Article pubs.acs.org/Langmuir
Novel SiO2/H2Ti2O5·H2O‑Nanochain Composite with High UV−Visible Photocatalytic Activity for Supertransparent Multifunctional Thin Films Lin Yao,† Junhui He,*,† Tong Li,†,‡ and Tingting Ren†,‡ †
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 the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: In the current work, a peroxo titanium complex (PTC) solution was used as a novel water-soluble precursor to fabricate H2Ti2O5·H2O and the SiO2/H2Ti2O5·H2O-nanochain composite at low temperature (90−100 °C). The average width of H2Ti2O5·H2O nanochains is 4.5 ± 1.5 nm. Under full-spectrum irradiation, the Si/Ti-nanochain composite showed good UV−visible light absorption and excellent photocatalytic activity, which is 2.8 times that of P25. In the composite, SiO2 not only contributes to the formation of nanochains and improves the catalytic performance of H2Ti2O5·H2O but also reduces the refractive index of the complex. When coated on transparent organic substrates, the composite thin film exhibited excellent antireflective (as high as 99.3% on PC and 98.9% on PMMA) and self-cleaning properties. Pencil hardness, washing, and tape adhesion tests showed favorable adhesion-to-substrate and mechanical robustness of thin films, which make them extremely attractive for applications as highly transparent and self-cleaning thin films on lenses, photovoltaic cells, and windows of high-rise buildings.
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INTRODUCTION Protonated layered titanates (PLTs), whose compositions are written as H2TimO2m+1 or H4TinO2n+2 (as expected for hydrous titania) are promising materials in various fields.1,2 Layers of PLTs consist of TiO6 octahedral units through edge and corner sharing with small positive ions and water molecules in the interlayer (Scheme S1).3−5 Because of their unique and alterable two-dimensional layered structures, PLTs with abundant interlayer surfaces show great capability in adsorption, ion exchange, and catalysis.1,6 Besides, PLTs are also promising candidates as a precursor for nanostructured TiO2 and metal titanate.7 Except for those fields (adsorbents, photocatalytic systems, Li-ion batteries, solar cells, etc.8−13) that have been reported, we believe that PLTs could have important applications in antireflective (AR) and self-cleaning thin films. PLTs with a large specific surface area and a porous structure may have a very low refractive index (RI). When nc of a PLT-based thin film meets the following two conditions, an AR thin film that effectively enhances the transmittance and removes the ghost image of the substrate could be obtained: (1) nc = (nans)1/2, in which na and ns are the refractive indices of the air and the substrate, respectively. (2) d = λ/4nc, in which d is the thickness of the coating and λ is the wavelength of the incident light.14 Furthermore, PLTs with outstanding photocatalytic activity could be applied in self-cleaning thin films. By © XXXX American Chemical Society
means of degrading organic pollutants on the surface of transparent substrates, PLT-based thin films could maintain outstanding optical performance of substrates outdoors in the long run. Unfortunately, rather few in-depth studies have been reported to date on the application of PLTs in antireflective and self-cleaning thin films. Conventionally, PLTs were synthesized by hydrothermal/ solvothermal methods, calcination, or ion-exchange with NaOH or NaCO3.4,15−17 The 2D exfoliated ultrathin titanate nanosheets can also self-assemble into 3D hierarchical structures, which are in favor of separation and recycling in practical applications. The obtained materials mainly include nanotubes, nanosheets, and spheroidal nanocrystallites.6,11,18−20 The prime controlling factor for the properties of these materials with varying shapes is the arrangement of the layers. Direct synthesis of different 3D architectures through the hierarchical assembly of low-dimensional building blocks (layer or sheet) through bottom-up approaches is a very challenging task. Most of the 3D hierarchical PLTs were synthesized at high temperature (≥200 °C) in the presence of hazardous organic surfactants and/or solvents such as ethylenediamine, N,N-dimethylformaReceived: September 27, 2016 Revised: November 16, 2016
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DOI: 10.1021/acs.langmuir.6b03532 Langmuir XXXX, XXX, XXX−XXX
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under vigorous stirring. The mixed sol was refluxed at 90−100 °C for 1−10 h. The Si/Ti hybrid sol was then obtained by cooling to room temperature. A PTC solution (50 mL) was refluxed at 100 °C for 4 h and then cooled to room temperature. Assembly of the Si/Ti Hybrid Thin Film. A transparent organic substrate was sonicated in deionized water for at least 15 min and then treated with oxygen plasma (84 W, 3 min) under an 800 mL min−1 flow of oxygen. The cleaned substrate was immersed in the Si/Ti hybrid sol for 30 s and withdrawn from the sol at 50 mm/min. The asprepared thin film was first dried at room temperature for 0.5 h and then dried at 80 °C for 12 h. Characterization. Scanning electron microscopy (SEM) observations were carried out on a Hitachi S-4800 field emission scanning electron microscope. The specimens were coated with a layer of gold by ion sputtering before SEM observations. For transmission electron microscopy (TEM) observations, powder products were dispersed in ethanol by sonication for 10 min and added to carbon-coated copper grids. After drying at 60 °C overnight, they were observed on a JEOL JEM-2100F transmission electron microscope at an acceleration voltage of 150 kV. The roughness and morphology of film surfaces were characterized by atomic force microscopy (AFM) on an MM8SYS scanning probe microscope (Bruker AXR). Transmission and reflection spectra in the wavelength range of 300−2500 nm were recorded using a Varian Cary 5000 UV−vis−NIR spectrophotometer. X-ray diffraction (XRD) patterns of prepared products, recorded on a Bruker D8 Focus X-ray diffractometer using Cu Kα radiation (λ = 0.154184 nm), were used to identify phase constitutions in the products. Ultraviolet−visible diffuse reflection spectra against BaSO4 were recorded on a Varian Cary 300 UV−vis spectrophotometer. Xray photoelectron spectra (XPS) analyses of samples were recorded on an ESCALab220i-XL. Adsorption and photocatalytic oxidation of RhB on the Si/Ti nanocomposite were investigated in air in a quartz vessel at room temperature. Thirty milliliters of an RhB aqueous solution and 87 mg of the finely ground catalyst powder (Ti wt = 7 mg, Si wt = 35 mg) were placed in the quartz vessel, which formed a suspension under stirring. The suspension was vigorously stirred first in the dark for a certain time to evaluate the adsorption performance. After the adsorption−desorption equilibrium was establishing, the photocatalytic reaction was carried out under irradiation by a 50 W xenon lamp without a light filter (λ = 300−2500 nm). The distance between the light source and quartz vessel was 20 cm. Three milliliter samples of the suspensions were taken at 10 min intervals and centrifuged at 15 000 rpm for 10 min, and the concentration of RhB was analyzed using a TU-1901 spectrophotometer. For comparison, the performances of 15 mg of H2Ti2O5·H2O (Ti wt = 7 mg), 12 mg of P25 (Ti wt = 7 mg), and 75 mg of SiO2 (Si wt = 35 mg) were also measured. All of the experiments were carried out under identical conditions. Three tests were used to assess the mechanical property of the Si/ Ti-nanochain composite thin film. A pencil scratching test was carried out with an Elcometer 3086 motorized pencil hardness tester according to the State Standard Testing Method (GB/T6739-1996, equivalent of ASTM D3363). The pencil was held firmly against the film at a 45° angle and pushed away by the tester in a 6.5 mm stroke at a speed of 0.5 mm/s. A tape adhesion test was carried out according to ASTM D3359-93. A scalpel was used to cut an X on the thin film. 3M scotch tape (cat. 600) was pressed onto and peeled off of the thin film surface. The residual glue was removed by washing with ethanol. The thin film surface was finally observed by optical microscopy. In the scrubbing resistance test, the Si/Ti-nanochain composite thin film was washed with a sponge brush (ASTM D4828-92) for up to 100 cycles in 2 min.
mide (DMF), diethylenetriamine, isopropyl alcohol (IPA), and ethylene glycol.5,21−23 Very few reports on the direct synthesis of 3D-assembled structures of PLTs present environmentally friendly processes, including at low temperature (≤100 °C), in all-water-based solution, without a large amount of toxic emissions and alkali waste. Sutradhar et al. used titanium peroxo carbonate as a precursor with ammonium hydroxide to synthesize hierarchical spheres of protonated-layered hollow titanates.19 The controlled hydrolysis of the titanium precursor and pressure developed as a result of the in situ formation of excess NH3 are the critical factors for the 3D assembly of the nanosheets into a spherical morphology. Besides, Lin et al. developed a simple, low-temperature (90 °C), urea-modulated method for the synthesis of PLTs.24 The urea concentration is a key factor in controlling the morphology and crystalline structure. PLTs nanosheets could be obtained with a high urea concentration and prolonged aging time. Although the reaction temperature has been lowered to 110 and 90 °C, there are still some defects in the two synthesis routes, including high pressure and a long reaction time (7 days). Herein, we report a new method for the low-temperature synthesis of PLTs in all-water-based solution. Peroxo titanium complex (PTC) solution was used as a novel water-soluble precursor and refluxed for several hours at 90−100 °C for the synthesis of PLTs. When PTC is the only precursor, there is no fixed morphology of the obtained PLTs. However, when the SiO2 nanoparticle hydrosol was mixed with PTC, H2Ti2O5H2O nanochains were obtained after reflux. To the best of our knowledge, the synthesis of ultrathin H2Ti2O5H2O nanochains (average width 4.5 nm) has never been reported. The process of PLT assembly is environmentally friendly, requiring no posttreatment such as calcination or hydrothermal treatment with NaOH. The Si/Ti-nanochain composite showed good UV− visible light absorption and excellent photocatalytic activity under full-spectrum irradiation for the degradation of rhodamine B (RhB). Its catalytic rate is 2.8 times that of P25. The Si/Ti-nanochain composite could be successfully coated onto PC and PMMA substrates. In the composite, SiO2 not only contributes to the formation of nanochains and improves the catalytic efficiency but also reduces the refractive index of the composite. Therefore, the flexible thin film demonstrated excellent optical and mechanical performance.
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EXPERIMENTAL SECTION
Materials. Titanium tetrachloride (TiCl4, 99.0%), sodium hydroxide (NaOH, 96.0%), hydrogen peroxide (H2O2, 30%), and Rhodamine B (RhB) were purchased from Beijing Chemical Co. (3-Aminopropyl) triethoxysilane (APTS, 98%) and silicon oxide hydrosol (SiO2, 40 wt %) were purchased from Alfa Aesar. TiO2 (P-25) was obtained from Acros. All chemicals were analytical grade and used without further purification. Ultrapure water was obtained from a Millipore Milli-Q purification system (>18 MΩ cm). Preparation of the Peroxo Titanium Complex (PTC) Solution. In a typical procedure, 2−10 mL of pure TiCl4 liquid was carefully diluted with 370 mL of ice water under vigorous magnetic stirring. After 0.5 h, a 75 g/L NaOH solution was added to adjust the pH of the solution to 7−10. A white precipitate was obtained. The precipitate was washed with ultrapure water by centrifugation three times. Then, the precipitate was dispersed in 185 mL of ultrapure water. After vigorous stirring for 0.5 h, 10−15 mL of a H2O2 (30%) solution was then added. After continuous stirring for 1 h, an orange transparent solution of PTC was obtained. Preparation of the Si/Ti Hybrid Sol and H2Ti2O5·H2O Sol. In a typical experiment, 25 mL of ultrapure water, 200 μL of APTS, and 3− 5 mL of SiO2 hydrosol were mixed with 30 mL of a PTC solution
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RESULTS AND DISCUSSION Characterization of Structure and Composition. Because the constitution of products is uncertain, we temporarily named the product generated by direct reflux from PTC the Ti compound and that generated by reflux from B
DOI: 10.1021/acs.langmuir.6b03532 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) XRD patterns of H2Ti2O5·H2O powder without thermal treatment and after calcination for 3 h at 200 and 400 °C, respectively. (b) TG and DTG analyses of H2Ti2O5·H2O. (c) Survey XPS spectra of H2Ti2O5·H2O and the Si/Ti nanocomposite. (d) Ti 2p high-resolution XPS spectrum of H2Ti2O5·H2O. (e) O 1s high-resolution XPS spectrum of H2Ti2O5·H2O. (f) O 1s high-resolution XPS spectrum of the Si/Ti nanocomposite.
tively. When the environmental temperature increased to 200 °C, there was only a small loss of crystal and adsorbed water and no significant color change occurred (Figure S2), indicating that the H2Ti2O5·H2O compound was stable at T ≤ 200 °C. When the calcination temperature increased to 400 °C, H2Ti2O5 underwent inner chemical dehydration and turned into white TiO2. These agree well with the results by TG/DTG observations. There are two prominent and separate steps of weight loss shown in Figure 1b. The first weight loss (25−275 °C) is the loss of crystal and adsorbed water, and the second (275−500 °C) is the intramolecular dehydration. These data suggest that the as-prepared SiO2/H2Ti2O5·H2O nanocomposite and H2Ti2O5·H2O could be obtained by in situ nanoassembly at relatively low temperature without post-treatment. These characteristics benefit the coating of Si/Ti hybrid multifunctional films on transparent organic substrates (such as PC and PMMA), which cannot withstand high temperature. X-ray photoelectron spectroscopy (XPS) analysis was performed to further confirm the chemical compositions of the Ti compound and Si/Ti nanocomposite. Before the analyses, the samples were dried at 200 °C for 3 h to get rid of adsorption and crystallization water. As displayed in Figure 1c, major peaks for the O 1s, Ti 2p1/2, Ti 2p3/2, and Si 2p states
PTC and SiO2 hydrosol the Si/Ti nanocomposite. The crystal phases of SiO2 and the as-synthesized Ti compound and Si/Ti nanocomposite were determined by XRD. As shown in Figure S1, both of SiO2 and Si/Ti nanocomposite were amorphous. The latter is mainly attributed to the fact that the content of Si was much greater than the content of Ti in the Si/Ti nanocomposite. When the mass ratio of Si/Ti decreased from 4.9 to 2.5, several diffraction reflections (2θ ≈ 39, 47, and 63°) were observed, which are coincident with those of the Ti compound. Therefore, we consider the crystal phase of the titanyl compound in the Si/Ti nanocomposite to be the same as that of the Ti compound. Figure 1a shows the typical powder XRD patterns of the as-prepared Ti compound and its products upon calcination for 3 h at 200 and 400 °C, respectively. The diffraction patterns of the Ti compound were indexed as an orthorhombic H2Ti2O5·H2O (JCPDS 47-0124) with lattice parameters of a = 18.03° Å, b = 3.78° Å, and c = 3.00° Å. All diffraction lines (2θ from 10 to 50) are broad and weak peaks corresponding to (200), (110), (600), (501), and (020) crystal planes, respectively. The diffraction patterns of the products upon calcination for 3 h at 200 and 400 °C were indexed as an orthorhombic structure (H2Ti2O5·H2O, JCPDS 47-0124) and a tetragonal structure (anatase TiO2, JCPDS 21-1272), respecC
DOI: 10.1021/acs.langmuir.6b03532 Langmuir XXXX, XXX, XXX−XXX
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Langmuir clearly exist in the wide-scan XPS spectrum. The Ti 2p highresolution XPS spectra (Figures 1d and S3a) show that the binding energies (BE) of Ti 2p3/2 and Ti 2p1/2 are 458.8 and 464.4 eV, respectively, indicating that Ti exists in the Ti4+ form in both the Ti compound (H 2Ti2 O 5) and the Si/Ti nanocomposite.25 The Si 2p core region of the Si/Ti nanocomposite can be fitted mainly as a single doublet at a BE of 103.4 eV corresponding to the Si−O−Si bond26 (Figure S3b). As shown in Figure 1e,f, O 1s signals in the H2Ti2O5 and Si/Ti nanocomposite are quite different. Regarding H2Ti2O5, the peak at 530.3 eV corresponds to lattice oxygen (Ti−O−Ti) in H2Ti2O5, and another peak centered at 532.5 eV is assigned to surface hydroxides.27 As for the Si/Ti nanocomposite, the BE values of O in Si−O−Si (532.8 eV) and −OH (532.5 eV) are too close to distinguish. Therefore, the strong peak centered at 532.7 eV is observed on behalf of the O atoms in both SiO2 and −OH. A relatively weak peak at 530.3 eV corresponding to lattice oxygen (Ti−O−Ti) is also observed because of the addition of a large number of SiO2 nanoparticles. The atomic percentage compositions of the H2Ti2O5 and Si/Ti nanocomposite are shown in Table S1. The atomic ratio of titanium and oxygen is 2/5, which becomes additional evidence for the generation of H2Ti2O5. The atomic ratio of titanium and silicon in the Si/Ti nanocomposite is 7.0, which is very close to the theoretical value of 7.3. Morphologies of the purchased SiO2 hydrosol, as-prepared H2Ti2O5·H2O hydrosol, and Si/Ti nanocomposite hydrosol were characterized by TEM. The mean diameter of SiO2 nanoparticles was measured to be ca. 21 ± 8 nm (Figure 2a).
The chainlike morphology of H 2 Ti 2 O 5 ·H 2 O was well documented by a series of high-magnification TEM images of the obtained product at different reaction times. According to Figure S4, in the process of Ti-nanochain formation H2Ti2O5· H2O-based nanocrystallites (d = 2−4 nm) and ultrafine nanochains (d = 2−4 nm) were first formed and further assembled to stable nanochains (d = 3−6 nm) after refluxing for 4−10 h at 90−100 °C. The preparation of H2Ti2O5·H2O nanochains is of great significance. It is beneficial for increasing the porosity and specific surface area and reducing the refractive index of the H2Ti2O5·H2O compound. Thus, the H2Ti2O5·H2O nanochain compound may have high photocatalytic activity and might be coated on transparent organic substrates without reducing the light transmittance of substrates. To further analyze the morphology change of H2Ti2O5·H2O before and after the combination with SiO2 nanoparticles, H2Ti2O5·H2O, SiO2, and the Si/Ti-nanochain composite were characterized with a BET surface area analyzer. The corresponding powder samples were prepared by drying at 80 °C for 12 h, which was the same procedure as the later preparations of thin films. The nitrogen adsorption and desorption isotherms and corresponding pore size distributions are shown in Figure 3. The isotherms of H2Ti2O5·H2O are
Figure 2. TEM images of the (a) SiO2 hydrosol and (b) H2Ti2O5·H2O hydrosol. HRTEM images of (c) the H2Ti2O5·H2O hydrosol and (d) the Si/Ti-nanochain composite hydrosol.
Figure 3. (a) Nitrogen adsorption−desorption isotherms and (b) pore size distributions of H2Ti2O5·H2O, SiO2, and the Si/Ti-nanochain composite.
Figure 2b,c shows TEM and HRTEM images of the sample taken from the H2Ti2O5·H2O hydrosol after refluxing at 90− 100 °C for 4 h. Clearly, PTC converted to H2Ti2O5·H2O with no particular morphology, including nanoparticles and nanosheets. The interlayer spacing of synthesized H2Ti2O5·H2O is ca. 0.8 nm, which is close to the value of the [200] lattice plane of H2Ti2O5·H2O (JCPDS 47-0124, 0.90 nm, spacing of the layers). The selected area electron diffraction pattern shows two diffraction halos, indicating poor crystallinity of the product, which is consistent with the XRD observations. The general morphology of the obtained Si/Ti hybrid hydrosol is shown in Figure 2d. It could be observed that H2Ti2O5·H2O nanochains were successfully obtained by in situ nanoassembly.
identified as type III, which is characteristic of imperforate powder with an extremely low specific surface area (Figure 3a).28 The isotherms of SiO2 and Si/Ti nanocomposites are, however, characteristic as a type IV isotherm with a type H1 hysteresis loop, indicating that they are mesoporous materials and the pores in the samples are interstitial voids formed by accumulated particles with uniform size.29 BJH calculation for the pore size distribution, which is derived from the desorption data, reveals a narrow pore-size distribution centered at 3−4 nm for SiO2 (Figure 3b). For the Si/Ti-nanochain composite, the pore-size distribution is wider than that for SiO2 because of the coexistence of nanoparticles with different morphologies (nanochains and nanospheres). And the pore-size distribution D
DOI: 10.1021/acs.langmuir.6b03532 Langmuir XXXX, XXX, XXX−XXX
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where C0 is the initial concentration of RhB solution and k is a rate constant related to the reaction. As shown in Figure 4b, the degradation kinetics exhibits pseudo-first-order behavior, and the apparent photocatalytic degradation rate constants are 0.065, 0.023, 0.004, and 5.4 × 10−4 min−1 when using the Si/Ti-nanochain composite, P25, H2Ti2O5·H2O, and SiO2, respectively. The catalytic rates of pure and nonporous H2Ti2O5·H2O are rather slow. Nevertheless, when the content of titanium is consistent, the kinetics constant of the Si/Ti-nanochain composite (BET surface area 178.4 m2/g) is 16.3 times that of pure H2Ti2O5·H2O (BET surface area 1.6 m2/g) and 2.8 times that of P25 (BET surface area ca. 50 m2/g). Therefore, the formation of H2Ti2O5·H2O nanochains dramatically enhances the reaction efficiency. It may be attributed to two main reasons. On one hand, H2Ti2O5· H2O nanochains, spanning about 3−5 nm in width, could reduce the transfer distance of electrons and holes to the TiO2 surface and increase the density of active sites, leading to an improvement in photocatalytic efficiency. On the other hand, compared to P25, the Si/Ti-nanochain composite not only has strong absorption in the UV region but also has favorable absorption in the visible region. The optical properties of the as-prepared Si/Ti-nanochain composite, H2Ti2O5·H2O, and P25 were investigated by UV−vis diffuse reflection spectroscopy, and the results are shown in Figure. S7. P25 shows a band-edge absorption at around 410 nm, which is typical for P25 with a band gap energy of ca. 3.0−3.2 eV. In contrast, the Si/Ti-nanochain composite and H2Ti2O5·H2O show visible light absorption up to 600 nm, which could be the reason for their powder and solution appearing yellow. Because we cannot confirm whether H2Ti2O5·H2O is a direct or indirect semiconductor, the plots of the transformed Kubelka−Munk function versus the energy of light for both direct ((αhυ)2 vs hυ) and indirect ((αhυ)1/2 vs hυ) semiconductors were constructed (Figure S8).31 By extrapolating the linear portion of the curves to αhυ = 0, the band gap values for the Si/Tinanochain composite and H2Ti2O5·H2O were given. Assuming the H2Ti2O5·H2O to be a direct/indirect semiconductor, the band gaps for the Si/Ti-nanochain composite and H2Ti2O5· H2O obtained in such a way were approximately 2.82/2.06 and 2.63/1.99, respectively (Table S2). This result reveals that the band gap of the Si/Ti-nanochain composite is narrower than that of P25. The light-utilization efficiency and thus catalytic efficiency are improved under full-spectrum xenon lamp irradiation. Therefore, the Si/Ti-nanochain composite possesses excellent UV/visible-light catalytic activity and has good application prospects in the field of catalysis, including selective catalysis. The as-prepared Si/Ti-nanochain composite thin film also possesses excellent photocatalytic properties. When the rate of photocatalytic mineralization is higher than the contamination rate, the multifunctional film would keep the surface free from organic contamination under UV−vis illumination, which is usually called a self-cleaning property. Optical Performance of Si/Ti Nanochain Hybrid Thin Films. PMMA and PC substrates were dip-coated by the Si/Tinanochain composite hydrosol and then dried at 80 °C to obtain multifunctional thin films. The morphology, thickness, and uniformity of the thin films were observed by SEM. A topview SEM image (Figure 5a) shows that the substrate was homogeneously covered with spherical nanoparticles. The H2Ti2O5·H2O nanochains were too thin to observe by SEM. From a cross-sectional SEM image, it could be seen that spherical nanoparticles were connected to each other and were
is centered at 8−9.5 nm. The BET specific surface areas of H2Ti2O5·H2O, SiO2, and Si/Ti nanocomposite powders were estimated to be 1.6, 150.7, and 178.4 m2 g−1, respectively. Clearly, the nitrogen adsorption−desorption measurements are consistent with the above SEM and XRD observations. Usually, after the physical mixing of two materials with different specific surface areas, the surface area of the mixture is between the surface areas of the two materials. In the current work, the specific surface area of the as-prepared Si/Ti nanocomposite is even larger than that of SiO2 powder, indicating that the formation of the Ti-nanochain greatly increased the surface area of the composite and is thus beneficial to improving the light transmittance and catalytic properties of powder as well as thin film samples. Photocatalytic Performance. The adsorption and photocatalytic activity of as-synthesized H2Ti2O5·H2O, the Si/Tinanochain composite, SiO2, and P25 were tested by the degradation of rhodamine B (RhB) in aqueous solution under full-spectrum xenon lamp irradiation. As shown in Figure 4a,
Figure 4. (a) Adsorption and photocatalytic degradation of RhB before and under full-spectrum irradiation in the presence of samples, respectively. Here, C is the concentration of RhB at time t, and C0 is the initial concentration. (b) Photocatalytic degradation rates of RhB by the samples.
the adsorption of RhB on the four samples proceeded very quickly in the dark, taking only 5−10 min for them to essentially reach adsorption−desorption equilibrium. The large surface area of H2Ti2O5·H2O nanochains in the Si/Ti nanocomposite did not significantly enhance the adsorption on the catalyst surface. The saturated adsorption amount of the Si/Ti-nanochain composite was ca. 12%. The progress in photodegradation with irradiation time was monitored by analyzing the UV−vis spectra of reaction solutions, taken out at 10 min intervals. As shown in Figure S5, the intensity of the characteristic absorption peak at 553 nm, which is proportional to the concentration of RhB, underwent a decrease to different extents. It is clear that Si/Tinanochain composite had the highest photocatalytic activity for the degradation of RhB among the four samples, and the photocatalytic activity of all of the samples is in the following order: Si/Ti-nanochain composite > P25 > H2Ti2O5·H2O > SiO2. As shown in Figures S5c and S6, nearly all RhB in the solution was degraded by the Si/Ti-nanochain composite after full-spectrum xenon lamp irradiation for 50 min. It is well known that the photocatalytic oxidation of organic pollutants follows the Langmuir−Hinshelwood kinetics.30 Therefore, the reaction process can be described as follows C = C0 exp( −kt )
(1) E
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antireflective thin film is that both the preparation of the precursor sol and the coating of the thin film are all-water based. Thus, the preparation process is not only low-cost but also environmentally friendly, which is rather rare in the preparation of SiO2-based and TiO2-based antireflective thin films. The Fresnel equation offers a basic mathematical model of the refractive index (RI) of thin films.14 For an ideal homogeneous single-layer AR film, its refractive index nc has to meet the following two conditions: (1) nc = (nans)1/2 and (2) d = λ/4nc, where na and ns are the refractive indices of the air and the substrate, respectively, d is the thickness of the coating, and λ is the wavelength of incident light. If the thin film we prepared is single-layer and homogeneous, then we could simply calculate the refractive index of the thin film. The maximum transmittance was obtained at 660 nm with a 129nm-thick thin film on a PC substrate (ns = 1.58); therefore, the RI of the thin film was 1.28, which is very close to the ideal RI of 1.26. This also explains why the transmittance of the PC substrate could reach as high as 99.3% after coating the thin film. Mechanical Properties. The mechanical properties of thin films are a key issue, especially for outdoor applications.14 To assess the mechanical robustness of Si/Ti-nanochain composite thin films, three complementary tests were performed. First, the as-prepared thin films were assessed by the pencil hardness test (ASTM D3363-05 standard). The pencil hardness test measures the scratch resistance of a surface exposed to pencils of varied hardness from 6B to 6H, which are the softest to hardest pencils, respectively. Figure 5a shows, after thermal treatment at 80 °C for 12 h, that the hardness of the surface is significantly enhanced. Before thermal treatment, the thin film could not pass the 3H pencil hardness test, and 30% of surface area was damaged. After thermal treatment, the pencil hardness of the thin film could reach 4H. The SEM images of the thin film before and after scratching by the 4H pencil are shown in Figure 5b,c. After scratching, there are no significant changes in the surface and SiO2 nanoparticles are still clearly visible. After being tested by the 5H and 6H pencils; however, the surface was damaged to different degrees. The pencil hardness of the Si/Ti-nanochain composite thin film (4H) is much higher than those of thin films with a single component of SiO 2 nanoparticles (
