Photocatalysis based on semiconductors has attracted particular interest in the past four decades because of its ability to directly convert solar energy into chemical fuels. In recent years, much effort has been devoted to investigating the photocatalytic properties of potassium niobate oxides, K₄Nb₆O₁₇, K₂Nb₂O₆,  and KNbO₃, due to their unique crystal and energy band structures. In particular, K₄Nb₆O₁₇ with a layered anisotropic structure has high potential for photocatalytic applications. However, K₄Nb₆O₁₇, like other UV light responsive semiconductor photocatalysts, cannot exhibit any photocatalytic activity under visible light irradiation, due to its relatively large bandgap (3.2 eV).  Thus, it is highly desirable to develop an effective method to reduce the bandgap of K₄Nb₆O₁₇ for capturing more solar energy. Among various methods for reducing the bandgap of K₄Nb₆O₁₇, doping is the most commonly used method because it can induce intrinsic electronic and band structure changes.  

Although doping of metal ions into the lattice of K₄Nb₆O₁₇ can extend its visible light response, the introduction of dopants can also result in thermal or crystal instability and increased charge carrier recombination centers, and thus low photocatalytic efficiency. Recently, it has been demonstrated that self-doping, different from conventional impurity incorporation, is an effective and promising way to extend the visible light absorption of semiconductor photocatalysts. By introducing Ti³⁺ or oxygen vacancies, both anatase and rutile TiO₂ have shown enhanced visible-light-driven photocatalytic activities. Hu and coworkers found that Ta⁴⁺ self-doped in bulk NaTaO₃ can remarkably narrow its bandgap from 3.94 to 1.70 eV, and thus extend its photoresponse from the UV to the visible light region, leading to enhanced visible-light-driven photocatalytic activity. Nonetheless, to the best of our knowledge, there is no report on Nb⁴⁺ self-doped niobates with high solar absorption and improved photocatalytic H₂ production activity yet. 

Black Nb⁴⁺ self-doped K₄Nb₆O₁₇ microspheres were prepared for the first time through a facile UV light photoreduction method. By the introduction of Nb⁴⁺, the defective K₄Nb₆O₁₇ can harvest the full spectrum of visible light as well as near-infrared light. The black K₄Nb₆O₁₇ microspheres showed improved visible-light-driven photocatalytic H₂ production activity. Importantly, the present synthetic approach is also applicable to the preparation of other Nb⁴⁺ self-doped niobates.  

Chem. Commun.,2014