Hexagonal Surface Research Articles

Europium-doped MnO2 nanostructures for controlling optical properties and visible light photocatalytic activity

Pure and europium (Eu) doped manganese oxide (MnO2) photocatalysts were produced using a simple sol-gel technique, and their physical, anatomical, and optical characteristics were extensively studied by means of XRD, TEM, EDAX, Raman, UV, PL, and N2 adsorption-desorption analysis. The energy band gap and degree of crystallinity of Eu-doped MnO2 changedwith the presence of Eu. The Eu-doped MnO2 XRD pattern revealed a hexagonal surface morphology with an typical particle size of 30 nm. The occurrence of trivalent europium and its effective doping into the crystalline structure of MnO2 matrix were validated using the EDS method. The catalytic efficiency of Eu-doped MnO2 for methyl orange decomposition was investigated. Under infrared spectrum, photocatalytic studies revealed that phenol was degraded over a Eu doped MnO2 sample in 100 min (464 nm). The enhanced photocatalytic response by Eu doping was also thoroughly addressed.

Microwave synthesis of ZnO microcrystals with novel asymmetric morphology

This paper reports a discovery of novel morphological features of ZnO asymmetric micron-sized single crystals synthesized by microwave irradiation of W/O emulsions containing a Zn source in the inner aqueous phase. The influence of the zinc ion concentration on the size and shape is studied in detail. The morphological and structural properties show that the obtained ZnO powders are almost mono-dispersed ZnO single crystals with a novel morphology of truncated hexagonal pyramids ca. 1 μm in diameter. A scanning precession electron diffraction analysis reveals that the asymmetrically shaped ZnO single crystals thus obtained have a crystallographic c-axis perpendicular to the bottom hexagonal flat surfaces, and annular bright-field imaging indicates that the bottom surfaces are Zn-terminated ZnO(0001)-Zn polar faces. These new findings are led from a combination of well-controllable liquid-phase emulsion synthesis of the oxide powders and highly advanced atomic-resolution TEM imaging technique, suggesting for the first time a highly probable relationship between the polarity of the wurtzite-type crystal structure and the novel asymmetric morphology of the ZnO microcrystals obtained in this study.

Assessing effectiveness of Komagataeibacter strains for producing surface-microstructured cellulose via guided assembly-based biolithography

In this study, a medical device made of surface microstructured bacterial cellulose was produced using cellulose-producing acetic acid bacteria wild-type strains in combination with guided assembly-based biolithography. The medical device aims at interfering with the cell's focal adhesion establishment and maturation around implantable devices placed in soft tissues by the symmetrical array on its surface. A total of 25 Komagataeibacter strains was evaluated over a three-step selection. In the first step, the ability of strains to produce a suitable bacterial cellulose layer with high production yield was examined, then nine strains, with a uniform and smooth layer of bacterial cellulose, were cultured in a custom-made silicone bioreactor and finally the characteristics of the symmetrical array of topographic features on the surface were analysed. Selected strains showed high inter and intra species variability in bacterial cellulose production. The devices obtained by K2G30, K1G4, DSM 46590 (Komagataeibacter xylinus), K2A8 (Komagataeibacter sp.) and DSM 15973T (Komagataeibacter sucrofermentas) strains were pouched-formed with hexagonal surface pattern required for reducing the formation of fibrotic tissue around devices, once they are implanted in soft tissues. Our findings revealed the effectiveness of the selected Komagataeibacter wild-type strains in producing surface microstructured bacterial cellulose pouches for making biomedical devices.

Registry-Dependent Peeling of Layered Material Interfaces: The Case of Graphene Nanoribbons on Hexagonal Boron Nitride

Peeling of layered materials from supporting substrates, which is central for exfoliation and transfer processes, is found to be dominated by lattice commensurability effects in both low and high velocity limits. For a graphene nanoribbon atop a hexagonal boron nitride surface, the microscopic peeling behavior ranges from stick-slip, through smooth-sliding, to pure peeling regimes, depending on the relative orientation of the contacting surfaces and the peeling angle. The underlying mechanisms stem from the intimate relation between interfacial registry, interlayer interactions, and friction. This, in turn, allows for devising simple models for extracting the interfacial adhesion energy from the peeling force traces.

Epitaxial fabrication of topological Bi-Sb alloy films by surface alloying of Sb nanofilms

The bismuth-antimony (Bi-Sb) alloys with some specific Sb concentrations are three-dimensional topological insulators (3D TIs). For the thin Bi-Sb alloy films, the quantum confinement effect can suppress the bulk conduction channels and enhance the transport of the topological surface states. However, the fabrication of Bi-Sb films is still challenging. Here, we have successfully prepared the Bi-Sb alloy films by surface Bi-alloying of the epitaxial Sb nanofilm on highly oriented pyrolytic graphite (HOPG) substrate. The morphology, atomic structure, and electronic structure of the Bi-Sb alloy films were characterized by a combination of scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning tunneling microscopy/spectroscopy (STM/STS). The formation of hexagonal Bi-Sb surface alloy was confirmed by the atomic-resolution STM images. The quantum-well states (QWS) were directly observed in the STS spectra of Bi-Sb films due to the strong quantum confinement effect. The standing-wave patterns of QWS are further observed in the real-space spectra maps. Our work suggests a promising route to fabricate Bi-Sb alloy films and lays the foundation for further exploring their topological electronic structures.

Closure to “Discussion—Fabrication and Testing of Bioinspired Surface Designs for Friction Reduction at the Piston Ring and Liner Interface” (Maddox, S. R., Gangopadhyay, A., Ghaednia, H., Cai, J., Han, X., Meng, X., Goss, J. A., and Zou, M., 2021, ASME J. Tribol., 143(5), p. 051109)

Abstract We thank the discussant for their interest in our manuscript and their very helpful remarks. Existing tribological studies of biomimetic surfaces were mostly focused on dry friction and biological surfaces are highly deformable. Therefore, the learnings on the effects of textures may not be directly translate to fully lubricated interfaces. Nonetheless, we agree that we can still learn much from these studies. Investigating additional orientations of the elongated hexagon could possibly improve the frictional response of the lubricated surfaces. Given that existing literature indicates that orienting the hexagons with two edges perpendicular to the sliding direction yields lower friction than in the case of edges parallel to the sliding direction [1], the experimental conditions in the manuscript could be the worst-case scenario and thus a lower bound for frictional improvement. Additionally, the hexagon was designed based not only on the design of the frog toe (and other natural hexagonal surfaces such as snakes) but also on existing industrial piston cylinder liner designs, where the crossing grooves are oriented nearly perpendicular to the sliding direction [2, 3]. Hence, our tested design is an extension of that existing technology with learning from nature. However, the suggestions to expand the experiment with additional hexagonal orientations is well received and will be considered for future work.

DFT+U study of the electronic structure changes of WO3 monoclinic and hexagonal surfaces upon Cu, Ag, and Au adsorption. Applications for CO adsorption

A periodic DFT+U study about the adsorption of Cu, Ag, or Au on the monoclinic and hexagonal (001) surface of WO3 is presented. The scope is to elucidate the changes in the electronic structure of these materials upon adsorption of Cu, Ag and Au, and to provide some insights about the adsorption of CO. The adsorbed Cu, Ag, and Au atoms provokes the formation of positive charge on the surfaces by transferring electrons to the oxide. The charge is located at the bottom of the conduction band, which causes the Fermi level to move into the conduction band and the materials become semimetals. The CO molecule is weakly adsorbed on the clean surfaces. The adsorption provokes a compression of the surface's states at the conduction band due to the appearance of new states belonging to the CO molecule. This result is a slight stabilization of the bottom of the conduction band, which is translated into a reduction of the band gap upon CO adsorption on the clean surfaces. On the other hand, the CO molecule is also chemisorbed on the doped surfaces through a back-donation bond. The doped surfaces states at the conduction band are also compressed by the appearance of new states belonging to the CO molecule, and the states at the Fermi level seems to increase. The results provide good insights for the application of WO3 in gas sensing chemistry and photocatalysis.

Investigation of Interaction of Noble Metals (Cu, Ag, Au, Pt and Ir) with Nanosheets.

Two-dimensional nanomaterials, such as graphene and hexagonal boron nitride nanosheets, have attracted tremendous interest in the research community and as a starting point for the development of nanotechnology. Using classical applied mathematical modeling, we derive explicit analytical expressions to determine the binding energies of noble metals, including copper, silver, gold, platinum and iridium (Cu, Ag, Au, Pt and Ir) atoms, on graphene and hexagonal boron nitride nanosheets. We adopt the 6–12 Lennard–Jones potential function, together with the continuous approach, to determine the preferred minimum energy position of an offset metal atom above the surface of the graphene and hexagonal boron nitride nanosheets. The main results of this study are analytical expressions of the interaction energies, which we then utilize to report the mechanism of adsorption of the metal atoms on graphene and hexagonal boron nitride surfaces. The results show that the minimum binding energy occured when Cu, Ag, Au, Pt and Ir were set at perpendicular distances in the region from 3.302 Å to 3.683 Å above the nanosheet surface, which correspond to adsorption energies in the region ranging from 0.842 to 2.978 (kcal/mol). Our results might assist in providing information on the interaction energies between the metal atoms and the two-dimensional nanomaterials.

Evidence of room-temperature ferromagnetism in vertically aligned Bi–Co co-doped ZnO nanowires

We are the first to report Bi–Co co-doped zinc oxide nanowires (ZnO NWs) to investigate theoretically and experimentally the alignment of relative spin and the location of Bi3+ and Co2+ as the primary cause of magnetism. In the present study, we carefully performed theoretical and experimental studies of ZnO NWs prepared by the hydrothermal method and obtained exciting outcomes. The x-ray diffraction findings show that the NWs have a wurtzite crystal structure and high intensity at the (002) peak, indicating that most of the reflection comes from the hexagonal surface of the NWs. This means that the extended NWs are mainly aligned along the c-axis. The field-emission scanning electron microscope results show that, due to its anisotropic crystal existence, the grown NWs have a preferred path along the c-axis in the (001) direction. X-ray photoelectron spectroscopy studies established the presence of both Bi and Co in all the Bi–Co co-doped samples. They also showed that OH− groups exist on the surface of ZnO NWs, and increasing Co content tends to increase the O-deficient region in the ZnO matrix. UV–vis analysis showed a decreased optical band gap upon doping, which is probably due to the sp-d exchange interaction between localized d-electron bands of dopants. Optical studies also suggested that the doping induced an increase in electron concentration. The Raman results indicate that the ZnO NWs contain crystallization with few defects after Bi–Co-doping. From the magnetic studies, it was observed that the ferromagnetism probably originates from the defect-rich regions, i.e. NW surface and Bi and Co sit at ZnO’s substitutional site. In contrast, the core of the NWs remains under the other magnetic state. Our density functional theory studies are in line with the experimental results.

Sensitivity to crystal stacking in low-energy electron microscopy

In this work we demonstrate the general characteristics of hcp and fcc stacking in low-energy electron reflectivity for transition metal surfaces, by following the restacking during homoepitaxial growth in real-time. For this purpose, the stacking of a model system, single-crystalline Ag islands during layer-by-layer growth at high temperature on O/W(110), is chosen. Multiple scattering calculations are used to model the relation between electron reflectivity and the crystal geometry. The changes in the electron reflectivity are shown to derive from the changes in the stacking sequence of the topmost surface layers. The results allow to distinguish between the hcp and fcc crystalline arrangements at a surface based on typical differences in the reflectivity curves, making the Ag results relevant for a variety of materials with hexagonal surface geometry. In particular, the multiplet structure within the first Bragg peak in the very low electron energy regime is identified with the fcc structure and thus it can be utilized as a fingerprint to determine the stacking sequence.

Atomic layer deposited boron nitride nanoscale films act as high temperature hydrogen barriers

Hydrogen environmental barrier coatings reduce hydrogen diffusion and concomitant hydrogen embrittlement of materials such as those used in nuclear and fuel cell applications where hydrogen is used as a fuel source. In this work, atomic layer deposition (ALD) was used to coat substrates with boron nitride (BN) films of approximately 6, 8, and 15 nm thicknesses. Differential thermal analysis of the coated samples in hydrogen gas showed resistance to reaction with hydrogen to at least 1713 K. Diffusion of atomic hydrogen into the hexagonal BN (001) surface and between sheets as well as material stability were computationally studied using density functional theory. A high activation energy of 3.25 eV was calculated for atomic hydrogen diffusion into the (001) hexagonal BN surface through a sheet. However, lower activation energies of 1.35 eV, 1.11 eV, and 0.12 eV were computed for unique hydrogen diffusion pathways between sheets, suggesting that sheet orientation parallel to the substrate surface is vital for attaining desirable barrier film properties. A predicted positive nitrogen vacancy formation energy of 4.3 eV at 2773 K suggests that hexagonal BN is stable at nuclear thermal propulsion operating temperatures, and stability was confirmed experimentally up to 1773 K.

Simultaneous exfoliation and functionalization of hexagonal boron nitride in the aqueous phase for the synthesis of high-performance wastewater treatment membrane

A highly permselective membrane is needed for environmental applications due to current industrialization and environmental constraints. However, membrane fouling and firmness problems are encountered during operation in harsh working environments. Hexagonal boron nitride is an encouraging material due to its solidity under various working conditions, but exfoliation using environmentally friendly techniques is challenging. In this study, aqueous-phase exfoliation using tannic acid was demonstrated to be an easy, novel, and green method for preparing functionalized two-dimensional hexagonal boron nitride nanosheets. Microscopic and spectroscopic techniques were used to characterize the exfoliated nanosheets and the synthesized membrane. Tannic acid is adsorbed on the hexagonal boron nitride surface and gradually exfoliates the hexagonal boron nitride to release a few layers of functionalized boron nitride nanosheets; it behaves as a green surfactant by weakening the interlayer interactions. Ultimately, the functionalized exfoliated nanosheets are used to prepare a membrane for environmental applications. Fascinatingly, the prepared membrane is exceedingly stable in water and organic solvents and manifests adequate efficiency in transporting solvents with excellent screening of solutes and long-term antifouling performance. The synthesized membrane's rapid transport of solvents and ample separation efficiency of solutes may be due to the robust nanochannel and thin laminar networks of the nanosheets, which endow the membrane with favorable properties for separation and decontamination developments.

Approximate analytical method for hexagonal slot frequency selective surface analysis

In this article, an efficient approximate analytical method for hexagonal slot frequency selective surface (FSS) analysis is proposed. The main contribution of this article is to derive the tangential incident electric field along each oblique strip complementary to the hexagonal slot, so that Kirchhoff type circuit can be established according to the transmission line parameters and distributed sources. The current distribution is solved using Kirchhoff current law and continuity condition for potential. The transmission characteristic can then be calculated following Munk's scheme. Meanwhile, the physical nature of Munk's scheme is studied. The periodic circuit equations are derived from the corresponding electromagnetic field boundary conditions. The approximate closed-form expressions for aperture field and patch surface current distribution of common FSS elements can be calculated by the method of moments. From the analysis, it is known that the receiving current and transmitting current corresponds to the basis function and test function, respectively. Measured and simulated results of a symmetric hybrid radome composed of hexagonal slot are carried out to verify the accuracy and efficiency of the presented method. The CPU time is about 600 and 0.4 second for CST and the proposed method, respectively.

Catalytic Effect of Ti or Pt in a Hexagonal Boron Nitride Surface for Capturing CO2

We investigated the effect of doping a hexagonal boron nitride surface (hBN) with Ti or Pt on the adsorption of CO2. We performed first-principles molecular dynamics simulations (FPMD) at atmospheric pressure, and 300 K. Pristine hBN shows no interaction with the CO2 molecule. We allowed the Ti and Pt atoms to interact separately, with either a B-vacancy or an N-vacancy. Both Ti and Pt ended chemisorbed on the surface. The system hBN + Ti always chemisorbed the CO2 molecule. This chemisorption happens in two possible ways. One is without dissociation, and in the other, the molecule breaks in CO and O. However, in the case of the Pt atom as dopant, the resulting system repels the CO2 molecule.

Experimental Study of Parametric Dependency of ZnO Nanorods-based Vibration Sensor

Zinc oxide (ZnO) nanorods were grown on rigid substrate (fluorine doped tin oxide, FTO) with different molar concentrations (0.025, 0.075 and 0.125 M) using a low-temperature hydrothermal process. The XRD analysis revealed the formation of hexagonal wurtzite structured ZnO nanorods with (002) plane c-axis orientation of the samples. Morphological analysis by the FESEM method confirmed the formation of ZnO nanorods with hexagonal top surfaces. The electrical properties of the fabricated devices were identified using photoconductivity and impedance studies. Less internal resistance of 1.12 kΩ, turn on the voltage of 0.46 V for 0.075 M sample depicted better piezoelectric property. At 9 Hz resonant frequency, a maximum voltage of 2.13 V and for 1 g acceleration input, the output voltage obtained was 2.49 V for 0.075 M sample which is better than the other two precursors molar concentration varied samples (0.025 and 0.125 M). The sensitivity of this sample was found to be ∼3.05 V/g.

Sulfur doped In2O3-CeO2 hollow hexagonal prisms with carbon coating for efficient photocatalytic CO2 reduction

Developing integrated photocatalysts with advanced structures and desired components is an effective strategy for promoting solar-to-chemical energy conversion. Herein, a novel sulfur-doped In2O3-CeO2 hollow hexagonal prism with a thin carbon coating (S-C/In2O3-CeO2 HHP) is synthesized by directly pyrolyzing Ce-doped In-Materials of Institute Lavoisier-68 (In-MIL-68) hexagonal prism metal–organic frameworks under N2 atmosphere in the presence of sulfur powder, showing excellent photocatalytic carbon dioxide (CO2) reduction efficiency. This synthetic strategy realizes the regulation of structure, components, and surface defects of the final products. The synergistic effects of hollow porous hexagonal prism structure, In2O3-CeO2 heterojunction, and surface defects exhibit advantageous features for CO2 photoreactions with accelerated transfer and separation of charge carriers, enhanced visible-light utilization and CO2 chemisorption ability, and enlarged catalytic active sites. Benefiting from these advantages, the obtained optimized S-C/In2O3-CeO2 HHP exhibits superior gas phase photocatalytic performance for the conversion of CO2 into CH4 and CO, yielding a CH4 yield of 60.6 μmol g−1h−1 and a selectivity of 92.4% under AM1.5 irradiation without noble metal and organic photosensitizer. This work presents a strategy for the rational design and fabrication of efficient photocatalysts with advanced hollow architectures.

Sintesis Mg2+ doped ZnO dengan penambahan albumen ayam ras menggunakan gabungan metode sol-gel dan sonokimia

Mg2+ doped ZnO has been successfully synthesized using a combination of sol-gel and sonochemical methods. Zinc acetate dihydrate was used as a precursor, magnesium chloride hexahydrate as a source of Mg dopant, methanol as a solvent, and chicken albumen was used as an additive to replace monoethanolamine. The sol was heated at 110 °C for 1 hour. The gel formed was calcined at 600 °C for 3 hours. FTIR analysis shows that the absorbance band around 400-450 cm-1 indicates Mg-O stretching, the absorbance band around 500-550 cm-1 indicates Zn-O stretching, the absorbance band around 400-550 cm-1 shows Zn-O-Mg bonds. Mg. Measurements with UV-DRS, obtained the smallest ZnO band gap value doped Mg2+ around 2.7 eV with the addition of 10 mL albumen. The resulting crystal structure was wurtzite with crystal size with the addition of 10, 20, 30, 40 and 50 mL albumen were 25.22-55.91 nm, 32.99-75.87 nm, 45.92-83.91 nm, 45.92-75.89 nm and 46.15-71.47 nm respectively. SEM photo of Mg2+ doped ZnO with the addition of 10 mL of albumen has a hexagonal surface morphology.

Controllable Design of Bifunctional VO2 Coatings with Superhydrophobic and Thermochromic Performances.

The structure and functions of natural organisms provide great inspirational sources for designing and manufacturing bionic coatings, which hold a distinguished scientific promise to tackle challenges facing humans. In this work, we report a facile and controllable approach to prepare various hexagonal periodic array VO2 thin films by simply manipulating the speed of the dip-coating operation. The hexagonal cellular-structured VO2 surface delivered the best thermochromic performance with a Tlum of 79.34% and a ΔTsol of 9.87%. Impressively, superhydrophobic and thermochromic properties could be integrated into hexagonal semi-dome thin films (with a Tlum of 70.9%, a ΔTsol of 9.3%, and a water contact angle of 150°) without any post-treatment by low-surface-energy chemicals, which hold considerable potential for application in multifunctional smart windows. Moreover, based on the Cassie-Baxter mode and finite-difference time-domain calculations, the dependence of the thermochromic and wettability performances on the VO2 structure has been investigated in this study.

Striped electron fluid on (111) KTaO3

A recent study has revealed that the low carrier density electron gas (2DEG) induced at the interface of EuO and (111) KTaO$_3$ exhibits a broken symmetry phase with a strong in-plane anisotropy of the resistivity. We present a minimal tight binding model of this (111) 2DEG, including the large spin-orbit coupling from the Ta ions, which reveals a hexagonal Fermi surface with a highly enhanced 2$k_F$ electronic susceptibility. We argue that repulsive electronic interactions, together with a ferromagnetic EuO substrate, favor a magnetic stripe instability leading to a partially gapped Fermi surface. Such a stripe state, or its vestigial nematicity, could explain the observed transport anisotropy. We propose a $k\cdot p$ theory for the low energy $j=3/2$ states, which captures the key results from our tight-binding study, and further reveals the intertwined dipolar and octupolar modulations underlying this magnetic stripe order. We conclude by speculating on the relation of this stripe order to the superconductivity seen in this material.

Interwoven Hexagonal Frequency Selective Surface: An Application for WiFi Propagation Control

A sub-wavelength band stop unit cell frequency selective surface, comprising six continuously interwoven hexagonal unit cells and arranged as a tessellated surface, is proposed, analyzed, simulated, and measured. The broadband and sub-wavelength characteristics of this unit cell, below the typical $\lambda _{0}/2$ , are analyzed and discussed. It has been demonstrated that by using the unit cell proposed in this paper, a 22% increase in the fractional bandwidth can be achieved along with a substantial improvement in its angular stability at high angles of incidence, compared to similar works previously published. The analysis of this unit cell is performed using the equivalent circuit model that allows relating the number of interweaves with the equivalent capacitance and inductance, thus giving a better understanding of the unit cell characterization. In addition, a frequency selective surface arranged in a 2.5D configuration is also proposed. This 2.5D configuration provides a bandwidth similar to the conventional model but with a reduction in the resonance frequency by 44%. Simulations have been validated using two prototypes, each having an overall dimension of $400\times400$ mm. To demonstrate the scalability of the design, both prototypes are designed to be used as WiFi propagation control, working as a band stop filter at 2.4 and 5.8 GHz, respectively. Transmission coefficients have been measured for TE and TM-modes using an optical bench at angles of incidence from 0 to 60°. These compare favorably with simulations performed using the frequency domain solver in CST Microwave Studio and are found to have a maximum MSE of $2.3\times 10^{-3}$ .