Photoactive nanoarchitectures based on clays incorporating TiO2 and ZnO nanoparticles

[EN] 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.Authors thank the MINECO (Spain, project MAT2015-71117-R) for financial support

Organoclay hybrid materials as precursors of porous ZnO/silica-clay heterostructures for photocatalytic applications

In this study, ZnO/SiO2-clay heterostructures were successfully synthesized by a facile two-step process applied to two types of clays: montmorillonite layered silicate and sepiolite microfibrous clay mineral. In the first step, intermediate silica–organoclay hybrid heterostructures were prepared following a colloidal route based on the controlled hydrolysis of tetramethoxysilane in the presence of the starting organoclay. Later on, pre-formed ZnO nanoparticles (NP) dispersed in 2-propanol were incorporated under ultrasound irradiation to the silica–organoclay hybrid heterostructures dispersed in 2-propanol, and finally, the resulting solids were calcinated to eliminate the organic matter and to produce ZnO nanoparticles (NP) homogeneously assembled to the clay–SiO2 framework. In the case of montmorillonite the resulting materials were identified as delaminated clays of ZnO/SiO2-clay composition, whereas for sepiolite, the resulting heterostructure is constituted by the assembling of ZnO NP to the sepiolite–silica substrate only affecting the external surface of the clay. The structural and morphological features of the prepared heterostructures were characterized by diverse physico-chemical techniques (such as XRD, FTIR, TEM, FE-SEM). The efficiency of these new porous ZnO/SiO2-clay heterostructures as potential photocatalysts in the degradation of organic dyes and the removal of pharmaceutical drugs in water solution was tested using methylene blue and ibuprofen compounds, respectively, as model of pollutants.The authors thank the CSIC (i-COOP+ project COOPA20077) and the MINECO (project MAT2012-31759 and MAT2015-71117-R), Spain, and the EU (COST Action MP1202).Peer reviewe

Organoclay hybrid materials as precursors of porous ZnO/silica-clay heterostructures for photocatalytic applications

In this study, ZnO/SiO2-clay heterostructures were successfully synthesized by a facile two-step process applied to two types of clays: montmorillonite layered silicate and sepiolite microfibrous clay mineral. In the first step, intermediate silica–organoclay hybrid heterostructures were prepared following a colloidal route based on the controlled hydrolysis of tetramethoxysilane in the presence of the starting organoclay. Later on, pre-formed ZnO nanoparticles (NP) dispersed in 2-propanol were incorporated under ultrasound irradiation to the silica–organoclay hybrid heterostructures dispersed in 2-propanol, and finally, the resulting solids were calcinated to eliminate the organic matter and to produce ZnO nanoparticles (NP) homogeneously assembled to the clay–SiO2 framework. In the case of montmorillonite the resulting materials were identified as delaminated clays of ZnO/SiO2-clay composition, whereas for sepiolite, the resulting heterostructure is constituted by the assembling of ZnO NP to the sepiolite–silica substrate only affecting the external surface of the clay. The structural and morphological features of the prepared heterostructures were characterized by diverse physico-chemical techniques (such as XRD, FTIR, TEM, FE-SEM). The efficiency of these new porous ZnO/SiO2-clay heterostructures as potential photocatalysts in the degradation of organic dyes and the removal of pharmaceutical drugs in water solution was tested using methylene blue and ibuprofen compounds, respectively, as model of pollutants

ZnO/sepiolite heterostructured materials for photocatalytic degradation of emerging pollutants in wastewater

Oral presentation given at the XVI International Clay Conference, held in Granada (Spain) on July 17-21, 2017.The development of porous nanoarchitectures materials based on clays minerals is a topic of growing interest, being typically addressed to catalysis and adsorption applications [1,2]. Among clay minerals, sepiolite fibrous clay provides larger external surface area as well as the presence of external silanol groups in comparison to typical layered silicates (e.g., smectites), which favours the immobilization of diverse nanoparticles (NP). In this communication it will be introduced several porous ZnO-sepiolite nanostructured materials prepared using sepiolite or organosepiolite as intermediate to control the assembly and aggregation of the metal-oxide nanoparticles. On the one side, ZnO-sepiolite nanoarchitectures were prepared following a process based on the assembly of previously synthesized ZnO NP to sepiolite modified with hexadecyltrimethylammonium ions, both dispersed in 2-propanol medium, to produce an intermediate that after a thermal treatment consolidates the final ZnO/clay nanoarchitecture [3]. On the other side, the ZnO NP were incorporated to a sepiolite previously modified with SiO2 NP produced following a sol-gel methodology reported elsewhere [4]. In this case, it is intended to have a support of ZnO NP with a larger accessible surface area and also appropriate chemical reactivity of the clay surface. Finally, a novel heterostructure has been prepared using directly the sepiolite as substrate on which magnetite NP were in situ growth [5] and then ZnO NP were assembled as in the previously described materials. All types of samples have been characterized by diverse characterization techniques (XRD, FTIR, specific surface area and porosity determinations, TEM, FE-SEM-EDX, SQUID, etc.) and their activity in the photocatalytic degradation of emerging pollutants in water was tested. In this way, solutions of paracetamol, antypirine and ibuprofen drugs were exposed to a lamp that simulates solar light and the variation of their concentration with time of exposure was followed by HPLC. For instance, it has been observed that the ZnO/sepiolite heterostructure exhibits better photoactivity than ZnO/SiO2-sepiolite and ZnO/Fe3O4-sepiolite materials for the degradation of ibuprofen. However, the last material has an important advantage as it presents superparamagnetic afforded by the magnetite NP and it can be easily recovered from the reaction media by applying an external magnetic field, which makes very easy the reusability of the photocatalyst in consecutive cycles of treatment.MINECO (Spain, project MAT2015-71117-R) and CSIC (Spain, COOPA20077)

Sepiolite nanoplatform for the simultaneous assembly of magnetite and zinc oxide nanoparticles as photocatalyst for improving removal of organic pollutants

Novel ternary ZnO/FeO-sepiolite nanostructured materials were developed in a two-step procedure based on the incorporation of ZnO nanoparticles on a substrate composed by magnetite nanoparticles previously assembled to the sepiolite fibrous silicate (FeO-sepiolite). The structural and morphological characterization shows that both, ZnO and FeO nanoparticles, were homogeneously dispersed on the surface of sepiolite. Therefore, the resulting material is characterized as a multifunctional nanoplatform simultaneously providing magnetic and photoactive properties. ZnO/FeO-sepiolite materials exhibit superparamagnetic properties at room temperature, which is one of the sought properties in view to facilitate their recovery from the reaction medium after application as heterogeneous catalysts. ZnO/FeO-sepiolite materials were tested as photocatalysts using methylene blue dye in water as model of a pollutant molecule, showing full decolorization after 2 h of UV irradiation. Moreover, the photocatalytic activity of this nanoplataform may be maintained after reuse in several consecutive cycles of treatment. Remarkably, the ZnO/magnetite-sepiolite nanostructured material displays a similar activity as ZnO/sepiolite materials, but shows the additional advantage of easier recovery by means of a magnet which facilitates its reuse.Financial support from CSIC (project COOPA20077), MINECO (projects MAT2012-31759 and MAT2015-71117-R), Spain, as well as technical assistance by Mr. A. Valera, Mr. R. Barrios (ICMM-CSIC) and Dr. L. Pascual (ICP-CSIC) are gratefully acknowledged