Rational Design and Construction of Well-Organized Macro

May 11, 2017 - Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology and Key Laboratory of Photochemical Conversion and ...
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Rational Design and Construction of Well-Organized MacroMesoporous SiO2/TiO2 Nanostructure toward Robust HighPerformance Self-Cleaning Antireflective Thin Films Binbin Jin,†,‡ Junhui He,*,† Lin Yao,† Yue Zhang,† and Jing Li† †

Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Antireflection (AR) thin films on optical substrates are of great significance in high-performance optoelectronic devices. Here, we present a rational design and construction of well-organized macromesoporous nanostructure toward robust high-performance self-cleaning antireflective thin films on the basis of effective medium theory and finite difference time domain (FDTD) simulations that combine the optical design principle. A hierarchical macro-mesoporous SiO2 thin film with very high porosity and gradient refractive indexes works as a λ/4-wavelength AR layer and significantly suppresses the reflection in the range from 350 to 1200 nm. Even after dip-coating a layer of high refractive index TiO2 nanocrystals, the nanostructured thin film still exhibits broadband AR properties which are much superior to conventional flat SiO2/TiO2 thin films, especially in the range of 350−500 nm. In addition, the obtained thin film exhibits photocatalytic self-cleaning and durable superhydrophilicity. The advantages brought by the well-organized macro-mesoporous structure are also testified through comparing to the solely mesoporous SiO2/ TiO2 film counterpart. Moreover, the pencil hardness test and sandpaper abrasion test show favorable robustness and functional durability of the thin film, which make it extremely attractive for practical applications in optical devices, display devices, and photovoltaic cells. KEYWORDS: macro-mesoporous structure, SiO2/TiO2, thin film, antireflection, superhydrophilic, self-cleaning, robust

1. INTRODUCTION The reflection of light at the surface or interface of transparent substrates, in most cases, is undesirable. For example, reflection loss results in reduced efficiency of solar energy modules and poor visibility of the screen. Antireflection (AR) films, with precisely controlled thickness and refractive index, enable the light reflecting off the surfaces interfere destructively, thus minimizing the reflection loss.1 To realize antireflection, two well-known criteria are involved: (i) the thickness of the film should be λ/4nc, where λ is the incidence wavelength and nc is the refractive index of the film; (ii) nc = (nans)1/2, where na and ns are the refractive index of air and the substrate, respectively. For most optical substrates with refractive index of around 1.5, a single-layer with an unavailable low refractive index of 1.22 is required to satisfy the second requirement. This challenge is often overcome by introducing porous structure in films and the effective refractive index of which can be deduced by effective medium theory such as 2-dimensional Bruggeman model for air-material mixture.2−5 For example, porous polymers have been proposed as AR films through polymers phase separation followed by subsequent removal of one phase.6−8 Other methods include the fabrication of nanoparticle AR films by random packing, electrostatic interaction as © XXXX American Chemical Society

well as the controlled assembly of mesoporous or/and hollow silica nanoparticles, charged colloids and polyelectrolyte multilayers.9−13 Unfortunately, the obtained films usually suffer from poor mechanical durability. The main reason is that the packed nanoparticles are bound to each other and to the substrate only by van der Waals force. Moreover, the AR range of wavelength and incidence angle of most reported porous films need to be further broadened to meet the requirements of practical applications. An alternative method to realize broadband and omnidirectional antireflection takes inspiration from regular nanoarray of protuberances in moth eyes.14 Moth-eye nanostructured films appear to have a gradient refractive index and eliminate optical interface from air to substrate. A variety of nanoarrays such as nanopyramids,15 nanocones,16 nanograss,17 and nanorods18 have been achieved by nanoimprint lithography,19,20 electronbeam lithography,15,21 self-assembly,22 and phase separation.23 Although moth-eye nanostructured films demonstrate nearperfect antireflection properties, their high aspect ratio Received: March 23, 2017 Accepted: May 5, 2017

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DOI: 10.1021/acsami.7b04140 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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catalytic activity, both intrinsic and photoinduced superhydrophilicity and antifogging property. The pencil hardness test and sandpaper abrasion test showed excellent mechanical robustness and functional durability of the film. The advantages brought by the well-organized macro-mesoporous structure were also testified through comparing to the solely mesoporous SiO2/TiO2 film counterpart.

geometry raises problems such as being easy to destroy or the deformation in practical applications.24 Recently, our group had successfully fabricated a robust moth-eye-like structure through a simple two-step method.25 Despite exhibiting broadband and omnidirectional antireflection properties as well as structural integrity, the nanostructured film did not fully reflect the ideal variation of refractive index in terms of height. Moreover, the superior AR performance may readily disappear due to contamination. 11 As a result, the research trends in antireflection would be multifunctionality, functional integration, as well as robustness, which may be achieved by combining multiscale structures and chemical compositions. TiO2 materials, which are nontoxic, abundant, and easily available, possess fascinating photoinduced superhydrophilicity and photocatalytic self-cleaning properties.26,27 Introducing such materials into SiO2 films has been shown to be a promising way to improve the functional durability and mechanical robustness of AR films.28,29 Recent researches of SiO2/TiO2 materials have been focused on alternate coating of the two materials,30,31 codeposition of SiO2/TiO2 nanoparticles,32 construction of SiO2/TiO2 core−shell structures,33 as well as assembly of polymer-derived meso-macroporous TiO2/SiO2 film.34 Our group fabricated multifunctional films by successively depositing a mesoporous SiO2 layer, SiO2 nanoparticles, and TiO2 nanoparticles on a glass substrate.30 Although equipped with durable superhydrophilicity and high photocatalytic activity, the transmittance of films decreased significantly after dip-coating high refractive index TiO2 nanoparticles. In fact, improvement of film properties meets conflicting requirements. On one hand, high transmittance prefers a nearly flat and low roughness surface as well as an appropriate refractive index. On the other hand, large surface area, high porosity, and roughness are required for films with excellent wettability and photocatalytic activity. Therefore, it is necessary to finely balance these conflicting requirements toward high performance AR films. In the present study, SiO2/TiO 2 multilayered films possessing well-organized macro-mesoporous structures were constructed on glass substrates. The films consist of three layers: (i) a mesoporous SiO2 layer, (ii) a macro-mesoporous SiO2 layer, and (iii) a macro-mesoporous TiO2 layer. In order to acquire high photocatalytic activity, durable superhydrophilicity and mechanical robustness as well as high transmittance simultaneously in one film, the following design concepts are taken into consideration: (i) The first mesoporous SiO2 layer plays an important role in regulating optimal AR regions by varying its thickness to match a specific application. (ii) The well-organized macro-mesoporous structure constructs a gradient refractive index profile, which facilitates elimination of the optical interface between air and the substrate. (iii) The very high porosity of the macro-mesoporous SiO2 layer guarantees high transmittance of the film despite dip-coating of a high refractive index TiO2 layer. (iv) The third macromesoporous TiO2 layer offers large surface area and high roughness (in the range of critical conditions for AR effect),35 which significantly enhances the superwettability and photocatalytic properties of the film. Moreover, the outermost TiO2 layer endows the film with excellent mechanical performance. The obtained macro-mesoporous SiO2/TiO2 film exhibited broadband antireflection with an average reflectance of 3.45% in the wavelength range of 400−1200 nm, only 0.6% higher than that of macro-mesoporous SiO2 film. Furthermore, the macro-mesoporous SiO2/TiO2 film demonstrated high photo-

2. EXPERIMENTAL SECTION 2.1. Materials. Aqueous ammonia (25%), absolute ethanol (99.5%), cetyltrimethylammonium bromide (CTAB), Triton X-100, cyclohexane, acetylacetone, and methylene blue (MB, 98.5%) were purchased from Beihua Fine Chemicals. Tetraethyl orthosilicate (TEOS, 99+%) and titanium(IV) n-butoxide (TNBT, 98+%) were obtained from Alfa Aesar. All chemicals were of analytic grade and used without further purification. Polystyrene (PS) was prepared according to our previous studies.36 Ultrapure water with a resistivity higher than 18.2 MΩ cm was used in all experiments and was obtained from a three-stage Millipore Mill-Q Plus 185 purification system (Academic). 2.2. Preparation of SiO2 Precursor Solution. The SiO2 precursor solution was prepared by dissolving 0.08 g of CTAB in a mixture solvent of 35 mL of water and 15 mL of ethanol, followed by addition of 5 μL of aqueous ammonia and 40 μL of TEOS under stirring. 2.3. Preparation of TiO2 Precursor Solution. A total of 13.0 g of Triton and 75 mL of cyclohexane were mixed under vigorous stirring, followed by dropwise addition of 10 g of TNBT. The mixture was stirred at room temperature for 60 min. After that, 10 mL of acetylacetone was added to stabilize the sol. The sol was diluted by cyclohexane until the volume fraction of TNBT dropped to 2%. 2.4. Fabrication of Macro-Mesoporous SiO2/TiO2 Films. The well-organized macro-mesoporous structure was derived from the mesopore template of CTAB micelles and the macropore template of PS nanospheres. First, a glass substrate was sonicated in deionized water for 15 min followed by oxygen plasma (84 W, 5 min) treatment. The cleaned glass substrate was then immersed in the SiO2 precursor solution, allowing the growth of a mesoporous SiO2 layer under quiescent conditions in a Teflon container at 60 °C for 20 h. The substrate coated with mesoporous SiO2 layer was calcinated at 550 °C for 3 h to remove CTAB. After that, PS nanospheres were assembled on the surface of the mesoporous SiO2 layer with a concentration of about 0.5 wt % at constant temperature and humidity by a vertical deposition method. Subsequently, another mesoporous SiO2 layer was grown in a Teflon container containing the SiO2 precursor solution at 60 °C for 10 h, and the PS nanospheres as well as CTAB were removed by calcination at 550 °C for 3 h. Finally, the precoated substrate was immersed in the TiO2 precursor solution for 30 s and withdrawn at 150 mm min−1 followed by eventual crystallization at 550 °C for 3 h. 2.5. Characterization. The morphology of films were observed by scanning electron microscopy (SEM) on a Hitachi S4800 filedemission scanning electron microscope at 5 kV. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2100F at 200 kV. Small pieces were scratched from mesoporous SiO2 film and macro-mesoporous SiO2/TiO2 film, dispersed in ethanol under ultrasonication and casted on holey carbon grids for TEM observation. The roughness of films was characterized by atomic force microscopy (AFM) on a MM8-SYS scanning probe microscope (Bruker AXR). Transmission and reflection spectra in the wavelength range of 350− 1200 nm were recorded using a Varian Cary 5000 UV−vis-NIR spectrophotometer. Water contact angles (WCAs) of film surfaces were measured at ambient temperature on Kino SL200B/K automatic contact angle meter. Water droplets of 3 μL were dropped carefully on the film surfaces. Three different areas were measured for every sample surface. X-ray diffraction (XRD) patterns of the prepared TiO2 were recorded on a Bruker D8 Focus X-ray diffractometer using Cu Kα radiation (λ = 0.154184 nm) B

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Figure 1. (a) Structural model of the nanostructured film, (b) effective refractive index profile of the film, the unit cell is also shown, (c) schematic diagrams of two-dimensional views (xy, xz) of the simulation model, the applied boundary conditions (B.C), the direction of incident wave propagation (K) and electric field (E) for FDTD simulations are also shown. In simulations, we set S and L as constants of 160 and 75 nm, respectively. (d) 3D FDTD simulation of reflection spectrum for the different thickness of the mesoporous SiO2 layer. The photocatalytic properties of prepared films were evaluated through investigating their ability to decompose methyl blue (MB) under UV irradiation. Glass substrates with mesoporous SiO2/TiO2 film and macro-mesoporous SiO2/TiO2 film were immersed in a MB solution (2 mg L−1) for 30 s and then exposed to UV irradiation. The decomposition of MB was measured by recording the transmittance recovery at 665 nm.

where F (fill factor) is the area ratio of SiO2 in a slice and n1 and n0 are the refractive indexes of SiO2 and air, respectively. For the macro-mesoporous structure, where F increases gradually downward through the structure, and incident light will be reflected at each slice with a phase determined by the distance traveled through the structure. The neff can be modulated by manipulation of the fill factor, as shown in eq 2: ⎡ 6 A = ⎢⎢ F= A unit ⎣

3. RESULTS AND DISCUSSION 3.1. Calculation of Refractive Index and Optical Performance of Macro-Mesoporous SiO2 Film. Inspired by the moth-eye structure and our previous study, we proposed a unique architecture design including a layer of mesoporous SiO2 and a layer of hexagonal close-packed macro-mesoporous SiO2 as shown in Figure 1a. The geometrical parameters including the spacings (S, S1) and height (L) of the macromesoporous structure and the height (H) of the mesoporous layer are also shown. We used effective medium theory and finite difference time domain (FDTD) simulations to calculate the refractive index and reflectance spectrum of the nanostructured film. For the hexagonal close-packed macro-mesoporous SiO2 layer, it is available for us to reckon its effective refractive index (neff) by using effective medium theory involving 2dimensional Bruggeman equation for two material mixtures as shown in eq 1: ⎛ n − n1 ⎞ ⎛ n − n0 ⎞ 0 = F ⎜ eff ⎟ ⎟ + (1 − F )⎜ eff ⎝ neff + n1 ⎠ ⎝ neff + n0 ⎠

3 4

⎤ (2r )2 − 3πri 2 ⎥ ⎥ × (1 − f ) 3 6 4 (2r )2 ⎦

(2)

where Aunit is the area of unit cell as shown in Figure 1b in a dashed hexagon and A is the area of SiO2 material at a certain slice. r and ri are the radii of macropore at the height of 0 and i, respectively. f is the porosity of mesoporous SiO2. The hexagonal close-packed macro-mesoporous SiO2 layer was split into 15 slices and for each slice, the neff was calculated (please see the Supporting Information for more details). The obtained plot (Figure 1b) clearly shows that the neff increases gradually from the upper interface to the lower interface of macro-mesoporous SiO2 layer, which efficiently diminishes optical interface between air and macro-mesoporous SiO2 layer. 3D finite-difference time-domain (FDTD) was applied to study the optical characteristics of the hexagonal lattice structure. The top cross-section view and the middle crosssection view of the simulation model are shown in Figure 1c. The unit cell is marked in a white dotted rectangle, where the periodic boundary condition (B.C) is applied for the x axis and the y axis, and PML boundary condition for the z axis. The

(1) C

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Figure 2. Schematics of the fabrication of the macro-mesoporous SiO2/TiO2 film.

Figure 3. Titled-view SEM images of (a) the mesoporous SiO2 film, (b) the mesoporous SiO2/TiO2 film, (c) the macro-mesoporous SiO2 film, and (d) the macro-mesoporous SiO2/TiO2 film. The insets are magnified images of each film surface. The right inset in (a) is a TEM image of mesoporous SiO2 film. AFM images of (e) the mesoporous SiO2 film and (f) the macro-mesoporous SiO2 film.

the z axis direction and its polarization direction is parallel to the x axis. In order to predict the optimal AR region of

refractive indexes of the mesoporous layer and substrate are 1.24 and 1.52,25 respectively. The incident plane wave is along D

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Figure 4. (a) Comparison between the real (solid line) and simulated (dashed line) effective index of refraction profiles. The two curves were respectively obtained from the analysis of the AFM results and ideal theoretical model. (b) Measured and calculated reflection spectra for the macromesoporous SiO2 film. (c) UV−vis-NIR transmittance and reflectance spectra of the mesoporous SiO2 film and the mesoporous SiO2/TiO2 film. (d) UV−vis-NIR transmittance and reflectance spectra of the macro-mesoporous SiO2 film and the macro-mesoporous SiO2/TiO2 film.

nanospheres (diameter: 153 ± 4 nm). This is apparently attributed to the shrinkage of the macro-mesoporous structure during the calcination process. As shown in Figure 3d, the diameter of macropores reduced to 125 ± 8 nm associated with the addition of a TiO2 layer on the surface of the macromesoporous SiO2 film. These results suggested that the thickness of the outmost TiO2 layer was about 10 nm. AFM images of the mesoporous SiO2 film and macro-mesoporous SiO2 film are presented in Figure 3e,f. The root-mean-square (RMS) roughness was estimated from a scanning area of 10 μm × 10 μm of the two films and is 1.67 and 22.9 nm, respectively. The uniformity of macropores in Figure 3f is in accord with the SEM observation (Figure 3c). The higher roughness resulting from the presence of a subwavelength macroporous structure would make significant impacts on the optical performance, photocatalytic activity, and wettability of films. 3.3. Experimental Refractive Index and Optical Performance of Macro-Mesoporous SiO2 Film and Macro-Mesoporous SiO2/TiO2 Film. According to AFM results, the area ratio F of real macro-mesoporous structure could be exacted and substituted into eq 1 to calculate neff for each slice. The obtained plot (solid line in Figure 4a) clearly shows that the neff increases gradually from the upper interface to the lower interface of the macro-mesoporous SiO2 layer, which efficiently diminishes the optical interface between air and the macro-mesoporous SiO2 layer. Compared to the plot of the simulation structure of the same size (dashed line in Figure 4a), the neff of the real structure is a little larger, since the wall thickness (S1 in Figure 1a) can not to be neglected. The measured antireflective properties (Figure 4b) of the macromesoporous SiO2 film (H: 100 nm, L: 70 nm, S: 155 nm, Figure S2) is nearly consistent with the FDTD simulation of reflectance of the similar size (H: 100 nm, L: 75 nm, S: 160

nanostructured film, we systematically calculated the reflectance spectrum by varying H (50, 75, and 100 nm) in the simulations. The plots (Figure 1d) clearly show that the optimal AR region red-shifts as H increases from 50 to 100 nm while the spectral profiles are consistent. Therefore, these results may provide an AR design rule to guide specific manufacturing techniques for sunlight harvest and optical devices. 3.2. Characterization of Macro-Mesoporous SiO2/TiO2 Films. A schematic fabrication procedure of macro-mesoporous SiO2/TiO2 films is illustrated in Figure 2. A mesoporous SiO2 layer was first grown on a glass substrate by precursorderived self-assembly according to our previous reports.25,37 Hexagonal close-packed PS nanospheres were assembled on the surface and acted as template for the second mesoporous SiO2 layer growth. Then the PS nanospheres were removed by heat treatment. Finally, a layer of dense TiO2 was dip-coated on the surface of the macro-mesoporous SiO 2 layer. Control mesoporous SiO2/TiO2 films were also prepared to examine the effect of the macro-mesoporous structure. As shown in Figure 3a, the mesoporous SiO2 layer was very smooth and had nanoscaled cracks (left inset in Figure 3a). TEM image (right inset in Figure 3a) revealed closely packed mesopores with center-to-center distance between two adjacent mesopores and pore size roughly estimated to be 3.3 and 2.0 nm, respectively. After dip-coating a layer of TiO2, the obtained mesoporous SiO2/TiO2 film remained flat and the micelles formed between TiO2 nanocrystals endowed the surface with a porous structure.30 The formation of macropores resulting from PS nanospheres removal was clearly observed and is homogeneous over the entire surface of the macro-mesoporous SiO2 film (Figure 3c). The magnified SEM image (inset in Figure 3c) revealed that the diameter of macropores was 145 ± 5 nm, which was slightly smaller than that of the original PS E

DOI: 10.1021/acsami.7b04140 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) TEM image of the macro-mesoporous SiO2/TiO2 film, TiO2 nanocrystals are well dispersed on the surface, (b) XRD pattern of TiO2 powder, which was obtained by drying the TiO2 sol followed by calcination at 550 °C for 3 h. (c) Transmittance vs wavelength as a function of illumination time for photocatalytic degradation of MB on the macro-mesoporous SiO2/TiO2 film. (d) Percentage recovery vs UV illumination time of the mesoporous SiO2/TiO2 film and the macro-mesoporous SiO2/TiO2 film as a result of MB degradation.

transmittance of the macro-mesoporous SiO2/TiO2 film is even higher than that of the macro-mesoporous SiO2 film in the visible region. The very high porosity and gradient refractive index of the macro-mesoporous structure must contribute significantly to the enhancement of optical properties. 3.4. Photocatalytic Activity of the Macro-Mesoporous SiO2/TiO2 Film. Figure 5a shows a TEM image of the macromesoporous SiO2/TiO2 film, where well dispersed TiO2 nanocrystals of 3−4 nm in size are clearly visible. The high crystallinity of the resultant TiO2 was also confirmed by XRD measurements (Figure 5b). All of the identified peaks can be perfectly indexed to anatase TiO2. The photocatalytic activity of the macro-mesoporous SiO2/TiO2 film was evaluated by its ability of degrading methylene blue (MB) under UV irradiation. The film coated glass substrate was immersed in a MB aqueous solution (2 mg L−1) for 150 s and withdrawn at 150 mm min−1. The photocatalytic performance of the film toward MB degradation, measured by recording the transmittance decrease at 665 nm, is summarized in Figure 5c. Upon UV irradiation, the transmittance at 665 nm increased as a function of time. After 3 h of irradiation, MB was completely decomposed. For the mesoporous SiO2/TiO2 film, it also demonstrated a comparable photocatalytic activity as compared to that of the macro-mesoporous SiO2/TiO2 film in the initial UV irradiation stage, which might be attributed to direct UV decomposition of MB on the surface. While after 65 min of UV irradiation, the macro-mesoporous SiO2/TiO2 film showed much better photocatalytic activity than the mesoporous SiO2/TiO2 film (Figure 5d). The high photocatalytic activity of the macro-mesoporous SiO2/TiO2 film would mainly benefit from the synergistic effect

nm). The discrepancy between the measured and calculated plots is attributed to the neglect of the back side reflection in calculations and slight morphology difference between the real and simulation structure. Figure 4c,d shows the reflectance and transmittance spectra of the mesoporous SiO2 film and the macro-mesoporous SiO2 film before and after dip-coating a TiO2 layer. Clearly, the mesoporous SiO2 film and the macromesoporous SiO2 film exhibit both broadband antireflection and high transmittance, and the minimum reflectances are 2.2% at 601 nm and 1.6% at 681 nm, respectively. After dip-coating a layer of TiO2, the minimum reflectance of the mesoporous SiO2/TiO2 film increased to 4.0% at 649 nm. The reflectance is even higher than that of blank glass in the wavelength range of 350−400 nm. While for the macro-mesoporous SiO2/TiO2 film the minimum reflectance is 2.2% at 625 nm, i.e., 0.6% higher than that of the macro-mesoporous SiO2 film, the transmittance is higher than that of the macro-mesoporous SiO2 film over the wavelength range of 350−450 nm. This may be attributed to the combined effects of the high refractive index of TiO2 and the macroporous-mesoporous structure. The high refractive index of TiO2 would increase the reflection in the wavelength range of 350−450 nm, but the gradient refractive indexes brought by the macro-mesoporous structure would suppress the increase of reflection in the same wavelength range. Usually, displays prefer AR performance in the visible region (400−800 nm), while solar cells focus on the visible-infrared region (400− 1200 nm). Therefore, we summarized the average reflectance and transmittance of the above four films. As shown in Table S1, the average reflectance of the mesoporous SiO2/TiO2 film increased much more than that of the macro-mesoporous SiO2/TiO2 film after dip-coating a layer of TiO2. The average F

DOI: 10.1021/acsami.7b04140 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Time-dependent changes in instant water contact angle: (a) the mesoporous SiO2/TiO2 film and (b) the macro-mesoporous SiO2/TiO2 film. The insets are digital images of water contact angle tests at 500 ms. Time-dependent variations of water contact angles of (c) the mesoporous SiO2/TiO2 film and (d) the macro-mesoporous SiO2/TiO2 film. The insets are digital images of water contact angle change before and after UV irradiation after storing the samples in dark conditions for 24 days.

dependent variations of WCAs of the mesoporous SiO2/TiO2 film and the macro-mesoporous SiO2/TiO2 film, respectively. The mesoporous SiO2/TiO2 film lost its superhydrophilic behavior after storing in the dark for 6 days, and its WCA attained to 15° after 24 days storing. In contrast, the macromesoporous SiO2/TiO2 film maintained superhydrophilic and antifogging (Figure S3) behavior even after storing for 24 days in the dark. Additionally, upon UV irradiation for 1 h (16.8 mW cm−2), these two films restored their original superhydrophilicity. Digital images of the changes in WCAs before and after UV irradiation were also shown as insets in Figure 6c,d. These results clearly indicate that the macro-mesoporous structure could significantly enhance superhydrophilic and antifogging behavior as well as the durability of the superhydrophilic surface. 3.6. Abrasion Resistance of Macro-Mesoporous SiO2/ TiO2 Films. Abrasion resistance of AR films is essential in preserving consistent surface structure and optical performance. The abrasion resistance of the macro-mesoporous SiO2/TiO2 film was evaluated by sandpaper abrasion and pencil hardness tests. The sandpaper abrasion test was carried out as shown in Figure 7a. The specimen with a 100 g weight was placed on a piece of sandpaper and moved for 15 cm along the ruler, and then it was rotated by 90° and was again moved for 15 cm. After the specimen was turned over, it was moved for another 15 cm, and then it was rotated counter clockwise by 90° for the last 15 cm moving along the ruler. These steps constitute one cycle of the sandpaper abrasion test, which guarantees the double surfaces are abraded both longitudinally and transversely. The friction force between the specimen and the

of the macro-mesoporous structure and hybridization of SiO2/ TiO2. The large surface area and high accessibility of the macromesoporous structure would allow guest molecules to react sufficiently with well-dispersed TiO2 nanocrystals.38,39 As an adsorbent, mesoporous SiO2 could supply concentrated MB for TiO2 photoactive sites and prevent photogenerated holes and electrons from recombination.34,40 3.5. Wettability of Macro-Mesoporous SiO 2/TiO2 Films. Superhydrophilic films are of great importance in a variety of practical applications due to their fascinating selfcleaning and antifogging properties. To investigate the wettability of the macro-mesoporous SiO2/TiO2 film, WCA measurements were carried out. Time-dependent changes in WCAs of both mesoporous SiO2/TiO2 and macro-mesoporous SiO2/TiO2 films are shown in Figure 6a,b. The volume of water droplets for measurements was set as 3 μL. These two films both showed superhydrophilic property with a WCA at 0.5 s of 4.9° and 2.5°, respectively. Moreover, water spreading speed on the macro-mesoporous SiO2/TiO2 film is faster than that on the mesoporous SiO2/TiO2 film. According to early theoretical research by Wenzel41 and Quere42 et al., surface roughness can amplify the intrinsic wettability of a surface. AFM results have shown that the roughness of macro-mesoporous SiO2/TiO2 film is almost 14 times larger than that of the mesoporous SiO2/TiO2 film. Therefore, the enhancement of surface hydrophilicity of the macro-mesoporous SiO2/TiO2 film points to a positive impact of the macro-mesoporous structure. Usually, the superhydrophilic behavior would weaken after storage in dark conditions. We also investigated the durability of the hydrophilicity of the films. Figure 6c,d shows timeG

DOI: 10.1021/acsami.7b04140 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. (a) One cycle of sandpaper abrasion test. A macro-mesoporous SiO2/TiO2 film coated substrate was placed on a sandpaper, on which a 100 g weight was mounted. The coated substrate with the weight was first moved along the ruler for 15 cm and was then rotated clockwise by 90°, followed by moving along the ruler again for 15 cm. After that the coted substrate was turned over and moved along the ruler for another 15 cm. Finally, it was rotated counterclockwise by 90° for the last 15 cm move along the ruler. These steps constitute one cycle of the sandpaper abrasion test. SEM images of the macro-mesoporous SiO2/TiO2 film (b) before and (c) after 50 cycles of sandpaper abrasion test. (d) Reflectance spectra of the macro-mesoporous SiO2/TiO2 film coated substrate, which had been subjected to 10, 20, 30, 40, and 50 cycles of the sandpaper abrasion test. (e) Water contact angle as a function of the number of cycles of the sandpaper abrasion test. The inset shows nearly superhydrophilic of the surface after 50 cycles of the sandpaper abrasion test.

4. CONCLUSION In summary, effective medium theory and FDTD simulations were applied to guide the construction of the well-organized macro-mesoporous SiO2/TiO2 structure toward advanced antireflective thin films. The macro-mesoporous SiO2 layer of high porosity and gradient refractive index enabled the addition of TiO2 surface layer without compromising the refractive index required for high-quality AR thin films. The macro-mesoporous SiO2/TiO2 film coated substrate showed excellent AR performance with an average reflectance of 3.45% over the wavelength range of 400−1200 nm, which was only 0.6% higher than that of the macro-mesoporous SiO2 film coated substrate. In addition, 3−4 nm TiO2 nanocrystals that were well-dispersed in the macro-mesoporous structure endowed the thin film with high photocatalytic activity toward the degradation of MB. The intrinsic superhydrophilicity as well as the durability of the superhydrophilic surface was also improved by the presence of macro-mesoporous structure. The integration of superior optical performance, efficient photocatalytic activity, durable superhydrophilicity, as well as abrasion resistance in one film would promise great potentials for applications such as sunlight harvest, solar energy conversion, and optical devices.

sandpaper was 0.2 N (see the Supporting Information for more details). The surface morphologies before and after the sandpaper abrasion test are shown in Figure 7b,c. It is noted that some walls were destroyed but no patch was scratched off after the sandpaper abrasion test. The reflectance spectra of the macro-mesoporous SiO2/TiO2 film coated substrate after 10, 20, 30, 40, and 50 cycles of the sandpaper abrasion test are shown in Figure 7d. The reflectance increased slightly with increase of the number of abrasion test cycles. This was attributed to the tiny changes in the film thickness and surface structure as revealed in Figure 7b,c. The water contact angle after 50 cycles of the sandpaper abrasion test is shown in Figure 7e, and clearly, the superhydrophilicity was retained only with the WCA increased to 7° probably due to the tiny loss in surface composition and roughness. We also accessed the hardness of the film using a conventional pencil hardness test. As shown in Figure S5, although some macropores were buried after 4H, 5H, and 6H hardness tests, the film still maintained its structural integrity. We further measured water contact angles on the scratched surfaces to assess the influence of the pencil hardness test on the surface wettability. As shown in Figure S5c, f and i, to our surprise, the water contact angles of 4H, 5H, and 6H pencil scratched surfaces were 3.3°, 4.8°, and 6.7°, respectively. Therefore, the nanostructured film maintained enough roughness and surface chemistry to keep its superhydrophilicity (and thus antifogging performance) even after experiencing severe mechanical scratching. These results indicate favorable robustness and functional durability of the macro-mesoporous SiO2/TiO2 films, which would promise their applications in practical conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04140. Detailed calculation of refractive index; summary of average reflectance and transmittance of samples; H

DOI: 10.1021/acsami.7b04140 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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antifogging properties; the friction force between the sample and the sandpaper and pencil hardness tests (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 10 82543535. ORCID

Junhui He: 0000-0002-3309-9049 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant Nos. 21571182, 21271177, and 21607158), the Science and Technology Commission of Beijing Municipality (No. Z151100003315018), a Chinese Academy of Sciences Grant (CXJJ-14-M38), Youth Innovation Promotion Association of Chinese Academy of Sciences, and the National Natural Science Foundation of China (61307065) is greatly appreciated.

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DOI: 10.1021/acsami.7b04140 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX