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SiNWs arrays
Journal of Alloys and Compounds 635 (2015) 112–117
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Fabrication and photoelectrochemical study of vertically oriented TiO2/Ag/SiNWs arrays Bairui Tao a,b,c,⇑, Fengjuan Miao b,c, Paul K. Chu d a
Computer Center, Qiqihar University, Heilongjiang 161006, China College of Communications and Electronics Engineering, Qiqihar University , Heilongjiang 161006, China c National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China d Department of Physics and Materials Sciences, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China b
a r t i c l e
i n f o
Article history: Received 17 November 2014 Received in revised form 13 January 2015 Accepted 16 February 2015 Available online 23 February 2015 Keywords: Ag modified TiO2/Ag/SiNWs Photoelectrochemical Ordered channels
a b s t r a c t Ordered channeled and porous TiO2 and Ag modified silicon nanowires (TiO2/Ag/SiNWs) heterostructured nanocrystals arrays are synthesized by a two-step method based on an electrochemical etching procedure and a sol–gel process. The morphology and photoelectrochemical properties of the TiO2/Ag/ SiNWs are studied. The TiO2/Ag/SiNWs photocatalysts possess ordered channels and a porous structure with large specific surface area. UV–visible diffuse reflectance spectroscopy and ultraviolet Raman scattering demonstrate that the incorporated Ag significantly enhances light absorption by the TiO2/ SiNWs in the visible spectral range and improves the separation of photo-induced charge carriers in the TiO2/SiNWs. The photoelectrochemical properties of the TiO2/Ag/SiNWs are investigated by monitoring the degradation of pnitrophenol (PNP) and Ag enhances PNP photodegradation under UV–vis irradiation due to the Ag–TiO2 heterojunctions and surface texture. The photoelectrochemical properties of TiO2/Ag/SiNWs have promising applications in photoelectrochemical solar cells and other light-harvesting devices. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery by Fujishima and Honda [1], TiO2 as a key photocatalyst has received extensive attention due to its strong catalytic activity, high chemical stability, nontoxicity, low cost, and special optical and electronic properties [2,3]. The basic photocatalysis mechanism involves the creation of an electron–hole pair by exciting an electron from the valence band to the conduction band via light absorption. Unfortunately, since the rutile and anatase phases of TiO2 have band gaps of 3.1 and 3.2 eV, respectively, only a small fraction of the solar spectrum, primarily restricted to the UV range, is absorbed, thereby hindering broader applications to light energy conversion. Therefore, how to enhance the photocatalytic efficiency is very important to future application of TiO2 to photocatalysis. In this respect, considerable efforts have been made to narrow the band gap and reduce the recombination rate of electrons and holes in TiO2. Much effort has been attempted to enhance the charge separation efficiency and absorption in the ⇑ Corresponding author at: College of Communications and Electronics Engineering, Qiqihar University, Heilongjiang 161006, China. Tel.: +86 452 2742787; fax: +86 452 2738748. E-mail address: [email protected] (B. Tao). http://dx.doi.org/10.1016/j.jallcom.2015.02.125 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
visible spectrum by tuning the band gap of TiO2. These studies include introducing C, N, S, Fe, and V into the TiO2 lattice [4,5], dye photosensitization on the TiO2 surface [6], deposition of noble metals [7,8], and combination with other semiconductors [9]. It has been shown that deposition of noble metals on TiO2 may capture the photo-induced electrons or holes, reduce recombination of electron–hole pairs effectively, and extend the photo response of TiO2 to the visible light region [10,11]. Ag has a strong resonance wavelength of 300–1200 nm, high stability, and excellent electrical and thermal conductivity and is a suitable dopant in titania. Moreover, the Schottky barrier of the Ag–TiO2 interface plays the role of trapping the photo-generated electrons and recombination of photo-induced electrons and holes can be effectively inhibited. As a result, Ag decorated TiO2 has been widely studied in photoelectrochemistry and photocatalysis [12,13]. To meet the actual demands for diverse applications numerous efforts have been to modify the structural, optical, and electrical properties of TiO2-based materials. For example, Choi et al. and Wang et al. had prepared the graphene/TiO2 composite for stationary energy storage [14,15]. Zhao et al. had used well defined microsized hollow SiO2/TiO2 hybrid spheres as the photocatalytic matrix, in which ultrafine Ag nanoparticles are embedded to overcome the agglomerate of small TiO2 particles. But there has little report on
B. Tao et al. / Journal of Alloys and Compounds 635 (2015) 112–117
the study that combined both the hollow Si-based structure and TiO2 nanoparticles. Considering monolithic integration of Si-based microfabricated devices, ordered channeled and porous SiNWs heterostructure is used as the substrate to embed Ag while the hollow structure increases the surface area. In this work, TiO2/Ag/SiNWs are prepared by chemical etching and sol–gel process and the structural, optical, and photoelectrochemical characteristics are studied systematically. The doped photocatalyst shows significantly enhanced photoelectrochemical compared to the undoped samples under the same conditions. 2. Experimental details 2.1. Preparation of Ag/SiNWs The Ag decorated silicon nanowires were fabricated by chemical etching according to Ref. [16] and the detailed process can be found in Ref. [17]. Double side polished 100 mm (100) silicon wafers were used as the substrates to fabricate the SiNWs. Chemicals such as AgNO3, HF, and HNO3 were reagent grade and used without purification. The silicon wafers were cut into small chips with a rectangular shape, cleaned by a standard RCA process, rinsed with deionized water several times, and dried with nitrogen. The etchant for SiNWs consisted of AgNO3 (23 mM):HF (15%) = 1:1 (volume ratio). The samples were etched for 20 min. In the experimental, the composition of the etchant and etching time were adjusted to control the length of the Ag/SiNWs. The growth took place at room temperature (25 °C) and 1 atmospheric pressure. After etching, the chips were taken from the solution and rinsed with DI water.
2.2. Preparation of TiO2 films The TiO2 thin films were fabricated by the sol–gel method. Analytically pure titanium butoxide [Ti (OC4H9)4], anhydrous ethanol (C2H5OH), and acetic acid (CH3COOH) were the starting materials. Acetyl acetone and an equi-molar amount of titanium butoxide were added dropwise to the solution to the required volume ratio of C2H5OH:CH3COOH (16:1) under vigorous stirring at room temperature. Caution was exercised because the reaction was rather violent. Acetate and acetyl acetone were used to adjust the pH and stabilize the titanium butoxide, respectively and the solution was stirred for 1 h at 50 °C to increase the homogeneity. The 0.2 M precursor solution remained transparent without precipitates even after two months.
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2.3. Preparation of TiO2/Ag/SiNWs nanostructures The TiO2 thin films were deposited directly on the Ag/SiNWs by the sol–gel method. After drying under flowing nitrogen, the TiO2 films were deposited by spin coating the solution on the Ag/SiNWs samples at 4000 rpm for 20 s. The nanocomposites were dried at 180 °C for 200 s, pyrolyzed at 380 °C for 240 s to remove residual organic compounds, and annealed at 500 °C for 1 h in nitrogen. The deposition and annealing treatment procedures were repeated three times in order to obtain the desired thickness [18,19].
2.4. Characterization of TiO2/Ag/SiNWs nanostructures The morphology of the silicon TiO2/Ag/SiNWs nanostructure was characterized by scanning electron microscopy (SEM, JSM-6360LA) and the crystalline structure was determined by X-ray diffraction (XRD) using Cu Ka radiation (Rigaku, RINT2000, Japan). A vertical goniometer (Model RINT2000) was used and the continuous scanning mode (2h/h) with an interval of 0.02° and scanning rate of 10°/min was adopted. Ultraviolet Raman scattering was performed at room temperature on a micro-Raman spectrometer with a spectral resolution of 1.5 cm1 (Jobin-Yvon LabRAM HR 800 UV). The 325 nm (3.82 eV) line of a He–Cd laser with an output power of 30 mW was the excitation source. The light absorption properties were determined by monitoring the UV–visible diffuse reflectance spectra in the wavelength range between 200 and 800 nm.
2.5. Photoelectrochemical and photoelectrocatalytic degradation The photoelectrochemical experiments were carried out in a conventional three-electrode cell controlled by the electrochemical workstation with the TiO2/Ag/SiNWs nanocomposite electrode being the photo-anode. A platinum wire electrode was the counter electrode and a saturated calomel electrode (SCE) served as the reference electrode. The electrolytes consisted of 50 mL of 0.1 M Na2SO4 and 50 mL of a mixture containing 20 mg L1 of PNP and 0.1 M Na2SO4. A 350 W xenon lamp with an illumination intensity of 100 mW cm2 was the light source. All the experiments were performed at 25 °C. The photoelectrocatalytic degradation experiments were also conducted in the three-electrode cell controlled by the electrochemical workstation. The photo-anode, cathode, reference electrode, and electrolytes were the same as those in the photoelectrochemical experiments. The experiments were performed under magnetic stirring at room temperature and the pH of the solution was adjusted by H2SO4 or NaOH.
Fig. 1. SEM images for the Ag/SiNWs nanocomposite: (a) top-view, (b) magnified picture of the top-view, (c) cross-section, and (d) top-view after covered TiO2.
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3. Results and discussions Fig. 1 depicts the SEM images of the Ag/SiNWs. Fig. 1(a) is the top-view SEM image of Ag/SiNWs, and Fig. 1(b) is the magnified picture of the top-view. It is obviously, that the Ag/SiNWs forms a bundle-like structure and there are many Ag nanoparticles on the surface. The Ag nonoparticles rather flower like structure on the top of SiNWs. This means that there are more active materials per unit area. According to Fig. 1(c), the Ag/SiNWs are vertically aligned to the substrate and the interface between the Ag/SiNWs and bulk silicon is clear and layered. The length of the Ag/SiNWs is quite uniform and about 10 lm which can be controlled by the etching time. The diameter of Ag/SiNWs ranges from 60 to 300 nm. The top-view SEM image after cover by TiO2 is depicted in Fig. 1(d). It can be clearly observed that the surface of the Ag/SiNWs is coated and the surface of the SiNWs becomes much rougher. The XRD pattern of TiO2/Ag/SiNWs is shown in Fig. 2 and those of Ag/Si and TiO2/Si are also displayed for comparison. The TiO2/Ag/ SiNWs exhibit two characteristic peaks indicative of the (1 1 1) and (2 0 0) planes of face-centered cubic (fcc) Ag at 2h = 38.2° and 44.0° similar to those of the Ag nanoparticles [20,21]. There are also one intense (1 0 1) diffraction and several weaker diffraction peaks of (0 0 4), (2 0 0), (2 1 1), and (2 0 4) at about 25.3°, 37.8°, 48.0°, 55.1°, and 62.75°, respectively, but no impurity phases are observed confirming the pure anatase structure. The polycrystalline grains with different orientations are formed in the antase films and Ag exists in the TiO2 matrix. Anatase TiO2 is tetragonal with the space group D4h (I41/amd) and the primitive cell contains two units per cell. There are six first order Raman-active modes (Ag + 2B1g + 3Eg) at the C point of the Brillouin zone [22,23]. According to the Perdew–Burke–Ernzerbof (PBE) functional, the theoretical Raman frequencies of Eg, Eg, B1g, A1g, and Eg are 128, 158, 388, 514, 521, and 638 cm1, respectively. The 2Eg modes occurring at lower frequencies (<300 cm1) cannot be observed due to experimental limitations and Fig. 3 shows the spectra of TiO2/Ag/SiNWs as well as TiO2/Si. Compared to the Raman spectra, the shoulder structure cannot originate from the Ag. The change in the fundamental Raman-active phonon modes and appearance of the additional vibration mode can be attributed to Ag introduction. The peaks at 643, 520, and 400 cm1 are typical of TiO2 anatase whereas no peaks stemming from other TiO2 crystalline polymorphs are detected thus corroborating the XRD data. Fig. 4 shows the UV–visible absorption spectra of TiO2/Si, Ag and TiO2/Ag/SiNWs. The band gap absorption edges of the undoped
Fig. 2. XRD patterns of the TiO2/Ag/SiNWs, Ag/Si and TiO2/Si nanostructures.
TiO2/Si and TiO2/Ag/SiNWs are around 360 and 370 nm, respectively. The optical absorption of TiO2/Si at wavelengths shorter than 360 nm is mainly attributed to the O2+ ? Ti4+ charge transfer related to electron excitation from the valence band [24,25]. There is a red shift of about 10 nm with increasing Ag+ concentration and strong surface plasmon absorption is observed from the visible light region similar to Ag nanodendrites. The band gaps of TiO2/Ag/SiNWs, and TiO2/Si are estimated by DRS (diffuse reflectance spectra) by ploting the Kubelka–Munk functions (F(R)) versus the photon energy (Eph) [26,27]. The band gaps are calculated to be 3.20, 3.08 eV for TiO2/Si and TiO2/Ag/ SiNWs, respectively. The absorbance edge of TiO2/Ag/SiNWs shifts significantly to the visible region and Ag doping decreases the band gap of TiO2 subsequently increasing the visible light absorption of the photocatalyst. The photocatalytic activity of the TiO2/Ag/SiNWs is evaluated by monitoring the photodegradation of PNP under UV–visible irradiation. Fig. 5 shows the transient photocurrents of the TiO2/Ag/ SiNWs, TiO2/SiNWs, and TiO2/Si samples measured in 0.5 M Na2SO4 at an applied voltage of 0.5 V with 20 s light on/off cycles. The transient photocurrent responses of TiO2/Ag/SiNWs, TiO2/ SiNWs, and TiO2/Si are recorded for several on-off cycles of irradiation. Pronounced photocurrents are observed from TiO2/ Ag/SiNWs due to the generation and separation of photo-excited electron hole pairs at the TiO2/Ag/SiNWs electrolyte interface. The major reactions that cause the photocurrents are shown in the following:
þ
2Ag þ hm ! 2Agðe þ h Þ
þ
ð1Þ
þ
2Agðe þ h Þ þ 2TiO2 ! 2TiO2 ðe Þ þ 2Agðh Þ 2TiO2 ðe Þ þ 2Pt ! 2TiO2 þ 2Ptðe Þ þ
2Agðh Þ þ 2H2 O ! 2 OH þ 2Ag þ 2Hþ
2Ptðe Þ þ 2Hþ ! H2 þ 2Pt:
ð2Þ ð3Þ ð4Þ ð5Þ
According to this scheme, when the TiO2/Ag/SiNWs are irradiated by UV–visible light, photo-generated electron-hole pairs are formed in the Ag nanodendrites due to the effects of LSPR (localized surface plasmon resonance). The photo-excited electrons at the Ag nanoparticles are injected into the TiO2 so that most of these photo-excited electrons are transported from the TiO2 to the counter electrode (Pt) through the external circuit. Simultaneously, the oxidized Ag nanoparticles are reduced to metallic Ag and H2O is oxidized. The H+ ions are reduced to H2 on the Pt counter electrode and finally, photocurrents are produced.
Fig. 3. Raman spectra of the TiO2/Ag/SiNWs and TiO2/Si excited by the 325 nm line.
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Fig. 4. UV–vis DRS spectra acquired from the TiO2/Si, Ag and TiO2/Ag/SiNWs nanostructures.
Fig. 6. CV curves of TiO2/Ag/SiNWs and TiO2/SiNWs composite nanostructures in deionized water containing 0.5 M H2SO4.
The TiO2/Ag/SiNWs show higher photocurrents than TiO2/ SiNWs under UV–visible irradiation. The enhancement in the photocurrent may be ascribed to faster electron transport and more efficient separation of the plasmon-generated electrons and holes in the TiO2/SiNWs incorporated with Ag nanodendrites. In other words, the plasmon-induced electrons are transported from the surface of Ag nanodendrites to the conduction band of TiO2, whereas the holes are captured by reduced species in the electrolyte. TiO2/Ag/SiNWs and TiO2/SiNWs show higher activity than TiO2/Si partly because of the large surface area on the aligned SiNWs. In addition, the aligned SiNWs not only provide a shorter diffusion path for the PNP to be degraded on the active surface, but also reduce the electron-hole recombination rate [28,29]. The photocatalytic capability of the samples affected not only by the surface area of SiNWs but also the amount of incorporated Ag. CV is employed to determine the relationships between them. Fig. 6 shows the CV curves got from the TiO2/SiNWs and TiO2/Ag/ SiNWs nanostructures in a 0.5 M H2SO4 solution. The electrochemical active surface could be estimated by integrating the current under the CV curves. It is clearly the electrochemical active surface of TiO2/Ag/SiNWs is larger then that of TiO2/SiNWs, and TiO2/Ag/SiNWs shows the higher current density in the redox reaction, which indicating that the Ag nanodendritic could enhance the
reaction. The electrocatalytic activity may be determined by the overall electrochemically active surface area. Fig. 7 shows the I–V characteristics of the TiO2/Ag/SiNWs, TiO2/ SiNWs, and TiO2 in 0.5 M Na2SO4 with or without 20 mg L1 of PNP. The photocurrent increases with increasing applied voltage before leveling off, suggesting that a small external bias is beneficial to the electron transfer consequently reducing recombination of photoelectrons and holes in the TiO2/Ag/SiNWs and TiO2/SiNWs. The PNP increases the photocurrent density on both electrodes, because it can capture the photo-generated holes and be decomposed in the PEC oxidation process:
Fig. 5. Photocurrent responses in the light on-off process at an applied potential of 0.5 V (TiO2/Si (a), TiO2/Ag/SiNWs (b), and TiO2/SiNWs (c) samples).
g % ¼ ½ðtotal power output electrical power inputÞ=light power input 100 " # E0rev jEapp j ¼ jp 100 Io ð6Þ 2
In this equation, jp is the photocurrent density in (lA cm
) and
E0rev is the standard reversible potential (which is 1.23 V for the water splitting reaction at pH = 0). The external applied potential Eapp = Emeas Eaoc, where Emeas is the electrode potential of the working electrode at which the photocurrent is measured under
Fig. 7. Variation of the photocurrent density versus bias potential in 0.1 M Na2SO4 or 20 mg L1 PNP containing 0.1 M Na2SO4 solution under UV–visible light (100 mW cm2) irradiation of the TiO2, TiO2/SiNWs, and TiO2/Ag/SiNWs electrodes.
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shifts of the Raman modes are sensitive to the Ag concentration. The optical absorption properties of the TiO2/Ag/SiNWs films are studied by DRS and Ag doping decreases the band-gap of TiO2. The photoelectrochemical properties of the TiO2/Ag/SiNWs are studied and the photocurrent is dramatically enhanced. A maximum photo-conversion efficiency of 0.37% is achieved and the PEC degradation efficiency of PNP by the TiO2/Ag/SiNWs electrode is higher than that measured from the undoped materials. The TiO2/Ag/SiNWs films have large potential in applications such as water splitting, solar cells, as well as PEC degradation of pollutants.
Acknowledgements
Fig. 8. Photocatalytic degradation of PNP under UV–vis light irradiation of the TiO2, TiO2/SiNWs, and TiO2/Ag/SiNWs electrodes,respectively.
illumination and Eaoc is the electrode potential of the same working electrode under open circuit conditions and same conditions. I0 is the power density of the incident light (mW cm2) and jp E0rev is the total power output while jp|Eapp| is the electrical power input. According to this method, the photo-conversion efficiency of the electrodes can be calculated. A maximum photo-conversion efficiency of 0.37% is achieved on the TiO2/Ag/SiNWs electrode compared to 0.25% on the undoped one in the same electrolyte comprising 20 mg L1 of PNP and 0.5 M Na2SO4. These results indicate the Ag nanodendrites embedded SiNWs on TiO2 significantly improves the photocurrent density and photo-conversion efficiency. PNP was further used as the representative organic substances to evaluate the photoactivity of TiO2/Ag/SiNWs, TiO2/SiNWs and TiO2 samples (Fig. 8). The degradation of PNP could be described by first-order kinetic of ln(Ct/C0) vs reaction time (t):
ln
Ct C0
¼ kap t
ð7Þ
where kap is the apparent reaction rate constant, C0 and Ct are the initial concentration and the concentration of PNP at reaction time t, respectively. After 40 min of irradiation, 40.35% of PNP is degraded by the TiO2/SiNWs catalyst as compared to 30.35% by the TiO2 samples and 46.4% by the TiO2/Ag/SiNWs photocatalyst. This suggests that a large specific surface area may be an important factor, influencing the rate of photocatalytic degradation reaction, and the Ag doping enhances the photocatalytic activity of TiO2. The TiO2/Ag/ SiNWs are more efficient in the photocatalytic process due to the embedded Ag nanodendrites. It is believed that the Ag nanodendrites can induce localized surface plasmon with an absorption peak at around 420 nm. In addition, the embedded Ag nanodendrites can act as good electron acceptors with a favorable Fermi level (EF = 0.4 V). When the TiO2/Ag/SiNWs undergo charge separation under irradiation, the photo-generated electrons can be transferred to the Ag nanodendrites to reduce the electron–hole recombination rate. The transfer of electrons from the excited semiconductor to the metal is an important aspect affecting the photocatalytic efficiency. 4. Conclusion Well-defined Ag modified TiO2/SiNWs are prepared using an electrochemical etching procedure followed by a sol–gel process. The films are polycrystalline and show a strong (1 0 1) diffraction peak. Four Raman-active phonon modes are observed and the
This work was supported by the National Natural Science Foundation of China (Grant No. 61204127), New Century Excellent Talents In Heilongjiang Provincial University, (Grant No. 1253NECT025), Natural Science Foundation of Heilongjiang Province (Grant Nos. F201332, F201438) and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 112212.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication and photoelectrochemical study of vertically oriented TiO2/Ag/SiNWs arrays Bairui Tao a,b,c,⇑, Fengjuan Miao b,c, Paul K. Chu d a
Computer Center, Qiqihar University, Heilongjiang 161006, China College of Communications and Electronics Engineering, Qiqihar University , Heilongjiang 161006, China c National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China d Department of Physics and Materials Sciences, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China b
a r t i c l e
i n f o
Article history: Received 17 November 2014 Received in revised form 13 January 2015 Accepted 16 February 2015 Available online 23 February 2015 Keywords: Ag modified TiO2/Ag/SiNWs Photoelectrochemical Ordered channels
a b s t r a c t Ordered channeled and porous TiO2 and Ag modified silicon nanowires (TiO2/Ag/SiNWs) heterostructured nanocrystals arrays are synthesized by a two-step method based on an electrochemical etching procedure and a sol–gel process. The morphology and photoelectrochemical properties of the TiO2/Ag/ SiNWs are studied. The TiO2/Ag/SiNWs photocatalysts possess ordered channels and a porous structure with large specific surface area. UV–visible diffuse reflectance spectroscopy and ultraviolet Raman scattering demonstrate that the incorporated Ag significantly enhances light absorption by the TiO2/ SiNWs in the visible spectral range and improves the separation of photo-induced charge carriers in the TiO2/SiNWs. The photoelectrochemical properties of the TiO2/Ag/SiNWs are investigated by monitoring the degradation of pnitrophenol (PNP) and Ag enhances PNP photodegradation under UV–vis irradiation due to the Ag–TiO2 heterojunctions and surface texture. The photoelectrochemical properties of TiO2/Ag/SiNWs have promising applications in photoelectrochemical solar cells and other light-harvesting devices. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery by Fujishima and Honda [1], TiO2 as a key photocatalyst has received extensive attention due to its strong catalytic activity, high chemical stability, nontoxicity, low cost, and special optical and electronic properties [2,3]. The basic photocatalysis mechanism involves the creation of an electron–hole pair by exciting an electron from the valence band to the conduction band via light absorption. Unfortunately, since the rutile and anatase phases of TiO2 have band gaps of 3.1 and 3.2 eV, respectively, only a small fraction of the solar spectrum, primarily restricted to the UV range, is absorbed, thereby hindering broader applications to light energy conversion. Therefore, how to enhance the photocatalytic efficiency is very important to future application of TiO2 to photocatalysis. In this respect, considerable efforts have been made to narrow the band gap and reduce the recombination rate of electrons and holes in TiO2. Much effort has been attempted to enhance the charge separation efficiency and absorption in the ⇑ Corresponding author at: College of Communications and Electronics Engineering, Qiqihar University, Heilongjiang 161006, China. Tel.: +86 452 2742787; fax: +86 452 2738748. E-mail address: [email protected] (B. Tao). http://dx.doi.org/10.1016/j.jallcom.2015.02.125 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
visible spectrum by tuning the band gap of TiO2. These studies include introducing C, N, S, Fe, and V into the TiO2 lattice [4,5], dye photosensitization on the TiO2 surface [6], deposition of noble metals [7,8], and combination with other semiconductors [9]. It has been shown that deposition of noble metals on TiO2 may capture the photo-induced electrons or holes, reduce recombination of electron–hole pairs effectively, and extend the photo response of TiO2 to the visible light region [10,11]. Ag has a strong resonance wavelength of 300–1200 nm, high stability, and excellent electrical and thermal conductivity and is a suitable dopant in titania. Moreover, the Schottky barrier of the Ag–TiO2 interface plays the role of trapping the photo-generated electrons and recombination of photo-induced electrons and holes can be effectively inhibited. As a result, Ag decorated TiO2 has been widely studied in photoelectrochemistry and photocatalysis [12,13]. To meet the actual demands for diverse applications numerous efforts have been to modify the structural, optical, and electrical properties of TiO2-based materials. For example, Choi et al. and Wang et al. had prepared the graphene/TiO2 composite for stationary energy storage [14,15]. Zhao et al. had used well defined microsized hollow SiO2/TiO2 hybrid spheres as the photocatalytic matrix, in which ultrafine Ag nanoparticles are embedded to overcome the agglomerate of small TiO2 particles. But there has little report on
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the study that combined both the hollow Si-based structure and TiO2 nanoparticles. Considering monolithic integration of Si-based microfabricated devices, ordered channeled and porous SiNWs heterostructure is used as the substrate to embed Ag while the hollow structure increases the surface area. In this work, TiO2/Ag/SiNWs are prepared by chemical etching and sol–gel process and the structural, optical, and photoelectrochemical characteristics are studied systematically. The doped photocatalyst shows significantly enhanced photoelectrochemical compared to the undoped samples under the same conditions. 2. Experimental details 2.1. Preparation of Ag/SiNWs The Ag decorated silicon nanowires were fabricated by chemical etching according to Ref. [16] and the detailed process can be found in Ref. [17]. Double side polished 100 mm (100) silicon wafers were used as the substrates to fabricate the SiNWs. Chemicals such as AgNO3, HF, and HNO3 were reagent grade and used without purification. The silicon wafers were cut into small chips with a rectangular shape, cleaned by a standard RCA process, rinsed with deionized water several times, and dried with nitrogen. The etchant for SiNWs consisted of AgNO3 (23 mM):HF (15%) = 1:1 (volume ratio). The samples were etched for 20 min. In the experimental, the composition of the etchant and etching time were adjusted to control the length of the Ag/SiNWs. The growth took place at room temperature (25 °C) and 1 atmospheric pressure. After etching, the chips were taken from the solution and rinsed with DI water.
2.2. Preparation of TiO2 films The TiO2 thin films were fabricated by the sol–gel method. Analytically pure titanium butoxide [Ti (OC4H9)4], anhydrous ethanol (C2H5OH), and acetic acid (CH3COOH) were the starting materials. Acetyl acetone and an equi-molar amount of titanium butoxide were added dropwise to the solution to the required volume ratio of C2H5OH:CH3COOH (16:1) under vigorous stirring at room temperature. Caution was exercised because the reaction was rather violent. Acetate and acetyl acetone were used to adjust the pH and stabilize the titanium butoxide, respectively and the solution was stirred for 1 h at 50 °C to increase the homogeneity. The 0.2 M precursor solution remained transparent without precipitates even after two months.
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2.3. Preparation of TiO2/Ag/SiNWs nanostructures The TiO2 thin films were deposited directly on the Ag/SiNWs by the sol–gel method. After drying under flowing nitrogen, the TiO2 films were deposited by spin coating the solution on the Ag/SiNWs samples at 4000 rpm for 20 s. The nanocomposites were dried at 180 °C for 200 s, pyrolyzed at 380 °C for 240 s to remove residual organic compounds, and annealed at 500 °C for 1 h in nitrogen. The deposition and annealing treatment procedures were repeated three times in order to obtain the desired thickness [18,19].
2.4. Characterization of TiO2/Ag/SiNWs nanostructures The morphology of the silicon TiO2/Ag/SiNWs nanostructure was characterized by scanning electron microscopy (SEM, JSM-6360LA) and the crystalline structure was determined by X-ray diffraction (XRD) using Cu Ka radiation (Rigaku, RINT2000, Japan). A vertical goniometer (Model RINT2000) was used and the continuous scanning mode (2h/h) with an interval of 0.02° and scanning rate of 10°/min was adopted. Ultraviolet Raman scattering was performed at room temperature on a micro-Raman spectrometer with a spectral resolution of 1.5 cm1 (Jobin-Yvon LabRAM HR 800 UV). The 325 nm (3.82 eV) line of a He–Cd laser with an output power of 30 mW was the excitation source. The light absorption properties were determined by monitoring the UV–visible diffuse reflectance spectra in the wavelength range between 200 and 800 nm.
2.5. Photoelectrochemical and photoelectrocatalytic degradation The photoelectrochemical experiments were carried out in a conventional three-electrode cell controlled by the electrochemical workstation with the TiO2/Ag/SiNWs nanocomposite electrode being the photo-anode. A platinum wire electrode was the counter electrode and a saturated calomel electrode (SCE) served as the reference electrode. The electrolytes consisted of 50 mL of 0.1 M Na2SO4 and 50 mL of a mixture containing 20 mg L1 of PNP and 0.1 M Na2SO4. A 350 W xenon lamp with an illumination intensity of 100 mW cm2 was the light source. All the experiments were performed at 25 °C. The photoelectrocatalytic degradation experiments were also conducted in the three-electrode cell controlled by the electrochemical workstation. The photo-anode, cathode, reference electrode, and electrolytes were the same as those in the photoelectrochemical experiments. The experiments were performed under magnetic stirring at room temperature and the pH of the solution was adjusted by H2SO4 or NaOH.
Fig. 1. SEM images for the Ag/SiNWs nanocomposite: (a) top-view, (b) magnified picture of the top-view, (c) cross-section, and (d) top-view after covered TiO2.
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3. Results and discussions Fig. 1 depicts the SEM images of the Ag/SiNWs. Fig. 1(a) is the top-view SEM image of Ag/SiNWs, and Fig. 1(b) is the magnified picture of the top-view. It is obviously, that the Ag/SiNWs forms a bundle-like structure and there are many Ag nanoparticles on the surface. The Ag nonoparticles rather flower like structure on the top of SiNWs. This means that there are more active materials per unit area. According to Fig. 1(c), the Ag/SiNWs are vertically aligned to the substrate and the interface between the Ag/SiNWs and bulk silicon is clear and layered. The length of the Ag/SiNWs is quite uniform and about 10 lm which can be controlled by the etching time. The diameter of Ag/SiNWs ranges from 60 to 300 nm. The top-view SEM image after cover by TiO2 is depicted in Fig. 1(d). It can be clearly observed that the surface of the Ag/SiNWs is coated and the surface of the SiNWs becomes much rougher. The XRD pattern of TiO2/Ag/SiNWs is shown in Fig. 2 and those of Ag/Si and TiO2/Si are also displayed for comparison. The TiO2/Ag/ SiNWs exhibit two characteristic peaks indicative of the (1 1 1) and (2 0 0) planes of face-centered cubic (fcc) Ag at 2h = 38.2° and 44.0° similar to those of the Ag nanoparticles [20,21]. There are also one intense (1 0 1) diffraction and several weaker diffraction peaks of (0 0 4), (2 0 0), (2 1 1), and (2 0 4) at about 25.3°, 37.8°, 48.0°, 55.1°, and 62.75°, respectively, but no impurity phases are observed confirming the pure anatase structure. The polycrystalline grains with different orientations are formed in the antase films and Ag exists in the TiO2 matrix. Anatase TiO2 is tetragonal with the space group D4h (I41/amd) and the primitive cell contains two units per cell. There are six first order Raman-active modes (Ag + 2B1g + 3Eg) at the C point of the Brillouin zone [22,23]. According to the Perdew–Burke–Ernzerbof (PBE) functional, the theoretical Raman frequencies of Eg, Eg, B1g, A1g, and Eg are 128, 158, 388, 514, 521, and 638 cm1, respectively. The 2Eg modes occurring at lower frequencies (<300 cm1) cannot be observed due to experimental limitations and Fig. 3 shows the spectra of TiO2/Ag/SiNWs as well as TiO2/Si. Compared to the Raman spectra, the shoulder structure cannot originate from the Ag. The change in the fundamental Raman-active phonon modes and appearance of the additional vibration mode can be attributed to Ag introduction. The peaks at 643, 520, and 400 cm1 are typical of TiO2 anatase whereas no peaks stemming from other TiO2 crystalline polymorphs are detected thus corroborating the XRD data. Fig. 4 shows the UV–visible absorption spectra of TiO2/Si, Ag and TiO2/Ag/SiNWs. The band gap absorption edges of the undoped
Fig. 2. XRD patterns of the TiO2/Ag/SiNWs, Ag/Si and TiO2/Si nanostructures.
TiO2/Si and TiO2/Ag/SiNWs are around 360 and 370 nm, respectively. The optical absorption of TiO2/Si at wavelengths shorter than 360 nm is mainly attributed to the O2+ ? Ti4+ charge transfer related to electron excitation from the valence band [24,25]. There is a red shift of about 10 nm with increasing Ag+ concentration and strong surface plasmon absorption is observed from the visible light region similar to Ag nanodendrites. The band gaps of TiO2/Ag/SiNWs, and TiO2/Si are estimated by DRS (diffuse reflectance spectra) by ploting the Kubelka–Munk functions (F(R)) versus the photon energy (Eph) [26,27]. The band gaps are calculated to be 3.20, 3.08 eV for TiO2/Si and TiO2/Ag/ SiNWs, respectively. The absorbance edge of TiO2/Ag/SiNWs shifts significantly to the visible region and Ag doping decreases the band gap of TiO2 subsequently increasing the visible light absorption of the photocatalyst. The photocatalytic activity of the TiO2/Ag/SiNWs is evaluated by monitoring the photodegradation of PNP under UV–visible irradiation. Fig. 5 shows the transient photocurrents of the TiO2/Ag/ SiNWs, TiO2/SiNWs, and TiO2/Si samples measured in 0.5 M Na2SO4 at an applied voltage of 0.5 V with 20 s light on/off cycles. The transient photocurrent responses of TiO2/Ag/SiNWs, TiO2/ SiNWs, and TiO2/Si are recorded for several on-off cycles of irradiation. Pronounced photocurrents are observed from TiO2/ Ag/SiNWs due to the generation and separation of photo-excited electron hole pairs at the TiO2/Ag/SiNWs electrolyte interface. The major reactions that cause the photocurrents are shown in the following:
þ
2Ag þ hm ! 2Agðe þ h Þ
þ
ð1Þ
þ
2Agðe þ h Þ þ 2TiO2 ! 2TiO2 ðe Þ þ 2Agðh Þ 2TiO2 ðe Þ þ 2Pt ! 2TiO2 þ 2Ptðe Þ þ
2Agðh Þ þ 2H2 O ! 2 OH þ 2Ag þ 2Hþ
2Ptðe Þ þ 2Hþ ! H2 þ 2Pt:
ð2Þ ð3Þ ð4Þ ð5Þ
According to this scheme, when the TiO2/Ag/SiNWs are irradiated by UV–visible light, photo-generated electron-hole pairs are formed in the Ag nanodendrites due to the effects of LSPR (localized surface plasmon resonance). The photo-excited electrons at the Ag nanoparticles are injected into the TiO2 so that most of these photo-excited electrons are transported from the TiO2 to the counter electrode (Pt) through the external circuit. Simultaneously, the oxidized Ag nanoparticles are reduced to metallic Ag and H2O is oxidized. The H+ ions are reduced to H2 on the Pt counter electrode and finally, photocurrents are produced.
Fig. 3. Raman spectra of the TiO2/Ag/SiNWs and TiO2/Si excited by the 325 nm line.
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Fig. 4. UV–vis DRS spectra acquired from the TiO2/Si, Ag and TiO2/Ag/SiNWs nanostructures.
Fig. 6. CV curves of TiO2/Ag/SiNWs and TiO2/SiNWs composite nanostructures in deionized water containing 0.5 M H2SO4.
The TiO2/Ag/SiNWs show higher photocurrents than TiO2/ SiNWs under UV–visible irradiation. The enhancement in the photocurrent may be ascribed to faster electron transport and more efficient separation of the plasmon-generated electrons and holes in the TiO2/SiNWs incorporated with Ag nanodendrites. In other words, the plasmon-induced electrons are transported from the surface of Ag nanodendrites to the conduction band of TiO2, whereas the holes are captured by reduced species in the electrolyte. TiO2/Ag/SiNWs and TiO2/SiNWs show higher activity than TiO2/Si partly because of the large surface area on the aligned SiNWs. In addition, the aligned SiNWs not only provide a shorter diffusion path for the PNP to be degraded on the active surface, but also reduce the electron-hole recombination rate [28,29]. The photocatalytic capability of the samples affected not only by the surface area of SiNWs but also the amount of incorporated Ag. CV is employed to determine the relationships between them. Fig. 6 shows the CV curves got from the TiO2/SiNWs and TiO2/Ag/ SiNWs nanostructures in a 0.5 M H2SO4 solution. The electrochemical active surface could be estimated by integrating the current under the CV curves. It is clearly the electrochemical active surface of TiO2/Ag/SiNWs is larger then that of TiO2/SiNWs, and TiO2/Ag/SiNWs shows the higher current density in the redox reaction, which indicating that the Ag nanodendritic could enhance the
reaction. The electrocatalytic activity may be determined by the overall electrochemically active surface area. Fig. 7 shows the I–V characteristics of the TiO2/Ag/SiNWs, TiO2/ SiNWs, and TiO2 in 0.5 M Na2SO4 with or without 20 mg L1 of PNP. The photocurrent increases with increasing applied voltage before leveling off, suggesting that a small external bias is beneficial to the electron transfer consequently reducing recombination of photoelectrons and holes in the TiO2/Ag/SiNWs and TiO2/SiNWs. The PNP increases the photocurrent density on both electrodes, because it can capture the photo-generated holes and be decomposed in the PEC oxidation process:
Fig. 5. Photocurrent responses in the light on-off process at an applied potential of 0.5 V (TiO2/Si (a), TiO2/Ag/SiNWs (b), and TiO2/SiNWs (c) samples).
g % ¼ ½ðtotal power output electrical power inputÞ=light power input 100 " # E0rev jEapp j ¼ jp 100 Io ð6Þ 2
In this equation, jp is the photocurrent density in (lA cm
) and
E0rev is the standard reversible potential (which is 1.23 V for the water splitting reaction at pH = 0). The external applied potential Eapp = Emeas Eaoc, where Emeas is the electrode potential of the working electrode at which the photocurrent is measured under
Fig. 7. Variation of the photocurrent density versus bias potential in 0.1 M Na2SO4 or 20 mg L1 PNP containing 0.1 M Na2SO4 solution under UV–visible light (100 mW cm2) irradiation of the TiO2, TiO2/SiNWs, and TiO2/Ag/SiNWs electrodes.
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shifts of the Raman modes are sensitive to the Ag concentration. The optical absorption properties of the TiO2/Ag/SiNWs films are studied by DRS and Ag doping decreases the band-gap of TiO2. The photoelectrochemical properties of the TiO2/Ag/SiNWs are studied and the photocurrent is dramatically enhanced. A maximum photo-conversion efficiency of 0.37% is achieved and the PEC degradation efficiency of PNP by the TiO2/Ag/SiNWs electrode is higher than that measured from the undoped materials. The TiO2/Ag/SiNWs films have large potential in applications such as water splitting, solar cells, as well as PEC degradation of pollutants.
Acknowledgements
Fig. 8. Photocatalytic degradation of PNP under UV–vis light irradiation of the TiO2, TiO2/SiNWs, and TiO2/Ag/SiNWs electrodes,respectively.
illumination and Eaoc is the electrode potential of the same working electrode under open circuit conditions and same conditions. I0 is the power density of the incident light (mW cm2) and jp E0rev is the total power output while jp|Eapp| is the electrical power input. According to this method, the photo-conversion efficiency of the electrodes can be calculated. A maximum photo-conversion efficiency of 0.37% is achieved on the TiO2/Ag/SiNWs electrode compared to 0.25% on the undoped one in the same electrolyte comprising 20 mg L1 of PNP and 0.5 M Na2SO4. These results indicate the Ag nanodendrites embedded SiNWs on TiO2 significantly improves the photocurrent density and photo-conversion efficiency. PNP was further used as the representative organic substances to evaluate the photoactivity of TiO2/Ag/SiNWs, TiO2/SiNWs and TiO2 samples (Fig. 8). The degradation of PNP could be described by first-order kinetic of ln(Ct/C0) vs reaction time (t):
ln
Ct C0
¼ kap t
ð7Þ
where kap is the apparent reaction rate constant, C0 and Ct are the initial concentration and the concentration of PNP at reaction time t, respectively. After 40 min of irradiation, 40.35% of PNP is degraded by the TiO2/SiNWs catalyst as compared to 30.35% by the TiO2 samples and 46.4% by the TiO2/Ag/SiNWs photocatalyst. This suggests that a large specific surface area may be an important factor, influencing the rate of photocatalytic degradation reaction, and the Ag doping enhances the photocatalytic activity of TiO2. The TiO2/Ag/ SiNWs are more efficient in the photocatalytic process due to the embedded Ag nanodendrites. It is believed that the Ag nanodendrites can induce localized surface plasmon with an absorption peak at around 420 nm. In addition, the embedded Ag nanodendrites can act as good electron acceptors with a favorable Fermi level (EF = 0.4 V). When the TiO2/Ag/SiNWs undergo charge separation under irradiation, the photo-generated electrons can be transferred to the Ag nanodendrites to reduce the electron–hole recombination rate. The transfer of electrons from the excited semiconductor to the metal is an important aspect affecting the photocatalytic efficiency. 4. Conclusion Well-defined Ag modified TiO2/SiNWs are prepared using an electrochemical etching procedure followed by a sol–gel process. The films are polycrystalline and show a strong (1 0 1) diffraction peak. Four Raman-active phonon modes are observed and the
This work was supported by the National Natural Science Foundation of China (Grant No. 61204127), New Century Excellent Talents In Heilongjiang Provincial University, (Grant No. 1253NECT025), Natural Science Foundation of Heilongjiang Province (Grant Nos. F201332, F201438) and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 112212.
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