4.1.1.1 – Composition
Analysis of the elemental composition of the molybdenum-doped titanium dioxide films showed that titanium and oxygen were approximately
stoichiometric for individual films, despite small inter-film variation. Greater variation was noticed in the amount of oxygen present, but this was not surprising; as a light element (a ‘low-Z’ element, with fewer than 11 protons), oxygen has difficulty being accurately and precisely quantified by EDX systems (Ro et al., 1999; Osan et al., 2000; Ro et al., 2004). In addition, as EDX detects elements within a few microns of the surface, it may also include any molecular oxygen from the air that had adsorbed onto the of the sample surface; similarly, the EDX beam likely penetrated to the substrate, which would have then have caused the detector to receive information about the substrate. The average molybdenum concentration in the films was 7±2 %at. (Figure 22), therefore when stainless steel is used as a substrate, driving the dopant magnetron at power of 180 W yields a film of 7 %
molybdenum.
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Figure 22: Atomic percentage concentrations of the elements in the films. O= oxygen.
Ti = Titanium. Mo = molybdenum. The concentrations of titanium and oxygen were approximately stoichiometric, with a mean doped molybdenum concentration of 7 %at.
N=17
101 | P a g e 4.1.1.2 – Topography
The films deposited onto stainless steel substrates were imaged using SEM.
SEM inspection showed that the deposited films were uniform and non-homogenous, with a fragmented structure. The coatings were likely
conformal to the stainless steel, as evidenced by the native isle-and-channel structure visible in both uncoated stainless steel (Figure 23A) and the coated samples (Figure 23B and C).
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A x270 A x2500
B x270 B x2500
C x270 C x2500
50 µm 10 µm
50 µm 10 µm
50 µm 10 µm
Figure 23: Representative SEM images (x270 and x2500) of films deposited onto stainless steel substrates. The uncoated stainless steel coupons (A) appear similar to the Mo-doped titania samples (B and C), indicating conformal deposition along the stainless steel isle-and-channel structure.
103 | P a g e 4.1.1.3 – Profilometry
The similarities observed in SEM were not seen using profilometry. Coated samples had an average roughness of 376±108 nm, so were significantly rougher than uncoated samples, with an average difference of 190 nm (Figure 24, p<0.001).
Figure 24: Differences in roughness between uncoated control and molybdenum-doped titanium dioxide coatings on stainless steel substrates. There was a significant difference in the mean roughness between the coated substrate and control (190 nm, p<0.001).
This is not surprising, as although the magnetron sputter technique produces conformal coatings, the roughness is reliant on several factors, such as the condensation mechanism of coating material onto the substrate. This
condensation can occur around nucleation points, where atoms of deposited material first adsorbed onto the substrate – these are known as adatoms.
When additional adatoms are present, given sufficient energy, they will diffuse freely along the substrate and produce a homogenous, dense,
smooth coating. However, if they adatoms have insufficient energy to diffuse, then the nucleation points will become the primary site for target material to condense, leading to a granular, columnar structure. This structure is porous and has an increased roughness compared to the substrate. In addition, fine cracks or damage in the film caused by deposition stresses or thermal treatment can be observed to increase roughness, as can structures with a small grain size, polycrystalline texture, or many grain boundaries. The
104 | P a g e thickness of the coating could not be measured using the step-height method via optical profilometer, as the roughness obfuscated the step – a
representative sample is shown in Figure 25.
Figure 25: Side profile of a TiO2Mo coated stainless steel sample. Profile illustrates increase in roughness from the uncoated region (right) to the coated region (left), obfuscating the step-height and reducing the ability to determine thickness.
The profile shows that the coated area (0 µm-390 µm on the x-axis of the lower plot) is much rougher than the uncoated area (390 µm- 695 µm on the x-axis of the lower plot). An alternative technique to measure coating
thickness that has been used successfully in the literature is cross-sectional SEM – a cross-section of the coating and substrate is imaged and the coating thickness calculated from the total thicnkess minus the substrate thickness (Chun et al., 2001). This could have been used to determine the thickness of these coatings when the step-height method was unable to produce data.
105 | P a g e 4.1.1.4 – Crystallinity
Annealing the sample at 600 ⁰C for 30 minutes induced crystallinity in the molybdenum-doped titanium dioxide coatings (Figure 26).
Figure 26: Representative XRD pattern for uncoated stainless steel (blue) and Mo-doped titania coatings annealed at 600 ⁰C (orange). Characteristic rutile 110 (27.5⁰) and 101 (36.01⁰) peaks are displayed as dashed lines, while the typical peaks from stainless steel are displayed using brackets and denoted SS. The Mo-doped titania samples exhibited no anatase crystallinity, but did develop rutile crystallinity. The presence of the stainless steel peaks in the representative coating trace is likely due to X-rays penetrating the coating and interacting with the underlying substrate.
Characteristic peaks at 27.5⁰ (110) and 36.01⁰ (101) confirm the presence of the rutile phase. The peaks at around 47⁰ are characteristic of the underlying stainless steel substrate – this can be seen prominently in the uncoated control sample, but also in the TiO2Mo-coated sample. This is because the incident X-rays can pass through thin films, interacting with and reflecting from the structures in the underlying substrate; it is not indicative of stainless steel crystalline structures within the film. Interestingly, no anatase
crystallinity was detected. This implies that, at 600 ⁰C, the TiO2 crystals had completely progressed through the anatase phase and into rutile. While this is the typical transition (Kim et al., 2002), the absence is unusual, as several authors have found that titania exhibits high levels of anatase crystallinity after annealing at temperatures close 600 ⁰C and that rutile in isolation is found after annealing at higher temperatures, around 900 – 1100 ⁰C (Kim et al., 2002; Hou et al., 2003). However, more recent work by Hanaor and Sorrell (2011) considers the consensus to be that the non-reversible
transformation occurs at around 600 ⁰C, but can be encouraged to occur at
106 | P a g e temperatures around 400 ⁰C by the addition of rutile-promoting dopants, or suppressed up to around 1200 ⁰C by the addition of rutile-supressing
dopants. This is in agreement with Yang et al. (2018), who identified a range of temperatures from 400 ⁰C to 1000 ⁰C for the anatase to rutile
transformation. Therefore, the absence of an anatase phase after annealing at 600 ⁰C could be considered due to the addition of the molybdenum, which here acts as a rutile-promoting dopant.
107 | P a g e 4.1.1.5 – Activity
The activity of the samples was inferred by the breakdown of methylene blue. The rate constant at which breakdown occurred was used as an analogue for activity.
The average activity of the molybdenum-doped titanium samples was 3.0x10-5 s-1 ±1.6 x10-5 s-1, which was significantly greater than the undoped control samples (U=0, p=0.017), which had negligible degradation for the same conditions (2.33x10-7 s-1). This degradation shows that the
molybdenum-doped titania exhibited activity under fluorescent light (Figure 27).
The activity observed is encouraging, indicating that molybdenum-doped titania may degrade methylene blue.
Figure 27: Activity of molybdenum-doped titania coatings compared to uncoated stainless steel controls. Brackets and * indicate significant differences to control.
The coated samples exhibited an average activity of 3.4x10-5 s-1, which was significantly different to the uncoated control samples (p=0.017). N=7.
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