Photoactive nanoarchitectures based on clays incorporating TiO2 and ZnO nanoparticles

Abstract

Thought as raw materials clay minerals are often disregarded in the development of advanced materials. However, clays of natural and synthetic origin constitute excellent platforms for developing nanostructured functional materials for numerous applications. They can be easily assembled to diverse types of nanoparticles provided with magnetic, electronic, photoactive or bioactive properties, allowing to overcome drawbacks of other types of substrates in the design of functional nanoarchitectures. Within this scope, clays can be of special relevance in the production of photoactive materials as they offer an advantageous way for the stabilization and immobilization of diverse metal-oxide nanoparticles. The controlled assembly under mild conditions of titanium dioxide and zinc oxide nanoparticles with clay minerals to give diverse clay-semiconductor nanoarchitectures are summarized and critically discussed in this review article. The possibility to use clay minerals as starting components showing different morphologies, such as layered, fibrous, or tubular morphologies, to immobilize these types of nanoparticles mainly plays a role in i) the control of their size and size distribution on the solid surface, ii) the mitigation or suppression of the nanoparticle aggregation, and iii) the hierarchical design for selectivity enhancements in the catalytic transformation and for improved overall reaction efficiency. This article tries also to present new steps towards more sophisticated but efficient and highly selective functional nanoarchitectures incorporating photosensitizer elements for tuning the semiconductor-clay photoactivity.

Keywords: clays; nanoarchitectures; photocatalysts; titanium dioxide; zinc dioxide.

Figure 1

Schematic representation of the crystal structures of the following clay minerals: kaolinite (A), montmorillonite (B), sepiolite (C), halloysite nanotubes (HNT) (D), and the metal oxides, anatase (E) and wurtzite (F), obtained by applying the VESTA software using the following color codes: silicon oxide tetrahedron – blue: Si, red: O. In kaolinite and halloysite – aluminium oxide-hydroxide octahedron: green, red: O; magnesium oxide octahedron: brown, red: O. In montmorillonite – Al,Mg octahedron: pale blue; Ti: pink; Zn: gray. Panel D shows the nanotubular morphology of HNT resulting from the rolling of layers of 1:1 aluminium phyllosilicate with the same structural arrangement as kaolinite. TEM images of (G) montmorillonite and (H) sepiolite clay minerals.

Figure 2

TEM images of (A) ZnO NPs on montmorillonite with nanocrystal aggregation; reprinted with permission from [85], copyright 2004 American Chemical Society; (B) TiO₂@HNTs nanoarchitecture showing the titania NP assembled inside the lumen of the halloysite tubes; adapted with permission from [123], copyright 2011 American Chemical Society; and (C) ZnO@sepiolite where the ZnO NPs were generated on the external surface of the fibrous clay from zinc acetylacetonate following the protocol described in [87].

Figure 3

Synthesis of clay–semiconductor nanoarchitectures by the “organoclay colloidal route” involving either smectites (A) or fibrous clays (B) in the following steps: a) replacement of inorganic cations by alkylammonium ions forming the intermediate organoclay, which is treated with metal-oxide precursors being transformed (b) into intermediate compounds that after calcination (c) finally yield the nanoarchitecture containing the photoactive semiconductor. TEM Images (on the right) of A: ZnO@smectite from Gafsa, where ZnO NPs were previously prepared from Zn acetate [118], and B: TEM of TiO₂@sepiolite, where TiO₂ NPs were prepared from titanium isopropoxide; reprinted with permission from [109], copyright 2008 American Chemical Society.

Figure 4

ZnO-Fe₃O₄@sepiolite nanoarchitecture prepared in two steps: First, the fiber clay is modified by assembly of magnetite NPs. After that, the ZnO NPs are added yielding a magnetic photocatalyst. The STEM images on the right shows the silicate component (red), the magnetite NPs (green) and the ZnO NPs (blue) analyzed with an EDAX detector and a Gatan Tridiem energy filter; reprinted with permission from [133], copyright 2017 Elsevier.

Figure 5

(A) TEM image of the Pt–TiO₂@sepiolite clay nanoarchitectures prepared by a photodeposition procedure, reprinted with permission from [131], copyright 2015 Elsevier; (B) hydrogen production in methanol photoreforming using this Pt-doped clay nanoarchitecture as catalyst [131].

Figure 6

The structural arrangement of the [Ru(bpy)₃]²⁺–TiO₂@clay nanoarchitecture and its photocatalytic activity in the conversion of benzene to phenol. Adapted with permission from [167], copyright 2016, The Royal Society of Chemistry.