Function-switchable metal/semiconductor junction enables efficient photocatalytic overall water splitting with selective water oxidation products.

A novel metal/semiconductor photocatalyst, Cu nanoparticles (NPs) modified TiO2 hollow spheres (Cu/TiO2), was designed for efficient photocatalytic overall water splitting (POWS) under both ultraviolet (UV) and visible (Vis) light. This Cu/TiO2 photocatalyst possesses excellent POWS performance under Vis light at the highest level among the reported TiO2-based photocatalysts. Interestingly, the metal/semiconductor junction formed between Cu and TiO2 enables controlled water-oxidation product selectivity (H2O2 or O2) via different reaction pathways regulated by irradiation wavelengths. Under UV light, the electrons excited in TiO2 are captured by Cu NPs through the Cu/TiO2 Schottky interface for H2 production, with the photoholes in TiO2 producing H2O2 through a two-electron process; whilst under Vis light, Cu NPs act as plasmon to inject hot electrons to TiO2 for H2 production, while O2 is produced by hot holes on Cu NPs via a four-electron process. This rational design of function-switchable metal/semiconductor junction may be helpful to understand the mechanisms for POWS with desired gas/liquid water-oxidation products.

Trap-Assisted Charge Injection into Large Bandgap Polymer Semiconductors.

The trap-assisted charge injection in polyfluorene-poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) model systems with an Al or Al/LiF cathode is investigated. We find that inserting 1.3 nm LiF increases electron and hole injections simultaneously and the increase of holes is greater than electrons. The evolution of internal interfaces within polymer light-emitting diodes is observed by transmission electron microscopy, which reveals that the introduction of LiF improves the interface stability at both the cathode (cathode/polymer) and the anode (indium tin oxide (ITO)/PEDOT:PSS). Above-mentioned experimental results have been compared to the numerical simulations with a revised Davids model and potential physical mechanisms for the trap-assisted charge injection are discussed.

Electronically Controlled Chemical Stability of Compound Semiconductor Surfaces.

Effects of a humid environment on the degradation of semiconductors were studied to understand the role of the surface charge on material stability. Two distinctly different semiconductors with the Fermi level stabilization energy EFS located inside the conduction band (CdO) and valence band (SnTe) were selected, and effects of an exposure to 85 °C and 85% relative humidity conditions on their electrical properties were investigated. Undoped CdO films with bulk Fermi level EF below EFS and positively charged surface are very unstable. The stability greatly improves with doping when EF shifts above EFS, and the surface becomes negatively charged. This charge-controlled reactivity is further confirmed by the superior stability of undoped p-type SnTe with EF above EFS. These distinct reactivities are explained by the surface attracting either the reactive OH- or passivating H+ ions. The present results have important implications for understanding the interaction of semiconductor surfaces with water or, in general, ionic solutions.

A general and simple method for evaluating the electrical transport performance of graphene by the van der Pauw-Hall measurement.

Expected for many promising applications in the field of electronics and optoelectronics, a reliable method for the characterization of graphene electrical transport properties is desired to predict its device performance or provide feedback for its synthesis. However, the commonly used methods of extracting carrier mobility from graphene field effect transistor or Hall-bar is time consuming, expensive, and significantly affected by the device fabrication process other than graphene itself. Here we reported a general and simple method to evaluate the electrical transport performance of graphene by the van der Pauw-Hall measurement. By annealing graphene in vacuum to remove the adsorbed dopants and then exposing it in ambient surroundings, carrier mobility as a function of density can be measured with the increase of carrier density due to the dopant re-adsorption from the surroundings. Further, the relationship between the carrier mobility and density can be simply fitted with a power equation to the first level approximation, with which any pair of measured carrier mobility and density can be normalized to an arbitrary carrier density for comparison. We experimentally demonstrated the reliability of the method, which is much simpler than making devices and may promote the standard making for graphene characterization.

Surface engineered doping of hematite nanorod arrays for improved photoelectrochemical water splitting.

Given the narrow band gap enabling excellent optical absorption, increased charge carrier density and accelerated surface oxidation reaction kinetics become the key points for improved photoelectrochemical performances for water splitting over hematite (α-Fe2O3) photoanodes. In this study, a facile and inexpensive method was demonstrated to develop core/shell structured α-Fe2O3 nanorod arrays. A thin, Ag-doped overlayer of ~2-3 nm thickness was formed along α-Fe2O3 nanorods via ultrasonication treatment of solution-based ß-FeOOH nanorods in Ag precursor solution followed by high temperature annealing. The obtained α-Fe2O3/AgxFe2-xO3 core/shell nanorod films demonstrated much higher photoelectrochemical performances as photoanodes than the pristine α-Fe2O3 nanorod film, especially in the visible light region; the incident photon-to-current efficiency (IPCE) at 400 nm was increased from 2.2% to 8.4% at 1.23 V vs. RHE (Reversible hydrogen electrode). Mott-Schottky analysis and X-ray absorption spectra revealed that the Ag-doped overlayer not only increased the carrier density in the near-surface region but also accelerated the surface oxidation reaction kinetics, synergistically contributing to the improved photoelectrochemical performances. These findings provide guidance for the design and optimization of nanostructured photoelectrodes for efficient solar water splitting.

Co3O4 quantum dots: reverse micelle synthesis and visible-light-driven photocatalytic overall water splitting.

Co3O4 quantum dots were synthesized by a facile reverse micelle method for the first time, and were capable of splitting pure water into O2 and H2 stoichiometrically under visible-light irradiation (λ > 420 nm) without any cocatalyst.

Surface passivation of undoped hematite nanorod arrays via aqueous solution growth for improved photoelectrochemical water splitting.

A facile solution-based strategy was found to be effective for surface passivation of undoped hematite nanorod photoanodes by adding noble-metal chlorides such as HAuCl4 and H2PtCl6 in the Fe(3+) precursor solution. XPS and Raman spectra revealed that noble-metal ions would not be doped into, but lead to surface disorder of hematite. Incident photon to current efficiency (IPCE) of the hematite photoanode grown in HAuCl4 and H2PtCl6 contained precursor solution was increased from 3.6% to 11.5% and 12.9%, respectively, at 365 nm and 1.23 V vs. RHE (reversible hydrogen electrode). This photoelectrochemical (PEC) enhancement was ascribed to the surface passivation, which resulted in a reduced recombination of photogenerated carriers, as confirmed by ultrafast transient absorption spectroscopy.

Hydrogenation and disorder in engineered black TiO2.

A new form of TiO2 which is black in color has been shown to exhibit high efficiency for photocatalytic reactions under solar radiation [X. Chen, L. Liu, P. Y. Yu, and S. S. Mao, Science 331, 746 (2011)]. However, the mechanism behind this disorder-engineering process is not fully understood. In this Letter, based on density functional theory, we describe the role of hydrogen in producing lattice disorder in the anatase nanocrystals. We clarify further that the highly localized nature of the midgap states results in spatial separation of photoexcited electrons and holes in black TiO2, and that accounts for its high photocatalytic efficiency.

Physical and photoelectrochemical properties of Zr-doped hematite nanorod arrays.

This work examines the effect of Zr(4+) ions on the physical and photoelectrochemical (PEC) properties of hematite (α-Fe2O3) nanorod arrays grown in an aqueous solution containing zirconyl nitrate (ZrO(NO3)2) as a dopant precursor. The concentration of ZrO(NO3)2 in the precursor solution influenced both the film thickness and the Zr(4+) concentration in the resulting films. Zr doping was found to enhance the photocurrent for water splitting; the highest photocurrent at 1.0 V vs. Ag/AgCl (0.33 mA cm(-2)) for the Zr-doped α-Fe2O3 film was approximately 7.2 times higher than that for the undoped film (0.045 mA cm(-2)). Additionally, the incident photon to current efficiency (IPCE) at 360 nm and 1.23 V vs. the reversible hydrogen electrode (RHE) increased from 3.8% to 13.6%. Ultrafast transient absorption spectroscopy suggests that Zr doping may influence PEC performance by reducing the rate of electron-hole recombination.

On the orbital anisotropy in hematite nanorod-based photoanodes.

The orbital anisotropy of hematite (α-Fe2O3) nanorod arrays, an engineered structure commonly investigated for applications in solar water oxidation photoanodes, is probed using polarization-dependent soft X-ray absorption spectroscopy at the O K-edge and at the Fe L2,3-edge. Thereby the unoccupied states of α-Fe2O3 are examined. In the lowest energy region these are found to be strongly-hybridized Fe 3d (a1g) orbitals and O(2-) ligand 2p orbitals, oriented along the c-axis. For [110]-oriented α-Fe2O3 nanocrystals the observed direction of strong hybridization is parallel to the substrate surface (perpendicular to the direction of electron conduction and light propagation in operating electrodes). The Fe L3-edge line shape and aspects of polarization dependence can be reproduced by crystal field atomic multiplet calculations of 2p-to-3d transitions for Fe(3+) in the D3d point group symmetry of metal ions in the corundum structure. Both the O K-edge and Fe L3-edge spectra possess features that may be related to the high density of surface atoms in this nanoscale system. They are associated with partial coordination and therefore reduced symmetry compared to that for Fe(3+) in bulk crystals.

Properties of disorder-engineered black titanium dioxide nanoparticles through hydrogenation.

The recent discovery of "black" TiO2 nanoparticles with visible and infrared absorption has triggered an explosion of interest in the application of TiO2 in a diverse set of solar energy systems; however, what a black TiO2 nanoparticle really is remains a mystery. Here we elucidate more properties and try to understand the inner workings of black TiO2 nanoparticles with hydrogenated disorders in a surface layer surrounding a crystalline core. Contrary to traditional findings, Ti(3+) here is not responsible for the visible and infrared absorption of black TiO2, while there is evidence of mid-gap states above the valence band maximum due to the hydrogenated, engineered disorders. The hydrogen atoms, on the other hand, can undergo fast diffusion and exchange. The enhanced hydrogen mobility may be explained by the presence of the hydrogenated, disordered surface layer. This unique structure thus may give TiO2, one of the most-studied oxide materials, a renewed potential.

Nanomaterials for renewable energy production and storage.

Over the past decades, there have been many projections on the future depletion of the fossil fuel reserves on earth as well as the rapid increase in green-house gas emissions. There is clearly an urgent need for the development of renewable energy technologies. On a different frontier, growth and manipulation of materials on the nanometer scale have progressed at a fast pace. Selected recent and significant advances in the development of nanomaterials for renewable energy applications are reviewed here, and special emphases are given to the studies of solar-driven photocatalytic hydrogen production, electricity generation with dye-sensitized solar cells, solid-state hydrogen storage, and electric energy storage with lithium ion rechargeable batteries.

Soft X-ray characterization of Zn(1-x)Sn(x)O(y) electronic structure for thin film photovoltaics.

Zinc tin oxide (Zn(1-x)Sn(x)O(y)) has been proposed as an alternative buffer layer material to the toxic, and light narrow-bandgap CdS layer in CuIn(1-x),Ga(x)Se(2) thin film solar cell modules. In this present study, synchrotron-based soft X-ray absorption and emission spectroscopies have been employed to probe the densities of states of intrinsic ZnO, Zn(1-x)Sn(x)O(y) and SnO(x) thin films grown by atomic layer deposition. A distinct variation in the bandgap is observed with increasing Sn concentration, which has been confirmed independently by combined ellipsometry-reflectometry measurements. These data correlate directly to the open circuit potentials of corresponding solar cells, indicating that the buffer layer composition is associated with a modification of the band discontinuity at the CIGS interface. Resonantly excited emission spectra, which express the admixture of unoccupied O 2p with Zn 3d, 4s, and 4p states, reveal a strong suppression in the hybridization between the O 2p conduction band and the Zn 3d valence band with increasing Sn concentration.

Visible light-driven photocatalysis of doped SrTiO3 tubular structure.

SrTiO3 tubular structures co-doped with Cr and Ta were synthesized through a combination of solvothermal-hydrothermal processes. X-ray photoelectron spectroscopy (XPS) measurements of the oxidation state of Cr ions reveal that the formation of Cr6+ ions, which would serve as the non-radiative recombination centers for photogenerated electrons and holes, was suppressed without the process of high temperature hydrogen reduction. Compared to similar co-doped materials synthesized by solid-state reaction, (Cr, Ta) co-doped SrTiO3 tubular structures have significantly higher photocatalytic activity for hydrogen evolution as measured in an aqueous methanol solution under visible light irradiation.

Electron enrichment in 3d transition metal oxide hetero-nanostructures.

Direct experimental observation of spontaneous electron enrichment of metal d orbitals in a new transition metal oxide heterostructure with nanoscale dimensionality is reported. Aqueous chemical synthesis and vapor phase deposition are combined to fabricate oriented arrays of high-interfacial-area hetero-nanostructures comprised of titanium oxide and iron oxide nanomaterials. Synchrotron-based soft X-ray spectroscopy techniques with high spectral resolution are utilized to directly probe the titanium and oxygen orbital character of the interfacial region's occupied and unoccupied densities of states. These data demonstrate the interface to possess electrons in Ti 3d bands and an emergent degree of orbital hybridization that is absent in parent oxide reference crystals. The carrier dynamics of the hetero-nanostructures are studied by ultrafast transient absorption spectroscopy, which reveals the presence of a dense manifold of states, the relaxations from which exhibit multiple exponential decays whose magnitudes depend on their energetic positions within the electronic structure.

Solar light-driven photocatalytic hydrogen evolution over ZnIn2S4 loaded with transition-metal sulfides.

A series of Pt-loaded MS/ZnIn2S4 (MS = transition-metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS) photocatalysts was investigated to show various photocatalytic activities depending on different transition-metal sulfides. Thereinto, CoS, NiS, or MnS-loading lowered down the photocatalytic activity of ZnIn2S4, while Ag2S, SnS, or CuS loading enhanced the photocatalytic activity. After loading 1.0 wt.% CuS together with 1.0 wt.% Pt on ZnIn2S4, the activity for H2 evolution was increased by up to 1.6 times, compared to the ZnIn2S4 only loaded with 1.0 wt.% Pt. Here, transition-metal sulfides such as CuS, together with Pt, acted as the dual co-catalysts for the improved photocatalytic performance. This study indicated that the application of transition-metal sulfides as effective co-catalysts opened up a new way to design and prepare high-efficiency and low-cost photocatalysts for solar-hydrogen conversion.

Subnanometer porous thin films by the co-assembly of nanotube subunits and block copolymers.

Porous thin films containing subnanometer channels oriented normal to the surface exhibit unique transport and separation properties and can serve as selective membranes for separation and protective coatings. While molecularly defined nanoporous inorganic and organic materials abound, generating flexible nanoporous thin films with highly aligned channels over large areas has been elusive. Here, we developed a new approach where the growth of cyclic peptide nanotubes can be directed in a structural framework set by the self-assembly of block copolymers. By conjugating polymers to cyclic peptides, the subunit of an organic nanotube can be selectively solubilized in one copolymer microdomain. The conjugated polymers also mediate the interactions between nanotube and local medium and guide the growth of nanotubes in a confined geometry. This led to subnanometer porous membranes containing high-density arrays of through channels. This new strategy takes full advantage of nanoscopic assembly of BCPs and the reversibility of organic nanotube growth and circumvents impediments associated with aligning and organizing high aspect ratio nano-objects normal to the surface. Furthermore, the hierarchical coassembly strategy described demonstrates the feasibility of synchronizing multiple self-assembly processes to achieve hierarchically structured soft materials with molecular level control.

Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals.

When used as a photocatalyst, titanium dioxide (TiO(2)) absorbs only ultraviolet light, and several approaches, including the use of dopants such as nitrogen, have been taken to narrow the band gap of TiO(2). We demonstrated a conceptually different approach to enhancing solar absorption by introducing disorder in the surface layers of nanophase TiO(2) through hydrogenation. We showed that disorder-engineered TiO(2) nanocrystals exhibit substantial solar-driven photocatalytic activities, including the photo-oxidation of organic molecules in water and the production of hydrogen with the use of a sacrificial reagent.