Hammond Oliver S, Atri Ria S, Bowron Daniel T, de Campo Liliana, Diaz-Moreno Sofia, Keenan Luke L, Doutch James, Eslava Salvador, Edler Karen J
Department of Chemistry and Centre for Doctoral Training in Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, UK.
Nanoscale. 2021 Jan 28;13(3):1723-1737. doi: 10.1039/d0nr08372k.
Deep eutectic solvents (DES) and their hydrated mixtures are used for solvothermal routes towards greener functional nanomaterials. Here we present the first static structural and in situ studies of the formation of iron oxide (hematite) nanoparticles in a DES of choline chloride : urea where xurea = 0.67 (aka. reline) as an exemplar solvothermal reaction, and observe the effects of water on the reaction. The initial speciation of Fe3+ in DES solutions was measured using extended X-ray absorption fine structure (EXAFS), while the atomistic structure of the mixture was resolved from neutron and X-ray diffraction and empirical potential structure refinement (EPSR) modelling. The reaction was monitored using in situ small-angle neutron scattering (SANS), to determine mesoscale changes, and EXAFS, to determine local rearrangements of order around iron ions. It is shown that iron salts form an octahedral [Fe(L)3(Cl)3] complex where (L) represents various O-containing ligands. Solubilised Fe3+ induced subtle structural rearrangements in the DES due to abstraction of chloride into complexes and distortion of H-bonding around complexes. EXAFS suggests the complex forms [-O-Fe-O-] oligomers by reaction with the products of thermal hydrolysis of urea, and is thus pseudo-zero-order in iron. In the hydrated DES, the reaction, nucleation and growth proceeds rapidly, whereas in the pure DES, the reaction initially proceeds quickly, but suddenly slows after 5000 s. In situ SANS and static small-angle X-ray scattering (SAXS) experiments reveal that nanoparticles spontaneously nucleate after 5000 s of reaction time in the pure DES before slow growth. Contrast effects observed in SANS measurements suggest that hydrated DES preferentially form 1D particle morphologies because of choline selectively capping surface crystal facets to direct growth along certain axes, whereas capping is restricted by the solvent structure in the pure DES.
深共熔溶剂（DES）及其水合混合物被用于通过溶剂热法制备更绿色的功能纳米材料。在此，我们首次对在氯化胆碱∶尿素（其中(x_{尿素}=0.67)，即“reline”）的DES中形成氧化铁（赤铁矿）纳米颗粒这一典型溶剂热反应进行了静态结构和原位研究，并观察了水对该反应的影响。使用扩展X射线吸收精细结构（EXAFS）测量了DES溶液中(Fe^{3 +})的初始物种形成，同时通过中子和X射线衍射以及经验势能结构精修（EPSR）建模解析了混合物的原子结构。使用原位小角中子散射（SANS）监测反应以确定中尺度变化，并用EXAFS确定铁离子周围局部有序结构的重排。结果表明，铁盐形成八面体([Fe(L)_3(Cl)_3])配合物，其中((L))代表各种含O配体。由于氯化物被吸收到配合物中以及配合物周围氢键的扭曲，溶解的(Fe^{3 +})在DES中引起了微妙的结构重排。EXAFS表明该配合物通过与尿素热水解产物反应形成([-O-Fe-O-])低聚物，因此在铁方面为假零级反应。在水合DES中，反应、成核和生长迅速进行，而在纯DES中，反应最初进行迅速，但在5000 s后突然减慢。原位SANS和静态小角X射线散射（SAXS）实验表明，在纯DES中，反应5000 s后纳米颗粒自发成核，然后缓慢生长。SANS测量中观察到的对比效应表明，水合DES优先形成一维颗粒形态，因为胆碱选择性地覆盖表面晶面以沿某些轴引导生长，而在纯DES中封端受到溶剂结构的限制。
