摘要
二氧化钛(TiO_2)是一种重要的无机功能材料，具有化学性质稳定、抗光腐蚀、无毒和低成本等特点，在光电转换和光催化等领域已经受到越来越多的关注。然而，TiO_2是宽带隙半导体，只能被紫外光激发，且光生电子-空穴对容易复合，光量子效率低，限制了其在光电催化领域的应用。因此，开发具有可见光响应的纳米TiO_2光催化剂并拓展其应用领域是当前光催化研究的重要任务。
尽管传统纳米粉末和纳米颗粒薄膜具有优异的性能，但它们易团聚且光电转换效率低，因此在光电催化领域的应用受到限制，本文展开纳米材料的管/棒状阵列的研究。采用阳极氧化法在钛片上成功制备了分布均匀、排列整齐有序的TiO_2纳米管阵列并在其上热沉积Pt纳米颗粒得到Pt/TiO_2NTs催化剂，对样品进行了形貌、成分和结构的表征，并系统研究了Pt/TiO_2NTs对乙醇的电催化性能。采用水热法制备得到了不同微观结构的TiO_2纳米棒阵列，用不同方法在其上沉积CdS、Co_3O_4和In_2O_3纳米颗粒，得到CdS/TiO_2、Co_3O_4/TiO_2、In_2O_3/TiO_2复合纳米结构，并对这些复合纳米结构的光电化学性能进行了系统研究。采用贵金属沉积和半导体复合手段对金红石型的TiO_2纳米棒阵列进行了改性，得到了CdS-Pt/TiO_2光催化剂，研究了可见光（或紫外光）辐照下，改性TiO_2的光解水性能。本文的主要研究结果如下：
1）采用阳极氧化法在钛片上成功制备了分布均匀、排列整齐有序的TiO_2纳米管阵列，利用XRD、FESEM、TEM、UV-vis和XPS等手段对样品进行了表征，详细讨论了阳极氧化工艺对其形貌的影响，并对TiO_2纳米管阵列形成机理进行详细阐述。（第二章）
2）采用水热法在FTO上制备了TiO_2纳米棒阵列，利用XRD、FESEM、TEM、UV-vis和XPS等手段对制备的产物进行表征，系统地讨论了水热参数等对产物结构和形貌的影响，得出了TiO_2纳米棒阵列的生长规律。光电化学性能研究表明，TiO_2纳米棒阵列在光照瞬间产生光电流，20s左右达到稳定。TiO_2纳米棒的一维结构能为光生电子提供直线传输通路，促进光生电子-空穴对的分离。这种独特的棒状阵列结构，具有大的比表面积，具有较好的光电性能，有望成为新一代的半导体电极材料，具有良好的应用前景。（第三章）3）以TiO_2纳米棒阵列为载体，采用连续离子层吸附反应法在其表面沉积CdS纳米颗粒，制备了CdS/TiO_2复合纳米结构。FESEM和TEM表征表明CdS纳米颗粒为立方相，直径为3-5nm，均匀地沉积在TiO_2纳米棒表面上。经CdS量子点敏化后的纳米结构的光电化学性能大大提高，光电流密度达78.48μAcm~(-2)，是TiO_2纳米棒产生的光电流的5倍。TiO_2纳米棒阵列，CdS纳米颗粒和CdS/TiO_2复合纳米结构的光电流密度随电极电位的正移而增大，但CdS/TiO_2复合纳米结构增大得更多，表现出更高的光电化学性能。
以TiO_2纳米棒阵列为载体，采用光化学沉积法在其表面负载Co_3O_4纳米颗粒，制备了Co_3O_4/TiO_2复合纳米结构。XRD、FESEM和TEM表征表明Co_3O_4纳米颗粒像是“铆”在TiO_2纳米棒上的，且直径约为3-5nm。Co_3O_4纳米颗粒的沉积量是可以通过沉积溶液的浓度和光化学反应的时间等来进行调控的。UV-vis测试表明Co_3O_4/TiO_2异质结中TiO_2的禁带宽度减少至3.10eV，说明Co_3O_4的修饰降低了TiO_2的禁带宽度，这将有利于对可见光的吸收并增强其光电化学性能。XPS表征表明Co以+2价和+3价的形式存在。光电化学性能测试结果表明Co_3O_4/TiO_2复合纳米结构的光电性能优于TiO_2纳米棒阵列电极，由于Co_3O_4和TiO_2的耦合效应可以使得TiO_2的吸收光谱范围从紫外延伸到可见光区域所致。
以TiO_2纳米棒阵列为载体，采用浸渍提拉法在其表面沉积In_2O_3纳米颗粒，制备了In_2O_3/TiO_2复合纳米结构。XRD、FESEM和TEM表征表明In_2O_3纳米颗粒取向随机的粘附于纳米棒的边缘，尺寸为30-40nm之间，且是单晶的。In_2O_3纳米颗粒的厚度可以通过浸渍提拉的次数来控制。UV-vis测试表明TiO_2纳米棒阵列的带隙变窄，说明将宽带隙半导体(TiO_2)与窄带隙半导体(In_2O_3)复合能够起到带隙调变的作用。XPS表征表明In以+3价的形式存在。瞬态光电流谱和电流-电压谱测试结果表明经In_2O_3/TiO_2复合纳米结构的光电性能优于TiO_2纳米棒阵列电极。（第四章）
4）以TiO_2纳米棒阵列为载体，采用热沉积法制备了Pt/TiO_2，XRD、FESEM和TEM表征结果表明热沉积的Pt为纳米颗粒，尺寸为2-5nm，分布较均匀。采用连续离子吸附反应法循环5次在Pt/TiO_2纳米棒上沉积CdS纳米颗粒后，纳米棒的边缘形貌变得十分模糊，表面粗糙度变大，说明CdS量子点均匀地覆盖在Pt/TiO_2纳米棒的表面上，形成CdS-Pt/TiO_2复合纳米结构。通过稳态光电流谱、瞬态光电流谱和交流阻抗谱等的测量，研究TiO_2、Pt/TiO_2和CdS-Pt/TiO_2的光电解水性能。当测试偏压增加到1.0V时， CdS-Pt/TiO_2电极的光电流密度达到0.30mA/cm~2，是Pt/TiO_2(0.096mA/cm~2)的3.1倍和TiO_2(0.055mA/cm~2)的5.45倍。在可见光下CdS-Pt/TiO_2的最大光电效率为1.90％，为Pt/TiO_2(0.64％)的3.0倍和TiO_2电极（0.19％）的10倍。CdS-Pt/TiO_2较TiO_2和Pt/TiO_2的光电转换效率高，说明CdS-Pt/TiO_2具有更好的光电化学活性，这主要归因于沉积的金属Pt和CdS纳米颗粒。测试了各电极的电化学交流阻抗谱，并用等效电路进行模拟，实验结果表明CdS-Pt/TiO_2复合纳米结构的导电性明显高于TiO_2和Pt/TiO_2。对CdS-Pt/TiO_2复合纳米结构的光解水的机理进行了讨论，沉积Pt和CdS纳米颗粒能显著地减少光激发产生的电子-空穴对的复合，因而能增强光解水制氢的性能。（第五章）
（5）以电化学阳极氧化制备的TiO_2纳米管阵列为载体，采用光化学沉积法将Pt纳米颗粒负载在TiO_2纳米管阵列上，形成Pt/TiO_2NTs催化剂。FESEM和TEM表征显示光滑的TiO_2纳米管的内、外壁上均匀吸附了3~10nm大小的Pt纳米颗粒；EDS表征表明样品包含元素为Pt、Ti、O；XPS表征表明沉积在TiO_2纳米管上的Pt纳米颗粒为金属。通过循环伏安法、计时电流法和交流阻抗法等来研究Pt/TiO_2NTs催化剂对乙醇在酸性或碱性溶液中的电催化氧化特性。结果表明Pt/TiO_2NTs催化剂比Pt/C和Pt/Ti催化剂具有更高的催化活性和稳定性。（第六章）
Titanium dioxide (TiO_2) is an important semiconductor material and has attractedconsiderable attention in the fields of photoelectric conversion and photocatalysisbecause of its good chemical stability, anti-photocorrosion, nontoxic and low-cost.However, the wide band gap of TiO_2limits them to UV radiation, which is only4%ofthe solar spectrum energy. Moreover, the high recombination rate of photo-electron andhole limits its photoelectrochemical catalytic application. So, the development of TiO_2nano-photocatalysts with visible-light response as well as its new applications is one ofthe most important works in the photocatalytic fields．
Because traditional nanometer-scale powders are likely of aggregate and theconversion efficiencies of nanoparticle polycrystalline films is limited, TiO_2nanorodarray and nanotube array films are studied in this work. The vertically aligned TiO_2nanotube arrays (NTs) have been prepared by anodizing method on Ti foil and Pt/TiO_2electrocatalyst composite electrodes have been prepared and used for the electrooxidationof ethanol in acidic and alkaline media. Vertically aligned TiO_2nanorod arrays (NRs)were synthesized by a facile hydrothermal method, and followed by depositing CdS,Co_3O_4and In_2O_3nanoparticles on TiO_2NRs by different methods. The surfacemorphology, structure, optical and photoelectrochemical behaviors of these CdS/TiO_2、Co_3O_4/TiO_2、In_2O_3/TiO_2composite films are studied in detail. Modified rutile TiO_2NRswere synthesized by being treated using the noble metal (Pt) deposition andsemiconductor (CdS) compounding. The photocatalytic-activities for hydrogenevolution were firstly studied and compared with TiO_2NRs, Pt/TiO_2NRs andCdS-Pt/TiO_2NRs electrodes in the electrochemical system under the visible light (orUV-light) irradiation. The details are described as follows:
1) The highly ordered TiO_2nanotube arrays (NTs) have been prepared byanodizing method on Ti foil. The morphology and composition of the as-preparedsamples were characterized by X-ray diffraction, field-emission electron scanningmicroscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy,ultraviolet-visible reflection spectrophotometry, and X-ray photoelectron spectroscopy.The formation mechanism of TiO_2NTs was discussed in detail.(Chapter2)
2) TiO_2nanorod arrays (NRs) were synthesized by a facile hydrothermal methodand were characterized by XRD, FESEM, TEM, UV-vis spectrum, and XPS respectively. The influences of hydrothermic factor, such as solution concentration，hydrothermal temperature，and hydrothermal time on the structure and morphology ofthe samples were systematically studied. Then, the growth mechanisms of the sampleswere proposed. The TiO_2NRs was applied to construct photoelectric devices, by whichhighly sensitive and steady photocurrent responses were obtained. The electrodes showan instantaneous change in current upon light illumination and keep stable after20s.The current retracts to its original value almost instantaneously once the illumination isswitched off. The highly sensitive responses and excellent stability to the UV lightreveal that the TiO_2NRs can be used to fabricate the UV sensors/switches.(Chapter3)
3) CdS/TiO_2composite films have been fabricated via a two-step method.Vertically aligned TiO_2NRs were synthesized by a facile hydrothermal method, andfollowed by depositing CdS nanoparticles on TiO_2NRs by successive ionic layeradsorption and reaction (SILAR) method. The surface morphology, structure, opticaland photoelectrochemical behaviors of the CdS/TiO_2nanocomposite were considered.The CdS nanoparticles with a diameter of3-5nm are uniformly dispersed on the TiO_2NRs. The TiO_2NRs coated by CdS nanoparticles show higher photocurrent value thanthat of pure TiO_2NRs. The enhanced photoelectrochemical behaviors can be attributedto the coated CdS nanoparticles, which increase the probability of electron–holeseparation and extend the range of the TiO_2photoresponse from ultraviolet to visibleregion due to the low band gap of2.56eV.
A novel heterostructure of Co_3O_4/TiO_2was fabricated by Co_3O_4nanoparticlescoated on the TiO_2NRs using a photochemical coating method. The composition andmorphologies of samples were characterized. HRTEM showed that Co_3O_4nanoparticleswith mean size of2–3nm are coated on TiO_2nanorod, could not be ‘‘peeled’’ off fromTiO_2nanorod even under sonication for10min. The gap energy3.10and2.15eV isestimated corresponding to TiO_2and Co_3O_4in the Co_3O_4/TiO_2heterostructure,respectively, which is smaller than that of the reported pure TiO_2due to the interfacialincorporation and matched band edges between the semiconductor TiO_2and Co_3O_4. Thephotoelectrochemical properties have been investigated under visible-light irradiationand the results showed remarkably enhanced visible light PEC response of theCo_3O_4/TiO_2heterostructure due to the coupling effects of Co_3O_4and TiO_2incomparison with the pure TiO_2NRs, offering attractive perspectives for applications ofthe Co_3O_4/TiO_2heterostructure in solar cells or photocatalysts.
The In_2O_3/TiO_2nanocomposite films have been prepared by dip-coating process. The composition and morphologies of samples were characterized. In_2O_3nanoparticleswith mean size of30–40nm adhered to TiO_2NRs and the density of the nanoparticles isdramatically increased with the number of cycles of dip-coating process. Thephotoelectrochemical properties of the TiO_2NRs photoelectrodes before and after beingsensitized with In_2O_3nanoparticles were investigated under visible light illumination.The results show that the photocurrent of In_2O_3/TiO_2is dramatically enhancedcompared with that of the TiO_2NRs or In_2O_3nanoparticles under visible lightillumination. The enhanced photocurrent is attributed to the coupling effects of In_2O_3nanoparticles and TiO_2NRs. However, it also shows that when excess In_2O_3nanoparticles are deposited on the TiO_2NRs, there is a decrease of photocurrent undervisible light illumination.(Chapter4)
4) CdS-Pt/TiO_2composite nanostructure has been fabricated via a two-step method,which was used for the hydrogen production under visible-light irradiation. Pt/TiO_2wassynthesized by Pt nanoparticles deposited on TiO_2NRs, and followed by depositing CdSnanoparticles on Pt/TiO_2by SILAR method. The composition and morphologies ofobtained CdS-Pt/TiO_2nanocomposites were characterized by XRD, FESEM, TEM,UV-vis spectrum, and XPS respectively. The results show that the Pt nanoparticles with adiameter of2-5nm are uniformly dispersed on the TiO_2NRs and the CdS nanoparticles arealso uniformly dispersed on the Pt/TiO_2. It is found that the CdS nanoparticlesembedded in the interstices of the Pt/TiO_2formed intimate contacts between the NRs,which benefit to significantly enhance the charge separation and then the photocatalyticactivity. The photoactivity for the hydrogen production of CdS-Pt/TiO_2, Pt/TiO_2andTiO_2electrodes were assessed comparatively in terms of the photocurrent collected. TheCdS-Pt/TiO_2electrode shows the biggest photocurrent (0.30mA cm~(-2)) at1.0V undervisible-light illumination, which is about3.1times larger than that of the Pt/TiO_2electrode and5.45times larger than that of the TiO_2electrode, respectively. Thephotoelectricity efficiency CdS-Pt/TiO_2electrode is1.90％, which is about3.0timeslarger than that of the Pt/TiO_2electrode and10.0times larger than that of the TiO_2electrode, respectively. The electrochemical impedance spectroscopies were studied,and the results indicate that the electrical conductivity of CdS-Pt/TiO_2electrode is betterthan that of Pt/TiO_2and TiO_2electrodes. And the possible mechanism of the obtainedCdS-Pt/TiO_2nanocomposite as a photocatalyst for hydrogen production was proposed.(Chapter5)
5) The TiO_2NTs were prepared by anodizing method and Pt/TiO_2electrocatalyst composite electrodes were prepared by photodeposition method. FESEM and TEMobservations reveal that the Pt nanoparticles about3-10nm in diameter are uniformlydispersed on the TiO_2NTs, both inside and outside of the tubes. Electrocatalytic activityand stability for the electrooxidation of ethanol were studied by cyclic voltammetry,chronoamperometry and electrochemical impedance spectroscopy. Because of thethree-dimensional open nanostructure of the TiO_2support for well-dispersed Ptnanoparticles, the Pt/TiO_2electrocatalyst has a larger active surface area for faciletransport of ethanol in comparison with the Pt/C and Pt/Ti electrocatalysts. Meanwhile,Pt/TiO_2catalysts show a relatively higher activity and anti-poisoning capability forethanol electrooxidation in both acidic and alkaline media than the Pt/C and Pt/Tielectrocatalysts due to the stronger synergistic interaction between TiO_2support and Ptnanoparticles with smaller particle size.(Chapter6)
引文
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