Porous Titania Films Research Articles

The Effects of Annealing Temperatures and Dimethylformamide Doses on Porous TiO2 Films

In this study, we develop a facile and feasible synthetic technique for producing denser porous titanium dioxide (TiO2) films. The porous TiO2 films are effectively prepared using a sol–gel process with dimethylformamide (DMF). The TiO2 solution is synthesized by adjusting DMF doses ranging from 0 to 10 wt%, and the as-grown TiO2 films are further annealed at different temperatures from 300 to 500 °C. The TiO2 films exhibit an asymmetry anatase TiO2 phase as annealing temperatures increase, and a denser structure as DMF doses increase. The optical properties of all samples are studied, and the porous TiO2 obtained by 7.5 wt% DMF dose demonstrates a remarkable transmittance and reflectance of 51.87% and 27.55%, respectively, in the visible region from 350 to 850 nm when compared to the pure TiO2 films. The calculated band gap values range from 3.15 to 3.25 eV. Furthermore, the resistivity of 350 °C-annealed porous TiO2 thin film is determined by the Hall effect, revealing an increase from 4.46 to an of average 4.79 ohm · cm after injecting DMF solvent. These findings have the potential to assist a growing number of optoelectronic applications.

Design and Characterization of Nanostructured Titanium Monoxide Films Decorated with Polyaniline Species

The fabrication of nanostructured composite materials is an active field of materials chemistry. However, the ensembles of nanostructured titanium monoxide and suboxide species decorated with polyaniline (PANI) species have not been deeply investigated up to now. In this study, such composites were formed on both hydrothermally oxidized and anodized Ti substrates via oxidative polymerization of aniline. In this way, highly porous nanotube-shaped titanium dioxide (TiO2) and nano leaflet-shaped titanium monoxide (TiOx) species films loaded with electrically conductive PANI in an emeraldine salt form were designed. Apart from compositional and structural characterization with Field Emission Scanning Electron Microscopy (FESEM) and Raman techniques, the electrochemical properties were identified for each layer using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Based on the experimentally determined EIS parameters, it is envisaged that TiO-based nanomaterials decorated with PANI could find prospective applications in supercapacitors and biosensing.

A Novel Dye-Sensitized Solar Cell Structure Based on Metal Photoanode without FTO/ITO.

Traditional dye-sensitized solar cells (DSSC) use FTO/ITO containing expensive rare elements as electrodes, which are difficult to meet the requirements of flexibility. A new type of flexible DSSC structure with all-metal electrodes without rare elements is proposed in this paper. Firstly, a light-receiving layer was prepared outside the metal photoanode with small holes to realize the continuous oxidation-reduction reaction in the electrolyte; Secondly, the processing technology of the porous titanium dioxide (TiO2) film was analyzed. By testing the J–V characteristics, it was found that the performance is better when the heating rate is slow. Finally, the effects of different electrode material combinations were compared through experiments. Our results imply that in the case of all stainless-steel electrodes, the open-circuit voltage can reach 0.73 V, and in the case of a titanium photoanode, the photoelectric conversion efficiency can reach 3.86%.

Effect of pore morphology and surface roughness on wettability of porous titania films

Surface hydrophobicity/hydrophilicity of titania (TiO2) films, spin-coated on silicon wafers, were tuned by introducing surface mesopores with various morphologies using a triblock copolymer F38 as the template agent of different weight ratios via a sol-gel method. It is found that both the porosity (2.92 ∼ 33.03%) and the surface roughness (0.22 ∼ 0.43 nm for arithmetic mean roughness and 0.28 ∼ 0.58 nm for root mean square roughness) of the films increase monotonically as increasing F38 ratio from 5 to 25 wt%, accompanied by distinct changes of pore morphology from isolated mesopores with pore sizes of 5 ∼ 7 nm to longer worm-like pores (30 ∼ 100 nm in length). The apparent static contact angle (θ*) of the films with isolated mesopores is enhanced from ca. 90.6° to 100.1° as indicated by an increase of the roughness factor with incresing F38 from 5 to 15 wt%, which is in qualitative agreement with the Wenzel’s equation. Interestingly, the films with interconnected worm-like pores show obvious hydrophilicity (θ* = 80.7°) with further increasing F38 ratio higher than 20 wt%. The reversed surface wettability show that not only surface roughness but also pore morphology could significantly affect the wettability of the mesoporous TiO2 films.

Key Factors for Template‐Oriented Porous Titania Synthesis: Solvents and Catalysts

AbstractVarious types of titania nanostructures are synthesized with a polymer‐templated sol–gel method based on the amphiphilic diblock copolymer polystyrene‐b‐polyethylene oxide (PS‐b‐PEO) in combination with selective incorporation of the titania precursor titanium tetraisopropoxide. Custom tailoring of different types of titania morphologies is realized by changing the phase separation behavior of the PS‐b‐PEO template. Particularly, application of solvents from different categories is found to have a major impact upon the phase separation behavior of PS‐b‐PEO and the final titania film morphology. The amount of available hydrochloric acid catalyst during the gelation process is seen as an additional key factor to induce controllable morphological changes. Scanning electron microscopy and grazing incidence small angle X‐ray scattering measurements are carried out to study the surface and inner structure of porous titania films. Systematic analysis and comparison of different characterization results allow attributing the following three factors to the respectively formed titania nanostructure: the surface energy between PS blocks and surrounding solvent, the aggregation behavior of the titania nanoparticles, and the block‐specific selectivity of the used solvent. For all synthesized titania thin films, an anatase‐type crystallization is confirmed through X‐ray powder diffraction.

Sensitization of TiO2 by chemically deposited Cu2S for solar cell: Effect of deposition time on photoelectrochemical performance

In present investigation we have demonstrated Copper Sulphide (Cu2S) as an alternative light harvester for photoelectrochemical (PEC) application. A low-cost chemical route namely chemical bath deposition (CBD) method is efficiently employed for deposition of Cu2S nanoparticles over spin coated porous Titanium Dioxide (TiO2) films. Deposition time in CBD facilitates size tuning of Cu2S nanoparticles on TiO2 which influence the photoelectrochemical response and ultimately on photo conversion efficiency of device. The XRD study confirmed the formation of Cu2S nanoparticles on TiO2. The optical and surface morphological properties further confirm the formation of TiO2/Cu2S architecture and are systematically presented. The photovoltaic properties of Cu2S sensitized TiO2 solar cells are evaluated by constructing FTO/TiO2/Cu2S/polysulphide/carbon coated FTO device structure. The behaviour of electron in the device was study by electrochemical impedance spectroscopy (EIS) measurement. Furthermore, the J-V curve of TiO2/Cu2S hetrostructure photoanodes is used to measure the photo conversion efficiency and the maximum efficiency of 0.193 % is recorded under illumination of simulated sunlight at 50 mW/cm2 condition with maximum electron life time of 34 ms for 2 h deposited Cu2S.

In Situ Monitoring Mesoscopic Deformation of Nanostructured Porous Titania Films Caused by Water Ingression.

Nanostructured porous titania films are used in many energy-related applications. In this work, the temporal evolution of the mesoscopic deformation of mesoporous titania films synthesized via block copolymer-assisted sol-gel chemistry is investigated with in situ grazing incidence small-angle neutron scattering (GISANS) during exposure to D2O vapor. Two types of mesoporous titania films are compared, which have a different degree of structural stability, depending on the applied annealing temperature (400 °C vs 600 °C) in a nitrogen atmosphere. Water ingression causes a gradual structure deformation in terms of decreasing center-to-center distances and broadening of the size distribution of the titania nanostructures. Based on the evolution of the mesopore size obtained from in situ GISANS measurements, the results show that structures synthesized at lower temperature undergo a stronger deformation because of the lower elastic modulus originating from larger pores, despite having a higher degree of order.

Impact of the titania nanostructure on charge transport and its application in hybrid solar cells

Porous titania films are widely studied in a number of optoelectronic applications due to its favorable optical and electronic characteristics. Mesoporous titania thin films (MTTFs) with tunable pore size, pore order, accessibility and crystallinity are of interest in electronic devices due to the potential for optimization of the desired characteristics for charge separation and carrier transport. In this work, several MTTFs were prepared by sol–gel chemistry with different structural properties tuned by post-synthesis thermal treatment. The effect of the structural properties (pore diameter, order and accessibility) on the electrical properties of the material was studied by films fabrication onto a transparent conducting electrode, ITO, such that it enables optoelectronic applications. The performance as photoanode was explored by the fabrication of hybrid polymer (P3HT): titania solar cells. Not only does structural properties affect polymer impregnation inside the titania pores as expected and hence impacts charge separation at the interface, but also the thermal treatment affects crystallinity and the films electronic properties. A more complete picture about the electronic properties of the different MTTFs prepared in this work was studied by mobility measurement by space charge limited current and impedance spectroscopy.

Refractive index and scattering of porous TiO2 films

Porous titanium dioxide (TiO2) films are essential components of dye sensitized solar cells (DSSCs) as well as perovskite solar cells (PSCs). Unfortunately, porosity, refractive index, and scattering properties of these films are only roughly known. This induces uncertainties in modelling and understanding of these solar cells. Since the literature provides only descriptions of the optical properties of the porous TiO2 layers with unclear relevance to these solar cells, we investigate porous TiO2 films really used in DSSCs and potentially usable in PSCs. The effective refractive index and the film porosity for different nanostructures that were fabricated from solution-based techniques were determined. The found values are 1.7982 ± 0.005 for the effective refractive index of one kind of TiO2 films and 1.62 ± 0.002 for another one. These values lead to porosities of 53.5% and 65%, respectively. The scattering of the films can be described by a wavelength-independent effective scattering parameter for one film type and by effective scattering particles with a diameter of 46.5 nm for the other film type. The determined porosities are also of relevance for the ionic transport which is functionally crucial in DSSCs and a disturbance in PSCs.

Post-treatment of porous titanium dioxide film with plasmonic compact layer as a photoanode for enhanced dye-sensitized solar cells

Mesoporous TiO2 photoanode before dye adsorption was modified with a thin compact layer of plasmonic silver-doped anatase TiO2 (cTiO2-Ag) by a simple dip-coating method followed by thermal decomposition. Post-treatment process significantly enhanced the open-circuit photovoltage and short-circuit photocurrent density, resulting in an increase in the photoelectric conversion efficiency (η) of dye-sensitized solar cell (DSSC). Ag modification increased the optical absorption of dye through the localized surface plasmons and improved the interfacial connection between TiO2 particles for expediting the electron transport, but at the expense of decreasing dye adsorption. There was no appreciable change in the adsorption amount of dye after the formation of cTiO2-Ag on photoanode. The cTiO2-Ag layer could provide an effective approach to enhancing light harvesting due to the plasmonic effect, bridging the pathways for electron transport, and decreasing the charge recombination at fluorinated tin oxide/electrolyte and/or TiO2/electrolyte interfaces, all of which could result in superior photovoltaic performance of DSSC. The η value of DSSC using photoanode with cTiO2-Ag layer could reach as high as 9.45%, far better than that of DSSC using bare photoanode (6.27%).

Pore size control of block copolymer-templated sol–gel-synthesized titania films deposited via spray coating

We combine a sol–gel route with spray coating to obtain porous titania films, using the block copolymer polystyrene-block-polyethylene oxide (PS-b-PEO) as a structure-directing template of the titania nanostructures and titanium(IV) isopropoxide (TTIP) as the titania precursor. Due to the water-induced phase separation and self-assembly of PS-b-PEO and TTIP, a bicontinuous crystalline anatase titania foam structure results after a calcination step. At different sintering temperatures, ranging from 400 to 800 °C, the titania films are analyzed by X-ray diffraction to determine the crystal phase of the titania foams. This study demonstrates that the pore size of the titania foam films can be controlled by different amounts of TTIP used in the sol–gel solution and by the spray parameters of the applied spray coating. Homogenous porous films with a titania network architecture can be achieved from optimizing the number of spray shots used for the film deposition.

Preparation of gold and gold–silver alloy nanoparticles for enhancement of plasmonic dye-sensitized solar cells performance

A two-step method was applied to obtain gold–silver alloy nanoparticles (Au–Ag alloy NPs). First, Au and Ag colloids with average particle sizes of 14.80 and 8.47nm, respectively, were synthesized by pulsed laser ablation of pure metallic targets immersed in ethanol, subsequent mixing, and re-irradiation of individual Au and Ag colloids (volume ratio of 4:1). In this study, a dense layer of porous titanium dioxide (TiO2) film with a thickness of ∼11μm was deposited directly on the central area of the conductive face of a fluorine tin oxide (FTO) glass by blade doctor method. A specific amount of N719 dye molecules with and without the plasmonic-metal nanoparticles (NPs) was mixed, incorporated into the porous TiO2 film, and used as photoanode to fabricate dye-sensitized solar cells (DSSCs). The optical and structural properties of the prepared photoanodes before their application to DSSCs were investigated. The photocurrent density–voltage and external quantum efficiency characteristics of the DSSCs fabricated with these photoanodes were also analyzed and discussed. The experimental results confirmed the plasmonic enhancement effects on the performance of DSSCs, which indicated that cells loaded with the plasmonic-metal NPs yielded larger enhancement. The plasmonic effect of the Au–Ag alloy NPs significantly enhanced the broadband light absorption of dye molecules. The best cell was achieved using the photoanode loaded with Au–Ag alloy NPs, exhibiting a power conversion efficiency of 5.81%, which was ∼8.4% and 52.1% higher than that of the DSSCs with Au NPs and the reference cell, respectively.

Toward compositional design of reticular type porous films by mixing and coating titania-based frameworks with silica

A recently developed reticular type porous structure, which can be fabricated as the film through the soft colloidal block copolymer (e.g., PS-b-PEO) templating, is very promising as the porous platform showing high-performance based on its high surface area as well as high diffusivity of targeted organic molecules and effective accommodation of bulky molecules, but the compositional design of oxide frameworks has not been developed so enough to date. Here, I report reliable synthetic methods of the reticular type porous structure toward simple compositional variations. Due to the reproducibility of reticular type porous titania films from titanium alkoxide (e.g., TTIP; titanium tetraisopropoxide), a titania-silica film having similar porous structure was obtained by mixing silicon alkoxide (e.g., tetraethoxysilane) and TTIP followed by their pre-hydrolysis, and the mixing ratio of Ti to Si composition was easily reached to 1.0. For further compositional design, a concept of surface coating was widely applicable; the reticular type porous titania surfaces can be coated with other oxides such as silica. Here, a silica coating was successfully achieved by the simple chemical vapor deposition of silicon alkoxide (e.g., tetramethoxysilane) without water (with water at the humidity level), which was also utilized for pore filling with silica by the similar process with water.

Porous titania films fabricated via sol gel rout – Optical and AFM characterization

Mesoporous titania films of low refractive index ∼1.72 and thickness within the range of 57–96nm were fabricated via sol–gel rout and dip-coating technique on a soda–lime glass substrate. Tetrabutylorthotitanate Ti(OBu)4 was used as a titania precursor. High porosity and consequently low refractive index were achieved using the polyethylene glycol (PEG 1100) as a template. Based on transmittance, using Tauc’s relations, the optical energy band gaps and the Urbach energy were determined. The research shows that in the fabricated titania films there are two types of optical energy band gaps, connected with direct and indirect electron transitions and brought about by the presence of amorphous and crystalline phase respectively. Based on the quantum size effect, the diameters of nanocrystals versus film thickness were determined. AFM studies of the titania films have demonstrated that there are changes of surface morphology taking place with the change of thickness. We have demonstrated that the surface morphology of titania films has influence on wettability.

In-situ fabrication of dual porous titanium dioxide films as anode for carbon cathode based perovskite solar cell

We develop a dual porous (DP) TiO2 film for the electron transporting layer (ETL) in carbon cathode based perovskite solar cells (C-PSCs). The DP TiO2 film was synthesized via a facile PS-templated method with the thickness being controlled by the spin-coating speed. It was found that there is an optimum DP TiO2 film thickness for achieving an effective ETL, a suitable perovskite/TiO2 interface, an efficient light harvester and thus a high performance C-PSC. In particular, such a DP TiO2 film can act as a scaffold for complete-filling of the pores with perovskite and for forming high-quality perovskite crystals that are seamlessly interfaced with TiO2 to enhance interfacial charge injection. Leveraging the unique advantages of DP TiO2 ETL, together with a dense-packed and pinhole-free TiO2 compact layer, PCE of the C-PSCs has reached 9.81% with good stability.

The effects of gold colloid concentration on photoanode electrodes to enhance plasmonic dye-sensitized solar cells performance

Pulsed laser ablation of gold (Au) bulk immersed in ethanol and post-ablated size modification process were employed to generate homogeneous size distribution of nanoparticles (~15 nm) and different concentrations of Au colloids without using chemical reagents in the final preparation. A specific concentration of N719 dye molecules with or without Au colloidal solution at different concentrations was mixed and embedded onto the porous titanium dioxide films for photoanodes formation in dye-sensitized solar cells (DSSCs). Optical measurements confirmed the interaction between the dipole moments of the dye and the localized surface plasmon resonance (LSPR) of Au nanoparticles (AuNPs), thereby indicating that AuNPs were in close contact with the adsorbed dye molecules on the surface of the photoanode. The LSPR effects of AuNPs on the performance of DSSCs have been extensively studied by using different concentrations of Au colloids. It was found that, the performance of DSSCs is highly dependent on the concentration of AuNPs. For the highest concentration (9 × 1012 particles/mL), the best achieved plasmonic DSSC performance exhibited a power conversion efficiency of 5.36 %, whereas the reference DSSC (without AuNPs) showed an efficiency of 3.82 %. Thus, an enhancement factor of 40.31 % is indicated.

Guided in Situ Polymerization of MEH-PPV in Mesoporous Titania Photoanodes

Incorporation of conjugated polymers into porous metal oxide networks is a challenging task, which is being pursued via many different approaches. We have developed the guided in situ polymerization of poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) in porous titania films by means of surface functionalization. The controlled polymerization via the Gilch route was induced by an alkoxide base and by increasing the temperature. The selected and specially designed surface-functionalizing linker molecules mimic the monomer or its activated form, respectively. In this way, we drastically enhanced the amount of MEH-PPV incorporated into the porous titania phase compared to nonfunctionalized samples by a factor of 6. Additionally, photovoltaic measurements were performed. The devices show shunting or series resistance limitations, depending on the surface functionalization prior to in situ polymerization of MEH-PPV. We suggest that the reason for this behavior can be found in the orientation of the grown polymer chains with respect to the titania surface. Therefore, the geometry of the anchoring via the linker molecules is relevant for exploiting the full electronic potential of the conjugated polymer in the resulting hybrid composite. This observation will help to design future synthesis methods for new hybrid materials from conjugated polymers and n-type semiconductors to take full advantage of favorable electronic interactions between the two phases.

Hot plasmonic electrons for generation of enhanced photocurrent in gold-TiO2 nanocomposites.

In this manuscript, for the first time, we report a combination of electrophoretic and sintering approaches for introducing gold nanoparticles into nanoporous TiO2 films to generate ‘hot’ electrons resulting in a strong enhancement of photocurrent. The Au-TiO2 nanocomposite material was prepared by the electrophoretic deposition of gold nanoparticles into a porous nanoparticulate titanium dioxide film, creating a photoactive electrode. The composite film demonstrates a significant increase in the short circuit current (Isc) compared to unmodified TiO2 when excited at or close to the plasmon resonance of the gold nanoparticles. Then, we employed a thermal ripening process as a method of increasing the Isc of these electrodes and also as a method of tuning the plasmon peak position, with a high degree of selectivity. Photo-electrochemical investigations revealed that the increase in photocurrent is attributed to the generation and separation of plasmonically generated hot electrons at the gold/TiO2 interface and also the inter-band generation of holes in gold nanoparticles by photons with λ < 520 nm. Theoretical modelling outputs perfectly match our results obtained from photo-physical studies of the processes leading to enhanced photocurrent.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-014-0710-5) contains supplementary material, which is available to authorized users.

In situ observation of the evaporation-induced self-assembling process of PS-b-PEO diblock copolymers for the fabrication of titania films by confocal laser scanning microscopy.

The drying process of clear precursor solutions for fabricating highly porous titania films was observed by CLSM. Further, we succeeded in detecting the formation of micelles in the precursor solution containing high-molecular-weight PS-b-PEO diblock copolymers at the initial stage of the drying process as the direct evidence that this synthesis consisted of the EISA process.

Free-Standing High Surface Area Titania Films Grown at the Air–Water Interface

Free-standing titania films were grown at the air–water interface, a novel method to synthesize robust TiO2 nanowire/nanoparticle composite films. The calcined films contain an anatase crystal phase and have a high surface area with a structure composed of one-dimensional long nanowires and mesoporous nanoparticle branches. These suggest a promising way to manufacture large areas of thick porous titania films for many applications. As one possible application, use of these films in a dye-sensitized solar cell demonstrates the potential of these materials.