TiO2 Hybrid Networks for Superhydrophobic Coatings with

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Applications of Polymer, Composite, and Coating Materials

TDI/TiO2 hybrid networks for superhydrophobic coatings with superior UV-durability and cation adsorption functionality Zhiwei Huang, Robert S. Gurney, Yalun Wang, Wenjiao Han, Tao Wang, and Dan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00886 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Applied Materials & Interfaces

TDI/TiO2 Hybrid Networks for Superhydrophobic Coatings with Superior UV-Durability and Cation Adsorption Functionality Zhiwei Huang, Robert S. Gurney, Yalun Wang, Wenjiao Han, Tao Wang, Dan Liu* School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, China * E-mail: [email protected]

ABSTRACT Durability under UV illumination remains as a big challenge of TiO2-based superhydrophobic coatings, with the photocatalytic effect causing degradation of low surface energy material over time resulting in surfaces losing their hydrophobicity. We report surfaces made from tolylene-2, 4-diisocyanate (TDI)/TiO2 hybrid networks that demonstrate superhydrophobicity and superior UV-durability. Structural and morphological studies reveal that the TDI/TiO2 hybrid networks are composed of TiO2 nanoparticles interconnected with TDI bridges, and then encapsulated by a TDI layer. Through controlling the fraction of TDI in the synthesis process, the thickness of the TDI encapsulation layer around the TDI/TiO2 hybrid networks can be varied. When the weight ratio of TDI:TiO2 is 5:1, the superhydrophobicity of the hybrid network surface remains almost unchanged after a month of continuous UV illumination. This hybrid network surface can also clean methylene blue solution through the synergistic effects of cation adsorption and photocatalysis, holding promising potential for applications toward reducing cation pollutions in both liquid and air environments.

KEYWORDS: TiO2; hybrid; network; UV durability; cation adsorption

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INTRODUCTION Superhydrophobic (SH) surfaces are defined as those having a water contact angle (WCA) larger than 150o and a water sliding angle (WSA) of less than 10o.1 This anti-wetting attribute gives SH surfaces numerous functions such as self-cleaning,2 oil-water separation,3 anti-fogging/anti-icing4, 5 and friction reduction.6 Since Onda et al.7 firstly reported artificial SH surfaces in the mid-1990s, a variety of methodologies and fundamental theories have been developed to fabricate various anti-wetting surfaces. In the early stages of SH research, techniques such as hydrothermal synthesis,8 lithography,9 self-assembly,10 electro-spinning,11 chemical vapor deposition12 and etching13 were employed to create SH surfaces. However, these approaches are not ideal for large-scale fabrication, either due to the use of expensive raw materials (e.g. fluorochemicals and nanotubes), or in that special equipment and multistep procedures are involved. Some other methods are promising for large-scale fabrication, for example soft blasting demonstrated by Menga et al.,14 but only soft substrates can be used. For practical applications, simple and substrate-independent methods such as dip coating, blade coating, and spray coating are increasingly employed to fabricate SH surfaces.15-17 It is generally accepted that the excellent anti-wetting performance of SH surfaces results from the extremely low contact area between water droplets and the surfaces, that is to say, the water droplets rest on the SH surfaces are in fact mostly supported by air cushions underneath. To achieve this condition, low surface energy materials and micro- and nanometer-sized protrusions are vital.18 Fabricating SH surfaces by using modified nanoparticles has proved to be an effective method, as those nanoparticles can offer rough textures and multifunctionality.19 One typical example is modifying TiO2 nanoparticles with low surface energy materials,20, 21 and such SH surfaces have superiority in practical applications because they have photocatalytic degradation ability to degrade the adsorbed organic pollutant (such as oil), which otherwise will reduce the hydrophobicity of normal SH surfaces upon adsorption, and then recover the surface to SH properties and maintain the original anti-wetting and self-cleaning performance for a relatively long period. However, the design of long-lasting SH surfaces with photocatalytic degradation functionality remains as a big challenge, because the strong photocatalytic effect of TiO2 will gradually degrade those low surface energy materials.22 For example, many SH surfaces used fluorosilane (FAS) which contains -CF2-, -CF2H, and -CF3 groups to lower the surface energy of TiO2 nanoparticles, 2


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but the reactive free radicals generated by TiO2 upon UV illumination will degrade these FAS materials and quickly render the surfaces hydrophilic.23 It was reported that hydrophobic polymer like PDMS could be used to impart the long-term UV durability, but the photocatalytic degradation efficiency of the PDMS-encapsulated TiO2 nanoparticles was also found to be reduced.24 Many successful research efforts have been devoted to enhance the mechanical stability of SH surfaces,25-28 but few works have been done to increase the lifetime of SH surfaces under UV or sunlight illumination. Tolylene-2, 4-diisocyanate (TDI) is a chemical base component commonly used in the polyurethane (PU) industry. TDI consists of two active isocyanate –NCO groups, which can react with hydroxyl and epoxy groups thus enabling the production of a wide range of crosslinked polymers, which have been applied in various industrial products.29 In this work, TDI/TiO2 hybrid networks have been created through a two-step reaction. The synthesized TDI/TiO2 hybrid nanoparticles were dispersed with an environmentally friendly PU adhesive and coated on different substrates via a scalable spray-coating method to create SH surfaces with an average WCA of 168o and a WSA of 1°. PU adhesives used here can serve to increase both the anti-wetting and surface mechanical properties.30 By varying the weight ratio of TDI during synthesis, TDI/TiO2 hybrid networks with different TDI encapsulation layer thicknesses can be obtained. When the TDI:TiO2 ratio is 5:1, the superhydrophobicity of the TDI/TiO2 hybrid network remains almost unchanged after a month of continuous UV illumination. Moreover, the cleaning efficiency of cationic pollutants (in methylene blue (MB) solutions) was also increased through the synergistic effects of cation adsorption and photocatalysis. We therefore demonstrate a novel approach to dramatically enhance the UV-durability of TiO2 based SH surfaces through the creation of TDI/TiO2 hybrid networks. Furthermore, the TDI/TiO2 hybrid network surfaces show superior cation adsorption ability, extending the application of this promising coating to air and water cleaning.

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EXPERIMENTAL SECTION Materials. Titanium tetrachloride (99.9%, Aladdin), TDI (98%, Shenshi Chemical) and dibutyltindilaurate (95%, Aladdin) were used as received. PU adhesive (ESD-530) was offered by Dulub Co. Ltd. All solvents (anhydrous ethanol, anhydrous benzyl alcohol, diethyl ether, acetone and xylene) were obtained from Shenshi Chemical. All chemicals were analytical grade and used as received. Synthesis of TDI/TiO2 hybrid networks. The synthesis of TiO2 nanoparticles was adopted from previous report.31 Here, 2.3 mL titanium tetrachloride and 8 mL anhydrous ethanol were added into a beaker and magnetically stirred in an ice-water bath. The mixed solution was then transferred into a 100 mL flask which contained 40 mL anhydrous benzyl alcohol. The solution was stirred at 80 °C for 9 h and then the resulting translucent solution was collected for further use. 12 mL TiO2 solution (TiO2 solid content about 27 mg/mL) was precipitated twice using diethyl ether. After centrifugalization, TiO2 nanoparticles were dispersed in acetone and sonicated for 30 mins. The dispersion was then transferred into a 100 mL flask. Different weight ratios of TDI were dissolved into acetone and then introduced into the flask drop by drop with 1 wt. % dibutyltindilaurate as the catalyst. The mixed solution was stirred at 60 °C for 16 h. The TDI modified TiO2 nanoparticles were separated by centrifuge at 5000 r/min for 10mins and purified by repeated washing with toluene. The final pale hybrid networks were obtained after vacuum drying at 85 °C for 6 h. The synthesized TDI/TiO2 hybrid networks with different ratio of TDI to TiO2 from 1:1 to 5:1 were named as TDI/TiO2 (1:1), TDI/TiO2 (2:1), TDI/TiO2 (3:1), TDI/TiO2 (4:1) and TDI/TiO2 (5:1). Preparation of superhydrophobic coating. 150 mg TDI/TiO2 was dispersed in 3 mL mixed solvents (1:2 acetone:xylene) for 30 mins assisted by ultrasonication, and 3 wt. % PU adhesive was added into the mixture. After stirring and dispersing at 50 °C for an hour, the dispersion was sprayed on different substrates using a spray gun (air pressure is 15 Psi, nozzle diameter is 0.5 mm, and distance from the substrate is 20cm. The gun is operated by hand and moved evenly across the substrate surface.), and then heated at 80 °C for one hour to form the SH surfaces.

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Characterization. X-ray diffraction (XRD) analysis of synthesized TiO2 was carried out using an X-ray diffractometer (D8 Advance, Bruker, Germany) with Cu-Ka radiation operated at 40 kV and 40 mA. The chemical compositions of TDI/TiO2 networks were analyzed by a Fourier transform infrared spectrometer (FT-IR) (Nicolet 6700, Thermo, America) in the wave number range of 4000-400 cm-1 and an elemental analyzer (CHNS) (Vario EL cube, Germany). The elemental analyzer oxidizes the sample into simple compounds (via combustion), which are then detected with thermal conductivity detection or infrared spectroscopy. The surface morphology was examined by scanning electron microscope (SEM) (S4800, Hitachi, Japan) with an applied bias of 5.0 kV. UV irradiation was carried out using a 48 W mercury-xenon lamp with 365 nm wavelength at a distance of 25 cm from the SH surfaces. The structure of TiO2 nanoparticles and TDI/TiO2 (1:1), TDI/TiO2 (5:1) hybrid networks was characterized by transmission electron microscopy (TEM) (JEOL, Japan). An ultraviolet-visible spectrophotometer (UV-Vis) (U-3900H, Hitachi, Japan) was used to monitor the adsorption process of organic molecules. The degradation of MB molecules under sunlight was measured in ambient air under simulated sunlight (100 mW cm−2) using a Newport 3A solar simulator, and light intensity was calibrated with silicon solar cell certiﬁed by the National Renewable Energy Laboratory (NREL). Dynamic Light Scattering Instrument (DLS, Nano-ZS ZEN3600, Malvern, England) was used to measure the Zeta potential in solution. The surface wettability was examined by a WCA instrument (Theta Attension, Biolin Scientific, Finland) at room temperature, and recorded by OneAttension software. The water droplet volume for contact angle measurement was 4 μL. On each sample, four water droplets were deposited on different areas to acquire the average WCA. At least two samples were tested for each experiment. The error bars in the plots of WCA and SA represent the standard deviations. Dynamic monitoring of WCA and WSA was also performed from the time 6 μL water droplet was landing on the target surface.

RESULTS AND DISCUSSION Composition and structure of TDI/TiO2 hybrid networks We have used titanium tetrachloride as the precursor to synthesize TiO2 nanoparticles, with ethanol and benzyl alcohol as solvents. The TiO2 nanoparticles prepared via this route are highly crystalline 5



and of high purity, which is confirmed by their typical X-ray diffraction peaks which are shown in Figure S1. The crystal size of individual TiO2 nanoparticles can be calculated according to the Scherrer equation as ca. 4.5 nm using the (101) diffraction peak, and the size is consistent with a previous report.32

a

c

hydrogen bond

-NCO (2271cm-1)

Intensity (a.u.)

b

Intensity (a.u.)

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

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-OOC-NH- (1713 cm-1)

-NH-OC-NH- (1666 cm-1)

1600

1400

1200

TDI TDI-TiO2 TiO2

Wavenumber (cm-1)

TiO2

C

N

O

4000 3500 3000 2500 2000 1500 1000

lone pair electrons

H

Wavenumber (cm-1)

500

Figure 1. (a) Two-step reaction process of TDI/TiO2 networks and (b) molecular model of TDI/TiO2 hybrid network (the TDI particles in the scheme and figure are not drawn to scale). (c) FT-IR spectra of pure TDI, TiO2 and TDI/TiO2 (5:1) hybrid network.

The synthesis of TDI/TiO2 hybrid network can be divided into two steps. As shown in Figure 1a, when the TDI solution was added dropwise into the TiO2 solution, the –NCO unit from TDI will react with the –OH unit on the surface of TiO2, thus building up an interconnected network composed of TiO2 nanoparticles and TDI bridges. With more TDI added, excess –NCO units will react with the N-H unit generated during the first step, anchoring more TDI molecules and finally forming a TDI encapsulation layer. A molecular model of the TDI/TiO2 hybrid network is illustrated in Figure 1b, in which OyyyH hydrogen bonds are also highlighted. 6


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Surface chemical differences of the TiO2 nanoparticles before and after TDI modification were characterized using FT-IR (see Figure 1c). For the spectrum of the TDI/TiO2 (5:1) hybrid network, the wide characteristic absorption peak from 3700-3200 cm-1 pertains to the stretching vibration peak of the N-H bond with hydrogen bonding. The stretching vibration strength of –NCO at 2271 cm-1 in the TDI spectrum decreases significantly in the TDI/TiO2 (5:1) hybrid network spectrum, confirming the reaction between –NCO and –OH in the first step. We find only a small –NCO characteristic peak still exists in the spectrum of TDI/TiO2 (5:1) hybrid network, since these active – NCO groups are thought to remain after the synthesis of TDI/TiO2 hybrid networks as a result of the space steric effect. A carbonyl vibration peak appearing at 1713 cm-1 is attributed to the –CO– part of the dense and hydrogen-bonded carbamate group (–NH–COO–). The characteristic absorption peak at 1666 cm-1 originates from –CO– of the allophanate group (–NH–CO–NH–), which is generated during the second step of the reaction.33

Figure 2. (a) TEM images of our synthesized TiO2 nanoparticles and TDI/TiO2 (1:1), TDI/TiO2 (5:1) hybrid networks. (b) Schematics of TiO2 nanoparticles, TDI/TiO2 (1:1) and TDI/TiO2 (5:1) hybrid networks.

TEM characterization was performed to verify the structure of TiO2 nanoparticles and the TDI/TiO2 (1:1), TDI/TiO2 (5:1) hybrid networks. An average TiO2 nanoparticle size (that is, 7



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aggregated TiO2 crystals) of around 25 nm can be determined from Figure 2a. The TDI/TiO2 (1:1) hybrid network (Figure 2b) is made of TiO2 nanoparticles interconnected by TDI bridges, and a thin TDI encapsulation layer is around the TDI/TiO2 hybrid. As the TDI content increases (see TDI/TiO2 (5:1) hybrid network in Figure 2b), the thickness of the encapsulating TDI layer increases. Schematics of TiO2 nanoparticles, TDI/TiO2 (1:1) and TDI/TiO2 (5:1) hybrid networks are shown in Figure 2b. In order to further confirm that TDI has been introduced to construct the TDI/TiO2 hybrid network, C, H, N elemental analysis has been done with TDI/TiO2 (1:1) and TDI/TiO2 (5:1) hybrid networks. The element fractions are summarized in Table 1, here each test has been done twice and the final weight ratios are average values. The weight ratio of oxygen was calculated by 16/14 (relative atomic mass ratio) of nitrogen, as the introduction of one TDI molecule will add 2 nitrogen atoms and 2 oxygen atoms (oxygen atoms from TiO2 are not included here). By summing up the weight ratios of N, C, H, and O, we can get the content of the TDI component. From the elemental analysis results we can conclude that the final weight ratio of TDI to TiO2 in the hybrid network is very close to the original ratio we added during synthesis, suggesting the complete reaction between TDI and TiO2. Table 1. Elemental analysis of TDI/TiO2 (1:1) and TDI/TiO2 (5:1) hybrid networks.

Sample

N (wt. %)

C (wt. %)

H (wt. %)

O (wt. %)

TiO2 (wt. %)

TDI:TiO2

1:1

7.8

30.1

4.3

8.9

48.9

1.04:1

5:1

13.1

50.5

5.5

15

15.9

5.29:1

(*The elemental analyzer oxidizes the sample into simple compounds (via combustion), which are then detected with thermal conductivity detection or infrared spectroscopy. Here the weight ratio of oxygen is obtained by calculation and does not contain oxygen contribution from TiO2, the weight ratio of TiO2 is obtained by using 1 minus the percentage of all other elements.)

TDI/TiO2 hybrid network based superhydrophobic surface Figure 3a shows the general procedure of preparing the SH surfaces. By using the simple spray coating method, the as-prepared dispersions can be deposited onto various substrates, such as glass, metal and paper. The dyed water droplets can keep a spherical shape on the coated surfaces but spread on the uncoated surfaces (see Fig 3a), suggesting that our coatings are incredibly versatile 8


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and can be applied to a wide range of surfaces. With varying TDI content in the hybrid network, an average WCA of 168o and an average WSA of 1o are obtained (Figure 3c). The TDI weight ratio has only a minor influence on the SH properties of the TDI/TiO2 hybrid network surfaces, although a ratio of 5:1 does give the highest WCA approaching 170°. Two videos demonstrating the anti-wetting properties of these films are shown in Supporting Information. Video 1 shows water droplets bouncing on the SH surfaces and video 2 shows the contact process of water droplets with the SH surface. Both of the two videos demonstrate high anti-wetting properties, as little adhesion can be seen between the water droplets and the surface. The SEM surface morphologies of our fabricated SH surfaces prepared using hybrid networks of TDI/TiO2 (1:1), TDI/TiO2 (3:1) and TDI/TiO2 (5:1) are shown in Figure 3b. Rough structures on the surfaces are formed by the presence of different sized aggregates. We observe minor morphological differences among the three TDI/TiO2 hybrid networks, suggesting that the roughness is not strongly influenced by the component ratio. These similar chemical- and surface- structures of the various TDI/TiO2 hybrid networks lead to similar WCA and WSA values shown in Figure 3c.

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Figure 3. (a) The facile preparation procedure of the SH surfaces, and photos showing dyed water droplets on coated and uncoated substrates (glass, metal, paper). (b) SEM images showing surface morphologies of our fabricated SH surfaces prepared using hybrid networks of TDI/TiO2 (1:1), TDI/TiO2 (3:1) and TDI/TiO2 (5:1). Schematic of water supported by the rough SH surface is also presented. (c) WCA and WSA values of the spray-coated SH surfaces prepared using different TDI/TiO2 hybrid networks, without the presence of PU adhesive. (d) WCA and WSA values of the spray coated TDI/TiO2 (5:1) surfaces with different concentration of PU adhesive. (e) Schematic shows how PU adhesives work in the SH coatings.

Mechanical stability is one of the major challenges of SH surfaces. As the rough structures of SH surfaces are always inherently fragile, mechanical forces like friction and abrasion will easily break the surface structure and consequently decrease the SH properties. FT-IR analysis suggests that there are a small amount of active –NCO units left in the TDI/TiO2 hybrid networks, and this inspires us to use chemical reactions between adhesives and the –NCO unit to strengthen the mechanical stability of our SH coatings. PU adhesives have been widely applied for their excellent shear strength and impact resistance, and are suitable for various substrates like wood, glass, metal, paper, and ceramics. Here, we combined TDI/TiO2 (5:1) hybrid networks with an environmentally-friendly, PU adhesive to improve the mechanical properties of our SH surfaces. The WCA and WSA values of the TDI/TiO2 (5:1) hybrid network surfaces with different concentration of the PU adhesive are plotted in Figure 3d, from which we can conclude that good SH properties are maintained when the weight ratio of the PU adhesive is less than 3%. The evolution of contact and sliding angles with time when the SH surfaces are in contact with aqueous droplets have been monitored, and the results are shown in Figure S2. After one hour’s contact, the WCA is 154Ϩand the WSA is 11Ϩ. These results indicate that although the surface layer structure of the TDI/TiO2 particles contains bonds which are prone to hydration, the surface can still maintain SH properties after contact with aqueous droplets. The test was operated for only one hour because the volume of the water droplet decreased quickly as shown in Figure S2, which has been previously shown to influence WCA values.34 The retention of a high WCA is possible because the hydrophilic –OH groups from PU adhesives will react with the –NCO units that were left during the synthesis. FT-IR spectra of TDI/TiO2 (5:1) and its blend with PU are shown in Figure S3. We can see that the characteristic peak of –OH at 3431 cm-1, which appears in the spectrum of the pure PU adhesive, disappears in the spectrum of the blends (with 3 wt. % of PU). Furthermore the relative 11



strength of –NCO at 2271 cm-1 from TDI/TiO2 (5:1) decreases in the blend, indicating the reaction between –NCO from TDI/TiO2 (5:1) and –OH from the PU adhesives. With further increase of PU content, the WCA decreases quickly and the WSA increases. This is because with the addition of more adhesive, the hydrophilic –OH groups from PU accumulate on the surface to reduce the SH properties. Figure 3e is the schematic which showing how PU adhesives work with the hybrid networks. So, 3 wt. % of the adhesive was used to improve the mechanical properties of our SH surfaces. To confirm the improved mechanical properties of the TDI/TiO2 hybrid network surfaces, a surface sand paper abrasion test was carried out. Here, we placed a weighted (100 g) film face-down on sandpaper and dragged for a distance of 10 cm in two vertical directions (Figure S4a) This process is defined as one abrasion cycle. For the SH coating without the PU adhesive (Figure S4b, left photo), the coating was almost completely removed after 5 cycles’ abrasion test, with the bare glass exposed and thus the surface became hydrophilic. For the SH coating containing 3 wt. % PU adhesive (Figure S4b, right photo), the coating withstood the abrasion test and showed no mechanical damage, and the surface maintained the original SH properties, suggesting that the mechanical stability was improved by the addition of the adhesive. The WCA and the morphology differences have been shown in Figure S4c, we can see that after 10 cycles’ abrasion the WCA remains around 155o, though the surface structure was broken to some extent , the low surface energy materials and a rough structure kept still thus reserved the SH properties for the surfaces. UV durability The UV durability of SH surfaces prepared using different TDI/TiO2 hybrid networks were studied by monitoring the WCA values while exposing the SH surfaces to UV light. The results in Figure 4a show that, under the same UV conditions, the durability behaviors are different with varying TDI content. When the weight ratio of TDI to TiO2 is 1:1, the WCA decreased to ca. 140o after 100 h UV illumination. When the TDI content is increased, the UV durability increases. Encouragingly, the SH surfaces fabricated from the TDI/TiO2 (5:1) hybrid network can still maintain a WCA of 164o after 700 hours UV illumination, showing much stronger durability than comparable materials in other reports (see summary and comparison in Table 2). Table 2 lists the lifetime of hydrophobic surfaces fabricated by TiO2 based hybrids which were modified by FAS, octadodecylphosphonic acid (ODP), C6F12, [C12mim] Br, octadecyltrimethoxysilane (OTS), 12


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polytetrafluoroethylene (PTFE) or polydimethylsiloxane (PDMS). As there is no defined standard testing methodology for UV durability, the testing conditions for each report are also described alongside. In order to make a comparison with those literature reports, we put TDI/TiO2 (5:1) SH surfaces under 3 different testing conditions as follows: (i) 48W 365 nm mercury-xenon lamps with a lamp-sample distance of 25 cm (light intensity on the coating surface was measured to be 2 mW/cm2), (ii) 48W 365 nm mercury-xenon lamps with a lamp-sample distance of 1 cm (light intensity on the coating surface was measured to be 30 mW/cm2), and (iii) natural light outside (from September 2017 - July 2018, Wuhan, China). After all these tests, the WCA of our SH surfaces decreased by only 4o, showing dramatically improved UV durability in comparison to other reports. Table 2. Lifetime comparison of different TiO2-based hydrophobic surfaces. Film

Light illumination

Illumination time

WCA Change (before to after)

FAS modified TiO2 surface35

Solar lamp (~3.5 mW/cm2)

48 h

~145° to ~0°

UV light (~5 mW/cm )

60 mins

~165° to ~0°

C6F12 modified TiO2 surface

Natural light outside

150 days

~137° to ~101°

[C12mim]Br modified TiO2 surface38

UV light (300 W Hg lamp with a filter centered at 365 nm and located 18 cm)

120 mins

~152° to ~10°

OTS modified TiO2 surface39

UV light (~2 mW/cm2)

120 mins

~136° to ~65°

PTFE modified TiO2 surface

UV light (under 2*15W 352 nm Sankyo Denki BLB lamps)

60 mins

~152° to ~95°

PDMS modified TiO2 surface41

12w UV lamp with emission at 365 nm

318 h

~123° to ~115°

TDI/TiO2 (5:1) surface (this work)

UV light (48W 365 nm mercury-xenon lamps with a distance of 25 cm, ~2 mW/cm2) UV light (48W 365 nm mercury-xenon lamps with a distance of 1 cm, ~30 mW/cm2) Natural light outside

30 days

~168° to ~164°

30 days

~168° to ~164°

300days

~168° to ~163°

36

ODP modified TiO2 surface

37

40

2

13



a

b

170

Oleic acid added at t=0

180

160 150 140

TDI/TiO2 (1:1)

WCA (degree)

WCA (degree)

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

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TDI/TiO2 (3:1)

130 100

200

300

400

500

600

140 120 100 80 TDI/TiO2 (1:1)

60 UV switched on at 10 mins

40

TDI/TiO2 (5:1)

0

160

20

700

0

20

40

60

80

TDI/TiO2 (3:1) TDI/TiO2 (5:1)

100 120 140

Time (h)

Time (h)

Figure 4. (a) Durability tests of TDI/TiO2 hybrid network based SH surfaces under UV illumination (48W 365 nm mercury-xenon lamps with a lamp-sample distance of 25 cm, the light intensity on the coating surface is ~2 mW/cm2). (b) The loss of hydrophobicity after adding oleic acid, and UV light assisted self-healing ability. (48W 365 nm mercury-xenon lamps with a lamp-sample distance of 1 cm, the light intensity on the coating surface is ~30 mW/cm2).

The loss of hydrophobicity of our SH surfaces after adding oleic acid, and the UV illumination assisted self-healing properties are also evaluated and shown in Figure 4b. When one droplet of oleic acid (25ul) was dropped on the SH surface, for all TiO2/TDI ratios the WCA decreased significantly due to the presence of the hydrophilic –COOH group from oleic acid. After UV illumination, we can see that the polluted surface of the TDI/TiO2 (1:1) coating recovered its SH properties, with the WCA returning to 163° after ca. 15h, indicating the photocatalytic function of TiO2 degraded the oleic acid on the surface. Note that the reduced WCA (recovered 163° as opposed to original 168°) of the TDI/TiO2 (1:1) surface after 15 h UV illumination is due to the photocatalytic-induced degradation of organic TDI, and with UV illumination time going on, the surface WCA gradually lost its SH properties seen from the black dotted line in Fig 4b. For the polluted TDI/TiO2 (3:1) surfaces, the self-healing process was slower than that of TDI/TiO2 (1:1), with the WCA recovering to 160o after about 130 h. As for the polluted TDI/TiO2 (5:1) surfaces, the WCA self-healing was even slower, and the total recovery period took about two weeks. The results suggest that a thicker TDI encapsulation layer around TiO2 will strengthen the UV durability, the UV-induced self-healing ability is restrained, as a thicker TDI layer will block more UV from reaching the TiO2 surface, and thus the photocatalytic effects are hindered. 14


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Cation adsorption Normally, “self-cleaning properties” in the context of SH surfaces can be referenced with the classic example of water droplets rolling on lotus leaves and taking away dust from the leaf’s surface; the so-called “lotus effect”.42 Numerous researchers have done similar tests by using water droplets to clean dust from artificial SH surfaces to monitor the self-cleaning process. However, few reports have mentioned the adsorption cleaning performance of SH surfaces. For example, there are many organic molecules or (normally positively charged) metal ions in waste liquids from factories that are not easily cleaned by the aforementioned self-cleaning method. Here, we show a two-step cleaning process that demonstrates the superior adsorption ability of cations in MB pollution by the TDI/TiO2 hybrid networks. In this test, 6 mg TiO2, TDI/TiO2 (1:1) or TDI/TiO2 (5:1) hybrid nanoparticles were added in separate glass vials containing MB solutions with 10 mg MB dissolved in 20 mL acetone:water (1:2) mixed solvent solution. The use of acetone ensures the solution can wet the SH surface and interact with the MB.43 Each bottle was put in a dark room and stirred for one hour, before being moved to a solar simulator to receive constant light illumination of the comparative intensity of one sun. Analysis of the UV-Vis spectrum was used to determine the concentration changes of the MB solution, with the intensity of the spectrum corresponding to the solid content of MB in the solution (Beer-Lambert Law). As shown in Figure 5a-c, there is a big difference in MB concentration before (black curve) and after (red curve) the first hour of stirring in dark. The solution of TDI/TiO2 (5:1) is observed to have the strongest adsorption ability, as the intensity of the absorption spectrum reduces the most. From Figure 5d we can see that the relative concentration (C/C0) of MB after one hour adsorption by nanoparticles was only 6.1% left in the solution containing the (5:1) hybrid network, while there is 15.3% left for the solution with TDI/TiO2 (1:1) and 48.4% left for the solution with only TiO2 nanoparticles. For the next tests, the solutions were moved to a solar simulator, where we can observe the sunlight-assisted degradation of MB. After one hour of simulated sunlight illumination, the relative concentration C/C0 of MB remaining in the solution was just 3.4% for that containing the TDI/TiO2 (5:1) hybrid network compared to 6.3% for the pure TiO2. Under dark stirring conditions, the TDI/TiO2 network is more efficient in removing MB than pure TiO2. Hence, there is more MB remaining in the TiO2 sample, which is then efficiently removed during sunlight illumination as shown in figure 5a. However, the total efficiency (including dark stirring and sunlight illumination) is higher in the 15



TDI/TiO2 network systems, with the best efficiency seen in the solution containing the TDI/TiO2 (5:1) hybrid network, indicating that a thicker TDI encapsulation layer will enhance the MB pollution cleaning efficiency in a short period of time. In order to demonstrate the viability of this cation adsorption effect on a coating, we performed a similar test by placing a spray-coated SH film in a chamber filled with MB acetone/water in dark ambient. As shown in Figure S5, both TiO2/TDI (1:1) and TiO2:TDI (5:1) films were able to successfully remove MB from the solution, since the cleaning rate is slowed down due to the reduced contact area between the SH surfaces and MB pollution. Furthermore, when removed from solution and dried, the TiO2:TDI (5:1) SH film showed some damage due to being partly dissolved by acetone, a WCA of 152o (see Figure S5c) was determined. This demonstrates the promising function of these SH coatings to adsorb cations. In order to find out the adsorption mechanism, we used negatively charged methylene orange (MO) to repeat the adsorption tests. From Figure 5e we can see that even after 12 h stirring in the dark, there is no MO reduction in the solution, suggesting that an adsorption process as a result of electrostatic adsorption is the primary driving force for cation adsorption in MB solution. This can also be illustrated through the chemical structure of TDI/TiO2 (shown in Figure 1b), which contains a lot of oxygen atoms with free electrons, leading to the negatively charged nature of the synthesized TiO2/TDI networks. This is further proved by Zeta potential tests, in which the values for both the TiO2/TDI networks and TiO2 are less than -10 mW (Figure 5f).44 These negatively charged networks will bond to positively charged MB molecules (Figure 5g) because of strong electrostatic interactions.45, 46 Since the photocatalytic effect is bestowed by the TiO2, the question arises as to the mechanism by which MB molecules are able to interact with the TiO2 through the TDI encapsulation layer, i.e. the structure must be porous to some degree. We speculate that there may be some tunnels in the network structure which would allow a few MB molecules to diffuse into the network and contact with TiO2 nanoparticles. A further suggestion relates to the mechanism of the photocatalytic reaction: upon absorption of photons with energy larger than the band gap of TiO2, electrons are excited from the valence band to the conduction band, creating electron-hole pairs. Radicals such as •OH will then be produced on the surface of TiO2, which could diffuse out of the network and decompose MB molecules at the surface or within the encapsulation layer.47 We envisage that other positively charged organic particles such as dust (PM 2.5 particles) or bacteria in the air can also likely be adsorbed and gathered on these TDI/TiO2 hybrid network SH 16


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surfaces. Different to the “lotus effect”, electrostatic interactions give these SH surfaces the ability to capture dust which is retained on the surface (and later removed by natural liquids such as rain droplets). Following methodology used by Ramakrishna et al.48, a test simulating this dust retention and liquid droplet cleaning is shown in Figure S6 and video 3, where we found that magnesium powder stuck to the coating, but could easily be removed with water droplets. Cleaning environmental contaminants is a useful feature for a coating, and an additional beneficial feature over traditional self-cleaning surfaces.

17



Figure 5. UV–Vis absorption spectra over time for MB solutions containing a) TiO2, (b) TDI/TiO2 (1:1) hybrid network and (c) TDI/TiO2 (5:1) hybrid network particles (the illuminated samples were first stirred in dark for 1 hour before illumination). (d) Column chart of MB concentration changes over time. The values 18


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were calculated using the main absorption peaks of MB at ca. 654 nm. (e) UV–Vis absorption spectra of pure MO solution and those containing TiO2, TDI/TiO2 (1:1) and TDI/TiO2 (5:1) hybrid networks after 12 h stirring in dark ambient conditions. (f) Zeta potential measurement of nanoparticles in pure water. The hybrid networks and pure TiO2 were mixed with water and stirred for a whole night, then after standing still for one hour, the supernatant fractions were used as testing samples. (g) The chemical structures of MB and MO.

CONCLUSION In summary, TDI/TiO2 hybrid networks have been constructed by polyisocyanate TDI -modified TiO2 nanoparticles, and employed to fabricate SH surfaces via a scalable spray-coating technique on various substrates such as glass, metal and paper. A series of TDI/TiO2 hybrid networks with various thickness of the TDI encapsulation layer have been obtained through controlling the weight ratio of TDI:TiO2 during synthesis. 3% PU adhesive was used to strengthen the mechanical properties of these SH surfaces through chemical bonding between –OH from the adhesives and – NCO from the TDI/TiO2. An average WCA of 168° and a WSA of less than 2° can be obtained on these TDI/TiO2 hybrid network based SH surfaces. TDI/TiO2 (5:1) based SH surfaces can maintain a WCA of 164o after 30 days of strong UV illumination, which is a dramatic improvement in UV durability compared to previous reports. Furthermore, a thicker TDI encapsulation layer in the TDI/TiO2 hybrid networks is shown to enable a better adsorption ability of cationic pollutants. Our novel TDI/TiO2 hybrid network is shown to strengthen the UV durability of TiO2 based multifunctional SH surfaces, and holds promising potential for applications towards removing cationic pollutants in both liquid and air environments.

ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publication website at DOI:xxxxx Some supplementary experimental details, supplementary XRD date, time evolution of WCA and volume of water droplets, FT-IR spectra, abrasion tests, coating absorption tests and self-cleaning tests. 19



AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D.L.) ORCID: Dan Liu: 0000-0003-1312-9754 Tao Wang: 0000-0002-5887-534X Notes The authors declare no competing financial interest. REFERENCES: 1. Li, X. M.; Reinhoudt, D.; Crego-Calama, M. What Do We Need for a Superhydrophobic Surface? A Review on the Recent Progress in the Preparation of Superhydrophobic Surfaces. Chem. Soc. Rev. 2007, 36, 1350-1368. 2. Bhushan, B.; Jung, Y. C.; Koch, K. Self-Cleaning Efficiency of Artificial Superhydrophobic Surfaces. Langmuir 2009, 25, 3240-3248. 3. Xia, C. B.; Li, Y. B.; Fei, T.; Gong, W. L. Facile One-Pot Synthesis of Superhydrophobic Reduced Graphene Oxidecoated Polyurethane Sponge at the Presence of Ethanol for Oil-Water Separation. Chem. Eng. J. 2018, 345, 648-658. 4. Lomga, J.; Varshney, P.; Nanda, D.; Satapathy, M.; Mohapatra, S. S.; Kumar, A. Fabrication of Durable and Regenerable Superhydrophobic Coatings with Excellent Self-Cleaning and Anti-Fogging Properties for Aluminium Surfaces. J. Alloys.Compd. 2017, 702, 161-170. 5. Nguyen, T. B.; Park, S.; Lim, H. Effects of Morphology Parameters on Anti-Icing Performance in Superhydrophobic Surfaces. Appl. Surf. Sci. 2018, 435, 585-591. 6. Song, Y.; Nair, R. P.; Zou, M.; Wang, Y. A. Adhesion and Friction Properties of Micro/Nano-Engineered Superhydrophobic/Hydrophobic Surfaces. Thin Solid Films 2010, 518, 3801. 7. Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. J. L. Super-Water-Repellent Fractal Surfaces. Langmuir 1996, 12, 2125-2127. 8. Wang, D. A.; Guo, Z. G.; Chen, Y. M.; Hao, J.; Liu, W. M. In Situ Hydrothermal Synthesis of Nanolamellate CaTiO3 with Controllable Structures and Wettability. lnorg. Chem. 2007, 46, 7707-7709. 9. Kothary, P.; Dou, X.; Fang, Y.; Gu, Z. X.; Leo, S. Y.; Jiang, P. Superhydrophobic Hierarchical Arrays Fabricated by a Scalable Colloidal Lithography Approach. J. Colloid Interface Sci. 2017, 487, 484-492. 10. Liu, G. Y.; Wong, W. S. Y.; Nasiri, N.; Tricoli, A. Ultraporous Superhydrophobic Gas-Permeable Nano-Layers by Scalable Solvent-Free One-Step Self-Assembly. Nanoscale 2016, 8, 6085-6093. 11. Wu, J.; Li, X.; Wu, Y.; Liao, G. X.; Johnston, P.; Topham, P. D.; Wang, L. G. Rinse-Resistant 20


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TOC GRAPHICS

+ TDI

TDI Encapsulation layer TiO2

+ TDI

TDI bridge

TDI/TiO2 (1:1)

TiO2

TDI/TiO2 (5:1)

170

Oleic acid added at t=0

180

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130

WCA (degree)

WCA (degree)

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60 UV switched on at 10 mins

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TDI/TiO2 (5:1)

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