Hydrophilicity of amorphous TiO2 ultra-thin films

The UV-light-induced hydrophilicity of amorphous titanium dioxide thin films obtained by radio frequency magnetron sputtering deposition was studied in relation with film thickness. The effect of UV light irradiation on the film hydrophilicity was fast, strong and did not depend on substrate or thickness for films thicker than a threshold value of about 12 nm, while for thinner films it was weak and dependent on substrate or thickness. The weak effect of UV light irradiation observed for the ultra-thin films (with thickness less than 12 nm) is explained based on results of measurements of surface topography, UV-light absorption and photocurrent decay in vacuum. Comparing to thicker films, the ultra-thin films have a smoother surface, which diminish their real surface area and density of defects, absorb partially the incident UV light radiation, and exhibit a longer decay time of the photocurrent in vacuum, which proves a spatial charge separation. All these effects may contribute to a low UV light irradiation effect on the ultra-thin film hydrophilicity.     

Temperature-induced changes in morphology and structure of TiO2-Al2O3 fibers

Electrospinning of a sol-gel and polymer mixture is used to produce titania-alumina (TiO₂-Al₂O₃) fibers with diameters ranging from 200 to 800 nm. These composite metal-oxide fibers were calcined at various temperatures and their morphology is studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The decrease in the average diameter of the fibers with increasing temperature is observed. Powder X-ray diffraction (XRD) reveals that up to 800 °C the composite fibers have anatase titania structure whereas at 900 °C the fibers exhibit mixture of anatase and rutile phases. It is found that specific surface area decreases as a function of temperature in the 700-900 °C range. The change in phase (anatase-to-rutile) and the increase in crystallite size occur simultaneously. The presence of smaller amount of amorphous alumina in the primarily titania-based structure seems to play the role in stabilizing the anatase phase.     

Favorable recycling photocatalyst TiO2/CFA: Effects of loading percent of TiO2 on the structural property and photocatalytic activity

A series of photocatalysts TiO₂/CFA were prepared using coal fly ash (CFA), waste discharged from coal-fired power plant, as substrate, and then these photocatalysts were characterized by scanning electron microscope, X-ray diffraction analysis, nitrogen adsorption test and ultraviolet-visible absorption analysis. The effects of loading percent of TiO₂ on the photocatalytic activity and re-use property of TiO₂/CFA were evaluated by the photocatalytic decoloration and mineralization of methyl orange solution. The results show that the pore volume and the specific surface area of the TiO₂/CFA both increased with the increase in the loading percent of TiO₂, which improved the photocatalytic activity of TiO₂/CFA. However, when the loading percent of TiO₂ was too high (up to 54.51%), superfluous TiO₂ was easy to break away from CFA in the course of water treatment, which was disadvantaged to the recycling property of TiO₂/CFA. In this study, the optimal loading percent of TiO₂ was 49.97%, and the efficiencies of photocatalytic decoloration and mineralization could be maintained above 99% and 90%, respectively, when the photocatalyst was used repeatedly, without any decline, even at the sixth cycle.     

Influence of long-term aqueous exposure on surface properties of plasma sprayed oxides Al2O3, TiO2 and their mixture Al2O3-13TiO2

The surface properties of plasma sprayed Al₂O₃- and TiO₂-based coating materials were characterized in order to investigate the influence of surface strain and phase inhomogenity. The materials were water exposed up to 8 months. The bulk crystallographic structure, dissolution behaviour, effective charge (zeta potential, isoelectric point), surface compositions and oxidation states were determined. In addition, the properties of the aging solutions, such as conductivity, supernatant pH (point of zero charge), and redox potential, were monitored during aging.It was shown that the materials were stable under aging conditions, but that considerable surface rearrangements, such as dissolution-reprecipitation and surface site redistributions may occur. However, overall only minor changes in surface properties results from this restructuring process.     

Studies of sol–gel TiO2 and Pt/TiO2 catalysts for NO reduction by CO in an oxygen-rich condition

The textural properties, morphological features, surface basicity and oxygen reduction behaviours of titania and Pt supported titania catalysts synthesized via a sol–gel method were studied by means of N₂ physisorption, SEM, TEM, CO₂-TPD and H₂-TPR techniques. Mesostructured TiO₂ shows a very narrow pore size distribution that uniformly centred at about 4 nm. High resolution TEM images confirmed that most of Pt particles on Pt/TiO₂-SG had a size smaller than 2 nm. Both the titania support and Pt loaded catalysts chiefly contained weak basic sites with small amount of strong basic sites. Loading Pt did not significantly alter the surface reduction characters of titania, indicating a weak interaction between Pt metals and titania support. Catalytic evaluation revealed that the selectivity of NO reduction over titania was insensitive to variation of textural property. On the bare titania, low NO conversion but high selectivity to N₂O was obtained. However, the Pt/TiO₂-SG catalysts exhibited high NO conversion and high selectivity to N₂, which is assumed to relate to NO dissociation catalysed by the metallic Pt clusters. In addition, when the reaction temperature was above 200 °C, 3–11% NO₂ was yielded over the Pt/TiO₂-SG catalysts, which was discussed on a basis of reaction competition, metal-support interaction and NO dissociation.     

The comparison of photocatalytic activity of synthesized TiO2 and ZrO2 nanosize onto wool fibers

TiO₂ and ZrO₂ nanocrystals were successfully synthesized and deposited onto wool fibers using the sol-gel technique at low temperature. The photocatalytic activities of TiO₂-coated and ZrO₂-coated wool fibers were measured by studying photodegradation of methylene blue and eosin yellowish dyes. The initial and the treated samples were characterized by several techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and X-ray diffraction. The TEM study shows dispersed particles with 10-30 nm in size for TiO₂-coated and 20-40 nm in size for ZrO₂-coated samples on the fiber surface. Comparison of the photocatalytic activity of the coated samples reveals superiority of TiO₂ modified sample with respect to that of ZrO₂ for degradation of both dyes. Our observations indicate that by applying this technique to the fabrics, self-cleaning materials could be designed for practical application.     

The effects of SiO2 and TiO2 on the two-phase burning behavior of aqueous HAN propellant

Silica and titania nanoparticles were included at mass loadings of 1% and 3% in aqueous HAN propellants to evaluate their effects on liquid- and gas-phase decomposition and combustion. Both the liquid-phase and overall burning rates of propellant formulations were indirectly measured in a constant-volume strand burner filled with Argon from pressures of 3–22?MPa using a novel, pressure-based method developed by the authors in recent work. This approach provides overall burn times for propellants such as aqueous HAN which continue to burn beyond the disappearance of the liquid, making it superior to methods based solely on visual observation which only monitor the liquid surface regression. The presence of silica nanoparticles increased the liquid-phase burning rate in the low- and medium-pressure regimes (<10?MPa) and increased the overall burning rate at all pressures evaluated. The maximum amount of burning rate enhancement was realized at the lowest evaluated pressure (3?MPa) which corresponded to 80% and 670% increases in the liquid-phase and overall burning rates, respectively, for a silica loading of 1%, and 160% and 830% increases in the liquid-phase and overall burning rates, respectively, for a silica loading of 3%. The presence of titania did not measurably affect the liquid-phase burning rate, but it did increase the overall burning rate in the low-pressure regime (<5.7?MPa). This low-pressure overall burning rate enhancement was not amplified by further titania loading from 1% to 3% and was maximized at the lowest evaluated pressure (3?MPa) which corresponded to a 500% increase in the overall burning rate. The observed enhancements of the propellant's liquid-phase and overall burning rates were attributed to the presence of catalytic processes which diminish at higher pressures. This work represents the first time nanoparticle additives have been utilized to tailor the combustion of liquid HAN-based monopropellants.     

多种金属/TiO2核壳纳米棒阵列的通用制备方法

使用了一种具有较大通用性的方法制备了金属/二氧化钛(TiO₂)核壳纳米结构. 采用电沉积方法在多孔氧化铝模板(AAO)孔洞中沉积壁厚均一的TiO₂纳米管,TiO₂纳米管的壁厚可以通过沉积时间来控制,而纳米管的直径和长度则由模板孔洞大小和模板厚度决定. 采用这种方法制备的TiO₂纳米管顶端是开放的,而底端连接在电沉积前溅射在AAO模板背面的金膜上. 这种TiO₂纳米管阵列结构适合进行二次电沉积,以它为模板将Pd、Cu、Fe等金属沉积到纳米管中形成核壳纳米棒结构. 这是一种可以用于制备多种金属/TiO₂核壳纳米结构的通用方法,采用这种方法制备的金属/TiO₂核壳纳米棒结构具有填充率高和取向性好的特点,而且它们的壁厚和长度可以通过分别改变两步电沉积的时间来控制.     

Growth of ultra-thin TiO2 films by spray pyrolysis on different substrates

In the present study TiO₂ films were deposited by spray pyrolysis method onto ITO covered glass and Si (1 0 0) substrates. The spray solution containing titanium(IV) isopropoxide, acetylacetone and ethanol was sprayed at a substrate temperature of 450 °C employing 1-125 spray pulses (1 s spray and 30 s pause). According to AFM, continuous coverage of ITO and Si substrates with TiO₂ layer is formed by 5-10 and below 5 spray pulses, respectively. XPS studies revealed that TiO₂ film growth on Si substrate using up to 4 spray pulses follows 2D or layer-by-layer-growth. Above 4 spray pulses, 3D or island growth becomes dominant irrespective of the substrate. Only 50 spray pulses result in TiO₂ layer with the thickness more than XPS measurement escape depth as any signal from the substrate could not be detected. TiO₂ grain size remains 30 nm on ITO and increases from 10-20 nm to 50-100 nm on Si substrate with the number of spray pulses from 1 to 125.     

Synthesis and characterization of TiO2 nanotubes for humidity sensing

The highly ordered TiO₂ thin films are prepared by anodic oxidation and calcined at 300, 400, 500 and 600 °C, respectively. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) are employed to analyze the structure and the morphology of the TiO₂ films. We design a novel sensor to investigate the humidity sensing behaviours of the samples. The samples calcined at 600 °C show high sensitivity with nearly two orders change in the resistance and short response and recovery time (<190 s) during the relative humidity variation from 11 to 95%.     

Study on highly visible light active Bi-doped TiO2 composite hollow sphere

Bi-doped hollow titania spheres were prepared using carbon spheres as template and Bi-doped titania nanoparticles as building blocks. The Bi-doped titania nanoparticles were synthesized at low temperature. The prepared hollow spheres were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), UV-vis diffuse reflectance spectrum (DRS) and X-ray photoelectron spectroscopy (XPS). The effects of Bi content on the physical structure and photocatalytic activity of doped hollow titania sphere samples were investigated. Results showed that there was an optimal Bi-doped content (4%) for the photocatalytic degradation of methylene blue (MB).     

A study on photo-generated charges property in highly ordered TiO2 nanotube arrays

In this study TiO₂ nanotube arrays were fabricated by potentiostatic anodization of titanium sheet. The X-ray diffraction (XRD) pattern and field emission scanning electron microscopy (FE-SEM) image indicated the TiO₂ nanotube arrays were of pure anatase form and highly ordered. The properties of the photo-generated charges in the nanotube arrays were investigated by transient photovoltage (TPV) technique and surface photovoltage (SPV) technique based on lock-in amplifier with dc bias, in comparison with the commercial powder derived film. The separation processes of the photo-induced charges in the system of TiO₂ nanotubes on Ti have been demonstrated to be correlated with the incident light intensity, surface trapping states, and the interfacial electric field between Ti and TiO₂. The results also show that the highly ordered nanotube film could generate much stronger SPV responses under external electric field than the commercial powder derived film.     

Investigation of interface characteristics of Al2O3/Si under various O2 plasma exposure times during the deposition of Al2O3 by PA-ALD

Plasma-assisted atomic layer deposition (PA-ALD) is more suitable than thermal atomic layer deposition (ALD) for mass production because of its faster growth rate. However, controlling surface damage caused by plasma during the PA-ALD process is a key issue. In this study, the passivation characteristics of Al₂O₃ layers deposited by PA-ALD were investigated with various O₂ plasma exposure times. The growth per cycle (GPC) during Al₂O₃ deposition was saturated at approximately 1.4?Å/cycle after an O₂ plasma exposure time of 1.5?s, and a refractive index of Al₂O₃ in the range of 1.65–1.67 was obtained. As the O₂ plasma exposure time increased in the Al₂O₃ deposition process, the passivation properties tended to deteriorate, and as the radio frequency (RF) power increased, the passivation uniformity and the thermal stability of the Al₂O₃ layer deteriorated. To study the Al₂O₃/Si interface characteristics, the capacitance-voltage (C-V) and the conductance-voltage (G-V) were measured using a mercury probe, and the fixed charge density (Q_f) and the interface trap density (D_it) were then extracted. The Q_f of the Al₂O₃ layer deposited on a Si wafer by PA-ALD was almost unaffected, but the D_it increased with O₂ plasma exposure time. In conclusion, as the O₂ plasma exposure time increased during Al₂O₃ layer deposition by PA-ALD, the Al₂O₃/Si interface characteristics deteriorated because of plasma surface damage.     

Preparation, characterization and photocatalytic activity of the neodymium-doped TiO2 hollow spheres

Nd-doped titania hollow spheres were prepared using carbon spheres as template and Nd-doped titania nanoparticles as building blocks. The Nd-doped titania nanoparticles were synthesized at low temperature. The prepared hollow spheres were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflectance spectrum (DRS). The effects of Nd content on the physical structure and photocatalytic activities of doped titania hollow sphere samples were investigated. Results showed that there was an optimal Nd-doped content (3.9 at.%) for the photocatalytic degradation of dye X-3B (C.I. Reactive Red 2). The apparent rate constant of the best one was almost 9 times as that of P25 titania. The mechanism of photocatalytic degradation of dyes under visible light irradiation was also discussed.     

Applying the statistical experimental method to evaluate the process conditions of TiO2 nanotube arrays by anodization method

Vertically oriented TiO₂ nanotube arrays were successfully produced by the anodization technique in NH₄F/H₃PO₄ electrolyte. The structure and morphology were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). It is found that TiO₂ nanotube arrays annealed at 500 °C containing 100% anatase phase and entirely converted into rutile at 800 °C. The response surface methodology (RSM) and Box-Behnken design were applied to find the optimal factor conditions in production of TiO₂ nanotube arrays. Based on the results in preliminary experiments, we selected anodization time, anodization voltage and NH₄F concentration as the key factors to investigate their effects on responses. The regression models were built by fitting the experimental results with a second-order polynomial. By using the regression models, the optimal factor conditions were obtained as follows: anodization time of 300 min; anodization voltage of 15.39 V; NH₄F concentration of 0.50 M. Corresponding to the optimal factor conditions, the predicted average length and diameter of nanotube array were 1429 nm and 33 nm, respectively. Confirmation experiments using the optimized conditions were performed: TiO₂ nanotube arrays were obtained with an average tube length of 1420 nm and average tube diameter of 36 nm. The experimental results are in good agreement with the predicted results.     

Acetone gas phase condensation on alkaline metals doped TiO2 sol-gel catalysts

The vapor-phase condensation of acetone was studied over sol-gel prepared TiO₂ and TiO₂-doped with 1 wt.% of Li⁺ or Rb⁺. The acetone condensation was catalyzed by basic sites and the obtained products were mesityl oxide (MO), isomesityl oxide (ISMO), mesitylene (M) as well as isophorone (IP) and phorone (P). The TiO₂ crystalline structure was characterized by XRD and the basicity of catalysts by means of CO₂ chemisorption. The number of basic sites was 0.057 mmol CO₂/g for TiO₂ and Rb/TiO₂ and 0.676 mmol CO₂/g for Li/TiO₂ catalyst. CO₂-TPD showed that the strongest of the basic sites follows the order Rb/TiO₂ > TiO₂ > Li/TiO₂. Activity was found of the same order in the three catalysts. Selectivity patterns shown that MO (70%) was the main product in Li/TiO₂ while for TiO₂ and Rb/TiO₂ comparable amounts of MO (41 and 44%) and M (39 and 39%) were obtained.     

Mesoporous nanotube aggregates obtained from hydrothermally treating TiO2 with NaOH

Nanotube aggregates with high porosity were prepared from hydrothermal treatment of TiO₂ particles in NaOH at 130 °C, followed by HCl rinsing to different pH values. Pore structure of the aggregates, which were mainly mesoporous, was characterized by analyzing the N₂ sorption isotherm with different methods including the t-plot and density function theory. The surface area, pore volume and mean pore size of the aggregates increased with the rinsing acidity to reach a maximum (e.g. 400 m²/g in surface area) at pH 1.6 and then decreased with further increase of the acidity. The crystalline phase and composition of the aggregates were, as well, significantly affected by the acidity of the post-treatment rinsing. Large-surface area aggregates were of loosely-attached nanotubes, composed of both anatase TiO₂ and H₂Ti₂O₅·H₂O, obtained under a mildly acidic rinsing condition, while basic or highly acidic conditions resulted in the formation of closely coagulated dense structures consisting of different crystalline phases.     

衬底钝化处理对CsPbBr3 量子点薄膜发光稳定性影响

钙钛矿量子点因具有发光谱线窄、发光效率高、发光波长可调谐等优异的光学性能,在照明、显示、激光和太阳能电池等领域得到了广泛研究。然而,钙钛矿材料的稳定性问题,一直制约着其在光电器件中的应用。其中,钙钛矿材料在空气中受潮易分解的不稳定性尤为突出,这将严重影响其发光性质。为此,研究人员采用多种手段来改善钙钛矿材料的稳定性。目前,常见的方法是将一些具有疏水性的聚合物材料（例如POSS,PMMA等）引入到钙钛矿纳米晶中,或将钙钛矿纳米晶嵌入到介孔二氧化硅材料中,避免钙钛矿纳米晶暴露于空气中破坏其结构,以此来增强钙钛矿材料的发光稳定性。此外,钝化处理钙钛矿纳米晶表面,也是改善钙钛矿发光稳定性的一种常用方法。这些方法虽然在一定程度上可以改善钙钛矿的发光稳定性,但是在与有机物合成的过程中不免会引入其他有机官能团,介孔二氧化硅的引入,其处理方式相对复杂,而对钙钛矿纳米晶表面的钝化处理会破坏材料的原有结构。以上问题,都会影响钙钛矿的发光性质,不利于其在光电器件中的应用。硅（Si）具有低成本、大尺寸、高质量、导电好等优点,常被选作钙钛矿量子点光电器件的衬底材料。但是,由于Si衬底长时间暴露于空气,其表面易形成一层具有硅烷醇基团（Si-OH）的亲水性薄膜,这将对硅基钙钛矿器件的稳定性产生影响。因此,对Si表面进行钝化处理,破坏其表面Si-OH键,可以降低衬底表面的亲水性,增强疏水性,从而提高钙钛矿材料在器件中的稳定性。本研究使用氢氟酸（HF）对Si衬底表面进行钝化处理,发现钝化处理后的Si衬底表面与水的接触角由50.4°逐渐增大至87.7°,表明Si衬底表面由亲水性逐渐转变为疏水性。利用场致发射扫描电子显微镜(FE-SEM)测试发现,钝化处理后的Si衬底表面变粗糙,并且其表面上的CsPbBr₃量子点(CsPbBr₃ QDs)相对于未处理表面的分散性较好。利用光致发光(PL)光谱研究不同钝化处理时间的Si衬底表面上的CsPbBr₃ QDs薄膜的发光性质。其中,处理与未处理的Si衬底表面上CsPbBr₃QDs薄膜的PL积分强度随功率变化拟合值分别为1.12和1.203,表明其发光机制为激子发光。温度依赖性的PL光谱分析显示,随着温度的升高（10～300 K）,由于晶格热膨胀使CsPbBr₃ QDs带隙增大,发光峰位逐渐蓝移。并且,随着衬底钝化处理时间的增加,CsPbBr₃ QDs薄膜的发光热稳定性逐渐增强,最佳热稳定性可达220 K。而时间依赖性的PL光谱则进一步说明,钝化处理后的Si衬底表面CsPbBr₃QDs薄膜发光的时间稳定性逐渐增强,最高发光时间稳定性可达15 d。因此,通过简单而有效的对Si衬底表面进行钝化处理,可以有效减少了Si表面亲水基团,提高CsPbBr₃QDs薄膜的发光稳定性,为增强钙钛矿量子点在光电器件中的稳定性应用提供了新的研究思路。     

Characterization of TiO2/Au/TiO2 films deposited by magnetron sputtering on polycarbonate substrates

Transparent and conducting TiO₂/Au/TiO₂ (TAuT) films were deposited by reactive magnetron sputtering on polycarbonate substrates to investigate the effect of the Au interlayer on the optical, electrical, and structural properties of the films. In TAuT films, the Au interlayer thickness was kept at 5 nm. Although total thickness was maintained at 100 nm, the stack structure was varied as 50/5/45, 70/5/25, and 90/5/5 nm.In XRD pattern, the intermediate Au films were crystallized, while all TAuT films did not show any diffraction peaks for TiO₂ films with regardless of stack structure. The optical and electrical properties were dependent on the stack structure of the films. The lowest sheet resistance of 23 Ω/□ and highest optical transmittance of 76% at 550 nm were obtained from TiO₂ 90 nm/Au 5 nm/TiO₂ 5 nm films. The work function was dependent on the film stack. The highest work function (4.8 eV) was observed with the TiO₂ 90 nm/Au 5 nm/TiO₂ 5 nm film stack. The TAuT film stack of TiO₂ 90 nm/Au 5 nm/TiO₂ 5 nm films is an optimized stack that may be an alternative candidate for transparent electrodes in flat panel displays.     

From anodic TiO2 nanotubes to hexagonally ordered TiO2 nanocolumns

We report on the formation of hexagonally ordered TiO₂ nanocolumnar layers by electrochemical oxidation in a fluoride containing electrolyte, using self-organizing nanotube formation conditions at elevated potentials and low temperatures. The influence of the substrate temperature on the nanocolumn morphology and composition is investigated and characterized by FE-SEM and EDX. The origin of these nanocolumns can be attributed to a thickening of the inner tube wall of the double wall structure of self-organized TiO₂ nanotubes. Furthermore, a transition from nanocolumnar to nanotubular structure can be established by changing the applied voltage or applying a post-immersion treatment.