Qixin Guo

Artificial inorganic leafs for efficient photochemical hydrogen production inspired by natural photosynthesis.

Adv. Mater. 2010, 22, 951–956 2010 WILEY-VCH Verlag Gm Using sunlight to split water molecules and produce hydrogen fuel is one of the most promising tactics for controlling our carbon-based energy ‘‘habit’’. Of the various possible methods, nature provides a blueprint for converting solar energy in the form of chemical fuels. A natural leaf is a synergy of elaborated structures and functional components in order to produce a highly complex machinery for photosynthesis in which light harvesting, photoinduced charge separation, and catalysis modules combine to capture solar energy and split water into oxygen and ‘‘hydrogen’’ (in the form of reducing equivalents) efficiently. Thus, the design of efficient, cost-effective artificial systems by the coupling of leaflike hierarchical structures and analogous functional modules under the guidance of the key steps of natural photosynthesis—capture of sunlight photons, electron–hole separation with long lifetimes, and energy transduction into hydrogen—would be a major advance in the development of materials for energy conversion. Here, we present a general strategy to assemble man-made catalysts (Pt/N-doped TiO2) into leaf-shaped hierarchical structures, named artificial inorganic leaf (AIL), for efficient harvesting of light energy and photochemical hydrogen production. This concept may broaden the horizon for the design of artificial photosynthetic systems based on biological paradigms and provides a working prototype to exploit solar energy for sustainable energy resources. Many research efforts have been carried out to develop artificial photosynthetic systems by constructing a variety of analogous molecular systems consisting of electron donors and acceptors to mimic light-driven charge separation, which occur in photosynthetic reaction centers, or by assembling semiconductor photocatalysts into various nanostructures. Though significant progress has been achieved, most research only focused on the functional imitation of photosynthesis and neglected the structural effect. Actually, the whole structure of natural leaves strongly favors light harvesting: the focusing of light by the lenslike epidermal cells, the multiple scattering and absorbance of light within the veins’ porous architectures, the light propagation in the columnar cells in palisade parenchyma acting as light guides, the enhanced effective light pathlength and light scattering by the less regularly arranged spongy mesophyll cells, and the efficient light-harvesting and fast charge separation in the high surface area three-dimensional constructions of interconnected nanolayered thylakoid cylindrical stacks (granum) in chloroplast. Meanwhile, the photosynthetic pigments in chloroplast successfully perform electron transfer and energy transduction. Thus, in order to mimic a photosynthetic system, it may be necessary for an artificial system to both have similar hierarchical structures for efficient light-harvesting and charge-separation-analogous functional modules, which could i) absorb incident photons, generating excited states, ii) transfer this excitation energy to a donor/ acceptor interface, where photochemical charge separation takes place. Meanwhile, such a system should be able to transfer charge away from the interface in order to limit the rate of wasteful recombination reactions, and iii) it should couple the photochemically generated charges to appropriate catalysts for the production of hydrogen. So our approach for artificial photosynthesis is to construct an artificial leaf by copying the complex architecture of leaves, replace the natural photosynthetic pigments with man-made catalysts, and realize efficient light-harvesting and photochemical hydrogen production. We first demonstrate this new concept with N-doped TiO2, a widely used visible-light-responsive photocatalyst for hydrogen production. Recently, there has been a strong interest in doping TiO2 with anions such as N, S, B, P, C, and halogens. [22–25]

Temperature Dependence of Band Gap Change in InN and AlN.

The optical band gap of InN and AlN single crystal films was measured through absorption spectra in the temperature range of 4.2 to 300 K. The samples were grown on (0001)α-Al2O3 substrates by metalorganic vapor phase epitaxy. The Varshni equation and the Bose-Einstein expression have been compared with the experimental results. The nitride III-V compound semiconductors have smaller temperature dependence of band gap change. In III-V compound semiconductors, the variation of the band gap change increases in the order of N, P and As.

Molecular-beam epitaxial growth of GaAs and InGaAs/GaAs nanodot arrays using anodic Al2O3 nanohole array template masks

Highly ordered arrays of nanosized GaAs-based dots were successfully prepared on GaAs (001) substrates by molecular-beam epitaxy using selected area growth. Selected area growth employed alumina nanochannel array (NCA) templates formed by anodic oxidation, bonded to the GaAs substrates. Homogeneous GaAs dots, as well as compositionally modulated heterostructures within the nanosized dots, were demonstrated. In the latter case, multilayer InGaAs/GaAs heterostructured nanodot arrays were fabricated. Dot growth occurred only as defined by the template mask, resulting in a hexagonal lattice of dots with 100 nm period spacing, with dots retaining the circular lateral shape of the pores as determined by the NCA template pore size; dot diameters were adjustable from 45 to 85 nm for a lattice period of 100 nm. Cathodoluminescence spectra from an InGaAs/GaAs 10×10 dot array clearly showed an emission peak at 920 nm (5 K), confirming the formation of a high-quality InGaAs/GaAs quantum dot array.

Fabrication of ZnO microtubes with adjustable nanopores on the walls by the templating of butterfly wing scales

ZnO microtubes with adjustable arrayed nanopores on the walls are prepared by using butterfly wing scales as natural biotemplates. Flat butterfly wing scales used as templates rolled into tubes during the calcinations; all the windows on the scales were reserved and formed porous walls. Furthermore the density of the pores on the walls is partially determined by experimental conditions, such as temperature and time of treatment. The nano/microstructures obtained have been investigated by means of x-ray diffraction and field emission scanning electron microscopy. The room temperature (T = 300 K) cathodoluminescence of these unique ZnO microtubes is also studied. The spectrum shows a sharp near band edge emission and a broad deep level emission, which are similar to those for other previously synthesized ZnO microtubes.

Temperature dependence of Raman scattering in hexagonal indium nitride films

We report on Raman spectroscopy study of hexagonal InN thin films grown by metal-organic vapor-phase epitaxy, with the emphasis on frequencies and linewidths of E2(high) and A1(LO) modes in the temperature range from 93to443K. The present InN exhibits a fundamental band gap of ∼1.2eV from photoluminescence and optical transmission spectra, which is in good agreement with the recent suggested parameter for intrinsic InN. The temperature dependence of the E2(high) and A1(LO) phonons can be described well by a model which has taken into account the contributions of the thermal expansion of the crystal lattice, the strain between InN thin films and sapphire substrates, as well as three- and four-phonon coupling. Micro-Raman mapping results also demonstrate the high uniformity of the studied InN.

Existence and removal of Cu2Se second phase in coevaporated Cu2ZnSnSe4 thin films

The composition dependence of the electrical properties of Cu2ZnSnSe4 thin films synthesized by coevaporation and the results of phase analyses are reported. We found that the hole concentration depends on the Cu/(Zn + Sn) ratio and is on the order of 1017 cm−3 for the ratio of 0.7 and increases to over 1020 cm−3 when the ratio exceeds 0.9. Raman spectra indicate the coexistence of semimetallic Cu2Se second phase in the thin films with Cu/(Zn + Sn) ratio above 0.9. In order to remove the Cu2Se phase selectively, we attempted a KCN etching. After the KCN etching for 30 min, the Raman peak attributed to the Cu2Se phase disappeared, and the hole concentration decreased to about 1018 cm−3.

Grain refinement of Al–Si alloy (A356) by melt thermal treatment

Abstract In order to increase the casting quality and material plasticity of hypoeutectic Al–Si alloys, the effects of melt thermal treatment on the solidification structure of the A356 alloy were analyzed. Unlike the conventional melt addition treatment, no refinements are required during this novel processing. The structural characteristics and mechanical properties (plasticity and strength) were demonstrated. Results of quantitative metallorgraphic analysis showed that the primary dendrite size obviously reduces and the dendrite grains change into equiaxed ones. However, the secondary dendrite arm spacing changes little. Experimental results show that there is an optimal cooling rate range for thermal treatment, in which a maximum microstructure refining effect can be obtained. By the optimized parameters obtained with orthogonal experiments, the elongation ratio of the castings increases by 46.2% and the tensile strength by 8.6%. These results are explained by means of microheterogeneous melt theory, which is a result of the multiplication of the nuclei in the melt thermal treating procedure.

Wide bandgap engineering of (AlGa)2O3 films

Bandgap tunable (AlGa)2O3 films were deposited on sapphire substrates by pulsed laser deposition (PLD). The deposited films are of high transmittance as measured by spectrophotometer. The Al content in films is almost the same as that in targets. The measurement of bandgap energies by examining the onset of inelastic energy loss in core-level atomic spectra using X-ray photoelectron spectroscopy is proved to be valid for determining the bandgap of (AlGa)2O3 films as it is in good agreement with the bandgap values from transmittance spectra. The measured bandgap of (AlGa)2O3 films increases continuously with the Al content covering the whole Al content range from about 5 to 7 eV, indicating PLD is a promising growth technology for growing bandgap tunable (AlGa)2O3 films.

Growth of AlxIn1−xN single crystal films by microwave-excited metalorganic vapor phase epitaxy

Epitaxial layers of Al x In 1-x N have been grown on sapphire substrates by microwave-excited metalorganic vapor phase epitaxy. At growth temperature of 600 o C, single crystal layers of Al x In 1-x N with x up to 0,14 are obtained. The composition of Al x In 1-x N layers can be controlled, dependent on the ratio of the flow rates of the group III metalorganic sources in the vapor phase. The carrier concentration and Hall mobility of the layers are studied as a function of the composition. Optical measurements reveal that in all the layers, a direct band transition is observed, and its fundamental absorption edge varies continuously with composition

Iridescent large-area ZrO2 photonic crystals using butterfly as templates

Intact ZrO2 (with refractive index of 2.12) replica, which is large in size (about 3×4 cm2), has been synthesized by using natural butterfly wings as templates. Microstructure characters of original butterfly wing scales are maintained faithfully in this biomorphic ZrO2. All replicas can reflect iridescent visible lights, which can even be observed by naked eyes. Optical microscope investigations indicate that colors reflected by one single scale are different from those done by the overlapped two or even more scales. Colors are not only determined by materials’ refractive index, observation angle, and the structure of every single scale, but also by its piled number and modes. With the increase in the number of piled scales, the color is not simply redshifted or blueshifted, which is the most direct and powerful evidence for structural colors.