Ni-Mo-W Induced Codeposition
There are several mechanistic approaches used to describe induced codeposition of both Mo and W alloys, including our own work that assumes a mixed-metal intermediate adsorbed at the electrode surface.9 Since this intermediate is expected to be a partially reduced oxide species, it may be photoactive. If so, then the intermediate can be detected with modulated light techniques, such as intensity modulated photocurrent spectroscopy, IMPS, with an ac modulation of UV light. IMPS is more commonly used in semiconducting materials at an electrolyte interface,10and a simple impedance model that includes recombination and photocurrent used in reaction is adopted to interpret the results generated during codeposition. Ammonium ion electrolytes that are conventionally used for the electrodeposition of W and Mo alloys complicate the IMPS analysis since it can adsorb UV light. Therefore, electrolytes without ammonia are presented, and Ni-W codeposition is used as a typical example. Figure 1 shows an IMPS spectra during Ni-W deposition and confirms the presence of an adsorbed metal oxide intermediate. Also , this work represents the first example of using IMPS during electrodeposition.
X-ray photoelectron spectroscopy (XPS) was used to characterize the oxidation state of the metal species ex situ. XPS was conducted on Ni-W and Ni-Mo-W deposits fabricated from an equal molar tungstate and molybdate electrolyte with and without Auger gas etching. Before etching, no discernable Ni peaks were observed on the Ni-W surface, but both WO3 and W were detected. After a 30 s Auger gas etching, zero valence Ni and W were observed, as expected from the reduction of the nickel and W ions. On the Ni-Mo-W surface molybdenum is present in the following forms: Mo, MoO3 and MoO2, along with WO3and W. A small Ni 2p3/2 was also identified, indicating a small amount of Ni. After a 6 s and 30 s Auger gas etching, a part of the oxide layer was removed, including all the molybdenum oxides. In this region W oxides were still present. (Figure 2). In sum, this result shows that the tungsten oxide develops a thicker film within the deposit than molybdenum oxides with the same deposition conditions.
We gratefully acknowledge the NSF for support of this work under grant # CHE- 0957448, and the AESF Foundation.
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