Photodegradation of a dye can be understood as:

Dye Molecule ----> intermediates (2-partial degradation) ------> CO2 + H2O + X- + H+ (1-Total degradation)

1) Total degradation/decomposition: In this case all dye molecules are oxidized into CO2, H2O and anions from acids (like Cl-, SO42-, NO3-, etc). To calculate the amount of dye molecules that were totally degradated you can use TOC analysis (Total Carbon Content).

2) But it is know that the dye molecules can suffer just partial degradation leading to intermediate organic molecules. In this case you can calculate the amount of partial degradated molecules using UV-vis, GC, HPLC, etc analysis, while subproducts can be verified by GC, HPLC, NMR, etc.

The material must be able to generate electrons (e-) and holes (h+) due to irradiation, thats why normally people use semiconductors for these kind of process. Ideally the material must generate the charge carrier under visible illumination for Solar applications.

You can take a look at these two papers of my research group were we use mesoporous materials for photodegradation experiments:

Annealing temperature effects on sol-gel nanostructured mesoporous TiO 2/SiO 2 and its photocatalytic activity

Strauss, M., Maroneze, C.M., De Souza E Silva, J.M., Sigoli, F.A., Gushikem, Y., Mazali, I.O.

Materials Chemistry and Physics 2011 126 (1-2) , pp. 188-194

http://www.sciencedirect.com/science/article/pii/S0254058410009314

Size controlled synthesis of highly dispersed anatase/rutile nanoparticles with photocatalytic activity toward salicylic acid degradation

Silva, JMDE, Pastorello, M; Strauss, M; Maroneze, CM; Sigoli, FA ; Gushikem, Y ; Mazali, IO

RSC ADVANCES 2012 2(12) pp. 5390-5397

DOI: 10.1039/c2ra20453c

Degradation means that the dye caracteristic goes lost. This can be done by decomposition of the dye molecule (this means to crack the dye molecule in the way that you get smaller molecules) or changing the dye molecule by oxidtion, reduction, aglutination or other process, so that the dye caracteristic get lost. (not necessary crack the dye molecule).

Decomposition means that you will decompose the dye molecule (divide in parts).