Dye-sensitized solar cells based on a nanoparticle/nanotube bilayer structure and their equivalent circuit analysis

Dye-sensitized solar cells (DSSCs) were prepared by capitalizing on a TiO(2) bilayer structure composed of P-25 nanoparticles and freestanding crystalline nanotube arrays as photoanodes. After being subjected to sequential TiCl(4) treatment and O(2) plasma exposure, the bilayer photoanode was sensitized with N719 dye. DSSCs based on a 20 μm TiO(2) nanoparticle film solely and a bilayer of 13 μm TiO(2) nanoparticles and 7 μm TiO(2) nanotubes exhibited the highest power conversion efficiency, PCE, of 8.02% and 7.00%, respectively, compared to the devices made of different TiO(2) thicknesses. On the basis of J-V parameter analysis acquired by equivalent circuit model simulation, in comparison to P-25 nanoparticles, charge transport in nanotubes was found to be facilitated due to the presence of advantageous nanotubular structures, while photocurrent was reduced owing to their small surface area, which in turn resulted in low dye loading, as well as the lack of cooperative effect of anatase and rutile phases.     

Solid-state dye-sensitized solar cells based on ZnO nanoparticle and nanorod array hybrid photoanodes

The effect of ZnO photoanode morphology on the performance of solid-state dye-sensitized solar cells (DSSCs) is reported. Four different structures of dye-loaded ZnO layers have been fabricated in conjunction with poly(3-hexylthiophene). A significant improvement in device efficiency with ZnO nanorod arrays as photoanodes has been achieved by filling the interstitial voids of the nanorod arrays with ZnO nanoparticles. The overall power conversion efficiency increases from 0.13% for a nanorod-only device to 0.34% for a device with combined nanoparticles and nanorod arrays. The higher device efficiency in solid-state DSSCs with hybrid nanorod/nanoparticle photoanodes is originated from both large surface area provided by nanoparticles for dye adsorption and efficient charge transport provided by the nanorod arrays to reduce the recombinations of photogenerated carriers.     

Metal Oxides for Dye-Sensitized Solar Cells

The incessant demand for energy forces us to seek it from sustainable resources; and concerns on environment demands that resources should be clean as well. Metal oxide semiconductors, which are stable and environment friendly materials, are used in photovoltaics either as photoelectrode in dye solar cells (DSCs) or to build metal oxide p – n junctions. Progress made in utilization of metal oxides for photoelectrode in DSC is reviewed in this article. Basic operational principle and factors that control the photoconversion efficiency of DSC are briefly outlined. The d -block binary metal oxides viz. TiO₂, ZnO, and Nb₂O₅ are the best candidates as photoelectrode due to the dissimilarity in orbitals constituting their conduction band and valence band. This dissimilarity decreases the probability of charge recombination and enhances the carrier lifetime in these materials. Ternary metal oxide such as Zn₂SnO₄ could also be a promising material for photovoltaic application. Various morphologies such as nanoparticles, nanowires, nanotubes, and nanofibers have been explored to enhance the energy conversion efficiency of DSCs. The TiO₂ served as a model system to study the properties and factors that control the photoconversion efficiency of DSCs; therefore, such discussion is limited to TiO₂ in this article. The electron transport occurs through nanocrystalline TiO₂ through trapping and detrapping events; however, exact nature of these trap states are not thoroughly quantified. Research efforts are required not only to quantify the trap states in mesoporous metal oxides but new mesoporous architectures also to increase the conversion efficiency of metal oxide-based photovoltaics.     

Application of polymeric nanofibers in medical designs,part III: Musculoskeletal and urological tissues

Nanotechnology has potential applications in different sciences, especially in the biological sciences and medicine. The nanomaterials are applicable materials with different morphologies such as nanoparticles, nanotubes, nanowires, nanorods, and nanofibers. The development of nanofibers has greatly enhanced the scope for fabricating designs that can be potentially used in medical sciences. In part III the author summarizes the currently available applications of nanofibers in musculoskeletal and urologic tissues. The graphical abstract shows computed tomography analysis and macroscopic images of calvarial defects in rat, with the regeneration result, after four weeks, of bone healing upon implantation of scaffolds in the defect. (A and D) Control group; (B and E) poly (3-hydroxybutyrate-co-3-hydroxyvalerate)/nanohydroxyapatite (PHBV/nHAp) scaffold; and (C and F) PHBV/nHAp scaffold with unrestricted somatic stem cells precultured in vitro.     

Enhanced performance in dye-sensitized solar cells via carbon nanofibers-platinum composite counter electrodes

A composite counter electrode (CE) made of electrospun carbon nanofibers (ECNs) and platinum (Pt) nanoparticles has been demonstrated for the first time to improve the performance of dye-sensitized solar cells (DSCs). The new ECN-Pt composite CE exhibited a more efficient electro-catalytic performance with lower charge transfer resistance (R(ct)), larger surface area, and faster reaction rate than those of conventional Pt. It reduced the overall series resistance (R(se)), decreased dark saturation current density (J(0)) and increased shunt resistance (R(sh)) of the DSCs, thereby leading to a higher fill factor (FF) and larger open circuit voltage (V(oc)). The reduced electron transport resistance (R(s)) and faster charge transfer rate in the CE led to a smaller overall cell series resistance (R(se)) in the ECN-Pt composite based DSCs. The DSCs based on an ECN-Pt CE achieved a η of ～8%, which was improved over those of pure Pt or ECN based cells.     

Nanostructure control of graphene-composited TiO2 by a one-step solvothermal approach for high performance dye-sensitized solar cells

We present a one-step solvothermal approach to prepare uniform graphene-TiO(2) nanocomposites with delicately controlled TiO(2) nanostructures, including ultra-small 2 nm nanoparticles, 12 nm nanoparticles and nanorods. Using three composites as photoanode materials, the effect of nanostructure of graphene-composited TiO(2) on the performance of dye-sensitized solar cells was investigated, and results showed that the ultra-small 2 nm TiO(2)-graphene composite based photoanode exhibited the highest power conversion efficiency of 7.25%.     

Nanoscale semiconductor-insulator-metal core/shell heterostructures: facile synthesis and light emission

Controllably constructing hierarchical nanostructures with distinct components and designed architectures is an important theme of research in nanoscience, entailing novel but reliable approaches of bottom-up synthesis. Here, we report a facile method to reproducibly create semiconductor-insulator-metal core/shell nanostructures, which involves first coating uniform MgO shells onto metal oxide nanostructures in solution and then decorating them with Au nanoparticles. The semiconductor nanowire core can be almost any material and, herein, ZnO, SnO(2) and In(2)O(3) are used as examples. We also show that linear chains of short ZnO nanorods embedded in MgO nanotubes and porous MgO nanotubes can be obtained by taking advantage of the reduced thermal stability of the ZnO core. Furthermore, after MgO shell-coating and the appropriate annealing treatment, the intensity of the ZnO near-band-edge UV emission becomes much stronger, showing a 25-fold enhancement. The intensity ratio of the UV/visible emission can be increased further by decorating the surface of the ZnO/MgO nanowires with high-density plasmonic Au nanoparticles. These heterostructured semiconductor-insulator-metal nanowires with tailored morphologies and enhanced functionalities have great potential for use as nanoscale building blocks in photonic and electronic applications.     

One-dimensional organic-inorganic hybrid nanomaterials

This feature article presents the current research activities that concentrate on one-dimensional (1D) organic-inorganic hybrid nanostructures such as nanowires, nanorods, and nanotubes. The combination of organic and inorganic components in a 1D manner has been an increasingly expanding research field because the synergistic behavior of organic-inorganic materials is bound directly to the charming characteristics of 1D nanomaterials. These are responsible for the many novel optical and electrical properties, hierarchical superstructures, functions, and versatile applications that have been achieved. In this article, after justifying the interest in developing 1D organic-inorganic hybrid nanomaterials, we classify 1D hybrid nanostructures and review construction strategies that have been adopted, with a special focus on template-directed synthesis. In summary, we provide our personal perspectives on the future emphasis of the research on 1D organic-inorganic hybrid nanostructures.     

Nanoenergetic Composites of CuO Nanorods,Nanowires, and Al‐Nanoparticles

This paper reports on the synthesis of the nanoenergetic composites containing CuO nanorods and nanowires, and Al‐nanoparticles. Nanorods and nanowires were synthesized using poly(ethylene glycol) templating method and combined with Al‐nanoparticles using ultrasonic mixing and self‐assembly methods. Poly(4‐vinylpyridine) was used for the self‐assembly of Al‐nanoparticles around the nanorods. At the optimized values of equivalence ratio, sonication time, and Al‐particle size, the combustion wave speed of 1650 m s⁻¹ was obtained for the nanorods‐based energetics. For the composite of nanowires and Al‐nanoparticles the speed was increased to 1900 m s⁻¹. The maximum combustion wave speed of 2400 m s⁻¹ was achieved for the self‐assembled composite, which is the highest known so far among the nanoenergetic materials. It is possible that in the self‐assembled composites, the interfacial contact between the oxidizer and fuel is higher and resistance to overall diffusional process is lower, thus enhancing the performance.     

Enhancing Solar Cell Efficiencies through 1-D Nanostructures

The current global energy problem can be attributed to insufficient fossil fuel supplies and excessive greenhouse gas emissions resulting from increasing fossil fuel consumption. The huge demand for clean energy potentially can be met by solar-to-electricity conversions. The large-scale use of solar energy is not occurring due to the high cost and inadequate efficiencies of existing solar cells. Nanostructured materials have offered new opportunities to design more efficient solar cells, particularly one-dimensional (1-D) nanomaterials for enhancing solar cell efficiencies. These 1-D nanostructures, including nanotubes, nanowires, and nanorods, offer significant opportunities to improve efficiencies of solar cells by facilitating photon absorption, electron transport, and electron collection; however, tremendous challenges must be conquered before the large-scale commercialization of such cells. This review specifically focuses on the use of 1-D nanostructures for enhancing solar cell efficiencies. Other nanostructured solar cells or solar cells based on bulk materials are not covered in this review. Major topics addressed include dye-sensitized solar cells, quantum-dot-sensitized solar cells, and p-n junction solar cells.     

Carbonized electrospun polyvinylpyrrolidone/metal hybrid nanofiber composites for electrochemical applications

Electrospinning is a very versatile and efficient method of fabricating nanofibers with the desired properties. Polyvinylpyrrolidone (PVP) in ethanol solution was electrospun into nanofibers and used as a precursor for the preparation of carbon nanofibers. Cobalt chloride was also incorporated with PVP nanofibers to produce carbon nanofiber composites with enhanced electrical conductivity and electrochemical properties. The surface morphology and physical properties of the electrospun nanofibers, carbonized nanofibers, and their composites were observed by scanning electron microscopy, transmission electron microscopy, and X‐ray diffraction. The electrochemical behavior of the carbon nanofiber composites was studied by drop‐casting on a working surface of the screen‐printed carbon electrode and examined by cyclic voltammetry and electrochemical impedance spectroscopy. The results indicated that carbon nanofiber composites were decorated with cobalt nanoparticles and enhanced the charge‐transfer efficiency on the electrode surface. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45639.     

Inorganic nanostructures grown on graphene layers

This article presents a review of current research activities on the hybrid heterostructures of inorganic nanostructures grown directly on graphene layers, which can be categorized primarily as zero-dimensional nanoparticles; one-dimensional nanorods, nanowires, and nanotubes; and two-dimensional nanowalls. For the hybrid structures, the nanostructures exhibit excellent material characteristics including high carrier mobility and radiative recombination rate as well as long-term stability while graphene films show good optical transparency, mechanical flexibility, and electrical conductivity. Accordingly, the versatile and fascinating properties of the nanostructures grown on graphene layers make it possible to fabricate high-performance optoelectronic and electronic devices even in transferable, flexible, or stretchable forms. Here, we review preparation methods and possible device applications of the hybrid structures consisting of various types of inorganic nanostructures grown on graphene layers.     

N-Ion-implanted TiO2 photoanodes in quantum dot-sensitized solar cells

Hierarchical nanostructured titanium dioxide (TiO(2)) clumps were fabricated using electrostatic spray with subsequent nitrogen-ion doping by an ion-implantation technique for improvement of energy conversion efficiency for quantum dot-sensitized solar cells (QDSCs). CdSe quantum dots were directly assembled on the produced N-ion-implanted TiO(2) photoanodes by chemical bath deposition, and their photovoltaic performance was evaluated in a polysulfide electrolyte with a Pt counter electrode. We found that the photovoltaic performance of TiO(2) electrodes was improved by nearly 145% upon N-ion implantation. The efficiency improvement seems to be due to (1) the enhancement of electron transport through the TiO(2) layer by inter-particle necking of primary TiO(2) particles and (2) an increase in the recombination resistance at TiO(2)/QD/electrolyte interfaces by healing the surface states or managing the oxygen vacancies upon N-ion doping. Therefore, N-ion-doped photoanodes offer a viable pathway to develop more efficient QD or dye-sensitized solar cells.     

Carbon nanotubes filled with metallic nanowires

A facile method is proposed to use LaNi₂ hydrogen storage alloy as a catalyst precursor to produce metallic nickel filled carbon nanotubes. Multi-walled carbon nanotubes filled with long continuous nickel nanowire with several microns in length are synthesized through chemical vapor deposition at low temperature (550 °C). It is more efficient to fill Ni nanowires into nanotubes after the oxidation treatment of LaNi₂ alloy at low temperatures, while the oxidation treatment at high temperature results in the forming of herringbone carbon nanofibers with tips of Ni nanoparticles. The metallic Ni nanowires inside the cores of carbon nanotubes could not be eliminated during the purification process in concentrated hydrochloric acid. The analysis of transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray diffraction (XRD) reveals that the metallic nickel nanowires filled inside carbon nanotubes exist as a single crystalline with fcc structure.     

Solid-state thermal dewetted silver nanoparticles onto electrochemically grown self-standing vertically aligned ZnO nanorods for three-dimensional plasmonic nanostructures

Three dimensional (3D) plasmonic nanostructures composed of silver nanoparticles decorated ZnO NRs arrays, have been fabricated by a process combining the electrochemical growth of ZnO NRs and further formation of Ag nanoparticles by the solid-state thermal dewetting (SSD) process. The effect of SSD parameters on the morphological, structural and optical properties of the Ag NPs decorated ZnO NRs arrays has been investigated. It is possible to tune the bandgap of the Ag NPs@ZnO nanorods array 3D plasmonic nanostructure by tailoring the Ag nanoparticle sizes, allowing light manipulation at the nanoscale. The silver nanoparticles attached to the ZnO NRs arrays experienced surface plasmonic coupling effect, causing enhancement in the room temperature photoluminescence (PL) UV emission and quenching the corresponding visible light one. An enhancement in the near band edge emission PL intensity of ZnO to the deep level emission PL intensity ratio after Ag NPs decoration of the ZnO nanostructures corresponding to ca. 11 folds has been observed, indicating that the defect emission is obviously suppressed.     

A novel hybrid of carbon nanotubes/iron nanoparticles: iron-filled nodule-containing carbon nanotubes

A straightforward, one-step method for the preparation of novel carbon nanotube/iron nanoparticle hybrids with some degree of shape control is reported herein. These carbon nanostructures differ from those reported previously: the nanoparticles were not attached to or coated onto the surface of carbon nanotubes but embedded inside the carbon wall. They were synthesized in good yield by thermolysis of ferrocene and thiophene mixtures in a closed steel vessel. The shapes and compositions of these nanostructures can be simply controlled by adjusting the reaction temperature and relative amounts of the precursors. Iron-filled T-junction carbon nanotubes were also obtained easily by this procedure. These iron-filled nodule-containing carbon nanotubes (INCNTs) are either empty or filled with iron or iron carbide (Fe(C)) nanowires. The outer diameters of these nanotubes range from 70 to 150 nm and the lengths reach up to several micrometers. The average size of the Fe(C) nanoparticles (or empty cores) inside the nodules is about 50 nm in diameter. The carbon in the INCNTs is amorphous. Sulfur was found being responsible for the disordered structure and playing a unique role in promoting the growth of INCNTs as well as the formation of T- or Y-junctions.     

Morphology-dependent redox and catalytic properties of CeO2 nanostructures: Nanowires, nanorods and nanoparticles

The redox features and the catalytic activities of ceria nanowires, nanorods and nanoparticles were comparatively studied. The morphology-dependent phenomenon is closely related to the nature of the exposed crystal planes. The CeO₂ nanoparticles mainly expose the stable {1 1 1} plane on the surface, whereas the rod-shaped nanostructures preferentially expose the reactive {1 1 0} and {1 0 0} planes, giving higher oxygen storage capacity and catalytic activity for CO oxidation. Although both the CeO₂ nanorods and the CeO₂ nanowires predominantly expose the reactive {1 1 0} and {1 0 0} planes, the CeO₂ nanowires favor to expose a large proportion of active planes on the surface, resulting in a much higher activity for CO oxidation than the nanorods.     

Current–Voltage Characteristics in Individual Polypyrrole Nanotube, Poly(3,4-ethylenedioxythiophene) Nanowire, Polyaniline Nanotube, and CdS Nanorope

In this paper, we focus on current–voltage (I–V) characteristics in several kinds of quasi-one-dimensional (quasi-1D) nanofibers to investigate their electronic transport properties covering a wide temperature range from 300 down to 2 K. Since the complex structures composed of ordered conductive regions in series with disordered barriers in conducting polymer nanotubes/wires and CdS nanowires, all measured nonlinear I–V characteristics show temperature and field-dependent features and are well fitted to the extended fluctuation-induced tunneling and thermal excitation model (Kaiser expression). However, we find that there are surprisingly similar deviations emerged between the I–V data and fitting curves at the low bias voltages and low temperatures, which can be possibly ascribed to the electron–electron interaction in such quasi-1D systems with inhomogeneous nanostructures.     

Novel hollow mesoporous 1D TiO2 nanofibers as photovoltaic and photocatalytic materials

Hollow mesoporous one dimensional (1D) TiO(2) nanofibers are successfully prepared by co-axial electrospinning of a titanium tetraisopropoxide (TTIP) solution with two immiscible polymers; polyethylene oxide (PEO) and polyvinylpyrrolidone (PVP) using a core-shell spinneret, followed by annealing at 450 °C. The annealed mesoporous TiO(2) nanofibers are found to having a hollow structure with an average diameter of 130 nm. Measurements using the Brunauer-Emmett-Teller (BET) method reveal that hollow mesoporous TiO(2) nanofibers possess a high surface area of 118 m(2) g(-1) with two types of mesopores; 3.2 nm and 5.4 nm that resulted from gaseous removal of PEO and PVP respectively during annealing. With hollow mesoporous TiO(2) nanofibers as the photoelectrode in dye sensitized solar cells (DSSC), the solar-to-current conversion efficiency (η) and short circuit current (J(sc)) are measured as 5.6% and 10.38 mA cm(-2) respectively, which are higher than those of DSSC made using regular TiO(2) nanofibers under identical conditions (η = 4.2%, J(sc) = 8.99 mA cm(-2)). The improvement in the conversion efficiency is mainly attributed to the higher surface area and mesoporous TiO(2) nanostructure. It facilitates the adsorption of more dye molecules and also promotes the incident photon to electron conversion. Hollow mesoporous TiO(2) nanofibers with close packing of grains and crystals intergrown with each other demonstrate faster electron diffusion, and longer electron recombination time than regular TiO(2) nanofibers as well as P25 nanoparticles. The surface effect of hollow mesoporous TiO(2) nanofibers as a photocatalyst for the degradation of rhodamine dye was also investigated. The kinetic study shows that the hollow mesoporous surface of the TiO(2) nanofibers influenced its interactions with the dye, and resulted in an increased catalytic activity over P25 TiO(2) nanocatalysts.     

Oxide nanowires for solar cell applications

Oxide nanowire arrays were studied for their applications to solar cells. It was demonstrated that the nanowires could provide direct pathways for electron transport in dye-sensitized solar cells and therefore, while forming photoelectrode films, they offered better suppression of charge recombination than nanoparticles. However, the photoelectron films consisting of nanowires suffered a disadvantage in giving large surface area for dye adsorption. Such a shortcoming of nanowires had been exemplified in this paper illustrating that it could be well compensated by incorporating with nanoparticles to form a nanoparticle-nanowire array hybrid photoelectrode film. The oxide nanowires were also demonstrated to be able to enhance the performance of inverted structure polymer solar cells as a cathode buffer layer by establishing a large interface with the polymers so as to facilitate the transport of photogenerated electrons from the polymer to the electron collecting electrode. Such an enhancement effect could be further boosted while the nanowires were replaced with nanotubes; the latter may build up larger interface with the polymers than the former and therefore facilitates the electron transport more efficiently.