Ag-doping　TiO2　composite　nanotubes　(Ag-TNTs)　were　synthesized　by　alkaline　fusion　followed　by　hydrothermal　treatment.　The　microstructure　and　morphology　of　the　materials　were　characterized　by　XRD,　TEM,　XPS,　SPS　(surface　photovoltage　spectroscopy),　FISPS　(electric　field-induced　surface　photovoltage　spectroscopy)　and　Raman　spectroscopy.　First-principles　calculations　based　on　density-functional　theory　(DFT)　showed　the　formation　of　several　impurity　levels　near　the　top　of　the　valence　band　in　the　band　gap　(Eg)　of　rutile　TiO2　due　to　Ag　doping.　A　＇double　junction＇　is　proposed,　involving　a　Schottky　junction　and　p-n　junction　(denoted　as　＇Ag-p-n　junction＇)　occurring　between　the　Ag　particles　and　the　nanotube　surface,　as　well　as　forming　inside　TiO2　nanotubes,　respectively.　The　strongly　built-in　electric　field　of　the　junctions　promotes　the　separation　of　photo-holes　and　photoelectrons,　enhancing　the　photocatalytic　efficiency.　XRD　results　indicated　that　the　composite　Ag-TNTs　exist　as　a　mixture　of　anatase　and　rutile　phases.　XPS　results　showed　that　Ti4+　is　the　primary　state　of　Ti.　Raman　spectral　analysis　of　Ag-TNTs　revealed　the　presence　of　a　new　peak　at　271　cm-1.　The　red-shift　of　the　absorption　light　wavelength　of　Ag-TNTs　was　0.16　eV　(20　nm)　due　to　a　considerable　narrowing　of　Eg　by　the　existing　impurity　levels.　©　2013　Elsevier　Ltd.