Black TiO2 nanobelts/g-C3N4 nanosheets Laminated Heterojunctions with Efficient Visible-Light-Driven Photocatalytic Performance
Black TiO2 nanobelts/g-C3N4 nanosheets laminated heterojunctions (b-TiO2/g-C3N4) as visible-light-driven photocatalysts are fabricated through a simple hydrothermal-calcination process and an in-situ solid-state chemical reduction approach, followed by the mild thermal treatment (350 °C) in argon atmosphere. The prepared samples are evidently investigated by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption, and UV-visible diffuse reflectance spectroscopy, respectively. The results show that special laminated heterojunctions are formed between black TiO2 nanobelts and g-C3N4 nanosheets, which favor the separation of photogenerated electron-hole pairs. Furthermore, the presence of Ti3+ and g-C3N4 greatly enhance the absorption of visible light. The resultant b-TiO2/g-C3N4 materials exhibit higher photocatalytic activity than that of g-C3N4, TiO2, b-TiO2 and TiO2/g-C3N4 for degradation of methyl orange (95%) and hydrogen evolution (555.8 μmol h−1g−1) under visible light irradiation. The apparent reaction rate constant (k) of b-TiO2/g-C3N4 is ~9 times higher than that of pristine TiO2. Therefore, the high-efficient laminated heterojunction composites will have potential applications in fields of environment and energy.
The samples are characterized by XRD to identify the phase composition of the samples. Figure 1 shows the XRD patterns of TiO2, b-TiO2, g-C3N4, TiO2/g-C3N4, b-TiO2/g-C3N4 composites. For pure TiO2, the peaks at around 25.3, 37.8, 47.9, 53.8, 55.1, 62.7, and 68.7° are ascribed to the (101), (004), (200), (105), (211), (204), and (116) crystal planes of anatase TiO226,36, respectively. Moreover, the b-TiO2 still keeps the pristine crystal phase after the process of treatment with NaBH4, indicating that the crystal phase can’t be influenced by NaBH4. However, the XRD pattern of b-TiO2 shows a slightly extending characteristic peak at 25.3°, which may be ascribed to the effect of oxygen vacancies (Ov), leading the disorder-induced lattice37. The component g-C3N4 is characterized by two diffraction peaks at around 13.1° and 27.4° are attributed to the (100) plane and (002) plane, which correspond to in-planar structural packing and inter-planar stacking peaks of the aromatic system38,39, respectively. For the TiO2/g-C3N4 and b-TiO2/g-C3N4 samples, the XRD patterns show the characteristic diffraction peaks of both anatase and g-C3N4, indicating that the composites consisted of both anatase TiO2 and g-C3N4. No other characteristic peaks are found, revealing the high purity of the as-prepared samples.
Photocatalytic hydrogen evolution
Photocatalytic hydrogen evolution tests were carried out in an online photocatalytic hydrogen generation system (AuLight, Beijing, CEL-SPH2N) at room temperature. The experiments were carried out by taking 50 mg of photocatalysts in a 100 mL of aqueous solution containing the 80 mL of deionized water and 20 mL of methanol used as the sacrificial reagent in closed-gas circulation reaction cell. Prior to the reaction, the system was vacuumized completely to remove O2 and CO2 dissolved in water. Then, the mixture solution was irradiated by a 300 W Xeon-lamp equipped with an AM 1.5 G filter (Oriel, USA). The hydrogen was periodically analyzed using an on-line gas chromatography with the interval of each 1 h (SP7800, TCD, molecular sieve 5 Å, N2 carrier, Beijing Keruida, Ltd).