Ran Long

Toward Enhanced Photocatalytic Oxygen Evolution: Synergetic Utilization of Plasmonic Effect and Schottky Junction via Interfacing Facet Selection

DOI: 10.1002/adma.201501200 participate in oxidation reaction. As long as a semiconductor with appropriate bandgap (i.e., wide bandgap) is selected, the redox abilities of electrons or holes can be maintained as high as those in wide-bandgap semiconductors despite the use of incident visible light with relatively low energy. Unfortunately, the reported photocatalytic effi ciencies purely offered by the plasmonic hot carrier injection effect in the absence of semiconductor photoexcitation are negligible in contrast to those by semiconductor photoexcitation. [ 11,19 ] The major reason for this limitation is the lack of a driving force to steer the migration of injected electrons or holes to semiconductor surface for reduction or oxidation reactions. The low charge migration rates and uncertain charge diffusion directions make the charge carriers randomly walk in the semiconductor, so only a small portion of plasmonic hot carriers can arrive at the catalytic sites. We have thus decided to develop a new approach to better harness the utilization of plasmonic hot carriers. Thus far, use of a Schottky junction has been recognized as the most wellestablished strategy for steering the fl ow of the carriers that are photogenerated in semiconductor. It is well known that metal (especially for nonplasmonic metal, Pt and Pd) can serve as an sink for the photogenerated electrons or holes when forming a Schottky junction with n-type or p-type semiconductor, respectively (Figure S1, Supporting Information). [ 20,21 ] The formed Schottky barrier can inhibit the backfl ow of electrons or holes from metal to semiconductor. As a result, the charge “pump” role of the Schottky junction ensures the effi cient unidirectional transfer of charge carriers across the interface of metal– semiconductor (M–S) junction. Naturally we consider the possibility whether this Schottky-junction effect may be extended to the utilization of plasmonic hot carriers in photocatalysis through guiding their migration directions. However, this idea can be hardly accomplished by a single M–S junction between plasmonic metal and semiconductor. When a plasmonic metal is used for both the Schottky junction and hot carrier injection, the injection of plasmonic hot carriers would follow an opposite direction to the carrier fl ow driven by the Schottky junction (Figure S1, Supporting Information). [ 11,14,15,18,22 ] This severe competition dramatically reduces the effi ciency of carrier trapping on metal and e–h separation, particularly when metal and semiconductor are both excited under full-spectrum irradiation. In this communication, we report a new design for synergizing the plasmonic effect with the Schottky junction. The core concept of this work is to separate the Schottky junction from the plasmonic hot carrier injection by building two M–S interfaces based on the selection of semiconductor facets and metals. The functions of these two interfaces are synergized by Photocatalytic water splitting represents a highly important approach to addressing current energy and environmental demands. Photocatalysis requires effi cient separation of photo generated electron–hole (e–h) pairs in semiconductor to undergo redox reactions. [ 1 ] The reduction and oxidation capabilities of photogenerated electrons and holes in a semiconductor are determined by the positions of conduction band (CB) and valence band (VB) edges, respectively. Only when the CB edge lies at a higher position (more negative) than the redox potential of reduction half reaction, and meanwhile, the VB edge is at a lower position (more positive) than the potential of oxidation half reaction, can an overall photocatalytic reaction take place. [ 2 ] Thus wide-bandgap semiconductors with higher CB and lower VB edges generally show higher redox abilities as well as more promising photocatalytic performance in comparison with narrow-bandgap ones. However, semiconductors with wide bandgaps can only absorb light in the UV region which accounts for ≈5% of solar spectrum, thereby limiting their solar energy conversion effi ciency for practical applications. [ 3 ] For this reason, the relationship between absorption of long-wavelength light and high redox abilities of charge carriers is essentially an irreconcilable contradiction for bare semiconductor photocatalysts. Most recently, integration of surface plasmon into photocatalysis has been widely explored by composing hybrid structures between noble metals and semiconductors, which may potentially circumvent this situation. [ 4–10 ] As demonstrated by many research groups, [ 11–18 ] the metal with surface plasmon (e.g., Ag and Au) that directly contacts a semiconductor can be excited under visible light illumination to generate and inject hot carriers into the semiconductor. Specifi cally, hot electrons may fl ow into the CB of n-type semiconductor [ 13 ] and in the case of p-type semiconductor, instead hot holes are injected into the VB of semiconductor (Figure S1, Supporting Information). [ 16 ]

Surface Facet of Palladium Nanocrystals: A Key Parameter to the Activation of Molecular Oxygen for Organic Catalysis and Cancer Treatment

In many organic reactions, the O(2) activation process involves a key step where inert ground triplet O(2) is excited to produce highly reactive singlet O(2). It remains elusive what factor induces the change in the electron spin state of O(2) molecules, although it has been discovered that the presence of noble metal nanoparticles can promote the generation of singlet O(2). In this work, we first demonstrate that surface facet is a key parameter to modulate the O(2) activation process on metal nanocrystals, by employing single-facet Pd nanocrystals as a model system. The experimental measurements clearly show that singlet O(2) is preferentially formed on {100} facets. The simulations further elucidate that the chemisorption of O(2) to the {100} facets can induce a spin-flip process in the O(2) molecules, which is achieved via electron transfer from Pd surface to O(2). With the capability of tuning O(2) activation, we have been able to further implement the {100}-faceted nanocubes in glucose oxidation. It is anticipated that this study will open a door to designing noble metal nanocatalysts for O(2) activation and organic oxidation. Another perspective of this work would be the controllability in tailoring the cancer treatment materials for high (1)O(2) production efficiency, based on the facet control of metal nanocrystals. In the cases of both organic oxidation and cancer treatment, it has been exclusively proven that the efficiency of producing singlet O(2) holds the key to the performance of Pd nanocrystals in the applications.

Unraveling Metal-insulator Transition Mechanism of VO2 Triggered by Tungsten Doping

Understanding the mechanism of W-doping induced reduction of critical temperature (TC) for VO2 metal-insulator transition (MIT) is crucial for both fundamental study and technological application. Here, using synchrotron radiation X-ray absorption spectroscopy combined with first-principles calculations, we unveil the atomic structure evolutions of W dopant and its role in tailoring the TC of VO2 MIT. We find that the local structure around W atom is intrinsically symmetric with a tetragonal-like structure, exhibiting a concentration-dependent evolution involving the initial distortion, further repulsion, and final stabilization due to the strong interaction between doped W atoms and VO2 lattices across the MIT. These results directly give the experimental evidence that the symmetric W core drives the detwisting of the nearby asymmetric monoclinic VO2 lattice to form rutile-like VO2 nuclei, and the propagations of these W-encampassed nuclei through the matrix lower the thermal energy barrier for phase transition.

Control Over the Branched Structures of Platinum Nanocrystals for Electrocatalytic Applications

Structural control of branched nanocrystals allows tuning two parameters that are critical to their catalytic activity--the surface-to-volume ratio, and the number of atomic steps, ledges, and kinks on surface. In this work, we have developed a simple synthetic system that allows tailoring the numbers of branches in Pt nanocrystals by tuning the concentration of additional HCl. In the synthesis, HCl plays triple functions in tuning branched structures via oxidative etching: (i) the crystallinity of seeds and nanocrystals; (ii) the number of {111} or {100} faces provided for growth sites; (iii) the supply kinetics of freshly formed Pt atoms in solution. As a result, tunable Pt branched structures--tripods, tetrapods, hexapods, and octopods with identical chemical environment--can be rationally synthesized in a single system by simply altering the etching strength. The controllability in branched structures enables to reveal that their electrocatalytic performance can be optimized by constructing complex structures. Among various branched structures, Pt octopods exhibit particularly high activity in formic acid oxidation as compared with their counterparts and commercial Pt/C catalysts. It is anticipated that this work will open a door to design more complex nanostructures and to achieve specific functions for various applications.

Implementing Metal‐to‐Ligand Charge Transfer in Organic Semiconductor for Improved Visible‐Near‐Infrared Photocatalysis

The coordination of organic semiconductors with metal cations can induce metal-to-ligand charge transfer, which broadens light absorption to cover the visible-near-infrared (vis-NIR) spectrum. As a proof-of-concept demonstration, the g-C3 N4 -based complex exhibits dramatically enhanced photocatalytic H2 production with excellent durability under vis-NIR irradiation.

Tunable Oxygen Activation for Catalytic Organic Oxidation: Schottky Junction versus Plasmonic Effects

The charge state of the Pd surface is a critical parameter in terms of the ability of Pd nanocrystals to activate O2 to generate a species that behaves like singlet O2 both chemically and physically. Motivated by this finding, we designed a metal-semiconductor hybrid system in which Pd nanocrystals enclosed by {100} facets are deposited on TiO2 supports. Driven by the Schottky junction, the TiO2 supports can provide electrons for metal catalysts under illumination by appropriate light. Further examination by ultrafast spectroscopy revealed that the plasmonics of Pd may force a large number of electrons to undergo reverse migration from Pd to the conduction band of TiO2 under strong illumination, thus lowering the electron density of the Pd surface as a side effect. We were therefore able to rationally tailor the charge state of the metal surface and thus modulate the function of Pd nanocrystals in O2 activation and organic oxidation reactions by simply altering the intensity of light shed on Pd-TiO2 hybrid structures.

Isolation of Cu Atoms in Pd Lattice: Forming Highly Selective Sites for Photocatalytic Conversion of CO2 to CH4

Photocatalytic conversion of CO2 to CH4, a carbon-neutral fuel, represents an appealing approach to remedy the current energy and environmental crisis; however, it suffers from the large production of CO and H2 by side reactions. The design of catalytic sites for CO2 adsorption and activation holds the key to address this grand challenge. In this Article, we develop highly selective sites for photocatalytic conversion of CO2 to CH4 by isolating Cu atoms in Pd lattice. According to our synchrotron-radiation characterizations and theoretical simulations, the isolation of Cu atoms in Pd lattice can play dual roles in the enhancement of CO2-to-CH4 conversion: (1) providing the paired Cu-Pd sites for the enhanced CO2 adsorption and the suppressed H2 evolution; and (2) elevating the d-band center of Cu sites for the improved CO2 activation. As a result, the Pd7Cu1-TiO2 photocatalyst achieves the high selectivity of 96% for CH4 production with a rate of 19.6 μmol gcat-1 h-1. This work provides fresh insights into the catalytic site design for selective photocatalytic CO2 conversion, and highlights the importance of catalyst lattice engineering at atomic precision to catalytic performance.

Composition-dependent activity of Cu–Pt alloy nanocubes for electrocatalytic CO2 reduction

A protocol has been developed for the synthesis of Cu–Pt alloy nanocubes with a relatively broad range of composition ratios. It enables the investigation of the composition-dependent activity of Cu–Pt alloys for electrocatalytic CO2 reduction.

Investigation of Size‐Dependent Plasmonic and Catalytic Properties of Metallic Nanocrystals Enabled by Size Control with HCl Oxidative Etching

Particle size is one important parameter of nanocrystals that need to be tightly controlled, owing to its versatility for tailoring the properties and functions of nanocrystals towards various applications. In this article, oxidative etching by hydrogen chloride is employed as a tool to control the size of metallic nanocrystals. As a result of the size control, investigations into the size-dependent plasmonic and catalytic properties of metallic nanocrystals can be investigated. Given that the shape can be kept consistent when tuning the particle size in this system, it enables the systematic investigation of size-dependent properties free of the influence of other factors such as shape effect.

Efficient coupling of solar energy to catalytic hydrogenation by using well-designed palladium nanostructures.

A Ru(3+)-mediated synthesis for the unique Pd concave nanostructures, which can directly harvest UV-to-visible light for styrene hydrogenation, is described. The catalytic efficiency under 100 mW cm(-2) full-spectrum irradiation at room temperature turns out to be comparable to that of thermally (70 °C) driven reactions. The yields obtained with other Pd nanocrystals, such as nanocubes and octahedrons, are lower. The nanostructures reported here have sufficient plasmonic cross-sections for light harvesting in a broad spectral range owing to the reduced shape symmetry, which increases the solution temperature for the reaction by the photothermal effect. They possess a large quantity of atoms at corners and edges where local heat is more efficiently generated, thus providing active sites for the reaction. Taken together, these factors drastically enhance the hydrogenation reaction by light illumination.