Are you asking through photoreduction process? Then Cu is best one.

Dear sir thanks for your answer. But I want to know which transition metal easily doped with TiO2 particle by photochemical method.

Cu, Ni, Pt, Au, Ag can easily doped in metallic form on the semiconductor surface. However what you wants to dope is more important and the purpose of doping?

Zr, Zn, W, Fe can be doped into lattice of TiO2.

Journal of Photochemistry and Photobiology A: Chemistry Volume 63, Issue 3, 28 February 1992, Pages 367–375

The main purpose of doping is know about the change that the doped TiO2will undergo in its band gap .The mechanism of doped TiO2 is not clearly understood because it requires to know whether the doped metal ions are located on the surface or in the lattice as they affect the binding with the Ti (IV) to different extents and therby affect the band gap energies to different extents. Yet the other factors like temperature, mole % of dopant and the particle sizes of both also play very crucial role in determining the band gap energies.That is why two different views- one given by KARAKITSOU & VERIKIOS and the other by MU ET AL are mentioned here .The former says that doping by a metal ion of valency higher than Ti(IV) should increase the photoactivity while the latter proclaimed that doping with trivalent and pentavalent metal ion are detrimental to the photo activity of TiO2. And both have examples to prove their respective view points.

Yet a few broad inferences may be deived as:

[1] Some dopants are good for some specific reactions like Fe (III) increases the photoreduction of N2 while it hardly affects phenol degradation.

[II] Enhanced photoreactivity for water cleavage and N2 reduction was noticed in Cr (III) doping while opposite effect was noticed with Mo and V doping.

In this way one can find many examples in literature where the dopants are helpful in increasing/ decrearing the rates of the reactions.

[III]Now coming to the effect of temperature- five different sets of same mole % of V dopant at 25.0, 200.0, 400.0, 600.0 and 800.0 C each for 4 hours were considered to prove that the photo oxidation of 4-chloro phenol reduced with increasing temperature.

[IV] Continuing, the effect of change of mole % was studied by changing the mole %

of Zn Fe2O4 in TIO2 which showed the following changes in the color of the light emitted and thereby indicating the change in band gaps as:

TiO2- X of ZnFe2O4 (X= molar conc.)

0.01(dark green), 0.05 (light green), 0.1(chocolate), 0.15 (pink), 0.2(orange) while pure TiO2(dry) emitted violet and wet TiO2 emitted blue light.

[V] All said, the Ni (II) doped TiO2 showed the highest photoactivity which is 1.9 times the photacactivity of pure TiO2 in the 0.00002-0.2 mole % of Ni (II)

Metal doping on TIO2 is a complex phenomenon. Particularly is size-dependent. In any case, the doping/solubility limit can divide metales in three groups; low (below 2-3 at. %), intermedidiate (below 5.6 at. %) and high (above last pont uo to 25-35 at. %). In the last group yo can find W, Mo, Nb and others but the doping structural and electronic properties vary in terms of (doping atom + electroneutrality defect) local/ long range structure as well as electronic effects on localized and band type states. A more complete description of the issue can be found at: https://www.researchgate.net/publication/51820731_Advanced_nanoarchitectures_for_solar_photocatalytic_applications?ev=prf_pub

Article Advanced Nanoarchitectures for Solar Photocatalytic Applications