Enhanced visible-light absorption from Ag2O nanoparticles in nitrogen-doped TiO2 thin films

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Abstract

Ion-beam-assisted deposition was used to prepare TiON thin film with ultrafine Ag2O semiconductor nanoparticles. These Ag2O nanoparticles were about 3 nm. The obtained thin films showed an obvious “red-shift” in the optical absorption spectrum. The absorption in the visible-light range is resulting from the absorption by metallic-like nanoparticles as silver changed its chemical state from Ag+ to Ag in nitrogen-doped titanium oxide matrix under visible-light illumination.

Research Highlight

► Ag2O/TiON thin film was prepared by using ion-beam-assisted deposition technique. ► Ag2O particle in TiON thin film was several nanometers. ► Ag2O/TiON thin film showed obvious “red-shift” in optical absorption spectrum. ► In situ XPS study was used to understand the enhanced visible-light absorption.

Introduction

Titanium dioxide (TiO2) thin film has been extensively investigated as a photocatalyst since the discovery of its photosensitization effect by Honda and Fujishima in 1972 [1]. Its strong photo-oxidizing potential, high chemical stability, nontoxicity, etc., have motivated the study for environmental applications ranging from deodorization to the purification of air and water [1], [2], [3], [4], [5], [6], [7], [8]. However, pure TiO2 has very low photocatalytic efficiency outdoors, where only ~5% of the solar energy in the ultraviolet range is capable of activating the photocatalytic reaction, due to its wide energy band gap (3.2–3.8ev). To use the solar energy effectively, it is essential to extend the absorption spectrum of TiO2 into the visible-light region, where a much higher proportion (45%) of the solar energy may be used. To address this requirement, early studies had focused on alloying TiO2 with transition metals [2] and doping TiO2 with carbon [3], sulfur [4], fluorine [5], or nitrogen [6], [7], [8], [9], [10]. Research results showed that nitrogen doping TiO2 had an excellent effect on extending the absorption spectrum into the visible-light region.

The assembly of metal or semiconductor nanoparticles into dielectric thin films has been investigated extensively [11], [12], [13], [14] because of their unique optical and electrical properties compared with their counterparts in bulk forms. Noble-metal nanoparticles, such as Au and Pd, are known to exhibit characteristic optical absorption in the UV–visible region caused by the surface plasmon resonance (SPR) originating from collective oscillations of free electrons [15], [16], [17], [18], [19], [20], [21]. For semiconductor nanoparticles, their optical spectra may display a quantum size effect. As semiconductor nanoparticles decrease in size, their excitation energy generally increases, resulting in a shift of their absorption band to a shorter wavelength region (“blue-shift”). Ag2O is a noble-metal oxide of widespread technological interests, whose reported band gap ranges from 0.49 to 3.1 eV [22], [23], [24]. As the Ag2O nanoparticle size goes down to a couple of nanometers, a “red-shift” may occur in the optical absorption spectrum because of the quantum size effect.

The present study focused on the improvement of the utility of solar energy of TiO2 thin film. With the help of ion-beam-assisted deposition (IBAD) technique [25], we prepared nitrogen-doped TiO2 thin film with ultrafine Ag2O semiconductor nanoparticles and attempted to determine if Ag2O nanoparticles might have effected on the “red-shift” in the optical absorption spectrum of TiO2 (TiON) thin film.

Ag2O/TiON and Ag2O/TiO2 thin films were deposited by electron beam evaporation of TiO2/Ag2O pellet with/without simultaneously nitrogen ions bombarding the growing film, respectively. For comparison purpose, TiON and TiO2 thin films were also prepared by electron beam evaporation of pure TiO2 pellet with/without nitrogen ion bombardment, respectively, under the same experimental conditions.

Section snippets

Experimental procedure

The ion-beam-assisted deposition (IBAD) system used in this study consisted of a 3-cm-diameter Kaufman-type ion source, a 3-kW electron-beam evaporator, and a substrate holder with a heater made of tantalum, as shown schematically in Fig. 1. A quartz-crystal monitor was used to control the film thickness. The film thickness was confirmed after deposition by Gaertner Ellipsometer (Model L116c, Wavelength: 633 nm). Micro-slides glasses were used as substrate in this study. They were ultrasonically

Results and discussion

Fig. 2 is the XRD pattern of Ag2O/TiON thin film obtained by IBAD technique. The pattern shows that an anatase phase exists in the film. A very weak peak belonging to Ag2O is observed in this pattern, which suggests that Ag dopant exists as Ag2O in the obtained film at a very small quantity.

Fig. 3 shows the curve of XPS high-resolution scan over Ti(2p) and Ag(3d) peaks on Ag2O/TiON thin film. There are two main peaks in the Ti(2p) binding energy region [Fig. 3a]. The binding energy of Ti 2p3/2

Conclusions

Ion-beam-assisted deposition was used to prepare ultrafine Ag2O semiconductor nanoparticles into TiON thin film. The obtained Ag2O/TiON thin film has a dense surface and the grain size in the film is about 20 nm, and the average size of Ag2O nanoparticles is about 3 nm, while the grain size of the Ag2O/TiO2 thin film is about 70 nm and the average size of Ag2O nanoparticles is about 20 nm. The chemical state transition of silver in Ag2O/TiON thin film results in an obvious optical red-shift in its

Acknowledgments

The authors would like to thank C. H. Lei in the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, for the help on STEM. This work was supported by the Center of Advanced Materials for the Purification of Water with Systems, National Science Foundation (grant no. CTS-0120978) and was funded by the China Scholarship Council (grant no. 2006A45003) and Jiangsu Province University—Industry Cooperation Project (grant no. BY2009154). XPS and XRD measurements

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