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Hierarchical Ag/TiO2/Si Forest-Like Nano/Micro- Architectures as Antireflective, Plasmonic Photocatalytic, and Self-Cleaning Coatings Bhaskar Dudem, L. Krishna Bharat, Jung Woo Leem, Dong Hyun Kim, and Jae Su Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02220 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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ACS Sustainable Chemistry & Engineering

Hierarchical Ag/TiO2/Si Forest-Like Nano/MicroArchitectures as Antireflective, Plasmonic Photocatalytic, and Self-Cleaning Coatings Bhaskar Dudem, L. Krishna Bharat, Jung Woo Leem, Dong Hyun Kim, and Jae Su Yu* Department of Electronic Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea. *Corresponding author. Email address: [email protected] (Prof. J. S. Yu)

ABSTRACT We report the fabrication of hierarchical silver/titanium dioxide/silicon (Ag/TiO2/Si) structures with forest-like nano/micro-architectures and their physical (i.e., structural and optical) properties, surface wettability, and photocatalytic activities are studied. The TiO2 nanorods (NRs) are grown on micropyramidal Si (MP-Si) substrates by a facile and low-temperature chemical bath deposition technique. To obtain the optimum structure with low reflectivity, we systematically investigate the optical properties by varying the growth conditions of TiO2-NRs such as chemical concentrations, growth temperature, and growth time. The optimized TiO2NRs/MP-Si exhibit efficient antireflective characteristics, indicating the much lower average reflectance (Ravg) and solar weighted reflectance (Rsw) values of ∼ 3.5% and ∼ 3.3%, respectively, in a broad wavelength range of 300-1000 nm compared to the bare Si and the MP-Si (i.e.,

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Ravg/Rsw ∼ 41.5%/39.2% and 25.6%/24%). Furthermore, the TiO2-NRs/MP-Si show a superhydrophilic surface with water contact angles below 5°, which can act as a protecting layer against mechanical damages and ensure high water wetting (i.e., antifogging and removal of surface dust particles). It also reveals the relatively good photocatalytic activity under ultraviolet (UV) and solar light illuminations. However, to further induce the degradation of dye molecules (i.e., rhodamine B), the synthesized Ag nanoparticles are integrated on the surface of TiO2NRs/MP-Si, resulting in dye degradation rates of 94% (UV) and 91% (solar) after 230 min, respectively (e.g., removal of organic pollutants).

KEYWORDS: titanium dioxide nanorods, micropyramidal silicon, antireflective coatings, photocatalysis, superhydrophilicity

INTRODUCTION Titanium dioxide (TiO2) has been widely used as a thin-film antireflective coating (ARC) material in semiconductor-based solar cells because of its lower refractive index (n) of ~ 2.1-2.6 compared to silicon (Si) and gallium arsenide (n > 3.5).1,2 However, the thin-film ARCs exhibit a narrow band of low reflection in the light incidence of wavelengths and angles.3 As an alternative, biomimetic double-scale nano/micro-structures like ‘moth eye’ have been developed to suppress the surface reflection losses1,4,5 and to increase the probability of light trapping and scattering in the active layer of solar cells6,7 because they form linear effective gradientrefractive-index profiles between air and the bulk surface and extend effective optical path lengths due to the diffracted and rebounded lights between microstructure arrays, resulting in the enhanced device performance. However, the fabrication of these nano/micro-structures often


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involves expensive and complex processes including electron-beam,8,9 laser interference,10 nanoimprint,11,12 or photo lithography,13 followed by the dry etching process. Herein, we adopted a facile, cost-effective, and low-temperature chemical bath deposition (CBD) technique and KOH wet etching process to produce these nano/micro-structures on Si substrate. Additionally, the TiO2 has drawn great attention for a variety of applications such as energy conversion devices, supercapacitors, photocatalysis, and antibacterial coatings because of its excellent long-term stability, ease of availability, non-toxicity, and environmentally friendly nature.14-17 Besides, the TiO2 film/nanostructures have been also utilized as a surface passivation layer to suppress the surface recombination losses in textured Si-based silicon solar cells.18,19 Especially, the TiO2 shows superior photocatalytic self-cleaning property that can effectively decompose the organic pollutants adsorbed on its surface into water and carbon dioxide in the presence of ultraviolet (UV) light (i.e., photocatalytic chemical-reaction self-cleaning).20,21 However, the photocatalytic activity of TiO2 is only limited under UV light irradiation (< 400 nm) due to its high band gap energy (3.0-3.2 eV) and quick recombination of the photogenerated electron and hole pairs.22,23 In order to improve the efficiency of photocatalytic property, several strategies have been studied on increasing the active surface area, modifying the band-gap by doping with metals or non-metals, and incorporation of noble metals, and so on.23-25 Among these, the metal (e.g., gold and silver) nanoparticle-integrated TiO2 composites can decrease considerably the electron-hole pair recombination rate and enhance the interfacial charge-transfer reaction rate, resulting in the improved photocatalytic activity of TiO2.26,27 Besides, the nano/micro-structured TiO2 with a large roughened surface shows a high wetting behavior can remove the dust particles on its surface owing to its super-hydrophilic surface. Since, such


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surface can be utilized to produce a water sheet instead of water droplets and thus it carries the dirt contaminants away (i.e., physical-reaction self-cleaning).28,29 Recently, several researchers have developed multifunctional (i.e., self-cleaning and antireflective) coatings by double-layered TiO2-SiO2 nanostructured films, layer by layer assembly of TiO2 nanoparticles and SiO2 sub-micrometer particles as building blocks, and by synthesizing the raspberry-like SiO2-TiO2 core-shell nanoparticles, etc.30-32 Also, the nanostructured TiO2 (e.g., nanopores, nanorods, and hierarchical architectures) has been studied as an multifunctional coating since it offers a large effective surface area, which can reduce the surface reflectance losses and enhance the possibility of collecting the incident light for photocatalytic reaction.28,32-34 To further improve the photocatalytic activity of TiO2, metal nanoparticles have been used for plasmonics. Metal nanoparticles can be easily synthesized and coated on the surface of various substrates by simple and cost-effective dip-coating method.35-37 Furthermore, for the high-roughened TiO2, the super-hydrophilic surface might help the metal nanoparticles in aqueous solution to relatively well form the surface due to its strong affinity to water.38 To our best knowledge, however, there is no fabrication and characterization of metal nanoparticle-integrated TiO2/Si with forest-like hierarchical nanorods/micropyramid arrays for antireflective and self-cleaning (i.e., plasmonic photocatalysis and super-hydrophilicity) coatings via a facile, cost-effective, and low-temperature chemical bath deposition (CBD) technique. In this work, we successfully demonstrated the excellent antireflective, high water wetting, and photocatalytic activities of the hierarchical silver (Ag)/TiO2/Si with forest-like nanorods (NRs)/micropyramid (MP) architectures, which were fabricated by a CBD method and a potassium hydroxide (KOH)-based chemical wet etching process. However, various ARCs such as Al2O3/TiO2 double layer,39 SiC-SiO2 nanocomposite,40 ZnO/Al2O3core/shell nanorods array,41


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sub-wavelength,42 and hybrid nano structures,43 etc. are reported to enhance the efficiency of solar cell. However, these reports only focused on optical properties. In our case, the forest-like Ag NPs/TiO2-NRs/MP-Si also shows superhydrophilic and photocatalytic effects, together with efficient AR property. Thus, it is very meaningful to analyze the optical property, water wettability, and photocatalytic activity by applying the Ag NPs and TiO2 NRs into the MP-Si. To attain the optimum structure of TiO2-NRs/MP-Si with efficient antireflective characteristics, we investigated its reflectance characteristics by varying the growth parameters of TiO2-NRs such as concentrations of titanium trichloride (TiCl3) and hydrogen chloride (HCl), growth temperature, and growth time. The surface wettability was also explored. Finally, the photocatalytic activity was studied on the photodegradation of Rhodamine B (RhB) dye molecules under UV and solar light illuminations, together with an integration of Ag-NPs, including the antimicrobial test.

EXPERIMENTAL AND OPTICAL SIMULATION DETAILS Fabrication of MP-Si substrates. Randomly-distributed micropyramidal arrays were fabricated on the Si surface by an anisotropic wet-chemical etching treatment (Fig. 1a). Initially, Si substrates with a size of 2 × 2 cm2 were subsequently cleaned by acetone, methanol, and deionized (DI) water for 5 min, respectively. After that, to remove unwanted oxide and organic impurities on the surface, the samples were dipped into buffer oxide etchant solution and 5 wt % hydrofluoric (HF) acid for 1 min and then were washed with DI water. Next, the cleaned Si substrates were immersed into a mixture solution of potassium hydroxide (KOH) and isopropyl alcohol to form the micro-scale pyramid structures on the Si surface (i.e., MP-Si). In general, the irregular micropyramidal arrays could be fabricated on the surface of (100)-oriented monocrystalline Si by the alkaline anisotropic wet-etching treatment. Herein, the wet etching process using an alkaline KOH etchant solution under low temperatures (< 100 °C) and


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concentrations (< 0.5 M), resulting in a four-sided pyramidal geometry through exposed {111} faces. The detailed formation mechanism of irregular MPs on the Si surface can be found in our previous work.44 At KOH:IPA of 10:3 vol%, the MP-Si showed the lowest reflectance spectrum in the wavelength range of 300-1000 nm (Fig. S1 of the Supporting Information). Thus, in this experiment, we chose KOH:IPA of 10:3 vol % for the MP-Si. Finally, the wet-etched MP-Si samples were dipped into a mixture solution of HCl, hydrogen peroxide (H2O2), and DI water (HCl:H2O2:H2O = 1:1:5 vol%) at 80 °C for 30 min to remove residual potassium impurities on the surface of samples.

Growth of TiO2-NRs on MP-Si. In order to enhance the adhesion between the Si and TiO2 seed layer and also to make the hydrophilic surface of the MP-Si, the samples were treated with UV/ozone plasma for 20 min. We prepared a TiO2 colloidal solution by dropwise adding 25 mL ethanol and 1 mL of DI water into a mixture solution containing 17 mL of titanium (IV) butoxide and 4.1 mL of diethanolamine in 53 mL absolute ethanol.45 A thin TiO2 seed layer was spin-coated on the surface of MP-Si using 5 µL of TiO2 colloidal solution and then the samples were annealed at 500 °C for 1 h in a furnace. To grow the TiO2-NRs by a CBD method (Fig. 1a),46,47 the TiO2 seed layer/MP-Si samples were soaked in a glass container with a reaction solution consisting of 50 mL of DI water, 1.25-2.5 mL of TiCl3, and 0.25-1.25 mL of HCl. Subsequently, the glass container was tightly packed and placed into the oven, to perform the CBD process at 80 °C for different growth times of 1-20 h, which results in the forest-like TiO2NRs on MP-Si (as shown in the SEM image of Fig. 1a) with further washing the samples using DI water.

Formation of Ag-NPs on TiO2-NRs/MP-Si. Silver nanoparticles (Ag-NPs) were formed on the surface of forest-like TiO2-NRs/MP-Si by soaking the samples for 40 min in Ag


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NP-aqueous solution, synthesized in 100 mL of DI water containing 3 mM tannic acid and 1 mM silver nitrate with adjustment of pH ~ 7 by adding potassium carbonate, in a dark room. After that, the samples were dried at room temperature.

Characterization. Structures of the fabricated samples were observed by using a fieldemission scanning electron microscope (FE-SEM: LEO SUPRA 55, Carl Zeiss) and transmission electron microscope (TEM: JEM-2100F, JEOL) systems. Crystallinity, energy dispersive X-ray (EDX), and selected area electron diffraction (SAED) analyses of TiO2 nanorods and Ag-NPs were additionally carried out by the TEM system. X-ray diffraction (XRD) patterns were examined by using a M18XHF-SRA, Mac Science X-ray diffractometer using Cu Kα-radiation (λ= 0.15418 nm) in a 2θ range of 3-90° (step size: 0.02°) to explore crystallinity and phase of samples. The composition and chemical states of samples were also characterized by using an Xray photoelectron spectroscopy (XPS) system (Thermo Multi-Lab 2000 System). The optical characteristics were investigated by using a UV-vis-NIR spectrophotometer (Cary 5000, Varian) with an integrating sphere at normal incidence and a spectroscopic ellipsometry (V-VASE, J. A. Woollam Co. Inc.) at incident angles (θinc) of 20-70° under the un-polarized light. Surface wettability was characterized by using a contact angle measurement system (Phoenix-300, SEO Co., Ltd.). Time-correlated single photon counting (TCSPC) or fluorescence lifetime spectra were measure by using a time-resolved luminescence spectrometer (QM-4/2005SE).

Evaluation of photocatalytic activity. Photocatalytic activity of forest-like TiO2NRs/MP-Si with and without Ag-NPs towards the degradation of rhodamine B (RhB) as a model dye was explored under the UV or solar light illumination at ambient temperature. Samples with a size of 1×2 cm2 were horizontally immersed into the RhB solution of 10 mL (1 µM) in a glass container. After that, the glass container was placed under the UV or solar light irradiation for 0-


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230 min. The distance between the light source and sample was maintained at 20 cm to minimize the heat effect. The solar light irradiation was provided by using a solar simulator (SUN 3000, ABET) with a 300 W Xe short arc lamp, while the UV light was provided by the same Xe short arc lamp (300 W) with a short pass 400 nm filter (Edmund Optics, Barrington). After a certain period of light irradiation on the suspension, the absorption spectrum of the RhB solution was measured in a wavelength range of 350-700 nm by using a UV-vis-NIR spectrophotometer (Cary 5000, Varian). For recyclability and stability of the samples, the photocatalytic activity was repeated 10 times under the UV and solar light illuminations for 230 min. We further tested the photocatalytic effect of Ag-NPs/TiO2-NRs/MP-Si via the inactivation of bacteria. For this, Escherichia coli (E. coli) model bacteria DH10b strain, also known as TOP 10 E. coli strains categorized in a Gram-negative bacterium, was employed. Luria broth (LB) with agar medium after autoclave was used. 100 µl of DH10b E. coli strain was equally spread on these LB agar plates and it was kept completely dry for 2 min. We placed the Ag-NPs/TiO2-NRs/MP-Si substrate on an agar plate in contact with the bacterial lawn. Finally, these plates were incubated for 14 h at 37 °C, following which the zone of E. coli inhibition was observed under the microscope.

RESULTS AND DISCUSSION Figure 1b shows the reflectance spectra of the bare Si, MP-Si, and TiO2 seed layer/MP-Si. As shown in Fig. 1b, the bare Si with a high refractive index (n ≥ 3.4) exhibited a high reflectivity of > 35% over a wavelength region of 300-1000 nm, showing a high average reflectance (Ravg) value of ∼ 41.5%. However, the MP-Si (i.e., height = 3-6 µm and bottom size = 5-10 µm from the SEM image of Fig. 1b) had a lower reflectance (i.e., Ravg ∼ 24%) than that of the bare Si due


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to the extension of effective optical path lengths by the diffracted and rebounded lights between MP arrays.48,49 On the other hand, the TiO2 seed layer-coated MP-Si showed a further reduction of reflectance (i.e., Ravg ∼ 16.5%). This is attributed to the TiO2 layer with lower refractive index (i.e., nTiO2 ~ 2.1-2.6) compared to the bare Si, showing a linear step gradient-refractive-index distribution of air (n = 1)/TiO2 seed layer (n ~ 2.1-2.6)/Si (n ~ 3.4).50 From the SEM images of Fig. 1b, it can be observed that the TiO2 seed layer with a thickness of ~ 100±30 nm was coated on the surface of MP-Si substrate. Using the TiO2 seed layer/MP-Si, the TiO2-NRs were grown to further suppress the surface reflectivity. Also, for the optimum structure with the outstanding antireflective property, we adjusted the growth parameters such as chemical concentrations, growth temperature, and growth time and investigated their structural and optical properties.

Structural properties of TiO2-NRs. The XRD pattern of the TiO2-NRs/MP-Si is shown in Fig. 2a. The TiO2-NRs were grown at 80 °C in 0.77 mM TiCl3 solution for 10 h. Most of the diffraction peaks are well matched with one of the tetragonal TiO2 anatase phase, and they are in good agreement with the standard XRD data (JCPDS 84-1286). However, the two diffraction peaks located at 2θ = 27.99° and 42.03° correspond to the standard XRD data (JCPDS 88-1175), showing the presence of rutile along with anatase phase in TiO2-NRs. From the XRD data, it can be noted that the TiO2-NRs grown on the MP-Si substrate were well crystallized and there were no impurity peaks except a broad diffraction peak at 2θ = 69.62°, which is associated with (400) plane of the silicon substrate (JCPDS 80-0018). The width of the TiO2-NRs is determined from the TEM image (Fig. 2b) and found to be in the range of ~ 25-30 nm. The high-resolution TEM measurement (Fig. 2c) indicates that the materials are crystalline with a d-spacing of 0.32 nm related to the (110) plane (JCPDS 81-1175). In addition, a set of well-defined diffraction spots were observed in the selected area electron diffraction (SAED) patterns, which can also verify


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the crystallinity of TiO2-NRs (Fig. 2d). The constituent elements of a TiO2-NR were characterized by elemental mapping. The titanium (Ti), oxygen (O), and Si elements in a single TiO2-NR appeared shown in Fig. 2e-2h.

Effect of growth parameters on structural and optical properties. To obtain the optimum structure of TiO2-NRs/MP-Si with efficient antireflective characteristics, the growth process of TiO2-NRs was carried out at different TiCl3 concentrations, HCl amounts, growth temperatures, and growth times. TiCl3 concentration: Figure 3 shows the (a) 30°-tilted and cross-sectional SEM images and the (b) reflectance spectra of the TiO2-NRs/MP-Si samples for different TiCl3 concentrations of 0.43, 0.51, 0.77, and 0.86 mM. The HCl amount, growth temperature, and growth time were kept at 1 mL, 80 °C, and 10 h, respectively. The growth morphology of TiO2-NRs is strongly dependent on the TiCl3 concentration, as can be seen in Fig. 3a. As the concentration of TiCl3 was increased, the density of the TiO2-NRs became larger. The height of the NRs was also increased, indicating 300, 460, 702, and 760 nm for different TiCl3 concentrations of 0.43 to 0.51, 0.77, and 0.86 mM, respectively. However, the NRs were sparsely grown at low TiCl3 concentrations (i.e., 0.43 and 0.51 mM) while the highly-packed forest-like TiO2-NRs began to grow uniformly on the MP-Si substrate at the TiCl3 concentrations of 0.77 and 0.86 mM. This result indicates that TiO2-NRs with higher densities and heights can be obtained at relatively larger amounts of TiCl3 due to the growth rate enhancement of TiO2-NRs by TiCl3.45,51 The reflectance depends on the surface morphologies of TiO2-NRs on the MP-Si (Fig. 3b). As the concentration of TiCl3 was increased from 0.43 to 0.77 mM, the Ravg of TiO2-NRs/MP-Si was reduced from 6.5 to 4.9% and then increased to 5.9% at 0.86 mM. This is ascribed to the formation of TiO2-NRs with relatively high height and density at 0.77 mM, leading to the effective gradient-refractive-index profile between


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the air and Si via the TiO2-NRs. On the other hand, at high TiCl3 concentration of 0.86 mM, the reflectance was increased due to the mismatch of refractive indices between air/TiO2-NRs/seed layer/MP-Si caused by much denser TiO2 NRs (especially, bottom area) and thicker seed layer on MP-Si. HCl and growth temperature: A strong aqueous acidic medium is more beneficial to slow down the hydrolysis reaction rate of Ti precursor, thus resulting in the formation of vertically welloriented TiO2-NRs.51,52 To figure out the effect of HCl, we investigated the growth morphologies and optical properties of the TiO2-NRs on the MP-Si at different HCl amounts in the range of 0.25-1.25 mL (Fig. 4 and Fig. S2a). The TiCl3 concentration, growth temperature, and growth time were maintained at 0.77 mM, 80 °C, and 10 h, respectively. As shown in the SEM images of Fig. S2a, at the low HCl amount (0.25 mL), porous film-like TiO2-NRs, which have hardly the growth orientation, were formed on the MP-Si substrate due to the less acidic medium. On the other hand, at the high HCl amount of 1.25 mL, denser TiO2 film consisting of highly-packed vertically-grown NRs was coated. However, when appropriate amount of acid (i.e., 0.5 mL of HCl) was added to the growth solution, vertically-oriented TiO2-NRs with the better porosity between NRs were formed on the MP-Si, showing a lowest reflectivity (Ravg ∼ 3.5%) compared to the other samples (i.e., Ravg ∼ 4.0, 4.9, and 12.6% at 0.25, 1, and 1.25 mL of HCl, respectively), as can be seen in Fig. 4a. The effect of growth temperature on the growth morphologies and optical properties of TiO2-NRs/MP-Si was also explored (Fig. 4b and Fig. S2b). The TiCl3 concentration, HCl amount, and growth time were kept at 0.77 mM, 0.5 mL, and 10 h, respectively. At a low growth temperature of 60 °C, the TiO2-NRs were unevenly grown on the surface of MP-Si substrate due to the low reaction rate.51,52 On the contrary, at higher temperatures of ≥ 80 °C, the uniformly


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well-oriented TiO2-NRs were formed, indicating the lower Ravg value of ∼3.5% at 80 °C compared to the other samples (Ravg ∼ 7.9 and 5.3% at 60 and 100 °C, respectively). Growth time: Figure 5 shows the (a) 10°-tilted SEM images and (b) reflectance spectra of TiO2NRs/MP-Si at different growth times. The growth temperature, TiCl3 concentration, and HCl amount were fixed at 80 °C, 0.77 mM, and 0.5 mL, respectively. From the SEM images (Fig. 5a), it can be observed that the density and height of TiO2-NRs on the MP-Si were increased with increasing the growth time. The TiO2-NRs began to grow on the surface of seed layer/MP-Si after 1 h and were formed like a forest with highly-packed NRs after 10 h. For the TiO2NRs/MP-Si samples, as the growth time was increased from 1 to 10 h, the Ravg value was reduced from 13.7 to 3.5% at wavelengths of 300-1000 nm. This is attributed to the increased height and uniform distribution of NRs on the MP-Si. On the other hand, at the longer growth time of 20 h, the Ravg value was increased to be ~ 5% owing to the highly-packed taller NRs (i.e., ~ 1.5 µm) which look like a bulk film, leading to the increase of reflectivity. From these results, the optimum TiO2-NRs/MP-Si with the efficient AR property (i.e., Ravg ∼3.5%) was finally obtained at a TiCl3 concentration of 0.77 mM, HCl amount of 0.5 ml, the growth temperature of 80 °C, and the growth time of 10 h. For comparison, the TiO2-NRs were also fabricated on the flat-Si substrate (i.e., non-forest-like TiO2-NRs/flat-Si) by the same TiO2-NRs growth conditions and its optical reflectance was investigated (Fig. S3). The forest-like TiO2-NRs/MP-Si had superior antireflective characteristics compared to the non-forest-like TiO2-NRs/flat-Si over the wide range of wavelengths (300-1000 nm), showing the lowest Ravg value of ~ 3.5% (i.e., Ravg ~ 7.2% TiO2-NRs/flat-Si). The efficient antireflective characteristics of forest-like TiO2-NRs/MPSi can be also verified from the photograph in the inset of Fig. 5b. The florescent white light is clearly observed on the surface of the bare Si. For the TiO2-NRs/flat-Si, the white light is partly


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reflected on its surface despite a black surface. On the other hand, the white light is not reflected on the surface of MP-Si because of the light-scattering effect, but its surface still looks like bright (i.e., not perfect black). However, for the TiO2-NRs/MP-Si, there is no any reflected white light on its surface (i.e., perfect dark black). These results indicate that the forest-like TiO2NRs/MP-Si strongly absorb visible lights. In addition, for solar cell applications, we estimated the solar weighted reflectance (RSW), which is defined as the ratio of the usable photons reflected to the total usable photons, by normalizing the reflectance spectrum integrated with the terrestrial AM1.5 global spectrum in a wavelength range of 300-1000 nm.53-55 For the optimum TiO2-NRs/MP-Si sample, the smaller RSW value of ~ 3.3% is evaluated compared to the TiO2-NRs/flat-Si, TiO2 seed layer/MP-Si, MPSi, and bare Si (i.e., RSW= 7.48, 15.6, 23.4, and 39.2%, respectively). Therefore, it is noted that the TiO2-NRs/MP-Si can be used as an efficient ARC to enhance the efficiency of Si-based solar cells, considering that the ARCs with lower RSW values can further improve the device performance of solar cells.56

Light-scattering and angle-dependent characteristics of TiO2-NRs/MP-Si. To demonstrate the light trapping and scattering characteristics of the TiO2-NRs/MP-Si structure, we performed the numerical optical analysis using the finite-difference time-domain (FDTD) simulations. The simulation model of TiO2-NRs/MP-Si used to calculate the electric field distributions is shown in Fig. S4. Figure 6a shows the contour plots of calculated electric field (Ey) intensity distribution for optical light-propagation properties of bare Si, TiO2-NRs/flat-Si, and TiO2-NRs/MP-Si via FDTD simulations at λ = 530 nm. As shown in Fig. 6a, for the bare Si, there is no scattered propagation of light inside the Si. For the TiO2-NRs/flat-Si, weak light scattering was observed. However, the TiO2-NRs/MP-Si exhibits a strong light-scattering


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behavior with the wide angular spread. This is mainly caused by the extended effective optical path lengths due to the rebound lights between the micro pyramids on Si. Thus, these theoretical results show the increased photons transmitted through the surface of the TiO2-NRs/MP-Si, suppressing light losses escaping back to air. In other words, the simulation results clarify that the TiO2-NRs/MP-Si structures are more beneficial (i.e., not a disadvantage) to enhance the probability of light trapping and scattering due to the formation of linear effective gradientrefractive-index profiles and the extension of effective optical path lengths caused by the diffracted and rebounded lights between microstructure arrays. Therefore, the forest-like TiO2NRs/MP-Si structures have a strong light-scattering behavior inside Si, which can cause the efficiency enhancement of solar cells.57,58 In addition, the light incident angle-dependent reflectance characteristics of both samples (i.e., forest-like and non-forest-like structures) were also investigated (Fig. 6b). The ARCs with omnidirectional characteristics are essential for various Si-based solar cells. The bare Si exhibited the high reflectivity (i.e., > 35%) at θinc = 2070° in a wavelength range of 300-1000 nm. On the other hand, the TiO2 NRs/flat-Si considerably decreased the reflectance of bare Si in wide ranges of incident angles and wavelengths. But, for the forest-like TiO2-NRs/MP-Si, the reflectance was further reduced, showing the lower Ravg value of ~ 0.1 % at θinc = 20-70° in a wavelength range of 300-1000 nm (i.e., Ravg ~ 40% and ~ 4.5% for bare Si and TiO2-NRs/flat-Si, respectively). From these results, the forest-like TiO2NRs/MP-Si is a superior ARC compared to the planar-type ARCs in broad ranges of wavelengths and incident angles.

Surface wettability of TiO2-NRs/MP-Si. For outdoor applications, the surface wetting behavior of the TiO2-NRs/MP-Si was studied (Fig. 6c). The bare Si had a hydrophilic surface with a water contact angle (θCA) of ~ 71.2°. On the other hand, the TiO2-NRs/MP-Si exhibited a


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super-hydrophilic wetting behavior with θCA values of < 5°. The TiO2 film with a planar surface revealed the hydrophilic property with relatively larger θCA values of > 20°.59 However, when the surface roughness is increased by nano- or micro-scale structures, the super-hydrophilic surface can be achieved, as explained by Wenzel model.60,61 The super-hydrophilicity makes water droplets to spread out like a sheet on the surface. When a water sheet flows on the superhydrophilic surface, it can wedge into the space between the dust particles and the surface and move the dust particles away (i.e., self-cleaning effect). Besides, the super-hydrophilic TiO2NRs/MP-Si can be also employed for anti-fogging and quick-drying applications.61,62 Thus, the efficient antireflective TiO2-NRs/MP-Si with the additional self-cleaning ability would increase the practical feasibility to maintain the device performance (i.e., photovoltaic systems) in outdoor dusty environments.

Photocatalytic activity of TiO2-NRs/MP-Si. If organic contaminants are adsorbed on the surface of the super-hydrophilic ARCs, the device performance can be degraded since they strongly stick on the surface of ARCs and would interfere with the incident light absorbed into the solar cells. Therefore, it is necessary to remove the organic pollutants on the surface of devices. This can be solved from the photocatalytic self-cleaning ability of TiO2-NRs by decomposing the organic contaminants under UV/solar light illuminations. TiO2 is one of the best-known photocatalyst. Therefore, we simply tested the photocatalytic activity of forest-like TiO2-NRs/MP-Si on the degradation of RhB dyes in aqueous solution. The TiO2-NRs/MP-Si sample was immersed into the RhB-dye solution and irradiated by the UV and solar lights. The absorption spectra of RhB dye solutions as a function of UV and solar light irradiation time in the presence of TiO2-NRs/MP-Si are shown in Fig. 6a and 6b. From both the absorption spectra, it can be clearly observed that the absorption spectrum of RhB dye is gradually decreased with


15


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increasing the UV and solar irradiation time. Meanwhile, the mismatch of absorption profiles between UV and solar lights is observed. The absorption intensity is gradually varied without any large shift of two absorption peaks (i.e., around 520 and 554 nm) under UV light irradiation. On the other hand, there are variations in the shape of spectra, showing that the absorption peaks were shifted towards the shorter wavelengths at solar (i.e., UV and visible) light irradiation. This spectral variation or wavelength shift is mainly attributed to the de-ethylation of RhB and the similar tendency can be also found in previous works.63-65 It could be assumed that when light was illuminated on RhB (Tetraethyldiamino-o-carboxyphenyl xanthenyl chloride), it loses the ethyl groups gradually and causes a shift or change in the absorption peaks. Figure 6c shows the photodegradation rate (C/Co) in the presence and absence of the TiO2-NRs/MP-Si. From the photodegradation rate (C/C0, where C0 and C are the initial and later absorption peak intensities of RhB dye at the maximum absorption wavelength of 553 nm), the RhB dye is more rapidly decomposed under UV light after 230 min, exhibiting the higher photodegradation rate of 82% compared to that (i.e., 66%) under solar light. However, in the absence of photocatalyst, the negligible photodegradation rates (i.e., 6 and 5%) of RhB dye were observed after 230 min under the UV and solar irradiations. In addition, the recyclable photocatalytic characteristics of TiO2NRs/MP-Si under UV light were also studied (Fig. 6d). Up to 10 cycles, the photodegradation rate of RhB dye was nearly identical. This indicates that the TiO2-NRs/MP-Si can serve as highly effective and sustainable photocatalyst with recyclability. However, the lower photocatalytic efficiency of TiO2-NRs/MP-Si is observed at solar light irradiation compared to that at UV light irradiation. Thus, it is necessary to further enhance the photocatalytic activity under solar lights.

Photocatalytic activity of TiO2-NRs/MP-Si incorporated with Ag-NPs. To enhance the photocatalytic activity of TiO2, we integrated the Ag-NPs on the surface of TiO2-


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NRs/MP-Si. The presence of Ag-NPs on the surface of TiO2-NRs/MP-Si is verified from the XRD pattern of Ag-NPs/TiO2-NRs/MP-Si (Fig. 7a). For the Ag-NPs/TiO2-NRs/MP-Si, an additional small diffraction peak at 2θ of 77.4°, which is perfectly matched with the facecentered cubic phase (fcc) of Ag (JCPDS 04-0783) and also indexed to (311) reflection, was observed, along with the diffraction peaks of TiO2. Moreover, it can be noted that the incorporation of Ag-NPs did not cause any shift in the peaks of TiO2-NRs/MP-Si and change its crystalline behavior. The XPS spectra (Fig. S5) of the TiO2-NRs/MP-Si incorporated with AgNPs also confirmed the presence of Ag in metallic form (Ago) on the surface of TiO2-NRs.66 Additionally, to observe the shape and size of Ag-NPs, TEM analyses were performed (Fig. 8b). It can be observed that a spherical-like Ag-NP with diameters of approximately 13 nm is located on the surface of TiO2-NR, showing two lattice fringes with interplanar spacing of 0.32 nm and 0.23 nm corresponded to the crystallographic (110) plane of TiO2 and the (111) plane of the fcc phase of Ag, respectively.67 From the EDX spectrum (Fig. S6), the Ag in a single TiO2-NR integrated with Ag-NPs is also detected with 1.65 weight% and 0.32 atomic%, respectively. As shown in Fig. 8c and 8d, the introduction of Ag-NPs into the TiO2-NRs/MP-Si leads to a significant improvement on the photodegradation of RhB dye compared to the TiO2-NRs/MP-Si without Ag-NPs under UV and solar light irradiations. Especially, at the solar light, the photodegradation of RhB dye in the presence of Ag/TiO2-NRs/MP-Si is faster than that of the TiO2-NRs/MP-Si without Ag nanoparticles, showing an exponential decay in the RhB dye concentration (Fig. S7a). Therefore, the Ag-NPs on the surface of TiO2-NRs further enhances photodegradation of RhB dye, exhibiting the larger photodegradation rates of 94 and 91% than those (i.e., 82 and 66%) of TiO2-NRs/MP-Si without Ag-NPs under UV and solar light


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irradiations after 230 min, respectively. The reaction kinetics of photocatalytic degradation is found by following an apparent pseudo-first order rate equation:

ln(C C0 ) = − kt , where k is the rate constant and t is the irradiation time. The relation between ln(C/C0) and UV or solar light irradiation time is plotted and fitted linearly in Fig. S7b. The calculated rate constant (k) and linear correlation coefficient (R2) are shown in the Table S1 of Supporting Information, and the R2 values (> 0.89164) clearly support the pseudo-first order rate kinetics. Additionally, the methylene blue (MB) dye was also employed to study the photocatalytic activity of all the samples under UV and solar light irradiations (Fig. S8). The photodegradation trend of MB dye is similar to the RhB. In general, the TiO2 generates the electron-hole pairs under the light irradiation, but the lifetime of the photogenerated charge carriers and the time scale of chemical interactions of TiO2 with the adsorbed organic contaminants (i.e., RhB dyes) may not be matched, which causes the low photocatalytic activity. However, in the case of TiO2 incorporated with Ag-NPs, the photogenerated electrons in TiO2 under UV light are locked by Ag-NPs or the carriers generated from Ag-NPs, which can absorb the visible (or solar) light in the wavelength range of > 400 nm (Fig. S9 and S10), are reversely injected into the conduction band of TiO2, thus improving the photocatalytic reactions.68-70 To monitor the charge carrier dynamics or the electron transfer in between semiconductors (i.e., TiO2-NRs) and metallic nanoparticle (i.e., Ag-NPs), the time-correlated single photon counting (TCSPC) or fluorescence lifetime spectra were measured (Figure S11), under an excitation wavelength of 295 nm and emission wavelength at 465 nm. As shown in Figure S11, the Ag-NPs/TiO2-NRs/MP-Si shows a prolonged lifetime of 9.732 µs compared to the pure TiO2-NRs/MP-Si (7.615 µs) without AgNPs. The increase in lifetime was mainly due to the spatially separated electron and holes, which


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extends the electron-hole recombination and is advantageous for photocatalysis, as described above. Furthermore, the excitation of localized plasmon polaritons on the surface of Ag-NPs considerably enhances the near-field amplitude at each corresponding resonant wavelength. This can also significantly boost the photocatalytic activity of TiO2, known as plasmonic photocatalysis.71,72 Additionally, to explore the long-term stability of Ag-NPs, the recyclable photocatalytic activity of Ag-NPs/TiO2-NRs/MP-Si under UV light was also examined (Fig. S12). The Ag-NPs/TiO2-NRs/MP-Si show similar performance on the photodegradation of RhB dye up to 10 cycles. These results indicate that the Ag-NPs/TiO2-NRs/MP-Si can also serve as highly effective and sustainable photocatalyst with recyclability. Furthermore, it can be confirmed that Ag-NPs are strongly adhered onto the surface of TiO2-NRs. Therefore, the AgNPs/TiO2-NRs/MP-Si can be employed to decompose the organic contaminants as a selfcleaning layer, together with efficient antireflection and super-hydrophilicity.

Antimicrobial effect of Ag-NPs /TiO2-NRs/MP-Si. For photocatalytic applications, most of the researchers have tested the bacteria inactivation of photocatalyst.73,74 The antimicrobial effect of the Ag-NPs/TiO2-NRs/MP-Si is determined in terms of E. coli inhibition zone formed on agar medium. As shown in Fig. S13, for the plate without any substrate, the E. coli colonies were uniformly distributed in a plate. On the other hand, it can be observed that there are no any E. coli colonies in an area placed by the Ag-NPs/TiO2-NRs/MP-Si, indicating the zone of inhibition (i.e., weight dotted line area) for E. coli. The microscopic images also clarify the formation of inhibition zone for E. coli (Fig. S13c). This zone of inhibition for E. coli was mainly attributed to the negative oxygen ion species generated from Ag-NPs/TiO2-NRs/MP-Si photocatalyst.74,75


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CONCLUSION We successfully fabricated the forest-like nano/micro-architectures on Si substrate (i.e., TiO2NRs/MP-Si) by utilizing a facile, cost-effective, and low-temperature CBD technique and KOH wet etching process and investigated their optical, surface wetting, and photocatalytic behaviors. To obtain the optimum structure of the TiO2-NRs/MP-Si with efficient antireflection, the growth morphologies and optical properties were systematically observed at different growth conditions such as TiCl3 concentration, HCl amount, growth temperature, and growth time. The resulting optimized TiO2-NRs/MP-Si had efficient AR characteristics in the wide wavelength range of 300-1000 nm, exhibiting a much lower Ravg value of ∼ 3.5% than those (i.e., Ravg ∼ 25.6 and 41.5%) of MP-Si and bare Si. The structure also showed a super-hydrophilic surface with θCA values of < 5°. By integrating the Ag-NPs into the surface of TiO2-NRs/MP-Si using a simple dip-coating method, the significantly enhanced photocatalytic activity under UV and solar light irradiations

was

observed

compared

to

the

TiO2-NRs/MP-Si,

indicating

the

high

photodegradation rates of 94 and 91% at UV and solar lights after 230 min, respectively. From these results, the TiO2-NRs/MP-Si integrated with Ag-NPs can be employed as sustainable antireflective, plasmonic photocatalytic, and self-cleaning coatings for Si-based device applications.

Supporting Information This Supporting Information is available free of charge on the ACS Publication Website at DOI: xxxxxxxxxx.


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SEM images and reflectance of MP-Si samples at different KOH:IPA ratios, SEM images of the TiO2-NRs/MP-Si at various HCl concentrations and growth temperatures, measured reflectance of TiO2-NRs/flat-Si and TiO2-NRs/MP-Si, FDTD simulation model, XPS spectra of TiO2-NRs/MP-Si with and without Ag-NPs, EDX spectrum of single TiO2-NR coated with Ag-NPs, photodegradation and Pseudo-first order kinetic plots for RhB and MB dyes, reflectance/absorption and fluorescence lifetime spectra of TiO2-NRs/MP-Si and Ag-NPs/TiO2-NRs/MP-Si,

absorption

photocatalytic

Ag-NPs/TiO2-NRs/MP-Si,

test

for

spectrum

of

Ag-NPs

solution,

antibacterial

activity

Recycling test

of

photocatalysts. Corresponding Author *Email: [email protected] (Prof. J. S. Yu)

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4011998 and No. 2017H1D8A2031138).

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Figure 1. (a) Schematic diagram of the fabrication procedure of forest-like TiO2-NRs on the MP-Si substrate using a chemical bath deposition technique. (b) Reflectance spectra of the bare Si, MP-Si, and TiO2 seed layer/MP-Si. SEM images of the TiO2-NRs/MP-Si and TiO2 seed layer/MP-Si are shown in the insets of (a) and (b), respectively.


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Figure 2. (a) XRD pattern of TiO2-NRs on the MP-Si. (b) TEM image, (c) HRTEM image, and (d) SAED pattern of a single TiO2-NR. (e) SEM image and (f-h) EDX elemental mapping images of the TiO2-NRs/MP-Si. The insets of (e-g) show the TEM image and elemental mapping images of constituent elements for a single TiO2-NR, respectively. The sample is prepared at 80 °C for 10 h in 0.77 mM TiCl3 solution.


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Figure 3. (a) 30°-tilted and cross-sectional SEM images and (b) reflectance spectra of the TiO2NRs/MP-Si samples for different TiCl3 concentrations of 0.43, 0.51, 0.77, and 0.86 mM at HCl amount of 1 mL, growth temperature of 80 °C, and growth time of 10 h.


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(a) TiCl3 concentration: 0.77 mM

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60 °C 80 °C 100 °C

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Wavelength (nm) Figure 4. Reflectance spectra of the TiO2-NRs/MP-Si at different (a) HCl amounts and (b) growth temperatures.


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Figure 5. (a) 10°-tilted [i-v] and cross-sectional [vi] SEM images and (b) reflectance spectra of the TiO2-NRs/MP-Si samples at different growth times of 1, 2.5, 5, 10, and 20 h. Photographic images of the bare Si, MP-Si, TiO2-NRs/flat-Si, and optimized TiO2-NRs/MP-Si are shown in the inset of (b).


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Figure 6. (a) Contour plots of calculated Ey-field intensity distributions for the optical lightpropagation properties of bare Si, TiO2 NRs/flat-Si, and TiO2 NRs/MP-Si via numerical FDTD calculations at λ = 530 nm. (b) Measured reflectance spectra for bare-Si, TiO2-NRs/flat-Si, and TiO2-NRs/MP-Si as a function of light incident angle (θinc) from 20 to 70° under the unpolarized light. (c) Photographs of a water droplet on the surface of the bare Si and optimized TiO2-NRs/MP-Si.


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Figure 7. (a,b) Absorption spectra of rhodamine B (RhB) dyes as a function of irradiation time under UV and solar lights in the presence of TiO2-NRs/MP-Si. (c) Photodegradation of RhB dyes under UV and solar light irradiations. (d) Recycling properties of photodegradation of RhB dyes over 10 cycles in the presence of TiO2-NRs/MP-Si under UV light irradiation.


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Figure 8. (a) XRD patterns of the TiO2-NRs/MP-Si and Ag-NPs/TiO2-NRs/MP-Si samples. (b) TEM and HRTEM images of the Ag NP-incorporated TiO2-NR. (c,d) Absorption spectra of RhB in the presence of TiO2-NRs/MP-Si with and without Ag-NPs under the UV and solar light irradiations after 230 min.


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Table of Contents (TOC) Synopsis: Forest-like Ag-NPs/TiO2-NRs/MP-Si with omnidirectional broadband antireflective, photocatalytic, self-cleaning properties could be useful for sustainable high-performance solar applications.


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Micro-Architectures as

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