The development of artificial photosynthesis system that can efficiently catalyze water oxidation to generate oxygen remains one of the most important challenges in solar energy conversion to chemical energy. In photosystem II (PSII), the Mn 4 CaO 5 cluster adopts a distorted coordination geometry and every two octahedra are linked by di-μ-oxo (edge-shared) or mono-μ-oxo bridges (corner-shared), which is recognized as a critical structure motif for catalytic water oxidation. These structural features provide guidance on the design and synthesis of new water oxidation catalysts. Herein we synthesized a new layered organic cobalt phosphonate crystal, Co 3 (O 3 PCH 2 -10 NC 4 H 7 -CO 2 ) 2 ·5H 2 O (1), and demonstrate it as a heterogeneous catalyst for water oxidation. Its catalytic activity was compared to those of cobalt phoshonates with different structures (2-4) in terms of O 2 evolution rate and O 2 yield under the same reaction condition. The compound with both mono-and di-µ-oxo bridged octahedral cobalt displays superior catalytic activity. In contrast, the presence of only mono-µ-oxo bridged cobalt in the structure results in lower O 2 yield and O 2 evolution rate. Further structural analysis reveals that the presence of longer Co-N bond induces distorted dissymmetry coordination geometry, and consequently facilitates water oxidation. These 15 results provide important insight into the design of water oxidation catalysts. Broader context Splitting water into hydrogen and oxygen is a promising pathway20 for solar energy conversion to chemical energy. Water oxidation to oxygen (O 2 ) is regarded as a major challenge toward artificial photosynthesis. Recently, most water oxidation systems have been developed based on μ-oxo bridging cobalt heterogeneous water oxidation catalysts. Major advances have been made on 25 electrochemically deposited catalytic films or colloidal suspensions of nanoparticles. Though these cobalt-based materials have been shown as promising catalyst candidates for water oxidation, it is difficult to establish the structureperformance relationship owing to their ill-defined chemical 30 structures. In this work, we synthesized a cobalt phosphonate crystal and demonstrate its high catalytic performance for water oxidation. Comparative analysis of a series of cobalt phosphonates with different types of crystal structures reveals the potential structural features for efficient water oxidation. This 35 study provides valuable insight into the design and synthesis of efficient water oxidation catalysts. 65 polyoxometalate (POM) cobalt complexes, 18-20 Co 3 O 4 , 21 and Li 2 Co 2 O 4 . 22 The cubic Co 4 O 4 core is regarded as the crucial structural feature for efficient catalysis of water oxidation. Interestingly, some cobalt solid catalysts without Co 4 O 4 topology have also been reported toward catalyzing water oxidation. For 70 example, the Co-based perovskite catalyst displays an electrocatalytic activity in water oxidation reaction in alkaline solution. 23, 24 Recently, Co-Fe Prussian blue...

Metallic Ag nanoparticles deposited on BiVO4promote the separation of photo-induced electron/hole pairs for the generation of reactive oxygen species, leading to excellent photocatalytic disinfection activity under both visible light and natural sunlight.

We introduced photon-trapping microstructures into GeSn-based photodetectors for the first time, and achieved high-efficiency photo detection at 2 µm with a responsivity of 0.11 A/W. The demonstration was realized by a GeSn/Ge multiple-quantum-well (MQW) p-i-n photodiode on a GeOI architecture. Compared with the non-photon-trapping counterparts, the patterning and etching of photon-trapping microstructure can be processed in the same step with mesa structure at no additional cost. A four-fold enhancement of photo response was achieved at 2 µm. Although the incorporation of photo-trapping microstructure degrades the dark current density which increases from 31.5 to 45.2 mA/cm2 at −1 V, it benefits an improved 3-dB bandwidth of 2.7 GHz at bias voltage at −5 V. The optical performance of GeSn/Ge MQW photon-trapping photodetector manifests its great potential as a candidate for efficient 2 µm communication. Additionally, the underlying GeOI platform enables its feasibility of monolithic integration with other photonic components such as waveguide, modulator and (de)multiplexer for optoelectronic integrated circuits (OEICs) operating at 2 µm.

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