Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage

Redox mediators play a major role determining the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the reduction of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO₂ and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu^(II/I)(dmby)₂TFSI_2/1 (0.97 V vs SHE, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu^(II/I)(tmby)₂TFSI_2/1 (0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu^(II/I)(dmp)₂TFSI_2/1 (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 μs) was observed for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu^(II/I)(tmby)₂TFSI_2/1 as evidenced by longer lifetimes. The solar-to-electrical power conversion efficiencies for [Cu(tmby)₂]^2+/1+, [Cu(dmby)₂]^2+/1+, and [Cu(dmp)₂]^2+/1+ based electrolytes were 10.3%, 10.0%, and 10.3%, respectively, using the organic Y123 dye under 1000 W m^-2 AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.

Promoting oxygen vacancy formation and p-type conductivity in SrTiO3via alkali metal doping: a first principles study

Strontium titanate (SrTiO₃, STO) is a prototypical perovskite oxide, widely exploited in many technological applications, from catalysis to energy conversion devices. In the context of solid-oxide fuel cells, STO has been recently applied as an epitaxial substrate for nano-sized layers of mixed ion-electron conductive catalysts with enhanced electrochemical performances. To extend the applications of such heterogeneous nano-cathodes in real devices, also the STO support should be active for both electron transport and oxide diffusion. To this end, we explored using first-principles calculations the strategy of doping of STO at the Sr site with sodium and potassium. These two ions fit in the perovskite structure and induce holes in the STO valence band, so as to obtain the desired p-type electronic conduction. At the same time, the doping with alkali ions also promotes the formation of oxygen vacancies in STO, a prerequisite for effective oxide diffusion. Analysis of electron density rearrangements upon defect formation allows relating the favorable vacancy formation energies to an improved electronic delocalization over the oxide sub-lattice, as observed in closely related materials (e.g. Sr₂Fe_1.5Mo_0.5O₆). Overall, our results suggest the alkali-doped STO as a new potential substrate material in nanoscale heterogeneous electrodes for solid oxide electrochemical cells.

Ab initio study of PbCr(1-x)S x O4 solid solution: an inside look at Van Gogh Yellow degradation

Van Gogh Yellow refers to a family of lead chromate pigments widely used in the 19th century and often mixed with lead sulfate to obtain different yellow hues. Unfortunately, some paintings, such as the famous Sunflowers series, suffered degradation problems due to photoactivated darkening of once bright yellow areas, especially when irradiated with UV light. Recent advanced spectroscopic analyses have proved that this process occurs mostly where the pigment presents a sulfur-rich orthorhombic phase of a PbCr(1-x)S x O4 solid solution, while chromium-rich monoclinic phases are lightfast. However, the question of whether degradation is purely a surface phenomenon or if the bulk properties of sulfur-rich pigments trigger the process is still open. Here, we use first-principles calculations to unveil the role of sulfur in determining important bulk features such as structure, stability, and optical properties. From our findings, we suggest that degradation occurs via an initial local segregation of lead sulfate that absorbs at UV light wavelengths and provides the necessary energy for subsequent reduction of chromate ions into the greenish chromic oxide detected in experiments. In perspective, our results set reliable scientific foundations for further studies on surface browning phenomena and can help to chose the best strategy for the proper conservation of art masterpieces.

Effects of simultaneous palatal expansion and mandibular advancement in a child suffering from OSA

This clinical report describes a child suffering from obstructive sleep apnoea (OSA) and class II skeletal malocclusion with maxillary contraction and anterior open bite. He presented moderate obstructive sleep apnoea with large impact on quality of life of patient and parents. He was treated using an innovative orthodontic device (Sleep Apnea Twin Expander) to simultaneously carry out palatal expansion and mandibular advancement. After orthodontic therapy, the OSA-18 questionnaire demonstrated an improvement of the main respiratory symptoms, while cardiorespiratory sleep study revealed a reduction in obstructive sleep apnoea events. Post-treatment, clinical assessment and cephalometric analysis showed a reduction of sagittal maxillary discrepancy and an extension of upper airway space. In conclusion, this case report suggests that orthodontic treatment might be a valuable alternative treatment in children with obstructive sleep apnoea related to craniofacial anomalies.

Cost-effective solar concentrators based on red fluorescent Zn(ii)–salicylaldiminato complex

A red light and concentrated zinc complex embedded in PMMA yields a cost-effective and efficient luminescent solar concentrator.

Origin and Electronic Features of Reactive Oxygen Species at Hybrid Zirconia-Acetylacetonate Interfaces

The hybrid sol-gel zirconia-acetylacetonate amorphous material (HSGZ) shows high catalytic activity in oxidative degradation reactions without light or thermal pretreatment. This peculiar HSGZ ability derives from the generation of highly reactive oxygen radical species (ROS) upon exposure to air at room conditions. We disclose the origin of such unique feature by combining EPR and DRUV measurements with first-principles calculations. The organic ligand acetylacetonate (acac) plays a pivotal role in generating and stabilizing the superoxide radical species at the HSGZ-air interfaces. Our results lead the path toward further development of HSGZ and related hybrid materials for ROS-based energy and environmental applications.

Structure and energy level alignment at the dye–electrode interface in p-type DSSCs: new hints on the role of anchoring modes from ab initio calculations

The thermodynamic driving force for hole injection in p-DSSCs is maximized by the dye anchoring modes that minimize electrode surface distortions.

Oxygen transport in perovskite-type solid oxide fuel cell materials: insights from quantum mechanics

CONSPECTUS: Global advances in industrialization are precipitating increasingly rapid consumption of fossil fuel resources and heightened levels of atmospheric CO2. World sustainability requires viable sources of renewable energy and its efficient use. First-principles quantum mechanics (QM) studies can help guide developments in energy technologies by characterizing complex material properties and predicting reaction mechanisms at the atomic scale. QM can provide unbiased, qualitative guidelines for experimentally tailoring materials for energy applications. This Account primarily reviews our recent QM studies of electrode materials for solid oxide fuel cells (SOFCs), a promising technology for clean, efficient power generation. SOFCs presently must operate at very high temperatures to allow transport of oxygen ions and electrons through solid-state electrolytes and electrodes. High temperatures, however, engender slow startup times and accelerate material degradation. SOFC technologies need cathode and anode materials that function well at lower temperatures, which have been realized with mixed ion-electron conductor (MIEC) materials. Unfortunately, the complexity of MIECs has inhibited the rational tailoring of improved SOFC materials. Here, we gather theoretically obtained insights into oxygen ion conductivity in two classes of perovskite-type materials for SOFC applications: the conventional La1-xSrxMO3 family (M = Cr, Mn, Fe, Co) and the new, promising class of Sr2Fe2-xMoxO6 materials. Using density functional theory + U (DFT+U) with U-J values obtained from ab initio theory, we have characterized the accompanying electronic structures for the two processes that govern ionic diffusion in these materials: (i) oxygen vacancy formation and (ii) vacancy-mediated oxygen migration. We show how the corresponding macroscopic oxygen diffusion coefficient can be accurately obtained in terms of microscopic quantities calculated with first-principles QM. We find that the oxygen vacancy formation energy is a robust descriptor for evaluating oxide ion transport properties. We also find it has a direct relationship with (i) the transition metal-oxygen bond strength and (ii) the extent to which electrons left behind by the departing oxygen delocalize onto the oxygen sublattice. Design principles from our QM results may guide further development of perovskite-based MIEC materials for SOFC applications.

Origin of the energy barrier to chemical reactions of O2 on Al(111): evidence for charge transfer, not spin selection

Dissociative adsorption of molecular oxygen on the Al(111) surface exhibits mechanistic complexity that remains surprisingly poorly understood in terms of the underlying physics. Experiments clearly indicate substantial energy barriers and a mysteriously large number of adsorbed single oxygen atoms instead of pairs. Conventional first principles quantum mechanics (density functional theory) predicts no energy barrier at all; instead, spin selection rules have been invoked to explain the barrier. In this Letter, we show that correct barriers arise naturally when embedded correlated electron wave functions are used to capture the physics of the interaction of O(2) with the metal surface. The barrier originates from an abrupt charge transfer (from metal to oxygen), which is properly treated within correlated wave function theory but not within conventional density functional theory. Our potential energy surfaces also identify oxygen atom abstraction as the dominant reaction pathway at low incident energies, consistent with measurements, and show that charge transfer occurs in a stepwise fashion.

Insufficient Hartree-Fock Exchange in Hybrid DFT Functionals Produces Bent Alkynyl Radical Structures

Density functional theory (DFT) is often used to determine the electronic and geometric structures of molecules. While studying alkynyl radicals, we discovered that DFT exchange-correlation (XC) functionals containing less than ∼22% Hartree-Fock (HF) exchange led to qualitatively different structures than those predicted from ab initio HF and post-HF calculations or DFT XCs containing 25% or more HF exchange. We attribute this discrepancy to rehybridization at the radical center due to electron delocalization across the triple bonds of the alkynyl groups, which itself is an artifact of self-interaction and delocalization errors. Inclusion of sufficient exact exchange reduces these errors and suppresses this erroneous delocalization; we find that a threshold amount is needed for accurate structure determinations. Below this threshold, significant errors in predicted alkyne thermochemistry emerge as a consequence.

Halogen bonds between 2,2,6,6-tetramethylpiperidine-N-oxyl radical and CxHyFzI species: DFT calculations of physicochemical properties and comparison with hydrogen bonded adducts

Structural, thermodynamic, and magnetic properties of adducts between the 2,2,6,6-tetramethylpiperidine-N-oxyl radical and representative hydrogen and halogen bond donors in solution have been investigated by an integrated computational tool including hybrid density functionals and discrete-continuum solvent models. From a quantitative point of view, the computed values show a fair agreement with experiment when environmental effects are taken into the proper account. From a more general point of view, our analysis points out a number of analogies, but also some difference, between hydrogen and halogen bond, which have been interpreted in terms of the various effects tuning thermodynamic and spectroscopic parameters.

Interplay of Intrinsic, Environmental, and Dynamic Effects in Tuning the EPR Parameters of Nitroxides: Further Insights from an Integrated Computational Approach

The role of stereoelectronic, environmental, and short-time dynamic effects in tuning the hyperfine and gyromagnetic tensors of a prototypical nitroxide spin probe has been investigated by an integrated computational approach based on extended Lagrangian molecular dynamics and discrete-continuum solvent models. Trajectories were generated in two protic solvents as well as in the gas phase for reference; structural analysis of the dynamics, and comparison with optimized solute-solvent clusters, allowed for the identification of the prevailing solute-solvent hydrogen-bonding patterns and helped to define the strategy for the computation of magnetic parameters. This was performed in a separate step, on a large number of frames, by a high-level DFT approach coupling the PBE0 hybrid functional with a tailored basis set and with proper account of specific and bulk solvent effects. Remarkable changes in solvation networks are found on going from aqueous to methanol solution, thus providing a rationalization of indirect experimentally available evidence. The computed magnetic parameters are in satisfactory agreement with the available measured values and allow for an unbiased evaluation of the role of different effects in tuning the overall EPR observables. Apart from their intrinsic interest, our results pave the route toward the development of tunable detection protocols based on specific spectroscopic signatures.

Evidence of Variable H-Bond Network for Nitroxide Radicals in Protic Solvents

This letter presents the results of a thorough computational investigation of two prototypical nitroxide spin probes in two different protic solvents, namely water and methanol, based on the combined use of Car-Parrinello molecular dynamic simulations and static cluster/continuum quantum chemical computations. Remarkable changes in solvation networks were found on going from aqueous to methanolic solutions. Moreover, despite their structural similarity, the two nitroxide probes display quite different behaviors in water. This provides a rationalization of indirect experimentally available indications. Eventually, the combination of static and dynamical ab initio methods exploited in the present study allows to dissect many subtle features of the nitroxide-solvent interaction, and also allows for an analysis of solvent effects on magnetic parameters (hyperfine coupling constants and g-tensor shift).

Interplay of Stereoelectronic and Enviromental Effects in Tuning the Structural and Magnetic Properties of a Prototypical Spin Probe: Further Insights from a First Principle Dynamical Approach

The nitrogen isotropic hyperfine coupling constant (hcc) and the g tensor of a prototypical spin probe (di-tert-butyl nitroxide, DTBN) in aqueous solution have been investigated by means of an integrated computational approach including Car-Parrinello molecular dynamics and quantum mechanical calculations involving a discrete-continuum embedding. The quantitative agreement between computed and experimental parameters fully validates our integrated approach. Decoupling of the structural, dynamical, and environmental contributions acting onto the spectral observables allows an unbiased judgment of the role played by different effects in determining the overall experimental observables and highlights the importance of finite-temperature vibrational averaging. Together with their intrinsic interest, our results pave the route toward more reliable interpretations of EPR parameters of complex systems of biological and technological relevance.

Spectroscopic Properties in the Liquid Phase: Combining High-Level Ab Initio Calculations and Classical Molecular Dynamics

We present an integrated computational tool, rooted in density functional theory, the polarizable continuum model, and classical molecular dynamics employing spherical boundary conditions, to study the spectroscopic observables of molecules in solution. As a test case, a modified OPLS-AA force field has been developed and used to compute the UV and NMR spectra of acetone in aqueous solution. The results show that provided the classical force fields are carefully reparameterized and validated, the proposed approach is robust and effective, and can also be used by nonspecialists to provide a general and powerful complement to experimental techniques.

Adenotonsillectomy for obstructive sleep apnea in children with Prader-Willi syndrome

The aim of our study was to evaluate the efficacy of adenotonsillectomy for the treatment of obstructive sleep apnea syndrome (OSA) in pediatric patients with Prader-Willi syndrome (PWS), and to describe the postoperative complications. Five patients (4 males; median age, 4.4 years; range, 1.6-14.2 years) were studied. All patients underwent an overnight cardiorespiratory sleep study. All patients had adenotonsillar hypertrophy (ATH), and two were also obese. The preoperative obstructive apnea/hypopnea index (AHI; median and range) was 12.2 (9.0-19.9) events/hr; the mean oxygen saturation was 95 (79-96)%; the nadir oxygen saturation was 71 (58-78)%; and the oxygen desaturation index (ODI) was 15.8 (11.4-35.9) events/hr. Preoperatively, patients were classified as having moderate to severe OSA. A second sleep study, performed 16 (3-43) months after adenotonsillectomy, showed a significant decrease in AHI (P = 0.009) and ODI (P = 0.009). Mean and nadir oxygen saturation did not differ significantly postsurgery (P = 0.188, P = 0.073, respectively). Four out of five children showed at least one postoperative complication. Difficult awakening from anesthesia, hemorrhages, and respiratory complications requiring reintubation and/or supplemental oxygen administration were observed. In conclusion, patients with PWS and OSA who underwent adenotonsillectomy showed a significant decrease in AHI and number of oxygen desaturations.

Paediatric burns in the acute phase: specific aspects

This paper deals with specific aspects of paediatric burns in the acute phase and considers how the treatment of burned children differs from that of burned adults. The epidemiology of paediatric burns is reviewed. Particular aspects of the treatment of burned children are presented, with regard to treatment at the site of the accident, first aid, resuscitation, and local treatment. The importance of the accurate assessment of paediatric burns is stressed.

Solvent Effects on the UV (n → π*) and NMR (13C and 17O) Spectra of Acetone in Aqueous Solution. An Integrated Car−Parrinello and DFT/PCM Approach

The accurate reproduction of ultraviolet and nuclear magnetic resonance spectra of acetone in aqueous solution is used as a test of an integrated computational tool rooted in the density functional theory, the polarizable continuum model, and the Car-Parrinello dynamics. The analysis and solution of several conceptual and practical issues results in a robust and effective approach, which also can be used by nonspecialists and provides a general and powerful complement to experimental techniques.

Preliminary European results of local microwave hyperthermia and chemotherapy treatment in intermediate or high risk superficial transitional cell carcinoma of the bladder

INTRODUCTION

Superficial bladder cancer can be treated by transurethral resection (TUR) and adjuvant intravesical therapy. Intravesical bacillus Calmette-Guérin (BCG) has been proven to be more efficacious with respect to recurrence prevention than intravesical chemotherapy, although at the cost of more severe side effects. There is a need for a new treatment modality with higher efficacy and less toxicity. The subject of this study is the efficacy of local microwave hyperthermia and chemotherapy treatment in intermediate or high risk superficial transitional cell carcinoma (TCC) of the bladder.

PATIENTS AND METHODS

Ninety eligible patients received adjuvant treatment with a combination of mitomycin-C (MMC) and local microwave hyperthermia. All patients had multiple or recurrent Ta or T1 TCC of the bladder and were classified as intermediate or high risk according to EAU criteria. In total, 41 patients were BCG failures. The treatment regimen included 6 to 8 weekly sessions followed by 4 to 6 monthly sessions. Follow-up consisted of video-cystoscopy and urine cytology every 3 months. All patients were observed for 2 years.

RESULTS

Kaplan-Meier analyses of the total group (N = 90) indicated that 1 year after treatment only 14.3% (SE 4.5%) of all patients experienced a recurrence. After 2 years of follow-up the risk of recurrence was 24.6% (SE 5.9%). No progression in stage and grade was observed.

CONCLUSION

Microwave induced hyperthermia combined with MMC has promising value in intermediate or high risk superficial bladder cancer patients compared to literature data of BCG and/or intravesical chemotherapy, particularly where other treatments, i.e. BCG, have failed.

Reliable NMR chemical shifts for molecules in solution by methods rooted in density functional theory

The conceptual and numerical problems involved in the computation of reliable NMR chemical shifts for molecules in condensed phases are analyzed with reference to a number of case studies ranging from aromatic compounds in low-polarity solvents to carbonyl and amidic models in aqueous solution and to large polypeptides. The results show that an integrated tool including the most recent density functionals, mixed discrete-continuum solvent models, hybrid QM/MM approaches and, when needed, averaging from molecular dynamics simulations are becoming an invaluable complement to experimental results.