Huiying Xu

Effect of TiO2 morphology on photovoltaic performance of dye-sensitized solar cells: nanoparticles, nanofibers, hierarchical spheres and ellipsoid spheres

Different morphologies of anatase TiO2 from nanoparticles, nanofibers, and hierarchical spheres to ellipsoid spheres are successfully fabricated via solvothermal reactions of titanium n-butoxide and acetic acid. The variations of morphology, size and crystal phase of TiO2 micro-/nanostructures are investigated in detail by XRD, FTIR, SEM and TEM. Effects of different TiO2 morphologies on the photovoltaic performance of dye-sensitized solar cells (DSSCs) are also discussed based on I–V, IPCE, IMPS, IMVS and UV-vis absorption and diffuse spectra. The DSSC based on the hierarchical anatase TiO2 sphere photoelectrode shows an overall light-to-electricity conversion efficiency of 9.35% accompanying a short-circuit current density of 17.94 mA cm−2, an open-circuit voltage of 803 mV and fill factor of 0.65, which is much higher than that of nanoparticles (7.37%), nanofibers (8.15%) and ellipsoid TiO2 spheres (7.93%). The significant enhancement of short-circuit current density and power conversion efficiency for the hierarchical sphere-based DSSC compared to other nanostructures is mainly attributed to the larger dye loading, superior light scattering ability, and/or faster electron transport and longer electron lifetime.

Dinuclear Zn(II) complex catalyzed phosphodiester cleavage proceeds via a concerted mechanism: A density functional theory study

Density functional theory (DFT) calculations were used to study the mechanism for the cleavage reaction of the RNA analogue HpPNP (HpPNP = 2-hydroxypropyl-4-nitrophenyl phosphate) catalyzed by the dinuclear Zn(II) complex of 1,3-bis(1,4,7-triazacyclonon-1-yl)-2-hydroxypropane (Zn(2)(L(2)O)). We present a binding mode in which each terminal phosphoryl oxygen atom binds to one zinc center, respectively, and the nucleophilic 2-hydroxypropyl group coordinates to one of the zinc ions, while the hydroxide from deprotonation of a water molecule coordinates to the other zinc ion. Our calculations found a concerted mechanism for the HpPNP cleavage with a 16.5 kcal/mol reaction barrier. An alternative proposed stepwise mechanism through a pentavalent oxyphosphorane dianion reaction intermediate for the HpPNP cleavage was found to be less feasible with a significantly higher energy barrier. In this stepwise mechanism, the deprotonation of the nucleophilic 2-hydroxypropyl group is accompanied with nucleophilic attack in the rate-determining step. Calculations of the nucleophile (18)O kinetic isotope effect (KIE) and leaving (18)O KIE for the concerted mechanism are in reasonably good agreement with the experimental values. Our results indicate a specific-base catalysis mechanism takes place in which the deprotonation of the nucleophilic 2-hydroxypropyl group occurs in a pre-equilibrium step followed by a nucleophilic attack on the phosphorus center. Detailed comparison of the geometric and electronic structure for the HpPNP cleavage reaction mechanisms in the presence/absence of catalyst revealed that the catalyst significantly altered the determining-step transition state to become far more associative or tight, that is, bond formation to the nucleophile was remarkably more advanced than leaving group bond fission in the catalyzed mechanism. Our results are consistent with and provide a reliable interpretation for the experimental observations that suggest the reaction occurs by a concerted mechanism (see Humphry, T.; Iyer, S.; Iranzo, O.; Morrow, J. R.; Richard, J. P.; Paneth, P.; Hengge, A. C. J. Am. Chem. Soc. 2008, 130, 17858-17866) and has a specific-base catalysis character (see Yang, M.-Y.; Iranzo, O.; Richard, J. P.; Morrow, J. R. J. Am. Chem. Soc. 2005, 127, 1064-1065).

Mechanistic Investigation of Dirhodium-Catalyzed Intramolecular Allylic C–H Amination versus Alkene Aziridination

The reaction mechanisms and chemoselectivity on the intramolecular allylic C-H amination versus alkene aziridination of 4-pentenylsulfamate promoted by four elaborately selected dirhodium paddlewheel complexes are investigated by a DFT approach. A predominant singlet concerted, highly asynchronous pathway and an alternative triplet stepwise pathway are obtained in either C-H amination or alkene aziridination reactions when mediated by weak electron-donating catalysts. A singlet stepwise C-H amination pathway is obtained under strongly donating catalysts. The rate-determining step in the C-H amination is the H-abstraction process. The subsequent diradical-rebound C-N formation in the triplet pathway or the combination of the allylic carbocation and the negative changed N center in the singlet pathway require an identical energy barrier. A mixed singlet-triplet pathway is preferred in either the C-H insertion or alkene aziridination in the Rh2(NCH3CHO)4 entry that the triplet pathway is initially favorable in the rate-determining steps, and the resultant triplet intermediates would convert to a singlet reaction coordinate. The nature of C-H amination or alkene aziridination is estimated to be a stepwise process. The theoretical observations presented in the paper are consistent with the experimental results and, more importantly, provide a thorough understanding of the nature of the reaction mechanisms and the minimum-energy crossing points.

Mechanism and Enantioselectivity of Dirhodium-Catalyzed Intramolecular C–H Amination of Sulfamate

The mechanisms and enantioselectivities of the dirhodium (Rh2L4, L = formate, N-methylformamide, S-nap)-catalyzed intramolecular C-H aminations of 3-phenylpropylsulfamate ester have been investigated in detail with BPW91 density functional theory computations. The reactions catalyzed by the Rh2(II,II) catalysts start from the oxidation of the Rh2(II,II) dimer to a triplet mixed-valent Rh2(II,III)-nitrene radical, which should facilitate radical H-atom abstraction. However, in the Rh2(formate)4-promoted reaction, as a result of a minimum-energy crossing point (MECP) between the singlet and triplet profiles, a direct C-H bond insertion is postulated. The Rh2(N-methylformamide)4 reaction exhibits quite different mechanistic characteristics, taking place via a two-step process involving (i) intramolecular H-abstraction on the triplet profile to generate a diradical intermediate and (ii) C-N formation by intersystem crossing from the triplet state to the open-shell singlet state. The stepwise mechanism was found to hold also in the reaction of 3-phenylpropylsulfamate ester catalyzed by Rh2(S-nap)4. Furthermore, the diradical intermediate also constitutes the starting point for competition steps involving enantioselectivity, which is determined by the C-N formation open-shell singlet transition state. This mechanistic proposal is supported by the calculated enantiomeric excess (94.2% ee) with the absolute stereochemistry of the product as R, in good agreement with the experimental results (92.0% ee).

DFT investigation of Ni-doped graphene: catalytic ability to CO oxidation

Herein, CO oxidation on Ni-doped graphene (Ni-Gr) is investigated by first-principle calculations. The strong binding energy (−7.57 eV) of the Ni atom at a single vacancy in graphene and high energy barrier (3.41 eV) for Ni atom mobility in graphene suggest that graphene is stable even after Ni doping, which avoids the problem of metal clustering. The stronger binding interaction between Ni-Gr and O2 than that between Ni-Gr and CO can prevent CO poisoning to Ni-Gr. To explore the catalytic effect of CO oxidation on Ni-Gr, both the Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms are investigated. The overall energy barrier at 0 K for the LH and ER mechanisms is 0.63 and 0.77 eV, respectively. At 298.15 K, the overall energy barrier for the LH mechanism decreases to 0.58 eV, whereas that for the ER mechanism increases to 0.88 eV, which implies that CO oxidation on Ni-Gr prefers to proceed via the LH mechanism kinetically. Our results show that the studied system, Ni-Gr, has chemical stability against metal clustering and CO poisoning, and it is a promising catalyst for CO oxidation at mild temperatures. This study provides a good theoretical guideline for the development of Ni-Gr based CO oxidation catalysts.

Removal of NO with silicene: a DFT investigation

Removing or reducing NO is meaningful for environment protection. Herein, the investigation of the probability of NO reduction on silicene is presented utilizing DFT calculations. Two mechanisms for NO reduction on silicene are provided: a direct dissociation mechanism and a dimer mechanism. The direct dissociation mechanism is characterized as the direct breaking of the N–O bond. The calculated potential energy surfaces show that the total energy barrier in the favored direct dissociation pathway is 0.466 eV. On the other hand, the dimer mechanism is identified to undergo a (NO)2 dimer formation on silicene, which then decomposes into N2O + Oad or N2 + 2Oad. The (NO)2 dimer formation on silicene is found to be feasible both in thermodynamics and kinetics. The formation energy barriers for (NO)2 dimer are lower than 0.231 eV. The calculation results indicate that the (NO)2 dimers can be readily reduced into N2O or N2. The energy barriers in the favored decomposition pathways to produce N2O are quite low (<0.032 eV). The energy barrier for the release of N2 is calculated to be 0.156 eV. The further reduction of N2O to N2 on silicene is also investigated. The results indicate it is easy to reduce N2O to N2 with an energy barrier of only 0.445 eV. NO reduction on silicene hence prefers to generate N2 via the dimer mechanism when compared to the direct dissociation. NO reduction on silicene with silicane as substrate is further proved to proceed via the same reduction mechanism as compared with the free-standing model. Hence, our results presented here suggest that silicene can be a potential material in NO removal, which will reduce NO into environmentally-friendly gases.

Density functional theory study of the mechanism of zinc carbenoid promoted cyclopropanation of allenamides

The mono- and bis-cyclopropanation of allenamides with the zinc carbenoid Zn(CH2Cl)2 have been studied using density functional theory calculations employing the M06 functional. The monomeric and dimeric precursor complexes were both constructed to model the reaction processes. In the monomeric reaction, the formation of the endo-monocyclopropyl species takes place via a methylene transfer pathway rather than a carbometalation pathway. The formation of the exo-monocyclopropyl species does not readily occur via a methylene transfer pathway due to a high activation barrier. The corresponding carbometalation pathway was not able to be found. Following the monocyclopropanation step, the biscyclopropanation of the endo-monocyclopropyl species is facile to form amidospiro[2.2]pentane. In the aggregation model, the allenamides and the zinc carbenoid form a dimer aggregate that is then followed by two pathways. One pathway takes place via transition states inside the aggregate structure (denoted here as a closed-mode process) while the other pathway introduces another zinc carbenoid molecule from outside the aggregation species (denoted here as an open-mode process). The aggregate mechanisms are not favored because the dimeric reactant of the open-mode process is not stable to coexist with the monomer and the activation barriers of the two aggregate pathways are higher than those of the monomeric pathways. The calculation results show that the key factors in the reaction mechanisms are the co-planarity of the allenic moiety with the oxazolidinone ring, the torsional strain in the butterfly-type transition state, the ring strain in the substrate–carbenoid complexes and the coordination between the carbenoid-Zn and O(CO) atoms and other long-distance interactions.

Platinum(II)-catalyzed cyclization sequence of aryl alkynes via C(sp3)-H activation: a DFT study.

The mechanism and intermediates of hydroalkylation of aryl alkynes via C(sp(3))-H activation through a platinum(II)-centered catalyst are investigated with density functional theory at the B3LYP/[6-31G(d) for H, O, C; 6-31+G(d,p) for F, Cl; SDD for Pt] level of theory. Solvent effects on reactions were explored using calculations that included a polarizable continuum model for the solvent (THF). Free energy diagrams for three suggested mechanisms were computed: (a) one that leads to formation of a Pt(II) vinyl carbenoid (Mechanism A), (b) another where the transition state implies a directed 1,4-hydrogen shift (Mechanism B), and (c) one with a Pt-aided 1,4-hydrogen migration (Mechanism C). Results suggest that the insertion reaction pathway of Mechanism A is reasonable. Through 4,5-hydrogen transfer, the Pt(II) vinyl carbenoid is formed. Thus, the stepwise insertion mechanism is favored while the electrocyclization mechanism is implausible. Electron-withdrawing/electron-donating groups substituted at the phenyl and benzyl sp(3) C atoms slightly change the thermodynamic properties of the first half of Mechanism A, but electronic effects cause a substantial shift in relative energies for the second half of Mechanism A. The rate-limiting step can be varied between the 4,5-hydrogen shift process and the 1,5-hydrogen shift step by altering electron-withdrawing/electron-donating groups on the benzyl C atom. Additionally, NBO and AIM analyses are applied to further investigate electronic structure changes during the mechanism.

Mechanistic Insight into the Intramolecular Benzylic C-H Nitrene Insertion Catalyzed by Bimetallic Paddlewheel Complexes: Influence of the Metal Centers.

The intramolecular benzylic C-H amination catalyzed by bimetallic paddlewheel complexes was investigated by using density functional theory calculations. The metal-metal bonding characters were investigated and the structures featuring either a small HOMO-LUMO gap or a compact SOMO energy scope were estimated to facilitate an easier one-electron oxidation of the bimetallic center. The hydrogen-abstraction step was found to occur through three manners, that is, hydride transfer, hydrogen migration, and proton transfer. The imido N species are more preferred in the Ru-Ru and Pd-Mn cases whereas coexisting N species, namely, singlet/triplet nitrene and imido, were observed in the Rh-Rh and Pd-Co cases. On the other hand, the triplet nitrene N species were found to be predominant in the Pd-Ni and Pd-Zn systems. A concerted asynchronous mechanism was found to be modestly favorable in the Rh-Rh-catalyzed reactions whereas the Pd-Co-catalyzed reactions demonstrated a slight preference for a stepwise pathway. Favored stepwise pathways were seen in each Ru-Ru- and Pd-Mn-catalyzed reactions and in the triplet nitrene involved Pd-Ni and Pd-Zn reactions. The calculations suggest the feasibility of the Pd-Mn, Pd-Co, and Pd-Ni paddlewheel complexes as being economical alternatives for the expensive dirhodium/diruthenium complexes in C-H amination catalysis.

Mechanistic investigation into the cleavage of a phosphomonoester mediated by a symmetrical oxyimine-based macrocyclic zinc(II) complex.

Density functional calculations are utilized to explore the hydrolysis mechanisms of the phosphomonoester 4-nitrophenyl phosphate catalyzed by a symmetrical zinc(II) complex. The formation process and properties of the active catalyst are verified. Eight plausible mechanisms are proposed and categorized into three groups. All of the proposed mechanisms, except for Mechanism 7 (see text), are S(N)2-type addition-substitution reaction pathways. Nucleophilic attack at the ortho position occurs in Mechanism 7 with a relatively high reaction barrier. Mechanisms 1 and 2 in the monocatalyst model, Mechanisms 5 to 7 in the sandwich-dual-catalyst model, as well as the nucleophilic addition-substitution step in Mechanism 8 are concerted reaction pathways, whereas the rest appear to occur in a stepwise manner. Meanwhile, the explicit solvent model is utilized to consider direct hydrogen bonds and solvation interactions and these results indicate that the added water molecule is involved in the hydrolysis process, but does not change the mechanisms significantly. Mechanism 8, with the lowest reaction barrier, is the most favored reaction pathway of the eight proposed mechanisms, although Mechanisms 1, 4, and 6 are in competition with Mechanism 8. In consideration of the zinc(II) complex concentration, Mechanism 1 is only the predominant reaction pathway at a low zinc(II) complex concentration; Mechanisms 4 and 6 tend to be more competitive with increasing concentration of the zinc(II) complexes, and Mechanism 8 is favored at high zinc(II) complex concentrations. Our calculated results are consistent with, and can be used to systematically interpret, experimental observations. More importantly, insightful suggestions are made regarding the catalyst design and selection of the reaction environment.