Ultra-Broadband High-Entropy Oxide Absorber Layer for Enhanced Photodetector Performance.

A novel light-absorbing material of high-entropy oxide (HEO) has been synthesized using the hydrothermal method. The HEO has six metals, namely, Fe, Ni, Mn, Cr, Mg, and Cu. The obtained HEO light absorber is demonstrated to show unprecedented broadband absorption, ranging from 310 to 1400 nm. The photodetector having a structure of Ag/HEO/n-Si has been evaluated for its performance. Under the illumination of various light wavelengths, the photodetector exhibits a remarkably wide range of photoresponse from 365 to 1050 nm, giving wide-spectrum photocurrent densities in the order of 1 mA/cm2, a responsibility as high as 3.5 A/W (850 nm), and an external quantum efficiency (EQE) of more than 700% (850 nm), outperforming all of the reported oxide-based photodetectors. The superior device performance is attributed to the excellent light absorbance and EQE of the oxygen vacancy-containing HEO. Moreover, a number of tests, including the abrasion test, temperature endurance, acidic resistance, on-off switching cycling, and 3 dB bandwidth measurement, show the excellent reliability of the obtained HEO-based photodetector.

Continuous Production of Functionalized Graphene Inks by Soft Solution Processing.

The continuous production of high-quality, few-layer graphene nanosheets (GNSs) functionalized with nitrogen-containing groups was achieved via a two-stage reaction method. The initial stage produces few-layer GNSs by utilizing our recently developed glycine-bisulfate ionic complex-assisted electrochemical exfoliation of graphite. The second stage, developed here, uses a radical initiator and nitrogen precursor (azobisisobutyronitrile) under microwave conditions in an aqueous solution for the efficient nitrogen functionalization of the initially formed GNSs. These nitrile radical reactions have great advantages in green chemistry and soft processing. Raman spectra confirm the insertion of nitrogen functional groups into nitrogen-functionalized graphene (N-FG), whose disorder is higher than that of GNSs. X-ray photoelectron spectra confirm the insertion of edge/surface nitrogen functional groups. The insertion of nitrogen functional groups is further confirmed by the enhanced dispersibility of N-FG in dimethyl formamide, ethylene glycol, acetonitrile, and water. Indeed, after the synthesis of N-FG in solution, it is possible to disperse N-FG in these liquid dispersants just by a simple washing-centrifugation separation-dispersion sequence. Therefore, without any drying, milling, and redispersion into liquid again, we can produce N-FG ink with only solution processing. Thus, the present work demonstrates the 'continuous solution processing' of N-FG inks without complicated post-processing conditions. Furthermore, the formation mechanism of N-FG is presented.

High Entropy Oxide (CrMnFeNiMg)3 O4 with Large Compositional Space Shows Long-Term Stability as Cathode in Lithium-Sulfur Batteries.

The repeated formation and irreversible diffusion of liquid-state lithium polysulfides (LiPSs) are the primary challenges in the development of high-energy-density lithium-sulfur battery (LSB). An effective strategy to alleviate the resulting polysulfide loss is critical for the stability of LSBs. In this regard, high entropy oxides (HEOs) appear as a promising additive for the adsorption and conversion of LiPSs owing to the diverse active sites, offering unparalleled synergistic effects. Herein, we have developed a (CrMnFeNiMg)3 O4 HEO as a functional polysulfide trapper in LSB cathode. The adsorption of LiPSs by the metal species (i. e., Cr, Mn, Fe, Ni, and Mg) in the HEO takes place through two different paths and leads to enhanced electrochemical stability. We demonstrate that the optimal sulfur cathode with the (CrMnFeNiMg)3 O4 HEO attains a high peak and reversible discharge capacities of 857 mAh g-1 and 552 mAh g-1 , respectively, at a cycling rate of C/10, a long cycle life of 300 cycles, and a high rate performance at the cycling rates from C/10 to C/2.

3D hierarchical cobalt vanadate nanosheet arrays on Ni foam coupled with redox additive for enhanced supercapacitor performance.

Room-temperature synthesized 3D hierarchical cobalt vanadate (Co3V2O8) nanosheet arrays on Ni foam for use as supercapacitor electrode is presented. In a 3 M KOH electrolyte, the electrode exhibits a capacitance of 109.9 mA h g-1 (878.9 F g-1) at a current density of 1 A g-1. The capacitance is enhanced to 198.1 mA h g-1 (1584.5 F g-1) at 1 A g-1 through the addition of 0.05 M redox-additive K3[Fe(CN)6] into the KOH electrolyte. Furthermore, the Co3V2O8/activated carbon asymmetric supercapacitor cell with the advanced electrolyte outperforms most reported Co3V2O8-based electrodes with a remarkable energy density of 55.5 W h kg-1 at an 800 W kg-1 power density. Combining a facile synthetic strategy and excellent electrochemical performance, the obtained Co3V2O8 exhibits potential for practical application.

Carbon clusters on substrate surface for graphene growth- theoretical and experimental approach.

Growth morphology of carbon clusters deposited on different substrates were investigated by theoretical and experimental approach. For theoretical approach, molecular dynamics was employed to evaluate an adsorptive stability of different size of carbon clusters placed on different substrates. The adsorptive stability was estimated by the difference of total energy of supercell designed as carbon cluster placed on a certain crystal plane of substrate. Among the simulations of this study, carbon cluster flatly settled down on the surface of SrTiO[Formula: see text](001). The result was experimentally verified with layer by layer growth of graphene by pulsed laser deposition in carbon dioxide atmosphere. The absorptive stability can be useful reference for screening substrate for any target material other than graphene.

Sputter-Deposited High Entropy Alloy Thin Film Electrocatalyst for Enhanced Oxygen Evolution Reaction Performance.

Thin film catalysts, giving a different morphology, provide a significant advantage over catalyst particles for the gas evolution reaction. Taking the advantages of sputter deposition, a high entropy alloy (HEA) thin film electrocatalyst is hereby reported for the oxygen evolution reaction (OER). The catalyst characteristics are investigated not only in its as-deposited state, but also during and after the OER. For comparison, unary, binary, ternary, and quaternary thin film catalysts are prepared and characterized. The surface electronic structure modification due to the addition of a metal is studied experimentally and theoretically using density functional theory calculation. It is demonstrated that sputtered FeNiMoCrAl HEA thin film exhibits OER performance superior to all the reported HEA catalysts with robust electrocatalytic activity having a low overpotential of 220 mV at 10 mA cm-2 , and excellent electrochemical stability at different constant current densities of 10 and 100 mA cm-2 for 50 h. Furthermore, the microstructure transformation is investigated during the OER, which is important for the understanding of the OER mechanism provided by HEA electrocatalyst. Such a finding will contribute to future catalyst design.

Charge-Discharge Mechanism of High-Entropy Co-Free Spinel Oxide Toward Li+ Storage Examined Using Operando Quick-Scanning X-Ray Absorption Spectroscopy.

Transition metal high-entropy oxides (HEOs) are an attractive class of anode materials for high-performance lithium-ion batteries (LIBs). However, owing to the multiple electroactive centers of HEOs, the Li+ storage mechanism is complex and debated in the literature. In this work, operando quick-scanning X-ray absorption spectroscopy (XAS) is used to study the lithiation/delithiation mechanism of the Cobalt-free spinel (CrMnFeNiCu)3 O4 HEO. A monochromator oscillation frequency of 2 Hz is used and 240 spectra are integrated to achieve a 2 min time resolution. High-photon-flux synchrotron radiation is employed to increase the XAS sensitivity. The results indicate that the Cu2+ and Ni2+ cations are reduced to their metallic states during lithiation but their oxidation reactions are less favorable compared to the other elements upon delithiation. The Mn2+/3+ and Fe2+/3+ cations undergo two-step conversion reactions to form metallic phases, with MnO and FeO as the intermediate species, respectively. During delithiation, the oxidation of Mn occurs prior to that of Fe. The Cr3+ cations are reduced to CrO and then Cr0 during lithiation. A relatively large overpotential is required to activate the Cr reoxidation reaction. The Cr3+ cations are found after delithiation. These results can guide the material design of HEOs for improving LIB performance.

A New High Entropy Glycerate for High Performance Oxygen Evolution Reaction.

Herein, a new high entropy material is reported, i.e., a noble metal-free high entropy glycerate (HEG), synthesized via a simple solvothermal process. The HEG consists of 5 different metals of Fe, Ni, Co, Cr, and Mn. The unique glycerate structure exhibits an excellent oxygen evolution reaction (OER) activity with a low overpotential of 229 and 278 mV at current densities of 10 and 100 mA cm-2, respectively, in 1 m KOH electrolyte, outperforming its subsystems of binary-, ternary-, and quaternary-metal glycerates. The HEG also shows outstanding stability and durability in the alkaline electrolyte. The result demonstrates the significance of synergistic effect that gives additional freedoms to modify the electronic structure and coordination environment. Moreover, HEG@HEG electrolyzer shows a good overall water splitting performance and durability, requiring a cell voltage of 1.63 V to achieve a current density of 10 mA cm-2.

Exploring the First High-Entropy Thin Film Libraries: Composition Spread-Controlled Crystalline Structure.

Thin films of two types of high-entropy oxides (HEOs) have been deposited on 76.2 mm Si wafers using combinatorial sputter deposition. In one type of the oxides, (MgZnMnCoNi)Ox, all the metals have a stable divalent oxidation state and similar cationic radii. In the second type of oxides, (CrFeMnCoNi)Ox, the metals are more diverse in the atomic radius and valence state, and have good solubility in their sub-binary and ternary oxide systems. The resulting HEO thin films were characterized using several high-throughput analytical techniques. The microstructure, composition, and electrical conductivity obtained on defined grid maps were obtained for the first time across large compositional ranges. The crystalline structure of the films was observed as a function of the metallic elements in the composition spreads, that is, the Mn and Zn in (MgZnMnCoNi)Ox and Mn and Ni in (CrFeMnCoNi)Ox. The (MgZnMnCoNi)Ox sample was observed to form two-phase structures, except single spinel structure was found in (MgZnMnCoNi)Ox over a range of Mn > 12 at. % and Zn < 44 at. %, while (CrFeMnCoNi)Ox was always observed to form two-phase structures. Composition-controlled crystalline structure is not only experimentally demonstrated but also supported by density function theory calculation.

MoS2 /MoOx -Nanostructure-Decorated Activated Carbon Cloth for Enhanced Supercapacitor Performance.

MoS2 /MoOx nanostructures were grown on activated carbon cloth through a facile one-step microwave-assisted hydrothermal method. The growth of MoS2 /MoOx on activated carbon cloth creates a unique structure that favors ion intercalation. The conductive activated carbon cloth, MoO3-x , and monoclinic MoO2 provide fast electron transport, whereas the MoS2 nanosheets/MoO3-x nanoparticles structure improves the capacitance. As a result, MoS2 /MoOx -nanostructure-decorated activated carbon cloth shows a high specific capacitance of 230 F g-1 at a scan rate of 5 mV s-1 with a low contact resistance of approximately 1.91 Ω. Moreover, the activated carbon cloth acts as a template for the growth of a perpendicular MoS2 layer, which gives an excellent utilization rate of the active MoS2 /MoOx material. We also demonstrate that the MoS2 /MoOx /activated carbon cloth nanocomposite shows excellent electrochemical stability with retention up to 128 % after 1500 cycles. Finally, we show the use of a microwave-assisted hydrothermal method for the synthesis of the MoS2 /MoOx /activated carbon cloth nanocomposite as an alternative and clean route to improve the kinetics of the intercalation redox reaction.

A Composite Photocatalyst Based on Hydrothermally-Synthesized Cu2ZnSnS4 Powders.

A novel composite photocatalyst based on Cu2ZnSnS4 (CZTS) powders was synthesized and investigated for use as a photocatalyst. CZTS powders were first made using a conventional hydrothermal method and were then used to grow silver nanoparticles hybridized onto the CZTS under various conditions through a microwave-assisted hydrothermal process. After the obtained samples were subsequently mixed with 1T-2H MoS2, the three synthesized component samples were characterized using X-ray diffractometry (XRD), scanning electron microscopy, transmission electron microscopy (FE-SEM, FE-TEM), UV-visible spectroscopy (UV-Vis), Brunauer-Emmet-Teller (BET), photoluminescence spectroscopy (PL), and X-ray photoelectron spectroscopy (XPS). The resulting samples were also used as photocatalysts for the degradation of methylene blue (MB) under a 300 W halogen lamp simulating sunlight with ~5% UV light. The photodegradation ability was greatly enhanced by the addition of Ag and 1T-2H MoS2. Excellent photodegradation of MB was obtained under visible light. The effects of material characteristics on the photodegradation were investigated and discussed.

Hybridized 1T/2H MoS2 Having Controlled 1T Concentrations and its use in Supercapacitors.

Molybdenum disulfide (MoS2 ) nanoflowers consisting of hybridized 1T/2H phases have been synthesized by using a microwave-assisted hydrothermal (MTH) method. The concentration of the 1T phase, ranging from 40 % to 73 %, is controlled by simply adjusting the ratio of the Mo and S precursors. By using the hybridized 1T/2H MoS2 as an electrode material, it was demonstrated that the resulting supercapacitor performance is dominated by the 1T phase concentration. It was found that a supercapacitor with 73 % 1T phase exhibits excellent capacitance of 259 F g-1 and great cyclic stability after 1000 cycles. The formation mechanism of the MHT-synthesized hybridized 1T/2H MoS2 is also reported. More importantly, the mechanism also explains the observed relationship between the 1T phase concentration and the ratio of the Mo and S precursors.

Direct Growth of MoS2 Nanowalls on Carbon Nanofibers for Use in Supercapacitor.

Direct growth of MoS2 nanowalls on vapor grown carbon nanofibers (VGCNFs) has been achieved using a microwave-assisted hydrothermal (MAH) method under an acidic condition. The acidic condition was obtained through the addition of an HCl aqueous solution. We demonstrate that the HCl not only modifies the pH value for limiting the growth rate but also leads to the formation of NaCl, which is the key for the direct and unique growth of MoS2 on the VGCNF surface. A growth mechanism is therefore proposed. The growth of MoS2 onto the high electrically conducting VGCNF creates a unique structure that not only reduces the aggregation of MoS2 but also improves the electrical conductivity of the resulting composite electrode. The MoS2 nanowall/VGCNF composite shows Csp as high as 248 F g-1 at 5 mV s-1 and excellent electrochemical stability with a retention of 96% after 1,000 cycles at a high charge rate of 200 mV s-1. The ease of composite fabrication and electrochemical stability suggest that the MoS2 nanowall/VGCNF composite is a promising candidate electrode material for supercapacitor.

The effect of silver nanoparticles/graphene-coupled TiO2 beads photocatalyst on the photoconversion efficiency of photoelectrochemical hydrogen production.

In this work, a novel configuration of the photoelectrochemical hydrogen production device is demonstrated. It is based on TiO2 beads as the primary photoanode material with the addition of a heterostructure of silver nanoparticles/graphene. The heterostructure not only caters to a great improvement in light harvesting efficiency (LHE) but also enhances the charge collection efficiency. For LHE, the optimized cell based on TiO2 beads/Ag/graphene shows a 47% gain as compared to the cell having a photoanode of commercial P25 TiO2 powders. For the charge collection efficiency, there is a pronounced improvement of an impressive value of 856%. The reason for the improvement in light absorption is attributed to either the light scattering of TiO2 beads or the surface plasmonic resonance on the Ag nanoparticles/graphene. The photoconversion efficiency (PCE) of the resulting cells is also presented and discussed. The PCE of the TiO2 beads/Ag/graphene cell is approximately 2.5 times than that of pure P25 cell.

Cu2ZnSnS4 absorption layers with controlled phase purity.

We report the synthesis and characterization of Cu2ZnSnS4 (CZTS) with controlled phase purity. The precursor was first prepared using sequential electrodeposition of Cu, Zn, and Sn in different orders. The Cu/(Sn+Zn) ratio in each stacking order was also varied. The precursor was subjected to annealing at 200°C and sulfurization at 500°C in a 5%-H2S/Ar atmosphere for the formation of CZTS. The phase evolutions during the electrodeposition and annealing stages, and the final phase formation at the sulfurization stage were examined using both x-ray diffractometry and Raman spectroscopy, both of which are shown to be complimentary tools for phase identification. Detailed growth path is therefore reported. We also demonstrate by controlling the stacking order and the Cu/(Sn+Zn) ratio, CZTS with a phase purity as high as 93% is obtained.

Continuous production of nitrogen-functionalized graphene nanosheets for catalysis applications.

This study reports the "continuous production" of high-quality, few-layer nitrogen-functionalized graphene nanosheets in aqueous solutions directly from graphite via a two-step method. The initial step utilizes our recently developed peroxide-mediated soft and green electrochemical exfoliation approach for the production of few-layer graphene nanosheets. The subsequent step, developed here, produces nitrogen-functionalized graphene nanosheets via selective alkylation/basic hydrolysis reactions using rather a simple nitrogen precursor bromoacetonitrile, which was never reported in the literature. A possible reaction mechanism of the nitrogen-functionalized graphene formation is proposed. The proposed method allows the quantification of the phenolic and hydroxyl functional groups of anodic few-layer graphene via the derivatization chemistry approach. Additionally, a nitrogen-functionalized graphene-gold nanocrystal hybrid is prepared using gold nanocrystals obtained via the microwave irradiation of H[AuCl4] and trisodium citrate solution. A systematic investigation demonstrates that the nitrogen-functionalized graphene-gold nanocrystal hybrid shows enhanced catalytic reduction of carbonyl compounds such as benzaldehyde.

Formation of beaded vapor grown carbon nanofibers.

Carbon nanofibers having interesting smooth, spherical beads are synthesized using a catalyst assisted vapor phase process under various methane/hydrogen mixtures. Fe(1-x)S(x) powders were used as the catalyst. The synthesis temperatures were 1100 degrees C and 1200 degrees C, which are lower than the ones used previously for the formation of beaded carbon nanofibers or nanotubes. Unlike the high temperature (> or = 1300 degrees C) grown beads that may exhibit a rough surface, all the beads obtained at both 1100 and 1200 degrees C with various methane concentrations have a smooth surface. The diameter and the linear density (no. of beads per unit fiber length) of the beads do not seem to correlate to the growth temperature and the methane concentration. The linear density is also independent of the growth time. However, the beads grow with time. Although the grown mechanism is not clear at the present time, it seems that a three-dimensional nucleation model plays a role.

The use of nanoscaled Al mid-layer for enhancing the electrical conductivity of ZnO.

A nanoscaled Al thin film was placed between two ZnO thin films to form a ZnO/Al/ZnO multilayer thin film structure. Individual Al and ZnO thin films with difference thicknesses were first prepared and characterized for the optical and electrical properties. The multilayer structure was then obtained by depositing individual layers with desired thicknesses in sequence. We show that by appropriate selections of layer thickness, the use of a nanoscaled Al mid-layer in ZnO enhances the electrical conductivity of the ZnO without scarifying its optical transmittance.

Unprecedented re-growth of carbon nanotubes on in situ re-activated catalyst.

A simple-stepped growth process for the synthesis of carbon nanotubes that exhibit excellent field emission properties is reported. In the process, the growth was interrupted, and during the interruption the catalyst was re-activated in situ, resulting in enhanced growth of the CNTs after the interruption. A film of CNTs re-grows on top of an existing CNT film at much higher rates, which can be up to 669% higher. The tubular structure continues during the re-growth. The structural continuity creates an opportunity for the fabrication of junction CNTs for nano-electronic applications. The resulting CNTs also have excellent field emission properties, exhibiting an extremely low turn-on field of 0.10 V microm(-1).

Electrospun ZnO Nanowires as Gas Sensors for Ethanol Detection.

ZnO nanowires were produced using an electrospinning method and used in gas sensors for the detection of ethanol at 220 °C. This electrospinning technique allows the direct placement of ZnO nanowires during their synthesis to bridge the sensor electrodes. An excellent sensitivity of nearly 90% was obtained at a low ethanol concentration of 10 ppm, and the rest obtained at higher ethanol concentrations, up to 600 ppm, all equal to or greater than 90%.