Efficiency enhancements in Ag nanoparticles-SiO2-TiO2 sandwiched structure via plasmonic effect-enhanced light capturing
© Xu et al.; licensee Springer. 2013
Received: 19 December 2012
Accepted: 18 January 2013
Published: 12 February 2013
TiO2-SiO2-Ag composites are fabricated by depositing TiO2 films on silica substrates embedded with Ag nanoparticles. Enhancement of light absorption of the nanostructural composites is observed. The light absorption enhancement of the synthesized structure in comparison to TiO2 originated from the near-field enhancement caused by the plasmonic effect of Ag nanoparticles, which can be demonstrated by the optical absorption spectra, Raman scattering investigation, and the increase of the photocatalytic activity. The embedded Ag nanoparticles are formed by ion implantation, which effectively prevents Ag to be oxidized through direct contact with TiO2. The suggested incorporation of plasmonic nanostructures shows a great potential application in a highly efficient photocatalyst and ultra-thin solar cell.
KeywordsPlasmonic effect Ion implantation Ag nanoparticles Photocatalysis
Titanium dioxide (TiO2) has strong photocatalytic activity, high chemical stability, a long lifetime of photon-generated carriers, nontoxicity, and low cost, which make it one of the most widely used photocatalysts for hydrogen production and solar cells, as well as water and air remediation [1–3]. At modern times, TiO2 becomes a hot research topic because of the potential applications in the field of environment and energy [4–6]. Unfortunately, owing to its wide band gap of 3.2 eV (at 390 nm), only approximately 4% solar spectrum can be utilized. During the last decades, great efforts have been made to modify the TiO2 to enhance the visible light response. A considerable increase in the photocatalytic activity in the visible region has been observed by doping [7–10]. However, to date, the doping structure lacks reliable controllability. Recently, metallic nanostructures have been introduced into a semiconductor film (e.g., ZnO, InGaN quantum wells) for enhancement of light emission, photocurrent solar cells [11–14], and photocatalysts [15–17] by a strong plasmonic effect of metallic nanostructures. In order to maximize the utilization rate of the UV region of the sunlight, in this letter, we design a new composite structure to enhance the light absorption efficiency by coupling TiO2 to Ag nanoparticles (NPs) embedded in SiO2 formed by low-energy Ag ion implantation. Ag NPs show a very intense localized surface plasmon resonance (SPR) in the near-UV region , which strongly enhances the electric field in the vicinity of the Ag NPs. This enhanced electric field at the near-UV region could increase the UV light absorption to boost the excitation of electron–hole pairs in TiO2 and thus increase the photoelectric conversion efficiency. In this kind of structure, the Ag NPs embedded in SiO2 serve two purposes. Firstly, SiO2 as a protective layer prevents Ag to be oxidized through direct contact with TiO2. Secondly, the size and depth distributions of the embedded Ag NPs can be controlled by choosing implantation parameters and post-implantation thermal treatment , which can tune the SPR spectrum of Ag NPs to match the absorption edge of TiO2. Thus, it is possible to design nanostructures that concentrate the light surrounding near Ag NPs, which enhance the light absorption of the TiO2 film.
Ag ion implantation parameters for all samples
Fluence of ion implantation (ions/cm2)
Energy of ion implantation (kV)
5 × 1016
5 × 1016
1 × 1017
5 × 1016
The photocatalytic efficiencies of TiO2 and TiO2-SiO2-Ag nanostructural composites with an area of 4 cm2 were evaluated by measuring the degradation rates of 5 mg/L methylene blue (MB) solution under UV–vis irradiation. A mercury lamp (Osram 250 W (Osram GmbH, Munich, Germany) with a characteristic wavelength at 365 nm) was used as a light source. The TiO2 and the TiO2-SiO2-Ag composite films were placed in 40 mL of MB solution with a concentration of 5 mg/L. Before irradiation, the samples were put in 40 mL of MB solution for 30 min in the darkness to reach absorption equilibrium. The decolorization of the MB solution was measured by an UV–vis spectrometer (Shimadzu UV 2550, Shimadzu Corporation) 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
The near-field enhancement in the TiO2 layer due to the presence of the Ag NPs is also simulated using the finite-difference time-domain (FDTD) method as shown in Figure 4b. In our structure, we consider x as the light incident direction, the illuminating plane wave with a wavelength of 420 nm is y polarized, an Ag NP with a diameter of 20 nm is embedded in SiO2, and the distance to the surface of the SiO2 substrate is 7 nm. An amplitude enhancement to 3 can be observed. Theoretical and experimental results show that an enhancement of the near field is induced by the SPR of Ag NPs. The SPR excitations cause a large increase in electromagnetic field in the vicinity of metal NPs. The localized amplification can increase the incident excitation field and boost the creation of hole–electron pairs, which results in the enhancement of the photocatalytic activity of TiO2.
In conclusion, we have successfully demonstrated a plasmonic effect by simply incorporating Ag NPs with TiO2 film. Optimum ion implantation conditions for Ag NPs synthesis in SiO2 were experimentally estimated. The plasmonic effect occurring near the interface of TiO2 and silica glass has effectively enhanced the light trapping. Both the experimental data and the simulations show that the enhancement effect is attained from the near-field enhancement induced by the SPR of Ag NPs. Our results have shown that the plasmonic effect has great potential in the application of increasing the UV light absorption in TiO2 photocatalysts and opening up opportunities for highly efficient ultra-thin film solar cells.
The authors thank the National Basic Research Program of China (973 Program, 2009CB939704), the NSFC (10905043, 11005082, 91026014, 11175133, 51171132, 11004052, U1260102), the foundations from the Chinese Ministry of Education (311003, 20100141120042, 20110141130004 ), NCET, the Young Chenguang Project of Wuhan City (201050231055), the Fundamental Research Funds for the Central Universities, Hubei Provincial Natural Science Foundation (2011CDB270, 2012FFA042), and the Russian Foundation for Basic Research for the partial support.
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