Suspensions Of TiO2 Nanoparticles Research Articles

Titanium Dioxide Nanoparticles Induce Root Growth Inhibition in Soybean Due to Physical Damages

Titanium dioxide (TiO2) nanoparticles (NPs) are the most widely released nanomaterials in the environment and are considered an emerging contaminant. Although the phytotoxicity mechanism of TiO2 NPs on plants involves the elevated generation of reactive oxygen species (ROS), it is still not well established in soybean. Herein, we evaluated the effects of 250–1000 mg L−1 TiO2 NPs on seed germination, growth, content of ROS, lipid peroxidation, and activity of antioxidant enzymes in roots of soybean plants. Our data revealed that up to 1000 mg L−1 TiO2 NPs did not affect soybean seed germination. Transmission electron microscopy images and determinations of zeta potential and hydrodynamic diameter of a suspension of TiO2 NPs demonstrated that they form aggregates, favoring their adsorption to the root surface with consequent physical damage. The main deleterious effects noted on roots were reduced cell viability, reduced root hair number, striated aspect of root apexes, and reduced fresh and dry weights of roots. In disagreement with other studies, plant exposure to TiO2 NPs reduced the level of total ROS and lipid peroxidation, probably due to increased superoxide dismutase (SOD) activity. Altogether, our data suggest that the toxicity mechanism of TiO2 NPs on soybean roots involves physical damage resulting from their adsorption to the root surface, but not the generation of ROS.

Effect of the suspension of Ag-incorporated TiO2 nanoparticles (Ag-TiO2 NPs) on certain growth, physiology and phytotoxicity parameters in spinach seedlings.

In this work, the effect of the inoculation of silver-incorporated titanium dioxide nanoparticles (Ag-TiO2 NPs) in spinach seeds was evaluated on certain growth, physiology and phytotoxicity parameters of the plants. This is an important crop for human consumption with high nutritional value due to their low calorie and fat content, providing various vitamins and minerals, especially iron. These NPs were obtained by means of the sol-gel method and heat treatment; the resulting powder material was characterized using X-ray diffraction and scanning electron microscopy and the influence of these NPs on plants was measured by estimating the germination rate, monitoring morphological parameters and evaluating phytotoxicity. The photosynthetic activity of the spinach plants was estimated through the quantification of the Ratio of Oxygen Evolution (ROE) by the photoacoustic technique. Samples of TiO2 powder with particle size between 9 and 43 nm were used to quantify the germination rate, which served to determine a narrower size range between 7 and 26 nm in the experiments with Ag-TiO2 NPs; the presence of Ag in TiO2 powder samples was confirmed by energy-dispersive X-ray spectroscopy. The analysis of variance showed that the dependent variable (plant growth) could be affected by the evaluated factors (concentration and size) with significant differences. The statistical trend indicated that the application of the Ag-TiO2 NPs suspension of lowest concentration and smallest particle size could be a promoting agent of the growth and development of these plants. The inoculation with NPs of 8.3 nm size and lowest concentration was related to the highest average ROE value, 24.6 ± 0.2%, while the control group was 20.2 ± 0.2%. The positive effect of the Ag-TiO2 NPs treatment could be associated to the generation of reactive oxygen species, antimicrobial activity, increased biochemical attributes, enzymatic activity or improvements in water absorption.

Effect of isopropanol co-product on the long-term stability of TiO2 nanoparticle suspensions produced by microwave-assisted synthesis

In this paper we evaluate the effect of isopropanol arising from the reagent Ti(O-iPr)4 on the long-term stability of nanoparticle size of TiO2 produced during a microwave assisted sol-gel synthesis. Different configurations of microwave apparatus have been considered: the synthesis was carried out both in vessel or flask, and both performing or not isopropanol distillation through a modified microwave apparatus. Results revealed that isopropanol distillation after a short reaction time provided suspensions with slightly smaller particle size, that also show longer stability over time, especially for syntheses carried out at certain temperatures. Reactions performed distilling isopropanol from vessel and flasks showed comparable results, superior to the corresponding suspensions obtained without distilling isopropanol. This study is intended to provide a useful contribution to those applications where the need for stable TiO2 suspensions is critical.

Sustainable Bleaching Process of Raw Cotton by TiO2 Light-Activated Nanoparticles

Nanoparticles (NPs) of titanium dioxide (TiO2) are promising for the photocatalytic degradation of organic compounds, arising as an excellent alternative to conventional bleaching processes. Within this background, aqueous suspensions of TiO2 NPs were impregnated by the pad-dry method in raw cotton. The hydrophilic properties and whiteness index (WI) of Berger were the main parameters examined before and after the TiO2 treatment. In the absence of water, using a UV-Vis lamp, high hydrophilicity was found for raw cotton. Diversely, under UV-LED radiation, the presence of water molecules in the coated cotton was revealed to be a crucial parameter in enhancing its whiteness. A comparable WI of Berger with conventional bleaching methods was obtained using 0.5 g L-1 TiO2 suspension under UV-LED radiation, suggesting that the impregnation of TiO2 NPs can be a promising alternative for the industrial bleaching methods, as it proves to be more sustainable and with low operating costs.

Role of laser fluence in decoration of graphene nanosheets with TiO2 nanoparticles by pulsed laser ablation method

A simple and green method to decorate graphene nanosheets is introduced, according to laser ablation of graphite target in TiO2 nanoparticle suspension. TiO2 nanoparticles were synthesized by laser ablation of a high purity titanium target in distilled water with the fundamental wavelength of pulsed Nd:YAG laser at 7 ns pulse width. 4 samples of graphene/TiO2 nanocomposites were produced by laser ablation of graphite target in TiO2 nanoparticle suspension with different laser fluence. The TiO2 nanoparticles and graphene/TiO2 nanocomposites were characterized by X-ray diffraction, field emission scanning electron microscope, transmission electron microscope, UV–Vis–NIR absorption spectroscopy, FTIR and Raman spectroscope. Micro images of nanocomposite confirm that TiO2 nanoparticles have placed and fixed on graphene nanosheets during their exfoliation from graphite target in TiO2 nanoparticles suspension. It was found that absorption intensity of graphene/TiO2 nanocomposite is higher than the pure TiO2 nanoparticles which can be beneficial to the photogeneration of electron–hole pair charge carriers for improving the photocatalytic performance. The change in the intensity ratios of Raman peaks of graphene nanosheets show that charge transfer between them and TiO2 nanoparticles has taken place. Results show that the number and size of the TiO2 nanoparticles on the graphene nanosheets can be easily tuned by laser fluence.

Continuum Model Analysis of QCM Nanotribological Data to Obtain Friction Coefficients for 304SS Contacts Lubricated by Water and TiO2 Nanoparticle Suspensions

We report a study of the response of a Quartz Crystal Microbalance (QCM) to rubbing contacts in air, water and aqueous suspensions of 40nm TiO2 nanoparticles. Measurements were performed with a contact comprised of 3 close-packed 304SS ball bearings situated symmetrically about the center of a 304SS QCM electrode with 2nm rms roughness. Two continuum methods were employed to infer macroscale friction coefficients μ employing QCM nanotribological data recorded in the Cattaneo-Mindlin (CM) slip regime at vibrational amplitudes that varied between 1nm and 17nm. The “slope” Method 1 involved sweeps of the QCM amplitude of vibration as ball bearings were held in continuous contact with the oscillating electrode. The “contact” Method 2 obtained μ by analyzing the shifts in frequency and bandwidth that occur at a fixed uo to solve for μ. when ball bearings were brought in and out of contact with the QCM’s electrode. The results for dry and water lubricated contacts compared favorably with macroscale friction coefficients reported in the literature. The model failed to adequately describe contacts lubricated with the NP suspension, but its continuum nature did not appear to be the dominant factor underlying failure. The failure was more likely attributable to either a lack of a CM slip regime when NP were present at the interface and/or the fact that the amplitude of vibration was close in size to the individual NP contacting regions, in violation of a key underlying assumption of the model.

Porous Layered Double Hydroxide/TiO2 Photocatalysts for the Photocatalytic Degradation of Orange II

Layered Double Hydroxide (LDH)/TiO2 nanocomposites with photocatalytic properties were synthesized by both impregnation and the direct coprecipitation of LDH matrices using a colloidal suspension of TiO2 nanoparticles. While the two methods led to an efficient TiO2 nanoparticle immobilization, the direct coprecipitation allowed us to tune the amount of immobilized TiO2 within the materials. The LDH/TiO2 nanocomposites obtained were deeply characterized by chemical analysis (ICP-AES), Powder X-ray diffraction (XRD), Fourier Transformed Infra-Red (FTIR), Thermogravimetric analysis (TGA), and High-Resolution Transmission Electron Microscopy (HRTEM). Clearly, the immobilization of TiO2 by direct coprecipitation promoted a modification of the textural properties and a net increase in the surface area. The crystallized TiO2 nanoparticles can be distinctly visualized by HRTEM at the surface of the layered material. Several parameters, such as the nature of the chemical composition of LDH (ZnAl and MgAl), the method of immobilization and the amount of TiO2, were shown to play a crucial role in the physicochemical and photocatalytic properties of the nanocomposites. The photocatalytic efficiency of the different LDH/TiO2 nanocomposites was investigated using the photodegradation of a model pollutant, the Orange II (OII), and was compared to a pure TiO2 colloidal solution. The degradation tests revealed that the nanocomposite obtained from MgAl LDH at a low MgAl LDH/TiO2 ratio was the most efficient for the photodegradation of OII leading to complete mineralization in 48 h.

Effects of colloidal TiO2 and additives on the interfacial polymerization of thin film nanocomposite membranes

Highly stable, well suspended TiO2 nanoparticles (NPs) were synthesized by a sol-gel method and directly mixed with an aqueous amine solution to react at the interface with trimesoyl chloride, forming polyamide (PA) thin film nanocomposite (TFN) membranes with a uniform distribution of the nanoparticles in the PA film. Besides the presence of TiO2 NPs, the influence of remaining chemicals such as ethanol and acid from the synthesis of the TiO2 colloids on the film formation was also investigated. The residual ethanol and acid were found to play a role in thin film formation and properties of the final membranes. The presence of TiO2 NPs, ethanol, and acid had a positive impact on membrane wettability, water flux, and salt rejection. Compared to the unmodified membrane, the TiO2-TFN membranes achieved a good separation performance with around 12% and 19% improvement in water permeability and NaCl rejection, respectively. In addition, the developed TFN membranes possessed an improved anti-fouling property with low flux decay and high flux recovery up to 94%. This simple one-pot synthesis procedure of TiO2 NPs combined with the in-situ integration during the interfacial polymerization showed great potential for economic scaling up in industrial production of high-performance TFN membranes.

Enhanced chlorine evolution from dimensionally stable anode by heterojunction with Ti and Bi based mixed metal oxide layers prepared from nanoparticle slurry

This study reports enhanced current (CERCS) and energy efficiency (EERCS) of reactive chlorine species (RCS) generation on Ir7Ta3Oy anode by Ti/Bi mixed metal oxide heterojunction layers despite reductions in pseudo-capacitance and film conductivity. In potentiostatic electrolysis of 50 mM NaCl solutions, dramatic improvement (0.61 mmol cm−2 hr−1 at 2.5 V NHE) was noted by simple coating of thin (~2 μm) TiO2 layer from ball-milled TiO2 nanoparticle (80–100 nm) suspension, even with moderate elevation in voltammetric wave. Decoration of Bi2O3 particles (1 – 2 μm) showed limited or adverse effects for RCS generation and stability. However, Bi-doped TiO2 layers prepared from polyol-mediated or co-precipitation methods marked the highest CERCS (~100%) and EERCS (8.16 mmol Wh−1 at 2.5 V NHE) by increased mixing level and effective shift in surface charge. Surface ·OH exclusively mediated the RCS generation whose further transformation to higher oxide could be restrained by the heterojunction layer.

Enhanced polyethylene wetting properties by a simple dipping process

Functional surfaces with special wettability have drawn the researchers’ attention in recent years due to the numerous benefits they present in their various applications, especially in the field of polymeric materials. Several works were published concerning various kinds of polymers, but data on polyolefins, specifically polyethylene, are rather scarce, and all are involved and onerous processes. Herein we report a simple dipping method in a suspension of TiO2 nanoparticles combined with a silane coupling agent. The main result reported is the obtainment of superhydrophobicity in polyethylene with the addition of n-hexadecane to the suspension. A systematic study aiming at the increase in surface hydrophobicity was carried out on the most commercially used types of polyethylene. The surfaces were coated with titanium dioxide nanoparticles functionalized with an organosilane in order to obtain a superhydrophobic material. The main variation regarding the published data dwelled in the addition of an alkane to the treatment solution. The technique consisted of a single layer immersion (dip-coating) of the polymer in the treatment solution, at an optimally defined working temperature, and with variations in the parameters time of immersion and concentration of the alkane additive. The treated surfaces were analyzed and characterized by static contact angle measurement (WCA), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Raman spectroscopy, and optical profilometry. A progressive increase in hydrophobicity was found with increasing alkane concentration, reaching WCA values higher than 150° for all types of polyethylene used, characterizing superhydrophobic behavior.

Morphological characterization and photocatalytic efficiency measurements of pure silica transparent open‐cell sponges coated with TiO2

AbstractTransparent glass sponges are a new class of materials that can potentially be used for effective light distribution in photocatalytic applications. In this work, transparent glass sponges are prepared by the polymer replica technique and coated with TiO2 to obtain a highly efficient photocatalytic material. Necessary conditions for obtaining transparent open‐celled glass sponges are presented. The structure is characterized by employing light microscopy and scanning electron microscopy. In addition, µ‐computer tomography (µ‐CT) volume image analysis was performed that allows the calculation of geometrical parameters like pore size and specific surface area (SSA) of the structure, required for application of the new material. For photocatalytic application, the sponges have been coated with titanium dioxide (TiO2) by immersion in a suspension of TiO2 nanoparticles. The activity of the resulting coatings has been tested with the model reaction of transformation of Cr(VI) to Cr(III) in aqueous solutions in the presence of EDTA at pH 2. The reaction kinetics could be adjusted to a pseudo‐first order reaction and the obtained rate constants have been employed to evaluate the photocatalytic activity. The activity of the sponges has been compared with that of samples of glass plates coated with TiO2; it has been found that the sponges exhibited a higher catalytic activity than the glass plates. The photocatalytic activity increased with the porosity of the sponges, and the optimum sample was determined based on its photocatalytic performance and its mechanical stability. The results are promissory for application in photoreactors for transformation of pollutants in water and air decontamination.

Performance Assessment of Micropollutants Removal From Water Using Advanced Oxidation Processes

In a global climate change scenario, reliable access to clean and safe water for all remains a great worldwide challenge for the twenty first century, as one of the most ambitious targets of several Sustainable Development Goals (SDG) established in the UN Agenda 2030. The increasing presence in the urban aquatic ecosystems of priority pollutants and contaminants of emerging concerning (CECs) have brought new challenges to the existing water treatment systems (WTS) concerning with public health protection and the of drinking water sources preservation. Advanced oxidation processes (AOPs) have been widely studied because of their potential as a complementary or alternative process to conventional wastewater treatment. Several AOPs using nanomaterials as photocatalyst can be particularly effective in the degradation of many toxic micropollutants, and enhance the multifunctionality, versatility and sustainability of WTS. This work presents a synthesis of the major results obtained in several pilot and lab-scale studies aiming to assess the performance of different low-cost catalytic processes used for antibiotic and pesticide removal. For each photo-oxidation reactors, different test scenarios are defined in order to evaluate the effects of several abiotic and hydraulic parameters on process kinetics and removal efficiencies. The experimental results were very promising, because antibiotic removal efficiencies achieved the maximum value of 96% for the photo-oxidation by water columns with suspended TiO2 nanoparticles, and 98% for the photocatalytic filtration performed by a porous medium coated with TiO2. In the photoelectrocatalytic reactor, the atrazine concentration has been fully removed for reaction times between 35 and 95 minutes.

Photocurrent amplification of graphene intercalation with titanium dioxide in photoelectrochemical devices

As a promising solution to renewable energy source and self-powered sensing, photoelectrochemical (PEC) devices have attracted extensive research interests in recent years. In this work, a PEC device is developed by sequentially incorporating graphene and titanium dioxide (TiO2) nanoparticle multilayers using layer-by-layer (LbL) self-assembly. As a mediation layer between the TiO2 nanoparticle film and the gold substrate, compact graphene layer effectively facilitates the separation of electrons and holes, yielding significantly higher quantum efficiency. Dependent on the concentration of TiO2 nanoparticle suspension, graphene intercalation layer enhances more than one order of magnitude photocurrent amplification under UV light illumination, ranging from 12 to 43 times. The low-cost and facile fabrication processes to generate a stable photocurrent output of this device implies its promising potential applications to sensing and energy harvesting.

Titania-Coated Alumina Foam Photocatalyst for Memantine Degradation Derived by Replica Method and Sol-Gel Reaction.

In the present work, alumina (Al2O3) foam was prepared by the replica method where a polyurethane (PU) foam (30 pores per inch (ppi)) template was impregnated with a 60 wt.% Al2O3 suspension. Sintered Al2O3 foam was used as substrate for the deposition of sol-gel derived titania (TiO2) film using dip coating. For the preparation of TiO2 sol, titanium(IV) isopropoxide (Ti-iPrOH) was used as the precursor. The common problem of qualification and quantification of a crystalline coating on a highly porous 3D substrate with an uneven surface was addressed using a combination of different structural characterization methods. Using Powder X-ray Diffraction (PXRD) and synchrotron Grazing Incidence X-ray Diffraction (GIXRD) on bulk and powdered Al2O3 foam and TiO2-coated Al2O3 foam samples, it was determined Al2O3 foam crystallizes to corundum and coating to anatase, which was also confirmed by Fourier Transformed Infrared Spectroscopy (FTIR). Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM/EDS) revealed the structural and microstructural properties of the substrate and coating. Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA) were used to clarify the evolution of the porous microstructure. The Al2O3-TiO2 composite was evaluated as a photocatalyst candidate for the degradation of the micropollutant medication memantine. The degradation rate was monitored using a light-emitting diode (LED) lamp operating at electromagnetic (EM) wavelength of 365 nm. The photocatalytic activity of sol-gel-derived TiO2 film immobilized on the Al2O3 foam was compared with commercially available TiO2 nanoparticles, P25-Degussa, in the form of a suspension. The levels of memantine were monitored by High-Performance Liquid Chromatography–Tandem Mass Spectrometry (HPLC–MS/MS). The efficiency and rate of the memantine photodegradation by suspended TiO2 nanoparticles is higher than the TiO2-coated Al2O3 foam. But, from the practical point of view, TiO2-coated Al2O3 foam is more appropriate as a valuable photocatalytic composite material.

Tuning friction and slip at solid-nanoparticle suspension interfaces by electric fields

We report an experimental Quartz Crystal Microbalance (QCM) study of tuning interfacial friction and slip lengths for aqueous suspensions of TiO2 and Al2O3 nanoparticles on planar platinum surfaces by external electric fields. Data were analyzed within theoretical frameworks that incorporate slippage at the QCM surface electrode or alternatively at the surface of adsorbed particles, yielding values for the slip lengths between 0 and 30 nm. Measurements were performed for negatively charged TiO2 and positively charged Al2O3 nanoparticles in both the absence and presence of external electric fields. Without the field the slip lengths inferred for the TiO2 suspensions were higher than those for the Al2O3 suspensions, a result that was consistent with contact angle measurements also performed on the samples. Attraction and retraction of particles perpendicular to the surface by means of an externally applied field resulted in increased and decreased interfacial friction levels and slip lengths. The variation was observed to be non-monotonic, with a profile attributed to the physical properties of interstitial water layers present between the nanoparticles and the platinum substrate.

Unpredictable Nanoparticle Retention in Commonly Used Plastic Syringes Introduces Dosage Uncertainties That May Compromise the Accuracy of Nanomedicine and Nanotoxicology Studies

In recent animal experiments with suspensions of radiolabeled TiO2 nanoparticles large and highly variable radioactivity fractions were retained in disposable plastic syringes. After unloading between 10% and up to 70% of the loaded dose were still present in the syringes. As a consequence the effectively delivered nanoparticle dose to the animals was frequently much smaller than the nominal dose of the nanoparticles loaded into the syringe. The high variability of this nanoparticle retention challenges the application of a precise, predefined dose and creates a major error source when normalizing organ and tissue contents to the dose loaded into the syringe, which is usually set as the applied dose. A control study was performed employing six commonly used syringe types with seven types of radiolabeled oxide and metallic nanoparticles. For this purpose the syringes were loaded with a given volume of nanoparticle suspension, the radioactivity was measured, the syringe was unloaded and the activity measurement was repeated with the empty syringe. The highest retention values were found when using TiO2 nanoparticle suspensions with Tuberkulin type syringes. In the worst case between 6.6% and 79.1% of the nanoparticles were retained in the syringe. When using the same nanoparticle suspension with an insulin-type syringe the retention was reduced to 1.4% to 20.6%. For amorphous silica nanoparticles the maximum observed retention was 8% and for Au nanoparticles it was 5.1%. Further data gathered from in vivo animal imaging studies show that nanoparticle retention in syringes also affects experiments with nanoparticles such as exosomes, polymersomes, and protein-based nanoparticles investigated for possible applications in nanomedicine. Since the retention is highly variable the effectively applied dose cannot be determined by applying a simple syringe retention factor. The present work shall alert to the problem and illustrate its possible magnitude and unpredictable variability. As mitigation strategy adequate checks with different syringe types are proposed in order to find out whether a given combination of syringe type and nanoparticle suspension is affected by nanoparticle retention and, if necessary, to select a different syringe type that minimizes retention.

Efficient Photocatalytic Reduction of Aqueous Perrhenate and Pertechnetate.

The pertechnetate anion (99TcO4-) is a long-lived radioactive species that is soluble in aqueous solution, in contrast to sparingly soluble 99TcO2. Results are reported for photocatalytic reduction and removal of perrhenate (ReO4-), a nonradioactive surrogate for 99TcO4-, using a TiO2 (P25) nanoparticle suspension in formic acid under UV-visible irradiation. Re(VII) removal up to 98% was achieved at pH = 3 under air or N2. The proposed mechanism is Re(VII)/Re(IV) reduction mediated by reducing radicals (·CO2-) from oxidation of formic acid, not direct reduction by photogenerated electrons of TiO2. Recycling results indicate that photocatalytic reduction of ReO4- exhibits excellent regeneration and high activity with >95% removal even after five cycles. 99Tc(VII) is more easily reduced than Re(VII) in the presence of NO3- with very slow redissolution of reduced 99Tc. This study presents a novel method for the removal of ReO4-/99TcO4- from aqueous solution, with potential application for deep geological disposal.

Removal of arsenic with reduced graphene oxide-TiO2-enabled nanofibrous mats

Arsenic contamination is found at dangerous levels in drinking water around the world and is many times treated with an oxidation step from arsenite [As(III)] to the more easily adsorbed and less toxic arsenate [As(V)]. Herein, electrospun nanofibrous mats made of poly(lactic-co-glycolic) acid and chitosan are coated with reduced graphene oxide (rGO) and titanium dioxide (TiO2) for enhanced oxidation of arsenic. We compared the oxidative properties of UV-irradiated rGO-TiO2 nanocomposite-enabled fibers (rGO-TiO2@fibers) to both their non-reduced counterpart (GO-TiO2@fibers) and TiO2 nanoparticles in suspension. The rGO-TiO2@fibers have a rate constant an order of magnitude larger, and oxidize 2.5 times more As(III), per mass of TiO2 as compared to GO-TiO2@fibers. When bound to UV-illuminated TiO2, GO—and especially the more-conductive rGO—accept electrons from TiO2’s valence band, inhibiting electron-hole recombination and increasing production of hydroxyl radicals (⋅OH) that oxidize As(III). Hydroxyl radical generation was correlated with As(III) oxidation and used to determine radical-scavenging of the fibers. TiO2 paired with rGO achieved the highest levels of radical production both when bound to fibers and when in suspension, illustrating a synergistic relationship between rGO and TiO2. Reuse properties and titanium leaching of the rGO-TiO2@fibers and GO-TiO2@fibers were evaluated over the course of six oxidative cycles, illustrating limited leaching and consistent enhanced oxidative performance by the rGO-TiO2@fibers. Nanocomposites of rGO-TiO2 have not yet been reported as a surface coating on electrospun fibrous scaffolds, and their applications are typically in dye degradation, bacteria inactivation, and electrochemistry. Through their superior oxidative performance, “green” synthesis, and facile recyclability, rGO-TiO2@fibers herein are an example of nano-enabled technology being strategically leveraged to address water treatment challenges around the world.

Conformational transitions of poly(ethylene oxide) in supersonic jets of supercritical CO2

The supersonic gas jets are proposed as a tool for researching conformational transitions of polymer chain molecules in untraditional (supercritical) fluids. The working fluid of the jet and the solvent for polymers in this case are supercritical fluids. This method is used for the microencapsulation of TiO2 nanoparticles in poly(ethylene oxide) (Mv = 8,000,000). The suspension of TiO2 nanoparticles in the polymer solution in supercritical carbon dioxide is expanded into vacuum or background gas. The substrate of the jet resulting from rapid expansion was investigated by SEM. Along the axis of the jet, zones of coil-globule-coil transitions were found. The position and extent of these zones can be controlled by varying the conditions in the reactor (pressure, temperature, co-solvent concentration) and background gas pressure in the nozzle chamber.

Unusual photorheological properties of TiO2 nanoparticle suspensions under UV light irradiation

Precise control of the rheological properties of nanofluids in a non-contact way is of particular interest for its application in various nanotechnologies such as in microfluidic chips. In this study, for the first time, it was found that the viscosity of TiO2 nanoparticle suspensions can be precisely controlled by UV light irradiation of certain time. After 20 min irradiation of UV light, the viscosity increment of the suspensions compared to the base fluid was observed to reduce by 63.2%. This photorheological effect is irreversible unless ultra-sonication treatment was applied. More interestingly, the viscosity of the suspensions first decreases and then reversely increases with the irradiation time, namely, there is a critical value for the irradiation time. However, the UV irradiation has negligible influence on the thermal conductivity of the suspensions. It turns out that the UV irradiation can cause both increased particle size and hydrophilcity of TiO2 nanoparticles in the suspensions. Accordingly, molecular dynamics (MD) simulation revealed that the increased hydrophilcity of particle surface can result in the decrease in the viscosity of solid–liquid systems. It can be concluded that the observed unusual photorheological properties should be attributed to the combined effect of particle aggregation process and hydrophilcity increase. Our study should be of great value for nanoparticle suspensions in practical applications related to fluid flow and heat transfer.