P-O linked g-C3N4/TiO2-nanotubes (TNTs) Z-scheme composites were fabricated using a solid sublimation and conversion means, followed by an impregnation method for the first time, and they were characterized by scanning electrons microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and UV-visible diffuse reflection spectroscopy. The photocatalytic activity was investigated by CO2 reduction and the charge separation was investigated by surface photovoltage spectroscopy and produced (OH)-O-center dot radicals. Results showed that CO2 reduction products of acetic, methanol and formic acids with yields of 46.9 +/- 0.76 mg L-1 h(-1), 38.2 +/- 0.69 mg L-1 h(-1) and 28.8 +/- 0.64 mg L-1 h(-1) were obtained using the optimized sample, which were 3.3, 3.5 and 3.8 times of bare TNTs. The optimized sample also showed a transient photocurrent of 0.85 mA cm(-2). The enhanced performance was due to the synergistic effect of g-C3N4 and P-O links. The modified g-C3N4 enhanced visible light absorption and charge separation. The P-O links between g-C3N4 and TNTs were benefit for charge transfer. This work contributes to help us comprehend the mechanism of charge transfer and separation in Z-scheme composites, and presents a workable approach for preparing efficient TNTs-based photocatalysts for energy production. (C) 2019 Elsevier Ltd. All rights reserved.