Molecular mechanisms of metal-oxide sol-gel and challenges in the single-source precursor approach
by
Vadim Kessler
Department of Molecular Sciences, Swedish University of Agricultural Sciences, Uppsala, Sweden
Metal oxide sol-gel processes differ principally from that of silica due to the predominantly ionic nature of bonding in metal alkoxide and hydroxide species. This leads to high reaction speeds and reversibility of processes in homogeneous media. Metal oxide sols emerge thus not as polymer colloids but via nucleation of metal oxides in the form of poly-oxo-metalate like species, resulting from coordination equilibria. The emerging species have a typical structure consisting of well-ordered and in many cases even crystalline cores surrounded by an amorphous shell, bearing residual organic ligands or anions. Their typical size is between 2 and 5 nm. This mechanism has been reliably verified for Ti, Zr, Hf, Rare Earth, Nb and Ta oxides. An insight into structure and reactivity of the sol particles, which we once upon a time proposed to call Micelles Templated by Self-Assembly of Ligands (MTSALs), can be obtained from studies of the intermediates in their formation via hydrolysis of, often modified, metal alkoxides. The latter are non-cluster oligonuclear oxo alkoxide complexes
that remain highly reactive and quickly respond to changes in the surrounding medium by changing their structure and even composition. This constitutes a principal challenge in using such intermediates as single source precursors – they easily can transform into something else before the desired phase can be formed, if they stay in contact with solution. An attractive alter- native in using oxo-alkoxide complexes, especially heterometallic ones, as single source precursors lies in their separation from solution and further transformation in solid phase via Metal-Organic Decomposition (MOD). Application of this combined pre-sol-gel and MOD approach permitted us recently to access a number of “impossible” oxide phases such as titanium molybdate, TiMoO5 – an attractive material for alkali ion battery an- odes, or highly uniformly Rare Earth doped titania, REE:TiO2 – attractive solar light photocatalyst.
It is our pleasure to invite you to join the ISGS eSeminar for June 2022, which will be delivered by Professor Vadim Kessler on Wednesday 15 June. Please click here to learn more about his many achievements and interesting career journey, including his many contributions to both the professional and social fabric of our sol- gel community.
Date: Wednesday 15 June
Format:
- ZOOM (eSeminar link), Meeting ID: 307 291 6432 Passcode: 3ez@xwbM
Time:
- 09:00 – USA, East Coast
- 10:00 – Argentina, Brazil (São Paulo)
- 14:00 – UTC
- 14:00 – UK, Portugal
- 15:00 – Central Europe, Sweden, Norway
- 16:00 – Romania, Israel, Eastern Europe, Ukraine
- 16:00 – Russia (Moscow, St Petersburg)
- 21:00 – China, Singapore, Malaysia
- 22:00 – Japan
- 23:00 – Australia (East Coast)
