Initial surface reactions of TiO2 atomic layer deposition onto SiO2 surfaces: density functional theory calculations

We present a density functional theory (DFT) study of the initial surface reactions of TiO2 deposition onto a SiO2 substrate using atomic layer deposition (ALD). The precursors for the deposition process were chosen to be TiCl4 and H2O, and several cluster models were used for the SiO2 substrate. We predict the activation barriers, transition states, and reaction pathways of the surface reactions, and we investigate the effect of surface heterogeneity (such as the presence of siloxane bridges) on the reactivity of the SiO2 surface. Our study suggests that the concentration and arrangement of different reactive groups on the substrate will strongly dictate the process of film growth during ALD, including the film morphology and the growth rate.     

Role of water in the atomic layer deposition of TiO(2) on SiO(2)

Atomic layer deposition (ALD) of TiO(2) on SiO(2) powder using sequential addition of TiCl(4) and H(2)O vapors has been investigated by infrared spectroscopy. In the first cycle, TiCl(4) reacts monofunctionally or bifunctionally with surface silanols forming (Si-O-)(n)Ti-Cl(4)(-)(n) (n = 1, 2) species. Subsequent addition of water vapor leads to the hydrolysis of the (Si-O-)(n)Ti-Cl(4)(-)(n) to form a Ti-O-Ti network, and at the same time, some cleavage of Si-O-Ti bonds occurs, regenerating Si-OH in the process. It is shown that the species formed on the surface in the first TiCl(4) dose are temperature dependent. However, after addition of H(2)O vapor, the amount of TiO(2) deposited in the first complete cycle is independent of reaction temperature. In the second and above cycles, the amount of TiO(2) deposited as a function of ALD cycles strongly correlates with the amount of water on the surface. This, in turn, led to a temperature dependence of the growth rate of the TiO(2) per cycle.     

Atomic/molecular layer deposited thin-film alloys of Ti-4,4'-oxydianiline hybrid-TiO(2) with tunable properties

By combining atomic layer deposition (ALD) and molecular layer deposition (MLD) thin-film techniques, the latter being a variant of the former in which organic precursors are used, it is possible to deposit thin films containing precisely controlled portions of inorganic and organic constituents. This in turn enables the adjustment of material properties by changing the number of ALD and MLD cycles applied during the deposition. In this work, the properties of such thin-film "alloys" prepared by varying the portions of Ti-4,4'-oxydianiline (Ti-ODA) inorganic-organic hybrid and TiO(2) in the structure were investigated. The films were deposited at 280 °C using TiCl(4) and water as precursors for TiO(2), and TiCl(4) and ODA for the Ti-ODA hybrid. The results demonstrate excellent tunability of the film properties such as degree of crystallinity, roughness, refractive index, and hardness depending on the relative number of TiO(2) and Ti-ODA cycles employed.     

ZrO2在Si(100)-2×1表面原子层淀积反应机理的密度泛函理论研究

用密度泛函方法研究了ZrO₂在羟基预处理的Si(100)-2×1表面原子层淀积(ALD)初始反应过程的反应机理, ZrO₂的ALD过程包括两个前体反应物ZrCl₄和H₂O交替的半反应. 两个半反应都经历一个相似的吸附中间体反应路径. 比较单羟基Si表面反应的反应焓变, 可以发现双羟基Si表面反应, 由于相邻羟基的存在, 对ZrCl₄的半反应影响较大, 尤其是化学吸附能增加明显. 而对于H₂O的半反应, 单、双羟基Si表面反应的能量变化不是很明显. 使用内禀反应坐标(IRC)方法, 验证了两个半反应存在相似的过渡态结构和反应机理. 另外, 发现随着温度的升高, 吸附络合物的稳定性降低, 其向反应物方向的解吸附变得容易, 而向产物方向的解离难度增加.     

Ta2O5在Si(100)表面原子层沉积反应机理的密度泛函研究

采用密度泛函方法研究了以TaCl5和H2O作为前驱体在硅表面原子层沉积(ALD) Ta2O5的初始反应机理. Ta2O5的原子层沉积过程包括两个连续的“半反应”, 即TaCl5和H2O“半反应”. 两个“半反应”都经历了一个相似的吸附中间体反应路径. 通过H钝化和羟基预处理硅表面反应能量的比较发现, TaCl5在羟基预处理硅的表面反应是热力学和动力学都更加有利的反应. 另外, 从能量上看, H2O的“半反应”不容易向生成产物的方向进行.     

First principles simulation of reaction steps in the atomic layer deposition of titania: dependence of growth on Lewis acidity of titanocene precursor

It is a common finding that titanocene-derived precursors do not yield TiO(2) films in atomic layer deposition (ALD) with water. For instance, ALD with Ti(OMe)(4) and water gives 0.5 ?/cycle, while TiCp*(OMe)(3) does not show any growth (Me = CH(3), Cp* = C(5)(CH(3))(5)). From mass spectrometry we found that Ti(OMe)(4) occurs in the gas phase practically exclusively as a monomer. We then used first principles density functional theory (DFT) to model the ALD reaction sequence and find the reason for the difference in growth behaviour. Both precursors adsorb initially via hydrogen-bonding. The simulations reveal that the Cp* ligand of TiCp*(OMe)(3) lowers the Lewis acidity of the Ti centre and prevents its coordination to surface O ('densification') during both of the ALD pulses. The effect of Cp* on Ti seems to be both steric (full coordination sphere) and electronic (lower electrophilicity). This crucial step in the sequence of ALD reactions is therefore not possible in the case of TiCp*(OMe)(3) + H(2)O, which means that there is no deposition of TiO(2) films.     

The importance of dye chemistry and TiCl(4) surface treatment in the behavior of Al(2)O(3) recombination barrier layers deposited by atomic layer deposition in solid-state dye-sensitized solar cells

Atomic layer deposition (ALD) was used to fabricate Al(2)O(3) recombination barriers in solid-state dye-sensitized solar cells (ss-DSSCs) employing an organic hole transport material (HTM) for the first time. Al(2)O(3) recombination barriers of varying thickness were incorporated into efficient ss-DSSCs utilizing the Z907 dye adsorbed onto a 2 μm-thick nanoporous TiO(2) active layer and the HTM spiro-OMeTAD. The impact of Al(2)O(3) barriers was also studied in devices employing different dyes, with increased active layer thicknesses, and with substrates that did not undergo the TiCl(4) surface treatment. In all instances, electron lifetimes (as determined by transient photovoltage measurements) increased and dark current was suppressed after Al(2)O(3) deposition. However, only when the TiCl(4) treatment was eliminated did device efficiency increase; in all other instances efficiency decreased due to a drop in short-circuit current. These results are attributed in the former case to the similar effects of Al(2)O(3) ALD and the TiCl(4) surface treatment whereas the insulating properties of Al(2)O(3) hinder charge injection and lead to current loss in TiCl(4)-treated devices. The impact of Al(2)O(3) barrier layers was unaffected by doubling the active layer thickness or using an alternative ruthenium dye, but a metal-free donor-π-acceptor dye exhibited a much smaller decrease in current due to its higher excited state energy. We develop a model employing prior research on Al(2)O(3) growth and dye kinetics that successfully predicts the reduction in device current as a function of ALD cycles and is extendable to different dye-barrier systems.     

On the electrodeposition of titanium in ionic liquids

The ability to electrodeposit titanium at low temperatures would be an important breakthrough for making corrosion resistant layers on a variety of technically important materials. Ionic liquids have often been considered as suitable solvents for the electrodeposition of titanium. In the present paper we have extensively investigated whether titanium can be electrodeposited from its halides (TiCl(4), TiF(4), TiI(4)) in different ionic liquids, namely1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIm]Tf(2)N), 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl-sulfonyl)amide ([BMP]Tf(2)N), and trihexyltetradecyl-phosphonium bis(trifluoromethylsulfonyl)amide ([P(14,6,6,6)]Tf(2)N). Cyclic voltammetry and EQCM measurements show that, instead of elemental Ti, only non-stoichiometric halides are formed, for example with average stoichiometries of TiCl(0.2), TiCl(0.5) and TiCl(1.1). In situ STM measurements show that-in the best case-an ultrathin layer of Ti or TiCl(x) with thickness below 1 nm can be obtained. In addition, results from both electrochemical and chemical reduction experiments of TiCl(4) in a number of these ionic liquids support the formation of insoluble titanium cation-chloride complex species often involving the solvent. Solubility studies suggest that TiCl(3) and, particularly, TiCl(2) have very limited solubility in these Tf(2)N based ionic liquids. Therefore it does not appear possible to reduce Ti(4+) completely to the metal in the presence of chloride. Successful deposition processing for titanium in ionic liquids will require different maybe tailor-made titanium precursors that avoid these problems.     

A New Method for Preparing Ti-Si Mixed Oxides

Ti-Si mixed oxides with different TiO2 content were prepared by sol-gel one step hydrolysis, using TiCl4 as the precursor. The samples were characterized by BET, FT-IR and XRD. The results indicated that titanium is in fourfold coordination with oxygen in the SiO4^4- in mixed oxides, form the bond of Ti-O-Si, and the low titania materials are mixed on an atomic scale.The phase of anatase appeared when TiO2 content is up to 80%. The mixed oxides had high specific surface areas up to 681.5 m^2/g. 10TiSi is a better support than SiO2 in the reaction of CO oxidation.     

溶胶-凝胶法制备复合MxOy-TiO2光催化剂

以无机盐TiCl4为原料制备TiO2溶胶,并利用溶胶胶粒修饰法制备SiO2 TiO2、ZrO2 TiO2、WO3 TiO3、MoO3 TiO2及Pt/TiO2光催化剂,用于光催化氧化甲醛反应,考察添加物对 催化剂光催化性能的影响.其中,SiO2 TiO2催化剂的光催化氧化降解甲醛性能最好,而MoO3 TiO2的催化性能最差.SiO2 TiO2催化剂优良的光催化性能可归于SiO2 TiO2催化剂的高 比表面积,高空隙率,小晶粒粒径和强吸光性能等性质的综合影响.     

多重射流燃烧反应制备的TiO2/ SiO2纳米复合颗粒的形态和结构

TiO₂-SiO₂ nanocomposite particles were prepared in premixed H₂ / Air flame, and the morphology and structure of these nanocomposites were characterized by FTIR, XRD, TEM and HRTEM. The morphology of SiO₂ / TiO₂ nanocomposites was different from that of pure TiO₂ or SiO₂ nanoparticles, and the chemical bond of Ti-O-Si was found in the nanocomposites indicating that the TiO₂ / SiO₂ nanocomposites were not merely a physical mixture of TiO₂ and SiO₂. TiO₂ nanocrystalline grains with sizes of 1～2 nm were homogeneously dispersed in the amorphous SiO₂ matrix when TiCl₄ and SiCl₄ were mixed at molecular level in the flame. The particle size and rutile content decreased with increasing of SiO₂ molar ratio.     

Microwave-solvothermal synthesis of various polymorphs of nanostructured TiO2 in different alcohol media and their lithium ion storage properties

The various polymorphs (anatase, rutile, and brookite) of TiO(2) with different nanomorphologies have been synthesized by a facile microwave-assisted solvothermal process without surfactants, employing TiCl(4) or TiCl(3) as precursors in various alcohol (ethanol, propanol, butanol, and octanol) media. The samples have been characterized by X-ray diffraction (XRD), electron microscopy, and Brunauer-Emmett-Teller (BET) surface area analysis. The Ti/Cl ion concentration, reaction pH, and size of the alcohol molecule are found to control the morphology, crystal structure, and crystallite size of the TiO(2) particles. Among the various TiO(2) polymorphs synthesized, the rutile TiO(2) spheres built up of nanorods that were synthesized with TiCl(4) in octanol have an average pore size and surface area of, respectively, 5 nm and 404 m(2)/g and exhibit the best electrochemical performance with a capacity of >200 mAh/g after 100 cycles and high rate capability. The excellent electrochemical properties originate from the nanorod-building morphology and mesoporosity of TiO(2) spheres that provide good electrical contact, accommodates the strain smoothly, and facilitates facile lithium-ion diffusion.     

Double-layer coating of SrCO3/TiO2 on nanoporous TiO2 for efficient dye-sensitized solar cells

Surface modification plays a crucial role in improving the efficiency of dye-sensitized solar cells (DSSCs), but the reported surface treatments are in general superior to the untreated TiO(2) but inferior to the typical TiCl(4)-treated TiO(2) in terms of solar cell performance. This work demonstrates a two-step treatment of the nanoporous titania surface with strontium acetate [Sr(OAc)(2)] and TiCl(4) in order, each step followed by sintering. An electronically insulating layer of SrCO(3) is formed on the TiO(2) surface via the Sr(OAc)(2) treatment and then a fresh TiO(2) layer is deposited on top of the SrCO(3) layer via the TiCl(4) treatment, corresponding to a double layer of Sr(OAc)(2)/TiO(2) coated on the TiO(2) surface. As compared to the typical TiCl(4)-treated DSSC, the Sr(OAc)(2)-TiCl(4) treated DSSC improves short-circuit photocurrent (J(sc)) by 17%, open-circuit photovoltage (V(oc)) by 2%, and power conversion efficiency by 20%. These results indicate that the Sr(OAc)(2)-TiCl(4) treatment is better than the often used TiCl(4) treatment for fabrication of efficient DSSCs. Charge density at open circuit and controlled intensity modulated photocurrent/photovoltage spectroscopy reveal that the two electrodes show almost same conduction band level but different electron diffusion coefficient and charge recombination rate constant. Owing to the blocking effect of the SrCO(3) layer on electron recombination with I(3)(-) ions, the charge recombination rate constant of the Sr(OAc)(2)-TiCl(4) treated DSSC is half that of the TiCl(4)-treated DSSC, accounting well for the difference of their V(oc). The improved J(sc) is also attributed to the middle SrCO(3) layer, which increases dye adsorption and may improve charge separation efficiency due to the blocking effect of SrCO(3) on charge recombination.     

复合载体TiO2/SiO2的气相吸附法制备及MoO3在其表面上的分散状态

用气相流动吸附法制备了TiO_2/SiO_2复合载体,用浸渍法制备了MoO_3/(TiO_2/SiO_2)催化剂。应用XRD和LRS等技术研究了TiO_2在SiO_2表面及MoO_3在TiO_2/SiO_2表面上的分散状态。结果表明,TiO_2在复合载体中的含量低于其分散阈值时,它在SiO_2表面呈单层但非密置单层分散;MoO_3在催化剂中的含量低于其分散阈值时,它在复合载体表面亦呈单层但非密置单层分散。TiO_2与SiO_2之间、MoO_3与TiO_2/SiO_2之间相互作用都较弱。TiO_2在SiO_2表面的分散可改善MoO_3的分散状态,提高MoO_3在SiO_2表面上的分散阈值。     

TiO2纳米粒子膜的制备、表面态性质和光催化活性

在酸性和碱性条件下,用TiCl4水解法制备了TiO2纳米粒子膜催化剂.采用原子力显微镜(AFM),X射线衍射(XRD),表面光电压谱(SPS)和场诱导表面光电压谱(EFISPS)测定了催化剂表面的微结构及能级结构.对催化剂进行了光催化降解苯酚实验,测定了其光催化活性.结果表明,酸性条件下制备的TiO2膜催化剂的光催化活性较高,其结果接近于P25.用能带理论解释了TiO2纳米粒子膜催化剂光催化活性的差异,分析了膜厚对光催化活性的影响.     

NiO／TiO_2－SiO_2催化剂上甲烷与二氧化碳转化制合成气

ＮｉＯ／ＴｉＯ_２－ＳｉＯ_２催化剂上甲烷与二氧化碳转化制合成气邓存，路勇，丁雪加（中国科学院兰州化学物理研究所ＯＳＳＯ国家重点实验室，兰州７３００００）关键词甲烷，二氧化碳，合成气，负载型Ｎｉ催化剂己有报道表明［１～４］，Ａｌ２Ｏ３担载的Ｎｉ金属催化...     

Influence of a TiCl4 post-treatment on nanocrystalline TiO2 films in dye-sensitized solar cells

In this study, the influence of the TiCl(4) post-treatment on nanocrystalline TiO(2) films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO(2) particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO(2) films are already more than sufficient without treatment, the increase in short circuit current density J(SC) cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO(2) upon TiCl(4) treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO(2).     

Controlled synthesis of self-assembled 3D nanostructures using metastable atomic layer deposition

An important feature of atomic layer deposition (ALD) is the fact that the coating that has been deposited is conformal to the substrate surface. Therefore, prepatterned substrates are usually used for the fabrication of 3D nanostructures using ALD. This article presents a new method to generate 3D silver-silica nanostructures using plasma-enhanced atomic layer deposition of silica with tri(dimethylamino)silane (TDMAS) and oxygen plasma as precursors. For this method, silver nanoparticles are used as templates, and during the deposition of silica, the repeatable process of the formation of metastable silver oxides and their decomposition is involved, leading to strong side reactions and the formation of 3D silver-silica hybrid nanostructures. This method is known as metastable atomic layer deposition (MS-ALD). Unlike the conventional ALD, the coating of MS-ALD is not conformal to the substrate surface. Rather, the 3D nanostructures are self-assembled because of side reactions. The geometry of the formed nanostructures can be easily adjusted by tuning the deposition parameters, such as dose time of both precursors and cycle numbers. In our study, we observed nanosponges with features sizes of up to 4 for less than 45 MS-ALD cycles. Nanowire-like silver-silica hybrid nanostructures are generated at higher cycle numbers with feature sizes of up to 10 μm. A similar trend could be observed for changing the dose time of both precursors of TDMAS and oxygen plasma. The height of the nanostructures increases with dose time of both precursors. In contrast to this trend, the surface coverage declines when the investigated parameters (number of cycles, TDMAS, and oxygen plasma dose time) are increased.     

Influence of precursors chemistry on ALD growth of cobalt-molybdenum oxide films

Cobalt molybdenum compounds are important catalytic materials in many processes, e.g. in splitting of ammonia to form CO free hydrogen fuel. We here report on deposition of such cobalt molybdenum oxides by atomic layer deposition (ALD) using different types of metal precursors CoCp(2) (Cp = cyclopentadienyl), Co(thd)(2) (Hthd = 2,2,6,6-tetramethylheptan-3,5-dione), Mo(CO)(6) and oxygen precursors O(3), H(2)O, and (O(3) + H(2)O). The growth dynamics have been investigated using quartz crystal microbalance (QCM) methods. It is evident that mixing of the different precursor chemistries affect the growth patterns. When water is introduced to the reactions, a surface controlled mechanism takes place which guides the deposited stoichiometry towards the CoMoO(4) phase over a wide range of cobalt rich pulsed compositions. This is a rare example of how surface chemistry can control stoichiometry of depositions in ALD. The deposited films have been investigated by X-ray diffraction, Raman spectroscopy and atomic force microscopy. The catalytic activity of selected films have been characterized by temperature programmed ammonia decomposition, proving the films to be catalytically active and lowering the decomposition temperature by some 200 °C.     

Resolving the Heat Generated from ZrO2 Atomic Layer Deposition Surface Reactions

In situ pyroelectric calorimetry and spectroscopic ellipsometry were used to investigate surface reactions in atomic layer deposition (ALD) of zirconium oxide (ZrO₂). Calibrated and time-resolved in situ ALD calorimetry provides new insights into the thermodynamics and kinetics of saturating surface reactions for tetrakis(dimethylamino)zirconium(IV) (TDMAZr) and water. The net ALD reaction heat ranged from 0.197 mJ cm⁻² at 76 °C to 0.155 mJ cm⁻² at 158 °C, corresponding to an average of 4.0 eV/Zr at all temperatures. A temperature dependence for reaction kinetics was not resolved over the range investigated. The temperature dependence of net reaction heat and distribution among metalorganic and oxygen source exposure is attributed to factors including growth rate, equilibrium surface hydroxylation, and the extent of the reaction. ZrO₂-forming surface reactions were investigated computationally using DFT methods to better understand the influence of surface hydration on reaction thermodynamics.