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J. Phys. Chem. C 2009, 113, 148–152
Superhydrophilic and Antireflective Properties of Silica Nanoparticle Coatings Fabricated via Layer-by-Layer Assembly and Postcalcination Xiangmei Liu†,‡ and Junhui He*,† Functional Nanomaterials Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancun Beiyitiao 2, Haidianqu, Beijing 100190, China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100864, China ReceiVed: September 18, 2008; ReVised Manuscript ReceiVed: NoVember 5, 2008
Superhydrophilic and antireflective coatings were fabricated from silica nanoparticles of 30 nm (S-30) and 150 nm (S-150) and polyelectrolytes of poly(diallyldimethylammonium) chloride (PDDA) and sodium poly(4styrenesulfonate) (PSS) via layer-by-layer assembly and postcalcination. The time for a droplet to spread flat decreases to as short as 0.28 s by applying a coating of (PDDA/S-150)3/(PDDA/S-30)2 while the maximum transmittance reaches as high as 98.5% by applying a coating of (PDDA/S-30)8. Factors that affect the superhydrophilic and antireflective properties of coatings, such as the number of deposition cycles, size of nanoparticles and surface roughness, were investigated in details by observing their surface morphologies and by measuring their water contact angles, water spreading time, and transmittances. Systematic investigation gave an optimal structure of calcinated (PDDA/S-30)8/(PDDA/S-150)2/(PDDA/S-30)2 for the superhydrophilic and antireflective coating. Its maximum transmittance and the time for a droplet to spread flat reached 97.1% and “2 + 0” > “2 + 2” > “3 + 2”, that is, decrease with an increase of the number of deposition cycles
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Figure 7. Time-dependent changes in instant contact angle as a function of the number of deposited bilayers: (PDDA/S-150)n/(PDDA/ S-30)m (abbreviated as “n + m”) and (PDDA/S-30)8/(PDDA/S150)2(PDDA/S-30)2 (abbreviated as “8 + 2 + 2”). Water droplets of 1 µL were applied in all of the measurements.
of either S-150 or S-30. In this series, the minimum time from initial contact to spreading flat (WCA: ca. 0°) is 0.42 s when using a water droplet of 3 µL (Figure 6b, line “3 + 2”). It is clearly attributed to the increase of surface roughness from “1 + 1” to “2 + 0” to “2 + 2” to “3 + 2”. Effects of the volume of water droplets for measurements were also investigated. The volumes of water droplets used for the measurements in Figure 6b and Figure 7 were 3 and 1 µL, respectively. By comparing Figure 6b and Figure 7, the time from initial contact to spreading flat using 3 µL of water droplets (Figure 6b) is longer than that using 1 µL of water droplets (Figure 7) for identical coatings. The minimum time from initial contact to spreading flat (WCA: ca. 0°) is 0.28 s when using a water droplet of 1 µL (Figure 7, line “3 + 2”), which doubtlessly met the requirements of antifogging coatings as reported in the literature.5,6 The WCAs of the coatings before calcination (not shown here) are much larger than those of the coatings after calcination. All of the polyelectrolytes were removed after calcination, exposing the more hydrophilic SiO2 surface and evacuating the interstitial voids.9 Thus, the coatings became much more hydrophilic. From the above results, it is clear that coatings containing larger nanoparticles (S-150) can be more effective in enhancing hydrophilicity and reducing the spreading time than coatings containing many layers of very small nanoparticles. Conclusions Superhydrophilic and antireflective coatings were fabricated from silica nanoparticles and polyelectrolytes via LbL assembly and postcalcination. The transmittance of coated substrates increases with the coating thickness, reaching a maximum value of 98.5% for the (PDDA/S-30)8 multilayer coated slide glass, and then decreases for even thicker coatings. Hydrophilicity can be enhanced either by increasing the number of deposition cycles
Liu and He or by introducing larger nanoparticles. The time from initial contact to spreading flat is related to the volume of water droplets applied. Also, the minimum spreading time (0.28 s) was achieved by the (PDDA/S-150)3/(PDDA/S-30)2 multilayer coated slide glass using a water droplet of 1 µL. Systematic investigation gave an optimal structure of calcinated (PDDA/ S-30)8/(PDDA/S-150)2/(PDDA/S-30)2 for the superhydrophilic and antireflective coating. It has a transmittance as high as 97.1% and a time to spread flat as short as
