Synergy of low-energy {101} and high-energy {001} TiO₂ crystal facets for enhanced photocatalysis

Step edge structures on the anatase TiO2 (001) surface studied by atomic-resolution TEM and STM

Atomic arrangements in oxide surfaces can be uncovered by combining side view imaging using transmission electron microscopy and top view imaging using scanning tunnelling microscopy.

Phase and morphology transformation from assembled hexagonal HTiOF3 prisms to {001} faceted TiO2 nanosheets

A three-stage synthesis method was developed for the preparation of TiO₂ nanosheets with exposed {001} facets by a solvothermal process.

Highly efficient catalytic activity for the hydrogen evolution reaction on pristine and monovacancy defected WP systems: a first-principles investigation

A deep understanding of HER catalytic activity of tungsten phosphide at the atomic level and its effective improvement by introducing a monovacancy.

Chiral α-hydroxy acid-coadsorbed TiO2 photocatalysts for asymmetric induction in hydrogenation of aromatic ketones

In enantioselective photohydrogenation of aromatic ketones on TiO₂, the enantioselectivity is strongly affected by not only chiral reagents but also the crystalline phase, surface structure, and morphology of TiO₂.

Supercritical CO2-assisted deposition of NiO on (101)-anatase-TiO2 for efficient facet engineered photocatalysts

Deposition of NiO on the (101) facet of anatase nanocrystals by the SFCD route yields nanocomposites more efficient than pure anatase TiO₂ for the photodecomposition of both anionic and cationic dyes.

Effect of electron transfer on the photocatalytic hydrogen evolution efficiency of faceted TiO2/CdSe QDs under visible light

Higher photocatalytic hydrogen evolution efficiency of {001}-TiO₂/CdSe QDs was obtained because of their faster electron transfer rate, compared to that of {101}-TiO₂/CdSe QDs.

Modulating the External Facets of Functional Nanocrystals Enabled by Two-Dimensional Oxide Crystal Templates

The nature of the external crystal facets is critical to control the (photo)catalytic properties. Two-dimensional materials can expose only one type of crystal facet among zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) materials. Controllable tuning of the nature of the external crystal facets of 2D materials is highly desirable but very challenging. Here, we show that 2D particles with the desirable crystal facet for high-performance photocatalytic H₂ generation can be obtained by using 2D metal oxide materials (i.e., nanosheets of Ca₂Nb₃O₁₀ and Ti_0.87O₂) as templates taking consideration of the crystal configuration of 2D oxide and target compounds. We demonstrate that anatase TiO₂ crystals grown on different nanosheet templates exhibit variations in photocatalytic hydrogen production rates from aqueous phase methanol solutions (6.7%), which can be attributed to the nature of the main crystal facet exposed. In view of the large number of 2D materials that have already been synthesized, this work offers a key to design and synthesize nanoparticles with the desirable specific external crystal facet for (photo)catalysis application.

Efficient photoelectrochemical water splitting on ultrasmall defect-rich TaOx nanoclusters enhanced by size-selected Pt nanocluster promoters

Formation of nanoclusters has attracted a lot of attention in recent years because of their distinct properties from isolated atoms and bulk solids. Here, we focus on the catalytic properties of supported transition metal oxide nanoclusters, such as TaO₂, with a well-defined size distribution below 10 nm. We show that their catalytic performance can be greatly enhanced by introducing a reaction promoter such as Pt. Different combinations of precisely size-selected, defect-rich TaO_x and Pt nanoclusters are produced by a gas-phase aggregation technique in a special DC magnetron sputtering system. Argon flow rate and aggregation length are carefully optimized to control the sizes of these ultrasmall TaO_x and Pt nanoclusters by using a quadrupole mass filter, and TEM studies reveal the different crystalline nature of TaO_x (amorphous) and Pt (crystalline) nanoclusters. We have further demonstrated the size-dependent photoanode activity of (TaO_x, Pt) nanocluster systems in a photoelectrochemical water splitting reaction, where the Pt nanocluster promoters are found to provide a significant enhancement in the photocurrent density, approximately tripled that was observed from just TaO_x nanocluster catalysts alone. The photocurrent density and photoconversion efficiency tend to reduce when Pt nanoclusters become overpopulated due to blocking of the photosensitive TaO_x surface. Reducing the Pt nanocluster size resolves this problem by incorporating a greater number of smaller nanocluster promoters without blocking TaO_x, which leads to further enhancement in the photocurrent density. The enhanced photocatalytic activity is attributed to synergetic effects introduced by the Pt nanoclusters that act as temporary charge storage sites to facilitate effective separation of a large number of electron-hole pairs, generated from a large number of active sites on the defect-rich amorphous TaO_x nanoclusters upon illumination.

Face-to-Face Interfacial Assembly of Ultrathin g-C3N4 and Anatase TiO2 Nanosheets for Enhanced Solar Photocatalytic Activity

Face-to-face interfacial assembly of TiO₂-g-C₃N₄ hybrid (2D TCN-A) is developed by surfactant-assisted hydrothermal treatment forming a sandwich structure of anatase TiO₂ nanosheets (TiO₂-A, 5-6 monolayers) and g-C₃N₄ nanosheets (∼3 monolayers). Post air-annealing is found effective for insertion of oxygen to the hybrid, which remedies the oxygen vacancies of TiO₂ (B) nanosheets and converts it to anatase nanosheets. The enhanced light adsorption, increased donor density, and prolonged life of charge carries are achieved by variation of bandgap and the formation of heterojuction between the two kinds of nanosheets, facilitating separation and transfer of charge carriers. The 2D TCN-A-70 nanosheets show a high photodegradation rate of methyl orange (k_app ≈ 0.189 min^-1) and photocatalytic evolution rate of hydrogen (18200 μmol g^-1 h^-1). This 2D nanosheets hybrid is potentially useful in alleviating environmental and energy issues.

Citrate-Capped Hybrid Au-TiO2 Nanomaterial for Facile and Enhanced Electrochemical Hydrazine Oxidation

Effective and facile electrochemical oxidation of chemical fuels is pivotal for fuel cell applications. Herein, we report the electrocatalytic oxidation of hydrazine on a citrate-capped Au-TiO₂-modified glassy carbon electrode, which follows two different oxidation paths. These two pathways of hydrazine oxidation are ascribed to occur on Au and the activated TiO₂ surface of the Au-TiO₂ hybrid electrocatalyst. This activation was achieved through molecular capping of the Au-TiO₂ surface by citrate, which leads to favorable hydrazine oxidation with a lower Tafel slope compared to that of the clean surface of the respective materials, that is, Au and TiO₂.

Achieving High Current Density of Perovskite Solar Cells by Modulating the Dominated Facets of Room-Temperature DC Magnetron Sputtered TiO2 Electron Extraction Layer

The short circuit current density of perovskite solar cell (PSC) was boosted by modulating the dominated plane facets of TiO₂ electron transport layer (ETL). Under optimized condition, TiO₂ with dominant {001} facets showed (i) low incident light loss, (ii) highly smooth surface and excellent wettability for precursor solution, (iii) efficient electron extraction, and (iv) high conductivity in perovskite photovoltaic application. A current density of 24.19 mA cm^-2 was achieved as a value near the maximum limit. The power conversion efficiency was improved to 17.25%, which was the record value of PSCs with DC magnetron sputtered carrier transport layer. What is more, the room-temperature process had a great significance for the cost reduction and flexible application of PSCs.

Same titanium glycolate precursor but different products: successful synthesis of twinned anatase TiO2 nanocrystals with excellent solar photocatalytic hydrogen evolution capability

Exploiting a synthesis protocol to tailor TiO₂ with a unique morphology and crystal phase has received considerable interest in the energy and environmental fields.

Controlling the shape of anatase nanocrystals for enhanced photocatalytic reduction of CO2 to methanol

Anatase titania with tunable shapes has been synthesized by a facile hydrothermal method using peroxotitanic acid as a precursor. The results of investigations showed that the photocatalytic activity in the reduction of CO₂ to methanol increased with an increase in the percentage of {001} surfaces.

Modulating the photocatalytic redox preferences between anatase TiO2{001} and {101} surfaces

The surface protonation/deprotonation can change the photocatalytic redox preferences of TiO₂{001} and {101} surfaces.

Ultrathin TiO2nanosheets synthesized using a high pressure solvothermal method and the enhanced photoresponse performance of CH3NH3PbI3–TiO2composite films

Highly crystalline ultrathin (2–3 nm) TiO₂nanosheets are synthesized using a high pressure solvothermal method. The perovskite–TiO₂films exhibit strikingly enhanced photoresponse performance.

Facet-Engineered Surface and Interface Design of Photocatalytic Materials

The facet-engineered surface and interface design for photocatalytic materials has been proven as a versatile approach to enhance their photocatalytic performance. This review article encompasses some recent advances in the facet engineering that has been performed to control the surface of mono-component semiconductor systems and to design the surface and interface structures of multi-component heterostructures toward photocatalytic applications. The review begins with some key points which should receive attention in the facet engineering on photocatalytic materials. We then discuss the synthetic approaches to achieve the facet control associated with the surface and interface design. In the following section, the facet-engineered surface design on mono-component photocatalytic materials is introduced, which forms a basis for the discussion on more complex systems. Subsequently, we elucidate the facet-engineered surface and interface design of multi-component photocatalytic materials. Finally, the existing challenges and future prospects are discussed.

Preparation of TiO₂-Decorated Boron Particles by Wet Ball Milling and their Photoelectrochemical Hydrogen and Oxygen Evolution Reactions

TiO₂-coated boron particles were prepared by a wet ball milling method, with the particle size distribution and average particle size being easily controlled by varying the milling operation time. Based on the results from X-ray photoelectron spectroscopy, transmission electron microscopy, energy dispersive X-ray analysis, and Fourier transform infrared spectroscopy, it was confirmed that the initial oxide layer on the boron particles surface was removed by the wet milling process, and that a new B-O-Ti bond was formed on the boron surface. The uniform TiO₂ layer on the 150 nm boron particles was estimated to be 10 nm thick. Based on linear sweep voltammetry, cyclic voltammetry, current-time amperometry, and electrochemical impedance analyses, the potential for the application of TiO₂-coated boron particles as a photoelectrochemical catalyst was demonstrated. A current of 250 μA was obtained at a potential of 0.5 V for hydrogen evolution, with an onset potential near to 0.0 V. Finally, a current of 220 μA was obtained at a potential of 1.0 V for oxygen evolution.

Recent progress of arsenic adsorption on TiO2 in the presence of coexisting ions: A review

Arsenic (As)-contaminated wastewater and groundwater pose a pressing environmental issue and worldwide concern. Adsorption of As using TiO₂ materials, in combination with filtration, introduces a promising technology for the treatment of As-contaminated water. This review presents an overview on the recent progress of the application of TiO₂ for removal of As from wastewater and groundwater. The main focus is on the following three pressing issues that limit the field applications of TiO₂ for As removal: coexisting ions, simulation of breakthrough curves, and regeneration and reuse of spent TiO₂ materials. We first examined how the coexisting ions in water, especially high concentrations of cations in industrial wastewater, affect the efficacy of As removal using the TiO₂ materials. We then discussed As breakthrough curves and the effect of compounded ions on the breakthrough curves. We successfully simulated the breakthrough curves by PHREEQC after integrating the CD-MUSIC model. We further discussed challenges facing the regeneration and reuse of TiO₂ media for practical applications. We offer our perspectives on remaining issues and future research needs.

Enhancement in Rate of Photocatalysis Upon Catalyst Recycling

Recyclability is an important aspect for heterogeneous photo-catalysts. Ease of recovery and stability of the photo-catalyst in terms of efficiency over the number of cycles are highly desired and in fact it is ideal if the efficiency is constant and it should not decrease marginally with each cycle. Presented here is a seminal observation in which the photocatalytic activity is shown to improve with increasing number of catalytic cycles (it is 1.7 times better after the 1^st cycle and 3.1 times better after the 2^nd cycle). Specifically, nanorods of pure TiO₂ and TiO₂ doped with controlled amount of tungsten have been used to degrade two model pollutants: Phenol and Rhodamine B under exclusive visible light illumination. It was found that, in case of 1 mol.% W incorporation, rate of photocatalysis and also the range of visible light absorption of the photocatalyst increased after the photocatalysis as compared to before photocatalysis. This aspect is unique for doped TiO₂ and hence provides an intriguing way to mitigate low photoactivity.

Facet-dependent trapping and dynamics of excess electrons at anatase TiO2 surfaces and aqueous interfaces

Excess electrons from intrinsic defects, dopants and photoexcitation play a key role in many of the properties of TiO2. Understanding their behaviour is important for improving the performance of TiO2 in energy-related applications. We focus on anatase, the TiO2 polymorph most relevant in photocatalysis and solar energy conversion. Using first-principles simulations, we investigate the states and dynamics of excess electrons from different donors near the most common anatase (101) and (001) surfaces and aqueous interfaces. We find that the behaviour of excess electrons depends strongly on the exposed anatase surface, the environment and the character of the electron donor. Whereas no electron trapping is observed on the (101) surface in vacuo, an excess electron at the aqueous (101) interface can trigger water dissociation and become trapped into a stable surface Ti(3+)-bridging OH complex. By contrast, electrons avoid the (001) surface, indicating that oxidation reactions are favoured on this surface. Our results provide a bridge between surface science experiments and observations of crystal-face-dependent photocatalysis on anatase, and support the idea that optimization of the ratio between {101} and {001} facets could provide a way to enhance the photocatalytic activity of this material.

Facet-controlled anatase TiO2 nanoparticles through various fluorine sources for superior photocatalytic activity

Reactive surface-exposed anatase TiO2 (a-TiO2) is highly desirable for applications requiring superior photocatalytic activity. In order to obtain a favorable surface, morphology control of the a-TiO2 using capping agents has been widely investigated. Herein, we systematically study the effects of different F sources (HF, TiF4, and NH4F) as the capping agent on the morphology control and photocatalytic activities of a-TiO2 in a hydrothermal process. When either HF or TiF4 was added, large truncated bipyramids formed with the photocatalytically active {001} facet, whereas the NH4F was not effective for facet control, yielding nanospheres similar to the pure a-TiO2. The morphology changes were related to the decomposition behaviors of the F sources in the solvent material: HF and TiF4 decomposed and supplied F(-) ions before a-TiO2 nucleation, which changed the nucleation rate and growth direction, leading to the resultant a-TiO2 morphology. On the other hand, NH4F supplied F(-) ions after a-TiO2 nucleation and could not change the growth behavior. In terms of the photocatalytic effect, the HF- and TiF4-treated a-TiO2 effectively decomposed ∼90% and ∼80% of methylene blue, respectively, in 1 h, while ∼60% was decomposed for the NH4F-treated a-TiO2. Note that pure a-TiO2 photocatalytically decomposed only ∼10% of methylene blue over the same time. These results pave the way to precise control of the facet of TiO2 through using different capping agents.

Efficiently Enhancing Visible Light Photocatalytic Activity of Faceted TiO2 Nanocrystals by Synergistic Effects of Core-Shell Structured Au@CdS Nanoparticles and Their Selective Deposition

Integrating wide bandgap semiconductor photocatalysts with visible-light-active inorganic nanoparticles (such as Au and CdS) as sensitizers is one of the most efficient methods to improve their photocatalytic activity in the visible light region. However, as for all such composite photocatalysts, a rational design and precise control over their architecture is often required to achieve optimal performance. Herein, a new TiO2-based ternary composite photocatalyst with superior visible light activity was designed and synthesized. In this composite photocatalyst, the location of the visible light sensitizers was engineered according to the intrinsic facet-induced effect of well-faceted TiO2 nanocrystals on the spatial separation of photogenerated carriers. Experimentally, core-shell structured Au@CdS nanoparticles acting as visible light sensitizers were selectively deposited onto photoreductive {101} facets of well-faceted anatase TiO2 nanocrystals through a two-step in situ photodeposition route. Because the combination of Au@CdS and specific {101} facets of TiO2 nanocrystals facilitates the transport of charges photogenerated under visible light irradiation, this well-designed ternary composite photocatalyst exhibited superior activity in visible-light-driven photocatalytic H2 evolution, as expected.

Unraveling the Multiple Effects Originating the Increased Oxidative Photoactivity of {001}-Facet Enriched Anatase TiO2

Crystal shape control on a series of anatase photocatalysts was achieved by varying the amount of HF employed as a capping agent in their hydrothermal synthesis. A systematic comparison between their physicochemical properties, determined by several complementary surface and bulk techniques before and after thermal treatment at 500 °C, allowed one to discern the influence of the relative amount of exposed {001} crystal facets among a series of effects simultaneously affecting their oxidative photocatalytic activity. The results of both formic acid and terephthalic acid photo-oxidation test reactions point to the primary role played by calcination in making {001} facets effectively photoactive. Annealing not only removes most of the residual fluorine capping agent from the photocatalyst surface, thus favoring substrate adsorption, but also produces morphological modifications to a crystal packing that makes accessible a larger portion of surface {001} facets due to the unpiling of platelike crystals. The photocatalyst bearing the highest amount of exposed {001} facets (60%) shows the highest photoactivity in both the direct and the (•)OH-radical-mediated photocatalytic test reaction.

Direct Observation of Charge Separation on Anatase TiO2 Crystals with Selectively Etched {001} Facets

Synchronous illumination X-ray photoelectron spectroscopy (SIXPS) was employed for the first time to directly identify the photogenerated charge separation and transfer on anatase TiO2 single-crystals with selectively etched {001} facets. More specifically, for the TiO2 crystals with intact {001} and {101} facets, most of photogenerated charge carriers rapidly recombined, and no evident electron-hole separation was detected. With selectively etching on {001} facets, high efficient charge separation via hole transfer to titanium and electron to oxygen was clearly observed. However, when the {001} facets were completely etched into a hollow structure, the recombination for photogenerated electron-hole pairs would dominate again. These demonstrations clearly reveal that the appropriate corrosion on {001} facets could facilitate more efficient electron-hole separation and transfer. As expected, the optimized TiO2 microcrystals with etched {001} facets could achieve a hydrogen generation rate of 74.3 μmol/h/g, which is nearly 7 times higher than the intact-TiO2 crystals.

Charge Separation in TiO2/BDD Heterojunction Thin Film for Enhanced Photoelectrochemical Performance

Semiconductor photocatalysis driven by electron/hole has begun a new era in the field of solar energy conversion and storage. Here we report the fabrication and optimization of TiO2/BDD p-n heterojunction photoelectrode using p-type boron doped diamond (BDD) and n-type TiO2 which shows enhanced photoelectrochemical activity. A p-type BDD was first deposited on Si substrate by microwave plasma chemical vapor deposition (MPCVD) method and then n-type TiO2 was sputter coated on top of BDD grains for different durations. The microstructural studies reveal a uniform disposition of anatase TiO2 and its thickness can be tuned by varying the sputtering time. The formation of p-n heterojunction was confirmed through I-V measurement. A remarkable rectification property of 63773 at 5 V with very small leakage current indicates achieving a superior, uniform and precise p-n junction at TiO2 sputtering time of 90 min. This suitably formed p-n heterojunction electrode is found to show 1.6 fold higher photoelectrochemical activity than bare n-type TiO2 electrode at an applied potential of +1.5 V vs SHE. The enhanced photoelectrochemical performance of this TiO2/BDD electrode is ascribed to the injection of hole from p-type BDD to n-type TiO2, which increases carrier separation and thereby enhances the photoelectrochemical performance.

Synthesis of {110}-faceted rutile TiO2 nanocrystals from tetratitanate nanoribbons for improving dye-sensitized solar cell performance

{110}-faceted rutile TiO₂ nanocrystals with nanorods and nanoflower morphologies were synthesized through simple hydrothermal treatment of a tetratitanate nanoribbons precursor.

A sol-hydrothermal route to truncated tetragonal bipyramid nanocrystals and hierarchical hollow microspheres of anatase TiO2 for application in dye-sensitized solar cells

DSSCs based on TiO₂ bilayer photoanodes made of truncated tetragonal bipyramids and hierarchical hollow microspheres delivered a PCE of 9.06%.

Light-induced spatial separation of charges toward different crystal facets of square-like WO3

Light-induced preferential migration of electrons and holes to the minor (200) and (020) facets and the dominant (002) facets of square-like WO₃, respectively, resulted in the square-like WO₃ nanoplates with Pt loaded mainly on dominant (002) facets shows higher photocatalytic activity than that Pt loaded on the minor facets.

Etching treatment of vertical WO3 nanoplates as a photoanode for enhanced photoelectrochemical performance

An innovative etching method was developed to increase surface voids, active crystal facets and surface groups, which led to improved photocurrent performance.

Synthesis, characterization and enhanced photocatalytic CO2 reduction activity of graphene supported TiO2 nanocrystals with coexposed {001} and {101} facets

Enhanced CO₂ photoreduction over graphene supported TiO₂ nanocrystals with co-exposed {001} and {101} facets.