All Publications

  • Sulfur-treated TiO2 shows improved alcohol dehydration activity and selectivity. Nanoscale R Riscoe, A., Oh, J., Cargnello, M. 2022


    The dehydration of alcohols is an important class of reactions for the development of fossil-free fuel and chemical industries. Acid catalysts are well known to enhance the reactivity of alcohols following two main pathways of either dehydration to olefins or dehydrogenation to ketones/aldehydes. TiO2 surfaces have been well documented for primary and secondary alcohol dehydration with selectivity ranging from 1-100% towards dehydration products based on process conditions and catalyst structure. In this work we document the effects of various sulfur treatments of TiO2 surfaces which induce higher activity and, more importantly, higher selectivity for alcohol dehydration than untreated surfaces. The increase in activity and >99% dehydration selectivity is coupled with demonstrated stability for several hours on stream at high conversion. Using temperature programmed reaction studies, XPS and FT-IR spectroscopy, we identify Lewis acidic sites correlated with sulfate species on TiO2 surfaces as active sites for the reaction.

    View details for DOI 10.1039/d1nr06029e

    View details for PubMedID 35137741

  • Steam-created grain boundaries for methane C-H activation in palladium catalysts. Science (New York, N.Y.) Huang, W., Johnston-Peck, A. C., Wolter, T., Yang, W. D., Xu, L., Oh, J., Reeves, B. A., Zhou, C., Holtz, M. E., Herzing, A. A., Lindenberg, A. M., Mavrikakis, M., Cargnello, M. 2021; 373 (6562): 1518-1523


    [Figure: see text].

    View details for DOI 10.1126/science.abj5291

    View details for PubMedID 34554810

  • Transparent Pressure Sensor with High Linearity over a Wide Pressure Range for 3D Touch Screen Applications ACS APPLIED MATERIALS & INTERFACES Choi, H., Oh, J., Kim, Y., Pyatykh, M., Yang, J., Ryu, S., Park, S. 2020; 12 (14): 16691-16699


    The demand for display technology is expected to increase with the continuous spread of portable electronics and with the expected emergence of flexible, wearable, and transparent display devices. A touch screen is a critical component in display technology that enables user interface operations, and the future generation of touch screens, the so-called 3D touch screens, is expected to be able to detect multiple levels of pressure. To enable 3D touch screens, transparent pressure sensors with high linearity over a working range that encompasses the pressure range of human touch (10-100 kPa) are required. In this work, a transparent and linear capacitive pressure sensor is reported with a transmittance over 85% and high linearity (R2 = 0.995) over 5-100 kPa of pressure. To render the sensor transparent, a microstructured "hard" elastomer layer was filled in with a refractive index matching a "soft" elastomer layer, through which light scattering was minimized. High linearity was attained from the sensor's unique architecture that increases the effective area of the capacitor with applied pressure. These attributes render our sensor highly suitable for future 3D touch screen applications.

    View details for DOI 10.1021/acsami.0c00267

    View details for Web of Science ID 000526583500075

    View details for PubMedID 32180401

  • Highly Uniform and Low Hysteresis Piezoresistive Pressure Sensors Based on Chemical Grafting of Polypyrrole on Elastomer Template with Uniform Pore Size SMALL Oh, J., Kim, J., Kim, Y., Choi, H., Yang, J., Lee, S., Pyatykh, M., Kim, J., Sim, J., Park, S. 2019; 15 (33): e1901744


    Sensor-to-sensor variability and high hysteresis of composite-based piezoresistive pressure sensors are two critical issues that need to be solved to enable their practical applicability. In this work, a piezoresistive pressure sensor composed of an elastomer template with uniformly sized and arranged pores, and a chemically grafted conductive polymer film on the surface of the pores is presented. Compared to sensors composed of randomly sized pores, which had a coefficient of variation (CV) in relative resistance change of 69.65%, our sensors exhibit much higher uniformity with a CV of 2.43%. This result is corroborated with finite element simulation, which confirms that with increasing pore size variability, the variability in sensor characteristics also increases. Furthermore, our devices exhibit negligible hysteresis (degree of hysteresis: 2%), owing to the strong chemical bonding between the conductive polymer and the elastomer template, which prevents their relative sliding and displacement, and the porosity of the elastomer that enhances elastic behavior. Such features of the sensor render it highly feasible for various practical applications in the near future.

    View details for DOI 10.1002/smll.201901744

    View details for Web of Science ID 000481733900018

    View details for PubMedID 31192540

  • Microstructured Porous Pyramid-Based Ultrahigh Sensitive Pressure Sensor Insensitive to Strain and Temperature ACS APPLIED MATERIALS & INTERFACES Yang, J., Kim, J., Oh, J., Kwon, S., Sim, J., Kim, D., Choi, H., Park, S. 2019; 11 (21): 19472-19480


    An ultrahigh sensitive capacitive pressure sensor based on a porous pyramid dielectric layer (PPDL) is reported. Compared to that of the conventional pyramid dielectric layer, the sensitivity was drastically increased to 44.5 kPa-1 in the pressure range <100 Pa, an unprecedented sensitivity for capacitive pressure sensors. The enhanced sensitivity is attributed to a lower compressive modulus and larger change in an effective dielectric constant under pressure. By placing the pressure sensors on islands of hard elastomer embedded in a soft elastomer substrate, the sensors exhibited insensitivity to strain. The pressure sensors were also nonresponsive to temperature. Finally, a contact resistance-based pressure sensor is also demonstrated by chemically grafting PPDL with a conductive polymer, which also showed drastically enhanced sensitivity.

    View details for DOI 10.1021/acsami.9b03261

    View details for Web of Science ID 000470034700064

    View details for PubMedID 31056895

  • Highly Ordered 3D Microstructure-Based Electronic Skin Capable of Differentiating Pressure, Temperature, and Proximity ACS APPLIED MATERIALS & INTERFACES Kim, J., Kwon, S., Kim, Y., Choi, H., Yang, J., Oh, J., Lee, H., Sim, J., Ryu, S., Park, S. 2019; 11 (1): 1503-1511


    Electronic skin are devices that mimic the functionalities of human skin, which require high sensitivity, large dynamic range, high spatial uniformity, low-cost and large-area processability, and the capacity to differentiate various external inputs. We herein introduce a versatile droplet-based microfluidic-assisted emulsion self-assembly process to generate three-dimensional microstructure-based high-performance capacitive and piezoresistive pressure sensors for electronic skin applications. Our technique can generate uniformly sized micropores that are self-assembled in an orderly close-packed manner over a large area, which results in high spatial uniformity. The size of the micropores can easily be tuned from 100 to 500 μm, through which sensitivity and dynamic range were controlled as high as 0.86 kPa-1 and up to 100 kPa. Our device can be printed on curvilinear surfaces and be molded into various shapes. We furthermore demonstrate that by simultaneously utilizing capacitive and piezoresistive pressure sensors, we can distinguish between pressure and temperature, or between pressure and proximity. These demonstrations make our process and sensors highly useful for a wide variety of electronic skin applications.

    View details for DOI 10.1021/acsami.8b19214

    View details for Web of Science ID 000455561200161

    View details for PubMedID 30565915

  • Intrinsically stretchable multi-functional fiber with energy harvesting and strain sensing capability NANO ENERGY Ryu, J., Kim, J., Oh, J., Lim, S., Sim, J., Jeon, J. S., No, K., Park, S., Hong, S. 2019; 55: 348-353
  • A Highly Sensitive Bending Sensor Based on Controlled Crack Formation Integrated with an Energy Harvesting Pyramid Layer ADVANCED MATERIALS TECHNOLOGIES Lee, S., Oh, J., Yang, J., Sim, J., Ryu, J., Kim, J., Park, S. 2018; 3 (12)
  • Pressure Insensitive Strain Sensor with Facile Solution-Based Process for Tactile Sensing Applications ACS NANO Oh, J., Yang, J., Kim, J., Park, H., Kwon, S., Lee, S., Sim, J., Oh, H., Kim, J., Park, S. 2018; 12 (8): 7546–53


    Tactile sensors that can mechanically decouple, and therefore differentiate, various tactile inputs are highly important to properly mimic the sensing capabilities of human skin. Herein, we present an all-solution processable pressure insensitive strain sensor that utilizes the difference in structural change upon the application of pressure and tensile strain. Under the application of strain, microcracks occur within the multiwalled carbon nanotube (MWCNT) network, inducing a large change in resistance with gauge factor of ∼56 at 70% strain. On the other hand, under the application of pressure to as high as 140 kPa, negligible change in resistance is observed, which can be attributed to the pressure working primarily to close the pores, and hence minimally changing the MWCNT network conformation. Our sensor can easily be coated onto irregularly shaped three-dimensional objects (e.g., robotic hand) via spray coating, or be attached to human joints, to detect bending motion. Furthermore, our sensor can differentiate between shear stress and normal pressure, and the local strain can be spatially mapped without the use of patterned electrode array using electrical impedance tomography. These demonstrations make our sensor highly useful and important for the future development of high performance tactile sensors.

    View details for DOI 10.1021/acsnano.8b03488

    View details for Web of Science ID 000443525600011

    View details for PubMedID 29995382