| Based on the current rate of energy consumption, our world, relying heavily on non-renewable energy, could not survive by the end of this century. Solar energy is a free, inexhaustible resource, while hydrogen provides us with energy without producing greenhouse gases, which makes it the best choice for sustainable development. Solar fuels production via the photoelectrochemical (PEC) way is an attractive alternative. In PEC water splitting cells, the photoanode is one of the most critical parts of them. (Oxy) nitride is emerging as a new class of suitable photoanode material. Having a theoretical 18.5% solar-to-hydrogen efficiency, LaTaON2 is anticipated to be an efficient photoanode material. However, LaTaON2 has been reported to exhibit very low solar photocurrent of 0.15 mA cm-2 at 1.6 VRHE in the literature, which suggest there may be serious carrier recombination. Therefore, there is plenty room for improvement.This work is focus on improving the solar photocurrent of LaTaON2 granular electrodes. According to the experimental results, the factors that hamper the performance of LaTaON2 granular electrode have been found. The density of solid interfaces, including interface between particles and interface between grains in the particle, became the main barrier for charge transport, while the slow process of water oxidation adds insult to injury. In this work, appropriate strategies have been made to overcome these problems. The solar photocurrent of LaTaON2 achieves 2.1 mA cm2 at 1.6 VRHE, an order of magnitude higher than the previously-reported value. This dramatic enhancement is ascribed to hybrid structure of nanoparticles and porous microparticles, as well as CoOx modification. The main contents are as follows:Increasing the photocurrent of the LaTaON2 granular electrode via interfacial engineering. The LaTaON2 powders with different morphologies and sizes were prepared by flux-assisted ammonolysis derived from the precursor sintering at various temperatures. The granular electrodes were fabricated by electrophoretic deposition with a subsequent necking procedure. The granular electrodes (PC1350) derived from precursor sintered at 1350℃ have the highest photocurrent (0.58 mA cm-2 atl.6 VRHE) under visible light irradiation among the sample granular electrodes. For the granular electrodes derived from the precursor sintered at no higher than 1250℃, the total density of the solid interfaces, including interface between the particles and interface between the grains within the particle, is the limiting factor for charge transport. The precursors sintered at no lower than 1350℃ underwent topological chemical transformation to form porous smples. Thus, part of the solid interfaces change into solid-liquid interface, which facilates the photo-generated holes separation. It should be noted that the transparent conductive substrate should be covered by dense semiconductor layer or insulator layer. Otherwise, there would be severe back reaction, in the case of 1450℃, on the bare transparent conductive substrate, which would greatly impair the photocurrent.Increasing the photocurrent of the LaTaON2 granular electrode via loading of CoOx oxygen evolution catalyst with post heat treatment under NH3. The CoOx oxygen evolution catalyst was loaded on LaTaON2 granular electrodes via (photo)electrodeposition. The CoOx loaded granular electrode has a solar photocurrent of 1.6 mA cm-2 at 1.6 VRHE, but with an increased onset potential. After post heat treatments under different atmospheres, air and nitrogen are found to be harmful for the CoOx loaded granular electrodes, while the ammonia is in favor of the improvement of the photocurrent. Therefore, the solar photocurrent of the CoOx loaded granular electrodes increased from 0.8 mA cm"2 to 2.1 mA cm-2, after a post heat treatment under ammonia. The CoOx nanospheres, recrystallizing from the as-deposited amorphous layer after annealed under NH3, are observed by HRTEM (High Resolution Transmission Electron Microscopy). These CoOx nano-spheres on LaTaON2 particles facilitate the formation of the Schottky nano-junctions accelerating the hole transport, thereby leading to ca.160% increase in the photocurrent at 1.6 VRHE. Meanwhile, some possible reasons for the decay of photocurrent and the improvement strategies have also been proposed.This work sheds light on on the construction of photoanodes, especially for (oxy)nitrides. In the near future, practical PEC water splitting cells could be emerged, as well as the systems of gas separation, hydrogen strorage&transport, the utilization of hydrogen. More deep insight into the carriers transport and the mechanism of surface reaction should be provided. Moreover, exploreing new territory of photoelectrochemical synthesis would be refreshing. |
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