Conversion enhancement of flexible dye-sensitized solar cells based on TiO2 nanotube arrays with TiO2 nanoparticles by electrophoretic deposition

We prepared highly ordered titanium dioxide nanotube arrays (TNAs) by anodizing Ti foils in F⁻ containing electrolyte. The thickness and dye loading amount of TNAs were 26 μm and 1.06 × 10⁻⁷ mol cm⁻², respectively. TiO₂ nanoparticles (TNPs) were electrophoretically deposited on the inner wall of nanotube to produce coated nanotube arrays (TNAP). The dye loading was increased to 1.56 × 10⁻⁷ mol cm⁻², and the electron transport rate improved. TNAs and TNAP were sensitized with ruthenium dye N3 to yield dye-sensitized TiO₂ nanotube solar cells. The power conversion efficiency of TNA-based dye-sensitized solar cells (DSSCs) was 4.28%, whereas the efficiency of TNAP-based DSSCs increased to 6.28% when illuminated from the counter electrode. The increase of power conversion efficiency of TNAP-based DSSCs is ascribed to the increased surface area of TNAs and the faster electron transport rate.     

The improvement of electron transport rate of TiO2 dye-sensitized solar cells using mixed nanostructures with different phase compositions

Dye-sensitized solar cells (DSCCs) in the form of mixed nanostructures containing TiO₂ nanoparticles and nanowires with different weight ratios and phase compositions are reported. X-ray diffraction and field emission scanning electron microscopy analyses revealed that the synthesized TiO₂ nanoparticles had average crystallite size in the range 21–39 nm, whereas TiO₂ nanowires showed diameter in the range 20–50 nm. The indirect optical band gap energy of TiO₂ nanowires, anatase- and rutile-TiO₂ nanoparticles was calculated to be 3.35, 3.28 and 3.17 eV, respectively. The power conversion efficiency of the solar cells changed with nanowire to nanoparticle weight ratio, reaching a maximum at a specific value. An increase of 4.3% in cell efficiency was achieved by introducing 10 wt% nanowire into the as-synthesized TiO₂ nanoparticles (WP1 cell). Furthermore, an increase of 27.6% in cell efficiency was achieved by using crystalline anatase-TiO₂ nanoparticles rather than as-synthesized TiO₂ nanoparticles in WP1 solar cell. It was found that the power conversion efficiency and short circuit current of WP1 cell were decreased down to around 30.8% and 39.1%, respectively using rutile nanoparticles rather than anatase nanoparticles. The improvement of cell efficiency was related to rapid electron transport and less recombination of photogenerated electrons, as confirmed by electrochemical impedance spectroscopy.     

Tertiary hierarchically structured TiO2 for CdS quantum-dot-sensitized solar cells

A tertiary hierarchically structured mesoporous spherical TiO₂ (with a diameter of 1190 ± 60 nm) was synthesized by combining the sol–gel and the subsequent solvothermal treatment, and applied to CdS quantum-dot-sensitized solar cells (QDSSCs). This mesoporous spherical (MS) TiO₂ offers a high surface area (76.02 m² g⁻¹), a high internal reflectance in the visible region and a pore accessibility. A conversion efficiency of 1.9% was achieved by CdS QDSSCs composed of the MS TiO₂ photoanode, which corresponds to ∼58% improvement as compared with the values obtained from the conventional devices made with 20-nm-sized nanocrystalline TiO₂ under AM 1.5 illumination of 100 mW cm⁻². Thus, the MS TiO₂ can be a promising candidate for the photoanode material of QDSSCs.     

Large-diameter titanium dioxide nanotube arrays as a scattering layer for high-efficiency dye-sensitized solar cell

Large-sized titanium dioxide (TiO₂) nanotube arrays with an outer diameter of approximately 500 nm have been successfully synthesized by potentiostatic anodization at 180 V in a used electrolyte with the addition of 1.5 M lactic acid. It is found that the synthesized large-diameter TiO₂ nanotube array shows a superior light scattering ability, which can be used as a light scattering layer to significantly enhance the efficiency of TiO₂ nanoparticle-based dye-sensitized solar cells from 5.18% to 6.15%. The remarkable light scattering ability makes the large-diameter TiO₂ nanotube array a promising candidate for light management in dye-sensitized solar cells (DSSCs).     

All-solid-state quantum-dot-sensitized solar cells with compact PbS quantum-dot thin films and TiO2 nanorod arrays

To improve the electron injection efficiency from PbS quantum dots to TiO₂ nanorods and prevent the direct contact of spiro-OMeTAD and TiO₂ nanorods, a compact PbS quantum-dot thin film can be successfully obtained on TiO₂ nanorod arrays 360 nm in length by repeated spin coating of Pb(Ac)₂, Na₂S and 1,2-ethanedithiol solution in a step-by-step process. The corresponding solid-state quantum-dot-sensitized solar cells are fabricated using a novel structured FTO/compact PbS quantum-dot thin film sensitized TiO₂ nanorod array/spiro-OMeTAD/Au that achieves a photoelectric conversion efficiency of 3.57% under AM 1.5 G illumination (100 mW cm⁻²), which represents a high value among all-solid-state PbS quantum-dot-sensitized TiO₂ nanorod array solar cells.     

Enhanced photoelectric efficiency by surface modification of TiO<Subscript>2</Subscript> thin film using various acidic species

This study examined the photoelectric conversion efficiency of the dye-sensitized solar cell (DSSC) when the surface of a nanometer-sized TiO₂ film, which was prepared using the solvothermal method, was modified by five acid compounds. The TiO₂ film exhibited an anatase structure with an average particle size in the range of 10–15 nm, and the maximum absorption band was shown in the UV-visible spectrum around 360 nm. The surface colors of the carboxylic acid-modified TiO₂ films were changed to light or dark with differing energy conversion efficiencies. Particularly, the conversion efficiency was considerably enhanced from approximately 6.25% for the non-modified TiO₂ film to approximately 7.50% for the film treated by acetic acid of 1.0 mole, with the N719 dye under 100 mW/cm² of simulated sunlight. FT-IR analysis of the films after N719 dye adsorption confirmed that the IR spectrum of the modified TiO₂ showed a sharp and strong band at 500 cm⁻¹, which was assigned to a metal-O bond, due to the formation of a new Ti-O bond between the O of COO⁻ and the Ti atom, which was relatively weaker in the non-modified TiO₂. Furthermore, these results were in agreement with an electrostatic force microscopy (EFM) study showing that the electrons were transferred rapidly to the surface of the acetic acid-modified TiO₂ film, compared with that on the nonmodified TiO₂ film.     

Comparison of the photovoltaic efficiency on DSSC for nanometer sized TiO2 using a conventional sol–gel and solvothermal methods

When the two types of TiO₂ coatings prepared by sol–gel and solvothermal methods were applied to dye-sensitized solar cell (DSSC) in this study, the energy conversion efficiency of the solvothermal-modified TiO₂ was considerably higher than that on the sol–gel modified TiO₂; approximately 8.51 (solvothermal) and 5.93% (sol–gel) with the N719 dye under 100 mW/cm² of simulated sunlight, respectively. These results are in agreement with an electrostatic force microscopy (EFM) study showing that the electrons were transferred rapidly to the surface of the solvothermal-modified TiO₂ film, compared with that on a sol–gel modified TiO₂ film. Furthermore, FT-IR analysis of the films after N719 dye adsorption showed that the solvothermal-modified TiO₂ had a strong band at 500 cm^?1, which was assigned to metal–O, due to a new Ti–O bond between the O of COO^? and a Ti atom. This peak was considerably weaker in the sol–gel modified TiO₂.     

Synthesis of CuInS2 quantum dots on TiO2 porous films by solvothermal method for absorption layer of solar cells

Copper indium disulfide (CuInS₂) nano-particles have been synthesized by solvothermal method for absorption layer of solar cells. The CuInS₂ nano-particles can be adsorbed in pores of TiO₂ porous films. The effects of heat-treatment on crystalline structures and sizes of the CuInS₂ nano-particles were investigated. Crystalline structures and sizes were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) investigations. Surface morphologies and optical properties were studied by field-emission scanning electron microscope (FESEM) and UV–vis spectra when CuInS₂ were absorbed on TiO₂ films.The results show that the CuInS₂ quantum dots (size is smaller than 10 nm) can be synthesized by solvothermal method at 150 °C. CuInS₂ particles sizes increase with the rise of reaction temperature and time. The CuInS₂ quantum dots can be adsorbed on TiO₂ films well and high-absorptive anodic electrode of solar cells can be prepared. Blue shift of absorption edge was observed as the sizes of CuInS₂ quantum dots decreased.     

Controlled fabrication of Sn/TiO2 nanorods for photoelectrochemical water splitting

In this work, we investigate the controlled fabrication of Sn-doped TiO₂ nanorods (Sn/TiO₂ NRs) for photoelectrochemical water splitting. Sn is incorporated into the rutile TiO₂ nanorods with Sn/Ti molar ratios ranging from 0% to 3% by a simple solvothermal synthesis method. The obtained Sn/TiO₂ NRs are single crystalline with a rutile structure. The concentration of Sn in the final nanorods can be well controlled by adjusting the molar ratio of the precursors. Photoelectrochemical experiments are conducted to explore the photocatalytic activity of Sn/TiO₂ NRs with different doping levels. Under the illumination of solar simulator with the light intensity of 100 mW/cm², our measurements reveal that the photocurrent increases with increasing doping level and reaches the maximum value of 1.01 mA/cm² at −0.4 V versus Ag/AgCl, which corresponds to up to about 50% enhancement compared with the pristine TiO₂ NRs. The Mott-Schottky plots indicate that incorporation of Sn into TiO₂ nanorod can significantly increase the charge carrier density, leading to enhanced conductivity of the nanorod. Furthermore, we demonstrate that Sn/TiO₂ NRs can be a promising candidate for photoanode in photoelectrochemical water splitting because of their excellent chemical stability.     

Reduced graphene oxide-TiO2 nanocomposite as a promising visible-light-active photocatalyst for the conversion of carbon dioxide

Photocatalytic reduction of carbon dioxide (CO₂) into hydrocarbon fuels such as methane is an attractive strategy for simultaneously harvesting solar energy and capturing this major greenhouse gas. Incessant research interest has been devoted to preparing graphene-based semiconductor nanocomposites as photocatalysts for a variety of applications. In this work, reduced graphene oxide (rGO)-TiO₂ hybrid nanocrystals were fabricated through a novel and simple solvothermal synthetic route. Anatase TiO₂ particles with an average diameter of 12 nm were uniformly dispersed on the rGO sheet. Slow hydrolysis reaction was successfully attained through the use of ethylene glycol and acetic acid mixed solvents coupled with an additional cooling step. The prepared rGO-TiO₂ nanocomposites exhibited superior photocatalytic activity (0.135 μmol g_cat⁻¹ h⁻¹) in the reduction of CO₂ over graphite oxide and pure anatase. The intimate contact between TiO₂ and rGO was proposed to accelerate the transfer of photogenerated electrons on TiO₂ to rGO, leading to an effective charge anti-recombination and thus enhancing the photocatalytic activity. Furthermore, our photocatalysts were found to be active even under the irradiation of low-power energy-saving light bulbs, which renders the entire process economically and practically feasible.     

Self-ordered TiO2 quantum dot array prepared via anodic oxidation

The template-based methods belong to low-cost and rapid preparation techniques for various nanostructures like nanowires, nanotubes, and nanodots or even quantum dots [QDs]. The nanostructured surfaces with QDs are very promising in the application as a sensor array, also called ''fluorescence array detector.'' In particular, this new sensing approach is suitable for the detection of various biomolecules (DNA, proteins) in vitro (in clinical diagnostics) as well as for in vivo imaging.The paper deals with the fabrication of TiO₂ planar nanostructures (QDs) by the process of titanium anodic oxidation through an alumina nanoporous template on a silicon substrate. Scanning electron microscopy observation showed that the average diameter of TiO₂ QDs is less than 10 nm. Raman spectroscopic characterization of self-organized titania QDs confirmed the presence of an anatase phase after annealing at 400°C in vacuum. Such heat-treated TiO₂ QDs revealed a broad emission peak in the visible range (characterized by fluorescence spectroscopy).     

TiO2 films prepared by micro-plasma oxidation method for dye-sensitized solar cell

Nanocrystalline TiO₂ films are widely investigated as the electrodes of dye-sensitized solar cell(s) with different preparation methods. In this paper, thin titanium dioxide films have been prepared on titanium plates by the micro-plasma oxidation method in the sulfuric acid solution. The thin TiO₂ films were sensitized with a cis-RuL₂(SCN)₂·2H₂O (L = cis-2,2′-bipyridine-4,4′-dicarboxylic acid) ruthenium complex and implemented into a dye-sensitized solar cell configuration. The influence of reaction current density (10, 15, 20, 25 and 30 A dm⁻²) on the structural and the surface morphology of the films was investigated by X-ray diffraction, scanning electron microscopy, atom force microscopy and X-ray photoelectricity spectroscopy. Impedance analysis for dye-sensitized solar cells was carried out by electrochemical impedance spectroscopy. The results show that the rise of current density leads to the increase in the amount of rutile and the thickness of the TiO₂ film, which makes the TiO₂ films have different photovoltages and photocurrents. The relatively higher photoelectricity properties were obtained in the TiO₂ films prepared at a current density of 20 A dm⁻². The open-circuit voltage and the short-circuit current are 605 mV and 165 μA cm⁻², respectively.     

An approach to laminated flexible Dye sensitized solar cells

We have built TiO₂ Dye sensitized solar cells (DSSCs) that combined flexible TiO₂ photoanodes coated on ITO/PET substrates with a gel electrolyte based on PVDF-HFP-SiO₂ films. Titanium isopropoxide (TiP₄) was used as additive to TiO₂ nanoparticles for increasing power conversion efficiency in Dye sensitized solar cell electrodes prepared at low-temperature (130 °C). An efficiency η_AM1.5G = 3.55% on ITO/PET substrates is obtained at 48 mW/cm² illumination with a standard liquid electrolyte based on methoxypropionitrile. Among several solvents forming gels with PVDF-HFP-SiO₂, N-methyl (pyrrolidone) (NMP) was found to enable the most stable devices. A power conversion efficiency η_AM1.5G = 2% was obtained under 10 mW/cm² with flexible TiO₂-ITO-PET photoanodes and the PVDF-HFP-SiO₂ + NMP gel electrolyte.     

Naturally inspired SERS substrates fabricated by photocatalytically depositing silver nanoparticles on cicada wings

Densely stacked Ag nanoparticles with an average diameter of 199 nm were effectively deposited on TiO₂-coated cicada wings (Ag/TiO₂-coated wings) from a water-ethanol solution of AgNO₃ using ultraviolet light irradiation at room temperature. It was seen that the surfaces of bare cicada wings contained nanopillar array structures. In the optical absorption spectra of the Ag/TiO₂-coated wings, the absorption peak due to the localized surface plasmon resonance (LSPR) of Ag nanoparticles was observed at 440 nm. Strong Surface-enhanced Raman scattering (SERS) signals of Rhodamine 6G adsorbed on the Ag/TiO₂-coated wings were clearly observed using the 514.5-nm line of an Ar⁺ laser. The Ag/TiO₂-coated wings can be a promising candidate for naturally inspired SERS substrates.     

Facile synthesis of mesoporous Ag-loaded TiO2 thin film and its photocatalytic properties

Ag nanoparticles highly dispersed into TiO₂ thin films are synthesized via a remarkably simple one-pot route in the presence of a P123 triblock copolymer as template directing and reducing agents, where the reduction of Ag⁺ to Ag⁰ by in situ heat-induced reduction through the oxidation of template at 400 °C and the controlled polymerization of TiO₂ take place simultaneously. The obtained mesoporous Ag/TiO₂ films deposited on soda-lime glass were optically transparent and crack-free. SEM and Kr adsorption clearly prove that Ag/TiO₂ films at different Ag contents are mesoporous with large surface area and regularly ordered mesopores and the thickness of the obtained films is ∼280 ± 20 nm. The pristine TiO₂ film exhibits a specific surface area of 63 cm²/cm² and specific pore volume of 0.013 mm³/cm² that it is decreased to 42 cm²/cm² and 0.010 mm³/cm² respectively as a result of Ag-loaded mesoporous TiO₂. The newly prepared photocatalysts Ag/TiO₂ films were evaluated for their photocatalytic degradation of 2-chlorophenol as a model reaction. It was found that the meso-ordered Ag/TiO₂ films are more photoactive 8 times than nonporous commercial photocatalysts Pilkington Glass Activ™. The recycling tests indicated that Ag/TiO₂ films was quite stable during that liquid-solid heterogeneous photocatalysis since no significant decrease in activity was observed even after being used repetitively for 10 times, showing a good potential in practical application. In general, the cubic mesoporous Ag/TiO₂ nanocomposites are stable and can be recycled without loss of their photochemical activity.     

Onset coarsening-coalescence kinetics of ZrO2 and less refractory TiO2 nanoparticles: Effect of homologous temperature and phase transformation

Specific surface area change of ZrO₂ (predominant tetragonal - (t) symmetry, 30-50 nm) and less refractory TiO₂ anatase nanoparticles (20-50 nm) upon isothermal firing at 700-1000 °C in air was determined by N₂ adsorption-desorption hysteresis isotherm. The nanoparticles underwent onset coarsening-coalescence within minutes without appreciable phase transformation for TiO₂, but with extensive transformation into monoclinic (m-) symmetry for ZrO₂. The apparent activation energy of such a process being not much higher for ZrO₂ (77 ± 23 kJ/mol) than TiO₂ (56 ± 3 kJ/mol) nanoparticles can be attributed to transformation plasticity. The minimum temperature for coarsening/coalescence of the present ZrO₂ and TiO₂ nanoparticles was estimated as 710 and 641 °C, respectively.     

Filter-membrane treatment of continuous-flow tetracycline through photocatalysis-assisted peroxydisulfate oxidation

Herein, we craft a high-performance advanced oxidation process (AOP) of photocatalysis-coupled peroxydisulfate (PDS) system toward practical applications in organic wastewater treatment. CoFe₂O₄ nanoparticles well grow onto TiO₂ nanotube arrays (TNAs) mesh (CoFe₂O₄/TNAs). The artful CoFe₂O₄/TNAs composite plays a bifunctional role in the proposed AOP. On one hand, CoFe₂O₄/TNAs itself is a visible-light-response photocatalyst. On the other hand, the Co(II) or Fe(II) in CoFe₂O₄ can heterogeneously activate PDS to generate energetic active species (^•OH, SO₄˙^–, O₂˙^–, and ¹O₂). Meanwhile, the photo-induced electrons can be also captured by electron traps Co(III) (or Fe(III)) which are further reduced to Co(II) (or Fe(II)) for continuous activation of PDS. As a result, the robust AOP of photocatalysis-coupled PDS can almost completely degrade 10 mg/L refractory tetracycline within 40 min. Moreover, the proposed process shows a stable continuous-flow treatment performance. This work provides basic understanding and support for continuous-flow treatment of organic wastewater by photocatalysis-coupled PDS system.     

Investigation into Photoconductivity in Single CNF/TiO<Subscript>2</Subscript>-Dye Core–Shell Nanowire Devices

A vertically aligned carbon nanofiber array coated with anatase TiO₂ (CNF/TiO₂) is an attractive possible replacement for the sintered TiO₂ nanoparticle network in the original dye-sensitized solar cell (DSSC) design due to the potential for improved charge transport and reduced charge recombination. Although the reported efficiency of 1.1% in these modified DSSC’s is encouraging, the limiting factors must be identified before a higher efficiency can be obtained. This work employs a single nanowire approach to investigate the charge transport in individual CNF/TiO₂ core–shell nanowires with adsorbed N719 dye molecules in dark and under illumination. The results shed light on the role of charge traps and dye adsorption on the (photo) conductivity of nanocrystalline TiO₂ CNF’s as related to dye-sensitized solar cell performance.     

Enhanced electron collection efficiency of nanostructured dye‐sensitized solar cells by incorporating TiO2 cubes

Herein, enhancement of dye‐sensitized solar cell (DSC) performance is reported by combining the merits of the dye loading of TiO₂ nanoparticles and light scattering, straight carrier transport path, and efficient electron collection efficiency of TiO₂ cubes. We fabricate DSC devices with various arrangement styles and compositions of the electrodes in the forms of monolayer and double layer films. For this purpose, the solvothermal synthesized TiO₂ cubic particles (100‐600 nm) are employed as the scattering layer, whereas TiO₂ nanoparticles (15‐30 nm) synthesized via a combination of solvothermal and sol‐gel routes are used as the active layer of devices. We improve the photovoltaic characteristics of DSCs by two mechanisms. First, the light harvesting of DSC devices made of nanoparticles is improved by controlling the thickness of monolayer films, reaching the highest efficiency of 7.0%. Second, the light scattering and electron collection efficiency are enhanced by controlling the composition of double layer films composed of mixtures of TiO₂ nanoparticles and cubes, obtaining the maximum efficiency of 8.21%. The enhancements are attributed to balance between charge transfer resistance and charge recombination of photo‐generated electrons as well as dye loading and light scattering.     

The effect of the inner particle structure on the electronic structure of the nano-crystalline Li-Ti-O spinels

The effect of the inner particle structure on Li insertion activity and electronic structure of the nano-crystalline Li-Ti-O spinels was studied on materials prepared by solid state and solvothermal synthesis. The high temperature prepared materials of composition corresponding to Li₄Ti₅O₁₂ feature particles with characteristic size of ca. 200 nm with randomly distributed defects. The products of solvothermal synthesis with composition Li_1.1Ti_1.9O_4+δ, feature cubic particles of characteristic dimension of ca. 50 nm; the characteristic particle size differs from that of the coherent domain determined by X-ray diffraction. The reduction of the solvothermal and high temperature synthesized nano-crystalline spinels in Li containing solutions leads according to ⁶Li MAS NMR spectra to Li insertion into tetrahedral 8b and octahedral 16c position, respectively. Additional broad NMR signal attributable to a Knight shift was observed in spectra of partially reduced high temperature spinels. In the case of solvothermal spinels is the Knight shift signal less pronounced and appears only in spectra of samples in which the phase transition occurs on the local level. The UV-vis-NIR spectra of the partially reduced Li-Ti-O spinel samples correspond to expected semiconductor character of Li-Ti-O spinels. Both types of materials are characterized by band gap of 3.8 eV (high temperature spinel) and 3.5 eV (solvothermal material). Partial reduction accompanied with Li insertion causes additional optical transition in the visible to near infrared region, which can be attributed to formation of trivalent Ti, character of which changes with degree of reduction. The behavior observed for partially reduced high temperature spinels is similar to that reported for TiO₂ (anatase). The spectral behavior of the partially reduced solvothermal spinels is more complex and reflects suppressed phase transition.