Improvement Of Photocatalytic Activity Research Articles

Photocatalytic activity of Ti3+ self-doped dark TiO2 ultrafine nanorods, grey SiO2 nanotwin crystalline, and their composite under visible light

Efficient electron-holes separation is of crucial importance for the improvement of photocatalytic activity for photocatalytic reaction. In this work, dark TiO2 (D-TiO2) nanorods, grey SiO2 (G-SiO2) and D-TiO2/G-SiO2 composite with surface defects are synthesized. We report that the efficiency of photo-generated electrons and holes separation is well enhanced by introducing G-SiO2 into D-TiO2 lattice. Using first-principles method, we find that surface defects (O or Si vacancy) can be conducive to improving the optical absorption under visible-light region. Combination of the experimental results, for D-TiO2/G-SiO2 composite, the surface defects of TiO2 nanocrystallines can significantly improve the photocatalytic efficiency.

Halloysite-derived mesoporous g-C3N4 nanotubes for improved visible-light photocatalytic hydrogen evolution

Abstract Hydrogen evolution via visible-light photocatalytic water splitting is an ideal solution to the worldwide energy and environmental crisis, while the key challenge is to develop efficient and cheap photocatalysts. Here, mesoporous graphitic carbon nitride (g-C3N4) nanotubes were synthesized using modified halloysite as template via a facial vapor deposition method. The halloysite-derived mesoporous g-C3N4 nanotubes exhibited an excellent photocatalytic hydrogen evolution efficiency of 633 μmol g−1 h−1 under visible light irradiation, which was 14.3 times that of bulk g-C3N4. The improvement of photocatalytic activity mainly benefits from the enlarged specific surface area and the reduced recombination rate of photoinduced electron-hole pairs of the mesoporous g-C3N4 nanotubes. The strategy established here may be valuable for synthesizing efficient g-C3N4 photocatalysts with other tailor nanostructures utilizing diverse raw clay minerals.

Potassium ions intercalated into g-C3N4-modified TiO2 nanobelts for the enhancement of photocatalytic hydrogen evolution activity under visible-light irradiation

Solar-to-chemical energy conversion is a challenging photochemical reaction for renewable energy storage. In recent decades, photocatalytic H2 evolution has been studied extensively. TiO2 is a well-established semiconductor in the field of photocatalytic H2 production; however, its low efficiency for solar energy utilization, and high photocarrier recombination rate, restrict its photocatalytic efficiency. Here, a series of K-intercalated g-C3N4-modified TiO2 nanobelts (TCN–Kx) with different dosages of K atoms were fabricated using a hydrothermal method followed by a calcination process. XRD, TEM and XPS tests indicate that a tight interfacial connection is formed between K–g-C3N4 and the TiO2 nanobelts. DFT calculations indicated that K dopants prefer to be at the interlayer sites of g-C3N4, suggesting increased charge transfer efficiency. The H2 production efficiency of the TCN–Kx composite materials from water splitting under visible-light irradiation was clearly improved. Steady fluorescence spectroscopy and photocurrent measurements confirmed that the improvement in photocatalytic H2 production activity was due to the superior charge separation and electron transfer efficiency of TCN–Kx composite materials.

Self-modified breaking hydrogen bonds to highly crystalline graphitic carbon nitrides nanosheets for drastically enhanced hydrogen production

Highly crystalline graphitic carbon nitride (g-C3N4) possesses the high separation efficiency of photogenerated electron-hole pairs owing to the significantly decreased intralayer hydrogen bonds, which leads to drastic improvement of photocatalytic activity. However, the preparation of such g-C3N4 material remains a challenge by a simple and economic thermal-treatment in a furnace. Herein, we report a novel and effective strategy for high-yield synthesis of extremely active crystalline carbon nitride nanosheets (CCNNSs) by two-step calcination without the assistance of any additive or salt intercalation. As expected, the as-prepared CCNNSs exhibit a remarkably high hydrogen evolution rate of 9577.6 μmol h−1 g−1 under simulated solar light irradiation, which is 15.5 times than that of bulk g-C3N4, as well as higher than most of the reported crystalline g-C3N4. Moreover, a highly apparent quantum efficiency of 9.01% at 420 nm for hydrogen evolution can be achieved, which is also superior to the reported crystalline g-C3N4. Such two-step calcination approach not only provides an economical way to effectively regulate the crystallinity of bulk g-C3N4, but also achieves the preparation of CCNNSs with high yield. Our research opens up a new window to self-modification and fabrication of highly active metal-free photocatalysts for solar light-driven hydrogen production.

Passivated Codoping Can Improve the Solar-to-Hydrogen Efficiency of Graphitic Carbon Nitride

Passivated codoping has never been reported to enhance the solar-to-hydrogen activity of g-C3N4, although it is an effective approach for other materials. In this letter, we use many-body Green’s function theory to analyze the electronic structures, optical absorption spectra, and spatial distribution of electron–hole pair of the doped g-C3N4. Our results suggest that the passivated codoping, such as B+O, could not only extend absorption toward visible range but also promote the separation of photogenerated hole and electron to different parts of g-C3N4. Improvement of photocatalytic activity can be realized only when the n-type and p-type dopants reside in different tris-triazine units of g-C3N4. This manifests the important role of codoping microstructure on solar-to-hydrogen efficiency. These results could provide a guideline for the design of more efficient artificial photocatalysts.

Noninvasively Modifying Band Structures of Wide-Bandgap Metal Oxides to Boost Photocatalytic Activity.

Although doping with appropriate heteroatoms is a powerful way of increasing visible light absorption of wide-bandgap metal oxide photocatalysts, the incorporation of heteroatoms into the photocatalysts usually leads to the increase of deleterious recombination centers of photogenerated charge carriers. Here, a conceptual strategy of increasing visible light absorption without causing additional recombination centers by constructing an ultrathin insulating heterolayer of amorphous boron oxynitride on wide-bandgap photocatalysts is shown. The nature of this strategy is that the active composition nitrogen in the heterolayer can noninvasively modify the electronic structure of metal oxides for visible light absorption through the interface contact between the heterolayer and metal oxides. The photocatalysts developed show significant improvements in photocatalytic activity under both UV-vis and visible light irradiation compared to the doped counterparts by conventional doping process. These results may provide opportunities for flexibly tailoring the electronic structure of metal oxides.

Preparation of new ferroelectric Li0.95Ta0.57Nb0.38Cu0.15O3 materials as photocatalysts in microbial fuel cells

AbstractA microbial fuel cell (MFC) is a device for both the generation of bioelectricity and the treatment of wastewater. However, its performance is affected by several parameters, and more particularly the materials used as the active phase at the cathode. In order to develop new, more efficient cathodes, two new non‐stoichiometric ferroelectric cathode materials are studied in this work. These electrodes have been synthesized according to two heat treatment modes (slow cooling and rapid cooling) from non‐stoichiometric ferroelectric materials with the formula Li0.95Ta0.57Nb0.38Cu0.15O3. The synthesized phases were characterized by X‐ray diffraction (XRD), transmission electronic microscopy (TEM), particle size distribution (PSD), and differential scanning calorimetry (DSC). The main characteristics of these phases are the Curie temperatures, 1217 and 1197 °C, and the specific surfaces of 0.572 and 0.801 m2/g for the slow and rapid cooling phases, respectively. These materials were subsequently tested as photocathodes in a single chamber MFC in terms of the bioenergy production and wastewater treatment, by measuring the output power density and the rate of removal of COD in the presence of a light source. For the samples prepared by slow and rapid cooling, the values of maximum power density were 20.10 and 205.35 mW/m3, respectively. The COD removal rates were 74 and 80 %, respectively. Accordingly, the phase prepared by rapid cooling was shown to be more efficient in terms of power generation and wastewater treatment with a significant improvement in photocatalytic activity.

Spaced Titania Nanotube Arrays Allow the Construction of an Efficient N-Doped Hierarchical Structure for Visible-Light Harvesting.

Regularly spaced TiO2 nanotubes were prepared by anodizing a titanium substrate in triethylene glycol electrolyte at elevated temperature. In comparison to conventional TiO2 nanotubes, spaced nanotubes possess an adjustable spacing between the individual nanotubes; this allows for controlled buildup of a hierarchical nanoparticle‐on‐nanotube structure. Here, we use this principle for layer‐by‐layer decoration of the tubes with TiO2 nanoparticles. The hierarchical structure after N doping and NH3 treatment at 450 °C shows a significant enhancement of visible‐light absorption, although it only carries a low doping concentration of nitrogen. For optimized N‐doped and particle‐decorated spaced TiO2 nanotubes, a considerable improvement in photocatalytic activity is obtained in comparison with conventional N‐doped TiO2 nanotubes or comparable N‐doped nanoparticle films. This is attributed to an enhanced visible‐light absorption through the N‐doped nanoparticle shell and a fast charge separation between the shell and the one‐dimensional nanotubular core.

Photoreactivity and mechanism of BiPO4/WO3 heterojunction photocatalysts under simulant sunlight irradiation

Considering the practical use, it is of important value to design full-spectrum photocatalysts that can absorb natural sunlight for photodegrading organic contaminants. A series of BiPO4/WO3 heterojunction catalysts with different WO3 contents were readily synthesized via a facile hydrothermal route. Under simulant sunlight irradiation, the photocatalytic activity of the as-prepared products was evaluated by degrading rhodamine B (RhB) solution. Results demonstrated that the most enhanced RhB degradation of 77.2% could be achieved in the BiPO4/WO3 composite with molar ratio of WO3 to BiPO4 to be 2:1, which was about 4.9 and 2.1 times higher than pure BiPO4 (15.9%) and WO3 (37.8%), respectively. During the photocatalytic process of BiPO4/WO3 composite, the photoinduced electrons in BiPO4 would transfer to WO3 while the photoinduced holes in WO3 could easily inject to BiPO4 through their intimate contact interfaces, thus leading to an efficient transfer and separation of interfacial charge carriers. Moreover, the interaction of BiPO4 and WO3 could also extend the region of the absorption spectrum, which then benefited the improvement of photocatalytic activity.

Visible-light responsive BiOBr nanoparticles loaded on reduced graphene oxide for photocatalytic degradation of dye

This work reports the preparation of reduced graphene-based bismuth oxybromide (G-BiOBr) nanocomposite using a facile synthesis procedure. The synthesized nanocomposite material was characterized using a powder X-ray diffractometer, transmission electron microscope, X-ray photoelectron spectroscopy and UV–vis diffuse reflectance spectroscopy. The photocatalytic efficiency of the as-prepared BiOBr and G-BiOBr nanocomposite were examined through photodegradation of rhodamine-B dye (RhB) under a visible light irradiation at 420 nm. Experimental factors (such as contact time, the dye concentration and dosage of the catalyst) were investigated using a Central Composite Design. The results revealed that G-BiOBr nanocomposite exhibits improved photocatalytic activity, ≈100% degradation, far above the as-prepared BiOBr material. The improvement in photocatalytic activity may be ascribed to improved light absorption and charge separation. Also, a mechanism was proposed to describe the improved photocatalytic degradation of RhB dye using G-BiOBr where the graphene plays a key role in electron transfer and in reducing the rate of electron-hole recombination.

Large enhanced photocatalytic activity of g-C3N4 by fabrication of a nanocomposite with introducing upconversion nanocrystal and Ag nanoparticles.

A novel heterostructured nanocomposite UCNPs@SiO2@Ag/g-C3N4 was developed for the first time to substantially boost the solar-light driven photocatalytic activity of g-C3N4. Its photocatalytic properties and photocatalytic mechanism were investigated. The as-synthesized photocatalyst with excellent improvement in the solar absorption and separation efficiency of photoinduced electron–hole pairs exhibited optimum solar-induced photocatalytic activity in dye degradation and hydrogen production. The experimental results showed that the rates of degradation of Rhodamine B (RhB) and hydrogen evolution were about 10 and 12 times higher than that of pristine g-C3N4, respectively. The excellent photocatalytic activity was attributed to the synergetic effect of upconversion nanoparticles (UCNPs) and Ag nanoparticles (NPs) on the modification of the photocatalytic properties of g-C3N4, resulting in a broad light response range for g-C3N4 as well as the fast separation and slow recombination of photoinduced electron–hole pairs. This study provides new insight into the fabrication of g-C3N4-based nanocomposite photocatalysts with high catalytic efficiency through the artful assembly of UCNPs, Ag NPs and g-C3N4 into a hetero-composite nanostructure. The prominent improvement in photocatalytic activity enables the potential application of g-C3N4 in the photocatalytic degradation of organic pollutants and hydrogen production utilizing solar energy.

The improvement of photocatalytic activity of monolayer g-C3N4 via surface charge transfer doping.

Graphite-like carbon nitride (g-C3N4) has attracted much attention due to its peculiar photocatalytic performance as a visible-light-responsive photocatalyst. However, its insufficient sunlight absorption is not conducive to the photocatalytic activity of the g-C3N4. Herein, by using first-principles density functional theory (DFT) calculations, we demonstrated a simple yet efficient way to achieve improvement of photocatalytic activity of monolayer g-C3N4via surface charge transfer doping (SCTD) using the electron-drawing tetracyanoquinodimethane (TCNQ) and electron-donating tetrathiafulvalene (TTF) as surface dopants. Our calculations revealed that the electronic properties of monolayer g-C3N4 can be affected by surface modification with TCNQ and TTF. These dopants are capable of drawing/donating electrons from/to monolayer g-C3N4, leading to the accumulation of holes/electrons injected into the monolayer g-C3N4. Correspondingly, the Fermi levels of monolayer g-C3N4 were shifted towards the valence/conduction band regions after surface modifications with TCNQ and TTF, along with the increase/decrease of work functions. Moreover, the optical property calculations demonstrated that the TCNQ and TTF modifications could significantly broaden the optical absorption of monolayer g-C3N4 in the visible-light regions, yielding an improvement in the photocatalytic activity of monolayer g-C3N4. Our results unveil that SCTD is an effective way to tune the electronic and optical properties of monolayer g-C3N4, thus improving its photocatalytic activity and broadening its applications in splitting water and degrading environmental pollutants under sunlight irradiation.

Improvement of photocatalytic activity of surfactant modified In2O3 towards environmental remediation

TX-100 surfactant modification of In2O3 semiconductor thin films with better surface coverage and crystallinity for photoelectrochemical applications with improved catalytic properties.

Facile synthesis of ZnO/g-C3N4 composites with honeycomb-like structure by H2 bubble templates and their enhanced visible light photocatalytic performance

ZnO/g-C3N4 composite photocatalyst has been successfully prepared through hydrothermal reaction in the presence of Zn(NO3)2·6H2O-NH3·H2O-NaBH4-g-C3N4 at 60 °C for 2 h, followed by annealing at 400 °C in air for 2 h. As-synthesized composite photocatalyst was characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV-vis) spectroscopy, photoluminescence spectroscopy (PL) and X-ray photoelectron spectroscopy (XPS). The composite photocatalyst exhibits honeycomb-like superstructure decorated with ZnO (28.3%) nanoparticles, and its photocatalytic activity increased by 70% compared to g-C3N4 itself evaluating by photodegardation of Rhodamine B under visible light. The improvement of photocatalytic activity could be attributed to the adsorption ability and effective separation of electron-hole pairs between g-C3N4 and ZnO. A possible mechanism was proposed to explain the formation of ZnO/g-C3N4 composite with honeycomb-like superstructure.

Construction of magnetic AgBr/Cu/CuFe2O4 Z-scheme photocatalyst with improved photocatalytic performance

A novel magnetic AgBr/Cu/CuFe2O4 composite was prepared by a two-step synthesis method, and this ternary composite exhibited an excellent photocatalytic performance for rhodamine B (RhB) and tetracycline (TC) degradation under visible light. The photocatalytic activity improvement of AgBr/Cu/CuFe2O4 composite could be attributed to the efficient space separation of photogenerated charge carriers through a Z-scheme configuration. As-synthesized AgBr/Cu/CuFe2O4 could be easily recovered by a magnetic field along with good reusability.

Plasmonic Cu nanoparticle on reduced graphene oxide nanosheet support: An efficient photocatalyst for improvement of near-infrared photocatalytic H2 evolution

Cu nanoparticles (NPs) are low-cost plasmonic metals and have been used for photocatalytic hydrogen evolution due to their surface plasmon resonance (SPR) effect. An efficient photocatalyst consisting of plasmonic Cu NPs on reduced graphene oxide (rGO) nanosheets support (Cu/rGO) is successfully prepared by a facile in-situ photoreduction process to form multiple nanoscale junctions for enhancement of photocatalytic hydrogen evolution. Compared to individual Cu NPs, the coupling to rGO nanosheets produces a higher photocatalytic ability with the highest H2 evolution rate of 59mmolg−1h−1 for sample C, which contains 1.0wt% of rGO nanosheets. The rGO nanosheet possesses an extremely high conductivity and, can accept and transfer photogenerated electrons with no barrier. Therefore, recombination of photoinduced charges can be efficiently suppressed and the accepted electrons can be rapidly transferred to reactive sites for H2 evolution across its two-dimensional plane. More importantly, a near-infrared photocatalytic activity for sample C was obtained under monochromatic light irradiation at 800 and 900nm due to the broad spectrum response of plasmonic Cu NPs. Furthermore, a possible reaction mechanism is proposed for the photocatalytic activity improvement as well as the detailed charge transfer condition.

In situ fabrication of CDs/g-C3N4 hybrids with enhanced interface connection via calcination of the precursors for photocatalytic H2 evolution

Novel carbon dots (CDs)/graphitic carbon nitride (g-C3N4) hybrids were fabricated via an in situ thermal polymerization of the precursors, urea and glucose. This heterojunction catalyst exhibited enhanced photocatalytic H2 evolution activity under visible-light (λ > 420). A sample of CDs/g-C3N4 hybrids, CN/G0.5, which was prepared from 0.5 mg of glucose in 6.0 g of urea (8.3 × 10−3 wt% glucose), exhibited the best photocatalytic performance for hydrogen production from water under visible light irradiation, which is about 4.55 times of that of the bulk g-C3N4 (BCN). The improvement of photocatalytic activity was mainly attributed to the construction of built-in electric field at the interface of CDs and g-C3N4, which could improve the separation of photogenerated electron-hole pair. Moreover, the tight connection of CDs with g-C3N4 would serve as a well electron transport channel, which could promote the photocatalytic H2 evolution ability.

Enhancing photocatalytic H2 evolution from water on CuO-Co3O4/TiO2: The key roles of Co3O4 loading amounts

Spatially controlled co-loading of both oxidation and reduction cocatalysts on semiconductor photocatalysts has been demonstrated to be favorable for photogenerated charge separation and thus enhanced photocatalytic activity. Herein, we presented a simple ball-milling method to improve photocatalytic H2 production of TiO2 by co-loading of oxidation cocatalyst Co3O4 with reduction cocatalyst CuO, wherein the Co3O4 loading amount was significant especially for TiO2 loaded with sufficient or optimum CuO. In comparison with that on TiO2 loaded with only CuO, the photocatalytic H2 production from water over TiO2 photocatalyst co-loaded with CuO and trace Co3O4 was enhanced by approximately 5.5 times, even though complex spatial control was not adopted. In contrast, remarkable improvement of photocatalytic activity after Co3O4 co-loading was only observed on TiO2 photocatalysts loaded with less CuO in the presence of methanol. Better separation of photogenerated charges on the surface of dual-cocatalysts modified TiO2 was further demonstrated by decreased luminescence as well as increased photogenerated electrons and hydroxyl radicals as compared to those loaded with only CuO.

Synthesis of Au nanoparticle-decorated carbon nitride nanorods with plasmon-enhanced photoabsorption and photocatalytic activity for removing various pollutants from water

Herein we have developed Au nanoparticle-decorated carbon nitride (Au-CN) nanorods as novel and efficient photocatalysts. Au-CN with different Au/CN precursor molar ratios (0.5%, 1% and 2%) have been prepared by a solvothermal-hydrothermal two-step method, where CN nanorods have diameters of 20–30nm and length of 0.5–1μm while Au nanoparticle have diameter of ∼13nm. Au-CN nanorods exhibit a broad photoabsorption from ultraviolet to near-infrared with edge at ∼790nm, revealing an obvious red-shift compared with g-C3N4 bulk (∼460nm), CN nanorods (∼715nm). Under visible-light irradiation, 1%Au-CN nanorods exhibit the highest photocatalytic activity, and they can degrade 98.2% rhodamine B (RhB), 77.2% 4-chlorophenol (4-CP), 83.9% tetracycline (TC) and reduce 43.6% hexavalent chromium (Cr(VI)) in 120min, higher than those by pure CN nanorods (70.3% RhB, 36.6% 4-CP, 54.6% TC, 23.1% Cr(VI)) and g-C3N4 bulk (31.5% RhB, 17.2% 4-CP, 36.9% TC, 11.8% Cr(VI)). Compared with CN nanorods, the obvious improvement of photocatalytic activity of 1%Au-CN nanorods should be attributed to the plasmon-enhanced photoabsorption and efficient separation of hole-electron pairs due to the introduction of Au nanoparticles.

Low temperature preparation of oxygen-deficient tin dioxide nanocrystals and a role of oxygen vacancy in photocatalytic activity improvement

The introduction of oxygen vacancies (Vos) into tin dioxide crystal structure has been found as an effective method to improve its photocatalytic performance. Herein, oxygen-deficient tin dioxide (SnO2−x) nanocrystals were successfully prepared via a facile, one-step hydrothermal method at the temperature lower than those reported previously. The effect of hydrothermal temperature on phase composition and Vos content was also firstly investigated. Due to its high oxygen vacancy concentration, the SnO2−x prepared at 80 °C provides the best photocatalytic degradation of methyl orange under UV–visible light. Scavenger trapping and nitroblue tetrazolium experiments also show that the Vos act as electron trapped sites and molecular oxygen adsorption sites, therefore increasing the production of active O2− radical which is the main species governing the photocatalytic activity of SnO2−x nanocrystals. Raman spectroscopy, X-ray photoelectron spectroscopy, photoluminescence measurement and electron spin resonance investigation clearly indicate that increasing the hydrothermal temperature results in the coexistence of SnO2−x and Sn3O4 phases and the reduction of Vos concentration which are detrimental to the photocatalytic performance. Density functional theory calculations also reveal that the presence of Vos is responsible for the upshift of valence band maximum and an extended conduction band minimum, hence a valence band width broadening and band gap narrowing which consequently enhance the photocatalytic performance of the oxygen-deficient SnO2−x.