Shift Of Light Absorption Research Articles

Improving photocatalytic efficiency of ZnO nanoflowers through gold incorporation for Rhodamine B photodegradation

In this study, we reported the synthesis and characterization of Au-doped ZnO hybrid nanostructures via a wet chemical approach and assessed their photocatalytic efficiency for the removal of Rhodamine (Rh–B). X-ray powder diffraction (XRD) has been done to verify the crystal structure, i.e. phase identification, crystallite size, texture, stress, and strain. The XRD patterns demonstrate that the wurtzite structure has been retained and that Au nanoparticles were effectively ingested into the ZnO nanoflowers lattice. This demonstrates that Au nanoparticles are effectively absorbed into the ZnO nanoflowers lattice, considering that peaks shifted in the direction of smaller Bragg's angles. As a result, the size of the crystallites increases from (20.7–25.3) nm, and their microstrain decreases from (0.92–0.72) x 10−3 A0 for pure-ZnO to Au-doped ZnO heterostructures, respectively. Scanning Electron Microscopy (SEM) investigation reveals flower-like structures of pure ZnO while a further closed look indicates that these flowers are made up of a variety of oriented, thorn-like branched NWs with diameters ranging from (50–90) nm and lengths ranging from a few microns. The corresponding EDS evaluation of pure ZnO nanoflowers and Au-doped ZnO heterostructures clearly shows that the noble metal Au NPs are deposited, and enormously safeguard the surface of ZnO nanoflowers. Reducing the Au-doped ZnO band gap which generated a red shift in light absorption, was confirmed by the PL spectra which in turn increased the photocatalytic efficiency and reduced the dye degradation time of Rh–B. The photocatalytic ability of Au-doped ZnO heterostructures considerably increased the photodegradation efficiency of Rh–B molecules from 28 min for pure ZnO to 10 min for Au-doped ZnO heterostructures. Due to their wide surface area and distinct morphology as compared to pure ZnO nanoflowers.

A facile impregnation synthesis of Ni-doped TiO2 nanomaterials for dye-sensitized solar cells

This study reports a facile impregnation method for synthesizing Ni-doped TiO2 nanomaterials using P25-TiO2 as a starting material. The as prepared nanomaterials were subjected to structural and optical characterizations and subsequently employed in photovoltaic studies. X-ray diffraction (XRD) and Raman studies confirmed that Ni doping did not alter the anatase and rutile contents of P25-TiO2. Also, the presence of the constituent dopants and their ionic states were confirmed by Energy-Dispersive X-ray (EDX) and X-ray photoelectron (XPS) spectroscopies. Topographic Atomic Force Microscopic (AFM) images illustrated that Ni doping had increased the surface roughness of the TiO2. Optical characterization by UV-Visible spectroscopy revealed that the Ni doping had caused red shift in light absorption due to reduced TiO2 bandgap and improved the dye adsorption on TiO2 films. Then, the photocurrent–photovoltage property of the fabricated devices was investigated and the optimized 0.10 wt% Ni-doped TiO2 photoanode based device exhibited pronounced power conversion efficiency (PCE) of 6.29% under air mass (AM) 1.5 conditions (100 mWcm−2, 1 sun). Improved charge transport properties were also observed by the electrochemical impedance spectroscopic (EIS) study for the device with optimized Ni-doped TiO2 compared to the control device.

Ni/N co-doped P25 TiO2 photoelectrodes for efficient Dye-Sensitized Solar Cells

This study focuses, synthesising and characterising Ni/N co-doped P25–TiO2 nanostructure electrodes for photovoltaic application. The X-ray diffraction (XRD) patterns confirm the presence of major anatase and minor rutile phases of TiO2 for un-doped, doped and co-doped nanoparticles. The UV–Visible spectra of un-doped, Ni-doped, N-doped and Ni/N co-doped nanoparticles reveal gradual red shift in light absorption and reduction in TiO2 bandgap. Enhanced light absorption observed for dye-coated Ni/N co-doped TiO2 electrode relative to the doped and un-doped electrode may be attributed to the increased surface area for dye adsorption/dye loading capacity due to synergistic effect of Ni and N doping. The improved surface roughness of Ni/N co-doped TiO2 electrode, confirmed by AFM, leads to high dye adsorption. The photovoltaic performance of the fabricated DSSCs was examined under simulated irradiation intensity of 100 mWcm−2 with AM 1.5 filter. The Ni/N co-doped device exhibited 35% higher than that of the control device (un-doped TiO2 based DSSC). The improvement in the PCE of the co-doped device is predominantly due to the increase in short circuit current density (JSC) as a result of higher light absorption ability and reduced charge transport resistance of the co-doped electrode.

Hydrophilic Properties Study of Mn-TiO2 Thin Films Deposited by Dipping Technique

in this paper, the current work was devoted to the manufacture of TiO2 nanoparticles doped with manganese, synthesis by the sol-gel technique using a dip-conting device, for their hydrophilic properties and photocatalytic activity, and the products were characterized by X-ray diffraction, scanning electron microscopy, and Uv-Visible absorption, and the results XRD showed an phase Anatase , and the results of the SEM Explained the shape of the morphology of the samples after the doping process compared with pure TiO2, and the results of a shift in light absorption from ultraviolet rays to visible light were evident. The results showed that the thin films have a high wettability under visible rays proven to have excellent optical stimulation in the visual area, making the application of a thin film as a self-cleaning material an attractive option.

First Principles Modelling of Surface Modified TiO2 for Water Activation and Oxygen Evolution

The photocatalytic oxidation of water to molecular oxygen is a key step in the water splitting reaction. The development of robust catalysts with high efficiency in this half-reaction is crucial for achieving technological application of water splitting. Density Functional Theory simulations can provide key insights into potential catalysts for this reaction. In this contribution we describe our work on modelling of surface modified TiO2 focusing on water activation and the oxygen evolution mechanism on these heterostructures. Rutile and anatase TiO2 are modified with nanostructures of metal oxides, namely alkaline earth oxides, cerium oxide and manganese oxides; these are also being prepared and characterised at collaborators laboratories in the framework of an EU H2020 M-ERA.net 2 project RATOCAT.We find that nanoclusters of these oxides with different compositions and adsorption modes are strongly adsorbed and can induce a red shift in light absorption towards the visible region of the electromagnetic spectrum. A simple model of the photoexcited state allows the localisation of electrons and holes produce by excitation to be determined and understood. Typically, DFT studies neglect key issues issues such as reducibility of the oxide or the role of adsorbed hydroxyls on the properties and the water activation processes in OER. We include a detailed analysis of reduction of the heterostructures, in which we find that loss of oxygen is generally favourable. In addition, the surface hydroxylation (through adsorption and dissociation of water) and the cluster hydroxylation are examined. We find that a single water molecule will dissociate at the supported oxide nanocluster, with large energy gain that indicates hydroxyl groups will be present and must be accounted for.With a suitable model of a hydroxylated oxide heterostructure, including oxygen vacancies, we explore water activation and find for those structures with hydroxyls and single oxygen vacancies, the water adsorption is dissociative but with moderate energy gains. Too high an energy gain and the subsequent OER steps are not favourable. We determine the step with the highest free energy cost and use this to estimate an overpotential for these heterostructures as well as determining key aspects of these structures that drive favourable OER.Support for this work from Science Foundation Ireland through the H2020 M-ERA.net 2 co-fund program, Grant Number 16/M-ERA/3418 is acknowledged. We also acknowledge access to computing resources through the SFI funded Irish Centre for High End Computing, ICHEC.

Enhanced photocatalytic activities of ZnO dumbbell/reduced graphene oxide nanocomposites for degradation of organic pollutants via efficient charge separation pathway

In the present work, a well-defined dumbbell shaped ZnO was synthesized by hydrothermal method and ZnO dumbbell/reduced graphene oxide (ZnO/rGO) nanocomposites were prepared by a simple solution mixing method with the different rGO loading amount. The formation of nanocomposites, crystal structure, shape, size and optical properties of the ZnO/rGO nanocomposites were investigated using various analytical techniques. The photocatalytic degradation efficiency of synthesized materials was evaluated by the degradation of aqueous Methyl Orange (MO) and Methylene Blue (MB) under UV–Visible light irradiation. The results revealed that 3 wt% rGO loaded ZnO dumbbell (ZnO-3% rGO) exhibited the higher photocatalytic degradation efficiency than pure ZnO dumbbell and rGO. The enhancement in the photocatalytic dye degradation efficiency of ZnO-3% rGO is attributed to the efficient dye adsorption nature, a red shift in light absorption and inhibition of photo-excited electron-hole pair recombination rate. The photocatalytic dye degradation efficiency also evaluated for various catalyst dosage, dye concentration and pH of the solution. Moreover, it was demonstrated the stability of catalyst over a repeated cycle of dye treatment.

Hydrothermal Synthesis of Rare-Earth Modified Titania: Influence on Phase Composition, Optical Properties, and Photocatalytic Activity.

In order to expand the use of titania indoor as well as to increase its overall performance, narrowing the band gap is one of the possibilities to achieve this. Modifying with rare earths (REs) has been relatively unexplored, especially the modification of rutile with rare earth cations. The aim of this study was to find the influence of the modification of TiO2 with rare earths on its structural, optical, morphological, and photocatalytic properties. Titania was synthesized using TiOSO4 as the source of titanium via hydrothermal synthesis procedure at low temperature (200 °C) and modified with selected rare earth elements, namely, Ce, La, and Gd. Structural properties of samples were determined by X-ray powder diffraction (XRD), and the phase ratio was calculated using the Rietveld method. Optical properties were analyzed by ultraviolet and visible light (UV-Vis) spectroscopy. Field emission scanning electron microscope (FE-SEM) was used to determine the morphological properties of samples and to estimate the size of primary crystals. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical bonding properties of samples. Photocatalytic activity of the prepared photocatalysts as well as the titania available on the market (P25) was measured in three different setups, assessing volatile organic compound (VOC) degradation, NOx abatement, and water purification. It was found out that modification with rare earth elements slows down the transformation of anatase and brookite to rutile. Whereas the unmodified sample was composed of only rutile, La- and Gd-modified samples contained anatase and rutile, and Ce-modified samples consisted of anatase, brookite, and rutile. Modification with rare earth metals has turned out to be detrimental to photocatalytic activity. In all cases, pure TiO2 outperformed the modified samples. Cerium-modified TiO2 was the least active sample, despite having a light absorption tail up to 585 nm wavelength. La- and Gd-modified samples did not show a significant shift in light absorption when compared to the pure TiO2 sample. The reason for the lower activity of modified samples was attributed to a greater Ti3+/Ti4+ ratio and a large amount of hydroxyl oxygen found in pure TiO2. All the modified samples had a smaller Ti3+/Ti4+ ratio and less hydroxyl oxygen.

First principle study of electronic structures and optical absorption properties of O and S doped graphite phase carbon nitride (g-C3N4)6 quantum dots

Graphite phase carbon nitride (g-C3N4) quantum dots have received much attention due to its good stability, water solubility, biological compatibility, non-toxicity as well as strong fluorescence characteristics. In order to enhance the light absorption and improve photocatalytic activities of the g-C3N4 quantum dots, theoretical studies are carried out on the O and S atoms doped (g-C3N4)6 quantum dots. First-principles calculations based on the density functional theory and time dependent density functional theory are performed to investigate the geometries, electronic structures and ultraviolet visible absorption spectra of O and S atoms doped (g-C3N4)6 quantum dots. The results show that the highest electron occupied molecular orbital-the lowest electron unoccupied molecular orbital (HOMO-LUMO) energy gap of doped (g-C3N4)6 quantum dots is significantly reduced though the CN bond lengths closely related to the impurities only change slightly. The calculated formation energies indicate that the O-doped (g-C3N4)6 quantum dots are more stable, and the O atom tends to substitute for N atom at the N3-site, while the S atoms prefer to substitute for N atom at the N8-site. The simulated spectra indicate that the doping of O and S in (g-C3N4)6 could improve the light absorption. Not only the absorption peaks are extended from the UV to the infrared region (e.g. 200-1600 nm), but also the corresponding absorption intensities are enhanced significantly by doping the O or S atoms with the appropriate concentration. The increase of proper impurity concentration will lead to a pronounced red shift in light absorption. The effect of doping site on the optical absorption property of (g-C3N4)6 quantum dots shows that the absorption intensity is mainly affected in the visible range, however, besides the influence on the absorption intensity, the light absorptions of some structures are also affected beyond 800 nm. Overall, the O atoms and S atoms have a substantially similar effect on the light absorption of the (g-C3N4)6 quantum dots, while the effects of these impurity atoms are different in the long wavelength region. Oxygen doping is better than sulfur doping in the absorption of (g-C3N4)6 quantum dots by comparing the doping of O and S. These first-principles studies give us a method to effectively improve the light absorption of g-C3N4 quantum dots, and could provide a theoretical reference for tuning its electronic optical properties and applications.

Synthesis of 3,3′′,5,5′′‐Tetraphenyl‐4,4′′‐terphenoquinone and Its Reductive Disproportionation Reaction

AbstractBased on our recent reinvestigation of the disproportionation reaction of 3,3′,5,5′‐tetraphenyl‐4,4′‐biphenoquinone, which exhibited drastic color change, we performed the synthesis and characterization of 3,3′′,5,5′′‐tetraphenyl‐4,4′′‐terphenoquinone with the aim of obtaining enhanced physical properties, such as lower reduction potentials and bathochromic shift in light absorption, and realizing a disproportionation reaction at a lower temperature. To construct the terphenyl and/or terphenoquinone skeletons, the Suzuki‐Miyaura coupling, which involves coupling between 1,4‐diiodobenzene and the boronic acid ester with phenyl and oxygen moieties attached was carried out. The subsequent hydroquinone formation reaction and oxidation reaction yielded a brilliant ultramarine‐blue terphenoquinone. The quinone showed properties typical of π‐expanded quinones, namely, positively shifted reduction potential, small on‐site Coulomb repulsion, and lower excitation energy. DSC measurement of this product showed an exothermic process at 212.5 °C and this temperature was much lower than that of the corresponding biphenoquinone, 289.5 °C. The products of this reaction were similar to those of the biphenoquinone reaction, i. e., two benzofurans and the corresponding hydroquinone.

Degradation of organophosphorus flame retardant tris (1-chloro-2-propyl) phosphate (TCPP) by visible light N,S-codoped TiO2 photocatalysts

N and N,S co-doped TiO2 catalysts were prepared by a simple sol–gel method and were characterized by various techniques such as X-ray diffraction, scanning electron microscopy, UV–vis spectroscopy, porosimetry and particle size distribution. The photocatalytic efficiency of the prepared catalysts was evaluated for the degradation of organophosphorus flame retardant TCPP in aqueous solutions at environmental relevant concentrations under UV–vis and visible light irradiation. All the prepared N and N,S doped catalysts showed lowered crystallite size and higher visible-light response compared to the undoped TiO2 catalyst. An increased surface acidity was observed for N,S co-doped catalysts as a result of surface sulfate species which play a crucial role in the photocatalytic performance. N-doped TiO2 was found to be the most photoactive catalyst for the removal of TCPP, under UV–vis irradiation. Among N,S co-doped TiO2 catalysts, the one prepared with equimolar Ti:N,S ratio showed the higher performance for the removal of TCPP under both UV–vis and visible irradiation. Its higher activity was attributed to the higher surface area, larger pore volume, well-crystallized anatase and red shift in light absorption. Under visible light irradiation the degradation kinetics of TCPP were slower than under UV–vis irradiation due probably to reduced production of hydroxyl radicals.

Localization of Photoexcited Electrons and Holes on Low Coordinated Ti and O Sites in Free and Supported TiO2 Nanoclusters

Photocatalysis is being intensively studied for reactions such as water splitting and CO2 reduction, where absorption of light in a semiconductor such as TiO2 produces electrons and holes that can drive chemical reactions. Efficiencies on bulk materials, however, are too low to be of practical use. In recent years, low dimensional structures such as nanoclusters or nanotubes, displaying metal and oxygen coordination environments very different from the bulk, show promise for improved photocatalytic activities. Key to this is the presence of low coordinated metal and oxygen sites which can act as both charge carrier trapping sites and active sites with target molecules such as water or CO2. This paper presents the results of a density functional theory, with Hubbard U correction (DFT+U), study of electron and hole localization in free and metal oxide-supported TiO2 nanoclusters that display low coordinated titanium and oxygen sites. In free TiO2 nanoclusters, electrons and holes preferentially localize at 4-fold coordinated Ti and titanyl, i.e., singly coordinated, oxygen sites in TiO2. For TiO2 nanoclusters supported on rutile (110) electrons preferentially trap on a Ti site in the rutile surface and holes are trapped on the nanocluster–preferentially on titanyl oxygen, if present, and a 2-fold coordinated oxygen otherwise. Our analysis of La2O3 and SiO2 surfaces modified with a Ti5O10 nanocluster shows that electrons and holes are trapped on the TiO2 nanocluster, with electrons residing on a low coordinated Ti site at the interface between TiO2 and the support. Holes are trapped on low coordinated oxygen sites. An important finding is that the modification of the support oxides with these TiO2 nanoclusters induces a band gap narrowing over the unmodified oxide, which will induce a red shift in light absorption. Overall, these studies provide important insights for the design of improved photocatalysts, in that we should seek structures with low coordinated metal and/or oxygen sites that can trap electrons and holes and be useful in adsorbing and activating molecules of interest in photocatalysis.

First principles investigation of anion-controlled red shift in light absorption in ZnX (X = O, S, Se) nanocluster modified rutile TiO2

There is significant interest in finding new approaches to modifying TiO2 in order to endow it with visible light driven photocatalytic activity. To date, the most widely studied approach is to dope TiO2 with metals, non-metals or both simultaneously. However, there are some serious issues associated with doping of TiO2. We have studied an alternative approach in which TiO2 surfaces are modified with nanoclusters of metal oxides and have shown that this can introduce new states into the original TiO2 band gap that permits a red shift to induce visible light absorption while enhancing the photocatalytic activity over unmodified TiO2. In this paper we extend this idea to modification of rutile TiO2 with nanoclusters of ZnO, ZnS and ZnSe, which allows us to explore the possibility of tuning the magnitude of the red shift in modified TiO2 through the choice of anion in the nanocluster modifier. We find that the adsorption energies of the nanoclusters and the interfacial geometry are strongly influenced by the nature of the anion, with the ZnSe nanoclusters the most weakly bound, arising from the large ionic radius of Se giving longer surface–nanocluster distances and thus weaker binding. The magnitude of the red shift compared to unmodified rutile (110) can be over 1 eV and is tuned by the size (or surface loading) of the nanocluster and the anion in the nanocluster. Finally, a model of the photoexcited state shows that holes localised onto low coordinated anion sites on the adsorbed nanoclusters and electrons localise onto the TiO2 surface which will reduce charge recombination and enhance the photocatalytic activity. This concept of an anion controlled red shift in light absorption in nanocluster-modified TiO2 should be extendable to other metals such as tin or lead chalcogenides.

Graphene-wrapped hierarchical TiO2 nanoflower composites with enhanced photocatalytic performance

Graphene-wrapped titanium dioxide nanoflower composites (G–TiO2) consisting of nanosheets and nanoparticles were synthesized using a two-step solvo/hydrothermal process. Materials were characterized using SEM, TEM, high-resolution TEM (HRTEM), XRD, Raman spectroscopy, and FTIR. Further analysis was performed using Branauer–Emmett–Teller (BET) specific surface area analysis, electrochemical impedance spectroscopy (EIS), UV-Vis spectroscopy, and diffuse reflectance UV-Vis spectroscopy. Photocatalytic activity was determined by the photo-degradation of methylene blue under UV irradiation. Results show that the TiO2 nanoflower exhibits a higher photocatalytic activity than commercial P25 by a factor of 1.49. This is attributed to the highly crystalline, hierarchical nature of the nanoflower structure, which provides improved charge transport and a reduced recombination rate of photo-generated electron–hole pairs. After wrapping with graphene, the G–TiO2 composite can further improve the photocatalytic performance by providing a planar conjugated surface for dye adsorption, by further reducing recombination through accepting electrons from TiO2, and by causing a red shift in light absorption. The highest photocatalytic performance was found using a graphene loading of 5 wt%, which outperforms commercial P25 by a factor of 3.4.

Ordered Mesoporous CeO2 Synthesized by Nanocasting from Cubic Ia3d Mesoporous MCM-48 Silica: Formation, Characterization and Photocatalytic Activity

Ordered nanocrystalline mesoporous cerium oxide was successfully synthesized using MCM-48 molecular sieves as a hard template. The long-range ordered mesostructure was characterized by low-angle and wide-angle X-ray diffraction (XRD), N2 adsorption−desorption technique, and transmission electron microscopy (TEM), and the obtained cerium oxide exhibited high similarity to the cubic Ia3d symmetry of the silica template. Due to smaller crystal size, the mesoporous materials have a blue shift in light absorption as compared with nonporous materials. X-ray photoelectron spectrum (XPS) and energy-dispersive X-ray spectrum (EDS) patterns confirm that mesoporous materials possess more surface vacancies than bulk CeO2. Photocatalytic degradation of azodye acid orange 7 under visible light illumination was used as a catalytic test reaction. The activity of mesoporous product in dye decolorization is substantially higher than those for a nonporous sample and TiO2 P25.

Preparation, Characterisation, and Photocatalytic Behaviour of Co‐TiO2 with Visible Light Response

The preparation of cobalt‐modified TiO2 (Co‐TiO2) was carried out by the incipient impregnation method starting from commercial TiO2 (Degussa, P‐25) and cobalt acetate. XPS data show that cobalt is incorporated as divalent ion, and it is likely present within few subsurface layers. No appreciable change in structural‐morphologic properties, such as surface area and anatase/rutile phase ratio, was observed. Conversely, Co addition brings about conspicuous changes in the point of zero charge and in surface polarity. Diffuse reflectance spectra feature a red shift in light absorption that is dependent on the amount of cobalt. The influence of cobalt addition on the performance of TiO2 as a photocatalyst in the degradation of 4‐chlorophenol and Bisphenol A is investigated. The results show that the modified oxide presents a higher photoactivity both for illumination with UV‐visible (λ > 360 nm) and visible light (λ > 420 nm; λ > 450 nm), and that this enhancement depends on the amount of the added species and on the final thermal treatment in the preparation step. We also show that Co‐TiO2 is a more active catalyst than pure TiO2 for the reduction of O2 in the dark, which is an important reaction in the overall photocatalytic processes.

Characterization of Two Light‐Harvesting Subunits Isolated from the Brown Alga Pelvetia canaliculata: Heterogeneity of Xanthophyll Distribution

AbstractThe main light‐harvesting fraction from Pelvetia canaliculata was isolated on a sucrose density gradient from digitonin‐solubilized chloroplasts. After further solubilization by dodecyl maltoside, the bulk fraction was separated into two subunits by preparative isoelectric focusing. The more acidic brown fraction was mainly composed of 22 kDa polypeptides having an apparent pI of 4.55. Its pigment composition was very simple, containing chlorophyll (Chi) a, Chi c and fucoxanthin. The in vivo spectral properties of fucoxanthin, namely a shift in light absorption to the green and efficient energy transmission to Chi a, were conserved in this subunit. No xanthophyll associated with photoprotection was found in this band, even when obtained from photoinhibited thalli. The less acidic green band contained predominantly 22 kDa polypeptides that were resolved into numerous components by denaturing isoelectric focusing. Its pigment composition was more complex, containing, in addition, pigments of the so‐called xanthophyll cycle. In photoinhibited thalli, about half of the violaxanthin was converted into antheraxanthin and zeaxanthin. All the pigments of the xanthophyll cycle were specifically associated with this subunit, and it may thus have a central role in the thermal dissipation of the absorbed light energy as postulated for light‐harvesting complex II isolated from green plants.

Hot spot locations and temperature distributions in a forced convection photochemical reactor

We have computed steady axisymmetric temperature distributions in a tubular photoreactor in which fluid in laminar flow absorbs light from an azimuthally uniform, radially incident light source. In addition to the primary on-axis temperature maximum, a second hot spot forms off the centerline as the optical density γ increases. This results from a shift of light absorption toward the wall, which is held at a fixed temperature (e.g. by forced convection cooling on the outside). At still larger γ the centerline hot spot disappears, leading to a decrease in the maximum temperature and accompanied by an increase in the temperature near the wall. There is a critical value of γ, depending on the Peclet number Pe, below which the hot spot is located on the centerline and above which a second hot spot forms and moves off the centerline. The reduction in maximum centerline temperature may be advantageous for product selectivity and yield in the central core of the reactor, but may be disadvantageous with respect to the formation of light-absorbing deposits on the interior of the tube wall. Axial heat conduction has significant effects for Pe < 50, including displacement of the hot spot upstream from the exit plane of the illuminated section, as well as an increase in the maximum temperature.