Adsorption configurations and energetics of BClx (x=0-3) on TiO2 anatase (101) and rutile (110) surfaces

This study investigates the adsorption and reactions of boron trichloride and its fragments (BClx) on the TiO2 anatase (101) and rutile (110) surfaces by first-principles calculations. The results show that the possible absorbates on the TiO2 anatase and rutile surfaces are very similar. The single- and double-site adsorption configurations are found for both anatase and rutile surfaces. The particular adsorbate feature on the anatase surface is its in-plane double-site adsorption by Ti and O from its sawtooth surface. The potential energy surface shows that BCl3 can be adsorbed on the O site for both the anantase and rutile surfaces and the most of the BClx reaction on both anatase and rutile surfaces are endothermic, except for the dissociative reaction on the rutile surface. The energy levels of the BClx reactions between the anatase and rutile surfaces show that the rutile surface has lower energy levels than those of anatase surface. This result reveals that the BClx dissociative adsorption more easily occurs on rutile surface than on anatase surface.     

Functionalization of beta-Ga2O3 nanoribbons: a combined computational and infrared spectroscopic study

The adsorption of H 2O, alcohols (CH 3OH and 1-octanol), and carboxylic acids (formic, acetic, and pentanoic) on beta-Ga 2O 3 nanoribbons has been studied using infrared reflection-absorption spectroscopy (IRRAS) and/or ab initio computational modeling. Adsorption energies and geometries are sensitive to surface structure, and hydrogen bonding plays a significant role in stabilizing adsorbed species. On the more stable (100)-B surface, computation shows that the physisorption of H 2O or CH 3OH is weakly exothermic whereas chemisorption via O-H bond dissociation is weakly endothermic. Experiment finds that a large fraction of a saturation coverage of adsorbed 1-octanol is displaced by exposure to acetic acid vapor. This is consistent with computational results showing that acids adsorb more strongly than methanol on this surface. The remaining alcohol, not displaced by acetic acid, suggests the presence of defects and/or (100)-A regions because computation shows that this less-stable surface adsorbs methanol more strongly than does the (100)-B. The nu(C-H) modes of adsorbed 1-octanol are easily detected whereas no adsorbed H 2O is observed even though H 2O and CH 3OH exhibit similar adsorption energies. It is inferred from this that the failure to detect H 2O on the dominant (100)-B surface results from the orientation of the physisorbed H 2O essentially parallel to the surface. Computation shows that this configuration is stabilized by H bonding. For chemisorbed formic acid, computation shows that a bridging carboxylate structure is favored over a bidentate or monodentate configuration. Computation also shows that chemisorption is favored on the (100)-A surface but physisorption is favored on the more stable (100)-B. Analysis of IRRAS data for acetic and pentanoic acids finds evidence for both types of adsorption. The carboxylate resists displacement by H 2O vapor, which suggests that carboxylic acids may be useful for functionalizing beta-Ga 2O 3 surfaces. The results provide insight into the interplay between surface structure and reactivity on an oxide surface and about the importance of hydrogen bonding in determining adsorbate structure.     

Structural studies of C-amidated amino acids and peptides: crystal structures of Z-Gly-Phe-NH2, Tyr-Lys-NH2, and Asp-Phe-NH2

As part of the series investigating the structural features of C-terminal amidated amino acids and peptides, three crystal structures of Z-Gly-Phe-NH2, Tyr-Lys-NH2, and Asp-Phe-NH2 were analyzed by the X-ray diffraction method, and their molecular conformations and intermolecular interactions were investigated. Although the respective dipeptides exhibited an energetically allowable torsion angle concerning each backbone or side chain, the observed extended (Z-Gly-Phe-NH2, Asp-Phe-NH2) and folded (Tyr-Lys-NH2) conformations were considerably different from those of the corresponding unamidated peptides, due to the conformational flexibility of the respective dipeptides. The comparison between the crystal packings of the amidated and unamidated dipeptides indicated that the C-terminal amides tend to associate with the same neighboring group through hydrogen bonds, in which both the amide NH and O=C groups participate, while the unamidated peptides prefer a linear molecular connection, where both or either of the two carboxyl oxygens participate in the hydrogen bond formation. The difference in hydrogen bonding ability between the C-terminal amide and carboxyl groups has been considered to be based on the structural data of the related peptides analyzed so far.     

Unique adsorption behaviors of NO and O2 at hydrogenated anatase TiO2(101)

The extra electron on the hydrogenated anatase TiO₂(101) is localized at the nearest Ti_5c only, and the chargetransfer promoted NO and O₂ adsorptions are also site-selective. These results are totally different from those at hydrogenated rutile TiO₂(110).     

Comparison of polyethylene glycol adsorption to nanocellulose versus fumed silica in water

The recent intensification of industrially produced cellulose nanocrystals (CNCs) and cellulose nanofibrils has positioned nanocelluloses as promising materials for many water-based products and applications. However, for nanocelluloses to move beyond solely an academic interest, a thorough understanding of their interaction with water-soluble polymers is needed. In this work, we address a conflicting trend in literature that suggests polyethylene glycol (PEG) adsorbs to CNC surfaces by comparing the adsorption behaviour of PEG with CNCs versus fumed silica. While PEG is known to have strong hydrogen bonding tendencies and holds water tightly, it is sometimes (we believe erroneously) presumed that PEG binds to cellulose through hydrogen bonding in aqueous media. To test this assumption, the adsorption of PEG to CNCs and fumed silica (both in the form of particle films and in aqueous dispersions) was examined using quartz crystal microbalance with dissipation, isothermal titration calorimetry, rheology and dynamic light scattering. For all PEG molecular weights (300–10,000 g/mol) and concentrations (100–10,000 ppm) tested, strong rapid adsorption was found with fumed silica, whereas no adsorption to CNCs was observed. We conclude that unlike silanols, the hydroxyl groups on the surface of CNCs do not readily hydrogen bond with the ether oxygen in the PEG backbone. As such, this work along with previous papermaking literature supports the opinion that PEG does not adsorb to cellulose surfaces.     

Effect of Hydrogen on O2 Adsorption and Dissociation on a TiO2 Anatase (001) Surface

The effect of hydrogen on the adsorption and dissociation of the oxygen molecule on a TiO₂ anatase (001) surface is studied by first‐principles calculations coupled with the nudged elastic band (NEB) method. Hydrogen adatoms on the surface can increase the absolute value of the adsorption energy of the oxygen molecule. A single H adatom on an anatase (001) surface can lower dramatically the dissociation barrier of the oxygen molecule. The adsorption energy of an O₂ molecule is high enough to break the O?O bond. The system energy is lowered after dissociation. If two H adatoms are together on the surface, an oxygen molecule can be also strongly adsorbed, and the adsorption energy is high enough to break the O?O bond. However, the system energy increases after dissociation. Because dissociation of the oxygen molecule on a hydrogenated anatase (001) surface is more efficient, and the oxygen adatoms on the anatase surface can be used to oxidize other adsorbed toxic small gas molecules, hydrogenated anatase is a promising catalyst candidate.     

Dye sensitization of the anatase (101) crystal surface by a series of dicarboxylated thiacyanine dyes

Dye sensitization of the single crystal anatase (101) surface was studied using a structurally similar series of dicarboxylated thiacyanine dyes that bind to the oxide surface through their carboxylate groups. An ultraviolet (UV) light treatment of the anatase (101) surfaces, immediately prior to dye adsorption, improved both the reproducibility of dye coverage and the incident photon-to-current efficiencies (IPCE) for sensitization. The UV treatment does not pit or roughen the anatase surface and results in high IPCEs of more than 1% in some cases and absorbed photon current efficiencies (APCE) from 5 to 100%. The photocurrent spectra showed features associated with surface-bound dye monomers and H-dimers that could be followed as a function of the dye surface coverage. Models for the surface structures of the adsorbed dye layers that are consistent with the measurements are presented, along with a discussion of adsorption isotherms.     

Packing of ruthenium sensitizer molecules on mostly exposed faces of nanocrystalline TiO2: crystal structure of (NBu4+)2[Ru(H2tctterpy)(NCS)3]2−·0.5 DMSO

An X‐ray crystal study of the new ‘black dye’ sensitizer tri(thiocyanato)(4,4′,4″‐tricarboxy‐2,2′:6′,2″‐terpyridine)ruthenium(II) is reported. In the crystal, strong hydrogen bonds form chains of ruthenium complex dianions with the O···O distances of 2.48–2.54 Å. From the molecular geometry of the dianions, structural models of their close packing on the (101) and (001) crystal surfaces of TiO₂ (anatase) have been built. The maximum possible density of molecular packing noticeably exceeds the experimental value. The hydrogen bonding between the anions in monolayers, located on the TiO₂ surface, is discussed. Copyright © 2002 John Wiley & Sons, Ltd.     

The Case of Formic Acid on Anatase TiO2(101): Where is the Acid Proton?

Carboxylic‐acid adsorption on anatase TiO₂ is a relevant process in many technological applications. Yet, despite several decades of investigations, the acid‐proton localization—either on the molecule or on the surface—is still an open issue. By modeling the adsorption of formic acid on top of anatase(101) surfaces, we highlight the formation of a short strong hydrogen bond. In the 0 K limit, the acid‐proton behavior is ruled by quantum delocalization effects in a single potential well, while at ambient conditions, the proton undergoes a rapid classical shuttling in a shallow two‐well free‐energy profile. This picture, supported by agreement with available experiments, shows that the anatase surface acts like a protecting group for the carboxylic acid functionality. Such a new conceptual insight might help rationalize chemical processes involving carboxylic acids on oxide surfaces.     

Molecular recognition at the air-water interface and two-dimensional molecular organization —Supermolecules Project in Kurume

The research activity of the Supermolecules Project, which is administered by the Research Development Corporation of Japan, is described. This Japan-France collaborative project is made of three research groups at Kurume Research Park in Kyushu, Keihanna Research Park in Kyoto, and Université Louis Pasteur in Strasbourg. Major research topics of the Kurume group are molecular recognition by interfacial hydrogen bonding, formation of molecular patterns, and layer-by-layer adsorption. In the first topic, it was found that complementary hydrogen bonding provided efficient host-guest interactions at the macroscopic and mesoscopic interfaces. In the same vein, aqueous dipeptides are selectively bound to monolayers bearing peptide chains. In the second field, designed arrangement of component molecules in monolayers was realized through either specific interaction with aqueous templates or complementary hydrogen bonding among monolayer components. Preparation of ultrathin films by alternate adsorption was extended from the original use of linear polyelectrolytes by other groups to biopolymers and inorganic macroions. The extension to small molecules also expanded the usefulness of this technique.     

FTIR spectroscopy of alcohol and formate interactions with mesoporous TiO2 surfaces

The effects of pH and ultraviolet (UV) light with ligated formic acid on mesoporous TiO2 were characterized by transmission Fourier transform infrared (FTIR) spectroscopy and compared with adsorbed formate complexes. Surface-modified anatase thin films were prepared from acidic aqueous nanoparticulate anatase suspensions diluted with methanol and ethanol. Bands assigned to carboxylic acid groups displayed unique bonding character in the ligated formic acid on the anatase surface. For increased proton concentrations in the films, separation in -COO stretching bands (delta nu) for formic acid increased (increase in frequency for nuC=O and decrease in frequency for nuC-O). With UV exposure, surface-bound organics were rapidly removed by photocatalytic oxidation at 40 degrees C and 40% relative humidity (RH). In addition, the delta nu of the formic acid bands decreased as organics were mineralized to carbonates and CO2 with UV light. Aqueous formic acid adsorption experiments showed a distinctly different bonding environment lacking carbonate, and the delta nu for the carboxylic groups indicated a bridging bidentate coordination. The delta nu of the bands increased with increasing proton concentration, with both bands shifting to higher wavenumbers. The shifts may be ascribed to the influence of protonation on surface charge and the effect of that charge on the electronegativity of carboxylate groups bound to the surface. As alcohols are used in the mesoporous TiO2 solar cell preparation, implications of these surface modifications to dye-sensitized photovoltaics are discussed.     

Computational study on the reactions of H2O2 on TiO2 anatase (101) and rutile (110) surfaces

This study investigates the adsorption and reactions of H(2)O(2) on TiO(2) anatase (101) and rutile (110) surfaces by first-principles calculations based on the density functional theory in conjunction with the projected augmented wave approach, using PW91, PBE, and revPBE functionals. Adsorption mechanisms of H(2)O(2) and its fragments on both surfaces are analyzed. It is found that H(2)O(2) , H(2)O, and HO preferentially adsorb at the Ti(5c) site, meanwhile HOO, O, and H preferentially adsorb at the (O(2c))(Ti(5c)), (Ti(5c))(2), and O(2c) sites, respectively. Potential energy profiles of the adsorption processes on both surfaces have been constructed using the nudged elastic band method. The two restructured surfaces, the 1/3 ML oxygen covered TiO(2) and the hydroxylated TiO(2), are produced with the H(2)O(2) dehydration and deoxidation, respectively. The formation of main products, H(2)O(g) and the 1/3 ML oxygen covered TiO(2) surface, is exothermic by 2.8 and 5.0 kcal/mol, requiring energy barriers of 0.8 and 1.1 kcal/mol on the rutile (110) and anatase (101) surface, respectively. The rate constants for the H(2)O(2) dehydration processes have been predicted to be 6.65 × 10(-27) T(4.38) exp(-0.14 kcal mol(-1)/RT) and 3.18 × 10(-23) T(5.60) exp(-2.92 kcal mol(-1)/RT) respectively, in units of cm(3) molecule(-1) s(-1).     

Modeling competitive interactions in proteins: vibrational spectroscopy of M+(n-methylacetamide)1(H2O)n=0-3, M=Na and K, in the 3 microm region

To properly understand the preferred structures and biological properties of proteins, it is important to understand how they are influenced by their immediate environment. Competitive intrapeptide, peptide...water, ion...water, and ion...peptide interactions, such as hydrogen bonding, play a key role in determining the structures, properties, and functionality of proteins. The primary types of hydrogen bonding involving proteins are intramolecular amide...amide (N-H...O=C) and intermolecular amide...water (O-H...O=C and H-O...H-N). n-Methylacetamide (NMA) is a convenient model for investigating these competitive interactions. An analysis of the IR photodissociation (IRPD) spectra of M+(n-methylacetamide)1(H2O)n=0-3 (M=Na and K) in the O-H and N-H spectral regions is presented. Ab initio calculations (MP2/cc-pVDZ) are used as a guide in identifying both the type and location of hydrogen bonds present. In larger clusters, where several structural isomers may be present in the molecular beam, ab initio calculations are also used to suggest assignments for the observed spectral features. The results presented offer insight to the nature of ion...NMA interactions in an aqueous environment and reveal how different ion...ligand pairwise interactions direct the extent of water...water and water...NMA hydrogen bonding observed.     

水介质中氢键吸附与疏水吸附协同作用的研究

研究了丙烯酸型树脂(D152)在水、乙醇和正己烷中对苯胺、N-甲基苯胺和N,N-二甲基苯胺的吸附行为.在水中D152树脂对3种吸附质的吸附亲合性随N上甲基数的增加而增大,说明疏水作用是主要的吸附机理,但其吸附焓己超出范德华力的范围而在氢键的键能范围内,故氢键吸附也同时在起作用.在正己烷中,D152树脂对3种吸附质的吸附亲合性随N上甲基数的增加而减小,与水中呈相反的趋势,说明氢键作用是主要的吸附机理.在乙醇中,D152树脂对3种吸附质均无吸附,因为疏水作用和氢键作用均受到的乙醇抑制.在水中,吸附质与树脂间的氢键作用同样受到水的抑制,但氢键吸附却依然存在,说明水介质中氢键吸附和疏水吸附可能存在一种协同作用.在热力学上对水介质中氢键吸附和疏水吸附的协同作用给于合理的解释.     

大孔交联聚(对乙烯基苄基苯胺)树脂对苯酚的吸附

由氯甲基化聚苯乙烯合成了大孔交联聚(对乙烯基苄基苯胺)树脂, 测定了其对正己烷和水中苯酚的吸附等温线, 计算了吸附焓. 结果表明, 苯胺基树脂主要是通过氢键吸附正己烷中苯酚的, 树脂负载的功能基氮原子和苯环都作为氢键受体与苯酚的羟基氢原子形成氢键, 而其对水中苯酚的吸附是基于氢键和疏水作用.     

氧化钼表面不同氧位对氢吸附性能的从头计算研究

 用从头计算Hartree－Fock方法研究了MoO3（010）和（100）晶面上几种结构不等价氧的成键特征和电子结构,并考察了H＋在不同氧位上的吸附性能以及吸附后形成的OH从表面脱附的性质．结果表明,在氧化钼晶体中,钼氧原子间的成键具有离子性和共价性相结合的特性,且几种不等价氧与钼之间的成键性质各不相同：端氧或不对称桥氧与钼的成键具有较强的共价性,而对称桥氧具有较强的离子性;H＋在MoO3（010）和（100）晶面上几种不等价氧位都能形成稳定的吸附,而在端氧位的吸附最稳定;H＋吸附形成的OH都与表面有较强的作用,端氧位的OH最难脱附,而桥氧位的OH在表面的活动性较大,故桥氧位很可能是丙烯选择氧化过程中脱氢反应的活性中心．     

Interaction of coadsorbed CH3Cl and D2O layers on Pd(111) studied by sum frequency generation

Adsorption of methyl chloride and coadsorption of CH3Cl and D2O on Pd(111) surfaces at T=100 K have been studied under ultrahigh-vacuum conditions using femtosecond sum frequency generation (SFG) spectroscopy in the spectral regions of CH and OD bands. On the bare Pd(111) substrate, the CH3Cl coverage dependence of the resonant SFG signal is consistent with a progressive molecular rearrangement starting at half saturation followed by the growth of two ordered monolayers in which the molecular axes are perpendicular to the surface. When CH3Cl is adsorbed on top of predeposited D2O on Pd(111), the SFG signals as a function of the CH3Cl exposure indicate that methyl chloride is adsorbed onto D2O through hydrogen bonding. On the contrary when the adsorption order is reversed the strong decrease of the CH3 signal as a function of the D2O exposure is explained by assuming that water molecules penetrate inside the CH3Cl layers, leading to the formation of disordered CH3Cl clusters. In all cases a nonresonant contribution due to molecular adsorption is observed and it shows a dependence upon surface structure and coverage significantly different from that of the resonant vibrational bands.     

Chemistry of O‐ and H‐Containing Species on the (001) Surface of Anatase TiO2: A DFT Study

The chemistry of oxygen, hydrogen, water, and other species containing both oxygen and hydrogen atoms on the anatase TiO₂ (001) surface is investigated by DFT. The adsorption energy of atoms and radicals depends appreciably on the position and mode of adsorption, and on the coverage. Molecular hydrogen and oxygen interact weakly with the clean surface. However, H₂O dissociates spontaneously to give two nonidentical hydroxyl groups, and this provides a model for hydroxylation of TiO₂ surfaces by water. The mobility of the hydroxyl groups created by water splitting is initially impeded by a diffusion barrier close to 1 eV. The O₂ adsorption energy increases significantly in the presence of H atoms. Hydroperoxy (OOH) formation is feasible if at least two H atoms are present in the direct vicinity of O₂. In the adsorbed OOH, the O? O bond is considerably lengthened and thus weakened.     

Adsorption behavior of statherin and a statherin peptide onto hydroxyapatite and silica surfaces by in situ ellipsometry

The salivary protein statherin is known to adsorb selectively onto hydroxyapatite (HA), which constitutes the main mineral of the tooth enamel. This adsorption is believed to be crucial for its function as an inhibitor of primary (spontaneous) and secondary (crystal growth) precipitation of calcium phosphate salts present in saliva. A fragment corresponding to the first 21 N-terminus amino acids of statherin (StN21) was previously found to reduce the rate of demineralization of HA. Therefore, the interfacial properties of this peptide and statherin onto silica, hydrophobized silica and HA discs was studied by in situ ellipsometry. Their reversibility induced by dilution and elutability induced by buffer and sodium dodecyl sulfate (SDS) was also determined. The results revealed that statherin adsorbed at a greater extent onto the HA as compared to StN21, suggesting that the hydrogen bonding between the uncharged polar residues at the C-terminal region of statherin and HA contributes to its adsorption. However, on both silica surfaces the peptide adsorption appeared to proceed in a similar way. Onto the hydrophobized silica the adsorption of both peptides was suggested to occur either via multilayer formation or adsorption of aggregates from solution, while onto the hydrophilic silica adsorption of peptide aggregates from solution was the suggested mechanism. Further, both peptides were observed to be strongly adsorbed onto HA, even after SDS treatment, in comparison to the layers adsorbed onto hydrophobized silica. Both peptide layers were found to be weakly adsorbed onto the hydrophilic silica surface as they were totally removed by buffer dilution.     

Peptide/TiO2 surface interaction: a theoretical and experimental study on the structure of adsorbed ALA-GLU and ALA-LYS

The adsorption on the TiO(2) surface of two dipeptides AE (L-alanine-L-glutamic acid) and AK (L-alanine-L-lysine), that are "building blocks" of the more complex oligopeptide EAK16, has been investigated both theoretically and experimentally. Classical molecular dynamics simulations have been used to study the adsorption of H-Ala-Glu-NH(2) and H-Ala-Lys-NH(2) dipeptides onto a rutile TiO(2) (110) surface in water solution. Several peptide conformers have been considered simultaneously upon the surface. The most probable contact points between the molecules and the surface have been identified. Carbonyl oxygens as well as nitrogen atoms are possible Ti coordination points. Local effects are responsible for adsorption and desorption events. Self-interaction effects can induce molecular reorientations giving less strongly adsorbed species. The chemical structure and composition of thin films of the two dipeptides AE and AK on TiO(2) were investigated by XPS (X-ray photoelectron spectroscopy) measurements at both O and N K-edges. Theoretical ab initio calculations (DeltaSCF) were also performed to simulate the spectra, allowing for a direct comparison between experiment and theory.