Development of high quantum efficiency CdS/ZnS core/shell structured photocatalyst for the enhanced solar hydrogen evolution

Development of co-catalyst free, core/shell structured photocatalyst with ultra-thin shell is of great importance towards the stable and continuous hydrogen (H₂) production, where the shell prevents photo-corrosion of the core for longer stability with continuous H₂ generation. Accordingly, herein, we report a one-step, surfactant free hydrothermal process for synthesis of high-efficient CdS/ZnS core/shell structured catalyst for H₂ evolution under natural solar light. The structural and morphological characterizations using XRD and TEM techniques revealed the formation of phase pure CdS/ZnS system, with core and shell thickness of 395 and 15 nm, respectively. XPS studies revealed that the constituted elements in system exist in their native oxidation states, which indicated the stable structural integrity of the individual phase in the core/shell structure. The synergistic optical properties of CdS/ZnS showed the absorption edge around 500 nm and the decreased PL intensity indicated the improved charge recombination resistance in the system. The parametric studies such as synthesis time, core diameters and shell thickness optimization were conducted to study the formation kinetics of the core/shell structure and their photocatalytic efficiencies. Accordingly, the optimized core/shell catalyst showed around 763 and 2.4 folds superior activity when compared to the pristine CdS and ZnS, respectively. Further, the catalyst showed excellent stability for over 100 h with quantum efficiency of 8.78% under the irradiation of 20 W LED light at 420 nm. Based on the obtained results, the observed improved photocatalytic quantum efficiency could be ascribed to their synergistic effects of CdS and ZnS towards increased charge separation and spatial distributions of the carriers due to their core/shell configuration of the materials.     

Direct methanol fuel cells using Se/Ru core/shell cathodes provide high catalytic activity and stability

We have synthesized Se/Ru core/shell nanoparticles (NPs) as cathode electrocatalysts. By controlling the concentration of RuCl₃, we obtained Se/Ru_0.2, Se/Ru_0.7, and Se/Ru_1.1 core/shell NPs. The mass activities of as-prepared catalysts were 58.0, 32.1, and 12.5 A/g, respectively; part of these values are higher than that (2.2 A/g) of a commercial PtRu electrode and those (22.8, 28.0, and 24.5 A/g) of traditional carbon-supported Ru_xSe_y electrodes. The Se/Ru_1.1 electrode generated the greatest current density and was more tolerant toward poisoning in the presence of 1 M MeOH; cyclic voltammetry measurements revealed that this electrode was stable for at least 100 cycles. This is the first example demonstrating preparation of Se/Ru core/shell NPs that provided excellent catalytic activity and stability toward oxidation of MeOH. With their ease of large-scale preparation, low cost, high electroactivity, and high stability, such Se/Ru core/shell NPs have great potential for use as cathode electrocatalysts in direct MeOH fuel cells.     

Facile preparation of ZnS/CdS core/shell nanotubes and their enhanced photocatalytic performance

In this paper, ZnS/CdS core/shell nanotubes were successfully synthesized by combining hydrothermal treatment and ion exchange conversion, and the significant influence of CdS content in the shell on photo absorption and photocatalytic activity was also investigated. The core/shell nanotubes structure of CdS deposition on both sides of ZnS nanotube was confirmed by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The room temperature PL spectra of ZnS/CdS core/shell nanotubes indicated that CdS on the shell can reduce the recombination of photon-generated electron and hole. The photocatalytic activity tests prove that ZnS/CdS nanotubes have much higher photocatalytic hydrogen production activity than ZnS nanotube and CdS nanotube. Under the irradiation of visible light, the highest photocatalytic hydrogen production rate of 110 μmol h⁻¹ g⁻¹ is observed over the ZnS/CdS core/shell nanotubes with CdS/ZnS molar ratio of 1:4, which is about 11.02 and 5.56 times more active than ZnS nanotube and CdS nanotube, respectively. The improved performance of ZnS/CdS samples can be due to the strong photo response in the visible light region and the efficient separation of electron–hole pairs.     

Phase separated thermotropic layers based on UV cured acrylate resins - Effect of material formulation on overheating protection properties and application in a solar collector

This paper focuses on the effect of material composition on the overheating protection properties of thermotropic systems with fixed domains for solar thermal collectors. Numerous functional layers were prepared by a variation of base resin (polyester-, epoxy- or urethane-acrylate) and of thermotropic additives (non-polar and polar waxes) as well as by additive concentration (5 and 7 wt%). A detailed investigation of optical properties, switching temperature and switching process was performed applying UV/Vis/NIR spectroscopy. Thermal transitions of both the thermotropic layers and the additives used were determined by Differential Scanning Calorimetry (DSC). The capability of the produced thermotropic layers to reduce stagnation temperatures in an all-polymeric flat plate collector was evaluated by theoretical modeling. The thermotropic layers showed a hemispheric solar transmittance between 76% and 87% in clear state. Above the switching threshold this transmittance changed by 1-16% to values between 62% and 85%. The layers exhibited switching temperatures between 33 and 80 °C. The transition is fully completed within a temperature frame of 10-25 °C. Resin types with higher glass transition temperatures were detected to benefit the reduction of the hemispheric solar transmittance above the switching threshold. This reduction was also found to increase with increasing molecular weight of the non-polar additive types. The comparison of the switching performance with the thermal transitions of the additives revealed a good correlation. Theoretical modeling showed that by the use of selected thermotropic layers in the glazing the maximum absorber temperatures can be limited to temperatures below 130 °C.     

Chicken nuggets-like core/shell nanosheet arrays as high performance flexible all-solid-state supercapacitor cathode and buckwheat shell as anode

The energy density of a flexible all-solid-state supercapacitor (ASC) requires new electrode material with special structure and morphology as a prerequisite for its secured improvement. In this paper, a new morphological exploration of chicken nuggets-like core/shell NiCo₂O₄/MnO₂ (NCM) nanosheet arrays on Ni foam was employed. The application of this special morphology aims to greatly improve the electrochemical performance of the cathode electrode. Additionally, Buckwheat Biochar (BBC) is utilized as the anode while the PVA/KOH thin film is prepared as the separator. The chicken nuggets-like core/shell NCM nanosheet arrays were obtained by a two-step hydrothermal method. A series of characterization methods were carried out to further support the core/shell's well-designed structure and precise composition. The tests exhibited excellent specific capacitance of 593.3 F g^?1 at 5 mA cm^?2 and outstanding cycling stability with a retention of 90% after 10000 cycles. Furthermore, the assembled NCM//BBC ASC device indicated a high specific capacitance (239 F g^?1 at the current density of 5 mA cm^?2), this is in due part of the unique architecture of NCM nanosheet arrays and interconnected special porous structure of the BBC and the thin film PVA/KOH. Hence, the assembled ASC device exhibited high energy density (an energy density of 58 Wh·kg^?1 at 3263 W kg^?1) and remarkable cycling stability.     

Directed electrochemical synthesis of ZnO/PDMcT core/shell nanorod arrays with enhanced photoelectrochemical properties

In this paper, we demonstrate a simple two-step electrochemical deposition strategy for synthesizing ZnO/Poly(2,5-dimercapto-1,3,4-thiadiazole) (PDMcT) core/shell nanorod arrays. The as-synthesized ZnO/PDMcT samples are characterized by Fourier-transform infrared (FTIR), Raman spectroscopy, power X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The ZnO/PDMcT nanorod arrays are found to exhibit significantly enhanced photocurrent density in photoelectrochemical cell applications as compared to the prinstine ZnO nanorod arrays.     

Well-defined core/shell pDA@Ni-MOF heterostructures with photostable polydopamine as electron-transfer-template for efficient photoelectrochemical H2 evolution

Here, novel core/shell polydopamine@Ni-MOF (pDA@Ni-MOF) heterogeneous nanostructures are synthesized via a simple one-pot nucleation-growth technique. This rational core/shell design method provide a uniform Ni-MOF shell thickness (shell: ～ 10 nm) as well as homogeneous wrapping of pDA templates with quite narrow size distributions. The obtained band properties of bare pDA (E_CB = ?0.35 eV and E_VB = 2.95 eV vs normal hydrogen electrode (NHE)) and bare Ni-MOF (E_CB = ?0.49 eV and E_VB = 2.85 eV vs NHE) clearly revealed charge separation is occurred on pDA by absorbing light due to π-π1 transition, and photogenerated electrons on conduction band (CB) of pDA was migrated to CB of Ni-MOF. Specifically, the photoelectrochemical (PEC) water performance of pDA@Ni-MOF photoanodes with highest current density is recorded as 8.61 mA/cm² at 0.77 V vs. RHE under visible LED irradiation, which is significantly higher than bare pDA (0.008 V vs. RHE) and bare Ni-MOF (0.011 V vs. RHE) at the same conditions. Note that, the higher photon absorption properties of pDA in core together with high interaction valence bond between two semiconductors could generate electron rich state giving rise to faster electron transfer kinetics as next generation of MOF based hybrid materials with regular morphologies.     

Photocatalytic hydrogen production using TiO2 coated iron-oxide core shell particles

Photocatalytic water splitting plays a challenging role as it is one of the most important reactions for solving energy, environmental problems and sustainability. Photocatalytic water splitting was improved by using a novel kind of magnetically separable core shell nano photocatalyst TiO₂/Fe₂O₃, prepared by co-precipitation method. It was characterised for particle size (XRD), band gap (UV-DRS), morphology (SEM), particle size (HRTEM), elemental composition (EDS) and electrochemical studies. Photocatalytic splitting of water was examined in tubular reactor of 500 mL capacity with various sacrificial agents viz., methanol, ethanol, acetic acid, lactic acid, EDTA and triethanolamine. To enhance the hydrogen production, various operating parameters viz., effect of sacrificial agents, catalytic dosage, light irradiation and recycle flow rate were optimized. With the optimized operating parameters (0.2 g catalyst dosage, 60 mL/min recycle flow rate, 96 W/m² light irradiation and EDTA as sacrificial agent) the maximum hydrogen achieved was 2700 μmol/h for the quantum yield of 3.86% at 550 nm. The reusability studies were conducted and the TiO₂ coated Fe₂O₃ core shell particles were found to be stable than the plain TiO₂ nano particles. Effective charge transfer from TiO₂ to Fe₂O₃ and the suppression of e^?/h⁺ pair recombination attributed significant enhancement in photoactivity, thereby increasing the hydrogen production.     

ZnO/CuInS2 core/shell heterojunction nanoarray for photoelectrochemical water splitting

ZnO/CuInS₂ core/shell nanorods array thin film was synthesized on conducting glass substrates for photoelectrochemical water splitting via a simple hydrothermal and cation exchange reaction, using ZnO nanorods array as reactive template. Uniform CuInS₂ films were obtained on the surface of ZnO nanorods, based on the ion-by-ion growth mechanism. The optical property of core/shell nanoarray was characterized, and enhanced absorption spectrum was observed. Hydrogen generation efficiency of 3.2% at 0.29 V versus saturated calomel electrode was achieved with synthesized ZnO/CuInS₂ core/shell nanoarray electrode due to the improved absorption and appropriate energy gap structure. The synthesized core/shell nanoarray has potential application in photoelectrochemical water splitting.     

Enhanced photocatalytic H2 production from glycerol solution over ZnO/ZnS core/shell nanorods prepared by a low temperature route

A series of ZnO/ZnS core/shell nanorods with different ZnS/ZnO molar ratios was synthesized via a new water bath route. The nanorods have a diameter of about 100 nm and a length ranging from a few hundred nanometers to several micrometers. They are formed by coating ZnO nanorod with a layer of porous ZnS shell mainly consisting of crystals which are about 12 nm in diameter. The results showed that the deposition thickness of the ZnS shell layer strongly affected the morphologies, surface area, structure, photo absorption and photocatalytic performance of the ZnO/ZnS core/shell nanorods. The as-prepared ZnO/ZnS core/shell nanorods exhibited a higher photocatalytic activity for H₂ evolution from the glycerol/water mixtures compared with the ZnO nanorods under the same conditions. The maximum H₂ production was 2608.7 and 388.4 μmol h⁻¹ gcat⁻¹ under UV and solar-simulated light irradiation and the corresponding quantum efficiencies were 22% and 13%, respectively. The deposition thickness of the ZnS shell and the interaction between the ZnO rod and ZnS shell and the core/shell structure with n-p heterojunction substantially influence the optical and catalytic performance of the ZnO/ZnS core/shell nanorods.     

Pd/NiO core/shell nanoparticles on La0.02Na0.98TaO3 catalyst for hydrogen evolution from water and aqueous methanol solution

Novel Pd/NiO core/shell nanoparticles (NPs) have been synthesized by a simple impregnation method with low temperature processing, in a ‘green’, scalable process using nontoxic chemicals. The cocatalyst consisting of a Pd core and a NiO shell formed simultaneously on the surface of La-doped NaTaO₃ photocatalyst. The Pd core both induces migration of photogenerated electrons from the bulk of the La_0.02Na_0.98TaO₃ and transfers electrons to the NiO shell. Without the NiO shell, Pd NPs show negligible H₂ production from water splitting, due to the rapid reaction between hydrogen and oxygen on the surface. On the other hand, the NiO shell allows the permeation of hydrogen and enables hydrogen reduction on Pd. The incorporation of NiO shell onto Pd remarkably enhances the photocatalytic performance of La-doped NaTaO₃ for hydrogen production from pure water. In addition, the core/shell structure can significantly enhance the stability of Pd during the photocatalytic reaction. Similar concepts could be extended to other applications, where the catalytic activity and stability are of concerns. The formation mechanism of the core/shell photocatalyst is proposed based on the high resolution transmission electron microscopy (HRTEM) images and X-ray absorption near-edge structure (XANES) analyses.     

Preparation of supported Ni catalysts with a core/shell structure and their catalytic tests of partial oxidation of methane

Synthesis of supported Ni catalysts with a core/shell structure at the multibubble sonoluminescence (MBSL) condition and their catalytic tests for methane decomposition by partial oxidation were performed in this study. The catalysts prepared were analyzed by XRD, TEM and XPS. Without doping the third components, the supported catalyst of core/shell structure made with 10% Ni loading on Al₂O₃ yields 96% conversion efficiency of methane at reaction temperature of 800 °C and shows excellent thermal stability for the first 40 h. It turns out that coexistence of NiO and NiOx species on the surface of the catalysts play a very important role in the partial oxidation of methane. In addition, the uniform layer of Ni particles on the surface of support material hindered coke formation and sintering process, which enhances thermal stability for the catalysts.     

Photocatalytic hydrogen production using Fe2O3-based core shell nano particles with ZnS and CdS

Stability and efficiency of photocatalysts are important to realize the practical applications of them for photocatalytic hydrogen production from industrial sulfide effluent. Novel, magnetically separable core–shell nano photocatalysts viz., CdS/Fe₂O₃, ZnS/Fe₂O₃ and (CdS + ZnS)/Fe₂O₃ were prepared and their hydrogen evolution activity under visible light was examined. The XRD result shows that CdS and ZnS were very well coated on the surface of the iron oxide core shell particles. The HR-TEM result also confirms the core shell formation. (CdS + ZnS)/Fe₂O₃ evolved higher volume of hydrogen than the other catalysts. It is ascribed to rapid migration of excited electrons from (CdS + ZnS) toward Fe₂O₃ suppressing electron hole annihilation compared to other catalysts. The catalysts can be easily recovered from the reaction medium using external magnetic bar and so the photocatalyst can be reused without any mass loss. Hence, it can be a potential catalyst for recovery of hydrogen from industrial sulfide containing waste streams.     

Heterostructured (Ba,Sr)TiO3/TiO2 core/shell photocatalysts: Influence of processing and structure on hydrogen production

Heterostructured powders composed of microcrystalline (mc-) BaTiO₃ and SrTiO₃ cores coated with nanostructured (ns-) TiO₂ shells were prepared using a sol–gel method. The influences of annealing temperature, coating thickness, cocatalyst loading, and core size on photocatalytic hydrogen production were experimentally determined. The amount of hydrogen produced depends on the annealing temperature, which influences the interface, phase composition, light absorption, crystallinity, mesoporosity, and surface area. The heterostructured powders produced more hydrogen than ns-TiO₂ alone when annealed between 500 °C and 800 °C. The amount of hydrogen produced by heterostructures with 100–150 nm thick nanostructured titania coatings was greater than for thicker or thinner coatings. The optimum Pt loading was determined to be 1% by weight. Heterostructured powders consisting of mc-BaTiO₃/ns-TiO₂ produce more hydrogen than those with nano-sized BaTiO₃ cores, suggesting a size effect that is counter to the conventional relationship between catalytic activity and particle size.     

Improved performance of dye-sensitized solar cells with TiO2/alumina core-shell formation using atomic layer deposition

Alumina (Al₂O₃) shell formation on TiO₂ core nanoparticles by atomic layer deposition (ALD) is studied to suppress the recombination of charge carriers generated in a dye-sensitized solar cell (DSSC). It is relatively easy to control the shell thickness using the ALD method by controlling the number of cycles. An optimum thickness can be identified, which allows tunneling of the forward current while suppressing recombination. High-resolution TEM measurements show that a uniform Al₂O₃ shell is formed around the TiO₂ core particles and elemental mapping of the porous TiO₂ layer reveals that the Al₂O₃ distribution is uniform throughout the layer. The amount of dye absorption is increased with increase in the shell thickness but electrochemical impedance spectroscopic (EIS) measurement shows a drastic increase in the resistance. With an optimum Al₂O₃ thickness of 2 nm deposited by ALD, a 35% improvement in the cell efficiency (from 6.2 to 8.4%) is achieved.