Modified Sol-Gel Synthesis of Vanadium Oxide Nanocomposites ...

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100 90 Weight % 80 70 60 50 40 100 200 300 400 500 600 Temperature o C Figure 3. TGA of initial sample undergoing calcination in oxygen [13]. Thermal gravimetric analysis (TGA) in oxygen (Figure 3) or nitrogen gave a similar curve. This graph revealed a weight loss of around 10% for H 2 O, and about 45% for the hydrocarbon chain of the surfactant. The TGA of the surfactant itself showed a steep weight loss around 200°C, which is also seen in the TGA of the sample. The electron microprobe images showed a layered morphology for the material. For use as a reference in the elemental analysis for the electron microprobe, VOPO 4 was synthesized [14] and then furnace dried at 500°C for several days. VOPO 4 exhibited a ratio of 1.01 P to 1.00 V. Elemental analysis, by WDS and EDS, of the initial compound showed only 4 elements with the resulting atomic ratio of V 1 P 1.5 Na 0.4 K 0.3 . The standard deviation is large for Na and K so there is probably more than what is written for the ratio given. Solid state NMR analyses were done on the material for both 51 V and 31 P. Lapina et. al. extensively investigated 51 V solid state NMR of vanadium compounds [12]. They discovered that the coordination of vanadium oxides may be determined, together with the number of adjacent vanadium polyhedra from the chemical shift anisotropy (csa) of the static solid state NMR of 51 V. Figure 4. The 51 V static solid state NMR of the initial sample showing the chemical shift anisotropy. The vertical line is caused by the transmitter offset [13]. -100 -300 -500 -700 ppm -900 -1100 -1300 -1500 GG6.16.4
The initial compound had a peak with a range of about 350 ppm for static solid state 51 V NMR. This means a VO 4 tetrahedron has one of the oxygens shared with another vanadium oxide tetrahedron, forming a V 2 O 7 group. A compound with a known structure containing V 2 O 7 groups was also run, and it had a similar range [15]. The MAS NMR for 51 V gave one peak indicating 1 type of vanadium nuclei. Also performed on the sample was 31 P MAS NMR, which indicated 2 types of phosphate nuclei with shifts of 0.6 and 25 ppm. The surfactant dodecylphosphate was analyzed by 31 P MAS NMR, and showed no shift relative to phosphoric acid. Phosphates are always tetrahedra, and here they form a chain with the vanadia tetrahedra. FTIR was also used to help elucidate the structure. Vibrations near 2900 cm –1 for the aliphatic C-H stretch and a P-O stretch at 1111 cm –1 [16] were seen. No vibrations were observed at 3000 cm –1 for hydroxyl bonded to a P, nor was a peak seen for the O-H stretch for a hydrogen bonded hydroxyl (which is also bonded to a P) at 2600 cm –1 [16]. The P=O stretch, which was seen for the dodecylphosphate at 1239 cm –1 , was not seen in the material. So no P=O or P-O-H bond vibrations were observed. Unfortunately some of the vanadium oxygen bonds occur in the region where the P-O exhibits a strong wide band from 900-1200 cm –1 , so they were not able to be observed. TEM images showed a layered morphology with a repeat distance of 33 Å. The layers are stacked on top of one another in what appears to be a random method, like sheets of paper stacked, with the sheets at different orientations. Thus the crystals have order in one direction only, as was seen in the XRD pattern. Several crystals were analyzed and all had a similar layered appearance. The proposed structure of the initial compound is a bilayer of surfactant molecules aligned with their hydrophobic hydrocarbon ‘tails’ together, and their phosphate ‘heads’ sticking out and bonded with a layer of vanadium oxide giving a 37Å interlayer distance. The experimental evidence is consistent with the formula V 2 O 3 (PO 4 C 12 H 25 ) 3 Na 2-x K x (H 2 O) 3.2 . The hydrocarbon chains are covalently bonded to the phosphate tetrahedra, and most likely in the ‘plane’ of the tetrahedra plane (Figure 5). Sodium and potassium cations are bonded to the other oxygens of the phosphate. The few water molecules can hydrogen bond to the edges of the polyhedra. Figure 5. The structure of the initial compound with the hydrocarbon chains removed for clarity; the dark tetrahedra are V, and the light tetrahedra are P [13]. GG6.16.5
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Page 3: layered structure with approximate

Modified Sol-Gel Synthesis of Vanadium Oxide Nanocomposites ...

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