Research groups led by Prof. LI Can and Prof. LI Rengui from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), together with Prof. YAN Jianchang’s group from the Institute of Semiconductors of CAS, achieved new advances in spatial charge separation for artificial photosynthesis. They revealed that surface polarity-induced surface electric field can effectively promote spatial separation of photogenerated charges and greatly enhance the activity of photocatalytic overall water splitting (OWS).
Artificial photosynthesis for solar fuel is considered as a "Holy Grail" in scientific research, the core issue of the field is the photogenerated charge separation. It remains a hot and challenging topic to develop effective strategies to promote the separation and transportation of photogenerated charges. Prof. LI Can's group has been dedicated to this cutting-edge issue for artificial photosynthetic solar fuels and achieved a series of progresses.
They have proposed phase-junction-promoted charge separation (Angew. Chem. Int. Ed., 2008; Angew. Chem. Int. Ed., 2012), unraveled the spatial charge separation between different crystal faces (Nat. Commun., 2013), designed a strategy for charge separation on high-symmetric semiconductor materials (Energy Environ. Sci., 2016), and developed novel techniques to image photogenerated charges (Angew. Chem. Int. Ed., 2015; Nat. Energy, 2018), the researchs are well recognized by the academic community.
Although the crystal morphology and crystal facets can be used to modulate the separation of photogenerated, the surface polarity is a common and general intrinsic property of materials. Yet, it is unclear how to rationally design and tune the intrinsic polarity of semiconductor materials and how the surface polarity influence the charge separation.
In this work, a typical semiconductor gallium nitride (GaN) was chosen as a mode. GaN nanorod arrays with definite surface polarity was fabricated through a metal organic vapor deposition combined with ion inductively coupled plasma-assisted etching method. The nanorod arrays are exposed with top polar surface and side nonpolar surface, obvious spatial charge separation can take place between the two surfaces.
The photogenerated electrons are accumulated on the nonpolar surface, while the holes on the polar surface. It is supposed to originate from different surface band bending induced by the different surface dipole moment between the polar and nonpolar surfaces, which drives the photogenerated electrons and holes to migrate selectively, promoting the charge separation effectively.
Furthermore, the photoelectrochemical tests and photocatalytic reactions reveal that the efficiency of charge separation between the polar and nonpolar surface can exceed 80%, about ten times higher than the film counterpart, which is the highest value in this kind of material. Based on this unique charge separation, reduction cocatalyst and oxidation cocatalyst can be deposited on the nonpolar and polar surface respectively. These can enhance the quantum efficiency of photocatalytic overall water splitting from 0.9% to 6.9%.
This work proposes a novel universal strategy to promote charge separation and laid a theoretical foundation of the construction of highly efficient artificial photosynthesis systems. It also provides new insights to understand the intrinsic driving force for photogenerated charge separation.
This work entitled "Surface polarity-induced spatial charge separation boosting photocatalytic overall water splitting on GaN nanorod arrays" was published online in Angew. Chem. Int. Ed. as a Full Article. This work was financially supported by the "Transformational Technologies for Clean Energy and Demonstration", Strategic Priority Research Program of the Chinese Academy of Sciences, Key Research Program of Frontier Sciences of Chinese Academy of Sciences and National Science Foundation of China. Meanwhile, it is also dedicated to 70th anniversary of DICP. (Text by LI Zheng and LI Rengui)