Although [Ni(S 2 CNBu i 2) 2 ] is stable at high temperatures in a range of solvents, solvotherma... more Although [Ni(S 2 CNBu i 2) 2 ] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145°C in oleylamine to give pure NiS nanoparticles, while in n-hexylamine at 120°C a mixture of Ni 3 S 4 (polydymite) and NiS results. A combined experimental and theoretical study gives mechanistic insight into the decomposition process and can be used to account for the observed differences. Upon dissolution in the primary amine, octahedral trans-[Ni(S 2 CNBu i 2) 2 (RNH 2) 2 ] result as shown by in situ XANES and EXAFS and confirmed by DFT calculations. Heating to 90−100°C leads to changes consistent with the formation of amide-exchange products, [Ni(S 2 CNBu i 2){S 2 CN(H)R}] and/or [Ni{S 2 CN(H)R} 2 ]. DFT modeling shows that exchange occurs via nucleophilic attack of the primary amine at the backbone carbon of the dithiocarbamate ligand(s). With hexylamine, amide-exchange is facile and significant amounts of [Ni{S 2 CN(H)Hex} 2 ] are formed prior to decomposition, but with oleylamine, exchange is slower and [Ni(S 2 CNBu i 2){S 2 CN-(H)Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100°C to afford nickel sulfides, even in the absence of primary amine, as shown from thermal decomposition studies of [Ni{S 2 CN(H)Hex} 2 ]. DFT modeling of [Ni{S 2 CN(H)R} 2 ] shows that proton migration from nitrogen to sulfur leads to formation of a dithiocarbimate (S 2 CNR) which loses isothiocyanate (RNCS) to give dimeric nickel thiolate complexes [Ni{S 2 CN(H)R}(μ-SH)] 2. These intermediates can either lose dithiocarbamate(s) or extrude further isothiocyanate to afford (probably amine-stabilized) nickel thiolate building blocks, which aggregate to give the observed nickel sulfide nanoparticles. Decomposition of the single or double amide-exchange products can be differentiated, and thus it is the different rates of amideexchange that account primarily for the formation of the observed nanoparticulate nickel sulfides.
An in situ XAS study of the solvothermal decomposition of iron and nickel dithiocarbamate complex... more An in situ XAS study of the solvothermal decomposition of iron and nickel dithiocarbamate complexes was performed in order to gain understanding of the decomposition mechanisms. This work has given insight into the steps involved in the decomposition, showing variation in reaction pathways between the iron and nickel dithiocarbamates, and the non-innocent role of oleylamine as the solvent and capping agent in the reaction.
It is generally accepted that cation dehydration is the rate-limiting step to crystal growth from... more It is generally accepted that cation dehydration is the rate-limiting step to crystal growth from aqueous solution. Here we employ classical molecular dynamics simulations to show that the water exchange frequency at structurally distinct calcium sites in the calcite surface varies by about two orders of magnitude. The decrease in water exchange frequency with progressive embedding of surface calcium ions is thought to be rate limiting to subsequent attachment of carbonate ions during calcite growth. Therefore, a process-based calcite growth kinetics model, reparameterized using the water exchange frequencies computed from molecular dynamics simulations, is used to illustrate the impact of these variations on kink-formation rate, step velocities and bulk growth rate. The calculated frequencies of kink formation show a strong variation with surface structures, which can be amplified depending on the saturation state and calcium to carbonate ratio of the solution. Modelled and measured step velocities and bulk growth rates are generally in agreement, showing that variations in calcite growth rates and step velocities observed experimentally might be at least partially induced by surface topography.
Redox behaviour of magnetite Fe3O4nanoparticles in thermodynamic equilibrium conditions enclosed ... more Redox behaviour of magnetite Fe3O4nanoparticles in thermodynamic equilibrium conditions enclosed by non-dipolar reconstructed surfaces.
Quaternary phosphate-based glasses in the P 2 O 5-CaO-Na 2 O-TiO 2 system with a fixed P 2 O 5 an... more Quaternary phosphate-based glasses in the P 2 O 5-CaO-Na 2 O-TiO 2 system with a fixed P 2 O 5 and CaO content of 40 and 25 mol% respectively have been successfully synthesised via sol-gel method and bulk, transparent samples were obtained. The structure, elemental proportion, and thermal properties of stabilised solgel glasses have been characterised using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), 31 P nuclear magnetic resonance (31 P NMR), titanium K-edge X-ray absorption near-edge structure (XANES), fourier transform infrared (FTIR) spectroscopy, and differential thermal analysis (DTA). The XRD results confirmed the amorphous nature for all stabilized sol-gel derived glasses. The EDX result shows the relatively low loss of phosphorus during the sol-gel process and Ti K-edge XANES confirmed titanium in the glass structure is in mainly sixfold coordination environment. The 31 P NMR and FTIR results revealed that the glass structure consist of mainly Q 1 and Q 2 phosphate units and the Ti 4+ cation was acting as a cross-linking between phosphate units. In addition DTA results confirmed a decrease in the glass transition and crystallisation temperature with increasing Na 2 O content. Ion release studies also demonstrated a decrease in degradation rates with increasing TiO 2 content therefore supporting the use of these glasses for biomedical applications that require a degree of control over glass degradation. These sol-gel glasses also offer the potential to incorporate proactive molecules for drug delivery application due to the low synthesis temperature employed.
We have applied density functional theory calculations with Hubbard corrections (DFT+U) to invest... more We have applied density functional theory calculations with Hubbard corrections (DFT+U) to investigate the structural, electronic, and redox properties of Ti-substituted zirconia (111) surfaces. The calculations show that titanium dopants are likely to accumulate at the oxide surface, where an isolated dopant is 0.25 eV more stable than in the bulk. We have investigated in detail the relative distribution of dopants and oxygen vacancies at the surface and report the most stable configurations for each composition. It is found that the formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. The analysis of the electronic structure of the doped and reduced surfaces reveals that, when an O vacancy is created around an isolated Ti dopant, a Ti 4+ f Ti 2+ reduction takes place, with the reduced cation in a high-spin configuration. However, if the vacancy is created in the vicinity of a pair of dopants, each Ti atom adopts a +3 oxidation state with an additional decrease in the vacancy formation energy.
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2011
Using a combination of density functional theory calculations and statistical mechanics, we show ... more Using a combination of density functional theory calculations and statistical mechanics, we show that a wide range of intermediate compositions of ceria–zirconia solid solutions are thermodynamically metastable with respect to phase separation into Ce-rich and Zr-rich oxides. We estimate that the maximum equilibrium concentration of Zr in CeO 2 at 1373 K is approximately 2 per cent, and therefore, equilibrated samples with higher Zr content are expected to exhibit heterogeneity at the atomic scale. We also demonstrate that in the vicinity of the (111) surface, cation redistribution at high temperatures will occur with significant Ce enrichment of the surface, which we attribute to the more covalent character of Zr–O bonds compared with Ce–O bonds. Although the kinetic barriers for cation diffusion normally prevent the decomposition/segregation of ceria–zirconia solid solutions in typical catalytic applications, the separation behaviour described here can be expected to occur in mode...
The failure response to uniaxial tension and compression of a nanorod of SiO 2 , in the form of a... more The failure response to uniaxial tension and compression of a nanorod of SiO 2 , in the form of a stack of sixmembered rings (Si 6 O 18) with appropriate termination at the two ends, is investigated at the molecular level. A 144-atom rod is shown to exhibit meaningful mechanical instability that can be compared with simulations of bulk quartz; at the same time it is small enough for the interatomic forces to be treated quantum mechanically. The present work is a prelude to the study of the effects of water on the mechanical strength of silica, a fundamental issue in understanding the dislocation mechanisms of hydrolytic weakening.
In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue an... more In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue and in synthetic biomaterials, we have investigated the interaction of three constituent amino acids of the collagen matrix with two major hydroxyapatite surfaces. We have employed electronic structure techniques based on the density functional theory to study a range of different binding modes of the amino acids glycine, proline, and hydroxyproline at the hydroxyapatite (0001) and (0110) surfaces. We have performed full geometry optimizations of the hydroxyapatite surfaces with adsorbed amino acid molecules to obtain the optimum substrate/adsorbate structures and interaction energies. The calculations show that the amino acids are capable of forming multiple interactions with surface species, particularly if they can bridge between two surface calcium ions. The binding energies range from 290 kJ mol(-1) for glycine on the (0001) surface to 610 kJ mol(-1) for hydroxyproline on the (0110) surface. The large adsorption energies are due to a wide range of interactions between the adsorbate and surface, including proton transfer from the adsorbates to surface OH or PO(4) groups. Hydroxyproline binds most strongly to the surfaces, but all three amino acids should be good sites for the nucleation and growth of the hydroxyapatite (0110) surface at the collagen matrix.
The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO2(111) surface... more The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO2(111) surface have been investigated in detail by means of periodic density functional theory-based calculations, using the LDA+U and GGA+U potentials for a broad range of U values, complemented with calculations employing the HSE06 hybrid functional. In addition, the effects of the lattice parameter a0 and of the starting point for the geometry optimization have also been analyzed. From the present results we suggest that the oxidation state of single Au atoms on CeO2(111) predicted by LDA+U, GGA+U, and HSE06 density functional calculations is not conclusive and that the final picture strongly depends on the method chosen and on the construction of the surface model. In some cases we have been able to locate two well-defined states which are close in energy but with very different electronic structure and local geometries, one with Au fully oxidized and one with neutral Au. The energy difference bet...
... Neyvis Almora-Barrios*a and Nora H. de Leeuw*ab. a Department of Chemistry, University Colleg... more ... Neyvis Almora-Barrios*a and Nora H. de Leeuw*ab. a Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ. ... 27, S. Hamad, CE Hughes, CRA Catlow and KDM Harris, J. Phys. Chem. B, 2008, 112, 72807288 Article ChemPort . ...
Bulletin of the American Physical Society, Mar 19, 2010
We use density functional theory calculations with Hubbard corrections (DFT+ U) to investigate th... more We use density functional theory calculations with Hubbard corrections (DFT+ U) to investigate the electronic and redox properties of Ti-substituted zirconia (111) surfaces. It is found that titanium dopants are more likely to segregate at the surface than to migrate to the zirconia bulk. The formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. If an O vacancy is created around an isolated Ti dopant, a Ti $^{4+}\ quad\ to $ Ti $^{2+} $ reduction takes place, while if the ...
We present density functional theory calculations of the adsorption of arsenic acid (AsO(OH)3) an... more We present density functional theory calculations of the adsorption of arsenic acid (AsO(OH)3) and arsenous acid (As(OH)3) on the Al(iii)-modified natural zeolite clinoptilolite under anhydrous and hydrated conditions.
h i g h l i g h t s Uranium substitution in ThO 2 is found to increase the covalent nature of the... more h i g h l i g h t s Uranium substitution in ThO 2 is found to increase the covalent nature of the ionic bonding. The (111), (110), and (100) surfaces of ThO 2 are studied and the particle morphology is proposed. STM images of the (111), (110), and (100) surfaces of ThO 2 are simulated. Uranium adsorption on the major surfaces of ThO 2 is studied.
Although [Ni(S 2 CNBu i 2) 2 ] is stable at high temperatures in a range of solvents, solvotherma... more Although [Ni(S 2 CNBu i 2) 2 ] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145°C in oleylamine to give pure NiS nanoparticles, while in n-hexylamine at 120°C a mixture of Ni 3 S 4 (polydymite) and NiS results. A combined experimental and theoretical study gives mechanistic insight into the decomposition process and can be used to account for the observed differences. Upon dissolution in the primary amine, octahedral trans-[Ni(S 2 CNBu i 2) 2 (RNH 2) 2 ] result as shown by in situ XANES and EXAFS and confirmed by DFT calculations. Heating to 90−100°C leads to changes consistent with the formation of amide-exchange products, [Ni(S 2 CNBu i 2){S 2 CN(H)R}] and/or [Ni{S 2 CN(H)R} 2 ]. DFT modeling shows that exchange occurs via nucleophilic attack of the primary amine at the backbone carbon of the dithiocarbamate ligand(s). With hexylamine, amide-exchange is facile and significant amounts of [Ni{S 2 CN(H)Hex} 2 ] are formed prior to decomposition, but with oleylamine, exchange is slower and [Ni(S 2 CNBu i 2){S 2 CN-(H)Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100°C to afford nickel sulfides, even in the absence of primary amine, as shown from thermal decomposition studies of [Ni{S 2 CN(H)Hex} 2 ]. DFT modeling of [Ni{S 2 CN(H)R} 2 ] shows that proton migration from nitrogen to sulfur leads to formation of a dithiocarbimate (S 2 CNR) which loses isothiocyanate (RNCS) to give dimeric nickel thiolate complexes [Ni{S 2 CN(H)R}(μ-SH)] 2. These intermediates can either lose dithiocarbamate(s) or extrude further isothiocyanate to afford (probably amine-stabilized) nickel thiolate building blocks, which aggregate to give the observed nickel sulfide nanoparticles. Decomposition of the single or double amide-exchange products can be differentiated, and thus it is the different rates of amideexchange that account primarily for the formation of the observed nanoparticulate nickel sulfides.
An in situ XAS study of the solvothermal decomposition of iron and nickel dithiocarbamate complex... more An in situ XAS study of the solvothermal decomposition of iron and nickel dithiocarbamate complexes was performed in order to gain understanding of the decomposition mechanisms. This work has given insight into the steps involved in the decomposition, showing variation in reaction pathways between the iron and nickel dithiocarbamates, and the non-innocent role of oleylamine as the solvent and capping agent in the reaction.
It is generally accepted that cation dehydration is the rate-limiting step to crystal growth from... more It is generally accepted that cation dehydration is the rate-limiting step to crystal growth from aqueous solution. Here we employ classical molecular dynamics simulations to show that the water exchange frequency at structurally distinct calcium sites in the calcite surface varies by about two orders of magnitude. The decrease in water exchange frequency with progressive embedding of surface calcium ions is thought to be rate limiting to subsequent attachment of carbonate ions during calcite growth. Therefore, a process-based calcite growth kinetics model, reparameterized using the water exchange frequencies computed from molecular dynamics simulations, is used to illustrate the impact of these variations on kink-formation rate, step velocities and bulk growth rate. The calculated frequencies of kink formation show a strong variation with surface structures, which can be amplified depending on the saturation state and calcium to carbonate ratio of the solution. Modelled and measured step velocities and bulk growth rates are generally in agreement, showing that variations in calcite growth rates and step velocities observed experimentally might be at least partially induced by surface topography.
Redox behaviour of magnetite Fe3O4nanoparticles in thermodynamic equilibrium conditions enclosed ... more Redox behaviour of magnetite Fe3O4nanoparticles in thermodynamic equilibrium conditions enclosed by non-dipolar reconstructed surfaces.
Quaternary phosphate-based glasses in the P 2 O 5-CaO-Na 2 O-TiO 2 system with a fixed P 2 O 5 an... more Quaternary phosphate-based glasses in the P 2 O 5-CaO-Na 2 O-TiO 2 system with a fixed P 2 O 5 and CaO content of 40 and 25 mol% respectively have been successfully synthesised via sol-gel method and bulk, transparent samples were obtained. The structure, elemental proportion, and thermal properties of stabilised solgel glasses have been characterised using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), 31 P nuclear magnetic resonance (31 P NMR), titanium K-edge X-ray absorption near-edge structure (XANES), fourier transform infrared (FTIR) spectroscopy, and differential thermal analysis (DTA). The XRD results confirmed the amorphous nature for all stabilized sol-gel derived glasses. The EDX result shows the relatively low loss of phosphorus during the sol-gel process and Ti K-edge XANES confirmed titanium in the glass structure is in mainly sixfold coordination environment. The 31 P NMR and FTIR results revealed that the glass structure consist of mainly Q 1 and Q 2 phosphate units and the Ti 4+ cation was acting as a cross-linking between phosphate units. In addition DTA results confirmed a decrease in the glass transition and crystallisation temperature with increasing Na 2 O content. Ion release studies also demonstrated a decrease in degradation rates with increasing TiO 2 content therefore supporting the use of these glasses for biomedical applications that require a degree of control over glass degradation. These sol-gel glasses also offer the potential to incorporate proactive molecules for drug delivery application due to the low synthesis temperature employed.
We have applied density functional theory calculations with Hubbard corrections (DFT+U) to invest... more We have applied density functional theory calculations with Hubbard corrections (DFT+U) to investigate the structural, electronic, and redox properties of Ti-substituted zirconia (111) surfaces. The calculations show that titanium dopants are likely to accumulate at the oxide surface, where an isolated dopant is 0.25 eV more stable than in the bulk. We have investigated in detail the relative distribution of dopants and oxygen vacancies at the surface and report the most stable configurations for each composition. It is found that the formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. The analysis of the electronic structure of the doped and reduced surfaces reveals that, when an O vacancy is created around an isolated Ti dopant, a Ti 4+ f Ti 2+ reduction takes place, with the reduced cation in a high-spin configuration. However, if the vacancy is created in the vicinity of a pair of dopants, each Ti atom adopts a +3 oxidation state with an additional decrease in the vacancy formation energy.
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2011
Using a combination of density functional theory calculations and statistical mechanics, we show ... more Using a combination of density functional theory calculations and statistical mechanics, we show that a wide range of intermediate compositions of ceria–zirconia solid solutions are thermodynamically metastable with respect to phase separation into Ce-rich and Zr-rich oxides. We estimate that the maximum equilibrium concentration of Zr in CeO 2 at 1373 K is approximately 2 per cent, and therefore, equilibrated samples with higher Zr content are expected to exhibit heterogeneity at the atomic scale. We also demonstrate that in the vicinity of the (111) surface, cation redistribution at high temperatures will occur with significant Ce enrichment of the surface, which we attribute to the more covalent character of Zr–O bonds compared with Ce–O bonds. Although the kinetic barriers for cation diffusion normally prevent the decomposition/segregation of ceria–zirconia solid solutions in typical catalytic applications, the separation behaviour described here can be expected to occur in mode...
The failure response to uniaxial tension and compression of a nanorod of SiO 2 , in the form of a... more The failure response to uniaxial tension and compression of a nanorod of SiO 2 , in the form of a stack of sixmembered rings (Si 6 O 18) with appropriate termination at the two ends, is investigated at the molecular level. A 144-atom rod is shown to exhibit meaningful mechanical instability that can be compared with simulations of bulk quartz; at the same time it is small enough for the interatomic forces to be treated quantum mechanically. The present work is a prelude to the study of the effects of water on the mechanical strength of silica, a fundamental issue in understanding the dislocation mechanisms of hydrolytic weakening.
In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue an... more In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue and in synthetic biomaterials, we have investigated the interaction of three constituent amino acids of the collagen matrix with two major hydroxyapatite surfaces. We have employed electronic structure techniques based on the density functional theory to study a range of different binding modes of the amino acids glycine, proline, and hydroxyproline at the hydroxyapatite (0001) and (0110) surfaces. We have performed full geometry optimizations of the hydroxyapatite surfaces with adsorbed amino acid molecules to obtain the optimum substrate/adsorbate structures and interaction energies. The calculations show that the amino acids are capable of forming multiple interactions with surface species, particularly if they can bridge between two surface calcium ions. The binding energies range from 290 kJ mol(-1) for glycine on the (0001) surface to 610 kJ mol(-1) for hydroxyproline on the (0110) surface. The large adsorption energies are due to a wide range of interactions between the adsorbate and surface, including proton transfer from the adsorbates to surface OH or PO(4) groups. Hydroxyproline binds most strongly to the surfaces, but all three amino acids should be good sites for the nucleation and growth of the hydroxyapatite (0110) surface at the collagen matrix.
The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO2(111) surface... more The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO2(111) surface have been investigated in detail by means of periodic density functional theory-based calculations, using the LDA+U and GGA+U potentials for a broad range of U values, complemented with calculations employing the HSE06 hybrid functional. In addition, the effects of the lattice parameter a0 and of the starting point for the geometry optimization have also been analyzed. From the present results we suggest that the oxidation state of single Au atoms on CeO2(111) predicted by LDA+U, GGA+U, and HSE06 density functional calculations is not conclusive and that the final picture strongly depends on the method chosen and on the construction of the surface model. In some cases we have been able to locate two well-defined states which are close in energy but with very different electronic structure and local geometries, one with Au fully oxidized and one with neutral Au. The energy difference bet...
... Neyvis Almora-Barrios*a and Nora H. de Leeuw*ab. a Department of Chemistry, University Colleg... more ... Neyvis Almora-Barrios*a and Nora H. de Leeuw*ab. a Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ. ... 27, S. Hamad, CE Hughes, CRA Catlow and KDM Harris, J. Phys. Chem. B, 2008, 112, 72807288 Article ChemPort . ...
Bulletin of the American Physical Society, Mar 19, 2010
We use density functional theory calculations with Hubbard corrections (DFT+ U) to investigate th... more We use density functional theory calculations with Hubbard corrections (DFT+ U) to investigate the electronic and redox properties of Ti-substituted zirconia (111) surfaces. It is found that titanium dopants are more likely to segregate at the surface than to migrate to the zirconia bulk. The formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. If an O vacancy is created around an isolated Ti dopant, a Ti $^{4+}\ quad\ to $ Ti $^{2+} $ reduction takes place, while if the ...
We present density functional theory calculations of the adsorption of arsenic acid (AsO(OH)3) an... more We present density functional theory calculations of the adsorption of arsenic acid (AsO(OH)3) and arsenous acid (As(OH)3) on the Al(iii)-modified natural zeolite clinoptilolite under anhydrous and hydrated conditions.
h i g h l i g h t s Uranium substitution in ThO 2 is found to increase the covalent nature of the... more h i g h l i g h t s Uranium substitution in ThO 2 is found to increase the covalent nature of the ionic bonding. The (111), (110), and (100) surfaces of ThO 2 are studied and the particle morphology is proposed. STM images of the (111), (110), and (100) surfaces of ThO 2 are simulated. Uranium adsorption on the major surfaces of ThO 2 is studied.
Uploads
Papers by N. Leeuw