Debnath Bharati, Salunke Hemant G, Shivaprasad Sonnada M, Bhattacharyya Sayan
Department of Chemical Sciences and Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India.
Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India.
ACS Omega. 2017 Jun 28;2(6):3028-3035. doi: 10.1021/acsomega.7b00622. eCollection 2017 Jun 30.
The intrinsic physical properties of nanostructures of metals and their oxides are altered when they are prone to surface oxidation in ambient atmosphere. To overcome this limitation, novel synthesis methodologies are required. In this study, solid octahedral shapes of MnO limit the inward oxygen diffusion compared to that of the MnO-nanoparticle-assembled octahedra. In addition to morphology control, which restricts the thickness of the MnO surface layer, the binding chemistry of the surfactants plays an essential role. For example, the MnO surface layer is 0.4 nm thinner with trioctylphosphine oxide than with trioctylamine as the surfactant. The nanostructures were prepared by varying the surfactants, surfactant-to-precursor molar ratio, accelerating agent, and reaction heating rate. The surface oxidation of MnO nano-octahedra was probed by Rietveld analysis of X-ray diffraction patterns and X-ray photoelectron spectroscopy and characterized by magnetic measurements, as the presence of ferrimagnetic MnO shell on the antiferromagnetic MnO core provides an exchange coupling at the core-shell interface. Thicker the MnO shell, higher is the exchange-biased hysteresis loop shift.
当金属及其氧化物的纳米结构在环境大气中易于发生表面氧化时，其固有物理性质会发生改变。为克服这一限制，需要新颖的合成方法。在本研究中，与MnO纳米颗粒组装的八面体相比，MnO的固态八面体形状限制了氧向内扩散。除了形态控制（其限制了MnO表面层的厚度）外，表面活性剂的键合化学也起着至关重要的作用。例如，以三辛基氧化膦作为表面活性剂时，MnO表面层比以三辛胺作为表面活性剂时薄0.4nm。通过改变表面活性剂、表面活性剂与前驱体的摩尔比、促进剂和反应加热速率来制备纳米结构。通过对X射线衍射图谱进行Rietveld分析和X射线光电子能谱来探测MnO纳米八面体的表面氧化，并通过磁性测量进行表征，因为在反铁磁性MnO核上存在亚铁磁性MnO壳会在核壳界面处提供交换耦合。MnO壳越厚，交换偏置的磁滞回线偏移就越大。
