Examination of Niw Induced Codeposition By Intensity Modulated Photocurrent Spectroscopy (IMPS)

Introduction NiW is of interest for their outstanding catalytic property of hydrogen evolution reaction, 1 , 2 robust corrosion resistance 3 , 4 and improved hardness. 5 NiW exhibits induced codeposition, introduced by Brenner, 6 where the reduction of tungstate in aqueous electrolytes occurs by the aid of other elements being codeposited, such as nickel. Several mechanisms to describe induced codeposition have been raised, 7-11 some that include an adsorbed intermediates 9,11 or oxide intermediates 8 that may be responsible for tungstate reduction. In the related NiMo system, adsorbed oxide intermediates have been reported using in situ Raman spectroscopy. 12 , 13 Since the NiW intermediate may contain partially reduced nickel and tungsten oxide species at the working electrode surface during codeposition, a photoactive response would be expected while deposition is taking place. Intensity modulated photocurrent spectroscopy (IMPS), a technique used to characterize semiconductor electrode-electrolyte interfaces in situ , 14 is used here to probe the NiW intermediate during electrodeposition to help better understand the induced codeposition behavior. Experimental The NiW electrolyte contained 0.375 M sodium citrate, 1 M boric acid, 0.075 M sodium tungstate, and 0.15 M nickel sulfate at a pH was 7. The electrodeposition of pure Ni was conducted in the same electrolyte without tungstate. A three electrode cell was used with a gold working electrode, Pt counter electrode and Ag/AgCl reference. IMPS and the pretreatment before it were conducted using a Zahner IM6ex and PP210 systems. IMPS was performed in a frequency range of 500 Hz to 3000 kHz, with a UV monochromatic light source having a steady state intensity of 30 W/m 2 with a 10 % amplitude of the light intensity. The working electrode potential was constant during the IMPS experiment, and a range of working electrode potentials examined in the region of NiW codeposition. Results and Conclusions In Figure 1, IMPS of NiW is presented. A typical IMPS response was generated as expected from a photoactive surface, suggesting that there are intermediates formed on the electrode surface during NiW codeposition and that they exhibit semiconducting behavior. The low frequency intercept at the origin, indicates that the electron-hole recombination process dominates the semiconducting behavior created by adsorbed intermediates. The IMPS of NiW induced codeposition was also influenced by the applied potential. The