In this study, reduced graphene oxide (RGO) with a hollow nanostructure was successfully synthesized by layer-by-layer self-assembly using electrostatic interactions and van der Waals forces between ...building blocks, and its lithium storage characteristics were investigated. After 800 cycles at a current density of 1 A/g, the microwave-irradiated RGO hollow spheres (MRGO-HS) maintained a capacity of 626 mA h/g. In addition, when the charge/discharge capacity was measured stepwise in the current density range of 0.1-2 A/g, the discharge capacity of the RGO rapidly decreased to 156 mA h/g even at the current density of 2 A/g, whereas MRGO-HS provided a capacity of 252 mA h/g. Even after the current density was restored at a current density of 0.1 A/g, the MRGO-HS capacity was maintained to be 827 mA h/g at the 100th cycle, which is close to the original reversible capacity. Thus, MRGO-HS provides a higher capacity and better rate capability than those of traditionally synthesized RGO.
Ge/RuO2 nanocomposites were successfully fabricated as anode materials for lithium-ion batteries using RuO2 nanosheets and Ge/GeO2 nanoparticles (NPs). X-ray diffraction (XRD) and X-ray absorption ...spectroscopy (XAS) analyses showed that elemental Ge nanoparticles were distributed onto the rutile-type RuO2. Transmission electron microscopy images showed well-dispersed Ge nanoparticles embedded in rutile-type RuO2. The Ge/RuO2 nanocomposite maintained higher discharge capacities (471 mA h g−1) after the 90th cycle at 0.1 A g−1 than that (211 mA h g−1) of Ge/GeO2 nanoparticles. The Ge/RuO2 nanocomposite exhibited a higher capacity retention than Ge/GeO2 NPs. These results suggest that the well-dispersed Ge nanoparticles within RuO2 matrices enhance the cycle stability and capacity retention of the anode material.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Sodium‐ion batteries (SIBs), which are being considered as possible alternatives to lithium‐ion batteries, have attracted great scientific and industrial interest because sodium is inexpensive and ...more abundant than lithium. Among various anode materials for SIBs, Na2Ti3O7, with a layered structure, has a high potential due to its stable electrochemical sodiation/desodiation behavior. However, Na2Ti3O7 has a large band‐gap energy and a zigzag framework, which results in low electrical conductivity and poor sodium‐ion diffusion. Thus, to circumvent such drawbacks, in this study, Na2Ti3O7 was successfully exfoliated by the soft‐chemical exfoliation technique via a two‐step intercalation reaction. Transmission electron microscopy (TEM) studies show that the obtained exfoliated material is composed of thin nanosheets. Electrochemical sodium‐ion storage tests showed that the exfoliated nanosheets had a much higher discharge capacity than bare bulk Na2Ti3O7, revealing that the exfoliation process could be utilized to develop new electrode materials with enhanced energy‐storage properties.
Exfoliation of Na2Ti3O7 into colloidal nanosheet.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Emotion recognition research has been conducted using various physiological signals. In this paper, we propose an efficient photoplethysmogram-based method that fuses the deep features extracted by ...two deep convolutional neural networks and the statistical features selected by Pearson’s correlation technique. A photoplethysmogram (PPG) signal can be easily obtained through many devices, and the procedure for recording this signal is simpler than that for other physiological signals. The normal-to-normal (NN) interval values of heart rate variability (HRV) were utilized to extract the time domain features, and the normalized PPG signal was used to acquire the frequency domain features. Then, we selected features that correlated highly with an emotion through Pearson’s correlation. These statistical features were fused with deep-learning features extracted from a convolutional neural network (CNN). The PPG signal and the NN interval were used as the inputs of the CNN to extract the features, and the total concatenated features were utilized to classify the valence and the arousal, which are the basic parameters of emotion. The Database for Emotion Analysis using Physiological signals (DEAP) was chosen for the experiment, and the results demonstrated that the proposed method achieved a noticeable performance with a short recognition interval.
Silver oxide nanoparticles with controlled particle size were successfully obtained utilizing citrate-intercalated layered double hydroxide (LDH) as a substrate and Ag
as a precursor. The lattice of ...LDH was partially dissolved during the reaction by Ag
. The released hydroxyl and citrate acted as a reactant in crystal growth and a size controlling capping agent, respectively. X-ray diffraction, X-ray photoelectron spectroscopy, and microscopic measurements clearly showed the development of nano-sized silver oxide particles on the LDH surface. The particle size, homogeneity and purity of silver oxide were influenced by the stoichiometric ratio of Ag/Al. At the lowest silver ratio, the particle size was the smallest, while the chemical purity was the highest. X-ray photoelectron spectroscopy and UV-vis spectroscopy results suggested that the high Ag/Al ratio tended to produce silver oxide with a complex silver environment. The small particle size and homogeneous distribution of silver oxide showed advantages in antibacterial efficacy compared with bulk silver oxide. LDH with an appropriate ratio could be utilized as a substrate to grow silver oxide nanoparticles with controlled size with effective antibacterial performance.
Hydrocalumite structured layered double hydroxides (LDHs) with various Fe3+ ratios were prepared through a coprecipitation method. In order to control the Fe3+ content in LDH, binary Ca–Fe LDHs were ...first synthesized with various Ca/Fe ratios. The X-ray diffraction pattern showed that only a limited Ca/Fe ratio resulted in LDH formation. The Fe3+ content in LDH was controlled by applying Al3+ while the divalent and trivalent metal ratio was set to 2. Through X-ray diffraction patterns, ternary LDHs with Ca–Al–Fe composition were successfully synthesized without significant impurities, with the Al increasing crystallinity. Quantification showed that Al moiety participated in the formation of the LDH framework more than Ca and Fe, implying a structural stabilization in the presence of Al. In order to investigate the global and local structure of Fe moiety in the LDH, both solid state UV-vis and X-ray absorption spectroscopies were carried out. Both spectroscopies revealed that the existence of Al induced slight local distortion in coordination but global crystal stabilization.
The stability and postcritical behaviour of a horizontal flag undergoing gravity-induced deformation and periodic contact with a nearby horizontal rigid wall are experimentally investigated. The ...results elucidate the combined effects of gravity and contact on flutter, and reveal design principles for application to triboelectric energy harvesting. By varying the free-stream velocity, flag thickness and distance between the flagpole and the wall, the dynamics of the flag are classified into quasistatic equilibrium, flutter, partial contact and saturated contact modes. Considering the significance of gravitational effects, a new dimensionless flow velocity is proposed to identify the distribution of the dynamic modes, and its definition varies according to whether the wall is placed above or below the flag. The critical conditions for transitions between the dynamic modes are determined from the balance of fluid dynamic and gravitational effects. The distance from the flagpole to the wall is found to be more critical for transitions in the lower-wall configuration than in the upper-wall configuration. The peak contact force as well as the oscillation amplitude and frequency at postequilibrium exhibits remarkably different trends depending on the location of the wall. The peak contact force imposed on the wall by the fluttering flag weakens as the distance to the wall increases in the case of an upper wall, whereas it becomes stronger in the case of a lower wall.
The dynamics of a stack of inverted flags under a free stream are experimentally investigated, in which multiple flags are initially placed parallel to the free stream with no gaps between them. With ...increasing free-stream velocity, the stacked inverted flags transition from an initial straight mode to a quasi-static mode where their leading edges are spread out with significant deflection. An irregular flapping mode then occurs, followed by a collective in-phase flapping mode. Within the regime of the irregular flapping mode, an interesting phenomenon appears: the flags clap periodically near a centerline. By varying the number, height, and thickness of the flags, the critical conditions for the mode transitions are identified. While the critical flow velocity is insensitive to the number of flags in the transition to irregular flapping, it increases with the number of flags in the transition to the in-phase flapping mode. The observation that the in-phase flapping mode of the stacked flags is similar to the large-amplitude flapping mode of a single flag leads us to propose a dimensionless flow velocity, which accounts for the effect of the number of flags, to predict the stability thresholds. Using this dimensionless flow velocity, the dynamic characteristics of in-phase flapping, such as the dimensionless amplitude, frequency, and strain energy, are examined, and these parameters are found to be almost independent of the number of flags.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
While development of a sodium-ion battery (SIB) cathode has been approached by various routes, research on compatible anodes for advanced SIB systems has not been sufficiently addressed. The anode ...materials based on titanium oxide typically show low electrical performances in SIB systems primarily due to their low electrical/ionic conductivity. Thus, in this work, layered titanium oxides were hybridized with covalent organic nanosheets (CONs), which exhibited excellent electrical conductivity, to be used as anodes in SIBs. Moreover, to enlarge the accessible areas for sodium ions, the morphology of the hybrid was formulated in the form of a hollow sphere (HS), leading to the highly enhanced ionic conductivity. This synthesis method was based on the expectation of synergetic effects since titanium oxide provides direct electrostatic sodiation sites that shield organic components and CON supports high electrical and ionic conductivity with polarizable sodiation sites. Therefore, the hybrid shows enhanced and stable electrochemical performances as an anode for up to 2600 charge/discharge cycles compared to the HS without CONs. Furthermore, the best reversible capacities obtained from the hybrid were 426.2 and 108.5 mAh/g at current densities of 100 and 6000 mA/g, which are noteworthy results for the TiO2-based material.
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IJS, KILJ, NUK, PNG, UL, UM
Owing to their unique molecular structure and chemical reactivity, Keggin-Al
13
(AlO
4
Al
12
(OH)
24
(H
2
O)
12
7+
) ions demonstrate versatility in various chemical reactions. Herein, ruthenium ...oxide nanosheets are introduced as a promising host material for the intercalation of Keggin-Al
13
ions with the aim to enhance electrochemical energy storage. Ruthenium oxide, known for its high energy density as an anode material in lithium-ion batteries, faces limitations in terms of cycling stability caused by volume expansion during lithiation. To address these limitations, an approach involving the intercalation of Keggin-Al
13
ions into ruthenium oxide nanosheets is developed. The resulting Al
13
-treated RuO
2
(AR-150), heated at 150 °C, maintained the increased interlayer spacing, compared to that of the pristine layered ruthenium oxide. The AR-150 consisting of restacked nanosheets exhibits a considerably increased pseudocapacitance contribution (83.8% at 0.8 mV s
−1
). In addition, the expanded lamellar structure of AR-150 effectively mitigates volume expansion during repeated lithiation, demonstrating impressive cycling stability. It maintains a reversible capacity of 379.0 mA h g
−1
with a capacity retention of 75.0% after 120 cycles at 100 mA g
−1
. This strategy based on the intercalation chemistry utilizes the unique properties of ruthenium oxide nanosheets to advance their applications in electrochemical energy storage.
Ruthenium oxide nanosheets were used as hosts for the Keggin-Al
13
. The synthesized AR-150 has stable cycling performance as an anode material for lithium-ion batteries.
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