Nanoporous TiO2 Film Research Articles

Synergy of sodium and WO3 for the formation of anatase-based TiO2 nanoporous films with enhanced photoactivity

This study examines the structural evolution and photoactivity of sputter-deposited TiO2-WO3 films upon annealing in terms of the role played by sodium contained in glass substrates. Even though it is generally agreed that sodium contaminates TiO2 photocatalysts, we discover that the sodium content of the glass is a key factor in drawing out the incorporated WO3 and retarding TiO2 grain growth, ultimately forming a nanoporous film with an anatase matrix. It is found that an alkylsiloxane monolayer on this film decomposes much faster than the layer on a film that is free of sodium uptake. The enhancement of the film′s photocatalytic activity is elucidated by structural analysis.

Influence of co-existing alcohol on charge transfer of H2 evolution under visible light with dye-sensitized nanocrystalline TiO2

The dye-sensitized nanocrystalline TiO2 colloidal suspension in aqueous systems has been applied for H2 evolution in the photocatalytic water splitting under visible irradiation. We have reported that the conversion efficiency is strongly influenced by the combination of dye-sensitizers and co-existing species in the suspension system. We herein analyze the fluorescence quenching of the dye-sensitized TiO2 nanoparticles in the suspensions and photoelectrochemical properties of dye-sensitized nanoporous TiO2 films. The combination of dye-sensitizers and co-existing species dominates the charge recombination resulting in significant differences in the efficiencies of the proton reduction into H2.

Immobilization of glucose oxidase into a nanoporous TiO2 film layered on metallophthalocyanine modified vertically-aligned carbon nanotubes for efficient direct electron transfer

Glucose oxidase (GOD) was adsorbed into a nanoporous TiO2 film layered on the surface of an iron phthalocyanine (FePc) vertically-aligned carbon nanotube (CNT) modified electrode. A Nafion film was then dropcast on the electrode's surface to improve operational and storage stabilities of the GOD-based electrode. Scanning electron microscopy (SEM) micrographs revealed the formation of FePc and nanoporous TiO2 nanoparticles along the sidewall and the tip of CNTs. Cyclic voltammograms of the GOD electrode in neutral PBS exhibited a pair of well-defined redox peaks, attesting the direct electron transfer of GOD (FAD/FADH2) with the underlying electrode. The potential of glucose electro-oxidation under nitrogen was ∼+0.12V with an oxidation current density of 65.3μAcm−2 at +0.77V. Voltammetric and amperometric responses were virtually unaffected by oxygen, illustrating an efficient and fast direct electron transfer. The modification of the CNT surface with FePc resulted in a biosensor with remarkable detection sensitivity with an oxygen-independent bioelectrocatalysis. In deaerated PBS, the biosensor displayed average response time of 12s, linearity from 50μM to 4mM, and a detection limit of 30μM (S/N=3) for glucose.

Preparation of nanoporous TiO2 films for DSSC application by a rapid atmospheric pressure plasma jet sintering process

We investigate the nanoporous TiO2 films sintered by atmospheric pressure plasma jets (APPJs) and their applications as photoanodes of dye-sensitized solar cells (DSSCs). A 30-s APPJ-sintered nanoporous TiO2 layer exhibits an additional absorption band between 400 and 500 nm in wavelength, attributed to incomplete removal of the organic solvents in the pastes. For TiO2 layers sintered by APPJs for 60 s and beyond, the absorption spectra are nearly identical to those of a conventional 15 min, 510 °C calcined sample. The XRD and XPS results indicate similar characteristics for APPJ-sintered and furnace-sintered TiO2 films. A DSSC with a 30-s APPJ-sintered TiO2 photoanode shows poor cell efficiency with an extremely large TiO2/dye/electrolyte electron transport interfacial resistance and a short carrier lifetime. As the APPJ treatment time reaches 60 s and beyond, the power conversion efficiencies become comparable to that of a sample with a 510 °C conventionally calcined TiO2 photoanode. Our experimental results verify that a 60-s APPJ sintering process is sufficient to replace a conventional 15 min, 510 °C furnace calcination process for TiO2 photoanodes of DSSCs. The ultra-short sintering process is made possible by the synergistic effect of the temperature and the reactivity of the APPJ, which can lower the fabrication cost.

Effect of Counter-ion and Solvent on the Morphology and Barrier Layer Properties of Nanoporous/Nanotubular TiO2 Films Grown by Anodization in Fluoride Containing Media

Nanoporous TiO2 films were formed by anodization in three different electrolytes fluoride-containing, at different formation potentials for 2.5 hours. Film topography and morphology were characterized by AFM and SEM images, respectively. Aditionally, an in-situ study of the anodic film growth was carried out by electrochemical impedance spectroscopy. The growth ratio for the pore size obtained from SEM images was not directly proportional to the barrier layer growth ratio derived from EIS. The main role of the counter-ion seems to be insertion of fluoride ions altering the properties of the formed oxide, decreasing the film resistance and causing a more aggressive chemical attack, observed in the increase in pore size and film roughness. The solvent tunes the chemical attack of the anodic layer, and could also be establishing the titanium oxide stoichiometry, thus altering oxygen vacancies in the film and the migration of fluoride ions through the oxide lattice.

Optical coatings on f iber for relative-humidity sensing applications

A fiber-optic relative-humidity (RH) sensor composed of multilayer of porous dielectric films is proposed. The transducer deposited on fiber end-face is multilayer coating consisting of nano-porous TiO2 and SiO2 films, which forms a low-fineness Fabry-Perot (F-P) filter with one of minimum reflections at about 1 350-nm wavelength. The dielectric thin films realized by e-beam evaporation without ion-source assistance have columnar and porous structures, which exhibit sensitivity to RH change of environments. When the sensor is exposed to an environment of RH change from 10.9% to 92.8%, experimental results demonstrate 77.9-nm shift of characteristic wavelength.

Effect of pH on the Barrier Layer of TiO2 Nanoporous Films Potentiostatically Grown in Aqueous Media Containing Fluoride Ions

The effect of pH on the potentiostatic anodization growth of TiO2 film in 0.1 M HClO4/0.05 M NH4F solution was electrochemically characterized by EIS measurements. The increase in electrolyte pH modified considerably the voltammetric behavior of Ti electrode by diminishing the active dissolution region of the material and giving rise to two current plateaus that indicate modification of properties of the formed oxide. Five different formation potentials (EF) were selected from this study to potentiostatically grow oxide films, and to evaluate their morphology, the modification of their properties, and their interaction with the electrolyte. SEM images of the formed films showed that the EF increase resulted in a larger diameter of the pores formed, but an increase in pH led to a decrease in this parameter. In-situ characterization by EIS indicated that pH increase in the electrolyte used in anodization causes lower interaction between the formed oxide and F− ions in the solution due to the change in the surface charge excess of Ti oxide, resulting in an increase in the compact layer resistance and a decrease in both the time constant associated with F− ion adsorption and F− ion diffusion coefficient within the compact oxide layer.

Collaborational effect of heterolytic layered configuration for enhancement of microwave heating

Microwave irradiation efficiently heats up the microwave-inert materials in the range of applied frequencies when two microwave-inert materials are brought into contact in the layered configuration. This heating is applied for annealing TiO2 nanoporous films for dye-sensitized solar cells achieving a one order of magnitude more rapid annealing process for comparable performances.

Preparation of Photoanode of Dye-Sensitized Solar Cell by Electrospining

The dye-ability and electron transport property of TiO2 film are the two critical factors in determining efficiency of the the dye sensitized solar cell (DSSC). Increasing dye adsorption which increases the light harvesting is usually achieved by using nanoporous or nanoparticle TiO2 films. Electron transport is determined by the inter-particle resistance of TiO2 film. Electrospinning is a viable method for forming porous structure materials with high surface area and less immersing time. In this study, it was found that electrospinning is able to achieve good solar cell performance due to the high electron transport caused by the nano-pores in the TiO2 film.

Study on Properties of Dye-Sensitized TiO<sub>2</sub> Films

TiO2nanoporous thin films were fabricated by the hydrothermal method using tetrabutyl titanate as the precursor and used to assemble dye-sensitized solar cells. The properties which include the short-circuit photocurrent(ISC),open-circuit voltage(VOC) and the fill factor(FF)of the solar cells were tested. The microstructure of titania films were modulated by changing the hydrothermal reaction temperature and the amount of PEG and the influence on the properties of the solar cells by titania films weas studied. The results indicate that there are better photoelectric properties for TiO2films at the hydrothermal temperature fo 230°C and the additive amount of PEG being 5% of the mass of TiO2. ISC,VOCand FF was 12.42mA, 681mV, and 0.52 respectively, the photoelectric conversion efficiency(η)was 4.37%．

Fabrication of nanoporous TiO2 films with novel surface morphology on conducting glass (FTO) substrate

The crystalline structure and surface morphology of TiO2 semiconductor coating play an important role in the conversion efficiency of dye-sensitized solar cells. In order to obtain TiO2 coating with controllable morphology and high porosity, nanoporous TiO2 films were fabricated on conducting glass (FTO) substrates, Ti thin films (1.5–2 μm) were deposited on conducting glass (FTO) substrates via the DC sputtering method, and then electrochemically anodized in NH4F/ethylene glycol solution. The crystalline structure and surface morphology of the samples were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. The influences of anodizing potential, electrolyte composition, and pH value on the surface morphology of nanoporous TiO2 films were extensively studied. The growth mechanism of nanoporous TiO2 films was discussed by current density variations with anodizing time. The results demonstrate that nanoporous TiO2 films with high porosity and three-dimensional (3D) networks are observed at 30 V, when the NH4F concentration in ethylene glycol solution is 0.3% (mass fraction) and the electrolyte pH value is 5.0.

(Photo)Electrochemical characterization of nanoporous TiO2 and Ce-doped TiO2 sol–gel film electrodes

Focusing on effects of charge separation limitations and doping on the photoactivity of TiO2, different nanoporous TiO2 film electrodes were prepared by the sol–gel or the suspension methods, respectively. X-ray diffraction (XRD) characterization revealed for all undoped TiO2 electrodes (TiO2-SG and TiO2-P25) mixed phases of anatase and rutile, whereas the 2%CeTiO2-SG electrode showed only anatase phase pattern, revealing that the incorporation of Ce ions prevented phase transformation. The (photo)electrochemical characterization of the nanoporous TiO2 film electrodes was performed by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in the dark and under UV irradiation, respectively. A photocurrent maximum depending on the film thickness was observed at about 13μm for all TiO2 electrodes investigated. The photocurrent responses of Ce-doped sol–gel TiO2 electrodes were comparably low, indicating an enhanced electron–hole recombination at the Ce ion levels within the band gap of TiO2. A porous electrode model was adapted for the fitting of the experimental EIS data. The potential and incident radiant power density dependence of the heterogeneous charge transfer reaction (RCT) and the coupled transport of photogenerated electrons to the collector electrode (ZS) were ascribed to promoted charge carrier separation and migration, i.e. electron drift dominated over diffusion. Consequently, the dependence of different discrete impedance elements was discussed, supposing a macroscopic electric field across the 3D array of interconnected TiO2 nanoparticles that form the film electrodes. Besides photocurrent doubling, addition of methanol as a hole scavenger revealed the limitation of the overall reaction by the heterogeneous charge transfer reaction, in particular at high radiant power densities.

Influence of sintering on the structural and electronic properties of TiO2 nanoporous layers prepared via a non-sol–gel approach

In this work, a nonaqueous method is used to fabricate thin TiO2 layers. In contrast to the common aqueous sol–gel approach, our method yields layers of anatase nanocrystallites already at low temperature. Raman spectroscopy, electron microscopy and charge extraction by linearly increasing voltage are employed to study the effect of sintering temperature on the structural and electronic properties of the nanocrystalline TiO2 layer. Raising the sintering temperature from 120 to 600 °C is found to alter the chemical composition, the layer’s porosity and its surface but not the crystal phase. The room temperature mobility increases from 2 × 10−6 to 3 × 10−5 cm2/Vs when the sinter temperature is increased from 400 to 600 °C, which is explained by a better interparticle connectivity. Solar cells comprising such nanoporous TiO2 layers and a soluble derivative of cyclohexylamino-poly(p-phenylene vinylene) were fabricated and studied with regard to their structural and photovoltaic properties. We found only weak polymer infiltration into the oxide layer for sintering temperatures up to 550 °C, while the polymer penetrated deeply into titania layers that were sintered at 600 °C. Best photovoltaic performance was reached with a nanoporous TiO2 film sintered at 550 °C, which yielded a power conversion efficiency of 0.5 %. Noticeably, samples with the TiO2 layer dried at 120 °C displayed short-circuit currents and open circuit voltages only about 15–20 % lower than for the most efficient devices, meaning that our nonaqueous route yields titania layers with reasonable transport properties even at low sintering temperatures.

Adsorption dynamics of the N719 dye on nanoporous titanium oxides studied by resonance Raman scattering and Fourier transform infrared spectroscopy

The adsorption dynamics of the (Bu4N)2[Ru(dcbpyH)2(NCS)2] (N719) dye on nanoporous TiO2 films were examined. Resonant Raman scattering and Fourier transform infrared absorption spectroscopy were used to study TiO2 thin films that had been sensitized in ethanol solutions for different immersion times. Our experimental results suggested that the N719 dyes were anchored to the surface of nanoporous TiO2 through bidentate chelating or bridging forms between the carboxylate groups of the N719 dyes and the TiO2 at all stage of adsorption, while the strength of intermolecular π–π interactions between adjacent bipyridine rings became stronger as the density of the adsorbed dyes increased.

Effect of MgO-coated TiO2 thin film on ITO/TiO2/MEH–PPV/Au solar cells

In this study, the effects of magnesium oxide-coated titanium dioxide (MgO–TiO2) thin film on the performance of ITO/TiO2/MEH–PPV/Au solar cells were investigated. The ITO/MgO–TiO2/MEH–PPV/Au solar cells were fabricated using nano-porous MgO–TiO2 layer infiltrated with poly (2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH–PPV) to form an active layer on the solar cells. The TiO2 films were prepared by a sol-gel dip-coating technique onto indium tin oxide (ITO) coated glass substrates. To vary the MgO loading content, the TiO2 films were dip-coated in 1 × 10−1, 1 × 10−2, 1 × 10−3, 1 × 10−4 and 1 × 10−5 M solutions of magnesium acetate. After sintering at 450 °C, thin insulating MgO layers were formed on the nano-porous TiO2 films. The ITO coated glass and gold (Au) were used as cathode and anode, respectively. The results revealed that the MgO–TiO2 modification increased open circuit voltage, fill factor, and power conversion efficiency of the ITO/TiO2/MEH–PPV/Au solar cells. These improvements resulted from negative shifts of flat-band potentials of the nano-porous TiO2 films once coated with a thin insulating MgO layer. Moreover, the MgO coating was found to improve the life time of the solar cells.

Porous TiO2 Photonic Band Gap Materials by Anodization

Self-organized TiO2 nanomaterials grown by anodic oxidation of Ti foils have attracted broad scientific interest due to their wide potential applications. The majority of anodic TiO2 nanostructures studied to date are in the form of self-ordered nanotube arrays. Here we report an exotic type of multilayered nanoporous TiO2 films that are conveniently fabricated by anodizing Ti foils of high imperfection levels using a novel multipulse anodization method. The fabricated TiO2 films feature tens of well-defined layers whose long-range structural periodicity leads to photonic band gaps in the visible wavelengths and vivid film colors. Moreover, the optical responses (or the film color) of the fabricated TiO2 multilayers not only are sensitive to environmental chemicals but also can be electrically switched on and off repeatedly, demonstrating their novel potential applications for, e.g., colored smart windows or electric display boards.

Distinctive Optical Properties of Polysilane/TiO2 Nanocomposite Films

Optimal technological conditions for the fabrication of nanocomposite films based on the poly(di-n-hexylsilane) (PDHS) incorporated into a TiO2 nanoporous film have been developed, and such composites are fabricated. Their optical spectra have been investigated in a wide temperature range (15–330) K. It is shown that the distinctive feature of nanocomposite films is related to the predominant fluorescence band of aggregates with respect to the fluorescence bands corresponding to the gauche- and trans-conformations of polymer chains and to the observation of the intense fluorescence band of aggregates even at room temperature.

Improvement in Performances of Dye-Sensitized Solar Cell with SiO<SUB>2</SUB>-Coated TiO<SUB>2</SUB> Photoelectrode

The pure TiO2 and the nano-porous SiO2-coated TiO2 (STO) films were deposited on the FTO substrates by spray technique for the application of dye-sensitized solar cells (DSSCs). XRD pattern shows the pure TiO2 and STO films exhibits the same structure. We found that there is no much difference in dye absorption between the STO and the pure TiO2 films. The electrochemical impedance spectra reveal that insulating nature of the porous SiO2 increases surface resistance of the TiO2 film and supresses back transfer of the photogenerated electrons to the electrolyte. The field-emission scanning electron microscopy (FE-SEM) and energy dispersion X-ray spectroscopy (EDS) reveal that the surface morphology and the existence of SiO2 layer on the surface of the TiO2 films, respectively. The photoelectrochemical results show that the short-circuit photocurrent (J(SC)) increased from 16.73 mA cm(-2) to 18.31 mA cm(-2) and the open-circuit voltage (V(OC)) value changed from 0.71 V to 0.74 V for the STO films. The efficiency of cell has been greatly improved from 8.25 to 9.3%.

Photoelectricity performance research based on the sol-modified thin film electrode of dye-sensitized solar cells

Nanoporous film electrode is a crucial composition of dye-sensitized solar cells, which influences the photoelectric conversion performance. To improve the property of the photoelectrode, different modification methods by using different concentrations of TiO2 sol are investigated. The crystallite size and phase of the nanoporous TiO2 particles and the TiO2 sol after sintering are studied with X-ray diffraction. The microstructure morphologies of the conductive glass and the films are determined by the high resolution transmission electron microscopy and the field emission scanning electron microscopy. The influences on electron lifetime n and the electron transit time d are analyzed by intensity-modulated photocurrent spectroscopy and photovoltage spectroscopy from the mechanisms of electron transport and back reaction kinetics. It is found that the back reactions are well suppressed under dark conditions after sol modifications. n is effectively extended and d is also shorten correspondingly by any kind of sol treatment. The short-current density and the photoelectric conversion efficiency are increased by 10.9% and 11.9% separately, when 0.10 molL-1 sol modification is applied both to the conductive glass and to the nanoporous TiO2 film at the same time.

Effect of ZnS and CdS coating on the photovoltaic properties of CuInS2-sensitized photoelectrodes

This paper presents a facile route to prepare CuInS2 (CIS) as a light absorber for solar cells. CIS was effectively deposited inside the pores of the nanoporous anatase titanium dioxide photoelectrodes with a high coverage degree. The performance of the CIS-sensitized TiO2 photoelectrodes could be substantially enhanced through a sequential coating with a CdS shell and a ZnS layer via successive ionic layer adsorption and reaction (SILAR). Electrochemical impedance spectrum (EIS), cyclic voltammetry (CV), and photocurrent experiments were employed to study the effect of CdS and ZnS coating. The CdS shell provided high surface coverage to passivate surface states. The ZnS layer acted as a potential barrier which was similar to an insulating layer employed in the conventional metal–insulator–semiconductor solar cells, allowing the adjustment of the electric field and potential distribution in the interface between the electrodes and the electrolyte and suppressing the dark current, resulting in further increase in the photoresponse. We found that the thickness of the nanoporous TiO2 films had a dramatic impact on the optical response and efficiency of CIS-sensitized photoelectrodes, with the 5 μm sample performing much better than the 11 μm sample, which underlined the importance of charge (or ion) transport kinetics.