Silicon Moth-Eye Structures with Antireflection, pân

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Titanium Oxide/Silicon Moth-eye Structures with Antireflection, p-n Heterojunctions and Superhydrophilicity Gang Shi, Jie Chen, Likui Wang, Dawei Wang, Jingguo Yang, Ying Li, Liping Zhang, Caihua Ni, and Lifeng Chi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03117 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016

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Titanium Oxide/Silicon Moth-eye Structures with Antireflection, p-n Heterojunctions and Superhydrophilicity

Gang Shi,† Jie Chen,† Likui Wang,† Dawei Wang,† Jingguo Yang,† Ying Li,*,†,‡ Liping Zhang,† Caihua Ni,† and Lifeng Chi*,§,∥

†

The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of

Chemical and Material Engineering, Jiangnan University, Wuxi, 214122, China ‡

National Engineering Laboratory for Cereal Fermentation Technology, Jiangnan University,

Wuxi, 214122, China §

Physikalisches Institut and Center for Nanotechnology (C198eNTech), Westfälische

Wilhelms-Universität Münster, Münster, D-48149, Germany ∥

Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory for

Carbon-Based Functional Materials & Devices; Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China

Corresponding Authors * Ying Li. E-mail: [email protected]. * Lifeng Chi. E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: With employing KOH etching silicon and hydrothermal growth of titanium oxide (TiO2), TiO2 nanorods assembled on the silicon micro-pyramids to form biomimetic composite coating, similar to moth eyes. The biomimetic composite coating possessed not only the micro-nano hierarchical structures but also the p-n heterojunctions, resulting in a decrease in the reflection of incident light and an increase in the separation efficiency of photon-generated carriers. Meanwhile, the structures showed excellent superhydrophilicity, making for the self-cleaning of the material surface. We further demonstrate that, by exploiting the advantages of this method, the application of such structures in the photocatalysis field is thus straightforward.


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INTRODUCTION In 1972, the titanium oxide (TiO2) was applied in the fields of photocatalysis by Fujishima and Honda, marking the beginning of heterogeneous catalysis.1 As a n-type semiconductor, TiO2 has attracted wide attentions in the photocatalytic degradation of organic pollutants,2 the purification of water and air,3 the dye-sensitized solar cells,4 and the solar-based hydrogen production.5 However, TiO2 cannot absorb sun light effectively due to its large band gap of 3.0 eV for rutile.6 Therefore, to exploit effective visible light active TiO2 with a slower recombination rate of charge carriers and higher stability, many research activities mainly focused on the modification of the surface or bulk properties of TiO2 materials by doping with nonmetal atoms,7 noble metals,8 quantum-dot sensitization,9 and narrow-band gap semiconductors.10 Especially, TiO2 modified by narrow-band gap semiconductors forms heterojunctions at their interfaces, enhancing the electron and hole separation and broadening the absorbing range of sun-light. Silicon (Si), a narrow-band gap semiconductor (Eg = 1.1 eV), is the most commonly used material for solar energy conversion.11 However, due to the high refractive index (4.09) of pristine Si, more than 30% of incident light is reflected off its surface.12 In nature, the moth-eye corneas, consisting of combined micro- and nanostructures, can reduce the reflection at a broad range of wavelengths and angles of incidence, owing to the continuous gradient transition of the refractive index from air to the substrate.13-15 Therefore, a lot of reports showed that researchers strove to fabricate artificial moth eyes with various methods16-18 to suppress reflection of


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incident light, including block copolymer micelle lithography,19-21 phase separation,22 photolithography,23 electron-beam lithography,24,25 laser interference lithography,26,27 reactive-ion etching lithography,28,29 nanoimprint lithography,30,31 wet etching lithography32,33 and self-assembly.34,35 However, the above-mentioned reports only focused on suppressing the light reflection to increase the absorption of incident light, neglecting the utilization efficiency of absorbed light. Additionally, single Si material applied to artificial moth eyes, compared to composite material, is not beneficial to the multifunction. Here, combining wet etching and hydrothermal synthesis, the composite coating with artificial moth-eye structures was fabricated (TiO2 nanorods were coated on the Si pyramids). Especially, the artificial moth-eye structures led to an optimized graded refractive index and hence prominent excellently antireflective properties. Moreover, n-type TiO2 with a wide band and p-type silicon with a narrow band formed p-n heterojunctions, retarding the recombination of photo-generated electron and hole pairs and broadening the wavelength regions of the absorbing light. The application of such structures in the photocatalysis area is thus straightforward.

EXPERIMENTAL SECTION Chemicals and Instruments. Ethanol, hydrochloric acid, potassium hydroxide (KOH), titanium n-butoxide [Ti(OC4H9)4], methylene blue (MB) were purchased from Beijing Chemical Reagent Plant in the highest available purity and used without further purification. The Si wafers [p-type (100)] were obtained from Youyan Guigu


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Beijing, China. Wet Etching of the Silicon Wafer.36, 37 Firstly, the Si wafer was submerged in a HF (1% electronic grade) aqueous solution for 30 s, to remove the native oxide layer. The wafer was then washed in an ultrasonic bath with deionized water for 5 min and dried under nitrogen gas flow. At last, pyramidal structures were generated by etching the Si wafer in a solution of KOH (pH=14) for 5-35 min at 85 °C, and washed in an ultrasonic bath with deionized water for 5 min. Hydrothermal Growth of the TiO2 Nanorods.38 The Si substrate with pyramidal structures was immersed into 0.075 M Ti(OC4H9)4 in ethanol solution for 30 s. Then, the above sample was sintered at 450 °C for 30 min to yield layers of TiO2 nanoparticles. Finally, the TiO2 nanorods were grown from the TiO2 nanoparticles in the solution of 10 mL purified water, 10 mL hydrochloric acid (37%) and 0.5 mL Ti(OC4H9)4 at 130 °C for 8 h. Photocurrent Measurement. The photoelectric conversion of the sample was investigated using electrochemical method. Under the irradiation of a xenon light source, the photoelectric property measurements were performed in a Na2SO4 (0.5 M) electrolyte in a glass cell with a quartz window, where the sample served as the working electrode, Pt gauze as the counter electrode, and Ag/AgCl as the reference electrode. Photocatalytic Activity Measurement. Photocatalytic activities were performed by the degradation of MB under the irradiation from 300 W xenon light source. The sample (1.0×1.5 cm2) was put into the 3 mL MB solution (2 mg/L, neutral condition)


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and stirred in dark for 30 min to ensure the adsorption equilibrium before the light illumination. After light irradiation, the solution was collected hourly and immediately measured with a UV–vis spectrophotometer at the wavelength of 663 nm to calculate the MB concentration changes. Characterization. Scanning electron microscopy (SEM) measurements were carried out by using a Hitachi S4800 (Japan) microscope. X-ray diffraction (XRD) patterns were recorded by a Rigaku D/Max-2550 diffractometer with Cu Kα radiation (λ = 1.54056 Å) (40 kV, 350 mA) in the range of 20-70° (2θ). The ultraviolet-visible (UV-vis)

diffuse

reflectance

spectra

were

carried

out

using

TU-1901

spectrophotometer (Beijing Puxitongyong, China). Photocurrent response was recorded using a Gamry instruments framework electrochemical station using the irradiation of a Hal-320 compact Xenon light source. The water contact angle of the sample was determined by using a contact angle analyzer (DCA315, USA). The photocatalytic degradation of MB was conducted under the illumination of a 300 W xenon lamp.

RESULTS AND DISCUSSION Fabrication of TiO2/Si Moth-eye Structures. The fabrication process is schematically shown in Figure 1. The Si wafer [p-type, (100)] was initially etched by immersing it into KOH aqueous solution. Because of the different atom densities on


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Figure 1. Schematic illustration of the procedure for fabricating TiO2/Si moth-eye structures.

Si (100) and Si (111), the etching rate of Si (100) is much higher than that of Si (111), resulting in the formation of pyramidal shapes with the characteristic 54.7° sidewalls.36,37 Figure 2 shows the progression of the microscale Si pyramids (Si-p) generated with KOH chemical etching. It can be observed that the size and density of the pyramids increased with erosion time extending from 5 min to 30 min, as seen in Figure 2A-2C. In the process of the etching, V-shaped grooves between the pyramids became wider until the exposed (100) surface of Si disappeared (etching for 30 min), as seen in Figure 2C. Due to the defects on the (111) surfaces, the original pyramids split into smaller ones with further etching for 35 min, seeing Figure 2D. The Si-p for 30 min of etching was selected as the substrate for the growth of TiO2 nanorods, due to large lateral-surface and closely arrays of pyramids.


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Figure 2. Cross-sectional (45o tilt view) SEM photographs of Si-p fabricated with KOH etching time: A) 5 min, B) 15 min, C) 30 min and D) 35 min.

Secondly, to form artificial moth-eye structures, TiO2 nanorods were grown on Si-p via a hydrothermal method. The advantages of this design structure are in two areas. On the one hand, TiO2/Si moth-eye structures, consisting of combined microand nanostructures, could show a superior antireflection than the traditional two-dimensional coating.16 On the other hand, these structures could enhance the electron and hole separation by combining Si and TiO2 to fabricate the p–n junctions. Figures 3A-3C show the TiO2 nanorods SEM images with different scale and angle. The large-area TiO2 nanorods were uniformly and densely coated on the Si-p, as seen in Figure 3A. Figure 3B shows that the average height and width of TiO2 nanorods were 1.1±0.2 µm and 170±32 nm, respectively. The cross section of TiO2 nanorods was square in line with the ideal rutile TiO2, seeing Figure 3C. Figure 3D shows the sharp X-ray diffraction (XRD) measurements of TiO2 nanorods, confirming that the samples are highly pure and well crystallized rutile-structure.


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Figure 3. A), B) and C) TiO2 nanorods SEM images with different scale and angle, D) XRD pattern of TiO2 nanorods.

Photoelectric

and

Super-hydrophilic

Properties

of

TiO2/Si

Moth-eye

Structures. To demonstrate multifunction of TiO2/Si moth-eye structures, the reflection spectrum, light current and contact angle were studied. From the photographs of the un-etched Si (Si-u), the Si-pyramid (Si-p), TiO2 nanorod-coated Si-u (TiO2/Si-u) and TiO2 nanorod-coated Si-p (TiO2/Si-p), as shown in Figure 4A, TiO2/Si-p appeared darkest black, demonstrating that the reflection of TiO2/Si-p was the lowest among the four samples. The reflectivity was further measured using a spectrometer over a wavelength ranging from 250 to 800 nm. Figure 4B shows the measured reflection for Si-u, Si-p, TiO2/Si-u and TiO2/Si-p, respectively. More than 30% of the incident light was reflected on the Si-u, due to its high refractive index. The reflectance of Si-p with micro-sized geometry and TiO2/Si-u with nano-sized geometry was reduced with different degrees. Most significantly, TiO2/Si-p, with


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micro-sized and nano-sized geometry, reflected merely 5% of the incident light, which could be attributed to the biomimetic morphology of TiO2/Si-p and the refractive index of TiO2. TiO2/Si-p similar to moth-eye structures, according to effective medium theory,13,39 could dramatically reduce the reflection at the surfaces. Meanwhile, the TiO2 refractive index (2.8)40 between the ambient media and the Si was also contributing to suppressing the reflection of the substrate.

Figure 4. A) Photographs of Si-u (a), Si-p (b), TiO2/Si-u (c) and TiO2/Si-p (d), respectively. B) Antireflection spectra of Si-u (a), Si-p (b), TiO2/Si-u (c) and TiO2/Si-p (d), respectively.

The photoelectric conversion of TiO2/Si-p, contrasted with that of TiO2/Si-u, was investigated using electrochemical method. Figure 5A illustrates the charge flow in the junctions of n-type TiO2 and p-type Si under light illumination. Selecting TiO2/Si-p or TiO2/Si-u as working electrode, a saturated Ag/AgCl electrode and a platinum electrode as the reference and counter electrode, respectively, linear sweep voltammetry curves were measured in a glass cell with a quartz window with 0.5 M Na2SO4 electrolyte. Figure 5B shows that the photocurrent density of TiO2/Si-p (curve d) was always higher than that of TiO2/Si-u (curve c) from -0.2 to +1.5 V under the illumination of xenon lamp. The reason is that TiO2/Si-p performed the lowest


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reflection, namely the highest absorption of incident light, contributing to photocurrent promotion. More significantly, there was no saturation of photocurrent observed in the photoelectrode even at high positive potentials, indicative of the efficient charge separation in the hybrid structure, since the photocurrent was limited by the number of holes and electrons excited by illumination. The efficient charge separation in the TiO2/Si-p is due to the n–p heterojunctions at the contact surfaces of two materials.

Figure 5. A) Scheme of photoinduced charge separation in TiO2/Si system. B) Liner sweep voltammograms collected at an applied potential from -0.2 to 1.5 V from TiO2/Si-u and TiO2/Si-p electrodes.

Figure 6 showed the measured water contact angle (CA) data of the sample surface before and after the light illumination. Under the illumination of a xenon light source for 30 min, the static water CA dates of TiO2/Si-u and TiO2/Si-p all decreased, as shown in Figure 6. This is due to the preferential adsorption of water molecules on the photo-generated TiO2 surface defective sites.41 What is more, TiO2/Si-p (Figure 6B) exhibited apparently super-hydrophilicity with a lower static water CA of about 0° than that of TiO2/Si-u (Figure 6A). TiO2 nanorods, prepared via hydrothermal process,


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with a large number of hydroxyl groups on their surfaces, so their CA is below 90°. According to Cassie model,42,43 the actual CA decreases for hydrophilic materials with increasing surface roughness. Such super-hydrophilicity is ascribed to the composite micro- and nanostructures of TiO2/Si-p, the surface of which is rougher than TiO2/Si-u.44

Figure 6. Changes of water contact angles on the TiO2/Si-u substrate A) and on the TiO2/Si-p substrate B) before and after the light illumination.

Photocatalytic Applications of TiO2/Si Moth-eye Structures. Due to its excellent antireflection, efficient charge separation and apparent super-hydrophilicity, TiO2/Si-p is an ideal photocatalyst. In the photocatalysis experiment, methylene blue (MB) was selected as a typical dye to investigate the photocatalytic degradation. Before degradation, the samples were dispersed in the MB solution by stirring in the dark for 30 min without light irradiation. After that, the photocatalysts could reach adsorption equilibrium. Figure 7A presents the absorption spectra subsequent to the photocatalytic degradation of MB by TiO2/Si-p, at the end of which most of the dye was degraded. Meanwhile, the evolution of the degradation processes of Si-p, Si-u, TiO2/Si-u and TiO2/Si-p was compared in Figure 7B, revealing that TiO2/Si-p


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displayed the best photocatalytic capability and decomposed almost 100% of MB after 6 h. The high-efficiency photocatalysis of TiO2/Si-p is due to that its antireflection increases much absorption of incident light, its p-n heterojunctions improves the utilization efficiency of absorbed light and its super-hydrophilicity enhances the active sites of the chemical reaction.

Figure 7. A) UV-vis spectra of MB under the irradiation of a xenon light source for different times for the TiO2/Si-p substrate. B) Photodegradation of MB under irradiation for different times with different substrates.

CONCLUSION A biomimetic composite coating (TiO2/Si-p) with p-n heterojunctions and antireflection was fabricated by anisotropic etching and surface assembly. TiO2/Si-p showed

excellent

antireflection,

efficient

charge

separation

and

apparent

super-hydrophilicity. Compared with Si-u, Si-p and TiO2/Si-u, TiO2/Si-p exhibited the best photocatalytic performance on the degradation of MB.

ACKNOWLEDGEMENTS


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This work was supported by the National Basic Research Program of China (21401079, 21501069, 21671081 and 51302109), Research Fund for the Doctoral Program of Higher Education (20130093120003), Fundamental Research Funds for the Central Universities (JUSRP51626B) and Jiangsu Province Science Foundation for Youths (BK20140158, BK20130144 and BK20161128).

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Figures Captions Figure 1. Schematic illustration of the procedure for fabricating TiO2/Si moth-eye structures. Figure 2. Cross-sectional (45o tilt view) SEM photographs of Si-p fabricated with KOH etching time: A) 5 min, B) 15 min, C) 30 min and D) 35 min. Figure 3. A), B) and C) TiO2 nanorods SEM images with different scale and angle, D) XRD pattern of TiO2 nanorods. Figure 4. A) Photographs of Si-u (a), Si-p (b), TiO2/Si-u (c) and TiO2/Si-p (d), respectively. B) Antireflection spectra of Si-u (a), Si-p (b), TiO2/Si-u (c) and TiO2/Si-p (d), respectively. Figure 5. A) Liner sweep voltammograms collected at an applied potential from -0.2 to 1.5 V from TiO2/Si-u and TiO2/Si-p electrodes, B) scheme of photoinduced charge separation in TiO2/Si system. Figure 6. Changes of water contact angles on the TiO2/Si-u substrate A) and on the TiO2/Si-p substrate B) before and after the white light Figure 7. A) UV-vis spectra of MB under the irradiation of a white light for different times for the TiO2/Si-p substrate. B) Photodegradation of MB under irradiation for different times with different substrates.


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Titanium Oxide/Silicon Moth-eye Structures with Antireflection, p-n Heterojunctions and Superhydrophilicity

Gang Shi, Jie Chen, Likui Wang, Dawei Wang, Jingguo Yang, Ying Li, Liping Zhang, Caihua Ni, and Lifeng Chi


Silicon Moth-Eye Structures with Antireflection, pân

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