Photocatalytic hydrogen evolution on the hetero-system polypyrrol/TiO_2 under visible light

摘要

The semiconducting properties of the hetero-system polypyrrol (PPy)/TiO_2 are investigated for the first time to assess its photo catalytic hydrogen evolution under visible light. The hetero-system PPy/TiO_2 is stable up to -345 K, with a weight loss accounting for ～1.2%. The X-ray diffraction shows mixed phases with the rutile TiO_2 variety. Optical transitions at 0.42 and 3.01 eV, directly allowed, are determined for PPy and TiO_2 respectively. The p-type conductivity of PPy is evidenced from the Mott-Schottky plot; In neutral solution, a flat band potential of 0.11 V_(Sce) and a holes density of 4.57 × 10~(21) cm~(-3) are obtained. The electrochemical impedance spectroscopy, measured over a wide frequency range (1 mHz-10~5 Hz), reveals the contribution of the bulk and grain boundaries with a constant phase element (CPE). The energetic diagram predicts the electron injection from PPy into TiO_2 and the best photoactivity is achieved at pH ～7. A hydrogen liberation rate of 1.15 mmol h~(-1) (g catalyst)~(-1) and a quantum efficiency of 0.17% under full light (29 mW cm~(-2)) are determined in presence S_2O_3~(2-). The photoactivity is completely restored during the second cycle, indicating a zero deactivation effect.