Synthesis of cubic and monoclinic hafnia nanoparticles by pulsed plasma in liquid method

Hafnia (HfO₂, hafnium dioxide) is a wide band gap and high-κ material, and the metastable cubic hafnia has a much higher permittivity compared with the normal monoclinic hafnia. Here, we employ a one-step process, the pulsed plasma in liquid (PPL) method to synthesize two types of hafnia nanoparticles (NPs): one which is mainly in cubic phase (cubic: 81.7 at%, monoclinic: 18.3 at%) and the other which is in monoclinic phase. High-resolution transmission electron microscopy images showed that the particles were small (particle size ~3 nm). X-ray absorption fine structure analysis showed no chemical shifts, indicating that the synthetic hafnia NPs contained no oxygen vacancy. The synthetic hafnia NPs mainly in cubic phase showed a much higher relative permittivity than that of the commercial hafnia (monoclinic), and have a larger band gap than the synthetic monoclinic hafnia NPs.     

Structural evolutions of CdSe nanocrystals in ripening process

A size-selective separation method is used to synthesize CdSe nanocrystals (NCs) with different grain sizes. The structural evolutions of CdSe NCs in the ripening process are investigated by high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) spectroscopy. The results indicate that the structural disorder of the Se–Cd bond in CdSe NCs rises from 0.0005, 0.0012 and 0.0034 Å² as the size of NCs increases from 1.9 to 2.7 and 3.1 nm. The small of the 1.9 nm NCs unambiguously reveals that at this stage the CdSe NCs are well crystallized and almost free of interior defects. The unusual increase of with size can only be interpreted by the interior defects rather than surface defects of NCs. The interior defects produced in the core of the CdSe NCs through the ripening process are accumulated. This leads to a rapid increase in their structural disorders for the large CdSe NCs.     

Reversible structure change of one-atomic layer GeO2 on SiO2 surface under the interaction with Rh particles by in situ XAFS studies

Structures of Rh/1 AL (atomic layer) GeO₂/SiO₂ were studied during reduction and oxidation processes by means of in situ XAFS (X-ray absorption fine structure). It was found that RhGe bimetallic particles with Rh--Rh and Rh--Ge distances at 0.266 and 0.242 nm were formed after reduction at 723 K, respectively. Subsequent oxidation at 673 K regenerated 1 AL GeO₂ structure on the SiO₂ surface. This reversible structure transformation is discussed in relation to reactivity and volatility of Ge oxide species.     

高温原位XAFS研究熔态Sb的局域结构

利用高温原位XAFS技术研究了半金属Sb在固态和熔态时的局域结构特点。结果表明在893K高温时固态Sb的局域结构与常温(298K)时晶态Sb的相似,但其热无序度较大。随着温度再升高20K到熔化后的913K时,熔态Sb的主配位峰形状有较大变化,其强度仅为熔化前的70％左右。这一结果表明熔化导致Sb的结构无序显著增加,我们认为这是熔态Sb中Sb原子的第一近邻配位的大部分共价键断裂造成的。模型无关的Reverse Monte Carlo方法拟合计算结果也表明熔化引起Sb样品的第一近邻配位数由固态的6增加到熔态的9。当温度升高至1058K时,Sb样品的Sb原子的径向结构函数曲线的形状与913K的Sb样品的相似,说明在熔化后的100至200K的温度范围,熔态Sb中Sb原子的局域结构差别不大。     

分子束外延生长ZnO薄膜的XAFS研究

利用同步辐射广延X射线吸收精细结构(EXAFS),研究在不同条件下分子束外延制备的ZnO薄膜,如分别在蓝宝石(0001)、Si(100)衬底上,生长温度为200℃或300℃下得到样品的局域结构。发现这些ZnO薄膜的EXAFS函数(k^2x(k))谱形状相似,说明各个样品都具有较为相近的基本局域结构。对生长温度为200℃的ZnO／Al2O3(0001)和ZnO／Si(100)样品,其Zn-O第一配位峰的无序度仃。分别为0．0054A^2和0．0080A^2,当生长温度从200℃提高到300℃时,ZnO／Al2O3(0001)样品的Zn-O第一配位峰的无序度仃。降为0．0039A^2。结果表明衬底与ZnO的晶格失配度和生长温度对ZnO薄膜的配位数、Zn-O键长影响不大,但较小的晶格失配度和较高的生长温度下得到的ZnO薄膜局域有序性较高;且样品的局域结构越有序,相应的配位峰幅度也越高。     

X射线吸收精细结构在古陶瓷研究中的应用

主要简述了XAFS的基本原理,国内外学者应用XAFS研究古陶瓷结构信患的相关工作,旨在推动我国XAFS在古陶瓷研究领域中的应用,挖掘更多的"潜"信息,丰富古陶瓷研究的内涵,更好地探讨古陶瓷呈色机制和烧成制度.     

用X射线吸收谱对多元物质精细结构的研究(英文)

基于EXFS的物理机制 ,我们用EXAFS研究多元物质中任一组元临近结构时 ,首先由测定的吸收谱求出精细结构函数X(k) ,根据X(k)确定结构参数 .这种方法在结构研究中有其独到之处 ,本文根据“短程序理论”所构造的物理模型 ,对用透射法的结果的处理方法进行了深入讨论 .     

蓝色发光GaN纳米晶和晶体的XAFS研究

利用XSAFS（X－射线吸收精细结构）方法研究六方的纳为晶和晶体GaN在78K和300K温度下Ga原子的局域配位环境结构,对于第一近邻Ga－N配位,纳米晶GaN的平均增长R、配位数N、热无序度σT和结构无序度σs与晶体GaN的相近,分别为0．194nm、4．0、0．0052nm、0．0007nm;当温度从78K增加到300K,GaN样品中Ga－N配位的σT增加不多,小于0．0005nm,表明第一近邻Ga－N配位的共价键作用力较强,几乎不受温度和晶体状态的影响。对于第二近邻Ga-Ga配位,R为0．318nm,纳米晶GaN的σs（0．0057NM）比晶体GaN的（0．001nm )大0．0047nm;在78K和300K时,纳米晶GsA样品的Ga-Ga位位的σT分别为0．0053nm和0.0085nm,这一结果表明Ga-Ga配位的σT受温度变化产生很大影响,纳米晶GaN中Ga原子的局域配位环境与晶体GaN的差别主要表现在第二近邻Ga-Ga配位的σs相对较大,可能是由于纳米晶GaN内部缺陷及存在较多的表面不饱和配位原子所致。     

On the contribution of X-ray absorption spectroscopy to explore structure and activity relations of Pt/ZrO2 catalysts for CO2/CH4 reforming

X-ray absorption spectroscopy (XAS) has proven to be a very useful technique in characterizing metal-based catalysts exposed to extreme operating conditions. The technique allows in situ evaluation of structural parameters (XAFS) and electronic properties (XANES). The elucidation of the nature and state of Pt-based catalysts in dry reforming of methane with carbon dioxide is presented as case study to show the contribution and potential of XAS to explore property/performance relationships for heterogeneous catalysts. Pt/ZrO₂ is an active and stable catalyst for the reaction between CH₄ and CO₂ to synthesis gas (H₂/CO). The activity and stability of the catalyst is strongly influenced by the catalyst pretreatment (calcination/reduction). The combination of hydrogen chemisorption, IR spectroscopy, XPS and XAS is shown to be suitable to track the changes of the state of the catalyst. In particular, it will be demonstrated, how XAFS helped to correctly attribute variations in the chemisorptive properties of Pt/ZrO₂ after severe temperature treatment to partial and reversible decoration of the small Pt particles with fragments of the oxide support. In situ tracking of the reduction of the catalysts by XANES additionally helped to semiquantitatively assess the partial reduction of the ZrO₂. Finally, XANES helped to demonstrate that CO₂ exposure under these severe conditions did not lead to detectable levels of surface oxidation of Pt. Based on XANES, IR spectroscopy and kinetic measurements it is concluded that in dry reforming activation of methane occurs on Pt, while CO₂ is activated on the support and the two entities react at the metal–support interface. This revised version was published online in June 2006 with corrections to the Cover Date.     

XAFS characterization of the binuclear iron complex in overexchanged Fe/ZSM5 – structure and reactivity

We have characterized, with emphasis on XAFS spectroscopy, one of the most promising DeNOx catalysts, i.e., Fe/ZSM5 prepared through the FeCl₃sublimation technique. XAFS is a very useful tool for this purpose since it is element specific and can be used in situ, namely, in the presence of the reactants and at reaction temperature. In this communication it will be pointed out that the as-synthesized Fe/ZSM5 catalyst contains stable binuclear iron oxo/hydroxo-complexes. The reaction of these complexes with the probe molecule CO clearly shows that only one of the oxygen shells around iron changes while the iron contribution is not affected, the iron complex remaining binuclear. This revised version was published online in July 2006 with corrections to the Cover Date.