Theoretical Study On The Structures And Properties Of Gold And Gold-based Binary-alloy Clusters

In the later period of the 1980s,Haruta found that supported nano-gold clusters had unusual catalytic properties and novel physical properties,such as the excellent catalytic activity to CO oxidation at a low temperature,the good water resistance and stability,the good humidity enhancing effect etc.This discovery changes the traditional concept that gold does not have the catalytic activity,so a research fever is raised all over the world on the nano-gold and its alloy clusters.The studies on their structures and properties are the foundation on which the studies on the chemical and physical properties of gold and its alloy clusters are based and have very significant scientific meanings and application values for conducting related experimental and theoretical studies.With the development of modern experiment technologies,more and more gold and its alloy clusters are developed successfully.Experimentally,the studies on the gold and its alloy clusters are focused on ion clusters,getting related structural data by measuring the ion mobility and photoelectron spectra of the ion clusters.Due to the restrictions on the experimental conditions and measurement technologies,it is relatively difficult to conduct experimental studies on the neutral gold and its alloy clusters and it is difficult to obtain comprehensive microcosmic information of the geometric and electronic structures of these clusters.The understandings of the structures and performances of the neutral gold and its alloy clusters are mainly gained by theoretical studies;therefore,conducting the theoretical studies becomes the most efficient approach to obtaining the structural information of the clusters, especially the calculation based on DFT can get the calculation results of the middle and small sized systems with a high accuracy.At present,on one hand,the theoretical studies on the clusters are the basic studies on small gold clusters,which explore how the clusters evolve to the building blocks in terms of their structures and characteristics as the sizes of the clusters are increasing;on the other hand,the aim of the theoretical studies on the clusters is to find and predict special properties of special materials so as to conduct a theoretical design on functional materials and to promote the development of the nanometer cluster materials.Based on the above two points,this thesis theoretically designs and predicts the geometric and electronic structures of the gold and its alloy clusters and finds out the basic structural units,bonding rules and growth mechanisms of these clusters by performing the DFT method.This thesis researches the adsorption properties of the gold and platinum-doped gold clusters on methanol as the sizes are increasing, explores relevant microcosmic interaction mechanisms,gains the key factors of controlling adsorptions and provides a certain theoretical guidance to the experimental studies.At the same time,this thesis also does theoretical studies on the adsorption properties of lithium,sodium,cesium and platinum-doped gold clusters on oxygen, compares different kinds of binary-alloys and proportioning adsorption properties and obtains good adsorption conditions.The major innovative results in this thesis are as follows:1.The structures and properties of the neutral gold clusters are researched by performing the DFT method and the calculation results show that the most stable structure of Au_n(n=2-10) clusters is a two-dimensional planar and the clusters are basically small triangle since Au₅.The second chapter designs all possible configurations of the Au clusters and researches the relative stability and geometric and electronic properties of these structures,all of which do not have any imaginary frequency after being optimized and maintain high symmetry structures.The calculation shows that the average bond lengths of the clusters are gradually increasing as the sizes of the clusters are increasing and that because the Coordinate Number(CN) of Au₈ is smaller and the average bond lengths are shortened,the average bond lengths of the clusters are inclined to be stable since AU₁₀.The calculated fragmentation energy D_e and the second difference of binding energyΔ²E_b show that an odd-even oscillation happens to the D_e andΔ²E_b as the sizes are increasing and that even-numbered clusters are more stable than their adjacent odd-numbered clusters if they have higher values.The HOMO-LUMO gaps of all stable structures are calculated.When the size is more than 4,an obvious odd-even oscillation happens to the HOMO-LUMO gaps and the chemical stability of even-numbered clusters is better than that of the adjacent odd-numbered clusters if the HOMO-LUMO gaps of the even-numbered clusters are bigger than that of the adjacent odd-numbered clusters.The calculation of the stable configuration of Au₁₀ finds out a new stable structure,which has not been reported before.The calculation shows that Au_n(n=2-10) clusters are preferred to be the two-dimensional planar and that the clusters have the obvious oscillation and the stability of the even-numbered clusters is obviously stronger than that of the odd-numbered clusters.2.This thesis deeply explores the adsorption of the Au_n(n=2-10) clusters on methanol and separately researches the hydroxyl oxygen and hydrogen on methyl which can be possibly adsorbed to the Au.It is showed that Au atoms coordinate number and electric charge are the major factors of affecting the methanol adsorption, that the hydroxyl oxygen and hydrogen on methyl are more preferred to be adsorbed to the Au atoms with the CN being equal to 2 or 3 and that the adsorption energy of the Au to adsorb the oxygen is obviously higher than that of hydrogen on methyl.When the hydrogen on methyl is adsorbed to the Au,the Au-O distance and C-O bond length are related to the adsorption site activities.The distance of the complex Au-O formed by the Au with the high adsorption activity and the hydrogen on methyl is short and the C-O bond length is longer than C-O bond length in isolated methanol, so the adsorption energy is strong.Oxygen shares the similar case with the hydrogen adsorbed to the Au atoms,that is,the distance of Au-H in complex formed by the Au with the high adsorption activity and the hydrogen is short and the elongated amplitude of the C-H bond is bigger.When CN is less than 4,the CN of Au is the major factor of determining the activities of the adsorption sites;however,when the CN is more than and equal to 5,the charge of the adsorption sites is the major factor of determining the activities of the adsorption sites.In the state of having the same CN,the charge on Au decides the height of the adsorption energy.The obvious odd-even oscillation happens to the Au clusters adsorbing the methanol;the adsorption energy of the even-numbered clusters is higher than that of the adjacent odd-numbered clusters;the adsorption activity of Au₄ is the strongest and the adsorption energy is the biggest;the complex with methanol is the most stable.When the CN is equal to 2 or 3,Au possesses a higher adsorption activity and is all located at the edge of the clusters,which is helpful to the adsorption of the methanol.3.The stable structures and properties of the Au_nPt(n=1-8) alloy clusters are researched by performing the DFT method at B3LYP/LANL2DZ level with the findings that Au/Pt alloy binding energy is more than that of pure Au clusters and that Pt -doped Au clusters are more stable.The most stable configuration of the Au₈Pt is a 3D structure which shows that a 2D-3D structural transition of Au_nPt alloy occurs when the size is equal to 9.Pt is more preferred to be located at the center of the alloy clusters in the plane structure,while the electronic structure of the clusters changes in the 3D configuration with Pt preferring to a low CN.By comparing the average bond lengths of the pure Au and Au_nPt(n=1-8) alloy clusters,it is found that when the stable configurations are the same,the average bond length of the Au/Pt alloy is much shorter and the Au-Pt bond length is much shorter. In the plane structure,due to the higher electronegative Au,Pt transfers charge to Au to make itself carrying positive charge and as the increase of the size the CN of Pt is increasing and the charge transferred to Au is increasing.By researching the most stable configuration of Au₈Pt,it is found that in the 3D structure,the electronic structure of Au/Pt alloy is changing and Pt is more preferred to the low CN site and is located at the edge of the configuration.By comparing the binding energies of the pure Au and Au/Pt alloy clusters,it is found that the binding energy of the Au/Pt alloy clusters is more than that of pure Au clusters,which shows that Au/Pt alloy clusters are more stable;when the sizes of the clusters are 7 and 8,the binding energy of Au₆Pt is relatively higher but the binding energy of Au₇ is relatively lower and the binding energy of Au₆Pt is relatively lower but the binding energy of Au₈ is relatively higher;the higher symmetry can make the clusters more stable and the interactions between atoms are easier to balance,so the binding energy is higher.By studying the second difference of binding energyâ–³²E_b except Au₅Pt,it is found that Au/Pt alloy clusters show an opposite parity vibration to pure Au clusters;Pt being mixed into changes the odd-even oscillation of the Au clusters;Pt interacts with Au's d-d orbit; the electrons of Pt are transferred to the Au,which strengthens the interactions between atoms in the clusters and affects the odd-even oscillation changes of the clusters.The interactions between Pt and Au enhance the stability of the clusters and reduce the fluctuation amplitude of the odd-even oscillation.Theâ–³²E_b value of the Au6Pt is obviously higher than that of other alloy clusters,which refers to a higher stability mainly because Au₆Pt has a high level of symmetry;Pt is located at the center and bonded with the Au;as Pt transfers an electron to the Au,the positive charge becomes more to make the Au-Pt bonding energy big,which is favorable to the stability of the clusters.By studying fragment energies of D_Pt and D_Au.it is found that the energy needed to break away Pt is obviously bigger than the energy needed to dissociate Au out of the pure Au and Au/Pt alloy clusters and that the D_Pt has a certain odd-even oscillation which means that the fragment energy with even size is higher than that of odd size.By comparing the binding energies of the 2D and 3D structures, it is found that as the sizes are increasing,the binding energy in the plane configuration prefers to be stable after Au₆Pt,while the stability of the stereo configuration is gradually increasing and that the stability of Au₈Pt exceeds that of the plane configuration and the binding energies are maintained at the peak.4.This thesis deeply discusses Au_nPt(n=1-8) alloy clusters adsorbing methanol. According to the possibilities of the adsorption,four kinds are determined.The hydroxyl oxygen and hydrogen on methyl are separately adsorbed to the Pt and Au to discover that the complex formed by oxygen adsorbed with alloy clusters is more stable with a lower energy,which shows that methanol is more preferred to the oxygen being adsorbed to the alloy clusters;the adsorption activity of Pt is higher than that of Au which is favorable to the adsorption of the hydroxyl oxygen and hydrogen;but when the CN of Pt is equal to 6,the methanol is inclined to be adsorbed to the Au.When the adsorption energy of Au₄Pt is the maximum,the adsorption activity of Pt is the most active;while when the Au adsorption activity in Au3Pt is the most active,the adsorption energy is the maximum.The research shows that the distance of Pt(Au)-H and the C-H bond distance are related to the adsorption activity of Pt(Au) as the sizes are changing.When being adsorbed to the atoms with a high activity,the distance of Pt(Au)-H is short and the amplitude of extending the C-H bond is large.The result of the oxygen being adsorbed to the Au/Pt alloy clusters is the same as that of hydrogen on methyl;when being adsorbed to the atoms with the high activity,the distance of Pt(Au)-O is short and the amplitude of extending the C-O bond is large.The adsorption activity of Pt in the Au/Pt alloy clusters is higher than that of Au;Pt adsorbing the hydrogen on methyl has an obvious odd-even oscillation,which means the adsorption energy of odd-numbered clusters is higher than that of the adjacent even-numbered clusters, which is opposite to the pure Au adsorbing hydrogen on methyl.Au/Pt alloy clusters adsorbing oxygen has more complicated situations:when the CN is less than 6,the adsorption activity of Pt is higher than that of Au;when the CN of Pt is equal to 6,the adsorption activity is lower than that of Au with the low CN;and the oxygen is inclined to be adsorbed to the Au.When the sizes of the Au/Pt alloy clusters are smaller than 5,the adsorption activity of Pt is obviously higher than that of Au clusters;when the sizes are larger than 5,the adsorption activity decreases to have a smaller difference with the pure Au clusters;the adsorption energy of Au₄Pt is the ultimate,which is more than the maximum adsorption energy of the pure Au clusters. Pt being mixed adds the adsorption activity of the Au clusters,which is more favorable to the methanol adsorption.5.Binary clusters of Au adsorption of O₂ are studied by performing the DFT method.The research shows that the alloy of alkali-metal doped Au is helpful to the adsorption of O₂,wherein the alloy of Au and Li can obviously enhance the adsorption activity which is higher than the adsorption activity recorded in previous reports and documents and provides the theoretical basis to the experimental studies.It is calculated that the alloy of Au with Li,Na,Cs and Pt can enhance the adsorption ability of Au of O₂;the adsorption energy of AuM₂(M=Li,Na,Cs,Pt) is the ultimate and AuLi₂＞AuNa₂＞AuPt₂＞AuCs₂.The adsorption energies of pure and doped Au clusters dimer and alkali-metal doped Au are larger than the adsorption energy of pure Au clusters,but smaller than the adsorption energy of pure alkali-metal and the adsorption property of AuPt is higher than the adsorption properties of Au₂ and Pt₂.In trimer,because of the too high activities,the interactions between the Li₃ and Na₃ with O₂ are no longer the adsorption interaction.The adsorption order of the Cs and Pt being mixed into the Au is AuM₂＞M₃＞Au₂M(M=Cs,Pt) which is the same as the one reported in the former document.6.we have reexamined the reaction of 4-CP with·OH radical to ascertain the mechanism details for forming intermediates 4-CC and HQ at the initial stage of the degradation of 4-CP initiated by·OH by performing DFT calculations.The calculated results show that abstracting hydrogen atom in the hydroxyl of 4-chlorophenol by the·OH is the most plausible process for forming 4-chlorocaechol intermediate,while adding·OH to the aromatic ring is the dominant pathway for forming the hydroquinone.The 4-CP·radical is found to be a energetically most stable species among the radical intermediates involved in the reaction and thus is expected to play a crucial role for the hydroxyl-initiated 4-CP degradation.The present results are in good agreement with the experimental finding and provide aids to some extent for improving our understanding for the 4-CP degradation initiated by·OH radical.