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Exploration and Exploitation of Water in Colloidal Crystals Francisco Gallego-Gómez,* Alvaro Blanco, and Cefe López performance is essential, but challenging, all the more so the smaller the system scales. Water (or another liquid) sorption and capillary phenomena at the nanoscale are certainly fundamental in fields related to nanomaterials science, like wetting and spreading,[8] micro-/nanofluidics,[9] nanotribology (friction and lubrication, wear),[10] or superhydrophobicity.[11] Many of these subjects are of enormous impact in different industries, such as cements, food, pharmaceutics, soil, oil recovery, mining, civil engineering, catalysis, rubber and tires, fuel cells, wear, lubricants, paper, paints, coatings, membranes, etc. Also, many meteorological and biological processes are governed by such topics. However, in spite of the impressive progress in recent decades, most of these topics are still in need of a much deeper understanding. An advanced example is that of colloidal crystals (CCs), which are also commonly named artificial opals, with important applications in not only, for example, photonics,[12,13] energy conversion,[14] or sensing,[15] but also as models for atomic systems[16] or as templates.[17] CCs refer to solid 3D ordered arrangements formed by the self-assembly of sub-micrometer, monodisperse spheres from colloidal suspensions upon solvent evaporation.[18] Many of the CC properties depend on the arrangement and surroundings of the colloidal particles, which may be crucially affected by their surface properties. In particular, more- or less-hydrophilic spheres adsorb water from moisture in the ambient environment, which greatly enriches the physics of the system. The way water interacts with particle surfaces determines, in a non-trivial manner, its morphology and distribution in such a nanoconfinement (sometimes leading to intriguing structures), and critically influences the collective behavior of the ensemble. Some of these features are gathered in Figure 1. However, less attention has been paid to this matter. The presence of water strongly complicates the CC picture, and it could be considered an inconvenient problem to deal with. However, far from that, recent work conducted in our laboratory has proven it to be a great opportunity to study water-related phenomena at the nanoscale with innovative approaches. Probably the most-prominent feature of CCs is their photonic properties, which, as will be stated, are drastically affected not only by the amount of water taken up, but also by its distribution. We will show how such a system lends itself to suitable probes with optical properties that are sensitive to the exact
Water on solid surfaces is ubiquitously found in nature, in most cases due to mere adsorption from ambient moisture. Because porous structures have large surfaces, water may significantly affect their characteristics. This is particularly obvious in systems formed by separate particles, whose interactions are strongly influenced by small amounts of liquid. Water/solid phenomena, like adsorption, condensation, capillary forces, or interparticle cohesion, have typically been studied at relatively large scales down to the microscale, like in wet granular media. However, much less is known about how water is confined and acts at the nanoscale, for example, in the interstices of divided systems, something of utmost importance in many areas of materials science nowadays. With novel approaches, in-depth investigations as to where and how water is placed in the nanometer-sized pores of self-assembled colloidal crystals have been made, which are employed as a well-defined, versatile model system with useful optical properties. In this Progress Report, knowledge gained in the last few years about water distribution in such nanoconfinements is gathered, along with how it can be controlled and the consequences it brings about to extract new or enhance existing material functionalities. New methods developed and new capabilities of standard techniques are described, and the water interplay with the optical, chemical, and mechanical properties of the ensemble are discussed. Some lines for applicability are also highlighted and aspects to be addressed in the near future are critically summarized.
1. Introduction Liquids interacting with solid surfaces in a gaseous environment are present far and wide, not only in nature, but also in man-made items, including many advanced materials;[1,2] from sandcastles and pottery to modern ceramics and micro-/nanoelectromechanical devices, interactions at interfaces are crucial for the processing and functionality of many materials. Divided media like colloidal aggregates,[3] powders,[4] granular materials,[5,6] and soils,[7] as highly porous systems formed by individual solid particles, are particularly affected by the interplay of liquid present at the large solid surface area, which vastly determines the interparticle forces. In most cases, gas and liquid phases, respectively, consist of air and water adsorbed from ambient moisture. A deep knowledge of how water (and, subsequently, air) is confined in such systems and how it affects their Dr. F. Gallego-Gómez, Dr. A. Blanco, Prof. C. López Instituto de Ciencia de Materiales de Madrid c/ Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain E-mail: [email protected]
DOI: 10.1002/adma.201405008
Adv. Mater. 2015, DOI: 10.1002/adma.201405008
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distribution of the co-existing phases. This will provide a new, powerful tool to infer the water morphology at the nanoscale, even allowing monitoring in situ under varying conditions. On the other hand, a CC containing water between the spheres actually constitutes a “wet” divided system, only greatly simplified since it possesses, as an ordered structure, a definite configuration with a known arrangement of spheres and specificsurface area demarcated by the opal interstices. This avoids the extraordinarily complex problem of identifying how water distributes between random solid particles, a fact that strongly limits research on powders and granular media.[4–6] A wet CC offers a handy approach to investigate the water-dependent micromechanical behaviors of particles ensembles. Imaging at the nanoscale will complete a set of novel methods for a multidisciplinary study of nanoconfined water in CCs. Here, we critically review recent advances made in our laboratory toward not only exploring the behavior of water within CCs, but also exploiting its presence in such systems to gain new functionalities. The use of CCs has allowed us to benefit from their simple and inexpensive preparation and versatility (with even a controllable degree of disorder[19]), among the large theoretical and experimental background collected in recent decades. Undoubtedly, their photonic properties, being responsive to the presence of water, represent the crucial advantage over any other wet porous material, in which water merely changes the mechanical or chemical properties. Silica CCs, in particular, constitute a special subject in the study of water-containing opals due to the silica’s high hydrophilicity and the possibility of modifying its surface chemistry and tune the water adsorption. Thus, they constitute an appropriate test system with well-defined topology, large porosity, adjustable surface chemistry, and modifiable water content, and provide an excellent playground to study the morphology and geometric location of water in sub-micrometer environments. This Progress Report is organized as follows. Section 2 briefly introduces some fundamentals of the water–CC interplay, on which the procedures employed later are based: firstly, the nature of water contained in a silica opal and, secondly, the wet CC as either a photonic crystal or granular-like material. Section 3 describes new methods developed and the main results achieved: i) monitoring of the CC photonic properties and modeling of the water distribution; ii) scanning electron microscopy (SEM) to visualize the water structures, previously “stabilized”, within the sub-micrometer interstices; and iii) nanoindentation to examine the CC mechanical behavior. Section 4 shows some examples of how the knowledge acquired can be directly used to obtain new powerful functionalities suitable for real applications. Finally, in Section 5, we critically analyze results that need further investigation (including discrepancies with the theoretical framework to be solved) and give a perspective of the next tasks and future research to be faced.
2. Colloidal Crystals and Water In this Section, we first describe how water adsorbs on the surface of the spheres of a CC, and discuss whether capillary condensation between the opal spheres occurs or not. Since silica CCs were chosen as the main investigation subject, we focus
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Francisco Gallego-Gómez graduated in Physics from the University of Seville and received his Ph.D. (2005) in Physical Chemistry in Germany. He has carried out work at the universities of Munich (LMU), Cologne, Alicante and Madrid (Autónoma), and now is a postdoctoral fellow at the Spanish Scientific Research Council (CSIC). His expertise covers different aspects of material science, from organic and hybrid composites for holography and photoconductivity, to self-assembled structures for photonic applications. Currently, his research interest focuses on solid–liquid physics at the nanoscale in divided media like colloids, and on the extrapolation to macroscopic phenomena in granular media. Alvaro Blanco graduated in Physics from the Universidad Autónoma de Madrid in 1994, obtained his Ph.D. in Physics in 2001, investigating the fabrication and characterization of 3D photonic crystals, at the Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC). After a period of two years in the Institut für Nanotechnologie in Forschungszentrum Karlsruhe working on holographic photonic crystals as a postdoctoral fellow, he moved to ICMM-CSIC as a Ramon&Cajal researcher where he has been a Permanent Scientist since 2008. His current research lies on the preparation and characterization of self-assembled 3D heterostructures with applications in photonics. Prof. Cefe López’s alma mater is the Universidad Autónoma de Madrid where he graduated in Physics and received his Ph.D. (1989) in optical properties of semiconductors. After a postdoctoral period at the University of Oxford and a teaching stint at the Universidad Carlos III de Madrid he gained tenure in the Spanish Scientific Research Council (CSIC) in 1993. His main research interest is focused on self-assembled photonic structures and related systems. In particular, his multidisciplinary group covers synthesis and processing of materials – such as photonic glasses, composites, nanoparticles, and quantum dots – as well as the study of light transport, generation, and interference in ordered and disordered dielectric structures.
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on silica surfaces and how annealing changes their hydrophilic properties. Secondly, we briefly discuss how water generally affects the properties of a CC, considering its two main aspects: photonic crystal and granular properties. Both kinds of properties not only depend on the water content but also on its exact distribution between the interstices of the spheres.
2.1. Water in Silica CCs: Chemistry of the Silica Surface The rich chemistry of silica greatly expands the physics of large-surface-area systems, such as colloids, including CCs.[20] The behavior of water at the silica surface, often anomalous, has attracted a great interest for many years.[21] The surface of silica has a large number of hydroxyl (−OH) groups (surface silanols), bound to the Si atoms of incomplete siloxane
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Figure 1. SEM images of CCs illustrating different aspects of the water− solid interaction at the interfaces. a) The collective mechanical behavior of silica spheres upon indentation recalls common footprints in wet sand – see Section 3.4. b) The interparticle cohesion greatly depends on the specific arrangement. A defect in the CC (due to, for example, an air bubble) led to detachment of many spheres from the layers of a silica CC with high disorder, indicating less cohesion. As the order in the arrangement increased (in this case, in deeper spheres layers), less spheres were missing; the higher cohesion of the well-ordered layers finally avoided any loss of spheres – see Section 5.2. c) Complex trails of adsorbed water are formed on the surface of the polystyrene spheres – see Section 5.3. d) Detailed shape of water menisci formed between silica spheres and a silica sphere-substrate – see Section 5.2. The water structures in (c) and (d) have previously been transformed to allow imaging (Section 3.2). d) Adapted with permission.[80] Copyright 2013, American Chemical Society.
bridges (≡Si−O−Si≡), which have great affinity for water through hydrogen bonds, and endow silica with high hydrophilicity.[22–24] In a humid atmosphere, water is readily adsorbed by way of the surface silanols building a monolayer of water molecules, potentially followed by multilayer adsorption on the already-adsorbed water molecules, until a hydration layer (or wetting film) is formed on the silica surface. This type of water, whose content depends on both the surface silanol density (αOH) and the ambient conditions (temperature, T, and relative humidity, RH), is usually referred to as physically adsorbed, or physisorbed, water. This water is loosely bound to the surface, so it easily begins to desorb (dehydration or dewetting) by, for example, lowering the ambient pressure or raising the temperature, although complete elimination is only achieved in vacuum and/or at about 120 °C.[22] Physisorbed water is readily measurable from weight changes by water-adsorption isotherms or thermogravimetric analysis (TGA), by infrared (IR) spectroscopy,[25,26] solid-state nuclear magnetic resonance (NMR),[27] etc. CCs made of silica spheres are thus water-rich mesoporous structures (the voids between the spheres are in the range of tens of nanometers in size). This is an intrinsic characteristic given by the adsorption on the silica surface and not by aqueous residues from CC fabrication[28] or in suspension (free water[22]); these potential remnants are safely removed before any measurement by heating at ca. 120 °C. In particular, we deal with sub-micrometer-sized spheres of amorphous silica prepared by Stöber synthesis,[29] which are customary used to fabricate inexpensive, high-quality face-centered-cubic (fcc) structures.[30] We primarily use CCs made of Stöber spheres with a diameter (D) of 335 nm, synthesized according to ref. [31]. Bigger spheres (D = 905 nm), fabricated by a regrowth process,[32] are also employed. The synthesis parameters mainly determine the size of the spheres, but they may also affect some properties of the resulting silica, like the microporosity or surface roughness.[33] Silica aging, in which continued polycondensation and shrinkage of the pore network are involved, may be a factor to be considered as well.[20,21,23] As-grown (that is, untreated) silica CCs indeed exhibit the outstanding ability to physisorb water from the environment, as measured by adsorption isotherms (Figure 2).[34] The initial rise corresponds to the adsorption of a monolayer of water molecules and the following steady increase is a signature of the formation of multilayers.[35] The convex shape of the isotherm at RH < 10% (type II) indicates the existence of narrow micropores ( √2D). This latter aspect, which would occur if water were allocated between the spheres, is especially relevant,
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as the photonic properties are very sensitive to lattice changes. In addition, while Equation 1 is oversimplified and the spatial distribution of the different media is averaged, the exact solutions for the Bragg peak do depend on the specific location of each material. In practical terms, it is necessary to know the extent of such changes in hydrophilic CCs subjected to a varying environment (e.g., inside a heating-up optoelectronic device or in vacuum). This, in turn, offers a straightforward means of tuning the photonic properties of the opals by manipulating the amount of water, as seen in the previous section. Much more ambitiously, at a fundamental level, it is possible to access the structural information of the water within the CC from its optical response. In the last instance, this arises from the regularity of the crystalline structure, so any alteration in the geometry or composition induced by the adsorbed water is manifested straightaway in the PBG properties. By using a full description of the Bragg peak properties, which requires a rigorous solution of Maxwell’s equations, we were able to infer a great deal of information about the way water is adsorbed and confined in the nanometer interstices of the opal (Section 3.1). In particular, we employed the MPB package,[49] which allows the calculation of the photonic bands in heterogeneous periodic structures (in our case, formed by the spheres plus the air/water in the voids), even having a non-uniform distribution (air and water). These exact solutions are developed for infinite and perfectly ordered systems. Nevertheless, size effects in real samples with more than 10–15 spheres layers are negligible,[50,51] while disorder effects, although leading to a less-intense peak, do not affect the width.[51] On the other hand, stacking faults along the (111) direction do not affect the Bragg peak properties.[50,51] In any case, we have used high-quality silica CCs made by vertical deposition[30] with 20–25 layers. Bragg peak intensities were typically over 80% and no deformation in the (111) planes was assessed by diffraction.
2.3. CCs as Wet Granular Materials One can deal with the presence of water within the opal using a different point of view, that is, by considering an assembly of spheres in a wet environment. It is well known from research on granular media that even small amounts of liquid between grains can strongly change the physical properties of the system.[52,53] Independently of the origin of this liquid, it forms, in partially saturated systems, a meniscus (or neck, or capillary bridge) around every contact point or narrow gap between two grains (pendular regime – Figure 6a), which imparts an attractive force between them. This capillary force, Fcap, which is typically stronger than the van der Waals attraction, plays a fundamental role in the mechanical properties of any liquidcontaining granular assembly (a wet granular material) via both capillary cohesion and increased interparticle friction. Fcap is given by the exact shape and size of the meniscus (eventually, by its external surface), which depends on the amount of liquid, the geometry of the interstice, and the chemistry of the surfaces (see Section 5.2). The external surface of the neck approximates a toroid (circular approximation) with two radii of curvature, R (the half-width of the neck) and r (the meniscus radius), which
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are geometrically related to the other characteristic parameters: the separation of the surfaces, η, the filling angle, β, and the contact angle, θ (see Figure 6a).[2] These relationships must be conveniently modified if a wetting film is present on the surfaces.[54] From these considerations, capillary forces essentially depend on how much liquid is building the menisci between the particles and how much is just wetting their surfaces. In spite of the paramount importance of the liquid in granular systems, wet conditions have only been studied since relatively recently,[55–57] and there is still much to understand. In ordinary granular materials, determining the way water is placed between the randomly distributed particles constitutes an extraordinarily complex problem. At best, partial information is obtained by demanding techniques and is usually restricted to scales above the micrometer level (see an example in Figure 6b). In a CC that takes up a non-negligible amount of moisture, like a silica one in the usual ambient conditions, water forms menisci between adjacent colloidal spheres and wets their surfaces. Thus, a silica CC represents a strong analogy to wet granular media (or, considering the small scale of the particles, wet powders). However, unlike them, a CC has a well-defined morphology, namely, the size and form of both grains (monodisperse spheres) and pores (opal voids), and their localization (fcc packing), which can even enable the distribution of water to be accurately known (Figure 4b). Moreover, features like the sphere size, the order degree of the ensemble, and the surface properties are easily adjustable. Thus, the study of the formation of capillary bridges and water-dependent mechanical properties of such a simplified, versatile system provides novel insight into topics of granular and powder science. In addition, given the sub-micrometer dimensions of the colloidal spheres, fundamental phenomena like menisci morphology and capillary forces can be directly investigated at the nanoscale, where aspects of classical theories may fail.
3. Exploring the Water in Colloidal Crystals As shown, water is an important part of silica CCs that significantly affects their photonic and micromechanical properties. In this Section, we show how to take advantage of this fact to study the fundamental features of water in the pore network.
Adv. Mater. 2015, DOI: 10.1002/adma.201405008
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Figure 6. a) Characteristic parameters of a liquid meniscus formed between two equal spheres. b) Irregular water bridges formed in a granular material of polydisperse sub-millimeter glass spheres, obtained by X-ray tomography. b) Adapted with permission.[53] Copyright 2008, Nature Publishing Group.
Firstly, a central point in our work is reviewed: the influence on the PBG of dehydration of opals with different surface properties is monitored in situ by mere UV–vis spectroscopy, and an analytical method is developed to connect the bandgap properties with the actual water distribution. As a result, a simple optical tool is turned into a powerful method to study nanoconfined water, such as between the CC spheres. The potential of this technique, to which we will refer in this report as the PBG method, relies on the sensitivity of the photonic performance, as a collective feature, to specific changes in the structure. In this sense, this new method bears similarities to established techniques, such as small-angle X-ray/neutron scattering (SAXS/SANS)[58] or NMR spectroscopy, in which information about liquid inside a porous environment is obtained from the collective properties by applying a set of assumptions. Secondly, the nanometer-sized water morphology is (quasi-directly) visualized. This not only allows comparison with the PBG method, but also the appraisal of particular features, to which CC properties are rather insensitive. Thirdly, the comprehensive micromechanical characterization of the CC, tackled for the first time, gives relevant insights into the interparticle forces and the decisive role of the structural and dynamic behavior of water.
3.1. Water-Dependent PBG: Simple Optics to Reveal the Nanoconfined Water Distribution 3.1.1. Model for the Water Pattern and Desorption Experiment As discussed above, the presence of physisorbed water in the silica CC affects both the average refractive index of the opal voids and the packing of the spheres. Since the PBG characteristics critically depend on these factors, which, in any case, can be easily simulated, simply measuring the optical spectrum provides reliable information about both the CC structure and the water distribution, just by assuming a few simple, realistic hypotheses. A simple experimental procedure,[31,60] in which the CC is gently heated to induce desorption, allowed the water content in the CC to be modified in a controlled fashion, while the reflectance spectrum was measured in situ. This informed us about the progress of the CC (PBG position and width) as a function of the water content (Figure 7, for as-grown and annealed silica opals). Temperature rise (in steps of 5–10 °C) primarily induced spectral changes within a few seconds, which were fully reversible by cooling as long as T < 150 °C.[60] This is the approximate temperature at which physisorbed water is completely desorbed and dehydroxylation begins, and therefore it was taken as the upper limit in desorption experiments. Now, the goal is to obtain the distribution of the water and its evolution from the behavior of the CC properties. We consider a simple model for water distribution[31,60] (Figure 8a), which distinguishes between: i) water placed in the voids, as a wetting film around the spheres surface, and ii) water between nearest-neighbor spheres forming a pendular neck. Some assumptions must be made. Firstly, the wetting film is ideally homogeneous with uniform thickness d; in the case of inhomogeneities due to, for example, the presence of water clusters or patches, d corresponds to the average thickness. Secondly, according to the circular approximation, the
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Figure 7. Evolution of the PBG parameters, λBragg (a) and width (b), during water desorption in as-grown (black) and annealed (colors) silica CCs. The bottom axis represents the decreasing content of physisorbed water, as this is the change direction in the heating experiment. The symbols represent the experimental data; the lines are the fitting curves of the model. The open symbols are values at RT. Adapted with permission.[31] Copyright 2012, American Chemical Society.
neck is modeled as a toroid with azimuthal radius, R, but, for simplicity, with no meniscus curvature r (flat edges in Figure 8a) as it barely affects the neck volume. r can be readily calculated from the other parameters. Thirdly, the model assumes η > 0, so some separation is allowed to exist between the spheres; that is, a non-close-packed fcc structure is allowed for the CC. This is an important assumption in our model, although it might be controversial (see discussion in Section 5.3). Fourthly, the minor amount of water filling the Stöber micropores is considered to be immobile in our experiments (with time scales of seconds) given the much slower water diffusion in very narrow spaces,[61,62] especially those having hydrophilic surfaces.[63] Explicitly, water diffusion in Stöber silica CCs slowed down to two orders of magnitude in the micropores of the spheres;[64] and water uptake times of hours have been measured for micropores of annealed Stöber spheres.[38] Nevertheless, this internal water needs higher temperatures to be desorbed (above 180 °C[65] than those employed here, as the very negative Laplace pressure in such narrow gaps must be overcome. Finally, the water density in both the wetting film and the neck is taken as that of bulk liquid water, although, rigorously, this is incorrect for the first layers adjacent to the surfaces.[61] With these assumptions, any triad of parameters {d, R, η} that is compatible with a certain amount of water defines a possible distribution (pattern) for this water in the CC. The
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Figure 8. a) Left: Model for the water distribution between the CC spheres adopted for the PBG method, which distinguishes between a wetting film of thickness d around the spheres and capillary necks between them, described by R and η. Right: Limiting cases for the ratio ρ: water is entirely distributed as wetting film (ρ = 0, top) or as necks (ρ = 1, bottom). b) Evolution of the model parameters depicted in (a) upon water desorption in as-grown (black symbols) and annealed silica CCs (colored symbols) as obtained from fitting of experimental data (Figure 7). The open symbols are the values at RT, graphically shown in Figure 9, left column (the solid lines are a guide to the eye). The gray shaded area corresponds to the second desorption regime of the as-grown CC (see text). The dashed line marks the water patterns graphically represented in Figure 9, right column, at the same water contents (1.5 wt%). Adapted with permission.[31] Copyright 2012, American Chemical Society.
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3.1.2. Hydrophilic CCs As shown in Figure 7, hydrophilic silica CCs (as-grown opals – black symbols) exhibited a blueshift as large as 23 nm upon desorption (by heating from room temperature (RT) to ca. 150 °C). Indeed, the voids emptying due to desorption (nv diminishes in Equation 1) was expected to lead to a decrease of λBragg decrease, but it cannot account for the experimental value. By considering a water content of 7.5 wt% at RT (Figure 3a), MPB simulation gives a maximum blueshift of 14 nm after complete dewetting. On the other hand, the PBG width narrowed by 9% during dehydration. This finding was particularly puzzling because water elimination implies a higher spheres/ voids refractive-index contrast, which would lead to a broader bandgap. In order to explain this unexpected behavior, we considered the possibility of having a non-close-packed assembly of fcc spheres, that is, spheres with finite separation (η > 0), which additionally varies with the amount of water. For constant η, which also includes the usually assumed close-packed arrangement (η = 0), MPB calculations always predict a broadening of the PBG during desorption (as a result of nv decrease). In contrast, if η is allowed to decrease upon dewetting, the PBG narrowing can be reproduced (in spite of the increased refractive index contrast).[60] That is to say, the experimental behavior of the PBG width denotes the contraction of the opal structure due to water desorption. This, moreover, explains the large blueshift measured, as the structure compaction implies a decrease of d111, which adds a further contribution to blueshift (see Equation 1). Using our model, experimental data were fitted with excellent agreement (black lines in Figure 7) to
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MPB simulation software allows us to calculate the PBG corresponding to each combination. At a given T, the water content is known (from, for example, thermogravimetric analysis (TGA), Figure 3a), as are λBragg and the width of the PBG (from the optical spectrum, e.g., Figure 4a). These constitute three pieces of experimental data that enable the three unknown variables to be found (the triad {d, R, η}) using an iterative process for best agreement.[60] Note that we do not impose the pattern to obey the circular approximation (in our case, in the presence of a wetting film[54]). Therefore, an interesting aspect provided by the resulting pattern is how homogeneously the water distributes in the CC. We define a further parameter, the ratio of the volume of water necks to the total water volume (ρ), which is a function of {d, R, η}. ρ illustrates whether the water is uniformly distributed over the spheres (i.e., forming only a wetting film but no necks, ρ → 0) or accumulates between the spheres (exclusively building necks, ρ → 1) – see Figure 8a (right). Thus, the water pattern that fits best the experimental spectrum is inferred at each T (each value of water content). In practice, and as a crucial advantage, fitting is made using the whole data set from the desorption experiment, which minimizes analysis errors, allowing, for example, the adjustment of the nominal value D within its nominal error (about 10%). Simultaneously, this informs us about the water-pattern evolution during desorption, i.e., how {d, R, η}, and thus ρ, vary as a function of the water content, which is the essential variable in our experiment (Figure 8b).
determine the exact evolution of the water pattern (black symbols in Figure 8b).[31] At RT, the obtained water pattern consisted of necks of η = 12 nm and R = 64 nm, and a wetting film of d = 4.5 nm. This means that the physisorbed water significantly accumulated between nearest neighbor spheres (ρ = 0.6). The spheres being separated by such an appreciable η form a non-close fcc packing with a filling fraction of 0.66, far below the f = 0.74 of a close-packed fcc arrangement. Our value is in accordance with the low estimates in other silica opals (obtained by different means): f = 0.60 in ref. [39] and 0.61 in ref. [66], although no satisfactory explanation for the non-close packing has been given (these values, lower than ours, may be affected by some opal disorder). Regarding the wetting film, the large value of d is consistent with the vast multilayer adsorption observed in the water isotherm (Figure 2). However, such a thickness clearly exceeds the literature values: although the possibility of stable multilayer water films has been sufficiently assessed at similar RHs on silica and other hydrophilic surfaces,[46,67,68] their thickness was typically ≤1 nm (at most, ca. 2 nm[69]). Nonetheless, such a comparison must be taken with caution since these studies were performed on flat, open surfaces (see discussion in Section 5.1). Upon water desorption, the progress of {d, η, R} and ρ showed two marked regimes (Figure 8b). In the first stage, both d and η quickly decreased until vanishing, almost simultaneously, at T ≈ 60–70 °C, i.e., with a water content of ca. 2–3 wt%. In contrast, R remained constant; that is to say, the toroidal water necks became significantly thinner, but not smaller (in width). In the second stage (at T 70 °C), once the wetting film has disappeared and the spheres have reached contact (closepacked fcc), R markedly diminished. The evolution of the ratio ρ is revealing: it steadily increased in the heating experiment (up to 1, as the wetting film rapidly evaporated), indicating that water, above 60−70 °C, resided exclusively in the necks between spheres (building an annulus around each contact point). This means that water in the gaps between the spheres desorbed less easily than in the more “open” spaces. This is in agreement with the dehydration behavior generally observed in porous systems, in which water in narrow pores, where the Laplace pressure is lower, requires higher temperatures to be removed.[1,2] In addition, the interrelated evolution of d and R suggests water transport from the wetting film into the neck (see Section 5.3). The described evolution of the water pattern reproduces the experimental data with remarkable accuracy (Figure 7), in which the “non-closeness” of the CC, given by η, plays a special role. In the presence of adsorbed water, the behavior inferred from the model implies that the spheres are separated in order to allow significant lattice reduction upon dewetting (needed to account for such PBG changes). η linearly decreases with the water content and vanishes at low (but finite) amounts, so the spheres separation appears as a natural consequence of the (profuse) presence of water, which, in addition, tends to accumulate between the spheres (ρ > 0). These features will be discussed in more detail in Sections 5.1 and 5.4. Finally, note that a possible overestimate of the amount of water outside the spheres due to water inside the micropores would not affect any of our qualitative results. In any case, such an overestimate is expected to be small, as discussed above.
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3.1.3. Hydrophobic CCs In order to study how the surface hydrophilicity affects the water distribution in the CC, the experiment was extended to progressively hydrophobic opals.[31] To that effect, we employed opals annealed at values of Ta between 400 and 600 °C, a range in which the change in the surface hydrophilicity is more pronounced (Figure 3b). An identical procedure followed with each annealed sample: the experimental data and the deduced evolution of the water pattern {d, η, R} are shown in Figure 7 and 8b (colored symbols and lines). As a result of the increasing hydrophobicity, the data begin at lower water contents: the lower the content, the higher the Ta. A direct conclusion can be drawn: the PBG data (λBragg and width in Figure 7) for the different opals, plotted against the water content, did not overlap with each other at the same water contents. That is, the different surface chemistry strongly influenced the PBG and, subsequently, the water distribution and its evolution upon desorption, with the exception of η (Figure 8b). In other words, a CC with two different surfaces cannot present the same water pattern, even if same amounts of water are achieved by controlled dewetting: the water distribution is unique and determined by the surface hydrophilicity. Since decreasing surface hydrophilicity is necessarily accompanied by a decreasing presence of physisorbed water, we separately consider both effects as follows. First, we consider the data at room temperature (RT) (the first points of each data set in Figure 8b), that is, when CCs are fully hydrated. The values of {d, R, η} decreased with increasing Ta in well-defined trends toward 0 (when the ability to adsorb water vanishes). The corresponding water patterns are graphically shown in Figure 9, left column. It is observed that the decreasing hydrophilicity of the surfaces predominantly affected the water bridge between the spheres (with a 20-fold volume loss) rather than the wetting film (3-fold volume decrease). This fact is conveniently captured by the progress of ρ, which reduced from 0.60 to 0.15 after dehydroxylation: water distributes more uniformly in a hydrophobic opal. In particular, the wetting film thickness d diminished (from 4.5 to 1.5 nm) on increasingly hydrophobic surfaces, a trend which is consistent with the literature.[46,70] Moreover, spheres progressively approached each other (η → 0), so annealed CCs tend to adopt a close-packed arrangement. This result agrees with the high filling fractions (f ≈ 0.70−0.74) found in hydrophobic, both polymer[19,71,72] or annealed silica,[71] opals, near the ideal closepacked fcc value. The compactness of the hydrophobic CCs is consistent with our previous conclusion that the separation of the spheres is caused by the presence of abundant water in the ensemble. Note that the PBG narrowing upon water desorption diminishes in the less-hydrophilic CCs, and the width even increased in the highly dehydroxylated 560 °C-annealed samples (Figure 7b): this strongly supports our deduction that the bandgap narrowing, when it occurs, is due to the lattice contraction upon dewetting. Secondly, we consider how water is desorbed in each opal. Figure 8b shows that the parameters of all the CCs evolve nearly in parallel to one another. This indicates that, no matter the silica surface treatment and the initial water content or distribution (at RT), desorption was essentially identical in all opals: d and η rapidly decreased toward 0 while R only 10
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Figure 9. Off-cut side view of two adjacent spheres and the corresponding water patterns (in black) according to the parameters in Figure 8b (all proportions are maintained). From top to bottom, the water patterns of progressively dehydroxylated CCs are compared. The left column shows patterns at RT (full hydration in all cases – solid lines in Figure 8b). The right column shows distributions at the same water content (1.5 wt% – dashed lines in Figure 8b). The corresponding water content/temperature and ρ are given for each sketch. The water necks are drawn with flat edges (instead of menisci) according to the simplifications of the model. Adapted with permission.[31] Copyright 2012, American Chemical Society.
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reduced in the last stage; the ratio ρ steadily increased, which means that water preferentially persisted in the necks. It is interesting to compare the CCs at the same amounts of water, e.g., 1.5 wt% water, indicated with the dashed line in Figure 8b (the water patterns are graphically shown in Figure 9, right column). Although having the same water content in all cases, the water bridges were drastically smaller in the hydrophobic CCs while, contrarily, wetting films were thicker. This behavior is most evident in ρ: for this water content, in the hydrophilic (as-grown) opal, water accumulated in big necks (ρ ≈ 1) while, in the hydrophobic (annealed) ones, water was distributed uniformly around the spheres, showing only small bridges (ρ → 0). Such different patterns of distribution must eventually rely on a balance between the potential energies of both the wetting film and the meniscus, which could be rational- Figure 10. Fluorescence microscopy images of liquid distribution on sub-millimeter glass ized in terms of the water potential for each spheres of 240 µm (a,b) and 375 µm (c). At small amounts of liquid, this distributes uniformly over the grain surface (a). When the amount is large enough, menisci are formed at the concondition, as discussed in Section 5.1. Note tact points between particles, where most of the liquid accumulates (b,c). In these cases, that Stöber spheres have smooth surfaces, the surface roughness is significant and plays an important role. Another example of menisci consistent with the polymerization proce- visualization, by X-ray tomography, is shown in Figure 6b. Images a,b) Adapted with permisdure.[73] Thus, the roughness should not sig- sion.[57] Copyright 1999, American Physical Society. c) Adapted with permission.[74] Copyright nificantly affect the water patterns. A drasti- 2004, Elsevier. cally distinct distribution behavior is found As the distribution of water between solid particles deterwhen the roughness is relevant, and a minimum amount of mines many of the physical properties of colloidal and granliquid is necessary to form bridges (see Figure 10). In any case, ular systems, direct visualization of its morphology in such rugosity cannot be responsible for the differences we observe environments has been pursued for decades. However, this when comparing between the same volumes of water (Figure 9, has proved very difficult, especially in 3D cases and at everright column). smaller scales.[6] Morphological features of bridges have been observed in, at best, the sub-millimeter range and when using wetting liquids different from water or with immersion fluids 3.2. Imaging of the Water Morphology at the Nanoscale replacing the air to achieve enough contrast[57,74,75] (Figure 10), 3.2.1. One-Pulse CVD and with demanding techniques like X-ray microtomography[53] (Figure 6b). The topology of water films has been (laterally) resolved at the micrometer scale by force-microscopy-based As can be seen, the PBG method may provide an important techniques.[76,77] At the nanoscale, a direct and available means, research tool for both colloidal science and photonics, as it predicts how water distributes through the solid particles of the such as conventional SEM, requires vacuum that removes the CC. As a particular merit, from simple optical spectra one is water. Environmental SEM (ESEM), typically with lower resoluable to infer the fraction of water building a wetting film over tion, works at both low temperature and pressure,[78] also prethe spheres and the one forming capillary bridges between cluding the study of water morphology at ambient conditions. them. From experimental data, the dimensions of both types Additionally, the electron gun can charge the water surface and of “morphologies” are deduced, as well as their dependence on lead to an outward pressure,[79] modifying the original condi[ 31,60 ] [ 31 ] conditions like temperature tions. Scanning tunneling microscopy (STM), on the other or the surface hydrophilicity. hand, needs of a conductive specimen. Obviously, the method is predictive and refers to the average We have demonstrated a novel, straightforward approach to behavior in the whole system. Nevertheless, some assumptions visualize the water distributed within the sub-micrometer pore were made and their validity may be disputed. Among them, network of the CC.[80] The idea relied on “fixing” the water patthe real topology of the wetting film (whether homogeneous or formed by patches/droplets), the exact shape of the capillary tern under ambient conditions to make it visible to SEM. This bridges, the uniformity of their distribution, or the high volume was achieved by transforming the water into silica by modiof water outside the spheres, must be verified. In this regard, it fied chemical vapor deposition (CVD) prior to SEM imaging. is of extraordinary interest to have a way to visualize the water Contrary to conventional CVD, which is typically used in morphology in the CC in order to: i) assess the predictions opals by alternating pulses of silicon tetrachloride (SiCl4) and made by the PBG method, and ii) gain insight into specific feawater vapor to successively grow silica and wet the surface,[81] tures that are indiscernible from the optical data. we used only a single SiCl4 cycle and omitted any water cycle.
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Figure 11. a) SEM image of internal cleavage of a CC with 335 nm silica spheres, after exposure to one-pulse CVD. b) Azimuthal view of the opal spheres. Silica necks connected to detached spheres (those belonging to the half-cleaved sample) are visible. The missing necks are due to imperfections in the arrangement. c) Ideal representation of the water distribution (internal (111) plane) according to the parameters obtained from the PBG method. d) Detail of an opal with 905 nm silica spheres, after exposure to one-pulse CVD, whereby both the neck and the wetting film can be appreciated on each sphere. c) Adapted with permission.[80] Copyright 2013, American Chemical Society.
Subjecting the CC to this one-pulse CVD, SiCl4 reacted strictly where the pre-existing (native) water in the opal was present, resulting in a faithful silica replica of the original water pattern at the ambient conditions used (except for a volume reduction of ca. 25%, as a result of the stoichiometric reaction 2H2O + SiCl4 → SiO2 + 4HCl). We used this method on the same CCs employed in the optical studies described above to facilitate cross-checking of the previous results. Moreover, their photonic properties allowed easy monitoring of the modifications made by the one-pulse CVD since the transformation of water into silica redshifts the optical spectrum according to the amount of native water contained in the opal.
3.2.2. Visualizing the Water Morphology SEM images of an as-grown CC subjected to one-pulse CVD at normal laboratory conditions showed a well-defined silica structure on the spheres, corresponding to the transformed native water pattern (Figure 11a). This pattern indeed resembles the model adopted for the PBG method (Figure 11b,c), which confirms the predicted accumulation of water in the necks. Consistent with the fcc arrangement, the number of necks approached 12 per sphere (i.e., 3 per quadrant are visible in the images). Missing necks denote failed contacts to neighbors due to imperfect assembly. Coexisting with the capillary bridges, the modeled wetting film covering the spheres is also confirmed under normal conditions. This is clearly discernible on 905-nm silica spheres as a homogeneous, nanometer-thick conformal
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film of silica (Figure 11d). The resulting silica transformation in the CC was also monitored by subsequent PBG redshifts, which were well reproduced by MPB calculations (assuming reaction with only the adsorbed water). This and further tests supported that the native water (without contribution of any extraneous humidity source) completely reacted with SiCl4 and that no significant distortion of the water pattern occurred during the one-pulse CVD.[80] Direct inspection of the SEM images provided a wealth of quantitative information about the water morphology to the nanometer scale, in particular, regarding the azimuthal radius of the water necks, which is the most-visible feature. The dimensions of the silica replicas were volume-corrected to obtain those of the original water morphologies (isotropic volume reduction was assumed). Statistical scrutiny yielded a mean radius R = 67 ± 5 nm, which is in excellent agreement with the prediction of R ≈ 64 nm from the PBG method. A similar neck/sphere size ratio was found in as-grown opals of 905 nm spheres, where R = 187 ± 21 nm. Such proportionality with the diameter of the spheres is expected for bridges obeying the circular approximation.[2] The wetting film thickness can be quantitatively, albeit roughly, estimated on 905 nm spheres to be d ≈ 15 nm. Such an observation serves as quasi-direct verification of nanometer-thick adsorbed water films, as inferred from the optical measurements, although at odds with theoretical expectations (Section 5.1). More-detailed morphological analysis is made in Section 5.2. The influence of the ambient environment or the surface on the water morphology is readily proven by performing one-pulse CVD using different opals and conditions. As expected, smaller neck sizes were measured by increasing T (R = 54 ± 6 nm at at 55 °C, Figure 12a) or in annealed CCs (R = 45 ± 5 nm in a 560 °C-annealed opal, Figure 12b). This demonstrates a relatively low sensitivity of R with the amount of water over a broad range, since it decreased much less than the adsorbed water did (respectively in these examples, the water contents were two and four times less than in as-grown opals at normal conditions – see Figure 7). Only very hydrophobic spheres (the PS opal in Figure 12c), with a very low water content (
PROGRESS REPORT
Exploration and Exploitation of Water in Colloidal Crystals Francisco Gallego-Gómez,* Alvaro Blanco, and Cefe López performance is essential, but challenging, all the more so the smaller the system scales. Water (or another liquid) sorption and capillary phenomena at the nanoscale are certainly fundamental in fields related to nanomaterials science, like wetting and spreading,[8] micro-/nanofluidics,[9] nanotribology (friction and lubrication, wear),[10] or superhydrophobicity.[11] Many of these subjects are of enormous impact in different industries, such as cements, food, pharmaceutics, soil, oil recovery, mining, civil engineering, catalysis, rubber and tires, fuel cells, wear, lubricants, paper, paints, coatings, membranes, etc. Also, many meteorological and biological processes are governed by such topics. However, in spite of the impressive progress in recent decades, most of these topics are still in need of a much deeper understanding. An advanced example is that of colloidal crystals (CCs), which are also commonly named artificial opals, with important applications in not only, for example, photonics,[12,13] energy conversion,[14] or sensing,[15] but also as models for atomic systems[16] or as templates.[17] CCs refer to solid 3D ordered arrangements formed by the self-assembly of sub-micrometer, monodisperse spheres from colloidal suspensions upon solvent evaporation.[18] Many of the CC properties depend on the arrangement and surroundings of the colloidal particles, which may be crucially affected by their surface properties. In particular, more- or less-hydrophilic spheres adsorb water from moisture in the ambient environment, which greatly enriches the physics of the system. The way water interacts with particle surfaces determines, in a non-trivial manner, its morphology and distribution in such a nanoconfinement (sometimes leading to intriguing structures), and critically influences the collective behavior of the ensemble. Some of these features are gathered in Figure 1. However, less attention has been paid to this matter. The presence of water strongly complicates the CC picture, and it could be considered an inconvenient problem to deal with. However, far from that, recent work conducted in our laboratory has proven it to be a great opportunity to study water-related phenomena at the nanoscale with innovative approaches. Probably the most-prominent feature of CCs is their photonic properties, which, as will be stated, are drastically affected not only by the amount of water taken up, but also by its distribution. We will show how such a system lends itself to suitable probes with optical properties that are sensitive to the exact
Water on solid surfaces is ubiquitously found in nature, in most cases due to mere adsorption from ambient moisture. Because porous structures have large surfaces, water may significantly affect their characteristics. This is particularly obvious in systems formed by separate particles, whose interactions are strongly influenced by small amounts of liquid. Water/solid phenomena, like adsorption, condensation, capillary forces, or interparticle cohesion, have typically been studied at relatively large scales down to the microscale, like in wet granular media. However, much less is known about how water is confined and acts at the nanoscale, for example, in the interstices of divided systems, something of utmost importance in many areas of materials science nowadays. With novel approaches, in-depth investigations as to where and how water is placed in the nanometer-sized pores of self-assembled colloidal crystals have been made, which are employed as a well-defined, versatile model system with useful optical properties. In this Progress Report, knowledge gained in the last few years about water distribution in such nanoconfinements is gathered, along with how it can be controlled and the consequences it brings about to extract new or enhance existing material functionalities. New methods developed and new capabilities of standard techniques are described, and the water interplay with the optical, chemical, and mechanical properties of the ensemble are discussed. Some lines for applicability are also highlighted and aspects to be addressed in the near future are critically summarized.
1. Introduction Liquids interacting with solid surfaces in a gaseous environment are present far and wide, not only in nature, but also in man-made items, including many advanced materials;[1,2] from sandcastles and pottery to modern ceramics and micro-/nanoelectromechanical devices, interactions at interfaces are crucial for the processing and functionality of many materials. Divided media like colloidal aggregates,[3] powders,[4] granular materials,[5,6] and soils,[7] as highly porous systems formed by individual solid particles, are particularly affected by the interplay of liquid present at the large solid surface area, which vastly determines the interparticle forces. In most cases, gas and liquid phases, respectively, consist of air and water adsorbed from ambient moisture. A deep knowledge of how water (and, subsequently, air) is confined in such systems and how it affects their Dr. F. Gallego-Gómez, Dr. A. Blanco, Prof. C. López Instituto de Ciencia de Materiales de Madrid c/ Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain E-mail: [email protected]
DOI: 10.1002/adma.201405008
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distribution of the co-existing phases. This will provide a new, powerful tool to infer the water morphology at the nanoscale, even allowing monitoring in situ under varying conditions. On the other hand, a CC containing water between the spheres actually constitutes a “wet” divided system, only greatly simplified since it possesses, as an ordered structure, a definite configuration with a known arrangement of spheres and specificsurface area demarcated by the opal interstices. This avoids the extraordinarily complex problem of identifying how water distributes between random solid particles, a fact that strongly limits research on powders and granular media.[4–6] A wet CC offers a handy approach to investigate the water-dependent micromechanical behaviors of particles ensembles. Imaging at the nanoscale will complete a set of novel methods for a multidisciplinary study of nanoconfined water in CCs. Here, we critically review recent advances made in our laboratory toward not only exploring the behavior of water within CCs, but also exploiting its presence in such systems to gain new functionalities. The use of CCs has allowed us to benefit from their simple and inexpensive preparation and versatility (with even a controllable degree of disorder[19]), among the large theoretical and experimental background collected in recent decades. Undoubtedly, their photonic properties, being responsive to the presence of water, represent the crucial advantage over any other wet porous material, in which water merely changes the mechanical or chemical properties. Silica CCs, in particular, constitute a special subject in the study of water-containing opals due to the silica’s high hydrophilicity and the possibility of modifying its surface chemistry and tune the water adsorption. Thus, they constitute an appropriate test system with well-defined topology, large porosity, adjustable surface chemistry, and modifiable water content, and provide an excellent playground to study the morphology and geometric location of water in sub-micrometer environments. This Progress Report is organized as follows. Section 2 briefly introduces some fundamentals of the water–CC interplay, on which the procedures employed later are based: firstly, the nature of water contained in a silica opal and, secondly, the wet CC as either a photonic crystal or granular-like material. Section 3 describes new methods developed and the main results achieved: i) monitoring of the CC photonic properties and modeling of the water distribution; ii) scanning electron microscopy (SEM) to visualize the water structures, previously “stabilized”, within the sub-micrometer interstices; and iii) nanoindentation to examine the CC mechanical behavior. Section 4 shows some examples of how the knowledge acquired can be directly used to obtain new powerful functionalities suitable for real applications. Finally, in Section 5, we critically analyze results that need further investigation (including discrepancies with the theoretical framework to be solved) and give a perspective of the next tasks and future research to be faced.
2. Colloidal Crystals and Water In this Section, we first describe how water adsorbs on the surface of the spheres of a CC, and discuss whether capillary condensation between the opal spheres occurs or not. Since silica CCs were chosen as the main investigation subject, we focus
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Francisco Gallego-Gómez graduated in Physics from the University of Seville and received his Ph.D. (2005) in Physical Chemistry in Germany. He has carried out work at the universities of Munich (LMU), Cologne, Alicante and Madrid (Autónoma), and now is a postdoctoral fellow at the Spanish Scientific Research Council (CSIC). His expertise covers different aspects of material science, from organic and hybrid composites for holography and photoconductivity, to self-assembled structures for photonic applications. Currently, his research interest focuses on solid–liquid physics at the nanoscale in divided media like colloids, and on the extrapolation to macroscopic phenomena in granular media. Alvaro Blanco graduated in Physics from the Universidad Autónoma de Madrid in 1994, obtained his Ph.D. in Physics in 2001, investigating the fabrication and characterization of 3D photonic crystals, at the Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC). After a period of two years in the Institut für Nanotechnologie in Forschungszentrum Karlsruhe working on holographic photonic crystals as a postdoctoral fellow, he moved to ICMM-CSIC as a Ramon&Cajal researcher where he has been a Permanent Scientist since 2008. His current research lies on the preparation and characterization of self-assembled 3D heterostructures with applications in photonics. Prof. Cefe López’s alma mater is the Universidad Autónoma de Madrid where he graduated in Physics and received his Ph.D. (1989) in optical properties of semiconductors. After a postdoctoral period at the University of Oxford and a teaching stint at the Universidad Carlos III de Madrid he gained tenure in the Spanish Scientific Research Council (CSIC) in 1993. His main research interest is focused on self-assembled photonic structures and related systems. In particular, his multidisciplinary group covers synthesis and processing of materials – such as photonic glasses, composites, nanoparticles, and quantum dots – as well as the study of light transport, generation, and interference in ordered and disordered dielectric structures.
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on silica surfaces and how annealing changes their hydrophilic properties. Secondly, we briefly discuss how water generally affects the properties of a CC, considering its two main aspects: photonic crystal and granular properties. Both kinds of properties not only depend on the water content but also on its exact distribution between the interstices of the spheres.
2.1. Water in Silica CCs: Chemistry of the Silica Surface The rich chemistry of silica greatly expands the physics of large-surface-area systems, such as colloids, including CCs.[20] The behavior of water at the silica surface, often anomalous, has attracted a great interest for many years.[21] The surface of silica has a large number of hydroxyl (−OH) groups (surface silanols), bound to the Si atoms of incomplete siloxane
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Figure 1. SEM images of CCs illustrating different aspects of the water− solid interaction at the interfaces. a) The collective mechanical behavior of silica spheres upon indentation recalls common footprints in wet sand – see Section 3.4. b) The interparticle cohesion greatly depends on the specific arrangement. A defect in the CC (due to, for example, an air bubble) led to detachment of many spheres from the layers of a silica CC with high disorder, indicating less cohesion. As the order in the arrangement increased (in this case, in deeper spheres layers), less spheres were missing; the higher cohesion of the well-ordered layers finally avoided any loss of spheres – see Section 5.2. c) Complex trails of adsorbed water are formed on the surface of the polystyrene spheres – see Section 5.3. d) Detailed shape of water menisci formed between silica spheres and a silica sphere-substrate – see Section 5.2. The water structures in (c) and (d) have previously been transformed to allow imaging (Section 3.2). d) Adapted with permission.[80] Copyright 2013, American Chemical Society.
bridges (≡Si−O−Si≡), which have great affinity for water through hydrogen bonds, and endow silica with high hydrophilicity.[22–24] In a humid atmosphere, water is readily adsorbed by way of the surface silanols building a monolayer of water molecules, potentially followed by multilayer adsorption on the already-adsorbed water molecules, until a hydration layer (or wetting film) is formed on the silica surface. This type of water, whose content depends on both the surface silanol density (αOH) and the ambient conditions (temperature, T, and relative humidity, RH), is usually referred to as physically adsorbed, or physisorbed, water. This water is loosely bound to the surface, so it easily begins to desorb (dehydration or dewetting) by, for example, lowering the ambient pressure or raising the temperature, although complete elimination is only achieved in vacuum and/or at about 120 °C.[22] Physisorbed water is readily measurable from weight changes by water-adsorption isotherms or thermogravimetric analysis (TGA), by infrared (IR) spectroscopy,[25,26] solid-state nuclear magnetic resonance (NMR),[27] etc. CCs made of silica spheres are thus water-rich mesoporous structures (the voids between the spheres are in the range of tens of nanometers in size). This is an intrinsic characteristic given by the adsorption on the silica surface and not by aqueous residues from CC fabrication[28] or in suspension (free water[22]); these potential remnants are safely removed before any measurement by heating at ca. 120 °C. In particular, we deal with sub-micrometer-sized spheres of amorphous silica prepared by Stöber synthesis,[29] which are customary used to fabricate inexpensive, high-quality face-centered-cubic (fcc) structures.[30] We primarily use CCs made of Stöber spheres with a diameter (D) of 335 nm, synthesized according to ref. [31]. Bigger spheres (D = 905 nm), fabricated by a regrowth process,[32] are also employed. The synthesis parameters mainly determine the size of the spheres, but they may also affect some properties of the resulting silica, like the microporosity or surface roughness.[33] Silica aging, in which continued polycondensation and shrinkage of the pore network are involved, may be a factor to be considered as well.[20,21,23] As-grown (that is, untreated) silica CCs indeed exhibit the outstanding ability to physisorb water from the environment, as measured by adsorption isotherms (Figure 2).[34] The initial rise corresponds to the adsorption of a monolayer of water molecules and the following steady increase is a signature of the formation of multilayers.[35] The convex shape of the isotherm at RH < 10% (type II) indicates the existence of narrow micropores ( √2D). This latter aspect, which would occur if water were allocated between the spheres, is especially relevant,
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as the photonic properties are very sensitive to lattice changes. In addition, while Equation 1 is oversimplified and the spatial distribution of the different media is averaged, the exact solutions for the Bragg peak do depend on the specific location of each material. In practical terms, it is necessary to know the extent of such changes in hydrophilic CCs subjected to a varying environment (e.g., inside a heating-up optoelectronic device or in vacuum). This, in turn, offers a straightforward means of tuning the photonic properties of the opals by manipulating the amount of water, as seen in the previous section. Much more ambitiously, at a fundamental level, it is possible to access the structural information of the water within the CC from its optical response. In the last instance, this arises from the regularity of the crystalline structure, so any alteration in the geometry or composition induced by the adsorbed water is manifested straightaway in the PBG properties. By using a full description of the Bragg peak properties, which requires a rigorous solution of Maxwell’s equations, we were able to infer a great deal of information about the way water is adsorbed and confined in the nanometer interstices of the opal (Section 3.1). In particular, we employed the MPB package,[49] which allows the calculation of the photonic bands in heterogeneous periodic structures (in our case, formed by the spheres plus the air/water in the voids), even having a non-uniform distribution (air and water). These exact solutions are developed for infinite and perfectly ordered systems. Nevertheless, size effects in real samples with more than 10–15 spheres layers are negligible,[50,51] while disorder effects, although leading to a less-intense peak, do not affect the width.[51] On the other hand, stacking faults along the (111) direction do not affect the Bragg peak properties.[50,51] In any case, we have used high-quality silica CCs made by vertical deposition[30] with 20–25 layers. Bragg peak intensities were typically over 80% and no deformation in the (111) planes was assessed by diffraction.
2.3. CCs as Wet Granular Materials One can deal with the presence of water within the opal using a different point of view, that is, by considering an assembly of spheres in a wet environment. It is well known from research on granular media that even small amounts of liquid between grains can strongly change the physical properties of the system.[52,53] Independently of the origin of this liquid, it forms, in partially saturated systems, a meniscus (or neck, or capillary bridge) around every contact point or narrow gap between two grains (pendular regime – Figure 6a), which imparts an attractive force between them. This capillary force, Fcap, which is typically stronger than the van der Waals attraction, plays a fundamental role in the mechanical properties of any liquidcontaining granular assembly (a wet granular material) via both capillary cohesion and increased interparticle friction. Fcap is given by the exact shape and size of the meniscus (eventually, by its external surface), which depends on the amount of liquid, the geometry of the interstice, and the chemistry of the surfaces (see Section 5.2). The external surface of the neck approximates a toroid (circular approximation) with two radii of curvature, R (the half-width of the neck) and r (the meniscus radius), which
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are geometrically related to the other characteristic parameters: the separation of the surfaces, η, the filling angle, β, and the contact angle, θ (see Figure 6a).[2] These relationships must be conveniently modified if a wetting film is present on the surfaces.[54] From these considerations, capillary forces essentially depend on how much liquid is building the menisci between the particles and how much is just wetting their surfaces. In spite of the paramount importance of the liquid in granular systems, wet conditions have only been studied since relatively recently,[55–57] and there is still much to understand. In ordinary granular materials, determining the way water is placed between the randomly distributed particles constitutes an extraordinarily complex problem. At best, partial information is obtained by demanding techniques and is usually restricted to scales above the micrometer level (see an example in Figure 6b). In a CC that takes up a non-negligible amount of moisture, like a silica one in the usual ambient conditions, water forms menisci between adjacent colloidal spheres and wets their surfaces. Thus, a silica CC represents a strong analogy to wet granular media (or, considering the small scale of the particles, wet powders). However, unlike them, a CC has a well-defined morphology, namely, the size and form of both grains (monodisperse spheres) and pores (opal voids), and their localization (fcc packing), which can even enable the distribution of water to be accurately known (Figure 4b). Moreover, features like the sphere size, the order degree of the ensemble, and the surface properties are easily adjustable. Thus, the study of the formation of capillary bridges and water-dependent mechanical properties of such a simplified, versatile system provides novel insight into topics of granular and powder science. In addition, given the sub-micrometer dimensions of the colloidal spheres, fundamental phenomena like menisci morphology and capillary forces can be directly investigated at the nanoscale, where aspects of classical theories may fail.
3. Exploring the Water in Colloidal Crystals As shown, water is an important part of silica CCs that significantly affects their photonic and micromechanical properties. In this Section, we show how to take advantage of this fact to study the fundamental features of water in the pore network.
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Figure 6. a) Characteristic parameters of a liquid meniscus formed between two equal spheres. b) Irregular water bridges formed in a granular material of polydisperse sub-millimeter glass spheres, obtained by X-ray tomography. b) Adapted with permission.[53] Copyright 2008, Nature Publishing Group.
Firstly, a central point in our work is reviewed: the influence on the PBG of dehydration of opals with different surface properties is monitored in situ by mere UV–vis spectroscopy, and an analytical method is developed to connect the bandgap properties with the actual water distribution. As a result, a simple optical tool is turned into a powerful method to study nanoconfined water, such as between the CC spheres. The potential of this technique, to which we will refer in this report as the PBG method, relies on the sensitivity of the photonic performance, as a collective feature, to specific changes in the structure. In this sense, this new method bears similarities to established techniques, such as small-angle X-ray/neutron scattering (SAXS/SANS)[58] or NMR spectroscopy, in which information about liquid inside a porous environment is obtained from the collective properties by applying a set of assumptions. Secondly, the nanometer-sized water morphology is (quasi-directly) visualized. This not only allows comparison with the PBG method, but also the appraisal of particular features, to which CC properties are rather insensitive. Thirdly, the comprehensive micromechanical characterization of the CC, tackled for the first time, gives relevant insights into the interparticle forces and the decisive role of the structural and dynamic behavior of water.
3.1. Water-Dependent PBG: Simple Optics to Reveal the Nanoconfined Water Distribution 3.1.1. Model for the Water Pattern and Desorption Experiment As discussed above, the presence of physisorbed water in the silica CC affects both the average refractive index of the opal voids and the packing of the spheres. Since the PBG characteristics critically depend on these factors, which, in any case, can be easily simulated, simply measuring the optical spectrum provides reliable information about both the CC structure and the water distribution, just by assuming a few simple, realistic hypotheses. A simple experimental procedure,[31,60] in which the CC is gently heated to induce desorption, allowed the water content in the CC to be modified in a controlled fashion, while the reflectance spectrum was measured in situ. This informed us about the progress of the CC (PBG position and width) as a function of the water content (Figure 7, for as-grown and annealed silica opals). Temperature rise (in steps of 5–10 °C) primarily induced spectral changes within a few seconds, which were fully reversible by cooling as long as T < 150 °C.[60] This is the approximate temperature at which physisorbed water is completely desorbed and dehydroxylation begins, and therefore it was taken as the upper limit in desorption experiments. Now, the goal is to obtain the distribution of the water and its evolution from the behavior of the CC properties. We consider a simple model for water distribution[31,60] (Figure 8a), which distinguishes between: i) water placed in the voids, as a wetting film around the spheres surface, and ii) water between nearest-neighbor spheres forming a pendular neck. Some assumptions must be made. Firstly, the wetting film is ideally homogeneous with uniform thickness d; in the case of inhomogeneities due to, for example, the presence of water clusters or patches, d corresponds to the average thickness. Secondly, according to the circular approximation, the
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Figure 7. Evolution of the PBG parameters, λBragg (a) and width (b), during water desorption in as-grown (black) and annealed (colors) silica CCs. The bottom axis represents the decreasing content of physisorbed water, as this is the change direction in the heating experiment. The symbols represent the experimental data; the lines are the fitting curves of the model. The open symbols are values at RT. Adapted with permission.[31] Copyright 2012, American Chemical Society.
neck is modeled as a toroid with azimuthal radius, R, but, for simplicity, with no meniscus curvature r (flat edges in Figure 8a) as it barely affects the neck volume. r can be readily calculated from the other parameters. Thirdly, the model assumes η > 0, so some separation is allowed to exist between the spheres; that is, a non-close-packed fcc structure is allowed for the CC. This is an important assumption in our model, although it might be controversial (see discussion in Section 5.3). Fourthly, the minor amount of water filling the Stöber micropores is considered to be immobile in our experiments (with time scales of seconds) given the much slower water diffusion in very narrow spaces,[61,62] especially those having hydrophilic surfaces.[63] Explicitly, water diffusion in Stöber silica CCs slowed down to two orders of magnitude in the micropores of the spheres;[64] and water uptake times of hours have been measured for micropores of annealed Stöber spheres.[38] Nevertheless, this internal water needs higher temperatures to be desorbed (above 180 °C[65] than those employed here, as the very negative Laplace pressure in such narrow gaps must be overcome. Finally, the water density in both the wetting film and the neck is taken as that of bulk liquid water, although, rigorously, this is incorrect for the first layers adjacent to the surfaces.[61] With these assumptions, any triad of parameters {d, R, η} that is compatible with a certain amount of water defines a possible distribution (pattern) for this water in the CC. The
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Figure 8. a) Left: Model for the water distribution between the CC spheres adopted for the PBG method, which distinguishes between a wetting film of thickness d around the spheres and capillary necks between them, described by R and η. Right: Limiting cases for the ratio ρ: water is entirely distributed as wetting film (ρ = 0, top) or as necks (ρ = 1, bottom). b) Evolution of the model parameters depicted in (a) upon water desorption in as-grown (black symbols) and annealed silica CCs (colored symbols) as obtained from fitting of experimental data (Figure 7). The open symbols are the values at RT, graphically shown in Figure 9, left column (the solid lines are a guide to the eye). The gray shaded area corresponds to the second desorption regime of the as-grown CC (see text). The dashed line marks the water patterns graphically represented in Figure 9, right column, at the same water contents (1.5 wt%). Adapted with permission.[31] Copyright 2012, American Chemical Society.
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3.1.2. Hydrophilic CCs As shown in Figure 7, hydrophilic silica CCs (as-grown opals – black symbols) exhibited a blueshift as large as 23 nm upon desorption (by heating from room temperature (RT) to ca. 150 °C). Indeed, the voids emptying due to desorption (nv diminishes in Equation 1) was expected to lead to a decrease of λBragg decrease, but it cannot account for the experimental value. By considering a water content of 7.5 wt% at RT (Figure 3a), MPB simulation gives a maximum blueshift of 14 nm after complete dewetting. On the other hand, the PBG width narrowed by 9% during dehydration. This finding was particularly puzzling because water elimination implies a higher spheres/ voids refractive-index contrast, which would lead to a broader bandgap. In order to explain this unexpected behavior, we considered the possibility of having a non-close-packed assembly of fcc spheres, that is, spheres with finite separation (η > 0), which additionally varies with the amount of water. For constant η, which also includes the usually assumed close-packed arrangement (η = 0), MPB calculations always predict a broadening of the PBG during desorption (as a result of nv decrease). In contrast, if η is allowed to decrease upon dewetting, the PBG narrowing can be reproduced (in spite of the increased refractive index contrast).[60] That is to say, the experimental behavior of the PBG width denotes the contraction of the opal structure due to water desorption. This, moreover, explains the large blueshift measured, as the structure compaction implies a decrease of d111, which adds a further contribution to blueshift (see Equation 1). Using our model, experimental data were fitted with excellent agreement (black lines in Figure 7) to
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MPB simulation software allows us to calculate the PBG corresponding to each combination. At a given T, the water content is known (from, for example, thermogravimetric analysis (TGA), Figure 3a), as are λBragg and the width of the PBG (from the optical spectrum, e.g., Figure 4a). These constitute three pieces of experimental data that enable the three unknown variables to be found (the triad {d, R, η}) using an iterative process for best agreement.[60] Note that we do not impose the pattern to obey the circular approximation (in our case, in the presence of a wetting film[54]). Therefore, an interesting aspect provided by the resulting pattern is how homogeneously the water distributes in the CC. We define a further parameter, the ratio of the volume of water necks to the total water volume (ρ), which is a function of {d, R, η}. ρ illustrates whether the water is uniformly distributed over the spheres (i.e., forming only a wetting film but no necks, ρ → 0) or accumulates between the spheres (exclusively building necks, ρ → 1) – see Figure 8a (right). Thus, the water pattern that fits best the experimental spectrum is inferred at each T (each value of water content). In practice, and as a crucial advantage, fitting is made using the whole data set from the desorption experiment, which minimizes analysis errors, allowing, for example, the adjustment of the nominal value D within its nominal error (about 10%). Simultaneously, this informs us about the water-pattern evolution during desorption, i.e., how {d, R, η}, and thus ρ, vary as a function of the water content, which is the essential variable in our experiment (Figure 8b).
determine the exact evolution of the water pattern (black symbols in Figure 8b).[31] At RT, the obtained water pattern consisted of necks of η = 12 nm and R = 64 nm, and a wetting film of d = 4.5 nm. This means that the physisorbed water significantly accumulated between nearest neighbor spheres (ρ = 0.6). The spheres being separated by such an appreciable η form a non-close fcc packing with a filling fraction of 0.66, far below the f = 0.74 of a close-packed fcc arrangement. Our value is in accordance with the low estimates in other silica opals (obtained by different means): f = 0.60 in ref. [39] and 0.61 in ref. [66], although no satisfactory explanation for the non-close packing has been given (these values, lower than ours, may be affected by some opal disorder). Regarding the wetting film, the large value of d is consistent with the vast multilayer adsorption observed in the water isotherm (Figure 2). However, such a thickness clearly exceeds the literature values: although the possibility of stable multilayer water films has been sufficiently assessed at similar RHs on silica and other hydrophilic surfaces,[46,67,68] their thickness was typically ≤1 nm (at most, ca. 2 nm[69]). Nonetheless, such a comparison must be taken with caution since these studies were performed on flat, open surfaces (see discussion in Section 5.1). Upon water desorption, the progress of {d, η, R} and ρ showed two marked regimes (Figure 8b). In the first stage, both d and η quickly decreased until vanishing, almost simultaneously, at T ≈ 60–70 °C, i.e., with a water content of ca. 2–3 wt%. In contrast, R remained constant; that is to say, the toroidal water necks became significantly thinner, but not smaller (in width). In the second stage (at T 70 °C), once the wetting film has disappeared and the spheres have reached contact (closepacked fcc), R markedly diminished. The evolution of the ratio ρ is revealing: it steadily increased in the heating experiment (up to 1, as the wetting film rapidly evaporated), indicating that water, above 60−70 °C, resided exclusively in the necks between spheres (building an annulus around each contact point). This means that water in the gaps between the spheres desorbed less easily than in the more “open” spaces. This is in agreement with the dehydration behavior generally observed in porous systems, in which water in narrow pores, where the Laplace pressure is lower, requires higher temperatures to be removed.[1,2] In addition, the interrelated evolution of d and R suggests water transport from the wetting film into the neck (see Section 5.3). The described evolution of the water pattern reproduces the experimental data with remarkable accuracy (Figure 7), in which the “non-closeness” of the CC, given by η, plays a special role. In the presence of adsorbed water, the behavior inferred from the model implies that the spheres are separated in order to allow significant lattice reduction upon dewetting (needed to account for such PBG changes). η linearly decreases with the water content and vanishes at low (but finite) amounts, so the spheres separation appears as a natural consequence of the (profuse) presence of water, which, in addition, tends to accumulate between the spheres (ρ > 0). These features will be discussed in more detail in Sections 5.1 and 5.4. Finally, note that a possible overestimate of the amount of water outside the spheres due to water inside the micropores would not affect any of our qualitative results. In any case, such an overestimate is expected to be small, as discussed above.
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3.1.3. Hydrophobic CCs In order to study how the surface hydrophilicity affects the water distribution in the CC, the experiment was extended to progressively hydrophobic opals.[31] To that effect, we employed opals annealed at values of Ta between 400 and 600 °C, a range in which the change in the surface hydrophilicity is more pronounced (Figure 3b). An identical procedure followed with each annealed sample: the experimental data and the deduced evolution of the water pattern {d, η, R} are shown in Figure 7 and 8b (colored symbols and lines). As a result of the increasing hydrophobicity, the data begin at lower water contents: the lower the content, the higher the Ta. A direct conclusion can be drawn: the PBG data (λBragg and width in Figure 7) for the different opals, plotted against the water content, did not overlap with each other at the same water contents. That is, the different surface chemistry strongly influenced the PBG and, subsequently, the water distribution and its evolution upon desorption, with the exception of η (Figure 8b). In other words, a CC with two different surfaces cannot present the same water pattern, even if same amounts of water are achieved by controlled dewetting: the water distribution is unique and determined by the surface hydrophilicity. Since decreasing surface hydrophilicity is necessarily accompanied by a decreasing presence of physisorbed water, we separately consider both effects as follows. First, we consider the data at room temperature (RT) (the first points of each data set in Figure 8b), that is, when CCs are fully hydrated. The values of {d, R, η} decreased with increasing Ta in well-defined trends toward 0 (when the ability to adsorb water vanishes). The corresponding water patterns are graphically shown in Figure 9, left column. It is observed that the decreasing hydrophilicity of the surfaces predominantly affected the water bridge between the spheres (with a 20-fold volume loss) rather than the wetting film (3-fold volume decrease). This fact is conveniently captured by the progress of ρ, which reduced from 0.60 to 0.15 after dehydroxylation: water distributes more uniformly in a hydrophobic opal. In particular, the wetting film thickness d diminished (from 4.5 to 1.5 nm) on increasingly hydrophobic surfaces, a trend which is consistent with the literature.[46,70] Moreover, spheres progressively approached each other (η → 0), so annealed CCs tend to adopt a close-packed arrangement. This result agrees with the high filling fractions (f ≈ 0.70−0.74) found in hydrophobic, both polymer[19,71,72] or annealed silica,[71] opals, near the ideal closepacked fcc value. The compactness of the hydrophobic CCs is consistent with our previous conclusion that the separation of the spheres is caused by the presence of abundant water in the ensemble. Note that the PBG narrowing upon water desorption diminishes in the less-hydrophilic CCs, and the width even increased in the highly dehydroxylated 560 °C-annealed samples (Figure 7b): this strongly supports our deduction that the bandgap narrowing, when it occurs, is due to the lattice contraction upon dewetting. Secondly, we consider how water is desorbed in each opal. Figure 8b shows that the parameters of all the CCs evolve nearly in parallel to one another. This indicates that, no matter the silica surface treatment and the initial water content or distribution (at RT), desorption was essentially identical in all opals: d and η rapidly decreased toward 0 while R only 10
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Figure 9. Off-cut side view of two adjacent spheres and the corresponding water patterns (in black) according to the parameters in Figure 8b (all proportions are maintained). From top to bottom, the water patterns of progressively dehydroxylated CCs are compared. The left column shows patterns at RT (full hydration in all cases – solid lines in Figure 8b). The right column shows distributions at the same water content (1.5 wt% – dashed lines in Figure 8b). The corresponding water content/temperature and ρ are given for each sketch. The water necks are drawn with flat edges (instead of menisci) according to the simplifications of the model. Adapted with permission.[31] Copyright 2012, American Chemical Society.
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reduced in the last stage; the ratio ρ steadily increased, which means that water preferentially persisted in the necks. It is interesting to compare the CCs at the same amounts of water, e.g., 1.5 wt% water, indicated with the dashed line in Figure 8b (the water patterns are graphically shown in Figure 9, right column). Although having the same water content in all cases, the water bridges were drastically smaller in the hydrophobic CCs while, contrarily, wetting films were thicker. This behavior is most evident in ρ: for this water content, in the hydrophilic (as-grown) opal, water accumulated in big necks (ρ ≈ 1) while, in the hydrophobic (annealed) ones, water was distributed uniformly around the spheres, showing only small bridges (ρ → 0). Such different patterns of distribution must eventually rely on a balance between the potential energies of both the wetting film and the meniscus, which could be rational- Figure 10. Fluorescence microscopy images of liquid distribution on sub-millimeter glass ized in terms of the water potential for each spheres of 240 µm (a,b) and 375 µm (c). At small amounts of liquid, this distributes uniformly over the grain surface (a). When the amount is large enough, menisci are formed at the concondition, as discussed in Section 5.1. Note tact points between particles, where most of the liquid accumulates (b,c). In these cases, that Stöber spheres have smooth surfaces, the surface roughness is significant and plays an important role. Another example of menisci consistent with the polymerization proce- visualization, by X-ray tomography, is shown in Figure 6b. Images a,b) Adapted with permisdure.[73] Thus, the roughness should not sig- sion.[57] Copyright 1999, American Physical Society. c) Adapted with permission.[74] Copyright nificantly affect the water patterns. A drasti- 2004, Elsevier. cally distinct distribution behavior is found As the distribution of water between solid particles deterwhen the roughness is relevant, and a minimum amount of mines many of the physical properties of colloidal and granliquid is necessary to form bridges (see Figure 10). In any case, ular systems, direct visualization of its morphology in such rugosity cannot be responsible for the differences we observe environments has been pursued for decades. However, this when comparing between the same volumes of water (Figure 9, has proved very difficult, especially in 3D cases and at everright column). smaller scales.[6] Morphological features of bridges have been observed in, at best, the sub-millimeter range and when using wetting liquids different from water or with immersion fluids 3.2. Imaging of the Water Morphology at the Nanoscale replacing the air to achieve enough contrast[57,74,75] (Figure 10), 3.2.1. One-Pulse CVD and with demanding techniques like X-ray microtomography[53] (Figure 6b). The topology of water films has been (laterally) resolved at the micrometer scale by force-microscopy-based As can be seen, the PBG method may provide an important techniques.[76,77] At the nanoscale, a direct and available means, research tool for both colloidal science and photonics, as it predicts how water distributes through the solid particles of the such as conventional SEM, requires vacuum that removes the CC. As a particular merit, from simple optical spectra one is water. Environmental SEM (ESEM), typically with lower resoluable to infer the fraction of water building a wetting film over tion, works at both low temperature and pressure,[78] also prethe spheres and the one forming capillary bridges between cluding the study of water morphology at ambient conditions. them. From experimental data, the dimensions of both types Additionally, the electron gun can charge the water surface and of “morphologies” are deduced, as well as their dependence on lead to an outward pressure,[79] modifying the original condi[ 31,60 ] [ 31 ] conditions like temperature tions. Scanning tunneling microscopy (STM), on the other or the surface hydrophilicity. hand, needs of a conductive specimen. Obviously, the method is predictive and refers to the average We have demonstrated a novel, straightforward approach to behavior in the whole system. Nevertheless, some assumptions visualize the water distributed within the sub-micrometer pore were made and their validity may be disputed. Among them, network of the CC.[80] The idea relied on “fixing” the water patthe real topology of the wetting film (whether homogeneous or formed by patches/droplets), the exact shape of the capillary tern under ambient conditions to make it visible to SEM. This bridges, the uniformity of their distribution, or the high volume was achieved by transforming the water into silica by modiof water outside the spheres, must be verified. In this regard, it fied chemical vapor deposition (CVD) prior to SEM imaging. is of extraordinary interest to have a way to visualize the water Contrary to conventional CVD, which is typically used in morphology in the CC in order to: i) assess the predictions opals by alternating pulses of silicon tetrachloride (SiCl4) and made by the PBG method, and ii) gain insight into specific feawater vapor to successively grow silica and wet the surface,[81] tures that are indiscernible from the optical data. we used only a single SiCl4 cycle and omitted any water cycle.
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Figure 11. a) SEM image of internal cleavage of a CC with 335 nm silica spheres, after exposure to one-pulse CVD. b) Azimuthal view of the opal spheres. Silica necks connected to detached spheres (those belonging to the half-cleaved sample) are visible. The missing necks are due to imperfections in the arrangement. c) Ideal representation of the water distribution (internal (111) plane) according to the parameters obtained from the PBG method. d) Detail of an opal with 905 nm silica spheres, after exposure to one-pulse CVD, whereby both the neck and the wetting film can be appreciated on each sphere. c) Adapted with permission.[80] Copyright 2013, American Chemical Society.
Subjecting the CC to this one-pulse CVD, SiCl4 reacted strictly where the pre-existing (native) water in the opal was present, resulting in a faithful silica replica of the original water pattern at the ambient conditions used (except for a volume reduction of ca. 25%, as a result of the stoichiometric reaction 2H2O + SiCl4 → SiO2 + 4HCl). We used this method on the same CCs employed in the optical studies described above to facilitate cross-checking of the previous results. Moreover, their photonic properties allowed easy monitoring of the modifications made by the one-pulse CVD since the transformation of water into silica redshifts the optical spectrum according to the amount of native water contained in the opal.
3.2.2. Visualizing the Water Morphology SEM images of an as-grown CC subjected to one-pulse CVD at normal laboratory conditions showed a well-defined silica structure on the spheres, corresponding to the transformed native water pattern (Figure 11a). This pattern indeed resembles the model adopted for the PBG method (Figure 11b,c), which confirms the predicted accumulation of water in the necks. Consistent with the fcc arrangement, the number of necks approached 12 per sphere (i.e., 3 per quadrant are visible in the images). Missing necks denote failed contacts to neighbors due to imperfect assembly. Coexisting with the capillary bridges, the modeled wetting film covering the spheres is also confirmed under normal conditions. This is clearly discernible on 905-nm silica spheres as a homogeneous, nanometer-thick conformal
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film of silica (Figure 11d). The resulting silica transformation in the CC was also monitored by subsequent PBG redshifts, which were well reproduced by MPB calculations (assuming reaction with only the adsorbed water). This and further tests supported that the native water (without contribution of any extraneous humidity source) completely reacted with SiCl4 and that no significant distortion of the water pattern occurred during the one-pulse CVD.[80] Direct inspection of the SEM images provided a wealth of quantitative information about the water morphology to the nanometer scale, in particular, regarding the azimuthal radius of the water necks, which is the most-visible feature. The dimensions of the silica replicas were volume-corrected to obtain those of the original water morphologies (isotropic volume reduction was assumed). Statistical scrutiny yielded a mean radius R = 67 ± 5 nm, which is in excellent agreement with the prediction of R ≈ 64 nm from the PBG method. A similar neck/sphere size ratio was found in as-grown opals of 905 nm spheres, where R = 187 ± 21 nm. Such proportionality with the diameter of the spheres is expected for bridges obeying the circular approximation.[2] The wetting film thickness can be quantitatively, albeit roughly, estimated on 905 nm spheres to be d ≈ 15 nm. Such an observation serves as quasi-direct verification of nanometer-thick adsorbed water films, as inferred from the optical measurements, although at odds with theoretical expectations (Section 5.1). More-detailed morphological analysis is made in Section 5.2. The influence of the ambient environment or the surface on the water morphology is readily proven by performing one-pulse CVD using different opals and conditions. As expected, smaller neck sizes were measured by increasing T (R = 54 ± 6 nm at at 55 °C, Figure 12a) or in annealed CCs (R = 45 ± 5 nm in a 560 °C-annealed opal, Figure 12b). This demonstrates a relatively low sensitivity of R with the amount of water over a broad range, since it decreased much less than the adsorbed water did (respectively in these examples, the water contents were two and four times less than in as-grown opals at normal conditions – see Figure 7). Only very hydrophobic spheres (the PS opal in Figure 12c), with a very low water content (
