It is likely due to oxygen vacancy developed during heat treatment to 350 deg C. Please refer to the following article for further details. Hope it is useful

Journal of Physics and Chemistry of Solids 61 (2000) 1237–124

http://144.206.159.178/ft/639/11944/242439.pdf

Dear IDRIS AKYUZ,

I think the following witness may be satisfy your problem.

Titanium Dioxide

Color Index Pigment White 6 CI 77891

Formula TiO2

Structure

Titanium dioxide exists in three crystal forms:

brookite that is not used as a pigment,

anatase that is used occasionally

and rutile, the most commonly used crystal form.

Two different processes are used to manufacture the rutile grade: the sulphate method (the older one) and the chloride process (more modern).

Pigments produced by the sulphate method used to be slightly stained by the iron salt, giving it a slightly yellow color.

Properties

Titanium Dioxide contains most of the properties of an ideal white pigment ie. clarity of color, high resistance to most chemicals, organic solvents and heat, and most importantly, a high refractive index. This last property allows the production of highly opaque coatings. Further, titanium dioxide has good durability and is unaffected by industrial atmospheres, however, its photoreactivity reduces the light fastness of some colored pigments and nearly all organic pigments. Despite its excellent properties TiO2 is reasonably priced. Nevertheless, attempts are made to replace or partially replace it with cheaper pigments or semi-opaque extenders.

Because of its tighter network, rutile has in general better properties than anatase.

Compared properties of rutile and anatase

Property Rutile Anatase

Refractive Index 2.72 2.55

Density 4.2 3.9

Surface Area 12-17 m2/g 10 m2/g

Oil Absorption 13-24 19-20

Heat Stability > 1000°C < 700°C

Hiding power

The hiding power is influenced by the refractive index and the granulometry. Rutile has a higher refractive index and is therefore more opaque.

The hidding power of TiO2 is superior to that of other white pigments.

Whiteness

Anatase is softer in texture and has a slightly better white color. Indeed, there is a good diffusion of the visible spectrum of light except in blue where there is an absorption. This phenomenon is more marked for rutile than for anatase. Therefore, the rutile appears yellower than the anatase.

However, rutile has much less tendency to chalk. Modern grades have much better chalk resistance imparted by treating the photoreactive sites on the surface of the crystals with inorganic oxides, such as those of aluminium, silicon and zinc.

Chemical Stability

It is the most chemically inert pigment. It has a neutral pH, a good resistance to alcali solutions

Uses

The balance between dispersibility and durability generally requires products that are specific to a range of applications, such as water-based paints or solvent based stoving finishes. TiO2 has favorable physiological properties due to its inert composition. This makes it suitable for use in food packaging, toys, and other sensitive applications, provided that it meets purity criteria.

Titanium Dioxide can be used in all coatings formulations for building and industry.

Tio2 is resist toward corrision juct becoz of ths oxide layer.

What is the chemcal identity of your substrate? TiO2 is very acute to the surroundings. One reason for the change of culor for this phenomenon is the TiO2 was doped (or contaminated) by some element of the substrate.

Titanium dioxide exists in three crystal forms:

brookite that is not used as a pigment,

anatase that is used occasionally

and rutile, the most commonly used crystal form.

Two different processes are used to manufacture the rutile grade: the sulphate method (the older one) and the chloride process (more modern).

Pigments produced by the sulphate method used to be slightly stained by the iron salt, giving it a slightly yellow color.

Anatase is softer in texture and has a slightly better white color. Indeed, there is a good diffusion of the visible spectrum of light except in blue where there is an absorption. This phenomenon is more marked for rutile than for anatase. THEREFORE, THE RUTILE APPEARS YELLOWER THAN THE ANATASE.

i thing used crystal form.

Dear Idris Akyuz,

I think that there are some impurties on your substrate that interact with TiO2 layer just at the interface of your film. Have you tried to clean several time and by different procedures the surface of the substrate?

Is it possible for you to upload a picture? It sounds like contamination or doping to me, or possibly if it is just a yellow 'tint' then perhaps you are producing rutile.

Ti2O3 has black colour. May be admixture of Ti3+.

It is likely due to oxygen vacancy developed during heat treatment to 350 deg C. Please refer to the following article for further details. Hope it is useful

Journal of Physics and Chemistry of Solids 61 (2000) 1237–124

http://144.206.159.178/ft/639/11944/242439.pdf

I worked in titanium (ultra pure) refining, pure titanium will get oxides of either blue or yellow. Quite brilliant colors too. From what I could tell, all samples I saw were refined the same way. If anything you may be getting different crystal structures during cooling. BUT I've also seen blue and yellow crystals oxide films on the same sample LECO analysis would give slightly higher oxygen levels on the colored areas...but since it is only surface oxide it isn't significant enough to change 5-9's to 4-9's. They were noticeably higher, 0.1178 ppm vs 0.0748 ppm perhaps.

The oxides only formed on smooth surfaces and were definitely associated with grain boundaries. May be seeing more active surfaces in certain planes 011 or 111, I can't say for sure.

Dear Ìdris,

To complete the good answer from Abdul, the yellow color (580 nm) rises from valence electrons. So, this could suggest, for example, interaction on surface of compound.

Another possibility: Dehydroxylated rutile when reacted with aqueous H2O2 can also take on a yellow color (depends on your rutile). The color persists in air and exhibits a strong ESR signal (can't remember all of the species present, but superoxide, peroxy radical are likely).

you need to determine the crystal structure of this film, possibly glancing/grazing incidence XRD. with results at hand, it is easy to predict.

I do not know from what precurssor are you starting. But various titanium oxochlorides are yellow to orange colured.

I have one question

why my tio2 turned to yellow after annealing process (2h, 600 C, 5 C/min)

the first one is before annealing and the second one after annealing

when I checked with ICP, TiO2 content is >96% by weight.

Can someone explain to me?

@kenneth - TQ for your thought. I also wonder why my TiO2 did not have photocatalytic property after i have done methylene Blue test.

kenneth: i am using Titanium oxysulfate (TiOSO4) as precursor by slow addition of Ammonium hydroxide until white precipitation (metatitanic acid). This is my XRD data. I wonder what other factors that influence tio2 to become photocatalyst active?

Kenneth: well crystallized but still no photocatalytic activity. I dried at 70 C for 12 hours before calcination process .. please do give a suggestion =)