Facet Growth Research Articles

Strongly facet-dependent activity of iron-doped β-nickel oxyhydroxide for the oxygen evolution reaction.

Iron (Fe)-doped β-nickel oxyhydroxide (β-NiOOH) is a highly active, noble-metal-free electrocatalyst for the oxygen evolution reaction (OER), with the latter being the bottleneck in electrochemical water splitting for sustainable hydrogen production. The mechanisms underlying how the Fe dopant modulates this host material's water electro-oxidation activity are still not entirely clear. Here, we combine hybrid density functional theory (DFT) and Hubbard-corrected DFT to investigate the OER activity of the most thermodynamically favorable (and therefore, expected to be the majority) crystallographic facets of β-NiOOH, namely (0001) and (101̄0). By considering active sites involving both oxidation and reduction of the transition-metal active center during the redox cycle on these two different facets, we show that six-fold-lattice-coordinated Fe in β-NiOOH is redox inactive towards both oxidation and reduction while five-fold-lattice-coordinated Fe in β-NiOOH does exhibit redox activity. However, the determined redox activity of Fe (or lack of it) is not indicative of good (or bad) performance as a dopant on these two facets. Three of the four active sites investigated (oxo and hydroxo sites on (0001) and a hydrated site on (101̄0)) exhibit only a marginal (<0.1 V) decrease or increase in the thermodynamic overpotential upon doping with Fe. Only one of the redox-active sites investigated, the hydroxo site on (101̄0), exhibits a large attenuation in the thermodynamic overpotential upon doping (to ∼0.52 V from 0.86 V), although the doped overpotential is larger than that observed experimentally for Fe-doped NiOOH. Thus, although pure β-NiOOH facets containing four-, five-, or six-fold lattice-coordinated Ni sites have roughly equal OER activities, yielding similar OER onset potentials (shown in A. Govind Rajan, J. M. P. Martirez and E. A. Carter, J. Am. Chem. Soc., 2020, 142, 3600-3612), only those facets containing four-fold lattice-coordinated Fe (e.g., as shown in J. M. P. Martirez and E. A. Carter, J. Am. Chem. Soc., 2019, 141, 693-705) would be active under analogous conditions for the Fe-doped material. It follows that, while undoped β-NiOOH demonstrates a roughly facet-independent oxygen evolution activity, the activity of Fe-doped β-NiOOH strongly depends on the crystallographic facet. Our study further motivates the investigation of strategies for the selective growth of facets with low iron coordination number to enhance the water splitting activity of Fe-doped β-NiOOH.

Efficient catalyst for VOCs obtained by loading active species on metal-doped cordierite

The carrier plays a critical role in dispersing the active species and enhancing their catalytic performances. Cordierite is one of the most widely adopted industrial catalyst carriers. This work investigated the possibility of increasing catalytic performance of MnCu-loaded cordierite by doping transition metals during cordierite production. In brief, Zn, Pb, Cu, Cr, Cd, and Ni were doped into cordierites before MnCu was loaded onto the cordierite. The obtained catalysts were compared in catalytic oxidization of toluene and various characterizations. As a result, Cu doping obviously created lattice distortion of the cordierite, which promoted preferential growth of (−202) facet in CuO phase and (111) facet in Cu2O phase after loading the active species. The preferential growth resulted in a bigger concentration of lattice oxygen as well as more balanced distributions of Cu+/Cu2+ and Mn3+/Mn4+. All these properties helped to decompose toluene by the lattice oxygen, which was subsequently replenished by gaseous O2. Therefore, doping-free cordierite only reached a toluene removal of 35.2 % at 270 °C, while metal-doped one attained a removal close to 100 %. The main result of this work is in favor of producing a more effective industry catalyst carrier.

Developing centimeter-sized CuZr-based metallic glass composites via multi-element microalloying

A multi-element microalloying strategy has been implemented to manipulate the microstructure of CuZr-based bulk metallic glass composites (BMGCs) and, in turn, to improve their mechanical properties. It is found that microalloying with a combination of Ti, Hf, Co, Ni and/or Nb/Sn can effectively refine the austenitic B2–CuZr phase, resulting in finely dispersed B2 crystallites embedded in the BMG matrix while enlarging the sizes of BMGCs. Particularly, an optimum BMGC, i.e., Cu40.75Zr42Al4Ag5Sn0.75Co0.5Ti5Hf1Ni1, can be made by copper-mold casting in the range of diameter from 6.5 to 13.5 mm, much wider than those of BMGCs obtained from microalloying with merely one or two elements. Meanwhile, this centimeter-sized BMGC possesses good mechanical properties with a fracture strength as high as 1960 MPa and a compressive plastic strain up to 6.8 %. Our results demonstrate that multi-element microalloying would reduce the driving force for nucleation of B2 by lowering the melting entropy, and slow down the B2-growth kinetics by promoting faceted crystal growth while retarding multi-component diffusion, both accounting for the making of centimeter-sized BMGCs that contain finely dispersed B2 crystallites in the glass matrix.

Reducing GaN crystal dislocations through lateral growth on uneven seed crystal surfaces using the Na-flux method

In recent years, we have achieved low threading dislocation density in GaN wafers by using the Na-flux multi-point seed technique. However, the resulting wafers exhibit regions of high dislocation density, exceeding 105 cm−2 at the coalescence boundary where pyramidal crystals merge. In this study, we discovered that annealing seed crystals at 900 °C generated an uneven surface with approximately 50 μm of GaN decomposition, and growing GaN on the thus-generated uneven surfaces induced lateral growth composed of facets. We then investigated the effect of the uneven surface on dislocation reduction and found that the average threading dislocation density of the grown crystal was reduced from cm−2 in the seed crystal to cm−2. We confirmed that the reduction in threading dislocation density was due to the termination of dislocations by means of inclusions, and to mergers or annihilation as they encountered one another during facet growth.

The effect of polyvinylpyrrolidone (PVP) on the growth of (1 1 0) crystal facet of BiOCl for enhanced photodegradation of RhB under LED

The effect of polyvinylpyrrolidone (PVP) on the growth of (1 1 0) crystal facet of BiOCl for enhanced photodegradation of RhB under LED

Phase field modelling of hopper crystal growth in alloys

Here we use phase field to model and simulate “hopper” crystals, so named because of their underlying cubic structure but with a hopper-like depression on each of the six faces. Over the past three decades simulations of single phase solidification have successfully explored dendritic structures, in two and three dimensions, formed under high undercooling from a slight perturbation in anisotropy. More recently we see the modelling of faceted structures at near equilibrium, and also, under high undercooling, the formation of dendritic-like structures in two dimensions which retain some faceting in the dendrite arms. A cubic hopper crystal appears to be a hybrid structure, somewhere between a perfect cube and a dendrite, and, to date, has not appeared in the modelling literature. In this paper we describe a model for faceted cubic growth and explore results, necessarily in three dimensions, that include perfect cube, hopper and dendritic. We also touch briefly on one other morphology—octahedral.

Melt Diffusion-Moderated Crystal Growth and its Effect on Euhedral Crystal Shapes

Abstract Crystal growth is often described as either interface-controlled or diffusion-controlled. Here, we study crystal growth in an intermediate scenario where reaction rates at the crystal-melt interface are similar to the rates of diffusive transport of ions through the melt to the advancing crystal surface. To this end, we experimentally investigated euhedral plagioclase crystal shapes in dry mafic (basaltic) and hydrous silicic (haplodacitic) melts. Aspect ratios and inferred relative growth rates of the 3D short (S) and intermediate (I) crystal dimensions vary significantly between mafic and silicic melts, with δS:δI = 1:6–1:20 in basalt and 1:2.5–1:8 in hydrous haplodacite. The lower aspect ratios of plagioclase grown in the silicic melt coincide with 10 to 100× lower melt diffusion rates than in the mafic melt. Using an anisotropic growth model, we show that such differences in melt diffusivity can explain the discrepancy in plagioclase aspect ratios: if interface reaction and melt diffusion rates are of similar magnitude, then the growth of a crystal facet with high interfacial reaction rates may be limited by melt diffusion, while another facet of the same crystal with lower interfacial reaction rates may grow uninhibited by melt diffusivity. This selective control of melt diffusion on crystal growth rates results in progressively more equant crystal shapes as diffusivity decreases, consistent with our experimental observations. Importantly, crystals formed in this diffusion-moderated, intermediate growth regime may not show any classical diffusion-controlled growth features. The proposed model was developed for plagioclase microlites but should be generalisable to all anisotropic microlite growth in volcanic rocks.

Self-organized, periodic nanostructures in epitaxially grown SrRuO3 thin-film

The three-dimensional (3D) nanosized compactness of devices is in high demand for the spintronic-device applicability. However, very few self-organized, periodic magnetic 3D nanostructures of transition metal oxides are known. Here, we report the direct microscopic evidence of a self-assembled, square-shaped, periodic array of coherent orthogonal nanoblock-type structure in an epitaxial SrRuO3 (SRO) thin film (∼90 nm thickness) grown on (001) LaAlO3 (LAO) using pulsed-laser deposition technique. The evolution of different growth facets of SRO over the LAO single crystalline substrate with different thickness are observed. The combined effect of substrate-induced strain and misfit is utilized to grow such self-assembled, ordered nanostructure. The spontaneous formation of such ordered ferromagnetic nano-devices can fill the gap of a miniaturized spintronic device for magnetic memory-based applications.

NV-doped microstructures with preferential orientation by growth on heteroepitaxial diamond

For the wafer-scale fabrication of diamond devices, the growth of diamond substrates by heteroepitaxial chemical vapor deposition is the most promising option currently available. However, the transfer of growth and also structuring processes from small homoepitaxial to larger heteroepitaxial samples is not straightforward and requires adaptation. In this study, we present an approach for the fabrication of functional microstructures including pyramids and mesas as well as more complex structures with hollow centers. The associated methods were previously demonstrated by homoepitaxial growth and are now evaluated on heteroepitaxially grown diamond films. After optimizing the growth procedures to ensure a sufficient quality of the bare diamond substrates, precursor structures for overgrowth were fabricated by e-beam lithography and plasma etching. In the overgrowth of nanopillars, a truncated pyramidal shape was achieved. The characterization with scanning electron microscopy revealed the growth of higher-index facets. Nevertheless, photoluminescence spectroscopy reveals localized doping on the sides of the microstructures. In addition, optically detected magnetic resonance reaches a contrast of 6% of one preferred nitrogen vacancy orientation per facet and a transverse relaxation time T2∗ of 96 ns.

Numerical modeling of faceted crystal growth using a lattice Boltzmann-phase field model with a new interfacial energy function

A lattice Boltzmann-phase field model is proposed to study faceted crystal growth in a supersaturated solution, taking into account a new anisotropic interfacial energy function to reproduce the facet characteristics. In this model, a flow field solver and a concentration field solver are coupled with the phase field equation to study faceted crystal growth with fluid convection and solute transfer. Qualitative and quantitative comparisons are carried out with the experimental observation and theoretical analysis of snowflakes to demonstrate the validity and accuracy of the present numerical method. Then crystallization of sodium chloride in a supersaturated solution is investigated in a 3D space at a constant temperature. The results show that the initial supersaturation, solute depletion rate, and solution flow can greatly change the faceted crystal growth by affecting solute transport and distribution. The mechanism of the formation of morphologies and interface velocity evolution is studied and concluded, which shows that the present model has instructional significance for studying and understanding the regulation mechanism of faceted crystal growth.

Hierarchical structure formation by crystal growth-front instabilities during ice templating.

Directional solidification of aqueous solutions and slurries in a temperature gradient is widely used to produce cellular materials through a phase separation of solutes or suspended particles between growing ice lamellae. While this process has analogies to the directional solidification of metallurgical alloys, it forms very different hierarchical structures. The resulting honeycomb-like porosity of freeze-cast materials consists of regularly spaced, lamellar cell walls which frequently exhibit unilateral surface features of morphological complexity reminiscent of living forms, all of which are unknown in metallurgical structures. While the strong anisotropy of ice-crystal growth has been hypothesized to play a role in shaping those structures, the mechanism by which they form has remained elusive. By directionally freezing binary water mixtures containing small solutes obeying Fickian diffusion, and phase-field modeling of those experiments, we reveal how those structures form. We show that the flat side of lamellae forms because of slow faceted ice-crystal growth along the c-axis, while weakly anisotropic fast growth in other directions, including the basal plane, is responsible for the unilateral features. Diffusion-controlled morphological primary instabilities on the solid-liquid interface form a cellular structure on the atomically rough side of the lamellae, which template regularly spaced "ridges" while secondary instabilities of this structure are responsible for the more complex features. Collating the results, we obtain a scaling law for the lamellar spacing, [Formula: see text] , where [Formula: see text] and [Formula: see text] are the local growth rate and temperature gradient, respectively.

Temperature Gradient Control with an Air-Pocket Design for Growth of High Quality SiC Crystal

The hot-zone design using an air-pocket was adopted to produce uniform temperature gradient in horizontal direction. In order to investigate the change of temperature gradient toward horizontal direction with growth time, the front shape of SiC growing crystal was measured with different growth stages such as initial, growing and finished stage. While SiC ingot grown in conventional hot-zone design exhibited inhomogeneous growth front in the initial stage of growth and multi facet formation in final stage, which could result in increased defect density, a homogeneous temperature gradient and improved crystal quality was obtained in the modified hot-zone design. Based on the mapping measurement of FWHM (Full width at half maximum) value in X-ray rocking curve, the crystal quality of SiC crystals grown with the modified hot-zone design was observed to be definitely better than conventional design.

Synthesis of Petitjeanite Bi3O(OH)(PO4)2 Photocatalytic Microparticles: Effect of Synthetic Conditions on the Crystal Structure and Activity toward Degradation of Aqueous Perfluorooctanoic Acid (PFOA)

The discovery of synthetic Bi3O(OH)(PO4)2 [BOHP] and its application toward photocatalytic oxidation of the water contaminant perfluorooctanoic acid (PFOA) have prompted further interest in development. Despite its high activity toward PFOA degradation, the scarce appearance in the literature and lack of research have left a knowledge gap in the understanding of BOHP synthesis, formation, and photocatalytic activity. Herein, we explore the crystallization of BOHP microparticles via hydrothermal syntheses, focusing on the influence of ions and organics present in the reaction solution when using different hydroxide amendments (NaOH, NH4OH, NMe4OH, and NEt4OH). To better understand the unique structure-activity aspects of BOHP, the related bismuth oxy phosphate (BOP) structural family was also explored, including A-BOP (A = Na+ and K+) and M-BOP derivatives (M = Ca2+, Sr2+, and Pb2+). Results from materials characterization, including X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy, indicated that the crystal structure, morphology, and atomic composition were significantly influenced by solution pH, inorganic metal cations (Na+, K+, Ca2+, Sr2+, and Pb2+), and organic amines. Experiments involving ultraviolet photocatalytic destruction of PFOA by a BOHP suspension revealed that catalytic activity was influenced by the choice of reagents and their variable effect on surface facet growth and crystal defects in the resulting microparticles. Together, this work provides a strategy for crystal facet and surface defect engineering with the potential to expand to other metal oxides within the hydrothermal system.

Insight into the complex coupled growth behavior of Al2O3/YAG eutectic ceramic based on the evolutions of microstructure and crystallographic texture

Clarifying the dependence of microstructure and crystallographic texture on growth kinetics is critical concerns for mechanical properties prediction in complex faceted eutectic growth. In this study, we present the comprehensive investigations of microstructure and crystallographic texture characteristic in directionally solidified Al2O3/YAG eutectic that rely on its coupled growth mechanism. The growth mechanism of eutectic spacing is also well depicted by the Magnin-Kurz (MK) model associated with morphological criteria for branching in faceted phase. As increasing the solidification rate (10 µm/s∼200 µm/s), the regularization tendency for irregular three-dimensional interpenetrated (TDI) structure in geometric category is stronger. Whereas, the enhanced kinetic undercooling increases the misorientation and interfacial energy, which is attributed to the fact that the growth kinetics is more dominant than the minimum interface energy in this work. Additionally, the fluctuation tendency of eutectic spacing and crystallographic orientation with the solidification rate is diametrically opposite. The colony structure consisting of coupled irregular eutectic is formed by the planar interface instability, and its formation mechanism is revealed by the misorientation distribution. This comprehensive research contributes to the design of high-performance eutectic ceramics considering the influence of microstructural features and crystallographic growth modes.

Crack initiation mechanism in a high-strength Ti-5Al-7.5V alloy subjected to high cycle fatigue loading

Fatigue crack initiation mechanism at high cycle regime in a bimodal Ti-5Al-7.5V alloy are investigated by fracture observations, focused-ion-beam cross-section and electron back-scattered diffraction characterizations. The aged Ti-5Al-7.5V exhibits superior tensile and fatigue properties, alongside a wide range of fatigue life when tested at the stress levels of ∼ 700–800 MPa. Fatigue failure was dominated by surface and subsurface crack initiation at facets matching basal planes. These faceted primary α (αp) grains generally have a medium or high Schmid factor (SF) for basal < a > slip. Moreover, a basal faceting process also occurred in the “hard” grains, in which the c-axis of the HCP-α grains was approximately parallel to the loading axis so that the < a > type slips were thought to be suppressed. The observed internal microcracks were formed along the grain boundaries located in clusters of αp grains, especially along (0001) twist boundaries. Furthermore, transmission electron microscope observations evidenced the absence of intense slip bands within faceted grain and fatigued αp grains, which indicates that the facet growth should not along the pre-existing slip bands resulting from high cycle fatigue loadings. The pre-existing structural defects including dislocation networks and subgrain boundaries within αp grains are relatively stable configurations under fatigue loadings. The observed fine (∼500 nm in diameter) silicides were widely dispersed, whether the silicide can act as the crack nucleation site still needs further more detailed studies and this remains an open question.

Surface engineering of Bi2MoO6 as an efficient photoanode in tandem water splitting system by pulsed sonoelectrodeposition

The systematic and controlled synthesis of surface vacancies in semiconductors consider as a promising rout to modify their properties. Therefore, synthetic methods that can create these surface defects in nanomaterials are of great interest. Bismuth molybdate with engineered surface defect growing on its different crystal faces can be an excellent candidate for photoelectrochemical (PEC) water splitting. Simultaneously engineering the growth of the desired crystal face and creating surface vacancies on it would be very difficult. Herein, sonoelectrodeposition (SED) applied with continuous and pulse modes. The two different modes of ultrasound led to create two unexpectedly morphologies with different exposed crystal facets. Pulsed sonoelectrodeposition (PSED) method caused raspberry-like morphology with {010} desired exposed facet including high amounts of stabilized Oxygen vacancies (OVs). High efficiencies of PEC and dye sensitized solar cell (DSSC)/PEC with prepared photoanode originated from high amount of stabilized OVs, different exposed crystal facets and morphologies. The solar to hydrogen (STH) conversion in unassisted water splitting system (DSSC)/PEC was about 3.62%. Results confirmed that pulse mode of ultrasound acts as a proper method for simultaneously engineering of desired facet growth and creation of high amount of stabilized OVs on its surface.

Bismuth Coordination Polymers with Fluorinated Linkers: Aqueous Stability, Bivolatility, and Adsorptive Behavior.

Bismuth metal-organic frameworks and coordination polymers (CP) are challenging to synthesize, given the poor solubility of bismuth precursors and asymmetric and labile ligation of Bi3+ due to its intrinsic lone pair. Here, we synthesize and structurally characterize three Bi3+-CPs, exploiting a tetrafluoroterephtalate (F4BDC) linker to determine the effect of high acidity on these synthesis and coordination challenges. Single-crystal X-ray diffraction characterization showed that pi-pi stacking of linkers directs framework arrangement and generally deters open porosity in the three structures, respectively featuring Bi chains (Bi chain -F 4 BDC), Bi dimers (Bi 2 -F 4 BDC) linked into chains, and Bi tetramers (Bi 4 -F 4 BDC). Powder X-ray diffraction and microscopic imaging show the high purity and stability of these compounds in water. Naphthalenedisulfonate (NDS) was used as a mineralizer in the synthesis of (Bi chain -F 4 BDC) and (Bi 4 -F 4 BDC), and studies of its role in assembly pathways yielded two additional structures featuring mixed NDS and F4BDC, respectively, linking monomer and octamer Bi nodes, and confirmed that F4BDC is the preferred (less labile) linker. Methylene blue (MB) adsorption studies show differing efficacies of the three Bi-F4BDC phases, attributed to surface characteristics of the preferential growth facets, while generally most effective adsorption is attributed to the hydrophobicity of fluorinated ligands. Finally, thermogravimetric analysis of all three Bi-F4BDC phases indicates simultaneous ligand degradation and in situ formation of volatile Bi compounds, which could be exploited in the chemical vapor deposition of Bi-containing thin films.

Electrostructural and morphological features of etch pits in boron-doped HPHT-diamond single crystals and multisectoral plates

Morphological, structural, and nano-electrical features of the growth defects and sectoral boundaries revealed by selective etching in synthetic boron-doped single crystals of diamond (BDD) and multisectoral plates are characterized by a set of scanning probe microscopy (SPM) methods, confocal micro-Raman, and micro-FTIR spectroscopy. Diamond single crystals were grown under high pressure and high temperature conditions (HPHT) in the FeAlBC system. Morphology of dislocation etch pits and micro-defect related pits/protrusions compared both on the growth facets and multisectoral plates cut parallel to the {110} facet of BDD single crystals. Ex situ AFM observation of the etched surface revealed non-homogeneously distributed pits and protrusions over the growth facets correlated with irregular macroscopic growth defects. There is a center-to-periphery gradient of dislocation density in multisectoral plates. The protrusions are homogeneously distributed over plates; some often decorate dislocation pits. Micro-Raman mapping on dislocation pits (boron-rich regions) decorated by low-boron content regions revealed crystalline imperfections in three dimensions, with apparent compressive/tensile strain. The defects were mainly electrically neutral under surface potential mapping by Kelvin probe force microscopy, opposite to the inter-sectoral boundaries having pronounced surface potential steps. The scanning spreading resistance microscopy shows resistivity variations at etching-reviled surface defects and sectoral boundaries if the appropriate DC bias is applied to the SPM tip.

Initial stages of growth of the GaN(11\=22) layer on a nano-structured Si(113) substrate

Scanning electron microscopy was used to study of the initial stages of the formation of a semipolar GaN(1122) layer during Metalorganic Chemical Vapor Deposition on Si(113) substrates, on the surface of which U-shaped grooves with element sizes &amp;lt;100 nm (NP-Si(113)) were formed. It was found that NP-Si(113) substrates with a buffer AlN layer stimulate the formation of islands faceted by the planes m-GaN, c-GaN. It is shown that there is a predominant growth of the m-GaN facet in comparison with c-GaN.The experimental results correspond to the Gibbs--Curie--Wolff selection principle, but taking into account elastic stresses in the c-GaN plane. Keywords: semipolar gallium nitride, nano-structured substrate, silicon.

Highly Monodisperse PbS Quantum Dots with Facet Engineering for High-Radiance Light-Emitting Diodes in the NIR-II Window

Solution-processed PbS quantum dot light-emitting diodes (QLEDs) with emission in the second near-infrared window (NIR-II, 1100–1700 nm) are excellent candidates as light sources for optical communication, night vision, and biomedical monitoring. However, it is still a tremendous challenge to achieve high-radiance PbS QLEDs due to serious QD surface traps and unbalanced charge injection. Herein, highly monodisperse PbS QDs with tailored facet growth were successfully synthesized by the continuous precursor injection method. The synthesized PbS QDs revealed a tunable absorption peak from 1200 to 1700 nm with a supernarrow full width at half-maximum (fwhm; <105 nm). The tailored surface facet growth effectively decreased the nonpolar (100) facet and surface defects. Furthermore, with a multilayered organic–inorganic hybrid architecture of indium tin oxide (ITO)/poly(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)/poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))-diphenylamine) (TFB)/PbS QDs/ZnO/Al, the novel infrared PbS QLED exhibited an excellent maximum radiance of 16.14 W sr–1 m–2 at 6.15 V with a shortwave-infrared electroluminescence at ∼1530 nm. Importantly, the NIR-II QLEDs using facet-tailored PbS QDs with the EL peak between 1100 and 1700 nm represented extremely high radiances. The excellent performances are ascribed to the passivated PbS QDs with a tailored surface facet and the hybrid device structure for charge injection balance. This work provides an effective approach for the preparation of high-quality PbS QDs and is expected to significantly boost the potential commercial applications of PbS NIR-II QLEDs.