Carbon-doped BiOI (C-BiOI) photocatalysts were successfully synthesized via a hydrothermal method with Bi(NO3)(3)center dot 5H(2)O, KI, and glucose as raw materials for the first time. The synthesized samples had excellent photocatalytic activities in the degradation of methyl orange (MO) and the reduction of N-2 to NH3. To reveal the origin of the superior photoactivity, the C-BiOI was examined by multi techniques, including N-2-adsorption, XRD, SEM, TEM, Raman, XPS, DRS, PL, EIS and transient photocurrent response. The characterization results indicated that the carbon clusters entered the interlayers of BiOI crystal during preparation. The doped carbon interfered the lattice periodicity and generated vacancies in the BiOI structure, resulting in the decreased band gap and increased efficiency in charge separation, both of which could significantly hasten the photocatalytic reaction. Additionally, the introduced carbon affected the morphology of BiOI and increased its specific surface area, which may also benefit the photocatalytic process. The carbon content was crucial to the promotion effect. Under visible light, the optimized carbon-doped BiOI (C-BiOI-2) presented an MO degradation rate of 0.136 min(-1), which was 4.44 times higher than that of pure BiOI. However, for the photocatalytic N-2 fixation, due to the contribution of surface carbon in N(2 )adsorption, the C-BiOI sample containing higher carbon content (C-BiOI-3) displayed superior performance than C-BiOI-2. The NH3 generation rate under simulated sunlight reached 311 mu mol g(-1) h(-1), which was about 3.7-fold of that of BiOI. This work may shed some insight into the designing and understanding of carbon-doped semiconductor photocatalysts. (C) 2018 Elsevier Inc. All rights reserved.