Dye-sensitized solar cells (DSSCs) made from oriented, one-dimensional semiconductor nanostructures such as nanorods, nanowires, and nanotubes are receiving attention because direct connection of the point of photogeneration with the collection electrode using such structures may improve the cell performance. Specifically, oriented single-crystalline TiO2 nanorods or nanowires on a transparent conductive substrate would be most desirable, but achieving these structures has been limited by the availability of synthetic techniques. In this study, a facile, hydrothermal method was developed for the first time to grow oriented, single-crystalline rutile TiO2 nanorod films on transparent conductive fluorine-doped tin oxide (FTO) substrates. The diameter, length, and density of the nanorods could be varied by changing the growth parameters, such as growth time, growth temperature, initial reactant concentration, acidity, and additives. The epitaxial relation between the FTO substrate and rutile TiO2 with a smal...

Growth of oriented single-crystalline rutile TiO(2) nanorods on transparent conducting substrates for dye-sensitized solar cells.

https://chemistry.beloit.edu/classes/nanotech/ZnO/mattoday10_05_40.pdf

ZnO - nanostructures, defects, and devices

We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

Optical properties of ZnO nanostructures.

Flexible nanogenerators that efficiently convert mechanical energy into electrical energy have been extensively studied because of their great potential for driving low-power personal electronics and self-powered sensors. Integration of flexibility and stretchability to nanogenerator has important research significance that enables applications in flexible/stretchable electronics, organic optoelectronics, and wearable electronics. Progress in nanogenerators for mechanical energy harvesting is reviewed, mainly including two key technologies: flexible piezoelectric nanogenerators (PENGs) and flexible triboelectric nanogenerators (TENGs). By means of material classification, various approaches of PENGs based on ZnO nanowires, lead zirconate titanate (PZT), poly(vinylidene fluoride) (PVDF), 2D materials, and composite materials are introduced. For flexible TENG, its structural designs and factors determining its output performance are discussed, as well as its integration, fabrication and applications. The latest representative achievements regarding the hybrid nanogenerator are also summarized. Finally, some perspectives and challenges in this field are discussed.

Flexible Nanogenerators for Energy Harvesting and Self-Powered Electronics.

Semiconductor nanowires represent an important class of nanostructure building block for photovoltaics as well as direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. In this talk, I will highlight several recent examples in this lab using semiconductor nanowires and their heterostructures for the purpose of solar energy harvesting. In addition, we have also discovered that the thermoconductivity of the silicon nanowires can be significantly reduced due to phonon scattering, pointing to a very promising approach to design better thermoelectrical materials. It is important to note that the engines that generate most of the world's power typically operate at only 30–40 per cent efficiency, releasing roughly 15 terawatts of heat to the environment. If this “wasted heat” could be recycled, the impact globally would be enormous. Our silicon nanowire thermoelectric technology could have a significant impact in alternative energy generation.

Semiconductor nanowires for energy conversion.

A method for growing vertical ZnO nanowire arrays on arbitrary substrates using either gas-phase or solution-phase approaches is presented. A ∼10 nm-thick layer of textured ZnO nanocrystals with their c axes normal to the substrate is formed by the decomposition of zinc acetate at 200−350 °C to provide nucleation sites for vertical nanowire growth. The nanorod arrays made in solution have a rod diameter, length, density, and orientation desirable for use in ordered nanorod−polymer solar cells.

/pdf/general-route-to-vertical-zno-nanowire-arrays-using-textured-37vpv215yi.pdf

General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds

Both melt viscosity (� o) and elasticity (correlated here with the longest melt relaxation time � 1) were found to control the diameter distribution of meltblown fibers. Fibers were formed by melt blowing binary polystyrene (PS) blends containing widely differing component molecular weights using a custom-built laboratory apparatus. Varying the concentration and molecular weight of a high molecular weight PS provided independent control overo and � 1. These rheological parameters influence the average diam- eter (dav) and the distribution of diameters (coefficient of variation, CV) of meltblown fibers in different ways. Increasingo leads to an increase in dav but has little impact on CV. On the other hand, increas- ing � 1 beyond a threshold value reduces CV while simultaneously increasing dav. A one-dimensional slender-jet theoretical model with both upper convected Maxwell and Phan-Thien and Tanner constitu- tive equations was developed to investigate the influence of viscoelasticity and processing parameters on the properties of meltblown fibers. This model predicts a strong dependence of fiber diameter on the air shear stress and variations in fiber diameter with viscoelasticity that are in qualitative agreement with the experimental results. We believe these results suggest that carefully controlling the viscoelastic profile of polymers used in melt blowing is a viable approach for producing nanofibers with narrow fiber diameter distributions using current commercial equipment. Published by Elsevier B.V.

Meltblown fibers: Influence of viscosity and elasticity on diameter distribution

Nanofibers were generated by melt blowing three sets of polymer blends, each comprised of pairs of immiscible components. Blends containing minority phases (25% by volume) of poly(ethylene-co-chlorotrifluoroethylene) (PECTFE) in poly(butylene terephthalate) (PBT), PECTFE in poly(styrene) (PS), and PBT in PS were dispersed as droplets in a continuous majority phase and melt blown into long (>100 μm) fibers with average diameters of several micrometers. Electron microscopy experiments revealed that melt blowing transformed the initial spherical dispersions into a nanofiber-in-fiber morphology. Macroscopic mats of nonwoven PBT and PECTFE nanofibers, with average diameters as small as 70 nm, were isolated by selectively removing the majority phase with a solvent. This method provides a potentially inexpensive, high throughput, one-step route to scalable quantities of polymeric nanofibers.

/pdf/nanofibers-from-melt-blown-fiber-in-fiber-polymer-blends-4sp8ftxy1v.pdf

Dawud H. Tan

Papers

General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds

Meltblown fibers: Influence of viscosity and elasticity on diameter distribution

Nanofibers from Melt Blown Fiber-in-Fiber Polymer Blends

Influence of Laval nozzles on the air flow field in melt blowing apparatus

Modeling the melt blowing of viscoelastic materials