Light-excited superhydrophilicity of amorphous TiO2 thin films deposited in an aqueous peroxotitanate solution

We report on the photoinduced superhydrophilicity of the surface of amorphous TiO2. Amorphous TiO2 thin films were prepared on self-assembled monolayers by the peroxotitanate-complex deposition (PCD) and liquid-phase deposition (LPD) methods. The surface morphology and topography were characterized in detail. The contact angles were 34 degrees and 66 degrees for the as-deposited thin films through the PCD and LPD methods, respectively, which slowly increased to about 70 degrees and 73 degrees after being stored in air. After irradiation by UV light, the contact angle vanished and the surface exhibited superhydrophilicity. The superhydrophilicity and hydrophobicity could be switched by alternatively exposing the surface to UV light and drying in an atmosphere filled with organic gases. Although the oxidation of the contamination on the surface has effects on the increase in hydrophilicity, the X-ray photoelectron spectroscopy results suggested that the superhydrophilicity was also related to the transformation of the Ti-OH groups to groups that have dangling bonds. This paper indicates that an amorphous TiO2 thin film does not need to be heated to obtain superhydrophilicity; such a self-cleaning surface can be achieved at room temperature by our newly developed environmentally friendly method.     

Ultrathin SiO(x) Film Coating Effect on the Wettability Change of TiO(2) Surfaces in the Presence and Absence of UV Light Illumination

Wettability of sol-gel TiO(2) film surfaces has been studied by following the H(2)O contact angle (straight theta) as functions of illumination time and subsequent dark storage time. Upon illumination of the TiO(2) surface (lambda(ex)>300 nm), the value of straight theta rapidly decreased to reach approximately zero (photoprocess). When the resultant superhydrophilic sample was stored in the dark, the straight theta value increased slowly with increasing storage time (dark process). Ultrathin SiO(x) films were formed on the surface of TiO(2) by repeating chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane and its photooxidation. With increasing thickness of the SiO(x) monolayer coating, the rate for the dark process significantly decreased, while that for the photoprocess also decreased. Both X-ray photoelectron spectroscopic studies and product analyses during the photoprocess clarified that photocatalytic oxidation of organic adsorbates in air causes the superhydrophilicity of the TiO(2) surfaces. Copyright 2000 Academic Press.     

Reactive uptake of HONO to TiO2 surface: "dark" reaction

The interaction of HONO with TiO(2) solid films was studied under dark conditions using a low pressure flow reactor (1-10 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake of HONO to TiO(2) was studied as a function of HONO concentration ([HONO)(0) = (0.3-3.3) × 10(12) molecules cm(-3)), water concentration (RH = 3 × 10(-4) to 13%), and temperature (T = 275-320 K). TiO(2) surface deactivation upon exposure to HONO was observed. The measured initial uptake coefficient of HONO on TiO(2) surface was independent of the HONO concentration and showed slight negative temperature dependence (activation factor = -1405 ± 110 K). In contrast, the relative humidity (RH) was found to have a strong impact on the uptake coefficient: γ(0) = 1.8 × 10(-5) (RH)(-0.63) (calculated using BET surface area, 40% uncertainty) at T = 300 K. NO(2) and NO were observed as products of the HONO reaction with TiO(2) surface with sum of their yields corresponding to nearly 100% of the nitrogen mass balance. The yields of the NO and NO(2) products were found to be 42 ± 7% and 60 ± 9%, respectively, independent of relative humidity, temperature, and concentration of HONO under experimental conditions used. The contribution of aerosol to the total HONO loss in the boundary layer (calculated with initial uptake data for HONO on TiO(2) surface) showed the unimportance of this process in the atmosphere. In addition, the diffusion coefficient of HONO in He was determined to be D(HONO-He) = 490 ± 50 Torr cm(2) s(-1) at T = 300 K.     

Heterogeneous Interaction of H(2)O(2) with TiO(2) Surface under Dark and UV Light Irradiation Conditions

The heterogeneous interaction of H(2)O(2) with TiO(2) surface was investigated under dark conditions and in the presence of UV light using a low pressure flow tube reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of gaseous H(2)O(2) ([H(2)O(2)](0) = (0.17-120) × 10(12) molecules cm(-3)), irradiance intensity (J(NO(2)) = 0.002-0.012 s(-1)), relative humidity (RH = 0.003-82%), and temperature (T = 275-320 K). Under dark conditions, a deactivation of TiO(2) surface upon exposure to H(2)O(2) was observed, and only initial uptake coefficient of H(2)O(2) was measured, given by the following expression: γ(0)(dark) = 4.1 × 10(-3)/(1 + RH(0.65)) (calculated using BET surface area, estimated conservative uncertainty of 30%) at T = 300 K. The steady-state uptake coefficient measured on UV irradiated TiO(2) surface, γ(ss)(UV), was found to be independent of RH and showed a strong inverse dependence on [H(2)O(2)] and linear dependence on photon flux. In addition, slight negative temperature dependence, γ(ss)(UV) = 7.2 × 10(-4) exp[(460 ± 80)/T], was observed in the temperature range (275-320) K (with [H(2)O(2)] ≈ 5 × 10(11) molecules cm(-3) and J(NO(2)) = 0.012 s(-1)). Experiments with NO addition into the reactive system provided indirect evidence for HO(2) radical formation upon H(2)O(2) uptake, and the possible reaction mechanism is proposed. Finally, the atmospheric lifetime of H(2)O(2) with respect to the heterogeneous loss on mineral dust was estimated (using the uptake data for TiO(2)) to be in the range of hours during daytime, i.e., comparable to H(2)O(2) photolysis lifetime (～1 day), which is the major removal process of hydrogen peroxide in the atmosphere. These data indicate a strong potential impact of H(2)O(2) uptake on mineral aerosol on the HO(x) chemistry in the troposphere.     

Stick-slip phenomenon in measurements of dynamic contact angles and surface viscoelasticity of poly(styrene-b-isoprene-b-styrene) triblock copolymers

In this paper, a series of poly(styrene-b-isoprene-b-styrene) triblock copolymers (SIS), with different chemical components, was synthesized by anionic polymerization. The relationships between surface structures of these block copolymers and their stick-slip phenomena were investigated. There is a transition from stick-slip to a closely smooth motion for the SIS films with increasing PS content; the patterns almost vanish and the three-phase line appears to move overall smoothly on the film surface. The results show that the observed stick-slip pattern is strongly dependent on surface viscoelasticity. The jumping angle Δθ, which is defined as θ(1) - θ(2) (when a higher limit to θ(1) is obtained, the triple line "jumps" from θ(1) to θ(2) with increases in drop volume), was employed to scale the stick-slip behavior on various SIS film surfaces. Scanning force microscopy/atomic force microscopy (AFM) and sum frequency generation methods were used to investigate the surface structures of the films and the contributions of various possible factors to the observed stick-slip behavior. It was found that there is a linear relationship between jumping angle Δθ and the slope of the approach curve obtained from AFM force measurement. This means that the stick-slip behavior may be attributed mainly to surface viscoelasticity for SIS block copolymers. The measurement of jumping angle Δθ may be a valuable method for studying surface structure relaxation of polymer films.     

Surface tension and its temperature coefficient of molten tin determined with the sessile drop method at different oxygen partial pressures

The surface tension of molten tin has been determined by the sessile drop method at temperatures ranging from 523 to 1033 K and in the oxygen partial pressure (P(O(2))) range from 2.85 x 10(-19) to 8.56 x 10(-6) MPa, and its dependence on temperature and oxygen partial pressure has been analyzed. At P(O(2))=2.85 x 10(-19) and 1.06 x 10(-15) MPa, the surface tension decreases linearly with the increase of temperature and its temperature coefficients are -0.151 and -0.094 mN m(-1) K(-1), respectively. However, at high P(O(2)) (3.17 x 10(-10), 8.56 x 10(-6) MPa), the surface tension increases with the temperature near the melting point (505 K) and decreases above 723 K. The surface tension decrease with increasing P(O(2)) is much larger near the melting point than at temperatures above 823 K. The contact angle between the molten tin and the alumina substrate is 158-173 degrees, and the wettability is poor.     

Tin oxide-surface modified anatase titanium(IV) dioxide with enhanced UV-light photocatalytic activity

[Sn(acac)(2)]Cl(2) is chemisorbed on the surfaces of anatase TiO(2)via ion-exchange between the complex ions and H(+) released from the surface Ti-OH groups without liberation of the acetylacetonate ligand (Sn(acac)(2)/TiO(2)). The post-heating at 873 K in air forms tin oxide species on the TiO(2) surface in a highly dispersed state on a molecular scale ((SnO(2))(m)/TiO(2)). A low level of this p block metal oxide surface modification (~0.007 Sn ions nm(-2)) accelerates the UV-light-activities for the liquid- and gas-phase reactions, whereas in contrast to the surface modification with d block metal oxides such as FeO(x) and NiO, no visible-light response is induced. Electrochemical measurements and first principles density functional theory (DFT) calculations for (SnO(2))(m)/TiO(2) model clusters (m = 1, 2) indicate that the bulk (TiO(2))-to-surface interfacial electron transfer (BS-IET) enhances charge separation and the following electron transfer to O(2) to increase the photocatalytic activity.     

Growth and structure of water on SiO2 films on Si Investigated by Kelvin probe microscopy and in Situ X-ray spectroscopies

The growth of water on thin SiO2 films on Si wafers at vapor pressures between 1.5 and 4 Torr and temperatures between -10 and 21 degrees C has been studied in situ using Kelvin probe microscopy and X-ray photoemission and absorption spectroscopies. From 0 to 75% relative humidity (RH), water adsorbs forming a uniform film 4-5 layers thick. The surface potential increases in that RH range by about 400 mV and remains constant upon further increase of the RH. Above 75% RH, the water film grows rapidly, reaching 6-7 monolayers at around 90% RH and forming a macroscopic drop near 100%. The O K-edge near-edge X-ray absorption spectrum around 75% RH is similar to that of liquid water (imperfect H-bonding coordination) at temperatures above 0 degrees C and is ice-like below 0 degrees C.     

Photoinduced electron transfer from the excited J-aggregate state of a thiacarbocyanine dye to TiO2 colloids

The photophysical properties of the J-aggregate of 3,3'-di(3-sulfopropyl)-4,5,4',5'-dibenzo-9-phenyl-thiacarbocyanine triethyl-ammonium salt in the absence and presence of TiO(2) colloids have been studied using UV-visible absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and ESR spectroscopy. The fluorescence emission of the J-aggregate decreases with increasing concentration of TiO(2) colloids. The average fluorescence lifetime of the J-aggregate in the presence of TiO(2) colloids is shorter than that in the absence of TiO(2) colloids. A strong photoinduced ESR signal has been observed during illumination by light with lambda=633 nm in the presence of TiO(2) and the ESR signal can be attributed to the J-aggregate radical cation. From the above results, it is concluded that photoinduced electron transfer from the excited singlet state of the J-aggregate to the conduction band of TiO(2) takes place and the electron transfer rate is about 1.5 x 10(8) s(-1).     

Photoinduced phase separation and miscibility in the condensed phase of a mixed Langmuir monolayer

We report our studies on the mixed Langmuir monolayer of mesogenic molecules, p-(ethoxy)-p-phenylazo phenyl hexanoate (EPPH) and octyl cyano biphenyl (8CB), employing the techniques of surface manometry and Brewster angle microscopy. Our studies show that the mixed monolayer exhibits higher collapse pressures for certain mole fractions of EPPH in 8CB as compared to individual monolayers. Also, a considerable reduction in the area per molecule is seen in the mixed monolayer, indicating a condensed phase. We have also studied the photostability of the mixed monolayer at different initial surface pressures. The mixed monolayer, under alternate cycles of UV and visible illumination, exhibits changes in surface pressures. This is due to the photoinduced transformation of EPPH isomers in the mixed monolayer. Our in-situ Brewster angle microscope studies for 0.5 mole fraction of EPPH in 8CB show a phase separation in the UV and a miscible phase in the visible, at low surface pressures ( approximately 5 mN/m). At higher surface pressures ( approximately 10 mN/m), under UV illumination, we find a phase separation which does not revert to a miscible phase under visible illumination.     

UV-manipulated wettability between superhydrophobicity and superhydrophilicity on a transparent and conductive SnO2 nanorod film

A smart surface with wettability that can be switched between superhydrophobicity and superhydrophilicity has been realized on a transparent and conductive SnO2 nanorod film by the alternation of UV-irradiation and dark storage.     

Photoenhanced uptake of NO2 by pyrene solid films

We report uptake kinetics measurements of the heterogeneous reaction of gas phase NO2 with solid films of pyrene. By using a coated flow tube equipped with several near-ultraviolet (UV) emitting lamps (range 300-420 nm), we examined the effect of actinic radiation on the heterogeneous loss kinetics of nitrogen dioxide. With atmospherically relevant concentrations of NO2, (20-119 ppbv), the uptake ranged from below 10(-7) in the dark to 3.5 x 10(-6) under near-UV irradiation. Under illuminated conditions, the uptake coefficient decreased markedly with increasing gas-phase concentration, suggestive of a Langmuir-Hinshelwood-type surface reaction mechanism. The NO2 reactivity was not a function of deposited Pyrene mass or of the relative humidity (in the range 10-89%) and depended linearly on the intensity of illumination. Gas-phase product analysis indicated that approximately 50% of the NO2 loss could be accounted for by HONO and NO release. These experimental results are discussed along with a possible nitration mechanism.     

Neutron reflection and small-angle neutron scattering studies of a fluorocarbon telomer surfactant

Dilute aqueous phase behavior of a novel tris(hydroxymethyl)acrylamidomethane (THAM)-derived telomer bearing a perfluorohexyl hydrophobic chain, F6THAM6, has been investigated. Fluorinated polyhydroxy surfactants of this kind find use in emerging biomedical applications. Neutron reflection (NR) and drop volume surface tension (DVT) methods have been used to determine the critical micelle concentration (cmc=4.7 x 10(-4) mol x dm(-3)) and surface adsorption parameters (at the cmc NR gives a molecular area a(cmc)=67.4 and 62 A(2) and surface excess gamma(cmc)=2.46 x 10(-6) mol x m(-2)). The aggregation structures were determined by small-angle neutron scattering (SANS), indicating globular (polydisperse spheres) micelles of radius approximately 30 A are present. These findings are compared with literature on surfactants with related structures, to identify how the unusual molecular structure of F6THAM6 affects surfactant properties.     

Photoinduced interaction between fluorescein ester derivatives and CdS colloid

The photoinduced interaction of fluorescein ester derivatives, fluorescein's butyl ester (FL4) and fluorescein's anthraquinone-methyl ester (FL-AQ), and colloidal CdS was examined by absorption, fluorescence spectroscopy, and photoinduced ESR spectroscopy. It is found that FL4 and FL-AQ molecules are adsorbed on the surfaces of CdS colloid by an electrostatic interaction forming the surface complex of the type CdS-FL4 or CdS-FL-AQ. The apparent association constant (K(app)) and the degree of association (alpha) of CdS-FL4 obtained from absorption spectra are 2.25 x 10(4) M(-1) and 0.78, respectively. The values of K(app) and alpha of CdS-FL4 as determined from fluorescence spectra are 1.54 x 10(4) M(-1) and 0.82, respectively, which matches well with that determined from the absorption spectra changes. And the values of K(app) and alpha of CdS-FL-AQ obtained from absorption spectra are 4.18 x 10(4) M(-1) and 0.83, respectively. These data indicate that there is a strong interaction between the dye and the CdS particle surface. But there was no evidence for interfacial electron transfer from FL4 or FL-AQ to colloidal CdS by photoinduced ESR experiments. The fluorescence quenching is due to the formation of a nonfluorescent complex. The related phenomena are discussed in this paper.     

Synthesis, structures, and photoinduced electron transfer reaction in the 9,9'-spirobifluorene-bridged bipolar systems

[reaction: see text] A series of 9,9'-spirobifluorene-bridged bipolar compounds DnAm bearing various n:m ratios for triarylamine (D) versus 1,3,4-oxadiazole-conjugated oligoaryl moiety (A) have been synthesized to investigate the corresponding photoinduced electron transfer (PET) property. The excitation behaviors were probed by steady-state absorption, emission, fluorescence solvatochromism, and femtosecond fluorescence up-conversion spectroscopy. The overall reaction dynamics can be rationalized by the rate of PET, in combination with solvent relaxation dynamics. It was found that the rate of PET is dependent on the anchored D/A ratio. The rate of D1A1 and D2A1 was resolved to be approximately 2.44 x 10(12) and 2.32 x 10(12) s(-)(1), respectively, while it is irresolvable in D1A2 and D2A2 (>6.6 x 10(12) s(-)(1)). In another approach, based on the comprehensive X-ray data, cyclic voltammetry, and absorption/emission spectra, the rate of photoinduced electron transfer was also qualitatively estimated. Fair comparisons were made between experimental and theoretical approaches to gain detailed insight into the PET for the titled systems.     

Cathodic adsorptive stripping voltammetric determination of uranium with potassium hydrogen phthalate

The adsorption properties of dioxouranium (II)-Phathalate complexes onto hanging mercury drop electrode are exploited in developing a highly sensitive and selective stripping voltammetric procedure for the determination of uranium (VI). The reduction current of adsorbed complex ions of U(VI) was measured by both linear sweep (LSCSV) and differential pulse cathodic stripping voltammetry (DPCSV), preceded by a period of preconcentration onto the electrode surface. As low as 2x10(-9) mol dm(-3) (0.5 mug/l) and 2x10(-8) mol dm(-3) (4.8 mug/l) with accumulation time 240 and 120 s using DPCSV and LSCSV, respectively, have been determined successfully. The relative standard deviation of 2.2% at the 5 ppm level was obtained. The interferences of some metal ions and anions were studied. The application of this method was tested in the determination of uranium in superphosphate fertilizer.     

Heterogeneous uptake and reactivity of formic acid on calcium carbonate particles: a Knudsen cell reactor, FTIR and SEM study

The heterogeneous uptake and reactivity of formic acid (HCOOH), a common gas-phase organic acid found in the environment, on calcium carbonate (CaCO(3)) particles have been investigated using a Knudsen cell reactor, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). FTIR measurements show that the adsorption of formic acid on the surface of calcium carbonate results in the formation of calcium formate. Besides calcium formate, carbonic acid is also a reaction product under dry conditions (<1% RH). Under dry conditions and at low pressures, the initial uptake coefficient of formic acid on CaCO(3) particles is measured to be 3 +/- 1 x 10(-3) and decreases as the surface saturates with adsorbed products. The maximum surface coverage of formic acid under dry conditions is determined to be (3 +/- 1)x 10(14) molecules cm(-2). Under humidified conditions (RH >10%), adsorbed water on the surface of the carbonate particles participates in the surface reactivity of these particles, which results in the enhanced uptake kinetics and extent of reaction of this organic acid on CaCO(3) as well as opens up several new reaction pathways. These reaction pathways include: (i) the water-assisted dissociation of carbonic acid to CO(2) and H(2)O and (ii) the formation of calcium formate islands and crystallites, as evident by SEM images. The results presented here show that adsorbed water plays a potentially important role in the surface chemistry of gas-phase organic acids on calcium carbonate particles.     

Cyanobacterial chlorophyll as a sensitizer for colloidal TiO2

Chlorophyll has been extracted from cyanobacteria. The adsorption of chlorophyll on the surface of colloidal TiO(2) through electrostatic interaction was observed. The apparent association constant (K(app)) of chlorophyll-TiO(2) obtained from absorption spectra is 3.78x10(4)M(-1). The K(app) value of chlorophyll-TiO(2) as determined from fluorescence spectra is 1.81x10(4)M(-1), which matches well with that determined from the absorption spectra changes. These data indicate that there is an interaction between chlorophyll and colloidal TiO(2) nanoparticle surface. The dynamics of photoinduced electron transfer from chlorophyll to the conduction band of colloidal TiO(2) nanoparticle has been observed and the mechanism of electron transfer has been confirmed by the calculation of free energy change (DeltaG(et)) by applying Rehm-Weller equation as well as energy level diagram. Lifetime measurements gave the rate constant (k(et)) for electron injection from the excited state chlorophyll into the conduction band of TiO(2) is 4.2x10(8)s(-1).     

Heterometal alkoxides as precursors for the preparation of porous Fe- and Mn-TiO2 photocatalysts with high efficiencies

Transition-metal-doped titanium glycolates (M-TG, with M=Fe, Mn), which are the first non-stoichiometric heterometal alkoxides, have been synthesised through a solvothermal doping approach. X-ray diffraction, UV/Vis diffuse reflectance and ESR spectroscopy revealed that the dopant ion (Fe(3+) or Mn(2+)) is substituted for Ti(4+) in the TG lattice. Fe(3+) prolongs the crystallisation time of Fe-TG, whereas Mn(2+) has a smaller effect on the crystallisation time in comparison with Fe(3+). The as-synthesised M-TG materials were used directly as single-source precursors for the preparation of metal-doped titania (M-TiO(2)) through a simple thermal treatment process. The as-prepared M-TiO(2) materials maintain the rod-like morphology of the precursors and possess a mesoporous structure with high crystallinity. It has been proved that the dopant ions are incorporated into the TiO(2) lattice at the Ti(4+) positions. The photocatalytic activities of the M-TiO(2) materials obtained were evaluated by testing the degradation of phenol under UV irradiation. From the photocatalytic results, it was concluded that high crystallinity, a large surface area and appropriate transition-metal-doping are all beneficial to the enhancement of the photocatalytic performance of the doped TiO(2) material. In addition, it was noted that in comparison with Mn-TiO(2), Fe-TiO(2) shows higher photocatalytic activity due to the better inhibition effect of Fe(3+) on recombination of photogenerated electron-hole pairs. In contrast to the conventional nanosized TiO(2) photocatalyst, the micrometre-sized M-TiO(2) particles we obtained can be easily separated and recovered after the photocatalytic reactions.     

Ellipsometry and Infrared Reflection Absorption Spectroscopy of Adsorbed Layers of Soluble Surfactants at the Air-Water Interface

Optical techniques play an increasingly important role in the characterization of microstructure and surface densities of thin films at various interfaces. In this study, ellipsometry and infrared reflection absorption spectroscopy (IRRAS) were used for determining the surface densities of adsorbed layers of cationic surfactants in situ at the air-water interface. The surfactants were N(alpha)-lauroyl-arginine methyl ester (LAM) and N(alpha), N(omega)-bis(N(alpha)-lauroyl-arginine)-alpha,omega-alkylidenediamide (C(6)(LA)(2)). In ellipsometry, the ellipsometric phase angle Delta was obtained at various surfactant concentrations and was referenced to that of the solvent. Three algorithms were used for analyzing the data. The surface densities are 3.3+/-0.3x10(-6) mol/m(2) at 1 mM for LAM and 1.5+/-0.3x10(-6) mol/m(2) at 0.1 mM for C(6)(LA)(2) by using an algorithm for which the monolayer thickness was estimated from molecular modeling. The corresponding surface densities from literature surface tension data and the Gibbs adsorption isotherm procedure are 2.2+/-0.4x10(-6) mol/m(2) and 1.2+/-0.2x10(-6) mol/m(2), respectively. In addition, IRRAS spectra were obtained from monolayers of LAM and C(6)(LA)(2) at the air-water interface. The frequencies of the methylene stretching vibration bands indicate that the monolayers are liquid-like. The surface densities were determined from the reflectance-absorbance data by using the model of either an isotropic film or an anisotropic film on the aqueous subphase. The IRRAS-based surface densities from either model, by using DPPC monolayers for calibration, are 2.4+/-0.7x10(-6) mol/m(2) at 1 mM for LAM and 1.5+/-0.6x10(-6) mol/m(2) at 0.1 mM for C(6)(LA)(2), which are in fair agreement with the ellipsometry- and the surface-tension-based surface densities. Copyright 2001 Academic Press.