Niobate Photocatalysts Research Articles

Schinus molle extract mediated green synthesis of iron niobate photocatalyst for the degradation of methyl orange dye under visible light

FeNbO4 monoclinic nanocomposite semiconductors were synthesised using hydrothermal and sol gel methods; photocatalysts were then calcined at 800 °C. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis spectroscopy (UV/Vis) and X-ray diffraction (XRD) technologies were used to investigate crystallinity, morphology and optical properties of the photocatalysts. Fourier transform infrared spectroscopy (FTIR) was used to determine the functional groups of both treated and untreated FeNbO4. It was found that the S. molle extract-treated FeNbO4 prepared using the hydrothermal method (FeNbO4-HT + S. molle) has the smallest nanoparticles (22.8 nm) with the smallest band gap energy (2.78 eV). X-ray photoelectron spectroscopy verified the presence of the elements of FeNbO4 as well as their oxidation states. The photodegradation reactions of 10-ppm methyl orange dye solutions using FeNbO4-sol gel, FeNbO4-HT and FeNbO4-HT + S. molle were carried out under visible light (>420 nm) for 50 min. The reactions resulted in degradation percent of 74 %, 78 % and 96 % by using FeNbO4-sol gel, FeNbO4-HT and FeNbO4-HT + S. molle respectively. The photocatalytic activity of FeNbO4 treated with S. molle extract demonstrated superior light absorption and photostability, which is attributed to the consistency in the photocatalysts’ morphology, optical band gap, particle size distribution, and porosity. The photocatalysts remained stable and effective for five degradation cycles.

Hydrothermal preparation of crystalline lithium niobate photocatalysts for effective degradation of dye-containing contaminated wastewater by ultraviolet to visible light irradiation

Hydrothermal preparation of crystalline lithium niobate photocatalysts for effective degradation of dye-containing contaminated wastewater by ultraviolet to visible light irradiation

Effects of Anionic Polymer Modification of Dye‐Sensitized Niobate Photocatalysts on Solar‐Driven Z‐Scheme Overall Water Splitting

Effects of Anionic Polymer Modification of Dye‐Sensitized Niobate Photocatalysts on Solar‐Driven Z‐Scheme Overall Water Splitting

Effects of Anionic Polymer Modification of Dye‐Sensitized Niobate Photocatalysts on Solar‐Driven Z‐Scheme Overall Water Splitting

Pt‐intercalated calcium niobate nanosheets (Pt/HCa2Nb3O10) sensitized by a Ru(II) complex dye are good photocatalysts for producing H2 from aqueous solutions containing I− as a reversible electron donor. These materials are applicable to Z‐scheme overall water splitting in combination with a WO3‐based O2‐evolving photocatalyst under simulated sunlight. In this work, the effects of anionic polymer modification of the dye‐sensitized nanosheets are examined by adsorbing sodium poly(styrenesulfonate) (PSS), sodium polyacrylate, sodium polymethacrylate (PMA), or sodium poly(4‐styrenesulfonic‐co‐maleic acid) onto the dye‐sensitized nanosheet surface. For half‐cell H2‐evolution reaction in the presence of NaI, all of the polymers have a positive impact on the activity under visible light at lower light intensity, whereas only PMA is effective under high light‐intensity condition. For Z‐scheme overall water splitting with PtOx/H‐Cs‐WO3, PSS and PMA give almost the same solar‐to‐hydrogen energy conversion efficiencies (0.12% ± 0.01%) under optimized conditions. However, PMA operates better than PSS at relatively low and high NaI concentrations, which are in general disadvantageous for the H2‐ and O2‐evolving components of the Z‐scheme, respectively.

Nitrogen-doped, proton-exchanged Dion-Jacobson layered niobate perovskites for photocatalytic hydrogen generation in solar light

Wide band gap semiconductor niobate photocatalysts with Dion-Jacobson layered perovskite structure were nitrogen-doped via simple gas–solid reaction to extend their absorption into the visible light range. Nitrogen doping was performed using ammonia as precursor, resulting in decreased band gaps of doped AB2Nb3O10 compounds (A = Cs, Rb, K; B = Ca, Sr) down to 2.5 eV. The resulting materials were investigated concerning their chemical and electronic structures. Nitrogen-doped AB2Nb3O10 crystals showed a clear red shift in absorption. Photocatalytic performance tests for the doped materials evaluated the capability of H2 production under simulated solar irradiation. The addition of carbonates to the gas–solid reaction turned out to be advantageous for the reduction of defects and the preservation of photocatalytic activity of nitrogen-doped layered niobates AB2Nb3O10.Graphical

New Modified Sol–Gel Method for the Preparation KNb3O8 as a Hydrogen Evolution Photocatalyst in Z-Scheme Overall Water Splitting

The layered potassium niobate, KNb3O8, is evident as a photocatalyst for hydrogen production from water splitting under UV light. We hereby reported a new modified sol–gel method for the preparation of potassium niobate photocatalyst. Photocatalytic water splitting has been improved in this work by loading Pt nanoparticles as cocatalyst. The catalysts were characterized by powder X-ray diffraction (XRD), Ultraviolet–Visible spectroscopy (UV–Vis), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Their photocatalytic activities were in (i) an aqueous MeOH, used as sacrificial hole scavengers for hydrogen production and (ii) a new Z-scheme photocatalytic water splitting system where Pt/KNb3O8 was used for H2 evolution and Pt/WO3 was used for O2 evolution. The I−/IO3− aqueous solution was used as redox couple for the photocatalytic Z-scheme over all water splitting under UV light irradiation. The photocatalysts were prepared by loading platinum on sol–gel synthesis KNb3O8 and commercial WO3 via impregnation method. We find that, this KNb3O8 belonging to a orthorhombic structure with rod-like morphology. The Pt nanoparticles were sufficiently well-dispersed on the surface of photocatalyst, and enhanced photocatalytic properties. Our results show that I− concentration significantly influenced the photocatalytic activity. The combination of Pt/KNb3O8 with Pt/WO3 achieves a high H2 evolution rate (539 μmol g−1 h−1) and O2 evolution rate (140 μmol g−1 h−1) in 2 mM NaI solution.

Novel band gap engineered Bi5Nb3O15/N-rGOcomposite catalyst for photo degradation of reactive dyes

Niobate photo catalysts draw significant attention in the fields of both pollutant degradation and H2 generation. However, their chemically inert nature limits the synthetic routes available to obtain these semiconductors. In this work, we report the synthesis of bismuth niobate (Bi5Nb3O15) by a simple solvothermal method and incorporation of this material with nitrogen doped reduced graphene oxide for the first time. The synthesized catalysts were characterized by XRD, Raman, N2 adsorption-desorption isotherm, DRS-UV, FESEM and TEM analyses. XRD patterns exhibited single crystalline orthorhombic structure for Bi5Nb3O15. Electron microscopy showed plate-like structure for Bi5Nb3O15/N-rGO. UV–vis diffuse reflectance spectra revealed red shifting of reduced graphene oxide and nitrogen doped reduced graphene oxide composite catalysts indicating its visible activity. This novel Bi5Nb3O15/N-doped reduced graphene oxide showed higher photocatalytic decolorization of reactive dyes namely reactive blue 19 and reactive orange 16 under visible light than bare Bi5Nb3O15orBi5Nb3O15/rGO.

Improved visible-light activity of nitrogen-doped layered niobate photocatalysts by NH3-nitridation with KCl flux

In the present study, nitrogen doping into layered perovskite niobites, KCa2Nan–3NbnO3n+1 (n = 3 or 4), was attempted to enhance their response to visible light during photocatalytic water reduction and oxidation. Although conventional nitridation, i.e., heating in NH3 stream at a high temperature, produced black-colored samples due to the undesirable reduction of Nb5+ to Nb4+ (and/or Nb3+), the use of KCl flux with NH3 stream effectively prevented the reduction of Nb5+, resulting in yellow-colored samples that had a much lower concentration of reduced Nb species. The samples prepared with KCl flux exhibited stronger absorption (at λ = 350–550 nm) when compared to those prepared without KCl flux. This was due to the higher concentration of N3– anions, as confirmed by elemental analysis and the observed peak shift in XRD patterns. On the other hand, the absorption band with λ > 550 nm, which indicated the presence of reduced species or anion vacancies, were astonishingly smaller for the samples prepared with KCl than for those without KCl. This indicated that the formation of reduced Nb species was inhibited by KCl. XPS analysis also supported this effect. The use of KCl flux also prevented the problematic decrease in the concentration of potassium cations in the interlayer space during high-temperature treatment in NH3 flow. The samples prepared with KCl flux exhibited much higher O2 and H2 evolution rates under visible-light irradiation in the presence of an electron acceptor and a donor, respectively as compared to those prepared without KCl. This clear improvement in O2 and H2 evolution was probably due to the larger amount of doped nitrogen introduced and/or the suppressed formation of reduced Nb species.

Facile Fabrication and Mechanism of Single-Crystal Sodium Niobate Photocatalyst: Insight into the Structure Features Influence on Photocatalytic Performance for H2 Evolution

Single-crystal sodium niobate with wire- and cube-like structures has been created by a facile and eco-friendly route including a hydrothermal as well as in-site self-assembled process via optimizing thermal treatment temperature. Comparative studies were carried out to evaluate the structure and phase transformation during the fabrication course. A possible growth mechanism of NaNbO3 with controllable morphologies was elucidated based on the time- and temperature-dependent evolution experiments. In addition, special attention was paid to figure out the mutual relationship between the unique structure features and photocatalytic performance toward photocatalytic H2 evolution. The results illustrated that the NaNbO3 nanowires formed at 500 °C calcinations exhibited the highest photocatalytic activity compared with other materials and presented super stability as well. The superior photocatalytic performance could be interpreted in terms of the better crystalline, fewer defects, and perfect 1D nanowire morp...

A silver niobate photocatalyst AgNb7O18 with perovskite-like structure

Silver-containing niobate AgNb7O18 nanoparticles were prepared via sol-gel polymerized complex synthesis. The sample was given a good XRD Rietveld refinement in a typical perovskite-like structure. The detailed surface properties of AgNb7O18 nanoparticles were tested by SEM, EDS, TEM, nitrogen adsorption-desorption isotherms and XPS measurements. AgNb7O18 is characterized via a direct transition and the band energy is 2.76 eV. The hybridization between O 2p and Ag 3d states in valence band contributes to the narrowed band gap. The first principles electronic structure elucidation supports the narrow band gap of AgNb7O18. The sample shows an efficient degradation of methylene blue dye (MB) solutions. The trapping tests were performed to elucidate the real active species in the photocatalytic reactions. The results indicate that the superoxide radical is the main active species in the photocatalytic system, and the photocatalyst also shows a good stability and durability.

A new silver niobate photocatalyst AgNb13O33: Synthesis, structure and photochemical properties

Ag-containing niobate AgNb13O33 has been prepared by the sol–gel polymerized complex method. The structure was investigated via a good XRD Rietveld refinement in a typical perovskite-like framework. The morphological properties of the AgNb13O33 nanoparticles were tested via transmission electron microscopy (TEM), scanning electron microscope (SEM), and energy dispersive spectra (EDS) measurements. AgNb13O33 is a typical direct allowed semiconductor with band energy of 2.81 eV, which is much narrower than 3.4 eV of Nb2O5. This silver niobate can harvest both UV and near UV light, which is benefited from the (O-2p/Ag-4d) hybridization in the valence band. The photocatalytic abilities of AgNb13O33 nanoparticles were reported. The sample presents more efficient photocatalytic effects on photo-degradation of methylene blue (MB) dye solutions than that of Nb2O5. The photocatalytic mechanism was discussed on band energy levels, trapping experiments, photoluminescence and the decay lifetime.

New AgNbO4−δ compound with high visible light photocatalytic activity

A new visible light responsive silver niobate photocatalyst AgNbO4−δ was synthesized by using a low-temperature chemical precipitation method. X-ray diffraction (XRD) results indicated that the AgNbO4−δ compound has a tetragonal scheelite (ABO4) type structure. It has also an optical absorption edge at about 596nm, corresponding to a bandgap of about 2.08eV. In addition, its microstructure and element composition were studied by using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). Furthermore, its photocatalytic activities were evaluated in terms of degradation of Rhodamine B, showing that the as-prepared AgNbO4−δ compound has higher photocatalytic activity than P25.

Preparation-method-dependent morphological, band structural, microstructural, and photocatalytic properties of noble metal–GaNbO4 nanocomposites

We report the distinct physicochemical and photophysical properties of gallium niobate photocatalysts (bandgap: ∼3.1 eV), prepared by a solid-state (SS) reaction and sol–gel (SG) method and dispersed with a noble metal (∼0.5% of Pt, Au, or RuOx) cocatalyst. SG–GaNbO4 comprised smaller size particles (∼20–50 nm) and a larger surface area (∼160 m2 g−1) compared to SS–GaNbO4 (particle size ∼30–150 nm, surface area ∼27 m2 g−1). XRD patterns revealed a preparation-dependent variation in the relative intensity of prominent reflections. In TEM examination, SG samples exhibited small-range grain boundaries and heterogeneous metal/substrate interfacial contacts, while SS–GaNbO4 had long-range ordering. Laser-Raman and thermoluminescence investigations revealed that lattice distortion, defect-induced inter-bandgap charge trapping states, and the local environment around the metal/semiconductor interfaces may also depend on the preparation method. Metal–GaNbO4 nanocomposites showed no activity for the dissociation of pure water under UV (>250 nm) irradiation, despite the favourable conduction and valence band potentials. This was attributed to the sharp Ga and Nb d-levels in the narrow conduction band of GaNbO4, as confirmed by ab initio electronic structure calculation. These photocatalysts, however, showed good activity for semiconductor-mediated photo-dissociation of aqueous methanol to produce H2; a cocatalyst-dependent activity trend, Pt > RuOx > Au, was observed. Doping of S at ∼5% of the oxygen sites led to decreased photoactivity, ascribed to the presence of localized S 3p states just above the O 2p valence level. In conclusion, besides band characteristics, certain morphological and microstructural properties play a crucial role in the photoactivity of the metal/oxide nanocomposites.

Photocatalytic Degradation of Methyl Orange on the Composite of TiO<sub>2</sub> and Lanthanum Niobate

In order to improve the photocatalytic activity of TiO2, the optimal catalytic conditions of TiO2 and niobate as well as the composite TiO2 and niobate photocatalyst were studied. 10 mg/l of methyl orange solution was taken as imitated waste water. The results indicated that the optimum dosage of TiO2 was 0.8 g/l. The optimal calcination temperature to prepare lanthanum niobate was 900 oC. n(La):n(Nb) was in the range of 0.6:1−3:1, and the photocatalyst prepared in the ratio of 1.4:1 showed the highest activity. The results also showed that, TiO2-lanthanum niobate composite prepared by solid-state reaction method showed better photocatalytic activity on methyl orange degradation compared with pure TiO2. After 120 minutes of irradiation, methyl orange degradation rate reached 94.2 % when using the composite photocatalyst.

Nanostructured niobate photocatalysts

Nanostructured niobium-contained semiconductors with special [NbO 6 ] octahedral structure have been proven to be effective photocatalysts on splitting pure water into hydrogen and oxygen or photodegradation organic toxic dyes into small molecules under UV or visible light irradiation. This paper reviews the recent research and development progresses of nanostructured niobate photocatalysts, including the synthetic methods for niobate nanostructures and their characterisation, and the photocatalytic abilities under light irradiation.

Water Splitting over New Niobate Photocatalysts with Tungsten‐Bronze‐Type Structure and Effect of Transition Metal‐Doping

Photophysical properties and photocatalytic activities for water splitting over KM(2)Nb(5)O(15) (M = Sr and Ba) and K(2)LnNb(5)O(15) (Ln = La, Pr, Nd, and Sm) with tungsten bronze-type structure were investigated. Single phases of KM(2)Nb(5)O(15) and K(2)LnNb(5)O(15) were successfully prepared by solid-state reaction (SSR) method and polymerizable complex (PC) method. The band gaps of these niobates were estimated to be 3.1-3.5 eV. These metal oxides loaded with an activated NiO(x) cocatalyst showed photocatalytic activities for water splitting into H(2) and O(2) under UV irradiation. When K(2)LaNb(5)O(15) was doped by Rh ions, a new visible-light absorption band was observed around 400-500 nm in addition to the band gap absorption band of the K(2)LaNb(5)O(15) host. K(2)LaNb(5)O(15):Rh showed photocatalytic activities for H(2) or O(2) evolution from an aqueous solution containing a sacrificial reagent under visible-light irradiation.

Photocatalytic Degradation of Methyl Orange on Iron Niobate Prepared by Solid-State Reaction

Iron niobate photocatalyst as a new type of photocatalyst was prepared by solid-state reaction of Fe3O4 and Nb2O5 and its activity was evaluated using photocatalytic degradation of methyl orange. Preparation conditions such as calcination temperature and time, and irradiation time were investigated according to photocatalytic efficiencies. FeNb2O6 was produced during calcination below 700 oC and FeNbO4 was produced above 800 oC. Iron niobate with optimum activity could be prepared after calcination at 700 oC for 8 h when Fe:Nb molar ratio was 0.8:1. Methyl orange degradation rate was 72.7% after 180 min of irradiation at photocatalyst concentration of 4 g/l.

A one-pot method to grow pyrochlore H4Nb2O7-octahedron-based photocatalyst

Cubic pyrochlore niobates (A2Nb2O7, Fd-3m) have been shown to exhibit excellent photocatalytic performance and solid niobic acids show great advantages over these niobate photocatalysts. But no pyrochlore niobic acid has been reported. Here we develop a new and one-pot hydrothermal route for high-yield controlled synthesis of pyrochlore H4Nb2O7 with no aid of additives. Combined with the intrinsic loosely pyrochlore structure, the “acidity-promoted effect”, and the high crystallinity, the novel H4Nb2O7 exhibits outstanding photocatalytic activity, i.e., the reaction rate constant of H4Nb2O7 is almost 3 times of that for its amorphous precursor Am–Nb2O5.nH2O, and almost 2 times that of its corresponding calcium pyroniobate. This study discloses insights for the designed strategy of efficient photocatalysts with synergistic features, and this route provides flexibility and selectivity for the deliberate synthesis of other functional materials.

Visible light responsive bismuth niobate photocatalyst: enhanced contaminant degradation and hydrogen generation

Niobate photocatalysts are of considerable interest in both contaminant degradation and H2 generation. However, the chemically inert nature limits the synthetic routes available to obtain these materials. Within this paper, single-crystalline Bi3NbO7 nanoparticles have been successfully synthesized by combining polymerized complex-assisted and hydrothermal methods. The reaction temperature was as low as 140 °C and the reaction time was only 12 h. The morphology, physical properties, and band structure of the sample was systematically investigated. The as-prepared Bi3NbO7 nanocrystallites in our experiment possessed higher photocatalytic activities in the photocatalytic degradation of both phenol and aqueous ammonia under visible-light irradiation than those of samples such as commercial Nb2O5, amorphous Nb2O5·nH2O, nitrogen-doped TiO2, P25 and Bi3NbO7 synthesized by solid solution routes. Moreover, the single-crystalline Bi3NbO7 nanoparticles were also able to efficiently evolve hydrogen from water under visible light, which provides a superior candidate for visible-light responsive photocatalysts.

Hydrothermal synthesis of potassium niobate photocatalysts under subcritical and supercritical water conditions

Hydrothermal synthesis of potassium niobate powders was carried out under various subcritical and supercritical water conditions using crystalline Nb2O5 powder as a starting material. A single phase of K4Nb6O17 was formed under subcritical water conditions, while mixed phases of K4Nb6O17 and KNbO3 were obtained under supercritical water conditions where KNbO3 was predominant over K4Nb6O17 as the heating duration was increased. Characterization of these hydrothermally synthesized potassium niobates by XRD, SEM, and TG-DTA analyses revealed that fine hydrated powders can be obtained under subcritical and supercritical water conditions. The hydrothermally synthesized potassium niobate powders were used for photocatalytic hydrogen evolution from water decomposition. The crystallinity is responsible for the high photocatalytic performance of the hydrothermally synthesized potassium niobate powders. The maximal hydrogen evolution rate was achieved for the potassium niobate hydrothermally synthesized at 400 °C for 4 hours. Besides, the hydrogen evolution rate was enhanced more than 10-fold by Ni loading for the hydrothermally synthesized potassium niobate powder which was much higher in comparison with the Ni loaded solid-state synthesized photocatalyst.