Band gap of the TiO2 (rutile and anatase) phase is 3.2-3.35 eV. It is approximately equal to the ZnO band gap. Due to an oxygen defect, ZnO is considered as an N-type Semiconductor.
Does TiO2 also follow the same properties in terms of carrier flow as ZnO?
While TiO2 bulk is a n-type semiconductor (as previously noted here), there is no obvious answer for nanostructured-TiO2. There is a connection of primary particle size, band gap and n or p-type behavior for particle size above and below 5nm. This is at least the current understanding which means that you can have "pure" n or p-type TiO2. No need of doping although doping agents obviously can alter the behavior of the native system . A summary of such can be found in the below recent review (2012).
http://www.researchgate.net/publication/235970767_Design_of_Advanced_Photocatalytic_Materials_for_Energy_and_Environmental_Applications
Book Design of Advanced Photocatalytic Materials for Energy and E...
Energy band gap is rather similar as TiO2 however, Quantum effect with nano particle size of ZnO appeared different trend then TiO2.
Yes, the Titanium Dioxide is both intrinsic and N-type (non-intentionally doped), due to oxygen vacacies in the TiO2 lattice, its properties are similar to ZnO. Currently, studies are made to dope the titanium dioxide and get a P-type conductivity (http://www.sciencedirect.com/science/article/pii/S0167577X10000753).
Intrinsically, nature in a semiconductor depends in the difference of its effective masses for different charge carriers: the lighter ones go faster. However, in general the defects in a semiconductor, such as intrinsic vacancies, or doped foreign atoms form defect bands in the energy gap and determine the charges carriers. For a TiO2 crystal at elevated temperature, the O vacancies form defect states (occupied) below the conduction band. And the occupied defect states provide electrons as charge carriers (n-type) . If you dope other atoms to form unoccupied states just above the valence bands, then you get p-type crystals.
Experiments on titanium dioxide show that it is an electronic semi-conductor in which the the current carriers are actually free electrons, as contrasted with the hole conduction of the other type of semi-conductor. It is found that the variation with oxygen pressure is that which would be expected if the titanium dioxide decomposes in the following manner: TiO2-->Ti++O2+e-. The deviation of the curves at low pressures is probably due to the presence of small impurities in the samples used. It is found that the variation of conductivity with temperature is represented by the formula σ=Ae-ɛkT. The activation energy ɛ is about 1.7 electron volts. Transport measurements show that the ionic conductivity is less than that which can be measured in these experiments. Measurements of the Hall effect, although not very quantitative, show that the mean free path for the conduction electrons is very small.
In a practical sense, TiO2 is a conductor as sputtered onto semiconductors (maybe in a highly rich O2 plasma - not sure).
Al is a a conductor as well - but as a doapant in Si or Ge it a p-type dopant as it supplies only 3 3electrons and thus produces a hole.....
You may also want to look at this link
http://answers.yahoo.com/question/index?qid=20110114084615AAimgrF
oxygen vacancy produces a filled mid gap state, with small shifting below conduction band edge. The formation of oxygen vacancy, creates a filled Ti 3d band at the Fermi level and hence, the possibility of Stoner splitting of the 3d band. Thus, It produce n-type.
Sanaa, I think you are referring to a general semiconductor. To determine whether it's p- or n-type you have to measure the position of the Fermi level inside the forbidden gap or which are the charge carriers (holes or electrons) that are responsible of current transport. This is a step forward of knowing the band gap energy. There are several techniques to determine the doping type, based on different phenomena. Hall effect is one of them and is widely used for this purpose.
Regards.
While TiO2 bulk is a n-type semiconductor (as previously noted here), there is no obvious answer for nanostructured-TiO2. There is a connection of primary particle size, band gap and n or p-type behavior for particle size above and below 5nm. This is at least the current understanding which means that you can have "pure" n or p-type TiO2. No need of doping although doping agents obviously can alter the behavior of the native system . A summary of such can be found in the below recent review (2012).
http://www.researchgate.net/publication/235970767_Design_of_Advanced_Photocatalytic_Materials_for_Energy_and_Environmental_Applications
Book Design of Advanced Photocatalytic Materials for Energy and E...
Sanaa, you'r welcome. If the Fermi level is near the top of the valence band (bottom of the band gap) the semiconductor is p-type since holes are responsible for current transport. In the other case of a Fermi level near the bottom of the conduction band (top of the band gap) the current transport is due to electrons and the semiconductor is n-type.
@Sanaa Hayel
Hello! It seems that you did not get a straight answer to your query, " can i determine while the semiconductor is P-type or N-type by determining the band gap?".
The answer is, no, it is not possible to determine the conductivity type of a semiconductor by determining its band gap. For example, both p- and n-type silicon have the same energy band gap.
I may also add that the conductivity type is not determined by the position of the Fermi level, which is a computational (theoretical) construct rather than a physical quantity. This is quite a widespread confusion. On the contrary, the position of the Fermi level is determined (calculated) from the concentration of charge carriers (electrons and holes) in a semiconductor. It is, however, correct that in an n-type semiconductor (electrons are majority charge carriers) the Fermi level is near the conduction band edge, and in p-type semiconductors (holes are majority charge carriers) the Fermi level is near the valence band edge.
Free carrier concentrations in semiconductors are measured most popularly by the Hall voltage measurement technique, which includes measurement of the electrical conductivity.
Ti3+ doped TiO2 (oxygen vacancy) is N-type. Non-metal doped TiO2 generally is P-type.
previously i have also many doubt regarding how i can say whethere a photocatalytic material is a p-type or n-type. now i am little bit relaxed by reaading the above discussions. many many thanks to all and research gate.
the type of conductivity in titanium dioxide is strongly dependent on crystal lattice defects. If we have oxygen vacancies then this is an n-type semiconductor, if we have metal deffects - then this is a p-type semiconductor
Article Titanium-Defected Undoped Anatase TiO 2 with p-Type Conducti...
I have been working on TiO2 for more than 10 years now.
In principle, bulk TiO2 is considered as n-type semiconductor and you can determine it by performing Hall Effect Measurements rather than optical band-gap. However, it is recently found that even undoped TiO2 can also show p-type response, this happens when you move from bulk to nano-level crystals. Moreover, the surface of TiO2 is very sensitive to oxygen which can get adsorbed on the surface of TiO2 to give unique electrical properties. See the new published paper in SNB: https://authors.elsevier.com/a/1XTnA3IQMPAqJy
Article Cost-effective fabrication of polycrystalline TiO 2 with tun...
But I am getting more n-type nature of TiO2 with decreasing concentration of oxygen vacancy. What should be the reason? Please suggest !
Thank you all for discussion and found to be useful.
Undoped TiO2 showing p-type conductivity is interesting to explore further.
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