Influence of Size and Shape on the Photocatalytic Properties of SnO2 Nanocrystals

Tuning the functional properties of nanocrystals is an important issue in nanoscience. Here, we are able to tune the photocatalytic properties of SnO₂ nanocrystals by controlling their size and shape. A structural analysis was carried out by using X‐ray diffraction (XRD)/Rietveld and transmission electron microscopy (TEM). The results reveal that the number of oxygen‐related defects varies upon changing the size and shape of the nanocrystals, which eventually influences their photocatalytic properties. Time‐resolved spectroscopic studies of the carrier relaxation dynamics of the SnO₂ nanocrystals further confirm that the electron–hole recombination process is controlled by oxygen/defect states, which can be tuned by changing the shape and size of the materials. The degradation of dyes (90 %) in the presence of SnO₂ nanoparticles under UV light is comparable to that (88 %) in the presence of standard TiO₂ Degussa P‐25 (P25) powders. The photocatalytic activity of the nanoparticles is significantly higher than those of nanorods and nanospheres because the effective charge separation in the SnO₂ nanoparticles is controlled by defect states leading to enhanced photocatalytic properties. The size‐ and shape‐dependent photocatalytic properties of SnO₂ nanocrystals make these materials interesting candidates for photocatalytic applications.     

Modelling a Linker Mix‐and‐Match Approach for Controlling the Optical Excitation Gaps and Band Alignment of Zeolitic Imidazolate Frameworks

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO₂ reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.     

Hybrid Density Functional Study on Mono‐ and Codoped NaNbO3 for Visible‐Light Photocatalysis

Monodoping with Mo, Cr, and N atoms, and codoping with Mo?N and Cr?N atom pairs, are utilized to adjust the band structure of NaNbO₃, so that NaNbO₃ can effectively make use of visible light for the photocatalytic decomposition of water into hydrogen and oxygen, as determined by using the hybrid density functional. Codoping is energetically favorable compared with the corresponding monodoping, due to strong Coulombic interactions between the dopants and other atoms, and the effective band gap and stability for codoped systems increase with decreasing dopant concentration and the distance between dopants. The molybdenum, chromium, and nitrogen monodoped systems, as well as chromium–nitrogen codoped systems, are unsuitable for the photocatalytic decomposition of water by using visible light, because defects introduced by monodoping or the presence of unoccupied states above the Fermi level, which promotes electron–hole recombination processes, suppress their photocatalytic performance. The Mo?N codoped NaNbO₃ sample is a promising photocatalyst for the decomposition of water by using visible light because Mo?N codoping can reduce the band gap to a suitable value with respect to the water redox level without introducing unoccupied states.     

Carbon-dot-modified TiO<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript> mesoporous single crystals with enhanced photocatalytic activity for degradation of phenol

We introduce a facile method to synthesize carbon quantum dots/titanium dioxide mesoporous single crystals (CQDs–MSCs). CQDs were synthesized using a pyrolysis method with citric acid as carbon precursor. V-CQDs–MSCs composite was prepared using a vacuum activation method and exhibited enhanced photocatalytic activity. The composites were characterized by diffuse reflectance spectroscopy (DRS), electron paramagnetic resonance (EPR), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) analysis, and thermogravimetry (TG) analysis. XPS results indicated that CQDs were bonded on the surface of MSCs by formation of Ti–O–C bonds, which played an important role in transfer of photogenerated electrons from Ti³⁺–TiO₂ to CQDs. V-CQDs–MSCs obtained using CQD loading content of 2% and vacuum activation temperature of 600 °C showed the highest photocatalytic activity for degradation of phenol under simulated solar light irradiation.     

Visible‐Light Photocatalytic Hydrogen Production Activity of ZnIn2S4 Microspheres Using Carbon Quantum Dots and Platinum as Dual Co‐catalysts

ZnIn₂S₄ microspheres (ZIS MSs) were for the first time decorated with carbon quantum dots (CQDs) and platinum nanoparticles (NPs) as dual co‐catalysts of for photocatalytic H₂ production. The ZIS MSs co‐loaded with CQDs and Pt exhibited a high photocatalytic H₂ production rate of 1032.2 μmol h^?1 g^?1 with an apparent quantum efficiency of 2.2 % (420 nm) in triethanolamine aqueous solution under visible‐light irradiation, which was much higher than the respective photocatalytic rates of pure ZIS, Pt loaded ZIS, and CQDs‐decorated ZIS. Such a great enhancement was attributed to the integrative effect of good crystallization, enhanced light absorption, high electrical conductivity of CQDs, and the vectorial electron transfer from ZIS to CQDs and Pt NPs (ZIS→CQDs→Pt).     

Surface Plasmon Resonance‐Enhanced Visible‐NIR‐Driven Photocatalytic and Photothermal Catalytic Performance by Ag/Mesoporous Black TiO2 Nanotube Heterojunctions

Ag/mesoporous black TiO₂ nanotubes heterojunctions (Ag‐MBTHs) were fabricated through a surface hydrogenation, wet‐impregnation and photoreduction strategy. The as‐prepared Ag‐MBTHs possess a relatively high specific surface area of ≈85 m² g^?1 and an average pore size of ≈13.2 nm. The Ag‐MBTHs with a narrow band gap of ≈2.63 eV extend the photoresponse from UV to the visible‐light and near‐infrared (NIR) region. They exhibit excellent visible‐NIR‐driven photothermal catalytic and photocatalytic performance for complete conversion of nitro aromatic compounds (100 %) and mineralization of highly toxic phenol (100 %). The enhancement can be attributed to the mesoporous hollow structures increasing the light multi‐refraction, the Ti³⁺ in frameworks and the surface plasmon resonance (SPR) effect of plasmonic Ag nanoparticles favoring light‐harvesting and spatial separation of photogenerated electron–hole pairs, which is confirmed by transient fluorescence. The fabrication of this SPR‐enhanced visible‐NIR‐driven Ag‐MBTHs catalyst may provide new insights for designing other high‐performance heterojunctions as photocatalytic and photothermal catalytic nanomaterials.     

Enhanced visible‐light‐driven photocatalytic activity of F doped reduced graphene oxide ‐ Bi2WO6 photocatalyst

The reduced graphene oxide‐Bi₂WO₆ (rGO‐BWO) photocatalysts with the different R_F/O values (molar ratio of the F molar mass and the O's molar mass of Bi₂WO₆) had been successfully synthesized via one‐step hydrothermal method. The F‐doped rGO‐BWO samples were characterized by X‐ray diffraction patterns (XRD), field‐emission scanning electron microscopy (FE‐ESEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area (BET), X‐ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectra (DRS). The results indicate that F^? ions had been successfully doped into rGO‐BWO samples. With the increasing of the R_F/O values from 0 to 2%, the evident change of the morphology and the absorption edges of F‐doped rGO‐BWO samples and the photocatalytic activities had been enhanced. Moreover, the photocatalytic activity of F‐doped rGO‐BWO with R_F/O = 0.05 were better than rGO‐BWO and the other F‐doped rGO‐BWO under 500 W Xe lamp light irradiation. The enhanced photocatalytic activity can be attributed to the morphology of the intact microsphere that signify the bigger specific surface area for providing more possible reaction sites for the adsorption–desorption equilibrium of photocatalytic reaction, the introduction of F^? ions that may cause the enhancement of surface acidity and creation of oxygen vacancies under visible light irradiation, the narrower band gap which means needing less energy for the electron hole pair transition.     

Photocatalytic degradation of amoxicillin by carbon quantum dots modified K2Ti6O13 nanotubes: Effect of light wavelength

Novel carbon quantum dots modified potassium titanate nanotubes (CQDs/K₂Ti₆O₁₃) composite was synthesized and exhibited high photocatalytic activity for degradation of amoxicillin under UV and visible lights with nine wavelengths. Better amoxicillin removal was achieved at lower wavelength irradiation due to its higher photo energy.     

Preparation of high‐activity AgBr/Ag3PO4 photocatalyst based on hexadecyltrimethylammonium bromide and mechanism of photocatalytic enhancement

A high‐activity AgBr/Ag₃PO₄ heterojunction photocatalyst was synthesized based on hexadecyltrimethylammonium bromide. Its microspheres were characterized using X‐ray diffractometry, transmission electron microscopy and ultraviolet–visible diffuse reflectance spectroscopy. The new photocatalyst with high photocatalytic activity exceptionally outperforms pure Ag₃PO₄ and AgBr in methyl orange degradation. The enhancement of photocatalytic activity is attributed to the efficient separation of electron–hole pairs. In this photocatalytic reaction, h⁺ and ^?O^2? are the main reactive species that induce visible‐light‐driven degradation.     

Engineering the Atomic Interface with Single Platinum Atoms for Enhanced Photocatalytic Hydrogen Production

It is highly desirable but challenging to optimize the structure of photocatalysts at the atomic scale to facilitate the separation of electron–hole pairs for enhanced performance. Now, a highly efficient photocatalyst is formed by assembling single Pt atoms on a defective TiO₂ support (Pt₁/def‐TiO₂). Apart from being proton reduction sites, single Pt atoms promote the neighboring TiO₂ units to generate surface oxygen vacancies and form a Pt‐O‐Ti³⁺ atomic interface. Experimental results and density functional theory calculations demonstrate that the Pt‐O‐Ti³⁺ atomic interface effectively facilitates photogenerated electrons to transfer from Ti³⁺ defective sites to single Pt atoms, thereby enhancing the separation of electron–hole pairs. This unique structure makes Pt₁/def‐TiO₂ exhibit a record‐level photocatalytic hydrogen production performance with an unexpectedly high turnover frequency of 51423 h^?1, exceeding the Pt nanoparticle supported TiO₂ catalyst by a factor of 591.     

Template‐free Fabrication of ZnS/TiO2 Photocatalyst with Macrochannels

Constructing a porous structure in photocatalysts is an effective strategy for improving the photocatalytic activity because of its enhanced molecule transfer capability and light capturing efficiency. In this work, a hierarchical macro‐/mesoporous ZnS/TiO₂ composite with macrochannels was successfully synthesized without using templates by the simple dropwise addition of an ethanol solution of tetrabutyl titanate and zinc acetate into a sodium sulfide aqueous solution, which was then calcined at 450°C. Compared with pure TiO₂, the ordered porous ZnS/TiO₂ composite exhibited an enhanced photocatalytic activity on methylene blue removal under UV‐light irradiation. The results indicate that the macro‐/mesoporous structure, the large specific surface area, and the heterostructure combination between ZnS and TiO₂ play a synergistic effect on the enhanced photocatalytic activity via improving the light absorption and the diffusion of organic molecules, providing more reactive sites for the photocatalytic reaction and improving the separation of photogenerated electron–hole pairs, respectively. Radical trapping experiments demonstrated that holes (h⁺) and superoxide anion radicals (O₂⁻) play an important role in the photocatalytic oxidation process.     

First‐Principles Calculations on Electronic Structures of N/V‐Doped and N‐V‐Dodoped Anatase TiO2 (101) Surfaces

The energetic and electronic properties of N/V‐doped and N‐V‐codoped anatase TiO₂ (101) surfaces are investigated by first‐principles calculations, with the aim to elucidate the relationship between the electronic structure and the photocatalytic performance of N‐V‐codoped TiO₂. Several substitutional and interstitial configurations for the N and/or V impurities in the bulk phase and on the surface are studied, and the relative stability of different doping configurations is compared by the impurity formation energy. Systematic calculations reveal that N and V impurities can be encapsulated by TiO₂ to form stable structures as a result of strong N‐V interactions both in the bulk and the surface model. Through analyzing and comparing the electronic structures of different doping systems, the synergistic doping effects are discussed in detail. Based on these discussions, we suggest that N_OV_Ti codoping cannot only narrow the band gap of anatase TiO₂, but also forms impurity states, which are propitious for the separation of photoexcited electron–hole pairs. In the case of N_OV_Ti‐codoped TiO₂ (101) surfaces, this phenomenon is especially prominent. Finally, a feasible synthesis route for N_OV_Ti codoping into anatase TiO₂ is proposed.     

Enhanced Photocatalytic Activity of MIL‐125 by Post‐Synthetic Modification with CrIII and Ag Nanoparticles

NH₂‐MIL‐125, [Ti₈O₈(OH)₄(bdc‐NH₂)₆] (bdc^2?=1,4‐benzene dicarboxylate) is a highly porous metal–organic framework (MOF) that has a band gap lying within the ultraviolet region at about 2.6 eV. The band gap may be reduced by a suitable post‐synthetic modification of the nanochannels using conventional organic chemistry methods. Here, it is shown that the photocatalytic activity of NH₂‐MIL‐125 in the degradation of methylene blue under visible light is remarkably augmented by post‐synthetic modification with acetylacetone followed by Cr^III complexation. The latter metal ion extends the absorption from the ultraviolet to the visible light region (band gap 2.21 eV). The photogenerated holes migrate from the MOF’s valence band to the Cr^III valence band, promoting the separation of holes and electrons and increasing the recombination time. Moreover, it is shown that the MOF’s photocatalytic activity is also much improved by doping with Ag nanoparticles, formed in situ by the reduction of Ag⁺ with the acetylacetonate pendant groups (the resulting MOF band gap is 2.09 eV). Presumably, the Ag nanoparticles are able to accept the MOF’s photogenerated electrons, thus avoiding electron–hole recombination. Both, the Cr‐ and Ag‐bearing materials are stable under photocatalytic conditions. These findings open new avenues for improving the photocatalytic activity of MOFs.     

Effect of calcination temperature on the photocatalytic activity of carbon‐doped titanium dioxide revealed by photoluminescence study

Carbon‐doped titania (C‐TiO₂) nanoparticles were synthesized by the sol–gel method at different calcination temperatures (300–600°C) employing titanium tetraisopropoxide (TTIP) as the titanium source and polyoxyethylene sorbitan monooleate (Tween 80) as the carbon source. The physical properties of C‐TiO₂ samples were characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The photocatalytic activities were checked through the photodegradation of phenolphthalein (PHP) under ultraviolet irradiation. The UV spectrum showed that the carbon doping extends the absorption range of TiO₂ to the visible region. However, the photocatalytic activity is affected by the electron–hole recombination phenomenon, as revealed by the photoluminescence (PL) study. According to the PL spectra, carbon doping reduces the edge‐to‐edge electron–hole recombination. Nevertheless, the number of defect sites is greatly influenced by the calcination temperature of C‐TiO₂. C‐TiO₂ that was calcined at 400°C showed the highest photodegradation percentage of PHP, which was mainly attributed to the synergic effect of the low direct edge‐to‐edge electron–hole recombination, high content of defect sites, and retention of active electrons on the surface hydroxyl group.     

Visible‐light‐driven photocatalytic degradation using Mn‐doped SrMoO4 nanoparticles

Mn‐doped SrMoO₄ nanocrystals were synthesized by thermal decomposition of metal–organic salt in an organic solvent with the doping content in the range 0–12 mol%. The structures, morphologies and optical properties were characterized using various techniques. The results suggest that Mo sites in the SrMoO₄ lattice are substituted by the Mn dopant, the adsorption bands are found to be shifted toward the visible light region and the band gap becomes narrower correspondingly. The photocatalytic performance of the as‐synthesized product was determined using the degradation of methylene blue by visible light irradiation. The photocatalytic performance is enhanced with Mn doping, and the optimal degradation rate is 85% in 140 min for 5 mol% Mn doping. The enhanced photocatalytic activity with Mn doping may be ascribed to the energy band adjustment and effective photogenerated electron–hole separation caused by the Mn doping. A possible photocatalytic mechanism is also discussed.     

Carbon quantum dots-based semiconductor preparation methods,applications and mechanisms in environmental contamination

Photocatalyst is the most widespread method in advanced oxidation technologies, but due to the photo-induced electron combine easily with hole and the wavelength of adsorption is limited which will affect some practical applications. Carbon quantum dots (CQDs) is non-toxic and harmless green materials, it has the ability to improve the photocatalytic effect which is attributed to its good electrical and optical properties. Their up-conversion effect, photosensitization and electrical conductivity are assistants which help promote the photocatalytic effect in environmental applications. The key mechanisms of CQDs to improve photocatalysis can be roughly divided into three categories: 1) Up-conversion effect converts the incident light into the emitted light with high energy to solve the problem which is the light absorption range; 2) CQDs act as a photosensitizer instead of valence band to provide electrons to the conduction band of semiconductor; 3) CQDs can be used as the internal or external electronic conductor in materials to alleviate the trend of electron and hole separation. However, CQDs and CQDs-based photocatalysts have different views to solve environmental problems, so it is necessary to integrate different views. Therefore, this review is mainly aimed at the recent researches about the preparation processes of CQD, CQDs-based photocatalysts, and their ability to remove environmental pollutants, with a special emphasis on the mechanism for depredating pollutants. Furthermore, this paper analyzes and discusses the prospects and challenges of CQDs in the environmental field.     

Visible‐Light Photocatalytic Activity and Deactivation Mechanism of Ag3PO4 Spherical Particles

Ag₃PO₄ spherical particles were synthesized by a facile precipitation method using silver nitrate and Na₂HPO₄ as precursors. The as‐prepared samples had a high photocatalytic activity toward Rhodamine B (RhB) degradation under visible‐light illumination. With increasing recycling times the photocatalytic activity first increased and then decreased. Based on systematic characterization of particles by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), UV/Vis absorption spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), a possible mechanism responsible for the improvement and subsequent decline of the photocatalytic performance of Ag₃PO₄ is proposed. Ag₃PO₄ spherical particles recycled for four times showed the highest photocatalytic activity because, according to our mechanism, Ag nanoparticles deposited on Ag₃PO₄ acted as electron trapping centers to prevent photogenerated electron‐hole pairs from recombination. A further increase in the recycle times decreases the photocatalytic activity owing to the shielding effect by Ag layers on the surface of Ag₃PO₄. The results presented herein shed new light on the photostability of Ag₃PO₄ spherical particles and are potentially applicable to other photocatalytically active composites.     

Preparation of porous g‐C3N4/Ag/Cu2O: a new composite with enhanced visible‐light photocatalytic activity

From previous reports, graphitic carbon nitride (g‐C₃N₄) can be used as a photocatalyst, although the low efficiency of solar energy utilization, small specific surface area and high recombination rate of photogenerated electron–hole pairs limit its practical application. For the purpose of increasing photocatalytic activity, especially under irradiation of visible light, we successfully synthesized a new composite, namely porous g‐C₃N₄/Ag/Cu₂O, through chemical adsorption of Ag‐doped Cu₂O on porous g‐C₃N₄, which has not been investigated carefully worldwide. The composition, morphology and optical properties of the composite were investigated through methods including X‐ray diffraction, energy‐dispersive X‐ray, Fourier transform infrared, UV–visible and photoluminescence spectroscopies and transmission electron microscopy. Using rhodamine B as organic pollutant to be degraded under the irradiation of visible light, different mass ratios of Ag/Cu₂O doped on porous g‐C₃N₄ led to enhanced photocatalytic performance of the composite compared to pure porous g‐C₃N₄. When the mass ratio of Ag/Cu₂O is 15%, porous g‐C₃N₄/Ag/Cu₂O exhibits a degradation rate 2.015 times higher than that of pure porous g‐C₃N₄. The reasons for this phenomenon may be attributed to the increased utilization efficiency of visible light, high‐speed separation of photogenerated electron–hole pairs, accelerated interfacial transfer process of electrons and increased surface area of the composite. Copyright © 2016 John Wiley & Sons, Ltd.     

Improved photocatalytic performance of anatase TiO2 synthesized through ethanol supercritical drying technique

Anatase titania (TiO₂) nanoparticles were synthesized via a self‐developed ethanol vapor‐thermal method at 240°C (T240) and 250°C (T250), i.e. at temperatures lower and higher, respectively, than the supercritical temperature (243.5°C, 7.0 MPa) of ethanol. Compared to T240, T250 exhibited a higher ratio of exposed (001) facets, oxygen vacancies, and concomitant TiO_x. The specific surface area of T250 was 119.0 m² g^?1, smaller than that of T240 (144.2 m² g^?1). During the degradation of methylene blue, T250 exhibited a high apparent rate constant (K_app) of 14.5 × 10^?2 min^?1, which was 6.3 times larger than that for T240. Furthermore, compared to T240, T250 exhibited better performance toward degradation of phenol. Results of electron spin resonance spectroscopy and photoluminescence indicated that the photogenerated electron–hole pairs possessed higher separation efficiency for T250 than for T240. In summary, the excellent photocatalytic performance of T250 originates from the higher ratios of exposed (001) facets, oxygen vacancies or TiO_x, C═O groups adsorbed at the surface of particles, and higher separation efficiency of photogenerated electron–hole pairs. By employing this self‐developed vapor‐thermal method, a variety of catalysts and their composites can be synthesized, which may exhibit novel morphological characteristics and properties as well as excellent photocatalytic performance.     

A Bi-based oxychloride PbBiO2Cl as a novel efficient photocatalyst

A novel visible-light-driven layered photocatalyst of Bi-based PbBiO₂Cl is prepared by solid-state reaction. The optical band gap of PbBiO₂Cl is determined to be 2.45 eV by UV–vis diffuse reflectance spectroscopy. Generally, the photocatalytic activity for degrading methyl orange (MO) over PbBiO₂Cl is higher than that over anatase-type TiO₂ under UV light and visible light illumination. The RuO₂ loading over PbBiO₂Cl leads to an obvious increase in photocatalytic performance. The internal electric fields between [PbBiO₂] and [Cl] slabs are considered to be useful for the efficient separation of electron–hole pairs upon photoexcitation.