Characterization Of Solgel Coated SnO2 Thin Films

Typical SEM micrograph of nanocrystaUine SnO2 thin film coated on a Pyrex glass substrate is shown in Figure 4 [77]. The SnO2 thin film appears to be relatively smooth. At some locations, few cracks are observed on the film surface, which are characteristic features of the sol-gel dip-coated thin films.

Typical broad-scan XPS spectrum, within the B.E. range of 0-1100 eV, obtained for the nanocrystalline SnO2 thin film dip-coated on the Pyrex glass substrate is shown in Figure 5a [77], which primarily shows the presence of Sn and O on the glass surface after the dip-coating process. Typical narrowscan analysis of Sn (3d) spectra, within the B.E. range of 480-500 eV, is presented in Figure 5b. Sn 3d5/2 B.E. level of 485.8 eV is observed in Figure 5b, which is in between the Sn 3d5/2 B.E. values of 484.9 eV and 486.7 eV reported for pure Sn and SnO2, respectively [79]. Hence, the observed Sn 3d5/2 B.E. value of 485.8 eV is attributed to the presence of Sn-oxidation state less than +4. The O:Sn relative atomic ratio of ~1.6 is calculated from the survey spectrum and is in agreement with the Sn 3d5/2 B.E. value. The nonstoichio-metric O:Sn ratio suggests the presence of oxygen vacancies [80], which is responsible for the n-type semiconductor property of the film.

Typical TEM images, obtained from the FIB-milled TEM sample of the nanocrystalline SnO2 thin film dip-coated on the Pyrex glass substrate, are shown in Figure 6 at different magnifications [77]. The thickness of the nanocrystalline SnO2 thin film can be directly measured via these micrographs. Various regions corresponding to Pt, Au-Pd (both originating from the FIB-milling procedure), SnO2 (originating from the sol-gel dip-coating process), and the glass substrate are marked appropriately. The SnO2 thin film appears to be smooth and highly continuous. No cracks are visible within the film, although slight variation in the film thickness is noted in these images. The average SnO2 thin-film thickness is observed to be within the range of ~ 100-150 nm.

Typical AFM images of the nanocrystalline SnO2 thin-film sol-gel dip-coated on the Pyrex glass substrate, at low and high magnifications, are presented in Figure 7a and 7b, respectively [77]. The nanocrystalline SnO2 thin film is observed to be made up of nanoparticles having near-spherical shape and uniform particle size distribution. Very dense packing of nanoparticles is noted in these micrographs. The average nanoparticle size is estimated to be

Cracks

Cracks

Figure 4. SEM micrograph of SnO2 thin-film gas sensor [47].

Sn 3d

1200 1000 800 600 400 200 0 500 495 490 485 480

Figure 5. Broad-scan (a) and narrow-scan (b) XPS analysis of SnO2 thin-film gas sensor [47].

1200 1000 800 600 400 200 0 500 495 490 485 480

Figure 5. Broad-scan (a) and narrow-scan (b) XPS analysis of SnO2 thin-film gas sensor [47].

within the range of 15-20 nm and is comparable with the resolution limit of AFM determined by the AFM-tip radius.

Typical HRTEM images obtained from the FIB-milled TEM sample are presented in Figures 8a and b at different magnifications. Selected-area electron diffraction (SAED) pattern obtained from the center of the nanocrystalline SnO2 thin film is presented in Figure 8c [77]. From these images, it is obvious that the sol-gel dip-coated SnO2 thin film is highly dense and nanocrystalline in nature. The nano-crystalline SnO2 thin film-glass substrate interface is visible in Figure 8a. No porosity or cracks are visible at the interface, which suggests very adherent nature of the sol-gel dip-coated nanocrystalline SnO2 thin film. Porosity and cracks are not observed within the nanocrystalline SnO2 thin film at nanoscale level, indicating very dense packing of the SnO2 nanocrystallites within the thin film, which is in agreement with the AFM analysis. The average SnO2 nanocrystallite size of ~6-8 nm is measured from these HRTEM images. The nanocrystallite size distribution is observed to be very narrow. SAED pattern, Figure 8c, shows continuous and diffused ring patterns, which suggests the nanocrystalline nature of SnO2 thin film.

The variation in the average nanocrystallite size as a function of calcination temperature, within the range of 100-1000 °C, reported for the sol-gel derived nano-crystalline SnO2 thin films or powders is presented in Figure 9 [9, 12-14, 21, 22, 25]. It can be observed that, within the calcination temperature range of 100-700 °C, the grain growth is marginal and lies within the range of 3-20 nm. However, above 700 °C, rapid grain growth is observed. Grain size as high as 300 nm is observed at the calcination temperature of 1000 ° C. The activation energy for the grain growth, for high (700-1000 °C) and low temperature ranges (100-700 °C), can be determined from the

Figure 6. TEM micrographs of SnO2 thin-film gas sensor at low (a) and high (b) magnifications [47].

Figure 4. SEM micrograph of SnO2 thin-film gas sensor [47].

Figure 6. TEM micrographs of SnO2 thin-film gas sensor at low (a) and high (b) magnifications [47].

Sn 3d

Figure 7. AFM micrographs of SnO2 thin-film gas sensor at low (a) and high (b) magnifications [47].

activation energy plot, Figure 10. The activation energy values of 91 kJ/mol and 9 kJ/mol are determined for high and low calcination temperature regions, respectively. Very low activation energy value (9 kJ/mol) observed in the lower temperature range (100-700 °C) is attributed to the possible generation of excess oxygen-ion vacancy concentration in the SnO2 nanoparticles, as reported for other oxide particles [81].

0 0

Post a comment