Enhanced photocatalysis by coupling of anatase TiO2 film to triangular Ag nanoparticle island
© Xu et al.; licensee Springer. 2012
Received: 14 February 2012
Accepted: 2 May 2012
Published: 2 May 2012
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.
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.
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
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.
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.
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).
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