Xiangchao Ma

Energy transfer in plasmonic photocatalytic composites

Among the many novel photocatalytic systems developed in very recent years, plasmonic photocatalytic composites possess great potential for use in applications and are one of the most intensively investigated photocatalytic systems owing to their high solar energy utilization efficiency. In these composites, the plasmonic nanoparticles (PNPs) efficiently absorb solar light through localized surface plasmon resonance and convert it into energetic electrons and holes in the nearby semiconductor. This energy transfer from PNPs to semiconductors plays a decisive role in the overall photocatalytic performance. Thus, the underlying physical mechanism is of great scientific and technological importance and is one of the hottest topics in the area of plasmonic photocatalysts. In this review, we examine the very recent advances in understanding the energy transfer process in plasmonic photocatalytic composites, describing both the theoretical basis of this process and experimental demonstrations. The factors that affect the energy transfer efficiencies and how to improve the efficiencies to yield better photocatalytic performance are also discussed. Furthermore, comparisons are made between the various energy transfer processes, emphasizing their limitations/benefits for efficient operation of plasmonic photocatalysts.

Efficient Separation of Photogenerated Electron‐Hole Pairs by the Combination of a Heterolayered Structure and Internal Polar Field in Pyroelectric BiOIO3 Nanoplates

Semiconductor photocatalysts are important for new energy exploitation and decontamination of hazardous organic pollutants because they can photoconvert solar energy into chemical energy, photodecompose organic materials, and photoreduce carbon dioxide. However, their practical applications still remain beyond our grasp due to their poor quantum yield, which arises from the rapid recombination of photogenerated electron-hole (e-h) pairs. In principle, heterostructures can separate photogenerated electrons and holes efficiently because the different components possess different valence band (VB) and conduction band (CB) edges. However, the electron-hole separation becomes inefficient if there is no good contact between the different components. It is challenging to find a simple method that efficiently separates photogenerated electrons and holes. Bismuth-based layered semiconductors have long been an important subject for photocatalysis. Nanostructures of Bi2WO6, [4] Bi2MoO6, [5] BiOX (X=Cl, Br, I), Bi2S3, [7] Bi2O2CO3 [8] have been prepared by various methods and used as the photocatalysts for decomposing organic pollutants. However, bismuth-based layered materials with high photocatalytic activity have not been found yet. The heterolayered bismuth oxide iodate, BiOIO3, [9] is pyroelectric, and its stereoactive lone-pairs on both Bi and I cations are located at the BiO6 hexagonal and IO3 trigonal pyramidal sites, respectively. The BiO6 pyramids edge-share to form the Bi2O4 layers (Figure 1a), the two surfaces of which are corner-shared with IO3 pyramids to form BiOIO3 slabs (Figure 1b). The pyroelectricity of BiOIO3 does not arise from the BiO6 pyramids, but from the IO3 pyramids. The local dipole moments of the BiO6 pyramids are practically canceled out, but those of the IO3 pyramids are not, thereby leading to a pyroelectric polarization along the c-axis direction (Figure 1b). Under an electric field, electrons and holes move in opposite directions. Bastard et al. reported that the CB and VB electrons in GaAs quantum wells can be spatially separated by applying an external electric field. Thus, it is reasonable to suppose that photogenerated e-h pairs of a pyroelectric compound can be effectively separated by the internal polar field and hence exhibit a good photocatalytic activity. In this Communication we verify this hypothesis by studying the heterolayered pyroelectric compound BiOIO3 to find that it is an efficient photocatalyst under ultraviolet (UV) light irradiation, far superior to that of TiO2 (P25). We synthesize BiOIO3 nanoplates by using a simple hydrothermal method. Bi ACHTUNGTRENNUNG(NO3)3·5 H2O and KIO3 were used as the sources of Bi and IO3, which are environmentally friendly and inexpensive. Figure 2 shows the typical XRD pattern of the as-prepared BiOIO3 product. All the XRD peaks are indexed to orthorhombic BiOIO3 (space group: Pca21; a= 5.6584(4), b=11.0386(8), c=5.7476(4) ), which indicates a high purity of the obtained product. The intense and sharp diffraction peaks demonstrate that the product is well crys[a] W. Wang, Prof. Dr. B. Huang, Z. Wang, X. Qin, X. Zhang State Key Laboratory of Crystal Materials Shandong University, Jinan 250100 (P. R. China) Fax: (+86)0531-88365969 E-mail : bbhuang@sdu.edu.cn [b] X. Ma, Prof. Dr. Y. Dai School of Physics Shandong University, Jinan 250100 (P. R. China) [c] Prof. Dr. M.-H. Whangbo Department of Chemistry North Carolina State University Raleigh, North Carolina, 27695-8204 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201302884. Figure 1. a) A perspective view of the Bi2O4 layer in BiOIO3, in which the large cyan and small white circles represent the Bi and O atoms, respectively. b) A perspective view of the BiOIO3 slab, in which a Bi2O4 layer shares O corners with IO3 units. The arrows along the c-direction indicate the direction of the pyroelectric polarization, which results from the dipoles of the individual IO3 units.

Controlled synthesis of Ag2O microcrystals with facet-dependent photocatalytic activities

Ag2O microcrystals with different morphologies have been successfully synthesized by using various complexing agents. To achieve kinetic control of the growth of the Ag2O microcrystals, [Ag(NH3)2]+ complexing ions are required to restrict the release rate of silver ions before adding NaOH solution. The complexing anions play an important role in the growth process of the Ag2O microcrystals. This kinetic control leads to five morphologies of Ag2O microcrystals (cubic, octahedral, rhombic dodecahedra, polyhedra with 18 faces and rhombicuboctahedral), which exhibit facet-dependent photocatalytic activity for the degradation of methyl orange (MO) under visible light irradiation. The cubic Ag@Ag2O photocatalyst with exposed {100} facets showed the greatest activity of all the other morphologies of the photocatalysts. The mechanism of dramatic enhancement of the photocatalytic activity of Ag@Ag2O with exposed {100} facets was discussed in detail from three aspects, including the highest surface energy of the {100} facet, the larger difference value between the weighted average of the effective mass of holes and electrons along the [100] direction, and the suitable redox potentials of the (100) surface.

The Role of Effective Mass of Carrier in the Photocatalytic Behavior of Silver Halide‐Based Ag@AgX (X=Cl, Br, I): A Theoretical Study

The recent discovery of Ag@AgX (X=Cl, Br, I) plasmonic photocatalysts motivates us to elucidate the origin of the higher photocatalytic performance compared to commonly used TiO(2) -based materials. Herein, the electronic structure and effective masses of electrons at the conduction band minimum (CBM) and holes at the valence band maximum (VBM) are studied along different directions in the silver halide for the first time by means of first-principles calculations. It is revealed that the smaller effective mass of electrons at the CBM in silver halides contributes to the higher photocatalytic performance. The remarkable dependence of the effective mass of holes on the direction and the anion of the silver halide explains well the experimental observed morphology and anion dependence of photocatalytic activities of Ag@AgX. The crystal field splitting of the Ag 4d bands in the valance band of silver halides is found to be a main factor leading to the large effective mass of the photogenerated holes and consequently to a weaker transfer ability. A new crystal design and exerting strain along the coordinate axis are proposed as solutions to decrease the effective mass of holes. The present work may be helpful in exploring this novel class of silver halide-based photocatalysts.

Relative photooxidation and photoreduction activities of the {100}, {101}, and {001} surfaces of anatase TiO2.

The photoredox ability of the TiO2 {100}, {101}, and {001} surfaces is investigated by examining the trapping energies, trapping sites, and relative oxidation and reduction potentials of simulated photogenerated holes and electrons in the form of more realistic polaronic states on the basis of density functional electronic structure calculations. Our results enable us to re-estimate their relative photooxidation ({100} > {101} > {001}) and photoreduction ({100} > {101} > {001}) activities, which rectify the conventional understanding. The dual functions of the surface under coordinated atoms acting as active adsorption sites for adsorbates and hindering the population of electrons to the outermost surface layer are identified, and the specific surface geometric structures also play an important role in trapping holes and electrons through the ease of lattice distortion. In addition, we attribute the commonly low photocatalytic performance of the {101} surface to the large and similar trapping energies and adjacent trapping sites for electrons and holes, which result in high electron-hole recombination rates. However, the large difference in trapping energies for electrons and holes on different surfaces allows us to spatially gather electrons and holes on different surfaces by artificially designing the exposing facets of nanocrystals without resorting to the energy band potential difference between surfaces, thus expanding the ideas to improve the photocatalytic properties of materials through the regulation of crystal facets. Our present work can provide a helpful message for the design of more reactive photocatalytic TiO2 nanocrystals and the fabrication of other reactive photocatalysts.

Hydrogen Doped Metal Oxide Semiconductors with Exceptional and Tunable Localized Surface Plasmon Resonances

Heavily doped semiconductors have recently emerged as a remarkable class of plasmonic alternative to conventional noble metals; however, controlled manipulation of their surface plasmon bands toward short wavelengths, especially in the visible light spectrum, still remains a challenge. Here we demonstrate that hydrogen doped given MoO3 and WO3 via a facile H-spillover approach, namely, hydrogen bronzes, exhibit strong localized surface plasmon resonances in the visible light region. Through variation of their stoichiometric compositions, tunable plasmon resonances could be observed in a wide range, which hinge upon the reduction temperatures, metal species, the nature and the size of metal oxide supports in the synthetic H2 reduction process as well as oxidation treatment in the postsynthetic process. Density functional theory calculations unravel that the intercalation of hydrogen atoms into the given host structures yields appreciable delocalized electrons, enabling their plasmonic properties. The plasmonic hybrids show potentials in heterogeneous catalysis, in which visible light irradiation enhanced catalytic performance toward p-nitrophenol reduction relative to dark condition. Our findings provide direct evidence for achieving plasmon resonances in hydrogen doped metal oxide semiconductors, and may allow large-scale applications with low-price and earth-abundant elements.

Noble-metal-free plasmonic photocatalyst: hydrogen doped semiconductors

The unique capacity of localized surface plasmon resonance (LSPR) offers a new opportunity to overcome the limited efficiency of semiconductor photocatalyst. Here we unravel that LSPR, which usually occurs in noble metal nanoparticles, can be realized by hydrogen doping in noble-metal-free semiconductor using TiO2 as a model photocatalyst. Moreover, its LSPR is located in infrared region, which supplements that of noble metal whose LSPR is generally in the visible region, making it possible to extend the light response of photocatalyst to infrared region. The near field enhancement is shown to be comparable with that of noble-metal nanoparticles, indicating that highly enhanced light absorption rate can be expected. The present work can provide a key guideline for the creation of highly efficient noble-metal-free plasmonic photocatalysts and have a much wider impact in infrared bioimaging and spectroscopy where infrared LSPR is essential.

A 3D AgCl hierarchical superstructure synthesized by a wet chemical oxidation method.

A novel 3D AgCl hierarchical superstructure, with fast growth along the 〈111〉 directions of cubic seeds, is synthesized by using a wet chemical oxidation method. The morphological structures and the growth process are investigated by scanning electron microscopy and X-ray diffraction. The crystal structures are analyzed by their crystallographic orientations. The surface energy of AgCl facets {100}, {110}, and {111} with absorbance of Cl(-) ions is studied by density functional theory calculations. Based on the experimental and computational results, a plausible mechanism is proposed to illustrate the formation of the 3D AgCl hierarchical superstructures. With more active sites, the photocatalytic activity of the 3D AgCl hierarchical superstructures is better than those of concave and cubic ones in oxygen evolution under irradiation by visible light.

Electron-Rotor Interaction in Organic-Inorganic Lead Iodide Perovskites Discovered by Isotope Effects.

We report on the carrier-rotor coupling effect in perovskite organic-inorganic hybrid lead iodide (CH3NH3PbI3) compounds discovered by isotope effects. Deuterated organic-inorganic perovskite compounds including CH3ND3PbI3, CD3NH3PbI3, and CD3ND3PbI3 were synthesized. Devices made from regular CH3NH3PbI3 and deuterated CH3ND3PbI3 exhibit comparable performance in band gap, current-voltage, carrier mobility, and power conversion efficiency. However, a time-resolved photoluminescence (TRPL) study reveals that CH3NH3PbI3 exhibits notably longer carrier lifetime than that of CH3ND3PbI3, in both thin-film and single-crystal formats. Furthermore, the comparison in carrier lifetime between CD3NH3PbI3 and CH3ND3PbI3 single crystals suggests that vibrational modes in methylammonium (MA(+)) have little impact on carrier lifetime. In contrast, the fully deuterated compound CD3ND3PbI3 reconfirmed the trend of decreasing carrier lifetime upon the increasing moment of inertia of cationic MA(+). Polaron model elucidates the electron-rotor interaction.

Insights into How Fluorine-Adsorption and n-Type Doping Affect the Relative Stability of the (001) and (101) Surfaces of TiO2: Enhancing the Exposure of More Active but Thermodynamically Less Stable (001)

The stability of both the pure and fluorine (F)-adsorbed surface of TiO2 is examined on the basis of density functional calculations. For pure surfaces, both the beneficial local geometric structures and local potential strengthen the Ti-O binding in (101), rendering it the most stable surface. For F-adsorbed surfaces, F-adsorption significantly weakens the Ti-O bonds in (101) but strengthens them in (001), so that (001) becomes more stable than (101) for the F-adsorbed surfaces. On the basis of this observation, we further show that the n-type doping in TiO2 can significantly decrease the ability of F-adsorption in switching the relative stability of the two surfaces. The present work not only provides new insights into the physical and chemical properties about both pure and F-adsorbed surfaces of TiO2 and conclusively explains related experimental results but also suggests viable ways to prepare TiO2 samples with a high percentage of (001).