Modification of TiO2 for enhanced surface properties: finite Ostwald ripening by a microwave hydrothermal process

The effect of microwave modification of colloidal TiO2 suspensions under extended periods of treatment is presented. The nanoparticulate TiO2 is compared and contrasted to similar convection hydrothermally treated TiO2 and a commercial titania product, namely Degussa P25. Microwave-treated samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy to determine their physicochemical characteristics. Comparative surface area analyses were performed by N2 adsorption and calculated from a Brunauer-Emmett-Teller (BET) isotherm. The complementary techniques of XRD and TEM showed good correlation between observed and calculated particle sizes (by application of the Scherrer equation), with the material being highly crystalline anatase TiO2, as identified by XRD and Raman. This investigation identified that extended periods of microwave hydrothermal treatment do not greatly enhance the crystallinity and primary grain size. Treatment of >180 min has a negative effect on crystallite growth; however, treatment up to this time had a significant effect on the material's surface area. The limiting regime of Ostwald ripening for hydrothermal treatment is discussed in relation to the mechanism of microwave treatment, that is, rapid heating to temperature and extremely rapid rates of crystallization. The effect of these property modifications are further discussed in relation to photocatalytic and photoelectrochemical applications of TiO2 nanoparticles.     

量子顺电体La1/2Na1/2TiO3纳米晶的制备与谱学表征

用改进硬脂酸溶胶-凝胶法制备了量子顺电体La_1/2Na_1/2TiO₃纳米晶,用差热-热重分析和X射线衍射测试了样品的晶化过程及物相结构,用FTIR和Raman光谱表征了其谱学特性.     

Synthesis and photocatalytic activity of mesoporous TiO(2) with the surface area, crystallite size, and pore size

Mesoporous TiO(2) materials with various pore-size distributions were synthesized by using diblock copolymers via a sol-gel process in aqueous solution. The properties of these materials were characterized by FE-SEM, HR-TEM, XRD, DRS, BET, and BJH analysis. All particles have spherical morphology with a diameter range of 1-3 mum. The mesoporous TiO(2) materials calcined at 400 degrees C were found to have different specific surface areas - 186, 210, and 192 m(2) g(-1) - and average pore sizes depending on the type of diblock copolymer-5.1, 6.1, and 6.4 nm-and their crystallite sizes were found to be 8.1, 8.3, and 8.8 nm. The photocatalytic activity of each sample was investigated by measuring the photodecomposition of methylene blue (MB), and the small crystallite size, large surface area, and small pore size were found to exhibit better photocatalytic activities. In addition, the photocatalytic activities of all the mesoporous TiO(2) materials were found to be better than that of commercial TiO(2).     

Vibrational structure of titanium silicate catalysts. A spectroscopic and theoretical study.

A thorough analysis of the vibrational features of the titanium silicalite-1 (TS-1) catalyst is presented, based on quantitative IR measurements, Raman and resonant Raman experiments, quantitative XANES, and quantum chemical calculations on cluster and periodic models. The linear correlation of the intensity of the IR and Raman bands located at 960 and 1125 cm(-1) and the XANES peak at 4967 eV with the amount of tetrahedral Ti are quantitatively demonstrated. Raman and resonant Raman spectra of silicalite and TS-1 with variable Ti content are presented, showing main features at 960 and 1125 cm(-1) associated with titanium insertion into the zeolite framework. The enhancement of the intensity of the 1125 cm(-1) feature and the invariance of the 960 cm(-1) feature in UV-Raman experiments, are discussed in terms of resonant Raman selection rules. Quantum chemical calculations on cluster models Si[OSi(OH)(3)](4) and Ti[OSi(OH)(3)](4) at the B3LYP/6-31G(d) level of theory provide the basis for the assignment of the main vibrational contributions and for the understanding of Raman enhancement. The resonance-enhanced 1125 cm(-1) mode is unambiguously associated with a totally symmetric vibration of the TiO(4) tetrahedron, achieved through in-phase antisymmetric stretching of the four connected Ti-O-Si bridges. This vibration can also be described as a totally symmetric stretching of the four Si-O bonds pointing toward Ti. The resonance enhancement of this feature is explained in terms of the electronic structure of the Ti-containing moiety. Asymmetric stretching modes of TO(4) units show distinct behavior when (i) T is occupied by Si as in perfect silicalite, (ii) T is occupied by Ti as in TS-1, or (iii) the oxygen atom belongs to an OH group, such as in terminal tetrahedra of cluster models and in real defective zeolites. Asymmetric SiO(4) and TiO(4) stretching modes appear above and below 1000 cm(-1), respectively, when they are achieved through antisymmetric stretching of the T-O-Si bridges, and around 800 cm(-1) (in both SiO(4) and TiO(4)) when they involve symmetric stretching of the T-O-Si units. In purely siliceous models, the transparency gap between the main peaks at 800 and 1100 cm(-1) contains only vibrational features associated with terminal Si-OH groups, while in Ti-containing models it contains also the above-mentioned asymmetric TiO(4) modes, which in turn are strongly coupled with Si-OH stretching modes. Calculations on periodic models of silicalite and TS-1 free of OH groups using the QMPOT embedding method correctly reproduce the transparency gap of silicalite and the appearance of asymmetric TiO(4) vibrations at 960 cm(-1) in TS-1. Finally, we demonstrate, for the first time, that the distortion of the tetrahedral symmetry around Ti caused by water adsorption quenches the UV-Raman enhancement of the 1125 cm(-1) band.     

Enhanced photocatalytic activity of mesoporous TiO2 aggregates by embedding carbon nanotubes as electron-transfer channel

Mesoporous multiwalled carbon nanotubes/titanium dioxide (CNTs/TiO(2)) nanocomposites with low loading amounts (0-0.5 wt%) of CNTs embedded inside mesoporous TiO(2) aggregates has been prepared by a simple one-pot hydrothermal method using titanium sulfate as titanium source. The as-prepared CNTs/TiO(2) samples are carefully characterized, analyzed and discussed. In contrast to previous reports with high CNT loading, our results indicate that a low CNT loading slightly influences the textural properties (including crystallite size, degree of crystallinity, specific surface areas, and pore volume etc.) and UV-light absorption of the mesoporous TiO(2) aggregates. The SEM and TEM results demonstrate that the CNTs are mostly embedded in the mesoporous TiO(2) aggregates. Moreover, chemical bonds are formed at the interface between CNTs and TiO(2), which is confirmed by the Raman, IR and XPS analyses. Significantly, we point out that PL analysis in terms of intensity of PL signals seems to not be a reliable way to monitor the recombination rate in the CNTs/TiO(2) composite, due to the quenching effect of CNTs. Instead, the analysis of transient photocurrent responses is introduced, which definitely reflects CNTs as fast electron transfer channels in chemically-bonded CNTs/TiO(2) composites with low CNT loading. Notably, the positive synergy effects of CNTs and TiO(2) depend on both the CNT loading amount and the state of interfacial contacts. In our study, only these chemically bonded CNTs/TiO(2) nanocomposites with appropriate loading amounts (<0.1 wt%) favor the separation of photogenerated electron-hole pairs and decrease their recombination rate and thus display significantly enhanced photocatalytic activity for degrading acetone in air under UV irradiation, as compared with pristine TiO(2) counterparts and commercial P25 photocatalyst. In contrast, a high CNT loading (>0.1 wt%) results in a decrease in photocatalytic activity; a simple mechanical mixing of CNTs and TiO(2) without forming chemical bonds at the interface also results in inferior photocatalytic performance.     

Wet chemical synthesis of Cu/TiO2 nanocomposites with integrated nano-current-collectors as high-rate anode materials in lithium-ion batteries

Using a soft-template assisted method, well-organized Cu/TiO(2) nanoarchitectured electrode materials with copper nanowires as their own current collectors are synthesized by controlled hydrolysis of tetrabutyl titanate in the presence of Cu-based nanowires, and investigated by SEM, TEM, XRD, Raman spectroscopy and electrochemical tests towards lithium storage. Two types of Cu/TiO(2) nanocomposites with different TiO(2) grain sizes are obtained by using different thermal treatments. The two types of Cu/TiO(2) nanocomposites show much enhanced rate performances compared with bare TiO(2). A high-rate capability (reversible capacity at 7500 mA g(-1) still accounts for 58% of its initial capacity at 50 mA g(-1)) is observed for the Cu/TiO(2) nanocomposite with smaller TiO(2) grain size. The improvements can be attributed to the integrated Cu nanowires as mechanical supports and efficient current collectors. A cell made from the Cu/TiO(2) nanoarchitectured electrodes exhibits promise as an energy storage device with both high energy and high power densities.     

Sb掺杂SnO_2(ATO)纳米晶的水热合成和导电性能

以Sn和SbI_3为主要原料，在120-170 ℃温和水热条件下合成了具有导电能力 的Sb掺杂SnO_2(ATO)透明导电纳米粉体，运用FT-IR， XRD， BET， TEM等手段对 粉体的形成过程进行了分析表征。实验结果表明，所合成的纳米ATO粉体均为四方 锡石结构，无其他杂相存在，晶粒大小在4-7nm之间，粉体呈单分散状态。比表面 积在137-184m~2·g~(-1)之间，随水热温度的升高，晶粒长大，比表面积下降，粉 体导电性能提高。该方法对于其他透明导电氧化物纳米粉体的合成具有借鉴意义。     

Mesoporous TiO2 core-shell spheres composed of nanocrystals with exposed high-energy facets: facile synthesis and formation mechanism

A facile new method that combines electrospray and hydrothermal treatment is used to prepare mesoporous core-shell TiO(2) spheres with high specific surface areas and high pore volumes. Interestingly, the resulting TiO(2) spheres are composed of anatase TiO(2) nanocrystals with exposed step-like {001} and smooth {010} facets. The percentage of exposed {001} facets can be adjusted by changing the experimental parameters used in the electrospray and hydrothermal treatment processes, such as the contents of poly(N-vinyl-2-pyrrolidone) and acetic acid. The combination of high specific surface area (>100 m(2) g(-1)), high pore volume (>0.30 cm(3) g(-1)), useful pore size (10-15 nm), spherical core-shell structure, and exposed high energy facets makes these TiO(2) spheres an important candidate for use in many photoelectrochemical applications. The formation mechanism of the mesoporous TiO(2) spheres is also studied. The great advantage of this method is that interesting and complicated mesoporous superstructures can be prepared using electrospray technology.     

Effect of crystallite size on the thermal phase change and porous properties of boehmite

The effect of crystallite size on the thermal phase change and porous properties of boehmite was investigated using boehmites with crystallite sizes of 2.9 to 24.4 nm and boehmite gels prepared by precipitation and hydrothermal methods. The dehydroxylation temperature of boehmite increases, its phase transition temperature from gamma- to theta-Al(2)O(3) decreases and the theta- to alpha-Al(2)O(3) transition temperature increases as the crystallite size of the boehmite increases. Boehmite with a crystallite size of approximately 5 nm shows the highest specific surface area and greatest thermal stability. This boehmite contains pores of about 2-3 nm radius, suggested to be responsible for the superior porous properties and thermal stability.     

不同原料合成的TiO2的变温拉曼光谱分析

分别以钛酸四丁酯、钛酸四异丙酯、钛酸四乙酯为原料, 用溶胶凝胶法合成出TiO₂, 利用变温拉曼及变温XRD研究了不同原料相同方法合成得到的TiO₂的相转变温度. 结果表明, 所合成TiO₂的相转变温度与原料的反应活性存在对应关系, 根据锐钛矿拉曼特征峰半峰宽与温度的关系图, 将锐钛矿晶粒尺寸与半峰宽关联. 分析对比原料水解活性与产品相转变温度之间的关系对实验结果进行了解释.     

Relationship between formation conditions, properties, and crystallite size of boehmite

Boehmites (gamma-AlOOH) with a variety of crystallite sizes were prepared under various precipitation conditions from solutions of aluminum nitrate and sodium aluminate and also by hydrothermal treatment of a commercial boehmite powder. The relationships between the crystallite size and other properties of the boehmites were investigated. Crystalline boehmite was formed in the pH range 7 to 10 while gels precipitated below pH 7 and bayerite (Al(OH)3) formed at pH 11. The crystallite size of the boehmite increased at higher pH, higher temperature of the precursor solution, and longer aging time. As the crystallite size of the boehmite increased, the specific surface area, pore volume, and excess water decreased. Boehmites of small crystallite size, called pseudoboehmite, showed broadened X-ray peak widths and increased d-spacing of the 020 reflection. These changes are attributed to the small crystallite size (smaller numbers of stacking layers) and also to excess water in the interlayers of the boehmite structure.     

DBS包覆TiO2 纳米粒子的表面光伏及光致发光性能

The TiO2 nanoparticles capped with sodium dodecylbenzenesulfonate (DBS) were synthesized by a sol-hydrothermal method, and were also characterized by XRD, TEM, Surface Photovoltage Spectroscopy (SPS) and Photoluminescence (PL). The effects of capping DBS on photovoltage and photoluminescence performances of TiO2 nanoparticle as well as appropriate capping conditions were principally investigated. The results show that the capping situation is desirable when the pH value and adding DBS amount are in the range of 4.5-5.5 and 1.0%-3.0% of TiO2 weight in advance of the hydrothermal process, respectively. The added DBS could inhibit the growth of anatase crystallite during the hydrothermal process. Moreover, the intensities of SPS and PL of TiO2 nanoparticle decreased after DBS was capped, which is possibly attributed to the electrophilic property of sulfonic acid group (-SO3-) as well as the decrease of surface defect.     

Influence of crystal size on the electron–phonon coupling in ZnO nanocrystals investigated by Raman spectroscopy

We have synthesized ZnO nanocrystals of different sizes (25–41 nm) using the sol–gel method and characterized them using different techniques such as: transmission electron microscopy (TEM) and X-ray diffraction (XRD). Raman spectra of different sizes of ZnO nanocrystals were recorded at two excitation wavelengths, 514 and 647 nm, in the spectral range 300–1200 cm⁻¹. The vibrational modes were assigned on the basis of group theory analysis. The influence of mean crystallite size on the strength of the electron–phonon coupling is experimentally estimated by the variation of relative intensities of second order Raman band and the first order Raman band for ZnO nanocrystals of different sizes. We found that the intensity ratio of the 2E₂ and 1E₂ Raman bands decreases almost linearly for both excitation wavelengths with decreasing crystallite size, which reveals that the Fröhlich interaction plays a dominant role in the electron–phonon coupling of the ZnO nanocrystals.     

静电纺丝-碱热法制备高活性由纳米片组装的TiO2纳米纤维（英文）

半导体光催化有望解决日益严峻的环境污染与能源危机,因而得到广泛重视.纳米TiO2因为其强的氧化能力和良好的(光)化学稳定性与生物相容性,成为了最受欢迎的半导体光催化材料.到目前为止,材料科学家们制备了多种形貌的TiO2光催化材料,如纳米棒(线)、纳米片和空心微球等.作为染料太阳能电池的光阳极材料,小颗粒尺寸的纳米TiO2具有大的比表面积,有利于敏化剂的吸附,从而增强太阳能电池的光电转换性能.但是尺寸太小的TiO2颗粒不利于光散射,导致入射的太阳光直接穿透光阳极薄膜而不利于吸收和利用太阳光.为了解决敏化剂吸附和增强光散射这对矛盾,本文设计制备了由纳米片组装的TiO2纳米纤维:(1)首先通过静电纺丝法制备TiO2纳米纤维前躯体;(2)将TiO2纳米纤维前驱体在500°C焙烧,去除有机物,得到晶化度良好的由纳米颗粒组装的TiO2纳米纤维;(3)将TiO2纳米纤维进行NaOH碱热处理,使TiO2纳米颗粒转化成钛酸盐纳米片,然后经历酸洗和焙烧,得到由纳米片组装的TiO2纳米纤维.染料敏化太阳能电池的性能测试结果显示,碱热2.5 h所得TiO2样品的光阳极薄膜的光电转化效率提升了2.3倍;同时,利用丙酮光催化分解的活性来评价纳米纤维的光催化活性,发现碱热2.5 h所得纳米纤维上光催化降解丙酮的活性提升了3.1倍.结构表征结果显示,随着碱热时间的延长,从纤维表面生长出来的纳米片逐渐变长,催化剂的比表面积和孔容不断增加.大的比表面积有利于底物的吸附,纳米片结构有利于增强光散射,通过延长光程增强对光的利用效率,从而提升纳米纤维的光活性.光电流测试的结果显示,与碱热前的TiO2纳米纤维相比,碱热后的TiO2纳米纤维光电流显著增强,这是由于纳米片结构减小了扩散距离,有利于光生载流子快速转移到催化剂表面,引发丙酮的光催化氧化.     

Sb掺杂纳米TiO2光催化剂的晶体结构与光催化性能

Nanoscale Sb doped titanium dioxide thin films photocatalyst (Ti1-xSbO2) were obtained from dip-coating sol-gel method. The influence of dopant Sb density on the crystal structure and the phase transformation of the thin films were characterized by X-ray diffraction (XRD) and Raman spectra. The results of XRD showed that as-prepared films were not only in anatase state but also in brookite. The crystalline size was estimated to be around 13.3-20 nm. Raman spectra indicated there coexisted other phases and a transformation from brookite to anatase in the samples doped with 0.2% Sb. After doping a proper amount of Sb, the crystallization rate and the content of the anatase Ti1?xSbO2 in the thin films was clearly enhanced because Sb replaced part of the Ti of TiO2 in the thin films. The anode current density (photocurrent density) and the first order reaction speed constant (k) of thin films doped with 0.2% Sb reached 42.49 1A/cm2 and 0.171 h/cm2 under 254 nm UV illumination, respectively, which is about 11 times and 2 times that of the non-doped TiO2 anode prepared by the same method respectively.     

制备均一形貌的长二氧化钛纳米管

在温和的水热条件下,用碱溶液处理不同粒径的锐钛矿相和金红石相二氧化钛 纳米粉体,得到了不同形貌的二氧化钛纳米管,并用TEM,XRD,FT-Raman和BET等 对其进行了表征。金红石相的超细纳米晶有利于形成均一形貌的纳米管,用粒径仅 为7.2 nm的金红石相纳米粉体为前驱体得到了长度为500 nm的长二氧化钛纳米管。 用纳米晶反应活性对晶粒尺寸的依赖性及晶相稳定性解释了长纳米管的形成机理。     

钬掺杂提高TiO2纳米晶光催化活性的光谱性能机制研究

采用酸催化的溶胶-凝胶法制备了未掺杂和掺杂0.5%Ho(摩尔分数)的纳米TiO2光催化剂,以甲基橙的光催化降解为探针反应,评价了其光催化活性。运用XRD,TEM,EDS,FT-IR,UV-Vis DRS以及PL光谱表征技术考察了Ho掺杂对纳米TiO2的微晶尺寸、晶体结构、表面组成与光学性能的影响,并对改性机制作了探讨。结果表明,Ho掺杂可以抑制TiO2由锐钛矿相向金红石相的转变,阻碍TiO2晶粒增长,使TiO2的光吸收带边发生蓝移,增加表面羟基含量,促进光生载流子分离,从而使Ho掺杂TiO2样品光催化降解甲基橙的能力显著增强。     

Facile fabrication of anatase TiO2 microspheres on solid substrates and surface crystal facet transformation from {001} to {101}

Anatase TiO(2) microspheres with controlled surface morphologies and exposed crystal facets were directly synthesized on metal titanium foil substrates by means of a facile, one-pot hydrothermal method without use of any templating reagent. The obtained products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelecron spectroscopy (XPS), and the focused ion beam (FIB) technique. The sizes of the resultant microspheres ranged from 1.1 to 2.1 μm. The transformation of anatase TiO(2) microspheres with exposed {001} facets surface to nanosheets surface with {101} facets was achieved by simply controlling the hydrothermal reaction time. The anatase TiO(2) microspheres with exposed square-shaped plane {001} facets were obtained by controlling the reaction time at 1 h. The prolonged reaction time transforms the anatase TiO(2) microspheres with exposed square-shaped plane {001} facets to eroded {001} facets then to a nanosheet surface with exposed {101} facets. With hydrothermal synthesis, the surface morphological structure and crystal facets formation are highly dependent on dissolution/deposition processes, which can be strongly influenced by attributes, such as pH of the reaction media, the total concentration of dissolved and suspended titanium species, and the concentration of fluoride in the reaction solution. The changes of these attributes during the hydrothermal process were therefore measured and used to illustrate the morphology and crystal-facet transformation processes of anatase TiO(2) microspheres. The surface morphologies and crystal-facet transformations during hydrothermal processes were found to be governed by the compositional changes of the reaction media, driven by dynamically shifted dissolution/deposition equilibria. The photocatalytic activities of the photoanodes made of anatase TiO(2) microspheres were evaluated. The experimental results demonstrated that the photocatalytic activity of anatase TiO(2) microspheres with exposed {001} facets was found to be 1.5 times higher than that of the anatase TiO(2) microspheres with exposed {101} facets.     

Nanoparticles of amorphous ruthenium sulfide easily obtainable from a TiO2-supported hexanuclear cluster complex [Ru6C(CO)16]2-: a highly active catalyst for the reduction of SO2 with H2

TiO(2)-supported ruthenium-metal particles were derived from an anionic hexanuclear carbido carbonyl cluster [Ru(6)C(CO)(16)](2-) and compared with those prepared conventionally by impregnation of TiO(2) with a solution of RuCl(3) followed by reduction with H(2). The average sizes of the metal particles in both systems are similar, that is, 12 A for molecular cluster-derived particles and 15 A for those derived from the RuCl(3) precursor, although the size distribution is sharper in the former case. These supported particles efficiently promote the reduction of SO(2) with H(2) to give elemental sulfur. Their active form is ruthenium sulfide as confirmed by EXAFS and X-ray diffraction measurements. The nanoscale ruthenium sulfide particles, which originated from the cluster complex, have an amorphous character and show activity even at low temperature (463 K), whereas ruthenium sulfide formed from RuCl(3)-derived metal dispersion is a pyrite-type RuS(2) crystallite and needs a temperature above 513 K to effect the same catalysis. Amorphous ruthenium sulfide maintains its nano-sized scale (approximately 14 A) regardless of the reaction temperature, while RuS(2) crystallite aggregates to form larger nonuniform particles.     

介孔TiO2的水热法制备及其光催化性能

以二钛酸钾(K2Ti2O5)经离子交换得到的无定形水合二钛酸(H2Ti2O5·xH2O)为原料, 与葡萄糖溶液在220 ℃下进行水热反应, 再在空气中520 ℃焙烧, 制备出介孔TiO2. 用扫描电子显微镜(SEM)、X射线衍射(XRD)、N2吸附、透射电子显微镜(TEM)等技术对样品进行了表征. 结果表明, 该介孔TiO2具有微米级棒状或针状形貌, 晶粒大小为12.3 nm, 比表面积为106 m2·g-1, 孔容为0.31 cm3·g-1, 孔径为8.06 nm, 焙烧处理后晶型仍是锐钛矿相. 水热生成的碳抑制了晶粒的团聚生长和晶型的转变, 提高了介孔TiO2的热稳定性. 甲基橙降解实验评价了介孔TiO2的光催化性能, 结果发现其活性与商用TiO2催化剂P25相当, 而其较大的粒径更容易回收再利用. 以碘化钾为探针反应, 表明介孔TiO2的光催化机制以光生空穴氧化为主.