Effects of Interface Layer on Photocatalyst of NaTaO_3:La Composite Film and Its Performance of Photocatalytic Water Splitting

边畅

杨海滨

硕士

吉林大学

光解水制氢;;NaTaO_3:La;;复合薄膜光催化剂;;p-n结;;可回收利用

随着经济的迅速发展,能源过度消耗伴随着环境污染等现象成为人类面临的巨大挑战之一。因此为了减缓资源危机和减弱环境污染,找到清洁、可循环的能源迫在眉睫。氢作为一种具备清洁低碳、应用面广、便与储存等优点的二次能源,引起了全球范围的高度关注。另外,为了降低生产成本,诸多科学家利用广泛的太阳能作为产氢的一次能源成为当下热门研究,其中使用半导体光催化材料裂解水生产出H_2是比较理想的办法之一。由于光催化剂作为光裂解水制氢中的核心材料,因此人们不断探索更优质的光催化剂。在半导体光催化材料中,具备钙钛矿结构的NaTaO_3由于其较高的稳定性和在紫外灯照射下优异的光催化活性引发各国学者的大量研究。而为了进一步提高NaTaO_3催化剂的光解水活性,本文通过对NaTaO_3掺杂La使其表面形成具有序阶梯结构的颗粒粉末。不过,传统的粉末光催化剂有如下几点不足:首先,在光解水实验中,需将催化剂(粉末状)添加到溶液里,这使得粉末光催化材料散布在液体中,不方便回收再次利用;其次,光催化分解水过程中产生的是氢气和氧气的混合气体,很难单独分离出两种物质;最后,在反应过程中产氢位和产氧位同时在催化剂表面,并且距离很近,这就使得载流子的复合率较高,进而降低了产氢效率。因此为避免传统粉末光催化颗粒的不足,本文在Ti箔基底上制备了薄膜状Ti/NaTaO_3:La光催化剂,此膜状催化剂方便回收循环使用,大大降低了对环境的污染,且使氢气和氧气分别在薄膜两侧产生。为提升薄膜Ti/NaTaO_3:La光催化剂的效率,形成TiO_2纳米管阵列/n型TiO_2纳米颗粒膜/p型NiO薄膜/NaTaO_3:La颗粒薄膜所构成的复合薄膜Ti/TNT/N-P/NaTaO_3:La光催化剂,并优化复合薄膜光催化剂的界面。具体的研究内容如下:(1)采用Ta_2O_5,Na_2CO_3和La_2O_3为原始材料,通过高温固相反应制备粉末状NaTaO_3:La光催化剂。然后选择Ti箔片为基底通过丝网印刷技术制备膜状Ti/NaTaO_3:La样品,并探讨退火温度的差异对其活性的影响。光裂解水实验反应中光源为300 W高压汞灯,测试条件为NaOH水溶液并且pH值等于13。测试结果表明随着退火温度的增高,膜状Ti/NaTaO_3:La光催化剂的光解水效率呈先增大后减小的规律,当退火条件为400 ~oC时产生的H_2和O_2量最多,达到156μmol/4 h,表明退火条件对样品的效率有影响。(2)在Ti基底表面,制备TiO_2纳米管阵列作为电子传输层(TNT),并在其上旋涂NiO薄膜作为p型材料(P)。为提高薄膜界面质量,在TiO_2纳米管阵列表面通过浸渍提拉溶胶凝胶的办法形成同种锐钛矿型TiO_2薄膜作为n型材料(N),构成了具有较大异质结区域的p-n结。最后,丝网印刷NaTaO_3:La光催化剂层,经400 ~oC退火得到复合膜状Ti/TNT/N-P/NaTaO_3:La光催化剂。并分别探究电子传输层和n型层,显示当退火条件为450 ~oC时,形成的TiO_2纳米管最有利于电子纵向运输;当形成4层TiO_2纳米晶薄膜时,表面平坦均匀、致密且几乎无孔洞存在,有利于和p型层紧密结合形成更优质的p-n结,此时复合薄膜Ti/TNT/N-P/NaTaO_3:La光催化剂,产气量高达348μmol/4 h。另外同Ti/NaTaO_3:La光催化剂相比,Ti/TNT/N-P/NaTaO_3:La薄膜光催化剂性能显著优于前者。这归因于光照下Ti/TNT/N-P/NaTaO_3:La薄膜光催化剂中的p-n结所产生的内建电场能使更多的光生电子用于产氢。同时薄膜光催化剂可以在Ti箔两侧分别产生氢气和氧气,克服了传统粉末光催化剂所存在的部分缺点。该型光催化剂将在高效光解水制氢方面有很好的应用前景。

With the rapid development of the economy,problems such as energy shortage and environmental pollution have become one of the great challenges facing humankind.Therefore,in order to mitigate the energy crisis and reduce environmental pollution,it is urgent to find clean and recyclable energy.Hydrogen energy,as a clean and low-carbon secondary energy with wide application,convenience and storage,has attracted worldwide attention.Second,in order to reduce the cost of hydrogen production,many scientists use a wide range of solar energy as a primary source of hydrogen production has become a current research focus,and the use of semiconductor catalysts to produce hydrogen and oxygen from water is one of the most ideal technologies.As the photocatalyst is the core material in the process of hydrogen production by photolysis of water,people are constantly exploring high-quality photocatalysts.Among semiconductor photocatalysts,NaTaO_3 with perovskite structure has caused extensive research by scholars in various countries due to its relatively high stability and excellent photocatalytic performance under ultraviolet light irradiation.In order to further improve the photolysis water activity of NaTaO_3 catalyst,the structure with sequence ladder was formed on the surface of NaTaO_3 by doping La.While,traditional powder photocatalysts have the following defects:in the process of hydrogen production from photolysis water,the powder photocatalyst needs to be dispersed in the water to participate in the reaction,which makes the powder photocatalytic material dispersed in the liquid.And the dispersed catalyst pollutes water resources,making it difficult to recycle and reuse;Second,a mixture of hydrogen and oxygen is produced during the photocatalytic decomposition of water,and it is difficult to directly separate hydrogen and oxygen;Finally,during the reaction process,hydrogen and oxygen production sites are located on the catalyst surface at the same time and close to each other,which leads to a higher carrier recombination rate and a lower hydrogen production rate.Therefore,in order to avoid the disadvantages of powder catalytic particles,a film-like Ti/NaTaO_3:La was prepared on the Ti foil substrate.The thin film catalyst was convenient for recycling and reuse,greatly reduced environmental pollution,and generated hydrogen and oxygen on both sides of the film.In order to improve the efficiency of the film-shaped photocatalyst on the original basis,a composite Ti/TNT/N-P/NaTaO_3:La film photocatalyst composed of TiO_2 nanotube array/n-type TiO_2 nanoparticle film/NiO film/NaTaO_3:La particle film was formed,and the interface layer which were in direct contact with the N layer in the composite film material was optimized.The detailed inquiry process was described below:(1)Selecting Ta_2O_5,Na_2CO_3 and La_2O_3 as original reagents,powdered NaTaO_3:La samples were made by high-temperature solid-phase reaction.Then select Ti foil as the substrate to prepare Ti/NaTaO_3:La film-like catalyst by screen printing technology,and explore the effect of the difference in annealing temperature on the performance of film-like catalyst.In the photocracking water experimental reaction,300 W high pressure mercury lamp was selected as the illuminant,and experimental circumstances were NaOH alkaline solution and the pH value was equal to 13.The test results show that as the annealing temperature increases,the photolytic water activity of the Ti/NaTaO_3:La thin film photocatalyst increases first and then decreases.When the temperature was 400 ~oC,the amount of H_2 and O_2 generated reaches the maximum,reaching 156μmol/4 h,indicating that the temperature has effect on the efficiency of the thin film photocatalyst.(2)On the surface of Ti substrate,prepare TiO_2 nanotubes which as an electron transport layer(TNT),a NiO layer is prepared thereon as a p-type material(P).In order to increase the area of the heterojunction region directly formed by the combination of TiO_2 nanotubes and NiO films,the same anatase type TiO_2 layer was formed on its surface by dipping the titrasol gel as a n-type material(N),and the p-n junction was formed with a large heterojunction region.Finally,the NaTaO_3:La photocatalyst layer was screen printed and annealed at 400°C to obtain the composite thin film Ti/TNT/N-P/NaTaO_3:La photocatalyst.In addition,the electron transport layer and n-type were explored separately.It shows that when the annealing condition is 450°C,the TiO_2 nanotubes formed are most conducive to the vertical transport of electrons;When four layers of TiO_2 thin films are formed,the surface is flat,uniform,dense and almost no holes exist,which is conducive to the close combination with the p-type layer to form a better quality p-n junction.At this time,the composite film Ti/TNT/N-P/NaTaO_3:La photocatalyst can produce up to 348μmol/4 h.In addition,compared with Ti/NaTaO_3:La photocatalyst,the composite Ti/TNT/N-P/NaTaO_3:La thin film photocatalyst is significantly better than the former.This is due to the built-in electric field generated by the p-n junction in the Ti/TNT/N-P/NaTaO_3:La thin film photocatalyst under light,which can enable more photo-generated electrons to be used for hydrogen production;Secondly,the thin film photocatalyst can produce hydrogen and oxygen on the different sides of the Ti foil,which overcomes some of the shortcomings of the traditional powder photocatalysts.This kind of photocatalyst will have a good application prospect in the efficient water splitting to produce hydrogen.