Published papers

Energy band alignment of a thin-film heterojunction of molybdenum disulfide (MoS2) and polymeric carbon nitride (g-C3N4) has been investigated, and can be regarded as a model system of heterojunction photocatalysts. A bilayer of MoS2 and g-C3N4 (MoS2/g-C3N4) with an exposed platinum (Pt) bottom electrode was fabricated by thermal chemical vapor deposition and sulfur hexafluoride-based dry etching, followed by deposition of MoS2 nanosheets on top of that via tape exfoliation and elastomer-assisted transfer techniques. Simultaneous imaging of topography and surface potential over MoS2/g-C3N4 on Pt was performed by using Kelvin probe force microscopy (KPFM) in the dark. The energy band alignment of MoS2/g-C3N4 on Pt was estimated based on the KPFM imaging results and other physical properties of individual MoS2 and g-C3N4 samples examined with ultraviolet photoelectron spectroscopy and ultraviolet-visible absorption spectroscopy. In addition, the photo-induced change in the surface potential over MoS2/g-C3N4 on Pt was measured to consider the behavior of photo-induced carriers therein. The depicted energy band alignment having band bending in the MoS2 layer suggests the possibility of S-scheme heterojunction formation at the MoS2/g-C3N4 interface.

Photocatalysis offers solutions to a variety of problems, including the treatment of wastewater, the degradation of volatiles, and the removal of dyes, etc. The graphitic carbon nitride (g-C3N4) is one of the candidates to be used as a photocatalyst. The g-C3N4 was synthesized by the polycondensation method with a synthesis temperature from 450 to 650 °C. The crystalline structure, composition, morphology and optical properties were analyzed by XRD, FTIR, FESEM, XPS, BET, DRS, and PL. The synthesis of g-C3N4 was confirmed at temperatures of 550 °C, 600 °C, and 650 °C. The calcination temperature influences morphological and optical properties, resulting in a significant decrease in the bandgap of g-C3N4 synthesized at 600 °C. Further, the photocatalytic degradation of Rhodamine B (RhB) using g-C3N4 was evaluated under visible irradiation. The synthesis temperature has a profound effect on the photocatalytic activity of g-C3N4. The sample prepared at a temperature of 600 °C shows an apparent rate constant of 0.01387, with 70.85% photodegradation efficiency observed even after 5 cycles for the degradation of RhB, indicating g-C3N4’s potential for the photocatalytic degradation of RhB.

Composite photocatalysts have many advantages, such as visible light response, charge separation efficiency, and the lack of precious metal use, and are attracting attention as high-performance photocatalytic materials that can solve many of the problems associated with single photocatalysts. On the other hand, there remain many unknowns regarding charge transfer, the understanding of which is key to improving the performance of composite photocatalysts. In particular, elucidating the details of the charge transfer process at the photocatalyst interface is an urgent and important issue that must be understood to improve the performance of future composite photocatalysts. In this study, we have focused on composite formation of tungsten trioxide (WO3) and nitrogen-rich carbon nitride (C3N5), and have investigated the band bending occurring at the interface during complexation and the charge transfer before and after light irradiation over the composite photocatalysts using various experimental techniques and simulations. The results support the presence of the ideal S-scheme mechanism and the overall photocatalytic performance of WO3/C3N5 was superior to that of the other individual photocatalysts. In addition, C3N5 particle size was found to decrease during annealing treatment when complexed with WO3, which is surmised to be due to the loss of bridging nitrogen.

Plastic waste poses significant environmental and health challenges. Photocatalytic degradation using sunlight offers a promising solution; however, many previous studies have relied on impractical conditions, such as employing ultraviolet light irradiation and hot-pressed mixture of plastics and photocatalysts. In this study, we explored a simple and practical method for plastic degradation under visible light. A visible-light-responsive tungsten trioxide (WO3) powder with a nanoflower structure was simply mixed with polyethylene (PE) powder without hot pressing and then irradiated with visible light. Effective degradation of PE was observed, and the reaction proceeded stably under prolonged irradiation. In contrast, a hot-pressed WO3–PE sample exhibited reduced degradation efficiency, likely due to the collapse of the nanoflower structure, which diminished light absorption and surface activity. These findings demonstrate that a simple powder mixture can enable efficient plastic degradation under visible light, presenting a practical strategy for photocatalytic plastic waste treatment and contributing to sustainable environmental solutions.

The present investigation describes the effect of deposition potential and annealing on copper oxide (CuO) thin film as a supercapacitor electrode. CuO thin films were grown on conducting glass substrate (ITO) by electrodeposition at room temperature and annealed for 3 hrs. at 300° C. The formation of polycrystalline CuO was confirmed by X-ray diffraction (XRD) & X-ray photoelectron spectroscopy (XPS). Significant change in morphology of CuO thin films after annealing has been observed. Annealing has improved crystallinity, surface coverage and grain connectivity of CuO thin films. CuO thin films deposited at −450 mV vs. Ag/AgCl exhibits essential porosity for electrochemical reactions. Higher deposition potential of −500 mV vs. Ag/AgCl leads to formation of compact CuO thin films. Electrochemical performance of as deposited and annealed CuO thin films were compared using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in 1 M aqueous KOH electrolyte. The maximum specific capacitance of 57.44 F/g at 20 mV/s was measured. The CuO thin film shows specific energy of 2.67 Wh/kg with a coulombic efficiency of 77.27 % at 0.5 mA/cm2 current density. These results showed that the electrodes prepared by simple, low-cost electrodeposition method can be promising electrode materials for supercapacitor application.

4-(N,N-Dimethylamino)phenyl-substituted 1,3-dimethyl-2,3-dihydro-1H-benzimidazole (N-DMBI-H) has been utilized as a solution-processable n-type dopant in organic electronics. In this study, a dimethyl-substituted N-DMBI-H derivative (DMe-N-DMBI-H), in which two methyl groups are attached at the terminal 5- and 6-positions of the benzimidazole moiety of N-DMBI-H molecule, has been examined to control its electron-donating ability. The effectiveness of DMe-N-DMBI-H as a solution-processable donor dopant has been clarified by evaluating electrical characteristics of DMe-N-DMBI-H-doped PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) thin-film transistors, such as field-effect mobility, gate threshold voltage, and contact resistance. Our electrochemical and electrical characterizations as well as quantum chemical calculations have suggested that DMe-N-DMBI-H works as a donor dopant somewhat stronger than N-DMBI-H.

Photoreduction of carbon dioxide (CO2) in gas phase was investigated over composites of Cu2O nanoparticles (CNPs) and anodized TiO2 nanotube arrays (TNAs). CNPs with an average size of 90 nm were pulse-electrodeposited on vertically aligned hollow TiO2 nanotubes having average pore size of 90 nm. Radical trapping experiments with electron spin resonance spectroscopy suggest that photoinduced charge separation occurs via direct Z-scheme mechanism in Cu2O-loaded TNA (CNP/TNA) sample. Successively, CO2 photoreduction for CNP/TNA from water/CO2 mixture was monitored using a quadrupole mass analyzer in high vacuum, where the partial pressures of intermediate and final reaction species were measured in real time under ultraviolet-visible (UV–VIS) light irradiation (300–600 nm). The photoreduction of CO2 on CNP/TNA happens through preferred generation of formaldehyde (HCHO) and methanol (CH3OH), while platinum-loaded TNA (Pt/TNA) produced methane (CH4) and hydrogen (H2) instead of HCHO and CH3OH. Carbon monoxide (CO) formation was commonly observed for both CNP/TNA and Pt/TNA specimens. These results reveal that CO2 photoreduction occurs through hydrogenation in gas phase over CNP/TNA even in high vacuum, although CO2 deoxygenation to CH4 is conventionally dominant for gas phase reactions.

The production of energy sources by metal-free photocatalysts based on graphitic carbon nitride (g-C3N4) has garnered substantial attention. In this study, nitrogen-rich carbon nitride (C3N5) was successfully synthesized through the thermal polycondensation of 3-amino-1,2,4-triazole. The structural and physical characterization has suggested that a portion of the triazine rings, which constitute the structural framework of g-C3N4, may be substituted with five-membered rings in C3N5. Furthermore, the polymerization of C3N5 proceeded more extensively than that of g-C3N4 from melamine precursors. The increased nitrogen content in C3N5 resulted in a heightened number of π-electrons and a narrowed energy bandgap, with the potential of the valence band maximum being negatively shifted. Additionally, photocatalytic assessments encompassing nitro blue tetrazolium reduction, H2 production from triethanolamine aqueous solution, and CO2 reduction in the liquid phase were performed. All findings demonstrated that C3N5 exhibits significantly superior photocatalytic properties compared to g-C3N4. It is particularly noteworthy that C3N5 selectively generates methanol and H2 from oversaturated CO2 solutions under visible light irradiation, while g-C3N4 selectively generates formaldehyde. These outcomes strongly indicate that C3N5 serves as a metal-free, visible-light-responsive photocatalyst, capable of contributing to both the production of renewable energy sources and the reduction of greenhouse effect gases.

The performance of perovskite solar cells (PSCs) has been greatly influenced by the surface morphology and orientational growth of organometal halide perovskite, which can be controlled by buffer layers located underneath the perovskite layer. In this study, organic semiconductors such as rubrene and pentacene were selected as the buffer layer materials. We deposited CH3NH3PbI3 (MAPbI3) layers by the laser evaporation method onto the rubrene/pentacene bilayer, pentacene single layer, and rubrene single layer, respectively. The MAPbI3-based solar cell with the rubrene/pentacene bilayer showed a better cell performance compared to other PSCs with rubrene and pentacene single buffer layers. The better PSC performance can be presumably attributed to an orientational growth behavior and a smoother surface of MAPbI3 thin film on the rubrene/pentacene bilayer, as well as a more efficient hole transport in the organic bilayer.

Fast carrier dynamics in organic thin-film transistors (OTFTs) was investigated by time-resolved electrostatic force microscopy (tr-EFM). We found that the carrier diffusion in the OTFTs proceeded in two stages: fast diffusion and slow diffusion. By applying the instantaneous frequency method to EFM, the temporal evolution of the spatial distribution of fast carriers in the channel region of the OTFTs, which took place on the timescale of several hundreds of nanoseconds, was evaluated. The inhomogeneous distribution of the local decay time constant showed that the carrier diffusion of the OTFTs was limited by the grain boundaries between each crystalline region. The quantitative capability of the method was verified by comparing the values of the carrier mobility estimated by the tr-EFM measurement and a numerical simulation. The mobility estimated from the experiment and the simulation showed good agreement, showing the possibility of the tr-EFM to evaluate the time evolution of dynamic phenomena in semiconductor devices.

Electrochemically synthesized composites of vertically aligned titanium dioxide (TiO2) nanotube arrays (TNAs) and cuprous oxide (Cu2O) nanoparticles (CNPs) are used for studying gas phase CO2 photoreduction behaviors. Anodized TNA surfaces with an average aperture size of 60 nm are decorated with CNPs using galvanostatic pulse electrodeposition. The nucleation and growth of CNPs are investigated with the help of cyclic voltammetry and potential-time transients. The number of CNPs and their distribution on TNA surfaces are widely altered by adjusting the ON/OFF time, the number of applied current pulse, and the bath temperature. After characterizing structural and physical properties of the prepared CNPs/TNAs samples, in situ observation of CO2 photoreduction in gas phase over CNPs/TNAs is carried out in a high vacuum. The enhancement in CO2 photoreduction over CNPs/TNAs samples is observed for the optimized size and the number of CNPs on TNAs. The reaction route of the same is ascertained from the reaction products. The experimental results indicate that the size of CNPs should be comparable to the average pore size of TNAs for promoting CO2 photoreduction, and the relationship between CO2 photoreduction and the structural properties of CNPs is further discussed.

TiO2 nanotube arrays (NTA) have attracted much attention among photocatalysts because of their large specific surface area and easy surface transfer of excited electrons, and in recent years, attempts have been made to further improve their properties by forming Z-schemes when they are composited with other photocatalysts. However, as the spacing within and between nanotubes is only a few nanometers, the formation of heterojunctions is extremely difficult when TiO2-NTA is composited with other photocatalytic materials with larger grain sizes. Creating nanoparticle photocatalysts with dimensions smaller than those of the nanotube system is thus required to effectively form heterojunctions. We have constructed an original vacuum chemical vapor deposition (CVD) system with fine temperature control, an attribute that we believe is necessary for the preparation of small nanoparticles. Using this system, it is possible to greatly reduce the polymerization rate of melamine, the precursor of the carbon nitride (g-C3N4) photocatalyst, which offers the benefits of increased reduction power and a metal-free composition. As a result, g-C3N4 small nanoparticles with particle sizes of about 10 nm were successfully prepared, and heterojunctions could be formed even inside TiO2-NTA. The fabricated TiO2-NTA/g-C3N4 composite structure exhibited significantly improved redox power and photocatalytic hydrogen production compared to TiO2-NTA and g-C3N4 alone. In addition, while the hydrogen production rates for TiO2-NTA and g-C3N4 were almost constant, TiO2-NTA/g-C3N4 showed a rapid increase in the hydrogen production rate after a certain period of light irradiation, which was presumably caused by oxygen desorption from g-C3N4. The results of this study provide a method for supporting small nanoparticle materials on nanotube substrates and their importance in improving photocatalytic properties, and will also make a significant contribution not only to the field of photocatalysis but also to other fields requiring small nanoparticle materials.

Optimizing the underlayer used as an active layer in perovskite solar cells is important for improving their cell performance. We previously demonstrated the usefulness of a rubrene/pentacene bilayer as an underlayer in CH3NH3PbI3 (MAPbI3) cells prepared by alternative vapor deposition of PbI2 and CH3NH3I (MAI). In the present work, to examine the applicability of this rubrene/pentacene bilayer for the deposition of MAPbI3 via a new method involving immersing a PbI2 evaporated film into an MAI solution is used to prepare MAPbI3 films; this method is referred to as a two-step method. Adjustment of the parameters of the two-step method used to prepare MAPbI3 films on rubrene/pentacene bilayers led to cells with a higher power conversion efficiency compared with that of cells with MAPbI3 films deposited directly onto poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), without rubrene/pentacene bilayers. The rubrene/pentacene presumably promotes the suppression of recombination at the interface between MAPbI3 and the hole transport layer.

Post-annealing effects in low-voltage organic thin-film transistors (TFTs) based on vacuum-deposited films of the liquid-crystalline small-molecule semiconductor 3,10-didecylnaphtho[2,1-b]naphtho[1′,2′:4,5]thieno[2,3-d]thiophene (S-DNTT-10) were investigated. Bottom-contact (BC) and top-contact (TC) TFTs with gold source and drain contacts were prepared by using a high-capacitance gate dielectric based on aluminum oxide and an alkylphosphonic acid self-assembled monolayer. During S-DNTT-10 deposition, the substrate was at room temperature. In BC devices, post-annealing at a temperature of 160 °C leads to a reduction in the number of three-dimensional aggregates on the surface and to a larger grain size of the S-DNTT-10 films, leading to smaller contact resistance, larger intrinsic channel mobility, and better long-term stability in comparison with the TFTs prepared without post-annealing. In TC devices, post-annealing performed before the deposition of the source/drain contacts is found to benefit the initial TFT performance, while post-annealing performed after the source/drain deposition turns out to be better for the long-term stability of the TFTs, presumably due to the fact that the thermally induced morphological changes in the S-DNTT-10 thin films are affected by the presence of the metal contacts (or lack thereof). This study suggests that post-annealing can be of great use for improving the characteristics of organic TFTs based on vacuum-deposited liquid-crystalline organic semiconductors.

In organic thin-film transistors (OTFTs), carrier doping is a versatile technique to control and improve their device characteristics. In this study, bottom-gate, bottom-contact (BGBC) p-channel transistors comprised by contact electrodes of highly p-doped layer (p+) and a non-doped, intrinsic semiconductor layer (i), so-called p+-i-p+ type transistors, were newly fabricated by using the transfer printing of poly(3-hexylthiophene-2,5-diyl) (P3HT) films. A uniform P3HT thin layer was formed onto a poly(dimethylsiloxane) (PDMS) elastomer by the push coating method, followed by the peel-off and transfer process of P3HT micropatterns via elastomer stamping with the aid of prepatterned master substrates. Source-drain p+ layers were prepared by liquid-phase doping of the P3HT micropatterns with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) as an acceptor dopant. Then, undoped i stripes were deposited onto the p+ contact electrode patterns by the transfer printing, which enabled us to construct p+-i-p+ type polymer transistors with hydrophobic CYTOP-coated SiO2 gate insulators. The prepared devices presented p-channel, normally-off operation with no gate-sweep hysteresis, suggesting that the appropriate combination of printing and molecular doping techniques can pave the way toward the device design of all-organic transistors.

Thermal transport properties in acceptor-doped thiophene-based polymer films such as poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) are investigated using the cross-plane 3-omega method. A bilayer structure consisting of polymer semiconductors sequentially doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and poly(methylmethacrylate) (PMMA) on a Si substrate is originally prepared by spin coating and employed to prevent leakage current from a heater wire during the 3-omega measurements. Nondoped PBTTT thin film reveals an intrinsic cross-plane thermal conductivity of 0.49 W m−1 K−1, which is higher than that for nondoped P3HT film (0.20 W m−1 K−1). X-ray diffraction (XRD) analysis suggests that higher crystallinity of PBTTT films may result in higher thermal conductivity. The intrinsic thermal conductivity of P3HT and the interfacial thermal resistance between PMMA and P3HT are slightly increased at a low doping concentration (0.01 mg mL−1) and decreased at a higher doping concentration of 0.1 mg mL−1, whereas F4TCNQ doping reduces these values monotonously in the case of PBTTT layers. Furthermore, excess doping occurs only for P3HT and increments both intrinsic thermal conductivity and thermal resistance at PMMA/P3HT interface. The observed trend in the cross-plane thermal transport caused by the sequential doping can be attributed to the differences between P3HT and PBTTT in their crystallinity and effective doping levels.

Visible light-assisted hydrogen (H2) generation was attempted for tungsten trioxide (WO3) nanorods (NRs) loaded with platinum (Pt). WO3 NRs were synthesized with a simple hydrothermal method using sodium tungstate as a precursor and oxalic acid as a structure-directing reagent. A variety of structural analyses clarify that both the end parts of a single hexagonal WO3 NR are transformed into triclinic WO3 when oxalic acid is added during the NR synthesis. The WO3 NR synthesized without oxalic acid (WO3(0)) displayed site-selective photodeposition of Pt cocatalysts on the (1 0 0) plane, whereas the Pt photodeposition occurred preferentially at the end (triclinic) parts of the WO3 NR synthesized with 0.05 M of oxalic acid (WO3(0.05)). These site-selective Pt deposition allows us to consider an inversion of the redox reaction field in WO3(0) and WO3(0.05) sample. Pt-loaded WO3(0.05) showed better photocatalytic activity ascertained in both methylene blue decomposition and H2 generation from water and methanol mixture, which can be brought about by a direct Z-scheme formed at heterojunctions between hexagonal and triclinic WO3 in a single NR. Finally, photoassisted H2 generation from water including ammonia borane with Pt-loaded WO3(0.05) was demonstrated, where a H2 generation rate under visible light irradiation was about 2.7 times higher than that in the dark. This result suggests a new strategy of efficient H2 generation based on synergetic effects of the WO3 NR photocatalysis and the Pt catalysis.

Gas phase photocatalysis over composites of electrochemically anodized titania nanotube (TNT) arrays and graphitic carbon nitride (g-C3N4) nanoparticles were examined with a home-made, gas-circulating reactor system. g-C3N4 nanoparticles were deposited onto anodized TNT surfaces by facile thermal polymerization using melamine and urea as precursors. The existence of g-C3N4 on TNT arrays clearly improved hydrogen production from gas phase photocatalytic decomposition of water/methanol mixture under visible light (VIS) irradiation (370-800 nm). The enhanced production of hydroxyl radicals via water photooxidation for TNT/g-C3N4 composites was also ascertained by photoluminescence measurement using terephthalic acid as a radical trap. These results deriving from synergetic effects of TNT and g-C3N4 can be attributed to the photogenerated hole-electron separation enhanced at TNT/g-C3N4 interface, suggesting that g-C3N4 is a metal-free sensitizer for enabling VIS responsive photocatalysis in anodized TNTs even for gas phase reactions.