Experimental2.1. Preparation of photocatalystsGaN:ZnO catalyst was prepared by heating a mixture of 2.16 g -Ga2O3(Acros, 99.99%) and 1.88 g ZnO (J.T. Baker 99%) at 1125 K for15 h under NH3flow [20]. Prior to the nitridation process, -Ga2O3and ZnO were pulverized in a mortar for 30 min. After the nitri-dation process, the resulting powder was pulverized for another30 min. For the final step, GaN:ZnO was calcined under air flow at873 K for 2 h. GaN:ZnO–Ni/NiO catalyst was prepared by incipientwetness impregnation as described previously [23]. Firstly, 0.3 gof GaN:ZnO powder was impregnated with 3–4 mL of de-ionizedwater containing 2.5 wt.% of Ni precursor (Ni(NO3)2·6H2O, NacalaiTesque Inc.) over an evaporating dish. After that, the slurry wasplaced in an oven at 353 K to ensure complete dryness. Lastly, theresulting powder was reduced with H2for 2 h at 573 K and then oxi-dized by air for 1 h at 473 K to form a core-shell structure of Ni/NiOcluster.Pt/WO3catalyst was prepared by a photo-deposition process.Firstly, H2PtCl6solution of desired concentration was mixed withcommercial WO3(Aldrich 99.995%) powder to give a 0.5 wt.%of Pt-loaded. The well-mixed solution was next irradiated byUV lamp with an intensity of 5 W/cm2for 1.5 h to perform thephoto-deposition process. The resulting powder was subsequentlywashed by de-ionized water and centrifuged several times toensure that there was no residual Cl−remaining on the photocata-