Fig. 3

SEM images obtained at high magnification (50,000 ×) using the secondary electron signal from Norway spruce wood subjected to different pretreatments. A STEX_180°C/auto, B STEX_210°C/auto, C STEX_210°C/HAc, D STEX_210°C/H3PO4, E STEX_210°C/H2SO4, and F STEX_210°C/SO2