Dear Qaiser

First of all what was the electrolyte  that you were using in the electrochemical study of TiO2.   Also, you can measure Poteniodynamic polarization curves in order to extract the necessary parameters of the above study.   You may begin the measurement of the Poteniodynamic polarization curves with a potential -450 mV from the open circuit potential (Voc)of the TiO2 in that particular electrolyte up to +1600 mV from the Voc, with a potential scan rate of 0.5mV/sec.   It should take over one hour to obtain the curves.  

Dear Qaiser

Yes, the over voltage  is equal to the applied potential - the open circuit potential,  So If the OCP is -150mV , then you should start from -600mV to +1450mV to get almost same graph.

When your over voltage is equal to 0V (Applied potential-the open circuit potential), it is expected that the exchange current net is equal zero, but in your case you got some currents.   This occurred because TiO2 ( as a semiconductor) was tested in 0.6M NaCl solution without taking into the consideration of a deduction of the IR drop from the total over potential of the TiO2/0.6M NaCl electrochemical system.   The IR drop is a potential which forms due to an ionic current that is present in an Ohmic (solution ) resistance. So to get a zero current at the zero over potential , one must compensate for the IR drop.   In many instruments of potentialstat/Galvanostat, you can compensate for the IR drop during the measurement of the  Poteniodynamic  polarization curves.

The rest of your questions will find answers after the above steps. Try to use a new sample in a fresh solution. 

Dear Qaiser

What you have done was correct.   But in Gamry poteniostat Interface 1000E, it should have an IR drop compensation.   You need to get rid off the IR drop potential from your total over potential in order to get a correct corresponding current to the over potential.   Because, You are running your test in an aqueous solution in order to get the bulk (characteristic)parameters of the semiconductor, therefore, you need to get rid off any unnecessary interfacial parameters, i.e., IR drop, from your measurements.

Your Ti-electrode might suffer from possible breakdown[1] of the TiO2 passivity. Recently, the development of several techniques for surface analysis and the observation has been accelerated the understanding of mechanisms leading to the breakdown[1] of (the electrode-surface) passivity. If you reduce drastically the concentration of the NaCl, then these complex[2] corrosion phenomena will be minimized e.g. you will increase the electrochemical stability of Titanium surfaces. So you will be able to measure the semiconductor // electrolyte interface in a (one) day time window.

1. Correlation of aging precipitates with the corrosion behaviour of Cu-4 wt.% Ti alloys in 3.5 wt.% NaCl solution http://www.sciencedirect.com/science/article/pii/S0010938X16302426

2. Crystallographic orientation of single grains of polycrystalline titanium and their influence on electrochemical processes http://www.sciencedirect.com/science/article/pii/S0257897203001634

I propose to normalize the current with the mean value of current in the semi-constant[1] current regime (near the j-minimum value).

1. Error index note in Fig. 11.5. The showed current[2] was normalized by joc  (not to jov , as it appears).

2. https://www.uni-ulm.de/fileadmin/website_uni_ulm/nawi.inst.251/Lehre/Electrochemistry/semiconductors.pdf

Try a proposal concentration of the NaCl in the range : (0.1M,  0.08M).

Should I measure Mott schottky plots with respect to reference or OCP?

thanks

A reference electrode (Vref) is preferable[1] for (stable and linear-V) measurements. Also,  for the plots (as in Fig. 11.5[2]) use a voltage difference V'plot,  so that : V'plot(j0)=0,  where  j0=min( abs(current) ).

1. The OCP is not very stable. If you do not have a reference electrode, then make a fast measurement(s) (total time < 10min).

2. https://www.uni-ulm.de/fileadmin/website_uni_ulm/nawi.inst.251/Lehre/Electrochemistry/semiconductors.pdf

The polarization range is good for aging studies. For your (avoid aging) studies use the safe V-window : -200mV to 1100mV. However, your electrode is, already, quite aged from corrosion in the high NaCl concentration (and V

The proposal advise was, only, for the safe[1] polarization window e.g. the range :  -200mV to 1100mV.   My next proposal is to : start with a (re-)new Ti-electrode and study, only, the safe[1,2] polarization window.

1. The polarization range Vp-r of the (last) graph is useful, only, for another study : the Ti-metal "surface aging studies".  So, this Vp-r is, not only, out of your focus, but, over the time, you destroy (precipitate) the TiO2 (initially on the Ti-metal surface), e.g. always, use the safe range Vp: -200mV to 1100mV [2].  I was thinking that you want to study : (your quotation) :  the electronic properties of the TiO2 n-type semiconductor and electrolyte interface.

2 Always, use as Minimum polarization this value: VMin = - 200mV, in order to protect the TiO2 on the Ti-metal.

Dear Qaiser

I agree with Professor Samaras as far as your initial enquiry  about "the electronic properties of the TiO2 n-type semiconductor and electrolyte interface".   A safe polarization range is essential as professor Samaras has proposed.   It is significant to release that you are after bulk electronic properties of the TiO2 n-type semiconductor by using an interfacial electrochemical test, i.e., anodic potential dynamic curve. The test is an imposed, invasive, test in order to estimate the bulk electronic properties of the   TiO2 n-type semiconductor.   

Dear Qaiser Ali Khan,

start, please, with a (re-)new Ti-electrode and study, only, the safe polarization[1] window : -200mV to +1100mV.

1. The figure is related with another topic.

Dear Qaiser,

I am afraid that I can not help you here much more. Ioannis and Khaled have more experience with this type of measurements (single frequency dc voltage sweep ac measurements). My approach would be different, but at this time I do not know enough about the method (dc/linear/cyclic voltammetry) in order to come up with something useful. 

However, I am really worried about your TiO2 valence and conduction band edges. If your 250 mV and 3 mV refer to your reference electrode, then you have a real problem, because the sum is ~0.253 eV and that IS NOT the band gap of TiO2 which is more like 3.2 eV. But then again, I might be misunderstanding something.

With best regards

Petr

Dear Qaiser,

yes, Ioannis has mentioned this possibility and now you are confirming this. Then everything is all right as far as this quantity is concerned.

Greetings

Petr

Thanks Professor,

But I think I will have to chnage the voltage window as for my bare samples , The OCP is above -200mV , I mean -500mV , so i think i have to start from somehow -700 till 1100mV. 

Thanks

Dear Qaiser,

if I would be you, I would use the entire possible dc bias voltage range available, the whole way up to the dielectric breakdown voltage (well, perhaps not so high).

But be aware that the essential information about the electrical behaviour of your system lies NOT in the Gerisher diagram but rather in the frequency dependence of the complex impedance/capacitance of your system.

best regrads

Petr

Dear Professor Petr Viscor:

Thank you very much for this information, Yes I am analysing the impedence /capacitance data from measurments, But Can you please advise me about how to know about dielectric breakdown voltage for titanium?

I was searching but didnot find.

Best reagrds

Qaiser Ali Khan

Dear Qaiser,

if the OCP is near -500mV then it means[1] that your electrode (surface layer) started an unwanted pit corrosion process. So, this is my proposal schedule :

1. Stay (static) at Vp = +1000mV for at least 1hour : it is for a partial surface layer restoration (hopefully[2]).

2. Do your LSV studies in the safe Vp-range :  -200mV  to  +1100mV.

3. Repeat your LSV studies in the (semi-)safe range : -200mV  to  +3500mV

4. Repeat your LSV studies in the (semi-)safe range : -200mV to +4000mV

I hope it helps.

1. You did   N O T   re-new very well your electrode (surface OXIDE layer restoration) . Next time re-new it better.

2. You might stay at Vp = +1000mV over night for a better surface layer restoration.

Dear Qaiser,

Titanium has no breakdown voltage because it is a metal. With increasing applied voltage It will melt through Joule heating (its electrical resistance). I think you meant TiO2. Here, the TiO2 has a breakdown voltage of 0.02*10*6 volt/cm (parallel to optical axis) and 0.12*10*6 volt/cm (perpendicular to optical axis).

With best regards

Petr

Dear Prof. Ioannis Samaras 

Thank you very much for continuous support. I will try to do same as suggested and then will let you know about the results.

I have small query that steps 1,2,3 and 4 should be one after other on same electrode area in same solution?.

Thanks 

In the same solution steps 1,2,3. Also, step 4 is optional.