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13693 KiB

Open AccessArticle

Induced Mesocrystal-Formation, Hydrothermal Growth and Magnetic Properties of α-Fe₂O₃ Nanoparticles in Salt-Rich Aqueous Solutions

by Erik Brok, Jacob Larsen, Miriam Varón, Thomas W. Hansen and Cathrine Frandsen

Crystals 2017, 7(8), 248; https://doi.org/10.3390/cryst7080248 - 8 Aug 2017

Cited by 4 | Viewed by 5144

Abstract

Iron oxide nanoparticles are widely prevalent in our aqueous environment (e.g., streams, seawater, hydrothermal vents). Their aggregation and crystal growth depend on their chemical surroundings, for instance just a change in pH or salt concentration can greatly affect this. In turn this influences [...] Read more.

Iron oxide nanoparticles are widely prevalent in our aqueous environment (e.g., streams, seawater, hydrothermal vents). Their aggregation and crystal growth depend on their chemical surroundings, for instance just a change in pH or salt concentration can greatly affect this. In turn this influences their properties, mobility, fate, and environmental impact. We studied the growth of α-Fe₂O₃ (hematite), starting from 8 nm hematite particles in weakly acidic (HNO₃) aqueous suspension with different states of particle aggregation, using salt (NaCl and NaH₂PO₄) to control their initial aggregation state. The samples were then subject to hydrothermal treatment at 100–140 °C. We followed the development in aggregation state and particle size by dynamic light scattering, X-ray diffraction, small angle neutron scattering and transmission electron microscopy, and the magnetic properties by Mössbauer spectroscopy. The addition of NaCl and NaH₂PO₄ both led to aggregation, but NaCl led to linear chains of hematite nanoparticles (oriented parallel to their hexagonal c-axis), such that the crystalline lattice planes of neighboring hematite particles were aligned. However, despite this oriented alignment, the particles did not merge and coalesce. Rather they remained stable as mesocrystals until heat-treated. In turn, the addition of NaCl significantly increases the rate of growth during hydrothermal treatment, probably because the nanoparticles, due to the chain formation, are already aligned and in close proximity. With hydrothermal treatment, the magnetic properties of the particles transform from those characteristic of small (aggregated) hematite nanoparticles to those of particles with more bulk-like properties such as Morin transition and suppression of superparamagnetic relaxation, in correspondence with the growth of particle size. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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3876 KiB

Open AccessArticle

Reversed Crystal Growth of Calcite in Naturally Occurring Travertine Crust

by Heather F. Greer, Wuzong Zhou and Li Guo

Crystals 2017, 7(2), 36; https://doi.org/10.3390/cryst7020036 - 28 Jan 2017

Cited by 10 | Viewed by 10211

Abstract

A microstructural investigation by electron microscopy on a travertine specimen collected from Munigou National Park, Sichuan Province, China revealed evidence of a non-classical reversed crystal growth route previously only discovered in synthetic materials. Examination of the travertine specimen suggests that the presence of [...] Read more.

A microstructural investigation by electron microscopy on a travertine specimen collected from Munigou National Park, Sichuan Province, China revealed evidence of a non-classical reversed crystal growth route previously only discovered in synthetic materials. Examination of the travertine specimen suggests that the presence of organic matter initiates the oriented aggregation of calcite nanocrystallites. Surface re-crystallisation of the aggregates leads to a single crystalline rhombohedral shell with a polycrystalline core. This core-shell structure carries a strong resemblance to synthetic calcite prepared in the presence of chitosan, where the growth of calcite was found to follow the so-called reversed crystal growth process. It is proposed that the similar roles of biomolecules in naturally occurring travertine and chitosan in the synthetic system are based on their isoelectric points and the polymerizable property of long chain chemical structures. This study is important so that the structural similarities between naturally occurring biominerals and biomimetic materials can be further understood. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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1843 KiB

Open AccessArticle

Applicability of Effective Medium Approximations to Modelling of Mesocrystal Optical Properties

by Oleksandr Zhuromskyy

Crystals 2017, 7(1), 1; https://doi.org/10.3390/cryst7010001 - 22 Dec 2016

Cited by 17 | Viewed by 6461

Abstract

Rigorous superposition T-matrix method is used to compute light interaction with mesocrystalline structures. The results are used to validate the applicability of effective medium theories for computing the effective optical constants of mesocrystal structures composed of optically isotropic materials. It is demonstrated that [...] Read more.

Rigorous superposition T-matrix method is used to compute light interaction with mesocrystalline structures. The results are used to validate the applicability of effective medium theories for computing the effective optical constants of mesocrystal structures composed of optically isotropic materials. It is demonstrated that the Maxwell-Garnett theory can fit the rigorous simulation results with an average accuracy of 2%. The thus obtained refractive indexes can be used with any electromagnetic simulation software to represent the response of mesocrystals composed of optically small primary particles arranged into a cubic type lattice structures. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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2539 KiB

Open AccessArticle

In Situ Studies on Phase Transitions of Tris(acetylacetonato)-Aluminum(III) Al(acac)₃

by Nicole Pienack, Laura Ruiz Arana, Wolfgang Bensch and Huayna Terraschke

Crystals 2016, 6(12), 157; https://doi.org/10.3390/cryst6120157 - 28 Nov 2016

Cited by 20 | Viewed by 8543

Abstract

In situ investigations on the nucleation and crystallization processes are essential for understanding of the formation of solids. Hence, the results of such experiments are prerequisites for the rational synthesis of solid materials. The in situ approach allows the detection of precursors, intermediates, [...] Read more.

In situ investigations on the nucleation and crystallization processes are essential for understanding of the formation of solids. Hence, the results of such experiments are prerequisites for the rational synthesis of solid materials. The in situ approach allows the detection of precursors, intermediates, and/or polymorphs, which are mainly missed in applying ex situ experiments. With a newly developed crystallization cell, simultaneous in situ experiments with X-ray diffraction (XRD) and luminescence analysis are possible, also monitoring several other reaction parameters. Here, the crystallization of the model system tris(acetylacetonato)-aluminum(III) Al(acac)₃ was investigated. In the time-resolved in situ XRD patterns, two polymorphs of Al(acac)₃, the α- and the γ-phase, were detected at room temperature and the influence of the pH value onto the product formation was studied. Moreover, changes in the emission of Al(acac)₃ and the light transmission of the solution facilitated monitoring the reaction by in situ luminescence. The first results demonstrate the potential of the cell to be advantageous for controlling and monitoring several reaction parameters during the crystallization process. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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10750 KiB

Open AccessArticle

Controllable Hydrothermal Conversion from Ni-Co-Mn Carbonate Nanoparticles to Microspheres

by Yanqing Tang, Yangcheng Lu and Guangsheng Luo

Crystals 2016, 6(11), 156; https://doi.org/10.3390/cryst6110156 - 23 Nov 2016

Cited by 2 | Viewed by 6495

Abstract

Starting from Ni-Co-Mn carbonate nanoparticles prepared by microreaction technology, uniform spherical particles of Ni_1/3Co_1/3Mn_1/3CO₃ with a size of 3–4 μm were obtained by a controllable hydrothermal conversion with the addition of (NH₄)₂CO [...] Read more.

Starting from Ni-Co-Mn carbonate nanoparticles prepared by microreaction technology, uniform spherical particles of Ni_1/3Co_1/3Mn_1/3CO₃ with a size of 3–4 μm were obtained by a controllable hydrothermal conversion with the addition of (NH₄)₂CO₃. Based on characterizations on the evolution of morphology and composition with hydrothermal treatment time, we clarified the mechanism of this novel method as a dissolution-recrystallization process, as well as the effects of (NH₄)₂CO₃ concentration on the morphology and composition of particles. By changing concentrations and the ratio of the starting materials for nano-precipitation preparation, we achieved monotonic regulation on the size of the spherical particles, and the synthesis of Ni_0.4Co_0.2Mn_0.4CO₃ and Ni_0.5Co_0.2Mn_0.3CO₃, respectively. In addition, the spherical particles with a core-shell structure were preliminarily verified to be available by introducing nano-precipitates with different compositions in the hydrothermal treatment in sequence. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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2099 KiB

Open AccessArticle

A Hierarchically Micro-Meso-Macroporous Zeolite CaA for Methanol Conversion to Dimethyl Ether

by Yan Wang, Fen-fen Ren, Da-hai Pan and Jing-hong Ma

Crystals 2016, 6(11), 155; https://doi.org/10.3390/cryst6110155 - 23 Nov 2016

Cited by 11 | Viewed by 6185

Abstract

A hierarchical zeolite CaA with microporous, mesoporous and macroporous structure was hydrothermally synthesized by a ”Bond-Blocking” method using organo-functionalized mesoporous silica (MS) as a silica source. The characterization by XRD, SEM/TEM and N₂ adsorption/desorption techniques showed that the prepared material had well-crystalline [...] Read more.

A hierarchical zeolite CaA with microporous, mesoporous and macroporous structure was hydrothermally synthesized by a ”Bond-Blocking” method using organo-functionalized mesoporous silica (MS) as a silica source. The characterization by XRD, SEM/TEM and N₂ adsorption/desorption techniques showed that the prepared material had well-crystalline zeolite Linde Type A (LTA) topological structure, microspherical particle morphologies, and hierarchically intracrystalline micro-meso-macropores structure. With the Bond-Blocking principle, the external surface area and macro-mesoporosity of the hierarchical zeolite CaA can be adjusted by varying the organo-functionalized degree of the mesoporous silica surface. Similarly, the distribution of the micro-meso-macroporous structure in the zeolite CaA can be controlled purposely. Compared with the conventional microporous zeolite CaA, the hierarchical zeolite CaA as a catalyst in the conversion of methanol to dimethyl ether (DME), exhibited complete DME selectivity and stable catalytic activity with high methanol conversion. The catalytic performances of the hierarchical zeolite CaA results clearly from the micro-meso-macroporous structure, improving diffusion properties, favoring the access to the active surface and avoiding secondary reactions (no hydrocarbon products were detected after 3 h of reaction). Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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12568 KiB

Open AccessArticle

Facile Synthesis, Characterization, and Visible-light Photocatalytic Activities of 3D Hierarchical Bi₂S₃ Architectures Assembled by Nanoplatelets

by Tiekun Jia, Xiaofeng Wang, Fei Long, Jili Li, Zhihua Kang, Fang Fu, Guang Sun and Jian Chen

Crystals 2016, 6(11), 140; https://doi.org/10.3390/cryst6110140 - 31 Oct 2016

Cited by 12 | Viewed by 5030

Abstract

3D hierarchical Bi₂S₃ architectures have been successfully synthesized via a simple and effective hydrothermal process. The as-prepared Bi₂S₃ samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption- desorption isotherms, [...] Read more.

3D hierarchical Bi₂S₃ architectures have been successfully synthesized via a simple and effective hydrothermal process. The as-prepared Bi₂S₃ samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption- desorption isotherms, and UV-vis diffuse reflectance spectrum (DRS). The observation of field emission scanning electron microscope (FESEM) images showed that numerous nanoplatelets are randomly arranged and interconnected with each other, which are assembled into 3D hierarchical Bi₂S₃ architectures. The photocatalytic activity of the as-prepared Bi₂S₃ samples was evaluated by the degradation of Rhodamine B (RhB) under visible light irradiation. The effect of hydrothermal temperature, reaction time, pH value and shape on the photocatalytic efficiency of the as-prepared Bi₂S₃ samples was investigated. The results showed that 3D hierarchical Bi₂S₃ architectures prepared at 165 °C for 12 h at a pH of 2.4 exhibits high photocatalytic efficiency, which could be ascribed to the synergetic effect of the shape, surface area, crystallinity, band gap and crystalline size. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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4644 KiB

Open AccessArticle

Effect of HNO₃ Concentration on the Morphologies and Properties of Bi₂WO₆ Photocatalyst Synthesized by a Hydrothermal Method

by Wenjie Wang, Minna Guo, Dongliang Lu, Weimin Wang and Zhengyi Fu

Crystals 2016, 6(7), 75; https://doi.org/10.3390/cryst6070075 - 20 Jul 2016

Cited by 20 | Viewed by 6224

Abstract

Bi₂WO₆ samples were prepared by a hydrothermal method using Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O as raw materials. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman and Brunauer–Emmett–Teller (BET) were [...] Read more.

Bi₂WO₆ samples were prepared by a hydrothermal method using Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O as raw materials. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman and Brunauer–Emmett–Teller (BET) were employed for sample characterization. The photocatalytic activity of the samples was evaluated by the degradation of Rhodamine B under visible-light irradiation. The photocatalytic activity of Bi₂WO₆, as well as the effect of varying HNO₃ concentrations on the morphologies of Bi₂WO₆, was investigated. The HNO₃ concentration significantly affected the structure and morphology of the Bi₂WO₆. The photocatalytic performance varied with the structure, morphology, and surface area of the Bi₂WO₆ samples. The results indicated that the H10 sample exhibits uniform morphology and excellent photocatalytic performance; using this sample, the degradation of Rhodamine B reached 96% in 90 min under visible-light irradiation. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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Review

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5201 KiB

Open AccessReview

Mesocrystals: Past, Presence, Future

by Elena V. Sturm (née Rosseeva) and Helmut Cölfen

Crystals 2017, 7(7), 207; https://doi.org/10.3390/cryst7070207 - 9 Jul 2017

Cited by 77 | Viewed by 10936

Abstract

In this review, we briefly summarize the history of mesocrystal research. We introduce the current structural definition of mesocrystals and discuss the appropriate base for the classification of mesocrystals and their relations with other classes of solid state materials in terms of their [...] Read more.

In this review, we briefly summarize the history of mesocrystal research. We introduce the current structural definition of mesocrystals and discuss the appropriate base for the classification of mesocrystals and their relations with other classes of solid state materials in terms of their structure. Building up on this, we comment on the problems in mesocrystal research both fundamental and methodological. Additionally, we make the short overview of the mesocrystal formation principles and synthetic routes used for their fabrications. As an outlook into the future, we highlight the most notable trends in mesocrystal research and developments. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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10505 KiB

Open AccessReview

A Review on the Fabrication of Hierarchical ZnO Nanostructures for Photocatalysis Application

by Yi Xia, Jing Wang, Ruosong Chen, Dali Zhou and Lan Xiang

Crystals 2016, 6(11), 148; https://doi.org/10.3390/cryst6110148 - 16 Nov 2016

Cited by 92 | Viewed by 11850

Abstract

Semiconductor photocatalysis provides potential solutions for many energy and environmental-related issues. Recently, various semiconductors with hierarchical nanostructures have been fabricated to achieve efficient photocatalysts owing to their multiple advantages, such as high surface area, porous structures, as well as enhanced light harvesting. ZnO [...] Read more.

Semiconductor photocatalysis provides potential solutions for many energy and environmental-related issues. Recently, various semiconductors with hierarchical nanostructures have been fabricated to achieve efficient photocatalysts owing to their multiple advantages, such as high surface area, porous structures, as well as enhanced light harvesting. ZnO has been widely investigated and considered as the most promising alternative photocatalyst to TiO₂. Herein, we present a review on the fabrication methods, growth mechanisms and photocatalytic applications of hierarchical ZnO nanostructures. Various synthetic strategies and growth mechanisms, including multistep sequential growth routes, template-based synthesis, template-free self-organization and precursor or self-templating strategies, are highlighted. In addition, the fabrication of multicomponent ZnO-based nanocomposites with hierarchical structures is also included. Finally, the application of hierarchical ZnO nanostructures and nanocomposites in typical photocatalytic reactions, such as pollutant degradation and H₂ evolution, is reviewed. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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