Wide-range tuning of the titanium dioxide flat-band potential by adsorption of fluoride and hydrofluoric acid

The adsorption of fluoride, (HF)_nF_- ions, and hydrofluoric acid at the surface of n-type TiO_2-xF_x (x ≈ 0.001) electrodes was studied by electrochemical methods. Adsorption of F_- from acetonitrile/tetraethylammonium fluoride (TEAF) solution follows a Langmuir isotherm above a fractional coverage of about 0.5, and the equilibrium constant for adsorption is 8300 M_-1. Specific adsorption of fluoride ions, at millimolar concentration in acetonitrile, results in a large negative shift in the flat-band potential, as manifested by capacitance measurements and cyclic voltammetry of Ru(bpy)³²⁺. Fluoride ions are strongly complexed by HF in acetonitrile to form (HF)_nF_- ions (n = 1, 2), which are also specifically adsorbed at the TiO_2-xF_x(001 ) surface. The flat-band potential can be shifted by ca. 2 V, from -1.8 V vs Cu/CuF₂ in pure acetonitrile/0.5 M TEAF to +0.2 V in acetonitrile/0.9 M TEAF/36 M HF. Addition of relatively small amounts (ca. 2 M) HF to acetonitrile/0.5 M TEAF causes the valence band edge to shift to a sufficiently positive potential that elemental fluorine can be evolved photoelectrochemically.