*2.2. Preparation of TiO2 Mesospheres*

TiO2 mesospheres were prepared through a combined route of sol-gel and solvothermal synthesis. In the first step (sol-gel), 3 mL of TTIP was slowly added into 150 mL of ethylene glycol, and the solution was continuously stirred for 12 h at room temperature. After a 12 h stirring process, 300 mL of acetone with 2 mL of de-ionized water (DI) were added into the solution and stirring was continued for 2 h to obtain a white suspension. Subsequently, the white suspension was collected and subjected to several centrifugations with ethanol and DI water, respectively. The obtained product was dried at 60 ◦C for 10 h to remove the impurities and form titanium glycolate. In the second step of the procedure (solvothermal route), 1 g of titanium glycolate was dispersed into 60 mL of a mixed solvent of ethanol and DI water under stirring. The prepared white solution was then transferred into a 100 mL autoclave and kept at 180 ◦C for 12 h. Finally, the TiO2 mesospheres were obtained after annealing at 350 ◦C for 1 h.

#### *2.3. Preparation of Graphene Oxide (GO)*

Graphene oxide (GO) was prepared under room-temperature conditions by the oxidation of natural graphite powder using a modified Hummer's method [29]. Briefly, 1 g of natural graphite powder and 0.5 g of sodium nitrate were blended in 23 mL of concentrated sulfuric acid under vigorous stirring for 30 min. A total of 3 g of well-ground potassium permanganate was then slowly added into the mixed solution and stirred for 30 min under ice-bath conditions at 7 ◦C. The mixed solution was then stirred at 35 ◦C for 30 min. To quench the vigorous oxidation process in the solution, 3 mL of hydrogen peroxide was added to 60 mL of DI water, and the solid GO powder was obtained after washing several times with 5% of HCL and DI water, then dried overnight at 60 ◦C.
