Reversible Photoredox Processing of Transition Metal Oxides
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4WO3 yellow
•
2W 205 + 0 2(g) blue
(1)
WO3 has been the subject of extensive study because of its potential application as electrochromic displays 5 and in "smart windows"6 . Sputtered WO3 films are known to have a sufficiently open structure such that exchange/incorporation of cations from solution, and other species from the gas phase, occurs readily. Bechinger and coworkers 7 have recently discussed the reversible photochromism of WO3 films in the context of displays, and active optical filters. We are interested in evaluating nonlinear interface optical switches (NIOSs)8 and have used laser chemistry with organometallic reactants 9 ,10 ,1 ! to produce films composed of metal and metal oxide (W0O3 ) cluster materials. Such clusters have been predictedl 2 ,13 to have large optical and electrical nonlinearities. In fact, we observed very strong optical nonlinearity using a variety of measurements involving the nonlinear interface configuration. The simplest experiment involved measuring either the transmitted light or the reflected light as a function of the incident light power. The origin of the nonlinearity, its relation to the structure of the materials comprising the interface, and the source of variations in the results with varying film fabrication and treatment procedures 7 , are still not well understood. The NIOS configuration is the same situtation exploited in Brewster Angle Microscopy 14 and is particularly sensitive. We considered the possibility that the observed large optical nonlinearities involved some interaction between the ambient atmosphere and the films because of the known sensitivity of WO3 films to adsorbed species; the basis for their catalytic properties' 5 and their application as gas 16 and temperature 7 sensors. Thermally-induced oxygen exchange involving
347 Mat. Res. Soc. Symp. Proc. Vol. 397 ©1996 Materials Research Society
bulk transition metal oxides and a surrounding gas phase is well known 17 . Part of the problem with attempting to rationalize the results of our NIOS experiments on cluster films involved the lack of knowledge of the microscopic structure of those species. While there is a significant ongoing effort to characterize the clusters themselves, and then to correlate the distribution of the sizes of the clusters 18 with the conditions which produced them, much remains to be discovered. For this reason we chose to perform some exploratory experiments on an authentic, well characterized material, W0 3 , which could provide a basis for continuing our optimization of materials for NIOS design and fabrication. We report the results of experiments on bulk W0 3 powder revealing a possible photoinduced structural modification which may bear on our previous thin film experiments. In the present study we use Raman spectroscopy to demonstrate the purely photoinduced, totally reversible exchange of oxygen with the ambient gas phase. Characterizing the process using Raman spectroscopy allows clear observation of the photoinduced change between corner sharing octahedra formi
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