We report on the fabrication of Ag nanoparticle (NP) decorated mesoporous Si nanowire (NW) heterostructure (HS) by a simple and low cost chemical process. The as-grown Si NWs are mesoporous in nature and the Ag NP decorated Si NWs (Ag@Si NWs) exhibit broadband light emission, ultralow reflectance, efficient photocatalytic degradation of organic dyes and excellent sensitivity for the detection of organic molecules over a wide range of concentration. The broadband white light photoluminescence emission from the bare Si NWs is explained on the basis of quantum confinement effect in Si NCs/NWs and the nonbridging oxygen hole center defects in the Si-SiOx interface. High work function of the noble metal NPs facilitates the effective separation of the photoinduced electron-hole pairs in Si NWs, which enables the Ag@Si NWs to exhibit high photocatalytic efficiency for the degradation of organic dye. The Ag@Si NWs exhibited high potential and sensitivity for the selective and quantitative detection of different organic molecules at extremely low concentration down to 10(-12) M by surface-enhanced Raman scattering and 10(-11) M by fluorescence-based detection. These versatile properties of the Ag@Si NWs open up opportunities for a variety of energy and environmental applications, such as white light emission, solar cell, artificial photosynthesis, disposal of organic pollutant and bio-chemical sensors etc. (C) 2018 Elsevier Inc. All rights reserved.