Band alignment and enhanced photocatalytic activation of alpha/beta-Bi2O3 heterojunctions via in situ phase transformation

The assembling heterojunction, one of the key topics in photocatalysts and semiconductors (SCs), is generally accomplished in at least two steps, of which the first step is the synthesis of a matrix, and then the growth of the second phase on the matrix. Herein we present the preparation of alpha/beta-Bi2O3 heterojunctions by an in situ phase transformation technique. Under normal pressure, a facile citrate method was used to synthesize beta-Bi2O3 nanosheets and alpha/beta-Bi2O3 heterojunctions. The novel features of the process are the mild operating conditions by an appropriate selection of heat treatment temperature and time. Using transmission electron microscopy (TEM), we found that a number of nano-sized alpha-Bi2O3 form on the beta-Bi2O3 nanosheet via a controlled beta ->alpha phase transition, generating a large number of heterojunctions. The CM1 (calcining beta-Bi2O3 precursor at 363 degrees C for 4 h) heterojunction achieves a strong visible light absorption and dye absorption capacity and produces a very high reaction rate for Rhodamine B (RhB) photodegradation. Electrochemical impedance spectroscopy (EIS) revealed excellent charge transfer characteristics of the heterojunction, which accounts for its high photoactivity. Using the X-ray electron valence band spectra, it is found that the valence band of alpha-Bi2O3 is more negative than that of beta-Bi2O3. Thus, in heterojunctions, the photogenerated holes in beta-Bi2O3 are transferred to alpha-Bi2O3 with good charge transport characteristics by the intrinsic driving force in the interface field. Furthermore, a separated hole can accomplish a transfer process from alpha-Bi2O3 to the aqueous solution within its lifetime due to the diameter of a-Bi2O3 being less than 17.6 nm.