Nanorod-Direct Oriented Attachment Growth and Promoted Crystallization Processes Evidenced in Case of ZnWO4

Preparation and characterization of CdWO4:Cu nanorods with enhanced photocatalytic performance under sunlight irradiation

The objective of this work is to convert an ultraviolet active photocatalyst to a visible active photocatalyst and investigate the effect of copper (Cu²⁺) doping on the morphology and photocatalytic activity of CdWO₄.

High temperature-mediated rocksalt to wurtzite phase transformation in cadmium oxide nanosheets and its theoretical evidence

Herein, a high temperature-induced phase transformation (PT) in chemically grown CdO thin films is demonstrated, and its corresponding electronic origin further investigated by density functional theory. In particular, the cubic rocksalt to hexagonal wurtzite PT in the CdO thin film annealed at 900 °C was confirmed by X-ray diffraction (XRD), which was consistent with the high-resolution transmission electron microscopy (TEM) results. Moreover, atomic force microscopy and scanning electron microscopy clearly evidenced the morphological evolution via the formation of a nanosheet network in the wurtzite-phase CdO film. The high temperature treatment also led to a significant enhancement in the optical band gap from 2.2 to 3.2 eV, as manifested by UV-visible spectroscopy. The enhanced surface roughness of the nanosheet caused a deviation in the net dipole moment, which may break the polarizable bonds and help in reducing the average dielectric constant, resulting in a band gap opening for the transformed phase. Furthermore, X-ray absorption spectroscopy at the oxygen k-edge revealed a notable shift in the inflection point of the absorption edge, while the X-ray photoelectron spectroscopy (XPS) Cd 3d and O 1s spectra suggested a gradual reduction in the CdO2 phase with an increase in annealing temperature. In addition, different complementary techniques including Rutherford backscattering and Raman spectroscopy were exploited to understand the aforementioned PT and its structural correlation. Finally, molecular dynamics simulation together with density functional theory calculation suggested that the symmetry modification at the Brillouin zone boundary provides a succinct signature for the PT in the CdO thin film.

A high energy-density P2-Na2/3[Ni0.3Co0.1Mn0.6]O2 cathode with mitigated P2-O2 transition for sodium-ion batteries

High voltage P2-Na2/3[Ni1/3Mn2/3]O2 is regarded as a promising cathode for high-energy sodium-ion batteries (SIBs). However, the undesired P2-O2 phase transition at high voltages above 4.0 V leads to a large volume change and further causes the rapid decay of capacity. Herein, an electrochemical-active Co3+ substitution is introduced to suppress the P2-O2 phase transformation but not at the cost of capacity. The spherical, Co3+ substituted P2-Na2/3[Ni0.3Co0.1Mn0.6]O2 with a high tap-density of 1.86 g cm-3 is successfully synthesized by co-precipitation and solid-state reactions. As anticipated, it delivers a large specific capacity of 161.6 mA h g-1 with a high median-voltage of 3.64 V (vs. Na/Na+), translated into a high energy-density of ∼590 W h kg-1, which is comparable to that of the commercialized LiCoO2 cathode in lithium-ion batteries. Apart from improved cycling stability ascribed to the mitigated P2-O2 transition, this cathode also shows a better rate property compared with those modified by Li+, Mg2+, Zn2+, Cu2+, and Ti4+ doping and Al2O3 coating. Besides, the P2-Na2/3[Ni0.3Co0.1Mn0.6]O2|hard carbon full-cells deliver a reversible capacity of 150.6 mA h g-1 and have enhanced cycle-life and high-rate capability. These gratifying achievements indicate that this P2-Na2/3[Ni0.3Co0.1Mn0.6]O2 is a very promising candidate as a high energy-density cathode for SIBs.

Study on micro-nanocrystalline structure control and performance of ZnWO4 photocatalysts

ZnWO₄ micro/nanocrystals with different sizes and well-developed crystals were synthesized by the molten salt method at low temperature.

Facile synthesis of hierarchical Mn3O4 superstructures and efficient catalytic performance

The development of novel materials with excellent performance depends not only on the constituents but also on their remarkable micro/nanostructures. In this work, manganese oxide (Mn₃O₄) hausmannite structures with a uniform three-dimensional (3D) flower-like hierarchical architecture have been successfully synthesized by a novel chemical route using surfactants as structure-directing agents. Microstructure analysis indicates that the obtained 3D flower-like Mn₃O₄ superstructure consists of a large number of two-dimensional (2D) Mn₃O₄ nanosheets, which is different from the reported 3D Mn₃O₄ hierarchical structures based on zero-dimensional nanoparticles or one-dimensional nanowires and nanorods. This 3D Mn₃O₄ hierarchical architecture provides us with another type of manganese oxide with different superstructural characteristics, which may have potential practical applications in the catalytic degradation of organic pollutants. The catalytic performance of this hierarchical Mn₃O₄ superstructure, which was prepared by three different types of structure-directing agents, including cetyltrimethylammonium bromide (CTAB), poly(vinylpyrrolidone) (PVP), and poly(ethylene oxide)-poly(propylene oxide) (P123), was evaluated for the catalytic degradation of organic pollutants, e.g. methylene blue. Interestingly, the hierarchical Mn₃O₄ superstructure prepared using CTAB as a template showed efficient catalytic degradation. The formation processes and possible growth mechanism of this novel 3D Mn₃O₄ hierarchical superstructure assembled by 2D Mn₃O₄ nanosheets are discussed in detail.

The chemistry of ZnWO4 nanoparticle formation

The need for a new approach to describing nanoparticle nucleation and growth different from the classical models is highlighted. In and ex situ total scattering experiments combined with additional characterization techniques are used to unravel the chemistry dictating ZnWO4 formation.

The need for a change away from classical nucleation and growth models for the description of nanoparticle formation is highlighted. By the use of in situ total X-ray scattering experiments the transformation of an aqueous polyoxometalate precursor mixture to crystalline ZnWO₄ nanoparticles under hydrothermal conditions was followed. The precursor solution is shown to consist of specific Tourné-type sandwich complexes. The formation of pristine ZnWO₄ within seconds is understood on the basis of local restructuring and three-dimensional reordering preceding the emergence of long range order in ZnWO₄ nanoparticles. An observed temperature dependent trend in defect concentration can be rationalized based on the proposed formation mechanism. Following nucleation the individual crystallites were found to grow into prolate morphology with elongation along the unit cell c-direction. Extensive electron microscopy characterization provided evidence for particle growth by oriented attachment; a notion supported by sudden particle size increases observed in the in situ total scattering experiments. A simple continuous hydrothermal flow method was devised to synthesize highly crystalline monoclinic zinc tungstate (ZnWO₄) nanoparticles in large scale in less than one minute. The present results highlight the profound influence of structural similarities in local structure between reactants and final materials in determining the specific nucleation of nanostructures and thus explains the potential success of a given synthesis procedure in producing nanocrystals. It demonstrates the need for abolishing outdated nucleation models, which ignore subtle yet highly important system dependent differences in the chemistry of the forming nanocrystals.

Synthesis of ZnWO4/CdWO4 core–shell structured nanorods formed by an oriented attachment mechanism with enhanced photocatalytic performances

We have described the oriented attachment mechanism in which CdWO₄ nanorods obviously act as an epitaxial ‘substrate’ and guide the ZnWO₄ aggregation process for the formation of CdWO₄ nanorod based aggregated structures.

In situ synthesis and photocatalytic performance of WO3/ZnWO4 composite powders

The WO₃/ZnWO₄ composite powders were synthesized through an in situ reaction process with tunnel structure K₁₀W₁₂O₄₁·11H₂O filiform crystallites used as a precursor.

Synthesis of high aspect ratio CuO submicron rods through oriented attachment and their application in lithium-ion batteries

High aspect ratio CuO submicron rods were synthesized via the polymer-assisted oriented attachment of nanocrystal building blocks along with calcination.

Yolk–shell ZnWO4 microspheres: one-pot synthesis, characterization and photocatalytic properties

Novel yolk–shell ZnWO₄ microspheres have been synthesized via a mild and one-pot hydrothermal route by using l-aspartic acid as the chelating agent and shape modifier.

Growth mechanisms and size control of FePt nanoparticles synthesized using Fe(CO)x (x < 5)-oleylamine and platinum(ii) acetylacetonate

By using Fe(CO)x-OAm (oleylamine, x < 5) as the Fe precursor to slow down the formation rate of FePt nanoparticles (NPs), a time dependence of the NPs' nucleation and growth process was observed by transmission electron microscopy (TEM). The complexing temperature of OAm and Fe(CO)5 at which Fe(CO)x-OAm was formed has a strong influence on the nucleation rate and growth process of the NPs. TEM analyses indicated that the NPs with isotropic shape were single crystalline throughout the synthesis and were formed by a diffusion-controlled Ostwald-ripening (OR) growth mechanism. The nanorod particles were first formed via joining of arbitrarily oriented single crystals and the two crystals formed a uniform particle afterwards, as described by the oriented-attachment (OA) mechanism. The ratio of OAm to Fe(CO)5 used in the preparation of Fe(CO)x-OAm has a significant influence on the growth process, and subsequently the shape, size and size distribution of the FePt NPs. By adjusting the ratio and its complexing temperature, single-crystal FePt NPs with controllable size and isotropic shape were obtained. The insight into the exploration of the specific roles of the reaction conditions and the formation mechanisms provided important information for controlling the morphology of the nanoparticles.

Synthesis of micro-nano hierarchical structured LiFePO₄/C composite with both superior high-rate performance and high tap density

Efforts were made to synthesize LiFePO(4)/C composites showing both high rate capability and high tap density. First, monoclinic phase FePO(4)·2H(2)O with micro-nano hierarchical structures are synthesized using a hydrothermal method, which are then lithiated to LiFePO(4)/C also with hierarchical structures by a simple rheological phase method. The primary structures of FePO(4)·2H(2)O are nanoplates with ∼30 nm thickness, and the secondary structures of the materials are intertwisted micro-scale rings. The LiFePO(4)/C materials lithiated from these specially structured precursors also have hierarchical structures, showing discharge capacities of more than 120, 110, and 90 mAh g(-1) at rates of 5 C, 10 C and 20 C, respectively, and high tap density of 1.4 g cm(-3) as cathode materials for lithium ion batteries. Since tap density is an important factor that needs to be considered in fabricating real batteries in industry, these hierarchical structured LiFePO(4)/C moves closer to real and large-scale applications.

Progress of nanocrystalline growth kinetics based on oriented attachment

The crystal growth mechanism, kinetics, and microstructure development play a fundamental role in tailoring the materials with controllable sizes and morphologies. The classical crystal growth kinetics-Ostwald ripening (OR) theory is usually used to explain the diffusion-controlled crystal growth process, in which larger particles grow at the expense of smaller particles. In nanoscale systems, another significant mechanism named "oriented attachment (OA)" was found, where nanoparticles with common crystallographic orientations directly combine together to form larger ones. Comparing with the classical atom/molecular-mediated crystallization pathway, the OA mechanism shows its specific characteristics and roles in the process of nanocrystal growth. In recent years, the OA mechanism has been widely reported in preparing low-dimension nanostructural materials and reveals remarkable effects on directing and mediating the self-assembly of nanocrystals. Currently, the interests are more focused on the investigation of its role rather than the comprehensive insight of the mechanism and kinetics. The inner complicacy of crystal growth and the occurrence of coexisting mechanisms lead to the difficulty and lack of understanding this growth process by the OA mechanism.In this context, we review the progress of the OA mechanism and its impact on materials science, and especially highlight the OA-based growth kinetics aiming to achieve a further understanding of this crystal growth route. To explore the OA-limited growth, the influence of the OR mechanism needs to be eliminated. The introduction of strong surface adsorption was reported as the effective solution to hinder OR from occurring and facilitate the exclusive OA growth stage. A detailed survey of the nanocrystal growth kinetics under the effect of surface adsorption was presented and summarized. Moreover, the development of OA kinetic models was systematically generalized, in which the "molecular-like" kinetic models were built to take the OA nanocrystal growth behavior as the collision and reaction between molecules. The development of OA growth kinetics can provide a sufficient understanding of crystal growth, and the awareness of underlying factors in the growth will offer promising guidance on how to control the size distribution and shape development of nanostructural materials.

General synthesis and phase control of metal molybdate hydrates MMoO4.nH2O (M = Co, Ni, Mn, n = 0, 3/4, 1) nano/microcrystals by a hydrothermal approach: magnetic, photocatalytic, and electrochemical properties

Different phases and morphologies of molybdate hydrates MMoO 4. nH 2O (M = Co, Ni, Mn, n = 0, 3/4, 1) nano/microcrystals, which include NiMoO 4.H 2O microflowers, MnMoO 4.H 2O microparallelogram plates, and CoMoO 4.3/4H 2O microrods, can be selectively synthesized by a hydrothermal process. The pH and reaction temperature have a crucial influence on the synthesis and shape evolution of the final products. Uniform CoMoO 4.3/4H 2O and NiMoO 4.H 2O nanorod bundles can be produced by a hydrothermal process with the assistance of PEG-400. The calcination of CoMoO 4.3/4H 2O and NiMoO 4.H 2O at 500 and 550 degrees C, respectively, allows the formation of monoclinic beta-CoMoO 4 and alpha-NiMoO 4. The antiferromagnetic property of MnMoO 4.H 2O, MnMoO 4, and CoMoO 4.3/4H 2O has been studied for the first time. The photocatalytic activity of metal molybdate particles with different morphologies has been tested by degradation of acid fuchsine under visible light. Electrochemical performances of MMoO 4 (M = Ni, Co) nanorod bundles and MnMoO 4 microrods have been evaluated.

Controlled Synthesis of the ZnWO4 Nanostructure and Effects on the Photocatalytic Performance

ZnWO4 photocatalysts with various morphologies were synthesized by a hydrothermal process. The effects of hydrothermal temperature and time on the crystallinity and morphology of ZnWO4 catalyst were investigated. The crystallinity was enhanced with the increase of hydrothermal temperature and hydrothermal time. The formation of ZnWO4 nanoparticles was controlled via kinetic process above 160 degrees C, and ZnWO4 nanorods with a highly [100]-preferred orientation formed via the thermodynamically control process in the temperature range of 120-140 degrees C. The morphology and crystallinity of ZnWO4 photocatalyst have a significant influence on the photocatalytic activity for aqueous Rhodamine B and gaseous formaldehyde degradation. ZnWO4 nanorod catalyst showed a much higher photocatalytic activity than the nanoparticle one. The enhanced photocatalytic activity can be attributed to the anisotropic structure of nanorod.

Evolution of single crystalline dendrites from nanoparticles through oriented attachment

Single crystalline PbMoO4 dendrites were prepared by a simple hydrothermal method in the presence of surfactants. The formation and evolution of these dendrites was investigated by transmission electron microscopy, and the results clearly showed that the dendritic structure was achieved through oriented attachment of nanoparticles along crystallographically specific direction while the traditional Ostwald ripening mechanism also acted to form the initial particles before attachment and smooth the morphology of the dendrites after attachment.

Environmentally friendly methodologies of nanostructure synthesis

Environmentally friendly synthetic methodologies have gradually been implemented as viable techniques in the synthesis of a range of nanostructures. In this work, we focus on the application of green-chemistry principles to the synthesis of complex metal oxide and fluoride nanostructures. In particular, we describe advances in the use of the molten-salt synthetic methods, hydrothermal protocols, and template-directed techniques as environmentally sound, socially responsible, and cost-effective methodologies that allow us to generate nanomaterials without the need to sacrifice sample quality, purity, and crystallinity, while allowing control over size, shape, and morphology.

Controllable size, shape and morphology of molybdic acid self-aggregated with rhodamine B to construct functional material

Controllable size, shape and morphology of rhodamine B/molybdic acid (RBMA) aggregates were prepared from a self-aggregation reaction in a molybdic acid and rhodamine B (RhB) coexisting solution. Nanodisks, as well as microcrystal rods and polyhexagonal microcrystal rods, have been obtained in conventional bulk solutions at different temperatures. Large-sized network microcrystal rods and branched fractal aggregates constructed with nanosubunits after the nucleation duration of an ice-water-cooled process have also been achieved under the evaporation-enhanced conditions on glass substrates. The factors affecting the size, shape and morphology of RBMA aggregates including temperature, nucleation and growth, and processing conditions are discussed. The results show that photofunctional molecules (RhB) modified the surface of the molybdic acid particles and influenced their self-aggregation. The temperature and nucleation play key roles in the formation of RBMA aggregates. The structures of RBMA aggregates were characterized by X-ray diffraction, infrared spectra and elemental dispersive spectroscopy. The results indicate that the aggregates show the characteristics of RhB-mediated hydrated ammonium molybdenum bronze with the metastable hexagonal phase. Visible-light-induced electrons transfer reactions in the RBMA aggregates from rhodamine B molecules to MoO3 matrixes were measured by UV-vis spectra and X-ray photoelectron spectra, and the fluorescent image was observed by laser scanning confocal microscopy.

Size- and Shape-Dependent Transformation of Nanosized Titanate into Analogous Anatase Titania Nanostructures

A size- and shape-dependent morphological transformation was demonstrated during the hydrothermal soft chemical transformation, in neutral solution, of titanate nanostructures into their anatase titania counterparts. Specifically, lepidocrocite hydrogen titanate nanotubes with diameters of approximately 10 nm were transformed into anatase nanoparticles with an average size of 12 nm. Lepidocrocite hydrogen titanate nanowires with relatively small diameters (average diameter range of < or = 200 nm) were converted into single-crystalline anatase nanowires with relatively smooth surfaces. Larger diameter (>200 nm) titanate wires were transformed into analogous anatase submicron wire motifs, resembling clusters of adjoining anatase nanocrystals with perfectly parallel, oriented fringes. Our results indicate that as-synthesized TiO2 nanostructures possessed higher photocatalytic activity than the commercial titania precursors from whence they were derived.

Monoclinic Structured BiVO4 Nanosheets: Hydrothermal Preparation, Formation Mechanism, and Coloristic and Photocatalytic Properties

Bismuth vanadate (BiVO(4)) nanosheets have been hydrothermally synthesized in the presence of sodium dodecyl benzene sulfonate (SDBS) as a morphology-directing template. The nanosheets were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with an X-ray energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), IR spectroscopy, transmission electron microscopy (TEM), and high-resolution TEM (HR-TEM). The BiVO(4) nanosheets had a monoclinic structure, were ca. 10-40 nm thick, and showed a preferred (010) surface orientation. The formation mechanism and the effects of reaction temperature and time on the products were investigated. UV-visible diffuse reflection spectra indicated that the BiVO(4) nanosheets had outstanding spectral selectivity and improved color properties compared with the corresponding bulk materials. Furthermore, the nanosheets showed good visible photocatalytic activities as determined by degradation of N,N,N',N'-tetraethylated rhodamine (RB) under solar irradiation.

Two-Step Self-Assembly of Nanodisks into Plate-Built Cylinders through Oriented Aggregation

Oriented aggregation-based self-assembly of hexagonal LaF3 nanodisks with cavities into plate-built cylinders proceeding in acidic solution in the absence of any organic additive was disclosed. The self-assembly consists of two steps. First, the nanodisks sequentially aggregated together by coalescence mainly through {100} planes to form larger monocrystalline plates, followed by Ostwald ripening to smooth their surfaces. The holes on the primary nanodisks should be responsible for this intriguing growth. Second, the surface-smoothed plates were stacked face-to-face with each other along the [001] direction to construct the cylinders. The acidic condition was found to be a prerequisite for the oriented aggregations in this system.