Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 16 de 16
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
Langmuir ; 2020 Aug 03.
Artigo em Inglês | MEDLINE | ID: mdl-32664735

RESUMO

To bridge the gap between the cleanliness of a freshly cleaved surface of 2D BiOI and that available from a purely chemical-etching means, we subjected single-crystal BiOI to a series of surface treatments and quantified the resulting chemical states and electronic properties. Vapor transport syntheses included both physical vapor transport from single-source BiOI, as well as chemical vapor transport from Bi2O3 + BiI3 and from Bi + I2 + Bi2O3. Surface treatments included tape cleaving, rinsing in water, sonication in acetone, an aqueous HF etch, and a sequential HF etch with subsequent sonication in acetone. X-ray diffraction, XRD, and X-ray photoelectron spectroscopy, XPS, probed the resulting bulk crystalline species and interfacial chemical states, respectively. In comparison with overlayer models of idealized oxide-terminated or iodide-terminated BiOI, angle-resolved XPS elucidated surface terminations as a function of each treatment. Ultraviolet photoelectron spectroscopy, UPS, established work-function, and Fermi-level energies for each treatment. Data reveal that HF etching yields interfacial BiI3 at BiOI steps that is subsequently removed with acetone sonication. UPS establishes n-type behavior for the vapor-transport-synthesized BiOI, and surface work function and Fermi level shifts for each chemical treatment under study. We discuss the implications for processing BiOI nanofilms for energy-conversion applications.

2.
Photochem Photobiol Sci ; 18(12): 2849-2853, 2019 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-31729503

RESUMO

Recently, we demonstrated that triphenylacetic acid could be used to seal dye molecules within MOF-5, but guest release required the digestion of the framework by treatment with acid. We prepared the sterically bulky photocapping group [bis-(3-nitro-benzyl)-amino]-(3-nitro-phenyl)-acetic acid (PC1) that can prevent crystal violet dye diffusion from inside MOF-5 until removed by photolysis.

3.
Phys Chem Chem Phys ; 21(32): 17880-17892, 2019 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-31380540

RESUMO

Zeolites have recently attracted attention for upgrading renewable resources in the presence of liquid water phases; however, the stability of zeolites in the presence of liquid-phase water is not completely understood. Accordingly, the stability of the ZSM-5 framework and its acid sites was studied in the presence of water at temperatures ranging from 250 to 450 °C and at pressures sufficient to maintain a liquid or liquid-like state (25 MPa). Treated samples were analyzed for framework degradation and Al content and coordination using a variety of complementary techniques, including X-ray diffraction, electron microscopy, N2 sorption, 27Al and 29Si NMR spectroscopy, and several different types of infrared spectroscopy. These analyses indicate that the ZSM-5 framework retains >80% crystallinity at all conditions, and that 300-400 °C are the most aggressive. Decrystallization appears to initiate primarily at crystal surfaces and share many characteristics in common with alkali promoted desilication. Liquid water treatment promotes ZSM-5 dealumination, following a mechanism analogous to that observed under steaming conditions: initiation by Al-O hydrolysis, Al migration to the surface, and finally deposition as extra framework Al or possibly complete dissolution under some conditions. As with the framework, dealumination is most aggressive at 300-400 °C. Several models were evaluated to capture the non-Arrhenius effect of temperature on decrystallization and dealumination, the most successful of which included temperature dependent values of the water auto-ionization constant. These results can help interpretation of previous studies on ZSM-5 catalysis in hot liquid water and suggest future approaches to extend catalyst lifetime.

4.
Langmuir ; 35(29): 9352-9363, 2019 Jul 23.
Artigo em Inglês | MEDLINE | ID: mdl-31260325

RESUMO

We functionalized chemically oxidized Si(111) and TiO2 surfaces with covalently attached rylene molecules and demonstrated further chemical conversion of the attached species. Base-catalyzed activation of perylene tetracarboxylic dianhydride (PTCDA) preceded reaction with phenylaminosilane-terminated surfaces, yielding surface-bound perylene via an imide linkage. Transflection infrared (IR) spectroscopy of the carbonyl vibrational region elucidated the presence of anhydride, imide, and ester species following each reaction stage. The presence of both anhydride and imide IR features following reaction with PTCDA validates successful perylene attachment. Subsequent functionalization of the surface-attached perylenes yielded IR spectra with little or no detectable anhydride features that indicate successful conversion to ester or imide species based on respective reactions with alkyl bromides or aryl amines. X-ray photoelectron spectroscopy quantified fractional coverages of surface-attached perylene species following a post-deposition derivatization with fluorine-containing alkyl bromides and with anilines. Overlayer model interpretation of the photoelectron results determined a perylene surface coverage of ∼15% relative to the surface density of Si(111) atop sites and a ∼10% surface coverage of imide-terminated perylene species. The interpreted coverage data yield an approximate conversion efficiency for the anhydride-to-imide derivatization at surface-attached perylenes of ∼66%. We discuss the present results in terms of possible coverage and packing on oxide-free silicon surfaces and the utilization of covalently attached rylene species as electron-transporting and hole-blocking connecting layers in molecular electronics and tandem-junction photovoltaic designs.

5.
Dalton Trans ; 48(14): 4520-4529, 2019 Apr 02.
Artigo em Inglês | MEDLINE | ID: mdl-30888344

RESUMO

Luminescent metal-organic frameworks (MOFs) have been explored extensively as potential probes for nitroaromatic molecules, which are common constituents of explosive devices. Guest encapsulation within MOF pores is often cited as the prerequisite for emission changes, but the evidence for this signal transduction mechanism is often inadequate. Using the unique bipyridyl ligand AzoAEpP (2,2'-bis[N,N'-(4-pyridyl)ethyl]diaminoazobenzene), we constructed two luminescent pillared paddle-wheel Zn2+ MOFs using aryl dicarboxylate ligands 1,4-naphthalenedicarboxylic acid (ABMOF-1) and benzene 1,4-dicarboxylic acid (ABMOF-2). While both MOFs exhibit luminescence, 2,4-dinitrophenol only extinguishes ABMOF-1 emission. Since the size of the pores in ABMOF-1 precludes guest inclusion, we used X-ray photoelectron spectroscopy (XPS) to confirm the surface interaction and obtain insight into the nature of the quenching process. XPS experiments utilized a fluorinated nitroaromatic molecule, 4-trifluoromethyl-2,6-dinitrophenol, that extinguishes ABMOF-1 emission, and verified surface adsorption through a series of angle-resolved (ARXPS) and argon-ion sputter depth profile experiments. By further developing these techniques, we hope to develop a general approach for distinguishing between the various intermolecular interactions between MOFs and analytes that lead to changes in luminescence.

6.
Nat Commun ; 10(1): 16, 2019 01 03.
Artigo em Inglês | MEDLINE | ID: mdl-30604757

RESUMO

There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn0.5Ge0.5I3) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N2 atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.

7.
J Am Chem Soc ; 140(47): 16042-16047, 2018 11 28.
Artigo em Inglês | MEDLINE | ID: mdl-30415539

RESUMO

Framework nitrogen atoms of carbon nitride (C3N4) can coordinate with and activate metal sites for catalysis. In this study, C3N4 was employed to harvest visible light and activate Co2+ sites, without the use of additional ligands, in photochemical CO2 reduction. Photocatalysts containing single Co2+ sites on C3N4 were prepared by a simple deposition method and demonstrated excellent activity and product selectivity toward CO formation. A turnover number of more than 200 was obtained for CO production using the synthesized photocatalyst under visible-light irradiation. Inactive cobalt oxides formed at relatively high cobalt loadings but did not alter product selectivity. Further studies with X-ray absorption spectroscopy confirmed the presence of single Co2+ sites on C3N4 and their important role in achieving selective CO2 reduction.

8.
ACS Appl Mater Interfaces ; 9(39): 34377-34388, 2017 Oct 04.
Artigo em Inglês | MEDLINE | ID: mdl-28880534

RESUMO

We investigated synthetic strategies for the functionalization of Si(111) surfaces with organic species containing amine moieties. We employed the functionalized surfaces to chemically "glue" perovskites to silicon with efficient electron transfer and minimal oxidation leading to deleterious recombination at the silicon substrate. A two-step halogenation-alkylation reaction produced a mixed allyl-methyl monolayer on Si(111). Subsequent reactions utilized multiple methods of brominating the allyl double bond including reaction with HBr in acetic acid, HBr in THF, and molecular bromine in dichloromethane. Reaction with ammonia in methanol effected conversion of the bromide to the amine. X-ray photoelectron spectroscopy (XPS) quantified chemical states and coverages, transient-microwave photoconductivity ascertained photogenerated carrier lifetimes, atomic force microscopy (AFM) quantified perovskite-silicon adhesion, and nonaqueous photoelectrochemistry explored solar-energy-conversion performance. The HBr bromination followed by the amination yielded a surface with ∼10% amine sites on the Si(111) with minimal oxide and surface recombination velocity values below 120 cm s-1, following extended exposures to air. Importantly, conversion of amine sites to ammonium and deposition of methylammonium lead halide via spin coating and annealing did not degrade carrier lifetimes. AFM experiments quantified adhesion between perovskite films and alkylammonium-functionalized or native-oxide silicon surfaces. Adhesion forces/interactions between the perovskite and the alkylammonium-functionalized films were comparable to the interaction between the perovskite and native-oxide silicon surface. Photoelectrochemistry of perovskite thin films on alkylammonium-functionalized n+-Si showed significantly higher Voc than n+-Si with a native oxide when in contact with a nonaqueous ferrocene+/0 redox couple. We discuss the present results in the context of utilizing molecular organic recognition to attach perovskites to silicon utilizing organic linkers so as to inexpensively modify silicon for future tandem-junction photovoltaics.

9.
ACS Appl Mater Interfaces ; 9(2): 1459-1470, 2017 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-27991759

RESUMO

Metal oxides with moderate band gaps are desired for efficient production of hydrogen from sunlight and water via photoelectrochemical (PEC) water splitting. Here, we report an α-SnWO4 photoanode synthesized by hydrothermal conversion of WO3 films that achieves photon to current conversion at wavelengths up to 700 nm (1.78 eV). This photoanode is promising for overall PEC water-splitting because the flat-band potential and voltage of photocurrent onset are more negative than the potential of hydrogen evolution. Furthermore, the photoanode utilizes a large portion of the solar spectrum. However, the photocurrent density reaches only a small fraction of that which is theoretically possible. Density functional theory based thermodynamic and electronic structure calculations were performed to elucidate the nature and impact of defects in α-SnWO4 prepared by this synthetic route, from which hole localization at Sn-at-W antisite defects was determined to be a likely cause for the poor photocurrent. Measurements further showed that the photocurrent decreases over time due to surface oxidation, which was suppressed by improving the kinetics of hole transfer at the semiconductor/electrolyte interface. Alternative synthetic methods and the addition of protective coatings and/or oxygen evolution catalysts are suggested to improve the PEC performance and stability of this promising α-SnWO4 material.

10.
Angew Chem Int Ed Engl ; 54(39): 11428-32, 2015 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-26184365

RESUMO

Solar water splitting in acidic solutions has important technological implications, but has not been demonstrated to date in a dual absorber photoelectrochemical cell. The lack of functionally stable water-oxidation catalysts (WOCs) in acids is a key reason for this slow development. The only WOCs that are stable at low pH are Ir-based systems, which are typically too expensive to be implemented broadly. It is now shown that this deficiency may be corrected by applying an ultra-thin monolayer of a molecular Ir WOC to hematite for solar water splitting in acidic solutions. The turn-on voltage is observed to shift cathodically by 250 mV upon the application of a monolayer of the molecular Ir WOC. When the molecular WOC is replaced by a heterogeneous multilayer derivative, stable solar water splitting for over 5 h is achieved with near-unity Faradaic efficiency.

11.
J Phys Chem B ; 114(7): 2435-49, 2010 Feb 25.
Artigo em Inglês | MEDLINE | ID: mdl-20108956

RESUMO

Several complementary experimental and theoretical methodologies were used to explore water uptake on sodium chloride (NaCl) particles containing varying amounts of sodium dodecyl sulfate (SDS) to elucidate the interaction of water with well-defined, environmentally relevant surfaces. Experiments probed the hygroscopic growth of mixed SDS/NaCl nanoparticles that were generated by electrospraying aqueous 2 g/L solutions containing SDS and NaCl with relative NaCl/SDS weight fractions of 0, 5, 11, 23, or 50 wt/wt %. Particles with mobility-equivalent diameters of 14.0(+/-0.2) nm were size selected and their hygroscopic growth was monitored by a tandem nano-differential mobility analyzer as a function of relative humidity (RH). Nanoparticles generated from 0 and 5 wt/wt % solutions deliquesced abruptly at 79.1(+/-1.0)% RH. Both of these nanoparticle compositions had 3.1(+/-0.5) monolayers of adsorbed surface water prior to deliquescing and showed good agreement with the Brunauer-Emmett-Teller and the Frenkel-Halsey-Hill isotherms. Above the deliquescence point, the growth curves could be qualitatively described by Kohler theory after appropriately accounting for the effect of the particle shape on mobility. The SDS/NaCl nanoparticles with larger SDS fractions displayed gradual deliquescence at a RH that was significantly lower than 79.1%. All compositions of SDS/NaCl nanoparticles had monotonically suppressed mobility growth factors (GF(m)) with increasing fractions of SDS in the electrosprayed solutions. The Zdanovskii-Stokes-Robinson model was used to estimate the actual fractions of SDS and NaCl in the nanoparticles; it suggested the nanoparticles were enhanced in SDS relative to their electrospray solution concentrations. X-ray photoelectron spectroscopy (XPS), FTIR, and AFM were consistent with SDS forming first a monolayer and then a crystalline phase around the NaCl core. Molecular dynamics simulations of water vapor interacting with SDS/NaCl slabs showed that SDS kinetically hinders the initial water uptake. Large binding energies of sodium methyl sulfate (SMS)-(NaCl)(4), H(2)O-(NaCl)(4), and SMS-H(2)O-(NaCl)(4) calculated at the MP2/cc-pVDZ level suggested that placing H(2)O in between NaCl and surfactant headgroup is energetically favorable. These results provide a comprehensive description of SDS/NaCl nanoparticles and their properties.

12.
J Phys Chem A ; 114(3): 1411-9, 2010 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-19848399

RESUMO

We investigate the Rayleigh discharge and evaporation dynamics of highly charged two-component droplets consisting principally of methanol with 2-methoxyethanol, tert-butanol, or m-nitrobenzyl alcohol. A phase Doppler anemometer (PDA) characterizes droplets generated by electrospray ionization (ESI) according to size, velocity, and charge as they move through a uniform electric field within an ion mobility spectrometer (IMS). Repeated field reversals result in droplet "ping-pong" through the PDA. This generates individual droplet histories of solvent evaporation behavior and the dynamics of charge loss to progeny droplets during Rayleigh discharge events. On average, methanol droplets discharge at 127% their Rayleigh limit of charge, q(R), and release 25% of the net charge. Charge loss from methanol/2-methoxyethanol droplets behaves similarly to pure 2-methoxyethanol droplets which release approximately 28% of their net charge. Binary methanol droplets containing up to 50% tert-butanol discharge at a lower percent q(R) than pure methanol and release a greater fraction of their net charge. Mixed 99% methanol/1% m-nitrobenzyl alcohol droplets possess discharge characteristics similar to those of methanol. However, droplets of methanol containing 2% m-nitrobenzyl evaporate down to a fixed size and charge that remains constant with no observable discharges. Quasi-steady-state evaporation models accurately describe observed evaporation phenomena in which methanol/tert-butanol droplets evaporate at a rate similar to that of pure methanol and methanol/2-methoxyethanol droplets evaporate at a rate similar to that of 2-methoxyethanol. We compare these results to previous Rayleigh discharge experiments and discuss the implications for binary solvents in electrospray mass spectrometry (ESI-MS) and field-induced droplet ionization mass spectrometry (FIDI-MS).


Assuntos
Termodinâmica , Álcoois Benzílicos/química , Etilenoglicóis/química , Metanol/química , Espectrometria de Massas por Ionização por Electrospray , Tensão Superficial , Volatilização , terc-Butil Álcool/química
13.
J Phys Chem A ; 113(10): 2060-9, 2009 Mar 12.
Artigo em Inglês | MEDLINE | ID: mdl-19173586

RESUMO

A combination of experiments and molecular dynamic (MD) simulations has been applied to elucidate the nature of water on organic self-assembled monolayers (SAMs) before and after oxidation. SAMs mimic organics adsorbed on environmental urban surfaces. Water on clean or SAM-coated borosilicate glass surfaces was measured at equilibrium as a function of relative humidity (RH), using transmission Fourier transform infrared (FTIR) spectroscopy at 1 atm and 22 +/- 1 degrees C. The SAMs included C18 and C8 alkanes, as well as the C8 terminal alkene. Oxidation of the terminal alkene SAM was carried out with either KMnO(4) solution or gaseous O(3). The FTIR data showed at least two distinct peaks due to water on these surfaces, one at approximately 3200 cm(-1), which dominates at low RH (20%), and one at approximately 3400 cm(-1) at high RH (80%), which is similar to that in bulk liquid water. Temperature-programmed desorption (TPD) experiments showed that oxidation leads to more strongly adsorbed water. However, the amount of water in equilibrium with water vapor on the oxidized alkene was not significantly different from that on the unoxidized SAM, although there was a change in the relative intensities of the two contributing infrared peaks at 80% RH. MD simulations with hydrogen bond analysis suggest that molecules on the surface of small water clusters that dominate on SAM surfaces at low RH have fewer hydrogen bonds, while those in the interior of the clusters have three and four hydrogen bonds similar to bulk liquid water. Taken together, the experimental infrared data and MD simulations suggest a correlation between the relative intensities of the 3200 cm(-1)/3400 cm(-1) bands and the hydrogen-bonding patterns of the water on the surface and in the interior of clusters on the SAM surfaces. These studies suggest that water clusters will be present even on hydrophobic surfaces in the atmosphere and hence are available to participate in heterogeneous chemistry. In addition, oxidation of organic coatings on atmospheric particles or surfaces in the boundary layer may not lead to enhanced water uptake as is often assumed.

14.
Anal Chem ; 78(11): 3800-6, 2006 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-16737240

RESUMO

The recently developed technique of field-induced droplet ionization (FIDI) is applied to study interfacial chemistry of a single droplet. In a new variation of the FIDI method, 1-2-mm-diameter droplets hang from a capillary and undergo heterogeneous reactions between solution-phase analytes and gas-phase species. Following a specified reaction time, the application of a high electric field induces FIDI in the droplet, generating fine jets of highly charged progeny droplets that are characterized by mass spectrometry. Sampling over a range of delay times following exposure of the droplet to gas-phase reactants, the spectra yield the temporal variation of reactant and product concentrations. We illustrate the technique with three examples: the adsorption of the polycyclic aromatic hydrocarbon naphthalene into a water-methanol droplet, the ozonolysis of oleic acid, and localization of the carbon-carbon double bond within a lysophosphatidic acid. Gas-phase naphthalene reacts with 80% methanol-20% water droplets containing 100 microM silver nitrate. Positive ion mass spectra show increasing concentrations of silver ion-naphthalene adducts as exposure times increase. To examine the ozonolysis of organic molecules, gas-phase ozone generated by a mercury pencil-style lamp reacts with either 10 microM oleic acid or 100 microM oleoyl-L-alpha-lysophosphatidic acid (LPA; 18:1). Negative ion spectra from the ozonolysis of oleic acid show azelaic acid and 9-oxononanoic acid as the principle reaction products. Ozonolysis products from LPA (18:1) unambiguously demonstrate the double bond position in the original phospholipid.

15.
J Phys Chem B ; 109(16): 8244-50, 2005 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-16851963

RESUMO

We recently reported that strong electric fields may be employed to directly extract positive and negative ions for mass analysis, including intact proteins, from neutral droplets. The present study investigates the dynamics of this process using switched high electric fields to enable time-resolved studies of droplet distortion, Taylor cone formation, and charged progeny droplet extraction from neutral and charged 225 microm methanol droplets. After a specific time in the field, a flashlamp is triggered to record droplet distortions using shadow photography. At a critical field strength E(c)0 corresponding to the Taylor limit, neutral droplets exhibit a prolate elongation along the field axis forming symmetric cone-jets of positive and negatively charged progeny droplets, approximately 10 microm in diameter. This process is termed field-induced droplet ionization (FIDI). Because the time scale of FIDI is related to the frequency of shape oscillations that occur below the Taylor limit, models of field-dependent oscillation become an important predictor of the time scale for progeny jet formation. Droplets with a net charge q distort into asymmetric tear shapes and emit a single charged jet of progeny at a critical field E(c)(q) that is less than E(c)0. The measured decrease in droplet stream charge indicates that total charge loss can be greater than the original charge on the droplet, resulting in oppositely charged droplets. Interestingly, above E(c)0, charged droplets sequentially emit a jet of the same polarity as the net charge followed by a jet of reverse polarity emitted in the opposite direction. For both neutral and charged droplets, increasing the electric field decreases the time to form jets and the combination of net charge and higher-than-critical fields has a compound effect in accelerating progeny formation. The implications of our results for using switched fields in FIDI-mass spectrometry for on-demand ion sampling from neutral and charged droplets are discussed.


Assuntos
Eletroquímica , Metanol/química , Condutividade Elétrica , Modelos Químicos , Fatores de Tempo
16.
Anal Chem ; 74(24): 6291-7, 2002 Dec 15.
Artigo em Inglês | MEDLINE | ID: mdl-12510751

RESUMO

We report studies of the evaporation and discharge dynamics of highly charged droplets generated by electrospray ionization from n-heptane, n-octane, and p-xylene doped with Stadis-450, a conductivity-enhancing agent. A phase Doppler anemometer (PDA) characterizes individual droplets moving through the uniform electric field within an ion mobility cell according to size, velocity, and charge. Repeated reversal of the electric field allows multiple PDA measurements on selected droplets with diameters ranging from 3 to 60 microm and up to 10(7) elementary positive charges. This "ping-pong" technique provides individual droplet histories from which we determine the dynamics of solvent evaporation and charge loss. On average, n-heptane discharges at 101% of the Rayleigh limit of charge, while n-octane and p-xylene droplets discharge at 87% and 89% of their respective limits. Discharge events release an average of 19% of the charge in n-heptane and 17% of the charge in both n-octane and p-xylene. Within the limits of the measurements, no detectable change in droplet diameter accompanies observed discharge events, indicating the loss of a relatively small fraction of the total volume. We compare these results to previous experiments, theoretical models for droplet evaporation and discharge, and predictions from the Rayleigh model. We report both Stadis-450 and triethylamine mass spectra in octane and discuss issues regarding the use of hydrocarbon solvents in electrospray mass spectrometry.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA