Investigation of thermal and optical properties of thin WO₃films by the photothermal Deflection Technique

Nanostructured photoelectrodes based on WO3: applications to photooxidation of aqueous electrolytes

“…Thus the vertically oriented and porous structures encountered in these electrodes are suitable for improving light harvesting and to achieve a good electrolyte permeation of the oxide substrate, possibly leading to a shortened hole diffusional path, as demonstrated by the good photon to electron conversion in correspondence to those wavelengths which have a long penetration depth in the semiconductor (i.e. B7 mm at 440 nm) 48 and create electronhole pairs far from the outermost layer of the oxide. It can also be appreciated that the spectral extension reaches its maximum by prolonging the accelerated anodization to 6-7 hours, which constitute the optimal values in terms of maximization of the electroactive surface.…”

Physical Properties of Macroporous Tungsten Oxide Thin Films and Their Impact on the Photocurrent Density

“…Calculating the thermal diffusion length ( ), at the chopping frequency , by the expression = √ / , and using the previous obtained values of , we determined around 200 m. This value is much higher than the sample thickness. Additionally, the absorption coefficient at near the band edge for WO 3 samples is about 10 6 m −1 ; hence, the optical penetration length is about a micron [13]. In this case, the PA measurements were performed under thermally thin condition, and the PA signal was proportional to the product of absorption coefficient and sample thickness.…”

“…The PA signal is generated by sample heating during the absorption of intensity modulated light in a gas enclosed with the sample in the PA chamber, becoming a noninvasive and contactless method. In previous work photothermal techniques have been applied to determine optical band gap and thermal properties of nonporous WO 3 thin films [12,13]. On the other hand, the photoluminescence (PL) technique is well known as a technique which allows the detection of radiative transition processes [14].…”

Enhanced Photocatalytic Water Splitting on Very Thin WO3 Films Activated by High-Temperature Annealing

“…Besides generally too positive values of the photocurrent onset potentials, another major limitation of several metal oxide semiconductors used as photoanodes is the indirect nature of optical transition, resulting in relatively low absorption coefficients, especially close to the semiconductor band edge . Such is also the case of WO 3 films that, in order to absorb a significant portion of blue visible light, must attain thicknesses of the order of several micrometers . Although nanostructuring of the semiconductor films allows one to partly circumvent the mismatch existing between the photon penetration depth and the distance over which the photogenerated charge carriers can be separated and collected, other unavoidable consequences are substantial ohmic drops and uneven current distribution within the pores of thicker nanostructured photoanodes .…”

“…24 Such is also the case of WO 3 films that, in order to absorb a significant portion of blue visible light, must attain thicknesses of the order of several micrometers. 25 Although nanostructuring of the semiconductor films allows one to partly circumvent the mismatch existing between the photon penetration depth and the distance over which the photogenerated charge carriers can be separated and collected, other unavoidable consequences are substantial ohmic drops and uneven current distribution within the pores of thicker nanostructured photoanodes. 26 Nevertheless, a particularly important advantage of the WO 3 as a water splitting photoanode is a relatively long hole diffusion length (L D ) of ∼150 nm, 20,27,23 which, in the case of nanostructured WO 3 films, allows one to reduce bulk recombination of photogenerated charge carriers, resulting in high incident photon-to-current conversion efficiencies (IPCEs).…”