Browsing by Subject "Xps"

Now showing 1 - 4 of 4

Open Access
The formation and characterization of cyclodextrin functionalized polystyrene nanofibers produced by electrospinning
(2009) Uyar, Tamer; Havelund, R.; Hacaloglu J.; Zhou X.; Besenbacher F.; Kingshott P.
Polystyrene (PS) nanofibers containing the inclusion complex forming beta-cyclodextrin (β-CD) were successfully produced by electrospinning aimed at developing functional fibrous nanowebs. By optimization of the electrospinning parameters, which included varying the relative concentration of PS and β-CD in the solutions, bead-free fibers were produced. Homogeneous solutions of β-CD and PS in dimethylformamide (DMF) were used with concentrations of PS varying from 10% to 25% (w/v, with respect to DMF), and β-CD concentrations of 1% to 50% (w/w, with respect to PS). The presence of β-CD facilitated the production of bead-free PS fibers even from lower polymer concentrations as a result of the higher conductivity of the PS/CD solutions. The morphology and the production of bead-free PS/CD fibers were highly dependent on the β-CD contents. Transmission electron microscope (TEM) and atomic force microscope (AFM) images showed that incorporation of β-CD yielded PS fibers with rougher surfaces. Thermogravimetric analysis (TGA) and direct insertion probe pyrolysis mass spectroscopy (DP-MS) results confirmed the presence of β-CD in the PS fibers. X-ray diffraction (XRD) spectra of the fibers indicated that the β-CD molecules are distributed within the PS matrix without any phase separated crystalline aggregates up to 40% (w/w) β-CD loading. Furthermore, chemical analyses by Fourier transform infrared (FTIR) spectroscopy studies confirm that β-CD molecules are located within the PS fiber matrix. Finally, preliminary investigations using x-ray photoelectron spectroscopy (XPS) and time-of-flight static secondary ion mass spectrometry (ToF-static-SIMS) show the presence of the cyclodextrin molecules in the outer molecular layers of the fiber surfaces. The XPS and ToF-SIMS findings indicate that cyclodextrin functionalized PS webs would have the potential to be used as molecular filters and/or nanofilters for the purposes of filtration/purification/separation owing to surface associated β-CD molecules which have inclusion complexation capability. © 2009 IOP Publishing Ltd.
Open Access
Response of polyelectrolyte layers to the SiO2 substrate charging as probed by XPS
(2009) Conger, C. P.; Süzer, Şefik
A single layer of the Cationic polyelectrolyte poly(allyamine) hydrochloride (PAH) deposited, using the layer-by-layer technique, on a silicon substrate containing 5 nm oxide layer is investigated by XPS while applying an external potential bias to the sample to control and manipulate the charge built-up on the oxide layer. Under application of a -10 V bias, the oxide layer is positively charged due to Photoemission process, evidenced by the measured Si2p binding energy of 104.4 eV. Application of a +10 V bias attracts the low energy neutralizing electrons, stemming from a hot filament, and leads to a negatively charged oxide layer, also evidenced by the measured Si2p binding energy of 102.9 eV. The single polyelectrolyte overlayer also responds to this polarity change of the oxide layer underneath by displaying a somewhat larger shifts both in the C1s and Nls peaks. In addition to the shifts in the positions, the N1s peaks undergo a significant intensity depletion, mostly on the positively charged -N+ component. We interpret this intensity depletion to be the result of reorientation of some of the dangling positively charged groups by moving toward the negatively charged oxide underlayer. To our knowledge this is the first time that a chemically specific response to an electrical stimuli is reported using XPS. A bilayer LbL film consisting of PAH and PSS, exhibits even a larger charging shift, but this time no intensity alteration is observed, most probably due to locking of the -N+ groups by the -SO3 + counterions of the second layer. © 2009 American Chemical Society.
Open Access
Role of the exposed Pt active sites and BaO2 formation in nox storage reduction systems: a model catalyst study on BaOx/Pt(111)
(American Chemical Society, 2011) Vovk, E. I.; Emmez, E.; Erbudak, M.; Bukhtiyarov, V. I.; Ozensoy, E.
BaOx(0.5 MLE - 10 MLE)/Pt(111) (MLE: monolayer equivalent) surfaces were synthesized as model NOx storage reduction (NSR) catalysts. Chemical structure, surface morphology, and the nature of the adsorbed species on BaOx/Pt(111) surfaces were studied via X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). For θBaOx < 1 MLE, (2 2) or (1 2) ordered overlayer structures were observed on Pt(111), whereas BaO(110) surface termination was detected for θBaOx = 1.5 MLE. Thicker films (θBaOx g 2.5 MLE) were found to be amorphous. Extensive NO2 adsorption on BaOx(10 MLE)/Pt(111) yields predominantly nitrate species that decompose at higher temperatures through the formation of nitrites. Nitrate decomposition occurs on BaOx(10 MLE)/Pt(111) in two successive steps: (1) NO(g) evolution and BaO2 formation at 650 K and (2) NO(g) + O2(g) evolution at 700 K. O2(g) treatment of the BaOx(10 MLE)/ Pt(111) surface at 873 K facilitates the BaO2 formation and results in the agglomeration of BaOx domains leading to the generation of exposed Pt(111) surface sites. BaO2 formed on BaOx(10 MLE)/Pt(111) is stable even after annealing at 1073 K, whereas on thinner films (θBaOx = 2.5 MLE), BaO2 partially decomposes into BaO, indicating that small BaO2 clusters in close proximity of the exposed Pt(111) sites are prone to decomposition. Nitrate decomposition temperature decreases monotonically from 550 to 375 K with decreasing BaOx coverage within θBaOx = 0.5 to 1.0 MLE. Nitrate decomposition occurs at a rather constant temperature range of 650700 K for thicker BaOx overlayers (2.5 MLE < θBaOx < 10 MLE). These two distinctly characteristic BaOx-coveragedependent nitrate decomposition regimes are in very good agreement with the observation of the so-called “surface” and “bulk” barium nitrates previously reported for realistic NSR catalysts, clearly demonstrating the strong dependence of the nitrate thermal stability on the NOx storage domain size.
Open Access
X-ray photoelectron spectroscopy for resistance-capacitance measurements of surface structures
(AIP Publishing, 2005-04-29) Ertas, G.; Demirok, U. K.; Atalar, Abdullah; Süzer, Şefik
In x-ray photoemission measurements, differential charging causes the measured binding energy difference between the Si 2p of the oxide and the silicon substrate to vary nonlinearly as a function of the applied external do voltage stress, which controls the low-energy electrons going into and out of the sample. This nonlinear variation is similar to the system where a gold metal strip is connected to the same voltage stress through an external 10 Mohm series resistor and determined again by x-ray photoelectron spectroscopy (XPS). We utilize this functional resemblance to determine the resistance of the 4 nm SiO2 layer on a silicon substrate as 8 Mohm. In addition, by performing time-dependent XPS measurements (achieved by pulsing the voltage stress), we determine the time constant for charging/discharging of the same system as 2.0 s. Using an equivalent circuit, consisting of a gold metal strip connected through a 10 Mohm series resistor and a 56 nF parallel capacitor, and performing time-dependent XPS measurements, we also determine the time constant as 0.50 s in agreement with the expected value (0.56 s). Using this time constant and the resistance (8.0 Mohm), we can determined the capacitance of the 4 nm SiO2 layer as 250 nF in excellent agreement with the calculated value. Hence, by application of external do and pulsed voltage stresses, an x-ray photoelectron spectrometer is turned into a tool for extracting electrical parameters of surface structures in a noncontact fashion. (c) 2005 American Institute of Physics.