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The most straightforward method to substitute maximal vanadium into rutile TiO2 with mixed valence states was to use commercially available mixed-phase V2O5 powders and anatase TiO2 (which transitions to rutile above ~ 600 °C in the presence of vanadium227, 252, 414_{) as the} raw materials. The raw materials were combined in acetone (not ethanol used in other chapters, as V2O5 was found to react and forms side products with this solvent) and ball milled in order to obtain a homogenous mixture with a small particle size. While V2O5 has a natural yellow/brown colouration, after milling the mixtures appeared blue, then dried to a green colour. There is a possibility that the process of ball milling induced a valence state change in the vanadium, as noted by others in V2O5415 _{and in V2O5 and TiO2 mixtures under high energy} circumstances416, 417_{, which was then partially re-oxidized on standing in this experiment. This} might imply some useful pre-reactions. The dried powders were then pressed into pellets without binders and prepared for sintering.

The sintering process required some optimizing. In order to obtain a rutile product after sintering, the temperature had to be high enough for the raw TiO2 phase to transition from anatase to rutile. It also needed to be high enough to react but not so high as to separate out due to the low melting point of V2O5 (~ 690 °C). Finally, an atmosphere reducing enough to prevent V2O5 remaining a separate oxidized phase was required. Two distinct sintering methods are presented in this chapter, the Low Oxygen method and the Moderate Oxygen method.

7.1.1.1 Low Oxygen Method

The first method (Low Oxygen method, ‘-LO’) involved a sintering event (or events) on a platinum foil boat in a tube furnace under flowing nitrogen gas, which provided a moderately reducing atmosphere. Nitrogen gas is a cheap and safe to handle reducing agent which has not been explored thoroughly in syntheses of these kinds of materials before in the available literature and avoids use of sealed ampules and vacuum furnaces. The tube furnace used in these experiments has a quartz inner tube, limiting the maximum operating temperature for an extended dwell time to 1050 °C.

In Figure 7.1 XRPD patterns of the best sintering results for each composition synthesized using the ‘LO’ method are presented. The specific sintering conditions are outlined in Table 7.1. At low levels of doping (1% V, VTO1-LO) unreacted materials are seen, which imply

the 1050 °C temperature limit is insufficient for this composition. From 2 – 50% V using the LO method, a rutile structured phase is evident. Despite 1050 °C being hot enough to react and form a single phase, the 2 – 10% samples do not densify well after a single sintering. A regrinding and resintering did help in densification 5 and 10%. For samples with 20% substitution and beyond melting behaviour was seen which did aid in densification, but also distorted the macroscopic shape of the samples. This is particularly relevant for VTO50-LO type samples, the synthesis for which is explored in later sections. The 70% sample (VTO70-LO) showed appreciable melting even at 750°C and is notably two-phase in the XRPD patterns, with the additional phase matching to V2O5.

Figure 7.1. XRPD patterns of vanadium substituted rutile samples produced by the ‘LO’ method. Replacement levels of 1 – 70% were explored, but show the temperature is insufficient at 1% and a solubility limit has been reached by 70%. Rutile peak locations are indicated by dashed lines.

Table 7.1. Summary of ‘LO’ method sintering temperatures and times for each VTO

composition.

Sample Sintering 1 (tube furnace) Sintering 2 (tube furnace) VTO1-LO 1050 °C/ 20 hours -

VTO2-LO 1050 °C/ 20 hours -

VTO5-LO 1050 °C/ 13 hours 1050 °C/ 15 hours* VTO10-LO 1050 °C/ 13 hours 1050 °C/ 15 hours* VTO20-LO 1050 °C/ 20 hours -

VTO50-LO 900-1050 °C/ 20 hours** - VTO70-LO 750 °C/ 12 hours -

*necessary for densification

**optimization presented later in this section

These results imply that the ‘LO’ reducing conditions are sufficient for a broad range of vanadium incorporation levels but there is a limit is somewhere between 50 and 70%. This limit is higher than the 10% limit found when synthesized in air250-253 _{and lower than the 80-}

90% using VO2 as a raw material259-261, 286 _{. However, this method only allowed a narrow range} over which sintering temperature could be optimized for good sample quality. This led to an exploration of a second method using a higher temperature furnace.

7.1.1.2 Moderate Oxygen Method

The second method attempted (moderate oxygen method, ‘-O’) utilized a muffle furnace for a second sintering which had a higher attainable temperature. This was connected to a nitrogen supply line, but did not flow, providing only minimal reducing effect beyond the natural effect of high temperature, and as such there is ‘moderate’ oxygen available. The samples were sintered initially using a tube furnace, reground, then sintered a second time in the muffle furnace to ensure densification and the detailed conditions are presented in Table 7.2. Applying the ‘O’ method, it was found that denser, phase pure rutile samples with 1 – 10% vanadium could be made (Figure 7.2). However, incorporations of 20% and beyond resulted in the appearance of V2O5, similar to literature studies using an air atmosphere. This supports the prediction that the muffle furnace conditions were indeed more oxidizing than the tube furnace conditions, and the solubility of vanadium in the samples is evidently decreased. The fact that some vanadium was reoxidized in the two step ‘O’ method trials implies that a proportion of the vanadium may have been on the surface of crystal grains rather than deeply substituted into the rutile framework, as bulk vanadium is not found to be easily oxidized237_{, an} important note of discussion in later sections.

Figure 7.2. XRPD patterns of samples produced by the ‘O’ method. Peaks associated with V2O5 were seen at 20% incorporation and beyond, suggesting the solubility is dependent on oxygen partial pressure. Rutile peak locations are indicated by dashed lines.

Table 7.2. Best sintering temperatures and times for ‘O’ method VTO samples. Sample Sintering 1 (tube furnace) Sintering 2 (muffle furnace) VTO1-O 1050 °C/ 15 hours 1300 °C/ 10 hours

VTO2-O 1050 °C/ 15 hours 1300 °C/ 10 hours VTO5-O 1050 °C/ 13 hours 1200 °C/ 10 hours VTO10-O 1050 °C/ 18 hours 1200 °C/ 5 hours VTO20-O 1050 °C/ 18 hours 1050 °C/ 10 hours VTO50-O 1050 °C/ 18 hours 850 °C/ 10 hours

Across these synthesis trials, the maximum amount of vanadium introduced was 50% using the ‘LO’ method. This composition had the most potential for a large range of vanadium valence states, and so further optimization of the synthesis was conducted for generation of high quality samples suitable for electrical, electronic and magnetic analysis.