Xiao-Na Li

Injection, transport, absorption and phosphorescence properties of a series of blue-emitting Ir(III) emitters in OLEDs: a DFT and time-dependent DFT study.

Quantum-chemistry methods were explored to investigate the electronic structures, injection and transport properties, absorption and phosphorescence mechanism of a series of blue-emitting Ir(III) complexes {[(F(2)-ppy)(2)Ir(pta -X/pyN4)], where F(2)-ppy = (2,4-difluoro)phenylpyridine; pta = pyridine-1,2,4-triazole; X = phenyl(1); p-tolyl (2); 2,6-difluororophenyl (3); -CF(3) (4), and pyN4 = pyridine-1,2,4-tetrazolate (5)}, which are used as emitters in organic light-emitting diodes (OLEDs). The mobility of hole and electron were studied computationally based on the Marcus theory. Calculations of Ionization potentials (IPs) and electron affinities (EAs) were used to evaluate the injection abilities of holes and electrons into these complexes. The reasons for the lower EL efficiency and phosphorescence quantum yields in 3-5 than in 1and 2 have been investigated. These new structure-property relationships can guide an improved design and optimization of OLED devices based on blue-emitting phosphorescent Ir(III) complexes.

CO oxidation catalyzed by single gold atoms supported on aluminum oxide clusters.

The single gold atom doped aluminum oxide clusters AuAl3O3(+), AuAl3O4(+), and AuAl3O5(+) have been prepared and mass-selected to react with CO, O2, and mixtures of CO and O2 in an ion trap reactor under thermal collision conditions. The reactions have been characterized by mass spectrometry with isotopic substitution ((16)O2 → (18)O2) and density functional theory calculations. The AuAl3O5(+) cluster can oxidize two CO molecules consecutively to form AuAl3O4(+) and then AuAl3O3(+), the latter of which can react with one O2 molecule to regenerate AuAl3O5(+). The AuAl3(16)O3(+) ions interact with a mixture of C(16)O and (18)O2 to produce the fully substituted (18)O species AuAl3(18)O3-5(+), which firmly identifies a catalytic cycle for CO oxidation by O2. The oxidation catalysis is driven by electron cycling primarily through making and breaking a gold-aluminum chemical bond. To the best of our knowledge, this is the first identification of catalytic CO oxidation by O2 mediated with gas-phase cluster catalysts with single-noble-metal atoms, which serves as an important step to understand single-atom catalysis at strictly a molecular level.

CO oxidation promoted by gold atoms supported on titanium oxide cluster anions.

Laser ablation generated Au(x)(TiO2)(y)O(z)(-) (x = 0, 1; y = 2, 3; z = 1, 2) oxide cluster anions have been mass-selected using a quadrupole mass filter and reacted with CO in a hexapole collision cell. The reactions have been characterized by time-of-flight mass spectrometry and density functional theory calculations. Gold-titanium bimetallic oxide clusters Au(TiO2)(y)O(z)(-) are more reactive in CO oxidation than pure titanium oxide clusters (TiO2)(y)O(z)(-). The computational studies identify the dual roles that the gold atom plays in CO oxidation: functioning as a CO trapper and electron acceptor. Both factors are important for the high reactivity of Au(TiO2)(y)O(z)(-) clusters. To the best of our knowledge, this is the first example of CO oxidation by gold-containing heteronuclear oxide clusters, which provides molecular-level insights into the roles of gold in CO oxidation over oxide supports.

Hierarchically structured Fe3O4 microspheres: morphology control and their application in wastewater treatment

Novel three-dimensional (3D) flower-like Fe3O4 microspheres were synthesized by a facile precursor-templated conversion method. The precursor was prepared by an ultrasound-assisted hydrothermal method without the help of any surfactant. The possible formation mechanism of the precursor was proposed, and it was found that the synthetic parameters for the precursor such as the time of ultrasonic pretreatment and NH4Ac concentration are crucial for the formation of the flower-like hierarchical precursor structure. The flower-like Fe3O4 microspheres obtained by calcining the precursor in N2 exhibit superparamagnetic behaviour and show relative high saturation magnetization at room temperature. Furthermore, the as-obtained product, with high BET surface area, has been used as an absorbent in wastewater treatment and exhibit a strong capability to remove organic pollutants.

Thermal Methane Conversion to Formaldehyde Promoted by Single Platinum Atoms in PtAl2O4− Cluster Anions†

Identification and mechanistic study of thermal methane conversion mediated by gas-phase species is important for finding potentially useful routes for direct methane transformation under mild conditions. Negatively charged oxide species are usually inert with methane. This work reports an unexpected result that the bi-metallic oxide cluster anions PtAl2 O4 (-) can transform methane into a stable organic compound, formaldehyde, with high selectivity. The clusters are prepared by laser ablation and reacted with CH4 in an ion trap reactor. The reaction is characterized by mass spectrometry and density functional theory calculations. It is found that platinum rather than oxygen activates CH4 at the beginning of the reaction. The Al2 O4 (-) moiety serves as the support of Pt atom and plays important roles in the late stage of the reaction. A new mechanism for selective methane conversion is provided and new insights into the surface chemistry of single Pt atoms may be obtained from this study.

Reactivity of Atomic Oxygen Radical Anions Bound to Titania and Zirconia Nanoparticles in the Gas Phase: Low-Temperature Oxidation of Carbon Monoxide

Titanium and zirconium oxide cluster anions with dimensions up to nanosize are prepared by laser ablation and reacted with carbon monoxide in a fast low reactor. The cluster reactions are characterized by time-of-flight mass spectrometry and density functional theory calculations. The oxygen atom transfers from (TiO(2))(n)O(-) (n = 3-25) to CO and formations of (TiO(2))(n)(-) are observed, whereas the reactions of (ZrO(2))(n)O(-) (n = 3-25) with CO generate the CO addition products (ZrO(2))(n)OCO(-), which lose CO(2) upon the collisions (studied for n = 3-9) with a crossed helium beam. The computational study indicates that the (MO(2))(n)O(-) (M = Ti, Zr; n = 3-8) clusters are atomic radical anion (O(-)) bonded systems, and the energetics for CO oxidation by the O(-) radicals to form CO(2) is strongly dependent on the metals as well as the cluster size for the titanium system. Atomic oxygen radical anions are important reactive intermediates, while it is difficult to capture and characterize them for condensed phase systems. The reactivity pattern of the O(-)-bonded (TiO(2))(n)O(-) and (ZrO(2))(n)O(-) correlates very well with different behaviors of titania and zirconia supports in the low-temperature catalytic CO oxidation.

Activation of Multiple CH Bonds Promoted by Gold in AuNbO3+ Clusters†

Single, double, triple: Highly selective double H atom abstraction (HAA) from ethane and triple HAA from n-butane by AuNbO(3)(+) clusters have been characterized by mass spectrometry and DFT calculations. The multiple HAAs are initiated by oxygen-centered radicals and promoted by gold. The gold atoms act as electron acceptors during the triple HAAs and help to store a pair of valence electrons between Au and Nb atoms.

CO Oxidation Promoted by Gold Atoms Loosely Attached in AuFeO3– Cluster Anions

Time-of-flight mass spectrometry experiment shows that upon the interactions with carbon monoxide, the mass-selected AuFeO3(-) oxide cluster anions can evaporate neutral gold atoms in a hexapole collision cell and oxidize CO into CO2 in an ion trap reactor. The computational studies identify that the gold atom is loosely attached in the AuFeO3(-) cluster, and the different reaction channels can be attributed to different cluster velocities. The structure of the AuFeO3(-) cluster is very flexible, and the approach of CO induces significant geometrical and electronic structure changes of AuFeO3(-), which facilitates the exposure of the positively charged gold atom to trap and oxidize CO. The CO oxidation by the AuFeO3(-) cluster follows the Au-assisted Mars-van Krevelen mechanism, in which the direct participation of the surface lattice oxygen (O(2-)) is proposed.

Experimental and theoretical study of hydrogen atom abstraction from n-butane by lanthanum oxide cluster anions.

Lanthanum oxide cluster anions are prepared by laser ablation and reacted with n-C(4)H(10) in a fast flow reactor. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. (La(2)O(3))(m=1-3)OH(-) and La(3)O(7)H(-) are observed as products, which suggests the occurrence of hydrogen atom abstraction reactions: (La(2)O(3))(m=1-3)O(-) + n-C(4)H(10) → (La(2)O(3))(m=1-3)OH(-) + C(4)H(9) and La(3)O(7)(-) + n-C(4)H(10) → La(3)O(7)H(-) + C(4)H(9). Density functional theory (DFT) calculations are performed to study the structures and bonding properties of La(2)O(4)(-), La(3)O(7)(-), and La(4)O(7)(-) clusters. The calculated results show that each of La(2)O(4)(-) and La(4)O(7)(-) contains one oxygen-centered radical (O(-•)) which is responsible for the high reactivity toward n-C(4)H(10). La(3)O(7)(-) contains one oxygen-centered radical (O(-•)) and one superoxide unit (O(2)(-•)), and the O(-•) is responsible for its high reactivity toward n-C(4)H(10). The O(-•) and O(2)(-•) can be considered to be generated by the adsorption of an O(2) molecule onto the singlet La(3)O(5)(-) with electron transfer from a terminally bonded oxygen ion (O(2-)) to the O(2). This may help us understand the mechanism of the formation of O(-•) and O(2)(-•) radicals in lanthanum oxide systems. The reaction mechanisms of La(2)O(4)(-) + n-C(4)H(10) and La(3)O(7)(-) + n-C(4)H(10) are also studied by the DFT calculations, and the calculated results are in good agreement with the experimental observations.

Thermal Conversion of Methane to Formaldehyde Promoted by Gold in AuNbO3+ Cluster Cations

In addition to generation of a methyl radical, formation of a formaldehyde molecule was observed in the thermal reaction of methane with AuNbO3(+) heteronuclear oxide cluster cations. The clusters were prepared by laser ablation and mass-selected to react with CH4 in an ion-trap reactor under thermal collision conditions. The reaction was studied by mass spectrometry and DFT calculations. The latter indicated that the gold atom promotes formaldehyde formation through transformation of an Au-O bond into an Au-Nb bond during the reaction.