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Photochromism in Composite and Hybrid Materials Based on Transition-Metal Oxides and Polyoxometalates

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Photochromism in Composite and Hybrid Materials Based on Transition-Metal Oxides and Polyoxometalates Progress in Materials Science 51 (2006) 810–879 www.elsevier.com/locate/pmatsci Photochromism in composite and hybrid materials based on transition-metal oxides and polyoxometalates Tao He, Jiannian Yao * CAS Key Laboratory of Photochemistry, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China Received 25 April 2005; accepted 5 December 2005 Abstract Photochromic materials are attractive and promising for applications in many fields. One subject in this area is to prepare and study the photochromism in composite or hybrid materials based on transition-metal oxides or polyoxometalates. Their properties not depend only on the chemical nature of each component, but also on the interface and synergy between them. Since the charge transfer plays a key role in the photochromism of these materials, it is very important to increase the charge (electrons, holes, and protons) interactions between the two components in either com- posites or hybrids. To realize this, one big challenge is to optimize the two components on a molec- ular or nanometer scale, which is closely relevant to the constituents, sample history (pre-treatment, preparation, and post-treatment), environment (humidity, presence of reducible or oxidizible mat- ters, light-irradiation wavelength, intensity, time, etc.). Based on these, many novel composite or hybrid materials with improved photochromism, visible-light coloration, reversible photochromism, multicolor photochromism or, possibly, fast photoresponse, have been prepared during the last two decades or three. This may underscore the opportunity of using these composite and hybrid mate- rials as the photonic applications. In present paper, we summarize thoroughly all the recent progress in these subjects. Ó 2005 Elsevier Ltd. All rights reserved. * Corresponding author. Tel./fax: +86 10 82616517. E-mail address: [email protected] (J. Yao). 0079-6425/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.pmatsci.2005.12.001 T. He, J. Yao / Progress in Materials Science 51 (2006) 810–879 811 Contents 1. Introduction . 812 2. Inorganic/inorganic composite materials . 813 2.1. Semiconductor/metal composite materials . 814 2.1.1. Improved photochromism of WO3 and MoO3 ................... 814 2.1.2. Multicolor photochromism ................................. 817 2.2. Semiconductor/semiconductor composite materials ...................... 818 2.2.1. Improved photochromism in WO3/MoO3 system . 818 2.2.2. Improved photochromism in other composites containing WO3 ...... 822 2.2.3. Improved photochromism in MoO3/TiO2 system ................. 825 2.2.4. Visible-light coloration in MoO3/TiO2 system . 827 2.2.5. Visible-light coloration induced by CdS . 828 2.3. Photochromic materials with dopants . 830 2.3.1. Doped TiO2 systems . 830 2.3.2. Transition-metal-doped titanates . 833 2.3.3. Doped molybdates and tungstates . 835 2.3.4. Field-assisted photochromism . 836 2.4. Miscellaneous composite materials . 837 2.4.1. a-WO3/Si heterostructure . 837 2.4.2. H3PW12O40/TiO2 system . 839 2.4.3. Other composite systems . 839 3. Inorganic/organic hybrid materials . 840 3.1. Model molecules, photochromic mechanism, and preparation methods . 840 3.1.1. Model molecules and photochromic mechanism . 840 3.1.2. Preparation methods. 842 3.1.3. Summary . 843 3.2. Hybrids at molecular level . 843 3.2.1. Donor–acceptor systems prepared from POMs and aromatic organic molecules . 843 3.2.2. Alkylammonium POMs. 845 3.2.3. Hybrids prepared from POMs and small biological molecules . ..... 847 3.2.4. Visible-light coloration in CT complexes. 847 3.3. Hybrids at nanometer level . 848 3.3.1. Multilayer thin films prepared by electrostatic layer-by-layer method . 848 3.3.2. POMs embedded in polymeric matrices . 856 3.3.3. POMs embedded in organic/silica matrices . 862 3.3.4. POMs anchored to organic polymeric backbone . 863 3.3.5. Self-organized hybrids based on POMs and DODA . 864 3.4. Miscellaneous hybrid materials. 867 3.4.1. TMOs/DMF systems . 867 3.4.2. Molybdenum-oxide cluster/citric acid complexes . 868 3.4.3. Molybdenum (VI) oxalate complexes . 870 4. Concluding remarks. 870 Acknowledgements . 871 References . 872 812 T. He, J. Yao / Progress in Materials Science 51 (2006) 810–879 1. Introduction The word ‘‘photochromism’’ derives from two Greek words meaning light and color, which refers to such a phenomenon that the material can change color in a reversible way by electromagnetic radiation (UV, visible, and IR illumination) [1–6]. The reverse process can take place by exposure to the light at a different frequency [1–3], by heating in the dark [1–3], by electrochemical polarization [7], or by chemical oxidation [8]. Photo- chromic materials exhibit a wide range of optical properties, which makes them attractive and promising for a variety of applications. In the book edited by Brown [1], Bertelson has given an excellent review about these applications. The sensitivity of the materials to light radiation makes them useful for self-developing photography, protective materials, dosim- etry and actinometry. The materials with good photochromic reversibility have potential applications in reusable information storage media, data display, optical signal processing, chemical switch for computer, smart window (control of radiation intensity), and the like. These materials can also be used as the reagents for photomasking, photoresist, camou- flage, and so on. The photochromism was first phenomenologically observed in both inorganic and organic materials, which dates back to the 19th century [2,9–14]. However, subsequent developmental work has proliferated the number of organic materials considerably. Although so far most photochromic materials are organic, inorganic materials have some advantages over the organic counterparts. They have better thermal stability, strength, chemical resistance, and macroscopic shape molding (can be easily shaped as thin films, coatings, monoliths, or other suitable forms). In the first half of 20th century, the research on inorganic photochromic materials was mainly focused on alkali halides, alkaline earth halides, alkali metal azides, TiO2, titanates, complex minerals, and complex mercury salts, and so on [1,2,10,15–18]. After Deb’s pioneering work [19–22], a widespread interest [7–9,23–38] has been motivated in the photochromism of transition-metal oxides (TMOs) and polyoxometalates (POMs), specifically in MoO3 and WO3. The photochro- mic response of MoO3 [7] and WO3 [36] thin film was extended from ultraviolet (UV) light to visible light after the cathodic polarization pre-treatment. The photochromic activity of TMO films can be improved when irradiated in reducible atmosphere [30,31], though from which only a few applications can be benefited. For single inorganic photochromic species, they usually exhibit poor reversibility (e.g., thermal bleached WO3 or MoO3 cannot be photocolored again), small change in optical density after coloration (low photochromic activity), narrow response in the spectrum (all the TMOs except V2O5 response only to the blue or UV light), low fatigability (cannot be cycled many times while maintaining per- formance), monotonic coloration (usually blue color for most TMOs), low thermal stabil- ity of virgin or colored states, slow response time, and sometimes a high-cost preparation with difficulty in tailor-make, and so forth. During the last two decades or three, specifically due to the development of nanotech- nology and nanoscience, a considerable promising potential of molecular materials has been lying on the possibility to create composite and hybrid materials. The technological drive in the quest for this novel class of multifunctional materials is the desire to secure a property or a combination of properties not available in any of the individual component of the composites or hybrids and/or at least to improve some properties of the active com- ponent [39–42]. Accordingly, the trend in photochromism begins to concern the materials combining several properties in a synergistic way. In most cases, it is to develop T. He, J. Yao / Progress in Materials Science 51 (2006) 810–879 813 photochromic systems of inorganic/inorganic composites [23,28,29,38] and inorganic/ organic hybrids [43–47] based on TMOs or POMs. These systems are very interesting from the perspectives of basic science and technology. The properties expected in them do not depend only on the chemical nature of each component, but also on the interface and syn- ergy between them. The latter usually plays a key role in tuning the photochromic behav- ior. Thus, a critical point for the design of these materials is the tuning of nature, extension and accessibility of the inner interfaces [48]. The general tendency in research is to create intimate mixing and/or interpenetration at molecular or nanometer level between the two components in order to obtain strong interfacial interactions, specifically a strong charge (electron, hole, proton) communication. Accordingly, the preparation methods transfer from pretty high-cost physical techniques (such as thermal evaporation or sputtering deposition) to readily and relatively low-cost ‘‘soft’’ chemical (chimie douce) ones. Hope- fully most of the aforementioned issues for single species, though not all, may be sur- mounted or at least ameliorated in these systems. In the composite systems, the photogenerated electrons and holes can transfer between the two inorganic constituents due to the different energy levels,
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