Low Dye Loading Research Articles

Fast cascade neutralization of an oxidized sensitizer by an in situ-generated ionic layer of I− species on a nanocrystalline TiO2 electrode

A novel coadsorbent including three terminal double bonds reacted with I2 and generated ionic complexes on the surface of TiO2. The in situ-generated ionic layer accelerated the neutralization rate of dye+ and decreased the recombination reactions with dye+ and I3− due to coadsorbent effects, resulting in 22% higher power conversion efficiency, even at 15% low dye loading, compared to the conventional device.

Nile-Red–Nanoclay Hybrids: Red Emissive Optical Probes for Use in Aqueous Dispersion

Water-dispersible and (bio)functionalizable nanoclays have a considerable potential as inexpensive carriers for organic molecules like drugs and fluorophores. Aiming at simple design strategies for red-emissive optical probes for the life sciences from commercial precursors with minimum synthetic effort, we systematically studied the dye loading behavior and stability of differently functionalized laponites. Here, we present a comprehensive study of the absorption and emission properties of the red emissive hydrophobic and neutral dye Nile Red, a well-known polarity probe, which is almost insoluble and nonemissive in water. Adsorption of this probe onto disk-shaped nanoclays was studied in aqueous dispersion as function of dye concentration, in the absence and presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB) assisting dye loading, and as a function of pH. This laponite loading strategy yields strongly fluorescent nanoclay suspensions with a fluorescence quantum yield of 0.34 at low dye loading concentration. The dye concentration-, CTAB-, and pH-dependent absorption, fluorescence emission, and fluorescence excitation spectra of the Nile-Red-nanoclay suspensions suggest the formation of several Nile Red species including emissive Nile Red monomers facing a polar environment, nonemissive H-type dimers, and protonated Nile Red molecules that are also nonfluorescent. Formation of all nonemissive Nile Red species could be suppressed by modification of the laponite with CTAB. This underlines the great potential of properly modified and functionalized laponite nanodisks as platform for optical probes with drug delivery capacities, for example, for tumor and therapy imaging. Moreover, comparison of the Nile Red dimer absorption spectra with absorption spectra of previously studied Nile Red aggregates in dendrimer systems and micelles and other literature systems reveals a considerable dependence of the dimer absorption band on microenvironment polarity which has not yet been reported so far for H-type dye aggregates.

Hydrothermal Synthesis of a Crystalline Rutile TiO2 Nanorod Based Network for Efficient Dye‐Sensitized Solar Cells

One-dimensional (1D) TiO2 nanostructures are desirable as photoanodes in dye-sensitized solar cells (DSSCs) due to their superior electron-transport capability. However, making use of the DSSC performance of 1D rutile TiO2 photoanodes remains challenging, mainly due to the small surface area and consequently low dye loading. Herein, a new type of photoanode with a three-dimensional (3D) rutile-nanorod-based network structure directly grown on fluorine-doped tin oxide (FTO) substrates was developed by using a facile two-step hydrothermal process. The resultant photoanode possesses oriented rutile nanorod arrays for fast electron transport as the bottom layer and radially packed rutile head-caps with an improved large surface area for efficient dye adsorption. The diffuse reflectance spectra showed that with the radially packed top layer, the light-harvesting efficiency was increased due to an enhanced light-scattering effect. A combination of electrochemical impedance spectroscopy (EIS), dark current, and open-circuit voltage decay (OCVD) analyses confirmed that the electron-recombiantion rate was reduced on formation of the nanorod-based 3D network for fast electron transport. As a resut, a light-to-electricity conversion efficiency of 6.31% was achieved with this photoanode in DSSCs, which is comparable to the best DSSC efficiencies that have been reported to date for 1D rutile TiO2 .

ZnO–TiO2 Nanocomposite Films for High Light Harvesting Efficiency and Fast Electron Transport in Dye-Sensitized Solar Cells

Electron transport and recombination are the essential processes that determine the charge collection efficiency in dye-sensitized solar cells (DSSC). While nearly 100% of charges are collected in well-built ordinary DSSCs, this value can be sharply reduced by the use of redox couples other than iodide/triiodide due to fast electron recombination. To compensate, structures capable of fast electron transport are needed. Nanorod arrays that have this attribute tend to suffer from low surface area, resulting in low dye loading and reduced light harvesting. We have therefore developed a novel nanocomposite structure consisting of zinc oxide (ZnO) nanorods coated with titanium dioxide (TiO2) nanoparticles using an electrostatic layer-by-layer (LbL) deposition technique. The titanium dioxide nanoparticle coating can add an order of magnitude of surface area and is compatible with known high-performance dyes. This composite nanostructure has been designed to take advantage of the improved electron transport along the nanorods and surface area provided by the nanoparticles, yielding good charge collection and light harvesting. Transient measurements indicate that the composite film can transport electrons at least 100 times faster than a nanoparticulate TiO2 film. In tests using ferrocene/ferrocenium as a model alternative redox couple with fast recombination, current–voltage measurements indicate that the ZnO–TiO2 hybrid films generate much higher currents than conventional TiO2 nanoparticulate films. However, not all charges successfully transfer from TiO2 to ZnO due to an energy barrier between the materials.

TiO2 flowers and spheres for ionic liquid electrolytes based dye-sensitized solar cells

Micro-sized 3D branched flower like and sphere TiO2 have been synthesized through a controlled hydrolysis followed by hydrothermal process. The resultant flower and sphere TiO2 were characterized by SEM, XRD, BET and light scattering measurements. Overwhelming anatase and strong light scattering were exhibited for the flower sample. Pure sphere and flower and sphere-mixed (flower/sphere) films were formed and applied as photoelectrodes for dye-sensitized solar cells (DSCs) with both ionic liquid and gelled ionic liquid electrolytes. The devices exhibited considerably enhanced overall energy conversion efficiency in comparison with that of those derived from TiO2 nanoparticles. The superior performances were attributed to their fast electrolyte diffusion and good interfacial contact of TiO2/electrolyte as validated by EIS investigations as well as light scattering. The participation of flowers with the sphere films further improved the electrolyte penetration and light scattering, making the flower/sphere device the most efficient with a low dye loading.

Transformation of Organic–Inorganic Hybrid Films Obtained by Molecular Layer Deposition to Photocatalytic Layers with Enhanced Activity

We present the transformation of organic-inorganic hybrid titanicone films formed by TiCl(4) as metal precursor and ethylene glycol (EG) using solvent-free MLD to highly active photocatalytic films. The photocatalytic activities of the films were investigated using hydroxyl-functionalized porphyrin as a spectroscopic marker. TEM imaging and electron diffraction, XPS, UV-vis spectroscopy, and spectroscsopic ellipsometry were employed for structural and composition analyses of the films. The photocatalytic activity of Ti-EG films was investigated for different anneal temperatures and compared to TiO(2) films prepared by ALD using TiCl(4) as metal precursor and H(2)O (TiO(2) films). Overall, our results indicate that the photocatalytic activity of the thermally annealed Ti-EG film is about 5-fold increased compared to that of the TiO(2) film prepared by ALD for optimal process conditions. The combined results indicate that the structural and photocatalytic properties can be assigned to three states: (I) amorphous state, intermediate dye loading, low photocatalytic activity, (II) intermediate film state with both crystalline and amorphous regions, high dye loading, high catalytic activity, and (III) highly crystalline film with low dye loading and low photocatalytic activity. The formation of photocatalytic nanotubes (NTs) is demonstrated using sacrificial Ge nanowires (NWs) scaffolds to yield Ti-EG NT structures with controllable wall thickness structures and enhanced dye loading capacity. Our results demonstrate the feasibility and high potential of MLD to form metal oxides with high photocatalytic activity.

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.

Direct observation of single flexible polymers using single stranded DNA().

Over the last 15 years, double stranded DNA (dsDNA) has been used as a model polymeric system for nearly all single polymer dynamics studies. However, dsDNA is a semiflexible polymer with markedly different molecular properties compared to flexible chains, including synthetic organic polymers. In this work, we report a new system for single polymer studies of flexible chains based on single stranded DNA (ssDNA). We developed a method to synthesize ssDNA for fluorescence microscopy based on rolling circle replication, which generates long strands (>65 kb) of ssDNA containing "designer" sequences, thereby preventing intramolecular base pair interactions. Polymers are synthesized to contain amine-modified bases randomly distributed along the backbone, which enables uniform labelling of polymer chains with a fluorescent dye to facilitate fluorescence microscopy and imaging. Using this approach, we synthesized ssDNA chains with long contour lengths (>30 μm) and relatively low dye loading ratios (~1 dye per 100 bases). In addition, we used epifluorescence microscopy to image single ssDNA polymer molecules stretching in flow in a microfluidic device. Overall, we anticipate that ssDNA will serve as a useful model system to probe the dynamics of polymeric materials at the molecular level.

Ordered Mesoporous SnO2−Based Photoanodes for High-Performance Dye-Sensitized Solar Cells

Ordered mesoporous SnO2 with three-dimensional bicontinuous cubic mesostructure, high surface area, and crystalline frameworks was synthesized by the solvent-free infiltration of tin precursor in KIT-6 silica template and employed as a photoanode in dye-sensitized solar cells (DSSCs). It is shown that coating an ultrathin TiO2 or Al2O3 layer on mesoporous SnO2 photoanode greatly improves the open-circuit voltage, short-circuit current, and fill factor, leading to more than a 3-fold improvement in the energy conversion efficiency. The superior photovoltaic performance of surface modified mesoporous SnO2 photoanode is mainly the result of inhibited electron recombination caused by passivation of reactive surface states and increased dye loading. Control cells fabricated with conventional nanoparticle SnO2/TiO2 photoanode exhibit a similar trend but with about 30% lower energy conversion efficiency, which is mainly because of limitations due to low dye loading and poor light scattering.

Effect of dye coverage on photo-induced electron injection efficiency in N719-sensitized nanocrystalline TiO 2 films

The effect of dye coverage on the photo-induced electron injection efficiency in N719-sensitized nanocrystalline TiO 2 films was studied by transient absorption spectroscopy. We evaluated the electron injection efficiency of films with several dye concentrations on the surface and found no significant difference between them. We also studied the suppression of the efficiency by the addition of tBP (4- tert-butylpyridine) and found that the effect of tBP was more pronounced for films with lower dye loading. This suggests that sensitizer dyes adsorbed densely onto the surface suppress the adsorption of tBP.

On the Arrangements of R6G Molecules in Organophilic C12TMA/Lap Clay Films for Low Dye Loadings

Absorption and fluorescence spectroscopies with linearly polarized light are applied to characterize the adsorbed species of rhodamine 6G (R6G) laser dye in ordered organophilic laponite (Lap) clay films for low dye loadings. The organophilic character of the clay was controlled by the number of organic surfactant dodecyl-trimethylammonium (C12TMA) cations intercalated into the interlayer space of the clay. Experimental results suggest that for moderate to high surfactant contents (>70% of the total cation exchange capacity, CEC, of the clay) the accessibility of the interlayer space for R6G molecules is reduced. In these cases, the first stage in the adsorption of R6G molecules is at the external surface, where dye molecules can self-associate even for very low dye loading (around 0.1%CEC), probably for the limitation of the external surface area. The presence of a nonfluorescent H-type aggregate with a short-displaced coplanar structure and a fluorescent oblique head-to-tail J aggregate is reported. The inclined structure of this J aggregate is stabilized by the surfactant molecules at the external surface.

Dye-sensitization of self-assembled titania nanotubes prepared by galvanostatic anodizationof Ti sputtered on conductive glass

Self-organized porous TiO2 nanotubes (NTs) were prepared on conductive glass by galvanostatic anodizing of sputtered titaniumin an NH4F /glycerol electrolyte. DC magnetron sputtering at an elevated substrate temperature (500 °C) was used to deposit 650 nm thick titanium films. After anodizing, NTs, 830 nm long, withan average external diameter of 92 nm, were grown; this gave a high conversion rate ofoxide from titanium (1.9), with a 220 nm thick layer of titanium, which was notoxidized, located at the base of the tubes. The NTs revealed a mainly amorphousstructure, which transformed mostly to anatase upon thermal treatment in air at450 °C.The tubes were sensitized by the N719 complex and the resultant photoelectrodes were incorporatedinto liquid dye solar cells (DSCs) and further tested under back-side illumination. High values ofVoc (714 mV) were obtained under 1 sun (AM 1.5), assigned to low dark current magnitude andlarge recombination resistance and electron lifetime. In addition, typical values of fillfactors (of the order of 0.62) were attained, in agreement with the estimated ohmicresistance of the cells in combination with low electron transfer resistance at theplatinum/electrolyte interface. The overall moderate power conversion efficiency(of the order of 0.3%) was mainly due to the low short-circuit photocurrents (Jsc = 0.68 mA cm−2),which was confirmed further by the corresponding IPCE values (5.2% at 510 nm). The magnitudeof Jsc was attributed to absorbed light losses due to back-side illumination of the cells, the low dye loading(due to the limited thickness of anodic titania) and the high charge transfer resistance at theTiO2 /conductive substrate due to the presence of barrier layer(s) underneath the tubes.These preliminary results encourage the DSC community to explore further thegalvanostatic anodizing of titanium in order to produce highly efficient porousTiO2 NTs directly on conductive glass. Current work is focusing on achieving complete anodizingof the metal substrate and full transparency for the photoelectrode in order to increase andoptimize the resultant cell efficiencies.

Improving the fluorescence polarization method to evaluate the orientation of fluorescent systems adsorbed in ordered layered materials

The fluorescence polarization method, recently developed for the evaluation of the preferential orientation of fluorescent dyes adsorbed in layered materials [F. López Arbeloa, V. Martínez Martínez, J. Photochem. Photobiol. A: Chem. 181 (2006) 44], is readapted to improve its application. Fluorescence polarization was previously obtained by recording the emission intensity for two orthogonal orientations of the emission polarizer (i.e., the horizontal and vertical polarized light) after excitation with vertical or horizontal polarized light. In the method proposed in this work, samples are excited with unpolarized light, reducing the polarization effect of the excitation light scattering at those emission wavelengths close to the excitation wavelength. Moreover, the present method decreases the effect of the orientation of other non-fluorescent species present in the system, which are active in the excitation process. Consequently, the new method is more simple, precise and sensitive. It is applied to evaluate the orientation of rhodamine 6G dye adsorbed in ordered laponite clay films with low and moderated dye loadings.

Photoactive Hybrid Nanomaterials: Indocyanine Immobilized in Mesoporous MCM-41 for “In-Cell” Bioimaging

Mesoporous silica nanoparticles are being explored as versatile tools for various biomedical and biotechnological applications including disease diagnosis, drug delivery, and intracellular imaging. In this paper, the synthesis and characterization of a fluorescent hybrid mesoporous silica nanomaterial, which is noncytotoxic and shows great potential for "in-cell" bioimaging applications, will be described. The hybrid mesoporous material has been obtained by confining highly fluorescent organic dyes, belonging to the indocyanine family, within the channels of mesoporous MCM-41. To explore the dispersion of the dye inside the mesoporous channels and the formation of dye aggregates, several hybrid samples with increasing dye/MCM-41 loading (up to 100 mg/g) were prepared. A uniform distribution of monomeric 1,1'-diethyl-3,3,3',3'-tetramethylindocarbocyanine iodide has been achieved at low dye loading (1 mg/g), as evidenced by photoluminescence spectra and lifetime, while a progressive formation of J-aggregates is induced by an increase in the dye loading. To elucidate the properties of the dye immobilized in mesoporous MCM-41, a detailed physical chemical characterization by structural (X-ray diffraction), volumetric and optical (Fourier transform infrared, diffuse-reflectance UV-vis and photoluminescence) techniques has been performed. By ultrasonication of the bulk material, nanoparticles of 2-20 nm diameter were obtained. Biocompatibility, endocytic uptake, and intracellular compartmentalization of such fluorescent nanoparticles were investigated in mammalian cultured cells.

Deposition potential dependence of ZnO–eosin Y hybrid thin films prepared by electrochemical deposition and their photoelectrochemical properties

Well crystallined ZnO–eosin Y hybrid thin films were deposited on ITO substrate by two-step cathodic electrodeposition from aqueous mixture of zinc nitrate and esoin Y sodium salt. Effects of the deposition potential on the crystal structure, morphology, and optical properties of the hybrid films were investigated. Results showed that the deposition potential had significant influences on the preferred orientation and the dye loading efficiency of the hybrid film. The nanoporous film with high dye concentration (0.79 mol L −1) was obtained at −0.8 V, while compact films with entrapping eosin Y molecules of relatively lower dye loading (0.28–0.10 mol L −1) were deposited at more negative potentials. The prototype of dye sensitized solar cells utilizing these hybrid films were fabricated, and their photoelectrochemical properties were characterized. It revealed that the hybrid film with porous nature had better photoelectrochemical property, exhibiting the conversion efficiency of 0.21%, open circuit photovoltage of 0.64 V, and short-circuit photocurrent density of 0.67 mA cm −2 under AM 1.5 illumination.

Spectral properties of tetraanionic porphyrin in formamide colloids of layered double hydroxides

Abstract Formamide colloidal suspensions of layered double hydroxides (LDHs) with anionic dye, meso-tetrakis(4-sulfonatophenyl)porphine (TPPS) were prepared. Optical properties of these suspensions were investigated by means of absorption and fluorescence spectroscopies in the visible region. For more detailed characterization, second derivative curves of the spectra were calculated. The adsorption of the dye on LDH nanosheets led to partial molecular aggregation, reflected in significant changes of the dye optical properties. The Soret band in the absorption spectra was split into two peaks and decreased in intensity. Changes were observed also for the Q-bands. The molecular aggregation significantly quenched the emission of the dye. The low intensity emission from dye H-aggregates was identified at slightly longer wavelengths. The molecular aggregation increased with the dye/LDH ratio. At low dye loadings, the optical properties were very similar to the dye solution. The yields of the dye molecular aggregation increased with LDH surface charge.

Nonlinear dielectric response of poled amorphous polymer dipole glasses

Temperature-dependent spectra of the linear, second- and third-order nonlinear dielectric permittivities are reported for an amorphous polymethylmethacrylate/Disperse Red 1 guest–host polymer and a poly(styrene maleic anhydride)-Disperse Red 1 side-chain polymer glass. Both polymer systems contain Disperse Red 1 chromophores, a very strong molecular dipole. In guest–host polymers with low dye loading, the dipole density and dipole moment of the chromophores can be determined from the linear and third-order nonlinear dielectric relaxation strength, associated with the micro-Brownian motion of the chromophore dipoles. The second-order nonlinear dielectric permittivity is non-vanishing in the glassy state only in poled polymers. Contributions to the second-order dielectric permittivity arise from piezoelectricity and from the elasto-optical and electronic electro-optical Pockels effect. In poled polymer dipole glasses with nonlinear optically active chromophores, the electronic electro-optical response is the dominant source for the second-order nonlinear dielectric permittivity. Therefore, electrical current versus voltage measurements enable a measurement of the electro-optical Pockels effect in poled polymer chromophore dipole glasses.

Photoprocesses of Thiacarbocyanine Monomers, Dimers, and Aggregates Bound to Polyanions

The photophysical and photochemical properties of thiacarbocyanine 1 and the methyl 2 and ethyl 3 meso derivatives in aqueous solution were studied in the presence of poly(styrenesulfonate) (PSS), poly(acrylic acid) (PAA), or poly(methacrylic acid) (PMA). When the ratio of polyanion residue to dye concentrations (r) is increased, a monomer/dimer equilibrium in neat water is shifted toward bound H-aggregates (high dye loading conditions), dimers, and monomers (low dye loading). The photodeactivation properties of 1−3 depend significantly on these ground-state equilibria. The fluorescence quantum yield (Φf) is strongly enhanced for low dye loading and reduced when bound aggregates and dimers are present, r = 3−30 and (0.2−2) × 103, respectively. For 1 the quantum yield of trans → cis photoisomerization (Φt-c) is also reduced by aggregation. The quantum yield of intersystem crossing (Φisc) is enhanced for bound dimers, the value is largest for 3. In the redox reactions via the triplet state of 3, ascorbic acid and p-benzoquinone serve as electron donor and acceptor, respectively. The photoinduced electron transfer is less efficient for bound monomers than for bound dimers. For the three polyanions comparable effects were found; especially, the ground-state equilibria and the Φf, Φisc, and Φt-c values depend in a well-defined manner on the parameter r.

Photophysics of aggregated 9-methylthiacarbocyanine bound to polyanions

The photophysical properties of 3,3 ′-diethyl-9-methylthiacarbocyanine (DTC) were studied in the presence of polystyrene sulfonate (PSS), polyacrylic acid (PAA) and polymethacrylic acid (PMA). The absorption spectra reflect a monomer/dimer equilibrium in neat aqueous solution and a shift towards bound H-aggregates, bound dimers and bound monomers on increasing the ratio of polyanion residue to dye concentrations ( r). These equilibria also determine the photodeactivation modes of DTC. The fluorescence intensity is reduced, when dimers and aggregates are present and strongly enhanced for low dye loading ( r=10 4). In contrast, the quantum yield of intersystem crossing is enhanced for bound dimers ( r=10 3).

Adsorption Study of Cationic Dyes Having a Trimethylammonium Anchor Group on Hectorite Using Electrooptic and Spectroscopic Methods

Clay minerals are natural or synthetic material of colloidal dimensions. Due to the sheetlike structure clay minerals offer a huge specific surface area and hence optimal properties for modification through adsorption. The current work studies the adsorption of five cationic dyes on the synthetic clay mineral hectorite. All dyes have a trimethylammonium anchoring group in common. The adsorbed dye molecules are characterized by means of pulsed electric linear dichroism and UV-VIS spectroscopy. With increasing dye loading a continuous shift in the absorption spectra is observed. But there is no occurrence of a new absorption band. Therefore we conclude that the dyes preferentially adsorb as amorphous aggregates on the clay surface. At low dye loadings the dye molecules lie flat on the clay mineral surface. Increasing dye concentration leads to a continuous increase in average tilt angle. However the orientation of the dye molecules is very sensitive to functional groups. The introduction of a nitro group to a particular dye increases significantly the tendency to lie flat on the surface whereas the introduction of a methoxy group at the same position has the opposite effect.