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Bismuth telluride (Bi2Te3), as an emerging two-dimensional (2D) material, has attracted extensive attention from scientific researchers due to its excellent optoelectronic, thermoelectric properties and topological structure. However, the application research of Bi2Te3 mainly focuses on thermoelectric devices, while the research on optoelectronic devices is scarce. In this work, the morphology evolution and growth mechanism of 2D Bi2Te3 nanosheets with a thickness of 12 ± 3 nm were systematically studied by solvothermal method. Then, the Bi2Te3 nanosheets were annealed at 350 °C for 1 h and applied to self-powered photoelectrochemical-type broadband photodetectors. Compared with the as-synthesized Bi2Te3 photodetector, the photocurrent of the photodetector based on the annealed Bi2Te3 is significantly enhanced, especially enhanced by 18.3 times under near-infrared light illumination. Furthermore, the performance of annealed Bi2Te3 photodetector was systematically studied. The research results show that the photodetector not only has a broadband response from ultraviolet to near-infrared (365–850 nm) under zero bias voltage, but also obtains the highest responsivity of 6.6 mA W−1 under green light with an incident power of 10 mW cm−2. The corresponding rise time and decay time are 17 ms and 20 ms, respectively. These findings indicate that annealed Bi2Te3 nanosheets have great potential to be used as self-powered high-speed broadband photodetectors with high responsivity. [ABSTRACT FROM AUTHOR]