Ceria film deposition

Ozer, N. 2001. Optical properties and electrochromic characterization of sol-gel deposited ceria films. Solar Energy Mater. Solar Cells 68 391-400. 

Results are similar for films deposited on YSZ however, there appears to be a difference between films deposited on ceria vs YSZ in terms of interfacial electrochemical resistance. As shown previously in Figure 6c, LSC films on YSZ often exhibit a second high-frequency impedance associated with oxygen-ion exchange across the electrode/electrolyte interface.That this difference is associated with the solid—solid interface has been confirmed by Mims and co-workers using isotope-exchange methods. As discussed in greater detail in sections 6.1—6.3, this interfacial resistance appears to result from a reaction between the electrode and electrolyte, sometimes detected as a secondary phase at the interface. 

In addition to the nonuniformity in the oxide polish rate, non-uniformity in the oxide deposition process leads to variations in the final thickness of the oxide. Deposition nonuniformities are compounded by the fact that thick oxides must be deposited prior to CMP. For example, if the oxide CMP process must remove 5(X) nm in order to planarize the surface, with a final target oxide thickness of 500 nm, a 1 pm thick film must be deposited. A 10% variation in film deposition rates transfers to a final thickness variation of 100 nm or 20% of the final film thickness (assuming no variation in CMP rate across the wafer). Alternatively, if the CMP process must remove 1 pm of oxide in order to planarize, the deposited oxide must be 1.5 pm thick. The same 10% variation in film deposition rates now results in a 150 nm thickness variation or 30% of the final thickness. Thus, the CMP process affects the final oxide thickness uniformity by virtue of the planarization rate as well as polish rate uniformity. This nonuniformity is a second reason that ceria-based slurries discussed in Section 5.1.3 are undesirable. 

Hardacre el al. (7 75, 174) investigated the properties, structure, and composition of cerium oxide films prepared by cerium deposition on Pt(lll), finding that the activity for CO oxidation is enhanced on Pt(lll) that is partially covered by ceria. It was suggested that new sites at the Pt-oxide interface become available for reaction. A remarkable observation is the high activity for CO oxidation when the Pt(lll) sample is fully encapsulated by ceria (Pt was undetectable by XPS and AES). It was proposed that an ultrathin, disordered ceria film becomes the active catalyst. It was also demonstrated by XPS and AES that Pt dramatically increases the reducibility of cerium oxide that is in intimate contact with Pt. This result suggests that intimate contact between the noble metal and oxide phases is indeed crucial to facile oxygen release from ceria. High-resolution electron microscopy demonstrated the presence of direct contact between ceria and noble metal for supported Pt-Rh catalysts (775). Hardacre et al. (173,174) related the catalytic activity of the ceria phase to partially reduced cerium oxide. 

Growth by vapor deposition and oxidation (VDO) of Ce onto a substrate has been used successfully. The simplicity of this approach and its ability to be integrated into UHV systems designed for multiple surface diagnostic methods makes this a common technique for surface studies of chemisorption and surface reaction studies on model catalytic surfaces. Many of the ceria films used in work described below were produced in this way. Ce deposition and oxygen exposure (oxidation) may be performed simultaneously or sequentially. - Single crystal metals (Pt, Cu, Pd, Ni, and Ru ) and oxides, including yttrium-stabilized zirconia (YSZ), and sapphire, have been used as substrates for this approach. Such films have been... 

Polycrystalline films can be prepared by spray pyrolysis. In this technique a spray of an aqueous solution of cerium salt is nebulized and directed by a stream of compressed gas onto a heated substrate. Various parameters are important for determining the resulting structure of the ceria film. The effects of the spray solution and of substrate temperature for films deposited upon silica substrates have... 

The adsoqjtion of NO on metal loaded ceria has been examined for Pt, and Pd, As known from work on single crystals, NO dissociates to some extent on each of these metals. The amount of dissociation is dependent upon the structure of the metal surface. Gorte considered Pt and Pd particles deposited on rough, poly crystal line ceria films grown by spray pyrolysis.For Pt they found variation in the TPD results (amount of NO uptake and shape of N2 desorption profile) that varied with the size of the Pt particles. However, the results were comparable to NO TPD results from Pt grown on sapphire. It was concluded that no unusual interaction existed between Pt and the (oxidized) ceria. For Pd it was found that a pronounced difference in the TPD product ratio, NO/N2, occurred for Pd on ceria compared to Pd on sapphire. They attributed the difference to NO adsorption on reduced ceria. 

Sol-gel technique has been used to deposit solid electrolyte layers within the LSM cathode. The layer deposited near the cathode/electrolyte interface can provide ionic path for oxide ions, spreading reaction sites into the electrode. Deposition of YSZ or samaria-doped ceria (SDC, Smo.2Ceo.8O2) films in the pore surface of the cathode increased the area of TPB, resulting in a decrease of cathode polarization and increase of cell performance [15],... 

The catalysts were synthesized as films, with ceria prepared by spray pyrolysis of 0.1 M solutions of Ce(N03)3 onto nonporous alumina wafers held at 250 °C. The ceria was then calcined at 300 °C, resulting in a crystallite size of 10 nm. Pt, Pd, or Rh was vapor deposited onto the oxide film. For kinetics testing, the temperature was 300 °C. To determine the reaction order of H20, Pco was maintained constant at 0.026 atm. For the reaction order on CO, Ph2o was maintained constant at 0.02 atm. The kinetic parameters are tabulated in Table 69. 

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