Characteristics of Magnetic Mesoparticles Fabricated by Electroless

Apr 5, 2008 - ... a superconducting quantum interference device (SQUID), and a vibrating sample magnetometer (VSM) were employed to characterize these...
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Ind. Eng. Chem. Res. 2008, 47, 3021-3029

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Characteristics of Magnetic Mesoparticles Fabricated by Electroless Nickel Deposition Ming-Kai Chang, Chun-Han Chen, and Bing-Hung Chen* Department of Chemical Engineering, National Cheng Kung UniVersity, 1 UniVersity Road, Tainan 70101, Taiwan

A low-cost wet chemical process, viz., the electroless nickel deposition, was successfully developed to produce magnetic nickel-tungsten-phosphorus (NiWP) mesoparticles with diameters ranging from 100 to 500 nm on silica templates of ca. 102 nm and gold templates of ca. 3 nm. An X-ray diffractometer (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDX), a superconducting quantum interference device (SQUID), and a vibrating sample magnetometer (VSM) were employed to characterize these particles. The averaged sizes of the NiWP particles could be conveniently regulated with the duration of the electroless plating process, in which exponential growth kinetics of the NiWP particles was observed. Both XRD and electrodiffraction (ED) patterns of these NiWP alloys indicate existence of diffraction peaks close to the (111), (200), (220), and (311) reflections of bulk face-centered cubic (fcc) Ni. The zero-field-cooled (ZFC) and field-cooled (FC) measurements also indicate the irreversibility temperatures of magnetization of these particles are in between 300 and 380 K. Dependent on the plating baths, the Curie temperatures of the resulting NiWP particles are around 330 K and higher than 380 K. Hence, weak ferromagnetic behaviors were evidently observed on all of these NiWP particles. Their saturation magnetization and coercivity were found in the range from 6 to 28 emu/g of sample and from 26 to 50 Oe, respectively. 1. Introduction In recent years, magnetic nanoparticles have attracted considerable attention because of their unique characteristics and potential applications in electronic devices, biomedical devices, such as enhanced contrast agents for magnetic resonance imaging (MRI) and magnetic delivery vehicles for drugs and genes, catalysts and extractants in chemical processes, environmental remediation, and decontamination, etc.1-9 These nanoparticles often exhibit very interesting physicochemical properties that are quite different from those possessed by their bulk materials.8-13 For instance, their magnetic properties vary,3,10,14 and their surface becomes so chemically active, even so on those materials originally having inert nature in bulk forms, to be quite vulnerable to various environmental factors.8-11 Hence, these materials become more susceptible to damage and degradation triggered by environmental factors. 8-11 As a result, passivation of materials, such as with encapsulation by carbon, is often invoked to protect these magnetic nanoparticles.15,16 There are various types of magnetic materials having significant magnetism,14,17 which include not only metal-based materials (Fe, Ni, Co, Gd, etc.17) but also organic compounds, such as families of p-nitrophenyl nitronyl nitroxide and vanadium tetracyanoethylene.14 Still, at room temperature, only metal-based magnets retain high magnetism.14,17 Thus far, as found in the open literature, the most popular nanomagnets are the iron-based alloys, followed by the nickel-based materials. To fabricate magnetic nanoparticles for applications at room temperature, various physical and chemical synthesis methods have been employed.8 Among these methods, chemical synthesis in the liquid phase, such as the direct reduction of metal ions by strong reductants like NaBH4, could provide a relatively economic scheme to produce magnetic particles in nanoscale.8 * To whom correspondence should be addressed. Tel.: +886-6-2757575ext.62695.Fax: +886-6-234-4496.E-mail: [email protected].

However, such a direct reduction of metal ions in liquids often gives a wide size distribution of resulting particles and could not grow particles to a few tens of nanometers. That is, obtaining nanoparticles and mesoparticles with the desired size distribution is not easily attainable by direct chemical reduction of metal ions in the liquid phase. Instead, a technique similar to direct reduction of metal ions, called electroless plating (EP), which with great simplicity and capability in mass production has been commonly utilized in industries to deposit protective layers or conductive wires on various subjects of interest, was introduced as an alternative to fabricate magnetic nickel mesoparticles with diameters ranging from 100 to 300 nm.18 In this work, outcomes from a further study of this technique on size tunability and crystal structures of produced particles as well as their magnetic characteristics are reported. The EP process could proceed on any object with proper pretreatments, regardless of its size and geometry.19 However, one intriguing characteristic of the EP process is that it only takes place on a conductive surface because electrons required for metal reduction could only be supplied from oxidation of reducing agents only absorbed on this conductive surface.20,21 Alternatively speaking, if the deposition of the very first metal layer is initiated in the EP process, the subsequent metal deposition will be undergone automatically on the surface of the as-deposited metal front until depletion of any active constituents in the plating baths.19-21 Hence, the EP process is often referred to as a self-catalytic process. The well-known silver-mirror reaction is just one of the typical EP examples. Briefly speaking, the EP process is a surface reaction, in contrast to the bulk reaction of direct reduction of metal ions. The electroless nickel-phosphorus (NiP) deposits, especially in amorphous form, have been known for their excellent properties of anticorrosion, wear and thermal resistance, and hardness.19 Namely, magnetic particles made by such electroless nickel plating (ENP) processes may not require any protective

10.1021/ie071030e CCC: $40.75 © 2008 American Chemical Society Published on Web 04/05/2008

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coatings to prevent themselves from degradation and attacks by environmental factors during applications.18,19 Although nickel is well-known for its ferromagnetic behavior, a content of phosphorus, as an unavoidable byproduct and codeposit in ENP processes, with more than 7 to ∼8 wt % in the NiP deposits weakens the magnetism of the alloys and turns them into having paramagnetic behavior.18 A remedy to retain the strong magnetism is the introduction of tungsten codeposited into the alloys, resulting from reduction of tungstate stabilizers in the plating bath.18 The resulting nickel-tungsten-phosphorus (NiWP) alloys have been shown as soft ferromagnetic materials and possess good magnetic properties.18 In this report, the manufacture methodology of these NiWP particles along with their magnetic properties and structures are summarized. 2. Experimental Section 2.1. Reagents and Apparatus. NiSO4‚6H2O, NaH2PO2‚H2O, and sodium citrate dehydrate were purchased from Riedel-de Hae¨n. 3-Aminopropyltrimethoxysilane, (97%, APTMS) and tetraethoxysilane (98%, TEOS) were acquired from Fluka. NH4OH (28.0% to ∼30.0%) and absolute alcohol were obtained from J. T. Baker. Na2WO4‚2H2O and HAuCl4 were procured from Alfa Aesar. Tetrakis(hydroxymethyl)-phosphonium chloride solution (80%, THPC) was supplied from Aldrich. Deionized water from a Milli-Q purification system (Millipore, U.S.A.) having resistivity greater than 18.2 MΩ‚cm was used in sample preparation. All chemicals were of reagent grade and used as received without further purification. Various instruments were employed to characterize the NiWP particles and deposits. These include a JEOL JSM-6700F scanning electron microscope (SEM) equipped with an energydispersive X-ray spectrometer (EDX) (Oxford Inca 400), a Hitachi HF-2000 transmission electron microscope (TEM), a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, model MPMS7), a vibrating sample magnetometer (VSM) from Princeton Measurements (Princeton, NJ), and a Rigaku RINT-2000 X-ray diffractometer (XRD) with Cu KR1 radiation, λ ) 1.5406 Å, as the incident X-ray source. No thermal treatments were attempted on the NiWP nanoparticles and film prior to any measurements by instruments in this work. 2.2. Electroless Deposition of NiWP Particles and Film. Preparations of silica and gold templates as well as the deposition method of NiWP alloys onto these substrates were reported earlier.18 For the sake of keeping this report as concise as possible, the experimental methodology is briefly described as follows. A conductive surface is always required in the electroless deposition method for depositing the very first layer of alloys, on which the subsequent deposition process could take place continuously. Hence, in this work the dielectric silica nanoparticles of ca. 102 nm diameter, synthesized according to the Sto¨ber method,22 were coated with gold seeds of ca. 3 nm, reduced from HAuCl4 solution using the method reported by Duff et al.,23 prior to the electroless deposition process (Figure 1). For comparison, electroless NiWP alloys were also grown on the gold seeds and brass substrates (20 × 10 × 0.17 mm3) as well. The onset of the ENP process began right after the silica or gold templates were submerged into the plating baths at the desired temperatures (Table 1). It is of note that the formula of the one noted with the lower-magnetism (LM) bath has been reported earlier.18 Consequently, the electroless NiWP nanoparticles or films were fabricated on these substrates, of which

Figure 1. TEM micrograph of silica seeds coated with gold nanoparticles, shown as little dots, for subsequent electroless nickel plating (ENP).

Table 1. Composition of Electroless NiWP Plating Baths chemicals

lower-magnetism (LM) bath

higher-magnetism (HM) bath

NiSO4‚6H2O Na2WO4‚2H2O NaH2PO2‚H2O sodium citrate water temperature

26 g/L 25 g/L 19 g/L 44 g/L add up to 1 liter 70 °C

26 g/L 25 g/L 19 g/L 28 g/L add up to 1 liter 70 °C

NiWP nanoparticles and film are denoted as NiWP/silica, NiWP/ Au, and NiWP film, respectively, according to the substrates used. Moreover, the sizes of these electroless NiWP nanoparticles could be regulated with durations of the ENP process. In addition, these NiWP particles were easily separated and collected with magnets or by centrifugation. 3. Results and Discussion The silica template with an average diameter near 102 nm used in this work is shown as Figure 1. The little dots on the templates are gold nanoparticles of ca. 3 nm attached to the silica surface, which work as the conducting layer and provide a catalytic surface for subsequent electroless nickel deposition. Two different plating baths studied in this work produce NiWP particles possessing different magnetic properties and growth rates. Briefly speaking, the main difference between these two baths is only reflected in the content of sodium citrate (Table 1). To deposit the desired metal on the objects without supply of external current, the EP process involves several simultaneous and complicated redox reactions, mainly oxidation of reducing agents and simultaneous reduction of metal ions. For example, the ENP requires adsorption of active species and electron transfer on the just-resulted nickel coatings.19-21 Once the nickel alloys are plated, the deposition process continues to take place on the newly plated NiWP fronts. In this work, it is believed that the NiWP alloys were first grown around the gold seeds on the silica templates and, then, spread out progressively to cover all the surface of the silica templates. At ambient temperature, nickel is well-known for its ferromagnetic property. The Curie temperature and saturation magnetization of pure bulk nickel are 627 K and 54.5 emu/

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g.17,25 In contrast, tungsten and phosphorus are paramagnetic and diamagnetic, respectively. The existence of the phosphorus in the obtained nickel alloys could greatly change the magnetic properties of the alloys, for example, from ferromagnetics to weak paramagnetics.18 Consequently, it is pivotal to control the phosphorus content in the electroless nickel alloys, should the magnetic nickel alloys be desired. However, the presence of the unwanted phosphorus, a byproduct in the ENP process, is unavoidable. The overall reactions of the ENP process in the common acidic type of EN plating bath, as employed in this work, could be expressed by the following two reactions that proceed simultaneously:19 plating front

Ni2+ + 2H2PO2- + 2H2O 98 Ni + 2H2PO3- + 2H+ + H2 (1) plating front

3H2PO2- + 2H+ 98 2P + H2PO3- + 3H2O (2) Importantly, the ENP process only occurs on the conductive surface, for instance, the newly deposited NiWP fronts, and differs from normal chemical reduction of metal ions in an aqueous solution where metal blacks are produced everywhere. In general, trace amounts of stabilizers, such as Pb2+, Sn2+, or tungstate anions, must be added in the ENP baths to prevent the baths from collapse. However, stabilizers in the plating baths are often reduced along with nickel ions and affect the phosphorous contents in the resulting electroless nickel alloys,21 which may alter the magnetic properties of these electroless nickel deposits.18 NiWP mesoparticles were successfully fabricated with a plating duration of 4 min. These NiWP particles differ significantly in size with plating baths formulated differently. For example, the averaged size of the NiWP/silica particles made from the higher-magnetism (HM) bath is around 248 nm, in contrast to that of ca. 392 nm plated from the LM bath. Likewise, the averaged diameter of the NiWP/Au nanoparticles harvested from the HM bath is ca. 130 nm, in contrast to that of 170 nm obtained from the LM bath. It is interesting to observe that the ENP process occurred so rapidly on gold templates after a certain induction period, which probably ascribes to the very large specific surface area, available to the ENP process, of the very small gold seeds. In this work, the NiWP/Au nanoparticles appeared suddenly in both plating baths after an induction period of ca. 3 min from the onset of the ENP process. Hence, it is almost impossible to have a good control on the sizes of the NiWP/Au nanoparticles by adjustment in the plating duration. As aforementioned, the main difference between HM and LM baths is only the content of sodium citrate, namely, the major complexing agent in the plating baths. Surprisingly, not only the magnetic properties, but also the deposition rates and compositions of the resulting NiWP alloys, are all affected. For example, the plating duration to deposit NiWP films from both plating baths onto brass substrates was set equally at 1 h. The thickness of the consequent NiWP film plated from the HM bath is 11.8 µm, contrary to 4.6 µm for the NiWP film deposited from the LM bath. That is, the hourly averaged deposition rate from the LM bath is only about 40% of that from the HM bath. Still, both NiWP films possess a shiningly bright appearance. The compositions of the electroless NiWP deposits obtained from both plating baths are tabulated in Table 2. The nickel contents of NiWP particles are slightly less than those in the NiWP films. Interestingly, the nickel contents in all NiWP mesoparticles produced from both plating baths appeared quite close to ca. 90 wt %. That is, the collective contents of

Table 2. Composition of Electroless NiWP Deposits by EDX Analysis lower-magnetism (LM) bath

higher-magnetism (HM) bath

Ni (wt %) W (wt %) P (wt %) Ni (wt %) W (wt %) P (wt %) NiWP film 94.9 ( 0.1 3.0 ( 0.5 2.1 ( 0.5 96.2 ( 0.1 2.5 ( 0.3 1.3 ( 0.3 NiWP/silica 89.7 ( 1.8a 5.1 ( 1.6a 5.2 ( 0.8a 91.1 ( 0.8 6.4 ( 0.7 2.5 ( 0.3 NiWP/Au 87.6 ( 1.6a 5.8 ( 1.5a 6.6 ( 0.3a 90.0 ( 1.6 6.2 ( 0.7 3.6 ( 0.4 a

Data have been reported in ref 18.

phosphorus and tungsten in all NiWP mesoparticles deposited from both plating baths are nearly the same at around 10 wt %. The fact of the replacement of phosphorus in NiWP alloys by tungsten implies that the tungstate anions not only play a role of stabilizing the plating bath but also compete for electrons against the side reduction reaction of hypophosphite anions, which is also consistent with our previous observation.18 The tungsten contents in the NiWP mesoparticles fabricated from the HM bath are around 6.3 wt %, slightly higher than those around 5.5 wt % on the NiWP particles plated from the LM bath. However, the phosphorus contents differ notably in NiWP mesoparticles deposited from both plating baths. The NiWP particles produced from the HM bath contain only about 3 wt % phosphorus, only a half of those plated from the LM bath. In addition, Chang et al.18 previously pointed out the magnetic properties of electroless nickel particles were greatly influenced by their phosphorous contents. It is, thus, desirable to have tungsten anions as stabilizers in the plating bath, if the ferromagnetism of the resulting NiWP particles is retained. A TEM micrograph exhibits a chainlike aggregate behavior on NiWP/silica nanoparticles deposited from the HM bath (Figure 2a). Their electron diffraction pattern, shown as Figure 2b, implies that there exists a face-centered cubic (fcc) structure on these particles, which is also confirmed with the XRD data (Figure 3). Four diffraction peaks appear in the vicinity at 2θ = 44.4°, 52°, 76.3°, and 92.8° for both NiWP/silica and NiWP/ Au nanoparticles plated from the HM bath. These four peaks are, coincidently, close to diffraction peaks, i.e., (111), (200), (220), and (311), of bulk nickel metal with an fcc crystal structure. Additionally, these characteristic peaks also appeared in the XRD patterns of NiWP/silica, NiWP/Au, and NiWP film deposited from the LM bath.18 Furthermore, these diffraction peaks of our NiWP nanoparticles are consistent with those of NiWP films reported by Valova et al.24 Magnetic properties of these NiWP nanoparticles and films were measured with a SQUID at 300 K. Figure 4 shows the reversible magnetization curves of NiWP mesoparticles and film deposited from the HM bath. Hysteresis loops observed on these magnetization curves clearly denote ferromagnetic characteristics of these NiWP deposits. Their magnetic properties, such as saturation magnetization MS, could be obtained from fitting the curves of the measured magnetization M, expressed as a function applied magnetic field H, with a function made up of a ferromagnetic (FM) and a paramagnetic (PM) part:26,27

M(H) )

[

( )]

2MS -1 H ( HC πMR tan tan π HC 2MS

+ χH

(3)

where HC and MR stand for the coercivity and the remnant magnetization of the hysteresis loop, respectively; χ is the magnetic susceptibility. Moreover, the first and the second terms in eq 3 account for the FM hysteresis loop and the PM components at high magnetic fields, respectively. Reasonably good fits to magnetization curves have been obtained (Figure 4b).

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Figure 2. TEM micrograph (a) and electron diffraction pattern (b) of NiWP/silica nanoparticles prepared from the HM bath.

Figure 3. X-ray diffraction (XRD) data of NiWP/silica and NiWP/Au nanoparticles deposited from the HM bath.

The magnetic characteristics of these NiWP deposits are tabulated in Table 3, along with those plated from the LM bath. The saturation magnetization of NiWP film, NiWP/silica, and NiWP/Au produced from the HM bath are 26.77, 19.54, and 14.52 emu/g of sample, respectively, which are comparable to the saturation magnetization of pure bulk nickel material near 54.5 emu/g.25 Their corresponding coercivity and remnant magnetization are 95.9 Oe and 2.35 emu/g of sample for NiWP film, 32.8 Oe and 2.34 emu/g of sample for NiWP/silica, as well as 49.6 Oe and 2.94 emu/g of sample for NiWP/Au, respectively. These values are significantly greater than the corresponding values for the NiWP deposits fabricated from the LM bath (Table 3). Moreover, values listed in Table 3 are comparable to those of thin nickel films. Miller et al.28 studied the magnetic properties of thin nickel films with thicknesses ranging from 30 to 150 nm. They reported film coercivities and saturation magnetization in the ranges from 2 to 290 Oe and from 31.4 to 56.1 emu/g, depending on the fabrication conditions of these films.28

The thermal dependence of the magnetization characteristics of these NiWP nanoparticles deposited from both HM and LM baths were further investigated with a SQUID under an applied field of 100 Oe between 4 and 380 K. The sample was initially cooled in a zero field to 4 K. An applied field of 100 Oe was employed, and the magnetization was recorded as a function of increasing temperature to 380 K, of which the curve is called zero-field-cooled (ZFC). Likewise, the field-cooled (FC) magnetization was measured by progressively cooling the sample again down to 4 K in the presence of 100 Oe. At a given temperature, called the blocking temperature TB, a maximum in the ZFC curve is observed. Also, ZFC and FC magnetization curves may bifurcate at a different temperature, often called the irreversibility temperature TIRV (Figure 5). This field-dependent TIRV marks the metastable nature of magnetization and is often exhibited by various magnetically disordered systems.27,29 For magnetic nanoparticles of a particle volume, TB almost coincides with TIRV.14,27,30 The difference between TB and TIRV is mainly attributed to the wide size distribution of these magnetic particles and relative orientation of magnetic particles having different TB.14,27,30 At a temperature below TB, these nanomagnets are blocked in a ferromagnetic state with an irreversible magnetization and behave as spin-glass-like materials.29 In addition, another important indicator on thermal magnetic property, called the Curie temperature or the Curie point TC, separates the disordered PM phase of a ferromagnet at T < TC from its ordered FM phase at T > TC.14,17,29 Typically, the Curie temperature is higher than TB and TIRV. Explicitly, as the temperature of a ferromagnetic system is lowered, the sequence of magnetic phase transition goes from a PM state to an FM phase at TC, subsequently from an FM state to a re-entrant state at TIRV, in which ferromagnetic order coexists with cluster spinglass order, and then, at TB, to a state in which even the smallest spin-glass cluster is frozen.29 Figure 5a indicates the existence of high TIRV at 335 K and at a temperature even higher than 380 K, respectively, for NiWP/ silica and NiWP/Au plated from HM baths. For these two samples, TB is 120 and 245 K. Their Curie temperatures must be higher than 380 K, as their saturation magnetizations have

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Figure 4. Reversible magnetization curves for various NiWP particles and deposits plated from the HM bath measured by SQUID at T ) 300 K: (a) various NiWP deposits; (b) magnetization curve of NiWP/silica particles fitted with eq 3. Table 3. Coercivity HC, Remnant Magnetization MR, and Saturation Magnetism MS of Electroless NiWP Deposits higher-magnetism (HM) bath lower-magnetism (LM) bath HC (Oe) NiWP film 95.9 NiWP/silica 32.8 NiWP/Au 49.6

MR (emu/g of sample) 2.35 2.34 2.94

MS MS MR (emu/g of HC (emu/g of (emu/g of sample) (Oe) sample) sample) 26.77 19.54 14.52

12.7 27.4 26.1

0.02 0.46 0.60

7.72 5.53 4.88

not diminished to zero yet at 380 K. Likewise, TIRV and TB for NiWP/silica and NiWP/Au particles deposited from the LM bath

are 315 and 195 K, as well as 295 and 275 K, respectively (Figure 5b). Interestingly, their Curie temperatures are lowered to around 330 K, which is significantly lower than that of pure bulk nickel at 627 K. It is clear that presence of diamagnetic phosphorus in the electroless nickel alloys significantly suppresses the Curie temperatures, the saturation magnetization, and the coercivities of the alloys. Controlled growth of the NiWP alloys on silica and gold templates was attempted mainly by adjusting the plating time of the ENP processes. Figure 6 displays the effect of the plating duration on the averaged sizes of the NiWP particles grown on

Figure 5. Magnetization curves as a function of temperature for various NiWP particles and deposits measured by SQUID with an applied magnetic field equal to 100 Oe: (a) NiWP deposited from the HM bath; (b) NiWP deposited from the LM bath.

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Figure 6. Effect of deposition times on the sizes of the NiWP/silica nanoparticles.

For NiWP/silica particles plated from the HM bath:

DHM ) 86.9 + 432.1(1 - e-0.146t)

(4)

For NiWP/silica particles deposited from the LM bath:

DLM ) 101.6 + 206.2(1 - e-0.491t) + 216.6(1 - e-0.155t) (5)

Figure 7. SEM micrographs of NiWP/silica nanoparticles harvested from the HM bath with various deposition times.

silica templates, whereas the SEM micrographs of the harvested NiWP/silica particles from the HM bath are exhibited as Figure 7. Interesting chainlike patterns, similar to those shown in Figure 2, were observed again on these NiWP particles. The growth curves of the NiWP/silica particles were fit well with an exponential growth model (Figure 6). The predicted timedependent particle sizes of these NiWP/silica particles could be expressed as follows.

where D indicates the averaged particle size (in nm) and t stands for plating time (in min). Apparently, both empirical equations imply that the growth kinetics of the NiWP particles could be suitably modeled with a first-order mass-transfer model process. Furthermore, this exponential time dependence observed in the growth of these NiWP particles could be ascribed to the limited flux of reactive species provided by surface diffusion over the plated front.31 It is of note to mention that, in eq 5, two exponential functions, instead of a single one, were used to obtain the best-fit of the predicted time-dependent sizes of NiWP particles deposited from the LM bath. The reason is that the initial average size of silica particles, i.e., that corresponding to the value at t ) 0, predicted with one exponential trial function, is far from the value of initial average size of silica templates used in this work. Instead, with two exponential trial functions it gives 101.6 nm, i.e., obtained from eq 5 with t ) 0, which is coincidentally close to the initial average diameter of silica particles used in this work near 102 nm. Interestingly, both plating baths would give an ultimate size of NiWP particles near 520 nm. However, the growth rates of the NiWP particles in both plating baths differ. For example, the initial growth rate of the NiWP/silica particles deposited in the LM bath is larger than that in the HM bath only during the first 3.5 min of plating processes. Afterward, the HM bath renders a higher deposition rate, as high as 3 times that of the LM bath. As aforementioned, in planar configuration, the averaged deposition rate of NiWP film, estimated over 1 h of plating process, was larger in the HM bath than in the LM bath. More importantly, these empirical models of growth curves enable us with great convenience to predict the deposition time needed for the desired sized of the NiWP particles.

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Figure 8. Magnetization curves and hysteresis loop of NiWP nanoparticles measured by VSM at T ) 300 K: (a) prepared from the HM bath with various deposition times; (b) prepared from the LM bath with various deposition times.

The effects of different plating durations on magnetic properties of these NiWP particles were also investigated with a VSM at 300 K (Figure 8). The saturation magnetization of these particles increases with a longer plating duration, which also results in larger particles. For example, an increase in saturation magnetization from 5 to 26 emu/g of sample was found for the NiWP/silica particles deposited in the HM bath with plating duration increased from 2 to 12 min. In the HM bath, a plating duration of 8 to ∼10 min seems sufficient to produce NiWP particles with saturation magnetization comparable to their ultimate values adopted from those found in the NiWP films. On the contrary, the NiWP particles in the LM

bath grow large enough in about 2 min to have saturation magnetizations comparable to those of their film counterpart. Noteworthily, ferromagnetic hysteresis loops are clearly observed in all magnetization curves of these samples plated from HM and LM baths. As aforementioned, phosphorus present in electroless nickel alloys obviously deteriorates the ferromagnetic properties of nickel alloys. Moreover, tungsten and phosphorus appear in the electroless alloys as byproducts from two simultaneously competing but unwanted reduction reactions, i.e., the reduction of hypophosphite anions (reducing agent) and tungstate anions (stabilizer). To keep hold of good ferromagnetic properties of

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nickel alloys, such as a higher saturation magnetization, coercivity, and a suitable Curie temperature, a strategy on proper replacement of phosphorus with tungsten might be desirably developed. 4. Conclusions In this work, magnetic NiWP particles with tunable sizes from 100 to 500 nm were successfully fabricated with a low-cost and wet chemical process, namely, the EP technique. Exponential time dependence was observed on the size of the NiWP particles, which indicates the growth kinetics of these particles are regulated by the surface diffusion transport of reactive species to the plated fronts. The nickel contents of all NiWP particles deposited from both plating baths are about 90 wt %, whereas the main difference in composition is reflected in the collective contents of phosphorus and tungsten in alloys. The XRD and electron diffraction patterns indicate fcc as the preferred crystalline structure on these NiWP particles. The irreversibility temperature of magnetization of the obtained NiWP particles ranges from ca. 300 K to a temperature higher than 380 K. Weak ferromagnetic behaviors were observed on these NiWP particles. The saturation magnetizations of these NiWP particles ranging from ca. 5 to 27 emu/g of sample, only 10-50% of the value of 54.5 emu/g of sample possessed by bulk nickel metals, were obtained, whereas the coercivities of these NiWP particles were from 26 to 50 Oe. To retain a higher coercivity as well as the remnant and saturation magnetization, the development of a strategy on replacement of phosphorus with the tungsten is desired. Acknowledgment This study was financially supported by the National Science Council of Taiwan. Literature Cited (1) Bromberg, L.; Hatton, T. A. Decomposition of Toxic Environmental Contaminants by Recyclable Catalytic, Superparamagnetic Nanoparticles. Ind. Eng. Chem. Res. 2007, 46, 3622. (2) Xu, J.; Bhattacharyya, D. Fe/Pd Nanoparticle Immobilization in Microfiltration Membrane Pores: Synthesis, Characterization, and Application in the Dechlorination of Polychlorinated Biphenyls. Ind. Eng. Chem. Res. 2007, 46, 2348. (3) Sun, S. H. Recent Advances in Chemical Synthesis, Self-Assembly, and Applications of FePt Nanoparticles. AdV. Mater. 2006, 18, 393. (4) Dobson, J. Gene Therapy Progress and Prospects: Magnetic Nanoparticle-Based Gene Delivery. Gene Ther. 2006, 13, 283. (5) Wickline, S. A.; Neubauer, A. M.; Winter, P. M.; Caruthers, S. D.; Lanza, G. M. Molecular Imaging and Therapy of Atherosclerosis with Targeted Nanoparticles. J. Magn. Reson. Imaging 2007, 25, 667. (6) McCarthy, J. R.; Kelly, K. A.; Sun, E. Y.; Weissleder, R. Targeted Delivery of Multifunctional Magnetic Nanoparticles. Nanomedicine 2007, 2, 153. (7) Weitschies, W.; Kosch, O.; Mo¨nnikes, H.; Trahms, L. Magnetic Marker Monitoring: An Application of Biomagnetic Measurement Instrumentation and Principles for the Determination of the Gastrointestinal Behavior of Magnetically Marked Solid Dosage Forms. AdV. Drug DeliVery ReV. 2005, 57, 1210. (8) Li, L.; Fan, M.; Brown, R. C.; van Leeuwen, J. H.; Wang, J.; Wang, W.; Song, Y.; Zhang, P. Synthesis, Properties, and Environmental Applications of Nanoscale Iron-Based Materials. Crit. ReV. EnViron. Sci. Technol. 2006, 36, 405. (9) Huber, D. L. Synthesis, Properties, and Applications of Iron Nanoparticles. Small 2005, 1, 482.

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ReceiVed for reView July 27, 2007 ReVised manuscript receiVed February 21, 2008 Accepted February 24, 2008 IE071030E