Regular ArticleFabrication of floating colloidal crystal monolayers by convective deposition
Graphical abstract
Introduction
Colloidal lithography is a powerful tool to prepare two-dimensional ordered nanostructures, which have potential applications in the areas of photonics, plasmonics, sensing, and solar cells [1], [2], [3], [4]. Colloidal crystal monolayers (CCM) are the templates for colloidal lithography [5], [6], [7], [8], [9]. Therefore, numerous effort has been devoted to assembling colloids into CCM. The methods developed up to now include convective deposition [10], drop casting [11], spin coating [12], and electrophoretic deposition [13]. However, the major drawbacks of the aforementioned methods are the requirement of (super-)hydrophilic substrates onto which colloids assemble, and the difficulty to transfer the formed colloidal crystals to other substrates. Therefore, a variety of approaches have been attempted to prepare floating CCM [14], [15], [16], [17], [18]. As the term suggests, floating CCM are not attached to a substrate but can freely float in the continuous phase. The most commonly employed approach to prepare floating CCM are the liquid interface mediated methods. Instead of using solid substrates like glass and mica, a liquid interface is used in these methods, such as gas–liquid interface and liquid–liquid interface [14], [15], [16]. For example, Kondo et al. [16] obtained floating CCM by first spreading monodisperse hydrophobic alkoxyl chains coated silica particles at the air-benzene interface and subsequent picking the formed monolayer up with a mica substrate. However, to increase the ordering and packing density in monolayer by the liquid interface mediated methods, the control of the hydrophobicity of colloids [16], the utility of Langmuir-Blodgett trough [19] or the addition of various polymers [20] or surfactants [21] are usually required. Besides the commonly employed liquid interface mediated methods, other methods have been reported. Ramos et al. uses a surfactant-mediated method to prepare floating CCM [17]. By mixing aqueous charge-stabilized polystyrene latex particles with a mixture of an oppositely charged and a neutral surfactant which self-assembled into vesicles, 2D colloidal crystal monolayers were formed on the vesicles. In that system, besides 2D colloidal crystal monolayers, there also were many free particles and random clusters present. Furthermore, the requirement of two types of surfactants as well as the formation of vesicles makes the system complicated and difficult to improve. Tang et al. reported the spontaneous formation of floating CCM of CdTe nanoparticles with tetrahedral shape. The authors ascribed the formation of floating CCM to a combination of electrostatic repulsion and anisotropic hydrophobic attraction [18]. However, the requirement of anisotropic shape limits the potential applications of this method. In addition, optical binding can be another tool to prepare floating CCM which required complicated optical experimental setup and was limited to very small numbers of colloids [22], [23].
As mentioned before, convective deposition is a widely employed method to prepare CCM. In a classical convective deposition, (super-)hydrophilic substrate is inserted into an aqueous colloidal dispersion and subsequent slowly withdrawn out of the dispersion. The colloids nucleate at the drying front via attractive capillary interactions. Once a crystallization nucleus is formed, a convective flow sets in that contains additional colloids, resulting in the formation of CCM [10]. In this article, a modified convective deposition method to prepare floating CCM is presented. As illustrated in Scheme 1, cross-linked polymeric colloids are first dispersed in a volatile dispersing solvent. Instead of withdrawing the substrate out of dispersion, we let the solvent evaporate for approximately 20 min in a fume hood. During evaporation, monolayers are formed onto the inner wall of the centrifugal tube. After removal of the remaining bulk dispersion, an extracting solvent is added. Subsequently, the formed monolayers are peeled off by manual shaking, and eventually dispersed in the extracting solvent. There are three crucial differences between the method reported here and earlier reported procedures that make use of convective deposition. The first difference is the dispersing solvent. While water is generally the solvent of choice, here volatile organic solvents are used which apparently accelerates the formation of CCM. Second, the hydrophobic inner wall of the centrifugal tube instead of (super-)hydrophilic glass is used as a substrate, hence the tedious pretreatment of the substrate is avoided. The last and most important difference is that by using the method present here, CCM can be easily peeled off, and freely float in the dispersion, which is ascribed to the use of appropriate extracting solvents.
In this work, we first demonstrate that using our new method, the formed assemblies are indeed monolayers and these monolayers can freely float in the dispersion. Subsequently, we systematically investigate the experimental parameters in terms of the dispersing solvent, the extracting solvent, the wall materials, and the colloid on the formation of the floating CCM. Eventually, a possible mechanism is proposed which includes CCM formation and peeling off: the CCM formation step involves the capillary attractions that originated from the deformation of a meniscus structure of the air–liquid interface between the colloids, while the CCM peeling off is caused by the penetration of the extracting solvent into the gap between the CCM and the substrate.
Section snippets
Materials
Styrene (St, 99%), divinylbenzene (DVB, 55% mixture of isomers, tech. grade), 1-(chloromethyl)-4-ethenylbenzene (VBC, ≥90%, tech. grade), acrylic acid (AA, 99%), methanol (anhydrous, 99.8%), N,N-dimethylformamide (DMF, ≥99%), acetone (AR, ≥99.5%), methyl acetate (anhydrous, 99.5%), acetonitrile (GC, ≥99.5%), 1,4-dioxane (DOX, ACS reagent, ≥99.0%), 1-methylpyrrolidin-2-one (NMP, anhydrous, 99.5%), 1-ethenylpyrrolidin-2-one (PVP, K30, Mw = 40 kg/mol), polyvinyl alcohol (PVA, Mw = 85–124 kg/mol,
Experimental proof of floating CCM
Chlorinated cross-linked colloids, further abbreviated as CPS-Cl, with a diameter of 418 ± 8 nm and a zeta potential of −38 ± 4 mV were first used as the building blocks. The colloids were synthesized by seeded emulsion polymerization and fluorescent labeling in combination with confocal microscopy indicates that the particles are chemically isotropic (S2, Supporting Information). THF was introduced as both the dispersing solvent and the extracting solvent for CPS-Cl, which were kept in a
Conclusion
We report a modified convective deposition method to prepare floating colloidal crystal monolayers (CCM) consisting of submicron polymeric building blocks. The method involves two major steps. In the first step, we make plausible that evaporation of the dispersing solvent results in the formation of a meniscus structure of the air–liquid interface between the colloids on the substrate. The deformation of the meniscus gives rise to capillary attraction, driving the colloids towards the crystal
CRediT authorship contribution statement
Yong Guo: Methodology, Investigation, Formal analysis, Writing - original draft, Writing - review & editing, Funding acquisition. Willem K. Kegel: Supervision, Project administration, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to thank Kanvaly Lacina for taking the scanning electron microscopy images. Hans Meeldijk and Chris Schneijdenberg are thanked for helping with freeze-drying samples. Yong Guo is supported by a scholarship under State Scholarship Fund (File No. 201306200056) from the Chinese government.
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