Synthesis of {100} Facet Dominant Anatase TiO2 Nanobelts and the Origin of Facet‐Dependent Photoreactivity

Sword‐like anatase TiO₂ nanobelts exposed with 78 % clean {100} facets were synthesized and the facet‐dependent photoreactivity of anatase TiO₂ was investigated. By quantitative comparison with the reference {001} facets, the {100} facets possessed about ten‐times higher active sites density than that on {001} facets, resulting in higher photoreaction efficiency. After the active sites density normalization, the {100} and {001} facets exhibited distinct wavelength‐dependent photocatalytic performance, attributed to the anisotropic electronic structures in TiO₂ crystals.     

Facet-Control versus Co-Catalyst-Control in Photocatalytic H2 Evolution from Anatase TiO2 Nanocrystals

Titanium dioxide (TiO₂) and, in particular, its anatase polymorph, is widely studied for photocatalytic H₂ production. In the present work, we examine the importance of reactive facets of anatase crystallites on the photocatalytic H₂ evolution from aqueous methanol solutions. For this, we synthesized anatase TiO₂ nanocrystals with a large amount of either {001} facets, that is, nanosheets, or {101} facets, that is, octahedral nanocubes, and examined their photocatalytic H₂ evolution and then repeated this procedure with samples where Pt co-catalyst is present on all facets. Octahedral nanocubes with abundant {101} facets produce >4 times more H₂ than nanosheets enriched in {001} facets if the reaction is carried out under co-catalyst-free conditions. For samples that carry Pt co-catalyst on both {001} and {101} facets, faceting loses entirely its significance. This demonstrates that the beneficial role of faceting, namely the introduction of {101} facets that act as electron transfer mediator is relevant only for co-catalyst-free TiO₂ surfaces.     

Graphite-like carbon deposited anatase TiO<Subscript>2</Subscript> single crystals as efficient visible-light photocatalysts

Graphite-like carbon deposited single-crystal anatase TiO₂ with exposed {001} facets was fabricated through a two-step solvothermal process by using glucose as carbon source. The physicochemical properties of the as-prepared samples were investigated by X-ray diffraction, Brunauer-Emmett-Teller, transmission electron microscopy, Raman, UV–vis diffuse reflectance spectra, electrochemical impedance spectroscopy and surface photovoltage spectroscopy. These results demonstrated that graphite-like carbon layers were deposited on the surface of TiO₂ single-crystal nanosheets with exposed highly reactive {001} facets via the dehydration of glucose during the process of hydrothermal treatment. The loading of the graphite-like carbon layers could effectively extend the light absorption edge of the single-crystal anatase TiO₂ nanosheets to visible light region and accelerate the separation of photo-generated electrons and holes, contributing an excellent visible-light driven photocatalytic performance to the graphite-like carbon deposited single-crystal anatase TiO₂ nanosheets for the degradation of methyl orange.     

Morphology‐Engineered Highly Active and Stable Ru/TiO2 Catalysts for Selective CO Methanation

Ru/TiO₂ catalysts exhibit an exceptionally high activity in the selective methanation of CO in CO₂‐ and H₂‐rich reformates, but suffer from continuous deactivation during reaction. This limitation can be overcome through the fabrication of highly active and non‐deactivating Ru/TiO₂ catalysts by engineering the morphology of the TiO₂ support. Using anatase TiO₂ nanocrystals with mainly {001}, {100}, or {101} facets exposed, we show that after an initial activation period Ru/TiO₂‐{100} and Ru/TiO₂‐{101} are very stable, while Ru/TiO₂‐{001} deactivates continuously. Employing different operando/in situ spectroscopies and ex situ characterizations, we show that differences in the catalytic stability are related to differences in the metal–support interactions (MSIs). The stronger MSIs on the defect‐rich TiO₂‐{100} and TiO₂‐{101} supports stabilize flat Ru nanoparticles, while on TiO₂‐{001} hemispherical particles develop. The former MSIs also lead to electronic modifications of Ru surface atoms, reflected by the stronger bonding of adsorbed CO on those catalysts than on Ru/TiO₂‐{001}.     

Oxygen-Deficient Dumbbell-Shaped Anatase TiO2−x Mesocrystals with Nearly 100 % Exposed {101} Facets: Synthesis,Growth Mechanism,and Photocatalytic Performance

The development of hierarchical TiO₂ superstructures with new morphologies and intriguing photoelectric properties for utilizing solar energy is known to be an effective approach to alleviate the serious problems of environmental pollution. Herein, unique oxygen-deficient dumbbell-shaped anatase TiO_2−x mesocrystals (DTMCs) enclosed by nearly 100 % {101} facets were readily synthesized by mesoscale transformation in TiCl₃/acetic acid (HAc) mixed solution, followed by calcination under vacuum. These mesocrystals exhibited much higher photoreactivity toward removing the model pollutants methyl orange and Cr^VI than truncated tetragonal bipyramidal anatase nanocrystals (TNCs), anatase mesocrystals built from truncated tetragonal bipyramidal anatase nanocrystals (TTMCs), and anatase mesocrystals constructed by anatase nanocrystals with nearly 100 % exposed {101} facets (TMCs), revealing that both the oxidation and reduction abilities of anatase TiO₂ were simultaneously enhanced upon fabricating an oxygen-deficient mesocrystalline architecture with about 100 % exposed {101} facets. Further characterization illustrated that such an enhancement of photoreactivity was mainly due to the strengthened light absorption, boosted charge carrier separation, and nearly 100 % exposed {101} facets of the oxygen-deficient dumbbell-shaped anatase mesocrystals. This work will be useful for guiding the synthesis of oxygen-deficient ordered superstructures of metal oxides with desired morphologies and exposed facets for promising applications in environmental remediation.     

Enhancing the Photocatalytic Activity of Anatase TiO2 by Improving the Specific Facet‐Induced Spontaneous Separation of Photogenerated Electrons and Holes

Recently, it has been proven that directional flow of photogenerated charge carriers occurs on specific facets of TiO₂ nanocrystals. Herein, we demonstrate that the photocatalytic activity of anatase TiO₂ nanocrystals in both photoreduction and photooxidation processes can be enhanced by selectively depositing Pt nanoparticles on the {101} facets, which strengthens spontaneously surface‐induced separation between photogenerated electrons and holes in the photocatalysis process. An optimal ratio of the oxidative {001} facets to the reductive {101} facets exists with regard to the photocatalysis of the faceted TiO₂ nanocrystals, and this is crucial for balancing the recombination and redox reaction rates of photogenerated electrons and holes. The present work might help us gain deeper insight into the relation between the specific surface of semiconductor photocatalysts and their photocatalytic activities and provides us with a new route to design photocatalysts with high photocatalytic activity.     

Synthesis of nano-sized anatase TiO2 with reactive {001} facets using lamellar protonated titanate as precursor

Nano-sized anatase TiO(2) with exposed {001} facets was synthesized from lamellar protonated titanate precursor. Owing to small size (ca. 11 nm) and high surface area (155 m(2) g(-1)), the crystals with 26.1% {001} facets exhibited markedly superior photoactivity to reference ca. 76 nm anatase TiO(2) nanosheets with 88.4% {001} facets.     

Axis‐Oriented,Continuous Anatase Titania Films with Exposed Reactive {100} Facets

Homogeneous TiO₂ single crystals with high exposure of {100} reactive facets were constructed as a seed monolayer on transparent conductive substrates with the desired orientation of reactive facets. A secondary growth process was subsequently carried out on the monolayer seed film to form an axis‐oriented continuous reactive film. Performing secondary growth with different precursors led to optimized conditions for high‐performance photoelectrochemical activity of anatase TiO₂ films. Experimental techniques such as UV/Vis absorption spectroscopy, X‐ray diffraction, high‐resolution SEM, and photoelectrochemistry were used to characterize the structural, optical, and photoelectrochemical properties of the as‐synthesized films. As a photoanode in a photoelectrochemical cell, the axis‐oriented reactive film shows a maximum photocurrent density of 0.3 mA cm^?2, as opposed to 0.075 mA cm^?2 for non‐axis‐oriented (randomly oriented) TiO₂ film.     

具有丰富(001)面的锐钛矿TiO2晶体的无氟合成

采用一种全新的方法实现了无氟、在阳离子聚合物参与下合成具有丰富(001)面的锐钛矿TiO₂晶体(TiO₂-P), 并通过X射线衍射、扫描电镜、透射电镜、紫外-可见光谱等手段对样品进行了表征. 光催化测试显示, 该样品对甲基橙的降解具有比常规锐钛矿TiO₂和商业P25更好的活性, 这可归因于TiO₂-P具有大量的暴露(001)晶面.     

Microwave-induced synthesis of porous single-crystal-like TiO2 with excellent lithium storage properties

Porous anatase TiO(2) single crystal architectures with large specific surface area and remarkable crystalline phase-stability were fabricated via a green microwave-assisted process. Ionic liquid was chosen as both an essential structure-directing agent for the formation of the {001} facets exposed TiO(2) and an etching agent source for selective erosion of the exposed {001} facets, leading to robust porous framework with exposed {101} facets. These porous anatase single crystals were thermally stable up to 800 °C, indicating excellent structure stability. The product showed stable cyclability at high current rate, better reversibility, and high Coulumbic efficiency of 100% for lithium storage.     

Effects of anatase TiO2 morphology and surface fluorination on environmentally relevant photocatalytic reduction and oxidation reactions

Aiming at clarifying the interplay on TiO₂ photoactivity between particle morphology and surface fluorination, the photocatalytic performance of anatase nanocrystals, characterized by a pseudo-spherical shape or a nanosheet structure, is investigated in both a reduction and an oxidation reaction, either in the absence or in the presence of added fluoride anions. Cr(VI) photocatalytic reduction is strongly favored by a large exposure of anatase {001} facets; however, surface fluorination leads in this case to a morphology-independent photoactivity decrease, due to the decreased adsorption of the reaction substrate. More interestingly, a beneficial synergistic effect between the platelet-like anatase morphology and TiO₂ surface fluorination is clearly outlined in Rhodamine B photocatalytic degradation, possibly resulting from the intrinsic ability of fluorinated {001} anatase facets of boosting ^?OH radical mediated oxidation paths, due to their larger amount of surface –OH groups, as revealed using Fourier-transform infrared spectroscopy.     

锐钛矿(001)与(101)晶面在光催化反应中的作用

采用水热法制备了(001)和(101)晶面暴露的单晶锐钛矿TiO₂颗粒. 利用光还原沉积贵金属(Au, Ag, Pt)和光氧化沉积金属氧化物(PbO₂, MnO_x)的方法研究了暴露的锐钛矿(001)和(101)晶面在光催化中的作用. 通过透射电子显微镜(TEM)、扫描电子显微镜(STM)、能量色散X射线光谱仪(EDX)和X射线光电子能谱(XPS)的表征, 发现发生光还原反应生成的贵金属粒子主要沉积在暴露的锐钛矿(101)晶面上, 而发生光氧化反应产生的金属氧化物颗粒主要沉积在暴露的锐钛矿(001)晶面上. 此结果表明光激发产生的电子与空穴主要并分别分布在单晶锐钛矿TiO₂的(101)与(001)晶面上, 并在其上参与光催化还原反应和氧化反应. 同时也表明暴露的不同晶面对光生电荷具有分离效应. 基于本研究可以认为同时暴露分别进行氧化和还原反应的晶面可以有效促进光催化反应.     

铂修饰的{001}面暴露的二氧化钛纳米片阵列的制备和光催化性能

采用有机溶剂热法在FTO衬底上制备{001}面暴露的单晶锐钛矿相TiO2纳米片阵列,通过FESEM和XRD研究样品的形貌和晶体结构. 与水热法制备的纳米片阵列相比,有机溶剂热法制备的样品取向性更好. 采用光沉积方法在纳米片阵列上沉积Pt,所得到的Pt纳米颗粒粒径更为均匀,并且更容易沉积在{001}面上. 所负载的Pt 纳米颗粒增强了TiO2纳米片的光吸收性能,同时大大减弱了光致发光强度. 在光催化性能测试中,具有最优负载量的样品催化性能提高了一倍. 与传统的Pt负载相比,{001}面的最优负载量显得相当小,这可能源于高活性{001}面的原子结构.     

Prolonging charge-separation states by doping lanthanide-ions into {001}/{101} facets-coexposed TiO2 nanosheets for enhancing photocatalytic H2 evolution

Ultrathin TiO₂ nanosheets with coexposed {001}/{101} facets have attracted considerable attention because of their high photocatalytic activity. However, the charge-separated states in the TiO₂ nanosheets must be extended to further enhance their photocatalytic activity for H₂ evolution. Herein, we present a successful attempt to selectively dope lanthanide ions into the {101} facets of ultrathin TiO₂ nanosheets with coexposed {001}/{101} facets through a facile one-step solvothermal method. The lanthanide doping slightly extended the light-harvesting region and markedly improved the charge-separated states of the TiO₂ nanosheets as evidenced by UV-vis absorption and steady-state/transient photoluminescence spectra. Upon simulated sunlight irradiation, we observed a 4.2-fold enhancement in the photocatalytic H₂ evolution activity of optimal Yb³⁺-doped TiO₂ nanosheets compared to that of their undoped counterparts. Furthermore, when Pt nanoparticles were used as cocatalysts to reduce the H₂ overpotential in this system, the photocatalytic activity enhancement factor increased to 8.5. By combining these results with those of control experiments, we confirmed that the extended charge-separated states play the main role in the enhancement of the photocatalytic H₂ evolution activity of lanthanide-doped TiO₂ nanosheets with coexposed {001}/{101} facets.     

Study of Oxygen Vacancies on Different Facets of Anatase TiO2

Anatase TiO₂ samples with different ratios of {101} to {001} facets were prepared with hydrothermal method and further treated under three specific calcination atmospheres (air, H₂, N₂). The characterization results indicate that {001} facets may generate more oxygen vacancies and more Ti³⁺ species than {101} facets.     

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.     

Hierarchical TiO2 microspheres: synergetic effect of {001} and {101} facets for enhanced photocatalytic activity

Well-faceted nanocrystals of anatase TiO(2) with specific reactive facets have attracted extraordinary research interest due to their many intrinsic shape-dependent properties. In this work, hierarchical TiO(2) microspheres consisting of anatase nanosheets or decahedrons were synthesized by means of a facile hydrothermal technique; meanwhile, the percentage of {001} facets can be tuned from 82 to 45%. Importantly, by investigating the photo-oxidation reactions for ˙OH radical generation and photoreduction reactions for hydrogen evolution, the TiO(2) microspheres consisting of nano-decahedrons with 45% {001} facets show superior photoreactivity (more than 4.8-times) compared to the nanosheets with 82% {001} facets. By analyzing the results of scanning electron microscopy (SEM), photoluminescence (PL) and first-principles density functional theory (DFT) calculations, a model of charge separation between the well-formed {001} and {101} facets is proposed, and the enhanced photocatalytic efficiency is largely attributed to the efficient separation of photogenerated charges among the crystal facets co-exposed.     

Searching for facet-dependent photoactivity of shape-controlled anatase TiO2

Improving the performance of mostly employed anatase TiO₂ photocatalysts by properly controlling their crystal shape represents a big challenge to improve their efficiency in photocatalytic applications. After the synthesis, reported in 2008, of anatase microcrystals enriched in high-energy {001} facets, many efforts have been made aimed at tuning the crystal morphology of anatase, by means of either fluorine-mediated or more environmentally friendly methods, producing a deviation from its regular crystal growth. In this relatively new field of investigation, controversial opinions emerged concerning the role of each type of facet and its relative amount in relation to photoefficiency optimization. This review addresses this topic by presenting a critical survey of selected literature reports. After a brief introduction on the main synthetic strategies adopted to obtain shape-controlled anatase photocatalysts, the attention is focused on the methods employed for their comprehensive characterization, including the identification and quantification of exposed facets and the assessment of their influence on bulk and surface properties relevant to photoactivity. Potential interferences, derived from synthetic routes and possibly affecting the conclusions of facet-dependent photoactivity investigations, are also discussed. Key examples of test reactions actually demonstrating how both the type and/or the amount of specific facets influence photocatalytic activity are finally reported, aiming at providing rational bases for the design of better performing shape-controlled anatase photocatalysts.     

Synthesis of Cs2.5H0.5PW12O40/TiO2 Nanocomposites with Dominant TiO2 {001} Facets and Related Photocatalytic Properties

TiO₂ nanosheets with dominant {001} facets, coupled with Cs_2.5H_0.5PW₁₂O₄₀, were successfully synthesized by a one‐step hydrothermal reaction. The photocatalytic activity of nanocatalysts was evaluated by the degradation of Rhodamine B under UV light irradiation. The results showed that both the addition of Cs_2.5H_0.5PW₁₂O₄₀ and the exposed {001} facets of TiO₂ have a positive effect on the photocatalytic activity. The improved photoactivity of nanocomposites in comparison with that of TiO₂ nanosheets could be attributed to the synergistic effect between Cs_2.5H_0.5PW₁₂O₄₀ and TiO₂ which facilitates the separation of photo‐induced hole‐electron pairs.     

Controlled Synthesis and Enhanced Catalytic and Gas‐Sensing Properties of Tin Dioxide Nanoparticles with Exposed High‐Energy Facets

A morphology evolution of SnO₂ nanoparticles from low‐energy facets (i.e., {101} and {110}) to high‐energy facets (i.e., {111}) was achieved in a basic environment. In the proposed synthetic method, octahedral SnO₂ nanoparticles enclosed by high‐energy {111} facets were successfully synthesized for the first time, and tetramethylammonium hydroxide was found to be crucial for the control of exposed facets. Furthermore, our experiments demonstrated that the SnO₂ nanoparticles with exposed high‐energy facets, such as {221} or {111}, exhibited enhanced catalytic activity for the oxidation of CO and enhanced gas‐sensing properties due to their high chemical activity, which results from unsaturated coordination of surface atoms, superior to that of low‐energy facets. These results effectively demonstrate the significance of research into improving the physical and chemical properties of materials by tailoring exposed facets of nanomaterials.