Solution processed nanostructured hybrid materials based on PbS quantum dots and reduced graphene oxide with tunable optoelectronic properties.

Nanostructured hybrid materials (NHMs) are promising candidates to improve the performance of several materials in different applications. In the case of optoelectronic technologies, the ability to tune the optical absorption of such NHMs is an appealing feature. Along with the capacity to transform the absorbed light into charge carriers (CC), and their consequently efficient transport to the different electrodes. In this regard, NHM based on graphene-like structures and semiconductor QDs are appealing candidates, assuming the NHMs retain the light absorption and CC photogeneration properties of semiconductor QDs, and the excellent CC transport properties displayed by graphene-like materials. In the current work a solution-processed NHM using PbS quantum dots (QDs) and graphene oxide (GO) was fabricated in a layer-by-layer configuration by dip-coating. Afterwards, these NHMs were reduced by thermal or chemical methods. Reduction process had a direct impact on the final optoelectronic properties displayed by the NHMs. All reduced samples displayed a decrement in their resistivity, particularly the sample chemically reduced, displaying a 107 fold decrease; mainly attributed to N-doping in the reduced graphene oxide (rGO). The optical absorption coefficients also showed a dependence on the rGO's reduction degree, with reduced samples displaying higher values, and sample thermally reduced at 300 °C showing the highest absorption coefficient, due to the combined absorption of unaltered PbS QDs and the appearance of sp2 regions within rGO. The photogenerated current increased in most reduced samples, displaying the highest photocurrent the sample reduced at 400 °C, presenting a 2500-fold increment compared to the NHM before reduction, attributed to an enhanced CC transfer from PbS QDs to rGO, as a consequence of an improved band alignment between them. These results show clear evidence on how the optoelectronic properties of NHMs based on semiconductor nanoparticles and rGO, can be tuned based on their configuration and the reduction process parameters.

Tuning the optoelectronic properties of PEDOT:PSS-PVP core-shell electrospun nanofibers by solvent-quantum dot doping and phase inversion.

In the present study core-shell PEDOT:PSS-polyvinylpirrolidone nanofibers were synthesized by coaxial electrospinning. These fibers were doped with different solvents (dimethylsulphoxide, dimethyl sulfoxide (DMSO), isopropyl alcohol (IPA), and ethylene glycol), and PbS nanoparticles at different concentrations; additionally, the coaxial electrospinning setup process was inverted in order to exchange the phases comprising the core-shell morphology. Experimental results showed that DMSO and IPA solvents produced a change in the PEDOT:PSS phase from its benzoid structure to a more conjugated (quinoid) one. The synthesized samples displayed an increment in the conductance of the composite nanofibers, based on a more conjugated structure of the PEDOT:PSS phase, and a better dispersion of the PbS nanoparticles within the nanofibers; this increment was, under certain synthesis conditions, up to three orders of magnitude higher than in the case of the nanofibers with no solvent, nor nanoparticles, added. Photoresponse also showed a clear increment in the value of the photogenerated current as the concentration of the nanoparticles increased. Inverting the arrangement of the core-shell phases in the nanofibers increased the conductance and the photogenerated current in the cases analyzed. These results show novel evidence on the capability of tuning the conductance and photoresponse of composite core-shell nanofibers, based on the doping of the PEDOT:PSS phase with different solvents and PbS nanoparticles, and the arrangement of the core-shell phases. Tailoring the optoelectronic properties of conductive, flexible nanofibers is a desirable competence in technological areas such as transparent flexible conductors, biosensors and tissue engineering.

Simultaneous intercalated assembly of mesostructured hybrid carbon nanofiber/reduced graphene oxide and its use in electrochemical sensing.

Polyacrylonitrile nonwovens intercalated with graphene oxide (GO) sheets were prepared by a simultaneous electrospinning-spray deposition system. These hybrid nonwovens were carbonized in a two-stage process to obtain a mesostructured hybrid carbon containing carbon nanofibers (CNF) and reduced GO sheets (CNF/RGO). During the carbonization process, the CNF act as spacers between the RGO layers to prevent their compactation and restacking resulting in a three-dimensional structure. The presence of RGO increases the electrical conductivity in the CNF/RGO material. The resulting hybrid carbon is nitrogen-doped as indicated by x-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy. This N-doped porous carbon was used to prepare electrodes with improved sensitivity for the electrochemical detection of L-cysteine.

Eco-friendly isolation of cellulose nanoplatelets through oxidation under mild conditions.

Agave is recognized as a low recalcitrant material, which makes it a potential source to obtain nanocellulose. Aqueous dispersions (in water, H2O2, H2O2/H2SO4) of agave powder were heated at 120°C under vapor pressure (1kg/cm2). The resultant materials were observed with an optical microscope (OM), a laser scanning microscope (LSM) to obtain the thickness measurement and a scanning electron microscope (SEM) to observe morphology. Raman spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to obtain the chemical structure. Cellulose nanoplatelets (CNPs) from Agave salmiana were successfully isolated under mild conditions. Physicochemical analysis indicates that lignin was removed in a single step oxidation with hydrogen peroxide in presence of sulfuric acid at low concentration (0.17M). The CNPs images revealed the presence of entangled cellulose nanofibrils (Ø≈14nm) along the nanoplatelets (thickness ≈80nm).

Biocatalytic synthesis of polypyrrole powder, colloids, and films using horseradish peroxidase.

Polypyrrole was synthesized in high yield by a biocatalytic method in mild aqueous media using hydrogen peroxide as oxidizer. A redox mediator, 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) diammonium salt, was used to oxidize the pyrrole. ABTS is a very effective peroxidase substrate, which was enzymatically oxidized to generate a radical cation that in turn was able to chemically oxidize pyrrole. This indirect biocatalytic method was implemented because pyrrole is not a substrate of horseradish peroxidase, however, the polymerization process was successfully optimized and later adapted to prepare also polypyrrole thin films and water dispersible polypyrrole colloids. The polypyrrole powder and colloids were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, electrical conductivity, and thermogravimetric analysis. In addition, the deposition of the polypyrrole thin film was monitored using a quartz-crystal microbalance and its morphology studied by optical and scanning electron microscopy. The biocatalytic polymerization of pyrrole results in a polymer spectroscopically very similar to chemically synthesized polypyrrole.

Three-layer core/shell structure in Au-Pd bimetallic nanoparticles.

This paper describes the internal structure of Au-Pd nanoparticles exhibiting newly discovered three-layer core/shell morphology, which is composed of an evenly alloyed inner core, an Au-rich intermediate layer, and a Pd-rich outer shell. By exploitation of spatially resolved imaging and spectroscopic and diffraction modes of transmission electron microscopy (TEM), insights were gained on the composition of each one of the observed three layers, indicating a significant extent of intimate alloy among the monometallic elements.