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Ind. Eng. Chem. Res. 2009, 48, 352–360
In situ synthesis of TiO2-functionalized metal nanoparticles David M. King,† Yun Zhou,† Luis F. Hakim,† Xinhua Liang,† Peng Li,‡ and Alan W. Weimer*,† Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309, and Department of Earth and Planetary Sciences, UniVersity of New Mexico, Albuquerque, New Mexico 87131
The ability to prepare and functionalize passivated nanosized metal powders was demonstrated using TiO2 films that were deposited via atomic layer deposition (ALD). Metal nanopowders were synthesized from the dehydration and subsequent decomposition of bulk quantities of metal oxalates in a fluidized-bed reactor. The gas-phase TiO2 ALD coating process was used to passivate these particles in situ, which alleviated the need to expose the oxygen-sensitive materials to air. Metal oxalate size reduction was achieved using a cryogenic milling process that reduced the micrometer-sized oxalate feed powder and yielded metal nanopowders with an average primary diameter of ∼50 nm. The metal oxalates that have received the most attention are those of iron, nickel, cobalt, and copper. Al2O3-based ALD films have been deposited on each of these metals, whereas TiO2:metal nanocomposite powder research has been performed here and is the basis for this work. 1. Introduction The ability to fabricate nanosized metal particles requires a passivation step to prevent degradation of the core material. Most metal particles react in air to form a native oxide. This oxidation reaction can be fast if the ratio of surface area to volume is high, as it is with nanosized particles. Metal nanoparticles can be synthesized via the decomposition reaction of a metal salt, and the metal oxalate pathway has received significant attention.1–7 Metal nanoparticles must be passivated by a film that serves as an oxygen barrier, which can prevent potentially hazardous explosions when synthesizing and processing bulk quantities of powder.8–10 The properties of nanosized materials can be vastly different than their bulk-material counterparts.11,12 With metal nanoparticles, however, an opportunity to study or utilize these small-grain-size properties diminishes or may be nonexistent, because of the inability to passivate the high-surface-area oxide-free nanostructures.13 Both organic and inorganic in situ passivation techniques have gained attention recently, although most are only available with liquidphase deposition techniques, which may be inconvenient for powdersthatareintendedforuseindryorsolid-stateapplications.14,15 Titanium dioxide (TiO2) is a medium-bandgap (∼3.0-3.2 eV) semiconductor material that finds extensive usage in powder form, because of its photostability, high refractive index, biocompatibility, ultraviolet (UV) absorptivity, relative chemical inertness, and inexpensive manufacture.16–18 TiO2 is also a wellknown photocatalyst with a large propensity to photodegrade surrounding media, because of free-radical generation in the presence of UV light irradiation.19,20 Advanced nanostructured assemblies that consist of metal:TiO2 heterojunctions have been shown to offer improvement in photodegradation efficiency, relative to TiO2 nanopowders alone.18 Atomic layer deposition (ALD) is a technique that has been demonstrated recently as a viable strategy to deposit nanothick Al2O3 passivating layers on metal nanoparticles.21 ALD is an analog of chemical vapor deposition (CVD), which is a process that administers reactive vapor-phase precursors to a system * To whom correspondence should be addressed. Tel: 1-303-4923759. Fax: 1-303-492-4341. E-mail address: alan.weimer@ colorado.edu. † Department of Chemical and Biological Engineering, University of Colorado. ‡ Department of Earth and Planetary Sciences, University of New Mexico.
simultaneously.22,23 ALD is a gas-phase adsorption technique that relies on self-limiting surface reactions to construct precision-thickness films.24 This coating technique was first developed for use in the semiconductor industry as an answer to the demand for thinner gate insulating films caused by shrinking dimensions, in accordance with Moore’s Law.25 Many film types have been deposited using ALD methods, but insulating metal oxide films have received the most attention, because of the desire for high-k dielectric materials that can be deposited with precision thickness control on a variety of metal or ceramic surfaces.22 ALD films can also act as effective barrier layers to inhibit metal migration across layers, which would be detrimental to chip performance, whereas the dielectric films, which are typically amorphous metal oxides such as Al2O3, operate as effective oxygen barriers.26,27 Most TiO2-based materials are not known to be good oxygen barriers, because of their normally crystalline structure in bulk materials. Amorphous TiO2 films, on the other hand, may provide some additional resistance to oxygen diffusion. Relatively inexpensive processes for the growth of metal oxides can be developed on the industrial scale, because of the availability of low-cost metal chloride precursors, process developments in the manufacture of organometallic precursors, and an abundance of oxygen-containing precursors that are readily available, including H2O and O2. Two ALD routes to deposit amorphous TiO2 films on surfaces with precision thickness utilize titanium tetrachloride (TiCl4) or the organometallic, titanium tetraisopropoxide (Ti(OCH(CH3)2)4, TTIP) at 100 °C.28–33 The binary CVD reactions are as follows: TiCl4 + 2H2O f TiO2 + 4HCl
(1)
Ti(OCH(CH3)2)4 + 2H2O f TiO2 + 4CH(CH3)2)OH
(2)
Each of these binary CVD reactions can be divided into two half-reactions that occur solely on a surface [S], at active sites denoted by asterisks in the written reaction to define TiO2 ALD using the precursors shown in eq 2: [S] : (OH)*x + Ti(OCH(CH3)2)4 f * + x · CH(CH3)2OH(3) [S] : (O)xTi(OCH(CH3)2)4-x
[S] : (O)xTi(OCH(CH3)2)4 + 2H2O f [S] : TiO2 : (OH)*x + (4 - x) · CH(CH3)2OH(4)
10.1021/ie800196h CCC: $40.75 2009 American Chemical Society Published on Web 07/03/2008
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The sequential application of the reactions in eqs 3 and 4 can produce atomic-layer-controlled TiO2 deposition using the organometallic TTIP precursor and H2O, which evolves isopropanol vapor as the reaction product. Analogous half-reactions can be written for the chloride route to TiO2 ALD film growth, with each evolving hydrochloric acid vapor. Abatement systems for gaseous HCl byproducts are readily available and commonly used in industrial-scale chloride processes. Each half-reaction exhibits self-limiting growth behavior under proper operating conditions, because like-kind ligands do not react with each other.34 These types of sequential surface reactions have been studied extensively with Fourier transform infrared (FTIR) vibrational spectroscopy, growth on a quartz crystal microbalance, and other in situ techniques, and they have shown to be self-limiting. Therefore, there is a finite number of reactive surface sites available, and the concentration of these sites can be thought of as a nucleation site pattern. Equations 3 and 4 represent the “A” and “B” half-reactions, respectively, where each ALD cycle consists of one AB sequence. The growth rate of each TiO2 ALD cycle is dependent primarily on the operating temperature and pressure, but also has a tendency to decrease as the precursor size increases, because of steric hindrance at the substrate surface. TiO2 growth rates using these precursors are typically in the range of 0.15-0.40 Å/cycle, with the smaller, more-reactive chloride precursor offering faster growth. Another method to increase the ALD-based growth of metal oxide layers is to increase the oxidation potential of the oxidant. This method may also be utilized to lower operating temperatures. Concentrated H2O2 (50% in H2O) can be used to increase the oxidation potential of the second half-reaction significantly and has been done so here with each of these precursors.28 TiO2 growth rates can be increased from 0.15 Å/cycle to upward of 0.4 Å/cycle, using TTIP as the titanium-containing precursor, whereas TiO2 growth rates for TiCl4-H2O2 processes can be increased from 0.4 Å/cycle up to 0.6 Å/cycle, depending on the particle surface area, morphology, and degree of saturation of the doses.33 Switching to a stronger oxidant is usually a measure that is taken if the deposition temperature must be reduced, as in the case of operating too near the decomposition temperature of the metalcontaining species.35 ALD is not a line-of-sight dependent process. Any exposed particle surface with the ability to support functional groups can be coated.22 At high turnover frequencies, which are characteristic of well-fluidized beds, the entering precursor molecules will be exposed to available nucleation sites prior to escaping from the top of the bed, until a certain stochastic surface conversion is attained.36 The surface conversion at the point of breakthrough is a function of flow rates, precursor reactivity, and nucleation site density. It has been demonstrated that nanoparticles fluidize as micrometer-sized aggregates, but the aggregation tendency is dynamic in nature.37–39 Primary particles are continuously shed from their aggregates and are up taken by others, thereby exposing the entire particle surface to reactive vapor-phase precursors at some point throughout the dosing step. It has been shown that, when dosing time requirements increase, because of the need to coat large overall surface areas, precursor adsorption instability can occur and ultimately prohibit ALD growth on nanopowder surfaces.33 This observation has only recently been reported, despite ALD techniques having been developed ∼40 years ago.40 Because nanopowder coating processes in laboratory-scale fluidized beds can possess surface areas that are several orders of magnitude greater than those found in wafer coating processes in the semiconductor industry, the dose time requirements increase
significantly from fractions of a second to periods on the order of minutes. Adsorption/desorption kinetics can allow for the deterioration of adsorbed monolayers when using bulky precursors, or while operating near the decomposition temperature of relatively unstable precursors. Industrial-sized reactors would be scaled with the cross-sectional area to achieve desired precursor flow rates and powder manufacturing throughput, and as such, processing times do not scale directly with bed mass. ALD of nanothick TiO2 films on bulk quantities of metal powders is presented here to augment the magnetic properties of the core materials with those inherent to TiO2. Iron and nickel powders are synthesized from their respective hydrated oxalatebased powders, which can be cryogenically milled to achieve size reduction on the nanoscale. Amorphous TiO2 ALD films are beneficial here to fabricate superior oxidation-resistant metal nanoparticles, which, in turn, can be used for magnetic TiO2 particles in the as-deposited state, or can be further processed into metal titanates or homogeneous TiO2:metal heterojunctionladen composite structures via standard metallurgical techniques. A nanostructuring process to apply TiO2 films on metal nanoparticle surfaces would be technologically desirable to a variety of industries, as long as a feasible pathway toward scaleup exists. 2. Experimental Methods A fluidized-bed reactor (FBR) was used to dehydrate and decompose hydrated metal oxalate powders under reduced pressures in an observable and controlled fashion. A schematic of the reactor is shown in Figure 1, and the details of this configuration have been described elsewhere.36 Mechanical agitation was provided by a magnetically coupled stirring unit, which was used to assist powder fluidization. Proper fluidizing conditions are imperative during the decomposition portion of the synthesis process, because fine metal powders can easily sinter together at elevated temperatures. The hydrated oxalates of iron (FeC2O4), nickel (NiC2O4), and cobalt (CoC2O4) were purchased in the powder form from Sigma-Aldrich, and hydrated copper oxalate (CuC2O4) was obtained from Alfa Aesar. The primary particle size of the unprocessed metal oxalates was in the range of 1-10 µm. The powder masses used for each experiment were 5-10 g in a 2.5-cm-diameter FBR and 20-60 g in a 6.3-cm-diameter FBR. Dehydration steps occurred at 150 °C, and the decomposition temperatures varied from 230 °C (copper oxalate) to 430 °C (iron oxalate). To aid the surface reduction, a 30% H2 in N2 mixture was used as the fluidizing gas throughout the decomposition of the various oxalate powders. It has been demonstrated elsewhere that this ratio yields optimal results in a balance between decomposition temperature, sintering prevention, and surface reduction rate.21 The ultimate expected mass losses for the full conversion hydrated oxalates of cobalt, copper, nickel, and iron to their metal nanopowders are 53.2%, 60.4%, 67.9%, and 69.0%, respectively. After the dehydration and subsequent decomposition steps were complete, the temperatures were returned to 100 °C, which was the operating temperature for TiO2 deposition. The same FBR was then used to deliver reactive ALD precursors to the surfaces of primary powders, without exposing the powders to oxygen in the atmosphere. Two different delivery strategies were required to administer the two titanium-containing precursors, because the room-temperature vapor pressure of TTIP is much lower than that of TiCl4. A bubbler was used to deliver a constant flow rate of TTIP effectively to the reactor system, using N2 as the entraining gas, whereas TiCl4 utilized the driving force of its vapor pressure alone. Pressures were
354 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009
Figure 1. Schematic of the fluidized-bed reactor (FBR) used for TiO2 film deposition on metal nanopowders.
easily attainable using these strategies to provide a pressure drop across the particle bed that was sufficient to promote and maintain the fluidizing conditions. The benefit of this FBR configuration is that it allows for gaseous precursors to flow into and out of the system in a continuous fashion via a porous distributor plate and porous filters, respectively, while preventing particles from being permanently elutriated out of the system. The demonstrated notion that nanoparticles fluidize as micrometersized aggregates prevents the primary particles from escaping through the ∼10 µm porous filters, such that no powder is lost during the coating process. A residual gas analyzer (RGA) with an internal quadrupole mass spectrometer (QMS200, Stanford Research Systems) was used for real-time exit stream gas analysis. This is an ideal technique for ALD reactors in a continuous-flow system, because all reactants and products, other than the chemisorbed metal oxide films, are in the gas phase. The m/z peaks of interest (i.e., primary and fragmentation peaks) for the precursors were 41, 43, and 45 for TTIP; 35 and 36 for TiCl4; and 17, 18, 33, and 34 for H2O2. Higherorder TTIP fragments were only sporadically visible, from a combination of the high volatility of the precursor ligands and the fact that the incoming TTIP stream concentration was low, because of dilution by N2 from the bubbler. Reaction products were visible at m/z ) 41, 43, and 45 for TTIP-H2O2 processes, corresponding to isopropanol, or CH(CH3)2OH, and m/z ) 17 and 18 for runs that utilized TiCl4-H2O2 and generated HCl vapor. H2O2 decomposition was observed, to a small degree, between oxidizing steps, as evidenced by a release of diatomic oxygen (m/z ) 32) at the onset of the H2O2 dose. Extreme caution must be used when handling concentrated H2O2, and material compatibility must be ensured prior to operating processes that utilize this strong oxidant. Particle size reduction was performed via a solvent-based cryogenic milling technique, using a Union Process 01-HD attritor mill. The milling media consisted of 2–mm MgOstabilized ZrO2 spheres. Before the milling process, ethanol was mixed with the oxalate powders and milling media, in a volume ratio of 2:1:4, to form a viscous slurry. The solvent-based method for particle grinding changes the attrition mechanism from collision to that of friction, which has been demonstrated to be more effective in milling ultrafine particles.21,41,42 The mixture was milled at a rotational speed of 600 rpm. Liquid N2 was added to the attrition tank in regular intervals throughout the milling process, which is a critical feature of this method.
The liquid N2 can eliminate the heat generated during the milling process, and it also can generate a low-temperature milling situation to increase the brittleness of the oxalate particles. Higher powder brittleness helped to achieve smaller particle sizes. To study the optimum milling result, different attrition times (ranging from 3 h to 15 h) were performed. After grinding, the mixture in the attrition tank was removed and dried at 100 °C, and the milling media was separated using a 1.4–mm sieve tray. The particle size reduction effect was analyzed by measuring the Brunauer-Emmett-Teller (BET) surface area, using a Quantachrome Autosorb 1 system, with N2 as the adsorbate species. A thermogravimetric analysis (TGA) system (Theta Industries, Inc.) was used to obtain the appropriate dehydration and decomposition temperature ranges. The powders were held using a ZrO2 crucible suspended by a platinum wire. The chamber was evacuated using a vacuum pump to minimize the amount of oxygen present in the system prior to beginning each experiment. Transmission electron microscopy (TEM) analysis was performed using a Model CM10 Philips transmission electron microscope operating at 100 kV, and high-resolution transmission electron microscopy (HRTEM) was conducted using a JEOL Model 2010F 200 kV Schottky field-emission TEM system. The particles were deposited directly onto copper grids with a holey carbon overlay film. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used to measure the composition of ALD films. One hundred to two hundred milligrams of the powders were placed into vials that contained 10 mL of H2SO4 with 1 mL of 5% Triton X-100 surfactant, maintained at 95 °C using a heating block. The particles were allowed to digest for ∼4 h with periodic agitation, which was sufficient time for complete dissolution of the TiO2 films and the metal substrate. Samples were removed from the heating block, cooled, and diluted to 50 mL with H2O; 1 mL of this solution was then drawn off the top of the digest tubes and further diluted to 10 mL with H2O. The dissolved titanium and iron or nickel content, expressed in terms of parts per million (ppm), was measured using an Applied Research Laboratories model ICP-AES 3410+ system. 3. Results and Discussion The in situ thermal decomposition of metal oxalates was performed on small samples in a TGA system and on larger
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Figure 3. Bed pressure response of gaseous products released during the synthesis of iron metal powder from iron oxalate dihydrate in an FBR.
Figure 2. SEM images of the unprocessed oxalates of (a) nickel and (b) iron.
batches in laboratory-scale FBRs. The micrometer-sized feed powder varied in morphology across metal types, with nickel and copper oxalate more frequently being comprised of round or spherical structures, whereas the iron and cobalt oxalates were relatively rodlike with high aspect ratios. Scanning electron microscopy (SEM) images of the nickel and iron oxalate feed material are shown in Figure 2. The use of TGA allowed for exploration of the kinetics of the dehydration and decomposition steps, as well as the exploration of various H2:N2 ratios that maximized the conversion to metal nanopowder. In-depth discussions of the decomposition kinetics of various metal oxalate powders can be reviewed elsewhere.43–49 The equations that describe the dehydration and decomposition of the dihydrated metal oxalates (i.e., Fe, Ni, and Co) are as follows, where M represents the metal: dehydration
M(CO2)2 · 2H2O 98 M(CO2)2 + 2H2O
(5)
decomposition
M(CO2)2 98 M + 2CO2
(6)
During the dehydration step in eq 5, conjugated water molecules were desorbed from the surface of the particles. After the water was purged from the system, the reactor temperature was increased to the appropriate decomposition temperature, which was predetermined using a much smaller sample in the TGA apparatus and a constant temperature ramp rate. It was impera-
tive that the presence of water was minimized prior to further increasing to the decomposition temperature, because residual water can react with the decomposed metal nanoparticles to form the metal oxide. Equation 6 shows the decomposition of the nonhydrated metal oxalate, which yields the metal powder and two moles of carbon dioxide. An FBR with an attached mass spectrometer was used to synthesize bulk quantities of metal powder from their metal oxalates. With this unique reactor configuration, gases that were administered to the system or were evolved via chemical reactions can be monitored throughout the synthesis and coating process. The pressure drop across the bed (P2 - P1 from Figure 1) was measured to determine the proper carrier gas flow rate to fluidize the particle bed effectively, and the temperature was then increased at the desired ramp rate. Because the dehydration and decomposition reactions evolved water and carbon dioxide, respectively, the pressure at the outlet of the reactor was expected to increase significantly during each step. The pressure response observed during the synthesis of iron from its oxalate powder is shown in Figure 3. The base pressure for each of the steps was normalized to 1 Torr (or, for clarity, 133 Pa) as the pressure drifted upward with increasing operating temperatures. The temperature was held at 150 °C for a significant amount of time, to allow the reaction in eq 5 to proceed to completion. After the bed pressure had returned to its original steady-state value, which signified that H2O generation was complete, the temperature was again increased at the desired ramp rate up to the decomposition temperature. The pressure rose due to CO2 generation from the iron oxalate, as eq 6 suggests. After decomposition, the temperature was reduced to 100 °C to prepare for the TiO2 ALD coating process. It has been shown in previous work that the decomposition of iron oxalate partially proceeds through the mixed oxidation state iron oxide, or Fe3O4, prior to ultimately being fully reduced to iron metal powder.21 This oxalate system is much more complex than other metal oxalates, as those of Ni, Cu, and Co did not seem to require the reduction of an oxide state to obtain the metal powder. The TGA mass loss curve showed a significantly slower reaction that occurred after the majority of the mass was lost due to decomposition, as shown in Figure 4. At the point of this “knee” in the mass loss trace, the total mass lost was 57%, which corresponded to complete conversion of FeC2O4 · 2H2O to Fe3O4. The temperature ramp rate was 10 °C/ min here, and the conversion to Fe3O4 occurred over a 10-min time span. In previous work, the reaction mechanism changed after 60% mass loss and powder XRD verified the combined
356 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009
Figure 4. Mass loss curve with temperature during the dehydration and decomposition of a small quantity of iron oxalate powder in a TGA apparatus.
Figure 5. Mass spectrometry plot of the H2O and CO2 evolved during the synthesis of nickel metal powder from nickel oxalate dihydrate. 21
presence of both Fe3O4 and elemental iron. A ramp rate of 5 °C/min was used in that work, and the mechanism changed after 20 min. It was previously unclear whether the oxygen present in the Fe3O4 phase was due to uptake of oxygen from water or carbon dioxide emitted from the dehydration and decomposition steps, i.e., decomposition to pure iron with localized oxidation to Fe3O4. With the current results, it is much more plausible that all iron oxalate particles proceed through the Fe3O4 state and full reduction is required to obtain pure iron. Equations 5 and 6 can be expanded to reflect the observed mechanisms for iron oxalate decomposition as follows: dehydration
Fe(CO2)2 · 2H2O 98 Fe(CO2)2 + 2H2O
(7)
decomposition
3Fe(CO2)2 98 Fe3O4 + 4CO + 2CO2
(8)
reduction
Fe3O4 + 4H2 98 3Fe + 4H2O
(9)
Care must be taken to further reduce the powder to pure iron after the CO2 evolution reaction is complete. The complete conversion to iron nanopowder is shown in Figure 4, as the ultimate mass lost during this TGA run was 69%. The mass spectrometry data obtained throughout the synthesis of nickel metal powder from nickel oxalate are shown in Figure 5. The water signal increased slowly with temperature prior to the dehydration temperature, likely because of the removal of adsorbed water throughout the reactor system. A significant rise in the water peak was observed at the onset of dehydration,
Figure 6. Mass spectrometry plot of (a) TiCl4 and (b) H2O2 surface reactions that generate HCl as a byproduct.
which then increased and subsequently decreased normally. As the deaeration of water from the bed continued, the temperature was increased to the decomposition temperature that was determined from TGA experiments. This temperature was reached at approximately the 2.5 h mark on the plot, and the evolution of CO2 proceeded rapidly. The reactor temperature was increased by 10 °C after a maximum CO2 signal was obtained, corresponding to the five local maxima observed on the plot. Each increment increased the oxalate decomposition kinetics, and the reaction product was generated at a much faster rate. The reactor was then cooled to the TiO2 film deposition temperature after the decomposition approached completion. ALD of Al2O3 films has been performed on ultrafine iron powder in previous work, and these amorphous insulating films served as effective oxygen barriers that prevented oxidation of the core powder.21 TiO2 films were deposited here to fabricate multifunctional barrier films on magnetic metal powders. Two TiO2 ALD film synthesis routes were used, based on the titanium-containing precursors shown in the CVD reactions in eqs 1 and 2. TiCl4-H2O2 was used at 100 °C to deposit TiO2 on iron powder. The reaction product of these precursors is HCl, which is a very hazardous and corrosive material. Special care is required when generating large quantities of this strongly acidic gaseous byproduct. In situ mass spectrometry was used to monitor the evolution of HCl, as well as the breakthrough of unreacted TiCl4 or H2O2 throughout each entire ALD cycle. The mass spectrometry response for one representative ALD cycle is shown in Figure 6. Figure 6a clearly shows that the breakthrough of unreacted TiCl4 occurred after 180 s, but the dose was continued through to 300 s, solely for academic purposes. A relatively high surface conversion was achieved during this half-reaction, which can be quantified by the area
Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 357
Figure 8. XRD spectra of TiO2-coated nickel and iron particles decomposed from their oxalates.
Figure 7. HRTEM images and EDS spectra of TiO2-coated (a) nickel and (b) iron particles.
under the cumulative generated HCl curve at the point of breakthrough, relative to the total amount produced. A clear indication of breakthrough was not observed during the H2O2 dose; however, the oxidizing agent was preferentially underdosed to prevent the fully reduced iron particle surfaces from having any unnecessary exposure to oxygen. TTIP-H2O2 binary sequence reaction chemistry was used at 100 °C to deposit TiO2 onto nickel and iron powders that were decomposed from their oxalate powders in the as-received state. The mass spectrometer was used to monitor the reaction progress via the generation of isopropanol and its fragmentation species. Precursor breakthrough was observed prior to the completion of each half-reaction, which signified that full surface conversion was not achieved at that point. This was expected, because the reactivity of the TTIP precursor is relatively low and the inlet stream was also diluted by N2 from the delivery mechanism. Because ALD half-reactions are self-limiting and self-terminating when operated under ideal conditions, subsaturating doses will directly correspond to lower stochastic growth rates per cycle. Thus, an economic tradeoff is available, because inexpensive precursors with low reactivity can be overdosed without significant cost implications, while more-expensive precursors that are engineered for higher reactivity may be preferentially underdosed. A strategy that involves precursor underdosing requires additional cycles to achieve a desired film thickness, but complete uptake of the precursor occurs prior to the point of unreacted precursor breakthrough, because of the apparent plug-flow nature of the FBR. Highly reactive precursors have demonstrated almost-complete surface conversions at the point of breakthrough in other ALD work performed in an FBR.35,36 TiO2 films were deposited on the surfaces of primary nickel particles using 100 TTIP-H2O2 cycles at 100 °C. The ICP-AES elemental content of the TiO2 film was 4.1% of the TiO2:Ni nanocomposite system. An HRTEM image and the EDS spectrum of the ceramic-metal nanocomposite structure are shown in Figure 7a. The film thickness was measured to be 3 nm, which corresponded to a stochastic growth rate of 0.3 Å/cycle. This is less than the maximum observed growth rate of 0.4 Å/cycle, which was achieved when depositing TiO2 films
onto 65-nm ZnO nanoparticles in another work.33 The EDS spectrum also confirmed the presence of both nickel and titanium. An HRTEM and corresponding EDS spectrum is also shown for the TiO2:Fe nanocomposite system in Figure 7b. The presence of both metals was apparent using energy-dispersive spectroscopy (EDS), as was the ∼9 nm film, which was deposited using 150 TiCl4-H2O2 cycles at 100 °C. The growth rate was 0.6 Å/cycle, and TiO2 growth using these precursors at 100 °C has never been reported. The elemental content of the film in the TiO2:Fe composite was 18.4%. Residual chlorine is also present to a significant degree, primarily because of the low growth temperatures. After a full TiO2 film has been established, the H2O2 dosing time should be increased, to minimize the presence of unreacted -Cl ligands. The XRD spectra for the TiO2-coated nickel and iron powders are shown in Figure 8. The decomposition of nickel oxalate in the FBR clearly did not run to completion, as evidenced by the two peaks at low diffraction angles, which correspond to the dehydrated nickel oxalate material.50 Nonetheless, despite the ∼90% conversion from oxalate, no peaks were present for NiO, which was the expected oxide form of the metal. There were no kinetic or thermodynamic limitations found during the nickel oxalate decomposition performed in the TGA system, because that procedure resulted in pure nickel metal. When comparing these results to the mass spectrometry data recorded during the decomposition (Figure 5), the decrease in CO2 evolution was relatively abrupt, which indicated that the soak time at the decomposition temperature was inadequate. The primary reason for not maintaining the reactor temperature above the decomposition temperature for an extended period of time was to avoid sintering of the ultrafine metal powder. Since the TiO2 ALD films were amorphous at this deposition temperature, no TiO2 signal was present, as expected. A portion of the TiO2:Ni composite powder was thermally annealed in air to 700 °C, to determine how the film would interact with the core metal particle. Based on the XRD spectrum from the annealed material, the powder consisted of a combination of nickel titanate, the mixed-metal oxide of nickel and titanium, and nickel oxide. The ratio of nickel titanate to nickel oxide was congruent with complete uptake of the total TiO2 present in the TiO2:Ni composite powder. The iron oxalate decomposition did run to completion in the FBR, although the XRD spectrum showed a significant final presence of Fe3O4. The presence of oxygen in the core iron particle was due to the deposition process itself, specifically when H2O2 was dosed during the first few ALD cycles. Because
358 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 Table 1. Particle Size Reduction Efficiency of Cryogenic Milling Process with Timea milling time [h]
surface area [m2/g]
particle size [µm]
Zr contaminationb [%]
0 3 6 9 15
0.6 12.9 20.2 22.0 28.8
4.69 0.20 0.13 0.12 0.09
0.00 0.03 0.15 0.22 0.48
a Particle sizes represent the spherical equivalent diameter based on the specific surface area. b As determined using ICP-AES analysis.
the growth rate of TiO2 ALD was relatively low, on the order of 0.5 Å/cycle, 5-10 ALD cycles may be required to provide one monolayer of surface coverage. There was ample opportunity for the core iron particle to be partially oxidized during these first cycles by the concentrated H2O2. This was not observed when the nickel metal surfaces were exposed to H2O2, because the oxidation of nickel to NiO is much slower at 100 °C, relative to the near-instantaneous surface reaction of Fe to Fe3O4.13 An optimal strategy would be either to deposit a tie layer of a fast-growing ALD metal oxide on the iron surface, such that one monolayer can be deposited by the first half-cycle prior to oxygen exposure, or to deposit a nonoxide on the iron particle surface. For this system, a titanium nitride (TiN) tie layer may be preferable, in which case a TiN:TiO2 graded multilayer coating could be fabricated for a more-congruous transition to TiO2. The flexibility of the particle ALD system allows for the
Figure 10. TEM images of iron particles synthesized from decomposed iron oxalate powders (a) in the as-received state and (b) after cryogenic milling.
Figure 9. SEM images of iron oxalate powder after (a) 3 h and (b) 9 h of cryomilling.
deposition of a wide array of film types, including metal oxynitrides with tunable O:N ratios and mixed-metal ceramic films with tunable metal ratios. Particle size reduction of iron oxalate powders was performed using the cryogenic milling method described previously.21 The size reduction effect with cryogenic milling time can be seen in Table 1, with the original feed powder starting at a surface area with an equivalent particle diameter of ∼4.7 µm. After the 15 h milling time process was complete, the average equivalent diameter was reduced to
