Rough Crystal Surface Research Articles

Synthesis of Cu2O micro/nanocrystals for catalytic combustion of high-concentration CO: The crucial role of glucose

Cubic Cu2O micro/nanocrystals were successfully synthesized by liquid-phase reduction using copper salt of CuSO4 or CuCl2·2H2O, and glucose or ascorbic acid as reducing agent, respectively. The activity of the catalysts was evaluated by light-off curves of CO self-sustained catalytic combustion via temperature-programmed oxidation of CO (CO-TPO), with the results showing the activity of catalysts following the order of Cu2O–Cl-GLU > Cu2O–S-GLU > Cu2O–S-AA > Cu2O–Cl-AA, (Cl denotes CuCl2·2H2O, GLU denotes glucose, S denotes CuSO4 and AA denotes ascorbic acid, respectively), corresponding to the ignition temperature of 109 °C, 122 °C, 137 °C and 186 °C, respectively. The crystal structure, elemental valence, morphology and redox property of the prepared catalysts were analyzed by using various characterization techniques. Combined with in situ infrared spectrum, the CO self-sustained catalytic combustion over Cu2O catalysts mainly follows the Mars-van-Krevelen (M-v-K) mechanism: the adsorbed and activated CO reacts with lattice oxygen to yield CO2 and oxygen vacancy, and then the oxygen vacancy can be replenished by gaseous oxygen. Combined with catalytic performance of high-concentration CO, it is found that the catalysts prepared using glucose as reducing agent are more angular compared with ascorbic acid. The Cu2O–Cl-GLU synthesized with glucose and CuCl2·2H2O exhibits the best catalytic activity among all the catalysts tested, attributing to its more obvious edge and rough crystal surface. The unique structure of Cu2O–Cl-GLU leads to the high exposure rate and coordination unsaturation of atoms on the cubic Cu2O micro/nanocrystals that can improve the ability of activating gaseous O2 and low temperature reducibility, and consequently facilitating the catalytic activity.

Analysis of a fourth-order exponential PDE arising from a crystal surface jump process with Metropolis-type transition rates

We analytically and numerically study a fourth order PDE modeling rough crystal surface diffusion on the macroscopic level. We discuss existence of solutions globally in time and long time dynamics for the PDE model. The PDE, originally derived by the second author, is the continuum limit of a microscopic model of the surface dynamics, given by a Markov jump process with Metropolis type transition rates. We outline the convergence argument, which depends on a simplifying assumption on the local equilibrium measure that is valid in the high temperature regime. We provide numerical evidence for the convergence of the microscopic model to the PDE in this regime.

Electronic structure tailoring of BiOBr (0 1 0) nanosheets by cobalt doping for enhanced visible-light photocatalytic activity

Generally, pristine semiconductor materials are difficult to own excellent photocatalytic performance, and doping is considered an effective strategy to improve their performance. Herein, Co-doped BiOBr (0 1 0) was successfully synthesized. The improved performance could be attributed to the enhanced charge separation efficiency and the expanded light absorption region. Moreover, Co-doped BiOBr (0 1 0) owned relatively rough crystal surface with rich atomic defects and large specific surface area, which leaded to its excellent adsorption performance that contributed to the hole-dominated degradation process. Through density functional theoretical (DFT) calculation, the effect of Co doping on electronic structure of BiOBr was investigated, indicating that an additional energy level inserted into the band gap of Co-doped BiOBr, the band structure of Co-doped BiOBr was more intensive than that of BiOBr, and the CBM and VBM of the Co-doped BiOBr shifted towards lower energy regions. Tetracycline hydrochloride was chosen to further evaluate the photocatalytic performance of as-prepared Co-doped BiOBr (0 1 0), and 83% of tetracycline hydrochloride was degrade within 30 min. Finally, the visible-light driven catalytic mechanism of Co-doped BiOBr (0 1 0) was elucidated. Thus, a feasible strategy was proposed for the fabrication of excellent visible-light-driven photocatalytic materials.

Self-Generation of Surface Roughness by Low-Surface-Energy Alkyl Chains for Highly Stable Superhydrophobic/Superoleophilic MOFs with Multiple Functionalities.

We transformed the hydrophilic metal-organic framework (MOF) UiO-67 into hydrophobic UiO-67-Rs (R=alkyl) by introducing alkyl chains into organic linkers, which not only protected hydrophilic Zr6 O8 clusters to make the MOF interspace superoleophilic, but also led to a rough crystal surface beneficial for superhydrophobicity. The UiO-67-Rs displayed high acid, base, and water stability, and long alkyl chains offered better hydrophobicity. Good hydrophobicity/oleophilicity were also possible with mixed-ligand MOFs containing metal-binding ligands. Thus, a (super)hydrophobic MOF catalyst loaded with Pd centers efficiently catalyzed Sonogashira reactions in water at ambient temperature. Studies of the hydrophobic effects of the coordination interspace and the outer surface suggest a simple de novo strategy for the synthesis of superhydrophobic MOFs that combine surface roughness and low surface energy. Such MOFs have potential for environmentally friendly catalysis and water purification.

Self‐Generation of Surface Roughness by Low‐Surface‐Energy Alkyl Chains for Highly Stable Superhydrophobic/Superoleophilic MOFs with Multiple Functionalities

AbstractWe transformed the hydrophilic metal–organic framework (MOF) UiO‐67 into hydrophobic UiO‐67‐Rs (R=alkyl) by introducing alkyl chains into organic linkers, which not only protected hydrophilic Zr6O8 clusters to make the MOF interspace superoleophilic, but also led to a rough crystal surface beneficial for superhydrophobicity. The UiO‐67‐Rs displayed high acid, base, and water stability, and long alkyl chains offered better hydrophobicity. Good hydrophobicity/oleophilicity were also possible with mixed‐ligand MOFs containing metal‐binding ligands. Thus, a (super)hydrophobic MOF catalyst loaded with Pd centers efficiently catalyzed Sonogashira reactions in water at ambient temperature. Studies of the hydrophobic effects of the coordination interspace and the outer surface suggest a simple de novo strategy for the synthesis of superhydrophobic MOFs that combine surface roughness and low surface energy. Such MOFs have potential for environmentally friendly catalysis and water purification.

Advanced optical simulation of scintillation detectors in GATE V8.0: first implementation of a reflectance model based on measured data

Typical PET detectors are composed of a scintillator coupled to a photodetector that detects scintillation photons produced when high energy gamma photons interact with the crystal. A critical performance factor is the collection efficiency of these scintillation photons, which can be optimized through simulation. Accurate modelling of photon interactions with crystal surfaces is essential in optical simulations, but the existing UNIFIED model in GATE is often inaccurate, especially for rough surfaces. Previously a new approach for modelling surface reflections based on measured surfaces was validated using custom Monte Carlo code. In this work, the LUT Davis model is implemented and validated in GATE and GEANT4, and is made accessible for all users in the nuclear imaging research community. Look-up-tables (LUTs) from various crystal surfaces are calculated based on measured surfaces obtained by atomic force microscopy. The LUTs include photon reflection probabilities and directions depending on incidence angle. We provide LUTs for rough and polished surfaces with different reflectors and coupling media. Validation parameters include light output measured at different depths of interaction in the crystal and photon track lengths, as both parameters are strongly dependent on reflector characteristics and distinguish between models. Results from the GATE/GEANT4 beta version are compared to those from our custom code and experimental data, as well as the UNIFIED model. GATE simulations with the LUT Davis model show average variations in light output of <2% from the custom code and excellent agreement for track lengths with R2 > 0.99. Experimental data agree within 9% for relative light output. The new model also simplifies surface definition, as no complex input parameters are needed. The LUT Davis model makes optical simulations for nuclear imaging detectors much more precise, especially for studies with rough crystal surfaces. It will be available in GATE V8.0.

Solution-grown aligned crystals of diketopyrrolopyrroles (DPP)-based small molecules: Rough surfaces and relatively low charge mobility

Abstract Field-effect transistors (FETs) of three diketopyrrolopyrroles (DPP)-based small molecules, 3,6-bis(5-phenylthiophene-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (PDPPP), 3,6-bis(5-(4-fluorophenyl)thiophene-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (FPDPPPF) and 3,6-bis(5-(4- n -butylphenyl)thiophene-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (BuPDPPPBu), have been studied in this work. Well aligned crystals of the three molecules were grown from para -xylene by droplet-pinned crystallization method. FETs based on these aligned crystals exhibit a hole mobility up to 0.19 cm 2 V −1 s −1 and electron mobility up to 0.008 cm 2 V −1 s −1 . The achieved hole mobility is of the same order of magnitude as reported highest hole mobility for DPP-based small molecules, but it is much lower than that of the high-performance DPP-based polymers. The relative low mobility is mainly attributed to the rough crystal surfaces with steps and, thus, non-smooth charge transport channels at the interfaces between the crystals and the dielectrics. This work has implications for understanding the low charge mobility of DPP-based small molecules.

Some effects of ice crystals on the FSSP measurements in mixed phase clouds

Abstract. In this paper, we show that in mixed phase clouds, the presence of ice crystals may induce wrong FSSP 100 measurements interpretation especially in terms of particle size and subsequent bulk parameters. The presence of ice crystals is generally revealed by a bimodal feature of the particle size distribution (PSD). The combined measurements of the FSSP-100 and the Polar Nephelometer give a coherent description of the effect of the ice crystals on the FSSP-100 response. The FSSP-100 particle size distributions are characterized by a bimodal shape with a second mode peaked between 25 and 35 μm related to ice crystals. This feature is observed with the FSSP-100 at airspeed up to 200 m s−1 and with the FSSP-300 series. In order to assess the size calibration for clouds of ice crystals the response of the FSSP-100 probe has been numerically simulated using a light scattering model of randomly oriented hexagonal ice particles and assuming both smooth and rough crystal surfaces. The results suggest that the second mode, measured between 25 μm and 35 μm, does not necessarily represent true size responses but corresponds to bigger aspherical ice particles. According to simulation results, the sizing understatement would be neglected in the rough case but would be significant with the smooth case. Qualitatively, the Polar Nephelometer phase function suggests that the rough case is the more suitable to describe real crystals. Quantitatively, however, it is difficult to conclude. A review is made to explore different hypotheses explaining the occurrence of the second mode. However, previous cloud in situ measurements suggest that the FSSP-100 secondary mode, peaked in the range 25–35 μm, is likely to be due to the shattering of large ice crystals on the probe inlet. This finding is supported by the rather good relationship between the concentration of particles larger than 20 μm (hypothesized to be ice shattered-fragments measured by the FSSP) and the concentration of (natural) ice particles (CPI data). In mixed cloud, a simple estimation of the number of ice crystals impacting the FSSP inlet shows that the ice crystal shattering effect is the main factor in observed ice production.

Anisotropic etching of the silicon crystal-surface free energy model

Mechanism of the silicon crystal wet chemical etching is still debated. Some aspects related with the fast etching convex corners are not yet completely understood and explained by the birth-and-spread mechanism. Simple model based on the new physical hypothesis consisting two independent mechanisms is proposed. The first one is related to the electrostatic charge presence at the sample interface. This charge is supposed to be responsible for the Si–Si bonds simultaneous breaking in the specific locations of the crystal lattice and the whole mono atom layer (or several layers) release to the etching solution. Etching vector associated with this mechanism is vertically oriented to the sample surface. Second mechanism of the model has etching vector laterally oriented to the (1 1 1) crystal plane and applies to the rough crystal surfaces and to the crystal planes other than (1 0 0) and (1 1 1). In the model atoms which are highly exposed to the etching solution over the sample surface have lower activation energy and are etched off with much higher rates than with the vertical vector mechanism. This can be observed as a rough surface planarization and crystal surface quality improvement with the etching progress. Anisotropic etching model deals with the two stable silicon crystal planes (1 1 1) and (1 0 0), and five semi-stable crystal planes (1 1 0), (1 1 2), (1 1 3), (2 2 1), (3 3 1), which are present at the sample face, during the convex corner mask etching.

Chemical contrast in scanning near‐field optical microscopy

Repeatable and stable scanning near-field optical microscopy (SNOM) images are obtained under shear-force control if uncoated, i.e. cold and sharp, tapered fiber tips are used. True chemical contrast is seen for the first time on organic crystals due to the different near-field reflectivities of different chemical species on partly oxidized anthracene. The topography spans a Z-range of 500 nm. The surface roughness is tolerable and the sites of reaction can be related to crystal structure data. A submicroscopic local resolution of 18 nm has been obtained for chemical contrast, discriminating different chemical species even with poorly reflecting organics on rough crystal surfaces. Thus, reflection-back-to-the-fiber SNOM will find wide applications for both transparent and opaque samples. All drawbacks with blunt and hot metal-coated tips at a <10 nm distance from the surface in previous SNOM setups are overcome. © 1997 John Wiley & Sons, Ltd.

From vicinal to rough crystal surfaces

One generally expects the properties of a vicinal surface to be independent of the existence of steps as soon as these steps overlap, i.e. when their mutual distance is smaller than their width. By using the roughening theory by Nozieres and Gallet [1], we show that, at least for surfaces weakly coupled to the lattice, this overlap occurs for distances significantly larger than the commonly defined width. Our prediction is supported by an analysis of the various measurements of the angular variation of the surface stiffness of helium crystals, which were performed by Wolf et al. [2], Andreeva et al. [3] and Babkin et al. [4]. As a consequence, the interaction between crystal steps should be studied on vicinal surfaces with a much smaller tilt angle than previously thought. This article is also an opportunity to return to the relation between the step width and the correlation length on smooth surfaces, as well as to the treatment of the various finite size effects which occur in the problem of roughening. We finally reconsider how the weak coupling hypothesis applies to the case of helium crystals

Crystalline surfaces in helium

The particular properties of helium at low temperature recently allowed important progress in the general understanding of the structure and dynamics of crystal surfaces. Excellent agreement has been found between experimental results in helium 4 and the renormalization group theories of the roughening transition, which predict that it is an infinite order (Kosterlitz-Thouless) phase transition. The dynamic roughening of a crystal surface, i.e. the influence of a finite growth rate, has been particularly clarified. The dominant growth process of smooth faces close to their roughening transition has been shown to be the two-dimensional nucleation of terraces. In both helium 3 and helium 4, it also appeared possible to measure and analyze the three interface coefficients which describe the dissipation at rough crystal surfaces is non-equilibrium situations. The latter studies provide interesting information on the elementary processes (heat and mass flow) which occur in quantum liquids during their crystallization. A general review of all these phenomena is given.

Elastic scattering of light from a rough crystal surface in spectral region of exciton resonance

The cross-section for elastic scattering of light from CdS crystals is measured in exciton-resonance spectral region. The scattering is shown to result from surface roughness with r.m.s. height ∼10 A ̊ . The effects of correlation between roughness and surface excitonic potential are found which are explained by theory proposed.

Deterioration of Hardened Palm Oil

Hardened palm oil was found to deteriorate with an abnormal increase in acid value (AV) during storage. Therefore, a study was carried out to determine the causes of this and how to prevent it.1) The increase in AV of hardened palm oil depended on the degree of hardening and storage temperature. The increase was marked when the oil had an iodine value of 1030 and contained more elaidic acid than oleic acid. Also it was more marked during storage at 25°C than that at 15°C or 37°C.2) The addition of lipophilic sucrose fatty acid ester (SE-1) or soybean lecithin was effective for the most part in preventing deterioration. However, no effects were observed by the addition of monoglyceride or polyoxyethylene sorbitan fatty acid ester.3) Hardened palm oil without emulsifiers storage at 25°C showed a rough crystal surface with a pile of irregular crystal size under scanning electron microscope. The surface of palm oil samples emulsified with SE-1 or soybean lecithin was smooth.

Role of internal boundary scattering and modifications in Casimir ’s theory

The assumption of perfect roughness of crystal boundaries is modified by considering a partially reflecting, absorbing and rough crystal surface which would make changes in crystal microboundaries and develop the strains inside the crystals to grow internal boundaries. The development of microboundaries and internal strains would shorten the phonon mean free paths and demand a modification in Casimir’s theory. The modified Casimir length describes larger thermal resistance. The heat conductivity of germanium and Mg2Sn in the low-temperature range is calculated. in encountering the internal-boundary scattering, and successful agreements with experimental data are obtained.

Deterioration of Oils and Fats of Hardened Coconut Oil Series. III

Palm kernel oil was purified and hardened, and the samples at each stage of the process including crude oil were stored at -20, 5, 15 and 30°C for 6 months, in the same way as done with the coconut oil, of which A.V., P.O.V. and Co.V. were measured in every month. Correlation of these values with crystal growth was examined by electron microscope.1) Palm kernel oil showed the same tendency as the coconut oil ; increase in A.V. was in the order of those at 15 and 5°C, and the increase was more marked in the hardened oil. No change was recognized in those stored at-20 and 30°C.2) Crystal surface of the hardened palm kernel oil under electron microscope showed the presence of “whiskers” in the oil stored at 5°C and that stored at 15°C showed a very rough crystal surface, both being larger than those of hardened coconut oil. Single crystal was small and few in the oil stored at 20°C and the surface was smooth in the stored at 30°C.3) These experimental results indicated that the changes of A.V. were greater in those with rough crystal surface.

Effects of Electrode Materials and Surface Preparation on CdS-Metal Contacts

Experimental studies on the effects of contact materials and crystal surface preparation on the voltampere characteristics of contacts to CdS single crystals are presented. Rectifying contacts are influenced by the surface preparation prior to electrode application. An artificially roughened or naturally rough crystal surface tends to reduce the rectification ratio of a contact. Two kinds of current fluctuations that may be attributed to the contacts are observed.