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Klaus Fischer - One of the best experts on this subject based on the ideXlab platform.
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Adsorption of aliphatic polyhydroxy carboxylic acids on gibbsite: pH dependency and importance of Adsorbate Structure
Environmental Sciences Europe, 2018Co-Authors: Tatjana Schneckenburger, Jens Riefstahl, Klaus FischerAbstract:Background Aliphatic (poly)hydroxy carboxylic acids [(P)HCA] occur in natural, e.g. soils, and in technical (waste disposal sites, nuclear waste repositories) compartments . Their distribution, mobility and chemical reactivity, e.g. complex formation with metal ions and radionuclides, depend, among others, on their adsorption onto mineral surfaces. Aluminium hydroxides, e.g. gibbsite [α-Al(OH)_3], are common constituents of related solid materials and mimic the molecular surface properties of clay minerals. Thus, the study was pursued to characterize the adsorption of glycolic, threonic, tartaric, gluconic, and glucaric acids onto gibbsite over a wide pH and (P)HCA concentration range. To consider specific conditions occurring in radioactive wastes, adsorption applying an artificial cement pore water (pH 13.3) as solution phase was investigated additionally. Results The sorption of gluconic acid at pH 4, 7, 9, and 12 was best described by the “two-site” Langmuir isotherm, combining “high affinity” sorption sites (adsorption affinity constants $$K_{\text{L1}}$$ K L1 > 1 L mmol^−1, adsorption capacities
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adsorption of aliphatic polyhydroxy carboxylic acids on gibbsite ph dependency and importance of Adsorbate Structure
Environmental Sciences Europe, 2018Co-Authors: Tatjana Schneckenburger, Jens Riefstahl, Klaus FischerAbstract:Aliphatic (poly)hydroxy carboxylic acids [(P)HCA] occur in natural, e.g. soils, and in technical (waste disposal sites, nuclear waste repositories) compartments . Their distribution, mobility and chemical reactivity, e.g. complex formation with metal ions and radionuclides, depend, among others, on their adsorption onto mineral surfaces. Aluminium hydroxides, e.g. gibbsite [α-Al(OH)3], are common constituents of related solid materials and mimic the molecular surface properties of clay minerals. Thus, the study was pursued to characterize the adsorption of glycolic, threonic, tartaric, gluconic, and glucaric acids onto gibbsite over a wide pH and (P)HCA concentration range. To consider specific conditions occurring in radioactive wastes, adsorption applying an artificial cement pore water (pH 13.3) as solution phase was investigated additionally. The sorption of gluconic acid at pH 4, 7, 9, and 12 was best described by the “two-site” Langmuir isotherm, combining “high affinity” sorption sites (adsorption affinity constants $$K_{\text{L1}}$$ > 1 L mmol−1, adsorption capacities < 6.5 mmol kg−1) with “low affinity” sites ( $$K_{\text{L2}}$$ < 0.1 L mmol−1, adsorption capacities ≥ 19 mmol kg−1). The total adsorption capacities at pH 9 and 12 were roughly tenfold of that at pH 4 and 7. The S-shaped pH sorption edge of gluconic acid was modelled applying a constant capacitance model, considering electrostatic interactions, hydrogen bonding, surface complex formation, and formation of solved polynuclear complexes between Al3+ ions and gluconic acid. A Pearson and Spearman rank correlation between (P)HCA molecular properties and adsorption parameters revealed the high importance of the size and the charge of the Adsorbates. The adsorption behaviour of (P)HCAs is best described by a combination of adsorption properties of carboxylic acids at acidic pH and of polyols at alkaline pH. Depending on the molecular properties of the Adsorbates and on pH, electrostatic interactions, hydrogen bonding, and ternary surface complexation contribute in varying degrees to the adsorption process. Linear distribution coefficients Kd between 8.7 and 60.5 L kg−1 (1 mmol L−1 initial PHCA concentration) indicate a considerable mineral surface affinity at very high pH, thus lowering the PHCA fraction available for the complexation of metal ions including radionuclides in solution and their subsequent mobilization.
R Calarco - One of the best experts on this subject based on the ideXlab platform.
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in gan 0001 3 3 r 30 Adsorbate Structure as a template for embedded in ga n gan monolayers and short period superlattices
Applied Physics Letters, 2017Co-Authors: C Cheze, F Feix, Mariia Anikeeva, Tobias Schulz, M Albrecht, H Riechert, O Brandt, R CalarcoAbstract:We explore an alternative way to fabricate (In, Ga)N/GaN short-period superlattices on GaN(0001) by plasma-assisted molecular beam epitaxy. We exploit the existence of an In Adsorbate Structure manifesting itself by a ( 3 × 3 ) R 30 ° surface reconstruction observed in-situ by reflection high-energy electron diffraction. This In adlayer accommodates a maximum of 1/3 monolayer of In on the GaN surface and, under suitable conditions, can be embedded into GaN to form an In0.33Ga0.67N quantum sheet whose width is naturally limited to a single monolayer. Periodically inserting these quantum sheets, we synthesize (In,Ga)N/GaN short-period superlattices with abrupt interfaces and high periodicity as demonstrated by x-ray diffractometry and scanning transmission electron microscopy. The embedded quantum sheets are found to consist of single monolayers with an In content of 0.25–0.29. For a barrier thickness of 6 monolayers, the superlattice gives rise to a photoluminescence band at 3.16 eV, close to the theoretic...
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in gan 0001 boldsymbol mathsf left sqrt 3 times sqrt 3 right r30 circ Adsorbate Structure as a template for embedded in ga n gan monolayers and short period superlattices
arXiv: Materials Science, 2017Co-Authors: C Cheze, F Feix, Mariia Anikeeva, Tobias Schulz, M Albrecht, H Riechert, O Brandt, R CalarcoAbstract:We explore an alternative way to fabricate (In,Ga)N/GaN short-period superlattices on GaN(0001) by plasma-assisted molecular beam epitaxy. We exploit the existence of an In Adsorbate Structure manifesting itself by a $(\sqrt{3}\times\!\sqrt{3})\text{R}30^{\circ}$ surface reconstruction observed in-situ by reflection high-energy electron diffraction. This In adlayer accommodates a maximum of 1/3 monolayer of In on the GaN surface and, under suitable conditions, can be embedded into GaN to form an In$_{0.33}$Ga$_{0.67}$N quantum sheet whose width is naturally limited to a single monolayer. Periodically inserting these quantum sheets, we synthesize (In,Ga)N/GaN short-period superlattices with abrupt interfaces and high periodicity as demonstrated by x-ray diffractometry and scanning transmission electron microscopy. The embedded quantum sheets are found to consist of single monolayers with an In content of 0.25-0.29. For a barrier thickness of 6 monolayers, the superlattice gives rise to a photoluminescence band at 3.16 eV, close to the theoretically predicted values for these Structures.
Tatjana Schneckenburger - One of the best experts on this subject based on the ideXlab platform.
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Adsorption of aliphatic polyhydroxy carboxylic acids on gibbsite: pH dependency and importance of Adsorbate Structure
Environmental Sciences Europe, 2018Co-Authors: Tatjana Schneckenburger, Jens Riefstahl, Klaus FischerAbstract:Background Aliphatic (poly)hydroxy carboxylic acids [(P)HCA] occur in natural, e.g. soils, and in technical (waste disposal sites, nuclear waste repositories) compartments . Their distribution, mobility and chemical reactivity, e.g. complex formation with metal ions and radionuclides, depend, among others, on their adsorption onto mineral surfaces. Aluminium hydroxides, e.g. gibbsite [α-Al(OH)_3], are common constituents of related solid materials and mimic the molecular surface properties of clay minerals. Thus, the study was pursued to characterize the adsorption of glycolic, threonic, tartaric, gluconic, and glucaric acids onto gibbsite over a wide pH and (P)HCA concentration range. To consider specific conditions occurring in radioactive wastes, adsorption applying an artificial cement pore water (pH 13.3) as solution phase was investigated additionally. Results The sorption of gluconic acid at pH 4, 7, 9, and 12 was best described by the “two-site” Langmuir isotherm, combining “high affinity” sorption sites (adsorption affinity constants $$K_{\text{L1}}$$ K L1 > 1 L mmol^−1, adsorption capacities
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adsorption of aliphatic polyhydroxy carboxylic acids on gibbsite ph dependency and importance of Adsorbate Structure
Environmental Sciences Europe, 2018Co-Authors: Tatjana Schneckenburger, Jens Riefstahl, Klaus FischerAbstract:Aliphatic (poly)hydroxy carboxylic acids [(P)HCA] occur in natural, e.g. soils, and in technical (waste disposal sites, nuclear waste repositories) compartments . Their distribution, mobility and chemical reactivity, e.g. complex formation with metal ions and radionuclides, depend, among others, on their adsorption onto mineral surfaces. Aluminium hydroxides, e.g. gibbsite [α-Al(OH)3], are common constituents of related solid materials and mimic the molecular surface properties of clay minerals. Thus, the study was pursued to characterize the adsorption of glycolic, threonic, tartaric, gluconic, and glucaric acids onto gibbsite over a wide pH and (P)HCA concentration range. To consider specific conditions occurring in radioactive wastes, adsorption applying an artificial cement pore water (pH 13.3) as solution phase was investigated additionally. The sorption of gluconic acid at pH 4, 7, 9, and 12 was best described by the “two-site” Langmuir isotherm, combining “high affinity” sorption sites (adsorption affinity constants $$K_{\text{L1}}$$ > 1 L mmol−1, adsorption capacities < 6.5 mmol kg−1) with “low affinity” sites ( $$K_{\text{L2}}$$ < 0.1 L mmol−1, adsorption capacities ≥ 19 mmol kg−1). The total adsorption capacities at pH 9 and 12 were roughly tenfold of that at pH 4 and 7. The S-shaped pH sorption edge of gluconic acid was modelled applying a constant capacitance model, considering electrostatic interactions, hydrogen bonding, surface complex formation, and formation of solved polynuclear complexes between Al3+ ions and gluconic acid. A Pearson and Spearman rank correlation between (P)HCA molecular properties and adsorption parameters revealed the high importance of the size and the charge of the Adsorbates. The adsorption behaviour of (P)HCAs is best described by a combination of adsorption properties of carboxylic acids at acidic pH and of polyols at alkaline pH. Depending on the molecular properties of the Adsorbates and on pH, electrostatic interactions, hydrogen bonding, and ternary surface complexation contribute in varying degrees to the adsorption process. Linear distribution coefficients Kd between 8.7 and 60.5 L kg−1 (1 mmol L−1 initial PHCA concentration) indicate a considerable mineral surface affinity at very high pH, thus lowering the PHCA fraction available for the complexation of metal ions including radionuclides in solution and their subsequent mobilization.
C Cheze - One of the best experts on this subject based on the ideXlab platform.
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in gan 0001 3 3 r 30 Adsorbate Structure as a template for embedded in ga n gan monolayers and short period superlattices
Applied Physics Letters, 2017Co-Authors: C Cheze, F Feix, Mariia Anikeeva, Tobias Schulz, M Albrecht, H Riechert, O Brandt, R CalarcoAbstract:We explore an alternative way to fabricate (In, Ga)N/GaN short-period superlattices on GaN(0001) by plasma-assisted molecular beam epitaxy. We exploit the existence of an In Adsorbate Structure manifesting itself by a ( 3 × 3 ) R 30 ° surface reconstruction observed in-situ by reflection high-energy electron diffraction. This In adlayer accommodates a maximum of 1/3 monolayer of In on the GaN surface and, under suitable conditions, can be embedded into GaN to form an In0.33Ga0.67N quantum sheet whose width is naturally limited to a single monolayer. Periodically inserting these quantum sheets, we synthesize (In,Ga)N/GaN short-period superlattices with abrupt interfaces and high periodicity as demonstrated by x-ray diffractometry and scanning transmission electron microscopy. The embedded quantum sheets are found to consist of single monolayers with an In content of 0.25–0.29. For a barrier thickness of 6 monolayers, the superlattice gives rise to a photoluminescence band at 3.16 eV, close to the theoretic...
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in gan 0001 boldsymbol mathsf left sqrt 3 times sqrt 3 right r30 circ Adsorbate Structure as a template for embedded in ga n gan monolayers and short period superlattices
arXiv: Materials Science, 2017Co-Authors: C Cheze, F Feix, Mariia Anikeeva, Tobias Schulz, M Albrecht, H Riechert, O Brandt, R CalarcoAbstract:We explore an alternative way to fabricate (In,Ga)N/GaN short-period superlattices on GaN(0001) by plasma-assisted molecular beam epitaxy. We exploit the existence of an In Adsorbate Structure manifesting itself by a $(\sqrt{3}\times\!\sqrt{3})\text{R}30^{\circ}$ surface reconstruction observed in-situ by reflection high-energy electron diffraction. This In adlayer accommodates a maximum of 1/3 monolayer of In on the GaN surface and, under suitable conditions, can be embedded into GaN to form an In$_{0.33}$Ga$_{0.67}$N quantum sheet whose width is naturally limited to a single monolayer. Periodically inserting these quantum sheets, we synthesize (In,Ga)N/GaN short-period superlattices with abrupt interfaces and high periodicity as demonstrated by x-ray diffractometry and scanning transmission electron microscopy. The embedded quantum sheets are found to consist of single monolayers with an In content of 0.25-0.29. For a barrier thickness of 6 monolayers, the superlattice gives rise to a photoluminescence band at 3.16 eV, close to the theoretically predicted values for these Structures.
D P Woodruff - One of the best experts on this subject based on the ideXlab platform.
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Adsorbate Structure determination using photoelectron diffraction methods and applications
Surface Science Reports, 2007Co-Authors: D P WoodruffAbstract:Abstract Photoelectron diffraction is the consequence of the coherent interference of the directly-emitted component of the photoelectron wavefield emitted from a core level of a surface atom with other components elastically scattered by surrounding atoms. The resulting variations of photoemission intensity with emission angle and energy provide a means of determining the local structural environment on the surface in an element-specific fashion. Through exploitation of chemical shifts in the photoelectron binding energy, additional chemical-state specificity can also be achieved. The underlying physics, methodology and applications of this method are reviewed with emphasis on the determination of Adsorbate Structures on surfaces, mainly exploiting the backscattering of low energy photoelectrons, but also including some use of near-forward scattering at higher energies. Examples of applications of the methods are mostly of atomic and molecular Adsorbates on metal surfaces, but also include studies on semiconductor and oxide surfaces.
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an integrated approach to Adsorbate Structure determination using photoelectron diffraction direct imaging and quantitative simulation
Surface Science, 1996Co-Authors: D P Woodruff, A M Bradshaw, K M Schindler, R Davis, N A Booth, Carol J Hirschmugl, O Schaff, V Fernandez, A Theobald, Ph HofmannAbstract:Abstract Using scanned-energy mode photoelectron diffraction (PhD) we have developed a two-stage methodology for the quantitative determination of the local geometry of molecular Adsorbates on surfaces. The first stage involves the inversion of the experimental Adsorbate photoelectron diffraction spectra using a direct method to obtain an “image” of the nearest neighbour substrate atoms. The underlying physics is essentially the same as that of inversion in photoelectron holography, but our method has been shown to be effective for many cases and does not require the collection of additional data. The second stage is to optimise the detailed Structure indicated by this “image” using an iterative trial-and-error comparison of the same experimental spectra with the results of simulations based on multiple scattering calculations. Specific examples of this approach which relate to ammonia, CO and hydrocarbon surface chemistry are outlined in this short review; NH3 and NHx (NH or NH2) adsorbed on Cu(110), CH3O on Ni(111) and C2H2 on Ni(111).
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Adsorbate Structure determination on surfaces using photoelectron diffraction
Reports on Progress in Physics, 1994Co-Authors: D P Woodruff, A M BradshawAbstract:Photoelectron diffraction is the name given to the phenomenon resulting from the coherent interference of the directly emitted component of an electron wavefield, emerging from an atom as a result of core level photoemission, with other components elastically scattered by surrounding atoms. Experimental characterization of this effect provides information which can be used to provide quantitative determinations of the Structure of surfaces, and particularly of adsorbed species on surfaces, in an element-specific fashion. Since the initial. Demonstration of the phenomenon in the late 1970s, an extensive methodology for surface Structure determination has been developed. In this review the background physics of the process, and the development of the technique is described. A brief discussion of the high energy forward scattering version of the technique (X-ray photoelectron diffraction-XPD), which utilizes zero-order diffraction effects, is included, but the most of the review is concerned with the lower energy backscattering method more relevant to the determination of detailed adsorption sites on surfaces. In addition to the general theoretical, experimental and methodology background, a number of the more recent developments are described including use of 'direct inversion' methods for (approximate) Structure determination, including a survey of photoelectron holography, and the realization of chemical shift photoelectron diffraction to allow Structure determinations of surfaces including atoms of one element in more than one inequivalent site. All of the developments are illustrated with specific examples, mainly of molecular and atomic Adsorbates on metal surfaces.
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experimental tests of new direct methods for Adsorbate Structure determination using photoelectron diffraction
Journal of Vacuum Science and Technology, 1994Co-Authors: Ph Hofmann, A M Bradshaw, K M Schindler, V Fritzsche, S Bao, D P WoodruffAbstract:Two recently proposed methods for approximate but direct Adsorbate site determination using scanned‐energy mode photoelectron diffraction data have been tested on experimental data from three different adsorption systems. The three systems studied, O, CO, and C2H4 on Ni(111) involve three different local adsorption sites (single hollow, mixed hollows and near‐atop) and have all been subjected to separate full quantitative Structure determinations. Both the simple Fourier transform approach of Fritsche and Woodruff, and the projection method of Hofmann and Schindler are found to correctly identify all the adsorption sites. The precision of these approximate Structure determinations, and the size of the data bases used, are compared with each other and with other proposed methods.