the dye/perovskite to TiO2 . Consequently, when the flame-annealed mesoporous TiO2 film is used to fabricate DSSCs and PSCs, both exhibit enhanced charge transport and higher power conversion efficiencies than those fabricated using furnace-annealed TiO2 films. Finally, when the ultrafast flame-annealing method is combined with a fast dye-coating method to fabricate DSSC devices, its total fabrication time is reduced from over 3 h to ≈10 min.

    View details for DOI 10.1002/smll.201702260

    View details for PubMedID 28940949

  • Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide NATURE COMMUNICATIONS Shi, X., Siahrostami, S., Li, G., Zhang, Y., Chakthranont, P., Studt, F., Jaramillo, T. F., Zheng, X., Norskov, J. K. 2017; 8: 701


    Electrochemical production of hydrogen peroxide (H2O2) from water oxidation could provide a very attractive route to locally produce a chemically valuable product from an abundant resource. Herein using density functional theory calculations, we predict trends in activity for water oxidation towards H2O2 evolution on four different metal oxides, i.e., WO3, SnO2, TiO2 and BiVO4. The density functional theory predicted trend for H2O2 evolution is further confirmed by our experimental measurements. Moreover, we identify that BiVO4 has the best H2O2 generation amount of those oxides and can achieve a Faraday efficiency of about 98% for H2O2 production.Producing hydrogen peroxide via electrochemical oxidation of water is an attractive route to this valuable product. Here the authors theoretically and experimentally investigate hydrogen peroxide production activity trends for a range of metal oxides and identify the optimal bias ranges for high Faraday efficiencies.

    View details for PubMedID 28951571

  • Electroless Deposition and Ignition Properties of Si/Fe2O3 Core/Shell Nanothermites. ACS omega Huang, S., Deng, S., Jiang, Y., Zhao, J., Zheng, X. 2017; 2 (7): 3596-3600


    Thermite, a composite of metal and metal oxide, finds wide applications in power and thermal generation systems that require high-energy density. Most of the researches on thermites have focused on using aluminum (Al) particles as the fuel. However, Al particles are sensitive to electrostatic discharge, friction, and mechanical impact, imposing a challenge for the safe handling and storage of Al-based thermites. Silicon (Si) is another attractive fuel for thermites because of its high-energy content, thin native oxide layer, and facile surface functionality. Several studies showed that the combustion properties of Si-based thermites are comparable to those of Al-based thermites. However, little is known about the ignition properties of Si-based thermites. In this work, we determined the reaction onset temperatures of mechanically mixed (MM) Si/Fe2O3 nanothermites and Si/Fe2O3 core/shell (CS) nanothermites using differential scanning calorimetry. The Si/Fe2O3 CS nanothermites were prepared by an electroless deposition method. We found that the Si/Fe2O3 CS nanoparticles (NPs) had a lower reaction onset temperature (∼550 °C) than the MM Si/Fe2O3 nanothermites (>650 °C). The onset temperature of the Si/Fe2O3 CS nanothermites is also insensitive to the size of the Si core NP. These results indicate that the interfacial contact quality between Si and Fe2O3 is the dominant factor for determining the ignition properties of thermites. Finally, the reaction onset temperature of the Si/Fe2O3 CS NPs is comparable to that of the commonly used Al-based nanothermites, suggesting that Si is an attractive fuel for thermites.

    View details for DOI 10.1021/acsomega.7b00652

    View details for PubMedID 31457677

    View details for PubMedCentralID PMC6641388

  • Three-Dimensional Hetero-Integration of Faceted GaN on Si Pillars for Efficient Light Energy Conversion Devices. ACS nano Kim, D. R., Lee, C. H., Cho, I. S., Jang, H., Jeon, M. S., Zheng, X. 2017


    An important pathway for cost-effective light energy conversion devices, such as solar cells and light emitting diodes, is to integrate III-V (e.g., GaN) materials on Si substrates. Such integration first necessitates growth of high crystalline III-V materials on Si, which has been the focus of many studies. However, the integration also requires that the final III-V/Si structure has a high light energy conversion efficiency. To accomplish these twin goals, we use single-crystalline microsized Si pillars as a seed layer to first grow faceted Si structures, which are then used for the heteroepitaxial growth of faceted GaN films. These faceted GaN films on Si have high crystallinity, and their threading dislocation density is similar to that of GaN grown on sapphire. In addition, the final faceted GaN/Si structure has great light absorption and extraction characteristics, leading to improved performance for GaN-on-Si light energy conversion devices.

    View details for DOI 10.1021/acsnano.7b01967

    View details for PubMedID 28514135

  • Methanol Photo-Oxidation on Rutile TiO2 Nanowires: Probing Reaction Pathways on Complex Materials JOURNAL OF PHYSICAL CHEMISTRY C Crampton, A. S., Cai, L., Janvelyan, N., Zheng, X., Friend, C. M. 2017; 121 (18): 9910-9919
  • Electrochemical generation of sulfur vacancies in the basal plane of MoS2 for hydrogen evolution NATURE COMMUNICATIONS Tsai, C., Li, H., Park, S., Park, J., Han, H. S., Norskov, J. K., Zheng, X., Abild-Pedersen, F. 2017; 8


    Recently, sulfur (S)-vacancies created on the basal plane of 2H-molybdenum disulfide (MoS2) using argon plasma exposure exhibited higher intrinsic activity for the electrochemical hydrogen evolution reaction than the edge sites and metallic 1T-phase of MoS2 catalysts. However, a more industrially viable alternative to the argon plasma desulfurization process is needed. In this work, we introduce a scalable route towards generating S-vacancies on the MoS2 basal plane using electrochemical desulfurization. Even though sulfur atoms on the basal plane are known to be stable and inert, we find that they can be electrochemically reduced under accessible applied potentials. This can be done on various 2H-MoS2 nanostructures. By changing the applied desulfurization potential, the extent of desulfurization and the resulting activity can be varied. The resulting active sites are stable under extended desulfurization durations and show consistent HER activity.

    View details for DOI 10.1038/ncomms15113

    View details for Web of Science ID 000399985300001

  • Enhancing ignition and combustion of micron-sized aluminum by adding porous silicon PROCEEDINGS OF THE COMBUSTION INSTITUTE Parimi, V. S., Huang, S., Zheng, X. 2017; 36 (2): 2317-2324
  • Facile Thermal and Optical Ignition of Silicon Nanoparticles and Micron Particles. Nano letters Huang, S. n., Parimi, V. S., Deng, S. n., Lingamneni, S. n., Zheng, X. n. 2017; 17 (10): 5925–30


    Silicon (Si) particles are widely utilized as high-capacity electrodes for Li-ion batteries, elements for thermoelectric devices, agents for bioimaging and therapy, and many other applications. However, Si particles can ignite and burn in air at elevated temperatures or under intense illumination. This poses potential safety hazards when handling, storing, and utilizing these particles for those applications. In order to avoid the problem of accidental ignition, it is critical to quantify the ignition properties of Si particles such as their sizes and porosities. To do so, we first used differential scanning calorimetry to experimentally determine the reaction onset temperature of Si particles under slow heating rates (∼0.33 K/s). We found that the reaction onset temperature of Si particles increased with the particle diameter from 805 °C at 20-30 nm to 935 °C at 1-5 μm. Then, we used a xenon (Xe) flash lamp to ignite Si particles under fast heating rates (∼103 to 106 K/s) and measured the minimum ignition radiant fluence (i.e., the radiant energy per unit surface area of Si particle beds required for ignition). We found that the measured minimum ignition radiant fluence decreased with decreasing Si particle size and was most sensitive to the porosity of the Si particle bed. These trends for the Xe flash ignition experiments were also confirmed by our one-dimensional unsteady simulation to model the heat transfer process. The quantitative information on Si particle ignition included in this Letter will guide the safe handling, storage, and utilization of Si particles for diverse applications and prevent unwanted fire hazards.

    View details for PubMedID 28873319

  • Tuning properties of MoS2 by mechanical strain Li, H., Park, S., Zheng, X., IEEE IEEE. 2017
  • Sulfur-Modulated Tin Sites Enable Highly Selective Electrochemical Reduction of CO2 to Formate Joule Zheng, X., De luna, P., de Arquer, F., Zhang, B., Becknell, N., Cui, Y., Du, X., Yang, P., Sargent, E. 2017
  • Molybdenum disulfide catalyzed tungsten oxide for on-chip acetone sensing APPLIED PHYSICS LETTERS Li, H., Ahn, S. H., Park, S., Cai, L., Zhao, J., He, J., Zhou, M., Park, J., Zheng, X. 2016; 109 (13)

    View details for DOI 10.1063/1.4962946

    View details for Web of Science ID 000384747900042

  • One-Step Hydrothermal Deposition of Ni:FeOOH onto Photoanodes for Enhanced Water Oxidation ACS ENERGY LETTERS Cai, L., Zhao, J., Li, H., Park, J., Cho, I. S., Han, H. S., Zheng, X. 2016; 1 (3): 624-632
  • High-Performance Ultrathin BiVO4 Photoanode on Textured Polydimethylsiloxane Substrates for Solar Water Splitting ACS ENERGY LETTERS Zhao, J., Guo, Y., Cai, L., Li, H., Wang, K. X., Cho, I. S., Lee, C. H., Fan, S., Zheng, X. 2016; 1 (1): 68-75
  • Kinetic Study of Hydrogen Evolution Reaction over Strained MoS2 with Sulfur Vacancies Using Scanning Electrochemical Microscopy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Li, H., Du, M., Mleczko, M. J., Koh, A. L., Nishi, Y., Pop, E., Bard, A. J., Zheng, X. 2016; 138 (15): 5123-5129


    Molybdenum disulfide (MoS2), with its active edge sites, is a proposed alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Recently, the inert basal plane of MoS2 was successfully activated and optimized with excellent intrinsic HER activity by creating and further straining sulfur (S) vacancies. Nevertheless, little is known about the HER kinetics of those S vacancies and the additional effects from elastic tensile strain. Herein, scanning electrochemical microscopy was used to determine the HER kinetic data for both unstrained S vacancies (formal potential Ev0 = −0.53 VAg/AgCl, electron-transfer coefficient αv = 0.4, electron-transfer rate constant kv0 = 2.3 × 10(–4) cm/s) and strained S vacancies (Esv0= −0.53 VAg/AgCl, αsv = 0.4, ksv0 = 1.0 × 10(–3) cm/s) on the basal plane of MoS2 monolayers, and the strained S vacancy has an electron-transfer rate 4 times higher than that of the unstrained S vacancy. This study provides a general platform for measuring the kinetics of two-dimensional material-based catalysts.

    View details for DOI 10.1021/jacs.6b01377

    View details for PubMedID 26997198

  • Quasi-ballistic Electronic Thermal Conduction in Metal Inverse Opals. Nano letters Barako, M. T., Sood, A., Zhang, C., Wang, J., Kodama, T., Asheghi, M., Zheng, X., Braun, P. V., Goodson, K. E. 2016; 16 (4): 2754-2761


    Porous metals are used in interfacial transport applications that leverage the combination of electrical and/or thermal conductivity and the large available surface area. As nanomaterials push toward smaller pore sizes to increase the total surface area and reduce diffusion length scales, electron conduction within the metal scaffold becomes suppressed due to increased surface scattering. Here we observe the transition from diffusive to quasi-ballistic thermal conduction using metal inverse opals (IOs), which are metal films that contain a periodic arrangement of interconnected spherical pores. As the material dimensions are reduced from ∼230 nm to ∼23 nm, the thermal conductivity of copper IOs is reduced by more than 57% due to the increase in surface scattering. In contrast, nickel IOs exhibit diffusive-like conduction and have a constant thermal conductivity over this size regime. The quasi-ballistic nature of electron transport at these length scales is modeled considering the inverse opal geometry, surface scattering, and grain boundaries. Understanding the characteristics of electron conduction at the nanoscale is essential to minimizing the total resistance of porous metals for interfacial transport applications, such as the total electrical resistance of battery electrodes and the total thermal resistance of microscale heat exchangers.

    View details for DOI 10.1021/acs.nanolett.6b00468

    View details for PubMedID 26986050

  • Enhancing Low-Bias Performance of Hematite Photoanodes for Solar Water Splitting by Simultaneous Reduction of Bulk, Interface, and Surface Recombination Pathways ADVANCED ENERGY MATERIALS Cho, I. S., Han, H. S., Logar, M., Park, J., Zheng, X. 2016; 6 (4)
  • Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies NATURE MATERIALS Li, H., Tsai, C., Koh, A. L., Cai, L., Contryman, A. W., Fragapane, A. H., Zhao, J., Han, H. S., Manoharan, H. C., Abild-Pedersen, F., Norskov, J. K., Zheng, X. 2016; 15 (1): 48-?


    As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs ,,,,), molybdenum disulphide (MoS2) is known to contain active edge sites and an inert basal plane. Activating the MoS2 basal plane could further enhance its HER activity but is not often a strategy for doing so. Herein, we report the first activation and optimization of the basal plane of monolayer 2H-MoS2 for HER by introducing sulphur (S) vacancies and strain. Our theoretical and experimental results show that the S-vacancies are new catalytic sites in the basal plane, where gap states around the Fermi level allow hydrogen to bind directly to exposed Mo atoms. The hydrogen adsorption free energy (ΔGH) can be further manipulated by straining the surface with S-vacancies, which fine-tunes the catalytic activity. Proper combinations of S-vacancy and strain yield the optimal ΔGH = 0 eV, which allows us to achieve the highest intrinsic HER activity among molybdenum-sulphide-based catalysts.

    View details for DOI 10.1038/NMAT4465

    View details for Web of Science ID 000366690600019

    View details for PubMedID 26552057

  • General Characterization Methods for Photoelectrochemical Cells for Solar Water Splitting CHEMSUSCHEM Shi, X., Cai, L., Ma, M., Zheng, X., Park, J. H. 2015; 8 (19): 3192-3203


    Photoelectrochemical (PEC) water splitting is a very promising technology that converts water into clean hydrogen fuel and oxygen by using solar light. However, the characterization methods for PEC cells are diverse and a systematic introduction to characterization methods for PEC cells has rarely been attempted. Unlike most other review articles that focus mainly on the material used for the working electrodes of PEC cells, this review introduces general characterization methods for PEC cells, including their basic configurations and methods for characterizing their performance under various conditions, regardless of the materials used. Detailed experimental operation procedures with theoretical information are provided for each characterization method. The PEC research area is rapidly expanding and more researchers are beginning to devote themselves to related work. Therefore, the content of this Minireview can provide entry-level knowledge to beginners in the area of PEC, which might accelerate progress in this area.

    View details for DOI 10.1002/cssc.201500075

    View details for Web of Science ID 000362729800001

    View details for PubMedID 26365789

  • Highly Efficient Solar Water Splitting from Transferred TiO2 Nanotube Arrays. Nano letters Cho, I. S., Choi, J., Zhang, K., Kim, S. J., Jeong, M. J., Cai, L., Park, T., Zheng, X., Park, J. H. 2015; 15 (9): 5709-5715


    We report a synergistic effect of flame and chemical reduction methods to maximize the efficiency of solar water splitting in transferred TiO2 nanotube (TNT) arrays on a transparent conducting oxide (TCO) substrate. The flame reduction method (>1000 °C) leads to few oxygen vacancies in the anatase TNT arrays, but it exhibits unique advantages for excellent interfacial characteristics between transferred TNT arrays and TCO substrates, which subsequently induce a cathodic on-set potential shift and sharp photocurrent evolution. By contrast, the employed chemical reduction method for TNT arrays/TCO gives rise to an abrupt increase in photocurrent density, which results from the efficient formation of oxygen vacancies in the anatase TiO2 phase, but a decrease in charge transport efficiency with increasing chemical reduction time. We show that flame reduction followed by chemical reduction could significantly improve the saturation photocurrent density and interfacial property of TNT arrays/TCO photoanodes simultaneously without mechanical fracture via the synergistic effects of coreducing methods.

    View details for DOI 10.1021/acs.nanolett.5b01406

    View details for PubMedID 26261876

  • Bridging combustion and nanotechnology Zheng, X. AMER CHEMICAL SOC. 2015
  • Enhancing Catalytic CO Oxidation over Co3O4 Nanowires by Substituting Co2+ with Cu2+ ACS CATALYSIS Zhou, M., Cai, L., Bajdich, M., Garcia-Melchor, M., Li, H., He, J., Wilcox, J., Wu, W., Vojvodic, A., Zheng, X. 2015; 5 (8): 4485-4491
  • Optoelectronic crystal of artificial atoms in strain-textured molybdenum disulphide NATURE COMMUNICATIONS Li, H., Contryman, A. W., Qian, X., Ardakani, S. M., Gong, Y., Wang, X., Weisse, J. M., Lee, C. H., Zhao, J., Ajayan, P. M., Li, J., Manoharan, H. C., Zheng, X. 2015; 6


    The isolation of the two-dimensional semiconductor molybdenum disulphide introduced a new optically active material possessing a band gap that can be facilely tuned via elastic strain. As an atomically thin membrane with exceptional strength, monolayer molybdenum disulphide subjected to biaxial strain can embed wide band gap variations overlapping the visible light spectrum, with calculations showing the modified electronic potential emanating from point-induced tensile strain perturbations mimics the Coulomb potential in a mesoscopic atom. Here we realize and confirm this 'artificial atom' concept via capillary-pressure-induced nanoindentation of monolayer molybdenum disulphide from a tailored nanopattern, and demonstrate that a synthetic superlattice of these building blocks forms an optoelectronic crystal capable of broadband light absorption and efficient funnelling of photogenerated excitons to points of maximum strain at the artificial-atom nuclei. Such two-dimensional semiconductors with spatially textured band gaps represent a new class of materials, which may find applications in next-generation optoelectronics or photovoltaics.

    View details for DOI 10.1038/ncomms8381

    View details for Web of Science ID 000357175300014

    View details for PubMedID 26088550

  • Interwoven Three-Dimensional Architecture of Cobalt Oxide Nanobrush-Graphene@NixCo2x(OH)(6x) for High-Performance Supercapacitors NANO LETTERS Qu, L., Zhao, Y., Khan, A. M., Han, C., Hercule, K. M., Yan, M., Liu, X., Chen, W., Wang, D., Cai, Z., Xu, W., Zhao, K., Zheng, X., Mai, L. 2015; 15 (3): 2037-2044


    Development of pseudocapacitor electrode materials with high comprehensive electrochemical performance, such as high capacitance, superior reversibility, excellent stability, and good rate capability at the high mass loading level, still is a tremendous challenge. To our knowledge, few works could successfully achieve the above comprehensive electrochemical performance simultaneously. Here we design and synthesize one interwoven three-dimensional (3D) architecture of cobalt oxide nanobrush-graphene@Ni(x)Co(2x)(OH)(6x) (CNG@NCH) electrode with high comprehensive electrochemical performance: high specific capacitance (2550 F g(-1) and 5.1 F cm(-2)), good rate capability (82.98% capacitance retention at 20 A g(-1) vs 1 A g(-1)), superior reversibility, and cycling stability (92.70% capacitance retention after 5000 cycles at 20 A g(-1)), which successfully overcomes the tremendous challenge for pseudocapacitor electrode materials. The asymmetric supercapacitor of CNG@NCH//reduced-graphene-oxide-film exhibits good rate capability (74.85% capacitance retention at 10 A g(-1) vs 0.5 A g(-1)) and high energy density (78.75 Wh kg(-1) at a power density of 473 W kg(-1)). The design of this interwoven 3D frame architecture can offer a new and appropriate idea for obtaining high comprehensive performance electrode materials in the energy storage field.

    View details for DOI 10.1021/nl504901p

    View details for Web of Science ID 000351188000090

    View details for PubMedID 25710223

  • Laminar flame speeds, counterflow ignition, and kinetic modeling of the butene isomers PROCEEDINGS OF THE COMBUSTION INSTITUTE Zhao, P., Yuan, W., Sun, H., Li, Y., Kelley, A. P., Zheng, X., Law, C. K. 2015; 35: 309-316
  • Fabrication of Nanowire Electronics on Nonconventional Substrates by Water-Assisted Transfer Printing Method MICRO- AND NANOTECHNOLOGY SENSORS, SYSTEMS, AND APPLICATIONS VII Lee, C. H., Kim, D. R., Zheng, X. 2015; 9467

    View details for DOI 10.1117/12.2178057

    View details for Web of Science ID 000357259000007

  • Titanium incorporation into hematite photoelectrodes: theoretical considerations and experimental observations ENERGY & ENVIRONMENTAL SCIENCE Kronawitter, C. X., Zegkinoglou, I., Shen, S., Liao, P., Cho, I. S., Zandi, O., Liu, Y., Lashgari, K., Westin, G., Guo, J., Himpsel, F. J., Carter, E. A., Zheng, X. L., Hamann, T. W., Koel, B. E., Mao, S. S., Vayssieres, L. 2014; 7 (10): 3100-3121

    View details for DOI 10.1039/c4ee01066c

    View details for Web of Science ID 000342884300001

  • Transfer Printing Methods for Flexible Thin Film Solar Cells: Basic Concepts and Working Principles ACS NANO Lee, C. H., Kim, D. R., Zheng, X. 2014; 8 (9): 8746-8756


    Fabricating thin film solar cells (TFSCs) on flexible substrates will not only broaden the applications of solar cells, but also potentially reduce the installation cost. However, a critical challenge for fabricating flexible TFSCs on flexible substrates is the incompatibility issues between the thermal, mechanical, and chemical properties of these substrates and the fabrication conditions. Transfer printing methods, which use conventional substrates for the fabrication and then deliver the TFSCs onto flexible substrates, play a key role to overcome these challenges. In this review, we discuss the basic concepts and working principles of four major transfer printing methods associated with (1) transfer by sacrificial layers, (2) transfer by porous Si layer, (3) transfer by controlled crack, and (4) transfer by water-assisted thin film delamination. We also discuss the challenges and opportunities for implementing these methods for practical solar cell manufacture.

    View details for DOI 10.1021/nn5037587

    View details for Web of Science ID 000342184400005

  • Sol-flame synthesis of cobalt-doped TiO2 nanowires with enhanced electrocatalytic activity for oxygen evolution reaction. Physical chemistry chemical physics Cai, L., Cho, I. S., Logar, M., Mehta, A., He, J., Lee, C. H., Rao, P. M., Feng, Y., Wilcox, J., Prinz, F. B., Zheng, X. 2014; 16 (24): 12299-12306


    Doping nanowires (NWs) is of crucial importance for a range of applications due to the unique properties arising from both impurities' incorporation and nanoscale dimensions. However, existing doping methods face the challenge of simultaneous control over the morphology, crystallinity, dopant distribution and concentration at the nanometer scale. Here, we present a controllable and reliable method, which combines versatile solution phase chemistry and rapid flame annealing process (sol-flame), to dope TiO2 NWs with cobalt (Co). The sol-flame doping method not only preserves the morphology and crystallinity of the TiO2 NWs, but also allows fine control over the Co dopant profile by varying the concentration of Co precursor solution. Characterizations of the TiO2:Co NWs show that Co dopants exhibit 2+ oxidation state and substitutionally occupy Ti sites in the TiO2 lattice. The Co dopant concentration significantly affects the oxygen evolution reaction (OER) activity of TiO2:Co NWs, and the TiO2:Co NWs with 12 at% of Co on the surface show the highest OER activity with a 0.76 V reduction of the overpotential with respect to undoped TiO2 NWs. This enhancement of OER activity for TiO2:Co NWs is attributed to both improved surface charge transfer kinetics and increased bulk conductivity.

    View details for DOI 10.1039/c4cp01748j

    View details for PubMedID 24820239

  • Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano letters Rao, P. M., Cai, L., Liu, C., Cho, I. S., Lee, C. H., Weisse, J. M., Yang, P., Zheng, X. 2014; 14 (2): 1099-1105


    We report a scalably synthesized WO3/BiVO4 core/shell nanowire photoanode in which BiVO4 is the primary light-absorber and WO3 acts as an electron conductor. These core/shell nanowires achieve the highest product of light absorption and charge separation efficiencies among BiVO4-based photoanodes to date and, even without an added catalyst, produce a photocurrent of 3.1 mA/cm(2) under simulated sunlight and an incident photon-to-current conversion efficiency of ∼ 60% at 300-450 nm, both at a potential of 1.23 V versus RHE.

    View details for DOI 10.1021/nl500022z

    View details for PubMedID 24437363

  • Rapid and Controllable Flame Reduction of TiO2 Nanowires for Enhanced Solar Water-Splitting NANO LETTERS Cho, I. S., Logar, M., Lee, C. H., Cai, L., Prinz, F. B., Zheng, X. 2014; 14 (1): 24-31


    We report a new flame reduction method to generate controllable amount of oxygen vacancies in TiO2 nanowires that leads to nearly three times improvement in the photoelectrochemical (PEC) water-splitting performance. The flame reduction method has unique advantages of a high temperature (>1000 °C), ultrafast heating rate, tunable reduction environment, and open-atmosphere operation, so it enables rapid formation of oxygen vacancies (less than one minute) without damaging the nanowire morphology and crystallinity and is even applicable to various metal oxides. Significantly, we show that flame reduction greatly improves the saturation photocurrent densities of TiO2 nanowires (2.7 times higher), α-Fe2O3 nanowires (9.4 times higher), ZnO nanowires (2.0 times higher), and BiVO4 thin film (4.3 times higher) in comparison to untreated control samples for PEC water-splitting applications.

    View details for DOI 10.1021/nl4026902

    View details for Web of Science ID 000329586700005

    View details for PubMedID 24295287

  • Flash ignition of freestanding porous silicon films: effects of film thickness and porosity. Nano letters Ohkura, Y., Weisse, J. M., Cai, L., Zheng, X. 2013; 13 (11): 5528-5533


    We report the first successful xenon flash ignition of freestanding porous Si films in air. The minimum flash ignition energy (Emin) first decreases and then increases with increasing the porous Si film thickness due to the competition between light absorption and heat loss. The Emin is lower for higher porosity film because high porosity reduces both the heat capacity and the thermal conductivity, facilitating the temperature rise. These results are important for initiating controlled porous Si combustion and preventing their unwanted combustion for safety reasons.

    View details for DOI 10.1021/nl403114g

    View details for PubMedID 24175629

  • Peel-and-Stick: Mechanism Study for Efficient Fabrication of Flexible/Transparent Thin-film Electronics SCIENTIFIC REPORTS Lee, C. H., Kim, J., Zou, C., Cho, I. S., Weisse, J. M., Nemeth, W., Wang, Q., van Duin, A. C., Kim, T., Zheng, X. 2013; 3


    Peel-and-stick process, or water-assisted transfer printing (WTP), represents an emerging process for transferring fully fabricated thin-film electronic devices with high yield and fidelity from a SiO2/Si wafer to various non-Si based substrates, including papers, plastics and polymers. This study illustrates that the fundamental working principle of the peel-and-stick process is based on the water-assisted subcritical debonding, for which water reduces the critical adhesion energy of metal-SiO2 interface by 70 ~ 80%, leading to clean and high quality transfer of thin-film electronic devices. Water-assisted subcritical debonding is applicable for a range of metal-SiO2 interfaces, enabling the peel-and-stick process as a general and tunable method for fabricating flexible/transparent thin-film electronic devices.

    View details for DOI 10.1038/srep02917

    View details for Web of Science ID 000325469500004

    View details for PubMedID 24108063

    View details for PubMedCentralID PMC3794378

  • Electroassisted Transfer of Vertical Silicon Wire Arrays Using a Sacrificial Porous Silicon Layer NANO LETTERS Weisse, J. M., Lee, C. H., Kim, D. R., Cai, L., Rao, P. M., Zheng, X. 2013; 13 (9): 4362-4368


    An electroassisted method is developed to transfer silicon (Si) wire arrays from the Si wafers on which they are grown to other substrates while maintaining their original properties and vertical alignment. First, electroassisted etching is used to form a sacrificial porous Si layer underneath the Si wires. Second, the porous Si layer is separated from the Si wafer by electropolishing, enabling the separation and transfer of the Si wires. The method is further expanded to develop a current-induced metal-assisted chemical etching technique for the facile and rapid synthesis of Si nanowires with axially modulated porosity.

    View details for DOI 10.1021/nl4021705

    View details for PubMedID 23919596

  • Morphological control of heterostructured nanowires synthesized by sol-flame method NANOSCALE RESEARCH LETTERS Luo, R., Cho, I. S., Feng, Y., Cai, L., Rao, P. M., Zheng, X. 2013; 8


    Heterostructured nanowires, such as core/shell nanowires and nanoparticle-decorated nanowires, are versatile building blocks for a wide range of applications because they integrate dissimilar materials at the nanometer scale to achieve unique functionalities. The sol-flame method is a new, rapid, low-cost, versatile, and scalable method for the synthesis of heterostructured nanowires, in which arrays of nanowires are decorated with other materials in the form of shells or chains of nanoparticles. In a typical sol-flame synthesis, nanowires are dip-coated with a solution containing precursors of the materials to be decorated, then dried in air, and subsequently heated in the post-flame region of a flame at high temperature (over 900°C) for only a few seconds. Here, we report the effects of the precursor solution on the final morphology of the heterostructured nanowire using Co3O4 decorated CuO nanowires as a model system. When a volatile cobalt salt precursor is used with sufficient residual solvent, both solvent and cobalt precursor evaporate during the flame annealing step, leading to the formation of Co3O4 nanoparticle chains by a gas-solid transition. The length of the nanoparticle chains is mainly controlled by the temperature of combustion of the solvent. On the other hand, when a non-volatile cobalt salt precursor is used, only the solvent evaporates and the cobalt salt is converted to nanoparticles by a liquid-solid transition, forming a conformal Co3O4 shell. This study facilitates the use of the sol-flame method for synthesizing heterostructured nanowires with controlled morphologies to satisfy the needs of diverse applications.

    View details for DOI 10.1186/1556-276X-8-347

    View details for Web of Science ID 000323330600001

    View details for PubMedID 23924299

    View details for PubMedCentralID PMC3750428

  • Sol-Flame Synthesis: A General Strategy To Decorate Nanowires with Metal Oxide/Noble Metal Nanoparticles NANO LETTERS Peng, Y., Cho, I. S., Rao, P. M., Cai, L., Zheng, X. 2013; 13 (3): 855-860


    The hybrid structure of nanoparticle-decorated nanowires (NP@NW) combines the merits of large specific surface areas for NPs and anisotropic properties for NWs and is a desirable structure for applications including batteries, dye-sensitized solar cells, photoelectrochemical water splitting, and catalysis. Here, we report a novel sol-flame method to synthesize the NP@NW hybrid structure with two unique characteristics: (1) large loading of NPs per NW with the morphology of NP chains fanning radially from the NW core and (2) intimate contact between NPs and NWs. Both features are advantageous for the above applications that involve both surface reactions and charge transport processes. Moreover, the sol-flame method is simple and general, with which we have successfully decorated various NWs with binary/ternary metal oxide and even noble metal NPs. The unique aspects of the sol-flame method arise from the ultrafast heating rate and the high temperature of flame, which enables rapid solvent evaporation and combustion, and the combustion gaseous products blow out NPs as they nucleate, forming the NP chains around NWs.

    View details for DOI 10.1021/nl300060b

    View details for Web of Science ID 000316243800001

    View details for PubMedID 22494023

  • Reducing minimum flash ignition energy of Al microparticles by addition of WO3 nanoparticles APPLIED PHYSICS LETTERS Ohkura, Y., Rao, P. M., Cho, I. S., Zheng, X. 2013; 102 (4)

    View details for DOI 10.1063/1.4790152

    View details for Web of Science ID 000314723600076

  • Flame synthesis of WO3 nanotubes and nanowires for efficient photoelectrochemical water-splitting PROCEEDINGS OF THE COMBUSTION INSTITUTE Rao, P. M., Cho, I. S., Zheng, X. 2013; 34: 2187-2195
  • Codoping titanium dioxide nanowires with tungsten and carbon for enhanced photoelectrochemical performance. Nature communications Cho, I. S., Lee, C. H., Feng, Y., Logar, M., Rao, P. M., Cai, L., Kim, D. R., Sinclair, R., Zheng, X. 2013; 4: 1723-?


    Recent density-functional theory calculations suggest that codoping TiO2 with donor-acceptor pairs is more effective than monodoping for improving photoelectrochemical water-splitting performance because codoping can reduce charge recombination, improve material quality, enhance light absorption and increase solubility limits of dopants. Here we report a novel ex-situ method to codope TiO2 with tungsten and carbon (W, C) by sequentially annealing W-precursor-coated TiO2 nanowires in flame and carbon monoxide gas. The unique advantages of flame annealing are that the high temperature (>1,000 °C) and fast heating rate of flame enable rapid diffusion of W into TiO2 without damaging the nanowire morphology and crystallinity. This is the first experimental demonstration that codoped TiO2:(W, C) nanowires outperform monodoped TiO2:W and TiO2:C and double the saturation photocurrent of undoped TiO2 for photoelectrochemical water splitting. Such significant performance enhancement originates from a greatly improved electrical conductivity and activity for oxygen-evolution reaction due to the synergistic effects of codoping.

    View details for DOI 10.1038/ncomms2729

    View details for PubMedID 23591890

  • Morphological control of heterostructured nanowires synthesized by sol-flame method. Nanoscale research letters Luo, R., Cho, I. S., Feng, Y., Cai, L., Rao, P. M., Zheng, X. 2013; 8 (1): 347-?


    Heterostructured nanowires, such as core/shell nanowires and nanoparticle-decorated nanowires, are versatile building blocks for a wide range of applications because they integrate dissimilar materials at the nanometer scale to achieve unique functionalities. The sol-flame method is a new, rapid, low-cost, versatile, and scalable method for the synthesis of heterostructured nanowires, in which arrays of nanowires are decorated with other materials in the form of shells or chains of nanoparticles. In a typical sol-flame synthesis, nanowires are dip-coated with a solution containing precursors of the materials to be decorated, then dried in air, and subsequently heated in the post-flame region of a flame at high temperature (over 900°C) for only a few seconds. Here, we report the effects of the precursor solution on the final morphology of the heterostructured nanowire using Co3O4 decorated CuO nanowires as a model system. When a volatile cobalt salt precursor is used with sufficient residual solvent, both solvent and cobalt precursor evaporate during the flame annealing step, leading to the formation of Co3O4 nanoparticle chains by a gas-solid transition. The length of the nanoparticle chains is mainly controlled by the temperature of combustion of the solvent. On the other hand, when a non-volatile cobalt salt precursor is used, only the solvent evaporates and the cobalt salt is converted to nanoparticles by a liquid-solid transition, forming a conformal Co3O4 shell. This study facilitates the use of the sol-flame method for synthesizing heterostructured nanowires with controlled morphologies to satisfy the needs of diverse applications.

    View details for DOI 10.1186/1556-276X-8-347

    View details for PubMedID 23924299

    View details for PubMedCentralID PMC3750428

  • Flame synthesis of 1-D complex metal oxide nanomaterials PROCEEDINGS OF THE COMBUSTION INSTITUTE Cai, L., Rao, P. M., Feng, Y., Zheng, X. 2013; 34: 2229-2236
  • Sol-flame synthesis of hybrid metal oxide nanowires PROCEEDINGS OF THE COMBUSTION INSTITUTE Feng, Y., Cho, I. S., Cai, L., Rao, P. M., Zheng, X. 2013; 34: 2179-2186
  • Peel-and-Stick: Fabricating Thin Film Solar Cell on Universal Substrates SCIENTIFIC REPORTS Lee, C. H., Kim, D. R., Cho, I. S., William, N., Wang, Q., Zheng, X. 2012; 2


    Fabrication of thin-film solar cells (TFSCs) on substrates other than Si and glass has been challenging because these nonconventional substrates are not suitable for the current TFSC fabrication processes due to poor surface flatness and low tolerance to high temperature and chemical processing. Here, we report a new peel-and-stick process that circumvents these fabrication challenges by peeling off the fully fabricated TFSCs from the original Si wafer and attaching TFSCs to virtually any substrates regardless of materials, flatness and rigidness. With the peel-and-stick process, we integrated hydrogenated amorphous silicon (a-Si:H) TFSCs on paper, plastics, cell phone and building windows while maintaining the original 7.5% efficiency. The new peel-and-stick process enables further reduction of the cost and weight for TFSCs and endows TFSCs with flexibility and attachability for broader application areas. We believe that the peel-and-stick process can be applied to thin film electronics as well.

    View details for DOI 10.1038/srep01000

    View details for PubMedID 23277871

  • Shrinking and Growing: Grain Boundary Density Reduction for Efficient Polysilicon Thin-Film Solar Cells NANO LETTERS Kim, D. R., Lee, C. H., Weisse, J. M., Cho, I. S., Zheng, X. 2012; 12 (12): 6485-6491


    Polycrystalline Si (poly-Si) thin-film, due to its low Si consumption, low substrate cost, and good stability, is an attractive candidate for cost-effective solar cells, but the as-deposited poly-Si typically has a columnar structure with grain boundaries in between, severely limiting the efficiency of the poly-Si. Here, we report a micropillar poly-Si solar cell that utilizes the columnar structure of the as-deposited poly-Si grains. We first formed submicrometer diameter poly-Si pillars, smaller than the initial grain sizes, and used these pillars as the seeds for the subsequent epitaxial growth of Si, which effectively reduces grain boundary density in the final poly-Si crystal. In addition, the vertically aligned micropillar arrays form radial p-n junctions that further mitigate the grain boundary recombination losses by improving the light absorption and charge-carrier collection efficiencies. Consequently, the maximum efficiency of micropillar poly-Si thin-film solar cells is 6.4%, that is, ∼1.5 times higher than that of the planar cells.

    View details for DOI 10.1021/nl3041492

    View details for Web of Science ID 000312122100073

    View details for PubMedID 23167740

  • Thermal conductivity in porous silicon nanowire arrays NANOSCALE RESEARCH LETTERS Weisse, J. M., Marconnet, A. M., Kim, D. R., Rao, P. M., Panzer, M. A., Goodson, K. E., Zheng, X. 2012; 7


    The nanoscale features in silicon nanowires (SiNWs) can suppress phonon propagation and strongly reduce their thermal conductivities compared to the bulk value. This work measures the thermal conductivity along the axial direction of SiNW arrays with varying nanowire diameters, doping concentrations, surface roughness, and internal porosities using nanosecond transient thermoreflectance. For SiNWs with diameters larger than the phonon mean free path, porosity substantially reduces the thermal conductivity, yielding thermal conductivities as low as 1 W/m/K in highly porous SiNWs. However, when the SiNW diameter is below the phonon mean free path, both the internal porosity and the diameter significantly contribute to phonon scattering and lead to reduced thermal conductivity of the SiNWs.

    View details for DOI 10.1186/1556-276X-7-554

    View details for Web of Science ID 000311320400001

    View details for PubMedID 23039084

    View details for PubMedCentralID PMC3494563

  • Copper Ion Enhanced Synthesis of Nanostructured Cobalt Oxide Catalyst for Oxidation of Methane CHEMCATCHEM Feng, Y., Zheng, X. 2012; 4 (10): 1551-1554
  • Fabrication of Flexible and Vertical Silicon Nanowire Electronics NANO LETTERS Weisse, J. M., Lee, C. H., Kim, D. R., Zheng, X. 2012; 12 (6): 3339-3343


    Vertical silicon nanowire (SiNW) array devices directly connected on both sides to metallic contacts were fabricated on various non-Si-based substrates (e.g., glass, plastics, and metal foils) in order to fully exploit the nanomaterial properties for final applications. The devices were realized with uniform length Ag-assisted electroless etched SiNW arrays that were detached from their fabrication substrate, typically Si wafers, reattached to arbitrary substrates, and formed with metallic contacts on both sides of the NW array. Electrical characterization of the SiNW array devices exhibits good current-voltage characteristics consistent with the SiNW morphology.

    View details for DOI 10.1021/nl301659m

    View details for Web of Science ID 000305106400114

    View details for PubMedID 22594496

  • Nanowire electronics that can be shaped to fit any surface and attach to any material developed at Stanford SENSOR REVIEW Zheng, X., Bergeron, L. 2012; 32 (3): 256-256
  • Flash ignition of Al nanoparticles: Mechanism and applications COMBUSTION AND FLAME Ohkura, Y., Rao, P. M., Zheng, X. 2011; 158 (12): 2544-2548
  • Branched TiO2 Nanorods for Photoelectrochemical Hydrogen Production NANO LETTERS Cho, I. S., Chen, Z., Forman, A. J., Kim, D. R., Rao, P. M., Jaramillo, T. F., Zheng, X. 2011; 11 (11): 4978-4984


    We report a hierarchically branched TiO(2) nanorod structure that serves as a model architecture for efficient photoelectrochemical devices as it simultaneously offers a large contact area with the electrolyte, excellent light-trapping characteristics, and a highly conductive pathway for charge carrier collection. Under Xenon lamp illumination (UV spectrum matched to AM 1.5G, 88 mW/cm(2) total power density), the branched TiO(2) nanorod array produces a photocurrent density of 0.83 mA/cm(2) at 0.8 V versus reversible hydrogen electrode (RHE). The incident photon-to-current conversion efficiency reaches 67% at 380 nm with an applied bias of 0.6 V versus RHE, nearly two times higher than the bare nanorods without branches. The branches improve efficiency by means of (i) improved charge separation and transport within the branches due to their small diameters, and (ii) a 4-fold increase in surface area which facilitates the hole transfer at the TiO(2)/electrolyte interface.

    View details for DOI 10.1021/nl2029392

    View details for Web of Science ID 000296674700082

    View details for PubMedID 21999403

  • Fabrication of Nanowire Electronics on Nonconventional Substrates by Water-Assisted Transfer Printing Method NANO LETTERS Lee, C. H., Kim, D. R., Zheng, X. 2011; 11 (8): 3435-3439


    We report a simple, versatile, and wafer-scale water-assisted transfer printing method (WTP) that enables the transfer of nanowire devices onto diverse nonconventional substrates that were not easily accessible before, such as paper, plastics, tapes, glass, polydimethylsiloxane (PDMS), aluminum foil, and ultrathin polymer substrates. The WTP method relies on the phenomenon of water penetrating into the interface between Ni and SiO(2). The transfer yield is nearly 100%, and the transferred devices, including NW resistors, diodes, and field effect transistors, maintain their original geometries and electronic properties with high fidelity.

    View details for DOI 10.1021/nl201901z

    View details for Web of Science ID 000293665600065

    View details for PubMedID 21696196

  • Hybrid Si Microwire and Planar Solar Cells: Passivation and Characterization NANO LETTERS Kim, D. R., Lee, C. H., Rao, P. M., Cho, I. S., Zheng, X. 2011; 11 (7): 2704-2708


    We report an efficient hybrid Si microwire (radial junction) and planar solar cell with a maximum efficiency of 11.0% under AM 1.5G illumination. The maximum efficiency of the hybrid cell is improved from 7.2% to 11.0% by passivating the top surface and p-n junction with thin a-SiN:H and intrinsic poly-Si films, respectively, and is higher than that of planar cells of the identical layers due to increased light absorption and improved charge-carrier collections in both wires and planar components.

    View details for DOI 10.1021/nl2009636

    View details for Web of Science ID 000292849400024

    View details for PubMedID 21609002

  • Unique Magnetic Properties of Single Crystal gamma-Fe2O3 Nanowires Synthesized by Flame Vapor Deposition NANO LETTERS Rao, P. M., Zheng, X. 2011; 11 (6): 2390-2395


    Single crystal γ-Fe(2)O(3) nanowires with 40-60 nm diameters were grown for the first time by single-step atmospheric flame vapor deposition (FVD) with axial growth rates up to 5 μm/minute. Because of their superior crystallinity, these FVD γ-Fe(2)O(3) nanowires are single magnetic domains with room temperature coercivities of 200 Oe and saturation magnetizations of 68 emu/g.

    View details for DOI 10.1021/nl2007533

    View details for Web of Science ID 000291322600034

    View details for PubMedID 21563788

  • Vertical Transfer of Uniform Silicon Nanowire Arrays via Crack Formation NANO LETTERS Weisse, J. M., Kim, D. R., Lee, C. H., Zheng, X. 2011; 11 (3): 1300-1305


    Vertical transfer of silicon nanowire (SiNW) arrays with uniform length onto adhesive substrates was realized by the assistance of creating a horizontal crack throughout SiNWs. The crack is formed by adding a water soaking step between consecutive Ag-assisted electroless etching processes of Si. The crack formation is related to the delamination, redistribution, and reattachment of the Ag film during the water soaking and subsequent wet etching steps. Moreover, the crack facilitates embedding SiNWs inside polymers.

    View details for DOI 10.1021/nl104362e

    View details for Web of Science ID 000288061500068

    View details for PubMedID 21322602

  • Morphology-Controlled Flame Synthesis of Single, Branched, and Flower-like alpha-MoO3 Nanobelt Arrays NANO LETTERS Cai, L., Rao, P. M., Zheng, X. 2011; 11 (2): 872-877


    We report an atmospheric, catalyst-free, rapid flame synthesis technique for growing single, branched, and flower-like α-MoO(3) nanobelt arrays on diverse substrates. The growth rate, morphology, and surface coverage density of the α-MoO(3) nanobelts were controlled by varying the flame equivalence ratio, the source temperature, the growth substrate temperature, and the material and morphology of the growth substrate. This flame synthesis technique is a promising, alternative way to synthesize one-dimensional metal oxide nanostructures in general.

    View details for DOI 10.1021/nl104270u

    View details for Web of Science ID 000287049100095

    View details for PubMedID 21261293

  • Methane oxidation over catalytic copper oxides nanowires PROCEEDINGS OF THE COMBUSTION INSTITUTE Feng, Y., Rao, P. M., Kim, D. R., Zheng, X. 2011; 33: 3169-3175
  • Synthesis and ignition of energetic CuO/Al core/shell nanowires PROCEEDINGS OF THE COMBUSTION INSTITUTE Ohkura, Y., Liu, S., Rao, P. M., Zheng, X. 2011; 33: 1909-1915
  • Flame synthesis of tungsten oxide nanostructures on diverse substrates PROCEEDINGS OF THE COMBUSTION INSTITUTE Rao, P. M., Zheng, X. 2011; 33: 1891-1898
  • Orientation-controlled alignment of axially modulated pn silicon nanowires. Nano letters Lee, C. H., Kim, D. R., Zheng, X. 2010; 10 (12): 5116-22


    We demonstrate orientation-controlled alignment of axially modulated pn SiNWs by applying dc electric fields across metal electrodes. The as-aligned pn SiNWs exhibit rectifying behaviors with a 97.7% yield, and about 35% of them exhibit no hysteresis in their current-voltage curves that can be directly used to construct AND/OR logic gates. Moreover, the as-aligned pn SiNWs can be packaged either with polydimethylsiloxane or additional metal layer to protect and even improve the quality of these NW diodes.

    View details for DOI 10.1021/nl103630c

    View details for PubMedID 21043492

  • Orientation-Controlled Alignment of Axially Modulated pn Silicon Nanowires NANO LETTERS Lee, C. H., Kim, D. R., Zheng, X. 2010; 10 (12): 5116-5122


    We demonstrate orientation-controlled alignment of axially modulated pn SiNWs by applying dc electric fields across metal electrodes. The as-aligned pn SiNWs exhibit rectifying behaviors with a 97.7% yield, and about 35% of them exhibit no hysteresis in their current-voltage curves that can be directly used to construct AND/OR logic gates. Moreover, the as-aligned pn SiNWs can be packaged either with polydimethylsiloxane or additional metal layer to protect and even improve the quality of these NW diodes.

    View details for DOI 10.1021/nl103630c

    View details for Web of Science ID 000284990900058

  • Plasma-Enhanced Catalytic CuO Nanowires for CO Oxidation NANO LETTERS Feng, Y., Zheng, X. 2010; 10 (11): 4762-4766


    We report the first experimental study of catalytic CO oxidation over copper oxide (CuO) nanowires (NWs) grown directly on copper meshes. The catalytic activity of CuO NWs is significantly improved by a brief argon or hydrogen radio frequency plasma treatment. The plasma enhancement effect comes from the generation of grain boundaries and the reduction of Cu(II) to the more active oxidation state Cu(I) according to our TEM, XPS, and kinetic study.

    View details for DOI 10.1021/nl1034545

    View details for Web of Science ID 000283907600079

    View details for PubMedID 20964283

  • Characterization of the wettability of thin nanostructured films in the presence of evaporation JOURNAL OF COLLOID AND INTERFACE SCIENCE Rogacs, A., Steinbrenner, J. E., Rowlette, J. A., Weisse, J. M., Zheng, X. L., Goodson, K. E. 2010; 349 (1): 354-360


    Vapor chambers using conventional porous membrane wicks offer limited heat transfer rates for a given thickness. This limitation can be addressed through wick nanostructuring, which promises high capillary pressures and precise control of the local porosity. This work develops a measurement technique for the wettability of nanostructured wicks based on optical imaging. Feasibility is demonstrated on a hydrophilic silicon nanowire array (SiNW) synthesized using the Vapor-Liquid-Solid (VLS) growth mechanism followed by surface plasma treatment. The wettability is determined by comparing the time-dependent liquid interface rise with a model that accounts for capillary, viscous, and gravitational forces and for evaporation. This model is demonstrated to be useful in extracting internal contact angle from thin ( approximately 10microm) porous films.

    View details for DOI 10.1016/j.jcis.2010.05.063

    View details for Web of Science ID 000279966700045

    View details for PubMedID 20579656

  • Fabricating nanowire devices on diverse substrates by simple transfer-printing methods PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Lee, C. H., Kim, D. R., Zheng, X. 2010; 107 (22): 9950-9955


    The fabrication of nanowire (NW) devices on diverse substrates is necessary for applications such as flexible electronics, conformable sensors, and transparent solar cells. Although NWs have been fabricated on plastic and glass by lithographic methods, the choice of device substrates is severely limited by the lithographic process temperature and substrate properties. Here we report three new transfer-printing methods for fabricating NW devices on diverse substrates including polydimethylsiloxane, Petri dishes, Kapton tapes, thermal release tapes, and many types of adhesive tapes. These transfer-printing methods rely on the differences in adhesion to transfer NWs, metal films, and devices from weakly adhesive donor substrates to more strongly adhesive receiver substrates. Electrical characterization of fabricated NW devices shows that reliable ohmic contacts are formed between NWs and electrodes. Moreover, we demonstrated that Si NW devices fabricated by the transfer-printing methods are robust piezoresistive stress sensors and temperature sensors with reliable performance.

    View details for DOI 10.1073/pnas.0914031107

    View details for Web of Science ID 000278246000010

    View details for PubMedID 20479263

    View details for PubMedCentralID PMC2890492

  • Direct Growth of Nanowire Logic Gates and Photovoltaic Devices NANO LETTERS Kim, D. R., Lee, C. H., Zheng, X. 2010; 10 (3): 1050-1054


    Bottom-up nanowires are useful building blocks for functional devices because of their controllable physical and chemical properties. However, assembling nanowires into large-scale integrated systems remains a critical challenge that becomes even more daunting when different nanowires need to be simultaneously assembled in close proximity to one another. Herein, we report a new method to directly grow nanowire devices consisting of different nanowires. The method is based on the epitaxial growth of nanowires from the sidewalls of electrodes and on the matching of electrode design with synthesis conditions to electrically connect different nanowires during growth. Specifically, the method was used to grow silicon nanowire-based AND and OR diode logic gates with excellent rectifying behaviors, and photovoltaic elements in parallel and in series, with tunable power output.

    View details for DOI 10.1021/nl100011z

    View details for Web of Science ID 000275278200053

    View details for PubMedID 20178355

  • Rapid Catalyst-Free Flame Synthesis of Dense, Aligned alpha-Fe2O3 Nanoflake and CuO Nanoneedle Arrays NANO LETTERS Rao, P. M., Zheng, X. 2009; 9 (8): 3001-3006


    This paper describes a simple and yet rapid flame synthesis method to produce one-dimensional metal oxide nanostructures by directly oxidizing metals in the postflame region of a flat flame. Single and bicrystal alpha-Fe(2)O(3) nanoflakes and CuO nanoneedles were grown in the postflame region by a solid diffusion mechanism and were aligned perpendicularly to the substrate with a surface coverage density of 10 nanostructures per square micrometer. The alpha-Fe(2)O(3) nanoflakes reached lengths exceeding 20 microm after only 20 min of growth. This rapid growth rate is attributed to a large initial heating rate of the metal substrate in the flame and to the presence of water vapor and carbon dioxide in the gas phase that together generate thin and porous oxide layers that greatly enhance the diffusion of the deficient metal to the nanostructure growth site and enable growth at higher temperatures than previously demonstrated.

    View details for DOI 10.1021/nl901426t

    View details for Web of Science ID 000268797200034

    View details for PubMedID 19588968

  • Probing Flow Velocity with Silicon Nanowire Sensors NANO LETTERS Kim, D. R., Lee, C. H., Zheng, X. 2009; 9 (5): 1984-1988


    We report our experimental efforts to quantify the impact of fluidic and ionic transport on the conductance level of silicon nanowire (SiNW) sensors configured as field effect transistors (FETs). Specifically, the conductance of SiNW FETs placed in a microfluidic channel was observed to change linearly with the flow velocity of electrolytic solutions. The direction of conductance change depends on the doping type of the SiNWs and their location inside the microfluidic channel, and the magnitude of the conductance change varies with the ionic strength and compositions of the electrolytic solution. Our quantitative analysis suggests that the flow velocity sensing is a consequence of the streaming potential that is generated by the movement of counterions inside the electrical double layer (EDL) of the silica substrate. The streaming potential, which varies with the flow velocity and the ionic properties of the electrolytic solution, acts in the same way as the charged analytes in affecting the conductance of SiNWs by changing the surface potential. This study highlights the importance of considering the ionic transport in analyzing and optimizing nanowire FET sensors, which can significantly change the conductance of NWs. Moreover, SiNWs were demonstrated for the first time to be able to detect the streaming potential, the flow velocity and the ionic strength, opening up their new application potentials in microfluidics.

    View details for DOI 10.1021/nl900238a

    View details for Web of Science ID 000266157100045

    View details for PubMedID 19331420

  • Single and Tandem Axial p-i-n Nanowire Photovoltaic Devices NANO LETTERS Kempa, T. J., Tian, B., Kim, D. R., Hu, J., Zheng, X., Lieber, C. M. 2008; 8 (10): 3456-3460


    Nanowires represent a promising class of materials for exploring new concepts in solar energy conversion. Here we report the first experimental realization of axial modulation-doped p-i-n and tandem p-i-n(+) -p(+)-i-n silicon nanowire (SiNW) photovoltaic elements. Scanning electron microscopy images of selectively etched nanowires demonstrate excellent synthetic control over doping and lengths of distinct regions in the diode structures. Current-voltage (I-V) characteristics reveal clear and reproducible diode characteristics for the p-i-n and p-n SiNW devices. Under simulated one-sun solar conditions (AM 1.5G), optimized p-i-n SiNW devices exhibited an open circuit voltage (Voc) of 0.29 V, a maximum short-circuit current density of 3.5 mA/cm(2), and a maximum efficiency of 0.5%. The response of the short-circuit current versus Voc under varying illumination intensities shows that the diode quality factor is improved from n=1.78 to n=1.28 by insertion of the i-type SiNW segment. The temperature dependence of Voc scales as -2.97 mV/K and extrapolates to the crystalline Si band gap at 0 K, which is in excellent agreement with bulk properties. Finally, a novel single SiNW tandem solar cell consisting of synthetic integration of two photovoltaic elements with an overall p-i-n(+) -p(+)-i-n structure was prepared and shown to exhibit a Voc that is on average 57% larger than that of the single p-i-n device. Fundamental studies of such well-defined nanowire photovoltaics will enable their intrinsic performance limits to be defined.

    View details for DOI 10.1021/nl8023438

    View details for Web of Science ID 000259906800070

    View details for PubMedID 18763836

  • Numerical Characterization and Optimization of the Microfluidics for Nanowire Biosensors NANO LETTERS Kim, D. R., Zheng, X. 2008; 8 (10): 3233-3237


    The present study aims to enhance the analyte transport to the surface of nanowires (NWs) through optimizing the sensing configuration and the flow patterns inside the microfluidic channel, and hence to reduce the response time of NW biosensors. Specifically, numerical simulations were carried out to quantitatively investigate the effects of the fundamental surface reaction, convection, and diffusion processes on the sensing performance. Although speeding up all these processes will reduce the sensing response time, enhancing the diffusional transport was found to be most effective. Moreover, the response time of NW biosensors is inversely proportional to the local concentration of the analyte in the vicinity of the NWs, which suggests that the sensing response time can be significantly reduced by replenishing the local analyte rapidly. Therefore, the following three optimization strategies were proposed and their effects on the time response of NWs were characterized systematically: device substrate passivation, microfluidic channel modification, and suspending NWs. The combination of these three optimization methods was demonstrated to be able to reduce the response time of NW biosensors by more than 1 order of magnitude.

    View details for DOI 10.1021/nl801559m

    View details for Web of Science ID 000259906800029

    View details for PubMedID 18788786

  • Coaxial silicon nanowires as solar cells and nanoelectronic power sources NATURE Tian, B., Zheng, X., Kempa, T. J., Fang, Y., Yu, N., Yu, G., Huang, J., Lieber, C. M. 2007; 449 (7164): 885-U8


    Solar cells are attractive candidates for clean and renewable power; with miniaturization, they might also serve as integrated power sources for nanoelectronic systems. The use of nanostructures or nanostructured materials represents a general approach to reduce both cost and size and to improve efficiency in photovoltaics. Nanoparticles, nanorods and nanowires have been used to improve charge collection efficiency in polymer-blend and dye-sensitized solar cells, to demonstrate carrier multiplication, and to enable low-temperature processing of photovoltaic devices. Moreover, recent theoretical studies have indicated that coaxial nanowire structures could improve carrier collection and overall efficiency with respect to single-crystal bulk semiconductors of the same materials. However, solar cells based on hybrid nanoarchitectures suffer from relatively low efficiencies and poor stabilities. In addition, previous studies have not yet addressed their use as photovoltaic power elements in nanoelectronics. Here we report the realization of p-type/intrinsic/n-type (p-i-n) coaxial silicon nanowire solar cells. Under one solar equivalent (1-sun) illumination, the p-i-n silicon nanowire elements yield a maximum power output of up to 200 pW per nanowire device and an apparent energy conversion efficiency of up to 3.4 per cent, with stable and improved efficiencies achievable at high-flux illuminations. Furthermore, we show that individual and interconnected silicon nanowire photovoltaic elements can serve as robust power sources to drive functional nanoelectronic sensors and logic gates. These coaxial silicon nanowire photovoltaic elements provide a new nanoscale test bed for studies of photoinduced energy/charge transport and artificial photosynthesis, and might find general usage as elements for powering ultralow-power electronics and diverse nanosystems.

    View details for DOI 10.1038/nature06181

    View details for Web of Science ID 000250230600042

    View details for PubMedID 17943126

  • Experimental counterflow ignition temperatures and reaction mechanisms of 1,3-butadiene PROCEEDINGS OF THE COMBUSTION INSTITUTE Zheng, X. L., Lu, T. F., Law, C. K. 2007; 31: 367-375
  • Thermochemical and kinetic analyses on oxidation of isobutenyl radical and 2-hydroperoxymethyl-2-propenyl radical JOURNAL OF PHYSICAL CHEMISTRY A Zheng, X. L., Sun, H. Y., Law, C. K. 2005; 109 (40): 9044-9053


    In recognition of the importance of the isobutene oxidation reaction in the preignition chemistry associated with engine knock, the thermochemistry, chemical reaction pathways, and reaction kinetics of the isobutenyl radical oxidation at low to intermediate temperature range were computationally studied, focusing on both the first and the second O2 addition to the isobutenyl radical. The geometries of reactants, important intermediates, transition states, and products in the isobutenyl radical oxidation system were optimized at the B3LYP/6-311G(d,p) and MP2(full)/6-31G(d) levels, and the thermochemical properties were determined on the basis of ab initio, density functional theory, and statistical mechanics. Enthalpies of formation for several important intermediates were calculated using isodesmic reactions at the DFT and the CBS-QB3 levels. The kinetic analysis of the first O2 addition to the isobutenyl radical was performed using enthalpies at the CBS-QB3 and G3(MP2) levels. The reaction forms a chemically activated isobutenyl peroxy adduct which can be stabilized, dissociate back to reactants, cyclize to cyclic peroxide-alkyl radicals, and isomerize to the 2-hydroperoxymethyl-2-propenyl radical that further undergoes another O2 addition. The reaction channels for isomerization and cyclization and further dissociation on this second O2 addition were analyzed using enthalpies at the DFT level with energy corrections based on similar reaction channels for the first O2 addition. The high-pressure limit rate constants for each reaction channel were determined as functions of temperature by the canonical transition state theory for further kinetic model development.

    View details for DOI 10.1021/jp058116a

    View details for Web of Science ID 000232482400014

    View details for PubMedID 16332010

  • Experimental and computational study of nonpremixed ignition of dimethyl ether in counterflow PROCEEDINGS OF THE COMBUSTION INSTITUTE Zheng, X. L., Lu, T. F., Law, C. K., Westbrook, C. K., Curran, H. J. 2005; 30: 1101-1109
  • Experimental determination of counterflow ignition temperatures and laminar flame speeds of C-2-C-3 hydrocarbons at atmospheric and elevated pressures PROCEEDINGS OF THE COMBUSTION INSTITUTE Jomaas, G., Zheng, X. L., Zhu, D. L., Law, C. K. 2005; 30: 193-200
  • Nonpremixed ignition of H-2/air in a mixing layer with a vortex PROCEEDINGS OF THE COMBUSTION INSTITUTE Zheng, X. L., Yuan, J., Law, C. K. 2005; 30: 415-421
  • Ignition of premixed hydrogen/air by heated counterflow under reduced and elevated pressures COMBUSTION AND FLAME Zheng, X. L., Law, C. K. 2004; 136 (1-2): 168-179
  • Ignition of premixed hydrogen/air by heated counterflow PROCEEDINGS OF THE COMBUSTION INSTITUTE Zheng, X. L., Blouch, J. D., Zhu, D. L., Kreutz, T. G., Law, C. K. 2002; 29: 1637-1643
  • Experimental and Computational Study of Non-premixed Ignition of Dimethyl Ether in Counterflow Proceedings of the Combustion Institute Zheng, X. L., Lu, T. F., Law, C. K., Westbrook, C. K., Curran, H. J. 2005: 1101–1109