Anatase Thin Film Growth—Optimizing Grains and Facets for Photoelectrochemical Applications

Reducing our dependence on fossil fuels for energy is important to eliminate greenhouse gas emissions. First-generation solar cell production is not sustainable, as manufacturing crystalline silicon is resource intensive. Anatase TiO₂ is a wide band-gap semiconductor oxide that has applications in third-generation solar cells and photoelectrocatalysis for water splitting and may become part of the portfolio of materials for clean energy production. One approach to improve devices based on TiO₂ thin films is to use crystallographic anisotropy to induce space-charge separation of photogenerated charge carriers. Exploiting the space-charge anisotropy intrinsic to the crystallographic directions and facets of anatase may improve both the reactivity and charge transport for photocatalytic and charge transport applications. It has been found that the (001) and (101) anatase facets are preferentially oxidizing and reducing, respectively. In addition, charge transport has lower resistance along the [001] direction. Therefore, anatase thin films for PEC should be designed with the c-axis normal to the conducting substrate and (001) facets at the surface to improve both charge transport and reactivity, respectively. Previously, we reported a synthesis for anatase thin films with [001] texture (Ichimura 2012). In this study, we examine thin film growth systematically to determine the conditions that maximize the [001] orientation and (001) facet expression of anatase thin films.<br/><br/>This work aims to optimize a one-pot hydrothermal synthesis of anatase TiO₂ thin films as a function of reagent concentration, pH, and temperature for photoelectrochemical applications. In this research, TiF₄ is the titanium source and HF is a structure-directing agent used to promote (001) facet growth. The TiO₂ thin films are synthesized in batches to reliably control concentration and pH. Preliminary syntheses have revealed that pH is an important factor to control grain size while temperature has a large effect on the growth of the (001) facets. The thin films are characterized by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and grazing incidence X-ray diffraction (GIXRD). The TiO₂ film parameters (grain size and statistics, film thickness, and extent of orientation) will be correlated with reagent concentration, pH, and temperature. Preliminary PEC measurements that employ a Ni(II) based catalyst for the oxygen evolution reaction will also be reported. Additionally, ¹⁹F NMR will be used to correlate film structure and solution composition.