Rapid Synthesis and Screening of Zn_xCd_1−xS_ySe_1−y Photocatalysts by Scanning Electrochemical Microscopy

Quaternary-alloyed Zn x Cd 1− x S y Se 1− y arrays were prepared from four precursor aqueous/glycerol (3:1) solutions with a robotic piezoelectric dispenser on fluorine-doped tin oxide (FTO) conductive glass. Hydrazine (N 2 H 4 ) was used for protecting the selenium source, dimethylselenourea, from oxidation so that fabrication of the arrays could be carried out under ambient air conditions. The absorption spectra of the photocatalyst spots (230-μm size) on the array were measured with a fiber-optic/CCD detector system. The band gaps were a function of the elemental composition and ranged from 1.8 to 3.6 eV. The photoelectrochemical properties were evaluated by scanning electrochemical microscopy in the photoelectrochemical mode using an optical fiber tip attached to a Xe lamp as the excitation source. The spot with a precursor composition Zn 0.3 Cd 0.7 S 0.8 Se 0.2 (elemental ratio, 1:2.12:1.75:0.81) showed the highest photocurrent under 150 W Xe lamp irradiation. The difference of the photocurrent onset indicated that addition of Zn raises the conduction band position of CdS y Se 1− y .