Manipulating the Assembly of Spray-Deposited Nanocolloids: In Situ

Apr 12, 2016 - Drying of a nanoparticle solution droplet and development of film fabrication method using ultra-sonic mist. Shin OKUBO , Go MURASAWA...
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Manipulating the Assembly of Spray-Deposited Nanocolloids: In Situ Study and Monolayer Film Preparation Peng Zhang,*,§ Gonzalo Santoro,§ Shun Yu,§ Sarathlal K. Vayalil,§ Sebastian Bommel,§ and Stephan V. Roth§,¶ §

Deutsches Elektronen-Synchrotron (DESY), Notkestrasse 85, D-22607 Hamburg, Germany Department of Fiber and Polymer Technology, Royal Institute of Technology, Teknikringen 56-58, SE-10044 Stockholm, Sweden



S Supporting Information *

ABSTRACT: Fabrication of nanoparticle arrays on a substrate is one of the most concerned aspects for manipulating assembly of nanoparticles and preparing functional nanocomposites. Here, we studied in situ the assembly kinetics of polystyrene nanocolloids by using grazing incidence small-angle X-ray scattering. The structure formation of the nanoparticle film is monitored during air-brush spraying, which provides a rapid and scalable preparation. By optimizing the substrate temperature, the dispersion of the nanocolloids can be tailored to prepare monolayer film. The success of the monolayer preparations is attributed to the fast solvent evaporation which inhibits the aggregation of the nanocolloids. The present study may open a new avenue for the manufacturefriendly preparation of well-dispersed nanoparticle thin films.

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compared to the spheres, the ellipsoids would disturb the flow route and prevent the strong aggregation of suspended particles at the pinning lines. To reach a more general protocol, further investigation of the structure formation process in real time and at high spatial resolution is necessary. To study the structure formation process in thin liquid film, grazing incidence smallangle X-ray scattering (GISAXS) in combination with a synchrotron source is a reliable method because of its high spatial resolution and fast time-resolution ability.11−16 Herzog et al. studied the drying process of polystyrene nanoparticles solution with in situ GISAXS and clarified that the thin film formation experienced three different stages: solvent evaporation, nanoparticle ordering, and final drying stage.17 For the particle aggregation on the substrate, it is proved that the occurrence of vertical layering occurs later than the lateral ordering on the basis of time-resolved GISAXS in situ study.18 Moreover, Kao et al. successfully disclosed the kinetics of rapid fabricating of supramolecular structures with GISAXS.7 In this contribution, we show that a scalable nanocolloidal monolayer film can be prepared with controlled fast evaporation of solvent. This is achieved via spray deposition, which is a cost-efficient technique to atomize and achieve well dispersion of the liquid and the nanoparticles.13,19−24 Spray deposition has been widely used to study the colloidal selfassembly and prepare large-scale multilayer thin film structures in solar cells.13,20−22,24 Some recent work illustrate that the

INTRODUCTION Droplet evaporation and corresponding solute assembly are of the fundamental interest for the solution-based thin film processing. With the evaporation of solvents, the solute will aggregate at the pinning lines and form the ring-like structures, i.e., so-called “coffee ring”.1 Plenty of scientific investigations have been striving to understand the structure formation process and exploring the applications of “coffee-ring”-like structures.2−4 It is generally accepted that the formation of the ring-like structure is due to that the capillary flow outward from the droplet center bring the suspended materials to the pinning lines. On the other hand, manipulating the capillary flow is the main way to alleviate or counterbalance the coffee-ring effect and prepare homogeneously deposited thin films.5 However, fragmentation and coalescence are encountered in the late stage of droplet drying, which can be envisaged by the fluctuation of contact angle value.6 These processes bring further complexity to the capillary flow. As a result, much more complex dried structures, like dendrites, stacked rings, etc., are observed. We are interested in further manipulating the capillary flow and get well-ordered structures on the basis of knowledge obtained from time-resolved study of the drying kinetics. The related study is expected to be attractive for the fast and scalable production of nanoparticle thin films.7−10 A few attempts have been tried to inhibit the inhomogeneous deposition of solutes due to “coffee-ring” effect, namely manipulating the nanoparticles’ assembly to reach a thin film rather than the vertical growth at pinning lines. Yunker et al. reported that the “coffee-ring” effect could be eliminated by controlling the shape of the suspended particles.3 For example, © XXXX American Chemical Society

Received: March 5, 2016 Revised: April 10, 2016

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Figure 1. Schematic drawing of the in situ GISAXS study of the controlled assembly of spray-deposited PS nanoparticles. The combination with a heater (indicated by the red symbol) is designed to manipulate the solvent evaporation rate. The flow directions of the high-pressure Ar gas and PS solution are marked with white arrows and yellow bars, respectively. ki and kf, αi and αf are the incident and scattered wavevector, incident and exit angles, respectively. The time-resolved GISAXS study of structure formation represented by the 2D GISAXS images marked with stages 0 to 5 are corresponding to before spraying, spraying, droplet coalescence, flat thin film, fragmentation, and dried film, respectively. temperature controlling was achieved by a hot stage (DHS 1100, Anton Paar GmbH, Austria). Four different substrate temperatures, i.e., 25, 54, 100, and 120 °C, were selected, referring to be lower and higher than the boiling point of water/ethanol mixture. The size of the spray-deposited zone was calibrated with ink solution and ruler under the glass substrate. It should be noted that only the size value of central region is taken into consideration since the outer region is consisting of sparsely dispersed fine droplets which are hard to be distinguished.23,24 The environment conditions during the spray deposition are temperature 25 ± 0.5 °C and humidity 47 ± 5%. Grazing Incidence Small-Angle X-ray Scattering (GISAXS). The GISAXS measurements were performed at the microfocus endstation of P03 beamline at PETRA III of the Deutsches Elektronen Synchrotron (DESY), Germany.34 The X-ray wavelength and the beam size were 0.1088 nm and 28 × 24 μm2 (horizontal × vertical), respectively. A Pilatus 300 K detector (Dectris Ltd., Switzerland) was employed to collect in situ 2D GISAXS patterns. The sample-todetector distance was 4057 ± 1 mm. As shown in Figure 1, an incident wave vector ki is scattered in the direction of kf; αi and αf represent incident and exit angles, respectively. αi was set to 0.45°, which is above the critical angles for total reflection of polymer and silicon substrate. The corresponding X-ray illuminated area in the beam direction is ca. 3 mm. To reach high temporal resolution, the typical exposure and readout times for in situ measurement were set at 45/95 and 5 ms, respectively. The analysis of the data was performed with the software DPDAK.35 Atomic Force Microscopy. The atomic force microscopy (AFM) data were acquired in air at 25 °C with an NTEGRA Aura AFM (NTMDT Co., Russia), operated in tapping mode. AFM probes (HA_NC, Etalon, NT-MDT Co.) with a resonance frequency of ca. 140 Hz and a spring constant of 3.5 N/m were used. Image processing was performed by Nanotec WSxM.36 Scanning Electron Microscopy. A scanning electron microscopy (FEI Quanta 400F, FEI Europe, Eindhoven, Netherlands) was used to characterize the surface structure of the PS colloidal thin films. The operation voltage of 5 kV has been used. The PS colloidal thin films are coated with Au using a sputter coater. The estimated Au layer thickness is ca. 3 nm by controlling the sputtering time. The mean value of the particle size is taken by statistically calculating the

spray deposition is compatible with industrial-style highthroughput, roll-to-roll applications.25−27 By manipulating the substrate temperature and solvent evaporation rate, we can further tune the thin film structure, since it is closely related to the competition between the particle movement and liquid evaporation.4 The kinetics of structure formation was studied in situ with GISAXS measurements with a time resolution of 50 ms. These findings offer a nice model pursuing study of the structure formation process of nanoparticle thin film under manipulated solvent evaporation as well as having applications in electrical, optical, and ultrasensitive sensors and solar cells.20−22,28−31

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EXPERIMENTAL SECTION

Materials. The polystyrene nanoparticles (PPs-0.1) with nominal diameter of 100 nm (measured with dynamic light scattering) were purchased from Kisker Biotech GmbH & Co. KG, Germany. The nanoparticles are well dispersed in water with concentration 2.5 wt %. For the spray experiment, the colloid solution was diluted with equal volume ethanol. The addition of the ethanol is designed to fasten the evaporation of solvents. The single-crystal silicon wafers were supplied by Si-Mat, Germany. They were cut into strips of 20 × 70 mm and then carefully treated with “piranha” solution (caution: “piranha” solution sulfuric acid:hydrogen peroxide = 70:30 v/v, dangerously attacks organic matter!) for about 30 min at 120 °C to generate a clean, hydrophilic oxide surface. The substrate was then rinsed with a large amount of deionized water and then purged with dry nitrogen flow as mentioned in the previous publication.32 Air-Brush Spray Deposition. The spray deposition was achieved through the atomization of the PS nanoparticle solutions with compressed Ar gas at pressure 1 bar as shown in Figure 1. The atomized droplet size is expected to be around 10 μm, which has been illustrated in the previously published work.33 A commercial sprayer (Grafo T3, Harder & Steenbeck GmbH & Co. KG, Germany) is modified to adapt to the beamline environment and the in situ X-ray measurements. The spray time was set at 0.1 s, and a ca. 50 pL solution is deposited. The nozzle-to-substrate distance was fixed to 10 cm. Acid-cleaned silicon wafers were used as substrates. The substrate B

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Figure 2. (a) Typical 2D GISAXS pattern of PS nanoparticle thin film at 54 °C. The inserted scale bar indicates 0.1 nm−1. SBS represents the specular beam stop. The existence of the black streaks is due to the intermodular detector gaps (IDG). (b) Horizontal integration result along the horizontal line at Yoneda peak position of silicon substrate (qz = 0.59 nm−1) in panel a. The peak maxima qy = 0.069 and −0.069 nm−1 are marked with arrows. (c) Vertical integration result along the vertical line at qy = 0.069 nm−1 in panel a. diameters of no less than 15 colloidal particles in different locations of the substrate. Contact Angle Measurement. The contact angle and its change with time for PS nanoparticle solutions (volume 2 μL) on cleaned silicon wafers were determined with the OCA 20 contact angle measuring system (Data Physics Instruments GmbH, Germany), using the sessile drop method. For each temperature, at least five data points on clean substrates were collected. Two video systems allow observing the droplet drying in situ. The frequency for the contact angle data collection is 25 Hz. The video images were revisited with the OCA 20 software supplied by the manufacture get the contact angle values. The substrate temperature control was achieved with a hot stage controlled by the Julabo circulating water bath (World Precision Instruments, Inc., USA). To reach thermal equilibrium, the silicon wafers were placed on the heater for at least 10 min before the measurements.

single stages of structure formation in the spray-deposited thin film are collected with a time resolution of 50 ms. To explain the structure information extracted from the GISAXS patterns, a typical 2D GISAXS pattern of PS nanoparticles is shown in Figure 2a. Enhanced visibility of the inherent peaks in Figure 2a is achieved by summing up 60 frames with individual exposure time of 95 ms. It should be mentioned that the signals were obtained at varied sample positions with a pitch of 0.1 mm to avoid possible beam damage. In Figure 2b, there are two side streaks symmetrically distributed regarding the scattering plane, i.e., qy = 0.069 and −0.069 nm−1. The appearance of the streaks in the vertical direction reflects that the nanoparticles in the thin film have regular in-plane structure. The periodicity value, 91 nm, is calculated via the equation d = 2π/q. It should be noted that this value is slightly lower than the nominal diameter value (100 nm) of the PS nanoparticle in solution. This can be rationalized by the slight shrinkage of the colloids in dried state. By integrating the signal along the vertical cut at qy = 0.069 nm−1, we can get structural information in the direction normal to the substrate. From that we can also ascertain the Yoneda peak positions of the silicon substrate (YSiOx) and PS nanoparticles (YPS) as shown in Figure 2c. The Yoneda peak, showing up at the critical angle of the scattering materials, is a material specific peak which relates to the average electron density of the material.39 Thus, the Yoneda peak signal can be used to track the formation of the nanoparticle thin film due to the change in electron contrast. This is especially helpful to study the structure formation at the liquid/substrate interface. Within the distorted wave born approximation, there will be intensity

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RESULTS AND DISCUSSION Various in situ study protocols, including video microscopy,1,3 bright field optical microscopy,5 and reflection interference microscopy,6 were applied to track the multiphase structures in the solution and their time-related evolutions. Most of them are working based on the reflection or absorption of the light signals. However, the initial stage of the formation of nanoparticle thin film may occur at the liquid/substrate interfaces. X-ray-based characterization techniques are especially helpful in detecting the inner/buried structure of materials. Among them, grazing incidence small-angle X-ray scattering (GISAXS) is powerful in detecting the surface and interface structure in thin film.7,12,14,15,37,38 By combining with the high flux in synchrotron facility, GISAXS enables us to do in situ study with high temporal resolution. As shown in Figure 1, C

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Figure 3. 2D map of the GISAXS patterns, in-plane cut at αf = αc,Si at different substrate temperatures: (a) 54, (b) 100, and (c) 120 °C. The five different structure formation stages corresponding to (1) spraying, (2) coalescence of droplets, (3) flat thin film, (4) fragmentation, and (5) dried state are marked in panel a. The starting times for spray and formation of stable colloidal thin film are marked with solid and dashed arrows, respectively. The scale bar indicates 1.2 s. Panel d showing typical 2D GISAXS patterns of PS colloidal thin film at 54 °C and the time-related structural change from stage 0 (s0, before spraying) to s5 (dried state).

changes and peak position shifting of the Yoneda peak due to the change of the interface roughness.17,18,40 In the following, we extract the signal at αc,Si to study the structure formation at varied substrate temperatures. To get real-time information about the drying process, the collected single stages of PS nanoparticle thin film are integrated into a 2D map using the software DPDAK.35 The time-resolved horizontal cuts between qy = −0.195 nm−1 and qy = 0.20 nm−1 at YSiOx (qz = 0.59 nm−1) at different substrate temperatures are shown in Figure 3. The set of the qz position is corresponding to αf = αc,Si. For clarification, the spray and formation of dried nanoparticle thin film are marked with solid and dashed arrows. As shown in Figure 3a, five different stages according to the Yoneda peak intensity change with time can be distinguished. The first 0.1 s is attributed to the spraying stage (stage 1), indicated by the broadening of the YSiOx peak signal. Two pixels with time resolution 50 ms/pixel corresponds to the spray time 0.1 s. This can also be found from the 2D images in Figure 1. Before spraying (stage 0), only weak scattering signal from substrate is observed because of the low roughness in this case. During spraying (stage 1), clear broadening of the scattering pattern is found. This is because the deposition of colloidal droplets distorts the mirror reflection signal and enhances the diffuse scattering signal. Subsequently, the two maximum intensity stages from left to right are related to the coalescence of the droplets of atomized solution (stage 2) and fragmentation of the thin film (stage 4), respectively. In between, the thinner streak is due to the formation of flat liquid film on the substrate (stage 3). This phenomenon agrees with the scattering signal change in Figure 3d. As shown in Figure 3d, broad scattering rods are found in stages 2 and 4 which indicates long-range ordering like droplet coalescence and fragmentation, respectively. In between, stage 3 only shows slim scattering rod which is comparable to the stage 0. This can be rationalized by the metastable structure by forming flat liquid film on the substrate. The wetting of colloidal solution on the substrate can be confirmed by the contact angle measurement mentioned in the following. In addition, in stage 2, the coalescence of the droplets is due to the fact that the formation of large droplet will stabilize the system due to lower energy.6

The decrease of the YSiOx intensity in stage 3 can be rationalized by the fact that the formation of the flat liquid film decreases the electron density fluctuations at the liquid/substrate interface. Finally, the dried state (stage 5) is observed as the appearance of constant Yoneda peak intensity signal. In addition, time-resolved GISAXS study of the structure formation of colloidal thin film in vertical direction at 54 °C is also observed by doing the vertical integration along the vertical line at qy = 0.069 nm−1, and five stages are clearly distinguished (see Supporting Information). Weak regular fringes only observed in the dried state (stage 5) indicates that the packing of particles in the vertical direction occurs close to the drying state. A similar structure formation phenomenon has been illustrated in the drying of colloidal and polymer thin films.11,17 This phenomenon combined with structure formation in the lateral direction indicates that the time-related structural change as mentioned above is a threedimensional feature of the spray-deposited colloidal thin film. For a quantitative analysis, the time to form stable colloidal thin films at 54, 100, and 120 °C are counted, and they are 1890, 720, and 50 ms, respectively, indicated by the time gap between the solid and dashed arrows. By comparing the drying time, it is found that the higher the substrate temperature, the faster is the drying process. As a consequence, stages 2 to 4 are squeezed together which are hard to be distinguished in Figures 3b and 3c. Especially, as shown in Figure 3c, when the temperature is 120 °C, the drying time is close to the resolution of the measurement, i.e., 50 ms. Therefore, higher temperature are not taken into consideration. The surface morphology of the as-prepared (ca. 30 min at designed temperatures) PS nanocolloidal thin film was characterized with SEM and AFM, and the results are shown in Figure 4. In Figure 4a, for the room temperature prepared sample, the nanoparticles with spherical diameter of 100 ± 5 nm are observed. The spray-deposited PS colloids are packing together with clear defects, e.g., holes, and no sign of hexagonal particle packing is observed. Moreover, the sample has a dominant height fluctuation due to the vertical growth of the nanoparticles, as shown in the inserted AFM image. With the increase of substrate temperature to 54 °C as shown in Figure D

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corresponding inhomogeneous deposition of suspended materials are suppressed in the high temperature prepared sample. Although it is blurred by the instability of PS colloids at 120 °C, it is interesting to point out that the PS colloidal monolayer without any vertical stacking growth is confirmed by the AFM data in the inset of Figure 4d. In addition, it should be noted that the GISAXS measurements took around 30 min. So the thermally induced structural rearrangement is unavoidable at high temperatures like 120 °C.32,41 It should be pointed out that the as-prepared monolayer does not show high surface coverage. However, as illustrated in the previous publications, the surface coverage can be increased by simply repeating the spray cycle without sacrificing the monolayer structure.13,23,24 The homogeneity of the spraydeposited colloidal thin films was investigated by performing a lateral GISAXS scan after the spray. There is no variation in the scattering intensity and peak positions (marked by the arrows) observed (see Supporting Information). This indicates that the as-prepared thin film show homogeneous structure in the scanned area (7.4 × 3 mm2). Moreover, with the substrate temperature increased from 54 to 100 °C, the high order peaks of the scattering pattern disappear which indicates that the asprepared colloidal thin film does not have close packing anymore. This result agrees with the structure change observed from SEM images in Figure 4. To explore the mechanism of the suppression of coffee-ring effect, the following three points are taken into consideration: changing of the wetting properties with temperature, Marangoni effect, and fast evaporation. Friedman et al. reported that with the increase of substrate temperature there were wetting transitions for water on solid substrates including quartz, sapphire, and graphite.42 Typically, water tends to wet the substrate better with the increase of

Figure 4. Typical SEM images showing surface morphologies of spraydeposited colloidal thin film prepared at different substrate temperatures: (a) 25, (b) 54, (c) 100, and (d) 120 °C. The insets in panels a and d are the typical AFM height images showing the height fluctuation of the as-prepared colloidal thin film.

4b, similar irregular packing of PS colloids is found. Moreover, as shown in the Figures 4c and 4d, the as-deposited PS nanocolloids show separated structures at high temperatures like 100 and 120 °C instead of the irregular packing in the lower temperatures as shown in Figures 4a and 4b. In Figure 4c, most particles attach to each other in the in-plain direction, typical feature of monolayer film structure (see Supporting Information). This means the “coffee-ring” effect and the

Figure 5. (a) Plots of contact angle values of PS colloid solution at different temperatures. (b) Optical microscopy image showing the droplet morphology just after deposition onto the substrate at 54 °C. (c) Optical microscopy image showing the ring-like structure of the dried droplet at 54 °C. E

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of YSiOx intensity (∼50 ms) after spray indicates that there is no detectable droplet evolution at 120 °C. The fast evaporation of solvent will inhibit the movements of the nanoparticles are subsequently jammed into disorder states and further collapsed at 120 °C as shown in Figure 4d. Thus, we also exclude the possibility of Marangoni effect on dominating the formation of PS nanocolloidal monolayer at elevated temperature. Air-brush spray deposition is a technique to atomize the solution and improve the contacts between solution and substrate by forming filmwise structure. Here, the formation of a thin liquid film is due to the external force from high Ar pressure rather than the wetting property of the solution. The increased contact area of the solution and substrate will promote the heat transport between substrate and solution. Therefore, fast evaporation of the solvent is expected in spray-deposited liquid thin film.47 As shown in Figure 6b, the diameter of the central region of spray cone Φ = 30 mm is used as the size of the triple phase contact line of spray-deposited PS colloids. The triple phase contact line in spray-deposited PS nanoparticles is about 7.5 times larger than that of sessile droplets (Φ = 4 mm) for the same solution volume. With the increase of the substrate temperature, the evaporation of the solvents will be promoted, and this will inhibit the formation of thin solution film and following splitting into droplets. This can be proved by the in situ GISAXS data shown in Figure 3. Especially, when the solvent evaporation rate is higher than that of the aggregation rate of two most adjacent nanoparticles, the capillary flow dominated ring-like structure will be suppressed.4 To give a quantitative analysis, we calculated the Peclet number with Pe = R2/Dτ, where R is the averaged distance of the most adjacent two PS nanoparticles, calculated by the colloid concentration. D is the diffusion coefficient of PS nanoparticle in distilled water, and τ is the liquid thin film drying time. If Pe ≤ 1, the most adjacent two PS nanoparticles cannot aggregate together. In the present system, the R is 1.01 μm (see Supporting Information), and the PS nanoparticle diffusion constant is D = 4.3 μm2/s.48 By assuming Pe = 1, a maximum drying time τmax to prevent the nanoparticle aggregation is 0.24 s. By comparing with in situ GISAXS data, we find the τmax lies between the drying times of PS colloids at 100 °C (0.72 s) and 120 °C (0.05 s). This offers theoretical support for the success of preparing monolayer PS film by controlling the solvent evaporation. Another proof for the suppression of the capillary flow and disappearing of ring-like structure with increasing substrate temperature from 85 to 120 °C can be found in the optical microscopy results (see Supporting Information). Therefore, as shown in Figure 4d, PS nanoparticle monolayer film is achieved. In addition, the present findings rationalized the formation of Au nanoparticle monolayer mentioned by Al-Hussein et al.,13 i.e., the much diluted Au nanoparticle solution (bigger R) can be applied to fabricate monolayer film structure at longer drying time (longer τ) like at room temperature.

substrate temperature, characterized by the monotonic decrease of contact angle values. To address this point, the contact angle values of the PS nanoparticle solution at different temperatures are measured. The corresponding results are shown in Figure 5. The values shown in Figure 5 are the statistical averaged maximum values. This reflects the wetting properties of solution after depinning of the triple phase contact line.43 In general, the contact angle values in the studied temperature range are below 45°, which indicates the colloidal solution wets the substrate. As shown in Figure 5a, the contact angle values increase with the temperature, show a peak at 78 °C, and then decrease. The change of the contact angle values for PS nanoparticle solution agrees with the report of the ethanol/water mixture.44 However, the value at 100 °C is still larger than that of 25 °C. Hence, compared to that at 25 °C, there is no improved wetting for PS nanocolloid solution at 100 °C. With the video system, we also collected the droplet morphology before and after drying at 54 °C. As shown in Figure 5b, a typical droplet is observed on the substrate. When the droplet is dried, a typical coffee-ring structure of the PS nanoparticles is observed in Figure 5c. The schematic drawing showing the capillary-flowinduced aggregation of the PS nanoparticles at the pinning lines is shown in Figure 6a.

Figure 6. Schematic drawing showing the evaporation-driven nanocolloidal assembly in (a) sessile droplet and (b) air-brush spray-deposited thin film with triple phase contact lines represented by diameter (Φ) 4 and 30 mm, respectively. The sketches in right sides of panels a and b showing the cross-section scenarios for capillary force and fast evaporation dominated transport of colloids, respectively. The presence of pinning line and triple phase contact lines are indicated.

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The Marangoni effect is well-known in heated solution systems.45,46 Different from the droplet center, the fast evaporation of the solvent at the triple phase contact line will cause temperature and surface tension gradients between the center and edge. As a result, recirculating flow of the suspended materials between the edge and the center will occur, and it is expected to counteract the outward capillary flow. However, as shown in the in situ GISAXS data in Figure 3c, the drying time at 120 °C is equal to or less than 50 ms, which means that large-scale recirculating flow is hard to occur. The rapid change

CONCLUSIONS In this work, we study in situ the drying process of air-brush spray-deposited PS nanoparticles with GISAXS. By tracking the YSiOx intensity change with time, the structure evolution of deposited PS nanoparticles with a time resolution as high as 50 ms is explored. The structure formation process is divided into five stages: (1) spraying, (2) coalescence of droplets, (3) flat liquid film, (4) fragmentation, and (5) dried state. By increasing the substrate temperature to 120 °C, we proved that the drying F

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(7) Kao, J.; Thorkelsson, K.; Bai, P.; Zhang, Z.; Sun, C.; Xu, T. Rapid fabrication of hierarchically structured supramolecular nanocomposite thin films in one minute. Nat. Commun. 2014, 5, 4053. (8) Han, W.; Lin, Z. Q. Learning from “Coffee Rings″: Ordered Structures Enabled by Controlled Evaporative Self-Assembly. Angew. Chem., Int. Ed. 2012, 51, 1534−1546. (9) Choi, H. W.; Zhou, T. L.; Singh, M.; Jabbour, G. E. Recent developments and directions in printed nanomaterials. Nanoscale 2015, 7, 3338−3355. (10) Derby, B. Inkjet Printing of Functional and Structural Materials: Fluid Property Requirements, Feature Stability, and Resolution. Annu. Rev. Mater. Res. 2010, 40, 395−414. (11) Zhang, J.; Posselt, D.; Smilgies, D.-M.; Perlich, J.; Kyriakos, K.; Jaksch, S.; Papadakis, C. M. Lamellar Diblock Copolymer Thin Films during Solvent Vapor Annealing Studied by GISAXS: Different Behavior of Parallel and Perpendicular Lamellae. Macromolecules 2014, 47, 5711−5718. (12) Ree, M. Probing the Self- Assembled Nanostructures of Functional Polymers with Synchrotron Grazing Incidence X- Ray Scattering. Macromol. Rapid Commun. 2014, 35, 930−959. (13) Al-Hussein, M.; Schindler, M.; Ruderer, M. A.; Perlich, J.; Schwartzkopf, M.; Herzog, G.; Heidmann, B.; Buffet, A.; Roth, S. V.; Muller-Buschbaum, P. In Situ X-ray Study of the Structural Evolution of Gold Nano-Domains by Spray Deposition on Thin Conductive P3HT Films. Langmuir 2013, 29, 2490−2497. (14) Muller-Buschbaum, P. Grazing incidence small-angle X-ray scattering: an advanced scattering technique for the investigation of nanostructured polymer films. Anal. Bioanal. Chem. 2003, 376, 3−10. (15) Hexemer, A.; Mü ller-Buschbaum, P. Advanced grazingincidence techniques for modern soft-matter materials analysis. IUCrJ. 2015, 2, 106−125. (16) Hu, S.; Rieger, J.; Roth, S. V.; Gehrke, R.; Leyrer, R. J.; Men, Y. GIUSAXS and AFM Studies on Surface Reconstruction of Latex Thin Films during Thermal Treatment. Langmuir 2009, 25, 4230−4234. (17) Herzog, G.; Benecke, G.; Buffet, A.; Heidmann, B.; Perlich, J.; Risch, J. F. H.; Santoro, G.; Schwartzkopf, M.; Yu, S.; Wurth, W.; Roth, S. V. In Situ Grazing Incidence Small-Angle X-ray Scattering Investigation of Polystyrene Nanoparticle Spray Deposition onto Silicon. Langmuir 2013, 29, 11260−11266. (18) Roth, S. V.; Autenrieth, T.; Grubel, G.; Riekel, C.; Burghammer, M.; Hengstler, R.; Schulz, L.; Muller-Buschbaum, P. In situ observation of nanoparticle ordering at the air-water-substrate boundary in colloidal solutions using x-ray nanobeams. Appl. Phys. Lett. 2007, 91, 091915. (19) Kannappan, S.; Palanisamy, K.; Tatsugi, J.; Shin, P.-K.; Ochiai, S. Fabrication and characterizations of PCDTBT: PC71BM bulk heterojunction solar cell using air brush coating method. J. Mater. Sci. 2013, 48, 2308−2317. (20) Girotto, C.; Rand, B. P.; Genoe, J.; Heremans, P. Exploring spray coating as a deposition technique for the fabrication of solutionprocessed solar cells. Sol. Energy Mater. Sol. Cells 2009, 93, 454−458. (21) Sarkar, K.; Braden, E. V.; Pogorzalek, S.; Yu, S.; Roth, S. V.; Muller-Buschbaum, P. Monitoring Structural Dynamics of In situ Spray-Deposited Zinc Oxide Films for Application in Dye-Sensitized Solar Cells. ChemSusChem 2014, 7, 2140−2145. (22) Chen, L. M.; Hong, Z. R.; Kwan, W. L.; Lu, C. H.; Lai, Y. F.; Lei, B.; Liu, C. P.; Yang, Y. Multi-Source/Component Spray Coating for Polymer Solar Cells. ACS Nano 2010, 4, 4744−4752. (23) Izquierdo, A.; Ono, S. S.; Voegel, J. C.; Schaaf, P.; Decher, G. Dipping versus Spraying: Exploring the Deposition Conditions for Speeding Up Layer-by-Layer Assembly. Langmuir 2005, 21, 7558− 7567. (24) Buffet, A.; Kashem, M. M. A.; Perlich, J.; Herzog, G.; Schwartzkopf, M.; Gehrke, R.; Roth, S. V. Stripe-Like Pattern Formation in Airbrush-Spray Deposition of Colloidal Polymer Film. Adv. Eng. Mater. 2010, 12, 1235−1239. (25) Chang, S.-H.; Lu, M.-D.; Tung, Y.-L.; Tuan, H.-Y. Gram-Scale Synthesis of Catalytic Co9S8 Nanocrystal Ink as a Cathode Material

time of the droplets decreased to be 2.6% of that at room temperature. Moreover, we find that the as-deposited thin film at 120 °C has formed a monolayer structure. Hence, a rapid and scalable preparation of the nanoparticle monolayer film is first reported. For the mechanism analysis, it is attributed to that the solvent evaporation rate being faster than the aggregation rate of the two most adjacent nanoparticles. The present findings can help not only to understand the fast drying kinetics of nanoparticle solutions but also to explore their potential applications in preparing wet-based nanoparticle thin films and functional devices like solar cells.20−22

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00892. Time-resolved GISAXS study of structure formation in vertical direction (S1), high-resolution SEM image showing separated structure of colloids (S2), lateral scanning GISAXS maps (S3), particle numbers in colloidal solution (S4), and optical microscope images showing the T-dependent structural change (S5) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (P.Z.). Present Addresses

P.Z.: INM−Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbruecken, Germany. G.S.: Instituto de Ciencia de Materiales de Madrid, ICMMCSIC, sor Juana Inés de la Cruz 3, Cantoblanco, 28049 Madrid, Spain. S.Y.: Fiber and Polymer Technology, Royal Institute of Technology, Teknikringen 56-58, 10044 Stocklem, Sweden. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.Y. acknowledges the kind financial support from Knut and Alice Wallenberg Foundation. Portions of this research were carried out at the light source PETRA III at DESY. DESY is a member of the Helmholtz Association (HGF).

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DOI: 10.1021/acs.langmuir.6b00892 Langmuir XXXX, XXX, XXX−XXX