Simultaneous spectroscopic and topographic near-field imaging of TiO2 single surface states and interfacial electronic coupling

We have probed single surface states and the involved interfacial charge transfer coupling on the TiO(2) surface using confocal as well as tip-enhanced near-field topographic-spectroscopic imaging analysis on a niobium-doped rutile TiO(2)(110) surface. The confocal images excited with a radially polarized donut mode render ring-shaped excitation patterns typical for quantum systems with two perpendicular transition dipole moments. The tip-enhanced near-field optical images of single surface states are visualized by the strong exciton plasmon-polariton coupling localized at the subdomain boundaries with a spatial resolution of ～15 nm (far beyond the optical diffraction limit). We suggest that the abundant surface states in the doped TiO(2) generate excitons under laser excitation which are strongly coupled to the surface plasmon-polaritons of the Au tip. Moreover, the interfacial electronic molecule-substrate coupling has been characterized by probing the molecule-perturbed surface states distribution and the associated specific Raman vibrational modes. The imaging and characterization of the surface states and their distributions on TiO(2) surfaces at nanoscale are critically relevant to a deep understanding of interfacial electron transfer dynamics and energetics involving in solar energy conversion, photocatalysis, and mechanistic understanding of surface-enhanced Raman scattering spectroscopy.     

End-group functionalization of poly(3-hexylthiophene) as an efficient route to photosensitize nanocrystalline TiO2 films for photovoltaic applications

Bulk heterojunction (BHJ) and dye-sensitized solar cells (DSSCs) have seen increased popularity over recent years and each technology has experienced tremendous improvements in power conversion efficiencies (PCEs), reaching 8 and 12%, respectively. The two technologies have been on independent improvement pathways, and this work establishes a link between them by using the archetypical hole conductor (poly-3-hexylthiophene, P3HT) in BHJs as a sensitizer on TiO(2) for DSSC applications. Three polymers were synthesized and examined as potential TiO(2) sensitizers in DSSCs under AM1.5 solar radiation. Using Grignard metathesis, regioregular P3HT was synthesized then functionalized with either one or two cyanoacrylic acid linker moieties to bind to the TiO(2) surface. End-group modification resulted in minimal changes to the optical and electronic properties as compared to pristine P3HT. Cyclic voltammetry (CV) experiments at anodic potentials of adsorbed sensitizer quantified the amount of alkylthiophene adsorbed on the TiO(2), whereas under reductive sweeps, cyanoacrylic acid end-group binding was determined. CVs of each polymer indicated that loading was drastically different as compared to pristine P3HT with the lowest loading on TiO(2) and monofunctionalized P3HT exhibited the highest loading. The DSSCs showed power conversion efficiencies (PCEs) of 0.1%, 0.2 and 2.2% for the polymer-sensitized TiO(2) of the unfunctionalized, monofunctionalized and difunctionalized polymers, respectively. DSSCs were then subjected to electrochemical impedance spectroscopy (EIS) in the dark and under monochromatic light radiation. The large variance in performance for the functionalized-P3HT sensitizers is attributed to differences in the adsorption modes of sensitizer on the TiO(2) surface, which in the difunctionalized case limits electrolyte recombination and favors forward charge transfer reactions.     

溅射—阳极氧化法在釉面砖上制备多彩TiO_2膜

采用直流磁控溅射法先在釉面砖上镀制一层 Ti膜 ,然后采用阳极氧化法制备透明的 Ti O2 薄膜。根据光的干涉原理 ,控制 Ti O2 膜的厚度 ,制备了土黄、深兰、浅兰、金黄、粉红、紫红、绿色等十余种颜色的金属化装饰面砖     

Interface architecture determined electrocatalytic activity of Pt on vertically oriented TiO(2) nanotubes

The surface atomic structure and chemical state of Pt is consequential in a variety of surface-intensive devices. Herein we present the direct interrelationship between the growth scheme of Pt films, the resulting atomic and electronic structure of Pt species, and the consequent activity for methanol electro-oxidation in Pt/TiO(2) nanotube hybrid electrodes. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements were performed to relate the observed electrocatalytic activity to the oxidation state and the atomic structure of the deposited Pt species. The atomic structure as well as the oxidation state of the deposited Pt was found to depend on the pretreatment of the TiO(2) nanotube surfaces with electrodeposited Cu. Pt growth through Cu replacement increases Pt dispersion, and a separation of surface Pt atoms beyond a threshold distance from the TiO(2) substrate renders them metallic, rather than cationic. The increased dispersion and the metallic character of Pt results in strongly enhanced electrocatalytic activity toward methanol oxidation. This study points to a general phenomenon whereby the growth scheme and the substrate-to-surface-Pt distance dictates the chemical state of the surface Pt atoms, and thereby, the performance of Pt-based surface-intensive devices.     

Template-based fabrication of nanowire-nanotube hybrid arrays

The fabrication and structure characterization of ordered nanowire-nanotube hybrid arrays embedded in porous anodic aluminum oxide (AAO) membranes are reported. Arrays of TiO(2) nanotubes were first deposited into the pores of AAO membranes by a sol-gel technique. Co?nanowires were then electrochemically deposited into the TiO(2) nanotubes to form the nanowire-nanotube hybrid arrays. Scanning electron microscopy and transmission electron microscopy measurements showed a high nanowire filling factor and a clean interface between the Co nanowire and the TiO(2) nanotube. Application of these hybrids to the fabrication of ordered nanowire arrays with highly controllable geometric parameters is discussed.     

Spectroscopic and nonlinear photophysical characterization of organic octupolar-compounds supported by anodic-alumina nanotube-arrays

Amorphous anodic alumina membranes (AAM) comprising highly ordered nanometric porous arrays (porous anodic aluminas: PAA) with 1D-nanotube dimensions of ∼75 nm in diameter and 45 microns in depth were successfully prepared and used as nanostructured host networks for different functionalized octupolar chromophores (named here Oct-(n)). Atomic force microscopy (AFM) studies performed on the developed hybrid systems confirmed a homogeneous insertion of these organic molecules into the PAA nanotube-arrays. Samples with high structural quality were selected for several photophysical characterizations: Comprehensive X-ray diffraction (XRD) and optical spectroscopic characterizations performed according to UV-vis absorption, photoluminescent (PL) and Raman measurements revealed the structural and optical performance of these molecules within the PAA-confinement. Since the implemented optical chromophores were specifically functionalized for nonlinear optical (NLO) applications, the obtained Oct-(n)/PAA-based amorphous hybrids were also characterized according to cubic NLO-techniques such as third harmonic generation (THG) and the Z-Scan method. PAA-confined octupolar chromophores have shown interesting linear and NLO optical properties which have not yet been intensively investigated in bulk hybrid systems; hence, the obtained hybrid nanostructures represent a promising field of investigation in the route to functional octupolar-based materials, where different self-assembled molecular structures may be formed, giving rise to enhanced linear and NLO-properties.     

不同阳极氧化条件下TiO_2纳米管阵列的制备及表征

分别在三种不同的电解液中,以钛为基体采用阳极氧化的方法制备了TiO2纳米管阵列薄膜,用SEM观察纳米管阵列薄膜的形貌、测量纳米管管径大小;用XRD、拉曼光谱检测TiO2纳米管阵列薄膜热处理前后的晶型.结果表明:不同的电解液体系和氧化电压下得到的纳米管形貌各不相同.在0.24wt%HF水溶液中得到的TiO2纳米管排列整齐,管径为110nm;在0.5wt%NaF+2.7wt%Na2SO4水溶液和0.88wt%NH4F的丙三醇-水(体积比1:1)混合溶液中得到的纳米管排列不规整,管径为100nm;在0.24wt%HF条件下生成的TiO2纳米管管径与氧化电压成线性关系:d=k×U+b,其中,系数k=5.2nm/V,b=2.2nm,0≤U≤25V.经450℃热处理2h后TiO2纳米管结构由无定形态转变为锐钛矿.     

Energy Level Alignment at Metal/Solution‐Processed Organic Semiconductor Interfaces

Energy barriers between the metal Fermi energy and the molecular levels of organic semiconductor devoted to charge transport play a fundamental role in the performance of organic electronic devices. Typically, techniques such as electron photoemission spectroscopy, Kelvin probe measurements, and in‐device hot‐electron spectroscopy have been applied to study these interfacial energy barriers. However, so far there has not been any direct method available for the determination of energy barriers at metal interfaces with n‐type polymeric semiconductors. This study measures and compares metal/solution‐processed electron‐transporting polymer interface energy barriers by in‐device hot‐electron spectroscopy and ultraviolet photoemission spectroscopy. It not only demonstrates in‐device hot‐electron spectroscopy as a direct and reliable technique for these studies but also brings it closer to technological applications by working ex situ under ambient conditions. Moreover, this study determines that the contamination layer coming from air exposure does not play any significant role on the energy barrier alignment for charge transport. The theoretical model developed for this work confirms all the experimental observations.     

Alignment of the dye's molecular levels with the TiO(2) band edges in dye-sensitized solar cells: a DFT-TDDFT study

We present a theoretical study of the lineup of the LUMO of Ru(II)-polypyridyl (N3 and N719) molecular dyes with the conduction band edge of a TiO(2) anatase nanoparticle. We use density functional theory (DFT) and the Car-Parrinello scheme for efficient optimization of the dye-nanoparticle systems, followed by hybrid B3LYP functional calculations of the electronic structure and time-dependent DFT (TDDFT) determination of the lowest vertical excitation energies. The electronic structure and TDDFT calculations are performed in water solution, using a continuum model. Various approximate procedures to compute the excited state oxidation potential of dye sensitizers are discussed. Our calculations show that the level alignment for the interacting nanoparticle-sensitizer system is very similar, within about 0.1?eV, to that for the separated TiO(2) and dye. The excellent agreement of our results with available experimental data indicates that the approach of this work could be used as an efficient predictive tool to help the optimization of dye-sensitized solar cells.     

Comparative study of photocatalytic and photoelectrocatalytic properties of alachlor using different morphology TiO2/Ti photoelectrodes

Wormhole-shaped TiO(2)/Ti (WT) and nanotube-shaped TiO(2)/Ti (TNT) photoelectrodes were prepared by anodic oxidation method. The morphology and structure were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was found that both crystal types of WT and TNT photoelectrodes were composed of anatase and rutile TiO(2) phases; however TNT photoelectrodes had highly ordered nanostructure. The photoelectrochemical (PECH) and photoelectrocatalytic (PEC) properties of WT and TNT photoelectrodes were investigated by photocurrent transient, open-circuit potential and degradation rate of alachlor under the artificial solar light illumination. All results showed that TNT photoelectrodes prepared in NaF-Na(2)SO(4) solution have more excellent photoelectron properties than WT photoelectrodes prepared in H(2)SO(4) solution. The photocatalytic (PC) and PEC experiments of alachlor showed that PC and PEC activities of TNT photoelectrodes were superior to WT photoelectrodes. At applied bias potentials the degradation rate of alachlor at TNT photoelectrodes increased significantly to 94.5%. The higher PC and PEC performance of TNT photoelectrodes were ascribed to the long-range ordered structure and short-orientation diffusion distance of photogenerated carries.     

CdS and CdTeS quantum dot decorated TiO2 nanowires. Synthesis and photoefficiency

An easy process was developed to synthesize TiO(2) nanowires sensitized with CdS and CdTeS quantum dots (QDs) requiring no pretreatment of the TiO(2) nanowires prior to nanoparticle generation. CdS and CdTeS nanoparticles were firstly grown by an in situ colloidal method directly onto the TiO(2) surface, hence not requiring subsequent functionalization of the QDs. The resulting nanostructure assembly and composition was confirmed by transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Successful decoration of the TiO(2) nanowires by the QDs was observed by TEM, while XPS spectra provided clear evidence for the coexistence of CdS and CdTeS QDs and TiO(2) nanowires. The electronic structure of the TiO(2) nanowires was preserved as indicated by Raman spectroscopy. Preliminary photocurrent measurements showed that inclusion of Te in CdS QDs improved the photocurrent efficiency. Compared to bare TiO(2) nanowires, CdS/TiO(2) nanoassemblies showed an enhancement in photocurrent efficiency of 300% while CdTeS/TiO(2) presented an improvement of 350%. This study indicates that the generation of strongly anchored CdS and CdTeS QDs on a TiO(2) nanowire surface is achievable without introduction of a linker molecule, whose presence is known to decrease the electron injection efficiency.     

Preparation and application of nanoglued binary titania-silica aerogel

Nanoglued binary titania (TiO2)-silica (SiO2) aerogel, as a novel type of photocatalyst, has been synthesized on glass substrates. Using an about-to-gel SiO2 sol as nanoglue, anatase TiO2 aerogel was immobilized into a three-dimensional mesoporous network of the SiO2. Factorial designs were employed to optimize both TiO2 aerogel and binary TiO2-SiO2 aerogel synthesis. Characterization of the as-prepared TiO2 and binary samples by surface area, porosity, and surface chemical composition showed that the photocatalysts were high-surface-area nanoporous materials, with a Ti4+ valency. The binary aerogel exhibited high photocatalytic activity for the degradation of methylene blue (MB) under simulated solar light; the reaction followed the pseudo first-order Langmuir-Hinshelwood (L-H) kinetic model. Fluorescence spectroscopy revealed that the hydroxyl (*OH) radical was formed during the illumination of the binary TiO2-SiO2 aerogel in a solution of probe molecules, which corroborates the probable mechanism of hydroxyl radical oxidation of contaminants in photocatalytic reactions.     

Crystalline characterization and photodecomposition properties of rod-shaped Na2Ti6O13 powder prepared by molten salt process

To prepare one-dimensional nanostructured Na2Ti6O13 powder, the starting materials of TiO2, NaCl and Na2CO3 were mixed and then heat-treated at 1000 degrees C for 2 hrs in air under molten state of NaCl. Changes in shape and phase, photo absorbance and photocatalytic ability of TiO2 particle were observed controlling added amount of Na2CO3 under constant weight ratio of TiO2 to NaCl using SEM, X-ray diffractometer, Raman spectroscopy, and UV-Vis spectroscopy. The TiO2 particle was changed into rod-shape Na2Ti6O13 with the addition of Na2CO3, showing increase in optical energy band-gap of the powder as well as gradual decrease of the photo-decomposition ability.     

Detection and stability of nanoscale space charges in local oxidation nanolithography

We report on the stability of space charges within nanoscale silicon oxide patterns generated by atomic force microscope tip-induced local anodic oxidation of alkyl-terminated silicon. Surface potentials of these structures are investigated using two different approaches: Kelvin probe force microscopy and the spectroscopy of adsorbed charge-sensitive dye molecules. Both techniques prove that there is no decay of the space charge itself at least for several days. The apparent decrease of the surface potential measured with the Kelvin probe method is known to be influenced by the ambient humidity. It is supposed to be caused by a screening effect through the formation of a water layer. This is confirmed by our investigation of the surface potential decrease kinetics, which could be well fitted with an adapted model of water condensation. The fluorescence of the charge-sensitive dye di-4-ANEPPS, which is applied to the structures, shows a spectral shift of about 270 meV compared to an uncharged silicon oxide surface. The high stability of the charges supports the use of local anodic oxidation patterns as templates for selective immobilization of cationic species.     

Polarization mapping: a method to improve sum frequency generation spectral analysis

A "polarization mapping" method has been applied to improve the fitting quality of sum frequency generation (SFG) vibrational spectra of complicated systems and validate the data analysis of simple SFG spectra. Using such a method, two-dimensional SFG spectra can be constructed, from which more reliable spectral information can be obtained from a surface/interface. Model calculations as well as experiments have been employed to illustrate the power of the polarization mapping method for spectral analysis of SFG spectra. By using a deuterated polystyrene surface and interfacial protein molecules as examples, we demonstrate that this method is especially important for complicated molecules, such as polymers and proteins.     

Ab initio density functional simulation of structural and electronic properties of MgO ultra-thin adlayers on the (001) Ag surface

To simulate the properties of ultrathin layers of magnesium oxide epitaxially grown on silver (001), we have adopted a periodic slab model, consisting of six layers of Ag covered on both sides with an MgO monolayer. All calculations have been performed with the 98 program. Several DFT functionals were tried and a rich basis set was adopted. The electronic and structural properties of the two bulk materials (Ag and MgO) were quite accurately reproduced. The presence of the metallic substrate was found to have an appreciable influence on the structural and electronic features of the oxide surface. In the most stable configuration (O ions directly above Ag atoms, Mg ions in the hollow sites), the surface is corrugated, and there is a net transfer of electrons from the overlayer to the metal, leading to a substantial reduction of the work function of the metal and to a decrease of the electrostatic field at the surface. The reactivity properties of the supported oxide surface have been investigated by studying the interaction of the composite material with water molecules.     

Functionalization of silicon dioxide surface with 3-aminopropyltrimethoxysilane for fullerene C60 immobilization

Formation of self-assembled monolayers (SAM) of 3-aminopropyltrimethoxysilane (APTMS), chemically bonded to silicon dioxide surface, using a new solvent free process, has been studied by contact angle measurements, ellipsometry, ATR-FTIR spectroscopy and AFM imaging. The possibility of using as-obtained APTMS SAMs for anchoring functional molecular moieties is then studied with fullerene C60. In a first part we have analyzed the grafting kinetics of APTMS SAMs in order to control the formation of a single monolayer. Results show that about four hours are needed to obtain a complete APTMS single monolayer. In parallel, the ordering kinetics of the SAM has been monitored by ATR-FTIR spectroscopy, showing that the monolayer reaches its final order before grafting. We show that those APTMS SAMs can be used to graft C60 molecules deposited from a solution and forming about one monolayer anchored on amine terminal moieties. Such results could help paving the way to the preparation of hybrid C60-based molecular devices on silicon through a bottom-up approach.     

Enhanced photocatalytic activity by the construction of a TiO<Subscript>2</Subscript>/carbon nitride nanosheets heterostructure with high surface area via direct interfacial assembly

A TiO2 heterostructure modified with carbon nitride nanosheets (CN-NSs) has been synthesized via a direct interfacial assembly strategy.The CN-NSs,which have a unique two-dimensional structure,were favorable for supporting TiO2 nanoparticles (NPs).The uniform dispersion of TiO2 NPs on the surface of the CN-NSs creates sufficient interfacial contact at their nanojunctions,as was confirmed by electron microscopy analyses.In comparison with other reported metal oxide/CN composites,the strong interactions of the ultrathin CN-NSs layers with the TiO2 nanoparticles restrain their re-stacking,which results in a large specific surface area of 234.0 m2·g-1.The results indicate that the optimized TiO2/CN-NSs hybrid exhibits remarkably enhanced photocatalytic efficiency for dye degradation (with k of 0.167 min-1 under full spectrum) and H2 production (with apparent quantum yield =38.4％ for,λ =400 ± 15 nm monochromatic light).This can be ascribed to the improved surface area and quantum efficiency of the hybrid,with a controlled ratio that reaches the appropriate balance between producing sufficient nanojunctions and absorbing enough photons.Furthermore,based on the identification of the main active species for photodegradation,and the confirmation of active sites for H2 evolution,the charge transfer pathway across the TiO2/CN-NSs interface under simulated solar light is proposed.     

Effects of 5-(3-aminophenyl)-tetrazole on the inhibition of unalloyed iron corrosion in aerated 3.5% sodium chloride solutions as a corrosion inhibitor

The effects of 5-(3-aminophenyl)-tetrazole (APT) on the inhibition of unalloyed iron corrosion in aerated 3.5% NaCl solutions as a corrosion inhibitor have been studied using open circuit potential (OCP), cyclic potentiodynamic polarization (CPP), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) measurements. The inhibited iron surface was characterized by scanning electron spectroscopy (SEM) and energy dispersive X-ray (EDS) investigations. The OCP showed positive shifts of potential in the presence of APT and the increase of its concentration. CPP and CA measurements indicated that APT molecules decrease the pitting and uniform corrosions through decreasing the pitting and absolute currents, and corrosion rate as well as shifting the corrosion and pitting potentials of iron towards the noble values. EIS plots revealed that APT increases the surface and polarization resistances of iron. SEM/EDS investigations proved that the inhibition of iron corrosion in NaCl containing APT solutions is achieved by the adsorption of APT molecules onto iron to preclude the dissolution process by blocking the active sites on its surface.     

Enhancing the photoelectric conversion of dye-sensitized solar cell via nitrogen-doped nanocrystalline titania electrode

A high efficient dye-sensitized solar cell (DSC) was fabricated using nitrogen-doped nanocrystalline titania(TiO2) photoanode. X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), X-ray powder diffraction (XRD), zeta potentials, nitrogen adsorption-desorption and elemental analysis experiments were employed to characterize the nitrogen-doped nanocrystalline TiO2 photoanode. An obvious enhancement of the optical absorption in the range of 380-550 nm was observed for nitrogen-doped TiO2, which was attributed to both the substitutional N and the chemisorbed N2 molecules. A conversion efficiency of 9.04% was obtained on the DSC based on nitrogen-doped TiO2 photoanode annealed in a flow of NH3 at 550 degrees C, with an increase of 15.6% improvement in comparison with pure TiO2 (7.82%). The mechanism for the enhanced photovoltaic performance was discussed.