Constructing porous carbon nitride nanosheets for efficient visible-light-responsive photocatalytic hydrogen evolution

Effective nitrogen doping of TiO2 polymorphs at mild temperatures for visible-light-responsive hydrogen evolution

Herein, a mild-temperature nitrogen doping route with the urea-derived gaseous species as the active doping agent is proposed to realize visible-light-responsive photocatalytic hydrogen evolution both for the anatase and rutile TiO2. DFT simulations reveal that the cyanic acid (HOCN), derived from the decomposition of urea, plays a curial role in the effective doping of nitrogen in TiO2 at mild temperatures. Photocatalytic performance demonstrates that both the anatase and rutile TiO2 doped at mild temperatures exhibit the highest hydrogen evolution rates, although the ones prepared at high temperatures possess higher absorbance in the visible range. Steady-state and transient surface photovoltage characterizations of these doped TiO2 polymorphs prepared at different temperatures reveal that harsh conditions (high temperature reaction) typically result in the formation of intrinsic defects that are detrimental to the transport of the low-energy visible-light-induced electrons, while the mild-temperature nitrogen-doping could flatten the pristine upward band bending without triggering the formation of Ti3+, thus achieving enhanced visible-light-responsive hydrogen evolution rates. We anticipate that our findings will provide inspiring information for shrinking the gap between the visible-light-absorbance and the visible-light-responsiveness in the band engineering of wide-bandgap metal-oxide photocatalysts.

Polymeric Carbon Nitride with Chirality Inherited from Supramolecular Assemblies

The facile synthesis of chiral materials is of paramount importance for various applications. Supramolecular preorganization of monomers for thermal polymerization has been proven as an effective tool to synthesize carbon and carbon nitride-based (CN) materials with ordered morphology and controlled properties. However, the transfer of an intrinsic chemical property, such as chirality from supramolecular assemblies to the final material after thermal condensation, was not shown. Here, we report the large-scale synthesis of chiral CN materials capable of enantioselective recognition. To achieve this, we designed supramolecular assemblies with a chiral center that remains intact at elevated temperatures. The optimized chiral CN demonstrates an enantiomeric preference of ca. 14 %; CN electrodes were also prepared and show stereoselective interactions with enantiomeric probes in electrochemical measurements. By adding chirality to the properties transferrable from monomers to the final product of a thermal polymerization, this study confirms the potential of using supramolecular precursors to produce carbon and CN materials and electrodes with designed chemical properties.

Skin B/N-doped anatase TiO2 {001} nanoflakes for visible-light photocatalytic water oxidation

The limited visible-light-responsive photoactivities of most doped wide-bandgap photocatalysts with widened absorption range have long been the obstacles for the efficient conversion of solar energy to chemical energy by photocatalysis. The weak transport ability of visible-light-induced low-energy charge carriers, and numerous recombination centers arising from the energy-band modifiers along the transport path are two major factors responsible for such a mismatch. A potential solution is to shorten the transport path of photo-induced charges in well-modulated light absorbers with low-dimensional structure and the spatially concentrated dopants underneath their surfaces. As a proof of concept, skin B/N-doped red anatase TiO₂ {001} nanoflakes with the absorption edge up to 675 nm were synthesized in this study. Experimental results revealed that boron dopants in the TiO₂ nanoflakes from the hydrolysis of nanosized TiB₂ played a crucial role in controlling nitrogen doping in the surface layer of the nanoflakes. As visible light excitation occurs at the surface layer, the photons can be sufficiently absorbed by the formed energy levels at the surface layers, and the photogenerated charge carriers can effectively migrate to the surface, thus leading to efficient visible-light-responsive photocatalytic oxygen evolution activity from water oxidation.