Low Temperature Synthesis of Large‐Size Anatase TiO2 Nanosheets with Enhanced Photocatalytic Activities

TiO₂ nanosheets have continuously been intriguing due to their high surface activities as photocatalyst but still challenging to synthesis large‐scale 2D nanostructures. A special microstructure evolution of TiO₂, ripening in aqueous solution at low temperature (≈4 °C), is found for the first time, i.e., from the initial aperiodic atom‐networks gradually into low crystallized continuous spongy structure with small crystal facets and ultimately forming large‐size anatase nanosheets with exposed (101) and (200) facets. Based on this finding, the synthesized anatase TiO₂ nanosheets possess monodispersed large‐scale 2D nanostructure so as to exhibit appreciable quantum size effects and remarkable enhanced optical absorption capacity. Using photocatalytic reduction of Cr (VI) to Cr (III) as the probe reaction to evaluate photocatalytic activities of the TiO₂ nanosheets, the reductivity of Cr (VI) achieves 99.8% in 15 min under irradiation of 200–800 nm light. At the same time, an in situ Cr (III)‐doping occurs spontaneously and triggers pronounced visible light driven photocatalysis, reducing 99% of Cr (VI) in 100 min under irradiation of 400–800 nm light.     

Ultrathin Anatase TiO2 Nanosheets for High‐Performance Photocatalytic Hydrogen Production

An ethanol solvothermal route has been developed to prepare ultrathin anatase TiO₂ nanosheets with dominant {001} facets (≈97%), a thickness of ≈2.5 nm, and a side length of ≈200 nm. The introduction of ethanol solvent significantly enhances the content of surface chemisorbed F^? on the TiO₂ nanosheet, which has a higher stability and further lowers the surface energy of {001} facets, giving rise to the large percentage of active {001} facets. Adopting well‐defined morphology, such nanosheets loaded with 1 wt% Pt exhibit an H₂ evolution rate as high as 17.86 mmol h^?1 g^?1, and the corresponding apparent quantum efficiency has been determined to be 34.2%.     

A Photoresponsive Rutile TiO2 Heterojunction with Enhanced Electron–Hole Separation for High‐Performance Hydrogen Evolution

Rutile titanium dioxide (TiO₂) is a promising photocatalyst due to its high thermodynamic stability and few intragrain defects. However, it has not yet achieved photocatalytic activity comparable to that of anatase TiO₂ owing to its higher recombination rate of electron–hole pairs. To effectively separate the electron–hole pairs in rutile TiO₂, a facet heterojunction (FH) structure to prolong the lifetime of the photogenerated electrons is proposed. Ultrathin TiO₂ nanosheets with different facets are coated in situ onto TiO₂ nanorod (NR) substrates, where FHs are built among the nanosheets as well as between the nanosheets and NR substrates. The as‐prepared rutile TiO₂, with an FH structure (FH‐TiO₂), serves as an effective photocatalyst for water splitting. More than 45 and 18 times higher photogenerated current density and H₂ production rate, respectively, are obtained compared to those of pure rutile TiO₂ NRs. Moreover, FH‐TiO₂ delivers a 0.566 mmol g^?1 h^?1 H₂ production rate even in pure water. This study offers important insights into the rational design of rutile TiO₂ structures for highly efficient photocatalytic reactions.     

Highly Selective Photoreduction of CO2 with Suppressing H2 Evolution by Plasmonic Au/CdSe–Cu2O Hierarchical Nanostructures under Visible Light

Here, the photocatalytic CO₂ reduction reaction (CO₂RR) with the selectivity of carbon products up to 100% is realized by completely suppressing the H₂ evolution reaction under visible light (λ > 420 nm) irradiation. To target this, plasmonic Au/CdSe dumbbell nanorods enhance light harvesting and produce a plasmon‐enhanced charge‐rich environment; peripheral Cu₂O provides rich active sites for CO₂ reduction and suppresses the hydrogen generation to improve the selectivity of carbon products. The middle CdSe serves as a bridge to transfer the photocharges. Based on synthesizing these Au/CdSe–Cu₂O hierarchical nanostructures (HNSs), efficient photoinduced electron/hole (e^?/h⁺) separation and 100% of CO selectivity can be realized. Also, the 2e^?/2H⁺ products of CO can be further enhanced and hydrogenated to effectively complete 8e^?/8H⁺ reduction of CO₂ to methane (CH₄), where a sufficient CO concentration and the proton provided by H₂O reduction are indispensable. Under the optimum condition, the Au/CdSe–Cu₂O HNSs display high photocatalytic activity and stability, where the stable gas generation rates are 254 and 123 µmol g^?1 h^?1 for CO and CH₄ over a 60 h period.     

Low‐Energy Facets on CdS Allomorph Junctions with Optimal Phase Ratio to Boost Charge Directional Transfer for Photocatalytic H2 Fuel Evolution

Low‐energy facets on CdS allomorph junctions with optimal phase ratio are designed to boost charge directional transfer for photocatalytic H₂ fuel evolution. Fermi energy level difference between low‐energy facets as driving force promotes electrons directional transfer to hexagonal CdS(102) facet and holes to cubic CdS(111) facet. The optimal allomorphs CdS presents superior photocatalytic H₂ evolution rate of 32.95 mmol g^?1 h^?1 with release in a large amount of visible H₂ bubbles, which is much higher than single‐phase CdS with high‐energy facets and even supports noble metal photocatalysts. This scientific perspective on low‐energy facets of allomorph junctions with optimal phase ratio breaks the long‐held view of pursuing high‐energy crystal surfaces, which will break the understanding on surface structure crystal facet engineering of photocatalytic materials.     

On the Controlled Loading of Single Platinum Atoms as a Co-Catalyst on TiO2 Anatase for Optimized Photocatalytic H2 Generation

Single-atom (SA) catalysis is a novel frontline in the catalysis field due to the often drastically enhanced specific activity and selectivity of many catalytic reactions. Here, an atomic-scale defect engineering approach to form and control traps for platinum SA sites as co-catalyst for photocatalytic H₂ generation is described. Thin sputtered TiO₂ layers are used as a model photocatalyst, and compared to the more frequently used (001) anatase sheets. To form stable SA platinum, the TiO₂ layers are reduced in Ar/H₂ under different conditions (leading to different but defined Ti³⁺-O_v surface defects), followed by immersion in a dilute hexachloroplatinic acid solution. HAADF-STEM results show that only on the thin-film substrate can the density of SA sites be successfully controlled by the degree of reduction by annealing. An optimized SA-Pt decoration can enhance the normalized photocatalytic activity of a TiO₂ sputtered sample by 150 times in comparison to a conventional platinum-nanoparticle-decorated TiO₂ surface. HAADF-STEM, XPS, and EPR investigation jointly confirm the atomic nature of the decorated Pt on TiO₂. Importantly, the density of the relevant surface exposed defect centers—thus the density of Pt-SA sites, which play the key role in photocatalytic activity—can be precisely optimized.     

Glycine assisted synthesis of flower-like TiO2 hierarchical spheres and its application in photocatalysis

Flower-like anatase TiO₂ hierarchical spheres assembled by nanosheets were synthesized by glycine assistant via a simple hydrothermal approach and after-annealing process. These flower-like spheres are about 2 μm in diameter with sheet thickness about 20 nm. Results showed reaction time, temperature, solution pH and glycine dosage all played an important role in control of shape and size of the as-synthesized TiO₂ nanocrystals. The photocatalytic activity of this nano-TiO₂ was evaluated by the photocatalytic oxidation decomposition of methyl orange under sunlight illumination in the presence of hydrogen peroxide (H₂O₂). The photocatalytic activity of the obtained TiO₂ was higher than that of commercial TiO₂.     

Graphdiyne-hybridized N-doped TiO<Subscript>2</Subscript> nanosheets for enhanced visible light photocatalytic activity

In this study, graphdiyne (GD)-hybridized nitrogen-doped TiO₂ nanosheets with exposed (001) facets (GD-NTNS) have been prepared via a hydrothermal reaction and utilized as photocatalyst for the photodegradation of rhodamine B (RhB) under visible light illumination. The resultant GD-NTNS composites exhibit superior visible light photocatalytic activity than that of the bare TiO₂ nanosheets (TNS) and nitrogen-doped TiO₂ nanosheets (NTNS). The enhanced photoactivity can be attributed to the synergistic effects of GD and nitrogen doping with efficient electron transfer and strong visible light absorption. It has been revealed that ^·O^2? and h⁺ are the major species for the enhanced photoactivity under visible light. Our work will facilitate the potential for future design of hybrid materials for practical applications beyond photocatalysts.     

Noble metal nanoclusters and their <Emphasis Type="Italic">in situ</Emphasis> calcination to nanocrystals: Precise control of their size and interface with TiO<Subscript>2</Subscript> nanosheets and their versatile catalysis applications

In this work, we present a new versatile strategy to prepare noble metal (Au, Ag and Cu) nanoclusters on TiO₂ nanosheets in large scales with exposed (001) facets with controlled size, crystalline interface, and loading amount. By precise in situ calcination, the metal (M = Au, Ag, and Cu) nanocrystals with controllable size and better crystalline interface with the TiO₂ support have been prepared. The potential application of the as-prepared Au, Ag, and Cu nanoclusters on TiO₂ nanosheets as potential heterogeneous catalysts for organic synthesis, such as catalytic reduction of 4-nitrophenol to 4-aminophenol, has been demonstrated. After calcination, Au, Ag, and Cu nanocrystals were found to be proficient cocatalysts for photocatalytic H₂ evolution, particularly the Au cocatalyst. Based on precise high-resolution transmission electron microscopy (HRTEM) and inductively coupled plasma optical emission spectrometry (ICP-OES) analyses, the flexible control of their size and loading amount as well as their intimate contact with the TiO₂ nanosheet enhanced the photocatalytic H₂ evolution activity and the sensitivity of the photocurrent response of the film. Furthermore, this aqueous-directed synthesis of metal nanoclusters on a support will generate further interest in the field of nanocatalysis.

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金红石二氧化钛纳米片的性质及其光催化活性

采用溶胶-凝胶、质子交换和层状剥离的方法, 制备出金红石TiO₂纳米片。利用X射线电子衍射谱(XRD)、透射电子显微镜(TEM)、紫外-可见吸收光谱(UV-Vis)、X光电子能谱(XPS)的价带谱和荧光光谱(PL)等对样品进行了表征, 研究了光生载流子的转移过程。结果证明: 金红石TiO₂纳米片具有较大的比表面积(185.7 m²/g), 厚度约5 nm, 与金红石TiO₂样品相比, 金红石TiO₂纳米片的禁带宽度增加, 氧化还原能力增强; 此外, 纳米片结构能够促使光生载流子快速转移到纳米片的表面并产生有效分离, 阻止了光生电子和空穴的复合, 提高了光催化反应中光生载流子的利用率。金红石纳米片的这些特性导致其具有较高的光催化活性, 紫外光催化降解对氯苯酚的实验表明: 金红石TiO₂纳米片的光催化活性高于金红石TiO₂和锐钛矿TiO₂样品。     

Controllable synthesis and photocatalytic activities of water-soluble TiO2 nanoparticles

Water-soluble anatase, mixed-phase (anatase and rutile) and rutile TiO₂ nanoparticles (NPs) or nanorods were synthesized under mild solution conditions using polyethylene glycol 400 (PEG 400) as a stabilizer and HCl as a phase controlling reagent. The photocatalytic properties of these NPs with different crystal phases were evaluated by photocatalytic degradation experiments of methyl orange (MO). As-prepared pure anatase TiO₂ NPs show a higher photocatalytic activity than other samples and commercial P25, which may be related to the high crystallinity, the pure anatase phase, small size and the enhanced absorbability associated with the existence of PEG 400 on the NP surface.     

Supporting Ultrathin ZnIn2S4 Nanosheets on Co/N‐Doped Graphitic Carbon Nanocages for Efficient Photocatalytic H2 Generation

Ultrathin ZnIn₂S₄ nanosheets (NSs) are grown on Co/N‐doped graphitic carbon (NGC) nanocages, composed of Co nanoparticles surrounded by few‐layered NGC, to obtain hierarchical Co/NGC@ZnIn₂S₄ hollow heterostructures for photocatalytic H₂ generation with visible light. The photoredox functions of discrete Co, conductive NGC, and ZnIn₂S₄ NSs are precisely combined into hierarchical composite cages possessing strongly hybridized shell and ultrathin layered substructures. Such structural and compositional virtues can expedite charge separation and mobility, offer large surface area and abundant reactive sites for water photosplitting. The Co/NGC@ZnIn₂S₄ photocatalyst exhibits outstanding H₂ evolution activity (e.g., 11270 µmol h^?1 g^?1) and high stability without engaging any cocatalyst.     

Photocatalytic degradation of dyes and organic contaminants in water using nanocrystalline anatase and rutile TiO2

Nanocrystalline TiO₂ was synthesized by controlled hydrolysis of titanium tetraisopropoxide. The anatase phase was converted to rutile phase by thermal treatment at 1023 K for 11 h. The catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), Fourier-transform infrared absorption spectrophotometry (FT-IR) and N₂ adsorption (BET) at 77 K. This study compare the photocatalytic activity of the anatase and rutile phases of nanocrystalline TiO₂ for the degradation of acetophenone, nitrobenzene, methylene blue and malachite green present in aqueous solutions. The initial rate of degradation was calculated to compare the photocatalytic activity of anatase and rutile nanocrystalline TiO₂ for the degradation of different substances under ultraviolet light irradiation. The higher photocatalytic activity was obtained in anatase phase TiO₂ for the degradation of all substances as compared with rutile phase. It is concluded that the higher photocatalytic activity in anatase TiO₂ is due to parameters like band-gap, number of hydroxyl groups, surface area and porosity of the catalyst.     

From UV to Near‐Infrared Light‐Responsive Metal–Organic Framework Composites: Plasmon and Upconversion Enhanced Photocatalysis

The exploitation of photocatalysts that harvest solar spectrum as broad as possible remains a high‐priority target yet grand challenge. In this work, for the first time, metal–organic framework (MOF) composites are rationally fabricated to achieve broadband spectral response from UV to near‐infrared (NIR) region. In the core–shell structured upconversion nanoparticles (UCNPs)‐Pt@MOF/Au composites, the MOF is responsive to UV and a bit visible light, the plasmonic Au nanoparticles (NPs) accept visible light, whereas the UCNPs absorb NIR light to emit UV and visible light that are harvested by the MOF and Au once again. Moreover, the MOF not only facilitates the generation of “bare and clean” Au NPs on its surface and realizes the spatial separation for the Au and Pt NPs, but also provides necessary access for catalytic substrates/products to Pt active sites. As a result, the optimized composite exhibits excellent photocatalytic hydrogen production activity (280 µmol g^?1 h^?1) under simulated solar light, and the involved mechanism of photocatalytic H₂ production under UV, visible, and NIR irradiation is elucidated. Reportedly, this is an extremely rare study on photocatalytic H₂ production by light harvesting in all UV, visible, and NIR regions.     

Heterostructured TiO2 Nanorod@Nanobowl Arrays for Efficient Photoelectrochemical Water Splitting

Heterostructured TiO₂ nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO₂ nanorods grown on the inner surface of arrayed anatase TiO₂ nanobowls are designed and fabricated as a new type of photoanodes for photoelectrochemical (PEC) water splitting. The unique heterostructures with a hierarchical architecture are readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two‐dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO₂ NR@NB arrays provide multiple scattering centers and hence exhibit an enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated electron transfer. Accordingly, the PEC measurements of the TiO₂ NR@NB arrays on the fluoride‐doped tin oxide (FTO) substrate show significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the unmodified TiO₂ NR@NB array photoelectrode yields a photocurrent density of 1.24 mA cm^–2 at 1.23 V with respect to the reversible hydrogen electrode, which is almost two times higher than that of the TiO₂ nanorods grown directly on the FTO substrate.     

Photocatalytic degradation of dyes and organic contaminants in water using nanocrystalline anatase and rutile TiO2

Nanocrystalline TiO₂ was synthesized by controlled hydrolysis of titanium tetraisopropoxide. The anatase phase was converted to rutile phase by thermal treatment at 1023 K for 11 h. The catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), Fourier-transform infrared absorption spectrophotometry (FT-IR) and N₂ adsorption (BET) at 77 K. This study compare the photocatalytic activity of the anatase and rutile phases of nanocrystalline TiO₂ for the degradation of acetophenone, nitrobenzene, methylene blue and malachite green present in aqueous solutions. The initial rate of degradation was calculated to compare the photocatalytic activity of anatase and rutile nanocrystalline TiO₂ for the degradation of different substances under ultraviolet light irradiation. The higher photocatalytic activity was obtained in anatase phase TiO₂ for the degradation of all substances as compared with rutile phase. It is concluded that the higher photocatalytic activity in anatase TiO₂ is due to parameters like band-gap, number of hydroxyl groups, surface area and porosity of the catalyst.     

Ligand-free Au nanoclusters/g-C3N4 ultra-thin nanosheets composite photocatalysts for efficient visible-light-driven photocatalytic H2 generation

In this paper, we report the synthesis of ligand-free Au nanoclusters (NCs)/g-C₃N₄ ultra-thin nanosheets (NSs) composite via a facile wet-impregnation method with post-annealing. On the one hand, post-annealing was used for the exfoliation of multi-layered g-C₃N₄ to obtain ultra-thin NSs; on the other hand, after Au₂₅(Cys)₁₈ NCs were loaded, post-annealing was further adopted to remove the ligands to obtain clean surface on Au NCs. It is demonstrated that the loaded Au NCs were aggregating resistant by post-annealing. Constructing heterojunctions with appropriate inter-band structures between the ligand-free Au NCs and the ultra-thin g-C₃N₄ NSs, along with the mono-distribution of the Au NCs and their intimate contact with g-C₃N₄ NSs ensured the smooth interfacial charge transfer. As a result, the composite photocatalysts exhibited efficient visible-light-induced photocatalytic H₂ generation, mainly due to the local electric field enhancement induced by excitation of Au NCs under visible light and the improved charge separation in g-C₃N₄. This work provides a general strategy for the synthesis of noble metal NCs based composites with clean surface as the efficient photocatalysts for solar energy conversion.

Graphical Abstract

A stepwise post-annealing strategy is exploited to prepare g-C₃N₄ ultra-thin nanosheets modified with highly dispersed ligand-free Au nanoclusters for efficient photocatalytic hydrogen production.

     

The effect of the H2 flow rate on the structure and optical properties of TiO2 films deposited by inductively coupled plasma assisted chemical vapor deposition

The optical and photocatalytic properties of TiO₂ are closely related to crystalline structures, such as rutile and anatase. In this paper, TiO₂ films were produced by inductively coupled plasma (ICP) assisted chemical vapor deposition (CVD) without extra heating of the substrate, and the effect of H₂ addition on the structure and optical properties of the films was investigated. After increasing the partial pressure of H₂, the structure of the TiO₂ films changed from anatase to rutile, which usually appears at high temperatures (> 600 °C). The light transmittance decreased with increasing the H₂ flow rate due to the increased surface roughness. The photocatalytic activity of the anatase TiO₂ film was better than that of the rutile TiO₂ film.     

Enhanced H2 Generation of Au‐Loaded,Nitrogen‐Doped TiO2 Hierarchical Nanostructures under Visible Light

Hierarchical N‐doped TiO₂ nanostructured catalysts with micro‐, meso‐, and macroporosity are synthesized by a facile self‐formation route using ammonia and titanium isopropoxide precursor. UV–vis diffuse reflectance spectra confirm the red shift and band gap narrowing due to the doping of N species in the TiO₂ nanoporous catalyst. Hierarchical macroporosity with fibrous channel patterning is observed and well preserved even after calcination at 800 °C, indicating thermal stability, whereas micro‐ and mesoporosity are lost after calcination at 500 °C. The photocatalytic activity of hiearchical N doped TiO₂ catalysts loaded with Au is evaluated for H₂ production reaction in visible light. The enhanced photocatalytic activity is attributed to the combined synergetic effect of N doping for visible light absorption, micro‐ and mesoporosity for an increase of effective surface area and light harvestation, and hierarchical macroporous fibrous structure for multiple reflection and effective charge transfer.     

Preparation of helical TiO2/CMC microtubes and pure helical TiO2 microtubes

Helical TiO₂/CMC (carbon microcoils) microtubes and helical TiO₂ microtubes were obtained by making TiO₂ layer coatings on the surface of CMC templates using a sol-gel and chemical vapor deposition (CVD) processes. The preparation conditions, morphologies and some properties were examined. Uniform TiO₂ (anatase) layers were obtained on the CMC templates by a CVD process using vapor phase hydrolysis of titanium tetra-isopropoxide at 300°C followed by heat treatment in N₂ or by calcination in air at 500–;650°C. The helical TiO₂/CMC microtubes showed good photocatalytic activity. It was considered that the helical structure activated and enhanced the photocatalytic activity of TiO₂, probably caused by the generation of inductive microelectric current induced by the irradiation of UV light, resulting in the generation of micromagnetic fields around the tubes.