Electron Spin Resonance Technique Research Articles

Incorporation of Manganese (II) in Beta-Tricalcium Phosphate from EPR and ENDOR Measurements for Powders

Powders of β-tricalcium phosphate (β-TCP, Ca3PO4) doped with manganese (Mn2+) are comprehensively analyzed with electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques. The modeling of the spectra permitted to calculate the values of zero-field splitting (B20 = −904 MHz; B40 = −1.41 MHz and B43 = 195.2 MHz) and explain the origin of the low-field hyperfine structures as the allowed spin transitions of fine structure. Three structurally inequivalent positions for Mn2+ in the β-TCP crystal lattice are identified and their g-factors and hyperfine constants are quantified. The obtained results can serve as fundamental background to the study of structurally disordered matrices with high spin (S ≥ 1) impurities which are important for catalytic systems.

Facile synthesis of CaWO4 nanoparticles incorporated on porous carbons with improved photocatalytic degradation of tetracycline

The construction of an efficient and recyclable photocatalyst for the degradation of antibiotics is still a challenge. In this work, we successfully synthesized porous carbon-supported CaWO4 nanoparticles (CaWO4 @C) by solidifying sodium alginate (SA) mixed with Na2WO4 by Ca2+ ions and subsequent high-temperature carbonization. The introduction of porous carbon significantly reduced the size of CaWO4 particles, increased the specific surface area for mass transport, and improved visible light utilization. The influence factors of photocatalytic activity were extensively explored through photocatalytic degradation of tetracycline (TC). As a result, the optimized CaWO4 @C exhibited excellent photocatalytic performance with the TC degradation rate of 94.56% after 60-min visible light irradiation. The TC photodegradation rate constant was calculated to be 0.051 min−1, which was 30.19-time higher than the pristine CaWO4. Besides, the photocatalytic reaction mechanism for degradation of TC was revealed in terms of free radical trapping experiments and electron spin resonance (ESR) techniques.

Large-Scale Conformational Changes of FhaC Provide Insights Into the Two-Partner Secretion Mechanism.

The Two-Partner secretion pathway mediates protein transport across the outer membrane of Gram-negative bacteria. TpsB transporters belong to the Omp85 superfamily, whose members catalyze protein insertion into, or translocation across membranes without external energy sources. They are composed of a transmembrane β barrel preceded by two periplasmic POTRA domains that bind the incoming protein substrate. Here we used an integrative approach combining in vivo assays, mass spectrometry, nuclear magnetic resonance and electron paramagnetic resonance techniques suitable to detect minor states in heterogeneous populations, to explore transient conformers of the TpsB transporter FhaC. This revealed substantial, spontaneous conformational changes on a slow time scale, with parts of the POTRA2 domain approaching the lipid bilayer and the protein’s surface loops. Specifically, our data indicate that an amphipathic POTRA2 β hairpin can insert into the β barrel. We propose that these motions enlarge the channel and initiate substrate secretion. Our data propose a solution to the conundrum how TpsB transporters mediate protein secretion without the need for cofactors, by utilizing intrinsic protein dynamics.

Microwave-assisted synthesis and photodynamic activity of tris-heteroleptic Ru(II) complexes with asymmetric polypyridyl ligands

Ru(II) polypyridyl complexes are attracting particular attention as possible photodynamic therapy (PDT) agents due to their interesting photophysical properties, which can be easily tunable through the appropriate selection of attached ligands. However, blue-wavelength absorption of the complexes limited their potential application. In this study, three tris-heteroleptic Ru(II) polypyridyl complexes were synthesized to shift the absorption of the compounds toward longer wavelengths. Their photophysical, biological, and photodynamic properties were evaluated. The mechanism of the photoinduced toxicity was investigated by fluorescence and electron paramagnetic resonance (EPR) techniques in vitro and in aqueous solutions. The results revealed the photodynamic activity of the studied compounds is based mainly on the photogeneration of 1O2 and H2O2. Additionally, the synthesized Ru complexes demonstrated high activity in inhibiting the detachment of cancer cells. This effect was additionally enhanced by light activation of Ru compounds. The presented findings demonstrate a rational strategy for the efficient synthesis of tris-heteroleptic Ru(II) polypyridyl complexes to be used as promising photodynamic agents for cancer and antimetastatic treatments.

X-Band Parallel-Mode and Multifrequency Electron Paramagnetic Resonance Spectroscopy of S = 1/2 Bismuth Centers.

The recent successes in the isolation and characterization of several bismuth radicals inspire the development of new spectroscopic approaches for the in-depth analysis of their electronic structure. Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for the characterization of main group radicals. However, the large electron–nuclear hyperfine interactions of Bi (209Bi, I = 9/2) have presented difficult challenges to fully interpret the spectral properties for some of these radicals. Parallel-mode EPR (B1∥B0) is almost exclusively employed for the study of S > 1/2 systems but becomes feasible for S = 1/2 systems with large hyperfine couplings, offering a distinct EPR spectroscopic approach. Herein, we demonstrate the application of conventional X-band parallel-mode EPR for S = 1/2, I = 9/2 spin systems: Bi-doped crystalline silicon (Si:Bi) and the molecular Bi radicals [L(X)Ga]2Bi• (X = Cl or I) and [L(Cl)GaBi(MecAAC)]•+ (L = HC[MeCN(2,6-iPr2C6H3)]2). In combination with multifrequency perpendicular-mode EPR (X-, Q-, and W-band frequencies), we were able to fully refine both the anisotropic g- and A-tensors of these molecular radicals. The parallel-mode EPR experiments demonstrated and discussed here have the potential to enable the characterization of other S = 1/2 systems with large hyperfine couplings, which is often challenging by conventional perpendicular-mode EPR techniques. Considerations pertaining to the choice of microwave frequency are discussed for relevant spin-systems.

Electron Paramagnetic Resonance Study of PbSe, PbTe, and PbTe:In Semiconductors Obtained by the Pulsed Laser Deposition Method.

The magnetic properties of lead selenide (PbSe) and indium-doped lead telluride (PbTe:In) composites have been studied by using the electron paramagnetic resonance (EPR) technique. The samples were obtained by using the pulsed laser deposition method (PLD). Temperature dependences of the EPR spectra were obtained. The analysis of the temperature dependencies of the integral intensity of the EPR spectra was performed using the Curie–Weiss law. In these materials, the paramagnetic centers of Pb1+ and Pb3+ ions were identified. The results are discussed.

Understanding Redox Reaction Mechanisms of a Flavin Mononucleotide By in Situ NMR and EPR Techniques

There are many types of energy storage technologies available at present, such as batteries and supercapacitors.1,2 Common battery chemistries include lead3 and lithium-ion4; however, scaling them up to the grid level is challenging.2 Recently, redox flow batteries (RFBs) have attracted much attention, as they can overcome the intermittent nature of renewable energy sources. We have previously developed a new approach to study RFBs by using in-situ nuclear magnetic resonance (NMR) and in-situ electron paramagnetic resonance (EPR), allowing the real time tracking of the redox state of various species in a system. As a result of this work, the electrochemical processes can be better understood. Investigating and understanding the electrolyte properties is a major goal in flow battery research and this will help to develop further RFB systems.Here, we report the analysis of the redox mechanism of a flavin mononucleotide-based anolyte (FMN) in a RFB system with the in-situ NMR and EPR setups. It represents one of the safest families of organic molecules for flow battery applications as it is a molecule derived from riboflavin (vitamin B2) and commonly used as an orange-red food additive.5–7 Nevertheless, the redox properties of flavins in strongly alkaline solution have yet to be thoroughly studied. In particular, the mechanism of the redox reaction for this system is not well understood.To understand the chemistry of FMN in strongly alkaline solution, first the NMR properties were studied. It was revealed that the chemical structure of FMN changed over time, i.e., additional signals were observed to grow in, and, additionally, the existence of a paramagnetic species was confirmed. The paramagnetic species formed on dissolution and was responsible for significant line broadening in the NMR spectrum. To examine how these changes affect the redox properties of FMN, a RFB was cycled over multiple cycles. Here we observed a second charge plateau developed as we continued to cycle the battery that was not mirrored during discharge. The in-situ EPR and NMR analysis reveal that the second plateau was likely caused by a side product derived from FMN. Therefore, possible degradation mechanisms of flavins, such as hydrolysis, need to be considered. Herein we propose that the observed changes over time are caused by the hydrolysis of FMN and provide an explanation for the different redox properties.References Wang, Y. & Zhong, W. H. Development Of Electrolytes Towards Achieving Safe And High-Performance Energy-Storage Devices: A Review. ChemElectroChem 2, 22–36 (2015).Divya, K. C. & Østergaard, J. Battery Energy Storage Technology For Power Systems - An Overview. Electr. Power Syst. Res. 79, 511–520 (2009).Ruetschi, P. Review On The Lead-Acid Battery Science And Technology. J. Power Sources 2, 3–120 (1977).Goodenough, J. B. & Park, K. S. The Li-Ion Rechargeable Battery: A Perspective. J. Am. Chem. Soc. 135, 1167–1176 (2013).Orita, A., Verde, M. G., Sakai, M. & Meng, Y. S. A Biomimetic Redox Flow Battery Based On Flavin Mononucleotide. Nat. Commun. 7, 1–8 (2016).Grajek, H., Zurkowska, G., Drabent, R. & Bojarski, C. Hanna Grajek. Biochem. Biophys. Acta 881, 241–247 (1986).Grajek, H., Drabent, R., Zurkowska, G. & Bojarski, C. Absorption Of The Flavin Dimers. BBA - Gen. Subj. 801, 456–460 (1984). Figure 1

In-situ construction of Bi2S3/Bi2MoO6 hollow microsphere with regulable oxygen vacancies for full-spectrum photocatalytic performance

Novel Bi2S3/Bi2MoO6 hollow microsphere composites with oxygen vacancies were successfully synthesized through in-situ anion exchange and hydrothermal method. After introducing sulfur into the Bi2MoO6 substrate with oxygen vacancies, Bi2S3 was close distributed on the surface of the Bi2MoO6 hollow microsphere to form Bi2S3/Bi2MoO6 heterojunctions and led to inducing the more oxygen vacancies. Compared to pristine Bi2MoO6 and Bi2S3, the Bi2S3/Bi2MoO6-0.03 exhibited higher photocatalytic efficiency toward Rhodamine B (RB) degradation under the full-spectrum (visible and near-infrared) light illumination. Meanwhile, it showed high near-infrared photocatalytic activity for antibiotics. The enhanced catalytic performance was attributed to the synergetic effect of heterojunction and the oxygen vacancies between Bi2MoO6 and Bi2S3. Meanwhile, the active species were carefully investigated by the active species capture tests and the electron spin resonance technique. Finally, the reasons for the improved photocatalytic performance and reaction mechanism were also uncovered in detail, and degradation products obtained by N-de-ethylation by the cleavage of CN of RB were demonstrated. The findings in this work provide insight into the potential applications in treating wastewater.

Yttrium -barium oxide as a robust photocatalyst for photocatalytic degradation of organic dyes under visible light

Based on the structure related to the high-temperature superconductor yttrium-barium-copper oxide, two novel high-efficiency visible light photocatalysts were created in this study. The yttrium-barium oxide (YBO) semiconductors Y2Ba3O6 (YB3O) and Y2Ba4O7 (YB4O) were prepared by a copper-free solid-phase sintering method. They were applied for the effective treatment of dye-containing wastewater by photocatalysis under visible light irradiation. The degradation efficiency of methylene blue (MB) reached more than 95% within 10 min. Stable visible light degradation of methyl orange (MO) was achieved in the presence of YB3O and YB4O. The electron spin resonance technique and active substance capture technique confirmed the presence of superoxide radicals (·O2−), hydroxyl radicals (·OH) and holes (hVB+) under visible light illumination. UV–Vis diffuse reflectance spectroscopy analysis showed that the direct optical band gaps of YB3O and YB4O were 2.550 eV and 2.583 eV, respectively, which resulted in their high visible absorption at 486.27 nm and 480.06 nm. After five cycles, the recoveries of YB3O and YB4O reached 67.15% and 72.98%. Therefore, YB3O and YB4O are considered as powerful semiconductor catalysts for the photocatalytic degradation of organic dyes in wastewater.

Low-temperature recombination luminescence of La-doped Ca2SnO4

Low-temperature ultraviolet-excited photoluminescence (PL) and recombination luminescence (RL) properties of La-doped Ca2SnO4 have been investigated by luminescence, electron paramagnetic resonance (EPR) and optically-detected magnetic resonance (ODMR) techniques. Two PL and RL bands at 340 nm and 450 nm have been observed. PL excitation spectra measurements with a synchrotron source showed a significant difference between the 450 nm and the 340 nm PL bands. The 450 nm band has a long-lasting hyperbolic decay, while the 350 nm band shows a fast decay. Assuming an excitonic nature of the 340 nm band, the band gap of the Ca2SnO4:La has been estimated to be approximately 5.5 eV. ODMR measurements suggest that the low-temperature RL band at 450 nm is caused by tunnelling recombination of electron trap and hole trap centres, and the recombination energy is transferred to Sn2+ luminescence centres.

Influence of the preparation method and silver content on the nature of active sites in Ag/CeO2 catalysts used for propylene oxidation

Silver supported on ceria solids (1 and 4 wt% Ag) are prepared by three different methods: Impregnation–Reduction with Citrate (IRC), Deposition–Precipitation with Urea (DPU), and Wetness Impregnation (WI). The catalysts are tested in the total oxidation of propylene. Presence of Ag2+ with Ag+ improves the catalytic activity. The solid prepared by the (IRC) method with a silver loading of 4 wt%, shows a better catalytic activity compared to the other solids. H2-Temperature Programmed Reduction (TPR) and Electron Paramagnetic Resonance (EPR) techniques confirm that the presence of three different redox couples: Ag2+/Ag+, Ag2+/Ag0, and Ag+/Ag0 in the (IRC) solid containing 4 wt% of silver is responsible for its superior catalytic performance.

Mono- and binuclear copper(II) complexes with different structural motifs and geometries: Synthesis, spectral characterization, DFT calculations and superoxide dismutase enzymatic activity

Several new copper(II) mono- and binuclear complexes, [Cu(HL)(ClO4)(H2O)]CH3OH (1), [Cu(HL)(DMF)](ClO4)CH3OH (2), [Cu2(HL)2(pyrazine)](ClO4)2(3) and [Cu2(HL)2(4,4-bipy)(ClO4)2]2H2O (4) (HL = 4-bromo-2-(pyridine-2-ylhydrazonomethyl)-phenol, 4,4-bipy = 4,4-bipyridyl) have been synthesized and characterized by elemental analysis, FTIR, UV–vis and electron paramagnetic resonance (Epr) spectroscopy. Molecular structures of all complexes have been obtained using single crystal X-ray analysis. The geometry of copper(II) in 1 is distorted square pyramidal and distorted square planar in 2. In binuclear complexes each copper(II) centre has distorted square planar and distorted square pyramidal geometry in 3 and 4, respectively. The complexes 1–4 possess the self-assembled supramolecular structures via different molecular interactions (hydrogen bondings, CH ⋯π and π⋯π stacking interactions). The possible intermolecular interactions were explored using 3D Hirshfeld surface and related 2D fingerprint plots, spectroscopic properties were studied using UV–visible and electron paramagnetic resonance techniques. Optimized geometry, frontier molecular orbital (FMO) and natural bond order (NBO) analysis were performed using density functional theory (DFT) calculations. To predict the chemical reactivity of complexes some global reactivity descriptors (chemical potential μ, electronegativity χ, hardness η and electrophilicity index ω) have been evaluated using DFT. The low temperature magnetic susceptibility measurements of binuclear complexes have been obtained predicts the magnetic exchange coupling value (J) by variable temperature measurements. The magnetic exchange coupling value (J) obtained are −8.1 and −2.4 cm−1 for 3 and 4, respectively. The superoxide dismutases (SOD) of all complexes were measured using NBT assay method. The complexes show moderate superoxide dismutase (SOD) mimetic activity.

Peroxymonosulfate activation by concave porous S/N co-doped carbon: Singlet oxygen-dominated non-radical efficient oxidation of organics

Developing high-efficient, low-cost and environmentally friendly catalysts is essential for activating peroxymonosulfate (PMS) but remains a challenge. Recently, novel catalysts of transition metal-nitrogen co-doped carbon materials, especially Fe-N-C, have attracted much attention due to their superior performance for PMS activation. Herein, FeSO4 and ZnSO4 were added during the synthesis of ZIF-8, obtained concave N, S co-doped iron-based carbon materials (Fe-S@NC) with mesoporous structure through high-temperature pyrolysis. This porous carbon material with a depressed surface not only promoted the exposure of active sites, but also accelerated mass transfer. The synergistic effect of Fe atoms and heteroatoms significantly enhanced the activation ability of the material to PMS. Benefiting from these advantages, only 2 mg/L Fe-S1@NC and 0.5 mM PMS was required to achieve the rapid degradation of acetaminophen (ACT) (99.7%, 0.7290 min-1). Besides, the activation system has a wide pH adaptability and low leaching iron concentration. Then the reaction mechanism of Fe-S1@NC/PMS system was investigated by quenching experiments and electron paramagnetic resonance technique (EPR), it was proposed that singlet oxygen (1O2) was the primary reactive species responsible for the degradation of ACT. This study provides new insight for the design of carbon catalysts in advanced oxidation field.

Effects of X-Rays, Electron Beam, and Gamma Irradiation on Chemical and Physical Properties of EVA Multilayer Films

Gamma-ray irradiation, using the cobalt-60 isotope, is the most common radiation modality used for medical device and biopharmaceutical products sterilization. Although X-ray and electron-beam (e-beam) sterilization technologies are mature and have been in use for decades, impediments remain to switching to these sterilization modalities because of lack of data on the resulting radiation effects for the associated polymers, as well as a lack of education for manufacturers and regulators on the viability of these sterilization alternatives. For this study, the compatibility of ethylene vinyl acetate (EVA) multilayer films with different ionizing radiation sterilization (X-ray, e-beam, and gamma irradiation) is determined by measuring chemical and physical film properties using high performance liquid chromatography, differential scanning calorimetry, Fourier-Transform InfraRed spectroscopy (FTIR), surface energy measurement, and electron spin resonance techniques. The results indicate that the three irradiation modalities induce no differences in thermal properties in the investigated dose range. Gamma and X-Ray irradiations generate the same level of reactive species in the EVA multilayer film, whereas e-beam generates a reduced quantity of reactive species.

Visible light driven robust photocatalytic activity in vanadium-doped ZnO/SnS core-shell nanocomposites for decolorization of MB dye towards wastewater treatment

Vanadium metal oxide (V2O5) doped ZnO/SnS core-shell nanocomposites are fabricated by hydrothermal method at 2200C for 10 hours. All synthesized Nanocomposites were optically, structurally, morphologically and magnetically examined by different techniques such as UV-DRS, XRD, TEM, electron paramagnetic resonance (EPR), XPS and PL techniques. The photocatalytic activity (PCA) was studied for methylene blue (MB) model pollutant dye under 300 W visible light illumination. XRD peaks confirmed the mixed phase abundance with hexagonal ZnO and orthorhombic SnS. TEM micrographs inferred uneven surface with more agglomeration. XPS conclude V4+ oxidation state for vanadyl atoms along with constitutes elements. The UV-DRS inferred red shift in bandgap. EPR argued that vanadyl ions perform tetragonal compression distorted octahedral site symmetry. PL study described quantum confinement effect. The achieved MB dye degradation under visible light illumination is more than 96% in 2 hours. Enhanced PCA argued owing to plasmonic effect and tunable energy bandgap nature of heterogeneous semiconducting materials.

Electron spin resonance (ESR) spectroscopy study of gamma-irradiated dried white mulberry (Morus Alba L.) fruits

Two different groups of dried fruits of white mulberry (Morus Alba L.) sold in Turkish markets were investigated by the X-band ESR spectroscopy technique for irradiation detection purposes. No resonance signals were detected in the ESR spectra of the un-irradiated samples. After irradiation, identical multi-line resonance signals were induced in the ESR spectra of both types of samples, except for the intensities. At doses of 2 and 10 kGy, the long-term stabilities of the radiation-induced radicals at room temperature were investigated over a storage period of about 100 days. The results obtained show that the ESR technique can be used to distinguish irradiated white mulberry fruits from un-irradiated ones.

An EPR study of gamma irradiated mixed materials based on sucrose for dosimetric purposes

Since the recent past it was known that sucrose showed some good results as dosimeter using Electron Paramagnetic Resonance (EPR) technique. However there is still considerable interest in improving its accuracy and precision as well as in developing new systems with improved characteristics. In view of this fact, materials consisting of mixture at two different ratios (70:30 and 50:50%) between sucrose and ascorbic acid, sucrose and cysteine and sucrose and diphenylamine were studied by EPR spectroscopy. The obtained new materials were irradiated by γ-rays having doses varying from 0.5 up to 20 kGy. Before irradiation no EPR spectra were recorded. After irradiation complex EPR spectra with dominants of the sucrose spectrum in them were observed. The influence of the instrumental settings on the EPR signal intensity was reported. It was found out that the dose response of sucrose has linear dependence up to 20 kGy. The dose response of the new materials is compared with those in case of sucrose. The results showed that the addition of ascorbic acid, cysteine and diphenylamine to sucrose were leading to decrease in its sensitivity, but the EPR dose response remains unchanged. It was supposed that the decrease in the sensitivity was probably due to the antiradical activity of the added substances. To check this assumption the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity (FRSA) of the used substances was studied. It was concluded that these mixed materials can be used for dosimetric purposes, however they have lower sensitivity compared to sucrose.

Impact of structural defects on the photocatalytic properties of ZnO

ZnO photocatalytic materials have been prepared using three precursors (Zinc Acetate, Zinc Hydroxide, and Zinc Peroxide), and the effect of calcination temperature on structural and photocatalytic properties of ZnO has been investigated. XRD, UV–Vis, BET, XPS, Time-Resolved Microwave Conductivity (TRMC), Raman, and Electron Paramagnetic Resonance (EPR) have been used to correlate the impact of the surface area, the introduction of oxygen and/or zinc vacancies, and the charge carrier dynamics, with their photocatalytic properties. The main objective is to provide a new approach to the impact of structural defects on ZnO semiconductors determined by using Raman and EPR techniques, and by coupling with the important role of the surface area considered as one of the most relevant characteristics in photocatalysis in terms of performance. The fewer surface defects, the more photoactive the catalyst is. The results are discussed for the model degradation of formic acid and phenol under UV-irradiation.

The MOF/LDH derived heterostructured Co3O4/MnCo2O4 composite for enhanced degradation of levofloxacin by peroxymonosulfate activation

In this study, metal–organic framework interfacing layered double hydroxide (MOF/LDH) derived heterostructured Co3O4/MnCo2O4 composite was synthesized and employed to activate peroxymonosulfate (PMS) for levofloxacin (LEV) degradation. With a greater pseudo-first-order reaction rate constant than those previously reported, approximately 96.9% of LEV (10 mg/L) could be decomposed within 30 min by the Co3O4/MnCo2O4/PMS catalytic system, which exhibited superior performance over single Co3O4 and MnCo2O4 catalyst. The influences of critical reaction parameters on LEV removal were evaluated and the experimental results indicated that Co3O4/MnCo2O4 possessed broad adaptability to solution pH, as well as appreciable practicality, universality and stability. By exposing more active sites through higher specific surface area and enhancing the electron transfer rate with heterostructure, the Co3O4/MnCo2O4 catalyst remarkably promoted the generation of reactive oxygen species (ROS) including SO4−, OH, O2− and 1O2, which were further verified by scavenger tests and electron paramagnetic resonance (EPR) technique. The transformation routes of LEV were elucidated according to the identified degradation intermediates. Finally, the underlying reaction mechanism was revealed on the basis of the results of characterizations and experiments. This work not only provided a novel strategy to improve the catalytic ability of conventional active components for PMS activation, but also brought new insights into the huge potential of mixed transition metal oxides, which would motivate future development of catalysts fabrication with advantageous microstructures and favorable properties for water pollution remediation.

Facile ion-exchange synthesis of Gd-doped K2Ta2O6 photocatalysts with enhanced visible light activity

One of the well-known ways of increasing the visible light absorption capability of semiconducting materials is cation doping. This study aims to use Gd doping to tailor the bandgap energy of K2Ta2O6 (KTO) for photocatalytic degradation of organic pollutants under visible light irradiation. Accordingly, the parent KTO and Gd-doped KTO with different Gd concentrations (K2-3xGdxTa2O6; x = 0.025, 0.05, 0.075 and 0.1 mol%) were synthesized by hydrothermal and facile ion-exchange methods, respectively. The powder XRD, FT-IR, SEM-EDS, TEM-SAED, N2 adsorption-desorption, XPS, UV–Vis DRS, PL and ESR techniques were used to investigate the effect of Gd dopant concentration on the structural and photocatalytic properties of KTO. The photocatalytic activity of these samples was investigated for the photocatalytic degradation of methylene blue (MB) dye in an aqueous solution at room temperature under visible light irradiation. The experimental results show that all Gd-doped KTO samples exhibit enhanced photocatalytic activity compared with parent KTO toward MB degradation. In particular, Gd-KTO obtained by doping of 0.075 mol% shows the highest photocatalytic activity among the Gd-doped samples and the degradation efficiency of MB was 79% after 180 min of visible light irradiation, which is approximately 1.5 times as high as that by parent KTO (53%). In addition, trapping experiments and electron spin resonance (ESR) analysis demonstrated that the hydroxyl radicals (•OH) have played a crucial role in the photocatalytic degradation of MB. The reusability and stability of Gd doped-KTO with a Gd content of 0.075 mol% against MB degradation were examined for five cycles. Based on the present study results, a visible light induced photocatalytic mechanism has been proposed for Gd0075-KTO sample.