Surface chemistry of nanoporous carbon and the effect of pH on adsorption from aqueous phenol and 2,3,4-trichlorophenol solutions

Granular nanoporous activated carbon prepared from polyacrylonitrile (PAN) was investigated as a means of removing weak aromatic acids from aqueous solutions. This carbon is highly nanoporous, the BET surface area being 544 m²/g with V_tot=0.278 cm³/g, and V_micro=0.266 cm³/g. Aqueous treatment reduced the surface area to 364 m²/g.

Granular nanoporous carbon prepared from PAN contains O and N related surface functional groups. The surface concentration of both oxygen and nitrogen atoms was found by XPS to be 5.3%. Surface groups containing these hetero atoms are responsible for the acid–base character of this carbon in aqueous solutions. The pH_PZC of the carbon is 8.4. The microporous pore network produces a wide hysteresis loop, observed when the granular carbon was studied by continuous titration. This loop, which was found to diminish, but not to disappear completely when the particle size was reduced, is attributed to irreversible hydrolysis of surface esters and/or lactones. The surface concentration of the functional groups titrated by the equilibrium Boehm method is 449.2 μeq./g, of which 112.9 μeq./g have acidic and 336.3 μeq./g have basic character. Within the acidic species three subgroups, namely the regions pK_a<6.37, 6.37K_a<10.25 and 10.25K_a<15.74.

The acid–base behavior influences the adsorption performance of the granular carbon in aqueous solutions of weak aromatic acids, in this case phenol and 2,3,4-trichlorophenol. Both the adsorption capacity and the overall interaction parameter, K (both derived from a fit to the Langmuir equation) depend on the adsorbed species and on the pH. The former is a consequence of the different water solubilities of the solute molecules, while the latter stems from the pH sensitivity of both the surface functional groups and these weak acids. The K values show a sequence pH=3K values indicate different adsorption mechanisms. The effect of pH is more marked for 2,3,4-trichlorophenol.     

High Resolution Scanning Tunneling Microscopy of a 1D Coordination Polymer with Imidazole‐Based N,N,O Ligands on HOPG

Novel κ³‐N,N,O ligands tend to form 1D coordination polymer strands. Deposition of 1D structures on highly oriented pyrolytic graphite (HOPG) was achieved from diluted solutions and polymer strands have been studied on HOPG by AFM/STM. Single strands were mapped by STM and their electronic properties were subsequently characterized by current imaging tunneling spectroscopy (CITS). Periodic density functional calculations simulating a polymer strand deposited on a HOPG surface are in agreement with the zig‐zag structure indicated by experimental findings. Both the observed periodicity and the Zn–Zn distances can be reproduced in the simulations. Van der Waals interactions were found to play a major role for the geometry of the isolated polymer strand, for the adsorption geometry on HOPG, as well as for the adsorption energy.     

Low temperature adsorption of CO on plycrystalline molybdenum,studied by X-ray and vacuum UV photoelectron spectroscopy

The adsorption of CO on a polycrystalline Mo film at ～80°K has been studied by X-ray and UV photoelectron spectroscopy (XPS and UPS). Two adsorption states were revealed by XPS, the ratio of the O(1s) intensities from these two states was about 4 : 1 at saturation coverage (P_CO ～ 10^?6 torr). Broad resonances corresponding to the MO's of CO(ads) are observed in the UPS spectrum. On warming to room temperature chemical shifts of about 1.2 and 1.9 eV to lower binding energies were observed for the O(1s) and C(1s) signals of the major CO(ads) component. The minor state desorbed as anticipated from previous adsorption studies. The XPS spectrum observed at room temperature was virtually identical to that previously reported for CO adsorbed at room temperature. On warming the ‘orbital’ of adsorbed CO revealed by UPS is replaced by a structure apparently characteristic of carbon and oxygen atoms. These results suggest that low temperature adsorption takes place predominantly into a molecular state which on warming to room temperature dissociates.     

The impact of cellulose structure on binding interactions with hemicellulose and pectin

Four cellulose substrates including highly crystalline cellulose nanowhiskers (CNWs) from Gluconacetobacter xylinus (cellulose Iα) or cotton (cellulose Iβ) and amorphous cellulose derived from CNWs (phosphoric acid swollen cellulose nanowhiskers, PASCNWs) were used to explore the interaction between cellulose and well-defined xyloglucan, xylan, arabinogalactan and pectin. The binding behavior was characterized by adsorption isotherm and Langmuir models. The maximum adsorption and the binding constant of xyloglucan, xylan and pectin to any CNWs were always higher than to PASCNWs derived from the same source. The binding affinity of xyloglucan, xylan and pectin to G. xylinus cellulose was generally higher than to cotton cellulose, showing that binding interactions depended on the biological origin of cellulose and associated differences in its structure. The surface area, porosity, crystal plane and degree of order of cellulose substrate may all impact the interactions.     

Submonolayer-Deposition of Mass-Selected Au+ and Au n + (n = 3 and 7) on HOPG and Amorphous Carbon

Ions of gold monomer and clusters emitted from a liquid metal ion source were mass-selected, and deposited on cleaved HOPG (highly oriented pyrolytic graphite) surfaces and on amorphous carbon thin films at room temperature with the impinging energy E _i from 0 to 500 eV. The coverage of deposited ions were 1/100 and 1/1000 monolayers on HOPG surfaces and 1/3 monolayers on carbon films. Scanning tunneling microscopy of the HOPG surfaces deposited with low impinging energy (E _i<50 eV) revealed that large clusters with diameters ranging from 2 to 5 nm and height of 1–2 layers were present instead of isolated monomers and original clusters. When E _i was higher than 100 eV, HOPG surfaces were damaged and only bumpy surfaces were observed by STM. Transmission electron microscopy of Au⁺-deposited carbon films showed the formation of clusters with diameter 0.5–20 nm, depending on the E _i and the time elapsed after deposition.     

Kinetics and dynamics in physisorption of CH3Cl on HOPG: surface temperature and molecular orientation dependence

We report a study of kinetics and dynamics in physisorption of CH(3)Cl on a highly-oriented pyrolytic graphite (HOPG). Thermal energy atom scattering (TEAS) was used to probe the kinetics of thermal CH(3)Cl adsorption on HOPG during the coverage evolution. The desorption energy of CH(3)Cl on HOPG changes from 0.25 to 0.30 eV with increasing surface coverage, suggesting the attractive interaction between CH(3)Cl molecules on the surface. On the other hand, the oriented molecular beam scattering was used to monitor the dynamical interaction of CH(3)Cl with HOPG at zero coverage, demonstrating that the CH(3)Cl scattering intensity depends on the molecular orientation of the incident CH(3)Cl. The observed steric preference is not sensitive to the surface temperature. These results suggest that the moderate anisotropy in the interaction potential induces the molecular-orientation dependence of energy dissipation during the transient trapping into the physisorption well.     

NEXAFS study of a Pt-containing rod-like organometallic polymer (Pt-DEBP): molecular orientation onto HOPG, Au/Si(1 1 1), Cr/Si(1 1 1) and Si(1 1 1) surfaces

The influence of different substrates on the molecular orientation of organometallic polymer Pt-DEBP, [Pt(PBu₃)₂CCC₁₂H₈CC]_n, has been investigated by NEXAFS spectroscopy. Thin films were deposited on HOPG, Au/Si(1 1 1), Cr/Si(1 1 1), Si(1 1 1) and stainless steel. The assignment of the spectral features has been carried out on the basis of previous STEX calculations performed on phenylacetylene model molecule in gas phase and adsorbed on Pt(1 1 1) and Cu(1 0 0). Angular dependent analysis of the π* resonance occurring at 285.50 eV photon energy deriving by the benzene carbon orbitals showed a polarisation effect for all substrates. A preferential molecular orientation at nearly 40° to the surface normal was observed. This result might be explained by the strong interaction between sp and sp² carbons of the organic diethynylbiphenyl DEBP moiety contained in close chains, leading to polymer self-assembling.     

Underpotential deposition and involved alloy formation of cadmium on silver particles modified HOPG substrates

Cadmium underpotential deposition (UPD) on Ag particles modified highly ordered pyrolytic graphite (HOPG) surfaces, and the involved alloy formation were studied by conventional electrochemical techniques. Voltammetric results indicated that the Cd UPD followed an adsorption behavior different from that observed for massive Ag electrodes and Ag particles supported on vitreous carbon. Nanometer-sized bimetallic Cd–Ag particles were characterized by ex situ atomic force microscopy (AFM). Initially, AFM images show Ag deposits of similar size distributed preferably on HOPG step edges. No remarkable morphological changes are observed on the surface after the subsequent Cd deposition, suggesting that the Cd particles are deposited selectively over the Ag crystals. From the analysis of desorption spectra, employing different polarization times, and density functional theory (DFT) calculations, the formation of a Cd–Ag surface alloy could be inferred.     

Structure and reactivity of Ru nanoparticles supported on modified graphite surfaces: a study of the model catalysts for ammonia synthesis

Supported ruthenium metal catalysts have higher activity than traditional iron-based catalysts used industrially for ammonia synthesis. A study of a model Ru/C catalyst was carried out to advance the understanding of structure and reactivity correlations in this structure-sensitive reaction where dinitrogen dissociation is the rate-limiting step. Ru particles were grown by chemical vapor deposition (CVD) via a Ru(3)(CO)(12) precursor on a highly oriented pyrolytic graphite (HOPG) surface modified with one-atomic-layer-deep holes mimicking activated carbon support. Scanning tunneling microscopy (STM) has been used to investigate the growth, structure, and morphology of the Ru particles. Thermal desorption of dissociatively adsorbed nitrogen has been used to evaluate the reactivity of the Ru/HOPG model catalysts. Two different Ru particle structures with different reactivities to N(2) dissociation have been identified. The initial sticking coefficient for N(2) dissociative adsorption on the Ru/HOPG model catalysts is approximately 10(-6), 4 orders larger compared to that of Ru single-crystal surfaces.     

Lipoic acid-palladium complex interaction with DNA, voltammetric and AFM characterization

The mechanism of interaction of lipoic acid-palladium complex (LAPd) with double-stranded DNA (dsDNA), as well as the adsorption process and the redox behaviour of LAPd, of its ligand lipoic acid (LA), and of the LAPd-containing dietary supplement, Poly-MVA™, were studied using atomic force microscopy (AFM) and voltammetry at highly oriented pyrolytic graphite (HOPG) and glassy carbon electrodes. In the presence of small concentrations of LAPd molecules, the dsDNA molecules appeared less knotted and bended, and more extended on the HOPG surface, when compared with the dsDNA molecules adsorbed from the same dsDNA solution concentration. The voltammetric results demonstrated the interaction of both LAPd and Poly-MVA™ with dsDNA, but no oxidative damage caused to dsDNA was detected. AFM images revealed different adsorption patterns and degree of surface coverage and correlation with the structure, the concentration of the solution, the applied potential, and the voltammetric behaviour of the LA, LAPd and Poly-MVA™ was observed. The application of a negative potential caused the dissociation of the LAPd complex and Pd(0) nanoparticle deposition, whereas the application of a positive potential induced the oxidation of the LAPd complex and the formation of a mixed layer of LA and palladium oxides.     

Separation and permeation characteristics of a DD3R zeolite membrane

Permeation of various gases (carbon dioxide, nitrous oxide, methane, nitrogen, oxygen, argon, krypton, neon) and their equimolar mixtures through DD3R membranes have been investigated over a temperature range of 220–373 K and a feed pressure of 101–400 kPa. Helium was used as sweep gas at atmospheric pressure. Adsorption isotherms were determined in the temperature range 195–298 K, and modelled by a single and dual site Langmuir model. The permeation flux is determined by the size of the molecule relative to the window opening of DD3R, and its adsorption behaviour. As a function of temperature, bulky molecules (methane) show activated permeation, weakly adsorbing molecules decreasing permeation behaviour and strongly adsorbing molecules pass through a maximum. Counter diffusion of the sweep gas (helium) ranged from almost zero up to the order of the feed gas permeation and was strongly influenced by the adsorption of the feed gas.

DD3R membranes have excellent separation performance for carbon dioxide/methane mixtures (selectivity 100–3000), exhibit good selectivity for nitrogen/methane (20–45), carbon dioxide and nitrous oxide/air (20–400), and air/krypton (5–10) and only a modest selectivity for oxygen/nitrogen (2) separation. The selectivity of mixtures of a strongly and a weakly adsorbing component decreased with increasing temperature and pressure. The selectivity of mixtures of weakly adsorbing components was independent of pressure.

The permeation and separation characteristics of light gases through DD3R membranes can be explained by taking into account: (1) steric effects introduced by the window opening of DD3R leading to molecular sieving and activated transport, (2) competitive adsorption effects, as observed for mixtures involving strongly adsorbing gases, and (3) interaction between diffusing molecules in the cages of the zeolite.     

Growth Properties of Carbon Nanowalls on Nickel and Titanium Interlayers

This research is conducted in order to investigate the structural and electrical characteristics of carbon nanowalls (CNWs) according to the sputtering time of interlayers. The thin films were deposited through RF magnetron sputtering with a 4-inch target (Ni and Ti) on the glass substrates, and the growth times of the deposition were 5, 10, and 30 min. Then, a microwave plasma-enhanced chemical vapor deposition (PECVD) system was used to grow CNWs on the interlayer-coated glass substrates by using a mixture of H₂ and CH₄ gases. The FE-SEM analysis of the cross-sectional and planar images confirmed that the thickness of interlayers linearly increased according to the deposition time. Furthermore, CNWs grown on the Ni interlayer were taller and denser than those grown on the Ti interlayer. Hall measurement applied to measure sheet resistance and conductivity confirmed that the electrical efficiency improved significantly as the Ni or Ti interlayers were used. Additionally, UV-Vis spectroscopy was also used to analyze the variations in light transmittance; CNWs synthesized on Ni-coated glass have lower average transmittance than those synthesized on Ti-coated glass. Based on this experiment, it was found that the direct growth of CNW was possible on the metal layer and the CNWs synthesized on Ni interlayers showed outstanding structural and electrical characterizations than the remaining interlayer type.     

Charge injection barriers at a ribonucleic acid/inorganic material contact determined by photoemission spectroscopy

Ribonucleic acid (RNA) homopolymer thin films on highly oriented pyrolytic graphite (HOPG) were prepared in ultrahigh vacuum (UHV) directly from aqueous solution by electrospray (ES) injection. The polyadenosine (poly rA) films were prepared in several steps of increasing thickness without breaking the vacuum. Before deposition and between deposition steps, the samples were characterized with photoemission spectroscopy (PES). Both X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) were employed. XPS enabled the detailed measurement of core level peaks, giving insight into the chemical interaction at the interface and the layer morphology. The corresponding UP-spectra sequence allowed us to directly follow the transition from HOPG valence bands to the poly rA highest occupied molecular orbital (HOMO) structure. This enabled the determination of the poly rA ionization energy and work function as well as the charge injection barriers between the Fermi level of the HOPG substrate and the poly rA HOMO. The injection barrier between the lowest unoccupied molecular orbital (LUMO) and the HOPG Fermi level was determined using the HOMO-LUMO gap value determined by optical absorption. The results indicate that significant injection barriers exist between HOPG and the poly rA overlayer, limiting conductivity across this interface.     

Ionization energy and electronic structure of polycytidine

Ribonucleic acid (RNA) polycytidine (poly rC) homopolymer thin films were prepared on highly oriented pyrolytic graphite (HOPG) substrates. The films were grown from aqueous solution directly in a vacuum in multiple steps with use of an electrospray (ES) deposition system. Before poly rC deposition and after each deposition step the sample was characterized with X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). Evaluation of the UP-spectra sequence allowed the determination of ionization energy and highest occupied molecular orbital (HOMO) electronic structure, as well as the charge injection barriers between HOPG and poly rC. Comparison with earlier results on polyadenosine (poly rA) indicates significant differences between ionization energies (poly rC: 8.1 eV; poly rA: 6.8 eV) and orbital alignment at the graphite interface. The larger ionization energy of poly rC results in a larger hole injection barrier and a smaller electron injection barrier relative to the HOPG Fermi level.     

Repair of defects created by Ar+ sputtering on graphite surface by annealing as confirmed using ToF‐SIMS and XPS

Defects were created on the surface of highly oriented pyrolytic graphite (HOPG) by sputtering with an Ar⁺ ion beam, then characterized using X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) at 500°C. In the XPS C1s spectrum of the sputtered HOPG, a sp³ carbon peak appeared at 285.3 eV, representing surface defects. In addition, 2 sets of peaks, the C_x⁻ and C_xH⁻ ion series (where x = 1, 2, 3...), were identified in the ToF‐SIMS negative ion spectrum. In the positive ion spectrum, a series of C_xH₂^+• ions indicating defects was observed. Annealing of the sputtered samples under Ar was conducted at different temperatures. The XPS and ToF‐SIMS spectra of the sputtered HOPG after 800°C annealing were observed to be similar to the spectra of the fresh HOPG. The sp³ carbon peak had disappeared from the C1s spectrum, and the normalized intensities of the C_xH⁻ and C_xH₂^+• ions had decreased. These results indicate that defects created by sputtering on the surface of HOPG can be repaired by high‐temperature annealing.     

Self-assembly of [Et,Et]-bacteriochlorophyll cF on highly oriented pyrolytic graphite revealed by scanning tunneling microscopy

The chlorosomal light-harvesting antennae of green phototrophic bacteria consist of large supramolecular aggregates of bacteriochlorophyll c (BChl c). The supramolecular structure of (3(1)-R/S)-BChl c on highly oriented pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2) has been investigated by scanning tunneling microscopy (STM). On MoS2, we observed single BChl c molecules, dimers or tetramers, depending on the polarity of the solvent. On HOPG, we observed extensive self-assembly of the dimers and tetramers. We propose C=O...H-O...Mg bonding networks for the observed dimer chains, in agreement with former ultraviolet-visible and infrared spectroscopic work. The BChl c moieties in the tetramers are probably linked by four C=O...H-O hydrogen bonds to form a circle and further stabilized by Mg...O-H bondings to underlying BChl c layers. The tetramers form highly ordered, distinct chains and extended two-dimensional networks. We investigated semisynthetic chlorins for comparison by STM but observed that only BChl c self-assembles to well-structured large aggregates on HOPG. The results on the synthetic chlorins support our structure proposition.     

A new method for the voltammetric determination of molybdenum(VI) using carbon paste electrodes modified in situ with cetyltrimethylammonium bromide

Molybdenum(VI) is determined by anodic stripping voltammetry using a carbon paste electrode modified in situ with cetyltrimethylammonium bromide (CTAB). The preconcentration of molybdenum is performed by adsorption and reduction of ion-pairs of cetyltrimethylammonium and molybdenum(VI) oxalate at a potential of −0.4 V vs. the saturated calomel electrode (SCE). The supporting electrolyte contains 0.01 M oxalic acid and 0.075 mM CTAB. Differential pulse anodic stripping voltammetry exploiting the reoxidation signal is used for the determination of trace levels of molybdenum(VI). Linearity between current and concentration exists for a range of 0.5–500 μg 1⁻¹ Mo with proper preconcentration times; the limit of detection (calculated as 3σ) is 0.04 μg 1⁻¹ with an accumulation period of 10 min.     

UV photoelectron spectroscopy of carbonmonoxide adsorbed on small palladium particles

The adsorption of carbonmonoxide (CO) on palladium clusters evaporated onto a carbon substrate has been studied by photoelectron spectroscopy (UPS). Variations in the electron binding energy of the CO valence levels and an attenuation of the d-electron emission of Pd as a function of particle size has been observed.     

Comparison of ribonucleic acid homopolymer ionization energies and charge injection barriers

Thin films of guanosine and uridine ribonucleic acid (RNA) homopolymers (poly rG, poly rU) were grown in high vacuum in several steps on highly oriented pyrolytic graphite (HOPG) using electrospray deposition. Between deposition steps, the sample surface was characterized with X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). The resulting spectra series allowed the determination of the orbital alignment at the HOPG interface, as well as the ionization energies of the homopolymer thin films. Comparison with earlier results on cytidine and adenosine RNA homopolymers (poly rC, poly rA) indicates significant ionization energy and charge injection barrier differences between purines and pyrimidines.     

β-联碳酰基类衍生物有序自组装膜的STM研究

在大气条件下, 利用扫描隧道显微镜研究了四个β-联碳酰基类衍生物在高定向裂解石墨(HOPG)表面的自组装结构. 研究分子的结构中均包含π电子共轭体系和烷基链. 实验研究了分子结构对自组装结构的影响, 并利用分子结构的变化实现了自组装膜结构的调控. 结果表明, 在甲苯溶剂中制备的这些自组装结构均长程有序, 分子间氢键和偶极相互作用是影响自组装膜结构变化的重要因素.