Zinc oxide can be called a multifunctional material thanks to its unique physical and chemical properties. The first part of this paper presents the most important methods of preparation of ZnO divided into metallurgical and chemical methods. The mechanochemical process, controlled precipitation, sol-gel method, solvothermal and hydrothermal method, method using emulsion and microemulsion enviroment and other methods of obtaining zinc oxide were classified as chemical methods. In the next part of this review, the modification methods of ZnO were characterized. The modification with organic (carboxylic acid, silanes) and inroganic (metal oxides) compounds, and polymer matrices were mainly described. Finally, we present possible applications in various branches of industry: rubber, pharmaceutical, cosmetics, textile, electronic and electrotechnology, photocatalysis were introduced. This review provides useful information for specialist dealings with zinc oxide.

Zinc Oxide-From Synthesis to Application: A Review.

As an alternative to the gold standard TiO2 photocatalyst, the use of zinc oxide (ZnO) as a robust candidate for wastewater treatment is widespread due to its similarity in charge carrier dynamics upon bandgap excitation and the generation of reactive oxygen species in aqueous suspensions with TiO2. However, the large bandgap of ZnO, the massive charge carrier recombination, and the photoinduced corrosion–dissolution at extreme pH conditions, together with the formation of inert Zn(OH)2 during photocatalytic reactions act as barriers for its extensive applicability. To this end, research has been intensified to improve the performance of ZnO by tailoring its surface-bulk structure and by altering its photogenerated charge transfer pathways with an intention to inhibit the surface-bulk charge carrier recombination. For the first time, the several strategies, such as tailoring the intrinsic defects, surface modification with organic compounds, doping with foreign ions, noble metal deposition, heterostructuring with other semiconductors and modification with carbon nanostructures, which have been successfully employed to improve the photoactivity and stability of ZnO are critically reviewed. Such modifications enhance the charge separation and facilitate the generation of reactive oxygenated free radicals, and also the interaction with the pollutant molecules. The synthetic route to obtain hierarchical nanostructured morphologies and study their impact on the photocatalytic performance is explained by considering the morphological influence and the defect-rich chemistry of ZnO. Finally, the crystal facet engineering of polar and non-polar facets and their relevance in photocatalysis is outlined. It is with this intention that the present review directs the further design, tailoring and tuning of the physico-chemical and optoelectronic properties of ZnO for better applications, ranging from photocatalysis to photovoltaics.

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Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications

Self-assembled hierarchical MoO2/graphene nanoarchitectures have been fabricated on a large scale through a facile solution-phase process and subsequent reduction of the Mo-precursor/graphene composite. The as-formed MoO2/graphene nanohybrid as an anode material for lithium-ion batteries exhibits not only a highly reversible capacity but also an excellent cycling performance as well as good rate capability. Results show that the hierarchical rods made of primary MoO2 nanocrystals are uniformly encapsulated within the graphene sheets. The synergistic effect of the hierarchical nanoarchitecture and the conducting graphene support may contribute to the enhanced electrochemical performances of the hybrid MoO2/graphene electrode. This work presents a facile synthetic strategy that is potentially competitive for scaling-up industrial production. Besides, the MoO2/graphene hybrids with a well-defined hierarchical topology not only provide flexible building blocks for advanced functional devices, but are also ide...

Self-assembled hierarchical MoO2/graphene nanoarchitectures and their application as a high-performance anode material for lithium-ion batteries.

The development of advanced energy storage devices is at the forefront of research geared towards a sustainable future. Nanostructured materials are advantageous in offering huge surface to volume ratios, favorable transport features, and attractive physicochemical properties. They have been extensively explored in various fields of energy storage and conversion. This review is focused largely on the recent progress in nanostructured Mo-based electrode materials including molybdenum oxides (MoOx, 2 ≤ x ≤ 3), dichalconides (MoX2, X = S, Se), and oxysalts for rechargeable lithium/sodium-ion batteries, Mg batteries, and supercapacitors. Mo-based compounds including MoO2, MoO3, MoO3−y (0 < y < 1), MMoxOy (M = Fe, Co, Ni, Ca, Mn, Zn, Mg, or Cd; x = 1, y = 4; x = 3, y = 8), MoS2, MoSe2, (MoO2)2P2O7, LiMoO2, Li2MoO3, etc. possess multiple valence states and exhibit rich chemistry. They are very attractive candidates for efficient electrochemical energy storage systems because of their unique physicochemical properties, such as conductivity, mechanical and thermal stability, and cyclability. In this review, we aim to provide a systematic summary of the synthesis, modification, and electrochemical performance of nanostructured Mo-based compounds, as well as their energy storage applications in lithium/sodium-ion batteries, Mg batteries, and pseudocapacitors. The relationship between nanoarchitectures and electrochemical performances as well as the related charge-storage mechanism is discussed. Moreover, remarks on the challenges and perspectives of Mo-containing compounds for further development in electrochemical energy storage applications are proposed. This review sheds light on the sustainable development of advanced rechargeable batteries and supercapacitors with nanostructured Mo-based electrode materials.

Nanostructured Mo-based electrode materials for electrochemical energy storage

ZnO nanoparticles via Moringa oleifera green synthesis: Physical properties & mechanism of formation

Among all the functional materials, ZnO plays an outstanding role in terms of chemical and physical properties, but also in terms of morphological variety and the number of reported synthesis approaches. Complex shapes and hierarchical architectures make ZnO a perfect example to study chemical and crystallization mechanisms. In this review article, we will discuss the nucleation and growth of ZnO nanostructures in liquid media by classical and non-classical (i.e., particle-based) crystallization pathways. We elaborate the chemical conditions and parameters that are responsible for the occurrence of one or the other pathway.

Zinc oxide nanoparticles: chemical mechanisms and classical and non-classical crystallization

A detailed study is presented on the formation of MoO2 nanoparticles from the dissolution of the precursor to the final rodlike product, with a focus on the exploration of the inorganic reaction occurring ahead of the nucleation step, and interplay between size and crystal structure of MoO2. In situ X-ray absorption spectroscopy experiments show that the crystallization and the growth process of MoO2 nanorods is initiated by rapid reduction of the MoO2Cl2 precursor in benzyl alcohol and acetophenone. This reaction triggers the nucleation of 2 nm MoO2 particles with spherical shape and hexagonal crystal structure. The transformation from spheres into rods emerges as a complex process driven by oriented attachment. High-resolution transmission electron microscopy and X-ray diffraction results provide evidence that the 2 nm particles first aggregate into 5–20 nm-large oriented assemblies. The increase in particle size induces the phase transition from hexagonal to the less symmetrical monoclinic crystal structure, and finally the transformation into rods. Is it shown that electrodes for lithium-ion batteries based on MoO2 nanorods have a long-term cycling life. The specific discharge capacity even after 200 cycles at a discharge rate of 1 C is about 300 Ah kg−1.

Interplay between size and crystal structure of molybdenum dioxide nanoparticles--synthesis, growth mechanism, and electrochemical performance.

Zinc oxide nanostructures are known to exist in a great variety of morphologies. However, the underlying mechanisms leading to these architectures are far from being fully understood. Here, we present a time dependent study of the generation of zinc oxide nanorods, which arrange into bundles with a fan- or bouquet-like structure, using the benzyl alcohol route. The structural evolution of the nanoparticles was monitored by electron microscopy techniques, whereas the progress of the chemical reaction was followed by quantification of the organic by-products using gas chromatography. With this study we give a detailed insight into the formation of the zinc oxide structures, which involves a complex pathway based on many in parallel occurring processes such as crystallization of primary particles, their oriented attachment and surface reconstruction inside the nanoparticulate agglomerates. However, in spite of such an intricate growth behavior, the ZnO nanostructures are surprisingly uniform in size and shape.

Mechanistic aspects of molecular formation and crystallization of zinc oxide nanoparticles in benzyl alcohol

Extension of the benzyl alcohol route to metal sulfides: "nonhydrolytic" thio sol-gel synthesis of ZnS and SnS2.

We present a detailed study on the nonaqueous synthesis of tungstite nanostructures with the focus on crystallization processes and the evolution of particle morphology. Time-dependent transmission electron microscopy (TEM) revealed a complex, particle-based crystallization mechanism involving first the formation of spherical and single-crystalline primary particles of 2–8 nm, which are cross-linked to large and unordered agglomerates, followed by their organization into rod-like structures of 40 × 200–400 nm. These rods undergo an internal ordering process, during which crystallographically oriented stacks of platelets develop. In situ small angle X-ray scattering (SAXS) experiments confirm this pathway of particle formation. The scattering intensity is dominated by the fast formation of rod-like particles, which cause an inter-platelet peak in the SAXS pattern with ongoing internal ordering. With continuous reaction time, the platelet stacks start to fall apart forming shorter assemblies of just a few platelets or even single platelets.

Bettina Ludi

Papers

Zinc oxide nanoparticles: chemical mechanisms and classical and non-classical crystallization

Interplay between size and crystal structure of molybdenum dioxide nanoparticles--synthesis, growth mechanism, and electrochemical performance.

Mechanistic aspects of molecular formation and crystallization of zinc oxide nanoparticles in benzyl alcohol

Extension of the benzyl alcohol route to metal sulfides: "nonhydrolytic" thio sol-gel synthesis of ZnS and SnS2.

A comprehensive study of the crystallization mechanism involved in the nonaqueous formation of tungstite