Imogolite-like nanotubes: structure, stability, electronic and mechanical properties of the phosphorous and arsenic derivatives

Abstract

Imogolite is a single-walled aluminosilicate nanotube (NT) found in nature that can be easily synthesized, as well as its analogue aluminogermanate NT. Based on geometrical assumptions and pKa values, species such as H3PO4, H3PO3, H3AsO3, H3AsO4 could also be candidates to form imogolite-like structures. In the present work, we provide insights about the stability, electronic, structural and mechanical properties of possible imogolite like NTs by means of self-consistent charge density-functional tight-binding method (SCC-DFTB). Similarly to aluminogermanate, where the tetrahedral silicate groups are replaced by germanate, in this work tetrahedral silicate groups are substituted by phosphate, phosphite, arsenate and arsenite units in the imogolite structure. Detailed analysis is focused on structural properties, strain energy, band gap and Mulliken charges distribution. The calculated strain energy curves for all studied zigzag imogolite-like NTs present well-defined minima, which change as a consequence of composition variation. Moreover, the strain energy curves of armchair imogolite-like NTs also present minima, although in all cases less stable than zigzags by at least 2.2 meV per atom. The insulating NT behaviour changes after internal modification from silicate to phosphate, phosphite, arsenate and arsenite, as well as the charge distribution inside and outside the nanotubes.

Large-scale density functional theory simulation of inorganic nanotubes: a case study on Imogolite nanotubes

“…atomic basis set, 48,50,52,55 exploiting the system symmetry, 53 or resting on the transferability of TB-DFT parameterisations for analogous systems, 49,54 best practice for accurate simulation of these systems in the presence of defects, dopants, termination effects, electronic excitations and solvation by media of non-negligible ionic strength is far from being established.…”

“…not 1D-periodic as used so far for Imo-NTs [48][49][50][51][52][53][54][55] ) and large enough to allow accurate description of both the core and terminal regions of the NT model. Although real Imo-NTs are 20-100-nm long depending on their composition, 26 it is reasonable to expect the relaxation of the tube terminations to involve no more than a few nanometres from the tube end.…”

“…Although density functional theory (DFT) modelling and derived tight-binding approximations (TB-DFT) have assisted research in the growth mechanisms of AlSi NTs, 29,30 been used to elucidate the properties of periodic defect-free AlSi and AlGe NTs models [48][49][50][51][52][53] and to investigate possible phosphorous-and arsenic-based functionalisation, 54 the recent experimental progress in the field of Imogolite-like open-ended nanotubes (Imo-NTs) has yet to be matched by the material modelling community. With the exception of three DFT contributions focused on point defects and Fe-doping in AlSi and AlGe NTs, 50,51,55 the structural and electronic characterisation of defects, dopant and termination effects in Imo-NTs and their role in tuning the NT electronic and spectroscopic properties as well as its chemical reactivity is to date unexplored.…”

“…With a periodic unit cell containing over 300 atoms for the smallest system of the Imo-NT family, the AlSi (N524) NT: 336 atoms, 34 the computational efforts required for standard plane-wave or atomic basis set simulation of these systems are, at the very least, challenging. As a result, DFT investigations of Imo-NTs have been limited to periodically repeated models of no more than one periodic ring [48][49][50][51][52][53][54][55] or fractions of the ImoNTs. 29,30 Although reduction of the computational cost of the simulations can be obtained by using minimal (fixed) 1 a Front view of the AlSi NT (N524) structure with the adopted, layer-resolved, radial atomic labelling; b the AlSi wall structural motif together with the zigzag periodic unit (black line) along the nanotube axis as seen from outside the NT cavity; c front view of the AlSiMe NT (N524) with substitution of the inner hydroxyls (ZOH) by methyl groups (ZCH 3 ); the AlSiMe NT wall structure is displayed in d. Al: green; Si: yellow; O: red; H: silver; C: cyan 2 Front view of the real-space separation between highenergy VB-edge (green) and low-energy CB-edge (red) in AlSi NTs.…”

Development of biobased polymer/clay nanocomposites

“…Because imogolite material can absorb more organic materials (Boudot, 1992), Farmer's method becomes very popular for synthesizing INTs (Koenderink et al, 1999;Marz an and Philipse, 1994;Barrett et al, 1991;Lee et al, 2013;McCutcheon et al, 2005). INTs possess hydrophilic properties with OH groups on the outer layer surface, which can be easily dissolved in organic solvents and polymer matrices (Guimaraes et al, 2013). INTs can be employed as a stabilizer for platinum particles (Marz an and Philipse, 1994), biodegradation adsorbent (Boudot, 1992;Ma et al, 2011a), catalyst (Imamura et al, 1996(Imamura et al, , 1993, as well as nanofillers of polymer nanocomposites (Lee et al, 2013;Ma et al, 2011b;Yamamoto et al, 2005;Kang et al, 2011;Ma et al, 2012).…”