Nanoparticles into Fractal Patterns on Glass Surface - ACS Publications

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Organization of SiO2 and TiO2 Nanoparticles into Fractal Patterns on Glass Surface for the Generation of Superhydrophilicity Nainsi Saxena, Tapaswinee Naik, and Santanu Paria J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09519 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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The Journal of Physical Chemistry

Organization of SiO2 and TiO2 Nanoparticles into Fractal Patterns on Glass Surface for the Generation of Superhydrophilicity Nainsi Saxena, Tapaswinee Naik, Santanu Paria* Interfaces and Nanomaterials Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela 769008, Orissa, India. *

Corresponding author. E–mail addresses: [email protected], or. [email protected]; Fax: +91 661 246 2999

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Abstract The superhydrophilic surfaces have many important practical applications such as antifogging, antifouling, self-cleaning, etc. The present study demonstrates a simple and facile template-assisted dip-coating approach for the organization of silica (SiO2) and titania (TiO2) nanoparticles (NPs) into fractal patterns on the glass surface. The pure NPs suspension showed “coffee ring effect” and did not form any organized pattern on the glass surface after drying. In this reported method, NPs were organized into fractal patterns using a template consist of sodium carboxymethyl cellulose (CMCNa) and oxalic acid mixture in the presence of a cationic surfactant (CTAB). The presence of surfactant plays a major role to alter the coffee ring effect because of a Marangoni flow in the direction of droplet edge to its centre induced by the surface tension gradient and surface potential of the particles, which eventually helps to get a uniform fractal pattern. Finally, the fractal patterns of only SiO2 and TiO2 NPs were attained on the glass surface after calcining the CMCNa template at 450 °C. The obtained fractal patterns of SiO2 and TiO2 coated glass surfaces showed the average water contact angle of ~ 6° and ~8° respectively, whereas, coating of only NPs without pattern could not achieved such low average contact angle. These coated surfaces were found to have an excellent antifogging property (transparency of the surface) in presence of water vapor.


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1. Introduction: The wettability of the solid surfaces is an attractive and emerging research field of study in recent years because of its many advanced practical applications and academic interests.1–5 The term superhydrophilicity refers to the strong affinity of water towards any surface with water droplet contact angle (θ ) 150°. The superhydrophilic and superhydrophobic states of the solid surfaces are very important from the practical perspective than the intermediate stage between these two (10°< θ < 150°). The topic of superhydrophilicity is younger comparative to that of superhydrophobicity. The superhydrophilic surfaces are having unique properties such as fast water spreading and drying, antifogging,6 antifouling,7 etc, which are mainly useful for automobile mirror and glasses,8,9 dental mirror,10 bioactive implantation,11,12 biocompatible materials,13,14 humidity sensor,15 microfluidic devices,16,17 heat transfer enhancement,18,19 and so on. Because of these wide varieties of applications of superhydrophilic surfaces, the topic has been drawing significant attention in recent years. There are mainly two strategies for generating a superhydrophilic surface. The first strategy is to develop surface modifications by various treatments (mentioned below) along with the surface roughness. The second one is by creating only micro/nanometer level roughness on the surface. There are many techniques available for the surface modification along with creation of surface roughness. Among these surface modification techniques, some important techniques are photo induced hydrophilicity (PIH) using UV light20 and ion irradiation,21 plasma treatment,22 laser ablation,23 and fluorine induced superhydrophilicity (FIS)24. Photo induced hydrophilicity was first reported in 1997 using polycrystalline TiO2 thin film on glass surface.25 When the TiO2 coated surface was exposed to the UV radiation, the coating with sub-micron roughness was generated on the surface, and it became superhydrophilic. Later, coatings of other inorganic nanomaterials such as SnO2, ZnO, and WO3 made through PIH 3 ACS Paragon Plus Environment


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were also reported by several researchers.

26–29

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The disadvantage of this method is the

functionalization can be made under irradiation of a specific light condition only. Plasma and corona treatments are also important techniques mainly applicable in the oxidation of polymer surfaces.22 Recently, FIS based surfaces via oxy-fluoridation were also reported, in which fluorine atoms were linked to metal and semimetal atoms and a superhydrophilic surface was obtained.24 In spite of the fact that the FIS surfaces show excellent antifogging property but the fabrication steps are tedious and complex. As surface roughness plays a vital role in modification or controlling the surface wettability, another universal approach for creating superhydrophilic surface is to create micro or nano level roughness.5 The first study of wetting phenomena on rough surfaces was conducted by R. N. Wenzel in 1939 and it was reported that the liquid can wet all the surface irregularities if the surface is rough.30,31 The surface roughness enhancement by the hierarchical structures is an effective and well-known method. Water can be absorbed on such rough surface into the internal spacing due to the 3D capillary flow. Till now various nanoparticles such as SiO2,32,33 TiO2,34,35 ZnO,36 SnO237 coated surfaces have been reported. Various methods have been frequently used for depositing the nanoparticles on a flat substrate such as sol-gel,33,38 structure growth in solution,39 ink-jet printing,40 chemical vapor deposition,27,41

chemical

and

hydrothermal

methods,1

etching,42

electrochemical,43

electrospinning,44 and phase separation of polymers.45 The disadvantages of above processes are some processes require multi-steps, expensive techniques, and some processes are not useful for the larger area. Some other approaches for generating different micro to nano patterns on the surfaces can also be directly achieved by using lithography and self-assembly techniques. The lithographic based structured micro-pillars array with switchable wettability has been reported earlier.46,47 However, the process is again expensive and complex. To overcome such drawbacks, researchers have shown continuously increasing interest into self4 ACS Paragon Plus Environment

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assembly-based processes to generate superhydrophilic surfaces during the last few years. Various self-assembly process like layer by layer assembly,6,33,48 electrostatic selfassembly,15 template-assisted assembly 49,50 of colloidal and nanoparticles are mainly used for generating roughness which in turn effectively change the wettability of a surface. Various types of hierarchical structure can be generated using these self-assembly processes. Among several organized structures, the organization of nanoparticles into the fractal pattern is drawing researchers attention in recent years because of their several promising applications such as solar cells,51 surface enhanced Raman spectroscopy,52 catalysis,53 electronics,54 and so on. A well-organized fractal pattern on the solid surface can also be used easily to generate higher roughness factor of a surface. Using this concept, in this work, we attempt to organize the NPs into fractal pattern over a large area of a flat surface using an inexpensive and easy route which can be utilize for generating superhydrophilic surfaces or other potential applications. The fractal patterns originated from the mixture of CMCNa and oxalic acid were used as a template to organize SiO2 and TiO2 NPs in the presence of CTAB (Cetyltrimethylammonium bromide). These fractal patterns can be generated either over a small area from an evaporating sessile droplet or over a large area from the evaporation film after dip coating of the surface. This process can also be used for the organization of other NPs. In this process, we explore an easy and inexpensive methodology for the generation of fractal patterns over a small as well as a large surface area to produce a superhydrophilic surface which is probably reported for the very first time. 2. Experimental 2.1 Materials The (following) materials used in this study were purchased from the following companies, Oxalic acid (H2C2O4.2H2O), 99% from Merck. (India); Cetyltrimethylammonium bromide



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(CTAB) 99%, from Fluka Analytical, Sodium carboxymethylcellulose (CMCNa) 99% from Loba Chemie Pvt. Ltd, Tetraethylorthosilicate (TEOS) 99% from Sigma-Aldrich, aqueous ammonia (25%) Merck (India), absolute ethanol (99.5%), Merck (India). The TiO2 NPs (particle size 21 nm) were purchased from Sigma-Aldrich. All these chemicals were used as received, without any further purification. Glass slides were purchased from Borosil Glass Works Ltd. Ultra-pure water of 18.2 MΩ. cm resistivity was used for all experiments. 2.2

Methods

The Silica nanoparticles used in this experiment were synthesized by modified Stober’s method 55 at 25 ± 3 °C using TEOS in ethanol, in the presence of ammonia as a condensation catalyst. In this method, 7.5 ml ethanol, 0.4 ml (0.6 M) aqueous ammonia and 1.5 ml of deionised water were mixed together and stirred well for 5 min. Then a 0.6 ml of TEOS was added drop wise into the reaction mixture under magnetic stirring condition and continued stirring for 3 h, finally, a white colour 0.2 M silica suspension was obtained. The glass slides were washed with ultrapure water, ethanol, and then dried in a hot air oven for 2 h. For the generation of self-assembled structure, a mixture of oxalic acid (40 mM) and CMCNa (0.4 wt. %) was prepared and stirred well for 30 min at room temperature and denoted as D. Then the as synthesized NPs suspension was added (5:1 v/v mixture: NPs) and termed as D5. Finally, the cleaned glass slide was dipped for 10 min in the D5 suspension and dried inside the closed chamber of a dry block heater (Genetix Biotech Asia) for 1h at 35 °C. After that, the slides were heated at 450 °C for 2 h in a muffle furnace to remove the organic molecules and observed under both optical (Leica-DM-2500M) and electron microscopes (JEOLJSM6480LV, SEM and FEI-NOVA Nano SEM, FE-SEM). The particle size and Zeta potential were measured using dynamic light scattering (DLS, Malvern Zeta size analyser, Nano ZS). The contact angle measurements were done using a video-based contact angle


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meter, (Data Physics, OCA 30). The height profile was measured by an atomic force microscope (AFM), Veeco-diCP II (Model No. AP-100). 3 3.1

Results and discussion Organization of Silica NPs on glass surface

In this work, our final objective is to produce fractal patterns of silica NPs on the glass surface. However, when a particle suspension is dropped on a flat surface, in most of the cases the ‘coffee ring’ structure is formed at the periphery of the droplet after drying. Coffee ring structure is defined as the formation of a ring like pattern on a flat surface after drying of a coffee drop (or a colloidal suspension).56 For the understanding of the organization behaviour of silica NPs suspension, a sessile droplet (5 µl) of silica NPs suspension (average size ~270 nm Fig. S1, Supporting Information) was gently placed on the glass surface with the help of a micropipette and kept for drying at 35 °C inside a dry block heater for 10 min. After evaporation of the droplet, the surface was examined microscopically and it was observed that the formation of a ring structure at the initial pinned pining line of the contact line as shown in Fig. 1a. This ring type structure formation was because of the coffee ring effect (CRE), which occurs after deposition of the particles at the triple point of drop periphery.57 When a droplet of non-zero contact angle is placed on a flat surface, the rate of evaporation of liquid at the edge is higher compared to that of centre of the droplet. Because of the evaporative flux, the silica NPs moved towards the edge from the interior of the drop. The particles continued to move towards the drop periphery until the particle diameter was less than the liquid film thickness or height from the surface. When the particles were very close to the drop boundary, the adhering liquid film brought the particles to a closer distance because of the capillary force and finally at the closer distance van der Waals force of attraction helped them to deposit at the drop periphery to form a coffee ring structure. During



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evaporation of the droplet, the height of the droplet was reduced gradually with the shrinking of the drop volume compared to that of the initial stage. In this study, our prime objective is to generate uniform organized structure over a large area. However, initially, we tested the organization ability of NPs using a small evaporating drop of NPs suspension then it was tested for the large area via dip-coating. Now, to know the behaviour of the SiO2 NPs suspensions on a large area, a glass slide was dipped into the NPs suspension for 10 min and dried at 35 °C inside a dry block heater. The microscopic structure presented in Fig. 1b clearly shows that the NPs are deposited randomly without formation of any organized patterns on the surface. In this case, we believe that during the evaporation of liquid film, the SiO2 particles were not organized as it was showing the CRE during the evaporation droplet.

Fig. 1 (a) Optical microscopic image of a full drop of SiO2 NPs suspension after drying scale bar=500 µm, (b) SEM image SiO2 after dip-coating of glass slide (inset magnified view of NPs) scale bar= 10µm (c) optical microscopic image of SiO2 NPs suspension drop in the presence of 0.1 × CMC (critical micellar concentration) of CTAB (0.1 mm) after drying scale bar=500µm. To eliminate the CRE as well as to organize the NPs on the flat surface, the role of surfactant was also studied. Surfactant molecules are generally used to get better dispersion of nanoparticles in aqueous media, which in turn also affect the particle deposition behaviour 8 ACS Paragon Plus Environment

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on a flat surface. So, a cationic surfactant (CTAB) solution of 0.1 mM concentration (0.1 × CMC) was mixed and the evaporating drop study was performed. The optical microscopic characterization depicts a uniform deposition of particles without the formation of coffee ring structure, as shown in Fig. 1c. The reasons for the elimination of CRE can be attributed as follows: (i) negative charges of both particles and glass surface neutralizes in the presence cationic surfactant, which in turn helps the hydrophobic (van der Waals) interaction between the surface and particles. (ii) The presence of surfactant induces a Marangoni flow in the direction of droplet edge to its centre because of the surface tension gradient. The surface tension gradient arises because of the higher concentration of the surfactant molecules at the drop periphery (low surface tension) than that at the central region (high surface tension) during the evaporation of drop. The presence of Marangoni flow favours de-pinning of the contact line, and finally elimination of the coffee ring effect.

3.2

Organization of SiO2 nanoparticles into fractal pattern on glass surface

It has been observed in our previous studies58,59 and several other studies that different additives have a significant role in organizing the nanoparticles on a flat surface from the evaporating drops. In our work, we have reported that sulphur nanoparticles can be organized into the different fractal patterns in the presence of different organic and inorganic acid salts.59 In addition, recently we have also observed in our ongoing unpublished work about the formation of fractal pattern by CMCNa after evaporation of a sessile drop in the presence of oxalic acid. Here, it is noteworthy to mention that only CMCNa is not forming any organized structure after drying (details are given in figure S2, Supporting Information), however, the presence of oxalic acid helps to form the organized structure because of similar reasons mentioned in our previous study.58 In this study, we found pure SiO2 NPs were unable to generate any patterned structure in the presence of only oxalic acid. So, we attempt



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to use the previously studied CMCNa pattern in the presence of oxalic acid as a template to organize the SiO2 NPs. The optical and electron microscopic images of the mixture of CMCNa and oxalic acid (D) after drying of a sessile droplet are presented in Figure 2. The Figure 2a shows that during drying of the droplet, the drop was squeezed towards the centre from the original periphery without pinning or forming any structure up to a certain distance. After some time at a certain concentration of solute, the drop was started pinning with the formation of fractal patterns. The patterns of CMCNa were started from pinning line and converge towards the central region of the drop. The magnified view of the branches is shown in Figure 2b.

Fig. 2 (a) Optical microscopic image of CMCNa + oxalic acid after drying of a sessile droplet (Arrow indicating the drop periphery), scale bar = 500 µm. (b). Magnified view of the selfassembled pattern on the glass surface through SEM, scale bar =10 µm. Further, we mixed SiO2 NPs with D to see whether the NPs can also be organized along with the CMCNa pattern. A sessile droplet of 5 µL volume of D5 suspension was gently placed on the glass surface with the help of a micropipette and allowed to dry at 35 °C inside the dry bath heater. A full drop view (Fig. 3a) clearly shows that the drop was pinned at the first contact line, and then the fractal patterns formation was started from the second contact line


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and oriented towards the centre. There was no organized structure between the first and second contact lines. Generally, the formation of the second ring is called as inner coffee ring deposits (ICRD).60 As our main aim is to organize the silica nanoparticles over the full drop area, we studied the effect of surfactant again on the elimination of ICRD and the results are presented in (b) and (c) parts of Fig. 3. From the Figs. 3(b, c) it can be seen that in the presence of 0.1 and 1.5 mM of CTAB concentrations full drop area is covered by the fractal patterns after elimination of ICRD. Finally, to check the organization pattern on a larger area of a surface due to the evaporation of the liquid film, the glass surface was dipped in both the above-mentioned conditions in the presence and absence of CTAB. Later, the film was dried at 35° C. It can be seen from the optical microscopic images that under both conditions fractal patterns are formed on the surface. The Fig. 3d shows that the pattern does not cover the surface fully in the absence of CTAB, while, the particles are agglomerated on the surface in the presence of 0.1 mM CTAB (below CMC) as illustrated in Fig. 3e. A comparison between these two indicate there is no significant difference in the appearance. However, in the presence of 1.5 mM CTAB (Fig. 3f), the patterns are uniform and dense without any agglomeration of particles in comparison to that of 0.1 mM CTAB (Fig. 3e). From these observations, it is very clear that the SiO2 NPs are organized along the pathway of CMCNa and formed similar fractal patterns. The Mullins – Sekerka instability during the evaporation of droplet because of the presence of salt in the mixed media is also important for the formation of organized structure.61 This instability is mainly attributed to the crystallization of formed sodium oxalate salt. The presence of sodium oxalate salt in the mixture leads to the crystallization process and act as a building block for the arrangement of SiO2 NPs. As surface charge (ζ – potential) of the particles is an important parameter in the organization process, we measured the ζ – potential under different conditions. The ζ – potential values of SiO2 NPs in aqueous media, CMCNa + oxalic acid mixture, CMCNa + 11 ACS Paragon Plus Environment


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oxalic acid + 0.1 mM CTAB mixture (D5S0.1), CMCNa + oxalic acid + 1.5 mM CTAB mixture (D5S1.5) are -41, -0.96, +0.32, +0.83 mV, respectively. The reduction of surface charge of SiO2 NPs in the presence of CMCNa is mainly because of the adsorption of CMCNa molecules onto the nanoparticles surface. In the presence of CTAB, the surface charge becomes positive but because of the neutralization of the positively charged head group (Trimethylammonium group) of CTAB with the negatively charged oxalic acid, the value of surface charge is low. Since the electrostatic repulsive forces between the particle-particle and particle-surface are reduced after suppression of the surface charge, the particles are easily organized along with the CMCNa pattern. From the surface charge analysis, it is also clear that better dispersion of particles in the presence of surfactant is mainly because of the steric repulsion. We have also measured the surface tension and contact angle of all prepared suspensions for the better understanding of the surfactant effect on nanoparticles suspension. Table-1 shows the surface tension and contact angle values on glass surface under different conditions. Table -1: Surface Tension of Different Mixtures and Respective Contact Angle Values on Glass Surface. Component D D5 D5S1.5 1.5 mM of CTAB

Surface tension (mN/m)

Contact angle (°)

56.37 43.84 34.70 31.74

39.5 24.5 46.3 71.0

From the Table-1 it is clear that the surface tension of D, D5, D5S1.5 decreases gradually and the value of D5S1.5 is slightly higher than pure CTAB. The value of surface tension of solution D is lower than pure water (72 mN/m) because of the presence of organic molecules like CMCNa, the contact angle is also less than pure water because of lowering of surface tension. After addition of NPs into the mixture D (denoted as D5), both surface tension and contact angle reduces and the reason for the reduction was well explained in our previous 12 ACS Paragon Plus Environment

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study.

62

In brief, the presence of the particles affects the cohesive force among the water

molecules at the interface, which causes the surface tension reduction at the interface. The reduction in contact angle on hydrophilic glass surface in the presence of SiO2 NPs is attributed to surface tension reduction and development of the disjoining pressure. Later, when CTAB is added to the suspension, the surface tension of D5S1.5 further reduces because of the presence of surfactant. In spite of surface tension reduction, the contact angle increases because of strong adsorption of cationic surfactant molecules onto the particles and glass surfaces through the positively charged head group of the surfactant molecules. Finally, all these combined effects help to organize the NPs on glass surface.

Fig. 3 Optical microscopic images of: (a) full drop view of suspension D5, (b) full drop view of suspension D5S0.1, (c) full drop view of suspension D5S1.5, (d) dip-coated glass surface of suspension D5, (e) dip-coated glass surface of suspension of D5S0.1 (marked region indicates agglomerated particles), (e) dip-coated glass surface of suspension of D5S1.5. Scale bars are 500 µm for (a to c) and 200 µm for (d to f).



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After the formation of organized structure, the surface was heated at 450 °C for 2 h to burn all the associated organic molecules in order to get the patterns of only SiO2 NPs. To support this, we performed thermo-gravimetric analysis (TGA) and the weight loss (%) vs. temperature graph is presented in Fig.4 for the template forming mixture (CMCNa +oxalic acid + CTAB).

Fig. 4 (a) TGA curve for reaction mixture (b) SEM image (magnified view) of selfassembled SiO2 NPs after burning of coated slide, scale bar = 5 µm. From the TGA curve, it can be observed that there is a sharp change within 100 °C because of the evaporation of unbound moisture, and then between 100-350 °C almost 90% decomposition of organic molecules occurs gradually. Finally, between 350-450 °C there is a slow change in decomposition and beyond that the change is negligible. The trace amount of residue present is mainly because of the formation of sodium superoxide (NaO2) or sodium carbonate (Na2CO3).63 We also observed the coated glass surface with template mixture without NPs after heating at 450 °C through SEM and it was found that most of the areas were clean and there were some residues on some areas. The elemental analysis (EDS) also shows the presence of only Na and O. (SEM and EDS analysis are provided in Fig. S3, 14 ACS Paragon Plus Environment

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supporting information). From the TGA study it can be concluded that the pattern shown after calcination is mainly because of the particles. After burning, organized structure of SiO2 NPs was confirmed from the microscopic analysis (Fig. 4b) with trace amount of the salt. We have also studied the effect of multiple coatings with the same suspension of D5S1.5 and the microscopic images of the surfaces are presented in Fig. 5(a-d). It is observed that the fractal density increases after double coatings compared to that of single.

Fig 5: FE-SEM images of CMCNa (D5S1.5) mediated fractal patterns of SiO2 NPs after burning at 450 °C: (a,b) single coating, (b,c) double coating, scale bar = 300 µm for (a,c) and



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scale bar = 5 µm for (b,d). (e) AFM image of double coated SiO2 NPs on glass surface, (f) height profile of the image. Further to get an idea about the height profile of the deposited particles a small portion of SiO2 double coated surface was analysed using AFM (Fig. 5e). The height profile obtained through a line from the surface topography is presented in (Fig. 5f). The line profile shows that in most of the places the height is close to a single particle layer (~300 nm). It can also be concluded that after double coating, particles are organized in empty places of the surface instead of multiple layers, which can also be seen from the SEM images. 3.3

Organization of TiO2 Nanoparticles using CMCNa template on glass surface

To see the versatility of the method TiO2 NPs was also used to organize on glass surface by CMCNa template. The water suspension of 21 nm TiO2 NPs (0.798 g/L concentration) was mixed with solution D (5:1 v/v D: NPs) noted as D5. In this process, CTAB was also mixed with D5 to maintain the final concentration of 1.5 mM. Then the glass surface was dip coated with the final TiO2 suspension and analyzed under optical and electron microscopes, after drying and calcined at 35°C and 450 °C respectively. It can be observed from the images presented in Fig. 6 that similar types of fractal patterns are also formed in the presence of TiO2 NPs. The patterns are also retained after calcinations and dense after double coatings. From this study, it can be concluded that similar fractal pattern can be generated over a large surface area using other NPs also.


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Fig. 6 Microscopic images of organized TiO2 NPs on glass surface mediated by CMCNa in the presence of CTAB (1.5 mM). (a) Optical microscopic image of single coated surface before calcination, scale bar = 200 µm (b)SEM image of single coated surface after calcination, scale bar = 50 µm, (c) magnified view of the same surface, scale bar = 1 µm, (d) Optical microscopic image of double coated surface before calcinations, scale bar = 200 µm, (e) SEM image of double coated surface after calcinations, scale bar = 50 µm, (f) magnified view of the same surface, scale bar = 1 µm. HRTEM image (g) stem part of assembled pattern (h) magnified view of small area (i) magnified view of TiO2 nanoparticle (size of particles 21 nm d-spacing of 0.352 nm). Since the particle size of TiO2 is more than 10 times smaller than SiO2 the presence of particles is not clearly visible from the FE-SEM images. That reason the formed fractal



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patterns were also characterized using HR-TEM to assure the branches of the fractal pattern are made with TiO2 NPs. Formed pattern on the glass surface after calcination was scratched with a sharp blade and dispersed in isopropanol, and then dropped on a carbon coated copper TEM grid. The low and high magnification TEM images of a small portion of a branch are presented in Fig. 6(g-h). Fig. 6h clearly shows the branch is made of small TiO2 particles the sizes of the particles are also matching with that mentioned by Sigma Aldrich. A HR-TEM image (Fig. 6i) clearly indicates the size of the particle is 21.03 nm with spherical morphology. The lattice fringes with an interlayer spacing of 0.352 is matching with the (101) planes of anatase TiO2. 3.4

Effect of organized structure on wettability of the glass surface

In spite of several possibilities of practical applications of these fractal patterns, we tested the effect of these structures on the wettability of glass surface. In general, wettability of a flat surface can be changed by creating the roughness on the surface. In this study, we used these patterns as an inexpensive technique to enhance the roughness factor at the nanoscale to improve the wettability of the surface. For determining the wettability of these NPs assembled glass surface, pure water contact angle measurements were done and the results are presented in Table 2. Pure water shows the contact angle of 42° on glass surface. After simple dip coating of NPs on glass surface, when the particles are unorganized on the surface the contact angle reduces for both particles, but the reduction is more in the case of SiO2 NPs. When the particles were organized into the fractal pattern after single coating, further reduction of contact angle was observed (~ 5° lower) than that of the normal dip coated surface into the NPs suspension. Finally, further significant reduction of contact angle was observed for both particles after double coating. Keeping in mind the residue after burning may also change the hydrophilicity of the surface, we measured the contact angle of glass surface after formation of pattern with mixture D + CTAB and calcination at 450 °C. The 18 ACS Paragon Plus Environment

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water contact angle was decreased on the coated surface to 27° from that of 42° on normal glass slide. So, the change is significant but little higher than the silica coated unpattern surface. However, it was also found that the deposited residue having high solubility in water, so the mixture D + CTAB coated surface was dipped in water and taken out immediately than the contact angle again increased to 38.11°. So, the change in contact angle is not significant when the residue salts are removed. We also measured the contact angle after washing on the fractal surface, but no significant change in contact angle was found. Fig.7 shows the pure water drop view on glass surfaces under different mentioned coatings conditions. Noteworthy to mention that, both cases after double coating the surfaces become superhydrophilic as the contact angle is < 10°. The maximum lowering of contact angle after double coated fractal pattern is mainly because of the formation of dense structure as mentioned before.

Fig. 7 Drop view of pure water on glass surfaces (a) untreated surface (CA =42°), (b) double coated SiO2 NPs (without organization) (CA =21.85°), (c) double coated TiO2 NPs (without organization) (CA =23.50°), (d) double coated organized SiO2 NPs (CA =6.22°), (e) double



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coated organized TiO2 NPs (CA =8.02°). Antifogging test of the (f) Silica coated glass surface (g) normal glass (h) TiO2 coated glass surface. Table-2 Contact Angle of Pure Water on Glass Surfaces Under Different Conditions. Contact angle (°) NPs SiO2 TiO2

Double coated NPs 21.85. 23.50

Single coated fractal

Double coated fractal

17.47

6.22

19.50

8.02

In general, there are two models offered theoretically for the prediction of contact angle on the rough surfaces, Wenzel’s 30 and Cassie-Baxter’s 64 models. In Wenzel’s model a rough surface is entirely wetted by liquid without any trapped air in between the roughness. However, in Cassie-Baxter model air is trapped between the rough asperities of the solid surface. cos θ* = r cos θ

(1)

Where θ* is the contact angle on the rough surface, θ is equilibrium contact angle on flat smooth surface, and r is roughness factor defined as the ratio between the actual surface area of surface to the projected area. From the equation, the roughness factor values on double coated fractal surfaces are 1.338 and 1.332 for SiO2 and TiO2 coating respectively. To see the practical applicability of these superhydrophilic surfaces we performed antifogging test. For the analysis of the antifogging property the coated glass samples were placed above a beaker filled with hot water. Figure 7 (f, g, h) shows the physical appearances of treated and untreated glass surfaces in the presence of water vapor. From Fig. 7g, the foggy appearance over the untreated glass surface is clearly visible, whereas, the appearance


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of SiO2 and TiO2 coated fractal patterned surfaces are very clear because of the formation of continuous thin liquid films on these superhydrophilic surfaces. 4

Conclusions

In summary, well organized fractal patterns of SiO2 NPs on hydrophilic glass surface were generated using CMCNa and Oxalic acid mixture template via dip coating method. Used SiO2 NPs alone were unable to form any patterns because of the coffee ring effect. However, when these particles were mixed with CMCNa and oxalic acid, gave fractal patterns at the central region of the droplet leaving the drop peripheral region blank because of ICRD. The formation of fractal structure is mainly because of the dendritic crystallization of sodium oxalate salt, Marangoni flow, and van der Waals attractive force between the particles. The ICRD was eliminated by adding a cationic surfactant (CTAB) into the mixture. Additionally, the structure was also uniform throughout the surface without any agglomeration of particles. Organized fractal patterns of only SiO2 were successfully obtained after calcination of the CMCNa + oxalic acid template at 450 °C. The same methodology was also applied to get organized fractal pattern of TiO2 NPs. Furthermore, these organized structures on glass surface showed superhydrophilic nature with water contact angles ~6 and ~8° for SiO2 and TiO2 NPs respectively. The fabricated superhydrophilic surfaces were also showed the antifogging property in the presence of water vapor. Supporting information FE-SEM image of as synthesized SiO2 NPs, microscopic images of oxalic acid and CMCNa solution after drying, microscopic analysis of surface coated by mixture D + CTAB after burning. (PDF) Acknowledgment



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We acknowledge Indian Association for the Cultivation of Science, Kolkata, India, for giving us the opportunity to access their AFM facility.

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