Electrochemical and Photoelectrochemical Investigation of Single-Crystal Anatase

Abstract

Single crystals of TiO2 anatase containing 0.22% of Al and traces of V, Zr, Nb, and La were grown by chemical transport reactions employing TeCl4 as the transporting agent. Electrodes having the (101) face exposed doped by reduction with hydrogen were employed. The electrochemical and photoelectrochemical behavior of a single crystal of anatase were scrutinized for the first time. Properties were compared to those of single-crystal rutile having the (001) face exposed. Impedance analysis established that the flatband potential of anatase (101) is shifted negatively by 0.2 V with regards to that of rutile (001). Interfacial capacitance measurements under forward bias indicate smaller density of surface states on anatase. Photoelectrochemical oxidation of water occurs on both rutile and anatase with incident photon-to-current conversion efficiencies close to unity at λ = 300 nm. From the comparison of U fb and E g, it follows that anatase (101) and rutile (001) electrodes differ mainly in the position of the conduction band edge. The complete photoelectrolysis of water to H2 and O2 is thermodynamically possible on anatase only. Photosensitized electron injection from adsorbed cis-Ru[L2(SCN)2] (L = 2,2‘-bipyridyl-4,4‘-dicarboxylic acid) proceeds with similar efficiency on both types of electrodes. However, light-induced charge separation on the single-crystal electrodes is about three times less efficient compared with nanoscopic anatase films. Anatase (101) is strikingly more active for electrochemical insertion of Li+ than rutile (001). The diffusion coefficients for Li+ insertion and extraction were estimated to be 2 × 10-13 and 6 × 10-13 cm2/s, respectively.

Electric-dipole effect of defects on the energy band alignment of rutile and anatase TiO2

“…Second, the Kohn-Sham valence band edges from the DFT+U calculations are aligned by subtracting Evac. Third, the conduction band edges is aligned based on the above aligned valence band edges by adding the commonly accepted band gaps of anatase (3.2 eV) and rutile (3.0 eV) TiO2 37 .…”

Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4

“…C harge separation has a key role in determining solar energy conversion efficiency of semiconductor-based systems for producing solar electricity and solar fuels through solar cells [1][2][3][4] , photoelectrocatalysis [5][6][7][8] and photocatalysis [9][10][11][12][13] . As a key step in energy conversion, electron-hole pairs generated by light absorption need to be separated and transferred to the surface of the semiconductors 1,9,[14][15][16][17] .…”

Nanoscale Effects in Water Splitting Photocatalysis

“…Because the dimensions of nanomaterials are only a fraction of the space charge layer thickness in solid materials, screening effects from electrolytes and specifically adsorbed ions are dominant. For pure water, the most important ions are hydroxide and hydronium ions, and their effect on metal oxides, including TiO 2 [106] and Fe 2 O 3 [107,108] gives rise to the well-known 59 mV pH À1 variation of the flatband potential with the solution pH [109]. In surface water, phosphate, silicate, and fluoride ions are often strongly adsorbing [110], which determines the redox stability of many minerals [109].…”

“…In the literature, proton adsorption is usually considered as an effect on the chemical potential μ. This leads to the known Nernstian dependence of the semiconductor flatband potential on solution [106,107,109]. Equation (5) works because redox reactions with protons are fast, allowing the protons to be in electrochemical equilibrium with the electrons in the particle.…”