Tunable hydrodynamic chromatography of microparticles localized in short microchannels.

This paper describes a new way to perform hydrodynamic chromatography (HDC) for the size separation of particles based on a unique recirculating flow pattern. Pressure-driven (PF) and electro-osmotic flows (EOF) are opposed in narrow glass microchannels that expand at both ends. The resulting bidirectional flow turns into recirculating flow because of nonuniform microchannel dimensions. This hydrodynamic effect, combined with the electrokinetic migration of the particles themselves, results in a trapping phenomenon, which we have termed flow-induced electrokinetic trapping (FIET). In this paper, we exploit recirculating flow and FIET to perform a size-based separation of samples of microparticles trapped in a short separation channel using a HDC approach. Because these particles have the same charge (same zeta potential), they exhibit the same electrophoretic mobility, but they can be separated according to size in the recirculating flow. While trapped, particles have a net drift velocity toward the low-pressure end of the channel. When, because of a change in the externally applied PF or electric field, the sign of the net drift velocity reverses, particles can escape the separation channel in the direction of EOF. Larger particles exhibit a larger net drift velocity opposing EOF, so that the smaller particles escape the separation channel first. In the example presented here, a sample plug containing 2.33 and 2.82 microm polymer particles was introduced from the inlet into a 3-mm-long separation channel and trapped. Through tuning of the electric field with respect to the applied PF, the particles could be separated, with the advantage that larger particles remained trapped. The separation of particles with less than 500 nm differences in diameter was performed with an analytical resolution comparable to that of baseline separation in chromatography. When the sample was not trapped in the separation channel but located further downstream, separations could be carried out continuously rather than in batch. Smaller particles could successfully pass through the separation channel, and particles were separated by size. One of the main advantages of exploiting FIET for HDC is that this method can be applied in quite short (a few millimeters) channel geometries. This is in great contrast to examples published to date for the separation of nanoparticles in much longer micro- and nanochannels.

Dye-sensitized solar cells based on a single layer deposition of TiO 2 from a new formulation paste and their photovoltaic performance

A new strategy for enhancing the efficiency and reducing the production cost of TiO 2 solar cells by design of a new formulated TiO 2 paste with tailored crystal structure and morphology is reported. The conventional three- or four-fold layer deposition process was eliminated and replaced by a single layer deposition of TiO 2 compound. Different TiO 2 pastes with various crystal structures, morphologies and crystallite sizes were prepared by an aqueous particulate sol-gel process. Based on simultaneous differential thermal (SDT) analysis the minimum annealing temperature to obtain organic-free TiO 2 paste was determined at 400°C, being one of the lowest crystallization temperatures of TiO 2 photoanode electrodes for solar cell application. Photovoltaic measurements showed that TiO 2 solar cell with pure anatase crystal structure had higher power conversion efficiency (PCE) than that made of pure rutile-TiO 2. However, the PCE of solar cells depends on the anatase to rutile weight ratio, reaching a maximum at a specific value due to the synergic effect between anatase and rutile TiO 2 nanoparticles. Moreover, it was found that the PCE of solar cells made of crystalline TiO 2 powders was much higher, increasing in the range 32-84% depending on anatase to rutile weight ratio, than that of prepared by amorphous powders. TiO 2 solar cell with the morphology of mixtures of nanoparticles and microparticles had higher PCE than the solar cell with the same phase composition containing TiO 2 nanoparticles due to the role of TiO 2 microparticles as light scattering particles. The presented strategy would open up new insight into fabrication and structural design of low-cost TiO 2 solar cells with high power conversion efficiency. © 2012 Elsevier Ltd.