Inorganic core-shell assemblies for closing the artificial photosynthetic cycle.

Co oxide (Co(3)O(4)) nanotubes are shown to act as an efficient water oxidation catalyst when driven with a visible light sensitizer (pH 7). The nanotubes form the core of a Co(3)O(4)-SiO(2) core-shell nanotube design for separating the carbon dioxide photoreduction from the oxygen evolution reaction. Amorphous dense phase silica of a few nanometers depth is shown to conduct protons while blocking molecular oxygen. Organic molecular wires embedded in the silica shell provide controlled charge transport between the light absorber on one side and the Co(3)O(4) catalyst on the other side. Hence, the silica shell is suitable as a membrane of an assembly for closing the photosynthetic cycle on the nanometer scale under product separation.

Light induced carbon dioxide reduction by water at binuclear ZrOCo(II) unit coupled to Ir oxide nanocluster catalyst.

An all-inorganic polynuclear unit consisting of an oxo-bridged binuclear ZrOCo(II) group coupled to an iridium oxide nanocluster (IrO(x)) was assembled on an SBA-15 silica mesopore surface. A photodeposition method was developed that affords coupling of the IrO(x) water oxidation catalyst with the Co donor center. The approach consists of excitation of the ZrOCo(II) metal-to-metal charge-transfer (MMCT) chromophore with visible light in the presence of [Ir(acac)3] (acac: acetylacetonate) precursor followed by calcination under mild conditions, with each step monitored by optical and infrared spectroscopy. Illumination of the MMCT chromophore of the resulting ZrOCo(II)-IrO(x) units in the SBA-15 pores loaded with a mixture of (13)CO2 and H2O vapor resulted in the formation of (13)CO and O2 monitored by FT-IR and mass spectroscopy, respectively. Use of (18)O labeled water resulted in the formation of (18)O2 product. This is the first example of a closed photosynthetic cycle of carbon dioxide reduction by water using an all-inorganic polynuclear cluster featuring a molecularly defined light absorber. The observed activity implies successful competition of electron transfer between the IrO(x) catalyst cluster and the transient oxidized Co donor center with back electron transfer of the ZrOCo light absorber, and is further aided by the instant desorption of the CO and O2 product from the silica pores.

Efficient and sustained photoelectrochemical water oxidation by cobalt oxide/silicon photoanodes with nanotextured interfaces.

Plasma-enhanced atomic layer deposition of cobalt oxide onto nanotextured p(+)n-Si devices enables efficient photoelectrochemical water oxidation and effective protection of Si from corrosion at high pH (pH 13.6). A photocurrent density of 17 mA/cm(2) at 1.23 V vs RHE, saturation current density of 30 mA/cm(2), and photovoltage greater than 600 mV were achieved under simulated solar illumination. Sustained photoelectrochemical water oxidation was observed with no detectable degradation after 24 h. Enhanced performance of the nanotextured structure, compared to planar Si, is attributed to a reduced silicon oxide thickness that provides more intimate interfacial contact between the light absorber and catalyst. This work highlights a general approach to improve the performance and stability of Si photoelectrodes by engineering the catalyst/semiconductor interface.

Forming heterojunctions at the nanoscale for improved photoelectrochemical water splitting by semiconductor materials: case studies on hematite.

In order for the future energy needs of humanity to be adequately and sustainably met, alternative energy techniques such as artificial photosynthesis need to be made more efficient and therefore commercially viable. On a grand scale, the energies coming to and leaving from the earth are balanced. With the fast increasing waste heat produced by human activities, the balance may be shifted to threaten the ecosystem in which we reside. To avoid such dire consequences, it is necessary to power human activities using energy derived from the incoming source, which is predominantly solar irradiation. Indeed, most life on the surface of the earth is supported, directly or indirectly, by photosynthesis that harvests solar energy and stores it in chemical bonds for redistribution. Being able to mimic the process and perform it at high efficiencies using low-cost materials has significant implications. Such an understanding is a major intellectual driving force that motivates research by us and many others. From a thermodynamic perspective, the key energy conversion step in natural photosynthesis happens in the light reactions, where H2O splits to give O2 and reactive protons. The capability of carrying out direct sunlight-driven water splitting with high efficiency is therefore fundamentally important. We are particularly interested in doing so using inorganic semiconductor materials because they offer the promise of durability and low cost. In this Account, we share our recent efforts in bringing semiconductor-based water splitting reactions closer to reality. More specifically, we focus on earth-abundant oxide semiconductors such as Fe2O3 and work on improving the performance of these materials as photoelectrodes for photoelectrochemical reactions. Using hematite (α-Fe2O3) as an example, we examine how the main problems that limit the performance, namely, the short hole collection distance, poor light absorption near the band edge, and mismatch of the band edge energetics with those of water redox reactions, can in principle be addressed by adding nanoscale charge collectors, forming buried junctions, and including additional light absorbers. These results highlight the power of forming homo- or heterojunctions at the nanoscale, which permits us to engineer the band structures of semiconductors to the specific application of water splitting. The key enabling factor is our ability to synthesize materials with precise control over the dimensions, crystallinity, and, most importantly, the interface quality at the nanoscale. While being able to tailor specific properties on a simple, earth-abundant device is not straightforward, the approaches we report here take significant steps towards efficient artificial photosynthesis, an energy harvesting technique necessary for the well-being of humanity.

Spontaneous formation of hierarchically structured curly films of nickel carbonate hydrate through drying.

Novel curly nickel carbonate hydrate film superstructures can be prepared for the first time via a facile drying process of the films formed on air/solution interface in the presence of double hydrophilic copolymer or polyelectrolyte additives. As-prepared curly film patches with average edge sizes of several hundred micrometers display adjustable curly features along different orientation. The coiling up degree of the film edges is strongly dependent on the polymer concentration in bulk solution. Most of these curly structures have a relatively smooth outer surface; however, the microstructures of the outer surface of curly films formed show porous network-like features. In addition, using different kinds of nickel salts can produce distinct curly film samples. A possible formation mechanism of the curly film structure has been proposed. The multiple interaction modes between nickel salt precursors and polymer can favor the self-organization of the film formed at the air/solution interface. This approach is expected to be extended for producing a variety of curly hierarchical structures.

Preparation of chitosan derivative with polyethylene glycol side chains for porous structure without specific processing technique.

The graft copolymer, chitosan-g-polyethylene glycol (PEG), was prepared through graft polymerization of PEG chains to chitosan due to the esterification reaction between PEG and 6-O-succinate-N-phthaloyl-chitosan (PHCSSA). The graft copolymer with porous structure was observed from scanning electron micrographs. It is a potential method to combine chitosan with the hydrophilic synthetic polymers. The graft reaction was carried out in homogeneous system and yielded copolymers with high grafting content. FTIR, NMR, XRD, DSC, spectrofluorophotometer and SEM were detected to characterize the copolymer.