Academic Appointments

  • Phys Sci Res Assoc, Mechanical Engineering

All Publications

  • Condensing water vapor to droplets generates hydrogen peroxide. Proceedings of the National Academy of Sciences of the United States of America Lee, J. K., Han, H. S., Chaikasetsin, S., Marron, D. P., Waymouth, R. M., Prinz, F. B., Zare, R. N. 2020


    It was previously shown [J. K. Lee et al., Proc. Natl. Acad. Sci. U.S.A, 116, 19294-19298 (2019)] that hydrogen peroxide (H2O2) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H2O2 is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water-air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H2O2 production in condensate microdroplets showed that H2O2 was generated from microdroplets with sizes typically less than 10 m. The spontaneous production of H2O2 was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H2O2 generation. We also found that the H2O2 production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H2O2 occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H2O2 from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry.

    View details for DOI 10.1073/pnas.2020158117

    View details for PubMedID 33229543

  • Spontaneous generation of hydrogen peroxide from aqueous microdroplets. Proceedings of the National Academy of Sciences of the United States of America Lee, J. K., Walker, K. L., Han, H. S., Kang, J., Prinz, F. B., Waymouth, R. M., Nam, H. G., Zare, R. N. 2019


    We show H2O2 is spontaneously produced from pure water by atomizing bulk water into microdroplets (1 mum to 20 m in diameter). Production of H2O2, as assayed by H2O2-sensitve fluorescence dye peroxyfluor-1, increased with decreasing microdroplet size. Cleavage of 4-carboxyphenylboronic acid and conversion of phenylboronic acid to phenols in microdroplets further confirmed the generation of H2O2 The generated H2O2 concentration was 30 M (1 part per million) as determined by titration with potassium titanium oxalate. Changing the spray gas to O2 or bubbling O2 decreased the yield of H2O2 in microdroplets, indicating that pure water microdroplets directly generate H2O2 without help from O2 either in air surrounding the droplet or dissolved in water. We consider various possible mechanisms for H2O2 formation and report a number of different experiments exploring this issue. We suggest that hydroxyl radical (OH) recombination is the most likely source, in which OH is generated by loss of an electron from OH- at or near the surface of the water microdroplet. This catalyst-free and voltage-free H2O2 production method provides innovative opportunities for green production of hydrogen peroxide.

    View details for DOI 10.1073/pnas.1911883116

    View details for PubMedID 31451646

  • 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

  • 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

  • Indium-Tin-Oxide Nanowire Array Based CdSe/CdS/TiO2 One-Dimensional Heterojunction Photoelectrode for Enhanced Solar Hydrogen Production ACS SUSTAINABLE CHEMISTRY & ENGINEERING Han, H. S., Han, G. S., Kim, J. S., Kim, D. H., Hong, J. S., Caliskan, S., Jung, H. S., Cho, I. S., Lee, J. 2016; 4 (3): 1161-1168
  • Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies (vol 15, pg 48, 2016) 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 (3)

    View details for DOI 10.1038/NMAT4564

    View details for Web of Science ID 000370967400024

  • 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)
  • A tree-like nanoporous WO3 photoanode with enhanced charge transport efficiency for photoelectrochemical water oxidation JOURNAL OF MATERIALS CHEMISTRY A Shin, S., Han, H. S., Kim, J. S., Park, I. J., Lee, M. H., Hong, K. S., Cho, I. S. 2015; 3 (24): 12920-12926

    View details for DOI 10.1039/c5ta00823a

    View details for Web of Science ID 000356022800043

  • TiO2 nanocrystals shell layer on highly conducting indium tin oxide nanowire for photovoltaic devices (vol 5, pg 3520, 2013) NANOSCALE Han, H. S., Kim, J. S., Kim, D. H., Han, G. S., Jung, H. S., Noh, J. H., Hong, K. S. 2013; 5 (24): 12674-12674
  • Nanowire-Based Three-Dimensional Transparent Conducting Oxide Electrodes for Extremely Fast Charge Collection ADVANCED ENERGY MATERIALS Noh, J. H., Han, H. S., Lee, S., Kim, J. Y., Hong, K. S., Han, G., Shin, H., Jung, H. S. 2011; 1 (5): 829-835
  • (020)-Textured tungsten trioxide nanostructure with enhanced photoelectrochemical activity JOURNAL OF CATALYSIS Han, H., Park, W., Hwang, S., Kim, H., Sim, Y., Surendran, S., Sim, U., Cho, I. 2020; 389: 328–36
  • Fabrication of an ingenious metallic asymmetric supercapacitor by the integration of anodic iron oxide and cathodic nickel phosphide APPLIED SURFACE SCIENCE Kim, H., Surendran, S., Chae, Y., Lee, H., An, T., Han, H., Park, W., Kim, J., Sim, U. 2020; 511
  • Retarded Charge-Carrier Recombination in Photoelectrochemical Cells from Plasmon-Induced Resonance Energy Transfer ADVANCED ENERGY MATERIALS Choi, Y., Lee, B., Jung, M., Han, H., Kim, S., Chen, K., Kim, D., Heinz, T. F., Fan, S., Lee, J., Yi, G., Kim, J., Park, J. 2020
  • Tunable Dielectric and Thermal Properties of Oxide Dielectrics via Substrate Biasing in Plasma-Enhanced Atomic Layer Deposition. ACS applied materials & interfaces Kim, Y., Kwon, H., Han, H. S., Kim, H. J., Kim, B. S., Lee, B. C., Lee, J., Asheghi, M., Prinz, F. B., Goodson, K. E., Lim, J., Sim, U., Park, W. 2020


    The ability to control the properties of dielectric thin films on demand is of fundamental interest in nanoscale devices. Here, we modulate plasma characteristics at the surface of a substrate to tune both dielectric constant and thermal conductivity of amorphous thin films grown using plasma-enhanced atomic layer deposition. Specifically, we apply a substrate bias ranging from 0 to ∼117 V and demonstrate the systematic tunability of various material parameters of Al2O3. As a function of the substrate bias, we find a nonmonotonical evolution of intrinsic properties, including density, dielectric constant, and thermal conductivity. A key observation is that the maximum values in dielectric constant and effective thermal conductivity emerge at different substrate biases. The impact of density on both thermal conductivity and dielectric constant is further examined using a differential effective medium theory and the Clausius-Mossotti model, respectively. We find that the peak value in the dielectric constant deviates from the Clausius-Mossotti model, indicating the change of oxygen fraction in our thin films as a function of substrate bias. This finding suggests that the increased local strength of plasma sheath not only enhances material density but also controls the dynamics of microstructural defect formation beyond what is possible with conventional approaches. Based on our experimental observations and modeling, we further build a phenomenological relation between dielectric constant and thermal conductivity. Our results pave invaluable avenues for optimizing dielectric thin films at the atomic scale for a wide range of applications in nanoelectronics and energy devices.

    View details for DOI 10.1021/acsami.0c11086

    View details for PubMedID 32915545

  • Photo-annealed amorphous titanium oxide for perovskite solar cells. Nanoscale Jeon, J. B., Kim, B. J., Bang, G. J., Kim, M., Lee, D. G., Lee, J. M., Lee, M., Han, H. S., Boschloo, G., Lee, S., Jung, H. S. 2019


    Electron selective layers are important to the efficiency, stability and hysteresis of perovskite solar cells. Photo-annealing is a low-cost, roll-to-roll-compatible process that can be applied to the post-treatment fabrication of sol-gel based metal oxide layers. Here, we fabricate an amorphous titanium oxide electron selective layer at a low temperature in a dry atmosphere using a UV light annealing system and compare it with a thermal annealing process. Active oxygen species are created by using UV light to promote hydrolysis and condense the TiO2 precursor, which removes organic ligands effectively. The photo-annealed TiO2-based perovskite solar cell has a power conversion efficiency of 19.37% without hysteresis.

    View details for DOI 10.1039/c9nr05776e

    View details for PubMedID 31552996

  • 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

  • Point defect-reduced colloidal SnO2 electron transport layers for stable and almost hysteresis-free perovskite solar cells RSC ADVANCES Ju, Y., Park, S., Han, H., Jung, H. 2019; 9 (13): 7334–37

    View details for DOI 10.1039/c9ra00366e

    View details for Web of Science ID 000462644300038

  • Facile and controllable surface-functionalization of TiO2 nanotubes array for highly-efficient photoelectrochemical water-oxidation JOURNAL OF CATALYSIS Kim, J., Han, H., Park, J., Park, W., Baek, J., Lee, J., Jung, H., Cho, I. 2018; 365: 138–44
  • 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)
  • 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
  • CdS-sensitized 1-D single-crystalline anatase TiO2 nanowire arrays for photoelectrochemical hydrogen production 1st International Conference on Nanotechnology, Nanomaterials and Thin Films for Energy Applications (Nano Energy) Kim, D. H., Han, H. S., Cho, I. S., Seong, W. M., Park, I. J., Park, J. H., Shin, S., Do Park, G., Park, S., Lee, S., Hong, K. S. PERGAMON-ELSEVIER SCIENCE LTD. 2015: 863–69
  • Ta-substituted SnNb2-xTaxO6 photocatalysts for hydrogen evolution under visible light irradiation JOURNAL OF MATERIALS CHEMISTRY A Lee, C. W., Park, H. K., Park, S., Han, H. S., Seo, S. W., Song, H. J., Shin, S., Kim, D., Hong, K. S. 2015; 3 (2): 825-831

    View details for DOI 10.1039/c4ta05885b

    View details for Web of Science ID 000346082100046

  • Facile Preparation of TiO2 Nanobranch/Nanoparticle Hybrid Architecture with Enhanced Light Harvesting Properties for Dye-Sensitized Solar Cells JOURNAL OF NANOMATERIALS Kim, J. S., Shin, S. S., Han, H. S., Shin, S., Suk, J. H., Kang, K., Hong, K. S., Cho, I. S. 2015
  • Nanostructured Ti-doped hematite (alpha-Fe2O3) photoanodes for efficient photoelectrochemical water oxidation INTERNATIONAL JOURNAL OF HYDROGEN ENERGY Lee, M. H., Park, J. H., Han, H. S., Song, H. J., Cho, I. S., Noh, J. H., Hong, K. S. 2014; 39 (30): 17501-17507
  • In2O3:Sn/TiO2/CdS heterojunction nanowire array photoanode in photoelectrochemical cells INTERNATIONAL JOURNAL OF HYDROGEN ENERGY Kim, J. S., Han, H. S., Shin, S., Han, G. S., Jung, H. S., Hong, K. S., Noh, J. H. 2014; 39 (30): 17473-17480
  • Heterojunction Fe2O3-SnO2 Nanostructured Photoanode for Efficient Photoelectrochemical Water Splitting JOM Han, H. S., Shin, S., Noh, J. H., Cho, I. S., Hong, K. S. 2014; 66 (4): 664-669
  • A Hierarchically Organized Photoelectrode Architecture for Highly Efficient CdS/CdSe-Sensitized Solar Cells ADVANCED ENERGY MATERIALS Park, J. H., Kim, D. H., Shin, S. S., Han, H. S., Lee, M. H., Jung, H. S., Noh, J. H., Hong, K. S. 2014; 4 (3)
  • Direct Printing Synthesis of Self-Organized Copper Oxide Hollow Spheres on a Substrate Using Copper(II) Complex Ink: Gas Sensing and Photoelectrochemical Properties LANGMUIR Choi, Y., Kim, D., Han, H. S., Shin, S., Hong, S., Hong, K. S. 2014; 30 (3): 700-709


    The direct printing synthesis of metal oxide hollow spheres in the form of film on a substrate is reported for the first time. This method offers facile, scalable, high-throughput production and device fabrication processes. The printing was carried out via a doctor-blade method using Cu(II) complex ink with controllable high viscosity based on formate-amine coupling. Following only thermal heating in air, well-defined polycrystalline copper oxide hollow spheres with a submicrometer diameter (≤1 μm) were formed spontaneously while being assembled in the form of a film with good adhesion on the substrate. This spontaneous hollowing mechanism was found to result from the Kirkendall effect during oxidation at elevated temperature. The CuO films with hollow spheres, prepared via direct printing synthesis at 500 °C, led to the creation of a superior p-type gas sensor and photocathode for photoelectrochemical water splitting with completely hollow cores, a rough/porous shell structure, a single phase, high crystallinity, and no organic/polymer residue. As a result, the CuO hollow-sphere films showed high gas responses and permissible response speeds to reducing gases and high photocurrent density compared to conventional CuO powder films and the values previously reported. These results exemplify the successful realization of a high-throughput printing fabrication method for the creation of superior nanostructured devices.

    View details for DOI 10.1021/la404098s

    View details for Web of Science ID 000330543700004

    View details for PubMedID 24422661

  • 1-D Structured Flexible Supercapacitor Electrodes with Prominent Electronic/Ionic Transport Capabilities ACS APPLIED MATERIALS & INTERFACES Kim, J. S., Shin, S. S., Han, H. S., Oh, L. S., Kim, D. H., Kim, J., Hong, K. S., Kim, J. Y. 2014; 6 (1): 268-274


    A highly efficient 1-D flexible supercapacitor with a stainless steel mesh (SSM) substrate is demonstrated. Indium tin oxide (ITO) nanowires are prepared on the surface of the stainless steel fiber (SSF), and MnO2 shell layers are coated onto the ITO/SSM electrode by means of electrodeposition. The ITO NWs, which grow radially on the SSF, are single-crystalline and conductive enough for use as a current collector for MnO2-based supercapacitors. A flake-shaped, nanoporous, and uniform MnO2 shell layer with a thickness of ~130 nm and an average crystallite size of ~2 nm is obtained by electrodeposition at a constant voltage. The effect of the electrode geometry on the supercapacitor properties was investigated using electrochemical impedance spectroscopy, cyclic voltammetry, and a galvanostatic charge/discharge study. The electrodes with ITO NWs exhibit higher specific capacitance levels and good rate capability owing to the superior electronic/ionic transport capabilities resulting from the open pore structure. Moreover, the use of a porous mesh substrate (SSM) increases the specific capacitance to 667 F g(-1) at 5 mV s(-1). In addition, the electrode with ITO NWs and the SSM shows very stable cycle performance (no decrease in the specific capacitance after 5000 cycles).

    View details for DOI 10.1021/am404132j

    View details for Web of Science ID 000329586300037

    View details for PubMedID 24397749

  • Surface-area-tuned, quantum-dot-sensitized heterostructured nanoarchitectures for highly efficient photoelectrodes NANO RESEARCH Park, S., Kim, D., Lee, C. W., Seo, S., Kim, H. J., Han, H. S., Hong, K. S., Kim, D. 2014; 7 (1): 144-153
  • Aligned Photoelectrodes with Large Surface Area Prepared by Pulsed Laser Deposition JOURNAL OF PHYSICAL CHEMISTRY C Noh, J. H., Park, J. H., Han, H. S., Kim, D. H., Han, B. S., Lee, S., Kim, J. Y., Jung, H. S., Hong, K. S. 2012; 116 (14): 8102-8110

    View details for DOI 10.1021/jp211233s

    View details for Web of Science ID 000302591300063

  • Facile hydrothermal synthesis of InVO4 microspheres and their visible-light photocatalytic activities MATERIALS LETTERS Noh, T. H., Kim, D. W., Seo, S. W., Cho, I. S., Kim, D. H., Han, H. S., Hong, K. S. 2012; 72: 98-100
  • Tin doped indium oxide core-TiO2 shell nanowires on stainless steel mesh for flexible photoelectrochemical cells APPLIED PHYSICS LETTERS Noh, J. H., Ding, B., Han, H. S., Kim, J. S., Park, J. H., Park, S. B., Jung, H. S., Lee, J., Hong, K. S. 2012; 100 (8)

    View details for DOI 10.1063/1.3684805

    View details for Web of Science ID 000300711200074

  • A Newly Designed Nb-Doped TiO2/Al-Doped ZnO Transparent Conducting Oxide Multi layer for Electrochemical Photoenergy Conversion Devices JOURNAL OF PHYSICAL CHEMISTRY C Noh, J. H., Han, H. S., Lee, S., Kim, D. H., Park, J. H., Park, S., Kim, J. Y., Jung, H. S., Hong, K. S. 2010; 114 (32): 13867-13871

    View details for DOI 10.1021/jp104247t

    View details for Web of Science ID 000280727500064

  • Tailoring the Morphology and Structure of Nanosized Zn2SiO4: Mn2+ Phosphors Using the Hydrothermal Method and Their Luminescence Properties JOURNAL OF PHYSICAL CHEMISTRY C An, J., Noh, J. H., Cho, I., Roh, H., Kim, J. Y., Han, H. S., Hong, K. S. 2010; 114 (23): 10330-10335

    View details for DOI 10.1021/jp911731s

    View details for Web of Science ID 000278479700002

  • Enhancing the Densification of Nanocrystalline TiO2 by Reduction in Spark Plasma Sintering JOURNAL OF THE AMERICAN CERAMIC SOCIETY Noh, J. H., Jung, H. S., Cho, I., An, J., Cho, C. M., Han, H. S., Hong, K. S. 2010; 93 (4): 993-997
  • Photoluminescence and electrical properties of epitaxial Al-doped ZnO transparent conducting thin films PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE Noh, J. H., Cho, I., Lee, S., Cho, C. M., Han, H. S., An, J., Kwak, C. H., Kim, J. Y., Jung, H. S., Lee, J., Hong, K. S. 2009; 206 (9): 2133-2138
  • Nb-Doped TiO2: A New Compact Layer Material for TiO2 Dye-Sensitized Solar Cells JOURNAL OF PHYSICAL CHEMISTRY C Lee, S., Noh, J. H., Han, H. S., Yim, D. K., Kim, D. H., Lee, J., Kim, J. Y., Jung, H. S., Hong, K. S. 2009; 113 (16): 6878-6882

    View details for DOI 10.1021/jp9002017

    View details for Web of Science ID 000265383300074

  • Functional Multilayered Transparent Conducting Oxide Thin Films for Photovoltaic Devices JOURNAL OF PHYSICAL CHEMISTRY C Noh, J. H., Lee, S., Kim, J. Y., Lee, J., Han, H. S., Cho, C. M., Cho, I. S., Jung, H. S., Hong, K. S. 2009; 113 (3): 1083-1087

    View details for DOI 10.1021/jp808279j

    View details for Web of Science ID 000262522000040