Electrochemically reduced TiO2 photoanode coupled with oxygen vacancy-rich carbon quantum dots for synergistically improving photoelectrochemical performance

[Display omitted] •Moderate OV are accurately induced into TiO2 by a simple electrochemical reduction.•Near-complete bulk charge separation is achieved for E-TiO2 (ηbulk = 94.7% at 1.23 VRHE).•OV–rich CQDs serve as OECs that sharply promote the OER kinetics (ηsurface = 82.5% at 1.23 VRHE).•OV–rich CQDs greatly enhance the visible light harvesting with increased Jabs (3.41 mA cm−2).•10-fold improvement of photocurrent is achieved for CQDs/E-TiO2 at 1.23 VRHE (2.55 mA cm−2). Severe bulk charge recombination, sluggish oxygen evolution reaction (OER) kinetics and poor visible light harvesting are still the technical bottlenecks of famous TiO2 photoanode for photoelectrochemical (PEC) water splitting. Here, a novel CQDs/E-TiO2 photoanode was designed based on the accurately electrochemical reduction of TiO2 (E-TiO2, Ti4+ + e− → Ti3+) and further modification of oxygen vacancy (OV)-rich carbon quantum dots (CQDs) for synergistically improving PEC performance. The electrochemical reduction creates moderate OV in TiO2, which increase the majority carrier density and provide photoinduced charge traps for sharply increasing the bulk charge separation efficiency (ηbulk). The CQDs modification dramatically improves the surface charge transfer efficiency (ηsurface) by serving as oxygen evolution catalysts (OECs), because the abundant OV in CQDs greatly promote the interfacial OER kinetics. Additionally, the visible light harvesting of TiO2 is significantly improved after CQDs modification. Near-complete bulk charge separation (ηbulk = 94.7%) is achieved for E-TiO2 at 1.23 V vs. RHE (VRHE), which is 4.0 times higher than that of TiO2. The CQDs/E-TiO2 shows the ηsurface of 56.0% at 0.40 VRHE, which is 7.2 times higher than E-TiO2. Therefore, the CQDs/E-TiO2 exhibits remarkable photocurrent densities of 1.50 mA cm−2 at 0.60 VRHE and 2.55 mA cm−2 at 1.23 VRHE, which are 27.0 and 10.0 times higher than TiO2, 3.5 and 1.5 times higher than E-TiO2, respectively.