Visible Light Photoactivity Research Articles

Construction of ternary hollow TiO2-ZnS@ZnO heterostructure with enhanced visible-light photoactivity

Through a simple fiber template method, we combined the n-type TiO2 catalysts with n-type ZnS@ZnO heterojunction to synthesize a high-efficiency ternary hollow TiO2-ZnS@ZnO n-n heterojunction. Under these modified conditions, the composite catalysts could broaden the visible light absorption range of the pure TiO2, and both the hollow structure and heterojunction could inhibit the recombination ability of the photon-generated carriers. Optical and Electrochemical characterizations indicated that the photogenerated electrons and holes can be separated efficiently and the composites have visible light absorption. The experimental results show that, the removal efficiency of the optimal composites for photocatalytic degradation of Tetracycline Hydrochloride (TC) under visible light (λ ≥ 420 nm) was 86.67% due to the ·O2− free radicals of efficient production, it exhibited 5 times higher degradation rate towards Tetracycline Hydrochloride (TC) than pure TiO2. The composites with high photocatalytic activity could also effectively degrade Methylthionine Chloride (MB), Xylenol Orange (XO), Acid Red 18 and Rhodamine B (RhB) under the driving of visible light. This work provided a simple method to construct ternary heterojunction, and also provided a promising strategy to solve the environmental pollution problem in the future.

Oxygen vacancies facilitated visible light photoactivity of CdWO4 prepared by ionic liquid assisted hydrothermal method

In this demonstration, CdWO4 with wonderful visible light photoactivity was synthesized by a hydrothermal method with the assistance of 1-ethyl-3-methylimidazolium dihydrogen phosphate ([Emim]H2PO4) ionic liquid (IL). The existence of oxygen vacancies (OVs) was verified by X-ray photoelectron spectroscopy (XPS) and low-temperature electron spin resonance (ESR). Scanning electron microscope (SEM), XPS and X-ray powder diffraction (XRD) results show that [Emim]H2PO4 affects the growth and surface properties of CdWO4. IL-CdWO4 has higher separation efficiency of photogenerated carriers under visible light irradiation than CdWO4, proven by surface photovoltage spectrum (SPS) and electrochemical measurements. Photoactivity of CdWO4 prepared was investigated using Rhodamine B (RhB) and tetracycline (TC) as simulated pollutants, the consequences exhibit that the visible light photoactivity of IL-CdWO4 toward destruction of RhB and TC is more than 3.45 and 6.78-fold of that of CdWO4, respectively. In view of the observations, photocatalytic mechanism of IL-CdWO4 was proposed.

Constructing broad spectrum response ROQDs/Bi2WO6/CQDs heterojunction nanoplates: Synergetic mechanism of boosting redox abilities for photocatalytic degradation pollutant

Rhodium oxide quantum dots (ROQDs) and carbon quantum dots (CQDs) were controllably assembled on [Bi2O2]2+ slices and [WO4]2- slices of thin layered Bi2WO6, respectively, to construct broad spectrum response ROQDs/Bi2WO6/CQDs heterojunctions by a two-step process including photodeposition ROQDs and hydrothermal anchoring CQDs. 0.1%ROQDs/Bi2WO6/4.2%CQDs exhibited optimized photoactivity for 4-chlorophenol (4-CP) degradation (Kapp=2.15×10−2 min−1) under the visible light (420 nm﹤λ﹤800 nm) irradiation, about 9.5 times as that of pristine Bi2WO6 (Kapp=2.26×10−3 min−1). ROQDs-modifying played a crucial role in enhancing photoactivity under the visible light of λ>420 nm, whereas CQDs- modifying played a decisive role under the visible light of λ>550 nm. Built-in electric fields at the ROQDs/Bi2WO6 and CQDs/Bi2WO6 interfaces and the intimate interfaces as charge transfer channels produced significant synergetic effects on promoting charge carrier separation and reducing charge transfer resistance. More importantly, the oxidative power of h+ was enhanced by forming Z-scheme ROQDs/Bi2WO6 heterojunction, thus making more contribution to 4-CP degradation by both h+ and ·OH than ·O2-. In addition, ROQDs and CQDs co-modification further improved light harvesting capacity, enlarged specific surface, pore volume, and average pore diameter, as well as increased surface hydroxyl and absorbed oxygen. Enhanced visible light photoactivity was attributed to the cooperation of all above factors. This work provides a viable avenue to develop efficient double heterojunction photocatalysts for environment purification and solar energy conversion.

Surface microenvironment engineering of black V2O5 nanostructures for visible light photodegradation of methylene blue

Utilization of photocatalysis as a promising strategy for environmental and energy applications has been widely considered. Herein, we report a novel black V2O5 material (bV2O5) synthesized using a controllable and environmentally benign physicochemical reduction method. HRTEM, ESEM, EDX. Raman, XPS, XRD, and BET textural characterization, as well as computational density functional theory (DFT) techniques were employed to understand the chemical and electronic changes obtained through modulation of the surface microenvironment. DFT analyses reveal that tuning a high degree of surface oxygen vacancies considerably ameliorated visible light photoactivity of practically inactive pristine V2O5. The optimized bV2O5 sample yielded 92% photodegradation of 20 mg/L cationic methylene blue (MB) in 60 min under visible light irradiation – corresponding to a 58-fold increase in photodegradation efficiency over pristine V2O5. Neutral quinoline yellow (QY) and anionic methyl orange (MO) photodegradation were also investigated to examine the photocatalytic efficacy of bV2O5 for degradation of other organic contaminants with different charges. DFT calculations show a clear thermodynamic stability towards reduction of the predominant polar (001) facet at 1-coordinated oxygen surface site. A staggered (type-II) heterostructure between pristine and reduced V2O5 was determined from band edge positions which is believed to promote the enhancement in photoactivity of the reduced sample by offering favorable electron-hole separation and allowing both hydroxyl and superoxide radical formation. The mechanism behind the formation of surface defects on bV2O5 was proposed based on configurational changes.

Cathodic shift of onset potential on TiO2 nanorod arrays with significantly enhanced visible light photoactivity via nitrogen/cobalt co-implantation* *Project supported by the National Natural Science Foundation of China (Grant No. 11875211), the Major Science and Technology Program of Changsha, China (Grant No. kq1902046), and the Fundamental Research Funds for the Central Universities, China.

Despite anionic doping has been widely implemented to increase the visible light activity of TiO2, it often gives rise to a dramatical anodic shift in current onset potential. Herein, we show an effective method to achieve the huge cathodic shift of TiO2 photoanode with significantly enhanced visible light photo-electrochemical activity by nitrogen/cobalt co-implantation. The nitrogen/cobalt co-doped TiO2 nanorod arrays (N/Co-TiO2) exhibit a cathodic shift of 350 mV in onset potential relative to only nitrogen-doped TiO2 (N-TiO2). Moreover, the visible-light (λ > 420 nm) photocurrent density of N/Co-TiO2 reaches 0.46 mA/cm2, far exceeding 0.07 mA/cm2 in N-TiO2 at 1.23 V versus reversible hydrogen electrode (RHE). Systematic characterization studies demonstrate that the enhanced photo-electrochemical performance can be attributed to the surface synergic sputtering of high-energy nitrogen/cobalt ions.

Sandwich-like Z-scheme g-C3N4/reduced graphene oxide@TiO2composite for enhanced visible light photoactivity

The sandwich-like Z-scheme g-C3N4/reduced graphene oxide@TiO2 (CN/RGO@T) composite with enhanced photocatalytic performance was successfully synthesized by hydrothermal method followed by a calcination process. It is interesting to note that TiO2 completely enclosed by RGO is presented in a nanoscroll morphology. Photocatalytic performance of the composite was evaluated in the propylene degradation. The results revealed that by introducing RGO as an interfacial mediator, Z-scheme g-C3N4-TiO2 nanocomposites could remarkably speed up the interfacial charge transmission and improve the separation efficiency of photogenerated carriers. Especially, 20%CN/RGO@T composite showed the highest visible light activity up to 21.9% for propylene oxidation. A lower photogenerated electron-hole recombination rate and higher carrier separation efficiency of the ternary complex 20%CN/RGO@T were confirmed by electron spin resonance, photoluminescence spectrum, and photocurrent-time characterization. This is due to the fact that strong π-π interaction between g-C3N4 and RGO leads to a carrier transmission channel between N-O and C-O-C bonds of g-C3N4 and RGO. This newly formed Z-scheme composite is having a possibility to be explored as a versatile photocatalyst to conduct various type photoreactions.

Enhanced visible light photoactivity of TiO2/SnO2 films by tridoping with Y/F/Ag ions

The Y, F, and Ag tridoped TiO2/SnO2 composite nanocrystalline film (YFAg–TS) with prominent photocatalytic performance was prepared by the modified sol–gel method and was characterized by utilizing X-ray diffraction (XRD), differential thermal and thermogravimetric (DTA–TG) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) method, ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), and photoluminescence (PL). The XRD and DTA–TG results expose that the YFAg–TS catalyst is a mixed phase consisting of anatase, rutile, and chlorargyrite, which is beneficial to improving the photocatalytic performance of TiO2. The SEM, TEM, and BET results disclose that the YFAg–TS film has smaller nanoparticles, higher specific surface area, and narrower pore size compared with pure TiO2 film. The XRD and TEM results exhibit that a part of yttrium can enter the TiO2 lattice to induce lattice distortion. The XPS results confirm the presence of Y3+ state in the YFAg–TS sample, and Y3+ ions can act as the trapping site of electrons to expedite the separation of electrons and holes. The UV–vis DRS results reveal that the YFAg–TS film has an obvious absorption edge shift and a narrower bandgap (2.70 eV) compared with pure TiO2 film. The PL results show that the YFAg–TS film has the highest photogenerated electrons and holes separation efficiency and charges transfer efficiency among all samples. The photocatalytic activity of the YFAg–TS was assessed by monitoring the degradation of methyl green and formaldehyde solution. The results manifest that the YFAg–TS film has high stability and excellent photocatalytic performance. The possible synergistic photocatalytic mechanism of YFAg–TS films has been discussed in this paper.

Effects of surface functionalization with alkylalkoxysilanes on the structure, visible light photoactivity and biocidal performance of Ag-TiO2 nanoparticles

Surface-modified Ag/TiO2 photocatalysts with enhanced activity under visible light and biocidal properties have been produced through a two-stage method involving the functionalization of TiO2 nanoparticles(NPs) with –SH or –NH terminated alkoxysilanes and deposition of AgNPs.The functionalization process increased the yield of the Ag deposition and promoted a more homogeneous distribution of the Ag over the TiO2 surface. The highest stability, homogeneity and visible range absorption was achieved by –NH functionalization. However, the modification with –SH groups alters the size of the AgNPs and decreases the effectiveness due to formation of Ag2S. The increased absorption in the visible spectrum accelerated the organic dye degradation under solar light, although aggregation processes affected their effectiveness under UV-A. The incorporation of AgNPs allowed for the elimination of phototrophic microorganisms under white light at very low Ag concentrations whereas TiO2 and –SH functionalized Ag/TiO2 showed no biocidal activity.

A straightforward one-pot approach to two new defect energy levels in ZnS

ZnS with double defect energy levels shows an amazing visible light photoactivity.

Enhanced visible-light photoactivities of porous LaFeO3 by synchronously doping Ni2+ and coupling TS-1 for CO2 reduction and 2,4,6-trinitrophenol degradation

TOC showing the enhanced visible-light photoactivities of porous LaFeO3 by synchronously doping with Ni2+ and coupling with TS-1 for CO2 reduction and 2,4,6-trinitrophenol degradation.

Dual functions of CO2molecular activation and 4f levels as electron transport bridges in erbium single atom composite photocatalysts therefore enhancing visible-light photoactivities

In the heterogeneous SAC system (SrTiO3:Er3+/g-C3N4), the single Er atoms perfectly play multiple roles as interfacial electronic transfer bridges and active sites for CO2activation.

Hollow 1D copper oxide nanostructures with enhanced activity for catalytic reduction and photocatalytic degradation of organic pollutants

Catalytic reduction and photocatalytic degradation are two important methods used for remediation of organic pollutants in waste water streams. Copper oxide is a fascinating material that can exhibit catalytic activity in both reactions. In this study we created various 1D copper oxide nanostructures using thermal oxidation of pre-synthesized copper nanowires. The process yield wire like, rod like and tubular wire like copper oxide nanostructures. The tubular morphology is formed by the classic Kirkendall effect. The BET surface area follows the order copper nanowire precursor > 400 °C annealed sample (tube structure) > 150 °C annealed sample (wire like structure)> 180 °C annealed sample (rod like structure). The efficiency of the synthesized materials in catalytic reduction reaction is tested using para nitrophenol reduction reaction. The visible light photocatalytic activity of the materials is tested using the degradation of methyl orange reaction. The synthesized materials show promising activity in both reactions. Interestingly, the copper oxide nanostructures outperform precursor copper nanowires, which possess the highest surface area, in both reactions. The sample calcined at 400 °C exhibits almost twice the catalytic activity than that of metallic copper in the hydride reduction reaction. The photocatalytic degradation rate exhibited by the same copper oxide nanostructure is 23 times higher than that of copper nanowires. The copper oxide nanostructure prepared in this study show catalytic activity almost an order of magnitude higher than copper oxide nanoparticles reported in literature. Catalytic cycling studies show that the synthesized materials show little reduction in property even after three cycles. The macroscopic wire like morphology, and high catalytic activity in reduction reaction and visible light photoactivity make the synthesized materials multipronged for applications like waste water remediation.

Surfactant- and template-free hydrothermal assembly of Cu2O visible light photocatalysts for trimethoprim degradation

A surfactant-free, low temperature hydrothermal synthesis of Cu2O nanostructures is demonstrated for application to the photocatalytic degradation of trimethoprim (TMP), an environmental xenobiotic. Photophysical properties of different size and shape Cu2O nanostructures were determined by bulk and surface microscopic and spectroscopic analyses. Visible light photoactivity for the oxidative degradation of TMP is sensitive to the rate of photoexcited charge carrier bulk recombination, and therefore the size of Cu2O crystallites. Optimum photodegradation activity was observed for a hierarchical Cu2O nanostructure, comprising 11 nm crystallites nucleated as 50−80 nm particles, themselves coalesced into 400 nm compact agglomerates. The specific activity of 1.12 μmol.g−1. min−1 for a 0.1 mM TMP aqueous solution is comparable to previous reports that required higher energy and/intensity UV irradiation. The stepwise hydroxylation and oxidative cleavage of TMP to form monocylic fragments is driven by hydroxyl radicals photogenerated over the hierarchical Cu2O nanostructure, which exhibits excellent catalytic stability for >25 h.

Size-dependent effects of ZnO nanoparticles on the photocatalytic degradation of phenol in a water solution

The effect of the ZnO nanoparticles (NPs) size on the photocatalytic degradation of phenol in a water solution under the influence of UV and Vis radiation was discussed. For the first time, research on photocatalytic degradation has used ZnO NPs produced by only one method (microwave solvothermal synthesis without heat treatment or other processes of reduction/oxidisation of the surface of NPs samples). ZnO NPs average size was determined using the Scherrer's formula, Nanopowder XRD Processor Demo web application, by converting the results of the specific surface area-density, and TEM tests. The ZnO NPs (average size between 23 nm and 71 nm) characterise by uniform morphology and narrow size distribution. The photocatalytic performance of ZnO NPs increases with the increase of the particle size and the decrease of the specific surface area. For both UV and Vis radiation, the highest reaction rate for phenol degradation was found for ZnO sample with the average particle size of 71 nm. Low concentrations of resorcine and hydroquinone (co-products of phenol degradation) were found for UV light photocatalytic test. However, high concentrations of hydro- and p-benzoquinone were observed for visible light photoactivity due to the slower decomposition of the main contamination associated with the utilized of other type of radiation. The photocatalytic activity was found to be attributable to the decrease in the charge carrier recombination rate.

Influence of ruthenium doping on UV- and visible-light photoelectrocatalytic color removal from dye solutions using a TiO2 nanotube array photoanode

The photocatalytic activity of TiO2 anodes was enhanced by synthesizing Ru-doped Ti|TiO2 nanotube arrays. Such photoanodes were fabricated via Ti anodization followed by Ru impregnation and annealing. The X-ray diffractograms revealed that anatase was the main TiO2 phase, while rutile was slightly present in all samples. Scanning electron microscopy evidenced a uniform morphology in all samples, with nanotube diameter ranging from 60 to 120 nm. The bias potential for the photoelectrochemical (PEC) treatment was selected from the electrochemical characterization of each electrode, made via linear sweep voltammetry. All the Ru-doped TiO2 nanotube array photoanodes showed a peak photocurrent (PP) and a saturation photocurrent (SP) upon their illumination with UV or visible light. In contrast, the undoped TiO2 nanotubes only showed the SP, which was higher than that reached with the Ru-doped photoanodes using UV light. An exception was the Ru(0.15 wt%)-doped TiO2, whose SP was comparable under visible light. Using that anode, the activity enhancement during the PEC treatment of a Terasil Blue dye solution at Ebias(PP) was much higher than that attained at Ebias(SP). The percentage of color removal at 120 min with the Ru(0.15 wt%)-doped TiO2 was 98% and 55% in PEC with UV and visible light, respectively, being much greater than 82% and 28% achieved in photocatalysis. The moderate visible-light photoactivity of the Ru-doped TiO2 nanotube arrays suggests their convenience to work under solar PEC conditions, aiming at using a large portion of the solar spectrum.

CQDs decorated oxygen vacancy-rich CeO2/BiOCl heterojunctions for promoted visible light photoactivity towards chromium (Ⅵ) reduction and rhodamine B degradation

Engineering a heterojunction with high photoactivity and better exciton separation in photocatalyst is a promising target for environmental purification. In this context, we fabricated CQDs decorated double-shell CeO2 hollow spheres(D-CeO2)/BiOCl heterojunction (D-CeO2: CQDs/BiOCl) composite by a simple method and scrutinized it photocatalytic performance toward chromium (Ⅵ) reduction and rhodamine B degradation. The resultant D-CeO2: CQDs/BiOCl composite is validated much more active under visible light irradiation with the high rhodamine B degradation rate of 0.12min−1 which is about 3 and 255 times, meanwhile, chromium (Ⅵ) reduction rate of 0.015 min−1 which is about 6 and 32 times higher than the neat BiOCl and D-CeO2, respectively. The interfacial domain of D-CeO2/BiOCl decorated with CQDs for efficient photoinduced charge carriers transfer and separation are the point to attractive photocatalytic performance which combines the double-shell hollow sphere morphology with highly efficient light harvesting, rich OVs for the charge carriers fast separation and accelerated charge transfer after the CQDs participating.

A Faster Path to Solar Water Splitting

Finding new wide-bandgap light absorbers that are stable in aqueous solutions is a long-standing challenge in photoelectrochemical water splitting research. Two papers in this issue describe recent advances in high throughput experimentation that may accelerate the discovery of suitable materials.

Controllable fabrication of highly visible-light responsive oxygen-rich Ti-based microspheres with the hierarchical surface through a facile and dopant-free hydrothermal route

The oxygen-rich Ti-based composite (ORTC) microspheres with high visible-light photoactivity were successfully synthesized by a facile and dopant-free hydrothermal method. The as-synthesized urchin-like microspheres possess a controllable hierarchical structure that is strongly dependent on the concentration of H2O2 and NH4Cl, and displayed an excellent visible light photoactivity without annealing activation, which is attributed to the formation of the oxygen excess defects. The ORTC prepared in optimal conditions gave the narrowest band gap of 2.31 eV due to its highest oxygen excess defects content, and showed a stable and high-efficiency visible-light photocatalytic performance for organic dyes degradation, exhibiting three-fold as that of the control sample. The synergistic effect of oxygen excess defects, hierarchical structure and strong adsorption capacity was identified as the key factor responsible for the prominent performance. Based on the obtained results, a plausible mechanism for fabricating urchin-like ORTC microspheres with nanofiber branches was proposed and discussed in detail.

Novel Fe2TiO5/reduced graphene oxide heterojunction photocatalyst with improved adsorption capacity and visible light photoactivity: experimental and DFT approach.

The design of high-efficiency materials is a major challenge for the degradation of organic pollutants. In this work, type II p-n heterojunction photocatalyst Fe2TiO5/rGO, with enhanced performance, was successfully prepared through simple process. The Fe2TiO5/rGO composites were prepared by hosting several amounts of reduced graphene oxide (rGO) into pseudobrookite nanocrystals (Fe2TiO5) which were priorly synthesized by a solid-state reaction. The morphology and the properties of the as-prepared composites were characterized through different techniques. The fixation of rGO sheets on Fe2TiO5 was proved using the X-ray diffraction analysis (XRD). The results of the scanning electron microscope (SEM) analysis showed a good mixing of rGO with Fe2TiO5. The X-ray fluorescence (XRF) confirmed the purity of the pristine Fe2TiO5. The dynamic light scattering (DLS) illustrated a strong tendency to aggregation. Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) analysis was performed to characterize the electronic aspect as the gap and the Urbach energies. Finally, computational density functional theory (DFT) calculations were carried out to confirm the experimental results. The adsorptive and photoactivity of Fe2TiO5/rGO heterojunction photocatalysts were evaluated by methylene blue (MB) degradation under visible light irradiation. The highest MB degradation rate was achieved for Fe2TiO5/rGO10% photocatalyst with the highest value of the elimination rate.

Vapor deposition of g-C3N4 on TiO2 nanosquares for efficient photodegradation of MB and Cr6+ under visible light

Herein, a novel composite photocatalyst was prepared by the vapor deposition of g-C3N4 on TiO2 nanosquare, dominated with {001} facets. The deposition method is robust, convenient, and effective. The surface topography and structure of the composites were analyzed by XRD, XPS, SEM, and TEM. The mechanism for enhanced photocatalysis was studied using PL, PC, UV–vis-NIR-DRS, TRPL, EIS and EPR. The photocatalytic activities of the fabricated composites were evaluated by the degradation of methylene blue (MB) and hexavalent chromium ions (Cr6+) under visible light irradiation. The photodegradation of MB and Cr6+ was completed in 50 and 40 min, respectively. Based on a pseudo-first-order kinetic model (ln(C0/C) = kt), the measured the degradation rate constant for MB using gCN/TO5 was 4.6 and 35.9 times higher than that of pure g-C3N4 and TiO2, respectively. Similar results were found for the Cr6+ degradation. The outstanding photocatalytic performance of the g-C3N4 on TiO2 composite was ascribed to the presence of the TiO2/g-C3N4 heterojunctions, which effectively improved the separation of the photoexcited electrons and holes hence the enhancement of visible-light photoactivity. Overall, this work showed a convenient and efficient method for fabricating the visible light photocatalysts with established possible mechanisms for the photoactivity of the hybrid photocatalytic composite material.