Uniform TiO₂ blocking layers (BLs) are fabricated using ultrasonic spray pyrolysis deposition (USPD) method. To improve the photovoltaic performance of dye-sensitized solar cells (DSSCs), the BL thickness is controlled by using USPD times of 0, 20, 60, and 100 min, creating TiO₂ BLs of 0, 40, 70, and 100 nm, respectively, in average thickness on fluorine-doped tin oxide (FTO) glass. Compared to the other samples, the DSSC containing the uniform TiO₂ BL of 70 nm in thickness shows a superior power conversion efficiency of 7.58±0.20% because of the suppression of electron recombination by the effect of the optimized thickness. The performance improvement is mainly attributed to the increased open-circuit voltage (0.77±0.02 V) achieved by the increased Fermi energy levels of the working electrodes and the improved short-circuit current density (15.67±0.43 mA/cm2) by efficient electron transfer pathways. Therefore, optimized TiO₂ BLs fabricated by USPD may allow performance improvements in DSSCs.

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• Flexible Dye-sensitized Solar Cell Using Titanium Gel at Low Temperature
  Seung Hwan Ji, Hyunsu Park, Doyeon Kim, Do Hyung Han, Hye Won Yun, Woo-Byoung Kim
  Korean Journal of Materials Research.2019; 29(3): 183.     CrossRef
• Surface tailoring of zinc electrodes for energy storage devices with high-energy densities and long cycle life
  Geon-Hyoung An, SeungNam Cha, Jung Inn Sohn
  Applied Surface Science.2019; 467-468: 1157.     CrossRef
• Crystallinity Control Effects on Vanadium Oxide Films for Enhanced Electrochromic Performances
  Kue-Ho Kim, Ju-Won Bae, Tae-Kuen Lee, Hyo-Jin Ahn
  Korean Journal of Materials Research.2019; 29(6): 385.     CrossRef

In this study, we synthesize tungsten oxide thin films by electrodeposition and characterize their electrochromic properties. Depending on the deposition modes, compact and porous tungsten oxide films are fabricated on a transparent indium tin oxide (ITO) substrate. The morphology and crystal structure of the electrodeposited tungsten oxide thin films are investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). X-ray photoelectron spectroscopy is employed to verify the chemical composition and the oxidation state of the films. Compared to the compact tungsten oxides, the porous films show superior electrochemical activities with higher reversibility during electrochemical reactions. Furthermore, they exhibit very high color contrast (97.0%) and switching speed (3.1 and 3.2 s). The outstanding electrochromic performances of the porous tungsten oxide thin films are mainly attributed to the porous structure, which facilitates ion intercalation/deintercalation during electrochemical reactions.

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• A fast-response electrochromic device based on a composite gel film comprising triphenylamine derivatives and WO3
  Xuejian Zhang, Jinming Zeng, Zipeng Xu, Mimi Zhu, Ping Liu
  New Journal of Chemistry.2021; 45(12): 5503.     CrossRef

Cost-effective functional phosphor nanoparticles are prepared by introducing low-cost SiO₂ spheres to rareearth phosphor (YVO₄:Eu³⁺, YVO₄:Er³⁺, and YVO₄:Nd³⁺) shells using a sol-gel synthetic method. These functional nanoparticles are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and general photoluminescence spectra. The SiO₂ sphere occupying the interior of the conventional phosphor is advantageous in significantly reducing the cost of expensive rare-earth phosphor nanoparticles. The sol-gel process facilitates the core–shell structure formation; the rare-earth shell phosphor has strong interactions with chelating agents on the surfaces of SiO₂ nanoparticles and thus forms layers of several nanometers in thickness. The photoluminescence wavelength is simply tuned by replacing the active materials of Eu³⁺, Er³⁺, and Nd³⁺. Moreover, the photoluminescent properties of the core–shell nanoparticles can be optimized by manipulating the specific contents of active materials in the phosphors. Our simple approach substitutes low-cost SiO₂ for expensive rare-earth-based phosphor materials to realize cost-effective phosphor nanoparticles for various applications.

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• Enhanced Energy-Transfer Properties in Core-Shell Photoluminescent Nanoparticles Using Mesoporous SiO₂ Intermediate Layers
  Woo Hyeong Sim, Seyun Kim, Weon Ho Shin, Hyung Mo Jeong
  Korean Journal of Metals and Materials.2020; 58(2): 137.     CrossRef

Cu-Fe alloys (CFAs) are much anticipated for use in electrical contacts, magnetic recorders, and sensors. The low cost of Fe has inspired the investigation of these alloys as possible replacements for high-cost Cu-Nb and Cu-Ag alloys. Here, alloys of Cu and Fe having compositions of Cu_100-xFe_x (x = 10, 30, and 50 wt.%) are prepared by gas atomization and characterized microstructurally and structurally based on composition and powder size with scanning electron microscopy (SEM) and X-ray diffraction (XRD). Grain sizes and Fe-rich particle sizes are measured and relationships among composition, powder size, and grain size are established. Same-sized powders of different compositions yield different microstructures, as do differently sized powders of equal composition. No atomic-level alloying is observed in the CFAs under the experimental conditions.

Porous Cu with a dispersion of nanoscale Al₂O₃ particles is fabricated by freeze-drying CuO-Al₂O₃/camphene slurry and sintering. Camphene slurries with CuO-Al₂O₃ contents of 5 and 10 vol% are unidirectionally frozen at -30°C, and pores are generated in the frozen specimens by camphene sublimation during air drying. The green bodies are sintered for 1 h at 700°C and 800°C in H₂ atmosphere. The sintered samples show large pores of 100 μm in average size aligned parallel to the camphene growth direction. The internal walls of the large pores feature relatively small pores of ~10 μm in size. The size of the large pores decreases with increasing CuO-Al₂O₃ content by the changing degree of powder rearrangement in the slurry. The size of the small pores decreases with increasing sintering temperature. Microstructural analysis reveals that 100-nm Al₂O₃ particles are homogeneously dispersed in the Cu matrix. These results suggest that a porous composite body with aligned large pores could be fabricated by a freeze-drying and H₂ reducing process.

The friction characteristics of Al-Fe alloy powders are investigated in order to develop an eco-friendly friction material to replace Cu fiber, a constituent of brake-pad friction materials. Irregularly shaped Al-Fe alloy powders, prepared by gas atomization, are more uniformly dispersed than conventional Cu fiber on the brake pad matrix. The wear rate of the friction material using Al-8Fe alloy powder is lower than that of the Cu fiber material. The change in friction coefficient according to the friction lap times is 7.2% for the Cu fiber, but within 3.8% for the Al-Fe alloy material, which also shows excellent judder characteristics. The Al-Fe alloy powders are uniformly distributed in the brake pad matrix and oxide films of Al and Fe are homogeneously formed at the friction interface between the disc and pad, thus exhibiting excellent friction and lubrication characteristics. The brake pad containing Al-Fe powders avoids contamination by Cu dust, which is generated during braking, by replacing the Cu fiber while maintaining the friction and lubrication performance.

In order to expand the application of oxide dispersion-strengthened (ODS) steel, a composite material is manufactured by adding mechanically alloyed ODS steel powder to conventional steel and investigated in terms of microstructure and wear properties. For comparison, a commercial automobile part material is also tested. Initial microstructural observations confirm that the composite material with added ODS steel contains i) a pearlitic Fe matrix area and ii) an area with Cr-based carbides and ODS steel particles in the form of a Fe-Fe3C structure. In the commercial material, various hard Co-, Fe-Mo-, and Cr-based particles are present in a pearlitic Fe matrix. Wear testing using the VSR engine simulation wear test confirms that the seatface widths of the composite material with added ODS steel and the commercial material are increased by 24% and 47%, respectively, with wear depths of 0.05 mm and 0.1 mm, respectively. The ODS steel-added composite material shows better wear resistance. Post-wear-testing surface and cross-sectional observations show that particles in the commercial material easily fall off, while the ODS steel-added material has an even, smooth wear surface.

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• First principles determination of formation of a Cr shell on the interface between Y–Ti–O nanoparticles and a ferritic steel matrix
  Ki-Ha Hong, Jae Bok Seol, Jeoung Han Kim
  Applied Surface Science.2019; 481: 69.     CrossRef
• Thermal Properties and Microstructural Changes of Fe-Co System Valve Seat Alloy by High Densification Process
  In-Shup Ahn, Dong-Kyu Park, Kwang-Bok Ahn, Seoung-Mok Shin
  Journal of Korean Powder Metallurgy Institute.2019; 26(2): 112.     CrossRef

Carbon nanofibers (CNF) are widely used as active agents for electrodes in Li-ion secondary battery cells, supercapacitors, and fuel cells. Nanoscale coatings on CNF electrodes can increase the output and lifespan of battery devices. Atomic layer deposition (ALD) can control the coating thickness at the nanoscale regardless of the shape, suitable for coating CNFs. However, because the CNF surface comprises stable C–C bonds, initiating homogeneous nuclear formation is difficult because of the lack of initial nucleation sites. This study introduces uniform nucleation site formation on CNF surfaces to promote a uniform SnO₂ layer. We pretreat the CNF surface by introducing H₂O or Al₂O₃ (trimethylaluminum + H₂O) before the SnO₂ ALD process to form active sites on the CNF surface. Transmission electron microscopy and energy-dispersive spectroscopy both identify the SnO₂ layer morphology on the CNF. The Al₂O₃-pretreated sample shows a uniform SnO₂ layer, while island-type SnOx layers grow sparsely on the H₂Opretreated or untreated CNF.

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• Atomic layer deposition of ZnO layers on Bi2Te3 powders: Comparison of gas fluidization and rotary reactors
  Myeong Jun Jung, Myeongjun Ji, Jeong Hwan Han, Young-In Lee, Sung-Tag Oh, Min Hwan Lee, Byung Joon Choi
  Ceramics International.2022; 48(24): 36773.     CrossRef
• Effects of SnO2 layer coated on carbon nanofiber for the methanol oxidation reaction
  Dong Ha Kim, Dong-Yo Shin, Young-Geun Lee, Geon-Hyoung An, Jeong Hwan Han, Hyo-Jin Ahn, Byung Joon Choi
  Ceramics International.2018; 44(16): 19554.     CrossRef

One-dimensional rutile TiO₂ is an important inorganic compound with applicability in sensors, solar cells, and Li-based batteries. However, conventional synthesis methods for TiO₂ nanowires are complicated and entail risks of environmental contamination. In this work, we report the growth of TiO₂ nanowires on a Ti alloy powder (Ti-6wt%Al-4wt%V, Ti64) using simple thermal oxidation under a limited supply of O₂. The optimum condition for TiO₂ nanowire synthesis is studied for variables including temperature, time, and pressure. TiO₂ nanowires of ~5 μm in length and 100 nm in thickness are richly synthesized under the optimum condition with single-crystalline rutile phases. The formation of TiO₂ nanowires is greatly influenced by synthesis temperature and pressure. The synthesized TiO₂ nanowires are characterized using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HR-TEM).

Sb-doped SnO₂ (ATO) transparent conducting films are fabricated using horizontal ultrasonic spray pyrolysis deposition (HUSPD) to form uniform and compact film structures with homogeneously supplied precursor solution. To optimize the molar concentration and transparent conducting performance of the ATO films using HUSPD, we use precursor solutions of 0.15, 0.20, 0.25, and 0.30 M. As the molar concentration increases, the resultant ATO films exhibit more compact surface structures because of the larger crystallite sizes and higher ATO crystallinity because of the greater thickness from the accelerated growth of ATO. Thus, the ATO films prepared at 0.25 M have the best transparent conducting performance (12.60±0.21 Ω/□ sheet resistance and 80.83% optical transmittance) and the highest figure-of-merit value (9.44±0.17 × 10^-3 Ω^-1). The improvement in transparent conducting performance is attributed to the enhanced carrier concentration by the improved ATO crystallinity and Hall mobility with the compact surface structure and preferred (211) orientation, ascribed to the accelerated growth of ATO at the optimized molar concentration. Therefore, ATO films fabricated using HUSPD are transparent conducting film candidates for optoelectronic devices.

The design and fabrication of photoelectrochemical (PEC) electrodes for efficient water splitting is important for developing a sustainable hydrogen evolution system. Among various development approaches for PEC electrodes, the chemical vapor deposition method of atomic layer deposition (ALD), based on self-limiting surface reactions, has attracted attention because it allows precise thickness and composition control as well as conformal coating on various substrates. In this study, recent research progress in improving PEC performance using ALD coating methods is discussed, including 3D and heterojunction-structured PEC electrodes, ALD coatings of noble metals, and the use of sulfide materials as co-catalysts. The enhanced long-term stability of PEC cells by ALD-deposited protecting layers is also reviewed. ALD provides multiple routes to develop improved hydrogen evolution PEC cells.

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• Improved Interface and Electrical Properties by Inserting an Ultrathin SiO2 Buffer Layer in the Al2O3/Si Heterojunction
  Doyeon Kim, Jae‐Young Choi, Sang Wook Ryu, Woo‐Byoung Kim
  Advanced Functional Materials.2019;[Epub]     CrossRef