Mesoporous CuO with full spectrum absorption for photothermal conversion in direct absorption solar collectors

•Facile prepared mesoporous CuO displays a narrower bandgap than non-porous CuO.•Mesoporous CuO shows obvious full and strong solar energy absorption.•Mesoporous CuO nanofluids exhibit good dispersibility in water.•The maximum growth rate of CuO-MS/water nanofluids can arrive as high as 70.05%.