Transparent Coatings Made from Spray Deposited Colloidal

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Transparent Coatings Made from Spray Deposited Colloidal Suspensions B. Fleury, G. Dantelle,* S. Darbe, J. P. Boilot, and T. Gacoin* Groupe de Chimie du Solide, Laboratoire de Physique de la Matière Condensée, UMR CNRS 7643, École Polytechnique, 91128 Palaiseau, France S Supporting Information *

ABSTRACT: The goal of this study is to elaborate fewmicrometer thick optically active coatings based on nanoparticles spray-deposited onto a substrate and to control their scattering properties through a progressive suppression of the coffee-ring effect. The modification of the aggregation state of the nanoparticles to be sprayed induces a change of the surface roughness of the films and consequently of their optical transmission. We draw the counterintuitive conclusion that a nonstable colloidal solution gives a smoother coating than a highly stabilized colloidal solution, leading to a more transparent coating. This phenomenon is demonstrated in the case of commercial TiO2 nanoparticles, as well as of homemade luminescent YVO4:Eu nanoparticles, and seems to be generalized to a large range of systems.

1. INTRODUCTION Optically active coatings based on the deposition of colloidal nanoparticles are highly appealing for many different applications, such as photocatalysis,1,2 photovoltaics,3−5 or opto-electronic devices6 (for lighting7 or displays for example). The basic principle of such film elaboration consists of first synthesizing nanoparticles through colloidal chemistry routes, and then depositing these nanoparticles onto a substrate. Compared to other methods of film depositions such as PVD or CVD, it is an easier and cheaper processing that can be used on variable substrates. Moreover, such a strategy allows an independent optimization of the physical properties of the nanoparticles. For example, it benefits from the possibilities offered by the colloidal/sol−gel chemistry to synthesize materials with a controlled micro/nanostructure exhibiting a high crystallinity at the nanometer scale,8 thus not requiring further thermal treatments for crystallization. In addition to the optimization of the properties of the active material itself (i.e., the nanoparticles), the development of highperformance coatings also requires to take into account several more extrinsic aspects associated with the film microstructure at various scales, whose importance depends on the targeted application. This mainly concerns the ability to deposit crackfree films with the appropriate thickness (typically several hundreds of nanometers to several micrometers) and to control the macroscopic physical properties (optical, mechanical, ...). Concerning the latter point, development of coatings with controlled light diffusion properties is important in many cases. Depending on the application, coatings should be transparent (glass coatings) or should have controlled diffusion properties (light extraction in OLEDs, dye sensitized solar cells9). Optical © 2012 American Chemical Society

properties are well-known to be determined by the homogeneity of the coating at length scales corresponding to visible wavelengths. Starting from nanoparticles whose dispersion state can be controlled to give negligible diffusion, the main issue is to control the particle distribution within the film and the film roughness and avoid the formation of cracks. All this may be controlled by the deposition process and also by the addition of a material that will act as a binder, either organic or inorganic.10 Different deposition methods can be used, such as spin- or dip-coating. Nevertheless, these methods exceptionally allow deposit of high thickness (>1 μm) films.11 Spray-deposition, which consists of spraying droplets of a solution containing the nanoparticles onto a substrate, is a good alternative as it combines the possibility of elaborating films with a thickness varying between ∼200 nm and several micrometers with the capability of depositing these films onto substrates with large dimensions, various shapes, and various rigidities (even flexible substrates). Another advantage of this technique is the fact that it does not lead to matter losses as the whole solution is sprayed onto the substrate. Finally, this process is widely used for industrial purposes as it can be automated. For these reasons, there is a renewed interest for this technique as evidenced by recent works such as the deposition of crack-free TiO2 nanocrystalline films for dyesensitized solar cells, with a thickness of 7−10 μm.12,13 Nevertheless, so far, the poor homogeneity and high rugosity of the films made by spray deposition has been considered as a Received: February 29, 2012 Revised: April 24, 2012 Published: April 24, 2012 7639

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Figure 1. (a) Transmission electron microscopy image of TiO2 nanoparticles. (b) Optical microscopy image of drops of TiO2 nanoparticles deposited by spray onto a cleaned silicon wafer, showing the superposition of deformed rings. Inset: A single drop deposit showing the ring effect. (c) Photograph of a 2-μm TiO2 thick film deposited on a glass substrate and held 2 cm above a page of text, evidencing the scattering properties of the film.

efficient phosphors.18 Similar results were obtained as for the TiO2 nanoparticles, showing that also in this case, the control of the aggregation state of YVO4:Eu nanoparticles in solution allowed us to master the scattering of these luminescent thick films. We may thus infer that our conclusions may be generalized to spray coating from many colloidal suspensions.

major drawback for many applications. The main reason comes from the principle of the technique itself, which involves the drying of large droplets whose distribution over the substrate may induce a poor homogeneity and high roughness. Moreover, the drying of the droplet leads to a so-called “coffee-ring effect” that has been documented both theoretically and experimentally in the literature for micrometer-sized particles.14−16 During the drying process, the drop edges are not free to move but are pinned to the substrate, leading to a solvent flow that drives the particles from the drop center toward the edges and thus to this ring formation. Recently, Yunker et al. studied the drying process of 1.3-μm polystyrene particles with different aspect ratios and have thus proved that the particle shape has a strong effect on the ring formation.15 Using elongated ellipsoids, they managed to suppress the “coffee-ring effect”. Such a result was explained by considering the strong attractive quadrupolar interactions between ellipsoidal particles17 which limit their mobility by creating lacunar aggregates at the interface and thus preventing them from migrating toward the edges of the drop. Starting from these results, the present work aims at revisiting the elaboration of nanoparticulate coatings by spray-deposition of colloidal suspensions. Our goal is to tackle and understand the issue of light scattering in those films, which is directly related to their homogeneity at various scales. For the first part of this study, we chose to investigate the spray deposition of 50-nm sized commercial TiO2 nanoparticles dispersed in water (Figure 1a). This system could be considered as a model system as the nanoparticles are nearly monodisperse and isotropic and present a high colloidal stability in water. As expected,15 spray deposition of such a commercial solution leads to the formation of a ring (Figure 1b). Films made from this solution appear to have a strong surface roughness, inducing high scattering (Figure 1c). The basic idea of our work was that the “coffee-ring effect” could be progressively suppressed by destabilizing the initial TiO2 colloidal solution prior to its deposition. In this case, the presence of aggregates with increased attractive interactions between the nanoparticles would reduce their mobility during the drying of the droplets and would permit a better control of the surface roughness and coatings with an improved transparency. Following this strategy, we succeeded in elaborating highly transparent 2-μm-thick films of TiO2 nanoparticles. The second part of this work was devoted to the application of the previously demonstrated concept in the case of the deposition of thick luminescent films based on YVO4:Eu nanoparticles doped with europium ions, which are very

2. EXPERIMENTAL SECTION 2.1. Nanoparticles. A colloidal suspension of titanium dioxide (TiO2 anatase) S300a from Crystal Global (concentration of 250 g/L) was used in this study. This aqueous solution contains almost spherical TiO2 nanoparticles (∼50 ± 5 nm) made of ∼4−5 nm primary grains (Figure 1a). For the experiments, the commercial solution was diluted by a factor of 10 with deionized water. This method was preferred to the addition of salts, which yields similar results but added a new component to the final film. A ζ potential of +62 mV and a conductivity of 6 mS/cm were measured using a Zeta-Sizer (Malvern Instruments) and a conductimeter, respectively. By dialyzing the colloidal solution against deionized water, the ζ potential could be decreased down to 0 mV as a result of the pH increase from 1.5 to 5.5 associated with the dialysis. Hence, the reduced stability led to nanoparticle aggregation and further flocculation. For this study, we focused our attention on three solutions of ∼5 × 10−3 volume fraction labeled A, B, and C, whose ζ potential was respectively 62, 25, and ∼0 mV and whose conductivity was measured to be respectively 6, 49, and

Table 1. Characteristics of the TiO2 and YVO4:Eu Colloidal Solutions Used for Spray-Depositiona solution solution A solution B solution C solution YVO4_PAA solution YVO4

nanoparticles

ζ potential

aggregate size

conductivity

TiO2 TiO2 TiO2 YVO4:Eu

+62 mV +25 mV ∼0 mV −50 mV

50 nm 500 nm ∼1 μm 70 nm

6 mS/cm 49 μS/cm 25 μS/cm 2.3 mS/cm

YVO4:Eu

+6 mV

900 nm

75 μS/cm

The value of the ζ potential allows the evaluation of the colloidal stability of the particles. a

25 μS/cm (Table 1). Solution A was free of aggregates and only contained well-dispersed 50-nm nanoparticles, as measured by Dynamic Light Scattering (DLS, Malvern Instruments). DLS also showed that solution C mainly consisted in ∼1-μm aggregates. Finally, solution B was intermediate, with 500-nm aggregates which precipitate and 50-nm nanoparticles not yet aggregated. After two months, all the nanoparticles from solution B were aggregated. YVO4 nanoparticles europium doped were synthesized by coprecipitation following the procedure reported in ref 19. After the reaction, the nanoparticles were dialyzed against deionized water and a colloidal solution of YVO4:Eu nanoparticles in water with a typical 7640

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concentration of 10 g/L (∼2 × 10−3 volume fraction) was obtained. YVO4:Eu nanoparticles are ovoid, with dimensions of 20 ± 5 nm by 40 ± 20 nm as determined by scanning electron microscopy. The ζ potential of this solution was measured to be +6 mV. DLS measurement gave a particle size of 900 nm, indicating particle aggregation (Table 1). We developed a procedure that consisted of adding a cationic polymer (poly(acrylic acid) , PAA) to the YVO4:Eu solution in order to increase the surface charge of the particles and also their steric environment. YVO4:Eu nanoparticles coated with PAA gave a much more stable colloidal solution, whose ζ potential is −50 mV. The particle size in solution was measured to be 70 nm by DLS, revealing the nonaggregated state of the particles (Table 1). A solution of as-prepared YVO4:Eu nanoparticles and a solution of YVO4:Eu nanoparticles coated with PAA were used to make YVO4:Eu coatings. 2.2. Single Droplet Drying Experiments. One-microliter drops were simply deposited with a pipet on a polished silicon wafer and dried at room temperature. 2.3. Spray Deposition. TiO2 solutions (labeled A, B, and C according to their colloidal stability) and YVO4:Eu solutions were deposited by spray onto different substrates: glass substrates (1 in.2) were cut out of soda lime microscope slides and silicon substrates were cut from silicon wafers polished on both sides. These substrates were washed with soap, deionized water, and ethanol. After drying under nitrogen, they were cleaned by using a UV-ozone treatment (Novascan). Solutions were deposited on both substrates. Substrates were heated at 90 °C during the spray deposition thanks to a hot plate

measurement, the scattered transmission (Tscat), corresponding to the fraction of light that is scattered in the 0°−87.5° and 92.5°−180° angles, can also be obtained. The direct transmission (Tdir) can be obtained by subtraction as Tdir = Ttot − Tscat. 2.5. Structural Characterization of the Films. Scanning electron microscopy (SEM) images were obtained with use of a Hitachi S4800 scanning electron microscope (FEG-SEM) at 1 or 3 kV of accelerating voltage. An optical microscope (Nikon OPTIPHOT-2, objective: ×100) was used to image the film surface. A Dektak 150 profilometer (Veeco Instruments) allowed us to obtain the roughness data. Surface roughness measurements were performed following 1mm long lines with a spatial resolution of 8 nm. Measurements were filtered with a cutoff at 4 μm (similar to the coherence length of the light sources) to separate the long-distance and the short-distance contributions to the roughness. Then, the standard deviation of the filtered roughness profile was calculated.

3. RESULTS AND DISCUSSION 3.1. TiO2 Nanoparticulate Coatings. 3.1.1. Study of the Single-Drop Drying Process. Figure 3a−c shows photographs of the three samples, prepared as in Table 1, in a glass vial illuminated from the back. One can observe that light becomes more and more scattered from solution A to solution C. Sample A exhibits the characteristic scattering properties of welldispersed TiO2 particles considering their size (50 nm), the high refractive index of TiO2, and concentration. The translucent aspect with reddish color is characteristic of the Tyndall effect. On the contrary, sample C appears milky white in color, which clearly shows light scattering from large aggregates. Sample B is intermediate: the reddish color can still be observed but the solution is milky and light transmission is limited as compared to sample A. For preliminary investigations, 1 μL samples of A, B, and C were deposited with a pipet onto silicon substrates and left to dry slowly in air. The optical images of the resulting drops, as well as their height profiles, are presented in Figure 3d−i. When solution A has dried out, it results in a drop that undergoes the typical “coffee-ring effect”: the drop measures 17 μm at the edges and only ∼200 nm at the center causing the blueish color on the image (Figure 3d,g). In the case of solution B, the ring effect is significantly reduced (Figure 3e). The height of the drop edges is reduced to ∼8 μm and the center of the ring is filled with particles up to a height of ∼5 μm (Figure 3h). However, the edges of the deposit remain very steep. Regarding solution C, the obtained deposit has the shape of the drop itself. Indeed the spatial distribution of the nanoparticles after drying mimics the drop itself (Figure 3i). Single drops of solutions deposited by spray were also observed (Figure 3j−l) in order to verify that the deposition technique and the solvent evaporation conditions do not induce too many differences in terms of drop profiles compared with our previous experiments on millimeter scale drop drying under ambiant conditions (the temperature is set to 90 °C for the spray deposition). The profiles are almost identical with those observed for the single drop drying (Figure 3d−f) with a coffee-ring shape for the very stable colloidal solution A (ζ = +62 mV) and a spread deposit for solution C (ζ ≈ 0 mV). These simple experiments demonstrate the influence of the aggregation state of the nanoparticles in solution on the drop shape. They can be understood considering that aggregated particles have a very limited diffusion when the solvent evaporates thus preventing the “coffee ring effect”. 3.1.2. Elaboration of TiO2 2-μm-Thick Films. On the basis of the results obtained on single droplets, we further evaluated

Figure 2. Scheme of the spray deposition setup. The substrate is taped onto an aluminum block heated by a hot plate. (Figure 2). Covering the backside of the substrates with double-sided tape allowed us to hold them on the heated massive aluminum block and provided a sufficient thermal transfer. Preliminary results, which are in agreement with the literature,11 showed that heating the substrates at 90 °C, i.e. at a temperature close to the evaporation temperature of the solvent, appears to be the best deposition temperature to obtain spatially homogeneous films and avoid uncontrolled spreading of the solution. The airbrush was a Paasche Talon gravity feed with a 0.38 mm nozzle, which is bigger than 50 times the size of the aggregates in solution to prevent clogging of the device. The air pressure was 1.5 atm, the air flow was ∼100 mL/s, and the flow of solution was ∼0.1 mL/min. Samples from single-drop deposit to several micrometer thick layers were made with use of these settings. To vary the thickness of the film, the deposition volume was varied. Elaboration of 2-μm-thick coatings was achieved with a volume of deposited solution of 0.5 mL. 2.4. Optical Characterization of the Films. Optical transmission spectra of the films were measured with a Cary 50 UV−visible spectrometer. An incident beam was sent onto a sample and the directly transmitted light (Tdir) was collected within a 5° aperture around the normal direction. To obtain a measurement that is representative of the whole sample, each spectrum presented here corresponds to the average of three spectra measured on three different spots on the films. Haze measurements, performed with a BYK Gardner haze-gard plus, yielded the total transmission (Ttot) through a sample, i.e. the amount of light transmitted through the sample considering the 0°−180° conic angle. From the same 7641

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Figure 3. (a−c) Photographs of the TiO2 colloidal solutions A, B, and C exhibiting various scattering properties as a result of their different stability. (d−f) Optical images of 1-μL drops dried at room temperature on silicon wafers. The observed cracks are induced by the drying method. (g−i) Height profile of the drops as measured with a Dektak 150 profilometer. Note that in part i, the artifact at the middle of the profile is due to sample tearing from the tip during the measurement. (j−l) Optical images of single drops of solutions A, B, and C spray-deposited onto a silicon wafer.

from solution A, whereas for a film resulting from solution C, the rms roughness is evaluated to be 33 nm. Considering the difference of aspect of the drops as seen in Figure 3, we may infer that the roughness of the films can be linked to the drop height profile (Figure 3g−i). In the case of solution A, during solvent elaboration, deposits with a ring-like shape are stacked on top of each other, leading to significant roughness. For solution C, the spread profile of the drops, discussed above, implies the formation of a less rough surface as the drops connect with each other. Hence, these experiments clearly show that the deposit obtained by drying a drop of solution A, which is a very stable colloidal solution (ζ = +62 mV), presents a relative roughness, whereas the same experiment performed by using an unstable colloidal solution (solution C, ζ ≈ 0 mV) leads to the counterintuitive observation that the deposit is more homogeneous. To prepare thicker films (thickness >2 μm), a larger volume of the TiO2 solutions A, B, and C was spray-deposited. Films with a thickness greater than 10 μm could be obtained (Figure S1 in the SI). However, regardless of the solution used, cracks appear above a certain thickness, which is nearly above 3 μm in the case of our colloidal TiO2 C sample (Figure S1 in the SI).

the application of this result in the context of spray deposition of thick nanoparticulate coatings. Figure 4 shows cross-section and top views of coatings obtained by spraying solutions A and C, respectively. In both cases, the film thickness is close to 2 μm. SEM observation of a cross-section of the three films does not show any significant difference in terms of particle distribution and density. This indicates that the difference of the aggregation state in the initial colloidal suspension is damped out during the drying process. This can be understood when considering that in the aggregated colloid, the Van der Waals interactions are not directed bonding, thus allowing a rearrangement of the particles during the drying process and a dense packing due to capillary forces. Although the cross sections of the films are similar, the surfaces differ according to the different aggregation states of the initial solutions. The spray-deposited micrometer-thick films obtained from solution A show significant roughness and cracks while the films made from sample C are homogeneous and not cracked (Figure 4b,c). This is quantitatively confirmed by the surface profile analysis (Figure 4d,e). After processing the height profiles, the rms roughness is evaluated to be 60 nm for a film resulting 7642

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Figure 4. (a) Typical cross-sectional view of a coating as seen by SEM. The overall aspect in terms of particle distribution and density is the same for coatings made from colloidal samples A, B, and C. Optical images of top-viewed 2-μm-thick films deposited by spraying solution A (part b) and solution C (part c). Insets show the corresponding SEM images evidencing the difference in roughness. (d, e) Roughness profiles of the corresponding films.

Figure 5. (Left) Transmission spectra of 2-μm-thick films of TiO2 nanoparticles made by spray deposition from solutions A, B, and C respectively. (Right) Pictures of three films made by spray deposition of the solutions A, B, and C, respectively. At the top, films are placed on a sheet of text and at the bottom, they are held at 2 cm above the paper in order to evaluate their scattering properties. (Bottom) Values of the total, direct, and scattered transmissions obtained by haze measurements between 400 and 800 nm (with Ttot = Tdir + Tscat).

from solution A is relatively scattering as its transmission spectrum reaches a maximum value of 38% at 800 nm. Comparatively, the film obtained from solution C is much more transparent, with a maximum transmission of 85% at 800 nm. As the TiO2 nanoparticles do not absorb light between 400 and 800 nm, the reason for the altered transmission of the coating is due to light-scattering properties. This difference in transmission is also displayed by looking at the films, either placed directly onto a sheet of paper or held at 2 cm from the paper (Figure 5, right). To find out the main parameter determining the scattering properties of the films, haze measurements were also performed on our samples. In these measurements, the total integrated transmission (Ttot) across the coated substrate and the scattered transmission (Tscat) are obtained as averages in the 400−800 nm range. The results for the different films are reported in Figure 5, bottom. The film resulting from spraying

The observation of a critical cracking thickness (CCT) is commonly observed for films deposited from suspensions, and is usually attributed to capillary stresses occurring during the solvent removal.20 Although our study was not quantitative, the comparison between our samples showed that films from solution A already present some cracks for a thickness of 2 μm, whereas no cracks were observed for samples from the flocculated solution C. We note that, although particles are much smaller in our case (which enhances capillary stresses), this observation is in agreement with the results from Chiu et al.20 and Tirumkudulu et al.,21 who reported some significant increases of the CCT values for flocculated particles in the 200−500 nm size range. 3.1.3. Understanding the Optical Properties of TiO2 Films. Figure 5 (left) shows the transmission spectra as measured by a UV−visible spectrometer of 2-μm TiO2 coatings made by spray deposition of colloidal solutions A, B, and C. The film obtained 7643

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The measured and calculated RR obtained by using the different techniques are consistent within 10%. Thus, the scattering in our spray-deposited coatings can be explained by considering only their surface roughness. Even though some cracks are present in the TiO2 films made from solution A, their contribution to the scattering properties remains low. Hence, through the detailed study of spray deposition of TiO2 colloidal solutions of different stabilities, we drew the conclusion that the most transparent films (in terms of nonscattering) were obtained with the less stable colloidal solutions, due to the suppression of the coffee-ring effect during the drying process. This conclusion has been obtained by studying a model system, with TiO2 nanoparticles highly homogeneous in size. 3.2. YVO4:Eu Nanoparticulate Coatings. To determine whether these conclusions could be or could not be generalized to other colloidal systems, we studied the same phenomenon in the case of YVO4:Eu nanoparticles synthesized in our laboratory. These particles are interesting for their luminescence properties and applications for lighting or displays require their deposition as coatings whose thickness should typically be in the micrometer range. Figure 6a shows transmission spectra of 2-μm-thick films deposited by spray from an as-made YVO4:Eu colloidal solution

solution A presents a total transmission of 85.6%, of which 67.1% correspond to scattered transmission and 18.5% to direct transmission. For the film coming from solution C, the total transmission corresponds to 92.5% with 23.1% corresponding to scattered light and 69.4% to direct transmission. To obtain a deeper understanding of the structural parameters that influence the optical quality of the films, we focused our attention on the origin of the scattering in these films. The three spray-deposited films present scattering effects that could a priori arise from different origins related to their microstructure: scattering from the particles themselves, and scattering from inhomogeneities in the particle density within the film thickness, cracks or surface roughness. As the films are made of closely packed TiO2 nanoparticles, which are small compared to the wavelength, they are seen by the light as dielectric media with an average refractive index nfilm = nnanoparticle(1 − x) + n0x, where x is the volume fraction of pores and n0 the refractive index of air. For this reason, light scattering can be separated into two contributions: volume and surface scattering associated with roughness. The former is caused by local variation of refractive index due to inhomogeneities in the particle density. However, we do not have evidence of any change in the film porosity with the colloidal stability of the solution. Moreover, 1- and 2-μm-thick films made from solution C present the same scattering properties and roughness, which experimentally confirms that the scattering does not arise from the volume but from the surface of the films (Figure S2 in the SI). Surface scattering can be directly linked to the surface roughness through the following equation:22 Tscat = T0(nfilm cos ifilm − n0 cos i0)2 (2πδ/λ)2

(1)

with Tscat corresponding to the scattered transmission, T0 the transmission of the corresponding planar sample, δ the rms roughness, and λ the wavelength. Note that this expression is based on incident light interference and is only valid for a relatively smooth surface and δ/λ ≪ 1. Equation 1 shows that the scattered transmission Tscat is proportional to the squared rms roughness. In a rough approximation, we may thus expect that the relative ratio of the square root intensity Tscat from the films A, B, or C averaged over all wavelengths should be proportional to their roughness ratio (RR): RR =

δi = δA

Tscat i Tscat A

Figure 6. (a) Transmission spectra of YVO4:Eu nanoparticulate coatings made by spray deposition of a highly stable YVO4:Eu solution (YVO4_PAA, gray dotted line) and of a low stability solution (YVO4, black solid line). (b) Picture of the luminescent coating resulting from the unstable YVO4:Eu solution, under 254 nm UV illumination. It shows both high transparency and fluorescence.

(2)

with i = B or C. Tscat can be estimated from UV−vis measurements (Tscat = 100 − Tdir (%)) averaged between 400 and 800 nm or it can be directly obtained from the haze measurements. From these two values of Tscat, we calculated two relative ratios, RR1 and RR2, respectively (Table 2).

(labeled YVO4, ζ = 9 mV) and a PAA-stabilized YVO4 solution (labeled YVO4_PAA, ζ = −50 mV). One can notice that, as in the case of the TiO2 colloidal solution, the less stable YVO4:Eu colloidal solution leads to the more transparent film with a transmission at 800 nm reaching 98%. Thus, as for TiO2 nanoparticles, in order to obtain highly transparent films, one needs to start from unstable YVO4:Eu colloidal solutions. The possibility of elaborating thick luminescent films with controlled scattering is a key requirement for many applications. A new range of applications from lighting devices to photovoltaics engineering is attainable by using this

Table 2. Roughness Ratio (RR) Measured Using the Profilometer and Calculated RR Using UV−Vis Measurements (RR1) and Haze Measurements (RR2) Using Equation 2 sample

measured RR

calcd RR 1

calcd RR 2

A B C

1 0.70 0.55

1 0.66 0.53

1 0.72 0.59 7644

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solar cells at room temperature. J. Photochem. Photobiol., A 2001, 145, 107. (6) Buonsanti, R.; Llordes, A.; Aloni, S.; Helms, B. A.; Milliron, D. J. Tunable infrared absorption and visible transparency of colloidal aluminum-doped zinc oxide nanocrystals. Nano Lett. 2011, 11, 4706. (7) Zhu, T.; Shanmugasundaram, K.; Price, S. C.; Ruzyllo, J.; Zhang, F.; Xu, J.; Mohney, S. E.; Zhang, Q.; Wang, A. Y. Mist fabrication of light emitting diodes with colloidal nanocrystal quantum dots. Appl. Phys. Lett. 2008, 92, 023111. (8) Mialon, G.; Gohin, M.; Gacoin, T.; Boilot, J. P. High temperature strategy for oxide nanoparticle synthesis. ACS Nano 2008, 2, 2505− 2512. (9) Zhang, Q.; Chou, T. P.; Russo, B.; Jenekhe, S. A.; Cao, G. A method for energy-conversion-efficiency enhancement in dyesensitized solar cells. Adv. Funct. Mater. 2008, 18, 1654−1660. (10) Pénard, A. L.; Gacoin, T.; Boilot, J. P. Functionalized sol-gel coatings for optical applications. Acc. Chem. Res. 2007, 40, 895. (11) Jiang, P.; McFarland, M. J. Large-scale fabrication of wafer-size colloidal crystals, macroporous polymers and nanocomposites by spincoating. J. Am. Chem. Soc. 2004, 126, 13778. (12) Halme, J.; Saarinen, J.; Lund, P. Spray deposition and compression of TiO2 nanoparticle films for dye-sensitized solar cells on plastic substrates Solar Energy Mater. Sol. Cells 2006, 90, 887. (13) Ranga Rao, A.; Dutta, V. Low-temperature synthesis of TiO2 nanoparticles and preparation of TiO2 thin films by spray deposition. Sol. Energy Mater. Sol. Cells 2007, 91, 1075. (14) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 389, 827. (15) Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. G. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature 2011, 476, 308. (16) Vermant, J. When shape matters. Nature 2011, 476, 286. (17) Loudet, J. C.; Alsayed, A. M.; Zhang, J.; Yodh, A. G. Capillary Interactions Between Anisotropic Colloidal Particles. Phys. Rev. Let. 2005, 94, 018301. (18) Buissette, V.; Giaume, D.; Gacoin, T.; Boilot, J. P. Aqueous routes to lanthanide-doped oxide nanophosphors. J. Mater. Chem. 2006, 16, 529. (19) Huignard, A.; Gacoin, T.; Boilot, J. P. Synthesis and luminescence properties of colloidal YVO4:Eu phosphors. Chem. Mater. 2000, 12, 1090. (20) Chiu, R. C.; Garino, T. J.; Cima, M. J. Drying of granular ceramic films: I, effect of processing variables on cracking behavior. J. Am. Ceram. Soc. 1993, 76, 2257−2264. (21) Singh, K. B.; Bhosale, L. R.; Tirumkudulu, M. S. Cracking in drying colloidal films of flocculated dispersions. Langmuir 2009, 25 (8), 4284−4287. (22) Amra, C. Diffusion de la lumière par les rugosités d’interface et les hétérogénéités de volume. Collect. SFO 2003, 8, 203−226.

deposition method as the nanoparticles are efficient phosphors (Figure 6b). To increase the quantity of optically active materials, one can deposit thicker YVO4:Eu films by increasing the volume of the colloidal solution sprayed onto the substrate. Nevertheless, as for TiO2, above ∼5 μm thickness, cracks start appearing within the film microstructure, leading to a drastic increase of the scattering properties.

4. CONCLUSIONS As demonstrated in the case of TiO2 and YVO4:Eu systems, the transparency of spray-deposited nanoparticulate coatings is governed by the film surface roughness, which can be controlled by the colloidal stability of the solution to be sprayed. The counterintuitive result is the fact that a nonstable colloidal solution leads to the formation of a smoother and thus more transparent film than a stabilized solution. This phenomenon can be explained by considering the dynamics of drop drying and the fact that aggregated particles have reduced mobility, preventing the “coffee-ring” effect responsible for high surface roughness in stabilized solution. Nevertheless, the existence of a critical thickness above which longitudinal cracks appear in the volume of the film seems to be the main limiting factor of this technique. Although the results presented in this paper concern the spray-deposition technique, other deposition techniques, such as inkjet, can also benefit from these conclusions.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and thierry. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank François Guillemot at Saint Gobain Recherche (Aubervilliers, France) for haze measurements. Lucio Martinelli from our lab is also warmly acknowledged for valuable discussions on the optical properties of the films.

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REFERENCES

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dx.doi.org/10.1021/la300872m | Langmuir 2012, 28, 7639−7645