Synthesis of rutile TiO2 powder by microwave-enhanced roasting followed by hydrochloric acid leaching

In this paper, the preparation of rutile TiO₂ powder from titanium slag by microwave-assisted activation roasting followed by hydrochloric acid leaching was investigated. The effects of the additive Na₂CO₃ on the crystal form, cell, crystallinity, phase transformation, surface functional groups and micro-surface structure of the calcined product were systematically studied using X-ray powder diffraction, Raman spectroscopy, Fourier-transform infrared spectroscopy, and scanning electron microscopy. The results confirmed that the strongest characteristic Raman bands of rutile TiO₂ and the weakest FT-IR bands of (CO₃)⁻² were found when the Na₂CO₃ mass ratio was 0.4. Accordingly, the crystallinity for the product, namely short rod structure rutile TiO₂ powder, reached its peak value of 99.21% with a corresponding average grain size of 43.5 nm. The excessive Na₂CO₃ was found to be disadvantageous for the crystallinity of the product, since it formed a coverage on the surface of titanium slag which prevented the oxidation reaction for the decomposition of anosovite.     

Ag nanoparticle-decorated SiO2@TiO2 hierarchical microspheres improve the efficiency of dye-sensitized solar cells

SiO₂@TiO₂-Ag (STA) microspheres decorated with Ag nanoparticles (Ag NPs) were prepared and assembled into the photoanode. The photoanode composed of STA microspheres and TiO₂ nanoparticles (P25) was prepared by doctor blade method. UV–vis measurement indicates that the introduction of a few STA microspheres observably enhances the light scattering and capturing ability of the photoanode. The photoelectric conversion efficiency of the DSSCs with 2wt% STA photoanode increased to 7.4% from 4.3% comparing with pure P25 TiO₂ nanoparticles. The configuration DSSCs have the maximum short circuit current density (J_sc) of 16.0 mA cm^?2 and open-circuit voltage (V_oc) of 0.780 V, which are significantly higher than the pure TiO₂ DSSCs. The significant improvement of the DSSCs performance can be due to the synergistic effect of the superior light scattering of STA and the localized surface plasma resonance (LSPR) effect of Ag NPs modified on the microspheres surface.

     

Performance improvement of dye-sensitized solar cells by surface patterning of fluorine-doped tin oxide transparent electrodes

The surface modification of fluorine-doped tin oxide (FTO) transparent electrodes was carried out by lithography and inductively coupled plasma etching to improve the conversion efficiency of dye-sensitized solar cells (DSSCs). The concentration of Cl₂ gas and dc-bias voltage to the substrate were varied as the main etch parameters. The transmittance and sheet resistance of the FTO electrodes were compared before and after etching. The DSSCs fabricated on the patterned FTO electrodes showed higher conversion efficiency than those fabricated on the ordinary FTO electrodes without patterns. Scanning electron microscopy showed that more TiO₂ particles could be involved in the DSSCs with patterned FTO electrodes, and that the contact between the TiO₂ layer and electrode were improved by patterning the FTO electrode. The current-voltage curves and incident photon to current efficiency spectra showed that a significantly higher photocurrent was produced in the DSSCs fabricated on the patterned FTO.     

Effect of atomic layer deposited ultra thin HfO2 and Al2O3 interfacial layers on the performance of dye sensitized solar cells

The objective of this work was to investigate the improvement in performance of dye sensitized solar cells (DSSCs) by depositing ultra thin metal oxides (hafnium oxide (HfO₂) and aluminum oxide (Al₂O₃)) on mesoporous TiO₂ photoelectrode using atomic layer deposition (ALD) method. Different thicknesses of HfO₂ and Al₂O₃ layers (5, 10 and 20 ALD cycles) were deposited on the mesoporous TiO₂ surface prior to dye loading process used for fabrication of DSSCs. It was observed that the ALD deposition of ultrathin oxides significantly improved the performance of DSSCs and that the improvement in the DSSC performance depends on the thickness of the deposited HfO₂ and Al₂O₃ films. Compared to a reference DSSC the incorporation of a HfO₂ layer resulted in 69% improvement (from 4.2 to 7.1%) in the efficiency of the cell and incorporation of Al₂O₃ (20 cycles) resulted in 19% improvement (from 4.2 to 5.0%) in the efficiency of the cell. These results suggest that ultrathin metal oxide layers affect the density and the distribution of interface states at the TiO₂/organic dye and TiO₂/liquid electrolyte interfaces and hence can be utilized to treat these interfaces in DSSCs.     

Structural and optical properties of Eu2O3 coated TiO2 nanoparticles and their application for dye sensitized solar cell

Specialized applications of dye sensitized solar cells (DSSCs) have attracted much attention as an economical substitute to first and second generation solar cells. Surface modification of TiO₂ nanoparticles with high band gap Eu₂O₃ coating has been carried out by chemical precipitation arrested solvothermal method. The characterizations have been done through X-ray diffraction, transmission electron microscopy, diffuse UV–visible, and photoluminescence spectroscopy. Scanning electron microscopy results reveal an increase in surface roughness in case of Eu₂O₃/TiO₂ electrode, which leads to enhancement in the dye loading capability. The synthesized nanoparticles have been employed for the fabrication of DSSCs. Effect of Eu₂O₃ coating on their performance has been studied. Eu₂O₃/TiO₂ electrode provides better surface area for the dye adsorption, which slows down the electron–hole recombination, and thus, improves the DSSC performance. The photocurrent density–voltage (J–V) characteristics reveal that the efficiency of DSSCs fabricated from Eu₂O₃/TiO₂ nanoparticles is 52 % higher than that from bare TiO₂ nanoparticles.     

LiFePO<Subscript>4</Subscript>/TiO<Subscript>2</Subscript>/Pt composite film used as effective and robust counter electrode for dye sensitized solar cells

A series of composite films based on LiFePO₄/TiO₂/Pt were synthesized and used as counter electrodes for dye sensitized solar cells (DSSCs). The composites are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET). These analysis results demonstrate that the crystal structure of LiFePO₄ in composite is not changed, and the prepared LiFePO₄/TiO₂/Pt composite films hold a rough surface and porous structure which provide more catalytic activity sites for I₃ ^? reduction and more space for I^?/I₃ ^? diffusion. The DSSC based on LiFePO₄/TiO₂/Pt composite CEs shows a high power conversion efficiency of 6.23% at a low Pt dosage of 2%, comparable to the conventional magnetron sputtering Pt CE (6.31%). The electrochemical analysis reveals that the presented composite CEs have good electrocatalytic activity and low charge transfer resistance. Furthermore, the DSSCs based on LiFePO₄/TiO₂/Pt composite CE exhibit high stability under the continuous tests condition and electrolyte soaking. The results suggest that this LiFePO₄-based composite film could be a perspective electrode for practical application of DSSCs and it maybe provide a potential for further research about photo-charging lithium-ion batteries.     

Tuning the dye aerosol impaction and TiO2 nanoparticle stacking structures for High-Efficiency Dye-Sensitized solar cells

The rapid manufacturing of high-efficiency dye-sensitized solar cells (DSSCs) is limited by the slow dye adsorption on TiO₂ nanoparticles (NPs)-accumulated photoelectrode using conventional dip-coating process. Therefore, we aim to accelerate the adsorption of dyes that are attached on TiO₂ NPs by employing an aerosol impactor. Herein the aerosolized dyes are designed to get deposited rapidly on the TiO₂ NPs-accumulated photoelectrode. In addition, to effectively trap the irradiated sunlight in DSSCs, we assemble the photoelectrodes incorporated with bilayered TiO₂ thin films comprising small TiO₂ NPs-based underlayer and large TiO₂ NPs-based overlayer as dye-supporting and light-scattering mediums, respectively. Furthermore, the effects of dye aerosol impaction and TiO₂ stacking structures on the efficiency of DSSCs are examined. The power conversion efficiency (PCE) of DSSCs comprising a N719 dye-supporting layer treated with dip-coating process was determined as ～ 5.67%; however, when the bilayered TiO₂ thin films with an optimized thickness ratio of light-scattering overlayer and dye-supporting underlayer were coated with N719 dyes using dye aerosol impactor, the resulting PCE increased to ～ 7.46%. This suggests that the photovoltaic characteristics of DSSCs can be enhanced considerably using the effective TiO₂ NP stacking structures coated with rapid, uniform, and strong aerosol dye adsorption throughout the TiO₂-based photoelectrodes.     

Analyzing the efficiency of nanostructured Sm3+- and Gd3+-doped TiO2 and constructing DSSCs using efficacious photoanodes

The rare earth elements, gadolinium and samarium, are doped with TiO₂ by hydrothermal synthesis technique to study the photoconversion performance of a photoanode in a dye-sensitized solar cell (DSSC). The obtained materials are subjected to the characterizations XRD, HR-TEM, UV–Vis spectroscopy, and XPS. DSSCs are fabricated using N719 dye, redox electrolyte, and platinum counter electrode. Charge-transfer ability was investigated using electrochemical impedance spectroscopy (EIS) on DSSCs. The efficiencies of DSSCs are influenced by the electron transport within the TiO₂–dye–electrolyte system. After the fabrication and simulation, among the two, Gd³⁺-doped TiO₂ gives the desired outcomes and higher efficiency (5.542%) than the pure and Sm³⁺-doped TiO₂ and thus it proves to be a superior solar cell anode material.

     

Anode growth of DSSCs by flat-flame chemical vapor deposition method

A unique method of flat-flame chemical vapor deposition, to synthesize nanostructured TiO₂ film used for dye-sensitized solar cells (DSSCs) is reported for the first time. This method produces nanostructures of TiO₂ exactly ideal for the anode of the DSSCs. Results show that cells have characteristic I–V curves with fill factor typically around 70% and efficiency higher than 4%. A dendrite-like microstructure of anodes accounts for the high efficiency of the cell. The successful formation of dendrite-like microstructure provides a chance for sintering of TiO₂ nanoparticles smaller than 20 nm, a possible cell efficiency enhancement by increased dye absorption due to the increased specific surface area of small particles.     

Effect of TiO2 nanotube length and lateral tubular spacing on photovoltaic properties of back illuminated dye sensitized solar cell

The main objective of this study is to show the effect of TiO₂ nanotube length, diameter and intertubular lateral spacings on the performance of back illuminated dye sensitized solar cells (DSSCs). The present study shows that processing short TiO₂ nanotubes with good lateral spacings could significantly improve the performance of back illuminated DSSCs. Vertically aligned, uniform sized diameter TiO₂ nanotube arrays of different tube lengths have been fabricated on Ti plates by a controlled anodization technique at different times of 24, 36, 48 and 72?h using ethylene glycol and ammonium fluoride as an electrolyte medium. Scanning electron microscopy (SEM) showed formation of nanotube arrays spread uniformly over a large area. X-ray diffraction (XRD) of TiO₂ nanotube layer revealed the presence of crystalline anatase phases. By employing the TiO₂ nanotube array anodized at 24?h showing a diameter ??80?nm and length ??1·5???m as the photo-anode for back illuminated DSSCs, a full-sun conversion efficiency (??) of 3·5 % was achieved, the highest value reported for this length of nanotubes.     

TiO2 hollow spheres as effective additives in oligomer electrolytes for dye-sensitized solar cells

TiO₂ hollow spheres are employed as an additive of oligomer electrolytes for dye-sensitized solar cells (DSSCs). The measurement of steady-state currents confirms that introducing TiO₂ hollow spheres can facilitate ionic diffusion in oligomer electrolytes. Even compared with conventional nanoparticle additives, the hollow spheres are more favorable to increase the diffusion coefficients of I^? and I₃^? in oligomer electrolytes. Furthermore, the hollow structure with a submicron size is effective to scatter incident light and thereby enhance the light-harvesting efficiency of DSSCs. The energy-conversion efficiency of the DSSCs with TiO₂ hollow sphere additives significantly improves up to 7.22% due to the facilitated ionic diffusion and the enhanced light-harvesting efficiency.     

The role of the TiO2 nanotube array morphologies in the dye-sensitized solar cells

Arrays of TiO₂ nanotubes were fabricated by the anodization of Ti foils and then used in assembling dye-sensitized solar cells (DSSCs). The role of the morphologies of the TiO₂ nanotubes in the photovoltaic performances of the DSSCs was studied in terms of the surface topography and the tube length. The necessity of removing the nanoporous films from the surface of the nanotube arrays for good DSSC performance has been demonstrated. Also, it has been shown that appropriately increasing the tube length was an effective measure for enhancing both the short-circuit current density and the conversion efficiency of the DSSCs.     

Composite electrolytes of polyethylene glycol methyl ether and TiO2 for dye-sensitized solar cells—Effect of heat treatment

For solid-state dye-sensitized solar cells (DSSCs), a composite electrolyte of polyethylene glycol methyl ether (PEGME) and titania (TiO₂) nanoparticles was prepared and characterized by Fourier transform-infra red (FT-IR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy. The heat treatment on PEGME and TiO₂ composite was found to be an essential step to improve morphology, amorphicity and ionic conductivity of PEGME–TiO₂ composite electrolytes. It was attributed to the increased bond strength of OC–O–Ti between PEGME and TiO₂ and increased surface roughness of composite materials, which may help to absorb a large amount of iodide couple and effective generation of I₃⁻ ions. A DSSC fabricated with heat treated PEGME–TiO₂ composite electrolyte showed significantly enhanced overall conversion efficiency of 3.1%, which was 20% higher than that of the DSSC fabricated with bare PEGME–TiO₂ composite electrolyte.     

Rutile TiO2 microspheres with exposed nano-acicular single crystals for dye-sensitized solar cells

Uniquely structured rutile TiO₂ microspheres with exposed nano-acicular single crystals have been successfully synthesized via a facile hydrothermal method. After calcination at 450 °C for 2 h, the rutile TiO₂ microspheres with a high surface area of 132 m²/g have been utilized as a light harvesting enhancement material for dye-sensitized solar cells (DSSCs). The resultant DSSCs exhibit an overall light conversion efficiency of 8.41% for TiO₂ photoanodes made of rutile TiO₂ microspheres and anatase TiO₂ nanoparticles (mass ratio of 1:1), significantly higher than that of pure anatase TiO₂ nanoparticle photoanodes of similar thickness (6.74%). Such a significant improvement in performance can be attributed to the enhanced light harvesting capability and synergetic electron transfer effect. This is because the photoanodes made of rutile TiO₂ microsphere possess high refractive index which improves the light utilisation efficiency, suitable microsphere core sizes (450–800 nm) to effectively scatter visible light, high surface area for dye loading, and synergetic electron transfer effects between nanoparticulate anatase and nano-acicular rutile single crystals phases giving high electron collection efficiency.      

Influence of growth time and annealing on rutile TiO2 single-crystal nanorod arrays synthesized by hydrothermal method in dye-sensitized solar cells

Vertically aligned TiO₂ nanorod (NR) arrays have been grown on the fluorine doped tin oxide (FTO) substrates by hydrothermal methods and the structures were employed to fabricate the dye-sensitized solar cells (DSSCs). The charge transport properties were investigated by the current to voltage curves and the electrochemical impedance spectroscopy. It was found that DSSCs containing the as-prepared and 500 °C annealed TiO₂ NRs exhibit different trends with the growth time (t). The DSSCs assembled with the un-sintered NR arrays showed the highest power conversion efficiency (η) of 1.87%. When DSSCs were assembled with the sintered NR arrays, nearly 400% enhanced efficiency were obtained, and the values (η) showed a positive correlation with t. This behavior may be attributed to the improved adhesion and electric contact between TiO₂ and FTO, as well as the reduced number of recombination sites.     

The optical properties of nanoporous structured titanium dioxide and the photovoltaic efficiency on DSSC

This study examined the characterization of nanoporous structured titanium dioxide and its application to dye-sensitized solar cells (DSSCs). TEM revealed nanopore sizes of 10.0 nm with a regular hexagonal form. When nanoporous structured TiO₂ was applied to DSSC, the energy conversion efficiency was enhanced considerably compared with that using nanometer sized TiO₂ prepared using a hydrothermal method. The energy conversion efficiency of the DSSC prepared from nanoporous structured TiO₂ was approximately 8.71% with the N719 dye under 100 mW cm⁻² simulated light. FT-IR spectroscopy showed that the dye molecules were attached perfectly to the surface and more dye molecules were absorbed on the nanoporous structured TiO₂ than on the nano-sized TiO₂ particles prepared using a conventional hydrothermal method. Electrostatic force microscopy (EFM) showed that the electrons were transferred rapidly to the surface of the nanoporous structured TiO₂ film.     

Effect of NaHCO3 on synthesis of anatase TiO2 nanopowders by thermal processing of the precursor

Titanium dioxide (TiO₂) nanopowders were successfully prepared by thermal processing of the precursor of titanium hydroxide, urea and sodium acid carbonate (NaHCO₃). The products were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The results show that NaHCO₃ has a certain effect on the average crystallite size and dispersity of TiO₂ nanopowders, at the same time other phases (Na₂SO₄ and Na₂CO₃) will be introduced. However, Na₂SO₄ has distinctive intercalation ability and catalytic activity. TiO₂ (anatase) powders can be prepared at ?600 °C for 2 h with addition of 2–10 wt% NaHCO₃, and the average crystallite size is 20.0–22.3 nm. The surface of the sample mainly consists of Ti, O, C, Na and S five species elements.     

Some electrical properties of sodium silicate glasses modified with titania

A number of glass samples were prepared from mixtures of SiO₂, TiO₂ and Na₂CO₃ (for Na₂O) and their electrical characteristics were measured and interpreted in terms of an electrical energy gap somewhat smaller than the optical gaps previously reported for these glasses. Capacitance measurements showed that one effect of increasing the TiO₂ content was to increase the permittivity. Traditional sodium silicate glasses conduct electricity by ions, but the admixture of TiO₂ brings a significant electronic contribution. Some measurements at high electric fields were made and evidence of the applicability of small polaron theory to the results is presented.     

Dye-sensitized solar cells using TiO<Subscript>2</Subscript> nanoparticles transformed from nanotube arrays

Dye-sensitized solar cells (DSSCs) were fabricated using TiO₂ mesoporous layers obtained by very simple method—transformation of TiO₂ nanotube (NT) films grown by electrochemical oxidation to nanoparticle (NP) films. This transformation is based on thermal annealing of TiO₂ NT arrays formed by anodization of titanium foil in fluorine ambient. Performance of DSSCs fabricated with different size NPs was studied in the range from 35 to 350 nm. Highest nominal efficiency (9.05%) was achieved for DSSC with NP size 65 nm while the lowest nominal efficiency (1.48%) was observed for DSSCs with NP size 350 nm. The dependence of the solar cell parameters with NP size is discussed.     

Microstructural and chemical variation of TiO2 electrodes in DSSCs after ethanol vapour treatment

TiO₂ based dye-sensitized solar cells (DSSCs) have great potential to solve many energy challenges, however, their low energy conversion rate is still a barrier for further applications. Ethanol vapour post-treatment can improve the DSSC's conversion efficiency without changing its architecture, and a stable 2–3% improvement was found in our experiments. Microstructural and chemical factors were investigated using scanning electron microscopy and analytical electron microscopy on treated and untreated electrodes. The vapour treatment improved the porosity and surface-to-volume ratio of the TiO₂ particles, decreased electron transport loss between TiO₂ and fluorine doped tin oxide, and increased hydroxyl sites on the TiO₂ particle's surface. The modification therefore enhanced the dye uptake and dye–TiO₂ coupling, and it reduced the energy loss during the carrier transfer.