Single Crystal Facets Research Articles

Facet-Dependent Lethality of a Contact Insecticide Crystal.

The activity of crystalline contact insecticides relies on the extraction of surface molecules by insect tarsi upon contact. Most crystals are inherently anisotropic, and surface molecules on symmetry independent faces are expected to have different free energies. The facet-dependent bioavailability and associated efficacy of insect lethality have not been investigated, however. We discriminate the bioactivity of various facets of single crystals of DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane), a well-known contact insecticide. Our findings reveal facet-dependent lethality differences of nearly 75% among four crystallographically unique facets. Furthermore, computations reveal that the respective lethalities of the facets are strongly correlated with the detachment energies of molecules from the crystal surfaces. This facet-dependent lethality suggests a pathway to enhance the efficacy of known contact insecticides through crystal habit control.

(Invited) Dissolution of Organic Overlayers Modified Platinum Single Crystal Interfaces in Acidic Medium

Electrocatalysts design has been an important researching discipline in the filed of electrochemical energy conversion and storage, among which, surface (organic) modification has becoming an efficient, selective, and flexible approach in electrocatalysis study and applications. In the focus of direct modulation on electrocatalysis interfaces, the functionalization of metallic catalysts with organic ligands (i. e. functional organic molecule, ionomers, ionic liquids) has demonstrated its capacity on improving catalytic activity and selectivity in electrochemical reactions such as ORR, CO2RR, as well as oxidation processes. However the electrocatalysts stability behavior and surface degradation dynamics of such modified systems are still not sufficiently studied.To fully decipher the potentials of surface/interface modification in electrocatalysis, we focus our studies on the dissolution behavior of functional organics (i. e. melamine, immidazolium ionic liquids) modified model Pt (hkl) single crystal facets, in acidic medium. In regard of the excellent system sensitivity of our inductively coupled plasma mass spectrometer (ICP-MS, detection limit down to 0.1-1 ng/L), we employed a customized scanning flow cell (SFC) coupled on-line monitoring system to probe the potential-resolved surface metal dissolution dynamics. With designed electrochemical system configuration, we acquired results demonstrating the delicate stability behavior changes over transient and accumulative dissolution study on (modified) Pt facets. In combination of other state-of-art operando spectroscopy methods (electrochemical infrared reflection absorption spectroscopy, electrochemical scanning tunneling microscopy, Raman spectroscopy, etc.), we further comprehended the surface oxidation process, surface blockage/modification mechanisms on electrocatalysis interface. Our research conferred a model stability study approach on surface modification of electrodes and electrocatalysts, providing important insights into the dynamic interfacial interaction between functional modifier and electrocatalysts.

Automated Growth Rate Measurement of the Facet Surfaces of Single Crystals of the β-Form of l-Glutamic Acid Using Machine Learning Image Processing.

Precision measurement of the growth rate of individual single crystal facets (hkl) represents an important component in the design of industrial crystallization processes. Current approaches for crystal growth measurement using optical microscopy are labor intensive and prone to error. An automated process using state-of-the-art computer vision and machine learning to segment and measure the crystal images is presented. The accuracies and efficiencies of the new crystal sizing approach are evaluated against existing manual and semi-automatic methods, demonstrating equivalent accuracy but over a much shorter time, thereby enabling a more complete kinematic analysis of the overall crystallization process. This is applied to measure in situ the crystal growth rates and through this determining the associated kinetic mechanisms for the crystallization of β-form l-glutamic acid from the solution phase. Growth on the {101} capping faces is consistent with a Birth and Spread mechanism, in agreement with the literature, while the growth rate of the {021} prismatic faces, previously not available in the literature, is consistent with a Burton-Cabrera-Frank screw dislocation mechanism. At a typical supersaturation of σ = 0.78, the growth rate of the {101} capping faces (3.2 × 10-8 m s-1) is found to be 17 times that of the {021} prismatic faces (1.9 × 10-9 m s-1). Both capping and prismatic faces are found to have dead zones in their growth kinetic profiles, with the capping faces (σc = 0.23) being about half that of the prismatic faces (σc = 0.46). The importance of this overall approach as an integral component of the digital design of industrial crystallization processes is highlighted.

Impact of Pt(hkl) Electrode Surface Structure on the Electrical Double Layer Capacitance.

The classical theory of the electrical double layer (EDL) does not consider the effects of the electrode surface structure on the EDL properties. Moreover, the best agreement between the traditional EDL theory and experiments has been achieved so far only for a very limited number of ideal systems, such as liquid metal mercury electrodes, for which it is challenging to operate with specific surface structures. In the case of solid electrodes, the predictive power of classical theory is often not acceptable for electrochemical energy applications, e.g., in supercapacitors, due to the effects of surface structure, electrode composition, and complex electrolyte contributions. In this work, we combine ab initio molecular dynamics (AIMD) simulations and electrochemical experiments to elucidate the relationship between the structure of Pt(hkl) surfaces and the double-layer capacitance as a key property of the EDL. Flat, stepped, and kinked Pt single crystal facets in contact with acidic HClO4 media are selected as our model systems. We demonstrate that introducing specific defects, such as steps, can substantially reduce the EDL capacitances close to the potential of zero charge (PZC). Our AIMD simulations reveal that different Pt facets are characterized by different net orientations of the water dipole moment at the interface. That allows us to rationalize the experimentally measured (inverse) volcano-shaped capacitance as a function of the surface step density.

In operando study of gypsum crystal growth through in-cell environmental SEM

Thanks to their setting ability, hydraulic binder materials (e.g., cements, plasters) are used for a wide variety of applications (e.g., construction, medical). The setting reaction occurs through a dissolution–precipitation mechanism during which initial particles dissolve releasing ionic species from which new crystals precipitate. The development of such a microstructure governs the final properties of the material and are thus of uttermost interest.In this paper, the in operando study of the precipitation of gypsum crystals from the dissolution of bassanite particles using a specific cell for scanning electron microscope (SEM) observation is presented. Image analysis based on segmentation enabled to give quantitative results on the growth of crystals at global and local scales. Single crystal facets are indexed based on crystallography and literature results, which enables the quantification of crystal growth rates for the indexed crystallographic planes. Different growth regimes are identified; they are attributed to the transition from unfavored to the most favored crystal facets known for gypsum. The {111} and {110} planes were the fastest growing planes, whereas the {120} and {010} planes were the most stable and grew slowly.

Hydrogenation effect on 2D ZnO single crystal/ZnO–SnO2 two phase ceramics and its enhanced mechanism in H2 gas sensing

In this work, a 1D/2D two-phase structure of ZnO single-crystal nanosheets/ZnO–SnO2 nanorods was constructed, and the hydrogen gas sensing properties including selectivity have been enhanced by this low-cost way without noble metal doping. It was proved that hydrogenation effect on ZnO (0001) single crystal facets was the key to forming the low resistance surface in hydrogen gas, leading significantly enhancement in response and selectivity. The response (Ra/Rg) to 200 ppm hydrogen gas exceeded 400 at a low operating temperature of 150 °C, response/recovery time in 100 ppm H2 was only 30 s/8 s. The sensor also has good selectivity for hydrogen detection. The enhancement in gas sensing properties due to dual mechanism of the hydrogenated low-resistance surface and the resistance modulation of ZnO/SnO2 heterojunctions, which proposed in this paper.

Unveiling Atomic-Scale Product Selectivity at the Cocatalyst-TiO2 Interface Using X-Ray Techniques: Insights into Interface Reactivity.

The microstructure at the interface between the cocatalyst and semiconductor plays a vital role in concentrating photo-induced carriers and reactants. However, observing the atomic arrangement of this interface directly using an electron microscope is challenging due to the coverings of the semiconductor and cocatalyst. To address this, multiple metal-semiconductor interfaces on three TiO2 crystal facets (M/TiO2 ─N, where M represents Ag, Au, and Pt, and N represents the 001, 010, and 101 single crystal facets). The identical surface atomic configuration of the TiO2 facets allowed us to investigate the evolution of the microstructure within these constructs using spectroscopies and DFT calculations. For the first time, they observed the transformation of saturated Ti6c ─O bonds into unsaturated Ti5c ─O and Ti6c ─O─Pt bonds on the TiO2 ─010 facet after loading Pt. This transformation have a direct impact on the selectivity of the resulting products, leading to the generation of CO and CH4 at the Ti6c ─O─Pt and Pt sites, respectively. These findings pinpoint the pivotal roles played by the atomic arrangement at the M/TiO2 ─N interfaces and provide valuable insights for the development of new methodologies using conventional lab-grade equipment.

Ni Ingress and Egress in SrTiO3 Single Crystals of Different Facets

Metal-ion surface interactions and/or doping in host perovskite-oxides are techniques that are widely employed for electronic structure tuning purposes and in developing novel heterogeneous catalysts; however, an in-depth understanding of the different elementary steps and factors involved in these processes is lacking. Herein, we use Atomic Force Microscopy (AFM), Scanning Transmission Electron Microscopy (STEM), and ab initio thermodynamics through density functional theory (DFT) to specifically investigate Ni surface adsorption, ingress, migration, segregation, and egress processes across different SrTiO3 (STO) single-crystal facets and terminations, specifically the (001), (110), and the (111). Under oxidizing and reducing conditions at different temperatures, Ni egress is observed on (110) STO samples, but not the (001). DFT results demonstrate Ni to have a higher thermodynamic egress propensity, specifically through an oxygen-terminated (110) facet in comparison to other (001) terminations, whereas for the (111)-Ti terminated facet, Ni is likely to remain in the bulk post ingress. We suggest that the observed uniqueness of the (110) surface facet toward Ni egress is possibly a consequence of a surface phase transition. These results can help guide design interests with regard to Ni surface stabilization, ingress/egress suppression, or facilitation in STO by elucidating the nuances involved across different facets.

The pervasive presence of oxygen in ZrC

Based on the recent interest in oxy-carbide materials in catalysis, we employ a thin film model concept to highlight that variation of key reaction parameters in the reactive magnetron sputtering of zirconium carbide films (sputtering power, template temperature or reactive plasma environment) under realistic preparation and application conditions often results in zirconium oxy-carbide films of varying stoichiometry. The composition of the films grown on silicon wafers and in vacuo - cleaved NaCl (001) single crystal facets was confirmed by depth profiling X-ray photoelectron spectroscopy and electron microscopy analysis. A correlation between methane-to-argon ratio, excess carbon and template temperature with elemental composition emphasizes the exclusive presence of oxygen-containing zirconium carbides. To generalize the approach, we also show that embedding of highly ordered Cu particles with uniform sizes in zirconium oxy-carbide matrices yields well-defined metal / oxy-carbide interfaces. As the presence of an oxy-carbide and its reactivity has been inextricably linked to enhanced activity and selectivity in a variety of processes, including hydrogenation, oxidation or reduction reactions, our model thin film approach provides the necessary well-defined catalysts to derive mechanistic details and to study the decomposition/re-carburization cycles of oxy-carbides. We have exemplified the concept for zirconium oxy-carbide, but deliberate extension to similar systems is easily possible.

Cover Feature: Strong Structural and Electronic Binding of Bovine Serum Albumin to ZnO via Specific Amino Acid Residues and Zinc Atoms (ChemPhysChem 2/2022)

The Cover Feature illustrates how bovine serum albumin binds to polar and non-polar ZnO single-crystal facets via its specific amino acids. Atomic scale calculations using density-functional tight-binding models are shown in the circles for specific binding cases, depending also on ZnO facet orientations. More information can be found in the Article by Hadi Hematian and co-workers.

Nanoscale Reactivity Mapping of a Single-Crystal Boron-Doped Diamond Particle.

Boron-doped diamond (BDD) is most often grown by chemical vapor deposition (CVD) in polycrystalline form, where the electrochemical response is averaged over the whole surface. Deconvoluting the impact of crystal orientation, surface termination, and boron-doped concentration on the electrochemical response is extremely challenging. To tackle this problem, we use CVD to grow isolated single-crystal microparticles of BDD with the crystal facets (100, square-shaped) and (111, triangle-shaped) exposed and combine with hopping mode scanning electrochemical cell microscopy (HM-SECCM) for electrochemical interrogation of the individual crystal faces (planar and nonplanar). Measurements are made on both hydrogen- (H-) and oxygen (O-)-terminated single-crystal facets with two different redox mediators, [Ru(NH3)6]3+/2+ and Fe(CN)64-/3-. Extraction of the half-wave potential from linear sweep and cyclic voltammetric experiments at all measurement (pixel) points shows unequivocally that electron transfer is faster at the H-terminated (111) surface than at the H-terminated (100) face, attributed to boron dopant differences. The most dramatic differences were seen for [Ru(NH3)6]3+/2+ when comparing the O-terminated (100) surface to the H-terminated (100) face. Removal of the H-surface conductivity layer and a potential-dependent density of states were thought to be responsible for the behavior observed. Finally, a bimodal distribution in the electrochemical activity on the as-grown H-terminated polycrystalline BDD electrode is attributed to the dominance of differently doped (100) and (111) facets in the material.

Spatial Anisotropy of Charge Transfer at Perfluoropentacene-Pentacene (001) Single-Crystal Interfaces and its Relevance for Thin Film Devices.

Archetypal donor-acceptor (D-A) interfaces composed of perfluoropentacene (PFP) and pentacene (PEN) are examined for charge transfer (CT) state formation and energetics as a function of their respective molecular configuration. To exclude morphological interference, our structural as well as highly sensitive differential reflectance spectroscopy studies were carried out on PFP thin films epitaxially grown on PEN(001) single-crystal facets. Whereas the experimental data supported by complementary theoretical calculations confirm the formation of a strong CT state in the case of a cofacial PFP-PEN stacking, CT formation is energetically less favorable and thus absent for the corresponding head-to-tail configuration as disclosed for the first time. In view of technological implementations, the knowledge gained on the single-crystal references is transferred to thin-film diodes composed of either stacked PFP/PEN bilayers or mixed PFP:PEN heterojunction interfaces. As demonstrated, their electronic and electroluminescent behavior can be consistently described by the absence or presence of interfacial CT states. Thus, our results hint at the thorough design of D-A interfaces to achieve the highest device performances.

Selective Cl-Decoration on Nanocrystal Facets of Hematite for High-Efficiency Catalytic Oxidation of Cyclohexane: Identification of the Newly Formed Cl-O as Active Sites.

Understanding the structure-reactivity relationship at the atomic scale is of great theoretical importance for rational design of highly active catalysts, which has long been a central concern in catalysis communities and interface science. Herein, we developed a high-efficiency catalyst for catalytic oxidation of C6H12 by poststructural decoration on well-defined single-crystal facets of hematite. Especially for Cl-decorated {012} facets, the conversion and KA oil selectivity are improved about 3.4 times and 2 times, respectively. A better catalytic performance of the newly formed active site is derived from the charge difference between Cl and the neighboring outmost O atoms, which is affected by the geometric and electronic structures of the original catalyst surface. Based on the experimental results and the theoretical analysis, we concluded that the contribution of various O terminations to Cl-decoration follows the order O(I) > O(III) > O(II). Cl-decorated {001} facets show the highest intrinsic activity, whereas Cl-decorated {012} facets show the best catalytic performance because of their more active sites for Cl-decoration.

Facile Strategy for Facet Competition Management to Improve the Performance of Perovskite Single-Crystal X-ray Detectors.

Methylammonium lead tribromide perovskite single crystals have been demonstrated to be good candidates as sensitive X-ray detectors in direct detection mode in recent years. However, its X-ray detection performance based on the orientation of different facets is still not clear. Here, we developed a facile strategy to chemically expose the [110] facet of single crystals from low-cost solution processes by tailoring the nonstoichiometry of feeding ions to selectively suppress the growth of the [100] facet. In contrast to physically cutting and sawing single-crystal ingots, this avoids damage to the fragile single crystals as well as orientation errors, more suitable for the naturally soft lattice. Compared to the [100] facet, the exposed [110] facet of perovskite single crystals exhibits a smaller trap density and excellent charge carrier transportation properties, leading to an improved sensitivity of 3928.3 μC/Gyair/cm2 to 120 keV hard X-rays, which potentially outperforms the currently dominating CsI scintillator of a commercial digital radiography (DR) medical imager for a routine health check.

Computational design of metal-supported molecular switches: transient ion formation during light- and electron-induced isomerisation of azobenzene

In molecular nanotechnology, a single molecule is envisioned to act as the basic building block of electronic devices. Such devices may be of special interest for organic photovoltaics, data storage, and smart materials. However, more often than not the molecular function is quenched upon contact with a conducting support. Trial-and-error-based decoupling strategies via molecular functionalisation and change of substrate have in many instances proven to yield unpredictable results. The adsorbate-substrate interactions that govern the function can be understood with the help of first-principles simulation. Employing dispersion-corrected density-functional theory (DFT) and linear expansion delta-self-consistent-field DFT, the electronic structure of a prototypical surface-adsorbed functional molecule, namely azobenzene adsorbed to (1 1 1) single crystal facets of copper, silver and gold, is investigated and the main reasons for the loss or survival of the switching function upon adsorption are identified. The light-induced switching ability of a functionalised derivative of azobenzene on Au(1 1 1) and azobenzene on Ag(1 1 1) and Au(1 1 1) is assessed based on the excited-state potential energy landscapes of their transient molecular ions, which are believed to be the main intermediates of the experimentally observed isomerisation reaction. We provide a rationalisation of the experimentally observed function or lack thereof that connects to the underlying chemistry of the metal-surface interaction and provides insights into general design strategies for complex light-driven reactions at metal surfaces.

Insights into Different Photocatalytic Oxidation Activities of Anatase, Brookite, and Rutile Single-Crystal Facets

For the understanding of the activity of TiO2 photocatalysts, knowledge of the activities of different crystal facets is necessary. This information can be achieved by the investigation of well-defined single-crystalline TiO2 surfaces. In this study, the photocatalytic activity of different anatase, brookite, and rutile single-crystal wafers with only one exposed surface has been investigated via the oxidation of methanol and the hydroxylation of terephthalic acid, respectively. X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements have shown that all surfaces are clearly defined and possess a smooth surface, which allows a reliable comparison of the photocatalytic activities. The investigated anatase surfaces show higher activity than the rutile surfaces, while the brookite surface is interestingly the least active one. To the best of our knowledge, there are no other reports based on the investigation and comparison of well-defined TiO2 anatase (100), anatase (001), a...

Complex oxide thin films: Pyrochlore, defect fluorite and perovskite model systems for structural, spectroscopic and catalytic studies

Well-ordered thin films of different defect fluorite and perovskite materials, namely lanthanum zirconate (La2Zr2O7), cerium zirconate (Ce2Zr2O7), lanthanum cerate (La2Ce2O7) and lanthanum strontium ferrite (La0.4Sr0.6FeO3), have been prepared by sputtering the respective powder targets onto vacuum-cleaved NaCl(0 0 1) single crystal facets. Characterization specifically also includes the sophisticated preparation of the initial target materials. For the defect fluorite materials, the target materials are the respective pyrochlore compounds, which reproducibly transform to the respective defect fluorite structures upon sputtering. At template temperatures of around 573 K, well-crystallized epitaxial thin films of the defect fluorite compounds La2Zr2O7 and Ce2Zr2O7 result, whereas for the perovskite compound post-annealing procedures at 973 K in air are necessary to obtain epitaxial ordering of orthorhombic structures. Epitaxial relations for the well-ordered defect fluorite thin films are determined to be La2Zr2O7(0 0 1)//NaCl(0 0 1) and Ce2Zr2O7(0 0 1)//NaCl(0 0 1), respectively. Structural and spectroscopic characterization of the films by (high-resolution) electron microscopy (HR-TEM), selected-area electron diffraction (SAED) and depth-profiling X-ray photoelectron spectroscopy (XPS) reveal that the structures and compositions of the initial target materials are well-preserved during the sputtering process. Following this preparation routine, access to well-ordered thin films of complex oxide materials with defined stoichiometry is therefore granted, which can subsequently be used as model systems for studies of their material’s or catalytic properties, as presented for simpler systems earlier. As a primary example, we show that for the defect fluorite thin films of Ce2Zr2O7 the defect fluorite-pyrochlore phase transformation can be observed at around 1123 K as detected by in situ electron diffraction.

Native oxide formation on pentagonal copper nanowires: A TEM study

Hydrothermally synthesized copper nanowires were allowed to oxidize in air at room temperature and 30% constant humidity for the period of 22 days. The growth of native oxide layer was followed up by high-resolution transmission electron microscopy and diffraction to reveal and understand the kinetics of the oxidation process. Copper oxides appear in the form of differently oriented crystalline phases around the metallic core as a shell-like layer (Cu2O) and as nanoscopic islands (CuO) on the top of that. Time dependent oxide thickness data suggests that oxidation follows the field-assisted growth model at the beginning of the process, as practically immediately an oxide layer of ∼2.8 nm thickness develops on the surface. However, after this initial rapid growth, the local field attenuates and the classical parabolic diffusion limited growth plays the main role in the oxidation. Because of the single crystal facets on the side surface of penta-twinned Cu nanowires, the oxidation rate in the diffusion limited regime is lower than in polycrystalline films.

Noble metal free photocatalytic H2 generation on black TiO2: On the influence of crystal facets vs. crystal damage

In this study, we investigate noble metal free photocatalytic water splitting on natural anatase single crystal facets and on wafer slices of the [001] plane before and after these surfaces have been modified by high pressure hydrogenation and hydrogen ion-implantation. We find that on the natural, intact low index planes, photocatalytic H2 evolution (in the absence of a noble metal co-catalyst) can only be achieved when the hydrogenation treatment is accompanied by the introduction of crystal damage, such as simple scratching and miscut in the crystal, or by implantation damage. X-ray reflectivity, Raman, and optical reflection measurements show that plain hydrogenation leads to a ≈ 1 nm thick black titania surface layer without activity, while a colorless, density modified, and ≈7 nm thick layer with broken crystal symmetry is present on the ion implanted surface. These results demonstrate that (i) the H-treatment of an intact anatase surface needs to be combined with defect formation for catalytic activation and (ii) activation does not necessarily coincide with the presence of black color.

Carbon tolerance of Ni–Cu and Ni–Cu/YSZ sub-μm sized SOFC thin film model systems

Thin films of YSZ, unsupported Ni–Cu 1:1 alloy phases and YSZ-supported Ni–Cu 1:1 alloy solutions have been reproducibly prepared by magnetron sputter deposition on Si wafers and NaCl(001) single crystal facets at two selected substrate temperatures of 298K and 873K. Subsequently, the layer properties of the resulting sub-μm thick thin films as well as the tendency towards carbon deposition following treatment in pure methane at 1073K has been tested comparatively. Well-crystallized structures of cubic YSZ, cubic NiCu and cubic NiCu/YSZ have been obtained following deposition at 873K on both substrates. Carbon is deposited on all samples following the trend Ni–Cu (1:1)=Ni–Cu (1:1)/YSZ>pure YSZ, indicating that at least the 1:1 composition of layered Ni–Cu alloy phases is not able to suppress the carbon deposition completely, rendering it unfavorable for usage as anode component in sub-μm sized fuel cells. It is shown that surfaces with a high Cu/Ni ratio nevertheless prohibit any carbon deposition.