Photoelectrochemical microscopy of oxide films on metals: Ti/TiO2 interface

Photo-electrochemical investigation of anodic oxide films on cast Ti–Mo alloys. I. Anodic behaviour and effect of alloy composition

“…In fact, optical gap values reported in Table 3 are compatible with those of pure component oxides, as well as with the formation of a mixed TiO 2 -MoO 3 phase: E g values of 3.0, 3.2 and 3.35 eV are reported for rutile, anatase and amorphous TiO 2 phase, respectively [12,17,20,21,[52][53][54][55][56], whilst for MoO 3 band gaps equal to 2.9 (␣ phase), 3.1 (␤ phase) and 3.3 (a-MoO 3 ) eV can be assigned [12,24,[57][58][59]. Similarity in the gap values of the two oxides arises from almost coincident electronegativities of both metal cations [12,60].…”

“…This result can be generalized to all cases investigated: when photocurrent sign inversion occurs, cathodic spectra give threshold energies close to or slightly higher than the corresponding E g values derived from the anodic spectra, suggesting that cathodic photocurrent arises from band-to-band excitation within the passive layer. For low Mo content in the alloy, the presence of only anodic photocurrent is in agreement with the n-type semiconductor behaviour generally observed for pure TiO 2 anodic films [17][18][19][20][21][53][54][55]; on the contrary, in the case of higher Mo contents the photocurrent sign inversion indicates electric field can be reversed into the anodic film, and thus its insulating behaviour. In fact, this inversion is not possible in semiconducting materials (only thin inversion layers can occur at high forward bias, which is not the case), but it is allowed in insulators.…”

Electrochemical oxidation of titanium by pulsed discharge in electrolyte

“…It has been previously shown that the structure of anodic films on titanium depends considerably on the conditions of their growth, although this relationship is still not clearly understood [26,[32][33][34][35][36][37][38]. In the present work, the structure of the anodic films on titanium obtained by the pulsed discharge and galvanostatic anodization was studied by the X-ray diffraction and Raman spectroscopy methods.…”

“…Since these properties are very important for possible practical applications of anodic films, many researchers have investigated them using different methods including impedance [26,33,34,36,[42][43][44][45] and photoelectrochemical techniques [7,26,33,37,38,[44][45][46][47][48][49][50]. Impedance studies showed that anodic oxide films on titanium are highly doped with donor densities, N d , ranging from 5 · 10 19 to 10 21 cm À3 [26,33,36,[42][43][44][45]47,50].…”

“…It is well known that the structure and electronic properties of anodic films on titanium as well as on other valve metals depend strongly on the film growth rate [26,[31][32][33][34][35][36][37][38]. Since these properties are very important for possible practical applications of anodic films, many researchers have investigated them using different methods including impedance [26,33,34,36,[42][43][44][45] and photoelectrochemical techniques [7,26,33,37,38,[44][45][46][47][48][49][50].…”