One of the reason, I am thinking, its band gap energy.  The energy gap between valence band and conduction band.  We can calculate it from UV-Visible absorption spectroscopy.  If it is low,  it is photochemically active.

Generally, TiO2 is the most compound have been used because easy to produce , non toxic and not expensive as well as the optical properties with chemical stability  of reaction condition. for the optical as band gap is 3.2 eV was interested for UV region and this band gap is related with the particle size of TiO2 and phase structure (anatase=3.2 eVand rutile=3 eV) if the TiO2 pure the the best for photochemically active is anatase=3.2 and decreasing of activity by rutile=3 eV have been reported due to the particle size increased and decrease the surface area.  at sane time only one case accepted with low band gap if using doping of impurities with TiO2 which helping to shift the absorption to visible light the has higher energy to make the reaction faster. good luck

TiO2 is a semiconductor, which absorb light of ultraviolet wavelength and generate electron hole pair...so it is photochemical active...

Before discussing the photo chemical activity and photo catalysis of Ti compounds, we first know these two terms and the mechanism of photo catalysis.

[I]A photoactive substance is one which is capable of responding to sunlight/ other electrochemical radiations(UV) photoelectrically by a chemical reaction.

[II].Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst.

[III] Mechanism of Photo chemical activity/Photo catalysis: When a photo catalyst absorbs UV radiation from sunlight or illuminated with light from fluorescent lamp, it will produce pairs of electrons and holes. The electron of the valence band of photocatalyst becomes excited to its conduction band ; thereby creating the negative-electron (e-)[in conduction band] and positive hole (h+)[in valence band] pair in photo catalyst. This stage is called the'photo-excitation' state’ of photocatalyst. The energy difference between the lower valence band and the higher conduction band is called the 'Band Gap'.

Wavelength of the light necessary for photo-excitation is:

1240 / (band gap energy in eV) = λ (nm) --- (A)

Conduction band of Photocatalyst (has electron)

--------------------------------------------

│

Band gap

│

---------------------------------------------

Valence band of Photocatalyst ( has +ve hole)

The positive hole of Photocatalyst breaks apart the water molecule to form H2 and hydroxyl radical. The electron reacts with O2 to form super oxide anion (O2^-1). This cycle continues as long as light is available.

O2(atmospheric) + e--- O2^-1.

OH^-1(moisture in air) +h^+1[the positive hole=OH* (free radical).

[IV]The photochemical nature of the compounds of titanium[ emphasis on TiO2] is explained as follows:

TiO2( titania), a photoactive material [occurring in nature in the rutile and anatase modifications, the latter having 10 times higher photochemical activity] is exposed to light, electrons are excited from its valence band to its condution band[ band gap 3.2eV] and can, for example, split water into its components oxygen and hydrogen. The hydrogen produced in that way is a "clean" energy source: No climate-killing greenhouse gases are generated but only water is produced during combustion.

[V] The reason for TiO2’s widespread use[ranked as one of the top 50 most available chemicals ] comes from its moderate band gap, nontoxicity, high surface area, low cost, recyclability, high photoactivity, wide range of processing procedures, and its excellent chemical and photochemical stability.

[VII] One of the greatest advantages of TiO2 as a photocatalyst is, unfortunately, also its largest disadvantage. TiO2 has one stable phase called rutile (tetragonal) and two meta stable phases called anatase (tetragonal) and brookite (orthorhombic). With an indirect band gap of 3.2 eV, a photon would need a wavelength equal to or shorter than 385nm [   Apply relation (A; given above] to electronically excite this semiconductor, meaning that it needs to be a UV-A or higher energy photon. TiO2’s band gap, although favorable for UV photocatalysis, subjects TiO2 to low efficiency yields in solar applications since less than 5% of the sun’s energy is emitted at wavelengths below 385nm.

[VIII]So, whereas anatase form of TiO2 is considered an ideal photocatalyst for UV applications, in its unmodified form, it is rendered highly inefficient for visible light.

applications.

I try another answer to come up to the expectations of my RG colleagues.

[I]Rutile and anatase, the most discussed semiconductors have a vairiety of large band gaps values [3.06eV(direct).3.10 ( indirect); appxo.413 nm ]and[ 3.23 eV(only indirect); appxo.387 nm] respectively.

[II] The varied values of band gaps makes it very suitable. The resons for the variation in values band gaps are given as:

[a] Along [001] direction in TiO2 crystal structure, ,there are rows of six-fold Ti alternate with five- fold Ti coordinated atoms with one dangling bond perpendicular to the surface.Also, there are two kinds of oxygen atoms.Oxygens with the main surface plane are three fold coordinated as in the bulk while the so called bridging oxygens are two fold coordinated. Because of their coordinative unsaturation, they can be easily removed to result a very large number of point defects.

[b] Oxygen ,being the second highest EN element ,we would expect a considerable polar character which acts as a stabilizing factor for the valence band of TiO2.

[c] Then there ia an involvement of d orbitals in the hybridization .

[d]It has a high( e -h) rate of recombination.

In order to understand this point, we first know the the mechanism as to what happens during the production of (e -- h) in a reaction; crystalline defects being the most prabable sites for e -- h recombination.

Species produced by the reduction of TiO2 during irradiation in the presence of strong trapping agent like MeOHor TEA(triethanolamine) in Ar atmosphere turn MeOH/TEA blue/ bluish grey due to the production of Ti^3 as:

TiO2+e---------Ti^3+2O(-2)

2O(-2)+3h -----O2(-1)[super oxide]

Superoxide, being short lived, will at once take up electrons to give O(-2) while Ti^3 will take up the positive hole to form Ti^4 to reform TiO2.

The reaction continues as long as light is available along withMeOH/TEA.

O2(-1) +3e----- 2O(-2)

Ti^3+h -----Ti^4

Ti^4+O2(-2)------ Ti O2 .