Electrochemically Shape-Controlled Synthesis Of Fe Nanoparticles, Their Structural Characterization And Properties

Catalytic performance of nanocrystals(NCs) can be finely tuned either by changing their composition,which mediates electronic structure,or by altering their shape,which determines their surface atomic arrangement and coordination.Therefore,the shape-controlled synthesis of nanocrystals presents an important way for tuning the activity,stability,and selectivity of nanocrystal catalysts.Although a variety of well-defined shapes of metal NCs with enhanced optical, electronic,and catalytic properties have been synthesized in the past decade,most of them are bounded by closest-packing facets because of the limit of crystal growth rule, which necessitates a minimization of the surface energy of the NCs.As the closestpacked facets are composed of atoms with high coordination number and accordingly have a low surface energy,they are stable but in general exhibit low catalytic activities for chemical reactions.The synthesis of NCs bounded by crystalline facets with an open structure,i.e.,with surface atoms of low coordination number and thus high surface energy,presents a promising direction in catalyst design and synthesis,although it is highly challengingAs the fourth most abundant element on earth.Fe NCs have been extensively investigated because of their wide applications.Especially Fe NCs are very important catalysts in denitrification,which is of very significant in today's society since the intensive use of fertilizers in agriculture and nitrates in some industries causes severe nitrate/nitrite pollution of water sources.In this thesis,we have developed an programed potential-step method to control the surface structure and growth of Fe nanocrystals,and prepared successfully nanocatalysts of Fe bounded by {110} or {100}planes.The study has demonstrated that the electrocatalytic activity of the Fe NCs is enhanced by increasing the fraction of {100} facets on the Fe NC surface.The main results are as following:1.Fe cuboid NCs supported on glassy carbon were prepared by electrochemical deposition under cyclic voltammetric(CV) and chronoamperometry(CA) conditions. The structure and composition of the Fe nanomaterials were characterized by scanning electron microscopy(SEM),selected area electron diffraction(SAED),X-ray diffraction (XRD) and energy dispersive X-ray analysis(EDX).The results demonstrated that the Fe cuboid nanoparticles are dispersed discretely on GC substrate,and the electrochemical synthesized nanocubes are single crystals of pure Fe.2.The size of the cube Fe NCs can be controlled by varying the growth time and the FeSO₄ concentrations in solution.3.The electrocatalytic activity of the synthesized Fe NCs was tested using nitrite reduction.The reduction current density(j) was normalized to the surface area of the nano-Fe/GC electrodes calibrated using a bulk Fe electrode,and used directly to compare the catalytic activities of different samples.4.Two series of Fe NCs enclosed by different crystalline facets were synthesized by means of the programed potential-step route.We demonstrated that the shape of the Fe NCs can be finely tuned systematically by varying the electrochemical conditions,i.e., the growth potential and the concentration of FeSO₄ in solution.For example,with the increase of deposition overpotential,the Fe NCs were tuned from rhombic dodecahedra or tetragonal bipyramids bounded by {110} facets to a series of 18-facets polyhedra enclosed by different combinations of {110} and {100} facets,and finally to cubes of {100} facets.This result is in agreement with the two-dimensional nuclei theory,which indicates that the rate of formation of two-dimensional nuclei of the type {hkl} is proportional to exp(-W_hkl/k_BT)(where W_hkl is the work of formation of the {hkl} nuclei, k_B is Boltzman's constant,and T is the absolute temperature).5.With the synthesized Fe NCs,the surface-structure functionality of the Fe NCs toward electrocatalytic reduction of nitrite was investigated.The result demonstrated clearly indicates that the electrocatalytic activity of the cubic Fe NCs with an opensurface {100} structure is much higher than that of the RD and TB Fe NCs enclosed with closest-packed {110} facets.The relationship between the surface structure of Fe NCs and their electrocatalytic activity toward nitrite reduction was analyzed by plotting the j value against the ratio of active surface atoms on an Fe NC(i.e.,R = N_active/N_total).The result illustrated clearly that j increases with R,demonstrating that the electrocatalytic activity of the Fe NCs is enhanced by increasing the fraction of {100} facets on the Fe NC surface.This structural dependence of Fe NC electrocatalytic activity was also confirmed by the steady catalytic activity,which was measured from the time-dependent current density of nitrite reduction at a fixed potential over a long reaction time.6.Fe octapods,dendritic and parallel intergrouth nanocrystals supported on glassy carbon were synthesized by means of the programed potential-step route.The surface structure and growth process of these Fe nanomaterials were characterized by SEM, SAED,HRTEM.The catalytic properties of the synthesized Fe NCs towards nitrite electroreduction were investigated,and enhanced electrocatalytic activity of the Fe octapods and dendritic NCs has been determined.By developing the programed potential-step method in this thesis to control surface structureand growth of metal nanocatalysts,we have prepared successfully two series of Fe NCs enclosed by different crystalline facets.The current study has enriched the contents of surface structure controlled growth of metal nanocrystals,and has deepened the understanding of the growth habits of metal nanocrystals.The study has illustrate an effective route to synthesis Fe nanocrystal catalysts with open surface structure,and made a significant progress in the design and preparation of practical metal nanocrystal catalysts.