Crystal Phase Control Research Articles

Crystal Phase Control Process of Anatase and Rutile TiO2 Nanopowder by Thermal Plasma

Crystal Phase Control Process of Anatase and Rutile TiO₂ Nanopowder by Thermal Plasma

Selective-Area MOCVD Growth and Carrier-Transport-Type Control of InAs(Sb)/GaSb Core-Shell Nanowires.

We report the first selective-area growth of high quality InAs(Sb)/GaSb core-shell nanowires on Si substrates using metal-organic chemical vapor deposition (MOCVD) without foreign catalysts. Transmission electron microscopy (TEM) analysis reveals that the overgrowth of the GaSb shell is highly uniform and coherent with the InAs(Sb) core without any misfit dislocations. To control the structural properties and reduce the planar defect density in the self-catalyzed InAs core nanowires, a trace amount of Sb was introduced during their growth. As the Sb content increases from 0 to 9.4%, the crystal structure of the nanowires changes from a mixed wurtzite (WZ)/zinc-blende (ZB) structure to a perfect ZB phase. Electrical measurements reveal that both the n-type InAsSb core and p-type GaSb shell can work as active carrier transport channels, and the transport type of core-shell nanowires can be tuned by the GaSb shell thickness and back-gate voltage. This study furthers our understanding of the Sb-induced crystal-phase control of nanowires. Furthermore, the high quality InAs(Sb)/GaSb core-shell nanowire arrays obtained here pave the foundation for the fabrication of the vertical nanowire-based devices on a large scale and for the study of fundamental quantum physics.

Mineralization of Calcium Carbonate on Multifunctional Peptide Assembly Acting as Mineral Source Supplier and Template

Crystal phase and morphology of biominerals may be precisely regulated by controlled nucleation and selective crystal growth through biomineralization on organic templates such as a protein. We herein propose new control factors of selective crystal growth by the biomineralization process. In this study, a designed β-sheet Ac-VHVEVS-CONH2 peptide was used as a multifunctional template that acted as mineral source supplier and having crystal phase control ability of calcium carbonate (CaCO3) during a self-supplied mineralization. The peptides formed three-dimensional nanofiber networks composed of assembled bilayer β-sheets. The assembly hydrolyzed urea molecules to one carbonate anion and two ammonium cations owing to a charge relay effect between His and Ser residues under mild conditions. CaCO3 was selectively mineralized on the peptide assembly using the generated carbonate anions on the template. Morphology of the obtained CaCO3 was fiber-like structure, similar to that of the peptide template. The mineralized CaCO3 on the peptide template had aragonite phase. This implies that CaCO3 nuclei, generated using the carbonate anions produced by the hydrolysis of urea on the surface of the peptide assembly, preferentially grew into aragonite phase, the growth axis of which aligned parallel to the direction of the β-sheet fiber axis.

Control of crystal phase of BiVO4 nanoparticles synthesized by microwave assisted method

Different crystalline phases BiVO4 nanoparticles (tetragonal-zircon, monoclinic-scheelite, and tetragonal-zircon/monoclinic-scheelite heterophase) have been prepared by fast microwave assisted method and annealing treatment. For the heterophase, the ratio of tetragonal-zircon and monoclinic-scheelite phases can be well controlled by controlling the annealing temperature. Furthermore, a tight interface junction has been formed between tetragonal-zircon BiVO4 and monoclinic-scheelite BiVO4 in a nanosize level. This tight interface junction can modify the electronic structure of BiVO4 nanoparticles, which would be very helpful for achieving high photocatalytic activity.

Crystal-Phase Control by Solution-Solid-Solid Growth of II-VI Quantum Wires.

A simple and potentially general means of eliminating the planar defects and phase alternations that typically accompany the growth of semiconductor nanowires by catalyzed methods is reported. Nearly phase-pure, defect-free wurtzite II-VI semiconductor quantum wires are grown from solid rather than liquid catalyst nanoparticles. The solid-catalyst nanoparticles are morphologically stable during growth, which minimizes the spontaneous fluctuations in nucleation barriers between zinc blende and wurtzite phases that are responsible for the defect formation and phase alternations. Growth of single-phase (in our cases the wurtzite phase) nanowires is thus favored.

Assembly of monoclinic ZrO2nanorods: formation mechanism and crystal phase control

The crystal phase and shape of ZrO2 nanoparticles were finely tuned by mediating the hydrolysis rate of zirconium cations and using sodium oleate as the capping agent under hydrothermal conditions. Pure monoclinic ZrO2 nanorods with a diameter of ~3 nm and length of 30–40 nm were obtained at a lower pH value of 9.4; whereas monodispersed ZrO2 particles of ~4 nm with mixed monoclinic and tetragonal phases were formed at a higher pH value of 11.4. Their formation mechanism was discussed in terms of the hydrolysis rate of the zirconium cations and the structure-directing role of the oleate species. The monoclinic ZrO2 nanorods showed prominent blue-green fluorescence under excitation by an ultraviolet lamp (365 nm) because of the presence of a large number of oxygen-vacancy defects.

ChemInform Abstract: Crystal Phase‐Controlled Synthesis, Properties and Applications of Noble Metal Nanomaterials

AbstractReview: 50 refs.

Control of crystal phase and morphology in hydrothermal synthesis of BiFeO3 crystal

Bismuth ferrite crystal is synthesized by hydrothermal method. The effect of hydrothermal temperature, KOH concentration, supersaturation, cooling rate, and compactedness on the crystal phase and the morphology of product were investigated. The results indicated that BiFeO3 microcrystalline with good quality could been synthesized at 140–240°C with KOH concentration of 4–14molL−1. In addition, it is favorable for improving the crystal quality to reduce the supersaturation and decrease the cooling rate. With the increasing of the compactedness, surface holes and cracks of the as-grown crystals would be reduced. The optimization of hydrothermal condition provides guidance to grow BiFeO3 crystal with a large size and high quality for practical application.

One‐Pot Synthesis of Mesoporous Anatase‐TiO2(B) Mixed‐Phase Nanowires Decorated with Sulfur and Fe2O3 Nanoparticles for Visible‐Light Photochemical Oxidation

AbstractWe present a facial one‐pot synthesis method for fabrication of mesoporous anatase‐TiO2(B) mixed‐phase nanowires (TNWs) decorated with sulfur and Fe2O3 nanoparticles (NPs). Through one‐pot calcination, control of morphology and crystal phase for TNWs was easily achieved. After calcined at 773 K, mixed‐phase heterostructured TNWs with mesoporous structure and high surface area were obtained. This can not only lead ultrafine NPs well‐disperse in porous matrix, but also greatly enhance interfacial separation and transfer of photo‐induced charges between various planes. Moreover, sulfur doping improved acid/light properties of solid and brought more orbital‐overlapping between active sites and H2O2. The prepared S‐Fe2O3/anatase‐TiO2(B) TNWs had excellent catalytic ability in visible‐light assisted Fenton oxidation at neutral pH, which was attributed to the synergistic effect of sulfur species, NPs and TNWs. The progresses are valuable in developing highly efficient and green heterogeneous catalysts.

A new strategy for achieving vertically-erected and hierarchical TiO2 nanosheets array/carbon cloth as a binder-free electrode for protein impregnation, direct electrochemistry and mediator-free glucose sensing

We present a new approach to directly grow uniform and highly-ordered TiO2 nanosheets array (NSA) on a low-cost flexible carbon cloth substrate while simultaneously fulfill precise TiO2 nanostructure tailoring and crystal phase control. The unique vertically-erected TiO2 NSA/carbon cloth with hierarchical structures was directly explored as electrode for enzyme immobilization and biosensing applications without suffering any influences of insulating binders usually used to fix nanomaterials on conductive substrates during sensor fabrications. Efficient direct electron transfer was successfully achieved for glucose oxidase (GOx) immobilized on the TiO2 NSA/carbon cloth, which produces a stable, mediator-free glucose sensor with good selectivity, high-sensitivity (52μA mM−1cm−2), low response time (<5s) and low detection limit (23.4μM, S/N=3). The mechanism of the superior direct electrochemical properties and sensing performance was investigated in detail, and discussed from the aspects of material nanostructure and crystalline form of TiO2 NSA, and an intimate contact between TiO2 and carbon cloth resulted from direct crystallization and growth of TiO2 nanosheets on the substrate.

Oleylamine‐Assisted Phase‐Selective Synthesis of Cu2−xS Nanocrystals and the Mechanism of Phase Control

Environmentally friendly Cu2−x S compounds exist in many different mixed phases in nature, while their nanoscale counterparts can be pure phase with interesting localized surface plasmon resonance properties. Because of the complexity of composition and phase, controllable synthesis of Cu2−x S nanocrystals becomes an important scientific issue in colloidal chemistry. In this work, a hot‐injection method is developed to synthesize Cu2−x S nanocrystals by injecting a sulfur precursor into a copper precursor using oleylamine and octadecene as solvents. By varying the reaction parameters (temperature, volume ratio of oleylamine/octadecene, molar ratio of Cu/S in the precursors), hexagonal CuS, monoclinic Cu1.75S, and rhombohedral Cu1.8S, nanocrystals can be selectively synthesized, providing a platform to illustrate the mechanism of crystal phase control. The crystal phase control of Cu2−x S nanocrystals is oleylamine‐determined by controlling the molar ratio of Cu/S in the reaction precursors as well as the ratio of Cu2−x S clusters/Cu+ in the subsequent reaction. More importantly, temperature plays an important role in varying the molar ratio of Cu/S and Cu2−x S clusters/Cu+ in the reaction system, which significantly influences the crystal phase of the resulting Cu2−x S nanocrystals. The understanding into crystal control provides a guideline to realize reproducible phase‐selective synthesis and obtain well‐defined high‐quality materials with precise control.

Crystal phase control in GaAs nanowires: opposing trends in the Ga- and As-limited growth regimes

Here we demonstrate the existence of two distinct regimes for tuning crystal structure in GaAs nanowires from zinc blende to wurtzite using a single process parameter: V/III-ratio, or variation of the group V precursor flow. Extensive previous studies have shown that crystal structure is sensitive to V/III-ratio, and even that it is possible to change structure entirely using this single parameter. However, an open question has remained about whether the observed dependencies are related to growth technique or types of precursors used. Specifically, opposite trends have been reported for molecular beam epitaxy (MBE) and metal organic vapour phase epitaxy (MOVPE): while wurtzite GaAs growth is reported for high nominal V/III-ratio in MBE, zinc blende GaAs is formed in MOVPE under apparently the same parameter change (increasing precursor V/III-ratio). Here we show that these observations are not necessarily contradictory, as it may first appear, by providing a consolidated picture covering all regimes in one MOVPE growth machine only. More precisely, we observe wurtzite formation for medium nominal V/III-ratios with a critical sensitivity to the balance between Ga and As supply. Slight deviations from wurtzite conditions will result in zinc blende formation for either low V/III-ratio in the As-limited regime or high V/III-ratio in the Ga-limited regime. Our observations strongly indicate that the applied growth conditions are the crucial ingredients for crystal structure control in GaAs nanowires rather than the growth technique or precursors used.

Controlling the crystal phase and structural quality of epitaxial InAs nanowires by tuning V/III ratio in molecular beam epitaxy

In this study, we demonstrated the control of crystal phase and structural quality of Au-catalyzed InAs nanowires grown on the GaAs {111}B substrates by tuning the V/III ratio in molecular beam epitaxy. It has been found that InAs nanowires can only be grown in a relatively narrow window of the V/III ratio. It is also demonstrated that the V/III ratio can be used to control the structural quality of wurtzite structured and zinc-blende structured InAs nanowires under low V/III ratios, and defect-free wurtzite structured and zinc-blende structured InAs nanowires were successfully achieved. This study provides an insight into the controlled growth of high-quality wurtzite structured and zinc-blende structured InAs nanowires through the V/III ratio engineering.

Complete Colloidal Synthesis of Cu2SnSe3 Nanocrystals with Crystal Phase and Shape Control

Here we report an investigation of systematic control of crystal phase in the ternary nanocrystal system, dicopper tin triselenide. Optimizing the synthetic parameters allows modulation between nucleation and growth in either the hexagonal or cubic phase. In addition to size controlled single crystals, the particles can be tuned to occur as 1D linear heterostructures or 3D tetrapods with growth in one phase and termination in the alternate.

Crystal-phase control of molybdenum carbide nanobelts for dehydrogenation of benzyl alcohol

Belt-shaped molybdenum carbides in α- and β-phases were synthesized by reducing and carburizing a nano-sized α-MoO3 precursor with hydrocarbon-hydrogen mixtures at appropriate temperatures; the β-Mo2C nanobelts with a higher fraction of coordinatively unsaturated Mo sites were more active than the α-MoC1-x nanobelts in dehydrogenation of benzyl alcohol to benzaldehyde.

Mixed-phase TiO2 nanorods assembled microsphere: crystal phase control and photovoltaic application

Here, mixed-phase TiO2 nanorods assembled microspheres were fabricated via a facile one-step hydrothermal process. The synthesized samples were investigated employing X-ray powder diffraction (XRD), Raman spectrometry, field emission-scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The study shows the interesting symbiotic phenomenon occurred in the mixed anatase and rutile phase TiO2. The mixed crystalline phase of anatase–rutile ratios were accurately tuned by adjusting the concentration of disodium edetate (EDTA). The crystalline phase has a great influence on their photovoltaic performance. The photoelectric conversion efficiency increases by 162% when the anatase weight ratio increases from 6% to 51%. While a 263% increase in the photoelectric conversion efficiency of the optimized mixed-phase TiO2 nanorods is achieved when compared to the pure rutile phase TiO2. Accordingly, the DSSCs based on the optimized mixed-phase TiO2 nanorods assembled microspheres show a high open circuit voltage of 0.85 V and a conversion efficiency of 8.85% with a relative low dye adsorption of 0.99 × 10−7 mol cm−2.

High-rate synthesis of Cu–BTC metal–organic frameworks

The reaction conditions for the synthesis of Cu-BTC (BTC = benzene-1,3,5-tricarboxylic acid) were elucidated using a continuous-flow microreactor-assisted solvothermal system to achieve crystal size and phase control. A high-rate synthesis of Cu-BTC metal-organic frameworks with a BET surface area of more than 1600 m(2) g(-1) (Langmuir surface area of more than 2000 m(2) g(-1)) and with a 97% production yield could be achieved with a total reaction time of 5 minutes.

Combinatorial Approaches to Understanding Polytypism in III–V Nanowires

Polytypism in III-V semiconductor nanowires is a topic that has received considerable attention in recent years. Achieving a pure nanowire crystal phase requires well-controlled and advanced parameter tuning for most III-V materials. Additionally, the new and unusual phases sometimes observed may present unique material properties if they can be controllably fabricated. With the prospect of using nanowires in applications within several different fields (including electronics, photonics, and life science), theoretical models are necessary to explain experimental trends and to attain a high level of crystal phase control. At present, there is no theoretical model (or combination of models) that fully explains how and why nanowire crystal structures commonly include several different polytypes. Here we use combinatorics and interlayer interactions to include higher order polytypes (4H and 6H) with the aim to explain nanowire crystal structure beyond the well-investigated zinc blende-wurtzite polytypism. Predictions from our theoretical models compare well with experimental results.

TiO2 Synthesis Inspired by Biomineralization: Control of Morphology, Crystal Phase, and Light-Use Efficiency in a Single Process

Hydroxyapatite is mineralized along the long axis of collagen fiber during osteogenesis. Mimicking such biomineralization has great potential to control inorganic structures and is fast becoming an important next-generation inorganic synthesis method. Inorganic matter synthesized by biomineralization can have beautiful and functional structures that cannot be created artificially. In this study, we applied biomineralization to the synthesis of the only photocatalyst in practical use today, titanium dioxide (TiO(2)). The photocatalytic activity of TiO(2) mainly relates to three properties: morphology, crystal phase, and light-use efficiency. To optimize TiO(2) morphology, we used a simple sequential peptide as an organic template. TiO(2) mineralized by a β-sheet peptide nanofiber template forms fiber-like shapes that are not observed for mineralization by peptides in the shape of random coils. To optimize TiO(2) crystal phase, we mineralized TiO(2) with the template at 400 °C to transform it into the rutile phase and at 700 °C to transform it into a mixed phase of anatase and rutile. To optimize light-use efficiency, we introduced nitrogen atoms of the peptide into the TiO(2) structure as doped elemental material during sintering. Thus, this biomineralization method enables control of inorganic morphology, crystal phase, and light-use efficiency in a single process.

Morphologies and Crystal Phase Control of Calcium Carbonate Using “Crew-Cut” Micelles of Poly (styrene)-&lt;i&gt;b&lt;/i&gt;-Poly (acrylic acid) as a Crystal Growth Modifier at Room Temperature

Cubic layer-structured calcite and spherical vaterite were successfully prepared in the present of PS-b-PAA at room temperature. The products were characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD) and flourier transform infrared spectrum (FI-IR). The characterized results revealed that the PS-b-PAA concentration had a considerable influence on the morphologies and crystal phase transformation. With the PS-b-PAA concentration increasing, the crystal phase of CaCO3particles would have a significant transition from calcite to vaterite, and the morphologies also showed remarkable changes from cubic layer-superstructure to spherical shape. Furthermore, cubic structured CaCO3 particles with a larger size of ca. 100 μm in side length could be prepared by gas diffusion method, indicating that the “crew-cut” micelles are able to result in long-live crystal active sites so that novel large-scale inorganic materials might be prepared.