APCVD过渡金属氧化物

1. Introduction  

Recently, the necessity for global environmental conservation and the reduced use of electrical energy  and decrease of CO2 have led to the development of energy-saving systems. Electrochromic devices  (ECDs) and especially “Smart windows” as energy efficiency-related have recently attracted  considerable attention because of their low switching voltage, the ability to display various colors,  high reflective contrast ratio, large viewing angle, memory effect and long-term functioning. The  properties of transition metal oxides, serving as functional layers in ECD are mainly determined by  their specific band structure and electron distribution. They possess d-electrons, which create d-band  partially overlapping s-band. The d-band provokes the interesting chemical, physical and optical  properties of transition metal oxides. Electrochromism is defined as effect of color change caused by  applied voltage and has been found in several different metal oxides, the metals belonging to transition  series. This experimental fact suggests close relation between electrochromic properties and electronic  structure of the functional material.

2. Experimental  

Using our experiences in WO3 and MoO3 film depositions, mixed films on their basis were deposited.  The precursors are the corresponding hexacarbonyls. The hexacarbonyl precursors (W(CO)6, Mo(CO)6 were mixed in a ratio Mo(CO)6:W(CO)6=1:4. The deposition proceeded in argon-oxygen rich ambient  at atmospheric pressure (APCVD process) in a horizontal cold walls CVD reactor. The precursor  powder placed in a sublimator immersed in silicon oil bath has been heated at temperature of 90o C and  has been controlled with an accuracy of ±1o C. This sublimator temperature provides a sufficient vapor  pressure of the hexacarbonyls. The gas lines (“Galtek” type Teflon) were heated up to the sublimator  temperature in order to assure successful transport of the vapors to CVD reactor. Argon (99.995%)  flow carries the precursor vapors to the reactor. The selected flow rate of Ar through the sublimator  assures a constant amount of the precursors vapor. Through a separate line, oxygen (99.95%) enters  the reactor. In the present study the ratio of flow rates of Ar/O2 is 1:32 [21-24].

3. Results and Discussions 

Optical transmittance was measured for the single oxides and the mixed oxide films. The results are  shown on figure 1. It is seen that after annealing the transmittance of single oxides increases, while for  the mixed WO3-MO3 it decreases. A possible explanation is mass transfer starting during annealing at  elevated temperatures, but the process is not accomplished (annealing time was accepted to be one  hour), the structure gets disordered, amorphous-like and the transmittance drops down.  

Fig1

XRD patterns of MoO3, WO3 and MoO3 -WO3 films, deposited on conductive glass substrates are  presented on figure 3. Most of the XRD lines found can be assigned to the corresponding oxides.  MoO3 film consists of orthorhombic Mo trioxide and a small fraction of sub-oxide. The triclinic WO3 phase is prevailed in the tungsten oxide film with W20O58 sub-oxide. The XRD spectrum of the mixed  oxide film is very similar in shape to that of WO3. Similar effect is observed for IR spectra. The XRD  peak positions are the same with enhanced intensity, which leads to suggestion that W atoms are partly  substituted by Mo atoms.

Figure 4 presents high resolution TEM micrograph of CVD as-deposited MoO3-WO3 film, grown  at gas flow ratio 1/32. The columnar structure of the film is clearly observed, it is typical for the CVD  grown films at temperatures below the melting point of the film material. These columns form grain  boundaries which facilitate the ions intercalation.

Fig4

Cyclic voltammetry is used as method to study insertion reactions and to receive information about  the electrochemical effects. Cyclic voltammetric experiments were performed in a standard threeelectrode arrangement. The electrodes were immersed in electrolytes of 1 mol/l LiClO4 in propylene  carbonate (PC). Current density vs. voltage voltammograms were registered between -1 V and +1.5 V.  The cyclic voltammetric (CV) curves manifest that the as-deposited and annealed MoO3-WO3 films.