Tae-Youl Yang

N-doped monolayer graphene catalyst on silicon photocathode for hydrogen production

Carbon-based catalysts have been attracting attention in renewable energy technologies due to the low cost and high stability, but their insufficient activity is still a challenging issue. Here, we suggest that monolayer graphene can be used as a catalyst for solar-driven hydrogen evolution reaction on Si-photocathodes, and its catalytic activity is boosted by plasma treatment in N2-ambient. The plasma treatment induces abundant defects and the incorporation of nitrogen atoms in the graphene structure, which can act as catalytic sites on graphene. The monolayer graphene containing nitrogen impurities exhibits a remarkable increase in the exchange current density and leads to a significant anodic shift of the onset of photocurrent from the Si-photocathode. Additionally, monolayer graphene shows the passivation effect that suppresses the surface oxidation of Si, thus enabling the operation of the Si-photocathode in neutral water. This study shows that graphene itself can be applied to a photoelectrochemical system as a catalyst with high activity and chemical stability.

Atomic migration in molten and crystalline Ge2Sb2Te5 under high electric field

Atomic migration under an electric field, electromigration, in molten and crystalline Ge2Sb2Te5 was studied using a pulsed dc stress to an isolated line structure. Under a single pulse (∼10−3 s), Ge2Sb2Te5 was melted by Joule heating, and an electrostatic force-induced drift of Ge and Sb toward the cathode and Te toward the anode was observed. Effective charge numbers were calculated to be 0.28, 0.38, and −0.29 for Ge, Sb, and Te, respectively. Electromigration in the crystalline state was studied by applying a 10 MHz pulsed dc; constituent elements migrated toward the cathode, which suggests a hole wind-force operating in this phase.

An iron oxide photoanode with hierarchical nanostructure for efficient water oxidation

Hematite (α-Fe2O3) has been attracting attention for photoelectrochemical water oxidation due to its visible light photon absorption capacity and high chemical stability, but the short-diffusion length of holes and the large overpotential are still challenging to overcome. Here, in an effort to address these challenges, we develop a hierarchically nanostructured photoanode composed of iron-oxides; Ti-doped hematite nanorods are decorated with an undoped hematite underlayer and β-FeOOH nano-branches. The Ti-doped hematite nanorod array is prepared by hydrothermal synthesis, and this nanostructure offers enhanced separation of photogenerated charges. The underlayer not only increases the photocurrent density but also improves the onset potential. The photocurrent further increases by the epitaxially grown β-FeOOH nano-branches on the hematite, but the onset potential is positively shifted by the β-FeOOH due to increasing flat-band potential. The analyses of the photocurrent transients and electrochemical impedance spectra reveal that β-FeOOH improves the photocurrent by decreasing the resistance to charge transfer through the anode/electrolyte. This study demonstrates a new possibility for improving the efficiency of a hematite photoanode with the interface of other iron-oxides.

Nanostructural dependence of hydrogen production in silicon photocathodes

Hydrogen production from solar power energy is an important energy and environmental issue. Silicon (Si) has been widely studied as a photocathode for hydrogen production from water splitting. In this study, the electrochemical behavior of a Si photocathode for water splitting is highly dependent on its nanostructure. The optimum nanostructure of a Si photocathode exhibits an enhanced photocurrent and a lower overpotential compared to the planar bulk Si. The limiting current density of nanostructured Si is 1.58 times greater than that of the planar structure for p-type Si/aqueous electrolyte solution. Nanostructured Si without any catalyst notably produced a current density of −10.65 mA cm−2 under Air Mass 1.5 Global conditions with a light intensity of 100 mW cm−2 at the reversible potential vs. reversible hydrogen electrode, which is about 43 times higher than that of the untreated Si structure. The solar-to-hydrogen conversion efficiency of the optimized Si nanowire without depositing any catalyst has reached up to about 70% of the efficiency of planar Si decorated with Pt. This significant enhancement achieved in this study emphasizes the importance of a controlled nanostructure in the development of highly efficient photoelectrochemical devices for hydrogen production.

Tyrosine-mediated two-dimensional peptide assembly and its role as a bio-inspired catalytic scaffold

In two-dimensional interfacial assemblies, there is an interplay between molecular ordering and interface geometry, which determines the final morphology and order of entire systems. Here we present the interfacial phenomenon of spontaneous facet formation in a water droplet driven by designed peptide assembly. The identified peptides can flatten the rounded top of a hemispherical droplet into a plane by forming a macroscopic two-dimensional crystal structure. Such ordering is driven by the folding geometry of the peptide, interactions of tyrosine and crosslinked stabilization by cysteine. We discover the key sequence motifs and folding structures and study their sequence-specific assembly. The well-ordered, densely packed, redox-active tyrosine units in the YYACAYY (H-Tyr-Tyr-Ala-Cys-Ala-Tyr-Tyr-OH) film can trigger or enhance chemical/electrochemical reactions, and can potentially serve as a platform to fabricate a molecularly tunable, self-repairable, flat peptide or hybrid film.

Engineering interface structures between lead halide perovskite and copper phthalocyanine for efficient and stable perovskite solar cells

Successful commercialization of perovskite solar cells (PSCs) in the near future will require the fabrication of cells with high efficiency and long-term stability. Despite their good processability at low temperatures, the majority of organic conductors employed in the fabrication of high-efficiency PSCs [e.g., 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) and poly(triaryl amine) (PTAA)] have low thermal stability. In order to fabricate PSCs with excellent thermal stability, both the constituent material itself and the interface between the constituents must be thermally stable. In this work, we focused on copper phthalocyanine (CuPC) as a model hole-transporting material (HTM) for thermally stable PSCs since CuPC is known to possess excellent thermal stability and interfacial bonding properties. The CuPC-based PSCs recorded a high power conversion efficiency (PCE) of ∼18% and maintained 97% of their initial efficiency for more than 1000 h of thermal annealing at 85 °C. Moreover, the device was stable under thermal cycling tests (50 cycles, −45 to 85 °C). The high PCE and high thermal stability observed in the CuPC-PSCs were found to arise as a result of the strong interfacial and conformal coating present on the surface of the perovskite facets, located between CuPC and the perovskite layer. These results will provide an important future direction for the development of highly efficient and thermally stable PSCs.

Designing thermal and electrochemical oxidation processes for δ-MnO2 nanofibers for high-performance electrochemical capacitors

To date, the phase of electrospun MnOx nanofibers (NFs) after thermal calcination has been limited to the low oxidation state of Mn (x < 2), which has resulted in insufficient specific capacitance. The organic contents in the as-spun MnOx NFs, which are essential for forming the NF structure, make it difficult to obtain the optimum phase (MnO2) to achieve high electrochemical performance. Herein, δ-MnO2 NFs, which were obtained by galvanostatic oxidation of thermally calcined MnOx NFs, were successfully fabricated while maintaining the 1-D nanoscale structure and inhibiting loss of the active materials. The galvanostatically oxidized Mn3O4 exhibited an outstanding performance of 380 F g−1 under a mass loading of 1.2 mg cm−2. The effect of galvanostatic oxidation was strongly dependent on the concentration and energetic stability of the Mn2+/3+ ions in the MnOx phases.

Enhanced conductivity of solution-processed indium tin oxide nanoparticle films by oxygen partial pressure controlled annealing

A highly conductive and transparent indium tin oxide (ITO) film was developed using a nanoparticle-based solution process through the control of oxygen partial pressure during annealing. At an oxygen partial pressure of 2.1 × 10−3 Torr, a maximum conductivity of 313 Ω−1 cm−1 was obtained: a great improvement over the conductivity of conventional ITO nanoparticle films (at this conductivity, the sheet resistance decreased to 30 Ω sq−1, and the transmittance reached 90%). By analyzing the electron concentration and mobility using Hall measurements, we determined that the main factor contributing to the enhanced conductivity is the increase in electron concentration that occurs due to the formation of oxygen vacancies under low oxygen partial pressures. However, if the oxygen partial pressure is too low, the removal of the organic ligands covering the ITO nanoparticles is incomplete, and the electron mobility is reduced. Microstructure control is also necessary for further improvement of the mobility.

Change of Damage Mechanism by the Frequency of Applied Pulsed DC in the Ge2Sb2Te5 Line

We investigated the damage on the Ge 2 Sb 2 Te 5 line structure by pulsed-dc stressing with various frequencies. The line immediately burnt out due to Joule heating under constant dc stress 2.5 MA/cm 2 . However, when pulsed dc 2.5 MA/cm 2 was stressed at the frequency of 5 MHz, failure due to thermal fatigue damage was observed. At higher frequency such as 10 MHz, no noticeable damage was observed, yet the compositional change in constitutive elements by electromigration was detected. The change of damage mechanism by varying of frequency is explained by the difference in thermal cycling extent in response to the pulsed-current operation at various frequencies, which is computed using a finite-difference method.

Investigation of crystallization behaviors of nitrogen-doped Ge2Sb2Te5 films by thermomechanical characteristics

It has been demonstrated that the crystallization behaviors of undoped and N-doped Ge2Sb2Te5 (GST) films can be evaluated by studying the thermomechanical behavior of the films. The crystallization temperatures (Tc) for undoped and 15 at. % N-doped GST films were determined to be about 150 and 250 °C, respectively. The activation energies for crystallization (Ea) were calculated to be 2.30, 3.29, and 3.72 eV for undoped, 10 at. % N-doped, and 15 at. % N-doped GST, respectively, by the Kissinger plot. For 15 at. % N-doped film, both the glass transition temperature (Tg) and Tc were determined to be about 150 °C, as determined by the thermomechanical characteristics.