In order to overcome the low utilization ratio of solar light and high electron-hole pair recombination rate of TiO2, the triangular Ag nanoparticle island is covered on the surface of the TiO2 thin film. Enhancement of the photocatalytic activity of the Ag/TiO2 nanocomposite system is observed. The increase of electron-hole pair generation is caused by the enhanced near-field amplitudes of localized surface plasmon of the Ag nanoparticles. The efficiently suppressed recombination of electron-hole pair caused by the metal-semiconductor contact can also enhance the photocatalytic activity of the TiO2 film.
plasmonphotocatalysisnanospheres lithographyAg nanoparticle island
TiO2, as a key photocatalyst, has received extensive attention during the past decades due to its strong catalytic activity, high chemical stability, nontoxicity, and low cost [1–5]. However, owing to its wide bandgap of 3.2 eV, only approximately 4% solar spectrum can be utilized and the conversion of photon to electron-hole pair is low. Furthermore, the high rate of electron-hole pair recombination limits the efficiency of photocatalytic activity. Therefore, how to enhance photocatalytic efficiency is very important for the widespread application of TiO2 as a photocatalyst. Recently, surface plasmon-mediated photocatalytic activity of TiO2 has become a hot research topic [6–9]. Surface plasmon resonance is produced by metal nanoparticles (NPs) due to photo-induced collective oscillation of conduction electrons on the surface of metal NPs when their size are smaller than the wavelength of the incident light beam (i.e., localized surface plasmon resonance, (LSPR)) . The bandgap of TiO2 is 3.2 eV; near UV light (irradiation) can excite electron-hole pairs . Ag NPs also show a very intense localized surface plasmon absorption in the near-UV region , which greatly enhances the electric field intensity in the vicinity of the Ag NPs. This enhanced near field at near-UV region could increase the light absorption to boost the excitation of electron-hole pairs in TiO2 and thus increase the efficiency of photocatalysis. This clearly indicates that the LSPR effect is a potential way for the enhancement of photocatalysis. The local field can be greatly enhanced in the vicinity of the triangular Ag NP array due to its unique morphology with sharp corners and edges. Theoretical studies on metal structures showed that the local field could be enhanced by several orders of magnitude in the NPs with special shape (e.g., triangular plates) . Yang et al.  demonstrated that the LSPR of triangular Ag NPs was much stronger than that of sphere Ag NPs. In this study, we can get triangular Ag NP island on the surface of TiO2 film. The Ag/TiO2 composite film shows higher photocatalytic activity than the pure TiO2.
TiO2 film with a thickness of about 680 nm was prepared by direct-current reactive magnetron sputtering deposition on silica glass and then was subsequently capped with triangular Ag NPs (schematically described in Figure 1d). Well-ordered latex sphere template of polystyrene (PS) nanospheres (460 nm in diameter) was prepared on the TiO2 surface [15, 16]. Metal Ag was deposited on the TiO2-PS surface by electron beam evaporation; the thickness of Ag film is approximately 100 nm. After the deposition of metal Ag, the PS nanospheres were removed by sonicating the sample in chloroform (CHCl3) for 20 s.
The structure of TiO2 film was investigated by grazing incidence X-ray diffraction (XRD). The morphologies of the self-assembled PS nanosphere mask and metal NPs were characterized by scanning electron microscopy (SEM) (FEI Sirion FEG, FEI Company, Eindhoven, The Netherlands). The microstructure of the samples was investigated using a JEOL JEM 2010 HT (JEOL Ltd., Akishima, Tokyo, Japan) transmission electron microscope (TEM) operated at 200 kV. Raman scattering spectra of all the samples were collected using a micro-Raman system. An Ar laser (488.0 nm) was used as the excitation source, and the laser power was kept at 10 mW.
The photocatalytic efficiency of TiO2 and Ag/TiO2 films with an area of 4 cm2 was evaluated by measuring the degradation rates of 5 mg/L methylene blue (MB) solution under UV irradiation. A mercury lamp (OSRAM AG, M ü nchen, Germany; 250 W with characteristic wavelength at 365 nm) was used as light source. Before irradiation, the samples were put in 40-mL MB for 30 min in darkness to reach absorption equilibrium. The decolorization of the MB solution was measured by a UV-vis spectrometer at the wavelength of 664.0 nm. The absorption spectrum of the MB solution was measured at a time interval of 30 min, and the total irradiation time was 4 h.
Results and discussion
XRD was used to investigate the crystal phase of the TiO2 films after annealing at 500°C for 2 h in oxygen atmosphere. As shown in Figure 2, it can be seen that the annealed TiO2 film shows a typical anatase structure without any other detectable phase (JCPDS no. 21-1272), indicating the formation of anatase TiO2 nanocrystals .
Figure 1a shows the self-assembled monolayer arrays of nanospheres with a typically ordered hexagonal pattern on the surface of the TiO2 film. The SEM image of the PS nanospheres coated with Ag film deposited by electron-beam evaporation is shown in Figure 1b, and the inset is the magnification SEM image of the same sample. It demonstrates that the Ag film wraps the PS spheres uniformly and tightly. The morphology of the Ag NPs exhibits ordered hexagonal periodic arrays formed on the surface of the TiO2 film with a large area after removing the PS sphere masks, as shown in Figure 1c. The Ag NP island is triangular due to the shape of the interstitial voids in the shadow mask. The inset in Figure 1c shows the magnified SEM image of Ag NPs on the TiO2 film. The formation of Ag NPs on the surface of TiO2 film can also be observed in the cross-sectional TEM image of the Ag/TiO2 nanocomposite film in the inset of Figure 1d. Thus, the triangular Ag NP island with a large uniform area can be obtained by this method.
In order to verify the influence of the Ag NP island on the photocatalytic activity of the TiO2 film, the photocatalytic activity of Ag/TiO2 composite system was evaluated by degradation of the MB solution under UV irradiation at room temperature. For comparison, the pure TiO2 film was carried out under the same experimental conditions. As shown in Figure 3 (inset), the Ag/TiO2 composite system obtained higher photocatalytic efficiency (81%) than the pure TiO2 film (60%). Meanwhile, the photodegradation of MB can be assumed to follow the classical Langmuir-Hinshelwood kinetics , and its kinetics may be expressed as follows:
Where k is the apparent first-order reaction rate constant (min-1), A0 and A represent the absorbance before and after irradiation for time t, respectively. From the plots of versus the irradiation time shown in Figure 3, the k values obtained from the slops of the simulated straight line are 3.875 × 10-3 and 6.372 × 10-3 min-1 for the pure TiO2 film and Ag/TiO2 system, respectively. The rate of MB decomposition for the Ag/TiO2 nanocomposite is more than 1.6-fold as fast as that of pure TiO2 film. The results indicate that the Ag/TiO2 composite system exhibits better photocatalytic performance than the pure TiO2 film.
The electronic structure of TiO2 plays a key role in TiO2 photocatalysis. The increasing number of electron-hole pairs and the separation of electron-hole pairs at the surface of TiO2 are the key factors to improve the photocatalytic abilities of TiO2. Based on our experimental results and literatures, the photocatalytic activity enhancement could be explained as follows.
Firstly, the LSPR can be enhanced at the corner of the triangular Ag NPs, the incident light field coupling to the LSPR might induce the enhancement of absorption of light, which boosts the excitation of electron-hole pairs in TiO2, and therefore increase the efficiency of photocatalysis. The local field intensity enhancement distribution E/E0 in the logarithmic scale due to the presence of Ag NPs was simulated using the finite-difference time-domain method, as shown in Figure 4a. In this method, we assume that 80-nm-thick Ag equilateral triangular NPs with 120-nm edge length are arranged in a hexagonal lattice and that the incident wavelength is 480 nm. For simplicity in simulation, we assume that the Ag NPs are isolated from each other. In our structure, we consider z as the light incident direction, and x is the polarization direction; the highest enhancement is close to 103 at the tip of the triangular Ag NP island. In order to prove the existence of the strong localized electric field induced by the triangular Ag NPs experimentally, we carried out a surface Raman scattering study. Figure 4b shows the Raman scattering spectra of the pure TiO2 film and the Ag/TiO2 nanocomposite film. All the samples observed in the Raman bands at 144, 199, 399, 516, and 640 cm− 1 can be assigned to the Eg, Eg, B1g, A1g or B1g, and an overtone Eg vibration mode, respectively . It is interesting to note that the Raman scattering is greatly enhanced in the Ag/TiO2 nanocomposite system compared with that in the pure TiO2 film. It is well known that Raman scattering intensity is proportional to the electric field intensity . The stronger Raman scattering attained from the Ag/TiO2 nanocomposite indicates that a stronger electric field was induced by introducing the Ag NPs, which is consistent with the simulated results. Theoretical and experimental results both show a strong local electric field induced by the Ag NP island. This enhanced local electric field induced by the Ag NPs will be used for the photoactivity enhancement of the TiO2 film.
Secondly, in the case of the Ag/TiO2 system, because of the lower work function of TiO2 (4.2 eV [21, 22]) compared with that of Ag (4.52 to 4.74 eV ), theirs is a Schottky contact and will result in the upward bending of the energy band near the contact region. When TiO2 is excited by the incident light, electrons will move to the surface and accumulate there, while photo-generated holes remain in the TiO2. However, considering the work function of Ag which is higher than that of TiO2, the photon-generated electrons will transfer from TiO2 to Ag NPs (as shown in Figure 5); the electrons and holes will separate at the surface region. Moreover, there are fewer electrons transferring from Ag to TiO2 due to the Schottky barrier. The accumulated electrons at Ag NPs could be transferred to the oxygen absorbed on the surface to form superoxide or . Accumulation of holes at the valence band of TiO2 leads to the production of surface hydroxyl radical ·OH , which is responsible for the oxidation decomposition of MB organics. Organic compounds are completely decomposed into water and carbon dioxide by reacting with the produced hydroxyl radicals . Thus, the Ag/TiO2 composite system can efficiently suppress the recombination of photo-generated carriers and enhance photocatalytic activity.
In conclusion, a highly ordered triangular Ag NP island on the surface of anatase TiO2 film was successfully prepared using the PS nanosphere lithography strategy. The Ag/TiO2 nanocomposite system can efficiently enhance photocatalytic activity. In addition, the Ag NP island will be of great significance to future applications in the fields of metal-semiconductor nanocomposite system photocatalysis for light-energy conversion.
Authors ’ contributions
JX participated in material preparation, data analysis, and drafted the manuscript. XX conceived and co-wrote the paper. FR, WW, ZD, GC, SZ, JZ, and FM participated in the sample characterization. CJ participated in its design and coordination. All authors read and approved the final manuscript.
The author thanks the NSFC (10905043, 11005082, 91026014, 11175133, 51171132), the Foundations from Chinese Ministry of Education (311003, 20100141120042, 20110141130004), the Open Research Fund of State Key Laboratory of Electronic Thin Films and Integrated Devices (UESTC) (KFJJ201004), Young Chenguang Project of Wuhan City (201050231055), and the Fundamental Research Funds for the Central Universities, Hubei Provincial Natural Science Foundation (2011CDB270).
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China
Center for Electron Microscopy and School of Physics and Technology, Wuhan University
School of Electrical & Electronic Engineering, Hubei University of Technology
Linsebigler AL, Lu G, Yates JT: Photocatalysis on TiO2surfaces: principles, mechanisms, and selected results. Chem Rev 1995, 95: 735. 10.1021/cr00035a013View Article
Tada H, Yamamoto M, Ito S: Promoting Effect of MgOxsubmonolayer coverage of TiO2on the photoinduced oxidation of anionic surfactants. Langmuir 1999, 15: 3699. 10.1021/la9816712View Article
Wang D, Zou Y, Wen S, Fan D: A passivated codoping approach to tailor the band edges of TiO2for efficient photocatalytic degradation of organic pollutants. Appl Phys Lett 2009, 95: 012106. 10.1063/1.3174917View Article
Han F, Kambala VSR, Srinivasan M, Rajarathnam D, Naidu R: Tailored titanium dioxide photocatalysts for the degradation of organic dyes in wastewater treatment: a review. Appl Catal A-Gen 2009, 359: 25. 10.1016/j.apcata.2009.02.043View Article
Jakob M, Levanon H, Kamat PV: Charge distribution between UV-irradiated TiO2 and gold nanoparticles: determination of shift in the Fermi level. Nano Lett 2003, 3: 353. 10.1021/nl0340071View Article
Tian Y, Tatsuma T: Mechanisms and applications of plasmon-induced charge separation at TiO2films loaded with gold nanoparticles. J Am Chem Soc 2005, 127: 7632. 10.1021/ja042192uView Article
Min BK, Heo JE, Youn NK, Joo OS, Lee H, Kim JH, Kim HS: Tuning of the photocatalytic 1, 4-dioxane degradation with surface plasmon resonance of gold nanoparticles on titania. Catal Commun 2009, 10: 712. 10.1016/j.catcom.2008.11.024View Article
Awazu K, Fujimaki M, Rockstuhl C, Tominaga J, Murakami H, Ohki Y, Yoshida N, Watanabe T: A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide. J Am Chem Soc 2008, 130: 1676. 10.1021/ja076503nView Article
Kumar MK, Krishnamoorthy S, Tan LK, Chiam SY, Tripathy S, Gao H: Field effects in plasmonic photocatalyst by precise SiO2thickness control using atomic layer deposition. ACS Catal 2011, 1: 300. 10.1021/cs100117vView Article
Evanoff DD, Chumanov G: Synthesis and optical properties of silver nanoparticles and arrays. Chem Phys Chem 2005, 6: 1221. 10.1002/cphc.200500113
Tang H, Prasad K, Sanjin è s R, Schmid PE, Lévy F: Electrical and optical properties of TiO. J Appl Phys 1994, 75:.
Kerker M: The optics of colloidal silver: something old and something new. J Colloid Interface Sci 1985, 105: 297. 10.1016/0021-9797(85)90304-2View Article
Yang WH, Schatz GC, Duyne RPV: Discrete dipole approximation for calculating extinction and Raman intensities for small particles with arbitrary shapes. J Chem Phys 1995, 103: 869. 10.1063/1.469787View Article
Yang Y, Matsubara S, Xiong L, Hayakawa T, Nogami M: Solvothermal synthesis of multiple shapes of silver nanoparticles and their SERS properties. J Phys Chem C 2007, 111: 9095. 10.1021/jp068859bView Article
Rybczynski J, Ebels U, Giersig M: Large-scale, 2D arrays of magnetic nanoparticles. Colloid. Surface. A: Physicochem. Eng Aspects 2003, 219: 1. 10.1016/S0927-7757(03)00011-6View Article
Zhang YJ, Li W, Chen KJ: Application of two-dimensional polystyrene arrays in the fabrication of ordered silicon pillars. J Alloys Compd 2008, 450: 512. 10.1016/j.jallcom.2006.11.184View Article
Gu DE, Yang BC, Hu YD: V and N co-doped nanocrystal anatase TiO2photocatalysts with enhanced photocatalytic activity under visible light irradiation. Catal Lett 2007, 118: 254. 10.1007/s10562-007-9179-5View Article
Al-Ekabi H, Serpone N: Kinetics studies in heterogeneous photocatalysis. I. Photocatalytic degradation of chlorinated phenols in aerated aqueous solutions over titania supported on a glass matrix. J Phys Chem 1988, 92: 5726. 10.1021/j100331a036View Article
Zhang WF, He YL, Zhang MS, Yin Z, Chen Q: Raman scattering study on anatase TiO2nanocrystals. J Phys D: Appl Phys 2000, 33: 912. 10.1088/0022-3727/33/8/305View Article
Garcia-Vidal FJ, Pendry JB: Collective theory for surface enhanced Raman scattering. Phys Rev Lett 1996, 77: 1163. 10.1103/PhysRevLett.77.1163View Article
Tan TY, Yip CK, Beydoun D, Amal R: Effects of nano-Ag particles loading on TiO2photocatalytic reduction of selenate ions. Chem Eng J 2003, 95: 179. 10.1016/S1385-8947(03)00103-7View Article
Breeze AJ, Schlesinger Z, Carter SA: Charge transport in TiO2/MEH-PPV polymer photovoltaics. Phys Rev B 2001, 64: 125205.View Article
Fang YJ, Sha J, Wang ZL, Wan YT, Xia WW, Wang YW: Behind the change of the photoluminescence property of metal-coated ZnO nanowire arrays. Appl Phys Lett 2011, 98: 033103. 10.1063/1.3543902View Article
Miyauchi M, Nakajima A, Hashimoto K, Watanabe T: A highly hydrophilic thin film under 1 μ W/cm2UV illumination. Adv Mater 1923, 2000: 12.
Alberic RM, Jardim WF: Photocatalytic destruction of VOCs in the gas-phase using titanium dioxide. Appl Catal B-Environ 1997, 14: 55. 10.1016/S0926-3373(97)00012-XView Article
Yu JG, Xiong JF, Cheng B, Liu SW: Fabrication and characterization of Ag-TiO2multiphase nanocomposite thin films with enhanced photocatalytic activity. Appl Catal B-Environ 2005, 60: 211. 10.1016/j.apcatb.2005.03.009View Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.