D max 2500

A Bruker D-MAX-2500 (Rigaku, Tokyo, Japan) X-ray diffractometer (XRD) was used for X-ray diffraction analysis of the samples. A Shimadzu UV-2770 UV-vis spectrophotometer was used for diffuse reflectance spectra analysis. A Bruker Equinox-55 Fourier-transform infrared spectrometer was used for FT-IR spectra analysis. The Kratos-AXIS ULTRA-DLD X-ray photoelectron spectrometer was used for X-ray photoelectron spectra analysis. A JEOL-JEM-2010 instrument was used for morphology analysis of the samples. The FEI-Tecnai (G2 S-Tw) equipped with energy-dispersive X-ray (EDX) sensor was used for elemental analysis. A home-built instrument connected to a lock-in amplifier (SR-830) coordinated to a light chopper (SR-540) was used for surface photovoltage spectra (SPS) analysis. A FLS-920 (Edinburgh) instrument was used for measuring the time-resolved photoluminescence (TR-PL) spectra. A model Bruker-EMX-plus spectrometer was used for conducting the electron spin resonance (ESR) tests. Other experimental details are provided in the supporting information.

The extent of ester, as defined by the succiniclic acid content (O-C=O %) and the sulfur content (S%), was measured using an X-ray photoelectron spectroscopy (D/max2500, Bruker AXS, Beijing, China). The DS was calculated according to Equation (1) and Equation (2), as follows [8 (link),25 (link)]: $$\mathrm{D}{\mathrm{S}}_{\mathrm{s}}=\frac{162{\mathsf{\omega }}_{\mathrm{s}}}{44-122{\mathsf{\omega }}_{\mathrm{s}}}\begin{array}{cccc}& & & \end{array}$$
$$\mathrm{D}{\mathrm{S}}_{\mathrm{c}}=\frac{162{\mathsf{\omega }}_c}{32-102{\mathsf{\omega }}_{\mathrm{c}}}\begin{array}{cccc}& & & \end{array}$$
where ω_s and ω_c are the mass fraction of the sulfonic and carboxyl groups; 122, 102 is the molar quantity of the substituted group; 162 is the molar quantity of a grape unit; 44 is the molar quantity of the O-C=O; 32 is the molar quantity of the S.