TiO2 Aggregates Research Articles

Enhanced charge transport properties by strengthened necks between TiO2 aggregates for dye sensitized solar cells

The electron diffusion in the TiO2 aggregate network was enhanced through the addition of TiO2 nanoparticles with preferential filling at the necks between adjacent TiO2 aggregates, which resulted in strengthening the connections, while retaining the porous structure of the TiO2 network. The fortified necks was found to reduce the transport resistance (Rt) by allowing facile transfer of electrons from one aggregate to another, while the scattering effect of the TiO2 aggregate network got weakened with adding the TiO2 nanoparticles as a result of reduction of the light scattering centers such as the necks and gaps between the aggregates. However, due to the increase in surface area as the TiO2 nanoparticles were added, the diminished light scattering effect of the aggregate network was compensated and even the highest performance was achieved when the 10% TiO2 nanoparticle was added into the TiO2 aggregate film, suggesting that widening necks between sub-micrometer sized light scatters such as an aggregate would be a good strategy in achieving further improvement of power conversion efficiency of dye sensitized solar cells through the improved charge transport property.

Scanning electrochemical cell microscopy platform for ultrasensitive photoelectrochemical imaging.

The development of techniques for nanoscale structure-activity correlations is of major importance for the fundamental understanding and rational design of (photo)electrocatalysts. However, the low conversion efficiency of characteristic materials generates tiny photoelectrochemical currents at the submicrometer to nanoscale, in the fA range, which are challenging to detect and measure accurately. Here, we report the coupling of scanning electrochemical cell microscopy (SECCM) with photoillumination, to create a submicrometer spatial resolution cell that opens up high resolution structure-(photo)activity measurements. We demonstrate the capabilities of the technique as a tool for: (i) high spatial resolution (photo)activity mapping using an ionic liquid electrolyte at a thin film of TiO2 aggregates, commonly used as a photoanode in dye sensitized solar cells (DSSCs) and (ii) in situ (photo)activity measurements of an electropolymerized conjugated polymer on a transparent Au substrate in a controlled atmospheric environment. Quantitative data, including localized (photo)electrochemical transients and external quantum efficiency (EQE), are extracted, and prospects for further technique development and enhancement are outlined.

Characterization of Nanoscaled TiO2 Produced by Simplified Sol–Gel Method Using Organometallic Precursor

This work is concerned with development of sol–gel method for preparation of nanoscaled TiO2 using organometallic precursor—titanium tetraisopropoxide (TTIP) and determination of the present crystalline phases depending on the temperature of further thermal treatment. The characteristic processes and transformations during the thermal treatment were determined by means of thermal gravimetric analysis and/or differential thermal analysis (TGA/DTA) method. The crystalline structure and size of the TiO2 crystallites were analyzed by means of Raman spectroscopy and X-ray powder diffraction (XRPD) method. At 250 °C, cryptocrystalline structure was detected, where amorphous TiO2 is accompanied with crystalline anatase. The anatase crystallite phase is stable up to 650 °C, whereas at higher temperature rutile transformation begins. It was observed that at 800 °C, almost the whole TiO2 is transformed to rutile phase. According to XRPD analysis, the increase of the temperature influences on the increase of the size of the crystalline particles ranging from 6 nm at 250 °C to less than 100 nm at 800 °C. The size and shape of the TiO2 crystalline particles were observed by transmission electron microscopy (TEM). The shape of the studied samples changes from nanospheres (250, 380, and 550 °C) to nanorods (650 and 800 °C). Morphology of the formed TiO2 aggregates was observed by scanning electron microscopy (SEM).

Experimental measurement and modelling of reactive species generation in TiO2 nanoparticle photocatalysis

The generation of reactive species in titanium dioxide (TiO2) nanoparticle photocatalysis was assessed in a laboratory scale setup, in which P25 Aeroxide TiO2 suspensions were photoactivated by means of UV-A radiation. Photogenerated holes and hydroxyl radicals were monitored over time by observing their selective reaction with probe compounds, iodide and terephthalic acid, respectively. TiO2 aggregate size and structure were characterized over the reaction time. Reactive species quenching was then described by a model, accounting for radiative phenomena, TiO2 nanoparticle aggregation and kinetic reactions. The interaction between iodide and photogenerated holes was influenced by iodide adsorption on TiO2 surface, described by a Langmuir–Hinshelwood mechanism, whose parameters were studied as a function of TiO2 concentration and irradiation time. Iodide oxidation was effectively simulated by modelling the reaction volume as a completely stirred two-dimensional domain, in which irradiation phenomena were described by a two-flux model and the steady state for reactive species was assumed. The kinetic parameters for iodide adsorption and oxidation were estimated and successfully validated in a different experimental setup. The same model was adapted to describe the oxidation of terephthalic acid by hydroxyl radicals. The kinetic parameters for terephthalic acid oxidation were estimated and validated, while the issues in investigating the interaction mechanisms among the involved species have been discussed. The sensitivity of operating parameters on model response was assessed and the most relevant parameters were highlighted.

Pyrolyzed titanium dioxide/polyaniline as an efficient non-noble metal electrocatalyst for oxygen reduction reaction

To overcome the prohibitive cost and poor durability of conventional Pt-based catalysts, TiO2/C was prepared by pyrolyzing a novel titanium dioxide/polyaniline (TiO2/PANI) composite. The prepared catalysts were characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, cyclic voltammetry (CV), and linear sweep voltammetry. Interaction between PANI and TiO2 was found to inhibit the aggregation of TiO2 and its transformation from anatase to rutile. The catalytic activity of the TiO2/C first increased with increasing PANI content and then decreased; the optimum was achieved when the PANI/TiO2 mass ratio was 35/100. CV and i–t curves showed that the prepared composite has a good catalytic stability.

Synthesis of mesoporous TiO2 spheres and aggregates by sol–gel method for dye-sensitized solar cells

Mesoporous microspheres of anatase TiO2 were synthesized by hydrolysis of titanium butoxide in butanol. The TiO2 micospheres were about 0.5μm in size, composed of primary nanoparticles of about 10nm, and the specific surface area increased by addition of triblock copolymer (Pluronic F127). TiO2 aggregates with irregular shape were formed by hydrothermal treatment. Before and after calcination, the crystallinity of TiO2 aggregates was high although the specific surface area decreased. The cell efficiency of TiO2 sphere electrode increased by F127 addition. Furthermore, TiO2 aggregate electrode exhibited an efficiency of 5.6%, which was much higher than those of TiO2 spheres with F127 and commercial TiO2 nanoparticles (P25) electrodes. It was confirmed that the high crystallinity of TiO2 was important to improve the photovoltaic properties as well as large surface area.

UV irradiation induced transformation of TiO2 nanoparticles in water: aggregation and photoreactivity.

Transformation of nanomaterials in aqueous environment has significant impact on their behavior in engineered application and natural system. In this paper, UV irradiation induced transformation of TiO2 nanoparticles in aqueous solutions was demonstrated, and its effect on the aggregation and photocatalytic reactivity of TiO2 was investigated. UV irradiation of a TiO2 nanoparticle suspension accelerated nanoparticle aggregation that was dependent on the irradiation duration. The aggregation rate increased from <0.001 nm/s before irradiation to 0.027 nm/s after 50 h irradiation, resulting in aggregates with a hydrodynamic diameter of 623 nm. The isoelectric point of the suspension was lowered from 7.0 to 6.4 after irradiation, indicating less positive charges on the surface. ATR-FTIR spectra displayed successive growth of surface hydroxyl groups with UV irradiation which might be responsible for the change of surface charge and aggregation rate. UV irradiation also changed the photocatalytic degradation rate of Rhodamine B by TiO2, which initially increased with irradiation time, then decreased. Based on the photoluminescence decay and photocurrent collection data, the change was attributed to the variation in interparticle charge transfer kinetics. These results highlight the importance of light irradiation on the transformation and reactivity of TiO2 nanomaterials.

New evidence for TiO2 uniform surfaces leading to complete bacterial reduction in the dark: Critical issues

This study presents new evidence for the events leading to Escherichia coli reduction in the absence of light irradiation on TiO2-polyester (from now on TiO2-PES. By transmission electron microscopy (TEM) the diffusion of TiO2 NP's aggregates with the E. coli outer lipo-polyssacharide (LPS) layer is shown to be a prerequisite for the loss of bacterial cultivability. Within 30min in the dark the TiO2 aggregates interact with E. coli cell wall leading within 120min to the complete loss of bacterial cultivability on a TiO2-PES 5% TiO2 sample. The bacterial reduction was observed to increase with a higher TiO2 loading on the PES up to 5%. Bacterial disinfection on TiO2-PES in the dark was slower compared to the runs under low intensity simulated sunlight light irradiation. The interaction between the TiO2 aggregates and the E. coli cell wall is discussed in terms of the competition between the TiO2 units collapsing to form TiO2-aggregates at a physiologic pH-value followed by the electrostatic interaction with the bacteria surface. TiO2-PES samples were able to carry repetitive bacterial inactivation. This presents a potential for practical applications. X-ray photoelectron spectroscopy (XPS) evidence was found for the reduction of Ti4+ to Ti3+ contributing to redox interactions between TiO2-PES and the bacterial cell wall. Insight is provided into the mechanism of interaction between the E. coli cell wall and TiO2 NP's. The properties of the TiO2-PES surface like percentage atomic concentration, TiO2-loading, optical absorption, surface charge and crystallographic phases are reported in this study.

Magnetizing of nano-materials on example of Degussa’s P-25 TiO2 photocatalyst: Synthesis of magnetic aggregates, characterization and possible use

This study deals with the production of a magnetic photocatalyst using a new magnetizing technique – growth of magnetite crystals onto P-25 TiO2 particles from Degussa. Magnetite (MT) crystals were grown via the synthesis of layered FeII–FeIII hydroxide sulfate (green rust) on the surfaces of TiO2. The magnetite/P-25 TiO2 aggregates (MT/P-25) were characterized with XRD and TEM. The structure of the P-25 TiO2 photocatalyst appeared to be unchanged by the treatment. The photocatalytic activity of MT/P-25 in aqueous suspensions were tested in the near neutral pH area where magnetite is stable and compared to the photoactivity of pure P-25 TiO2. Photooxidation of tartronic acid (TA) was first used to test the photooxidative efficiency of MT/P-25 followed by other more complex compounds: aqueous solution of 2,6 difluorobenzoic acid (2,6 DFBA) as well as the natural organic matter in water from the Hohloh Lake, Germany (NOM-HLW). For the TA photocatalytic oxidation with MT/P-25 similar oxidation rates were observed as for the P-25 TiO2, but for 2,6 DFBA and NOM-HLW the degradation rates were higher for pure P-25 TiO2. Two types of laboratory reactors were applied for photocatalytic degradation of 2,6 DFBA and NOM-HLW: a reactor (400mL) with an artificial UV radiation source and the other one (800mL) that was irradiated by natural solar radiation. Photonic efficiencies of organic substrates degradation processes for both types of the reactors were calculated and compared. The bigger reactor showed better results in degradation of organic substrates (photonic efficiency was more than 7 times higher for 2,6 DFBA).Magnetic separation using a simple separator (hand magnet, batch experiment) showed a mass loss less than 3% of MT/P-25 per experiment. However, using a high gradient magnetic separator (with continuous flow) less than 0.3% of MT/P-25 aggregates were lost with a suspension flow rate of 250mL/min. In both cases magnetically separated and reused MT/P-25 showed practically unchanged photoactivity.The mechanical stability of the MT/P-25 was checked comparing the photoactivity of MT/P-25 in TA solution without prior stirring and after stirring in the reactor “in the dark” for durations of 80 and 160h. No significant differences were observed in their photoactivity before and after stirring. However, ultrasonication of the MT/P-25 at 35kHz for 60min separated the MT/P-25 material into MT and TiO2. Nevertheless, for practical purposes the stability of the aggregates during the mechanical stirring and during the magnetic separation is of major importance.

Mapping nanoscale absorption of femtosecond laser pulses using plasma explosion imaging.

We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO2 aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma.

Cold Gas Sprayed Semiconductor-Based Electrodes for the Photooxidation of Water

Photoassisted water splitting for hydrogen generation requires the development of low cost, but highly efficient photoelectrodes. For the industrial hydrogen production with photoelectrochemical cells suitable catalysts and feasible photoelectrode preparation processes still need to be identified.This contribution explores the potential of cold gas spraying (CGS) for the production of photoanodes employing semiconductors for the water oxidation reaction (OER). Conventional large area coating techniques usually employ wet chemical methods with subsequent calcination steps to obtain enhanced binding between the catalyst particles and the substrate. In the cold gas spraying process, particles are accelerated to high velocities by a pressurized gas. The nitrogen used as process gas is preheated and then expanded in a De Laval type nozzle. On impact with the substrate, the particles deform and break up and thus can build an efficient interface to the back contact.For a first demonstration TiO2 aggregates delivered by EVONIK were probed for the preparation of TiO2 photoelectrodes. In photoelectrochemical experiments these cold gas sprayed TiO2 photoelectrodes showed seven times higher photocurrents in the photooxidation of water (at 1.23V(NHE)) than reference electrodes prepared by the established doctor blade technique. In systematic experiments it was observed that with increasing gas temperature in the coating process the obtained photocurrent is enhanced. The better performance can be mainly attributed to an improved bonding of the TiO2 particles to the substrate (back contact) due to their increased impact energy. Although the bulk characteristics of the particles remained unchanged incident photon to current efficiency (IPCE) and surface photovoltage measurements reveal the formation of surface-localized interband transitions at the TiO2 surface (1.2, 2 and 2.4 eV) during the CGS process. In order to study the influence of these defects on the photoreaction performance surface modification by plasma treatment is employed. For this structure-activity correlation analysis by Raman, XRD and XPS is performed.From these initial experiments the preparation is extended to other potential metal oxides (e.g. WO3, Fe2O3 and BiVO4) for the photooxidation of water. Besides the relevant particle-substrate bonding and defect chemistry of the semiconductor layer due to the process, morphology investigation by SEM, gas sorption and 3D topography are considered for the optimization of the coating technique.

A new architecture of bowl-type mesoporous TiO2 via facile electrospray method

We demonstrate here the fabrication of a novel morphology of bowl-shaped TiO2 aggregates via a facile electrospray technique using TiO2 nanowires. These have been prepared by using lauric acid via solvothermal synthetic route. The modulation of surface energies of the different crystallographic faces through the use of lauric acid is the key parameter for the anisotropic growth. These aggregates have high specific surface area of up to 93.10m2/g, and relatively large pores with a mean size of 4.68nm. These porous structures can provide a high number of active adsorption sites and act as photo-related reaction centers. The prepared bowl-shaped TiO2 are in micrometer size and have good morphology and high specific surface area. A possible mechanism for the formation of bowl-shaped TiO2 aggregates is also proposed, based on the results. The light scattering properties of bowl-shaped architectures are characterized by the UV–Vis spectrometry. The superior light scattering abilities of bowl-shape makes them promising architectures for applications in the photovoltaic devices.

Optimized TiO&lt;sub&gt;2&lt;/sub&gt; Nanocrystallites Aggregates for Enhanced Efficiency in Dye Solar Cells

TiO2 aggregates-based dye solar cells (DSCs) have gained an increasing attention due to their enhanced harvesting of light radiance. The capability of this photoelectrode material is attributed to the submicron spherical aggregates that introduce light scattering effect which can generate more electrons whilst high internal surface area for dye chemisorption is provided by nanocrystallites which made up the aggregates. Here, TiO2 aggregates (0.45-0.20 μm) composing of nanocrystallites (10-28 nm) with desired physicochemical properties for enhanced overall light conversion efficiency of DSC were synthesized by varying the water content in the hydrolysis of titanium alkoxide in ethanol and calcination temperature. TiO2 aggregates obtained were characterized using FESEM, XRD and UV-Vis spectroscopy. The assembled DSCs were then evaluated using solar simulator under AM 1.5 (100 mW/cm2) simulated sunlight. With higher water content in the hydrolysis process, the aggregates reduce in size and lose their spherical shapes resulting in lower absorption intensity indicating the occurrence of low light scattering in the TiO2 film. Nanocrystallites were found to have an increasing size of 12 nm to 28 nm with increasing calcination temperature of 400°C to 700°C. Sample of aggregates calcined at 450°C recorded the highest efficiency (~4%). Highest conversion efficiency was observed for DSC that used well-defined spherical TiO2 aggregates composing of nanocrystallites which were synthesized at optimum synthesis parameter which is by using ethanol with low water content (0.9 vol%) followed by calcination at 450°C. Thus, optimized TiO2 nanocrystallites which form spherical aggregate is critical in order to improve light harvesting efficiency of DSCs.

Hydrolysis Time on Rutile TiO&lt;sub&gt;2&lt;/sub&gt; White Pigment via Short Sulfate Process

Rutile TiO2 white pigment was prepared by self-seeded thermal hydrolysis route via short sulfate process from low concentration TiOSO4 solution. Hydrolysis time had significantly influenced the structure and pigment properties of rutile TiO2. The samples were characterized by XRD, particle size distribution and pigment properties test. The hydrolysis time had great influences on the hydrolysis rate, particle size distribution, crystal growth and particle aggregation of TiO2, eventually determined the pigment structure and properties. The optimized hydrolysis time after second boiling point was of 2.5h. And the prepared rutile TiO2 white pigment was with narrow particle size distribution and nice pigment properties.

Development of poly-(ethylene terephthalate) masterbatches incorporating highly dispersed TiO2 nanoparticles: Investigation of morphologies by optical and rheological procedures

Twin-screw melt extrusion was applied to disperse nanoscaled titania particles (TiO2) into poly(ethylene-terephthalate) (PET) for masterbatch (MB) development. Optical and rheological methods were applied to characterize the particle dispersions within the composite. Our studies show that highest impact on dispersion quality was obtained by the introduction of polymeric dispersing agents, whereas parameters such as screw rotation speed or adapted temperature profiles had only secondary influences. The dispersing agent forms highly dispersed polymer intercalated TiO2 nanoscaled particle morphologies, whereas for pure PET/TiO2 dense and considerable amounts of micron scaled TiO2 aggregates are found. These results are underpinned by scanning transmission electron microscopy (STEM) and by differential scanning calorimetry (DSC) data showing low polymer–particle interaction for pure PET/TiO2 systems and opposite properties for dispersing agent containing nanocomposites. It is shown that both optical as well as rheological analyses result in consistent and accurate informations on dispersion within the polymer.

TiO2–active carbon composites for wastewater photocatalysis

Titanium oxide–active carbon composites (TiO2–AC) with photocatalytic activity were prepared by sol–gel method at low temperature. Titanium isopropoxide and active carbon were used as precursors, ethanol as solvent and HNO3 as catalyst. The composition of the studied composites was accomplished by means of X-ray diffraction, and Fourier Transform-Infrared spectrometry. The rutile crystalline phase was present in all samples. In scanning electron microscopy images the TiO2 aggregates existed either on the surface of the active carbon or heterogeneously dispersed over the whole catalyst matrix. The contact angle and point of zero charge measurements were applied to characterise the surface energy and surface charge. The photocatalytic activities of the TiO2–AC composites were evaluated in the bleaching of methyl orange and methylene blue solution from ternary mixture of methyl orange, methylene blue and Triton X100. The double divisor ratio spectra derivative method (as a multivariable calibration method) was developed to overcome the spectral overlapping, for the simultaneous analysis of ternary mixture.

Wavelength dependence of Kubelka–Munk scattering spectra for studies of TiO2 microstructure and aggregation in paints

The wavelength exponent β is easily determined from diffuse reflectance spectra as the gradient of a log–log plot of the Kubelka–Munk scattering coefficient (S) against wavelength. Previous work suggests that β provides a useful relative measure of the size of scattering units applicable to many particulate systems. This article examines the applicability of β to paints, specifically studies of TiO2 aggregation, and comments on its wider use for pigmented coatings. Modelling using multisphere T-matrix Mie methods shows that β varies nearly linearly with the mean aggregate size. Experimental data shows good correlation between the state of aggregation and β. However, β depends strongly on the volume fraction of TiO2 owing to the effects of dependent scattering arising from crowding, therefore comparisons between paints can only be made at constant effective TiO2 volume fraction. A simple effective medium refractive index model in conjunction with Mie scattering appears to give a good account of the behavior of both S and β with increasing TiO2 volume concentration. When air is incorporated into the coatings at high pigment volume fractions, scattering is much more complicated. In this case it appears very difficult to relate β directly to coating microstructure properties such as pore diameter.

Contact behavior of size fractionated TiO2 nanoparticle agglomerates and aggregates

The size dependent contact behavior of nanoparticle agglomerates was studied. Size fractionated TiO2 nanoparticle agglomerates were synthesized with a combination of a flame spray reactor and a differential mobility analyzer resulting in four different agglomerate sizes (median values from 78 to 161nm). Contact behavior of the fractionated agglomerates was investigated using atomic force microscopy. A model of the contact behavior of agglomerates under tensile stress was established in which the aggregates within the agglomerates rearrange and form chains before breakage. Force curves show that the length of the chains and the amount of rearrangements depend on the agglomerate sizes. Larger agglomerates require more work for their aggregate rearrangement before the final breakage is induced.

Spatial point statistics for quantifying TiO2 distribution in paint

Spatial point statistics such as Clark-Evans R, Ripley’s K, and the neighborhood distribution function (NDF) have been used to study the spatial distribution of TiO2 particles in sections of paint films. Particle coordinates were obtained from electron micrographs. Ripley’s K and the NDF are particularly useful in detecting segregation, randomness, or aggregation at various length scales. Other statistics such as aggregate-size distributions were also calculated. The spatial distribution in well-dispersed paints studied here is essentially random and observed aggregates are explained by the spatial inhomogeneity inherent in the random distributions. The aggregate-size distributions are approximately logarithmic for well-dispersed paints. Mean aggregate size increases with increasing pigment volume concentration (15–30%PVC), even though the spatial distribution remains random, showing that crowding at high PVC is a heterogeneous process. Mean aggregate sizes determined by microscopy correlate well with opacity and other paint properties. Computer simulation shows that spatial point and aggregate-size analysis of points in 2-D slices taken from a 3-D volume underestimate absolute aggregate sizes, but that spatial statistics and aggregate distributions correlate strongly with those obtained from the original 3-D distribution. Therefore, results from analysis of micrographs are expected to translate to the 3-D paint film. As expected, coarse extenders crowd the TiO2 and increase aggregation, but highly anisodiametric particles appear less deleterious than blocky particles.

Texture change of TiN films due to anisotropic incorporation of oxygen

Results of comprehensive structural and microchemical analyses revealed the mechanisms responsible for changing the <111> to <001> competitive growth texture of TiN films when the level of oxygen doping was 20 at.%. It has been shown that the growth competition is related to the different way of segregation-incorporation of the oxygen species on the {001} and {111} crystal faces. Oxygen is preferentially segregated on the {111} crystal face and is growing in the form of a 2D TiO2 surface layer on the growth surface, limiting the growth of <111> oriented crystals. On the {001} faces, however, the major part of condensing oxygen species is dissolved into the crystal lattice and only a part forms TiO2 aggregates. These aggregates have a 3D morphology which do not hinder the growth of <001> oriented crystals. Consequently, the <001> oriented crystals win the growth competition leading to a film structure constituted of V-shaped columns.