Solvothermal Synthesis and Photoreactivity of Anatase TiO2 Nanosheets with Dominant {001} Facets

The inhibition mechanism of N2O generation in NH3-SCR process by water vapor

N₂O is a typical by-product in the NH3-SCR process, which requires urgent resolution due to its negative economic and environmental impacts. This study investigates in detail the mechanism of N2O generation on the surface of the Mn-Ce/TiO2 catalyst (Mn-Ce/TiO2-ZS) with anatase {001} facets preferentially exposed. The deep oxidation of NH3 and *NH2 capture of NO via O2 were proved to be the dominant N2O generation pathways. The production of N2O was remarkably reduced by the introduction of a low percentage of water vapor (H2O). The results revealed that low percentage of H2O was capable of enhancing the acid sites on the catalyst surface and facilitating the generation of active hydroxyl species. These active species inhibited the deep dehydrogenation of ammonia and the disintegration of nitrate species on the catalyst surface, as well as suppressing the generation of N2O.

Combined Role of {001} Facet-Enriched Morphology and Gold Nanoparticle Deposition on Anatase TiO2 Photoactivity

The interplay on anatase TiO2 photoactivity between particle morphology and gold nanoparticles (NPs) deposition, via either deposition-precipitation (DP) or photodeposition (P), is here investigated by evaluating the photoactivity of Au modified anatase (Au/TiO2) nanocrystals with either a pseudospherical shape or a nanosheet structure in both reduction and oxidation test reactions. The presence of Au NPs on the anatase surface only slightly affects its photoactivity in Cr(VI) reduction, which is kinetically limited by the anodic half-reaction, whereas a larger exposure of highly oxidant {001} facets is beneficial for overcoming this rate-determining step. In the photocatalytic oxidation of both formic acid, proceeding through a direct mechanism, and rhodamine B (RhB) on surface fluorinated photocatalysts, occurring through a hydroxyl-radical-mediated mechanism, the presence of gold NPs produces a significant photoactivity increase only with spherically shaped photocatalysts, mainly exposing {101} facets. These results are rationalized in light of the preferential migration of photogenerated, oppositely charged carriers toward different crystal facets. In fact, when the Au/TiO2 material mainly exposes the more oxidant {001} facets, where photoproduced holes preferentially migrate, recombination between these latter and the electrons captured by Au NPs is favored. Instead, Au NPs on {101} facets efficiently capture photopromoted electrons, preferentially migrating toward such facets with a consequent improvement of photoproduced charge separation.

Facet-Engineered BiVO4 Photocatalysts for Water Oxidation: Lifetime Gain Versus Energetic Loss

A limiting factor to the efficiency of water Oxygen Evolution Reaction (OER) in metal oxide nanoparticle photocatalysts is the rapid recombination of holes and electrons. Facet-engineering can effectively improve charge separation and, consequently, OER efficiency. However, the kinetics behind this improvement remain poorly understood. This study utilizes photoinduced absorption spectroscopy to investigate the charge yield and kinetics in facet-engineered BiVO4 (F-BiVO4) compared to a non-faceted sample (NF-BiVO4) under operando conditions. A significant influence of preillumination on hole accumulation is observed, linked to the saturation and, thus, passivation of deep and inactive hole traps on the BiVO4 surface. In DI-water, F-BiVO4 shows a 10-fold increase in charge accumulation (∼5 mΔOD) compared to NF-BiVO4 (∼0.5 mΔOD), indicating improved charge separation and stabilization. With the addition of Fe(NO3)3, an efficient electron acceptor, F-BiVO4 demonstrates a 30-fold increase in the accumulation of long-lived holes (∼45 mΔOD), compared to NF-BiVO4 (∼1.5 mΔOD) and an increased half-time, from 2 to 10 s. Based on a simple kinetic model, this increase in hole accumulation suggests that facet-engineering causes at least a 50-100 meV increase in band bending in BiVO4 particles, thereby stabilizing surface holes. This energetic stabilization/loss results in a retardation of OER relative to NF-BiVO4. This slower catalysis is, however, offset by the observed increase in density and lifetime of photoaccumulated holes. Overall, this work quantifies how surface faceting can impact the kinetics of long-lived charge accumulation on metal oxide photocatalysts, highlighting the trade-off between lifetime gain and energetic loss critical to optimizing photocatalytic efficiency.

Real-Time Multiscale Imaging of Heterogeneous Multistage Reactions: Insights into Nanoscale TiO2 Synthesis

Hydrothermal methods are widely used to synthesize functional inorganic materials. The interplay between the reactive species, solution chemistry, and the nanoscale product makes it challenging to control the reaction pathway to achieve a uniform product. Here, we resolve the heterogeneity that arises during hydrothermal synthesis across different length scales. We combine spatially resolved in situ X-ray pair distribution function (PDF) and small-angle X-ray scattering analysis, which are sensitive to structure on the atomic and nanoscale, with a novel time-lapse optical imaging strategy that reveals heterogeneity and phase separations across the entire reaction. For TiO2 synthesis via hydrothermal hydrolysis of TiCl4, we identify multiple cycles of TiO2 formation and separation that contribute to nonuniformity in the polymorphic product. The PDF data show that the characteristics of TiO2 formed during each formation-separation cycle differ, contributing to the ongoing challenge of precisely identifying reaction controls. The imaging strategy pioneered here provides an efficient in situ means to systematically compare how the reaction evolves under different chemical conditions, thereby advancing our understanding of functional inorganic material synthesis.

Mesoporous TiO2 Single-Crystal Particles from Controlled Crystallization-Driven Mono-Micelle Assembly as an Efficient Photocatalyst

Mesoporous materials with crystalline frameworks have been widely explored in many fields due to their unique structure and crystalline feature, but accurate manipulations over crystalline scaffolds, mainly composed of uncontrolled polymorphs, are still lacking. Herein, we explored a controlled crystallization-driven monomicelle assembly approach to construct a type of uniform mesoporous TiO2 particles with atomically aligned single-crystal frameworks. The resultant mesoporous TiO2 single-crystal particles possess an angular shape ∼80 nm in diameter, good mesoporosity (a high surface area of 112 m2 g-1 and a mean pore size at 8.3 nm), and highly oriented anatase frameworks. By adjusting the evaporation rate during assembly, such a facile solution-processed strategy further enables the regulation of the particle size and mesopore size without the destruction of the oriented crystallites. Such a combination of ordered mesoporosity and crystalline orientation provides both effective mass and charge transportation, leading to a significant increase in the hydrogen generation rate. A maximum hydrogen evolution rate of 12.5 mmol g-1 h-1 can be realized, along with great stability under solar light. Our study is envisaged to extend the possibility of mesoporous single crystal growth to a range of functional ceramics and semiconductors toward advanced applications.

Structure, Properties, Preparation, and Application of Layered Titanates

As a typical cation-exchangeable layered compound, layered titanate has a unique open layered structure. Its excellent physical and chemical properties allow its wide use in the energy, environmental protection, electronics, biology, and other fields. This paper reviews the recent progress in the research on the structure, synthesis, properties, and application of layered titanates. Various reactivities, as well as the advantages and disadvantages, of different synthetic methods are discussed. The reaction mechanism and influencing factors of the ion exchange reaction, intercalation reaction, and exfoliation reaction are analyzed. The latest research progress on layered titanates and their modified products in the fields of photocatalysis, adsorption, electrochemistry, and other applications is summarized. Finally, the future development of layered titanate and its exfoliated product two-dimensional nanosheets is proposed.

Insight into (Electro)magnetic Interactions within Facet-Engineered BaFe12O19/TiO2 Magnetic Photocatalysts

A series of facet-engineered TiO2/BaFe12O19 composites were synthesized through hydrothermal growth of both phases and subsequent deposition of the different, faceted TiO2 nanoparticles onto BaFe12O19 microplates. The well-defined geometry of the composite and uniaxial magnetic anisotropy of the ferrite allowed alternate interfaces between both phases and fixed the orientation between the TiO2 crystal structure and the remanent magnetic field within BaFe12O19. The morphology and crystal structure of the composites were confirmed by a combination of scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses together with the detailed study of BaFe12O19 electronic and magnetic properties. The photocatalytic activity and magnetic field effect were studied in the reaction of phenol degradation for TiO2/BaFe12O19 and composites of BaFe12O19 covered with a SiO2 protective layer and TiO2. The observed differences in phenol degradation are associated with electron transfer and the contribution of the magnetic field. All obtained magnetic composite materials can be easily separated in an external magnetic field, with efficiencies exceeding 95%, and recycled without significant loss of photocatalytic activity. The highest activity was observed for the composite of BaFe12O19 with TiO2 exposing {1 0 1} facets. However, to prevent electron transfer within the composite structure, this photocatalyst material was additionally coated with a protective SiO2 layer. Furthermore, TiO2 exposing {1 0 0} facets exhibited significant synergy with the BaFe12O19 magnetic field, leading to 2 times higher photocatalytic activity when ferrite was magnetized before the process. The photoluminescence emission study suggests that for this particular combination, the built-in magnetic field of the ferrite suppressed the recombination of the photogenerated charge carriers. Ultimately, possible effects of complex electro/magnetic interactions within the magnetic photocatalyst are shown and discussed for the first time, including the anisotropic properties of both phases.

Strategies for improving the stability of perovskite for photocatalysis: A review of recent progress

Photocatalysis is currently a hot research field, which provides promising processes to produce green energy sources and other useful products, thus eventually benefiting carbon emission reduction and leading to a low-carbon future. The development and application of stable and efficient photocatalytic materials is one of the main technical bottlenecks in the field of photocatalysis. Perovskite has excellent performance in the fields of photocatalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction reaction (CO2RR), organic synthesis and pollutant degradation due to its unique structure, flexibility and resulting excellent photoelectric and catalytic properties. The stability problems caused by perovskite's susceptibility to environmental influences hinder its further application in the field of photocatalysis. Therefore, this paper innovatively summarizes and analyzes the existing methods and strategies to improve the stability of perovskite in the field of photocatalysis. Specifically, (i) component engineering, (ii) morphological control, (iii) hybridization and encapsulation are thought to improve the stability of perovskites while improving photocatalytic efficiency. Finally, the challenges and prospects of perovskite photocatalysts are discussed, which provides constructive thinking for the potential application of perovskite photocatalysts.

Facile Hydrothermal Synthesis of BiVO4/MWCNTs Nanocomposites and Their Influences on the Biofilm Formation of Multidrug Resistance Streptococcus mutans and Proteus mirabilis

This study utilized a simple hydrothermal technique to prepare pure BiVO4 and tightly bound BiVO4/multiwalled carbon nanotubes (MWCNTs) nanocomposite materials. The surfactant was employed to control the growth, size, and assembly of BiVO4 and the nanocomposite. Various techniques including X-ray diffraction (XRD), Ultraviolet-visible (UV-vis), photoluminescence (PL), Raman, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) were utilized to analyze and characterize BiVO4 and the BiVO4/MWCNTs nanocomposite. Through XRD analysis, it was found that the carbon nanotubes were effectively embedded within the lattice of BiVO4 without generating any separate impurity phase and had no influence on the BiVO4 monoclinic structure. TEM images confirmed the presence of MWCNTs within BiVO4. Furthermore, adding MWCNTs in the BiVO4/MWCNTs nanocomposite resulted in an effective charge transfer transition and improved carrier separation, as evidenced by PL analysis. The introduction of MWCNTs also led to a significant reduction in the optical band gap due to quantum effects. Finally, the antibacterial activity of pure BiVO4 and the BiVO4/MWCNTs nanocomposite was assessed by exposing Proteus mirabilis and Streptococcus mutans to these materials. Biofilm inhibition and antibiofilm activity were measured using a crystal violet assay and a FilmTracer LIVE/DEAD Biofilm Viability Kit. The results demonstrated that pure BiVO4 and BiVO4/MWCNTs effectively inhibited biofilm formation. In conclusion, both pure BiVO4 and BiVO4/MWCNTs are promising materials for inhibiting the bacterial biofilm during bacterial infections.

Preparation and performance of aerogel-based BiOI/TiO2 heterojunction photoelectrocatalytic electrodes

TiO2/BiOI/CA electrodes with improved conductivity, reduced photoelectron-hole recombination rates, and increased reaction sites based on p-n type heterojunctions were constructed on carbon aerogels (CA) as photoelectrode substrates. Characterization based on ultraviolet-visible diffuse reflectance spectroscopy, photocurrent measurements, and impedance analysis showed that the TiO2/BiOI/CA photoelectrode with a Ti/Bi mole ratio of 0.4 exhibited the best visible light absorption, lowest photogenerated electron-hole pair recombination rate, and strongest photocatalytic degradation, with 90.4% degradation of phenol under 120 min of light. Moreover, the stability of this electrode remained at a high level. This was mainly because the energy levels of TiO2 and BiOI matched each other and the p-n heterojunction formed adjusted the energy band structure of the composite material, widened the electron transfer path, formed an internal electric field between the phase interfaces, had a higher electron transfer rate, and reduced the photogenerated electron-hole recombination rate. Since ˙OH and ˙O2- are the main active substances in the degradation of phenol, the TiO2/BiOI/CA photoelectrodes had higher degradation efficiency than BiOI/CA electrodes. This study provides a unique concept for the treatment of organic pollutant wastewater and electrode design for photoelectrocatalysis.

Electrospun silk nanofiber loaded with Ag-doped TiO2 with high-reactive facet as multifunctional air filter

Particulate matter (PM) and volatile organic compounds (VOCs) are air pollution that can cause high risk to public health. To protect individuals from air pollution exposure, fibrous filters have been widely employed. In this work, we develop silk nanofibers, which are loaded with Ag-doped TiO2 nanoparticles with exposed (001) (assigned as Ag-TiO2-silk), via electrospinning method and utilized them as multifunctional air filters that can efficiently reduce PM2.5, organic pollutants and microbials. The results showed that Ag-TiO2-silk with a loading of 1 wt% (1%Ag-TiO2-silk) exhibited the best performance among various different Ag-doped samples, as it performed the best as an air filter, which had the highest PM2.5 removal efficiency of 99.04 ± 1.70% with low pressure drop of 34.3 Pa, and also exhibited the highest photodegradation efficiency of formaldehyde. In addition, the Ag-TiO2-silk demonstrated antibacterial activity. These properties make silk composite nanofibers attractive for multifunctional and environmentally-friendly air filter application.

Facet Engineering of Advanced Electrocatalysts Toward Hydrogen/Oxygen Evolution Reactions

The crystal facets featured with facet-dependent physical and chemical properties can exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) attributed to their anisotropy. The highly active exposed crystal facets enable increased mass activity of active sites, lower reaction energy barriers, and enhanced catalytic reaction rates for HER and OER. The formation mechanism and control strategy of the crystal facet, significant contributions as well as challenges and perspectives of facet-engineered catalysts for HER and OER are provided.

The electrocatalytic water splitting technology can generate high-purity hydrogen without emitting carbon dioxide, which is in favor of relieving environmental pollution and energy crisis and achieving carbon neutrality. Electrocatalysts can effectively reduce the reaction energy barrier and increase the reaction efficiency. Facet engineering is considered as a promising strategy in controlling the ratio of desired crystal planes on the surface. Owing to the anisotropy, crystal planes with different orientations usually feature facet-dependent physical and chemical properties, leading to differences in the adsorption energies of oxygen or hydrogen intermediates, and thus exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, a brief introduction of the basic concepts, fundamental understanding of the reaction mechanisms as well as key evaluating parameters for both HER and OER are provided. The formation mechanisms of the crystal facets are comprehensively overviewed aiming to give scientific theory guides to realize dominant crystal planes. Subsequently, three strategies of selective capping agent, selective etching agent, and coordination modulation to tune crystal planes are comprehensively summarized. Then, we present an overview of significant contributions of facet-engineered catalysts toward HER, OER, and overall water splitting. In particular, we highlight that density functional theory calculations play an indispensable role in unveiling the structure–activity correlation between the crystal plane and catalytic activity. Finally, the remaining challenges in facet-engineered catalysts for HER and OER are provided and future prospects for designing advanced facet-engineered electrocatalysts are discussed.

Phase transition of individual anatase TiO2 microcrystals with large percentage of (001) facets: a Raman mapping and SEM study

TiO₂ has been extensively studied in many fields including photocatalysis, electrochemistry, optics, etc. Understanding the mechanism of the anatase-rutile phase transition (ART) process is critical for the design of TiO₂-based high-activity photocatalysts and tuning its properties for other applications. In this work, the ART process using individual anatase micro-particles with a large percentage of (001) facets was monitored and studied. Phase concentration evolution obtained via Raman microscopy was correlated with the morphological evolution observed in scanning electron microscope (SEM) images. The ART of anatase microcrystals is dominated by surface nucleation and growth, but the ART processes of individual anatase particles are distinctive and depend on the various rutile nucleation sites. Two types of transformation pathways are observed. In one type of ART pathway, the rutile phase nucleated at a corner of an anatase microcrystal and grew in one direction along the edge of the crystal firstly followed by propagation over the rest of the microcrystal in the orthogonal direction on the surface and to the bulk of the crystal. The kinetics of the ART follows the first-order model with two distinct rate constants. The fast reaction rate is from the surface nucleation and growth, and the slow rate is from the bulk nucleation and growth. In the other type of ART pathway, multiple rutile nucleation sites formed simultaneously on different edges and corners of the microcrystal. The rutile phase spread over the whole crystal from these nucleation sites with a small contribution of bulk nucleation. Our study on the ART of individual micro-sized crystals bridges the material gap between bulk crystals and nano-sized TiO₂ particles. The anatase/rutile co-existing particle will provide a perfect platform to study the synergistic effect between the anatase phase and the rutile phase in their catalytic performances.

The Photocatalytic Conversion of Carbon Dioxide to Fuels Using Titanium Dioxide Nanosheets/Graphene Oxide Heterostructure as Photocatalyst

Carbon dioxide (CO₂) photoreduction to high-value products is a technique for dealing with CO₂ emissions. The method involves the molecular transformation of CO₂ to hydrocarbon and alcohol-type chemicals, such as methane and methanol, relying on a photocatalyst, such as titanium dioxide (TiO₂). In this research, TiO₂ nanosheets (TNS) were synthesized using a hydrothermal technique in the presence of a hydrofluoric acid (HF) soft template. The nanosheets were further composited with graphene oxide and doped with copper oxide in the hydrothermal process to create the copper-TiO₂ nanosheets/graphene oxide (CTNSG). The CTNSG exhibited outstanding photoactivity in converting CO₂ gas to methane and acetone. The production rate for methane and acetone was 12.09 and 0.75 µmol h^-1 g_cat^-1 at 100% relative humidity, providing a total carbon consumption of 71.70 µmol g_cat^-1. The photoactivity of CTNSG was attributed to the heterostructure interior of the two two-dimensional nanostructures, the copper-TiO₂ nanosheets and graphene oxide. The nanosheets-graphene oxide interfaces served as the n-p heterojunctions in holding active radicals for subsequent reactions. The heterostructure also directed the charge transfer, which promoted electron-hole separation in the photocatalyst.

Inorganic Halide Perovskite Quantum Dots: A Versatile Nanomaterial Platform for Electronic Applications

Metal halide perovskites have generated significant attention in recent years because of their extraordinary physical properties and photovoltaic performance. Among these, inorganic perovskite quantum dots (QDs) stand out for their prominent merits, such as quantum confinement effects, high photoluminescence quantum yield, and defect-tolerant structures. Additionally, ligand engineering and an all-inorganic composition lead to a robust platform for ambient-stable QD devices. This review presents the state-of-the-art research progress on inorganic perovskite QDs, emphasizing their electronic applications. In detail, the physical properties of inorganic perovskite QDs will be introduced first, followed by a discussion of synthesis methods and growth control. Afterwards, the emerging applications of inorganic perovskite QDs in electronics, including transistors and memories, will be presented. Finally, this review will provide an outlook on potential strategies for advancing inorganic perovskite QD technologies.

Molecular-Level Zn-Ion Transfer Pump Specifically Functioning on (002) Facets Enables Durable Zn Anodes

The modification of metallic Zn anode contributes to solving the cycling issue of Zn-ion batteries (ZIBs) by restraining the dendrite growth and side reactions. In this regard, modulating (002) Zn is an effective way to prolong the lifespan of ZIBs with a parallel arrangement of Zn deposition. Herein, the authors propose to add trace amounts of Zn(BF₄ )₂ additive in 3 M ZnSO₄ to promote in-plane Zn deposition by forming a BF₄ ^- -[Zn(H₂ O)₆ ]²⁺ -[Zn(BF₄ )₃ ]^- transfer process and specifically functioning on (002) facets. In this way, the optimized electrolyte highly boosts the cycling stability of Zn anodes with a long lifespan at 34.2% Zn utilization (500 h/10 mA cm^-2 ) and 51.3% Zn utilization (360 h/10 mA cm^-2 ; 834 h/1 mA cm^-2 ). Moreover, the electroplated Zn on Cu substrate exhibits a competitive cumulative plating capacity (CPC) of 2.87 Ah cm^-2 under harsh conditions. The assembled Zn|(NH₄ )₂ V₆ O₁₆ ·3H₂ O full cells with a high cathode loading of 29.12 mg cm^-2 also realizes almost no capacity degradation even after 2000 cycles at 2 A g^-1 . With this cost-effective strategy, it is promising to push the development of aqueous ZIBs as well as provide inspiration for metal anode optimization in other energy storage systems.

Effect of non-ionic surfactant in the solvothermal synthesis of anatase TiO2 nanoplates with a high percentage of exposed {001} facets and its role in the photocatalytic degradation of methylene blue dye

The synthesis of anatase TiO₂ nanoparticles with controlled morphology and increased {001} facets exposed without the presence of fluorine-derived substances is a challenge. Herein, we report a highly effective approach to fabricate anatase TiO₂ nanoplates with exposed {001} facets and their exploitation as robust photocatalytic materials for dye remediation. These materials were synthesized under controlled hydrolysis and condensation reactions, using titanium (IV) n-butoxide in an ethanolic solution, with acetic and sulfuric acids, by a solvothermal method at 190 °C with or without the presence of the non-ionic surfactant Triton® X-100 and then characterized. During TiO₂ crystal synthesis, the effect of a non-ionic surfactant on the TiO₂ particle growth was investigated. Our results demonstrate that the proposed method can synthesize pure and crystalline anatase TiO₂ square nanoplates that form nanostructured spheres with high surface area, uniformly sized mesopores, and exposed {001} facets. The presence of non-ionic surfactant increased the exposed {001} facets percentage of the formed nanoplates from 69 to 80%, decreased the crystallite thickness, but unaffected its crystalline phase and band gap energy. The kinetic constants (K_a e K_b) for the synthesized TiO₂ anatase nanoplates are considerably higher than the commercial TiO₂ anatase constant (K_c). The synthesized photocatalysts show higher efficiency in the photocatalytic removal of methylene blue (MB) than commercial TiO₂ (for t = 120 min).

TiO2 nanosheet supported MnCeOx: a remarkable catalyst with enhanced low-temperature catalytic activity in o-DCB oxidation

Morphology engineering was an effective strategy for 1,2-dichlorobenzene (o-DCB) oxidation. Herein, TiO₂ nanosheet supported MnCeO_x (TiMn15Ce30-NS) showed excellent catalytic activity with T_50% = 156 °C and T_90% = 238 °C, which was better than the T_50% = 213 °C and T_90% = 247 °C for TiO₂ nano truncated octahedron supported MnCeO_x (TiMn15Ce30-NTO). TiMn15Ce30-NS also exhibited enhanced water resistance (T_50% = 179 °C, T_90% = 240 °C), and good stability with the o-DCB conversion retained at 98.9% for 12 h at 350 °C. The excellent catalytic activity of TiMn15Ce30-NS could be mainly ascribed to the preferentially exposed {001} crystal plane and Ce addition which favored the higher concentration of Mn⁴⁺ and surface active oxygen, along with stronger interaction between MnO_x and CeO_x. The present results deepen the understanding of the morphology-dependent effect on o-DCB oxidation.

Determination of Crystallographic Orientation and Exposed Facets of Titanium Oxide Nanocrystals

Titanium dioxide (TiO₂ ) nanocrystals have attracted great attention in heterogeneous photocatalysis and photoelectricity fields for decades. However, contradicting conclusions on the crystallographic orientation and exposed facets of TiO₂ nanocrystals frequently appear in the literature. Herein, using anatase TiO₂ nanocrystals with highly exposed {001} facets as a model, the misleading conclusions that exist on anatase nanocrystals are clarified. Although TiO₂ -001 nanocrystals are recognized to be dominated by {001} facets, in fact, anatase nanocrystals with both dominant {001} and {111} facets always co-exist due to the similarities in the lattice fringes and intersection angles between the two types of facets (0.38 nm and 90° in the [001] direction, 0.35 nm and 82° in the [111] direction). A paradigm for determining the crystallographic orientation and exposed facets based on transmission electron microscopy (TEM) analysis, which provides a universal methodology to nanomaterials for determining the orientation and exposed facets, is also given.

Hydrothermal Synthesis of Co-Exposed-Faceted WO3 Nanocrystals with Enhanced Photocatalytic Performance

In this paper, rod-shaped, cuboid-shaped, and irregular WO₃ nanocrystals with different co-exposed crystal facets were prepared for the first time by a simple hydrothermal treatment of tungstic acid colloidal suspension with desired pH values. The crystal structure, morphology, specific surface area, pore size distribution, chemical composition, electronic states of the elements, optical properties, and charge migration behavior of as-obtained WO₃ products were characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), fully automatic specific surface area and porosity analyzer, UV-vis absorption spectra, photoluminescence (PL) spectra, and electrochemical impedance spectroscopy (EIS). The photocatalytic performances of the synthesized pHx-WO₃ nanocrystals (x = 0.0, 1.5, 3.0, 5.0, and 7.0) were evaluated and compared with the commercial WO₃ (CM-WO₃) nanocrystals. The pH7.0-WO₃ nanocrystals with co-exposed {202} and {020} facets exhibited highest photocatalytic activity for the degradation of methylene blue solution, which can be attributed to the synergistic effects of the largest specific surface area, the weakest luminescence peak intensity and the smallest arc radius diameter.

Constructing porous carbon nitride nanosheets for efficient visible-light-responsive photocatalytic hydrogen evolution

The photocatalytic performance of polymeric carbon nitride (CN) is mainly restricted by the poor mass charge separation efficiency and poor light absorption due to its polymeric nature. The conventional strategies to address these problems involved constructing a nanosheets structure would result in a blue shifted light absorption and increased exciton binding energy. Here, with combination of ammonia etching and selectively hydrogen-bond breaking, holey carbon nitride nanosheets (hCNNS) were constructed, thus widening the light absorption range, and spontaneously shortening the migration distance of electrons and holes in the lateral and vertical directions, respectively. Further analysis also found out the reserved atomic structure order endowed hCNNS with the relatively high redox potential. When irradiated with visible light (λ > 420 nm) and loaded with 3 wt% Pt as the cocatalyst, the hydrogen evolution rate of hCNNS was about 40 times higher than the bulk CN, and the apparent quantum yield (AQY) of hCNNS is 1.47% at 435 ± 15 nm. We expect this research can provide a new sight for achieving highly efficient solar utilization of CN-based photocatalysts.

High Photoreactivity on a Reconstructed Anatase TiO2(001) Surface Predicted by Ab Initio Nonadiabatic Molecular Dynamics

Anatase TiO₂(001) surface with (4 × 1) reconstruction is proposed to be a highly active catalytic surface. In this work, using time-domain ab initio nonadiabatic molecular dynamics, we reveal that the ridge structure formed by anatase(001) surface reconstruction is the photoreactive site for hole migration and trapping. Moreover, the ridge structure is destroyed by low-coverage CH₃OH adsorption, leading to the suppression of its high photoreactivity. However, when the CH₃OH coverage is increased and intermolecular hydrogen bonds (H-bonds) form, the ridge structure and its high photoreactivity are restored. Furthermore, the hole trapping dynamics is strongly coherent with intermolecular proton transfer in structures with intermolecular H-bonds. Our study proves that anatase TiO₂(001)-(4 × 1) is a highly photoreactive surface where the ridge is the photoreactive site for hole trapping, which is coherent with the proton transfer process.

Versatile Synthesis of Mesoporous Crystalline TiO2 Materials by Monomicelle Assembly

Mesoscale TiO₂ structures have realized many technological applications-ranging from catalysis and biomedicine to energy storage and conversion-because of their large mesoporosities offering desirable accessibility and mass transport. Tailoring mesoporous TiO₂ structures with novel mesoscopic and microscopic configurations is envisaged to offer ample opportunities for further applications. In this Review, we explain how to synthesize novel mesoporous TiO₂ materials and present recent examples. An emphasis is placed on a "monomicelle assembly" strategy as an emerging and powerful approach to direct the formation of mesostructured TiO₂ with precise control over its structural orientations and architectures. Furthermore, typical examples of mesoporous TiO₂ for applications in batteries and photocatalysis are highlighted. The Review ends with an outlook towards the synthesis of mesoporous TiO₂ with tailored architectures by self-assembly, which could pave the way for developing advanced energy conversion and storage devices.

Surface-Terminated Hydroxyl Groups for Deciphering the Facet-Dependent Photocatalysis of Anatase TiO2

Understanding the relation between a crystal facet and photocatalytic performance is of great importance for the development of effective catalysts. In this work, we focus on anatase TiO₂ with controllable exposed facets toward photocatalytic hydrogen evolution by water splitting. By combining temperature-programmed desorption (TPD) and diffuse reflectance infrared spectroscopy (DRIFTS), we obtain that the adsorption of hydroxyl groups and the photo-driven breaking of hydroxyl groups depend strongly on the exposed facets. As a result, the higher catalytic hydrogen evolution activity of TiO₂ enclosed with (101) facets than that of (001) facets should be ascribed to the more favorable depletion of hydroxyl groups. Moreover, graphene quantum dots (GQDs) with rich surface functional groups are deliberately deposited on the TiO₂ surface. The determination of the states and dynamics of surface hydroxyl groups suggests that GQDs facilitate the reaction of hydroxyl groups on (001)TiO₂, thus leading to the activity enhancement. By contrast, the already active (101)TiO₂ become apparently less efficient after GQD deposition due to the restricted reaction of hydroxyl groups. Overall, our findings not only provide a unique guidance for understanding the crystal-plane-dependent photocatalysis but also present a powerful approach by which to tailor the photocatalytic performance.

Development of Monodisperse Mesoporous Microballs Composed of Decahedral Anatase Nanocrystals

Mesoporous monodisperse microballs of amorphous titania were prepared from solution of absolute ethanol, tetrabutyl titanate (TBOT) and potassium chloride via a sub-zero sol–gel route. The as-obtained microballs were used as the precursor in an alcohothermal (ethanol with a small amount of water) process to synthesize monodisperse mesoporous microballs built of decahedral anatase nanocrystals. FE-SEM observation and XRD analysis have confirmed that the formed decahedral anatase-rich powder retained the original spherical morphology of the precursor. Importantly, a hierarchical structure composed of faceted anatase has been achieved under “green” conditions, i.e., fluorine-free. Additionally, the hysteresis loops (BET results) have confirmed the existence of mesopores. Interestingly, faceted microballs show noticeable photocatalytic activity under UV/vis irradiation for hydrogen generation without any co-catalyst use, reaching almost forty times higher activity than that by famous commercial titania photocatalyst—P25. It has been proposed that enhanced photocatalytic performance is caused by mesoporous structure and co-existence of two kinds of facets, i.e., {001} and {101}, and thus hindered charge carriers’ recombination.

Ferrocene-Sensitized Titanium-Oxo Clusters with Effective Visible Light Absorption and Excellent Photoelectrochemical Activity

Sensitized Ti-oxo clusters have attracted growing attention as analogous molecular mode compounds of dye-sensitized titanium dioxide solar cells. However, reports on the introduction of metal complexes as photosensitizers into Ti-oxo...

A carbon nanowire-promoted Cu2O/TiO2 nanocomposite for enhanced photoelectrochemical performance

We started with earth abundant materials to design a heterojunction nanocomposite with excellent visible light photoelectrochemical performance and enhanced durability.

Low-Dimensional In2Se3 Compounds: From Material Preparations to Device Applications

Nanostructured In₂Se₃ compounds have been widely used in electronics, optoelectronics, and thermoelectrics. Recently, the revelation of ferroelectricity in low-dimensional (low-D) In₂Se₃ has caused a new upsurge of scientific interest in nanostructured In₂Se₃ and advanced functional devices. The ferroelectric, thermoelectric, and optoelectronic properties of In₂Se₃ are highly correlated with the crystal structure. In this review, we summarize the crystal structures and electronic band structures of the widely interested members of the In₂Se₃ compound family. Recent achievements in the preparation of low-D In₂Se₃ with controlled phases are discussed in detail. General principles for obtaining pure-phased In₂Se₃ nanostructures are described. The excellent ferroelectric, optoelectronic, and thermoelectric properties having been demonstrated using nanostructured and heterostructured In₂Se₃ with different phases are also summarized. Progress and challenges on the applications of In₂Se₃ nanostructures in nonvolatile memories, photodetectors, gas sensors, strain sensors, and photovoltaics are discussed in detail. In the last part of this review, perspectives on the challenges and opportunities in the preparation and applications of In₂Se₃ materials are presented.

Structure modulation of g-C3N4 in TiO2{001}/g-C3N4 hetero-structures for boosting photocatalytic hydrogen evolution

Structure design of photocatalysts is highly desirable for taking full advantage of their abilities for H2 evolution. Herein, the highly-efficient TiO2{001}/g-C3N4 (TCN) heterostructures have been fabricated successfully via an in situ ethanol-thermal method. And the structure of g-C3N4 in the TCN heterostructures could be exfoliated from bulk g-C3N4 to nanosheets, nanocrystals and quantum dots with the increase of the synthetic temperature. Through detailed characterization, the structural evolution of g-C3N4 could be attributed to the enhanced temperature of the ethanol-thermal treatment with the shear effects of HF acid. As expected, the optimal TCN-2 heterostructure shows excellent photocatalytic H2 evolution efficiency (1.78 mmol h-1 g-1) under visible-light irradiation. Except for the formed built-in electric field, the significantly enhanced photocatalytic activity of TCN-2 could be ascribed to the enhanced crystallinity of TiO2{001} nanosheets and the formed g-C3N4 nanocrystals with large surface area, which could extend the visible light absorption, and expedite the transfer of photo-generated charge carriers further. Our work could provide guidance on designing TCN heterostructures with the desired structure for highly-efficient photocatalytic water splitting.

Does Symmetry Control Photocatalytic Activity of Titania-Based Photocatalysts?

Decahedral anatase particles (DAPs) have been prepared by the gas-phase method, characterized, and analyzed for property-governed photocatalytic activity. It has been found that depending on the reaction systems, different properties control the photocatalytic activity, that is, the particle aspect ratio, the density of electron traps and the morphology seem to be responsible for the efficiency of water oxidation, methanol dehydrogenation and oxidative decomposition of acetic acid, respectively. For the discussion on the dependence of the photocatalytic activity on the morphology and/or the symmetry other titania-based photocatalysts have also been analyzed, that is, octahedral anatase particles (OAP), commercial titania P25, inverse opal titania with and without incorporated gold NPs in void spaces and plasmonic photocatalysts (titania with deposits of gold). It has been concluded that though the morphology governs photocatalytic activity, the symmetry (despite its importance in many cases) rather does not control the photocatalytic performance.

Photocatalysis and Li-Ion Battery Applications of {001} Faceted Anatase TiO2-Based Composites

Anatase TiO2 are the most widely used photocatalysts because of their unique electronic, optical and catalytic properties. Surface chemistry plays a very important role in the various applications of anatase TiO2 especially in the catalysis, photocatalysis, energy conversion and energy storage. Control of the surface structure by crystal facet engineering has become an important strategy for tuning and optimizing the physicochemical properties of TiO2. For anatase TiO2, the {001} crystal facets are the most reactive because they exhibit unique surface characteristics such as visible light responsiveness, dissociative adsorption, efficient charge separation capabilities and photocatalytic selectivity. In this review, a concise survey of the literature in the field of {001} dominated anatase TiO2 crystals and their composites is presented. To begin, the existing strategies for the synthesis of {001} dominated anatase TiO2 and their composites are discussed. These synthesis strategies include both fluorine-mediated and fluorine-free synthesis routes. Then, a detailed account of the effect of {001} facets on the physicochemical properties of TiO2 and their composites are reviewed, with a particular focus on photocatalysis and Li-ion batteries applications. Finally, an outlook is given on future strategies discussing the remaining challenges for the development of {001} dominated TiO2 nanomaterials and their potential applications.

Robust Hydrogen Production via Pickering Interfacial Catalytic Photoreforming of n-Octanol-Water Biphasic System

Pickering emulsion offers a promising platform for conducting interfacial reactions between immiscible reagents; it is particularly suitable for hydrogen production by photoreforming of non-water soluble biomass liquid and water. Herein, Pt-promoted (001)-facet-dominated anatase TiO2 nanosheets were synthesized by a hydrothermal route associated with microfluidic technology for high activity and metal dispersion, and selective surface modification was carried out for preparing Janus particles. Photoreforming hydrogen production through n-octanol and water that formed O/W microemulsion with an average diameter of 540 µm was achieved to obtain amphiphilic catalyst. The as-prepared 2D Janus-type catalysts exhibited remarkably stable emulsification performance as well as photocatalytic activity. This finding indicates that triethoxyfluorosilane had negligible impact on the catalytic performance, yet provided a remarkable benefit to large specific surface area at microemulsion interface, thereby enhancing the H2 yield up to 2003 μmol/g. The cyclic experiments indicate that the decrease in cyclic performance was more likely to be caused by the coalescence of the microemulsion rather than the decrease in catalytic activity, and the microemulsion could be easily recovered by simply hand shaking to more than 96% of the initial performance.

Electronic-State Manipulation of Surface Titanium Activates Dephosphorylation Over TiO2 Near Room Temperature

Dephosphorylation that removes a phosphate group from substrates is an important reaction for living organisms and environmental protection. Although CeO₂ has been shown to catalyze this reaction, cerium is low in natural abundance and has a narrow global distribution (>90 % of these reserves are located within six countries). It is thus imperative to find another element/material with high worldwide abundance that can also efficiently extract the phosphate out of agricultural waste for phosphorus recycle. Using para-nitrophenyl phosphate (p-NPP) as a model compound, we demonstrate that TiO₂ with a F-modified (001) surface can activate p-NPP dephosphorylation at temperatures as low as 40 °C. By probe-assisted nuclear magnetic resonance (NMR), it was revealed that the strong electron-withdrawing effect of fluorine makes Ti atoms (the active sites) on the (001) surface very acidic. The bidentate adsorption of p-NPP on this surface further promotes its subsequent activation with a barrier ≈20 kJ mol^-1 lower than that of the pristine (001) and (101) surfaces, allowing the activation of this reaction near room temperature (from >80 °C).

Metastable Facet-Controlled Cu2WS4 Single Crystals with Enhanced Adsorption Activity for Gaseous Elemental Mercury

Purposively designing environmental advanced materials and elucidating the underlying reactivity mechanism at the atomic level allows for the further optimization of the removal performance for contaminants. Herein, using well facet-controlled I-Cu₂WS₄ single crystals as a model transition metal chalcogenide sorbent, we investigated the adsorption performance of the exposed facets toward gaseous elemental mercury (Hg⁰). We discovered that the decahedron exhibited not only facet-dependent adsorption properties for Hg⁰ but also recrystallization along the preferential [001] growth direction from a metastable state to the steady state. Besides, the metastable crystals with a predominant exposure of {101} facets dominated the promising adsorption efficiency (about 99% at 75 °C) while the saturated adsorption capacity was evaluated to be 2.35 mg·g^-1. Subsequently, comprehensive characterizations and X-ray adsorption fine structure (XAFS), accompanied by density functional theory (DFT) calculations, revealed that it might be owing to the coordinatively unsaturated local environment of W atoms with S defects and the surface relative stability of different facets, which could be affected by the change in surface atom configuration. Hence, the new insight into the facet-dependent adsorption property of transition metal chalcogenide for Hg⁰ may have important implications, and the atomic-level study directly provides instructions for development and design of highly efficient functional materials.

A facile synthesis of Zn-doped TiO2 nanoparticles with highly exposed (001) facets for enhanced photocatalytic performance

It is a great challenge to simultaneously improve the visible light absorption capacity and enhance photon-generated carrier separation efficiency of photocatalysts. Herein, Zn-doped TiO2 nanoparticles with high exposure of the (001) crystal face were prepared via a one-step hydrothermal decomposition method. A detailed analysis reveals that the electronic structures were modulated by Zn doping; thus, the responsive wavelength was extended to 600 nm, which effectively improved the visible light absorption of TiO2. More importantly, the surface heterojunction of TiO2 was created because of the co-existing specific facets of (101) and (001). Therefore, the surface separation efficiency of photogenerated electron and hole pairs was greatly enhanced. So, the optimal TiO2 photocatalyst exhibited excellent photocatalytic activity, in which the Rhodamine B (RhB) degradation efficiency was 98.7% in 60 min, under the irradiation of visible light. This study is expected to provide guidance for the rational design of TiO2 photocatalysts.