Abstract
Based on TiO2 as a model system, the sol-gel one step facile method is established to fabricate the macro-porous morphology films on the basis of Marangoni effect. In this proposed mechanism, the binary mixture of hydrophilic CuCl2 and lipophilic Ti-O network is used in sol to produce phase separation. A suitable evaporation rate in the gel film process leads to the macro-porous film due to Marangoni effect. It is observed that the macro-porous morphology of the film sustains during the annealing process, which suggests the creation of porous surface morphology in gel film stage rather than due to annealing. To analyze the preparation mechanism, the sol-gel process and microstructure of films are examined using TG-DTA, SEM, TEM, XRD, Raman, UV-Vis, XPS and FTIR. Furthermore, the optical-thermal properties are studied for the potential applications of such porous surface films as solar selective absorber.
Highlights
Porous materials are getting great interest in various fields, Nakanishi and Soga (1991)[1] first reported the synthesis of macro-porous silica monoliths
The results of Thermogravimetry differential thermal analysis (TG-DTA) in different ambient are shown in Fig. 1 to analyze the pyrolysis process
In N2 atmosphere, i.e. an oxygen-free environment similar to the vacuum annealing condition, two wide and weak endothermic-like peaks are observed; featuring a typical pyrolysis process involved in the transformation from organics to carbon
Summary
The results of TG-DTA in different ambient are shown in Fig. 1 to analyze the pyrolysis process. The XRD results of samples annealed in air atmosphere are shown in Fig. 2b in the temperature range of 150–450 °C, where no crystalline phase of the film is observed, as in the case of vacuum annealing. The sample annealed in 150 °C in the air almost shows the same surface and cross-section results as the vacuum annealed sample, except reduced number of cracks This propose that low temperature in different annealing environment does not show a big Figure 5. It shows the oxidation states of Cu, one peak located at 932.5 eV belongs to Cu2O and other peak at 934 belongs to Cu which is in well agreement with XRD results of vacuumed anneal samples. The ε of the sample is slightly high which is an obstacle in practical application, since the emissivity contribution from presently used Cu substrate is high about 0.081 but the emissivity of film can be optimized by using better substrate or extra polished substrate
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