Shuxin Ouyang

Nano‐photocatalytic Materials: Possibilities and Challenges

Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.

An orthophosphate semiconductor with photooxidation properties under visible-light irradiation

The search for active semiconductor photocatalysts that directly split water under visible-light irradiation remains one of the most challenging tasks for solar-energy utilization. Over the past 30 years, the search for such materials has focused mainly on metal-ion substitution as in In(1-x)Ni(x)TaO(4) and (V-,Fe- or Mn-)TiO(2) (refs 7,8), non-metal-ion substitution as in TiO(2-x)N(x) and Sm(2)Ti(2)O(5)S(2) (refs 9,10) or solid-solution fabrication as in (Ga(1-x)Zn(x))(N(1-x)O(x)) and ZnS-CuInS(2)-AgInS(2) (refs 11,12). Here we report a new use of Ag(3)PO(4) semiconductor, which can harness visible light to oxidize water as well as decompose organic contaminants in aqueous solution. This suggests its potential as a photofunctional material for both water splitting and waste-water cleaning. More generally, it suggests the incorporation of p block elements and alkali or alkaline earth ions into a simple oxide of narrow bandgap as a strategy to design new photoelectrodes or photocatalysts.

Facet Effect of Single-Crystalline Ag3PO4 Sub-microcrystals on Photocatalytic Properties

We recently reported that Ag(3)PO(4) exhibits excellent photooxidative capabilities for O(2) evolution from water and organic dye decomposition under visible-light irradiation. However, very little is known about the shape and facet effects of Ag(3)PO(4) crystals on their photocatalytic properties. Herein we have developed a facile and general route for high-yield fabrication of single-crystalline Ag(3)PO(4) rhombic dodecahedrons with only {110} facets exposed and cubes bounded entirely by {100} facets. Moreover, studies of their photocatalytic performance have indicated that rhombic dodecahedrons exhibit much higher activities than cubes for the degradation of organic contaminants, which may be primarily ascribed to the higher surface energy of {110} facets (1.31 J/m(2)) than of {100} facets (1.12 J/m(2)).

MoS2/Graphene Cocatalyst for Efficient Photocatalytic H2 Evolution under Visible Light Irradiation

Exploiting noble-metal-free cocatalysts is of huge interest for photocatalytic water splitting using solar energy. Here we report a composite material consisting of CdS nanocrystals grown on the suface of a nanosized MoS2/graphene hybrid as a high-performance noble-metal-free photocatalyst for H2 evolution under visible light irradiation. Through the optimizing of each component proportion, the MoS2/G-CdS composite showed the highest photocatalytic H2 production activity when the content of the MoS2/graphene cocatalyst is 2.0 wt % and the molar ratio of MoS2 to graphene is 1:2. The photocatalytic H2 evolution activity of the proposed MoS2/G-CdS composite was tested and compared in Na2S-Na2SO3 solution and lactic acid solution. A 1.8 mmol/h H2 evolution rate in lactic acid solution corresponding to an AQE of 28.1% at 420 nm is not only higher than the case in Na2S-Na2SO3 solution of 1.2 mmol/h but also much higher than that of Pt/CdS in lactic acid solution. The relative mechanism has been investigated. It is believed that this kind of MoS2/G-CdS composite would have great potential as a promising photocatalyst with high efficiency and low cost for photocatalytic H2 evolution reaction.

Facile synthesis of rhombic dodecahedral AgX/Ag3PO4 (X = Cl, Br, I) heterocrystals with enhanced photocatalytic properties and stabilities

Herein, we have developed a facile and general method for the high-yield fabrication of AgX/Ag(3)PO(4) (X = Cl, Br, I) core-shell heterostructures with an unusual rhombic dodecahedral morphology, which exhibit much higher photocatalytic activities, structural stabilities and photoelectric properties than pure Ag(3)PO(4) crystals in environment and energy applications.

Ultrathin W18O49 Nanowires with Diameters below 1 nm: Synthesis, Near‐Infrared Absorption, Photoluminescence, and Photochemical Reduction of Carbon Dioxide

Inorganic nanowires with ultrathin diameters below the magic size (i.e., less than 2 nm) and even one unit cell size, have attracted much research attention in the past few years owing to their unique chemical and physical properties. As an important semiconductor material, tungsten oxide (WO3 x) nanowires and nanorods have attracted considerable attention because of their wide applications in gas sensors, electrochromic windows, optical devices, and photocatalysts. In particular, monoclinic W18O49 is of great interest owing to its unusual defect structure and promising properties in the nanometer regime. Early on, Park and co-workers reported the synthesis of W18O49 nanorods with a diameter of 4 nm by decomposing [W(CO)6] in Me3NO2·2 H2O and oleylamine. [16] Subsequently, Niederberger and co-workers synthesized hybrid W18O49/ organic nanowires with a very thin diameter of 1.3 nm by a bioligand-assisted method. Recently, Tremel and coworkers prepared W18O49 nanorods with a diameter of 2 nm by decomposing tungsten ethoxide in a mixture of oleic acid and trioctyl amine. Although good control over nanocrystal dimensions can be realized in these methods, removal of the surfactants or organic residues from the nanowire surface requires multiple washing steps. For fundamental investigations on the ultrathin oxide nanowire itself, as well as for technological applications (such as sensing and catalysis), the presence of residues on the nanowire surface from the synthesis may be a significant drawback. Herein, we report the preparation of ultrathin W18O49 nanowires that are efficient in the photochemical reduction of carbon dioxide by visible light. The ultrathin W18O49 nanowires were prepared by a very simple one-pot solution-phase method (see the experimental section in the Supporting Information). In a typical procedure, WCl6 was dissolved in ethanol, and the clear yellow solution was transferred to a teflon-lined stainless-steel autoclave and heated at 180 8C for 24 h. A blue flocculent precipitate was collected, washed, dried in air, and obtained in a yield of approximately 100 %. The product is insoluble in water and in acid (HCl, pH 0), and has a high specific surface area. W18O49 is a monoclinic structure type (P2 m) with lattice parameters of a = 18.318, b = 3.782, and c = 14.028 . Monoclinic W18O49 has a distorted ReO3 structure in which cornersharing distorted and tilt WO6 octahedra are connected in the a-, b-, and c-direction, thereby forming a three-dimensional structure (inset in Figure 1a). The X-ray diffraction (XRD) pattern of our sample demonstrates that the sample consists of monoclinic-phase W18O49 (Figure 1 a). The narrow (010) and (020) peaks strongly suggest that the possible crystal growth direction of the sample is [010], since the close-packed planes of the monoclinic W18O49 crystal are {010}, which will be further demonstrated by the direct observation of the highresolution transmission electron microscopy (HRTEM) image (see below). Energy-dispersive X-ray spectroscopy (EDS) confirms that the sample only contains W and O elements (Figure 1b). Furthermore, the Fourier transform infrared (FTIR) spectrum exhibits the clear surface of our sample (Figure S1 in the Supporting Information). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the as-synthesized sample is composed of nanowires with large aspect ratio and lengths of up to several micrometers (Figure 1c, d). Interestingly, higher-magnification TEM images (Figure 1e and Figures S2 and S3 in the Supporting Information) clearly reveal that the nanowires shown in Figure 1d are composed of a lot of individual, thinner nanowires. The diameter of the thinner nanowires is only about 0.9 nm. The HRTEM image and corresponding fast Fourier transform (FFT) pattern demonstrate that the ultrathin nanowires are crystalline and grow along [010] direction (Figure 1 f and Figure S4 in the Supporting Information). A comparison of the unit cell of W18O49 projected along the [010] direction with the typical diameter of the nanowires of 0.9 nm (red circle) allows [*] Dr. G. C. Xi, Dr. S. X. Ouyang, P. Li, Prof. J. H. Ye International Center for Materials Nanoarchitectonics (WPIMANA), and Environmental Remediation Materials Unit National Institute for Materials Science (NIMS) 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) E-mail: jinhua.ye@nims.go.jp

Photocatalytic and photoelectric properties of cubic Ag3PO4 sub-microcrystals with sharp corners and edges

Herein, we demonstrate a complex-precipitation strategy for high-yield fabrication of single-crystalline Ag(3)PO(4) sub-microcubes with sharp corners, edges, and smooth surfaces, which exhibit much higher photocatalytic activities and photoelectric conversion properties than spherical Ag(3)PO(4) particles and commercial N-doped TiO(2) under visible light irradiation.

Recent advances in TiO2-based photocatalysis

Semiconductor photocatalysis is a promising approach to combat both environmental pollution and the global energy shortage. Advanced TiO2-based photocatalysts with novel photoelectronic properties are benchmark materials that have been pursued for their high solar-energy conversion efficiency. In general, the photocatalytic efficiency is affected by the degree of light absorption, charge separation, and surface reactivity. Consequently, in this review we first discuss a series of interesting studies that aim to extend the light absorption of TiO2 from UV wavelengths into the visible or even the near-infrared region. We next focus on attempts to overcome the drawback that dopants usually act as charge recombination centres. We discuss the use of either selective local doping or the introduction of disorder together with doping, which aims to facilitate charge separation while preserving the visible-light response. We also show that crystal facet engineering can endow TiO2 with superior physicochemical properties, thus yielding high surface reactivity in photocatalytic reactions. Finally, we examine the recent theoretical advances of TiO2-based photocatalysis.

Surface-Alkalinization-Induced Enhancement of Photocatalytic H2 Evolution over SrTiO3-Based Photocatalysts

A strategy of reaction-environment modulation was employed to change the surface property of a semiconductor photocatalyst to enhance its photocatalytic performance. Surface alkalinization induced by a high alkalinity of the solution environment significantly shifted the surface energy band of a SrTiO(3) photocatalyst to a more negative level, supplying a strong potential for H(2)O reduction and consequently promoting the photocatalytic efficiency of H(2) evolution. This mechanism is also applicable for visible-light-sensitive La,Cr-codoped SrTiO(3) photocatalyst, which hence, could achieve a high apparent quantum efficiency of 25.6% for H(2) evolution in CH(3)OH aqueous solution containing 5 M NaOH at an incident wavelength of 425 ± 12 nm.

Enhanced incident photon-to-electron conversion efficiency of tungsten trioxide photoanodes based on 3D-photonic crystal design.

In this study, 3D-photonic crystal design was utilized to enhance incident photon-to-electron conversion efficiency (IPCE) of WO(3) photoanodes. Large-area and high-quality WO(3) photonic crystal photoanodes with inverse opal structure were prepared. The photonic stop-bands of these WO(3) photoanodes were tuned experimentally by variation of the pore size of inverse opal structures. It was found that when the red-edge of the photonic stop-band of WO(3) inverse opals overlapped with the WO(3) electronic absorption edge at E(g) = 2.6-2.8 eV, a maximum of 100% increase in photocurrent intensity was observed under visible light irradiation (λ > 400 nm) in comparison with a disordered porous WO(3) photoanode. When the red-edge of the stop-band was tuned well within the electronic absorption range of WO(3), noticeable but less amplitude of enhancement in the photocurrent intensity was observed. It was further shown that the spectral region with a selective IPCE enhancement of the WO(3) inverse opals exhibited a blue-shift in wavelength under off-normal incidence of light, in agreement with the calculated stop-band edge locations. The enhancement could be attributed to a longer photon-matter interaction length as a result of the slow-light effect at the photonic stop-band edge, thus leading to a remarkable improvement in the light-harvesting efficiency. The present method can provide a potential and promising approach to effectively utilize solar energy in visible-light-responsive photoanodes.