Superoleophilic Titania Nanoparticle Coatings with Fast Fingerprint

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Superoleophilic Titania Nanoparticle Coatings with Fast Fingerprint Decomposition and High Transparency Hyungryul J. Choi, Kyoo-Chul Park, Hyomin Lee, Thomas Crouzier, Michael F. Rubner, Robert E. Cohen, George Barbastathis, and Gareth H. McKinley ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14631 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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ACS Applied Materials & Interfaces

Superoleophilic Titania Nanoparticle Coatings with Fast Fingerprint Decomposition and High Transparency Hyungryul J. Choi1†‡, Kyoo-Chul Park1†‡, Hyomin Lee2‡, Thomas Crouzier3‡, Michael F. Rubner4, Robert E. Cohen2, George Barbastathis1,5*, and Gareth H. McKinley1* [1] Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA [2] Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA [3] Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA [4] Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA [5] Singapore-MIT Alliance for Research and Technology (SMART) Centre, Singapore ‡

Present Address

H. J. Choi

1 Infinite Loop, Cupertino, CA 95014, USA

K.–C. Park

Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208, USA

H. Lee

John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA

T. Crouzier

School of Biotechnology, KTH Royal Institute of Technology, Stockholm, Sweden.

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CORRESPONDING AUTHOR FOOTNOTE George Barbastathis 77 Massachusetts Ave. 3-461C, Cambridge, MA, USA 02139 Tel 617-253-1960 Fax 617-258-9346 E-mail [email protected] Gareth H. McKinley 77 Massachusetts Ave. 3-254, Cambridge, MA, USA 02139 Tel 617-258-0754 Fax 617-258-8559 E-mail [email protected]

Keywords: fingerprint degradation, smudge resistant, transparent nanoporous surfaces, superoleophilic surfaces, photocatalytic effects, titania nanoparticles

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Abstract Low surface tension sebaceous liquids such as human fingerprint oils are readily deposited on high energy surfaces such as clean glass, leaving smudges that significantly lower transparency. There have been several attempts to prevent formation of these dactylograms on glass by employing oil-repellent textured surfaces. However, nanotextured superoleophobic coatings typically scatter visible light and the intrinsic thermodynamic metastability of the composite superoleophobic state can result in failure of the oil repellency under moderate contact pressure. We develop titania-based porous nanoparticle coatings that are superoleophilic and highly transparent and which exhibit short time scales for decomposition of fingerprint oils under ultraviolet light. The mechanism by which a typical dactylogram is consumed combines wicking of the sebum into the nanoporous titania structure followed by photocatalytic degradation. We envision a wide range of applications because these TiO2 nanostructured surfaces remain photocatalytically active against fingerprint oils in natural sunlight and are also compatible with flexible glass substrates.

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1. Introduction

Undesired fingerprint oils (sebum) deposited on transparent surfaces such as the touchscreens of mobile phones and optical glass display significantly lower optical transmissivity, hamper visibility, and contaminate flat clean surfaces with dirt and bacteria.1-6 The micrometric pattern of a fingerprint or dactylogram results in reflection and scattering of incident light, and contaminants can easily adhere to the oil-wetted surfaces.1-6 Fingerprint oils are secreted from sebaceous glands located on top of the friction ridges that cover the tips of human fingers.1,2,7,8 The deposited print is predominantly water, but also contains a mixture of organic chemicals such as urea, uric acid, amino acid, ammonia, lipids, glucose and lactic acid resulting in an oily liquid with surface tension 23 < γ LV < 60 mN/m .1,2,7,9 Because of this low interfacial tension, previous studies on anti-fingerprint surfaces have mostly focused on developing transparent oil-repellent or superoleophobic surfaces.10-14 However, nanotextured superoleophobic coatings typically scatter visible light due to the length scale of the structures and the intrinsic thermodynamic metastability of the composite superoleophobic state can result in failure of the oil repellency under moderate contact pressure.15-23 In contrast to previous studies that have targeted an oil repellency mechanism, we develop and quantitatively characterize transparent fingerprint-eating (or dactylovorous) surfaces constructed from oleophilic and photocatalytic materials. This approach has been pioneered by Guldin et al.24 using block copolymer template assembly to construct an inverse opal silica morphology impregnated with TiO2 nanocrystals. In the present work, we use layer-by-layer assembly to construct our coating and we show how rationally-designed surface topography of titania coatings significantly reduce the characteristic time required for photocatalytic

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degradation of fingerprint oils, whilst also retaining high optical transparency in the range of visible light. Although photocatalytic decomposition using titanium dioxide (TiO2) films has been intensively studied for degrading low molecular weight organic materials into volatile components, studies on titania coatings that can achieve practical degradation times (≤ 1 hour under UV light) and low haze (≤ 1%, required for touchscreen applications) in the range of visible light are extremely rare.3,24-28 To overcome these limitations, we rely on the combined favorable effects of rapid capillary imbibition (or hemi-wicking17,29) of fingerprint oils into a porous oleophilic titania nanoparticle structure (see Methods for detailed fabrication process) that reduce the thickness of the deposited sebaceous film and increase the interfacial contact area with the photocatalytic TiO2 nanoparticles based on the mechanism illustrated in Figure 1A. To reduce scattering of visible light (λ), we select a very small length scale for the nanoparticle constituents (dp = 22 nm < λ/10). The cross-sectional SEM image of the nanoporous titania surface is shown in Figure 1B. The fast decomposition of the oils when exposed to UV light is visualized in the optical images shown in Figure 1C. The dactylogram deposited on the right hand side of the flexible glass slide (which is coated with the TiO2 nanoparticle film) is first imbibed into the nanoporous coating and then photocatalytically degraded when the surface is continuously exposed to UV light (1.5 ± 0.1 mW/cm2 at 300 nm < λ < 400 nm) for 1 hour.

2. Methods

2.1. Layer-by-Layer assembly and calcination of the TiO2 nanoparticle coating. Sequential adsorption of polymer and nanoparticle layers was performed using a StratoSequence VI spin dipper (nanoStrata Inc.), controlled by StratoSmart v6.2 software, at 120-130 rpm. The

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concentrations of poly(allylamine hydrochloride) (PAH, MW = 58,000 g/mol) and TiO2 nanoparticle dispersions (Svaya Nanotechnologies, Inc., with mean diameter = 20.5 nm) in the dipping solutions were 1 mg/mL and 0.03 wt% respectively. The PAH solutions and TiO2 dispersions and their respective rinse solutions were adjusted to pH 7.0 and pH 9.0 with either NaOH or HCl respectively. Distilled water (>18 MΩ•m, Millipore Milli-QTM) water (MQ water) was used in formulating the polymer solution, nanoparticle dispersion, and in all rinsing procedures. The dipping times in the PAH solutions and TiO2 dispersions were each 10 min interspersed by three successive rinse steps (of 2, 1, and 1 min respectively). This sequence of steps represents a single dipping cycle. The total number of dipping cycles (N) explored in this work is typically 10 ≤ N ≤ 70. Glass substrates were first degreased by sonication in a 4% (v/v) solution of Micro-90 cleaner (International Products Co.) for 15 min and subsequently sonicated twice in MQ water for 15 min. The substrates were blow-dried with dry air and treated for 2 min with an oxygen plasma (PDC-32G, Harrick Scientific Products, Inc.) at 150 mTorr before the LbL assembly. After the assembly process, the coated substrate was calcinated for 3 hours at 350 °C. This temperature is sufficient to remove the PAH and to sinter the particles together into a nanoporous coating, but ensures we retain the desired TiO2 crystal structure (anatase) that maximizes photocatalytic activity and prevents the anatase particles from being converted to a rutile structure. All of the experimental results presented in this paper represent the properties of nanoporous TiO2 surfaces after calcination. The porosity of the calcined nanoporous surfaces was measured using the setup introduced in Supporting Information (SI) Section 1.

2.2. Optical transmissivity and haze measurements. A spectrophotometer (Cary-500) was used for spectral transmissivity and haze measurements in the visible range (400 nm ≤ λ ≤ 800

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nm). Haze measurements were performed using the spectrophotometer with an integrating sphere, as described in ASTM D1003 “Standard Method for Haze and Luminous Transmittance of Transparent Plastics.”30 A stabilized diode laser (λprobe = 660 nm) was also used to measure the transmissivity at normal incidence angle.

2.3. Artificial sebum. At room temperature, 16 wt% oleic acid (Sigma-Aldrich, 99%), 12 wt% squalene (Alfa Aesar, 98%), 25 wt% jojoba oil (Sigma-Aldrich), and 41 wt% vegetable oil (Mazola)9 were introduced into a 200 mL glass bottle and then gently mixed using a vortex mixer for 1 hour. We used this mixture as an artificial sebum ( γ LV = 33.4 ± 0.7 mN/m , measured by a Krüss K10 tensiometer) after storing it at room temperature for 24 hours.

2.4. PDMS post arrays and experimental method. To minimize variability in successive dactylograms, we constructed a stamp-based method for depositing representative micro-droplet arrays of sebum. Polydimethylsiloxane (PDMS) post arrays were first fabricated using an SU-8 mold. The artificial sebum was spread over the backside of a silicon wafer by spin-coating at 7000 rpm for 60 seconds. The stamp was then pressed against the oil film with a contact force of 4 N and held in contact for 3 seconds. When the stamp was removed, a small amount of artificial sebum remains on the tops of the wetted PDMS posts. After having ‘inked’ the PDMS posts with artificial sebum, the stamp is first pressed into contact with a cleaned microscope slide (to ensure a uniform sebaceous coating on the microposts), and then used to stamp an array of sebum microdroplets onto the LbL-assembled and calcinated nanoparticle surface using a normal force of 4 N.

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2.5. Digital image distortion analysis. In addition to measuring haze and transmitted intensity we also monitored image distortion using a standard image resolution test target. Following previous publications31,32 we digitally cross-correlate two images taken through the nanoporous coating of the identical test target, before and after deposition of sebum through mechanical contact, or after exposure to a fog stream. The resulting image cross-correlation coefficient (0 ≤

α ≤ 1) indicates the level of image distortion with a value of zero representing complete loss of correlation due to blurriness and poor optical clarity, whilst a value of unity corresponds to no image distortion.

2.6. Contact angle measurements. Advancing contact angle measurements were performed using a Ramé-Hart model 590 goniometer, by dispensing liquid droplets of volume V = 5 µL and adding volume to the droplets at the flow rate of 1 µL/s.

2.7. Ultraviolet (UV) light absorption spectrum. The UV light absorption spectrum of a diluted titania nanoparticle suspension (0.05 vol% titania nanoparticle suspension (Svaya Nanotechnologies Inc.) / 99.95 vol% DI water) in a quartz cuvette (VWR Spectrophotometer cell 414004-078) was measured using a spectrophotometer (UV-1800 UV/vis Spectrophotometer, Shimadzu).

3. Results and Discussion

3.1. High transparency by the small length scale of the nanoparticles and overall thickness. Because the wavelength of the incident light is more than an order of magnitude greater than the components that constitute the nanostructured material, we can successfully model a

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representative volume element as a homogeneous medium with the refractive index determined by the volumetric ratio of the individual materials, according to effective medium theory.33-36 Designing the nanostructured coating to have a high porosity p = (1-ϕ), (where ϕ is the packing density of TiO2 nanoparticles) leads to a smaller refractive index than the corresponding value for flat silicate glass, thus reducing reflection and increasing transmissivity within a specific wavelength region when compared to a flat TiO2 film with the same overall thickness.33,37 The optical transmissivity value over the wavelength range of visible light was measured, as shown in Figure 2A. As the number of PAH/TiO2 dipping cycles deposited on the glass slides is increased, progressively lower optical transmissivity is measured over the spectral region of interest (400 nm ≤ λ ≤ 800 nm). However, the layered nanoparticle structures obtained with either N = 40 (thickness d40 = 69.0 ± 0.4 nm) or N = 50 (d50 =114.8 ± 6.2 nm) still have greater average transmissivity (over wavelengths 550 nm ≤ λ ≤ 800 nm) compared to a bare glass slide without a TiO2 nanoparticle coating. This reduced reflectivity arises because of destructive thinfilm interference at the air-nanoparticle-glass interfaces.37-39 Since optical haze induced by the nanoporous coating is also an important factor in selecting an optimal thickness, we also measured the haze (according to ASTM D1003) as the number of dipping cycles (N) was increased as shown in Figure 2B.30 The average haze value of a surface greater than 1% is red shaded because it exceeds the standard threshold desired in the touch screen industry.40

3.2. Rapid wicking of sebum by the porosity of oleophilic coatings. Rapid wicking of the low surface tension organic liquids that compose the sebum results in imbibition into the porous nanostructure and a blurry image of fingerprint as shown on the left hand side of Figure 3A, in contrast to the clear striped pattern of the viscous fingerprint oils (magnified Figure 3B). The

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spreading and imbibition of sebum into the nanoporous TiO2 coating shown in the microscopic images presented in Figure 3C can be quantified using bright field microscopy. The spreading of a stripe of sebum deposited from a single friction ridge over a thin porous titania film layer (N = 50, d50 = 114.8 ± 6.2 nm) can be modeled by considering wicking of a hemi-cylindrical droplet of sebum into a porous substrate (inset of Figure 3D).41 To derive the differential equation that describes the time change of position (lfront(t), in the x-direction) of sebum-air-titania interface through the thin porous layer, the volume inside the porous layer and outside the hemicylindrical droplet (colored in blue) is considered as the control volume of interest. Because the nanoporous film thickness (dfilm ~ O(10-1 µm)) is much smaller than the lateral spreading scale (the distance between two adjacent friction ridges is typically O(102 µm), wicking of the sebaceous liquid in the vertical direction (the z-direction) proceeds much more quickly than laterally; it is thus reasonable to simplify the problem as a quasi one-dimensional spreading problem supplied by a reservoir of fluid as shown in the inset of Figure 3D. Analysis of wicking into a nanoporous medium results in a Washburn-like spreading law with the front position of the sebum given by l front − l0

Dt .41,42 We checked the order of

magnitude of the characteristic diffusion coefficient of D = 2 K P Pc µ ~10 −10 m 2 s (see SI Section 2 for details of derivation), where KP is the permeability of the porous layer, Pc is the capillary pressure inside a pore, and µ is the viscosity of sebum, by comparing the value determined from curve fitting the experimental data shown in Figure 3D using the quadratic function l front = [ D(t − t 0 )] + l0 . We find that the estimated value of the diffusion coefficient is 12

in good agreement with the value determined from curve-fitting that gives

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The time required for complete imbibition and lateral capillary spreading over the characteristic distance between two friction ridges (

) is therefore only

seconds.

3.3. Fast photocatalytic degradation of fingerprint by the large accessible area of titania. The creation of a very high interfacial area between the oil and the photocatalytic titania nanoparticles that constitute the transparent nanoporous coating helps accelerate the ensuing photocatalytic decomposition (see SI Section 3 for detailed information about the photocatalytic decomposition). The greater the specific surface area, the shorter the exposure time required for photocatalytic decomposition of fingerprint oils. The porosity of the coating after calcination is determined using ellipsometry to measure the values of the effective refractive index of the nanoporous film when two different fluids of known refractive index are imbibed into the porous structure (see SI Section 1 for schematics and experimental setup).37 After calcination, the porosity of our LbL titania nanoparticle coatings was determined to be p = (1-ϕ) = 0.496. This relatively large porosity enhances capillary imbibition of fingerprint oils into the nanoporous surfaces. Taking a representative coating thickness of 100 nm after calcination, the accessible surface area of this photocatalytic nanostructure corresponds to approximately 14 cm2 per square centimeter of coated glass, and this value increases with the number of LbL deposition steps. To determine the effect of increased pore surface area for fingerprint decomposition, we additionally quantified the rate of sebum decomposition for different coated thicknesses of the porous TiO2 nanostructure using a polydimethylsiloxane (PDMS)-based stamp that consists of a square array of periodic posts (period = 200 µm) ‘inked’ with our artificial sebum ( γ LV = 33.4 ± 0.7 mN/m ) as shown in Figure 4A (see Methods for details). Figures 4B and 4C show the measured transmissivity of monochromatic light (λprobe = 660 nm from a diode laser)

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with respect to UV exposure time for an LbL film constructed from N = 50 dipping cycles. The sebum deposited on the surface triggers optical reflection and scattering, and the transmissivity initially drops to 85.5% from the original value of 92.6%. As the surface is exposed to UV light (with an average intensity of 1.5 ± 0.1 mW/cm2 averaged over the range 300 nm < λ < 400 nm) the transmissivity gradually recovers to its initial, sebum-free value, and the distortional effects of the sebum disappear after 150 minutes of UV exposure. The inset images of the standard test resolution targets further illustrate that image visibility through the N = 50 film (d50 = 114.8 ± 6.2 nm) is fully recovered after 150 minutes of UV light exposure (see Methods for details).

3.4. Optimization of coating thickness. To provide guidance for selecting the optimal thickness of the titania coating, we use the initial rate of recovery of the optical transmissivity as a means of quantifying the ability of the nanoporous coatings to photocatalytically decompose the artificial sebum deposits. This quantitative metric, in addition to transmissivity and haze values shown in Figure 2B, can then be used to choose an optimal thickness of the nanoporous titania layer amongst the films assembled after 40, 50, and 60 dipping cycles. The average rate of initial recovery of transmissivity (k = ∆T/∆t) shown on the left hand ordinate axis can be readily calculated from the initial slope of the change in transmissivity with exposure time (see Supporting Information, Fig S5) for each nanoporous titania multilayer film with deposited artificial sebum and for the initial 30 min of UV exposure (at constant incident intensity of I = 1.5 ± 0.1 mW/cm2 at 300 nm ≤ λ ≤ 400 nm). Higher values of k represent shorter times for decomposing fingerprint sebum, which is advantageous for maximizing the “dactylovorosity” of the surface.

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As shown in Figure 2B, as the number of dipping cycle increases, the average transmissivity of the coating steadily decreases but the decomposition rate constant k increases. The average transmissivity of an N = 40 dipping cycle titania nanoporous coating (d40 = 69.0 ± 0.4 nm) showed the highest value but the slowest decomposition rate (k = 3.62 x 10-5 /min). Although the artificial sebum film deposited on top of an N = 60 dipping cycle titania nanoporous coating (d60 = 138.2 ± 5.7 nm) is digested more rapidly (k = 1.29 x 10-3 /min), the average haze value of this coating (as measured by ASTM standard D1003) is greater than 1%, which exceeds the standard threshold desired in the touch screen industry.40 Based on the time required for degrading the stamped artificial sebum and the average values of optical properties summarized in Table 1 and Figure 2B, the thickness of the coating can be chosen depending on specific applications and their requirements. Transmissivity tests with artificial sebum deposited on a thinner (N = 40) TiO2 nanoparticle coating (d40 = 69.0 ± 0.4 nm) showed that the oils did not completely vanish even after 270 minutes of UV light exposure because of the lower pore area available for photocatalytic digestion of the sebum. Conversely, although the sebum film deposited on top of a thicker (N = 60) TiO2 nanoparticle coating (d60 = 138.2 ± 5.7 nm) disappears more rapidly (within 30 minutes of the same UV light exposure), the average optical transmissivity remains below that of a flat uncoated microscope glass slide as shown in Figure 2A. We therefore select an N = 50 bilayer coating as an optimal compromise between these competing demands.

3.5. Other properties associated with practical applications. Longevity tests of this TiO2 nanoparticle structure (N = 50 bilayers), measuring the transmissivity of the coating (λprobe = 660 nm) before and after ten successive depositions of the artificial sebum pattern, showed full

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recovery of the transmissivity. As shown in Figure 4C, the optical transmissivity initially drops to values below 86% after the artificial sebum is deposited on the nanoporous coating, but this recovers to the original transmissivity value after three hours of UV exposure. It should be noted that in comparison with a commercially-available material coated with TiO2 (SunClean™ glass) our layer-by-layer assembled N = 60 coating has an averaged transmissivity 16% greater than that of the commercial product and also shows thirty-fold reduction in degradation time to digest the artificial sebum stamp under the same UV exposure conditions (see SI Sections 4 and 5 for details). In addition to the fast photocatalytic degradation of sebum and superior optical transmissivity, we have also explored other benefits compared to typical flat glass surfaces that are conferred by this structural superwetting property and photocatalytic activity. These include anti-fogging properties, self-cleaning properties, and anti-microbial efficacy43 (see SI Sections 68) which can both be important in keeping a textured and porous surface clean and free of scattering defects. For practical uses, we confirmed that the TiO2 nanoporous surface developed in this study also showed mechanical robustness in resisting the average shear force (F ~ O(10-1) N) measured during human swiping motion (see SI Sections 9 and 10).44 However, studies on significantly improving the mechanical resistance against more extreme abrasion tests are still necessary. Finally we have also demonstrated that the calcined coatings can be deposited on flexible or bendable glass substrates, and do not delaminate for bending radii down to approximately 59 mm (see Figure 1C).

4. Conclusion

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In summary, we have designed and fabricated a nanoporous TiO2 coating using LbL assembly techniques and subsequent calcination that results in superoleophilic capillary imbibition when an oily liquid such as fingerprint sebum is deposited on the surface. Following imbibition, the photocatalytic properties and high specific surface area of the titania particles result in rapid degradation of the constituent oils that form the fingerprint or dactylogram. The specific surface area of the porous photocatalytic nanostructure, the speed of imbibition of the fingerprint oils and the optical properties of the coating can all be controlled systematically by varying the number of dipping cycles (N) used in the layer-by-layer assembly process. The nanoporous

and

photocatalytic

TiO2

coating

exhibits

unique

sebum-digesting

(or

‘dactylovorous’) properties but also has superior optical transmissivity and lower haze, as well as operational compatibility with flexible glass substrates and natural sunlight exposure. Such LBLassembled nanostructured coatings may thus find utility as an anti-smudge surface treatment for hand-held devices and touch-computing displays.

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Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Robust Omniphobic Surfaces. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 18200-18205. Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing Superoleophobic Surfaces. Science 2007, 318, 1618-1622. Deng, X.; Mammen, L.; Butt, H. J.; Vollmer, D. Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating. Science 2012, 335, 67-70. Kandjani, A. E.; Sabri, Y. M.; Field, M. R.; Coyle, V. E.; Smith, R.; Bhargava, S. K. Candle-Soot Derived Photoactive and Superamphiphobic Fractal Titania Electrode. Chem. Mater. 2016, 28, 7919-7927. Grynyov, R.; Bormashenko, E.; Whyman, G.; Bormashenko, Y.; Musin, A.; Pogreb, R.; Starostin, A.; Valtsifer, V.; Strelnikov, V.; Schechter, A.; Kolagatla, S. Superoleophobic Surfaces Obtained via Hierarchical Metallic Meshes. Langmuir 2016, 32, 4134-4140. Guldin, S.; Kohn, P.; Stefik, M.; Song, J.; Divitini, G.; Ecarla, F.; Ducati, C.; Wiesner, U.; Steiner, U. Self-Cleaning Antireflective Optical Coatings. Nano. Lett. 2013, 13, 53295335. Hashimoto, K.; Irie, H.; Fujishima, A. TiO2 Photocatalysis: A Historical Overview and Future Prospects. Jpn. J. Appl. Phys. 2005, 44, 8269-8285. Fujishima, A.; Tryk, D. A. Functionality of Molecular Systems Vol. 2; Springer: Japan, 1999. Kozawa, E.; Sakai, H.; Hirano, T.; Kohno, T.; Kakihara, T.; Momozawa, N.; Abe, M. Photocatalytic Activity of TiO2 Particulate Films Prepared by Depositing TiO2 Particles with Various Sizes. J. Microencapsulation 2001, 18, 29-40. Zhang, W.; Lu, X.; Xin, Z.; Zhou, C. A Self-Cleaning Polybenzoxazine/TiO2 Surface with Superhydrophobicity and Superoleophilicity for Oil/Water Separation. Nanoscale 2015, 7, 19476-19483. Cebeci, F.Ã.; Wu, Z.; Zhai, L.; Cohen, R.E.; Rubner, M.F. Nanoporosity-Driven Superhydrophilicity: A Means to Create Multifunctional Antifogging Coatings. Langmuir 2006, 22, 2856-2862. D1003 Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, ASTM Standards. Lee, H.; Alcaraz, M. L.; Rubner, M. F.; Cohen, R. E. Zwitter-Wettability and Antifogging Coatings with Frost-Resisting Capabilities. ACS Nano 2013, 7, 2172-2185. Chinga, G.; Syverud, K. Quantification of Paper Mass Distributions within Local Picking Areas. Nord. Pulp Pap. Res. J 2007, 22, 441-446. Sihvola, A. H. Electromagnetic Mixing Formulas and Applications; Institution of Electrical Engineers, 1999. Joannopoulos, J. D. Photonic Crystals: Molding the Flow of Light, 2nd ed; Princeton University Press, 2008. Maldovan, M.; Bockstaller, M. R.; Thomas, E. L.; Carter, W. C. Validation of the Effective-Medium Approximation for the Dielectric Permittivity of Oriented Nanoparticle-Filled Materials: Effective Permittivity for Dielectric Nanoparticles in Multilayer Photonic Composites. Appl. Phys. B: Lasers Opt. 2003, 76, 877-884. Yoldas, B. E. Investigations of Porous Oxides as an Anti-Reflective Coating for Glass Surfaces. Appl. Opt. 1980, 19, 1425-1429. Lee, D.; Rubner, M. F.; Cohen, R. E. All-Nanoparticle Thin-Film Coatings. Nano Lett. 2006, 6, 2305-2312.

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Supporting Information The detailed physical, mechanical, photocatalytic, chemical, and optical properties of the dactylovorous titania nanoporous surfaces are provided as Supporting Information.

Acknowledgements

The authors would like to thank Prof. N. X. Fang and Dr. H. Lee at Massachusetts Institute of Technology (MIT) for the use of the optical microscope, J.-G. Kim and Dr. Y. S. Lee at MIT for useful suggestions about the optical transmissivity measurements, and Prof. Cullen R. Buie, Dr. Z. Ge and A. Rajappan at MIT for the UV absorption spectrum measurements. We also gratefully acknowledge the staff and facility support from the Nano Structures Laboratory, Microsystems Technology Laboratory, and Center for Materials Science and Engineering at MIT for characterizing the nanoporous surfaces. This work was supported in part by the MIT Institute for Soldier Nanotechnologies (ISN) under Contract DAAD-19-02D-0002 with the U.S. Army Research Office and by the Singapore National Research Foundation (NRF) through the Singapore-MIT Alliance for Research and Technology (SMART) Centre and funded in part by a gift from the Xerox Research Center. H. J. Choi thanks STX Scholarship and Kwanjeong Educational Foundation Scholarship for financial support. K.-C. Park and H. Lee also thank the Samsung Scholarship for financial support.

Notes The authors declare no competing financial interest.

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Figures

A

Sebum (red color) TiO2 nanoparticles

Capillary imbibition (Hemi-wicking)

B

C

Photocatalytic effect

Flexible glass

Uncoated flexible glass

TiO2 coated flexible glass t = 0 min

5 mm

t = 10 min

t = 60 min 200 nm

2 cm

Figure 1. (A) Schematic illustration of the two-step mechanism that leads to degradation of the sebum on the nanoparticle coated surface: (step 1) capillary imbibition of low surface tension components into the permeable nanostructure and (step 2) photocatalytic decomposition of the imbibed liquids on TiO2 nanoparticles. (B) Cross-sectional SEM image of the nanoparticle coated surface. (C) Flexible glass substrate deposited with a TiO2 nanoparticle film (lower half) by the LbL fabrication method. The three sequential images on the right show that a fingerprint deposited on the flexible dactylovorous surface progressively disappears under sunlight irradiation (Isolar = 4 ± 1 mW/cm2 at 300 nm < λ < 400 nm) for 1 hour (see Supporting Information Section 11 for more details). The number of dipping cycles for all of the titania nanoparticle coating is N = 50.

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A

B 0.925

1.5

haze < 0.01

haze > 0.01 0.92

Transmissivity (%)

1.2 k (x10-3/min)

0.915

Glass 40 bilayers 50 bilayers 60 bilayers 70 bilayers

0.9

0.91

0.6

0.905 0.9

Average Transmissivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3 0.895 0

0.89 40

Wavelength (nm)

60 50 Number of Dipping Cycles

Figure 2. (A) Measured optical transmissivity over the visible range (400 nm ≤ λ ≤ 800 nm) for four different thicknesses of TiO2 nanoparticle multilayers. The black line represents the average transmissivity of a typical microscope (soda lime) glass slide. (B) Average transmissivity recovery rates (green) calculated from the initial 30 minutes of UV exposure (at a constant value of incident intensity I = 1.5 ± 0.1 mW/cm2 at 300 nm ≤ λ ≤ 400 nm) and average transmissivity values (blue) over the visible wavelength range for the nanoporous titania surfaces assembled from 40, 50 and 60 dipping cycles. The red shaded region represents coatings for which the haze of the TiO2 nanoparticle surface exceeds 1%, which is the standard threshold in the touchscreen industry.

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Figure 3. (A) Rapid wicking of the low surface tension organic liquids that compose the sebum results in imbibition into the porous nanostructure and a blurry image of fingerprint (left), in contrast to the clear stripe pattern of the viscous fingerprint oils (right). (B) Magnified view of the fingerprint oils, which reveals micrometric liquid droplets. (C) Sequential microscopic images taken by a bright field microscopy showing the spreading and imbibition of sebum into the porous nanostructure. (D) Plot showing the time evolution of the sebum-air-titania contact line position as a result of lateral capillary imbibition. The red solid line represents a result of curve fitting with a quadratic function form l front =  D ( t − t 0 )  line).

12

22


+ l0 (represented by the red solid

Page 23 of 25

UV exp osure

B Transmissivity (%)

A

UV Exposure Time (hour)

C Transmissivity without artificial sebum (92.6%)

0 min

150 min

5 mm Transmissivity (%)

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λ = 660 nm 5 mm

UV Exposure Time (min)

Figure 4. (A) Illustration of the experimental process for quantifying the dactylovorous effect (See Methods for details). The two microscope images correspond to the PDMS stamp with micrometric post array before depositing artificial sebum (left), and the same PDMS stamp with the absorbed sebum (right). (B) Longevity test of the 50 layer nanoporous dactylovorous coating. The hollow blue squares indicate the transmissivity values right after the deposition of the artificial sebum. The filled blue squares show full recovery with essentially no net change in the measured optical transmissivity (at λprobe = 660 nm) after repeated stamping with the artificial sebum pattern. The blue lines represent the recovery of transmissivity during each UV exposure period of 3 hours. The red shaded box highlights a single cycle of photocatalytic decomposition of the artificial sebum (measured during the first 3 hours of UV exposure) and is shown in greater detail in Figure 4C. (C) Time evolution of measured transmissivity (angle of incidence = 0°, λprobe = 660 nm) through an artificial sebum-stamped spot of a TiO2 nanoparticle coating (constructed from 50 LbL dipping cycles) during exposure to UV light (1.5 ± 0.1 mW/cm2 at 300 nm < λ < 400 nm). The insets show two images of a part of the resolution test chart viewed through the nanoparticle-coated surfaces and the artificial sebum deposits at t = 0 min (left) and 150 min (right) of UV exposure time. 23



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Tables

Table 1. Optical transmissivity and haze of the nanoporous titania coatings used for sebum digestion for films constructed from an increasing number of dipping cycles.

SunClean™ glass**

Average Thickness -

Photocatalytic effect* ~ 15 hours

Average transmissivity 72.5%

Average haze -

40 dipping cycles

69 nm

> 270 minutes

91.63%

0.34%

50 dipping cycles

115 nm

~ 150 minutes

91.13%

0.79%

60 dipping cycles

138 nm

~ 30 minutes

89.46%

1.08%

70 dipping cycles

141 nm

< 30 minutes

82.58%

1.77%

*Required exposure time until the moment that the optical transmissivity regains 99% of the original transmissivity measured under UV illumination (I = 1.5 ± 0.1 mW/cm2 at 300 nm < λ < 400 nm) without the artificial sebum. **See Supporting Information Sections 4 and 5 for details.

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Superoleophilic Titania Nanoparticle Coatings with Fast Fingerprint

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