Charge transfer from internal electrostatic fields is superior to surface defects for 2,4-dichlorophenol degradation in K3−xNaxB6O10Br photocatalysts

Abstract

Both surface oxygen vacancies and bulk motivation in a semiconductor play very important roles in the photocatalytic process. To distinguish between the roles of bulk motivation and the surface state in a photocatalytic process, two different phases of K–B–O–B (KBB) photocatalysts with built-in electric field or surface oxygen vacancies were fabricated via the addition of different amounts of Na⁺ substitute. The crystal structure, band structure, reactive species and photocatalytic performance of two types of photocatalysts were systematically investigated. For the six studied photocatalysts, K₃B₆O₁₀Br (KBB1), K_2.87Na_0.13B₆O₁₀Br (KBB2), and K_2.33Na_0.67B₆O₁₀Br (KBB3) with less Na⁺ content behaved as polar materials, while K_1.7Na_1.3B₆O₁₀Br (KBB4), K_0.80Na_2.20B₆O₁₀Br (KBB5), and Na₃B₆O₁₀Br (KBB6) with more Na⁺ content behaved as nonpolar materials. Among them, KBB3 exhibited the best photocatalytic activity, which was about 1.15, 1.07, 1.4, 1.25, and 1.18 times that of the KBB1, KBB2, KBB4, KBB5 and KBB6 samples, respectively. During the degradation process of 2,4-dichlorophenol (2,4-DCP), the dominant reactive oxidation species was mainly ˙O₂⁻, while the ˙OH and h⁺ played secondary roles. The oxygen vacancy concentration increased as the Na atoms increased for the polar materials, except for KBB1; the oxygen vacancy and the built-in electric field had a synergistic effect on the degradation process to nonpolar materials, with the main active species coming only from the high concentration of oxygen vacancies. Furthermore, the activity of KBB1 with only the built-in electric field was superior to that of KBB6 with surface oxygen vacancies, which further confirmed that for charge separation, the driving force from the bulk could provide more motivation than surface defects during the 2,4-DCP degradation process.