Solid Solutions of Rare Earth Cations in Mesoporous Anatase Beads and Their Performances in Dye-Sensitized Solar Cells.

Solid solutions of the rare earth (RE) cations Pr(3+), Nd(3+), Sm(3+), Gd(3+), Er(3+) and Yb(3+) in anatase TiO2 have been synthesized as mesoporous beads in the concentration range 0.1-0.3% of metal atoms. The solid solutions were have been characterized by XRD, SEM, diffuse reflectance UV-Vis spectroscopy, BET and BJH surface analysis. All the solid solutions possess high specific surface areas, up to more than 100 m(2)/g. The amount of adsorbed dye in each photoanode has been determined spectrophotometrically. All the samples were tested as photoanodes in dye-sensitized solar cells (DSSCs) using N719 as dye and a nonvolatile, benzonitrile based electrolyte. All the cells were have been tested by conversion efficiency (J-V), quantum efficiency (IPCE), electrochemical impedance spectroscopy (EIS) and dark current measurements. While lighter RE cations (Pr(3+), Nd(3+)) limit the performance of DSSCs compared to pure anatase mesoporous beads, cations from Sm(3+) onwards enhance the performance of the devices. A maximum conversion efficiency of 8.7% for Er(3+) at a concentration of 0.2% has been achieved. This is a remarkable efficiency value for a DSSC employing N719 dye without co-adsorbents and a nonvolatile electrolyte. For each RE cation the maximum performances are obtained for a concentration of 0.2% metal atoms.

High temperature stability of onion-like carbon vs highly oriented pyrolytic graphite.

The thermodynamic stability of onion-like carbon (OLC) nanostructures with respect to highly oriented pyrolytic graphite (HOPG) was determined in the interval 765-1030 K by the electromotive force (emf) measurements of solid electrolyte galvanic cell: (Low) Pt|Cr3C2,CrF2,OLC|CaF2s.c.|Cr3C2,CrF2,HOPG|Pt (High). The free energy change of transformation HOPG = OLC was found positive below 920.6 K crossing the zero value at this temperature. Its trend with temperature was well described by a 3rd degree polynomial. The unexpected too high values of [Formula: see text] jointly to the HR-TEM, STEM and EELS evidences that showed OLC completely embedded in rigid cages made of a Cr3C2/CrF2 matrix, suggested that carbon in the electrodes experienced different internal pressures. This was confirmed by the evaluation under constant volume of [dP/dT by the α/κ ratio for OLC (0.5 MPa K(-1)) and HOPG (8 Pa K(-1)) where α and κ are the isobaric thermal expansion and isothermal compressibility coefficients, respectively. The temperature dependency of the pressure was derived and utilized to calculate the enthalpy and entropy changes as function of temperature and pressure. The highest value of the internal pressure experienced by OLC was calculated to be about 7 GPa at the highest temperature. At 920.6 K, ΔrH and ΔrS values are 95.8 kJ mol(-1) and 104.1 JK(-1) mol(-1), respectively. The surface contributions to the energetic of the system were evaluated and they were found negligible compared with the bulk terms. As a consequence of the high internal pressure, the values of the enthalpy and entropy changes were mainly attributed to the formation of carbon defects in OLC considered as multishell fullerenes. The change of the carbon defect fraction is reported as a function of temperature.

Synthesis and characterization of multiwalled carbon nanotube/FeCo nanocomposites.

Multiwalled carbon nanotube/FeCo nanocomposites were produced by Catalytic Chemical Vapour Deposition using highly porous FeCo-SiO2 aerogels with different loadings and dimensions of FeCo nanoparticles as catalysts. Multiwalled carbon nanotubes with average number of walls depending on the size of the catalyst nanoparticles were obtained. Inside the nanotubes spherical or elliptical FeCo nanoparticles are retained, and the magnetic properties of the resulting nanocomposites were characterized in detail.

Experimental thermodynamics of high temperature transformations in single-walled carbon nanotube bundles.

The thermodynamic quantities associated to the transformation of carbon in single-walled carbon nanotube (SWCNT) bundles to carbon in graphite were determined from 750 to 1015 K by a CaF2 solid electrolyte galvanic cell: (-) Mo | Cr3C2,CrF2,C'' | CaF2s.c. | Cr3C2,CrF2,C' | Mo (+). The trend with temperature of the electromotive force of the cell was found to be greatly dependent on temperature and fully reversible with it. The standard enthalpy DeltaH(T)(o) and entropy DeltaS(T)(o) changes are 7.1, 6.0, and 60.2 kJ mol(-1) and 8.6, 14.7, and 72.8 J K(-1) mol(-1) at 778, 883, and 975 K, respectively. This most likely correlates with the different arrangements and shapes of tubes that deviate from the ideal triangular closed packed structure of the SWCNT bundles. The constraints to the thermal expansion of bundles in the electrode containing them generated high internal pressures that were responsible for deformations of tube shape and lattice. Stable bundle states were formed that interconvert as a function of temperature. Comparative analyses by low angle XRD, microRaman, and HR-TEM of SWCNT bundles before and after experiments support this scenario. The cohesion energy and associated entropy changes are also reported for such states. The formation enthalpy of unbundled SWCNTs was calculated equal to 9.5 +/- 0.4 kJ mol(-1).

The thermodynamics of the transformation of graphite to multiwalled carbon nanotubes.

The thermodynamic quantities associated to the transformation from graphite to multiwalled carbon nanotubes (MWCNTs) were determined by electromotive force (emf) and differential scanning calorimetry (DSC) measurements. From the emf versus T data of galvanic cell Mo|Cr(3)C(2), CrF2, MWCNTs|CaF2 s.c.|Cr(3)C(2), CrF2, graphite|Mo with CaF2 as solid electrolyte, Delta(r)H(T) degrees= 8.25 +/- 0.09 kJ mol(-1) and Delta(r)S(T) degrees= 11.72 +/- 0.09 JK(-1) mol(-1) were found at average temperature T = 874 K. The transformation enthalpy was also measured by DSC of the Mn(7)C(3) formation starting from graphite or MWCNTs. Thermodynamic values at 298 K were calculated to be: Delta(r)H(298) degrees = 9.0 +/- 0.8 kJ mol(-1) as averaged value from both techniques and Delta(r)S(298) degrees approximately Delta(r)S(T) degrees. At absolute zero, the residual entropy of MWCNTs was estimated 11.63 +/- 0.09 JK(-1) mol(-1), and transformation enthalpy Delta(r)H(0) degrees approximately Delta(r)H(298) degrees. The latter agrees satisfactorily with the theoretical calculations for the graphite-MWCNTs transformation. On thermodynamic basis, the transformation becomes spontaneous above 704 +/- 13 K.