(i) mass loss
below 100uC, which corresponds to water loss; (ii) mass loss around 300uC, which corresponds to cellulose/hemicellulose/lignin degradation;
and (iii) mass loss between 350–550uC, corresponding to the
burning of carbonous residues32. 23% of the mass remains as the SiO2
product. Differential thermal analysis (DTA, Fig. 2e) confirms these
three stages during heating, with an additional endothermic peak for
the raw RH sample at 635uC (highlighted in the black square in
Fig. 2e). This peak is due to the melting of silica catalyzed by metal
impurities (such as K1, Na1, and Ca21) in raw RHs7 and and
formation of bulk SiO2, as shown in Fig. 2a. After optimization,
heating leached RHs at 700uC in air produces amorphous SiO2
nanoparticles with diameters distributed around 80 nm; these
nanoparticles are loosely interconnected and are macroscopically
preserved in the shape of the RHs (Fig. 2f and Supplementary Fig.
S2). In the next step, this type of nano-SiO2 is further reduced to
obtain nano-Si.