Visible Light Photoactivity Research Articles

Crafting carbon sphere-titania core–shell interfacial structure to achieve enhanced visible light photocatalysis

Anchoring titania on carbon sphere forms a core–shell structure (TiO2@CS) which shows high-performance visible light response photocatalysis. In order to further improve such photocatalysis, herein acrylic acid (AA) is employed to modify the surface chemistry of CS so as to craft the interfacial structure between CS and TiO2. It was found that AA addition leads to morphology change from smooth to rough surface or to aggregates of smaller primary nanoparticles which, however, do not impede formation of the close contact core–shell structure with TiO2. Detailed analysis demonstrates a complex change in surface functional groups of CS in which the acidic OH is the major type whose quantity shows a peak maximum and a continuous increase at small and large AA addition ranges, respectively. Photocatalysis, photocurrent, and DFT calculation certificate that rather than COOH, the acidic OH, particularly, that related to sufficiently conjugated backbone carbon contributes to the formation of the interfacial structure which facilitates better visible light responsive photocatalysis. Benefiting from the improved interfacial charge separation, the visible light photoactivity reaches the highest when the quantity of the conjugated carbon-related OH becomes the maximum. An explicit charge transfer mechanism is proposed based on both experimental and theoretical calculation results.

Eliminating the isolated states from the band structure of doped TiO2: A first principle study

Band structure of anatase [Formula: see text] could be modified by doping foreign atoms. With ab-initio calculations, considering praseodymium, nitrogen and gadolinium as potential dopants, some mono-, co- and tri-doped models are introduced. The structure modification due to each model is compared with a standard [Formula: see text] system. Pr doping reduced the band gap of [Formula: see text] by introducing Pr 4f states below the conduction band, and the Fermi level appears below the conduction band minimum. The addition of nitrogen in Pr-doped [Formula: see text] pushed back the Fermi level to its intrinsic position while reducing the band gap. The tri-doped system comprising of Pr, N and Gd as dopants provided a populated band gap. The impurity states associated with Gd appeared in the middle of the band gap. The density of states analysis reveals that Pr 4f states change the location of N 2p states in the band structure. Instead of appearing as isolated states, the N 2p states are mixed with O 2p states while the Pr 4f states are coupled with the Ti 3d states in Pr, N-[Formula: see text]. The Pr, N, Gd tri-doped [Formula: see text] system provided strong visible light absorption due to the stepwise transition of electrons by Gd f states. While providing reasonable visible light absorption, the Pr, N-[Formula: see text] is expected to provide strong visible light photoactivity due to the clear band gap. Due to the absence of isolated states, the generated photo-excited carriers would be efficiently utilized in the oxidation/reduction processes.

Metal-doped ZnFe2O4 nanoparticles derived from Fe-bearing slag with enhanced visible-light photoactivity

M-ZnFe2O4 (M = Al, Zn, Cu, Mn) nanoparticles, which are potential photocatalysts, were prepared from Fe-bearing solid waste, namely Waelz slag. The valuable elements, namely Fe, Al, Zn, Cu, and Mn, were extracted from the slag simultaneously through sulfuric acid leaching. The extracted elements were converted into sediments in a controlled fashion by manipulating the pH of the lixivium and subsequently transformed into metal-doped ZnFe2O4 nanoparticles using the sol-gel method. The as-synthesised products were characterised using X-ray diffraction, Brunauer-Emmett-Teller analysis, field emission scanning electron microscopy, energy dispersive spectroscopy, UV–vis spectroscopy, and photoluminescence spectroscopy. The photocatalytic performances of the slag-derived M-ZnFe2O4 nanoparticles in photocatalytic methyl orange degradation under visible light were compared with that of undoped ZnFe2O4 nanoparticles. The results indicated that the slag-derived nanoparticles gave better photocatalytic performances than did the undoped sample, with the M-ZnFe2O4 (M = Al, Zn, Cu) sample, which was prepared with precipitates obtained at pH 7, exhibiting the best photocatalytic activity. The enhancement can be attributed to grain refinement and doping.

Surface polar charge induced Ni loaded CdS heterostructure nanorod for efficient photo-catalytic hydrogen evolution

Designing of artificial heterostructure photo-catalysts to crop solar energy for H2 evolution from water is of great importance nowadays. The ultrafine Ni (0.5, 1.0, 2.0 and 5.0 wt%) particles loaded CdS nanorods were synthesized by a simple chemical process. XRD shows the crystalline phase of CdS with increase in size from 17 to 28 nm with 10.19% and 10.06% enhancement in the lattice strain and the dislocation density for Ni (0.5–5.0 wt%). The XPS peaks observed at 854.88 eV and 861.07 eV for Ni2+ with energy separation of 6.18 eV confirmed the existence of NiO on Ni surface. The Raman bands for pure CdS and Ni (1.0 wt%)-CdS nanorods were observed at 300 cm−1 and 293 cm−1 for 1LO phonon and 601 cm−1 and 586 cm−1 for 2LO phonon, respectively. The Ni loading tuned the CdS band gap from 2.36 to 2.20 eV. The eight fold enhancement in the CdS specific surface area i.e., from 4.19194 m2 g−1 to 34.8343 m2 g−1 was achieved. After Ni loading, the synergetic effect of efficient electron separation and transportation was observed by the continuous quenching of luminescence emission intensity and the reduction of charge transfer resistance from 706 Ω for CdS to 484 Ω of CdS. The Ni (1.0 wt%)@ NiO optimal loading on CdS results highest photo-catalytic H2 evolution of 9.0 mmol at rate of 1.8 mmol h−1, which is about 50 times higher than that of 180 μmol at rate of 36 μmol h−1 for pure CdS. A thin layer of NiO on plasmonic Ni surface could be the promising system for photo-catalytic H2 evolution due to visible light photo-activity.

Oxygen vacancies enable the visible light photoactivity of chromium-implanted TiO2 nanowires

The introduction of oxygen vacancies activates the visible-light photoactivity of chromium-doped TiO 2 by enhancing the efficiency of charge separation and transfer. Although computational studies have demonstrated that metal ion doping can effectively narrow the bandgap of TiO 2 , the visible-light photoactivity of metal-doped TiO 2 photoanodes is still far from satisfactory. Herein, we report an effective strategy to activate the visible-light photoactivity of chromium-implanted TiO 2 via the incorporation of oxygen vacancies. The chromium-doped TiO 2 activated by oxygen vacancies (Cr-TiO 2 -vac) exhibited an incident photon-to-electron conversion efficiency (IPCE) of ~6.8% at 450 nm, which is one of the best values reported for metal-doped TiO 2 . Moreover, Cr-TiO 2 -vac showed no obvious photocurrent decay after 100 h under visible-light illumination.

Synthesis of Ti3+ self-doped mesoporous TiO2 cube with enhanced visible-light photoactivity by a simple reduction method

Through a simple reduction method, mesoporous anatase TiO2 cube was synthesized and the Ti3+ species and oxygen vacancies were successfully introduced into the anatase TiO2. CO2 and CO gases were produced by the thermal decomposition of oxalic acid and acted as a soft template to form the mesoporous structure, wherein CO acted as a reducing agent for reducing Ti4+ to Ti3+. The X-ray photoelectron spectroscopy spectra confirmed the presence of Ti3+ and oxygen vacancies in the samples. The UV–vis diffuse reflection spectra showed that the Ti3+ and oxygen vacancies shift the absorption edges of TiO2 from UV to the visible light region, which leaded to the enhanced photocatalytic activity. The charge carrier recombination rates of as-prepared samples were monitored by PL spectroscopy. The results showed that the trend was TiO2-3.70 < TiO2-1.85 < TiO2-0 < TiO2-7.41 < TiO2-11.11 (The X in the sample number TiO2-X refers to the ratio of the amount of H2C2O4 to Ti(SO4)2 (mole ratio)), implying that the charge carrier recombination rate of TiO2-3.70 was the lowest, which was responsible for enhancing the photocatalytic activity in the degradation of methylene orange under visible light irradiation. The Ti3+ and oxygen vacancies content, crystal size, specific area were strongly affected by the amount of oxalic acid. The oxalic acid content had an optimum value for the highest photocatalytic activity. It was worth noting that the sample exhibits excellent photocatalytic stability even after five recycles. This work demonstrates a new way to fabricate Ti3+ self-doped mesoporous TiO2 cube that have multiple channel structure and has potential to be used as in environmental photocatalysis.

Effective Visible Light Exploitation by Copper Molybdo-tungstate Photoanodes.

The need for stable oxide-based semiconductors with a narrow band gap, able to maximize the exploitation of the visible light portion of the solar spectrum, is a challenging issue for photoelectrocatalytic (PEC) applications. In the present work, CuW1–xMoxO4 (Eg = 2.0 eV for x = 0.5), which exhibits a significantly reduced optical band gap Eg compared with isostructural CuWO4 (Eg = 2.3 eV), was investigated as a photoactive material for the preparation of photoanodes. CuW0.5Mo0.5O4 electrodes with different thicknesses (80–530 nm), prepared by a simple solution-based process in the form of multilayer films, effectively exhibit visible light photoactivity up to 650 nm (i.e., extended compared with CuWO4 photoanodes prepared by the same way). Furthermore, the systematic investigation on the effects on photoactivity of the CuW0.5Mo0.5O4 layer thickness evidenced that long-wavelength photons can better be exploited by thicker electrodes. PEC measurements in the presence of NaNO2, acting as a suitable hole scavenger ensuring enhanced photocurrent generation compared with that of water oxidation while minimizing dark currents, allowed us to elucidate the role that molybdenum incorporation plays on the charge separation efficiency in the bulk and on the charge injection efficiency at the photoanode surface. The adopted Mo for W substitution increases the visible light photoactivity of copper tungstate toward improved exploitation and storage of visible light into chemical energy via photoelectrocatalysis.

Rapid visible-light degradation of EE2 and its estrogenicity in hospital wastewater by crystalline promoted g-C3N4

Metal-free, chemically activated crystalline graphitic carbon nitride (g-C3N4) nanorods with enhanced visible-light photoactivity demonstrated rapid photodegradation of 17α-ethinylestradiol (EE2) in water and real hospital wastewater. Pure g-C3N4 and another three crystalline promoted g-C3N4 photocatalysts developed by hydrothermal method were characterized by, High-Resolution Transmission Electron Microscopy (HRTEM), X-ray diffraction (XRD), Fourier-Transform Infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET), Photoluminescence (PL), Electron spin resonance (ESR), X-ray Photoelectron Spectroscopy (XPS) and Diffuse Reflectance Spectroscopy (DRS). Hydrothermal-based chemical activation did not alter the crystal structure, functional group or surface morphology, but it enhanced the specific surface area of activated g-C3N4 due to intralayer delamination and depolymerization of g-C3N4. Compared to pure g-C3N4, the activated g-C3N4-3 demonstrated efficient degradation of EE2 (<30 min, 3 mg/l) by visible wavelengths of the solar spectrum. This work provides advanced insight into the construction of heterojunction visible-light photocatalysts and production of O2− via reduction of O2 with photogenerated electrons. Proposed and derived mechanism for photodegradation of EE2 by g-C3N4-3 using gas chromatography-mass spectrometry (GCMS). Yeast Estrogen Screen (YES) was performed to evaluate the estrogenicity of treated water samples. Efficient removal of EE2 estrogenic activity (<45 min, 3 mg/l) was achieved using the visible light-activated g-C3N4. Estrogenicity removal rate corresponded well with EE2-degradation rate.

Solid-state synthesis of ultrathin MoS2 as a cocatalyst on mesoporous g-C3N4 for excellent enhancement of visible light photoactivity

Abstract An ultrathin MoS2 nanosheet was synthesized using a simple solid-state method with sulfur powder as the sulfur source and SiO2 as a template. The confinement effect of SiO2 spheres inhibited the growth of MoS2 along the z-axis to produce a MoS2 nanosheet composed of a few layers (2–9 layers). The SiO2 template method was simultaneously used to prepare mesoporous g-C3N4, which exhibited a higher specific area and faster charge transfer than bulk g-C3N4 and thus higher photocatalytic activity for RhB degradation. The ultrathin MoS2 nanosheets loaded onto mesoporous g-C3N4 provide active sites to obviously promote photocatalytic RhB degradation. The unique ultrathin structure of the MoS2 nanosheet decreases the distance of electron transfer in MoS2, inhibits the recombination probability of carriers, and strongly adsorbs RhB, thus facilitating the photocatalytic reaction for RhB degradation. Control experiments confirmed that ∙O2− is the most important active species, and CO2 is the dominant degraded product during the photocatalytic reaction. This study provides guidance for the design of photocatalysts and an in-depth understanding of the reaction mechanism of pollutant photodegradation.

Microwave-Assisted Ion-Exchange Synthesis and Enhanced Visible-Light Photoactivity of Graphene-CdS/CuS Nanoplates.

A novel reduced graphene oxide graphene-CdS/CuS nanoplate (G-CdS/CuS NP) was fabricated using a microwave method of loading CdS nanoplates onto graphene sheets (G-CdS NP) and subsequent cation exchange between G-CdS NP and Cu(NO₃)₂. The prepared G-CdS/CuS NP exhibited excellent photocatalytic activity which was attributed to the synergy between graphene and CdS/CuS NP. The heterostructure in the CdS/CuS NP lead to an efficient charge transport across the interface. Graphene facilitated the transformation of electrons and reduced the recombination of electron-hole pairs. These results provide a fabrication method for highly active photocatalysts and will lead to future rational design of photocatalytic materials.

Synthesis of SnO2/yolk-shell LaFeO3 nanocomposites as efficient visible-light photocatalysts for 2,4-dichlorophenol degradation

Herein, yolk-shell LaFeO3 (YS-LFO) microsphere has been successfully prepared by a carbon microsphere (CMS) template-adsorption-calcination method. It was confirmed that the YS-LFO microspheres showed high visible-light photoactivities for 2,4-dichlorophenol degradation as compared to bare LaFeO3 (LFO) nanoparticles, attributing to the unique yolk-shell structure and the increased surface area. The visible-light photocatalytic activities of YS-LFO microspheres could be further promoted by coupling with SnO2 (SO) nanoparticles. Based on the surface photovoltage spectra, photoelectrochemical measurements and other characterization, it was demonstrated that the coupled SO afforded a thermodynamically suitable platform for accepting the high-level energy electrons from LaFeO3 so as to enhance the charge separation. Compared with YS-LFO microspheres and LFO nanoparticles, the amount optimized (10SO/YS-LFO) nanocomposites displayed 1.6 and 4.7-time enhancement for 2,4-dichlorophenol degradation under visible-light irradiation, respectively. This efficient strategy of morphology controlling and photoelectron modulating could pave the approach to design high-activity visible-light driven photocatalysts.

Liquid-Plasma Hydrogenated Synthesis of Gray Titania with Engineered Surface Defects and Superior Photocatalytic Activity.

Defect engineering in photocatalysts recently exhibits promising performances in solar-energy-driven reactions. However, defect engineering techniques developed so far rely on complicated synthesis processes and harsh experimental conditions, which seriously hinder its practical applications. In this work, we demonstrated a facile mass-production approach to synthesize gray titania with engineered surface defects. This technique just requires a simple liquid-plasma treatment under low temperature and atmospheric pressure. The in situ generation of hydrogen atoms caused by liquid plasma is responsible for hydrogenation of TiO2. Electron paramagnetic resonance (EPR) measurements confirm the existence of surface oxygen vacancies and Ti3+ species in gray TiO2−x. Both kinds of defects concentrations are well controllable and increase with the output plasma power. UV–Vis diffused reflectance spectra show that the bandgap of gray TiO2−x is 2.9 eV. Due to its extended visible-light absorption and engineered surface defects, gray TiO2−x exhibits superior visible-light photoactivity. Rhodamine B was used to evaluate the visible-light photodegradation performance, which shows that the removal rate constant of gray TiO2−x reaches 0.126 min−1 and is 6.5 times of P25 TiO2. The surface defects produced by liquid-plasma hydrogenation are proved stable in air and water and could be a candidate hydrogenation strategy for other photocatalysts.

Enhancing DSSC Photoanode Performance by Using Ni-Doped TiO2 to Fabricate Scattering Layers

Titanium dioxide powder (TDP) and nickel-doped titanium dioxide powder (NiTDP) were synthesized by simple processes based on the sol–gel method. Nickel can be doped into titanium dioxide to narrow its energy bandgap; as a result, the visible-light photoactivity can be improved. In this study, the particle size of the fabricated TDP and NiTDP were larger than 100 nm, which is suitable for fabricating the light scattering layer (LSL) of the photoanode of dye-sensitized solar cells (DSSCs). The photoanode of our DSSCs comprised two layers: (1) a light absorbent layer made of commercial P25 titanium dioxide nanoparticles and (2) an LSL made of TDP or NiTDP. The DSSC used an organic MK-2 dye sensitizer, iodine-based redox electrolyte, and a multiwalled carbon nanotube counter electrode. Experimental results show that the photocurrent was significantly enhanced when the LSL was used. The DSSC with the absence of the LSL exhibited a conversion efficiency of 2.15%, while those of the DSSCs with the TDP-based LSL and NiTDP-based LSL were 2.37% and 2.78%, respectively. These results combined with UV–Vis spectra data, confirm the effectiveness of the extended visible-light photoactivity of the NiTDP-based LSL on the performance of the photoanode.

Nickel as recyclable catalyst for effective photocatalytic degradation of methyl orange

The reusability study of EGNiO catalyst for degradation of methyl orange (MO) dye was carried out and investigated. The overall degradation rate shows only small decreases which is less than 18% after run for seventh cycles, showing that the catalyst is more stable. The heat treatment and scouring of catalyst surface became the major reason and also plays an important role during the reusability process. No ions dissolution was observed during photoreaction as also confirmed by AAS analysis. It is reveals that the generation of hydroxyl radicals were fully used during the photocatalytic reaction due to no deposition of MO on the catalyst surface. Good photoactivity and photoresponse in visible light make these EGNiO catalyst as potential catalyst to be applied in wastewater treatment system.

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Co-MOF as an electron donor for promoting visible-light photoactivities of g-C3N4 nanosheets for CO2 reduction

A possible mechanism for boosting the visible-light photoactivities of graphitic carbon nitride (g-C3N4) nanosheets for CO2 reduction via coupling with the electron donor Co-metal-organic framework (MOF) is proposed in this study. Specifically, Co-MOF as an electron donor is capable of transferring the photogenerated electrons in the lowest unoccupied molecular orbital (LUMO) to the conduction band of g-C3N4 to facilitate charge separation. As expected, the prepared Co-MOF/g-C3N4 nanocomposites display excellent visible-light-driven photocatalytic CO2 reduction activities. The CO production rate of 6.75 µmol g−1 h−1 and CH4 evolution rate of 5.47 µmol g−1 h−1 are obtained, which are approximately 2 times those obtained with the original g-C3N4 under the same conditions. Based on a series of analyses, it is shown that the introduction of Co-MOF not only broadens the range of visible-light absorption but also enhances the charge separation, which improves the photocatalytic activity of g-C3N4 to a higher level. In particular, the hydroxyl radical (•OH) experiment was operated under 590 nm (single-wavelength) irradiation, which further proved that the photogenerated electrons in the LUMO of Co-MOF can successfully migrate to g-C3N4. This work may provide an important strategy for the design of highly efficient g-C3N4-based photocatalysts for CO2 reduction.

Enhanced Visible-Light-Induced Photocatalytic Performance of g-C3N4/ZnS/CuS Ternary Composite for Environmental Remediation

The inherent drawback of high photo-induced electron-hole pair recombination rate restricts the photocatalytic performances of graphitic carbon nitride (g-C3N4).Here, in this work, g-C3N4/ZnS/CuS ternary nanocomposites with different CuS concentration were successfully fabricated by a hydrothermal process and subsequent followed by a cation exchange reaction. The as-synthesized samples were characterized by the techniques of XRD, SEM, DRS, FT-IR and PL. Characterization results show a much more efficient electron-hole pair separation rate of g-C3N4/ZnS/CuS, the g-C3N4/ZnS/CuS (8%) sample shows the best photocatalytic performance for degradation of Rhodamine B (RhB), which is about 1.6 times higher than that of g-C3N4/ZnS and 6.2 times higher than that of pure g-C3N4, respectively. The high photodegraded activity is attributed to the important role of ZnS and CuS acted as electron co-catalysts, which retard the fast recombination of photo-induced holes and electrons, resulting in higher photodegraded activity than pure g-C3N4.Our research work indicates that the heterostructured formation ofg-C3N4, ZnS and CuS is helpful for the fabrication of g-C3N4-based photocatalysts with enhanced visible light photoactivity.

Tailoring the visible light photoactivity of un-doped defective TiO2 anatase nanoparticles through a simple two-step solvothermal process

Anatase TiO2 has become a material of great interest for photocatalytic production of hydrogen, environmental purification and solar energy conversion. Among the key parameters boosting the photocatalytic efficiency of the anatase nanoparticles, an increased light absorption to expand its optical response to the visible region, together with an improved charge separation of the photo-generated electrons and holes, can be enumerated. In this work, yellow-coloured, single-phase anatase nanoparticles have been obtained using a simple two-step solvothermal routine which requires no external addition of dopants, nor the use of a harassing/aggressive synthesis atmosphere. The obtained powders display a lowered bandgap (<3.0 eV) and significantly reduce the recombination processes, eventually leading to an improved photocatalytic performance under visible light, as exemplified by an enhanced degradation of phenol. This exceptional response is linked to the presence of intrinsic defects in the yellowish particles and, hence, the specific conditions of the proposed methodology become crucial to produce a propitious TiO2-defective nanomaterial capable of photo-degrade the phenol molecule, in contrast with the lack of photocatalytic activity currently exhibited by commercial photocatalysts under visible light.

Anionic Dopant Delocalization through p-Band Modulation to Endow Metal Oxides with Enhanced Visible-Light Photoactivity.

An N-doped TiO2 model reveals a conceptually different mechanism for activating the N dopant based on delocalized orbital hybridization through O vacancy incorporation. Synchrotron-based X-ray absorption spectroscopy, time-resolved fluorescence, and DFT studies revealed that O vacancy incorporation can effectively stimulate the delocalization of N impurity states through p-band orbital modulation, which leads to a significant enhancement in photocarrier lifetime. Consequently, this effect also results in a remarkable increase in the incident photon-to-electron conversion efficiency in the range of 400-550 nm compared to that of conventional N-incorporated TiO2 (15 % versus 1 % at 450 nm). This work reveals the fundamental necessity of orbital modulation in the band engineering of metal oxides for driving solar water splitting and beyond.

Highly efficient visible-light photoactivity of Z-scheme MoS2/Ag2CO3 photocatalysts for organic pollutants degradation and bacterial inactivation

Here, a novel Z-scheme MoS2/Ag2CO3 heterojunction photocatalyst was assembled from two-dimensional MoS2 nanosheets and Ag2CO3 nanoparticles through facile hydrothermal and in-situ precipitation method. The MoS2/Ag2CO3 heterojunction exhibited much enhanced visible-light photocatalytic performance in probe experiment for organic pollutants degradation and Escherichia coli (E. coli) inactivation compared to pristine Ag2CO3 and MoS2. The degradation rates of Lanasol Red 5B, rhodamine B, ciprofloxacin, and metronidazole reached 95%, 90%, 80%, and 72%, respectively. On the other hand, E. coli was completely inactivated in 80 min in the presence of 5%-MoS2/Ag2CO3. The improved photocatalytic performance was ascribed to the enhanced photogenerated charge separation efficiency and increased lifetime of the charge carriers, proved by photoluminescence spectra, time-resolved fluorescence emission decay spectra, and electrochemical measures. In addition, the active species trapping and ESR experiments all indicated that holes (h+) exhibited a significant contribution and superoxide radicals (O2−) acted as assistants. Based on experiment results, the photocatalytic enhancement mechanism for organic pollutants degradation and E. coli inactivation was discussed. The effect of representative environmental factors on the degradation of Lanasol Red 5B was investigated. The experiment results indicated that the degradation efficiency was partially influenced in the presence of inorganic salt. Furthermore, the appearance of a small amount of Ag nanoparticles not only enhanced the charge transfer, but also improved the stability of photocatalyst. Overall, MoS2/Ag2CO3 heterojunction has a great application potential for future water purification.