A family of visible-light responsive photocatalysts obtained by dispersing CrO6 octahedra into a hydrotalcite matrix.

Increasing serious environmental pollution and energy shortage have become two intractable problems in the 21st century; it is, therefore, urgent to develop new materials and techniques to settle these issues. The chemistry of semiconductors has received huge attention owing to their wide application in photocatalysis, solar energy conversion, water splitting, optics, electrochemical devices and sensing. Titanium dioxide, the most promising photocatalyst, has been applied in areas of environment and energy. However, the efficient use of this kind of material is only achievable upon UV excitation in 5% solar energy, therefore, many attempts have been made to sensitize TiO2 for the much larger visible fraction by doping with transition metal and nonmetal ions. These TiO2-doped materials, in general, show limited absorption in the visible-light region, which leads to low activity. In addition, traditional visible-light photocatalysts are either unstable upon illumination (e.g., CdS, CdSe) or display low activity (e.g., WO3, Fe2O3, In(OH)ySz and ZnSn(OH)6). [16] As a result, novel and efficient visible-light photocatalysts are highly essential to meet the requirements of future environment and energy technologies driven by solar energy. Chromium-based inorganic materials have been widely used as catalysts for selective oxidation of various organic compounds. The precipitation of chromium hydroxide is an important procedure that occurs in soil and natural water. The considerable interest in chromium hydroxides is focused on their wide technological applications in catalysis, pigment and colloid science. The structure of chromium hydroxide is based on an octahedral layer, in which onethird of the octahedral CrO6 sites are occupied. Its structure is complementary to that of bayerite (a-Al(OH)3) and similar to brucite-like Mg(OH)2. [17g] It has been reported that chromium hydroxide shows a strong absorption in the visible-light region, which implies that it could serve as potential photocatalytic material. However, no photocatalytic activity with visible-light irradiation has been found for chromium hydroxide. This is possibly related to the low efficiency of electron–hole separation or excited electron transfer. It has been reported that high dispersion of transitional metal octahedron would facilitate the electron transfer and avoid the recombination of electron and hole. This motivated us to take the challenge of distributing the CrO6 octahedron unit within an inorganic matrix to enhance the efficiency of charge separation and improve photoconversion capability with visible-light irradiation. Herein, we explored the idea of dispersing the CrO6 octahedron unit in an inorganic hydroxide matrix using hydrotalcite. Hydrotalcites, commonly known as layered double hydroxides (LDHs), are a large class of typical layered clays, which can be described by the general formula [M1 xM III x> (OH)2] ACHTUNGTRENNUNG(A n )x/n·mH2O (M II and M are diand trivalent cations, A is an n-valent anion). The host structure is based on brucite-like Mg(OH)2 layers of edge-sharing M(OH)6 octahedra; isomorphous substitution of part of the divalent M cations by trivalent M cations generates positively charged sheets with charge-balancing anions in the interlayer gallery. Recently, pioneering work by Grey et al. with the assistance of multinuclear NMR spectroscopy demonstrated that the M and M cations are distributed orderly in LDH layers. This inspired us to disperse the CrO6 octahedron unit with another metal octahedron at the molecular level by the formation of LDH materials; this could promote the efficiency of electron–hole separation and facilitate the transfer of excited electrons under visible light. In this work, visible-light responsive MCr X–LDHs (M= Cu, Ni, Zn; X=NO3 , CO3 2 ) were synthesized by a simple and scale-up co-precipitation method developed by our group; these were demonstrated to be effective and recyclable photocatalysts for the decomposition of organic dyes and colorless pollutants. The intrinsically electronic nature and the band gap of LDH materials were explored, which is essential for understanding the semiconductor property of Cr-containing LDHs and for their effective application in solar energy transformation. It was found that the CuCr NO3–LDH exhibited pronounced visible-light-response activity as well as excellent recycle ability for dye and phenols. [a] Y. Zhao, S. Zhang, B. Li, Dr. H. Yan, S. He, L. Tian, W. Shi, Prof. M. Wei, Prof. D. G. Evans, Prof. X. Duan State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology 100029, Beijing (P. R. China) Fax: (+86)10-64425385 E-mail : [email protected] [b] Prof. J. Ma School of Chemistry and Chemical Engineering Key Laboratory of Mesoscopic Chemistry of MOE Nanjing University, 210093, Nanjing (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101874.