| 1. |
- Abderrazek, K., et al.
(author)
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Photocatalytic degradation of indigo carmine using [Zn-Al] LDH supported on PAN nanofibres
- 2015
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In: Clay minerals. - : Mineralogical Society. - 0009-8558 .- 1471-8030. ; 50:2, s. 185-197
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Journal article (peer-reviewed)abstract
- Zn-Al layered double hydroxides (LDH), before and after calcination, were tested for the removal of indigo carmine (IC) dye from solution. These LDH photocatalysts were characterized by powder x-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetry/differential thermogravimetry (TG/DTG), nitrogen physisorption at -196 degrees C, scanning electron microscopy (SEM) and diffuse reflectance spectrophotometry (DRS). The different photocatalysts were supported on polyacrylonitrile (PAN) nanofibres, so that filtration was unnecessary. The PXRD and FTIR analyses showed that the IC adsorption on c-Zn-Al-3-500 (LDH calcined at 500 degrees C) was enhanced by construction of the hydrotalcite matrix intercalated with the dye. The intercalation was clearly evidenced by the appearance of a peak at low degrees 2 theta values. All of the materials prepared exhibited photocatalytic activity, which for the c-Zn-Al-3-500 was comparable to that of commercial PAN-supported ZnO nanoparticles (100% degradation after 180 min). Kinetic studies showed that the degradation of the IC followed a pseudo-first order rate. The high activity and the ease of both synthesis and separation processes rendered this photocatalyst a promising candidate for environmental remediation.
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| 2. |
- Hillier, Stephen
(author)
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Adsorption of phosphate by halloysite (7 angstrom) nanotubes (HNTs)
- 2020
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In: Clay Minerals. - : Mineralogical Society. - 0009-8558 .- 1471-8030. ; 55, s. 184-193
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Journal article (peer-reviewed)abstract
- The adsorption and retention of phosphates in soil systems is of wide environmental importance, and understanding the surface chemistry of halloysite (a common soil clay mineral) is also of prime importance in many emerging technological applications of halloysite nanotubes (HNTs). The adsorption of phosphate anions on tubular halloysite (7 angstrom) has been studied to gain a greater understanding of the mechanism and kinetics of adsorption on the surface of HNTs. Two well-characterized tubular halloysites with differing morphologies have been studied: one polygonal prismatic and one cylindrical, where the cylindrical form has a greater surface area and shorter tube length. Greater phosphate adsorption of up to 42 mu mol g(-1) is observed on the cylindrical halloysite when compared to the polygonal prismatic sample, where adsorption reached a maximum of just 15 mu mol g(-1) compared to a value for platy kaolinite (KGa-2) of 8 mu mol g(-1). Phosphate adsorption shows strong pH dependence, and the differences in phosphate sorption between the prismatic and cylindrical morphologies suggest that phosphate absorption does not occur at the same pH-dependent alumina edge sites and that the lumen may have a greater influence on uptake for the cylindrical form.
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| 3. |
- Hillier, Stephen
(author)
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Correlations among the mineralogical and physical properties of halloysite nanotubes (HNTs)
- 2016
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In: Clay Minerals. - : Mineralogical Society. - 0009-8558 .- 1471-8030. ; 51, s. 325-350
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Journal article (peer-reviewed)abstract
- Twenty one samples of relatively pure tubular halloysites (HNTs) from localities in Australia, China, New Zealand, Scotland, Turkey and the USA have been investigated by X-ray diffraction (XRD), infrared spectroscopy (IR) and electron microscopy. The halloysites occur in cylindrical tubular forms with circular or elliptical cross sections and curved layers and also as prismatic tubular forms with polygonal cross sections and flat faces. Measurements of particle size indicate a range from 40 to 12,700 nm for tube lengths and from 20 to 600 nm for diameters. Size distributions are positively skewed with mean lengths ranging from 170 to 950 nm and mean diameters from 50 to 160 nm. Cylindrical tubes are systematically smaller than prismatic ones. Features related to order/disorder in XRD patterns e.g. as measured by a 'cylindrical/prismatic' (CP) index and IR spectra as measured by an 'OH-stretching band ratio' are related to the proportions of cylindrical vs. prismatic tubes and correlated with other physical measurements such as specific surface area and cation exchange capacity. The relationships of size to geometric form, along with evidence for the existence of the prismatic form in the hydrated state and the same 2M(1) stacking sequence irrespective of hydration state (i.e. 10 vs. 7 angstrom) or form, suggests that prismatic halloysites are the result of continued growth of cylindrical forms.
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| 4. |
- Hillier, Stephen
(author)
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Effect of pH on the cation exchange capacity of some halloysite nanotubes
- 2016
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In: Clay Minerals. - : Mineralogical Society. - 0009-8558 .- 1471-8030. ; 51, s. 373-383
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Journal article (peer-reviewed)abstract
- The cation exchange capacity (CEC) of seven well characterized halloysite nanotubes (HNTs) in the dehydrated 7 angstrom form has been measured using a method based on cobalt hexammine exchange. In addition to unbuffered measurements, which varied between 2.9 and 9.3 cmol(+)kg(-1), CECs were also determined over a wide pH range and proton titration measurements were conducted on two samples. The data were fitted using a constant capacitance model based on the presence of two sites: permanently charged sites and pH-dependent variable charged sites. Normalization of CEC to the average specific surface area (BET) of the halloysite samples reduces considerably the variation of CEC values for the different samples particularly over the intermediate pH range (5-9) with the average value at pH 7 equal to 8.5 cmol(+)kg(-1) and a standard deviation of 1.17. Overall the CEC behaviour of the seven samples appears reasonably consistent throughout the set. Calculations based on proton titrations suggest a ratio of variable charge to basal sites for the dehydrated halloysite nanotubes of similar to 3:1.
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| 5. |
- Hillier, Stephen
(author)
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Morphological features of halloysite nanotubes as revealed by various microscopies
- 2023
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In: Clay Minerals. - 0009-8558 .- 1471-8030. ; 58, s. 395 - 407
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Journal article (peer-reviewed)abstract
- Nine halloysite nanotubes (HNTs) have been examined using scanning electron microscopy (SEM), atomic force microscopy (AFM) and (cross-sectional) transmission electron microscopy (TEM) to evaluate details of their external and internal morphologies. The samples span morphologies within the cylindrical to prismatic-polygonal framework proposed by Hillier et al. (2016). The 'carpet roll' model assumed in the conceptualization of most technological applications of HNTs is shown to be far too simplistic. Both cylindrical and prismatic forms have abundant edge steps traversing their surfaces that, by analogy with plates of kaolinite, correspond to prism faces. The mean value for the diameter of the central lumen of the tubes is 12 nm. Numerous slit-like nanopores, with diameters up to 18 nm, also occur between packets of layers, particularly in prismatic forms at the junction between a central cylindrical core and outer packets of planar layers. These pores expose aluminol and siloxane surfaces, but unlike the lumen, which is assumed only to expose an aluminol surface, they do not extend along the entire length of the nanotube. Edge steps seen most clearly by AFM correspond in height to the packets of layers seen in TEM. TEM cross-sections suggest that tube growth occurs by accretion of a spiralled thickening wedge of layers evolving from cylindrical to polygonal form and reveal that planar sectors may be joined by either abrupt angular junctions or by short sections of curved layers. A more realistic model of the internal and external morphologies of HNTs is proposed to assist with understanding of the behaviour of HNTs in technological applications.
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| 6. |
- Hillier, Stephen
(author)
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On the mechanism of exfoliation of 'Vermiculite'
- 2013
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In: Clay Minerals. - : Mineralogical Society. - 0009-8558 .- 1471-8030. ; 48, s. 563-582
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Journal article (peer-reviewed)abstract
- Six samples of 'Vermiculite' have been studied to investigate the mechanism of its well known but poorly understood property to exfoliate. The samples were analysed quantitatively by XRD to determine their precise mineralogical composition. Electron microprobe methods, including elemental mapping of native potassium and of caesium (introduced by cation exchange) were used to examine variation in the chemical composition of the particles. Most of the samples examined show heterogeneous mineralogical compositions which occur as distinct zones within the volume of individual particles, presenting a mosaic texture. Exfoliation is related to this mosaic distribution of the different mineral phases within the particles. Lateral phase boundaries between vermiculite and mica layers, or vermiculite and chlorite layers are postulated to prevent or impede the escape of gas from a particle, resulting in exfoliation when the pressure exceeds the interlayer bonding forces that hold the layers together. This mechanism provides a common explanation for the exfoliation of 'Vermiculite' by thermal methods or by treatment with H2O2. Paradoxically, one sample which consists of pure vermiculite, in the mineralogical sense of the term, demonstrates that pure vermiculite does not and should not exhibit the property of exfoliation. Our explanation of the mechanism of exfoliation explains the commonly observed particle size dependence of exfoliation and the tendency for obviously poly-phase 'Vermiculite' samples to show the largest coefficients of expansion.
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| 7. |
- Hillier, Stephen
(author)
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Phase and structural features of tubular halloysite (7 Å)
- 2018
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In: Clay Minerals. - : Mineralogical Society. - 0009-8558 .- 1471-8030. ; 53, s. 691-720
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Journal article (peer-reviewed)abstract
- Despite many reviews and original articles, the actual crystal structure of tubular halloysites remains unclear. Analysis of the structural features of defect-free kaolinite, refined by Bish & von Dreele (1989), shows that the ordered 1Tc kaolinite structure can be described equally well by the orthogonal layer cell {a(0), b(0), gamma(0)} (gamma(0) = 90 degrees) or by two enantiomorphic oblique layer cells {a(1), b(1), gamma(1)} and {a(2), b(2), gamma(2)}, related to each other by a mirror plane. To simulate diffraction effects for tubular halloysite, the parameters and atomic coordinates of the orthogonal layer unit cell and the layer-displacement vectors t(1) and t(2) responsible for formation of the kaolinite enantiomorphs were deduced by transformation of the parameters of the defect-free kaolinite refined by Bish & von Dreele (1989). Modelling X-ray diffraction patterns show that the samples consist of either single, two or three phases, with the number and their structural features depending on the morphology of the particles. Samples formed of prismatic particles consist of halloysite-like structure (HLS), kaolinite-like structure (KLS) and halloysite cylindrical structure (HCS) phases occurring in various proportions. Samples of proper cylindrical tubes consist of a single HCS phase, whilst samples formed by particles having morphologies intermediate between proper cylindrical and well-developed prismatic forms consist of the KLS and HCS phases. The KLS phase is comparable to low-ordered platy kaolinite with identical unit-cell parameters, layer-displacement vectors and arbitrary stacking faults, except that the layer displacements are not random as in kaolinite, but are distributed at R = 1 such that t(1) and t(2) displacements have a strong tendency to be segregated. Structural parameters describing the HLS and KLS phases are identical, but in the HLS phase there is a strong tendency to the regular alternation of the t(1) and t(2) displacements, and the HLS phases do not contain arbitrary stacking faults. A characteristic feature of the three-phase prismatic samples is that the stacking of the layers along the c* axis is periodic and the layer thicknesses are similar to those of platy kaolinite. In contrast, in the KLS phase formed in samples with particles of intermediate morphologies, the hydrated 10 and 7.25 angstrom layers are interstratified. The relationship between the structural and morphological features of the coexisting phases suggests a sequence of phase formation from the centre to the surface of halloysite tubes that progresses from the HCS to the HLS via the KLS phase. The results of this study demonstrate that all kaolinite and halloysite (7 angstrom) varieties are built by the same fundamental structural units.
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| 8. |
- Hillier, Stephen
(author)
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The rise and rise of halloysite
- 2016
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In: Clay Minerals. - : Mineralogical Society. - 0009-8558 .- 1471-8030. ; 51, s. 303-308
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Journal article (peer-reviewed)abstract
- Up until about 2005, the main application of halloysites had been as an alternative raw material to kaolinite for ceramics. Since then, however, there has been an exponential increase in studies aimed at applications of halloysite nanotubes, now widely referred to as HNTs. The readily available and relatively cheap nanotubular forms of halloysite have potential uses in nanocomposites with polymers, as carriers for active agents, e.g. in medicine, agriculture, cosmetics and environmental remediation, as well as in nanotemplating, as supports for catalyst immobilization and as heterogeneous catalysts.
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| 9. |
- Johansson, U., et al.
(author)
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Adsorption of silane coupling agents onto kaolinite surfaces
- 1999
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In: Clay minerals. - : Mineralogical Society. - 0009-8558 .- 1471-8030. ; 34:2, s. 239-246
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Journal article (peer-reviewed)abstract
- The adsorption of the silane coupling agents N-(2-amino-ethyl)-3-aminopropyltrimethoxysilane (Z-6020), N-(2-(vinylbenzyl-amino)-ethyl)-3-amino-propyl-trimethoxysilane (Z-6032) and 3-glycidoxy-propyl-trimethoxysilane (Z-6040) onto the surface of kaolinite has been studied using the solvents water and ethanol. The adsorbed silanes were analysed using diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy and FT-Raman spectroscopy. When the silane coupling agents are dissolved in water, silanol groups are formed. Raman spectra of aqueous solutions of the silanes show that condensation of the silanols into oligomers occurred. The peak at 1000 cm(-1) assigned to Si-O-Si vibrations indicates that polymerization has occurred. A similar behaviour is observed when the silanes are dissolved in ethanol. The DRIFT spect-ra show that the silane coupling agents adsorb onto the surface of kaolinite, It is proposed that the oligomers are bonded to the kaolinite surface only by hydrogen bondings with no covalent bonds formed.
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| 10. |
- Mahmoud E., Awad, et al.
(author)
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Adsorption of 5-aminosalicylic acid on kaolinite surfaces at a molecular level
- 2019
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In: Clay minerals. - : Cambridge University Press. - 0009-8558 .- 1471-8030. ; 54:1, s. 49-56
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Journal article (peer-reviewed)abstract
- The application of clay minerals in therapeutics is becoming important due to their structural and surface physicochemical properties. 5-aminosalicylic acid (5-ASA) is a very common pharmaceutical drug and is used worldwide. The interactions between the 5-ASA molecule and both the aluminol and siloxane surfaces of kaolinite are studied by means of atomistic calculations using force fields based on empirical interatomic potentials and quantum mechanics calculations based on density functional theory. A conformational analysis of 5-ASA has been performed and the anion of 5-ASA was also studied. The calculated adsorption energy values indicate that 5-ASA is likely to be adsorbed on the kaolinite surfaces with greater affinity to the aluminol surface. Hence, kaolinite may be considered as a promising pharmaceutical carrier of 5-ASA.
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