Pettinger Natasha W, Williams Robert E A, Chen Jinquan, Kohler Bern
Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, USA.
Center for Electron Microscopy and Analysis, Department of Materials Science and Engineering, Ohio State University, 1305 Kinnear Rd., Columbus, OH 43212, USA.
Phys Chem Chem Phys. 2017 Feb 1;19(5):3523-3531. doi: 10.1039/c6cp08227k.
A stable sol of cerium oxide nanoparticles forms spontaneously when cerium(iv) ammonium nitrate (CAN) is dissolved in room-temperature water at mM concentrations. Electron microscopy experiments reveal the formation of highly crystalline cerium oxide particles several nm in diameter and suggest that they are formed from amorphous particles that are similar in size. Under the low pH conditions of the experiments, the nanoparticles form a stable dispersion and show no evidence of aggregation, even many months after synthesis. The absence of particles large enough to scatter light significantly makes it possible to observe the crystallization kinetics through dramatic changes in the UV-visible absorption spectra that occur during solution aging. Measurements show that the cerium oxide nanocrystals are formed roughly an order of magnitude more slowly in DO than in HO solution. This large solvent kinetic isotope effect (k/k ∼ 10), which is reported here for the first time for the crystallization of a solid metal oxide phase, indicates a rate-determining proton transfer reaction, which is assigned to the conversion of hydroxy to oxo bridges. In DO solution, the absorption per mole of cerium ions increases by over 400% at 290 nm as the weakly absorbing precursor phase is transformed into nanocrystalline cerium oxide. An isosbestic point is detected at 368 nm, and the absorption spectra can be modeled throughout aging by the sum of spectra of just two interconverting species. Preliminary ultrafast transient absorption experiments confirm that the optical properties of the amorphous precursors differ greatly from those of the final, nanocrystalline phase. Crystallization of CeO from CAN in water has much in common with the crystallization of iron oxides from iron(iii) salts, including the importance of non-classical nucleation and growth pathways. It is an outstanding system for studying the poorly understood events that cause molecularly solvated ions to self-assemble into nanocrystals, following hydrolysis. At the same time, the strong susceptibility of CAN to spontaneously form CeO nanocrystals under the mildest of reaction conditions indicates that caution is needed when working with this common sacrificial oxidant.
当硝酸铈铵（CAN）以毫摩尔浓度溶解于室温水中时，会自发形成稳定的氧化铈纳米颗粒溶胶。电子显微镜实验揭示了直径为几纳米的高度结晶的氧化铈颗粒的形成，并表明它们是由尺寸相似的无定形颗粒形成的。在实验的低pH条件下，纳米颗粒形成稳定的分散体，即使在合成后数月也没有聚集的迹象。没有足够大到能显著散射光的颗粒，使得通过溶液老化过程中紫外可见吸收光谱的显著变化来观察结晶动力学成为可能。测量表明，氧化铈纳米晶体在重水溶液中的形成速度比在轻水溶液中慢大约一个数量级。这种大的溶剂动力学同位素效应（kH/kD ∼ 10），首次在此报道了固体金属氧化物相的结晶情况，表明存在一个速率决定质子转移反应，该反应被归因于羟基桥向氧桥的转化。在重水溶液中，随着弱吸收的前驱体相转变为纳米晶氧化铈，每摩尔铈离子在290 nm处的吸收增加超过400%。在368 nm处检测到一个等吸收点，并且在整个老化过程中，吸收光谱可以通过仅两种相互转化物种的光谱之和来建模。初步的超快瞬态吸收实验证实，无定形前驱体的光学性质与最终的纳米晶相有很大不同。在水中由CAN结晶CeO与由铁（III）盐结晶氧化铁有许多共同之处，包括非经典成核和生长途径的重要性。它是研究导致分子溶剂化离子在水解后自组装成纳米晶体的、尚未充分理解的过程的一个杰出体系。同时，CAN在最温和的反应条件下极易自发形成CeO纳米晶体，这表明在使用这种常见的牺牲性氧化剂时需要谨慎。
