Hydroxide Monolayers Research Articles

Layer‐by‐Layer Assembly of Na9[EuW10O36]⋅32 H2O and Layered Double Hydroxides Leading to Ordered Ultra‐Thin Films: Cooperative Effect and Orientation Effect

Well-ordered, ultra-thin films (UTFs) based on the hybrid assembly of Na(9)[EuW(10)O(36)]·32H(2)O (denoted as EuW(10)) and exfoliated MgAl layered double hydroxide (LDH) monolayers have been fabricated by utilising a layer-by-layer (LBL) technique. The assembly process was monitored by UV/Vis absorption and fluorescence spectroscopy measurements to get a stepwise and regular growth of the UTFs upon increasing the deposited cycles. In the resulting (EuW(10)/LDH)(n) UTFs, the intercalation of EuW(10) anions into LDH monolayers allows the UTFs to retain their optical properties. Meanwhile, LDH monolayers provide EuW(10) with a confined and protective microenvironment to isolate them from each other between adjacent layers. The UTFs exhibit a periodic, long-range, ordered structure. Anisotropic luminescence spectroscopy measurements show that the (EuW(10)/LDHs)(n) UTFs display well-defined red luminescence with an anisotropy value, r, of about 0.15. Therefore, the cooperative and orientation effects between the host layers and guest anions play significant roles for the improved properties of the (EuW(10)/LDHs)(n) UTFs. This work could benefit the design and fabrication of novel electro-optical devices based on the UTFs of hybrid materials.

Near-Infrared Absorption and Polarized Luminescent Ultrathin Films Based on Sulfonated Cyanines and Layered Double Hydroxide

Organic−inorganic hybrid ultrathin films (UTFs) were fabricated by alternate assembly of sulfonated carbocyanine derivate (Scy) and exfoliated Mg−Al-layered double hydroxide (LDH) monolayers via layer-by-layer (LbL) method, which show uniform near-infrared (NIR) absorption and photoluminescence properties. UV−vis absorption and fluorescence spectroscopy indicate a stepwise and regular growth of the UTFs upon increasing deposition cycles. X-ray diffraction demonstrates that the UTFs possess periodical layered structure with the basal spacing of 3.3 nm; scanning electron microscopy and atomic force microscopy show that the UTFs surface are microscopically smooth and uniform. The Scy/LDH UTFs exhibit well-defined polarized photoemission character with the maximum luminescence anisotropy as high as ∼0.8. Moreover, the Scy/LDH UTFs display higher stability than the pristine Scy, suggesting that the existence of LDH nanosheets enhance the thermal performance of the Scy dye. A molecular dynamics study was carrie...

Anionic Poly(p-Phenylenevinylene)/Layered Double Hydroxide Ordered Ultrathin Films with Multiple Quantum Well Structure: A Combined Experimental and Theoretical Study

The sulfonated phenylenevinylene polyanion derivate (APPV) and exfoliated Mg-Al-layered double hydroxide (LDH) monolayers were alternatively assembled into ordered ultrathin films (UTFs) employing a layer-by-layer method, which shows uniform yellow luminescence. UV-vis absorption and fluorescence spectroscopy present a stepwise and regular growth of the UTFs upon increasing deposited cycles. X-ray diffraction, atomic force microscopy, and scanning electron microscopy demonstrate that the UTFs are orderly periodical layered structure with a thickness of 3.3-3.5 nm per bilayer. The APPV/LDH UTFs exhibit well-defined polarized photoemission characteristic with the maximum luminescence anisotropy of approximately 0.3. Moreover, the UTF exhibit longer fluorescence lifetime (3-3.85-fold) and higher photostability than the drop-casting APPV film under UV irradiation, suggesting that the existence of a LDH monolayer enhances the optical performance of the APPV polyanion. A combination study of electrochemistry and periodic density functional theory was used to investigate the electronic structure of the APPV/LDH system, illustrating that the APPV/LDH UTF is a kind of organic-inorganic hybrid multiple quantum well (MQW) structure with a low band energy of 1.7-1.8 eV, where the valence electrons of APPV can be confined into the energy wells formed by the LDH monolayers effectively. Therefore, this work not only gives a feasible method for fabricating a luminescence ultrathin film but also provides a detailed understanding of the geometric and electronic structures of photoactive polyanions confined between the LDH monolayers.

Thin film of sulfonated zinc phthalocyanine/layered double hydroxide for achieving multiple quantum well structure and polarized luminescence

Ordered thin films based on alternate assembly of zinc phthalocyanine complex and layered double hydroxide monolayers have been fabricated, which exhibit multiple quantum well structure and well-defined polarized luminescence.

Ordered Poly(p‐phenylene)/Layered Double Hydroxide Ultrathin Films with Blue Luminescence by Layer‐by‐Layer Assembly

Lavender layers: A poly(p-phenylene) anionic derivate and exfoliated Mg-Al layered double hydroxide monolayers were assembled into ultrathin films with well-defined blue fluorescence (see picture; the numbers indicate the number of bilayers), long-range order, and high photostability. These films work as multiple quantum-well structures for valence electrons.

Layer-by-layer assembly of ruthenium(ii) complex anion/layered double hydroxide ordered ultrathin films with polarized luminescence

Ordered ultrathin films of tris(1,10-phenanthroline-4,7-diphenylsulfonate)ruthenium(ii) anions and cationic layered double hydroxide monolayers have been fabricated by the layer-by-layer assembly technique and show well-defined polarized red luminescence.

In situ Microstructure Control of Oriented Layered Double Hydroxide Monolayer Films with Curved Hexagonal Crystals as Superhydrophobic Materials.

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In Situ Microstructure Control of Oriented Layered Double Hydroxide Monolayer Films with Curved Hexagonal Crystals as Superhydrophobic Materials

Oriented NiAl-layered double hydroxide (LDH) films with micro-/nanometer scale binary structures are prepared by in situ crystallization, without using any external aluminum source or shape-directing surfactant, on a porous anodic alumina/aluminum substrate. The NiAl-LDH film structures can be controlled by tuning crystallization temperature and time. Facile hydrophobic modification of the film surface leads to superhydrophobicity, as shown in the figure.

In situ cleaning and characterization of oxygen- and zinc-terminated, n-type, ZnO{0001} surfaces

A layer containing an average of 1.0 monolayer (ML) of adventitious carbon and averages of 1.5 ML and 1.9 ML of hydroxide was determined to be present on the respective O-terminated (0001̄) and Zn-terminated (0001) surfaces of ZnO. A diffuse low-energy electron diffraction pattern was obtained from both surfaces. In situ cleaning procedures were developed and their efficacy evaluated in terms of the concentrations of residual hydrocarbons and hydroxide and the crystallography, microstructure, and electronic structure of these surfaces. Annealing ZnO(0001̄) in pure oxygen at 600–650 °C±20 °C reduced but did not eliminate all of the detectable hydrocarbon contamination. Annealing for 15 min in pure O2 at 700 °C and 0.100±0.001 Torr caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits (∼0.03 ML=∼0.3 at. %) of our x-ray photoelectron spectroscopy instrument. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO(0001̄) surface to a remote plasma having an optimized 20% O2/80% He mixture for the optimized time, temperature, and pressure of 30 min, 525 °C, and 0.050 Torr, respectively, resulted in the desorption of all detectable hydrocarbon species. Approximately 0.4 ML of hydroxide remained. The plasma-cleaned surface possessed an ordered crystallography and a step-and-terrace microstructure and was stoichiometric with nearly flat electronic bands. A 0.5 eV change in band bending was attributed to the significant reduction in the thickness of an accumulation layer associated with the hydroxide. The hydroxide was more tightly bound to the ZnO(0001) surface; this effect increased the optimal temperature and time of the plasma cleaning process for this surface to 550 °C and 60 min, respectively, at 0.050 Torr. Similar changes were achieved in the structural, chemical, and electronic properties of this surface; however, the microstructure only increased slightly in roughness and was without distinctive features.

Formation of a nickel hydroxide monolayer on Au through a self-assembled monolayer of 5,5′-dithiobis(2-nitrobenzoic acid): voltammetric, SERS and XPS investigations of the modified electrodes

The formation of self-assembled monolayers (SAM) of 5,5'-dithiobis (2-nitrobenzoic acid), DNBA on gold has enabled further derivatization of the electrode surface with functional moieties anchored to the surface bound molecules. A SAM of DNBA was formed on the Au surface. Nickel ions tethered to the SAM-covered Au surface, were subsequently derivatized electrochemically to yield nickel hydroxide overlayers, thereby showing the possibility of preparing ultra-thin films of metal oxides through solution chemistry. The nickel hydroxide surface coverage obtained on bare and SAM-covered electrodes was estimated from voltammetric peaks and it varied from one monolayer to about 300 monolayers. The formation of a monolayer of nickel hydroxide has been achieved for the first time by electrochemical modification. Further, the modified electrodes were subjected to SERS and XPS studies to understand their surface characteristics. Modified electrodes provide a catalytic pathway involving nickel hydroxide for the electro-oxidation of glucose in alkaline solutions.

Local decomposition of NiO ultra-thin films formed on Ni(111)

AES, LEED and STM have been combined to study the decomposition of NiO(111) and NiO(100) thin films formed on Ni(111) after annealing between 500 and 700K in UHV. The annealing of the two to four layer thick films leads to their partial decomposition and produces stable NiO(111) crystallites and the Ni(111)-(2×2) phase of adsorbed oxygen. The decomposition of the NiO(111) thin films produces an intermediate (3×2)rect. phase assigned to 2D arrangements of residual NiO rows on reduced Ni(111) planes, before the formation of the (2×2) adsorbed phase. The loss of oxygen of the (2×2) adsorbed phase leads to the uncovering of the substrate terraces preferentially on the upper side of the steps, indicating a preferential removal of the oxygen atoms by adsorption in the metal or reactive desorption at the ledges of the superstructure. The formation of a surface monolayer of nickel hydroxide on the NiO(111) thin film shifts the observation of the early stages of decomposition from 550 to 600K with respect to the NiO(111) surface covered by a monolayer of hydroxyl groups. This is attributed to the increase of the film thickness resulting from the surface hydroxidation. The NiO(111) orientation has a higher resistance to decomposition than the NiO(100) one, which is already totally decomposed into the adsorbed phase at 550K. This is assigned to the higher stability of the NiO(111)/Ni(111) interface.

Kinetics and Mechanism of Passivation of Zinc in Dilute Sodium Hydroxide Solutions

AbstractPrevious workers have reported the formation of a monolayer of zinc hydroxide or oxide on zinc amalgam and amalgamated zinc, respectively. In this work the potential step method has been used to detect the number of monolayers formed on solid zinc before passivation occurs in dilute sodium hydroxide solutions. Analysis of the first peak shows two-dimensional formation of a passivating film with instantaneous nucleation. Dependence of the rate constant on hydroxyl ion concentration and potential shows that the rate determining step is the discharge of OH−ions.

Effect of Crystal Structure on the Anodic Oxidation of Nickel

The formation of and on different crystal planes of nickel, and of polycrystalline nickel, was studied in alkaline solution. Oxide films of less than two monolayers were reduced potentiostatically, then current‐potential curves were measured in the range −200 to 1600 mv at different rates of the linearly increasing potential. Electron diffraction and microscopy were used to develop a suitable experimental procedure for dealing with bare and smooth nickel surfaces. The extent of oxide film formation in air on different planes was in the order (110) > (100) > (111). The reactivity on anodization, characterized by the potential at which the equivalent of one monolayer of hydroxide is formed, was in the same order. On continued oxidation the structure of the crystal surface is lost; subsequent reduction forms a strongly disordered surface. The results are discussed with regard to the mechanism of anodization, the extent of differences between crystal planes, and the conditions under which they are maintained or lost.

The Anode Behavior of Germanium in Aqueous Solutions

The anode characteristics of n‐ and p‐type germanium are different. A large voltage barrier is observed at about 0.8 ma/cm2 current density at room temperature on n‐type but not on p‐type electrodes. The voltage barrier on 3 ohm‐cm n‐type germanium anodes breaks down at about 9 volts in many electrolytes. During anodic dissolution the germanium surface appears to be covered with about a monolayer of oxide or hydroxide. This suggests that germanium goes into solution as a complex ion with the hydroxyl or oxide radicals attached. A mechanism is proposed for the over‐all anode dissolution process involving two holes and two electrons for each germanium atom dissolving.