Stoichiometric Clusters Research Articles

Electronic Structure of Lithium Peroxide Clusters and Relevance to Lithium–Air Batteries

The prospect of Li–air(oxygen) batteries has generated much interest because of the possibility of extending the range of electric vehicles due to their potentially high gravimetric density. The exact morphology of the lithium peroxide formed during discharge has not been determined yet, but the growth likely involves nanoparticles and possibly agglomerates of nanoparticles. In this article, we report on density functional calculations of stoichiometric lithium peroxide clusters that provide evidence for the stabilization of high spin states relative to the closed shell state in the clusters. The density functional calculations indicate that a triplet state is favored over a closed shell singlet state for a dimer, trimer, and tetramer of lithium peroxide, whereas in the lithium peroxide monomer, the closed shell singlet is strongly favored. Density functional calculations on a much larger cluster, (Li2O2)16, also indicate that it similarly has a high spin state with four unpaired electrons located on the surface. These results have been confirmed by higher level G4 theory calculations that indicate that the singlet and triplet states of the dimer are nearly equal in energy and that the triplet state is more stable than the singlet for clusters larger than the dimer. The high spin states of the clusters are characterized by O–O moieties protruding from the surface, which have superoxide-like characteristics in terms of bond distances and spin. The existence of these superoxide-like surface structures on stoichiometric lithium peroxide clusters may have implications for the electrochemistry of formation and decomposition of lithium peroxide in Li–air batteries including electronic conductivity and charge overpotentials.

Enhanced solar photocatalytic activity of TiO2 by selenium(IV) ion-doping: Characterization and DFT modeling of the surface

TiO2 was doped with Se(IV) ions to obtain a visible light active photocatalyst with high photocatalytic activity. In order to characterize and describe the effect of Se(IV)-doping on the electronic and structural properties of TiO2, a combination of experimental structural methods and DFT calculations were used. A series of Se(IV)-doped photocatalysts with different Se(IV) contents were prepared by an incipient wet impregnation method, followed by calcination at 623, 723 and 823K for 1h, 3h and 5h. The Se(IV)-doped photocatalysts were characterized by XRD, XPS, UV-DRS, FESEM, EDX, HR-TEM and surface area (BET) measurements. A high visible light absorption band from 420nm extending up to 650nm in the UV-DRS was obtained. The photocatalytic activity of the Se(IV)-doped TiO2 photocatalyst was also determined by investigating the kinetics of the photocatalytic degradation of 4-nitrophenol under both UV-A and sunlight irradiation. Se(IV)-doped TiO2 enhanced the degradation rate of 4-NP. It also exhibited substantial photocatalytic activity under direct sunlight irradiation. In the computational part of the study, neutral, stoichiometric clusters Ti7O18H8 and Ti25O55H10 cut from the anatase bulk structure and two new models for the Se(IV)-doped TiO2 were developed. The DFT calculations were carried out by the hybrid B3LYP functional, by using double-zeta, LanL2DZ basis set. The experimental results combined with DFT calculations indicated that Se(IV)-doping of TiO2 enhances the visible-light photocatalytic activity by the introduction of additional electronic states originating from the Se 3p orbitals in the band gap.

Ab initio study of neutral (TiO2)n clusters and their interactions with water and transition metal atoms

We have systematically investigated the growth behavior and stability of small stoichiometric (TiO2)n (n = 1–10) clusters as well as their structural, electronic and magnetic properties by using the first-principles plane wave pseudopotential method within density functional theory. In order to find out the ground state geometries, a large number of initial cluster structures for each n has been searched via total energy calculations. Generally, the ground state structures for the case of n = 1–9 clusters have at least one monovalent O atom, which only binds to a single Ti atom. However, the most stable structure of the n = 10 cluster does not have any monovalent O atom. On the other hand, Ti atoms are at least fourfold coordinated for the ground state structures for n ≥ 4 clusters. Our calculations have revealed that clusters prefer to form three-dimensional structures. Furthermore, all these stoichiometric clusters have nonmagnetic ground state. The formation energy and the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap for the most stable structure of (TiO2)n clusters for each n have also been calculated. The formation energy and hence the stability increases as the cluster size grows. In addition, the interactions between the ground state structure of the (TiO2)n cluster and a single water molecule have been studied. The binding energy (Eb) of the H2O molecule exhibits an oscillatory behavior with the size of the clusters. A single water molecule preferably binds to the cluster Ti atom through its oxygen atom, resulting an average binding energy of 1.1 eV. We have also reported the interaction of the selected clusters (n = 3, 4, 10) with multiple water molecules. We have found that additional water molecules lead to a decrease in the binding energy of these molecules to the (TiO2)n clusters. Finally, the adsorption of transition metal (TM) atoms (V, Co and Pt) on the n = 10 cluster has been investigated for possible functionalization. All these elements interact strongly with this cluster, and a permanent magnetic moment is induced upon adsorption of Co and V atoms. We have observed gap localized TM states leading to significant HOMO–LUMO gap narrowing, which is essential to achieve visible light response for the efficient use of TiO2 based materials. In this way, electronic and optical as well as magnetic properties of TiO2 materials can be modulated by using the appropriate adsorbate atoms.

Binary Neutral Metal Oxide Clusters with Oxygen Radical Centers for Catalytic Oxidation Reactions: From Cluster Models Toward Surfaces

We present theoretical results based on DFT calculations of the reactivity of binary neutral stoichiometric Zrn-1ScO2n clusters with oxygen radical centers toward CO and acetylene with the aim to build a bridge between clusters and surface models. Following the concept of the same total valence electron count, the studied series of binary clusters mimics cationic stoichiometric (ZrO2)n+ species characterized also by the presence of an oxygen radical center. In the case of Zr11ScO24, representing a section of the (ZrO2)n bulk, in addition to the oxygen radical center, the presence of neighboring Zr atoms effectively promotes oxidation reactions toward CO and acetylene, showing the importance of the surrounding of active centers.

Multi-timescale investigation of radiation damage near TiO2 rutile grain boundaries

To understand the interactions between defects and grain boundaries (GBs) in oxides, two atomistic modeling methods were used to examine the role of GBs in a model system, rutile TiO2, in modifying radiation-induced defect production and annealing. Molecular dynamics was used to investigate defect production near a symmetric tilt GB at both 300 K and 1000 K. The damage production is found to be sensitive to the initial distance of the primary knock-on atom from the GB. We find three distinct regimes in which GBs have different effects. Similar to GBs in metals, the GB absorbs more interstitials than vacancies at certain distances while this behavior of biased loading of interstitials diminishes at other distances. Further, we obtain the statistics of both interstitial and vacancy clusters produced in collision cascades in terms of their compositions at two temperatures. Perfectly stoichiometric defect clusters represent a small fraction of the total clusters produced. Moreover, a significant reduction in the number of interstitial clusters at 1000 K compared to 300 K is thought to be a consequence of enhanced migration of interstitials towards the GB. Finally, the kinetic properties of certain defect clusters were investigated with temperature accelerated dynamics, without any a priori assumptions of migration mechanisms. Small interstitial clusters become mobile at high temperatures while small vacancy clusters do not. Multiple migration pathways exist and are typically complex and non-intuitive. We use this kinetic information to explain experimental observations and predict their long-time migration behavior near GBs.

Density Functional Theory Study of Ti<sub>n</sub>O<sub>2n-m </sub>Clusters (n=1-4, m=0,1)

Using density functional theory, the equilibrium geometries of TinO2n and TinO2n-1 clusters (n=1-4) have been obtained. It suggests that the structures of these two corresponding clusters are changed slightly, except for the number of terminal oxygen atoms. The electronic properties have also been investigated. The bond between Ti and terminal oxygen atom is found to be more covalent than other Ti-O bonds. It also indicates that by deleting one terminal oxygen atom, HOMO is mainly derived from titanium atoms with least coordination, but not from singly-coordination oxygen atoms as that in the stoichiometric clusters. Highest energy levels of least-coordination Ti atoms shift highly and they become more reactive. In addition, HOMO-LUMO energy gaps and binding energies were observed. The calculated results show that the energy gaps decrease quickly, except for Ti4O7 clusters and all the binding energies are relatively large.

A DFT + U study of structure and reducibility of CenO2n−x (n ⩽ 4, 0 ⩽ x ⩽ n) nanoclusters

Equilibrium structures, stability and reducibility of CenO2n−x (n⩽4, x=0∼4) nanoclusters have been studied using first principles DFT+U method. The planar rhombus Ce2O2 structure is found to be the building block for the most stable CenO2n−x clusters. The normalized binding energy of the cluster decreases linearly with increasing cluster size. The most stable stoichiometric CenO2n clusters are electronically in closed-shell configuration (singlet), while the non-stoichiometric CenO2n−x clusters are in a high spin state (triplet or quintet). The reduction energy, i.e., the energy required to remove an oxygen atom from a cluster, increases with the size and the extent of reduction. On the other hand, per electron based reduction energy for the cluster to reach the same formal oxidation state is independent of the cluster size.

Influence of Surfactants and Charges on CdSe Quantum Dots

Surface effects significantly influence the functionality of semiconductor nanocrystals. High quality nanocrystals can be achieved with good control of surface passivation by various hydrophobic ligands. In this work, the chemistry between CdSe quantum dots and common surface capping ligands is investigated using density functional theory (DFT). We discuss the electronic structures and optical properties of small CdSe clusters controlled by their size of particle, self-organization, capping ligands, and positive charges. The chosen model ligands reproduce good structural and energetic description of the interactions between the ligands and quantum dots. In order to capture the chemical nature and energetics of the interactions between the capping ligands and CdSe quantum dots, we found that PMe3 is needed to adequately model trioctylphosphine (TOP), NH3 is sufficient for amines, while OPH2Me could be used to model trioctylphosphine oxide. The relative binding interaction strength between ligands was found to decrease in order Cd–O > Cd–N > Cd–P with average binding energy per ligand being −25 kcal/mol for OPH2Me, −20 kcal/mol for NH3 and −10 kcal/mol for PMe3. Charges on studied stoichiometric clusters were found to have a significant effect on their structures, binding energies, and optical properties.

Atomistic and electronic structure of (X 2 O 3 ) n nanoclusters; n =1–5, X=B, Al, Ga, In and Tl

The stable and metastable, as measured using an all-electron density functional theory approach, stoichiometric clusters of boron, aluminium, gallium, indium and thallium oxide are reported. Initial candidate structures were found using an evolutionary algorithm to search the energy landscape, defined using classical interatomic potentials, for alumina and india followed by data mining or rescaling. Characterization of the refined structures was performed by electronic structure techniques at the hybrid density functional and many-body GW levels of theory. We make accurate predictions of the spectroscopic properties represented by mean ionization potentials of 11.4, 9.9, 9.8, 8.8 and 8.4 eV and electron affinities of 0.05, 1.1, 1.6, 1.9 and 2.5 eV for boria, alumina, gallia, india and thallia, respectively. The changes in the global minima, atomistic and electronic properties with respect to the cluster and cation size are discussed.

Unusual structure and magnetism in manganese oxide nanoclusters

We report an unusual evolution of structure and magnetism in stoichiometric MnO clusters based on an extensive and unbiased search through the potential energy surface within density functional theory. The smaller clusters, containing up to five MnO units, adopt two-dimensional structures; and regardless of the size of the cluster, magnetic coupling is found to be antiferromagnetic in contrast to previous theoretical findings. Predicted structure and magnetism are strikingly different from the magnetic core of Mn-based molecular magnets, whereas they were previously argued to be similar. Both of these features are explained through the inherent electronic structures of the clusters.

Defect studies in small CdTe clusters

We study Cd vacancy formation in prototype stoichiometric and non-stoichiometric CdTe clusters with and without passivation. For certain clusters like Cd13Te16, vacancy leads to severe distortion of the geometry due to propagation of defect. Annealing of the vacancy out of the cluster is observed in all unpassivated clusters. Passivated clusters retain their initial geometry and vacancy induced structural distortions are not seen in these clusters since the defect gets localized. Vacancy also induces intragap states. However, it was observed that the passivation of the dangling bonds created by the vacancy removes the intragap states. In an attempt to have CdTe clusters with extrinsic carriers, we substituted a Cd atom by its adjacent atoms Pd/Ag/In/Sn in these CdTe clusters. Substitutional doping of Cd by metal atoms increases the stability of unpassivated clusters. For certain clusters, metal atom doping leads to a half-metallic character. Pd/Ag-doped clusters are p-type semiconductors whereas In-doped clusters are n-type semiconductors. Sn doping in these clusters does not result in n-type semiconductors.

Origin of the size-dependence of the polarizability per atom in heterogeneous clusters: The case of AlP clusters

An analysis of the atomic polarizabilities α in stoichiometric aluminum phosphide clusters, computed at the MP2 and density functional theory (DFT) levels, the latter using the B3LYP functional, and partitioned using the classic and iterative versions of the Hirshfeld method, is presented. Two sets of clusters are examined: the ground-state Al(n)P(n) clusters (n=2-9) and the prolate clusters (Al(2)P(2))(N) and (Al(3)P(3))(N) (N≤6). In the ground-state clusters, the mean polarizability per atom, i.e., α/2n, decreases with the cluster size but shows peaks at n=5 and at n=7. We demonstrate that these peaks can be explained by a large polarizability of the Al atoms and by a low polarizability of the P atoms in Al(5)P(5) and Al(7)P(7) due to the presence of homopolar bonds in these clusters. We show indeed that the polarizability of an atom within an Al(n)P(n) cluster depends on the cluster size and the heteropolarity of the bonds it forms within the cluster, i.e., on the charges of the atoms. The polarizabilities of the fragments Al(2)P(2) and Al(3)P(3) in the prolate clusters were found to depend mainly on their location within the cluster. Finally, we show that the iterative Hirshfeld method is more suitable than the classic Hirshfeld method for describing the atomic polarizabilities and the atomic charges in clusters with heteropolar bonds, although both versions of the Hirshfeld method lead to similar conclusions.

Hydrogen‐Atom Abstraction from Methane by Stoichiometric Vanadium–Silicon Heteronuclear Oxide Cluster Cations

Vanadium-silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O(2) background. Reactions of the heteronuclear oxide cations with methane in a fast-flow reactor were studied with a time-of-flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V(2)O(5))(n)(SiO(2))(m)](+) (n=1, m=1-4; n=2, m=1), and the estimated first-order rate constants for the reactions were close to that of the homonuclear oxide cluster V(4)O(10) (+) with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal-oxygen-centered radicals (O(t)*) were found in all of the stable isomers. These O(t)* radicals are active sites of the clusters in reaction with CH(4). The O(t)* radicals in [V(2)O(5)(SiO(2))(1-4)](+) clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH(4) activation on silica-supported vanadium oxide catalysts.

On the optical, electronic, and structural properties of zinc sulfide nanoclusters

The optical, electronic, and structural properties of stoichiometric zinc sulfide clusters ((ZnS)n, n � 10) have been systematically investigated using the density functional approach. The size evolution of several reactivity descriptors such as ionization potential, electron affinity, chemical hardness, and static dipole polarizability has been determined for ZnS clusters. Energetically, the relative stability of ZnS clusters at different sizes is studied by calculating their binding energy, the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap, and the second-order difference in total energy. In addition to energetic analysis, minimum polarizability principle and principle of maximum hardness are used to characterize the magic number clusters. Moreover, it is shown that the differential mean polarizability can also be a useful quantity to characterize the stability of the studied clusters. Also, it is found that there is a strong inverse correlation between the static dipole polarizability and the ionization potential of the ZnS clusters. Similarly, the softness has also been shown to mostly correlate with the dipole polarizability of these clusters. Thus, this work will have some important implications for the calculation of polarizability of ZnS clusters in terms of the corresponding ionization potentials directly. Finally, the similarity function has been also used to investigate the extent that the clusters are similar to the pure ZnS or to each other. V C 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3841-3850, 2011

Cluster generation under pulsed laser ablation of zinc oxide

Neutral and cationic Zn n O m clusters of various stoichiometry have been produced by nanosecond laser ablation of ZnO in vacuum and investigated by time-of-flight mass spectrometry. Particular attention was paid to the effect of laser wavelength (in the range from near-IR to UV) on cluster composition. Under 193-nm laser ablation, the charged clusters are essentially substoichiometric with $\mathrm{Zn}_{n}\mathrm{O}_{n-1}^{+}$ and $\mathrm{Zn}_{n}\mathrm{O}_{n-3}^{+}$ being the most abundant series. Both sub- and stoichiometric cationic clusters are generated in abundance at 532- and 1064-nm ablation whose composition depends on the cluster size. The reactivity of small stoichiometric $\mathrm{Zn}_{n}\mathrm{O}_{n}^{+}$ clusters (n<11) toward hydrogen is found to be high, while oxygen-deficient species are less reactive. The neutral plume particles are mainly stoichiometric with Zn4O4 tetramer being a magic cluster. It is suggested that the Zn4O4 loss is the dominant fragmentation channel of large zinc oxide clusters upon electron impact. Plume expansion conditions under ZnO ablation with visible and IR laser pulses are shown to be favorable for stoichiometric cluster formation.

Oxidation of Small Silver Clusters: A Density Functional Theory Study

The oxidation of small silver clusters (Agn, n ≤ 9) was investigated through electronic structure calculations based on density functional theory. The adsorption energies of molecular and dissociated adsorption show a pronounced odd/even alternation, with lower energies calculated for even-sized clusters. Molecular adsorption is favored for n ≤ 5, whereas dissociation is preferred for the larger sizes. Molecular oxygen is adsorbed in atop (Ag, Ag2, Ag6, Ag8) or bridge (Ag3, Ag4, Ag5, Ag7, Ag9) configurations, and atomic oxygen is preferably adsorbed in 3-fold hollow positions. Results for stoichiometric (Ag2nOn) clusters were compared to O2 adsorption on Ag(111), and ab initio thermodynamics was used to estimate the temperature for the oxide-to-metal phase transition. The barrier for O2 dissociation on Ag8 was calculated to be higher than the corresponding barrier on Ag(111), which indicates a slower oxidation process. Adsorption of NOx onto the oxidized clusters was found to proceed through a formal reduction of the clusters; that is, NOx is adsorbed as NOx+1 with x = 1, 2.

Fe+3-doped TiO2: A combined experimental and computational approach to the evaluation of visible light activity

Abstract The visible light activity of TiO2 particles was improved by Fe3+-doping. In order to characterize and describe the effect of Fe3+-doping on the electronic and structural properties of TiO2, a combination of experimental structural methods and density functional theory (DFT) calculations was used. A series of Fe3+-doped photocatalysts with different Fe3+ contents were prepared by an incipient wet impregnation method, in order to prevent penetration of the dopant cations into the bulk of TiO2. An obvious decrease in the band-gap and a red shift of the absorption threshold were observed by UV-DRS. The Fe3+-doped photocatalysts were characterized by FT-IR, XRD, Raman and XPS. The morphological structure of the photocatalysts was examined by SEM. Energy-dispersive X-ray analysis (EDX) in the SEM was also taken for the chemical analysis of the doped samples. The results indicate substitutional Fe3+-doping of TiO2. In the computational part of the study, a neutral, stoichiometric cluster Ti3O8H4 cut from the anatase bulk structure and three new models for the substitutional Fe3+-doped TiO2 were developed. The DFT calculations were carried out by the hybrid B3LYP functional, by using double-zeta, LanL2DZ basis set. A higher photocatalytic activity for the degradation of 4-nitrophenol was obtained for the Fe3+-doped TiO2 compared to the undoped TiO2. The results of the DFT calculations indicate that the origin of the visible light activity of the Fe3+-doped TiO2 is due to the introduction of additional electronic states within the band-gap.

Hydrogen-atom abstraction from methane by stoichiometric early transition metal oxide cluster cations

Stoichiometric early transition metal oxide cations (TiO(2))(1-5)(+), (ZrO(2))(1-4)(+), (HfO(2))(1-2)(+), (V(2)O(5))(1-5)(+), (Nb(2)O(5))(1-3)(+), (Ta(2)O(5))(1-2)(+), (MoO(3))(1-2)(+), (WO(3))(1-3)(+), and Re(2)O(7)(+) are able to activate the C-H bond of methane under near room temperature conditions.

The Role of Non-Metal Doping in TiO2 Photocatalysis

AbstractThe visible light activity of TiO

Evolutionary structure prediction and electronic properties of indium oxide nanoclusters

Indium sesquioxide is widely used as a transparent conducting oxide in modern optoelectronic devices; the rising cost of indium has generated interest in the nanoscale properties of In(2)O(3), and questions arise as to the nature of its physicochemical properties below the bulk regime. We report the stable and metastable stoichiometric clusters of (In(2)O(3))(n), where n = 1-10, as predicted from an evolutionary search within the classical interatomic potential and quantum density functional energy landscapes. In contrast to the paradigm set by ZnO, which favours high symmetry bubble-like structures, the In(2)O(3) nanoclusters are found to tend towards dense, low symmetry structures approaching the bulk system at remarkably small molecular masses. Electronic characterisation is performed at the hybrid density functional and many-body GW levels to obtain accurate predictions of the spectroscopic properties, with mean values of the ionisation potentials and electron affinities calculated as 7.7 and 1.7 eV, respectively.