TiO2 Photocatalytic MaterialsView this Special Issue
Editorial | Open Access
Jiaguo Yu, Mietek Jaroniec, Gongxuan Lu, " Photocatalytic Materials", International Journal of Photoenergy, vol. 2012, Article ID 206183, 5 pages, 2012. https://doi.org/10.1155/2012/206183
Since the discovery of photocatalytic splitting of water on TiO2 electrodes in 1972 by Honda and Fujishima, a great deal of effort has been devoted in recent years to the development of highly active heterogeneous photocatalysts for environmental applications including air purification, water disinfection, hazardous waste remediation, and water purification, as well as for the energy-related applications, such as, hydrogen production and solar cells. Among the various oxide and nonoxide semiconductor photocatalysts, the photocatalytic performance of titania has been most intensively studied because of its biological and chemical inertness, strong oxidizing power, cost effectiveness, and long-term stability against photocorrosion and chemical corrosion, and especially its energy band edges, which well match the redox potentials of water. However, the photocatalytic performance of TiO2 must be further enhanced from the practical and commercial viewpoints, mainly due to the high recombination rate of photogenerated conduction band electrons and valence band holes and narrow light-response range resulting from the wide band gap. To resolve these problems, many methods have been proposed to enhance the photocatalytic activity of TiO2, including crystal and textural modification, band gap (electronic structure) engineering, interfacial heterostructuring, noble metal loading, metal ion doping, carbon and nitrogen doping, dye sensitization, and also the usage of sacrificial reagents (electron donors or hole scavengers).
This special issue contains thirty nine papers, which mainly deal with environmental purification, hydrogen production, and dye-sensitized solar cells. Among them 30 papers are related to environmental photocatalysis, 4 papers deal with photoelectrocatalysis and photoelectrochemistry, 3 papers are devoted to photocatalytic hydrogen production, and 2 papers focus on dye-sensitized solar cells. Furthermore, this special issue contains two papers related to the photocatalytic degradation of endocrine disrupting chemicals (EDCs) present in wastewater by TiO2 and to the preparation of film-type TiO2 nanotube photocatalysts by liquid-phase deposition, respectively. Ten of the aforementioned papers are devoted to the doping or codoping TiO2 photocatalysts. Eight papers deal with the composite photocatalysts. Two papers are related to the preparation, photocatalytic activity, and recovery of magnetic photocatalysts. Two papers report data on the modification of Degussa-P25 TiO2 powders. Another two contributions describe the removal of indoor formaldehyde pollutants. Three papers are devoted to bactericidal and antibacterial activity of TiO2. One paper deals with photocatalytic selectivity of TiO2. Another paper discusses the plasmon photocatalytic selectivity of Au-TiO2. Finally, two other papers are related to the low-temperature preparation of TiO2 and photocatalytic treatment of textile wastewater, respectively. We wish to express our thanks to all the authors for submitting interesting contributions to this special issue. A brief summary of all thirty nine accepted papers is provided below.
In “Degrading endocrine disrupting chemicals from wastewater by TiO2 photocatalysis: a review,” the authors discuss the heterogeneous photocatalysis, the abatement of endocrine-disrupting chemicals (EDCs), phthalates, bisphenol A and chlorophenols in particular, using TiO2-based catalysts. Degradation mechanisms, pathways, and intermediate products of various EDCs on TiO2 photocatalysis are described in detail. The influence of experimental conditions on the photocatalytic degradation of various EDCs on TiO2 is covered with a special attention. Finally, the future prospects and challenges for the photocatalytic degradation EDCs on titania are summarized and discussed.
Another contribution “A review on TiO2 nanotube film photocatalysts prepared by liquid-phase deposition,” provides a concise appraisal of studies on the formation of TiO2 nanotube films by liquid-phase deposition based on the template-assisted growth. The formation mechanisms of anodic alumina templates and TiO2 nanotube films are discussed. The morphology of TiO2 is influenced by the morphology of anodic alumina. This work shows that the proper concentration of deposition solution (0.1 mol/L (NH4)2TiF6) and proper calcination temperature (400°C) are favorable for the production of TiO2 nanotube films.
In “Photocatalytic properties of nitrogen-doped Bi12TiO20 synthesized by urea addition sol-gel method,” the undoped and nitrogen-doped Bi12TiO20 materials were synthesized by sol-gel method in the presence of urea and subjected to annealing at 600°C. The UV-Vis absorption spectra indicate that the absorbance band of N-doped samples is shifted from 420 to 500 nm due to the substitution of oxygen with nitrogen and the formation of Ti–N and N–O bonds. The optimal N-doping amount was determined. The resulting photocatalyst Bi12TiO20−y Ny (y = 0.03) with N/(N+O) mole ratio of about 3% showed better performance than those strongly doped (Bi12TiO20−z Nz; z = 0.06), undoped (Bi12TiO20), and slightly doped (Bi12TiO20−x Nx; x = 0.01).
In “Hydrothermal synthesis of nitrogen-doped titanium dioxide and evaluation of its visible light photocatalytic activity,” the N-doped TiO2 was synthesized from nanotube titanic acid (NTA) precursor via a hydrothermal route in ammonia solution. The N-doped TiO2 catalyst showed much higher activity towards degradation of methylene blue and p-chlorophenol under visible light irradiation than Degussa P25. This is due to the enhanced absorption of N-doped TiO2 in visible light region associated with the formation of single-electron-trapped oxygen vacancies and the inhibition of recombination of photogenerated electron-hole pair by doped nitrogen.
In “The synthetic effects of iron with sulfur and fluorine on photoabsorption and photocatalytic performance in codoped TiO2,” the structural and electronic properties of iron-fluorine (Fe-F-) and iron-sulfur (Fe-S-) codoped anatase TiO2 are investigated by first-principle calculations based on density functional theory. The formation energy of codoped system is lower than that of single-element doping, which indicates the synergic effect of codoping on the stability of the structure. The codopants introduced impurity gap states resulting in the reduction of electron transition energy and thus, the visible light absorption by the samples. It is concluded that Fe-S should be a better codoping pair because it introduces extended impurity states resulting in stronger visible light absorption than those generated by Fe-F codoping compounds. This work explains the recent experiments and guides the selection of of the more effective codopants in TiO2.
In “The photocatalytic inactivation effect of Fe-doped TiO2 nanocomposites on leukemic HL60 cells-based photodynamic therapy,” the photocatalytic inactivation of Fe-doped TiO2 on leukemic HL60 cells was investigated using photodynamic therapy (PDT) reaction chamber based on LED light source; the viability of HL60 cells was examined by Cell Counting Kit-8 (CCK-8) assay. The growth of leukemic HL60 cells was significantly inhibited by adding TiO2 nanoparticles, and the inactivation efficiency was effectively enhanced by the surface modification of TiO2 nanoparticles by Fe doping.
In “Photocatalytic decomposition of amoxicillin trihydrate (AMOX) antibiotic in aqueous solutions under UV irradiation using Sn/TiO2 nanoparticles,” the effects of Sn-doping on the crystal structure, surface area, adsorption properties, pore size distribution, and optical absorption properties of the catalysts were investigated. The resulting Sn/TiO2 nanoparticles exhibited high photocatalytic activity during mineralization of AMOX under UV light due to the increase of generated hydroxyl radicals, band gap energy, specific surface area, and decrease in the crystallite size. The kinetics of AMOX mineralization was explained in terms of the Langmuir-Hinshelwood model.
In “Enhanced visible light photocatalytic activity of mesoporous anatase TiO2 codoped with nitrogen and chlorine,” anatase mesoporous TiO2 photocatalysts codoped with N and Cl (N-Cl-TiO2) were synthesized by one-step sol-gel process in the presence of ammonium chloride. N-Cl-TiO2 catalyst exhibited higher visible light photocatalytic activity than P25 TiO2 and N-TiO2, which was attributed to the small crystallite size, intense light absorption in visible region, and narrow band gap.
In “Neodymium-doped TiO2 with anatase and brookite two phases: mechanism for photocatalytic activity enhancement under visible light and the role of electron,” the titania samples doped with neodymium (Nd) were synthesized by a sol-gel method and examined for the photocatalytic degradation of rhodamine-B under visible light. Anatase and brookite phases were detected in these samples. Additionally, Nd as Nd3+ may enter into the lattice of TiO2 and the presence of Nd3+ substantially enhances the photocatalytic activity of TiO2 under visible light. It was found that hydroxyl radicals produced by Nd-doped TiO2 under visible light are reactive species for Rh-B degradation and the photogenerated electrons are mainly responsible for the formation of the reactive species.
In “Mechanistic study of visible-light-induced photodegradation of 4-chlorophenol by TiO2−xNx with low nitrogen concentration,” the TiO2−x Nx powders with low N-doping concentrations (0.021 < x < 0.049) were prepared by annealing commercial TiO2 (P-25) under NH3 flow at 550°C. Regardless of UV or visible light, the photoactivities of the samples decreased as x increased, and TiO1.979N0.021 showed the highest activity for the 4-chlorophenol (4-CP) decomposition under visible-light irradiation. The visible-light response for N-doped TiO2 could arise from the N-induced midgap level, formed above the valence band (O 2p). Electron spin resonance (ESR) measurements and the radical scavenger methods provided the evidence that the active species (OH• and ) are responsible for the photodecomposition of 4-CP over TiO2−x Nx under the visible irradiation. A possible photocatalytic mechanism was discussed in detail.
In “TiO2:(Fe, S) thin films prepared from complex precursors by CVD, physical chemical properties, and photocatalysis,” the TiO2 thin films were prepared using Ti(dpm)2(OPri)2 and Ti(OPri)4 (dpm = 2,2,6,6-tetramethylheptane-3,5-dione, Pri = isopropyl) as the precursors. The volatile compounds Fe[(C2H5)2NCS2]3 and [(CH3)C]2S2 were used to prepare doped TiO2 films. The synthesis was done in vacuum or in the presence of Ar and O2. Physical, chemical, and photocatalytic properties of the (Fe, S)-doped TiO2 films were studied. The TiO2:(Fe, S) films prepared from the Ti(OPri)4 precursor showed higher photocatalytic activities, very close to that of Degussa P25 powder in UV region.
In “fabrication of Al-doped TiO2 visible-light photocatalyst for low-concentration mercury removal,” the high-quality Al-doped TiO2 visible-light photocatalyst was prepared via a single-step direct combination of vaporized Ti, Al, and O2 using a 6 kW thermal plasma system. The formed Al-doped TiO2 nanoparticles were a mixture of anatase and rutile phases and had a size between 10 and 105 nm. The absorption spectra of these nanoparticles are shifted towards the visible light region, depending on the Al2O3 addition. Hg0 breakthrough tests revealed that the nanoparticles showed an appreciable Hg0 removal capability under visible-light irradiation. Nevertheless, the moisture reduced Hg removal by the nanoparticles, especially when visible-light irradiation was applied, suggesting that the competitive adsorption between H2O and Hg species on the active sites of TiO2 surface occurred.
In “Effect of Ag-Cu bimetallic components in a TiO2 framework for high hydrogen production on methanol/water photo-splitting,” the TiO2 photocatalysts doped with Ag, Cu, and Cu-Ag were prepared for the production of H2 from methanol/water photodecomposition. As compared to monometal-incorporated TiO2, the H2 production was markedly enhanced in bimetal-incorporated photocatalyst.
In “Photoelectrocatalytic degradation of sodium oxalate by TiO2/Ti thin film electrode,” the photocatalytically active TiO2 thin films were deposited on the titanium substrate plate by chemical vapor deposition (CVD) method, and the photoelectrocatalytic degradation of sodium oxalate was investigated. The additional applied potential in photocatalytic reaction could prohibit recombination of electron/hole pairs, but the photoelectrocatalytic effect was decreased when the applied electric potential was over 0.25 V.
In “Anodization parameters influencing the growth of titania nanotubes and their photoelectrochemical response,” the TiO2 nanotubes (TNTs) were fabricated by electrochemical oxidation of Ti foil in a standard two-electrode cell-containing NH4F. The tube length decreased with bath temperature, which can be attributed to the faster chemical dissolution rate at high temperatures. However, nanotubes growth rate was enhanced by ~260% with the addition of EDTA as the complexing agent. Meanwhile, the nanotubes diameter was found to be proportionally dependent on bath temperature but independent of the voltage ramp and addition of EDTA. The photoelectrochemical response under illumination was enhanced by using the calcined TNT.
In “Effect of N,C-ITO on composite N,C-TiO2/N,C-ITO/ITO electrode used for photoelectrochemical degradation of aqueous pollutant with simultaneous hydrogen production,” the composite Ti/TO electrode was simultaneously used for hydrogen production and degradation of organic pollutants. The N,C-TiO2 layer in this electrode enhanced not only the photocurrent response at entire applied potentials but also the flat band potential; a shift of about 0.1 V towards cathode was observed, which is beneficial for the PEC process.
In “Investigation on the photoelectrocatalytic activity of well-aligned TiO2 nanotube arrays,” the well-aligned TiO2 nanotube arrays were fabricated by anodizing Ti foil in viscous F− containing organic electrolytes, and their photocatalytic activity was evaluated in the photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of methylene blue (MB) dye in different supporting solutions. The excellent performance of ca. 97% for color removal was reached after 90 min in the PEC process compared to that of PC process which indicates that a certain external potential bias favors the promotion of the electrode reaction rate on the TiO2 nanotube array when it is under illumination. In addition, it was found that the PEC degradation of MB conducted in supporting solutions was accelerated at low pH and in the presence of Cl−.
In “Photocatalytic oxidation of triiodide in UVA-exposed dye-sensitized solar cells,” the UVA irradiation of glass mounted dye-sensitized solar cells without UV filtration caused a failure within 400 hours of light exposure. The failure mode was shown to relate to the consumption of , which was directly related to TiO2 photocatalysis. The device failure was more rapid for the cells under electrical load indicating that the degradation of the electrolyte is related to photogenerated hole production by excitation of TiO2. Once depleted by UV exposure, the was regenerated by simple application of a reverse bias, which can restore severely the UV-degraded devices to near original working conditions.
In “Effects of homogenization scheme of TiO2 screen-printing paste for dye-sensitized solar cells,” the TiO2 screen-printing paste was prepared by two methods to disperse the nanocrystalline TiO2 powder: a “ball-milling route” and a “mortal-grinding route.” The TiO2 ball-milling (TiO2-BM) route gave monodisperse TiO2 nanoparticles, resulting in high photocurrent density (14.2 mA cm−2) and high photoconversion efficiency (8.27%). On the other hand, the TiO2 mortal-grinding (TiO2-MG) route gave large aggregates of TiO2 nanoparticles, resulting in low photocurrent density (11.5 mA cm−2) and low photoconversion efficiency (6.43%).
In “Enhancing photocatalytic performance through tuning the interfacial process between TiO2-assembled and Pt-loaded microspheres,” the Pt-TiO2 microspheres were prepared in a solution with subsequent Pt loading through a photoreduction. The Pt loading led to an efficient separation of photogenerated electron-hole pairs. Meanwhile, electrons stored in the “microcapacitor” of TiO2 microspheres contributed to the enhancement of photocatalytic activity under UV light irradiation.
In “Facile synthesis and photocatalytic property of titania/carbon composite hollow microspheres with bimodal mesoporous shells,” the titania/carbon composite hollow microspheres with bimodal mesoporous shells were fabricated by one-pot procedure involving acidic (NH4)2TiF6 aqueous solution in the presence of glucose at 180°C for 24 h and then calcined at 450°C for 2–4 h. All TiO2/C composite hollow spheres generally exhibited bimodal mesopore size distribution with peaks in the range of 2.3–4.5 nm and 5.7–12.7 nm. The TiO2-2 sample showed the highest daylight-induced photocatalytic activity and greatly exceeded that of Degussa P25.
In “Performance of Ag-TiO2 photocatalysts towards the photocatalytic disinfection of water under interior-lighting and solar-simulated light irradiations,” the characterization and photoactivity of Ag-TiO2 materials was studied using different amounts of silver during the hydrothermal synthesis. The resulting photocatalysts were tested towards photocatalytic disinfection of water using a solar light simulator and an interior-light irradiation setup. The obtained data indicate an increase in the photooxidative effect of TiO2, while dark processes show that the partial inactivation is due to the Ag-TiO2 surface bactericidal effect and possible lixiviated Ag+.
In “Preparation and photocatalytic property of TiO2/diatomite-based porous ceramics composite materials,” the nano-TiO2/diatomite-based porous ceramics composite materials were prepared by hydrolysis deposition method using titanium tetrachloride as the precursor of TiO2 and diatomite as the porous support. After calcination at 550°C, the thin TiO2 anatase film was formed on the diatomite with the average grain size of TiO2 of about 10 nm. The degradation ratio for 5 mg/L malachite green solution reached 86.2% after irradiation for 6 h under ultraviolet.
In “Anatase TiO2 nanospindle/activated carbon (AC) composite photocatalysts with enhanced activity in removal of organic contaminant,” a novel TiO2/AC composite photocatalyst was prepared by coating of anatase TiO2 nanospindles onto the surface of AC particles, which showed better photocatalytic activity than pure TiO2 due to the synergistic effect between supporting materials and TiO2 nanospindles. Most importantly, this composite photocatalysts effectively reduced the lixiviation of TiO2 from the surface of AC.
In “Preparation and photocatalytic activity of TiO2-deposited fabrics,” the nanoscale TiO2 photocatalytic films were formed on the surface of polyester nonwovens by using direct current reactive magnetron sputtering. The test results proved that the grain sizes of the sputtered clusters increased and the coating layer became more compact with increasing film thickness, but the composition of films did not change significantly. The photocatalytic activity of TiO2-coated fabrics mostly depended on the film thickness.
In “Synthesis of Fe3O4/C/TiO2 magnetic photocatalyst via vapor phase hydrolysis,” a core/multishell-structured Fe3O4/C/TiO2 magnetic photocatalyst was prepared via vapor phase hydrolysis process. The size and crystallinity of anatase TiO2 crystallite in the shell could be tuned by temperature and duration of the process. The photocatalytic activity of Fe3O4/C/TiO2 was higher than that of the commercial anatase TiO2 in photocatalytic degradation of methylene blue (MB), and its recycling property was significantly improved. The intermediate carbon layer can effectively eliminate the electron interaction or photodissolution of Fe3O4 occurring at the point of contact.
In “Preparation, photocatalytic activity, and recovery of magnetic photocatalyst for decomposition of benzoic Acid,” the optimal experimental parameters of TiO2 preparation were pH 3, weight ratio of TiO2/SiO2(Fe3O4) at 1 and calcination temperature of TiO2/SiO2/Fe3O4 at 350°C. Furthermore, the paramagnetic behaviors of the prepared TiO2/SiO2/Fe3O4 gave rise to the magnetic photocatalyst, which could be separated more easily through the application of a magnetic field.
In “Enhanced photocatalytic activity of TiO2 powders (P25) via calcination treatment,” the authors showed that calcination influenced the microstructures and photocatalytic activity of the P25 TiO2 powders. An optimal calcination temperature (500°C) was determined, and the photocatalytic activity of TiO2 powders calcined at 500°C was nearly twice higher than that of the uncalcined P25 TiO2.
In “Photocatalytic degradation of NOx using Ni-containing TiO2,” a nickel-modified titania photocatalyst was prepared by photodeposition method using Degussa-P25 TiO2 particle and nickel chloride as raw materials. Ni did not enter into the TiO2 crystal lattice and was uniformly dispersed onto the TiO2 surface. The modified titanium dioxide with 0.1 mol% of nickel exhibited twice higher the NOx-removal activity in comparison to the bare TiO2 under ultraviolet illumination. The nickel content in this photodeposition process plays an important role in controlling affinity towards NOx molecules, recombination rate of electron-hole pair, and the content of active sites on the TiO2 surface and therefore, affects the optical and photocatalytic properties.
In “Effects of calcination temperatures on photocatalytic activity of ordered titanate nanoribbon (TNR)/SnO2 films fabricated during an EPD process,” the highest photocatalytic activity was obtained for the calcined TNR/SnO2 film at 600°C due to the formation of well-crystallized anatase phase, the unique morphology, and the fast charge carrier separation and transfer at the interface of TiO2 and SnO2.
In “Synergistic effect of nanophotocatalysis and nonthermal plasma on the removal of indoor HCHO,” the effect of combination of photocatalytic oxidation (PCO) with nonthermal plasma technology (NTP) on the removal of indoor HCHO was investigated. The effects of plasma discharge configuration, the applied voltage, the flow velocity, and the humidity on the HCHO removal were studied. The HCHO removal was shown to be more effective in the line-to-plate electrode discharge reactor; the HCHO reaction rate was enhanced and the amount of air that needs to be cleaned was enlarged. The synergistic effect was observed for the indoor air purification by combining PCO with NTP.
In “Degradation of gaseous formaldehyde by visible light-responsive titania photocatalyst filter,” a method is proposed by using electrophoretic deposition (EPD) to fabricate the titania (TiO2) photocatalyst filter, which after modification with lithium nitrate (LiNO3) became responsive to visible light and effectively degraded gaseous formaldehyde. The best total average degradation performance of thisphotocatalyst filter was about 9.2% and 16.3% higher than that of the original photocatalyst filter (P-25, Degussa) at the UVA and visible irradiation, respectively, at 26°C.
In “Preparation of antibacterial color-coated steel sheets,” a simple way to manufacture antibacterial color-coated sheet using Ag-loaded TiO2 is developed. The optimal technical parameters are 2% for silver-loaded titanium dioxide, which is dispersed well as color-coated sheets, reaching the antibacterial efficiency of 99.99%. The efficiency of methyl orange degradation reached 88% in 4 h.
In “Highly selective deethylation of rhodamine B on TiO2 prepared in supercritical fluids,” a pure phase anatase TiO2 nanoparticles with sizes of 5–8 nm and different crystallinity were synthesized in supercritical isopropanol/water using a continuous flow reactor. Their photodegradation of rhodamine B (RhB) was evaluated under visible light irradiation. The as-prepared TiO2 nanoparticles showed much higher photodegradation efficiencies than commercial Degussa P25 TiO2. Moreover, the photodegradation of RhB on the as-prepared TiO2 follows a different process than that on P25 TiO2, quicker N-deethylation and slower cleavage of conjugated chromophore structure.
In “Enhancement of visible-light photocatalytic activity of mesoporous Au-TiO2 nanocomposites by surface plasmon resonance,” the Au-TiO2 nanocomposites were prepared by a simple spray hydrolytic method with photoreduction at 90°C. The light absorption, the formation rates of hydroxyl radicals, and photocatalytic decolorization of RhB aqueous solution were significantly enhanced by the embedded Au nanoparticles in the Au-TiO2 nanocomposites due to the surface plasmon resonance. The composite -0.015 sample exhibited the best visible-light photocatalytic activity for decolorization of RhB aqueous solution due to the synergistic effects of the absorption shift into visible and improved efficiency of interfacial charge transfer process.
In “Ta/TiO2- and Nb/TiO2-mixed oxides as efficient solar photocatalysts: preparation, characterization, and photocatalytic activity,” Ta/TiO2- and Nb/TiO2-mixed oxide photocatalysts were prepared by simple impregnation method using different TiO2 : Nb mass ratios, followed by calcination at 500°C. Nb/TiO2- and Ta/TiO2-mixed oxides showed higher activity than the untreated TiO2 under natural solar light. The maximum activity was observed for Nb/TiO2 sample (at mass ratio of 1 : 0.1), characterized by the smallest crystalline size (17.79 nm). As compared to the untreated TiO2, the solar decolorization and mineralization rates improved by about 140% and 237%, respectively, and the band gap was reduced to 2.80 eV.
In “Removal of a cationic dye by adsorption/photodegradation using electrospun PAN/O-MMT composite nanofibrous membranes coated with TiO2,” the polyacrylonitrile (PAN)/organic-modified montmorillonite (O-MMT) nanofibrous composite membranes were firstly prepared by electrospinning and then coated with titanium dioxide (TiO2) using spin coating technique. With the increase of O-MMT amount, the diameters of the nanofibers decreased, and the adsorption rate of MB was evidently improved. Besides, with the increase of TiO2 film thickness, the photocatalytic properties were enhanced while the adsorption process was slowed down.
In “Low-temperature reverse microemulsion synthesis, characterization, and photocatalytic performance of nanocrystalline titanium dioxide,” the nanosized TiO2 nanoparticles were synthesized in n-hexanol/CTAB/water reverse microemulsions. Among all the as-synthesized photocatalysts by aging method, the nanoparticles aged at 65°C for 90 h showed the highest photocatalytic activity, which was higher than that of commercial P25.
In “TiO2-based photocatalytic treatment of raw and constructed-wetland pretreated textile wastewater,” a combination of photocatalytic and biological degradation of wastewaters generated in textile production (simulation of real textile effluent) is presented.
Copyright © 2012 Jiaguo Yu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.