Structural and optical characterisation of silanised Dy-doped-Gd2O3 NPs.

In this work, we studied the optical properties of Dy-doped Gd2O3 nanoparticles (NPs) before and after their APTES functionalisation. We obtained luminescent Dy@Gd2O3 NPs (0.5, 1, and 5% mol) using a modified polyol method. Our work describes their detailed structural analysis using FT-IR, XRD, HRTEM, TGA and XAS techniques. The results show that these systems present a crystalline structure with a body-centred cubic cell and particle sizes of 10 nm. The dopant position was inferred as substitutional, through XAS analysis at the M4,5-edges of Gd and Dy and K-edge of O, and in C2 sites, based on photoluminescence studies. There was sensitization of the luminescence by the matrix as shown by the emission increase of the hypersensitive transition (6F9/2 → 6H13/2, 572 nm) and also a broadband appears around 510 nm attributed to defects in Gd2O3. An enhanced emissive lifetime of 398 µs was found for the sample doped at 1%. We functionalised the Dy@Gd2O3 (at 1%) NPs with 3-aminopropiltrietoxisilane (APTES) for further application as a biomarker sensor. We found that these NPs conserved their luminescence after adding the surface agent (avoiding quenching effects) making them potential materials for biosensing.

Re-evaluating the role of phosphinic acid (DINHOP) adsorption at the photoanode surface in the performance of dye-sensitized solar cells.

Dineohexyl phosphinic acid (DINHOP) is a popular amphiphilic molecular insulator considered as the most efficient co-adsorbent (co-grafter) for the improvement of the photovoltaic performance of TiO2 based hybrid solar cells. Although the effect of its incorporation on the improvement of cell performance has been well demonstrated, the mechanisms through which it affects the photovoltaic and electrodynamic parameters of the cells are not yet clear. Here we re-examine the mechanism through which the DINHOP co-adsorbent affects the photovoltaic and electrodynamic parameters of dye-sensitized solar cells. Although DINHOP is widely believed to inhibit (passivate) recombination across the TiO2/electrolyte interface, we demonstrate that this is sublte, noticeable only for a very high concentration (e.g. 750 µM) of DINHOP, co-sensitized with a dye. For the most frequently used DINHOP concentrations (e.g. 75 µM and 375 µM), an observed increase of the diffusion coefficient and recombination rate could be directly associated with a decrease of total intra-gap states in TiO2. For a DINHOP concentration as low as 75 µM, the conduction band edge of TiO2 moves upward due to the combined effect of charge accumulation and a decrease in the total number of intra-gap states leading to an effective enhancement of the DCCS VOC, where the decrease in total intra-gap states does not contribute positively. The decrease of total intra-gap states enhances both the transport and recombination rates of charge carriers by the same fraction due to a transport-limited recombination process. On the other hand, adsorption of DINHOP molecules at higher concentrations such as 375 µM and 750 µM additionally modifies the distribution of intra-gap states, affecting the nonlinear recombination parameter of charge carriers at the anode-electrolyte interface, leading to an overall enhancement of the DSSC VOC. In all cases, incorporation of DINHOP results in an overall improvement of the solar cell efficiency (∼14% compared with the reference one), with a maximum for a concentration of 375 µM, where no inhibition of recombination was observed. Interestingly, for this DINHOP concentration, we estimate that 1 DINHOP molecule per every 12 molecules of dye occupies the intra-gap states of the TiO2 surface. The results presented in this work elucidate the physical phenomena involved in the interaction of co-adsorbents, pre-treatments or additives with the electrolyte at the surface of the TiO2 photoanode of dye-sensitized solar cells and can be easily adapted to study other electrochemical systems.

Toward an ideal polymer binder design for high-capacity battery anodes.

The dilemma of employing high-capacity battery materials and maintaining the electronic and mechanical integrity of electrodes demands novel designs of binder systems. Here, we developed a binder polymer with multifunctionality to maintain high electronic conductivity, mechanical adhesion, ductility, and electrolyte uptake. These critical properties are achieved by designing polymers with proper functional groups. Through synthesis, spectroscopy, and simulation, electronic conductivity is optimized by tailoring the key electronic state, which is not disturbed by further modifications of side chains. This fundamental allows separated optimization of the mechanical and swelling properties without detrimental effect on electronic property. Remaining electronically conductive, the enhanced polarity of the polymer greatly improves the adhesion, ductility, and more importantly, the electrolyte uptake to the levels of those available only in nonconductive binders before. We also demonstrate directly the performance of the developed conductive binder by achieving full-capacity cycling of silicon particles without using any conductive additive.

Compositional Evolution of Individual CoNPs on Co/TiO2 during CO and Syngas Treatment Resolved through Soft XAS/X-PEEM.

The nanoparticle (NP) redox state is an important parameter in the performance of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. Here, the compositional evolution of individual CoNPs (6-24 nm) in terms of the oxide vs metallic state was investigated in situ during CO/syngas treatment using spatially resolved X-ray absorption spectroscopy (XAS)/X-ray photoemission electron microscopy (X-PEEM). It was observed that in the presence of CO, smaller CoNPs (i.e., ≤12 nm in size) remained in the metallic state, whereas NPs ≥ 15 nm became partially oxidized, suggesting that the latter were more readily able to dissociate CO. In contrast, in the presence of syngas, the oxide content of NPs ≥ 15 nm reduced, while it increased in quantity in the smaller NPs; this reoxidation that occurs primarily at the surface proved to be temporary, reforming the reduced state during subsequent UHV annealing. O K-edge measurements revealed that a key parameter mitigating the redox behavior of the CoNPs were proximate oxygen vacancies (Ovac). These results demonstrate the differences in the reducibility and the reactivity of Co NP size on a Co/TiO2 catalyst and the effect Ovac have on these properties, therefore yielding a better understanding of the physicochemical properties of this popular choice of FTS catalysts.

Resolving the Effect of Oxygen Vacancies on Co Nanostructures Using Soft XAS/X-PEEM.

Improving both the extent of metallic Co nanoparticle (Co NP) formation and their stability is necessary to ensure good catalytic performance, particularly for Fischer-Tropsch synthesis (FTS). Here, we observe how the presence of surface oxygen vacancies (Ovac) on TiO2 can readily reduce individual Co3O4 NPs directly into CoO/Co0 in the freshly prepared sample by using a combination of X-ray photoemission electron microscopy (X-PEEM) coupled with soft X-ray absorption spectroscopy. The Ovac are particularly good at reducing the edge of the NPs as opposed to their center, leading to smaller particles being more reduced than larger ones. We then show how further reduction (and Ovac consumption) is achieved during heating in H2/syngas (H2 + CO) and reveal that Ovac also prevents total reoxidation of Co NPs in syngas, particularly the smallest (∼8 nm) particles, thus maintaining the presence of metallic Co, potentially improving catalyst performance.

Photocharging and Band Gap Narrowing Effects on the Performance of Plasmonic Photoelectrodes in Dye-Sensitized Solar Cells.

The incorporation of plasmonic nanostructures in active electrodes has become one of the most attractive ways to enhance the photoconversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). Although an enhancement of PCE because of the incorporation of plasmonic nanostructures of different sizes, either bare or coated, has been demonstrated, the fundamental mechanisms associated to such enhancement are still unclear. Besides, the photocurrent enhancement of plasmonic DSSCs is frequently associated to the strong surface plasmon resonance (SPR) absorption of metal nanoparticles. In this work, through oxygen K-edge soft X-ray absorption and emission spectroscopies of plasmonic electrodes and electrodynamical characterization of the fabricated cells, we demonstrate a band gap narrowing and photocharging effect on the plasmonic electrodes that definitely contribute to the PCE enhancement in plasmonic DSSCs. The incorporation of bare metal nanoparticles in active metal-oxide semiconductor electrodes such as TiO2 in optimum concentration causes an upward shift of its valence band edge, reducing its effective band gap energy and enhancing the short-circuit current of DSSCs. On the other hand, small perturbation-based stepped light-induced transient measurements of photovoltage and photocurrent of the operating DSSCs revealed an upward shift of quasi-Fermi level of photoelectrodes because of the photocharging effect induced by the incorporated metal nanoparticles. The upward shift of the quasi-Fermi level causes an increase in open-circuit voltage ( VOC), nullifying the effect of band gap reduction. The short-circuit photocurrent enhancement was controlled by the band gap narrowing, screening the SPR contribution. The results presented in this work not only clarify the contribution of SPR absorption in plasmonic DSSCs, but also highlight the importance of considering the corrections in the effective base voltage because of the quasi-Fermi level band shift during the estimation of the transport and recombination parameters of an assembled DSSC.

Reducing exciton binding energy by increasing thin film permittivity: an effective approach to enhance exciton separation efficiency in organic solar cells.

Photocurrent generation in organic solar cells requires that excitons, which are formed upon light absorption, dissociate into free carriers at the interface of electron acceptor and donor materials. The high exciton binding energy, arising from the low permittivity of organic semiconductor films, generally causes low exciton separation efficiency and subsequently low power conversion efficiency. We demonstrate here, for the first time, that the exciton binding energy in B,O-chelated azadipyrromethene (BO-ADPM) donor films is reduced by increasing the film permittivity by blending the BO-ADPM donor with a high dielectric constant small molecule, camphoric anhydride (CA). Various spectroscopic techniques, including impedance spectroscopy, photon absorption and emission spectroscopies, as well as X-ray spectroscopies, are applied to characterize the thin film electronic and photophysical properties. Planar heterojunction solar cells are fabricated with a BO-ADPM:CA film as the electron donor and C60 as the acceptor. With an increase in the dielectric constant of the donor film from ∼4.5 to ∼11, the exciton binding energy is reduced and the internal quantum efficiency of the photovoltaic cells improves across the entire spectrum, with an ∼30% improvement in the BO-ADPM photoactive region.