In the typical synthesis, 0.439 g Zn(CH3COO)2·2H2O (2 mmol) and 0.9532 g HEPES (4 mmol) were added into 30 mL TEG under rigorous stirring and sonication, respectively. Then the mixture was transferred into a Teflon-lined stainless steel autoclave to perform a solvothermal process at 150 °C for 9 h. After cooling down to room temperature, the product was collected by centrifugation and washed with deionized water for five times, and finally dried in a desiccator for a few days at room temperature for further characterization (S1). Other ZnO samples (S2–S6) were prepared under identical conditions in the absence of HEPES (S2), in the presence of HEPES by varying the amount of NaOH (S3–S5) or using deionized water as a solvent (S6) (Supplementary materials).