摘要
本论文研究了不同类型TiO_2纳米材料(纳米颗粒、纳米管和纳米带)的光转换效率及其它特性。这些纳米材料是通过溶胶凝胶法、水热法和阳极氧化法合成得到,它们形貌上的差别取决于它所采用的合成方法。材料的表征主要通过X射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和选区电子衍射(SAED)来完成。由这些TiO_2纳米材料组装成的染料敏化太阳能电池(DSSC),其光电转换参数是利用1.5 AM的太阳模拟器(Oriel91192)测量得到。
利用溶胶凝胶法制备了TiO_2纳米颗粒,并比较了制备过程中有无表面活性剂对结果的影响。在实验中发现,十六烷基三甲基溴化铵(CTAB)和聚乙烯吡咯烷酮(PVP)可控制粒径的大小。分析了这些纳米颗粒的大小、粒径的分布和相的组成,结果表明,在不添加表面活性剂和添加PVP表面活性剂的情况下,样品出现不同金红石相浓度的二元相结构。测量了样品对甲基橙的光催化性质,并与公司产TiO_2作了对比。结果表明,以PVP作为表面活性剂制备时,样品的降解效率最高,而以CTAB作为表面活性剂时,降解效率最低。二元混合相的样品比纯锐钛矿相具有更高的降解效率。这可能是因为金红石相和锐钛矿相之间的协同作用效应增强了电子空穴对的分离,导致提高了样品的光催化活性。样品N-TiO_2、CTAB-TiO_2、PVP-TiO_2和Com.TiO_2的总转换效率分别为2.81%、1.57%、0.09%和1.21%。比较由这些纳米颗粒组成的染料敏化太阳能电池时可明显发现,当电池是由不添加表面活性剂所制得的TiO_2纳米颗粒(N-TiO_2)组装时,它比其它组装电池有着更高的短路电流值(I_(SC))和总转换效率。这是因为它具有高比表面和多孔结构,使得更多的敏化染料能够储存在电极中,从而增强了捕获光的能力。
通过水热法,利用粒径约为50-400nm的商用锐钛矿相纳米颗粒和NaOH合成了TiO_2纳米管和纳米带。发现水热合成的温度和反应时间对最终产物的形貌有着强烈的影响。当合成温度固定在180度时,样品的形貌会随着反应时间(5-72h)发生变化。当反应时间在5-20h时,会得到外径约为100nm的空心纳米管。当反应时间为72h时,会得到由纳米带组成的纳米束,这些纳米带的宽度约为50-500nm之间,长度约为几个微米。实验结果还表明,由不同反应温度(135℃-195℃)和反应时间下得到的样品,在特定温度下通过不同的反应时间,可发生由纳米管向纳米带的转变。当染料敏化太阳能电池(DSSC)是由这些TiO_2纳米管、纳米带和两者的混合体所组装时,它们的转换效率分别为0.03%、0.02%和0.21%。与纯纳米管或纯纳米带相比,两者混合体组装电池有着最高的转换效率和更优的开路电压(V_(OC))和短路电流(I_(SC))。
合成了自组装TiO_2纳米管阵列,并研究了合成中NH_4F浓度对其维度(长度、直径和厚度)的影响。结果表明,NH_4F浓度强烈影响着纳米管的维度。纳米管的长度随着浓度的增加会逐步增加,当浓度增到1.00 wt%时,长度达到最大值。继续增加浓度时,其长度会缓慢减小。同时也观察到纳米管的直径和壁厚也会受到NH_4F浓度的影响,当浓度增加时,直径会在80-140nm间变化,壁厚会减小。
制备了由自组装纳米管阵列组成的染料敏化太阳能电池(DSSC),这些纳米管阵列分别生长在包覆有厚度为400nm导电玻璃的钛膜和6μm厚的钛箔上。它们的转换效率分别为0.43%和0.89%。研究发现,为了提高电池的效率,必须通过增加纳米管阵列电极的厚度来获得更多的染料数目。
In this research,a comparison study was performed between the properties and photoconversion efficiency of different types of titanium dioxide(TiO_2) nanomaterials(nanoparticles,nanotubes and nanoribbons),synthesized by various synthesis techniques including;sol-gel,hydrothermal and anodic oxidation depending on the required TiO_2 nanomaterial type.Characterization of the synthesized nanomaterials was performed by means of X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM) and selected area electron diffraction (SAED).The synthesized TiO_2 nanomaterials were assembled into dye-sensitized solar cells(DSSC) and their photoelectrochemicai parameters were measured using 1.5 AM solar simulator(Oriel 91192).
Titanium dioxide(TiO_2) nanoparticles were synthesized by sol-gel technique with and without the addition of surfactant.Cetyltrimethylammonium bromide(CTAB) and polyvinylpyrolidone(PVP) were used as particle size inhibitors.Characterization results show that the prepared nanoparticles show different particle sizes,particle size distribution and phase composition,where powders prepared without surfactant and with PVP show a bi-phase structure with different rutile concentrations.The photocatalytic activity of the prepared powders in methyl orange was measured and compared to commercial TiO_2 nanopowder(Com.TiO_2).The highest degradation efficiency was obtained by the photocatalyst prepared with PVP as surfactant;while the lowest degradation efficiency was obtained by the photocatalyst prepared using CTAB.The bi-phase powders show higher degradation efficiency compared to pure anatase phase powders,probably due to a synergetic effect between anatase and rutile powders,which enhances the electron-hole separation and thus increases the photocatalytic activity.The overall conversion efficiency of the samples(N-TiO_2), (CTAB-TiO_2),(PVP-TiO_2) and(Com.TiO_2) was calculated to be 2.81,1.57,0.09 and 1.21% respectively.Observations showed that the DSSC assembled with TiO_2 nanoparticles prepared without the introduction of surfactant(N-TiO_2) presented much higher values of short-circuit current and overall conversion efficiency in comparison to other cells,which was attributed to its high surface area and porous structure,allowing more sensitizer dye to be chemically anchored in the electrode and thus increasing the light harvesting.
Titanate nanotubes and nanoribbons have been synthesized hydrothermally using commercial anatase nanopowder(particle size≈50-400nm) and NaOH.It has been found that the hydrothermal temperature and the treatment duration have a strong effect on the morphology control of the resulting products.At a fixed temperature of 180℃,the morphology of the products changes with the hydrothermal duration(5-72 h).Hollow nanotubes with an outer diameter of about 10nm,were formed at treatment durations between 5-20 h.Bundles of nanoribbons with widths ranging from 50nm - 500nm and lengths up to several tens of micrometers were produced at treatment duration of 72 h.Experimental results on samples treated at different temperatures(135℃-195℃) for different durations revealed in the transformation of the nanotubes into nanoribbons after specific treatment duration at any selected temperature.The DSSC measurement for the titanate nanotubes nanoribbons, and mixed tubes-ribbons structure,revealed conversion efficiencies of 0.03,0.02 and 0.21% respectively.The mixed structure gave higher conversion efficiency with improved open-circuit voltage(V_(OC)) and short-circuits current(I_(SC)) when compared to pure nanotubes or nanoribbons.
The effect of ammonium fluoride(NH_4F) concentration on the dimensions(length,diameter,and wall thickness) of the self-organized nanotube arrays has been investigated.Results show that varying the concentration of NH_4F exerts a strong effect on changing the dimensions of the as-grown nanotube arrays.The length of the nanotube arrays increases gradually by increasing the concentration up to a maximum length at a concentration of 1.00 wt%,after which the length decreases slightly with the increase in NH_4F concentration.It was also observed that the diameter and wall thickness of the nanotube arrays vary with the change in concentration of NH_4F,where the diameter was found to alter between 80 and 140 nm,and the wall thickness decreases by increasing the NH_4F concentration.
Self-organized nanotube arrays were grown on titanium thin film coated on conducting glass (400 nm thick) and Ti foil(6μm thick) and their DSSC measurements were performed.The overall conversion efficiency of the nanotube arrays grown on Ti foil electrode and titanium film coated on glass was calculated to be 0.89 and 0.43%respectively.It was found that to obtain highly efficient cell,the thickness of nanotube array electrode must be increased to attain high amount of dye.
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