Hongjin Lv

In the last five years and currently, research on solar fuels has been intense and no sub-area in this field has been more active than the development of water oxidation catalysts (WOCs). In this timeframe, a new class of molecular water oxidation catalysts based on polyoxometalates have been reported that combine the advantages of homogeneous and heterogeneous catalysts. This review addresses central issues in green energy generation, the challenges in water oxidation catalyst development, and the possible uses of polyoxometalates in green energy science.

Semiconductor-metal nanoheterostructures, such as CdSe/CdS dot-in-rod nanorods with a Pt tip at one end (or CdSe/CdS-Pt), are promising materials for solar-to-fuel conversion because they allow rational integration of a light absorber, hole acceptor, and electron acceptor or catalyst in an all-inorganic triadic heterostructure as well as systematic control of relative energetics and spatial arrangement of the functional components. To provide design principles of such triadic nanorods, we examined the photocatalytic H2 generation quantum efficiency and the rates of elementary charge separation and recombination steps of CdSe/CdS-Pt and CdS-Pt nanorods. We showed that the steady-state H2 generation quantum efficiencies (QEs) depended sensitively on the electron donors and the nanorods. Using ultrafast transient absorption spectroscopy, we determined that the electron transfer efficiencies to the Pt tip were near unity for both CdS and CdSe/CdS nanorods. Hole transfer rates to the electron donor, measured by time-resolved fluorescence decay, were positively correlated with the steady-state H2 generation QEs. These results suggest that hole transfer is a key efficiency-limiting step. These insights provide possible ways for optimizing the hole transfer step to achieve efficient solar-to-fuel conversion in semiconductor-metal nanostructures.

An all-inorganic, oxidatively and thermally stable, homogeneous water oxidation catalyst based on redox-active (vanadate(V)-centered) polyoxometalate ligands, Na10[Co4(H2O)2(VW9O34)2]·35H2O (Na101-V2, sodium salt of the polyanion 1-V2), was synthesized, thoroughly characterized and shown to catalyze water oxidation in dark and visible-light-driven conditions. This synthetic catalyst is exceptionally fast under mild conditions (TOF > 1 × 10(3) s(-1)). Under light-driven conditions using [Ru(bpy)3](2+) as a photosensitizer and persulfate as a sacrificial electron acceptor, 1-V2 exhibits higher selectivity for water oxidation versus bpy ligand oxidation, the final O2 yield by 1-V2 is twice as high as that of using [Co4(H2O)2(PW9O34)2](10-) (1-P2), and the quantum efficiency of O2 formation at 6.0 μM 1-V2 reaches ∼68%. Multiple experimental results (e.g., UV-vis absorption, FT-IR, (51)V NMR, dynamic light scattering, tetra-n-heptylammonium nitrate-toluene extraction, effect of pH, buffer, and buffer concentration, etc.) confirm that the polyanion unit (1-V2) itself is the dominant active catalyst and not Co(2+)(aq) or cobalt oxide.

Floriated ZnFe2O4 with porous nanorod structures were successfully synthesized via mild hydrothermal and calcination processes by using cetyltrimethylammonium bromide (CTABr) as a template-directing reagent. The resulting ZnFe2O4 was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS) and nitrogen adsorption measurement. It was found that the floriated ZnFe2O4 nanostructures were composed of porous nanorods with an average length of 122 nm and diameter of 29 nm. The obtained ZnFe2O4 with a bandgap of ∼1.94 eV was firstly used as a visible-light-driven photocatalyst for hydrogen production, and exhibits remarkable photostability in an aqueous suspension by using CH3OH as a sacrificial reagent. Moreover, the possible photo-reaction mechanism for the hydrogen production from CH3OH aqueous solution was proposed for better understanding the photocatalytic behavior of ZnFe2O4 without Pt-loading.

A tetra-nickel-containing polyoxotungstate, Na6K4[Ni4(H2O)2(PW9O34)2]·32H2O (Na6K4-Ni4P2), has been synthesized in high yield and systematically characterized. The X-ray crystal structure confirms that a tetra-nickel cluster core [Ni4O14] is sandwiched by two trivacant, heptadentate [PW9O34](9-) POM ligands. When coupled with (4,4'-di-tert-butyl-2,2'-dipyridyl)-bis(2-phenylpyridine(1H))-iridium(III) hexafluorophosphate [Ir(ppy)2(dtbbpy)][PF6] as photosensitizer and triethanolamine (TEOA) as sacrificial electron donor, the noble-metal-free complex Ni4P2 works as an efficient and robust molecular catalyst for H2 production upon visible light irradiation. Under minimally optimized conditions, Ni4P2 catalyzes H2 production over 1 week and achieves a turnover number (TON) of as high as 6500 with almost no loss in activity. Mechanistic studies (emission quenching, time-resolved fluorescence decay, and transient absorption spectroscopy) confirm that, under visible light irradiation, the excited state [Ir(ppy)2(dtbbpy)](+)* can be both oxidatively and reductively quenched by Ni4P2 and TEOA, respectively. Extensive stability studies (e.g., UV-vis absorption, FT-IR, mercury-poison test, dynamic light scattering (DLS) and transmission electron microscopy (TEM)) provide very strong evidence that Ni4P2 catalyst remains homogeneous and intact under turnover conditions.