Abstract
We investigate termination effects in aluminosilicate (AlSi) and aluminogermanate (AlGe) imogolite nanotubes (NTs) by means of semi-local and range-corrected hybrid Density Functional Theory (DFT) simulations. Following screening and identification of the smallest finite model capable of accommodating full relaxation of the NT terminations around an otherwise geometrically and electrostatically unperturbed core region, we quantify and discuss the effects of physical truncation on the structure, relative energy, electrostatics and electronic properties of differently terminated, finite-size models of the NTs. In addition to composition-dependent changes in the valence (VB) and conduction band (CB) edges and resultant band gap (BG), the DFT simulations uncover longitudinal band bending and separation in the finite AlSi and AlGe models. Depending on the given termination of the NTs, such longitudinal effects manifest in conjunction with the radial band separation typical of fully periodic AlSi and AlGe NTs. The strong composition dependence of the longitudinal and radial band bending in AlSi and AlGe NTs suggests different mechanisms for the generation, relaxation and separation of photo-generated holes in AlSi and AlGe NTs, inviting further research in the untapped potential of imogolite compositional and structural flexibility for photo-catalytic applications.
Highlights
Termination-induced structural relaxation and reconstruction can alter, and be used to tailor, the mechanical, electronic and optical properties of nanostructured materials [1,2,3,4,5]
To quantify the geometrical relaxation of the finite AlSi and AlGe NT models we resorted to two structural descriptors, namely the ring-resolved average displacement (h∆ri), and the layer-resolved average ring-diameter. h∆ri is defined as: h∆r(R)i = N−1
A given ring contains all the longitudinally equivalent saturating the terminal (Si) (Al) atoms and bridging hydroxyls (OH), in addition to the closest one extra hydroxyl (OH) groups pointing to the NT ends
Summary
Termination-induced structural relaxation and reconstruction can alter, and be used to tailor, the mechanical, electronic and optical properties of nanostructured materials [1,2,3,4,5]. Physical truncation of a solid-state material leaves the terminal atoms under-coordinated and incompletely bonded, disrupting the crystalline periodicity by introducing dangling bonds on the medium-exposed surfaces of the material. These dangling bonds and their additional interactions with the surrounding medium can strongly alter the properties of finite-sized materials leading to structural reconstruction as well as the emergence of unexpected optical, charge-carrier and chemical properties [6,7,8,9,10]. To contribute to progress in this area, here we investigate modern, favorably scaling density functional theory (DFT)
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