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Development of highly efficient and robust electrocatalysts for oxygen evolution reaction (OER) under specific electrolyte is a key to actualize commercial low-temperature water electrolyzers. Herein, a rational catalyst design strategy is first reported based on amorphous-crystalline (a-c) interfacial engineering to achieve high catalytic activity and durability under diverse electrolytes that can be used for all types of low-temperature water electrolysis. Abundant a-c interface (ACI) is implemented into a hollow nanocubic (pre)-electrocatalyst which is derived from Ir-doped Ni-Fe-Zn Prussian blue analogues (PBA). The implemented c-a interface is well maintained during prolonged OER in alkaline, alkalized saline, and acidic electrolytes demonstrating its diverse functionality for water electrolysis. Notably, the final catalyst exhibits superior catalytic activity with excellent durability for OER compared to that of benchmark IrO2 catalyst, regardless of chemical environment of electrolytes. Hence, this work can be an instructive guidance for developing the ACI engineered electroctalyst which can be diversely used for different types of low-temperature electrolyzers.
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This paper presents the successful synthesis of AgInS2nanocrystals (NCs) double-shelled with GaSxand ZnS for emitting bright and narrow excitonic luminescence from AgInS2core NCs. Additionally, the AgInS2/GaSx/ZnS NCs with a core/double-shell structure have demonstrated high chemical and photochemical stability. The AgInS2/GaSx/ZnS NCs were prepared via three steps: (i) synthesis of AgInS2core NCs by solvothermal method at 200 °C for 30 min, (ii) shelling GaSxon AgInS2core NCs at 280 °C for 60 min to produce the AgInS2/GaSxcore/shell structure, and (iii) the outermost ZnS shelling at 140 °C for 10 min. The synthesized NCs were characterized in detail by using appropriate techniques such as x-ray diffraction, transmission electron microscopy, and optical spectroscopies. The luminescence evolution of the synthesized NCs is as follows: from the broad spectrum (peaking at 756 nm) of the AgInS2core NCs to become the narrow excitonic emission (at 575 nm) prominent beside the broad one after shelling with GaSx, then only the bright excitonic luminescence (at 575 nm) without broad emission after double-shelling with GaSx/ZnS. The double-shell has made the AgInS2/GaSx/ZnS NCs not only remarkably enhance their luminescence quantum yield (QY) up to â¼60% but also maintain the narrow excitonic emission stably for a long-term storage over 12 months. The outermost ZnS shell is believed to play a key role in enhancing QY and protecting AgInS2and AgInS2/GaSxfrom certain damage.
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This paper presents for the first time the systematic synthesis of AgInS2(AIS) nanocrystals (NCs) with different sizes of 2.6-6.8 nm just by controlling only the reaction temperature. The synthesis of AIS core NCs was carried out in 2 steps: (i) synthesis of Ag2S NCs and then (ii) partial exchange of Ag+with In3+in the template Ag2S NCs. For step (i), Ag2S NCs of different sizes were synthesized by reaction of the Ag and S precursors at different temperatures of 30 °C to 130 °C, for the same reaction time of 30 min. For step (ii), AIS NCs were created by the exchange of Ag+with In3+at 120 °C for 60 min. Finally, GaSxwas shelled on AIS core NCs to produce the AgInS2/GaSxcore/shell structures. The synthesized AIS/GaSxNCs demonstrate the clear excitonic absorptions and strong, narrow excitonic luminescence peaking at 530-606 nm depending on the size of AIS core NCs.
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Transition metal (TM)-based carbon hybrids have numerous applications in the field of regenerative electrochemical energy. The synergetic effects of high conductivity of carbon supports and abundant catalytic active sites in TMs make these hybrids promising oxygen evolution reaction (OER) electrocatalysts. However, strategies for modulating the catalytic active species in the above hybrids are limited despite being highly sought after. Furthermore, the exact roles of chemical species in the hybrids (e.g., N, C, or TM) mainly responsible for this high OER performance remain unknown. Herein, an innovative approach based on atomic layer deposition is developed to tune the true active species in Co nanoparticle/N-doped carbon nanotube (Co/N-CNT) hybrids. Specifically, the configuration predominantly promoting water oxidation in an alkaline medium is identified as pyridinic N-Co-C. Furthermore, a physicochemical intact interface between metallic Co nanoparticles and conductive N-CNTs is demonstrated to induce synergetic effects for accelerating charge transfer and enhancing electrocatalytic activity as well as stability in the hybrid catalysts. The optimized hybrid catalyst is revealed to exhibit outstanding alkaline OER activity and stability, outperforming RuO2 , a benchmark novel OER electrocatalyst.
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The pulsed laser fragmentation in liquid (PLFL) process is a promising technique for the synthesis of carbon-based functional materials. In particular, there has been considerable attention on graphene quantum dots (GQDs) derived from multiwalled carbon nanotubes (MWCNTs) by the PLFL process, owing to the low cost and rapid processing time involved. However, a fundamental deep understanding of the formation of GQDs from MWCNTs by PLFL has still not been achieved despite the high demand. In this work, a mechanism for the formation of GQDs from MWCNTs by the PLFL process is reported, through the combination of experimental and theoretical studies. Both the experimental and computational results demonstrate that the formation of GQDs strongly depends on the pulse laser energy. Both methods demonstrate that the critical energy point, where a plasma plume is generated on the surface of the MWCNTs, should be precisely maintained to produce GQDs; otherwise, an amorphous carbon structure is favorably formed from the scattered carbons.
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Dielectric spectroscopy was performed on a Nb and In co-doped rutile TiO2 nano-crystalline ceramic (n-NITO) synthesized by a low-temperature spark plasma sintering (SPS) technique. The dielectric properties of the n-NITO were not largely affected by the metal electrode contacts. Huge dielectric relaxation was observed at a very low temperature below 35 K. Both the activation energy and relaxation time suggested that the electronic hopping motion is the underlying mechanism responsible for the colossal dielectric permittivity (CP) and its relaxation, instead of the internal barrier layer effect or a dipolar relaxation. With Havriliak-Negami (H-N) fitting, a relaxation time with a large distribution of dielectric relaxations was revealed. The broad distributed relaxation phenomena indicated that Nb and In were involved, controlling the dielectric relaxation by modifying the polarization mechanism and localized states. The associated distribution function is calculated and presented. The frequency-dependent a.c. conductance is successfully explained by a hopping conduction model of the localized electrons with the distribution function. It is demonstrated that the dielectric relaxation is strongly correlated with the hopping electrons in the localized states. The CP in SPS n-NITO is then ascribed to a hopping polarization.
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Titanium dioxide (TiO2) with exposed (001) facets (TiO2(001)) has attractive photocatalytic properties. However, the high recombination rate of the photo-excited charge carriers on this surface often limits its application. Here, we report that a few-layered 1T-MoS2 coating on TiO2(001) nanosheets (abbreviated as MST) can be a promising candidate that overcomes some of the challenges of TiO2(001). Computational and experimental results demonstrate that MST as a photocatalyst exhibits a significantly low-charge recombination rate as well as excellent long-term durability. The synthesized MST 2D nanocomposites show a 31.9% increase in photocatalytic activity for hydrogen (H2) production relative to the counterpart TiO2(001). MST offers a new route for further improvement of the photocatalytic activity of TiO2 with exposed high energy facets.
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Nb and In co-doped rutile TiO2 nanoceramics (n-NITO) were successfully synthesized through a chemical-solution route combined with a low temperature spark plasma sintering (SPS) technique. The particle morphology and the microstructure of n-NITO compounds were nanometric in size. Various techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric (TG)/differential thermal analysis (DTA), Fourier transform infrared (FTIR), and Raman spectroscopy were used for the structural and compositional characterization of the synthesized compound. The results indicated that the as-synthesized n-NITO oxalate as well as sintered ceramic have a co-doped single phase of titanyl oxalate and rutile TiO2, respectively. Broadband impedance spectroscopy revealed that novel colossal permittivity (CP) was achieved in n-NITO ceramics exhibiting excellent temperature-frequency stable CP (up to 10(4)) as well as low dielectric loss (â¼5%). Most importantly, detailed impedance data analyses of n-NITO compared to microcrystalline NITO (µ-NITO) demonstrated that the origin of CP in NITO bulk nanoceramics might be related with the pinned electrons in defect clusters and not to extrinsic interfacial effects.
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Titanium dioxide (TiO2) is a widely studied material with many attractive properties such as its photocatalytic features. However, its commercial use is limited due to issues such as deactivation in the visible spectrum caused by its wide bandgap and the short lifetime of photo-excited charge carriers. To overcome these challenges, various modifications could be considered. In this study, we investigated copper doping and electron beam treatment. As-spun TiO2 nanofibers were fabricated by electrospinning a TiO2 sol, which obtained viscosity through a polyvinylpyrrolidone (PVP) matrix. Cu-doped TiO2 nanofibers with varying dopant concentrations were synthesized by adding copper salts. Then, the as-spun nanofibers were calcined for crystallization. To evaluate photocatalytic performance, a photodegradation test of methylene blue aqueous solution was performed for 6 h. Methylene blue concentration was measured over time using UV-Vis spectroscopy. The results showed that Cu doping at an appropriate concentration and electron-beam irradiation showed improved photocatalytic efficiency compared to bare TiO2 nanofibers. When the molar ratio of Cu/Ti was 0.05%, photodegradation rate was highest, which was 10.39% higher than that of bare TiO2. As a result of additional electron-beam treatment of this sample, photocatalytic efficiency improved up to 8.93% compared to samples without electron-beam treatment.
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Alkaline water electrolysis is a vital technology for sustainable and efficient hydrogen production. However, the oxygen evolution reaction (OER) at the anode suffers from sluggish kinetics, requiring overpotential. Precious metal-based electrocatalysts are commonly used but face limitations in cost and availability. Carbon nanostructures, such as carbon nanotubes (CNTs), offer promising alternatives due to their abundant active sites and efficient charge-transfer properties. Surface modification of CNTs through techniques such as pulsed laser ablation in liquid media (PLAL) can enhance their catalytic performance. In this study, we investigate the role of surface-modified carbon (SMC) as a support to increase the active sites of transition metal-based electrocatalysts and its impact on electrocatalytic performance for the OER. We focus on Co3O4@SMC heterostructures, where an ultrathin layer of Co3O4 is deposited onto SMCs using a combination of PLAL and atomic layer deposition. A comparative analysis with aggregated Co3O4 and Co3O4@pristine CNTs reveals the superior OER performance of Co3O4@SMC. The optimized Co3O4@SMC exhibits a 25.6% reduction in overpotential, a lower Tafel slope, and a significantly higher turnover frequency (TOF) in alkaline water splitting. The experimental results, combined with density functional theory (DFT) calculations, indicate that these improvements can be attributed to the high electrocatalytic activity of Co3O4 as active sites achieved through the homogeneous distribution on SMCs. The experimental methodology, morphology, composition, and their correlation with activity and stability of Co3O4@SMC for the OER in alkaline media are discussed in detail. This study contributes to the understanding of SMC-based heterostructures and their potential for enhancing electrocatalytic performance in alkaline water electrolysis.
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Proton-exchange-membrane water electrolysis (PEMWE) requires an efficient and durable bifunctional electrocatalyst for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, Ir-based electrocatalyst is designed using the high entropy alloy (HEA) platform of ZnNiCoIrX with two elements (X: Fe and Mn). A facile dealloying in the vacuum system enables the construction of a nanoporous structure with high crystallinity using Zn as a sacrificial element. Especially, Mn incorporation into HEAs tailors the electronic structure of the Ir site, resulting in the d-band center being far away from the Fermi level. Downshifting of the d-band center weakens the adsorption energy with reaction intermediates, which is beneficial for catalytic reactions. Despite low Ir content, ZnNiCoIrMn delivers only 50 mV overpotential for HER at -50 mA cm-2 and 237 mV overpotential for the OER at 10 mA cm-2 . Furthermore, ZnNiCoIrMn shows almost constant voltage for the HER and OER for 100 h and a high stability number of 3.4 × 105 nhydrogen nIr -1 and 2.4 × 105 noxygen nIr -1 , demonstrating the exceptional durability of the HEA platform. The compositional engineering of ZnNiCoIrMn limits the diffusion of elements by high entropy effects and simultaneously tailors the electronic structure of active Ir sites, resulting in the modified cohesive and adsorption energies, all of which can suppress the dissolution of elements.
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Cobalt-promoted molybdenum sulfide (CoMoS) is known as a promising catalyst for H2 evolution reaction and hydrogen desulfurization reaction. This material exhibits superior catalytic activity as compared to its pristine molybdenum sulfide counterpart. However, revealing the actual structure of cobalt-promoted molybdenum sulfide as well as the plausible contribution of a cobalt promoter is still challenging, especially when the material has an amorphous nature. Herein, we report, for the first time, on the use of positron annihilation spectroscopy (PAS), being a nondestructive nuclear radiation-based method, to visualize the position of a Co promoter within the structure of MoS at the atomic scale, which is inaccessible by conventional characterization tools. It is found that at low concentrations, a Co atom occupies preferably the Mo-vacancies, thus generating the ternary phase CoMoS whose structure is composed of a Co-S-Mo building block. Increasing the Co concentration, e.g., a Co/Mo molar ratio of higher than 1.12/1, leads to the occupation of both Mo-vacancies and S-vacancies by Co. In this case, secondary phases such as MoS and CoS are also produced together with the CoMoS one. Combining the PAS and electrochemical analyses, we highlight the important contribution of a Co promoter to enhancing the catalytic H2 evolution activity. Having more Co promoter in the Mo-vacancies promotes the H2 evolution rate, whereas having Co in the S-vacancies causes a drop in H2 evolution ability. Furthermore, the occupation of Co to the S-vacancies leads also to the destabilization of the CoMoS catalyst, resulting in a rapid degradation of catalytic activity.
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Oxygen evolution reaction (OER) under acidic conditions becomes of significant importance for the practical use of a proton exchange membrane (PEM) water electrolyzer. In particular, maximizing the mass activity of iridium (Ir) is one of the maiden issues. Herein, the authors discover that the Ir-doped calcium copper titanate (CaCu3Ti4O12, CCTO) perovskite exhibits ultrahigh mass activity up to 1000 A gIr -1 for the acidic OER, which is 66 times higher than that of the benchmark catalyst, IrO2 . By substituting Ti with Ir in CCTO, metal-oxygen (M-O) covalency can be significantly increased leading to the reduced energy barrier for charge transfer. Further, highly polarizable CCTO perovskite referred to as "colossal dielectric", possesses low defect formation energy for oxygen vacancy inducing a high number of oxygen vacancies in Ir-doped CCTO (Ir-CCTO). Electron transfer occurs from the oxygen vacancies and Ti to the substituted Ir consequentially resulting in the electron-rich Ir and -deficient Ti sites. Thus, favorable adsorptions of oxygen intermediates can take place at Ti sites while the Ir ensures efficient charge supplies during OER, taking a top position of the volcano plot. Simultaneously, the introduced Ir dopants form nanoclusters at the surface of Ir-CCTO, which can boost catalytic activity for the acidic OER.
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With respect to the detection of Fe3+ ions, graphene quantum dots (GQDs) have limitations for commercialization owing to their high limit of detection (LOD). Here, we report a one-step pulsed laser ablation (PLA) process to fabricate amino-functionalized GQDs (FGQDs) for the efficient detection of Fe3+ using polypyrrole (PPy) both as a precursor (amine N) and as a surfactant and also using graphite as a carbon precursor. Using this method, the amine N groups were easily incorporated into the carbon network of the GQDs. Additionally, compared to pristine GQDs, FGQDs showed smaller particle sizes and narrower size distributions owing to the surface passivation effects of the PPy surfactant. Due to the synergistic effect of surface passivation and incorporation of amine N groups, FGQDs exhibited a sensitive response to Fe3+ ions in the concentration range of 500 nM to 50 µM, which is lower than the quality standards for Fe3+ ions (â¼5.36 µM) as suggested by the World Health Organization (WHO). Furthermore, the processing time for synthesizing FGQDs by the PLA process was less than 30 min, thus allowing successful practical applications of GQDs in the sensing field.
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The superior chemical and electrical properties of TiO2 are considered to be suitable material for various applications, such as photoelectrodes, photocatalysts, and semiconductor gas sensors; however, it is difficult to commercialize the applications due to their low photoelectric conversion efficiency. Various solutions have been suggested and among them, the increase of active sites through surface modification is one of the most studied methods. A porous nanostructure with a large specific surface area is an attractive solution to increasing active sites, and in the electrospinning process, mesoporous nanofibers can be obtained by controlling the composition of the precursor solution. This study successfully carried out surface modification of TiO2 nanofibers by mixing polyvinylpyrrolidone with different molecular weights and using diisopropyl azodicarboxylate (DIPA). The morphology and crystallographic properties of the TiO2 samples were analyzed using a field emission electron microscope and X-ray diffraction method. The specific surface area and pore properties of the nanofiber samples were compared using the Brunauer-Emmett-Teller method. The TiO2 nanofibers fabricated by the precursor with K-30 polyvinyl pyrrolidone and diisopropyl azodicarboxylate were more porous than the TiO2 nanofibers without them. The modified nanofibers with K-30 and DIPA had a photocatalytic efficiency of 150% compared to TiO2 nanofibers. Their X-ray diffraction patterns revealed anatase peaks. The average crystallite size of the modified nanofibers was calculated to be 6.27-9.27 nm, and the specific surface area was 23.5-27.4 m2/g, which was more than 150% larger than the 17.2 m2/g of ordinary TiO2 nanofibers.
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Exploring bifunctional electrocatalysts to lower the activation energy barriers for sluggish electrochemical reactions for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are of great importance in achieving lower energy consumption and higher conversion efficiency for future energy conversion and storage system. Despite the excellent performance of precious metal-based electrocatalysts for OER and ORR, their high cost and scarcity hamper their large-scale industrial application. As alternatives to precious metal-based electrocatalysts, the development of earth-abundant and efficient catalysts with excellent electrocatalytic performance in both the OER and the ORR is urgently required. Herein, we report a core-shell CoFeS2@CoS2 heterostructure entangled with carbon nanotubes as an efficient bifunctional electrocatalyst for both the OER and the ORR. The CoFeS2@CoS2 nanocubes entangled with carbon nanotubes show superior electrochemical performance for both the OER and the ORR: a potential of 1.5 V (vs. RHE) at a current density of 10 mA cm-2 for the OER in alkaline medium and an onset potential of 0.976 V for the ORR. This work suggests a processing methodology for the development of the core-shell heterostructures with enhanced bifunctional performance for both the OER and the ORR.
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We investigated the effect of specific surface area on the electrochemical properties of NiCo2O4 (NCO) for glucose detection. NCO nanomaterials with controlled specific surface areas were prepared by additive-assisted hydrothermal synthesis, and self-assembled nanostructures with urchin-, pine-needle-, tremella-, and flower-like morphologies were obtained. The novelty of this method is the systematic control of chemical reaction routes assisted by the addition of different additives during synthesis, which results in the spontaneous formation of various morphologies without any difference in the crystal structure and chemical states of the constituent elements. Such morphological control of NCO nanomaterials leads to considerable changes in the electrochemical performance for glucose detection. Combined with materials characterization, the relationship between the specific surface area and the electrochemical performance is discussed for glucose detection. This work can provide scientific insights for tailoring the surface area of nanostructures, which determines their functionality for potential applications in glucose biosensors.
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Cobalto , Níquel , Níquel/química , Cobalto/química , GlucosaRESUMEN
Thin film-based optical sensors have been attracting increasing interest for use in developing technologies such as biometrics. Multilayered dielectric thin films with different refractive indices have been utilized to modulate the optical properties in specific wavelength bands for spectral selectivity of Thin Film Narrow Bandpass Filters (TFNBFs). Progress in TFNBF design has been made with the incorporation of metallic thin films. Narrower bandwidths with higher transmittance have been achieved in specific spectral bands. In this work, Ti/TiO2/SiO2 based multilayer thin films were prepared using pulsed-DC reactive sputtering. Computer simulations using the Essential Macleod Program allowed the optimal number of layers and thickness of the multilayer thin films to be determined to efficiently tailor the optical path transmitting specific wavelength bands. The addition of Ti metal layers within dielectric (TiO2/SiO2) multilayer thin films significantly changes the cutoff frequency of transmittance at specific wavelengths. Representative 26 multilayer films consisting of Ti, TiO2, and SiO2 show lower transmittance of 10.29% at 400 nm and 10.48% at 680 nm. High transmittance of 80.42% at 485 nm was observed, which is expected to improve the spectral selectivity of the TFNBF. This work provides a contribution to future simulation based design strategy based on experimental thin film engineering for potential industrial development opportunities such as optical biometrics.
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The design of atomically dispersed single atom catalysts (SACs) must consider high metal-atom loading amount, effective confinement, and strong interactions with matrix, which can maximize their catalytic performance. Here reported is a promising method to synthesize SACs on highly conductive multiwall carbon nanotube (MWCNT) supports using pulsed laser confinement (PLC) process in liquid. Atomic cobalt (Co) and phosphorus (P) with a high loading density are homogeneously incorporated on the outer wall of the MWCNT (Co-P SAC MWCNT). Density functional theory (DFT) calculations in combination with systematic control experiments found that the incorporated Co and P adatoms act as an adsorption energy optimizer and a charge transfer promoter, respectively. Hence, favorable kinetics and thermodynamics in Co-P SAC MWCNT can be simultaneously achieved for water oxidation resulting in a superior catalytic performance than the benchmark RuO2 catalyst. Crucially, total processing time for assembling Co-P SAC MWCNT via PLC process is less than 60 min, shedding light on the promising practical applications of our SAC design strategy.
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An amendment to this paper has been published and can be accessed via a link at the top of the paper.