Exfoliated graphene into highly ordered mesoporous titania films: highly performing nanocomposites from integrated processing

Bidimensional SnSe2-Mesoporous Ordered Titania Heterostructures for Photocatalytically Activated Anti-Fingerprint Optically Transparent Layers

The design of functional coatings for touchscreens and haptic interfaces is of paramount importance for smartphones, tablets, and computers. Among the functional properties, the ability to suppress or eliminate fingerprints from specific surfaces is one of the most critical. We produced photoactivated anti-fingerprint coatings by embedding 2D-SnSe₂ nanoflakes in ordered mesoporous titania thin films. The SnSe₂ nanostructures were produced by solvent-assisted sonication employing 1-Methyl-2-pyrrolidinone. The combination of SnSe₂ and nanocrystalline anatase titania enables the formation of photoactivated heterostructures with an enhanced ability to remove fingerprints from their surface. These results were achieved through careful design of the heterostructure and controlled processing of the films by liquid phase deposition. The self-assembly process is unaffected by the addition of SnSe₂, and the titania mesoporous films keep their three-dimensional pore organization. The coating layers show high optical transparency and a homogeneous distribution of SnSe₂ within the matrix. An evaluation of photocatalytic activity was performed by observing the degradation of stearic acid and Rhodamine B layers deposited on the photoactive films as a function of radiation exposure time. FTIR and UV-Vis spectroscopies were used for the photodegradation tests. Additionally, infrared imaging was employed to assess the anti-fingerprinting property. The photodegradation process, following pseudo-first-order kinetics, shows a tremendous improvement over bare mesoporous titania films. Furthermore, exposure of the films to sunlight and UV light completely removes the fingerprints, opening the route to several self-cleaning applications.

Mesoporous ordered films via self-assembly: trends and perspectives

The synthesis of ordered mesoporous films via self-assembly represents one of the main accomplishments in nanoscience. In fact, controlling the complex chemical-physical phenomena that govern the process triggered by the solvent's fast evaporation during film deposition has represented a challenging task. Several years after the first articles on the subject, the research in the field entered a new stage. New advanced applications based on the peculiar properties of mesoporous films are envisaged while basic research is still going on, especially to clarify the mechanism behind self-organization in a spatially defined environment and the physics and chemistry in mesoscale porosity. This review has been dedicated to analysing the main trends in the fields and the perspective for future developments.

Improving the Photocatalytic Activity of Mesoporous Titania Films through the Formation of WS2/TiO2 Nano-Heterostructures

Heterostructures formed by anatase nanotitania and bidimensional semiconducting materials are expected to become the next-generation photocatalytic materials with an extended operating range and higher performances. The capability of fabricating optically transparent photocatalytic thin films is also a highly demanded technological issue, and increasing the performances of such devices would significantly impact several applications, from self-cleaning surfaces to photovoltaic systems. To improve the performances of such devices, WS₂/TiO₂ heterostructures obtained by incorporating two-dimensional transition metal dichalcogenides layers into titania mesoporous ordered thin films have been fabricated. The self-assembly process has been carefully controlled to avoid disruption of the order during film fabrication. WS₂ nanosheets of different sizes have been exfoliated by sonication and incorporated in the mesoporous films via one-pot processing. The WS₂ nanosheets result as well-dispersed within the titania anatase mesoporous film that retains a mesoporous ordered structure. An enhanced photocatalytic response due to an interparticle electron transfer effect has been observed. The structural characterization of the heterostructure has revealed a tight interplay between the matrix and nanosheets rather than a simple additive co-catalyst effect.

Comparative Evaluation of Graphene Nanostructures in GERS Platforms for Pesticide Detection

Graphene-enhanced Raman scattering (GERS) produces enhancement of the Raman signal, which is based on chemical rather than electromagnetic mechanism such as in the surface-enhanced Raman scattering. Graphene oxide, amino- and guanidine-functionalized graphene oxide, exfoliated graphene, and commercial graphene nanoplatelets have been used to investigate the GERS response with the change of graphene properties. Different graphene nanostructures have been embedded into organic-inorganic microporous films to build a platform for the fast and sensitive detection of pesticides in water. The graphene nanostructures vary in the number of layers, lateral size, degree of oxidation, and surface functionalization. The GERS performances of the graphene nanostructures cast on silicon substrates and embedded in the nanocomposite films have been comparatively evaluated. After casting a few droplets of the pesticide aqueous solution on the graphene nanostructures, the Raman band enhancements of the analytes have been measured. In the nanocomposite films, the characteristic Raman bands originating from pesticides such as paraoxon, parathion, and glyphosate could be traced at concentrations below 10^-7, 10^-5, and 10^-4 M, respectively. The results show that the surface functionalization reduces the GERS effect because it increases the ratio between the sp³ carbon and sp² carbon. On the other hand, the comparison among different types of graphenes shows that the monolayers are more efficient than the few-layer nanostructures in enhancing the Raman signal.

Mesoporous thin films on graphene FETs: nanofiltered, amplified and extended field-effect sensing

The ionic screening and the response of non-specific molecules are great challenges of biosensors based on field-effect transistors (FETs). In this work, we report the construction of graphene based transistors modified with mesoporous silica thin films (MTF-GFETs) and the unique (bio)sensing properties that arise from their synergy. The developed method allows the preparation of mesoporous thin films free of fissures, with an easily tunable thickness, and prepared on graphene-surfaces, preserving their electronic properties. The MTF-GFETs show good sensing capacity to small probes that diffuse inside the mesopores and reach the graphene semiconductor channel such as H⁺, OH^-, dopamine and H₂O₂. Interestingly, MTF-GFETs display a greater electrostatic gating response in terms of amplitude and sensing range compared to bare-GFETs for charged macromolecules that infiltrate the pores. For example, for polyelectrolytes and proteins of low MW, the amplitude increases almost 100% and the sensing range extends more than one order of magnitude. Moreover, these devices show a size-excluded electrostatic gating response given by the pore size. These features are even displayed at physiological ionic strength. Finally, a developed thermodynamic model evidences that the amplification and extended field-effect properties arise from the decrease of free ions inside the MTFs due to the entropy loss of confining ions in the mesopores. Our results demonstrate that the synergistic coupling of mesoporous films with FETs leads to nanofiltered, amplified and extended field-effect sensing (NAExFES).

Boron Nitride-Titania Mesoporous Film Heterostructures

The fabrication of optically active heterostructures in the shape of mesostructured thin films is a highly challenging task. It requires an integrated process to allow in one-step incorporating the two-dimensional materials within the mesoporous ordered host without disrupting the pore organization. Hexagonal boron nitride (BN) nanosheets have been successfully introduced into titania mesoporous films using a template-assisted sol-gel synthesis and evaporation-induced self-assembly. Two types of BN sheets have been used, with and without defects, to investigate the role of defects in heterostructure properties. It has been found that the defects increase the ultraviolet radiation A (UVA) absorbance and enhance the photocatalytic response of the film. The BN sheets are optically transparent and do not exhibit any photocatalytic property but contribute to anatase crystallization via heterogeneous nucleation.

Silica-graphene porous nanocomposites for environmental remediation: A critical review

With the increase of industrialization, there is an urgent need for developing technologies to detect and remove toxic pollutants from water bodies. The pollutants are often released to the environment due to the consumption of raw materials that are necessary for the production of technological goods (such as chemical and pharmaceutical compounds, metals, and alloys or foods). Amongst all the remediation techniques, adsorption is considered as one of the preferred techniques, due to its fast and efficient removal of contaminants. Novel materials, which are engineered for selective and responsive water remediation, have also recently revealed a strong potential in the detection of pollutants. Here, current trends of silica-graphene (SG) porous composites for the removal of oils, organic solvents, heavy metals, and dyes are reviewed in detail. Insights on the modifications of composites to enhance their sorption performance have been highlighted. In addition, the detection of pollutants using porous SG nanocomposites is also critically reviewed. Overall, SG composites reveal a strong potential as nanostructure materials with improved efficiency for environmental-based applications.

Nanographene oxide-TiO2 photonic films as plasmon-free substrates for surface-enhanced Raman scattering

The development of nanostructured semiconductors with tailored morphology and electronic properties for surface-enhanced Raman scattering (SERS) has been attracting significant attention as a promising alternative to conventional coinage metal SERS substrates. In this work, functionalized TiO₂ photonic crystals by graphene oxide nanocolloids (nanoGO) are demonstrated as highly sensitive, recyclable, plasmon-free SERS substrates that combine slow-photon amplification effects with the high adsorption capacity and surface reactivity of GO nanosheets. Comparative evaluation of photonic band gap engineered nanoGO-TiO₂ inverse opal films was performed on methylene blue SERS detection under different laser excitations in combination with rigorous theoretical simulations of the photonic band structure. A very low detection limit of 6 × 10^-7 M and an enhancement factor of 5 × 10⁴ along with excellent self-cleaning performance and reusability could be achieved by the interplay of slow-photon effects assisted by interfacial charge transfer between the analyte and the nanoGO-TiO₂ semiconducting substrate. Slow-photon management in combination with judicious engineering of chemical enhancement in photonic nanostructures is accordingly proposed as an advanced approach for the design of efficient dielectric SERS substrates.

Sol-Gel Chemistry for Carbon Dots

Carbon dots are an emerging class of carbon-based nanostructures produced by low-cost raw materials which exhibit a widely-tunable photoluminescence and a high quantum yield. The potential of these nanomaterials as a substitute of semiconductor quantum dots in optoelectronics and biomedicine is very high, however they need a customized chemistry to be integrated in host-guest systems or functionalized in core-shell structures. This review is focused on recent advances of the sol-gel chemistry applied to the C-dots technology. The surface modification, the fine tailoring of the chemical composition and the embedding into a complex nanostructured material are the main targets of combining sol-gel processing with C-dots chemistry. In addition, the synergistic effect of the sol-gel precursor combined with the C-dots contribute to modify the intrinsic chemo-physical properties of the dots, empowering the emission efficiency or enabling the tuning of the photoluminescence over a wide range of the visible spectrum.

Improving the Selective Efficiency of Graphene-Mediated Enhanced Raman Scattering through Molecular Imprinting

Enhancement of Raman scattering signal through graphene is an important property that could be exploited for producing innovative sensing devices with advanced properties. Because the enhancement of Raman scattering is due to only a chemical mechanism, the amplification of the signal is lower than that one produced by excitation of localized surface plasmons. The combination of a highly selective technique, which is molecular imprinting, with graphene-mediated enhanced Raman scattering, represents a new synergistic approach that we have developed in the present work. The careful material design has allowed obtaining a porous composite embedding exfoliated graphene and molecular cavities specifically designed for recognizing Rhodamine 6G. The molecularly imprinted porous samples have shown a signal enhancement that increases as a function of the number of molecular cavities, which are also accountable for the molecular recognition properties. Environmental ellipsometric porosimetry has shown no substantial difference between molecularly imprinted and not-imprinted films confirming that the signal enhancement of the imprinted samples is due to the molecular cavities. Interestingly, the most efficient sample has shown a Raman enhancement per cavity that exceeds the value of 1 × 10¹² and a remarkable molecular selectivity allowing for a Rhodamine 6G signal amplification 4.5 higher than structural analogues such as Rhodamine B and methylene blue. The robust and flexible matrix ensures a good recyclability of the samples without lack of linear response. These results prove the great potential of molecular imprinting as a general strategy to provide selectivity to GERS-active substrates for a new generation of sensing devices.

Incorporation of graphene into silica-based aerogels and application for water remediation

Graphene/silica nanocomposites in the form of highly porous aerogels are obtained for the first time by integrating a novel approach for the production of low defectivity graphene with a two-step route for the synthesis of a silica-based monolith.

Graphene and carbon nanodots in mesoporous materials: an interactive platform for functional applications

The present review is focused on a specific class of nanocomposites obtained through integration of graphene or carbon-based nanomaterials (such as carbon nanodots) with mesoporous inorganic or hybrid materials, obtained via template assisted self-assembly. The task of integrating graphene and carbon nanodots with a self-assembly process is still very challenging and this review shows some of the solutions which have been envisaged so far. These nanocomposite materials are an ideal interactive platform for developing innovative functional applications; they have a high capability of undergoing integration into advanced devices, which well exploits the advantage of tuning the wide properties and flexibility of the soft-chemistry route. A wide range of applications have been developed so far which span from sensing to electronics up to optics and biomedicine. Even though a large number of proof-of-concepts have been reported to date, an even greater expansion of applications in the field is expected to happen in the near future.

Graphene-mediated surface enhanced Raman scattering in silica mesoporous nanocomposite films

Silica mesoporous nanocomposite films containing graphene nanosheets and gold nanoparticles have been prepared via a one-pot synthesis using silicon tetrachloride, gold(III) chloride tetrahydrate, a 1-N-vinyl-2-pyrrolidone dispersion of exfoliated graphene and Pluronic F127 as a structuring agent. The composite films have shown graphene-mediated surface-enhanced Raman scattering (G-SERS). Graphene has been introduced as dispersed bilayer sheets while gold has been thermally reduced in situ to form nanoparticles of around 6 nm which preferentially nucleate on the surface of the graphene nanosheets. The presence of graphene and gold nanoparticles does not interfere with the self-assembly process and the formation of silica mesoporous films ordered as 2D hexagonal structures. The material has shown a remarkable analytical enhancement factor ranging from 80 up to 136 using rhodamine 6G as a Raman probe. The films have been characterised by grazing incidence X-ray diffraction, FTIR and UV-vis spectroscopy studies; transmission electron microscopy and spectroscopic ellipsometry have been used to study the morphology, thickness and porosities of the samples. Raman spectroscopy has been employed to characterise the graphene nanosheets embedded into the mesoporous films and the enhanced Raman scattering.

Sol–gel chemistry for graphene–silica nanocomposite films

Graphene–silica films with high optical transparency have been obtained by adding a dispersion of exfoliated graphene into a silica sol.