Rational Design Of Nb₂O₅ And G-C₃N₄-Based Nanostructures For Photocatalytic Water Treatment Application

Visible light-driven photocatalysis has demonstrated significant promise in the fields of clean and renewable energy,as well as water treatment.However,due to limited light absorption efficiency and significant electron-hole recombination,modifications on semiconductor structures are required in order to enhance their photocatalytic performance.In this sense,the synthesis,characterization,and applicability of a visible-light active photocatalyst,Niobium Oxide（Nb₂O₅）,and graphitic carbon nitride（g-C3N4）are described in this thesis.Nb₂O₅ is a prospective alternative to other semiconductors used in water treatment because it possesses catalytic properties such as surface groups capable of supporting catalytic processes and high chemical stability,making it an excellent choice for water treatment.However,the actual application of Nb₂O₅ as a photocatalyst is limited due to its wide band gap（3.4 eV）which lies in UV region,the main goal of this dissertation is developing strategies to achieve the following purposes:（1）Increase the absorption spectrum of Nb₂O₅ to the visible region;（2）Provide a large specific surface area for light irradiation,which is beneficial in creating more active sites for the adsorption of reactants,and（3）decrease the recombination rate of the photoinduced generated charges,as it is a factor that directly influences photocatalytic activity.Given these challenges,this dissertation investigated the modification of Nb₂O₅ and g-C3N4 nanostructures with non-metal elements,constructing a unique morphology with a large specific surface area,forming an isotype heterojunction between two distinct crystal phases of the same semiconductor material,and constructing an S-scheme heterojunction between Nb₂O₅ and g-C3N4,which are alternative techniques to improve their photocatalytic properties.The main contributions of our thesis can be summarized as follows:（1）Nb₂O₅ surfaces were modified,either by adding non-metal heteroatom doping such as C and N to the surface or altering the surface molecules,significantly improves electron transport from the valence band（VB）to the conduction band（CB）by shortening the bandgap of the targeted semiconductor.The first activated substance only by ultraviolet radiation can thereafter become active under visible light.In this sense,we constructed a C and N comodified Nb₂O₅ 2D nanonet structure（C-N/Nb₂O₅NNs）from niobium oxalate applying a straightforward hydrothermal technique and 2-methylimidazole（Hmim）as the C and N source.The resulting nanonets are strong and cost-efficient,with great recycling durability.In comparison to N-doped TiO2（N-TiO2）and Nb₂O₅ control samples（Nb₂O₅-CS）,the resultant nanonets exhibited the highest photocatalytic degradation activity of Rhodamine B（RhB）under light illumination（λ>420 nm）.Through this study,we discovered that the synergetic consequences of C and N on the nanonets surface,which were effectively incorporated into the surface of the Nb₂O₅ nanonets structure,significantly improved not only the visible light response by reducing the bandgap to 2.9 eV,but also the light utilization efficiency and photoinduced electron-hole pair separation efficiency of our nanonets structure.As a result,we suggested that the existence of carbonate species（COx）and nitrogen species（NOx）increase the population of produced holes（h+）,which played a critical role in the photocatalytic decomposition mechanism of RhB.This implied that the modification of Nb₂O₅ with C and N was of reasonable importance.This synergism provides a novel perspective on the origins of photodegradation processes by adding h+ as a critical intermediate in the process.Our method offers a unique perspective on the creation of 2D nanostructures,which may find applications in catalysis,solar energy conversion,and environmental protection,among other fields.（2）Unique morphology with a large specific surface area is advantageous because it allows for a higher concentration of active sites per square meter,which can lead to enhanced reactivity.The smaller the particle size,the larger the surface area and the more activity is expected.This can be explained by increasing the number of active sites per square meter and a higher pollutant absorbance on the catalyst surface.In this regard,in addition to modified Nb₂O₅ with carbon and nitrogen,we designed and engineered 3D flower-like hierarchical N-doped Nb₂O₅/C nanostructures with high specific surface areas to prevent further recombination of charge carriers and prolong their lifetimes.The as-prepared N-NBO/C possesses a 260.37 m2·g-1 of specific surface area and is smaller than 10 nm of single wire diameter.The effect of reaction parameters such as hydrothermal reaction time,temperature,and Hexamethylenetetramine（Hmta）concentration on NBO morphology have been systematically investigated to elucidate the growth mechanism’s formation.The carbon on the surface and the nitrogen in the framework of NBO have benefited the light harvest,visible light absorption,oxygen vacancies formation,and electron-hole separation.The photocatalytic performance of the as-fabricated N-NBO/C nanostructures was estimated via the photodegradation of RhB 30 mg/L as well as above 98%of RhB was decomposed during 30 min upon visible-light radiation.Hence,the obtained NNBO/C nanostructure exhibits much higher activity in photocatalytic decomposition of RhB subject to visible-light radiation than those of pure niobium oxide NBO,Nitrogen-doped titanium oxide（N-TIO）,and Nitrogen-doped niobium oxide（N-NBO）.This work supplies a versatile route to synthesize nitrogen-doped and carbon sensitized metal-oxide nanostructures for possible utilization in solar energy transformation and environmental cleaning.（3）Heterojunction modification is necessary to overcome the high recombination rates of photoinduced generated electron-hole pairs as well as their low reduction and oxidation abilities in a single photocatalyst.In this sense,well-defined morphologies of 1D nanotube and 2D nanosheet O-doped g-C3N4 isotype heterojunction architecture were easily established by directly calcinating a combined precursor of melamine,cyanuric acid and urea.A plausible formation mechanism supported by XRD and FT-IR analyses is proposed and investigated.The obtained results show that the fabricated materials exhibit a high surface area（114.4 m2 g-1）while the P.L.technique confirmed the rapid charge separation.Compared with the asfabricated 2D g-C3N4 nanosheets,1D/2D O-doped g-C3N4 isotype heterojunctions possess superior photocatalytic activity for RhB degradation under visible light illumination.The degradation pathway was investigated using LC-MS analysis.The improved photocatalytic degradation ability of 1D/2D O-doped g-C3N4 isotype heterojunctions is attributed to the enhanced absorption capability,high surface area,and separation efficiency of photo generated charge carriers.（4）Another heterojunction mechanism was constructed on the principle of S-scheme heterojunction between two different semiconductor materials in order to address the low redox potential and recombination rate of photoinduced electrons and hole pairs in the type-II heterojunction photocatalytic system（OCN）.In this work,a novel S-scheme heterojunction system consisting of 2D O-doped g-C3N4（OCN）nanosheets and 3D N-doped Nb₂O₅/C（NNBO/C）nano flowers is constructed via ultrasonication and vigorous agitation technique followed by heat treatment for the photocatalytic degradation of RhB.Detailed characterization and decomposition behavior of RhB showed that the fabricated material shows excellent photocatalytic efficiency and stability towards RhB photodegradation under visible-light illumination.The enhanced performance could be attributed to the following factors:fast charge transfer,highly-efficient charge separation,extended lifetime of photoinduced charge carriers,and the high redox capability of the photoinduced charges in the S-scheme system.Various trapping experiment conditions and electron paramagnetic resonance（EPR）provide clear evidence of the S-scheme photogenerated charge transfer path,whereas the RhB mineralization degradation pathway was also examined using LC-MS.