Fabrication of PbS nanoparticle-sensitized TiO₂ nanotube arrays and their photoelectrochemical properties

TiO(2) nanotube arrays (NTAs) are modified with PbS nanoparticles by successive ionic layer adsorption and reaction (SILAR) or electrodeposition, with an aim towards tuning the photoelectrochemical cell to the visible region. The PbS modification of the TiO(2) NTAs results in an increase in the visible light adsorption, however the increase in photocurrent is dependent on the modification method. PbS/TiO(2) NTAs prepared by SILAR and electrodeposition show, respectively, photocurrents of 11.02 and 5.72 mA/cm(2). The increase in photocurrent is attributed to enhanced charge separation efficiency and improved electron transport.     

Photochemical reduction of CO₂ using TiO₂: effects of organic adsorbates on TiO₂ and deposition of Pd onto TiO₂

Reduction of CO(2) using semiconductors as photocatalysts has recently attracted a great deal of attention again. The effects of organic adsorbates on semiconductors on the photocatalytic products are noteworthy. On untreated TiO(2) (P-25) particles a considerable number of organic molecules such as acetic acid were adsorbed. Although irradiation of an aqueous suspension of this TiO(2) resulted in the formation of a significant amount of CH(4) as a major product, it was strongly suggested that its formation mainly proceeded via the photo-Kolbe reaction of acetic acid. Using TiO(2) treated by calcination and washing procedures for removal of the organic adsorbates, CO was photocatalytically generated as a major product, along with a very small amount of CH(4), from an aqueous suspension under a CO(2) atmosphere. In contrast, by using Pd (>0.5 wt %) deposited on TiO(2) (Pd-TiO(2)) on which organic adsorbates were not detected, CH(4) was the main product, but CO formation was drastically reduced compared with that on the pretreated TiO(2). Experimental data, including isotope labeling, indicated that CO(2) and CO(3)(2-) are the main carbon sources of the CH(4) formation, which proceeds on the Pd site of Pd-TiO(2). Prolonged irradiation caused deactivation of the photocatalysis of Pd-TiO(2) because of the partial oxidation of the deposited Pd to PdO.     

Environment-friendly biomimetic synthesis of TiO₂ nanomaterials for photocatalytic application

We have demonstrated an environment-friendly biomimetic synthesis method for the preparation of TiO(2) nanomaterials with different crystal phases and morphologies. This is the first time that it has been found that the crystal phase of TiO(2) can be controlled just by using different biotemplates, and cannot be changed by calcination up to 750?°C. In our experiment, anatase TiO(2) was obtained by using yeast and albumen templates, while rutile TiO(2) was formed by using dandelion pollen as the template.     

TiO₂ photoanode structure with gradations in V concentration for dye-sensitized solar cells

V-TiO?(GC) photoanode film with graduated structure was prepared in a dye-sensitized solar cell work electrode by layer-by-layer method using TiO? precursor with gradations in V concentration on the indium tin oxide transparent conducting glass from substrate to surface. The effects of the gradient in V concentration on the structures and properties of TiO? photoanode film were discussed. The structure of the gradient V concentration has remarkable influence on the final performance of the DSSCs and I-V characteristic measurement indicates an enhanced efficiency by 31% as compared to pure TiO? nanoparticles film samples due to abundant solar light, fast injection and transmission velocity and the slowdown recombination of photoexcited electrons, simultaneously.     

Sodium fluoride-assisted modulation of anodized TiO₂ nanotube for dye-sensitized solar cells application

This work reports the use of sodium fluoride (in ethylene glycol electrolyte) as the replacement of hydrofluoric acid and ammonium fluoride to fabricate long and perpendicularly well-aligned TiO? nanotube (TNT) (up to 21 μm) using anodization. Anodizing duration, applied voltage and electrolyte composition influenced the geometry and surface morphologies of TNT. The growth mechanism of TNT is interpreted by analyzing the current transient profile and the total charge density generated during anodization. The system with low water content (2 wt %) yielded a membrane-like mesoporous TiO? film, whereas high anodizing voltage (70 V) resulted in the unstable film of TNT arrays. An optimized condition using 5 wt % water content and 60 V of anodizing voltage gave a stable array of nanotube with controllable length and pore diameter. Upon photoexcitation, TNTs synthesized under this condition exhibited a slower charge recombination rate as nanotube length increased. When made into cis-diisothiocyanato-bis(2,2?-bipyridyl-4,4?-dicarboxylato) ruthenium(II) bis (tetrabutyl-ammonium)(N719) dye-sensitized solar cells, good device efficiency at 3.33 % based on the optimized TNT arrays was achieved with longer electron time compared with most mesoporous TiO? films.     

Multilayer self-assembly of TiO₂ nanoparticles and polyaniline-grafted-chitosan copolymer (CPANI) for photocatalysis

A photocatalytic thin film of TiO? nanoparticles and polyaniline-grafted-chitosan (CPANI) was fabricated by layer-by-layer (LbL) approach. The growth of the self-assembly of polymer nanocomposite was monitored by UV-vis spectroscopy and the thin film morphology was analyzed from scanning electron microscopy (SEM). Poly(styrene sulfonate) (PSS) was used as a bridging layer between TiO? nanoparticles and CPANI. Incorporation of CPANI within the LbL self-assembly of polymer nanocomposites enhanced the dye degradation ability of the thin film. These results indicate that the presence of CPANI improves the adsorption of dye in the self-assembly. The effect of surface area and the amount of catalyst was also examined. The reusability of the thin films for dye degradation study ensures the stability of the self-assembly.     

Switching mechanisms in flexible solution-processed TiO₂ memristors

Memristors are emerging as unique electrical devices with potential applications in memory, reconfigurable logic and biologically inspired computing. Due to the novelty of these devices, the complete details of their switching mechanism is not yet well established. In this work, the switching mechanism of our solution-processed titanium dioxide-based memristor is investigated by studying how variations in the device area and film thickness affect electrical behavior and correlating these behavioral changes to proposed switching mechanisms. The conduction path of the switching is also investigated through electrical characterization of devices both before and after physically cutting the devices in half, as well as through infrared imaging of the devices during operation. The results suggest that the electrical behavior of these devices is dominated by a localized, charge-based phenomenon that exhibits a dependence on device area.     

Solution fabrication and photoelectrical properties of CuInS₂ nanocrystals on TiO₂ nanorod array

One-dimensional semiconductor architectures are receiving attention in preparing photovoltaic solar cells because of its superior charge transport as well as excellent light-harvesting efficiency. In this study, vertically aligned single-crystalline TiO(2) nanorods array was grown directly on transparent conductive glass (FTO), and then CuInS(2) nanocrystals were deposited on nanorods array by spin coating method to form TiO(2)/CuInS(2) heterostructure films. The resulting nanostructure assembly and composition was confirmed by field-emission scanning electron microscope (FESEM) , transmission electron microscopy (TEM), high-resolution TEM, and X-ray diffraction(XRD). Ultraviolet-visible absorption spectroscopy (UV-vis) data indicates that the absorbance of the nanocomposite film extended into the visible region compared with bare TiO(2) nanorod arrays. The surface photovoltage spectra (SPS) also showed a new and enhanced response region corresponding to the absorption spectrum. These results suggest that the novel CuInS(2) nanocrystals sensitized TiO(2) nanorod array on FTO photoelectrodes has a potential application in photovoltaic devices.     

Electrospray preparation of hierarchically-structured mesoporous TiO₂ spheres for use in highly efficient dye-sensitized solar cells

We report a simple method to prepare hierarchically structured TiO(2) spheres (HS-TiO(2)), using an electrostatic spray technique, that are utilized for photoelectrodes of highly efficient dye-sensitized solar cells (DSSCs). This method has an advantage to remove the synthesis steps in conventional sol-gel method to form nano-sized spheres of TiO(2) nanoclusters. The fine dispersion of commercially available nanocrystalline TiO(2) particles (P25, Degussa) in EtOH without surfactants and additives is electro-sprayed directly onto a fluorine-dopoed tin-oxide (FTO) substrate for DSSC photoelectrodes. The DSSCs of HS-TiO(2) photoelectrodes show high energy conversion efficiency over 10% under illumination of light at 100 mW cm(-2), AM1.5 global. It is concluded from frequency-dependent measurements that the faster electron diffusion coefficient and longer lifetime of HS-TiO(2) than those in nonstructured TiO(2) contribute to the enhanced efficiency in DSSCs.     

Design and development of novel linker for PbS quantum dots/TiO₂ mesoscopic solar cell

A novel bifunctional linker molecule, bis(4-mercaptophenyl)phosphinic acid, is designed to be used in a QDs solar cells. The linker anchors to TiO(2) mesoporous film through the phosphinic acid functional group and to the PbS QDs through the two thiol groups. The way of attachment of this new linker molecule in a photovoltaic PbS QDs/TiO(2) mesoporous device was studied by FTIR measurements. The photovoltaic performance of this new linker in a heterojunction PbS QDs solar cell show high V(oc) relative to QDs based solar cells, which will allow to receive high power conversion efficiency using this novel designed linker. This novel bifunctional linker molecule should pave the way for enhancing binding strength, and efficiency of QDs solar cells compared to the state-of-the-art linkers.     

Self-assembled TiO₂ with increased photoelectron production, and improved conduction and transfer: enhancing photovoltaic performance of dye-sensitized solar cells

Highly crystalline mesoporous anatase TiO(2) is prepared through supramolecular self-assembly and by utilizing cetyltrimethylammonium bromide (CTAB) as templating material. Photoanodes of dye-sensitized solar cells (DSSCs) made from these TiO(2) nanoparticles are found to have a high specific surface area of 153 m(2)/g and high surface roughness. Optical absorption spectroscopy studies reveal that the photoanode films adsorb four times more dye than films made of commercial P25 TiO(2). Mercury porosimetry and field emission scanning electron microscope (FESEM) studies show hierarchical macro- and meso-porosity of the photoanode films leading to better dye and electrolyte percolation, combined with improved electron conduction pathways compared to P25 films. Electrochemical impedance studies confirm lower impedance and higher electron lifetime in the synthesized mesoporous TiO(2) films compared to P25 films. Higher photovoltaic efficiency was recorded of cells made from the synthesized mesoporous TiO(2) in comparison to the corresponding cells made from P25. Incident-photon-to-current efficiency data provided critical understanding of recombination kinetics, and provided proof of Mie scattering by the self-assembled submicrometer sized TiO(2) aggregates and the macropores in their structure. The scattering phenomenon was further corroborated by diffused reflectance studies. An in-depth analysis of CTAB-templated mesoporous TiO(2) has been conducted to show how it can be a good candidate photoanode material for enhancing the performance of DSSCs.     

Control of electron transfer from lead-salt nanocrystals to TiO₂

The roles of solvent reorganization energy and electronic coupling strength on the transfer of photoexcited electrons from PbS nanocrystals to TiO(2) nanoparticles are investigated. We find that the electron transfer depends only weakly on the solvent, in contrast to the strong dependence in the nanocrystal-molecule system. This is ascribed to the larger size of the acceptor in this system, and is accounted for by Marcus theory. The electronic coupling of the PbS and TiO(2) is varied by changing the length, aliphatic and aromatic structure, and anchor groups of the linker molecules. Shorter linker molecules consistently lead to faster electron transfer. Surprisingly, linker molecules of the same length but distinct chemical structures yield similar electron transfer rates. In contrast, the electron transfer rate can vary dramatically with different anchor groups.     

Growth of single-crystalline rutile TiO2 nanorods on fluorine-doped tin oxide glass for organic–inorganic hybrid solar cells

Single-crystalline TiO₂ nanorods (TiO₂ NRs) are grown directly on FTO substrates by hydrothermal methods. The diameters and lengths of TiO₂ NRs are easily controlled by growth conditions. When used in hybrid solar cells, TiO₂ NRs function as the continuous pathway for fast electron transport to charge collecting electrode, demonstrating a high power conversion efficiency (PCE) of 3.21% with 140?nm long TiO₂ NRs. The bilayer polymer coating are introduced into 500?nm long TiO₂ NRs to reduce the surface roughness, resulting in the improved contact between the polymer blend and silver electrode and an enhanced PCE from 2.70 to 3.07%.     

CdS/CdSe-cosensitized TiO₂ photoanode for quantum-dot-sensitized solar cells by a microwave-assisted chemical bath deposition method

A CdS/CdSe quantum-dot (QD)-cosensitized TiO(2) film has been fabricated using a microwave-assisted chemical bath deposition technique and used as a photoanode for QD-sensitized solar cells. The technique allows a direct and rapid deposition of QDs and forms a good contact between QDs and TiO(2) films. The photovoltaic performance of the as-prepared cell is investigated. The results show that the performance of the CdS/CdSe-cosensitized cell achieves a short-circuit current density of 16.1 mA cm(-2) and a power conversion efficiency of 3.06% at one sun (AM 1.5 G, 100 mW cm(-2)), which is comparable to the one fabricated using conventional successive ionic layer adsorption and reaction technique.     

One-step hydrothermal synthesis of mesoporous anatase TiO₂ microsphere and interfacial control for enhanced lithium storage performance

Mesoporous TiO(2) anatase microspheres consisting of self-assembled nanocrystals have been synthesized by a one-step hydrothermal method at 120 (o)C using titanium-peroxo complex, without a post-calcination process. Transmission and scanning electron microscopic imaging reveal that diamond-shaped nanocrystals as primary particles, which are 20 nm in average width and 50 nm in length and oriented with (101) plane of anatase phase, are aggregated to form a secondary microsphere particle with 0.5-1 μm in diameter. BET analysis data show that the TiO(2) anatase particles possess significantly large surface area of 254 m(2) g(-1) with the pore size of ～14 nm. Mesoporous TiO(2) anatase anode shows an enhanced lithium storage performance in pyrrolidinium-based ionic liquid electrolyte diluted with ethyl methyl carbonate, delivering 195 - 150 mAhg(-1) at the C/2 rate with 77 % capacity retention and 98-99 % Coulombic efficiencies over 50 cycles despite the absence of surface carbon-coating. AC impedance analysis results reveal that the formation of a stable solid electrolyte interphase (SEI) layer in diluted ionic liquid electrolyte induces the enhanced cycling performance. Control of electrode-electrolyte interfacial compatibility enables the enhancement of cycling performance and the preservation of microstructure. The data contribute to provide cost-efficient synthetic method for the TiO(2) and the interfacial control aspect of performance control for safer batteries.     

Thermal conductivity and secondary porosity of single anatase TiO₂ nanowire

Single anatase TiO? nanowire is synthesized using the electrospinning technique with the sol-gel method and is suspended over a pre-processed 100 μm-wide TEM grid for further characterization. The diameters of the nanowires fall in the range of 250-400 nm. The transient electrothermal (TET) method is adopted to acquire the voltage-time (U-t) profile of the Ir-coated nanowire under step Joule heating. The intrinsic thermal diffusivity of single anatase TiO? nanowires varies from 1.3 to 4.6 × 10?? m2 s?1, and the thermal conductivity changes distinctly from 1.3 to 5.6 W m?1 K?1, much lower than the value of the bulk counterpart: 8.5 W m?1 K?1. The density and thermal conductivity increase significantly with the diameter, largely because at larger diameters less secondary porosity is left by decomposition of organic composites and their escape from the wire during calcination. The density of TiO? nanowires is found to be much lower than that of the bulk counterpart. This is supported by the SEM image of the secondary porous surface. High secondary porosity is observed for TiO? nanowires, ranging from 18% to 63%. This very high secondary porosity confirms that the decomposition of PVP content may distort the fibrous matrix and leave vacancies. In addition, the transition from amorphous to anatase phase could also create a porous state due to crystal particle aggregation.     

Construction of TiO₂ hierarchical nanostructures from nanocrystals and their photocatalytic properties

Anatase TiO(2) mesoporous structures with high specific surface areas are of special significance in various applications. In this work, hierarchical anatase TiO(2) materials with flowerlike morphologies have been prepared via a one-step template-free hydrothermal method, by using titanocene dichloride as precursor and EDA as chelating agent in aqueous solution. Particularly, the hierarchical structures are assembled from very thin TiO(2) nanosheets, which are composed of numerous highly crystallized anatase nanocrystals. In addition, the assembled materials own relatively large specific surface areas of 170 m(2)/g, and uniform mesopores of 7 nm. We further demonstrate that the hierarchical TiO(2) materials show very good photocatalytic performance when applied in photodegradation of methylene blue, which should be related to the unique features of hierarchical structures, large specific surface areas and high crystallization degree of the obtained TiO(2) materials. With these features, the hierarchical TiO(2) may find more potential applications in the fields such as dye-sensitized solar cells and lithium ion batteries.