Pristine Halloysite Nanotubes Research Articles

Hydrodeoxygenation of guaiacol over halloysite nanotubes decorated with Ru nanoparticles: Effect of alumina acid etching on catalytic behavior and reaction pathways

Halloysite nanotubes (HNTs) is a promising support for liquid-phase hydrodeoxygenation (HDO) catalysts due to a wide abundance, low cost and tunable properties. Acid etching of halloysite by sulfuric acid was found to enlarge lumen diameter (24.3 nm), enhance specific surface area (154 m2/g) and acidity (0.343 mmol/g). Based on pristine and dealuminated HNTs, Ru-containing catalysts were synthesized via impregnation under microwave irradiation. The catalytic properties of synthesized catalysts in HDO of guaiacol were studied in batch reactors at 120–180 °C, under hydrogen pressure of 3 MPa, with guaiacol/Ru molar ratio of 200 in water as a solvent. Comparing the results of catalytic tests, we have concluded that the catalysts based on etched halloysite enhanced greater activity (TOF – 211 h−1) in the HDO of guaiacol compared their analogue supported on pristine HNTs. Moreover, the Ru-catalyst based on etched HNTs contributed to an increase selectivity to hydrogenolysis with effective oxygen removal.

Mussel-inspired grafting pH-responsive brushes onto halloysite nanotubes for controlled release of doxorubicin

The development of stimuli-responsive drug nanocarriers is an increasingly important area in nanomedicine because efficient delivery of toxic drugs to targeted tissues minimizes side effects. The specific objective of this study was to synthesize and characterize a novel pH-responsive drug carrier based on halloysite nanotubes for the controlled release of the anticancer drug doxorubicin. Poly(N,N-dimethylaminoethyl methacrylate) brushes were grafted from the surface of halloysite nanotubes using the combination of mussel-inspired polydopamine surface modification and activators regenerated by electron transfer in atom transfer radical polymerization. The chemical structure and morphology of the modified halloysite nanotubes were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermal gravimetric analysis as well as scanning and transmission electron microscopies. Dynamic light scattering and zeta potential analysis were carried out to evaluate the pH-responsivity of the functionalized halloysite nanotubes. The results of the drug loading and release study of pristine and functionalized halloysite nanotubes showed that grafting of poly(N,N-dimethylaminoethyl methacrylate) brushes on the polydopamine-modified halloysite nanotubes surface leads to a drastic increase in doxorubicin loading capacity and a highly pH-sensitive release behaviour. Less than 10 % of the loaded doxorubicin was released from poly (N,N-dimethylaminoethyl methacrylate)-grafted halloysite nanotubes at pH 7.4 after 24 h; in contrast, at pH 5.5 there was a continuous release of doxorubicin totalling 13 % in the first 30 min, i.e. lower than for the pristine halloysite nanotubes (32 %), but reaching 48 % after 24 h. Poly (N,N-dimethylaminoethyl methacrylate)-grafted halloysite nanotubes can hence be considered as a potential candidate for delivering highly toxic drug molecules to the acidic target sites.

Metal-free pristine halloysite nanotubes: Electrochemically active and stable oxygen evolution reaction

Halloysite nanotubes (HNT) are naturally occurring one-dimensional porous clay minerals and have a unique chemical structure of kaolinite. The presence of hydroxyl functional groups on the surface of HNT is more suitable for O2 evolution reaction (OER). The HNT based electrocatalyst coated on Ni foam substrate shows excellent water oxidation activity with an overpotential of 287 mV to reach a current density of 10 mA cm−2 and have excellent stability in alkaline electrolyte solution over 32 h without any decay. The adsorption of OH− species occurs on the surface of HNT from the alkaline electrolyte via one-electron transfer, the metal cation underwent repeated oxidation and reduction reactions during the OER. This result outperforms many of the transition metal-based electrocatalysts. This work provides a new platform to use naturally abundant clay minerals as an anode material in fuel cell applications.

Effect of pristine halloysite nanotubes on dynamic mechanical and thermal behavior of reinforced epoxy nanocomposites

Effect of pristine halloysite nanotubes on dynamic mechanical and thermal behavior of reinforced epoxy nanocomposites

Physico‐mechanical properties of poly (vinyl alcohol), poly (vinyl alcohol)/boric acid, and poly (vinyl alcohol) nanocomposites incorporated with amino‐functionalized and pristine halloysite nanotubes films

AbstractEco‐friendly poly(vinyl alcohol) (PVA), PVA/boric acid, PVA/halloysite nanotubes (HNTs), and PVA/amino‐functionalized HNTs (APTES‐HNTs) films were fabricated by a solution casting technique. The samples were characterized by fourier transform infrared, X‐ray diffraction, differential scanning calorimetry, scanning electron microscope, and energy‐dispersive spectroscopy. The characterization results proved the chemical and physical interactions between the PVA and different additives. The viscoelastic behavior of the films was evaluated by DMA and creep analysis. The storage modulus, loss factor, and both αα and ββ transitions affected by APTES‐HNTs as a potential filler to form effective cross‐links. APTES‐HNTs existence enhanced creep‐recovery beyond expectations. Tensile and impact strength were measured to understand samples' mechanical stability. PVA/APTES‐HNTs and PVA/boric acid showed more yield behavior after the elastic limit. Furthermore, the subsequent rupture and impact strength were increased significantly compared with neat PVA and PVA/HNTs. The viscoelastic and mechanical behaviors were linked to each other by the area under Tanδ curve and the work of rupture and impact strength, which their linear correlation coefficient is statistically significant at 95% confidence limits. It seems that the presence of APTES‐HNTs provides new cross‐links, which altered (improved) the physico‐mechanical properties of PVA, offering a bionanocomposite suitable for further applications. From the literature, possible explanations are provided for these observations.

Exploring halloysite nanotubes as catalyst support for methane combustion: Influence of support pretreatment

Catalytic combustion of methane is considered an environmentally friendly route for energy generation owing to the high H/C ratio of methane and low CO2 emissions compared to other hydrocarbons fuels. The utilization of heterogeneous catalysts permits combustion at lower temperatures compared with thermal combustion. Herein, we developed new catalysts of palladium-supported halloysite nanotubes (HNTs) for methane combustion. This was performed via chemical modification of HNTs using four different species i.e. H2SO4 (HNTs-H2SO4), NaOH (HNTs-NaOH), sodium dodecyl sulfate (HNTs-SDS), and cetyltrimethylammonium bromide (HNTs-CTAB). After chemical modification, Pd was deposited on the pretreated HNTs and the impact of chemical treatment on the morphology, crystal structure, textural properties, and methane oxidation activity was studied. All catalysts based on chemically modified HNTs exhibited enhanced catalytic performance towards methane combustion compared to Pd-supported on pristine HNTs, in particular, Pd/alkali-treated HNTs (Pd/HNTs-NaOH) revealed the highest catalytic activity towards methane combustion with the temperature of complete conversion (T100) equals 385 °C and activation energy of 79.15 kJ mol−1. Furthermore, the same sample displayed enhanced stability compared to other counterparts. This was assigned to the confinement of Pd nanoparticles at the inner surface which enhances the catalyst-support interaction, increases the number of surface active sites, and enhances the resistance to sintering at high temperatures.

Halloysite nanotubes as a carrier of cornelian cherry (Cornus mas L.) bioactives

Cornelian cherry fruit extract rich in anthocyanins and iridoids was encapsulated in the halloysite nanotubes in order to obtain a stable nanoscale system for better delivery and prolonged release of bioactive constituents. The cyclic vacuum technique was used for halloysite nanotubes-cornelian cherry composite preparation and the loading of 8.5 wt% was achieved. Pure cornelian cherry extract exhibited antiproliferative effect on HT-29, MCF7, and MRC-5 cells, pristine halloysite nanotubes affected the growth of MCF7 cells, while halloysite nanotubes-cornelian cherry composites demonstrated proliferative activity in all tested cells. The sustained release of anthocyanins was achieved by this encapsulation strategy. Additional implementation of halloysite nanotubes-cornelian cherry into yogurt prolonged the release of iridoid molecules. Obtained results indicate that halloysite nanoclay is a suitable nanocarrier for cornelian cherry constituents that can be utilized in food and pharmaceutical industries.

Silver nanoparticles incorporated in dicarboxylic/TEPA modified halloysite nanotubes for the degradation of organic contaminants

Silver supported on organo-modified halloysite nanotubes (Ag-HNTX/T) were fabricated through three steps of modifications; (i) functionalization of pristine halloysite nanotubes (HNT) with dicarboxylic acids, namely succinic acid (SA), oxalic acid (OX), or sebacic acid (SB), (ii) addition of tetraethylenepentamine (TEPA), and finally (iii) incorporation of silver particles (Ag). The modification with dicarboxylic acid and TEPA on HNT was proven by the presence of COOH and NH functional groups in the FTIR spectra. Upon introduction of Ag particles on the organo-modified HNT, the Ag-HNTSA/T sample exhibits UV-DRS absorption at 460 nm, corresponding to the presence of Ag particles with the average size of 12.1 ± 6.5 nm. The HRTEM result reveals that Ag particles are deposited on the surface and inside the lumen of HNT, which is in-line with slight decrease in SBET attributed to some pore blocking in Ag-HNTSA/T. The catalytic activity of the fabricated Ag-HNTX/T (X = SA, OX or SB) was evaluated through the borohydride-assisted degradation of organic contaminants, i.e. methyl orange (MO), methylene blue (MB), p-nitrophenol (p-NP), and bromocresol green (BG). The Ag-HNTX/T catalyst exhibited excellent catalytic activity and the process follows the pseudo-first order kinetics. The Ag-HNTX/T possesses sustainable catalytic performance up to 15 recycled applications.

Antibacterial properties and in vivo studies of tannic acid-stabilized silver–halloysite nanomaterials

AbstractTannic acid-stabilized silver nanoparticles were synthesized in situ on halloysite clay nanotubes. The synthesis strategy included simple steps of tannic acid adsorption on clay nanotubes and further particle formation from silver salt solution. Pristine halloysite nanotubes as well as amino-modified clays were used for silver stabilization in water or ethanol. The materials were tested for antibacterial performance using three different methods. All of the materials produced showed antimicrobial activity. The pristine halloysite-based material with ~5 nm particles produced using ethanol as the solvent and tannic acid as the reducing agent showed the greatest antibacterial activity against Serratia marcescens. The materials were tested in vivo on Caenorhabditis elegans nematodes to ensure their safety, and they showed no negative effects on nematode growth and life expectancy.

Ruthenium Catalysts Templated on Mesoporous MCM-41 Type Silica and Natural Clay Nanotubes for Hydrogenation of Benzene to Cyclohexane

Mesoporous ruthenium catalysts (0.74–3.06 wt%) based on ordered Mobil Composition of Matter No. 41 (MCM-41) silica arrays on aluminosilicate halloysite nanotubes (HNTs), as well as HNT-based counterparts, were synthesized and tested in benzene hydrogenation. The structure of HNT core-shell silica composite-supported Ru catalysts were investigated by transmission electron microscopy (TEM), X-ray fluorescence (XRF) and temperature-programmed reduction (TPR-H2). The textural characteristics were specified by low-temperature nitrogen adsorption/desorption. The catalytic evaluation of Ru nanoparticles supported on both the pristine HNTs and MCM-41/HNT composite in benzene hydrogenation was carried out in a Parr multiple reactor system with batch stirred reactors (autoclaves) at 80 °C, a hydrogen pressure of 3.0 MPa and a hydrogen/benzene molar ratio of 3.3. Due to its hierarchical structure and high specific surface area, the MCM-41/HNT composite provided the uniform distribution and stabilization of Ru nanoparticles (NPs) resulted in the higher specific activity and stability as compared with the HNT-based counterpart. The highest specific activity (5594 h−1) along with deep benzene hydrogenation to cyclohexane was achieved for the Ru/MCM-41/HNT catalyst with a low metal content.

Insights into grafting of (3-Mercaptopropyl) trimethoxy silane on halloysite nanotubes surface

Functionalization of halloysite nanotubes surface by using organosilanes is sensitive to the reaction conditions. Halloysite nanotubes (HNTs) were modified using (3-Mercaptopropyl) trimethoxy silane (MPTMS). The experiments were performed under different reaction conditions including, various solvents [Toluene, Tetrahydrofuran (THF), Ethanol, n-Hexane, 1,4 Dioxane and Acetonitrile], water content in the reaction media, volume of solvent, number of moles of silane and catalysts (triethyl amine, ammonia solution and tetra-ethoxy titanium). The elemental analysis, FT-IR analysis were used to identify the samples, which attained the highest percent of functionalization. SEM image and thermogravimetric analysis were provided for the pristine halloysite nanotubes and the modified halloysite nanotubes samples. This work provides a systematic investigation of the reaction conditions highlighting the best protocol to perform such a modification that represents a relevant strategy to control the surface properties of halloysite nanotubes as well as the optimization of the first step toward further modifications of this nano clay.

Surface chemistry of halloysite nanotubes controls the curability of low filled epoxy nanocomposites

A comparative study was performed on curing potential of low-filled epoxy/amine composites containing pristine halloysite nanotubes (P-HNTs), alkali-activated HNTs (A-HNTs), and silane-functionalized A-HNTs (F-HNTs). A-HNTs was obtained from P-HNTs in NaOH solution, then functionalized with 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane (ETMS) in isopropyl alcohol to obtain F-HNTs. X-ray spectroscopy (XPS) and thermogravimetric analysis (TGA) were employed to receive message from surface and bulk changes in nanoparticles, respectively. TGA results confirmed successful surface functionalization of HNT. In XPS spectra, as expected, Na1s appeared after surface activation. Growth in C1s form F-HNT in comparison with two other nanoparticles further approved successful modification. Harmoniously, oxygen graph was declined due to bonding of hydroxyl groups to the surface. Starting from 1 wt. % of HNTs with substantial aggregated HNTs domains seen in epoxy, we examined 0.5, 0.4, 0.3, 0.2, and then 0.1 wt.% of HNTs in 100 parts by weight of epoxy resin, where 0.2 wt.% loading was below the critical nanoparticle concentration for aggregation. Therefore, nanocomposites containing HNTs, A-HNTs, and F-HNTs were all prepared at such low loading level. Nonisothermal differential scanning calorimetry (DSC) at four different heating rates was performed for qualitative analysis of network formation by the aid of Cure Index dimensionless criterion. It was observed that the presence of low percentage of HNT and F-HNT led to Good cure situation in all heating rates. Whereas in low heating rates A-HNT led to Poor cure, which was compensated for at higher heating rates.

Isolating the effect of polymer-grafted nanoparticle interactions with matrix polymer from dispersion on composite property enhancement: The example of polypropylene/halloysite nanocomposites

Interfacial effects can significantly perturb the properties of nanocomposites. Modification of filler surfaces can improve filler dispersion and compatibilize the polymer-filler interface, leading to property enhancements relative to composites made with unmodified filler. Here, we have isolated the effect of interactions between matrix polymer and polymer-grafted nanoparticles (PGNs) from dispersion quality and studied how the interactions affect the properties of polypropylene (PP)/halloysite nanotube hybrids in the absence of entanglements between matrix PP and grafted PP. We prepared PP hybrids with pristine halloysite nanotubes (p-HNT) or PP-grafted halloysite nanotubes (PP-g-HNT) up to 10 wt% filler using melt mixing or solid-state shear pulverization (SSSP). For melt-mixed samples, PGN-type filler yields better property enhancement than pristine filler, consistent with the combination of improved dispersion and polymer-filler interactions. As shown by microscopy and rheology, hybrids prepared by SSSP with p-HNT and PP-g-HNT fillers have very similar, high levels of dispersion, eliminating dispersion quality as a factor in property improvements. In comparison with well-dispersed PP/p-HNT nanocomposites at the same filler level, well-dispersed PP/PP-g-HNT nanocomposites prepared by SSSP exhibit enhanced tensile strength but the same values within error of Young's modulus and elongation at break when there is no entanglement between matrix polymer and grafted chains. Notably, at the same filler level, well-dispersed PP/PP-g-HNT nanocomposites exhibit higher crystallization rate and nucleation efficiency (NE) that is 2.5–4.5 times higher than well-dispersed PP/p-HNT hybrids. The impact of grafted PP chains on NE is so strong that at the same filler level but with worse dispersion, PP/PP-g-HNT hybrids made by melt mixing exhibit NE values 2.1 to 3.4 times higher than well-dispersed PP/p-HNT nanocomposites made by SSSP.

Study on the modification of hexamethylenediamine-phenol-based polybenzoxazine resins by halloysite nanotubes

In this study, hexamethylenediamine-phenol-based benzoxazine monomer (Ba-h) was synthesized using 1,6-hexamethylenediamine, phenol, and paraformaldehyde. Halloysite nanotubes (HNTs) were treated with silane coupling agent and succinic anhydride, respectively. Then the pristine and grafted HNTs were used as nano-fillers to modify Ba-h at different mass ratios ranging between 0 and 2.5 wt% via solution processing. Nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy confirmed the molecular structure of the monomer. The curing behaviors of the nanocomposites were examined by differential scanning calorimetry (DSC). The results of DSC, thermogravimetric analysis, and dynamic mechanical analysis displayed the obvious improvement in the thermal stability of the nanocomposites modified by the grafted HNTs. The results of impact and flexural tests revealed the improvement in the mechanical property of the nanocomposites modified by the pristine HNTs. These enhancements are related to the homogeneous dispersion of HNTs based on the scanning electron microscopy and transmission electron microscopy results.

Synthesis of Mesoporous/Macroporous Microparticles Using Three-Dimensional Assembly of Chitosan-Functionalized Halloysite Nanotubes and Their Performance in the Adsorptive Removal of Oil Droplets from Water.

Halloysite nanotubes (HNTs) were assembled into mesoporous/macroporous microparticles (c-g-HNTs MPs) using Pickering template-assisted approach. To unravel the stabilization mechanism in Pickering emulsion form, several emulsions and microparticles were prepared at various conditions and visualized using confocal laser scanning microscopy. The prepared c-g-HNTs MPs were used to treat emulsified oil solutions resulting in a maximum removal efficiency of 94.47%. The kinetics data of oil adsorption onto c-g-HNTs MPs was best fitted by the pseudo-second-order kinetic model ( R2 = 0.9983). The maximum monolayer adsorption capacity of oil onto c-g-HNTs MPs as predicted by the multilayer Brunauer-Emmett-Teller model was found to be 788 mg/g. Compared with pristine HNTs, c-g-HNTs MPs exhibited higher self-settleability rates in aqueous solutions as well as in emulsified oil solutions, demonstrating their candidacy for practical water treatment applications. The c-g-HNTs MPs were repeatedly used for five adsorption-desorption cycles with minimal losses noticed in their performance.

Toughening of brittle polyester with functionalized halloysite nanocomposites

This study presents the role of pristine halloysite nanotubes (HNT) and silane-functionalized halloysite (s-HNT) on toughening mechanisms and initiating plastic deformation in unsaturated polyester (UPE) nanocomposite. The critical stress intensity factor (KIc) and the critical strain energy release rate (GIc) as fracture toughness indications were measured and the relationship between the morphological structures and toughening mechanisms was identified. The results indicated that the fracture toughness values exhibited a steady-state increasing trend with the incorporation of up to 5 wt % HNT or s-HNT into the UPE resin. The 3% HNT or 3% s-HNT composites were found to obtain the highest toughness values supported with uniformly dispersed particles. The SEM observations showed different energy dissipation mechanisms are; zone shielding and shear yielding with a presence of full particle debonding with the HNT addition; and river line patterns, a tail-like structure and the formation of micro-cracks mechanisms were observed with the addition of s-HNT.

Halloysite Nanotubes Noncovalently Functionalised with SDS Anionic Surfactant and PS-b-P4VP Block Copolymer for Their Effective Dispersion in Polystyrene as UV-Blocking Nanocomposite Films

A simple and versatile method is reported for the noncovalent functionalisation of natural and “green” halloysite nanotubes (HNTs) allowing their effective dispersion in a polystyrene (PS) thermoplastic matrix via solvent mixing. Initially, HNTs (pristine HNTs) were modified with physically adsorbed surfactant molecules of sodium dodecyl sulphate (SDS) and PS-b-P4VP [P4VP: poly(4-vinylpyridine)] block copolymer (BCP). Hereafter, SDS and BCP modified HNTs will be indicated as SDS-m-HNT and BCP-m-HNT. Nanocomposite films with 1, 2, and 5 wt.% HNT loadings were prepared, abbreviated as PS-SDS-m-HNT1, PS-SDS-m-HNT2, and PS-SDS-m-HNT5 and PS-BCP-m-HNT1, PS-BCP-m-HNT2, and PS-BCP-m-HNT5 (where 1, 2, and 5 correspond to the wt.% of HNTs). All nanocomposites depicted improved thermal degradation compared to the neat PS as revealed by thermogravimetric analysis (TGA). Transmission electron microscopy (TEM) confirmed the good dispersion state of HNTs and the importance of modification by SDS and BCP. X-ray diffraction (XRD) studies showed the characteristic interlayer spacing between the two silicate layers of pristine and modified HNTs. The PS/HNT nanocomposite films exhibited excellent ultraviolent-visible (UV-vis) absorbance properties and their potential application as UV-filters could be envisaged.

Experimental and DFT Characterization of Halloysite Nanotubes Loaded with Salicylic Acid

Halloysite nanotubes (HNTs) and salicylic acid (SA) are natural substances widely used in different fields. HNTs are very promising as nanocarriers because of their biocompatibility, atoxicity, anti-inflammatory properties and capacity to maintain the biological activity of immobilized enzymes. Because of its bactericidal and antiseptic properties, salicylic acid (SA) is used in pharmaceutical formulations, and as an additive for preserving foods and cosmetics. In this study, we set up a procedure for the loading of HNTs with SA for their possible application in active food packaging. Pristine HNTs were studied together with acidic etched HNTs with enlarged internal lumen, and various pH values for the loading solutions were tested in order to obtain the maximum loading. The HNTs—empty and loaded with SA—were characterized by TG-FTIR, FTIR SEM, STEM, and nitrogen adsorption/desorption isotherms measurements. We obtained a maximum loading of 10.5% (w/w), using HNTs pretreated with H2SO4 2 M at 25 °C for 48...

Amine-Impregnated Mesoporous Silica Nanotube as an Emerging Nanocomposite for CO2 Capture.

Pristine halloysite nanotubes (HNTs) were pretreated to produce mesoporous silica nanotubes (MSiNTs), which was further impregnated with polyethenimine (PEI) to prepare an emerging nanocomposite MSiNTs/PEI (MP) for CO2 capture. Thermogravimetric analysis (TGA) was employed to analyze the influences of PEI loading amount and adsorption temperature on CO2 adsorption capacity of the nanocomposite. The Brunauer-Emmett-Teller (BET) surface area (SBET) of MSiNTs was six times higher than that of HNTs, and the corresponding pore volume was more than two times higher than that of HNTs. The well dispersion of PEI within the nanotubes of MSiNTs benefits more CO2 gas adsorption, and the adsorption capacity of the nanocomposite could reach 2.75 mmol/g at 85 °C for 2 h. The CO2 adsorption on the nanocomposite was demonstrated to occur via a two-stage process: initially, a sharp linear weight increase at the beginning, and then a relatively slow adsorption step. The adsorption capacity could reach as high as 70% within 2 min. Also, the nanocomposite exhibited good stability on CO2 adsorption/desorption performance, indicating that the as-prepared emerging nanocomposite show an interesting application potential in the field of CO2 capture.

Structure of Hybrid Materials Based on Halloysite Nanotubes Filled with Anionic Surfactants

The structures of pristine halloysite nanotubes (HNTs) and ones functionalized by anionic surfactants (sodium dodecanoate and sodium dodecyl sulfate) were investigated by small angle neutron scattering (SANS). These experiments evidenced the structural organization of the surfactants adsorbed onto the HNT cavity and the importance of the surfactant headgroup. Contrast matching experiments were employed in order to mask the dominant scattering effect of the clay hollow nanotubes and to focus on the surfactant organization within the lumen. Further investigation on the mesoscopic structure of the investigated materials was carried out by electric birefringence (EBR), which allowed study of the rotational mobility of both pristine and functionalized HNTs. The gained structural insights were used to deduce some relevant properties of the hybrids, such as their surfactant loading, charge, and solubilization ability toward hydrophobic compounds. For the latter, the HNT lumen hydrophobization was straightforward...