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Graphical abstract Highlights • A novel optical hollow-fiber with high photocatalytic activity was developed. • A photocatalyst of Er3+:YAlO 3 /SiO 2 /TiO 2 was graded on the surface of the fibers. • Photocatalytic activity of a single fiber toward degradation of 4-CP was tested. • Optimized operating conditions to enhance 4-CP degradation were investigated. • Photocatalytic fiber shows high activity toward degradation of 4-CP. Abstract We present a novel UV–visible-light photocatalytic optical hollow-fiber in which a hollow optical fiber (HOF) and TiO 2 -based composite are coupled for enhancement of photocatalytic activity toward degradation of 4-chlorophenol (4-CP). A HOF was coated with a TiO 2 -based composite composed of Er3+:YAlO 3 /SiO 2 /TiO 2 (EYST) and acting as the photocatalytic element. The EYST coating showed good light-inducing capacity at the fiber-coating interface and high photocatalytic activity driven by UV–visible light. The photocatalytic activity of the fabricated photocatalytic HOFs was investigated for degradation of 4-CP in aqueous solution in the wavelength spectrum of 360–780 nm. Examined were the effects of the EYST coating gradient and reactor operating conditions (pH and temperature) on the degradation of 4-CP. The EYST-coated HOF significantly enhanced the 4-CP degradation, dechlorination, and mineralization, compared to pure TiO 2 -coated HOF and EYST-coated solid optical fibers. Specifically, high photocatalytic activity was obtained when the EYST coating thicknesses at the HOF’s incident end and HOF’s hemispherical end were 10 and 160 μm, the pH was 6.0, and temperature was 55 °C. [ABSTRACT FROM AUTHOR]