nanoscale building blocks have attracted much interest due to their potential applications in replacement of traditional materials in the coming next few decades . however , most of the work in the synthesis of nanocrystals using solution - phase chemical method is focused on vi - ii semiconductor systems and noble metals . transition metal phosphides exhibit unique catalytic , optical , electronic , and magnetic properties depending on their phases . for example , fe3p is ferromagnet that has a high transition temperature ( tc = 716 k ) , while fep2 is a small - bandgap semiconductor with a tc of 15 k. some preparation methods have been developed for the interest of studying the size - property relations of nanoscale iron phosphides . for example , gu et al .   prepared 200 nm orthorhombic fep via solvothermal synthesis employing fecl3 and na3p at 180c . hu et al .   obtained a mixture of iron oxide nanoshells and hollow iron phosphide nanoparticles using sonichemistry method . prepared fep nanoparticles via a de - silylation route using a highly reactive phosphine source , p(sime3)3 . later , brock   found that trioctylphosphine ( top ) , which is a less - reactive reagent compared with p(sime3)3 , can be used as phosphine source as well as surfactant and solvent in the preparation of mnp and fep using metal carbonyls as metal source . liu et al .   synthesized fep nanorods by injecting fe(co)5/top into hot solution of top and trioctylphosphine oxide ( topo ) . hyeon et al .   studied the relation between the injection rate and length of fe2p nanorods synthesized using a syringe pump . fe2p nanorods can also be prepared without using a syringe pump as shown in the work of whitmire et al .   who developed a single - source organometallic method to control morphology by varying the ratio of tri - n - octylamine ( toa ) to oleic acid ( oa ) in a one - pot reaction . we recently reported the synthesis of ni2p nanowires in the system of toa / oa via an injection approach . in this work , we present a primary result on the synthesis of iron phosphide nanowires using a solution - phase chemical approach . instead of using toxic and unstable iron carbonyl compounds that were usually employed in most of previous work on the synthesis of iron phosphide nanowires , we developed a approach that uses a metal precursor of iron ( iii ) acetylacetonate ( fe(acac)3),which is low toxicity , inexpensive and rather stable at ambient condition . the structure and magnetic properties of as - synthesized nanowires are also presented in this work . in a typical synthesis , 0.5 mmol of fe(acac)3(96% , acros ) was added in 10 ml of oleylamine ( om , 8090% , acros ) and heated to 130c . the resulting solution was continuously injected into a stock solution of 5 ml of om , 5 mmol of top ( 97% , acros ) , and 2.5 mmol of topo ( 90% , acros ) at 340c using a syringe pump . the whole injection time varied from 3 to 6 h. a one - pot reaction was also attempted by reacting a mixture of 0.5 mmol of fe(acac)3 , 4 mmol of top , 2 mmol of topo , and 6 ml of om directly at 350c for 1 h. the obtained product precipitates were washed by hexane and acetone , and separated from solution by centrifugation . powder x - ray diffraction ( xrd ) data were collected on a panalytical xpert pro x - ray diffractometer using cu k radiation . transmission electron microscopy ( tem ) was performed on a tecnai f-30 transmission electron microscope . samples for tem analyses were prepared by sonicating the as - synthesized powders in hexane and dropping a small volume onto a carbon - coated copper tem grid followed by solvent evaporation . magnetic analysis was performed using a superconducting quantum interference device ( squid ) magnetometer ( mpms-5 ) . in a typical synthesis , 0.5 mmol of fe(acac)3(96% , acros ) was added in 10 ml of oleylamine ( om , 8090% , acros ) and heated to 130c . the resulting solution was continuously injected into a stock solution of 5 ml of om , 5 mmol of top ( 97% , acros ) , and 2.5 mmol of topo ( 90% , acros ) at 340c using a syringe pump . the whole injection time varied from 3 to 6 h. a one - pot reaction was also attempted by reacting a mixture of 0.5 mmol of fe(acac)3 , 4 mmol of top , 2 mmol of topo , and 6 ml of om directly at 350c for 1 h. the obtained product precipitates were washed by hexane and acetone , and separated from solution by centrifugation . powder x - ray diffraction ( xrd ) data were collected on a panalytical xpert pro x - ray diffractometer using cu k radiation . transmission electron microscopy ( tem ) was performed on a tecnai f-30 transmission electron microscope . samples for tem analyses were prepared by sonicating the as - synthesized powders in hexane and dropping a small volume onto a carbon - coated copper tem grid followed by solvent evaporation . magnetic analysis was performed using a superconducting quantum interference device ( squid ) magnetometer ( mpms-5 ) . figure 1a shows the low - resolution tem image of the iron phosphide wires prepared using injection method . the nanowires typically have a width of ~16 nm and a length of several hundred nanometers . the selected area diffraction ( saed ) pattern recorded from bundles of nanowires high - resolution tem ( hrtem ) images were taken on individual nanowires to observe their microstructures . the measured lattice fringes ( 0.344 nm ) that are perpendicular to the growth direction correspond well to the lattice spacing of ( 001 ) planes , indicating a growth direction of . park et al .   also reported a similar result for the growth direction of fe2p nanowires . typical xrd pattern of the as - prepared iron phosphide samples is shown in fig . the observed diffraction peaks also can be indexed with hexagonal fe2p structure , agreeing well with the saed analytic result . energy - dispersive x - ray spectroscopy ( eds ) analysis on individual nanowires ( fig . further quantitative analysis reveals a fe : p ratio of ~1.93:1 , which is close to the stoichiometric ratio of fe2p . normal tem image along with saed pattern ( a ) , hrtem image ( b ) , xrd pattern ( c ) and eds spectrum ( d ) of the as - prepared iron phosphide nanowires it can be inferred from the above results that the anisotropic shape of nanowires comes mainly from the intrinsically anisotropic nature of hexagonal crystalline structure . the monomer concentration of the injection route is relatively low , which favors the growth of existing seeds formed in the early stage of continuous delivery . another important prerequisite for the formation of anisotropic nanostructures is related to the surfactants used . usually , a multi - surfactant system favors the formation of an anisotropic nanostructure . in our case , the employed top and topo both can act as surfactants to cover different crystallographic surfaces of newly formed nanocrystal seeds , and eventually lead to the anisotropic growth toward c axis , forming a one - dimensional nanostructure . our studies found that if the experiments were conducted below 320c , the as - synthesized products were mainly iron oxide nanoparticles , which indicates that necessary thermal energy is needed to decompose top to form active phosphorus atoms to take part in the phosphide formation . for example , iron nanoparticles were obtained when top concentration was relatively low ( less than 2 mmol ) . however , when high concentration of top ( e.g. 12 mmol ) was used , the synthesized nanowires would have a curved morphology ( see fig . 2a ) , reflecting the insufficient capability of top for the protection of nanowires sidewalls . a small variation of top amount ( from 3 to 5 mmol ) did not make apparent difference in the products , showing the robustness of our synthetic approach . the employed surfactant topo also plays an important role in controlling the product s morphology . if no topo is added , the synthesized products mainly exhibit a nanoblock - like morphology resulting from the agglomeration of many soft - warped nanowires . this indicates that the existence of topo may function as a sidewall protector that can keep iron phosphide nanowires grow straightly over hundreds of nanometers . however , high concentration of topo on the contrary will reduce the quality of nanowires and lead to the formation of some big nanoblocks in the products . tem images of iron phosphide nanowires with a curved morphology synthesized using high concentration of top ( a ) , and products synthesized using one - pot reaction ( b ) the formation process of fe2p nanowires may be as follows . at first , the complex of fe(acac)3-om decomposes to iron clusters once added to hot solvent of om . then , these iron clusters will be attached to top molecules , and the p c bonds located at the particle surface may break and cause phosphorus to react with iron , forming iron phosphide . once fe2p seeds with an anisotropic structure forms , the growth along   direction will be favored due to the cooperation of anisotropic nature and the side - protection effect of surfactant molecules . 2b , the obtained products via one - pot reaction consist of nanowires plus nanosheets or nanoblocks . the shape singularity of the products from the one - pot reaction is not superior to the continuous - injection approach . considering the robustness of the reaction it is certain that continuous - injection route is crucial to keep the singularity of the products . a possible reason is that one - pot synthesis incurs a high concentration of nuclei , some of which are unable to develop into nanowires due to insufficient protections of surfactant molecules , whereas for the injection approach , the nuclei concentration at the beginning stage is restricted by the delivering rate , and the relatively low concentration of nuclei plus the continuous delivery of source materials will allow nuclei to fully grow into nanowires . the hysteresis loop of as - prepared fe2p nanowires measured at 5 k is shown in fig . it is clear that the magnetization curve is difficult to be saturated and the unsaturated magnetization moment is 45.5 emu / g ( 1.16 b ) , which is much lower than the saturation magnetization ( ms ) value of bulk fe2p ( 2.81 b ) . meanwhile , the measured coercive force is about 5,000 oe . a large coercive force has also been noticed in fe2p nanorod ( hc = 3.9 koe )   and nanoparticle samples ( ~5 koe ) . from the curve of fig . in fact , it looks like that the curve is a result of combination of two different loops . such a slightly twisted loop indicates that a small amount of impurities may exist in the synthesized products , although their amounts may beyond the detection limit of xrd . since squid is a characterization technology that is very sensitive to a trace amount of magnetic impurities , it can detect these impurities more easily . it is very likely that a small amount of fe or fe oxides constitute the impurities , since they were often found in the experiments performed at lower temperatures . such impurities were considered as the main reason accounting for the twisted hysteresis loops of fe2p nanorods . similar results have also been observed in fept nanoparticle samples , which also have a large coercivity . figure 3b shows the zero - field - cooled ( zfc ) and field - cooled ( fc ) curves of fe2p nanowires measured in a temperature range of 5300 k. a high blocking temperature ( ~275 k ) is observed , which is consistent with the hysteresis test at 300 k ( not included in this paper ) . a hysteresis loop measured at 5 k and b zfc / fc curves at an applied field of 100 oe for the as - prepared iron phosphide nanowires iron phosphide nanowires have been synthesized through a nonaqueous approach that utilizes fe(acac)3 as an iron source , top as a phosphorus source and oleylamine as solvent . the as - prepared nanowires have a hexagonal fe2p structure and grow along c axis . the dimension of nanowires can be tuned via reaction parameters such as injection rate , reaction temperature , and top / topo ratio . both injection method and one - pot reaction can produce iron phosphide nanowires , although the former can generate products with much better uniformity and yield . the magnetic tests reveal ferromagnetic characteristics below the blocking temperature , and a large coercive force of ~5,000 oe has been observed . the reported synthetic approach does not use commonly used toxic iron carbonyl compounds as iron precursor , which provides a convenient access to further studies of their special physical and chemical properties . this work was partially supported by the national natural science foundation of china ( grant nos . 50701036 and 50671087 ) and the national outstanding youth science foundation of china ( grant no . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
previous preparation of iron phosphide nanowires usually employed toxic and unstable iron carbonyl compounds as precursor . in this study , we demonstrate that iron phosphide nanowires can be synthesized via a facile nonaqueous chemical route that utilizes a commonly available iron precursor , iron ( iii ) acetylacetonate . in the synthesis , 
 trioctylphosphine ( top ) and trioctylphosphine oxide ( topo ) have been used as surfactants , and oleylamine has been used as solvent . 
 the crystalline structure and morphology of the as - synthesized products were characterized by powder x - ray diffraction ( xrd ) and transmission electron microscopy ( tem ) . 
 the obtained iron phosphide nanowires have a typical width of ~16 nm and a length of several hundred nanometers . 
 structural and compositional characterization reveals a hexagonal fe2p crystalline phase . 
 the morphology of as - synthesized products is greatly influenced by the ratio of top / topo . 
 the presence of topo has been found to be essential for the growth of high - quality iron phosphide nanowires . 
 magnetic measurements reveal ferromagnetic characteristics , and hysteresis behaviors below the blocking temperature have been observed .