Academic literature on the topic 'Catalytic nano-heterostructures'

Efficient CO₂ gas detection and visible light photocatalysis performance shown by interleaved CuO/ZnO heterostructures ascribed primarily to the high surface area, p/n nano-interfaces and catalytic role of Ag.

Abstract Stimulated by the attractive performance of multi-dimensional heterostructures involving hexagonal boron nitride (hBN), intense attentions have been paid to creation of sharp boundary/interface, which could bring hBN nano-structures additional appealing physical properties. However, the lack of controllable synthesis limits further experimental investigation on hBN nano-structures. Here, the directional etching of transitional metal nano-particles (NPs) on the surface of hBN to produce nano-trenches with sharp edges was systematic investigated. It is found that, only Pt and Ir NPs can produce armchair-oriented nano-trenches at low H2 partial pressure, while other transitional metals lead to zigzag oriented nano-trenches. The density and width of the nano-trenches always increase with etching temperature and the pre-treated solution concentration while the trench orientation depends on both H2 partial pressure and etching temperature. The aspect ratio of nano-trenches may reach several thousand under optimized conditions. The method exhibited here shines a light on edge-selective patterning of 2D crystals.

Background: The cost of effluent treatment is not affordable by small scale industries especially in developing countries. Hence the effluent is discharged without treatment into water bodies. The dyes do not degrade easily and possess a major concern to be addressed. The aquatic life is highly affected and also leads to bio magnification of the chemicals through the food chain. Objective: To synthesize a ternary hybrid structure for enhanced photocatalytic activity under visible light. It is intended to reduce toxicity caused by dyeing units. Methods: Synthesized nanomaterials are characterized and used as photocatlyst for the degradation of methylene blue. In degradation experiment known weight of catalyst was added to known volume of an aqueous solution of dye at various concentrations. The solutions are aerated in dark for about 30 min. At the time of irradiation of light, known aliquot of the aqueous mixture was collected at an interval of constant time each from the reaction solution. The catalyst in the mixture was separated by centrifuging the mixture and absorbance was measured. The % of degradation of the dye can be determined knowing initial and final dye concentration. Result: Heterostructures are characterized with analytical tools such as X-ray diffraction, Fourier transform infrared spectroscopy. Band gap of photocatlyst is calculated by application of UV-Vis spectroscopy. Morphology is seen using scanning electron microscopy and transmission electron microscopy. Distribution of constituent structures is observed with energy-dispersive X-ray (EDX) spectroscopy. The structures are used for photocatalytic degradation of methylene blue dye solution under UV and visible light irradiation. Heterostructures showed best performance under visible light. Conclusion: The ternary hybrid nanostructure ZnO-MnO2-Gd2O3 was effectively prepared by a simple solution combustion method. The ternary compound shows wide range of absorption by expanding absorption band both in UV and visible regions. Structures showed better catalytic property under visible light.

In this paper the multifariousness of SiC/Si heterostructures for device and sensor applications will be demonstrated. 3C-SiC based microelectromechanical resonator beams (MEMS) with different geometries actuated by the magnetomotive effect operating under ambient conditions were fabricated. The resonant frequency reaches values up to 2 MHz. The applications of these resonators are the measurement of the viscosity of liquids or mass detection. Furthermore, photonic devices in the form of SiC/Si infrared gratings for wavelength and polarization filters in infrared spectra are processed. SiC wear protection for a dosing system with the possibility to dose nano- or picoliter droplets of water based liquids as well as SiC nanomasking for catalytic agent nanostructures are demonstrated.

Semiconductor materials as photocatalysts hold great prospects for renewable energy substitutes and environmental protection. Nanostructured BiOX (X=Cl, Br, I) with favorable features of a unique layered crystal structure and suitable band gaps has been demonstrated to be a promising photocatalytic material. In this paper, a two-step synthesis route combining an electrospinning technique and SILAR reaction has been accepted as a straightforward protocol for the exploitation of BiOI/carbon nanofibers’ (CNFs) hierarchical heterostructures. As expected, the BiOI/CNFs presented a much higher degradation rate of methyl orange than that of the pure BiOI under visible light. The degradation rate of methyl orange reaches 85% within 210 min. The enhanced photocatalytic activity could be attributed to the fact that conductive CNFs as substrate could effectively improve the separation and transformation of photogenerated charges. Moreover, the fabricated BiOI/CNFs after five cycles could be easily recycled without a decrease in photocatalytic activity due to their ultra-long one-dimensional nano-structural property.

Colloidal quantum confined semiconductor-metal nano-heterostructures are a promising class of photocatalysts for solar energy conversion. In these photocatalysts, the semiconductor domain serves as the light absorber and the metal as the catalyst. Such photocatalysts combine the superior light absorption and charge transport properties of the semiconductor with the superior catalytic activity and selectivity of the metal. Furthermore, both domains can be independently tuned to enhance the photocatalytic performance of the heterostructure. Among various semiconductor/metal heterostructures, metal-tipped colloidal semiconductor nanorods (such as CdS-Pt), have attracted extensive interest because they have been reported to have high quantum efficiencies of light driven H2 generation and their morphology can be systematically tuned. The overall light driven H2 generation process involves multiple elementary charge separation and recombination steps in the semiconductor and across the semiconductor/metal interface as well as proton-coupled electron transfer reactions at the catalytic center. The change of the semiconductor or metal domains can often have effects on multiple competing processes involved in the overall reaction. As a result of these complexities, the mechanisms for the morphological dependence of the observed H2 generation efficiencies are not fully understood, hindering the rational design of these photocatalysts. In this talk, we use Pt tipped CdS nanorods (CdS-Pt) as a model system to examine the effect of Pt size and CdS rod length on their light driven H2 generation efficiency. We show that increasing the Pt particle size increases the overall H2 generation quantum efficiency through both increasing the rate of electron transfer from the CdS to Pt and enhancing the efficiency of water/proton reduction; H2 generation efficiency increases at longer CdS rod length by suppression of charge recombination across the Pt/CdS interface. Our work demonstrates that through systematic in situ study of elementary processes involved in the overall H2 generation, it is possible to rationally design and improve semiconductor-metal hybrid photocatalysts.

This talk summarizes R&D efforts in the author’s laboratory on the fabrication of oxide nano-heterostructures, exploiting intrinsic material properties, that are highly scalable and do not require use of lithography. One such process creates crystallographically oriented nanofiber arrays of single crystal TiO2 in H2/N2 environment. H2/N2 heat treatment was also used to grow nanofibers on polycrystalline SnO2, showing directional growth on grains with crystal facets. We have also developed a process to create nanofibers of TiO2 on Ti metal/alloys via oxidation under a limited supply of oxygen. In another process, SnO2 nanowires grown from commercial FTO slides using the vapor-liquid-solid (VLS) method were placed in a microwave-assisted hydrothermal chamber where TiO2 nanorods nucleated radially from the SnO2 nanowire cores. We developed yet another interesting nano-structure (nanoislands and/or nanobars) during thermal annealing of an oxide (GDC) on top of another oxide (YSZ) substrate that self-assembles along the softest elastic direction of the substrate. What is common about these structures is that they are fabricated without the use of lithographic techniques and involves simple processes such as gas-phase reactions and stress-driven processes. These nano-heterostructures can be used as platforms for chemical sensing, catalysis, photocatalysis, photovoltaics and biomedical applications. Sensing application presents opportunities and challenges that are presented including an Open access Database Of Resistive type gas Sensors (ODORS) that has been developed and can be used to select suitable sensing materials.

Hydrogen production via water dissociation under exposure to sunlight has emanated as an environmentally friendly, highly productive and expedient process to overcome the energy production and consumption gap, while evading the challenges of fossil fuel depletion and ecological contamination. Various classes of materials are being explored as viable photocatalysts to achieve this purpose, among which, the two-dimensional materials have emerged as prominent candidates, having the intrinsic advantages of visible light sensitivity; structural and chemical tuneability; extensively exposed surface area; and flexibility to form composites and heterostructures. In an abridged manner, the common types of 2D photocatalysts, their position as potential contenders in photocatalytic processes, their derivatives and their modifications are described herein, as it all applies to achieving the coveted chemical and physical properties by fine-tuning the synthesis techniques, precursor ingredients and nano-structural alterations.

博士
國立交通大學
材料科學與工程學系所
103
In this dissertation, we focused on the catalytic properties of semiconductor heterostructures, which included semiconductor-graphene: ZnO-rGO composites, semiconductor-metal: ZnSe0.5(N2H4)-Au nanowires, semiconductor- semiconductor- metal: In2O3-TiO2-Pt nanobelts and semiconductor-metal: Au@Cu7S4 yolk-shell nanoparticles. Due to the difference in band structure between the constituents, the excited electrons in one of the semiconductor domain would preferentially transfer to the other semiconductor or metal with lower conduction band or work function under light illumination, leading to remarkable charge separation property. The enchantment in photocatalytic activity of pollutant degradation or photoconversion efficiency in photoelectrochemical application therefore can be achieved. The charge transfer behavior at the interface of heterostructures was explored by using time-resolved photoluminescence (TRPL). Through quantitatively analyzing the electron transfer event with TRPL, the correlation between the charge transfer dynamic and the difference in band diagram for semiconductor heterostrucutres could be further established. In addition, the Au@Cu7S4 yolk-shell nanoparticles (NPs) were synthesized using Au@Cu2O core-shell NPs as a sacrificial template and served as peroxidase mimics to investigate their peroxidase-like catalytic properties by using 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide. First, the ZnO-rGO composites were synthesized by hydrothermal method. The graphene oxide (GO) sheets were first prepared by Hummers method, followed by the growth of ZnO on the sheets during the hydrothermal process; in the meantime, the graphene oxide could be reduced to reduced graphene oxide (rGO). By modulating the amount of GO nanosheets added, the content of rGO in the resultant ZnO–rGO composites can be readily controlled. Due to the difference in band structures for ZnO–rGO composites, the photoexcited electrons of ZnO would preferentially transfer to rGO, leading to charge carrier separation. TRPL spectra revealed that an increased electron-transfer rate constant was observed for ZnO–rGO with increasing rGO contents, suggesting that an increased number of photoexcited charge carriers were separated and available for photocatalysis utilization. The photocatalytic properties of the ZnO–rGO composites were investigated by using gaseous acetaldehyde (CH3CHO), a typical volatile organic compound (VOC), as the test pollutant. Furthermore, the photoactivity of ZnO–rGO toward electrolytic water oxidation was also evaluated, which revealed 50% increase in water oxidation current over pure ZnO under white light illumination. The demonstration from this work may facilitate the use of ZnO–rGO composites in photodegradation of VOCs as well as for photoelectrochemical applications. Second, ZnSe0.5(N2H4) inorganic-organic hybrids were synthesized using a facile hydrazine-assisted hydrothermal method. By modulating the volume ratio of hydrazine hydrate to deionized water employed in the synthesis, the morphology of the grown ZnSe0.5(N2H4) can be varied, which included nanowires, NBs and nanoflakes. With the relatively long exciton lifetime and highly anisotropic structure, ZnSe0.5(N2H4) nanowires performed much better in the photodegradation of rhodamine B (RhB) than the other two counterpart products. As compared to pure ZnSe NPs and single-phase ZnSe nanowires obtained from further processing ZnSe0.5(N2H4), the ZnSe0.5(N2H4) hybrid nanowires exhibited superior photocatalytic performance under visible light illumination. With further decorated Au particles, TRPL spectra showed that the photoexcited electrons in ZnSe0.5(N2H4) nanowires can be transported to the decorated Au, which enabled a fuller extent of participation of charge carriers in the photocatalytic process and thus conduced to a significant enhancement in the photocatalytic activity. Third, we investigated the interfacial charge carrier dynamics of the three-component semiconductor−semiconductor−metal heterojunction system. The samples were prepared by selectively depositing Pt NPs on the TiO2 surface of In2O3-decorated TiO2 nanobelts (In2O3−TiO2 nanobelts (NBs)). For In2O3−TiO2 NBs, because of the difference in band structures between In2O3 and TiO2, the photoexcited electrons of In2O3 nanocrystals would preferentially transfer to TiO2 NBs to cause charge carrier separation. With the introduction of Pt on TiO2 surface, a fluent electron transfer from In2O3, through TiO2, and eventually to Pt was achieved, giving rise to the increasingly pronounced charge separation property. TRPL spectra were measured to quantitatively analyze the electron transfer event between In2O3 and TiO2 for In2O3−TiO2 NBs and its dependence on Pt deposition. Upon the deposition of Pt, In2O3−TiO2 NBs showed an increased apparent electron-scavenging rate constant, fundamentally consistent with the result of their performance evaluation in photocatalysis. In the final part, we presented the peroxidase-like catalytic properties of Au@Cu7S4 yolk-shell NPs synthesized by using Au@Cu2O core-shell nanostructures as templates via Kirkendall effect. The TMB oxidation results revealed that the Au@Cu7S4 yolk-shell nanopartilces had remarkable activity in the generation of ∙OH radicals in the existence of H2O2. Due to their unique structure, the Au core can provide sufficient active sites for H2O2 adsorption as compared to Au@Cu2O core-shell samples. On the other hand, the Cu7S4 hollow structures for Au@Cu7S4 yolk-shell samples exposed higher surface area than that of Cu2O for Au@Cu2O core-shell samples. The RhB degradation test demonstrated that the present yolk-shell samples could practically be applied in degradation of organic pollutant with hydrogen peroxide as well.

Owing to its interesting chemistry and toxic nature, catalytic oxidation of CO is of both fundamental and practical interest. Two applications, viz. CO conversion in the catalytic converters and pre-treatment of CO-rich reformate feed for the polymer electrolyte membrane fuel cells (PEMFCs) stand out because of their focus on the current and future energy scenario, respectively. Both demand catalytic oxidation of CO to CO2 at low temperatures. To facilitate this, in this thesis, we address various aspects of the catalyst playing a role during CO oxidation. The heterogeneous catalysts generally employed for CO oxidation rely on the use of noble metals supported on reducible metal oxides, as this combination leads to exceptionally good synergistic properties for CO oxidation. Here, we have investigated different heterostructure systems for CO oxidation and preferential CO oxidation (PrOx) reactions The thesis addresses preferential CO oxidation (PrOx) to purify H2 for the fuel cell feed, at operating temperatures of PEMFCs. To develop such catalysts, along with introducing the oxygen vacancies in the support, engineering the supported-metal for their better selectivity is another feasible option. A facile nucleation of PtCu alloy nanoparticles is done over the reduced SrTiO3 support in four different compositions. These PtCu decorated supports are tested for their activity in preferential oxidation of CO under excess hydrogen. The effect of Cu dilution in the Pt catalyst is discussed with respect to the activity, selectivity and stability of the catalysts for preferential CO oxidation. In conclusion, various prospects of the catalytic nano-heterostructures presented in this thesis are presented. The insights obtained from this thesis work can be utilized in understanding fundamental aspects of the catalysts required to prepare commercially viable catalysts for CO oxidation as well as preferential CO oxidation

The synopsis part of this last chapter gives a brief summary of the book content. The outlook attempts to identify future areas of scientific activity, in which according to the authors´ visions nano-oxide materials may promote new developments. Among them are the controlled synthesis of oxide nanosheets and the experimental realization of oxide nanoribbons. The preparation of well-defined oxide heterostructures may reveal novel emergent states and new topological phases of matter. Mixed nano-oxides will be of interest for band structure engineering and to adjust band edges for photochemical reactivity. Programmable defect chemistry may open up new selective pathways for catalytic reactions. In parallel with experimental progress, advanced theoretical and simulation methods will take advantage of the ever-increasing computer power to tackle highly correlated materials and allow highthroughput computing. The interaction of nano-oxides with biological systems has great potential for opening up new avenues in the biotechnological area.