Layered-Defect Perovskite K3Bi2X9 (X = I, Br, and Cl) Thin Films for CO2 Photoreduction: An Analysis of Their Pseudocatalytic Behavior

Lead-free layered-defect perovskite K3Bi2X9 (X = I, Br, and Cl) films were proposed as efficient photocatalysts for the CO2 reduction reaction (CO2RR) to obtain clean and sustainable formic acid (HCOOH), a widely used feedstock in the industry. The films exhibited high crystallinity, hexagonal morphologies, and visible light absorption, which were modified by proportionally increasing the diameter of the X anion. The obtained photocatalytic activities showed values of 299 µmol h−1 (K3Bi2Br9), 283 µmol h−1 (K3Bi2I9), and 91 µmol h−1 (K3Bi2Cl9). However, the stability of the films is an important parameter that must be solved; therefore, three strategies were implemented—one with an intrinsic approach (solvent engineering) and two others with an extrinsic focus (substrate modification and heterojunction engineering). These modifications favored yields of up to 738 µmol h−1 and constant production over 6 h, demonstrating that the perovskite maintains continuous HCOOH generation. The analysis of the reaction medium showed the degradation of the material structure to BiOI and K+, which could have enhanced its affinity towards CO2. In this manner, the degraded perovskite (K3Bi2I9/BiOI) might still react with the CO2 to generate HCOOH in an aqueous medium under visible light, showing pseudocatalytic behavior.