The light absorption, transmittance, and reflection of the carbonized chitin and cellulose nanopapers were evaluated using an ultraviolet−visible−near-infrared (UV–vis–NIR) spectrometer (
UV-3600i Plus, Shimadzu Corp., Kyoto, Japan) equipped with an
ISR-603 integrating sphere (Shimadzu Corp., Kyoto, Japan). More than five samples were prepared and evaluated under each condition. Light absorption was calculated from the total light transmittance and reflection spectra. Solar absorption was calculated using Equation (1) [26 (
link)] as follows:
where
is the solar absorption (%);
λ is the wavelength (nm)
; λmin and
λmax are 300 and 2500 nm, respectively;
Isolar(
λ) is the solar spectral irradiance (AM1.5G) at
λ; and
αsolar(
λ) is the light absorption (%) at
λ. The optical bandgap values were also calculated from the UV–vis–NIR absorption spectra according to a previously reported method [15 (
link)] and Tauc’s equation [27 (
link)] (Equation (2)):
where α,
hν,
A, and
Eg are the absorbance, photon energy, constant, and optical band gap, respectively. The optical bandgap was estimated by plotting (
αhν)
1/n vs. photon energy (
hν) and extrapolating the linear region of the curve to the
X-axis (
Figure S1). The parameter
n was set to 2 for the indirect transition of the carbonized nanopapers because of their amorphous carbon structures.