求翻译：But no pores in nanoparticles are observed by SEM image (Figure 2d), suggesting that the pores observed by TEM are inside the nanoparticles, not on the surface. The pore size distribution of the TiO2 nanocrystals with internal pores reveals that their dominant size is 5−7 nm (Figure 2e). The N2 adsorption−desorption is是什么意思？

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