Cu(I/II)-Co(II/III) photocatalyst with intrinsic electron transport centre for photoreduction of chromium(VI) and photodegradation of methyl violet

A redox switchable photocatalyst consisting of mixed valent copper (CuI/II) sulfide (CuxS) and cobalt (CoII/III) oxide (CoyOz) designated as CuxS-CoyOz ( x=1,2, y=1,3 and z=1,4) was synthesized to execute the Z-scheme photocatalysis process for photoreduction of chromium(VI), and photodegradation of Methyl Violet (MV) dye. The synthesized photocatalyst with a band gap of 3.3eV was found to be highly effective to reduce 99.62% Cr(VI) and 99.32% of MV dye under UV-light irradiation. The possibility of switching the oxidation state in Cu and Co centre favored the band edge electron transfer on light irradiation and supported the Z-scheme electron transport chain. The photocatalytic reduction and degradation process was dependent on various reaction conditions like catalyst amount, pH of the reaction mixture, irradiation time. The reduction of Cr(VI) occurred with high efficacy at pH=2 while the MV dye degradation proceeded at high pH=10. The catalyst could reduce maximum concentration of Cr(VI) upto 250 ppm and MV dye upto 10-3M. The mechanism of the photocatalytic processes were well studied by different spectroscopic and electrochemical analysis. The photocatalytic activity of synthesized CuxS-CoyOz catalyst was compared with five other transition metal sulfides M-S (MnS, FeS, CoaSb, NiS and ZnS, where a=3, 4; b=3, 4) combined with cobalt oxide and it was found that CuxS-CoyOz exhibited the highest activity in both the photocatalytic process.