Cuiping Li

Low-temperature synthesis of heterogeneous crystalline TiO2–halloysite nanotubes and their visible light photocatalytic activity

One-dimensional heterogeneous crystalline TiO2–halloysite nanotubes with high visible-light photocatalytic activity have been successfully obtained by employing the low-temperature synthesis of crystalline TiO2 on halloysite nanotubes. The halloysite nanotubes can adsorb the TiO2 precursor and induce the growth of TiO2 nanocrystals on the support in situ. By simply adjusting the acidity of the TiO2 sol, crystalline TiO2 composite halloysite nanotubes with tunable crystalline phases of anatase or anatase/rutile mixed phases were achieved. The traditional thermal treatment for crystallite transformation was not required, thus an intact halloysite structure could be guaranteed. The as-obtained products were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption–desorption, UV-vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). The photocatalysis test results revealed that the obtained heterogeneous TiO2–halloysite showed enhanced visible-light photocatalytic activity for the degradation of rhodamine B and gentian violet compared with the pure halloysite nanotubes and Degussa-P25 owing to the heterogeneous TiO2 structure.

Effective Adsorption/Reduction of Cr(VI) Oxyanion by Halloysite@Polyaniline Hybrid Nanotubes

Halloysite@polyaniline (HA@PANI) hybrid nanotubes are synthesized by the in situ chemical polymerization of aniline on halloysite clay nanotubes. By facilely tuning the dopant acid, pH, and apparent weight proportion for aniline (ANI) and halloysite (HA) nanotubes in the synthesis process, PANI with tuned oxidation state, doping extent, and content are in situ growing on halloysite nanotubes. The reaction systems acidity is tuned by dopant acid, such as HCl, H2SO4, HNO3, and H3PO4. The adsorption result shows the fabricated HA@PANI hybrid nanotubes can effectively adsorb Cr(VI) oxyanion and the adsorption ability changes according to the dopant acid, pH, and apparent weight proportion for ANI and HA in the synthesis process. Among them, the HA@PANI fabricated with HCl as dopant acid tuning the pH at 0.5 and 204% apparent weight proportion for ANI and HA (HP/0.5/204%-HCl) shows the highest adsorption capacity. The adsorption capacity is in accordance well with the doping extent of PANI in HA@PANI. Furthermore, when HP/0.5/204%-HCl is redoped with HNO3, H2SO4, and H3PO4, the adsorption capacity declines, implying the dopant acid in the process of redoping exhibits a marked effect on Cr(VI) oxyanion adsorption for the HA@PANI hybrid nanotubes. HP/0.5/204%-HCl and HP/0.5/204%-H3PO4 have demonstrated good regenerability with an above 80% removal ratio after four cycles. Moreover, the HA@PANI adsorbent has better sedimentation ability than that of pure PANI. The adsorption behavior is in good agreement with Langmuir and pseudo second-order equations, indicating the adsorption of HA@PANI for Cr(VI) oxyanion is chemical adsorption. FT-IR and XPS of HA@PANI after Cr(VI) oxyanion adsorption indicate that the doped amine/imine groups (-NH+/═N+- groups) are the main adsorption sites for the removal of Cr(VI) oxyanion by electrostatic adsorption and reduction of the adsorbed Cr (VI) oxyanion to Cr(III) simultaneously.

Facile construction of ultrathin standing α-Ni(OH)2 nanosheets on halloysite nanotubes and their enhanced electrochemical capacitance

One-dimensional nanostructures of ultrathin standing α-Ni(OH)2 nanosheets@halloysite nanotubes are synthesized through one-step facile precipitation method. The nanocomposites exhibit high capacitance (1677 F g−1) and excellent cycling stability (100% capacity retention after 2000 cycles) due to their ultrathin and standing nanosheets and intense cation/anion exchange performance of halloysite nanotubes. The remarkable electrochemical performance will undoubtedly make the hybrid structures attractive for high-performance supercapacitors.

Halloysite nanotube supported Ag nanoparticles heteroarchitectures as catalysts for polymerization of alkylsilanes to superhydrophobic silanol/siloxane composite microspheres

Halloysite nanotube supported Ag nanoparticles heteroarchitectures have been prepared through a very simple electroless plating method. Robust Ag nanocrystals can be reproducibly fabricated by soaking halloysite nanotubes in ethanolic solutions of AgNO3 and butylamine. By simply adjusting the molar ratio of AgNO3 and butylamine, Ag nanoparticles with tunable size and quantity on halloysite nanotube are achieved. It reveals that the Ag nanoparticles are well-dispersed on the surface of halloysite nanotubes. The halloysite nanotube supported Ag nanoparticles heteroarchitectures can serve as active catalysts for the polymerization of an alkylsilane C18H37SiH3 with water to form silanol/siloxane composite microspheres and exhibit interesting superhydrophobicity ascribed to the micro/nanobinary structure.

Low temperature synthesis of polyaniline–crystalline TiO2–halloysite composite nanotubes with enhanced visible light photocatalytic activity

A series of one-dimensional polyaniline-crystalline TiO2-halloysite composite nanotubes with different mass ratio of polyaniline to TiO2 are facilely prepared by employing the low-temperature synthesis of crystalline TiO2 on halloysite nanotubes. The halloysite nanotubes can adsorb TiO2/polyaniline precursors and induce TiO2 nanocrystals/polyaniline to grow on the support in situ simultaneously. By simply adjusting the acidity of reaction system, PANI-crystalline TiO2-HA composite nanotubes composed of anatase, a mixed phase TiO2 and different PANI redox state are obtained. The XRD and UV-vis results show that the surface polyaniline sensitization has no effect on the crystalline structure of halloysite and TiO2 and the light response of TiO2 is extended to visible-light regions. Photocatalysis test results reveal the photocatalytic activity will be affected by the pH value and the volume ratio of ANI to TTIP. The highest photocatalytic activity is achieved with the composite photocatalysts prepared at pH 0.5 and 1% volume ratio of ANI and TTIP owing to the sensitizing effect of polyaniline and the charge transfer from the photoexcited PANI sensitizer to TiO2. Moreover, the PANI-TiO2-HA composite nanotubes synthesized by one-step at pH 0.5 with 1% volume ratio of ANI to TTIP exhibit higher visible light photocatalytic activity than those synthesized by the two-step. Heterogeneous PANI-TiO2-HA composite nanotubes prepared at pH 0.5 exhibit a higher degradation activity than that prepared at pH 1.5. The redoped experiment proves that the PANI redox state plays the main contribution to the enhanced visible light catalytic degradation efficiency of PANI-TiO2-HA prepared at pH 0.5. Furthermore, the heterogeneous PANI-crystalline TiO2-HA nanotubes have good photocatalytic stability and can be reused four times with only gradual loss of activity under visible light irradiation.

Large scale synthesis of Janus nanotubes and derivative nanosheets by selective etching

One-dimensional Janus nanotubes have been successfully synthesized in large quantity by selective etching of the interior Al2O3 from hydrophobically modified halloysite nanotube with the exterior surface preserved. By simply tuning the etching time, the colloid shape evolves from nanotubes to partially collapsed nanotubes and porous nanorods. The microstructure and chemical composition of the etched hydrophobically modified halloysite are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), FT-IR spectrophotometer and nitrogen adsorption-desorption. The results indicate the interior Al2O3 can be controlled etched. Labeling experiments demonstrate the hydrophilic nanoparticles are predominantly located onto the coarse region of the etched nanotubes. Selective etching of hydrophobically modified halloysite creates Janus nanotubes with a hydrophobic exterior surface and a hydrophilic interior surface. An enhanced capacity for emulsification of the Janus nanotubes toward immiscible mixture is shown. Furthermore, by a favorable growth of other functional materials, composition of the Janus nanotubes can be further extended, enabling different applications from magnetic separation to water purification, drug immobilization and controlled release. After crushing the Janus nanotubes, derivative nanosheets are derived. The method can be scaled up and economical compared with other method, which is the key to further exploit practical application.

Enhanced solvent-free selective oxidation of cyclohexene to 1,2-cyclohexanediol by polyaniline@halloysite nanotubes

One-dimensional polyaniline@halloysite (PANI@HA) nanotubes with enhanced selective oxidation activity of cyclohexene are fabricated by employing aniline (ANI) chemical polymerization on halloysite nanotubes in situ. By facilely controlling the doping acid, acidity, and ANI/HA weight ratio during the fabrication, PANI with a controllable doping degree, redox state, and content is grown on halloysite nanotubes. The cyclohexene selective oxidation result shows that PANI@HA nanotubes are effective catalysts in a solvent-free reaction system with H2O2 as the oxidant, and their catalytic activity relies on the doping acid, acidity, and ANI/HA weight ratio in the fabrication. PANI@HA synthesized with HCl as a doping acid to condition the acidity at 1 M and 2.04 ANI/HA weight ratio (PANI@HA/1 M/2.04-HCl) demonstrates highest catalytic activity (98.17% conversion and 99.50% selectivity to 1,2-cyclohexanediol). The cyclohexene selective catalytic activity matches well with the PANI doping degree in PANI@HA. In addition, the optimal reaction condition is 20 mg catalyst, 2.5 mL H2O2, 70 °C, and 24 h. Furthermore, PANI@HA/1 M/2.04-HCl exhibits superior dihydroxylation activity toward 2,3-dimethyl-2-butene and cycling performance with 99.11% conversion and 96.92% selectivity to 1,2-cyclohexanediol after five cycles. The CV of PANI@HA indicates that the cyclohexene selective oxidation is attributed to a reversible redox reaction of PANI in PANI@HA for catalytic decomposition of H2O2.