Efficient Photoelectrochemical Hydrogen Generation Based on Core Size Effect of Heterostructured Quantum Dots.

Colloidal quantum dots (QDs) are shown to be effective as light-harvesting sensitizers of metal oxide semiconductor (MOS) photoelectrodes for photoelectrochemical (PEC) hydrogen (H2) generation. The CdSe/CdS core/shell architecture is widely studied due to their tunable absorption range and band alignment via engineering the size of each composition, leading to efficient carrier separation/transfer with proper core/shell band types. However, until now the effect of core size on the PEC performance along with tailoring the core/shell band alignment is not well understood. Here, by regulating four types of CdSe/CdS core/shell QDs with different core sizes (diameter of 2.8, 3.1, 3.5, and 4.8 nm) while the thickness of CdS shell remains the same (thickness of 2.0 ± 0.1 nm), the Type II, Quasi-Type II, and Type I core/shell architecture are successfully formed. Among these, the optimized CdSe/CdS/TiO2 photoelectrode with core size of 3.5 nm can achieve the saturated photocurrent density (Jph) of 17.4 mA cm-2 under standard one sun irradiation. When such cores are further optimized by capping alloyed shells, the Jph can reach values of 22 mA cm2 which is among the best-performed electrodes based on colloidal QDs.

Surface Stoichiometry Control of Colloidal Heterostructured Quantum Dots for High-Performance Photoelectrochemical Hydrogen Generation.

Manipulating the separation and transfer behaviors of charges has long been pursued for promoting the photoelectrochemical (PEC) hydrogen generation based on II-VI quantum dot (QDs), but remains challenging due to the lack of effective strategies. Herein, a facile strategy is reported to regulate the recombination and transfer of interfacial charges through tuning the surface stoichiometry of heterostructured QDs. Using this method, it is demonstrated that the PEC cells based on CdSe-(Sex S1- x )4 -(CdS)2 core/shell QDs with a proper Ssurface /Cdsurface ratio exhibits a remarkably improved photocurrent density (≈18.4 mA cm-2 under one sun illumination), superior to the PEC cells based on QDs with Cd-rich or excessive S-rich surface. In-depth electrochemical and spectroscopic characterizations reveal the critical role (hole traps) of surface S atoms in suppressing the recombination of photogenerated charges, and further attribute the inferior performance of excessive S-rich QDs to the impeded charge transfer from QDs to TiO2 and electrolyte. This work puts forward a simple surface engineering strategy for improving the performance of QDs PEC cells, providing an efficient method to guide the surface design of QDs for their applications in other optoelectronic devices.

Use of tyrosinase-inorganic salt hybrid nanoflowers and tyrosinase-MOF hybrid composites for elimination of phenolic pollutants from industrial wastewaters.

Removal of phenolic pollutants from industrial wastewaters is always an important practical problem. Use of enzymes for dephenolization provides a green solution. In this work, enzymatic methods were developed by employing mushroom tyrosinase immobilized as enzyme-Cu3(PO4)2 hybrid nanoflowers and enzyme-metal organic framework (i.e., ZIF-8 and HKUST-1) hybrid composites, which were shown to be superior to processes mediated by tyrosinase immobilized on other supports in both dephenolization efficiency and reusability. Comparatively, tyrosinase@Cu3(PO4)2 and tyrosinase@HKUST-1 were better than tyrosinase@ZIF-8 in both specific activity and dephenolization efficiency. Typical phenolic pollutants, including 3 monophenols (phenol, p-cresol, p-chlorophenol) and 3 bisphenols (BPA, BPB, BPF), can be completely eliminated within 0.5-4 h. The dephenolization order was discussed based on the enzyme's substrate specificity. The operability and reusability of these hybrid biocomposites were highly improved by entrapping into alginate gels or by incorporating with modified magnetic Fe3O4 nanoparticles. Particularly, the magnetic biocatalyst was prepared via a facile one-pot/one-step de novo synthetic strategy, optimized by using response surface methodology (RSM). The as-prepared magnetic tyrosinase@mHKUST-1 retained a high dephenolization efficiency of 81% after 10 cycles and was effective for continuous dephenolization for at least 24 h. These hybrid biocomposites were also successfully applied to treatment of real industrial wastewater from a coke plant.