Fabrication and characterization of photoelectrode thin films with different morphologies of TiO2 nanoparticles for dye-sensitized solar cells

This study deals with the fabrication of three different morphologies of TiO2 nanoparticles to fabricate two-layer photoelectrode thin film for dye-sensitized solar cells (DSSC). The four different TiO2 morphologies are titania nanotubes (Tnt), TiO2 nanoparticles (H220), TiO2 nanoparticle (SP) and commercial DP-25 nanoparticles (P-25). To prepare the thin films of the photoelectrodes, the first layer is coated by H220 TiO2 nanoparticles, and the second is coated by 3 kinds of materials optimally proportionally mixed - P25, SP and Tnt. The photoelectric conversion efficiency of DSSCs with photoelectrodes fabricated using H220 reached 6.31%. Finally, the TiO2 nanaomaterials with four different morphologies were used to prepare a two layer photoelectrode with the structure of H220/P25-Tnt-SP which was combined with a Pt counter electrode to assemble DSSCs. These DSSCs had photoelectric conversion efficiencies of as high as 7.47%.     

Fabrication and characterization of nano TiO2 thin films at low temperature

Anatase TiO₂ thin films were successfully prepared on glass slide substrates via a sol–gel method from refluxed sol (RS) containing anatase TiO₂ crystals at low temperature of 100 °C. The influences of various refluxing time on crystallinity, morphology and size of the RS sol and dried TiO₂ films particles were discussed. These samples were characterized by infrared absorption spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission-scanning electron microscopy (FE-SEM) and UV–vis absorption spectroscopy (UV–vis). The photocatalytic activities of the TiO₂ thin films were assessed by the degradation of methyl orange in aqueous solution. The results indicated that titania films thus obtained were transparent and their maximal light transmittance exceeded 80% under visible light region. The TiO₂ thin films prepared from RS-6 sol showed the highest photocatalytic activity, when the calcination temperature is higher than 300 °C. The degradation of methyl orange of RS-6 thin films reached 99% after irradiated for 120 min, the results suggested that the TiO₂ thin films prepared from RS sol exhibited high photoactivities.     

Photoelectrochemical performance of Ag nanoparticles on TiO2 films prepared by aerosol pyrolysis

Titanium dioxide (TiO₂) layers were prepared by the pyrolysis of an ethanolic solution of di-iso-propoxy-titanium bis(acetylacetonate) in aerosol form, and then electrodeposited with Ag nanoparticles on their surface. The morphology and photoelectrochemical properties of the resulting Ag nanoparticles (NPs) on TiO₂ films were found to be significantly tuned by varying the electrodeposition time in an aqueous electrolyte containing AgNO₃ and KNO₃. Photocurrent density–voltage curves and electrochemical impedance spectra revealed that the Ag NPs remarkably improved the short-circuit current density and open circuit voltage, and considerably reduced the electrochemical impedance. Therefore, Ag NPs deposition enhanced the photo-absorption of the TiO₂ layer, excited photoelectrons by localised surface plasmon resonance, promoted photo-induced charge separation, and prevented electron–hole recombination.     

Morphological characterization of TiO2 thin films

Films prepared by reactive magnetron sputtering always present some structural and morphological heterogeneities.In this work, optical parameters, n(λ), k(λ) and E₀, of TiO₂ thin films were obtained, using only optical transmittance measurements. Films were described according to Abèles's model. Using a mono-oscillator type dispersion curve for the refractive index and a Lorentzian type curve for the absorption coefficient, we were able to demonstrate that the films were optically equivalent to a porous layer, with some dispersion in film thickness.The detailed analysis of the experimental transmittance data, fitted between 330 nm to 2200 nm, also enabled us to correlate the effective refractive index of each film with its deposition conditions.     

Absorption spectra and photocurrent densities of Ag nanoparticle/TiO2 composite thin films with various amounts of Ag

To study the absorption spectra and photocurrent densities of metallic Ag nanoparticle/titania (Ag NP)/TiO₂ composite thin films, COMP-Agn, with various amounts of Ag (10 mol% ≤n ≤80 mol%) were fabricated on a quartz glass substrate at 600 °C using the molecular precursor method. Respective precursor solutions for Ag-nanoparticles and titania were prepared from Ag salt and a titanium complex. Apart from a surface plasmon resonance (SPR) peak around 400 nm, additional wide-range absorption spread in the wide vis-region at wavelengths greater than 400 nm was observed in the composite thin films. The wide-range absorption is due to a tip–tip plasmon mode, intraparticle plasmonic coupling of tip and cavity resonances (LSPR). The absorption spectral patterns could be categorized into three types, depending on the Ag NP content. The photocurrent density of the TiO₂ thin film and COMP-Agn was measured under natural potential by a conventional three-electrode method using a Ag plate as a counter electrode. The photocurrent densities of COMP-Agn were comparable to those of the three patterns categorized according to their absorption spectra. The cathodic photocurrent densities generated by COMP-Agn with Ag content greater than 45 mol% could be observed under both UV- and visible (vis-) light irradiation. The enhanced photocurrent density was observed till the Ag content was increased up to 70 mol%. Under dark conditions, cathodic current densities were 1/5–1/20 of those under vis-light irradiation probable due to chemical redox reactions that may occur to the system. On the basis of photoexcited electron transfer from Ag NPs to the TiO₂ conduction band and the electrical conductivity of COMP-Agn, the excellent response to vis-light and major factors affecting the photoresponse and photocurrent polarity were clarified by LSPR.     

掺银TiO2薄膜的亲水特性

采用溶胶—凝胶法及浸渍提拉法在普通的载玻片上制得含不同掺银量的TiO2薄膜，通过对薄膜及相应粉体的XRD、XPS及薄膜致密度的测量，分析了银的掺杂量对TiO2薄膜亲水特性的影响。结果表明：TiO2薄膜中银的掺杂量≤0．635mol％时有利于TiO2薄膜亲水性能的改善；表面羟基和表面桥氧的含量对TiO2薄膜的亲水性能均有直接影响。     

Fabrication and properties of AgInTe2 thin films

AgInTe₂ thin films were deposited onto glass substrates by pulsed laser ablation. Their structure, composition, and transmission and reflection spectra in the range 0.5 to 2.5 Μm were studied. The absorption coefficient, energies of band-to-band transitions, and parameters of crystal-field and spin-orbit splittings were determined. The results obtained for the AgInTe₂ films are in good agreement with the data for bulk crystals.     

Photoelectrochemical properties of plasma-deposited TiO2 thin films

Photoanodes were fabricated from TiO₂ films deposited onto titanium substrates by plasma-enhanced chemical vapor deposition. The photocurrent-wavelength and photocurrent-voltage properties of the anodes were determined and compared with those of thermally grown TiO₂ photoanodes. The plasma-deposited photoanodes displayed quantum efficiencies higher than those for the thermally grown films and comparable with those reported for single-crystal rutile. The microstructure of the plasma-deposited films appeared to be primarily responsible for the high quantum efficiencies.     

Fabrication and self-cleaning properties of reduced graphene oxide/TiO2:Nd nanohybrid thin films

Self-cleaning is an important technique for environmental purification. In this work, a series of reduced graphene oxide (rGO)-TiO₂:Nd nanohybrid films were deposited and characterized by various analysis techniques. The rGO incorporation and Nd doping obviously enhanced the self-cleaning properties including photocatalytic activity, photo-induced superhydrophilicity, and conductivity of the TiO₂ film. The effect of Nd content and rGO proportion on the self-cleaning properties was also studied. The efficient synergistic effect of rGO and Nd on the self-cleaning properties was observed and mainly ascribed to interface-induced effect. The measurement of band energy level and optical conductivity confirmed the interface-induced effect.     

Fabrication of nanoporous TiO2 films through Benard-Marangoni convection

Fabrication of nanoporous TiO₂ thin films through Benard-Marangoni convection is reported. Variation of pore size and morphology in the sol-gel deposited metal-oxide films is investigated as a function of ambient humidity and air flow velocity during film drying, sol concentration, and addition of water/acid to the sol. Received: 8 August 2001 / Accepted: 18 August 2001     

Interfacial nature of Ag nanoparticles supported on TiO2 photocatalysts

Silver nanoparticles supported on anatase TiO₂ nanoparticles have been prepared by deposition–precipitation, and characterized by X-ray photoelectron spectroscopy, including scanning electron microscopy, X-ray diffraction crystallography, Raman, and UV–visible absorption spectroscopy. The Ag 3d peak and the X-ray diffraction patterns show characteristics of purely metallic Ag, with no indication of Ag oxide species. Depth-profiling X-ray photoelectron spectroscopy with Ar⁺ ion beam sputtering show a significant change in Ti 2p, and an asymmetric broadening of Ag 3d to a higher binding energy side. A decrease in major Ti 2p _3/2 at 459.2 eV and a significant increase in lower binding energy peak are due to change in oxidation state of Ti from +4 to +3/+2. A broadening of Ag 3d peak with sputtering time is tentatively assigned to a final state quantum size effect. Upon annealing the deposition–precipitation sample, no significant change in Ag 3d peak is observed, while Ti 2p and O 1s XPS intensities are reduced, plausibly due to change in analyzed surface area for TiO₂. The photocatalytic activity for the photodegradation of methyl orange is dramatically reduced upon high Ag-loading, compared to bare TiO₂. The X-ray photoelectron spectroscopy of Ag on TiO₂ prepared by an electrochemical deposition reveals that Ag is also metallic, with no evidence of an oxide form. Upon annealing the electrodeposited sample, the Ag 3d peak shifts by +0.3 eV, while the Ti 2p and O 1s show no critical change in intensity and peak position.     

Nb-doped TiO2 thin films for photovoltaic applications

Polycrystalline titania and Nb:TiO₂ thin films were deposited by RF magnetron sputtering. The influence of post-deposition annealing in vacuum and hydrogen atmosphere on the structure, morphology, oxidation states and optical properties was studied by X-ray diffraction, atomic force microscopy, XPS and UV–VIS spectroscopy. The heat treatment of titanium dioxide thin films in vacuum and H₂ atmosphere induces structural and morphological changes. The band gap narrowing was observed for the transparent as-deposited Nb:TiO₂ films, while annealing at 420 °C in H₂ atmosphere resulted in an enhancement of the electrical conductivity. Further on, TiO₂/p-CdTe photovoltaic devices with efficiency of 1.8% were fabricated and their characteristic ‘enhancement’ is discussed.     

Fabrication of BaTiO3 thin films through ink-jet printing of TiO2 sol and soluble Ba salts

BaTiO₃ thin films were prepared by ink-jet printing aqueous solutions of TiO₂ sol and soluble Ba salts. Higher pH values (pH > 13) as well as higher than stoichiometric Ba (Ba:Ti = 1.1:1) salt additions were required to compensate for the different aqueous solubilities of the Ba and the Ti. Impedance spectroscopy of the samples shows the thin film samples to have similar activation energy with bulk samples prepared through low temperature aqueous synthesis. The relative permittivity of the thin films (∼ 280) was lower than the bulk pellets (∼ 2750) which was attributed to the lower temperature heat treatment for the thin films.     

Chemical vapor deposition of doped TiO2 thin films

Niobium-, tantalum- and fluorine-doped TiO₂ films were made by atmospheric pressure chemical vapor deposition from titanium alkoxides mixed with niobium ethoxide, tantalum ethoxide and t-butyl fluoride respectively. 4% H₂ in N₂ was used as the carrier gas and the deposition temperatures were in the range 400–600°C. The resistivities of the films increased dramatically with film thickness. For highly doped films 1 μm thick resistivities as low as 0.01 Ω cm were achieved.     

阴极电沉积法同时制备TiO2薄膜和颗粒

采用阴极电沉积法电解钛酸四丁酯同时制备TiO2膜和颗粒,并讨论沉积条件对产物的影响。pH值是控制产物的主要参数。对比在紫外灯光照下不同产物不同时间下的光催化性能,比较其差异得出,酸性在pH值为1-2(加浓硫酸2-3mL)时的产物效果最好,光催化性能最好。并横向比较了加入硫酸体积相等的条件下,颗粒和薄膜的光催化效率。     

The fabrication of SnSe/Ag nanoparticles on TiO2 nanotubes

SnSe and silver (Ag) nanoparticles were sequentially deposited on TiO₂ nanotube (NT) by pulsed electrochemical deposition and polyol chemistry process, respectively. The morphological observation under scanning electron microscope (SEM) showed that the average size of SnSe was about 30 nm and the Ag was about 5 nm. Transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED) examination indicated that Ag nanoparticles exhibited a well-defined crystallinity. However, SnSe nanoparticles were amorphous and they turned to crystalline after being annealed at 300 °C in the atmosphere. The photocatalytic behavior of SnSe/Ag-TiO₂ NT was evaluated by UV–vis diffuse reflectance spectra (DRS). The results showed that the deposition of SnSe and Ag nanoparticles increased light absorption intensity in the wavelength range of visible light, which implied that the SnSe/Ag-TiO₂ NT is a promising ternary hybrid material in photocatalysis.     

Photocatalytic properties of nanocrystalline TiO2 thin film with Ag additions

In the present study, nanocrystalline TiO₂/Ag composite thin films were prepared by a sol–gel spin coating technique. While, by introducing polystyrene (PS) microspheres, porous TiO₂/Ag films were obtained after calcining at a temperature of 500 °C. The as-prepared TiO₂ and TiO₂/Ag thin films were characterized by X-ray diffractometry, and scanning electron microscopy to reveal the structural and morphological differences. In addition, the photocatalytic properties of these films were investigated by degrading methylene blue under UV irradiation.After 500 °C calcination, the microstructure of PS-TiO₂ film without Ag addition exhibited a sponge-like microstructure while significant sintering effect was noticed with Ag additions and the films exhibited a porous microstructure. Meanwhile, coalescence of nanocrystalline anatase-phase TiO₂ can be observed with respect to the sharpening of XRD diffraction peaks. The photodegradation of porous TiO₂ doped with 1 mol% Ag exhibited the best photocatalytic efficiency where 72% methylene blue can be decomposed after UV exposure for 12 h.     

Effect of TiO2 nanoparticles aggregation in silicate thin coating films on photocatalytic behavior for antifouling materials

Two types of disperser namely, a high speed agitation bead mill and a colloidal mill, were used for the dispersion behavior control of TiO₂ nanoparticles (20 nm in average primary diameter) in an aqueous suspension. A composite thin film raging in thickness from 90 to 400 nm was prepared from the coating suspension following the addition of a commercial ethyl silicate oligomer binder into the prepared suspension by means of a spray coating method. The mean aggregate size of TiO₂ nanoparticles in the aqueous suspension was found to be 80 nm and 290 nm in diameter, respectively, when using the agitated bead and colloidal milling methods. Large aggregates on the order of several hundred nanometers were found to remain in the suspension after colloidal milling. Further, a fine dispersion of TiO₂ nanoparticles in the thin film produced using the agitation bead milling process promoted the photocatalytic activity and enhanced transparency of the film for visible light. The aggregate structure of TiO₂ nanoparticles in an aqueous suspension was well maintained in the film prepared by a spray coating process.     

Low-temperature processing of thin films based on rutile TiO2 nanoparticles for UV photocatalysis and bacteria inactivation

Using a low-temperature, simple, and economic processing technique, TiO₂ nanoparticles (rutile phase) are immobilized in an inorganic matrix and then deposited on glass for bacteria inactivation in water. Using this low thermal budget method (maximum processing temperature of 220 °C), thin films of immobilized TiO₂ nanoparticles are obtained so that practical water decontamination after UV radiation is possible by avoiding the additional step of catalyst separation from treated water. In order to validate the photocatalytic activities of these TiO₂ nanoparticles (prepared as thin films), they were tested for bacteria inactivation in water under UV–A radiation (λ > 365 nm), while extensive characterizations by dynamic light scattering, X-ray diffraction, ultra violet–visible absorption spectroscopy, fourier-transform infra red spectroscopy, and profilometry were also carried out. Despite previous reports on the low or lack of photocatalytic activity of rutile-phase TiO₂, inactivation of Escherichia coli in water was observed when thin films of this material were used when compared with the application of UV radiation alone. Physical characterization of the films suggests that size and concentration-related effects may allow the existence of photocatalytic activity for rutile-TiO₂ as long as they are exposed under UV–A radiation, whereas no effect on bacteria inactivation was observed for thin films in the absence of TiO₂ or radiation. In brief, a low thermal budget process applied to thin films based on TiO₂ nanoparticles has shown to be useful for bacteria inactivation, while possible application of these films on widely available substrates like polyethylene terephthalate materials is expected.