【作者】 马德崇；

【导师】 赵敬哲；

【作者基本信息】 湖南大学 ， 无机化学， 2012， 博士

【摘要】 在过去的几十年里，由于金属微纳米材料独特的性能（与体材相比），使得各种金属微纳米结构在催化作用、摄影术、光子学、电子学、光电子学、信息储存、传感器、生物标记、成像、医学诊断和表面增强光谱等方面有巨大的应用潜力。金属微纳米材料因其独特的特性使得其相应的制备受到广泛研究。金属微纳米材料的独特性能依赖于它们的维度、成分、晶型、形貌和几何结构（如：核-壳，固态和孔结构）。另外，由于微纳米材料的形貌变化可以调整它们的性能在较宽的范围内变化，人们在不同结构形貌微纳材料的可控合成方面做出了许多努力。铋作为一种具有间接带隙能的半金属材料，具有高的各向异性费米面、低的载流子密度、小的有效质量和平均自由程，而成为研究物理现象的理想模型。铋微纳米材料因其在介观物理领域及构造纳米器件方面独特的应用，使得铋微纳米材料的合成也受到国内外许多研究小组的广泛关注。本文通过改变实验温度，溶剂，还原剂浓度及表面活性剂等因素，实现了微纳米级Bi的形貌和尺寸的控制，提出了低温液相合成微纳米Bi的新方法。系统的研究了液相下化学还原制备Bi微纳米材料形貌控制及纯度的制备过程。采用液相化学还原过程，在不同的介质中（碱性介质或酸性介质），用不同的还原剂合成了Bi微纳米材料，同时探讨了不同形貌和结构的Bi在可见光或无光下催化降解有机染料的活性。并应用X射线衍射仪（XRD），扫描电子显微镜（SEM），透射电子显微镜（TEM），高分辨透射电子显微镜（HRTEM）和紫外可见光谱（UV-visible）等手段对制备的Bi微纳米材料的结构，尺寸，形状，光学性质及催化性能等进行了系统的分析和表征。论文主要结果概括如下：首先，在90℃的碱性环境下，用水合肼做还原剂，通过简单快速的液相法成功制备了较大规模形貌可调的Bi微纳米结构。通过PEG种类的改变（如PEG20000，PEG6000或PEG400），选择性制备了多种一维（1D）Bi纳米结构和由1D纳米棒或片状结构组装的三维（3D）分级结构。这种3D结构是有纳米棒状或纳米盘的自组装结构。该微束状结构的长度主要集中在4–5μm，直径约0.5–1μm，其中组装成束状结构的纳米棒的直径大约在10–50nm。对于该束状结构的生长过程提出了可能的生长机理。通过适当的调节合成参数，如表面活性剂的种类，表面活性剂的用量，还原剂的浓度和反应温度等，可以选择性的制备束状，梭状，类球状，盘状和棒状的Bi微纳米结构。其次，本论文在碱性环境下，通过水相法用N₂H₄·H₂O作还原剂还原Bi（NO₃）₃，已选择性制备了不同形貌的Bi纳米结构。在不同有机物分子（柠檬酸或乙二醇）作为控制剂下，我们选择性制备了Bi纳米球和Bi纳米棒，通过向体系中引入不同控制剂也成功获得了Bi纳米管。依据合成条件的改变，成功合成了平均直径约100nm的Bi纳米球，长度约50nm或200nm的Bi纳米棒以及长达100–200nm的Bi纳米管。通过对不同实验条件影响的研究，提出了Bi纳米粒子生长的可能机理，并且对所得样品进行了紫外可见光谱的研究。紫外可见光谱显示，由于所合成Bi纳米结构尺寸的减小或形貌的改变，吸收峰发生移动或变宽。另外，在碱性体系中引入一定量的酒石酸（TA），用N₂H₄·H₂O作还原剂还原BiCl₃，选择性制备了Bi纳米立方体和Bi微米球。通过控制粉体的尺寸和分散性，成功合成了边缘长度约150–200nm的Bi纳米立方体和平均直径约10μm的Bi微米球。第三，在90℃的碱性介质中，通过简单温和的液相还原法，用次亚磷酸钠（NaH₂PO₂·H₂O）作还原剂来还原Bi（NO₃）₃，成功合成了不同形貌的Bi纳米结构，所得Bi纳米结构为粒子（大小约10–50nm）或带（长度达10μm，宽度达100nm）。在此实验中，酒石酸作为络合剂，通过调节反应中NaOH的量可以有效的控制Bi纳米结构的形貌，氢氧化钠可能影响[Bi₂（C₄H₂O₆）₂]^2-的还原速率，最终影响Bi纳米产物的形貌。低的还原速率导致1D Bi纳米带的生成，随后带状结构发生卷曲形成管状结构，并且所得的带状和管状结构的组成单元是由几纳米左右的Bi纳米粒子排列而成的。结果表明，TA和NaOH在制备过程中起到了至关重要的作用。由于Bi的光学特性，所得的Bi纳米结构在各种领域具有较大的潜在应用。据我们所知这是首次在碱性环境中利用NaH₂PO₂·H₂O做还原剂制备出金属Bi纳米材料，整个反应体系在低温条件下进行，因此可以扩展到其它金属的液相还原制备，例如，可以用于Au，Ag，Cu和Pt纳米结构的制备等。第四，本文在酸性介质中，通过低温液相还原法，利用NaH₂PO₂·H₂O做还原剂，已合成了大量的Bi微纳米级晶体。所得Bi晶体为盘状（大小约100nm，厚度约几纳米）和多面体（大小约500nm）形状。结果表明延长反应时间，Bi纳米晶体具有向多面体转变的趋势，自组装和导向生长可能是晶体形貌转变的原因。此外，在酸性介质中，用NaH₂PO₂·H₂O做还原剂，体系中TA的引入对1D铋纳米结构的制备起到了至关重要的作用。研究了体系中对Bi纳米结构制备的影响，TA作为形貌控制剂，所得的1D Bi纳米结构具有典型直径约30–50nm，长度可达5μm。通过调节反应温度，选择性制备了Bi纳米棒和纳米带/管状结构。最后，以所合成的不同形貌的Bi为研究对象，探讨形貌、结构对Bi在可见光或无光下催化活性的影响，并利用UV-vis结果对Bi样品的催化活性进行了讨论，结果显示，不同形貌结构的Bi微纳米材料在可见光或无光条件下对有机染料具有优异的催化活性。结果显示由1D组装的3D分级Bi束状结构（可见光照射70min后对罗丹明B的降解率达到99%）相对与1D结构的Bi（可见光照射150min后降解率可达99%）具有更为优异的催化活性，基本上可以完全使Rh B（罗丹明B）降解。具有相同形貌Bi微纳米结构，在不同的pH值下，对罗丹明B具有不同的催化活性，且随着体系中pH值的降低，催化活性明显提高。另外，考查不同尺度的铋微纳米材料作为催化剂对Rh B染料降解，Bi纳米粒子（20–50nm），Bi纳米立方体（150–200nm），Bi亚微米多体（400–500nm）及Bi微米球（10μm）对Rh B的降解率分别为100%，98%，97%，68.6%。因此实验结果表明，随着Bi样品尺寸的减少（从微米级到纳米级的转变），催化活性显著提高。而且，所得的Bi微纳米结构在可见光照射下表现出极好的循环稳定性，可以循环利用6次，且具有稳定的催化活性。此外，具有相同形貌的Bi微纳米结构，在不同的光源下，对罗丹明B表现出不同的催化活性。与暗室相比，Bi微纳米材料在可见光下显示了极好的催化活性，这可能是光强度引起的。通过本论文的研究，得到了具有不同维数及尺度的Bi微纳米材料，完善了开放体系下水相温和条件下制备Bi微纳米材料的方法。整个论文工作所选用的合成方法简单易行，所得到的Bi金属微纳米材料在光学和催化等领域具有广阔的应用前景。更多还原

【Abstract】 Fabrication of micro/nanostructures made of various metals has beenintensively studied in the last couple of decades due to their unique properties（compared with bulk materials）, which enables them to be promising in applicationsover very broad areas including catalysis, photography, photonics, electronics,optoelectronics, information storage, sensor, biological labeling, imaging, medicaltreatment, and surface-enhanced spectroscopies. The properties of metalmicro/nanostructures are strongly dependent on their dimension, composition,crystallinity, shape and construction geometry （e.g., core-shell, solid, and hollow）.The shape-controlled synthesis has received considerable attention recently becausevarying the shape of micro/nanostructures allows one to fine-tune their propertiesover a wide range. Bismuth （Bi）, as a semimetal with a very small band overlap,provides a very attractive model system for studying physical phenomena owing tohighly anisotropic fermi surface, low carrier densities, small effective mass andmean free path. Theoretical studies predicted that micro/nanostructured Bi waspotentially useful for mesoscopic physics and fabrication of nanoscale devices,which have stimulated great efforts to synthesize Bi micro/nanomaterials.In this thesis a newly rational low-temperature solution-synthetic route waspresented for the synthesis of metallic bismuth （Bi）. Results of the thesisdemonstrated that the shape and size of micro/nanoscale Bi could be controlled bychanging the synthetic conditions, such as reaction temperature, solvent,concentration of reductive and kind of surfactants. An aqueous chemical reductionmethod is proposed to prepare Bi micro/nano crystals using different reducing agentin different medium （alkaline medium or acid medium）. We also studies the effect ofmorphologies and structures of Bi samples on their catalytic activity to degradationof organic pollutants under visible-light or without light. The structures, sizes,shapes, optical and catalytic properties of the as-prepared Bi products were analyzedand characterized systemically by using XRD, SEM, TEM, HRTEM and UV-visiblespectroscopy. The major results of the thesis are outlined as follows:Firstly, Bi with tunable morphologies have been successfully prepared on alarge scale via a simple and rapid solution phase method at90℃with hydrazinehydrate （N₂H₄·H₂O） as reducting agent in alkaline medium. We selectively prepared different shaped1D rodlike nanostructures and3D architectures of Bi by changingthe variety of PEG （such as PEG20000, PEG6000or PEG400）. These3Darchitectures were self-assembled by1D nanorods or nanoplates. The micro-bundlestructures had a length of4–5μm, a diameter of0.5–1μm, and the individualnanorods composing the bundles are10–50nm in diameter. A possible formationmechanism for the interesting architectures was proposed to interpret the growthprocess. By rationally adjusting the synthetic parameters such as the variety ofsurfactant, the quantity of surfactant, reductant concentration and reactiontemperature, Bi micro/nanostructures with bundle-like, shuttle-like, quasi-spherical,plate-like and rod-like could be selectively synthesized.Secondly, the dissertation demonstrated a work for selective preparation ofdifferent shaped Bi nanostructures via an aqueous solution process by reducingbismuth nitrate with N₂H₄·H₂O in alkaline medium. Here, Bi nanospheres andnanorods were prepared with different organic molecules （citric acid or ethyleneglycol） as controlling agent. Bi nanotubes could also be obtained by introducingdifferent kinds of controlling agents to the reaction system. By changing thesynthetic conditions，Bi nanospheres with average diameter of100nm, Bi nanorodswith length of50nm or200nm, and Bi nanotubes with length of100–200nm havebeen successfully synthesized. The dissertation also proposed the possible growthmechanism of the Bi samples by studying the influences of different experimentalconditions. And the optical properties of the samples were studied by UV-visspectroscopy. The absorption peaks in UV-visible spectra shift or broaden due to thedecreased sizes or the changed shapes of the synthesized Bi nanostructures. Inaddition, we also demonstrated a work on the preparation of Bi nanocubes and mcirospheres by reducing bismuth chloride with N₂H₄·H₂O through introducing amountsof tartaric acid （TA） to the reaction system in alkaline medium. On controlling thesize and dispersibility of the project powders, Bi nanocubes with an edge length of150–200nm, and Bi mciro spheres with size of10μm have been successfullysynthesized.Thirdly, a simple and mild aqueous solution reduction method was successfullyused to synthesize different shaped Bi nanostructures at90℃by reducing bismuthnitrate pentahydrate with sodium hypophosphite （NaH₂PO₂·H₂O） in alkaline medium.The achieved Bi nanostructures exhibited nanoparticles （10–50nm in size） orbelt-like （lengths of up to10μm and widths of up to100nm） shapes. In theexperiments, TA acted as complexing reagent, and the morphologies of the Bi nanostructures can be controlled by adjusting the quantity of NaOH used in thereaction. The quantity of NaOH determined the reduction rates of [Bi₂（C₄H₂O₆）₂]^2-,which influenced the shape of Bi samples. Low reduction rates resulted in1D Binanostructures as nanobelts, which had tendency to roll to tube-like structures.Interestingly, we found that the belt-like and tube-like structures were constructedby aligned nanoparticles of several nanometers. Results indicated that tartaric acidand NaOH are key factors in our preparation. These Bi nanostructures are expectedto find potential applications in a variety of areas due to their optical characteristics.To the best of our knowledge, this work is the first concerning on the synthesis ofmetal Bi nanostructures in aqueous solution with NaH₂PO₂·H₂O as reductant. Webelieve that the rational low-temperature synthetic route is universal and can beadapted for the preparation of numerous metal materials in solution. For example, itmight be possible to prepare Au, Ag, Cu, and Pt nanostructures via methods similarto those described in this work.Fourthly, the thesis also reported a low temperature solution reduction methodemployed in the synthesis of large quantities of micro/nano-sized Bi crystals withNaH₂PO₂·H₂O as reductant in acidic solutions. The achieved Bi crystals exhibitedplate-like （100nm in size and few nanometers in thickness） or polyhedral （500nm insize） shapes. The experimental results suggest that the polyhedral Bi samples weresynthesized by prolonging the reaction time. The assembly and oriented growthshould be the reason. Optical properties of the Bi samples with different shapes werealso investigated by UV–vis method. In addition, it was found that the introductionof TA to the reaction system was essential for the preparation of1D bismuthnanostructures with NaH₂PO₂·H₂O as reducing agent in acidic solutions. Theachieved1D nanostructures have typical diameters of30–50nm and length of up to5μm. The effects of reaction parameters in the system on the preparation of Binanostructures were studied. TA was used as the shape controlling agent, which wasa key factor for getting1D structures in the preparation. Bi nanorods andnanobelt/tube-like structures can be selectively prepared by changing the reactiontemperature.Finally, based on the Bi samples with different structures obtained above, westudies the effect of morphologies and structures of Bi samples on their catalyticactivity to degradation of organic pollutants under visible-light or without light. Thecatalytic activity of the Bi samples was discussed via UV-vis results. Resultsindicated that the Bi samples exhibited highly activing for degradation of organic pollutants.3D hierarchical bundlelike Bi structures （the degradation rate of Rh B canreach up to99%after70min’s under visible-light irradiation） had betterperformance than the dispersed1D Bi nanostructures （the degradation rate of Rh Bcan reach up to99%after150min’s under visible-light irradiation）. The Rh Bsolution can be thoroughly degraded after70min with the sample of bundle-like Bistructures under visible light irradiation. Bi with the same morphology performeddifferent catalytic activities in Rh B solutions with different pH value. It was foundthat the degradation efficiency increased evidently when the pH value decreased. Inaddition, we also discussed the degradation of Rh B with the Bi nanostructures ascatalysts under various size.100%,98%,97%and68.6%degradation of Rh B isobserved after90min visible light irradiation for Bi nanoparticles （20–50nm）,nanocubes （150–200nm）, sub-micropolyhedra （400–500nm） and micro-spheres （10μm）, respectively. The results indicated that if the grain sizes of the Bi samples arereduced from micro-size to nano-size, their catalytic activities dramatically increase.Furthermore, the Bi samples could be easily recycled six times with little decrease ofthe catalytic activity under visible light irradiation. Bi samples exhibited differentcatalytic activities to Rh B solutions under different illumination. Bimciro/nano-structures under visible light irradiation showed higher catalytic activitythan that in the dark.In conclusion, Bi micro/nanomaterials with different dimensions are obtainedby aqueous synthesis method. Our work gives an improved progress on the mildsolution synthesis of Bi micro/nanomaterials. The obtained Bi micro/nanomaterialsare promising in optical and catalytic application.更多还原