Yunier Garcia-Basabe

Effects of the large distribution of CdS quantum dot sizes on the charge transfer interactions into TiO2 nanotubes for photocatalytic hydrogen generation

Hydrogen fuels generated by water splitting using a photocatalyst and solar irradiation are currently gaining the strength to diversify the world energy matrix in a green way. CdS quantum dots have revealed a hydrogen generation improvement when added to TiO2 materials under visible-light irradiation. In the present paper, we investigated the performance of TiO2 nanotubes coupled with CdS quantum dots, by a molecular bifunctional linker, on photocatalytic hydrogen generation. TiO2 nanotubes were obtained by anodization of Ti foil, followed by annealing to crystallize the nanotubes into the anatase phase. Afterwards, the samples were sensitized with CdS quantum dots via an in situ hydrothermal route using 3-mercaptopropionic acid as the capping agent. This sensitization technique permits high loading and uniform distribution of CdS quantum dots onto TiO2 nanotubes. The XPS depth profile showed that CdS concentration remains almost unchanged (homogeneous), while the concentration relative to the sulfate anion decreases by more than 80% with respect to the initial value after ∼100 nm in depth. The presence of sulfate anions is due to the oxidation of sulfide and occurs in greater proportion in the material surface. This protection for air oxidation inside the nanotubular matrix seemingly protected the CdS for photocorrosion in sacrificial solution leading to good stability properties proved by long duration, stable photocurrent measurements. The effect of the size and the distribution of sizes of CdS quantum dots attached to TiO2 nanotubes on the photocatalytic hydrogen generation were investigated. The experimental results showed three different behaviors when the reaction time of CdS synthesis was increased in the sensitized samples, i.e. similar, deactivation and activation effects on the hydrogen production with regard to TiO2 nanotubes. The deactivation effect was related to two populations of sizes of CdS, where the population with a shorter band gap acts as a trap for the electrons photogenerated by the population with a larger band gap. Electron transfer from CdS quantum dots to TiO2 semiconductor nanotubes was proven by the results of UPS measurements combined with optical band gap measurements. This property facilitates an improvement of the visible-light hydrogen evolution rate from zero, for TiO2 nanotubes, to approximately 0.3 μmol cm(-2) h(-1) for TiO2 nanotubes sensitized with CdS quantum dots.

Electronic structure and ultrafast charge transfer dynamics of phosphorous doped graphene layers on a copper substrate: a combined spectroscopic study

Graphene sheet layers were grown by chemical vapor deposition (CVD) under a polycrystalline copper substrate using methane (CH4) and triphenylphosphine (P(C6H5)3) as carbon and phosphorous precursors, respectively. The films obtained from the CH4 and P(C6H5)3 chemical precursors were labeled as G/Cu and GP/Cu, respectively. Electronic structure investigation was performed on these two graphene samples combining different spectroscopic techniques. Raman spectroscopy shows the presence of single and multilayers in G/Cu and GP/Cu, respectively. A blue shift of 30 cm−1 of the 2D band in the GP/Cu film with respect to G/Cu is evidence of the p-type doping of GP/Cu. X-ray photoelectron and reflection electron energy loss spectroscopy (REELS) confirm the bilayer formation in the GP/Cu film. REELS also shows that the presence of phosphorous does not open the electronic band gap of the graphene film. The work function of 4.3 eV for G/Cu and 4.8 eV for GP/Cu films were determined by ultraviolet photoelectron spectroscopy. The increase of the work function is attributed to the electron transfer to the Cu substrate. The partially unoccupied densities of states in phosphorous doped graphene (GP/Cu) were evaluated by X-ray photoabsorption spectroscopy. The core-hole clock approach using resonant Auger spectroscopy was employed for investigating the charge transfer dynamics around the P K-edge in GP/Cu. Ultrafast charge transfer delocalization on a time scale of femtoseconds was observed, demonstrating a strong electronic coupling between unoccupied states of the phosphorous and the conduction band of the copper substrate. The combined spectroscopic results suggest p-type doping in GP/Cu by the electron transfer mechanism.

Thermally induced anchoring of fullerene in copolymers with Si-bridging atom: Spectroscopic evidences

We use X-ray photoelectron spectroscopy (XPS), Near-edge X-ray absorption fine structure (NEXAFS), resonant Auger spectroscopy (RAS), Attenuation Total Reflection Infrared (ATR-IR) and Atomic Force Microscopy (AFM) to study the blend between the copolymer poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) and the fullerene derivative PC71BM submitted to different annealing temperatures. Those measurements indicate that there is an incidental anchoring of a fullerene derivative to the Si-bridging atoms of a copolymer induced by thermal annealing of the film. Insights about the physical properties of one possible PSiF-DBT/PC71BM anchored structure are obtained using Density Functional Theory calculations. Since the performance of organic photovoltaic based on polymer-fullerene blends depends on the chemical structure of the blend components, the anchoring effect might affect the photovoltaic properties of those devices.

Ultrafast interface charge transfer dynamics on P3HT/MWCNT nanocomposites probed by resonant Auger spectroscopy

The interfacial electronic structure and charge transfer dynamics of poly-3-hexylthiophene (P3HT) and multi-walled carbon nanotube (Fe-MWCNT) nanocomposites were investigated by near-edge X-ray absorption fine structure (NEXAFS) and resonant Auger (RAS) spectroscopies around the sulfur K-edge. Nanocomposites with 5 wt% (P3HT/Fe-MWCNT-5%) and 10 wt% (P3HT/Fe-MWCNT-10%) of Fe-MWCNT species were prepared and compared with pristine P3HT film. The quantitative NEXAFS analysis shows a strong π–π interchain interaction of the pristine P3HT polymer film, which is reduced by the presence of the Fe-MWCNT. S–KL2,3L2,3 RAS spectra were measured at photon energies corresponding to the main electronic transitions appearing in the S–K edge NEXAFS spectrum. Ultrafast charge transfer times were estimated from the RAS spectra using the core-hole clock approach with the S 1s core-hole lifetime as an internal clock. The π–π interchain charge transfer time increases from 4.7 fs on pristine P3HT polymer to 6.5 fs on the P3HT/Fe-MWCNT-5% nanocomposite. The electronic coupling between P3HT and Fe-MWCNT species occurs mainly through the P3HT π* molecular orbital. The increase of Fe-MWCNT concentration from 5 to 10 wt% reduces the charge transfer rate at the resonance maximum due probably to Fe-MWCNT aggregation, reducing the P3HT and Fe-MWCNT electronic coupling.

Femtosecond Electron Delocalization in Polymer:Fullerene Blend Films

Ultrafast electron dynamics in the low-femtosecond regime was evaluated for a low band-gap polymer and its blend with fullerene by resonant Auger spectroscopy. Remarkable changes developed by tuning the photon energy along the sulfur and silicon 1s absorption edges. The effect of thermal annealing and molecular orientation on the charge transfer delocalization times were probed and correlated with solar cells performance.