Sung-Ho Shin

Hemispherically Aggregated BaTiO3 Nanoparticle Composite Thin Film for High-Performance Flexible Piezoelectric Nanogenerator

We report high-performance flexible nanogenerators (NGs) based on a composite thin film, composed of hemispherically aggregated BaTiO3 nanoparticles (NPs) and poly(vinylidene fluoride-co-hexafluoropropene) P(VDF-HFP). The hemispherical BTO-P(VDF-HFP) clusters were realized by a solvent evaporation method, which greatly enhanced piezoelectric power generation. The flexible NGs exhibit high electrical output up to ∼75 V and ∼15 μA at the applied force normal to the surface, indicating the important role of hemispherical BTO clusters. Besides, the durability and reproducibility of the NGs were tested by cyclic measurement under bending stage, generating the output of ∼5 V and ∼750 nA. The approach we introduce here is simple, cost-effective, and well-suited for large-scale high-performance flexible NG fabrication.

Triboelectric Charging Sequence Induced by Surface Functionalization as a Method To Fabricate High Performance Triboelectric Generators

Two different materials, apart from each other in a triboelectric series, are required to fabricate high performance triboelectric generators (TEGs). Thus, it often limits the choices of materials and causes related processing issues for TEGs. To address this issue, we report a simple surface functionalization method that can effectively change the triboelectric charging sequence of the materials, broadening material choices and enhancing the performance of TEGs. Specifically, we functionalized the surfaces of the polyethylene terephthalate (PET) films either with poly-l-lysine solution or trichloro(1H,1H,2H,2H-perfluorooctyl) silane (FOTS). Consequently, the PET surfaces were modified to have different triboelectric polarities in a triboelectric series. The TEGs, fabricated using this approach, demonstrated the maximum Vopen-circuit (Voc) of ∼330 V and Jshort-circuit (Jsc) of ∼270 mA/m(2), respectively, at an applied force of 0.5 MPa. Furthermore, the functionalized surfaces of TEGs demonstrated superior stability during cyclic measurement over 7200 cycles, maintaining the performance even after a month. The approach introduced here is a simple, effective, and cost-competitive way to fabricate TEGs, which can also be easily adopted for various surface patterns and device structures.

Lithium-Doped Zinc Oxide Nanowires–Polymer Composite for High Performance Flexible Piezoelectric Nanogenerator

We present a method to develop high performance flexible piezoelectric nanogenerators (NGs) by employing Li-doped ZnO nanowires (NWs). We synthesized Li-doped ZnO NWs and adopted them to replace intrinsic ZnO NWs with a relatively low piezoelectric coefficient. When we exploited the ferroelectric phase transition induced in Li-doped ZnO NWs, the performance of the NGs was significantly improved and the NG fabrication process was greatly simplified. In addition, our approach can be easily expanded for large-scale NG fabrication. Consequently, the NGs fabricated by our simple method exhibit the excelling output voltage and current, which are stable and reproducible during periodic bending/releasing measurement over extended cycles. In addition, output voltage and current up to ∼ 180 V and ∼ 50 μA, respectively, were obtained in the large-scale NG. The approach introduced here extends the performance limits of ZnO-based NGs and their potentials in practical applications.

Piezoelectric properties of CH3NH3PbI3 perovskite thin films and their applications in piezoelectric generators

CH3NH3PbI3 (MAPbI3) perovskite thin films were applied to fluorine-doped SnO2 (FTO)/glass and Au/Ti/polyethylene terephthalate (PET) substrates via a two-step process, which involved depositing a CH3NH3I (MAI) solution onto PbI2 films via spin-coating followed by crystallization at temperatures of 100 °C. The 500 nm-thick crystallized MAPbI3 perovskite thin films showed a Curie temperature of ∼328 K, a dielectric permittivity of ∼52, a dielectric loss of ∼0.02 at 1 MHz, and a low leakage current density of ∼10−7 A cm−2 at ±3 V. The polarization–electric field (P–E) hysteresis loop and piezoresponse force microscopy (PFM) results showed that the films had well-developed ferroelectric properties and switchable polarization. Poling at an electrical field of 80 kV cm−1 enhanced the power density of the generator. The values for output voltage and current density of the poled films reached 2.7 V and 140 nA cm−2, respectively, which were 2.7-fold higher than those of the non-poled samples.

Piezoelectric performance enhancement of ZnO flexible nanogenerator by a CuO–ZnO p–n junction formation

The piezoelectric potential screening by large excess electrons in nominally undoped ZnO has limited the energy conversion efficiency of the ZnO nanogenerators (NGs). In this study, we report a simple and effective approach to enhance the piezoelectric output performance of the ZnO NGs by forming a CuO–ZnO heterostructure. By depositing a ZnO thin film on the pre-deposited CuO thin film, which forms a p–n junction, excess electrons in ZnO can be effectively reduced. Thus, the piezoelectric potential generated in ZnO by an applied force can be less affected. Using this approach, we obtained an output voltage up to ∼7.5 V and a maximum current of 4.5 μA cm−2 measured under the forward connection, which is a 7-fold higher output voltage and an approximately one order of magnitude higher current density by comparison to the ZnO NGs without a CuO layer. Our results clearly demonstrate the effectiveness of a CuO–ZnO heterostructure for realizing high performance flexible energy harvesting devices.

Solvent-assisted optimal BaTiO3 nanoparticles-polymer composite cluster formation for high performance piezoelectric nanogenerators

We report on an optimal BaTiO3-P(VDF-HFP) composite thin-film formation process for high performance piezoelectric nanogenerators (NGs). By examining different solvent ratios in a solvent-assisted composite thin film formation process, the BTO nanoparticle (NPs) clustering and related performance enhancements were carefully investigated. Using the optimal process, the fabricated BTO NGs exhibited an excelling output power performance. Under a compressive force of ∼0.23 MPa normal to the surface, the measured open-circuit output voltage and short-circuit current were over 110 V and 22 μA, respectively, with a corresponding peak output power density of 0.48 Wcm(-3). Our results clearly demonstrate the effectiveness of a solvent-assisted BTO cluster formation process for fabricating high performance piezoelectric energy harvesting devices.

Ferroelectric Zinc Oxide Nanowire Embedded Flexible Sensor for Motion and Temperature Sensing

We report a simple method to realize multifunctional flexible motion sensor using ferroelectric lithium-doped ZnO-PDMS. The ferroelectric layer enables piezoelectric dynamic sensing and provides additional motion information to more precisely discriminate different motions. The PEDOT:PSS-functionalized AgNWs, working as electrode layers for the piezoelectric sensing layer, resistively detect a change of both movement or temperature. Thus, through the optimal integration of both elements, the sensing limit, accuracy, and functionality can be further expanded. The method introduced here is a simple and effective route to realize a high-performance flexible motion sensor with integrated multifunctionalities.

Scalable and enhanced triboelectric output power generation by surface functionalized nanoimprint patterns.

We report nanoimprint lithographic submicron surface patterning for scalable output power generation and performance enhancement in triboelectric nanogenerators (TENGs). Specifically, one contact surface of a TENG is nanoimprinted with polyurethane acrylate (PUA) lines in different pitches and the counter contact surface is coated with perfluoropolyether (PFPE). The results show that a TENG with 200 nm pitch PUA lines exhibits voltage and current up to ∼430 V and ∼55 μA cm(-2), generating about a sixfold higher output power than that with a flat PUA surface at an applied force of 0.3 MPa. In addition, scalable output power was obtained by adjusting line pitches. Further enhancement in output power was also demonstrated by chemically functionalizing the PUA line patterns with poly (diallyldimethylammonium chloride) (PDDA). The PDDA functionalization boosted voltage and current up to ∼500 V and ∼100 μA cm(-2), respectively, which corresponds to ∼50% power density enhancement. The approach introduced here is a simple, effective, scalable and reproducible way to fabricate TENGs.

Spontaneously polarized lithium-doped zinc oxide nanowires as photoanodes for electrical water splitting

The key requirement for efficient water splitting is to form suitable band bending at the interface between the semiconductor and electrolyte. One intuitive approach is to directly induce dipoles inside photoelectrochemical cells (PECs) to render band bending favorable for water splitting. Ferroelectric materials, exhibiting spontaneous polarization, can be one promising material choice for this purpose as their polarization domains can be aligned in the desired direction by a poling process. In this work, we employed ferroelectric phase transformed Li-doped ZnO nanowires (NWs) as photoanodes for photoelectrochemical (PEC) water splitting and systematically investigated poling effects. Spontaneous polarization was induced in the NWs in the direction favorable for hole transfer to electrolytes, where the valence band bends upward at the electrolyte interface. Specifically, positively polarized PEC electrodes demonstrated ∼200% improved STH efficiency compared with negatively polarized ones.

Triboelectric Hydrogen Gas Sensor with Pd Functionalized Surface

Palladium (Pd)-based hydrogen (H2) gas sensors have been widely investigated thanks to its fast reaction and high sensitivity to hydrogen. Various sensing mechanisms have been adopted for H2 gas sensors; however, all the sensors must be powered through an external battery. We report here an H2 gas sensor that can detect H2 by measuring the output voltages generated during contact electrification between two friction surfaces. When the H2 sensor, composed of Pd-coated ITO (indium tin oxide) and PET (polyethylene Terephthalate) film, is exposed to H2, its output voltage is varied in proportion to H2 concentration because the work function (WF) of Pd-coated surface changes, altering triboelectric charging behavior. Specifically, the output voltage of the sensor is gradually increased as exposing H2 concentration increases. Reproducible and sensitive sensor response was observed up 1% H2 exposure. The approach introduced here can easily be adopted to development of triboelectric gas sensors detecting other gas species.