Assembly of large area crack free clay porous films

Porous materials with well-defined porosity have advantages in a wide range of applications, including filtration media, catalysis, and electrodes. The bottom-up fabrication of inverse opals have promised to provide those nanostructures, but fabrication of these materials is often plagued with large numbers of defects and macro-scale cracks. Here, we present a method for making nanostructured porous clay films with well defined pore size that are crack free over a large area (multiple cm2).

2][3][4][5][6] For some applications, such as automotive particulate lters, [7][8][9][10] well dened porosity is desired.Bottom up assembly of sub-micrometer colloidal spheres has long promised to lead to a better way to create such materials with new properties for a range of applications.One of the challenges associated with bottom up assembly, however, that has prevented this from becoming a reality, is that these types of self-assembled structures tend to have increasing amounts of uncontrolled defects as the size of the assembly increases. 11,12One problem associated with ordered lms from colloidal particles is that as the lms dry cracks form, 12 which limits the ability to fabricate large area or very thick photonic crystals or similar materials for other applications, such as integrated optics.These cracks develop during the lm formation due to the normal stress imposed by the solvent that creates a transverse tensile stress in the plane of the lm that is greater than the strength of the network of the particles making up the lm. 13Generally, for thicker lms, these forces are higher, so there is oen some critical thickness value above which lms begin to crack upon drying. 14When specically discussing the drying of lms made from latex particles, one source of stress is the shrinkage of the particles themselves as the lm dries, and the constraint imposed by the rigid substrate. 6 order to reduce and ideally eliminate crack formation during the fabrication of inverse opal porous lms, a coassembly method was rst developed by Meng et al. 15 in order to fabricate ordered porous colloidal crystal lms via a mixture of colloidal sphere and ultra-small particles in one step.Using an evaporation induced assembly by particles being driven to the meniscus and then ordering, the ultra-small particles inltrated into the voids of the colloidal crystal formed by the larger particles during the co-assembly.Other examples of binary colloidal crystals also have been reported. 16Going beyond the co-assembly of binary particles, studies of the cracking mechanism and development of crack free colloidal lms as large as square centimeters have been reported by the co-assembly of colloidal spheres and precursors, including polymer precursors 6,17,18 or inorganic material precursors. 1,17,19he spheres used to form the colloidal crystal are a sacricial template material that is later removed, and a large area inverse opal colloidal crystal lm can be achieved.Low crack inorganic lms can be achieved this way, but of low lm thickness. 20owever, utilizing 2D materials or particles with high aspect ratio as the inltrant into the voids of the colloidal crystal remains challenging, not only to fabricate large area and thick crack free lms, but in ensuring that the inltrant does not disrupt the nearly close-packed assembly of the larger spheres, retaining order and connectivity.
Here, we demonstrate a co-assembly method for making ordered inverse opal-like porous clay lms that are crack free over a large area (on the scale of square centimeters).To fabricate these inverse opal-like porous lms, laponite nanoplates are co-assembled into colloidal crystals with polystyrene latex particles, which are used as a sacricial template.A critical ratio of clay to polystyrene (PS) was determined to be necessary to create crack free versions of these lms.These materials have been fabricated with polystyrene opal colloidal templates either $500 nm or $1000 nm in size, but this is a generalizable method that can be made with colloidal particles of a much larger or smaller size.
Monodisperse polystyrene colloidal particles were synthesized (576 AE 7 nm and 1031 AE 15 nm) using as emulsion polymerization approach. 21Relative uniformity of particles size is required for their ability to pack, and to make a high quality colloidal crystal.The standard deviations of both particles are $3%.Using a co-assembly approach, laponite nanoplates, at the same time, were packed into the voids between polystyrene particles.Laponite is a kind of synthetic clay of trioctahedral smectitie of hectorite type, with a typical disk-shape platelet and the composition of Na 0.7 + [(Si 8 Mg 5.5 Li 0.4 )O 20 (OH) 4 ] 0.7 À . 22An individual Laponite disk has a thickness of 0.92 nm, a diameter of about 25 nm, and a negative surface charge density of 0.014 e À ÅÀ2 in water. 23The nanoplates prefer to have lamellae stacked in a parallel orientation along the (001) plane (Fig. 1).As shown in the inset image of nanobeam diffraction (NBD) pattern, the interspacing in between the lamella layers is 3.2 Å, corresponding to the 001 reection. 22omposite polystyrene/laponite (PS/Clay) lms were assembled by convective evaporative self-assembly of mixed aqueous dispersions.Both glass and silicon wafers were used as substrate, as well as the inside of glass capillary tubes.That is to say that it is possible to co-assemble these particles on at as well as curved substrates.Fig. 2A describes the process to create the clay inverse opal materials.A substrate is placed perpendicularly in a dispersion of laponite nanoplates and polystyrene particles.As the solution evaporates, the particles are carried to the meniscus.The larger polystyrene particles pack into a close packed conguration and the laponite lls in the interstitial voids between spheres.Aer the lm has dried, it is heated to 500 C in order to remove the polystyrene (Fig. S1 †), leaving a porous inverse opal-type structure.
Changes in lm quality resulted from varying the ratio of laponite to polystyrene.Films formed solely from polystyrene spheres under the assembly conditions used in this work contained macroscopic cracks, 15,24 as did lms containing only a small amount of laponite nanoplates (Fig. 2B).A sufficient amount of void lling material eliminates the formation of cracks.The macroscopic cracks are no longer observed when the weight ratio of laponite to polystyrene is above 0.15.Fig. 2B shows the calcined porous lms (with the polystyrene sphere template removed) formed with a range of laponite to polystyrene ratios.It can be clearly seen that a laponite/polystyrene ratio of 0.05 creates lms with cracks.A ratio of 0.1 forms lms that are still not entirely defect free, but at ratios above 0.15 (such as 0.15 and 0.30 shown in Fig. 2B and 0.90 shown in Fig. 3) the lms are totally crack free over the several square centimeters.The ratio of laponite to polystyrene in-between 0.15 and 0.9 leads to the open pores crack free colloidal crystal lm.Here, the laponite nanoplates tend to ll in the PS interconnected voids and totally then fullling the voids with raising the laponite concentration.A further increase in the amount of laponite, by increasing the weight ratio of laponite to polystyrene, leads to more and more laponite nanoplates inltrating the voids.At some point, however, there is an excess of  When the weight ratio of laponite to polystyrene is as low as 0.05, cracks are still present.The film formed with 0.01 ratio also shows some defects, but films formed with ratios of 0.15 and 0.30 are crack free over the scale of several square centimetre.The diameter of polystyrene spheres used here is $1000 nm.And the glass slide present here is 2.5 cm Â 2.5 cm.laponite compared to the void volume.At this point, the coassembly of laponite allows them to be stacked together (Fig. S2 †) in between or at the surface of the PS spheres, and eventually the laponite wall grow thicker and thicker and the result is a closed pore material with isolated pores aer removal of the polystyrene templates.The ordered porous lm is fragile, however.When formed on a exible substrate and deformed, one can see a homogenous distribution of cracks domains (Fig. S3 †).This is the result of by bending $10 degrees on a PS substrate.Although the lm is not very bendable, it is possible to form large area and thick lms without cracks.
Our results clearly indicate the possibility of introducing the multi-layered nanoplates aligned in the voids.The colloidal/ nanoplate co-assembly process also offer the signicant improvement of the lm quality, without cracking induced via dehydration induced contraction and associated local capillary force during drying. 25As the nanoplate suspension concentrates during the drying process it undergoes a liquid-sol-gel transition with an accompanying viscosity increase. 26This viscous suspension then provides a glue and necking to the interconnected colloid and prohibits the formation and propagation of the cracks, regardless of colloidal size.When decreasing the size of co-assembled colloidal sphere, the similar inverted opal and crack free porous lm was also fabricated by simply tuning the laponite/PS ratio (Fig. 3).At low laponite concentration, the cracks of the lm were observed; while the cracks were prohibited only and high quality lm with open interconnected pores was developed, when the concentration of laponite (the ratio of laponite to PS) is high enough.Further raising the laponite concentration to an extreme high value, the growing wall and non-interconnected pores were observed.
The high quality porous lm not only performs the crack lm and open interconnected pores, but also has a certain degree of periodicity.The selected area fast Fourier transform (FFT) of SEM image (Fig. 4A) presents the ordered hexagon pattern (insert image of Fig. 4A), meaning that the pores are ordered distributed with the same pore size and in the hexagon orientated close packing.The TEM image (Fig. 4B) of crashed aggregates showed that each pore was interconnected with each other and with the multi-layered aligned laponite walls lled inside the voids and perpendicular to the centre of the sphere.Although some periodicity is achieved, perfect single crystalline lms are not fabricated, and some point defects can be seen as well.One explanation for this may be the increase in viscosity of the solution during the drying process that might "freeze" the polystyrene templates in place before they can pack more perfectly.
As mentioned, the crack developed due to the normal stress imposed by the solvent evaporation and interfacial tension. 13ikely, the crack free lm was formed due to the smaller capillary stress imposed during solvent evaporation, which depends on lm thickness, particle shear modulus, particle packing conguration, and the liquid-air interfacial tension. 24nd among them, the lm thickness is one of the important factors inuencing the lm quality.In order to avoid cracking and obtain thicker colloidal lms, suspension chemistry, drying environment as well as the substrate material has been manipulated.However, for thicker lms, these forces are higher, so there is oen some critical thickness value above which lms begin to crack upon drying. 14To our knowledge the threshold thickness of crack free lm that begins to crack is in the range of few hundred nanometers, such as $500 nm reported by Aizenberg et al., 1 and $300 nm reported by Lee et al. 27 In our system, the thickness can be easily increased to $10 mm and thicker.The cross-sectional view of porous lm also showed the high quality crack free lm perpendicularly with the pores interconnected from the top to the bottom (Fig. 4C).One might speculate that the hygroscopic nature of the laponite might slow the lm drying and therefore mitigate crack formation.Another speculation is that the stacking and overlap of clay platelets distributes the stresses evenly, mitigating crack formation.Here, our thick lms are created by high solution concentrations.This and other factors such as substrate tilt angle are known to control lm thickness of colloidal crystals during convective self-assembly. 28,29g. 3 SEM images of porous film formed with $1000 nm PS and $500 nm PS.The weight ratio of laponite to polystyrene is 0.90.The images show clearly that after calcination, the porous structure is intact, having neither cracked nor collapsed.In summary, we have successfully developed a co-assembly method to fabricate crack free, inverse opal-like porous clay lms over a large area; on the scale of square centimeters and as thick as tens of microns.Laponite nanoplates are co-assembled with polystyrene spheres and driven into the voids of colloidal crystals, which results in crack free and inverse opal-like structured lms that can be fabricated at a critical ratio of clay to polystyrene (PS).This co-assembly approach is versatile and can be utilized to crack free lms with a variety of pore sizes (based on template size) and lm thickness (based on overall dispersion concentration).

Fig. 2
Fig.2Crack free polystyrene (PS)/clay co-assembly.(A) Schematic representation of porous material fabrication.Clay platelets and PS particles in a solution are drawn to a meniscus as the solvent evaporates, creating a crystal of spherical particles with clay particles in the interstitial spacing.The PS particles are then removed by calcination, leaving behind the inverse opal structure made of clay; (B) optical micrographs of porous clay films assembled with different weight ratios of laponite to polystyrene.The sufficient amounts of laponite prevent the formation of those cracks.When the weight ratio of laponite to polystyrene is as low as 0.05, cracks are still present.The film formed with 0.01 ratio also shows some defects, but films formed with ratios of 0.15 and 0.30 are crack free over the scale of several square centimetre.The diameter of polystyrene spheres used here is $1000 nm.And the glass slide present here is 2.5 cm Â 2.5 cm.

Fig. 4
Fig. 4 All clay porous film.(A) Top view SEM image of all clay porous film.The pores are ordered distributed and come out with hexagon FFT pattern (insert image).(B) TEM image of crashed aggregates.Each pore is interconnected with stacked laponite walls at the interface.(C) Cross-sectional view of all clay porous film, representing the crack free and pores interconnection is from the top to the bottom even the thickness is about 10 mm.