Main Research Interests

Main Research Interests

The general research interest of Sun group is to challenge the so called “Holy Grail” of artificial photosynthesis, including molecular solar cells and solar fuels, with the aim to accelerate fundamental research to drive a conceptual transition from fossil fuels to solar energy systems. For solar cells, the Sun group is interested in Grätzel type of solar cells with cheap molecular components such as organic dyes, and organic electrolytes; For solar fuels, the Sun group has been trying to create man-made devices with molecular components, including visible light driven water splitting to generate hydrogen. These include catalytic water oxidation and hydrogen production with molecular catalysts inspired from natural Photosystem II and Hydrogenases, interfacial studies of molecular components (such as dyes, catalysts etc) with nano-structured semiconductors (TiO₂, NiO etc), assembly of molecular devices for total water splitting. Reduction of CO₂ to CO, HCOOH, MeOH or even polycarbonates by electrochemical and photochemical methods is also within the interest of Sun group. The success of these projects will have a great impact on our society by offering sustainable energy systems without the costs of environment, and provides us with endless energy sources in the future.

On-going research projects

1. Dye-sensitized solar cells (DSSCs) based on organic chromophores

Design and synthesis of organic dyes for sensitization of n-type nanostructured semiconductors such as TiO₂ and p-type nanostructured semiconductors such as NiO.

2. New hole transport materials for solid state dye-sensitized solar cells (sDSSCs)

Design and synthesis of new hole transport materials to replace the commonly used Spiro-OMeTAD for solid state dye-sensitized solar cells (sDSSCs).
 

3. Bio(Photosystem II)-inspired molecular catalysts for water oxidation

Design, synthesis and catalytic properties of molecular catalysts based on transition metal complexes, such as Ru, Co, Cu and Fe complexes for chemical driven, electrochemical driven and visible light driven water oxidation.

4. Molecular complexes for catalytic hydrogen production

Synthesis and catalytic properties of Fe₂S₂ complexes as active site models of FeFe-hydrogenases for electrochemical and photochemical hydrogen generation. Co and Ni complexes as molecular catalysts for hydrogen generation.

5. Functional devices based on molecular catalysts for light-driven total water splitting

To immobilize the molecular water oxidation catalyst and molecular hydrogen generation catalyst on respective anode and cathode electrode for electrochemical and photochemical total water splitting.

Brief account of previous research activities in Sun group

1.   Photochemistry and photophysics of stilbene derivatives, using the techniques of laser flash photolysis and pulse radiolysis. For example, the protection of single or double strand DNA with ruthenium tris-bipyridyl complexes against OH^· radical attack studied by pulse radiolysis technique.

2.   Synthesis, characterization and photo-induced electron transfer studies of biomimetic model systems for the electron acceptor side of Photosystem II. For examples, tetraphenylporphyrins convalently linked to crown-ether quinones with different bridges (Angew. Chem. Int. Ed. Engl. 1994, 33, 2318-2320. Tetrahedron 1995, 51, 3535-3548. Solar Energy Mater. & Solar Cells 1995, 38, 91-110); photoinduced electron transfer and the subsequent charge separation studied in the nametic phase of liquid crystals (Magn. Reson. Chem. 1995, 33, Special Issue, S28-S33). This system could be used as light sensitive molecular probe to detect some metal ions such as Na⁺ and K⁺.

3.   Synthesis, characterization and photo-induced electron transfer studies of biomimetic systems for the electron donor side of Photosystem II. A group of ruthenium-manganese binuclear complexes and porphyrin-manganese complexes have been synthesized with variation of bridging ligands and distances between ruthenium and manganese (J. Am. Chem. Soc. 1997, 119, 6996-7004. J. Phys. Chem. A 1998, 102, 2512-2518. Biochim. Biophys. Acta 1998, 1365, 193-199. Eur. J. Inorg. Chem. 2001, 1019-1029. Inorg. Chem. 2002, 41, 1534-1544).

4.   Ru-complex covalently linked to L-tyrosine has been designed, synthesized and characterized. Photo-induced electron transfer studies showed that tyrosyl radical was generated in a similar way to the Tyrosine_Z radical in natural PS II (J. Am. Chem. Soc. 1997, 119, 10720-10725). It was also shown that the photo-generated tyrosyl radical could oxidize a Mn(III,III) dimer intermolecularly, resulting in the formation of high valence Mn(III,IV) complex (J. Am. Chem. Soc. 1999, 121, 89-96).

5.   It was found that hydrogen bonded tyrosine strongly promoted the electron transfer from the tyrosine to the photo-generated Ru(III), closely mimicking the natural system (J. Am. Chem. Soc. 1999, 121, 6834-6842. J. Am. Chem. Soc. 2000, 122, 3932-3936) of PSII.

6.   It was the first time that a Mn dimer has been covalently linked to the Ru complex, and a fast intramolecular electron transfer from the Mn(II,II) dimer to the photogenerated Ru(III) has been observed (J. Inorg. Biochem. 2000, 78, 15-22. Spectrochimica Acta A, 2001, 37, 2145-2160. J. Inorg. Biochem. 2002, 91, 159-172. Inorg. Chem. 2003, 42, 7502-7511).

7.   Electron donor-acceptor triads based on benzoquinone acceptor linked to a photosensitive Ru(bpy)₃²⁺ complex have been synthesized and characterized. A fairly long lived (lifetime 80 ns) charge separated state with a high yield (>90 %) was observed in one of the supramolecular complexes after photo-induced electron transfer processes. Also, a series of ruthenium(II) polypyridine complexes linked to naphthalene diimide have been synthesized and characterized. Following excitation of the chromophore, a relatively long-lived charge-separated state was observed in one dyad supermolecule (Inorg. Chem. 2003, 42, 5173-5184. Inorg. Chem. 2003, 42, 2908-2918).

8.   We have managed to synthesize phthalocyanines with different functional groups. Thus, a metal free and a Zn phthalocyanines with tetracarboxylate ethyl esters have been synthesized and characterized. By using a novel anchoring method, nanostructured TiO₂ electrodes sensitized with these phthalocyanines have been made (Langmuir 2001, 17, 2743-2747). The performance of the solar cells based on these electrodes have been measured. To improve the efficiency of these systems, another phthalocyanine dye ZnPcTyr was designed and synthesized (J. Am. Chem. Soc. 2002, 124, 4922-4932). The synthesis of this molecule was focused on avoiding aggregation on the TiO₂ oxide surface. We have managed to prepare phthalocyanine compounds with Ru(II) ion. Two substituted pyridines are coordinated to the Ru(II) ion as axial ligands (J. Porphyrins Phthalocyanines 2002, 6, 217-224). Absorption spectroscopy suggests that in ethanol solution of such a compound, no aggregations are found even at a high concentration.

9.   A self-assembly monolayer of carotenoid and pheophytin on the surface of nanostructured TiO2 has been achieved. Photoinduced electron transfers of this system has been studied by femtosecond pump-probe technique (J. Am. Chem. Soc. 2002, 124, 13949-13957. J. Am. Chem. Soc. 2004, 126, 3066-3067), showing very fast electron injection from the S2 excited state.

10. As a biomimetic model of active site of Fe-only hydrogenases, a Fe2S2 dinuclear complex with a functional amino group has been synthesized (Chem. Eur. J. 2003, 9, 557-560). For the first time, a Fe2S2 dinuclear complex has been linked to a redox active species for the study of light induced electron transfer processes. The synthetic procedure and characterization of this complex has been described (Angew. Chem. Int. Ed. 2003, 42, 3285-3288). By using the electrochemical method, we have been successfully demonstrated the hydrogen production catalysed by Fe2S2 dinuclear complexes which are close models to the active site of Fe-only hydrogenase (Angew. Chem. Int. Ed. 2004, 43, 1006-1009. Chem. Eur. J. 2004, 10, 4474-4479. Angew. Chem. Int. Ed. 2004, 43, 3571-3574.).

11. Photo-induced electron transfer has been demonstrated in a three component system containing a electron donor, a photosensitizer and Fe2S2 active site models of Fe-only hydrogenases as catalysts (Chem. Commun. 2006, 305-307. Inorg. Chem. 2006, 45, 9169-9171. Inorg. Chem. 2007, 46, 1981-1991. Inorg. Chem. 2007, 46, 3813-3815). In the presence of acids, visible light driven hydrogen generation has been achieved with the three component systems (Inorg. Chem. 2008, 47, 6948-6955. J. Phys. Chem. B 2008, 112, 8198-8202).

12. Supramolecular systems based on redox active Ru-tris-bipyridine complexes and CBs and CDs have been constructed and shown that they can functionalize as light driven molecular devices (Chem. Commun. 2006, 4195-4197. J. Phys. Chem. B. 2007, 111, 13357-13363. Chem. Commun. 2007, 4734-4736. J. Org. Chem. 2008, 73, 3775-3783, Eur. Org. Biomol. Chem. 2009, 7, 3605-3609, J. Org. Chem. 2009, 1163-1172, Chem. Commun. 2010, 46, 463-465. Chem. Eur. J. 2011, 17, 11604-11612).

13. Biomimetic active site models of FeFe-hydrogenases containing pendant amine ligands have been synthesized (Dalton Trans. 2009, 1919-1926). A iron hydride together with a proto on the pendant amine has been successfully isolated, and the isotope study showed that the hydride and the proton can be exchanged in solution, indicating the possible mechanism for H-H bond formation (Inorg. Chem. 2009, 48, 11551-11558).

14. A library of organic dyes have been designed and synthesized for applications in dye sensitized solar cells based on n-type semiconductors (TiO2) (Chem. Commun. 2006, 2245-2247. Chem. Mater. 2007, 19, 4007-4015. J. Phys. Chem. C. 2007, 111, 1853-1860. J. Org. Chem. 2007, 72, 9550-9556. J. Am. Chem. Soc. 2008, 130, 6259-6266). The efficiency of solar cells based on these organic dyes has reached more than 7%, and their photostability test gave satisfactory results (Angew. Chem. Int. Ed. 2009, 48, 1576-1580). Some of the organic dyes have also been applied to solid state solar cells with the efficiencies of more than 4.6% (J. Phys. Chem. C 2009, 113, 16816-16820. Synthetic Metals 2011, 161, 2280-2283. Adv. Funct. Mater. 2011, 21, 2944-2952. Phys. Chem. Chem. Phys. 2012, 14, 779-789. J. Phys. Chem. C 2012, 116, 18070-18078).

15. We have designed and synthesized organic chromophores for sensitization of p-type semiconductor NiO. A record high IPCE value has been achieved in solar cells based on these organic chromophores (J. Am. Chem. Soc. 2008, 130, 8570-8571; Adv. Mater., 2009, 21, 2993-2996. Adv. Mater. 2010, 22, 1759-1762), providing the possibilities for new generation of solar cells with tandem systems.

16. The electronic and molecular structures of organic dyes on TiO₂ interfaces for solar cell applications have been studied in a core level by photoelectron spectroscopy (J. Phys. Chem. C. 2007, 111, 8580-8586; Phys. Chem. Chem. Phys. 2010, 12, 1507-1517. Phys. Chem. Chem. Phys. 2011, 13, 3534-3546).

17. Several Quantum Rod Sensitized Solar Cells have been developed in our group with promising results (J. Am. Chem. Soc. 2011, 133, 8458-8460. Chem. Eur. J. 2011, 17, 6330-6333. ChemSusChem 2011, 4, 1741-1744. J. Mater. Chem. 2012, 22, 6032-6037), such as Solar Cells Sensitized with Type-II ZnSe-CdS Core/Shell Colloidal Quantum Dots (Chem. Commun. 2011, 47, 1536-1538).

18. A broad range of Iodine-Free Redox Couples for Dye-Sensitized Solar Cells has been developed in our group to replace the classical iodide/tri-iodide redox couple, they all showed better performance (Angew. Chem. Int. Ed. 2010, 49, 7328-7331. J. Am. Chem. Soc. 2011, 133, 9413-9422. Chem. Commun. 2011, 47, 10124-10126. Chem. Eur. J. 2011, 17, 6330-6333. J. Mater. Chem. 2011, 21, 10592-10601. J. Mater. Chem. 2011, 21, 5573-5575. Energy Environ. Sci. 2012, 5, 9752-9755. Angew. Chem. Int. Ed. 2012, 51, 9896-9899).

19. As bioinspired(Photosystem II) water oxidation catalysts, a series of Ru complexes containing carboxylate ligands have been synthesized. It has been shown that these complexes can work as good catalysts for water oxidation driven by chemical oxidants, electrochemistry and visible light (Inorg. Chem. 2009, 48, 2717-2719; Inorg. Chem. 2010, 49, 209-215; Chem. Eur. J. 2010, 16, 4659-4668. Angew. Chem. Int. Ed. 2010, 49, 8934-8937. ChemSusChem 2011, 4, 238-244. Chem. Eur. J. 2011, 17, 9520-9528). During the catalytic water oxidation process, a very important intermediate, a high valent Ru(IV)Ru(IV) dimer with water molecules as seventh ligands coordinated to the Ru ions has been successfully isolated and structurally characterized by single crystal X-ray (J. Am. Chem. Soc. 2009, 131, 10397-10399). DTF calculation showed that mechanism for O-O bond formation goes via Ru(IV)-O radical coupling reaction (Angew. Chem. Int. Ed. 2010, 49, 1773-1777. Chem. Eur. J. 2011, 17, 8313-8317. ).

20. Light-driven water oxidation catalyzed by a supramolecular assemblies (Angew. Chem. Int. Ed. 2012, 51, 2417-2420) and in functional devices (Chem. Commun. 2010, 46, 7307-7309. Chem. Commun. 2012, 48,10025-10027) have been demonstrated. A proof of concept has been published by us (Chem. Commun. 2012, 48, 988-990) to immobilize a water reduction molecular catalyst on the surface of p-type semiconductor NiO as the cathode for light driven hydrogen generation.

21. A series of advanced water oxidation catalysts, based on Ru complexes and non-covalent pi-pi interactions of axial ligands have been invented. They have shown catalytic water oxidation by Ce(IV) with record high turnover frequency of more than 300 per second (Nature Chem. 2012, 4, 418-423. J. Am. Chem. Soc. 2012, 134, 18868-18880) and high stability with turnover numbers of more than 55 000 (PNAS 2012, 109, 15584-15588).