This work reports a microfluidic reactor that utilizes gold nanoparticles (AuNPs) for the highly efficient photocatalytic degradation of organic pollutants under visible light. The bottom of microchamber has a TiO 2 film covering a layer of AuNPs (namely, TiO 2 /AuNP film) deposited on the F-doped SnO 2 (FTO) substrate. The rough surface of FTO helps to increase the surface area and the AuNPs enables the strong absorption of visible light to excite electron/hole pairs, which are then transferred to the TiO 2 film for photodegradation. The TiO 2 film also isolates the AuNPs from the solution to avoid detachment and photocorrosion. Experiments show that the TiO 2 /AuNP film has a strong absorption over 400-800 nm and enhances the reaction rate constant by 13 times with respect to the bare TiO 2 film for the photodegradation of methylene blue. In addition, the TiO 2 /AuNP microreactor exhibits a negligible reduction of photoactivity after five cycles of repeated tests, which verifies the protective function of the TiO 2 layer. This plasmonic photocatalytic microreactor draws the strengths of microfluidics and plasmonics, and may find potential applications in continuous photocatalytic water treatment and photosynthesis. The fabrication of the microreactor uses manual operation and requires no photolithography, making it simple, easy, and of low cost for real laboratory and field tests.2 of 11 resonance (LSPR) due to the collective oscillation of free electrons in response to the excitation of irradiant light. The LSPR effect can drastically enhance the visible response of TiO 2 photocatalysis for solar energy capture, environmental redemption, and selective organic photosynthesis [5,7,9,10]. Moreover, the direct physical contact of the noble metal NPs and the TiO 2 photocatalysts would form a Schottky junction to suppress the recombination of electron-hole pairs [8,11].Typical photodegradation systems involve the suspension of TiO 2 nanopowders in an aqueous solution of a bulky container. With the stirring, the TiO 2 nanopowders have full contact with the dissolved organic pollutants, resulting in a large specific surface area (SSA, defined as the total surface area per unit of mass) and high photodegradation efficiency. However, the suspended TiO 2 nanopowders absorb and scatter light, causing rapid decay, and thus an uneven distribution of the irradiant light. What is more problematic is the requirement of post processing, namely the nanopowders have to be separated from the solution after the reaction [12][13][14]. To avoid these problems, immobilized systems have been developed to fix the TiO 2 photocatalysts on a support, but they tend to have a small SSA and low efficiency [15].Microfluidic reactors have attracted much attention and have been proposed to tackle the drawbacks of photocatalytic processes [14,[16][17][18][19][20]. They inherit many advantages from microfluidics technology, such as small dimensions, high surface-to-volume (S/V) ratio, easy control of flow rates, short molecular diffusion distance,...

The rapid recombination of photogenerated electron-hole pairs is one of the main reasons for limiting the photocatalytic efficiency of semiconductor photocatalysts. This paper investigates and analyzes two methods to improve the separation and lifetimes of electronhole pairs: semiconductor heterojunction and metal electron sink. As a specific example, an Au-TiO 2 -ZnO composite film with a sandwich structure is prepared. The energy level gradient formed at the TiO 2 /ZnO heterojunction separates the electron-hole pairs effectively, and the AuNPs promote the photo-induced charge transfer. Moreover, a synergistic effect of TiO 2 /ZnO heterojunction and AuNPs, which can further improve the photocatalytic performance, has been found by the photoelectric performance measurement. The experimental results demonstrate that the UV photocurrent density of the Au-TiO 2 -ZnO composite film is almost 22 times that of the pure TiO 2 film. And a similar result has been observed in the UV light degradation of methylene blue. Such Au-TiO 2 -ZnO composite film combining semiconductor heterojunctions and metal electron sink has a potential prospect in the photo-degradation of organic pollutants.