Effect of Ni on the growth and photoelectrochemical properties of ZnS thin films

Undoped and Ni-doped ZnS thin film photoelectrodes were prepared using the chemical bath deposition process. X-ray diffraction patterns of a hexagonal wurtzite structure with preferential orientation along the (0 0 8) plane appeared on undoped ZnS films. An increase in the molar ratios of Ni, x, in the starting solution resulted in a decrease in the intensity of the (0 0 8) plane. Images from a scanning electron microscope revealed a drastic change of the surface morphology of the Ni-doped ZnS film due to ion-by-ion deposition. The energy band gaps of Ni-doped ZnS thin films shifted to lower energy levels between 3.34 and 3.01 eV. Moreover, increasing the Ni ratio led to a shift in the flat-band potential of the film towards a more positive value compared to that of ZnS. The Ni-doped ZnS films experienced a conversion from n-type to p-type when the molar ratio of Ni changed from 0.003 to 0.005. The photocurrent densities of Ni-doped ZnS film (x = 0.003) reached 3.74 mA cm−2 at an external potential of 1.5 V versus a Pt electrode and exhibited a threefold enhancement of photocurrent density compared to pure ZnS. A cathodic photocurrent of 0.82 mA cm−2 at an external potential of −1.5 V was obtained for a Ni concentration of x = 0.005.