Effect of TiO₂ Crystal Orientation on the Adsorption of CdSe Quantum Dots for Photosensitization Studied by the Photoacoustic and Photoelectron Yield Methods

Abstract

We describe the adsorption and growth of CdSe quantum dots (QDs) on single crystals of rutile TiO 2 with different crystal orientations. We used atomic force microscopy (AFM) to characterize the morphology of the QDs and photoacoustic (PA) spectroscopy to measure the optical absorption. Photoelectron yield (PY) spectroscopy was applied to characterize the valence band maximum (VBM) of the single crystal TiO 2 . The AFM images and the absorbance measurements showed that the number of CdSe QDs grown on the (111) surface was larger than those grown on the ( 110) and ( 001) surfaces. The absorbance measurements showed that the adsorption becomes linearly proportional to the adsorption time. However, the rate of adsorption is different for each crystal orientation. The crystals grow higher on (111) surfaces than on ( 110) and ( 001) surfaces. The position of the VBM for the (111) surface is higher than those for the (110) and (001) surfaces. Hence, the formation and growth of CdSe QDs on (111) surfaces is more active than on the other orientations. The increase in the average diameter of CdSe QDs with adsorption time is independent of the crystal orientation. Although the growth rate of CdSe QDs on (001) surfaces is lower than on (110) and (111) surfaces, the crystal quality is better on the former.

Electronic structures of two types of TiO2 electrodes: inverse opal and nanoparticulate cases

“…As the PY signal intensity has a good S/N ratio, an error bar is included for each point. 26 The positions of VBM and CBM in IO-TiO 2 are higher than those in NP-TiO 2 . We applied straight lines above a photon energy of 8.3 eV (linear region).…”

“…One promising approach in the development of third-generation solar cells is to exchange the organic dye by semiconductor quantum dots (QD-sensitized solar cells: QDSCs). [19][20][21][24][25][26] In sensitized solar cells, the nanostructured TiO 2 electrodes have a large surface area which increases the loading of the sensitizer thus enhancing light harvesting. 3 Secondly, they possess high extinction coefficients and large intrinsic dipole moments, leading to rapid charge separation.…”

Exponential optical absorption edge in PbS quantum dot-ligand systems on single crystal rutile-TiO2 revealed by photoacoustic and absorbance spectroscopies

“…Single-crystal R-TiO 2 wafers, (5×7×0.5 mm 3 ), with (001)-, (110)-, and (111)-cuts were obtained (Furuuchi Chemical Co., Ltd) [27], and they showed the basic unit cell structure of R-TiO 2 with lattice constants of a=b=0.4594 nm and c=0.2959 nm (figure S1 (available online at stacks.iop.org/MRX/9/025005/ mmedia)). The surface layers (light blue broken lines in figure S1) are anisotropic in these crystal orientations, especially the (111) orientation, indicating that the dielectric properties are different due to the different occupancies of the elements at the surface.…”

“…Hence, studies of the Urbach tail for QDs adsorbed on different surface orientation substrates provide us with comprehensive understanding of the optical absorption of QDs. The optical absorption intensity (P) in the Urbach tail region is given by [27,61]:…”

“…One of the most important step for understanding the electronic structure of QDs is to study the optical absorption of QDs and their dependencies on different well-characterized single crystal TiO 2 surfaces [19]. Despite this being crucial for advanced QDSCs, detailed optical absorption studies of QDs looking at the influence of the electrode surface has seldom been done [25][26][27][28][29][30][31][32][33].…”