Mesoporous ordered films via self-assembly: trends and perspectives

Abstract

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.

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1. Introduction

The possibility of fabricating thin films at controlled porosity through self-assembly techniques is one of the fascinating aspects of nanoscience. It is still a frontier research area, even though many years have passed since the first synthesis of mesoporous films in 1994.^1,2 Particularly attractive is the multidisciplinarity of the process in which supramolecular, sol–gel, and colloid chemistry gives the foundation for explaining a complex phenomenon. Research in the field has entered a mature stage where the basic chemical and physical aspects have been well studied, but several issues remain to be explored. Mesoporous films represent a narrow and confined environment of nanoscopic dimensions whose properties are still partially unknown. A better understanding of physics and chemistry in confined nano-scale porous environments would offer new opportunities for advanced functional applications.

This review focus on identifying the recent research trends in the sector that is fast moving.^3,4 It is clear from an analysis of the articles recently published in the scientific literature that research in the field is increasingly focused on the development of specific applications where the possibility of obtaining thin films with controlled porosity in terms of orientation and pore size represents an intrinsic advantage over other types of materials.⁵ In particular, the possibility of obtaining films oriented in specific directions with respect to the substrate has undoubtedly represented one of the most interesting results in recent years and has attracted great attention. The pore orientation engineering has paved the way for a whole series of new applications of mesoporous films. At the same time, the development of nanocomposites using mesoporous films as a matrix represents another challenging frontier. The fabrication of heterostructures based on materials with controlled porosity allows obtaining nanomaterials with unique properties.⁶ In particular, the possibility of integrating the new generation nanomaterials, in particular two-dimensional or zero-dimensional nanostructures within mesopores represents an opportunity not to be missed. As far as basic research is concerned, the deposition process of mesoporous thin films has reached a level of understanding now mature, thanks to the numerous studies that have been done previously. However, some aspects remain to be studied and deepened, such as mechanical properties and structure–property correlations in nanocomposites.

In drafting this review, the articles published in the sector in recent years have been considered, which number several hundred, testifying to the great interest that the material and the process still attract. Not all of them have been mentioned, a selection has been necessarily made according to the topics that emerge from the analysis of the literature, also to avoid the review becoming a mere list of articles. The article is restricted to the limited field of mesoporous films that show an organization of the porous phase. It should be underlined that most scientific literature focuses on mesoporous microparticles,⁷ the synthesis of which, however, does not take place in the critical conditions dictated by the fast evaporation of the solvent as in films. The review also does not report the fundamentals of the self-assembly process that lead to the formation of structures organized in the mesoscale during evaporation, a process referred to as Evaporation Induced Self-Assembly (EISA). These processes, together with the chemical-physical phenomena that govern them, are well described in numerous excellent reviews^8–12 and books¹³ to which the unfamiliar reader in the subject can refer.

2. Aligning the mesopores

The most common and historically the first method developed for fabricating mesostructured films is the deposition on a substrate under controlled conditions, typically via dip-coating,^14–16 spray coating,^17,18 or spin coating,^19,20 of a starting solution containing all the precursors. The method is relatively simple, and the only requirement is the proper control of the conditions in the deposition chamber, relative humidity, temperature and atmosphere. The self-assembly during the film deposition has been studied in detail and is now well understood. One of the main open issues is the possibility of controlling the orientation of the mesopores using a simple and reliable procedure. In the case of 3D structures that have no preferential orientation, the question is controlling the order on a long range rather than having ordered domains. 2D-hex mesoporous structures, instead, tend to align in a direction parallel to the substrate, with randomly orientated domains.²¹ However, many of the most exciting applications require precise pore orientation control with high accessibility from the external environment. In particular, obtaining a perpendicular alignment to the substrate allows for using mesoporous films as advanced sensors and filter membranes. Therefore, much attention has recently been paid to achieve a precise control of the pore orientation and size. Different strategies have been envisaged for such purpose in 2D-hex mesostructures, for instance application of external magnetic fields,^22,23 radio sputtering,²⁴ controlled fluxes of air,²⁵ epitaxial growth^26,27 and pre-patterning of the substrate.²⁸ More recently, alternative methodologies have been developed that have made it possible to open new perspectives using mesoporous films by aligning the mesopores perpendicularly to the substrate.

2.1. Seeding block-copolymer layers

In the last years, alternative processes to obtain ordered mesoporous films have been explored. One example is the two-step process based on the deposition of a first surfactant layer followed by of a second coating containing the metal oxide precursor (Fig. 1).²⁹ The method has been applied to obtain mesoporous TiO₂ films whose formation is governed by the interdiffusion of the two components, the surfactant and the oxide precursor, at the interface.

Fig. 1 Comparative pathways of the supported mesoporous film formation by: (A) conventional approach (EISA) using mixtures of precursors (alkoxides) and porogens (surfactants), and (B) alternative procedure using the components in two steps. Modified with permission from ref. 28.

The pre-deposition of a block-copolymer template layer has been proven to be a feasible technique for obtaining silica³⁰ and titania films with tilted and vertically aligned pores with respect to the substrates.³¹ The substrate is first immersed in a solution containing a surfactant (Pluronic P123), 1,6-diisocyanatohexane, and a small amount of glycerol used as a cross-linker to stabilize the layer. Onto this layer is deposited via-dip-coating the precursor sol with the surfactant, and during the evaporation, vertically aligned mesochannels form.^32,33 The epitaxial alignment of the surfactant hydrophobic blocks induces the orientation across the film. The aligned mesoporous films are particularly stable during calcination at temperatures up to 500 °C, without significant loss of the mesostructured order.³⁴ Silica thin membranes with uniform and large vertical mesochannels have been also synthesised using a two-step templated growth in solution.³⁵ At first, the substrate is immersed in a solution containing the surfactant, and then tetraethylorthosilicate (TEOS) in cyclohexane is added drop by drop. Free-standing mesoporous silica membranes have been finally obtained by the poly(methyl methacrylate) (PMMA) assistant transfer method.

2.2. Stöber method for the fabrication of vertically oriented mesochannels

Another innovative method has been developed starting from the well-known Stöber process that is used to fabricate silica particles of controlled shape and dimension.³⁶ The process has been also extended to obtain mesoporous silica particles whose mesochannels are radially oriented towards the surface.³⁷ The extension of the Stöber process to self-assembled mesoporous films allows for orienting the mesopores perpendicularly to the substrate.³⁸ The deposition of organized silica films is achieved by immersing the substrate into a Stöber solution that contains the surfactant, cetyltrimethylammonium bromide (CTAB), besides the silica precursor, (TEOS), water, ethanol and ammonia. Mesostructured films with ordered hexagonal packing of mesochannels perpendicular to the substrate are obtained by immersion in the Stöber solution for a controlled time that allows the spontaneous growth of the silica mesostructure (Fig. 2).

Fig. 2 Formation process of ordered mesoporous silica films with perpendicular mesochannels by the Stöber-solution spontaneous growth procedure. Reproduced with permission from ref. 36.

The slow hydrolysis of TEOS in highly basic conditions, with the formation of negatively charged oligomers, favours the electrostatic interaction with the positively charged micelle surface screening at the same time, the micelle–micelle repulsion. In addition, the diffusion of alcohol within the micelles lowers their curvature favouring the transition to cylindrical micelles. In addition, the diffusion of alcohol within the micelles lowers their curvature triggers the transition to cylindrical micelles. Ammonia also plays an important role promoting the hydrogen bonding between the CTAB micelles and the silica oligomers with further decreases of the curvature.

The method has some limitations because a hydrophilic substrate is necessary, and therefore, deposition on silicon is difficult. However, it opens the possibility of preparing membranes with the capability of filtering in the nanoscale. The silica film can be detached from the substrate and used as membranes, and several examples have been reported so far. The synthesis, with small modifications to control the swelling of the micelles during self-assembly, has been employed to obtain mesoporous silica membranes with a uniform thickness of 50 nm, vertical mesopore channels with p6mm 2D-hex symmetry, and pore diameters in the 2.8–11.8 nm range. Self-standing mesoporous silica membranes are obtained via polymethylmethacrylate (PMMA) assisted transfer that employs two polyethersulfone layers as the supports. The method has been extensively applied to prepare nano-filters and selective membranes for different applications.³⁹

2.3. Electrochemically assisted self-assembly (EASA)

An alternative method for the deposition of mesoporous films that has become very popular is the Electrochemically Assisted Self-Assembly (EASA).⁴⁰ This technique has been pioneered A. Walcarius and his coworkers⁴¹ and is now a well-established method for the deposition of vertically ordered mesoporous layers on conductive substrates. It has been applied to many applications ranging from electrochromic devices,⁴² to label free electrochemical sensors⁴³ and detection of doxorubicin in human whole blood⁴⁴ just to cite some.

The silica sol is initially kept at a pH of around 3–4, a value that makes hydrolysis faster than condensation.⁴⁵ The electrochemical process promotes an increase of silica pH at the solution–electrode interface, favouring the polycondensation on the electrode (Fig. 3). At the same time, the applied potential also directs the assembling of a surfactant template layer that, in turn, guides the growth of the mesoporous channels perpendicularly to the electrode. The negatively charged silica clusters that form at pH 9–10 interact at the interface of the transient cationic hemimicelles that form at the interface.⁴⁶ The proceed of silica condensation promotes the transformation of the hemicelles into cylindrical micelles forming the template for the vertically aligned mesopores. The mesoporous silica film deposited via EASA using CTAB as a cationic surfactant have a pore diameter typically around 2 nm. The pore dimension can be increased by swelling with mesitylene⁴⁷ or using surfactants with a longer alkyl chain.⁴⁸

Fig. 3 (A) Schematic illustration of the EASA process. (B) TEM images of the ordered and oriented mesoporous silica film (B₁ top view with electron diffraction pattern as inset; B₂ cross-section). (C) Typical GI-XD pattern confirming the hexagonal packing of vertically aligned mesopore channels. Reproduced with permission from ref. 40.

The potentialities of oriented mesochannels in silica films have been further exploited by covalent binding organo-functional species on the pore surface. One example of such applications is a surface modification with electroactive molecules, such as ferrocene. It can readily change its redox state via electron hopping transfer between adjacent sites, while the charge can propagate over a long distance even in the isolating environment of silica. Interestingly, the attachment on the surface of alkyne and alkene organic groups has been activated via click-chemistry.⁴⁹ Silica films with vertically oriented channels functionalized by bis(terpyridine) iron(II) have shown an electrochromic response with decolouration from violet within a response time to colouring and bleaching lower than 4 s.⁵⁰

2.4. Orientation by thermally induced film contraction

Phase transformations in mesoporous films are generally observed from drying to annealing stages as a result of the anisotropic stress induced by the shrinkage of the films deposited on a rigid substrate. These phase transformations must be allowed by the selection route. They can also result in a distortion of the pores from spherical to elliptical. Rankin and coworkers have shown an interesting application of these properties.⁵¹ That have fabricated vertically aligned mesochannels with a simple method based on the doping of silica with small amounts of titania (Fig. 4).

Fig. 4 (1) Schematic showing hypothesized incorporation of Ti (gold polyhedra) (a) throughout pore walls when only CTAB is used as the pore template and (b) at the pore surface due to complexation with dodecyl maltoside. (2) 2D GISAXS pattern of the CTAB-templated silica thin film with 1% titania doping and no sugar surfactant: (a) before calcination, showing a similar P6mm structure as undoped silica film with a background of an isotropic ring and (b) after calcination at 500 °C for 70 min. (3) Schematic of the mesopore fusion that transforms the porous structure from randomly oriented pores in titania-doped silica films with no sugar surfactant. Rearranged with permission from ref. 47.

Two different doping routes have been employed, dispersion of a small amount of titania (up to 6%) in the silica matrix and fixing of titania at the pore surface by complexation of titanium(IV) isopropoxide with a sugar-based cosurfactant, n-dodecyl β-D-maltopyranoside. Both routes produce alignment of the mesopores. Anisotropic stress and annealing of the films trigger the formation of vertically aligned mesopores formed by the merging of randomly oriented pores. The titania “doping” of silica makes the mesostructure more compliant to unidirectional contraction during the thermal shrinkage favouring the coalescence of the pores.

Another example of pore orientation triggered by the thermally induced film contraction is also related to titania mesoporous films.⁵² A low molecular weight block copolymer (PS-b-PEO), upon sequential evaporation of ethanol and tetrahydrofuran, form spherical micelles that self-assemble into a body-centered cubic Im3̄m mesostructure. The thermally induced contraction produces vertical channels derived from the coalescence of the pores in the orthogonal interconnected mesochannels (Fig. 5). A similar coalescence process has also been observed in titania films epitaxially grown layer by layer on a substrate initially modified by a block copolymer film (Pluronic F127) with a (011)-oriented Im3̄m cubic mesophase order.⁵³

Fig. 5 (A) High-magnification FE-SEM images of mesoporous TiO₂ films. Dashed circles indicate the locations of pores just below the surface. Inset: illustrative view of the bcc structure from the (011) direction. (B) Evolution of the d₀₁₁-spacing with temperature. The dotted curve is plotted qualitatively to describe the contraction mechanisms that could not be experimentally observed. The lower horizontal threshold line represents half the pore height (1/2h_pore, bottom), below which coalescence is expected to occur. (C) Illustration of the 3D pore distribution, along with 2D planar views, before and after coalescence. The side views use colours for the front pores and black/grey for the subsequent plane. The arrows in the lower-right magnified panel represent the possible occurrence of interchannel microporosity originating from the overlapping of neighbouring pores during vertical contraction. Reproduced with permission from ref. 48.

3. Understanding and controlling mechanical properties

The mechanical properties of mesoporous films have been the subject of numerous studies mostly focused on measuring elastic modulus (E) and hardness (H). These properties vary greatly as a function of material structure, such as the degree of crystallinity, porosity, composition, and topology of the mesophase.⁵⁴ Various techniques have been employed, such as surface acoustic waves,⁵⁵ nanoindentation,⁵⁶ ellipsometric porosimetry⁵⁷ and X-ray reflectivity⁵⁸ to evaluate the mechanical properties in mesoporous films. For a detailed review of the subject, see Soler-Illia and coworkers' work.⁵⁹ In general, it has been found, as expected, a direct relationship between porosity and E and H. However, the mechanical properties of thin films deposited by the liquid phase, such as sol–gel and mesoporous films, are highly dependent on the stress generated during heat treatment, which induces shrinkage in the direction perpendicular to the substrate. Such stress can induce fractures and delamination. The shrinkage in the case of mesoporous films can lead to both pore deformation and phase transformations induced precisely by mechanical stress.⁶⁰ Kozuka and coworkers⁶¹ have realized a detailed study devoted to understanding the stresses formed in mesoporous films. The mechanical properties of titania and Ce_xZr_1−xO₂ (x = 0, 0.5, 1), have been evaluated by the curvature method that measures the light deflection induced by the concave or convex bending of the films on a substrate (Fig. 6).

Fig. 6 (Left) Scheme of the setup of the thin film stress instrument used for curvature determination. It has a built-in heater for in situ measurements. (Right) Temperature dependence of the in-plane stress while heating (red dotted line) and the following natural cooling process (straight arrows). The different thermal expansion coefficients of the film and substrate directly affect the resulting residual stress. Rearranged with permission from ref. 57.

The mesoporous films show during the in situ process, in comparison to dense sol–gel films, remarkably lower absolute stress values. The block-copolymer template promotes stress relaxation during the process, reflected in a measured smaller intrinsic in-plane stress. The overall residual stress in titania mesoporous films is less than 50% of the one in dense films because the mesopores lower the intrinsic and thermal stress that arise during the heating and cooling stages during film processing.

On the other hand, the stress–strain deformation induced in the mesoporous films can also be used to design the material properties as a function of the required application; for instance, the thermoelectric properties of mesoporous ZnO thin films have been found to depend on the stress–strain in the material.⁶² Another example is the anisotropic and reversible deformation observed in mesoporous films deposited on elastomeric substrates⁶³ (Fig. 7). The effect of the mechanically induced stress on the optical properties of a mesoporous silica film has been measured by transferring the film onto an elastomeric poly(dimethylsiloxane) PDMS substrate. The compression of the substrate induced a reversible anisotropic deformation of the mesophase that produced a change in the refractive index.

Fig. 7 Left. (a) Schematic illustration of a mesostructured surfactant/silica hybrid film or a mesoporous silica film that has planar rectangular 2D mesostructures with C2mm symmetry. (b) SEM image of the mesoporous silica film showing a C2mm mesophase and pores of ∼7 nm diameter. (c) Concept of the present study: anisotropic and reversible deformation of the mesostructures of the mesostructured silica-based films by compression with accompanying deformation of the elastomeric substrates (PDMS). Right. GI-SAXS images (top) and the corresponding mesostructures (bottom) of the surfactant/silica hybrid film without (ε = 0%) and with (ε = 6%) compression (compressive strain ε (%), dL/L₀ × 100). Rearranged with permission from ref. 59.

4. Deposition and printing techniques

Developing advanced deposition and printing technologies for mesoporous films is a key issue for integrating the material into functional devices.⁶⁴ Several of the applied methods have underlined the high flexibility of film processing; the mesoporous layers can be easily patterned with different processes. The interest is still high and some new methods have been developed. One of these is the slot-die printing that has been successfully applied to deposit mesoporous titania^65,66 and hematite⁶⁷ (Fig. 8) mesostructured films. Slot-die printing is a process that is widely used in the industry for the deposition of liquid solutions on a substrate for large scale manufacturing. Extending the method to mesoporous films is, therefore, important for exploiting the full potentialities of industrial applications.

Fig. 8 Fabrication of mesoporous Fe₂O₃films via slot-die coating and high-temperature annealing: (a) PS-b-P4VP/FeCl₃ and PS-b-P4VP are dissolved in dimethylformamide. (b) Solutions are individually slot-die coated. (c) Film formation of the printed films is followed by in situ GISAXS. (d) Slot-die coated PS-b-P4VP/FeCl₃ thin film calcined at 500 °C for 2 h to finalize the α-Fe₂O₃ thin film. Reproduced with permission from ref. 63.

Another interesting method that has been developed as alternative to dip- and spin-coating for the deposition of mesoporous films is gravure printing (see Fig. 9 for the description of the method).⁶⁸

Fig. 9 Comparison of the two mesoporous film preparation strategies, evaporation-induced self-assembly (EISA) by dip-coating or gravure printing. (a) Gravure printing using a metal cylinder transferring the “ink” (sol–gel-solution) to the substrate. The ink is filled out of a reservoir into the cells of the gravure cylinder and directly transferred to the substrate. Excess ink is scraped off by a doctor blade. The cells are not completely emptied, as it comes to a film splitting. The settled fluid spots on the substrate extend to a surface at appropriate ratios of the surface tension of fluid and substrate. The arrangement of the cells specifies the layout to be printed film. The layout repeats after one cylinder revolution. (b) Conventional dip-coating process based on withdrawing a substrate with a defined speed from the sol–gel-solution. The film thickness and quality mainly depends on the used withdrawal speed, solution viscosity and substrate wetting. Reproduced with permission from ref. 64.

Gravure printing presents several advantages; ultrathin film of 20 nm can be deposited as a single layer while the film remains highly homogeneous even for small patterns. The film deposited via dip-coating can be fabricated with a sufficient quality only on a large substrate because of the rim effects.

Besides innovative fabrication methods, lithography in mesoporous films remains a key issue, especially for applications based on solid-state devices. To fabricate mesoporous silica pillars, lithographic etch masks have been used in combination with inductively coupled plasma (ICP) etching.⁶⁹ Large area ordered mesoporous silica nanopillar arrays with smooth vertical sidewall profiles have been fabricated on large areas and different channel depths.

Modifying the flat substrate by an array of linear microstructured grooves with a round cross-section has been used to achieve a total in-plane alignment of the mesopores.⁷⁰ The mesopores align perpendicularly to the groove long axis driven by the minimization of the elastic energy of the lyotropic liquid crystal phase that results from the self-assembly of the micelles.

Deposition of ordered mesoporous silica into patterned cylinders with pores oriented parallel is another innovative method developed by Hector and coworkers.⁷¹ On a silica on silicon substrate is deposited a TiN layer followed by a second silica film both of which are deposited by sputtering. The silica layer is finally patterned using e-beam lithography and reactive ion etching to fabricate nanoscale pillars with diameters of around 280 nm. The wafer has been then diced into small chips, and mesoporous silica structures have grown in the confined pillar space by drop-casting the EISA solution or via dip-coating. Interestingly, the mesopores orient perpendicularly to the pillar surface.

Controlling the thickness is also a critical issue, especially when ultrathin films are required.⁷² Wet-etching⁷³ has been applied to mesoporous silica films by drop-casting an aqueous solution of ammonium fluoride that decreases the film thickness. The interface tension of the solution governs the etching. The same process has also been applied to reduce the thickness of vertically aligned ferrocene-functionalized silica mesoporous films deposited via EASA.⁷⁴ Furthermore, a post-deposition process using NH₄ has been developed for controlling the porosity in mesoporous silica films.⁷⁵ The dissolution process, which proceeds in two well-distinct stages, allows tuning of the film porosity and thickness.

5. Negative electrodes and energy storage devices for lithium batteries

One of the areas of mesoporous materials research where researchers' attention has recently been focused is lithium batteries. The use of porosity-controlled materials offers the potential advantage of increased diffusion of the lithium ion while the flexibility of the synthesis method allows for the precise design of different types of devices.⁷⁶ Research in this area has focused both on the composition of mesoporous films, particularly silica, titania, lithium titanate, and molybdenum dioxide, but also on pore topology and control of the degree of crystallinity, which is one of the parameters that determines battery effectiveness. Research in the field also includes mesoporous microparticles specifically designed for applications in lithium batteries. Thin-film technology shows a ductility of use that makes it easily integrated into different configurations and types of lithium batteries, and several alternatives have been developed and tested.

MoO₂ mesoporous materials are an example of such application to lithium batteries.^77–79 Ordered mesoporous MoO₂ thin films have been tested to model the correlation between charge storage, crystallinity and porous topology.⁸⁰ The thin film configuration allows eliminating carbon additives or binders that can hinder the role of the MoO₂ nanostructure in modulating the charge storage properties. MoO₂ mesoporous films fired at 600 °C, which is an optimized temperature, can be charged and discharged in 24 h still keeping a Li⁺ storage capacity of 158 mA h g⁻¹. As the same authors have underlined, the thin film system does not represent a practical energy storage device but allows improving the design of the parameters that control the energy storage in porous materials.

Another material evaluated as an anode for Li-ion microbatteries is nanocrystalline mesoporous of lithium titanate, Li₄Ti₅O₁₂ (LTO), in thin films.⁸¹ LTO anodes potentially have higher stability in charge–discharge cycling, but the disadvantage of reduced electrical conductivity and Li⁺ diffusion. On the other hand, LTO as thin films should increase the cation's mobility, overcoming some of the intrinsic limitations of such materials as anode for Li batteries. Nanocrystalline mesoporous carbon-doped Li₄Ti₅O₁₂ thin-film with a pure spinel structure has been obtained via self-assembly using an in situ synthesis of Li–Ti double alkoxide.

Mesoporous silica films with vertical mesochannels have also been applied to develop anode-free metal lithium batteries⁸² that have no excess lithium metal.⁸³ The absence of the anode increases the battery energy density and avoids dendrite growth, one of the main drawbacks of lithium batteries. Lithium dendrites are metallic microstructures that form on the negative electrode during the charging process. The lack of the anode increases the life cycle and the safe use of the batteries. Mesoporous silica thin layers with vertically oriented mesopores have been used in anode-free devices to improve battery performances and effectively suppress Li dendritic structures (Fig. 10).

Fig. 10 Schematic illustrations of the Li electrodeposition process on the (a) stainless steel (SS) and (b) mesoporous silica thin films with channels perpendicular to the stainless steel substrate (MSTF⊥SS) in the anode-free Li metal batteries. Reproduced with permission from ref. 79.

The presence of the MSTF regulates the Li⁺ flux taking advantage of the ordered spacing between the mesopores, which also contributes to having a uniform Li⁺ distribution along in plan structure. Another advantage is the higher number of transferred lithium cations into the negatively charged mesochannels that limits the current density and the growth of dendritic structures. In addition, the mesoporous silica matrix is a suitable scaffold for lithium ions and has good wettability with respect to the electrolyte. On the other hand, the limited thickness of the film, around 30 nm, does not significantly enhance the resistance in the battery keeping the conductivity high.

The possibility of modulating the pore organization in terms of dimension, orientation, and pore wall thickness gives mesoporous films enough flexibility to be designed for different device configurations. Mesoporous titania has been widely studied as a material for anode lithium batteries⁸⁴ and several examples have also been reported for mesoporous films.⁸⁵ In particular, vertically oriented⁸⁶ and double diamond structured bicontinuous⁸⁷ porous organized structures have been studied for fabricating lithium battery anodes. Porous titania has the advantage with respect to the bulk to increase the Li⁺ ionic and electrical conductivity that is reflected in slow charge–discharge rates. Using the pre-deposition of a block-copolymer layer to vertically align titania mesochannels, films of thickness up to around 1.0 μm and accessible 7–9 nm nanopores have been obtained. A capacity of 254 mA h g⁻¹ is obtained after 200 cycles for a single-layer TiO₂ film, higher than dense titania films, has been measured. Good performances have also been measured in anodes obtained with bicontinuous titania mesoporous scaffolds with the capacity of 254 mA h g⁻¹ at the current density of 1 A g⁻¹.

Ordered mesoporous titania films, deposited by spray-coating, have also been fabricated as anode materials in lithium-ion batteries.⁸⁸ The material exhibits a high capacity of 680 mA h g⁻¹ at the first discharge, which, however, quickly decreases with the cycles to stabilize at around 170 mA h g⁻¹ after 50 cycles.

A solid-state battery–capacitor based on mesoporous silica films whose walls have been modified by a click chemistry reaction using graphene as the counter electrode has also been prepared⁸⁹ (Fig. 11). The device can deliver an energy density of 0.07 μW h cm⁻² at the high power density of 180 μW cm⁻². Optically transparent thin films with a vertical alignment of the mesopores have been deposited using EASA. The functionalization of the pore surface, to anchor redox-active species, has been done via click azide chemistry. This approach allows the introduction of large amounts of organic groups onto the silica walls. The charge propagation occur via an electron hopping process between adjacent redox centers.

Fig. 11 Schematic illustration of the preparation process of the ferrocene-functionalized vertically aligned mesoporous silica thin films on a flat ITO electrode. After hydrolysis of the silane precursors, the azide-functionalized silica thin films are further functionalized with ferrocene units by copper-catalyzed azide–alkyne Huisgen reaction (left). Structural characterization of the ferrocene-mesoporous modified with 40% of 3-azidopropyltrimethoxysilane film. TEM micrographs: top view, cross-sectional view (right top and bottom, respectively). Rearranged with permission from ref. 85.

6. Integrating the flatland with mesoporous films, advanced sensing

The integration of two-dimensional materials with a mesoporous matrix, which could be through the formation of a nanocomposite or as an active substrate, is one of the main trends observed in the development of functional applications based on mesoporous films.⁹⁰ Graphene, in particular, has been the most used two-dimensional material for this purpose for both applications, but other bidimensional materials, such as BN⁹¹ and W₂S,⁹² have also been considered recently. The development of mesoporous films with pore orientation perpendicular to the substrate, which has now reached an advanced state, has allowed the fabrication of different types of very versatile sensors that use graphene as a conductive substrate. Other graphene materials have been used as an electrode for electrochemical sensing platforms using vertically ordered mesoporous silica (VOMS) channels deposited via EISA. Several sensors with fast and highly sensitive responses have been developed using graphene-based architectures. In general, graphene is applied as an electrode on which, using different geometries, are deposited the mesoporous films. One example is the carbendazim (a pesticide) sensor developed using a BN-rGO substrate⁹³ or the uric acid in serum sensor that employs a 3D macroscopic graphene as substrate.⁹⁴

The ferrocene functionalized silica film coated onto an electro-exfoliated graphene electrode is another example of the integration of graphene with VOMS. The electroactive organically modified mesoporous silicates on graphene oxide-graphite 3D architectures operating via electron-hopping for high rate energy storage as pseudo-capacitive material.⁹⁵

An example of the different possibilities of designing a device based on the VOMF-graphene architecture is the fabrication is shown in Fig. 12.⁹⁶ On a gold substrate is cast a β-cyclodextrin-graphene layer that acts as a functional and conductive layer; the system is a sensor for the electrochemical detection of acetaminophen.

Fig. 12 Illustration of facile preparation of silica nanochannel array film (SNF)/β-cyclodextrin-graphene (CDG)/AuF by EASA method using CDG as nanoadhesive. SM = surfactant micelle. Reproduced with permission from ref. 92.

Other examples of mesoporous devices using a graphene layer as an electrode are the sensors for dihydroxybenezene isomers in environmental water samples,⁹⁷ for anti-fouling electrochemical detection of tert-butylhydroquinone in cosmetics and edible oils,⁹⁸ for the direct electrochemical analysis of human serum,⁹⁹ just to cite some.

Mesoporous thin films on graphene field-effect transistors (FETs) have shown the possibility of combining sensing with size-exclusion filtration.¹⁰⁰ The device show a size-excluded electrostatic gating response given by the pore size and a larger amplitude and sensing range compared to bare graphene FETs for charged macromolecules infiltrating the pores.

Incorporating graphene into mesoporous films is not easy because several strict requirements should be fulfilled. For example, graphene must be soluble and well dispersed in the precursor sol, should not disrupt the organization during self-assembly, and the films should remain optically transparent after deposition, which is required for several applications such as photocatalysis. On the other hand, the incorporation of graphene in titania mesoporous films significantly improves the functional properties and applications in sensing and photocatalysis have been demonstrated. One example is the incorporation of exfoliated graphene into ordered cubic mesoporous films¹⁰¹ (Fig. 13). The films show an enhanced photocatalytic effect also allowing patterning via deep X-ray lithography.

Fig. 13 Left. TEM images of representative areas taken from mesoporous graphene–titania nanocomposite films (2.5 vol% of EG dispersion). (a, b) Film fragments before calcination (thickness ≈ 280 nm); arrows indicate the graphene sheets. (c and d) Cross-section dark-field TEM images of the films after calcination. Right. (a) UV-vis spectroscopy of graphene mesoporous nanocomposite films before (solid line) and after (dotted line) thermal calcination (thicknesses ≈ 340 and 150 nm, respectively). Inset: picture of the film after thermal treatment. (b) Kinetics of photodegration of stearic acid deposited on pure titania (red squares) and graphene-titania (blue triangles) mesoporous nanocomposites (2.5 vol% of EG dispersion). Inset shows the pseudo-first-order fit of stearic acid decay at increasing time of UV excitation. Reproduced with permission from ref. 97.

Besides photocatalysis, graphene-titania films have shown a remarkable effect when used as a platform for Surface Enhanced Raman Scattering (SERS).¹⁰² Graphene can enhance the Raman scattering signal through what is indicated as graphene-mediated enhanced Raman scattering (GERS). The enhancement, however, is rather small (10–10²) because of the chemical mechanism (CM) which governs the charge transfer between graphene and the molecules to detect. This effect is, however, amplified when exfoliated graphene is dispersed in titania mesoporous film giving rise to Ti-GERS (titania-induced graphene-mediated ERS) that has been attributed to synergic interfacial interactions between graphene sheets and titania at the anatase crystallite edges within the nanocomposite.¹⁰³ The method to incorporate graphene into mesoporous films is flexible enough to allow fabricating complex heterostructures, for instance, growing inside the mesoporous matrix metal nanoparticles, such as silver¹⁰⁴ and gold.¹⁰⁵ Gold has been thermally reduced in situ with preferential nucleation on the surface of the graphene nanosheets.¹⁰¹ Silver nanoparticles have been instead grown in the graphene-mesoporous titania film by exposition to hard X-rays.¹⁰⁰ Also, in this case, the graphene sheets work as preferential nucleation sites, allowing faster nucleation of the nanoparticles.

Besides graphene, BN⁸⁷ and W₂S⁸⁸ nanosheets have also been successfully incorporated via one-pot self-assembly into mesoporous ordered titania films. The bidimensional materials can be easily dispersed in the precursor sol and remain homogeneously dispersed into the mesoporous films that keep the organization of the pores. The nanocomposites have been tested for photocatalysis showing improved performances. In the case of BN, the improvement is only observed when defective boron nitride sheets are incorporated in the mesoporous titania films.

Mesoporous films are a feasible matrix not only for graphene sheets but also for graphene¹⁰⁶ and carbon dots.^107–110 An example is the electrochemical sensing platform realized confining graphene quantum dots (GQD) into vertically ordered mesoporous silica channels through electrophoresis.¹⁰² The GQD plays multiple roles by enhancing selectivity as a recognition element and mediator for charge transfer. The integration expects interesting future developments of carbon dots into mesoporous films, which should allow extending the field of application and the construction of functional heterostructures.

7. Future outlook

The comprehension of the basic phenomena behind self-assembly in mesoporous ordered films is well established. This is particularly true in the case of silica and titania films, which are by far the most used and applied in devices and applications. However, despite the previous intense work, some basic knowledge remains to explore. In particular, mesoporous films represent an ideal nanoscale confined environment where the physics and chemistry of reactions and processes are still partly to be explored.

In contrast, research and applications on mesoporous films of different compositions than silica and titania are still quite limited, leaving much room for future investigations. A major trend is the integration of mesoporous films into industrial processes that are effectively applied in terms of scale, cost, material control, environmental impact, and reproducibility. Dip-coating and spin-coating deposition techniques have been the basis for the initial studies of mesoporous films. However, they have severe limitations when they need to be transposed to the market. Therefore, one of the strongest trends is developing sensors based on mesoporous films, mainly using mesostructures oriented vertically to the substrate.

Mesoporous films, when used as sensors, have the advantage that pore wall functionalization can be used to develop highly selective systems. Another area of great interest that is likely to be the subject of intense research in the future is batteries, in which mesoporous films can play an important role. One process that needs to be followed closely is the realization of complex heterostructures based on mesoporous films that may open up new scenarios in sensing and photocatalysis. Integrating two-dimensional materials together with nanoparticles is an example of heterostructures whose development is yet to be explored. Research on mesoporous films after the first phase of studying the basic phenomena governing the self-assembly process has entered a new phase; it is to follow in parallel the development of technologies based on mesoporous films and the innovation proceeding at great speed in nanoscience.

Author contributions

The author confirms sole responsibility for the contents and preparation of this manuscript.

Conflicts of interest

There are no conflicts to declare.

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93. Y. Zou, X. Zhou, L. Xie, H. Tang and F. Yan, Vertically-Ordered Mesoporous Silica Films Grown on Boron Nitride-Graphene Composite Modified Electrodes for Rapid and Sensitive Detection of Carbendazim in Real Samples, Front. Chem., 2022, 10, 939510.
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103. D. Carboni, B. Lasio, D. Loche, M. F. Casula, A. Mariani, L. Malfatti and P. Innocenzi, Introducing Ti-GERS: Raman Scattering Enhancement in Graphene-Mesoporous Titania Films, J. Phys. Chem. Lett., 2015, 6, 3149–3154.
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108. K. Suzuki, L. Malfatti, D. Carboni, D. Loche, M. Casula, A. Moretto, M. Maggini, M. Takahashi and P. Innocenzi, Energy Transfer Induced by Carbon Quantum Dots in Porous Zinc Oxide Nanocomposite Films, Phys. Chem. C, 2015, 119, 2837–2843.
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