Hydrothermal synthesis of 1D TiO2 nanostructures for dye sensitized solar cells

Mono-dimensional titanium oxide nanostructures (multi-walled nanotubes and nanorods) were synthesized by the hydrothermal method and applied to the construction of dye sensitized solar cells (DSCs). First, nanotubes (TiNTs) and nanotubes loaded with titanium oxide nanoparticles (TiNT/NPs) were synthesized with specific surface areas of 253 m²/g and 304 m²/g, respectively. After that, thermal treatment of the nanotubes at 500 °C resulted in their transformation into the corresponding anatase nanorods (TiNT-Δ and TiNT/NPs-Δ samples). X-ray diffraction and Raman spectroscopy data indicated that titanium oxide in the pristine TiNT and TiNT/NP samples was converted into anatase phase TiO₂ during the heating. Additionally, specific surface areas and water adsorption capacities decreased after the heat treatment due to the sample agglomeration and the collapse of the inner nanotube channels. DSCs were fabricated with the nanotube TiNT and TiNT/NP samples and with the anatase nanorod TiNT-Δ and TiNT/NPs-Δ samples as well. The highest power conversion efficiency of η = 3.12% was obtained for the TiNT sample, despite its lower specific surface compared with the corresponding nanoparticle-loaded sample (TiNT/NP).     

Fabrication of dye sensitized solar cell using TiO2 coated carbon nanotubes

We fabricated a dye sensitized solar cells (DSCs) using TiO₂ coated multi-wall carbon nanotubes (TiO₂-CNTs). Carbon nanotubes (CNTs) have excellent electrical conductivity and good chemical stability. We introduced CNTs in DSCs to improve solar cell performance through reduction of series resistance. TiO₂-CNTs were obtained by Sol-Gel method. Compared with a conventional TiO₂ cell, the TiO₂-CNTs content (0.1 wt.%) cell showed ∼ 50% increase in conversion efficiency, which is attributed to the increase in short circuit current density (J_sc). The enhancement in J_sc occurs due to improvement in interconnectivity between the TiO₂ particles and the TiO₂-CNTs in the porous TiO₂ film.     

The effect of Li intercalation on different sized TiO2 nanoparticles and the performance of dye-sensitized solar cells

The effect of Li⁺ insertion into different sized TiO₂ nanoparticles and their influences on the photoconversion efficiency of dye-sensitized solar cells (DSSC) were investigated. TiO₂ nanoparticles with different particle sizes (22 nm, 14 nm and 6 nm) doped with Li⁺ were employed to form thin film electrodes and their properties were characterized by X-ray diffraction (XRD) and electrochemical impedance spectroscopy analysis. XRD evidenced the presence of anatase as the main phase. From the XRD analysis, it was observed that the Li⁺ ions could be inserted into both the surface and bulk of the TiO₂ nanoparticles. In the larger particle size, the Li⁺ ions are inserted into the bulk anatase where as Li⁺ ions bounded on the TiO₂ surface for the smaller crystallite size. The photovoltaic properties were measured by a current-voltage meter under AM1.5 simulated light radiation. It exhibited that the overall photoconversion efficiency of DSSC was decreased in the larger particles while it was enhanced in the smaller nanoparticles when Li⁺ was doped into the TiO₂ nanoparticles. A nearly 40% decrease in the efficiency (η) of DSSC was observed upon intercalation of Li⁺ ions into 22 nm sized TiO₂ nanoparticles (P25). The 14 nm sized TiO₂ nanoparticles (P90) showed slightly less efficiency (η) upon Li⁺ doping than that of the undoped sample. However, the smallest sized TiO₂ nanoparticles (6 nm) showed higher efficiency than that of the undoped one. This phenomenon is explained based on electron trapping and charge recombination due to lithium doping.     

Structure induced optical properties of anodized TiO2 nanotubes

TiO₂ nanotubes were synthesized by means of anodization and investigated for their structure dependent optical properties. The anodization was conducted at operating voltages between 5 and 30 V for 3 h in a neutral, organic electrolyte consisting of 0.3 wt% NH₄F + 2 wt% H₂O + ethylene glycol and the resulting nanotubes were annealed at 450 °C for 2 h in air at atmospheric pressure. It is shown that an increase in the applied anodization voltage yielded an increase in the wall thickness, diameter and length of the nanotubes and that these varying morphologies have a direct influence on the crystallite size of the nanotubes during annealing. Photoluminescence spectra indicated that the optical bandgap of the TiO₂ nanotube film decreased with the increase in the anodization voltage, whereas supplementary Raman spectra showed a decrease in the confinement of the optical phonon modes as the crystallite sizes increased, in coherence with the phonon confinement model. These results present significant insights into the size-dependent properties of these novel nanostructured forms of TiO₂ and play an important role in their implementation in photovoltaic devices, such as the dye-sensitized solar cell.     

Surface treatment of TiO2 films for dye-sensitized solar cells using atmospheric-pressure non-equilibrium DC pulse discharge plasma jet

In this paper, we report the utilization of the DC pulse discharge plasma jet technique as a means for the preparation of titanium oxide (TiO₂) films on fluorine dope tin oxide (FTO) coated glass substrates used for dye-sensitized solar cells (DSCs). The TiO₂ film made on these experimental bases exhibited the BET specific surface area of 95 m²/g, the pore volume of 0.3 cm²/g and the TEM particle size of ∼25 nm. The DSCs made by the TiO₂ film exhibited an energy conversion efficiency of 5.7% at 100 mW/cm² light intensity. Consequently, we believe that the optimization between the specific surface area and photocurrent density of TiO₂ film was achieved by the plasma surface treatment which also contributed to the improvement of energy conversion efficiency of DSCs.     

Charge recombination reduction in dye-sensitized solar cells by depositing ultrapure TiO2 nanoparticles on “inert” BaTiO3 films

Ultrapure TiO₂ nanoparticles (∼5 nm in size) were supported on “inert” BaTiO₃ films by TiCl₄ treatment, which was used to fabricate dye-sensitized solar cells (DSSCs). The optimized electrode, designated as BaTiO₃/TiO₂(4), was obtained upon four cycles of TiCl₄ treatment. DSSC with BaTiO₃/TiO₂(4) electrode exhibits superior power conversion efficiency (PCE) compared to that with conventional anatase TiO₂ (∼25 nm in size) electrode. The interfacial charge recombination kinetics was investigated by electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). In contrast to DSSC with anatase TiO₂ electrode, the dramatically enhanced electron lifetime for DSSC with BaTiO₃/TiO₂(4) electrode could be attributed to the decrease of recombination reaction at the TiO₂ photoelectrode/electrolyte interface. It is proposed that the lower interfacial charge recombination can be related to the relatively shallower trap distributions in DSSC with BaTiO₃/TiO₂(4) electrode.     

Synthesis and electrophoretic deposition of hydrothermally synthesized multilayer TiO2 nanotubes on conductive filters

TiO₂ nanotubes were synthesized by the decomposition of titanium isopropoxide in water and the calcination at 450 °C for 2 h to form TiO₂ nanoparticles. The synthesized TiO₂ in anatase form nanoparticles were processed hydrothermally in 10 M NaOH solution at 130 °C for 24 h to obtain multilayer TiO₂ nanotubes. TEM analysis revealed that the diameters of the tubes were around 10 nm and they are in the length of 100 nm. Subsequently, colloidal suspensions containing 1% wt. Of TiO2 nanotubes were prepared with TEA and butanol and electrophoretic deposition (EPD) experiments were conducted in order to obtain coatings on Ni and carbon filters using a deposition time of 10 min. and an applied voltage of 65 V. It is also shown that multilayer TiO₂ nanotubes having outer diameter around 10 nm and inner diameters of 4.3 nm can be produced using the described technique. EPD is also shown to be an effective technique to coat three dimensional components, such as Ni and C filters for various applications including water and air purification systems.     

Synthesis of TiO2 nanotubes coupled with CdS nanoparticles and production of hydrogen by photocatalytic water decomposition

The CdS/TiO₂NTs composite was prepared by a simple two-step chemical solution routes to directly transfer trititanate nanotubes to TiO₂NTs and simultaneously coupled with CdS nanoparticles. The results of XRD, TEM, Diffuse reflectance UV-Visible absorption spectra revealed that the CdS nanoparticles were homogeneously embedded on the surface of TiO₂NTs and the absorption spectrum of TiO₂NTs was extended to visible region. The activity of hydrogen production by photocatalytic water decomposition for the CdS/TiO₂NTs composite was examined under visible light irradiation (λ > 400 nm) and the quantity of H₂ evolution was ca. 1708 μL/g for 6 h.     

Hydrothermal synthesis, characterization, photocatalytic activity and dye-sensitized solar cell performance of mesoporous anatase TiO2 nanopowders

Mesoporous anatase TiO₂ nanopowder was synthesized by hydrothermal method at 130 °C for 12 h. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), HRTEM, and Brunauer-Emmett-Teller (BET) surface area. The as-synthesized sample with narrow pore size distribution had average pore diameter about 3-4 nm. The specific BET surface area of the as-synthesized sample was about 193 m²/g. Mesoporous anatase TiO₂ nanopowders (prepared by this study) showed higher photocatalytic activity than the nanorods TiO₂, nanofibers TiO₂ mesoporous TiO₂, and commercial TiO₂ nanoparticles (P-25, JRC-01, and JRC-03). The solar energy conversion efficiency (η) of the cell using the mesoporous anatase TiO₂ was about 6.30% with the short-circuit current density (Jsc) of 13.28 mA/cm², the open-circuit voltage (Voc) of 0.702 V and the fill factor (ff) of 0.676; while η of the cell using P-25 reached 5.82% with Jsc of 12.74 mA/cm², Voc of 0.704 V and ff of 0.649.     

Solid state dye-sensitized solar cell with TiO2/NiO heterojunction: Effect of particle size and layer thickness on photovoltaic performance

The present work demonstrates the usefulness of nickel oxide as a hole transporting material in solid state dye-sensitized solar cells (SSDSSCs). We report on the photovoltaic performances of sensitized TiO₂/NiO heterojunctions, and demonstrate that the TiO₂ film thickness and morphology, as well as NiO film thickness, have significant effects on the photovoltaic behaviour of TiO₂/NiO SSDSSC. Under 1 sun AM1.5G simulated illumination, the SSDSSCs demonstrated best photovoltaic performance with a short circuit photocurrent density, open circuit voltage, fill factor and efficiency of 0.91 mA cm⁻², 780 mV, 40% and 0.3%, respectively. This study draws attention to the feasibility of enhancing the photovoltaic performance in SSDSSC devices through development of appropriately designed sensitized TiO₂/NiO heterojunctions.     

Preparation and characterization of regioregular poly(3-octylthiophene-2,5-diyl)/copper indium disenillide/titania heterojunction polymer solar cells

By optimizing the P3OT/CISe ratio, TiO₂ content in the P3OT/CISe active layer, annealing temperature and time, this study investigated hybrid Al/Ca/P3OT:CISe:TiO₂/PEDOT:PSS/ITO thin film solar cells with improved efficiency. Due to an increase in charge-carrier transport and a decrease of electron-hole recombination, it is possible to increase the efficiency of hybrid solar cells by adding TiO₂ nanoparticles to the P3OT:CISe active film. Also, performance enhancement of the solar cells can occur with an increase of CISe content in P3OT as well as the addition of a PEDOT:PSS layer to the cell structure. The optimum TiO₂ content in P3OT:CISe layer is 15 wt.%. The optimum annealing temperature and time are 125 °C and 30 min, respectively. The formation of large CISe and TiO₂ aggregates that reduce charge mobility may cause the decrease of efficiency. The rough surface may effectively reduce the charge-transport distance and provide nanoscale phase separation that further enhances internal light scattering and light absorption. The best results for the open circuit voltages (V_oc), short-circuit current density (J_sc), fill factor (FF), and efficiency (η_e) of Al/Ca/POCT15/PEDOT:PSS/ITO hybrid solar cells obtained at optimized conditions were V_oc = 0.49, J_sc = 3.20, FF = 42.96, and η_e = 0.674, respectively.     

Polythiophene thin films electrochemically deposited on sol-gel based TiO2 for photovoltaic applications

In this work, the influence of titanium dioxide (TiO₂) thin films on the efficiency of organic photovoltaic devices based on electrochemically synthesized polythiophene (PT) was investigated. TiO₂ films were produced by sol-gel methods with controlled thickness. The best TiO₂ annealing condition was determined through the investigation of the temperature influence on the electron charge mobility and resistivity in a range between 723 K and 923 K. The PT films were produced by chronoamperometric method in a 3-electrode cell under a controlled atmosphere. High quality PT films were produced onto 40 nm thick TiO₂ layer previously deposited onto fluorine doped tin oxide (FTO) substrate. The morphology of PT films grown on both substrates and its strong influence on the device performance and PT minimum thickness were also investigated. The maximum external quantum efficiency (IPCE) reached was 9% under monochromatic irradiation (λ = 610 nm; 1 W/m²) that is three orders of magnitude higher than that presented by PT-homolayer devices with similar PT thickness. In addition, the open-circuit voltage (V_oc) was about 700 mV and the short-circuit current density (J_sc) was 0.03 A/m² (λ = 610 nm; 7 W/m²). However, as for the PT-homolayer also the TiO₂/PT based devices are characterized by antibatic response when illuminated through FTO. Finally, the Fill Factor (FF) of these devices is low (25%), indicating that the series resistance (R_s), which is strongly dependent of the PT thickness, is too large. This large R_s value is compensated by TiO₂/PT interface morphology and by FTO/TiO₂ and TiO₂/PT interface phenomena producing preferential paths in which the internal electrical field is higher, improving the device efficiency.     

Photocatalytic hydrogen production over Na2Ti2O4(OH)2 nanotube sensitized by CdS nanoparticles

Na₂Ti₂O₄(OH)₂ nanotubes were obtained by hydrothermal reaction of TiO₂ with concentrated NaOH solution. CdS nanoparticles were then decorated on Na₂Ti₂O₄(OH)₂ nanotubes through partial ion-exchange method. The composite photocatalysts were characterized by X-ray diffraction (XRD), ultraviolet-visible spectra (UV-vis), transmission electron microscope (TEM), etc. The results showed that CdS nanoparticles of 5-6 nm were anchored on the surface of the Na₂Ti₂O₄(OH)₂ nanotubes. Under irradiation of visible light (λ ≥ 430 nm), the prepared CdS/Na₂Ti₂O₄(OH)₂ showed high photoactivity for hydrogen production.     

Comparison of photostability, optical and structural properties of TiO2/conjugated polymer hybrid composites prepared via different methods

In this work, we have synthesized high quality TiO₂ nanocrystallites by sol-gel method (TiO₂ white (w)) and compared its properties with the ones synthesized by the simple hydrolysis method in aqueous solution (TiO₂ transparent (t)). The TiO₂/MEH-PPV nanocomposites are formed mainly by two ways: (i) Prepared in the form of the colloidal solution by adding the known concentration of the TiO₂ in MEH-PPV and then sonicate it well; (ii) In the thin film form by depositing the above solution over a glass substrate by spin coating. The properties of the resulting dispersions could be tailored by varying the composition and concentration of TiO₂ nanoparticles in CP's. The TiO₂ nanoparticles prepared by both methods show anatase character of TiO₂ as elucidated by X-ray diffraction (XRD) studies. Transmission electron microscopic (TEM) studies reveal that the transparent colloidal suspension of TiO₂ exhibits agglomeration of TiO₂ nanoparticles (size ~ 150-300 nm) and this trend is maintained in the MEH-PPV matrix for TiO₂/MEH-PPV composites as well. However, the composite obtained by mixing MEH-PPV with sol-gel prepared TiO₂(w) shows uniform nanoscale dispersion of TiO₂ (size ~ 20 nm) in MEH-PPV matrix. The UV-VIS absorption, photoluminescence (PL) and lifetime studies confirm the presence of dynamic quenching effect indicating efficient photoinduced charge transfer in TiO₂/MEH-PPV hybrid composites particularly with white TiO₂. It is conjectured that the devices containing TiO₂/MEH-PPV composites for TiO₂ prepared by the sol-gel method should lead to significant improvement in the photovoltaic performance of TiO₂/MEH-PPV hybrid solar cells.     

A new method for surface modification of TiO2/Al2O3 nanocomposites with enhanced anti-friction properties

In this paper, the TiO₂/Al₂O₃ composite nanoparticles were prepared by a hydrothermal method and in situ modified with acrylic acid. It was found that the mean particle size of modified TiO₂/Al₂O₃ composite nanoparticles was about 80 nm with a uniform distribution by the particle size analysis. The modified TiO₂/Al₂O₃ composite nanoparticles can disperse in lubricating oil homogenously for several weeks. The dispersion stabilization of modified TiO₂/Al₂O₃ composite nanoparticles in lubricating oil was significantly improved in comparison with the as-prepared nanoparticles, which was due to the introduction of grafted polymers by surface modification. The formation of covalent bands was identified by Fourier transform infrared spectrum. Under an optimized concentration of 0.1 wt%, the averaged friction coefficient was reduced by 14.75%, when the modified TiO₂/Al₂O₃ composite nanoparticles were used as lubricating oil additivities.     

Photocatalytic characteristics of TiO2 nanotubes with different microstructures prepared under different pulse anodizations

In this work, different positive voltages of a pulsed waveform, time intervals and electrolyte stirring were used to prepare by anodization of pure titanium plates, a series of TiO₂ thin films based on nanotubes with different morphologies and dimensions. Electrolyte stirring results in large size TiO₂ nanotubes with uneven surface morphology and less oriented directions. The titanium plates containing the TiO₂ thin films were further annealed at 450 °C for 30 min under air. It was found that only crystalline anatase was formed under this condition. Both methylene blue degradation and antibacterial tests against Escherichia coli were performed to evaluate the photocatalytic performance of these TiO₂ films. Better methylene blue degradation ability was achieved by TiO₂ nanotubes prepared under electrolyte stirring. However, the antibacterial ability of the annealed TiO₂ nanotubes was affected by their inner diameter rather than their length. It is also concluded that the anodized TiO₂ nanotube arrays fabricated in this work are promising for photo-induced methylene blue degradation and bacteria killing applications.     

Flame-made single phase Zn2TiO4 nanoparticles

Using zinc naphthenate and titanium tetra isopropoxide (1:1 mol.%) dissolved in ethanol as precursors, single phase Zn₂TiO₄ nanoparticles were synthesized by the flame spray pyrolysis technique. The Zn₂TiO₄ nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The BET surface area (SSA_BET) of the nanoparticles was measured by nitrogen adsorption. The average diameter of Zn₂TiO₄ spherical particles was in the range of 5 to 10 nm under 5/5 (precursor/oxygen) flame conditions. All peaks can be confirmed to correspond to the cubic structure of Zn₂TiO₄ (JCPDS No. 25-1164). The SEM result showed the presence of agglomerated nanospheres with an average diameter of 10-20 nm. The crystallite sizes of spherical particles were found to be in the range of 5-18 nm from the TEM image. An average BET equivalent particle diameter (d_BET) was calculated using the density of Zn₂TiO₄.     

NO treated TiO2 as an efficient visible light photocatalyst for NO removal

In this study, we report that nitrogen doped TiO₂ could be achieved via thermal treatment of Degussa P25 TiO₂ in NO atmosphere directly (P25-NO). The samples were characterized with XRD, XPS, and FT-IR. The characterization results suggested that nitrogen species were interstitially doped in P25-NO during the NO thermal treatment process. In comparison with P25, the P25-NO exhibited significantly enhanced photocatalytic activities under visible light irradiation (λ > 420 nm) for gaseous NO removal. On the basis of electronic band structure theory, we proposed a possible mechanism for the enhanced visible light driven photocatalytic oxidation process over the interstitial N doping P25-NO samples. This work could not only deepen understanding of the enhanced photoactivity originated from interstitial N doping in TiO₂, but also provide a facile route to prepare nitrogen doped TiO₂ for environmental and energy applications.     

Sputter deposition and surface treatment of TiO2 films for dye-sensitized solar cells using reactive RF plasma

Sputter deposition followed by surface treatment was studied using reactive RF plasma as a method for preparing titanium oxide (TiO₂) films on indium tin oxide (ITO) coated glass substrate for dye-sensitized solar cells (DSCs). Anatase structure TiO₂ films deposited by reactive RF magnetron sputtering under the conditions of Ar/O₂(5%) mixtures, RF power of 600 W and substrate temperature of 400 °C were surface-treated by inductive coupled plasma (ICP) with Ar/O₂ mixtures at substrate temperature of 400 °C, and thus the films were applied to the DSCs. The TiO₂ films made on these experimental bases exhibited the BET specific surface area of 95 m²/g, the pore volume of 0.3 cm²/g and the TEM particle size of ∼ 25 nm. The DSCs made of this TiO₂ material exhibited an energy conversion efficiency of about 2.25% at 100 mW/cm² light intensity.     

Solution growth of KDP single crystals doped with titanium dioxide nanoparticles

Pure KDP single crystals and KDP crystals doped with TiO₂ nanocrystals were grown by the method of temperature reduction from aqueous solutions. Adsorption of the phosphate-ions on the surface of TiO₂ particles was studied by FTIR spectroscopy. It was shown that the nanoparticles with adsorbed H₂PO₄^– and (H₂PO₄)₂^2–anions were incorporated predominantly into the positively charged face (1 0 1) of the pyramidal sector of KDP. High-resolution X-ray three-crystal diffractometry (TCD) investigation of the as grown samples of KDP + TiO₂ revealed the presence of the turns of the growth layer “stacks” up to 3 arcsec in the growth sectors {1 0 0} and {1 0 1}. The observed thickness of these “stacks” was of the order of 20–30 μm. For KDP + TiO₂ crystals there was found a relative change of the crystal lattice parameter (Δd/d) caused by incorporation of TiO₂ nanoparticles into the boundaries of the growth layers. This gave rise to the formation of a semicoherent binding on the interface between the captured TiO₂ and the matrix. No essential influence of the nanoparticles on the laser damage threshold of KDP with 10⁻⁵ wt.% of TiO₂ was established.