Cu ( I ) Modification during γ-Fe 2 O 3 Nanoparticles Synthesis and Subsequent Characterization

During the synthesis of the γ-Fe 2 O 3 nanoparticles via the chemically induced transition method, Cu(I) modification has been attempted by adding CuCl/NaOH to the treatment solution. The experimental results showed that, under the condition of a NaOH content equal to 0.04 moles, when the content of CuCl is as low as 1.25 × 10−3 or 2.50 × 10−3 moles, the products are single γ-Fe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O composite nanoparticles, whereas when the content of CuCl is higher, 5 × 10−2 moles, the product is a mixture consisting of γ-Fe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O nanoparticles and Cu(II)(OH)Cl nanoparticles. For the γFe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O composite nanoparticles, the Cu(I)FeO 2 interface layer is not thick enough to form one unit cell, but it can modify the formation of a FeCl 3 ⋅6H 2 O surface layer and the effective magnetization of the γ-Fe 2 O 3 core.


Introduction
By definition, nanomaterials have one or more dimensions in the nanometer scale range (<100 nm) and consequently show novel properties when compared to bulk materials [1].A nanocomposite is a material composed of two or more phases.Nanoparticles are typically defined as solids measuring less than 100 nm in all the three dimensions; composite nanoparticles are generally coatings in which the combination of different physical and chemical properties may lead to completely novel materials with modified properties [1,2].Significant research effort has shown that the surface modification of the particles can be easily accomplished in postsynthesis steps or during the synthesis, thereby providing the nanoparticles with additional functionalities [3].Magnetic nanoparticles constitute an important class of functional materials and can be categorized, based on single or multiple materials, into simple and core/shellor composite-nanoparticles, which are gaining increasing interest because of their novel properties and the numerous applications in many diverse fields [3][4][5].Composite magnetic iron oxide nanoparticles have applications ranging from ferrofluids to separation science and technology [6][7][8].
Studies on nanoparticles have focused on the development of simple and effective methods for fabricating nanomaterials with controlled size and morphology and hence tailoring their properties [9].Liquid-phase synthesis is often used to prepare inorganic nanoparticles [10]; the conventional aqueous synthesis of the -Fe 2 O 3 particles involves three or more steps [11,12].We have proposed a method to synthesize -Fe 2 O 3 nanoparticles by thermally treating the FeOOH/Mg(OH) 2 precursor in FeCl 2 treating solution [13,14].Through this method, known as chemically induced transition (CIT) method, FeOOH species were transformed into -Fe 2 O 3 nanocrystallites by dehydration and Mg(OH) 2 was dissolved to assist the precipitation of the nanoparticles.Besides, the Fe 2+ ions in the FeCl 2 solution were oxidized to Fe 3+ to form a FeCl 3 ⋅6H 2 O coating on the -Fe 2 O 3 nanocrystallites [14].During the synthesis of the -Fe 2 O 3 nanoparticles, the surface modification was performed by adding a salt solution composed of metal ions with a valency of two, such as Zn(II)Cl 2 [15,16] and Co(II)(NO 3 ) 2 [17]

Preparation.
The preparation of Cu(I)-modified -Fe 2 O 3 nanoparticles by the so-called chemically induced transition method can be divided into two steps.First, the FeOOH/Mg(OH) 2 precursor was synthesized by coprecipitation, as described in detail elsewhere [13].Second, 5 g of the precursor was added to a boiling FeCl 2 treating solution (0.25 M, 400 mL) and kept boiling under reflux for 20 min.Then, both CuCl solution (50 mL), with a varying concentration, and NaOH solution (2 M, 20 mL) were added simultaneously to the solution, and the resulting mixture was boiled continuously for 10 min.After cooling naturally to room temperature, the products precipitated from the solution; subsequently, they were washed with acetone and allowed to dry.The concentrations of the CuCl solutions were 0.025, 0.050, and 1.000 M, corresponding to samples (1), (2), and (3), respectively.For comparison, unmodified particles were also prepared by adding the precursor to the FeCl 2 solution and boiling for 30 min, producing sample (0).

Characterization.
The bulk chemical compositions of the as-prepared products were obtained by energy dispersive X-ray spectroscopy (EDS, Quanta-200).The morphologies were observed by transmission electron microscopy (TEM, G20ST).The crystal properties were analyzed by highresolution TEM (HRTEM, JEM-2100F) and X-ray diffractometry (XRD, D/Max-RC).The surface chemical compositions were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi).The specific magnetization curves were measured at room temperature by vibrating sample magnetometry (VSM, HH-15).

Results and Discussion
3.1.Results.The EDS measurements revealed that samples (1), (2), and (3) contained not only O, Fe, and Cl and no Mg or Na-as sample (0)-but also Cu. Figure 1 shows the EDS spectra.The atomic percentages of the Fe, Cl, and Cu elements (  values), listed in Table 1, show that the amount of Cu increases from sample (1) to sample (3).
Typical TEM images are shown in Figure 2. All the samples are made of approximately spherical nanoparticles involving hexagonal particles.The size of the nanoparticles for both samples (1) and ( 2) is clearly larger than that of the unmodified sample (0).Additionally, sample (3) contained two kinds of particles, larger size and smaller size, as the arrows marked with "A" and "B," respectively, indicate.
The HRTEM revealed these particles to be small crystallites.Figure 3 is a typical HRTEM image of sample (2), which shows that the interlaced lattice fringes of two sets of planes have the same spacing of approximately 0.29 nm.As shown by the XRD spectra in Figure 4, the modified samples can be divided into two types, with samples (1) and ( 2 For samples (1), (2), and (3), the XPS results confirmed the presence of the same chemical elements as determined by EDS.The quantitative results for the Fe, Cl, and Cu elements are listed in Table 1. Figure 5 shows the O 1s, Fe 2p 3/2 , Cl 2p, and Cu 2p spectra for samples (1) to (3).Similar to the XRD results, the XPS spectra can also be divided into two categories and are analyzed as follows.
Notably, the collection depth of the signal in the EDS analysis largely exceeds the dimensions of the nanoparticles, whereas the signal collection depth in the XPS experiment is ∼3, where  = 1.24 nm and 1.51 nm for Fe 2p and Cu 2p electrons, respectively [23,24].After statistical analysis [25], the TEM results revealed that the size of the particles of both samples (1) and (2) followed a lognormal distribution similar to that of sample (0).Their median diameter   and standard deviation ln   are listed in Table 3.Consequently, the average size of the particles ⟨⟩ is calculated by the formula ⟨⟩ = exp[ln   + 0.5ln 2   ] [25], and the results show the size of samples ( 1) and ( 2) larger than 11 nm, as also listed in Table 3.Thus, for the nanoparticles in samples ( 1) and ( 2), the EDS results reflected the average ratio of the elements in the nanoparticles, whereas the XPS results reflected the ratio of the elements near the nanoparticle surface.Therefore, for the nanoparticles having a multiple layer structure, as shown in Figure 7, as the depth of the XPS detection is smaller than the radius of the particles (as ⟨⟩/2), the ratio of the elements in the internal layer to the elements in the outer layer, from XPS measurement, is lower than the average ratio in the total nanoparticle measured by EDS.For samples (1) and ( 2), the experimental results (see Table 1) show that the ratio of Cu to Cl from XPS measurements is far lower than that obtained from EDS measurements.Accordingly, for samples (1) and ( 2 where  Fe ,  Cu , and  Cl are the atomic percentages of Fe, Cu, and Cl, respectively, and  Cu is equal to zero for sample (0).
Thus, the molar percentages of every phase in samples (0), (1), and ( 2) can be obtained from the values of  Fe ,  Cu , and  Cl as measured by EDS (see Table 1).As a consequence, the mass percentages of these phases can be derived from where   is the molar percentage and   is the molar mass of the  phase.Accordingly, the mass percentages of each phase in samples (0), (1), and (2) were calculated from the values of   and the molar masses of -Fe 2 O 3 , Cu(I)FeO 2 , and FeCl 3 ⋅6H 2 O.The values of both   and   are listed in Table 4. ( For samples (1) and ( 2), the specific magnetization  can be described by where   ,  Cu-Fe , and  Fe-Cl are the specific magnetization of the -Fe 2 O 3 , Cu(I)FeO 2 , and FeCl 3 ⋅6H 2 O phase, respectively, and   /100 is the mass fraction of the  phase.As -Fe 2 O 3 and Cu(I)FeO 2 are ferrimagnetic and FeCl 3 ⋅6H 2 O is paramagnetic, the specific magnetization of both samples (1) and ( 2) results mainly from the mass percentages of the ferrimagnetic phase,   =   + Cu-Fe .From the results listed in Table 4,   = 95.40 for sample (1) and   = 95.52 for sample (2).Thus, the specific magnetization of sample ( 1) is slightly less than that of sample (2).In addition, sample (3) contained Cu(II)(OH)Cl particles, so the specific magnetization of sample ( 3) is lower than that of both samples (1) and ( 2).The magnetization (moment per unit volume)  is an important parameter used to characterize magnetic materials.For a particle system, the magnetization can be obtained generally from  =  ⋅ , where  is the specific magnetization (moment per unit mass) and  is the density of the material.For composite nanoparticles containing many phases with different densities, the density of the particles should be taken as the average density ⟨⟩, which can be derived from where   is volume percentage and   is density of the  phase.
For samples (1) and ( 2), the amount of Cu(I)FeO 2 listed in Table 5 is so small that the thickness of the Cu(I)FeO 2 in the -Fe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O nanoparticles is no more than one unit cell, and Cu(I) is not in the same state as in bulk Cu(I)FeO 2 ; this case is similar to that of -Fe 2 O 3 nanoparticles having CoFe 2 O 4 layer less thick than one unit cell [26].The presence of Cu(I)FeO 2 could modify the formation of FeCl 3 ⋅6H 2 O and make the FeCl 3 ⋅6H 2 O layer assume a three-dimensional (3D) oriented arrangement [27], so as to have a certain crystallized orientation relatively to the -Fe 2 O 3 crystallites.Therefore, both -Fe 2 O 3 and FeCl 3 ⋅6H 2 O are identified in the HRTEM stripe images showing the two sets of planes.Also, the saturation magnetization of both the modified samples (1) and ( 2) is higher than that of the unmodified sample (0), although the ferrite volume percentages of the formers are lower than that of the latter.It is known that the spins close to the surface to be pinned by surfactant molecules, which cause anomalously large magnetic anisotropy, would result in the less apparent saturation magnetization of nanoparticles than that of the bulk [28].Accordingly, it is judged that the Cu(I)FeO 2 thin layer in the -Fe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O nanoparticles may modify the magnetically silent "dead layer" [29], which existed at the interface between the -Fe 2 O 3 and FeCl 3 ⋅6H 2 O phases of the -Fe 2 O 3 /FeCl 3 ⋅6H 2 O nanoparticles of the unmodified sample (0) and did not provide any contribution to the effective magnetization.

Conclusions
When the FeOOH/Mg(OH) 2 precursor was thermally treated in FeCl 2 solution, the Mg(OH) 2 dissolved, FeOOH transformed into -Fe 2 O 3 nanocrystallites, and Fe 2+ in the FeCl 2 treating solution was simultaneously oxidized to Fe 3+ .The nanocrystallites absorbed Fe For the -Fe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O composite nanoparticles, the average thickness of the Cu(I)FeO 2 layer is not enough to form one unit cell, and Cu(I) is not in the same state in bulk Cu(I)FeO 2 .The experimental results show that the Cu(I)FeO 2 intermediate layer could modify the formation of the FeCl 3 ⋅6H 2 O layer to stimulate a 3D oriented attachment of the layer relatively to the -Fe 2 O 3 crystallites; it may also modify the magnetic "dead layer" between the -Fe 2 O 3 core and FeCl 3 ⋅6H 2 O surface layer to enhance the effective magnetization.Besides having high magnetization, such nanoparticles have an inert FeCl 3 ⋅6H 2 O surface; therefore they could possess a relatively good chemical stability and can be used directly to synthesize ionic ferrofluids without ferric nitric treatment as the Massart method [30].
Note: standard data for Fe 2 O 3 , CuFeO 2 , FeCl 3 , and H 2 O from the NIST X-ray Photoelectron Spectroscopy Database at http://www.nist.gov, in which no O 1s, Fe 2p 3/2 , and Cu 2p 1/2 data for CuFeO 2 and no Cu(OH)Cl data were provided.Standard data for CuCl 2 from [18].The binding energy of both O 1s and Fe 2p 3/2 for CuFe 2 O could not be determined in the present investigation.
energy data of sample (3) are listed in Table 2(b).As a comparison, the data of CuCl 2 are also listed in Table 2(b).

3. 2 .
Discussion.According to the experimental results, every sample modified by CuCl/NaOH contained same -Fe 2 O 3 phase as the unmodified sample (0).Also, the modified samples have Cu-containing compounds.This result indicates that the precursor first transformed into -Fe 2 O 3 crystallites in the FeCl 2 treating solution and, then, following the addition of CuCl/NaOH, the Cu-containing compounds were formed.Under an additional specific content of NaOH equal to 4 × 10 −2 moles (20 mL, 2 M), the composition of the asprepared products depended on the content of additional CuCl.For lower contents of CuCl-1.25 × 10 −3 moles (50 mL, 0.025 M) and 2.50×10 −3 moles (50 mL, 0.050 M), that is, samples (1) and (2), respectively-the Cu-containing compound is Cu(I)FeO 2 , whereas when the content of CuCl is higher-5.00× 10 −2 moles (50 mL, 1.000 M), that is, sample (3)-the product exhibits two different Cu-containing compounds: Cu(I)FeO 2 and Cu(II)(OH)Cl.These results reveal that, by adding CuCl/Na(OH) to the treating solution, the oxidation of the Cu + ions and the formation of the Cu-containing compounds could depend on the concentration of CuCl under a certain content of NaOH.At low CuCl concentration, all the Cu + ions formed Cu(I)FeO 2 and no Cu(II) compound was created, whereas, at higher CuCl concentration, part of the Cu + ions formed Cu(I)FeO 2 , and the remaining Cu + ions were oxidized to Cu 2+ ions to produce Cu(II)(OH)Cl.The formation of Cu(OH)Cl rather than CuFe 2 O 4 could be due to a reason similar to that behind the formation of Cu 2 (OH) 3 NO 3 , where Cu 2+ does not coprecipitate with Fe 3+ as the pH of the reaction is too low[22].

For
the unmodified sample (0), it is known that FeCl 3 ⋅6H 2 O forms on the -Fe 2 O 3 crystallites [14].XPS results revealed the presence of FeCl 3 ⋅6H 2 O in the modified samples (1), (2), and (3).For both samples (1) and (2), based on the -Fe 2 O 3 , Cu(I)FeO 2 , and FeCl 3 ⋅6H 2 O phases, the nanoparticles contained -Fe 2 O 3 and FeCl 3 ⋅6H 2 O, as confirmed by the lattice fringes of the two sets of planes observed in the HRTEM images, which precisely correspond to the (220) plane of -Fe 2 O 3 (spacing of 0.2953 nm) and (002) plane of FeCl 3 ⋅6H 2 O (spacing of 0.2927 nm), respectively.The Cu(I)FeO 2 could be an intermediate layer formed between the -Fe 2 O 3 core and the FeCl 3 ⋅6H 2 O surface layer, as the ratio of Cu to Cl obtained from the XPS measurement is lower than that obtained from the EDS measurement.As a consequence, a schematic model of the particle structure of both samples (1) and ( ), the Cu(I)FeO 2 grows between the -Fe 2 O 3 core and the FeCl 3 ⋅6H 2 O surface layer to form -Fe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O composite nanoparticles.Furthermore, sample (3) exhibited strong diffraction peaks of Cu(II)(OH)Cl in the XRD spectra.Therefore, sample (3) is a mixture of -Fe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O nanoparticles and Cu(II)(OH)Cl nanoparticles, which may correspond to the larger and smaller particles, respectively, observed in the TEM image.

Figure 7 :
Figure 7: Schematic diagram of the nanoparticles structures of samples (1) and (2);   indicates the depth detected by XPS;  Cl is the thickness of FeCl 3 ⋅6H 2 O;  Cu is the thickness of Cu(I)FeO 2 .
3+ and Cl − to form -Fe 2 O 3 /FeCl 3 ⋅6H 2 O nanoparticles, in which the -Fe 2 O 3 core was coated with the FeCl 3 ⋅6H 2 O layer.By adding Cu(I)Cl/NaOH to the FeCl 2 solution during the synthesis, the compositions of the as-prepared products can be modified.For a certain content of NaOH, 0.04 moles, using a low content of Cu(I)Cl (1.25 × 10 −3 moles or 2.50 × 10 −3 moles), single Cu(I) modified composite nanoparticles can be prepared.The structure of such composite nanoparticles can be described as -Fe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O and consisted of three parts as follows: -Fe 2 O 3 core, a Cu(I)FeO 2 intermediate layer, and an outermost FeCl 3 ⋅6H 2 O layer.For a higher content of CuCl, 5 × 10 −2 moles, Cu + was partially oxidized to Cu 2+ , and the as-prepared product was a mixture of -Fe 2 O 3 /Cu(I)FeO 2 /FeCl 3 ⋅6H 2 O nanoparticles and Cu(II)(OH)Cl nanoparticles.The Cl and Cu spectra of Cu(OH)Cl measured by XPS have the same structure as those of CuCl 2 , and the binding energies of Cl 2p 3/2 , Cl 2p 1/2 , Cu 2p 3/2 , and Cu 2p 1/2 for the Cu(OH)Cl compound are approximately 198.6, 200.1, 934.6, and 954.5 eV, respectively.
, to fabricate Zn(II)Fe 2 O 4 and Co(II)Fe 2 O 4 epitaxial layers, respectively, on the -Fe 2 O 3 crystallites.Additional NaOH can enhance these modifications to produce more ZnFe 2 O 4 or CoFe 2 O 4 .In this work, a surface modification