**2. Experimental**

## *2.1. Substrate Modification*

We used *piranha* solution, which is a kind of superoxidant, to modify the hydrophilic substrates. The *piranha* solution consisted of a mixture of concentrated sulfuric acid (98%, analytical reagent, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) and hydrogen peroxide (30%, analytical reagent, Shanghai Macklin Reagent Co. Ltd., Shanghai, China) with a volume ratio of 7:3. Quartz substrates (20.0 mm × 15.0 mm × 1.00 mm) were treated with *piranha* solution at 353 K for 24 h and then rinsed repeatedly with deionized water followed by ethanol (analytical reagent, Shanghai Macklin Reagent Co. Ltd., Shanghai, China); after that, the substrate was stored in acetone (analytical reagent, Shanghai Macklin Reagent Co. Ltd., Shanghai, China). After the acetone is completely volatilized, the quartz substrate can be used. A silicon wafer (20.0 mm × 15.0 mm × 1.00 mm) was treated with the same procedure for AFM and XPS characterization. A KBr pellet (radius of 10.0 mm, thickness of 2.00 mm) was formed from KBr powder (analytical reagent, Shanghai Macklin Reagent Co. Ltd., Shanghai, China) by using a tablet press and was used for the FTIR experiments.

#### *2.2. Preparation of the PMoA*/*PANI Hybrid Thin Film*

The double crystal H3PMo12 O40 (PMoA, analytical reagent, Shanghai Reagents Co. Ltd., Shanghai, China) was stored until use. A 0.02 g aliquot of polyaniline (PANI 98%, Shanghai Macklin Biochemical Co. Ltd., Shanghai, China) was mixed with 20 mL of a mixture of *<sup>N</sup>*,*<sup>N</sup>*-dimethylformamide (DMF, analytical reagent, Shanghai Macklin Reagent Co. Ltd., Shanghai, China) and ethanol to form solution No. 1 with a density of 0.1 mg/mL. A 0.04 g aliquot of H3PMo12 O40 (PMoA, analytical reagent, Shanghai Reagents Co. Ltd., Shanghai, China) was mixed with 20 mL of a mixture of *<sup>N</sup>*,*<sup>N</sup>*-dimethylformamide (DMF, analytical reagent, Shanghai Reagents Co. Ltd.) and ethanol (analytical reagent, Shanghai Reagents Co. Ltd.) to form solution No. 2 with a density of 0.2 mg/mL. Solution No. 1 and solution No. 2 were mixed to form solution No. 3. A pipetting gun was used to drop this mixed solution onto a quartz substrate to form a PMoA/PANI hybrid thin film with a amount of 100 μL, as shown in Figure 1. To test the thin film thickness, solution No. 3 was dropped on a quartz substrate to form a thin film, than the quartz substrate was put on the test bed of an thin film thickness measurement system (FCT-1030, Changchun Institute of Optics, Fine Mechanics and Physics, LCD Lab, Chinese Academy of Science. Changchun, China). When visible light is vertically illuminated on the measured film, one part of the light is reflected on the surface of the film, the other part penetrates into the film and then reflects at the interface between the film and the bottom layer. The thickness of the hybrid thin films was nearly 1.8 μm, as calculated by the supporting software of the thin film thickness measurement system.
