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New Sol-Gel NiO Precursors
Inorgtech Press release

There is a growing requirement for chemical routes (e.g. sol-gel ) for the deposition of nickel oxide (NiO) and complex oxides containing nickel, which have a number of industrial applications. For instance, NiO is a component of electrochromic devices such as switchable automobile mirrors and smart windows [1], whist nickel ferrite and nickel zinc ferrite have applications in high frequency microwave devices [2]

The sol-gel deposition of NiO has been severely restricted by a lack of suitable precursors which have sufficient solubility and stability in alcohol solvents [3]. Simple nickel alkoxides are polymeric and insoluble, whilst those containing donor functionalised alkoxide groups are difficult to obtain halide free. Although [Ni(N0₃)₂(H₂0)₆] has been used to deposit NiO [4], serious doubts about the thermal stability of metal nitrates are likely to restrict its use to small scale applications.

In collaboration with the University of Cranfield and the University of Amiens on an EU-funded scheme (LANDSEC) we have now developed the new precursors [Ni(acac)₂(dmaeH)] and [Ni(acetate)₂(dmaeH)₂] (dmaeH = dimethylamino ethanol, HOCH₂CH₂NMe₂) as improved NiO sol-gel precursors.

The complexes are mononuclear complexes (see Figures 1 and 2) containing six-coordinate Ni(ll) atoms in distorted octahedral environments [5]. The [dmaeH] ligands adduct with the Ni(Il) centres, preventing intermolecular aggregation and leading to alcohol-soluble mononuclear complexes.

Both complexes dissolve readily in dmaeH to form stable solutions which are ideal for sol-gel applications, and researchers at Cranfield University have successfully used the complexes to deposit electrochromic NiO thin films on conductive glass substrates [5]. The films were of good optical quality and demonstrated reversible electrochromic behaviour showing increasing contrast and colour efficiency upon cycling, making them suitable for application in tungsten-oxide based electrochromic stacks.

Figure 3 Crystal structure of [Ni(acac)2(dmaeH)]	Figure 4 Crystal structure of [Ni(acetate)2(dmaeH)2]