This is likely related to the high concentration of oxygen vacancies you are creating in the TiO2 matrix at high temperature in reducing atmosphere and/or to the hydrogenation of the the nanoparticles surfaces. I think that this review by X. Chen et al. could be very helpful for this topic http://pubs.rsc.org/en/content/articlelanding/2015/cs/c4cs00330f#!divAbstract

after some days in air, it becomes transparent again ... ?

no it has been a week and they are still black, but when heating again in air they become transparent. But heating in air is not good for the other components of the devise, so this is not a solution.

I saw the convertion of Ti4+ in Ti3+ in reducing atmosphere. So you should look for XRD analysis to see if changed the structure of the material. Did you done it? 

Maybe because you are working in an atmosphere poor in oxigen, this convertion may occur. 

Hello, 

no I did not tried the XRD. Thank you for that hint.

so,  dear Carsten, 

AFM and/or iR-visual spectroscopy and/or XRD analysis, whatever is more easy for your specific sample, will help your understanding.

@Carsten Kortz: Defect rich TiO2 turns black. O2 deficiency created in your samples due to vacuum heating may be responsible for this. Actually this is a very good material for visible photo catalysis (TiO2 is only known for UV photocatalysis). To confirm take DRS you can see broad absorption of visible photons. If interested think in this way.    

This is likely related to the high concentration of oxygen vacancies you are creating in the TiO2 matrix at high temperature in reducing atmosphere and/or to the hydrogenation of the the nanoparticles surfaces. I think that this review by X. Chen et al. could be very helpful for this topic http://pubs.rsc.org/en/content/articlelanding/2015/cs/c4cs00330f#!divAbstract