Bismuth oxides have important catalytic properties in oxidation reactions such as ammoxidation for the synthesis of acrylonitrile from propylene, oxygen, and ammonia. Such powerful catalytic activity appears to be correlated with the existence of variable oxidation states of bismuth and with the high oxygen mobility on the surface. Bismuth(r1r) oxide also has many different technical applications such as Schottky barrier solar cells, optical coatings, and dielectric materials in metal insulator semiconductor capacitors."] Therefore, taking into account both the reactivity of the surface and the electrical properties, bismuth oxide in the form of films or thin films can also find important applications in gas sensor devices.However, since the phase and morphology of the films strongly affect the functional properties, we carried out the synthesis of pure Biz03 thin films with the aim of studying the influence of the substrate and of the process parameters on the growth modality and structure of the coatings.The Bi-0 system is rather complicated and can have four different main crystalline phases, usually indicated with the letters a, p, y, and 6. Only the aand P-Bi2O3 phases are well characterized, since they are stable at room temperature, while the y and 6 phases can be obtained only at high temperatures ( T > 1003 K for 6 and T > 908 K for y). Each of these phases has different electrical properties, ranging from a typical electronic conduction behavior in the a phase to a perfectly ionic conduction mechanism in the 6
phase.[*]Because of the different electrical properties found in each of the Bi203 phases it is clear that a possible use of bismuth oxide films in gas sensing devices is connected with the possibility of controlling the stoichiometry and the morphology in the deposition procedures. 238 0 VCH Verlagsgcsellschaft mbH, 0-69469 Weinheim, 1996
Amorphous WO 3 films are obtained by using the oxoalkoxide precursor WO(O t Bu) 4 . This compound is easily evaporated at 80 C, and reacts with water at the surface of the substrate (Si(100) or soda lime glass (KGlass)) to give films of tungsten oxide in a combined CVD sol±gel process. The films deposited at low temperatures (150 C, Si(100), or 100 C, KGlass) can be defined as tungstic species (of general formula WO n (OH) m ), having interesting electrochromic properties. Thermal treatment at temperatures over 400 C converts the material to a more typical WO 3 film.
Amorphous WO3 films, doped with phosphorus, have been synthesized by chemical vapor
deposition of volatile, low-melting P-substituted tungsten carbonyls. The presence of a small
quantity of dopant released by the precursor during its decomposition is sufficient to inhibit
the crystallization of the tungsten oxide on the matrix (P/W 2−4 atom % on Si(100) and ca.
10 atom % on KGlass). The nature of the film is scarcely affected by the experimental
conditions of deposition (namely p
O
2
partial pressure) and the quantity of the P dopant is
properly tuned by an appropriate choice of the molecular precursor, being in the order
W(CO)4[P(OEt)3]2 (P/W 1/1) > W2(μ-PR2)2(CO)8 (P/W 1/4) > W(CO)4(PEt3)2 (P/W 1/10 on
KGlass, 1/30 on Si). The films on KGlass exhibit interesting electrochromic properties with
a maximum efficiency of 66 cm2/C.
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