Au 4f Peak Research Articles

Calibration of binding energy and clarification of interfacial band bending for the Al2O3/diamond heterojunction

Due to the presence of an intrinsic C 1s peak in diamond, it is impossible to calibrate its binding energies using the adventitious C 1s peak (284.8 eV) during x-ray photoelectron spectroscopy measurement. The absence of accurate binding energy measurement makes it challenging to determine the interfacial band bending for the oxide/diamond heterojunction. To overcome this issue, a net-patterned gold (Au) mask is applied to the boron-doped diamond (B-diamond) to suppress the charge-up effect and calibrate the binding energy using the standard Au 4f peak (83.96 eV). The B-diamond epitaxial layer shows downward band bending toward the surface with a valence band maximum of 0.85 eV. Upon the formation of Al2O3 using an ozone precursor through the atomic layer deposition technique, the B-diamond continues to exhibit downward band bending toward the Al2O3/B-diamond interface. However, the bending energy has reduced, potentially attributed to the modification of the oxygen vacancies on the B-diamond surface by the ozone precursor during the Al2O3 deposition.

Avoiding Sabatier’s conflict in bifunctional heterogeneous catalysts for the WGS reaction

Summary A new strategy for designing efficient bifunctional heterogeneous catalysts for the water-gas-shift (WGS) reaction (CO + H2O → CO2 + H2) is presented. We avoid conflicting tasks that require ∗OH or ∗O from dissociated water to adsorb on and then desorb from the substrate of a catalyst. Instead, the CO on metal directly obtains OH from water that is connected by weak hydrogen bonds to the substrate. The task of the substrate is to efficiently channel ∗H on its surface and release them as H2 gas. Experimental and theoretical results show that bifunctional catalysts with weakly reactive substrates have significantly higher CO conversion rates compared with highly reactive substrates that follow either the LH-associative or LH-redox process.

(Invited) Dilute Cu Alloying in Au Nanostructures for Highly Selective and Long-Term Stability of CO2 Reduction

Electrochemical CO2 reduction can produce value-added chemical or fuels in moderate reaction environment. For efficient CO2 reduction to a target product with high selectivity, vast research works have been carried out over a few decades. Particularly, CO2 conversion performance using Au and Ag based catalysts for CO production has reached to economically viable level according to recent techno-economic analysis [1]. In various routes to improve catalytic property, metal alloys are shown effective by tailoring intermediate binding energy of CO2 reduction reaction (CO2RR). For example, bimetallic AuCu alloys, such as AuCu, AuCu3 and Au3Cu, have shown synergetic effects which lead to higher catalytic activities (i.e. overpotential, stability) although CO selectivity is still lower than state-of-art records (over 95%) [2-4]. Herein, we demonstrate the dilute Cu alloying in Au nanostructure for highly selective CO production with long-term stability. To synthesize nanostructured Au with controlled Cu alloying, we firstly formed Cu/Au bilayers by employing the electrochemical deposition (ED) and underpotential deposition (UPD) of Cu on 200-nm-thick Au thin films on Si substrates. ED and UPD processes were conducted at applied potentials of –0.1 and 0.3 V (vs. Ag/AgCl), respectively, in 50 mM H2SO4 electrolyte with 50 mM CuSO4. We carried out electrodepositon for 10s and 60s while underpotentially deposited Cu sample were only prepared for 60s. The Cu-covered Au surfaces were then electrochemically treated for nanostructuring with the same method previously reported by our research group [5]. Specifically, we electrochemically oxidized the Cu/Au bilayers at 2.5 V (vs. RHE) for 40 minutes in 0.2 M KHCO3 solution (pH = 8.5). Then electro-reduction was sequentially followed under constant current density of –0.5 mA cm–2 for about 10 minutes in the same solution. We denote the prepared samples by Nano-ED-CuAu-10s, -60s and Nano-UPD-CuAu in accordance with the type of Cu deposition processes. Additionally, nanostructured Au without Cu was prepared for the comparison. As a result of the electrochemical treatment, nanoporous structures were formed for all samples. The heights of the nanostructures were 200, 215, 225 and 240 nm for Nano-Au, Nano-UPD-CuAu, Nano-ED-CuAu-10s and 60s. XPS investigations clearly show the appearance of Cu 2p peak and slight shift of Au 4f peak position to the higher binding energy (~0.1 eV) when Cu was incorporated in Au nanostructures. In addition, the surface concentration of Cu were found to be 2, 11 and 13% for Nano-UPD-CuAu, Nano-ED-CuAu-10s and 60s, respectively. Our nanostructured CuAu and Au exhibit low CO2RR overpotential and high selectivity for CO in CO2 saturated 0.2 M KHCO3. For instance, the selectivity of CO2RR for all electrodes is around 50 – 70% (Nano-Au : 47%, Nano-UPD-CuAu : 65% Nano-ED-CuAu-10s : 69%, Nano-ED-CuAu-60s : 57%) at low overpotential (–0.34 V (vs. RHE)). It is noted that bare Au thin film shows almost no CO production. At higher applied potential (–0.59 V), the CO selectivity increases over 90% and notably, Nano-UPD-CuAu shows almost unity CO production (~99.5%) with nearly completely suppressing H2 evolution. In addition, Cu-Au nanostructures show the remarkably improved durability as compared to nanostructured Au catalysts. Whereas the CO selectivity at –0.49 V for Nano-Au drops from 92% at 30 minutes to below 80% in about 4.5 hours, Nano-UPD-CuAu retains the same performance for about 7 hours. Surprisingly, Nano-ED-CuAu samples are stable for 12 hours showing the selective CO production over 80%. These results indicate that dilute Cu-Au alloy nanostructures can be effective to improve stability with highly selective CO production. Additionally, in the presentation, we will introduce an in-situ tailoring technique to re-active the degraded Au catalysts by utilizing Cu impurities in electrolyte during CO2 electroreduction process for robust and efficient CO2 reduction. [1] Jouny et al., Ind. Eng. Chem. Res. 57, 2165 (2018) [2] Kim et al., Nat. Commun. 5, 4948 (2014) [3] Kim et al., Appl. Catal. B, 213, 211 (2017) [4] Kim et al., ACS Energy Lett., 3, 2144 (2018) [5] Kim et al., J. Mater. Chem. A., 6, 5119 (2018)

Investigation of Ligand-Stabilized Gold Clusters on Defect-Rich Titania

Chemically synthesized atomically precise gold clusters stabilized by triphenylphosphine ligands [Au9(PPh3)8](NO3)3] were deposited onto the surface of titania fabricated via atomic layer deposition. The titania surface was pretreated by heating and sputtering. After deposition of the clusters onto pretreated titania, the samples were heated at 200 °C for 20 min under ultrahigh vacuum and subsequently investigated using metastable-induced electron spectroscopy to study the electronic structure of the outermost layer of the sample and X-ray photoelectron spectroscopy to determine the chemical composition of the surface of the sample. The former study revealed that two reference spectra are needed to explain the electronic structure of the sample. One reference spectrum is related to the titania substrate, while the second spectrum is related to the presence of the Au cluster cores and the ligands removed from the cluster cores. The latter study found that the Au 4f peak is shifted to lower binding energy a...

In-Situ XPS Monitoring and Characterization of Electrochemically Prepared Au Nanoparticles in an Ionic Liquid.

Gold nanoparticles (Au NPs) have been electrochemically prepared in situ and in vacuo using two different electrochemical device configurations, containing an ionic liquid (IL), N-N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, that serves both as reaction and as stabilizing media for the NPs. It was observed in both devices that Au NPs were created using an anodically triggered route. The created Au NPs are relatively small (3–7 nm) and reside within the IL medium. X-ray photoelectron spectroscopy is utilized to follow not only the formation of the NPs but also their charging/discharging properties, by monitoring the charging shifts of the Au4f peak representing the electrodes and also the Au NPs as well as the F1s peak of the IL after polarizing one of the electrodes. Accordingly, DC polarization across the electrodes leads to a uniform binding energy shift of F1s of the IL along with that of Au4f of the NPs within. Moreover, this shift corresponds to only half of the applied potential. AC polarization brings out another dimension for demonstrating further the harmony between the charging/discharging property of the IL medium and the Au NPs in temporally and laterally resolved fashions. Polarization of the electrodes result in perfect spectral separation of the Au4f peaks of the NPs from those of the metal in both static (DC) and in time- and position-dependent (AC) modes.

Design and Fabrication of Pt-Au Nanocatalyst with High Performance

Design and Fabrication of Pt-Au Nanocatalyst with High Performance Fanpeng Kong[a], Chunyu Du[a]*, Jinyu Ye[b], Lei Du[a], Guangyu Chen[a], Geping Yin[a]* [a] MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, Harbin Institute of Technology. Email: cydu@hit.edu.cn, yingphit@hit.edu.cn. [b] State Key Lab of Physical Chemistry of Solid Surfaces, Xiamen University. : Direct formic acid fuel cell (DFAFC), as a promising power source, can greatly improve energy conversion efficiency of small organic molecules and dramatically reduce the environment pollution.[1-2] Pt is a key catalytic component at both anode and cathode in DFAFC. However, the scarcity and high cost of Pt require the design and fabrication of Pt-based nanostructure with high performance and reduced usage of Pt. Here, we found that the formic acid oxidation (FAO) activity of Pt and Au nanoparticles supported on carbon (Pt1-Au1/C) increases more than 40 fold relative to that of Pt/C. Generally, ensemble and electronic effects are proposed to explain the enhanced FAO performance.[3-4] However, we have to exclude the ensemble effect since there is no Au adatoms on Pt. From X-ray photoelectron spectroscopy (XPS) patterns, the binding energy of Pt4f and Au4f peak in Pt1-Au1/C shifts positively and negatively compared to that in Pt/C and Au/C, respectively, indicating that Au modifies the electronic structure of adjacent Pt. This electronic modification is further confirmed by the higher onset potential of CO oxidation on Pt1-Au1/C than that on Pt/C in the CO stripping voltammetry, demonstrating stronger binding of CO on Pt1-Au1/C. Unfortunately, the pathway of formic acid oxidation on Pt-Au system is ambiguous because Pt and Au are closely coupled together, so that the in-situ information on the respective role of Au and Pt is difficult to obtain. To this end, a novel selective surface engineering method is proposed to probe the main active sites. We separately employed CO adsorption and selective under potential deposition to engineer respectively Pt and Au surfaces. However, the activity of Pt1-Au1/C with engineered surfaces is obviously lower than that of Pt1-Au1/C. Therefore, the coexistence of Pt and Au, rather than electronic effect, is indispensable for the high FAO activity. In-situ FTIR measurement of Pt/C, Au/C and Pt1-Au1/C is carried out to probe more detailed molecular information. Formate on Pt1-Au1/C is more than that on Pt/C and Au/C, which should result from the coverage of CO on Au, instead of Pt, at low potentials. Since Pt has higher affinity to H than Au, the increased formate is attributed to the accelerated rupture of O-H bond in formic acid on Au by Pt. However, formate is not an active intermediate but a spectator species, so that it cannot account for the increased activity. Recently, it is proved that –COOH is the active species during FAO. In view of strong affinity between Pt and H, we can rationally assume that Pt ruptures the C-H bond of formic acid on Au to produce –COOH, which decomposes to CO2 quickly, so that Pt1-Au1/C possesses a remarkable FAO activity. Reference: J. V. Perales-Rondon, A. Ferre-Vilaplana, J. M. Feliu, E. Herrero, J. Am. Chem. Soc. 2014, 136, 13110-13113.M. E. Scofield, C. Koenigsmann, L. Wang, H. Liu, S. S. Wong, Energy Environ. Sci. 2015, 8, 350-363.Q. S. Chen, Z. Y. Zhou, F. J. Vidal-Iglesias, J. Solla-Gullon, J. M. Feliu, S. G. Sun, J. Am. Chem. Soc. 2011, 133, 12930-12933.G. R. Zhang, D. Zhao, Y. Y. Feng, B. S. Zhang, D. S. Su, G. Liu, B. Q. Xu, ACS Nano 2012, 6, 2226-2236.

Synthesis of Au-SnO<SUB align="right">2 nanocomposites by two-step laser ablation

Au-SnO2 nanocomposites have been synthesised by two-step laser ablation in liquid medium. The method involves synthesis of gold colloids followed by ablation of tin target in a freshly prepared gold colloidal solution. The effect of varying ablation time shows a decrease in average particle size and formation of core-shell structure. Optical absorption spectra of Au-SnO2 exhibit a red-shift in the localised surface plasmon resonance (LSPR) peak of pure gold nanoparticle as a result of electron transfer from tin oxide to gold. The XPS of the Sn 3d peak of the sample confirms the formation of tin oxide. A negative shift in the binding energy of Au 4f peak in the XPS further confirms charge transfer process.

In situ study of an oxidation reaction on a Pt/C electrode by ambient pressure hard X-ray photoelectron spectroscopy

We have constructed an ambient pressure X-ray photoelectron spectroscopy instrument that uses hard X-ray radiation at the high-performance undulator beamline BL36XU of SPring-8. The dependence of the Au 4f peak intensity from Au foil on the ambient N2 pressure was measured. At a photon energy of 7.94 keV, the Au 4f peak intensity maintained 40% at 3000 Pa compared with that at high vacuum. We designed a polymer electrolyte fuel cell that allows us to perform X-ray photoelectron spectroscopy measurements of an electrode under working conditions. The oxidized Pt peaks were observed in the Pt 3d5/2 level of Pt nanoparticles in the cathode, and the peaks clearly depended on the applied voltage between the anode and cathode. Our apparatus can be applied as a valuable in situ tool for the investigation of the electronic states and adsorbed species of polymer electrolyte fuel cell electrode catalysts under the reaction conditions.

New insights into micro/nanoscale combined probes (nanoAuger, μXPS) to characterize Ag/Au@SiO2 core-shell assemblies.

This work has examined the elemental distribution and local morphology at the nanoscale of core@shell Ag/Au@SiO2 particles. The characterization of such complex metal/insulator materials becomes more efficient when using an initial cross-section method of preparation of the core@shell nanoparticles (ion milling cross polisher). The originality of this route of preparation allows one to obtain undamaged, well-defined and planar layers of cross-cut nano-objects. Once combined with high-resolution techniques of characterization (XPS, Auger and SEM), the process appears as a powerful way to minimize charging effects and enhance the outcoming electron signal (potentially affected by the topography of the material) during analysis. SEM experiments have unambiguously revealed the hollow-morphology of the metal core, while Auger spectroscopy observations showed chemical heterogeneity within the particles (as silver and gold are randomly found in the core ring). To our knowledge, this is the first time that Auger nano probe spectroscopy has been used and successfully optimized for the study of some complex metal/inorganic interfaces at such a high degree of resolution (≈12 nm). Complementarily, XPS Au 4f and Ag 3d peaks were finally detected attesting the possibility of access to the whole chemistry of such nanostructured assemblies.

Selenium Adsorption on Au(111) and Ag(111) Surfaces: Adsorbed Selenium and Selenide Films

Results of a high resolution photoemission and electrochemistry study of Se adsorption on Au(111) and Ag(111) surfaces performed by immersion of pristine samples into an aqeuous solution of Na2Se are presented. Cyclic voltammetry on Au shows formation of selenium adsorbed species and the structures observed in reductive desorption are related to the atomic and polymeric species observed in XPS. In the case of Au(111) XPS spectra in the Se(3d) region indeed show two main features attributed to Se chemisorbed atomically and polymeric Se8 features. Smaller structures due to other types of Se conformations were also observed. The Au(4f) peak line shape does not show core level shifts indicative of Au selenide formation. In the case of silver, XPS spectra for the Ag(3d) show a broadening of the peak and a deconvolution into AgB bulk like and AgSe components shows that the AgSe is located at a lower binding energy, an effect similar to oxidation and sulfidation of Ag. The Se(3d) XPS spectrum is found to be subs...

Chemically-synthesised, atomically-precise gold clusters deposited and activated on titania

Synchrotron XPS was used to investigate a series of chemically-synthesised, atomically-precise gold clusters Au(n)(PPh(3))(y) (n = 8, 9, 11 and 101, with y depending on cluster size) immobilized on titania nanoparticles. The gold clusters were washed with toluene at 100 °C or calcined at 200 °C to remove the organic ligand. From the position of the Au 4f(7/2) peak it is concluded that cluster size is not altered through the deposition. From the analysis of the phosphorous spectra, it can be concluded that the applied heat treatment removes the organic ligands. Washing and calcination leads to partial oxidation and partial agglomeration of the clusters. Oxidation of the clusters is most likely due to the interaction of the cluster core with the oxygen of the titania surface after removal of ligands. The position of the Au 4f(7/2) peak indicates that the size of the agglomerated clusters is still smaller than that of Au(101).

Negatively Charged Gold Atoms in Subnanometric Particles: Experimental Evidence from an X-Ray Photoelectron Spectroscopy Study

The results of an X-ray Photoelectron Spectroscopy study conducted on a series of gold nanoparticles recently reported by us, stabilized by monodentate, bidentate, tridentate and tetradentate thiolate calix[n]arene ligands, are presented here. By virtue of the different denticity of the ligands, the nuclearity of the resulting particles can be tuned down to the subnanometric range. From the present XPS results, a clear correlation among the experimental binding energy of single Au 4f peak components and the specific Au state of charge is proposed, where the smaller (i.e., nanometer) fraction of the series selectively shows negatively charged Au atoms. Our findings are relevant for the open discussion of a specific role played by negatively charged Au atoms in catalytic reactions, especially at low temperatures.

Effect of oxygen mobility in the lattice of Au/TiO2 on formaldehyde oxidation

Two Au catalysts supported on TiO2 were prepared by impregnation method followed by sodium borohydride reduction or calcination in air (Au/TiO2-R and Au/TiO2-C, respectively). The 1 wt % Au/TiO2-R sample was found to be highly efficient for the oxidation of low concentrated formaldehyde at room temperature. A HCHO conversion of 98.5% was achieved with this catalyst, whereas the Au/TiO2-C sample showed almost no activity under the same conditions. Highly dispersed metallic Au nanoparticles with small size (∼3.5 nm) were identified in the 1 wt % Au/TiO2-R catalyst. A significant negative shift of Au4f peak in XPS spectra with respect to bulk metallic Au was observed for the 1 wt % Au/TiO2-R but no similar phenomena was found for the heat-treated catalyst. More Au nanoparticles and higher content of surface active oxygen were identified on the surface of the Au/TiO2-R in comparison with the Au/TiO2-C, suggesting that the Au/TiO2-R catalyst can enhance the amount of active sites and species involved in for HCHO oxidation. The reduction treatment by sodium borohydride promotes the formation of dispersed metallic Au nanoparticles with small size because it facilitates the electron transfer and increases the content of surface Au nanoparticles and activated oxygen. All these factors are responsible for a high activity of this catalyst in the oxidation of HCHO.

In situ XPS study of the adsorption and reactions of NO and O 2 on gold nanoparticles deposited on TiO 2 and SiO 2

Ambient pressure photoelectron spectroscopy (APPES) has been used to study the adsorption of nitric oxide (NO) and molecular oxygen (O 2) over gold-based model catalysts consisting of mono-dispersed gold nanoparticles with different diameters (2–5 nm) and oxide supports (including polycrystalline silica and titania thin films). APPES is an in situ technique that makes possible to monitor via XPS chemical changes occurring on the catalyst surface and to identify adsorbed species under reaction conditions. In our experiments, no changes were observed on the Au/SiO 2 samples during exposure to 0.5 Torr of NO, while adsorbed NO and several N-containing species were detected on Au/TiO 2 model catalysts under the same conditions. In addition, shifts in the Ti 3p and O 1s peaks in TiO 2 were observed relative to the Au 4f peak. Similar behavior, although to a lesser extent, was observed on Au/TiO 2 samples when O 2 was used. In both cases, the shifts of the Ti 3p and O 1s peaks could be attributed to band bending effects on the TiO 2 substrate caused by chemisorption of the gases.

Packing density of HS(CH2)nCOOH self-assembled monolayers

Self-assembled monolayers (SAMs) of HS(CH(2))(n)COOH, n = 5, 10, 15 deposited from ethanol solution onto gold are prepared by five approaches, and their packing densities are evaluated by X-ray photoelectron spectroscopy (XPS) measurements. The five approaches are: (1) direct deposition; (2) acetic-acid-assisted deposition; (3) butyl-amine-assisted deposition; (4) displacement of a preformed HS(CH(2))(n)CH(3) (n = 5, 10, 15) SAMs; and (5) co-deposition with HS(CH(2))(n)CH(3) (n = 5, 10, 15). Packing density metrics are calculated from measurements of SAM and substrate photoemission intensities and their attenuations by two methods. In one case the attenuated photoemissions are expressed as a ratio relative to comparable measurements on an experimental HS(CH(2))(n)CH(3) model system. In the other case a new method is introduced where a calculated attenuation based on theoretical random coil and extended chain models is used as the reference to determine a packing density fraction. Packing densities are also correlated with the S2p(Au-bonded):Au4f peak area ratios and with shifts in the C1s binding energies. SAMs prepared by the direct deposition are a partial multilayer where a second molecular layer is physisorbed onto the SAM and not removable by solvent washing. The addition of acetic acid to the deposition solution disrupts dimer associations of HS(CH(2))(n)COOH in solution and at the surface of the monolayer and yields the most ordered monolayer with the highest density of -COOH groups. The addition of butyl amine results in a labile ammonium carbonate ion pair formation but results in a lower packing density in the SAM. The displacement of the preformed HS(CH(2))(n)CH(3) SAM and the co-deposition of HS(CH(2))(n)CH(3) with HS(CH(2))(n)COOH result in SAMs with little incorporation of the -COOH component.

Gold/Tin Oxide Nanocomposite by Nanojoining

In this paper we report synthesis of Au-SnO2 nanocomposite by nano-joining. It involves synthesis of Au and SnO2 colloids separately followed by post irradiation of mixed Au-SnO2 colloids. We have used 532 nm and 1064 nm wavelengths of a pulsed Nd:YAG laser and different post irradiation time. The post irradiation by 532 nm laser light results in formation of Au-SnO2 nanocomposite by nanojoining. Distinct gold nanoparticles along with the SnO2 aggregates are observed in case of post irradiation by 1064 nm showing no nanojoining effect. Our results indicate that nanojoining is a wavelength selective process. The X-ray Photoelectron Spectroscopy (XPS) indicates negative binding energy shift of Au 4f peak whereas the position of Sn 3d peak remains unaffected after the post irradiation process. The invariance of Sn 3d peak after post irradiation indicates no change in the valence state of Sn. A red shift with respect to the plasmon resonance peak of gold colloids is observed in Au-SnO2 nanocomposite.

Characterization of Au-Rh/TiO<SUB>2</SUB> Bimetallic Nanocatalysts by CO and CH<SUB>3</SUB>CN Adsorption: XPS, TEM and FTIR Measurements

On X-ray photoelectron spectra of the Au-Rh/TiO2 catalysts the position of Au4f peak was practically unaffected by the presence of rhodium, the peak position of Rh3d, however, shifted to lower binding energy with the increase of gold content of the catalysts. Rh enrichment in the outer layers of the bimetallic crystallites was experienced. The bands due to Au0-CO, Rh0-CO and (Rh0)2-CO were observed on the IR spectra of bimetallic samples, no signs for Rh+-(CO)2 were detected on these catalysts. The band due to CH3CN on Lewis acid centers shifted to lower wavenumbers with the increase of Rh content, which shows that the strength of Lewis acid sites weakens with the increase of Rh content of the catalysts. CH3CN, on the other hand, dissociates producing CN(a) species even at this temperature. From the shift to higher wavenumbers of the band due to CN(a) the strengthening of the C-N bond with increasing Rh content has been established. The results were interpreted by electron donation from titania through gold to rhodium and by the higher particle size of bimetallic crystallites. The effects of catalyst composition, reaction temperature and composition of the reacting gas mixtures have been studied on the oxidation of CO in the presence of hydrogen (PROX process). The presence of both O2 and H2 reduced the surface concentration of CO adsorbed on metallic sites. Mass spectroscopic analysis of the gas phase showed that gaseous CO2 formed in the highest amount in CO+O2 mixture, the presence of H2 suppressed the amount of CO2 produced. This negative effect of hydrogen was the lowest on 1% Rh/TiO2, the highest inhibition was observed on Au/TiO2 systems.

Oxidation of Al–Au intermetallics and its consequences studied by x-ray photoelectron spectroscopy

Three common Al–Au intermetallics, Al2Au, AlAu2, and AlAu4, were oxidized in the air and characterized using x-ray photoelectron spectroscopy in terms of the elemental chemical state. It was found that there is an increasing trend of oxidation in these intermetallics as the Au content increases. AlAu4 shows the greatest tendency to oxidize with two extra peaks appearing on the Au 4f spectra after long exposure time in air. The surface of AlAu2, although fully oxidized, reveals only one Au 4f peak shift as depth increases. Al2Au was the least oxidizing compound, and the oxide is thin. The binding energies of Al 2p and Au 4f peaks were measured and reported. The Au atoms trapped in the oxide layers exhibit higher binding energy emissions compared to those of elemental Au.

Characterization of gold nanoclusters deposited on HOPG by atmospheric plasma treatment

AbstractThe interest of gold nanoparticles in the field of nanocatalysis and nanosensors is growing. For example, carbon nanotubes covered with gold nanoclusters could present interesting properties in catalysis or/and in devices based on catalytic reaction such as chemical gas sensor. Unfortunately, the characterization of nanoparticles deposited onto a substrate is generally not a trivial exercise. Indeed, in many cases, the amount and size of material deposited is in the range of the sensitivity limits and the spatial resolutions of the techniques used. In that respect, our system, i.e. gold deposited onto a carbon support, is a favourable case, due to the high difference between the atomic numbers of gold and carbon.In this work, gold nanoparticles were deposited onto a HOPG (Highly Oriented Pyrolitic Graphite) substrate using an atmospheric plasma. In this preliminary study, HOPG sample has been used as a model surface that could present the same chemical properties as multiwall carbon nanotubes (MWCNTs).The surface composition was analysed using X‐ray photoelectron spectroscopy (XPS). Narrow region electron spectra were used to extract the chemical‐state information from the C 1s, O 1s and Au 4f peaks. The Au 4f spectral line shape was also analysed using the QUASES–Tougaard software package in order to obtain the in‐depth concentration profile of the resulting nanomaterial. This result evidences a good agreement between experiment and modelling. Indeed, field emission scanning electron microscope (FE‐SEM) and atomic force microscopy (AFM) images highlight a homogeneous distribution of 10 nm‐size gold clusters with a surface coverage of 12% on the HOPG surface, both in agreement with the results obtained with the Tougaard and coworkers method. Copyright © 2008 John Wiley & Sons, Ltd.

X-ray photoelectron spectra and structure of composites prepared via deposition of Au, Ni, and Au + Ni nanoparticles on SiO2 from colloidal solutions in triethylamine

Monometallic (Au and Ni) and bimetallic (Au + Ni) nanoparticles deposited on SiO2 from colloidal solutions in triethylamine have been studied by x-ray photoelectron spectroscopy (XPS). The solutions were prepared through vapor-phase metal synthesis. We have determined the basic parameters of the core-level and valence-band XPS spectra of the Au/SiO2, Ni/SiO2, and Au-Ni/SiO2 systems. The results indicate that the Au in these systems is in the Au(0) state, while Ni is oxidized to Ni(II). The Au/SiO2 and Au-Ni/SiO2 samples differ little in the shape of the Au 4f peak. In the Au-Ni/SiO2 system, the Ni 2p 3/2-Au 4f 7/2 binding energy difference is 0.3 eV larger and the Ni 2p 3/2 peak is narrower than those in the monometallic samples. The effects characteristic of the bimetallic system may be due to the interaction between Ni and Au or may be interpreted as evidence that the Au particles level off the potential relief on the SiO2 surface.