Trends in hot carrier distribution for disordered noble-transition metal alloys.

We developed and tested an approach for predicting trends for efficient hot carrier generation among disordered metal alloys. We provide a simple argument for the importance of indirect transitions in the presence of disorder, thus justifying the use of joint density of states (JDOS)-like quantities for exploring these trends. We introduce a newJDOS-like quantity,JDOSK, which heuristically accounts for longer lifetimes of quasiparticles close to the Fermi energy. To demonstrate the efficacy of this new quantity, we apply it to the study of Cu50X50where X = Ag, Au, Pd and Y50Pd50where Y = Au, Ni. We predict that Ni50Pd50produces the most hot carriers among the alloys considered. The improvement in the density of excited photocarriers over the base alloy used, Cu50Ag50, is 3.4 times for 800 nm and 19 times for 1550 nm light. This boost in hot-carrier generation is consequence of the ferromagnetic nature of the Ni alloy. We argue that our method allows efficient material-specific predictions for low bias photoconductivity of alloys.

Role of Electronegativity in Environmentally Persistent Free Radicals (EPFRs) Formation on ZnO.

Environmentally persistent free radicals (EPFRs), a group of emerging pollutants, have significantly longer lifetimes than typical free radicals. EPFRs form by the adsorption of organic precursors on a transition metal oxide (TMO) surface involving electron charge transfer between the organic and TMO. In this paper, dihalogenated benzenes were incorporated to study the role of electronegativity in the electron transfer process to obtain a fundamental knowledge of EPFR formation mechanism on ZnO. Upon chemisorption on ZnO nanoparticles at 250 °C, electron paramagnetic resonance (EPR) confirms the formation of oxygen adjacent carbon-centered organic free radicals with concentrations between 1016 and 1017 spins/g. The radical concentrations show a trend of 1,2-dibromobenzene (DBB) > 1,2-dichlorobenzene (DCB) > 1,2-difluorobenzene (DFB) illustrating the role of electronegativity on the amount of radical formation. X-ray absorption spectroscopy (XAS) confirms the reduction of the Zn2+ metal center, contrasting previous experimental evidence of an oxidative mechanism for ZnO single crystal EPFR formation. The extent of Zn reduction for the different organics (DBB > DCB > DFB) also correlates to their polarity. DFT calculations provide theoretical evidence of ZnO surface reduction and exhibit a similar trend of degree of reduction for different organics, further building on the experimental findings. The lifetimes of the EPFRs formed confirm a noteworthy persistency.

Emergent properties from CuPd alloy films under near-infrared excitation.

Noble-transition metal alloys offer emergent optical and electronic properties for near-infrared (NIR) optoelectronic devices. We investigate the optical and electronic properties of CuxPd1-x alloy thin films and their ultrafast electron dynamics under NIR excitation. Ultraviolet photoelectron spectroscopy measurements supported by density functional theory calculations show strong d-band hybridization between the Cu 3d and Pd 4d bands. These hybridization effects result in emergent optical properties, most apparent in the dilute Pd case. Time-resolved terahertz spectroscopy with NIR (e.g., 1550 nm) excitation displays composition-tunable electron dynamics. We posit that the negative peak in the normalized increment of transmissivity (ΔT/T) below 2 ps from dilute Pd alloys is due to non-thermalized hot-carrier generation. On the other hand, Pd-rich alloys exhibit an increase in ΔT/T due to thermalization effects upon ultrafast NIR photoexcitation. CuxPd1-x alloys in the dilute Pd regime may be a promising material for future ultrafast NIR optoelectronic devices.

CoCrFeNi High-Entropy Alloy as an Enhanced Hydrogen Evolution Catalyst in an Acidic Solution.

High-entropy alloys (HEAs) have intriguing material properties, but their potential as catalysts has not been widely explored. Based on a concise theoretical model, we predict that the surface of a quaternary HEA of base metals, CoCrFeNi, should go from being nearly fully oxidized except for pure Ni sites when exposed to O2 to being partially oxidized in an acidic solution under cathodic bias, and that such a partially oxidized surface should be more active for the electrochemical hydrogen evolution reaction (HER) in acidic solutions than all the component metals. These predictions are confirmed by electrochemical and surface science experiments: the Ni in the HEA is found to be most resistant to oxidation, and when deployed in 0.5 M H2SO4, the HEA exhibits an overpotential of only 60 mV relative to Pt for the HER at a current density of 1 mA/cm2.

Formation of Environmentally Persistent Free Radicals (EPFRs) on the Phenol-Dosed α-Fe2O3(0001) Surface.

Environmentally persistent free radicals (EPFRs) are a class of toxic air pollutants that are found to form by the chemisorption of substituted aromatic molecules on the surface of metal oxides. In this study, we employ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) to perform a temperature-dependent study of phenol adsorption on α-Fe2O3(0001) to probe the radical formation mechanism by monitoring changes in the electronic structure of both the adsorbed phenol and metal oxide substrate. Upon dosing at room temperature, new phenol-derived electronic states have been clearly observed in the UPS spectrum at saturation coverage. However, upon dosing at high temperature (>200 °C), both photoemission techniques have shown distinctive features that strongly suggest electron transfer from adsorbed phenol to Fe2O3 surface atoms and consequent formation of a surface radical. Consistent with the experiment, DFT calculations show that phenoxyl adsorption on the iron oxide surface at RT leads to a minor charge transfer to the adsorbed molecule. The experimental findings at high temperatures agree well with the EPFRs' proposed formation mechanism and can guide future experimental and computational studies.

A Noble-Transition Alloy Excels at Hot-Carrier Generation in the Near Infrared.

Above-equilibrium "hot"-carrier generation in metals is a promising route to convert photons into electrical charge for efficient near-infrared optoelectronics. However, metals that offer both hot-carrier generation in the near-infrared and sufficient carrier lifetimes remain elusive. Alloys can offer emergent properties and new design strategies compared to pure metals. Here, it is shown that a noble-transition alloy, Aux Pd1- x , outperforms its constituent metals concerning generation and lifetime of hot carriers when excited in the near-infrared. At optical fiber wavelengths (e.g., 1550 nm), Au50 Pd50 provides a 20-fold increase in the number of ≈0.8 eV hot holes, compared to Au, and a threefold increase in the carrier lifetime, compared to Pd. The discovery that noble-transition alloys can excel at hot-carrier generation reveals a new material platform for near-infrared optoelectronic devices.

Vibrational and Structural Studies of Environmentally Persistent Free Radicals Formed by Phenol-Dosed Metal Oxide Nanoparticles.

Environmentally persistent free radicals (EPFRs) are formed by the adsorption of substituted aromatic precursors on the surface of metal oxides and are known to have significant health and environmental impact due to their unique stability. In this article, the formation of EPFRs is studied by adsorption of phenol on ZnO, CuO, Fe2O3, and TiO2 nanoparticles (∼10-50 nm) at high temperatures. Electron paramagnetic resonance indicates the formation of phenoxyl-type radicals. Fourier transform infrared spectroscopy provides further evidence of EPFR formation by the disappearance of -OH groups, indicating the chemisorption of the organic precursor on the metal oxide surface. These results are further confirmed by inelastic neutron scattering, which shows both ring out-of-plane bend and C-H in-plane bend motions characteristic of phenol adsorption on the studied systems. Also, the changes in the oxidation state of the metal cations are investigated by X-ray photoelectron spectroscopy, which shows that the direction of electron transfer (redox) during phenol chemisorption is strongly dependent on surface properties as well as surface defects of the metal oxide surface.

Electronic signatures of a model pollutant-particle system: chemisorbed phenol on TiO2(110).

Environmentally persistent free radicals (EPFRs) are a class of composite organic/metal oxide pollutants that have recently been discovered to form from a wide variety of substituted benzenes chemisorbed to commonly encountered oxides. Although a qualitative understanding of EPFR formation on particulate metal oxides has been achieved, a detailed understanding of the charge transfer mechanism that must accompany the creation of an unpaired radical electron is lacking. In this study, we perform photoelectron spectroscopy and electron energy loss spectroscopy on a well-defined model system-phenol chemisorbed on TiO2(110) to directly observe changes in the electronic structure of the oxide and chemisorbed phenol as a function of adsorption temperature. We show strong evidence that, upon exposure at high temperature, empty states in the TiO2 are filled and the phenol HOMO is depopulated, as has been proposed in a conceptual model of EPFR formation. This experimental evidence of charge transfer provides a deeper understanding of the EPFR formation mechanism to guide future experimental and computational studies as well as potential environmental remediation strategies.

EPFR Formation from Phenol adsorption on Al2O3 and TiO2: EPR and EELS studies.

We have examined the formation of environmentally persistent free radicals (EPFRs) from phenol over alumina and titania using both powder and single-crystal samples. Electron paramagnetic resonance (EPR) studies of phenol adsorbed on metal oxide powders indicates radical formation on both titania and alumina, with both oxides forming one faster-decaying species (lifetime on the order of 50-100 hours) and one slower-decayng species (lifetimes on the order of 1000 hours or more). Electron energy loss spectroscopy (EELS) measurements comparing physisorbed phenol on single-crystal TiO2(110) to phenoxyl radicals on the same substrate indicate distinct changes in the π-π* transitions from phenol after radical formation. The identical shifts are observed from EELS studies of phenoxyl radicals on ultrathin alumina grown on NiAl(110), indicating that this shift in the π-π* transition may be taken as a general hallmark of phenoxyl radical formation.

The role of interstitial sites in the Ti3d defect state in the band gap of titania.

Titanium dioxide (TiO2) has a number of uses in catalysis, photochemistry, and sensing that are linked to the reducibility of the oxide. Usually, bridging oxygen (Obr) vacancies are assumed to cause the Ti3d defect state in the band gap of rutile TiO2(110). From high-resolution scanning tunneling microscopy and photoelectron spectroscopy measurements, we propose that Ti interstitials in the near-surface region may be largely responsible for the defect state in the band gap. We argue that these donor-specific sites play a key role in and may dictate the ensuing surface chemistry, such as providing the electronic charge required for O2 adsorption and dissociation. Specifically, we identified a second O2 dissociation channel that occurs within the Ti troughs in addition to the O2 dissociation channel in O(br) vacancies. Comprehensive density functional theory calculations support these experimental observations.