Applied Surface Science (v.339, #C)

Structure of CO2 monolayer on KCl(1 0 0) by Taiquan Wu; Dan Cao; Xinyan Wang; Zhiwei Jiao; Zhouting Jiang; Miaogen Chen; Honglei Luo; Ping Zhu (1-8).
The first-principle technique has been employed to determine the structure of carbon dioxide (CO2) dimers, molecular chains, monolayers and the CO2/KCl(1 0 0) system. Their potential structures have been proposed. CASTEP calculation shows that CO2 molecular chains and monolayers based on two stable dimers by the electric interaction are all self-assembly system. At the coverage of 1.00 ML, two stable structures have been proposed when CO2 monolayer on the KCl(1 0 0) surface. The best one is the monolayer adsorbed on the surface with the C atom in the bridge site, the angle α between the molecular bond and the surface is 24°. The better one is the monolayer horizontally adsorbed on the surface with the C atom in the top-Cl site. The structural parameters in the adsorption system are similar to those in the monolayer.
Keywords: Molecular self-assembly; CO2 monolayer; First-principle theory; CASTEP; KCl(1 0 0) surface;

Adsorption analysis of thin films of terephthalic acid on Au and Al studied by MIES, UPS and XPS by Marcel Marschewski; Christian Otto; Lienhard Wegewitz; Oliver Höfft; Andreas Schmidt; Wolfgang Maus-Friedrichs (9-14).
The adsorption behavior of thin films of terephthalic acid (TPA) evaporated on a gold surface as well as on an aluminum foil was studied. The orientation of the molecules was characterized by metastable induced electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS). To make sure that the evaporation of TPA is nondestructive, additional X-ray photoelectron spectroscopy (XPS) was performed. These measurements also exclude any radiation damage.TPA on the gold surface shows a well-ordered layer growth up to 7.5 nm. Since the MIES spectra show both the acid structure and the phenyl group, a flat-laying orientation is assumed. In contrast, the phenylic carbon structure could not be observed while evaporating TPA on the oxidized aluminum foil. The MIES/UPS spectra only show the COOH group. To exclude a random arrangement of the molecules we also performed low temperature measurements. It can be concluded from these measurements, in addition to the fact that the work function increases during the evaporation, that TPA has a perpendicular arrangement on the aluminum foil.
Keywords: Metastable induced electron spectroscopy; Photoelectron spectroscopy; Adsorption; Organic molecules;

Quantifying the effects of surface quality on texture measurements of tantalum by H.Y. Fan; S.F. Liu; Y. Guo; C. Deng; Q. Liu (15-21).
Surface quality plays an important role in texture measurements of the materials with high linear absorption coefficient. In this article, three different surface preparation methods were used and the effects of surface quality on texture measurements were quantitatively investigated. A bulk annealed tantalum was selected as a model material, whose surface was subjected to grinding, electropolishing and etching, respectively. In the case of the three surface states, macrotexture analyses were performed on an X-ray diffraction instrument and features of the surfaces were examined by optical microscope and surface profiler. Comparisons were made based on matching the percentages of two primary textures ({1 0 0}<uvw> and {1 1 1}<uvw>) obtained by X-ray diffraction (XRD) and electron back-scattering diffraction (EBSD) methods. It was revealed that states of sample surface, like roughness and damaged region caused by grinding, have a significant impact on texture results. Electropolishing was proved to be suitable for metals with high linear absorption coefficient for XRD texture measurements.
Keywords: Texture measurement; Surface quality; Linear absorption coefficient; X-ray diffraction (XRD);

Charge transfer behavior of graphene-titania photoanode in CO2 photoelectrocatalysis process by Md. Rakibul Hasan; Sharifah Bee Abd Hamid; Wan Jeffrey Basirun (22-27).
In the present study, a graphene-titania composite photoelectrode was synthesized, characterized and examined for the photoelectrocatalytic (PEC) response. The charge transfer process on the semiconductor/electrolyte interface was investigated via electrochemical impedance spectroscopy (EIS) and voltammetry. In addition, the influence of pH toward the photoanode performance was also investigated and it was noticed that a high pH condition was favorable higher photocurrent response from the EIS measurements. The main reason could be attributed to the decrease of recombination process at the photoanode with fast quenching of the photogenerated holes with OH ions at high pH. The experiment was also run for CO2 photoreduction and increased photocurrent was observed.
Keywords: Charge transfer; Recombination rate; CO2 conversion; Photoelectrocatalysis;

A systematic first-principles model study of Pb(II) adsorption on the basal hydroxylated (0 0 1) surface of kaolinite in aqueous environment was carried out using periodic DFT slab calculations. The effective coordination number, coordination geometry, preferred adsorption position and adsorption type of Pb(II) complex were examined, with two different types of surface sites (Ou with “up” hydrogen and Ol with “lying” hydrogen) considered. Complexes in mono-, bi- and tri-dentate modes exhibit coordination number of 3–5 within the hemidirected tendency geometry. The monodentate complex on “Ou” site and the tridentate complex on “Ou–Ou–Ol” site of two neighboring Al centers are found to be the major adsorption species. Hydrogen bonding interaction of surface Ol with hydrogen of aqua ligands acts as the key factor for the stability of complex. Partial density of state (PDOS) projections combining with Mulliken bond populations show that coupling of Pb 6p with the antibonding Pb 6s–O 2p states is the dominant orbital interaction of Pb(II) with aqua oxygen (Ow) or surface oxygen (Os). Different from the covalent Pb―Ow, bond of Pb―Os displays strong ionic characters.
Keywords: Kaolinite; Adsorption; Pb(II); Water environment; Density functional theory;

We use dispersion corrected DFT calculations (DFT + D3) to investigate the selectivity of Ni-based surface alloys toward hydrazine adsorption. A series of Ni–M (M = Fe, Pt, Ir, Pd and Rh) alloy films were investigated, namely Ni15/M1/Ni(1 1 1), Ni14/M2/Ni(1 1 1), Ni12/M4/Ni(1 1 1) and Ni8/M8/Ni(1 1 1). Our results show that the doped atoms of Ir, Rh and Fe provide stronger adsorption sites than the Ni atom on the Ni(1 1 1) surface, while the doped atoms of Pt and Pd provide weaker adsorption sites. By analyzing the most favorable adsorption of hydrazine on Ni–M alloy surfaces we found that Ni8Fe8/Ni(1 1 1), Ni8Rh8/Ni(1 1 1), Ni15Ir1/Ni(1 1 1) and Ni14Ir2/Ni(1 1 1) present enhanced adsorption properties if compared to the pure Ni(1 1 1) surface, and seem to be better candidates for hydrazine catalysis, which are in agreement with that found by experiments. The correlation between d-band center position and adsorption energies of top modes in the Ni or doped atom has been calculated at DFT + D3 level to provide further insight into the Ni-based surface alloy properties for hydrazine adsorption.
Keywords: Ni-based surface alloy; DFT + D3; Hydrazine; Adsorption; Doped atoms;

XPS study of surface chemistry of tungsten carbides nanopowders produced through DC thermal plasma/hydrogen annealing process by Pavel V. Krasovskii; Olga S. Malinovskaya; Andrey V. Samokhin; Yury V. Blagoveshchenskiy; Valery А. Kazakov; Artem А. Ashmarin (46-54).
X-ray photoelectron spectroscopy (XPS) has been employed to characterize the surface composition and bonding of the tungsten carbides nanopowders produced through a DC thermal plasma/hydrogen annealing process. The XPS results were complemented with those from Raman spectroscopy, high-resolution transmission electron microscopy, and evolved gas analysis. The products of the DC plasma synthesis are the high-surface-area multicarbide mixtures composed mainly of crystalline WC1−x and W2C. The materials are contaminated with a pyrolitic carbonaceous deposit which forms ∼1 nm thick graphitic overlayers on the nanoparticles’ surface. The underlying carbides are not oxidized in ambient air, and show no interfacial compounds underneath the graphitic overlayers. When annealed in hydrogen, the multicarbide mixtures undergo transformation into the single-phase WC nanopowders with an average particle size of 50–60 nm. The surface of the passivated and air-exposed WC nanopowders is stabilized by an ultrathin, no more than 0.5 nm in thickness, chemically heterogeneous overlayer, involving graphitic, carbon-to-oxygen, and WO3 bonding. Oxygen presents at coverages above a monolayer preferentially in the bonding configurations with carbon. The surface segregations of carbon are normally observed, even when the bulk content of carbon is below the stoichiometric level.
Keywords: Surface chemistry; Nanoparticle; Nanopowder; Tungsten carbide; X-ray photoelectron spectroscopy;

Adsorption and oxidation of NO on graphene oxides: A dispersion corrected density functional theory investigation by Meiling Hou; Wanglai Cen; Huijuan Zhang; Jie Liu; Huaqiang Yin; Fusheng Wei (55-61).
Carbonaceous materials have been found to be active for the catalytic oxidation of NO into NO2 at room temperature, but the mechanism is unclear. Calculations based on density functional theory were employed to investigate the effects of hydroxyl and epoxy groups on the adsorption of NO, and then the oxidation to NO2. It is found that the surface hydroxyl groups of carbon materials can enhance the adsorption of NO. The enhancement is derived from a weak covalent interaction between the nitrogen atom of the NO molecule and the carbon surface. NO can be oxidized by epoxy groups with rather low barrier, which helps to explain the low temperature catalytic oxidation. The hydroxyl group possesses no significant effects on the oxidation barrier. The low oxidation barrier is ascribed to the unique electronic structure NO, the highest occupied molecular orbit (HOMO) of which is partially filled.
Keywords: First principles; Denitration; Graphene; H-bonding interaction; Epoxy group;

The goal of this paper was to develop a new composite obtained in mild hydrothermal conditions starting from fly ash (a waste raising significant environmental problems), and TiO2. The composite was characterized through XRD, SEM/EDX, AFM, and BET surface measurements. The composite was further used for the advanced treatment of wastewaters with multiple-pollutants load. The photocatalytic efficiency of the powder composite was tested on synthetic solutions containing a heavy metal cation (copper), a dye (methyl orange), and a surfactant (sodium dodecylbenzenesulfonate), under UV and simulated solar radiation. Comparative experiments were done in systems with and without H2O2 showing a significant increase in efficiency for methyl orange removal from mono-, bi-, and tri-pollutants solutions. The process parameters were optimized and the adsorption mechanisms are discussed, outlining that adsorption is the limiting step. Experiments also outlined that homogeneous photocatalysis (using H2O2) is less efficient then the heterogeneous process using the novel composite, both under UV and simulated solar radiation.
Keywords: Fly ash; Composite; Multi-pollutant wastewater; Copper removal; Dye; Surfactant;

Room temperature ammonia sensor based on copper nanoparticle intercalated polyaniline nanocomposite thin films by U.V. Patil; Niranjan S. Ramgir; N. Karmakar; A. Bhogale; A.K. Debnath; D.K. Aswal; S.K. Gupta; D.C. Kothari (69-74).
Thin films of copper nanoparticles intercalated-polyaniline nanocomposites (NC) have been deposited at room temperatures by in situ oxidative polymerization of aniline in the presence of different concentrations of Cu nanoparticles. The response characteristics of the NC thin films toward different gases namely NH3, CO, CO2, NO and CH4 were examined at room temperature. Both pure polyaniline (PANI) and NC films exhibited a selective response toward NH3. Incorporation of Cu nanoparticles resulted in an improvement of the sensors response and response kinetics. The response and the recovery times of composite film toward 50 ppm of NH3 were 7 and 160 s, respectively. Additionally, the NC sensor film could reversibly detect as low as 1 ppm of NH3 concentrations. The enhanced response of NC films toward NH3 is attributed to the deprotonation and reprotonation processes as also supported by Raman investigations.
Keywords: Polyaniline; Nanocomposites, thin film; Ammonia, sensor;

This paper reports that 3D crater-like microdefects form on the metal surface when laser shock processing (LSP) is applied. LSP was conducted on pure copper block using the aluminum foil as the absorbent material and water as the confining layer. There existed the bonding material to attach the aluminum foil on the metal target closely. The surface morphologies and metallographs of copper surfaces were characterized with 3D profiler, the optical microscopy (OM) or the scanning electron microscopy (SEM). Temperature increases of metal surface due to LSP were evaluated theoretically. It was found that, when aluminum foil was used as the absorbent material, and if there existed air bubbles in the bonding material, the air temperatures within the bubbles rose rapidly because of the adiabatic compression. So at the locations of the air bubbles, the metal materials melted and micromelting pool formed. Then under the subsequent expanding of the air bubbles, a secondary shock wave was launched against the micromelting pool and produced the crater-like microdefects on the metal surface. The temperature increases due to shock heat and high-speed deformation were not enough to melt the metal target. The temperature increase induced by the adiabatic compression of the air bubbles may also cause the gasification of the metal target. This will also help form the crater-like microdefects. The results of this paper can help to improve the surface quality of a metal target during the application of LSP. In addition, the results provide another method to fabricate 3D crater-like dents on metal surface. This has a potential application in mechanical engineering.
Keywords: Laser shock processing; Microdefects; Temperature increase; Aluminum foil; Shock wave;

The effect of fibronectin on structural and biological properties of single walled carbon nanotube by Fatemeh Mottaghitalab; Mehdi Farokhi; Fatemeh Atyabi; Ramin Omidvar; Mohammad Ali Shokrgozar; Majid Sadeghizadeh (85-93).
Despite the attractive properties of carbon nanotubes (CNTs), cytoxicity and hydrophobicity are two main considerable features which limit their application in biomedical fields. It was well established that treating CNTs with extracellular matrix components could reduce these unfavourable characteristics. In an attempt to address these issues, fibronectin (FN) with different concentrations was loaded on single walled carbon nanotubes (SWCNTs) substrate. Scanning electron microscope, atomic force microscopy (AFM), contact angles and X-ray photoelectron spectroscopy (XPS) were preformed in order to characterize FN loaded SWCNTs substrates. According to XPS and AFM results, FN could interact with SWCNTs and for this, the hydrophilicity of SWCNTs was improved. Additionally, SWCNT modified with FN showed less cytotoxicity compared with neat SWCNT. Finally, FN was shown to act as an interesting extracellular component for enhancing the biological properties of SWCNT.
Keywords: Single walled carbon nanotube; Fibronectin; Hydrophilicity; Cytotoxicity;

Transparent, durable and thermally stable PDMS-derived superhydrophobic surfaces by Xiaojiang Liu; Yang Xu; Keyang Ben; Zao Chen; Yan Wang; Zisheng Guan (94-101).
We reported a novel, simple, modification-free process for the preparation of transparent superhydrophobic surfaces by calcining candle-soot-coated polydimethylsiloxane (PDMS) films. Though a calcination process, a candle soot template was gradually removed while robust fibrous and network structures were created on glass. Owing to these structures, the glass substrates were durable and highly transparent with an average transmittance (400–800 nm) of 89.50%, very closed to the bare glass slides (89.70%). These substrates exhibited a water contact angle (WCA) of 163° and a sliding angle (SA) of ∼1°. Importantly, the superhydrophobicity of these surfaces can thermally recover after oil-contamination due to their high thermal stability below 500 °C. Based on these, superhydrophobic fiberglass cotton was also prepared for optimized oil-water separation and air filtration. This method is suitable for large-scale production because it uses inexpensive and environmentally friendly materials and gets rids of sophisticated equipment, special atmosphere and harsh operations.
Keywords: Superhydrophobic; Transparent; Durable; PDMS; Template; Thermally stable;

PECVD synthesis, optical and mechanical properties of silicon carbon nitride films by Evgeniya Ermakova; Yurii Rumyantsev; Artur Shugurov; Alexey Panin; Marina Kosinova (102-108).
SiC x N y thin films were synthesized at a temperature of 700 °C by the PECVD process, using trimethylphenylsilane C6H5Si(CH3)3 (TMPhS) and ammonia as a reactive mixture. The effect of NH3 dilution on the structure and chemical bonding of SiC x N y films was investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, high-resolution transmission electron microscopy and energy-dispersive X-ray analysis. The influence of deposition conditions on the transmittance, the optical band gap, the hardness and the Young's modulus of SiC x N y films was studied. It was shown that the chemical composition and the functional properties of the films are governed by the initial pressure ratio of NH3 to TMPhS. The variation of the ratio enables the film of different composition to be deposited, e.g. SiC x , SiC x N y and SiN y . It was shown that the films deposited from a reactive mixture with the highest ammonia dilution had a transmittance comparable to that of SiO2 and hardness of 23 GPa.
Keywords: SiCN films; PECVD; Optical band gap; Hardness; Young's modulus;

This study focuses on the preparation of superhydrophobic films by crosslinkable polymer material-Poly(methyl methacrylate-butyl acrylate-hydroxyethyl methacrylate)-b-Poly(perfluorohexylethyl methacrylate) (P (MMA-BA-HEMA)-b-PFMA) with a simple one-step casting process. Nanoscale micelle particles with core-shell structure was obtained by dissolving the polymer and curing agent in the mixture of acetone and 1H, 1H, 5H octafluoropentyl-1,1,2,2 tetrafluoroethyl ether (FHT). Superhydrophobic films were fabricated by casting the micelle solution on the glass slides. By controlling the polymer concentration and acetone/FHT volume ratio, superhydrophobic polymer film with water contact angle of 153.2 ± 2.1° and sliding angle of 4° was obtained. By introducing a curing agent into the micelle solution, mechanical properties of the films can be improved. The adhension grade and hardness of the crosslinked superhydrophobic films reached 2 grade and 3H, respectively. The hydrophobicity is attributed to the synergistic effect of micro–submicro–nano-meter scale roughness by nanoscale micelle particles and low surface energy of fluoropolymer. This procedure makes it possible for widespread applications of superhydrophobic film due to its simplicity and practicability.
Keywords: Crosslinked superhydrophobic films; Block polymer; Micelle solution; Mixed solvent;

The investigation on the stratification phenomenon of aluminum rear alloyed layer in silicon solar cells by Xi Xi; Xiaojing Chen; Song Zhang; Zhengrong Shi; Guohua Li (116-121).
A stratification phenomenon of aluminum rear alloyed layer was found in the study of aluminum rear emitter N-type solar cells. It is related to the composition of the paste. The outer aluminum alloyed layer can be called as aluminum doped emitter, and it gives the contribution to the junction formation. The inner layer is only the Al/Si mixed layer. The aluminum atoms in this layer are not bonded with silicon atoms. This inner layer will ruin the quality of the rear junction. The shunt resistance, reverse current density and the junction electric leakage value are getting worse when the thickness of the inner layer increases. The thickness of the inner Al/Si mixed layer increases with the increasing of firing temperature, while the depth of the aluminum doped emitter almost does not change. From the analyses, the inner Al/Si mixed layer is redundant and deleterious. Only a single deep aluminum doped rear emitter is needed for N-type solar cells. The highest power conversion efficiency of 19.93% for aluminum rear emitter N-type cells without the stratification phenomenon has been obtained.
Keywords: Solar cells; Aluminum rear alloyed layer; Aluminum doped emitter; Stratification phenomenon;

Large-scale patterned ZnO nanorod arrays for efficient photoelectrochemical water splitting by Yaping Hu; Xiaoqin Yan; Yousong Gu; Xiang Chen; Zhiming Bai; Zhuo Kang; Fei Long; Yue Zhang (122-127).
Nowadays, the fabrication of photoanodes with high light-harvesting capability and charge transfer efficiency is a key challenge for photoelectrochemical (PEC) water splitting. In this paper, large-scale patterned ZnO nanorod arrays (NRAs) were designed and fabricated via two-beam laser interference lithography and hydrothermal synthesis, which were further applied as PEC photoanodes for the first time. By adopting the ZnO NRA photoanodes with square pattern, the PEC cells achieved a maximum efficiency of 0.18%, which was improved 135% compared to the control group with no patterned ZnO NRAs. The large-scale highly ordered ZnO NRAs have enhanced light-harvesting ability due to the light-scattering effect. In addition, the enlarged surface area of the patterned ZnO NRAs accelerated the charge transfer at the photoanode/electrolyte interface. This research demonstrates an effective mean to realize the efficient solar water splitting, and the results suggest that large-scale highly ordered nanostructures are promising candidates in the field of energy harvesting.
Keywords: ZnO; Nanorod array; Patterned; Photoelectrochemical; Water splitting;

Chitosan (CS) was used as a template to prepare a hybrid chitosan–phthalocyanine-TiO2 (PC/CS-TiO2) photocatalyst at room temperature without any calcination treatment. The as-prepared hybrid photocatalyst (PC/CS-TiO2) was characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and UV–vis diffuse reflectance spectroscopy (DRS). The results of the photodegradation of aniline, used as a model pollutant, revealed that the hybrid photocatalyst (PC/CS-TiO2) exhibited a photocatalytic activity under visible-light irradiation. The enhanced activity of the hybrid catalyst is attributed to the cooperative role of the three components of the photocatalyst; chitosan as a template for the immobilization crystalline TiO2 nanoparticles, phthalocyanine that promote the light absorption in the visible range and TiO2 acting as an acceptor of electrons generated by the photons absorption to produce superoxide radicals.
Keywords: Sol–gel processes; Raman and IR spectroscopy; Chitosan-TiO2;

Surface hydrophobicity is primarily attained through the use of low surface energy materials. Experimental attempts to turn hydrophilic surfaces to hydrophobic have consisted of coating and thin film deposition. However, in many applications low surface energy materials and coatings are not practical, though hydrophobicity is still desired. In this paper, we demonstrate the transition from hydrophilic to hydrophobic wetting states on an intrinsically hydrophilic surface (contact angle less than 45°) using only surface microstructuring. The surface microstructures consist of re-entrant microcavities which interfere with the complete wetting of the surface, causing a liquid droplet to sit on the surface in a Cassie wetting state. The microstructures were fabricated on a silicon-on-insulator (SOI) wafer through steps of photolithography, etching, and bonding. Contact angle measurements demonstrated the ability of the microfabricated surfaces to sustain large contact angles above 100°, compared to a bare silicon surface which has a contact angle of 40°. Energy-dispersive X-ray spectroscopy showed silicon to be the only chemical element on the surface, while optical observations with an inverted microscope hinted to the existence of a Cassie wetting state.
Keywords: Hydrophobic; Re-entrant microcavities; Wetting transition; Air entrapment;

Comparison of KrF and ArF excimer laser treatment of biopolymer surface by I. Michaljaničová; P. Slepička; J. Heitz; R.A. Barb; P. Sajdl; V. Švorčík (144-150).
The goal of this work was the investigation of the impact of two different excimer lasers on two biocompatible and biodegradable polymers (poly-l-lactide and poly hydroxybutyrate). Both polymers find usage in medical and pharmaceutical fields. The polymers were modified by KrF and ArF excimer lasers. Subsequently the impact on surface morphology, surface chemistry changes, and thermal properties was studied by means of confocal and AFM microscopy, FTIR and XPS spectroscopy and DSC calorimetry. Under the same conditions of laser treatment it was observed that ArF laser causes more significant changes on surface chemistry, surface morphology and pattern formation on the polymers under investigation. The data obtained in this work can be used for a wide range of possible applications, in tissue engineering or in combination with metallization in electronics, e.g. for biosensors.
Keywords: Biopolymers; Excimer laser; Nanostructuring; Surface modification; Surface chemistry;

Durable superhydrophobic PTFE films through the introduction of micro- and nanostructured pores by Yao-Yao Zhang; Quan Ge; Long-Lai Yang; Xiao-Jun Shi; Jiao-Jiao Li; De-Quan Yang; Edward Sacher (151-157).
A superhydrophobic surface, highly water repellant and self-cleaning, is typically made by introducing micro- and nanoscale roughness onto the surface of a low surface energy material. Herein, we offer a new process of superhydrophobic film formation, accomplishing the same thing through the production of micro- and nanoscale surface porosities. Such a material is prepared by introducing zinc acetate (ZnAc2) and sodium chloride (NaCl) into a commercially available PTFE (polytetrafluoroethylene) emulsion. On drying, baking and washing with acetic acid, the PTFE film produced from the emulsion had both micro- and nanoscale surface porosities, and demonstrated superhydrophobic properties, with a static contact angle >150° and a slide angle <10°. From SEM observation, NaCl contributes microscale porosity, while ZnAc2 decomposes to ZnO, contributing nanoscale porosity. Using either ZnAc2 or NaCl alone produces a surface with a static contact angle >150°, but with a slide angle >10°. Based on XPS and SEM data, we explore herein the affect of chemistry and porosity on the mechanism of superhydrophobic surface formation, and the durability of that surface under abrasion.
Keywords: PTFE; Porous film; Self-cleaning; Superhydrophobic;