Aqueous Niobium Chemistry at Low pH: New Precursors for Oxide Thin Films

Phosphate and peroxide stabilize new oxo-hydroxo niobium clusters in water at low pH. The clusters open a new chapter in aqueous niobium chemistry under acidic conditions. The clusters also produce atomically smooth, amorphous niobium oxide phosphate (NbPOx) thin films. Reaction pathways from cluster solutions to amorphous niobium oxide phosphate solids are elucidated.Four new peroxoniobium phosphate clusters – H6Nb5P1O13(O2)5 (Nb5P1), H10Nb7P3O23(O2)7 (Nb7P3), H6Nb4P2O14(O2)4 (Nb4P2), and H10Nb6P4O24(O2)6, (Nb6P4) – were synthesized at pH < 2. Crystal structures of two clusters as tetramethylammonium (TMA) salts, TMA5[HNb4P2O14(O2)4]·9H2O and TMA3[H7Nb6P4O24(O2)6]·7H2O, were determined by X-ray crystallography. Solution speciation and cluster stability as a function of pH, phosphate concentration, and counterion (H+ and TMA+) were investigated with electrospray ionization mass spectrometry, dynamic light scattering, and small-angle X-ray scattering. Pair distribution function analysis of niobium oxide phosphate gel powders providesinsights about how cluster condensation leads to amorphous niobium oxide phosphate solids.Thermochemical and physical properties of NbPOx thin films were explored as a function of Nb:P ratios and anneal temperature via temperature programmed desorption mass spectrometry, X-ray diffraction and reflectivity, spectroscopic ellipsometry, transmission and scanning electron microscopies, optical transmission and reflection spectroscopy, and atomic force microscopy. Wide tunability in refractive index and band gap of the films was demonstrated by controlling the phosphate concentration in precursor solutions. The NbPOx thin films may also be directly patterned via electron-beam exposures, and as an ultra-thin capping layer they enhance dehydration of solution-processed aluminum oxide phosphate (AlPO) thin films.