Synthesis And Properties Of Nitrogen-doped Titanium Oxo Clusters

Titanium dioxide(TiO₂)nanomaterials have become the most extensive photocatalyst due to its excellent performance in catalysis,such as low cost,stable chemical properties,non-toxic,non-polluting and light stability.However,the band gaps of bulk anatase and rutile are 3.20 and 3.02 e V,respectively,and they can only absorb ultraviolet light(5%)in the solar spectrum,which greatly limits their practical application in the visible light region.Doping TiO₂with nitrogen(N-TiO₂)can reduce the band gap values and enhance the absorption capacity of visible light,and thereby improve the photocatalytic activity.However,the reason why the band gap of N-TiO₂ decreases is still unclear.it is due to the N-Ti bond,the change in the valence of titanium,or the generation of oxygen vacancies,also needs to be further clarified.However,Because of the inaccuracy of structures,unclear information of interface,and uncertain surface composition,N-TiO₂have greatly hindered the in-depth studies of such photocatalytic materials.Crystalline nitrogen-doped titanium oxide clusters(N-TOCs)can obtain precise atomic structures by X-ray single crystal diffraction technology,which can be used as structural and reaction models for N-TiO₂ materials.Therefore,synthesizing N-TOCs and studying their band gaps can clarify whether the N-Ti bond is the reason for the reduction of the band gap of N-TiO₂ materials.Moreover,N-TOCs also belong to semiconductor materials and have good photocatalytic properties.The number of N-TOCs is still small than other transition metal complexes.In this work,We mainly focuses on the synthesis of N-TOCs,the regulation of band gaps and photocatalytic degradation of organic dyes.Twenty compounds have been synthesized based on different chromophore ligands,they are divided into three parts,The Band gap regulation of N-TOCs can provide basic experimental datas for the synthesis of N-TiO₂ materials.The work content of this paper is divided into the following five aspects:1.In the introduction,we first introduce the research background and synthesis methods of titanium oxide clusters,and then summarize the research status of N-TiO₂ and then describe the synthesis progress of N-TOCs based on non-chromophore and chromophore ligands briefly,and the N-TOCs with N-Ti bond type were introduced in terms of photocatalytic degradation of organic dyes.,and finally explain the significance of the topic selection of the paper,and make a introduction of the primary contents.2.We selected pyridine nitrogen-containing ligands of different structures to synthesize 6cases of N-TOCs(compounds 1-6).All clusters have direct N-Ti bonds,and the ratio of nitrogen to titanium in the structures is 1:4.The ultraviolet-visible diffuse reflectance experiment shows that the band gap value of these N-TOCs is in the range of 3.74–3.38 e V,which is not significantly lower than that of TiO₂.This shows that the N-Ti bond is not the reason for the decrease of the band gaps of N-TiO₂ materials.At the same time,we tested the performance of catalytic degradation of organic dyes under simulated sunlight for this series of titanium oxide clusters.The results showed that they all showed good photodegradation activities for methyl orange(MO)and rhodamine B(Rh B).3.To synthesize the narrow band gaps of N-TOCs,we use catechol and2,2'-dihydroxybiphenyl as chromophore ligands,and nitrogen-containing phenanthroline,pyrazole,2,2'-bipyridine as nitrogen source ligands,4 cases of N-TOCs(complexes 7-10)were synthesized by a simple solvothermal method.Interestingly,there are 2 isostructural Ti₁₀clusters and 2 mononuclear titanium.they provide a basis for us to study the effect of the number of nuclei of N-TOCs on the band gaps.UV-visible absorption and diffuse reflectance analysis showed that the compounds 7-10 absorb in the visible region of 500-600 nm,and the band gap is about 2.0 e V.this shows that the introduction of phenolic ligands into the N-TOCs structure can significantly reduce the band gaps,but the number of nuclei of titanium oxide clusters has almost no effect on the band gaps.This series of complexes have good photoelectric response performances,among which the maximum can reach 0.21?A.cm^-2.4.The third part of the work shows that the use of phenol ligands can synthesize N-TOCs with a band gap of about 2.0 e V,but materials with a band gap of 2.0 e V are likely to cause photo-generated carriers due to the small band gap during photocatalysis.Therefore,it is necessary to select suitable ligands to further regulate the band gaps of N-TOCs.Based on this,we selected the nitrogen-containing ligands salicylaldoxime,benzhydroxamic acid,pyridine-2-carbaldoxime and other oxime ligands to synthesize 10 cases of novel N-TOCs(compounds 11-20).UV-visible absorption and diffuse reflectance analysis showed that the11-20 complexes absorb in the visible region,and the band gaps are in the range of 2.94–1.85e V.In addition,we have compared the band gap values from two dimensions,One is based on four isomorphic Ti₈ clusters based on benzhydroxamic acid and aromatic carboxylic acid ligands,and the other is based on different chromophores.The results show that the structure of the chromophore ligands is the key to influence the band gaps of N-TOCs,while the aromatic carboxylic acid ligands have little effects on the band gaps.This is the first time that oxime and hydroxamic acid ligands are used to control the band gaps of TOCs in a wide range of 2.94–1.85 e V,which can provide experimental basic datas for studying the ideal band gaps control of N-TiO₂ materials.In addition,the series of N-TOCs are not only rich in structures,but also have good photoelectric response performances and appropriate conduction bands and valence bands for potential values,and have potential photocatalytic activities.5.A systematic summary of the overall work in the paper,and an outlook for the future work of the subject.