Photocatalysts DOI: 10.1002/anie.200802483 Ag@AgCl: A Highly Efficient and Stable Photocatalyst Active under Visible Light** PengWang,BaibiaoHuang,*XiaoyanQin,XiaoyangZhang,YingDai,JiyongWei,andMyung- Hwan Whangbo Dedicated to Professor Minhua Jiang Nanoparticles (NPs) of noble metals can strongly absorb visible light because of their plasmon resonance, [1–4] which is greatly influenced by their morphology and size. [5–10] The phenomenon of plasmon resonance gives rise to important applications such as colorimetric sensors, [11,12] photovoltaic devices, [13,14] photochromic devices, [15] and photocatalysts. [16] Noble metal NPs exhibit characteristic optical and physical properties that are substantially different from those of the corresponding bulk materials. [17–19] In particular, silver NPs show efficient plasmon resonance in the visible region, which Awazu et al. [16] recently utilized to develop a plasmonic photocatalyst. In their study, TiO 2 was deposited on NPs consisting of a silver core covered with a silica (SiO 2 ) shell to prevent oxidation of Ag by direct contact with TiO 2 . Under UV illumination, this plasmonic photocatalyst exhibits enhanced catalytic activity, which increases with decreasing thickness of the SiO 2 shell. To enhance the activity of a plasmonic photocatalyst, it is desirable to deposit silver NPs directly onto the surface of an active dielectric substrate without a protective shell, because the near-field effect of the NPs will be more strongly felt by the substrate. Herein we show that such a photocatalyst can be obtained from silver chloride by exploiting its photosensitivity, and the resulting plasmonic photocatalyst is highly efficient and stable under visible-light illumination. Silver halides are photosensitive materials extensively used as source materials in photographic films. On absorbing a photon, a silver halide particle generates an electron and a hole, and subsequently the photogenerated electron combines with an Ag + ion to form an Ag 0 atom. Ultimately, a cluster of silver atoms is formed within a silver halide particle upon repeated absorption of photons. Due to this instability under sunlight, which provides the very basis for chemical photog- raphy, silver halides are seldom used as photocatalysts. Nevertheless, there have been reports that under UV/Vis illumination AgCl deposited on a conducting support photo- catalyzes O 2 production from water in the presence of a small excess of silver ions in solution, [20] and that under UV illumination AgBr dispersed on a silica support photocata- lyzes H 2 production from CH 3 OH/H 2 O solution. [21] In their study on the AgBr/SiO 2 photocatalyst, Kakuta et al. [21] observed that Ag 0 species are formed on AgBr in the early stage of the reaction, and AgBr is not destroyed under successive UV illumination. As suggested by Kakuta et al., electron–hole separation may occur smoothly in the presence of Ag 0 species, and the latter may catalyze H 2 production from alcohol radicals formed by photo-induced holes. If so, silver NPs formed on silver halide particles might be expected to be a stable photocatalyst under visible-light illumination due to their plasmon resonance. This expectation led us to prepare a new photocatalyst active and stable under visible light, namely, AgCl particles with silver NPs formed on their surface, by first treating Ag 2 MoO 4 with HCl to form AgCl powder and then reducing some Ag + ions in the surface region of the AgCl particles to Ag 0 species (for details, see the Experimental Section). For convenience, these are referred to as Ag@AgCl particles. The X-ray diffraction (XRD) pattern of the Ag@AgCl product clearly shows that the cubic phase of Ag with lattice constant a = 4.0861  (JCPDS file: 65-2871) coexists with the cubic phase of AgCl with lattice constant a = 5.5491  (JCPDS file: 31-1238; see Figure 1). Scanning electron microscopy (SEM) images of the Ag@AgCl product (Figure 2) reveal that silver NPs with diameters in the range of 20–150 nm are deposited on the surface of AgCl particles with diameters in the range of 0.2–1.3 mm. The UV/Vis diffuse-reflectance spectra of Ag@AgCl, AgCl, and N-doped TiO 2 (used as reference photocatalyst) are compared in Figure 3. In contrast to AgCl and N-doped TiO 2 , Ag@AgCl has a strong adsorption in the visible region which is almost as strong as that in the UV region. This is attributed to the plasmon resonance of silver NPs deposited on AgCl particles. To evaluate the photooxidation capability of Ag@AgCl, we examined the decomposition of methylic orange (MO) dye in solution over the Ag@AgCl sample under visible-light irradiation as a function of time (Figure 4). For comparison, we also carried out decomposition of the MO dye in solution over the N-doped TiO 2 reference photocatalyst under visible- [*] P. Wang, Prof.Dr. B. Huang, X. Qin, Prof. X. Zhang, Dr. J. Wei State Key Lab of Crystal Materials Shandong University, Jinan 250100 (China) E-mail: bbhuang@sdu.edu.cn Homepage: http://www.icm.sdu.edu.cn/index.php Prof.Dr. Y. Dai School of Physics, Shandong University Prof. Dr. M.-H. Whangbo Department of Chemistry, North Carolina State University Raleigh, NC 27695-8204 (USA) [**] This work was financially supported by the National Basic Research Program of China (973 Program, Grant 2007CB613302) and the National Natural Science Foundation of China under Grants 50721002 and 10774091. M.-H.W. is thankful for support by the OfficeofBasicEnergySciences,DivisionofMaterialsSciences,U.S. Department of Energy, under Grant DE-FG02-86ER45259. Angewandte Chemie 7931 Angew. Chem. Int. Ed. 2008, 47, 7931–7933 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim