LAUSR

Abstract Harvesting full spectrum sunlight for direct absorption solar collectors (DASCs) has attracted extensive attention and a number of potential nanofluids have been reported. In the current work, oxygen-deficient TiO2 (TiO2-x) improves the defects of conventional TiO2, making it fascinating optical absorption in the full solar spectrum. In the current work, oxygen-deficient TiO2 is obtained by NaBH4-reduction. The lower the reduction temperature is, the poorer the full solar absorption possesses. 600 °C is an appropriate reducing temperature from the results of XRD and UV–Vis–NIR. TiO2-x is stably dispersed in two oil system (heat transfer oil and silicone oil), leading to good medium-temperature solar thermal conversion. Au nanoparticles can further enhance the full solar absorption of oxygen-deficient TiO2. The highest temperature can be arrived at 91 °C for 100 ppm 5% Au/TiO2-x, 26.6 °C higher than base silicone oil. The excellent full solar absorption properties of TiO2-x and Au/TiO2-x in oil nanofluids bring new paradigms for working fluids in DASCs with medium temperatures.