Molecular mechanisms of photoinduced oxygen evolution, PL emission, and surface roughening at atomically smooth (110) and (100) n-TiO2 (rutile) surfaces in aqueous acidic solutions

The success in preparing atomically smooth and stable (110) and (100) TiO2 (rutile) surfaces, combined with in situ photoluminescence (PL) and photocurrent measurements as well as atomic force microscopic (AFM) inspection, has enabled us to make systematic studies on molecular mechanisms of oxygen photoevolution and related processes on TiO2 (rutile), which are important for solar water splitting and photocatalytic environmental cleaning. The studies have revealed that various surface processes and properties, such as the flat-band potential (Ufb), the spectrum and intensity of the PL from a precursor of the oxygen photoevolution reaction, and photoinduced surface roughening, have all strong dependences on the atomic-level structure of the TiO2 surface. Importantly, all the results have been explained on the basis of our recently proposed new mechanism that the oxygen photoevolution reaction is initiated by a nucleophilic attack of an H2O molecule to a surface-trapped hole, thus giving confirmative evidence to it. The molecular mechanisms for photoinduced primary processes at the TiO2 surface, clarified in the present work, will provide a typical model for photoreactions on metal oxides in contact with aqueous solutions.     

In situ FTIR studies of primary intermediates of photocatalytic reactions on nanocrystalline TiO2 films in contact with aqueous solutions

Multiple internal reflection infrared spectroscopy was applied to in situ investigations of surface intermediates of photocatalytic reactions on nanocrystalline TiO(2) films in contact with aqueous solutions. UV irradiation in the presence of dissolved O(2) caused the appearance of new bands peaked at 943, 838, and 1250-1120 cm(-)(1) together with intensity changes in other bands. Investigations of influences of the solution pH, the presence or absence of hole and electron scavengers, and isotopic H(2)O --> D(2)O exchange on the spectral changes have revealed that the primary step of photocatalytic O(2) reduction is the formation of the surface peroxo species, Ti(O(2)), giving the 943 cm(-)(1) band, probably with the surface superoxo species, TiOO., as a precursor, in neutral and acidic solutions. The surface peroxo species is then transformed to the surface hydroperoxo, TiOOH, giving the 838 and 1250-1120 cm(-)(1) bands, by protonation in the dark. This is, to our knowledge, the first direct in situ spectroscopic detection of primary intermediates for the photocatalytic O(2) reduction in aqueous solutions. On the basis of the assignment, a possible reaction scheme for various processes of the photocatalytic O(2) reduction is proposed, which is in harmony with other spectral changes induced by the UV irradiation.     

Oxygen photoevolution on a tantalum oxynitride photocatalyst under visible-light irradiation: how does water photooxidation proceed on a metal-oxynitride surface?

The mechanism of water photooxidation (oxygen photoevolution) on a TaON photocatalyst was studied on the basis of our previous studies on the mechanism of this reaction on TiO(2) and N-doped TiO(2). We have confirmed that photocatalytic O(2) evolution occurs from an aqueous TaON suspension in the presence of Fe(3+), as reported. In-situ MIR-IR experiments have indicated that the TaON surface is slightly oxidized under visible-light irradiation, indicating that the oxygen photoevolution on TaON actually occurs on a thin Ta-oxide overlayer. The in-situ MIR-IR experiments have also shown that a certain surface peroxo species, tentatively assigned to adsorbed HOOH, is formed as an intermediate of the O(2) photoevolution reaction. Studies on the effect of addition of reductants to the electrolyte on the IPCE have shown that photogenerated holes at the TaON surface cannot oxidize reductants such as SCN(-), Br(-), methanol, ethanol, 2-propanol, and acetic acid, though they can oxidize H(2)O into O(2). Detailed considerations of these results have strongly suggested that the water photooxidation reaction on TaON proceeds by our recently proposed new mechanism, that is, the reaction is initiated by a nucleophilic attack of a water molecule (Lewis base) on a surface-trapped hole (Lewis acid).     

Mechanism of water photooxidation reaction at atomically flat TiO2 (rutile) (110) and (100) surfaces: dependence on solution pH

The mechanism of water photooxidation reaction at atomically flat n-TiO(2) (rutile) surfaces was investigated in aqueous solutions of various pH values, using photoluminescence (PL) measurements. The PL bands, which peaked at around 810 and 840 nm for the (110) and (100) surfaces, respectively, were assigned to radiative transitions between conduction-band electrons and surface-trapped holes (STH), [Ti-O=Ti(2)](s)+, formed at triply coordinated (normal) O atoms at the surface lattice. The PL intensity (I(PL)) decreased stepwise with increasing solution pH, namely, it sharply decreased at around pH 4, near the point of zero charge of TiO(2) (about 5), and then rapidly decreased to zero near pH 13. The first sharp decrease around pH 4 is attributed to the increased rate of nucleophilic attack of a water molecule to a hole at a site of surface bridging oxygen (Ti-O-Ti), the density of which increases with increasing pH. The nucleophilic attack is regarded as the main initiating step of the water oxidation reaction in low and intermediate pH. The high PL intensity at low pH is ascribed to slow nucleophilic attack owing to a very low density of Ti-O-Ti by its protonation at the low pH. The second sharp decrease near pH 13 is attributed to formation of surface anionic species like Ti-O- which can be readily oxidized by photogenerated holes. Interrelations between reaction intermediates proposed in this work and those reported by time-resolved laser spectroscopy are discussed.     

Computational study on the reactions of H2O2 on TiO2 anatase (101) and rutile (110) surfaces

This study investigates the adsorption and reactions of H(2)O(2) on TiO(2) anatase (101) and rutile (110) surfaces by first-principles calculations based on the density functional theory in conjunction with the projected augmented wave approach, using PW91, PBE, and revPBE functionals. Adsorption mechanisms of H(2)O(2) and its fragments on both surfaces are analyzed. It is found that H(2)O(2) , H(2)O, and HO preferentially adsorb at the Ti(5c) site, meanwhile HOO, O, and H preferentially adsorb at the (O(2c))(Ti(5c)), (Ti(5c))(2), and O(2c) sites, respectively. Potential energy profiles of the adsorption processes on both surfaces have been constructed using the nudged elastic band method. The two restructured surfaces, the 1/3 ML oxygen covered TiO(2) and the hydroxylated TiO(2), are produced with the H(2)O(2) dehydration and deoxidation, respectively. The formation of main products, H(2)O(g) and the 1/3 ML oxygen covered TiO(2) surface, is exothermic by 2.8 and 5.0 kcal/mol, requiring energy barriers of 0.8 and 1.1 kcal/mol on the rutile (110) and anatase (101) surface, respectively. The rate constants for the H(2)O(2) dehydration processes have been predicted to be 6.65 × 10(-27) T(4.38) exp(-0.14 kcal mol(-1)/RT) and 3.18 × 10(-23) T(5.60) exp(-2.92 kcal mol(-1)/RT) respectively, in units of cm(3) molecule(-1) s(-1).     

The importance of bulk Ti3+ defects in the oxygen chemistry on titania surfaces

The role of bulk defects in the oxygen chemistry on reduced rutile TiO(2)(110)-(1 × 1) has been studied by means of temperature-programmed desorption spectroscopy and scanning tunneling microscopy measurements. Following O(2) adsorption at 130 K, the amount of O(2) desorbing at ～410 K initially increased with increasing density of surface oxygen vacancies but decreased after further reduction of the TiO(2)(110) crystal. We explain these results by withdrawal of excess charge (Ti(3+)) from the TiO(2)(110) lattice to oxygen species on the surface and by a reaction of Ti interstitials with O adatoms upon heating. Important consequences for the understanding of the O(2)-TiO(2) interaction are discussed.     

氧缺位型TiO2的制备、表征及其光催化析氧活性

采用69 ℃饱和水蒸气和H2混合气, 于927 ℃下处理金红石型TiO2, 得到不同氧缺位的光催化剂, 并用X射线衍射(XRD)、比表面(BET)、电子顺磁共振(EPR)、紫外-可见漫反射(DRS)、光电子能谱(XPS)对其进行了表征. 考察了热处理时间对氧缺位型TiO2光催化分解水析氧活性的影响. 结果表明, 适量的氧缺位能显著提高金红石型TiO2光催化分解水的析氧活性, 其最大析氧速率达222 μmol·L-1·h-1.     

Electron spin resonance spin-trapping detection of radical intermediates in N-doped TiO2-assisted photodegradation of 4-chlorophenol

The electron spin resonance (ESR) spin-trapping technique using 5,5-dimethyl-1-pyrroline-N-oxide as the spin-trap reagent has been applied to detect free radical intermediates generated during in situ ultraviolet or visible irradiation of aqueous 4-chlorphenol (4-CP)/N-doped TiO(2) suspensions. ESR measurements gave the first direct evidence that the active species ((*)OH and O(2*-)) are responsible for the photodecomposition of 4-CP over N-doped TiO(2) under visible-light irradiation, strongly suggesting that the photocatalytic reaction of organic compounds in powdered N-doped TiO(2) systems proceed via surface intermediates of oxygen reduction or water oxidation, not via direct reaction with holes trapped at the N-induced midgap level. These results have important implications for the evaluation of the oxidative powder of TiO(2-x)N(x) catalysts.     

O2 interaction and reactivity on a model hydroxylated rutile(110) surface

Recently several theoretical studies have examined oxygen adsorption on the clean, reduced TiO2(110) surface. However the photocatalytic behavior of TiO2 and the scavenging ability of oxygen are known to be influenced by the presence of surface hydroxyls. In this paper the chemistry of O2 on the hydroxylated TiO2 surface is investigated by means of first-principles total energy calculations and molecular dynamics (MD) simulations. The MD trajectories show a direct, spontaneous reaction between O2 and the surface hydroxyls, thus supporting the experimental hypothesis that the reaction does not necessarily pass through a chemisorbed O2 state. Following this reaction, the most stable chemisorbed intermediates are found to be peroxide species HO2 and H2O2. Although these intermediates are very stable on the short time scale of MD simulations, the energetics suggests that their further transformation is connected to a new 300 K feature observed in the experimental water temperature programmed desorption (TPD) spectrum. The participation of two less stable intermediate states, involving terminal hydroxyls and/or chemisorbed water plus oxygen adatoms, to the desorption process, is not supported by the total energy calculations. Analysis of the projected density of states, however, suggests the possibility that these intermediates have a role in completing the surface oxidation immediately before desorption.     

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.     

金红石/锐钛矿混晶结构的TiO2薄膜光催化活性

采用磁控溅射法制备出一组金红石/锐钛矿混晶结构的纳米TiO2薄膜催化剂,并通过光催化降解苯酚实验考察该薄膜的催化性能. 光催化实验证明, 随着催化剂中金红石含量减少, 催化剂的光催化活性逐渐提高. X射线衍射(XRD)、X射线光电子能谱(XPS)、表面光电压谱(SPS)和原子力显微镜(AFM)结果表明, 催化剂为金红石和锐钛矿混晶结构, 并随着金红石含量减少, 催化剂的表面羟基(OH)和桥氧(—O—)的含量逐渐增加, 而且费米能级逐渐提高. 表面羟基和桥氧是有利于光催化的“活性物种”; 费米能级的提高使TiO2/H2O 面处TiO2的表面带弯增大, 导致了价带光生空穴参加光催化反应的几率增大, 有效地促进了光生载流子的分离; 这些因素是催化剂光催化活性逐渐提高的主要原因.     

Ag和Pd及其离子对TiO2光催化分解CH3CHO的影响

当用能量大于其禁带宽度的光照射通有氧气的TiO₂悬浮液时,在TiO₂微粒表面会产生反应活性很高的空穴和O₂^-、H₂O₂等多种活性氧.在上一篇文章中^[1]我们已报道了在通氧气和紫外光照的条件下,向TiO₂悬浮液中加入少量Ag⁺或Pd²⁺,将会大幅度提高体系中H₂O₂的生成量.另外,蔡汝雄等人也曾通过向TiO₂悬浮液中加入SOD的方法来提高其中H₂O₂的生成量^[2],而且证明了H₂O₂生成量的增多有助于杀死子宫癌细胞^[3].另一方面,利用TiO₂光催化来分解处理工业废水中的有机物已多见报道^[4～7].因此,为考察H₂O₂含量的增加是否有助于TiO₂催化分解有机物,我们以CH₃CHO为氧化对象,测定了经Ag和Pd表面修饰以及直接向悬浮体系添加Ag⁺或Pd²⁺离子前后,TiO₂光催化氧化分解CH₃CHO效率的变化,并对氮气和氧气气氛的实验结果进行了测定和比较.     

Phase structure and luminescence properties of Eu3+-doped TiO2 nanocrystals synthesized by Ar/O2 radio frequency thermal plasma oxidation of liquid precursor mists

Eu3+-doped TiO2 luminescent nanocrystals have been synthesized in this work via Ar/O2 thermal plasma oxidizing mists of liquid precursors containing titanium tetra-n-butoxide and europium(III) nitrate, with varied O2 input in the plasma sheath (10-90 L/min) and Eu3+ addition in the precursor solution (Eu/(Ti + Eu) = 0-5 atom%). The resultant nanopowders are mixtures of the anatase (30-36 nm) and rutile (64-83 nm) polymorphs in the studied range, but the rutile fraction increases steadily at a higher Eu3+ addition, as revealed by X-ray diffraction (XRD) and Raman spectroscopy, because of the creation of oxygen vacancies in the TiO2 gas clusters by substitutional Eu3+ doping. The amount of Eu3+ that can be doped into a TiO2 lattice was limited up to 0.5 atom%, above which Eu2Ti2O7 pyrochlore was formed in the final products. High resolution transmission electron microscopy (HRTEM) observation indicates that the particles are dense and have sizes ranging from several nanometers up to 180 nm. Efficient nonradiative energy transfer from the TiO2 host to Eu3+ ions, which was seldom reported in the wet-chemically derived nanoparticles or thin films of the current system, was confirmed by combined studies of excitation, UV-vis (ultraviolet-visible), and PL (photoluminescence) spectroscopy. As a consequence of this, bright red emissions were observed from the plasma-generated nanopowders either by exciting the TiO2 host with UV light shorter than 405 nm or by directly exciting Eu3+ at a wavelength beyond the absorption edge (405 nm) of TiO2.     

一步低温法制备可见光响应的棕色纳米TiO2及其光催化性能

光催化作为一种具有前景的技术,被广泛运用于有机物降解、废水处理、空气净化、抗菌、太阳能电池等领域.在众多的光催化材料中,纳米TiO2因具有性质稳定、耐腐蚀、廉价和无毒等优点而受到广泛关注.但纳米TiO2禁带宽度较大(3.2 eV)、只对紫外光有响应及电子-空穴对易复合等特性限制了它的应用.因此,提高纳米TiO2的可见光响应一直是研究的热点.本文发展了一种在低温下制备棕色纳米TiO2的改良溶胶-凝胶法.该法以钛酸四丁酯为钛源,无水乙醇为溶剂,形成溶胶后无需陈化和高温高压,在简单温和的条件下即可制备出棕色纳米TiO2.比较了低温干燥和高温焙烧两种处理方法,结果表明,随着制备温度的升高,样品的粒子尺寸增大,比表面积减小,颜色从白色转变为棕色,在更高的温度又变浅.样品的可见光吸收在180℃时达到最大,随后减弱.在优化温度180℃下制备的TiO2-180℃纳米粒子不仅具有较小的粒径(5.0 nm),较大的比表面积(213.45 m2/g),且在整个紫外-可见光区都具有较强的吸收,其禁带宽度低至1.84 eV.X-射线光电子能谱结果表明,TiO2粒子表面的–OH/H2O含量随制备温度升高而先增加后下降.Raman光谱中Eg峰的移动和变宽表明TiO2晶格可能存在缺陷或氧空位,而TiO2-180℃纳米粒子的电子顺磁共振图谱的g值在2.003左右,对应氧空位中的未成对电子,验证了以上推测.其中TiO2-180℃纳米粒子呈现为最强的EPR信号,表明其晶格内存在最高浓度的氧空位,这是其具有强可见光吸收的原因.光催化实验结果表明,在可见光照射下,TiO2-180℃可高效降解亚甲基蓝(MB).当C(MB)=10 mg/L,pH=4,催化剂添加量为0.07 g时,TiO2-180℃催化剂的光催化活性达到最佳,光照1 h后MB降解率达到99.33%,反应速率常数(0.08287 mg/(L·min))约为同条件下P25(0.01342 mg/(L·min))的6倍.同时,TiO2-180℃催化剂在不同单色光下的光催化活性与它对单色光的光响应大致相符.循环降解实验证明TiO2-180℃催化剂具有很好的稳定性.光猝灭实验表明,·OH在光催化降解过程占主导作用,而TiO2-180℃样品表面含有较多的–OH,有利于·OH的产生,乃至光催化反应.研究表明,晶格内高浓度的氧空位导致的强可见光响应,得益于低温制备条件而保留了大量–OH/H2O的纳米粒子表面以及更大的比表面积,共同促成了TiO2-180℃优越的光催化活性.所制备的棕色纳米TiO2经过进一步修饰后有望运用于实际应用中.     

Facile synthesis and characterization of functionalized, monocrystalline rutile TiO2 nanorods

Functionalized, monocrystalline rutile TiO2 nanorods were prepared from TiCl4 in aqueous solution under acidic conditions in the presence of dopamine, followed by aging and hydrothermal treatment at 150 degrees C. The surface-bound organic ligand controls the morphology as well as the crystallinity and the phase selection of TiO2. The presence of monocrystalline rutile TiO2 was confirmed by X-ray powder diffraction and HRTEM investigations. The as-prepared nanorods are soluble in water at pH <3. The surface functionalization was analyzed by IR and 1H NMR, confirming the presence of dopamine on the surface. The surface amine groups can be tailored further with functional molecules such as dyes. Confocal laser scanning microscopy (CLSM) was used to characterize the binding of the fluorescent dye 4-chloro-7-nitrobenzofurazan (NBD) to the functionalized surface of the TiO2 nanorods.     

锐钛矿、金红石和板钛矿降解罗丹明B光催化活性的比较

详细考察并比较了采用低温制备的锐钛矿、金红石和板钛矿氧化钛降解罗丹明B的光催化活性.与传统高温焙烧制备的金红石相比,低温制备的金红石粒径小,比表面积大,表面羟基数目多,因此光催化效率明显增加.更重要的是,当锐钛矿和金红石具有相似粒径和比表面积时,金红石具有较高的光降解罗丹明B活性.对于板钛矿氧化钛而言,虽然其表观光催化...     

Effects of surface oxygen vacancies on photophysical and photochemical processes of Zn-doped TiO2 nanoparticles and their relationships

In this paper, TiO(2) nanoparticles doped with different amounts of Zn were prepared by a sol-gel method and were mainly characterized by means of X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and surface photovoltage spectrum (SPS). The effects of surface oxygen vacancies (SOVs) of Zn-doped TiO(2) nanoparticles on photophysical and photocatalytic processes were investigated along with their inherent relationships. The results show that the SOVs easily bind photoinduced electrons to further give rise to PL signals. The SOVs can result in an interesting sub-band SPS response near the band edge in the TiO(2) sample consisting of much anatase and little rutile, except for an obvious band-to-band SPS response. Moreover, the intensities of PL and SPS signals of TiO(2), as well as the photocatalytic activity for degrading phenol solution, can be enhanced by doping an appropriate amount of Zn. These improvements are mainly attributed to the increase in the SOV amount. It can be suggested that the SOVs should play an important role during the processes of PL, surface photovoltage, and photocatalytic reactions, and, for the as-prepared TiO(2) samples doped with different amounts of Zn by thermal treatment at 550 degrees C, the larger the SOV amount, the stronger the PL and SPS signal, and the higher the photocatalytic activity.     

表面相结结构在促进光催化电荷分离中的应用

二十世纪八十年代以来,特别是近十年,光催化研究在利用可再生能源太阳能的道路上飞速发展。越来越多的研究表明,相结结构的构筑是有效提高半导体光催化剂性能的重要策略。其中, TiO2作为重要的模型光催化剂,其相关研究成果呈现出指数增长的趋势。本综述围绕TiO2模型光催化剂,主要介绍TiO2表面相结的研究成果,包括TiO2表面相的表征、锐钛矿：金红石TiO2相结用于光催化产氢研究、TiO2相结在光催化中作用的最新认识等。在表征方面,通过表面灵敏的紫外拉曼光谱研究了TiO2相变过程中表面相结构的变化,结合可见拉曼以及XRD表征揭示了TiO2独特的相变过程,即相变始于锐钛矿粒子的界面处,小粒子逐渐团聚为大粒子,致其相变从大粒子体相开始最终扩展到整个粒子。使用CO, CO2探针红外光谱,根据锐钛矿和金红石表面吸附物种的差异,进一步证实了锐钛矿：金红石表面相结结构,为紫外拉曼光谱的表面表征特性提供坚实证据。同时,利用发光光谱观察到锐钛矿晶相的可见发光带和金红石晶相的近红外发光带,并基于此给出了TiO2材料表面相结结构的荧光表征新方法。此外荧光光谱还提供了锐钛矿、金红石相中载流子动力学信息,揭示了束缚态在光催化中的作用。在光催化应用方面,观察到混相结构TiO2较单独锐钛矿及金红石相具有更高的光催化产氢活性,通过在较大金红石颗粒上担载纳米锐钛矿粒子,证明了相结结构在提高光催化活性中的核心作用,并首次提出了锐钛矿：金红石表面异相结结构概念,推断其对电荷分离的促进作用是最终提高反应活性的原因。之后将此概念应用到改善商品TiO2(P25)光催化活性中,通过可控热处理精细调控P25的表面相结构,在光催化重整生物质衍生物产氢实验中,成功将P25光催化产氢活性提高3?5倍。之后发展了新的TiO2表面控制方法,通过加入Na2SO4等相变控制剂,延缓了TiO2从锐钛矿向金红石的相变过程,在较高温度下实现TiO2相结结构的调控,最终可将P25光催化重整甲醇制氢的活性提高6倍,同时通过高分辨电镜清晰观察到锐钛矿：金红石相结的原子层生长接触。在相结作用机理方面,多种时间分辨光谱技术以及理论计算被用作探索锐钛矿：金红石相结处的电子转移机理。通过时间分辨红外光谱对TiO2表面相结结构作用的研究,特别是利用锐钛矿、金红石不同的瞬态吸收光谱特征,证明了锐钛矿：金红石相结处的载流子转移过程,存在锐钛矿向金红石的电子转移过程。模型光催化剂TiO2相结的研究成果,加深了对光催化机理的认识,促进新型高效光催化体系的设计合成。     

空气电极/AC作载体对TiO2光催化性能的影响

The photo catalytic degradation of activated red in the aqueous solution was studied using TiO2 supported on air electrode and active carbon (AC) as photo catalysts. It was found that the photo catalytic reaction rate of TiO2 was obviously increased by the presence of air electrode and AC supported. The air electrode which has functions of synthesizing H2O2 in situ and photocatalysis was reported. The results also implied that biasing of the electrode at +0.5V led to efficient charge separation. The current density of air (oxygen) electrode had effect on the oxidation rate of azo dye molecule, i=15 mA/cm2, and the rate could reach maximum. With AC mass fraction of about 21% the oxidation rate for TiO2/AC was obviously larger than that for TiO2, but the result was contrary to this for higher AC mass fraction (>30%). The experiment results showed that because TiO2 was supported on active carbon, the effective surface area of the photo catalysis and their absorbability for organic molecules can be increased. The pH in solution had effect on the oxidation rate of organic molecules.     

Synthesis and characterization of nano titania powder with high photoactivity for gas-phase photo-oxidation of benzene from TiOCl(2) aqueous solution at low temperatures

Nano rutile, anatase, and bicrystalline (anatase + brookite) titania powders with an average crystal size of below 10 nm are prepared from aqueous TiOCl(2) solution at low temperatures by adjusting pH values of the starting solution and adding different additives. Adding a small amount of octyl phenol poly(ethylene oxide) into aqueous TiOCl(2) solution leads to the change of particle morphologies of obtained nano titania from needlelike to nano spherical rutile crystals. Amorphous-anatase transformation of titania could proceed in liquid-solid reaction at low temperatures, even at room temperature. A formation mechanism of rutile, anatase, and brookite titania was proposed. It is found that H(+) or H(3)O(+) plays a catalytic role in the phase transformation from amorphous to anatase titania and that the presence of a small amount of SO(4)(2)(-) ion is unfavorable to the formation of both rutile and brookite. By carefully adjusting preparation conditions, nano pure anatase with higher surface area, good crystallinity, and a lower recombination rate of photoexcited electrons and holes was obtained. This nano pure anatase showed a very good photocatalytic activity for gas-phase photo-oxidation of benzene.