Dynamics of Semiconducting Nanocrystals Uptake Into Mesoporous TiO2 Thick Films Through Electrophoretic Deposition and its Application in Solar Energy

Abstract

Quantum dots (QDs), have been recognized as one type of the most promising photosensitizer for photoanode fabrication due to their unique and scalable optical properties where photoluminescence and absorption of the QDs are size-tunable. However, there are several major concerns with QD sensitized photoanodes: First, QDs should be deposited into mesoporous films effectively and without aggregation; Second, QDs have to be bound strongly onto mesoporous films; Last but not least, the charge transfer kinetics between QDs and wide-band-gap semiconductor has to be fast in order to achieve a highly efficient photoelectrode. The search for methods to achieve these requirements is attractive and challenging. Electrophoretic deposition (EPD), which is a straightforward technique for the uptake of nanoparticles into mesoporous films, has demonstrated highly effective in the preparation of high-efficiency QD-sensitized photoanodes. EPD consists in the application of an external potential between two electrodes immersed into a solution of the particles to be uploaded. Charged particles (both positive and negative) undergo a driving force that induces their penetration and grafting into the mesoporous photoanode. Unlike prior chemical linker-based methods, EPD does not require pretreatment of the TiO2, and deposition time as short as 1 h is sufficient for effective coating. After EPD, the photoanode typically exhibits a high optical density, leading to complete absorption of solar radiation in the absorption range of the QDs. Despite the remarkable results obtained in terms of photoconversion efficiency (PCE), no systematic investigation is reported in literature on EPD of nanoparticles in mesoporous films so far. In the present thesis, we addressed this open issue and gave an exhaustive description of the phenomenon from the experimental point of view as well as from the point of view of the modeling of the physical chemical processes occurring during EPD. We investigated the uptake of colloidal QDs into TiO₂ films using EPD. We examined PbS@CdS core@shell QDs, which are optically active in the NIR region of the solar spectrum and exhibit much increased long-term stability toward oxidation compared to their pure PbS counterpart, as demonstrated by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). We applied Rutherford backscattering spectrometry (RBS) to obtain Pb depth profile into the TiO2 matrix. The applied electric field induces the fast anchoring of QDs to oxide surface. Consequently, QD concentration in the solution contained in the mesoporous film drastically decreases, inducing a Fick-like diffusion of QDs. We modelled the entire process as a QD diffusion related to the formation of QD concentration gradient, and a depth-independent QD anchoring. EPD duration and applied voltages in the range 5 to 120 mins and 50 to 200 V were considered. Furthermore, we were able to determine the electric field-induced diffusion coefficient D for QDs and the characteristic time for QD grafting, in very good agreement with experiment. D increases from (1.5±0.4)×10^-5 μm² s^-1 at 50 V to (1.1±0.3) ×10^-3 μm² s-1 at 200 V. These results quantitatively describe the process of QD uptake during EPD, and can be used to tune the optical and optoelectronic properties of composite systems, which determine, for instance, the PCE of the photoanodes. The dynamics of EPD could also be applied to other different colloidal nanoparticles and quantum rod materials for sensitization of mesoporous films. In addition, we also demonstrated the increased stability of the core@shell structure compared to PbS QDs after EPD in terms of structure and optical properties. Based on our previous studies that confirmed a fast charge transfer from PbS@CdS to TiO₂, PbS@CdS sensitized photoanodes can be strong candidates for the development of highly efficient and stable photoanodes in PV devices and photoelectrochemical (PEC) H₂ generation.