Visible Light Photoactivity Research Articles

Synthesis of nitrogen doped faceted titanium dioxide in pure brookite phase with enhanced visible light photoactivity

Brookite titanium dioxide (TiO2) is rarely studied, as compared with anatase and rutile phases TiO2, due to its comparatively lower photoactivity. It has been recently reported that brookite TiO2 with active facets exhibits excellent performance, however, synthesis of such faceted brookite TiO2 is difficult because of its low thermodynamic phase stability and low structural symmetric. Furthermore, like faceted anatase and rutile TiO2, faceted brookite TiO2 is not responsive to visible light due to its wide bandgap. In this study, a novel dopant, hydrazine, was introduced in the development of nitrogen doping. By applying this dopant, nitrogen doped brookite nanorods with active {120}, {111} and {011¯} facets were successfully synthesized. The resultant materials exhibited remarkably enhanced visible-light photoactivity in photodegradation.

Degradation of refractory pollutants under solar light irradiation by a robust and self-protected ZnO/CdS/TiO2 hybrid photocatalyst

Photocatalyst plays a vital role in the photochemical water treatment. To improve the visible-light photoactivity of TiO2 for refractory pollutant degradation, CdS/TiO2 hybrids with different nanostructures have been prepared, but usually suffer from a low photocatalytic degradation efficiency and a rapid photocorrosion. In this work, we developed a synergistic ZnO/CdS/TiO2 hybrid, which could act as a robust and self-protected photocatalyst for water purification without additional sacrificial reagents. This was attributed to the two different junction mechanisms in one single hybrid. Photons were selectively adsorbed by ZnO and CdS, then, the electrons with a low reductive activity in ZnO recombined directly with the holes with a low oxidative activity in CdS, whereas the holes with a high oxidative activity in ZnO and the electrons with a high reductive activity in CdS were captured for catalytic reaction. The superiority of the novel ZnO/CdS/TiO2 hybrid over the traditional CdS/TiO2 hybrid in both photocatalytic activity and anti-photocorrosion capacity was demonstrated in the degradation of Atrazine and Rhodamine B, two typical refractory organic pollutants, and the treatment of real textile wastewater under solar light irradiation. The developed ZnO/CdS/TiO2 hybrid exhibited an excellent potential for the degradation of refractory pollutants, and provided a new way to advance intrinsically solar-susceptible catalyst for photochemical wastewater treatment.

A facile synthesis of g-C3N4/TiO2 hybrid photocatalysts by sol–gel method and its enhanced photodegradation towards methylene blue under visible light

The g-C3N4/TiO2 hybrid photocatalysts were synthesized through a modified sol–gel technique with varying the weight ratio of g-C3N4 under facile conditions. The microstructure and interface properties of the obtained photocatalysts were systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR) and UV–visible diffused reflectance spectroscopy (UV–vis DRS). It is indicated that the heterojunction between g-C3N4 and TiO2 formed and the TiO2 nanoparticles well dispersed on g-C3N4 sheets. Among as synthesized hybrid photocatalysts, the g-C3N4/TiO2-80% sample exhibited the highest photocatalytic activity under visible light irradiation using methylene blue as the target pollutant. Compared with pure g-C3N4 and TiO2, the g-C3N4/TiO2-80% hybrid photocatalyst exhibited enhanced visible-light photoactivity, which is around 3.5 times as high as that of the pure g-C3N4. The enhancement of the g-C3N4/TiO2 hybrid photocatalysts can be attributed to the relatively higher adsorption capacity and effective separation of photogenerated electron–hole pairs. The g-C3N4/TiO2 hybrid photocatalysts exhibited stable photocatalytic performance with a very low activity loss after going through five cycles, which shows that it is a promising material for the photodegradation of dyes in wastewater.

Facile preparation of NPs based on Ag–AgCl immobilized in porous PVA sphere with high visible light photoactivity and good photostability under Cl− condition

Porous PVA sphere immobilized with nanoparticles (NPs) based on Ag–AgCl was synthesized for the first time. The approach can be simply divided into three steps. Firstly the in-situ synthesis of Ag NPs in PVP/N, N-dimethylacetamide (DMAc) solution, secondly the phase inversion of Ag NPs/PVA aqueous solution in acetone (AC) to get porous Ag/PVA sphere, finally the crosslinking of Ag/PVA porous sphere by glutaraldehyde using HCl as catalyst, and AgCl NPs were formed through the reaction between Ag NPs and HCl simultaneously. BET and SEM results showed that AgCl/PVA sphere had a porous structure, and its surface area was about 192m2/g. XRD and XPS data proved the formation process of NP from Ag ions to Ag NPs and then to AgCl NPs. TEM and EDX images revealed that AgCl NPs were mainly buried inside PVA matrix and their sizes were about 50–90nm. Even for 5th cycles of photodegradation of methyl orange (MO) in xenon lamp, AgCl/PVA porous sphere still showed a high photoactivity and a good photostability in the presence of chloride ions. The possible mechanism of photocatalysis was proposed finally.

Surface co-modification of TiO2 with N doping and Ag loading for enhanced visible-light photoactivity

A composite of N-doped TiO2 with Ag loading (Ag/N-TiO2) was successfully synthesized by a facile in situ calcination process using titanium nitride (TiN) and silver nitrate (AgNO3) as the starting materials.

What if the Electrical Conductivity of Graphene Is Significantly Deteriorated for the Graphene-Semiconductor Composite-Based Photocatalysis?

The extraordinary electrical conductivity of graphene has been widely regarded as the bible in literature to explain the activity enhancement of graphene-semiconductor composite photocatalysts. However, from the viewpoint of an entire composite-based artificial photosynthetic system, the significant matter of photocatalytic performance of graphene-semiconductor composite system is not just a simple and only issue of excellent electrical conductivity of graphene. Herein, the intentional design of melamine resin monomers functionalized three-dimensional (3D) graphene (donated as MRGO) with significantly deteriorated electrical conductivity enables us to independently focus on studying the geometry effect of MRGO on the photocatalytic performance of graphene-semiconductor composite. By coupling semiconductor CdS with graphene, including MRGO and reduced graphene oxide (RGO), it was found that the CdS-MRGO composites exhibit much higher visible light photoactivity than CdS-RGO composites although the electrical conductivity of MRGO is remarkably much lower than that of RGO. The comparison characterizations evidence that such photoactivity enhancement is predominantly attributed to the restacking-inhibited 3D architectural morphology of MRGO, by which the synergistic effects of boosted separation and transportation of photogenerated charge carriers and increased adsorption capacity can be achieved. Our work highlights that the significant matter of photocatalytic performance of graphene-semiconductor composite is not a simple issue on how to harness the electrical conductivity of graphene but the rational ensemble design of graphene-semiconductor composite, which includes the integrative optimization of geometrical and electrical factors of individual component and the interface composition.

Visible light photoactivity of Polypropylene coated Nano-TiO2 for dyes degradation in water.

The use of Polypropylene as support material for nano-TiO2 photocatalyst in the photodegradation of Alizarin Red S in water solutions under the action of visible light was investigated. The optimization of TiO2 pastes preparation using two commercial TiO2, Aeroxide P-25 and Anatase, was performed and a green low-cost dip-coating procedure was developed. Scanning electron microscopy, Atomic Force Microscopy and X-Ray Diffraction analysis were used in order to obtain morphological and structural information of as-prepared TiO2 on support material. Equilibrium and kinetics aspects in the adsorption and successive photodegradation of Alizarin Red S, as reference dye, are described using polypropylene-TiO2 films in the Visible/TiO2/water reactor showing efficient dyes degradation.

A novel photocatalyst AgBr/ZnO/RGO with high visible light photocatalytic activity

A novel ternary photocatalyst AgBr/ZnO/RGO, where AgBr/ZnO is supported on reduced graphene oxide, is synthesized via a facile hydrothermal–impregnation method. The resultant composite presents a lamellar structure with AgBr nanoparticles homogeneously dispersing on the surface. The photocatalytic experiment for methyl orange dye degradation under visible light irradiation shows that ternary composite AgBr/ZnO/RGO has an activity 12.8 times and 2.3 times higher than binary photocatalysts ZnO/RGO and AgBr/ZnO respectively. More importantly, the ternary composite also demonstrates a good photostability. Metallic Ag is produced during the photocatalytic process, which may serve as the electron transfer mediator in the vectorial Z-scheme transfer of photogenerated charge carriers at the interface of AgBr/ZnO/RGO. The effective separation of photogenerated electrons and holes was proposed to be responsible for the enhancement of visible light photoactivity.

Facile synthesis and enhanced visible-light photoactivity of DyVO4/g-C3N4I composite semiconductors

DyVO4/iodine modified graphitic carbon nitride (DyVO4/g-C3N4I) composite semiconductors with different weight percents of DyVO4 were successfully synthesized by a facile heating method, and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), UV–vis diffuse reflection spectroscopy (UV–vis DRS), photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra, N2 adsorption–desorption analysis and photo-electrochemical measurement. The resulting DyVO4/g-C3N4I semiconductor with a suitable weight percents of 6.3% DyVO4 showed the highest visible-light photoactivity, and its degradation ratio for methylene blue was more than 1.8 time higher than that of DyVO4, g-C3N4 and g-C3N4I. The H2 evolution rate of 6.3% DyVO4/g-C3N4I was 10.6, 4.7 and 1.7 times higher than that of DyVO4, g-C3N4 and g-C3N4I, respectively, while still having excellent reusability and stability. The obviously enhanced photoactivity of 6.3% DyVO4/g-C3N4I is mainly ascribed to the fact that the proper DyVO4 modified g-C3N4I increase its specific surface area, decrease band-gap energy, enhance absorption in the 400–700nm region and promote efficient separation of photo-generated carriers. The mechanism on the improvement of visible-light photoactivity is discussed.

Investigation of the synergistic effect of porphyrin photosensitizer on graphene–TiO2 nanocomposite for visible light photoactivity improvement

To use the visible light in photodegradation reactions efficiently, graphene–TiO2 nanocomposite was photosensitized using tetrakis(4‐carboxyphenyl)porphyrin. To investigate the effect of graphene as well as dye sensitization on the photoactivity of the synthesized photocatalyst, photocatalytic properties and photocurrent responses of the prepared samples were examined under visible light irradiation. A twofold increase of photocurrent response in the TG nanocomposite photosensitized with porphyrin (TGP) compared to TG (3%) nanocomposite provides an evidence of the effective influence of the porphyrin photosensitizer in the visible light photoactivity enhancement. There is a complete correspondence between the photocatalytic activity and photocurrent response of the photocatalysts. The photocatalytic properties of the photocatalysts were carried out in the degradation of methyl orange and bisphenol A under visible light irradiation. It was determined that TGP compound was stable after photocatalytic process and degraded 93% of methyl orange and 85% of bisphenol A in 240 min under visible light irradiation. Furthermore, the mechanism of investigation of the photocatalytic process determines that and OH. are the main reactive species for degradation of methyl orange and bisphenol A over the TGP compound under visible light irradiation. © 2015 American Institute of Chemical Engineers Environ Prog, 35: 642–652, 2016

Atmospheric pressure synthesis of nitrogen doped graphene quantum dots for fabrication of BiOBr nanohybrids with enhanced visible-light photoactivity and photostability

A facile bottom-up approach for producing nitrogen doped graphene quantum dots (N-GQDs) was carried out by the carbonization of ammonium citrate under atmospheric pressure. And further, N-GQDs/BiOBr nanohybrids with N-GQDs uniformly distributed on BiOBr nanoplates were fabricated by a one-pot solvothermal method using the proposed N-GQDs as the precursor. As an excellent enhancer, the doped N-GQDs could improve the photoactivity and photostability of BiOBr nanoplates in the N-GQDs/BiOBr nanohybrids. Moreover, the photoactivity of N-GQDs/BiOBr nanohybrids was influenced remarkably by the content of the doped N-GQDs, and the results indicated that 3 wt% N-GQDs/BiOBr nanohybrids showed the optimal photoactivity. The N-GQDs/BiOBr nanohybrids showed excellent performance in the photoelectrochemical detection of glutathione as well as photocatalytic degradation of rhodamine B. And a possible mechanism of N-GQDs on the enhancement photoactivity and photostability of N-GQDs/BiOBr nanohybrids was also proposed. This gave insights into N-GQDs/BiOBr nanohybrids and described a novel method to design and fabricate high photoactivity nanohybrids related to N-GQDs.

Pt–TiO2-assisted photocatalytic degradation of the cytostatic drugs ifosfamide and cyclophosphamide under artificial sunlight

The aim of this work was to develop an effective photocatalytic technique for the removal of two cytostatic drugs from water. To this end, the kinetics, efficiency and decomposition pathway of the photocatalytic decay of ifosfamide (IF) and cyclophosphamide (CF) was investigated. The samples of Pt-doped (0.15at.%) and undoped TiO2 photocatalysts were prepared by a simple sol–gel method and then structurally characterized. The effects of operating parameters such as loading of photocatalyst, drug concentration, and photoactivity in solar and visible light were studied. The degradation processes in both drugs followed a pseudo-first-order kinetics. A degradation mechanism was proposed based on the identification of inorganic intermediates (NH4+, Cl−, PO43−) and organic by-products. The results showed that PO43− ions released during the decomposition of drugs were adsorbed on the photocatalyst, but this phenomenon had no negative effect on its photoactivity. The experimental results of the reactive species quenching showed that OH radicals were the predominant oxidant species participated in reaction, and the oxidation of drugs occurred in the bulk solution. Moreover, Pt-doped TiO2 was more effective at generating OH radicals than TiO2, probably due to the significantly promoted electron/hole separation in the modified photocatalyst. The results also demonstrated that the decomposition pathways of IF and CF differed.

Thickness-dependent photocatalytic activity of bismuth oxybromide nanosheets with highly exposed (0 1 0) facets

A series of BiOBr nanosheets was hydrothermally synthesized through pH adjustment of precursor suspensions. X-ray diffraction, X-ray photoelectron spectroscopy, electronic microscopy and UV–vis diffuse reflectance spectroscopy analyses indicate that these nanosheets possess similar crystallinity, surficial composition, structures, band gaps and highly exposed (010) facets, but different lateral sizes and thicknesses. The visible-light photoactivity of these nanosheets in photodegradation of salicylic acid increases with reduced nanosheet thickness. An inversely linear relationship was obtained between the pseudo-first-order rate constant (k) and the nanosheet thickness. The photoactivity enhancement of the samples with reduced nanosheet thickness is ascribed to the decreasing recombination efficiency of photogenerated charge carriers. This study indicates that the photoactivity of the BiOBr nanosheets with dominant exposed (010) facets decisively depend on the nanosheet thickness, rather than the percentage of exposed active facets ((001) facets), which may be universal for other two-dimensional photocatalysts.

Facile formation of mesoporous BiVO4/Ag/AgCl heterostructured microspheres with enhanced visible-light photoactivity.

In this study, we demonstrate a facile and novel dual-ion-exchange method together with subsequent visible-light induced reduction for synthesis of mesoporous BiVO4/Ag/AgCl ternary heterostructured microspheres (HSMSs) with uniform size distribution. Using flower-like BiOCl microspheres as the starting material, and introducing NaVO3 and AgNO3 by a facile impregnation method, mesoporous BiVO4/AgCl HSMSs have been obtained through solid-phase dual-ion-exchange reactions at 400 °C for 2 h. Interestingly, it has been found that Ag(+) ions play an indispensable role on the dual-ion-exchange reactions, and then the BiVO4/AgCl HSMSs are converted into BiVO4/Ag/AgCl ternary HSMSs by a facile visible-light illumination for 2 h. The as-prepared mesoporous BiVO4/Ag/AgCl ternary HSMSs manifest high photocatalytic activity in degrading methyl orange (MO) and phenol under visible-light illumination, and a possible Z-scheme photocatalytic mechanism is proposed to understand the enhanced photochemical properties.

N2/Ar plasma induced doping of ordered mesoporous TiO2 thin films for visible light active photocatalysis

This study reports the rapid and effective nitrogen doping of ordered, mesoporous TiO2 thin films using nitrogen/argon (N2/Ar) plasma. The resulting nitrogen-doped TiO2 (N–TiO2) films show significant enhancement in both visible light absorption and photocatalytic activity. The cubic ordered mesoporous TiO2 thin films are prepared via a sol–gel method using titanium tetrachloride (TiCl4) as precursor and triblock copolymer Pluronic F127 as the template. Following brief calcination, the TiO2 films are treated with N2/Ar plasma under controlled conditions of reactive gas pressure, microwave power, and plasma exposure duration. To vary the degree of nitrogen doping, the plasma exposure time was varied from zero up to 210 min. The nitrogen content of the films increases with plasma exposure duration, up to over 3 at% N. X-ray photoelectron spectroscopic (XPS) analyses and UV–vis absorbance spectra of N–TiO2 films indicate that the incorporated N atoms reduce the band gap of TiO2 and thus enhance the absorption of visible light. Finally, the visible-light photocatalytic activity of N–TiO2 films is determined from the photocatalytic degradation of methylene blue under visible-light illumination (with a 455 nm LED). The N–TiO2 films prepared by 150 min treatment show the optimum photocatalytic activity with a pseudo-first order rate coefficient of 0.24 h−1, which is six times greater than that of undoped TiO2 films. Treatment for excessive time (e.g. 210 min) leads to a decline in photocatalytic activity due to coarsening of the porous structure. The present study suggests that plasma-induced doping is a promising approach to enable the efficient incorporation of heteroatoms into surfactant-templated TiO2 thin films while maintaining their nanostructures, thereby leading to the significant enhancement of visible-light photoactivity.

Visible-light driven generation of reactive radicals over BiFeO3/TiO2 nanotube array: experimental evidence and energetic mechanism

BiFeO3-based materials have attracted broad interests due to their multiferric and photoactive properties, but there remains some controversial uncertainty about the electronic structures of various nanoparticles of BiFeO3. In this work, BiFeO3 nanoparticles smaller than 100 nm have grown into TiO2 nanotube array by an ultrasonic-immersion method. Some evidences on the energy band position of BiFeO3 nanoparticles in BiFeO3/TiO2 nanotube array were from the visible-light driven photo responses. Positive surface photovoltage polarity of the composite is the same as that of the pure TiO2 nanotube array, which reveals the photo-induced electron transfer from BiFeO3 nanoparticles to TiO2 nanotubes. Furthermore, both ·OH and ·O2 − radicals could be detected by the electron paramagnetic resonance (EPR) spin-trapping method. According to the EPR result, the valence band edge of BiFeO3 nanoparticles could be deduced as more positive than the potential at which ·OH radical could be generated. A scheme on the energy band alignment and photo-induced charges transfer processes is then proposed based on the observed visible-light photo activity of BiFeO3/TiO2 nanotube array.

Significantly Enhanced Visible Light Photoelectrochemical Activity in TiO₂ Nanowire Arrays by Nitrogen Implantation.

Titanium oxide (TiO2) represents one of most widely studied materials for photoelectrochemical (PEC) water splitting but is severely limited by its poor efficiency in the visible light range. Here, we report a significant enhancement of visible light photoactivity in nitrogen-implanted TiO2 (N-TiO2) nanowire arrays. Our systematic studies show that a post-implantation thermal annealing treatment can selectively enrich the substitutional nitrogen dopants, which is essential for activating the nitrogen implanted TiO2 to achieve greatly enhanced visible light photoactivity. An incident photon to electron conversion efficiency (IPCE) of ∼10% is achieved at 450 nm in N-TiO2 without any other cocatalyst, far exceeding that in pristine TiO2 nanowires (∼0.2%). The integration of oxygen evolution reaction (OER) cocatalyst with N-TiO2 can further increase the IPCE at 450 nm to ∼17% and deliver an unprecedented overall photocurrent density of 1.9 mA/cm(2), by integrating the IPCE spectrum with standard AM 1.5G solar spectrum. Systematic photoelectrochemical and electrochemical studies demonstrated that the enhanced PEC performance can be attributed to the significantly improved visible light absorption and more efficient charge separation. Our studies demonstrate the implantation approach can be used to reliably dope TiO2 to achieve the best performed N-TiO2 photoelectrodes to date and may be extended to fundamentally modify other semiconductor materials for PEC water splitting.

Synthesis of acidified palygorskite/BiOI with exceptional performances of adsorption and visible-light photoactivity for efficient treatment of aniline wastewater

The acidified palygorskite/BiOI composites with different amounts of acidified palygorskite were synthesized using an ethylene glycol-assisted solvothermal method. The morphology, structure, surface area and pore-size distribution of the composites were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and N2 adsorption–desorption isotherms. The BiOI microspheres consisting of numerous nanosheets were uniformly dispersed on the surface of acidified palygorskite. The composite with doping amount of 50wt.% acidified palygorskite manifested a large BET area of 118.5m2/g with mesoporous structure of 10.8nm. The surface chemistry was obtained by the determination of the point of zero charge. The introduction of acidified palygorskite made the composites have more acidic character. The adsorption of aniline onto acidified palygorskite/BiOI followed the pseudo-second-order model and the Freundlich equation. As pH increased, equilibrium adsorption concentration showed an upward trend, and reached a maximum at pH=7. The 50wt.%-acidified palygorskite/BiOI exhibited the optimal adsorption performance and visible-light photoactivity for 50mgL−1 of aniline solution. About 90wt.% of aniline was removed through combination of high adsorption and efficient photodegradation processes.

Facile hydrothermal synthesis of TiO2/Bi2WO6 hollow microsphere with enhanced visible-light photoactivity

In this paper, novel TiO2/Bi2WO6 heterojunction photocatalysts with hollow microsphere structures were successfully fabricated by a one-pot glucose-assisted hydrothermal route followed by the calcination at 450°C. The photocatalytic activities of the as-prepared photocatalysts were evaluated on the degradation of rhodamine B (RhB) aqueous solution under visible light irradiation and the results indicated that the as-prepared TiO2/Bi2WO6 composite exhibited higher photocatalytic activity than bare TiO2 and Bi2WO6. In addition, the TiO2/Bi2WO6 composite with TiO2 loading of 20wt.% showed the highest photocatalytic activity and retained the high photocatalytic performance after four recycles for the photodegradation of RhB, confirming its stability and practical usability. The enhanced photocatalytic activity of TiO2/Bi2WO6 composite was attributed to well matched energy band positions of TiO2 and Bi2WO6, which facilitated the efficient separation of photogenerated electron–hole pairs and thereby led to the increasing photocatalytic activity. On the basis of calculated energy band positions, the results of photoluminescence (PL) and active species trapping experiments, the mechanism of the obviously enhanced photocatalytic performance was proposed.

Boosting the visible-light photoactivity of Bi2WO6 using acidic carbon additives

We have explored the role of the physicohemical properties of carbon materials as additives to bismuth tungstate on its structure, optical properties, and photocatalytic activity for the degradation of rhodamine B under visible light. For this purpose, C/Bi2WO6 hybrid composites were prepared following two different routes: (i) physical mixture of the catalyst components, and (ii) one-pot hydrothermal synthesis of the semiconductor in the presence of the carbon additive. Three carbons with different properties were selected as additives: biomass-derived activated carbon, carbon nanotubes and carbon spheres obtained from polysaccharides. Data has shown the outstanding role of the acidic/basic nature of the carbon additive, and of the synthetic method on the photocatalytic performance of the resulting composites. For a given additive, the degradation rate of RhB is greatly improved for the catalysts prepared through a one-step hydrothermal synthesis, where there is low shielding effect of the carbon matrix. Carbon additives of acidic nature boost the surface acidity of the hybrid photocatalyst, thereby enhancing the photodegradation of RhB under visible light via a coupled mechanism (photosensitization, semiconductor photocatalysis and carbon-photon mediated reactions).