Environmental transmission electron microscopy (ETEM) opens a new field for exploring dynamic behavior of the materials in-situ in response to varying experimental stimuli in TEM. The electron beam itself is included as a stimuli; electron interacts with the sample and the environment and thus electron probing of the reacting materials also increase the chance of driving the material far from its steady state reactions. For example, the reduction mechanism evoked by the electron beam in metal

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... ide [1-3] is often different from the reduction that is solely driven by the chemical potential changes. In order to understand the genuine sample reaction mechanism with the environment, the beam interaction with the sample and the environment has to be addressed first. Here, we studied the reduction mechanism of rutile (TiO2) nanocrystals at high temperatures. To address the electron beam effect, we varied the electron dose rate from low (~ 10 e -/Å 2 s) to high over 400 e -/Å 2 s) at 550 ⁰C and recorded the structure change with time. The observation was done using an ETEM (Hitachi 9500, LaB6) operated at 300 kV. This instrument combines the sample heating and gas environment with a CMOS-based, high-fidelity and high-transfer rate direct electron detection camera (K2 Summit, Gatan) [4] . The TiO2 nanocrystals of average size ~50 nm were supported on the SiN membrane of MEMS heating holder (Blaze, Hitachi Canada) [5, 6] . The combination of the stable heating stage at high temperature and the direct electron detection unable us to achieve the atomic resolution at low dose by recording the images with the long exposure time (4-10 s). The electron beam acts as a strong reducing agent in the TEM vacuum (~10 -5 Pa). Under the electron beam and in vacuum, the reconstructed the (110) surface structure of TiO2 is shown in Figure 1 . Further reduction leads to the formation of the crystallographic shear planes (CSP) inside the nanocrystals ( Figure 2 ). With oxygen injection to the column up to the pressure of 3x10 -3 Pa, the defective CSPs moved or even disappeared, and the nanocrystals surface were also re-oxidized. Once TiO2 nanocrystals were oxidized under the oxygen partial pressure, the surface structure was stable and not affected by the electron beam even at the high dose rate. We also found that the dose rate was more critical than the total dose for the reduction and defect formation of TiO2 nanocrystals. At high dose rate, the electron beam introduced CSPs to the nanocrystals in a few seconds. At low dose rate below the empirical threshold rate of 100 e -/Å 2 /s, the defects did not form regardless total illumination time. Comparing with the control experiments performed at room temperature, we concluded the reduction mechanism is governed by the competition between the reduction rated by the electron beam and the recovery rate at high temperatures [7] . 1680