Sensitive photocatalytic reduction of high concentration chromium-containing wastewater driven by one-step hydrothermal synthesis of bi-element modified TiO2

•One step synthesis of N-TiO2-x@C by novel low-temperature hydrothermal.•Nitrogen doping process is prior to carbon coating.•Ti3+/Ov defects and positive charge surface in the N-TiO2-x@C are formed due to interface bonding.•High catalytic performance is reflected in the treatment of g/L level Cr(VI).