Morphology-controlled Synthesis Of W₁₈O₄₉ Nanostructures And Their Photocatalytic Activity

As one of the important n-type semiconductors,monoclinic W₁₈O₄₉(WO_2.72)has the largest amount of oxygen vacancies which can act as traps and tightly catch reactant molecules.Moreover,the W₁₈O₄₉ has been reported as the only oxide that can exist in a pure form.Especially,because of the distinctive defect structure and broad absorption extending to near infrared?NIR?regions,W₁₈O₄₉ has been extensively studied.However,the W₁₈O₄₉ is easily oxidized to WO3 under the condition of elevated temperature.It is found that the W surface has the lowest oxidization degree when the concentration of oxygen is low.Therefore,we conclude that reducing the content of oxygen will be conducive to the synthesis of the non-stoichiometric W₁₈O₄₉ in the process of solvothermal reaction.Moreover,the addition of a reducing agent to the reaction system is also advantageous for the preparation of W₁₈O₄₉.Particularly,W₁₈O₄₉ has strong anisotropic growth behavior along the [010] direction,and this feature easily results in the formation of onedimensional form,such as nanowires,nanorods,nanobundles.So it still remains a significant challenge to develop a facile method to organize well-defined 2D or 3D morphology-controlled W₁₈O₄₉ microstructures with more oxygen deficiencies and better performance.?1?Hierarchical W₁₈O₄₉ such as spindle-like structures,flowers with sharp petals and nanowire structures are successfully synthesized via a solvothermal reduction method.By reducing the content of oxygen,the phase of WO3/W₁₈O₄₉ changes to pure W₁₈O₄₉ and the morphology of the W₁₈O₄₉ shows an obvious evolution from nanowire to flower-like W₁₈O₄₉ assembled by nanoneedle.This enhanced photocatalytic activity of W₁₈O₄₉ is attributed to smaller crystallite size and larger surface area.The increasing of oxygen vacancy results in a narrowing bandgap and strong light absorption performance.By combining the results of the transient photocurrent and electrochemical impedance measurements,the electron-hole pair separation rates and carrier migration rates of the W₁₈O₄₉ havebeen greatly improved.?2?Octadecylamine?ODA?has an important influence on the phase compositions of samples.Pure W₁₈O₄₉ has been successfully prepared by the introduction of octadecylamine.Results show that octadecylamine has an important effect on the phase composition.The octadecylamine can accelerate the formation of W5+and promote the formation of the non-stoichiometric tungsten oxide.Moreover,the morphologies of the samples are highly correlative with the content of the octadecylamine and the octadecylamine was effectively utilized as reducing agent.When the molar ratio of ODA and WCl6 reaches 0.5:1,the prepared W₁₈O₄₉ with narrow band gap and strong optical absorption spectrum can harvest more solar light and more electron hole pairs generated,which results in high photocurrent and enhanced photocatalytic activity.?3?Pure W₁₈O₄₉ with nanoparticle structure has been successfully prepared by the introduction of oxalic acid?OA?.The surface of a W₁₈O₄₉ particle is negatively charged with anions?WOn?-and these anions can form strong hydrogen bonding with the carboxyl of oxalic acid,which makes the W₁₈O₄₉ colloid granules segregate from each other and inhibit the aggregation of W₁₈O₄₉ particles.When the molar ratio of OA and WCl6 reaches 0.1:1,the prepared W₁₈O₄₉ shows enhanced Photocatalytic performance,which can be attribute to the strong light absorption properties.