On the Effect of Thermal Processing on Sn Diffusion and Efficiency Enhancement in Hematite/FTO Photoanodes

Abstract

The frequently underestimated effects of “in air” thermal treatment processing conditions such as temperature, duration, and heating and cooling rates in the design and efficiency of photoelectrodes fabricated for academic studies onto the most common commercial transparent conductive glass substrate i.e. fluorine-doped tin oxide (FTO) were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, conductive atomic force microscopy, electrochemical impedance spectroscopy, and direct current and photoelectrochemical (PEC) measurements. The PEC response of Hematite photoanode thin films consisting of short nanorods thermally treated at 400 and 800°C upon fast or extended time conditions is inhibited by factors such as crystallinity, Sn diffusion, or substrate integrity. A “fast” thermal treatment in air at 750°C provided the best synergy between charge transfer resistance, Sn-diffusion from the FTO substrate, nanorod dimensions, reduced recombination, improved charge separation and minimized substrate damage. This study offers valuable fundamental and practical insights for a better understanding of the benefits and drawbacks of photoelectrode thermal processing, which is critical for the improvement of the PEC performance-reproducibility relationship for FTO-based solar water splitting systems and devices.

Selective placement of modifiers on hematite thin films for solar water splitting

“…The furnaces were preheated at employed temperatures and the region where the samples were placed had their temperatures veried by an external thermocouple. Temperatures and duration of thermal treatments adopted in this work were optimized as reported elsewhere 44,45 to preserve the substrate integrity and conductivity. Fig.…”

Advances in Engineered Metal Oxide Thin Films by Low-Cost, Solution-Based Techniques for Green Hydrogen Production

“…After the thermal treatment, some interface mismatches are also created [98]. Therefore, the optimization of this process is essential to enhance the hematite PEC performance and to minimize substrate damage [70,[99][100][101][102]. Some studies have been devoted to understanding how the thermal treatment influences the hematite electronic structure, the chemistry of defects in hematite, FTO, and the hematite-FTO interface, as well as the substrate damage caused by the element diffusion from FTO to hematite.…”

Self-Diffusion versus Intentional Doping: Beneficial and Damaging Impact on Hematite Photoanode Interfaces

“…15−17 Even at higher temperatures, FTO can maintain acceptable conductivities as long as the treatment duration is limited to less than 30 min. 16,17 Undeniably, the substrate selection is impacted by the requirement for thermal treatments in the design of oxidebased photoelectrodes. These treatments can play a strategic role in inducing crystal phase transitions and activating the photoabsorber surface for PEC applications.…”

“…In the quest to find a thermal treatment that provides a synergic interaction between the TCO substrate and photoabsorber properties, several studies have been reported. ,,, These studies have highlighted the significance of thermal treatment variables, such as temperature, duration, atmosphere, and even the type of thermal treatment employed in the photoelectrode design. Further insights related to the performance of photoelectrodes composed of TCOs and photoabsorbers such as TiO 2 , CuO, WO 3 , Ta 3 N 5 , and α-Fe 2 O 3 , as well as the influence of thermal treatments and potential interface modifications, can be found elsewhere. − …”

“…Antimony-doped tin oxide (ATO), indium tin oxide (ITO), and fluorine-doped tin oxide (FTO) are widely recognized as notable substrate examples commonly employed in most electrochemical applications including PEC systems. − From the available TCOs, FTO is frequently preferred over other options because of its superior thermal and chemical stability. This stability becomes particularly advantageous when high-temperature treatments are required. ,− Unlike ITO, which experiences a significant decrease in conductivity when exposed to temperatures above 350 °C, , FTO remains stable up to 800 °C. − Even at higher temperatures, FTO can maintain acceptable conductivities as long as the treatment duration is limited to less than 30 min. , …”