Growth and thermal properties of tin oxide (Sn02) nanomaterials prepared via horizontally vapor phase growth (HVPG) deposition

• Main Author: De Los Reyes, Ronald B.
• Format: text
• Language: English
• Published: Animo Repository 2009
• Subjects: Thermal properties; Nanostructured materials; Nanomaterials; Physics
• Online Access: https://animorepository.dlsu.edu.ph/etd_masteral/3723; https://animorepository.dlsu.edu.ph/context/etd_masteral/article/10561/viewcontent/CDTG004462_P.pdf
• Institution: De La Salle University

Tin Oxide (SnO2) nanomaterial was synthesized using Horizontal Vapor Phase Growth deposition without a seed or catalyst at a temperature of 1200oC and growth times of 3 hours, 4 hours and 6 hours. Minimal amount of 99.99% purity Merck SnO2 powder was loaded onto a quartz tube with one end closed. The tube is then connected to a Thermionics High-Vacuum System and sealed with an appropriate amount of LPG and oxygen gas. The sealed quartz tube capsule is sectioned into three zones, namely zone 1, 2 and 3. The quartz tube is then loaded onto a Thermolyne Horizontal Tube Furnace and heated at the pre-set growth conditions with zone 1 of the tube completely inside the furnace, zone 2 is near the opening and zone 3 is completely outside. During heating, a thermal gradient was established along the length of the tube, with zone 1 having a temperature of 1200oC, zone 2 is at 1000oC and zone 3 is at 700oC. The tube is then allowed to cool down to room temperature by itself. Upon breaking, fluffy white deposit are observed in the inner walls of the quartz tube. The products are then subjected to scanning electron microscopy (SEM) analysis, energy dispersive X-ray (EDX) analysis, differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The resulting nanostructures, as shown by the SEM images, include the following: nanorods, nanowires, nanobelts, tadpole-like structure, and nanoparticles; with nanorods being the dominant nanostructure observed in the samples. The growth orientation of the nanomaterials is random. There is no distinguishable difference between the resulting nanomaterials synthesized at different growth times, except that it is observed that there are less nucleations of nanoparticles in zone 1 of the sample prepared at 6 hours. In addition, some novel structures were observed for the sample prepared for 6 hours. This includes straight and bent rods, zigzag nanorods, cantilever-like structure and furball-like agglomeration of different nanomaterials. The ii shapes of the rods are tetragonal which suggest that the resulting nanostructures have rutile crystal structure. Widest variety of nanostructures is found in zone 2 for all the prepared samples. Energy dispersive X-ray analysis reveals that different nanostructures differ in elemental concentration of Sn and O as supported by backscattered electron images. The nanobelt structure was found to be oxygen-deficient which suggests that it is made from SnO. The nanorods, nanowires and nanoparticles on the other hand were found to be oxygen-rich which suggests that it is made of SnO2. Differential thermal analysis and differential scanning calorimetry results show an endothermic peak for the sample prepared for 6 hours. There are no satisfactory peaks for analysis obtained for the samples prepared for 3 hours and 4 hours. This result suggests that there are a lot more nanomaterials present in the sample prepared for 6 hours compared with the first two. The glass transition temperature is measured to be at 1092oC, the latent heat of fusion (enthalpy) equal to 2586 J/g and the specific heat capacity is equal to 2.876 J/g.Co. The endothermic peak is observed at 1200oC and the melting point is measured as 1022oC. The measured melting point of the SnO2 nanomaterial is considerably lower than the standard melting point of SnO2 which is 1620oC. Looking back at the growth conditions of the SnO2 while on the quartz tube, zone 1 has a temperature of 1200oC, zone 2 is at 1000oC and zone 3 is at 900oC. The endothermic peak matches the temperature of zone 1 and the melting point almost equals the zone 2 temperature. It is worth note taking that most of the nanomaterials are deposited in zone 2 of the quartz tube. These findings suggest that the SnO2 source powder, which is initially located in zone 1, sublimes to vapor phase and was transported to cooler regions in the quartz tube. The SnO2 undergoes dissociation to SnO and O2 with the O2 acting as a carrier gas for SnO and aided by iii the thermal gradient. Some of the SnO will undergo disproportionation reaction to form SnO2 and Sn. This process results to the formation of nanowires and nanobelts. The process proceeds via Vapor-Liquid-Solid (VLS) transition, as shown by the nucleation tips found on the nanorods. On the other hand, some SnO that wont undergo dispropotionation reaction will be deposited directly as SnO. There are no evidences of nucleation tips for the nanobelts and it is suggested that nanobelt formation undergoes Vapor-Solid (VS) transition. Energy dispersive X-ray analysis and backscatterd electron images of the nanomaterials confirm that the nanowires and nanorods are oxygen rich which suggest that it is composed of SnO2. On the other hand, it is found that nanobelts are oxygen deficient, which suggest that it is made of SnO. The substrates EDX analysis confirms that is mostly made up of SiO2 and there are isolated Sn metal deposited to it. Kinetics calculation was carried out using the DSC thermogram of SnO2 nanomaterial under a DTA-DSC temperature ramp. The reaction profile was plotted first by calculating it from the DSC thermogram. The function of the plot is approximated by finding a polynomial function determined using Mathematica software. The polynomial function was used to calculate numerically the Borchardt-Daniels equation at different temperatures corresponding to the temperature under the endothermic peak. The results of these numerical calculations are used as input for a multilinear regression procedure done using Microsoft Excel. Kinetics calculations show that the SnO2 nanomaterial has activation energy of 174.620 kJ/mol, pre-exponential constant of 2.184/s and order of reaction equal to 0.915.