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}.     

{001} facets dominated anatase TiO2: Morphology, formation/etching mechanisms and performance

Controllable growth of anatase TiO2 crystals with exposed high reactive crystal facets has aroused great attention in the fields of science and technology due to their unique structure-dependent properties. Recently, much effort has been paid to synthesize anatase TiO2 crystals with exposed high reactive {001} facets. Herein, we review the recent progress in synthesizing {001} facets dominated anatase TiO2 crystals with different morphologies by various synthetic methods. Furthermore, our review is mainly focused on the formation/etching mechanisms of {001} facets dominated anatase TiO2 crystals based on our and other studies. The extensive application potentials of the anatase TiO2 crystals with exposed {001} facets have been summarized in this review such as photocatalysis, photoelectrocatalysis, solar energy conversion, lithium ion battery, and hydrogen generation. Based on the current studies, we give some perspectives on the research topic. We believe that this comprehensive review on anatase TiO2 crystals with high reactive {001} facets can further promote the relative research in this field.     

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.     

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.     

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.     

Methanol Conversion into Dimethyl Ether on the Anatase TiO2(001) Surface

Exploring reactions of methanol on TiO₂ surfaces is of great importance in both C1 chemistry and photocatalysis. Reported herein is a combined experimental and theoretical calculation study of methanol adsorption and reaction on a mineral anatase TiO₂(001)‐(1×4) surface. The methanol‐to‐dimethyl ether (DME) reaction was unambiguously identified to occur by the dehydration coupling of methoxy species at the fourfold‐coordinated Ti⁴⁺ sites (Ti_4c), and for the first time confirms the predicted higher reactivity of this facet compared to other reported TiO₂ facets. Surface chemistry of methanol on the anatase TiO₂(001)‐(1×4) surface is seldom affected by co‐chemisorbed water. These results not only greatly deepen the fundamental understanding of elementary surface reactions of methanol on TiO₂ surfaces but also show that TiO₂ with a high density of Ti_4c sites is a potentially active and selective catalyst for the important methanol‐to‐DME reaction.     

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.     

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.     

Unprecedented Eighteen‐Faceted BiOCl with a Ternary Facet Junction Boosting Cascade Charge Flow and Photo‐redox

Exposure of anisotropic crystal facets allows the directional transfer of photoexcited electrons (e^?) and holes (h⁺), for spatial charge separation. High‐index facets with a high density of low‐coordinated atoms always serve as reactive catalytic sites. However, preparation of multi‐facets or high‐index facets is highly challenging for layered bismuth‐based photocatalysts. Herein, we report the preparation of unprecedented eighteen‐faceted BiOCl with {001} top facets and {102} and {112} oblique facets via a hydrothermal process. Compared to the conventional BiOCl square plates with {001} top facets and {110} lateral facets, the eighteen‐faceted BiOCl has highly enhanced photocatalytic activity for H₂ evolution and hydroxyl radicals (^.OH) production. Theoretical calculations and photodeposition results disclose that the of eighteen‐faceted BiOCl has a well‐matched {001}/{102}/{112} ternary facet junction, which provides a cascade path for more efficient charge flow than the binary facet junction in BiOCl square plates.     

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.     

The Effects of Exposed Specific Facets and Sulfation on the Surface Acidity of Cu2O Solids

Cuprous oxide microcrystals with {111}, {111}/{100}, and {100} exposed facets were synthesized. ³¹P MAS NMR using trimethylphosphine as the probe molecule was employed to study the acidic properties of samples. It was found that the total acidic density of samples increases evidently after sulfation compared with the pristine cuprous oxide microcrystals. During sulfation, new {100} facets are formed at the expense of {111} facets and lead to the generation of two Lewis acid sites due to the different binding states of SO₄²⁻ on {111} and {100} facets. Moreover, DFT calculation was used to illustrate the binding models of SO₄²⁻ on {111} and {100} facets. Also, a Pechmann condensation reaction was applied to study the acidic catalytic activity of these samples. It was found that the sulfated {111} facet has better activity due to its higher Lewis acid density compared with the sulfated {100} facet.     

Different Atomic Terminations Affect the Photocatalytic Nitrogen Fixation of Bismuth Oxybromide: A First Principles Study

We have systematically investigated the electronic structures and activation capacities of BiOBr {001} facets with different atomic terminations by means of DFT methods. Our calculations reveal that oxygen vacancies (OVs) give a significant boost in band edges of the O‐terminated BiOBr {001} facets, and excess electrons induced by OVs could exceed the reduction potentials of high‐energy N₂ intermediates. Interestingly, the Bi‐terminated BiOBr {001} facets may be good candidates for photocatalytic nitrogen fixation due to the stronger activation ability of N₂ molecules comparing with O‐terminated BiOBr {001} facets with OVs. Moreover, the Bi‐terminated BiOBr {001} facets may tend to yield NH₃ instead of N₂H₄.     

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.     

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.     

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.     

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.     

Visible‐Light‐Induced Efficient Selective Oxidation of Nonactivated Alcohols over {001}‐Faceted TiO2 with Molecular Oxygen

In the presence of molecular oxygen, a {001}‐faceted nanocrystalline anatase TiO₂ catalyst enabled the selective oxidation of nonactivated aliphatic alcohols to the corresponding aldehydes or ketones under visible light. The reaction shows excellent conversion and selectivity towards the formation of the carbonyl products without over‐oxidation to the corresponding carboxylic acids. The exceptional reactivity of the catalyst is possibly due to the absorption of visible light originating from a stronger interaction of alcohol with the {001} facet, which facilitates the modification of the band structure of TiO₂, thus facilitating the photogenerated hole transfer and subsequent oxidation processes. The experimental results have also been corroborated by first‐principles quantum chemical DFT calculations.     

Platinum Multicubes Prepared by Ni2+‐Mediated Shape Evolution Exhibit High Electrocatalytic Activity for Oxygen Reduction

Pt(100) facets are generally considered less active for the oxygen reduction reaction (ORR). Reported herein is a unique Pt‐branched structure, a multicube, whose surface is mostly enclosed by {100} facets but contains high‐index facets at the small junction area between the adjacent cubic components. The synthesis is accomplished by a Ni²⁺‐mediated facet evolution from high‐index {311} to {100} facets on the frameworks of multipods. Despite the high {100} facet coverage, the Pt multicubes exhibit impressive ORR activity in terms of half‐wave potential and current density nearly to the level of the most active Pt‐based catalysts, while the durability of catalysts is well retained. The facet evolution creates a set of samples with tunable ratios of high‐index to low‐index facets. The results reveal that the excellent ORR performance of Pt multicubes is a combined result of active sites by high‐index facets and low resistance by flat surface. It is anticipated that this work will offer a new approach to facet‐controlled synthesis and ORR catalysts design.     

Crystal‐Facet Engineering of Ferric Giniite by Using Ionic‐Liquid Precursors and Their Enhanced Photocatalytic Performances under Visible‐Light Irradiation

In the work presented here, well‐dispersed ferric giniite microcrystals with controlled sizes and shapes are solvothermally synthesized from ionic‐liquid precursors by using 1‐n‐butyl‐3‐methylimidazolium dihydrogenphosphate ([Bmim][H₂PO₄]) as phosphate source. The success of this synthesis relies on the concentration and composition of the ionic‐liquid precursors. By adjusting the molar ratios of Fe(NO₃)₃ ? 9H₂O to [Bmim][H₂PO₄] as well as the composition of ionic‐liquid precursors, we obtained uniform microstructures such as bipyramids exposing {111} facets, plates exposing {001} facets, hollow spheres, tetragonal hexadecahedron exposing {441} and {111} facets, and truncated bipyamids with carved {001} facets. The crystalline structure of the ferric giniite microcrystals is disclosed by various characterization techniques. It was revealed that [Bmim][H₂PO₄] played an important role in stabilizing the {111} facets of ferric giniite crystals, leading to the different morphologies in the presence of ionic‐liquid precursors with different compositions. Furthermore, since these ferric giniite crystals were characterized by different facets, they could serve as model Fenton‐like catalysts to uncover the correlation between the surface and the catalytic performance for the photodegradation of organic dyes under visible‐light irradiation. Our measurements indicate that the photocatalytic activity of as‐prepared Fenton‐like catalysts is highly dependent on the exposed facets, and the surface area has essentially no obvious effect on the photocatalytic degradation of organic dyes in the present study. It is highly expected that these findings are useful in understanding the photocatalytic activity of Fenton‐like catalysts with different morphologies, and suggest a promising new strategy for crystal‐facet engineering of photocatalysts for wastewater treatment based on heterogeneous Fenton‐like process.