Chromium is a metal commonly used in the manufacturing industry. However, when present in industrial wastewater and refuse, chromium causes serious water pollution. In this study, we synthesized a high-efficiency low-cost SnOx (SnO/SnO2) nanodot (ND) photocatalyst using monobutyltin trichloride (MBTC) using a one-step thermal decomposition process. SnOx NDs not only exhibit strong broadband and multicolor fluorescence emission (λem ranging from 450 nm to 585 nm), but also emit white light under ultraviolet irradiation. We also found that, under visible-light irradiation, SnOx NDs exhibited a photocurrent response that improved photocatalytic activity. In addition, we used electrochemical impedance spectroscopy (EIS) to detect Cr(VI) over a concentration range of 5–1000 ppb, with a detection limit of 0.006 ppb under light. This detection limit is lower than the maximum limit of 50 ppb for human water consumption, as stipulated by the European Union.