【作者】 李厚深；

【导师】 夏海兵；

【作者基本信息】 山东大学 ， 材料物理与化学， 2014， 博士

【摘要】 银纳米晶由于其优异的光学性质,以及在表面等离子体共振和表面增强拉曼散射等方面的潜在应用已经得到了广泛的关注。银纳米晶所具备的独特的性质是由其尺寸和形貌决定的,因此不同尺寸和形貌的银纳米晶的合成一直备受关注,例如球形、立方体、三棱柱、片状、棒状和线状等银纳米晶。其中,类球形银纳米晶已经被广泛研究并应用于诸多领域,例如表面增强拉曼散射、单分子的识别与标定和抗菌试剂的合成等。尽管在油相体系中,有一系列合成单分散银纳米晶的方法,并且重复性很好,但是在油相体系中,银纳米晶表面包覆的有机配体大大限制了其在生物医学和催化方面的应用。因此,为了实现其在生物和催化方面的应用,在水相中利用硼氢化钠、抗坏血酸和柠檬酸钠等还原剂还原可溶性银盐是最简便和通用的制备贵金属纳米晶的方法。在水相合成中,硼氢化钠还原银盐制备小尺寸银纳米晶的方法已经有十几年的发展,但采用此方法得到的银纳米晶尺寸的分散性较差。目前,由于柠檬酸钠还原法的操作简便、无毒害性、易于配体交换和调控尺寸,而被广泛应用于制备贵金属纳米晶。然而,采用柠檬酸钠合成银纳米晶时,同样会产生银纳米晶的尺寸不均匀和形貌复杂化的问题。到目前为止,还没有有效的、可控的方法在水相中直接合成高质量的单分散类球形银纳米晶。本论文在室温下,将一定浓度的柠檬酸钠、硝酸银和阴离子(Cl-、Br-、I-、 SO42-、CO32-、PO43-或S2-)的盐溶液在搅拌状态下,连续加入到一定体积的水中,预混一定时间后,再加入到含有抗坏血酸还原剂的沸水中,在水相中合成了单分散类球形的银纳米晶。并系统地探讨了抗坏血酸、柠檬酸钠和不同阴离子(Cl-、 Br-、I-、SO42-、CO32-、PO43或S2-)对合成单分散类球形银纳米晶的影响。深入研究了不同阴离子(Cl-、Br-、I-、SO42-、CO32-、PO43-或S2-)与Ag+结合形成不同银前驱物导致的银纳米晶尺寸变化的反应机理。另外,利用电化学置换方法,以单分散类球形银纳米晶为牺牲模板,在水相中合成了高质量和高产率的笼状金／银纳米晶。研究了笼状金／银纳米晶对过氧化氢检测和4-硝基苯酚催化的性能。本论文的主要内容如下：(1)我们利用抗坏血酸／柠檬酸钠还原法合成了单分散类球形的银纳米晶并对其反应机理进行了探讨。在传统柠檬酸钠还原法合成的银纳米晶中,各种形貌的银纳米晶共存,比如球形、棒状和三角片,且银纳米晶的尺寸范围在38-84nm之间。由于银纳米晶具有相对较宽的粒径和形貌分布,导致银纳米晶存在较宽的且非对称的表面等离子体共振吸收峰,表面等离子体共振吸收峰的最大吸收峰的峰位在460nm；最大半峰宽是150nm。有报道已经证明,柠檬酸钠在还原Ag+离子到Ag0原子的过程中,柠檬酸钠能够与Ag2+结合形成相对稳定的带正电的络合物,这种络合物的存在能够抑制银纳米晶成核-生长过程中必不可少的前驱物Ag42+的形成。因此在银纳米晶生长的反应初期,银纳米晶的成核速度相对较慢。而较慢的成核速度导致了在银纳米晶的生长阶段有大量的Ag+离子的存在,随着反应的进行,不可避免的发生了二次成核,因此得到的银纳米晶形成了较宽的粒径和形貌分布。为了在反应初期大量消耗Ag+而快速成核,我们在溶液中加入了还原能力更强的抗坏血酸。利用抗坏血酸／柠檬酸钠还原法合成了单分散类球形的银纳米晶,银纳米晶的尺寸为31士3nm。银纳米晶的粒径和形貌分布均匀,其表面等离子体共振吸收峰的峰形趋于对称且半峰宽较窄；表面等离子体共振吸收峰的最大吸收峰的峰位在405nm且最大半峰宽仅为50nm。由于抗坏血酸的络合能力较柠檬酸钠要弱但其还原能力较强,因此在沸水中加入抗坏血酸后,大量的一价银离子快速的被还原成零价的银原子,限制了Ag2+的形成。这样不仅容易形成Ag42+用于成核而且大量的消耗掉溶液中存在的Ag+,因此抑制了二次成核且加速了银纳米晶的生长。(2)我们利用抗坏血酸／柠檬酸钠法,通过在预混合液中加入不同的阴离子(Cl-、Br-、I-、SO42-、CO32-、[P43-和S2-)与Ag+形成不同的银前驱物,通过化学电势的不同,得到了粒径尺寸在16-30nm的单分散类球形银纳米晶,实现了银纳米晶尺寸的精细调控。我们根据不同阴离子与Ag+形成的银化合物在25℃下的溶度积常数,分别计算了这些银化合物在100℃下的溶度积值以及不同的阴离子与Ag+在硝酸银溶液中形成银沉淀物的最大浓度。根据加入的不同阴离子的浓度,我们发现：加入到反应溶液中的SO42-、CO32-、PO43和CI-的浓度足够低(与形成银沉淀物的最大浓度相比),以至于不能在溶液中与Ag+形成银的相应的沉淀物。在没有阴离子加入时,抗坏血酸／柠檬酸钠还原法合成的银纳米晶的尺寸为31nm,而SO42-、CO32-、PO43-和C1-这些阴离子加入后,得到的银纳米晶的尺寸分别为30nm、27nm、25nm和23nm。银纳米晶的尺寸随着这些阴离子与Ag+形成的银化合物的化学电势的降低(NO3->SO42*->CO32->PO43->Cl-)而减小。根据LaMer模型的晶体生长理论,银纳米晶的粒径尺寸主要应该由最初在成核阶段所生成的成核数目所决定。因此,阴离子与Ag+形成银化合物后的化学电势越低,越容易被还原,即越容易快速成核并且在成核阶段生成更多数目的银核,因此得到的最终的银纳米晶的尺寸越小。而溶液中加入的I-、Br-和S2-离子的浓度比在溶液中形成银的沉淀物的最大浓度要高很多,因此,这些阴离子在硝酸银溶液中能与Ag+形成银的沉淀物。这些银沉淀物在反应中发生非均匀成核,导致银纳米晶的尺寸逐渐减小,分别为23nm、21nm和16nm。(3)在阴离子辅助抗坏血酸／柠檬酸钠还原法中,柠檬酸钠的浓度在0.58mM-0.85mM之间时,能够得到单分散类球形银纳米晶。在这个范围内改变柠檬酸钠浓度,对合成的银纳米晶的尺寸和形貌影响不大。当柠檬酸钠的浓度低于0.34mM时,由于柠檬酸钠的稳定能力下降,在银纳米晶的生长过程中,柠檬酸钠不能稳定足够数目的银核,致使得到的银纳米晶的尺寸呈现多分散且形貌不规则。当柠檬酸钠的浓度大于1.02mM时,得到的银纳米晶的尺寸仍旧呈现多分散且形貌不规则。这是由于柠檬酸钠的浓度过高,柠檬酸钠的还原能力增强,在反应过程中柠檬酸钠将Ag+逐渐还原为Ag0,导致在反应中形成二次成核,从而影响了银纳米晶的尺寸和形貌的均一性。因此,为了得到单分散类球形的银纳米晶,在银纳米晶的生长过程中既要适当增强柠檬酸钠的稳定能力也要适当减弱柠檬酸钠的还原能力。(4)本论文以水相合成的单分散类球形银纳米晶为模板,通过电化学置换反应,在室温下合成了高质量和高产率的笼状金／银纳米晶。笼状金／银纳米晶的紫外-可见光谱的吸收峰在近红外区域,峰位在820nm。并且利用笼状金／银纳米晶修饰电极对过氧化氢进行了检测,其线性检测范围在0.2mM-26.5mM(R2=0.999),最低检测限(LOD)为11μM(S/N=3)。由于笼状纳米晶具有更高的比表面积、更多的催化活性位点且更利于电子传导,所以基于笼状金／银纳米晶的过氧化氢传感器的灵敏度更高,检测范围更广且检测限更低。此外,笼状金／银纳米晶进行过氧化氢检测时的电流响应时间很短(<6秒),对抗坏血酸、葡萄糖和尿酸有很好的抗干扰性且稳定性也很好。为了更好的说明笼状金／银纳米晶其特殊的结构引起的性能改变,利用笼状金／银纳米晶作为催化剂对硼氢化钠还原的4-硝基苯酚的催化性能进行了研究。结果发现,笼状金／银纳米晶催化4-硝基苯酚的活性参数为4000s-1g-,比其他形貌的金／银纳米晶的催化活性更好。与同样尺寸的实心金纳米晶的催化性能相比,实心金纳米晶的活性参数仅为283s-1g-1,因此笼状金/银纳米晶对4-硝基苯酚的催化效果更好。更多还原

【Abstract】 Silver nanocrystals (Ag NCs) have attracted considerable attention due to their remarkable optical properties and potential applications in surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS). Since the properties of Ag NCs are dependent on their sizes and shapes, Ag NCs with different sizes and shapes such as spheres, cubes, prisms, plates, disks, rods, and wires have been synthesized. Currently, spherical Ag NCs are widely studied in many areas for applications, such as SERS, single-molecule labeling and recognition, antimicrobial agents and so on.Although monodisperse Ag NPs can be prepared in an organic solvent, the organic ligand surface coating has largely limited their technical applicability in biomedicine and catalysis. In the myriad of synthetic methods for spherical Ag NCs, the chemical reduction of silver salt by reducing agents, such as NaBH4, ascorbic acid and citrate, is the facile and most commonly used one. Although it has been developed for decades to reduce silver ions to small Ag NCs by NaBH4in water, the resulting NCs usually have broad size distributions. For instance, citrate is commonly used for the preparation of noble metal NCs due to its simple protocol, nontoxicity and easy to exchange ligands, and flexibility to tune their size. However, most of citrate approach easily results in Ag NCs with fairly broad size and shape distribution. To date, no reliable techniques are available to directly produce Ag NCs with monodisperse sizes and truly quasi-spherical shapes in water.In this dissertation, a given number of sodium citrate, AgN03, and anions (Cl-, Br-, I-, SO42-, CO32-, PO43-or S2-) were consecutively added to water with stirring, and then the mixture solution was added into the boiling water of ascorbic acid (AA). The monodisperse, quasi-spherical Ag NCs were synthesized in water. The factors of AA, citrate and various anions were studied systematically. The reaction mechanism of the size change of Ag NCs due to adding anions in mixture was studies in detail. Then high quality and yield Au nanocages were synthesized in water used monodisperse, quasi-spherical Ag NCs as sacrifice template by galvanic replacement reaction. The performance of H2O2detection and4-nitro phenol catalysis of Au/Ag nanocages were also studied. The main content is as follows:(1) We synthesized monodisperse, quasi-spherical Ag NCs via AA/citrate reduction protocol and discussed the reaction mechanism in detail. In the conventional citrate reduction protocol, the NCs with different shapes such as spheroid, rod, and triangle coexist, and the NC sizes are in the range of38-84nm. Due to their fairly broad size and shape distribution, the resulting Ag NCs exhibit a broad and considerably asymmetric SPR band, centered at ca.460nm; the full width at half-maximum (fwhm) is ca.150nm. It has been demonstrated that, during citrate reduction of Ag+ions to Ag0, citrate can form relatively stable complexes with positively charged Ag2+ions, which suppresses the formation of Ag42+ions. The precursors, Ag42+ions, are essential for nucleation during Ag NC growth. Thus, the nucleation rate at the early stage during Ag NC growth via citrate reduction is rather slow. Owing to the slow nucleation, the concentration of Ag+ions remaining at the NC growth stage is expected to be fairly high, so the secondary nucleation is unavoidable, thus leading to a broad size and shape distribution. In order to rapidly consume a large amount of Ag+ions for fast nucleation, we added additional reducing agent, AA to water. Monodisperse, quasi-spherical Ag NCs were synthesized via AA/citrate protocol; the size of Ag NCs is31±3nm. This significantly improved size uniformity and shape sphericity makes the SPR band of the resulting Ag NCs fairly narrow and symmetric; its maximum absorption is centered at405nm and its fwhm is ca.50nm. AA is known to have a stronger reducing ability but weaker complexation ability than citrate. Thus, the presence of AA allows fast reduction of a considerable amount of Ag+ions to Ag0with a limited amount of Ag2+ions formed. This should not only facilitate the formation of Ag42+ions for nucleation but also significantly reduce the amount of Ag+ions leftover, thus suppressing secondary nucleation and accelerating the growth of Ag NCs.(2) Monodisperse, quasi-spherical Ag NCs with the size range from16to30nm were synthesized by adding different anions (Cl-, Br-, I-, SO42-, CO32-,PO43-and S2) into premixture solution via AA/citrate reduction protocol. Here we calculated the solubility product constants (Ksp) of different silver compounds at100℃and the maximum concentrations of the corresponding anions in water at the AgNC>3concentration used. The concentrations of SO42-, CO32-, PO43-, or Cl-ions are sufficiently low so as not to precipitate Ag+ions. In the AA/citrate reduction protocol, Ag NCs were31nm. Ag NCs obtained in the presence of SO42-, CO32-, PO43-, and Cl-ions were30nm,27nm,25nm and23nm, respectively. The sizes of as-prepared Ag NPs decrease with the following potential decrease in the order of anions:NO3-> SO42-> CO32-> PO43-> Cl-. According to the LaMer model of NP growth, the sizes of final NPs ought to be determined dominantly by the numbers of nuclei formed in the nucleation stage. As such, silver compounds with small potentials are easily reduced, allowing fast nucleation and the formation of a large number of nuclei upon nucleation, thus leading to the small size of final Ag NPs. The concentration of I", Br-, and S2" ions were far larger than the maximum concentration needed to form silver precipitates as a result of the exceedingly low solubility of silver compound in water, which makes heterogeneous nucleation inevitable during the growth of Ag NPs. Thus, the sizes of Ag NCs in the presence of I-, Br-, and S2-ions were decreased from23nm,21nm to16nm, respectively.(3) The optimal citrate concentration in AgNO3/citrate mixture was found in the range of0.58-0.85mM for formation of monodisperse, quasi-spherical Ag NCs. Polydisperse, elongated Ag NPs are obtained when the citrate concentration is adjusted to below0.34mM, which should be attributed to the fact that there are not sufficient citrate ions to stabilize the growing NPs. However, polydisperse, elongated Ag NPs are also obtained at citrate concentrations greater than1.02mM. This is possibly because the citrate reduction of Ag+ions to Ag0at that high concentration inevitably takes place, thus leading to undesirable nucleation in the reaction. To guarantee the formation of monodisperse, quasi-spherical Ag NPs, one ought to strengthen the stabilizing role of citrate and, at the same time, weaken its reducing role.(4) Monodisperse, spherical Ag nanocrystals are used as sacrificial template to synthesis of high quality and yield Au/Ag nanocages by galvanic replacement reaction with HAuCl4in water at room temperature. The UV-vis spectrum of Au/Ag nanocages is in NIR. region and the peak is820nm. The linearity of the Au/Ag nanocages biosensor for the detection of H2O2spans from0.2mM to26.5mM (R2=0.999) with a detection limit of11uM (S/N=3). Due to the high surface-to-volume radios, Au/Ag nanocages can be used in biosensor, and show more excellent performance than solid Au nanocrystals. Besides, the enzyme-free Au/Ag nanocages H2O2biosensor has a short response time (<6s), and good anti-interference and stability. In order to explore the effect of the structure of Au/Ag nanocages, Au/Ag nanocages are also found to serve as an effective catalyst for the reduction of4-nitrophenol to4-aminophenol in the presence of NaBH4. The results show that the activity parameter of Au/Ag nanocages is4000s-1g-1, which is better than catalytic performance of other structure NCs. Compared with the catalysis of the same number of solid Au NCs, the activity parameter is only283s-1g-1. Thus, Au/Ag nanocages had the superior performance in catalysis.更多还原