CNx was deposited by chemical vapor deposition of urea pyrolysis products according to our previous report. Two pieces of TiO2 electrodes were placed in a Schlenk tube connected to a round-bottom flask containing 1 g of urea. Before the CNx deposition was started, the muffle oven (Carbolite, Germany) was preheated to 425 °C. Then, the reactor was directly placed into the muffle oven and heated at 425 °C for 30 min. Finally, the reactor was cooled down to room temperature in air. The resulting electrodes are denoted as CNx-TiO2. Material characterization: The electronic absorption spectra were measured using a UV-Vis spectrophotometer (UV-2600, Shimadzu, Japan) equipped with the integrating sphere and the absorptance (Abs.) was calculated by the equation: